The Composite Insertion Electrode: Theoretical Part. Equilibrium in the Insertion Compound and Linear Potential Dependence

The specific energy obtainable by discharge of porous insertion electrodes is limited by electrolyte depletion in the pores. This can be overcome using a solid ion conductor as electrolyte. The term "composite" is used to distinguish these electrodes from porous electrodes with liquid electrolyte. The theoretical basis for such electrodes is discussed and, using a simplified model, equations are derived to describe the distribution of potential and current during discharge/charge operation. Under the assumption that the insertion compound particles are small enough to ensure equilibrium, and that the local electrode potential depends linearly on the degree of insertion, these equations are solved to obtain analytical expressions for the discharge curve. It is shown that the parameters which determine the discharge behavior for a given discharge current are simply related to the effective ionic and electronic conductivities, the thickness of the electrode, the volume fractions, and the slope of the potential curve.