Ni(OH)₂ anodes for secondary alkaline batteries

Nickel hydroxide is commonly used as an active material for nickel positiveelectrodes in Ni-based rechargeable batteries, electrochromic devices, and as promising catalysts for oxygen evolution reaction (OER). Some properties of nickel hydroxide electrodes (such as high power density, very good cyclability, and high specific energy) have made them very viablefor an extended range of applications. It is reported that there are four phases produced over the lifetime of a nickel hydroxide electrode, namely, a-Ni(OH)₂,
g-NiOOH, b-NiOOH, and b-Ni(OH)₂ and the electrochemical reactions of nickel hydroxides can be shown simply by:

In a battery application a « g cycling has a couple of advantages over b « b cycling. First, more electrons are exchanged per nickel atom in the a « g cycling because the oxidation state of nickel in g-NiOOH is higher than that of b-NiOOH.

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. It was reported that a stabilized a-Ni(OH)₂ electrode has a much larger charge capacity than a b-Ni(OH)₂ electrode.

Second, on prolonged charging, b-NiOOH is known to be converted to g-NiOOH, causing a mechanical deformation which results in an irreversible damage to the electrode. On the other hand, a-Ni(OH)₂ can be cycled to g-NiOOH reversibly without any mechanical deformation. Therefore, a-Ni(OH)₂ is expected to be a better electroactive material for a high performance nickel positive electrode.

The presence of certain metal cations in hydrated nickel oxides and oxyhydroxides, generically referred as nickel hydrous oxides, can strongly affect the electrochemical characteristics of this interesting electrode material. Cobalt, cadmium and zinc are commonly used as beneficial additives in nickel battery electrodes.

The addition of several percent of additives, such as Cd, Co, and Zn, to the nickel hydroxide is a very effective method of suppressing the formation of g-NiOOH. Although there are numerous studies on these topics, the results are still not complete. In our laboratory the research about the influence on different additives on the optical and electrochemical response is currently under study.