Sulfur-Microporous Carbon Composite Positive Electrodes for Rechargeable Lithium Sulfur Batteries
Authors: Ishikawa Masashi , Takahashi Takuya , Matsui Yukiko , Yamagata Masaki , Uchida Satoshi 
Published: February 2020

Over the decades, researching on sulfur as a positive electrode material for the lithium-sulfur (Li-S) battery has widely been studied. The sulfur has a high theoretical capacity (1672 mAh g-1) and reasonable discharge voltage (ca. 2 V vs Li/Li+), and is an abundant material as a by-product of fossil fuel. However, it is well known that a sulfur positive electrode has some crucial problems for realistic application as Li-S battery, which are mainly dissolution of intermediate product species in its charge-discharge processes, insulation property of sulfur and lithium sulfide, and relatively large volume change of a sulfur positive electrode. Upon discharge process of a sulfur positive electrode, S8 molecule goes through a series of lithium polysulfides (Li2S
n

[n =1-8]). At this time, Li2S8, Li2S6, and Li2S4 can easily dissolve to a liquid electrolyte. Moreover, insoluble Li2S2 and Li2S also transform to a soluble lithium polysulfide by being oxidized during a charge process. These processes cause a redox shuttle reaction in a liquid electrolyte, which may induce rapid capacity decay and low charge-discharge efficiency. Also, a dissolved lithium polysulfide into a liquid electrolyte reacts with various electrolyte components (carbonate solvent, BF4 anion, and bis(fluorosulfonyl)imide (FSI) anion, etc.) and hence generates unnecessary products. Therefore, a simple sulfur positive electrode limits the choice of liquid electrolytes, which is one of some problems preventing from practical using Li-S batteries. 

There have been at least two approaches to overcome the above issue. One is embedding sulfur into less than about 1 nm-sized micro pores by a diffusion melt method; the other is covalently bonding sulfur onto de-hydrogenated polyacrylonitrile (S-PAN) by heat treatment. These types of sulfur composite electrodes show better cycle stability and higher charge-discharge efficiency than those of other types of sulfur composite positive electrodes and avoid the necessity to use a special electrolyte because they can completely prevent dissolution of a lithium polysulfide into a liquid electrolyte. This should be an advantage for a Li-S battery. It is interesting to note that these types of composites show similar charge-discharge behavior in spite of undergoing different synthesis routes, where their mechanisms have not been clarified yet. Sulfur loading to the composites depends on micro-pore volume of a porous material; for instance, the most suitable content of sulfur were about < 50 % in S-PAN composites, which should be improved. Therefore, in order to enhance the energy density of a Li-S cell, we think that sulfur-microporous material composites would be more investigated as a potentially better choice.

In this study, we synthesized a sulfur-microporous activated carbon (AC) composite material and systematically investigated fundamental battery performance of the sulfur-microporous AC composite positive electrodes with different particle size AC, electrolyte, and negative electrode material. We confirmed that the sulfur-AC positive electrode shows relatively better low-temperature performance and prevents self-discharge at 60ºC by blocking dissolution of lithium polysulfide, which induces a redox shuttle reaction. According to the present results, a micro pore structure of 1 nm-sized strongly retains sulfur inside the pores, which indicates that some problems of sulfur as positive electrode material should be overcame by this simple synthesis method. Moreover, safety and reliability of the battery containing a sulfur positive electrode will be further improved by changing negative electrode material from alkali metal (lithium or sodium) to silicon or carbon material. Further research on sulfur-microporous AC positive electrode is currently proceeding in our laboratory and will be reported in this presentation. In our opinion, our efforts would open up a possibility of a sulfur-microporous composite as positive electrode material for a next generation rechargeable battery.