Innovative Investigation of Ordered Mesoporous Carbon Modified Separator for High Performance Lithium-Sulfur Batteries
Authors: Kwon Yelim , Lee Jungho , Cui Chenglin , Kim Hansol , Kim JI Man 
Published: February 2024

These days, many portable electronic devices are used widely, and it makes secondary batteries important for future innovation. Li-ion battery which is most commonly used has the high specific energy and long cycle life. But it has problems with high price and possibility of explosion. Also it reaches the limitations of energy density for electric vehicles (EVs) and energy storage systems (ESS). Lithium-Sulfur battery is becoming a one of the promising alternatives. It has high theoretical capacity (1,675 mAh g-1) and specific energy density (~2600 Wh kg-1) and low cost. Also it is eco-friendly and safe. However, there is the primary problem to be commercialized. That one is called shuttle effect, which means dissolution of polysulfide into the electrolyte in the repetitive charging and discharging. It makes capacity decrease and internal resistance increase. And the active material and the discharge products, which are sulfur and Li2S2, Li2S, have low conductivity. The irreversible volume expansion during electrochemical reaction reaches to 80%. [1,2] To alleviate these problems, various sulfur host materials and modified separators have been investigated. [3,4]

One of the representative approach is to apply carbon materials as sulfur hosts. Sulfur must be dispersed throughout the carbon conductor because of its low intrinsic electrical conductivity. The carbon matrix functions as a network for rapid electron transport. Various carbon materials, such as porous carbons, carbon nanotubes, or carbon materials with heteroatoms or chemical groups such as N, O, and S, have been employed to minimize polysulfide migration. One of the most effective sulfur hosts for limiting polysulfide diffusion is mesoporous carbon. By virtue of their inherent porosity and conductive carbon architecture, mesoporous carbon hosts have demonstrated better cycling performance.

Another method for dealing with the shuttle effect is to form an interlayer between the cathode and the separator. Because the separator is an important role in battery safety and electrochemical performance, designing a high quality separator is critical for the development of high performance Li-S batteries. Separators made of polypropylene (PP) or polyethylene (PE) are commercially used and have some physical and electrochemical advantages. However, these separators have drawbacks such as low ionic conductivity and poor electrolyte compatibility. Various materials have been studied and used for modifying materials on the separator. For example, carbon materials, graphene oxides, and metal-organic frameworks are excellent at suppressing the shuttle effect of polysulfide. On the other hand, characterizing these structures and comprehending polysulfide behavior in these systems during electrochemical reaction, remain substantial challenges.

Herein, we report on a dual-functional modified separator by applying ordered mesoporous carbon (OMC) on a commercial separator.The mesopores and the micropores of the OMC own a specific physical adsorption capability for polysulfide, thus it can adsorb a large amount of polysulfide. In addition, it is thermally and chemically stable. Also, the OMC can be detected by operando small-angle scattering (SAXS). In recent years, SAXS approach was applied to explain the ordered porous structure of sulfur host materials for Li-S batteries. These research provide information on how sulfur is adsorbed by porous carbon. This analysis identifies the changes of sulfur species as to whether polysulfide is adsorbed in mesopores or solid discharge products are formed on the surface, or infiltrated into micropores. The use of in-depth characterization techniques allows for more precise and independent assessment of the rapid electrochemical reactions in Li-S batteries.

In brief, the highly ordered mesoporous carbon was synthesized, and it was used as an interlayer that served not only as a physical barrier to limit the polysulfide shuttle phenomena but also as an additional current collector to reuse active materials. By promoting the electrochemical reaction of active materials, it exhibited better electrochemical performance such as high capacity, stable cycle performance, and improved rate performance. The adsorbed polysufide and the behavior of sulfur species in the OMC during cycling were investigated using operando SAXS to acquire insight into the role of the OMC modified separator.

[1] Li, Z.; He, Q.; Xu, X.; Zhao, Y.; Liu, X.; Zhou, C.; Ai, D.; Xia, L.; Mai, L. Adv. Mater. 2018, 30 (45), 1804089.

[2] Zeng, L.-C.; Li, W.-H.; Jiang, Y.; Yu, Y. Rare Met. 2017, 36 (5), 339-364.

[3] Liu, Z.; Liu, B.; Guo, P.; Shang, X.; Lv, M.; Liu, D.; He, D. Electrochim. Acta 2018, 269, 180-187.

[4] Schuster, J.; He, G.; Mandlmeier, B.; Yim, T.; Lee, K. T.; Bein, T.; Nazar, L. F. Angew. Chem. 2012, 124 (15), 3651-3655.

Figure 1