Development of Highly Sulfur-Loadable Microporous Activated Carbon from Azulmic Acid for Lithium-Sulfur Battery
Authors: Tonoya Takeshi , Hinago Hidenori , Ishikawa Masashi 
Published: December 2020

Introduction
The lithium-Sulfur battery shows ca. 10 times higher energy density than that of the lithium ion battery. However, a major problem is the behavior of lithium polysulfide intermediates, which may dissolve into an electrolyte. We have investigated microporous activated carbon as matrix stabilizing S and realized stable cycling performance.1,2 However, the activated carbon with only micropores has a relatively low sulfur loading (ca.30 wt.%) and needs to be improved. We previously reported azulmic carbon (AZC), which has many micropores that suppress the dissolution of lithium polysulfides and is capable of high sulfur loading (ca.60 wt.%).3 In this study, we have successfully produced AZC capable of further high sulfur loading by improving a typical activated carbon production method. The sulfur cathode using this improved AZC and sulfur composite showed a better charge-discharge capacity than the previous AZC.
Method
AZC was carbonized from azulmic acid (AZA) at 800°C for 1h under N2 atmosphere (the obtained product is denoted by AZA-800 hereafter). AZA-800 and KOH or K2CO3 were mixed by a ratio of 1:2. Those mixtures were heated at a predetermined temperature of 800 to 900 ℃ for 1h under Ar atmosphere in a silica tube for alkaline activation of AZA-800. The AZC-S composite was prepared by mixing various AZC with S, and then applying heat treatment: 155°C for 5 h, then 300°C for 2 h. The AZC-S composite cathode was prepared by mixing the AZC-S composite, acetylene black, carboxymethyl cellulose, and styrene-butadiene rubber at a respective weight ratio of 89:5:3:3. The electrolyte was prepared by mixing lithium bis(trifluoromethylsulfonyl)imide with tetraglyme and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether at a ratio of 10:8:40 (by mol). All the cell components were installed into a demountable cell with a lithium anode. A typical charge-discharge cycling test was carried out at 167.2 mA g-1 (0.1 C) with cutoff voltages of 3.0 and 1.0 V at 25°C.
Major results and conclusion
Figure 1 shows the pore size distribution of AZC-KOH and AZC-K2CO3. It was confirmed that the pore volume of AZC-K2CO3 increased as the activation temperature increased. When K2CO3 was used as an activator, the proportion of micropores tended to be higher than that with KOH. AZC-K2CO3 can confine up to 70 wt.% sulfur, which is ca.10 wt.% higher than the previous AZC-KOH. Figure 2 shows the results of constant current charge and discharge tests of the cathode prepared using AZC-KOH and AZC-K2CO3 with sulfur composites. AZC-K2CO3 has a lower capacity decay rate with charge / discharge cycles than AZC-KOH, which may be due to a large proportion of micropores that suppresses the elution of lithium polysulfides. The activation procedure using K2CO3 provides activated carbon with a lot of micropores and hence high sulfur loading. It was suggested that AZC-K2CO3 is a promising activator for achieving high energy density in positive electrodes of lithium-sulfur batteries.
This work was supported by “Advanced Low Carbon Technology Research and Development Program, Specially Promoted Research for Innovative Next Generation Batteries (ALCA-SPRING)” from Japan Science and Technology Agency (JST); Grant#: JPMJAL1301.
References

T. Takahashi et al., Prog. Nat. Sci.: Mater. Int., 25, 612 (2015).

S. Okabe et al., Electrochemistry, 85, 671 (2017).

S. Usuki et al., Electrochemistry, 85, 650 (2017).

Figure 1