Duracell Ni-MH manual Composition and Chemistry

Ni-MH Rechargeable Batteries

Composition and Chemistry(cont.)

The sealed nickel-metal hydride cell uses the “oxygen-recombination” mechanism to prevent a build- up of pressure that may result from the generation of oxygen towards the end of charge and overcharge. This mechanism requires the use of a negative electrode (the metal hydride/metal electrode) which has a higher effective capacity than the positive (nickel oxyhydrox- ide/nickel hydroxide electrode) electrode. A schematic drawing of the electrodes is shown in Figure 3.3.1.

During charge, the positive electrode reaches full charge before the negative electrode which causes the evolution of oxygen to begin:

2OH- _____> H2O + 12 O2 + 2e-

The oxygen gas diffuses through the separator to the negative electrode, a process which is facilitated by the “starved-electrolyte” design and the selection of an appropriate separator system.

At the negative electrode, the oxygen reacts with the metal hydride and oxidizes or discharges the metal hydride to produce water:

2MH + 12 O2 _____> 2M + H2O

FIGURE 3.3.1

Positive Electrode

NiOOH/Ni(OH)2

Useful Capacity

	MH/Alloy
Charge	Discharge
Reserve	Reserve

Negative Electrode

Schematic representation of the electrodes, divided into useful capacity, charge reserve and discharge reserve.

Thus, the negative electrode does not become fully charged and pressure does not build up.

The charge current, however, must be con- trolled at the end of charge and during overcharge to limit the generation of oxygen to below the rate of recombination. Thus, charge control is required to pre- vent the build-up of gases and pressure. Duracell rec- ommends that continuous overcharge not exceed C/300 for optimal performance.

As shown in Figure 3.3.1, the nickel-metal hydride cell is designed with a discharge and charge reserve in the negative electrode. The discharge reserve minimizes gassing and degradation of the cell in the event of overdischarge. The charge reserve ensures that the cell maintains low internal pressure on over- charge.

The negative electrode has excess capacity compared to the positive electrode and is used to handle both overcharge and overdischarge. Thus, the useful capacity of the battery is determined by the positive electrode.

3