Garnets Initiate Grafting of Methyl Methacrylate Brushes onto Fluoropolymers for Electrochemically Stable and Fast‐Ion‐Conducting Composite Solid‐State Electrolytes

Fluoropolymer‐based solid‐state electrolytes (SSEs) promise next‐generation all‐solid‐state Li metal batteries but suffer poor stability against Li metal anodes and sluggish Li+ transport. Here, we propose garnet‐type Li6.4La3Zr1.4Ta0.6O12 (LLZTO) as a bifunctional mediator to enable the in‐situ grafting and compositing for poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVH), aiming at electrochemically stable and superionic SSEs. The LLZTO not only induces the formation of C=C bonds as active sites for effectively grafting methyl methacrylate (MMA) brush chains to PVH, but also acts as an ion‐conducting filler to enhance mechanical properties and ion transport. In addition, the grafted MMA brush chains improve electrochemical stability against Li metal anodes and weaken polymer crystallinity to create amorphous domains for Li+ transport. Therefore, the resulting composite SSEs, PVH‐graft‐MMA/LLZTO (PVHML), achieves an impressive ionic conductivity of 0.94 mS cm−1 at 25 °C, high mechanical strength (2.02 MPa), and exceptional cycling stability in Li symmetric cells (2800 h at 0.1 mA cm−1, 25 °C). Furthermore, PVHML‐based all‐solid‐state LiFePO4|Li full cells demonstrate superior cyclability with 89.8% capacity retention at 0.2C after 200 cycles (25 °C). This strategy provides an efficient solution to the challenges of fluoropolymer‐based SSEs, paving the way for their practical applications in high‐performance all‐solid‐state lithium metal batteries.