Revealing heterogeneous electric double layer (EDL) structures of localized high-concentration electrolytes (LHCEs) and their impact on solid–electrolyte interphase (SEI) formation in lithium batteries

Abstract

Recently, we have proposed micelle-like structures to fully understand microstructures in localized high-concentration electrolytes (LHCEs) that provide many benefits to high-capacity electrodes. It is critical to understand the electric double layer (EDL) structures of LHCEs, how they differ from the bulk electrolyte structure, and how they impact solid electrolyte interphase (SEI) formation on a Li–metal electrode with a combined DFT-MD-data approach. In this work, we reveal the heterogeneous EDL structures of a prototypical LHCE consisting of lithium bis(fluorosulfonyl)imide (LiFSI) salt, dimethoxyethane (DME) solvent, and tris(2,2,2-trifluoroethyl)orthoformate (TFEO) diluent at a concentration of LiFSI–1.2DME–2TFEO for lithium batteries. We find that the 1D EDL Stern model with an adsorbed cation layer and a more diffused layer with ions, solvents, and diluents missed the heterogeneity of this type of electrolyte. The micelle-like structures in the LHCE are maintained in its EDL that is divided into the Li⁺-rich salt-solvent cluster region and the Li⁺-poor diluent region. The diluent region contains Li⁺ ions in the EDL but has no ions in the bulk of the LHCE. This is because TFEO cannot form a complete solvation shell in the bulk electrolyte due to the steric effect; however, only a partial solvation shell is needed near the surface. The appearance of Li⁺ ions in the diluent region is also necessary for the EDL, as TFEO alone cannot screen the charge. Thus, the reduction voltage of TFEO will be increased by the association of Li⁺ ions in the EDL, highlighting one of the impacts of the heterogeneous EDL structures on SEI formation. This work helps extend the development of the EDL theory and guides the design of more effective LHCEs for high-performance rechargeable batteries.