A Flame Retardant PVDF-HFP-Based Composite Electrolyte Enabled by Yttrium-Doped NASICON for Stable Lithium Metal Batteries

Composite polymer electrolytes (CPEs) that integrate the advantages of both inorganic and polymer electrolytes show considerable scope for all-solid-state lithium metal batteries (ASSLMBs) by virtue of their enhanced ionic conductivity, excellent mechanical strength, and low interfacial resistance. Here, NASICON-type Li_1.5Al_0.43Y_0.07Ti_1.5(PO₄)₃ (YLATP) is used as an inorganic filler for incorporation into a poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) organic matrix. The 7 wt % YLATP CPE exhibits a wide electrochemically stable potential window of over 5 V and the highest ionic conductivity of 6.28 × 10^–4 S cm^–1 at room temperature, with a lithium-ion transference number of 0.78. It also exhibits a self-extinguishing phenomenon, which suggests fireproof properties in comparison to the prevalent lithium-ion battery separator. Consequently, the ASSLMB using LiFePO₄ as the cathode demonstrates a high initial discharge capacity of 168 mAh g^–1 at a 0.1 C-rate, with a capacity retention of 94% after 50 cycles and improved rate capability in comparison to the pristine (Li_1.5Al_0.5Ti_1.5(PO₄)₃) inorganic filler. Post-cycling analysis confirms the formation of a favorable solid electrolyte interphase (SEI) in the 7 wt % YLATP-based CPE, which exhibits a significantly lower concentration of CO₃^2–. This indicates that the low lithium-ion-conducting inorganic byproduct, Li₂CO₃, does not substantially contribute to the interfacial processes. The synergistic effects of the YLATP filler, namely its ability to reduce the crystallinity of the PVDF-HFP matrix and to enhance Li⁺ transport via immobilization of TFSI^– anions, collectively contribute to the development of a high-performance CPE suitable for advanced all-solid-state lithium metal batteries (ASSLMBs).