Enhancement effect of trace H₂O on the charge-discharge cycling performance of a Li metal anode

The effects of a small amount of H₂O with and without CO ₂ in an electrolyte of 1 M LiPF₆/ethylene carbonate and diethyl carbonate on the cycling life of a Li metal anode is investigated in this paper using charge-discharge cycling. A low cycling performance, which is less than 55%, is observed with the electrolyte with trace H₂O but without CO₂; however, when the trace H₂O is accompanied by CO₂, performance drastically improves and coulombic efficiency reaches a maximum of 88.9%. In the presence of CO₂, the cycling performance is found to be strongly affected by the H₂O content in the electrolyte, and increases with an increase in H₂O content of up to 35 ppm. From an X-ray photoelectron spectroscopy analysis, trace H ₂O is found to affect the compounds of the solid electrolyte interphase (SEI) on the lithium surface and produces an Li₂CO ₃ and LiF layer on the upper part of the SEI, both known to be good passivation layers for preventing side reactions during charge-discharge cycling.