Cycle Life Study and Failure Analysis of the Rechargeable Porous Zinc Electrode in Alkaline Electrolyte
Authors: D'Ambrose Michael , Messinger Robert J. , Banerjee Sanjoy , Yadav Gautam Ganapati , Turney Damon , Gallaway Joshua W , Nyce Michael 
Published: February 2020

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 and a high theoretical gravimetric charge capacity of 820 mAh/g. 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 1000 or more charge-discharge cycles. This corresponds to a Zn electrode charge density in the range of 650 mAh/mL.

The percentage of charge capacity of Zn accessed is known as the depth of discharge (DOD). Battery energy density of 200 Wh/L motivates zinc electrode improvement to 20% Zn DOD. However, increased Zn DOD leads to a decrease in cycle life. A negative exponential relationship between cycle life and Zn DOD for the range of 1% to 15% Zn DOD for zinc-manganese dioxide (Zn-MnO2) batteries was observed experimentally. Over 2800 cycles were achieved in a Zn-MnO2 battery at 1% Zn DOD. The effect of additives on the Zn electrode at Zn DOD values in the range of 15% to 30% was studied in zinc-nickel (Zn-Ni) battery cycle tests. Additives used were surfactant cetyltrimethylammonium bromide (CTAB), bismuth oxide (Bi2O3), synthetic layered silicate Laponite, and calcium hydroxide (Ca(OH)2). A cycle life of 542 at 15% Zn DOD was observed for a Zn electrode containing CTAB, bismuth oxide, and Laponite. In both Zn-MnO2 and Zn-Ni batteries, absolute value of Zn potential vs. Hg/HgO on discharge decreased as cycle number increased, even before discharge capacity faded.

The Zn electrode is associated with multiple failure mechanisms. Electrolyte depletion is due to a lack of hydroxide ions (OH-), either because of OH- consumption during battery discharge or an insufficient electrolyte reservoir. Pore plugging results from decreased electrode porosity associated with ZnO precipitation. Zn passivation occurs when precipitated ZnO films completely surround Zn. Shape change of the Zn electrode is a phenomenon in which Zn is not deposited uniformly during charge. The Zn electrode is also susceptible to gassing, in which the hydrogen (H2) evolution reaction occurs during charge. Water is reduced to gaseous H2, thus the flow of electrons into the electrode is not used to deposit metallic Zn, resulting in lower Zn plating efficiency.

In order to further understand underlying physical, chemical, and electrochemical behavior of the Zn electrode as well as improve energy density and cycle life, future experimental and theoretical work is proposed. The effect of additives on the cycle life of Zn-MnO2 batteries will be studied. Additionally, electrode performance for Zn DOD values in the range of 15% to 30% Zn DOD will be investigated. A theoretical mathematical model of the Zn electrode will be formulated based on the governing equations of the system. Numerical solutions will be obtained for the spatial profiles of Zn(OH)4
2- concentration, OH- concentration, current density, potential, and porosity at extended cycle number.

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