TTE‐Based Localized High‐Concentration Electrolytes in Li Metal Cells: Effects on Li Reactivity and Solid Electrolyte Interphase Formation

The use of localized high‐concentration electrolytes (LHCEs) has emerged as a promising strategy to improve the stability of Li metal electrodes while overcoming the high viscosity and cost associated with high‐concentration electrolytes (HCEs). In this work, we investigate how electrolyte solvation structure and further physicochemical properties, like Li electrodeposition and SEI formation, evolve across a wide concentration range—from diluted electrolytes to HCEs and LHCEs—and how these changes influence Li reactivity and solid electrolyte interphase (SEI) formation in NCM ‖ Li cells. Ether‐based electrolytes composed of LiTFSI as conducting salt and 1,2‐dimethoxyethane (DME) as solvent are systematically modified using 1,1,2,2‐tetrafluoroethyl 2,2,3,3‐tetrafluoropropylether (TTE) as a non‐ion‐coordinating diluent to form LHCEs. Surface‐sensitive characterization, including time‐of‐flight secondary ion mass spectrometry, is employed to analyze the SEI composition. By correlating electrolyte concentration, solvation structure, and interfacial chemistry, this study focuses on the investigation of the Li metal surface after spontaneous formation of the SEI with the electrolytes and after electrodeposition/‐dissolution in Cu ‖ Li as well as high‐energy NCM ‖ Li cells.