All-solid-state batteries (ASSBs) have garnered significant attention for their potential use in high-performance Li-ion batteries for next-generation electric vehicles. Warm isostatic pressing (WIP) is a promising technique for the densification of ASSB electrodes. In this study, composite cathodes consisting of a mixture of Li(Ni_1/3Mn_1/3Co1/3)O₂ and Li₆PS₅Cl were processed using WIP and assembled into test cells to investigate the correlation between the structure of the electrodes and their electrochemical performance. The structures of the electrodes were analyzed using X-ray computed tomography (CT) with synchrotron radiation at SPring-8. Electrochemical performance was evaluated through electrochemical impedance spectroscopy and charge-discharge measurements. The voids in the electrodes decreased as the WIP temperature increased, resulting in improved charge-discharge properties, which suggests a reduction in charge transfer resistance. Thus, WIP was found to be effective in the production of ASSBs.