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Sustainable Energy

Startup Gets $30 Million to Bring High-Energy Silicon Batteries to Market

Amprius’s silicon-based batteries are starting to appear in electronics.

Cheaper, higher energy batteries are needed for electric cars.

Amprius, a startup working on a new type of long-lasting lithium-ion batteries for laptops and electric vehicles, has started to sell its batteries for use in portable electronics. The company recently raised $30 million in venture capital to develop its next-generation batteries, which use high-energy silicon electrodes. The company says the batteries will store about 50 percent more energy than the battery cells in today’s electric vehicles.

The funding is a rare piece of good news for lithium-ion battery startups (see “What Happened to A123?” “A123’s Technology Wasn’t Good Enough,” and “The Sad Story of the Battery Breakthrough That Proved Too Good to Be True”). The company, founded in 2008, has raised $61 million to date.

Silicon electrodes can store more lithium than conventional graphite ones, but they swell and shrink as the battery is used and recharged, and that causes the battery to fall apart. Amprius founder Yi Cui, a materials science and engineering professor at Stanford, showed how to avoid this problem by making silicon nanowires that don’t fall apart (see “Doubling Lithium-Ion Battery Storage”).

That material, however, proved difficult to bring to market since it requires custom manufacturing equipment. So for its first product, Amprius developed another type of resilient nanoparticle with silicon at the core surrounded by a layer of carbon. This material stores less energy than silicon nanowires, but it can be used in existing factories.

Kang Sun, Amprius’s CEO, says the company has made hundreds of thousands of the core-shell batteries, mostly via contract manufacturers in Asia. He says the batteries are now being used in portable electronics devices made by some Chinese manufacturers. They can store 650 watt-hours per liter or 280 watt-hours per kilogram (a measurement preferred by automakers, who care more about weight than volume), which gives them a slight, but significant, edge over the competition. Conventional high-energy batteries for electronics typically store between 400 and 620 watt-hours per liter; electric car battery cells store about 200 to 240 watt-hours per kilogram.

The performance of the version using silicon nanowires is even more impressive. Prototypes of the cells store 750 watt-hours per liter and 350 watt-hours per kilogram.

Demonstrating that the batteries can be recharged enough times to last the lifetime of a device or car will be important. Sun says its current batteries can be recharged over 500 times and still retain 80 percent of their original capacity—enough for portable electronics, but not for electric vehicles. The next-generation batteries are expected to last from 700 to 1,000 cycles, Sun says. That might be enough to last the life of some electric cars—those with batteries so large that they don’t need to be recharged daily. (A car with a 300-mile battery range, for example, could go a week on a charge for typical U.S. drivers. Even with weekend trips, a 1,000-cycle battery could last 10 years.)

Other manufacturers have tried to bring silicon batteries to market without much success. For example, in 2009, Panasonic made headlines after it announced a new lithium-ion battery product with a silicon electrode that could store even more energy than Amprius hopes to store with its silicon nanowire batteries (see “Tesla to Use High-Energy Batteries from Panasonic”). Now, several years later, that battery still hasn’t made it to market, reportedly as a result of technical problems.

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