Mechanism of solid electrolyte interphase film formation using ethylene carbonate-based local high concentration electrolyte in sodium-ion batteries

Sodium-ion batteries (SIBs) have the advantages of abundant resources and low cost, making them potential candidates for the next-generation large-scale energy storage technology. However, the capacity fade during cycling used in sodium-ion batteries is a major challenge. The rational design of the electrolyte is one of the ways to solve these problems. In this work, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (HFE) is introduced into a sodium hexafluorophosphate (NaPF₆)/ethylene carbonate (EC) electrolyte to design a locally high concentration electrolyte (LHCE), which helps stabilize the solid electrolyte interphase (SEI) in sodium-ion batteries (SIBs). By modulating the solvation structure of the electrolyte, a NaF-rich SEI is formed on the surfaces of electrodes. With sodium iron phosphate (NFPO) as the cathode, the cell maintained a capacity retention rate of 91.5 % after 300 cycles at 0.5C. In addition, a sodium nickel iron manganese oxide (NFMO)||Hard carbon (HC) pouch cell achieves a capacity retention of 84.2 % after 500 cycles at 1C. This study provides a new perspective for the understanding and design of locally high concentration electrolytes for sodium-ion batteries.