Reversible extraction of lithium from LiFePO4 (triphylite) and insertion of lithium into FePO4 at 3.5 V vs. lithium at 0.05 mA/cm(2) shows this material to be an excellent candidate for the cathode oi a low-power, rechargeable Lithium battery that is inexpensive, nontoxic, and environmentally benign. Electrochemical extraction was limited to similar to 0.6 Li/formula unit; but even with this restriction the specific capacity is 100 to 110 mAh/g. Complete extraction of lithium was performed chemically; it gave a new phase, FePO4, isostructural with heterosite, Fe0.65Mn0.35PO4. The FePO4 framework of the ordered olivine LiFePO4 is retained with minor displacive adjustments. Nevertheless the insertion/extraction reaction eless proceeds via a two-phase process, and a reversible loss in capacity with increasing current density appears to be associated with a diffusion-limited transfer of lithium across the two-phase interface. Electrochemical extraction of lithium from isostructural LiMPO4 (M = Mn, Co, or Ni) with an LiClO4 electrolyte was not possible; but successful extraction of lithium from LiFe1-xMnxPO4 was accomplished with maximum oxidation of the Mn3+/Mn2+ occurring at x = 0.5. The Fe3+/Fe2+ couple was oxidized first at 3.5 V followed by oxidation of the Mn3+/Mn2+ couple at 4.1 V vs. lithium. The Fe3+-O-Mn2+ interactions appear to destabilize the Mn2+ level and stabilize the Fe3+ level so as to make the Mn3+/Mn2+ energy accessible.