From Sodium Storage Mechanism to Design of High-Capacity Carbon-Based Anode: A Review

Sodium-ion batteries (SIBs) have emerged as a viable alternative to lithium-ion technologies, with carbon-based anodes playing a pivotal role in addressing key challenges of sodium storage. This review systematically examines hard carbon as the premier anode material, elucidating its dual sodium storage mechanisms: (1) sloping capacity (2.0-0.1 V vs. Na⁺/Na) from surface/defect adsorption and (2) plateau capacity (<0.1 V) via closed-pore filling and pseudo-graphitic intercalation. Through critical analysis of recent advancements, we establish that optimized hard carbon architectures delivering 300-400 mAh/g capacity require precise coordination of pseudo-graphitic domains (d₀₀₂ = 0.36-0.40 nm) and <1 nm closed pores. This review ultimately provides a design blueprint for next-generation carbon anodes, proposing three research frontiers: (1) machine learning-guided microstructure optimization, (2) dynamic sodiation/desodiation control in sub nm pores, and (3) scalable manufacturing of heteroatom-doped architectures with engineered pseudo-graphitic domains. These advancements position hard carbon anodes as critical enablers for high-performance, cost-effective SIBs in grid-scale energy storage applications.