Oversaturated Li-FeOF solid solutions developed using LiPON interfacial coating

Abstract

FeOF is a promising high-capacity cathode material for Li-ion batteries (LIBs). However, lithiation of FeOF triggers a conversion reaction once the Li content reaches approximately 0.6Li per FeOF, limiting its reversible capacity in LIB applications. Our previous work demonstrated that FeOF with LiPON coating could significantly enhance the cycling stability of FeOF electrodes (from 29% to 89% over 100 cycles). In this study, the phase transformation and phase separation of FeOF upon lithiation were tracked using (S)TEM, EELS, and in situ XAS under the effect of interfacial LiPON coating. We quantified that the onset potentials of the conversion reaction for LiPON-coated FeOF were delayed for ∼0.3 V upon the discharged process compared with that for bare FeOF, where the thresholds of lithiation amount for the conversion reaction extends from ∼0.6Li to ∼1.1Li per formula unit due to the coating to form an oversaturated (supercooled) Li-FeOF solid solution. It is evidenced that the interfacial kinetics offered by the LiPON coating effectively drive the Li-FeOF system away from thermodynamic equilibrium.