LITHIUM ION LIKELY TO POWER HYBRIDS OF THE FUTURE

Toyota Motor Corporation indicated recently that the company would migrate to lithium ion battery technology for its future hybrid vehicles. Hybrid Synergy View visited with Gary E. Smith, national service technology manager at Toyota Motor Sales, U.S.A., Inc., about lithium ion technology.

---

Q: What are the advantages of lithium ion batteries that would make Toyota select this technology for its hybrids?

Gary Smith (GS): Lithium ion batteries permit higher energy density than the nickel-metal hydride batteries we're now using. That means they can be made in smaller, lighter sizes with the same wattage capacity. So the savings would be mainly in a vehicle's weight, which is key to efficiency and economy. Second, it could provide more interior space for passengers and cargo.

Q: Granted that the lithium ion batteries can be smaller and lighter -- do they perform as well in charging and discharging?

GS: They will, I'm sure. The traction batteries we're talking about -- the kind used for vehicle propulsion -- will have little in common with the lithium ion batteries commonly used in cell phones and laptop computers. Most consumer electronics' Li-ion [lithium ion] batteries operate under light load, typically run from full charge to various levels of discharge over long periods of time and are charged to full capacity. Additionally, they are frequently replaced after a relatively short service life or discarded with the device at the end of its life.

But vehicle traction batteries have a much different use pattern. They are charged to perhaps 85 percent of their capacity and only allowed to discharge down to about 30 percent. That is regulated by the battery management system, which -- among several other functions -- prolongs the life of the batteries. Another difference is that the batteries in consumer electronics are rarely pressed to deliver more than their rated amperage, whereas a car's traction battery is designed to deliver several times its rated output for short periods and to be recharged rapidly.

Q: How does the life of lithium ion batteries compare to nickel-metal hydride types?

GS: It will be at least equivalent to our present batteries. As you know, Toyota warranties hybrid batteries for 100,000 miles -- and in some states, 150,000 -- so you can be sure that no battery will ever get into the market that can't meet those standards.

Q: And at the end of that service life...

GS: They're recyclable.

Q: Are there safety concerns with lithium ion batteries, such as overheating or crashworthiness?

GS: Our vehicles have forced-air ventilation systems to make certain the batteries are kept cool for both safety and long battery life. And of course, the battery management system monitors and regulates the temperature and flow of current in and out to make sure the batteries are not stressed from heat or excessive charge/discharge. A well-designed battery management system is key to the long-term success of all rechargeable batteries, and it is more critical on Li-ions. As to crashworthiness, all batteries are enclosed in metal cases and are situated in protected areas of the vehicles. Computers monitor the entire system status several times each second, and if there are any anomalies, such as a collision or circuit problems, all high-voltage circuits are disabled and contained inside of the protective case.

Q: Would arrays of off-the-shelf consumer-type lithium batteries work for this purpose?

GS: It's possible. However, that is a significant compromise. Some of the earliest non-U.S.-specification Prius cars had racks of rechargeable D batteries, the kind you'd use in a flashlight. These were bulky and were not optimized for the specific use case. So the battery makers optimized the design and formed them into wafer-style packs/collections that make up today's Prius battery packs. I'm sure the lithium ion packs that the battery makers come up with for hybrid cars will likewise be optimized for the special demands that come with automotive use.

Q: Are there important differences between lithium ion and lithium ion polymer batteries?

GS: Lithium ion batteries contain a gelatinous material; lithium ion polymer batteries -- usually called lithium polymer -- have the lithium ions deposited on a hard, polymer material. The particular "recipe" that the battery makers will select for hybrid traction batteries will depend on the battery's configuration, placement and desired service life as well as on the demands of constant charging and recharging.

Q: How would today's Prius have to be changed to accommodate lithium ion batteries?

GS: The difference would be mainly in the battery management system that regulates the charging and discharging processes.

Q: How do the costs compare to nickel-metal hydride batteries?

GS: I would expect the first lithium ion traction batteries to be more expensive than today's nickel-metal hydride types, but when they get into large-scale production, their costs will continue to get lower.

Q: Would lithium ion batteries be required if Toyota develops a plug-in hybrid?

GS: If not required, certainly desirable. The same energy density advantages would be especially beneficial to plug-ins, which would impose greater demands during all-electric range.

Q: Is lithium ion the solution?

GS: Lithium is certainly what's next in this type of energy storage. I am optimistic that lithium will offer compelling advantages in weight, size, cost and range that will have a positive effect on the ownership experience in future hybrids, plug-ins or electric vehicles.