Arrays of tiny carbon rods may build tomorrow's lithium cells.
A method for growing forests of miniature carbon pillars could lead to a new generation of lithium batteries, which could power everything from mobile phones to laptop computers.
Normal mobile-phone batteries generate current when lithium ions flow between two terminals. The new technique boosts the power they generate by creating an array of multiple terminals out of carbon rods, and floating a sea of lithium ions between them.
The method, developed by physicist Marc Madou of the University of California, Irvine, and his colleagues, effectively crams hundreds of batteries into the space usually taken up by a single battery cell. "It's like having a multiplicity of batteries in a single space," says Madou.
Madou's co-worker, Chunlei Wang, made the new batteries possible by figuring out how to grow the tiny rods - something that had stumped the group for some time. "I gave up on the problem about seven years ago," says Madou.
Wang succeeded by adopting a plastic polymer that is used to help build computer chips and hardens when it comes into contact with light. She created neat rows of upright rods by illuminating the polymer through a mask peppered with holes, and then applying a chemical that etched away the unreacted polymer.
Wang then cooked the rods at around 900 °C in an oxygen-free atmosphere. This burns off atoms such as hydrogen from within the polymer, leaving behind a shrunken version of the plastic. The resulting array had thousands of tiny carbon rods, each less than half a millimetre high. Wang's breakthrough, says Madou, was to find a polymer that could survive the heating process without falling apart.
To complete the battery, Wang and Madou poured lithium ions into the space between the rods and wired lines of them together so that alternative rows acted as the positive and negative terminals of a battery. The researchers have submitted their work to the Journal of the Electrochemical Society.
The new battery could have big advantages over existing lithium cells. Because they combine many positive and negative terminals, the batteries can generate larger bursts of current than today's cells provide.
The batteries might also allow the levels of current to be changed by adjusting the number of rows that are connected together. This could be useful in devices such as laptop computers, says Madou, which need more power to start up than when running normally.
Before their battery hits the market, the team needs to find a cheaper surface to house the rods. Madou and Wang used silicon, as the computer industry has already developed the technology needed to grow structures on it. But this would be too expensive for the battery to compete with existing power cells, says Madou.