Reaction Mechanism of the Olivine-Type Li[sub x](Mn[sub 0.6]Fe[sub 0.4])PO[sub 4] (0≤x≤1)

Abstract

The charge-discharge reaction mechanism of the olivine-type Lix(Mn0.6Fe0.4)PO4Lix(Mn0.6Fe0.4)PO4 (0⩽x⩽1.0),(0⩽x⩽1.0), a possible 4 V class cathode material for lithium batteries, was investigated using equilibrium voltage measurements, X-ray diffraction, Mössbauer spectroscopy, and X-ray absorption spectroscopy. The flat two-phase region with an open-circuit voltage (OCV) of ca. 4.1 V (region I: 0⩽x⩽0.6,0⩽x⩽0.6, Mn3+/Mn2+)Mn3+/Mn2+) and the S-curved single-phase region with OCV≈3.5VOCV≈3.5V (region II: 0.6⩽x⩽1.0,0.6⩽x⩽1.0, Fe3+/Fe2+)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 LixFePO4LixFePO4 (0⩽x⩽1),(0⩽x⩽1), in which the whole Fe3+/Fe2+Fe3+/Fe2+ reaction proceeds in a two-phase manner (LiFePO4­FePO4)(LiFePO4­FePO4) with a flat voltage profile at 3.4 V. © 2001 The Electrochemical Society. All rights reserved.