Modeling the influence of water on the performance of non-aqueous Li-O2 batteries

Abstract

Unresolved controversy regarding the influence of water (H2O) on the action mechanisms and performance of lithium-oxygen (Li-O2) batteries hinders the ultimate goal of operating the Li-O2 batteries in ambient air instead of pure O2. Herein, we explored the influence of H2O on the discharge/charge behavior and cycling performance of Li-O2 batteries by establishing a mathematical model. In the presence of H2O, the generation reaction of lithium hydroxide (LiOH) lowers the volume fraction of lithium peroxide (Li2O2) in the cathode near the O2 side. Although the presence of H2O with a higher concentration enhances the initial discharge capacity of Li-O2 batteries, the discharge capacity gradually decreases with cycles due to the continuous deposition of undecomposed LiOH in the cathode. The cycle termination of the Li-O2 batteries at lower H2O concentrations is primarily attributed to the porous cathode blocking caused by LiOH accumulation, but the effect of H2O crossover on Li anode corrosion becomes more crucial as H2O concentration increases. The initial discharge capacity increment of Li-O2 batteries is limited by increasing H2O diffusivity, while the battery cycle life decreases obviously with the increase of H2O diffusivity. For high H2O diffusivities, the cycle termination of the Li-O2 batteries is no longer controlled by LiOH accumulation in the cathode but by Li anode corrosion due to H2O crossover. The proposed model can correctly predict the performance of the Li-O2 batteries incorporating H2O and is conducive to designing protection strategies to enhance the cycle life of Li-O2 batteries working in ambient air.