The composite solid electrolytes (CSEs) have attracted considerable attention because of their excellent flexibility and good compatibility with electrode materials. Our CSE was prepared by incorporating LLZO ceramic filler, lithium salt, and different concentrations of a solid plasticizer in a poly (vinylidene fluoride hexafluoropropylene) (PVDF-HFP) polymer matrix. Afterwards, we examined the ionic conductivity and electrochemical stability of the CSE, and its interfacial compatibility with a Li-metal electrode. Optimized wt.% plasticizer with CSE exhibits a high ionic conductivity of 4.23 × 10^–4 and 8.14 × 10^–4 S cm^–1 at 25 and 60 °C, respectively, with an excellent stability of up to 4.76 V vs. Li/Li⁺. The CSE is sandwiched between the Li-metal anode and NCM811 cathode for the fabrication of a solid-state battery (SSB), which provides a discharge capacity (163 mAh g^–1) at a rate of 0.2 C (60 °C). The LiNbO₃ (LNO) surface modification over the NCM811 cell provides a high discharge capacity of 198 mAh g^–1, excellent rate capability, and good cycle life. Moreover, the LNO coating on the surface of the NCM811 cathode, which effectively increases the contact between the cathode and electrolyte, eventually leads to an increase in the discharge capacity. Hence, the prepared CSE is a good choice for use in SSB; moreover, the LNO coating noticeably improved the cycling stability, reversibility, and rate capability compared to a pristine NCM811 cathode in the SSB configuration.