Coal-based hard carbon anodes for sodium-ion batteries: A review on fabrication, sodium storage mechanism, and defect engineering

Sodium-ion batteries (SIBs) are a cost-effective alternative to lithium-ion systems for grid-scale energy storage, yet their commercial viability depends on high-performance anodes. Hard carbon, with its disordered structure, expanded interlayer spacing, and tunable porosity, is the most promising candidate. Among various precursors, coal offers high carbon yield, low cost, abundant supply, and structural tunability. This review critically analyzes coal-based hard carbon anodes for SIBs, covering historical development, battery components, and sodium storage mechanisms including adsorption, intercalation, and pore-filling models. Synthesis routes (direct carbonization, indirect carbonization, and rapid pyrolysis) are compared for their effects on microstructure and electrochemical performance. Key challenges low initial Coulombic efficiency, poor rate capability, voltage hysteresis, and precursor heterogeneity are critically examined. Advanced modification strategies are highlighted: porous structure engineering, heteroatom doping (N, P, B, S), oxygen-containing functional group regulation, and molecular crosslinking. The role of theoretical calculations (DFT and MD) in guiding defect engineering is also emphasized. Finally, we outline future perspectives, including precise microstructure engineering, high ICE strategies, high-mass-loading electrodes, sustainable manufacturing, and low-temperature performance, establishing a comprehensive framework for rational design of coal-derived hard carbon anodes.