In Situ X-Ray Absorption Studies of a High-Rate LiNi[sub 0.85]Co[sub 0.15]O[sub 2] 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 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 leads to the oxidation of to . Ni atoms oxidize during the initial stages of charge and attain a maximum oxidation state of 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 to 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 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. © 2000 The Electrochemical Society. All rights reserved.