Electrolytic preparation of two-dimensional layered sulphur nanomaterials and their application in lithium-sulphur batteries

Currently, two-dimensional (2D) layered materials have gained much attention for their outstanding properties. Among these materials, layered sulphur, which is an emerging 2D material, has shown great potential for various applications due to its unique chemical and physical properties. In this study, we present a new electrolytic method for synthesizing layered sulphur using thiourea as the sulphur source and poly-3,4-ethylenedioxy thiophene/polystyrene sulfonate (PEDOT: PSS) as the stabilizer. This method is simple, fast, and has a high yield. Furthermore, a composite material was prepared containing carbon nanotubes (CNTs) and layered sulphur, which was used a cathode material in lithium-sulphur batteries. The prepared cathode exhibited a discharge specific capacity of 450 mAhg−1 and a reversible capacity of 46.6% after 40 cycles of reciprocation at 0.1 C cycling current. Electrode design based on layered sulphur composites provides a new idea for the structural design of lithium-sulphur battery cathodes.


Introduction
Two-dimensional (2D) nanomaterials have become one of the focal points in basic physics and chemistry for their unique optoelectronic properties [1][2].However, graphene has a zero band gap [3], multilayer molybdenum disulfide [4] is an indirect band gap, and black phosphorus [5] is unstable and not easy to be prepared.Therefore, much researchers are devoted to explore new 2D materials.among which layered sulphur nanomaterials have attracted increasing attention.Among these, layered sulphur are a novel 2D layered nanomaterial with stable optical properties [6], excellent photoelectric conversion and photocatalytic properties.In our previous work [7], layered sulphur were prepared by liquid-phase ultrasonic exfoliation of sublimated sulphur using bovine serum protein (BSA) as stabilizer.The prepared layered sulphur have good dispersion, wide range of optical absorption, remarkable multi-band emission photoluminescence and electrochemiluminescence properties.The layered sulphur brings new opportunities for the research of 2D nanomaterials, however, the current methods for the preparation of layered sulphur are time-consuming (～100 h) and are limited by the low yields.
Currently, sulphur cathode material for lithium-sulphur batteries has become a promising battery system because of its high specific capacity [8], abundant resources [9] and environmental friendliness [10][11].However, the sulphur cathode has poor electrical conductivity, the phenomenon of body expansion, and the "shuttle effect" of the intermediate lithium polysulfide in the charging and discharging process [12][13][14].Improving the conductivity and stability of sulphur-based cathode materials is an urgent requirement for exploring new sulphur-based cathode materials.Yeon et al reported that a rod-like nanosulfurs immobilized onto the radially oriented open-porous spherical reduced graphene oxide architectures are constructed through controllable spray frozen assembly, melt recrystallization, and ozonation chemistry, which showed improved rate and cyclic capabilities of Li-S battery cathodes [15].However, there are a few studies that use 2D Layered sulphur materials as sulphur cathode materials.Up to now, only Jin et al [16] reported the preparation of layered sulphur by solidphase grinding of sublimated sulphur using dopamine as a stabilizer, and the prepared layered sulphur was used as a cathode active material for lithium-sulphur batteries, which exhibited good cycling performance.The difficulty of preparation of layerd sulphur is the main reaon.Therefore, the development of the new preparation method for layered sulphur cathode materials is significant.Due to the controlled potential or current of the electrolysis method, it is widely used in the preparation of nanomaterials.The electrolysis method was used for preparation of layered sulphur.Taking advantage of the superior conductivity of CNTs, layered sulphur was loaded by CNTs in this paper in order to improve the electrochemical activity of batteries.
Herein, a simple, fast and high-yield electrolysis method for synthesis of layered sulphur was developed, in which thiourea were used as sulphur sources and poly-3,4-ethylenedioxy thiophene/polystyrene sulfonate (PEDOT:PSS) was used as stabilizer.Using CNTs as a sulphur carrier, the synthesized layered sulphur materials were electrostatically interacted with CNTs to form layered sulphur/CNTs composite materials, which was used as cathode materials for lithium-sulphur batteries.The electrochemical behavior of the layered sulphur/CNTs composite cathode was investigated.The first discharge specific capacity of the layered sulphur/CNTs composite cathode is 888.5 mAhg -1 , which is nearly three times higher than the first discharge specific capacity (355.0 mAhg -1 ) of the sublimated sulphur S/CNTs composite cathode.The discharge capacity of the layered sulphur/CNTs composite cathode after 40 cycles of reciprocation is 450mAhg -1 , and the retention rate is 46.6 %.Thiourea (1 g) and NaCl (0.5 g) were added to 40 mL of 0.01 g/mL -1 PEDOT: PSS and was electrolyzed under direct current for 1 h, and the originally clear solution gradually turned blue and turbid, as shown in Fig. 1a.Here the precipitation is obtained by centrifugation and washing, and precipitates can reach the milligram level.Compared to other reported methods the proposed method in our text is simple, fast and high-yield.

Preparation of Layered sulphur/CNTs composites or sublimated sulphur S/CNTs
Cetyltrimethylammonium bromide (CTAB) (40 mg) and CNTs (20 mg) were added to 20 mL of ultrapure water and sonicated for 30 min to obtain dispersed CNTs.The prepared layered sulphur or sublimated sulphur (3 mL) were mixed with the dispersed CNTs (1 mL), and the mixture was sonicated for 3 min.The final solution was centrifuged at 8000 r/min for 30 min and the precipitate was washed and dried under vacuum at 60 °C for 24 h to obtain layered sulphur/CNTs composites, as shown in Fig. 1b.

Fabricatio of the positive electrode
The lithium-sulphur battery electrolyte was fabricated as follows. 1 M bistrifluoromethanesulfonimide lithium salt (LiTFSI)/1,2-Dimethoxyethane (DME): 1,3-Dioxolane (DOL) =1:1 (Vol%) with 1% LiNO3, layered sulphur/CNTs or sublimated sulphur S/CNTs (60 mg), conductive carbon black (Super P, 30 mg), and polyvinylidene fluoride (PVDF, 10 mg) were mixed together.At the same time, nmethylpyrrolidone (300 µL) was added and mixed for 40 min.The mixed slurry was then evenly brushed on aluminum foil (200 um) and dried under vacuum at 60 °C for 24 h.The transferred material was cut into 12 mm diameter circular working electrode sheets and a 13 mm lithium sheet was punched from the lithium foil as a counting/reference electrode.The TEM images of layered sulphur prepared by electrolysis are shown in Fig. 2a, which show a clear layered structure.Fig. 2b shows the SEM image of layered sulphur, which shows a clear lamellar stacking effect, reflecting the two-dimensional lamellar structure of layered sulphur.In Fig. 2c TEM image of layered sulphur/CNTs composite shows the lamellar structure belonging to layered sulphur with as well as the tubular structure of CNTs, in which the tubular materials are interlaced to form a spatial mesh structure.In Fig. 2d layered sulphur/CNTs SEM image can be seen that the mesh structure and the lamellar structure are covered with each other, indicating that CNTs can load layered sulphur well and they are well combined.The full XPS spectra of Layered sulphur and layered sulphur/CNTs are shown in Fig. 3a, b.Both show the four elements S2p, C1s, N1s and O1s.The S2p high-resolution energy spectra of layered sulphur as shown in Fig. 3c include 160.9 eV, 163.9 eV, 164.9 eV, and 167.7 eV.Where the binding energy at 163.9 eV is attributed to S 2-and the binding energy at 164.9 eV is attributed to zero-valent sulphur.The binding energy at 167.7 eV is attributed to -SO2.Thus, the layered sulphur are mainly composed of atomic sulphur and surface-abundant sulfonyl/sulfonate/S 2-.In the C1s high-resolution XPS energy spectrum of layered sulphur/CNTs (Fig. 3e), the binding energies of 284.7 and 285.3 eV correspond to the C-C/C=C bond and the C-O/C-S/C-N bond, respectively.The S2p high-resolution XPS energy spectra (Fig. 3d) also included 161.9 eV, 163.9 eV, 167.8 eV and 168.8 eV, and there was a subtle change in the binding energy of S after binding layered sulphur to CNTs compared with the high-resolution XPS energy spectra of layered sulphur.

Results and Discussion
Fig. 3f shows the XRD spectra of layered sulphur/CNTs, with obvious diffraction peaks at 23.0°, 26.0°, 27.8° and 31.8°,etc.Compared with the XRD diagram of layered sulphur (inset of Fig. 3f), the peak positions match with the sublimated sulphur (JCPDS 83-2285) diffraction peaks, where the diffraction peaks at 23° and 27.8° correspond to the sublimated sulphur ( 222) and (313) crystallographic planes of the crystals; therefore, the structure of layered sulphur does not change after binding to CNTs.Constant current charge/discharge test was performed on assembled coin cell batteries at room temperature.In the voltage range of 1.6~2.8V, the first charge/discharge current for the first circle of layered sulphur/CNTs and sublimated sulphur S/CNTs composites at 0.1 C multiplier were investigated.The results are shown in Fig. 4a.Both of them have two potential plateaus during electrolysis at 2.05 V and 2.35 V, respectively.The initial charging and discharging capacities of layered sulphur/CNTs are 888.5 and 876.7 mAhg -1 , respectively, which are much higher than those of S/CNTs (355.0 and 333.3 mAhg -1 ).This indicates that layered sulphur with two-dimensional layered structure have better charging and discharging performance than sublimated sulphur.The charge/discharge performance of layered sulphur with two-dimensional layered structure is much better than that of sublimated sulphur.
The multiplicative performance of sublimated sulphur S/CNTs and layered sulphur/CNTs was tested at different condition of 0.1 C, 0.2 C, 0.5 C, 0.8 C and 1.0 C. The comparison of the multiplicative performance Fig. 4b shows that layered sulphur/CNTs consistently exhibit higher charge/discharge capacity than than S/CNTs.After 40 cycles, the discharge capacity of layered sulphur/CNTs composite is 450 mAhg -1 .The excellent electrochemical performance of layered sulphur/CNTs materials is attributed to the synergistic effect of the special layered structure and the good electrical conductivity of CNTs.
The comparation of the cyclic voltammogram (CV) of the Li-S battery based on the positive materials of layered sulphur/CNTs composites and sublimated sulphur S/CNTs composites (inset Figure4c) is shown Fig. 4c.The CV curves of both electrode materials showed significant reduction and oxidation peaks around 2.1 V and 2.45 V.The reduction peak at 2.10 V is attributed to the reduction of sulphur to intermediate polysulfides (Li2Sx, 4 < x < 8), and the oxidation peak at 2.45 V is due to the oxidation of insoluble Li2S/Li2S2 to soluble polysulfides which was further oxidized elemental sulphur in a continuous oxidation reaction.The difference between them is that layered sulphur /CNTs have a small reduction peak at 2.30 V because the intermediate polysulfide is further reduced to insoluble Li2S or Li2S2.In contrast, because the reduction process of S/CNTs electrode is a continuous reduction state, no obvious reduction peak at 2.30 V apprared.
Electrochemical impedance spectroscopy (EIS) for different materials electrode was carried out and the results were shown in Figure 4d.Obviously, the order of conductivity of these electrodes is CNTs > layered sulphur /CNTs > Black FTO > layered sulphur, which indicates that the layered sulphur has a weak conductivity, while when it combined with the CNTs, the composite of layered sulphur/CNTs exhibited improved conductivity.Combined with the high conductivity of CNTs, layered sulphur with the unique layered structure can effectively alleviate the shuttle dissolution of lithiumsulphur batteries, increase the reaction rate of the battery, and improve the electrochemical performance of the battery.

Conclusion
A new method for the preparation of 2D nanomaterials of layered sulphur has been developed using thiourea as the sulphur source and PEDOT: PSS as the stabilizer by DC electrolysis technique.When CNTs were used as sulphur carriers, and the electrochemical performance of layered sulphur/CNTs composite as sulphur cathode electrode in lithium sulphur battery was investigated.Due to high conductivity of CNTs, and the unique layered structure of layered sulphur, which can increase the reaction rate of the battery, and improve the electrochemical performance of the battery.The discharge specific capacity of the layered sulphur/CNTs composite cathode is 888.5 mAhg -1 , which is nearly three times higher than the first discharge specific capacity (355.0 mAhg -1 ) of the S/CNTs composite cathode.The discharge capacity of the layered sulphur/CNTs composite cathode after 40 cycles of reciprocation is 450 mAhg -1 , and the retention rate is 46.6 %.The capacity decay rate is only 0.193 % for every cycle.Due to the excellent electrical conductivity of CNTs, layered sulphur loaded with CNTs effectively alleviated the shuttle dissolution of lithium ions and improved the electrochemical performance of batteries.The new synthesize method of layered sulphur provides convenience for the further study of the potential properties of 2D layered sulphur materials.The application of 2D layered sulphur in batteries provide a new idea for the structural design of lithium-sulphur battery cathodes.

Fig 1 .
Fig 1.Schematic illustration of the preparation process of layered sulphur nanomaterial preparation (a) and layered sulphur/CNTs composite (b)Thiourea (1 g) and NaCl (0.5 g) were added to 40 mL of 0.01 g/mL -1 PEDOT: PSS and was electrolyzed under direct current for 1 h, and the originally clear solution gradually turned blue and turbid, as shown in Fig.1a.Here the precipitation is obtained by centrifugation and washing, and precipitates can reach the milligram level.Compared to other reported methods the proposed method in our text is simple, fast and high-yield.

Fig 2 .
Fig 2. TEM images of layered sulphur (a) and layered sulphur/CNTs (c) and SEM images of layered sulphur (b) and layered sulphur/CNTs (d).The TEM images of layered sulphur prepared by electrolysis are shown in Fig.2a, which show a clear layered structure.Fig.2bshows the SEM image of layered sulphur, which shows a clear lamellar stacking effect, reflecting the two-dimensional lamellar structure of layered sulphur.In Fig.2cTEM image of layered sulphur/CNTs composite shows the lamellar structure belonging to layered sulphur with as well as the tubular structure of CNTs, in which the tubular materials are interlaced to form a spatial mesh structure.In Fig.2dlayered sulphur/CNTs SEM image can be seen that the mesh structure and the lamellar structure are covered with each other, indicating that CNTs can load layered sulphur well and they are well combined.

Fig 3 .
Fig 3. XPS energy spectra of layered sulphur (a) and layered sulphur/CNTs(b).(c) S2p highresolution spectra of layered sulphur.High-resolution spectra of S2p(d), C1s(e) of layered sulphur/CNTs.(f) XRD spectra of layered sulphur and layered sulphur/CNTs.The full XPS spectra of Layered sulphur and layered sulphur/CNTs are shown in Fig.3a, b.Both show the four elements S2p, C1s, N1s and O1s.The S2p high-resolution energy spectra of layered sulphur as shown in Fig.3cinclude 160.9 eV, 163.9 eV, 164.9 eV, and 167.7 eV.Where the binding energy at 163.9 eV is attributed to S 2-and the binding energy at 164.9 eV is attributed to zero-valent

Fig 4 .
(a) The initial charge-discharge profiles performed of electrode materials of S/CNTs (red line) and layered sulphur/CNTs (green line), scan rate, 50 mV/s.(b) S/CNTs and layered sulphur/CNTs of comparison of magnification performance.(c) Cyclic voltammogram of layered sulphur/CNTs and S/CNTs.(d) EIS for different materials electrode.