GITT Analysis of Lithium Insertion Cathodes for Determining the Lithium Diffusion Coefficient at Low Temperature: Challenges and Pitfalls

Understanding the thermodynamic and kinetic properties of electrode materials for Li-ion batteries is of great importance for their knowledge-based optimization and development of novel materials and cell designs. The galvanostatic intermittent titration technique (GITT) is widely applied in battery research to study the state-of-charge (SOC) dependent kinetics of Li ion insertion in anode and cathode materials. While kinetic data at high and ambient temperatures are largely available, low temperature diffusion and rate coefficients are hardly reported in the literature and can vary by orders of magnitude for identical active materials. Herein, we demonstrate and discuss several challenges and pitfalls in the application and evaluation of GITT measurements for determining kinetic coefficients of Li-insertion electrodes, which become especially important at low temperature. This includes theoretical considerations and an experimental analysis of the promising cathode material LiNi0.5Co0.2Mn0.3O2 (NCM523) in the wide temperature range of -40 to 40 °C. We show how the choice of experimental conditions for the GITT measurements and of the subsequent mathematical evaluation procedure to determine the lithium diffusion coefficient significantly influence the derived value of the diffusion coefficient in NCM523. The results suggest that the large scattering of reported values of the diffusion coefficient could be caused by the use of different evaluation procedures. Simple calculation methods appear to be less suited the lower the temperature is. We outline that the proper estimation of the apparent Li ion chemical diffusion coefficient from GITT data requires an additional impedance analysis and a detailed knowledge of the cathode microstructure.