Structural and Chemical Evolutions of Li/Electrolyte Interfaces in Li-Metal Batteries: Tracing Compositional Changes of Electrolytes under Practical Conditions

Despite the promises in high-energy-density batteries, Li-metal anodes (LMAs) have suffered from extensive electrolyte decomposition and unlimited volume expansion owing to thick, porous layer buildup during cycling. It mainly originates from a ceaseless reiteration of the formation and collapse of solid-electrolyte interphase (SEI). This study reveals the structural and chemical evolutions of the reacted Li layer after different cycles and investigates its detrimental effects on the cycling stability under practical conditions. Instead of the immediately deactivated top surface of the reacted Li layer, the chemical nature underneath the reacted Li layer can be an important indicator of the electrolyte compositional changes. It is found that cycling of LMAs with a lean electrolyte (approximate to 3 g Ah(-1)) causes fast depletion of salt anions, leading to the dynamic evolution of the reacted Li layer structure and composition. Increasing the salt-solvent complex while reducing the non-solvating diluent retards the rate of depletion in a localized high-concentration electrolyte, thereby demonstrating prolonged cycling of Li||NMC622 cells without compromising the Li Coulombic efficiencies and high-voltage stability.