In SituStructural Study of 4V-Range Lithium Extraction/Insertion in Fluorine-Substituted LiMn₂O₄

The structural features accompanying the lithium extraction/insertion process in a fluorine-substituted spinel oxide with formula LiMn₂(O_3.74F_0.26) 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 Li_xMn₂O₄.