The aluminum rechargeable battery is a desirable device for large-scale energy storage owing to the high capacity derived from the properties of the aluminum metal anode. The development of cathode materials is needed to compose battery systems. However, the design principles of the cathode materials have not been determined. We focus on the high capacity FeS2 cathode materials and investigate the discharge/charge reaction mechanisms in chloroaluminate ionic liquids as the electrolyte at 55°C. X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) measurements are performed for the discharged and charged samples. S 3p-orbitals are shown to play an important role in the redox reactions from the results of the S and Fe K-edge XANES spectra. As a result of the redox reaction, FeS2 is transformed into low crystalline FeS and amorphous Al2S3, as shown by the XRD and S, Al, and Fe K-edge XANES spectra. This reaction mechanism is different from the reaction observed with lithium ion.