Percolation investigation of organic radical battery electrodes and its crucial impact on capacity utilization

Abstract

Organic battery electrodes are usually composites consisting of an active material, a conductive additive, and a binder. The present work investigates the role of the conductive additive SuperP® (SP) in a standard composite electrode based on poly(2,2,6,6-tetramethyl-4-piperinidyl-N-oxylmethacrylate) (PTMA) and analyzes the relevance of percolation theory for the electrode’s capacity utilization. It is demonstrated by impedance spectroscopy (IS) and dielectric spectroscopy that at values between 5 wt.-% and 10 wt.-% of the conductive additive the composite shows a phase transition from dielectric behavior into a conducting state. This is caused by the creation of an interconnected network of conducting SP particles penetrating the dielectric polymer matrix as indicated by the measured Nyquist spectra. This percolation mechanism generates a 1,000-fold increase of conductivity from 10-7 to 10-4Sm-1. Evaluations of different composite parameters such as permittivity, dielectric loss, and conductivity as well as scanning electron microscopy (SEM) investigations verify the formation of conducting percolation paths in the organic electrode. Additionally, the first distribution of relaxation time (DRT) analysis of an electrode of an organic radical battery is presented to unravel conduction mechanisms in the dielectric non-percolated and conducting percolated electrodes. Subsequently, galvanostatic cycling with potentiostatic holding times revealed an increased capacity utilization with increasing amount of conductive additive. Electrodes below the percolation threshold do not show charge storage characteristics due to high ohmic overpotentials. Thus, the formation of electron conducting percolation paths by the additive is a crucial mechanism for enabling efficient accessibility of redox centers in the battery electrodes. Hence, IS is presented as a preliminary electrode characterization step prior to the cell assembly to ensure a high capacity utilization of the electrode within the battery.