Modeling of porous graphite electrodes of hybride electrochemical capacitors and lithium-ion batteries

Abstract

A model is proposed for the description of the discharging of porous graphite electrodes. The model takes into account the nonequivalence of the different layers of the internal porous surface, as well as change in the potential and current density in these layers depending on the amount of passed charge. The model uses a special “algorithmic approach” for calculating current distribution in such a non-stationary system, based on consistent computer formulation and solution of Kirchhoff’s algebraic equations for the equivalent electrical circuit that simulates a porous electrode with the given thickness on different time intervals. The analysis of the model enables a better understanding of the mechanism of current generation in porous graphite electrodes and offers and explanation of the effect of electrochemical process penetration into the thick electrodes during the discharge process. It is shown, particularly, that classical almost exponential current distributions in the initial quasi-stationary period of time changes considerably during the further discharge process. After some period of discharge, the current peaks originate in the electrode. These discharge currents travel into the depth of the electrode, involving again and again new layers the porous electrode. The effect of some design and technological parameters (like the electrode thickness, current density, resistor of separator, polymer binders, etc.) has been analyzed. This gives a possibility to estimate the influence of such parameters on the discharge curves and operating characteristics of lithium-ion batteries and hybrid electrochemical capacitors with negative graphite electrode.