Dual protective layer on lithium metal anodes for improved electrochemical performance – in-depth morphological characterization

Abstract

Lithium metal is a promising electrode material to increase the specific energy and energy density of rechargeable batteries. The high reactivity of lithium in contact with the electrolyte leads to the formation of a solid electrolyte interphase (SEI) that is inhomogeneous in composition and morphology. The SEI is prone to cracking due to volume changes and favors high surface area lithium growth, pit formation and an accelerated Li bulk consumption during electrodeposition/-dissolution due to continuous SEI regeneration. These drawbacks result in low coulombic efficiency and cycle life, and pose a safety risk to rechargeable lithium metal batteries with liquid electrolytes that must be addressed before commercialization. Protective layers by ex situ surface modifications are an attractive strategy to improve the cycle life and performance of lithium metal batteries as they mitigate Li metal degradation reaction and homogenize the Li ion transport from/into electrolyte. Herein, a dual protective layer consisting of an intermetallic LiZn and inorganic Li₃N layer deposited by thermal evaporation was investigated. A deeper insight into the durability of the dual protective layer was gained by cross-sections under cryogenic conditions before and after electrodeposition/-dissolution. Galvanostatic cycling experiments in symmetric Li||Li cells revealed an increase in cycle life of 80% and the state of health from LiNi_0.6Mn_0.2Co_0.2O₂||Li cells was improved by 50% (SOH 80) for Li electrodes with a dual protective layer compared to pristine Li metal.