(Invited) Bismuth Enables Formation of Disordered Birnessite in Rechargeable Alkaline Batteries
Authors: Bruck Andrea , Kim Matthew A , Gallaway Joshua W. 
Published: December 2020

Recent advances in rechargeable Zn/MnO2 alkaline batteries have shown promise for scalable energy storage systems which provide a safe, low-cost alternative to current Li-ion technologies and have a demonstrated lifetime over thousands of cycles. This cathode technology is based on a 2-electron Mn redox process where a layered birnessite-type phase has been shown to form after the first cycle with excellent reversibility between the discharge product, Mn(OH)2 when Bi2O3 and Cu constituents were added to the electrode.1 For this system to reach costs < $100/kWh, the complete reduction to Mn2+ must be reversible with high mass loading (> 20 mAh/cm2). To get a complete understanding of the role of Bi2O3 in the reversible formation of a birnessite phase a multimodal structural characterization strategy is necessary to identify all the various intermediates formed during battery cycling. Spectroscopic evidence including Raman and X-ray absorption spectroscopy during cycling provide crucial insights into intermediate 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.5,6
Figure 1 shows the birnessite vibrations at 510, 578 and 635 cm-1 form only at the top of charge with the Mn3O4 spinel vibration at 659 cm-1 present throughout the entire charging process. However, with the addition of the Bi2O3 a birnessite phase forms earlier in the charge process allowing for the facile conversion between the Mn(OH)2 and charged birnessite phase. This birnessite phase could not be identified through diffraction methods and is therefore considered a highly disordered phase that exists as a short-lived intermediate during battery operation. Using operando diffraction and X-ray near edge absorption spectroscopy (XANES) we characterize the disordered intermediate birnessite and the final crystalline phase formed as the final charge product. We expect the results will provide crucial insight into the development of aqueous, rechargeable battery systems utilizing MnO2.

Figure 1 Operando Raman spectroscopy. (A) Individual Raman spectra, (B) contour map, and (C) corresponding voltage profile of the battery with a K-birnessite electrode. (D) Individual Raman spectra, (E) contour map, and (F) corresponding voltage profile of the battery with a K-birnessite with Bi2O3 electrode.

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Figure 1