We performed a comparative study of the\ soft x-ray\ absorption spectroscopy\ of the LiMn2O4\ and Li1.15Mn1.85O4\ electrode\ materials with a quantitative\ analysis\ of Mn\ oxidation\ states. The revealed redox evolution of Mn upon\ electrochemical\ cycling clarifies the\ effect\ of excess Li in the materials, which naturally explains the different\ electrochemical\ performance. The spectral\ analysis\ perfectly agrees with different initial cycling capacities of the two materials. The results show unambiguously that Mn3+\ starts to dominate the\ electrode\ surface\ after only one cycle. More importantly, the data show that, while LiMn2O4\ electrodes\ follow the nominal Mn redox evolution, the formation of Mn3+\ on the\ electrode\ surface\ is largely retarded for Li1.15Mn1.85O4\ during most of the\ electrochemical\ processes. Such a different\ surface\ Mn redox behavior leads to differences in the detrimental\ effects\ of Mn2+\ formation on the\ surface,\ which is observed directly after only two cycles. Our results provide strong evidence that a key\ effect\ of the (bulk) excess Li\ doping\ is actually due to processes on the\ electrode\ surfaces.
}, year = {2017}, booktitle = {Applied Physics Letters}, journal = {Applied Physics Letters}, series = {Applied Physics Letters}, volume = {110}, pages = {093902}, month = {02/2017}, issn = {0003-6951}, doi = {10.1063/1.4977502}, language = {eng}, }