LiNi0.5Mn0.5O2 thin films have been deposited on the NASICON-type glass ceramics, Li1+x+y Al x Ti2−x Si y P3−y O12 (LATSP), by radio frequency (RF) magnetron sputtering followed by annealing. The films have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy. All-solid-state Li/PEO18-Li(CF3SO2)2N/LATSP/LiNi0.5Mn0.5O2/Au cells are fabricated using the LiNi0.5Mn0.5O2 thin films and the LATSP electrolyte. The electrochemical performance of the cells is investigated by galvanostatic cycling, cyclic voltammetry (CV), potentiostatic intermittent titration technique (PITT) and electrochemical impedance spectroscopy (EIS). Interfacial reactions between LiNi0.5Mn0.5O2 and LATSP occur at a temperature as low as 300°C with the formation of Mn3O4, resulting in an increased obstacle for Li-ion diffusion across the LiNi0.5Mn0.5O2/LATSP interface. The electrochemical performance of the cells is limited by the interfacial resistance between LATSP and LiNi0.5Mn0.5O2 as well as the Li-ion diffusion kinetics in LiNi0.5Mn0.5O2 bulk.