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A team of researchers at the University of Michigan has developed a new type of lithium-sulfur battery. To do this, they harnessed the properties of aramid nanofibers to significantly improve the performance of standard lithium-sulfur batteries.
The battery thus designed approaches, according to scientists, the theoretical limits of a battery’s capabilities. Its useful life would also be increased, to theoretically reach more than 3,500 charge cycles. In a real situation, however, it would probably be less, they admit, explaining that we would then prefer to go closer to 1000 cycles. As a reminder, the maximum number of “cycles” for a battery represents the number of times it can be fully charged and fully discharged before it fails or loses storage capacity.
It would also be resistant to heat, up to 80°C, making it an ideal candidate for developments in the field of electric vehicles, for example. In a statement, the University of Michigan says it would be capable of quintupling the range of electric cars. Thus, their new battery system appears to solve several recurring problems with existing lithium-sulfur batteries.
As the scientists point out in their article, published in the journal Nature Communications, “the high theoretical specific capacity of 1675 mAh g-1, respect for the environment and the terrestrial abundance of elements that make up lithium-sulfur batteries make them an attractive platform for energy.” storage in a variety of technology areas ranging from electric vehicles to robotics, power grids and aerospace engineering.” However, this type of battery also faces several key issues that greatly hamper its capabilities.
A protective membrane that filters molecules.
First, the “dendrites”. In a battery, dendrites are small outgrowths that form from the deposition of lithium, during cycles of discharge and recharge. They can cause overheating or even short circuits when they grow large enough to bridge the anode and cathode (the “+” and “-” terminals of a circuit).
This diagram shows how lithium ions can return to the lithium electrode while lithium polysulfides cannot cross the membrane separating the electrodes within the new battery architecture. Furthermore, the sharp dendrites of the lithium electrode cannot short-circuit it by piercing the membrane and reaching the sulfur electrode. © Ahmet Emre et al./Kotov Laboratory
The research team had previously managed to overcome this problem by using a separating membrane made of aramid nanofibers. In fact, the strength of these fibers is capable of preventing the formation of dendrites. The scientists specify that the “pores” left by this material are 1 nanometer, while the dendrites are 300 nanometers: a difference that also effectively blocks their development. This membrane has a structure close to cartilage, which inspired its use.
Another problem: the diffusion of lithium polysulfides from the cathode to the anode drastically reduces their capacities. In fact, the working principle of a lithium-ion battery is ion exchange. Therefore, the lithium ions must be able to pass freely from the cathode to the anode and vice versa. Only they are not the only ones who walk on the drums. Small lithium and sulfur molecules, called lithium polysulfides, also form and flow into the lithium. They stick to it and thus reduce its capacity.
To counteract this, the scientists once again rely on their famous aramid nanofiber membrane. First, the 1 micron pores provide very narrow “passages” and reduce the ability of polysulfides to slide. But that is not all. Scientists also charge the membrane by taking advantage of the properties of polysulfide molecules. These, negatively charged, tend to bind to the membrane. They thus form an additional “layer” that repels other molecules of the same type, while lithium ions, which have a positive charge, can pass through it without any problem.
Recycle bulletproof vests
This membrane, arranged in successive layers, is quite thin. Another advantage according to scientists, since it leaves more space for the other parts of the battery, which allows its capacity to be increased. Finally, according to them, aramid nanofibers are quite easy to manufacture, or even to recycle from Kevlar objects, such as bulletproof vests.
“Reaching record levels for various parameters and for various material properties is what is needed now for car batteries,” said Nicholas Kotov, who led the research. “It’s a bit like gymnastics for the Olympics: you have to be ‘perfect’ in every way.”
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