Structure And Electrochemical Performance Of Zinc Oxide-based Anodic Materials For Ni-Zn Rechargeable Batteries

As a kind of high-performance green secondary battery, zinc-nickel (Zn-Ni) battery has extensive applications because of its high energy density, high power density, high working voltage, facile raw material, no pollution, etc. However, its poor cycling performance limits the widespread commercialization, due to the Zn electrode shape change and Zn dendrite formation. In this paper, a series of ZnO-based oxides including ZnO-Bi2O3, ZnO-Bi2O3-YNbO4and ZnO-Bi2O3-Co2O3-Co3O4were synthesized by mechanical milling and solid state reactions. And Using these oxides as the nucleation sites, ZnO/ZnO-Bi2O3, ZnO/(ZnO-Bi2O3-YNbO4) and ZnO/(ZnO-Bi2O3-Co2O3-Co3O4) nanocomposites were synthetized by a simple homogeneous precipitation method. The phase composition, morphology and microstructure of the as-prepared samples were investigated. Furthermore, the effects of the nucleation agents on the electrochemical performances of ZnO nanocomposites were also discussed. The main conclusions are listed as following:Nanosized ZnO was prepared by homogeneous precipitation with Zn(NO3)2and CO(NH)2, the particles were pherical with uneven sizes of60-120nm in diameter. Electrochemical tests showed that nanosized ZnO as the active material had an average discharge capacity of467.5mAh g-1, but suffering from the rapid declining of capacity rapidly. The charge/discharge platform was unstable and the cyclic reversibility was bad. Due to its epitaxial growth, after60cycles, the nanosized ZnO grew from imperfect spherical particles with the initial size of90nm into hexagonal cylindrical (800nm in length,600nm in radial width), which reduced the surface area and the discharge capacity of the nanosized ZnO rapidly.Using ZnO-Bi2O3as the nucleation sites,.the ZnO/ZnO-Bi2O3nanocomposites had much smaller particle size of30-60nm, distributed more uniformly, and formed a tight structure. Electrochemical tests showed that the synthesized ZnO/ZnO-Bi2O3containing15wt.%ZnO-Bi2O3showed the best cycling performance, with an average discharge capacity of590.2mAh g-1, the lowest charge plateau voltage and highest discharge plateau voltage. The average charge/discharge mean voltages of ZnO/ZnO-Bi2O3were 1.87and1.678V, respectively. And the electrochemical reactivity and reversibility were significantly enhanced. After60cycles, a compact ZnO flake structure was formed. The particles were smaller than the nanosized ZnO through long-term cycling.Using ZnO-Bi2O3-YNbO4as the nucleation sites, the synthesized ZnO particles were not uniform with the sizes of about20-100nm, much smaller than nanosized ZnO. Electrochemical tests showed that ZnO/(ZnO-Bi2O3-YNbO4) with15wt.%had the best cycling performance with the average discharge capacity of555.9mAh g-1, active material utilization ratio of80.6%, the average charge/discharge mean voltage of1.87and1.679V. And also, the reactivity and reversibility were significantly enhanced. After60cycles, the ZnO in the nanocomposite remained rod-like structures in the nanometer sizes, and a high electrochemical activity.Using ZnO-Bi2O3-Co2O3-Co3O4(hereinafter referred to as the C1powder) as the nucleation sites, the ZnO particles with the sizes of30-60nm distributed more uniformly, and formed a tight structure. Electrochemical tests showed that the ZnO/C1with15wt.%had the best cycling performance with the average discharge capacity of595.8mAh g-1(90.9%of theoretical capacity), the capacity decay rate of0.5mAh g-1/cycle, had the lowest charge plateau voltage and highest discharge plateau voltage. After60cycles, the ZnO/C1nanocomposites showed a stable cluster-shaped structure, so it had good electrochemical properties.As above, three nucleating agents all can improve the electrochemical properties of zinc electrode. The reasons are contributed by the following facts:(1) During the charging/discharging processes, nucleating agents provided a stable growing center for active materials, so that ZnO was deposited uniformly on the surface of nucleating agents. The dissolution of ZnO was limited by close contact between the nucleating agents and ZnO.(2) The presence of nucleating agent destroyed the close packing of ZnO, increased the specific surface area of ZnO and maintained the electrochemical activity of ZnO.