Interface-Stabilized and Fire-Resistant Composite Polymer Electrolyte for Safe and Durable All-Solid-State Lithium Batteries

The multifaceted composition of the solid electrolyte interface (SEI), the low Li-ion conductivity, and the fire hazard sensitivity of poly(ethylene oxide) (PEO)-based solid-state-electrolytes (SSEs) restrict them from being used in cutting-edge all-solid-state lithium–metal batteries (SS-LMBs). Here, a multifunctional solid composite polymer electrolyte (SMB-CPE) was developed by using silica mesoball fillers, offering simultaneous improvements in ionic transport, interfacial stability, and thermal protection. The presence of fillers enabled the segmental motion of the polymer chains, thereby reducing the activation energy for Li-ion diffusion and empowering more efficient Li-ion transportation that exhibited Li-ion conductivity of 6.37 × 10^–3 S cm^–1 at 60 °C. Furthermore, the critical current density dramatically doubled it when compared to the unfilled system. Notably, the symmetric [Li/SMB-CPE/Li] cell showed excellent galvanostatic Li plating and stripping, exhibiting stability for 2000 h at 200 μA cm^–2. However, full-cell configurations with LiFePO₄ delivered an initial discharge capacity of ∼150 mAh g^–1 at 1 C and retained 81.5% capacity after 1000 cycles. Moreover, postcombustion studies reveal that the filler enhanced carbonization and suppressed the formation of hazardous byproducts. The formation of C–O, C═O, CF₃, LiF, and high-oxidation-state sulfur and nitrogen species was significantly lessened, suggesting mitigation of exothermic and toxic degradation pathways.