Toyota Selects NiMH Battery
Over the last couple of years, the mainstream media has been awash in reports of how automakers are lining up to build fleets of PHEVs and EVs using lithium-ion batteries as a principal power source. I've consistently argued that investing in objectively expensive lithium-ion battery company shares on the basis of testing decisions was dangerous. The reason for my caution is simple, a decision to test a new concept is very different from a decision to commercialize a proven concept and failures in the preliminary testing stages are far more common than successes. In other words, automakers frequently spend a huge amount of money to test a new technology before deciding, "this simply doesn't work for us."
Yesterday and this morning we learned that after secretly testing a fleet of 126 Prius Hatchbacks with lithium-ion battery packs for three years, Toyota Motors (TM) has decided to stick with its tried and true nickel metal hydride, or NiMH, battery technology for the foreseeable future (from SeekingAlpha)
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What is a NiMH battery?
The "metal" M in the negative electrode of a NiMH cell is actually an intermetallic compound. Many different compounds have been developed for this application, but those in current use fall into two classes. The most common is AB5, where A is a rare earth mixture of lanthanum, cerium, neodymium, praseodymium and B is nickel, cobalt, manganese, and/or aluminium. Very few cells use higher-capacity negative material electrodes based on AB2 compounds, where A is titanium and/or vanadium and B is zirconium or nickel, modified with chromium, cobalt, iron, and/or manganese, due to the reduced life performances . Any of these compounds serves the same role, reversibly forming a mixture of metal hydride compounds.
When overcharged at low rates, oxygen produced at the positive electrode passes through the separator and recombines at the surface of the negative. Hydrogen evolution is suppressed and the charging energy is converted to heat. This process allows NiMH cells to remain sealed in normal operation and to be maintenance-free.
NiMH cells have an alkalineelectrolyte, usually potassium hydroxide.