Free-Standing Flexible FeF_­­3-C Cathodes for Sodium-Ion Batteries

Abstract: Sodium-ion batteries (SIBs) have recently attracted great attention as a potential alternative to lithium-ion batteries due to ubiquity of sodium reserves. Conversion-type iron trifluoride (FeF3) may become a particularly interesting cathode due to low cost and abundance of Fe and extremely high theoretical capacity of this material (712 mAh g-1). Unfortunately, prior studies showed rather poor capacity and cycling performance of FeF3 due to significant volume changes, morphological changes and various side reactions. Here we demonstrate that by the confinement of FeF3 nanoparticles in flexible carbon nanofibers (CNFs) to mitigate structural changes during cycling and by utilizing a novel electrolyte salt, substantial performance improvements could be attained. This novel (for SIB) cell design facilitated electron and ion transport and additionally led to the formation of a thin cathode electrolyte interphase (CEI) layer on the FeF3-CNF surface. The assembled cells show high discharge capacity of up to ~200 mAh g-1 at the rate of 100 mA/g (up to ~230 mAh g-1 at 20 mA/g) and capacity retention up to ~ 70% after 100 cycles, which represent the best results reported on FeF3 in Na-ion electrolytes. We acknowledged the funding support from ARO (Grant No. W911NF-17-1-0053), W.F. M.L. and S. Luo acknowledge the fellowship support of China Scholarship Council.