ABSTRACT
Distinct facets of an electrocatalyst can promote polysulfide (Li2Sn (n = 4, 6, 8) and Li2Sm (m = 1, 2)) redox kinetics in lithium-sulfur (Li-S) battery chemistry. Herein, we report that the (100) facet of tungsten disulfide (e-WS2) generated in situ by electrochemical pulverization exhibits onset potentials of 2.52 and 2.32 V vs Li/Li+, respectively, for the reduction of polysulfides Li2Sn and Li2Sm, which is unprecedented till date. In a comparable study, bulk WS2 was synthesized ex situ. The transmission electron microscopy (TEM) analysis reveals that the (100) facet was dominant in e-WS2, while the (002) facet was pronounced in bulk WS2. The density functional theory (DFT) analysis indicates that the (100) facet displays metallic-like behavior, which is highly desired for enhanced polysulfide redox kinetics. We believe that the e-WS2 produced can potentially be an excellent electrocatalyst for other applications such as hydrogen evolution reaction (HER), photocatalysis, and CO2 reduction.
ABSTRACT
An economical and binder-free electrode was fabricated by impregnation of sub-5 nm MoS2 nanodots (MoS2 NDs) onto a three-dimensional (3D) nickel substrate using the facile dip-coating method. The MoS2 NDs were successfully synthesized by controlled bath sonication of highly crystalline MoS2 nanosheets. The as-fabricated high-surface area electrode demonstrated promising electrochemical properties. It was observed that the as-synthesized NDs outperformed the layered MoS2 peers as the electrode for supercapacitors. MoS2 NDs exhibited an excellent specific capacitance (C sp) of 395 F/g at a current load of 1.5 A/g in a three-electrode configuration. In addition, the fabricated symmetric supercapacitor demonstrated a C sp value of 122 F/g at 1 A/g and a cyclic performance of 86% over 1000 cycles with a gravimetric power and energy density of 10,000 W/kg and 22 W h/kg, respectively. Owing to its simple and efficient fabrication and high surface area, such 3D electrodes show high promise for various energy storage devices.
ABSTRACT
Here, we report that in situ MoS2 and S cathodes (MGC) prepared by simple decomposition of (NH4)2MoS4 facilitate direct formation of Li2S and suppress the long-term problem associated with polysulphide shuttling in Li-S batteries. For comparison, we prepared ex situ MoS2 and S cathodes (EMS) with a similar S/MoS2 mole ratio to that of in situ-prepared cathodes. Discharge capacity of EMS cathodes dropped by 80% after first few cycles, while assembled MGC cells demonstrated an initial discharge capacity of 1649 mA h/g, achieving close to theoretical capacity of elemental sulfur (1675 mA h/g) at C/3 and a reversible capacity of 1500 mA h/g was obtained in further cycles. The MoS2 nanostructure evolution after initial discharge helped in extending the cycle life of assembled cells even at a high C rate. Density functional theory (DFT) calculation was performed to understand the structural stability of intermediate MoS3 and possible electrochemical reactions pertaining to Li+ insertion in MoS2 and S. Based on DFT studies, MoS3 undergoes stoichiometric decomposition to stable MoS2 and S. Furthermore, electrochemical analysis confirmed the redox activity of MoS2 and S at 1.3 and 1.8 V against Li/Li+, respectively.