While batteries can operate in relatively cold climates, most perform at only 50% of their optimal level when the temperature hits -20ºC, and by -40ºC lithium-ion batteries only have about 12% of their room temperature capacity. This can be severely limiting when it comes to operating batteries in space, where temperatures can dip to -157ºC, or even in parts of Canada and Russia, where temperatures can be lower than -50ºC.
The team from Fudan University in Shanghai found a design that can function even where other batteries might fail. "It is well known that both the electrolyte and electrodes have great influence on the battery performance," says Dr. Yong-yao Xia, a battery researcher at the Department of Chemistry at Fudan.
When it gets cold, the conventional electrolytes that lithium-ion batteries often use become sluggish conductors. Instead the team used an ester (ethyl acetate)-based electrolyte which has a low freezing point that enables it to conduct a charge even at extremely low temperatures. For the electrodes, they used two organic compounds--a polytriphenylamine (PTPAn) cathode and 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA)-derived polyimide (PNTCDA) anode. Unlike the electrodes used in lithium-ion batteries, these organic compounds don't rely on intercalation--the process of continuously integrating ions into their molecular matrix, which slows down as the temperature drops.
"Benefitting from the ethyl acetate-based electrolyte and organic polymers electrodes, the rechargeable battery can work well at -70ºC," said Xia.
Xia and his team believe this may be a more elegant solution than other approaches such as additives that heat the batteries or by using a liquefied gas electrolyte. "Compared to the transition-metal-containing electrodes materials in conventional lithium-ion batteries, organic materials are abundant, inexpensive, and environmentally friendly," he said. He estimates the price of the electrode materials at about one third of the price of electrodes in a lithium-ion battery.
However, the specific energy is still low compared with commercialized lithium-ion batteries, and the assembly process needs to be further optimised. "But even though it has low specific