How the ultrasonic fast charging of electric cars could go on

Posted by Christian Pfäffli on

Several companies have already built lithium-ion batteries that can be fully charged in a few minutes - but only on a laboratory scale. Their next goal is, therefore, to bring the pilot batteries into electric cars as standard equipment. Wired magazine has summarized the current state of research and development from a U.S. perspective in an in-depth article that we have reproduced here. At the end of last year, those responsible for Formula E already announced promising specifications for the third generation of their all-electric racing cars: From 2022, the Formula E racers are to use extremely fast-charging stations that offer enough power to fully charge the battery of a Tesla Model S in just ten minutes. Although the racing drivers will only use these charging stations for short pit stops, this technology already allows a glimpse into the future beyond the race track: electric car batteries that can be charged in about the same time it takes to fill a combustion tank.

There are already some really fast-charging stations. Electric cars from Tesla and Porsche can be almost completely charged within 30 to 40 minutes on ultra-fast chargers with up to 350 kW power. This is almost because above about 80 per cent of the battery's capacity, the chargers reduce their energy output in order not to put too much strain on the battery. This is also why it takes relatively longer to completely fill the battery. To fully charge a battery, it is more gentle to charge it overnight on a much slower wallbox.

"More than 50 per cent of the US population live in apartments, condominiums or houses without a charging facility of their own," says Matthew Keyser, head of the electrochemical energy storage group at the National Renewable Energy Laboratory (NREL), in an interview with US magazine Wired. "In order to increase the acceptance of electric cars, we need to provide a means to create rapid charging options for this segment of our society," the researcher explains the urgency for further developments in rapid charging.

Lithium Plating wants to be banned
However, increasing the charge rate of a lithium-ion battery involves compromises. During charging, lithium ions flow from the cell's cathode to its anode, which is typically made of graphite, a type of carbon. In simple terms, the anode functions like a bucket that collects and stores the ions while the battery is being charged. Although thicker anodes - larger buckets - can absorb more energy in the form of lithium ions, allowing electric cars to continue driving on a single charge, the anode is also able to store the ions while the battery is charging. However, thicker anodes also make quick charging more difficult, as the ions have to move further inside the anode. If the ions cannot penetrate the anode fast enough during a charge, a molecular jam occurs and the lithium accumulates on the surface.

This phenomenon, known as lithium plating, can affect the performance of a battery. And if too many ions pile up on the surface of the anode, branched branches can form that break through 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 wrote a review article on fast rechargeable lithium-ion batteries, says one possible solution to lithium plating is to add silicon to the anode, according to Wired. Silicon is cheap, abundant and can alter the crystal structure of the anode to make lithium plating less likely. "Silicon is particularly popular with manufacturers because it can also improve the energy capacity of the battery," adds Tomaszewska.

Silicon as a solution?
In fact, many companies, including Tesla, have already added silicon or silicon oxide to graphite anodes to squeeze a little more energy out of their lithium-ion cells. But Enevate, an energy storage company based in Southern California, wants to do without graphite altogether. Over the past 15 years, the company has perfected an extremely fast rechargeable lithium-ion battery with an anode made of pure silicon, Wired writes: Earlier this year, the company's researchers announced that the latest generation of batteries could be charged up to 75 per cent in just five minutes - without compromising energy density. "We can charge quickly without losing energy density because we use a cost-effective pure silicon approach," said Ben Park, founder and chief technology officer of Enevate.

Sceptics rightly treat the news like this with caution. After all, battery companies are known for announcing performance breakthroughs in test cells that never make it to market. However, according to Jarvis Tou, the company's Executive Vice President, Enevate's technology is unique in that the anode material can be easily integrated into existing battery manufacturing processes. According to Tou, Enevate is already in discussions with lithium-ion manufacturers to integrate Enevate's anode into commercial batteries. The first applications for the rapid-charging batteries will be in power tools, but Enevate is also working with automakers to use them in electric cars by 2024.

Other companies are also working to bring special fast-charging anode chemicals to market. StoreDot, an Israeli energy storage company, is developing an electric car battery that can be charged in less than ten minutes. And last month, researchers at Echion, an English battery start-up, claimed to have built a lithium-ion battery that can be charged in as little as six minutes using an anode made of mixed niobium oxide, which was nanotechnologically produced to transport lithium ions efficiently. "We designed the material to have a specific crystal structure," says Wired, according to Jean de la Verpilliére, CEO and founder of Echion. "You can think of it as small tunnels on a molecular scale that allow lithium-ions to travel very quickly to the anode."

"It's hard, but it's not black magic either"
These tailor-made batteries have not yet made it out of the laboratory into the real world. Producing lithium-ion batteries on a large scale is a challenge, and manufacturers must also be convinced to include new materials in their assembly lines. This is why companies like Echion and Enevate have given priority to the development of those anode materials that can be easily integrated into existing battery production processes. Both say they are in talks with battery manufacturers to integrate their anode material into commercial cells. "We are not trying to reinvent the wheel," adds de la Verpilliére. "It's difficult to go from lab discovery to a product - but it's not black magic either."

However, building a cheap fast-charging battery may not require any new anode chemistry at all. At NREL, mentioned at the beginning, Keyser and his colleagues are concentrating on the optimization of graphite anodes, which are currently widely used in electric cars. According to Keyser, the team uses computer models to optimize the paths taken by lithium-ions on their way through an anode by influencing the size and shape of the graphite particles.

Nanoengineering anode structures are difficult to implement on a large scale, but Keyser's team is also investigating solutions for fast-charging batteries in which the structure or chemistry of a battery anode does not need to be changed at all. For example, intelligent algorithms could be implemented at charging stations to ensure that a battery is not overcharged at any time during the charging process, which could lead to lithium plating. Tesla is already doing this to a certain extent: the charging stations and cars communicate in such a way that the charging station delivers the right power for the age and model of the car to be charged.

Fast-charging batteries will help overcome the limited range and long charging times generally cited as two of the biggest obstacles to the mass introduction of electric cars. They can also accelerate the electrification of other vehicles such as long-distance buses and trucks in long-distance transport. Both industries need vehicles that can run for many hours in a day while meeting tight schedules. In the case of buses, strategically placed rapid charging stations could also be used for recharging, for example, while waiting at a bus stop. Long-distance truckers would not need to schedule extra time for recharging if it took as long as filling a diesel tank.

Source: Wired - Charge a Car Battery in 5 Minutes? That's the Plan

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