A nickel–cadmium battery is made up of a positive electrode with nickel oxyhydroxide as the active material and a negative electrode composed of metallic cadmium . These are separated by a nylon divider. The electrolyte, which undergoes no significant changes during operation, is aqueous potassium hydroxide.
The positive electrode in the discharged state is composed of nickel hydroxide, which has been doped and modified to meet the battery requirements, and graphite as the conductive medium. The nickel cycles between two oxidation states during charge and discharge; upon the charge, the nickel hydroxide is converted into nickel oxyhydroxide (NiOOH):
The specific gravity of the electrolyte is 1.2. Since the voltage produced by a single cell is very low, many cells are connected in series to get the desired voltage output and then this arrangement is known as the nickel cadmium battery. In these batteries, the number of positive plates is one more than that of negative plates.
The theoretical capacity of cadmium metal is 480 mAh g −1. However, cadmium is not usually applied as a metal to form a battery anode. The cadmium electrode may be formed starting with a mixed cadmium hydroxide, and/or cadmium oxide and a certain amount of cadmium powder. Two types of cadmium electrode are also widely used.
The most common failure modes in nickel–cadmium batteries are electrical shorts caused by the growth of cadmium dendrites and penetration through the separator, passivation, and wear of active materials, destruction of the separator, and swelling of positive active mass.
Other disadvantages of nickel–cadmium battery are the high rate of self-discharge, poor performance at high temperatures, and complex charging. During discharge reaction in N–Cd battery, cadmium is oxidized on the negative electrode to form Cd (OH) 2 and electrons (Fig. 4.1).