Lithium-ion batteries are not only required to be able to maintain the standards of securityresist long charge and discharge cycles with the minimum degradation, in addition to the maximum energy density possible. They also need to support the operating temperatures in extreme climates, as it can be in extreme cold. In this case, the batteries charge more slowly and reduce their energy capacity, because they consume it quickly, which translates into long waiting times to achieve lower autonomies. A team of researchers has published in ACS Central Science his works related to this problem, which are based on replace conventional graphite anode of lithium batteries by one built on the basis of a carbon-based material with an irregular surfacewhich maintains storage capacity up to -35ºC.
Temperatures below zero have very negative effects on batteries. Their storage capacity is significantly reduced, which means a reduction in autonomy, they slow down charging, requiring drivers to wait longer. In very extreme conditions of low temperatures, lithium batteries may not be able to transfer any load.
Manufacturers work to avoid this detrimental effect by designing complex thermal management systems that try to heat the battery compartment so that the battery always operates in its optimal temperature range. These systems require temperature sensors and other equipment that take up space and increase the weight of electric cars, in addition to requiring energy to operate, which inevitably has to come from the battery. They also require complex calculation algorithms that are capable of detecting changes in temperature and accurately regulating the state of the battery container.
That’s why researchers are also working to come up with ingenious solutions to this problem. based on systems that reduce the energy available in the battery itself. One of them, presented last year, explored how low temperatures could be taken into account in battery design. Scientists determined that the planar orientation of the anode graphitethat is, its extraordinarily regular surface, is responsible for the drop in the energy storage capacity of a lithium-ion battery that works in extremely cold conditions.
With this background study, a group of Chinese scientists led by Xi Wang and Jiannian Yao worked to modify the surface structure of the anode using a carbon-based material that improved the load transfer process. In a battery, the cathode is made up of different chemical materials such as nickel, cobalt, manganese or aluminum and the anode is usually graphitewhich forms a complex structure on which the lithium ions traveling through the electrolyte separating the two electrodes are deposited.
Building on recent research suggesting that the flat nature of these anodes contributes to a reduced ability of the battery to transfer charges when ambient temperatures are very cold, the team started experimenting with some alternative materials until reaching a very promising solution.
The new anode is made from a material known as cobalt-containing zeolite imidazolate framework (ZIF-67), which was heated to high temperatures to create 12 sided carbon nanospheres. These tiny structures feature irregular surfaces that have excellent electrical charge transfer capabilities. These spheres were arranged to work as the anode material in a coin-shaped battery with a lithium metal cathode.
Laboratory experiments showed that the battery could maintain stability when charged and discharged at temperatures ranging between 25°C (77°F) and -20°C (-4°F). In temperatures just below freezing, the battery kept 85.9% of its storage capacity. These tests were carried out in parallel with other lithium-ion battery designs with conventional graphite anodes, and also with graphite and carbon nanotubes. In these batteries, the result was that virtually no charge recovery at sub-zero temperatures.
When the researchers lowered the temperature to -35°C (-31°F), the irregular nanosphere anode still maintained its ability to recharge, and during discharge released nearly 100% of battery power. The incorporation of irregular nanosphere material in lithium-ion batteries suggests that a design based on this type of material could extend the functionality of lithium-ion batteries to extreme environments.
The research has been supported by the Fundamental Research Funds for 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 Laboratory of Chemistry and Chemical Engineering of Guangdong, and Beijing Jiaotong University.