Doubling Lithium-Ion Battery Storage

Battery startup Amprius says it has developed batteries capable of storing twice as much energy as anything on the market today, thanks to nanostructured silicon electrodes. The company says it is partnering with several as-yet unnamed major consumer electronics manufacturers to bring the batteries to market by early 2012. The batteries will allow portable electronics to run 40 percent longer without a recharge.

Amprius also says it is working with several major automakers who are evaluating the electrode materials for use in batteries for electric vehicles. The company is not yet disclosing these commercial partners, either.

When a lithium-ion battery is charged, lithium ions move from its cathode to its anode, while electrons flow in through an external electrical circuit. The process is reversed during discharge. The more lithium the anode can take in, the more total energy the battery can store, and the longer it can last. For the past 30 years, lithium-ion batteries have used carbon anodes. With no new materials, the total energy storage of these batteries has improved by only about 7 percent every year due to incremental engineering refinements.

Silicon has 10 times the theoretical lithium storage capacity of the carbon used to make battery anodes, but it’s been difficult for researchers to make it into a practical battery electrode. As large volumes of lithium ions move in and out of the material during charge and discharge, silicon swells and cracks.

In 2007, Yi Cui, a Stanford University materials science and engineering professor, demonstrated that nanostructured silicon films could be charged and discharged of lithium without experiencing these mechanical problems, making a potential anode material that could as much as double the energy storage of lithium batteries.

In the 18 months since Amprius was founded, researchers at the company have built on Cui’s research and have demonstrated that the silicon anodes can be used in practical batteries. Silicon nanowires, which are vertically arrayed and tethered but flexible, are used to make the battery anodes.

As the nanowires take in lithium, they can swell and bend to accommodate it, without breaking. However, this isn’t mechanically stable enough. Amprius has addressed this problem by giving the nanowires a thin, reinforcing metal core that the company likens to rebar (the steel strut used to reinforce concrete structures). This “rebar” prevents the anode from expanding and contracting too much. In testing, the silicon anodes can store three times more energy than carbon anodes by weight.

Prototype batteries have been tested through 250 charging cycles and have been shown to store twice the energy of a conventional battery. To be a serious contender for use in electric vehicles, however, the batteries will need to go through 3,000 charging cycles, says Ryan Kottenstette, the company’s director of business development.

Amprius CEO Kang Sun says the company is moving aggressively to bring its batteries to electric vehicles. “We are in a hurry, because electrification is moving forward faster than anyone thought,” he says. Sun, the former president of Chinese solar manufacturer JA Solar, notes that there are already about 80 electric-vehicle makers in China. “We have to be fast,” he says. The company expects to disclose some automaker partnerships in the next few months.

Conventional carbon anodes are made using large roll-to-roll systems. Kottenstette acknowledges that the vacuum deposition technique used for silicon nanowires will be more expensive. However, once potential manufacturing issues are ironed out, the company expects the boost in storage capacity to make up for the increased cost. The company is also working on a roll-to-roll vapor-deposition system. “Making it compatible with current processes is important to us,” says Kottenstette.