Application of thermodynamic activity models to the equilibrium potential for lithium intercalation process of a cobalt-free cathode material with a layered structure

Abstract

An electrode composed of a cobalt-free cathode material with a layered structure – lithium nickel manganese oxide (LiNi0.7Mn0.3O2) – is charged and the data are interpreted with thermodynamic activity models. Equilibrium potentials of the cathode material are measured with galvanostatic intermittent titration technique (GITT) and compared with the estimated results by Nernst equation and thermodynamic activity models. No phase transition is observed in the equilibrium potential curve of the cathode material. Non-random Two Liquids (NRTL) and Universal Quasi-chemical (UNIQUAC) model are used to calculate the activity coefficients of the components in the cathode material. The equilibrium potentials estimated by both the models show good agreement with the measured data. UNIQUAC with 5 adjustable parameters is better than NRTL with 4 parameters in fitting the experimental data. Excess Gibbs free energy and activity coefficients calculated by UNIQUAC are lower than those by NRTL indicating that non-ideality in the phase behavior of LiNi0.7Mn0.3O2 is estimated higher by UNIQUAC than NRTL.