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 Li2CO3, 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 pre-pared 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 Li2S2O7 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.

Automated summary

By characterizing the solid electrolyte interphase (SEI) using cryogenic electron microscopy (Cryo-EM), we investigated the influence of solvation structure on SEI formation and electrochemical performance. The SEI formed in low-concentration electrolytes exhibited a mosaic-type structure, leading to uneven lithium deposition and poor cycle stability. In contrast, high-concentration electrolytes generated an amorphous monolayer SEI, resulting in improved Coulombic efficiency and cycle stability.