­ Li-ion Batteries Come in from the Cold

One of the main drawbacks of Lithium ion batteries is that their storage capability is diminished in colder temperatures. Its a phenomenon that happens with many different types of battery, but Li-ion batteries are the currently the most widely used type of battery, and the applications in which Li-ion batteries are used also mean that the effects of cold are very noticeable, for example the drop in range in electric vehicles as the outside temperature drops, or a cell phone with much less operating time. In extreme temperatures, the batteries can even stop working completely.

The effect is caused by the change in the internal resistance of the battery. It is not only the efficiency of the battery storage that is affected by cold. The cold also damages the materials used in the batteries. This damage can include permanent structural damage to the materials and permanent loss of active material.

However, progress is being made to try to improve the electrical performance of Li-ion batteries in extreme cold conditions. Reporting in ACS Central Science, a publication from American Chemical Society, researchers have replaced the graphite anode in the Li-ion battery with a bumpy carbon-based material, which is capable of retaining its rechargeable storage capacity down to -31^oF. The flat orientation of graphite in the anode was discovered to be responsible for the drop in the battery’s energy storage capacity in the cold. Xi Wang, Jiannian Yao and colleagues modified the surface structure of a carbon-based material to improve the anode’s charge transfer process.

The new material was created by heating a cobalt-containing zeolite imidazolate framework (known as ZIF-67) at high temperatures. The resulting 12-sided carbon nanospheres had bumpy surfaces that showed excellent electrical charge transfer capabilities. The team then tested the electrical performance, using the new material as the anode, and lithium metal as the cathode, inside a coin-shaped battery. The new anode provided stable charging and discharging at temperatures from 77^oF to -4^oF and stored 85.9% of the room temperature energy storage capacity just below freezing. In comparison, Li-ion batteries made with other carbon-based anodes, including graphite and carbon nanotubes, held almost no charge at freezing temperatures. When the researchers dropped the air temperature to -31 F, the anode made with bumpy nanospheres was still rechargeable, and during discharge, released nearly 100% of the charge put into the battery. Using the bumpy material in Li-ion batteries has the potential to open up the possibility of using them at extremely low temperatures.

The authors were funded by Fundamental Research Funds for the Central Universities (China), the National Natural Science Foundation of China, the Ministry of Science and Technology of China, the Science and Technology Project of Guangdong Province, the Chemistry and Chemical Engineering Guangdong Laboratory and Beijing Jiaotong University.

https://www.acs.org/