Continuum modelling of transport and interface phenomena in magnesium electrolytes

Kurzfassung

Next-generation batteries based on metal anodes possess a considerably higher theoretical energy storage capacity than state-of-the-art Li-ion-technology. The major issue with many metal anodes is the formation of dendritic structures causing capacity loss and in the worst case short circuits. Magnesium prefers higher coordinated structures, which potentially enables a dendrite-free cycling of the battery. With its bivalent charge carrier ions rechargeable magnesium batteries can compensate the generally lower discharge potential compared to lithium metal electrodes. Another prominent advantage of magnesium is its natural abundance, which allows economic and sustainable large-scale applications of magnesium-based battery technologies. However, the processes in the electrolyte and at the electrode surface are not yet fully understood. The bivalency of the magnesium cations leads to strong coulomb interactions with the anion as well as with the solvent. However, a good solvation is important for the dissociation of the magnesium salts, which is in turn crucial for a high ionic conductivity of the electrolyte. At the same time the desolvation of magnesium ions close to the electrode surface plays an important role during the deposition process. Moreover, it was found [1,2], that magnesium salts are prone to form ion pairs and bigger clusters, which may adversely affect the transport in the electrolyte and the plating behavior at the electrode. The formation and size of these clusters strongly depend on the anion, the solvent, the concentration and the electric field strength [1,2]. In our contribution we will present results of a newly developed thermodynamic consistent continuum model [3] applied to symmetric magnesium cells. In a first step we analyze the thermodynamics of cluster formation in the electrolyte. The bivalent magnesium cations have a high charge density, which leads to strong electrostatic interactions. Moreover, the concentration of the magnesium salts in the electrolytes is usually quite high. Therefore, we need to take into account the non-ideal behavior of the electrolyte, which we describe by a modified Davis equation [4]. The effect of clusters on the activity of magnesium ions is included in our model and consistently coupled to the transport of the dissolved species in the electrolyte and the charge transfer reaction at the interface. Model parameters are either derived from experimental data [5] or results of DFT simulations. With our approach we are able to reproduce qualitative trends in electrochemical measurements by our partners [5] and provide more insights on the operation of magnesium batteries.