Construction of All-Solid-State Nickel-Zinc Rechargeable Cell with Hybrid Hydrogel Electrolyte

Introduction

All solid-state batteries with solid electrolytes attract attention because of their easy handling, high safety etc. The most important requirement for the solid electrolytes is high electrical conductivity. Polymer gel electrolytes using crosslinked polymers are a sort of solid electrolyte which can absorb and hold a great amount of liquid electrolyte. Now they are practically used for lithium-ion polymer batteries. Meanwhile, research and development on solid electrolytes for aqueous battery application have also been carried out.

We firstly prepared a new solid electrolyte, polymer hydrogel electrolyte, using crosslinked potassium polyacrylate (PAAK) which could hold great amount of KOH aqueous solution.¹ Polymer hydrogel electrolytes exhibited high electrical conductivity more than 10^-1 S cm^-1 which was comparable to the original KOH aqueous solutions. All solid state Ni-MH cells with polymer hydrogel electrolytes were quite similar to the conventional Ni-MH cells with KOH aqueous solutions in terms of discharge capacity, charge-discharge cycle durability, high-rate chargeability and dischargeability.^2,3 We also prepared inorganic hydrogel electrolyte by using hydrotalcite (HT), a clay with an inorganic layered double hydroxide structure, as an inorganic backbone, and  showed high conductivity and high diffusion coefficient of zinc ions which were comparable or close to that of liquid electrolyte.⁴

In terms of handling, a self-standing membrane is required for an electrolyte in electrochemical devices, but neither the polymer hydrogel electrolyte nor inorganic hydrogel electrolyte are sufficient in mechanical strength. Recently, we have prepared a new hydrogel electrolyte, organic-inorganic hybrid hydrogel electrolyte (HHE), by mixing hydrotalcite and PAAK with 6 M KOH solution.⁵ In this study, we assembled an all solid-state rechargeable Ni-Zn cell with a self-standing membrane of the hybrid hydrogel electrolyte, and evaluated its electrochemical characteristics.

Experimental

Three HHEs including 0.8, 1.6 or 2.4 g of HT were prepared as follows. HT was added to 10 mL of 6 M KOH aqueous solution. After being stirred for 5 min, 1 g of PAAK was added to the resultant suspension, followed by stirring for a few minutes and standing for 3 days, resulting in homogeneous milky hydrogel. After that, air bubbles in the gel were removed in vacuum. All procedures were performed at room temperature. Each hydrogel electrolyte is denoted HHE(0.8), HHE(1.6) and HHE(2.4), respectively.

The negative electrode was prepared as follows. 30 mg of ZnO powder was mixed with 2 wt.% PVA aqueous solution to prepare a paste. The paste was loaded on a Cu mesh and dried at 80 ^oC for 1 h. The electrolyte solution was 6 M KOH solution containing 0.4 M ZnO, and it was also used for preparing HHE.

Results and Discussion

For three HHEs, logarithm of ionic conductivity in the range of 0 to 70 °C showed linear dependence on the reciprocal of absolute temperature, suggesting that the HHEs followed the Arrhenius-type ionic conduction mechanism in this temperature range. Moreover, all HHEs are stable in the temperature range. All HHEs had ionic conductivity over 0.1 S cm^-1 which was close to that of the original solution and the ionic conductivity steadily decreased with an increase in HT content, while activation energy was independent of HT content, suggesting that the mobility of OH^- ion did not change with HT content.

Charge-discharge cycle performance of an all solid-state Ni-Zn cell with distance between positive and negative electrodes of 8 mm was similar to that of an aqueous Ni-Zn cell with the same distance. In contrast, the all solid-state Ni-Zn cell with an HHE membrane whose thickness is 1 mm demonstrated much better cycle performance than the corresponding aqueous Ni-Zn cell. In charging-discharging of the all solid-state Ni-Zn cell at 1 C, the discharge capacity at 1st cycle was 820 mAh g^-1, but both charge and discharge capacity steadily decreased with cycle.

Acknowledgement

This work was partly supported by Taiwan Textile Research Institute.

References

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