New zinc batteries for energy storage

Abstract

 In general, metallic iron, aluminum and zinc are considered as the candidates for the anodes of aqueous batteries. Among them, the metallic zinc is the most promising one since it is more stable in aqueous solution than aluminum, and more electropositive than iron. In addition, zinc is naturally abundant, low in cost, high in energy density and environment friendly. Traditional Zinc-air cells (ZACs) suffer from cathode-related issues (e.g., CO2 contamination and electrode flooding) and have relatively low voltage. In addition, rechargeable zinc batteries, namely Zn-Ni batteries and Zn-Br2 flow batteries, encounter inherent challenges, e.g., the negative effect of zincate ions on the electrochemistry of the cathode (nickel hydroxide) in Zn-Ni batteries, the bromine crossover to zinc, and the possible bromine emission to environment in Zn-Br2 flow batteries.

In this study, we develop two new types of zinc-air cells to improve the performance of traditional zinc-air cells, namely tri-electrolyte zinc-air cell (TE-ZAC) and tri-electrolyte microfluidic zinc-air cell (TEM-ZAC). The TE-ZAC, with a structure of zinc anode / aqueous 6 M KOH // cation exchange membrane (CEM) // aqueous saturated KCl // anion exchange membrane (AEM) // aqueous 6 M HCl / air-breathing cathode, delivers an open circuit voltage (Voc) of 2.0+ V and a maximum density of 130 mW cm-2. Compared to a traditional ZAC, the Voc and the maximum power density have been enhanced by ca. 33% and ca. 44%, respectively. The TEM-ZAC, where a microfluidic technology is utilized, exhibits a Voc as high as 2.18 V at an electrolyte flow rate of 0.075 ml min-1, which is, to the best of our knowledge, the highest voltage for ZACs. In addition, for both cells, the cathode-related issues encountered with traditional ZACs, are avoided since the alkaline for oxygen reduction reaction (ORR) is replaced with the acid. In this work, we have also developed three new concepts of rechargeable zinc aqueous cells, namely tri-electrolyte Zn-PbO2 (TE-Zn/PbO2) cell, dual-electrolyte Zn-LiCoO2 (DE-Zn/LiCoO2) cell, and hybrid Zn/active-carbon (H-Zn/AC) cell. The TE-Zn/PbO2 cell has low cost PbO2 as the cathode and delivers an average voltage as high as 2.1 V at a constant current of 25 mA (ca. 0.1C, where 1C means completing discharge / charge battery in 1 hour). The DE-Zn/LiCoO2 cell employs LiCoO2 as the cathode, delivers an average output voltage of 1.4 V at the discharge rate of 0.13C, and has a coulombic efficiency as high as 96%. Finally, the H-Zn/AC cell shows a 99.2% coulombic efficiency, and delivers an average output voltage of 0.8 V at a high discharge rate of 2.8C. The capacity retention is ca. 51% after 1700 cycles. Overall, although the cycle life of our cells is relatively low, the cycle performance can be greatly improved by either modifying the electrodes structure or adjusting the pH value of the catholyte. Most important of all, we demonstrate that our new cells work and have high potential for practical applications.