Active Components Positive and Negative Electrodes

Ni-MH Rechargeable Batteries

3CompositionandChemistry

A rechargeable battery is based on the principle that the charge/discharge process is reversible, that is, the energy delivered by the battery during discharge can be replaced or restored by recharging.

3.1 Active Components: Positive and Negative Electrodes

Nickel oxyhydroxide (NiOOH) is the active mate- rial in the positive electrode of the nickel-metal hydride battery in the charged state, the same as in the nickel- cadmium battery.

The negative active material, in the charged state, is hydrogen in the form of a metal hydride. This metal alloy is capable of undergoing a reversible hydrogen absorbing/desorbing reaction as the battery is charged and discharged, respectively.

The unique attribute of the hydrogen storage alloy is its ability to store hundreds of times its own volume of hydrogen gas at a pressure less than atmos- pheric pressure. Many different intermetallic com- pounds have been evaluated as electrode materials for nickel-metal hydride batteries. Typically, these fall into two classes: AB5 alloys, of which LaNi5 is an example,

and AB2 alloys, of which TiMn2 or ZrMn2 are examples. DURACELL nickel-metal hydride battery technol-

ogy is based on the use of AB5 instead of AB2 alloys. AB5 alloys offer better corrosion resistance characteris- tics, resulting in longer cycle life and better recharge- ability following storage. The composition of the metal alloy is formulated for optimal stability over a large number of charge/discharge cycles. Other important properties of the alloy include:

•Large hydrogen storage capability for high energy density and battery capacity.

•Favorable kinetic properties for high rate capability during charge and discharge.

•Low hydrogen pressure alloy and high purity mate- rials to minimize self-discharge.

3.2 Electrolyte

An aqueous solution of potassium hydroxide is the major component of the electrolyte of a nickel- metal hydride battery. A minimum amount of elec- trolyte is used in this sealed cell design, with most of

this liquid being absorbed by the separator and the electrodes. This “starved electrolyte” design facilitates the diffusion of oxygen to the negative electrode at the end-of-charge for the “oxygen recombination” reaction.

3.3 Cell Reactions

	During discharge, the nickel oxyhydroxide is		The overall reaction on discharge is:
reduced to nickel hydroxide
				MH + NiOOH ——> M + Ni(OH)2
	NiOOH + H2O + e- ——> Ni(OH)2 + OH-		The process is reversed during charge.
and the metal hydride (MH) is oxidized to the metal			The process is reversed during charge.
and the metal hydride (MH) is oxidized to the metal
alloy (M).

		MH + OH- ——> M + H2O + e-