Effects of Sonication on EIS Results for Zinc Alkaline Batteries

Abstract

Current battery technology relies on thin materials in order to limit the overpotential losses due to diffusion within the cell, which generally leads to increased cost and lower energy densities. This study analyzes the use of sonication as a means to reduce losses associated with diffusion using Electrochemical Impedance Spectroscopy (EIS) for Zinc-Manganese Oxide (Zn-MnO 2 ) cells. Impedance changes in the Warburg diffusion controlled regions are compared for different membrane materials.A growing interest in recent years has been to develop batteries which have higher capacities, higher power density, and shorter charge times. While it is important to study novel chemistries and materials, this paper focuses on using sonic energy to overcome some of the limitations inherent in diffusion-based cells. The optimization of battery technology has led to thin materials as a means to significantly reduce distances within the cell in order to reduce diffusion times, and therefore voltage losses due to diffusion limitations.Batteries rely on diffusion and migration for mass transfer of ions between electrodes. It is hypothesized that sonic energy waves can enhance this ionic mass transfer. Previous studies have shown that flow induced by peristaltic pump has shown to reduce overpotential losses. 1 Zinc-alkaline chemistry is used in this study, due to the chemistry's resilience outside controlled atmospheres such as are required for many battery chemistries.Previously, it has been shown that sonication can enhance mixing of solutions. 2 A growing field has emerged using sonic energy to enhance electrochemical systems called sonochemistry. Within this field, it has been shown that electrochemical reactions in Galvanictype cells experience an increase in mass-transfer limited currents under sonication. 3,4 Sonication can induce several phenomena that result in enhanced ion movement: acoustic streaming, acoustic cavitation (even bubble collapse), and microjet formation (uneven bubble collapse). 3 Acoustic streaming is an effect experienced on the scale of the electrodes that induces flow and is initiated and maintained by sonic waves. 5 Bubble formation occurs when pressure drops due to the sonic waves moving through solution. After the wave has passed, the bubble collapses leading to microjet and/or acoustic cavitation. However, intense cavitation may also cause erosion of electrodes. 6 The research reported in this paper uses electrochemical impedance spectroscopy (EIS) to provide insight into mass transfer mechanisms of a battery subjected to sonication. EIS applies alternating voltage at varying frequencies between electrodes to obtain an impedance related to the mass transfer processes. This impedance is composed of real and imaginary components due to the sinusoidal nature of the applied voltage. Plotting negative imaginary impedance vs. real impedance gives a Nyquist plot, which can be broken down into kinetic controlled regions and mass-transfer controlled regions. Previously, analysis of phase angles for high to ...

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