In spite of the great number of commercialized lithium-ion intercalating systems, the intrinsic ion-transfer kinetics of the intercalation reaction is still poorly understood. The process of Li-ion intercalation comprises several stages: i) diffusion of Li ion in solution; ii) partial or complete desolvation of Li⁺ at the interface; iii) diffusion of the ion through cathodic or anodic SEI at the electrode surface; iv) insertion of the Li ion into the lattice of the material, which is accompanied by the transition metal ion oxidation/reduction. Under these conditions, a pronounced solvent effect on the kinetics of the process is expected, as solvent determines both the structure of the interface (SEI composition) and the desolvation energy, which contributes to the activation barrier. Construction of a self-consistent kinetic model requires careful experimental examination of all of these stages and determination of the limiting stage of the process.

    In this work we focus on the solvent effect on the kinetics of lithium ion intercalation into model electrodes (LiCoO₂, LiMn₂O₄, V₂O₅, etc.), as these systems are characterized by different redox potentials and, correspondingly, by different SEI compositions.  We report the results of the electrochemical measurements for the model electrode materials in carbonate-based electrolytes (e.g. commercial electrolyte ethylene carbonate/dimethyl carbonate), protic solvents (water, ethylene glycol) and aprotic solvents (dimethyl sulfoxide, acetonitrile). For these groups of solvents we report the results of cyclic voltammetry numerical modeling, which allows for the determination of the intrinsic kinetic parameters of the intercalation reaction¹ (diffusion coefficient, rate constant and the parameters of the intercalation isotherm). To construct an accurate kinetic model of the intercalation process the real particle size distribution, as determined from scanning electron microscopy data, is taken into account.

    The observed tendencies in the rate constant values are compared with the results of quantum chemistry calculations on the DFT level of theory of the desolvation energies for the lithium ion in the solvents under study. The calculations were carried out using “cluster-continuum” approach, which allows separating the electrostatic and the non-electrostatic (specific) contributions to the overall desolvation barrier.  The combination of the electrochemical experimental data with the results of molecular modeling allowed us to draw conclusions on the solvent effect on the intercalation reaction kinetics and the contribution of the lithium ion desolvation energy to the overall activation  barrier.

1. S. Yu. Vassiliev, E. E. Levin, V. A. Nikitina. Kinetic analysis of lithium intercalating systems: cyclic voltammetry. Electrochim. Acta,  doi:10.1016/j.electacta.2015.12.172.