Rechargeable zinc-alkaline batteries are attractive alternatives to established battery technologies such as lithium-ion and lead-acid in certain applications because of their inherent low cost, safety, and environmental friendliness. Commercialization of zinc-alkaline batteries has been largely unsuccessful because of the limited cycle life of zinc electrodes at high depth of discharge. At high depths of discharge, dissolved zinc precipitates from the electrolyte as solid zinc oxide. Zinc oxide buildup causes passivation of zinc metal particles, which increases resistance within the battery and results in eventual cell death. It can also electrically isolate metal particles from the current collector, resulting in loss of active zinc mass. Zinc oxide in alkaline batteries has been studied for decades, but an understanding of the properties of the material and effects on battery performance is still not complete. A complete understanding of the discharge products of zinc electrodes is a necessary step toward wide-spread commercialization of zinc metal electrodes.

We demonstrate in this work that zinc oxide produced during discharge undergoes chemical changes as a function of electrode potential. In operando optical microscopy shows that the oxide reversibly changes color from blue at more negative zinc electrode potentials to white at more positive zinc electrode potentials. In operando Raman spectroscopy shows differences in Raman scattering between the blue and the white oxide species, indicating that additional bonds are present in the blue material, which are not observed in the white material. Solid-state ¹H magic angle spinning NMR spectroscopy on extracted oxide material suggests that changes to electrode potential result in simultaneous insertion or disinsertion of protons and electrons into the oxide material. More immobile species are present in the blue material compared to the white, indicating inserted protons may be more abundant in the blue oxide. Additionally, two-dimensional ¹H-¹H exchange spectroscopy shows more chemical exchange between sites in the white material compared to the blue, indicating protons may be more strongly bound in the blue oxide.

The behavior observed in these studies is similar to that observed for electrochromic materials. In electrochromic materials, inserted electrons can increase the electrical conductivity of the oxide by orders of magnitude. At more positive zinc potentials, an increase in the electrical resistance of the oxide may result in conductivity losses between zinc metal particles. Additionally, when protons are disinserted from the oxide, they result in a local pH decrease in the electrolyte surrounding the oxide, which affects the ionic conductivity of the electrolyte. The work outlined in this study demonstrates the effects of this behavior on electrode performance and explores avenues for controlling the behavior through electrode additives and operation controls.