Lithium batteries charge ahead - Researchers demonstrate cells that can power up in seconds.

Two researchers have developed battery cells that can charge up in less time than it takes to read the first two sentences of this article. The work could eventually produce ultra-fast power packs for everything from laptop computers to electric vehicles.

Byoungwoo Kang and Gerbrand Ceder of the Massachusetts Institute of Technology in Cambridge have found a way to get a common lithium compound to release and take up lithium ions in a matter of seconds. The compound, which is already used in the electrodes of some commercial lithium-ion batteries, might lead to laptop batteries capable of charging themselves in about a minute. The work appears in Nature¹ this week.

Lithium-ion batteries are commonplace in everything from mobile phones to hybrid vehicles. "They're essentially devices that move lithium ions between electrodes," says Ceder. The batteries generate an electric current when lithium ions flow out from a storage electrode, float through an electrolyte, and are chemically bound inside the opposing cathode. To recharge the battery, the process is reversed: lithium ions are ripped from the cathode compound and sent back to be trapped in their anode store.

The speed at which a battery can charge is limited by how fast its electrons and ions can move - particularly through its electrodes. Researchers have boosted these rates by building electrodes from nanoparticle clumps, reshaping their surfaces, and using additives such as carbon. But for most lithium-ion batteries, powering up still takes hours: in part because the lithium ions, once generated, move sluggishly from the cathode material to the electrolyte.

Tunnel vision

That seemed to be the case for lithium iron phosphate (LiFePO₄), a material that is used in the cathode of a small number of commercial batteries. But when Ceder and Kang did some calculations, they saw that the compound could theoretically do much better. Its crystal structure creates "perfectly sized tunnels for lithium to move through", says Ceder. "We saw that we could reach ridiculously fast charging rates."

So why hadn't anyone seen this speedy charging in practice? Ceder and Kang theorize that the lithium ions were having trouble finding their way to the crystal structure's express tunnels. The authors helped the ions by coating the surface of the cathode with a thin layer of lithium phosphate glass, which is known to be an excellent lithium conductor. Testing their newly-coated cathode, they found that they could charge and discharge it in as little as 9 seconds.

"As far as I know, this is the fastest yet for this material," comments Peter Bruce, a chemist at the University of St Andrews, UK. The researchers do not know exactly how the disordered glass helps lithium ions transfer between the electrolyte and the cathode.

Other materials, such as nickel oxide, have achieved similarly fast charging rates, says John Owen, a chemist at the University of Southampton, UK. "This is a nice demonstration of the concept in a lithium system," he says. Lithium, though, can store more energy for less weight than nickel compounds, and holds its charge better.

It's particularly important because lithium iron phosphate is already being used commercially, adds Bruce. Speeding lithium ion movement would vastly improve energy recovery in hybrid vehicles, which recharge their batteries when the vehicle brakes — a process that lasts only seconds. It could also eventually lead to fully electric vehicles that could charge reasonably quickly.

Ceder says that he thinks that improvements in modelling will allow researchers to find other candidates for ultra-fast batteries. "My guess is that there are more materials like this out there," he says.

I’ve heard that the lithium batteries of the coming $40K Chevy Volt have to be replaces at 100,000 miles at the cost of $10K. Credibility?

Considering the amount of research being done, I doubt in 10 years the price would be that much. That info is usually tossed around by folks who don't like the idea of electric cars.

I'd love to have an electric car to do errands around town.

The Prius’ traction battery is warranted for 150k miles in California as part of the emissions-related equipment. I’d imagine the same would be true of the Volt.

The same “$10,000” canard floated around about the Prius, and might even have been true back when it was brand new (and covered by warranty). In taxi service the batteries have routinely gone past 350k miles, and replacements nowadays run about $1500, or less than the cost of a transmission in a similarly well-used car. The batteries require no maintenance.

This type of battery, properly manufactured and used, is very reliable. There are some tricks to achieving close to unlimited life; for example the Prius’ computerized controls maintain charge between 40% and 80% of full. When run in that range, they go practically forever.

"Her car does get an honest 45 MPG which I attribute to two factors: The engine runs at a constant speed to charge the battery pack (at optimum efficiency) and the Prius wheel motors turn into generators when you slow down, using the car momentum to recharge the battery pack."

Close. It uses an Atkinson cycle variant of the common Otto-cycle internal combustion engine. That yields about a 15% efficiency gain right there. The peaky personality of these engines would render them undrivable if it weren't for the clever differential-like electronically continuously variable planetary gearset, which as you say maintains the engine in a narrow speed band for maximum efficiency. Regeneration is another advantage, as you also point out. But then there's another thing: the gas engine need only be sized for pulling the car at-speed. For burst-power needs, the electric motor contributes. So the gas engine doesn't need to be oversized in order to accommodate acceleration, passing and hill-climbs. As a consequence, the Prius is quite a drivable vehicle. It is also very roomy, seating four adults quite comfortably and five when needed, with a cavernous cargo area. And one last thing: the a/c is electric, so the engine doesn't have to run at idle at stops in order to keep the cabin livable. (Flip side: since the engine's coolant provides heat to the heater as with all other cars, turning the heater setting up can make the engine start to run!)

Lots of Freepers get all gruff and dismissive about the Prius because it's, well, kind of the ultimate Al Gore-mobile. But it's a nifty car, very easy to live with, and competitively priced compared to others with equally capacious cabins.

I drove a Honda Civic Hybrid for an afternoon, in both stop-and-go traffic and on the (urban) freeway. It was a little unnerving at first when the engine shut down at intersections. I got used to it.

It had a "capacity" charge meter, plus an "instant" meter that showed whether it was charging or discharging the battery. So, I could see how it was reacting.

It was on the freeway that I could see the real advantage: when I lifted my foot off the accelerator, the "engine braking" that we all expect was used to charge the battery. When I stepped on the accelerator to speed up slightly, the electric "assist" kicked in, then settled back to normal when I was cruising at the new speed.

There was no need to jam more gas into the engine to make minor speed upward speed adjustments. It was like running on cruise control on a flat, open road all the time, when I get my best mileage in a regular car.

I reset the "average gas mileage" meter when I got into the car. Over my entire trip, I exceeded 50 miles/gallon.

I don't know if the additional cost and complexity is worth it at current gas prices. But, as the technology becomes more common (and cost goes down), and the price of gas inevitably goes back up, it will probably be standard on almost every car -- just like automatic transmissions.

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Tunnel vision

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