Betting on a better battery
Some types of energy are easy to store. Petroleum, for example, holds a vast amount of chemical energy, which explains why we choose it to move our Hondas 'n Harleys. But the moving electrons we call electricity are tough to store in large quantities.
Sure, the batteries in your cellphone and laptop can store a bit of electrical energy once you recharge them for a few hours. But all-electric cars are limited by charging time. If you drive from Chattanooga to Cleveland, "gassing up" a spent battery with replacement electrons would take hours. And since nobody wants to while away the afternoon at an electric "gas station," all-electric cars have underwhelmed the market.
Recharging batteries takes hours because electrons and other charged particles move too slowly inside them. Recharging some batteries too quickly, in fact, will cause them to break or even explode! And so petroleum remains the king of the highway.
Now researchers at MIT have reengineered the lithium-ion battery, a high-tech model mainstay of cameras, cellphones and laptops, to remove roadblocks to the charged particles. The result is a battery that can be charged or discharged in seconds -- roughly 100 times faster than a commercial lithium-ion battery, says Gerbrand Ceder, a professor of materials science and engineering at MIT.
Flyweight is no racehorse
Lithium batteries are favored for many advanced uses because they are lightweight and store a lot of energy (measured in watt hours) and have relatively high power (rate of energy delivery, measured in watts).
The conventional explanation for the limit on power was that the lithium ions could not move quickly inside the battery's electrolyte. But several years ago, Ceder calculated that the ions should be moving much faster, which suggested that the sluggishness was occurring at the surface of the electrode (the component that exchanges electric charges with the electrolyte).
Through chemical-physical manipulations of a lithium-iron-phosphate battery, Ceder and his graduate student, Byoungwoo Kang, tampered with the usual formula and fabricated a nanoscale structure that removed the speed bumps that slow the lithium ions. Heating the material for 10 hours each at 350° C and the 600° C caused subtle alterations in the iron and phosphate concentrations, leading to a tight integration of the electrode and electrolyte that allows the lithium ions to reach their real speed limit.
The researchers reported that the battery had more power -- it could release its energy in 10 to 20 seconds, compared to six minutes for a conventional lithium ion battery. The next step, Kang says, is to remove the high concentration of carbon used in the test procedure, which would not be wanted in a commercial battery.
Battery's capacity rivals capacitor!
Electrical engineers can get high rates of discharge from a device called a capacitor, but while these can deliver a big current in fractions of a second, their energy storage is too low for most uses. In a sense, then, the new technology blends the best features of batteries and capacitors.
A further advantage is that the new design retained its charge-holding capacity better than existing lithium batteries.
Because the new battery technology is an outgrowth of the technology used to make existing lithium batteries, it could be pressed into production rather quickly, the MIT engineers say, and two companies have licensed the process. "Our process is not expensive, it's kind of a simple process, and it produces almost the same improvement for charge and discharge," says Kang. "Usually the rate of discharge is more important, but sometimes the limiting factor is the rate of charging."
Both rates could play a role in plug-in hybrid cars, Kang says, where the battery would, ideally, be able to power strong acceleration, and also to absorb the energy from hard braking -- both steps that challenge existing batteries.
If the technique succeeds, powerful, affordable, quick-charging batteries could also be used to store energy from intermittent, non-carbon energy sources like wind generators or solar cells, helping to usher out the age of high-polluting fossil fuels.
- David Tenenbaum
• Battery materials for ultrafast charging and discharging, Byoungwoo Kang and Gerbrand Ceder, Nature, Vol 458 | 12 March 2009 | doi:10.1038/nature07853.