Slow charging of electric cars and smartphones and the gradual decline in battery capacity are a well-known problem. Next-generation batteries with high energy density could speed up charging and extend service life, but during fast charging, deposits accumulate inside the cells, degrading performance and increasing the risk of fire. A group of researchers from Korea has proposed a technology that creates a kind of "expressway" for lithium ions.
A team led by Professors Dong Ki Yoon and Hye Ryeon Byeon of the Department of Chemistry at the Korea Advanced Institute of Science and Technology (KAIST) has developed a separator - a porous separating membrane - for lithium-metal batteries. The scientists learned to precisely control the paths along which lithium ions move by regulating the orientation of liquid crystal polymer molecules. The results were published in the journal ACS Nano.
Lithium-metal batteries can store more energy than conventional ones. However, during charging, lithium may deposit in the form of sharp branched structures - dendrites. They shorten service life and can damage the internal membrane, up to and including an explosion. The researchers used liquid crystals, which combine the order of a solid with the fluidity of a liquid. Surface treatment can change the orientation of their molecules, and this property was used to regularly align channels for lithium ions.
On the basis of this technology, a porous liquid crystal polymer separator with nanopores was created. Polyethylene glycol (PEG) chains - a polymer that facilitates the transfer of lithium ions - were incorporated into it. The scientists could freely set the vertical or horizontal direction of the molecules and compared how this affects ion transport.
It turned out that with vertical alignment of the molecules, the separator forms a kind of "highway" along which lithium ions move directly toward the electrodes. Compared with the horizontal version, the tortuosity of the ion path - an indicator of how much the route deviates from a straight line - decreased by about 2.5 times. The ions moved faster and more uniformly, which suppressed dendrite growth and markedly increased stability.
The new separator was tested in a real lithium-metal battery. Even under harsh fast-charging conditions at a high rate of 4C, after 100 charge-discharge cycles the cell retained 94% of its initial capacity. The C-rate reflects the charge and discharge rate: 1C corresponds to a full charge in about an hour, 2C in 30 minutes, and 4C in 15 minutes. In other words, this means charging in a quarter of an hour.
"The work shows that controlling the orientation of liquid crystal polymer molecules makes it possible to effectively design lithium-ion transport paths and thereby improve battery performance and stability," Yoon noted. "We expect that the approach will help the development of next-generation high-energy-density battery technologies, for which long range and fast charging are important."
This is a laboratory result, not a finished commercial product. The authors do not report a timeline for bringing the technology to market or the characteristics of future mass-produced batteries.