Improvement of Charge-Discharge Cycling Performances of Zinc Electrode By Using Porous-TiO2-Coated Zinc As a Negative Electrode Active Material

Abstract

Zinc electrodes are of great interest as a candidate for metal negative electrode that has high theoretical energy density, low cost and high safety owing to the use of aqueous electrolytes. However, rechargeable zinc electrodes still suffer from poor cycle life due to dendrite growth and shape change. These degradation phenomena of the zinc electrode are known to be promoted by the high solubility of the zinc species in the electrolyte. In an strong alkaline electrolyte usually employed in aqueous alkaline batteries, a discharge reaction of zinc takes place to produce Zn(OH)4 2-. Such soluble discharge products of zinc can migrate through the electrolyte, resulting in a redistribution of zinc. Therefore, inhibiting the migration of the discharge products of zinc are thought to be effective in minimizing dendrite growth and shape change. In this study, based on the concept of inhibition of the migration of the zinc discharge products into electrolyte, we report the synthesis, characterization and charge-discharge characteristics of porous-TiO2-coated Zn.We have newly synthesized zinc particles uniformly coated with porous TiO2 layers (porous-TiO2-coated Zn), as follows. Fine zinc powder (ca. 0.1-0.6 μm in diameter) was dispersed in the solution of titanium tetrabutoxide diluted with 1-butanol. While stirring and heating this solution, aqueous ammonia was added to form TiO2-precursor nanoparticles by the hydrolysis of titanium tetrabutoxide. Subsequently, in order to attach TiO2-precursor nanoparticles to the surface of zinc particles, urea as a linker molecule was added into this solution. After stirring and heating this solution, the samples obtained by filtration and washing with distilled water were dried and calcined in air. Core particles and uniform coating layers with the thickness of approximately 40 nm were observed in TEM images of the cross-section of the porous-TiO2-coated Zn particles. From a result of EDX analysis, it can be concluded that the coating layers mainly consist of titanium and oxygen. To further elucidate the characteristics of TiO2 coating layers, the pore-size distributions of uncoated-Zn and porous-TiO2-coated Zn were evaluated from nitrogen desorption isotherm. The porous-TiO2-coated Zn contains mainly pores with diameters smaller than 10 nm, in contrast to the uncoated-Zn which has no nano-sized pores. The peak pore diameter of porous-TiO2-coated Zn was 3.5 nm. This indicates that the porous-TiO2-coated Zn is composed of dense zinc particles and nanoporous TiO2coating layers.Comparison of the cycle life between uncoated-Zn and porous-TiO2-coated Zn was carried out. The zinc electrode was assembled in pouch cells with a large and a small sintered Ni(OH)2/NiOOH electrodes, which were respectively used as the counter and reference electrodes. 4 M KOH aqueous solution saturated with dissolved ZnO was used as electrolyte. Test cells were subjected to charge-discharge cycling test at 298 K. During the charge-discharge cycling test, the test cells were charged at 0.5 C rate for 1.5 h (615 mAh/g-Zn, zinc utilization of 75 %) and discharged at 0.5 C rate either for 1.5 h or until the zinc electrode potential reached at the cut-off potential of -0.9 V vs. Hg/HgO. From the charge-discharge cycling tests, reproducible results were obtained that the average values of the cycle life for 3 test cells employed each uncoated-Zn and porous-TiO2-coated Zn were 20 cycles and 117 cycles, respectively. It was found that the negative electrode using the porous-TiO2-coated Zn as an active material has achieved more than 5 times longer cycle life than that using uncoated-Zn in spite of a small TiO2 content.We propose the mechanism of improving the cycle life by using porous-TiO2-coated Zn that porous TiO2 coating layers act as a protection layer for the zinc oxides films formed through the discharge reactions and the pores of the TiO2 layers (< 10 nm) are sufficiently small to keep the zinc oxides. It should be noted that the effect of the TiO2 coating on the cycle performance cannot directly be attributed to the physical inhibition of the Zn(OH)4 2- migration because the average pore size of 3.5 nm is too large to block the migration of Zn(OH)4 2-.AcknowledgementThis work was supported by the Research and Development Initiative for Scientific Innovation of New Generation Batteries (RISING) project from NEDO in Japan.