In-Operando Surface Chemistry of Nickel and Copper Current Collectors During Zinc Electro-Deposition and Dissolution

Zinc electrodeposition from alkaline solutions is of renewed interest due to MnO2-Zn and Ni-Zn grid-scale energy storage devices. Control and understanding of the rechargeable zinc electrode remains difficult, and several Zn battery technologies suffer from a failure to deposit zinc after many cycles. The predominant substrates for zinc energy storage in zinc electrodes are copper and nickel. Here we study the performance of zinc electrodeposition onto nickel and copper substrates in an alkaline battery device, with the aim to identify the nanoscale surface chemistry that mediates the decreased electrodeposition performance. We find the coulombic-efficiency of the zinc cycling process to drop from 99% to ~50% under various operating conditions for electrodeposition, for both zinc and copper substrates. We present grazing-incidence x-ray diffraction measurements in-operando on the nickel surface, to identify the crystallography of nanoscale surface layers that form during cycling of the battery. Our results suggest the presence of zinc oxide and zinc peroxide layers on the nickel surface affects the coulombic efficiency of zinc cycling. We find that nanoscale layers of zinc oxide aids the zinc electrodeposition and allows charge-efficiency to be high. If the nickel anode surface is exposed to voltages ~0.5 V above zinc’s rest voltage this zinc oxide layer disappears, leaving behind a relatively bare nickel surface that has very poor zinc electrodeposition charge-efficiency due to nickel’s catalytic generation of hydrogen gas. The beneficial zinc oxide layer is seen to grow thicker during normal cycling and during open circuit rest over many hours. Similar performance problems are shown for copper current collectors. X-ray photoelectron spectroscopy and electron microscopy is performed on the copper current collectors, which uncovers the formation of a composite zinc-copper nanoscale layer that impedes zinc electrodeposition during battery charging. Electrochemical growth and dissolution of this zinc-copper layer is hypothesized to interact with shape change of the zinc electrode. In the case of both copper and nickel the formation of these layers is caused by undesired oxidation that arises due to microscopic variations in OH- and H2O concentration. Possible strategies for improving the long-cycle performance of zinc electrochemical energy storage are given.