Since 2008, Silicon Valley startup Amprius has been working on a multi-pronged effort in next-generation energy storage technology -- using silicon, rather than carbon, as a material for the electrodes within lithium-ion batteries.

The Stanford University spinout just announced a $30 million venture capital round aimed at supporting the first-generation batteries that Amprius has been producing at its Chinese facility since May, as well as continued development of the silicon nanowire technology behind the company’s founding.  

Amprius’ new C round comes on top of a $25 million B round in 2011, bringing the Sunnyvale, Calif.-based company’s investment to date to at least $55 million, not counting its undisclosed Series A round and federal grants. Monday’s round was led by Asian private equity firm SAIF Partners and joined by previous investors Kleiner Perkins Caufield & Byers, Trident Capital, VantagePoint Capital Partners, IPV Capital, Chinergy Capital, and Innovation Endeavors, the investment firm founded by former Google CEO Eric Schmidt.

As for the company’s technology, it’s centered on using silicon to replace carbon in lithium-ion battery electrodes and seeks to capture silicon’s much greater energy density potential -- theoretically up to ten times as much energy density. Stanford professor Yi Cui, whose work on silicon nanotubes is the core of Amprius’ intellectual property, has said that using anodes with this technology could increase the energy density of today’s lithium batteries by 40 percent.

Silicon-based electrodes have been a holy grail for battery makers and researchers for some time. But translating the advantages of silicon into real-world battery applications has proven difficult. The main challenge is that silicon, unlike carbon, expands by as much as 400 percent under lithium-ion insertion, and this expansion causes the silicon structures to fracture and degrade after only a handful of charging cycles.

Silicon nanowires can expand and contract without breaking, making them an ideal and practical battery material. But finding a cost-effective way to create these nanotube structures in a commercial production environment is a significant challenge, as Amprius laid out in a 2010 document (PDF) describing a multi-million-dollar R&D project it was undertaking in partnership with the National Institute of Standards and Technology:

Amprius currently makes silicon nanowires in a small-scale batch process using chemical vapor deposition (CVD), a process borrowed from the semiconductor industry. Mass consumer applications would require a far more efficient and low-cost manufacturing technique. The company hopes for a 1000-fold scale up of manufacturing capability, and the current project will explore two potential paths towards a large-scale process to produce silicon nanowire anodes "by the mile."

Amprius says that its “third-generation” silicon nanowire-technology-based batteries are expected to increase the energy density of batteries for consumer electronics like mobile phones, laptops and tablets to about 800 watt-hours per liter. That’s compared to today’s lithium-ion batteries that deliver energy densities between 400 and 530 watt-hours per liter.

But Amprius is not making these silicon nanowire-based batteries today, although CEO Kang Sun said that the NIST research has allowed the company to demonstrate a preliminary concept for a high-throughput manufacturing process for that technology. At present, Amprius is aiming at a 2015 date for putting those batteries on the market, he said.

Instead, Amprius has focused its first commercial production efforts on a set of nanostructure materials including both silicon and carbon, Sun said in a phone interview last week. That’s the mix of technology behind the Nanjing, China-based production facility that in May started producing battery components for a number of undisclosed end customers, which has been in volume production for the past three months or so.

“Volume manufacturing means we produce the batteries in the hundreds of thousands,” he said, with about 60,000 built so far for testing with customers that include Nokia, as well as a number of U.S. and China-based original equipment manufacturers. He declined to provide further details about which of those manufacturers might be preparing to use Amprius' batteries in commercial products.

“For the next half-year, our primary focus will continue to be on consumer electronics,” he said. Amprius’ current “first-generation” nanostructured batteries are able to store about 580 watt-hours per liter, which is 20 percent to 30 percent higher than any other battery in the marketplace.

In mid-2014, Amprius plans to start pilot production of its second-generation, 700 watt-hour per liter batteries, utilizing “a very unique silicon electrochemistry,” which includes methods to mitigate and control the expansion of the silicon contained in the anode nanostructure, as well as advances in electrolyte chemistry to support the battery, he said.

“Next year, the company will get into the electrical transportation market,” Sun added. That’s an application where the key performance metric isn’t watt-hours per liter, but watt-hours per kilogram, a reflection of the key importance of battery weight for vehicles.

Most of today’s lithium-ion EV and hybrid batteries have about 200 to 220 watt-hours per kilogram in energy density, he said. "Today, in practice, [we] can reach about 300 watt-hour per kilo -- and in theory, we can reach about 350 watt-hour per kilo, based on our existing cathode system,” Sun said.

But vehicle batteries also involve some far more complex challenges than consumer electronics batteries, he noted, including more robust charging cycle requirements, as well as the ability to perform under far higher temperatures. Right now, the company is working on “a very unique cathode” technology that’s expected to help its third-generation batteries reach 400 to 500 watt-hours per kilo, he said.

Amprius has been working on vehicle batteries via a $5 million Department of Energy grant (PDF), and expects to have a lab prototype of an EV battery ready in 2014, he said. But so far, the company’s vehicle battery progress has been solely in the preliminary modeling, testing and verification of materials, and development isn’t expected to begin until the third quarter of 2014, he said.

Fierce Competition, Difficult R&D Pathway for Silicon-Based Batteries

Amprius is far from alone in taking on the challenge of bringing silicon’s technical advantages to practical use in lithium-ion batteries, Sun said. “There are many competitors trying to figure out how to make this kind of technology work,” he noted. “There’s no doubt about it, almost every company has a silicon anode program.” Major battery and components manufacturers including 3M, Panasonic and LG Chem have made plans to bring batteries with silicon-based anodes to market (PDF), and they could create significant competition -- not only in terms of manufacturing might, but also in competing to license the technology emerging from university- and government-backed labs.

That means that Amprius has a lot to prove as it continues to seek customers for its current generation of technology, as well as moving toward the startup’s original promise of truly transformative improvements in lithium-ion battery technology. It's noteworthy that other companies pursuing silicon-based anode technology have largely concentrated on supplying components, rather than making batteries themselves. Some contenders here include Nexeon, a U.K.-based startup that has raised about $65 million in venture capital; Nanosys, a Silicon Valley startup working on a DOE-backed silicon-based anode R&D project (PDF); and XG Sciences, a Michigan State University spinout that supplies materials to U.S. and Asian manufacturers.

A further challenge lies in the highly technical and difficult-to-verify nature of nanotechnology improvements in battery chemistry. It’s important to note that another DOE-funded battery startup, Envia Systems, has been sued by former executives, claiming the company misappropriated some of its technology and misrepresented other technology licensed from another company as its own, in pursuit of delivering a high-energy density battery for General Motors. News like this is sure to cast doubts on the claims of rival startups, whether they’re deserved or not.

Meanwhile, Amprius’ core silicon nanowire technology is facing challenges from other technology development efforts. DOE’s Oak Ridge National Laboratory, for example, has published a description of its own silicon nanowire anode technology (PDF), which uses copper nanowires as a supporting structure, a technology it claims could offer more durability and faster charging than Amprius' approach.

A list of Amprius’ patent applications reveals a 2012 application for “novel electrolytes for use in rechargeable lithium-ion cells containing high capacity active materials, such as silicon, germanium, tin, and/or aluminum.” It also includes more than a dozen filings dealing with electrode development, indicating that the company is pursuing multiple pathways toward incorporating silicon into its batteries.

One filing from January 2013, for example, deals with template electrode materials that includes silicide nanowires in one arrangement, pointing to the company’s continuing work on its original technology.

At the same time, two of Amprius’ patent applications filed in December 2013 present alternative concepts. The first involves a “high surface area template containing a metal silicide and a layer of high capacity active material deposited over the template,” which could allow these super-thin layers to be “maintained below the fracture threshold of the active material used and preserve its structural integrity during battery cycling.” This appears to be similar to a concept described in another patent application from November 2012.

The other December patent application involves “interconnected hollow nanostructures that contain high capacity electrochemically active materials,” including silicon, in a way that could “provide space for the active materials of the nanostructures to swell into during battery cycling.”

That concept sounds similar to a research project, also led by Yi Cui at Stanford, which in June announced a novel electrode containing silicon particles in a conducting polymer hydrogel that could undergo expansion and contraction without breaking down for as many as 5,000 charging cycles.

In another development, Cui led another Stanford research team that in 2012 reported that its double-walled silicon nanotubes could withstand as many as 6,000 cycles without significant damage. It’s worth noting that Amprius’ 2012 report to the DOE (PDF) cites goals of maintaining performance through 1,000 cycles, a figure that’s relatively low compared to the cycle life that could be obtained through these more recent technology developments.

Of course, moving from lab results to manufacturing and deploying batteries in real-world applications is the true test of all of these technologies. On that front, Amprius “has demonstrated higher performance levels at this moment” than its would-be competitors, Sun said. Perhaps 2014 will be the year for the startup to get its consumer electronics customers to vouch for the real-world performance of its first generation of products, and lay the groundwork for its next generations to move from laboratories to cellphones and laptops -- and, perhaps, electric vehicles.