Influence of Li-salts on Cycle Durability of Sn-Ni Alloy Anode for Lithium-ion Capacitor

Tin-nickel (Sn-Ni) alloy is a promising candidate as an anode for the lithium-ion capacitor (LIC) because it is superior in volumetric energy density compared with that of the graphite anode. However, its cycle durability requires improvement, even with a higher utilization ratio of the anode. The effect of lithium salts, LiPF6 and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is investigated for usage in the LIC in severe conditions (utilization ratio of the anode: 20%). The LIC with LiTFSI delivered its initial capacity up to ∼400 cycles, which is 4 times longer than the LIC with LiPF6. The reason for the capacity decay in the LiPF6 system is attributed to the narrowing of the potential range of the activated carbon cathode due to a widening potential range of the Sn-Ni alloy anode during operation. This widening is attributed to the loss of the active material due to peeling-off from the substrate. However, when LiTFSI is used, no such decay is observed. It is suggested that a polymer-like solid electrolyte interphase derived from TFSI− may suppress the loss of the active material. This finding can encourage the development of an Sn-based anode for LICs in combination with a mild operating condition and electrolyte additives.