Cryo-TEM studies of binder free high performance FeF2 cathode based full cells enabled by surface engineering

Iron-based fluorides (IBFs, typically FeF3 and FeF2) are potential next generation high energy density cathodes for lithium-ion batteries (LIBs) due to their high specific capacity, cost effectiveness and crustal abundance. However, IBFs decompose electrolyte violently which impedes their practical implementation in LIBs. Herein we report a strategy to embed FeF2 nanoparticles in an interconnected cyclic polyacrylonitrile (cPAN) network to passivate the cathode/electrolyte interface. Moreover, cPAN affords strong adhesion to both the FeF2 nanoparticles and the current collector, essentially acting as a binder that is superior to polyvinylidene fluoride (PVDF). Cryo-transmission electron microscopy reveals that the amorphous cPAN was uniformly coated on the FeF2 surface with a thickness of 5 nm, acting as an effective (cathode electrolyte interphase) CEI layer that inhibited the growth of excessive CEI, thus achieved stable cycling in lean electrolyte. When paired with the high capacity SiO/C anode, the FeF2-cPAN|SiO/C full cell delivered a capacity of 400 mAh/g after 100 cycles at 0.5C. Density functional theory (DFT) calculation indicates that the cPAN is inert to the electrolyte, thus suppresses the catastrophic electrolyte decomposition caused by FeF2. DFT further suggests that the existence of vacancies on the carbon surface will cause H transfer reaction of the solvent, leading to the degradation of the electrolyte. This study provides a viable technology to enable high energy density FeF2 cathode-based LIBs for energy storage applications.