ИСТИНА

Ion intercalation reactions have attracted enormous attention due to the challenges of sodium-, potassium- and even magnesium-ion nonaqueous batteries implementation. The structural evolution of the materials in the course of intercalation/deintercalation stages, electronic structure of intercalation hosts and barriers for ionic diffusion as well as electrochemical charge/discharge characteristics of composite electrodes have been profoundly explored for the last two decades both for lithium/sodium and less elaborated potassium and magnesium intercalation processes. In most of battery prototype investigations the key parameter of interest is the ionic diffusion coefficient, as in the case of kinetically facile systems this parameter determines the rate at which the battery can be charged and discharged. The critical issue of the ion transfer rate across the material/solution interface is typically overlooked, despite this factor dictates both the general possibility of ion transfer and the actual rate of ion transfer, which will be reflected in kinetic polarization losses. This problem is not considered to be of primary importance for lithium ion intercalation into well-established layered or spinel structures, but becomes crucial when ion intercalation rates are addressed in pioneering studies of Na+-, K+- and Mg2+-intercalation. Two major problems that appear are related to the transfer of either a larger ion (K+) or of an ion with a higher charge, which is naturally more strongly bound to its solvation shell (Mg2+). In this case, the intercalation process may become either very sluggish (K+ intercalation into natural graphite) or even impossible (Mg2+ intercalation in nonaqueous solvents), when the activation barriers for ion incorporation into the structure or the barriers for desolvation become “unsurmountable”. The rationalization of the kinetic patterns of ion transfer process in the course of ion intercalation would shed light upon physical factors, which control the rate of ion transfer [1-3]. In this talk, the emphasis is placed on the differences and similarities of ion transfer and electron transfer reactions. For ion transfer reactions the concept of the reaction coordinate is introduced and illustrated by examples of ion adsorption and intercalation. Various contributions to the activation barrier, which is affected by the size and charge of the ion [2], solvation shell structure in a given solvent [3] and the presence of surface films [1] will be discussed in close connection to the practical cases of their effect on the overall intercalation rates. References: [1] V.A. Nikitina, M.V. Zakharkin, S.Yu. Vassiliev, L.V. Yashina, E.V. Antipov, K.J. Stevenson. Langmuir, 2017. 33(37): p. 9378. [2] V.A. Nikitina, S.M. Kuzovchikov, S.S. Fedotov, N.R. Khasanova, A.M. Abakumov, E.V. Antipov. Electrochim. Acta, 2017. 258: p. 814. [3] E.E. Levin, S.Yu. Vassiliev, V.A. Nikitina. Electrochim. Acta, 2017. 228: p. 114.