An NiMH battery is a rechargeable battery that uses nickel hydroxide for the positive electrode (cathode) and a hydrogen-absorbing alloy for the negative electrode. NiMH or Ni-MH is an abbreviation for nickel-metal hydride. The NiMH battery chemistry is a hybrid of the proven cathode chemistry of the sealed nickel-cadmium (NiCd) battery and the improved anode chemistry that features an advanced hydrogen energy storage concept instead of using cadmium, a highly toxic substance. While the removal of cadmium makes NiMH batteries more cost effective to recycle than NiCd batteries, a more prominent benefit of using NiCd batteries is that these batteries have twice to three times the capacity of equivalent size NiCd batteries. The design similarities between the two chemistries (e.g. similar operating voltages) also simplify battery replacement in products using NiCd batteries.
Charge reactions
At the positive electrode, nickel hydroxide is oxidized and converted to nickel oxyhydroxide.
Ni(OH)2 + OH⁻ ⇌ NiOOH + H2O + e⁻
At the negative electrode, water in the electrolyte is decomposed into hydrogen atoms which are absorbed into the alloy, thus forming a metal hydride.
H2O + M + e⁻ ⇌ OH⁻ + MH
Discharge reactions
At the negative electrode, hydrogen gas is oxidized from the hydrogen-absorbing alloy and is ombined with a hydroxyl ion to form water at the negative electrode while also contributing an electron to the circuit.
MH + OH⁻ ⇌ M + H2O + e⁻
The positive electrode material of the NiMH battery is in a charged state is nickel oxyhydroxide. During discharge, nickel oxyhyroxide is reduced to its lower valence state, nickel hydroxide (NiOOH).
NiOOH + H2O + e⁻ ⇌ Ni(OH)2 + OH⁻
Cell Construction
The fundamental components for a NiMH battery are a positive plate impregnated with nickel hydroxide, a negative plate impregnated with hydrogen-absorbing alloys, a separator made of fine fibers, an alkaline electrolyte, a metal housing, and a sealing plate provided with a safety vent. The plates may vary in the type of substrate and plaque, impregnation process, formation process, and termination technique. The positive and negative plates are sandwiched by the separator, wound into a coil, inserted into the metallic can that is sealed after injection of electrolyte. The plates and separator are soaked in an alkaline electrolyte made up of aqueous potassium hydroxide. The nickel-metal hydride battery is characterized by a vented design. Excess hydrogen and oxygen gases generated as a result of extended overcharge or incompatible battery/charger combinations are vented from the cell container, thus maintaining pressure equilibrium within the battery. The safety vent releases excess hydrogen and oxygen only during abusive conditions. The cells remain sealed during normal charging/operating conditions.Electrochemistry
At both electrodes, oxidation-reduction occurs in the aqueous potassium hydroxide. The positive electrode is electrochemically reversible between nickel hydroxide (Ni(OH)2) and nickel oxyhydroxide (NiOOH). Hydrogen-absorbing alloys form hydrides that could absorb and release hydrogen hydrogen in volumes to about a thousand times of their own volume.Charge reactions
At the positive electrode, nickel hydroxide is oxidized and converted to nickel oxyhydroxide.
Ni(OH)2 + OH⁻ ⇌ NiOOH + H2O + e⁻
At the negative electrode, water in the electrolyte is decomposed into hydrogen atoms which are absorbed into the alloy, thus forming a metal hydride.
H2O + M + e⁻ ⇌ OH⁻ + MH
Discharge reactions
At the negative electrode, hydrogen gas is oxidized from the hydrogen-absorbing alloy and is ombined with a hydroxyl ion to form water at the negative electrode while also contributing an electron to the circuit.
MH + OH⁻ ⇌ M + H2O + e⁻
The positive electrode material of the NiMH battery is in a charged state is nickel oxyhydroxide. During discharge, nickel oxyhyroxide is reduced to its lower valence state, nickel hydroxide (NiOOH).
NiOOH + H2O + e⁻ ⇌ Ni(OH)2 + OH⁻