ZnO@Ag microspheres used as the anodic materials of superior alkaline rechargeable Zn–Ni batteries

Abstract

Alkaline zinc-nickel batteries have attracted the attention of many researchers because of their safety, environmental friendliness and low cost. However, the low utilization rate of active substances and poor conductivity lead to the decrease of battery capacity and poor cycling stability, which hinder its further development. Herein, an excellent cyclic stability zinc-nickel battery system with a ZnO@Ag microspheres anode has been developed. The ZnO@Ag microspheres deliver a high specific capacity over 620 mAh g⁻¹ at 1C and of 94.08% coulombic efficiency before 700 cycles. The hydrothermal reaction time and the optimum amount of silver are also studied. This study demonstrates that the ZnO@Ag microsphere is emerging as a promising anodic material for the high-potential rechargeable aqueous zinc-nickel battery, and it may shed light on the high-performance anodic material for zinc-nickel battery storage devices.

Introduction

With the increase of energy demand, solar energy, wind energy, geothermal energy and other renewable energy sources have received extensive attention [[1], [2], [3], [4]]. The instability of these energy sources makes it difficult to integrate them into the grid and use them efficiently [5,6]. The development of various energy storage systems enables these renewable energies to be used more efficiently [7,8]. The high specific capacity and high energy density of lithium ion battery make it a wide choice of energy storage materials for many portable devices. However, lithium battery safety issues and high prices due to a shortage of lithium resources and other issues have greatly hindered the use of lithium batteries. Therefore, this is an urgent to develop a safety, environment friendliness, cheap and excellent performance batter [[9], [10], [11], [12], [13]]. Zinc-nickel secondary batteries have attracted extensive attention from researchers due to their advantages of high specific energy, high work voltage, environmentally friendly and low toxicity [14]. For a long time, the research on zinc negative electrode of alkaline zinc-nickel secondary battery mainly focuses on decreasing the solubility of zinc electrode in alkaline electrolyte and improving the electrical conductivity of zinc active substances. Toward this end, a large number of researchers have tried many methods [[15], [16], [17], [18]]. It can be roughly divided into the following three types: the additive is added, the active substance is modified and new materials are synthesized [19]. The addition of additives is the most common method [20]. such as some metal hydroxides, like Ca(OH)₂, Al(OH)₃ and Mg(OH)₂, etc., will react with ZnO and form the indissolvable compound like Ca(OH)₂∙2Zn(OH)₂∙2H₂O, that can reduce the solubility of zinc in alkaline electrolyte, relieve the formation of zinc dendrite and extend the zinc nickel battery cycle life. But it also reduces the activity of ZnO. In addition, some metal oxide or hydroxide, such as Bi, In, Cd, Pd, etc., have high hydrogen evolution overpotential, can be as additive [[21], [22], [23], [24], [25]]. These metals have a higher potential in the alkaline electrolyte than zinc, so the electrolytic deposition of these metals takes precedence over zinc in the charge process, and will not dissolve in the process of discharge, at the same time the metal substance is simply formed and generate the conductive network in the active substance to enhance the conductivity of zinc anode. Furthermore, the metal which have a hydrogen evolution overpotential is added in the zinc anode will increase the overall hydrogen evolution overpotential and inhibit the evolution of hydrogen. Modification of active material is to improve the whole shape or surface of active material by chemical method [[26], [27], [28], [29]]. In addition, a lot of exploration has been made on the synthesis of new materials as active components of zinc negative electrode, such as the synthesis of calcium zincate, zinc-based hydrotalcite and so on [[30], [31], [32]].

Spherical structure has a larger specific surface area than other structures of zinc oxide, which is conducive to increase the contact between the alkaline electrolyte and the zinc active substances, improving the utilization rate of the active substances, and increasing the actual discharge specific capacity in the charge and discharge process of ZnO electrode. And Ag as the most electrically conductive metal, coated on the surface of ZnO microspheres to form a conductive network, provides a conductive path for electron transfer between micro particles. In this paper, ZnO@Ag microspheres were prepared with SnCl₂ as sensitizing agent and triethanolamine as dispersing agent. However, since the silver body has no electrochemical activity, it has no capacity during charge and discharge in zinc-nickel secondary batteries. Although Nano Ag can improve the conductivity of zinc electrode and thus improve the electrochemical performance of zinc-nickel battery [33], it can also reduce the discharge capacity of the battery. Therefore, the optimal addition of Ag was studied in this paper.