Failure Analysis of the Rechargeable Porous Zinc Electrode in Alkaline Electrolyte

Abstract

Metallic zinc (Zn) is an attractive negative electrode material for rechargeable batteries because it is a non-toxic, earth abundant metal. It has a low equilibrium potential of -1.35 V vs. mercury-mercuric oxide (Hg|HgO) in concentrated alkaline electrolyte, a high gravimetric discharge capacity of 820 mAh/g, and a high volumetric discharge capacity of 5850 mAh/mL. During discharge, Zn oxidizes and forms a dissolved complex known as the zincate ion, Zn(OH)4 2-, from which zinc oxide (ZnO) precipitates as a solid. On charge, Zn(OH)4 2- is reduced to metallic Zn. Motivation to study the Zn electrode in rechargeable batteries is driven by a target overall battery energy density of 200 Wh/L that is sustainable for 500 or more charge-discharge cycles. This corresponds to a Zn electrode volumetric capacity of 650 mAh/mL. The cycle life of the porous Zn electrode in concentrated alkaline electrolyte is observed to decrease with increasing percentage of discharge capacity of Zn accessed. The percentage of discharge capacity of Zn accessed is known as the depth of discharge (DOD). Zn electrodes were cycled in the range of 1% to 15% Zn DOD in zinc-manganese dioxide (Zn-MnO2) batteries and in the range of 15% to 30% Zn DOD for zinc-nickel (Zn-Ni) batteries. Additives were incorporated into the zinc electrodes for the tests done on Zn-Ni batteries. Additives used were surfactant cetyltrimethylammonium bromide (CTAB), bismuth oxide (Bi2O3), synthetic layered silicate Laponite, and calcium hydroxide (Ca(OH)2). The potential values of the Zn electrodes vs. Hg|HgO reference electrodes were recorded. During healthy galvanostatic discharge, Zn electrodes displayed a nearly constant potential vs. Hg|HgO at approximately -1.35 V. Tests that failed due to Zn exhibited a shift from this equilibrium potential that corresponded to a loss of cell voltage. This shift may be attributed to a concentration overpotential as ZnO is precipitated, thereby inhibiting the hydroxide ions from reaching the reacting metal interface. The shift may also be due to formation of a passivated layer on the electrode which effectively blocks the remaining active material. There are multiple regimes observed in the potential curves of the Zn electrode vs. Hg|HgO prior to cell failure. Beyond the healthy plateau at -1.35 V, a sloping potential is observed. This is followed by an additional plateau. The second plateau shifts with increasing cycle number and precedes cell death. See the attached figure for discharge curves of a Zn electrode cycled at 11.3% Zn DOD. After the second plateau a steep slope is observed in the potential for later cycles, which is believed to be due to passivation of the Zn electrode. The additives were observed to modify the discharge potential curve of the Zn electrode vs. Hg|HgO. Figure 1