With the development of micro-nanopore controlled separators, the implementation of organic batteries has been accelerated
The results show to be utilized in many fields such as in electric vehicles that require high energy density at low costs
From left, Professor Kisuk Kang of the SNU Department of Materials Science and Engineering,
co-first authors Songyan Bai and Byunghoon Kim
A research team led by Professor Kisuk Kang in the SNU Department of Materials Science and Engineering received attention from academia with their development of post-lithium secondary batteries. The research paper was published in the world-renowned journal of <Nature Nanotechnology> on November 2.
As the demand for sustainable renewable energy is experiencing an unprecedented increase, the development of the energy storage system (ESS) that stores renewable energy has emerged to become a topic of global interest. Lithium secondary batteries can be repeatedly used to charge energy and discharge of stored energy; and are widely used as ESSs due to their high energy density.
The team has developed a way to overcome the problem of declining life expectancy in organic secondary batteries'*, which is being recognized as the next-generation battery material. The results have significantly increased the possibility of commercialization in the future. It can be used not just for electric vehicles that require high energy density, but also for energy storage systems (ESS) with substantially lower costs. It is expected to be an alternative to current transition-metal*-based cells in the long run.
* Organic secondary battery: A secondary battery in which the cell is composed of organic molecules capable of redox.
* Transition metal: Metal elements corresponding to periods 4-7, groups 3-12 in the periodic table. They are capable of easily carrying out redox.
While organic secondary batteries were in the spotlight as an alternative to current battery chemistry based on transition metals due to the cheap and accessible resources, the issue of low life-expectancy was a major obstacle. The research team revealed that the fundamental cause for the declining life expectancy is due to the shuttle phenomenon, where dissolved organic electrode materials move to the opposite electrode.
In order to suppress the shuttle phenomenon, a micro-nanopore-controlled separating membrane was developed that could block the movement of organic electrode molecules while the lithium ions and electrolyte molecules passed through the cell. By introducing this structure between the positive and negative electrodes, they have succeeded in drastically improving the life expectancy of organic secondary batteries.
The research team found that a material called Metal-Organic Framework (MOF) consists of a number of uniformly and regularly spread channels within the grid structure and that the proper selection and combination of metal and organic units, which are the components of MOFs, allows the size control of the channels to be adjusted at a sub-nanometer level.
Through the separation membrane's nano-processing, they fabricated a zeolite imidazole framework-8 (ZIF-8) that can inhibit the movement of organic electrode molecules while allowing the salt molecules to pass through. Through the introduction of the MOF-gel separators, it was confirmed that the electrode capacity remains at a minimum of 80% even after 2,000 of long-term charge-discharge cycles. This demonstrates that when compared to the conventional organic secondary batteries that had been reported to drop by more than half of their capacity after just a few hundred cycles, the new organic battery flaunts superior energy efficiency and increased life-expectancy.
Since organic electrode materials are composed of carbon, hydrogen, and oxygen that can be easily obtained through biomass, problems with the acquisition of resources are greatly reduced. Also, because it contains a minimal amount of transition metals, the price is very low and the production cost can be adjusted and easily lowered according to the process development.
Eco-friendly, low-cost organic electrode materials are a very important alternative. The ability to solve the biggest technical challenges arising from novel concepts such as eco-friendly anode materials is of great significance to the scientific community.
Professor Kang commented on the work as "… an interesting research finding with the successful development and the possibility of micro-nanopore-controlled separating materials, which proves the possibility of organic secondary cells being developed as energy storage devices that can be charged and discharged over a long cycle at a lower cost. It is expected to provide a solid basis for future developments."
The research was conducted with the support of the Ministry of Science and ICT and the Creative Materials Discovery Program from the National Research Foundation of Korea (NRF) under the Ministry of Science and ICT. The results were published in the international journal of Nature Nanotechnology on November 2.
Selective control of organic electrode movements with MOF-gel separator
(a) A schematic showing the self-assembly of microporous MOF-gel separators (ZIF-8), and a schematic illustrating the molecular and ionic sieving process of the MOF-gel separator. A metal unit and an organic unit were combined to design and synthesize a MOF separator with nano-level movement passages.
A sol-gel process has been introduced to successfully synthesize MOFs with uniform, dense, and shock-resistant channels.
(b) The intrinsic channel size in the designed MOFs separator are smaller than those of large organic electrode molecules and larger than those corresponding to other solvents or salts. Thus, the dissolved organic electrodes can be prevented from moving to other electrodes while allowing the movement of molecules necessary for charging and discharging.
Dramatic improvement in organic secondary battery performance through the
introduction of MOFs separator film.
(a) (Left) The introduction of manufactured MOFs separator film into organic secondary batteries has dramatically improved the life expectancy of the battery. In the case of using pre-existing separator membranes, the battery capacity was reduced by more than half (-43.0%) within 60 cycles, while the introduction of the new MOFs separation membranes preserved ~82.0% of the battery capacity for 100 cycles.
(Right) It was found that in the case of using the existing separator, surface particles were non-uniform due to the reaction between the transferred organic electrode molecules and the cathode by observing the surface of cathodes after it is repeatedly charged and discharged. On the other hand, the introduction of MOFs separator membrane inhibited the movement of organic electrode molecules, maintaining a clean cathode surface.
(b) With the introduction of the manufactured MOFs separator, it was confirmed that most battery capacities (~82.9%) were preserved, even after long cycles of charging and discharging that took place 2000 times. Here, the capacity reduction per cycle was only 0.008%; this is an amazing result that reveals that long-term charging and discharging can be done in organic secondary batteries.
For further information, please contact Prof. Kisuk Kang (email@example.com).