Thermodynamic model development for lithium intercalation electrodes

Staging is a characteristic phenomena observed in intercalation electrodes. During staging process the equilibrium potential of the electrode exhibits weak dependence on the solid phase Li concentration and does not follow the classical Nernst behavior. The coexistence of structurally different solid phases results in multiple plateaus in the equilibrium potential curve. Such complexities make the thermodynamic description of equilibrium potential as a function of concentration difficult, and so it is usually represented through an empirical expression. The objective of this work is to develop a frame work based on thermodynamic principles to describe the equilibrium potential of intercalation electrodes. Redlich-Kister thermodynamic equation was used to describe the excess Gibbs free energy, which in turn was used to evaluate the equilibrium potential as a function of concentration. The equilibrium potential expression for different lithium intercalation electrodes such as LiCoO2, LiNi0.8Co0.15Al0.05O2, graphite and hard carbon were developed based on Redlich-Kister equation. The thermodynamic model was also used to estimate the activity of species directly from excess Gibbs free energy. The developed thermodynamic expressions along with the activity correction are incorporated into a single particle diffusion model for a Li-ion cell consisting of a graphite and LiCoO2 electrode. The interactions between the Li-ions during intercalation/deintercalation process were incorporated into the present model by considering the chemical potential gradient corrected for activity as the driving force. The effect of inclusion of activity correction in the single particle model was studied for different discharge rates. It was observed that the activity correction term yielded increased capacity especially at higher rates. The effect of activity correction term was also found to be more significant in the LiCoO2 electrode compared to the carbon electrode. © 2008 Elsevier B.V. All rights reserved.