There is a growing demand for the development of high-energy-density lithium batteries for a number of applications including electric vehicles (EV), energy storage and space. The Li/composite polymer electrolyte (CPE)/pyrite battery, which has a high theoretical energy density (about 810 Wh kg−1 based on 2.8e/FeS2), and is made of cheap, non-toxic and green compounds is a good candidate for EV applications. Materials cost is estimated at 50$ kWh−1 five times lower than that of other lithium and lithium-ion batteries. Over 500 100% DOD cycles (at c 3 rate) with a capacity fading rate of less than 0.1% per cycle were carried out in a small (1 cm2 area) laboratory prototype cells with 7 μm-thick cathodes. Charge–discharge processes in the Li/LiI(PEO) n Al2O3-based CPE/pyrite battery during long-term cycle life have been analyzed with the use of dq/dV curves. These studies furnish insights into the electrochemical behavior of pyrite in polymer electrolyte-systems. Up to seven phases have been identified and found to change during the first 50–100 cycles. These phases do not change much over the subsequent 400 cycles. The major phases have been recently identified by EXAFS and NEXAFS measurements. It was proved that reduction of the ferrous disulfide proceeds as a multi-stage process, first to Li2FeS2 and finally to metallic iron. No evidence of FeS was found. When the battery is charged to 2.25 V, Li2−x FeS2 is formed.