The Ternary Metal Chalcogenides Electrodes with High Kinetics and Capacity for the Next-Generation Lithium Ion Batteries

Abstract

Transition metal dichalcogenide (TMDC) such as MoS2, WS2, and VS2 is one of the attractive materials for catalyst, sensor, and battery electrodes (anode/cathode) due to its large surface area and abundant defects. In lithium-ion batteries, most of the TMDCs have a high specific capacity of 430~930 mAh g-1 because they can react with a large number of lithium ions (~four) [1]. Nevertheless, because TMDC is unstable in terms of long-term cyclic performance due to its low electrical conductivity and large volume change, it is still difficult to be commercialized in the next-generation batteries. Accordingly, many researchers have focused on complementing stability and kinetic performance by adopting the strategies below: (i) the design of nanomaterials to reduce path length and accommodate strain (ii) the blend with conductive carbon to improve electrical conductivity [2].In this study, to improve kinetic performance without an additional conductive carbon, we replaced some of sulfur in Mo-S compounds with selenium with much higher conductivity. The selenium has a capacity of one-half that of sulfur, but has a similar volumetric capacity (3,253 mAh cm-3) and high electrical conductivity (10-3 S m-1). The substitution of selenium not only increases the electrical conductivity by a reduction of the band gap, but also improves redox kinetics by providing a large interlayer space [3]. Additionally, an increased electrical conductivity can enhance the utilization of chalcogens, contributing to the increase of capacity [4].Hence, we designed the ternary molybdenum selenosulfide (MoSSe), which has a layered nanostructure to suppress strain caused by a redox reaction. Fig. 1a showed the surface morphologies of the as-prepared MoSSe. As shown in the SEM and TEM images, the MoSSe showed a layered nanostructure with an average layer size of 8.95 nm, and well embedded in carbonized organic compounds, which might play a role of protective layer. Moreover, the selected area electron diffraction (SAED) pattern indicated that the MoSSe was composed of the mixture of polycrystalline of (100) and (110) phases and amorphous phase (Fig. 1b). Fig.1c shows the long-term electrochemical performance of MoSSe electrode tested at 0.25 A g-1. Although the capacity rapidly decreased during the initial 20 cycles because of the irreversible conversion reactions, the performance was recovered to 1,675 mAh g-1 during 200 cycles. It is presumably because the selenium contributes to the increases of kinetics and the interlayer space, thus improving the utilization rate of chalcogenides during formation cycle [5]. After the initial cycle, the capacity well maintained without a capacity decay for 600 cycles. Based on the above results, it is concluded that the substitution of selenium can improve the long-term stability as well as capacity in lithium-ion batteries. To clarify the effects of the substitution of selenium, we will be additionally discussed in this presentation.