The Effect of Water Contamination on the Aging of a Dual-Carbon Lithium-Ion Capacitor Employing LiFSI-Based Electrolyte

TL;DR

This study investigates how varying levels of water contamination affect the performance and aging of dual-carbon lithium-ion capacitors using LiFSI electrolyte, highlighting water's role in degradation mechanisms.

Contribution

It systematically examines water tolerance thresholds in LiFSI-based electrolytes and elucidates water-induced degradation pathways in lithium-ion capacitors.

Findings

950 ppm water has minimal impact on capacity retention

2300 and 6000 ppm water cause significant capacity fade

Water promotes decomposition of LiFSI and thickening of the SEI layer

Abstract

Fabricating electrochemical energy storage devices demands significant energy for drying cell components to ensure optimal performance. The development of new, water-tolerant materials would represent a tremendous advance in cost savings and sustainability. Although it is generally established that water deteriorates cell performance, there are few systematic studies on the maximum amount of tolerable water contamination, and most of the studies employed the electrolyte salt $\mathrm{LiPF_6}$, which inevitably decomposes upon exposure to humidity. In this work, the potential of using the non-hydrolyzing salt LiFSI is explored with respect to its performance in Li-ion capacitor cells based on activated carbon (AC) and pre-lithiated graphite (Gr). Water is deliberately added in various amounts (950, 2300, and 6000 ppm), and its effect on the electrochemical performance and aging of AC and Gr electrodes is systematically studied. While the addition of 950 ppm water has no evident impact on capacity retention (96% after 2000 cycles), the addition of 2300 ppm water or 6000 ppm water shows a distinct capacity fade, attributed to a significant loss of lithium inventory and increased resistivity due to irreversible reactions of the water with lithiated Gr. Post-mortem analysis reveals that water promotes the oxidative and reductive decomposition of LiFSI on AC and lithiated Gr, respectively. A significant thickening of the SEI on Gr is observed as the water concentration is increased.