Percolation Investigation of Polymer-Based Battery Electrodes and Its Influence on Capacity Utilization
Authors: Zimmer Philip , Stolze Christian , Muench Simon , Weisheng Xiao , Stumpf Steffi , Hoeppener Stephanie , Hager Martin D. , Schubert Ulrich S. 
Published: December 2024

Our oral presentation will offer a comprehensive exploration of the intricate dynamics governing organic radical battery electrodes, focusing on the percolation phenomena and its consequential influence on capacity utilization. Our latest investigations center around quantifying the role of conducting additives, notably the widely employed spherical SuperP® (SP), within a composite electrode based on poly(2,2,6,6-tetramethyl-4-piperinidyl-N-oxylmethacrylate) (PTMA). We recently reported a comprehensive study on the percolation investigation of SP in such composite electrodes in the Chemical Engineering Journal 2023, 477, 146882.

This quantitative analysis of the conducting additive in organic battery electrodes emphasizes the critical balance required for optimal electrode composition in industrial applications. Based on the percolation theory we experimentally investigate various electrode compositions and identify a pivotal phase transition at 8 wt.% SP, which is effectively transforming the electrode's behavior from dielectric to conducting.

To provide a comprehensive understanding, we incorporated a series of experimental analyses. The investigation begins with a detailed exploration of permittivities, shedding light on the dielectric properties of the composite electrode. Dielectric losses are scrutinized, providing insights into energy dissipation mechanisms within the electrode. Moreover, the characterization shows that the electrode exhibits so called epsilon-negative properties, which is commonly reported in literature for polymer/nanomaterial composites. However, a resonance type behavior of the permittivity measured at a relatively low frequency could not be assigned to any literature findings yet.

Furthermore, morphological assessments through scanning electron microscopy offer a visual, qualitative validation of the established conducting percolation paths within the PTMA-based composite electrode.

To the best of our knowledge, we are the first to apply distribution of relaxation times (DRT) analysis to organic radical battery electrodes, validating the hopping-type conduction mechanism for the dielectric electrodes below percolation threshold as well as assigning the conduction above percolation threshold to the SP phase.

The electrochemical characterization of the percolation phase transition's impact on the electrode’s capacity utilization was investigated in both coin cells (two-electrode setup) and Swagelok cells (three-electrode setup) via galvanostatic cycling experiments. These electrochemical analyses not only validate the percolation-induced shift in behavior but also provide insights into the charge storage capabilities of several electrode compositions, which provides a basis for electrode optimization. Furthermore, the cycling experiments elucidate the electrochemical performance variations when employing two-electrode setups as opposed to three-electrode setups.

Key findings underscore a direct correlation between the optimal electrode composition for enhanced capacity utilization and the observed percolation phase transition. Electrodes falling below the percolation threshold exhibit charge storage characteristics close to zero, primarily due to elevated ohmic overpotentials. In contrast, electrodes exceeding the threshold demonstrate a rapid saturation in capacity utilization, signifying the intricate interplay between conducting paths and efficient accessibility of redox centers within battery electrodes.

Additionally, our findings introduce the possible application of impedance spectroscopy (IS) measurements as a preliminary electrode characterization step. This new approach not only offers valuable insights into the charge-discharge dynamics of organic radical battery electrodes but also enables general electrode composition optimization prior to laborious cell assembly and time-consuming battery cycling experiments.

Furthermore, this work may be used as basis for a potential pre-quality control tool for electrodes in a high-throughput production in an industrial setting. Considering the graphical abstract, concentration changes could be identified before cell assembly. Additionally, the impact on cell performance due to the concentration changes may be estimated. By integrating IS into the production process, we envision real-time monitoring and adjustment during large-scale manufacturing processes, ensuring consistent electrodes with optimized performance.

Figure 1