Organic-Inorganic Composite Separator for Improving the Life Cycle Characteristics of Lithium-Sulfur Batteries
Authors: Cho Jinil , Gong Yong Jun , Pyo Seonmi , Yun Heejun , Kim Heebae , Yoo Jeeyoung , Kim Youn Sang 
Published: December 2020

Lithium-ion batteries (LIBs) have been widely used in many industries in recent years. In particular, the electrical vehicles (EVs) and energy storage system (ESS) markets are growing rapidly, and the requirements for LIBs are also growing. Under these circumstances, the active materials for LIBs that have been used are insufficient in various aspects. The most important characteristics of newly requested LIBs are economics and capacity. Until now, the industry sector where LIBs were mainly used was mainly devices with relatively small size and output, such as portable electronic devices. Therefore, if a certain level of capacity was secured in LIBs, there was no demand for an extremely large-capacity battery. However, due to changes in the industry, devices that require high power and long-term use, such as EVs and ESS, have emerged as new demands, and the requirements of LIBs have gradually changed to emphasize the capacity side. The cathode active materials used in commercially available LIBs are mostly metal oxides containing rare metals such as nickel and cobalt. These metal oxide-based cathode active materials have a relatively low theoretical capacity, unstable supply, and low production. So the price fluctuation is very high and the price is also very expensive. Anode has also been sideways for decades without significant changes in capacity since LIBs were commercialized, and new attempts at the active material level are needed. Research to introduce a new active material to secure these shortcomings has become more active in recent years.

Lithium-sulfur batteries (LSBs), which use sulfur as the cathode material and lithium metal as the anode active material, are receiving great attention as next-generation LIBs in this respect. In the case of sulfur used as a cathode active material, charging and discharging proceeds through a conversion reaction between sulfur and lithium, and thus has a very high theoretical capacity. This shows the potential to be a positive electrode active material with a significantly higher capacity than materials having a layered structure of a metal oxide series used in the conventional LIBs. Also, as a by-product of the petrochemical industry, a very large amount of supply is made, but the demand is not so abundant, so it has a very low price. Lithium metal, which is used as an anode active material, also has a very large capacity, making it one of the most actively studied materials in the LIBs field. Lithium metal has a theoretical capacity of over 3800mAh g-1, so its potential is endless. However, despite these great advantages, LSBs have not been commercialized because there are big disadvantages to be solved. When considering the problems caused by the cathode and those caused by the anode, the problem with cathode active material is the lack of life characteristics. In particular, the biggest problem is that the capacity of the system continuously decreases due to the dissolution of lithium polysulfide. Lithium polysulfide, an intermediate produced during the charging and discharging process, dissolves very well in the organic liquid electrolyte, so it moves freely between the cathode and the anode. At this time, since it is continuously reduced in the anode region to form an inactive layer, as the cycle progresses, the amount of active material decreases, so that life characteristics are not secured. Another major problem is that caused by lithium metal used as an anode. Lithium metal shows low coulombic efficiency by repeatedly generating an SEI layer on the surface as charging and discharging progress. Furthermore, it reacts with the liquid electrolyte and continuously consumes electrolyte and active material, showing very poor life characteristics.

In this study, we proposed a new type of organic-inorganic composite separator that can solve the above two problems at once. The proposed separator (PAAO) was fabricated by filling poly (vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) on the surface and inside of anodic aluminum oxide (AAO). Since AAO has vertical pores with regular spacing, it provides stability at the interface of lithium metal to improve the life characteristics of the anode. Also, PVdF-HFP provided a passage for lithium ions inside the pores of AAO and effectively prevented the movement of lithium polysulfide. AAO showed an improved lifespan of over 50% compared to a separator made of a commercially available polyolefin-based polymer. As a result, PAAO showed an improved lifespan of more than 50% and high coulomb efficiency in all cycles compared to the separator made of commercially available polyolefin-based polymers. We believe that PAAO has shown potential as a new separator that can be applied to next-generation LSBs.

Figure 1