In situ structural 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(O3.74F0.26) were studied in situ using 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 content x: (1) a single-phase range for 0.59 less than or equal to x less than or equal to 0.83, (2) a two-phase range for 0.23 less than or equal to x less than or equal to 0.59 with equilibrium potential 4.122 V, (3) a narrow single-phase range III for 0.18 less than or equal to x less than or equal to 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. (C) 1999 Academic Press.