THRIVE IN THE COLD?
Thermal runaway and cold-start challenges plague lithium-ion batteries. Researchers believe they have found a way to avoid both.
Written by Michael Abrams
In cold weather, it can take longer than average to charge an electric vehicle.
THE COLD SNAP THAT CROSSED THE COUNTRY in January 2024 sent Chicago into not-so-uncommon below-zero temperatures. More uncommon, however, were lines of electric vehicles waiting for a turn at charging stations. EV owners discovered that it took hours to charge their cars, or worse, that the lithium-ion batteries in their cars might run out of juice altogether, leaving them in need of a tow.
That isn’t the only weather-related challenge for electric vehicles. When temperatures rise to over 85 °F, the range of an EV shrinks, and when things get hotter, the risk of fire and explosions shoots up.
For battery operated vehicles are going to compete with the internal combustion sort, the problems of cold-weather sluggishness and hot-weather volatility must be solved.
“Right now electric vehicles are shifting from early adopters or luxury buyers to mainstream customers, so we have to pay attention to those kinds of extreme conditions,” said Chao-Yang Wang, a professor of mechanical engineering at Penn State. “Each Tesla supercharger is ready to give you 250-kilowatts of power. So why would people have to be stranded and wait in line for hours, and then have to charge for another few hours? The electricity is there—it’s not like you don’t have energy—and you’re already plugged in. It’s just not acceptable. We need more innovative thermal management solutions.”
Wang, with Guangsheng Zhang and Siyi Liu from the department of mechanical and aerospace engineering at the University of Alabama at Huntsville, authored the paper “Challenges and Innovations of Lithium-Ion Battery Thermal Management Under Extreme Conditions: A Review,” which appeared in the Journal of Heat and Mass Transfer. In it, they outline the hurdles that must be overcome to push electric vehicles fully into the mainstream, as well as some possible solutions.
Designing a battery-operated vehicle that performs well in biting cold and remains safe in blistering heat, has been an intractable proposition. Range and power requires high energy density, but the higher the energy density, the more dangerous the battery.
“You want to maintain high temperature stability and safety, but, at the same time, enable low temperature, performance,” Wang said. “There’s no way you can maintain both requirements at the same time, so it’s a dilemma.”
Battery researchers have long talked about “trade-offs” as if solving the problems at one end of the extreme temperature spectrum necessarily creates problems at the other end. Wang has some ideas, though. The most important thing, he said, is to address the potential for fire in hot weather.
“Safety comes first, no matter what,” he said. “I mean, at low temperatures the worst thing to happen is you just don’t drive. If the car is frozen it’s not going to blow people up.”
What does blow people up are the highly flammable electrolytes that can evaporate quickly in hot summers. If those vapors catch fire, it’s catastrophic for the battery.
“In a gasoline car you can separate the fuel from the oxygen, but for the battery, in terms of heat generation, it starts from the inside and does not need air,” Guangsheng Zhang said. “Cooling it from the outside is not very effective.”
EV fires can last for 24 hours and even when put out, they can easily reignite—it’s why a growing solution is to completely submerge burning vehicles in containers of water.
“Safety comes first, no matter what. I mean, at low temperatures the worst thing to happen is you just don’t drive. If the car is frozen it’s not going to blow people up.”
– Chao-Yang Wang
ELECTROLYTE ALTERNATIVES
“The number one thing we have to do is to remove all those volatile solvents,” Wang said. That will likely mean turning to something other than traditional electrolytes.
One option is lithium bis(fluorosulfonyl)imide as the salt. LiFSI, as it is called, remains stable up to 140-odd degrees Fahrenheit and could be used with the solvent DMOHC which is stable at temperatures as high as 185 °F. “These are unthinkable levels of temperature in the past,” Wang said. Another advantage is that thermally more stable DMOHC could be potentially cheaper than the mainstream battery solvents.
LiFSI and other alternatives don’t have to leave cars sluggish and slow-charging in the depths of winter. Those issues could be dealt with by embedding a thin foil—only 10 microns thick—within the battery cells.
“It takes only 30 seconds or so to warm up,” Wang said. “After that you can use the same energy from the charger to charge the battery materials—so the whole process, from pre-heating to fast charging, would take around 10 minutes.” The feasibility of the technique was demonstrated with 400 electric buses used in the 2022 winter Olympics.
The same foil could be used as a sensor to give drivers ample warning if temperatures or pressures inside the battery get dangerously high. Currently, electric battery sensors sit outside of battery cells, far from the actual reaction surface, creating a dangerous lag if there’s a problem with the battery.
“If you’re just measuring the surface temperature of the battery you cannot detect what went wrong inside the cell quick enough,” Zhang said.
For the short term, these solutions seem ample. “Science-wise, it’s all very clear, very simple,” Wang said. “But we need industrial practitioners to really implement and develop such a vehicle.” Eventually, though, we may need something more radical.
“The battery has been around for two centuries,” Wang said. “Always with the same structure: an anode and a cathode sandwiching an electrolyte between. At this point, the scientific society needs to rethink this structure, whether or not these three layers will allow us to develop novel batteries to meet our ever-increasing challenges and requirements.”
Michael Abrams is a science and technology writer in Westfield, N.J.
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