Why does heat shorten the battery capacity

Electric cars: charging in just ten minutes?

Hardly slower than refueling: Researchers have developed a technology that could rapidly accelerate the charging of electric cars. In just ten minutes, lithium-ion batteries can charge enough power for a range of up to 480 kilometers. This is made possible by the rapid, brief heating of the battery during rapid charging. This prevents the formation of obstructive lithium deposits in the battery and thus maintains the performance, as the scientists report.

Electromobility is an important building block for future traffic concepts. But so far there has been a lack of acceptance of electric cars. In addition to the lack of charging stations, this is primarily due to the short range and the long time it takes to recharge. Because while filling up with petrol is done in a few minutes, you can get away with electricity in less than an hour - even with fast charging stations.

Charging power limited - still

The problem: The lithium-ion batteries built into electric cars can only tolerate a limited amount of charging power. If this threshold is exceeded, extensive lithium deposits form on the graphite anode, which severely impair the functioning of the battery. "One of the main symptoms of this lithium build-up is the drastic drop in capacity," explain Xiao-Guang Yang and his colleagues from Pennsylvania State University.

However, in order to reduce the charging time of an electric car to around ten minutes, the batteries would have to withstand a charging power of 400 kilowatts - this has so far been impossible. “No current electric car battery can withstand that,” say the researchers. They were therefore looking for a method to prevent lithium build-up even at high charging capacities - and they found what they were looking for.

Almost full range in ten minutes

The solution seems astonishingly simple: "The key lies in an asymmetrical modulation of the temperature," explain Yang and his colleagues. Because if the lithium-ion batteries are quickly and briefly heated to around 60 degrees while charging, the lithium will not be deposited over a large area - even with 400 kilowatts of charging power. This means you could fill up with enough electricity in ten minutes to travel 320 to 480 kilometers in an electric car.

However, there is a catch: the heating must be quick and, above all, briefly to prevent other damage to the battery, as the researchers explain. Because if the heat lasts longer, the structure of the electrolyte changes and this in turn also affects the functionality of the battery. However, if you try to bring the battery to the right temperature from the outside, heating it to 60 degrees alone takes almost ten minutes.

Flash heating for batteries

So what to do The researchers solved this problem with a heater built into the battery itself. To do this, they embedded a thin nickel foil in the battery cell. "One end of the foil is soldered to the anode and thus the negative pole, the other end protrudes from the cell and forms a third pole - the activation pole (ACT)," report Yang and his team. When fast charging begins, the ACT pole is short-circuited with the positive pole of the battery and the current flows almost exclusively through the nickel foil.

The nickel foil heats up strongly within a few seconds and heats the battery to 60 degrees in less than a minute, as tests with appropriately modified electric car batteries have shown. If the battery is then hot enough, the nickel circuit is disconnected and the charging current is now completely used to charge the battery. As a result of this hot charging, the battery itself can handle 400 kilowatts without forming lithium deposits, as the researchers report.

The key here: once charging is complete, the battery quickly cools down to room temperature and stays cool even when discharging. This prevents the structure of the electrolyte from changing.

Stable even after 2,500 charging cycles

The first tests of this method were successful: A common battery for plug-in hybrid cars with 9.5 ampere hours and an energy density of 170 watt hours per kilogram could be charged up to 80 percent in ten minutes. After 1,700 charging cycles, it had only lost 20 percent of its capacity. An electric car battery with 209 kilowatt hours per kilogram even retained 91.7 percent of its capacity after 2,500 rapid charging cycles, as the researchers report.

“This technology not only allows us to charge quickly, it also only exposes the batteries to elevated temperatures for a short period of time - this extends their lifespan,” says senior author Chao-Yang Wang. Because even with normal, hour-long charging, the temperature in the battery rises, which means that its capacity decreases over time. In some models, external cooling is therefore used to keep the temperature of the car batteries.

The next goal is five minutes

"Overall, the great advantage of asymmetric temperature modulation (ATM) is that it offers a solution for accelerating electrochemical processes, but still effectively minimizes degradation of the material," the researchers state. According to their calculations, the method would also be financially worthwhile: the installation of the nickel foil makes the battery 0.47 percent more expensive, but at the same time there are no costs for battery cooling, as they explain.

The researchers are convinced that this charging technology could enable real quick charging of electric cars. The stay at the charging station would then hardly take longer than drinking a cup of coffee. But Wang and his team have even more ambitious goals: "We are working on charging an energy-dense battery in just five minutes without harming it," said Wang. "Because fast charging is the key to the success of electric vehicles." (Joule, 2019; doi: 10.1016 / j.joule.2019.09.021)

Source: Cell Press

October 31, 2019

- Nadja Podbregar