Structural Changes in the LiCrMnO4Cathode Material during Electrochemical Li Extraction and Insertion

Structural features of Li-extraction from the LiCrMnO4 cathode material in Li-electrochemical cells were investigated by in situ synchrotron powder diffraction and completed by ex situ neutron powder diffraction and magnetization measurements on delithiated products. The observed diffraction patterns reveal that both Li-extraction during the cell charge up to 5.15 V and the following Li-insertion upon the discharge down to 3.2 V vs. Li+/Li are one-phase solid-solution processes. According to the Rietveld structure refinement Li atoms were completely removed from the tetrahedral 8a site during the cell charge with the C/10 rate. However, only about 40% Li atoms were inserted during the cell discharge with the same rate. The unit cell parameter a and the average metal−oxygen distance (Cr,Mn-O) show non-linear dependence on the lithium content upon extraction, with the slope changing at 4.9 V and manifesting a change in the mechanism of redox behavior. While the first step between 3 V and 4.9 V is presumably accompanied by the oxidation of Cr3+ to Cr4+, the charging above 4.9 V requires a further oxidation of Cr4+ toward Cr6+. The paramagnetic effective moment of LixCrMnO4 compounds changes non-monotonously with decreasing Li-content.