In situ X-ray absorption studies of a high-rate LiNi_0.85Co_0.15O₂ cathode material

We have performed an in situ X-ray absorption spectroscopy (XAS) study to investigate the evolution of the local electronic and atomic structure of a high-surface-area Li_1-xNi_0.85Co_0.15O₂ (0≤x≤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 Li_1-xNi_0.85Co_0.15O₂ 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≈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.