The thermal and electrochemical behavior of pyrite as an electrode material for rechargeable lithium polymer electrolyte batteries has been investigated. The samples of pyrite from several different sources were characterized by thermogravimetric analysis (TGA), SEM, X-ray photoelectron (XPS) and electrochemical methods. As determined by thermogravimetric measurements, the pyrite samples of “vendors A and G” were highly stable up to 500°C. The weight loss of FeS2 at 500°C did not exceed 1.3%. The decomposition of the “vendor E” sample, including eight phase transitions, starts at about 100°C and is caused by the surface impurities of pyrite, such as iron oxides, hydroxides and sulfates. These influence the OCV and the first discharge of the Li/CPE/FeS2 cell. It is noteworthy that the performance characteristics, such as Li/Fe ratio, faradaic efficiency and charge–discharge overpotential of the Li/composite polymer electrolyte (CPE)/10-μm-thick cathode pyrite cells were found to be almost independent of the degree of contamination and, consequently, of the pyrite source during 30 cycles.