Bismuth Enables the Formation of Disordered Birnessite in Rechargeable Alkaline Batteries

Abstract

Recent advances in rechargeable Zn/MnO2 alkaline batteries have shown promise for scalable energy storage systems which provide a safe, low-cost alternative with 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. Herein, we investigate the reversible reaction between birnessite and Mn(OH)2 with and without a Bi2O3 additive using multimodal structural characterization techniques during active battery cycling. Diffraction results provide evidence of Bi3+ residing in the interlayer of birnessite which prevents irreversible Mn3O4 formation by limiting Mn3+ diffusion within the crystal lattice. Also, upon charge no MnOOH intermediate phases are observed. Instead, X-ray absorption and Raman spectroscopy indicate a disordered, non-crystalline birnessite-type phase consisting of mostly neutral H2O within the interlayer. Birnessite phases will reform without Bi2O3 present, but Mn3O4 formation severely polarizes the potential they are formed at, leading to capacity fade. Also, we discuss the reversible Bi2O3 conversion to Bi0 and its contribution to the observed capacity. We expect the results will provide crucial insight into the development of aqueous, rechargeable battery systems utilizing MnO2.