In Situ X-Ray Absorption Study of a Layered Manganese-Chromium Oxide-Based Cathode Material

Abstract

We have investigated the electronic and atomic structure of a manganese-chromium-based layered oxide material LiLi 0.2 Cr 0.4 Mn 0.4 O 2 during electrochemical cycling using in situ X-ray absorption spectroscopy. Our results indicate that charge compensation in the cathode material is achieved by the oxidation/reduction of octahedral CrIII ions to tetrahedral CrVI ions during delithiation/lithiation. Manganese ions are present predominantly in the MnIV oxidation state and do not appear to actively participate in the charge compensation process. To accommodate the large changes in coordination symmetry of the CrIII and CrVI ions, the chromium ions have to move between the regular octahedral sites in the R3 m-like lattice to interstitial tetrahedral sites during the charge/discharge process. The highly reversible at least after the first charge three-electron oxidation/ reductions and the easy mobility of the chromium between octahedral and tetrahedral sites are very unusual and interesting. Equally interesting is the fact that chromium is the active metal undergoing oxidation/reduction rather than manganese. Our results also suggest that in the local scale manganese and chromium ions are not evenly distributed in the as-prepared material, but are present in separate domains of Mn and Cr-rich regions.