A polychromatic simultaneous wavelength-dispersive X-ray fluorescence (PS-WDXRF) spectrometer can measure the valence changes of 3d transition metals with high precision in the laboratory. Adjustment and maintenance of the drive mechanism are unnecessary, and high-precision measurements are possible in a short time because the optical system has no moving parts and is compact. We have developed a PS-WDXRF spectrometer with improved analytical precision that can measure simultaneously the valence changes of three main elements, Mn, Co, and Ni, which are used as cathode materials in Li-ion batteries (LIBs). In this study, the analytical precision of the spectrometer is evaluated, and its precision is confirmed with actual battery cathodes. The identification precision of the fluorescent X-ray peak energy is <0.015 eV, and the valence identification precision is obtained to be <0.06. LiNi0.5Co0.2Mn0.3O2 (NCM523)-based LIB cathodes are analyzed under conditions maintaining this precision, and the valence changes of the 3d transition metals in NCM523 during charging and discharging are found to be 0.68 for Ni, 0.19 for Co, and 0.08 for Mn. These results indicate that Ni contributes the most to the redox process in NCM523-based LIBs, Co contributes slightly, and Mn does not contribute.