(Invited) Designing Interphase in Rechargeable Lithium Metal Batteries Via Liquid Electrolyte Additives

Abstract

The goal of reaching specific energy at 500 Wh kg-1 resurrects Li metal anode in recent years due to its high capacity (3860 mAh g-1) and low reductive potential (-3.04 V vs. standard hydrogen electrode). However, the reductive decomposition and chemical instability of the liquid electrolytes against Li metal result to unstable solid-electrolyte interphase (SEI) formation and porous Li deposition with low coulombic efficiency and severe volume expansion. Therefore, one strategy to enhance the cycle life of Li metal anode is to design robust SEI to improve the interfacial stability. Herein, we demonstrate two liquid electrolyte additives that can form and regulate desirable SEI on the Li metal anode. Acrylonitrile (AN) can be electrochemically polymerized as an additive to form a Li-ion conductive SEI during Li deposition. Electrochemical analyses, scanning electron microscopy, and X-ray photoelectron spectroscopy confirm the formation of the polymeric SEI, which induces uniform and dense Li deposition and reduced decomposition of the Li-ion electrolyte. Phosphorous pentoxide (P2O5) is also studied as an additive in the typical 1M LiPF6 electrolyte in carbonate solvents for Li metal batteries. P2O5 can effectively scavenges the active impurities in the carbonate electrolytes and form phosphorous-rich stable SEI. Significantly improved battery performance due to these two additives is demonstrated in 0.4 Ah pouch cells composed of LiNi0.6Mn0.2Co0.2O2 (NMC622) cathode (3 mAh cm-2) and Li anode (50 mm thick) with a lean electrolyte/capacity ratio of 3 g/Ah.