Experimental and theoretical investigation of Li-ion battery active materials properties: Application to a graphite/Ni0.6Mn0.2Co0.2O2 system

The knowledge of active materials properties and their evolution with aging is crucial to simulate and predict with a high reliability the electrochemical performance of lithium-ion batteries. In view of developing more accurate physics-based Lithium Ion Battery (LIB) models, this paper aims to present a consistent framework, including both experiments and theory, in order to retrieve the active material properties of commonly used electrodes made of graphite at the negative and Ni0.6Mn0.2Co0.2O2 (NMC 622) at the positive, as function of the active materials stoichiometry. To measure the equilibrium potential and the solid diffusion coefficient, Galvanostatic Intermittent Titration Technique (GITT) measurements were used. Electrochemical impedance spectroscopy (EIS) measurements with reference electrodes were performed to determine the exchange current density using the transmission line model. The measured stoichiometry dependence of these three active material properties has been further analyzed, based on thermodynamic considerations. For the positive material, a model is proposed highlighting the non-ideal behavior of lithium inside the host material. The thermodynamic relations available in the literature are not directly transposable to the Ni0.6Mn0.2Co0.2O2 material, suggesting the necessity to account for supplementary terms. Nevertheless, the proposed stoichiometry dependent laws determined with the same stoichiometry definition go already beyond most reported values for the Ni0.6Mn0.2Co0.2O2 and can be used to increase the predictability of multi-physics lithium-ion battery models.