Optimised sol-gel synthesis of Al-doped LLZO/PVDF–HFP/LiTFSI composite electrolytes with enhanced electrochemical performance

Composite solid electrolytes (CSEs) are promising alternatives to liquid electrolytes in lithium-ion batteries (LIBs). CSEs offer superior electrochemical stability, safety, and mechanical integrity. Herein, a flexible CSE film comprising aluminium-doped Li 6.28 La 3 Zr 2 Al 0.24 O 12 (LLZO – Al) and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF – HFP) is successfully fabricated via an optimised sol-gel synthesis and solution casting method. The LLZO – Al filler, with high cubic-phase purity, effectively reduces the crystallinity of the PVDF – HFP matrix, creating continuous Li + transport pathways. The optimised composite membrane containing 15 wt% LLZO – Al (CE15%) exhibits a high room-temperature ionic conductivity of 6.1 × 10 –4 S cm –1 and a broad electrochemical stability window up to 4.8 V vs. Li/Li + . Additionally, it demonstrates excellent mechanical flexibility (elongation at break: 162%) and enhanced thermal resistance. The Li/CE15%/Li symmetric cell maintains stable cycling for over 2000 h at 0.1 mA cm –2 , confirming outstanding interfacial stability. Furthermore, the LFP/CE15%/Li full cell delivers an initial discharge capacity of 166.1 mAh g –1 at 0.1 C and retains 86% of its capacity after 300 cycles. These results highlight the great potential of LLZO–Al/PVDF–HFP/LiTFSI composite electrolytes for high-performance solid-state Li-ion batteries.