Hard-Carbon Anodes for Sodium-Ion Batteries: Recent Status and Challenging Perspectives

Sodium-ion batteries (SIBs) hold great potential in the application of large-scale energy storage. With the coming commercialization of SIBs, developing advanced anode of particularly hard carbon is becoming increasingly urgent yet challenging. Hard carbon still suffers from unclear sodium storage mechanism, unsatisfactory performance, and low initial Coulombic efficiency (ICE). Herein, the current state-of-the-art advances in designing hard carbon anodes for high-performance SIBs is summarized. First, the formation process of hard carbon and typical sodium storage models of "insertion-adsorption," "adsorption-insertion," "adsorption-pore filling," and "adsorption-insertion-pore filling" are introduced systematically. Then, the key strategies including morphological engineering, heteroatom doping, and graphitic structure regulation are presented to enhance the capacity of hard carbon based on the in-depth understanding of sodium storage behaviors. Subsequently, to promote the practical application of hard carbon, more attention is paid to the methods of ICE improvement, including electrolyte optimization, defect and surface engineering, and presodiation. Whereafter, hard-carbon-based SIBs and their intriguing applications are briefly sketched. Finally, future directions and challenging perspectives of hard-carbon anodes for SIBs are proposed from the viewpoints of storage mechanisms, electrode structures, and presodiation techniques.