research & development

Super-Charging Lithium Batteries

Stanford University researchers have unveiled a nanowire electrode that could triple lithium batteries' energy storage capacity and improve their safety, reports Nature Materials. The research shows that nanowires of silicon just a few atoms across can function as high-capacity electrodes, absorbing and releasing 10 times more lithium ions than graphite electrodes. The development stems from the labs of nanowire innovator Yi Cui and battery expert Robert Huggins at Stanford's Materials Science and Engineering Department.

Charging a lithium battery usually means moving lithium ions from the battery's positive electrode or cathode into its negative electrode or anode. Silicon has the right electrochemical affinity for lithium ions to make it a promising material for anodes. Upon charging, the silicon anodes absorb too much lithium and swell to four times their previous volume, fracturing the material. After just a few charging cycles, the anodes are finished.

Nanowires, in contrast, take the swelling in stride. The researcher's silicon nanowires swell when charged from 89 to 141 nanometers wide and simultaneously elongate, thereby releasing the strain. They show no signs of mechanical failure after more than 20 cycles.

According to Cui, the silicon nanowires appear to be less susceptible as graphite to typical failure mechanisms that cause safety problems, including fires. "Potentially, silicon is going to be much safer than carbon," says Cui, who points out that improved safety could be key to lithium's future acceptance in vehicles. "It only takes an accident or two to destroy a technology." He says that testing over many more cycles is under way to confirm the silicon-nanowire anode's enhanced durability and safety.

The nanowire growth process that Cui uses, which feeds gaseous silicon to a liquid gold catalyst to make the solid electrode, is a high-temperature (600 to 900°C) process that could be costly to scale up. Cui believes that scale-up of the vapor-liquid-solid process is feasible, but says that he is also "exploring another approach."

While Cui's silicon nanowires make great anodes, lithium batteries have greater need for improved cathodes. Substituting an anode that stores more lithium ions has no impact without a corresponding cathode that can supply more charge. Both Cui and Yiying Wu, an Ohio State University chemist who recently developed his own lithium anode with a high-capacity cobalt-oxide nanowire, say their labs are working on novel cathode materials.