Rechargeable batteries perform poorly when it’s cold out. Now researchers have designed a new lithium-ion battery that still works at –70 °C. Such batteries could improve the performance of electric cars in winter, and help power high-altitude machinery, space stations, and planetary rovers.
On cold winter days, electric vehicles can lose half their driving range due to poor battery performance. At –40 °C, lithium-ion batteries retain just 12% of their capacity. So in the Arctic, at high altitudes, and in space, rechargeable batteries must be insulated and heated, or non-rechargeable batteries or supercapacitors must be used instead.
Lithium-ion batteries work poorly in extreme cold because their electrolyte solvents become viscous or even freeze, which hampers the movement of lithium ions between the anode and cathode during charging and use. Also when it’s cold, the ions can’t make their way inside the graphite anode—a process called intercalation. Instead, the lithium ions plate the electrode’s surface with flammable lithium metal.
To make a rechargeable battery that would operate safely and maintain performance in the extreme cold, Yongyao Xia, a physical chemist at Fudan University, selected ethyl acetate as a cold-tolerant electrolyte solvent. Ethyl acetate’s freezing point is –84 °C, and it doesn’t become viscous when it’s cold. A previous paper showed that ethyl acetate could be used in a lithium-ion battery, but those researchers didn’t explore its use at low temperatures. So Xia gave it a try. But when his team combined the solvent with conventional electrodes, the battery still performed poorly at low temperatures.
Xia’s group next combined the solvent with electrodes made of organic materials instead of the conventional inorganic ones. When the battery charges, the polyimide anode material undergoes a reaction that allows lithium ions to bind to it, while counter anions absorb onto a polytriphenylamine cathode. When the battery discharges, the reaction goes in reverse, lithium ions get released, and the anions desorb. The resulting organic battery works from 50 °C down to –70 °C. And at –70 °C, the battery maintains 70% of its room-temperature storage capacity.
Maintaining performance over such a wide temperature range is impressive, says Shirley Meng, a materials scientist at the University of California, San Diego. However, the Fudan battery works at only 1.2 V, which is a relatively low voltage, she says.
Xia says his group is further tailoring the electrolyte and electrode materials to improve performance.