Light My Volt!

Three new Chevrolet Volt electric/gas automobiles, the great white hope of GM and the Obama Administration that subsidized the Volt's development up to the hilt, recently caught fire during crash tests. Their lithium-ion battery packs burst into flame during the tests, creating minor consternation within GM which is frantically exercising damage control. This $41,000 vehicle is subsidized at about $8,000 a copy to get the price down(!) In addition, many $billions of taxpayer money (can you say that if it's all borrowed from China?) was "invested" in the development of the car and its battery pack.

Over two years ago, I wrote a lengthy blog post ("Batteries Not Included"--May 24, 2009) on the subject of electric automotive propulsion. (Some of the drawings are not reproduced in the archive version, for some reason.) In that post I discussed the heat problems inherent in large-scale lithium-ion batteries. Following is an excerpt from that prescient post. (Remember, it is dated 2009.)

There are three battery types in past, current and projected use for automotive propulsion: lead-acid, nickel-metal hydride (NiMH) and lithium-ion (Li-ion). The first is the familiar storage battery presently in your automobile(s). The second (NiMH) is used by Toyota in their hybrid Prius and in all other present-day hybrids. The third (Li-ion) is largely in the developmental stage for automotive propulsion, although commonly used in low-power applications like laptop computers and cameras. Let's look at these three technologies

Lithium-Ion

Small lithium-ion rechargeable batteries have been used for some time in laptop computers and other small electronic devices. They have performed well, except for an embarrassing problem with Sony laptops whose Li-ion batteries displayed a tendency to burst into flame. Sony determined that there was a defective lot from a Chinese manufacturer and recalled them all at considerable expense. The defect, in fact, was inadequate provision for heat elimination, a known problem with Li-ion batteries.

The advantages of Li-ion batteries include lighter weight, twice the efficiency and higher output voltage per cell--3.2 volts versus 1.25 volts--compared to NiMH. This is, of course, a great advantage for automotive propulsion where voltages in the range of 250v-500v are required. Another factor recommending Li-ion battery technology is the fact that it utilizes no rare-earth elements like the scarce (in the U.S.) Lathanium. All materials used in these batteries are fairly common. The disadvantages include an aging problem, where capacity decreases with time regardless of usage; heat sensitivity, reducing capacity; and safety issues including overtemperature (fire hazard) and overpressure (explosion hazard). These hazards require internal safety devices, which use up space and increase weight.

While progress is being made daily, there are still major development problems in creating a practical Li-ion automobile battery pack. There is, however, a frantic effort both here and in Japan and China to solve these problems because of the very substantial advantages of this technology. However, as of this writing, significant problems remain. How soon they will be solved to the extent that Li-ion batteries become practical for hybrid/electric automotive propulsion is impossible to tell at this time due to the secrecy surrounding much of the effort. My impression is the problems are very difficult and total solution is at least two years off. For now, NiMH is the only game in town, which has a Chinese name.

The internal electrodes of the Li-ion battery consist of a graphite anode and a complex two-layer cathode of lithium cobalt oxide (LiCoO₂). The electrolyte is a lithium salt in an organic solvent. In the event of overdischarge, the LiCoO₂ breaks down into lithium oxide (Li₂O), which is irreversible and destroys the battery. This is one of the problems with Li-ion technology. There is research underway to develop improved electrode materials, especially for the cathode.

A word about the much-ballyhooed Chevy Volt hybrid slated for production on some unspecified future date. It is a simple gas-electric hybrid with electric propulsion primary. Its yet-to-be developed plug-in rechargeable Li-ion battery pack will provide about 40 miles of all-electric operation, supposed to cover most commutes. After that, a 1.4L 4-cylinder gas engine will kick in to charge the battery via a 53KW generator, allowing further travel. The battery alone drives the wheels through an electric drive motor. There is no connection between the gas engine and the wheels.

The gas engine-powered generator is not designed to recharge the battery pack. This will be accomplished by plugging the car into a wall receptacle overnight and drawing electric power from coal-fired power plants. (O.K., maybe windmills or solar cells on your garage roof.)

Inquiring minds among my esteemed readers will have been wondering what was the reason for the long, somewhat torturous development process for the Li-ion automotive battery. The reason lies in basic chemistry, a subject somewhat lacking in the halls of Congress and the White House.

Essentially, a chemical-electric cell generates electricity as a byproduct of an electrolytic reaction between two dissimilar metals or metal analogs. The chemical reaction, which converts one constituent "metal" to another compound, produces excess electrons which flow through an external circuit, running motors, lights and stuff. The battery is discharged when the donor metal is exhausted. The cells are recharged by pumping electricity through them, reversing the reaction and reconstituting the original components.

Chemical reactions of this type typically produce heat proportional to the intensity of the reaction. Most battery cells generate in the neighborhood of 1.2 to 2.0 volts. Higher voltages are obtained by stacking cells, which is then called a battery. Your car battery consists of six-2 volt lead-acid cells, making a 12-volt battery. The lead-acid chemistry defines the 2-volt characteristic output. All lead-acid cells produce 2 volts.

The nickel-metal-hydride (NiMH) battery commonly used in hybrid automobiles produces about 1.25 volts per cell, requiring a large number to produce the typical 400 volts needed for electric automotive propulsion. The good thing is they produce little heat.

The Li-ion cell, on the other had, produces about 3.5 volts per cell, allowing for fewer cells and a smaller, lighter battery. The problem is, the more active chemical reaction that produces this higher voltage generates substantial heat. In small batteries for cell phones and laptops, dissipating this heat is not difficult. In a large automotive battery pack, it is a huge problem. Dealing with the heat problem was/is the major developmental stumbling block for the automotive Li-ion battery.

Obviously, GM thought their battery suppliers could solve the heat problem. There was a considerable shake-out of suppliers due to cost and development problems. They ended up with Li-ion cells made by LG Chem in Korea and assembled into the battery pack by GM. The pack has a sophisticated cooling system consisting of aluminum fin plates with a circulating dedicated cooling mechanism.

This is speculation, but I suspect the battery fires were caused by the crash tests disrupting the battery cooling system, resulting in a latent heat condition which raised temperatures to the flash point. I maintain this is a fragile arrangement in that the same thing obviously could occur in a relatively mild traffic accident. Personally, I wouldn't want to be anywhere hear a lithium fire. Better buy a Prius.