A critical analysis of chemical and electrochemical oxidation mechanisms in Li-ion batteries

Electrolyte decomposition in lithium-ion batteries (LIBs) remains a challenge, limiting the lifetime of commercial cells and slowing the adoption of next-generation energy storage technologies. Fundamental mechanistic understanding of electrolyte degradation is critical to enable the design of next-generation LIB technologies. To date, most explanations for electrolyte decomposition at LIB positive electrodes rely on either electrochemical oxidation of ethylene carbonate (EC) or chemical oxidation of EC by evolved singlet oxygen (1O2). In this work, we discuss the feasibility of these mechanisms based on density functional theory calculations. We find that electrochemical oxidation is thermodynamically unfavorable at any potential reached during normal LIB operation, while elementary reaction mechanism analysis suggests that previously reported reactions between EC and 1O2 are kinetically limited at room temperature. Our calculations suggest an alternative mechanism, in which EC reacts with superoxide (O2(-1)) and/or peroxide (O2(-2)) anions. This work provides a new perspective on LIB electrolyte decomposition and motivates further experimental and computational studies to fully understand reactivity at positive electrodes.