Sacrificial NH₄HCO₃ Inhibits Fluoropolymer/Garnet Interfacial Reactions Toward 1mS cm⁻¹ and 5V‐Level Composite Solid Electrolyte

Abstract Composite solid electrolytes (CSEs) integrate the fast ion conductivity of inorganic electrolytes and the excellent interfacial compatibility of polymer electrolytes. Typically, fluoropolymers and garnets are promising individuals to formulate cutting‐edge CSEs owing to their unique properties. However, the alkaline garnets can induce the dehydrofluorination of fluoropolymers, deteriorating their CSEs performance. Here, for the first time, NH₄HCO₃ is proposed as a sacrificial inhibitor to effectively prevent the garnet‐induced dehydrofluorination, using Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) and poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVH) as symbolic garnets and fluoropolymers, respectively. Various findings demonstrate that NH₄HCO₃ can buffer the alkalinity of LLZTO, thereby inhibiting the dehydrofluorination of PVH. In addition, NH₄HCO₃ can completely decompose to volatiles upon drying without compromising the properties of LLZTO and PVH. Additionally, a polymer‐in‐salt strategy is further introduced by adding high‐concentration LiTFSI salt to the above system, resulting in the PVH/LiTFSI/LLZTO (PLL) CSEs. Benefiting from the synergetic coupling of the sacrificial inhibitor and polymer‐in‐salt strategies, the PLL exhibits an exceptionally high ionic conductivity of 1.2 mS cm⁻¹ at 25 °C and stable voltage of 5.09 V, outperforming other reported CSEs. Consequently, the PLL delivers impressive high‐rate cyclability in solid‐state lithium‐metal batteries with an outstanding capacity retention of 95.4% after 240 cycles at 1 C (25 °C).