We have measured in situ the Ni K-edge X-ray absorption spectra of Li(1-z)Ni(1+z)O2 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 Li(1-x-z)Ni(1+z)O2. 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 Li0.86Ni1.14O2 (i.e., x = 0 and z = 0.14). That is, the material consists of Ni2+ (25%) and Ni3+ (75%) with half the Ni2+ atoms residing in Li sites and the other half in the NiO2 slabs. Upon charging, initially Ni2+ is oxidized to Ni3+ up to a state of charge which corresponds to x = 2z. Upon further charging to states corresponding to 2z < x less than or equal to 1 - z, Ni3+ is oxidized to Ni4+ with fractions being dependent on the values of x and z. Analysis of the edge energies for NiO, stoichiometric LiNiO2, and KNiIO6 as reference compounds for Ni2+, Ni3+, and Ni4+, 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 Li(1-x-z)Ni(1+z)O2 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 Ni2+, Ni3+, and Ni4+ extracted from extended X-ray absorption fine structure spectra.