Nickel Metal Hydride Battery
The nickel metal hydride (NiMH) battery is a type of rechargeable battery
similar to the nickel–cadmium battery, but using hydrogen-absorbing alloys for
the anode instead of cadmium. Compared to NiCd batteries, NiMH batteries have
higher energy densities, no memory effect, and less self-discharge.
Chemistry of NiMH Battery
Nickel
Metal Hydride Battery, the anode is made up of a hydrogen-absorbing alloy
typically made from rare earth metals such as lanthanum, cerium, praseodymium,
or mischmetal. During discharge, hydrogen is released from the anode alloy and
combines with oxygen from the cathode material to form water. The cathode is
typically made from nickel oxyhydroxide (NiOOH). At the anode, the reaction is:
MH + H2O ↔ M + H2 + OH-
While at the cathode, the reaction is:
NiOOH + H2O + e- ↔ Ni(OH)2 + OH-
The overall cell reaction is:
MH + NiOOH ↔ M + Ni(OH)2
Advantages of NiMH Batteries
Some key advantages of NiMH batteries include:
– Higher Energy Density: NiMH
batteries can store up to twice as much energy per unit weight as comparable
NiCad batteries. This allows devices to operate longer on a single charge.
– No Memory Effect: Unlike NiCad
batteries which exhibit a “memory effect”, NiMH batteries do not need
to be fully discharged before recharging. Partial discharges do not degrade the
capacity.
– Less Self-Discharge: NiMH batteries have a self-discharge rate of around
10% per month as compared to 25-30% for NiCad batteries. This results in a
longer shelf life.
– More Environment Friendly: NiMH
batteries do not contain heavy metals like cadmium and mercury which are toxic
and require special disposal. They can be safely recycled.
Applications of NiMH Batteries
Owing to their advantages over other battery technologies, NiMH batteries have
widespread applications:
– HEV/EV batteries: Primarily used
in the propulsion and auxiliary power systems of hybrid electric vehicles
(HEVs) and electric vehicles (EVs). Their high energy density makes them
suitable for mobility applications.
– Power tools: Common in cordless
power tools like drills, saws etc. where their ability to deliver high current
pulses is valuable.
– Consumer electronics: Popular
choice for portable devices like cameras, toys, mp3 players etc. where space is
a constraint.
– Remote controls: Widely used in TV
remote controls, garage door openers etc. due to long shelf life and safety.
– Medical equipment: Power medical devices like blood pressure monitors
where reliable operation is critical.
– Backup power: Used in
uninterruptible power supplies (UPS) and standby power systems for their
maintenance-free operation.
Types of NiMH Battery Chemistries
Over the decades, different cathode materials and anode alloy compositions have
been developed to improve performance characteristics:
– AB5 Alloy: Early commercial anodes
used AB5 hydrogen storing alloys of composition MmNi5. Provided capacity in
range of 150-300mAh/g.
– AA Alloy: Advanced alloy
containing aluminum, lanthanum and other rare earth elements. Demonstrated
capacity up to 320mAh/g. Better high-rate performance.
– AB2 Alloy: Contains lower nickel
content and higher concentration of rare earth metals. Capacity exceeds
400mAh/g. Good candidate for HEVs.
– NiCoO2: Cobalt doping of nickel
hydroxide cathode improves charge acceptance. Increases capacity to
250-300mAh/g.
– Nano-structured Materials: Use of
nano-sized alloy powders and flake-type NixCo1-x(OH)2 improve power density,
cyclability and low temperature characteristics.
Challenges and Future of NiMH Batteries
While NiMH batteries have clear advantages, scarcity and cost of raw materials
like cobalt remain a concern. Other challenges include:
– Capacity Fade: Repeated charging
and discharging causes capacity to decline by around 30% over battery life.
Improving cycle-life is important.
– Memory Effect: Although less
severe than NiCad, repeated shallow charge-discharge cycles can reduce capacity
of NiMH over time if not periodically fully charged.
– Cost: Higher manufacturing costs
compared to lead-acid batteries restricts use in some applications like utility
scale storage.
In address these challenges, advances in alloy engineering, surface coatings
and battery management systems can further enhance performance of nickel metal
hydride battery technology. With proper recycling processes, they can replace
toxic battery chemistries and support renewable energy integration.
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