NiMH Battery | Nickel Metal Hydride Rechargeable Battery

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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.

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⁻

Advantages

Compared to lead acid batteries, NiMH batteries have a significantly higher energy density (160-420 Wh/l) and specific energy (55-110 Wh/kg), which translates to either long autonomy periods or a drastic reduction in the space necessary for the battery. The high power density (100-500 W/kg) enables them to be rapidly charged. A long cycle life of 600 to 1200 cycles at 80% depth-of-discharge (DoD) means a NiMH battery is equivalent to hundreds of alkaline batteries in total service over its lifetime. NiMH batteries offer a discharge curve that is considerably flatter than alkaline batteries, particularly at higher current draw. The substitution of a hydrogen-absorbing negative electrode for the cadmium-based electrode not only eliminates the cadmium which raises toxicity concerns, but also contributes to an improvement in energy density over NiCd batteries. The memory effect is less of an issue with this chemistry than in nickel cadmium. Compared with lithium ion batteries, NiMH batteries are less prone to thermal runaways because the battery chemistry can withstand high temperatures and the electrolyte is highly constant over the entire electrochemical process.

Disadvantages

NiMH batteries are not fully immune from the memory effect. The battery must be discharged to 1V/cell every three or four months to prevent memory. Self discharge rates of this chemistry are higher than other chemistries. NiMH batteries are much more expensive than lead acid batteries and have not been considered for use in SLI applications in vehicles because of poor cold-cranking performance. NiMH batteries have reduced charge efficiencies at elevated temperatures due to oxygen evolution inside the cell. Although NiMH batteries are a safer and more reliable than lithium-ion batteries, they losing momentum in HEV and EV applications as they cannot compete with the lithium-ion technology in all key specifications such as energy density, power density, cycle life, and deep cycling capability.

Applications

The discharge behavior of the nickel-metal hydride chemistry is generally well suited to the needs of today’s electronic products which require a stable voltage for extended periods of operations, or high rate discharge. The transient effect and environmental conditions (notably discharge temperature and discharge rate) affects the discharge voltage profile. However, under most conditions the voltage curve of nickel-metal hydride batteries retains the flat plateau desirable for electronics applications. The NiMH battery is currently finding widespread application in portable electronics, power tools, integrated solar lights, emergency lighting, UPS (uninterrupted power supplies) and generator starting. However, lithium-ion batteries are gradually taking the market.