Initial stages of thermal decomposition of LiPF6-based lithium ion battery electrolytes by detailed Raman and NMR spectroscopy

Abstract

Independent of the specific electrode chemistry, the state-of-the-art lithium ion battery electrolytes based on LiPF₆ in organic solvents have a low thermal abuse tolerance and poor cycle life at elevated temperatures. We present here a detailed investigation of the initial stages of the thermal decomposition of LiPF₆ in EC/DMC stored at 85 °C using Raman and NMR spectroscopy. During storage (up to 160 h), significant amounts of CO₂ are evolved, as detected in the Raman spectra. Time-resolved ¹H, ³¹P, and ¹⁹F NMR spectra show the evolution of POF₃, POF(OH)₂, POF₂(OCH₂CH₂)_nF, and POF₂OMe as reactive decomposition products. Our unique ¹⁹F NMR approach, measuring while heating with both high energy and time resolution, allows for a first quantitative analysis of the evolved species and reveals several decomposition reactions during the first 30 min up to 72 h, where the rates of HF and POF₂OMe formation are surprisingly linear. EC is found to be much less reactive compared to DMC. All information is used in the formulation of an updated decomposition pathway chart for LiPF₆ based electrolytes.