Little Lithium Battery That Could

Inventive / by Michael Eisenstein /

By taking a second look at existing battery materials, researchers have found the secret to unleashing the electrical power of the common lithium-ion battery.

As anyone using a laptop on an international flight or trying to put the pedal to the metal in their electric car will tell you, batteries are not advancing at the rate of the rest of technology. While many researchers have been looking to high-energy devices, called supercapacitors, or to a total battery overhaul to solve our consumer gadget energy crises, researchers at the Massachusetts Institute of Technology have merely tweaked a well-known and widely-used battery material to make a lithium-ion battery capable of discharging and recharging electricity like a fire hose rather than a faucet. The advance means a powerful shift: recharging times of mere seconds for cell phones, laptops, and other consumer gadgets within a few years. Most promising, it may hold the key to giving electric cars the quick acceleration they need to compete with their gas-guzzling peers.

Scientists have long recognized lithium’s promise as the stuff of superior batteries — it’s highly electropositive, which means that it readily discharges ions, and as the lightest of metals, it enables lightweight gadgets that pack a lot of power. However, developing ideal conductive materials to shuttle current through lithium-ion batteries has put a damper on their promise. Some incorporate toxic or expensive elements, while others have a nasty tendency to burst into flames. These stumbling blocks are especially problematic in bigger batteries. “When you go from cell phone or laptop batteries to large car batteries, three things are important,” says Arumugam Manthiram, engineering professor at the University of Texas at Austin. “Cost, safety, and the charge-discharge rate.”

In 1997, Manthiram’s former mentor John Goodenough first demonstrated that lithium iron phosphate (yes, a name confusingly similar to that of the battery it serves) is useful as a battery material, but it seemed to represent a compromise. While delivering cost and safety attributes better than many competing materials, lithium ion phosphate falls dramatically short on the third count — it is very slow at releasing and absorbing power.

When a typical lithium-ion battery is discharging electricity, lithium ions flow from the anode (the negatively charged terminal made of graphite) through an electrolyte and into spaces within the crystals in the cathode (the positively charged terminal made of lithium iron phosphate). The rate at which a battery is able to charge and discharge electricity is determined by how readily electrons and ions shuttle through this system.
Researchers have been able to make considerable progress in improving the movement of electrons through the lithium iron phosphate cathodes, but ions had been stubborn to pick up the pace. The holdup, they found, was that in order for the ions to move into the cathode, they have to enter tiny tunnels in the cathode crystal at an impossibly precise angle. If they don’t get it right, they don’t get in, and potential electrical current is lost.

To help ensure a smoother ride, scientists developed cathodes made of nanoparticles, which create more surface area for the ions to access (much like creating a longer on-ramp to a highway) and a shorter distance for them to travel within the crystal, so they can enter and exit more quickly. In 2008, Manthiram’s team published findings for an improved manufacturing process that significantly reduces the time and effort involved in manufacturing such nanoparticle-based cells.

But the nanoparticle cathodes solve only part of the problem. Craig Fisher, a researcher at the Japan Fine Ceramics Center, developed a computer simulation in collaboration with University of Bath researcher M. Saifal Islam that finally revealed in atomic-scale detail why the interaction between cathode materials and lithium ions is so problematic. “The cathode is made up of thousands or millions of tiny crystallites, all at random angles,” his simulation confirmed. “You have to be very lucky to have all your little crystals aligned properly if you want the ions to go in and out of the electrolyte easily.” In other words, the basic rules of a carnival game were at play: Even with many more chances to win, the ball still has to go in the hole to win the prize.

Enter MIT’s Gerbrand Ceder and Byoungwoo Kang. By tinkering with the formulation of the cathode material, they were able to make lithium iron phosphate nanoparticles coated with a thin layer of conductive glass. Compared with the highly organized and structured lithium iron phosphate crystals, this glass is amorphous and disorderly at the atomic scale, creating lots of possible entry routes for ions and helping, essentially, to funnel the ions into the cathode material.

Ceder and Kang’s results, announced in the 12 March issue of Nature, are lithium-ion batteries able to achieve complete discharge in tens of seconds — more than 100 times faster than lithium-ion batteries currently on the market  — with relatively little loss in their capacity to charge after many cycles of recharging. Likewise, the batteries can theoretically be recharged as rapidly as one can safely transfer power into them.

Although these are preliminary results with laboratory prototypes, they show how an already useful battery technology could acquire considerably greater versatility. Many current designs for purely electric-powered cars assume that conventional batteries, which provide slow but steady power, will need supplementation via supercapacitors, which generate rapid bursts for quick acceleration. It now seems that this may not be the case. “Ceder has potentially come up with a material that makes the battery act the same as a supercapacitor, so that you get both the high energy storage and the high discharge in one device,” notes Fisher. “If this material can be scaled up and shown to be stable over a long period of time, it could revolutionize the fully electric car industry.”

Originally published March 30, 2009

Tags energy enhancement innovation technology

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