Abstract
We have measured in situ the Ni K‐edge X‐ray absorption spectra of 
cathode material charged in a nonaqueous cell. The material was charged to various states of charge (i.e., Li content) which corresponded to x = 0.0, 0.12, 0.24, 0.37, 0.49, and 0.86 in 
. We have determined variations in the Ni‒O and Ni‒Ni coordination numbers, bond lengths, and local disorders as well as the Ni K‐edge energies as a function of Li content. We show that in the pristine state, the composition of the material can be described by the formula 
(i.e., x = 0 and 
= 0.14). That is, the material consists of 
(25%) and 
(75%) with half the 
atoms residing in Li sites and the other half in the 
slabs. Upon charging, initially 
is oxidized to 
up to a state of charge which corresponds to x = 2
. Upon further charging to states corresponding to 2z <x≤1 − z, 
is oxidized to 
with fractions being dependent on the values of x and z. Analysis of the edge energies for NiO, stoichiometric 
, and 
as reference compounds for 
, 
, and 
, respectively, shows a quadratic dependence for edge energy vs. oxidation state. This type of correlation is consistent with variations observed in earlier studies for some Mn reference compounds in the same range of oxidation states. Oxidation‐state determination of Ni in 
as a function of state of charge (i.e., Li content or x) on the basis of edge energies yielded results which are in excellent agreement with oxidation state determinations made on the basis of the mole fractions for 
, 
, and 
extracted from extended X‐ray absorption fine structure spectra. © 1999 The Electrochemical Society. All rights reserved.