Origin of Low Performance of Lithium-Sulfur Batteries Under Lean Electrolyte Condition

Abstract

Lithium-sulfur (Li-S) battery is regarded as one of the most promising high-capacity rechargeable batteries, and the research and development on this technology has been active since the late 1960s. Benefiting from the resurrected efforts in the past decade or so, in-depth understanding and technologic know-how of Li-S technology have been accumulated thus leading to tremendous improvement of battery performance. However, wide deployment of Li-S batteries is still hindered by low specific energy and short cycle life. These challenges are dictated by the interplay of the Li anode, sulfur cathode and electrolyte in the battery. Particularly, implementing low mass ratio of electrolyte/sulfur (E/S ratio) is the most critical factor to realize high specific energy at cell level. In this study, we assembled a 800 mAh Li-S pouch cell with specific energy of 275 Wh/kg and E/S ratio of 2.8. The sulfur loading was 5.4 mg/cm2 on aluminum foil and the specific capacity of sulfur is 1050 mAh/g. With these cell parameters, a 5Ah Li-S pouch cell with 350 Wh/kg specific energy can be delivered by simply incorporating more electrode layers. However, this promising pouch cell could only be cycled for less than 20 cycles. To understand the origin of the low performance under low E/S ratio, the interfacial resistance during the discharge was measured with electrochemical impedance spectroscopy with different E/S ratio. It was found, comparing to relatively high E/S ratio, low E/S ratio at 2.4 induced drastically increasing charge transfer resistance during the first discharge plateau, i.e. generation of high order polysulfides, which terminated the discharge of Li-S cells with low capacity. This observation was also confirmed by the Galvanostatic Intermittent Titration Technique (GITT) experiments. Based the results, we propose that the rate limiting step during the overall charge transfer can either the dissolution (solvation) of polysulfide anions or adsorption of solvated polysulfide anions to the electrode. The potential strategies to address the problem are also discussed.