Utilizing Li-ion conductors as solid electrolytes is essential in solid- state lithium (Li) batteries (SSLBs), which is a promising solution for the next-generation electrochemical energy storage systems that require high energy and high levels of safety. Among various Li-ion conductors, Li1.5Al0.5Ti1.5(PO4)(3) (LATP), a NASICON-type ceramic, has attracted intensive attention due to its advantages of air stability and fast Li+ conductivity. However, to reach a decent ionic conductivity and reduce grain boundary resistance, LATP requires high temperatures for densification, which is time-consuming and expensive for large-scale applications. Herein, we report a simple solution-casting synthesis for new composite solid electrolytes by embedding LATP ceramic into a PVdF-HFP matrix In the LATP/PVdF -HFP composite solid membranes, the NASICON-type crystal structure of LATP is well maintained. Without taking any additional liquid electrolyte absorption, the prepared composite solid electrolytes with 10 wt % LATP show the highest ionic conductivity of 2.3 X 10(-4) S cm(-1) at room temperature, three times higher than that of polymer electrolyte (7.1 X 10(-5) S cm(-1)). In addition, the Li parallel to LiFePO4 (LFP) battery with LATP/PVdF-HFP composite electrolyte exhibits enhanced cycling performance of both capacity and stability as compared to the polymer electrolyte-based battery.