In situ x-ray absorption studies of a high-rate LiNi0.85Co0.15O2 cathode material

We have performed an ill situ X-ray absorption spectroscopy (XAS) study to investigate me evolution of the local electronic and atomic structure of a high-surface-area Li1-xNi0.85Co0.15O2 (0 less than or equal to x less than or equal to 1) cathode material during electrochemical delithiation. We hav e measured the changes in the oxidation state, bond distance, coordination number, and local disorder of Ni and Co absorbers as a function of the stat e of charge of the material. The X-ray absorption near edge spectra shows t hat delithiation of Li1-xNi0.85Co0.15O2 leads to the oxidation of Ni3+ to N i4+. Ni atoms oxidize during the initial stages of charge and attain a maxi mum oxidation state of Ni4+ well before the end of charge (x approximate to 0.85). On the other hand, Co atoms do not oxidize during the initial stage s of charge but oxidize close to the end of charge. Analysis of the extende d X-ray absorption fine structure (EXAFS) shows that the oxidation of Ni3to Ni4+ leads to the expected reduction in the Jahn-Teller effect. Also, to within the accuracy of the EXAFS technique, Co absorbers occupy Ni-type si tes in the NiO2 slabs. Furthermore, Co doping has a strong effect on the ov erall structural evolution and leads to a slight expansion of the a and b a xes close to the end of charge. (C) 2000 The Electrochemical at Society. S0 013-4651(00)01-042-9. All rights reserved.