On the Viability of Lithium Bis(fluorosulfonyl)imide as Electrolyte Salt for Use in Lithium-Ion Capacitors

Lithium-ion capacitors (LICs) represent promising high-power energy storage devices, most commonly composed of a lithium-ion intercalation anode (e. g., graphite or hard carbon), a supercapacitor activated carbon (AC) cathode, and an electrolyte with 1 M LiPF6 in carbonate solvents. LiPF6 is susceptible to hydrolysis, forming HF, which leads to challenges for disassembly and recycling, risks during hazardous events, and extensive energy consumption during production. Here, we report on the feasibility of replacing LiPF6 with the non-hydrolysing salt LiFSI for use with AC electrodes. Based on voltage hold measurements in a half-cell setup, good long-term stability is achieved with an upper cut-off voltage of 3.95 V vs. Li/Li+, potentially enabling cell voltages of similar to 3.8 V when combined with graphite or silicon-based anodes (operating at similar to 0.1 V vs. Li/Li+) in LIC full cells. The lower cut-off voltage was determined to be 2.15 V vs. Li/Li+. The systematic comparison of CV, leakage current analysis and capacity retention upon voltage hold highlights the importance of the latter method to provide a realistic assessment of the electrochemical stability window (ESW) of LiFSI on a commercial AC electrode. The morphological and surface-chemical post-mortem analysis of AC electrodes used with LiFSI revealed that the oxidation of the FSI anion, as evidenced by the presence of new S 2p and N 1s features in the XPS spectra, and an increasing number of oxygenated species on the AC were the main processes causing capacity fade at positive polarization.