# High-temperature chemical oxidation pathways in lithium-ion batteries: mechanistic insights into ethylene carbonate decomposition

## TL;DR

This study explores how ethylene carbonate in lithium-ion batteries breaks down at high temperatures, revealing two pathways that contribute to dangerous thermal events.

## Contribution

The paper identifies two distinct ethylene carbonate oxidation pathways under high-temperature conditions and links them to thermal events in lithium-ion batteries.

## Key findings

- Two EC oxidation pathways are identified: one at high voltages and another initiated by trace water impurities.
- Both pathways produce significant heat and release CO2 and H2 gases, contributing to thermal events.
- Density functional theory calculations confirm the thermodynamic favorability of these reactions.

## Abstract

A thermal event remains a safety challenge for lithium-ion batteries due to the self-reinforcing nature of the exothermic reactions occurring at elevated temperatures. Higher states of charge have been shown to exacerbate the onset and severity of a thermal event. For cells containing Ni-rich layered oxide-based electrodes, this has been attributed to the increased instability of the material leading to lattice oxygen release. The degradation reactions on the electrode/electrolyte interface triggered by this oxygen remain insufficiently understood. In this study, we investigate high-temperature degradation pathways of ethylene carbonate (EC)-based electrolytes in contact with Ni-rich positive electrode active materials up to 130 °C. By combining in situ high-temperature online electrochemical mass spectrometry with post-mortem analyses, we identify and validate key degradation intermediates and products. Two distinct EC oxidation pathways are revealed: one activated at high voltages, and the other one initiated by traces of water impurities. Complementary density functional theory calculations show the reactions are thermodynamically favorable and quantify the heat release associated with each pathway. Both pathways produce significant heat and lead to gassing of CO2 and H2. These findings suggest a significant contribution of EC to thermal gas evolution and exothermicity under abuse conditions, thereby establishing a mechanistic link between electrolyte chemistry and thermal events. This integrated experimental–computational approach provides critical insights to guide improved electrolyte formulations and predictive thermal models.

Under thermal abuse conditions, the interplay of state of charge and trace water impurities drives the degradation of ethylene carbonate-based electrolytes on Ni-rich positive electrodes in lithium-ion batteries.