XRD and XAFS study on structure and cation valence state of layered ruthenium oxide electrodes, Li2RuO3 and Li2Mn0.4Ru0.6O3, upon electrochemical cycling

Structure and valence state change of Li2RuO3 and ruthenium-substituted lithium manganese oxide, Li2Mn0.4Ru0.6O3 (LMR), with layered structure were investigated using Synchrotron X-ray diffraction (SXRD) and X-ray absorption spectroscopy measurements before and after electrochemical cycling. The charge-discharge voltage curves of both LMR and Li2RuO3 significantly vary in the subsequent cycle. The SXRD Rietveld structural refinements demonstrate that the LMR undergoes irreversible structural transition. The Mn K-edge spectra confirm the structural modification in the MnO6 octahedra with Li de-intercalation. The Ru L-edge spectra for LMR show similar behavior to Li2RuO3 during electrochemical cycling. These spectra appear reductive peak shift on the way to charging to 4.8 V. The phenomena are not attributed to the reduction of hexavalent Ru to pentavalent but a variation of the splitting between bonding and anti-bonding eg orbitals. The charge-discharge reactions mechanism of LMR and Li2RuO3 are discussed.