Precursor-Dependent Initial Coulombic Efficiency of Hard Carbon Anodes for Sodium-Ion Batteries: A Comparative Review

Hard carbon has been widely recognized as the most commercially viable anode material for sodium-ion batteries (SIBs); however, its inherently low initial Coulombic efficiency (ICE), typically 60-90%, remains a critical bottleneck constraining practical full-cell deployment. While extensive research has addressed ICE optimization, existing reviews have predominantly focused on individual precursor types or isolated strategies, lacking a unified cross-precursor comparative framework. This review systematically deconstructs the complete causal continua-from chemical composition through carbonization trajectories and microstructural evolution to ultimate ICE outcomes-across five major precursor categories: biomass, synthetic resins, pitches, coal-based materials, and saccharides. An "SSA-closed pore-defect" three-parameter trade-off framework is proposed to elucidate the microstructural origins of precursor-dependent ICE divergences. Cross-categorical benchmarking reveals that resin-based precursors achieve the highest ICE (95%) through ultra-low specific surface area and extensive closed porosity, pitch-based systems deliver the most consistent ICE distribution (86-91%), and coal-derived carbons are confined to the lowest tier (78-85%). The differentiated efficacy of carbonization conditions and post-treatment strategies across precursor types is critically evaluated, demonstrating that optimal process selection is inextricably linked to precursor taxonomy. Building upon these analyses, a precursor selection decision roadmap targeting three application-specific ICE thresholds is constructed, providing actionable guidance for matching precursor-process combinations to industrial requirements. The comparative framework is grounded in 25 representative studies selected through explicit inclusion criteria (detailed in the Introduction), and its predictive utility is illustrated for emerging precursor candidates beyond the five canonical categories. This cross-precursor perspective offers a systematic reference for accelerating the commercialization of hard carbon anodes in SIBs.