The Li–Co–O and Li–Ni–O systems, used as cathodes in lithium ion batteries, have been investigated by means of ab initio calculations and empirical methods. An approach based on ab initio calculations to obtain accurate enthalpies of formation for transition metal oxides is proposed. With the obtained enthalpies of formation and the empirical entropy data, the Gibbs energy functions of the binary and ternary oxides in the Li–Co–O and Li–Ni–O systems are determined. To prove the accuracy of this thermodynamic model, we calculate the cell voltages of lithium ion batteries. Compared to the previously calculated results, which underestimate the cell voltages of lithium ion batteries, our calculations are in good agreement with the experimental data. The present theoretical approaches are reliable to evaluate the thermodynamic and electrochemical properties of lithium-containing transition metal oxides.