The reaction of natural pyrite and lithium was investigated in solvent-free, polymer electrolytes at medium temperature (90°C). In such a system the full capacity of pyrite is accessed in the first discharge–charge cycle, however, the system is affected by a substantial capacity fading especially in low rate discharges. The capacity fading observed is attributed to the irreversible reaction of the sulfur formed at the end of charge step and the polymer in the electrolyte. The investigation showed that the irreversible process occurring at 1.3V is not the cause of the capacity fading but it is resulting from the reduction of the above mentioned oxidation products.