Local structure of LiNi0.5Mn0.5O2 cathode material probed by in situ x-ray absorption spectroscopy

The layered nickel manganese oxide of LiNi0.5Mn0.5O2 is a technologically important and promising cathode material for lithium-ion batteries. A study of the structural perturbation, charge compensation mechanism, and the valence of the constituent transition metal elements (Ni and Mn) during electrochemical cycling was performed using x-ray absorption spectroscopy (XAS) employing an in situ technique with an electrochemical cell designed for long term x-ray experiments. We present the changes in the oxidation state, bond distance, and coordination number of the Ni and Mn absorbers as a function of the state of charge of the material during electrochemical cycling at moderate rate through a typical Li-ion battery operating voltage range (2.9–4.7V). The oxidation states of the transition metals in LiNi0.5Mn0.5O2 are Mn4+ and Ni2+. The x-ray absorption near edge spectra reveal that on delithiation of Li1−xNi0.5Mn0.5O2 the Mn is electrochemically inactive and remains at Mn4+ whereas the Ni is oxidized from Ni2+ to almost Ni4+ through an intermediate stage of Ni3+. A quantitative picture of the Ni valence during the charging process was obtained employing Faraday’s law calculations in combination with the XAS results. The Faraday’s law calculation indicated that at the end of the charge Ni is at Ni3.91+ in close agreement with the XAS results where Ni is found to be at Ni3.88+ at the end of the charge. Analysis of the extended x-ray absorption fine structure shows that during cycling the material retains its crystallographic symmetry and good structural short-range order which should lead to stable cycling.