Cycle Life Enhancement of Metalic Li Anode By H2o and CO2 in Organic Electrolyte for Li-Air Battery

Li-Air battery is a promising system with high energy density, while both the cycle life of cathode as well as protection of anode against the impurity got mixed from the air should be concerened. Li metallic anode is well known to be damaged by trace H₂O in the electrolyte, hence in production of the lithium ion battery, water content is critically controlled. In this work, The effects of a small amount of H₂O with and without CO₂, both of them are the potential impurity coming from the air at the cathode, in an electrolyte of 1-M LiPF₆/ethylene carbonate and diethyl carbonate on the cycling life of a Li metal anode was investigated using charge–discharge cycling.

By bubbling Ar, CO₂ with H₂O content < 50 ppm, dry air with H₂O content < 21 ppm, or ambient air with humidity = 51±1% into the electrolyte solution with H₂O content < 20 ppm, the solved amount of H₂O and CO₂ was controlled. Li was electrodeposited with 5.1 C cm^-2 of charge onto Ni disc substrate for the charge-discharge testing. Charge and discharge cycling test was performed with 1.0 C cm^-2of charge amount for each step to exhaust the excess amount of Li with consumption by the side reaction of Li.

A low cycling performance with 54.4% of cycling efficiency was observed with the electrolyte with trace H₂O but without CO₂; however, when the trace H₂O is accompanied by CO₂, performance drastically improved and coulombic efficiency reaches a maximum value as 81.4%, higher than that achieved by trace H₂O alone. In the presence of CO₂, the cycling performance was 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.

To understand the effect of H₂O on cycling performance in detail, the charge–discharge test was conducted under several different conditions, by changing the bubbling time in order to vary the H₂O content in the electrolyte; ambient air was used throughout. The H₂O content in the electrolyte increases with an increase in bubbling time: the coulombic efficiency increases steeply with an increase of H₂O content until approximately 35 ppm, reaching a maximum coulombic efficiency of 88.9%, before decreasing gradually as H₂O content continues to increase. This remarkable enhancement affected by trace H₂O below 35 ppm is similar to the trend observed in the electrolyte with CO₂ bubbling. This strongly suggests that trace H₂O in the electrolyte affects the cycling performance of the lithium metal anode.

Investigation of the SEI layer is important in order to account for the enhancement effect on cycle performance. In order to clarify the effect of trace H₂O on the SEI’s chemical structure, XPS analysis was performed after an initial deposition of lithium of 5.1 C cm⁻². From an XPS 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.