Breakdown and Reformation of Birnessite in Rechargeable, Bismuth Modified MnO₂ Alkaline Batteries

Abstract

Rechargeable Zn-MnO2 alkaline batteries have been identified as a viable option for the modernization of grid scale energy storage due to their projected cost (<$100/kWh), scalability, and safer components when compared to non-aqueous alternatives.1 For this system to reach its maximum capability, the full Mn4+/2+ redox couple must be reversible over thousands of cycles with high mass loading. This was successfully demonstrated with the incorporation of various electrode constituents which alter the fundamental discharge and charge process of MnO2 to Mn(OH)2.1 Using non-destructive, synchrotron diffraction techniques, we detail the effect of Bi on the breakdown and reformation of birnessite (𝛿-MnO2) during cycling. Further spectroscopic evidence including Raman and X-ray absorption spectroscopy during cycling also provides crucial insights into amorphous phases and variations in redox activity, with and without additives. Early demonstrations of Bi3+ containing constituents with MnO2 in alkaline batteries increased the cyclability of these systems up to hundreds of cycles with low mass loading.2,3 Later investigations followed up by hypothesizing the presence of Bi3+ could potentially catalyze the reduction of Mn3+ to Mn2+ and that upon subsequent charge promotes the formation of a layered, birnessite, MnO2 structure.4 The quick reduction of Mn3+/2+ is critical because the presence of Mn3+ is associated with the irreversible phase change to the Mn3O4 spinel structure.5 Figure 1 shows the birnessite (001) reflection at ~7.1 Å rapidly shift to lower d-spacings upon discharge until ~6.9 Å then directly convert to Mn(OH)2 where the corresponding ~4.8 Å reflection appears. Notably, no crystalline intermediates such as MnOOH or Mn3O4 form during cycling. Also, upon charge the birnessite (001) reflection does not appear until the third potential plateau even though the Mn(OH)2 (001) reflection fades linearly until the end of charge. Due to these observations, further spectroscopic techniques were used to probe these reactions. Using operando Raman spectroscopy we observed a limited formation of an amorphous Mn3O4 during discharge and a disordered birnessite phase formation occurring on the second plateau of charge in the presence of Bi3+. These investigations provide new insight into the intermediates and formation mechanisms of Mn-based alkaline battery electrodes in the presence of Bi2O3. The presence of both Cu and Bi additives are necessary to stabilize the reversible processes occurring during electrochemical cycling. However, the synergistic relationship between Cu and Bi is not extensively investigated. Previous synchrotron XANES and XRF mapping showed that the Cu2+/0 redox couple was occurring during the Mn3+/2+ reduction. It was hypothesized that the Cu acts as a redox mediator and rapidly forms the Mn2+ species during discharge.6 Here we use a X-ray absorption spectroscopy quick scanning to monitor the Mn, Bi, and Cu redox activity which are all relating to the possible mediation of Mn reduction and oxidation. Using multiple techniques, we provide a complete understanding of the role of both bismuth and copper containing constituents on the redox transitions of MnO2 in alkaline electrolyte. Wholistic investigations combining multiple operando techniques are critical to the development of rechargeable alkaline batteries by advancing the fundamental design of these systems. Figure 1 (a) Operando X-ray diffraction data between 3-7.5 Å of the first cycle of Bi2O3-modified birnessite electrode in KOH electrolyte. (b) Corresponding galvanostatic discharge and charge at a rate of C/3. (c) Lattice parameter ‘c’, the interlayer spacing, of the birnessite phase during the first cycle. (d) The relative peak intensities of the birnessite and Mn(OH)2 (001) reflection during the first cycle. References: 1. Yadav, G. G.; Gallaway, J. W.; Turney, D. E.; Nyce, M.; Huang, J.; Wei, X.; Banerjee, S., Nature Communications 2017, 8, 14424. 2. Dzieciuch, M. A.; Gupta, N.; Wroblowa, H. S., Journal of The Electrochemical Society 1988, 135 (10), 2415-2418. 3. Wroblowa, H. S.; Gupta, N., Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 1987, 238 (1), 93-102. 4. Im, D.; Manthiram, A., Journal of The Electrochemical Society 2003, 150 (1), A68-A73. 5. Gallaway, J. W.; Hertzberg, B. J.; Zhong, Z.; Croft, M.; Turney, D. E.; Yadav, G. G.; Steingart, D. A.; Erdonmez, C. K.; Banerjee, S., Power Sources 2016, 321, 135-142. 6. Gallaway, J. W.; Yadav, G. G.; Turney, D. E.; Nyce, M.; Huang, J.; Chen-Wiegart, Y.-c. K.; Williams, G.; Thieme, J.; Okasinski, J. S.; Wei, X.; Banerjee, S., Journal of The Electrochemical Society 2018, 165 (13), A2935-A2947. Figure 1