Designing better anodes for sodium-ion batteries: Carbon materials, carbon–metal oxide composites and predictive modeling insights

Sodium-ion batteries are gaining attention as affordable and sustainable alternatives to lithium-ion batteries, largely due to sodium’s natural abundance and comparable electrochemical behavior. However, their widespread adoption remains limited by issues such as cathode instability, low initial coulombic efficiency of hard-carbon anodes, moderate cycle life, and sluggish kinetics. This review critically examines 3 key categories of hard-carbon anodes, including biomass-derived carbons, heteroatom-doped carbons (N, P, S), and carbon–metal oxide composites. It highlights the relationship between structural design and electrochemical performance through both experimental and computational insights. The discussion introduces major sodium storage models: intercalation–filling, adsorption–intercalation, and 3-stage, and explores how factors such as precursor type, synthesis route, and structural parameters of hard carbons (interlayer spacing, defect density, and pore structure) influence performance. Biomass-derived hard carbons are emphasized for their sustainability and storage efficiency, while heteroatom doping and metal oxide composites are shown to improve capacity and cycling stability by tuning electronic and defect structures. The review concludes with strategies for designing advanced hard carbon anode materials to overcome existing limitations. Emerging applications of artificial intelligence and machine learning are also discussed as transformative tools for predictive modeling and optimization of next-generation sodium-ion anodes.