In Situ Sulfur Reduction and Intercalation of Graphite Oxides for Li‐S Battery Cathodes

Abstract

S and edge-opened graphite oxide, chemical reduction of graphene oxide and deposition of S. [23][24][25] Because the S in these graphene-S composites is not completely encapsulated by graphene, the polysulfi de intermediates still slowly dissolve into electrolyte resulting in progressive cycling decay. The ideal structure for the carbon-S composites is to intercalate S atoms or molecules into a graphite interlayer to form S intercalated graphite compounds, thus maximizing S loading and minimizing the dissolution of polysulfi des. However, only a small amount of S can be intercalated into graphite even under the conditions of high pressure and/or high temperature due to small layer spacing (planar distance ≈0.34 nm). [ 26 ] Because of the large interlayer distance, expanded graphite has been investigated as host to embed S. The expanded graphiteembedded sulfur nanocomposites were normally synthesized by two-step reactions: thermal reduction of graphite oxides in H 2 /Ar at a high temperature (450 °C) and fl owed by S meltdiffusion at a low temperature of ≈155 °C. [ 27,28 ] Since S vapor can reduce graphite oxide (GO) at a high temperature, in this work we report a novel one-step method to synthesize S intercalated graphite by S in situ reducing GO and intercalating into the reduced graphite oxide (RGO) under vacuum at 600 °C. Figure 1 schematically depicts the preparation process of the RGO/S composite. At room temperature, sulfur exists mainly in the form of cyclooctasulfur (S 8 ), as heating the mixture of graphite oxide and S 8 to 600 °C, the large molecule S 8 will be broken into smaller chain species S 2 . [ 29,30 ] Due to the large interplanar distance of GO, these S 2 molecules can intercalate into GO to deoxygenate GO and form SO 2 gas. [ 31 ] Further S 2 intercalation into RGO will form S intercalated graphite compounds. By manipulating the interlayer distance of graphite oxide through controlling the degree of oxidation of graphite, [ 32 ] the S intercalation level can be maximized. However, the S 2 molecules deposited on the external surface and the edges of RGO interlayer may recombine to form cyclo-S 8 when the temperature is cooling down from 600 °C to room temperature. Due to the high solubility of CS 2 to S 8 , the surface S 8 can be removed using CS 2 solvent at ease. Here, we demonstrate that the RGO/S composites with 52% S loading show high capacities and long cycling stabilities. The CS 2 -wash treatment can further enhance the cycling stability of RGO/S composites. Almost no capacity decline can be observed for CS 2 -washed RGO/S composites in 225 cycles. Figure 2 a shows the X-ray diffraction (XRD) patterns of pure S, pristine graphite, GO and RGO/S composite. The XRD pattern of S exhibits very sharp and strong peaks throughout the entire diffraction range, indicating a well-defi ned crystal S 8 structure. Graphite exhibits a sharp peak at 2 θ = 26.6° corresponding to the diffraction of (002) plane with interlayer distance of ≈0.34 nm. [ 33 ]

One-pot self-assembly of graphene/carbon nanotube/sulfur hybrid with three dimensionally interconnected structure for lithium–sulfur batteries

“…The G band is usually assigned to the E 2g phonon of C sp 2 atoms, while the D band originates from a breathing-point phonon with A 1g symmetry and related to local defects and disorders. The intensity ratio of D band and G band (I D / I G ) of the GCS hybrid is higher than that of GO, which is attributed to the increase of edges and the unrepaired defects after the removal of oxygen-containing functional groups and this result is similar with the previous works [34,35]. FTIR spectra are shown in Fig.…”

Flexible one-dimensional carbon–selenium composite nanofibers with superior electrochemical performance for Li–Se/Na–Se batteries

“…To meet the requirements of large-scale energy storage devices such as electric vehicles (EV), plug-in hybrid vehicles (HEV) and grid energy storage, it is highly desired to develop LIBs with high energy density, long cycle life and high rate capability [4]. To further improve the cycle life and energy density of current Li-ion batteries, novel alternative electrode materials with high power density and high energy density should be used.…”

A porous nitrogen and phosphorous dual doped graphene blocking layer for high performance Li–S batteries

“…High natural abundance, low cost, and environmental-friendliness of sulfur bestows sulfur a promising raw material of cathode for Li-S rechargeable batteries 1,3,4 . However, commercialization of Li-S batteries is hindered by several barriers: (i) the insulating nature of elemental sulfur and intermediates Li 2 S 2 /Li 2 S results in low utilization of the active materials 4,5 ; (ii) the dissolution of sulfur and intermediate polysulfides (Li 2 S x , 2＜x≤8) into organic liquid electrolyte causes significant loss of the active mass and thus the passivation of the lithium anode, namely shuttling effect 5 ; and (iii) the huge volume expansion (as high as 80%) of the electrode, which leads to the poor integrity and stability of the electrode 5 .…”