Image credit: BBC
Imagine having a car with a small petrol tank, and it’s slowly shrinking after each fill-up. That’s how EV users must feel, if they know how their batteries behave. A new study analyses the processes.
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When lithium ions are forced rapidly through a battery, they might get stuck and turn into lithium metal, no longer able to move through the battery, says TechXplore.
Imagine being able to refuel your electric car while stopping for a quick snack or refill your phone while brushing your teeth.
“Fast charging is kind of the Holy Grail. It is what everyone who owns a lithium ion battery based device wants to be able to do,” says Senior Engineer David Wragg from Centre for Materials Science and Nanotechnology at the University of Oslo.
Inside the battery, however, there is a lot of complicated chemistry that can be sensitive to how fast it is charged. Things can go wrong.
“Capacity loss is the most critical one,” Wragg says to Titan.uio.no.
“It is possible to make batteries with very high capacity that might allow you to drive your electric car 1000 km, but after you’ve charged and discharged it a few times, you would lose about half of that capacity and range.
All rechargeable batteries deteriorate over time, but this negative effect is extra strong when the battery is subjected to fast charging. Wragg is one of the researchers behind a study that shows why.
They have been able to see that the lithium ions, which are so important for the capacity of a battery, are converted into pure lithium metal and are no longer useful. And most importantly: this effect is greatly enhanced by fast charging.
The battery is like a rocking chair
On one side of the battery is the anode, and on the other side is the cathode. Both of these electrodes can store electrons and ions. Between them is a separator and a liquid electrolyte that helps the ions from one side to the other.
Ions and electrons move from one side of the battery to the other when you use the current stored there and back again when you recharge it.
“They call this the rocking chair mechanism, where you rock the irons and the electrons from one side to the other. ”
“When they’re fresh and they’re working perfectly, batteries can store a certain amount of ions, and that’s the total capacity of the system,” Wragg says.
When the ions, which used to move back and forth, turn into metal, they are no longer able to move through the battery. The ions are charged and can be lured back and forth. The metal atoms are neutral, and can not be tempted in either direction.
“Once lithium is turned into metal it’s not really accessible for the electrochemical reaction anymore. This capacity is completely lost,” Wragg says.
This happens in all rechargeable lithium-ion batteries when you have charged them many enough times. But why does it get worse when you charge fast?
Bottlenecks during fast charging
During fast charging, the same number of ions move through the system, but much faster. All ions must find their place in the anode in a much shorter time.
“When you charge at double speed, you have to move the same amount of ions and electrons in half the time,” Wragg says.
If you charge four or six times as fast, it will naturally be even more difficult.
“It is difficult because there are certain limits on the chemistry that is going on when the you try put lithium ions into a solid electrode material really fast,” Wragg says.
Continued here.
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October 10, 2021 at 08:06AM
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