Electrolyte Engineering of Hard Carbon for Sodium-Ion Batteries: From Mechanism Analysis to Design Strategies

The hard carbon (HC) anodes with desirable electrochemical performances including high initial Coulombic efficiency, superior rate performance and long-term cycling play an indispensable role in the practical application of sodium ion batteries (SIBs), which are closely related to the electrolytes them matched. Fully analyzing the mechanism of electrolyte engineering for HC anodes is crucial for promoting the commercialization of SIBs, but is still lacking. In this review, the correlation between physicochemical properties of the electrolyte and the electrochemical performance of HC is first summarized. And point out the crucial role of electrolyte properties, including ion conductivity, de-solvation energy, and interface passivation ability for the Na+ storage in HC. Then, the formation process, composition, as well as structure of solid electrolyte interphase (SEI) on HC surface are mainly discussed, and the structure-activity relationship of SEI is analyzed in depth. Moreover, based on the mechanism analysis, relevant electrolyte design strategies have been summarized. Finally, the challenges and future development directions of the electrolyte engineering of HC are proposed. This review is expected to provide professional theoretical guidance for the development of electrolyte and contribute to the rational design of high-performance HC anodes, promoting the industrialization of SIBs.