A COMPOSITE

Abstract

There is provided a composite comprising a) a short chain sulfur; and b) a carbon-supported conductive polymer such as polyacrylonitrile, wherein sulfur atoms of said short chain sulfur are covalently linked to the conductive polymer of said carbon-supported conductive polymer via a C—S bond. A method of preparing said composite comprising polymerizing a plurality of monomers in the presence of a carbon scaffold, mixing elemental sulfur and heating the mixture to obtain said composite is also disclosed. An electrochemical cell comprising said composite as cathode, a sodium anode and a liquid electrolyte such as sodium trifluoromethanesulfonate dissolved in a mixture of solvents is disclosed.

Claims

1. A composite comprising: a) short chain sulfur; and b) a carbon-supported conductive polymer, wherein sulfur atoms of said short chain sulfur are covalently linked to the conductive polymer of said carbon-supported conductive polymer via a C—S bond.

2. The composite according to claim 1, wherein said composite is a two-component composite.

3. The composite according to claim 1, wherein said conductive polymer is a carbonized polymer.

4. The composite according to claim 1, wherein said conductive polymer comprises a plurality of monomers and said carbon-supported conductive polymer comprises a polymerized form of said plurality of monomers on or within a carbon support.

5. The composite according to claim 4, wherein said conductive polymer is selected from the group consisting of carbonized polyacrylonitrile (PAN), polyaniline, polypyrrole, polyacetylene, polyphenylene, polyphenylene sulfide, polythiophene, poly(fluorene)s, polypyrenes, polyazulenes, polynaphthalenes, polycarbazoles, polyindoles, polyazepines, poly(3,4-ethylenedioxythiophene) (PEDOT), poly(p-phenylene sulfide) (PPS), poly(p-phenylene vinylene) (PPV), and co-polymers mixtures thereof.

6. The composite according to claim 5, wherein the monomers of said conductive polymer is selected from the group consisting of acrylonitrile, aniline, pyrrole, acetylene, phenylene, phenylene sulfide, thiophene, (fluorene)s, pyrenes, azulenes, naphthalenes, carbazoles, indoles, azepines, 3,4-ethylenedioxythiophene, p-phenylene sulfide, p-phenylene vinylene, and mixtures thereof.

7. The composite according to claim 4, wherein said carbon support comprises a particulate porous carbon or a fibrous carbon.

8. The composite according to claim 7, wherein said particulate porous carbon has a surface area in the range of 100 m.sup.2/g to 2000 m.sup.2/g and an average pore size or average pore distribution size in the range of 2 nm to 500 nm.

9. The composite according to claim 7, wherein said fibrous carbon is a carbon cloth comprising fibers having a diameter in the range of 3 μm to 20 μm and having a cloth thickness in the range of 200 μm to 500 μm.

10. The composite according to claim 1, wherein said short-chain sulfur is selected from S.sub.2, S.sub.3, S.sub.4 or mixtures thereof.

11. The composite according to claim 1, wherein said short-chain sulfur is present in said composite in a concentration in the range of 20 wt % to 50 wt %, based on the total weight of said composite.

12. The composite according to claim 1, wherein said short-chain sulfur and said conductive polymer is present in said composite at a ratio in the range of 2:1 to 6:1.

13. A method of preparing a composite comprising: a) short-chain sulfur; and b) a carbon-supported conductive polymer, wherein sulfur atoms of the short-chain sulfur are covalently linked to the conductive polymer of the carbon-supported conductive polymer via a C—S bond, comprising the steps of: (a) polymerizing, in the presence of a carbon scaffold, a plurality of monomers making up the conductive polymer or a plurality of monomers making up a precursor of the conductive polymer; (b) mixing elemental sulfur with said carbon-supported conductive polymer or the carbon-supported conductive polymer precursor obtained in step (a); and (c) heating the mixture of the elemental sulfur with the carbon-supported conductive polymer precursor obtained in step (b).

14. The method according to claim 13, further comprising, before step (a), the steps of: (a1′) adding a polymerization initiator to a mixture of said plurality of monomers and said carbon scaffold in a solvent; or (a1″) adding said plurality of monomers to a mixture of a polymerization initiator and said carbon scaffold in a solvent; and (a2) heating said mixture from step (a1′) or step (a1″) to initiate polymerization.

15. The method according to claim 13, wherein said heating step (c) is undertaken at a temperature in the range of 400° C. to 600° C.

16. A cathode material comprising a composite comprising: a) short chain sulfur; and b) a carbon-supported conductive polymer, wherein sulfur atoms of said short chain sulfur are covalently linked to the conductive polymer of said carbon-supported conductive polymer via a C—S bond.

17. A sodium-sulfur electrochemical cell comprising a cathode material comprising a composite comprising: a) short chain sulfur; and b) a carbon-supported conductive polymer, wherein sulfur atoms of said short chain sulfur are covalently linked to the conductive polymer of said carbon-supported conductive polymer via a C—S bond.

18. An electrochemical cell comprising a pure sodium anode, a liquid electrolyte, and a cathode material comprising a composite comprising: a) short chain sulfur; and b) a carbon-supported conductive polymer, wherein sulfur atoms of said short chain sulfur are covalently linked to the conductive polymer of said carbon-supported conductive polymer via a C—S bond.

19. The electrochemical cell according to claim 18, wherein said liquid electrolyte comprises sodium trifluoromethanesulfonate dissolved in a mixture of solvents.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0103] 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.

[0104] FIG. 1 is a series of images showing the various composites produced in Example 1 below as well as the corresponding starting carbon material. In particular, FIG. 1a shows an image of pure polyacrylonitrile (PAN) polymer and PAN as polymerized on carbon nanotube (CNT) and porous Ketjenblack (KB) scaffolds respectively; FIG. 1b is an image of PAN polymerized on woven carbon fiber cloth; FIG. 1c is a scanning electron microscopy (SEM) image of pure S-PAN composite obtained from acrylonitrile polymerization and after carbonization with sulfur having a scale bar of 100 nm; FIG. 1d is a SEM image of bare unmodified CNT (starting material) having a scale bar of 1 μm; FIG. 1f is a SEM image of porous KB (starting material) having a scale bar of 100 nm; FIG. 1h is a SEM image of woven carbon fiber cloth (starting material) having a scale bar of 1 μm; FIG. 1e is a SEM image of CNT after acrylonitrile polymerization and S-PAN composite formation having a scale bar of 1 μm; FIG. 1g is a SEM image of porous KB after acrylonitrile polymerization and S-PAN composite formation having a scale bar of 1 μm; FIG. 1i is a SEM image of woven carbon fiber cloth after acrylonitrile polymerization and S-PAN composite formation having a scale bar of 10 μm; FIG. 1j is a SEM image of CNT after acrylonitrile polymerization and S-PAN composite formation having a scale bar of 100 nm; FIG. 1k is a SEM image of porous KB after acrylonitrile polymerization and S-PAN composite formation having a scale bar of 100 nm; and FIG. 1l is a SEM image of woven carbon fiber cloth after acrylonitrile polymerization and S-PAN composite formation having a scale bar of 1 μm.

[0105] FIG. 2 is a series of fourier-transform infrared (FTIR) spectra where FIG. 2a shows the FTIR spectra of pure polyacrylonitrile polymer and polyacrylonitrile synthesized on various carbon scaffolds and FIG. 2b shows the FTIR spectra of after their carbonization with sulfur to form the corresponding S-PAN composites.

[0106] FIG. 3 is a series of galvanostatic charge/discharge curves where FIG. 3a is for pure S-PAN cathode at 0.2 C; FIG. 3b is for S-PAN-CNT at 0.2 C; FIG. 3c is for S-PAN-KB at 0.2 C; FIG. 3d for S-Pan-Cloth at 0.2 C; and FIG. 3e shows cycling performance and Coulombic efficiencies of sodium-sulfur cells with pure and carbon-supported S-PAN composite cathodes.

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

[0108] Here, carbon-supported sulfur-polyacrylonitrile composites were formed, with the first step being the polymerization of acrylonitrile on carbon scaffolds, followed by the second step of carbonization with sulfur to form the composites as shown in Scheme 1 below.

##STR00001##

[0109] Three representative carbons were used here such as (1) multiwalled carbon nanotubes (CNTs; length: 0.5 to 2 μm, outer diameter: 20 to 30 nm, inner diameter: 1 to 2 nm), obtained from Sigma-Aldrich, Singapore (as a comparative example); (2) porous Ketjenblack (KB; pore width distribution about 2 to 100 nm), obtained from Lion Specialty Chemicals, Japan; and (3) a carbon cloth woven from individual non-porous carbon fibers (fiber diameter about 8 μm, total cloth thickness about 350 μm), obtained from NuVant Systems, Inc. of Indiana of the United States of America.

[0110] Acrylonitrile Polymerization on Carbon Scaffolds (First Step)

[0111] As-received acrylonitrile monomer (obtained from Sigma-Aldrich, Singapore) was first washed to remove polymerization inhibitors (e.g. monomethyl ether hydroquinone) by solvent extraction, sequentially using 1% sulfuric acid, 1% aqueous sodium hydroxide, and followed three times with deionized water, achieving a neutral pH after the final step. The pure monomer was prepared and promptly used each time to avoid self-polymerization.

[0112] 1.5 g of CNT or KB was first dispersed by ultrasonication in a 1:1 dimethyl sulfoxide (DMSO)-deionized water mixture (150 mL) for 30 minutes, and further purged with nitrogen in a sealed flask under stirring for 20 minutes. For the synthesis of pure PAN (as control), only the pure DMSO-water mixture was bubbled with nitrogen. Pure acrylonitrile monomer (30 mL) was subsequently injected using a syringe into the above carbon scaffolds or control accordingly. A solution of the radical initiator, azobisisobutyronitrile (AIBN, 12 wt % in acetone, 2.4 mL, obtained from Sigma-Aldrich, Singapore) was gradually introduced and the mixture slowly heated to 65° C., initiating the polymerization reaction. The sealed reaction flask was kept under nitrogen protection and stirred vigorously for 2 hours. After leaving to cool, the gel-like product was washed with methanol three times by centrifugation, and dried in vacuum overnight to give the pure PAN and carbon-supported PAN polymers.

[0113] In the case where the carbon scaffold is woven carbon cloth, cloths (ca. 5×5 cm) were first activated by refluxing in 9 M nitric acid at 110 to 120° C. for 20 hours. Each cloth was then washed with copious amounts of deionized water five times until a neutral pH was reached, followed lastly with methanol, and then dried in a 60° C. oven overnight before use. Cloths were briefly soaked in an AIBN solution (3 wt % in acetone) and rapidly dried under vacuum at room temperature, before placement in a sealed flask purged with nitrogen (one cloth per sealed flask). Separately, a DMSO-water mixture was also bubbled with nitrogen for 20 minutes before addition to the flask. Acrylonitrile monomer (10 mL) was introduced and quickly mixed to achieve homogeneity. The mixture was heated to 65° C. under quiescent condition, and kept for 3 hours. Cloths were then retrieved, washed with methanol three times, and stored in vacuum overnight.

[0114] Synthesis of Carbon-Supported Sulfur-Polyacrylonitrile Composites by Carbonization with Sulfur (Second Step)

[0115] The second step of sulfur carbonization is described here, yielding the four final composites (hereby termed as (i) pure S-PAN, (ii) S-PAN-CNT, (iii) S-PAN-KB, and (iv) S-PAN-Cloth).

[0116] Each PAN product (pure PAN, PAN-CNT, PAN-KB) obtained above was mixed with elemental sulfur by physical grinding in an agate mortar and pestle (weight ratio of five times sulfur to each PAN product) for approximately ten minutes to achieve a fine homogeneous powder. While other sulfur-to-PAN ratios were also investigated, the 5:1 weight ratio was determined to be optimal.

[0117] For the PAN-Cloth product, an excess of the ground sulfur-PAN powder (obtained by physically grinding elemental sulfur with pure PAN powder) was homogeneously distributed over each cloth.

[0118] Each precursor mixture was then transferred to alumina boats and carbonized in an argon-filled tube furnace (Ar-flow rate of 50 sccm, heating rate of 10° C. min.sup.−1) maintained at 450° C. for 6 hours before natural cooling to room temperature. Gram scale yields of S-PAN composites may be achieved using this method, with final composites over 1 g obtained.

[0119] Particulate-type S-PAN was produced under all conditions except on carbon cloth, where a uniform film was formed instead covering individual fiber surfaces.

[0120] Characterization of S-PAN Composites

[0121] Surface Area and Pore Information

[0122] Surface area and pore information of the carbon additives were first derived from nitrogen adsorption/desorption analysis, representative of low porosity (CNT), high porosity (KB), and macroporous (cloth) carbons respectively.

[0123] Table 1 summarizes the surface areas and porosities of three representative carbons. CNTs were employed firstly as a comparative example, with both low surface area and pore volume. In contrast, KB is a common mesoporous carbon produced large-scale for various industrial applications, exhibiting a high surface area and total pore volume more than three times that of the CNT type used herein. The majority of pores exist in the mesoporous region of 2 nm to 50 nm.

[0124] Lastly, the carbon fiber cloth has a low gravimetric surface area as it consists of thick fibers (diameter 8 μm) with a non-porous solid internal structure. However, its “porosity” differs from the CNT and KB scaffolds when viewed from a macroscopic perspective. The carbon cloth comprises of interwoven fibers, with large micrometer-sized voids (i.e. pores) existing between each longitudinally-ordered fiber. This unique morphology was found to be important as large voids allow for repeated radial expansion and contraction cycles of the deposited sulfur composite along each fiber, and is corroborated by electron microscopy in the following Section.

TABLE-US-00001 TABLE 1 Surface area, pore volumes, and pore width distribution of carbon scaffolds, based on nitrogen adsorption/desorption and Brunauer-Emmett-Teller (BET) surface area analysis. Modal Cumulative Total Surface pore width pore volume pore Carbon scaffold area distribution in modal range volume type (m.sup.2 .Math. g.sup.−1) (nm) (cm.sup.3 .Math. g.sup.-1) (cm.sup.3 .Math. g.sup.-1) CNT 91 25 - 120 0.6 0.9 KB 1507 2-71 2.2 2.9 Woven carbon cloth <10 55-115 0.7 1.0

[0125] Morphologies of Supported Sulfur-Polyacrylonitrile Composites

[0126] The bare-CNT, bare KB and bare cloth used as starting materials (before the two steps processing above) were assessed using scanning electron microscopy (SEM) and shown in FIG. 1d, FIG. 1f and FIG. 1h, respectively.

[0127] Physical appearances of the four PAN polymer materials produced above (first step) are provided in FIG. 1a and FIG. 1b Pure PAN polymer existed as a white particulate solid. In comparison, at the same weight loading of carbon additives (6 wt. % with respect to acrylonitrile precursor), PAN-CNT was obtained as a grey powder while PAN-KB was black (FIG. 1a). Polymerized PAN on nitric acid-activated carbon cloth was produced as a thin translucent film over the black cloth material (FIG. 1b).

[0128] After the subsequent carbonization process with sulfur (second step), the pure S-PAN composite showed a particulate morphology, as clusters of globular particles each approximately 100 to 200 nm in diameter (FIG. 1c). The S-PAN-CNT composite (FIG. 1e and FIG. 1j) similarly displayed particulate clusters, but with CNT strands interspersed throughout. Comparatively, unmodified KB carbon consists of porous particles roughly 100 nm in diameter, and was well dispersed within the S-PAN-KB composite (FIG. 1g and FIG. 1k). The carbon cloth is made up of interwoven carbon fibers of approximately 8 μm diameter (FIG. 1h), with an uneven but otherwise non-porous surface. After polymerization and carbonization however, a smooth layer of the composite was observed over the fibers (FIG. 1i and FIG. 1l). Individual fibers maintained their separation of at least several micrometers within the woven matrix, with this space consequently allowing for radial expansion of the S-PAN active layer on each fiber. As such, the synthesized S-PAN-CNT, S-PAN-KB, and S-PAN-Cloth serve as low porosity, high porosity, and macroporous composite variations contrasted against the pure S-PAN composite.

[0129] Chemical Structure of Carbon-Supported Polyacrylonitrile and Sulfur-Polyacrylonitrile Composites

[0130] Chemical structures of carbon-supported PAN polymers, and their consequent S-PAN composites were elucidated by Fourier-transform infrared (FTIR) spectroscopy (1) to confirm successful radical polymerization of the acrylonitrile monomer to PAN (through formation of C—H bonds along the polymer backbone), and (2) to ascertain chemical stability of the final S-PAN composites by cyclization to give an sp.sup.2-conjugated carbon and nitrogen backbone (as C═C and C═N bonds) along with covalent bonding between sulfur and carbon (observed as C—S bonds).

[0131] FIG. 2a displays the FTIR spectra of pure PAN polymer and carbon-supported PAN materials produced from the first step above. Most importantly, the absorption at 2935 cm.sup.−1 due to aliphatic sp.sup.3C—H stretching confirmed successful polymerization, together with peaks at 1450 cm.sup.−1 and 1360 cm.sup.−1 arising from C—H bending modes. The most prominent band at 2245 cm.sup.−1 corresponded to C≡N from the nitrile groups. Weaker absorptions at 1630 cm.sup.−1 and 1025 cm.sup.−1 may be attributed to C═N and C—N stretches from imine and amine-type structures respectively, suggesting a small extent of reaction on the nitrile moiety.

[0132] After the subsequent carbonization procedure (second step), S-PAN composites were produced from the PAN materials. FIG. 2b illustrates the FTIR spectra of the final composites, confirming both cyclization of the main carbon-nitrogen backbone and covalent bond formation between sulfur and carbon. The cyclization was first established with both symmetric and asymmetric C═N stretches at 1240 cm.sup.−1 and 1430 cm.sup.−1 between carbon and nitrogen, while strong symmetric and asymmetric C═C bands at 1500 cm.sup.−1 and 1550 cm.sup.−1 indicated sp.sup.2-hybridization characteristic of conjugation and therefore electrical conductivity in the composite. Additional bands were also observed at 800 cm.sup.−1 corresponding to C═N hexahydric ring breathing and at 1360 cm.sup.−1 associated with C—C deformations.

[0133] Covalent sulfur bonding was also ascertained with the 670 cm.sup.−1 band for C—S stretching between carbon and sulfur atoms in the composite. Bands at 513 cm.sup.−1 and 940 cm.sup.−1 respectively for S—S stretching and S—S ring breathing modes indicated that the bonded sulfur existed as short chains, typically 2 to 4 atoms in length.

[0134] Elemental Analysis of Sulfur-Polyacrylonitrile Composites

[0135] Elemental combustion analysis was used to determine exact sulfur compositions in each composite.

[0136] With sulfur itself being the active species contributing to the capacity of the sodium-sulfur battery, the exact sulfur content (by weight) of each S-PAN composite was determined using elemental combustion analysis. As tabulated in Table 2, pure S-PAN and S-PAN-CNT have fairly similar sulfur contents at 36% and 39% respectively. S-PAN-KB contained a marginally higher amount of carbon in comparison, and a lower sulfur composition of close to 30%. As a comparison, sulfur contents of pure particulate-type S-PAN composites range typically between 30% and 45%. Contrastingly, S-PAN-Cloth showed the least sulfur at 3.6%, but with significantly more carbon. This is nonetheless expected as the cloth-based composite consists primarily of woven carbon fibers, with the S-PAN existing as a thin layer over individual fibers as observed from SEM in FIG. 1i and FIG. 1l.

TABLE-US-00002 TABLE 2 Elemental compositions of carbon-supported and pure S-PAN composites by combustion analysis. Elemental composition wt. % Composite Material C H N S Pure S-PAN 41.9 0.7 15.1 36.3 S-PAN-CNT 42.6 0.4 13.5 39.4 S-PAN-KB 43.8 0.5 13.4 29.7 S-PAN-Cloth 60.2 1.5 14.2 3.6

Example 2

[0137] The preparation of battery cathodes and full cell assemblies is described here.

[0138] Cells were assembled using the carbonized samples obtained from Example 1, and tested in combination with sodium trifluoromethanesulfonate (NaCF.sub.3SO.sub.3) electrolyte in a 1:1 volume mixture of ethylene carbonate and diethyl carbonate.

[0139] For battery cathode preparation, the carbonized pure S-PAN and S-PAN composites were ground in an agate mortar with conductive carbon (Super P, obtained from Alfa Aesar, Singapore), and mixed with polymer binder (polyvinylidene fluoride, PVDF, obtained from Sigma-Aldrich, Singapore) in a weight ratio of 7:2:1 with N-methyl-2-pyrrolidone (NMP) solvent to yield a viscous slurry. Slurries were then coated onto carbon-coated aluminium foil (obtained from MTI Corporation, of California of the United States of America) with a doctor blade and allowed to dry completely at 70° C. For S-PAN-Cloth, the carbonized S-PAN-Cloth from Example 1 was used as-is, without further addition of binder or conductive carbon. Areal sulfur loadings for all four cathode materials were rigorously fixed between 0.5-0.6 mg.sub.(S).Math.cm.sup.−2.

[0140] Sodium-sulfur cells were fabricated as 2032-type coin cells. Assembly was done in an argon-filled glovebox with the respective S-PAN composites (11.28 mm diameter) used as the cathode. Freshly cut sodium blocks (99.9%) were rolled into sheets and cut into circular discs which served as the anode, separated by a Celgard membrane filled with 1 M sodium trifluoromethanesulfonate (NaCF.sub.3SO.sub.3) electrolyte in a 1:1 volume solvent mixture of ethylene carbonate (EC) and diethyl carbonate (DEC).

[0141] Sodium-Sulfur Battery Performance

[0142] The stability and performance of porous carbon-supported S-PAN composites after their integration as cathode material in sodium-sulfur batteries were investigated. The high porosity carbon-supported composite (S-PAN-KB) and the macroporous S-PAN-Cloth demonstrated the best cycling stabilities, with highest capacity retention after extended cycling.

[0143] Full cell fabrication was performed using each S-PAN composite as cathode in conjunction with a pure sodium anode, and tested using sodium trifluoromethanesulfonate (NaCF.sub.3SO.sub.3) in ethylene carbonate and diethyl carbonate as electrolyte.

[0144] FIG. 3a to FIG. 3d illustrates the galvanostatic charge/discharge profiles of the S-PAN composite cathodes prepared according to the cell assembly method described above where FIG. 3a applies to pure S-PAN cathode, FIG. 3b applies to S-PAN-CNT, FIG. 3c applies to S-PAN-KB and FIG. 3d applies to S-PAN-Cloth. Their performances were tested by charge/discharge cycling at 0.2 C (where 1 C=1673 mA.Math.g(s).sup.−1, as the theoretical specific capacity of sulfur is 1673 mAh.Math.g.sub.(S).sup.−1 The first discharge process started from ca. 1.6 V vs. Na/Na.sup.+ reaching just above 1600 mAh.Math.g.sub.(S).sup.−1 for the pure S-PAN, S-PAN-CNT, and S-PAN-Cloth cathodes, therefore indicating that the majority of the loaded sulfur had reacted. S-PAN-KB in contrast, had a higher first discharge capacity of 2150 mAh.Math.g.sub.(S).sup.−1, exceeding the theoretical capacity of sulfur. This added capacity arises however from sodiation of the S-PAN carbon-nitrogen backbone, which is itself an irreversible process, occurring simultaneously with the conversion of sulfur to sodium sulfide (Na.sub.2S). Additionally, the high surface area and porosity of the KB carbon scaffold indirectly contributed to the increased capacity by allowing a greater contact surface between the S-PAN active material and the electrolyte. Upon the first charge cycle, Na.sub.2S discharge products were reconverted back to sulfur. Although the initial charge profile of the S-PAN-Cloth experienced minor voltage drops, this eventually stabilized and was not observed in subsequent cycles from the 2.sup.nd charge onwards (FIG. 3d).

[0145] In all composites, the second discharge was initiated at ca. 2.1 V vs. Na/Na.sup.+ with capacities of 1200-1300 mAh.Math.g.sub.(S).sup.−1 recovered for the pure S-PAN, S-PAN-CNT, and S-PAN-Cloth Again, S-PAN-KB maintained a notably higher capacity ca. 1640 mAh.Math.g.sub.(S).sup.−1, contributed by its higher surface area. Average Coulombic efficiencies of all composites also remained high at >99.9% (FIG. 3e) on average over 50 cycles, indicating good chemical stability of the composites and their polysulfide intermediates in the presence of the reactive sodium anode.

[0146] Most notable however, is the difference in capacity retention of the porous carbon-supported composites. While the capacities of mesoporous S-PAN-KB and macroporous S-PAN-Cloth rapidly stabilised in the early cycles, the unsupported pure S-PAN and S-PAN-CNT (i.e. low surface area and porosity) counterparts continued on a gradual decline, maintaining only ca. 750 mAh.Math.g.sub.(S).sup.−1 and 550 mAh.Math.g.sub.(S).sup.−1 at their 50.sup.th cycles. Conversely, the mesoporous S-PAN-KB retained a high 1300 mAh.Math.g.sub.(S).sup.−1, and the macroporous S-PAN-Cloth with 1110 mAh.Math.g.sub.(S).sup.−1 (i.e. 80% and 91% capacity retention respectively).

[0147] These results correlate with the extent to which the carbon supports are able to provide for volume expansion, which can arise either from their (1) high surface area and pore volume, in the case of S-PAN-KB; or (2) unique morphologies such as S-PAN-Cloth, where the composite layer on each fiber has adequate space for radial expansion.

[0148] In S-PAN-KB, the high surface area of the mesoporous KB scaffold permits a greater contact surface between the active sulfur and the electrolyte, thus achieving a higher capacity than other substrates. Furthermore, its high total pore volume contributes to its high capacity retention with cycling.

[0149] Conversely for S-PAN-Cloth, its low surface area results in a lower initial capacity similar to the unsupported composite. Nonetheless, its unique morphology as a thin layer covering each fiber permits radial expansion during discharge cycles, thus avoiding structural degradation and maintaining the highest capacity retention of all materials.

[0150] Hence, the use of porous additives and structures as described herein to address cathode stability is an important strategy in the development of sodium-sulfur batteries.

INDUSTRIAL APPLICABILITY

[0151] The disclosed composite may be used as a cathode, which in turn can be used in an electrochemical cell. Therefore, the present application finds utility in electrochemistry and energy-related industries.

[0152] 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.