Anode materials for sodium-ion batteries

There are various types of anode materials for sodium-ion batteries, and carbon-based materials have the best comprehensive performance. The anode materials of sodium-ion batteries include metallic compounds, carbon-based materials, alloy materials and non-metallic elements. Among them, carbon-based materials have become the best anode materials for sodium-ion batteries due to their wide sources and strong sodium storage capacity. Other anode materials, such as metal compounds, alloy materials and non-metallic elements, all have the problem of volume expansion, which poses a risk of electrode fracture during charging and discharging. Due to thermodynamic reasons, sodium ions cannot reversibly intercalate/deintercalate between the graphite layers of the negative electrode. Therefore, traditional graphite cannot be used as the anode material for sodium-ion batteries. The reversible specific capacity of sodium ions in modified graphite materials remains relatively low, resulting in a significant increase in cost. Therefore, the application of graphite anodes in the field of sodium-ion batteries is currently limited.

1. Graphite type

Lithium ions are inserted into the graphite anode to form the LiC6 structure, with a theoretical capacity of 372mAh·g-1. Compared with lithium, the radius of sodium is much larger. The interlayer distance of graphite carbon (0.335nm) is not suitable for the insertion of sodium ions. Asher investigated the incorporation of sodium ions into graphite and the complete reaction of excess sodium pairs with graphite, but no NaC6 was obtained; only the higher-order NaC64 intercalation compound was produced. Fouletier et al. found that the intercalation amount of sodium ions in crystalline graphite was very small, with a capacity of only 35 mAh/g. Therefore, graphite is not suitable as the anode material for sodium-ion batteries. In practical research, non-graphite carbon materials are mainly used.

2. Disordered carbon materials

Disordered carbon materials mainly include two types: hard carbon (resin carbon, smoke dust, etc.) and soft carbon (coke, graphitized mesocarbon microspheres, carbon fibers, etc.). In lithium-ion batteries, the first specific capacity of these two carbon materials typically reaches 500-900 mah /g, but their cycle life varies depending on the specific structure and morphology of the carbon materials. Hard coal has received extensive attention because of its large interlayer spacing and irregular structure, which is suitable for sodium ion decalcification.

2.1 Hard carbon anode materials

Hard carbon refers to carbon materials that are difficult to graphitize even at temperatures above 2500℃. Hard carbon materials have a large interlayer spacing, and their internal structure is characterized by short-distance arrangement and long-distance study. The carbon microcrystals are randomly arranged. The storage of hard carbon sodium mainly comes from the defect structure of the material, the edges of the graphite microcrystals, the graphite interlayers and the internal pores. Hard carbon anode materials have advantages such as low sodium storage potential, high sodium storage specific capacity, wide range of precursors, and low cost, and are hailed as the most promising sodium storage anode materials.

Irregular Hard Carbon Powder XRD Pattern

2.2 Hard carbon precursors

The precursor materials of hard carbon are very extensive, mainly including biomass, resin and organic polymer. In recent years, in order to develop hard carbon precursor materials with excellent performance, many precursor materials have been studied and reported. Comprehensive comparison shows that corn starch has a relatively high first-round efficiency and reversible capacity at a current density of 30 mA/g, but its stability is poor under long-term cycling. Asphalt coating resin materials have the advantage of high stability, but their first-round Coulombic efficiency is low. The shell of mangosteen has good performance in all aspects.

2.4 Deficiencies and Optimization of Hard Carbon

Hard carbon is usually obtained from precursors with a strongly cross-linked structure. Graphite structures cannot be formed even at high temperatures. Many experiments have shown that increasing the graphitization degree of hard carbon materials and reducing defective graphite structures can improve the primary Coulombic efficiency (ICE). Hard carbon materials treated at high temperatures often have better ICE indicators. Hard carbon materials pyrolyzed at low temperatures usually have a large specific surface area and a relatively low degree of graphitization. A large number of defects lead to the formation of a relatively large SEI film on the surface of hard carbon materials when used as anode materials, resulting in a relatively low ICE. Therefore, increasing the carbonization temperature is the basic method to increase HC as the anode material.

3. Soft carbon

Soft carbon generally refers to carbon materials with a loose structure, significant deformability and excellent elasticity. Such carbon materials can undergo graphitization at temperatures as high as 2800℃, thereby exhibiting amorphous properties. Therefore, they are also known as graphitizable carbon. It has the characteristics of high surface area, porosity and high deformability. Usually, it has a higher carbon yield than hard carbon and shows more sp2 carbon bonds. Therefore, soft carbon exhibits better rate performance and electrical conductivity than hard carbon. Saurel et al. compared the X-ray diffraction patterns, microstructures and chargate-discharge curves of hard carbon, soft carbon and graphite. In the anode materials of sodium-ion batteries, the sodium storage capacity of hard carbon was significantly higher than that of soft carbon and graphite. However, due to its structural reasons, graphite showed extremely poor electrochemical performance and could not be used as the anode carbon material for sodium-ion batteries.

3.1. Sodium Storage Mechanism of Soft Carbon

Compared with hard carbon, the sodium storage behavior of soft carbon is relatively simple. On the one hand, the carbon layer spacing inside soft carbon is relatively short, and sodium ions cannot be embedded between the carbon layers. Therefore, the sodium storage behavior of soft carbon only involves the adsorption of defect sites and the filling of micropores. On the other hand, it is because the charge-discharge curve of soft carbon only has one section (the slope area), while that of hard carbon has two sections: the slope area and the platform area. Therefore, in recent years, researchers have basically held the same view on its sodium storage mechanism.

3.2 Deficiencies and Optimization of Soft Carbon

Soft carbon precursors mainly include products from the petroleum industry, such as coal, asphalt and petroleum coke. The shortcomings of soft carbon anode materials are also quite obvious. Their relatively low reversible capacity limits their commercialization. During the cycling process of sodium-ion battery soft carbon anodes, problems such as capacity attenuation and structural damage may also occur, which affect the cycle life and stability of the battery. Therefore, improving the cycle stability of soft carbon anode materials is a key challenge for commercialization

4. Nanostructured carbon materials

Due to the good structural stability and excellent electrical conductivity of structures such as nanowires, nanotubes and nanosheets, and the large specific surface area of nanomaterials, which can effectively reduce the diffusion path of ions, carbon-based materials with nanostructures can effectively improve electrochemical performance. Compared with the carbon-based materials reported previously, carbon materials with one-dimensional or two-dimensional nanostructures have more excellent lithium/sodium storage performance. Carbon nanotubes (CNTs), carbon nanofibers (CNFs), carbon derivatives (CDCs), and graphene have been widely studied in lithium-ion batteries and have high charge and discharge capacities. In sodium-ion battery systems, nanostructured carbon materials have also received attention.

5. Metal or alloy materials

Early research on the anode of sodium-ion batteries mainly focused on carbon-based materials. However, carbon-based materials generally have problems of low capacity and poor cycle performance. Researchers actively developed new anode materials to replace pure carbon-based materials. Metallic elements or alloy materials have become a research hotspot in recent years due to their high specific capacity. Lithium can form intermetallic compounds LixM(M =Si, Ge, Sn and Pb, etc.) with many metals at room temperature. Since the formation reactions of lithium alloys are usually reversible, they can be used as anode materials for lithium-ion batteries.

6. Metal oxides

Transition metal oxides have long been widely studied as anode materials for lithium-ion batteries due to their high capacity. This type of material can also be used as a promising sodium embedding material for sodium-ion batteries. Unlike the deintercalation reactions of carbon-based materials and the alloying reactions of alloy materials, transition metal oxides mainly undergo reversible REDOX reactions. So far, the transition metal oxides used as electrode materials for sodium-ion batteries are relatively few. The main cathode materials are: hollow γ-Fe2O3 and V2O5, and the main anode materials are TiO2, α-MoO3, SnO2, etc. TiO2 has the advantages of stability, non-toxicity, low cost and abundant content. It has low solubility in organic electrolytes and high theoretical energy density, and has always been a research hotspot in the field of lithium-intercalating materials.