Novel Poreless Inorganic Separator for Blocking Polysulfide Shuttle Effect in Lithium Sulfur Battery
Authors: Cho Jinil , Gong Yong Jun , Lee SeongHun , Yoo Gayeong , Yoo Jeeyoung , Kim Youn Sang 
Published: February 2020

Lithium ion batteries (LIBs) have been used in portable electronic devices due to high energy density, rechargeable characteristics, and tiny memory effect. However, advanced portable electronic device, electric vehicles (EVs), and large scale energy storage system (EES) require high specific capacity (energy density), light weight, and low cost LIBs. One of the important factors in determining the specific capacity of LIBs is the cathode active material. During the decades, general cathode active materials (lithium, cobalt, nickel, etc.) have some issue of rapid price rise as the demand increment for cathodic materials of EVs and EES. Especially, cobalt, which is the most essential cathode active material, the Democratic Republic of the Congo (DRC) accounts for more than 50% of the world's production. Cobalt supply is very unstable due to civil war in the DRC and exploitation of labor in the process of cobalt production. Therefore, a new cathode active material is necessary. Lithium sulfur (Li-S) batteries use an elemental sulfur as cathode and lithium metal as anode. The elemental sulfur is one of the most abundant materials on earth and has a high theoretical capacity. The theoretical capacity of 1675 mAh/g is up to seven times higher than that of LIBs. And elemental sulfur also has high theoretical gravimetric energy density of 2500 Wh/kg which is over four times higher than the ideal state of common LIBs. Additionally, sulfur is a stable and cost affordable material. Hence, the Li-S battery has attracted attention as a next-generation battery, and much researches have been reported. However, there are some problems that commercialization. The first problem is the dissolution of the polysulfide. The intermediate lithium polysulfides which was formed during the charge and discharge process are soluble in the typical organic electrolytes for LIBs. This phenomenon is called “polysulfide shuttle effect”. This leads to continuous cathode active material consumption, eventually the capacity is faded during cycle. The second problem is the safety and stability of the lithium metal used as the anode. Lithium metal surface control is a necessary step not only for the stability of Li metal anode, but also for a high capacity with a high coulombic efficiency. In this study, we developed poreless poly (vinylidene fluoride-co-hexafluoropropene) (PVdF-HFP) coated anodic aluminum oxide (PAAO) separator to blocking polysulfide shuttle effect and to control lithium metal interface. PAAO was prepared by coating PVdF-HFP solution on anodic aluminum oxide (AAO). AAO provides a highly ordered vertical pore structure and AAO has less tortuosity than conventional separator. The vertical pore structure of AAO evenly distributes the Li ion flux in battery system, so proposed separator effectively prevents the ununiformed growth of lithium. It is also mechanically, chemically, and thermally stable. The vertical aligned pores of AAO are filled with PVdF-HFP which has been used as gel polymer electrolyte, offer high lithium ion conductivity with conventional electrolyte. PVdF-HFP selectively passes lithium ions and effectively blocks dissolution of the polysulfide into the anode region. Consequently, the proposed system conserved high specific capacity with high coulombic efficiency during the cycle. Prepared PAAO showed high specific capacity (∼850 mAh/g, 0.2C), high coulombic efficiency (>98%), and long cycle life (>100 cycles, 0.5C). Reference polyethylene separator shoed low specific capacity (∼550 mAh/g, 0.2C), low coulombic efficiency (~95%) and fast capacity fade during cycle. Compared with a commercial polyethylene separator and bare AAO separator, PAAO separator showed better initial capacity, coulombic efficiency, and electrochemical stability. The suggested PAAO separator effectively block the polysulfide shuttle effect and improve the poor cyclability of the LI-S system. It suggests that this novel poreless separator can be a possible candidate of the powerful separator for LI-S batteries.