From microstructure to sodium-storage mechanism: Structure-performance insights into hard carbon

Hard carbon is the most promising anode material for sodium-ion batteries, yet its complex disordered structure has long obscured the actual sodium-ion storage behaviors. This review focuses on the critical structure-performance correlation that governs electrochemical performance. It examines how key microstructural features-including pseudo-graphitic domains, interlayer spacing, defects, heteroatom doping, and multi-scale pore architecture-collectively determine sloping/plateau capacities, initial Coulombic efficiency, and rate capability. The review highlights recent advances in decoupling these complex relationships through advanced characterization, predictive descriptors, and machine learning. Looking into the future, it is necessary to shift from empirical optimization methods toward quantifiable and predictable design principles for developing high-performance hard carbon anodes. • The structure-performance correlation of hard carbon anode materials for sodium-ion batteries has been summarized. • The classic sodium storage models of hard carbon and long-standing mechanism disputes have been discussed. • Microstructural parameters that determine the sodium storage behaviors have been discussed. • This review article aims to provide a universal design principle for the construction and optimization of hard carbon.