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 have measured the changes in the oxidation state, bond distance, coordination number, and local disorder of Ni and Co absorbers as a function of the state of charge of the material. The X-ray absorption near edge spectra shows that delithiation of Li1-xNi0.85Co0.15O2 leads to the oxidation of Ni3+ to Ni4+. Ni atoms oxidize during the initial stages of charge and attain a maximum 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 stages of charge but oxidize close to the end of charge. Analysis of the extended X-ray absorption fine structure (EXAFS) shows that the oxidation of Ni3+ to 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 sites in the NiO2 slabs. Furthermore, Co doping has a strong effect on the overall structural evolution and leads to a slight expansion of the a and b axes close to the end of charge. (C) 2000 The Electrochemical at Society. S0013-4651(00)01-042-9. All rights reserved.