Progress and Challenges in Sodium-Ion Battery Anodes: Mechanisms, Materials, and Design Strategies

Global energy storage demands are rising rapidly, driving interest in sustainable and low-cost battery technologies to complement lithium-ion systems. Sodium-ion batteries (SIBs) offer a promising route due to the wide availability and low cost of sodium. Their larger ionic radius, however, introduces challenges related to sluggish diffusion kinetics and structural instability, making continued anode development essential. This review summarizes recent progress in SIB anodes, focusing on the fundamental sodium storage mechanisms of intercalation, alloying, and conversion. Key material classes are examined, from hard carbons to high-capacity alloying systems (Sn, Sb, P) and conversion compounds (MoS₂, Fe₂O₃). Critical strategies for improving performance are discussed, including nanostructuring, carbon hybridization, doping, and electrolyte optimization. Despite laboratory advances, barriers to practical application remain, including low energy density, limited Coulombic efficiency, and solid electrolyte interphase instability. A roadmap for future work is provided, with emphasis on full-cell testing, computational tools, and scalable processing methods.