The Effects of Electrolytes and Cycling Protocols on Bi-Birnessite Cathode for Alkaline Batteries

Abstract

Stationary batteries need to be low-cost, highly energy-dense, safe, reliable, and made with abundant materials using simple manufacturing methods. MnO2 is abundant, low-cost, non-flammable, non-toxic, & has a high theoretical capacity of 617 mAh g-1. The phase changes of MnO2 during cycling are a significant determinant of the system’s reversibility. Electrolytic manganese dioxide, or γ-MnO2, is the most commonly used polymorph, frequently paired with Zn anodes in commercial primary batteries.1 In secondary batteries, long-lasting reversibility of γ-MnO2 cathodes requires limiting depth-of-discharge (DoD) to 5-10% of capacity to prevent the formation of additional MnO2 polymorphs. Alternatively, layered δ-MnO2, or birnessite, can deliver 60-80% of theoretical capacity but usually requires low active mass loading and has a low cycle-life due to the irreversible formation of Mn3O4. The use of copper and Bi2O3 additives was recently found to mitigate many birnessite problems, resulting in a cathode with high active mass loading that delivered full theoretical capacity for thousands of cycles.2 Furthermore, the birnessite was electrochemically synthesized from γ-MnO2 in situ, making the system even more attractive for commercialization. However, eliminating copper and replacing KOH with NaOH would make the system much more economically favorable. In this work, we investigated the sensitivity of Bi-birnessite’s cycle life to electrolyte type and cycling protocols. We compared the cycle life of Cu-intercalated Bi-birnessite in KOH to that of Bi-birnessite, with and without copper, in NaOH. We also evaluated the influence of depth-of-discharge on the capacity fade behavior of the cathode in NaOH. Finally, we assessed the performance of the cathode under a slow, shallow cycling protocol that simulates seasonal, long-term energy storage behavior. X-ray diffraction (XRD) was used at various points in the cathode’s cycle life to determine the phase evolution of the material. By extrapolating from the XRD and cycling data, we will compare the roles of KOH to NaOH and Cu2+ to Na+ in the system, respectively.References Shin, J., Seo, J. K., Yaylian, R., Huang, A. & Meng, Y. S. A review on mechanistic understanding of MnO2 in aqueous electrolyte for electrical energy storage systems. Int. Mater. Rev. 65, 356–387 (2020).10.1080/09506608.2019.1653520. Yadav, G. G. et al. Regenerable Cu-intercalated MnO2 layered cathode for highly cyclable energy dense batteries. Nat. Commun. 8, 14424 (2017). 10.1038/ncomms14424.