Fluorinated diluent electrolyte with designed solvation structure enables stable solid–electrolyte interphase for long-term lithium metal batteries

Electrolyte solvation chemistry critically affects lithium metal battery (LMB) performance. Developing functional diluents for local high-concentration electrolytes (LHCEs) is crucial yet constrained by conventional fluorinated ethers. Here we report a partially fluorinated aromatic diluent, 4-(1,1,2,2-tetrafluoroethoxy) toluene (TFTT), with negligible Li + coordination due to its distinct molecular structure. This unique property preserves the anionic coordination environment of the high-concentration network while strengthening Li + -FSI − interactions, promoting contact-ion pairs and aggregates. This tailored solvation structure, revealed by operando spectroscopy and DFT calculations, directs stable solid–electrolyte interphase (SEI) formation. Consequently, the TFTT-based LHCE enables highly reversible Li plating/stripping with 99.1% Coulombic efficiency over 600 cycles and stable cycling exceeding 1,500 h. Li||LiFePO 4 and Li||NCM811 cells show exceptional cyclability, and a practical 1.2 Ah pouch cell delivers 295 Wh kg −1 with 88.6% capacity retention after 100 cycles. This work provides a novel diluent design strategy for practical long-life LMBs.