ИСТИНА

In spite of the recent outburst in metal-ion battery research, the mechanisms of ion (de)intercalation reactions are still poorly understood. Experimental determination of the intercalation mechanisms is generally complicated by the large number of factors, which affect the reaction rate (material composition, the nature of the intercalating cation, solvent/cation interactions, surface layer structure, etc). In order to facilitate this task, one should vary the reaction conditions in a controllable way, which would allow for the elucidation of the nature of the slowest, rate-limiting step. In this case, the data on the rates of faster processes are typically lost, but the information on the slowest step kinetics allows for the unempirical reaction rate control, which is generally of interest for practical application of intercalation systems. Accurate electrochemical characterization of the intercalation reactions’ kinetics can be used to decipher complex pathways of the intercalating processes. In previous studies of our group, kinetic, thermodynamic and diffusional rate control regimes were addressed for lithium-ion intercalation reactions into oxide hosts [1, 2]. In this study, we focus on the description of kinetic patterns for the intercalation reactions of phase transformation materials [3]. In this case, the following reaction steps can be assumed: ion transfer across the electrolyte/material interphase, ion diffusion in the material, nucleation of a new phase and propagation of the phase boundary through the volume of a particle. Based on the electrochemical data, we demonstrate that under the conditions of fast charge transfer and fast diffusion processes slow nucleation of a new phase determines the reaction rate. In this study, we adopt a model approach to describe the electrochemical behavior of materials under slow nucleation conditions, which is based on the classical concept of critical nucleus in a multi-particle system. Intercalation of lithium-ion into vanadium pentoxide and lithium iron phosphate materials were chosen as model reactions to check the self-consistency of the applied approach. For the latter material, the effect of solvent nature on the nucleation rate was also explored. References: 1) Vassiliev, S. Y., Levin, E. E., & Nikitina, V. A. (2016). Kinetic analysis of lithium intercalating systems: cyclic voltammetry. Electrochimica Acta, 190, 1087-1099; 2) Levin, E. E., Vassiliev, S. Y., & Nikitina, V. A. (2017). Solvent effect on the kinetics of lithium ion intercalation into LiCoO2. Electrochimica Acta, 228, 114-124; 3) Oyama, G., Yamada, Y., Natsui, R. I., Nishimura, S. I., & Yamada, A. (2012). Kinetics of Nucleation and Growth in Two-Phase Electrochemical Reaction of Li x FePO4. The Journal of Physical Chemistry C, 116(13), 7306-7311.