Solid Electrolyte Interphase Composition Regulation via Coating AlF3 for a High-Performance Hard Carbon Anode in Sodium-Ion Batteries

Hard carbon (HC) is considered to be an anode material with great promise for sodium-ion batteries (SIBs), on account of the advantages of high reversible specific capacity and abundant source. Nevertheless, its poor high-rate and long-term cycle performance, which were caused by the unrestricted decomposition of the electrolyte and sustained formation of the solid–electrolyte interface (SEI) film, limit the practical use. Herein, the balance between high mechanical strength and good flexibility of the SEI layer formed in the discharge/charge process was achieved by modifying the surface of the HC anode with a proper amount of AlF3 coating. In this way, the reversible specific capacity increased from 322 to 346 mA h g–1 at 20 mA g–1. Furthermore, the capacity retention at 100 mA g–1 for 100 cycles of the modified sample considerably increased from 47.4 to 76.5%, as well as rate performance was significantly improved from 69 to 135 mA h g–1 at 200 mA g–1. The improvement of performance is mainly attributed to the rational distribution of organic–inorganic phase components in the SEI film (about 43% inorganic phase and 57% organic phase), which ensures the stability of the SEI layer in the continuous discharge/charge process, resulting in a superior performance. Therefore, this work not only provides an effective method to synthesize HC anode with high electrochemical performance in SIBs but also clearly illustrates the mechanism of how the coating affects the composition of the SEI film to improve the performance of materials.