The nickel–iron battery (NiFe battery) is a storage battery having a nickel(III) oxide-hydroxide cathode and an iron anode, with an electrolyte of potassium hydroxide. The active materials are held in nickel plated steel tubes or perforated pockets. It is a very robust battery which is tolerant of abuse, (overcharge, over discharge, and short-circuiting) and can have very long life even if so treated. It is often used in backup situations where it can be continuously charged and can last for more than 20 years.
Due to its low specific energy, poor charge retention, and its high cost of manufacture, other types of rechargeable batteries have displaced the nickel–iron battery in most applications. They are currently gaining popularity for off-the-grid applications where daily charging makes them an appropriate technology.
Durability of Nickel–iron battery
The ability of these batteries to survive frequent cycling is due to the low solubility of the reactants in the electrolyte. The formation of metallic iron during charge is slow because of the low solubility of the ferrous hydroxide. While the slow formation of iron crystals preserves the electrodes, it also limits the high rate performance: these cells charge slowly, and are only able to discharge slowly. Nickel–iron cells should not be charged from a constant voltage supply since they can be damaged by thermal runaway; the cell internal voltage drops as gassing begins, raising temperature, which increases current drawn and so further increases gassing and temperature.
Nickel-iron batteries have long been used in European mining operations because of their ability to withstand vibrations, high temperatures and other physical stress. They are being examined again for use in wind and solar power systems and for modern electric vehicles, especially boats, where the weight of the batteries is not an issue.
Electrodeposition of Ni Iron Battery
The half-cell reaction at the cathode:
2 NiOOH + 2 H2O + 2 e− ↔ 2 Ni(OH)2 + 2 OH−
and at the anode:
Fe + 2 OH− ↔ Fe(OH)2 + 2 e−
The open-circuit voltage is 1.4 volts, dropping to 1.2 volts during discharge. The electrolyte mixture of potassium hydroxide and lithium hydroxide is not consumed in charging or discharging, so unlike a lead acid battery the electrolyte specific gravity does not indicate state of charge. Lithium hydroxide improves the performance of the cell. The voltage required to charge the cells is between 1.45 and 1.65 volts. The equalization charge voltage is 1.65 volts.
