Infrared Spectroscopic Study of the Thermal Stability of Alkyl Carbonate Electrolytes

Abstract

Decomposition of ethylene carbonate (EC) in lithium ion batteries leads to battery swelling, off-gassing, and thermal runaway. Ethylene carbonate decomposes electrochemically by reduction at the anode or chemically in the bulk electrolyte at elevated temperature. The formation of a solid electrolyte interface (SEI) on the anode limits electrochemical decomposition so long as the SEI remains intact. While neat ethylene carbonate is thermally stable at temperatures exceeding 150°C, EC-based electrolytes containing lithium hexafluorophosphate (LiPF6) salt are known to chemically react at temperatures as low as 50°C. Decomposition rate has been shown to increase with higher salt content, indicating a salt-catalyzed thermal decomposition mechanism. Trace contaminants, especially moisture, have also been shown to strongly effect electrolyte thermal stability. Thus, small impurities in the manufacture of battery components can have dramatic effect on electrolyte stability and battery aging. Electrolyte decomposition mechanisms have been studied extensively. Differential scanning calorimetry (DSC) and accelerated rate calorimetry (ARC) can identify temperatures of reactions, kinetic parameters, and heat release rates. Decomposition mechanisms and reaction products are also identified indirectly by ex situ electrolyte and off-gas analysis. Despite numerous studies, much remains unknown about decomposition chemistry in lithium ion batteries, especially the role of trace impurities on stability. Further, little is known about electrolyte decomposition mechanisms in batteries, as few operando electrolyte diagnostics exist. In this work, the chemical stability of EC at elevated temperature is studied using infrared spectroscopy. Electrolyte samples are heated on an attenuated total reflection (ATR) spectrometer stage using both liquid samples and in operating half and full cell batteries. Liquid samples were prepared in an argon glove box with varying concentrations of LiPF6 salt, H2O, and in CO2 gas environments (to simulate cathode off-gassing). Accurate temperature readings up to 150°C were made using infrared thermometry. Operando spectroscopic measurements of electrolyte stability in graphite half cells, lithium cobalt oxide (LiCoO2) half cells, and lithium ion full cells were also collected. Reaction rates and activation energies are also reported using time-resolved infrared spectroscopy.