The effect of water contamination on the aging of a dual-carbon lithium-ion capacitor using LiFSI-based electrolyte

Non-hydrolyzing salts such as lithium bis(fluorosulfonyl)imide (LiFSI) offer a promising strategy to develop water-tolerant electrochemical energy storage devices, potentially reducing production costs and energy consumption during cell manufacturing. In this work, the effectiveness of a LiFSI-based carbonate electrolyte is evaluated in the presence of deliberately added trace water (950, 2300, and 6000 ppm H 2 O) within a Li-ion capacitor (LIC) composed of activated carbon (AC) positive and pre-lithiated graphite (Gr) negative electrodes. The electrochemical performance and aging behavior of the electrodes are systematically assessed and compared to the dry electrolyte ( < 12 ppm H 2 O). Remarkably, 950 ppm H 2 O does not significantly affect capacity retention or aging during galvanostatic cycling of the LIC, nor during voltage hold tests of AC electrodes at 3.95 V vs. Li/Li + . In contrast, higher water concentrations (2300 and 6000 ppm H 2 O) induce substantial parasitic processes, including reduced initial capacity and capacity retention, reduced coulombic efficiency, increased impedance on both electrodes, and extensive lithium inventory loss on the Gr electrode. Post-mortem XPS and SEM analyses attribute these side effects to water-promoted oxidative and reductive decomposition of the LiFSI salt, as well as solvent hydrolysis, which thickens and increases the resistance of the solid electrolyte interphase (SEI) on the Gr electrode.