The initial high hopes surrounding molybdenum disulfide’s (MoS2) potential in electronic applications were tempered somewhat when it was revealed that MoS2 contained traps—impurities or dislocations that can capture an electron or hole—that limit its electronic properties. Since then the two-dimensional material has been investigated for other applications, one of the most promising of which has been for use on the electrodes of lithium-ion (Li-ion) batteries where some research has indicated that it has three times the theoretical capacity of graphite.
However, even here MoS2 does not come without some challenges. Most notable among the problems with MoS2 anodes is the speed at which they begin to degrade and the low rate at which they discharge.
Now researchers at Nanyang Technological University in collaboration with a team at Hanyang University in South Korea have developed a solution that addresses these issues by using tubular structures of MoS2 that have been wired together by carbon nanotubes to enhance conductivity.
In research described in the journal Science Advances, the international team developed a new way of synthesizing tubular molybdenum disulfide.
The researchers claim that this approach is better than other proposed strategies for improving the electrochemical performance of molybdenum disulfide-based electrodes because it not only has high specific capacity (ampere-hours per unit of mass), but also outstanding rate capability (charging and discharging rate), and an ultra-long life of up to 1,000 cycles.
In addition to it imparting better performance characteristics, the researchers say that the process of synthesizing the CNT/MoS2 tubular nano-hybrids is simple and easily reproducible.
The process involves embedding the CNTs into polymer (polyacrylonitrile, or PAN) nanofibers through an electrospinning method. During the electrospinning process, the CNTs align along the lines of the electrospinning solution. This results in a flexible tube-in-fiber structure with CNTs aligned in polymer nanofibers. A protective layer of cobalt sulfide is grown onto the CNT/PAN tube-and-fiber composite to prevent damage to the structure during the synthesis process and maintain the one-dimensional (1D) morphology of the final materials.
The next step in the process involves growing ultrathin MoS2 nanosheets on the composite and simultaneously completely removing the PAN. This step forms the tubular MoS2 structure. Then, the composite is heated at 800°C for 2 hours to increase the crystallinity of MoS2.
Perhaps the greatest appeal of this approach is that by systematically changing some of the characteristics of the tubular structures—such as the shell thickness of the tubes and the number of carbon nanotubes inside each tube—it’s possible to change the electrochemical performance of the electrode. It is this adjustability that could make it adaptable to other materials for the development of high-performance Li-ion battery electrodes.