Late last year, Formula E officials announced the specs for the third generation of all-electric race cars that will debut on the motorway in 2022. The new Formula E cars will be the first to use extremely fast charging stations that pack enough power to fully charge a Tesla Model S battery in about 10 minutes. Although the racers will only use the charging stations for brief pit stops, they’ll provide a glimpse of the future beyond the racetrack: EV batteries that charge in the same amount of time it takes to fill a gas tank.
To be sure, fast EV chargers already exist. Tesla and Porsche have both recently deployed 250-kilowatt public charging stations, which can bring some EV battery packs close to full charge in around 40 minutes. That beats leaving the car to charge overnight in the garage, but it’s still a lot longer than filling up the tank on a gas guzzler. Plus, these are only available for a handful of new, high-end EVs. If we want to electrify our roads, we need affordable EV batteries that can be charged even faster.
“Over 50 percent of the US population lives in apartments, condos, or homes that don’t have access to charging,” says Matthew Keyser, who leads the electrochemical energy storage group at the National Renewable Energy Laboratory. “To increase EV adoption, we need to provide a means of charging quickly for this segment of our society.”
Boosting a lithium-ion battery’s charge rate involves trade-offs. During charging, lithium ions flow from the cell’s cathode to its anode, which is typically made from graphite, a type of carbon. The anode is like a bucket that collects and stores the ions while the battery is charging. Thicker anodes—bigger buckets—can hold more energy in the form of lithium ions, which allows electric cars to go further on a single charge.
But thicker anodes also make fast charging more difficult, because the ions must travel farther along twisted paths in the anode. If the ions can’t penetrate the anode fast enough during a charge, it causes a molecular traffic jam and the lithium gets bunched up on the surface. This phenomenon, known as lithium plating, can kill a battery’s performance. And if enough ions get stacked on the surface of the anode, they can form spindles that rupture the barrier between the battery’s anode and its electrolyte. These so-called lithium “dendrites” can cause the cell to short-circuit.
Anna Tomaszewska, a chemical engineer at Imperial College London who recently coauthored a review paper on fast-charging lithium-ion batteries, says one possible solution to lithium plating is to add silicon to the anode. Silicon is cheap, abundant, and can change the anode’s crystal structure in such a way that makes lithium plating less likely. “Silicon has been particularly popular with the manufacturers because it can also improve the energy capacity of the battery,” adds Tomaszewska.
Indeed, many companies, including Tesla, have added silicon or silicon oxide to graphite anodes to squeeze some more energy from their lithium-ion cells. But Enevate, an energy storage company based in Southern California, wants to take graphite out of the picture. For the last 15 years, the company has been perfecting an XFC, or extremely fast charging lithium-ion battery with a pure silicon anode.
Earlier this year, the company’s researchers announced that their latest generation of batteries could be charged to 75 percent in just five minutes—without sacrificing energy density. “We can have a fast charge without losing out on energy density because we’re using an inexpensive, pure-silicon approach,” says Ben Park, Enevate’s founder and chief technology officer.