The development of high-performance solid-state polymer electrolytes remains critical for enabling safe and durable lithium-ion batteries. In this work, PVA encapsulated Li7La3Zr2O12 (modified-LLZOPVA) was synthesized and confirmed through XRD, Raman analysis, and TEM. The modified-LLZOPVA powders were used as fillers in the PEO-LiCF3SO3 (O: Li ∼ 15) polymer matrix (PS15) to prepare an advanced composite electrolyte (CE) via a simple solution-casting approach. With a dc conductivity of 2.06 × 10−4 S cm−1, the optimised CE film (PS15 + 20 wt% modified-LLZOPVA), which is two orders of magnitude higher than the pristine polymer electrolyte (PS15) and one order higher than that containing unmodified LLZO. Finally, the electrochemical performance of the CE films, used as a separator-cum-electrolytes was evaluated in an all-solid-state Li-ion cell with LFP as the cathode. At 0.1C (60 °C), the LFP || PS15 + 20 wt% modified-LLZOPVA || Li cell provides a discharge capacity of 129.20 mA h g−1, a coulombic efficiency of 98.6%, and a capacity retention of 91.26%.On the other hand, under the same conditions, the LFP || PS15 || Li cell exhibits a lower discharge capacity of 80.01 mA h g−1, with 85.39% capacity retention and 95.2% coulombic efficiency. Moreover, the cell with the optimised PS15 + 20 wt% modified-LLZOPVA electrolyte demonstrates superior cycling stability, sustaining performance for up to 625 cycles, whereas the LFP || PS15 || Li cell maintains stability for only 280 cycles at 1C (60 °C). This study offers a valuable reference for the fabrication of composite electrolytes using simple processing routes while achieving high-voltage compatibility and superior performance.