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The effects of the current rate used during the first charging (pre-charging: so-called “formation”) on the cathodic deposition of metallic zinc (Zn) were analyzed for the high capacity (thick) zinc oxide (ZnO) electrode in rechargeable Zn-based batteries. Pre-charging at a lower current rate (1.875 mA cm−2) enabled greater electrode performances for the subsequent charge-discharge cycles. The Zn deposition profiles were investigated by conventional postmortem X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy using a scanning electron microscope, as well as in situ synchrotron XRD and ex situ synchrotron X-ray computed tomography. The results revealed significant differences in the deposition profiles of the metallic Zn depending on the current rates used during pre-charging. The higher rate (18.75 mA cm−2) resulted in an inhomogeneous deposition of Zn, whereas the lower rate yielded finer Zn particles dispersed homogeneously throughout the thick ZnO electrode. These morphological and spatial variations in the Zn deposition during pre-charging affected the subsequent cycling behavior of the thick ZnO electrode.

For the use of the zinc anode in secondary batteries, it is necessary to solve the “shape change” deterioration issue in that zinc species agglomerate in the center of the electrode to fade the available capacity. The local chemical compositions of the zinc electrodes during “shape change” were precisely analyzed using the synchrotron X-ray diffraction mapping analysis of practical zinc-nickel cells in a non-destructive manner. The in situ Zn/ZnO mapping shows that metallic Zn deposition chiefly occurs in the periphery of ZnO while ZnO are left in the center of electrode like a hill on charging. On discharging, the ZnO hill grows to the perpendicular direction on the electrode while metallic zinc is oxidized and dissolved. These findings allow us to propose a mechanism for the shape change; thus dissolved zincate species are decomposed on the ZnO hill during discharging to be accumulated in the center of the electrode. It is suggested that suppressing zincate dissolution and non-uniform zinc deposition slow the growth rate of the ZnO hill to enhance the cyclability of zinc-based secondary batteries.

Although considerable research efforts have been taken to minimize the dendrite growth of zinc in zinc-based secondary batteries, still dendrite growth is a severe issue, which needs further research in this field. In this paper, we have investigated that vanillin as an additive material either eliminates, or assist to minimize the formation of dendrite growth during the deposition of Zn. More than 300 cycles of charge-discharge curves are tested and even at relatively large current densities, dendrite-free zinc surfaces are observed with small amount of vanillin (0.5-5 mM). On the other hand, dendrite growth was observed in the vanillin-free solution. In order to observe the dynamic zinc deposition process on ultra-flat Au(111), we have used an advanced optical microscopy with differential interference microscopy which allowed us to follow the time dependent processes of deposition and dissolution of Zn at high speed accusation times (4~10 frames/sec even in large area of 100 x 100 μm). We also found that the formation of "black" films depends on the applied electrode potentials. It is anticipated that such insoluble films might be a key factor for the formation and growth of zinc dendrites in rechargeable zinc batteries. [ABSTRACT FROM AUTHOR]

We have studied in detail the electrochemical reaction of nickel in several kinds of molar ratio-controlled molten salts consisting of AlCl3 and 1-butylpyridinium chloride (BPC) at 40°C. We observed NiCl2 as an oxidation product from nickel on the surface of the electrode in slightly acidic AlCl3/BPC salts with molar ratios of 1.05/1.0 and 1.1/1.0. However, in strongly acidic salt with the ratio of 1.5/1.0, NiCl2 deposits on the electrode less than when in the above salts, and no NiCl2 is observed in basic and neutral salts with the ratio of 1.0/1.0 or less AlCl3 content. This suggests that [NiCl4]2- ions form as the oxidation of nickel in such neutral and basic AlCl3/BPC salts. [ABSTRACT FROM AUTHOR]