The Effect of Additives on the Performance and Failure Mechanisms of the Rechargeable, Low-Cost, Alkaline Zinc Electrode

Abstract

Metallic zinc (Zn) is an attractive negative electrode material for rechargeable batteries because it is a non-toxic, non-flammable, earth abundant metal that is compatible with aqueous electrolytes. Zinc metal is also energy dense, exhibiting a high gravimetric discharge capacity of 820 mAh/g, a high volumetric discharge capacity of 5850 mAh/mL, and a low equilibrium potential of -1.35 V vs. mercury-mercuric oxide (Hg|HgO) in concentrated alkaline electrolyte. Rechargeable aqueous Zn alkaline batteries have thus recently been the subject of intense investigations for safe, low-cost, stationary electrochemical energy storage. Our research aims to achieve Zn depth-of-discharge (DOD) of 20% that is sustainable for 500 or more charge-discharge cycles, corresponding to an overall battery energy density of 200 Wh/L at the cell level.However, the cycle life of the porous Zn electrode in concentrated alkaline electrolyte is observed to decrease strongly with increasing Zn DOD. For example, we recently demonstrated that metallic Zn electrodes cycled in the range of 1% to 16% Zn DOD had an exponential decrease in cycle life with Zn DOD, which was attributed mostly to the loss of active Zn material. Here, we study the effect of additives to increase the Zn DOD and cycle life of Zn electrodes. Zinc oxide (ZnO) was used as a starting material from which metallic Zn was formed in zinc-nickel (Zn-Ni) batteries. Additives were incorporated into the ZnO electrodes for the tests done on Zn-Ni batteries and these electrodes were cycled in the range of 15% to 30% Zn DOD. The potential values of the Zn electrodes vs. Hg|HgO reference electrodes were recorded to identify failure due to Zn. At 15% Zn DOD, ZnO electrodes with certain additives or combinations of additives achieved an increased cycle life compared to Zn electrodes without additives. This highlights the need to perform electrochemical and spectroscopic studies to determine the effects of the additives on the mechanisms of the Zn electrode. These additives, with further understanding, could be implemented in low-cost, energy-dense, rechargeable Zn alkaline batteries for grid-scale energy storage.