Unfolding the structural features of NASICON materials for sodium‐ion full cells

Abstract

Sodium‐ion batteries (SIBs) are potential candidates for the replacement of lithium‐ion batteries to meet the increasing demands of electrical storage systems due to the low cost and high abundance of sodium. Sodium superionic conductor (NASICON) structured materials have attracted enormous interest in recent years as electrode materials for safer and long‐term performance of SIBs for electric energy storage smart grids. These materials have a three‐dimensional robust framework, high redox potential, thermal stability, and a fast Na⁺‐ion diffusion mechanism. However, NASICON has low intrinsic electronic conductivity, which limits the electrochemical performance. This review describes the structural features of NASICONs to illustrate the ion storage mechanism and electrochemical performance of SIBs. Details of the NASICON crystal structure, the affiliated Na⁺‐ion diffusion mechanism, morphology, and electrochemical performance of these materials in sodium‐ion half‐cells as well as full cells are described. In addition to the application as electrode materials, the use of NASICONs as solid electrolytes is also elaborated in solid‐state SIBs. Based on these aspects, we have provided more perspectives in terms of the commercialization of SIBs and strategies to overcome the limitations of NASICONs. Hence, this review is expected to provide the researchers of energy storage with an in‐depth understanding of NASICON materials with the knowledge of structural features, which will provide a new avenue on the practicality of SIBs.

INTRODUCTION

Because of the rapid digitalization and increasing energy demand in energy sectors, along with the steady depletion of fossil fuels, there is an urgent need to exploit renewable and clean energy resources to overcome the environmental pollution and energy crisis all over the world. Energy storage systems have been extensively developed as primary candidates in the past decades to assimilate intermittent renewable energy into robust, clean, and uninterrupted efficient electrical grids.¹ A rechargeable electrochemical storage system is the most promising candidate by virtue of its high energy conversion efficiency, response time, ramp rate, and longevity, with excellent integration to the grid. Lithium-ion batteries (LIBs) are primary batteries dominating the commercial markets of portable electronics and electric vehicles since their first commercialization in the 1990s² due to the high specific energy density and large output power. Lithium has a low density (0.53 g cm⁻³) as well as the lowest redox potential (E = −3.04 V vs. SHE) and small ionic radii (0.76 A). Despite these advantages, LIBs are facing high pricing issues with time due to the scarcity of Li resources in the Earth's crust (20 ppm) and the uneven geographical distribution and toxicity of Li metal with its electrolytes, which hinders their uses in large-scale applications in the future.^3–6

Sodium-ion batteries (SIBs) are potential candidates to replace LIBs in energy storage devices due to their presence in the same periodic group, similar chemistry, and inexhaustible resources of Na metal in the Earth's crust (23,000 ppm).⁷ Despite these advantages, SIBs are facing problems of slow kinetics, insufficient specific power, low specific energy, and short cycling. As compared to Li, Na metal has a high molecular weight (23 g mol⁻¹ compared to 6.9 g mol⁻¹ for Li), a higher standard electrode potential (−2.71 V vs. SHE as compared to −3.02 V vs. SHE for Li), and a larger radius (1.02 Å as compared to 0.76 Å for Li), which pose limitations in various smart applications.⁸ To overcome these drawbacks, there is an urgent need to find suitable electrode materials with excellent structural and electrochemical properties.^9–11

Batteries consist of the main components: an anode, a cathode, an electrolyte, and a separator. It is generally accepted that it is important to determine the electrochemical properties, specific energy density, specific power, and cyclic performance of cathode materials for estimation of the performance and cost of SIBs. Cathode materials are generally characterized into three categories: layered metal oxide,^12–17 Prussian blue analogs,^18–25 and polyanions.^26–30 Layered metal oxides have a high specific capacity of 120–220 mAh g⁻¹. However, the operating potential is low, ranging from 2.6 to 3.2 V. Due to this drawback, they suffer from irreversible phase transition, which leads to a deterioration in their cyclic performance and energy density. Prussian blue analogs show reasonable voltage window and specific capacity, but they have thermal instability and lattice defect issues. Polyanion compounds have a three-dimensional (3D) structural arrangement A_xM_y(XO₄)_n (A = Na, Li, K), (M = transition metal V, Fe, Mn, Co, etc.) and (X = P, S, Si, Mo, etc.). It consists of a series of a polyanion tetrahedron unit (XO₄)_n and their derivatives, which have strong covalent bonds in the MO_x polyhedral, providing considerable structural and thermal stability, which allows small volumetric expansion and phase transition during the insertion/deinsertion of Na⁺ ions. The sodium superionic conductor (NASICON) belongs to the class of polyanionic compounds that have a 3D framework with excellent structural stability and strong covalent bonding with the small phase transition and volumetric expansion during cycling, which eventually leads to high safety and long cyclic stability. NASICON also has high thermal and oxidation stability due to strong covalent bonding with exceptional ionic conductivity for Na storage. Therefore, NASICON structured materials are considered promising cathode materials for large-scale grid applications.^31–37 However, practical applications are still limited because of the poor intrinsic electronic conductivity (10⁻⁶–10⁻⁸ S cm⁻¹).^38,39 Several methods have been used to improve the electronic conductivity of NASICON materials by combining with carbon-based materials,^40–42 elemental doping,^43–47 and designing 3D porous architectures.^48–51

In this review, we focus on the recent progress and limitations of NASICON structured electrode materials, which include phosphates (Na_xM₂(PO₄)₃), fluorophosphate (Na_xM₂(PO₄)₂F), binary transition-metal phosphates (Na_xMMʹ(PO₄)₂F), and mixed phosphates (Na_xM₂(PO₄)(SO₄)₂ and Na_xM₂(PO₄)₂P₂O₇) for SIBs. As only an electrochemical analysis is not enough to understand the performance of electrode materials, there is an additional focus on the Na storage mechanism, morphology, and crystal structure, coupled with different strategies, to improve the electrochemical performance. The full cell performance of NASICON materials for SIBs is important to driving the lab technology to commercialization. Hence, herein, we have presented a detailed discussion on the utilization of NASICONs in full cells. In addition, we report the best strategies used to tackle the challenges in the practical applications and prospects of commercialization of NASICON materials for SIBs. It is expected that this review will provide the reader with an understanding of NASICON structured materials for SIBs and ensure that in the future, they are considered potential candidates for large-scale grid applications^48,52 (Figure 1).

Figure 1. (A) Schematic illustration of NASICON materials and their features discussed in this article for SIB in an EES smart grid. (B) Number of publications on sodium-ion batteries, anode, cathode, and different NASICON materials (data were collected from the Web of Science). EES, electric energy storage; NASICON, sodium superionic conductor; and SIB, sodium-ion battery.

NASICON MATERIALS' CHARACTERISTICS

NASICON is a special type of polyanion compound that has been explored both as an electrode material and as an electrolyte after it was first reported, Na_1+xZr₂P_3−xSi_xO₁₂, as a solid electrolyte in 1976.⁵³ The formula of NASICON structured materials is Na_xMMʹ(XO₄)₃, where M and Mʹ are transition metals (V, Ti, Mn, etc.), X = P, S, and 0 ≤ x ≤ 4. It has a basic unit called a lantern, which consists of MO₆ octahedral interlinked with PO₄ tetrahedral units via corner-sharing, forming a tunable 3D structure with large tunnels for fast Na⁺-ion conduction. Na⁺ ions are distributed in two interstitial sites: M1 and M2. NASICON shows excellent structural stability, thermal stability, and high redox potential. One of the major concerns of NASICONs is their low intrinsic electronic conductivity, which limits their electrochemical performance. The intrinsic electrical conductivity and electrochemical performance can be enhanced by (1) combining with highly conductive carbon-based materials,^54–56 (2) doping with different elements,^57–59 and (3) nanostructuring, that is, by designing 3D architectures with high porosity and surface area, which provides fast electron transport^58–62 (Figure 2).

Figure 2. Schematic illustration of modification techniques to improve the electrical conductivity and electrochemical performance of sodium superionic conductor materials.

Thermodynamic stability and the voltage map of 28 single transition-metal NASICON as electrode materials for SIB using first-principal calculations with (M:Mʹ = 1:1) are represented in Figure 3. The average voltage range of NASICON is from 1.96 to 4.4 V. Nickel shows the highest theoretical redox voltage Na_xNi₂(PO₄)₃ (4.4 V), followed by Na_xNiCo(PO₄)₃ (4.36 V) and Na_xNiFe(PO₄)₃(4.34 V). There are new binary transition-metal phosphates that have not yet been reported that can be explored as electrode materials. Apart from Ni, Fe, and Co, Mn also shows high redox potential. The tunable structures provide exceptional ionic conductivity and stability, which improves the cyclic performance and rate capability, making NASICON structured materials potential candidates for next-generation electrode materials for grid applications.

Figure 3. Voltage profile of sodium superionic conductor NaxMMʹ(XO4)3 (M/Mʹ = transition metals (V, Ti, Fe, Cr, Co, Ni) representing redox pairs to voltage. Reproduced with permission: Copyright 2021, The Royal Society of Chemistry.63

NASICON CATHODE MATERIALS

To achieve high-performance SIBs for commercial applications, the investigation of suitable cathode materials is crucial. An ideal cathode material should have excellent structural and thermal stability, allowing reversible Na⁺-ion diffusion with minimal volume changes. Moreover, the cathode materials should be low-cost, abundant in nature, and synthesizable via scalable methods. NASICON cathode materials have the above-mentioned characteristics. Researchers have paid attention to the detailed investigation of various NASICON cathode materials for SIBs. Some of the notable NASICON cathode materials, including vanadium-based phosphates, iron-based phosphates, fluorophosphate, binary transition-metal phosphates, and mixed phosphates, are discussed below in detail (Tables 1 and 2).

Table 1 Overview of sodium superionic conductor structured electrode materials for sodium-ion batteries.

Materials	Synthesis method	Crystal structure	Space group	Morphology	Voltage (V)	Discharge capacity (mAh g−1)	Rate capability (mAh g−1)	Cyclic performance (retention, cycles, rate)	References
Cathode Na3V2(PO4)3
Na3V2(PO4)3/C	Solid-state reaction	Rhombohedral	R3C	Particles	2.7–3.8	93, 0.2 C	-	89%, 50, 0.2 C	[64]
Na3V2(PO4)3/C	Solution template based	Rhombohedral	R3C	Porous nanoparticles	2.3–3.9	114, 1 C	62, 40 C	50%, 30,000, 40 C	[65]
Na3V2(PO4)3/C	Spray-dried	Rhombohedral	R3C	Nanomicro holey spheres	2–4	106, 2 C	60, 50 C	50%, 5000, 50 C	[62]
Na3V2(PO4)3/C@rGO (reduced graphene oxide)	Freeze-drying	Rhombohedral	R3C	Porous nanoparticles array on rGO sheets	2.5–3.8	116, 1 C	103, 50 C	64%, 10,000, 100 C	[66]
Na3V2(PO4)3	Polymer-stabilized droplet template	Rhombohedral	R3C	Hierarchical mesoporous	2.3–3.9	100, 0.5 C	61, 100 C	99%, 10,000, 20 C	[67]
Na3V2(PO4)3/C	Membrane casting	Rhombohedral	R3C	Three-dimensional (3D) porous microspheres	2–4.3	116.1, 0.2 C	96, 30 C	80%, 10,000, 10 C	[49]
N-doped Na3V2(PO4)3/C	Solvothermal	Rhombohedral	R3C	3D hierarchical porous	2–4.3	117.2, 0.1 C	79.1, 200 C	73.4%, 10,000, 100 C	[58]
Na3Fe2(PO4)3
Na3Fe2(PO4)3/C	Solid-state reaction	Monoclinic	C2/c	Porous interconnected flakes	1.5–4.2	109, 0.1 C	75, 5 C	96%, 200, 1 C	[68]
Na3Fe2(PO4)3/C	Spray-dried	Monoclinic	C2/c	Porous flakes	1.5–3.5	100.8, 0.1 C	57.8, 10 C	90%, 1100, 5 C	[69]
Na3V(PO3)3N	Solid state	Cubic	P213	Particles	2.5–4.25	73, 1 C	-	67%, 3000, 1 C	[70]
Na3V(PO3)3N@NGO	Freeze-drying	Cubic	P213	Nanoparticles	3–4.25	78.9, 0.1 C	59.2, 30 C	75.9, 5000, 10 C	[71]
NaxM2−yMyʹ(XO4)
Na4MnV (PO4)3	Sol–gel	Trigonal	R3c	Worm-like nanoparticles	2.5–3.75	101, 1 C	90, 10 C	89, 1000, 1 C	[72]
Na3FeV (PO4)3	Sol–gel	Monoclinic	C12c1	Worm-like nanoparticles	2.5–3.75	103, 1 C	-	95, 1000, 1 C	[72]
Na4MnV (PO4)3	Solid-state reaction	Rhombohedral	R3c	-	2.5–4.3	156	-	-	[73]
Na3VCr (PO4)3	Sol–gel	Rhombohedral	R3c	-	2.5–4.3	93, 0.1 C	-	92%, 200, 0.5 C	[74]
Na3MnTi (PO4)3/C	Sol–gel	Rhombohedral	R3c	Nanoparticles	2.5–4.2	80, 0.1 C	40, 2 C	77.5%, 100, 0.1 C	[75]
Na3MnTi (PO4)3/C	Spray-dried	Rhombohedral	R3c	Hollow spheres	1.5–4.2	160, 0.2 C	129, 2 C	92%, 500, 2 C	[76]
Na3Mn1.15Ti0.85(PO4)3	Sol–gel	Rhombohedral	R3c	Particles	1.5–4.2	181, 0.1 C	100.4, 10 C	87.4%, 500, 1 C	[77]
rGO@Na3MnTi (PO4)3/C	Sol–gel	Rhombohedral	R3c	Nanoparticle-integrated rGO sheets	1.5–4.3	173, 0.1 C	92.4, 50 C	74.5%, 3500, 20 C	[78]
Na3MnTi (PO4)3/C @rGO	Spray-dried	Rhombohedral	R3c	Microspheres	2.5–4.2	114, 0.2 C	-	76.6%, 800, 1 C	[79]
Na4MnCr (PO4)3/C	Sol–gel	Rhombohedral	R3c	Nanoparticles	1.4–4.6	160, 0.05 C	70.3, 10 C	74%, 50, 0.2 C	[80]
Na4MnCr (PO4)3/C	Sol–gel	Rhombohedral	R3c	Nanoparticles	1.4–4.3	114, 0.1 C	43.2, 10 C	86.5%, 600, 5 C	[80]
Na4MnCr (PO4)3/C	Sol–gel	Rhombohedral	R3c	Particles	1.5–4.3	108.4, 0.1 C	70.8, 10 C	71.5%, 250, 2 C	[81]
Na3.41£0.59FeV (PO4)3	Sol–gel	Trigonal	R3C	Nanoparticles	1.5–4.4	119, 0.1 C	104, 10 C	94.73%, 700, 10 C	[82]
Na3.5Mn0.5V1.5(PO4)3/C	Solid-state reaction	Trigonal	R3C	Hydrangea hierarchical structure	2.5–3.8	113, 0.5 C	95.5, 50 C	76.6%, 5000, 10 C	[83]
Na2VTi(PO4)3	Sol–gel	Rhombohedral	R3c	Nanoparticles	1.5–4.5	147, 0.1 C	44, 20 C	77%, 500, 20 C	[84]
Fluorophosphates
Na3V2(PO4)2F3@rGO	Hydrothermal	Tetragonal	P42/mnm	Microcuboids	2–4.3	127.5, 0.2 C	73.7, 50 C	74.5%, 500, 5 C	[85]
Na3V2(PO4)2F3/C	Polyol Refluxing	Tetragonal	P42/mnm	Spherical nanoparticles	2–4.3	126.6, 1 C	105.9, 50 C	88%, 1500, 15 C	[86]
Na3V2(PO4)2F3@C/CNTs	Spray-dried	Tetragonal	P42/mnm	Microspherical	2–4.3	113, 1 C	85, 30 C	98.2, 300, 1 C	[87]
Na3V2(PO4)2F3@rGO	Freeze-drying	Tetragonal	P42/mnm	Microcubes	2–4.3	119, 0.5 C	71, 20 C	98%, 2000, 20 C	[88]
Na3V2O2(PO4)2F	Solvothermal	Tetragonal	I4/mmm	Nanoparticles	2.5–4.2	112, 0.5 C	73, 10 C	93.8, 1200, 2 C	[89]
Ti-doped Na3V2(PO4)2F3@C	Wet ball milling	Tetragonal	P42/mnm	Nano-sized globular particles	2–4.5	125, 0.2 C	104, 40 C	81.5, 200, 1 C	[90]
Ru-doped Na3V2O2(PO4)2F	Solvothermal	Tetragonal	I4/mmm	Hierarchical hollow microspheres	2–4.5	116.8, 0.5 C	84.1, 20 C	65%, 7500, 20 C	[91]
Graphene foam/Na3V2O2(PO4)2F	Solvothermal	Tetragonal	I4/mmm	Nanorod array	2–4.5	130, 0.5 C	80, 30 C	90%, 1000, 50 C	[92]
Carbon-coated Na3V2O2(PO4)2F	Scaleable liquid extraction separation	Tetragonal	I4/mmm	Multishelled hollow microspheres	2.5–4.3	111, 0.5 C	81, 15 C	70%, 3000, 15 C	[93]
Na3V2O2(PO4)2F/carbon black composite	Solid-state ball milling	Tetragonal	I4/mmm	Nanoparticles	2.5–4.2	141.9, 0.5 C	112.8, 20 C	98%, 10,000, 20 C	[94]
Na3V2O2(PO4)2F	Solvothermal	Tetragonal	I4/mmm	Hierarchical porous hollow spheres	2–4.3	127.1, 0.5 C	85.4, 50 C	62.2%, 2000, 20 C	[95]
N/p-doped Na3V2O2(PO4)2F/C	Hydrothermal	Tetragonal	I4/mmm	Microspheres	3–4.5	128, 0.5 C	122, 2 C	81.4%, 500, 2 C	[55]
Na3(VOPO4)2F/Na3V2(PO4)3/C	Carbothermal reduction	-	-	-	2.5–4.3	104, 1 C	83, 100 C	63%, 3000, 5 C	[96]
Fe-doped Na3V2−xFex(PO4)2F3/Na3V2(PO4)3/C	Sol–gel	-	-	Particles	2–4.5	119.8, 0.5 C	108, 10 C	91%, 2000, 10 C	[97]
Mixed phosphates
Na4Fe3(PO4)2(P2O7)/C	Solvothermal	Orthorhombic	Pn21a	Nanoplates	2–4	113, 0.05 C	80.3, 20 C	69.1%, 4400, 20 C	[98]
Na4Fe3(PO4)2(P2O7)/C	Template-assisted	Orthorhombic	Pn21a	Porous nanospheres	1.5-4.2	128.5, 0.2 C	79, 100 C	63.5%, 4000, 10 C	[99]
Na4Fe3(PO4)2(P2O7)/rGO	Spray-dried	Orthorhombic	Pn21a	microspheres	1.7–4.3	128, 0.1 C	35, 200 C	62.3%, 6000, 10 C	[98]
Na4Fe3(PO4)2(P2O7)/multi-wall carbon nanotubes (MWCNTs)	Spray-dried	Orthorhombic	Pn21a	Spherical nanoparticles	1.5–4.0	115, 0.1 C	62.8, 20 C	95%, 1200, 2 C	[100]
Na4Fe3(PO4)2(P2O7)/C	Spray-dried	Orthorhombic	Pn21a	Hollow spheres	1.5–4.2	107.7, 0.2 C	88, 10 C	92, 1500, 10 C	[101]
Anodes NaTi2(PO4)3
NaTi2(PO4)3/rGO	Polyol-assisted pyro synthesis	Rhombohedral	R3C	Nanoparticles	1–3	100, 2.3 C	91, 20 C	68%, 1000, 20 C	[102]
3D graphene@ NaTi2(PO4)3	Solvothermal	Rhombohedral	R3C	Cube-like porous structure	1.5–3	112, 1 C	67, 50 C	80%, 1000, 10 C	[103]
NaTi2(PO4)3/rGO	Solvothermal	Rhombohedral	R3C	Sea urchin microspheres	1.5–3	129.2, 0.1 C	99.4, 50 C	91%, 1000, 10 C	[104]
NaTi2(PO4)3/C	Solid-state reaction	Rhombohedral	R3C	Nanoparticles	1.5–3	132, 0.2 C	80, 50 C	87.5%, 1000, 50 C	[105]
NaTi2(PO4)3/C	Solvothermal	Rhombohedral	R3C	Hollow nanoparticles	0–1.2	124.4, 0.275 A g−1	108.3, 5.5 A g−1	97.2%, 3000, 5.5 A g−1	[106]
NaTi2(PO4)3/C	Solvothermal	Rhombohedral	R3C	Mesoporous nanoflowers	1.5–3	125, 1 C/133	95, 100 C	77.3%, 10,000, 20 C	[40]
Na1−2xTi2(PO4)3−xFx	Sol–gel	Rhombohedral	R3C	Nanoparticles	1.5–3	121, 0.2 C/133	62.5, 30 C	70%, 1000, 10 C	[107]

Table 2 Summary of sodium superionic conductor materials in sodium-ion full cells.

Full cell anode//cathode	Voltage (V)	Electrolyte	Discharge capacity (mAh g−1)	Rate capability	Cyclic performance (retention, cycles, rate)	Energy density (Wh kg−1)	Power density (W kg−1)	References
Asymmetric full cell
Sb/rGO//NVP/rGO	2.7–3.9	1 M NaClO4 in DMC-EC (1:1 wt) with 5 wt% FEC	481, 100 mA g−1	-	83.1%, 100, 100 mA g−1	-	-	[108]
HC//NVP	2–4	1 M NaClO4 in DMC-EC (1:1 wt) with 5 wt% FEC	103, 0.5 C	-	58.2%, 100, 0.5 C	350	-	[109]
NTP//NVP	0.7–2.4	1 M NaClO4 in DMC-EC (1:1 wt) with 5 wt% FEC	103, 1 C	80, 50 C	96.9%, 300, 5 C	90	6000	[110]
NVP/ECF)/NVP/ECF	1–2.5	1 M NaClO4 in DMC-EC (1:1 wt) with 5 wt% FEC	90, 0.25 C	33, 10 C	81%, 280, 2 C	162		[111]
Sb/C//NVP@rGO	1.5–3.5	1 M NaClO4 in DEC-EC (1:1 vol)	110, 0.1 C	46, 10 C	78%, 40, 0.5 C	242	4560	[112]
NTP@C//NVP@C-CC	0.7–2.4	1 M NaClO4 in EC-DMC (1:1 vol) with 5 wt% FEC	104.7, 1 C	61, 50 C	67.3%, 500, 5 C	94	8400	[113]
HP-NVP@C//HP-NVP@C	1.2–2.2	1 M NaClO4 in EC-DMC (1:1 vol)	103, 0.2 C	62, 10 C	91.2%, 1000, 5 C	164	-	[114]
Biocarbon//NVP	1–3.5	1 M NaClO4 in EC-PC (1:1 vol) with 3 wt% FEC	290, 100 mA g−1	-	91%, 50, 100 mA g−1	380	-	[115]
GDHC//NVP	2–3.8	1 M NaPF6 in EC/PC (1:1)	80, 0.05 C	-	-	189	-	[116]
Na2C6H2O6//NVP@C/MWCNTs	0.5–2.4	1 M NaClO4 in DMC-EC (1:1 wt) with 5 wt% FEC	67, 0.05 C	33.3, 10 C	67.8%, 100, 0.05 C	-	-	[117]
HC//NVP/NC	1.5–3.7	1 M NaClO4 in DEC-EC (1:1 vol) with 5 vol% FEC	113.9, 0.1 C	71.8, 10 C	60%, 200, 1 C	212		[58]
NTP//NNFC-PB	0.2–1.6	1 M Na2SO4	100, 1 C	86, 10 C	88%, 250, 5 C	42.5	130	[118]
NTP@MWCNTs//NMO	0.2–1.4	1 M Na2SO4	128, 2 C	23, 20 C	-	58.7	228	[119]
NTP@rGO//NVP/C	0.4–2	1 M NaClO4 in DEC-EC (1:1 vol)	128, 0.1 C	88, 50 C	80%, 1000, 10 C	73	7.6	[120]
NTP//AC	0.2–1.6	1 M Na2SO4	100, 1 C	55, 10 C	61%, 500, 2 C	33.4	33.4	[121]
NTP/C//NVPF/C	0–2	1 M Na2SO4	111, 0.5 C	69, 20 C	82%, 200, 1 C	-	-	[122]
NTP/C//NFC-PB	0–2	1 M Na2SO4	84, 0.5 C	70, 20 C	86.5, 500, 1 C	-	-	[122]
NTP/CNF//NVP//CNF	0.5–2	1 M NaPF6 in EC/PC (1:1 vol)	126, 0.05 C	68, 100 C	74.5%, 4000, 20 C	-	-	[123]
NTP//NVP	0.5–1.5	1 M NaPF6 in PC 2 wt% FEC	107, 0.5 C	85, 20 C	94%, 5000, 20 C	67.1	47.5	[124]
NTP@C NR//AC	0–2.5	1 M NaPF6 in EC/DEC (1:1 vol) 2 wt% FEC	-	-	94%, 7000, 1 C	37.1	9180	[125]
NVP//NVPF	1.5–3.0	1 M NaClO4 in EC-PC-FEC (1:1 vol) 10 wt% FEC	90.7, 0.5 C	31.2, 20 C	83%, 300, 2 C	-	-	[126]
HC//NVPF/C	2.2–4.5	1 M NaClO4 in EC-DMC (1:1 wt)-5 wt% FEC	109, 1 C	98.2, 10 C	71.8%, 600,	-	-	[127]
VO2//GF-NVOPF	1–4	1 M NaClO4 in EC-PC-FEC (1:1:0.03)	90, 1 C	48, 12 C	80%, 220, 4 C	215	320	[92]
N-doped carbon//NVPF@rGO	1.5–3.9	1 M NaClO4 in EC/DMC (1:1 vol) 5% FEC	98, 0.5 C	55, 20 C	95%, 400, 10 C	291	192	[88]
HC//NVPF	2–4.3	1 M NaPF6 dissolved in PC/EC/DMC (1:1:1)	111, 0.1 C	-	77%, 100, 0.1 C	418	-	[128]
HCC//NVPF/C	2–4.3	1 M NaClO4 in PC with 5% vol FEC	115.9, 0.1 C	87.3, 10 C	>90%, 200, 0.5 C	428		[129]
SC//NVPF@rGO	2–4	1 M NaClO4 in PC with 5% vol FEC	107, 0.5 C	32.8, 20 C	74.5%, 500, 5 C	338	225	[85]
SnPx//NVPF	1–4	1 M NaClO4 in EC/DEC 1:1 with 5% vol FEC	120, 0.1 C	-	78%, 200, 0.1 C	280		[130]
NVP//NVTPF/C	1.5–3.0	1 M NaClO4 in EC/DEC 1:1 with 2% FEC	117, 1 C	41, 200 C	91.3%, 500, 40 C	224		[90]
CuS//NVPF/NVP	0–4	1 M NaPF6 in diglyme	101, 1 C	81, 10 C	88%, 1200, 10 C	162	1800.5	[97]
HC//NVPN@NGO	1.5–4	1 M NaClO4 in EC-PC 1:1 and 5 vol% FEC	200, 100 mA g−1 (1 C = 80 mA g−1)	-	80%, 150, 100 mA g−1	-	-	[71]
PPy Fe3O4//NFPP/C	0.1–4	1 M NaClO4 in EC-PC 1:1 and 5 vol% FEC	225, 100 mA g−1 (0.5 C = 60 mAg−1)	-	76.9%, 500	-	-	[98]
HC//NFPP/MWCNTs	1–4	1 M NaClO4 in DEC-EC 1:1 and 5 wt% FEC	69.3, 0.1 C	-	74%, 100, 0.1 C	210	-	[100]
HC//NFPP/C	0.3–4	1 M NaClO4 in PC with 5 vol% FEC	79, 0.5 C	-	65.8%, 100, 0.5 C	108	-	[101]
SC//NMTP/C	0.5–4	1 M NaClO4 in EC–PC 1:1 and 5% FEC	139, 0.5 C	-	84.1%, 50, 0.5 C	-	-	[76]
HC//rGO@NMTP/C	1.3–4.2	1 M NaClO4 in PC with 5 vol% FEC	151, 0.5 C	61.9, 10 C	80.9%, 100, 2 C	-	-	[78]
HC//NMCP/C	1.2–4.6	1 M NaClO4 in PC with 10 vol% FEC	146, 0.1 C	75.9, 5 C	72.1%, 50, 0.2 C	-	-	[80]
HC//NMCP/C	1.2–4.3	1 M NaClO4 in PC with 5 vol% FEC	105.9, 0.1 C	50.1, 5 C	84.5%, 50, 0.2 C	-	-	[80]
HC//NMCP/C	1–4	1 M NaClO4 in PC/FEC (9:1) vol	220, 0.05 C	-	72.8, 200, 0.05 C	-	-	[81]
SC//NMTP/C	1.7–4.0	1 M NaClO4 in EC-PC 1:1 (w/w) and 5 wt% FEC	97, 0.2 C	-	73%, 100, 0.2 C	-	-	[79]
HC//NMVP/C	2–3.8	1 M NaClO4 in PC/FEC (1:0.05) vol	106.7, 200 mA g−1	-	88.7, 140, 200 mA g−1	-	-	[83]
Symmetric full cell
Na2VTi(PO4)3//Na2VTi(PO4)3	0.5–2.0	1 M NaClO4 in EC-PC (v/v)	78, 1 C	49, 20 C	74, 10,000, 10 C	-	-	[84]
rGO@NVPMK/C//rGO@NVPMK/C	1.2–2.2	1 M NaClO4 in PC	83.95, 0.2 C	52.1, 10 C	95%, 500, 10 C	147.75	937.8	[131]
NVP//NVP	0.5–2.5	1 M NaPF6 in DME	95, 0.1 C	17, 100 C	75.5%, 4000, 2 C	-	48,250	[132]
NVP/CGO//NVP/CGO	1–2.4	1 M NaClO4 in DMC-EC (1:1 wt) with 5 wt% FEC	103,1 C	-	75%, 100, 1 C	-	-	[133]
NTP@C-NS-FL//NVP@C-NS-FL	0.6–2.2	1 M NaClO4 in EC-PC (1:1 wt) with 5 wt% FEC	108, 0.2 C	85, 20 C	75%, 2000, 20 C	51	1400	[134]

Abbreviations: DEC, diethyl carbonate; EC, ethylene carbonate; ECF, elastic carbon foam; FEC, fluoroethylene carbonate; HC, hard carbon; MWCNT; multi-wall carbon nanotube; NFPP, Na₄Fe₃(PO₄)₂(P₂O₇); NMCP, Na₃MnCr(PO₄)₃; NMTP, Na₃MnTi(PO₄)₃; NTP, NaTi₂(PO₄)₃; NVP, Na₃V₂(PO₄)₃; NVPF, Na₃V₂(PO₄)₂F₃; NVPN, Na₃V(PO₃)₃N; rGO, reduced graphene oxide; SC, soft carbo. Na₃V₂(PO₄)₃ (NVP)

Vanadium-based NASICON NVP is the most widely explored material for SIBs. NVP has a stable rhombohedral framework with a space group of R3c first discovered by Delmas et al.¹³⁵ in 1978, which was later confirmed by Gopalakrishnan and Kasthuri Rangan¹³⁶ in 1992 using the oxidative intercalation method. The framework consists of VO₆ octahedra and PO₄ tetrahedral unit-formed lantern structure via corner-sharing (Figure 4A). Moreover, there are two interstitial sites for Na-ion diffusion: Na1 (sixfold coordination) and Na2 (eightfold coordination). The Na1 site can be filled with one Na ion (1.0 site occupancy) and the Na2 site can take on two Na ions (0.67 site occupancy). Na-ion migration pathways were explored by first principal calculations by evaluating the activation energy through (1) a pathway showing a diffusion channel between two tetrahedra with an activation energy of 0.090 eV, (2) the diffusion channel between tetrahedra and octahedra with a calculated activation energy of 0.1174 eV, and the (3) pathway between two adjacent octahedra that is least favorable due to a high activation energy of 2.434 eV (Figure 4E). The Na-ion insertion/extraction mechanism was studied by ex situ XPS analysis in Figure 4C, which shows the oxidation of V³⁺ to V⁴⁺ during charging and reduction of V⁴⁺ to V³⁺ upon discharging. Structural changes by Na-ion insertion/extraction upon charging and discharging were also examined by in situ X-ray diffraction (XRD) (Figure 4B). Electrochemical analysis of NVP was reported by Uebou et al.¹⁴¹ NVP shows two voltage plateaus at 3.4 V corresponding to V⁴⁺/V³⁺ and 1.6 V corresponding to V³⁺/V²⁺ (Figure 4D). Moreover, there is an additional peak at a low voltage of ~0.5 V, corresponding to the V³⁺/V²⁺ redox. The low-voltage plateau is due to additional Na⁺-ion insertion leading to the formation of Na₅V₂(PO₄)₃.^142,143 Besides the advantages, NVP has very low electronic conductivity (1.63 × 10⁻⁶ S cm⁻¹), limiting its reversibility and cyclability.³⁸ The electronic conductivity can be improved using the strategies discussed in the previous section.

Figure 4. (A) Crystal structure of Na3V2(PO4)3 (NVP). Reproduced with permission: Copyright 2014, The Royal Society of Chemistry.137 (B) In situ X-ray diffraction of the NVP in a voltage range of 2.7–3.7 V. Reproduced with permission: Copyright 2013, Wiley-VCH.138 (C) Possible ion migration pathways in NASICON structured NVP. Reproduced with permission: Copyright 2014, The Royal Society of Chemistry.139 (D) Ex situ X-ray photoelectron spectroscopy studies of NVP. Reproduced with permission: Copyright 2013, Wiley-VCH.65 (E) Cyclic voltammetry curves of NVP in a voltage range of 1.0–3.8 V versus Na+/Na. Reproduced with permission: Copyright 2012, Elsevier.140 (F) Schematic illustration of the micro–nano porous holey sphere architecture of NVP/C. (G,H) Scanning electron microscopy of nano–micro holey spheres of NVP. Reproduced with permission: Copyright 2019, Elsevier.62 (I) Long-term stability of NVP. Reproduced with permission: Copyright 2015, Wiley-VCH.66

Porous carbon-based NVP composites of different architectures, NVP/CNTs, NVP/C, and NVP/graphite, were explored to enhance the performance of SIBs (Figure 4F). Carbon-coated NVP/C was synthesized via a one-step solid-state reaction first reported by Jian et al.¹⁴⁰ in 2012. NVP/C shows 93 mAh g⁻¹ capacity at 0.1 C and 89% retention up to 50 cycles. Porous NVP/C was synthesized using a solution-based method, which shows superior cyclic performance and rate capability, and the material delivered 114 mAh g⁻¹ initial capacity at 1 C and 50% retention of capacity up to 30,000 cycles.⁶⁵ This long-term performance is attributed to nano-sized porous NVP particles embedded in the carbon matrix, allowing fast Na-ion diffusion. Nano arrays of NVP/C on rGO nanosheets were synthesized using the freeze-drying approach.⁶⁶ NVP/C@rGO delivered a discharge capacity of 116 mAh g⁻¹ at 1 C and excellent rate capability of 103 mAh g⁻¹ at 50 C as well as 64% capacity retention after 10,000 cycles at 100 C (Figure 4J). The superior performance of NVP/C@rGO is due to the unique concept of designing 3D hierarchical meso/macro porous architectures with high surface area and electrical conductivity. These nano architectures, combined with stable structural features, provide fast ionic diffusion and deliver remarkable electrochemical performance. 3D porous microspheres of NVP/C were also synthesized using the membrane casting process. NVP/C shows an initial discharge capacity of 116 mAh g⁻¹ at 0.2 C with a rate capability of 96 mAh g⁻¹ at 30 C, as well as an excellent cyclic performance by retaining 80% of the initial capacity after 10,000 cycles at 10 C.⁴⁹ Micro–nano porous hollow spheres of NVP/C were constructed using a spray-dried method (Figure 4G,H), which shows an excellent performance of 106 mAh g⁻¹ at 2 C and 44 mAh g⁻¹ at 200 C.⁶² Hou et al.⁵⁸ synthesized nitrogen-doped nano–micro NVP/C using the solvothermal approach. The unique structural design in which N-doped carbon-coated nanoflakes of NVP are interconnected to form a 3D hierarchical porous structure-doped NVP/C delivered 117 mAh g⁻¹ discharge capacity at 0.1 C and remarkable rate capability of 79.1 mAh g⁻¹ at 200 C, with outstanding long-term stability of 73.4% after 10,000 cycles at 100 C. Different hierarchical meso–micro porous architectures of NVP are constructed using the polymer-stabilized droplet template method.⁶⁷ The hierarchical porous structure of NVP delivered 100 mAh g⁻¹ discharge capacity at 0.5 C with a rate capability of 61 mAh g⁻¹ at 100 C, and so far, better long-term stability by retaining 99% capacity retention after 10,000 cycles at 20 C. Micro–nano channels enhance contact between the electrode and electrolytes, and the 3D structure provides resistance to large volume of changes, which overall improves rate capability, and it is one of the feasible approaches for practical electric energy storage (EES) grid applications. These above-mentioned strategies overcome the limitations of NVP cathode materials. 3D porous structures of NVP can be constructed using different facile approaches like spray-drying, solvothermal, template, and membrane casting methods. The porous design allows the electrolyte to penetrate through the pores, shorten the electronic path, and improve the surface area. Combining porous morphologies with stable structural features, NVP can deliver ultrafast and long-term Na storage in SIBs.

NVP shows excellent and stable performance as a cathode material for half-cell SIBs. It is further explored for practical applications in sodium-ion full cells (NIFCs). Carbon-based materials, alloying metals, and conversion materials including transition-metal oxides, sulfides, phosphides, and selenides are mostly considered as potential candidates for anode materials in SIBs. NIFC is comprised of an NVP/C cathode combined mostly with carbon-based anode materials because of their low cost, chemical stability, high conductivity, and high surface area.^144–149 The NVP cathode is combined with the biocarbon anode (Figure 5A–C). The discharge capacity of ~290 mAh g⁻¹ is obtained at 0.1 A g⁻¹ in a voltage window range of 1–3.5 V. Moreover, 93% retention of capacity is achieved after 50 cycles.¹¹⁵ Hou et al.⁵⁸ reported a pouch cell-assembled micro/nanoporous nitrogen-doped NVP as a cathode and HC as an anode (Figure 5D–F). Pouch cell delivers a capacity of 113.9 mAh g⁻¹ at the rate of 0.1 C based on a mass of cathode with an excellent energy density of 212 Wh kg⁻¹ based on the total mass of both electrodes. It also shows 60% retention of capacity up to 200 charge–discharge cycles at 1 C. Apart from carbon-based materials, organic electrode materials can also be used as anodes due to their structural diversity and low cost. A carbon-coated NVP cathode combines with ortho-disodium salts of a tetrahydroxyquinone (o-Na₂C₆H₂O₆) anode in an NIFC organic battery. NIFC delivered 67.8 mAh g⁻¹ initial capacity with good capacity retention of 98% after 100 charge–discharge cycles at 0.05 A g⁻¹.¹¹⁷

Figure 5. (A) Schematic representation of a biocarbon anode//Na3V2(PO4)3 (NVP) cathode. (B) Charge–discharge curves of a full cell. (C) Cyclic performance up to 50 charge–discharge cycles. Reproduced with permission: Copyright 2018, Elsevier.115 (D) Schematic representation of a pouch cell assembled with NMF-NVP/NC//HC. (E) Charge–discharge profile at 0.1 C. (F) Cyclic performance up to 200 cycles at 1 C. Reproduced with permission: Copyright 2021, American Chemical Society58

Alloying materials P, Sn, Sb, and Pb were also explored as anode materials. Sb is an attractive anode material that has 660 mAh g⁻¹ theoretical capacity. The electrochemical performance of Sb can be enhanced by carbon coating.^150,151 A flexible NIFC comprised of an Sb/rGO anode and an NVP/rGO cathode shows a discharge capacity of 481 mAh g⁻¹ with good cyclic performance. However, the Columbic efficiency is less due to the formation of SEI layer.¹⁰⁸ Zhang et al.¹¹² assembled NIFC with a graphene-scaffolded NVP cathode with an Sb/C alloy anode. NVP/C//Sb/C NIFC delivered a reversible capacity of 110 mAh g⁻¹ at 0.1 C. An energy density of 242 Wh kg⁻¹ was found at 0.1 C based on both the cathode and the anode, as well as 78% retention of capacity after 40 cycles at 0.5 C. The major disadvantage of using alloying materials is the sluggish kinetics, formation of an unstable SEI layer, and large volume change during alloying–dealloying, which leads to pulverization, limiting their large-scale practical feasibility for EES applications.¹⁵² Metal sulfides, selenides, and phosphides are also good candidates as anode materials for SIBs due to their good structural properties as well as high electronic conductivity, and form a stable SEI layer, which improves the storage capacity and rate capability.^153–159 He et al.¹⁶⁰ evaluated the NIFC performance of nickel sulfide (Ni₃S₂) assembled with an NVP/C cathode. NVP||Ni₃S₂ shows a high discharge capacity of 329.3 mAh g⁻¹. Ma et al.¹⁶¹ reported a full cell of a cobalt sulfide Co₉S₈@C anode and NVP@C as a cathode (Figure 6A–C). It showed 377.4 mAh g⁻¹ discharge capacity based on the mass of the anode and 92.1% retention capability after 500 cycles. This excellent full cell performance is attributed to the hybrid core–shell structures with uniform carbon coating via the scalable method. Another study was carried out on NIFC using an iron sulfide composite as an anode and NVP/C as a cathode.¹⁶² CoFeS@rGO||NVP/C NIFC showed a remarkable capacity of 536.1 mAh g⁻¹ based on the mass of the anode and rate capability as well as excellent long-term performance with 96.5% retention of capacity after 200 cycles (Figure 6D,E). The hollow structure wrapped in the graphene expands and contracts during cycling, and due to its porous nature, it relieves mechanical stresses, which improves the long-term stability. These results show that metal chalcogenides show superior electrochemical performance compared to alloys and carbonaceous materials due to their structural stability and stable SEI layer formation. When metal chalcogenides are combined with a NASICON structured NVP cathode in a full cell, the synergic stable structural features of both NASICONs and chalcogenides provide more ionic and electronic channels, which leads to good long-term stable performance with minimal volume changes during charging/discharging in NIFC.

Figure 6. (A) Schematic of a full SIB with the Na3V2(PO4)3 (NVP)/C as the cathode and Co9S8@C/3DNCF as the free-standing anode. (B) GCD profile of the NIFC CoS@C/NCF//NVP/C. (C) Cycling performance of NIFC of the CoS@C/NCF//NVP/C. Reproduced with permission: Copyright 2019, Wiley-VCH.161 (D) GCD profile of a CoFeS@rGO||NVP/C full battery. (E) Cyclic performance of a CoFeS@rGO||NVP/C full cell at 500 mA g−1. Reproduced with permission: Copyright 2019, Wiley-VCH.162 3D, three-dimensional; GCD, galvanostatic charge–discharge; NCF, nitrogen doped carbon foam; NIFC, sodium-ion full cell; and SIB, sodium-ion battery.

Symmetric NIFC consists of the same material for both an anode and a cathode. NASICON structure NVP has good structural and thermal properties due to strong covalent bonds and it also has high redox potential and voltage plateaus at 3.4, 1.6, and 0.5 V, so it can be used both as cathode and anode materials (Figure 7A). In an asymmetric full cell, an anode is based on carbon-based materials, alloys, and oxides that have practical limitations due to poor thermal stability, large volume change during Na-ion insertion/reinsertion, and SEI layer formation. Therefore, use of NASICON structured materials will prevent these issues. Moreover, fabrication of NIFC using NVP both as a cathode and as an anode is simple, which reduces its cost. Conventional NIFC comprises of Al and Cu current collectors. During the electrochemical process, Cu reacts with metallic Na, which decreases its efficiency. To improve cyclic performance, the bipolar electrode structure is used by using a single Al current collector, which also reduces the cost of the Cu current collector (Figure 7B).¹⁶³ NIFC with NVP/elastic carbon foam as bipolar self-supported electrodes has an output voltage of 0-1.8 V (Figure 7A). The NIFC shows a reversible capacity of 90.2 mAh g⁻¹ at 0.25 C based on the mass of the cathode. It also shows 162 Wh kg⁻¹ energy density based on the total mass of electrodes with 81% retention of capacity for 280 charge–discharge cycles at 2 C.¹¹¹ The self-supported electrodes without any addition of binders provide high conductivities and they are very lightweight. The unique scaffold design improves the electrical and ionic conductivity and prevents agglomeration, which overall provides long-term cyclic performance in full cell systems.

Figure 7. (A) Schematic illustration of the Na3V2(PO4)3 (NVP) symmetric full cell. Reproduced with permission: Copyright 2016, The Royal Society of Chemistry.111 (B) Bipolar electrode structure of a symmetric NVP full cell. Reproduced with permission: Copyright 2019, Springer Nature.163 (C) Cyclic voltammetry profile of a highly porous NVP symmetric full cell. (D) Rate performance of an NVPMK@C/RGO//NVPMK@C/RGO Symmetric full cell. Reproduced with permission: Copyright 2019, The Royal Society of Chemistry.114 (E) Cycling performance of a symmetric NVP full cell at a current density of 2 A g−1. Reproduced with permission: Copyright 2020, The Royal Society of Chemistry.132

Tu et al.¹¹⁴ synthesized highly porous NVP with sulfur-doped carbon to improve conductivity. NVP@SC NIFC electrochemical analysis shows a couple of redox peaks around 1.8 V. It delivered 103 mAh g⁻¹ capacity at 0.2 C and high energy density of 164 Wh kg⁻¹ at 1 C with 91.2% retention of capacity after 1000 cycles at 5 C (Figure 7C,D). The sulfur doping increases the interlayer spacing of the conducting carbon layer and the highly porous structure of NVP provides a dual strategy, which increases the number of active sites to improve the electronic and ionic diffusion for high-performance full cells. Zhang et al.¹³³ synthesized NVP nanospheres anchored on graphene using the freeze-casting method. The self-supported symmetric NIFC shows a capacity of 103 mAh g⁻¹ at 1.75 V output voltage and 75% retention of capacity after 100 cycles at 1 C. The NVP nanospheres have a diameter of around 20 nm anchored on the graphene to form a unique 3D porous structure. The graphene matrix improves the conductivity, while the small-diameter nanospheres provide fast Na⁺-ion diffusion by decreasing the length. Apart from the morphology and structural modification, the utilization of electrolytes is also important for full cell performance. Symmetric NIFC of NVP in the ether-based electrolyte (DME) delivered a capacity of 71.7 mAh g⁻¹ and 75.5% retention of capacity after 4000 cycles at 2 C (Figure 7E).¹³² It also shows the highest power density of 48,250 W kg⁻¹ among all the reports. The remarkable electrochemical performance of an NVP symmetric full cell in a DME electrolyte is due to favorable reaction kinetics due to fast Na⁺-ion diffusion through the 3D structural pathways.

We believe that NVP is a bipolar electrode that can be effectively utilized both as an anode and as a cathode. Due to its structural features, it can show excellent electrochemical performance in both half cells and full cells. The main disadvantage of NVP is the low intrinsic electronic conductivity, the high toxicity of vanadium, and the low operating voltage window, which limits its performance in practical applications. The electrical conductivity is enhanced by carbon coating and a porous structure. Thermal and structural stability is further enhanced by doping of different cations, whereas the capacity cannot be further enhanced at 3.4 V due to an immobile Na ion at the Na1 site and only two mobile Na ions at the Na2 site. For long-term stability, effective strategies for design are important, so designing the porous structure and a uniform carbon layer on the surface of NVP as well as the utilization of ether-based electrolytes provides a boost toward practical applications.

Na₃Fe₂(PO₄)₃

Na₃Fe₂(PO₄)₃ is another NASICON structured material that has been explored as an electrode for SIB with a robust open framework, a theoretical capacity of 105 mAh g⁻¹, and a high redox potential of 3.5 V. Delmas et al.¹⁶⁴ were the first to study Na₃Fe₂(PO₄)₃. Lyubutin et al.,¹⁶⁵ in 1988, found that NASICON structured Na₃Fe₂(PO₄)₃ shows three phases at different temperatures. It has an α phase of a monoclinic structure at room temperature, which transforms into a superionic rhombohedral (R3c space group), β Na₃Fe₂(PO₄)₃ at 368 K, and γ Na₃Fe₂(PO₄)₃ at 418 K, later confirmed by Kravchenko and Sigaryov.¹⁶⁶ in 1992. The crystal structure consists of a monoclinic system with a space group C2/c like NVP. It has a 3D open framework in which FeO₆ octahedra and PO₄ tetrahedra are connected via corner-sharing and two interstitial sites for Na ion: M1 and M2 (Figure 8A). Kuganathan and Chroneos investigated the Na-ion diffusion pathways based on activation energies in Na₃Fe₂(PO₄)₃.¹⁶⁸ There are three diffusion pathways for Na⁺, with activation energies of 0.44, 0.45, and 2.37 eV for pathways A, B, and C, respectively (Figure 8B). The lowest activation energies of 0.44 and 0.45 are the optimal pathways for Na-ion diffusion, which makes Na₃Fe₂(PO₄)₃ a potential candidate for cathode materials.

Figure 8. (A) Schematic illustration of the crystal structure of Na3Fe2(PO4)3. Reproduced with permission: Copyright 2019, Elsevier.167 (B) Na-ion diffusion pathway in Na3Fe2(PO4)3: A, B, and C in green, purple, and blue, respectively, show pathway trajectories. Reproduced with permission: Copyright 2019, Materials.168 (C) Cyclic voltammetry curve of Na3Fe2(PO4)3 at 0.1 mV s−1. (D) Cyclic performance of Na3Fe2(PO4)3/C for 200 cycles (insets show the transmission electron microscopy image and the X-ray absorption near-edge structure profile). Reproduced with permission: Copyright 2017, Wiley-VCH.68

Liu et al.¹⁶⁹ synthesized monoclinic-phase NASICON structured Na₃Fe₂(PO₄)₃ by a solid-state reaction as the cathode material for SIB. It showed a pair of redox peaks of Fe³⁺/Fe²⁺ at 2.5 V and reversible discharge capacity of 61 mAh g⁻¹ at 1 C, and excellent rate capability as well as 93% retention of capacity after 500 cycles. The drawback of Na₃Fe₂(PO₄)₃ is the low discharge capacity as compared to its theoretical capacity, which can be enhanced by different modification techniques, such as (1) carbon coating, (2) elemental doping, and (3) nano architecting, as discussed in previous sections. Dou et al.⁶⁸ enhanced the performance of Na₃Fe₂(PO₄)₃ by carbon coating, which activated an additional redox peak of Fe^3+/Fe⁴⁺ at 3.4 V, as shown in Figure 8C, which is confirmed by the ex situ x-ray absorption near-edge structure. NFP/C delivered a reversible discharge capacity of 109 mAh g⁻¹ at 0.1 C and rate capability of 75 mAh g⁻¹ at 5 C with 96% retention of capacity over 200 cycles at 1 C (Figure 8D). Multi-wall carbon nanotubes (MWCNTs) were incorporated with Na₃Fe₂(PO₄)₃ via the solid-state reaction method as the cathode material. The introduction of MWCNTs enhances the electrical conductivity. MWCNTs@NFP delivered a discharge capacity of 101 mAh g⁻¹ at 0.1 C with excellent rate capability, delivering 80.2 mAh g⁻¹ at 5 C.¹⁷⁰ Cation doping is another strategy to improve electrochemical performance. Na₃Fe₂(PO₄)₃ doped with potassium showed improved structural stability. It delivered 101.3 mAh g⁻¹ capacity at 0.1 C and maintained 93.7 mAh g⁻¹ capacity after 500 cycles with 100% Coulombic efficiency.¹⁷¹ The doping of the K atom increases the lattice distance, which improves the Na-ion diffusion kinetics. The improved structural stability overall improves the electrochemical performance.

Na₃Fe₂(PO₄)₃ shows good performance as a half-cell electrode material. However, to use it in practical applications, there is a need for detailed investigations of NIFC. Recently, porous flakes of Na₃Fe₂(PO₄)₃ were synthesized by the spray drying process.⁶⁹ The porous flakes increase the surface area and allow a path for electrolyte penetration. The NASICON structure of NFP stacked with a porous 2d flake morphology enhances its performance for SIB. The synthesized material delivered 100.8 mAh g⁻¹ discharge capacity at 0.1 C with high-rate capability and achieved 60 mAh g⁻¹ capacity after 1100 cycles at 5 C. Moreover, a full cell device was assembled by combining NFP as a cathode and hard carbon as an anode. NIFC shows 98.3 mAh g⁻¹ discharge capacity at 0.1 C based on the mass of the cathode with rate capability and 86.5% retention after 300 cycles at 1 C. NIFC delivered an energy density of 76 Wh kg⁻¹ and 760 W kg⁻¹ maximum power density and also showed good low-temperature performance at −20°C. From these studies, we believe that Na₃Fe₂(PO₄)₃ is a good candidate as a cathode material. Na₃Fe₂(PO₄)₃ has an open framework and high redox potential; it has a high-rate capability and long cyclic life, which makes it a potential candidate as an electrode material for SIB. Moreover, Fe is more abundant and nontoxic as compared to vanadium NFP, which has several drawbacks that limit its use towards commercialization, with a low theoretical capacity of 105 mAh g⁻¹ as compared to NVP. It has a comparatively high activation energy of 0.45 eV compared to NVP. To date, no enough research has been reported on NFP for commercialization, but the initial studies suggest that this material has good potential due to its structure as well as materials' cost and availability. To utilize NFP in practical applications, further research is needed to improve the performance in NIFCs.

Mixed transition-metal NASICON materials

Single transition-metal NASICON structured material Na_xMMʹ(XO₄)₃ can be used, where M = Mʹ has a limited redox reaction, which limits its energy density. To overcome this drawback for practical applications, mixed transition-metal NASICON structured material Na_xM_2−yM_yʹ(XO₄)₃, where M ≠ Mʹ, has been explored recently. Mixed transition-metal phosphates provide multiple electron reactions and increase the voltage window, which overall increases the discharge capacity and energy density of high-performance SIBs. Moreover, the synergic effect of transition metals enhanced the structural stability. The reversible Na⁺-ion intercalation in mixed transition-metal NASICON compounds was first explored in 1992 by Tillement et al.¹⁷² Na₂VTi (PO4)₃ is a mixed-metal phosphate that shows a high theoretical capacity of 147 mAh g⁻¹ due to redox coupling of V³⁺/V⁴⁺ and Ti⁴⁺/Ti³⁺. The crystal structure of NVTP is rhombohedral, in which [TiO₆, VO₆] octahedra and [PO₄] tetrahedra are joined to form a 3D framework (Figure 9A). The Na storage performance between 1.5 and 4.5 V and cyclic performance of NVTP are shown in Figure 9B,C. The improved electrochemical performance is due to V³⁺ and Ti⁴⁺ ions, which have similar ionic radii, can replace each other in the 3D framework, and increase the Na-ion diffusion as well as redox potential, improving the electrochemical storage overall. Zhou et al.⁷² synthesized Na₄MnV(PO₄)₃ and Na₃FeV(PO₄)₃ by replacing the more toxic V with highly abundant low-cost Mn and Fe transition metal and determining the crystal structure and electrochemical performance for cathode materials for SIBs. Na₃MnV (PO₄)₃ consists of a single-phase trigonal structure (space group R3c) in which the Mn/VO₆ octahedron and the PO₄ polyhedron share the corners. Na₃FeV (PO₄)₃ has a monoclinic structure (space group C12c1). Moreover, the half-cell electrochemical performance was examined, which showed multiple redox plateaus. In Na₃FeV (PO₄)₃, two redox plateaus of Fe^3+/Fe²⁺ and V^4+/V³⁺ appear at 2.5 and 3.3 V, respectively, whereas in Na₄MnV (PO₄)₃, there are two redox plateaus of Mn^3+/Mn²⁺ and V^4+/V³⁺ at 3.6 and 3.3 V, respectively. Na₃FeV (PO₄)₃ and Na₄MnV (PO₄)₃ show discharge capacities of 103 and 101 mAh g⁻¹ at 1 C and capacity retention of 95% and 89% after 1000 charge/discharge cycles at 1 C, respectively (Figure 9D,E). Single-phase Na₄MnV (PO₄)₃ cathode materials with improved discharge capacity of 156 mAh g⁻¹ at a cutoff voltage of 4.3 V were synthesized by a solid-state reaction. They found the third Na⁺-ion extraction, and the additional redox plateau observed at 3.86 V corresponds to V^4+/V⁵⁺, which increased its capacity and energy density but led to irreversible structural changes that caused unstable cyclic stability.⁷³ Another NASICON material, Na₃VCr (PO₄)₃, was investigated and the electrochemical performance was evaluated for SIB. NCVP shows a rhombohedral crystal structure (space group R3C). The electrochemical analysis confirmed the existence of two plateaus at 3.4 and 4.1 V, corresponding to V³⁺/V⁴⁺ and V⁴⁺/V⁵⁺, respectively.⁷⁴ NVCP shows a discharge capacity of 93 mAh g⁻¹ at 0.1 C and 92% capacity retention after 200 cycles at 0.5 C. Ex situ XRD shows that the diffraction peaks are shifted to a higher angle during charging. These peaks do not shift back to their original position upon discharge, which shows the irreversible transformation during sodiation/desodiation.

Figure 9. (A) Schematic illustration of the crystal structure of Na2VTi(PO4)3. (B) Galvanostatic charge–discharge profile of different cycles of Na2VTi (PO4)3 at 0.1 C. (C) Cyclic stability of Na2VTi(PO4)3 at 10 C for 500 cycles. Reproduced with permission: Copyright 2017, Nature.84 (D) Charge–discharge profile of Na3FeV(PO4)3 at different scan rates. (E) Cyclic performance of Na3FeV(PO4)3 and Na4MnV(PO4)3 for 1000 cycles at 1 C. Reproduced with permission: Copyright 2016, American Chemical Society.72 (F) Charge–discharge profile of Na3Mn1.2Ti0.8(PO4)3 (NMTP) at different rates. (G) Rate capability of NMTP with different compositions of x. (H) Long-term stability of NMTP with different compositions for 1000 cycles at 5 C. Reproduced with permission: Copyright 2021, Royal Society of Chemistry.77 (I) Charge–discharge profile of Na3MnCr(PO4)3/C at 0.05 C. (J) Rate capability of Na3MnCr(PO4)3/C. (K) Long-term cyclic performance of Na3MnCr(PO4)3/C at 5 C. Reproduced with permission: Copyright 2020, Wiley-VCH.80

Gao and Goodenough¹⁷³ reported Na₃MnTi(PO₄)₃ (NMTP) and investigated the Na⁺ insertion/extraction behavior. NMTP shows a rhombohedral structure, which is well maintained after Na-ion extraction/insertion. NMTP shows two redox peaks in the CV profile at 3.6 and 4.1 V, corresponding to Mn^3+/Mn²⁺ and Mn^4+/Mn⁴⁺, respectively, which is further confirmed by X-ray photoelectron spectroscopy analysis. It shows a discharge capacity of 80 mAh g⁻¹ at 0.1 C, but low Coulombic efficiency due to electrode instability. A carbon coating on NMTP/C with a hollow sphere morphology was synthesized using the spray-drying method.⁷⁶ NMTP/C shows a rhombohedral structure (space group R3C) in which isolated TiO₆/MnO₆ octahedra and PO₄ polyhedral are connected to form a stable structure. The electrochemical analysis shows that the three redox reactions at 2.12, 3.52, and 4.01 V correspond to Ti^3+/Ti⁴⁺, Mn^2+/Mn³⁺, and Mn^3+/Mn⁴⁺, respectively. It also shows a high discharge capacity of 160 mAh g⁻¹ at 0.2 C, much higher than that of the reported single transition-metal NASICON materials, and 92% retention of capacity after 500 cycles at 2 C. The structural changes in the hollow spheres of NMTP/C were further investigated by in situ XRD, which shows that two diffraction peaks (211) and (116) disappear during discharging and again appear during charging, which confirms the reversible two-phase reaction, indicating that three Na⁺ intercalate during sodiation and reversibly intercalate during dissociation. Na_{3 + 2x}Mn_{1 + x}Ti_1−x(PO₄)₃ is synthesized by varying the concentration of (x = 0, 0.1, 0.15, 0.2, 0.3, 0.4, 0.5) using the sol–gel method. This model shows three reversible electron reactions by increasing the Mn content, which increases the average voltage and discharge capacity. NM_1.15T_0.85P delivered a high reversible capacity of 181.4 mAh g⁻¹ at 0.1 C and even at a high rate of 10 C, and it shows 100.4 mAh g⁻¹ with 87.4% retention after 500 cycles at 1 C.⁷⁷ Moreover, the high energy density of 560.2 Wh kg⁻¹ is achieved when x = 0.15 at 0.1 C (Figure 9F–H). Ex situ XRD further examined the structural changes during the electrochemical process, which shows that two peaks (300) and (226) disappear during desodiation and again appear during sodiation; also, (113) and (116) peaks shift to higher angles during sodiation. Moreover, only a small lattice volume change (4.92%) is observed during ex situ XRD, which indicates that NMTP shows good cyclic stability. Recently, Zhang et al.⁸⁰ reported novel Cr-based binary metal NASICON structured phosphate Na₄MnCr(PO₄)₃. Na₄MnCr(PO₄)₃ shows rhombohedral crystal structures (space group R3C) with large lattice a = b = 8.919 Å, c = 21.522 Å, compared to NVP, enabling high Na⁺ diffusion. The 3D structural framework consists of a Mn/CrO₆ octahedral connected with a PO₄ tetrahedral via corner-sharing to form a lantern unit. It shows three electron reactions at Mn^2+/Mn³⁺ at 3.6 V, Mn^3+/Mn⁴⁺ at 4.2 V, and Cr^3+/Cr⁴⁺ at 4.4 V during electrochemical analysis. Ex situ X-ray absorption near-edge structure (XANES) spectroscopy also confirmed the oxidation states of Mn and Cr during the electrochemical process. For detailed analysis, extended X-ray absorption fine structure (EXAFS) is conducted, which shows that at 3.8 V, two Mn–O bond peaks split into two, confirming the oxidation state of Mn³⁺, and at 4.3 V, they further split into two peaks, which confirms the Mn⁴⁺ oxidation state. Moreover, it shows high discharge capacity of 160.5 mAh g−¹ at 0.05 C with high energy density of 566.5 Wh kg⁻¹ (Figure 9I–K). High discharge capacity and energy density are due to the reversible three-electron reaction. Novel sodium-deficient mixed transition Na_3.41£0.59FeV(PO₄)₃ (NFVP) cathode material was reported using the sol-gel method.⁸² NFVP shows a trigonal crystal structure (space group R3c). Electrochemical analysis shows three redox peaks at 2.55, 3.46, and 4 V, corresponding to Fe²⁺/Fe³⁺, V³⁺/V⁴⁺, and V⁴⁺/V⁵⁺, respectively. NFVP shows a reversible discharge capacity of 119 mAh g−¹ in the 1.5–4.4 V potential range at 0.5 C and rate capability of 111 mAh g⁻¹ at 5 C. Furthermore, NFVP shows 94.73% capacity retention after 700 charge/discharge cycles at 20 C and a high specific energy density of 372 Wh kg⁻¹ at 0.5 C. These studies show that this concept of utilizing binary transition-metal NASICON materials Na_xM_2−yM_y'(XO₄)₃ shows multielectron reactions and a high voltage window, improves the capacity, and meets the requirement of high-energy-density batteries of >500 Wh kg⁻¹. The utilization of low-cost and abundant transition metals with effective synthesis strategies will further open a new avenue of exploring new transition-metal materials for high-energy-density batteries for practical applications.

In full cell performance, the Goodenough group¹⁷³ used NASICON NMTP as an anode and a cathode in aqueous Symmetric SIB at a high output voltage of 1.4 V. NMTP shows two reversible redox couples of Mn³⁺/Mn²⁺ and Ti⁴⁺/Ti³⁺ at the anode and the cathode, respectively. NMTP symmetric NIFC delivered 57.9 mAh g⁻¹ at 0.5 C based on the mass of the cathode, with a good rate capability of 46.7 mAh g⁻¹ at 10 C, with 98% retention of the initial discharge capacity at 1 C after 100 cycles. Moreover, a symmetric full cell shows an energy density of 40 Wh kg⁻¹ based on the total mass of the cathode and the anode. Symmetric hybrid full cells of Na₂VTi(PO₄)₃/C both as an anode and as a cathode also showed excellent electrochemical performance.⁸⁴ NVTP was synthesized using a facile sol–gel method. Symmetric NIFC shows an initial discharge capacity of 78 mAh g⁻¹ and a high-rate capability of 49 mAh g⁻¹ at 20 C. NIFC delivered ultralong stability of 10,000 charge−discharge cycles with 74% retention at 10 C. Zhu et al.⁷⁶ fabricated the full cell by using hollow spheres of NMTP/C as a cathode and soft carbon as an anode. NIFC delivered 139 mAh g⁻¹ discharge capacity at 0.5 C based on the mass of the cathode and retained 84% of the initial capacity after 50 charge–discharge cycles. The remarkable long-term cyclic performance was attributed to the symmetric nature of cell, and due to the unique structure, the volume changes during Na⁺-ion insertion/extraction become negligible; also, with the carbon coating, it shows super electrochemical performance in a full cell.

Apart from the symmetric full cell performance, mixed transition-metal phosphates also show good electrochemical performance in asymmetric cells due to their structural features. An asymmetric full cell comprising rGO@NMTP/C as a cathode and hard carbon as an anode shows good performance for practical applications.⁷⁸ The electrochemical performance improved by encapsulation in graphene, and the in situ carbon coating provides fast electronic diffusion. NIFC shows a high discharge capacity of 151 mAh g⁻¹ at 0.5 C based on the mass of the cathode and 61.9 mAh g⁻¹ at 10 C as well as 80.9% retention of the initial capacity after 100 cycles at 2 C (Figure 10A–C). The excellent performance is due to the synergic effect of structural optimization and additional Na⁺-ion intercalation/deintercalation activated due to the Ti⁴⁺/Ti³⁺ redox couple. Dual carbon decorated NMTP microspheres as the cathode in a full cell with a soft carbon anode further improved the rate capability. NMTP/C//SC delivered 97 mAh g⁻¹ initial discharge capacity at 0.2 C with 70.1% retention of capacity after 100 cycles at 0.2 C.⁷⁹ Na_3.5Mn_0.5V_1.5(PO₄)₃/C was synthesized with an optimized structure and an asymmetric full cell was assembled using a cathode and hard carbon as an anode.⁸³ NMVP/C//HC delivered 102.7 mAh g⁻¹ discharge capacity at 0.2 C with 88.7% retention of capacity after 140 charge–discharge cycles. The inclusion of Mn in the framework increases the redox voltage and it will also allow additional Na⁺-ion at Na1 and Na2 sites, providing sufficient sodium supply. The hierarchical structure allow minimal volume change, which boost rate capability and ultra-long stability. Zhang et al.⁸⁰ synthesized a Na₃MnCr(PO₄)₃/C (NMCP/C) novel cathode material and assembled a full cell with a hard carbon anode for the practical feasibility of the SIB. The NMCP/C//HC full cell shows a high discharge capacity of 146.5 mAh g⁻¹ in voltage range of 1.2–4.6 V and 105.9 mAh g⁻¹ in the voltage range of 1.2–4.3 V at 0.1 C. Moreover, it shows 84.5% (1.2–4.3 V) and 72.1% (1.2–4.6 V) retention of capacity for 50 charge/discharge cycles at 0.2 C. They further reported NMCP/C//HC with full reversible activation of Mn⁴⁺/Mn³⁺ redox for long stability (Figure 10D,E).⁸¹ NMCP/C//HC delivered a discharge capacity of 220 mAh g⁻¹ at 0.05 C based on the mass of the anode and 72.8% retention of capacity after 200 charge/discharge cycles at 0.05 C. We believe that binary transition-metal NASICON has the potential to replace Li-ion batteries due to the high redox potential voltage as compared to single transition-metal NASICON. Due to multiple electron reactions, they show a high discharge capacity of >150 mAh g⁻¹ and theoretical energy density approaching >500 Wh kg⁻¹. The above studies show the performance of different binary transition-metal materials like Na₄MnCr(PO₄)_3, Na₃MnTi(PO₄), Na₃MnV(PO₄)₃, and Na₃FeV(PO₄)_3. All these materials show more than two electron reactions. Apart from the structural advantage of Mn, Fe-based materials are low-cost and abundant as compared to Vanadium-based materials. One major limitation of these materials is the reversibility of the redox reaction. Although several studies show the reversibility of a multielectron reaction in full cell systems, more optimization is still needed by designing binary metal transition phosphates with a reversible multi-election reaction and high voltage in full cells. We hope that in future, with further optimization, these materials will provide a pathway for high-energy-density SIB toward commercialization.

Figure 10. (A) Charge–discharge profile of full cell rGO/Na3MnTi(PO4)3 (NMTP)//HC at 0.5 C. Inset: cyclic voltammetry curve. (B) Rate capability of rGO/NMTP//HC at different C rates, inset charge–discharge profile. (C) Cyclic performance of rGO/NMTP//HC for 100 cycles at 2 C. Reproduced with permission: Copyright 2019, Elsvier.78 (D) Charge–discharge profile of full cell Na3MnCr(PO4)3 (NMCP)//HC at 0.5 C. (E) Cyclic performance of NMCP//HC for 200 cycles at 0.5 C. Reproduced with permission: Copyright 2020, Wiley-VCH.81

Fluorophosphates Na₃V₂(PO₄)₂F₃ (NVPF)

Another strategy to improve the performance of NASICON structured materials is to replace a (PO₄) anion with (PO₄)F. NASICON structured fluorophosphate with the formula unit NVPF_−2yO_2y (0 ≤ y ≤ 1) has been formulated. Fluorine is the most electronegative element that forms a strong bond (V–F and V–O) in the lattice, which improves the operating voltage, thermal stability, and retention of capacity, which improves the electrochemical performance overall. NVPF is the most widely studied NASICON structured vanadium-based fluorophosphate, delivering a theoretical capacity of 128 mAh g⁻¹ and a high theoretical energy density of 507 Wh kg⁻¹.^174–177 Meins et al.,¹⁷⁸ in 1999, first reported that the crystal structure of NVPF is tetragonal (space group P4₂/mnm), with a = 9.047 Å, c = 10.705 Å, and V = 876.2 ų, in which two octahedral units [V₂O₈F₃] are connected with the PO₄ tetrahedral unit via corner-sharing to form a 3D framework (V₂(PO₄)₂F₃)₃ with two Na⁺ diffusion sites Na1 and Na2 along the [110] and [1$$\bar{1}$$0] directions (Figure 11A). Barker et al.¹⁸¹ first reported the electrochemical performance of NVPF in hybrid ion batteries. Bianchini et al.,¹⁷⁴ in 2014, studied the crystal structure of (V₂(PO₄)₂F₃)₃ using high angular resolution synchrotron radiation diffraction (Figure 11B,C). They found the orthorhombic distortion in the crystal structure (space group Amam) with the lattice parameter (a = 9.02847 Å, b = 9.04444 Å, c = 10.74666 Å). Moreover, they also discovered a high-temperature disordered tetragonal (I4/mmm) crystal structure. The detailed crystal structure and sodiation mechanism were investigated using operando techniques.¹⁷⁹ The Na⁺-ion diffusion mechanism is shown in Figure 11D. Na⁺-ion deintercalation is very complicated; it shows four intermediate phases due to Na⁺ ordering–disordering in NVPF (Amam) at two voltage domains 3.7 and 4.2 V during Na⁺ extraction of Na_2.4VPF, Na_2.2VPF, Na₂VPF, and Na_1.8–1.3VPF of space group I4/mmm and final state Na₁VPF (Cmc2₁) fully occupied Na sites, as shown in Figure 11F,G. A small volume change is reported during the transition from the NVPF (219.20 Å) phase to the NaV₂(PO₄)₂F₃ (212.47 Å) phase, which predicts the long-term stability of NVPF. The electrochemical reaction of NVPF during charging–discharging is as follows: [Image Omitted. See PDF]

Figure 11. (A) Schematic illustration of the 3D crystal structure of Na3V2(PO4)2F3 (NVPF). Reproduced with permission: Copyright 2018, Wiley-VCH.88 (B) In situ X-ray diffraction (XRD) of NVPF between the 2θ range (17°–18°). (C) Galvanostatic intermittent titration technique curves of phase transition from NVPF to NaV2(PO4)2F3 mark different intermediate phases. Reproduced with permission: Copyright 2015, American Chemical Society.179 (D) Na+ diffusion sites in Na3V2(PO4)F3. Reproduced with permission: Copyright 2016, American Chemical Society.180 (E) Scanning electron microscopy image of spherical NVPF@C/CNTs. Reproduced with permission: Copyright 2018, The Royal Chemical Society.87 (F) Charge–discharge curves of NVPF between 1 and 4.8 V showing extraction amount of Na (Δx = 2, 2.25, 2.54, 2.75, and 3). (G) Schematic representation of the crystal structures of different states of NxVPF during charging. Reproduced with permission: Copyright 2019, Nature.128 (H) Long-term cyclic performance of NVPF@C at 15C. Reproduced with permission: Copyright 2019, Springer.86

Electrochemical properties of NVPF were determined by first-principle calculations. Ex situ XRD is used to investigate structural evolution during sodiation/desodiation. Ex situ XRD confirmed the single-phase reaction with very small volume (2%) and lattice change (1%). Moreover, NVPF delivered 120 mAh g⁻¹ at C/20 with a good rate capability of 94 mAh g⁻¹ at 4 C.¹⁸² Like other NASICON materials, NVPF also has the limitation of low intrinsic electronic conductivity, which can be enhanced by carbon coating, cation doping, and nanoarchitecture techniques.^{86–88,183–186} Carbon-coated NVPF as a cathode material for SIB was synthesized using the carbothermal reduction method.¹⁷⁶ NVPF shows a tetragonal crystal structure (space group P4₂/mnm). Electrochemical analysis shows that NVPF has two anodic peaks at 3.9 and 4.28 V and two cathodic peaks at 3.28 and 3.85 V. NVPF/C shows 111.5 mAh g⁻¹ discharge capacity at 0.091 C, with three discharge plateaus observed at 4, 3.5, and 3.3 V due to structural recording and the V^3+/V⁴⁺ redox reaction. Nano caged microcubes of NVPF/rGO were synthesized by hydrothermal method, followed by freeze-drying.⁸⁸ The nano microcubes, combined with graphene encapsulation, provide a boost to electrochemical storage. NVPF/rGO shows 119 mAh g⁻¹ discharge capacity at 0.5 C, with an excellent rate capability of 71 mAh g⁻¹ at 20 C and 114 mAh g⁻¹ when charged back at 0.5 C. It also shows ultralong stability by retaining 98% of the initial discharge capacity after 2000 cycles at 20 C. Besides rGO and carbon coating, CNTs were also utilized to enhance electrical conductivity. NVPF@C/CNT composites with a spherical morphology were also prepared using the spray-drying method (Figure 4E).⁸⁷ The electrochemical analysis shows that NVPF@C/CNTs delivered 113 mAh g⁻¹ capacity at 1 C and a high-rate capability of 85 mAh g⁻¹ at 30 C, as well as 98.2% retention of the initial discharge capacity after 300 charge–discharge cycles at 1 C and long-term cyclic stability up to 1400 cycles (Figure 4H). NVPF/C of uniform nanosized particles were synthesized using the polyol method.⁸⁶ The uniform particle morphology provides a shorter diffusion path for Na⁺ diffusion and better structural stability during sodiation/desodiation, which improves the electrochemical performance. It delivered 126.6 mAh g⁻¹ discharge capacity at 1 C and 105 mAh g⁻¹ even at a very high discharge rate of 50 C. The doping strategy used can also boost the performance of fluorophosphates. Na₃V_2−yTi_y(PO₄)₂F₃/C yielded Ti-doped NVPF using the wet ball milling process. NVTPF/C delivered a discharge capacity of 125 mAh g⁻¹ at 0.2 C and high-rate capability of 104 mAh g⁻¹ at 40 C with 81.5% capacity retention after 200 cycles at 1 C.⁹⁰ Microcuboids of NVPF/rGO with different crystal orientations were prepared using the hydrothermal process. These microcuboids are grown at a specific orientation with the addition of sodium halide. NVPF/rGO delivered 127.5 mAh g⁻¹ at 0.2 C with a rate capability of 73.7 mAh g⁻¹ at 50 C and 83.21% retention after 1000 cycles at 5 C.⁸⁵ The improved electrochemical performance is specifically attributed to the crystal growth along the Na-rich [110] direction, which enhances the diffusion of Na ions at the NVPF electrode and the electrolyte interface. This shows that by orienting NVPF particles along a specific direction, design of porous structures and a uniform carbon coating increase the Na-ion diffusion and electronic conductivity and improve the electrochemical performance.

NVPF shows better electrochemical performance due to a fluorine dopant, which improves the structural stability and redox potential voltage as compared to NVP in half-cell SIB. To explore NVPF in practical applications, it is used as a cathode in NIFC. NIFC was assembled by using microcubes of NVPF/rGO as a cathode and N-doped carbon as an anode. NVPF microcubes are caged in a graphene network, allowing favorable kinetics for Na-ion diffusion. NIFC delivered a discharge capacity of 98 mAh g⁻¹ at 0.5 C with a voltage range of 1.5–3.9 V. It shows good rate capability of 68 mAh g⁻¹ at 10 C and 95 mAh g⁻¹ when the discharge rate is set back to 0.5 C. NIFC shows 95.1% retention of its initial capacity at 10 C after 400 cycles. Moreover, NVPF/rGO//N-C delivered a specific energy density of 291 Wh kg⁻¹ and a maximum power density of 6144 W kg⁻¹⁸⁸ (Figure 12D–F). Yan et al.¹²⁸ assembled NIFC by using NVPF as a cathode and carbon as an anode. NIFC cycled between 2 and 4.3 V, and it delivered 111 mAh g⁻¹ at 0.1 C and 77% retention of capacity after 100 cycles. It also shows a high energy density of 418 Wh kg¹. The increase in energy density is related to the tetragonal structure due to the reversible removal of the third Na ion. The practicality of NVPF/C in full SIB was further investigated by using well-designed carbon-coated NVPF@C using a wet ball-milling strategy as a cathode and hard carbon cloth HCC as an anode. The NVPF@C//HCC delivered a discharge capacity of 115.9 mAh g⁻¹ at 0.1 C and a rate capability of 87.3 mAh g⁻¹ at 10 C between the voltage range 2 and 4.3 V. NIFC delivered 99.7 mAh g⁻¹ after 200 cycles, showing excellent capacity retention at 0.5 C. Moreover, it shows a high energy density of 428.5 Wh kg⁻¹ at 0.1 C.¹²⁹ The high-performance electrochemical results were due to the ultra-uniform carbon coating achieved by the ball-milling strategy, which increased the conductivity and diffusion kinetics. NIFC was fabricated with NVPF/CNTs as a cathode and a hard carbon sphere anode (HCS). NVPF/CNTs//HCS delivered 101.7 mAh g⁻¹ at 0.5 C and 82.4 mAh g⁻¹ at 5 C based on the mass of the cathode, with excellent retention of capacity 98.5% after 100 cycles at 0.5 C and 87.7% after 300 cycles at 5 C between the voltage range of 2 and 4.3 V.¹⁸⁴ Pi et al.¹²⁷ reported the development of NIFC for practical applications. NVPF/C was synthesized using a spray-dried process and was used as a cathode, whereas hard carbon (HC) was used as an anode. NIFC shows an excellent capacity of 114.1 mAh g⁻¹ at a 0.5 C with a good rate capability of 90.6 mAh g⁻¹ at 20 C and 71.8% retention of capacity after 600 cycles at 10 C rate between the voltage range 2.2 and 4.5 V. A NIFC device with NVPF/C as a cathode and synthesized nano architectures of SnP_x/carbon as an anode was obtained. NVPF//SnP/C delivered 120 mAh g⁻¹ at 0.1 C based on the cathode and an energy density of 280 Wh kg⁻¹ based on the total mass of the cathode and the anode with 78% retention of capacity after 200 cycles.¹³⁰ An NVPF/C//soft carbon full cell with an average voltage of 3.3 V delivered a discharge capacity of 107.9 mAh g⁻¹ at 0.5 C and 74.5% retention of capacity after 500 cycles at 5 C.⁸⁵ Moreover, NIFC showed a specific energy density of 338 Wh kg¹. NIFC was obtained by synthesizing NVPF/rGO using the sol–gel process as a cathode and NVP as an anode. NIFC delivered 90.7 mAh g⁻¹ at 0.1 C and rate capability of 61.5 mAh g⁻¹ at 5 C, with 83% retention of capacity after 300 cycles at 2 C¹²⁶ (Figure 12A–C). These studies show that all the good results in full cell sodium-ion batteries using NVPF as the cathode material are attributed to its unique structure, which is also stable due to a fluorine dopant. Apart from the crystal structure, these studies showed the development of different nano architectures of NVPF and uniform encapsulation of carbon-based material, which further enhance the kinetics of NVPF. We believe that as the NVPF has high redox potential with good electrochemical performance, it is the most viable material for commercialization. The only limitation is its high redox potential; it is hard to find a suitable electrolyte that is compatible at high potential. Therefore, more research work is needed to develop a suitable electrolyte for high potential to achieve long-term performance desired for practical applications.

Figure 12. (A) Charge–discharge curve of full cell Na3V2(PO4)2F3 (NVPF)@rGO//Na3V2(PO4)3 (NVP) at 0.5 C. (B) Rate capability of NVPF@rGO//NVP. (C) Cyclic performance of NVPF@rGO//NVP for 300 cycles at 2 C. Reproduced with permission: Copyright 2020, Elsevier.126 (D) Schematic representation of full cell NVPF@rGO//N-C. (E) Rate capability. (F) Cyclic performance for 400 cycles at 10 C. Reproduced with permission: Copyright 2018, Wiley-VCH.88

Na₃(VO)₂(PO₄)₂F (NVOPF)

NVOPF is another NASICON structured material that belongs to the same family of fluorophosphate (NVPF_3−2yO_2y where, y = 1). NVOPF is obtained from the oxygenation of NVPF, in which one fluorine atom is replaced by an oxygen atom. NVOPF shows a V³⁺/V⁴⁺ redox couple and a high voltage window of 3.6–4.1 V with a high theoretical capacity of 130 mAh g^–1 and energy density of 533 Wh kg⁻¹ due to the strong inductive effect of (PO₄)F anion.¹⁸⁷ Massa et al.¹⁸⁸ first explored the crystal structure of NVOPF, which has a tetragonal structure (space group I4/mmm) with lattice parameter of a = 6.3811 Å and c = 10.586 Å and unit cell volume V = 431.05 ų. The 3D crystal structure consists of bi-octahedra [V₂O₁₀F] and [PO₄] tetrahedra with two Na⁺ diffusion sites Na1 and Na2 at (8h) and (8i) and one F site at (2a) different from NVPF^187–190 (Figure 13A). Sauvage et al.¹⁸⁹ first explored the electrochemical properties of NVOPF. NVOPF shows two plateaus at 3.6 V and 4.0 V with a discharge capacity of 87 mAh g⁻¹ and poor retention of capacity. NVOPF/C was reported with an improved discharged capacity of 100 mAh g⁻¹ with the addition of carbon, but limited cyclic performance.¹⁸⁷ The presence of carbon and mixed-valence V³⁺/V⁴⁺ improves the conductivity and electrode mixture, which enhances battery performance. NVPOF with porous spherical structures was prepared using the solvothermal method, which shows better discharge capacity of 120 mAh g⁻¹ at 0.1 C, but the same limited cyclic performance up to 50 cycles.¹⁹² The long-term cyclic performance of NVOPF was improved by synthesizing nanoparticles of Na₃(VOPO₄)₂F using a facile solvothermal method at low temperatures.⁸⁹ NVOPF delivered 123.5 mAh g⁻¹ discharge capacity at 0.1 C and a good rate capability of 73 mAh g⁻¹ at 10 C, as well as 90% capacity retention after 1200 cycles at 2 C. The cyclic stability was further enhanced by synthesizing hierarchical porous spheres of Na₃(VOPO₄)₂F using a template-free solvothermal process. NVOPF delivered 127.1 mAh g⁻¹ at 0.5 C with an excellent rate capability of 85.4 mAh g⁻¹ at 50 C and 62.2% retention of capacity of 2000 charge–discharge cycles at 20 C.⁹⁵ Bianchini et al.,¹⁹¹ for the first time, reported the improved discharge capacity and energy density of Na₃(VOPO₄)₂F by enabling the insertion of a fourth Na⁺ ion and an additional redox peak at 1.57 V when discharged between 1 and 4 V. The phase transfer from Na₃(VOPO₄)₂F to Na₄(VOPO₄)₂F was further explored by operando XRD analysis. The (115) peak came back to its initial position after a complete cycle of Na⁺-ion extraction/insertion (Na⁺ extraction from Na₃VOPF to Na₁VOPF and then Na⁺ reinsertion to Na₄VOPF and final extraction to Na₃VOPF), which confirmed reversible intercalation and two biphasic reactions (Figure 13B,C). The intrinsic electronic conductivity of NVOPF improved with ionic doping, carbon coating, and porous nanostructuring (Figure 13D).^{55,91–95,193–195} Ru-doped porous hollow spheres of Na₃(VOPO₄)₂F were synthesized using a solvothermal method. It delivered 116.8 mAh g⁻¹ initial discharge capacity at 0.5 C, with an excellent rate capability of 84.1 mAh g⁻¹ at 20 C.⁹¹ Ru-doped NVOPF shows long-term cyclic stability by retaining 90.2% capacity after 1000 cycles and 65% retention after 7500 cycles at 20 C. The superior electrochemical performance is due to the RuO₂ conductive coating achieved by doping of Ru, and the hierarchical porous structure will also promote the good charge storage performance. Qi et al.⁹³ reported a scaleable liquid-liquid extraction room-temperature separation strategy to synthesize Na₃(VOPO₄)₂F for commercialization of SIB. NVOPF delivered an initial discharge capacity of 111 mAh g⁻¹ at 0.1 C and rate capability of 81 mAh g⁻¹ at 15 C. Moreover, it showed 70% retention of capacity after 3000 cycles at 15 C. The effect of the potential window and morphology on Na₃(VOPO₄)₂F was investigated by synthesizing both nano NVOPF and micro NVOPF using the hydrothermal method as the cathode material for SIB.¹⁹⁶ Nano-NVOPF delivered an initial discharge capacity of 121.5 mAh g⁻¹ between 3 and 4.5 V with excellent rate capability. Nano-NVOPF shows better electrochemical performance as compared to micro-NVOPF due to the small particle size, which promotes charge conduction during charging/discharging.

Figure 13. (A) Crystal structure of Na3(VOPO4)2F. (B) In situ X-ray diffraction patterns of Na3(VOPO4)2F during the full electrochemical cycle between 1 and 4.5 V. Reproduced with permission: Copyright 2021, Springer Nature.94 (C) Charge–discharge profile Na3(VOPO4)2F between different voltage windows; inset: cyclic voltammetry profile showing redox peaks. Reproduced with permission: Copyright 2017, Wiley-VCH.191 (D) Scanning electron microscopy image of multicore shell Na3(VOPO4)2F. Reproduced with permission: Copyright 2018, Cell Press Joule.93 (E) Rate capability of Na3(VO)2(PO4)2F (NVOPF) and NVOPF/CB composites. (F) Long-term cyclic performance of NVOPF and NVOPF/CB at 20 C. Reproduced with permission: Copyright 2021, Springer Nature.94

The full cell performance of NVOPF needs to be evaluated to achieve the commercial goals of SIBs. Arrays of Na₃(VOPO₄)₂F nanorods were fabricated on 3D graphene foam using the solvothermal method.⁹² NVOPF arrays delivered 130 mAh g⁻¹ capacity at 0.5 C with a good rate capability of 80 mAh g⁻¹ at 30 C, as well as remarkable ultralong stability of 90% retention after 10,000 charge–discharge cycles at 50 C. A flexible pouch cell was assembled using Na₃(VOPO₄)₂F/graphene foam as a cathode and VO₂ nanosheets as an anode. NIFC delivered a discharge capacity of 90 mAh g⁻¹ at 1 C and rate capability of 48 mAh g⁻¹ at 12 C. It also showed 80% retention of initial discharge capacity after 220 cycles at 4 C. Moreover, NIFC showed 215 Wh kg⁻¹ energy density at 320 W kg⁻¹ power density. The 3D unique structure of NVOPF combined with the 2D nano sheet of VO₂ leads to fast ionic transport and small volume change, which improves the rate performance. Moreover, the unique flexible structure is useful in flexible devices. The full cell performance was further investigated by coupling oxide anodes with NVOPF. The full battery of an SnO₂/Se/graphene composite as an anode and Na₃(VOPO₄)₂F as a cathode delivered 701.8 mAh g⁻¹ capacity based on the mass of the anode at 0.1 C with rate capability of 285.6 mAh g⁻¹ at 20 C as well as 88.6% retention of capacity after 200 cycles at 0.5 C. Moreover, NIFC delivered a specific energy density of 263.8 Wh kg⁻¹ (based on the total mass of the anode and the cathode) at 45.3 W kg⁻¹ power density and 96.3 Wh kg⁻¹ energy density at a power density of 7410 W kg⁻¹.¹⁹⁷ The NVOPF were fabricated by the solvent-free mechanochemical solid-state synthesis of Na₃(VOPO₄)₂F/carbon black composites for large-scale practical applications.⁹⁴ NVOPF/CB delivered an excellent discharge capacity of 141.9 mAh g⁻¹ at 0.1 C between 2.5 and 4.2 V, with a good rate capability of 112.8 mAh g⁻¹ at 20 C. Moreover, the long-term cyclic performance was tested for 10,000 cycles at 20 C; NVOPF/CB maintained 110.5 mAh g⁻¹, which is 98% retention of its initial discharge capacity (Figure 13E,F). This remarkable ultralong stability and low-cost manufacture were major strengths for the commercial application of SIB. Shen et al.⁹⁴ fabricated a full prototype cell (26,650 and 15,000 mAh) by using NVOPF/carbon black as a cathode and commercial hard carbon as an anode. NIFC delivered 1502 mAh at 0.1 C and rate capability of 1329 mAh at 5 C with 94.5% retention of capacity over 100 cycles at 3 C. We believe that, so far, from the commercial aspects, NVOPF delivers far better performance than other NASICON materials due to the synergic anion effects of fluorine and oxygen atoms with different carbon-coated nano porous structure. The full prototype devices also show exceptional storage capacity, and due to the excellent structural stability with high redox potential, it can combine with different anode materials in full devices. These initial studies show that NVOPF has excellent potential for the practical application of SIBs.

NVPF_−2yO_2y/NVP

In NVPF_−2yO_2y, the “F” atoms become unstable at high temperatures during heat treatment due to the presence of carbon and transform into NVP, Na₃VF₆, and V₂O₃. The new material consists of both vanadium phosphate and fluorophosphate Na₃(VOPO₄)₂F/NVP prepared using the carbothermal reduction method.⁹⁶ Due to the integration of stable NVP and high-energy-density Na₃(VOPO₄)₂F, long-term cyclic stability and high-rate capability can be achieved. NVPF/NVP/C delivered a discharge capacity of 104 mAh g⁻¹ at 1 C and 83 mAh g⁻¹ at a very high rate of 100 C with 63% retention for 3000 cycles at 5 C. A low-cost Fe-doped NVPF/NVP composite was synthesized using the sol–gel method. Vanadium is replaced with less toxic and low-cost Fe, which improves both ionic diffusion and electrochemical properties.⁹⁷ XRD analysis reveals that the three different phases of Na₃V_2−xFe_x (PO₄)₂F₃, Na₃V_2−xFe_x (PO₄)₃, and Na₅V_1−xFe_x(PO₄)₂F₂ correspond to P4₂/mmm, R3c, and P2₁/C, respectively. NVPF/NVP delivered an initial discharge capacity of 119.8 mAh g⁻¹ at 0.5 C in a potential window of 2–4.5 V and high-rate capability of 108 mAh g⁻¹ at 10 C, and also retained 91% of capacity after 2000 cycles at 10 C. Moreover, it delivered a high energy density of 433 Wh kg⁻¹ at 0.5 C. For practical applications, a full cell is assembled by using NVPF/NVP as a cathode and CuS as an anode. Electrochemical analysis is performed between 0 and 4.25 V. NIFC shows a discharge capacity of 101 mAh g⁻¹ at 1 C based on the mass of the cathode, with a high-rate capability of 81 mAh g⁻¹ at 10 C and 88% retention of capacity after 1200 cycles at 10 C. Moreover, NIFC delivered a high energy density of 162.2 Wh kg⁻¹ and a maximum power density of 1801 W kg⁻¹. These studies show that NASICON structured fluorophosphates have high discharge capacity, high redox potential, high theoretical energy density, and more structural stability than NVP due to the presence of highly electronegative fluorine. NVPF, NVPOF, and their composites are the most studied NASICON materials for SIB for full cell systems. These materials show good electrochemical performance in full sodium-ion devices. Furthermore, the low cost, scalable manufacture, and further optimization will keep these materials on track towards commercialization.

Na₃V(PO₃)₃N (NVPN)

NVPN is the new cathode material that belongs to the family of NASICON with the (PO₃)₃N anion group. It has a cubic crystal structure and space group (P2₁3) in which VO₆ octahedra are connected to three (PO₃)N tetrahedra that share the same N atom to form triphosphate (PO₃)₃N via corner sharing and build a 3D framework of [V(PO₃)₃N] with three Na⁺ diffusion sites Na1, Na2, and Na3 (Figure 14A). The nitrogen atom exerts a strong inductive effect by lowering the V–O bond covalency, which reduces the gap between bonding and antibonding orbitals and increases the potential voltage to 4 V.^70,199,200 Kim et al.⁷⁰ investigated the electrochemical properties and structural changes of NVPN. The first principal calculation shows the activation energies between diffusion pathways. Na⁺ diffusion from Na2 to Na3 is the fastest as it has the lowest activation energy of 0.287 eV. NVPN delivered a discharge capacity of 73 mAh g⁻¹ at 1 C between the voltage window of 2.5 and 4.25 V, with good rate capability 84% of capacity at 10 C. Moreover, it shows 67% capacity retention after 3000 cycles at 1 C. During the electrochemical process, almost zero structural changes (0.24%) were observed by ex situ XRD. Recently, Chen et al.⁷¹ synthesized N-doped graphene@NVPN composite using the freeze-drying approach. NVPN@NGO composites delivered 78.9 mAh g⁻¹ at 0.1 C between the voltage window of 3 and 4.25 V, with a rate capability of 59.2 mAh g⁻¹ at 30 C, as well as long-term cyclic performance by retaining 75.9% capacity after 5000 cycles at 10 C. NVPN@NGO were exposed to different environments, such as low temperatures, high temperatures, and air. NIFC shows good cyclic performance by retaining 92.3%, 90.3%, and 86.4% capacity after 800 cycles at 1 C at low temperatures, in air, and at high temperatures, respectively. Moreover, the full device is fabricated by using NVPN@NGO as a cathode and hard carbon as an anode. NIFC delivered 200 mAh g⁻¹ based on the anode at 100 mA g⁻¹ and 80% capacity retention after 150 cycles (Figure 14B,C). It is too early to say that NVPN will be useful for commercial applications as very few studies have been reported, and there is more room for research on the crystal structure and diffusion mechanism; also, uniform porous morphology from controlled synthesis can be achieved to increase the discharge capacity.

Figure 14. (A) Schematic illustration of the Na3V(PO3)3N (NVPN) crystal structure with Na+ ion migration pathways. (B) Charge–discharge curve of full cell NVPN//HC. (C) Cyclic performance of full cell NVPN//HC. Reproduced with permission: Copyright 2019, Wiley-VCH.71 (D) Schematic representation of the crystal structure of Na4Fe3(PO4)2(P2O7) (NFPP) with Na+ ion migration pathways. (E) Charge–discharge profile of an NFPP//Fe3O4 full cell. (F) Long-term cyclic performance of a full cell of NFPP//Fe3O4. Reproduced with permission: Copyright 2019, Springer Nature.98 (G) Schematic representation of the crystal structure of NaFe2(PO4)(SO4)2. (H) Charge–discharge profile of NaFe2(PO4)(SO4)2 at different rates. Reproduced with permission: Copyright 2018, Elsevier.198 (I) Rate capability of NaFeV(PO4)(SO4)2. Reproduced with permission: Copyright 2020, Elsevier.27

Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP)

NFPP is a mixed polyanion material that gained attention due to its NASICON-type structure with a 3D open framework, which has a high theoretical capacity of 129 mAh g⁻¹ and an average voltage of 3.2 V. It has an orthorhombic crystal structure (space group Pn2₁a). The 3D open framework consists of three FeO₆ octahedra interconnected to three tetrahedra PO₄ to form a [Fe₃P₂O₁₃] along the a axis. [Fe₃P₂O₁₃]_∞ and P₂O₇ diphosphate are connected along the a axis, which leads to the formation of large tunnels for Na⁺ ion diffusion along with the b axis^98,201,202 (Figure 14D). Kim et al.²⁰² first investigated the electrochemical mechanism of NFPP for SIB, which delivered a discharge capacity of 113.5 mAh g⁻¹ at 0.1 C. A tuneable NFPP/C composite was prepared using the solvothermal approach for SIB. NFPP/C delivered a discharge capacity of 113 mAh g⁻¹ at 0.05 C and a good rate capability of 80.3 mAh g⁻¹ at 20 C. The long-term cyclic performance shows 69.1% retention of capacity after 4400 charge/discharge cycles at 20 C.⁹⁸ Furthermore, it shows stability at low and high temperatures, at −20°C, delivering 95 mAh g⁻¹ at 0.1 C with 92.1% retention of capacity for 250 cycles at 0.5 C, and there is almost no change in the discharge capacity at a high temperature of 50°C, with 94.1% retention of capacity for 250 cycles at 0.5 C. NIFC was assembled by using NFPP/C as a cathode. NIFC delivered 225 mAh g⁻¹ at 100 mA g⁻¹(60 mA g⁻¹ = 0.5 C) based on the mass of the anode and 76.9% retention of capacity after 500 cycles at 100 mA g⁻¹ (Figure 14E,F). NFPP/C nanospheres with an average size of 30 nm were synthesized using the template method. NFPP/C shows a high discharge capacity of 128.5 mAh g⁻¹ at 0.2 C, with excellent rate capability of 79 mAh g⁻¹ at 100 C. Moreover, in terms of long-term cyclic performance, it shows 63.5% retention of the initial discharge capacity after 4000 cycles at 10 C.⁹⁹ 3D porous NFPP/microspheres decorated with rGO were synthesized using the spray-drying process.²⁰³ NFPP/rGO shows 128 mAh g⁻¹ discharge capacity at 0.1 C, with excellent rate capability of 35 mAh g⁻¹ at 200 C as well as capacity retention of 62.3% after 6000 charge/discharge cycles at 10C. Cao et al.¹⁰⁰ synthesized nanospheres of NFPP on MCNTs using the spray-drying method as a cathode for SIB. NFPP/MWCNTs show an initial discharge capacity of 115.7 mAh g⁻¹ at 0.1 C. It shows high-rate capability, with 62.8 mAh g⁻¹ at 20 C, and excellent capacity retention of 95% after 1200 cycles at 2 C. Moreover, the full device was assembled by using NFPP/MWCNTs as a cathode and HC as an anode. NIFC delivered 69.3 mAh g⁻¹ based on the mass of the cathode at 0.1 C between 1 and 4V, with 74% capacity retention after 100 cycles at 1 C. NFPP/MWCNTs//HC shows an energy density of 210 Wh kg⁻¹ based on the total mass of the electrodes and power density of 391 W kg⁻¹. Hollow spheres of NFPP/C were reported using a scaleable spray-drying method.¹⁰¹ NFPP/C delivers a discharge capacity of 107.7 mAh g⁻¹ at 0.2 C and a high-rate capability of 88 mAh g⁻¹ at 10 C, with 92% retention of the initial discharge capacity after 1500 charge/discharge cycles at 10 C. Moreover, the full cell was assembled by using NFPP/C as a cathode and HC as an anode. NIFC shows 79 mAh g⁻¹ capacity at 0.5 C based on the mass of the cathode in the potential voltage window of 0.3–4V with 65.8% capacity retention after 100 cycles at 0.5 C and an energy density of 108 Wh kg⁻¹ based on the cathode. These studies on mixed polyanion materials show the performance of NFPP as cathode materials for SIBs. NFPP shows good performance in a half-cell battery due to its 3D framework, which consists of one octahedra and three tetrahedra. Different porous nanostructures with carbon coatings that enhance the electrochemical performance have been discussed, but still, for full cells, very few studies have been reported and there is a need to improve the long-term performance in a full battery, so more research needs to be done on full cells of NFPP with different anode materials to enhance the cyclic performance and rate capability to meet the practical demands of SIBs.

Na_xFe₂(PO₄) (SO₄)₂

Na_xFe₂(PO₄)(SO₄)₂ is another mixed phosphate explored recently by replacing the PO₄ anion with a highly electronegative sulfate (SO₄) group. Shiva et al.²⁰⁴ first reported an NASICON framework NaFe₂PO₄(SO₄)₂ as a cathode material for SIB. The crystal structure is trigonal (space group R3c) (Figure 14G). Electrochemical analysis shows that NaFe₂PO₄(SO₄)₂ delivered 100 mAh g⁻¹ after 50 cycles at 0.1 C in a potential window of 2–4 V. The discharge capacity increases with cycles due to the unoptimized morphology of NaFe₂PO₄(SO₄)₂. Yahia et al.¹⁹⁸ further investigated the performance of NaFe₂PO₄(SO₄)₂ as a cathode material. NaFe₂PO₄(SO₄)₂ shows an activation energy of 0.60 eV, which is lower than that of the NASICON-type sulfate Fe₂(SO₄)₃: (0.89 eV).²⁰⁵ Moreover, it delivers 89 mAh g⁻¹ at C/20 and 96% capacity retention after 30 cycles at C/5 at a potential voltage of 3 V (Figure 14H). The electrochemical stability of NaFe₂PO₄(SO₄)₂ improved with optimization of the composition with the addition of vanadium NaFe_2−xV_xPO₄(SO₄)₂ (Figure 14I).²⁷ NaFe_2−xV_xPO₄(SO₄)₂ was synthesized using the sol–gel process with different compositions. XRD analysis and structural refinement show that NaFe_2−xV_xPO₄(SO₄)₂ has a trigonal crystal structure with the space group R-3. The 3D framework consists of Fe/VO₆ octahedra connected to a P/SO₄ tetrahedron. NaFe_2−xV_xPO₄(SO₄)₂ with x = 0.2, 0.4, and 0.6 shows a discharge capacity of >72 mAh g⁻¹ with 96% capacity retention after 50 cycles between the potential window of 1.5 and 4.5 V. NFPS is the new electrode material that has been investigated in recent years. The mixed phosphate and sulfate anion structures were explored in detail. We believe that the performance of this material is not up to the mark as compared to other NASICON materials as the discharge capacity and rate performance are comparatively low. The performance needs to be enhanced in the future with more research on internal structural features, and different refinement methods on nanostructures may improve the electrochemical performance in full cells.

NASICON ANODE MATERIALS NaTi₂(PO₄)₃ (NTP)

NASICON structured NTP is explored as an anode material for SIBs because of its 3D open framework. It has a rhombohedral structure and an R3C space group in which TiO₆ octahedra and PO₄ tetrahedra are interlinked via corner-sharing to form a lantern unit and two interstitial sites, M1 and M2, for Na ion diffusion.²⁰⁶ Owing to structural and thermal stability with fast Na ion diffusion and small expansion of volume during Na ion intercalation/deintercalation, it delivers a large theoretical capacity of 133 mA g⁻¹ and long cyclic stability. There are two possible Na ion migration pathways in NTP (Figure 15A): Pathway (I) between octahedra and tetrahedra along [100] and (II) between two tetrahedra along the [001] direction. The calculated activation energy barrier is shown in Figure 15B. The activation energy barrier is 0.79 eV along Pathway (I); the position of the Na⁺ ion is adjusted according to the strong electrostatic repulsion between two Na⁺ ions. However, the activation energy is reduced to 0.51 eV along Pathway (II), which is 35% less, which means that Na⁺ ions prefer this migration path between two tetrahedra during the sodiation process.²⁰⁷ Apart from the rhombohedral structure, senguttuvan et al.²⁰⁸ confirmed the presence of a triclinic structure with the P1 space group of Na₃Ti₂(PO₄)₃. Electrochemical analysis shows two redox couples Ti⁴⁺/Ti³⁺ and Ti³⁺/Ti²⁺ at 2.1 and 0.4 V, respectively, as shown in Figure 15C, with a 1.7 V difference. Delmas et al.,²¹¹ in 1987, first reported the reversible intercalation of sodium in NTP by a two-phase mechanism (Equation 2). They found that two sodium ions reversibly intercalated in NTP with the voltage plateau at 2.1 V in NTP, where intercalation of sodium starts and deintercalation starts at 2.15 V. Park et al.²¹² first reported NTP as an anode material for aqueous sodium-ion batteries. It shows a discharge capacity of 130 mAh g⁻¹, very close to the theoretical capacity. The rate capability of NTP is poor because of the low electrical conductivity of the phosphate group, which limits its practical applications. To overcome this limitation, mainly, there are three proposed strategies: (1) combining with carbon-based materials like carbon,^40,41 CNTs,²¹³ and graphene^{102,104,206,210} increases electrical conductivity, (2) elemental Doping,²¹⁴ (3) designing a 3D porous architecture as shown in Figure 15D–F to enlarge the surface area, which provides fast electronic transport that improves the overall electrochemical performance.^{103,104,106,125,215} [Image Omitted. See PDF]

Figure 15. (A) Schematic illustration of top and side views of the crystal structure of NaTi2(PO4)3 (NTP) showing pathways for Na ion diffusion along paths I and II. (B) Minimum energy calculation for Na ion diffusion along paths I and II. Reproduced with permission: Copyright 2020, Wiley-VCH.207 (C) Galvanostatic charge–discharge profile of Na3Ti2(PO4)3 versus Na between 0 and 2.5 V. Reproduced with permission: Copyright 2013, American Chemical Society.208 (D) Schematic illustration of a porous NTP/C framework; inset: crystal structure of NTP. Reproduced with permission: Copyright 2016, Elsevier.40 (E,F) Scanning electron microscopy images of a porous NTP/C array. Reproduced with permission: Copyright 2015, The Royal Society of Chemistry.209 (G) Rate capability of NTP-three-dimensional graphene at 1 C. Reproduced with permission: Copyright 2015, American Chemical Society.210 (H) Cyclic Stability of NTP/C for 10,000 charge–discharge cycles at 20 C. Reproduced with permission: Copyright 2016, Elsevier.40

NTP/rGO nanoparticles were synthesized using the polyol-assisted method. NTP/rGO delivered 100 mAh g⁻¹ discharge capacity at 2.3 C with a rate capability of 78 mAh g⁻¹ at 36.8 C and 68% retention of capacity after 1000 cycles at 20 C.¹⁰² In-depth studies on Na⁺ intercalation/deintercalation during charging and discharging were carried by ex situ X-ray absorption near-edge structure XANES. It was found that during discharging, the peak is shifted to lower energies, which represents Na⁺ intercalation, and during charging, the peak is shifted to higher energy, which shows Na⁺ ion deintercalation. The ex situ XANES analysis shows the reversible peak shifting representing the reversible Ti oxidation state transformation during electrochemical cycling. Porous NTP in a 3D graphene network delivers 112 mAh g⁻¹ capacity at 1 C and a high-rate capability of 67 mAh g⁻¹ at 50 C with 80% retention after 1000 cycles (Figure 15G).²¹⁰ Wang et al.¹⁰⁴ synthesized NTP/reduced graphene oxide as an anode material for NIB using the facile hydrothermal method. NTP/rGO delivers good discharge capacity and stability, with 129.2 mAh g⁻¹ discharge capacity at 0.1 C, with retention of 91% capacity after 250 cycles at 0.5 C and 82% retention at a high rate of 10 C after 1000 cycles.¹⁰⁴ Chang et al.⁴⁰ synthesized hierarchical mesoporous nanoflowers of NTP/C using a facile solvothermal method. It shows 125 mAh g⁻¹ capacity at 1 C and excellent rate capability by achieving 95 mAh g⁻¹ capacity at 100 C. The synthesized material shows exceptional long-term stability with 77.3% retention of capacity after 10,000 charge–discharge cycles at 20 C (Figure 15H). Recently, Zhang et al.¹⁰⁵ prepared low-temperature NTP/C with nanoarchitecture by a solid-state reaction. NTP/C delivered 132 mAh g⁻¹ at 0.2 C with excellent retention capacity of 87.5% after 1000 cycles at 50 C and 115 mAh g⁻¹ at a low temperature of −20°C. Rui et al.²⁰⁷ fabricated 3D hierarchal porous NTP/C foams of interconnecting pore channels. NTP/C delivers excellent Na⁺ ion diffusion of 2.94 × 10⁻⁹ cm² s⁻¹ at a low temperature of −20°C. It also shows 125 mAh g⁻¹ discharge capacity at 0.2 C at −20°C, with an excellent rate capability of 95 mAh g⁻¹ at 20 C, and maintained 116 mAh g⁻¹ after 500 cycles. Although the phosphate group limits the electronic conductivity, the above strategies show that the rate capability can be improved effectively by designing hierarchal porous architectures of NTP/C, which improve the electrochemical performance overall and represent a step forward towards the practical development of SIB for large-scale applications.

NASICON structured NTP exhibits good thermal stability, 3D structural framework and high theoretical capacity of 133 mAh g⁻¹ can be used in NIFC. The NASICON NTP anode combines with different cathodes of layered metal oxide, Prussian blue analog (PBA), and NASICON structured cathode materials. Li et al.²¹⁶ assembled NIFC of NTP/Na_0.44MnO₂. It shows a reversible capacity of 120 mAh g⁻¹ based on the mass of the NTP anode. It delivers an energy density of 127 Wh L⁻¹ and stable cyclic performance of >1000 cycles at 100 C. Pang et al.¹¹⁹ enhanced the performance by fabricating a full device using NTP@MWCNTs as an anode and Na_0.44MnO₂ nanorods as a cathode, 1 M Na₂SO₄ as electrolyte. NIFC delivered 128 mAh g⁻¹ reversible capacity at an operating voltage of 1.1 V and 58.7 Wh kg⁻¹ energy density, but cyclic stability is limited. The cyclic stability of the full cell was further enhanced by assembling hierarchal porous NTP/C//Na_0.44MnO₂.²⁰⁹ It shows long-term cyclic stability with 84% retention of its initial discharge capacity after 500 cycles at 1 C. However, the metal oxides NIFC have low energy density due to a small operating window limited to 1.1 V and low cyclic stability. PBAs have an open framework with large interstitial sites and are easy to fabricate with binder-free cathodes that can combine with a NASICON NTP anode for aqueous NIFC. Wu et al.²¹⁷ assembled aqueous NIFC by synthesizing copper hexacyanoferrate(II) Na₂CuFe(CN)₆ as a cathode material and carbon-coated NaTi₂(PO₄)₃/C as an anode. NIFC delivered an energy density of 48 Wh kg⁻¹ with excellent 97% capacity retention after 100 cycles and 88% capacity retention after 1000 cycles at 2 C. He et al.²¹⁸ fabricated a wearable aqueous SIB by synthesizing flexible nickel hexacyanoferrate KNiFe(CN)₆ on carbon nanofiber KNHCF@CNF as a cathode and NTP grown on CNF as an anode. Flexible binder-free SIB delivered a volumetric capacity of 34.21 mAh cm⁻³ at 0.2 mA cm⁻³ in a voltage window of 1.5 V and high energy density of 27.72 Wh g⁻¹ based on the total mass of the anode and cathode, with 84.7% retention of the initial capacity after 500 cycles.

As discussed earlier, the NASICON structured material NVP is an excellent candidate for using as a cathode due to the robust structure for Na ion diffusion and high redox potential of 3.4 V, so fabricating NIFC by using both NASICON structured NVP as a cathode and NTP represents a possible way to overcome issues related to the low operating voltage, long-term stability, and rate capability of aqueous SIB (Figure 16A). Zhang et al.²¹⁹ assembled all NASICON aqueous NIFC systems using an NTP anode and an NVP cathode. The NIFC delivered a high energy density of 29 Wh kg⁻¹ at a power density of 5145 W kg⁻¹. Self-standing NIFC was fabricated by using a novel morphology with a stable structure of NTP/CNF as an anode and NVP/CNF as a cathode.¹²³ NIFC delivered 120 mAh g⁻¹ at a high rate of 30 C. It also shows remarkable 74.5% retention of ultralong stability after 4000 cycles at 20 C. Zhang et al.¹²⁴ fabricated NIFC comprising a porous NVP/C cathode and an NTP/C anode for long-term stability. The NIFC delivered 104 mAh g⁻¹ capacity based on the mass of the cathode at 0.5 C with a high specific energy density of 67.1 Wh kg⁻¹ as well as remarkable 94% retention of capacity over 5000 charge–discharge cycles at 20 C with a good energy efficiency of 80%. Moreover, a prototype flexible pouch cell was assembled by stacking a pair of electrodes, which also shows a total cell capacity of 2.24 mA for two pairs and 4.37 mA for three pairs of electrodes, corresponding to 89.2% and 93.2% of the theoretical capacity, respectively. Apart from normal operating conditions, NASICON is also explored as a low-temperature and high-temperature SIB. Recently, Rui et al.²⁰⁷ fabricated NIFC by synthesizing porous foams of NTP/C as an anode and NVP/C as a cathode at low temperatures. NIFC delivered a reversible capacity of 60.5 mAh g⁻¹ at 0.2 C at −20°C, which is 94% of its theoretical capacity, and the excellent long-term stability with over 73% retention after 1000 cycles at low temperatures represents a step forward towards achieving Low temperature SIB (Figure 16B). NASICON structured materials for NIFC for high-temperature performance were fabricated by using a carbon-coated nanoarchitecture of NVP@C as a cathode and NTP@C as an anode.¹³⁴ The cell delivered 108 mAh g⁻¹ capacity at a high temperature of 60°C based on the mass of the cathode, with excellent rate capability of 91 mAh g⁻¹ at 10 C and 85 mAh g⁻¹ at 20 C. Moreover, NIFC shows long cyclic stability at a high temperature (60°C) and 75% retention after 1000 cycles at 20 C (Figure 16C,D).

Figure 16. (A) Schematic of a sodium-ion full cell of three-dimensional porous foam structures of NaTi2(PO4)3 (NTP)/C as an anode and Na3V2(PO4)3 (NVP)/C as a cathode. (B) Charge–discharge curves of a low-temperature sodium-ion battery (SIB) NTP/C//NVP/C at 0.2 C. Reproduced with permission: Copyright 2020, Wiley-VCH.207 (C) Rate capability of high-temperature SIB NTP@C//NVP at 1 C, 2 C, 5 C, 10 C, and 20 C. (D) Cyclic performance of an NTP/C//NVP//C full cell at 20 C. Reproduced with permission: Copyright 2020, Wiley-VCH.134

Titanium-based fluorophosphate Na_1−2xTi₂(PO₄)_3−xF_x were (0 ≤ x ≤ 1) explored as anode materials obtained by the doping of fluorine in NTP.¹⁰⁷ The crystal structure of Na_xTPF is rhombohedral (space group R3C), same as that of NTP, in which the TiO₆ octahedral shows corner sharing with PO₄ tetrahedra to form a 3D structure. Wei et al.¹⁰⁷ compared the electrochemical performance of NTP and NTPF and demonstrated that fluorine doping provides faster Na⁺ diffusion and improves the electrochemical performance. NTPF/C delivers 121 mAh g⁻¹ discharge capacity at 0.2 C, with a rate capability of 62.5 mAh g⁻¹ at 30 C. It also shows 70% retention of the initial discharge capacity after 1000 cycles at 10 C. Density functional theory calculations show that F atom replaces (PO₄)₃ in NTPF, which shifts the Fermi level upwards and improves the electronic conductivity. A full cell was assembled by using NTPF as an anode and NiHCF as a cathode. Na₃V₂(PO₄)₃ delivered 120 mAh g⁻¹ discharge capacity between 0.5 and 2.2 V at 66.5 mA g⁻¹, with 70% retention of capacity after 1000 cycles (Figure 16C,D).

Based on the studies, all NASICON structured materials show better performance in terms of reversible discharge capacity, rate capability, and long-term stability. We believe that among all the NASICON materials, NTP is the most suitable anode material due to the low redox couple. NTP has high ionic conductivity and exceptional structural stability, which allow almost zero volume change during Na⁺ ion insertion/deinsertion and inhibits the formation of SEI on the surface of the anode. In a full cell, when NTP is combined with the cathode material, it shows comparable performance with LIB. An NTP full cell is safer and provides long-term stability and large capacity. Furthermore, NTP shows good performance in all full cell devices at all temperatures. For practical applications, there is still a need to improve the energy density. NTP is a potential candidate for flexible wearable aqueous SIB, low-temperature SIB, and high-temperature SIB for practical applications.

NASICON SOLID ELECTROLYTES

NASICON structured solid electrolytes have been explored for the past 45 years after they were first studied by Goodenough et al. on Na_1+xZr₂P_3−xSi_xO₁₂ (0 ≤ x ≤ 3) in 1976^220,221 as a replacement for liquid electrolytes in SIB. Na_1+xZr₂P_3−xSi_xO₁₂ has two types of crystal structures: monoclinic (space group C2/c) when 1.8 ≤ x ≤ 2.2 and rhombohedral (space group R3C). The 3D robust framework consists of ZrO₆ octahedra connected to (Si/P)O₄ tetrahedra via corner sharing. In the rhombohedral structure, there are four Na⁺ diffusion sites, one Na1 and three Na2 sites, and these Na2 sites split into 1Na2 and 2Na3 sites in the monoclinic structure.^220,222 The ionic conductivity is strongly dependent on the composition and structure of the electrolyte. Na₃Zr₂PSi₂O₁₂ shows ionic conductivity of 0.2 S cm⁻¹ at 300°C with an activation energy of E = 0.29 eV and 6.7 × 10⁻⁴ S cm⁻¹ at room temperature. Jolly et al.²²³ investigated the phase transition from the monoclinic phase to the rhombohedral phase of Na₃Zr₂PSi₂O₁₂ at high temperatures. In situ XRD analysis reveals that the monoclinic phase exists at low temperatures and the stable rhombohedral phase only exists at high temperatures. Zou et al.²²⁴ investigated the Na⁺ diffusion in rhombohedral NASICON materials by initio molecular dynamics (AIMD) simulations. There are two Na⁺ ion migration channels possible in the rhombohedral structure: (1) Na1–Na3–Na2–Na3–Na1 and (2) Na2–Na3–Na3–Na2. These migration pathways consist of three segments: Na1–Na3, Na2–Na3, and Na3–Na3. AIMD simulation shows that diffusion in the Na1–Na3 channel is dominant as compared to Na3–Na3, which is very rare, so Na⁺ ion migration is more feasible in Na1–Na3–Na2–Na3–Na1. Those NASICON materials with a large Na1–Na3–Na2–Na3–Na1 channel will have high ionic conductivity. Moreover, the study shows that there are two types of migration of two Na⁺ ions at Na1 and Na2 sites: (1) along the same direction and (2) along different directions. The activation energy of a single Na⁺ ion is 0.472 eV, but in two Na⁺ ions, due to Coulombic interaction, it is 0.103 eV along the same direction and 0.242 eV along different directions. Doping and increasing the Na⁺ concentration lead to a decrease in the activation energy by facilitating concerted migration, which overall increases the ionic conductivity. Different dopant strategies and excess Na have been used to improve the ionic conductivity of Na_1+xZr₂P_3−xSi_xO₁₂. Na_3.3Zr₂Si₂PO₁₂ was synthesized using the solid-state sintering method. Na_3.3Zr₂Si₂PO₁₂ shows a conductivity of 0.12 S cm⁻¹ at 300°C.²²⁵ Ma et al.²²⁶ reported the fabrication of Na_3.4Zr₂Si_2.4P_0.6O₁₂ using a solution-assisted solid-state reaction method. Na_3.4Zr₂Si_2.4P_0.6O₁₂ showed conductivity of 5 × 10⁻³ S cm⁻¹ at room temperature. They also reported the fabrication of scandium-doped Na_3.4Sc_0.4Zr_1.6(SiO₄)₂(PO₄) by a solid-state reaction.²²⁶ Na_3.4Sc_0.4Zr_1.6(SiO₄)₂(PO₄) delivered ionic conductivity of 4 × 10⁻³ S cm⁻¹ at 25°C. Na₃Zr₂PSi₂O₁₂ was synthesized with different dopants Ce⁴⁺, Gd³⁺, and Yb³⁺ using the sintering method. Na₃Zr_1.9Yb_0.1Si₂PO₁₂ shows ionic conductivity of 1.7 × 10⁻³ S cm⁻¹, Na₃Zr_1.9Ce_0.1Si₂PO₁₂ shows ionic conductivity of 1.3 × 10⁻³ S cm⁻¹, and Na₃Zr_1.9Gd_0.1Si₂PO₁₂ shows ionic conductivity of 1.3 × 10⁻³ S cm⁻¹ as well as activation energy barriers of 0.38 eV, 0.29 eV, and 0.33 eV for Na₃Zr_1.9Yb_0.1Si₂PO₁₂, Na₃Zr_1.9Ce_0.1Si₂PO₁₂, and Na₃Zr_1.9Gd_0.1Si₂PO₁₂, respectively.²²⁷ Sun et al.²²⁸ reported doping of La into Na₃Zr₂PSi₂O₁₂ to improve the ionic conductivity. Na_3+xLa_xZr_2−xSi₂PO₁₂ with (0 ≤ x ≤ 0.5) has been synthesized using the sol–gel method. Na_3+xLa_xZr_2−xSi₂PO₁₂ with an optimal value of x = 0.3 shows an ionic conductivity of 1.34 × 10⁻³ S m⁻¹ at 25°C. Leng et al.²²⁹ reported the fabrication of Mg-doped Na_3.256Mg_0.128Zr_1.872Si₂PO₁₂ using a cold sintered approach. Na_3.256Mg_0.128Zr_1.872Si₂PO₁₂ shows a high ionic conductivity of 1.36 × 10^-3 S cm⁻¹ at room temperature. Composite Na_3.3Zr₂Si₂PO₁₂ electrolyte was obtained with 5% Na₂SiO₃ with liquid-phase sintering, which shows ionic conductivity of 1.45 × 10⁻³ S cm⁻¹.²³⁰ The porous architecture of Ca²⁺-doped Na₃Zr₂PSi₂O₁₂ was designed by sol–gel synthesis.²³¹ NZCSP shows a high ionic conductivity of 1.67 × 10⁻³ S cm⁻¹ at 25°C and an activation energy of 0.29 eV. Moreover, a full solid-state battery was assembled by using a Na metal anode, a dense layer of electrolyte, and an NVP composite as a cathode. NIFC delivered 103 mAh g⁻¹ at 0.2 C and 80.5 mAh g⁻¹ at 4 C, with >98% capacity retention after 450 cycles at 1 C. Lan et al.²³² assembled a full solid-state battery by using NVP as a cathode, Na_3.4Zr₂Si_2.4P_0.6O₁₂ as electrolyte, and Na metal as an anode. At room temperature, the full cell delivered 106 mAh g⁻¹ at 0.6 C with 90.57% capacity retention after 100 cycles at 0.6 C. These studies show that NASICON electrolytes with different modification strategies show a high ionic conductivity of 10⁻³ S cm⁻¹. These studies show that the ionic conductivity of NASICON solid electrolytes can be enhanced by changing the composition by doping, controlled sintering strategies, and porous structure design, which increase the Na diffusion. Besides these enhancements, there is still room for more research to improve the ionic conductivity of a NASICON solid electrolyte comparable to a liquid electrolyte for a full solid-state battery.

CONCLUSION AND OUTLOOK

In summary, NASICONs have a robust 3D framework with excellent ionic conductivity, thermal and structural stability, and suitable redox potential both as a cathode and as an anode. The 3D open framework provides exposed channels for Na⁺ diffusion and causes minimum volume changes during sodiation/dissociation, which improve the long-term cyclic performance and rate capability. This study provided a summary of the structural features of electrode materials, the Na⁺ ion diffusion mechanism, and detailed electrochemical analysis of half-cells as well as NIFCs. NASICON can be used as an anode, a cathode, or as an electrolyte because of its versatile structural properties. Despite these advantages, NASICON has low electronic conductivity, which limits its electrochemical performance in practical devices. This shortcoming of NASICON has been addressed by the use of various approaches like carbon coating, doping, and nanostructuring techniques. NVP is the most widely investigated material as a cathode for SIBs. However, there are two main drawbacks of using NVP: (1) the average potential is limited to 3.4 V, which limits the energy density, and (2) vanadium has a toxic nature. These drawbacks are addressed by the doping of F atoms, which increases its redox potential and improves energy density. NVPF has a high voltage potential of 4.3 V, and currently, it is considered as the best cathode material for NIFC in practical applications. To minimize the toxicity of V and cost, more abundant and less toxic elements (like Mn and Fe, etc.) have been explored. Mn- and Fe-based NASICONs have multi-electron reactions, less toxicity, high abundance, lower cost, and facile synthesizable approaches, which will prove to be more effective for practical applications. These transition-metal elements provide a new avenue to explore numerous other combinations of materials. However, a better understanding of the storage mechanism is still required to make these cathodes practically viable. In anode materials, the main challenge related to NASICON is the high redox potential of >1.5 V. To date, almost all the NASICONs, including, NVP, Na₃Fe₂(PO₄)₃, NVPF, and mixed-metal phosphates like Na₄MnCr(PO₄)₃, have higher voltage potential, except Na₃Ti₂(PO₄)_3, which stores charge at a lower voltage of 0.4 V. So, for practical SIB applications, NTP is the most promising choice as an anode. NASICON structured solid electrolytes also have several limitations, including comparatively low ionic conductivity at room temperature as compared to liquid electrolytes and compatibility issues with the electrode and the separator due to large interfacial resistance. To meet the requirement of high ionic conductivity (10⁻² S cm⁻¹), different modification techniques have been applied like doping of elements (Ce, Si, etc.) and a high-temperature sintering process, which will improve the overall ionic conductivity from 10⁻⁴ to 10⁻³ S cm⁻¹. Regardless of these improvements, there is still sufficient room for enhancements of ionic conductivity and stabilization of the interface between the electrode/electrolyte (Figure 17).

Figure 17. Summary of the advantages, limitations, and future outlook for different sodium superionic conductor (NASICON) materials.

Scientific insights discussed in this article relates structural features of NASICON materials with electrochemical performance for SIBs. Apart from the scientific research on a lab scale, there are industrial perspectives of SIBs. Despite the excellent performances of some of these materials, SIB technology is still facing practical limitations for commercialization. For the commercialization of SIBs, there are other factors that need to be considered, like thermal runaway, high safety design, manufacturing cost, fabrication feasibility of both the material and the device, and environmental compatibility. In the practical application of SIB batteries, major concerns are thermal runaway, leakage of the liquid electrolyte, unstable SEI layer formation, and major safety problems. Therefore, suitable selection of materials with high thermal stability is necessary. The ultimate goal is to fabricate low-cost SIBs with safer and nonexplosive materials. The cost of the battery depends on the type of electrode materials, electrolytes, binders, and separators. The material should be cost-effective and environmentally friendly as V and Cr are rare-earth transition metals, and V can be harmful because of toxicity; also, battery manufacturing costs will be high as compared to the use of Fe and Mn. Similarly, the synthesis method should be scalable in the industry. In the full cell prototype, the performance is mainly affected by the environmental conditions. To successfully commercialize the NASICONs, the material should be stable both at low temperature and high temperature as a slight change in temperature can cause thermal runaway problems. Moreover, the choice of the electrolyte is very important for prototype fabrication. Electrolytes should have high redox potential and can form a stable interface with the electrode; otherwise, an unstable solid electrolyte interface will form, which can cause dendrite formation and thermal runaway problems. All NASICON solid-state batteries are possible solution of abovementioned problems, because of their stable structural performance. All NASICON solid-state batteries have all the components cathode, anode, and electrolyte with a 3D stable framework.^232–235 Moreover, NASICON materials show good electrochemical performance at high and low temperatures, so these batteries can be used for all climates. There are two possible ways to fabricate NIFC: (1) using an asymmetric full cell (by using a different anode and cathode) and (2) using a symmetric full cell (using the same material for both electrodes). Asymmetric full cells have low-rate capability and long-term performance; also, the fabrication is difficult, and different types of current collectors Al and Cu have to be used, which increase the cost. By using the same electrode material, the safety concern can be addressed owing to the comparatively smaller volume expansion, which increases the cyclic performance. Moreover, in a symmetric cell, the same current collector can be used to save additional cost, and fabrication of the device will also become easier, which makes it the most viable choice for practical large-scale grid applications.

ACKNOWLEDGMENTS

This study was financially supported by the National Natural Science Foundation of China (52027801 and 51631001), the National Key R&D Program of China (2017YFA020N6301), the Natural Science Foundation of Beijing Municipality (2191001), and the China-Germany Collaboration Project (M-0199).

CONFLICTS OF INTEREST

The authors declare no conflicts of interest.