Silicon-anodes for lithium ion batteries are gaining traction for electric vehicles (EVs) as an alternative to traditional graphite-based designs, and their significant performance advantages make them appealing for other applications.
Alex Holland, research director with IDTechEx, told Fierce Electronics in an interview that graphite at the anode has been the go-to technology since the inception of lithium-ion batteries. “Cell designs and cathode materials have gotten a lot better and have allowed steady incremental improvement.” Density and cycle life rate capability have been advanced, he said, but there’s a limit to how much lithium you can store in graphite, he said.
Much like computer memory, density is a key characteristic of EV batteries. Holland said the appeal of silicon is that you can store more lithium per unit weight and per unit volume of anode material.
Silicon-based materials have been in development for many years, he said, with startups operating since 2010 but with little commercial deployment. “Some silicon-based material is already used in both consumer devices and electric vehicles, but it's used as an additive.”
Silicon can be difficult to wrangle
The primary hurdle for companies working with silicon materials in the context of batteries is that it expands and contracts when you cycle it, Holland said. “You charge the silicon, and it expands by up to 300%.” Discharging leads to contraction in volume, which creates issues for maintaining electrical contact, he said.
The effects of expansion and contraction combined with electrochemical implications when charging and discharging a silicon battery lead to low cycle life, Holland said. “How long it lasts has not been good enough today, and so you've got a bunch of solutions trying to mitigate against that issue.”
Until recently, these technical problems were the roadblock to using silicon-based materials in EVs that have higher lifetime cycle lives than a smartphone or a laptop, Holland said. “You can make these materials last long enough. It's now about proving some of the production processes, proving the scale, or getting the supply chain in place.”
Silicon batteries are good enough for consumer devices, Holland said, but the performance isn’t quite there for EVs. “The electric vehicle market is ultimately the key driver,” he said. “It's the big, big opportunity, given the size of the automotive market and electric vehicle demand.”
Early deployment of silicon-based materials has been in smaller, more niche markets, such as military applications as well as consumer devices such as smartphones, smart watches, and fitness wearables. “These are still useful markets in part because the performance criteria are lower,” Holland said.
IDTechEx has identified more than 30 startups in the silicon battery space. “They're all developing slightly different materials. It's a huge market opportunity.”
Startups are hitting notable milestones
One of these startups is StoreDot, which recently claimed to be the first company to produce
and validate silicon-dominant Extreme Fast Charging (XFC) EV battery cells for more than 2,000 cycles while maintaining more than 80% of their initial capacity. The company said this milestone means EV drivers can fast charge their cars in minutes every day, without worrying about the health of their battery.
StoreDot’s latest results follow on its March 2022 announcement that it had achieved 1250 cycles with smaller cells. Since then, the company has been scaling up and validating its technology while testing it with 15 EV car brands.
Another startup in the silicon battery space is Amprius, which is also targeting the massive EV market, as well as other niche applications. Ronnie Tao, VP of business development, told Fierce Electronics in an interview that EV OEMs are looking to drive down costs as low as possible. “That forces the battery innovation to go to low cost materials.”
Lower cost can mean lower performance, which means lower energy density, he said. The best that can be expected with cheaper materials is a commuter car that goes short distances.
Tao said eliminating range anxiety and reducing charging times means looking at advanced battery technologies and going beyond conventional graphite-based anode cells in conventional batteries today. “The benefit of having a silicon-based anode is that you essentially get increased energy density for the same amount of space or the same amount of weight.”
For EV applications, he said, that effectively increases range. Today, conventional technology has improved enough that EVs and can go more than 300 miles on a single charge, but Tao said Amprius’ 100% silicon anode technology doubles that, which is comparable to a conventional non-EV experience.
Familiarity favors silicon
Five years ago, lithium metal was all the rage, Tao said, but these solid state batteries have proven difficult to progress in terms of manufacturability. “Lithium ion using graphite is hitting the wall.” The challenge with using graphite as that anode material is that carbon within the graphite can only absorb so much lithium, he said, and silicon can absorb 10 times more lithium than graphite can.
Tao said silicon is a known material that’s scalable and easy to integrate. “The pendulum is swinging back towards silicon-based solutions because it's proven.” He said Amprius’ silicon technology can absorb as much lithium as the cathode can provide. “We need better and better cathode material to give us more lithium to be able to use in our systems.”
The challenge with silicon has been that it’s difficult to control within the battery and that what has slowed adoption, Tao said. Amprius has a partnership with Airbus that recognized the company had a proven technology and that it can scale, he said. “We are literally building up our production volume so that we can provide ourselves to the larger population.”
Tao said different types of batteries provide different types of performance characteristics, and that Amprius is in a sweet spot where most applications can use Amprius batteries. “There are the fringes that maybe another technology might be a little bit better,” he said. “The battery space is wide open in terms of different technologies for different segments.”