Fluorinated sodium aluminate main conducting salt boosting sodium storage of hard carbon

Tuning electrolyte components plays a very important role in improving the sodium storage of hard carbon (HC) anode. Herein, we unprecedentedly synthesize a fluorinated moisture resistant aluminum-centered sodium salt of sodium bis(perfluoropinacolato)aluminate (NaAl[OC(CF3)2]4, NaBPPA), which enhances the sodium storage and specific capacity of HC anode when serving as main conducting salt in electrolyte. It is revealed that NaBPPA induces the formation of a mechanically robust and fast Na+ conducting SEI layer on HC anode. In specific, the NaBPPA-based electrolyte facilitates the enrichment of unique inorganic species (such as NaAlF4, NaF, AlF3 and Al2O3), which endow the SEI layer with high Young's modulus and fast Na+ transportation. Thus, the problem of SEI layer dissolution is alleviated and the formation of quasi-metallic Na clusters at low potential plateau region is promoted by using NaBPPA as a main salt in electrolytes. This work reveals the great importance of novel sodium salt anions beyond conventional ones in boosting sodium storage of HC anode.