Reversible extraction of lithium from LiFePO4 (triphylite) and insertion of lithium into FePO4 at 3.5 V vs. lithium at 0.05 mA/cm2 shows this material to be an excellent candidate for the cathode of a low‐power, rechargeable lithium battery that is inexpensive, nontoxic, and environmentally benign. Electrochemical extraction was limited 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 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.

To understand the role of structure on the position of the octahedral Fe37Fe2 redox couple in compounds having the same polyanions, four iron phosphates: Li3Fe2(P04)3, LiFeP2O7, Fe4(P207)3, and LiFePO4 were investigated. They vary in structure as well as in the manner in which the octahedral iron atoms are linked to each other. The Fe3t/Fe2 redox couple in the above compounds lies at 2.8, 2.9, 3.1, and 3.5 eV, respectively, below the Fermi level of lithium. The reason for the difference in the position of the redox couples is related to changes in the P-O bond lengths as well as to changes in the crystafline electric field at the iron sites.

The position of several transition metal redox energies with respect to the Fermi energy of lithium in phosphates with sodium super ionic conductor (NASICON) framework were determined electrochemically upon lithium intercalation. The materials studied were Li2NaV2(P04)3, LiNa2FeV(P04)3, TiNb(P04)3, LiFeNb(P0j3, and Li2FeTi(P0j3 all having NASICON framework. The positions of the redox couples were found centered at the following energies: V4/V3 at 3.8 eV, Fe37Fe2 at 2.8 eV, Ti4/Ti3 at 2.5 eV, Nb57Nb4 at 2.2 eV, Nb47Nb3 at 1.8 eV, and V3/V2 at 1.7 eV below the Fermi energy of lithium. Several of these redox couples were found to overlap each other. Some of these materials look promising as positive electrode materials for Lit-ion batteries.

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