Experimental thermal runaway degradation and residual performance of fiber-reinforced PEEK and PPS composites for battery enclosure applications

High-powered lithium-ion batteries (LIBs) are adopted in many transport sectors to reduce carbon emissions. To ensure LIB safety, especially for electric vehicle (EV) applications, battery enclosures must be designed to withstand load cases and failure modes, such as thermal runaway (TR), while remaining lightweight to enhance vehicle range. Fiber-reinforced polymers (FRPs) represent a promising alternative to traditional materials like steel and aluminum for enclosures. This study investigates high-performance thermoplastic composites such as polyetheretherketone (PEEK) and polyphenylene sulfide (PPS), reinforced with glass (GF) and carbon fiber (CF) under multi-cell TR conditions. The samples are then subjected to a structural integrity assessment to quantify their damage. The testing method shows comparable open plies and residual compressive strength among samples from the same material. This supports its use for initial material screening prior to system-level evaluations. Structural integrity assessments show no clear correlation with the number of open plies and mass loss, emphasizing the need for subsequent mechanical testing methods. PPS samples showed increased resin degradation areas and lower residual compressive strength (CF-PPS retained 18.5 % while GF-PPS had 22.2 % residual compressive strength) compared to PEEK. GF reinforcement led to higher mass loss than CF and more open plies than CF-reinforced samples but showed higher residual compressive strength (20.7 and 23.2 % for CF-PEEK and GF-PEEK, respectively) and lower back-surface temperatures. The results herein can contribute to the assessment of suitable fiber–matrix systems under TR conditions for battery enclosure applications, by comparing failure mode and structural integrity.