The effect of Zn(OH) 2 addition on the electrode properties of nickel hydroxide electrodes
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The effect of Zn(OH)2 addition on the electrode properties of nickel hydroxide electrodes J. Chen, D. H. Bradhurst, S. X. Dou, and H. K. Liu Institute for Superconducting & Electronic Materials, University of Wollongong, Wollongong, New South Wales 2522, Australia (Received 30 June 1998; accepted 7 December 1998)
Nickel hydroxide powders currently used in the positive electrode of nickel-metal hydride (Ni–MH) batteries require cobalt or cobalt oxides to make them viable and attractive. As a step to eliminate the cobalt-containing materials, spherical nickel hydroxide powders coprecipitated with Zn(OH)2 were prepared by a spraying technique. These powders, which have a higher tapping density and a much smaller pore volume than conventional powders, were used as the active materials of nickel hydroxide electrodes. The effects of the Zn(OH)2 additions on the electrode properties, such as percentage utilization and cycle life, were studied, and the relationship between the electrode performance and the formation of g –NiOOH was investigated. The cycle life was increased because there was less electrode swelling due to much reduced formation of g –NiOOH.
I. INTRODUCTION
The nickel-metal hydride (Ni–MH) battery is a relatively new commercial product which meets the criteria for a more environmentally friendly battery than either the lead-acid or the nickel-cadmium battery.1 In current designs of Ni–MH batteries, the capacity of the nickel electrode limits the battery capacity. This is because the MH electrode is made larger than the nickel hydroxide electrode so the battery can be sealed safely.2 It follows that increasing the energy density of the nickel hydroxide electrode is essential to raise the energy density of such batteries. The overall electrochemical reactions responsible for the charge and discharge of the nickel hydroxide electrode are represented below3 : Charge
b-Ni(OH)2
Discharge
Thermal decay
Overcharge Charge
a-Ni(OH)2
b-NiOOH 1 H1 1 e2
Discharge
g-NiOOH 1 H1 1 e2
It can be seen that there are four phases observed over the lifetime of a nickel hydroxide electrode. The principal reaction in the nickel hydroxide electrode involves the charge and discharge of b –Ni(OH)2 to b –NiOOH which can be converted to g –NiOOH on overcharge. g –NiOOH discharges to form a –Ni(OH)2 , which is very unstable in the presence of water and alkali, and is converted to b –Ni(OH)2 . During the charge-discharge processes, it is observed that (i) the thickness of the nickel hydroxide electrode is increased and (ii) the surface area of the nickel hydroxide electrode is increased.4 1916
http://journals.cambridge.org
J. Mater. Res., Vol. 14, No. 5, May 1999
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The formation of g –NiOOH is associated with the volume expansion or swelling of the nickel hydroxide electrode. The phase change from b –NiOOH to g –NiOOH can be correlated to a 44% increase in volume. The increase in surface area is due to phase transformation from b – to g –NiOOH during cycli
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