PNNL

Abstract

Metal fluorides with high redox potential and capacity from strong metal-fluoride bonds and conversion reaction, makes them promising cathodic materials but detailed lithium insertion and extraction mechanisms have not been clearly understood and explained to date. Here we report low temperature synthesis of electrochemically active FeF3/FeF2 nanoparticles by catalytic decomposition of fluoropolymer [perfluoropolyether (PFPE)] using hydrated iron oxalate precursor in air and inert atmosphere. Freshly synthesized FeF3 nanoparticle delivered specific capacity over 210 mAh/g with decent cycling performance as a Li-ion battery cathode. Both in situ and ex situ characterization techniques were used to investigate the detailed PFPE decomposition and fluorination mechanism leading to FeF3/FeF2 formation as well as initial lithium insertion mechanism in FeF3 cathode. Specifically, a detailed understanding was investigated using thermogravimetric - mass spectroscopy (TGA-MS), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), transmission electron microscopy (TEM), scanning electron microscopy (SEM) - energy dispersive spectroscopy (EDS), and X-ray absorption near edge structure (XANES). The novel synthesis route developed not only offers an access to electrochemically active metal fluorides but also offers a catalytic approach for decomposing highly inert fluoropolymers for environmental protection.