With rapidly increasing demand for electric cars and large-scale energy storage systems, there has been a need for improvement in the current lithium-sulfur (Li-S) battery to satisfy the growing demand. The research team led by Professor Do Kyung Kim from the Department of Materials Science and Engineering has succeeded in developing an Li-S battery that uses the concept of capillary action. Similar to paper soaking up water with capillary action, the battery holds sulfur in carbon nanofibers.
Compared to traditional lithium-ion batteries, Li-S batteries are promising in that they offer higher specific capacity and energy density — ideal for energy storage devices. Nevertheless, the commercialization of such batteries has been limited due to the low electrical conductivity of sulfur, the variations in volume following charging and discharging, and the diffusivity of polysulfide intermediates. Hence, there have been attempts to encapsulate the sulfur with different types of materials in order to improve conductivity and inhibit the dissolving of the polysulfide. However, the usage of zero-dimensional carbon materials that have been widely used for encapsulation entails a complicated process and resistance induced from numerous contact between particles.
In order to overcome such limitations, the research team used electric radiation to synthesize one-dimensional carbon nanofibers, which were then soaked in a slurry of solid sulfur and dried. This relatively simple approach succeeded in producing Li-S electrodes with significantly reduced contact resistance. The research team observed how the solid sulfur was converted into its intermediate, liquid lithium polysulfide, which positioned itself between the carbon nanofibers and maintained its position throughout the process of charging and discharging. Thus, the study demonstrated how the sulfur could be locked between the carbon fibers without complex encapsulation procedures.
A further implication of this investigation is that the research achieved high mass loading of over 10 mg/cm2, which is much higher than the previous 2 mg/cm2. Consequently, the design showed an increased areal capacity of 7 mAh/cm2. PhD candidate Jong Hyuk Yun, a lead author of the study, commented that: “In our work, the very well-known ‘capillary action’ has been introduced to Li-S batteries to increase a sulfur loading above what is practical. The interwoven carbon nanofibers effectively absorb the sulfur species, thereby achieving high sulfur loading and high areal capacity. This research can open new avenues for the design of high performance Li-S batteries and their commercialization.”