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A lithium ion battery that can power a smart phone or tablet for up to 25 percent longer between charges than current alternatives is now out in the marketplace, from a venture capital-backed battery startup that has been very quiet for several years. The company, Amprius, is backed by a group of investors including Google chairman Eric Schmidt, VantagePoint Venture Partners, and Kleiner Perkins Caufield & Byers, among others.

We included Amprius, which was launched in 2008 as a spin out from Stanford University, on our list of 13 battery startups to watch in 2013. The startup has developed a battery based on research from Stanford’s Yi Cui, and its lithium ion batteries, announced Tuesday, use a nano-structured silicon material for the anode part of the battery.

A battery is made up of an anode on one side and a cathode on the other, with an electrolyte in between. Amprius’ nanostructured material allows the anode to be shrunk fourfold, delivering a fourfold increase in energy density.

Battery energy density is the amount of energy that can be stored in a battery per given volume. Amprius said its initial batteries can deliver 580 and 600 watt hours per liter, and its next-gen batteries can deliver 650 and 700 watt hours per liter. Traditional lithium ion batteries are operating at closer to 400 watt hours per liter.

Another one of the challenges that Amprius said it has overcome when building this battery is that it has had to engineer the silicon to make it stable enough to be charged and discharged repeatedly over time. The more stable the silicon, the longer the life time of the battery. Amprius said the anode can be charged and discharged more than 500 times while retaining 80 percent of the original capacity (a requirement for original equipment manufacturers, or OEMs).

Amprius is supplying its batteries to unnamed smartphone and tablet OEMs and is also working with OEMs to design its batteries in custom ways to fit into new consumer electronics, it said. The next-gen batteries are supposed to go into pilot production later this year.

Amprius has raised at least $25 million from investors including the ones listed above as well as IPV Capital, and Trident Capital. The company has an R&D lab in Sunnyvale, Calif., and an R&D lab and pilot production line in Nanjing, China.