Bismuth Enables the Formation of Disordered Birnessite in Rechargeable Alkaline Batteries

Recent advances in rechargeable Zn/MnO 2 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 Bi 2 O 3 additive using multimodal structural characterization techniques during active battery cycling. Diffraction results provide evidence of Bi 3+ residing in the interlayer of birnessite which prevents irreversible Mn 3 O 4 formation by limiting Mn 3+ 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 H 2 O within the interlayer. Birnessite phases will reform without Bi 2 O 3 present, but Mn 3 O 4 formation severely polarizes the potential they are formed at, leading to capacity fade. Also, we discuss the reversible Bi 2 O 3 conversion to Bi 0 and its contribution to the observed capacity. We expect the results will provide crucial insight into the development of aqueous, rechargeable battery systems utilizing MnO 2 .