Wide temperature cycling of Li-metal batteries with hydrofluoroether dilution of high-concentration electrolyte

The safe, stable cycling of Li-metal batteries (LMBs) over wide temperature ranges is crucial for practical applications, even in extreme environments. Although LMB performance has been enhanced using various high-concentration electrolytes (HCEs) with hydrofluoroether dilution, efficient operation over a wide temperature range remains elusive. This study elucidated the factors that enable LMB cycling in a wide temperature range (5–60 °C) by exploiting a model HCE composed of lithium bis(fluorosulfonyl)imide and 1,2-dimethoxyethane as well as an HCE diluted with 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE). Comprehensive analyses revealed that TTE dilution plays an essential role at lower temperatures by enhancing Li⁺ ion transport in the concentrated electrolyte while maintaining the original solvation structure. Furthermore, as the performance-determining factor for high-temperature cycling, TTE involvement in the solid–electrolyte interphase (SEI) reinforced the thermal stability. Thus, TTE dilution is crucial for both facile mass transport and thermally stable SEI formation. The resulting Li dendrite suppression and high Li Coulombic efficiency enable the realization of LMBs with a wide operating temperature range.