Structure and electrochemistry of Li2CrxMn2-xO4 for 1.0≤x≤1.5

Li2CrxMn2-xO4 samples with x = 1.0, 1.25, and 1.5 were prepared by heating sol-gel precursors to elevated temperatures in argon. Three crystalline phases were observed as a function of the final heating temperature and x, and these were plotted on a structure-composition-temperature diagram. The samples lose some oxygen as they are heated, and this is probably part of the cause for the structural changes observed. The structure of Li2CrMnO4 heated to 1000 degrees C has been determined to be a distorted-hexagonal layered structure, much like that of the layered LiMnO2 phase recently reported by Armstrong and Bruce.(19) This does not agree with previous work by Davidson et al.,(10,11) who claimed this phase was a tetragonal or orthorhombic distortion of the spinel structure. We show that Davidson's data are better explained by our model. At lower temperatures (e.g., 700 degrees C), the compounds are isostructural with LiCoO2, with the R-3m space group. Li/Li2CrxMn2-xO4 electrochemical cells were made for all the samples and voltage-composition profiles measured. Li2Cr1.25Mn0.75O4 heated at 700 degrees C has an attractive voltage profile and a reversible capacity near 150 mAh/g when measured at a low rate of discharge.