COMPOSITE FOR SODIUM BATTERIES

Abstract

A carbonized composite comprising a sulfur chain and a conductive network, wherein said sulfur chain is covalently bonded to said conductive network via one or more C—S bonds. The present disclosure also provides a method of preparing the carbonized composite disclosed herein. The carbonized composite may be used in electrochemical cells comprising a reactive metal anode.

Claims

1. A carbonized composite comprising: a) a sulfur chain; b) a conductive network; wherein said sulfur chain is covalently bonded to said conductive network via one or more C—S bonds; wherein said composite is substantially free of S.sub.8; and, wherein said composite is provided in the form of clusters of particles, each particle having a diameter of 1 um or less.

2. The carbonized composite of claim 1, wherein said sulfur chain comprises 2-7 sulfur atoms.

3. The carbonized composite of claim 1, wherein said sulfur chain comprises 2-4 sulfur atoms.

4. The carbonized composite of claim 1, wherein said conductive network is a carbon-based conductive network.

5. The carbonized composite of claim 1, wherein said conductive network comprises a plurality of C═C and C═N bonds.

6. The carbonized composite of claim 5, wherein sulfur content of the composite is about 20 wt. % to about 50 wt. % of a total weight of the composite.

7. The carbonized composite of claim 6, wherein sulfur content of the composite is about 20 wt. % to about 40 wt. % of a total weight of the composite.

8. The carbonized composite of claim 7, wherein carbon content of the composite is about 20 wt. % to about 50 wt. % of a total weight of the composite.

9. The carbonized composite of claim 8, wherein the composite comprises less than 1 wt. % of hydrogen based on a total weight of the composite.

10. A method of preparing the carbonized composite of claim 9, the method comprising the steps of: a) contacting elemental sulfur and a conductive network precursor to form a mixture, wherein an organic solvent is absent from the mixture; b) heating the mixture obtained in step (a) at a temperature of 300° C. to 600° C. to form a composite; and c) heating the composite under inert conditions under a temperature sufficient to remove bulk or unbonded sulfur to thereby obtain said carbonized composite.

11. The method of claim 10, comprising performing the heating step (b) at a temperature of about 550° C.

12. The method of claim 10, wherein the contacting step (a) comprises grinding particles of elemental sulfur and said polymer to form a homogenous mixture.

13. The method of claim 12, wherein comprising contacting the conductive network precursor and elemental sulfur at a weight ratio of 1:2 to 1:10.

14. The method of claim 10, wherein the conductive network precursor is a polymer.

15. The method of claim 14, wherein the conductive network precursor is a polymer comprising nitrile-functionalized monomer units.

16. The method of claim 14, wherein the polymer is polyacrylonitrile,

17. The method of claim 10, wherein comprising performing the heating step (b) for about 2 hours to about 10 hours.

18. The method of claim 10, comprising cooling the composite to room temperature after step (b) and before step (c).

19. The method of claim 10, wherein comprising carrying out the heating step (c) at a temperature of about 150° C. to about 300° C.

20. An electrochemical cell comprising: a) a sodium anode; b) a cathode comprising the carbonized composite as defined in claim 1; and c) an electrolyte.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0091] The accompanying drawings illustrate disclosed embodiments and serve to explain the principles of the disclosed embodiments. It is to be understood that the drawings are for purposes of illustration only and not as a definition of the limits of the invention.

[0092] FIG. 1 is a schematic illustration of a fabricated sodium sulfur battery comprising a sodium sheet as an anode and a sulfur-polyacrylonitrile composite cathode. The sodium sulfur battery of FIG. 1 was fabricated as a coin type cell. The illustrated electrochemical cell comprises a sodium anode and a cathode comprising the carbonized composite described herein coated on a conductive substrate. The sodium anode and cathode are separated by a porous membrane immersed in a suitable electrolyte which facilitates charge transfer.

[0093] FIGS. 2A, 2B, and 2C are a series of scanning electron micrographs of sulfur-polyacrylonitrile composites prepared by performing the first heating step at a temperature of A) 350° C., B) 450° C. and C) 550° C. The initial weight ratios of sulfur:polyacrylonitrile, which are used for the preparation of the composites is indicated in parentheses. The scanning electron micrographs demonstrate that the composite exists as clusters of globular or spherical particles, each particle having a diameter of less than 1 μm. Scale bars are drawn at 1 μm.

[0094] FIGS. 3A, 3B, and 3C are a series of Fourier Transform Infrared Spectroscopy (FTIR) spectra of sulfur-polyacrylonitrile composites prepared at A) 350° C., B) 450° C. and C) 550° C. The weight ratio of sulfur to polyacrylonitrile is indicated in parenthesis in each FTIR spectra. The absorptions at about 1500 cm.sup.−1 and about 1550 cm.sup.−1 corresponds to the symmetric and asymmetric stretches of the C═C double bond; while the absorptions at about 1240 cm.sup.−1 and 1430 cm.sup.−1 correspond to the symmetric and asymmetric stretches of the C═N bond. The presence of absorption bands at about 668 cm.sup.−1 correspond to C—S stretching modes which indicate that covalent C—S bonds are formed between the conductive network and sulfur chain. For clarity, the absorptions at about 2300 cm.sup.−1 to 2400 cm.sup.−1 correspond to the symmetric and asymmetric stretches of carbon dioxide which exists in the background.

[0095] FIG. 4A is a thermogravimetric analysis graph of composites prepared by conducting the first heating step at a temperature of 550° C. with an initial sulfur:polyacrylonitrile weight ratio of 3:1. The composite demonstrated good thermal stability up to about 650° C. before decomposing. This indicates the absence of elemental sulfur, S.sub.8, which is known to decompose at a temperature of 300° C. FIG. 4B (top) is a X-ray diffraction spectrum of the sulfur-polyacrylonitrile composite prepared from an initial 3:1 weight ratio of sulfur:polyacrylonitrile, and carbonized at a temperature of 550° C. The X-ray diffraction pattern of the composite displayed a broad peak at about 26°, characteristic of graphitized carbons. The X-ray diffraction pattern of elemental orthorhombic sulfur, S.sub.8, is also shown for comparison (bottom). Peaks corresponding to free orthorhombic sulfur S.sub.8 are not observed in the diffraction pattern of the composite, indicating the absence of S.sub.8. This also indicates that sulfur in the composite is bonded to the carbon-nitrogen conductive network.

[0096] FIGS. 5A and 5B demonstrate the performance of a battery prototype prepared with a sulfur polyacrylonitrile composite cathode with a pure sodium anode, with a 1 M sodium trifluoromethanesulfonate (NaOTf) electrolyte dissolved in a 1:1 volume/volume mixture of ethylene carbonate and diethyl carbonate. FIG. 5A shows the galvanostatic charge/discharge cycling curves at 0.2C, of a cathode comprising a sulfur-polyacrylonitrile composite prepared from an initial 3:1 weight ratio of sulfur:polyacrylonitrile, and carbonized at a temperature of 550° C. The discharge cycles began at about 2.1 V against the Na/Na.sup.+ electrode and the capacity of the cathode was found to have stabilized by the tenth charge/discharge cycle at about 1350 mAh.Math.g.sub.(s).sup.−1 of about. FIG. 5B shows the cycling performance of a sulfur polyacrylonitrile composite cathode produced from an initial 3:1 weight ratio of sulfur:polyacrylonitrile, and carbonized at a temperature of 550° C. The average Coulombic efficiency of the cathode is about 99.6% after about 30 cycles, indicating good stability of the intermediate polysulfide species even in the presence of a highly reactive sodium metal anode.

[0097] FIGS. 6A and 6B show two scanning electron micrographs of sulfur-polyacrylonitrile composites. FIG. 6A is a scanning electron micrograph of a sulfur-polyacrylonitrile composite prepared with a second heating step to remove residual sulfur. FIG. 6B is a scanning electron micrograph of a sulfur-polyacrylonitrile composite prepared without a second heating step. The composite of FIG. 6b therefore comprises residual sulfur S.sub.8. Scale bars are at 1 μm.

[0098] FIGS. 7A, 7B, and 7C compare the performance of a composite cathode prepared with the second heating step to remove sulfur (referred to as the “evaporated” sample), and without the heating step to remove sulfur (referred to as the “unevaporated” sample). Both the “evaporated” and “unevaporated” samples were prepared from an initial 3:1 sulfur:PAN weight ratio and carbonized at 550° C. A sodium-sulfur electrochemical cell was assembled using a cathode prepared with the “evaporated” and “unevaporated” sulfur-polyacrylonitrile composites. FIG. 7A shows the galvanostatic charge/discharge curves of the cell assembled with a cathode comprising the “evaporated” composite cathode. FIG. 7B shows the galvanostatic charge/discharge curves of the electrochemical cell assembled with a cathode comprising the “unevaporated” composite cathode. The discharge curves of FIG. 7B show a large decrease in capacity to below 400 mAh.Math.g.sub.(s).sup.−1 from the second cycle and an unstable voltage profile was observed in subsequent recharge cycles. FIG. 7C shows the Coulombic efficiencies of the sodium sulfur cells fabricated with cathodes prepared with the “evaporated” and “unevaporated” composites.

[0099] FIGS. 8A and 8Ba are plots summarizing the different molecular species present in the composite, obtained using time of flight secondary ion mass spectrometry methods. The plots illustrate the relative amounts of sulfur species and covalently bonded carbon-sulfur species present in an “ionized” sample of the sulfur-polyacrylonitrile composite cathode. The total counts of detected species are tabulated on a logarithmic intensity axis. FIG. 8A shows the relative amounts of sulfur chain fragments detected, with the majority of sulfur chains existing as short chain species, primarily S.sub.2, S.sub.3 and S.sub.4 chains. FIG. 8B demonstrates the relative contents of carbon fragments and carbon-sulfur species of varying lengths. These carbon-sulfur species further confirm covalent bonding between carbon and sulfur atoms in the composite.

EXAMPLES

[0100] 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.

General Experimental Section

[0101] Elemental combustion analysis was done on a Thermo Scientific Flash 2000 analyzer, with each sample individually sealed in a tin-foil capsule. Sulphanilamide was used as the analytical standard (Elemental Microanalysis, UK) for calibration. Scanning electron microscopy was completed on a JEOL 7600F field emission scanning electron microscope (JEOL, Japan) with samples directly mounted on a sample holder with conductive copper tape. FTIR spectra were obtained in transmittance mode on a Spectrum 2000 instrument (Perkin Elmer). Thermogravimetric analysis was performed on a TA Instruments Q500, using a temperature ramp rate of 10° C. min.sup.−1, under nitrogen gas flow. Powder X-ray diffraction was done using a Bruker D8 ADVANCE X-ray diffractometer, using a Cu Kα source at λ=1.5406 Å. Time of flight secondary ion mass spectrometry measurements were obtained with a TOF.SIMS 5 instrument (IONTOF, Germany) using a Bismuth primary ion beam at 30 keV, over a sample area of 100×100 μm.

Example 1

Gram-Scale Syntheses of Sulfur-Polyacrylonitrile Composites

[0102] The following outlines a process for the simplified gram-scale synthesis of particulate sulfur-polyacrylonitrile composites. The method described herein comprises three steps.

[0103] Elemental sulfur and polyacrylonitrile (PAN; average molecular weight=150,000 g mol-1) were first mixed by physical grinding in an agate mortar and pestle (sulfur:PAN weight ratios of 3:1, 4:1, or 5:1) for approximately ten minutes, to yield a fine light yellow powder. The ground sulfur-PAN mixtures (5 g each) were subsequently transferred into stainless steel autoclaves and sealed in an Argon-filled glovebox. Each autoclave was then removed and heated to 350° C., 450° C., or 550° C. from room temperature at a heating rate of 10° C. min.sup.−1, and held for 6 hours before being allowed to cool naturally. Typical yields of S-PAN composites are in the gram scale, ranging from approximately 3 g to 4 g. Finally, the black carbonized powders were transferred to alumina boats and placed in a tube furnace (Argon-flow rate of 100 sccm, heating rate of 10° C. min.sup.−1) maintained at 250° C. for 2 hours for removal of unreacted sulfur.

Example 2

Characterization of Sulfur-PAN Composites

[0104] The following describes the properties and chemical natures of Sulfur-PAN composites produced from the invented process. Composites were synthesized based on the described method in Example 1, at varying temperatures and initial weight ratios of sulfur:PAN at 3:1, 4:1, or 5:1.

[0105] Morphologies of the composites produced were first examined through microscopy, and found to have a particulate nature. Although composites with particulate morphologies have been applied in lithium-sulfur battery systems, they have not yet been employed with sodium-sulfur batteries as presented here.

[0106] Consequently, the chemical structure of the composites were probed with Fourier-transform infrared (FTIR) spectroscopy specifically to identify covalent bonding between sulfur as active species and the polymer framework, and conjugation within the carbon backbone, conferring chemical stability and electrical conductivity respectively.

[0107] Finally, while sulfur is the active species exploited in the sodium-sulfur battery system, it should not exist freely/unbound in its elemental form (i.e. orthorhombic sulfur, S.sub.8), due to detrimental effects associated with its high reactivity with sodium in the cell environment. To this end, time-of-flight secondary ion mass spectrometry was used to determine its absence, in addition to thermogravimetric analysis and X-ray diffraction. Elemental combustion analysis was also carried out to determine the total sulfur content present in the composite, for all forms of sulfur.

Morphology of Sulfur-Polyacrylonitrile Composites

[0108] All composites produced existed as particulate clusters, in globular/spherical form, each typically less than one micrometer in diameter (FIGS. 2A-2C). No distinct morphological differences were otherwise seen for composites prepared at the respective temperatures or precursor weight ratios. Additional methods were further employed to study the chemical nature of the composites, as outlined below.

Fourier-Transform Infrared Spectra of Sulfur-Polyacrylonitrile Composites

[0109] The chemical nature of the composites were then studied by Fourier-transform infrared (FTIR) spectroscopy for two reasons: (1) to ascertain chemical stability of the synthesized composite in the form of covalent bonding between sulfur and the composite (observed as C—S bonds), and (2) the prerequisite formation of an electrically conductive framework in the form of sp.sup.2-conjugated carbon and nitrogen (observed as C═C and C═N bonds).

[0110] All composites displayed C—S bonding which confirmed covalently-bonded sulfur. However, only composites synthesized at 550° C. (FIG. 3C) showed more intense C═C bands relative to C—C deformations and C═N stretches, indicating a more extensive sp.sup.2-carbon network associated with higher conductivity. Exact peak absorptions are as detailed below.

[0111] FIGS. 3A-3C illustrate the characteristic FTIR spectra of S-PAN composites synthesized at varying temperatures and precursor weight ratios. The overall structures of the composites were similar, with several bonding modes seen between carbon, nitrogen, and sulfur. Exact absorptions at 512 cm.sup.−1 and 940 cm.sup.−1 indicate S—S stretching and S— S ring breathing modes respectively, while the 668 cm.sup.−1 band for C—S stretching confirms new covalent bond formation between carbon and sulfur atoms in the composite. Bonding between carbon and nitrogen were also observed at 1240 cm.sup.−1 and 1430 cm.sup.−1 for symmetric and asymmetric C═N stretches, and at 800 cm.sup.−1 corresponding to C═N hexahydric ring breathing. Additional bands were seen at 1360 cm.sup.−1 associated with C—C deformations. A conjugated carbon backbone structure was also noted to exist with sp.sup.2-hybridized carbons with the presence of strong C═C symmetric and asymmetric bands at 1500 cm.sup.−1 and 1550 cm.sup.−1, suggestive of an sp.sup.2 conjugated network which may confer electrical conductivity in the composite, as compared to both its insulating sulfur and PAN precursors. Nevertheless, closer inspection reveals an important difference between the materials produced at the different temperatures, where only composites synthesized at 550° C. showed comparatively less intense C—C deformations and C═N stretches relative to the C═C bands, which is associated with a more extensive sp.sup.2-carbon network.

Elemental Analysis of Sulfur-Polyacrylonitrile Composites

[0112] As sulfur is itself the active species contributing to the capacity of the sodium-sulfur battery, the exact sulfur content was confirmed using elemental analysis. Elemental analysis reveals that the sulfur content of composites produced at 350° C. and 450° C. were fairly similar at approximately 40 wt. % regardless of the initial sulfur-to-PAN precursor ratio. However, composites synthesized at 550° C. had significantly lower sulfur contents overall, increasing slightly from 33.30% to 35.82% as the sulfur-to-PAN precursor ratio was increased. In addition, the hydrogen content of the composite carbonized at 550° C. from an initial 3:1 sulfur:PAN weight ratio was notably low at 0.32%, likely arising from a greater extent of sp.sup.2-hybridized carbons, in line with observations by FTIR spectroscopy. A monotonic increase in the hydrogen content was also noted for the 550° C. composites with increasing sulfur-to-PAN precursor ratios used.

TABLE-US-00001 TABLE 1 Elemental compositions of S-PAN composites by combustion analysis Carbonization Initial S-PAN Elemental composition Temperature weight ratio C H N S 350° C. 3:1 37.89 0.58 13.84 40.29 4:1 36.63 0.51 12.84 41.84 5:1 36.44 0.54 13.42 39.42 450° C. 3:1 35.62 0.55 12.68 40.07 4:1 38.00 0.69 14.97 39.67 5:1 37.52 0.63 14.98 42.06 550° C. 3:1 32.92 0.32 13.36 33.30 4:1 34.56 0.57 15.44 33.74 5:1 34.29 0.69 16.05 35.82

Stability of the Sulfur-Polyacrylonitrile Composite Under Optimised Conditions

[0113] It is imperative that no unbound sulfur (i.e. elemental orthorhombic sulfur, S.sub.8) exists in the synthesized composite, since its presence in a fabricated sodium-sulfur cell is detrimental to performance as a result of its high reactivity with the sodium anode. In this regard, thermogravimetric analysis was performed and the absence of free sulfur was confirmed, which would have otherwise been observed as a loss of sample weight at relatively low temperatures of about. 250-350° C.

[0114] The optimised composite carbonized at 550° C. from an initial 3:1 sulfur:PAN weight ratio displayed good thermal stability up to around 650° C. (FIG. 4A), with decomposition only starting above this temperature, confirming the absence of free sulfur. Additionally, X-ray diffraction patterns of the composite displayed a broad peak at ca. 26° typical of graphitized carbons (FIG. 4b), without any of the characteristic peaks of elemental sulfur. Both techniques confirm that no free sulfur exists in the composite and all sulfur present is therefore directly bonded to the carbon-nitrogen backbone.

[0115] The absence of elemental sulfur was also confirmed in the optimized composite, as determined by time-of-flight secondary ion mass spectrometry (FIG. 8A). The sulfur chains were noted to exist primarily as short chain species S.sub.2, S.sub.3 and S.sub.4.

Example 3

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

[0116] This Section describes a standard procedure for the preparation of batteries, but lends itself towards the fabrication of prototype sodium-sulfur full cells. Cells were assembled using cathode composites obtained from the invented synthetic method above in Section 1.1, and tested in combination with a new sodium trifluoromethanesulfonate (NaOTf) electrolyte salt, in various solvents.

[0117] S-PAN composites were ground in an agate mortar with conductive carbon (Super P), and mixed with polymer binder (polyvinylidene fluoride, PVDF) 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 with a doctor blade and allowed to dry completely at 70° C. Areal sulfur loadings were approximately between 0.4-0.6 mg.sub.(s).Math.cm.sup.−2.

[0118] 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 (NaOTf) electrolyte in a 1:1 volume solvent mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). Other solvent combinations tested are as detailed in Table 2 below.

Sodium-Sulfur Battery Performance

[0119] The stability of the optimised particulate composite has been demonstrated above. The stability and performance of the particulate S-PAN composite after its integration as cathode material in sodium-sulfur batteries is demonstrated below.

[0120] Cell fabrication and testing was done using a combination of sodium trifluoromethanesulfonate (NaOTf) as electrolyte salt, the S-PAN composite as cathode, in conjunction with a pure sodium anode. In light of the novel combination of the NaOTf electrolyte used with S-PAN cathodes, battery performances of the electrolyte were also evaluated in various solvents.

[0121] A battery prototype consisting of a S-PAN composite cathode was constructed with a pure sodium anode according to the method outlined in above section, and its performance tested by galvanostatic charge/discharge cycling at 0.2 C (where 1 C=1673 mA.Math.g(S)-1, and specific capacity of sulfur=1673 mAh.Math.g.sub.(S).sup.−1). The composite carbonized at 550° C. from an initial 3:1 sulfur:PAN weight ratio was determined to be most stable. FIG. 5A illustrates the charge/discharge profiles of the S-PAN cathode in a 1 M sodium trifluoromethanesulfonate (NaOTf) electrolyte dissolved in a 1:1 volume mixture of ethylene carbonate (EC) and diethyl carbonate (DEC).

[0122] The initial discharge process begins from 1.7 V vs. Na/Na.sup.+, and the capacity was found to exceed the theoretical capacity of sulfur, reaching above 2200 mAh-g.sub.(S).sup.−1. This additional capacity however, arises from the sodiation of the carbon-nitrogen backbone and is an irreversible process, occurring in conjunction with the conversion of sulfur to sodium sulfide (Na.sub.2S). Upon the first charge cycle, the Na.sub.2S discharge-product was reconverted back to sulfur.

[0123] The subsequent second discharge cycle then began at 2.1 V vs. Na/Na.sup.+, and a capacity of ca. 1600 mAh.Math.g.sub.(S).sup.−1 was recovered. Consequently, capacities were found to have stabilised by the tenth cycle at approximately 1350 mAh-g.sub.(S).sup.−1, further maintaining 1250 mAh-g.sub.(S).sup.−1 at the 30th cycle. Average Coulombic efficiencies also remained high at 99.6%, indicating good stability of sodium polysulfide intermediates in the presence of a highly reactive sodium metal anode.

[0124] Overall sodium-sulfur battery performance was further tested in different electrolyte solvents. In general for the S-PAN composites synthesized, better cycling performances were observed with carbonate-based solvents as compared to ether-based ones. As seen in FIGS. 5A-5C, a simple binary EC-DEC mixture allowed good capacity retention with high Coulombic efficiencies. No significant improvements were obtained with additives such as fluoroethylene carbonate (FEC) and vinylene carbonate (VC), or in mixtures with propylene carbonate (PC) solvent (Table 2). In particular, FEC and VC additives resulted in slightly lowered capacities, while unstable charge profiles were observed with PC-DEC or EC-PC mixtures along with reduced Coulombic efficiencies. Ether (glyme)-based solvents in general exhibited poor capacity retention, and unstable charge profiles.

TABLE-US-00002 TABLE 2 Battery cycling performance in various carbonate- and ether (glyme)-based solvents or solvent combinations, with 1M sodium trifluoromethanesulfonate electrolyte. Cycling Coulombic Solvent combinations stability Efficiency Note EC-DEC (1:1, v/v) Good ~100% — EC-DEC (1:1, v/v) with Fair ~100% Low initial discharge 5% FEC additive capacity EC-DEC (1:1, v/v) with Fair ~100% Low initial discharge 2% VC additive capacity EC-DMC (1:1, v/v) Fair ~100% Occasional instability during charge cycle EC-PC-DEC (1:1:2, v/v) Good ~100% — PC-DEC (1:1, v/v) Poor <100% Unstable charge (variable) cycles EC-PC (1:1, v/v) Poor <100% Unstable charge (variable) cycles.sup.a Tetraglyme with 0.1M Poor <100% Unstable charge NaNO.sub.3 additive (variable) cycles Tetraglyme Poor <100% Unstable charge (variable) cycles Diglyme Poor Low Unstable charge (variable) cycles Monoglyme Poor NA Unable to discharge Notes: .sup.aglass fiber separator used in place of Celgard membrane. Abbreviations: EC = ethylene carbonate, DEC = diethyl carbonate, DMC = dimethyl carbonate, FEC = fluoroethylene carbonate, VC = vinylene carbonate, PC = propylene carbonate, tetraglyme = tetraethylene glycol dimethyl ether, diglyme = diethylene glycol dimethyl ether, monoglyme = ethylene glycol dimethyl ether.

Example 4

Effects of Residual Elemental Sulfur, S.SUB.8

[0125] The effects of residual elemental sulfur were studied by comparing the morphology, chemical composition and electrical performance of composites which are substantially free of sulfur; and composites which comprise residual sulfur. The composite which is substantially free of sulfur was prepared according to the methods described herein (herein referred to as “evaporated” composite), while the composite comprising residual sulfur was prepared without the additional second heating step to evaporate sulfur (herein referred to as the “unevaporated” composite). Both composites were carbonized at 550° C. using an initial S:PAN weight ratio of 3:1.

[0126] The morphology of the S-PAN composites first carbonized at 550° C. (S:PAN weight ratio of 3:1), with and without an additional sulfur evaporation process is shown in FIGS. 6A and 6B. In both materials, no change to the particulate morphology was observed as a result of the sulfur evaporation. It should however be noted that the composite without sulfur evaporation (FIG. 6B) exhibited localized charging effects in the scanning electron micrographs, associated with electrically non-conductive materials (e.g. elemental sulfur as an electrical insulator).

[0127] Elemental combustion analysis was further performed to measure the sulfur content of the composites, and the results are shown in Table 3 below. The composite prepared without the second heating step contained a larger amount of sulfur of above 40 wt % before evaporation.

TABLE-US-00003 TABLE 3 Sulfur content of S-PAN composites with and without sulfur evaporation in inert gas flow, by elemental combustion analysis Composite Material T = 550° C., Sulfur content S-PAN ratio (3:1) (wt. %) Evaporated 33.30 Unevaporated 46.55

[0128] Finally, sodium-sulfur cells were assembled from both composites and tested FIGS. 7A-7C illustrate the typical galvanostatic charge/discharge profiles of surface-sulfur evaporated S-PAN in the sodium-sulfur system, with single sloping plateaus expected from the reaction of short-chain sulfur bonded to the composite. Contrastingly, the unevaporated composite displays a small initial plateau at a higher potential of 2.05 V vs. Na/Na.sup.+ in the first discharge cycle, followed by the usual sloping plateau. The high voltage plateau arises from the reaction of elemental sulfur in the composite with sodium ions, resulting in long-chain polysulfides that irreversibly dissolve into the electrolyte, resulting in an irreversible loss of capacity. As a result, a large decrease in capacity is observed from the second cycle, reaching below 400 mAh.Math.g.sub.(S).sup.−1. An unstable voltage profile was also seen in the subsequent re-charge, which might be attributed to the presence of reactive long-chain polysulfides in the electrolyte, and/or their passivation of the sodium anode. This instability could also be observed from the erratic Coulombic efficiencies of the first 5-7 cycles in the unevaporated composite (FIG. 7C), signifying the undesirable polysulfide shuttling effect. The average Coulombic efficiency over fifty cycles was also lower in the unevaporated composite at 99.2%, vs. 99.7% for the evaporated composite.

[0129] In light of the higher battery capacity and Coulombic efficiency of the surface-sulfur evaporated composite, it is thus demonstrated that the composite which is substantially free of elemental sulfur S.sub.8, prepared with an additional step of heating for sulfur evaporation, significantly contributes to the overall improved electrochemical performance of S-PAN composites, specifically in the sodium-sulfur battery system.

INDUSTRIAL APPLICABILITY

[0130] The disclosed carbonized composite may be used for the preparation of electrodes, such as cathodes which may be utilized in electrochemical cells. Due to its ease of manufacture, the carbonized composite disclosed herein may be conveniently prepared on an industrial scale.

[0131] The carbonized composite described herein may be used for the fabrication of sulfurized cathodes which are stable and operable at room temperature. Such cathodes are suitable for use in an electrochemical cell comprising an anode made from highly reactive metals such as sodium. In particular, the cathodes prepared with the carbonized composites may be coupled with a sodium anode, for the fabrication of sodium sulfur batteries. Sodium sulfur batteries are an alternative energy storage system to presently available technologies.