The chemical and physical properties of pyrite have been reviewed with reference to cathode material in lithium/polymer electrolyte batteries. The analysis of dQ/dV curves showed that the charge-discharge process in Li/polymer electrolyte/pyrite battery is more complicated than in non-aqueous and molten media. At least seven domains are distinguished on the dQ/dV discharge curve. The low-voltage step on discharge may be associated with the formation of a new phase causing from the reaction of metallic iron with the electrolyte components. This would explain the capacity fading of the cell. The high-voltage 1.85-2.25 V charge region may be attributed to the insertion of lithium into Li2-xFeS2. However the de-intercalation of lithium from Li2FeS2 in a LiI-CPE cell operating at 130 degrees C is not a pure topotactic one, but rather the Li2-xFeS2 undergoes some structural change on cycling. Iron oxides, hydroxides and sulfates contaminate the surface of pyrite from different sources. However the performance characteristics, such as reversible capacity and polarization of the Li/composite polymer electrolyte/FeS2 cells were found to be independent of the amount of impurities. The thin-cathode cell design has a projected energy density of 130 W h/kg at C/3 rate and specific power of 300 W/kg ton the basis of 5 mA/cm(2) demonstrated in experimental cells). Over 500 100% DOD cycles with a capacity fading rate of less than 0.1%/cycle have been demonstrated in a small laboratory prototype 7 mu m-thick modified cathode cell.