Effects of the maximum Li stoichiometry in electrode on peak current densities of cyclic voltammogram

Abstract

Cyclic voltammetry (CV) is widely used to study Li ion batteries. Most explanations of CV experiments, however, are based on traditional electrochemical kinetic models of all reactive species in electrolytes, possibly leading to non-physical lithium accumulation on electrode surfaces due to several orders of difference in magnitude between Li+ diffusion coefficients in electrode and electrolyte. Here, by considering size-effects of electrodes, we show that analytic expressions of the peak current density Ip of CV can be derived by combining Nernst diffusion layer model with conservation law of matter, leaving the coefficients of the analytic expressions undetermined. We then develop a complex model by considering the maximum stoichiometric intercalation sites (MSIS) of Li+ in electrode and name it as the MSIS model. Numerical simulations of the MSIS model on three shapes of electrodes confirm the validity of the analytic expressions, and the coefficients in the analytic expressions are determined from simulations. The plots of log(Ip) versus log(v) with v the scan rate show split lines, each of which can be further divided into low and high scan rate regions with two distinguished slopes, while the plots are always a single straight line in the MSIS-free model without the consideration of MSIS. The MSIS model and the present analytic and simulation results may provide a tool to characterize the MSIS effect and the Li+ diffusion coefficient in an electrode in experiments.