Page 65 - Handbook of Battery Materials

2.3 Nickel–Cadmium Batteries 31

Overall battery reaction:
Discharge
1.32 V (2.6)
←−−−−−−
2NiOOH + Cd + 2H 2 O −−−−−−→ 2Ni(OH) + Cd(OH) 2
2
Charge
Reactions take place at the positive electrode between nickel oxyhydroxide and
nickel hydroxide, and at the negative electrode between cadmium metal and
cadmium hydroxide. In addition, the H 2 O molecules, which are generated during
charging, are consumed during discharging. Therefore, variations in electrolyte
concentration are insignificant. Because of this reaction, the nickel–cadmium
battery excels in temperature characteristics, high-rate discharge characteristics,
durability, and so on [6]. Most significant is the fact that the amount of electrolyte
in the cell can be reduced enough to allow the manufacture of completely sealed
cells.
The nickel–cadmium battery was invented by Jungner in 1899. The battery used
nickel hydroxide for the positive electrode, cadmium hydroxide for the negative
electrode, and an alkaline solution for the electrolyte. Jungner’s nickel–cadmium
battery has undergone various forms of the development using improved materials
and manufacturing processes to achieve a superior level of performance.
In 1932, Shlecht and Ackermann invented the sintered plate. In those days,
conventional plates involved a system in which the active materials were packed
into a metal container called a pocket or tube. However, with the sintered-plate
method, the active materials are placed inside a porous electrode formed of
sintered nickel powder. In 1947, Neumann achieved a completely sealed structure.
This idea of protection against overcharge and overdischarge by proper capacity
balance is illustrated in Figure 2.5.
Focusing on the concept of the completely sealed system, the Sanyo Electric Co.
developed sealed-type nickel–cadmium batteries in 1961. This type of battery enjoys
a wide application range that is still expanding; a large variety of nickel–cadmium
batteries has been developed to meet user needs ranging from low-current uses like
emergency power sources and semiconductor memories to high-power applications
such as cordless drills.
Figure 2.6 shows the typical structural design of a cylindrical nickel–cadmium
battery. It has a safety vent, as illustrated in Figure 2.7, which automatically
opens and releases excessive pressure when the internal gas pressure increases.
Formation of hydrogen is avoided by ‘extra’ Cd(OH) 2 ; oxygen is removed by reaction
with Cd.
Figure 2.8 shows the charge characteristics when charging is performed at a
constant current. In nickel–cadmium batteries, characteristics such as cell voltage,
internal gas pressure, and cell temperature vary during charging, depending

Ni electrode

Discharge reserve Charge reserve
Cd electrode

Figure 2.5 Electrode capacity balance of a sealed Ni–Cd battery.