Scientific challenges faced by Mn‐based layered oxide cathodes with anionic redox for sodium‐ion batteries

Abstract

In the realm of sodium‐ion batteries (SIBs), Mn‐based layered oxide cathodes have garnered considerable attention owing to their anionic redox reactions (ARRs). Compared to other types of popular sodium‐ion cathodes, Mn‐based layered oxide cathodes with ARRs exhibit outstanding specific capacity and energy density, making them promising for SIB applications. However, these cathodes still face some scientific challenges that need to be addressed. This review systematically summarizes the composition, structure, oxygen‐redox mechanism, and performance of various types of Mn‐based cathodes with ARRs, as well as the main scientific challenges they face, including sluggish ion diffusion, cationic migration, O₂ release, and element dissolution. Currently, to resolve these challenges, efforts mainly focus on six aspects: synthesis methods, structural design, doped modification, electrolyte design, and surface engineering. Finally, this review provides new insights for future direction, encompassing both fundamental research, such as novel cathode types, interface optimization, and interdisciplinary research, and considerations from an industrialization perspective, including scalability, stability, and safety.

INTRODUCTION

The field of electrochemical energy storage, particularly lithium-ion batteries (LIBs), has witnessed significant advancements. This progress is attributed to the numerous advantages of LIBs, including their high energy density, minimal self-discharge, and extended lifespan. The widespread adoption of electric vehicles and the establishment of large-scale energy storage facilities in the future necessitate battery systems that offer increased weight-specific and volume-specific energy capacities, all while addressing cost-related challenges during development. Consequently, continuous optimization of LIBs is imperative to meet the demands for enhanced capacity, longevity, and cost-efficiency.¹ As appealing cathodes, the oxide and sulfur cathodes have received significant attention. Sulfur's multielectron reaction (S₈ + 16Li → 8Li₂S) grants a high theoretical specific capacity of 1673 mAh g⁻¹, with average voltage reaching 2.15 V, resulting in a theoretical energy density of 2500 Wh kg⁻¹.^2,3 Additionally, sulfur is abundant in the Earth's crust and relatively low cost, making lithium-sulfur batteries an attractive and cost-effective option for LIBs. However, the sulfur conversion type of cathodes faces three major challenges: the dissolution of intermediate lithium polysulfides into the electrolyte, low conductivity of sulfur and lithium sulfide, and large volume expansion of sulfur upon lithiation.⁴ These challenges have led to slow progress and a long road to application for lithium-sulfur batteries. Oxide cathodes are one of the most promising candidate cathodes in the short term. Oxide cathodes undergo Li⁺ intercalation and deintercalation reactions, resulting in minimal volume changes and good cycling stability. But oxide cathodes almost rely on single-electron reactions, resulting in theoretical specific capacities typically lower than 500 mAh g⁻¹.^5,6 Furthermore, the raw materials used (such as cobalt, nickel, etc.) tend to be costly, potentially increasing the cost of LIBs. So current research efforts are primarily focused on increasing the energy density of oxide cathodes and reducing costs.

Due to the greater abundance of sodium compared to lithium, sodium-ion batteries (SIBs) are considered as an alternative energy storage system to address the limitations of current battery technologies. SIBs have significant advantages in terms of resource abundance and cost. First, sodium is relatively abundant in the Earth's crust, at approximately 2.36%, which is much higher than lithium, indicating a relatively ample supply of sodium resources. Moreover, sodium is cost-effective, with the price of sodium carbonate, a sodium precursor, being less than $300 per metric ton, in contrast to lithium carbonate, priced at $29,000 per metric ton in 2023. This cost advantage positions SIBs as a more competitive option in terms of production costs. Additionally, SIBs can use low-cost cathodes, and they can substitute less expensive aluminum foil for the costly copper foil used as anode current collectors, resulting in SIBs having a total cost of at least 10% lower than LIBs. Furthermore, SIBs exhibit electrochemical charge and discharge behaviors similar to LIBs, making them a promising alternative for energy storage. Furthermore, these cost advantages and similarities in electrochemical behavior have positioned SIBs as a highly sought-after technology in the field of energy storage.⁷ This makes SIBs more economically competitive. Because the standard electrode potential of sodium metal is relatively high (the standard hydrogen electrode potential for sodium is −2.71 V, while that of lithium is −3.05 V), SIBs can be discharged to 0 V for storage without causing structural degradation, thus enhancing safety during operation and transportation. With various advanced electrode materials developed in recent years, SIBs hold the potential for increased energy density and cycle life, which will contribute to enhancing their competitiveness. As illustrated in Figure 1A, various types of inorganic sodium-ion cathodes have been developed in recent years, including layered oxides, Prussian blue, and polyanions.⁸ These works offer diverse options for the cathodes in SIBs, with each type of cathode having its own advantages and disadvantages. Furthermore, these research efforts contribute to the gradual improvement of the energy density of SIBs, enhancing their competitiveness.⁹

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As shown in Figure 1A and Table 1, compared to other types of sodium-ion cathodes, layered transition metal (TM) oxides exhibit a higher specific capacity, making them more suitable for high-energy-density applications. These intercalation-type layered TM oxides are typically represented as Na_xMO₂, where M stands for TMs, including but not limited to elements, such as Mn, Ni, Co, Fe, and others.¹⁸ The diversity in M element composition provides a broad research space for industrial applications and fundamental research. Whether they are O-type (where Na⁺ ions are situated in an octahedral coordination environment) or P-type (where Na⁺ ions are situated in a prismatic coordination environment), the charge compensation in these layered sodium-ion cathodes is typically achieved through the oxidation–reduction reactions of TM. Common TM redox couples include Mn⁴⁺/Mn³⁺, Ni⁴⁺/Ni³⁺, Co⁴⁺/Co³⁺, Fe⁴⁺/Fe³⁺, and others.¹⁹ In general, monovalent or multivalent layered oxides containing Mn exhibit higher reversible specific capacities. For instance, P2-Na_0.67MnO₂ can achieve an energy density of 360 Wh kg⁻¹ when the cell operates at a current density of 0.1 C within a voltage range of 1.5–4.3 V (vs. Na⁺/Na).²⁰ However, to catch the energy density of current LIBs, scientists have been inspired by high-capacity Li-rich layered oxide cathodes that rely on the accumulation of cation and anion redox processes.^21,22 They have been striving to design Na-rich layered oxides (Na(Na_yM_1 − y)O₂, where y < 1; M represents TM cations).²³ Typically, in layered oxides, anionic redox reactions (ARRs) are triggered by introducing alkali metals (AMs) into the MO₂ⁿ⁻ layer, generating nonbonding O 2p states related to oxygen lone-pair electrons.^22,24 However, due to the size mismatch between NaO₆ and MO₆ octahedra, applying this strategy to layered oxides in SIBs is not that straightforward. Scientists are attempting to fulfill the requirements of Na-rich layered oxides for anion redox by designing novel oxygen-redox cathodes. These efforts hold the promise of improving the energy density and performance of SIBs, bringing innovation to the field of battery technology. Among them, cathodes featuring low-cost 3d TMs like Mn (e.g., P2-Na_5/6Li_1/4Mn_3/4O₂²⁵ and P2/P3-Na_0.6Li_0.2Mn_0.8O₂²⁶) have become potential research subjects. As shown in Figure 1B, these materials exhibit energy densities close to or exceeding 600 Wh kg⁻¹.²⁷ Advanced characterization techniques have demonstrated that charge compensation primarily arises from the redox reactions between metal cations and oxygen anions. Anion redox in these materials benefits from the ability of electrons to be extracted from oxygen to form peroxo-like O–O dimers, enabling a reversible conversion of oxygen (O²⁻/Oⁿ⁻) and further contributing to capacity.²⁸ Recent research has further highlighted the significance of layered Na_xMnO₂ cathodes with anion redox activity. Therefore, it is essential to stay updated on the latest research developments. This review primarily focuses on summarizing the scientific challenges encountered in the development of layered Na_xMnO₂ cathodes with anion redox activity and explores potential modifications. Finally, this review provides insights into the future prospects of the Mn-based layered oxide cathodes with anionic redox for SIBs.

Table 1 Comparison of electrochemical performance, compacted density, and cost between typical Mn-based layered oxide cathodes and other popular cathodes.

Cathode type	Reversible capacity (mAh g−1)	Average voltage (V vs. Na+/Na)	Cycle performance	Compacted density (g cm−3)	Cost ($ ton−1)	References
Layered oxides	NaFe1/3Ni1/3Mn1/3O2	~130	3	73% (500 cycles at 1 C)	3.0–3.4	~5500	[10]
Na2/3Ni1/3Mn2/3O2	~140	3.6	83% (100 cycles at 1 C)	~6300	[11]
Na0.9Cu0.22Fe0.3Mn0.48O2	~100	3.2	87% (100 cycles at 1 C)	~6200	[12]
Prussian blue	Na2Fe[Fe(CN)6]	~120	3	70% (100 cycles at 1 C)	1.3–1.6	~690	[13]
Na2Mn[Mn(CN)6]	~100	3.4	75% (100 cycles at 1 C)	~1100	[14]
Polyanion compounds	NaFePO4	~100	2.5	90% (500 cycles at 1 C)	1.8–2.4	~3400	[15]
Na3V2(PO4)3	~105	3.4	86% (300 cycles at 1 C)	~11,000	[16]
Na3V2(PO4)2F3	~100	3.5	98% (300 cycles at 1 C)	~8700	[17]

ANIONIC REDOX IN MN-BASED LAYERED OXIDE CATHODES

The structure of layered cathodes in SIBs is significantly influenced by their composition. Unlike layered cathodes like LiMO₂ in LIBs, which are predominantly stable in the O3 structure, a slight excess of lithium may lead to the formation of spinel and other structures.²⁹ In contrast, layered cathodes tend to adopt O and P structures. The formation of layered Na_xMnO₂ structures is primarily achieved through the alternating arrangement of octahedrally coordinated TMO₆ and Na layers. In 1980, Delmas et al. first proposed four main types of layered structures, namely O2, O3, P2, and P3, where the letters O and P represent the positions of Na⁺ ions in octahedra and prisms, and the numbers indicate the number of Na layers in each repeating unit.³⁰ As shown in Figure 2A, the structure can be determined by measuring the TM–O distance. Typically, a ratio of the interlayer distance d(O–Na–O) to d(O–TM–O) is used to distinguish whether the material is more stable in the P2 or O3 phase.^31,32 A ratio higher than 1.62 usually leads to the formation of the P2 phase, while a ratio lower than 1.62 is more likely to result in the O3 phase. The variation in the interlayer distance is essentially a result of the electrostatic attractions and repulsions between NaO₂ and TMO₂ layers. An increase in Na content enhances the electrostatic attraction between Na layers, reducing d(O–Na–O) and favoring the O3 phase; conversely, in the P2 phase, electrostatic repulsion dominates.³¹

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Recently, the concept of cationic potential has been introduced to differentiate structure types.²³ The cationic potential can be defined as the sum of the TMs' weighted average ionic potentials multiplied by the Na-weighted average ionic potentials normalized to the ionic potential of the anion. As shown in Equation (1), this concept helps describe the electron cloud density and polarization of cations, reflecting the interaction between the AM layer (O–Na–O) and the TM layer (O–M–O) in layered oxides. As shown in Figure 2B, this parameter aids in explaining the competitive relationship between O3 and P2 structures. 1$<math altimg="urn:x-wiley:26379368:media:cey2605:cey2605-math-0001" display="block" wiley:location="equation/cey2605-math-0001.png" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mrow><msub><mi>\unicode{x003A6}</mi><mi mathvariant="italic">Cation</mi></msub><mo>\unicode{x0003D}</mo><mfrac><mrow><mover accent="true"><msub><mi>\unicode{x003A6}</mi><mi mathvariant="italic">TM</mi></msub><mo stretchy="true">\unicode{x000AF}</mo></mover><mo>\unicode{x000D7}</mo><mover accent="true"><msub><mi>\unicode{x003A6}</mi><mi mathvariant="italic">Na</mi></msub><mo stretchy="true">\unicode{x000AF}</mo></mover></mrow><mover accent="true"><msub><mi>\unicode{x003A6}</mi><mn mathvariant="italic">0</mn></msub><mo mathvariant="italic" stretchy="true">\unicode{x000AF}</mo></mover></mfrac></mrow></mrow></math>$

For Mn-based layered oxide cathodes with anion redox activity, O3 and P2 structures can also be distinguished using the cationic potential. Lattice oxygen redox reaction is closely related to the structure. In Li-rich layered oxide cathodes, lattice oxygen redox reactions are triggered by an excess of Li⁺ ions in the TM layer, providing additional capacity beyond the redox of the TM elements. In most sodium-ion layered oxide cathodes, almost 90% of Na⁺ ions can be extracted during charging without the need for excess sodium. This makes the lattice oxygen redox reactions easier to activate. In the following sections, we will categorize Mn-based layered oxide cathodes with ARR and introduce the structure, charge compensation mechanism, and electrochemical performance of each type of cathode.

Currently, there are many theories to describe the redox reactions of lattice oxygen, including localized O holes,^33,34 O–O dimerization,^35,36 reduction coupling,^37,38 ligand-to-metal charge transfer,^39,40 π redox,⁴¹ and O₂ trapped in bulk.⁴¹ Previous works conducted by research teams have suggested that the topological structure of the material significantly influences the behavior of lattice oxygen redox.^42–45

Na-deficient Mn-based layered oxide cathodes

AM element substituted system

In this system, exemplified by Na_2/3Ni_1/3Mn_2/3O₂ as a representative cathode, it has long been considered that the capacity contribution around the 4.2 V plateau is mainly attributed to the Ni³⁺/Ni⁴⁺ redox couple. However, during initial discharging, it undergoes significant irreversible capacity loss. Many works on Na_2/3Ni_1/3Mn_2/3O₂ propose that the capacity (~85 mA h g⁻¹) in the voltage range of 2.2–4.1 V is associated with the Ni²⁺/Ni³⁺ redox couple, and the long plateau capacity near 4.2 V (~80 mAh g⁻¹) is linked to the Ni³⁺/Ni⁴⁺ redox couple.^46–48 However, this viewpoint has recently been challenged. In 2017, Ma et al. designed a Na_0.78Ni_0.23Mn_0.69O₂ compound with TM defects, demonstrating that when charged to 4.1 V, Ni²⁺ is oxidized to Ni⁴⁺, and the plateau above 4.2 V is mainly due to O²⁻/O₂ⁿ⁻ coupling caused by TM vacancies.⁴⁹ In 2018, Tim and colleagues observed changes in the oxygen state at 4.5 V in Na_2/3Ni_1/3Mn_2/3O₂.⁵⁰ Additionally, online electrochemical mass spectrometry (OEMS) and density functional theory (DFT) calculations suggested that the capacity of the 4.2 V plateau was closely linked to irreversible O²⁻/O₂ⁿ⁻/O₂ evolution. The research also observed O₂ release and subsequent surface lattice densification, contributing significantly to irreversible capacity loss during the initial cycling period.⁵¹

As shown in Figure 3C and Table 2, efforts have been made to improve the electrochemical performance of layered compounds in the Na_xA_yMnO₂ (A = Li/Mg/Zn) series through the conventional substitution with AM cations. In 2016, Goodenough and colleagues first observed O redox activity in the electrochemical process of the P3-Na_0.6Li_0.2Mn_0.8O₂ cathode. Subsequently, a series of cathodes with O redox activity were reported.⁵³ Similar to Li₂MnO₃, Li⁺ ions within the TM layers in Na_0.6Li_0.2Mn_0.8O₂ assist in forming O 2p electron pairs or Na–O–Li configurations. The covalent bonding interaction between Li⁺–O²⁻ positions the electronic state of O 2p at a higher energy level, closer to the Fermi level than the Mn 3d electronic state. Consequently, electrons are more easily liberated from oxygen, imparting O redox activity in cathodes. Li-rich layered oxide cathodes typically undergo simultaneous oxidation–reduction of TM and O ions during cycling.^93,94 Using similar characterization methods like O K-edge mapping of resonant X-ray inelastic scattering (mRIXS) through the super-partial fluorescence yield (sPFY) analysis and Mn L-edge mRIXS through the inverse partial fluorescence yield (iPFY) analysis in Figure 3B, it has been confirmed that P3-Na_0.6Li_0.2Mn_0.8O₂ also exhibits highly reversible oxygen ion redox reactions.⁵² Hu and colleagues²⁶ characterized the charged P2-Na_0.6Li_0.2Mn_0.8O₂ cathode using neutron pair distribution function and observed the formation of O–O dimers similar to peroxides. Furthermore, X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure in Figure 3C confirmed that the charge compensation in the voltage range of 3.5–4.5 V is primarily contributed by anion redox activity.

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Table 2 Summary of the representative Mn-based layered oxide cathodes based on ARR.

Materials	Redox couples	Average voltage (V vs. Na+/Na)	Operating voltage (V vs. Na+/Na)	Current density (mA g−1)	Initial capacity (mAh g−1)	Reversible capacity (mAh g−1@cycles)	References
P2-Na2/3Ni1/3Mn2/3O2	Ni, O	~3.6	2.6–4.3	16	140	~80@100	[51]
P2-Na0.78Ni0.23Mn0.69O2	Ni, O	~3.5	2.0–4.5	12	140	~120@20	[49]
P3-Na0.6Li0.2Mn0.8O2	Mn, O	~3.0	2.0–4.5	15	120	~140@50	[53]
P2-Na0.6Li0.2Mn0.8O2	Mn, O	~2.7	2.0–4.5	10	162	~150@12	[52]
P3-Na0.6Li0.2Mn0.8O2	Mn, O	~4.0	3.5–4.5	10	70	~25@60	[26]
P2-Na5/6Li1/4Mn3/4O2	Mn, O	~2.5	1.5–4.4	10	190	~185@20	[25]
P2-Na0.72Li0.24Mn0.7O2	Mn, O	~2.5	1.5–4.5	10	271	~210@30	[54]
P2-Na2/3Mg1/3Mn2/3O2	Mn, O	~2.7	1.5–4.5	30	210	~150@50	[55]
P2-Na2/3Mg0.28Mn0.72O2	Mn, O	~2.5	1.5–4.4	10	225	~160@30	[56]
P2-Na0.67Mg0.28Mn0.72O2	Mn, O	~2.7	2.0–4.5	10	170	~160@50	[57]
P2-Na0.67Mg0.05Mn0.95O2	Mn, O	~2.7	1.5–4.2	12	180	~155@25	[58]
P3-Na0.67Mg1/3Mn2/3O2	Mn, O	~2.6	1.5–4.4	7.5	224	~150@30	[59]
P2-Na0.67Zn0.1Mn0.9O2	Mn, O	~2.4	1.5-4.4	12	203	~163@50	[60]
P2-Na2/3Zn2/9Mn7/9O2	Mn, O	~2.5	1.5–4.5	10	195	~105@50	[61]
P2-Na0.80Li0.08Ni0.22Mn0.67O2	Ni, Mn, O	~3.2	2.0–4.3	23.41170	12,094.8	~109@50~80@1000	[62]
P2-Na0.8Li0.12Ni0.22Mn0.66O2	Ni, O	~3.2	2.0–4.5	11.8	100	–	[27]
P2-Na0.66Li0.18Fe0.12Mn0.7O2	Mn, Fe, O	~2.6	1.5–4.5	10	180	~165@80	[63]
P2-Na0.66Li0.22Ti0.15Mn0.63O2	Mn, O	~2.6	1.5-4.5	10	225	~150@50	[64]
P2-Na0.72Li0.24Ti0.10Mn0.66O2	Mn, O	~2.5	1.5-4.5	10	194	~180@80	[65]
P2/O3-Na0.8Li0.27Ti0.05Mn0.68O2	Mn, O	~2.7	2.0–4.3	10	143	~110@50	[66]
P2/O3-Na2/3Li0.18Fe0.2Mn0.8O2	Mn, Fe, O	~2.8	1.5–4.2	24	125	~87@100	[67]
P2-Na0.66Li0.22Mn0.775Zr0.005O2	Mn, O	~2.5	1.5–4.5	10	197	~160@50	[68]
P2-Na0.66Li0.22Mn0.775Sn0.005O2	Mn, O	~2.5	1.5–4.5	10	198	~170@50	[68]
P2-Na0.6Mg0.2Mn0.6Co0.2O2	Mn, Co, O	~2.7	1.5-4.5	26	214	~170@10	[69]
P2-Na2/3Mn0.72Cu0.22Mg0.06O2	Mn, Cu, O	~3.2	1.5-4.5	17.4	110	~65@50	[70]
P2-Na2/3Mg1/3Ti1/6Mn1/2O2	Mn, O	~2.5	1.5−4.5	40	235	~160@50	[71]
P2-Na2/3Ni1/6Mn2/3Cu1/9Mg1/18O2	Mn, Ni, Cu, O	~3.2	2.5-4.5	60,600	8880	–~65@500	[72]
P2-Na0.67Mg0.28Mn0.6Ru0.12O2	Mn, O	~2.8	1.5–4.5	20	222	~192@50	[73]
P2-Na0.67Li0.11Mg0.12Mn0.77O2	Mn, O	~2.6	1.5–4.5	10	223	~137@50	[74]
P2-Na0.67Mg0.1Zn0.1Mn0.8O2	Mn, O	~2.7	1.5–4.5	15	233	165@50	[75]
O3-Na0.83Mg0.33Fe0.17Mn0.50O2	Mn, Fe, O	~2.7	1.5–4.3	10	230	~160@50	[76]
P2-Na0.7Mg0.05(Mn0.6Ni0.2Mg0.15)O2	Ni, Mn, O	~3.2	1.5−4.2	40	130	~100@160	[77]
P2-Na0.8Li0.1Mn0.6Ni0.2Cu0.1O2	Ni, Mn, O	~3.2	1.8–4.0	10	135	~110@100	[78]
P2-Na0.833Zn0.0375Li0.25Mn0.7125O2	Mn, O	~2.5	1.5–4.5	50	166	~162@100	[79]
P2/O3-Na0.67Ni0.33Mn0.66Sn0.01O2	Ni, Mn, O	~3.2	1.5–4.5	20	245	~200@50	[80]
Na4/7□1/7Mn6/7O2	Mn, O	~2.4	1.5–4.7	8	200	~190@20	[81]
Na4/7□1/7Mn6/7O2	Mn, O	~4.2	3.0–4.65	8	60	~40@30	[82]
P2-Na0.653Mn0.929□0.071O2	Mn, O	~2.2	1.5–4.3	20	202	~170@60	[83]
P2-Na2/3Zn1/9Mn7/9□1/9O2	Mn, O	~2.5	1.5–4.4	20	205	~88@100	[84]
P2-Na2/3Mn7/9Mg1/9□1/9O2	Mn, O	~2.5	1.5–4.4	15	213	–	[85]
P2-Na0.76Ca0.05(Ni0.23□0.08Mn0.69)O2	Ni, Mn, O	~3.2	2.0–4.3	12	160	~120@30	[86]
P2-Na0.8Mg0.13(Mn0.6Co0.2Mg0.07□0.13)O2	Co, Mn, O	~2.6	1.6–4.4	17	176	~102@120	[87]
O3-NaLi1/3Mn2/3O2	Mn, O	~2.8	1.2–4.7	35	190	~175@40	[88]
O3-Na1.2Mn0.4Ir0.4O2	Mn, O	~2.7	1.5–4.4	20	150	~110@50	[89]
O3-Na2Ru0.9Mn0.1O3	Mn, O	~2.5	1.5–4.3	20,100	160,150	~130@30~108@100	[90]
O3-Na2Ru0.8Mn0.2O3	Mn, O	~2.5	2.0–4.6	27	178	~120@100	[91]
O3-NaNi0.5Mn0.5O2	Mn, Ni, O	~2.6	2.0–4.0	12	145	~130@100	[92]

P2-Na_0.72Li_0.24Mn_0.76O₂ exhibits the high reversible specific capacity of approximately 220 mAh g⁻¹.⁵⁴ In the case of deep deintercalation of Na⁺, atomic rearrangement reactions help maintain the stability of the P2-type structure. As O²⁻ oxidizes, the radius of O ions decreases, leading to the formation of oxygen vacancies. However, as the oxidation reaction proceeds, the negative charge carried by O ions also decreases, reducing the electrostatic repulsion between O ions around the vacant Na layer and thus preserving a P2-type stacking with localized defects. This results in minimal volume changes (only 1.35% during the initial charging) and suppresses the occurrence of the P2–O2 phase transition. These properties have been confirmed by in situ X-ray diffraction (XRD) in Figure 4B. Furthermore, it was found that the cyclic voltage decay of P2-Na_0.72Li_0.24Mn_0.76O₂ in the voltage range of 1.5–4.5 V was not significant at that time. However, the electrochemical cycling stability of these materials is generally poor, especially regarding the issue of improving the reversibility of the oxygen ion redox reaction. Chen and colleagues first synthesized a new type of Fe-doped P2-Na_0.66Li_0.18Fe_0.12Mn_0.7O₂ cathode, which exhibited an initial discharge capacity of up to 210 mAh g⁻¹ within the voltage range of 1.5–4.5 V and maintained a discharge capacity of 165 mAh g⁻¹ after 80 cycles.⁶³ Additionally, previous researchers have demonstrated that Ti substitution can effectively adjust the O redox activity of both P2-Na_0.66Li_0.22Mn_0.78O₂⁶⁴ and P2-Na_0.72Li_0.24Mn_0.76O₂⁶⁵ cathodes.

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As shown in Figure 4A, AMs, Mg and Zn substitutions for Mn in Na_xMnO₂ materials (0 <x < 1), including Na_0.67Mg_xMn_1 − xO₂ and Na_0.67Zn_xMn_1 − xO_z, also exhibit O redox activity and reversible specific capacity.⁹⁵ Bruce's group first discovered that P2-Na_0.67Mn_0.72Mg_0.28O₂ possesses oxygen ion redox activity.⁵⁷ This is because the interaction between Mg 3s and O 2p orbitals forms weak ionic Mg–O bonds similar to Li–O bonds, resulting in O 2p nonbonding states.⁹⁶ The presence of O 2p lone-pair electrons in Na_2/3Mn_2/3Mg_1/3O₂ has been confirmed through the electron localization function (ELF). During the initial sodium extraction in P2-Na_0.67Mn_0.72Mg_0.28O₂⁵⁷ and P2-Na_2/3Mn_7/9Zn_2/9O₂,⁶¹ a slight variation in XRD peak positions is observed in Figure 4C,D, indicating the presence of a continuous solid solution. Yang and colleagues quantitatively analyzed the evolution of anion and cation redox during cycling in P2-Na_2/3Mg_1/3Mn_2/3O₂ using O K-edge mRIXS-sPFY and Mn L-edge mRIXS-iPFY spectra.⁵⁵ During the first cycle, the reversibility of its oxygen ion redox reaches as high as 79%, and after 100 cycles, the reversibility remains at 87%. This is attributed to the strong orbital overlap (covalent interaction) between O 2p and Co 3d or Cu 3d orbitals, which facilitates electron transfer and stabilizes the ARR. Based on this, researchers have modified the Na_xMg_yMn_1 − yO₂ cathodes through Co or Cu doping, such as P2-Na_0.6Mg_0.2Mn_0.6Co_0.2O₂,⁶⁹ P2-Na_2/3Mn_0.72Cu_0.22Mg_0.06O₂,⁷⁰ and P2-Na_2/3Ni_1/6Mn_2/3Cu_1/9Mg_1/18O₂.⁷² Furthermore, the substitution of Ti⁴⁺ (d0) forms Ti–O bonds (highly ionic), promoting the formation of more localized electrons around its connected O ions compared to Mn–O bonds, facilitating charge transfer for O redox, such as P2-Na_2/3Mg_1/3Ti_1/6Mn_1/2O₂.⁷¹ Regarding O3-Na_0.83Mg_0.33Fe_0.17Mn_0.50O₂, the addition of Mg to NaFe_0.5Mn_0.5O₂ activates the ARR, increasing the initial specific capacity to 230 mA h g⁻¹ and raising the plateau voltage to around 2.7 V.⁷⁶

Similar to Li, Zn in the Na_2/3Mn_7/9Zn_2/9O₂ lattice contributes to the ARR, which is the highly covalent Zn–O bond in the lattice.⁶¹ However, Zn²⁺ has a d¹⁰ orbital structure, which is entirely different from Li–O/Na–O/Mg–O bonds. Compared to the ionic Li–O/Na–O/Mg–O bonds and the covalent TM–O bonds, the Zn–O bonds lie between ionic and covalent bonds. Inert Zn²⁺ with 8-electron stability forms Zn–O bonds, which can improve the lattice oxygen stability. Taking Na_2/3Mn_2/3Zn_1/3O₂ as a theoretical model, characterization techniques such as partial density of states and ELF analysis have demonstrated that the ionic nature of Zn–O bonds and the presence of O 2p lone-pair electrons are the sources of the ARR mechanism.^56,58

TM vacancy-based system

By reducing TM to form TM defects, Na_0.653Mn_0.929O₂ was successfully synthesized as a model cathode. During the charge compensation process, this cathode can activate nonbonding O 2p orbital oxides, exhibiting a distinct voltage plateau at 4.2 V, corresponding to a total capacity of approximately 210 mAh g⁻¹ for the ARR. Its unique structure and chemical features can effectively increase the O 2p orbital near the Fermi level, triggering the oxygen oxidation reaction before the TM oxidizes to a higher valence state. It is worth noting that the proportion of oxygen redox in the charge compensation mechanism is relatively small in Na_0.653Mn_0.929O₂.⁸³

TM vacancies play roles like Li or Mg, encouraging the formation of isolated O 2p orbitals to participate in electron gain or loss. This phenomenon is observed in the Na₂Mn₃O₇ (Na_4/7□_1/7Mn_6/7O₂), which belongs to the triclinic P1 structure, consisting of alternating layers of Na and (□_1/7Mn_6/7) in Figure 5A.⁸¹ The Na⁺ ions exist in two positions: distorted (NaO₆) octahedra and (NaO₅) polyhedra. Simultaneously, the (□_1/7Mn_6/7) layer has inherent Mn vacancies, making it easy for O ions adjacent to the vacancies to form O 2p electron pairs. XRD reveals a highly reversible oxygen redox occurring at 4.1 V, with XRD patterns indicating the formation of a (□-Mn) arrangement that creates a superlattice. This cathode exhibits a very small voltage hysteresis, less than 50 mV. In this regard, Hu and colleagues characterized Na_4/7□_1/7Mn_6/7O₂ using the electron paramagnetic resonance (EPR) (shown in Figure 5B) and in situ synchrotron XRD (sXRD) techniques, concluding that the absence of hysteresis may be due to the absence of O-layer slip and the absence of out-of-plane migration of Mn ions.⁸² Bruce's group also suggested that the highly stable lattice-ordered superlattice structure in Na_4/7□_1/7Mn_6/7O₂ is one of the reasons for the formation of this small voltage hysteresis phenomenon.⁹⁷ Substituting low-valence elements for Mn or introducing vacancies in the TM layer can increase the Mn oxidation state in the original cathode, thereby triggering ARR, such as the Na_0.76Ca_0.05(Ni_0.23□_0.08Mn_0.69)O₂. Simultaneously, the presence of random vacancies at TM sites induces the appearance of nonbonding O 2p orbitals, thereby exciting the anionic redox processes.⁸⁶

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Na-rich Mn-based layered oxide cathodes

Do Na-rich layered oxide cathodes exhibit the ARR? Typically, we observe anionic redox in Li-rich cathodes like Li₂MnO₃. It is worth noting that more research on Na-rich cathodes with ARR is primarily focused on O3-type layered structures based on 4d/5d metals.^98–101 In contrast, there are relatively fewer reports on Na-rich cathodes based on 3d Mn. This could be due to the much larger radius of Na⁺ (approximately 1.06 Å) compared to 3d metal ions. Yamada's team successfully designed and synthesized the Na-rich material Na₂RuO₃ (Na(Na_1/3Ru_2/3)O₂) in 2013.⁹⁸ In this cathode, Na and Ru jointly occupy octahedral sites in the TM layer, similar to the Li₂RuO₃. However, this cathode exhibits a capacity of 135 mAh g⁻¹ at 0.2 C in the voltage range of 1.5–4.0 V, limited by the single-electron reaction of Ru⁴⁺/Ru⁵⁺. Further research revealed that the Na₂RuO₃ with a hexagonally ordered structure can reversibly insert approximately 1.3 Na⁺ within the voltage range of 1.5–4.0 V, while the Na₂RuO₃ cathode with a disordered arrangement of Na and Ru can only insert 1.0 Na⁺ in the same voltage range. The hexagonally ordered structure in Na₂RuO₃ likely exhibits excellent electrochemical performance because it includes a spontaneously ordered intermediate phase O1-NaRuO₃.⁹⁹ The presence of this intermediate phase helps reduce structural distortion, raises the antibonding σ* orbital Fermi level of the O–O bond, and thereby triggers the ARR. Thus, the hexagonally ordered superlattice structure may be a necessary condition for the Na₂TMO₃ cathodes with ARR. In 2016, Tarascon's team first reported the O3-Na₂IrO₃ cathode, which has a structure similar to α-Li₂IrO₃.¹⁰⁰ Through neutron diffraction (ND) and scanning transmission electron microscopy (STEM) characterization, it was demonstrated that the removal of 0.5 Na⁺ leads to distortion in the IrO₆ octahedral structure, resulting in the formation of O–O dimers. Using a two-step electrochemical synthesis approach, researchers synthesized a novel three-dimensional (3D) β-Na_1.7IrO₃ with charge compensation provided by the accumulated redox of Ir⁴⁺/Ir⁵⁺ and the redox of O²⁻/(O₂)ⁿ⁻ during 2.0–4.0 V.¹⁰¹

For Na-rich Mn-based layered oxide cathodes, as shown in Figure 6A, the activation of ARR can be achieved by doping with 5d metals like Ir, Cd, and 4d metals like Ru, as seen in cathodes like Na_1.2Mn_0.4Ru_0.4O₂ and Na_1.2Mn_0.4Ir_0.4O₂.^89–91 This is because 5d TMs can form strong covalent bonds with oxygen, causing the originally lower-lying Mn 3d states in the band to move to higher positions. This provides new bonding pathways for the unoccupied O 2p orbitals near the Fermi level, triggering ARR. This unstable nonbonded Na–O–Na structure offers additional electron transfer pathways, initiating extra redox reactions. Simultaneously, the Ru 4d and Ir 5d orbitals have significant overlap with O 2p orbitals, causing the Mn 3d band to rise, thereby reducing the likelihood of O₂ release.⁸⁹ In Figure 6B, upon the voltage reaching 3.8 V in the initial charging phase, the Raman peaks emerged around 1109 cm⁻¹, attributed to the O–O stretching mode within superoxide-related species (O²⁻) in Na_1.2Mn_0.4Ir_0.4O₂. With an increase in Mn content in Na₂Ru_1 − xMn_xO₃, the charge capacity of the first cycle significantly increases. However, a high concentration of Mn compromises the structural stability, resulting in undesired phase transitions and noticeable capacity decay at high voltage. Specifically, for Na₂Ru_1 − xMn_xO₃ with x = 0.1, the cathode exhibited the best capacity retention, with 81.0% after 30 cycles at 20 mA g⁻¹.^90,91

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Meanwhile, researchers are also working on increasing the Na content, for example, by preparing O3-NaLi_1/3Mn_2/3O₂.⁸⁸ It shows a specific capacity of up to 190 mA h g⁻¹ and remains stable even when exposed to humid air. As illustrated in Figure 6C, the Na occupies the octahedral positions between the Li_1/3Mn_2/3O₂ layers, while Li and Mn form an ordered “honeycomb” structure within the Li_1/3Mn_2/3O₂ layers. On full charge, a new peak emerges at 530.5 eV in XPS O 1s spectra (Figure 6D), indicating oxidized lattice oxygen Oⁿ⁻ (n < 2). This is confirmed by an additional feature in mRIXS, with excitation and emission energies of 531.0 and 523.7 eV, respectively, characteristic of Oⁿ⁻. For other O3-type Na-rich cathodes, Ni L₃-edge soft X-ray absorption spectroscopy (sXAS) in the PFY mode reveals that oxygen oxidation–reduction (~80%) constitutes the predominant contribution in the charge compensation mechanism of O3-NaFe_0.5N_i0.5O₂.¹⁰² Other iron-based layered oxide materials, such as O3-NaFe_0.5Mn_0.5O₂,¹⁰³ O3-NaFe_0.3Ni_0.7O₂,¹⁰⁴ and O3-NaFe_0.5Co_0.5O₂,¹⁰⁵ have also been reported, and their participation in the ARR process may warrant further examination. Nevertheless, more research is needed to address this challenge and explore modification methods suitable for enhancing the ARR in 3d Mn-based Na-rich cathodes. This can help broaden our understanding of Na-rich materials and promote their applications in the field of electrochemical energy storage.

SCIENTIFIC CHALLENGES AND MODIFICATION APPROACHES IN MN-BASED CATHODES WITH ANIONIC REDOX

Sluggish ion diffusion

The microstructures and macrostructures of Na_xMnO₂ cathodes are significantly influenced by the sodium ion content. Typically, when x ≤ 0.5, Na_xMnO₂ cathodes exhibit a tunnel-type microstructure and a rod-shaped macrostructure.^106–108 When prepared using methods such as solution processes, these materials can also exhibit a macrostructure resembling nanowires. In contrast, when x > 0.5, these materials typically exhibit a layered microstructure, with their macrostructure predominantly composed of lamellar polyhedral structures. Additionally, employing techniques such as solution processes allows for the preparation of macrostructures resembling microspheres and cubic formations.^20,109,110

Sodium ions undergo a periodic “flow” process between cathodes and anodes. In this process, sodium ions must continuously insert themselves into the electrode material while simultaneously being expelled. This oscillatory process of intercalation and deintercalation significantly affects the battery's performance. However, this oscillatory process is influenced by various factors, including the layered structure, morphology of the electrode material particles, and the arrangement of particles within the electrode material. These factors directly impact ionic mobility and electrochemical performance. Among these factors, particle size is particularly crucial. Particle size directly influences the material's packing state, subsequently affecting ion migration, thereby exerting a significant impact on performance. Controlling particle size proves to be an effective strategy. By reducing the characteristic diffusion length (as illustrated in Equation 2), it becomes possible to enhance the diffusion kinetics of sodium ions and the electrochemical reaction rates 2$<math altimg="urn:x-wiley:26379368:media:cey2605:cey2605-math-0002" display="block" wiley:location="equation/cey2605-math-0002.png" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mrow><msup><mi>L</mi><mn>2</mn></msup><mo>\unicode{x0003D}</mo><mn>2</mn><mi mathvariant="normal">D</mi><mo>\unicode{x000D7}</mo><mi mathvariant="normal">t</mi><mo>.</mo></mrow></mrow></math>$

It is worth noting that, despite the significant impact of particle size on performance, it has not received sufficient attention in current research. As illustrated in Figure 7A and Table 3, many Mn-based layered oxide cathodes for SIBs with anionic redox are frequently synthesized using the solid-state reaction method, leading to particle sizes typically in the micrometer range (3–5 µm). Cathodes with larger particles often have extended electron/ion transport pathways, leading to a deterioration in electrochemical performance and increased polarization. This limitation can significantly impact the battery's rate performance. Moreover, large-particle materials usually possess smaller specific surface areas, limiting the contact area with the electrolyte. This constraint hinders Na⁺ insertion and extraction, leading to a decrease in the specific capacity. Therefore, altering the synthesis methods to reduce the particle size of cathodes holds the potential to significantly enhance performance.

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Table 3 Summary of the synthesis methods, heat-treating atmosphere, and particle size of representative Mn-based layered oxide cathodes based on ARR.

Materials	Synthesis methods	Heat-treating atmosphere	Particle size (µm)	References
Na0.653Mn0.929O2	Solid-state reaction	O2	3–4	[83]
P2-Na0.8Mg0.13(Mn0.6Co0.2Mg0.07□0.13)O2	Solid-state reaction	O2	3–8	[87]
P2-Na2/3Zn1/9Mn7/9□1/9O2	Modified Pechini method	O2	2–5	[84]
P2-Na2/3Mn7/9Mg1/9□1/9O2	Sol-gel method	O2	2–5	[85]
O3-NaLi1/3Mn2/3O2	Solid-state reaction	Ar	3–4	[88]
Na2Ru0.9Mn0.1O3	Solid-state reaction	Ar	3–5	[90]
Na2Ru0.8Mn0.2O3	Solid-state reaction	Ar	3–8	[91]
O3-NaNi0.5Mn0.5O2	Sol-gel method	Air	3–8	[92]

The cathodes synthesized through co-precipitation, such as Na_0.67Mn_0.95Mg_0.05O₂⁵⁸ and Na_0.8Li_0.12Ni_0.22Mn_0.66O₂,²⁷ exhibit minimal changes in particle size, as depicted in Figure 7B. However, their thickness has been reduced to the submicron range. Another common method for synthesizing cathodes is the sol-gel method, as illustrated in Figure 7C,D. This method effectively reduces the particle size of Na_2/3Mg_1/3Mn_2/3O₂ to 1–3 µm, significantly enhancing the rate performance.⁵⁵ Na_0.80Li_0.08Ni_0.22Mn_0.67O₂ prepared by the sol-gel method shows plate-like particles with a size of 1–3 μm, exhibiting a reversible capacity of 94.8 mA h g⁻¹ at 10 C in the voltage range of 2.0–4.3 V, along with excellent long cycle life. The capacity retention after 1000 cycles at 10 C is 85.2%.⁶² As a novel method for preparing cathodes, the spray pyrolysis method, shown in Figure 8A, was employed by Li and colleagues to produce Na_2/3Ni_1/3Mn_2/3O₂ cathodes with particle sizes ~500 nm. This material delivers an initial discharge specific capacity of up to 228 mA h g⁻¹ within the voltage range of 1.5–4.5 V.⁵⁰ This approach provides a new perspective for controlling the particle size of cathodes to improve battery performance.

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Elemental doping is also considered a potential method to reduce particle size. In the Na₂RuO₃ lattice, doping with Mn⁴⁺ can effectively shrink the lattice parameters and decrease the synthesized particle size.⁹¹ As depicted in Figure 8B, the particle size of Na₂Ru_0.8Mn_0.2O₃ is reduced by ~10 µm compared to the original Na₂RuO₃. In comparison to the nondoped case, Na₂Ru_0.8Mn_0.2O₃ exhibits a high initial discharge specific capacity of 178 mA h g⁻¹ under a current density of 27 mA g⁻¹ in the voltage range of 1.5–4.0 V. Doping Zn²⁺ into Na_0.833Li_0.25Mn_0.75O₂, as depicted in Figure 8C, can reduce the particle size by approximately three times, while significantly improving the cycling performance.⁷⁹ While reducing particle size can improve specific capacity, larger particle surfaces are more susceptible to surface reactions, such as electrolyte decomposition or reactions between cathode and unstable substances in the electrolyte, potentially shortening the cycle life. Moreover, the preparation of uniform nanoscale cathodes may necessitate more complex processes and technologies, potentially increasing production costs. Furthermore, nanoscale particles may diminish the volumetric energy density of the battery. Therefore, selecting the appropriate particle size is crucial for high-capacity Mn-based layered oxide cathodes for SIBs with anionic redox. The optimal particle size ensures unhindered ion migration during the charge and discharge processes, simultaneously minimizing the increase in surface area, reducing the likelihood of surface reactions, and maintaining the volumetric energy density of SIBs.

Cationic migration

Some Mn-based layered oxide cathodes for SIBs with anionic redox undergo an exchange of ions between the TM and Na layers during charging and discharging. A work on the Na_0.6Li_0.2Mn_0.8O₂ cathode employed solid-state nuclear magnetic resonance (ss-NMR) to examine the migration of Li⁺ in different local environments.¹¹¹ These environments encompass the surface, Na layer, TM layer, and highly distorted TM layer during charging and discharging. As depicted in Figure 9A, Li⁺ migrates from the TM layer to the Na layer during charging. Li⁺ ions undergo gradual extraction throughout cycling. Li⁺ ions return to the TM layer during discharging, but this process is accompanied by some Li⁺ losses, which can be detrimental to maintaining structural stability during cycling. This phenomenon is supported by another work involving P2-Na_0.66Li_0.22Mn_0.78O₂.⁶⁸ As illustrated in Figure 9B, the position of ²³Na NMR signals is irreversibly influenced by Na–O–TM interactions. During charging, the ²³Na signals initially exhibit low chemical shift displacement. As the voltage rises to 4.5 V, the signals significantly broaden, indicating local disorder in the Na sites. During discharging, this disorder gradually diminishes but does not fully revert to the initially ordered P2 structure. Additional research utilizing ⁷Li NMR indicates that Li⁺ gradually migrates from the TM layer to the Na layer during charging, concurrently causing Mn to migrate within the layer and create vacancies in the TM layer. Li⁺ can migrate from the Na layer back to the TM layer during discharging, but it does not return to the original Li sites; instead, it fills the vacancies created by Mn migration within the layer. For other materials, such as Na_0.72Li_0.24Ti_0.10Mn_0.66O₂⁶⁵ and Na_0.78Li_0.25Mn_0.75O₂,¹¹² it is also suggested that after increasing the charging cut-off voltage, Na⁺ within the Na layer begins to migrate out instead of Li⁺. When the voltage reaches 5 V, the loss of Li⁺ becomes significant. Subsequent research utilizing ⁶Li NMR indicates that Li⁺ persists in Na_0.78Li_0.25Mn_0.75O₂ at 4.5 V. During this process, Li⁺ migrates from the TM layer to the Na layer, creating vacancies in the TM layer. However, beyond 5 V, the loss of Li⁺ becomes significant. Recent works indicate that during the charge and discharge process of the Na-rich structure O3-NaLi_1/3Mn_2/3O₂, local structural distortions occur, with the metal layers showing substantial variations in the interlayer distance.⁸⁸ As shown in Figure 9C, high-resolution high-angle annular dark field-STEM (HAADF-STEM) images and corresponding HAADF intensity profiles show that, at the early stages of cycling, the migration of Mn to the interlayer space is limited to the surface, not occurring in bulk, while in-plane migration increases with cycling. These research findings suggest that ion migration between the TM and Na layers during cycling leads to irreversible Li⁺ losses, potentially presenting challenges to battery performance and structural stability. Similar phenomena are also observed in other Mn-based layered oxide cathodes, such as NaNi_0.3Co_0.12Mn_0.18Fe_0.4O₂, where TM ions continuously migrate to the Na layer.¹¹³ The Fe ions migrate into the tetrahedral and octahedral sites in the Na layer, and Ni ions mainly migrate within the octahedral sites in the Na layer, while Mn and Co ions remain predominantly in the TM layer. Atomic-resolution energy-dispersive spectroscopy elemental mapping reveals that irreversible ion migrations (such as Fe and Ni) lead to the degradation of structure and performance. Some of the ions migrating to the Na layer can hinder the intercalation or deintercalation of other Na ions. Meanwhile, the migration-induced sliding of the TM layer enlarges the spacing in the Na layers, allowing solvent molecules from the electrolyte to potentially infiltrate into the interlayer spaces. Through ex situ XRD methods, the structural evolution during Na⁺ extraction from NaNi_0.5Mn_0.5O₂ was observed in which the O3 phase continuously transformed into O′3, P3, P′3, and then into the P3″ phase during charging, with an unusually large interlayer spacing (∼7.0 Å) observed.¹¹⁴ As a result, the impacts of cationic migration include hindering the deintercalation of Na⁺ from the Na layer, propagating lattice stress and strain, preventing complete intercalation of Na ions into the Na layer, and increasing reactions between the electrolyte and TM ions. Hindered deintercalation of Na ions and reactions with electrolyte infiltrating into the interlayer spaces lead to the partial loss of Na ions and TM elements. Consequently, this significantly reduces the charge transfer number and the utilization of active materials, potentially causing capacity decay and voltage decay in SIBs. The rate performance of the battery will also deteriorate due to the mentioned impacts.

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To suppress the occurrence of migration, current methods primarily focus on doped modification and structural design, which can lower the energy barrier for ion migration. A common strategy is to anchor inert ions with larger ionic radii (such as Mg, Ca, Zn) around (such as Li and Ni) elements prone to migration to form a more stable A@Mn6 superstructure in the TM layer, thus increasing the energy barrier for ion migration within the TM layer.^61,74,86 Pan and colleagues proposed a method to inhibit the migration of TMs in P2-Na_0.73Li_0.23Mn_0.77O₂ through the modulation of superstructure elements.⁷⁴ They introduced more stable Mg@Mn6 superstructure elements, partially replacing the unstable Li@Mn6 superstructure elements. As illustrated in Figure 10A, in P2-Na_0.67Li_0.11Mg_0.12Mn_0.77O₂, due to the stronger interaction between Mg²⁺ and O²⁻ compared to Li⁺ and O²⁻, the Mg@Mn6 superstructure element exhibits enhanced chemical stability. Its anchoring effect effectively suppresses the migration of TMs within the layers and the formation of uncoordinated oxygen. In Na_2/3Li_1/9[Ni_2/9Li_1/9Mn_2/3]₂, Li⁺ simultaneously occupies positions in both Na and TM layers. The ND results indicate that introducing some Li⁺ into the Na layer helps neutralize the charge difference of Ni²⁺.¹¹⁵ The cell shows a specific capacity of 64 mA h g⁻¹ at 2.0–4.2 V and 20 C. Even after 1500 cycles, the capacity retention rate remains remarkably high at 74.5%. When doping NaLi_0.25Mn_0.75O₂ with Zn²⁺, Zn²⁺ tends to occupy the Na layer rather than the TM layer.⁷⁹ Compared to the undoped Zn²⁺ sample, the P2-Na_0.833Zn_0.0375Li_0.25Mn_0.7125O₂ material demonstrates enhanced thermodynamic stability and reduces Mn³⁺, mitigating Jahn–Teller distortion. Characterization using HAADF-STEM and sXRD clarifies that P2-Na_2/3Mn_7/9Zn_2/9O₂ undergoes side reactions, such as cationic migration and surface reconstruction during charging.⁶¹ This differs from P2-Na_2/3Mn_0.72Mg_0.28O₂, where Zn²⁺ tends to migrate to tetrahedral sites rather than staying at octahedral sites due to the stable wurtzite crystal structure of ZnO. Furthermore, Ca²⁺ forms a stable and robust layered structure in the Na site. Doping Ca²⁺ into Na_0.76Ca_0.05(Ni_0.23□_0.08Mn_0.69)O₂ induces random vacancies at the TM sites, triggering ARR.⁸⁶ As shown in Figure 10B, in situ XRD demonstrates that a stable and robust layered structure suppresses the P2-O2 phase transition caused by cationic migration, thereby enhancing cycling stability. In Na_0.7Mg_0.05(Mn_0.6Ni_0.2Mg_0.15)O₂, Mg²⁺ is simultaneously doped into Na and TM sites, with Mg²⁺ in the Na layer acting as stabilizing “pillars” for the layered structure.⁷⁷ Mg²⁺ occupying positions in both the Na layer and TM layer contribute to the formation of “Na–O–Mg” and “Mg–O–Mg” bonds in the layered structure, facilitating reversible reactions, providing smoother voltage curves, and ultimately improving structural stability.

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However, from the current perspective of modification, although the issue of cationic migration has been alleviated to some extent, it has not fundamentally improved the cycling stability of the battery. Therefore, it is worth noting that cationic migration is not the primary cause of the performance decline in Mn-based layered oxide cathodes. The instability of anionic redox may play a more significant role in the deterioration of electrochemical performance.

O₂ release

For many Mn-based layered oxide cathodes for SIBs with anionic redox, a high charge cut-off voltage significantly influences the activity of anionic O²⁻. Generally, when the charge cut-off voltage is below 4.5 V, the issue of O₂ release is not prominent. The source of O₂ typically originates from two aspects: lattice oxygen atoms and surface oxygen atoms. Many Mn-based layered oxide cathodes maintain a charge cut-off voltage of 4.5 V or below. In these two typical cases, the Na_0.6Li_0.2Mn_0.8O₂ and Na_2/3Mg_1/3Mn_2/3O₂ systems both exhibit strong lattice oxygen redox activity but demonstrate unique electrochemical stability at charging voltages up to 4.5 V.^52,55 The mRIXS technique offers a comprehensive spectroscopic profile of lattice oxygen redox reactions across both excitation and emission energies. Utilizing sPFY, an intensity integration of the crucial mRIXS feature, enables the quantification of the variation in oxidized oxygen within electrodes.⁵⁵ As depicted in Figure 11A, employing ultrahigh-efficiency O-K mRIXS-sPFY technology unveiled that, during the capacity fading stage of Na_0.6Li_0.2Mn_0.8O₂, oxidation primarily involves nonlattice surface oxygen rather than lattice oxygen.⁵² Voltage decay mainly arises from the increasing contribution of Mn redox during cycling. Therefore, lattice oxygen redox reactions are not the root cause of stability issues. Instead, irreversible oxygen oxidation and changes in cation reactions lead to capacity and voltage decay. Moreover, Bruce's group made a groundbreaking discovery in 2020, revealing that Na_0.75Li_0.25Mn_0.75O₂ possesses a honeycomb-like superlattice structure akin to many redox materials.⁹⁷ In contrast, P2-Na_0.6Li_0.2Mn_0.8O₂ displays a distinctive ribbon-like superlattice configuration. As shown in Figure 11B, employing DFT calculations and high-resolution RIXS characterization, the investigation illustrated the development of molecular O₂ in the Na-deficient P2-Na_0.75Li_0.25Mn_0.75O₂ during discharging. Throughout the charging, the O–O bonds of the formed molecular O₂ break, resulting in the creation of O²⁻. However, due to Mn migration within the plane, the coordination environment surrounding the O²⁻ fails to revert to its initial state. Consequently, the honeycomb-like superlattice-ordered structure of P2-Na_0.75Li_0.25Mn_0.75O₂ diminishes after the first cycle. In contrast, the ribbon-like superlattice-ordered structure of P2-Na_0.6Li_0.2Mn_0.8O₂ effectively impedes Mn in-plane migration, suppressing the generation of molecular O₂ and facilitating the formation of stable electron holes on O²⁻. Consequently, the ribbon-like superlattice-ordered P2-Na_0.6Li_0.2Mn_0.8O₂ can sustain the oxygen redox reaction in the high-voltage region for a certain number of cycles. This ribbon ordering contributes to enhanced stability for high-voltage oxygen redox (calculated at 4.1 V) by preserving the degeneracy of the O 2p states.

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It is noteworthy that the challenge of lattice oxygen release appears less severe in Mg-based Na_0.67Mn_0.72Mg_0.28O₂,⁵⁷ Zn-based Na_2/3Mn_7/9Zn_2/9O₂,⁶¹ and Mn vacancy-based Na_4/7□_1/7Mn_6/7O₂.¹¹⁶ Rozier and colleagues revealed the presence of ARR in P2-Na_2/3Mn_7/9Zn_2/9O_2,⁶¹ and this material did not exhibit O₂ release behavior in the high voltage range (~4.5 V). For Na-rich Mn-based O3-NaLi_1/3Mn_2/3O₂, pressure tests and OEMS were conducted throughout the charging process.⁸⁸ Both methods verified the release of gas during the initial charging, predominantly O₂, commencing at approximately 0.58 Na⁺ removal (~3.8 V), as indicated by the OEMS (Figure 11C). The total amount of O₂ released during the first cycle reached 757 μmol g⁻¹. The composition transitioned to Na_0.09Li_1/3Mn_2/3O_1.86 by the end of charging, with almost no O₂ release (less than 1%) observed during the second charging, likely attributable to surface oxygen decomposition.

There is limited research on investigating the effects of increasing the charging cut-off voltage (>4.5 V). If the oxidation potential continues to rise, the oxygen oxidation–reduction process can trigger the release of lattice oxygen in many cathodes. ¹⁸O-labeled OEMS results (Figure 11D) for P2-Na_0.78Li_0.25Mn_0.75O₂ indicated that there was almost no O₂ loss when charged up to 4.5 V.¹¹² However, it occurred when the voltage was increased to 5 V, which is related to the substantial extraction of Li⁺ around the oxygen. When AM ions in the material are nearly depleted (extracted), the coordination number of O ions significantly decreases (not exceeding three coordinated cations), thereby triggering O₂ loss, primarily occurring in the near-surface region.

From the perspective of batteries with a cut-off voltage less than 4.5 V, lattice doping has been employed to enhance and maintain the reversibility of oxygen. As indicated by O K-edge sXAS and combined with Mn⁴⁺ EPR results for P2-Na_0.66Li_0.22Mn_0.775Sn_0.005O₂, a continuous decrease in the unoccupied state density during discharge was observed. Additionally, electron–electron dipole interactions between Mn⁴⁺ and oxidized O^2-α (0 < α < 2) indicated the reversibility of the oxygen anion redox process.⁶⁸ For Ti doping, after complete charging (cut-off voltage at 4.5 V), the O₂ⁿ⁻ EPR signal of Na_0.72Li_0.24Ti_0.1Mn_0.66O₂ could still be detected with no significant intensity attenuation, maintaining high reversibility of the oxygen redox.⁶⁵ Chen's team has revealed the causes of voltage decay in Na_0.8Li_0.24Mn_0.76O₂ cathodes by integrating electron energy loss spectroscopy and sXAS. They also have developed a layered oxide cathode, Na_0.8Li_0.24Al_0.03Mn_0.73O₂, with structural adjustments that exhibit no voltage decay. The introduction of robust Al–O bonds significantly weakens the covalency of Mn–O bonds, thereby enhancing the localization of oxygen electrons and eliminating the formation of O₂ release and surface oxygen vacancies.¹¹⁷

By simultaneously doping Mg²⁺ into Na and Mn sites, Na_0.7Mg_0.05(Mn_0.6Ni_0.2Mg_0.15)O₂, it was found that Mg²⁺ acted as “pillars” stabilizing the layered structure in the Na layer.⁷⁷ As illustrated in Figure 12A, the occupation of Mg²⁺ in both Na and Mn layers facilitated the formation of “Na–O–Mg” and “Mg–O–Mg” bonds, thereby promoting reversible oxygen redox reactions and enhancing structural stability. Furthermore, the Co²⁺ was introduced into the Na_xLi_yMn_1 − yO₂ system (Na_0.7Li_0.2Mn_0.7Co_0.1O₂), exhibiting lattice oxygen activity and reversibility, demonstrating an outstanding ultrahigh energy density of 729.7 Wh kg⁻¹.¹¹⁹ Theoretical calculations indicated that Co²⁺ enhanced the covalency between TM–O and reduced the bandgap, ensuring rapid electron transfer within the TMO₆ layer. Due to the adjusted electronic structure, EPR, in situ differential electrochemical mass spectrometry, and charge–discharge curves in Figure 12B, all indicated that oxidized lattice oxygen with electron holes in Na_0.7Li_0.2Mn_0.7Co_0.1O₂ was effectively stabilized, with no detection of O₂ release. In situ XRD revealed that Na_0.7Li_0.2Mn_0.7Co_0.1O₂, with a robust TMO₆ framework, exhibited better structural stability at the stage of high lattice oxygen oxidation. Finally, the modulated electronic structure and crystal structure for Na_0.7Li_0.2Mn_0.7Co_0.1O₂ ensured its excellent irreversible redox activity under high lattice oxygen activity. Building a dual-hexagonal superlattice structure in Na_2/3(Li_1/7Mn_5/14)(Mg_1/7Mn_5/14)O₂ achieves highly active and reversible lattice oxygen redox. Theoretical simulations and electrochemical tests indicate that the Li_1/7Mn_5/14 superlattice unit stimulates anionic redox activity, delivering a record high discharge capacity of 285.9 mAh g⁻¹ in SIBs.⁷ Li NMR and in situ XRD show that the Mg_1/7Mn_5/14 superlattice unit contributes to the reversibility of material structure and anionic redox, allowing Li⁺ to shuttle reversibly between Na and TM layers.¹²⁰ Chen and colleagues proposed an electron localization strategy to design a Na_0.8Li_0.24Al_0.03Mn_0.73O₂ cathode with no voltage decay.¹¹⁷ The introduction of a strong Al–O bond significantly weakens the covalency of Mn–O bonds, enhancing oxygen electron localization, eliminating O₂ release, and forming surface oxygen vacancies. Na_2/3Fe_2/9Ni_2/9Mn_5/9O₂, due to the formation of a unique Fe–(O–O) species, successfully suppresses oxygen release, effectively stabilizing the reversibility of O²⁻/O₂ⁿ⁻ oxidation–reduction at high working voltages.⁵¹ Therefore, increasing the covalency of TM–O bonds has been proven effective in suppressing O₂ release, thereby improving electrochemical performance. Moreover, the F-anion doping strategy for P2-type Na_0.6Mg_0.3Mn_0.7O_2 − xF_x also enhanced oxygen redox.¹²¹

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Under high cut-off voltages exceeding 4.5 V, there is currently limited research on the issue of O₂ release. At 5 V, in P2-Na_0.78Li_0.25Mn_0.75O₂, both Na⁺ and Li⁺ in the TM layer are released from the cathode.¹¹² As shown in Figure 12C, doping Mg²⁺ into Na_0.67Mg_0.28Mn_0.72O₂ proves effective in suppressing AM ion loss and stabilizing lattice oxygen at 5 V, thereby enhancing the material's structural stability. However, this strategy results in a slight reduction in cathode capacity. The STEM validation also has confirmed that Mg²⁺ does not migrate during charging in P2-Na_2/3Mn_0.72Mg_0.28O₂.⁹⁶ However, despite Mg's ability to restrain O₂ release, cyclic stability remains fundamentally unimproved, even at cut-off voltages below 4.5 V. Consequently, the precise role of Mg²⁺ in suppressing O₂ release and its associated negative effects remains unclear. The doping of Fe led to the preparation of intralayer disordered Na_0.67Li_0.2Fe_0.2Mn_0.6O₂ cathode and the elucidation of its crystal structure. Electrochemical tests demonstrated that the cycling stability of the Na_0.67Li_0.2Fe_0.2Mn_0.6O₂ was significantly superior to that of the Na_0.6Li_0.2Mn_0.8O₂. In Figure 12D, theoretical calculations and the O 1s etching XPS results indicated no O₂ release during charge/discharge processes.¹¹⁸

Currently, methods for suppressing lattice oxygen evolution in layered cathodes for SIBs under high voltage are relatively limited. However, inspiration can be drawn from lattice oxygen stabilization measures in layered cathodes for LIBs and applied to SIBs. Here are some potential strategies: mixing/replacing 3d metals Mn with 4d or 5d metals or d¹⁰/d⁰ non-TMs (e.g., Sn⁴⁺, Sb⁵⁺, Ti⁴⁺, Zr⁴⁺, Nb⁵⁺) and substituting O with S²⁻ or Se²⁻. Using d¹⁰/d⁰ metals to replace 3d metals, Mn can increase the overlap of M(nd)–O(2p) orbitals while reducing the impact of the U term, thereby lowering the risk of O₂ release.^43,122 The d-shell of these d¹⁰ and d⁰ metals is either fully filled or completely empty, helping to lower the directionality of the M–O bond, thereby enhancing the ability to stabilize the redox reaction. Substituting O with elements of lower electronegativity, such as S²⁻ or Se²⁻, increases the overlap of M(3d)–L(np) orbitals, reducing the risk of O₂ release while maintaining a high voltage.

By adopting common surface engineering, surface oxygen activity can be effectively suppressed. For example, in the P2-Na_0.66Li_0.22Mn_0.78O₂, a robust and stable fluorine-rich cathode electrolyte interphase (CEI) layer is constructed between the cathode and electrolyte.¹²³ This in situ formed CEI layer effectively reduces the irreversible dissolution of Li and Mn, as well as lattice oxygen loss during the redox process, significantly alleviating local structural degradation. This results in highly reversible redox reactions and excellent cycling stability. The cyclic performance of Na_0.6Li_0.2Mn_0.8O₂ is improved through surface coating with SnO₂.¹²⁴ The SnO₂ coating provides a uniform and stable ion transport interface, facilitating the deintercalation of Na⁺ and improving the rate performance. SnO₂ has abundant oxygen vacancies, which can suppress lattice oxygen loss during charging, thereby enhancing ARR reversibility. Other surface engineering aspects will be introduced in Section 3.4.

Element dissolution

After cycling, planar migration of Li⁺ ions and in-plane migration of Mnⁿ⁺ ions in the layered Na–Li–Mn–O systems will make the surface unstable, leading to irreversible Mn²⁺ dissolution. The ⁷Li NMR spectra of the P2-Na_0.66Li_0.22Mn_0.775Sn_0.005O₂ cathode show a sharp decrease in the intensities of all Li–TM and Li–AM peaks after 20 and 50 cycles, respectively, confirming irreversible Li losses.⁶⁸ Additionally, the EPR and Mn L-edge sXAS spectra in Figures 13A and 13B indicate the presence of dissolved Mn²⁺ during cycling. Furthermore, the transmission electron microscopy (TEM) images showing pronounced curvature and dislocations in the TM–O layer align with the above results, and the inductively coupled plasma (ICP) technique (Figure 13C) further confirms the dissolution of Li⁺ and Mnⁿ⁺ ions. Likewise, similar phenomena are also observed in Na_0.66Li_0.22Mn_0.775Zr_0.005O₂. The poor cycling performance may be due to the disproportionation reaction of surface Mn³⁺, generating Mn²⁺ soluble in the liquid electrolyte. The Mg²⁺ and Zn²⁺ also undergo planar migration in Na_0.67Mg_0.1Zn_0.1Mn_0.8O₂ during charging, and oxygen in the form of molecular O₂ is trapped in vacancy clusters in the fully charged state.⁷⁵ Mg²⁺ and Zn²⁺ migrating to tetrahedral sites contributes to enhance the redox activity of oxygen. However, the cycling stability is still constrained by potential issues such as the dissolution of metal elements (e.g., Mn^2/3+, Mg²⁺), which have not been resolved.

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The current methods to suppress metal element dissolution include the following:

Structural design: Adopting crystal structures like O3-NaLi_1/3Mn_2/3O₂, where increasing Na content forms an O-type phase, reduces the migration of TMs.⁸⁸ This design is expected to enhance the cycling stability of batteries, reduce Mn²⁺ dissolution, and maintain high specific capacity. Designing a core-shell composite structure as shown in Figure 14A, like the micrometer-scale O3/O′3-P2 core-shell composite structure, the O-type Ni-based core and P2-type Mn-rich shell combination can decrease Mn²⁺ dissolution.¹²⁵ This structure exhibits high specific capacity and good cycling performance. By the doping strategy, as seen in Na_0.72Li_0.24Ti_0.10Mn_0.66O₂, doping Ti⁴⁺ can suppress the in-plane migration of Li⁺ and increase the distorted coordination environment of Li⁺ in the TM layer. Consequently, the EPR spectra do not detect significant Mn³⁺ signals before discharging to 2.0 V.⁶⁵ The ICP and Mn 2p XPS results in Figure 14B revealed that the Na_0.66Li_0.22Ti_0.15Mn_0.63O₂ cathode showed a low dissolution content of Mn after cycling.⁶⁴ In Na_0.7Mg_0.05(Mn_0.6Ni_0.2Mg_0.15)O₂, Mg²⁺ in Na and TM layers can stabilize the crystal structure and contribute to improving cycling performance.⁷⁷ Using F⁻ doping, as in P2-type Na_0.6Mg_0.3Mn_0.7O_2 − xF_x, where strong interactions occur between F and Mn ions, can promote reversible conversion from Mn²⁺ to Mn⁴⁺ and inhibit Mn²⁺ dissolution, as demonstrated by the Mn L-edge XAS results in Figure 14C.¹²¹

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Electrolyte design: Sodium-ion electrolytes predominantly utilize sodium salts with weakly coordinating anion and ester-based solvents for Mn-based layered cathodes with ARR. The compatibility between electrolytes and cathodes primarily relies on the electrolyte's ability to facilitate rapid sodium ion transport (high ionic conductivity). Sodium salts with smaller lattice energy (NaPF₆ > NaClO₄ > NaTFSI > NaOTf > NaBF₄) exhibit the higher conductivity (>6.0 mS cm⁻¹).¹²⁷ The ionic conductivity of organic solvents, as a medium for soluble sodium salts, is also influenced by solvent chemistry. However, the magnitude of ionic conductivity is enough to ensure the rapid intercalation and deintercalation of sodium ions between the cathodes and electrolytes. Then, the compatibility of these sodium salts or solvents with layered oxide cathodes also depends on the interface between the cathodes and electrolytes and the overall operating voltage range of the SIBs. During the charge and discharge processes, electrolytes react with Mn-based layered cathodes to form a composite deposition of organic and inorganic decomposition products on surfaces (CEI). An ideal CEI possesses high ionic and electronic conductivity, dense and uniform coverage, and good stability. However, conventional ester-based solvents may not exhibit good film-forming properties with cathodes, leading to the disruption or instability of the interface layer during extended cycling. This can adversely affect the battery's cycling performance, particularly at high voltages. The electrochemical window of the electrolytes should exceed the redox potentials of both the cathode and anode. The thermodynamic highest occupied molecular orbital (HOMO) energy of sodium salts and solvent oxidation can to some extent limit the electrochemical window of electrolyte. When the operating voltage of Mn-based layered cathodes exceeds the electrochemical window of the electrolyte, electrolyte oxidation decomposition may occur, reducing the ionic conductivity. Moreover, these decomposition products may react with the surface of the layered cathodes, leading to capacity decay.¹²⁸ The type of anion in sodium salts has a significant impact on the electrochemical window. The oxidation potential follows the order of NaPF₆ > NaClO₄ > NaTFSI > NaFSI, indicating that PF₆⁻ ions in a mixed solvent of ethylene carbonate (EC)/diethyl carbonate (DEC) (with the lowest HOMO level (−11.67 eV)) are not easily prone to electron loss and decomposition.¹²⁹ But ClO₄⁻ ions have a HOMO level at −7.89 eV, indicating poor chemical stability during oxidation. Similar to lithium-ion electrolytes, sodium-ion electrolytes undergo decomposition when operating voltages exceed 4.2 V (vs. Na⁺/Na). This is because commonly used organic carbonates such as linear carbonates like dimethyl carbonate (DMC), ethyl methyl carbonate, and DEC, and cyclic carbonates like propylene carbonate and EC cannot remain stable at high voltages.^128,130 When the CEI destabilizes and interface rupture occurs, the accumulated reaction gas at the interface will escape, impacting battery safety. Firstly, the produced gas increases internal pressure, which may lead to battery swelling, deformation or even explosion. Therefore, conventional electrolytes are inadequate for high-voltage SIBs. Therefore, it is crucial to develop sodium-ion electrolytes or electrolyte additives that are resistant to high-voltage oxidation and exhibit good film-forming properties.

Electrolyte design has been explored as a strategy to enhance the battery performance. For instance, employing an electrolyte composed of sodium bis(fluorosulfonyl)imide (NaFSI) and triethyl phosphate (TEP) in a molar ratio of NaFSI:TEP of 1:1.5 has been shown to form a stable CEI layer.¹²⁶ This aims to minimize electrolyte oxidation, cathode surface reconstruction, and TM dissolution. After 50 cycles, the CEI layer formed by the NaFSI–TEP electrolyte shows a Mn atomic content of only 0.46% and a Ni atomic content of 0.11% (Figure 14D), significantly lower than the values without NaFSI–TEP (2.18% and 0.36%, respectively). Additionally, the thickness of this dense and uniform CEI layer is approximately 10 nm, formed by NaFSI–TEP with SO_x compounds that suppress solvent penetration and prevent TM migration to the electrode surface. The low solubility of TM is attributed to the high electrolyte concentration, resulting in fewer free solvent molecules, making it nearly impossible to dissolve other compounds. The 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14TFSI) ionic liquid is a widely used Li-O₂ battery electrolyte solvent known for its high stability with superoxide species and faster Li⁺ diffusion.⁵⁰ Using 1 M NaTFSI in Pyr14TFSI as the electrolyte, a significant reduction in initial cycle capacity drop in Na_2/3Ni_1/3Mn_2/3O₂ was achieved.¹³¹ Using 1.5M NaFSI as the electrolyte dissolved in a mixture of low-polarity solvents Tris (2, 2, 2-trifluoroethyl) phosphate (TFP) and DMC with a molar ratio of 1.5:2, enhances the battery performance. By reducing the content of free solvents in the electrolyte and increasing the concentration of anions in the first solvation shell, the cycling performance of the NaNi_0.68Mn_0.22Co_0.1O₂ was improved. This also led to the formation of an ultrathin CEI layer on the cathode surface, ensuring a highly reversible phase transition and significantly suppressing the Niⁿ⁺ dissolution. These effects were achieved by the low-polarity solvent TFP, reducing the content of free solvents and generating salt-derived components with low solubility.

Surface engineering: Surface modification strategies have been explored to address issues related to material degradation. In O3-NaLi_0.05Mn_0.50Ni_0.30Cu_0.10Mg_0.05O₂, significant surface layer loss and increased Mnⁿ⁺ dissolution were observed after 400 cycles.¹³² The presence of TM elements such as Ni and Mg also can be observed on the anode surface. To mitigate this phenomenon, Al(NO₃)₃·9H₂O and H₃PO₄ were added dropwise to form a protective layer on NaLi_0.05Mn_0.50Ni_0.30Cu_0.10Mg_0.05O₂. By using an inorganic phosphate compound (HF scavenger), it can combine an H⁺ without generating additional water molecules, avoiding secondary NaPF₆ hydrolysis and reducing HF generation. The addition of an AlPO₄ ion conductor layer and HF scavengers contributed to improved cycling stability, with the battery maintaining 95% capacity retention after 400 cycles. Using a fast Na⁺ conductor NaTi₂(PO₄)₃ coating layer on Na_0.67Ni_0.28Mg_0.05Mn_0.67O₂ also can effectively stabilize the interface and prevent HF corrosion, thereby inhibiting surface cracking and peeling phenomena in the electrode material.¹³³ Melal oxides such as Al₂O₃, TiO₂, MgO, and ZnO, were also applied as HF scavengers in other works.¹³⁴ Mg/Ti co-doping and MgO surface coating were introduced in Na_0.67Ni_0.17Co_0.17Mn_0.66O₂, while the MgO layer effectively prevents HF corrosion on the surface, leading to enhanced P2 structure stabilization and improved cycling performance.¹³⁵ This material has an initial discharge capacity of 111.6 mAh g⁻¹ and a high capacity retention of 90.6% in 2.0–4.5 V at a high current density of 100 mA g⁻¹. TiO₂ coating and Ti⁴⁺ doping in O3-NaMn_0.33Fe_0.33Ni_0.33O₂ were employed to prevent surface reactions, which can increase the Na⁺ diffusion coefficient and reduce the Mn³⁺/Mn⁴⁺ ratio.¹³⁶ This indicates that TiO₂ coating effectively prevents unnecessary surface reactions. Additionally, TiO₂ coating induces Ti⁴⁺ doping, increasing the bond energy of Na-O bonds and enlarging the interlayer spacing (d), thereby enhancing the Na⁺ diffusion coefficient. The influence of four different metal oxide coatings (Al₂O₃, TiO₂, SnO₂, and WO₃) applied through atomic layer deposition on P3/P2 dual-phase Na_2/3Ni_1/3Mn_2/3O₂ was examined.¹³⁷ This cathode consisted of a composite phase with a dominant P3 rhombohedral phase (~76%) and the remaining 24% being the P2 hexagonal phase. The fundamental cause of electrode surface failure was attributed to surface side reactions induced by high voltage and TM dissolution at low discharge voltage. Al₂O₃ shows the most effective protective effect for the cathode, which showed a high specific capacity of 105 mAh g⁻¹ at 5 C for 300 cycles, achieving a capacity retention of 87%. Additionally, lattice matching is more coordinated through the construction of an in situ heterogeneous structure on the surface. By utilizing a borate-based liquid–solid interface reaction, Na_0.65Mn_0.67Co_0.17Ni_0.17B_0.05O_2+x was designed with an interlocked spinel-like and layered heterogeneous structure.¹³⁸ The extrinsic spinel-like nanolayer effectively alleviates the instability issue on the surface at 4.1 V while also suppressing the accumulation of uneven stress and lattice distortion.

PERSPECTIVES

Mn-based layered oxide cathodes for SIBs exhibit significant application potential but also face a series of critical challenges. The problem of particle size (sluggish ion diffusion) results in capacity decay, necessitating suitable synthesis methods and structural design for resolution. Cationic migration issues limit the charge transfer and cycle life, while O₂ release and element dissolution additionally compromise the stability of batteries. Researchers have employed various modification strategies to tackle these challenges. The electrochemical performance of Mn-based cathodes has seen improvement through the implementation of suitable doping and structural design. Current research is mainly focused on how to control particle size, enhancing cationic migration pathways, reducing O₂ release, and inhibiting element dissolution.

Here, we will further analyze potential challenges and provide some directions for future research from both fundamental research and industrialization perspectives. For fundamental research, as depicted in Figure 15, several future development directions that need attention are listed below:

• 1.

  Novel mixed-conductive cathodes: Regarding particle size issues, current micrometer-scale electrode materials for Mn-based layered oxide cathodes for SIBs are typically a compromised choice with medium particle sizes, balancing energy density and rate performance. Future research directions may involve controlling particle size and shape through material engineering to achieve a more uniform distribution. Such nanocrystal microparticle design can be applied to synthesize cathode particles, further improving the performance of Mn-based cathodes. Complex phase transitions occur in the P2 and O3 phase structures during the extraction and insertion of Na⁺. Additionally, electrostatic interactions between Na–Na and Na–TM can induce charge and vacancy ordering, thereby increasing the Na⁺ diffusion barrier. Future development may involve the development of fast ionic and electron mixed-conductive cathodes by doping, structural regulation, and novel synthesis strategies to enhance the migration speed of ions or electrons and reduce structural changes. Strategies such as lattice engineering and alloy design may be adopted to enhance the cathode stability, which could address O₂ release issues at high voltage.

• 2.

  Optimizing the electrode–electrolyte interface: Further and more in-depth studies of interface mechanism will help address issues such as element dissolution and O₂ release. Furthermore, new coating materials and surface engineering may be used to suppress element dissolution and O₂ release. Ensuring a long cycle life for SIBs also requires considering the impact of electrolytes, as they may lead to dissolution and unstable SEI. By designing new electrolytes and adding appropriate additives, a broader voltage window can be achieved, reducing the risk of element dissolution and O₂ release. The interface between cathode and electrolyte has a significant impact on performance. Therefore, researchers need to focus on how to improve these interfaces to improve cell efficiency and cycle life.

• 3.

  Na-rich cathodes: Like Mn-based Li-rich cathodes for LIBs, Na-rich cathodes theoretically have higher specific capacity and energy density. Future research should delve deeper into the structural design and charge compensation mechanisms of Na-rich materials to elucidate their electrochemical performance and application potential. Therefore, it is important to emphasize some key points. First, understanding the electrochemical mechanism is crucial. Revealing the detailed mechanisms of reversible anionic redox will help optimize the material design to synthesize the Na-rich cathodes with high energy density, especially where oxygen plays a crucial role. Second, how to build a stable structure is another key point. Na-rich cathodes may undergo phase transitions, volume expansion, and other issues during cycling, leading to structural instability. It is necessary to design more stable structures or find additives to ensure that batteries have a longer cycle life. Both factors are crucial for practical applications. To achieve commercialization, Na-rich cathodes may also need to overcome challenges, such as O₂ release and surface element dissolution.

• 4.

  Interdisciplinary research: The development of SIBs requires interdisciplinary research covering materials science, electrochemistry, engineering, environmental science, and so forth. For example, computer simulation methods can be used to predict the structure and performance of Mn-based Na-rich cathodes. These simulations can guide experimental research, reducing the number of trial-and-error attempts. Physics and chemistry provide profound insights into the fundamental physical and chemical properties of materials, aiding in the synthesis of new materials. Environmental science focuses on the sustainability of materials, including resource utilization and waste disposal, ensuring that battery technology is environmentally friendly. Interdisciplinary research is expected to drive the development of SIB technology and expedite its application.

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For industrial and manufacturing aspects, two additional challenges need to be considered:

• 1.

  Manufacturing scalability: Scaling up the production of Mn-based layered oxide cathodes is necessary to meet the demands of large-scale SIB manufacturing. Cost is a significant factor in the commercialization of SIBs. Mn-based layered oxide cathodes must be manufactured using cost-effective processes and raw materials to remain competitive with other cathodes. Therefore, it is necessary to reduce manufacturing costs by optimizing material composition, synthesis routes, and methods without compromising battery performance. Reference LIB commercialization methods for lithium-ion layered oxide cathode, such as commonly used spray pyrolysis and co-precipitation methods, can also be adopted for layered sodium-ion cathode materials.¹³⁹ The key challenge lies in the need to develop or optimize process parameters suitable for high-performance sodium-ion Mn-based layered oxide cathodes, while ensuring consistent quality control during the manufacturing process.

• 2.

  Stability and safety: Ensuring the stability and safety of Mn-based layered oxide cathodes during battery manufacturing and operation also is crucial. Air sensitivity is a challenge faced by many sodium-ion Mn-based layered oxide cathodes. Air-sensitive cathodes are prone to react with air (mainly H₂O and CO₂) and form a series of harmful sodium compounds (Na_xH_yMO₂, NaHCO₃, Na_xH_y(H₂O)_zMO₂, etc.) on the surface.¹⁴⁰ These sodium compounds and Na⁺/H⁺ exchange can impede sodium diffusion and reduce capacity. Additionally, it damages the electrode-coating process. The slurry made from air-sensitive cathodes typically exhibits gelation, leading to electrode delamination and flaking after drying. The basicity of the formed sodium residue causes the commonly used polyvinylidene fluoride binder to defluorinate, resulting in poor adhesion and particle aggregation.¹⁴¹ Meanwhile, residual sodium compounds on the surface can lead to gas formation during cycling, causing unstable coulombic efficiency and safety issues. Decomposition of surface-residual salts during cycling can release gases such as CO₂ and CO, leading to battery swelling and posing an explosion risk. Air sensitivity hinders the commercial application of SIBs. Strict control of the atmosphere is required during material storage, transportation, and manufacturing, ultimately increasing engineering difficulty and cost. So, addressing these stability issues crucially relies on optimizing material design, electrode formulation, and electrode manufacturing technology.

CONCLUSIONS

The Mn-based layered oxide cathodes with anionic redox for SIBs have garnered considerable attention owing to their anionic redox activity. The ongoing research and innovation in these Mn-based cathodes aim to overcome scientific challenges, achieve higher performance, and contribute to sustainable battery technology. The trends in future development involve continuous modification and optimization of Mn-based cathodes, addressing issues such as sluggish ion diffusion, cationic migration, O₂ release, and element dissolution. Simultaneously, developing Na-rich cathodes with stable redox characteristics without increasing raw material costs or reducing energy density is a meaningful future research direction. These efforts are crucial for advancing clean energy technologies, reducing dependence on fossil fuels, and realizing the application of sustainable energy.

ACKNOWLEDGMENTS

The authors acknowledge the support of the National Key R&D Program of China (2022YFB2502000) and the National Natural Science Foundation of China (52207244).

CONFLICT OF INTEREST STATEMENT

The authors declare that there are no conflicts of interests.

References

Shi C, Yu M. Flexible solid‐state lithium–sulfur batteries based on structural designs. Energy Storage Mater. 2023;57:429‐459.

Shi C, Hamann T, Takeuchi S, et al. 3D asymmetric bilayer garnet‐hybridized high‐energy‐density lithium–sulfur batteries. ACS Appl Mater Interfaces. 2023;15(1):751‐760.

Shi C, Alexander GV, O'Neill J, Duncan K, Godbey G, Wachsman ED. All‐solid‐state garnet type sulfurized polyacrylonitrile/lithium‐metal battery enabled by an inorganic lithium conductive salt and a bilayer electrolyte architecture. ACS Energy Lett. 2023;8(4):1803‐1810.

Seh ZW, Sun Y, Zhang Q, Cui Y. Designing high‐energy lithium–sulfur batteries. Chem Soc Rev. 2016;45(20):5605‐5634.

Shi C, Wang T, Liao X, et al. Accordion‐like stretchable Li‐ion batteries with high energy density. Energy Storage Mater. 2019;17:136‐142.

Jiao S, Wang J, Hu Y‐S, Yu X, Li H. High‐capacity oxide cathode beyond 300 mA h/g: focus review. ACS Energy Lett. 2023;8(7):3025‐3037.

Usiskin R, Lu Y, Popovic J, et al. Fundamentals, status and promise of sodium‐based batteries. Nat Rev Mater. 2021;6(11):1020‐1035.

Liu Q, Hu Z, Chen M, et al. Recent progress of layered transition metal oxide cathodes for sodium‐ion batteries. Small. 2019;15(32): [eLocator: 1805381].

Wang K, Zhuo H, Wang J, Poon F, Sun X, Xiao B. Recent advances in Mn‐rich layered materials for sodium‐ion batteries. Adv Funct Mater. 2023;33(13): [eLocator: 2212607].

Jeong M, Lee H, Yoon J, Yoon W‐S. O3‐type NaNi_1/3Fe_1/3Mn_1/3O₂ layered cathode for Na‐ion batteries: structural evolution and redox mechanism upon Na (de) intercalation. J Power Sources. 2019;439: [eLocator: 227064].

Wang P‐F, Yao H‐R, Liu X‐Y, et al. Na⁺/vacancy disordering promises high‐rate Na‐ion batteries. Sci Adv. 2018;4(3): [eLocator: eaar6018].

Li Y, Yang Z, Xu S, et al. Air‐stable copper‐based P2‐Na_7/9Cu_2/9Fe_1/9Mn_2/3O₂ as a new positive electrode material for sodium‐ion batteries. Adv Sci. 2015;2(6): [eLocator: 1500031].

Liu Y, Qiao Y, Zhang W, et al. Sodium storage in Na‐rich Na_xFeFe(CN)₆ nanocubes. Nano Energy. 2015;12:386‐393.

Lee H‐W, Wang RY, Pasta M, Woo Lee S, Liu N, Cui Y. Manganese hexacyanomanganate open framework as a high‐capacity positive electrode material for sodium‐ion batteries. Nat Commun. 2014;5(1):5280.

Tang W, Song X, Du Y, et al. High‐performance NaFePO₄ formed by aqueous ion‐exchange and its mechanism for advanced sodium ion batteries. J Mater Chem A. 2016;4(13):4882‐4892.

Yao X, Zhu Z, Li Q, et al. 3.0 V high energy density symmetric sodium‐ion battery: Na₄V₂(PO₄)₃//Na₃V₂(PO₄)₃. ACS Appl Mater Interfaces. 2018;10(12):10022‐10028.

Shen C, Long H, Wang G, Lu W, Shao L, Xie K. Na₃V₂(PO₄)₂F₃@C dispersed within carbon nanotube frameworks as a high tap density cathode for high‐performance sodium‐ion batteries. J Mater Chem A. 2018;6(14):6007‐6014.

Wei F, Zhang Q, Zhang P, et al. Review—research progress on layered transition metal oxide cathode materials for sodium ion batteries. J Electrochem Soc. 2021;168(5): [eLocator: 050524].

Liu Q, Hu Z, Li W, et al. Sodium transition metal oxides: the preferred cathode choice for future sodium‐ion batteries? Energy Environ Sci. 2021;14(1):158‐179.

Luo C, Langrock A, Fan X, Liang Y, Wang C. P2‐type transition metal oxides for high performance Na‐ion battery cathodes. J Mater Chem A. 2017;5(34):18214‐18220.

Wang J, He X, Paillard E, Laszczynski N, Li J, Passerini S. Lithium‐ and manganese‐rich oxide cathode materials for high‐energy lithium ion batteries. Adv Energy Mater. 2016;6(21): [eLocator: 1600906].

Li B, Xia D. Anionic redox in rechargeable lithium batteries. Adv Mater. 2017;29(48): [eLocator: 1701054].

Zhao C, Wang Q, Yao Z, et al. Rational design of layered oxide materials for sodium‐ion batteries. Science. 2020;370(6517):708‐711.

Zheng H, Zhang C, Zhang Y, et al. Manipulating the local electronic structure in Li‐rich layered cathode towards superior electrochemical performance. Adv Funct Mater. 2021;31(30): [eLocator: 2100783].

Yabuuchi N, Hara R, Kajiyama M, et al. New O2/P2‐type Li‐excess layered manganese oxides as promising multi‐functional electrode materials for rechargeable Li/Na batteries. Adv Energy Mater. 2014;4(13): [eLocator: 1301453].

Rong X, Liu J, Hu E, et al. Structure‐induced reversible anionic redox activity in Na layered oxide cathode. Joule. 2018;2(1):125‐140.

Hirsh H, Li Y, Cheng J‐H, et al. The negative impact of transition metal migration on oxygen redox activity of layered cathode materials for Na‐ion batteries. J Electrochem Soc. 2021;168(4): [eLocator: 040539].

Assat G, Tarascon J‐M. Fundamental understanding and practical challenges of anionic redox activity in Li‐ion batteries. Nat Energy. 2018;3(5):373‐386.

Zhu Y‐F, Xiao Y, Dou S‐X, Kang Y‐M, Chou S‐L. Spinel/post‐spinel engineering on layered oxide cathodes for sodium‐ion batteries. eScience. 2021;1(1):13‐27.

Delmas C, Fouassier C, Hagenmuller P. Structural classification and properties of the layered oxides. Phys B+C. 1980;99(1‐4):81‐85.

Zhao C, Avdeev M, Chen L, Hu Y‐S. An O3‐type oxide with low sodium content as the phase‐transition‐free anode for sodium‐ion batteries. Angew Chem Int Ed. 2018;57(24):7056‐7060.

Clément RJ, Bruce PG, Grey CP. Review—manganese‐based P2‐type transition metal oxides as sodium‐ion battery cathode materials. J Electrochem Soc. 2015;162(14):A2589‐A2604.

Luo K, Roberts MR, Hao R, et al. Charge‐compensation in 3d‐transition‐metal‐oxide intercalation cathodes through the generation of localized electron holes on oxygen. Nat Chem. 2016;8(7):684‐691.

Tsuchimoto A, Shi X‐M, Kawai K, et al. Nonpolarizing oxygen‐redox capacity without O–O dimerization in Na₂Mn₃O₇. Nat Commun. 2021;12(1):631.

Chen Z, Li J, Zeng XC. Unraveling oxygen evolution in Li‐rich oxides: a unified modeling of the intermediate peroxo/superoxo‐like dimers. J Am Chem Soc. 2019;141(27):10751‐10759.

Kim B, Song J‐H, Eum D, et al. A theoretical framework for oxygen redox chemistry for sustainable batteries. Nat Sustain. 2022;5(8):708‐716.

Sathiya M, Abakumov AM, Foix D, et al. Origin of voltage decay in high‐capacity layered oxide electrodes. Nat Mater. 2015;14(2):230‐238.

Assat G, Iadecola A, Delacourt C, Dedryvère R, Tarascon J‐M. Decoupling cationic–anionic redox processes in a model Li‐rich cathode via operando X‐ray absorption spectroscopy. Chem Mater. 2017;29(22):9714‐9724.

Hong J, Gent WE, Xiao P, et al. Metal–oxygen decoordination stabilizes anion redox in Li‐rich oxides. Nat Mater. 2019;18(3):256‐265.

Li B, Kumar K, Roy I, et al. Capturing dynamic ligand‐to‐metal charge transfer with a long‐lived cationic intermediate for anionic redox. Nat Mater. 2022;21(10):1165‐1174.

Kitchaev DA, Vinckeviciute J, Van der Ven A. Delocalized metal–oxygen π‐redox is the origin of anomalous nonhysteretic capacity in Li‐ion and Na‐ion cathode materials. J Am Chem Soc. 2021;143(4):1908‐1916.

Eum D, Kim B, Song J‐H, et al. Coupling structural evolution and oxygen‐redox electrochemistry in layered transition metal oxides. Nat Mater. 2022;21(6):664‐672.

Seo D‐H, Lee J, Urban A, Malik R, Kang S, Ceder G. The structural and chemical origin of the oxygen redox activity in layered and cation‐disordered Li‐excess cathode materials. Nat Chem. 2016;8(7):692‐697.

House RA, Marie J‐J, Pérez‐Osorio MA, Rees GJ, Boivin E, Bruce PG. The role of O₂ in O‐redox cathodes for Li‐ion batteries. Nat Energy. 2021;6(8):781‐789.

Gao A, Zhang Q, Li X, et al. Topologically protected oxygen redox in a layered manganese oxide cathode for sustainable batteries. Nat Sustain. 2022;5(3):214‐224.

Zheng L, Li J, Obrovac MN. Crystal structures and electrochemical performance of air‐stable Na_2/3Ni_{1/3 – x}Cu_xMn_2/3O₂ in sodium cells. Chem Mater. 2017;29(4):1623‐1631.

Yang Q, Wang P‐F, Guo J‐Z, et al. Advanced P2‐Na_2/3Ni_1/3Mn_7/12Fe_1/12O₂ cathode material with suppressed P2–O2 phase transition toward high‐performance sodium‐ion battery. ACS Appl Mater Interfaces. 2018;10(40):34272‐34282.

Wang L, Sun Y‐G, Hu L‐L, et al. Copper‐substituted Na_0.67Ni_{0.3 − x}Cu_xMn_0.7O₂ cathode materials for sodium‐ion batteries with suppressed P2–O2 phase transition. J Mater Chem A. 2017;5(18):8752‐8761.

Ma C, Alvarado J, Xu J, et al. Exploring oxygen activity in the high energy P2‐type Na_0.78Ni_0.23Mn_0.69O₂ cathode material for Na‐ion batteries. J Am Chem Soc. 2017;139(13):4835‐4845.

Risthaus T, Zhou D, Cao X, et al. A high‐capacity P2 Na_2/3Ni_1/3Mn_2/3O₂ cathode material for sodium ion batteries with oxygen activity. J Power Sources. 2018;395:16‐24.

Zhang Y, Wu M, Ma J, et al. Revisiting the Na_2/3Ni_1/3Mn_2/3O₂ cathode: oxygen redox chemistry and oxygen release suppression. ACS Cent Sci. 2020;6(2):232‐240.

Wu J, Zhuo Z, Rong X, et al. Dissociate lattice oxygen redox reactions from capacity and voltage drops of battery electrodes. Sci Adv. 2020;6(6): [eLocator: eaaw3871].

Du K, Zhu J, Hu G, Gao H, Li Y, Goodenough JB. Exploring reversible oxidation of oxygen in a manganese oxide. Energy Environ Sci. 2016;9(8):2575‐2577.

Rong X, Hu E, Lu Y, et al. Anionic redox reaction‐induced high‐capacity and low‐strain cathode with suppressed phase transition. Joule. 2019;3(2):503‐517.

Dai K, Wu J, Zhuo Z, et al. High reversibility of lattice oxygen redox quantified by direct bulk probes of both anionic and cationic redox reactions. Joule. 2019;3(2):518‐541.

Yabuuchi N, Hara R, Kubota K, Paulsen J, Kumakura S, Komaba S. A new electrode material for rechargeable sodium batteries: P2‐type Na_2/3[Mg_0.28Mn_0.72]O₂ with anomalously high reversible capacity. J Mater Chem A. 2014;2(40):16851‐16855.

Maitra U, House RA, Somerville JW, et al. Oxygen redox chemistry without excess alkali‐metal ions in Na_2/3[Mg_0.28Mn_0.72]O₂. Nat Chem. 2018;10(3):288‐295.

Billaud J, Singh G, Armstrong AR, et al. Na_0.67Mn_1 − xMg_xO₂ (0 ≤ x ≤ 0.2): a high capacity cathode for sodium‐ion batteries. Energy Environ Sci. 2014;7(4):1387‐1391.

Song B, Hu E, Liu J, et al. A novel P3‐type Na_2/3Mg_1/3Mn_2/3O₂ as high capacity sodium‐ion cathode using reversible oxygen redox. J Mater Chem A. 2019;7(4):1491‐1498.

Zuo W, Qiu J, Liu X, et al. Highly‐stable P2‐Na_0.67MnO₂ electrode enabled by lattice tailoring and surface engineering. Energy Storage Mater. 2020;26:503‐512.

Bai X, Sathiya M, Mendoza‐Sánchez B, et al. Anionic redox activity in a newly Zn‐doped sodium layered oxide P2‐Na_2/3Mn_1 − yZn_iO₂ (0 < y < 0.23). Adv Energy Mater. 2018;8(32): [eLocator: 1802379].

Shen Q, Liu Y, Zhao X, et al. Unexpectedly high cycling stability induced by a high charge cut‐off voltage of layered sodium oxide cathodes. Adv Energy Mater. 2023;13(6): [eLocator: 2203216].

Yang L, Li X, Liu J, et al. Lithium‐doping stabilized high‐performance P2‐Na_0.66Li_0.18Fe_0.12Mn_0.7O₂ cathode for sodium ion batteries. J Am Chem Soc. 2019;141(16):6680‐6689.

Cao X, Li X, Qiao Y, et al. Restraining oxygen loss and suppressing structural distortion in a newly Ti‐substituted layered oxide P2‐Na_0.66Li_0.22Ti_0.15Mn_0.63O₂. ACS Energy Lett. 2019;4(10):2409‐2417.

Li C, Zhao C, Hu B, Tong W, Shen M, Hu B. Unraveling the critical role of Ti substitution in P2‐Na_xLi_yMn_1 – yO₂ cathodes for highly reversible oxygen redox chemistry. Chem Mater. 2020;32(3):1054‐1063.

Hu B, Geng F, Zhao C, et al. Deciphering the origin of high electrochemical performance in a novel Ti‐substituted P2/O3 biphasic cathode for sodium‐ion batteries. ACS Appl Mater Interfaces. 2020;12(37):41485‐41494.

Bianchini M, Gonzalo E, Drewett NE, et al. Layered P2–O3 sodium‐ion cathodes derived from earth abundant elements. J Mater Chem A. 2018;6(8):3552‐3559.

Zhao C, Li C, Yang Q, et al. Anionic redox reaction in Na‐deficient layered oxide cathodes: role of Sn/Zr substituents and in‐depth local structural transformation revealed by solid‐state NMR. Energy Storage Mater. 2021;39:60‐69.

Kim HJ, Konarov A, Jo JH, et al. Controlled oxygen redox for excellent power capability in layered sodium‐based compounds. Adv Energy Mater. 2019;9(32): [eLocator: 1901181].

Wang P‐F, Xiao Y, Piao N, et al. Both cationic and anionic redox chemistry in a P2‐type sodium layered oxide. Nano Energy. 2020;69: [eLocator: 104474].

Zhao C, Yao Z, Wang J, et al. Ti substitution facilitating oxygen oxidation in Na_2/3Mg_1/3Ti_1/6Mn_1/2O₂ cathode. Chem. 2019;5(11):2913‐2925.

Xiao Y, Zhu Y‐F, Yao H‐R, et al. A stable layered oxide cathode material for high‐performance sodium‐ion battery. Adv Energy Mater. 2019;9(19): [eLocator: 1803978].

Liu Z, Chu S, Wu J, et al. Long‐enduring oxygen redox enabling robust layered cathodes for sodium‐ion batteries. Chem Eng J. 2022;435: [eLocator: 134944].

Liu J, Qi R, Zuo C, et al. Inherent inhibition of oxygen loss by regulating superstructural motifs in anionic redox cathodes. Nano Energy. 2021;88: [eLocator: 106252].

Ji H, Ji W, Xue H, et al. Synergistic activation of anionic redox via cosubstitution to construct high‐capacity layered oxide cathode materials for sodium‐ion batteries. Sci Bull. 2023;68(1):65‐76.

Niu Y, Hu Z, Zhang B, et al. Earth‐abundant Na–Mg–Fe–Mn–O cathode with reversible hybrid anionic and cationic redox. Adv Energy Mater. 2023;13(27): [eLocator: 2300746].

Wang Q‐C, Meng J‐K, Yue X‐Y, et al. Tuning P2‐structured cathode material by Na‐site Mg substitution for Na‐ion batteries. J Am Chem Soc. 2019;141(2):840‐848.

Chen T, Guo J, Zhuo Y, et al. An inactive metal supported oxide cathode material with high rate capability for sodium ion batteries. Energy Storage Mater. 2019;20:263‐268.

Zhang K, Kim D, Hu Z, et al. Manganese based layered oxides with modulated electronic and thermodynamic properties for sodium ion batteries. Nat Commun. 2019;10(1):5203.

Li J, Risthaus T, Wang J, et al. The effect of Sn substitution on the structure and oxygen activity of Na_0.67Ni_0.33Mn_0.67O₂ cathode materials for sodium ion batteries. J Power Sources. 2020;449: [eLocator: 227554].

Mortemard de Boisse B, Nishimura S, Watanabe E, et al. Highly reversible oxygen‐redox chemistry at 4.1 V in Na_{4/7 − x}[□_1/7Mn_6/7]O₂ (□: Mn vacancy). Adv Energy Mater. 2018;8(20): [eLocator: 1800409].

Song B, Tang M, Hu E, et al. Understanding the low‐voltage hysteresis of anionic redox in Na₂Mn₃O₇. Chem Mater. 2019;31(10):3756‐3765.

Zhao C, Wang Q, Lu Y, et al. Decreasing transition metal triggered oxygen redox activity in Na‐deficient oxides. Energy Storage Mater. 2019;20:395‐400.

Yang L, Liu Z, Shen X, et al. Effect of vacancy‐tailored Mn³⁺ spinning on enhancing structural stability. Energy Storage Mater. 2022;44:231‐238.

Yang L, Liu Z, Liu S, et al. Superiority of native vacancies in activating anionic redox in P2‐type Na_2/3[Mn_7/9Mg_1/9□_1/9]O₂. Nano Energy. 2020;78: [eLocator: 105172].

Shen Q, Liu Y, Zhao X, et al. Transition‐metal vacancy manufacturing and sodium‐site doping enable a high‐performance layered oxide cathode through cationic and anionic redox chemistry. Adv Funct Mater. 2021;31(51): [eLocator: 2106923].

Li X‐L, Bao J, Shadike Z, et al. Stabilizing transition metal vacancy induced oxygen redox by Co²⁺/Co³⁺ redox and sodium‐site doping for layered cathode materials. Angew Chem Int Ed. 2021;60(40):22026‐22034.

Wang Q, Mariyappan S, Rousse G, et al. Unlocking anionic redox activity in O3‐type sodium 3d layered oxides via Li substitution. Nat Mater. 2021;20(3):353‐361.

Zhang X, Qiao Y, Guo S, et al. Manganese‐based Na‐rich materials boost anionic redox in high‐performance layered cathodes for sodium‐ion batteries. Adv Mater. 2019;31(27): [eLocator: 1807770].

Li X, Guo S, Qiu F, Wang L, Ishida M, Zhou H. Na₂Ru_1 − xMn_xO₃ as the cathode for sodium‐ion batteries. J Mater Chem A. 2019;7(9):4395‐4399.

Liu S, Lu F, Li H, et al. Na₂Ru_0.8Mn_0.2O₃: a novel cathode material for ultrafast sodium ion battery with large capacity and superlong cycle life. J Power Sources. 2019;421:14‐22.

Wang P‐F, You Y, Yin Y‐X, Guo Y‐G. An O3‐type NaNi_0.5Mn_0.5O₂ cathode for sodium‐ion batteries with improved rate performance and cycling stability. J Mater Chem A. 2016;4(45):17660‐17664.

Assat G, Foix D, Delacourt C, Iadecola A, Dedryvère R, Tarascon J‐M. Fundamental interplay between anionic/cationic redox governing the kinetics and thermodynamics of lithium‐rich cathodes. Nat Commun. 2017;8(1):2219.

Gent WE, Lim K, Liang Y, et al. Coupling between oxygen redox and cation migration explains unusual electrochemistry in lithium‐rich layered oxides. Nat Commun. 2017;8(1):2091.

Lai Y, Xie H, Li P, et al. Ion‐migration mechanism: an overall understanding of anionic redox activity in metal oxide cathodes of Li/Na‐ion batteries. Adv Mater. 2022;34(47): [eLocator: 2206039].

Ben Yahia M, Vergnet J, Saubanère M, Doublet M‐L. Unified picture of anionic redox in Li/Na‐ion batteries. Nat Mater. 2019;18(5):496‐502.

House RA, Maitra U, Pérez‐Osorio MA, et al. Superstructure control of first‐cycle voltage hysteresis in oxygen‐redox cathodes. Nature. 2020;577(7791):502‐508.

Tamaru M, Wang X, Okubo M, Yamada A. Layered Na₂RuO₃ as a cathode material for Na‐ion batteries. Electrochem Commun. 2013;33:23‐26.

Mortemard de Boisse B, Liu G, Ma J, et al. Intermediate honeycomb ordering to trigger oxygen redox chemistry in layered battery electrode. Nat Commun. 2016;7(1): [eLocator: 11397].

Perez AJ, Batuk D, Saubanère M, et al. Strong oxygen participation in the redox governing the structural and electrochemical properties of Na‐rich layered oxide Na₂IrO₃. Chem Mater. 2016;28(22):8278‐8288.

Pearce PE, Rousse G, Karakulina OM, et al. β‐Na_1.7IrO₃: a tridimensional Na‐ion insertion material with a redox active oxygen network. Chem Mater. 2018;30(10):3285‐3293.

Nanba Y, Iwao T, Boisse BM, et al. Redox potential paradox in Na_xMO₂ for sodium‐ion battery cathodes. Chem Mater. 2016;28(4):1058‐1065.

Zhou D, Huang W, Zhao F, Lv X. The effect of Na content on the electrochemical performance of the O3‐type Na_xFe_0.5Mn_0.5O₂ for sodium‐ion batteries. J Mater Sci. 2019;54(9):7156‐7164.

Wang X, Liu G, Iwao T, Okubo M, Yamada A. Role of ligand‐to‐metal charge transfer in O3‐type NaFeO₂–NaNiO₂ solid solution for enhanced electrochemical properties. J Phys Chem C. 2014;118(6):2970‐2976.

Yoshida H, Yabuuchi N, Komaba S. NaFe_0.5Co_0.5O₂ as high energy and power positive electrode for Na‐ion batteries. Electrochem Commun. 2013;34:60‐63.

Chae MS, Kim HJ, Bu H, et al. The sodium storage mechanism in tunnel‐type Na_0.44MnO₂ cathodes and the way to ensure their durable operation. Adv Energy Mater. 2020;10(21): [eLocator: 2000564].

Whitacre JF, Tevar A, Sharma S. Na₄Mn₉O₁₈ as a positive electrode material for an aqueous electrolyte sodium‐ion energy storage device. Electrochem Commun. 2010;12(3):463‐466.

Han D‐W, Ku J‐H, Kim R‐H, Yun D‐J, Lee S‐S, Doo S‐G. Aluminum manganese oxides with mixed crystal structure: high‐energy‐density cathodes for rechargeable sodium batteries. ChemSusChem. 2014;7(7):1870‐1875.

Lu D, Yao Z, Zhong Y, et al. Polypyrrole‐coated sodium manganate hollow microspheres as a superior cathode for sodium ion batteries. ACS Appl Mater Interfaces. 2019;11(17):15630‐15637.

Lu D, Yao ZJ, Li YQ, et al. Sodium‐rich manganese oxide porous microcubes with polypyrrole coating as a superior cathode for sodium ion full batteries. J Colloid Interface Sci. 2020;565:218‐226.

Yang L, Li X, Ma X, et al. Design of high‐performance cathode materials with single‐phase pathway for sodium ion batteries: a study on P2‐Na_x(Li_yMn_1 − y)O₂ compounds. J Power Sources. 2018;381:171‐180.

House RA, Maitra U, Jin L, et al. What triggers oxygen loss in oxygen redox cathode materials? Chem Mater. 2019;31(9):3293‐3300.

Chen C, Ding Z, Han Z, et al. Unraveling atomically irreversible cation migration in sodium layered oxide cathodes. J Phys Chem Lett. 2020;11(14):5464‐5470.

Komaba S, Yabuuchi N, Nakayama T, Ogata A, Ishikawa T, Nakai I. Study on the reversible electrode reaction of Na_1 – xNi_0.5Mn_0.5O₂ for a rechargeable sodium‐ion battery. Inorg Chem. 2012;51(11):6211‐6220.

Li Z‐Y, Ma X, Sun K, He L, Li Y, Chen D. Na_2/3Li_1/9[Ni_2/9Li_1/9Mn_2/3]O₂: a high‐performance solid‐solution reaction layered oxide cathode material for sodium‐ion batteries. ACS Appl Energy Mater. 2022;5(1):1126‐1135.

Li Y, Wang X, Gao Y, et al. Native vacancy enhanced oxygen redox reversibility and structural robustness. Adv Energy Mater. 2019;9(4): [eLocator: 1803087].

Sun L, Wu Z, Hou M, et al. Unraveling and suppressing the voltage decay of high‐capacity cathode materials for sodium‐ion batteries. Energy Environ Sci. 2024;17(1):210‐218.

Shi Y, Geng F, Sun Y, et al. Sustainable anionic redox by inhibiting Li cross‐layer migration in Na‐based layered oxide cathodes. ACS Nano. 2024;18(7):5609‐5621.

Lu H, Chu S, Tian J, et al. Ultra‐high‐energy density in layered sodium‐ion battery cathodes through balancing lattice‐oxygen activity and reversibility. Adv Funct Mater. 2024;34(2): [eLocator: 2305470].

Wang Q, Liao Y, Jin X, et al. Dual honeycomb‐superlattice enables double‐high activity and reversibility of anion redox for sodium‐ion battery layered cathodes. Angew Chem Int Ed. 2022;61(33): [eLocator: e202206625].

Hu H, He H‐C, Xie R‐K, et al. Achieving reversible Mn²⁺/Mn⁴⁺ double redox couple through anionic substitution in a P2‐type layered oxide cathode. Nano Energy. 2022;99: [eLocator: 107390].

Yamamoto K, Zhou Y, Yabuuchi N, et al. Charge compensation mechanism of lithium‐excess metal oxides with different covalent and ionic characters revealed by operando soft and hard X‐ray absorption spectroscopy. Chem Mater. 2020;32(1):139‐147.

Zhao C, Li C, Liu H, et al. Coexistence of (O₂)ⁿ⁻ and trapped molecular O₂ as the oxidized species in P2‐type sodium 3d layered oxide and stable interface enabled by highly fluorinated electrolyte. J Am Chem Soc. 2021;143(44):18652‐18664.

Zhou L, Zhang Z, Lv S, et al. SnO₂ coating to stabilize Mn‐based layered oxide cathode materials for sodium‐ion batteries. Mater Today Energy. 2023;38: [eLocator: 101450].

Chen C, Han Z, Chen S, et al. Core–shell layered oxide cathode for high‐performance sodium‐ion batteries. ACS Appl Mater Interfaces. 2020;12(6):7144‐7152.

Jin Y, Xu Y, Xiao B, et al. Stabilizing interfacial reactions for stable cycling of high‐voltage sodium batteries. Adv Funct Mater. 2022;32(40): [eLocator: 2204995].

Li C, Xu H, Ni L, et al. Nonaqueous liquid electrolytes for sodium‐ion batteries: fundamentals, progress and perspectives. Adv Energy Mater. 2023;13(40): [eLocator: 2301758].

Peljo P, Girault HH. Electrochemical potential window of battery electrolytes: the HOMO–LUMO misconception. Energy Environ Sci. 2018;11(9):2306‐2309.

Sun Y, Shi P, Xiang H, Liang X, Yu Y. High‐safety nonaqueous electrolytes and interphases for sodium‐ion batteries. Small. 2019;15(14): [eLocator: 1805479].

Chen X, Shen X, Li B, et al. Ion–solvent complexes promote gas evolution from electrolytes on a sodium metal anode. Angew Chem Int Ed. 2018;57(3):734‐737.

Jin Y, Le PML, Gao P, et al. Low‐solvation electrolytes for high‐voltage sodium‐ion batteries. Nat Energy. 2022;7(8):718‐725.

Zhang Q, Gu Q‐F, Li Y, et al. Surface stabilization of O3‐type layered oxide cathode to protect the anode of sodium ion batteries for superior lifespan. iScience. 2019;19:244‐254.

Tang K, Huang Y, Xie X, et al. The effects of dual modification on structure and performance of P2‐type layered oxide cathode for sodium‐ion batteries. Chem Eng J. 2020;384: [eLocator: 123234].

Li J, Hu H, Wang J, Xiao Y. Surface chemistry engineering of layered oxide cathodes for sodium‐ion batteries. Carbon Neutraliz. 2022;1(2):96‐116.

Wang J, Teng Y, Su G, Bao S, Lu J. A dual‐modification strategy for P2‐type layered oxide via bulk Mg/Ti co‐substitution and MgO surface coating for sodium ion batteries. J Colloid Interface Sci. 2022;608:3013‐3021.

Yu Y, Kong W, Li Q, et al. Understanding the multiple effects of TiO₂ coating on NaMn_0.33Fe_0.33Ni_0.33O₂ cathode material for Na‐ion batteries. ACS Appl Energy Mater. 2020;3(1):933‐942.

Ji H, Zhai J, Chen G, et al. Surface engineering suppresses the failure of biphasic sodium layered cathode for high performance sodium‐ion batteries. Adv Funct Mater. 2022;32(12): [eLocator: 2109319].

Yang Y, Feng Y, Chen Z, et al. Strain engineering by atomic lattice locking in P2‐type layered oxide cathode for high‐voltage sodium‐ion batteries. Nano Energy. 2020;76: [eLocator: 105061].

Liu J, Wang J, Ni Y, Zhang K, Cheng F, Chen J. Recent breakthroughs and perspectives of high‐energy layered oxide cathode materials for lithium ion batteries. Mater Today. 2021;43:132‐165.

Yao H‐R, Zheng L, Xin S, Guo Y‐G. Air‐stability of sodium‐based layered‐oxide cathode materials. Sci China Chem. 2022;65(6):1076‐1087.

Kubota K, Komaba S. Review—practical issues and future perspective for na‐ion batteries. J Electrochem Soc. 2015;162(14):A2538‐A2550.