A Functional PEO/LiTFSI-Coated Coated Separator for Electrochemical Lithium-Sulfur Battery
Authors: Chung Sheng-Heng , Chiu Li-Ling 
Published: July 2021

There is a strong interest in exploring next-generation rechargeable batteries that can deliver a high energy density for a long cycle life and a long shelf time at an affordable cost. The commercial lithium-ion battery systems have exhibited the advantages in satisfying all the above-mentioned requirements so that the lithium-ion technology has dominated the battery business for over 30 years. To obtain further upgrades in battery technologies, the current lithium-ion systems based on various lithium-insertion oxide cathodes are mature techniques and have faced the theoretical values in terms of their capacity and energy density values. Moreover, the increasing demands of commercial lithium-ion batteries have encountered the increasing prices and limited availability of the raw materials. To address these issues, the conversion-reaction cathodes featuring an enhanced energy density at an affordable cost garner significant attention.

Sulfur cathodes have become appealing as they offer an order of magnitude higher capacity and are abundant. The electrochemical conversion reactions of sulfur with lithium ions provide lithium sulfide, involving two electrons per sulfur and providing a high charge-storage capacity of 1,672 mA∙h g-1. The high-capacity sulfur cathodes enable the packaged lithium-sulfur cells to attain 2-3 times higher energy density than that offered by the current lithium-ion batteries. However, several challenges prevent the practical application of lithium-sulfur batteries. The high resistivity of solid-state sulfur and its end-discharged lithium sulfides as well as the dissolution and diffusion of intermediate liquid-state lithium polysulfides cause the poor electrochemical utilization of sulfur, fast capacity fade, and low Coulombic efficiency of the cell in a short cycle life.

Significant improvements have been made in the modification of the material and the structure of the sulfur cathodes and the separators in recent years. This presentation will focus on lithium-sulfur battery separator with a coating layer of mixtures containing poly(ethylene oxide) (PEO) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) on a polypropylene membrane, forming a functional gel polymer electrolyte (GPE) / separator composite. The PEO/LiTFSI-coated polypropylene membrane is configurated to face the sulfur cathode and functions as the restrict film to cease the fast diffusion of liquid-state polysulfides and to stabilize the liquid-state active materials within the cathode region of the cell as the catholyte for enhancing the electrochemical reaction kinetics and stability. As a result, our findings show that the lithium-sulfur cells with the PEO/LiTFSI-coated polypropylene membrane show a high discharge capacity of 1,200 mA∙h g-1, a long cycle life of 200 cycles, and high Coulombic efficiency of above 99%. As a comparison, the same cell with a PEO-coated polypropylene membrane reveals a poor electrochemical utilization and efficiency due to the lack of LiTFSI as the ion-diffusion matrix, resulting in low discharge capacity of 662 mA∙h g-1and poor capacity retention. On the other hand, the same cell with a regular unmodified separator membrane suffers the fast loss of the liquid-state active material, showing a short cycle life of less than 100 cycles. According to the material analysis and electrochemical comparison, the PEO/LiTFSI-coated polypropylene membrane proves its capability in blocking the uncontrolled migration of polysulfides, while allowing the smooth diffusion of lithium ions. In conclusion, our experimental and analytical results prove the PEO/LiTFSI-coated polypropylene membrane as an advanced and a facile separator design in the development of high-loading/content sulfur cathodes in lithium-sulfur batteries for realizing a high energy density.