Posted on April 09, 2017 by Charles Morris
Lithium-ion batteries represent a landmark technology that has made the current generation of electric vehicles possible. However, the day of their demise, while it still lies years in the future, is within view. Lithium-ion chemistries have a certain maximum energy density, dictated by those pesky laws of physics, and today’s batteries are not so far from that theoretical maximum. If drivers keep demanding longer ranges and faster charging times, then a better technology will have to be found.
Above: Panasonic's 18650 lithium-ion battery cell used in the Tesla Model S and X (Image: Daily Sabah)
Safety is also an issue. The spectacular explosions and fireballs that some documentary-makers revel in are not the norm (when was the last time your phone or computer caught fire?), but Li-ion batteries do have to be handled carefully, and necessary safety features add complexity and cost to battery packs. A new chemistry that is safer could also prove to be cheaper.
Researchers around the world are working on “beyond lithium” projects, and the past year has seen several significant breakthroughs. Of course, advances in the lab take years to make their way to the marketplace, but if and when one of these promising technologies can be commercialized, we could see game-changing improvements in the performance and cost of EVs.
One technology that’s been getting a tremendous amount of attention from researchers is the solid-state battery, which uses a solid electrolyte instead of the liquid electrolyte used today. Solid-state batteries could theoretically have double the energy density of current batteries, and last several times longer. They also use a non-flammable electrolyte - usually glass, polymer, or a combination - so they would eliminate the safety issues that plague Li-ion cells.
Above: Lithium-ion battery vs. solid state battery (Image: Toyota)
Lithium-air batteries likewise could offer far greater energy density - maybe as much as 10 times more - but they suffer from poor cycle life. In 2015, Cambridge scientists wowed the battery world with an announcement that they had demonstrated a highly efficient and long-lasting lithium-oxygen battery. Alas, researchers from several universities and national labs have since been unable to duplicate the original results.
Other promising battery chemistries use other elements in place of lithium. Sodium batteries powered Jules Verne’s futuristic submarine in “20,000 Leagues Under the Sea.” More recently, in 2015, researchers created a prototype sodium-ion battery in the industry-standard 18650 cylindrical format.
According to a recent article in the Nikkei Asian Review, battery research has seen a big shift in recent years. At one time, nearly half of the presentations at the Battery Symposium in Japan were about fuel cells and Li-ion battery cathode materials. But since 2012, these topics have been supplanted by presentations about solid-state, lithium-air and non-lithium batteries.
Above: How a lithium-air battery works (Image: Money Inc)
Toyota has been focusing on solid-state and Li-air batteries. At the latest Battery Symposium, battery researcher Shinji Nakanishi discussed a scenario for transitioning from Li-ion batteries to solid-state and then Li-air batteries. “We want our electric cars to go 500 km” on a single charge, he said. “And for this, we want rechargeable batteries that can generate 800 to 1,000 watt-hours per liter.” That would be two to three times the energy density of today’s best Li-ion batteries.
Panasonic, Tesla’s battery supplier, is also taking a hard look at solid-state technology. “We think the existing technology can still extend the energy density of Li-ion batteries by 20% to 30%,” President Kazuhiro Tsuga told Nikkei. “But there is a trade-off between energy density and safety. So if you look for even more density, you have to think about additional safety technology as well. Solid-state batteries are one answer.”
Engineers have been pushing the limits of Li-ion technology for decades. Today’s best Li-ion cells can reach an energy density of about 300 watts per kilogram, which is getting close to the theoretical maximum. “Existing Li-ion batteries still have room to improve their energy density because you can raise the density by introducing a nickel-based cathode material, so you can expect the batteries will still be used in the next few years,” said battery expert Naoaki Yabuuchi of Tokyo Denki University. He expects lithium-ion technology to reach its limits around 2020.
Above: Tesla Model X on display at Panasonic's booth at CES (Image: Business Wire)
Is Tesla working on any of these post-lithium chemistries? It would be strange if they were not. We know that the company is constantly evaluating new battery technologies. “Tesla has one of the largest cell characterization laboratories in the world - we have just about every cell you can imagine on test,” Tesla Product Planner Ted Merendino told me back in 2013. However, both Elon Musk and JB Straubel have said that so far, they’ve seen no viable replacement for lithium-ion, and believe me, they’ve been asked the question many times.
“We have yet to see even a single example… of a cell working at the laboratory level that is better than the one that we have, or the ones that we expect to come out with,” said Elon Musk in 2014. Now, the way I parse this statement, he isn’t saying that there’s no improved battery technology in the offing - on the contrary, he’s saying that Tesla will be the one to develop it.
When Model 3 was announced, some EV-watchers opined that, in order to deliver the new vehicle at the desired price point, Tesla would need to make a major battery breakthrough. In the event, Tesla has developed a new battery for Model 3, but it looks more like an incremental improvement than a paradigm shift. The new 2170 cell, which is now being produced at the Gigafactory, is slightly larger than the trusty 18650, and can store more energy. According to Elon Musk, it’s “the highest energy density cell in the world, and also the cheapest.” Advances in the way the cells are assembled into modules and packs are also expected to yield a significant reduction in battery costs.
Above: 2170 battery in production at the Tesla Gigafactory (Youtube: Tesla Garage)
So, it appears that lithium will continue its reign for a few more years at least. However, the post-lithium Holy Grail is still out there, and as likely as not, the knights of Tesla’s round table will be the ones to bring it home. Battery superstar Jeff Dahn and his colleagues aren’t working for Tesla just to make speeches at conferences. It’s entirely possible that, at some super-secret facility in California or Nevada, test mules are being powered by solid-state or lithium-air batteries even as we speak.