How fluorinated cyclic ether shapes localized high concentration electrolytes

Localized high concentration electrolytes (LHCEs) have emerged as a transformative choice, simultaneously exhibiting good electrochemical stability, sufficiently fast ionic transport, and improved rate capability. The design principle is to dilute the conventional electrolyte with a non-solvating diluent, forming extended ionic networks while keeping the salt concentration sufficiently low. Several fluorinated cyclic ethers have recently been reported as promising diluents. To better understand their success, we perform ab initio molecular dynamics simulations of LHCEs based on 2,2-bis(trifluoromethyl)-1,3-dioxolane (BTFMD) diluent, 1,2-dimethoxyethane (DME) solvent, and lithium bis(fluorosulfonyl)imide (LiFSI) salt. The Li + ions are found to be coordinated soley by FSI - and DME, whereas the concentration of the diluent dictates the competition between the two coordinating species in the Li + first solvation shell. The Li + solvation environment is linked to the electronic structure of the electrolyte: a higher concentration of the diluent decreases the density of electronic states in the vicinity of the valence band maxima, which arises from the depletion of non-solvating DME molecules. This observation is conceptually related to the improved oxidative stability measured experimentally and the chemical bonding in these systems. Our findings rationalize the performance of LHCEs and suggest the tunability of electrochemical responses by controlling the local Li + structure.