Background

Nickel Metal Hydride (NiMH) batteries started to enter the commercial market in the early 1990's as an alternative to Nickel Cadmium (NiCad) batteries. Nickel metal hydride batteries have several advantages over nickel cadmium batteries when used as a secondary battery. The two most important of these advantages of NiMH batteries include a nearly 100% greater energy stored per volume than NiCad batteries and the use of non-toxic metal hydrides rather than cadmium which make battery disposal easier. NiMH batteries have rapidly replaced NiCad batteries in many applications since their introduction with applications ranging from small electronics to electric vehicles.

NiMH batteries use metal hydride, hydrogen, and nickel hydroxide as their active materials. Table 1 list some common reactions used to describe the NiMH system.

Table 1

Overall (Discharge): NiOOH + (1/n)MHn Ni(OH)2 + (1/n)M

Anode (Discharge): (1/n)MHn + OH-
(1/n)M + H2O + e-

Cathode (Discharge): NiOOH + H2O + e-
Ni(OH)2 + OH-


NiCad batteries produced a equilibrium value of 1.32 V and NiMH batteries 1.34 V. This similarity in voltage makes replacing NiCad batteries with NiMH battereis simple. One last advantage of NiMH batteries is that they do not suffer from a memory effect like NiCad batteries. The memory effect refers to a phenomenon that occurs in NiCad batteries where the batteries appear to lose capacity after the battery experiences long periods under constant charging or without charging or numerous partial discharge cycles. This effect originates from the uneven growing of cadmium crystals within the battery. Deep discharges of NiCad batteries remove the memory effect.

Conventional NiMH batteries use stamped, foil-like pieces of metal for their collectors and a LaNi5 mis-match derivative for their anode material. During the charging process, the anode is hydrided and during discharge the stored hydrogen is released. An electrolyte barrier, usually a Nylon-Polypropylene blend, is used to separate the two anodes. Figure 1 shows a layered structure that is typically used.

Figure 1


Various types of collectors are used in NiMH batteries. A popular method for producing the collector involves placing foil like pieces of stamped nickel in a mold and sintering the foil with nickel tetracarbonyl. After the sintering process, the stamped nickel is left with rough, porous like nickel surface onto which the active material is placed. This particular process is easy to implement on a large scale and the porous surface produced increases the capacity of the electrode over a flat one.

After the collectors are made, the active material is then packed on. The metal hydride is typically grinded into a powder, combined with a binder and then spread and pressed onto the collector. The nickel hydroxide is deposited onto the collector using a chemical impregnation method. First the collectors are impregnated with nickel nitrate solution. Then the impregnated coils are placed in a tank filled with sodium hydroxide where a the nickel nitrate is turned into nickel hydroxide. During the process, cobalt is added to improve the conductivity of the nickel hydroxide. This impregnation method too is easy to implement on a large scale and produce a nickel hydroxide of high purity.

A wide range of cases are used for NiMH batteries. The requirements for the cases is that they be impermeable to hydrogen and water and be able to endure high internal pressures. Steel is typically used, but plastics have also been used.