In 2023, Haiji New Energy and Zhongke Haina successively released new sodium-ion battery products, with shapes including square and cylindrical, energy density of 115-155Wh/kg, and cycle life of 2000-6000 cycles.
Meanwhile, the application of sodium-ion batteries in low-speed vehicles and other small-power applications is accelerating. Haiji New Energy and Zhongke Haina's sodium battery products have been successfully applied in electric bicycles and electric vehicles, respectively.
It is expected that after 2025, with the further improvement in the energy density and cycle life of sodium-ion batteries, their large-scale application in the field of energy storage will also begin.
However, the current cost of sodium-ion batteries is around 0.84 yuan/Wh, which is higher than that of lithium iron phosphate batteries and lead-acid batteries, making it difficult for them to leverage their cost advantage in the energy storage field.
To further reduce the cost of sodium-ion batteries, anode materials are an important area for breakthrough.
Four major anode materials for sodium-ion batteries
The most common anode materials on the market are graphite and silicon, but both have low sodium storage capacity. In order to promote the industrial application of sodium-ion batteries, the market urgently needs to develop sodium anode materials with high safety and strong performance.
Currently, there are four main types of sodium-ion battery anode materials on the market, based on the charging and discharging mechanism in sodium batteries: intercalation materials, alloying materials, conversion materials, and organic materials.
Sodium battery intercalation anode material
Intercalation materials, as the name suggests, are negative electrode materials that undergo sodium ion intercalation reactions, such as carbon-based materials and titanium-based oxides.
The advantage of this type of material is that during the sodium ion insertion process, parameters such as bond distance, cell volume, crystal phase, and interplanar spacing do not change, and the material volume expansion is small during charging and discharging; the disadvantage is that this type of material has a low specific capacity.
Specifically, in terms of carbon-based materials, sodium-ion batteries choose carbon-based materials (graphite, expanded graphite, non-graphitized carbon, carbon nanomaterials, and carbon-based organometallic frameworks) as the negative electrode. The main reason is that sodium batteries and lithium-ion batteries have similar working principles, while graphite materials have been commercialized in lithium-ion batteries and have low operating voltage and stable chemical and thermodynamic properties.
Currently, carbon-based materials are the preferred anode materials for sodium-ion batteries. However, there is controversy in the market regarding their sodium storage mechanism, namely the debate between the "intercalation-adsorption" principle and the "adsorption-intercalation" principle.
The "intercalation-adsorption" principle suggests that the sodium storage process of carbon-based materials includes two stages. The first is the slope region of the charge-discharge curve, corresponding to sodium ions intercalating into the disordered graphene sheets; the second is the plateau region where sodium ions fill the nanopores.
The "adsorption-intercalation" mechanism suggests that the slope region of charging and discharging corresponds to sodium ions filling the nanopores of carbon-based materials, while the plateau region corresponds to sodium ions intercalating into disordered graphite microcrystals.
Titanium-based oxides have many advantages when used as anode materials for sodium-ion batteries, including reasonable operating voltage, low cost, and non-toxicity.
Research has found that among numerous titanium-based oxides, anatase TiO₂ with nano-processing, spinel lithium titanate with reduced particle size or carbon doping (Li₄Ti₅O₁₂), and Na₂Ti₂O₇ are all promising anode materials for sodium-ion batteries, with a maximum specific capacity of up to 311 mAh/g.
Sodium battery alloyed anode material
Alloyed materials mainly refer to elements in the periodic table IVA such as Si, Ge, Sn, and Pb, and elements in the VA such as P, As, Sb, and Bi. Sodium can react with these elements to form alloy compounds.
The advantage of this type of material is that each atom can react with multiple sodium ions, resulting in a high capacity of 300-2000 mAh/g; the disadvantage is that the material expands significantly during charging and discharging, resulting in poor cycle performance of sodium batteries.
Specifically, Si has an extremely low theoretical specific capacity, Ge's capacity retention drops sharply after cycling, and As is a carcinogen, making them all unsuitable as anode materials for sodium-ion batteries.
The theoretical capacity of Na15Sn4, formed by complete sodiumification of Sn, is as high as 847 mAh/g; the Pb sodium battery anode material has a reversible capacity of 477 mAh/g at a current density of 13 mA/g, and the capacity retention rate is as high as 98.5% after 50 cycles; the theoretical specific capacity of Sb is 660 mAh/g; and the theoretical specific capacity of P is as high as 2596 mAh/g. All of these are highly promising sodium-ion battery anode materials.
Sodium battery conversion anode materials
Conversion materials refer to metal oxides, metal sulfides, metal selenides, and metal phosphides that can undergo conversion reactions to store sodium.
The advantage of this type of material is that more electrons participate in the reaction, resulting in a high specific capacity of 200-1800 mAh/g; the disadvantage is that the material expands significantly during charging and discharging, leading to poor cycle performance of sodium batteries.
Metal oxides such as NiCo2O4, Sb2O3, Co3O4, Fe3O4, Fe2O3, SnO, SnO2, NiO, CuO, MoO3, and MnO2 generally need to be used in combination with nanotechnology, carbon coating, and composite technology to eliminate the mechanical stress caused by volume changes during the reaction process and enhance the intrinsic conductivity of the material.
Metal sulfides such as FeS, SnS2, CoS, Ni2S3, MoS2, ZnS, TiS2, WS2, and Sb2S3 have higher intrinsic conductivity and smaller volume expansion compared to metal oxides. Their sodium storage performance can be improved by combining nano-sizing, special morphology, and carbon coating techniques.
Metal selenides such as SnSe, Sb₂Se₃, MoSe₂, FeSe₂, ZnSe, and NiSe possess a layered structure, exhibiting better electrical conductivity, rate performance, and coulombic efficiency than sulfides. The material properties can be further enhanced by introducing carbon conductive networks and preparing specific morphologies.
Metal phosphides such as Se4P4 and Sn4P3 have good electrical conductivity. The presence of metal can effectively buffer the volume change of the negative electrode material during charging and discharging. The alloying reaction that occurs together with the conversion reaction can also improve the specific capacity of the material.
Sodium battery organic compound anode materials
Organic compound materials mainly include small organic molecule compounds and polymers (Schiff base compounds, polyamides and polyquinones, conductive polymers, etc.).
The advantages of this type of material are that it is widely available, low in cost, diverse in structure, and can undergo multi-electron reactions, resulting in excellent electrochemical performance. The disadvantages are that the material has extremely low electronic conductivity, its volume expands greatly during charging and discharging, which can lead to material pulverization, and it has poor stability in organic solvents.
Organic small molecule compounds, represented by copolycarboxylates, have high reversible specific capacity (sodium terephthalate can reach 250 mAh/g) and excellent cycling performance. The problem of low coulombic efficiency can be solved by coating the surface of sodium terephthalate with an Al2O3 nanolayer using atomic layer deposition.
Regarding the sodium storage performance of Schiff base compounds, the reversible specific capacity can reach 350 mAh/g under certain conditions, while the capacity retention rate of polyamides is relatively high after cycling, reaching 90% after 500 cycles under certain conditions.
Development direction of sodium-ion battery anode materials
While the anode material accounts for 14% of the total cost of sodium-ion batteries, its contribution to the overall performance of the battery is far more than 14%.
An ideal sodium-ion battery anode material should meet four conditions:
1. The elements in the negative electrode material should be lightweight and have low density so that more sodium ions can be stored per unit volume, enabling sodium-ion batteries to obtain stable high volumetric capacity and mass capacity.
2. To improve the working voltage of sodium batteries, the potential of the negative electrode material should be close to that of metallic sodium.
3. The negative electrode material must be stable in the electrolyte solvent.
4. The materials should be low in cost, environmentally friendly, and have high electronic and ionic conductivity.
Based on the above four conditions, embedded materials, alloyed materials, conversion materials, and organic materials each have their own advantages and disadvantages.
Among these, carbon-based materials, particularly those used in embedded materials, offer abundant raw materials, low cost, environmental friendliness, and stable properties. However, their specific capacity still has room for improvement, and their cycle stability and initial coulombic efficiency remain relatively low. A prerequisite for these three improvements is a deeper understanding of the sodium storage mechanism of carbon-based materials, which requires further research and development.
Furthermore, the theoretical specific capacity of conversion-type materials can reach 800-1200 mAh/g, while the theoretical specific capacity of phosphorus in alloyed materials is as high as 2569 mAh/g. However, the huge volume change of these materials during charge and discharge is far less than the 20% of the commercial standard. Nanomaterials, hollow or porous structures, and carbon coating are effective solutions to this problem.
