The ability to absorb hydrogen and store it as a metal hydride makes certain alloys of rare earth elements especially useful in the battery market.  Although the most capable hydrogen storage alloy, LaNi₅ was identified in the 1970s, it was 1986 before modifications to the crystal structure were identified that would make the alloy a practical material for rechargeable, or secondary, batteries.  Within 5 years, these batteries were entering the marketplace.  At that time, increasingly large numbers of consumers were looking to use rechargeable batteries, and the most prevalent chemistry relied upon the toxic nickel cadmium formulation.  Nickel metal hydride (Ni-MH) batteries were able to satisfy the demand for environmentally friendly rechargeable batteries while offering even greater energy capacity.  Despite competition from Li-ion technology, safety and affordability factors have kept Ni-MH in a healthy share of the rechargeable battery market for the past two decades, most notably powering the HEV vehicles that have recently become ubiquitous on American roadways.
NiMH battery components:

Batteries can be broken down into five components: cathode, anode, separators, electrolyte, and case.  Ni-MH batteries are named for their nickel hydroxide cathodes (or positive electrodes) and metal hydride anode (or negative electrode).  A KOH electrolyte is used to buffer the composition of water molecules which shuttle hydrogen atoms between the two electrodes.  The separator between electrodes must be permeable and un-obstructive to hydrogen gas while preventing the anode and cathode from short-circuiting.  These materials are packaged in a steel casing, which does not corrode in the alkaline electrolyte, and which features a vent to release hydrogen gas pressure before rupturing.

Rechargeable NiMH batteries function by combining two half-cell reactions that occur in concert.  During the charging process there is a reaction at the positive anode:

     Ni(OH)₂ + OH^- à NiOOH + H₂O + e^-       (1)

that is linked to the reaction at the negative:

     LaNi₅ + H₂O + e^- à LaNi₅H_ab + OH^-         (2)

and the reactions are linked via the electrolyte. Because these batteries use hydrogen derived from water molecules to transfer charge between the electrodes and provide electrons to the exterior circuit, using KOH as the electrolyte moderates the impact of hydroxide production or consumption. The energy that is stored in the cell during charging can be released to power a load, which can be expressed as the reverse of reactions (1) & (2).

Performance of NiMH compared to other batteries:

There are numerous ways to compare battery technologies: cost; power; weight; toxicity.  One popular form is the Ragone plot, which compares the energy density and the power density for different battery chemistries. This form not only describes how much energy can be stored, but also how quickly the energy can be delivered.  The difference between these preferences can be critical to applications such as hybrid electric vehicles (HEVs), which above all need power, and battery powered electric vehicles (BEVs), which above all need energy to travel extended distances without an internal combustion engine (ICE).

Market Overview

Secondary (rechargeable) batteries account for roughly 60% of the battery market, and portable rechargeable batteries account for slightly more than a third of the secondary battery market.  Portable rechargeable batteries fill needs in:

• Cellular phones
• Computers
• PDA
• Camcorders
• Digital Cameras
• MP3 Players
• HEV/PHEV/EV
• Others (energy storage, medical, UPS, backup systems)

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