A CORAL-LIKE COMPOSITE MATERIAL AND A METHOD OF PREPARING THE SAME

Abstract

There is provided a coral-like composite material comprising highly dispersed conductive metal nitride, metal carbide or metal carbonitride nanoparticles on mesoporous carbon nanosheets, and a method of preparing the same. There is also provided a coating material for a modified separator of a lithium-sulfur battery comprising the coral-like composite material as described herein, a conducting carbon material and a binder, and a method of preparing the same.

Claims

1. A coral-like composite material comprising highly dispersed conductive metal nitride, metal carbide or metal carbonitride nanoparticles on mesoporous carbon nanosheets.

2. The coral-like composite material of claim 1, wherein the size of the metal nitride, metal carbide or metal carbonitride nanoparticles is in the range of about 2 nm to about 20 nm.

3. The coral-like composite material of claim 1, wherein the metal element from the metal nitride, metal carbide or metal carbonitride nanoparticles is scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, cadmium, hafnium, tantalum, tungsten, rhenium, osmium, iridium, platinum, or their combinations thereof.

4. The coral-like composite material of claim 1, wherein the metal carbide is niobium carbide, titanium carbide, tungsten carbide, molybdenum carbide, vanadium carbide, hafnium carbide, niobium titanium carbide, chromium carbide, niobium tungsten carbide, niobium molybdenum carbide, niobium vanadium carbide, niobium hafnium carbide, titanium tungsten carbide, titanium molybdenum carbide, titanium vanadium carbide, titanium vanadium chromium carbide, titanium hafnium carbide, tungsten molybdenum carbide, tungsten vanadium carbide, tungsten hafnium carbide, molybdenum vanadium carbide, molybdenum hafnium carbide, vanadium hafnium carbide or their mixtures thereof.

5. The coral-like composite material of claim 1, wherein the metal nitride is titanium nitride, tungsten nitride, molybdenum nitride, vanadium nitride, niobium nitride, zirconium nitride, hafnium nitride, titanium tungsten nitride, titanium molybdenum nitride, titanium vanadium nitride, titanium niobium nitride, titanium vanadium chromium nitride, titanium zirconium nitride, titanium hafnium nitride, titanium chromium nitride, tungsten molybdenum nitride, tungsten vanadium nitride, tungsten niobium nitride, tungsten zirconium nitride, tungsten hafnium nitride, molybdenum vanadium nitride, molybdenum niobium nitride, molybdenum zirconium nitride, molybdenum hafnium nitride, vanadium chromium nitride, vanadium niobium nitride, vanadium zirconium nitride, vanadium hafnium nitride, niobium zirconium nitride, niobium hafnium nitride, zirconium hafnium nitride or their mixtures thereof.

6. The coral-like composite material of claim 1, wherein the metal carbonitride is vanadium carbonitride, titanium carbonitride, titanium vanadium chromium carbonitride, tungsten carbonitride, molybdenum carbonitride, niobium carbonitride, zirconium carbonitride, vanadium titanium carbonitride, vanadium chromium carbonitride, vanadium tungsten carbonitride, vanadium molybdenum carbonitride, vanadium niobium carbonitride, vanadium zirconium carbonitride, titanium tungsten carbonitride, titanium molybdenum carbonitride, titanium niobium carbonitride, titanium zirconium carbonitride, tungsten molybdenum carbonitride, tungsten niobium carbonitride, tungsten zirconium carbonitride, molybdenum niobium carbonitride, molybdenum zirconium carbonitride, niobium zirconium carbonitride or their mixtures thereof.

7. The coral-like composite material of claim 1, wherein the metal carbide, metal nitride or metal carbonitride nanoparticles further comprises surface metal oxides.

8. The coral-like composite material of am claim 1, wherein the coral-like composite material has: a) a surface area larger than 100 m.sup.2/g; b) a pore volume in the range of about 0.5 cm.sup.3/g to about 2 cm.sup.3/g; or c) a pore size in the range of about 2 nm to about 50 nm.

9. (canceled)

10. (canceled)

11. A method for preparing a coral-like composite material comprising the steps of a) mixing a mixture of a precursor of metal nitride, metal carbide or metal carbonitride material and a graphitic carbon nitride material; and b) drying the mixture and heating solids obtained from dried mixture at a first elevated temperature for a first time period and at a second elevated temperature for a second time period in an inert atmosphere.

12. The method of claim 11, wherein the precursor of the metal nitride metal carbide or metal carbonitride material is a transition metal alkoxide, a transition metal acetylacetonate or a transition metal chloride.

13. The method of claim 11, wherein the first elevated temperature of step b) is in the range of about 400° C. to about 700° C. and the first time period of step b) is more than 3 hours.

14. The method of claim 11, wherein the second elevated temperature of step b) is in the range of about 750° C. to about 1000° C. and the second time period of step b) is more than 2 hours.

15. A coating material for a modified separator of a lithium-sulfur battery comprising a coral-like composite material, a conducting carbon material and a binder, the coral-like composite material comprising highly dispersed conductive metal nitride, metal carbide or metal carbonitride nanoparticles on mesoporous carbon nanosheets.

16. The coating material of claim 15, wherein the conducting carbon material is selected from the group consisting of reduced graphene oxide, graphene, graphite, carbon nanotube, carbon fiber, acetylene black, and ketjenblack.

17. The coating material of claim 15, wherein the conducting carbon material has a diameter in the range of about 0.1 nm to about 100 μm.

18. The coating material of claim 15, wherein the coating material has a thickness in the range of about 5 μm to about 70 μm, or wherein the coating material has a mass density in the range of about 0.5 mg cm.sup.−2 to about 3 mg cm.sup.−2.

19. (canceled)

20. A method for preparing a modified separator for lithium-sulfur battery comprising the steps of a) mixing a mixture of a coral-like composite material, a conducting carbon material and a binder, wherein the coral-like composite material comprises highly dispersed conductive metal nitride, metal carbide or metal carbonitride nanoparticles on mesoporous carbon nanosheets; and b) coating the mixture on a porous and non-electrically-conductive membrane.

21. The method of claim 20, wherein the conducting carbon material is selected from the group consisting of reduced graphene oxide, graphene, graphite, carbon nanotube, carbon fiber, acetylene black, and ketjenblack.

22. The method of claim 20, wherein the porous and non-electrically-conductive membrane is a glass fiber membrane, a polypropylene and/or a polyethylene electrolytic membrane.

23. A lithium-sulfur battery comprising a coating material for a modified separator of a lithium-sulfur battery comprising a coral-like composite material, a conducting carbon material and a binder, the coral-like composite material comprising highly dispersed conductive metal nitride, metal carbide or metal carbonitride nanoparticles on mesoporous carbon nanosheets.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0085] The accompanying drawings illustrate a disclosed embodiment and serves to explain the principles of the disclosed embodiment. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention.

[0086] FIG. 1 is a schematic diagram showing the synthesis of coral-like nanomaterials and preparation of modified separator.

[0087] FIG. 2 shows XRD patterns of the coral-like nanomaterials (a) TiN/C, (b) V.sub.2CN/C, and (c) NbC/C.

[0088] FIG. 3 shows (a) nitrogen adsorption-desorption isotherm and (b) B.J.H. desorption pore size distribution of TiN/C, V.sub.2CN/C, NbC/C, and g-C.sub.3N.sub.4 template.

[0089] FIG. 4 shows a number of transmission electron microscopy (TEM) images of (a) g-C.sub.3N.sup.4, (b) TiN/C, (c) V.sub.2CN/C, and (d) NbC/C.

[0090] FIG. 5 shows TEM images (top) and HR-TEM (bottom) images of (a) TiN/C, (b) V.sub.2CN/C, and (c) NbC/C.

[0091] FIG. 6 shows (a) UV-vis spectrum of 2 mM Li.sub.2S.sub.4 solution before, and after LiPS adsorption with TiN/C, V.sub.2CN/C and NbC/C. (b, c, d) XPS spectra (top) before and (bottom) after LiPS adsorption.

[0092] FIG. 7 shows Raman spectrum of TiN/C, V.sub.2CN/C and NbC/C. D band at 1356 cm.sup.−1 and G band at 1583 cm.sup.−1 are associated with disorder and graphitic nature of carbon, respectively.

[0093] FIG. 8 shows (a) XPS spectrum, and (b) Ti 2p, (c) N 1s, (d) C 1s and (e) O 1s core scans of TiN/C.

[0094] FIG. 9 shows (a) XPS spectrum, and (b) V 2p, (c) N 1s, (d) C 1s and (e) O 1s core scans of V.sub.2CN/C.

[0095] FIG. 10 shows (a) XPS spectrum, and (b) Nb 3d, (c) N 1s, (d) C 1s and (e) O 1s core scans of NbC/C.

[0096] FIG. 11 shows calibration curve for Li.sub.2S.sub.4 solution at a wavelength of 320 nm.

[0097] FIG. 12 shows XPS S2p spectra of PS adsorbed on coral-like nanomaterials, (a) Li.sub.2S.sub.4—TiN/C, (b) Li.sub.2S.sub.4—V.sub.2CN/C, (c) Li.sub.2S.sub.4—NbC/C and (d) pure Li.sub.2S.sub.4.

[0098] FIG. 13 shows (a) CV of cycled cells with unmodified separator, and separators modified with TiN/C, V.sub.2CN/C and NbC/C. Arrows indicating redox onset potentials. (b) Nyquist plots of cycled cells with unmodified separator, and separators modified with TiN/C, V.sub.2CN/C and NbC/C. Inset: high-frequency region with electrochemically fitted circuit.

[0099] FIG. 14 shows (a) rate capability studies of cells containing unmodified separator, and separators modified with TiN/C, V.sub.2CN/C and NbC/C at various C rates. (b) First discharge cycle of cells containing unmodified separator, and separators modified with TiN/C, V.sub.2CN/C and NbC/C at 0.05 C.

[0100] FIG. 15 shows (a,b) long-term cycling performance of cells with unmodified separator, and separators modified with TiN/C, V.sub.2CN/C and NbC/C at 0.2 C. (c) Q.sub.H analysis of cells containing separators modified with TiN/C, V.sub.2CN/C and NbC/C at 0.2 C.

DETAILED DESCRIPTION OF FIGURES

[0101] As shown in FIG. 1, according to this disclosure, there is provide a precursor of a graphitic carbon nitride material 100, which was subjected to a heating step 10 at a temperature (for example, at 500° C.) for a period of time (for example, of 3 hours). A graphitic carbon nitride material 200 was obtained and was then mixed with metal alkoxides 300 in a solvent 400. The mixture was then subjected to a heating step 20 at a first elevated temperature (for example, at 650° C.) for a period of time (for example, of 4 hours) and at a second elevated temperature (for example, at 800° C.) for a period of time (for example, at 3 hours) in an inert atmosphere 30. The obtained solid was mixed with a conducting carbon material 500 and a binder 600 in a solvent 700, and the mixture was filtered through a glass fiber membrane to obtain a modified separator 800.

EXAMPLES

[0102] Non-limiting examples of the invention will be further described in greater detail by reference to specific Examples, which should not be construed as in any way limiting the scope of the invention.

Example 1: Synthesis of the Coral-Like Nanomaterials

[0103] Urea was purchased from Sigma-Aldrich Pte. Ltd. (Singapore). Urea (30 g) was placed in a covered alumina crucible and heated to 500° C. in a muffle furnace in air for 4 hours to obtain yellow graphitic carbon nitride (g-C.sub.3N.sup.4). g-C.sub.3N.sub.4(2 g) was dried in a 250-mL Schlenk flask overnight at 150° C. under vacuum. After cooling down to room temperature, anhydrous tetrahydrofuran (30 mL) was added into the flask and stirred for 30 minutes.

[0104] Precursors Ti(OnBu).sub.4, VO(OEt).sub.3 or Nb(OEt).sub.5 and Tetrahydrofuran (THF) were purchased from Sigma-Aldrich Pte Ltd. (Singapore). Ti(OnBu).sub.4, VO(OEt).sub.3 or Nb(OEt).sub.5 (5 mmol) were added dropwise to the g-C.sub.3N.sub.4/THF mixture under vigorous stirring, after which the mixture was further stirred for 30 minutes in a glovebox before solvent removal using a rotary evaporator under reduced pressure. Next, the solids were dried under high vacuum overnight before transferring to an alumina boat, which was then covered with a flat quartz plate and secured at both ends using copper wires. The boat was then placed inside a tube furnace, and heated under an argon flow (500 mL/min) at 650° C. for 4 hours (ramp rate: 2° C./min), followed by 800° C. for 3 hours (ramp rate: 1° C./min). The furnace was then allowed to cool down to room temperature. While maintaining an argon flow at a rate of 500 mL/min, passivation was conducted by first introducing nitrogen at a flow rate of 500 mL/min for 2 hours, followed by introducing air at a flow rate of 50 mL/min for 4 hours. 400-500 mg of black solids were obtained.

Example 2: Preparation of Sulfur Cathode

[0105] Cathodes were prepared via a two-step procedure involving scaffold formation, followed by vapor-phase sulfur deposition. Vapor grown carbon fiber was purchased from Beijing DK Nanotechnology Co. Ltd. (China) Graphitized carbon nanotube and LA-132 were purchased from XF nano Co. Ltd (China) and Chengdu Indigo Power Sources Co. Ltd (China), respectively. An aqueous slurry mixture of VGCF, graphitized carbon nanotube and LA-132 was prepared in a 60:30:10 weight ratio and casted on a carbon-coated aluminum foil. After drying at 60° C. for 4 hours, the carbon scaffold was punched into circular disks of 10 mm in diameter. The weight of each scaffold was 6.2 to 6.8 mg. Next, the scaffold was placed on a stainless steel mesh of about 1 mm above a heated S reservoir (175° C.) for about 25 minutes to obtain 5.2 to 5.7 mg of loaded S corresponding to a loading density of 6.0 to 7.0 mg S per cm.sup.2.

Example 3: Preparation of Modified Separator

[0106] PEO binder was purchased from Sigma-Aldrich Pte. Ltd. (Singapore).. TiN/C, V.sub.2CN/C or Nb.sub.2CN/C (12 mg), VGCF (4 mg) and PEO (M.sub.v of about 5×10.sup.6, 1 wt %, 200 μL) were dispersed in absolute ethanol (100 mL) under sonication for 30 minutes. Next, the dispersion was poured directly through a glass fiber membrane (GF/A, Whatman, 47 mm) under suction, dried under vacuum overnight, and chopped into circular disks of 16.2 mm to obtain the modified separator. The average thickness and mass density of the coating were about 50 μm and about 1.5 mg cm.sup.2, respectively.

Example 4: Preparation of the Modified Separator and Battery Cells

[0107] Sublimed sulfur (S), lithium sulfide (Li.sub.2S), dimethoxyethane (DME) were purchased from Sigma-Aldrich Pte. Ltd. (Singapore). Li.sub.2S.sub.4 solution (2 mM) was prepared in a glovebox by adding Li.sub.2S and S.sub.8 in appropriate amounts to 1,2-dimethoxyethane (DME) and subjected to overnight stirring at 50° C. Li.sub.2S.sub.4 solution (4 mL) was added to TiN/C, V.sub.2CN/C or Nb.sub.2CN/C (5 mg) and stirred overnight. The supernatant obtained via centrifugation was analyzed using UV-visible spectrophotometer. Residues were washed with DME and dried before XPS analysis.

Example 5: Characterization

[0108] Materials were characterized by field emission SEM (JEOL JSM-7400F), TEM (FEI Tecnai F20), energy-dispersive X-ray spectrometry (Oxford X-MaxN), XRD (Bruker D8 ADVANCE), thermogravimetric analysis (PerkinElmer Pyris 1 TGA), inductively-coupled plasma (ICP) optical-emission spectroscopy (PerkinElmer Optima 5300DV) and elemental analysis (Thermo Flashsmart elemental analyzer (CHNS)). Nitrogen adsorption-desorption isotherms at −196° C. were collected using Micromeritics ASAP 2460 physisorption analyzer. Samples (˜60 mg) were degassed at 120° C. for 12 hours before measurement. Specific surface areas were calculated using the BET (Brunauer-Emmet-Teller) method. Pore size distributions (PSD) were obtained by the Barrett, Joyner, and Halenda (BJH) method using the cylindrical pore model. XPS measurements were obtained using PHI Quantera SXM Scanning X-ray Microprobe with a Al Kα X-ray source, and the signals were collected at a take-off angle of 45°. XPS spectral fitting was done using the CasaXPS software. UV-vis spectra were obtained for 250-500 nm at a resolution of 1 nm using a Biotek Cytation 5 imaging reader with a sealed quartz. Raman spectroscopy was performed on a Horiba Jobin Yvon Modular Raman Spectrometer using an argon-ion laser at 514 nm calibrated with a silicon reference.

[0109] X-Ray Diffraction (XRD)

[0110] To determine the crystal phase and purity of the coral-like nanomaterials, X-ray diffraction (XRD) studies were conducted. XRD analysis revealed that the nanomaterials consisted of a single, pure phase of cubic (TiN).sub.0.88, V.sub.2CN and NbC.sub.0.87, respectively (FIG. 2). Applying Scherrer's formula to the (200) planes, the average crystallite sizes were calculated to be 3.1, 4.0 and 4.1 nm, respectively for TiN, V.sub.2CN and NbC.

[0111] Nitrogen Adsorption-Desorption Analysis

[0112] The surface area and pore volume of these coral-like nanomaterials were found to be much higher than its urea-derived g-C.sub.3N.sub.4 template, which decomposed during heat treatment (Table 1).

TABLE-US-00001 TABLE 1 Surface area, pore volume and pore size of the coral-like nanomaterials. Surface Area Pore Volume Pore Size Material [m.sup.2/g].sup.a [cm.sup.3/g].sup.b [nm].sup.c g-C.sub.3N.sub.4 54 0.19 40.3 TiN/C 277 1.15 34.2 V.sub.2CN/C 240 1.11 30.1 NbC/C 174 0.84 34.9 .sup.aCalculated using Brunauer-Emmet-Teller (B. E. T.) method. .sup.bObtained at P/P.sub.0 = 0.988. .sup.cDetermined at the peak of the Barrett-Joyner-Halenda (B. J. H.) pore size distribution.

[0113] Nitrogen adsorption-desorption analysis of the coral-like nanomaterials revealed a type III isotherm with H3 hysteresis loop (FIG. 3a). Pore sizes of TiN/C, V.sub.2CN/C and NbC/C were smaller than that of the g-C.sub.3N.sub.4 template, suggested that the metal alkoxides were impregnated within the template pores during synthesis (FIG. 3b).

[0114] Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM)

[0115] The particle morphology and nanostructure were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. SEM showed that TiN/C, V.sub.2CN/C and NbC/C have the similar coral-like structure of g-C.sub.3N.sub.4(FIG. 4). TEM revealed that these coral-like nanomaterials consisted of a high dispersion of nanoparticles on a sheet-like carbon structure (FIG. 5). High-resolution TEM (HR-TEM) further confirmed that these nanoparticles were 3 to 4 nm in size, which agreed well with the average crystallite sizes calculated from XRD peak widths. The presence of carbon was confirmed by broad XRD peak at 2θ of about 220 (002 plane of graphite), Raman spectroscopy and elemental analysis (FIG. 1 and FIG. 6, Table 2).

TABLE-US-00002 TABLE 2 Elemental analysis of g-C.sub.3N.sub.4, TiN/C, V.sub.2CN/C and NbC/C. Material C (wt %).sup.a H (wt %).sup.a N (wt %).sup.a Metal (wt %).sup.b g-C.sub.3N.sub.4 33.3 1.9 58.6 — TiN/C 36.9 0.6 10.9 Ti, 34.2 V.sub.2CN/C 35.5 0.6 5.3 .sup. V, 41.5 NbC/C 22.4 0.3 2.3 Nb, 62.2  .sup.aCHNS analysis. .sup.bICP analysis.

[0116] X-Ray Photoelectron Spectroscopy (XPS) and UV-Vis Spectroscopy

[0117] XPS revealed the rich surface bonding of the coral-like nanomaterials (FIG. 8 to FIG. 10). In general, the presence of XPS signals assigned to C—N, N—O, C—O bonds were likely to be due to unreacted g-C.sub.3N.sub.4 template or surface passivation in air. Surface passivation also resulted in the formation of metal oxide bonds, which were confirmed by the high intensity Ti—O, V—O and Nb—O XPS signals in TiN/C, V.sub.2CN/C and NbC/C, respectively. These metal oxides were amorphous since no crystalline oxide phases were detected by XRD (FIG. 2). The presence of surface metal oxides might be beneficial to suppressing PS shuttling since metal oxides are known to have excellent PS adsorption capabilities.

[0118] Ti—N bond in TiN/C was confirmed by XPS signals associated with Ti—N in the Ti 2p and N 1s regions at binding energies (BE) of 456.97 (Ti.sub.2p3/2) and 396.37 eV, respectively (FIGS. 7b and c). For V.sub.2CN/C, XPS signal at BE of 514.57 eV (V.sub.2p3/2) in the V 2p region could be assigned to both V—C and V—N bonds (FIG. 8b). This was corroborated with XPS signals in the N 1s region at a BE of 397.99 for V—N, and in the C 1s region at a BE of 283.07 for V—C(FIGS. 8c and d). For NbC/C, XPS signals at BE of 204.37 eV (Nb.sub.3d5/2) and 283.05 eV in the Nb 3d and C 1s regions, respectively, could be assigned to Nb—C bond (FIG. 9 b and d).

[0119] UV-vis spectroscopy and XPS were employed to evaluate the PS adsorption capability of the coral-like nanomaterials (FIG. 5). Li.sub.2S.sub.4 was selected for the study since it was reported to be the main PS in the dioxolane/dimethoxyethane electrolyte system. UV-vis spectrum revealed that the coral-like nanomaterials have reduced absorbance intensity as compared to the initial 2 mM PS curve, indicating PS adsorption (FIG. 5a). Using a calibration curve based on absorbance of standard solutions (FIG. 10), the amounts of Li.sub.2S.sub.4 adsorbed by V.sub.2CN/C, TiN/C and NbC/C were calculated to be 0.198, 0.144 and 0.119 mg/mg, respectively.

[0120] The nature of the adsorption was further studied using XPS. In general, XPS signals in the S 2p region of S corresponding to bridging S (S.sub.B), terminal S (S.sub.T) and sulfates were present in PS-adsorbed materials. BE of both S.sub.B and S.sub.T shifted to a higher energy as compared to that of pure Li.sub.2S.sub.4(FIG. 11), implying a decrease in electron density for S. Conversely, a shift to a lower BE was observed for the metal-nitrogen (M-N) (i.e. Ti—N, V—N) and metal-carbon (M-C) (i.e. V—C, Nb—C) signals, indicating an increase in electron density at the metal center, which could be attributed to strong metal-S interactions (FIG. 5b-d). These BE shifts indicated strong chemical interaction of these coral-like nanomaterials with PS. Notably, the shift in BE was more significant for the M-N and M-C bonds than the metal-oxygen (M-O) bonds, suggesting that PS had a higher affinity to M-N and M-C bonds than M-O bond, and that the amorphous M-O layer was thin and labile. In addition, the BE shifts of V—N and V—C in V.sub.2CN/C (0.86 eV) were found to be greater in magnitude as compared to Ti—N and Ti—N—O in TiN/C (0.40 eV) and Nb—C in NbC/C (0.24 eV), suggesting that PS interaction with V.sub.2CN/C was the strongest, followed by TiN/C and NbC/C. A stronger interaction would result in greater bond polarization, leading to a lower activation energy for electrochemical reactions in LSB. Although BE shifts were commonly reported in the literature, such comparison amongst metal compounds at this size range (3 to 4 nm) was not reported previously. Based on the above studies, V.sub.2CN/C was found to have the best PS adsorption ability, followed by TiN/C and NbC/C.

[0121] Cyclic Voltammetry (CV) and Electrical Impedance Spectroscopy (EIS)

[0122] CV curves of batteries containing both modified and unmodified separators revealed features typical of a LSB system: two sharp reduction peaks and a broader oxidation peak (FIG. 12a). These peaks were found to be sharper in batteries with modified separator. The area under the curve, for the modified separators appeared to be larger than that for the unmodified separator, indicating a greater charge storage capacity. In addition, as compared to the unmodified separator, the higher onset reduction potential and lower onset oxidation potential for the modified separator indicated a lower activation energy barrier for the electrochemical reactions in the LSB cells. EIS revealed that the charge transfer resistance (R.sub.CT) of the TiN/C, V.sub.2CN/C, NbC modified separator and the unmodified separator were 8.5, 11.4, 11.9 and 40.0, respectively (FIG. 12b). The reduced R.sub.CT values after separator modification could be due to the high electrical conductivity of TiN, V.sub.2CN and NbC, which could enhance the surface charge transfer reactions for better electrochemical performance.

[0123] Coin Cell Preparation and Electrochemical Testing

[0124] Standard 2032-type coin cells were used for cell cycling and rate capability tests. Assembly was done in an argon-filled glovebox, with the 10-mm cathodes and lithium foil as the anode/reference electrode. The electrolyte was prepared by adding 1 M LiTFSI and 2 wt % LiNO.sub.3 to a 1:1 volume mixture of 1,3-dioxolane (DOL) and DME. The modified separators, soaked with electrolyte, were inserted in between the cathode and the anode. Galvanostatic charge-discharge cycling was done using a LAND CT2001 battery tester (Wuhan LAND electronics) between 1.7 V and 2.8 V vs. Li/Li.sup.+. Cyclic voltammograms were obtained at a scan rate of 0.05 mV s.sup.−1, and electrochemical impedance spectra were collected with a 10 mV amplitude at open circuit potential between 1 MHz and 0.01 Hz on an M204 Autolab potentiostat (Metrohm) fitted with a frequency response analyzer module.

[0125] Electrochemical Performance

[0126] The electrochemical performance of the batteries was evaluated by subjecting them to rate capability tests at different C rates for 5 cycles each, and long-term cycling at a fixed C rate (1 C=1673 mA g.sup.−1) over 100 cycles. Rate capability studies showed that the specific discharge capacities of the batteries having separators modified with TiN/C, V.sub.2CN/C and NbC/C were 2.5 times higher than the unmodified control cell at all rates (FIG. 13a, Table 3), corroborating well with the larger area observed for the modified separators from CV analysis mentioned earlier.

TABLE-US-00003 TABLE 3 Average specific capacities of the modified separators at different C rates. Separator 0.1 C 0.2 C 0.5 C 1 C — 472 399 119 57 TiN/C 1203 1077 947 822 V.sub.2CN/C 1107 972 745 693 NbC/C 1118 993 823 727

[0127] The discharge curves of voltage versus specific capacity all showed two plateaus typical of LSB: the first plateau (Q.sub.H) at a higher voltage of about 2.35 V versus Li, and the second plateau (Q.sub.L) at a lower voltage of about 2.05 V versus Li (FIG. 13b). The overall specific capacity of LSB was determined to be the summation of the capacity contribution from Q.sub.H and Q.sub.L, associated with sulfur dissolution to soluble PS and its subsequent reduction to insoluble Li.sub.2S or Li.sub.2S.sub.2, respectively. Both Q.sub.H and Q.sub.L capacities were found to be larger for the modified separators due to the availability of the large surface areas and redox-active sites of TiN/C, V.sub.2CN/C and NbC/C for facile sulfur dissolution into PS and subsequent PS reduction to insoluble sulfides (FIG. 5b, Table 1).

[0128] Amongst the modified separators, the highest specific capacity was obtained for the TiN/C material, followed by NbC/C and V.sub.2CN/C at all rates based on the rate capability studies (FIG. 12b, Table 2). To determine if these high capacities could be sustained for repeated charge and discharge cycles, long-term cycling studies were conducted at 0.2 C. At 0.2 C, the initial specific discharge capacities of separator modified with TiN/C, V.sub.2CN/C and NbC/C and the unmodified separator were 1051, 963, 921, 401 mAh g.sup.−1 (FIG. 14a), corresponding to areal capacities of 6.73, 6.35, 5.99, 2.65 mAh cm.sup.−2, respectively (FIG. 14b). After 100 cycles, specific discharge capacities of 853, 877, 719, 294 mAh g.sup.−1, corresponding to areal capacities of 5.46, 5.79, 4.67, 1.94 mAh cm.sup.−2, were retained for separator modified with TiN/C, V.sub.2CN/C and NbC/C and the unmodified separator, respectively.

[0129] The areal capacities obtained using the separators modified with the coral-like nanomaterials exceeded that of current LIBs (4 mAh cm.sup.−2) even after 100 cycles, indicating their great potential as practical, high loading LSB. Capacity retention at 0.2 C was found to be the highest for V.sub.2CN/C (91.1%), followed by TiN/C (81.2%), NbC/C (78.1%) and unmodified separator (73.3%). Although the separator modified with the V.sub.2CN/C material had the lowest capacity, its ability to retain capacity, a serious issue in high loading cathodes in LSB, was superior as compared to TiN/C and NbC/C. Using the Q.sub.H values extracted from the discharge curves, a quantitative assessment on the PS-trapping ability (Q.sub.H retention) of each separator, could be obtained. The relative Q.sub.H retention for V.sub.2CN/C, TiN/C and NbC/C were 73.3%, 70.8% and 69.6%, respectively (FIG. 14c). Thus, V.sub.2CN/C has the best PS adsorption capability, followed by TiN/C and NbC/C, agreeing well with the PS adsorption studies conducted with UVS and XPS.

[0130] The use of conductive and highly dispersed nanoparticles of TiN, V.sub.2CN and NbC on a coral-like carbon structure have been demonstrated here as an effective PS barrier for high loading LSB. V.sub.2CN/C-modified separator was found to have the highest reversible specific capacity and capacity retention of 877 mAh g.sup.−1 (5.79 mAh cm.sup.−2) and 91.1%, respectively, after 100 cycles at 0.2 C. This could be attributed to its superior PS adorption capability as shown by UVS, XPS and Q.sub.H analysis studies. Although reversible capacities and capacity retention were found to be lower for TiN/C (853 mAh g.sup.−1 and 81.2%) and NbC/C (719 mAh g.sup.−1 and 78.1%), their areal capacities of 5.46 and 4.67 mAh cm.sup.−2, respectively, were still higher than 4 mAh cm.sup.−2—a value obtained by current state-of-the-art LIBs. As such, the design and construction of an effective PS barrier that has high PS adsorption and binding capabilities as demonstrated here, are parameters required to overcome the current limitations of LSB with high sulfur loadings.

INDUSTRIAL APPLICABILITY

[0131] In the present disclosure, the coral-like composite material, the coating material may be used for a modified separator of a lithium-sulfur battery with practical and high areal capacity. The design strategy of optimizing polysulfide adsorption via the use of novel highly dispersed and conductive nanoparticles on a high surface area, coral-like carbon matrix for separator modification represents an effective method to suppress polysulfide shuttling and improve electrochemical performance of lithium-sulfur battery with high sulfur loading.

[0132] It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.