Jan 1, 2026 08:47 PM
https://cen.acs.org/energy/energy-storag...eb/2025/12
KEY POINTS: Electric vehicle driving range drops by 25–40% at temperatures below 5 °C as chemical reactions in the battery become sluggish. Improving cold-weather performance often comes at the expense of energy density, lifetime, and other factors. Researchers are trying various methods to improve battery performance at extreme temperatures, which could also increase charging speed.
EXCERPT: . . . Solvents with lower freezing points conduct ions better at lower temperatures. Ester- and ether-based solvents fit the bill, and researchers have tried to add them as cosolvents with ethylene carbonate. Many car companies use ether cosolvents in newer battery packs, Yang says. “Every company has their own secret sauce, but we’re still stuck with carbonate solvent as the main component, with added ether-based solvent.”
Manufacturers can’t add too much of the lighter cosolvents, since they are more volatile and pose safety risks at high temperatures. They also form an unstable interface with the graphite anode. During the first few charge cycles of a battery, a small amount of electrolyte decomposes at the anode surface to form a solid electrolyte interphase (SEI), a thin film that allows lithium ions through but keeps out solvents. Without a stable SEI, the anode degrades and battery life declines.
Fluorinated ester–based solvents might be a solution. They are nonflammable and form a stable SEI, says Zhengcheng “John” Zhang, a senior chemist at Argonne National Laboratory. But not all fluorinated ester solvents work well. Zhang and his colleagues have found that you need a high degree of fluorination close to the ester group. Out of several fluorinated ester solvents they tested, 2,2,2-trifluoroethyl acetate worked best, storing and releasing as much charge at −40 °C as a carbonate solvent does at room temperature over the course of 400 charge cycles.
The University of Maryland’s Wang says it’s crucial to develop electrolytes that work over a wide temperature range. “We can’t just talk about low temperature,” he says. “In the summer you want to operate at high temperatures. So you want a small change in conductivity [over] a wide temperature range, from –40 to 60 °C. That is a challenge.”
He believes the remedy is a more inorganic SEI. The SEI typically contains organic and inorganic components. Organic components dissolve back into the electrolyte at higher temperatures, Wang says. “If you have more inorganic lithium fluoride, the solubility will not change too much with temperature. Then the SEI is more stable.”
This turns conventional wisdom on its head. The battery community believes that the SEI needs to be lithiophilic, or attract lithium, to reduce interfacial resistance. But this can also lead to metallic lithium depositing on the anode to form tiny spikes called dendrites that shorten battery life. Inorganic LiF, on the other hand, is lithiophobic. Wang believes that a lithiophobic SEI is better because lithium ions slide along the interface without penetrating it, like water droplets on nonstick coatings. “I suggest to the battery community that you need a lithiophobic SEI layer,” he says.
By carefully choosing a fluorine-containing lithium salt and ester solvent, his group has designed an electrolyte that forms an inorganic LiF-rich interface and remains conductive over a wide temperature range of –60 to 60 °C. The resulting battery retains 80% of its capacity after being recharged 400 times (Nature 2023, DOI: 10.1038/s41586-022-05627-8).
A team from Tsinghua University and the Beijing Institute of Technology took a different approach... (MORE - missing details)
KEY POINTS: Electric vehicle driving range drops by 25–40% at temperatures below 5 °C as chemical reactions in the battery become sluggish. Improving cold-weather performance often comes at the expense of energy density, lifetime, and other factors. Researchers are trying various methods to improve battery performance at extreme temperatures, which could also increase charging speed.
EXCERPT: . . . Solvents with lower freezing points conduct ions better at lower temperatures. Ester- and ether-based solvents fit the bill, and researchers have tried to add them as cosolvents with ethylene carbonate. Many car companies use ether cosolvents in newer battery packs, Yang says. “Every company has their own secret sauce, but we’re still stuck with carbonate solvent as the main component, with added ether-based solvent.”
Manufacturers can’t add too much of the lighter cosolvents, since they are more volatile and pose safety risks at high temperatures. They also form an unstable interface with the graphite anode. During the first few charge cycles of a battery, a small amount of electrolyte decomposes at the anode surface to form a solid electrolyte interphase (SEI), a thin film that allows lithium ions through but keeps out solvents. Without a stable SEI, the anode degrades and battery life declines.
Fluorinated ester–based solvents might be a solution. They are nonflammable and form a stable SEI, says Zhengcheng “John” Zhang, a senior chemist at Argonne National Laboratory. But not all fluorinated ester solvents work well. Zhang and his colleagues have found that you need a high degree of fluorination close to the ester group. Out of several fluorinated ester solvents they tested, 2,2,2-trifluoroethyl acetate worked best, storing and releasing as much charge at −40 °C as a carbonate solvent does at room temperature over the course of 400 charge cycles.
The University of Maryland’s Wang says it’s crucial to develop electrolytes that work over a wide temperature range. “We can’t just talk about low temperature,” he says. “In the summer you want to operate at high temperatures. So you want a small change in conductivity [over] a wide temperature range, from –40 to 60 °C. That is a challenge.”
He believes the remedy is a more inorganic SEI. The SEI typically contains organic and inorganic components. Organic components dissolve back into the electrolyte at higher temperatures, Wang says. “If you have more inorganic lithium fluoride, the solubility will not change too much with temperature. Then the SEI is more stable.”
This turns conventional wisdom on its head. The battery community believes that the SEI needs to be lithiophilic, or attract lithium, to reduce interfacial resistance. But this can also lead to metallic lithium depositing on the anode to form tiny spikes called dendrites that shorten battery life. Inorganic LiF, on the other hand, is lithiophobic. Wang believes that a lithiophobic SEI is better because lithium ions slide along the interface without penetrating it, like water droplets on nonstick coatings. “I suggest to the battery community that you need a lithiophobic SEI layer,” he says.
By carefully choosing a fluorine-containing lithium salt and ester solvent, his group has designed an electrolyte that forms an inorganic LiF-rich interface and remains conductive over a wide temperature range of –60 to 60 °C. The resulting battery retains 80% of its capacity after being recharged 400 times (Nature 2023, DOI: 10.1038/s41586-022-05627-8).
A team from Tsinghua University and the Beijing Institute of Technology took a different approach... (MORE - missing details)