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The structural features accompanying the lithium extraction/insertion process in a fluorine-substituted spinel oxide with formula LiMn2(O3.74F0.26) were studiedin situusing Bellcore-type plastic batteries directly placed in a synchrotron X-ray beam. The initial material contains two phases, a cubic spinel and a slightly orthorhombically distorted one (major phase). It becomes entirely cubic on extraction of 0.17 Li atoms per formula unit. The lithium extraction reaction around 4 V vs Li/Li+can be divided into three regions as a function of lithium contentx: (1) a single-phase range for 0.59≤x≤0.83, (2) a two-phase range for 0.23≤x≤0.59 with equilibrium potential 4.122 V, (3) a narrow single-phase range III for 0.18≤x≤0.23. These three domains are fully reversible. With the exception of the presence of the initial orthorhombic distortion, the main features of the so-called electrochemical 4 V plateau are very similar to those of stoichiometric LixMn2O4.

Neutron powder diffraction has been carried out on two series of Li–Mn–O samples with spinel structure, which have been shown to lose oxygen with respect to initial stoichiometric spinels: (i) Li1.05Mn2O4quenched from 800 or 925°C, (ii) “Li1.33Mn1.67O4” (nominal) treated with ammonia gas at 200°C. The structural refinements unambiguously show that both 925°C quenching and low-temperature ammonia treatment induce oxygen vacancies in the spinel lattice. However, the mechanism of oxygen loss is markedly different between quenchings at 800 and 925°C. While the latter actually corresponds to the introduction of anionic vacancies at constant cation composition, the former is due to a rearrangement of the Li/Mn ratio in the spinel phase and the formation of an additional lithium-rich phase Li2MnO3. The presence of vacancies induce a significant increase in atomic displacement parameters of oxygen.

Topotactic reactions in Li– M –O systems with M = Mn, Ti, V, Fe are described in the framework of composition–valence diagrams, using the Li/ M and 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 intercalation x in Li x M O y , 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 LiFe 5 O 8 , 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.