Understanding the Thermal Stability of Li-Rich Disordered Rock-Salt Oxyfluorides

Abstract

Li-rich disordered rock-salt oxyfluorides (DRS) are a new class of cathode material with unprecedented electrochemical properties. [1,2] DRS materials have already gained popularity by showing high reversible discharge capacity by substituting oxygen (O) with fluorine (F). They are a possible candidate of cathode materials for future high-energy Li-ion batteries with high capacity. Although DRS materials are explored widely for their structure and electrochemical performance, very little is known about their thermal stability. DRS oxyfluorides can be synthesized via solid state and mechanochemical ball milling. [3-5] The solid-state synthesis method is limited because of less fluorine solubility than the mechanochemical ball milling method; however, mechanochemical ball milling produces metastable materials. Few reports utilized thermal treatment on DRS to obtain structural stability and better capacity retention than the pristine DRS. [5,6] The in-depth understanding of the inherent stability of the DRS materials on thermal treatment remains poorly understood. [7] To address this gap, we investigated the thermal stabilities of Li2MO2F stoichiometries with a variety of M (M= Mn, Ni, and Co) and defined the structural transformations at different temperatures, from changes in the local structure of the oxyfluoride to decomposition into other phases, using X-ray diffraction, NMR and X-ray absorption spectroscopy. In this presentation, we will show how thermal treatments affect chemical and structural features by utilizing unique spectral characteristics. Additionally, we will discuss a complete understanding of these thermally treated F-rich phases' better electrochemical performance and capacity retention as battery cathode materials.