Impact of Lithium‐Ion Battery Separators on Gas Evolution during Temperature Abuse

Separators in lithium-ion batteries are typically considered to be electrochemically inert under normal operating conditions. Yet, temperature abuse tests at elevated temperatures of ca. 60 °C to 132 °C show that the choice of separator material has a decisive influence on battery behavior and degradation. Using online electrochemical mass spectrometry, we analyzed the evolution of cell voltage and gas products during and after thermal abuse for different separators. Polypropylene and polytetrafluoroethylene seem exhibited little change in gas evolution, producing only modest amounts of CO2 and POF3. In contrast, glass fiber and polyethylene terephthalate separators caused additional gas release, indicating electrochemical instability. Polyethylene terephthalate produced significantly more gas, resulting in the mechanical failure of the separator and drastic performance losses. The amount of CO2 evolved with polyethylene terephthalate is four times higher than that of the glass fiber separator. However, the amount of POF3 detected was five times higher for the glass fiber separator.