Effects of Pore Volume and Pore Size Distribution of Carbon Supports of Sulfur on Performance of Li-S Batteries with Solvate Ionic Liquid Electrolyte.

Abstract

1. Introduction Lithium Sulfur (Li-S) batteries have attracted much attention because of their high energy density. However, there are several issues that prevent the practical use of Li-S batteries. One of the critical problems is the dissolution of lithium polysulfides into the electrolyte during the electrode reactions, resulting in poor charge-discharge cycle stability. To overcome this, many recent reports use LiNO3 as it forms a SEI layer by the reaction at Li metal surface and inhibits the redox shuttle. On the other hand, we applied solvate ionic liquids (SILs) as an electrolyte, which achieved prevention of polysulfides-dissolution and stable cycles (more than 400 cycles) without LiNO3 [1] [2].With such polysulfides-insoluble electrolytes, the reaction process should occur in solid-state, which makes reaction process simpler. However, the charge-discharge capacity was still much lower than the theoretical capacity due to the insulating nature of sulfur and lithium polysulfides. Carbon supports have been widely used for sulfur composite cathode in a Li-S battery. To achieve a high capacity of the sulfur cathode, we applied various carbon supports and effects of pore structure and pore size distribution on performance of Li-S battery were elucidated. 2. Experiments The sulfur/carbon composite (S/C) were prepared by melt-diffusion strategy (Sulfur : carbon = 2:1). The S/C was dispersed in polyvinyl alcohol (PVA) solution N-methyl-2-pyrrolidone. The obtained slurry was coated onto Al foil and then dried at 80 ºC for 12 hours. The weight ratio was fixed at Sulfur : Carbon : PVA = 60:30:10. Electrochemical properties of sulfur cathodes were evaluated with 2032 type coin cells assembled in an Ar-filled glovebox. 3. Results and Discussion We used various carbon materials, Ketjenblack (KB), inverse opal carbons (IOCs), and mesoporous carbons (MPCs, Toyo Tanso Co. Ltd.), to investigate the charge and discharge capacities of carbon/sulfur composite cathodes. Among them, IOCs, which were prepared by silica colloidal template method, showed large capacity. IOCs have three-dimensionally ordered interconnected macropores with high specific surface area due to the presence of micropores. These features are favorable for high utilization of the sulfur in the carbon/sulfur composite cathode. Especially, the 2nd discharge capacity (mA h g-1) linearly become large as pore volume (cm3 g-1) of carbon increases (red plots in Fig. 1: KB and IOCs). However, in the case of MPC [3] (blue plots in Fig 1), the discharge capacity deviated from the correlation between pore volume and gravimetric capacity. The capacity was larger than that expected from the pore volume. MPC-based cathode showed 3rd plateau while KB-base cathode did not show such plateau (Fig.b). The 3rd plateau around 1.7 V is assumed to be appeared owing to the discharge reaction of the smaller sulfur molecules such as S2-4 confined in micropores [4]. Thus, the confinement of sulfur in micropores can be another methodology for achieving high utilization of sulfur in the cathode. 4. Acknowledge This research was supported by Japan Science and Technology Agency (JST) - Advanced Low Carbon Technology Research and Development Program - Specially Promoted Research for Innovative Next Generation Batteries (ALCA-SPRING) of Japan. Reference (1) K. Dokko, et al., J. Electrochem. Soc., 160, A1304 (2013)(2) N. Tachikawa, et al., Chem. Commun., 47, 8157 (2011)(3) T. Morishita, et al., Carbon, 48, 2670 (2010)(4) Z. Li, et al., ACS Nano, 8, 9295 (2014) Figure 1