Fluorine-regulated cathode electrolyte interphase enables high-energy quasi-solid-state lithium metal batteries

Lithium metal batteries (LMBs) enabled by quasi-solid electrolytes are under consideration for their prospect of reliable safety and high energy density. The limited oxidative stabilization and inferior chemical compatibility of quasi-solid electrolytes toward high-voltage cathodes are a long-standing challenge. Herein, we report that an additive level (0.05 M) of LiPF6 is introduced to a polymeric concentrated quasi-solid electrolyte (10 M LiFSI in poly-1,3-dioxolane [poly-DOL], ethylene carbonate [EC], and ethyl methyl carbonate [EMC]) to build in situ a fluorine-regulated cathode electrolyte interphase (CEI) on a highly catalytic LiNi0.8Co0.1Mn0.1O2 (NCM811) cathode. The CEI with a conformal thickness of ∼7 nm features a fluorine-rich outer layer and manipulative LiF/organofluorine species, which mitigates the detrimental side reactions between the quasi-solid electrolyte and NCM cathode and maintains the structure of cycled NCM, as demonstrated by the characterizations of SEM, TEM, XRD, Raman spectroscopy, AFM, EDS, and XPS. As a result, the LiPF6-contained polymeric concentrated quasi-solid electrolyte not only provides a superior ionic conductivity of 3.1 × 10−4 S cm−1 at 25 °C and a remarkable electrochemical stability window of 5.5 V vs Li/Li+, but also achieves an excellent capacity retention of 74% after 100 cycles for LiǁNCM811 quasi-solid-state LMB, bringing a quasi-solid electrolyte design strategy of engineered CEI chemistry for LMBs.