Electric vehicles are gaining popularity fast, but some prospective buyers remain hesitant. One big reason is that charging EVs is slow. While drivers today are accustomed to filling their gas tank in less than five minutes, EVs, depending on the size and specifications of the battery, typically take at least 30 minutes to get 80 percent charged at the fastest charging stations out there.
In five to 10 years, though, far faster charging might be possible. Companies are developing new lithium-ion battery materials, as well as new “solid state” batteries, which are more stable at faster charging speeds. They could place recharge rates of 20 minute or less within reach.
Meanwhile, a team of scientists recently designed a lithium battery prototype that, under laboratory conditions, can recharge more than 50 percent of its capacity in just three minutes—and do so thousands of times without significantly degrading. This, the researchers say, could pave a path toward batteries that can recharge fully in as little as 10 minutes.
However, there are still science and engineering challenges to overcome before ultra fast-charging EV batteries are both technically feasible and affordable. And some experts question whether EVs that can be charged so quickly are really the future we want—at least with the electric grid we have now.
Charging up
The batteries inside today’s EVs are composed of thousands of lithium-ion cells with the ability to store and release energy thousands of times. Each of those cells consists of two electrodes—a metal cathode and a graphite anode—separated by a liquid electrolyte. While the battery is charging, lithium ions flow through the liquid from the cathode to the anode, filling up spaces between the graphite layers like wooden blocks fitting into a Jenga tower.
The speed at which lithium ions move from the cathode into the anode dictates how quickly the battery charges. But just as placing blocks in a Jenga tower hastily can cause the structure to become unstable, if lithium is forced into the anode too fast, problems start to arise.
At high charging speeds lithium batteries can overheat, causing them to degrade over time. More problematically, lithium can start to build up on the surface of the anode instead of entering it, a phenomenon known as lithium plating. Not only can that drastically reduce the battery’s capacity, the lithium deposits eventually form filament-like structures known as dendrites. Once they start forming, those dendrites can grow across the electrolyte, touch the cathode and create a short circuit, causing the battery to catch fire or explode.
“Obviously that’s not particularly good from a safety point of view,” says Peter Slater, a professor of materials chemistry at the University of Birmingham in the U.K.
Because of the problems with fast charging, all EV batteries have built-in charging speed limits, set by the car’s on-board charge ports. A 350-kilowatt fast charging station—the most powerful public charger available in the U.S. today—might, in theory, be able to charge an Audi E-tron SUV’s 95 kilowatt-hour battery in about 16 minutes. But the battery itself can only accept about 150 kilowatts of power at most, placing its actual charging speed limit closer to 40 minutes.
Exactly how fast a battery will recharge in the real world depends not only on the charger or how many kilowatts of power the battery was designed to accept, but the battery’s size, how charged it is, and even the weather. Still, state-of-the art fast charging stations can often get an EV battery 80 percent full, potentially adding hundreds of miles of range, in about 30 minutes. (Once a battery is 80 percent full, the charging speed slows down to prevent the battery from being damaged.) Tesla owners can visit a supercharging station that will add up to 200 miles of range in 15 minutes.
An ultra-fast charging future?
While adding 200 miles of range in 15 minutes is fast, it’s a far cry from gassing up for a road trip in five minutes flat. Those hoping for an EV charging experience like that might want to hold out for the next generation of battery technologies.
One way to make a lithium-ion battery that can safely charge even more quickly is to use alternative anode materials. For instance, the U.K.-based startup Echion technologies has developed a niobium anode that doesn’t promote lithium plating or dendrite formation. Batteries made with this material can be charged “as fast as you want,” says CEO Jean De La Verpilliere. His prototype EV battery cells can be charged in six minutes “without impacting the safety or life of the battery,” he says.
However, that quick charge comes with a price: Niobium anodes store less energy per unit mass than conventional graphite ones. Because EV makers tend to prioritize energy-dense batteries (which can be driven longer on a single charge) over ultra-fast charging ones, Echion is currently targeting other markets for its batteries, like grid storage and power tools. Eventually, De La Verpilliere envisions that a version of these batteries might be used in vehicle fleets where any downtime to recharge costs the company money.
For individual drivers looking for a bigger jolt of kilowatts, emerging solid state battery designs offer promise. In such batteries, the lithium ions flow through a solid electrolyte, often a ceramic, rather than a liquid one. Because liquid electrolytes are flammable, this makes the battery safer. It also opens up the possibility of using different anode materials that are more resistant to lithium plating and can therefore be charged faster.
Solid Power, a company that is developing solid state batteries with funding from BMW Group and Ford, is working on a silicon anode battery cell that chief technology officer Joshua Buettner-Garre says can be charged halfway in 15 minutes, and it’s targeting 20-minute full recharge rates for a commercial version. It’s also developing batteries with lithium metal anodes, which can store ten times more energy per unit mass than graphite.
In a solid state design, lithium metal batteries should in theory be able to charge up very quickly. In practice, though, they too are prone to forming dendrites, causing them to fail quickly, especially at high charge speeds. Fast-charging lithium metal batteries would be the Holy Grail of high performance EVs batteries, but they are “still a work in progress,” Buettner-Garre says.
New research may be pushing these super batteries closer to reality. Recently, a team led by Harvard University materials scientist Xin Li designed a solid state lithium metal battery cell that uses several different layers of materials in the electrode to arrest lithium dendrite growth. In the journal Nature, the team described a prototype battery that could be charged in just three minutes, while retaining more than 80 percent of its capacity after 10,000 cycles. (Typical EV batteries degrade by a similar amount after 1,000 to 2,000 cycles.)
The research is still at an early stage. The team needs to demonstrate that the battery, currently the size of a coin, can be scaled up and mass-produced for automobiles.
Li says that a commercial version of this battery may be possible in about five years “if everything goes right.”
If the advantages of lithium metal can be harnessed, says Venkat Viswanathan, an engineer at Carnegie Mellon University whose lab also develops next-generation batteries,“a lot of the assumptions that you have made in terms of fast charging actually go out the window.”
Social speed limits
Even if EV batteries that can charge in less than 10 minutes are technically possible, it’s not clear that ultra-fast charging will ever be practical. At 400 volts and higher, today’s fast charging stations already draw much more power from the electric grid than the 120- and 240-volt outlets many EV owners use at home. If all Americans were driving EVs and everyone expected ever-faster charging to be available all the time, that could place some serious strain on the grid.
“There is another layer of consideration on the infrastructure,” Li says. “We have to see how much current the entire system can support.”
Buettner-Garrett says there’s a balance to be struck “both at the social impact level, and also at the charger level, to hit the right combination of convenience and cost.” EV makers, he says, recognize this and are eyeing 20- to 30- minute charge times for cars released in the mid-2020s.
Jenny Baker, a battery storage expert at Swansea University in the U.K., isn’t sure ultra-fast charging is the right goal . Charging up at home overnight when demand is lower, she notes, is more affordable and environmentally friendly, since grid operators have to draw less on backup power plants, which tend to burn dirtier fuels. Many EV owners, Baker included, also find that more convenient than stopping to recharge during the day.
“Charging at home, if you've got the ability, is the best for the environment,” Baker says. “I would be very disappointed if electric vehicles [become] just like gas cars, because that won't fulfill all of their potential.”
Will charging electric cars ever be as fast as pumping gas? - National Geographic
Read More
No comments:
Post a Comment