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More Energy in Batteries

Nanowire anodes could let lithium-ion batteries run twice as long.

A start-up based in Menlo Park, CA, plans to sell a new type of anode for lithium-ion batteries that, the company says, will let electric vehicles travel farther and mobile devices last longer without a recharge. Amprius’ lithium-ion anodes are made of silicon nanowires, which can store 10 times more charge than graphite, the material used for today’s lithium-ion battery anodes. According to the company, electric vehicles that run 200 miles between charges could go 380 miles on its batteries, and laptops that have four hours of run time could last for seven hours between charges.

Silicon charge: Mats of silicon nanowires hold 10 times more charge than conventional lithium-ion battery anode materials.

While other advanced battery companies are focused on power, which makes for fast charging and zippy acceleration, Amprius is trying to improve energy density, which enables longer run times. The more total energy a battery can store, the longer it can power a car or a phone between charges. As vehicle manufacturers look toward electric cars, and as mobile devices like iPhones run more energy-intensive applications, a battery’s energy density, and thus the time it can go without a recharge, becomes a more pressing issue.

When lithium-ion batteries are charged, lithium ions move from the cathode to the anode, while electrons flow in through an external electrical circuit; the process is reversed during discharge. Silicon has shown promise as an anode material because it can take up much more lithium than the carbon materials now used. Indeed, the theoretical maximum energy density of silicon is 10 times greater than carbon’s. But silicon is fragile and tends to swell and crack after just one charge cycle.

However, battery anodes made from silicon nanowires can be cycled over and over again without damage. This fall, Yi Cui, Amprius founder and assistant professor of materials science and engineering at Stanford, demonstrated nanostructured silicon anodes that meet silicon’s theoretical charge storage capacity without breaking. Mats of long, thin nanowires are pliable, which relieves the strain when the battery is charged and discharged. And collections of nanowires have a very high surface area, which means more sites for interacting with lithium.

Ryan Kottenstette, Amprius’ director of business development, says the company has made a number of improvements in the nanowire growth process to make it compatible with large-scale manufacturing. The nanowires are grown from a gas on a metal substrate coated with a catalyst. The company would not detail how the anodes are made, but it has developed a process that uses a more conductive substrate and a cheaper catalyst. “The anodes can be grown on a large scale at a fast speed in large areas on foil and with lower materials costs,” says Cui.

Amprius is in talks with vehicle and electronics manufacturers, and raised its first round of venture funding in March. The company hopes to raise more funds next summer to build a pilot manufacturing line.

No matter how good the anode is, the overall charge capacity of a battery depends on the cathode, too. The performance of today’s lithium-ion cathodes isn’t as good as that of the anodes Amprius is developing. The company’s initial battery designs make up for this mismatch by pairing a thin anode with a thick cathode. Compared to a conventional lithium-ion battery of equal size, this design stores 40 percent more charge. In order to further increase the energy density, however, the company will need new cathode materials.