Distribution of active materials in Bi-modified MnO2 electrodes

Abstract

There is a global need for batteries capable of buffering intermittent solar and wind power. Unlike Li-ion batteries, which are based on a flammable electrolyte, aqueous Zn-MnO2 batteries contain no flammable organic compounds. The low-cost basis materials lower the financial barrier for grid-scale battery integration with a potential for capital costs <$50/kWh. Traditionally, the Zn-MnO2 battery chemistry is considered non-rechargeable. However, though the mechanism is not well understood, adding bismuth compounds to the MnO2 cathode imparts rechargeability, making hundreds to thousands of cycles possible. This work seeks to uncover the physical effect on initial discharge of adding Bi2O3 to the MnO2 cathodes. The reduction reactions of MnO2 and Bi2O3 both involve a dissolution-precipitation reaction where Mn and Bi cations are transported away from the active material particles. By imaging electrodes discharged to several depths, with and without Bi2O3, the impact of Bi on the progressive morphologies of the electrodes is assessed. Scanning electron microscope (SEM) images are used to identify morphological changes to alkaline MnO2 cathodes discharged at a rate of C/20. To determine the effect of Bi2O3 or variation of the conductive additive on the morphology, three groups of batteries with varying cathode compositions were created, discharged, and embedded in epoxy slides. Cells in each group were discharged to 103 mA/g, 206 mA/g, 309 mAh/g, 411 mAh/g, and 514 mAh/g. SEM images of each slide were then processed with Fiji ImageJ software to determine the quantity, average size, and relative area of active material and discharge product particles in an electrode cross section. Results show significant differences between the groups of electrodes and suggest that both the presence of Bi2O3 and type of conductive additive impact the morphology of the discharging materials. This is apparent as different volume changes during discharge were observed for each group. The conductive additive appears to have a greater impact on this change than the presence of Bi2O3. Differences are observed during both the first (Mn4+/Mn3+) and second (Mn3+/Mn2+) electron reactions of Mn, indicating both Bi and conductive additive affect both segments of the discharge reaction.--Author's abstract