Lithiated-Polyamic Acid-Coated Glass Fiber As a Functional Separator for High-Performance Li-S Batteries

Abstract

Lithium sulfur batteries (LSB) can offer great opportunities for the next-generation energy storage systems required for electrical vehicles and grid energy storage applications due to its very high theoretical specific energy (2500 Wh kg−1). For the practical applications, the sulfur loading in the cathode should not be lower than 70wt% in order to achieve a high energy density. However, the electrical and/or ionic conductivity of the cathode can be reduced with a high S loading, respectively, leading to reduced sulfur utilization and slurry reaction kinetic. In order to overcome this issue, sulfur active species were first dissolved together with Li2S in the electrolyte to form lithium polysulfide (LiPolS) (e.g., Li2S8 and Li2S6) solution or catholyte. During cycling, the active sulfur (Li2S8) is deposited from the electrolyte into the S-free cathode conductive matrix, which could be made from carbon nanotube and carbon nanofiber. It has been demonstrated that the specific/areal capacity of Li – lithium polysulfide (Li-LiPS) cells show high specific capacity, indicating high S utilization as well as high reaction kinetic. However, the problem is the penetration of dissolved LiPSs through the separator to react with the Li metal anode and reduce to form short-chain LiPSs, resulting to the passivation of the anode with irreversible deposits on its surface. In order to mitigate this problem, a lithiated-polyamic acid functionalized glass microfibers (GF@PALi) was utilized as a functional separator to selectively transfer Li+ ions. The GF@PALi was prepared by simply dipping of commercial GF into aqueous solution of 5 wt% lithiated-polyamic acid and dried overnight at 60 oC in vacuum oven. Combined with CNTs on Ni foam electrode, catholyte of 0.4 M LiTFSI + 2.0 M Li2S8 + 1.0 M LiNO3 in DME/DOL and lithium anode, the fabricated LSB cell exhibits a high specific and areal capacities of 803 mAh g-1 and 2.43 mAh cm-2 at C/2 rate, respectively. In addition, the LSB cell with the GF@PALi separator presents a remarkable cycling durability until 200 cycles with 76% capacity retention, which is much more stable than the LSB cell with commercial Celgard PP separator working until 86 cycles. Such high durability could be attributed to the retarding polysulfide migration of the as-prepared GF@PALi separator. Figure 1