Optimization of Bi Additive Concentration and the Impact on the Performance of Secondary Zn/CuO Alkaline Batteries

Secondary Zn-CuO batteries containing a Bi2O3 additive represent a promising means of electrochemical energy storage using an aqueous chemistry and earth abundant materials but have previously required large quantities of Bi to operate. Here, we studied the effect of reduced Bi concentration on the electrochemical performance and physiochemical properties of CuO cathodes cycled in 35 wt.% KOH versus a Zn anode. Our results show that reversable CuO cycling is possible at Bi quantities as low as 0.5 wt.% Bi2O3 when dissolved in the electrolyte solution, and that the addition of 1 wt.% Bi2O3 to the CuO cathode with ball milling enables a more optimal combination of cycling behavior. Material characterization of ex situ cathodes in various states of charge shows that Bi concentration significantly impacts the physiochemical properties of the CuO cathode, contributing to the observed electrochemical behavior. Based on these results, we propose a mechanism to explain the process by which a soluble Bi reservoir influences both the discharge and charge processes of CuO in alkaline electrolyte. Finally, we demonstrate that the use of an alkaline polymer gel electrolyte enables improved capacity retention in the optimized 1 wt.% Bi2O3 cathodes.