(Invited) Microporous Activated Carbon and S Composite Cathode for Rechargeable Li-S Battery
Authors: Ishikawa Masashi , Tonoya Takeshi , Matsui Yukiko 
Published: December 2020

Sulfur (S) has an outstanding theoretical capacity (1,672 mAh g-1) and is abundant material. There are several problems limiting a realistic S positive electrode. A major issue is unstable properties of lithium polysulfide (Li2Sn) intermediates formed during charge and discharge processes; they dissolve easily into various electrolytes [1-3]. When dissolution of Li2Sn occurs, this leads to a rapid capacity decay by a redox shuttle reaction and to irreversible degradation of the overall Li-S system. To solve these problems S-composites for stabilization of S derivatives has been proposed. In previous work, S-composites with mesoporous activated carbon and conductive polymers with S and S-graphene have been extensively studied, and these systems show relatively high cycle stability. However, these electrodes still have a problem of insufficient charge-discharge efficiency. This is due to the redox shuttle of dissolving Li2Sn intermediates, and there is few practical methods that completely prevent the dissolution of Li2Snintermediates with these electrodes. Therefore, in recent years, other studies have been conducted to completely prevent the dissolution of Li2Sn by using new host materials for S and novel cell design. They are composites of S with microporous activated carbon or organic substances and all-solid Li-S batteries [1-5]. These technologies can completely suppress the dissolution of a Li2S intermediate, so that it is known that not only their capacity retention but also their charge-discharge efficiency is stabilized[6]. However, these electrodes have a problem in energy density because their micropore volume is still low, which is derived from low S loading.
We synthesized S-composites based on microporous alkali-activated carbon with high porous volume and tested positive electrode performance of the S-composites. As a result, we achieved both improvement of cycling characteristics by preventing Li2Sn dissolution and an increase in S loading amount[7]. To increase S loading into microporous carbon as well as limit dissolution of Li2S4-8 we designed micropore-rich activated carbon with high surface area and pore volume, which was obtained by activation of azulmic carbon (AZC) precursor as a nitrogen (N)-doped carbon[8]. The AZC was a carbonized product from azulmic acid (AZA) that was obtained by polymerization of hydrocyanic acid (H−C≡N) under a high temperature. We would suggest that our activated AZC is promising material as positive S electrode matrix for the next-generation rechargeable Li batteries.
Moreover, we increased S loading into micropores by preparing activated carbon with alkali activation from a N-doped carbon AZC-800 (carbonized at 800ºC) to enhance energy density of Li-S batteries. It was found that micropores are selectively formed by removing nitrogen from AZC-800 with alkali activation; hence AZC-800/1KOH and AZC-800/2KOH (the numerical before KOH is a KOH/AZC weight ratio) have high surface area. As a result, S can be loaded up to 55 and 62 wt.% in the respective activated carbon electrodes: AZC-800/1KOH@S55 and AZC-800/2KOH@S62. AZC-800/1KOH@S55 can cycle well in not only a glyme-based electrolyte, LiTFSI/G4/HFE but also 1M LiPF6 EC/DMC. AZC-800/2KOH@S62 can function in LiTFSI/G4/HFE but is incompatible with the EC/DMC-based electrolyte. We consider that these different characteristics are ascribed to a fine pore structure. The present microporous activated AZC with high S loading is promising positive electrode material for rechargeable Li-S batteries. Lately we have developed another activator instead of KOH. We have applied AZC-based materials also to a massive cathode based on Al-mesh-type current collector to increase a cell energy density.
This work was partly 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.
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