Sodium-ion batteries have emerged as a promising alternative to lithium-ion batteries due to the abundance and lower cost of sodium. A critical component influencing their performance is the anode material, which must efficiently store and release sodium ions while maintaining structural integrity. Three primary categories of anode materials are being explored: hard carbon, alloy-based anodes, and conversion-type materials. Each presents unique advantages and challenges in terms of capacity, stability, and kinetics.

Hard carbon stands out as the most advanced anode material for sodium-ion batteries. Its disordered microstructure with randomly oriented graphene layers creates ample interlayer spacing, facilitating sodium ion insertion and extraction. Hard carbon anodes typically deliver reversible capacities between 250 and 300 mAh/g, with some optimized variants reaching up to 350 mAh/g. The material exhibits a sloping voltage profile during sodiation and desodiation, which differs from the plateau behavior seen in graphite for lithium-ion batteries. A key advantage of hard carbon is its minimal volume expansion, usually below 10%, ensuring good cycling stability. However, initial Coulombic efficiency remains a challenge, often ranging from 70% to 85% due to irreversible sodium trapping and solid electrolyte interface formation. Strategies such as pre-sodiation, surface coating, and pore structure optimization have been employed to improve efficiency.

Alloy-based anodes, including materials like tin, antimony, and phosphorus, offer significantly higher theoretical capacities compared to hard carbon. For instance, phosphorus can reach a capacity of 2596 mAh/g by forming Na3P, while tin and antimony deliver 847 mAh/g and 660 mAh/g, respectively. These materials react with sodium through alloying mechanisms, enabling high energy density. However, the substantial volume expansion during cycling—often exceeding 300%—leads to particle pulverization and rapid capacity fade. To mitigate this, researchers have developed nanostructured composites, such as embedding active particles in a conductive carbon matrix or designing porous architectures. These approaches help accommodate mechanical stress and maintain electrical contact. Another challenge is the sluggish sodiation kinetics, which can be improved by reducing particle size and enhancing interfacial conductivity. Despite these efforts, alloy-based anodes still struggle with poor cycling stability and low initial Coulombic efficiency, typically below 80%.

Conversion-type anodes, including metal oxides, sulfides, and phosphides, operate through redox reactions where sodium ions react with the host material to form new compounds. For example, iron oxide (Fe2O3) can deliver a theoretical capacity of 1007 mAh/g, while molybdenum disulfide (MoS2) offers around 670 mAh/g. These materials often exhibit higher capacities than hard carbon but face challenges related to large voltage hysteresis and irreversible phase transformations. The conversion reactions are typically accompanied by significant volume changes and slow kinetics, leading to rapid performance degradation. Nanostructuring and hybridization with conductive materials like graphene have shown promise in enhancing cyclability and rate capability. However, conversion-type anodes generally suffer from low initial Coulombic efficiency, often below 70%, due to irreversible side reactions and electrolyte decomposition.

Comparing sodium-ion anode materials with lithium-ion graphite anodes highlights fundamental differences. Graphite, the dominant lithium-ion anode, has a layered structure that allows efficient lithium intercalation with minimal volume expansion (~10%) and high Coulombic efficiency (>90%). However, graphite performs poorly in sodium-ion systems due to insufficient interlayer spacing for sodium ions. Hard carbon, while less efficient than graphite, remains the most viable alternative for sodium-ion batteries. Alloy-based and conversion-type anodes offer higher capacities but face greater challenges in stability and kinetics compared to their lithium-ion counterparts, such as silicon or lithium metal anodes.

Volume expansion is a universal challenge for high-capacity sodium-ion anodes, particularly for alloy and conversion materials. The repeated expansion and contraction during cycling cause mechanical degradation, leading to electrode disintegration and loss of electrical contact. Strategies to address this include designing buffer structures, using elastic binders, and engineering porous electrodes to absorb strain. Sodiation kinetics also play a crucial role in determining rate capability. Materials with slow diffusion or high charge transfer resistance suffer from poor performance at high current densities. Enhancing ionic and electronic conductivity through doping, coating, or nanostructuring is essential for improving kinetics.

Coulombic efficiency is another critical parameter, especially for practical applications. Low initial efficiency results in sodium loss, reducing overall energy density. Pre-treatment methods, such as pre-sodiation or pre-cycling, can help mitigate this issue. Long-term cycling stability remains a hurdle for alloy and conversion anodes, where capacity fade often exceeds 0.5% per cycle. In contrast, hard carbon demonstrates better stability, with some studies reporting capacity retention above 80% after 500 cycles.

In summary, hard carbon is the leading anode material for sodium-ion batteries due to its balance of capacity, stability, and kinetics. Alloy-based and conversion-type materials offer higher capacities but require further development to overcome volume expansion and efficiency challenges. While sodium-ion anodes currently lag behind lithium-ion graphite in performance, ongoing research aims to close the gap through material engineering and electrode design. The choice of anode material will ultimately depend on the specific application requirements, balancing energy density, cycle life, and cost. Advances in these materials will be crucial for the commercialization of sodium-ion batteries as a competitive energy storage technology.