With $27 billion a year in sales, lithium-ion batteries already dominate the market for rechargeables. But there's always pressure to do better. Now researchers report that they've come up with a way to use nanotechnology to either significantly increase the energy storage capacity of lithium-ion batteries or reduce their weight while maintaining their current energy content. The new work could lead to everything from lighter laptops to electric cars with a considerably longer range.
In conventional rechargeables, positively charged lithium ions are stored in a carbon-based anode and flow into the cathode as the battery is discharged. Carbon is lightweight and durable over many cycles of charging and discharging. But it takes about six carbon atoms to hold each lithium ion. Recently, researchers have sought to make anodes from crystalline silicon because each silicon atom can hold about four lithium atoms, giving it the potential to store much more energy.
In 2007, researchers led by Yi Cui, a materials scientist at Stanford University in Palo Alto, California, did just that. They crafted their anode from crystalline silicon nanowires narrow enough to swell and shrink with less damage. The batteries could store as much as 10 times the charge as conventional rechargables, but they wore out over repeated charge and discharge cycles. The problem is that the crystalline material eventually breaks, destroying its ability to be recharged.
So for its current study, Cui's team replaced the fragile silicon nanowires with carbon nanofibers, which were then clad in amorphous silicon. The inherent stability of the carbon cores allowed the researchers to fully charge the amorphous silicon with lithium ions. The upshot, which they report in an upcoming issue of Nano Letters, is that the new carbon-silicon hybrid anodes have six times the charging capacity as conventional all-carbon anodes, but in early tests they appear to be more stable than their all-silicon counterparts. That could eventually allow battery companies to make more lightweight batteries, which could be key for future electric cars; companies could also keep the battery weight the same but increase the amount of energy they store by as much as 50%, Cui suggests.
Arumugam Manthiram, a materials chemist and battery expert at the University of Texas, Austin, praises the advance. But just how much the findings will improve future batteries is unclear, he says, because the new materials must still be integrated with other battery components and proven to be cheap, safe, and fast-charging. "It's a very, very challenging problem," Manthiram says. "That's why battery technology has been changing so slowly."