How and Why

Talk about battery life! Old device, new tricks.

By Curt Suplee
Tuesday, October 20, 2009

Since the opening bell of the wonder-stuffed 21st century, consumer technologies have evolved at an electrifying pace . . . batteries not included.

In recent years, microchip features have become vanishingly small, the amount of data on a disk has increased a billionfold, and you can order Cialis from a cellphone. But despite the promotional zeal of those drum-toting bunnies, the alkaline battery that powers most of our portable gizmos is not different in principle from the model that Alessandro Volta created in 1800. That fabled "voltaic pile"-- a stack of alternating zinc and copper disks separated by layers of brine-sodden-paper -- didn't look much like today's sleek cylinders. But it was the first battery as we understand the term -- that is, a device that turns stored chemical energy into electrical energy. And its successors are still using the same kind of system.

Is this retro-electro condition a failure of modern science? Well, no, for two reasons.

The first is that the newest versions of Volta's pile produce amazing boing for the buck and are poised to get better.

The second and most important reason that battery tech may seem to be lagging is that beyond a certain point you can't shrink chemistry. Digital data are almost infinitely compressible because the information is not a physical object. It can be embodied in the smallest difference between any two conditions -- on or off, 0 or 1 -- way down to subatomic scale.

But batteries aren't digital. "Energy is stored on stuff -- atoms, ions and so forth," says MIT professor Gerbrand Ceder. "This is done by transferring electrons from one atom to another. Their energy difference is the energy stored. That's why batteries will never scale in performance the way, say, semiconductors have done."

All electrochemical batteries work pretty much the same way. Reactions among different component materials cause negatively charged electrons to be displaced from atoms and to pile up in one location (the anode, marked with a minus sign), leaving the electron-deprived atoms, a.k.a. ions, to form a net positive charge at the cathode (marked with a plus). In between is a liquid, solid or goo called the electrolyte, through which the ions -- but not the electrons -- migrate in the course of the reaction. The negative electrons are attracted to the positively charged ions but can't get there until somebody places a conductor between the two populations and forms a circuit. Then the electrons flow in a somewhat organized stream and the bulb in your flashlight begins to glow.

In rechargeable batteries, that chemistry is reversible -- an increasingly desirable property in devices from power tools and motorized wheelchairs to cellphones. Hence the attraction of rechargeable nickel-cadmium, nickel-metal hydride and lithium-ion cells. The last is widely regarded as the most promising consumer battery technology for the next five to 10 years.

Lithium, the third-lightest element, is found way up in the nosebleed seats of the Periodic Table. It's an excellent conductor, is not outrageously rare and has drop-dead stats. A standard measure of battery performance is the number of watt-hours (Wh) that can be stored in a unit with a mass of one kilogram. Rechargeable lithium-ion batteries generally clock in around 120 to 160 Wh/kg, and some go to 200. By contrast, an auto lead-acid battery produces a wimpy 25 to 40 Wh/kg, and nickel-metal hydride of the sort commonly used in hybrid cars is about 40 to 90. (Interestingly, and here comes the bunny, some of today's disposable alkaline batteries can hit 200 Wh/kg, which explains their perennial popularity.)

As a practical matter, energy density for a rechargeable commercial battery probably can't get past 1,000 Wh/kg in the near future. That's plenty for most needs. But it's only one consideration. Also important is power density; that is, how fast you can get the energy out. (Or in, if you're recharging.) And that's why you don't typically see lithium-ion batteries in hybrid or plug-in electric cars. They tend to emit their energy slowly, limiting how fast the car could accelerate.

Ceder and colleagues appear to be on the verge of changing that. "There is no theoretical limit to how fast one can do the electron and ion transfer processes," he says.

A few years back, scientists found that a compound called lithium iron phosphate provides a very high mobility for charge carriers. Ceder's lab recently devised a way to use it to make lithium-ion batteries that can be fully charged or discharged in a matter of 10 or 20 seconds -- a stupefying notion when you consider the trillions of electrons and ions that have to be shuttled around when recharging even a single AAA cell.

If that and other current research projects make it to the market, we may soon be treated to a lot more jolt per volt as well as recharge times better suited for the Twitter-at-every-turn pace of modern life. If so, in the environmental and traffic congestion challenges of the 21st century, robust and replenishable batteries could be leading the charge.


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