Research progress on carbon materials as negative electrodes in sodium‐ and potassium‐ion batteries

Abstract

Carbon materials, including graphite, hard carbon, soft carbon, graphene, and carbon nanotubes, are widely used as high‐performance negative electrodes for sodium‐ion and potassium‐ion batteries (SIBs and PIBs). Compared with other materials, carbon materials are abundant, low‐cost, and environmentally friendly, and have excellent electrochemical properties, which make them especially suitable for negative electrode materials of SIBs and PIBs. Compared with traditional carbon materials, modifications of the morphology and size of nanomaterials represent effective strategies to improve the quality of electrode materials. Different nanostructures make different contributions toward improving the electrochemical performance of electrode materials, so the synthesis of nanomaterials is promising for controlling the morphology and size of electrode materials. This paper reviews the progress made and challenges in the use of carbon materials as negative electrode materials for SIBs and PIBs in recent years. The differences in Na⁺ and K⁺ storage mechanisms among different types of carbon materials are emphasized.

INTRODUCTION

Among the various energy storage devices available,^1–6 rechargeable batteries fulfill several important energy storage criteria (low installation cost, high durability and reliability, long life, and high round-trip efficiency, etc.).^7–12 Lithium-ion batteries (LIBs) are already predominantly being used in portable electronic devices.^13,14 However, the rapid development of mobile electronic devices and hybrid, electric vehicles has resulted in a surge in demand for LIBs. Lithium is known to be an element with insufficient reserves, making up only 0.065% of the Earth's crust and being unevenly distributed (mainly in South America). The demand for lithium resources is increasing, which is bound to cause the price of lithium to skyrocket.¹⁵ In addition, global lithium resources are limited, and lithium resources are difficult to recover. With the increasing demand for lithium resources and the decline in the supply capacity, eventually, human demands will not be met in the future.¹⁶ Therefore, there is an urgent need to develop new energy storage devices, such as sodium-ion batteries (SIBs), potassium-ion batteries (PIBs), and so on, to supplement LIBs for large-scale storage applications where energy density is not a major factor, thus minimizing concerns about LIB shortage.¹⁷ Also, it is expected that LIBs will be replaced in the future to achieve sustainable development in the field of energy storage.^15,18 This is why SIBs and PIBs are receiving more attention. Compared with lithium ions, sodium/potassium ions have very similar properties and they are abundant in the Earth's crust. In addition, both PIBs and SIBs have shown excellent performance and are even better than LIBs in some aspects.^19–21 It is anticipated that both PIBs and SIBs will be applied as large-scale fixed energy storage devices in the future.

Research into SIBs began in the 1970s, almost at the same time as research into LIBs.^7,22–24 Since the commercialization of LIBs, research focused on them has exploded, while research on SIBs has declined.^14,25 In recent years, many laboratories have shifted their research focus to SIBs/PIBs due to their advantages in terms of cost and storage, as well as their very similar structure to that of lithium materials.²⁶ SIBs and PIBs work in a similar way to LIBs, charging and discharging through a process in which sodium/potassium ions are inserted between the cathode (positive electrode) and the anode (negative electrode). During the charging process, Na⁺/K⁺ is precipitated from the positive electrode and inserted into the negative electrode via the electrolyte. At the same time, the compensating charge of the electrons is supplied to the negative electrode through the external circuit to ensure the charge balance between the negative electrode and the positive electrode. In contrast, Na⁺/K⁺ is removed from the negative electrode during discharge and embedded into the positive electrode via the electrolyte²⁷ (Figure 1). Although LIBs, SIBs, and PIBs have similar physical and chemical properties, as well as similar ion storage principles, the electrode materials of SIBs and PIBs do not have effective consistency with LIB electrode materials. Sodium ions have a larger radius than lithium ions (1.02 vs. 0.76 Å), resulting in slow diffusion dynamics, which makes graphite anode of LIBs unsuitable for SIBs. This is because lithium ions can be inserted between layers of graphite, but sodium ions cannot reach those places at atmospheric pressure.³⁰ The radius of the potassium ion is 1.38 Å, which is larger than that of the sodium ion (Table 1). Due to high steric hindrance, intercalation of potassium ions into the electrode material is difficult, leading to problems such as poor rate performance, low capacity, fast decay, and poor cycle stability of the electrode material.³² Therefore, to realize the commercialization of SIBs and PIBs, it is necessary to find suitable electrode materials for them, so that SIBs and PIBs have excellent electrochemical performance, such as high specific volume, long cycle life, and an appropriate operating voltage.^33–36

Figure 1. Illustration of a sodium/potassium-ion battery system. Reproduced under the terms of the creative commons license: Copyright 2017, the Royal Society of Chemistry.28 Reproduced with permission: Copyright 2021, Elsevier.29

Table 1 Physical and economic characteristics of lithium, sodium, and potassium.³¹

Alkalis	Lithium	Sodium	Potassium
Atomic number	3	11	19
Atomic weight	6.94	22.9898	39.0983
Density (g/cm3)	0.535	0.968	0.856
Melting point (°C)	180.54	97.72	63.38
Atomic radius (pm)	167	190	243
Covalent radius (pm)	128	166	203
Voltage versus SHE (V)	−3.04	−2.71	−2.93
Abundance in Earth's crust (%)	0.0017	2.3	1.5
Abundance in ocean (%)	0.000 018	1.1	0.042
Cost of industrial-grade metal (USD/mt)	492,457.57	3928.35	21,605.91

Due to their abundance, low cost, and stability, carbon materials have been widely studied and evaluated as negative electrode materials for LIBs, SIBs, and PIBs, including graphite, hard carbon (HC), soft carbon (SC), graphene, and so forth.^37–40 Carbon materials have different structures (graphite, HC, SC, and graphene), which can meet the needs for efficient storage of Na⁺/K⁺. Graphite is one of the most advanced negative electrode materials for LIBs, and its theoretical capacities for storing Na⁺ and K⁺ are 35 mAh g⁻¹ (Na⁺) and 279 mAh g⁻¹ (K⁺), respectively.^41,42 The high theoretical capacity indicates that graphite is a potential negative electrode material for PIBs. Due to the high specific surface area and a large number of defects, graphene materials (except monolayer graphene) are rich in Na⁺/K⁺ adsorption sites. Although HC has considerable capacity for both Na⁺ and K⁺, there are significant differences.^43–46 SC is also a potential SIB/PIB negative electrode. Its microstructure and composition, including graphitization degree and heteroatomic doping, directly affect the storage behavior of Na⁺/K⁺.^47,48 Although the internal composition and electrochemical reaction mechanism of the two alkaline ion cells are similar, the storage/release behavior of Na⁺ and K⁺ in carbon-based materials is not the same. Therefore, in this paper, the ion storage mechanism of carbon negative-electrode materials in SIBs and PIBs, and their influence on electrochemical performance will be compared, and the design of high-performance carbon negative electrodes will be proposed.

CARBON MATERIALS AS NEGATIVE ELECTRODES FOR ALKALI-METAL ION BATTERIES

Carbonaceous materials,^49,50 metal oxides,^51–54 and alloys^55,56 have been used as negative electrodes for SIBs and PIBs. However, metal oxides and alloy electrodes tend to swell during electrochemical reactions, leading to poor cycle durability. Among all negative electrode materials, carbonaceous materials have the advantages of abundant sources, environmentally friendly nature, low cost, good chemical inertia, and excellent electrochemical performance. In carbonaceous materials, C atoms usually have three kinds of hybrid orbitals: sp, sp², and sp³. In general, carbon materials based on sp or sp³ hybrid orbitals are difficult to form regular structures to insert/extract guest ions, such as polyacetylene and hydrocarbons, and therefore, they are rarely used as negative electrodes in ion batteries. The carbonaceous materials with the sp² hybrid are composed of planar carbon atoms in the honeycomb structure as basic units, which are piled up in an ordered or disordered manner.⁵⁷ Carbon materials with a unique combination of chemical and physical properties, such as cost-effectiveness, high abundance, excellent corrosion resistance, moderate conductivity, and a relatively high surface area, are considered to improve the performance of electrodes in energy storage devices.^58,59 The current research focuses on improving the specific capacitance, cycle efficiency, lifetime, power, and energy density.^60,61 Carbon materials have different structures, such as graphite, HC, SC, and graphene (Figure 2). In addition, carbon materials have different forms, such as 0D, 1D, 2D, and 3D (D refers to dimension). Table 2 lists the properties and parameters of carbon materials with different structures.

Figure 2. Schematic illustration of different structures of carbon materials.62–64 Reproduced with permission: Copyright 2014, Springer Nature.62 Reproduced with permission: Copyright 2021, Elsevier.63 Reproduced with permission: Copyright 2020, Elsevier.64

Table 2 Structural characteristics and parameters of different carbon materials.

	Structural diagram	Structural features	Layer spacing	Preparation	References
Graphite	[IMAGE OMITTED. SEE PDF.]	Graphite microcrystals are arranged regularly and have long-range order.	0.335 nm	Mainly with petroleum coke and needle coke as raw materials.	[41,65]
Hard carbon	[IMAGE OMITTED. SEE PDF.]	Graphite microcrystals are short and disordered, and have a micropore structure.	Decreases with an increase of the treatment temperature.	It can be prepared with waste paper, biological precursors, and so forth.	[65,66]
Soft carbon	[IMAGE OMITTED. SEE PDF.]	Graphite microcrystals are arranged in an orderly manner, but short-range order.	Close to 0.34 nm (900°C)	Can be carbonized into soft carbon coal asphalt, oil residue asphalt, mesophase asphalt, epoxy resin, phenolic resin, and so on.	[65,67]
Graphene	[IMAGE OMITTED. SEE PDF.]	A single layer of graphite is composed of atoms.	-	Graphite flake	[65,68]

Graphite is part of the most widely used negative electrode materials in commercial LIBs.^69–71 It is well known that its structure is a unique layered structure (Figure 3A–C) with hexagonal packing (AAA), Bernal packing (ABA), or rhombohedral packing (ABC).⁷² The structure forms strong covalent bonds within the graphene layer and weak covalent bonds through the van der Waals interaction in the vertical direction, resulting in a layer spacing of 3.35 Å, and is capable of inserting/extracting guest ions.^75–77 This structure allows alkali-metal ions to be inserted into the graphite to form fossilized ink. In general, the layered structure of graphite is not destroyed during ion insertion/extraction, thus ensuring the long cycle performance of graphite in ion batteries. Therefore, theoretically, the alkaline metal ion battery, using graphite as the negative electrode material, has higher stability and a longer cycle life.

Figure 3. (A) Schematic drawings of simple hexagonal (AAA), (B) Bernal (ABA), and (C) rhombohedral (ABC) stackings. Reproduced with permission: Copyright 2017, American Chemical Society.72 (D) Illustration of the structures for HC. Reproduced with permission: Copyright 2020, John Wiley and Sons.73 (E) Schematic illustrations of the ion storage in graphite and hard carbon. Reproduced with permission: Copyright 2018, John Wiley and Sons.74

HC and SC materials are also called amorphous carbon materials because of the lack of a long-range ordered structure on the plane and the ordered packing structure of graphite. HC can permanently maintain a disordered structure regardless of the temperature. In general, amorphous carbon materials consist of cavities, twisted graphene nanosheets, and randomly distributed graphitized microregions, so they tend to remain in amorphous structures, inhibiting the development of graphite structures (Figure 3D). The pore structure and good electrolyte compatibility of HC make it a potential alkali-ion storage material. As a result, HC has more Na⁺ and K⁺ storage sites than graphite (Figure 3E). Unfortunately, HC materials show largely irreversible capacity decay during the first charge and discharge. The most accepted structural model for HC is the “house of cards” model developed by Dahn et al.,⁷⁸ which shows the presence of small graphene sheets, amorphous regions with defects, and nanopores in HC structures. HC forms can be spherical, linear, or porous, which are often inherited from precursors. Small graphene sheets in HC (also known as graphite domains or pseudographite) are arranged in order within a short range, showing finite stacks of two to six layers and having a finite transverse size of ≈40 Å.^79,80 Studies have shown that parallel layers in the graphite domain are curved or vortex-layered rather than flat, as shown in the “house of cards” model.

Compared with HC, SC, such as petroleum coke, needle coke, and carbon microspheres, can be converted into graphite carbon after heat treatment (2000°C). Moreover, it has the advantages of fine grain size, large plane spacing, high cooperative efficiency of ion diffusion, and a stable charge–discharge platform. However, this description still fails to delineate the differences between SC and HC. In fact, when the SC precursors are treated at 1000°C, the graphite layer structure can be formed and the layer spacing is close to 0.34 nm. From the dimension, the size of the carbon layer is small, and the arrangement of graphite microcrystals is more orderly. With an increase in the temperature, these microcrystals are likely to graphitize. SC is usually obtained from aromatic substances, such as asphalt or tar, petroleum or coal refining, or plastic with low oxygen content.^81,82 Unlike HC, SC usually shows tilted potential energy during cultivation and fermentation.^83,84 HC negative electrodes alleviate concerns about dendrite formation, polarization, and high reactivity, and their reversible capacity is mainly derived from potentials where sodium plating can occur. Thus, the slanted profile enables SC to have better rate capability and a longer cycle life.

Graphene is a 2D carbon allotrope. As can be observed in Table 1, it is composed of carbon atoms in a hexagonal design. A single layer of carbon atoms is arranged in such a honeycomb structure to form a single layer of a graphene sheet. Graphene is a 2D layer of sp² hybrid carbon atoms packed tightly together in a honeycomb lattice; since the separation of graphene from lump graphite was proposed by Andre Geim and Konstantin Novoselov in 2004, it has received considerable attention from scientists.^85,86 Among carbon-based materials, graphene is lightweight and low-cost; moreover, it has good chemical stability, high electrical conductivity, special electronic and thermal conductivity, high light transmittance, mechanical strength, and high specific surface area resulting from its unique 2D structure. In addition, when integrated into the electrode with other nanomaterials, graphene can improve electrical conductivity, accommodate large volume changes, and enhance reaction kinetics.^87–90 Therefore, graphene has emerged as a potential candidate as a negative electrode material for achieving excellent battery performance.⁹¹ In the last decade, graphene and graphene-based materials have been extensively studied.⁹² In addition, due to the rapid development of graphene materials and huge potential for commercialization, their research has even evolved from the laboratory to the industry.⁹³

NA⁺/K⁺ STORAGE MECHANISM IN CARBON NEGATIVE-ELECTRODE MATERIALS

SIBs and PIBs have received considerable attention in current power supply systems.^94–98 Carbon materials have a variety of structures and morphologies, which have a crucial influence on their Na⁺/K⁺ storage mechanism.

Graphite

Graphite and related carbonaceous materials can reversibly intercalate metal atoms to store electrochemical energy in batteries.^29,64,99-101 Graphite, the main negative electrode material for LIBs, naturally is considered to be the most suitable negative-electrode material for SIBs and PIBs, but it is significantly different in graphite negative-electrode materials between SIBs and PIBs due to the differences between sodium and potassium ions themselves.^102,103 Therefore, researchers continue to explore the storage behavior of Na⁺ and K⁺ in graphite.

Earlier studies found that sodium ions could not be inserted into graphite to form graphite intercalation compounds (GICs), so the capacity of graphite-anodized SIBs was not ideal.^49,104 Wen et al.⁶² showed that Na⁺ could not be electro-intercalated into graphite due to the small layer spacing. Further studies showed that Na residues could be electrochemically inserted into GO due to the increased distance between the GO layers.^105,106 However, insertion is limited by steric hindering from large numbers of oxygen-containing groups. Therefore, a large amount of Na⁺ can be electrochemically inserted into graphite by appropriately reducing the interlayer distance and interlayer oxygen-containing groups (Figure 4A). Yoon et al.¹⁰⁷ found that [Na–linear–ether]⁺ complexes could be reversibly intercalated into graphite because of their high Na-solvent solvation energy and chemical stability in graphite (Figure 4B).

Figure 4. Na+/K+ storage mechanism in a graphite negative electrode. (A) Schematic illustration of sodium storage in graphite-based materials. Reproduced with permission: Copyright 2014, Springer Nature.62 (B) Schematic illustration of the conditions to realize Na co-intercalation in graphite. Reproduced with permission: Copyright 2016, John Wiley and Sons.107 (C) Structure diagrams of different K-GICs (graphite intercalation compounds), side view (up), and top view (down). Reproduced with permission: Copyright 2015, American Chemical Society.41

In 1997, Mizutani et al.¹⁰⁸ first found that K⁺ could intercalate into graphite in 2-methyltetrahydrofuran (MeTHF) or 2,5-dimethylte trahydrofuran (diMeTHF) solvents, and they confirmed that the binary K-GIC was formed. Also, in 2004, Zhu et al.¹⁰⁹ studied the adsorption of alkali metals (including Li, Na, and K) on a graphite substrate using a molecular orbital theoretical calculation method. They found three metal atoms attached to the hexagonal aromatic ring above the hollow site and observed the weakest attachment for Na⁺. Further study found that the single occupied molecular orbital (SOMO) of the Na⁺ was exactly halfway between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the graphite layer. As a result, the SOMO of Na⁺ cannot form stable interactions with the HOMO or LUMO of graphite. In addition, SOMO of Li⁺ and K⁺ can interact with HOMO or LUMO of graphite in a relatively stable manner. Due to the successful commercialization of LIBs, the above early exploration of PIBs did not attract widespread attention. In recent years, PIBs have attracted extensive attention in the quest to find alternatives to LIBs. In 2015, Jian et al.⁴¹ first reported electrochemical K⁺ intercalation into graphite in a nonaqueous electrolyte with a high reversible capacity of 273 mAh g⁻¹. It was confirmed by non-in-situ X-ray diffraction that KC_36, KC₂₄, and KC₈ were successively formed during the potassizing process, and they would be restored to graphite by reverse phase transition during the potassizing process (Figure 4C).

In conclusion, the graphite negative-electrode material may be more suitable for PIBs than SIBs.

Hard carbon

The storage of alkali-metal ions in HC is very complicated due to the diverse structures of HC.^110–113 The structure of HC at the molecular level is much more complex than the ordered layer structure of graphite. Its special structure indicates that it must have many kinds of storage sites of Na⁺/K⁺, and it is difficult to determine the capacity limit. HC materials have several different ion storage sites, including (1) defects and edges on the graphene, (2) graphene–graphene interlayers, and (3) nanopores.⁷⁴ Different storage mechanisms have been proposed, including adsorption on defects, edges, and functional groups, intercalation into graphitic layers, and filling into micropores.^{73,80,114–116} However, the distribution of charge and discharge voltage by the above three storage mechanisms is still controversial.

HCs typically show good capacity values and stable cycling properties over 250 mAh g⁻¹.¹¹⁷ HC negative electrodes have large storage capacity, low operating voltage, high cycle stability, and low cost. In addition, HC is obtained from a wide range of sources. So far, HCs from various raw materials have been prepared, such as converting corn stalk waste into high-performance negative-electrode material for SIBs without any chemical substances.¹¹⁸ Also, HC microfibers have been successfully prepared from recycled paper using a simple carbonization process.¹¹⁹ However, due to the residual uncertainties of complex HC structures, although several mechanisms for Na⁺ storage in HC have been proposed in recent years, there is still a lack of general mechanisms to explain the observed electrochemical processes that we observed.¹¹¹

The mechanism of HC Na⁺ storage in SIBs was first proposed by Stevens and Dahn¹²⁰ in 2000 and is called the “insertion-absorption” model, also known as the House of Cards model (Figure 5A). Since the introduction of HC materials by Stevens and Dahn in 2000, an increasingly more detailed understanding of Na⁺ storage mechanisms has been achieved. In 2015, Bommier et al.¹¹⁵ proposed a new HC Na⁺ storage mechanism. The inclined part is the storage site of Na⁺ in the defected part of carbon materials, and the platform part not only has Na⁺ intercalation in the graphite layer but also has Na⁺ filling in the nanopore. The mechanism is the filling of Na⁺ between the defective graphite-nanopore layers, as shown in Figure 5B, and Na⁺ inserted into the nanopores between the randomly stacked layers in the platform region, which is the “intercalation-filling” mechanism, as shown in Figure 5C. In 2018, Qiu et al.⁸⁰ established “intercalation–adsorption” and “adsorption–intercalation” mechanisms for sodium-ion storage (Figure 5D,E). Lu et al.¹²¹ synthesized a series of HC materials using a simple ball-milling method and clarified the mechanism of sodium-ion storage, as shown in Figure 5F,G. The defects of spheroidized HC negative electrodes are smaller than those of ordinary HC negative electrodes in terms of low voltage platform capacity and lower internal consumption, which further proves the mechanism of “adsorption–intercalation.”

Figure 5. Schematic illustrations of different Na+ storage mechanisms in hard carbon. (A) House of Cards model. Reproduced with permission: Copyright 2000, The Electrochemical Society.120 (B) “Adsorption–intercalation–filling” mechanism; (C) “intercalation-filling” mechanism. Reproduced with permission: Copyright 2015, American Chemical Society.115 (D) “Intercalation–adsorption” mechanism; (E) “adsorption–intercalation” mechanism. Reproduced with permission: Copyright 2017, John Wiley and Sons.80 (F,G) Schematic illustration of the sodiation process of the HC and ball-milled HC. Reproduced with permission: Copyright 2018, John Wiley and Sons.121 (H) Na+ storage into hard carbon: “Na adsorption,” “Na intercalation,” and “Na filling.” Reproduced with permission: Copyright 2019, John Wiley and Sons.122

To date, the storage mechanism of HC materials is still controversial. The storage mechanism of sodium in HC can be summarized into four modes: (1) “intercalation–filling” mode: Na⁺ ions are inserted into the graphite layer in the slope area and into the nanopores between the random stacking layers in the platform area; (2) “adsorption–intercalation” model: Na⁺ ions are adsorbed on the surface or defects of the carbon electrode in the slope area and embedded into the graphite layer in the plateau area; (3) “adsorption–filling” model: Na⁺ ions in the slope area are adsorbed at the defects, while Na⁺ ions in the plateau area are filled in the nanopores; and (4) the “three stages” model: sodium-ion defect adsorption occurs in the slope area, while sodium ions in the plateau area are first inserted into the graphite layer and finally filled with nanopores. Moreover, the differences between absorption, intercalation, and filling can be clearly seen in Figure 5H. Also, due to the complexity of the HC structure, a variety of mechanisms may be working together in the actual process. For example, Huang et al.¹²³ found that within a certain potential range, the physical adsorption/graphite layer intercalation or intercalation/pore filling of SIBs and PIBs coexisted, and they accurately determined the ion storage behavior in different charge and discharge stages (Figure 6A). In addition, HC with different fabric structures can be obtained by pyrolysis at different temperatures,¹¹¹ such as different layer spacings, defects, graphitization degree, micropores, and so forth. These factors will inevitably affect the sodium-intercalation behavior of HC.

Figure 6. (A) Schematic diagram of four different stages of the discharge process. Reproduced with permission: Copyright 2021, Elsevier.123 (B) Schematic illustration of capacitive and bulk storage in sulfur/nitrogen dual-doped hard carbon for potassium-ion batteries. Reproduced with permission: Copyright 2019, John Wiley and Sons.124 (C) Schematic structure illustration of initial (top), lithiated (bottom left), sodiated (bottom middle), and potassiated (bottom right) hard carbons. Reproduced with permission: Copyright 2021, American Chemical Society.125

For PIBs, first, the standard electrochemical potential of K⁺/K is lower than that of Na⁺/Na, which leads to higher working voltage and energy density for PIBs.¹²⁶ Second, potassium ions have higher ionic conductivity and mobility in organic electrolytes than lithium and sodium ions due to their weak Lewis acidity.¹²⁷ HC is one of the most popular negative electrodes for practical PIBs because of its high K-ion storage and relatively low material cost, allowing high specific capacity to be achieved at a low cost. Using simple manufacturing processes, the structure of HC can be adjusted to maximize the storage of different charge carriers.⁴⁰ However, due to co-embedding of K-solvent chelation, HC negative electrodes without surface modification often undergo severe structural damage and capacity decay during the cycle, which is a major obstacle to commercialization.⁴⁰ Li et al. synthesized HC through carbonization of rice husk and first studied its electrochemical performance as a PIB negative-electrode material. They found that HC shows high reversible capacity and stable cycling performance of 244.99 mAh g⁻¹ at a current density of 30 mA g⁻¹ and 103.77 mAh g⁻¹ after 500 cycles even at a high current density of 500 mA g⁻¹.⁷³

The K⁺ storage mechanism of HC in PIBs is similar to that of SIBs. As mentioned above, insertion and adsorption mechanisms are common K⁺ storage mechanisms in carbon-based materials. In general, K⁺ can be trapped into three active sites in carbonaceous materials: (I) defect sites, mainly from heteroatomic doping and vacancies; (II) porous sites, especially micropores/mesopores; and (III) interlaminar sites between adjacent graphite layers with short-range ordered periods.^46,128–130 The entire stored charge can be divided into two parts: the Faraday contribution due to K⁺ de-/intercalation and the capacitive contribution due to the double-layer capacitance/pseudocapacitance effect. Liu et al.¹²⁴ proved that potassium ions in HC show a better capacitive property, especially at higher current density (Figure 6B). Therefore, increasingly more researchers began to pay attention to the differences in energy storage mechanisms between SIBs and PIBs.

Recently, Huang et al.¹²⁵ proposed the storage mechanism of Li⁺, Na⁺, and K⁺ in HC anode, as shown in Figure 6C. They found that defect/heteroatom adsorption mainly determines the slope capacity of Li⁺, Na⁺, and K⁺ and the voltage lag is strongly dependent on the ion diameter. On the contrary, in the plateau area, there are obvious differences, namely, the pore filling process. In the micropores, lithium ions mainly bind to defective carbon/heteroatoms and functional groups at the edge or surface of the graphene film, forming microporous walls, and very few lithium ions overlap on the bonded lithium ions, as shown by the absence of low voltage platforms in the equilibrium profile. In contrast, sodium ions have a larger ionic radius and show better coordination with the pore wall, resulting in a higher storage potential during pore filling. In addition, larger K⁺ filling the micropores produces stronger coordination with the pore wall, similar to K⁺ intercalation in graphite.

In conclusion, although the storage mechanisms of sodium and potassium ions in HC are very similar, the differences between the storage mechanisms of sodium and potassium ions largely determine the performance of SIBs and PIBs.

Soft carbon

Although both HCs and SCs are amorphous carbons, the storage mechanisms of alkali-metal ions are quite different. Different from HC, SC can be graphitized at high temperatures.⁴⁸ Therefore, SC has both an ordered carbon layer and a disordered structure. This shows that the intercalation spacing of SC can be controlled to achieve an appropriate layer spacing, to facilitate rapid Na⁺/K⁺ transport kinetics.^83,131,132

In 2017, Jian et al.⁴⁸ investigated the mechanism of Na⁺ storage in SC and found that, first of all, there are more local structural defects in SC than in HC, so the overall dissolution potential of SC is higher than that of HC, especially in the quasi-plateau. Second, when the turbine-stratified domain of SC is intercalated with sodium ions, irreversible expansion of local and macroscopic structures occurs, so that some sodium ions are trapped in the intercalation position. Third, the irreversible quasi-platform of the first dissolution can be attributed to the irreversible structural expansion caused by intercalation. Fourth, this irreversibility of embedding may be related to the large binding energy of sodium ions in the structural domain of SC turbines. Fifth, SC has a higher reversible slope capacity than HC, which is due to the defects of its local structure. As shown in Figure 7A, the graphene layers in the HC are highly curved and misaligned, while the graphene layers in the SC have less curvature and better alignment. Because the SC has a relatively regular and ordered graphite microcrystalline structure and the layer spacing is basically close to that of a graphite material, the storage mechanism of the SC material is mainly reflected in the adsorption of sodium ions on the surface of the carbon layer, the microcrystalline space, and the edge of the carbon layer. The charge–discharge curve of SIBs with SC as a negative electrode shows a voltage slope, so the reaction potential is higher (higher than 0.5 V), and the 1st week's irreversible capacity is larger than that with HC as negative electrode. Gao et al.¹³⁵ prepared an SC fiber by industrialized wet spinning polyacrylonitrile at different heat treatment temperatures. The SC fiber (prepared at 1250°C) was used as the binder of SIBs and the negative electrode without a conductive agent. Its initial reversible capacity was 190 mAh g⁻¹, and there was still 93% capacity retention after 100 cycles. X-ray diffraction (XRD) results show that the (002) peak intensity is sensitive to the temperature at which carbon materials are prepared. With an increase in the preparation temperature, it becomes sharper, which is due to the widening and thickening of the carbon layer.¹³⁶ In addition, the small holes will collapse and merge into larger pores. Raman spectra show that the intensity ratio I_D/I_g gradually decreases with an increase in the temperature. The graphitization degree increases with an increase in the temperature. Finally, the ideal graphite structure is formed above 2000°C.

Figure 7. (A) Schematic illustrations for the structures and features of graphite, hard carbon, and soft carbon. Reproduced with permission: Copyright 2020, Elsevier.133 (B) Schematic illustration of the microstructure evolution of soft carbon derived at different annealing temperatures and the corresponding K+ storage behaviors. Reproduced with permission: Copyright 2021, Elsevier.134 (C) Schematic illustration of S/O co-doped soft carbon (SO–SC) preparation and (D) scanning electron microscopy image of SO–SC. Reproduced with permission: Copyright 2021, Elsevier.63

The structural flexibility of SC makes it easy to adjust the crystallinity and thus the storage behavior of large size Na⁺/K⁺ in the chaotic carbon lattice, so as to meet the needs of structural elasticity, low voltage characteristics, and high transfer dynamics.¹³⁷ Jian et al.⁵⁰ investigated three types of nongraphite carbon: hard carbon spheres (HCS), SC, and HCS–SC composites (HCS/SC: 80/20, mass ratio), and found that when HC and SC are connected to the same electrode particle, HCS–SC composite carbon can combine the advantages of the high rate of SC and long cycle life of HC. Tan et al.¹³⁴ prepared SC materials from asphalt by stepwise carbonization and studied their potassium-ion storage mechanism. The results show that there are two mechanisms for K storage: one is the adsorption of K⁺ on isolated graphene layers and defect centers, and the other is the formation of GICs by K⁺ inserted into graphite domains. As shown in Figure 7B, with large layer spacing, a large number of defects, and a rare graphite structure, the K⁺ storage in SC-800 follows the first mechanism. Furthermore, graphite SC (SC-2800) mainly follows the second mechanism, and SC-1400 and SC-2000 with turbocharged and graphite combined structures follow the second mechanism. The XRD patterns of SC-800 and SC-1400 show that the (002) peak decreases after cycling, as the interlayer distance increases to 4.0-4.5 nm and 3.9-4.4 nm, respectively. The deformation of the carbon structure is partly responsible for the attenuation of the cycle capacity of SC-800 and SC-1400. For SC-2000 and SC-2800, the (002) peak splits into two secondary peaks: one at a higher degree and the other at a lower degree. The resulting layer spacing is calculated to be 3.18 Å (27.9°). Also, during the long-term cycle, the capacities of SC-800, SC-1400, SC-2000, and SC-2800 after 100 cycles remain 61%, 75%, 62.5%, and 83%, respectively (compared with the capacity of the second cycle).

Shen et al.⁶³ recently prepared S/O co-doped soft carbon materials (SO-SC) using 3,4,9,10-perylene tetracarboxylic anhydride as precursors by a simple ball-milling and heat-treatment method (Figure 7C,D). Also, when used as a negative electrode material for SIBs and PIBs, SO–SC for SIBs' negative electrode provides 61.3% high initial Coulombic efficiency (ICE) and superior rate capacity (230.7 mAh g⁻¹, 2 A g⁻¹). The high-capacity retention rate after 4000 cycles is 86.9%. When used as the negative electrode for PIBs, SO–SC also shows 412.6 mAh g⁻¹ high capacity, significant rate performance, and cycling performance. The capacitance contribution of SO–SC increases with an increase in the scanning rate (both SIBs and PIBs). This is due to the transformation of nanoparticles from small size to large size during ball-milling, which is conducive to reducing the electrolyte/electrode contact area and thus improving the ICE.

During K⁺ intercalation/deintercalation, a larger K⁺ radius may lead to a larger expansion/shrinkage between SC layers, especially for SC layers with a higher degree of graphitization. This indicates that the storage behavior of K⁺ depends on the graphitization degree of SC. For SC materials with a high graphitization degree, K⁺ can be intercalated into the graphite layer and adsorbed on the amorphous carbon surface.² In addition, sulfur and oxygen doping sites are introduced into the carbon structure, which enlarges the interlayer distance and increases the Na⁺ or K⁺ coordination. Therefore, similar to HC and graphene, the heteroatomic doping strategy can improve the physical structure and sodium-ion storage capacity of SC materials.

Graphene

Graphene has a peeled planar lamellar structure, so it has a large specific surface area and more surface defects, thus providing storage sites for sodium-ion adsorption. The Na⁺ storage performance of graphene is related to many factors such as the preparation method, element doping, and so forth. The capacity distribution varies between 150 and 350 mAh g⁻¹. The charge–discharge curve of graphene presents a monotonous potential slope without the obvious potential platform, indicating that Na⁺ storage on graphene also shows similar surface adsorption behavior. As no sodium ions can be inserted into the carbon layer, they only show adsorption behavior at the active site or defect. Li⁺ and K⁺ can be electrochemically intercalated into graphite and reduced graphite oxide, while Na ions can only be electrochemically intercalated into reduced graphite oxide.¹⁰²

Luo et al.¹⁰² first used reduced graphene oxide (RGO) synthesized using the improved Hummer method as a negative electrode in PIBs, which showed a stable capacity of about 200 mAh g⁻¹ at a current density of 5 mA g⁻¹. When K⁺ is embedded with the RGO film electrode, the transparency of the RGO film increased from 29% to 84.3%. As K⁺ exits out of RGO film during charging, the transparency of the film decreases from 84.3% to the initial state. Using first-principles calculations, Gong and Wang¹³⁸ found that boron (B-)-doped graphene could be a high-capacity cathode material for PIBs. Computer simulations show that when K⁺ is embedded in a B₄C₂₈, the theoretical capacity can be as high as 546 mAh g⁻¹, the migration barrier of K⁺ is only 0.07 eV, and the possible discharge K⁺-immobilization platform is 0.82 V, which can effectively avoid the decomposition of the electrolyte and the formation of a solid electrolyte interphase (SEI), and improve the cycle stability and rate performance. Furthermore, the addition of element B may facilitate the dispersion of K⁺, making it difficult to agglomerate and avoid the formation of dendrites.

Graphene materials have fascinating chemical and physical properties, such as large surface area, excellent mechanical strength, and high electrical conductivity. However, the electrochemical performance of pure graphene materials is still not satisfactory. One of the optimization principles for graphene-based electrodes is to design new structures that can introduce more defects, edges, grain boundaries, and doped atoms.¹³⁹ For example, Peng et al.¹⁴⁰ reported that a novel bimetallic sulfide FeSb₂S₄ immobilized on ammoniated graphene (FeSb₂S₄/EN-rGO) has a reversible capacity of 782.5 mAh g⁻¹ when used as a negative electrode material for SIBs, and, after 500 cycles of 0.1 A g⁻¹, the high capacity of 515.7 mAh g⁻¹ was still maintained, and the capacity loss rate of each cycle was 0.04%. Figure 8A shows that the structural integrity of the composite is maintained after cycling. As shown in Figure 8B, the circulating negative electrode material remains firmly fixed to the EN-rGO plate, reducing the effective chemical attraction between them. FeSb₂S₄/EN-rGO has excellent Na⁺ storage capacity and high rate storage capacity, which thereby is especially suitable for long-term recycling (Figure 8C). Dong et al.⁶⁴ synthesized a 3D porous MoS₂/nitrogen-doped graphene aerogel (MoS₂/NGA) with different MoS₂ loads using the hydrothermal method (Figure 8D,E). As the negative electrode material of SIBs, the material has a long period of stability and a specific capacity of 673 mAh g⁻¹ when the current density is 100 mAh g⁻¹. This unique structure can provide more sulfur dioxide transport channels, and the 3D porous form can greatly accommodate the volume expansion of sulfur dioxide during the charging and discharging process (Figure 8F).

Figure 8. (A) Transmission electron microscopy and (B) high-resolution transmission electron microscopy images of the FeSb2S4/EN-rGO (reduced graphene oxide) negative electrode after cycles at 5 A g−1, and (C) comparison of Na+ storage performance at different current densities. Reproduced with permission: Copyright 2020, Elsevier.140 (D) The synthesis process of MoS2/NGA and (E) schematic diagram of one-step synthesis, and (F) schematic drawing of the graphene–MoS2 composite, demonstrating the synergistic effects of interface engineering on electrode materials. Reproduced with permission: Copyright 2020, Elsevier.64

Graphene can be used not only as an active electrode material in its own right but also as a functional additive. With the wide application of graphene functional additive, SIBs' graphene negative electrodes have been prepared. According to its' successful application of LIBs, graphene has unique structural variables, such as anchoring, wrapping, encapsulation, interlayer, and layered and mixed models (Figure 9A), and has been widely used in SIBs.¹⁴¹ The presence of graphene in the composite structure enhances the conductivity of the electrode and alleviates the problems caused by volume expansion. In addition, robust graphene matrices can provide multidimensional transport paths that effectively enhance Na⁺/electron diffusion. As shown in Figure 9B, Chu et al.¹⁴² reported a coral-like material that adhered NiSe₂ to a graphene substrate (NSG) through the colloid method and a subsequent coating process and they it as a negative electrode material for PIBs for the first time. The SEM image of NSG (Figure 9C) revealed that the coral-like NiSe₂ with a width of 500 nm was uniformly distributed and wrapped in graphene and the highly crystalline NiSe₂ and amorphous carbon were confirmed by transmission electron microscopy images (Figure 9D). NSG composite material has excellent K⁺ storage performance, achieving an unprecedented reversible specific capacity of 522 mAh g⁻¹ at 50 mA g⁻¹ over 50 cycles, excellent rate performance (272 mAh g⁻¹ at 1000 mA g⁻¹), and long life (259 mAh g⁻¹ over 1200 cycles at 1000 mA g⁻¹). This is due to the enhanced electronic conductivity and relief of volume expansion.

Figure 9. (A) Schematic of the different structures of graphene composite electrode materials. Reproduced with permission: Copyright 2014, Springer Nature.141 (B) Schematic illustration of the synthesis of the colloid-assisted strategy for the NSG, (C) FESEM images, and (D) transmission electron microscopy images of the NSG composite. Reproduced with permission: Copyright 2020, Elsevier.142 (E) Schematic diagram for the synthesis process of the rGO@p–FeS2@C composite, SEM images of (F) rGO@FeS2@C and (G) rGO@p–FeS2@C electrodes after 50 cycles for SIBs, and (H) SEM images of the rGO@p–FeS2@C electrode after 30 cycles for PIBs. Reproduced with permission: Copyright 2019, Elsevier.143 FESEM, field emission scanning electron microscopy; NSG, NiSe2 to graphene substrate; PIB, potassium-ion battery; rGO, reduced graphene oxide; SEM, scanning electron microscopy; SIB, sodium-ion battery.

Yao et al.¹⁴³ rationally constructed a 2D FeS₂–C structure, and as shown in Figure 9E, porous FeS₂ nanoparticles were distributed on the reduced GO matrix and the carbon coating surface with large internal voidings. Due to its structural characteristics of a conducting carbon net, a protective carbon layer, and an internal void chamber, the prepared rGO@p–FeS₂@C composite material has excellent electrochemical performance and can be used as the negative electrode material of SIBs and PIBs. In terms of Na⁺ storage, the capacity is 598 mAh g⁻¹ after 100 cycles at 0.1 A g⁻¹, and the rate performance is good (428 mAh g⁻¹ at 10 A g⁻¹). For K⁺ storage, it also maintains a superior rate capacity of 298 mAh g⁻¹ at 2 A g⁻¹. In Figure 9F–H, it can be clearly observed that the rGO@p–FeS₂@C electrode retains its original 2D shape well after several Na⁺ and K⁺ storage cycles, indicating that its structure is stable in the cycle process. In rGO@p–FeS₂@C composites, the rGO matrix and carbon coating are conducive to the rapid transfer of electrons and ions, while the large surface area and small size of FeS₂ increase the electrode/electrolyte contact area and shorten the ion diffusion path, which is conducive to the quasi-capacitive reaction with excellent rate performance.¹⁴⁴ In addition, the abundant internal voids can effectively accommodate the volume change of FeS₂, and the external carbon layer can prevent the agglomeration of FeS₂ nanoparticles, so that the composite has good structural stability and excellent cycling performance in SIBs and PIBs.

HOW TO DESIGN HIGH-PERFORMANCE CARBON-NEGATIVE ELECTRODE MATERIALS

Development of alkali-metal ion batteries represents one of the effective means to solve the problem of insufficient lithium resources. To realize the commercial application of alkali-metal ion batteries to supplement or even replace LIBs, high-performance anode materials must be designed. In recent years, researchers have made considerable efforts to improve the electrochemical performance of carbon-anode materials, such as heteroatomic doping, graphene modification, nanostructuring, and so forth.

Heteroatom doping

Heteroatomic doping is mainly applied in carbon materials of polyisobutylene polymers, which has the advantages of enhancing electrical conductivity, increasing active centers, and producing fast ion transport defects.^145,146 For example, by introducing different N dopants, it is possible to change the electron distribution of different electronegative carbon materials and thus add additional pseudocapacitance through the interaction between K⁺ and N dopants. In addition, it is possible to change the bonding length and bonding angle, which can increase the layer spacing and create surface defects, thus accelerating dynamics. Currently, the introduction of N, P, S, O, and other heteroatoms into carbon materials and their combinations is performed by the thermal reaction of carbon materials with corresponding heteroatomic gases or direct pyrolysis of corresponding heteroatomic organic compounds at high temperatures.

Chen et al.¹⁴⁷ prepared a core–shell-type Sb@Sb₂O₃ heterostructure coated in 3D nitrogen-doped carbon hollow spheres (Sb@Sb₂O₃@N-3DCHs) by means of spray drying and heat treatment (Figure 10A). It has a large number of heterogeneous interfaces, which are spontaneously formed at the core–shell contact through annealing and oxidation, and can promote rapid Na⁺/K⁺ transfer. This composite structure showed a high specific capacity of approximately 573 mA g⁻¹ for the SIB negative electrode and approximately 474 mA g⁻¹ for the PIB negative electrode at 100 mA g⁻¹ and superior rate performance (302 mA g⁻¹ at 30 A g⁻¹ for the SIB negative electrode and 239 mA g⁻¹ at 5 A g⁻¹ for the PIB negative electrode). Zhao et al.¹⁴⁸ proposed a simple solvothermal method combined with a dopamine coating and annealing strategy to synthesize a unique box-like NiS@C. The Ni–N bond encapsulates NiS particles in nitrogen-doped carbon cages, and the box-like NiS@C structure has excellent storage properties of sodium/potassium ions. Nitrogen-doped carbon and chemical bonds between NiS and carbon confer the composite with a highly conductive network and fast ion diffusion channels with excellent rate capability. NiS@C can provide a high Na⁺ storage capacity of 632 mA g⁻¹ at 5 A g⁻¹ cycles. After 300 cycles, a stable K⁺ storage capacity of 171 mA g⁻¹ can be maintained at 1 A g⁻¹. Cui et al.⁹⁹ constructed a N/O double-doped HC (NOHC) composite by carbonizing renewable sorghum straw (Figure 10B). As a PIB negative electrode, NOHC presents a high reversible capacity (304.6 mA g⁻¹ at 0.1 A g⁻¹ after 100 cycles) and superior cycling stability (189.5 mA g⁻¹ at 1 A g⁻¹ after 5000 cycles). At the same time, the fully charged battery lights up the wearable light-emitting diode watch (Figure 10C) and the white light-emitting diode bulb (Figure 10D); this shows that NOHC-800 has a broad application potential as a PIB negative electrode.

Figure 10. (A) Schematic illustration of the morphological and structural evolution process for the Sb@Sb2O3@N-3DCH composite during the preparation. Reproduced with permission: Copyright 2021, John Wiley and Sons.147 (B) Schematic illustration of the preparation of N/O double-doped hard carbon (NOHC) and (C) lighted light-emitting diode (LED) watch and (D) white LED bulb driven by the NOHC-800 potassium-ion full cell. Reproduced under the terms of the Creative Commons license: Copyright 2020, Cui et al.99

Nazir et al.¹⁴⁹ synthesized boron (B) and phosphorus (P) honeycomb-like carbon (BPC) through one-step pyrolysis of bromine ion salts containing both two elements (Figure 11A). The double doping of B and P in the carbon of BPC increases the layer spacing, defect, and conductivity, and improves the adsorption capacity, mass transport capacity, and charge diffusion capacity of the carbon of BPC. Moreover, the porous structure formed by BPC can promote electrolyte penetration and buffer the volume change in the circulation process and demonstrated excellent electrochemical properties: the discharge capacity was 245.3 mAh g⁻¹ after 80 cycles with a Coulombic efficiency of 99.6%. In terms of the rate performance of BPC-800, when the current density is 0.05 A g⁻¹, its reversible capacity is 247.5 mAh g⁻¹. Wang et al.¹⁵⁰ designed a new dense carbon material with high-density crosslinked bacterial cellulose (BC) and hexachlorocyclotriphosphazene (HCCP) co-doped with N and P (Figure 11B), which has high weight capacity and volume capacity. The prepared electrode has a high reversible capacity of 223 mAh g⁻¹ at 0.05 A g⁻¹ and 145 mAh g⁻¹ at 10 A g⁻¹ (Figure 11C). Pei et al.¹⁵¹ studied the interaction between dopants (N and S) and carbon vacancy defects and their influence on the electrochemical performance of HC materials for storing sodium ions by combining theoretical calculation and electrochemical testing. The results show that the doping–defect interaction plays a decisive role in the capacity and rate performance of carbon negative electrodes and N, S co-doping is an effective method. Based on this fact, they proposed a general in situ deformation method for the synthesis of N, S double-doped porous HC materials with the preferred properties for SIBs (Figure 11D,E). The results show that it has a large reversible capacity (0.05 A g⁻¹, 430 mAh g⁻¹) and unprecedented high-rate energy density energy (5 A g⁻¹, up to 277 mAh g⁻¹). In addition, it has good cycling stability in SIBs.

Figure 11. (A) Schematic illustration of the synthesis of B, P co-doped porous carbon. Reproduced with permission: Copyright 2020, Elsevier.149 (B) Schematic illustration of the fabrication process for the NPC samples. In these digital photos, bacterial cellulose BC is 825 mg, BC-HCCP is 974 mg, and C-800, NPC-700, NPC-800, and NPC-900 are all 200 mg. (C) Comparison of gravimetric rate capacities of the NPC-800 electrode at 10 A g−1 with those of state-of-the-art carbon negative-electrode materials of SIBs. Reproduced with permission: Copyright 2020, Elsevier.150 (D) Schematic diagram showing the synthesis of the in situ textured SNC-P material and (E) TEM images showing the typical porous structure and the thin wall of the SNC-P sample. Reproduced with permission: Copyright 2020, Elsevier.151 BC, bacterial cellulose; HCCP, hexachlorocyclotriphosphazene; NPC, N, P dual-doped carbon; SIB, sodium-ion battery; SNC, sulfur and nitrogen co-doped carbon; TEM, transmission electron microscopy.

Similarly, improving the ion storage capacity of carbon materials by doping heteroatoms and optimizing the structure of carbon materials to achieve cyclic stability is also applicable to PIBs. The molecular-scale copolymer pyrolysis strategy proposed by Zhang et al.,⁸ for example, can precisely control N doping at the edge of carbon-based materials. They prepared defection-rich, edge-N doped carbon (ENDC) using an optimized process, with N doping levels up to 10.5 at% and a high edge N content of 87.6%. The optimized ENDC has a high reversible capacity of 423 mAh g⁻¹, an initial crosslinking efficiency of 65%, excellent crosslinking capacity, and a long cycle life (retention rate of 93.8% after 3 months). All in all, heteroatomic doping is one of the efficient strategies to improve the storage properties of Na⁺ and K⁺.

Modification of graphene negative-electrode materials

Graphene has a good conductive network, because of which high performance is achieved when used in SIBs and PIBs. However, its disadvantage is that it is easy to stack in the reaction process and becomes a multilayer graphene structure or graphite structure, which is not conducive to the storage of sodium/potassium ions between layers.¹⁵² To overcome these limitations, currently, there are several ways to modify graphene.

Introduction of a multistage distributed aperture structure

The design of a multistage distributed pore size structure is an effective strategy to enhance the storage capacity of carbon-based materials by optimizing the transport pathway of Na⁺/K⁺, which can shorten the diffusion distance of ions. Thus, the electrochemical properties of carbon materials can be improved effectively. Yan et al.¹⁵³ prepared a sandwich composite nanomaterial using the ionic thermal method, overcoming the shortcomings of nongraphitized carbon materials. The transport mechanism of Na⁺ in a graphene/porous carbon complex is shown in Figure 12B. The porous carbon is evenly distributed on both sides of the graphene material, which promotes the diffusion of sodium ions in the material. The nanomaterials show high specific capacity, long cycle life, and high rate performance. The composite has a specific capacity of 400 mAh g⁻¹ at a current density of 50 mA g⁻¹. Wenzel et al.⁷¹ adopted the method of nanocasting and used nanoporous silica as a template to introduce nanoscale carbon materials into carbon materials successfully, which increased the ion transport speed and improved the storage performance of Na⁺.

Figure 12. (A) Schematic diagram for the formation mechanism of the three-dimensional MoS2–graphene composite microsphere. Reproduced with permission: Copyright 2015, John Wiley and Sons.154 (B) Transport mechanism of sodium ions in graphene/porous carbon composites. Reproduced with permission: Copyright 2014, John Wiley and Sons.153 (C) Illustration of the synthesis process of the S-G@HCS composite, where S-G represents S-doped graphene and HCS is hollow carbon spindles. (D) Transmission electron microscopy image of S-G@HCS composites. Reproduced with permission: Copyright 2021, John Wiley and Sons.98

Heterogeneous atom doping

Heteroatom incorporation has been demonstrated to be an effective strategy for material optimization in SIBs.³⁶ The introduction of heterogeneous atoms (such as N, B, etc.) into carbon materials can lead to the insertion of more electronegative functional groups on the surface, inducing sodium ions to undergo redox reactions on the surface. Ling et al.¹⁵⁵ prepared a negative electrode material for SIBs by doping B in graphene, which can adsorb sodium ions on its both sides during the reaction, with a specific capacity 2.54 times that of HC as the negative electrode material. In addition, it can be modified by coating metal oxides. Choi et al.¹⁵⁴ prepared 3D graphene microspheres coated with lamellar MoS₂ by single-pot spray pyrolysis (Figure 12A) and found that the accumulated lamellar MoS₂ reduced the diffusion resistance of sodium ions, and the graphene structure provided more space to relieve volume expansion, offering more diffusion channels in the cycle process. Yao et al.⁹⁸ fixed a hollow carbon spindle (HCS) onto the surface of graphene and then carried out the sulfur-doping treatment, aiming to integrate the high conductivity of graphene, the good structural stability of HCS, and a large number of active centers induced by S doping (Figure 12C,D). As a PIB negative electrode, the S-doped graphen@HCS composite showed a high capacity of 301 mAh g⁻¹ at 0.1 A g⁻¹ and a long cycle capacity of up to 1800 cycles at 2 A g⁻¹. Impressively, it can provide an excellent rate capacity of 215 mAh g⁻¹ at 10 A g⁻¹.

Expand the spacing between graphene layers

Wen et al.⁶¹ used the combined method of oxidation and partial reduction to expand the layer spacing of graphene from the initial 0.334 nm to 0.43 nm. This material still maintains the long-range ordered structure of graphite, and sodium ions can be disintercalated reversibly. The expanded graphite structure maintains a reversible capacity of 284 mAh g⁻¹ at a current density of 20 mA g⁻¹ and 184 mAh g⁻¹ at a current density of 100 mA g⁻¹. 73.92% of the initial capacity was maintained after 2000 cycles.

Solvent co-embedding

Cohn et al.¹⁵⁶ used diethylene glycol dimethyl ether as a solvent shell to coat sodium ions. This nonviscous coating facilitates rapid insertion and removal of sodium ions between multiple graphene layers and reduces solvation. The material maintains a reversible capacity of approximately 100 mA g⁻¹ at a current density of 30 mA g⁻¹, with no decay after 8000 cycles. Despite the significant progress made in graphene-based negative electrode materials for SIBs, there are still some areas that merit further investigation. Converting 2D graphene into a 3D porous structure increases the surface area and facilitates ion/electron diffusion but decreases the energy density. In addition, the mechanical strength of 3D porous graphene is poor, and the synthesis technology is usually very complex. Introduction of defects into graphene increases the active sites of Na⁺ storage, although it significantly reduces the electrical conductivity. The introduction of heteroatoms into graphene significantly changes its physicochemical and electronic properties, thus enhancing the storage capacity of Na⁺. However, precisely controlling the doping of graphene remains a challenge. Therefore, further research is needed to develop new and controlled syntheses of various graphene nanostructures with optimal electrochemical properties. It is necessary to explore how the interaction between graphene and nanomaterials affects the performance of the negative electrode. When designing graphene-based nanocomposites, the mass load, particle size, and morphology of the nanomaterials must be optimized to achieve higher performance. The development of graphene-based negative electrodes with high efficiency and long-term recyclability for implementation in real-world SIBs remains a challenge.

The working principle of LIBs, SIBs, PIBs, and other alkaline metal-ion batteries, and the ion storage mechanism of carbon materials are very similar. Therefore, the above methods can be used to optimize the graphene-negative electrodes used for PIBs.

Nanostructuring Carbon nanotubes (CNTs)

CNTs are promising materials for LIBs due to their unusual electrochemical and mechanical properties.¹⁵⁷ Similar to conventional carbon electrodes, doped electrodes can be used as conductive substrates with lower weight sensing. This provides a more effective strategy for the establishment of electroosmotic networks.¹⁵⁸ Like LIBs, CNTs can be assembled as free-standing electrodes without any binder as active sodium/potassium-ion storage materials or metal oxides^52,159-161 and high-capacity negative electrode materials, such as phosphorus, tin, germanium, antimony, and metal oxides^162-164 (Figure 13A). For example, Yang et al.¹⁶⁹ successfully prepared poplar flower-like N-doped carbon nanotubes (NCNT@VS₄) with a 3D structure by a solvothermal reaction using N-doped carbon nanotubes (NCNTs) as a template. NCNT@VS₄ provides excellent Na⁺ storage performance. After 2000 cycles, the reversible capacity is 430 mAh g⁻¹ (1 A g⁻¹), and the initial Coulombic efficiency is 81.6%. At the same time, the electrode shows excellent rate performance (460 mAh g⁻¹ at 5 A g⁻¹) and high current resistance.

Figure 13. (A) Common types of CNT-based negative electrode materials for SIBs. Reproduced with permission: Copyright 2021, Elsevier.165 (B) The electrochemical deoxidation strategy for the synthesis of the Bi/CNT composite and (C) the SEM image of electrolytic Bi/CNTs. Reproduced with permission: Copyright 2021, Elsevier.166 (D) Structure evolution of P-CNT and phosphorus and carbon nanotube composite electrodes during cycling. Reproduced with permission: Copyright 2020, American Chemical Society.167 (E) Schematic representation of the synthesis of N-rich hollow carbon nanotubes, (F) SEM image of NCNT, and (G) Na+ storage behaviors in the NCNT electrodes. Reproduced with permission: Copyright 2020, Elsevier.168 Bi/CNT, bismuth/carbon nanotube; CNT, carbon nanotube; NCNT, N-doped hollow carbon nanotubes; SEM, scanning electron microscopy.

Hu et al.¹⁶⁶ prepared a bismuth/carbon nanotube (Bi/CNT) composite (Figure 13B) negative electrode by one-step electrode oxidization. Electrolytic micro Bi/CNT composites (Figure 13C) reduce the impact of volume expansion, thus improving their cycling performance. The capacity of Bi/CNTs was 381.6 mAh g⁻¹ at 2.0 A g ⁻¹, and the capacity retention rate was 99.5% after 1000 cycles, which corresponds to a capacity decay of 0.0005% per cycle. The improved performance is due to the fact that carbon nanotubes increase the diffusion rate of sodium ions and act as a buffer to enhance the electrical conductivity of the Bi-based negative electrode. The lattice space of Bi is ~0.32 nm, which is identified as the Bi (012) crystal face. The electrolytic Bi/CNT composite negative electrode shows excellent rate performance at different current densities. Bi/CNT composite negative electrodes can achieve stable discharge capacities of 525, 407, 404, 404, 402.7, 402.1, 401.3, and 399 mA g⁻¹ at current densities of 0.1, 0.5, 1.0, 1.5, 2.0, 3.0, and 5.0 A, respectively. Charge–discharge curves at different current densities show similar potential curves. The charge–discharge curve of the Bi/CNT negative electrode at 0.1 A g⁻¹ during the initial cycle shows the maximum discharge/charge capacity (602/490 mAh g⁻¹). At different current densities, the capacity of Bi/CNTs remains at ~400 mAh g⁻¹. In addition, the charging/discharging capacity of Bi/CNT remains 400 mAh g⁻¹ at 1.0 A g⁻¹ after 200 cycles, with a capacity retention rate of 98.5%. In contrast, the capacity of electrolytic Bi is reduced to 350 mAh g⁻¹, after 200 cycles, at a current density of 1.0 A g⁻¹. The excellent cycling stability of micron-scale Bi/CNT indicates its application potential in SIBs and shows the potential of the micron-scale negative electrode.

Peng et al.¹⁶⁷ prepared three phosphorus/carbon (P/C) composites with different chemical bonding states using carboxyl carbon nanotubes, CNTs, and reduced carboxyl carbon nanotubes as raw materials using a simple ball-milling method; when used as the negative electrode of PIBs, the composite material (Figure 13D) has the best cycle stability (402.6 mAh g⁻¹, 110 cycles), and the capacity retention rate is 68.26% when the current density is 0.1 A g⁻¹. Zhong et al.¹⁶⁸ prepared hollow carbon nanotubes (NCNTs) with a length–aspect ratio using a template method and prepared N-rich hollow carbon nanotubes (up to 15.7%) with a length–aspect ratio by exploring polypyrrole as a high-nitrogen carbon precursor (Figure 13E,F). NCNT has excellent Na⁺ storage capacity, and SIBs with NCNT as negative electrodes show excellent electrochemical performance (Figure 13G). The above represents the application of carbon nanotubes in SIB negative electrodes and similar properties are also found in PIB systems.

In addition to using CNT composites as negative electrodes, CNTs are also used as electron conduction networks to facilitate the convenient transport of sodium/potassium ions^170-175 and lithium ions^176,177 in electron-to-cathode materials (Table 3). For example, Zhao et al.¹⁷⁸ incorporated Ni₂P nanoparticles into graphene and carbon nanotube to form multicomponent composites (Ni₂P/NPS/RGO/CNTs), aiming to develop flexible self-supporting entangled network membrane negative electrodes to improve the cycle life of SIBs. Ni₂P/NPS/RGO/CNTs composite electrodes maintain a capacity of 224 mAh g⁻¹ after 100 cycles. At a high current density of 0.5 A g⁻¹ after 500 cycles, the reversible capacity of Ni₂P/NPS/RGO/CNTs composite electrode remained stable at about 150 mAh g⁻¹, and the reversible capacity of the Ni₂P/NPS/RGO/CNTs remained stable after cycling at a rate of up to 1 A g⁻¹. It reached 91 mAh g⁻¹ after 2000 cycles. Even at 5 A g⁻¹, it showed a high multiplier capacity of 65 mAh g⁻¹. In addition, Wang et al.¹⁶⁴ designed and performed porous MnSe/FeSe (Mn–Fe–Se) adhesion/insertion to interlace CNTs by means of a simple chemical precipitation method, followed by one-step carbonization of Mn–Iron Prussian blue. Moreover, the characterization results showed that the prepared Mn–FeSe/CNTs with a high conductivity network structure were constructed by adhesion/insertion of Mn–Fe–Se spheres into interlaced CNTs. Each Mn–Fe–Se sphere was assembled using dozens of nanoparticles and a large number of voids. Individual nanoparticles were uniformly covered by a thin layer of nitrogen-doped amorphous carbon. Due to the synergistic effect of this unique porous structure and heterogeneous components, the Mn–Fe–Se/CNT negative electrode has reversible sodium/potassium storage, excellent rate performance, and excellent cycling stability.

Zhang et al.¹⁷⁹ synthesized a sandwich doped carbon nanotube @Nb₂C (N-CNT@Nb₂C), which showed excellent electrochemical performance in LIBs, SIBs, and PIBs, such as stable cycle performance of more than 500 cycles. Chen et al.¹⁸⁰ used S/N-doped carbon nanotube/carbon nanofiber composites (CNT/SNCF) as negative electrode materials for SIBs and PIBs. Independent CNT/SNCF electrodes show high discharge capacity (274.1 and 212.5 mAh g⁻¹ at 1 A g⁻¹ after 1000 cycles, respectively) and excellent cycle stability (150.4 and 100.1 mAh g⁻¹ after 5000 cycles at 5 A g⁻¹, respectively) and rate performance (109.3 mAh g⁻¹ at 10 A g⁻¹ and 108.7 mAh g⁻¹ at 5 A g⁻¹, respectively) (Table 3).

Table 3 Electrochemical properties of carbon nanotube composite negative electrode materials (sodium-ion batteries).

Electrode	Electrochemical performance	References
MWCNT (multi-walled carbon nanotube)-wrapped SiP2	925 mAh g−1 after 200 cycles at 0.2 A g−1	[225]
TiO2/CNT composite	223.4 mAh g−1 at 1 C 142.8 mAh g−1 at 10 C (3.35 A g−1)	[226]
Bismuth/carbon nanotube (Bi/CNT) composite	381.6 mAh g−1 at 2.0 A g−1 99.5% after 1000 cycles	[166]
MoS2-MSs/MWCNTs	519 mAh g−1 at 0.1 A g−1 after 100 cycles 327.1 mAh g−1 over 1000 cycles at 2 A g−1	[227]
FeS2@N-CNTs@rGO	(Approximately to 100%) over 750 cycles at 2 A g−1	[228]
TiO2@NC-NTs	158 mAh g−1 at 2 C (1 C = 335 mA g−1) for 2200 cycles 146 mAh g−1 at 5 C for 4000 cycles	[229]
Sb/HPACNTs	318 mAh g−1 after 4500 cycles at a current density of 1 A g−1	[230]
PNC-MeNTs	Over 10,000 cycles at 10 A g−1 and 3000 cycles at 1 A g−1 can be achieved	[231]
CNTs with porous and disordered structures (PCNTs)	255 mAh g−1 after 200 cycles at 0.1 A g−1	[232]
C-MoS2/CNT composites	100.0% capacity remaining after 1000 cycles	[233]
N-rich (up to 15.7%) hollow carbon nanotubes (NCNTs)	A favorable reversible capacity of 132 mAh g−1 after 5000 cycles at 4000 mA g−1	[168]
MS/CNTs	440 mAh g−1 at 0.5 A g−1, 400 mAh g−1 at 1 A g−1	[234]
NC@CoS2@CNTs	Reversible specific capacities of 669 and 334 mAh g−1 at the current densities of 200 mA g−1 and 5000 mA g−1, respectively, and unprecedented cycling stability of approximately 0.014% capacity decay per cycle over 1000 cycles at 3000 mA g−1	[235]
SnO2@hCNT	Reversible discharge capacity of 223 mAh g−1	[236]
TiO2/rGO@NC	The first charge specific capacity of 245.8 mAh g−1 (0.02 A g−1) along with 118.7 and 91.7 mAh g−1 at high current densities of 2.5 and 5 A g−1	[237]
Ni3S2/CNTs-10	The charge capacity retention of the Ni3S2/CNT-10 electrode is up to 82.2% after 200 cycles at the current density of 0.5 A g−1	[238]
SnS@CNT	Capacities of 666 and 615 mAh g−1 after 100 (100% retention) and 500 cycles (92% retention) at a current density of 1 A g−1	[239]

Other carbon nanostructures

So far, different methods have been developed for preparing negative electrode materials suitable for SIBs, but there is little mention of rate capabilities.¹ However, the ability to obtain attractive rates is one of the most important factors to obtain suitable electrodes for use in energy storage devices. There is little mention of the rate capacity of HC as currently reported negative electrodes for SIBs are not small enough and nanoscale materials are required to achieve high rate capacity.^71,181,182 Modification of morphology and size represents an effective strategy for improving the quality of transport and storage and can significantly improve the electrochemical properties of Na⁺, so the synthesis of nanomaterials represents a promising design for controlling the morphology and size of electrode materials.^183–186 Different nanostructures make different contributions toward improving the electrochemical properties of electrode materials.^187–189 In LIB research, the unique nanostructure can facilitate mass transfer by providing a larger surface area and a shorter diffusion distance.^190–192 This type of nanostructure design engineering has been extended to SIBs, with notable achievements.^193–196 Nanostructured materials,^{146,197–203} such as hollow nanostructures,²⁰⁴ nanofibers,^205–207 nanobubbles,^208,209 nanopores,²¹⁰ nanosheets,²¹¹ mesoporous carbon,²¹² and nanowires,²¹³ have recently been reported as SIB carbon negative electrode electrodes. For example, Bai et al.²¹⁴ prepared S and N co-doped (N by weight of 15.64%, S by weight of 3.1%) carbon nanosheets by treating N-rich carbon nanosheets with sublimated S (Figure 14A). S and N co-doping increased the interlayer spacing to 0.38 nm and resulted in a large number of active sites introduced by defects. These structural characteristics, combined with the advantages of nanosheets, confer SNC with a high Na⁺ storage capacity of 270 mAh g⁻¹ (0.1 A g⁻¹ after 1000 cycles) and 100 mAh g⁻¹ (1 A g⁻¹ after 1000 cycles). Moreover, kinetic analysis shows that S and N co-doping can improve the diffusion coefficient of Na⁺ and enhance the ion storage in the carbon negative electrode. Su et al.²¹⁵ developed a continuous and flexible porous carbon nanofiber membrane as a negative electrode material for SIBs by electrospinning a mixture of polyacrylonitrile and ultrafine zeolite–imidazolate skeleton (ZIF-8) nanoparticles and then carbonizing it at 1200°C (Figure 14B). The inherent porous crystal structure of ZIF-8 and its decomposition/evaporation at high temperatures contribute to the generation of fractional micropores that uniformly disperse along the nanofibers, resulting in good flexibility and excellent sodium-ion storage performance. By setting the ZIF-8 content to 45%, the flexible negative electrode can achieve an excellent Na⁺ capacity of 418 mAh g⁻¹ at 0.1 A g⁻¹ and a significant platform capacity of 278 mAh g⁻¹. As the negative electrode material of SIBs, the carbon nanostructure can provide excellent Na⁺ storage performance, which is one of the suitable negative electrode materials of SIBs.

Figure 14. (A) Schematic preparation process of S, N co-doped carbon nanosheets. Reproduced with permission: Copyright 2021, Elsevier.214 (B) Digital photographs of carbonized pure polyacrylonitrile nanofibers (CN, left) and porous carbon nanofibers (PCNs, right). Reproduced with permission: Copyright 2021, John Wiley and Sons.215 (C) Schematic diagram illustrating the synthesis of carbon particles and (D,E) transmission electron microscopy images of C-2100. Reproduced with permission: Copyright 2020, Elsevier.216

The radius of potassium ions is larger than that of sodium ions; to improve the storage performance of potassium ions in carbon negative electrodes, designing unique nanostructures is effective. For example, porous carbon materials, with large surface area and pore volume,²¹⁷–²¹⁹ can provide a large diffusion channel and a shorter diffusion distance to facilitate the rapid mass transfer and buffer the huge volume strain, thus improving the electrochemical performance of PIBs.^220,221 Choi et al.²¹⁶ synthesized porous carbon microspheres with a highly graphitized structure that enhanced the storage performance of potassium ions (Figure 14C–E). The carbon microspheres prepared by them had a low working potential of ~0.2 V, high Coulombic efficiency, and stable reversible capacity of 292.0 mAh g⁻¹ after 100 cycles.²¹⁶ Zhang et al. successfully fabricated a mesoporous carbon nanosheet assembled flower as a PIB negative electrode material with 381 mAh g⁻¹ at 50 mA g⁻¹, 101 mAh g⁻¹ at 2.0 A g⁻¹, and ultra-long cycle stability of over 600 cycles at 500 mA g⁻¹.²²²

Zhang et al.²²³ synthesized a high N-doped (26.7 at%) accordion-like carbon negative by direct pyrolysis, which was composed of thin carbon nanosheets and a turbine translation crystal structure (Figure 15A–C). Accordion-like carbon hierarchies appear in the self-assembly process during pyrolytic carbonization. The layered N-doped accordion construction yields a reversible capacity of up to 346 mAh g⁻¹ and superior cycle stability. Pei et al.²²⁴ prepared a series of hollow N-doped carbon nanofibers (HNCNFs) using polyaniline as the raw material (Figure 15D,E). As a negative electrode for PIBs, HNCNFs have an ultra-high-speed performance of 139.7 mAh g⁻¹ and ultralong cycle life of 188.4 mAh g⁻¹ at 30 A g⁻¹ after 4000 cycles.

Figure 15. Schematic illustrating the formation of the hierarchical structure of nitrogen-doped accordion-like carbon (N-ALC): (A) structure and morphology of the pristine uric acid crystal, (B) structure and morphology of the intermediate pyrolysis product from 380°C to 500°C, and (C) structure and morphology of the obtained N-ALC at temperatures above 700°C. Reproduced with permission: Copyright 2021, John Wiley and Sons.223 (D) Illustration of the fabrication process of HNCNFs and (E) field emission scanning electron microscopy image of HNCNFs-700. Reproduced under the terms of the creative commons license: Copyright 2020, the Royal Society of Chemistry.224

To sum up, both SIBs and PIBs require nanoscale materials to achieve high rate capacity as both sodium and potassium ions are larger than lithium ions. Nanostructured materials have unique physical and chemical properties, with a large surface area, increasing the electrode/electrolyte contact area and active center, making rapid ion transport at the electrode/electrolyte interface possible. In addition, the nanostructure design is often accompanied by the improvement of the electrical conductivity of the material, which can effectively shorten the path of electron transport, thus accelerating the diffusion of Na⁺/K⁺ in the bulk phase. For carbon materials, the development of carbon nanotubes, carbon nanowires, and other nanomaterials is one of the effective strategies to improve the electrochemical performance of carbon materials.

CONCLUSION

We have reviewed the recent progress of a large number of carbonaceous materials with different structures/textures as negative electrodes for SIBs and PIBs, focusing on the similarities and differences in Na⁺ and K⁺ storage mechanisms of different carbonaceous materials, suggesting that carbonaceous materials may be promising candidate negative electrodes for SIBs and PIBs. Na⁺ can be included in graphite intercalation by increasing layer spacing or co-intercalation between Na⁺ and solvent molecules. K⁺ has a larger radius than Na⁺, but it can be inserted into the graphite sandwich. The amorphous carbon materials have a higher specific capacity than graphite due to different microstructures and Na⁺ storage mechanisms. Due to the differences in the precursors and microstructures, including adsorption, nanopore filling, and insertion, researchers have different views on the mechanism of Na⁺ and K⁺ storage of amorphous carbon such as HC and SC. The results show that heteroatomic doping and nanostructure can effectively improve the performance of carbon materials as negative electrode materials for SIBs and PIBs. PIB has many potential advantages over SIB, such as higher battery voltage, better ion mobility, the use of aluminum as both cathode and negative electrode substrates, low cost, and availability of abundant potassium resources. In addition, many theories of LIBs have been proved to be similar to those of PIBs. Therefore, the knowledge of LIBs can be easily transferred to the PIB theory library, thus providing a direction for the rapid development of PIB technology.

Although considerable progress has been made in the research on SIBs and PIBs, there are still many obstacles and many challenges in their practical application. In terms of carbon negative-electrode materials, the research results are not enough and need to be further developed. The initial Coulombic efficiency and rate capacity of carbon materials are low and need to be further improved. The initial Coulombic efficiency of carbon materials rarely reaches more than 80%, and most of them are lower than 70%, but the higher initial Coulombic efficiency is required to exceed 90% in practical applications. Therefore, it is necessary to design carbon materials with special micro/nanostructures that are rich in active centers for reversible Na⁺ storage and shorten the diffusion path of sodium and potassium ions. To adjust the formation of SEI films and improve the initial Coulombic efficiency, optimized electrolyte and additives need to be explored. Reducing the processing cost and improving the storage capacity of Na⁺ and K⁺ in carbon materials are the key to practical applications. If low-cost and high-performance carbon materials can be produced by effective synthesis methods, they will have broad application prospects in SIBs and PIBs. Finally, SIBs and PIBs will certainly be excellent supplements or even substitutes for LIBs in the future.

CONFLICTS OF INTEREST

The authors declare no conflicts of interest.