Improving Kinetic in Redox Conversion By Adopting Cosse Catalytic Material in Li-S Battery

Abstract

The increasing demand of the energy sources and devices are facilitating the needs of the development in the energy storage systems. Lithium-sulfur (Li-S) battery is considered as one of the most promising energy storage systems because of its high theoretical energy density (2600 Wh kg-1), natural abundance leading to low cost, and their environmental friendliness. However, there are some obstacles to be solved in order to use the Li-S battery practically as follows. First, the sulfur and finally reduced product (Li2S) have a low conductivity, impeding the rate of reactions, and hence limiting total utilization the active sulfur. Second, the shuttling of the polysulfide intermediates (Li2Sx, 4 ≤ x ≤ 8) (Shuttle effect) leads to the low Coulombic efficiency and fast degradation of both electrodes of Li metal and Sulfur. Particularly, the transformation from liquid to solid state occurs unevenly on the surface of sulfur, leading to the aggregation, and hence a large overpotential [1]. To overcome such the problems, some researchers have adopted various methods to prevent the shuttling of the polysulfides. Though many effective structures of the carbon and heteroatom doped carbons have been presented, it is not enough to perfectly hold the polysulfides because of the low polarity of the carbon. Recently, metal sulfides have been reported for their strong affinity with the polysulfides. However, to enhance the kinetics in the Li-S battery, it is not only important to have a high interaction between polysulfides, but should have interfacial advantages to provides nucleation sites of Li2S2/Li2S, that can prevent aggregation during growth and increase electrical conductivity for fast redox reaction between polysulfide chain and solid products.In the context, we developed the cobalt sulfide selenide, CoSSe with high catalytic activity for redox conversion of sulfur. The material is formed on the porous carbon framework, which was also synthesized in this work, with as the large active surface.The as-prepared CoSSe particles show the average size of about 200 nm (Figure 1a). For those composite with the carbon (Figure 1b), it shows that the carbon has CoSSe particles on the surface and comprises the many pores with an average diameter of ~1µ, that can infiltrate the sulfur inside. The XRD results demonstrates that the CoSSe the CoS2 and CoSe2 are both in the pyrite group (Figure 1bTo verify the catalytic effects of the CoSSe/C, the chemical adsorption ability of CoSSe was tested by UV-vis spectroscopy (Figure 2a). The supernatant containing CoSSe clearly shows reduction in 340 nm and 420 nm, which are from the Li2S6, confirming the CoSSe has high binding capability of liquid-phase polysulfides [3]. Enhanced in the liquid-liquid phase kinetics of redox reactions was probed by the symmetric cells with Li2S6 containing electrolyte, which CoSSe/C exhibited a higher current density than that of Ketjenblack (KB) showing catalytic capability (Figure 2b). Galvanostatic charge-discharge profiles show the actual improvement in the Li-S cell when using CoSSe/C with sulfur infiltrated (CoSSe/C@S) (Figure 3). It shows that the CoSSe/C@S exhibits a higher initial capacity (1487 mAh g-1 vs. 1320 mAh g-1) and also has a lower polarization between charge and discharge plateau (0.16 V vs. 0.18 V) than C@S. Also, Figure 3c-d shows that the CoSSe/C@S has an overpotential of 0.021 V for the initiate charging, while C@S has 0.066 V, demonstrating that CoSSe facilitates the redox conversion in solid to liquid phase. From these results, it is concluded that the fabricated CoSSe with a pyrite structure has more effective properties in enhancing the redox reaction of polysulfide intermediates.