Co-MOF-74 Embedded Interlayer for Increasing Sulfur Utilization in Lithium-Sulfur Batteries
Authors: Sung SiHyeon , Yoon Woo Young 
Published: December 2020

Lithium-Sulfur (Li-S) battery is considered to be one of the most promising next generation battery because of its high theoretical capacity (1675 mAh/g) and high-energy density (2600 Wh/kg). However, Li-S battery still has challenges, such as electronic and ionic insulating property, shuttling effect, and sluggish reaction in Li2S2 to Li2S. Many researches have tried to improve electronic conductivity by using carbon materials such as Reduced Graphene Oxide and multi-wall carbon nano tube (MWCNT). MWCNT interlayer, which place between cathode and separator, demonstrated good electronic conductivity and physical barrier to prevent migration polysulfide cathode to anode. However, carbon materials are non-polar material so that it is hard to prevent polysulfide migration chemically. Therefore, many efforts have been attempted to mitigate shuttling effect chemically. Cobalt sulfides (CoS3, Co9S8...), Metal Organic Framework(MOF) facilitated adsorbing polysulfides chemically. Cobalt Sulfides not only mitigate shuttling effect by adsorb polysulfide anion but also promote the transformation of Li2S2 to Li2S. M-MOF-74, consisting of metal ion(M) and organic linkers (2,5-dioxido-1,4-benzenedicarboxylate, DOBDC), have been researched on Li-S battery system because of abundant unsaturated metal sites which adsorb polysulfide anion, and high specific surface area. In this report, Co-MOF-74 and MWCNT composites (Co-MOF-74@MWCNT) were synthesized by solvothermal method. Co-MOF-74@MWCNT interlayer was synthesized using a filtration method. Co-MOF-74@MWCNT interlayer placed between cathode and separator to act as physical and chemical barrier. This Co-MOF-74@MWCNT interlayer is compared to MWCNT interlayer. Prepared Co-MOF-74@MWCNT is confirmed by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy. SEM images confirmed that Co-MOF-74 particles distributed uniformly. Also, Co-MOF-74 particles surrounded by MWCNT. X-ray diffraction confirmed presence of Co-MOF-74 and MWCNT. Assembled batteries, which is prepared by Co-MOF-74@MWCNT interlayer and MWCNT interlayer, measured resistance by electrochemical impedance spectroscopy (EIS). Co-MOF-74@MWCNT interlayer cell (Co-MOF-74@MWCNT cell) showed higher resistance(Rct) than MWCNT interlayer cell (MWCNT cell). However, Co-MOF-74@MWCNT cell showed lower resistance (Rct) than without interlayer cell. According to CV test, Co-MOF-74 can promote redox reaction and showed small peak around 1.85V. Besides, through symmetric cell test, Co-MOF-74@MWCNT cell showed faster redox reaction than MWCNT cell. Furthermore, Co-MOF-74@MWCNT cell exhibited higher capacity than MWCNT cell. Co-MOF-74@MWCNT cell exhibited an initial capacity of 1415 mAh/g, 115% larger than the 1228 mAh/g of the MWCNT cell. Also, after 50 cycles Co-MOF-74@MWCNT cell showed 1202 mAh/g, 120% larger than the 996 mAh/g of the MWCNT cell. Voltage profile test exhibit the reason why Co-MOF-74@MWCNT cell showed higher capacity than MWCNT cell. The ratio of second plateau and first plateau of Co-MOF-74@MWCNT cell showed 2.79 and MWCNT cell showed 1.904. The ideal value is 3. Co-MOF-74@MWCNT cell showed 2.79 because Co-MOF-74 promote the utilization of sulfur. Besides, It can be proved that Co-MOF-74@MWCNT cell promotes Li2S2 to Li2S reaction through voltage profile test. In summary, various electrochemical analyses have confirmed that Co-MOF-74@MWCNT interlayer can be a physical and chemical barrier and promote transformation of Li2S2 to Li2S.

Figure 1