A team of Stanford University researchers, including former Energy Secretary Steven Chu, believes it has achieved the "holy grail" of lithium battery design: an anode of pure lithium that could boost the range of an electric car to 300 miles.
Lithium-ion batteries are one of the most common types of rechargeable batteries on the market today. But most of the batteries -- found in technologies like smartphones and electric cars -- use an anode made of graphite or silicon.
The lithium in a lithium-ion battery today is found in the electrolyte. The electrons in the electrolyte flow to the anode during recharging, and if the anode were also made of lithium, the battery would be able to generate much more power and weigh much less.
Until now, however, lithium anodes have been unusable. The material expands during charging, opening fissures on the surface that release lithium ions and form messy, hairlike growths called dendrites that reach out and short-circuit the battery. Lithium anodes are also highly chemically reactive with the lithium electrolyte and can overheat to the point of fire or even explosion.
The potential flammability of lithium-ion batteries has come under scrutiny after three electric cars made by Tesla Motors Inc. crashed and caught fire last year after hitting road debris (Greenwire, Nov. 8, 2013).
The Stanford team thinks it has solved these problems with a protective layer of tiny carbon domes, called nanospheres, that form a flexible honeycomb-styled shield over the anode. The nanosphere wall, just 20 nanometers thick, is strong and flexible enough to move up and down as the anode expands and contracts during the battery's charge-discharge cycle.
Taming a difficult metal
The researchers detailed their work in a report published in the journal Nature Nanotechnology this week.
"We're now looking for higher and higher energy density batteries, and graphite [anodes] can't do that anymore," said Yi Cui, a professor of material science and engineering and leader of the research team.
With its light weight and high energy density, lithium has the greatest potential as an anode, Cui said. The carbon nanosphere interface "increases the recycling efficiency and also decreases the side chemical reaction of the electrolytes," he added.
A lightweight, high-energy lithium anode could also go a long way in making lithium-ion batteries more commercially viable. According to the Stanford team, the nanosphere layer helps improve the coulombic efficiency of the battery -- a ratio of the amount of lithium extracted from the battery during use compared with the amount put back in during charging.
Previous lithium anodes achieved a 96 percent coulombic efficiency, but that dropped to less than 50 percent after 100 cycles. This new lithium anode achieves a 99 percent efficiency for 150 cycles.
Cui said that over the next few years, the team hopes to refine the battery design to both improve on the coulombic efficiency and sustain it for 500 to 1,000 cycles.
Competition for the internal combustion engine?
Chu, the former Energy secretary and a Nobel laureate, recently resumed his professorship at Stanford and is part of Cui's team. In a press release, he said the new lithium anode design could improve the battery's capacity fourfold.
"You might be able to have a cell phone with double or triple the battery life or an electric car with a range of 300 miles that cost only $25,000 -- competitive with an internal combustion engine getting 40 miles per gallon," Chu said.
Chu this year also joined an advanced battery startup called Amprius Inc., a company that Cui co-founded six years ago that specializes in high-capacity lithium-ion batteries (Greenwire, Jan. 20).
John Goodenough, who invented the original lithium-ion battery in the 1970s and is still working on the technology, called the Stanford design a "significant step."
But Goodenough -- now a professor of mechanical engineering and materials science at the University of Texas, Austin, who said he is also pursuing his own lithium anode design -- added that the technology was still a long way from commercial viability.
"I don't think it answers all the problems, by a long way," he said, "but it's something."