Decoupling Reaction, Diffusion and Ohmic Processes in Fast-Charging Electrodes: A New Model for CV Data Analysis

Abstract

Fast charging is critical for the efficient storage of energy in devices like electric vehicles and portable electronics. The rate performance of electrodes is limited by the electron transfer rate, the ohmic resistance of the electrode, the size and shape of the primary electrode particles, and the charge carrier diffusion process within the electrodes. However, separating the intrinsic contributions of mass transport and charge transfer from non-equilibrium data obtained under large interfacial polarization is very complex, which limits our understanding of high rate electrodes during charging and discharging. While it is possible to extract the diffusive and capacitive components of an electrode’s capacity from cyclic voltammetry (CV) data using the Dunn method, the Dunn model assumes a constant diffusional overpotential. In reality the ohmic resistance scales with the current, so that the ohmic potential drop across the electrode cannot be neglected a priori for fast charging electrodes in which high current densities are a requirement.We recently developed a new model based on the Butler-Volmer equation for the kinetics of the electron exchange process, and incorporated the charge carrier diffusion process and the internal ohmic losses, as well as the effect of the shape and dimensions of the primary electrode particles on the effective diffusion rate [1]. The model can provide the relation between current peak voltages and peak currents from a series of sweep rate CV measurements. The ohmic resistance, equilibrium voltage, exchange current (related to the electrochemical reaction rate) and limiting current (related to the charge carrier diffusion coefficient and Nernst diffusion layer thickness) can be extracted as independent variables from the fitted experimental data.By simulating a range of different high rate Li electrode materials (both cathodes and anodes), we identified influence of the reaction rate on the overpotential as a common significant factor, which explains the nonlinear relation between the potential and the current at peak position in different CV measurements. Our analysis suggests that a higher electrochemical reaction rate seems to be more important for the realization of faster charging batteries than a larger carrier diffusion coefficient.Finally, our extended model also enables us to predict the overpotential at any applied potential, which is not possible with other current models known till date. This insight helps us to gain more detailed understanding of the relationship between the intrinsic properties of the electrode materials and their rate performance in different reaction stages. This may facilitate further modification of electrode material towards enhanced high rate performance in next-generation batteries.[1] R. Xia et al., Energy Storage Mater. 53 (2022) 381-390.