Cycle performance of the zinc oxide electrode improves by mixing the organic solvent in the electrolyte. However, the organic solvent is concerned to be decomposed at the positive electrode. In order to solve the problem, the zinc oxide negative and the positive electrodes are separated by a membrane, and the organic solvent is added only in the negative electrode side. As a result, the organic solvent decomposition at the positive electrode is avoided and thus the cell shows good cycle performance.
Zinc is widely used as a negative electrode material for primary batteries with its strong reducing power and a high capacity. However, its use as a rechargeable electrode has been limited due to insufficient cycle life.
We have shown that water-organic co-solvent electrolytes are effective for restricting the dissolution of the zinc species and thus improving the cycle performance . On the other hand, organic solvents are generally vulnerable to oxidation and hence such water-organic co-solvent electrolytes can be incompatible with positive electrodes such as Ni(OH)2and air electrodes.
This study describes a cell setup that is effective for both zinc electrode rechargeability and positive electrode compatibility. Here a membrane that suppresses the transfer of the organic molecules was employed in the cell to show the Ni(OH)2electrode compatibility.
Zinc composite electrodes consisted of ZnO, PbO, carbon and a binder, pasted on a copper current collector. The potential of the zinc negative electrode was measured versus an Hg/HgO reference electrode. The positive electrode was a sintered nickel (Ni(OH)2) electrode. The electrolyte consisted of a 4 M KOH aqueous solution. In the negative electrode side, organic molecules such as alcohol were added to the KOH solution. A hydrophilic membrane was used as a separator to suppress the transfer of the organics to the positive electrode.
3. Results and Discussion
The membrane was permeable to small organics (MW<100) but can suppress the transfer of large molecules (i.e. oligomers or polymers). Accordingly, when this membrane is used as a separator between the negative and positive electrodes, the organic can be maintained at the negative electrode side while hydroxide ions are transferred through the membrane for charge neutrality during charging-discharging.
A large organic was selected and their effect on the cycle performance was examined. As shown in Fig. 1(a), the use of the organic is effective for improving the cycle life of the zinc electrode. The sintered nickel positive electrode was degraded when tested without a membrane, indicating the reactivity of the organic at the positive electrode. In contrast, when a proper membrane was used as the separator, the nickel electrode functioned without any problem, showing that the organic transfer through the membrane can be disregarded. Small organics were detrimental to the nickel electrode even if a membrane was set between the positive and negative electrodes.
The porous structure of the zinc electrode was observed by SEM analysis. Figure 1(b) shows that there are many pores in the composite electrode prior to the cycling test. After 40 charge-discharge cycles, the porous structure were maintained when the organic was used whereas the pores were mostly clogged without using the organic, as shown in Figs. 1 (b) and (c). These results show that the densification , which is a typical deterioration mode of the zinc electrode, was suppressed and there were on-site zinc dissolution and deposition with the addition of the organic. It is deduced that the water activity in the electrolyte was reduced in the presence of the organic species and thus the zincate dissolution was suppressed.
The shape change behavior , another typical deterioration mode of the zinc electrode, was observed by synchrotron X-ray diffraction mapping. The measurement using the zinc-nickel cells demonstrates the effect of the organic also on the shape change behavior.
This work was supported by RISING project of NEDO.
 A. Nakata, H. Arai, T. Yamane, T. Hirai and Z. Ogumi, J. Electrochem. Soc., 163, A50-A56 (2016).