Reaction Mechanism of the Olivine-Type Li x ( Mn0.6Fe0.4 ) PO 4 ( 0 ⩽ x ⩽ 1 )

The charge-discharge reaction mechanism of the olivine-type Li x (Mn 0.6 Fe 0.4 )PO 4 (0 ≤ x ≤ 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 flat two-phase region with an open-circuit voltage (OCV) of ca. 4.1 V (region 1: 0 ≤ x ≤ 0.6, Mn 3+ /Mn 2+ ) and the S-curved single-phase region with OCV 3.5 V (region 11: 0.6 ≤ x ≤ 1.0, Fe 3+ /Fe 2+ ) 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 Li x FePO 4 (0 ≤ x ≤ 1) ), in which the whole Fe 3+ /Fe 2+ reaction proceeds in a two-phase manner (LiFePO 4 -FePO 4 ) with a flat voltage profile at 3.4 V.