Nickel–metal hydride battery

Nickel–metal hydride battery (NiMH or Ni–MH) is a type of rechargeable battery. It is widely used in chargeable consumer electronics such as toothbrushes and digital cameras. Before 2020, Ni–MHs were also frequently used in electric and hybrid vehicles but by now lithium-ion (Li-ion) batteries have replaced them in this application.

How do nickel–metal hydride batteries work?

Ni–MH batteries consist of a negative electrode (anode), a positive electrode (cathode), a separator between them and an electrolyte. Similarly to the nickel-cadmium (Ni-Cd) battery, the cathode consists of nickel oxide hydroxide (NiO(OH)). But instead of cadmium, the anode consists of a metal hydrate. This metal is usually an alloy of several metals specially designed for capturing and releasing hydrogen to increase the energy capacity of the battery. For example, a mixture of lanthanum and cobalt can be used. When using the battery,  nickel oxide hydroxide reacts with water, producing nickel hydroxide and hydroxide ions. On the anode, the metal hydride reacts with a hydroxyl ion, forming water and releasing an electron into the circuit.

Reaction on the cathode during discharge:
NiO(OH) + H2O + e → Ni(OH)2 + OH

Reaction on the anode during discharge:
MH + OH → M + H2O + e

During charging, reverse reactions take place.

Advantages and disadvantages

Ni–MH batteries have a high cycle life (around 3000 cycles) and high life duration. They are mainly used as a substitute for regular non-chargeable AA batteries since they provide similar cell voltage while being less prone to leaking and explosions. These batteries are also quite robust – they tolerate overcharge and over-discharge and their operating temperature range is over 100 °C. Since NI-MH batteries do not use toxic cadmium, they have replaced Ni-Cd batteries in many applications. But with great advances in lithium-ion batteries, they are now replacing Ni-MH batteries. The main reasons for this are that Ni–MH batteries have high self-discharge, low cell voltage, and low energy capacity compared to Li-ion batteries.

  • NiMH battery
  • Ni–MH battery