Distinguishing Slow Charge Transfer from Slow Desolvation Rate-Determining Step in Ion Intercalation Processes

Abstract

The elucidation of the reaction rate-determining step nature in intercalation processes is essential for the development of approaches for a precise control of rate-limiting factors. Earlier work in the field of the lithium-ion intercalation kinetics has led to the establishment of a general pattern of Butler-Volmer type of charge transfer rate control for a wide range of intercalation materials. However, further research in the field of the development of aqueous Li-ion batteries, as well as alternative to Li+ battery chemistries (Na+, K+, Mg2+ intercalation) resulted in the accumulation of valuable experimental information on the electrochemical intercalation processes for a much wider range of solvents (aqueous solutions, glymes, nitriles, etc.), cations and electrode materials, which suggests that Butler-Volmer type potential dependence of the charge transfer rates is not characteristic for the intercalation processes in solvents, where surface layers formation is not observed, e.g. in aqueous solutions. In this work, we explore the kinetic patterns of lithium-ion intercalation into two model cathode materials (LiCoO2 and LiMn2O4) and develop criteria for distinguishing between Butler-Volmer slow charge transfer and slow chemical steps. A numerical model for the rate-limiting ion desolvation step is developed and the predictions of the model are compared with the experimental voltammetric and electrochemical impedance spectroscopy data. We show that slow desolvation step results in essential changes in the shape of both cyclic voltammetry and impedance responses with the kinetic resistance vs. potential dependencies being highly informative for the reaction rate control diagnostics. The consideration of the intercalation kinetics in four solvents (water, propylene carbonate, acetonitrile and dimethyl sulfoxide) allows concluding on the influence of the resistivity of surface layer/electrode material interface on the reaction slow step nature. Figure 1