Supercapacitor Safety: Overcharge-Induced Failure Mechanisms in Electrochemical Double-Layer Capacitors.

Electrochemical double layer capacitors (EDLCs) are widely considered to be reliable energy storage systems; however, their behavior when subjected to overcharge remains poorly understood. Using time-resolved, in situ synchrotron radiography combined with synchronized thermal and electrical measurements, we directly visualize the internal evolution of EDLCs under abusive overcharge. We reveal a progressive, thermally driven failure pathway, distinct from the chemically driven thermal runaway seen in lithium-ion batteries, in which initial electrical instability triggers electrolyte decomposition, vapour formation, and gas accumulation, generating heterogeneous internal pressures that drive electrode collapse, casing deformation, and structural destruction. Post-mortem X-ray Computed Tomography (X-ray CT) and cell tear-down confirm permanent electrode delamination and separator failure, while Gas Chromatography-Mass Spectrometry (GC-MS) analysis of vented gases identifies flammable and potentially toxic species, highlighting acute fire and chemical hazards. By linking internal structural dynamics to external failure signatures, this work provides the most detailed mechanistic picture of EDLC failure to date and reinforces the importance of acknowledging and mitigating underappreciated hazard modes in high-power, high-reliability system design.