A Method To Synthesize A Porous Carbon-Sulfur Composite Cathode For A Sodium-Sulfur Battery

Abstract

There is provided a method of synthesizing a porous carbon-sulfur composite comprising the step of carbonizing a carbon material having a metal-organic framework (MOF) at a temperature of 800-1000° C. to produce a porous carbon, mixing and heating the porous carbon with sulfur to infuse the sulfur (melt diffusion) into the pores of the porous carbon and removing excess sulfur not infused into the pores or present on the surface of the porous carbon. There is also provided a cathode comprising the porous carbon-sulfur composite and a method of preparing the cathode by mixing with conductive carbon and a polymer binder. The cathode finds use in an electrochemical cell comprising a sodium or lithium anode.

Claims

1. A method of synthesizing a porous carbon-sulfur composite, comprising the steps of: (a) carbonizing a carbon material having a metal-organic framework at a temperature in the range of 800° C. to 1000° C. to produce a porous carbon; (b) mixing and heating the porous carbon with sulfur to infuse said sulfur into the pores of said porous carbon; and (c) removing excess sulfur not infused into the pores or present on the surface of said porous carbon.

2. The method of claim 1, wherein the temperature for the carbonizing step (a) is at least 900° C. to less than 1000° C.

3. The method of claim 1, wherein the metal-organic framework of the carbon material is selected from the group consisting of zeolite-type metal-organic frameworks, micro-porous metal-organic framework (MMOFs), porous coordination networks (PCNs) and porous coordination polymers (PCPs).

4. The method of claim 3, wherein the zeolite-type metal-organic framework is selected from the group consisting of zeolitic imidazolate framework-3 (ZIF-3), zeolitic imidazolate framework-6 (ZIF-6), zeolitic imidazolate framework-8 (ZIF-8), zeolitic imidazolate framework-11 (ZIF-11), zeolitic imidazolate framework-14 (ZIF-14), zeolitic imidazolate framework-20 (ZIF-20), zeolitic imidazolate framework-60 (ZIF-60), zeolitic imidazolate framework-68 (ZIF-68) and zeolitic imidazolate framework-95 (ZIF-95).

5. The method of claim 1, wherein the method does not comprise a washing step after the carbonizing step (a).

6. The method of claim 1, wherein the sulfur infused into the pores of said porous carbon in step (b) is short-chain sulfur allotropes S.sub.2, S.sub.3 or S.sub.4 and the excess sulfur not infused into the pores or present on the surface of said porous carbon in step (c) is elemental sulfur S.sub.8.

7. The method of claim 1, wherein in the mixing and heating step (b), the sulfur and porous carbon are provided in a weight ratio in the range of 5:1 to 1:1.

8. The method of claim 1, wherein the removing step (c) comprises evaporating the excess sulfur under an inert gas flow in an open system.

9. A porous carbon-sulfur composite synthesized by a method comprising the steps of: (a) carbonizing a carbon material having a metal-organic framework at a temperature in the range of 800° C. to 1000° C. to produce a porous carbon; (b) mixing and heating the porous carbon with sulfur to infuse said sulfur into the pores of said porous carbon; and (c) removing excess sulfur not infused into the pores or present on the surface of said porous carbon.

10. The porous carbon-sulfur composite of claim 9, wherein the composite has a weight percentage of sulfur in the range of 30 weight % to 40 weight % based on the porous carbon-sulfur composite, or wherein the composite has a weight ratio of carbon and nitrogen in the range of 20:1 to 3:1.

11. (canceled)

12. The porous carbon-sulfur composite of claim 9, wherein the composite has a sulfur content consisting essentially of S.sub.2, S.sub.3, S.sub.4 and combinations thereof.

13. A cathode comprising a porous carbon-sulfur composite synthesized by a method comprising the steps of: (a) carbonizing a carbon material having a metal-organic framework at a temperature in the range of 800° C. to 1000° C. to produce a porous carbon; (b) mixing and heating the porous carbon with sulfur to infuse said sulfur into the pores of said porous carbon; and (c) removing excess sulfur not infused into the pores or present on the surface of said porous carbon.

14. A method of preparing a cathode comprising a porous carbon-sulfur composite synthesized by a method comprising the steps of: (i) carbonizing a carbon material having a metal-organic framework at a temperature in the range of 800° C. to 1000° C. to produce a porous carbon; (ii) mixing and heating the porous carbon with sulfur to infuse said sulfur into the pores of said porous carbon; and (iii) removing excess sulfur not infused into the pores or present on the surface of said porous carbon, comprising the steps of: (a) mechanically treating a mixture comprising the porous carbon-sulfur composite a conductive carbon, and a polymer binder; (b) adding an organic solvent into the mixture to yield a slurry; and (c) coating the slurry onto a carbon-coated aluminium foil.

15. The method of claim 14, wherein the mechanically treating step (a) comprises mixing the porous carbon-sulfur composite, the conductive carbon and the polymer binder at a weight ratio of about 7:2:1.

16. The method of claim 14, wherein the adding step (b) further comprises stirring the slurry overnight.

17. The method of claim 14, further comprising a drying step after the coating step (c); wherein the drying step is performed under a temperature in the range of 40° C. to 80° C.

18. An electrochemical cell comprising: (a) a cathode comprising a porous carbon-sulfur composite synthesized by a method comprising the steps of: (i) carbonizing a carbon material having a metal-organic framework at a temperature in the range of 800° C. to 1000° C. to produce a porous carbon; (ii) mixing and heating the porous carbon with sulfur to infuse said sulfur into the pores of said porous carbon; and (iii) removing excess sulfur not infused into the pores or present on the surface of said porous carbon; (b) an anode; and (c) an electrolyte in fluid communication with both said cathode and said anode.

19. The electrochemical cell of claim 18, further comprising a separator.

20. The electrochemical cell of claim 18, wherein the anode is a sodium anode or a lithium anode, or wherein the electrolyte is NaClO.sub.4 in tetraglyme, NaCF.sub.3SO.sub.3 in tetraglyme or a combination thereof.

21. (canceled)

22. The electrochemical cell of claim 18, wherein the electrolyte has a concentration in the range of 0.1 M to 2 M.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0098] 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.

[0099] [FIG. 1] shows X-ray diffraction (XRD) patterns of Carbon-900 and Carbon-900-S at various processing steps and sulfur powder. The curves are (a) Carbon-900, (b) pure sulfur powder (S.sub.8), (c) mixture of Carbon-900 with sulfur powder but before heating of the mixture, (d) product obtained after mixing and heating of Carbon-900 and sulfur powder, (e) Carbon-900-S.

[0100] [FIG. 2] shows TGA analysis of the Carbon-900-S composite indicating sulfur contents, after (a) mixing the porous carbon with sulfur at a weight ratio of 1:3 by grinding; (b) after melt diffusion but without surface sulfur removal by heating; (c) after heat removal of excess surface sulfur.

[0101] [FIG. 3] is a schematic diagram of a fabricated sodium-sulfur battery 300 comprising of 2032-type coin cell cases 2, a carbon-sulfur composite cathode 4, a separator 6 and a sodium anode 8.

[0102] [FIG. 4] shows (a) Galvanostatic charge/discharge curves at 0.1 C, (b) cycling performance of sodium-sulfur coin cells with the Carbon-900-S composite cathode using 1 M NaClO.sub.4 in tetraglyme as the electrolyte and (c) Cycling performances of sodium-sulfur coin cells using the Carbon-S-900 composite cathode and control samples.

[0103] [FIG. 5] shows the influence of the carbonizing temperature of the porous carbon on the sodium-sulfur battery performance.

[0104] FIG. 5a shows the cycling performance of sodium-sulfur coin cells with various Carbon-S composites cathode at 0.1 C in 1 M NaClO.sub.4 in tetraglyme electrolytes.

[0105] FIG. 5b shows the cycling performance of sodium-sulfur coin cells with various Carbon-S composites cathode at 0.2 C in 1 M NaOTf in tetraglyme electrolytes.

[0106] FIG. 5c shows cycling performance of sodium-sulfur batteries with various carbon-S composites cathodes at various current rates in 1 M NaClO.sub.4 in tetraglyme electrolyte.

EXAMPLES

[0107] Non-limiting examples of the invention and a comparative example 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 Porous Carbon-Sulfur Composites

[0108] In this example, a three-step procedure for the synthesis of porous carbon-sulfur composites is described.

[0109] In the first step, a precursor ZIF-8 was synthesized as follows. Typically, 20 g of 2-methylimidazole (purchased from Sigma Aldrich, Singapore) and 8.4 g of Zn(NO.sub.3).sub.2.6H.sub.2O (purchased from Sigma Aldrich, Singapore) were dissolved in 250 and 150 mL of methanol (purchased from Sigma Aldrich, Singapore) separately, followed by mixing with vigorous stirring at about 800 rpm for 5 minutes at 25° C.

[0110] The mixture was then left at room temperature overnight without disturbance to form a white product. The white product was purified by 3 times centrifugation in about 100 mL of methanol at 5000 rpm for 5 minutes and dried at 80° C. overnight in a vacuum oven.

[0111] The dried product was carbonized in a tube furnace at various temperatures and durations under argon flow at 200 sccm to produce a porous carbon. The carbonizing temperatures and durations used were 800° C. for 2 hours, 900° C. for 4 hours or 1000° C. for 8 hours with a heating rate of 2° C. min.sup.−1. A range between 800° C. and 1000° C. was selected to study the influence of the carbonizing temperature. The products synthesized at carbonizing temperatures of 800, 900 and 1000° C. were denoted as Carbon-800, Carbon-900 and Carbon-1000, respectively. Porosity and surface areas of the three products were obtained by gas sorption analysis, using both nitrogen (N.sub.2) gas at 77 K, and carbon dioxide (CO.sub.2) gas at 273 K. Surface area was determined by the Brunauer-Emmett-Teller (BET) method.

TABLE-US-00001 TABLE 1 Summary of porosity and surface areas of porous carbons produced for cathode preparation, based on gas sorption analysis. N.sub.2 sorption data, 77 K CO.sub.2 sorption data, 273 K BET Average Total area in surface area pore radius pores >0.4 nm T (° C.) (m.sup.2/g) (nm) (m.sup.2/g) Carbon-800 591 4.10 903 Carbon-900 753 3.89 864 Carbon-1000 800 1.38 945

[0112] An acid washing step was also used for removing residual Zn, specifically for the sample carbonized at 800° C. for 2 hours, as this carbonizing temperature was insufficient for complete removal of zinc. The acid washing step consisted of using 50 mL of HCl (purchased from Sigma Aldrich, Singapore) at 2 M to wash the product after carbonizing 1 time, followed by using 50 mL of NaOH (purchased from Sigma Aldrich, Singapore) at 2 M to wash the product 1 time and then rinsing with 50 mL of water 3 times. The product from the acid washing step was collected after drying overnight in an oven at 80° C.

[0113] In the second step, a sulfur melt diffusion was performed. Elemental sulfur (S.sub.8, purchased from Sigma Aldrich, Singapore) and the porous carbon were mixed in a weight ratio of 3:1 by pestle and mortar. The mixture was then transferred into stainless steel vessels and sealed, followed by heating at 155° C. for 16 hours to accomplish the sulfur melt diffusion.

[0114] In the third step, a black product after the sulfur melt diffusion step was placed in a tube furnace and heated to 250° C. and held for 3 hours with argon flow at a flow rate of 200 sccm to remove excess residual sulfur on the surface of the black product. The resulting porous carbon-sulfur composites synthesized at carbonizing temperatures of 800, 900 and 1000° C. were denoted as Carbon-800-S, Carbon-900-S and Carbon-1000-S, respectively.

[0115] Three control samples were also produced. Firstly, the Carbon-800-S-Control was produced, without the acid washing step to inspect the influence of residual Zn in the porous carbon. More importantly, for evaluating the importance of surface sulfur removal in the third step, two control samples Carbon-900-S-Control 1 and Carbon-900-S-Control 2 were also produced, in which the weight ratio of carbon to sulfur in the second step were set as 45/55 and 25/75, respectively. These mixtures were treated with the second step but not the third step.

TABLE-US-00002 TABLE 2 Summarization of porous carbon-sulfur composites produced for cathodes preparation. Acid Presence Presence Carbonizing Washing for of the of the Sample Name Temperature Residual Zinc Second Step Third Step Carbon-800-S 800° C. Yes Yes Yes Carbon-900-S 900° C. No Yes Yes Carbon-1000-S 1000° C. No Yes Yes Carbon-800-S- 800° C. No Yes Yes Control Carbon-900-S- 900° C. No Yes No Control 1 Carbon-900-S- 900° C. No Yes No Control 2

Example 2

Confirmation of Sulfur Embedding in Porous Carbon-Sulfur Composites

[0116] To exemplify the importance of the additional heating process for excess sulfur removal in the present disclosure, X-ray diffraction (XRD), elemental combustion analysis, and thermogravimetric analysis (TGA) were used to track the changes in sulfur content throughout the sulfur introduction process, based on the second and the third step as described in Example 1.

[0117] The confirmation of sulfur embedment within the porous carbon structure was achieved with XRD. For comparison, XRD spectra of the Carbon-900 sample are shown in FIG. 1 at the various processing stages: Carbon-900 mixed with sulfur in the second step before the heating step and after the heating step, and Carbon-900-S after the third step of sulfur removal. Pure Carbon-900 and pure elemental sulfur are also shown for comparison.

[0118] The Carbon-900 sample mixed with sulfur in the second step before the heating step showed the same distinctive pattern as pure elemental sulfur. After the heating step, new patterns appeared (marked with star symbol in FIG. 1) owing to the partial phase transition of sulfur from orthorhombic to monoclinic after 155° C. heating treatment. Nonetheless, the high intensity of the sulfur peaks indicate the presence of surface sulfur.

[0119] However, after the third step of surface sulfur removal, the XRD pattern of the Carbon-900-S composite showed two broad peaks at around 23 and 44°, corresponding only to the reflections of the (002) and (100) planes of graphitized carbon, similar to the starting Carbon-900. No diffraction peaks of sulfur were observed, confirming that all sulfur was embedded deep within the pores, and not existing on the surface.

[0120] Additionally, TGA was also useful in elucidating changes in the nature of sulfur, either embedded within the pores or otherwise on the surface. The processing of Carbon-900-S composite was selected as an example to study the features of sulfur via TGA investigation, which were illustrated in FIG. 2, the weight loss profiles of samples after the treatment procedure indicate that the heating process in the second step at 155° C. in a small vessel was essential for the sulfur injection, with sulfur outgassing starting at a significantly higher temperature (curve b) as compared to the sample from physical grinding (curve a). More importantly, two new gradient changes in curve b with higher outgas temperatures suggested the existence of new sulfur species due to stronger interaction with carbon, likely from confinement within the microporous structure. After surface sulfur removal (curve c), the sulfur content in the Carbon-900-S composite was calculated to be around 36%.

[0121] In this example, the disappearance of elemental sulfur from the porous carbon-sulfur composite was confirmed from XRD (FIG. 1). Despite the loss of spectral information for elemental sulfur in the composite (curve (e): Carbon-900-S), elemental combustion analysis indicated that sulfur was still present in the composite at about 35 weight %, albeit as a different allotrope (Table 3). This was corroborated with TGA data (curve (c) in FIG. 2) showing similar weight loss.

[0122] In addition, gradient changes in curve (b) of FIG. 2 with higher outgas temperatures were associated with short-chain allotropes, as they were confined within the porous structure and may only be outgassed from the composite at higher temperatures.

Example 3

Effect of the Carbonizing Temperature on Porous Carbon-Sulfur Composites

[0123] Porous carbon-sulfur composites were synthesized at various carbonizing temperatures based on Example 1. Targeting to understand the carbonizing temperature influence on the various carbon-sulfur composites as cathode materials, elemental analysis was performed on the carbon-sulfur composites. The sulfur contents were around 35% to 39%, consistent with the TGA measurement. As a reference, sulfur compositions in porous carbon composites typically ranged between 30% and 40%.

TABLE-US-00003 TABLE 3 Elemental compositions of porous carbon-sulfur composites determined by combustion elemental analysis. Carbon/ Elemental Composition (weight %) Nitrogen Ratio Sample Name C H N S (mol/mol) Carbon-800-S 40.95 0.74 13.01 35.6 3.67 Carbon-900-S 43.62 0.48 5.16 35.2 9.86 Carbon-1000-S 51.05 0.18 2.76 39.5 21.58

[0124] Specifically, the carbon/nitrogen ratio increased monotonically with the carbonizing temperature (Table 3). It has been shown in the prior arts that nitrogen doping could improve the conductivity of materials and hinder the shuttle effect of polysulfides, thus improving the cycling stability and rate performance. It was supposed that an optimal carbon/nitrogen ratio would lead to the best sodium-sulfur battery performance.

Example 4

Fabrication of Full Cell Consisting of Porous Carbon-Sulfur Composite Cathode and Sodium Anode

[0125] In this example, the composite material made in Example 1 was used to make a cathode.

[0126] To make the cathode, the porous carbon-sulfur composite, conductive carbon (Super P, purchased from Alfa Aesar, Singapore), and polyvinylidene fluoride (PVDF, purchased from Sigma Aldrich, Singapore) were ground in an agate mortar in a weight ratio of 7:2:1 with N-Methyl-2-pyrrolidone (NMP, purchased from Sigma Aldrich, Singapore solvent to yield a viscous slurry, which was stirred in a small vial at about 600 rpm at 25° C. overnight. Finally, the slurry was coated onto carbon-coated aluminium foil (purchased from MTI Corporation, USA) with a doctor blade (80 mm width, 50 μm to 400 μm variable coating thickness, purchased from MTI Corporation, USA) and allowed to dry completely at 60° C. overnight.

[0127] An electrochemical cell such as in FIG. 3 was made whereby the cell 300 comprises of 2032-type coin cell cases 2, a carbon-sulfur composite cathode 4, a separator 6 and a sodium anode 8.

[0128] Sodium-sulfur cells were fabricated as 2032-type coin cells in an argon-filled glovebox with the porous carbon-sulfur composite as the cathode (11.28 mm in diameter), freshly cut sodium circular discs (12.7 mm in diameter, 99.9% purity, purchased from Sigma Aldrich, Singapore) as the anode, and Celgard 2325 membrane (purchased from Celgard, USA) as the separator. Electrolytes used were 25 μL of 1 M sodium perchlorate (NaClO.sub.4, purchased from Sigma Aldrich, Singapore) or 1 M sodium triflate (NaCF.sub.3SO.sub.3 or NaOTf, purchased from Solvionic, France) in tetraglyme (an ether, purchased from Sigma Aldrich, Singapore).

Example 5

Importance of the Surface Excess Sulfur Removal of the Porous Carbon-Sulfur Composite on the Sodium-Sulfur Battery Performance

[0129] To investigate the performance of the batteries operating at room temperature, galvanostatic discharge-charge cycling tests were performed at 0.1 or 0.2 C (1 C=1673 mAg(s).sup.−1).

[0130] FIGS. 4a and 4b illustrated the charge/discharge profiles of the Carbon-900-S cathode in a 1 M NaClO.sub.4 in tetraglyme electrolyte, and the corresponding capacity profiles and Coulombic efficiencies. In the first discharge cycle, the measured specific capacity of the battery was 1585 mAh/g, close to the theoretical value of sulfur (1673 mAh/g). The specific capacity of the second discharge cycle dropped to 1439 mAh/g and gradually decayed with a decline rate of 0.49% per cycle in the following 50 discharge cycles. The specific capacity maintained a stable value of 1185 mAh/g at the 50.sup.th charge/discharge cycle. Thus, the Carbon-900-S composites worked well in the ether-based electrolyte.

[0131] In FIG. 4c, the coin cell using the Carbon-900-S-Control 1 and the Carbon-900-S-Control 2 cathodes were compared with the coin cell using the Carbon-900-S cathode. The Carbon-900-S-Control 2 cathode had the same initial carbon/sulfur weight ratio as the Carbon-900-S cathode and the Carbon-900-S-Control 1 cathode had a higher initial carbon/sulfur weight ratio. Both had residual sulfur on the surface of the porous carbon, resulting in poor battery performance.

[0132] Moreover, the additional step of heating under argon atmosphere to remove excess sulfur on the surface was important for the high cycling stability of the sodium-sulfur batteries. This step ensured that all the sulfur was encapsulated in the pores instead of being on the surface of the composite. As shown in FIG. 4c, without removal of excess surface sulfur, the capacity declined quickly even within the first two charge/discharge cycles. The more surface excess sulfur existed, the worse specific capacity achieved.

Example 6

Influence of Carbonizing Temperature of the Porous Carbon on the Sodium-Sulfur Battery Performance

[0133] To investigate overall sodium-sulfur battery performance, carbon-sulfur composites prepared under different carbonizing temperatures were tested in various electrolyte/solvent combinations at various current rates (FIG. 5). Overall, the Carbon-900-S composites showed the best performance under the various conditions tested (FIGS. 5a and 5b). At 0.1 C, the Carbon-900-S electrodes exhibited the highest specific capacity and cycling stability in the tetraglyme electrolyte, while Carbon-1000-S electrodes performed the poorest.

[0134] Moreover, the Carbon-900-S battery exhibited stable cycling performance over 200 charge/discharge cycles, and can maintain a specific capacity of 1050 mAh/g at the 200.sup.th charge/discharge cycle at 0.2 C in the tetraglyme electrolyte (FIG. 5b). Carbon-900-S also showed the highest specific capacity at various current rates (0.1 to 2 C) and maintained the best stability (FIG. 5c).

[0135] It should be emphasized that, at lower carbonizing temperature (800° C.), additional treatment of the carbon product with the acid washing step to etch away residual zinc was essential since the trace amount of zinc would degrade the battery performance badly, which could be overcome by increasing the carbonizing temperature to 900° C. (zinc will evaporate out from the porous carbon since the boiling point of bulk zinc is 907° C.) (FIG. 5a, 5c).

INDUSTRIAL APPLICABILITY

[0136] The porous carbon-sulfur composite may be used as an electrode and may be used in a variety of applications such as coin cells, batteries, energy storage devices, biosensors, implantable electrodes or an electrode in capacitors or organic microelectronics.

[0137] The sodium-sulfur battery may be used in smart windows, in electrochromic devices, in sensors for organic and bio-organic materials, in field effect transistors or in printing plates.

[0138] 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.