Characteristics of the 4 V plateau in LiMn2(O4−xFx) studied by in situ synchrotron X-ray diffraction

Fluorine-substituted LixMn2O4 was studied by synchrotron X-ray diffraction using Bellcore-type plastic batteries placed directly in the synchrotron beam. The results show that the ‘4 V plateauʹ actually contains three regions: (1) a single-phase range for 0.59≤x≤1, (2) a two-phase range for 0.23≤x≤0.59, (3) a narrow single-phase range for x≤0.23. These three domains are fully reversible, but the plastic battery undergoes extraneous oxidation reactions for voltages ≥4.4 V. The structural features of this compound are qualitatively similar to those of stoichiometric LixMn2O4. In particular, no suppression of the two-phase range such as that observed on non-stoichiometric or Ni-substituted LiMn2O4 was found.