Multi-scale impedance model for supercapacitor porous electrodes: Theoretical prediction and experimental validation

Abstract

In this paper, a multi-scale impedance model is developed for evaluating the charge storage capacity and ion-transfer rate of supercapacitor porous electrodes. This model can be used to theoretically understand the contributions of the intra-particle pore at nano-scale, the inter-particle pore at micron-scale, and the porous electrode at millimeter-scale to the performance of supercapacitor. Then, impedance spectrum characteristics of porous electrodes are screened via numerical simulation based on the developed multi-scale impedance model, especially for different electrode thicknesses and various faradaic processes with different time constants. Subsequently, seven supercapacitor samples, four of them with electrode thicknesses of 60, 100, 180, and 370 μm respectively, and three of them with different types of current collectors (nickel, carbon-coated stainless-steel, and stainless-steel), are fabricated and characterized to validate the developed model. The fitting results and residuals of the measured impedance data validate the linear relationship of the scaled equivalent resistance of electrolyte v. s. electrode thickness and the scaled characteristic time constant v. s. the square of electrode thickness. The validated multi-scale impedance model might offer an effective method to rationally balance or optimize charge storage and charge transfer via morphology design for porous electrodes.