Research and application progress of electrolyte solution for hard carbon anodes in sodium-ion batteries

Against the backdrop of the global energy transition and the scarcity of lithium resources, sodium-ion batteries (SIBs) have emerged as a crucial alternative energy storage technology. Graphite, the anode material used in commercialized lithium-ion batteries (LIBs), exhibits a Na⁺ storage specific capacity of less than 40 mAh g^-1 due to the difficulty of Na⁺ intercalation. In contrast, hard carbon, possessing a short-range ordered and long-range disordered structure, achieves a Na⁺ storage specific capacity of 200-350 mAh g^-1 along with excellent cycling and rate performance, making it the core anode material for the commercialization of SIBs. Moreover, by acting as an ion transport channel and participating in the construction of the electrolyte solution/hard carbon interface, the electrolyte formulation directly affects the performance of the hard carbon anode. This paper reviews the research progress of electrolyte solution for hard carbon (HC) anodes in SIBs and systematically compares five categories of electrolyte solution systems, namely classic carbonate ester electrolyte solution, ether-based electrolyte solution, high-concentration electrolyte solution, phosphate-based electrolyte solution, and new-type electrolyte solution such as carboxylate, sulfone, and nitrile-based electrolyte solutions. Meanwhile, it sorts out the characteristics and optimization paths of the aforementioned electrolyte solutions, which is of great significance for breaking through interface bottlenecks and promoting the development of SIBs.