Due to the physical and electrochemical properties of sodium, SIBs require different materials from those used for LIBs.
Anodes Carbons SIBs can use
hard carbon, a disordered carbon material consisting of a non-graphitizable, non-crystalline and amorphous carbon. Hard carbon's ability to absorb sodium was discovered in 2000. This anode was shown to deliver 300 mAh/g with a sloping potential profile above ~0.15 V vs Na/Na+. It accounts for roughly half of the capacity and a flat potential profile (a potential plateau) below ~0.15 V vs Na/Na+. Such capacities are comparable to 300–360 mAh/g of
graphite anodes in
lithium-ion batteries. The first sodium-ion cell using hard carbon was demonstrated in 2003 and showed a 3.7 V average voltage during discharge. Hard carbon was the preferred choice of
Faradion due to its excellent combination of capacity, (lower) working potentials, and cycling stability. Notably, nitrogen-doped hard carbons display even larger specific capacity of 520 mAh/g at 20 mA/g with stability over 1000 cycles.
Recent advances in hard carbon anodes Recent studies have focused on modifying the microstructure and surface chemistry of hard carbon to improve its performance as an anode material for sodium-ion batteries (SIBs). Hard carbon stores sodium through a combination of adsorption on defect sites, intercalation between turbostratic graphene layers, and filling of nanopores with sodium clusters. Its electrochemical behavior depends on the arrangement of pseudo-graphitic domains and the distribution of open and closed pores within the carbon matrix. To achieve high capacity and fast-charging performance, researchers have explored structural engineering approaches such as enlarging the interlayer distance and tuning the pore structure. Nitrogen doping and pore activation have been shown to increase interlayer spacing and create additional active sites for sodium storage, which improves rate capability and reversibility. Control over the size and volume fraction of closed pores affects sodium cluster formation, influencing the low-potential plateau capacity and cycling stability. Biomass-derived hard carbon with optimized pseudo-graphitic domains and tailored closed pores has been reported to reach a reversible capacity of 280 mAh/g at 1 A/g, retaining over 90% of its capacity after 1000 cycles. One drawback of carbonaceous materials is that, because their
intercalation potentials are fairly negative, they are limited to non-aqueous systems.
Graphene Janus particles have been used in experimental sodium-ion batteries to increase
energy density. One side provides interaction sites while the other provides inter-layer separation. Energy density reached 337 mAh/g.
Carbon arsenide (AsC5) mono/bilayer has been explored as an anode material due to high specific capacity (794/596 mAh/g), low expansion (1.2%), and ultra low diffusion barrier (0.16/0.09 eV), indicating rapid charge/discharge cycle capability, during sodium intercalation. After sodium adsorption, a carbon arsenide anode maintains structural stability at 300 K, indicating long cycle life.
Metal alloys Numerous reports described anode materials storing sodium via alloy reaction and/or conversion reaction. Wang,
et al. reported that a self-regulating alloy interface of nickel antimony (NiSb) was chemically deposited on Na metal during discharge. This thin layer of NiSb regulates the uniform electrochemical plating of Na metal, lowering overpotential and offering dendrite-free plating/stripping of Na metal over 100 h at a high areal capacity of 10 mAh cm−2. Many metals and semi-metals (Pb, P, Sn, Ge, etc.) form stable alloys with sodium at room temperature. Unfortunately, the formation of such alloys is usually accompanied by a large volume change, which in turn results in the pulverization (crumbling) of the material after a few cycles. For example, with
tin sodium forms an alloy , which is equivalent to 847 mAh/g specific capacity, with a resulting enormous volume change up to 420%. In one study, Li et al. prepared sodium and metallic tin /Na through a spontaneous reaction. This anode could operate at a high temperature of in a carbonate solvent at 1 mA/cm2 with 1 mAh/cm2 loading, and the full cell exhibited a stable charge-discharge cycling for 100 cycles at a current density of 2
C. (2
C means that full charge or discharge was achieved in 0.5 hour). Despite sodium alloy's ability to operate at extreme temperatures and regulate dendritic growth, the severe stress-strain experienced on the material in the course of repeated storage cycles limits cycling stability, especially in large-format cells. Researchers from
Tokyo University of Science achieved 478 mAh/g with nano-sized
magnesium particles, announced in December 2020. In 2024,
Dalhousie University researchers enhanced sodium-ion battery performance by replacing hard carbon in the negative electrode with lead (Pb) and single wall
carbon nanotubes (SWCNTs). This combination significantly increased volumetric energy density and eliminated capacity fade in half cells. SWCNTs endured active material connectivity, boosting capacity to 475 mAh/g and reducing losses, compared to 430 mAh/g in Pb cell without SWCNTs.
Oxides Some sodium
titanate phases such as Na2Ti3O7, or NaTiO2, delivered capacities around 90–180 mAh/g at low working potentials (+), though cycling stability was limited to a few hundred cycles.
Molybdenum disulphide In 2021, researchers from China tried layered structure as a new type of anode for sodium-ion batteries. A dissolution-recrystallization process densely assembled carbon layer-coated nanosheets onto the surface of
polyimide-derived N-doped
carbon nanotubes. This kind of C-/NCNTs anode can store 348 mAh/g at 2 A/g, with a cycling stability of 82% capacity after 400 cycles at 1 A/g. is another potential material for SIBs because of its layered structure, but has yet to overcome the problem of capacity fade, since suffers from poor electrochemical kinetics and relatively weak structural stability. In 2021, researchers from Ningbo, China employed pre-potassiated , presenting rate capability of 165.9mAh/g and a cycling stability of 85.3% capacity after 500 cycles.
Other anodes for Some other materials, such as
mercury,
electroactive polymers and sodium
terephthalate derivatives, have also been demonstrated in laboratories, but did not provoke commercial interest.
Oxides Many layered
transition metal oxides can reversibly intercalate sodium ions upon reduction. This is conventionally understood to occur through a change in oxidation state of the transition metal cations in the oxide lattice. However in recent years, the understanding of sodium insertion and removal in these lattices has shifted, and it is now appreciated that anionic redox plays a determining role in sodium-ion battery cathodes, particularly those containing Manganese, which does not change its oxidation state during cycling. Sodic transition metal oxides typically have a higher
tap density and a lower electronic
resistivity, than other cathode materials (such as phosphates). Due to a larger size of the Na+ ion (116 pm) compared to Li+ ion (90 pm), cation mixing between Na+ and first row transition metal ions (which is common in the case of Li+) usually does not occur. Thus, low-cost iron and manganese oxides can be used for Na-ion batteries, whereas Li-ion batteries require the use of more expensive cobalt and nickel oxides. The drawback of the larger size of Na+ ion is its slower intercalation kinetics compared to Li+ ion and the presence of multiple intercalation stages with different voltages and kinetic rates. However, its sodium deficient nature lowered energy density. Significant efforts were expended in developing Na-richer oxides. A mixed P3/P2/O3-type Na0.76Mn0.5Ni0.3Fe0.1Mg0.1O2 was demonstrated to deliver 140 mAh/g at an average discharge voltage of 3.2 V
vs Na/Na+ in 2015. In particular, the O3-type NaNi1/4Na1/6Mn2/12Ti4/12Sn1/12O2 oxide can deliver 160 mAh/g at average voltage of 3.22 V
vs Na/Na+, while a series of doped Ni-based oxides of the
stoichiometry NaaNi(1−x−y−z)MnxMgyTizO2 can deliver 157 mAh/g in a sodium-ion "full cell" with a hard carbon anode at average discharge voltage of 3.2 V utilising the Ni2+/4+ redox couple. Such performance in full cell configuration is better or on par with commercial lithium-ion systems. A Na0.67Mn1−xMgxO2 cathode material exhibited a discharge capacity of 175 mAh/g for Na0.67Mn0.95Mg0.05O2. This cathode contained only abundant elements. Copper-substituted Na0.67Ni0.3−xCuxMn0.7O2 cathode materials showed a high reversible capacity with better capacity retention. In contrast to the copper-free Na0.67Ni0.3−xCuxMn0.7O2 electrode, the as-prepared Cu-substituted cathodes deliver better sodium storage. However, cathodes with Cu are more expensive.
Oxoanions Research has also considered cathodes based on
oxoanions. Such cathodes offer lower tap density, lowering energy density than oxides. On the other hand, a stronger
covalent bonding of the polyanion positively impacts cycle life and safety and increases the cell voltage. Among polyanion-based cathodes, sodium vanadium phosphate and
fluorophosphate have demonstrated excellent cycling stability and in the latter, an acceptably high capacity (⁓120 mAh/g) at high average discharge voltages (⁓3.6 V
vs Na/Na+). Besides that, sodium manganese silicate has also been demonstrated to deliver a very high capacity (>200 mAh/g) with decent cycling stability. A French startup TIAMAT develops Na+ ion batteries based on a sodium-vanadium-phosphate-fluoride cathode material Na3V2(PO4)2F3, which undergoes two reversible 0.5 e-/V transitions: at 3.2V and at 4.0 V. A startup from Singapore, SgNaPlus is developing and commercialising Na3V2(PO4)2F3 cathode material, which shows very good cycling stability, utilising the non-flammable glyme-based electrolyte.
Prussian blue and analogues Numerous research groups investigated the use of
Prussian blue and various Prussian blue analogues (PBAs) as cathodes for Na+-ion batteries. The ideal formula for a discharged material is Na2M[Fe(CN)6], and it corresponds to the theoretical capacity of ca. 170 mAh/g, which is equally split between two one-electron voltage plateaus. Such high specific charges are rarely observed only in PBA samples with a low number of structural defects. For example, the patented rhombohedral Na2MnFe(CN)6 displaying 150–160 mAh/g in capacity and a 3.4 V average discharge voltage and rhombohedral Prussian white Na1.88(5)Fe[Fe(CN)6]·0.18(9)H2O displaying initial capacity of 158 mAh/g and retaining 90% capacity after 50 cycles. While Ti, Mn, Fe and Co PBAs show a two-electron electrochemistry, the Ni PBA shows only one-electron (Ni is not electrochemically active in the accessible voltage range). Iron-free PBA Na2MnII[MnII(CN)6] is also known. It has a fairly large reversible capacity of 209 mAh/g at C/5, but its voltage is unfortunately low (1.8 V versus Na+/Na).
Electrolytes Sodium-ion batteries can use
aqueous and non-aqueous electrolytes. The limited
electrochemical stability window of water results in lower voltages and limited energy densities. Non-aqueous
carbonate ester polar aprotic solvents extend the voltage range. These include
ethylene carbonate,
dimethyl carbonate,
diethyl carbonate, and
propylene carbonate. The most widely used salts in non-aqueous electrolytes are NaClO4 and
sodium hexafluorophosphate (NaPF6) dissolved in a mixture of these solvents. It is a well-established fact that these carbonate-based electrolytes are flammable, which pose safety concerns in large-scale applications. A type of
glyme-based electrolyte, with
sodium tetrafluoroborate as the salt is demonstrated to be non-flammable. In addition, NaTFSI (TFSI = bis(trifluoromethane)sulfonimide) and NaFSI (FSI = bis(fluorosulfonyl)imide, NaDFOB (DFOB = difluoro(oxalato)borate) and NaBOB (bis(oxalato)borate) anions have emerged lately as new interesting salts. Electrolyte additives can be used as well to improve the performance metrics.
Aqueous sodium-ion batteries Aqueous sodium-ion batteries (ASIBs) have gained significant attention in energy storage and conversion because they offer high safety, low cost, and improved environmental compatibility. Cathodes represent the primary constraint on ASIB performance. Intercalation-type materials offer only a finite number of Na+ storage sites, which limits the extent to which their specific capacity can be improved. Sodium transition-metal oxides (NaxMO2) are among the most extensively studied ASIB cathodes due to their open structures, electrochemical stability, high working voltage, and lower cost compared with lithium analogues (75). Their properties can be tuned by varying Na content, yielding layered oxides (x 0.5). Layered P2- and O3-type oxides offer high capacities and fast Na+ diffusion (76), illustrated by P2-Na2⁄3Ni1⁄3Mn2⁄3O2 delivering 157 mAh/g in Na2SO4 electrolyte (70) and P2-Na2⁄3Ni1⁄4Mn3⁄4O2 achieving a 1.2 V full-cell voltage when paired with NTP/graphite in a hybrid Na2SO4/Li2SO4 electrolyte. However, P2 phases undergo P2→O2 transitions at low Na content, while O3 phases such as NaMnO2 suffer from air/moisture instability; heteroatom substitution, such as Cu/Ti-doping in NaNi0.45Mn0.5O2, significantly improves air stability and cycling performance. Tunnel oxides like Na0.44MnO2 enable rapid Na+ transport and excellent cycling, achieving stable capacities in Na2SO4 and NaOH electrolytes, and their performance can be enhanced through Ti substitution and Na-rich compositions or even extended to potassium-based analogues such as K0.27MnO2. Prussian blue analogs (PBAs), with their open 3D framework, fast kinetics, and facile synthesis, offer capacities up to ~80 mAh/g, and some two-electron PBAs reach theoretical capacities of 170 mAh/g; reducing defects and adding Co2+ can greatly improve stability and capacity retention. Polyanionic compounds—including phosphates, fluorophosphates, and NASICON-type materials—provide stable 3D host structures and high voltage operation, though they often face interfacial resistance and transition-metal dissolution issues. Improvements through carbon coating and metal substitution have enabled materials like NaFePO4 to reach high reversible capacities and favorable high-temperature performance, while fluorophosphates such as Na3V2(PO4)2F3-SWCNT deliver higher working voltages. Recent advances in mixed phosphate–pyrophosphate frameworks, such as Na4Fe3(PO4)2P2O7, have demonstrated high power density, long cycle life, and even low-temperature operation. Graphite is generally not used as an anode in ASIBs, as the NaxC compounds it forms are thermodynamically unstable. This instability leads to low reversible capacity and an unfavorable reaction potential. Activated carbon (AC) is a structurally simple and easily manufactured carbon material that can be paired with suitable cathodes to form asymmetric hybrid capacitor–battery systems. For example, a Na4Mn9O18//Na2SO4/AC supercapacitor achieved an energy density of 34.8 Wh/kg and exhibited excellent cycling stability, retaining 84% of its capacity after 4,000 cycles at 18 C. In addition, other carbon-based materials—such as carbon microbeads and carbon fibers—have also been explored as potential anodes for sodium storage. However, many carbon materials still face challenges including low initial Coulombic efficiency and sluggish Na+ intercalation kinetics. Nanoengineering offers effective solutions to these limitations by shortening Na+ and electron diffusion pathways, creating reticular architectures that improve mechanical robustness and buffer volume changes during cycling, and increasing surface area and active sites. These structural advantages make nanoscale engineering an essential strategy for enhancing the electrochemical performance of carbon-based anodes in ASIBs. == Comparison ==