A Cathode Material and a Method of Preparing The Same

Abstract

There is provided a cathode material comprising a layer of sulfur species dispersed within or thereon a porous matrix comprising a first conducting carbon material, a second conducting carbon material and a binder, wherein the second conducting carbon material is carbon fiber or carbon nanotube. There is also provided a cathode material comprising a layer of sulfur species dispersed within or thereon a porous matrix comprising a first conducting carbon material, a second conducting carbon material and a binder, wherein said porous matrix is interconnected with uniform pores. There are also provided methods for preparing the above cathode material(s).

Claims

1. A cathode material comprising a layer of sulfur species dispersed within or thereon a porous matrix comprising a first conducting carbon material, a second conducting carbon material and a binder, wherein the second conducting carbon material is carbon fiber or carbon nanotube.

2. A cathode material comprising a layer of sulfur species dispersed within or thereon a porous matrix comprising a first conducting carbon material, a second conducting carbon material and a binder, wherein said porous matrix is interconnected with uniform pores.

3. The cathode material according to claim 1 or 2, wherein the sulfur species is a polysulfide or elemental sulfur.

4. The cathode material according to claim 3, where the polysulfide has a formula of Li.sub.2S.sub.n, wherein 2<n≤8.

5. The cathode material according to any one of the preceding claims, wherein the cathode material has a sulfur content in the range of about 30 wt % to about 80 wt % based on the total weight of the cathode material.

6. The cathode material according to any one of the preceding claims, wherein the first conducting carbon material is selected from the group consisting of reduced graphene oxide, graphene, graphite, carbon nanotube, carbon fiber, acetylene black, and ketjenblack.

7. The cathode material according to any one of the preceding claims, wherein the first conducting carbon material is different from the second conducting carbon material.

8. The cathode material according to any one of the preceding claims, wherein the first conducting carbon material is reduced graphene oxide.

9. The cathode material according to any one of the preceding claims, wherein the first conducting carbon material is doped with nitrogen, oxygen, sulfur, boron, phosphorus or their mixtures thereof.

10. The cathode material according to any one of the preceding claims, wherein the amount of the first conducting carbon agent is in the range of 20 wt % to 60 wt % based on the total weight of the cathode material.

11. The cathode material according to any one of the preceding claims, wherein the binder is a copolymer of acrylamide, lithium carboxylate and cyano group, polyvinylidene fluoride (PVDF), styrene/butadiene copolymer (SBR), carboxylmethyl cellulose (CMC), polysaccharides, or a polymer having a monomer selected from the group consisting of olefin, butadiene, carboxylate, carboxylate salt of Li and Na, styrene, amide, ester, acrylate, methacrylate, urethane and mixtures thereof.

12. The cathode material according to any one of the preceding claims, wherein the binder is a copolymer of acrylamide, lithium carboxylate and cyano group.

13. The cathode material according to any one of the preceding claims, wherein the binder is water soluble.

14. The cathode material according to any one of the preceding claims, wherein the amount of the binder is in the range of 5 wt % to 15 wt % based on the total weight of the cathode material.

15. The cathode material according to any one of the preceding claims, wherein the second conducting carbon material has a diameter in the range of about 0.1 nm to about 100 μm.

16. The cathode material according to any one of the preceding claims, wherein the second conducting carbon material is vapor grown carbon fiber (VGCF).

17. The cathode material according to any one of the preceding claims, wherein the amount of the second conducting carbon material is in the range of 5 wt % to 35 wt % based on the total weight of the cathode material.

18. The cathode material according to any one of the preceding claims, wherein the cathode material has a sulfur loading density in the range of 1.3 mg cm.sup.−2 to 15 mg cm.sup.−2.

19. The cathode material according to any one of the preceding claims, wherein the cathode material has a surface area in the range of 200 m.sup.2/g to 900 m.sup.2/g.

20. The cathode material according to any one of the preceding claims, wherein the cathode material has a pore volume in the range of 0.25 cm.sup.3/g to 3 cm.sup.3/g.

21. The cathode material according to any one of the preceding claims, wherein the cathode material has a pore size distrbution of mesopore size in the range of 2.0 nm to 50 nm and macropore size larger than 50 nm.

22. A method for preparing a cathode material comprising the steps of: a) coating a support with a slurry formed by mixing a mixture of a first conducting carbon material, a second conducting carbon material and a binder, wherein the second conducting carbon material is carbon fiber or carbon nanotube; and b) adding a sulfur source in fluid state to the coated support to thereby obtain the cathode material.

23. A method for preparing a cathode material comprising the steps of: a) coating a support with a slurry formed by mixing a mixture of a first conducting carbon material, a second conducting carbon material and a binder; and b) adding a sulfur source in fluid state to the coated support to thereby obtain the cathode material.

24. The method according to claim 22 or 23, further comprising, before said coating step (a), the step of (a1) stirring said mixture in a solvent overnight with a solid content in the range of 3 wt % to 10 wt %.

25. The method according to claim 24, wherein the solvent is water or water mixture with polar organic solvents.

26. The method according to any one of claims 22 to 25, wherein the first conducting carbon material has a concentration in the range of 60 wt % to 90 wt % based on the total weight of solid content in the slurry.

27. The method according to any one of claims 22 to 26, wherein the first conducting carbon material is reduced graphene oxide.

28. The method according to any one of claims 22 to 27, wherein the second conducting carbon material has a concentration in the range of 5 wt % to 50 wt % based on the total weight of solid content.

29. The method according to any one of claims 22 to 28, wherein the binder has a concentration in the range of 5 wt % to 20 wt % based on the total weight of solid content.

30. The method according to any one of claims 22 to 29, further comprising, after said coating step (a), the step of (a2) drying the coated support at a temperature in the range of 40° C. to 80° C. for more than 2 hours.

31. The method according to any one of claims 22 to 30, comprising the step of preparing a polysulfide (PS) solution as the sulfur source in fluid state by stirring a mixture of sulfur (S) and lithium sulfide (Li.sub.2S).

32. The method according to claim 31, wherein the mixture is stirred at a temperature in the range of 40° C. to 60° C. overnight in a glovebox.

33. The method according to claim 31 or 32, wherein the mixture has a S/Li.sub.2S mass ratio in the range of 2:1 to 5:1.

34. The method according to any one of claims 22 to 30, comprising the step of obtaining said sulfur source in fluid state by heating elemental sulfur solid at a temperature in the range of 160° C. to 190° C.

35. The method according to claim 34, wherein duration of the heating step is in the range of 5 minutes to 40 minutes.

36. A cathode material prepared by the method according to any one of claims 22 to 35.

37. An electrochemical cell comprising a cathode material according to any one of claim 1 to 21 or 36 and a liquid electrolyte.

38. A lithium-sulfur battery comprising one or more electrochemical cells according to claim 37.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0083] 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.

[0084] FIG. 1 is a schematic diagram showing the preparation of the cathode material as described herein (FIG. 1A) and the cathode material prepared by the conventional melt-diffusion method (FIG. 1B).

[0085] FIG. 2 shows SEM images under 5000× magnification of preforemd rGO cathodes before PS addtion: top view (FIG. 2a) and cross-sectional view (FIG. 2b).

[0086] FIG. 3 shows SEM image under 500× magnification of a preformed rGO cathode with elemental mapping: (FIG. 3a) SEM image, and (FIG. 3b) C, (FIG. 3c) N, and (FIG. 3d) O maps of preformed rGO cathode.

[0087] FIG. 4 is a number of graphs showing the electrochemical Performance of PS/rGO cathode. FIG. 4a: Rate capability at S=1.50 mg cm.sup.−2. FIG. 4b: Long-term cycling at S=1.50 mg cm.sup.−2 and 0.2 C, 0.5 C, 1.0 C, 2.0 C. FIG. 4c: Long-term cycling at 0.1 C and S=5.05 mg cm.sup.−2.

[0088] FIG. 5 is a graph showing the specific capacities of PS/rGO cathode and S(vapor)/rGO cathode prepared by sulfur vapor deposition.

[0089] FIG. 6 is a graph showing rate capability studies of PS/rGO and S/rGO cathodes at a S loading of 1.50 mg cm.sup.−2.

[0090] FIG. 7 is a number of graphs showing long-term cycling performance of PS/rGO and S/rGO cathodes at (a) 0.2 C, (b) 0.5 C, (c) 1.0 C and (d) 2.0 C with a S loading of 1.50 mg cm.sup.−2.

[0091] FIG. 8 shows SEM images of PS/rGO cathode (FIG. 8a, b) before and (FIG. 8c, d) after rate capability studies. FIGS. 8a and 8c are under 1000× magnification, and FIGS. 8b and 8d are under 5000× magnification.

[0092] FIG. 9 shows SEM images of S/rGO cathode (FIG. 9a, b) before (FIG. 9c, d) after rate capability studies. FIGS. 9a and 9c are under 1000× magnification, and FIGS. 9b and 9d are under 5000× magnification.

[0093] FIG. 10 is a number of graphs showing nitrogen adsorption which was performed on PS/rGO or S/rGO cathodes in the absence of sulfur or Li-PS. FIG. 10a: Nitrogen adsorption/desorption isotherm. FIG. 10b: BJH desorption pore size distribution of of PS/rGO and S/rGO.

[0094] FIG. 11 is a graph showing cyclic voltammograms of PS/rGO and S/rGO cathodes.

[0095] FIG. 12 is a graph showing Nyquist plots of cycled PS/rGO and S/rGO cells after rate capability studies. The inset shows a high-frequency region with electrochemically fitted circuit.

DETAILED DESCRIPTION OF FIGURES

[0096] As shown in FIG. 1a, according to this disclosure, there is provided a slurry-coated method 10 of forming a cathode material 600 comprising a polysulfide 500, a porous conducting material 100, a carbon fiber material 200 and a binder 300. Initially, a porous conducting material 100, a carbon fiber material 200 and a binder 300 were provided, which were then subjected to a slurry forming step 12 with the addition of water. The formed slurry was then subjected to a coating step 14 to form a preformed cathode host structure 400. A sulfur source in fluid state 500 was then added to the preformed cathode host structure 400 to form the cathode material 600.

[0097] In comparison, in FIG. 1b, there is provided a prior art melt-diffusion method 20 of forming a cathode material 800 comprising elementary sulfur 700, a porous conducting material 100, a carbon fiber material 200 and a binder 300. Initially, a porous conducting material 100 and elementary sulfur 700 were provided, which were then subjected to a melt-diffusion step 22 to impregnate the elementary sulfur 700 into the porous conducting material 100. After that, a carbon fiber material 200 and a binder 300 were added and subjected to a slurry forming step 24 with addition of water. The formed slurry was then subjected to a coating step 26 on a support to form the cathode material 800.

EXAMPLES

[0098] 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.

Materials and Methods

[0099] Materials: N-doped reduced graphene oxide was purchased from Nanjing JCNANO Technology Co. Ltd. (China). Vapor grown carbon fiber was purchased from Zhongke Leiming (Beijing) Science and Technology Co. Ltd. (China). LA-132 binder was purchased from Chengdu Indigo Power Sources Co. Ltd. (China). Sublimed sulfur (S), lithium sulfide (Li.sub.2S), dimethoxyethane (DME) and carbon disulphide (CS.sub.2) were purchased from Sigma Aldrich (Singapore).

[0100] Characterization: Field emission scanning electron microscopy (SEM) was performed on a JSM-7400F (JEOL) with energy-dispersive X-ray spectroscopy (Oxford Instruments) at an accelerating voltage of 6 kV. Fresh and spent cathode were washed with DME several times to remove LiTFSI, LiNO.sub.3 salt and polysulfide, and dried under vacuum before SEM imaging. Nitrogen adsorption-desorption isotherms at −196° C. were collected using Micromeritics ASAP 2420 physiorption analyzer. Samples (˜40-60 mg) were degassed at 60° C. for 12 hours before measurement. Specific surface areas were calculated using the Brunauer-Emmet-Teller (BET) method. Pore size and pore size distribution (PSD) were obtained by the BJH method using the cylindrical pore model. Pore volume was taken at P/P.sub.0=0.988. Samples for physisorption were prepared by removing cathode coated on an Al current collector. The melt-diffused sulfur host cathode was washed several times with CS.sub.2 to remove sulfur and dried under vacuum overnight before physisorption experiments. Elemental analysis of sulfur content was conducted on a Flashsmart elemental analyzer (Thermo Scientific).

Example 1: Cathode Preparation

[0101] The cathode formed in this example was prepared by FIG. 1A and FIG. 1B.

[0102] To prepare the preformed rGO host structure for the slurry-coated method (FIG. 1A), a mixture of 80 wt % reduced graphene oxide, 10 wt % vapor grown carbon fiber (VGCF) and 10 wt % LA-132 binder in water was stirred overnight before coating on carbon-coated Al current collector via the doctor's blade method. The solid content of the mixture is typically 4-7 wt %. Cathode host was then left to dry in a 60° C. oven for a few hours. A mass of 3.30-3.70 mg was used for the cathode.

[0103] The electrolyte was prepared by adding 1 M LiTFSI and 2 wt % LiNO.sub.3 to a mixture of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (volume ratio of 1:1). Li.sub.2S.sub.6 (PS) solution was prepared by stirring a mixture of S (160.5 mg) and Li.sub.2S (46.0 mg) at 50° C. in the electrolyte overnight in an Ar-filled glovebox. Sulfur concentrations of 2.85 M and 5.42 M were prepared by adding 2 mL and 1 mL of electrolyte, respectively.

[0104] For a sulfur loading of 1.50 mg cm.sup.−2, 21 μL of 2.85 M polysulfide solution, which was equivalent to 10 μL electrolyte/mg sulfur, was added to the preformed rGO host structure in a glovebox. Once the polysulfide solution is dropped onto the preformed rGO host structure, the preformed rGO host structure immediately absorbed the solution into its porous structure. The weight percentages of sulfur, reduced graphene oxide, VGCF and binder in the cathode were 35 wt %, 52 wt %, 6.5 wt % and 6.5 wt %, respectively.

[0105] For a sulfur loading of 5.05 mg cm.sup.−2, 37 μL of 5.42 M polysulfide solution was added, corresponding to 64 wt % of sulfur in the cathode. Electrolyte volume per sulfur weight was fixed at 8 μL/mg for a loading density of 5.05 mg cm.sup.−2.

[0106] Sulfur cathode via sulfur vapor depsition was prepared by placing the preformed carbon scaffold on a stainless steel mesh of about 1 mm above a heated (175° C.) reservoir of elemental sulfur for about 8 minutes corresponding to a sulfur loading density of about 1.5 mg cm.sup.2. Time can be prolonged to increase the sulfur loading.

[0107] To prepare the conventional melt-diffused sulfur host cathode structure (FIG. 1B), 87 wt % S-reduced graphene oxide composite, prepared by melt diffusion at 160° C. in a hydrothermal vessel overnight, 6.5 wt % of VGCF and 6.5 wt % of LA-132 were stirred in water with a solid content in the range of 5 to 10 wt % overnight and coated on carbon-coated Al current collector via the doctor's blade method. By controlling the wet film thickness, a sulfur loading density of ˜1.50 mg cm.sup.−2 was obtained. Cathode host was then left to dry in a 60° C. oven for a few hours. The sulfur content in the sulfur-reduced graphene oxide composite was ˜40 wt % based on elemental analysis. 20 μL of electrolyte (˜10 μL/mg sulfur) was added to the melt-diffused S cathode.

Example 2: Coin Cell Preparation and Electrochemical Testing

[0108] Standard 2032-type coin cells were used for cell cycling and rate capability tests. Assembly was done in an argon-filled glovebox, with the 12.7-mm cathodes and lithium foil as the anode/reference electrode. A glass fiber membrane (GF/A, GE Healthcare) and a Celgard membrane, soaked with electrolyte, were used as separator. Both membranes were soaked with electrolyte. Galvanostatic charge-discharge cycling was conducted with a LAND CT2001 battery tester (Wuhan LAND electronics) between 1.6 V and 3.0 V vs. Li/Li.sup.+ for the rate capability studies and at a high sulfur loading of 5.05 mg cm.sup.−2. For a sulfur loading of 1.50 mg cm.sup.−2, fixed rate cycling was performed between 1.8 V and 2.8 V. Cyclic voltammograms were obtained at a scan rate of 0.05 mV s.sup.−1, and EIS was conducted at 10 mV at open circuit potential between 1 MHz and 0.01 Hz on an M204 Autolab potentiostat (Metrohm) fitted with a frequency response analyzer module.

Example 3: Cathode Characterization

[0109] Porosity and Elemental Distribution

[0110] Scanning electron microscopy (SEM) of the preformed rGO cathode before PS addition revealed a highly porous, 3D structure of interconnected VGCF tubes and crumpled rGO sheets that were well separated (FIG. 2). Elemental mapping showed that carbon, oxygen and nitrogen were homogeneously distributed throughout the material, indicating that the components were well-dispersed during cathode preparation (FIG. 3). In comparison, the cathode material of the present invention appeared more porous and less densely-packed than other slurry-coated PS cathodes reported in the literature. High porosity is important to accommodate the volumetric changes during interconversion of sulfur and L.sub.2S, and provide the structure interconnectivity that is essential for long-range and rapid electron transfer. These features are important to achieve high electrochemical performance for Li-S batteries.

[0111] Robustness and Stability

[0112] To determine the robustness and stability of the PS/rGO electrode, rate capability study was conducted. The study involved increasing the charge rate from 0.1 C to 2.0 C, followed by lowering the charge rate to 0.2 C (FIG. 4a). The average discharge capacities of the PS/rGO electrode at 0.1 C, 0.2 C, 0.5 C, 1.0 C and 2.0 C were 1499, 1265, 1102, 999 and 879 mAh g.sup.−1, respectively. When the charge rate was abruptly reduced to 0.2 C, the capacity recovered to 1191 mAh g.sup.−1, indicating the high structural stability of the PS/rGO electrode. As shown in FIG. 5, the cathode material as prepared by the slurry coating method and sulfur vapor phase deposition showed similar specific capacities as compared to the cathode material as prepared by the slurry coating method and polysulfide solution addition. This indicates that the cathodes prepared by both methods are able to achieve high specific capacities and stability for lithium-sulfur battery performance due to the preformed cathode host structure.

[0113] Long Term Cycling

[0114] Long-term cycling at fixed C rates was also performed (FIG. 4b). The initial discharge capacities of the PS/rGO electrode were 1220, 1112, 1087 and 1007 mAh g.sup.−1 at 0.2, 0.5, 1.0 and 2.0 C, respectively. After 200 cycles, high discharge capacities of 999, 948, 906 and 866 mAh g.sup.−1 were retained at 0.2, 0.5, 1.0 and 2.0 C, respectively. Coulombic efficiencies are all larger than 98% throughout the 200 cycles at 0.2, 0.5, 1.0 and 2.0 C, respectively.

[0115] The above performance surpassed other slurry-coated PS reported in the literature. At 0.2 C, PS/rGO cathode gave a higher initial (1220 vs. 1000 mAh g.sup.−1) and retained discharge capacity (999 vs. 780 mAh g.sup.−1) at a higher S loading (1.50 vs. 1.21 mg cm.sup.−2) and larger number of cycles (200 vs. 100), as compared to the Pt/graphene PS electrode, which showed the best performance amongst the previously reported slurry-coated PS cathodes (Table 1). .sup.[20]

TABLE-US-00001 TABLE 1 Electrochemical performance of slurry-coated PS cathodes. Specific capacity Sulfur (mAh g.sup.−1): Cathode density Sulfur Cycle first cycle, material (mg cm.sup.−2) Concentration (M) C rate # last cycle Reference Super-P 1.3 2.25 ~0.1 C  50 610, 452 Previous carbon work Super-P 3.03 ~1.55 ~0.06 C  20 600, 550 Previous carbon work Hierarchical ~0.87 ~1.51 0.1 C 100 1100, 800  Previous silica-etch 0.2 C Average: work carbon ~1000 Pt/graphene 1.21 4.8 0.1 C 100 1100, 789  Previous 0.2 C 300 ~1000, 780   work 1.0 C 450, 350 TiN 0.32 or 1.6 0.1 C 100 1600, 1040 Previous nanoparticles 0.52.sup.a 1.0 C ~1200, 996   work WN 0.32 or 1.6 0.1 C 100 1768, 700  Previous nanoparticles 0.52.sup.a 1000, 573  work Mo.sub.2N 1068, 264  nanoparticles VN nanoparticles N-doped 1.50 2.85 0.2 C 100 1220, 1057 This work reduced 5.05 5.43 0.5 C 200 1112, 948  graphene 1.0 C 200 1087, 906  oxide with 2.0 C 200 1007, 866  vapor 0.1 C 50 grown carbon fiber .sup.aNot reported, estimated based on amount of catholyte added and area of typical coin cell cathode (12.7 mm or 10 mm in diameter).

[0116] Representative reports on free-standing cathodes based on reduced graphene oxide are shown in Table S2. Although these cathodes have excellent electrochemical performance, they are difficult to scale up and often involve a low sulfur concentration (i.e. require more electrolyte). The advantage of PS/rGO cathode lies in its high scalability, while maintaining excellent electrochemical performance.

TABLE-US-00002 TABLE 2 Electrochemical performance of various pure carbon-based PS cathodes. Specific capacity Sulfur (mAh g.sup.−1): Cathode density Sulfur C Cycle first cycle, material Preparation (mg cm.sup.−2) Concentration (M).sup.a rate # last cycle Reference N-doped Free-standing 0.53 1.2 0.2 C 100 ~1300, ~1000 Previous reduced 1.06 2.4 0.5 C 100 ~900, ~700 work graphene .sup. 1 C 100 Average: 600 oxide .sup. 2 C 100 Average: 400 0.2 C 100 ~1300, ~1000 N-doped Free-standing 6 2 0.25 C  100 1150, 881  Previous reduced 0.5 C 400 1150, 610  work graphene oxide with carbon nanotube aerogel N-doped Slurry- 1.50 2.85 0.2 C 100 1220, 1057 This reduced coated 5.05 5.43 0.5 C 200 1112, 948  work graphene 1.0 C 200 1087, 906  oxide with 2.0 C 200 1007, 866  vapor 0.1 C 50 858, 798 grown carbon fiber

[0117] Areal Capacity

[0118] To determine if the PS/rGO electrode could reach a practical areal capacity as LIB (4 mAh cm.sup.−2), sulfur loading density was increased. Low sulfur utilization was expected at high sulfur loadings due to a thicker layer of insulating sulfur on the cathode surface. At 0.1 C, a sulfur loading of 5.05 mg cm.sup.−2, and a high sulfur concentration of 5.43 M, the PS/rGO electrode gave an initial specific capacity of 858 mAh g.sup.−1, corresponding to an areal capacity of 4.33 mAh cm.sup.−2 (FIG. 4c). After 50 cycles, 798 mAh g.sup.−1 was retained, equivalent to an areal capacity of 4.03 mAh cm.sup.−2. Based on the electrochemical performance results, the PS/rGO cathode is capable of achieving a practical areal capacity comparable to current LIB technology, and a superior electrochemical performance to other slurry-coated PS electrode.

Comparative Example 1: Discharge Capacities

[0119] To address the difference between the distinctly different method of preparing PS (FIG. 1A) and melt-diffused (FIG. 1B) cathodes, the electrochemical performance of the S/rGO was also evaluated. The discharge capacities of S/rGO cathode were 1186, 920, 810, 739 and 650 mAh g.sup.−1 at 0.1, 0.2, 0.5, 1.0 and 2.0 C, respectively, and recovered to 872 mAh g.sup.−1 at 0.2 C (FIG. 6).

[0120] For long-term cycling of the S/rGO cathode, initial discharge capacities were 867, 821, 747 and 659 mAh g.sup.−1 at 0.2, 0.5, 1.0 and 2.0 C, respectively (FIG. 7). After 200 cycles, the discharge capacities became 653, 650, 605 and 533 mAh g.sup.−1 at 0.2, 0.5, 1.0 and 2.0 C, respectively.

[0121] Coulombic efficiencies are larger than 98% for both the PS/rGO cathode and the S/rGO cathode throughout the 200 cycles at 0.2, 0.5, 1.0 and 2.0 C, respectively.

[0122] Capacity fade, known to be positively correlated with PS shuttling effect, was also determined for the two electrodes. Capacity fade values for the PS/rGO electrode were 0.100%, 0.080%, 0.091%, 0.075% per cycle at 0.2, 0.5, 1.0 and 2.0 C, respectively. For the S/rGO electrode, the capacity fade values were 0.142%, 0.117%, 0.105% and 0.106% per cycle at 0.2, 0.5, 1.0 and 2.0 C, respectively.

[0123] PS/rGO electrode showed a 48% higher specific capacity and 26% lower capacity fade per cycle, on average, than the S/rGO electrode.

Comparative Example 2: SEM

[0124] The difference in electrochemical performance was found to be correlated to the difference in cathode structure. The PS/rGO cathode was highly porous and interconnected before (FIG. 8a, b) and after cycling (FIG. 8c, d). The pores were uniform in size and evenly distributed, suggesting that the reversible reaction of Li-S system during battery cycling did not affect the porous structure of the PS/rGO cathode.

[0125] On the other hand, fresh S/rGO cathode structure, although interconnected, consisted of a mixture of large and small pores (FIG. 9a, b). After the rate capability test, the cathode underwent a major structural transformation, forming a highly dense and closely packed structure (FIG. 9c, d). Structural changes were known to occur for composite sulfur cathodes due to the dissolution of sulfur to PS species during battery discharge.

Comparative Example 3: Nitrogen Adsorption Study

[0126] The difference in structure of the two electrodes was quantified by nitrogen adsorption which was performed on both cathodes in the absence of sulfur or Li-PS. Sulfur removal was necessary to simulate the effect of structural changes observed in SEM. For the PS/rGO cathode, analysis was conducted on the preformed rGO cathode, whereas the S/rGO cathode was washed with CS.sub.2 to remove the sulfur.

[0127] The nitrogen adsorption/desorption isotherm for both cathodes corresponded to a type II isotherm with H3 hysteresis loop (FIG. 10a). The surface area and pore volume of the preformed rGO cathode were 264 m.sup.2/g and 0.31 cm.sup.3/g, respectively. These values were higher than that of the washed S/rGO cathode (181 m.sup.2/g and 0.25 cm.sup.3/g, respectively).

[0128] In addition, as shown in FIG. 10b, pore size distribution analysis revealed the presence of mesopores (3-4 nm) and macropores (60-90 nm) for both cathodes. The rGO cathode had a comparable mesopore size (3.5 vs. 3.6 nm) and slightly larger macropore size (82 nm vs. 65 nm) than the washed S/rGO cathode.

[0129] Since both electrodes were essentially identical in terms of composition, electrolyte volume and sulfur loading density, the higher specific capacities and lower capacity fade values could be attributed to the higher surface area of the PS/rGO cathode as compared to the S/rGO cathode.

[0130] The higher surface area of the PS/rGO cathode led to an increased availability of electrochemically active sites for sulfur species, such as S, Li.sub.2S and PS, allowing both nucleation and binding to occur on the cathode surface, which led to higher specific capacities. This in turn led to a decrease in the concentration of dissolved PS in bulk, reducing the undesired PS shuttling effect. Therefore, the capacity fading of PS/rGO electrode was found to be lower than that of the S/rGO electrode.

Comparative Example 4: Electrochemistry

[0131] In addition to surface area difference, the structural change, or the lack thereof, was found to have a pronounced effect on ohmic resistance of the electrodes. Both S/rGO and PS/rGO electrodes were further examined using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), measured after rate capability studies.

[0132] CV curves of both electrodes revealed features typical of a Li-S battery system: two sharp reduction peaks and a broader oxidation peak (FIG. 11). Peak shape of both cathodes resembled one another as these electrodes consisted of essentially identical constituents, ratios and S loading (which is fixed at 1.50 mg cm.sup.−2). This implied that the structural difference between these cathodes did not affect the potential at which redox reactions occurred. The area under the curve, typically correlated with capacity, was found to be larger for the PS/rGO cathode as compared to the S/rGO cathode, which was consistent with the galvanostatic cycling results discussed earlier.

[0133] EIS data collected were mathematically transformed into Nyquist plots (FIG. 12). In general, these plots appeared to be an overlap of several semicircles ending with a steep upward slope. The semicircle at the high frequency region was fitted with an equivalent circuit (FIG. 9 inset). The first intercept at the high frequency region of the real (Z′) axis gives the value of electrolyte resistance, R.sub.e. The difference between the Z′ axis intercepts of the fitted semicircles gives the charge transfer resistance (R.sub.CT) value, which is associated with charge transfer process between S and the electrode.

[0134] The R.sub.e values of PS/rGO and S/rGO cells were found to be 4.7 Ω and 7.4 Ω, respectively. The R.sub.CTvalues of PS/rGO and S/rGO cells are 3.0 Ω and 7.8 Ω, respectively. Since PS/rGO cathode has a higher surface area than the S/rGO cathode, the amount of electrochemically active sites would be greater in PS/rGO than S/rGO. Therefore, for the same amount of electrolyte, the insulating S layer would be thinner in PS/rGO than S/rGO, resulting in a lower resistance.

[0135] In addition, structural changes that occurred in the S/rGO cathode could lead to disconnectivity between conductive elements within the cathode, contributing to the higher resistance as compared to the PS/rGO cathode. In the structurally intact PS/rGO cathode, the conductive elements within the structure remained interconnected, allowing continuous and unimpeded electron conduction pathways from the current collector throughout the entire 3D cathode structure.

[0136] The lower ohmic resistance of the PS/rGO electrode, as compared to the S/rGO cathode, suggested better redox kinetics that resulted in the improved rate and cycling performance of the Li-S batteries (FIGS. 3 and 4).

INDUSTRIAL APPLICABILITY

[0137] In the present disclosure, the cathode material can be used in a lithium-sulfur (Li-S) battery system for energy storage application. It offers potential advantages of high energy density, low material cost and high abundance of sulfur as compared to the conventional lithium battery. The cathode material and the method of preparing the same provide a strong case towards a paradigm shift away from conventional cathode preparation approaches to improve the electrochemical performance of lithium-sulfur batteries.

[0138] The lithium-sulfur batteries that use the cathode material as described in the present disclosure may be used as high density power sources for a wide variety of applications for example in automobile (electric vehicles including electric cars, hybrid vehicles, electric bicycles, personal transporters and advanced electric wheelchairs, radio-controlled models, model aircraft, aircraft), portable devices (mobile phone/smartphone, laptops, tablets, digital cameras and camcorders), in power tools (including cordless drills, sanders, and saws), or in healthcare (portable medical equipment such as monitoring devices, ultrasound equipment, and infusion pumps).

[0139] 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.