Stability of FeF-Based Sodium-Ion Batteries in Nonflammable Ionic Liquid Electrolytes at Room and Elevated Temperatures.

Iron trifluoride (FeF), a conversion-type cathode for sodium-ion batteries (SIBs), is based on cheap and abundant Fe and provides high theoretical capacity. However, the applications of FeF-based SIBs have been hindered by their low-capacity utilization and poor cycling stability. Herein, we report greatly enhanced performance of FeF in multiple types of ionic liquid (IL) electrolytes at both room temperature (RT) and elevated temperatures. The PyrFSI electrolyte demonstrated the best cycling stability with an unprecedented decay rate of only ∼0.023% per cycle after the initial stabilization and an average coulombic efficiency of ∼99.5% for over 1000 cycles at RT. The PyrFSI electrolyte demonstrated the best cycling stability with a capacity decay rate of only ∼0.25% per cycle at 60 °C. Cells using PyrFSI and EMIMFSI electrolytes also showed promising cycling stability with capacity decay rates of ∼0.039% and ∼0.030% per cycle over 1000 cycles, respectively. A protective and ionically conductive cathode electrolyte interphase (CEI) layer is formed during cycling in ILs, diminishing side reactions that commonly lead to gassing and excessive CEI growth in organic electrolytes, especially at elevated temperatures. Furthermore, the increased ionic conductivity and decreased viscosity of ILs at elevated temperatures help attain higher accessible capacity. The application of ILs sheds light on designing a protective CEI for its use in stable SIBs.