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• The novel internal lithium polymeric salt interfacial phase is well constructed • The in-situ polymerization and solvent-free methods are designed and investigated. • The interfacial phase construction process is suitable for various ceramic fillers. • Enhanced ionic conductivity properties and electrochemical stability of electrolytes. • The improved long-cycle performance of Li symmetric cells and full metal batteries. The solid-state lithium metal batteries are considered a promising direction for developing lithium-based energy storage devices because of their superiority over lithium-ion batteries containing liquid electrolytes in energy density and safety performance. However, the design, preparation, and optimization of solid-state electrolytes (SSEs), especially composite solid-state electrolytes (CSEs), still face critical construction design and composite process challenges. This work proposes and designs a method to construct a highly compatible and lithium-ion conductive interfacial phase by coating the inorganic phase with polymeric lithium salt in the composite solid electrolytes. The introduction of the lithium salt interfacial phase efficiently improves the cycle performance and stability of CSEs in lithium metal batteries by synergistic optimization of the composite interfacial phase. We investigate two different modification methods (solvent-free and in-situ methods) in two different inorganic systems (Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 (LLZTO) and LiTa 2 PO 8 (LTPO)) to demonstrate the practical value of this design. The highest ionic conductivity of the modified PEO/LTPO CSEs (50 wt%) can be improved to 9.3 × 10-4 S·cm−1 at 80℃ and 4.0 × 10-5 at RT (∼25℃, hereinafter inclusive), while those of the modified PEO/LLZTO CSEs can reach 1.4 × 10-3 S·cm−1 at 80℃ and 5.5 × 10-5 at RT. The results demonstrate that the interfacial phase also enhances the electrochemical and cycle stability of CSEs. [ABSTRACT FROM AUTHOR]

• Li 0.35 La 0.55 TiO 3 (LLTO) are synthesized by sol-gel method and two-step calcination. • LLTO-(PEO-FEC) possess high Li+ conductivity vs low electronic conductivity at 25 °C. • LLTO-(PEO-FEC) show a wide electrochemical window of 5.2 V (vs. Li+/Li). • FEC-Li+ coordination enables interface self-healing and dendrite-free Li deposition. Solid-state Li-metal batteries have attracted much attention owing to the high energy density and safety afforded by Li-metal anode and solid-state electrolytes. However, Li-dendrite penetration and unstable interface between solid-state electrolyte and Li-metal incumber their application. Herein, fluoroethylene carbonate (FEC) added Li 0.35 La 0.55 TiO 3 -Poly (ethylene oxide) (LLTO-(PEO-FEC)) composite solid-state electrolytes (CSSEs) are fabricated to address the bottlenecks. The CSSE possesses high Li+ conductivity of 1.13 × 10−4 S·cm−1 vs low electronic conductivity of 1.68 × 10−9 S·cm−1 at 25 °C, presents a wide electrochemical window of 5.2 V (vs. Li+/Li), and shows much more stable lithium deposition at current density of 0.2 mA·cm−2 for 800 h. FEC is driven by Li+ to the damaged interface to form new LiF-rich interface layer, which enables the self-healing of Li-metal and CSSE interface and enhances the electrochemical stability of CSSEs against Li-metal. Li| (LLTO-(PEO-FEC)) |LiFePO 4 cell presents discharge capability of about 115 mAh·g−1 after 100 cycles at 50 °C. This work reveals the mechanism of dendrite-free Li deposition via interface self-healing process under the effect of FEC–Li+ coordination and provides a promising way to prepare the high performances composite solid-state electrolytes and to fabricate the dendrite-free all-solid-state lithium battery. [ABSTRACT FROM AUTHOR]