Reliability-by-design for deployable next-generation supercapacitors: degradation, harsh-environment operation, sustainability, and AI-guided lifetime prediction

Supercapacitors (SCs) offer rapid charging, high power density, and long cycle life, yet their transition from laboratory prototypes to deployable technologies remains constrained by unresolved failure mechanisms, inconsistent benchmarking, voltage-holding instability, self-discharge, and uncertain lifetime prediction. This review advances a reliability-by-design perspective that shifts emphasis from capacitance and cycle retention toward full-device readiness. It critically examines degradation across electrodes, electrolytes, interfaces, current collectors, separators, binders, and encapsulation, including pore blockage, dissolution, electrolyte decomposition, gas evolution, corrosion, delamination, leakage current, self-discharge, and ESR growth. Device-relevant evaluation metrics are discussed, including energy density, floating-voltage stability, calendar aging, gas generation, thermal and mechanical robustness, and statistical lifetime analysis, with cross-study comparisons normalized for cell configuration, mass loading, operating conditions, and aging protocols. Insights from harsh-environment, flexible, self-healing, sustainable, and AI-enabled SCs are integrated into a five-gate readiness framework covering reliability, robustness, recyclability, realistic benchmarking, and reproducibility (5R). This framework links application-specific stresses to failure mechanisms, measurable reliability signatures, and mitigation strategies, and is operationalized through acceptance limits, screening thresholds, non-compensatory scoring, and retrospective device assessment. Finally, AI-compatible datasets are defined for state-of-health estimation, failure-mode identification, and remaining-useful-life prediction.