Composition–Valence Diagrams: A New Representation of Topotactic Reactions in Ternary Transition Metal Oxide Systems. Application to Lithium Intercalation

Abstract

Topotactic reactions in Li–M–O systems withM= Mn, Ti, V, Fe are described in the framework of composition–valence diagrams, using the Li/Mand the transition metal valence ν(M) as coordinates. We show that this representation is very convenient to depict and compare the various parameters associated with insertion/extraction reactions, i.e., the evolution of ν(M), the extent of intercalationxin LixMOy, the electrochemical potentials, and the cell parameter changes. New directions are suggested for topotactic reaction in oxides, especially in titanium ones. The composition–valence diagram helped in the detection of inconsistencies in the intercalation potentials in LiFe5O8, which are corrected using new voltammetric data. We conclude that lithium intercalation in octahedral sites of iron spinel oxides occurs at constant voltage around 1.6 V, whichever the Li–Fe–O host may be.