What Is a Nickel Metal Hydride Battery?
A nickel metal hydride battery is a type of rechargeable battery that can be charged and discharged, using a hydrogen storage alloy for the negative electrode and nickel hydroxide for the positive electrode.
Nickel metal hydride batteries are expensive because they use a hydrogen storage alloy instead of cadmium, but can be charged and discharged using a large current and have a large capacity per unit mass.
In addition, compared to other rechargeable batteries, nickel-metal hydride batteries have a relatively small memory effect (the voltage drop that occurs during discharge when batteries are repeatedly recharged without being fully discharged), and can be used repeatedly without performance degradation.
Uses of Nickel Metal Hydride Batteries
Nickel metal hydride batteries are used to take advantage of their high performance and long life, and are used in automotive batteries, notebook PCs, dry cell batteries, and other applications requiring high output and reliability.
In recent years, lithium-ion batteries, which do not have memory effect or self-discharge and have a larger capacity per unit mass and higher operating voltage, have come to be used.
Principles of Nickel Metal Hydride Batteries
1. Composition of Nickel Metal Hydride Battery
Nickel metal hydride batteries consist of electrodes (positive electrode: nickel oxyhydroxide, negative electrode: hydrogen storage alloy), a separator made of olefin nonwoven fabric, and potassium hydroxide solution as electrolyte. In the case of a dry cell, the structure wound around these components is contained in a can.
2. Charge-Discharge Reaction of Nickel Metal Hydride Battery
During discharge of a nickel metal hydride battery, at the positive electrode, nickel oxyhydroxide receives electrons in the presence of water, producing nickel hydroxide and hydroxide ions. At the anode, hydrogen ions and electrons are released from the hydrogen storage alloy in the presence of hydroxide ions to produce water.
During discharge, the reaction proceeds in the opposite direction: at the cathode, hydroxide ions react with nickel hydroxide to produce nickel oxyhydroxide, which releases electrons. At the anode, hydrogen is adsorbed by supplying electrons.
Figure 1. Charge-discharge reaction equation of a nickel-metal hydride battery
The charging and discharging of a nickel metal hydride battery occurs through a simple reaction that involves the adsorption of hydrogen and the production of water. For example, lead-acid batteries used in automobile batteries are charged and discharged through a precipitation-dissolution reaction of the electrodes, so repeated charging and discharging inevitably results in deterioration of the electrodes. A nickel metal hydride battery has no such degradation mode and can be used semi-permanently as long as the electrode itself does not deteriorate, making it a battery with a long service life.
3. Electrodes of Nickel Metal Hydride Battery
Co-alloys have been mainly used for the negative electrode in the past in order to achieve high capacity, but there has been a move toward Co-free electrodes mainly due to cost considerations. However, the use of Co-fewer alloys has been progressing, mainly due to cost considerations. As for the cathode, nickel oxyhydroxide in the charged state is highly conductive, but nickel hydroxide in the discharged state is an insulator, which causes problems such as loss of electron paths during discharge. For this reason, cobalt oxyhydroxide or other materials are added to give conductivity.
Other Information on Nickel Metal Hydride Batteries
Nickel Metal Hydride Battery Characteristics
1. Battery Characteristics
The nominal voltage of nickel metal hydride battery is 1.2V, which is the same as that of a nickel-cadmium battery. This is because the reactions used for charging and discharging are similar. Since the nominal voltage of lead-acid batteries is 2.0 V and the rated voltage of lithium-ion batteries is 3.7 V, they are relatively low voltage batteries. Since these batteries can easily carry a large current, they are used in equipment that requires high output, such as hybrid cars.
Nickel metal hydride batteries have a memory effect (the voltage of the battery drops as it is repeatedly recharged, resulting in a decrease in usable capacity and an inerting effect. Therefore, understanding the characteristics of the battery when using it will maximize its service life.
Figure 2. Memory effect
2. Safety
Basically, battery explosions and fires are caused by the ignition of organic solvents, which are electrolyte solvents, by sparks created by short circuits.
The electrolyte solvent in nickel metal hydride batteries is water, so even if a spark should occur, it will not ignite. Therefore, the current and voltage control mechanisms do not need to be designed as rigorously as in lithium-ion batteries, thus lowering the manufacturing cost. This low cost is one of the reasons why nickel metal hydride batteries are still widely used in industry.
Figure 3. Comparison of lithium-ion and nickel-metal hydride batteries
3. Environmental Impact
Lithium-ion batteries, lead-acid batteries, and nickel-cadmium batteries contain hazardous substances with a high environmental impact (e.g., cadmium in nickel-cadmium batteries is a causative agent of Itai-itai disease, one of the four major pollution diseases). The electrolyte is also an environmentally friendly storage battery because it does not use organic solvents.