The next challenge for solid-state batteries? Making lots of them
FOR DECADES, SCIENTISTS have wondered what to do with the liquid inside a lithium-ion battery. This electrolyte is key to how batteries work, shuttling ions from one end of the cell to the other. But it’s also cumbersome, adding weight and bulk that limit how far electric vehicles can go on a charge—on top of which, it can catch fire when a battery shorts. A perfect fix would be replacing that liquid with a solid—ideally one that’s light and airy. But the trick lies in making that switch while preserving all the other qualities a battery should have. A solid-state battery not only needs to send you farther down the road on each charge, it also has to juice up quickly and work in all sorts of weather. Getting all that right in one go is among the hardest questions in materials science.
In recent months, startups working on solid-state batteries have made steady progress towards those goals. Little battery cells that once sputtered after being charged are growing up into bigger ones that go much longer. There’s still a ways to go until those cells are road-ready, but progress is setting up the next challenge: Once you’ve built a good-enough battery under painstaking lab conditions, how do you build millions of them quickly? “These companies are going to have to have a massive mindset change, going from being R&D companies to manufacturing companies,” says Venkat Srinivasan, director of the Argonne Collaborative Center for Energy Storage Science. “It’s not going to be simple.”
In recent weeks, Solid Power, among the more lavishly funded of those solid-state companies, has fired up a pilot line in Colorado that it hopes will address that question. At full capacity, it will produce 300 cells per week, or about 15,000 per year. That’s a trickle compared with the millions of cells produced each year by gigafactories, and getting there will still take months of finessing tools and processes. But the goal, according to CEO Doug Campbell, is to start delivering cells to car makers like BMW and Ford for automotive testing by the end of the year.
Once the automakers are happy with how the batteries do on the road, the company plans to pass the baton to one of its gigafactory-owning battery partners, like the Korean battery behemoth SK Innovation. According to Campbell, that should be relatively simple. Solid Power has designed what he describes as a uniquely manufacturable “flavor” of solid-state design that allows battery makers to reuse existing processes and equipment designed for lithium-ion batteries. “In an ideal world, this is the last cell production line that’s operated by Solid Power,” he says of the Colorado facility.
In principle, that makes sense. A battery is a battery. Like their liquid-filled cousins, solid-state batteries require an anode, a cathode, and some way for ions to migrate between the two. That’s where the electrolyte comes in. But it’s not easy to make something that’s porous to ions, yet solid enough not to crack. Researchers have spent years looking for the right materials, eventually settling on a range of ideas that include ceramics and plasticky polymers. But not all of them are easy to make. Some are incredibly brittle, liable to fall apart when they’re made or when they’re slotted between the electrodes; others are softer and more pliant, but can’t be exposed to moisture. Plus, battery scientists don’t have a lot of practice producing the kinds of precursor materials that are required to make them. The history just isn’t there.
The second problem is the anode. The holy grail for solid-state involves changing up the anode from the typical graphite to lithium metal. Couple that with a solid electrolyte and it’s a recipe for immense amounts of energy. The trouble is the form that lithium metal takes. Battery makers are used to working with powdered materials for the anode and cathode that can be rolled out as a slurry. But lithium works best as a thin, free-standing foil—in the case of Solid Power’s, it’s 35 microns thick. “It has the consistency of wet tissue paper,” Campbell says. “And so you can imagine when you’re making literally kilometers of material, it gets very tricky.”
Lithium offers other kinds of trouble. Over time, and especially when the battery is forced to charge up fast, lithium ions can form dendrites—tendrils of metal that wind their way between the electrodes and eventually cause the battery to short. It sounds scary—and in an old-school lithium-ion battery it could be a recipe for a fire. But in lab tests of solid-state batteries, it hasn’t proven dangerous because the solid electrolyte isn’t flammable. Mostly, it’s just inconvenient, because it affects how many times the battery can be charged.
A few years ago, Solid Power set aside lithium in favor of an anode that’s mostly made of silicon. It was a practical move, Campbell says. No more messy foil, no more short circuits. Luckily for Solid Power, the sulfide that they chose starts off in a powder form too. For battery makers, it’s familiar stuff.
Those choices have trade-offs. Swapping out the lithium anode for silicon means adding more weight to the battery, putting a limit on how much energy it can pack. The design is still poised to be a big improvement over lithium-ion. But, well … it could be better. Campbell says the company is still working on a lithium design, though it will lag a year or two behind the silicon version. In the meantime, lithium metal manufacturing can catch up.
That kind of incremental approach is likely a smart idea, says Shirley Meng, a battery scientist at the University of Chicago. Large battery makers have gotten immensely better at making lithium-ion batteries over the past 30 years, she points out, designing massive factories and better automation that has driven down costs. “We don’t want to reinvent all the machines,” she says. “We want to drop in the solid-state and only make small tweaks. That’s the most ideal situation.”
But there’s a risk of being leap-frogged. Solid State’s archrival, QuantumScape, uses a different kind of proprietary ceramic and a lithium-based design that requires a distinct set of manufacturing processes. The company has suggested it plans to build its own factories, rather than try to retool or replicate ones already out there. The company, which is currently building out a pre-pilot production line in California, told investors in an earnings call last month that it hopes to deliver batteries to automakers for road testing sometime next year.
Both companies—and a slew of competitors—are still likely years away from putting their batteries in vehicles that are for sale. As the size of the batteries increases—measured in layers—tiny imperfections compound, which poses a particular problem for scaling up. A lithium-ion battery maker that’s really good at what it does might find that only 80 to 90 percent of its cells are actually usable. They’re constantly fighting to inch that number upwards. For solid-state batteries, expect that number to start off way lower. “This is probably the biggest challenge that everybody will be dealing with,” Srinivasan says.
For Solid Power, the current EV-size cells don’t do as well as they should in cold temperatures, and battery life declines too fast when the cells are repeatedly fast-charged. But Campbell says that working out the kinks in smaller versions of the battery gives him optimism. “It gives us the confidence that the chemistry is right,” he says. “This is not a chemistry problem. This is an engineering problem.”