Optimization of Bi₂O₃ Content in CuO-Bi₂O₃ Cathodes for Secondary Zn Alkaline Batteries Leads to Improved Capacity and Cycling

Secondary alkaline batteries pairing Zn anodes with metal oxide cathodes (MnO 2 , CuO, etc.) are a promising route to energy-dense, inexpensive batteries for use in grid-scale energy storage. However, the low cycle life and material stability of secondary alkaline batteries, especially when compared to competing chemistries like Li-ion batteries, must be addressed in order to further develop these technologies. In recent years, studies have shown that the addition of chemical additives like Bi 2 O 3 to the metal oxide cathodes can significantly improve their cyclability. Further work is needed, however, to better understand the mechanism of this improvement and further optimize cathode composition and performance. In this presentation, we will present our efforts to optimize the Bi 2 O 3 composition of a CuO-Bi 2 O 3 cathode for use in secondary Zn-CuO alkaline batteries. Earlier reports showed that relatively large quantities of Bi 2 O 3 additive (10 wt.%) allowed for improved cathode conductivity and promoted the electroreduction of CuO x to Cu 0 . However, the use of such large quantities of a heavy additive also reduced the effective capacity of the resulting cathode by up to 15%, relative to a CuO cathode without any Bi 2 O 3 . Here, we show that the use of as little as 1 wt.% Bi 2 O 3 is sufficient to affect a similar improvement in CuO cycling performance, and that even a Bi 2 O 3 -saturated KOH electrolyte solution is sufficient to impact the cyclability of a CuO cathode. Using a variety of physical and (electro)chemical techniques, we show that at these low Bi concentrations the Bi likely cycles between a sparingly soluble BiO x /Bi(OH) 4 2- reservoir contained in both the cathode and electrolyte in the charged state, and solid Bi 0 that is uniformly distributed on the surface of the CuO x /Cu 0 cathode in the discharged state. Various electron microscopy techniques (SEM and STEM) show that this has a notable impact on the physical morphology of the active material in the cathodes after cycling, while electrochemical impedance spectroscopy (EIS) shows how electron transfer resistance varies based on Bi 2 O 3 concentration. On a fundamental level, our results show that changes in Bi 2 O 3 concentration notably impact the charge-discharge behavior of the CuO cathodes and results in the suppression of metal hydroxide-related redox peaks on charge. The absence of these peaks is thus correlated with improved cyclability, suggesting that the addition of Bi and the suppression of these recharge pathways may lead to higher capacity, more stable CuO cathodes in the future. This work was supported by the U.S. Department of Energy, Office of Electricity and Sandia National Laboratories, a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA-0003525. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Dr. Imre Gyuk, Director of Energy Storage Research, Office of Electricity is thanked for his financial support. The views expressed herein do not necessarily represent the views of the U.S. Department of Energy or the United States Government.