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California, known for leading the United States in climate regulations, dropped a bombshell last month: By 2035, the state will ban sales of new gasoline powered cars and light trucks. Most new car sales are expected to shift to battery-powered electric vehicles (EVs). But along with high prices and modest range, current EVs have another big drawback: They are slow to recharge. Whereas filling a gas tank only takes a few minutes, recharging an EV takes anywhere from the better part of an hour to a day, depending on the charging equipment and the size of the battery.
“There will be a pushback [from car buyers] unless there is a faster charging solution,” says Sarah Tolbert, a battery expert at the University of California (UC), Los Angeles. Yi Cui, a materials scientist at Stanford University, agrees. He predicts the broad adoption of EVs will force a revolution in battery design. The need for fast charging, he says, “will definitely provide opportunities for new battery chemistries to emerge.” By using new materials for electrodes or charge-carrying ions, he and others have already come up with promising candidates.
Most EVs today use lithium-ion batteries in which one of the two electrodes, the anode, is made of graphite. Graphite has dominated the market because it’s cheap, abundant, and able to store enough lithium ions to give cars a range of about 500 kilometers. During charging, the applied voltage pushes electrons into the graphite, attracting lithium ions from the other electrode, the cathode. As the car drives, the lithium lets go of the electrons and travels back to the cathode, while the electrons are routed through the motor, which converts some of their energy into motion, before returning to the cathode.
But graphite anodes are difficult to charge quickly. Most chargers in the United States today use either a standard household voltage of 120 volts (an L1 charger) or 240 volts (L2). Even L2 chargers can require 10 hours or more to fully charge an EV with a typical 500-kilometer range. Still higher voltage L3 chargers, such as Tesla Superchargers, can charge an EV to 80% capacity within 45 minutes. But these nearly 500-volt chargers can cause lithium ions in the graphite to pile up into metal needles called dendrites that can short out the battery and cause it to catch fire. Even if that doesn’t happen, high-voltage charging can cause irreversible structural changes in the graphite that shorten the battery’s lifetime.
A partial solution may come from simply changing the rates at which graphite-containing batteries are discharged. In a 23 December 2021 Nature paper, Cui and his colleagues reported that doubling the discharge rate for the first 2 minutes a battery is in use essentially melts away any built-up lithium dendrites, which can extend a lithium-ion battery’s lifetime by 29% and make it stand up better to fast charging.
Another emerging option is to change the anode material altogether. Fifteen years ago, Cui and others showed anodes made from silicon can increase how much charge a battery can store and enable faster charging. Each silicon atom is able to bind four lithium ions, compared with only one for every six carbon atoms in graphite. But pushing so many lithium atoms into a silicon matrix can cause the anode material to swell up to four times in size. And repeatedly charging and discharging the battery typically pulverizes the silicon, killing the battery.
More recently, Cui and others have shown nanoscale modifications to the structure of the silicon, such as forging it into an array of nanowires, can allow the anode to swell and shrink without fracturing, thereby extending the battery life. Amprius, the company Cui spun out to commercialize the technology, reported in February it has developed a silicon-anode lithium-ion battery with a capacity of 450 watt-hours per kilogram, nearly double that of the 280 Wh/kg cells used in current Tesla EVs. What’s more, the new cells can charge to 80% of capacity in just 6 minutes. The company now sells the batteries for drones and other remote aircraft and is working to scale up the technology for EVs.
Other anode materials are also in the works. In 2013, Tolbert, along with UC Los Angeles colleague Bruce Dunn and others, reported that anodes made from the light, gray metal niobium would also enable higher capacity and faster charging than graphite. They processed niobium oxide into a spongelike form, made up of nanoscale tendrils shot through with micron-size pores. This material’s very high surface area enables it to hold lots of lithium, and the larger channels enable lithium ions to race through, resulting in faster charging. And unlike silicon, the structure of the niobium-oxide does not change when it grabs and releases lithium ions. Lithium ions nestle close to niobium atoms during charging and simply drift away during discharge, causing less damage to the battery as it goes through repeated charge/discharge cycles.
In 2017, UC Los Angeles licensed its technology to a California startup called Battery Streak. Last month, the company reported it has made palm-size “pouch” cells capable of charging to 80% of capacity in just 10 minutes. (Current EVs use thousands of similar-size cells.) During that fast charging, Battery Streak’s cells warm up by just 8°C, compared with graphite-based lithium-ion batteries, which heat by as much as 50°C during high-voltage charging. That should slow battery degradation and extend the life of Battery Streak cells more than 10-fold over current graphite-anode lithium batteries, says Dan Alpern, Battery Streak’s vice president of marketing. That increased battery life should offset niobium’s price, which is typically more than 30 times that of graphite. Like Amprius, Battery Streak is working to scale up its batteries for EVs.
Replacing the charge-carrying lithium ions with other materials can help as well. In the 24 August issue of Nature, for example, Donald Sadoway, a chemist at the Massachusetts Institute of Technology, and his colleagues reported a novel battery design that relies on aluminum ions. Their prototype has a capacity similar to conventional lithium-ion batteries but is capable of recharging in minutes. The battery must operate at near the boiling point of water to allow aluminum ions to move through the device’s molten salt electrolyte, which ferries ions between the electrodes. But Sadoway and his team are already working to reduce the operating temperature. If they’re successful, the battery could be a blockbuster because aluminum is cheap; compared with lithium batteries, the cost of materials for these batteries would be 85% lower.
Just how all these and other novel battery chemistries may shake out in the marketplace is anyone’s guess, says Gil Tal, a transportation technology expert at UC Davis. But he adds it’s a safe bet that large-scale EV adoption will cause the battery market to splinter, allowing users to choose their batteries based on whether they prioritize the lowest cost, the fastest charging, the largest capacity, or the longest life. By 2035, Tal says, “The market will be much more diverse.”