Scientists have developed rechargeable batteries that can operate at temperatures low enough to operate in hostile environments such as Antarctica or the warmer regions of Mars.
The performance of current lithium batteries declines below minus-20 degrees Celsius, dwindling to 12% by minus-40. This means, for example, that the batteries that power the Mars Rovers need to be heated to keep them from grinding to a halt.
However, research published in the journal Joule unveils a lithium battery design with good performance as low as minus-70 degrees Celsius. The design substitutes conventional metal electrodes for organic polymers and employs a low-freezing-point electrolyte – the chemical which carries the current within the battery.
The research team, led by Yong-yao Xia and Yonggang Wang from Fudan University in China, used a cathode (a negatively charged electrode) made out of polytriphenylamine and a positively charged anode made of snappily named 1,4,5,8-naphthalenetetracarboxylic dianhydride-derived polyimide. As an electrolyte the researchers used ethyl acetate, an ester with a freezing point of minus-84 degrees.
“Compared to the transition-metal-containing electrode materials in conventional lithium-ion batteries, organic materials are abundant, inexpensive, and environmentally friendly,” says Xia.
The organic polymer electrodes perform well at low temperatures because they use a different process to that used by metal-based electrodes for storing lithium atoms.
Metal-based electrodes integrate the lithium into the structure of the metal as they charge up, a process called intercalation. As they discharge, the lithium is released, but this slows to a snail’s pace at low temperatures.
Polymers, on the other hand, absorb the lithium onto their surface and into gaps between molecules, a fast process that is not as strongly affected by low temperatures.
As the temperature dropped, the performance of the team’s battery stayed unchanged to around minus-40 degrees. At minus-55 it performed at up to 90% of room temperature performance and at minus-70 was still strong, at up to 70% of room temperature performance.
Despite the polymer-based battery’s good performance, Xia cautions that it is no match for commercialised lithium batteries yet, because of its low specific energy (the energy per unit of mass). He suggests the design be used in combination with conventional technology “as an auxiliary power source to provide short-term power for the start-stop process”.