[EMIm]NO2-Modified PVDF–HFP Ionic Gel Electrolyte Membranes for Improved Lithium-Ion Transport and Interfacial Stability

Gel polymer electrolytes typically suffer from low ionic conductivity, limited oxidative stability, and thermal runaway. Nitrate additives can be beneficial, but they are limited by poor solubility. Herein, sandwich-structured ionic liquid gel electrolyte membranes were fabricated by incorporating a polyethylene (PE) interlayer between two poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF–HFP) layers loaded with LiTFSI and 1-ethyl-3-methylimidazolium nitrate ([EMIm]NO2). The PFENE3 membrane prepared with 22.9 wt % [EMIm]NO2 shows optimal overall performance. Specifically, the PE scaffold provides robust mechanical strength (36.7 MPa). The incorporation of [EMIm]NO2 induces multiple synergistic effects: (i) increasing ionic conductivity to 5.03 × 10–4 S cm–1 at 25 °C by suppressing PVDF–HFP crystallinity; (ii) achieving a high Li+ ion transference number (0.449) by confining other charge carriers within the TFSI-[EMIm]+-NO2 hydrogen-bonding network; and (iii) the sacrificial decomposition of NO2 to form robust protective interphases on both the anode and cathode. Consequently, Li/PFENE3/Li symmetric cells demonstrated stable cycling for over 600 h at 0.3 mA cm–2. Furthermore, the Li/PFENE3/NCM622 full cell delivered an initial capacity of 170.9 mAh g–1 at 0.1 C and 159.3 mAh g–1 at 0.5 C, with 78% retention after 100 cycles, significantly outperforming [EMIm]NO2-free systems. These findings provide a viable strategy for developing high-performance and safe Li-metal batteries.