Moisture-induced synergistic failure mechanisms in lithium-ion capacitors under 4 V floating aging

Electrode moisture, being a crucial parameter that requires stringent control during the production, possesses a critical impact on the performance of lithium-ion capacitors (LICs). In this work, the moisture-induced failure mechanisms of LICs has been investigated during high-voltage (4 V) float charging aging. By employing in situ differential electrochemical mass spectrometry (DEMS), electrochemical impedance spectroscopy (EIS), and post-mortem multi-scale characterization, it reveals that elevated moisture content (>1500 ppm) triggers a cascade of degradation pathways. The experimental results indicate that higher moisture content leads to accelerated aging of LICs. During high-temperature aging, the presence of moisture exacerbated chain-like side reactions in LICs (including gas evolution, SEI thickening, active lithium loss) and interfacial product accumulation, triggering multiscale synergistic degradation (electrolyte component imbalance, electrode/electrolyte interface destabilization, and hindered lithium-ion transport). These processes ultimately lead to capacitance attenuation and a sharp increase in impedance. This study provides theoretical and data support for the subsequent industrial production of LICs.