Unique double-layer solid electrolyte interphase formed with fluorinated ether-based electrolytes for high-voltage lithium metal batteries

Li metal batteries using high-voltage layered oxides cathodes are of particular interest due to their high energy density. However, they suffer from short lifespan and extreme safety concerns, which are attributed to the degradation of layered oxides and the decomposition of electrolyte at high voltage, as well as the high reactivity of metallic Li. The key is the development of stable electrolytes against both high-voltage cathodes and Li with the formation of robust interphase films on the surfaces. Herein, we report a highly fluorinated ether, 1,1,1-trifluoro-2-[(2,2,2-trifluoroethoxy) methoxy] ethane (TTME), as a co-solvent, which not only functions as a diluent forming a localized high concentration electrolyte (LHCE), but also participates in the construction of the inner solvation structure. The TTME-based electrolyte is stable itself at high voltage and induces the formation of a unique double-layer solid electrolyte interphase (SEI) film, which is embodied as one layer rich in crystalline structural components for enhanced mechanical strength and another amorphous layer with a higher concentration of organic components for enhanced flexibility. The Li||Cu cells display a noticeably high Coulombic efficiency of 99.28% after 300 cycles and Li symmetric cells maintain stable cycling more than 3200 h at 0.5 mA/cm² and 1.0 mAh/cm². In addition, lithium metal cells using LiNi_0.8Co_0.1Mn_0.1O₂ and LiCoO₂ cathodes (both loadings ∼3.0 mAh/cm²) realize capacity retentions of >85% over 240 cycles with a charge cut-off voltage of 4.4 V and 90% for 170 cycles with a charge cut-off voltage of 4.5 V, respectively. This study offers a bifunctional ether-based electrolyte solvent beneficial for high-voltage Li metal batteries.