The Return of the Lithium-Metal Battery
XNRGI nears commercialization of its porous silicon anode lithium-metal battery
Lithium-ion batteries are everywhere: You see them in gadgets, vehicles, robots, and power-grid storage. Worldwide production now stands at about 160 gigawatt-hours per year. The revolutionary technology earned three of its lead developers the Nobel Prize in Chemistry in 2019.
And yet the lithium-ion battery is far from perfect. It's still too pricey for applications requiring long-term storage, and it has a tendency to catch fire. Many forms of the battery rely on increasingly hard-to-procure materials, like cobalt and nickel. Among battery experts, the consensus is that someday something better will have to come along.
That something may well be the lithium-ion battery's immediate predecessor: the lithium-metal battery. It was developed in the 1970s by M. Stanley Whittingham, then a chemist at Exxon. Metallic lithium is attractive as a battery material because it easily sheds electrons and positively charged lithium ions. But Whittingham's design proved too tricky to commercialize: Lithium is highly reactive, and the titanium disulfide he used for the cathode was expensive. Whittingham and other researchers added graphite to the lithium, allowing the lithium to intercalate and reducing its reactivity, and they swapped in cheaper materials for the cathode. And so the lithium-ion battery was born. Batteries with lithium-metal anodes, meanwhile, seemed destined to remain an interesting side note on the way to lithium-ions.
But XNRGI, based in Bothell, Wash., aims to bring lithium-metal batteries into the mainstream. Its R&D team managed to tame the reactivity of metallic lithium by depositing it into a substrate of silicon that's been coated with thin films and etched with millions of tiny cells. The 3D substrate greatly increases the anode's surface area compared with a traditional lithium-ion's two-dimensional anode. When you factor in using metallic lithium instead of a compound, the XNRGI anode has up to 10 times the capacity of a traditional intercalated graphite-lithium anode, says Chris D'Couto, XNRGI's CEO.
The company expects to begin low-volume commercial production of its lithium-metal batteries this year, for shipment to electric-vehicle and consumer-electronics customers. XNRGI is also targeting grid storage. Last year, it signed an agreement to form a joint venture with the Canadian startup Cross Border Power, to sell and distribute its batteries to utility customers in North America.
Of course, new battery technologies are announced all the time, and tech news outlets, including IEEE Spectrum, are more than happy to tout their promising capabilities. But relatively few batteries that appear promising or even revolutionary in the lab actually make the leap to the marketplace.
Commercializing any new battery is a complicated prospect, notes Venkat Srinivasan, an energy-storage expert at Argonne National Laboratory, near Chicago. “It depends on how many metrics you're trying to satisfy," he says. For an electric car, the ideal battery offers a driving range of several hundred kilometers, charging times measured in minutes, a wide range of operating temperatures, a 10-year life cycle, and safety in collisions. And of course, low cost.
“The more metrics you have, the more difficult it will be for a new battery technology to satisfy them all," Srinivasan says. “So you need to compromise—maybe the battery will last 10 years, but the driving range will be limited, and it won't charge that quickly." Different applications will have different metrics, he adds, and “industry only wants to look at batteries that are at least as good as what's already available."
D'Couto acknowledges that commercializing XNRGI's batteries has not been easy, but he says several factors gave the company a leg up. Rather than inventing a new manufacturing method, it borrowed some of the same tried-and-true techniques that chipmakers use to make integrated circuits. These include the etching of the 20-by-20–micrometer cavities into the silicon and application of the thin films. Hence the battery's name: the PowerChip.
Each of those microscopic cells can be considered a microbattery, D'Couto says. Unlike the catastrophic failure that occurs when a lithium-ion battery is punctured, a failure in one cell of a PowerChip won't propagate to the surrounding cells. The cells also seem to discourage the formation of dendrites, threadlike growths that can cause the battery to fail.
Some flavors of lithium-ion batteries, such as those made by Enovix, Nexeon, Sila Nanotechnologies, and SiON Power, also achieve better performance by replacing some or all of the graphite in the anode with silicon. [See, for example, “To Boost Lithium-Ion Battery Capacity by up to 70%, Add Silicon."] In those batteries' anodes, the lithium is intercalated with the silicon, bonding to form Li15Si4.
In XNRGI's PowerChip, the silicon substrate has a conductive coating that acts as a current collector and a diffusion barrier that prevents the silicon from interacting with the lithium. D'Couto says that the lithium-metal anode's capacity is about five times that of silicon-intercalated anodes.
For most of its existence, XNRGI was known as Neah Power Systems, and it focused on developing fuel cells. The fuel cells used a novel porous silicon substrate. But the fuel-cell market didn't take off, and so in 2016, the company got a Department of Energy grant to use the same concept to build a lithium-metal battery.
XNRGI continues to experiment with cathode designs that can keep up with its supercharged anodes. For now, the company is using cathodes made from lithium cobalt oxide and nickel manganese cobalt, which could yield a battery with twice the capacity of traditional lithium-ions. It's also making sample batteries using cathodes supplied by customers. D'Couto says alternative materials like sulfur could boost the cathode performance even more. “Having a high-performing anode without a corresponding high-performing cathode doesn't maximize the battery's full potential," he says.
“People like me dream of a day where we've completely solved all the battery problems," says Argonne's Srinivasan. “I want everybody to drive an EV, everybody to have battery storage in their home. I want aviation to be electrified," he says. “Meanwhile, my cellphone battery is dying." In batteries as in life, there will always be room for improvement.