Nano-Scale In Situ TEM Observations of Electrodeposition/Dissolution Process of Zinc Metal onto a Platinum Electrode

Zinc-air batteries are one of the candidates as low-cost, high-energy-density batteries. Although primary (non-rechargeable) zinc-air batteries have been already commercialized, many problems remain for the practical application of secondary (rechargeable) zinc-air batteries. For example, the dendritic growth of zinc metal at the anode of a zinc-air battery can cause short circuits during charging processes. The detailed mechanism of dendrite formation has not been elucidated yet. It is thus necessary to clarify the deposition/dissolution behavior of zinc metal and its dependence on electrochemical conditions.<br/>In this study, we applied a liquid-phase transmission electron microscope (LP-TEM) using an electrochemical chip (e-chip) to investigate the initial stage of zinc electrodeposition. The ordinary zinc-air batteries involve a high-concentration potassium hydroxide aqueous solution containing saturated zinc oxide as an electrolyte. However, it is difficult to introduce such strong base electrolytes into LP-TEM. Therefore, we alternatively used an aqueous zinc sulfate solution for the first step toward the elucidation of electrochemical deposition/dissolution reactions of zinc metal in zinc-air batteries. We performed <i>in situ</i> TEM observation using electrochemical liquid cell holders (Protochips, Poseidon) with an on-chip three-electrode system. To minimize electron irradiation damage, the electron-beam current density during the observation was restricted to less than ~300 e⁻/(nm²s). The deposition and dissolution processes of zinc metal were conducted under constant-current mode by controlling the electric potential of the working electrode against the platinum reference electrode. The time-dependent electric potential (chronopotentiogram) of the working electrode was recorded by a potentiostat system.<br/>As a result, we succeeded in observing the dynamic growth of zinc dendrite from the working electrode in the electrochemical liquid cell holders. It should be noted that the quantitative electrochemical measurement corresponding to <i>in situ</i> TEM can be performed in this study. In a constant current mode, zinc grew on the platinum working electrode depending on the electric current. Changes in electric potential were observed depending on variations of the zinc deposits and the surface conditions of the working electrode. These results clarified the correlation between the precipitation morphology of zinc and the electric potential in the electrochemical deposition/dissolution reactions of zinc metal.<br/>In the presentation, we will show more details of the electric potential changes corresponding to the surface of the platinum working electrode.<br/>This presentation is based on results obtained from a project, JPNP16001 and JPNP21006, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).