Reaction mechanism of the olivine-type Lix(Mn0.6Fe0.4)PO4 (0 ≤ x ≤ 1)

The charge-discharge reaction mechanism of the olivine-type Li-x(Mn0.6Fe0.4)PO4 (0 less than or equal to x less than or equal to 1.0), a possible 4 V class cathode material for lithium batteries, was investigated using equilibrium voltage measurements, X-ray diffraction, Mossbauer spectroscopy, and X-ray absorption spectroscopy. The Rat two-phase region with an open-circuit voltage (OCV) of ca. 4.1 V (region I: 0 less than or equal to x less than or equal to 0.6. Mn3+/Mn2+) and the S-curved single-phase region with OCV approximate to 3.5 V (region II: 0.6 less than or equal to x less than or equal to 1.0, Fe3+/Fe2+) were clearly identified together with the corresponding change in the unit cell dimensions of the orthorhombic lattice. These features show significant differences from the reaction mechanism of LixFePO4 (0 less than or equal to x less than or equal to 1), in which the whole Fe3+/Fe2+ reaction proceeds in a two-phase manner (LiFePO4-FePO4) with a Rat voltage profile at 3.4 V. (C) 2001 The Electrochemical Society.