The Effects of the Carbon Materials and the Electrolyte Content on the Second-Electron Reaction of Electrolytic MnO2 in Alkaline Systems

Abstract

A low-cost battery with high energy density, high safety, and made with low scarcity material is needed for stationary storage applications. Electrolytic manganese dioxide (EMD), which is non-toxic, abundant, and low cost, is suitable for such applications. EMD has a high theoretical energy density (617 mAh g-1) for its two-electron reaction. However, most previous works focused on the first-electron reaction of EMD, which only provides an energy density of 308 mAh g-1. The dissolution-precipitation process during the second electron reaction of MnO2 based cathode was regarded as the main reason for the formation of Mn3O4 and will greatly affect the cycling stability of the electrode.1Recent works found that additives like Bi2O3 and Cu could help form a stable birnessite phase and suppress the dissolution of manganese during cycling, resulting in a very stable cycling performance.2However, the mass loading of the active material in these studies was still low and required additives like carbon nanotubes, which are not commercially favorable.In our work, we investigated the EMD second-electron reaction in an electrolyte-lean T-cell system, with very little excess electrolyte. We compared EMD performance in the T-cell with its performance in the electrolyte-flooded beaker cell system at different cycling rates in both KOH and NaOH electrolytes. We found a huge improvement in the stability of EMD when cycled in the electrolyte-lean T-cells especially with NaOH electrolyte (Fig. 1a). We also explored the influence of different carbon materials, including multi-walled carbon nanotube (MWCNT), graphite and Super P carbon blacks in the electrolyte-lean system (Fig. 1b). Different carbon materials were found to have a great impact on both capacity and cycling stability performance of the EMD cathode, possibly due to differences in the specific surface area and the pore sizes of the carbon materials. Electrodes with higher EMD loading and chemically synthesized Bi-doped birnessite material were also tested in the T-cell system to find a more economically favorable electrode composition. Scanning electron microscopy, X-ray diffractions, and other techniques were used to evaluate the phase change and morphology difference during the cycling process.Figure 1. (a) Comparison between T-cells with 37 wt.% KOH with 21 wt.% NaOH as electrolyte with a normal beaker cell with 37 wt.% KOH electrolytes. The electrodes composed of 60 wt.% EMD, 12 wt.% Bi2O3, 14 wt.% Cu, 10 wt.% MWCNT and 4 wt.% PTFE, NiOOH were used as anodes. The C-rate of all tests is C/3. (b) Comparison between different carbon material materials that used in the electrode in T-cells. The electrodes composed of 60 wt.% EMD, 12 wt.% Bi2O3, 14 wt.% Cu, 10 wt.% carbon material and 4 wt.% PTFE, NiOOH were used as anodes and 30 wt.% NaOH was used as electrolyte. The C-rate of all tests is C/3. G. S. Bell and R. Huber, Electrochim. Acta, 10, 509–512 (1965).G. G. Yadav, J. W. Gallaway, D. E. Turney, M. Nyce, J. Huang, X. Wei, and S. Banerjee, Nat. Commun., 8, 1–9 (2017). Figure 1