A Nanoconfined Iron(Iii) Fluoride Cathode In A Nadfob Electrolyte: Towards High-Performance Sodium-Ion Batteries

Iron(iii) fluoride (FeF3) is considered a potential cathode for sodium-ion batteries (SIBs) due to its high capacity and low cost. However, particle pulverization upon cycling generally results in rapid degradation of its structure and capacity. Here, we introduce a free-standing nanoconfined FeF3 cathode and a novel electrolyte salt, sodium-difluoro(oxalato)borate (NaDFOB), for SIBs. The assembled cells show a high discharge capacity up to similar to 230 mA h g(-1) at a rate of 20 mA g(-1) (similar to 200 mA h g(-1) at 100 mA g(-1)) and a capacity retention up to similar to 70% after 100 cycles, which represent the best results reported for FeF3 in Na-ion electrolytes. The achieved high performance can be attributed to the synergic protection provided by the nanoconfined FeF3 electrode and the NaDFOB electrolyte. Post-mortem analysis and quantum mechanics show that DFOB anions facilitated the formation of a thin cathode electrolyte interphase (CEI) on the surface of FeF3-carbon nanofibers (CNFs) via oligomerization.