Deciphering the mechanism of concentrated electrolyte for lithium metal anode via cryogenic electron microscopy

Increasing electrolyte concentration is a typical strategy to boost the stability of lithium (Li) metal anode, yet the fundamental mechanism remains a mystery. By virtue of comprehensive characterization of solid electrolyte interphase (SEI) via cryogenic electron microscopy (Cryo-EM), we revealed the effects of solvation structure in ether-based electrolytes on SEI formation as well as electrochemical performance. The SEI formed in the low-concentration electrolyte (LCE) adopts a Mosaic-type structure with randomly distributed LiCO, which leads to uneven Li deposition and poor cycle stability. The high-concentration electrolyte (HCE) with few free solvent molecules generates an amorphous monolayer SEI, contributing to significantly improved Coulombic efficiency and cycle stability. The addition of non-solvating diluent enables uniform Li deposition on Cu foils in the prepared local high-concentration electrolyte (LHCE), and brings about further enhancement in reversibility and stability. It is correlated with the dual-layer but thinner SEI consisting of an inner amorphous layer and an outer layer made up of mainly crystalline LiSO with high Li conductivity. This work points out the necessity to optimize the SEI structure as well as the solvation structure by altering the salt-to-solvent ratio or adding diluent to modify the viscosity and conductivity of electrolyte system.