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

Abstract

The effects of a small amount of H2O with and without CO2 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 H2O is found to affect the compounds of the solid electrolyte interphase (SEI) on the lithium surface and produces an Li2CO3 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.