Transition metal redox and Mn disproportional reaction in LiMn0.5Fe0.5PO4 electrodes cycled with aqueous electrolyte

We performed soft x-ray absorption spectroscopy (sXAS) and a quantitative analysis of the transition metal redox in the LiMn0.5Fe0.5PO4 electrodes upon electrochemical cycling. In order to circumvent the complication of the surface reactions with organic electrolyte at high potential, the LiMn0.5Fe0.5PO4 electrodes are cycled with aqueous electrolyte. The analysis of the transitional metal L-edge spectra allows a quantitative determination of the redox evolution of Mn and Fe during the electrochemical cycling. The sXAS analysis reveals the evolving Mn oxidation states in LiMn0.5Fe0.5PO4. We found that electrochemically inactive Mn2+ is formed on the electrode surface during cycling. Additionally, the signal indicates about 20% concentration of Mn4+ at the charged state, providing a strong experimental evidence of the disproportional reaction of Mn3+ to Mn2+ and Mn4+ on the surface of the charged LiMn0.5Fe0.5PO4 electrodes. Published by AIP Publishing.Guangdong Innovation Team Project [2013N080]; Shenzhen Science and Technology Research Grant [KYPT20141016105435850]; Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy [DE-AC02-05CH11231]SCI(E)EIARTICLEduanyd@pkusz.edu.cn; wlyang@lbl.gov; panfeng@pkusz.edu.cn210