Electrochemical Performances Of Nanosized ZnO, Surface-modified ZnO As Anodic Materials For Zinc-nickel Batteries

Zinc-nickel battery has the advantages of high energy density, high power density, high working voltage, wide working temperature, facile raw material, free of environment pollution in produce and application. However, the cycle life of Zn-Ni battery remains relatively low due to the Zn electrode shape change, Zn dendrite formation, etc. Thereby, the development of Zn-Ni battery is greatly limited. This work put forward material design and surface modification of ZnO to improve the cycle life of Zn-Ni battery. In the paper, the nanoscale plate-like ZnO was prepared and its electrochemical performances as the anode materials of Zn-Ni battery were systemically investigated. In addition, the influence of surface modification of silver on electrochemical performances of ZnO was also studied.Plate-like ZnO was prepared by a simple hydrothermal synthesis method with Zn(NO₃)₂·6H₂O and NaOH as raw materials. The dimension of ZnO powder ranged from 200 to 500 run and the average thickness was about 50 nm. The ZnO nanoplates distribute in the electrode randomly. During cycling, the rapidest growth direction determined by the crystal growth habit is vertical or inclined to the accelerated growth direction induced by concentration polarization which is aroused by liquid-side mass transfer, and the two growth modes compete and inhibit mutually, at last the zinc dendrite is impeded effectively. The morphology of ZnO nanoplates did not essentially change with increasing the cycle, and the morphology retention could stabilize electrochemical performance of the active material to a certain extent. The ZnO nanoplates had excellent plasticity and creep resistance, their volume change was small with cycling, thus the shape change of the zinc electrode could be restrained enormously. Compared to the conventional ZnO, the ZnO nanoplates showed better discharge stability. The discharge capacity of ZnO nanoplates still remained 427 mAh g^-1 until the 80th cycle, while that of conventional ZnO was only 178 mAh g^-1. The fading rate of ZnO nanoplates was only 0.19 %.Ag-modified ZnO was prepared by an electroless plating process with C₆H₁₂O₆·H₂O, NaOH and ZnO as raw materials. The average size of silver nanoparticles was about 45 nm and they were modified on part surface of ZnO particles. The silver nanoparticles modified on the surface of ZnO could decrease the resistance of the Zn electrode, and could also decrease the direct contact of core ZnO with electrolyte, then suppressed the dissolution of ZnO in the electrolyte and improved the utilization ratio of ZnO. Thus, the Ag-modified ZnO showed better discharge stability than the unmodified ZnO. The discharge capacity of Ag-modified ZnO still remained 456 mAh g^-1 until the 65th cycle, and the fading rate of the discharge capacity was only 0.89 mAh per cycle. From the typical charge-discharge curves it could be seen that the Ag-modified ZnO decreased the charge plateau voltage and increased the discharge plateau voltage, as resulted from the fact that the Ag-modified ZnO reduced the resistance of zinc electrode. The cyclic voltammetry curves showed that the reduce reaction of ZnO was suppressed by the modification of the silver nanoparticles, which was coincident with the decline in charge plateau voltage, but the surface modification did not influence the active material utilization. The impedance spectrum showed that the charge-transfer resistance (R_t) of Ag-modified ZnO was lower than that of unmodified ZnO, that was to say the electrochemical reaction was easier and the utilization of active material was enhanced.