Distribution of Zinc Oxide and Zinc Metal in ‘Shape Change’ Electrodes
Zinc metal is an ideal electrode material for a battery using aqueous electrolyte, owing to its high capacity of 820 mAh g-1 and high reducing power. However, zinc-based secondary batteries have not been established yet, because several deterioration modes appear during discharge-charge cycle, such as dendrite short-circuit, densification and shape changes. They are the main obstacles to prolong cycle life of zinc-based secondary batteries. Especially, the shape change is a specific phenomenon in that the active material is accumulated in the center of the current collector. This behavior can cause decrease in the discharge capacity, so we should suppress this phenomenon. For this purpose, the mechanism and the structure of the shape change should be studied in detail. In this study, non-destructive synchrotron XRD analysis with precise local resolution was carried out for observing practical zinc composite electrode.
We prepared a porous ZnO electrode (ca. 100 mAh), which consisted of ZnO and some additives, put on the Cu current collector. The electrode size were 18 mm width × 20 mm length. The ZnO electrode was sandwiched by two NiOOH positive electrodes, inserted with non-woven separators. The working electrode potential was monitored using the Hg/HgO reference electrode. The alkaline electrolyte used was a KOH aqueous solution saturated with ZnO.
Charge-discharge tests were employed under the following condition; 0.5C-rate at CC-CV charge and CC-discharge with 50% utilization. The cut-off potential was set at -1.5 V and -1.1 V against Hg/HgO potential for charging and discharging, respectively. The electrode structure was evaluated in a non-destructive manner at synchrotron beamline BL28XU, SPring-8 (Hyogo, Japan) in a transmission mode. The X-ray energy and the beam size were 30 keV (λ=0.4133 Å) and 1mm width ×0.4 mm length.
Results and Discussion
The XRD pattern of the initial electrode (before charging) consisted of peaks that were assigned to ZnO, Cu current collector, and the Ni(OH)2 positive electrode. After 1st charging, metallic zinc appeared uniformly. However, the ZnO intensity at the edge of the electrode was reduced after 5th charging, which supports the dissolving rate is high on the periphery of the electrode. Fig. 1 shows the results after 100 charge-discharge cycles in the charged state. Fig. 1a indicates ZnO was aggregated in the center of the electrode. This was much pronounced when compared with after 5 cycle charging. During the charging-discharging cycles, ZnO on the periphery of electrode gradually dissolved and after 100 charge-discharge cycles ZnO was left only in the center of electrode, which looks like a zinc oxide island. In addition, ZnO islands were surrounded by zinc metal deposits as shown in Fig.1b.
The distribution of zinc oxide and zinc metal depended on the alkaline concentration. In the concentrated alkaline electrolyte 8M KOH, ZnO islands grew in the out-of-plane direction of electrode. On the contrary, ZnO grew in the in-plane direction in the dilute alkaline electrolyte of 4M KOH. It is thus suggested that the shape change is originated from the zinc dissolution and deposition mechanism, and driven by the high solubility of zinc species in the alkaline aqueous electrolyte.