The LiNiO2–NiO2 pseudo-binary system has been studied using the CALPHAD approach coupled with ab initio calculations. The O1 phase, the H1-3 phase and the ordered and disordered O3 phases, are thermodynamically described using sublattice models. The phase equilibria and thermodynamic data are well reproduced by the present thermodynamic modeling. Based on the experimental information and present ab initio results, we consider the LiCoO2–CoO2 and LiNiO2–NiO2 systems to be ideally mixing. Using the extrapolated thermodynamic description of the Li(Co,Ni)O2–(Co,Ni)O2 system, the cell voltages of Li/O3- Li(Co,Ni)O2 cells are calculated and compared with experimental data. The good agreement shows the viability of the present approach.