Adsorption on electrodes: an extension of modeling intercalation and conversion reaction in batteries

Kurzfassung

Li ion batteries, which are based on a electrochemical lithium intercalation reaction, are the most common type of batteries with applications from costumer electronics to electromobility. A detailed understanding of insertion and conversion reaction mechanisms are essential for deriving predictive models of batteries. Although there are many studies regarding bulk properties of electrolytes and electrodes, like structural changes or diffusion and transport of ions in the electrode or the electrolyte, processes at the electrolyte/electrode interface, like adsorption and desolvation on electrode surfaces remain a big challenge for the formulation of a consistent theory for batteries. Bruce et al. [1] proposed a model to describe the intercalation process in an electrochemical environment. Figure 1 shows this so called adatom model, where the fully solvated Li-ion in bulk electrolyte partially loses its solvation shell before getting adsorbed on the surface. Then it diffuses to a site, at which it can intercalate into the electrode. During intercalation the adion loses the remaining solvent and gets incorporated in the lattice of the electrode. This proposed mechanism of a two-step-electrointercalation (desolvation/adsorption=adion formation and insertion) is confirmed for different insertion materials [2-4]. Also in conversion batteries, adsorption is most likely the first step before electron transfer reactions are possible. To the best of our knowledge there exists no model in literature, which includes adsorption and desolvation in the description of intercalation or conversion reaction. As a first step to model electrosorption we derive equations for surface charges from thermodynamic and electrostatic considerations. The model is implemented in a thermodynamic consistent reaction-transport model for Li-ion batteries [5]. We demonstrate in first 1D simulation that reasonable coverage on the surface of the electrode are obtained while maintaining the transport properties of the cell. [1] P.G. Bruce, et al., J. Electroanal. Chem., 322 (1992), pp 93-105. [2] S. Kobayashi, et al., J. Phys. Chem. B, 109 (2005), pp 13322-13326. [3] M. Nakayama, et al., J. Phys. Chem. C, 118 (2014), pp 27245-27251. [4] M. Nakayama, et al., J. Phys. Chem. B, 107 (2003), pp 10603-10607. [5] A. Latz, et al., Journal of Power Sources, 196 (2001), pp 3296-3302.