Abstract
A dual function cathode consisting of tungsten disulfide and porous carbon nanosheets (WS2/C) was synthesized to improve the performance of lithium sulfur batteries. The well-defined structure is composed of ≤5 layers with 0.62 nm interlayer spacing corresponding to the (002) facial plane of WS2. The composite depicted very strong affinity toward lithium polysulfides. Rapid transport of lithium ions was also revealed. The cathode demonstrated excellent cycling stability and rate capability by delivering a reversible specific capacity of 419mAh g−1 at 8C after 500 cycles with low capacity fading at 0.04% per cycle. At high sulfur loading of 4.7mg cm−2 the batteries delivered 3.4mAh cm−2 areal capacity after 100 cycles at 0.5C. The synergistic effect of strong chemical interaction between lithium polysulfides and WS2, and the superior electronic conductivity of carbon nanosheets are responsible for the enhanced performance. It also suppressed the self-discharge phenomenon by maintaining 94.5% of its initial capacity after 10 days resting. The electrochemical impedance spectroscopy (EIS) analysis demonstrated that even after 400 cycles, the interfacial and charge transfer resistances only increased by 1.2 and 1.7Ω, respectively, describing faster electrochemical kinetics by inhibiting the formation of insulating layer of lithium sulfide (Li2S) on the surface of the electrodes.
Highlights
In recent years tremendous attention has been focused on the development of alternative materials for the generation energy storage devices for their application in high performance electronic devices and electric vehicles
The peak corresponding to (002) plane of WS2/C composite shifts to lower Bragg angle compared to pure WS2, which indicates the increase in lattice d- spacing of WS2/C (d = 0.62 nm at 2θ = 14.23◦) with respect to pure WS2 (d = 0.61 nm at 2θ = 14.36◦).[35]
Raman peaks of WS2/C nanosheets and pure WS2 as shown in Fig. 2b, reveal two sharp peaks situated at 351 and 417 cm−1, which are attributed to the E12g and A1g vibrational modes of WS2.35,36 The first order optical modes of E12g represents the in- plane displacement of W and S atoms and A1g depicts the out of plane symmetric displacement of S atoms along the c-axis.[36,37]
Summary
In recent years tremendous attention has been focused on the development of alternative materials for the generation energy storage devices for their application in high performance electronic devices and electric vehicles. The spatial entrapment of polysulfide species is not sufficient for long period of cycling owing to the weak intermolecular interaction between the non-polar carbon structures and polar lithium polysulfides. Another effective approach to retain lithium polysulfides is the modification of porous carbon using oxygen/sulfur/nitrogen doping[20,21,22] and/or some polymer coating on the surface of porous carbon,[23] these strategies still need. Benefitting from the favorable architecture, the sulfur rich electrode of 70 wt% of active material delivers a high specific discharge capacity and excellent cycling performance, with a low capacity fade rate of 0.039%/cycle after 500 cycles at 2.0 C rate (80.50% capacity retention)
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