The kinetics of porous insertion electrodes

Abstract The kinetics of porous electrodes with an insertion compound as active material is a compound function of the kinetics of the insertion material and transport of inserted ions in the electrolyte-filled pores. The insertion compound and the electrolyte transport are first treated separately and then combined in the treatment of the porous electrode. The overvoltage and the materials utilization for the insertion compound are treated in terms of simple, solid state diffusion and a potential/composition dependence based on an idealized model with first order interactions. Transport in the electrolyte is treated using transport equations which take into consideration cross diffusion terms using the molar conductance. From this treatment an estimate of the conditions necessary to avoid depletion of electrolyte salt in the pores is derived. The concepts of load factors for the insertion compound and for the electrolyte part of the electrode are introduced to express the severity of the discharge in relation to the transport rates in the systems. The couplings between the insertion reaction and the transport in the pores are defined. Based on these the potentials and concentrations in the porous electrode during discharge can be found. As an example, results from such calculations on a typical porous TiS 2 electrode with liquid electrolyte are presented. The influence of electrode thickness and porosity is demonstrated and a contour representation of the influence of the load factors on the materials utilization is shown. It is demonstrated that a low mobility for the anion in the electrolyte is an advantage. In consequence, a theory for composite electrodes using solid electrolytes is developed. It is shown that this electrode behaves as a non-porous insertion electrode with a much enhanced transport rate.