Phospho‐olivines as Positive‐Electrode Materials for Rechargeable Lithium Batteries

Reversible extraction of lithium from LiFePO 4 (triphylite) and insertion of lithium into FePO 4 at 3.5 V vs. lithium at 0.05 mA/cm 2 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, FePO 4 , isostructural with heterosite, Fe 0.65 Mn 0.35 PO 4 . The FePO 4 framework of the ordered olivine LiFePO 4 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 LiMPO 4 (M = Mn, Co, or Ni) with an LiClO 4 electrolyte was not possible; but successful extraction of lithium from LiFe 1-x Mn x PO 4 was accomplished with maximum oxidation of the Mn 3+ /Mn 2+ occurring at x = 0.5. The Fe 3+ /Fe 2+ couple was oxidized first at 3.5 V followed by oxidation of the Mn 3+ /Mn 2+ couple at 4.1 V vs. lithium. The Fe 3+ -O-Mn 2+ interactions appear to destabilize the Mn 2+ level and stabilize the Fe 2+ level so as to make the Mn 3+ /Mn 2+ energy accessible.