(Invited) Catholyte-Based High Capacity Li-Sulfur Batteries
Authors: Lim Du Hyun , Nitze Florian , Aguilera Luis , Xiong Shizhao , Matic Aleksandar 
Published: February 2020

There is currently a considerable interest in the development of high capacity energy storage technologies. It is driven by the need in transport applications, such as electric vehicles (EV) where a capacity to go more than 500 km on one charge would be a desirable goal, as well as by renewable energy applications based on intermittent sources. The state-of-the-art for commercial rechargeable energy storage systems is the lithium ion battery (LIB) which has revolutionized the market for portable electronics. However, neither their capacity nor their price is currently matching the needs from the automotive or the renewable energy sectors. In addition, there is a need to improve the sustainability of the battery when considering mass implementation of large-scale energy storage systems.
Li-Sulfur (LiS) technology has great potential for large-scale energy storage applications due to the very high theoretical specific capacity (1675 mAh g-1) and high energy density (2500 Wh kg-1) [1]. In addition the technology has potential for being a sustainable solution due to the abundance, low price and low toxicity of sulfur. However, a much lower practical capacity, limited cyclability, and low rate capability prevent commercialization of the technology.
The origin of many of the problems can be traced to the intrinsic electrochemical process of the Li-Sulfur cell. During discharge sulfur is converted to Li2S through a series of steps involving the creation of lithium polysulfides (Li2Sx, 2<x≤8). A major issue is that these polysulfides have a relatively large solubility in standard electrolytes. In addition, the dissolved polysulfides cause degradation of the anode through shuttle reactions when they migrate through the electrolyte, forming insoluble layers on the anode surface. This leads to active material loss during the discharge-charge process gradually reducing the capacity and decreased Coulombic efficiency of the cell [7-10]. In addition, conducting material has to be added to sulfur (more than 30 wt%) when forming the cathode since sulfur has low electronic conductivity Consequently, the amount of active material in the cathode is further decreased, as is consequently the practical energy density of the cell.
The problems in the Li-Sulfur battery can be addressed by new materials concepts for both the electrolyte and the cathode. For the development of the electrolyte promising approaches include moving away from organic solvents, improving both performance and safety, and the use of polymer gel or solid polymer electrolytes or ceramic electrolytes [2]. It has also been shown that by deliberately adding polysulfides to the electrolyte a more stable cycling, high capacity and good utilization of active material can be obtained [3]. To mitigate the polysulfide dissolution, and improve the conductivity of the cathode at the same time, nanostructured porous-carbon/sulfur composites have been applied [1]. Even though these approaches have proven rather successful in preventing polysulfide dissolution the practical energy density of these cells remain limited due to rather low active material (sulfur) loading.
In this contribution we present different approaches towards Li-Sulfur cells with improved stability, safety, and high energy density. These include both new electrolyte and cathode solutions [2]. In particular we will discuss the application of the catholyte concept, where the active material is dissolved in the electrolyte. We show that by using a nanostructured porous-carbon cathode we can obtain a very high practical energy density, high sulfur utilization, and stable cycling. We also discuss the role of the presence of polysulfides in the electrolyte for the stabilization of the Li-metal anode in this cell [4].

References

[1] X. Ji and L. F. Nazar, J. Mater. Chem. 20, 9821 (2010)
[2] S. Xiong, J. Scheers, L. Aguilera, D-H. Lim, P. Jacobsson, and A. Matic, RSC Advances, 5, 2122 (2015)
[3] M. Agostini, S. Xiong, A. MAtic, and J. Hassoun, Chem. Mat. 27, 4604 (2015)
[4] S. Xiong, K. Xie, E. Blomberg, P. Jacobsson, and A. Matic, J. Pow. Sour., 252, 150 (2014)