SULFUR-FUNCTIONALIZED GRAPHENE, AND USE THEREOF AS LI-S BATTERY CATHODE

Abstract

The present invention provides a method for preparation of sulfur functionalized graphene which contains the following steps: a) providing a dispersion of fluorinated graphite; b) subjecting the dispersion of fluorinated graphite to sonication and/or mechanical treatment and/or thermal treatment; c) preparing a metal polysulfide, starting from a metal sulfide and sulfur; d) contacting the product from step b) with the product of step c) at a temperature within the range of 10-110° C.; e) separating the solid product formed in step d) from the solution. Further provided are sulfur functionalized graphene with high sulfur loading obtained by this method, and its use in electrical cells.

Claims

1. A method for preparation of sulfur-functionalized graphene which contains the following steps: a) providing a dispersion of fluorinated graphite; b) subjecting the dispersion of fluorinated graphite to sonication and/or mechanical treatment and/or thermal treatment; c) preparing a metal polysulfide, starting from a metal sulfide and sulfur; d) contacting the product from step b) with the product of step c) at a temperature within the range of 10-110° C.; e) separating the solid product formed in step d) from the solution.

2. The method according to claim 1, wherein the dispersion prepared in step a) is a dispersion of fluorinated graphite in an aprotic polar solvent.

3. The method according to claim 1, wherein in step a), the dispersion is subjected to mechanical treatment followed by sonication, wherein the mechanical treatment is selected from high-shear mixing, stirring, vigorous stirring, stirring with magnetic bar, stirring with a mechanical stirrer.

4. The method according to claim 1, wherein the metal in the polysulfide and sulfide in step c) is an alkali metal or an alkaline earth metal.

5. The method according to claim 1, wherein after contacting the product of step b) with the metal polysulfide reagent, the mixture is subjected to heating to a temperature within the range of 10-110° C. for at least 4 hours.

6. The method according to claim 1, wherein the weight ratio of the starting fluorinated graphite to the metal polysulfide is in the range of 1:2 to 1:20, more preferably 1:2 to 1:10.

7. Sulfur-functionalized graphene, containing graphene with covalently bound sulfur and having a sulfur loading of at least 60 wt. %, wherein the sulfur loading is determined by thermogravimetric analysis by measuring the weight loss in the temperature range of 200-350° C., wherein the covalently bound sulfur is in the form of S atoms and polysulfide chains covalently bonded on the graphene surface.

8. Sulfur-functionalized graphene according to claim 7, having the sulfur loading of at least 70 wt. %.

9. Sulfur-functionalized graphene according to claim 7, containing up to 10 at. % of fluorine, wherein the at. % are determined relative to the total atoms present in the sample and are determined by X-ray photoelectron spectroscopy using an Al—Kα source

10. Sulfur-functionalized graphene according to claim 7, consisting of an electrode having a cathode material in lithium sulfur batteries.

11. An electrical cell comprising at least two electrodes, a separator and an electrolyte, wherein one electrode contains or consists of the sulfur functionalized graphene according to claim 7.

12. The method according to claim 1, wherein the dispersion prepared in step a) is a dispersion of fluorinated graphite in an aprotic polar solvent in combination with a non-polar solvent.

13. The method according to claim 2, wherein the metal is selected from sodium, potassium and magnesium.

14. The method according to claim 1, wherein after contacting the product of step b) with the metal polysulfide reagent, the mixture is subjected to heating to a temperature within the range of 10-110° C. for at least 2 days.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0039] FIG. 1. S2p X-ray photoelectron spectra of the product of Example 1.

[0040] FIG. 2. X-ray photoelectron spectra of the starting material and the product of Example 2.

[0041] FIG. 3. C1s X-ray photoelectron spectra of the product of Example 2.

[0042] FIG. 4. Thermogravimetric analysis of the solid product from the reaction of Example 2. The analysis was performed in N.sub.2 atmosphere, up to 800° C. at 5° C. min.sup.−1.

[0043] FIG. 5. Infra-red spectra of (a) the starting fluorinated graphite and (b) the product from Example 2.

[0044] FIG. 6. Thermogravimetric analysis of the solid product from the reaction of Example 3, The analysis was performed in N.sub.2 atmosphere, up to 800° C. at 5° C. min.sup.−1.

[0045] FIG. 7. Thermogravimetric analysis of the solid product from the reaction of Example 4. The analysis was performed in N.sub.2 atmosphere, up to 800° C. at 5° C. min.sup.−1.

[0046] FIG. 8. Thermogravimetric analysis of the solid product from the reaction of Example 5. The analysis was performed in N.sub.2 atmosphere, up to 800° C. at 5° C. min.sup.−1.

[0047] FIG. 9. Thermogravimetric analysis of the solid product from the reaction of Example 6. The analysis was performed in N.sub.2 atmosphere, up to 800° C. at 5° C. min.sup.−1.

[0048] FIGS. 10A-10B. Scanning electron microscopy (SEM) images of the film of electrode material (prepared as described in Example 7, pasted on the aluminium foil.

[0049] FIGS. 11A-11B. Electrochemical characterization of the product from Example 2. FIG. 11) Cyclic voltammetry curves in 1 M LiTFSI in 1:1 DOL:TTE at different scan rates; FIG. 11B) galvanostatic charge-discharge profiles at 0.1 C (0.17 A g.sup.−1).

[0050] FIG. 12. Rate capability testing of the product from Example 2 in 1 M LiTFSI in 1:1 DOL:TTE at various C-rates (specific currents).

[0051] FIGS. 13A-13C. Specific capacity and coulombic efficiency of the product from Example 2 in 1 M LiTFSI in 1:1 DOL:TTE at FIG. 13A) 0.1 C (0.17 A g.sup.−1), FIG. 13B) 0.2 C (0.33 A g.sup.−1) and FIG. 13C) 1 C (1.7 A g.sup.−1)

[0052] FIGS. 14A-14B. Scanning electron microscopy (SEM) images of the battery tested material film from Example 2 after testing for 250 charge/discharge cycles.

[0053] FIG. 15. Specific capacity and coulombic efficiency of the product from Example 6 in 1 M LiTFSI in 1:1 DOL:TTE.

EXAMPLES OF CARRYING OUT THE INVENTION

Materials and Methods

[0054] Graphite fluoride (>61 wt % F), Na.sub.2S.9H.sub.2O, Sulfur and 1-Methyl-2-pyrrolidinone anhydrous, 99.5% were purchased from Sigma-Aldrich. Acetone (pure) and ethanol (absolute) were purchased from Penta, Czech Republic. All chemicals were used without further purification.

[0055] FT-IR spectra were measured on an iS5 FTIR spectrometer (Thermo Nicolet), using the KBr pellet accessory. Spectra were recorded by summing 50 scans, using pure KBr for the background acquisition. X-ray photoelectron spectroscopy (XPS) was performed on a PHI VersaProbe II (Physical Electronics) spectrometer, using an Al—Kα source (15 kV, 50 W). MultiPak (Ulvac—PHI, Inc.) software package was used for deconvolution of obtained data.

[0056] The samples were also analyzed with scanning electron microscopy using Hitachi SU6600 instrument with accelerating voltage of 5 kV. For these analyses, an electrode or a small droplet of a material dispersion in ethanol (concentration approximately 0.1 mg ml.sup.−1) was placed on a carbon-coated copper grid and left for drying.

[0057] Thermal analysis was performed with an STA449 C Jupiter Netzsch instrument.

[0058] Cyclic voltammetry (CV) and Galvanostatic Charge-Discharge with Potential Limitation (GCPL) were performed on a Bio-Logic battery tester (BCS-810) controlled with the BT-Lab software (version 1.64).

[0059] The following passage defines the battery metrics which are used in the present document, and generally accepted in the field. Specific capacity (C in mAh g.sup.−1) of the electrode material is calculated from galvanostatic charge-discharge curves according to the equations:

[00001]$C=\frac{I.Math.t}{m}\left[\mathrm{mAh}{g}^{-1}\right]$

[0060] The capacity was calculated with respect to the sulfur mass as it was measured through the TGA.

[0061] The coulombic efficiency (CE %) for each cycle is calculated according to the equation:

[00002]$\mathrm{CE}=\frac{\mathrm{Discharge}\mathrm{capacity}}{\mathrm{Charge}\mathrm{capacity}}\times 100\%$

[0062] The C-rate is calculated from the sulfur theoretical capacity (1672 mAh g.sup.−1), meaning that at 1 C (0.17 A g.sup.−1) a fully charged battery rated at 1 Ah should provide 1 A for one hour with respect to the sulfur mass. The current densities are calculated with respect to the sulfur mass and given in A g.sup.−1.

Example 1: Synthesis of Sodium Polysulfide (3 h Reaction)

[0063] In a glass spherical flask. 4.8 g of sodium sulfide nonahydrate (Na.sub.2S.9H.sub.2O) was dissolved in 20 ml water-ethanol mixed solvent (1:1 volume ratio) and 0.64 g of sulfur (S) was added subsequently. The solution was stirred with a teflon coated magnetic bar for 3 hours at 30° C. and turned from yellow to dark orange indicating the gradually increasing chain length of the polysulfides. The solvent was vacuum dried in a rotary evaporator (30° C.) and the residue was milled in a mortar to create a fine powder.

[0064] X-ray photoelectron spectroscopy on the product of Example 1 (FIG. 1) showed that the reaction of Na.sub.2S with S resulted in sodium polysulfide by displaying four doublets each one demonstrating different chemical state of sulfur. The first at 162 eV corresponds to the S2p.sub.3/2 of the terminal S, while the doublet at 1615 eV is ascribed to the central sulfur.

Example 2: Synthesis of Sulfur Functionalized Graphene (at 80° C., 1:8 Mass Ratio)

[0065] To prepare the sulfur functionalized graphene (GPS), firstly 250 mg of graphite fluoride was dispersed in 15 ml of NMP using a 50 ml glass spherical flask. The flask was covered and left stirring for 3 days. Then, it was sonicated for 4 hours to achieve better exfoliation. After the exfoliation, 2 g of the product from Example 1 was added and the mixture reacted at 80° C. in an oil bath under reflux for 48 h, in N.sub.2 atmosphere, using magnetic stirring with teflon coated magnetic bar. After the end of the reaction, the mixture was left to cool down and transferred to 50 ml falcon centrifuge tubes. The solid particles (the product) were separated from the solvent and the by-products by centrifugation at 15000 ref for ca. 10 minutes. The supernatant was discarded, and the tube was refilled with the next washing solvent. The sample was homogenized by shaking and sonication for at least 1 minute, in order to redisperse the precipitate in the new solvent. Washing was performed with different solvents: NMP (3×), acetone (3×), ethanol (3×), and distilled water (3×), then refilled back with distilled water. Finally, the aqueous mixture was frozen at −80° C. and after 3 hours it was inserted in a freeze dryer (−108° C., 0.4 mbar) for 2 days. The final product was a fine powder.

[0066] X-ray photoelectron spectroscopy on the starting Fluorographite (FG) and on the product of Example 2 (FIG. 2) shows the successful defluorination of GPS since the F1s peak is much lower than in FG and the S2p and Na1s peaks were emerged showing the insertion of sulfur and some remaining sodium from the polysulfide, probably at the one polysulfide edge, confirming the reaction between the PS and the FG. Moreover, in the deconvoluted C1s spectra of the GPS material a major peak at 284.8 eV corresponding to graphitic sp.sup.2 bonds is shown, while another peak at 285.6 eV emerged (FIG. 3), corresponding to carbons covalently bonded to S. The atomic ratio of each component is shown at Table 1.

[0067] The actual sulfur content for each example was determined via, thermogravimetric analysis (TGA) where the weight loss in the temperature range of 200-350° C. is due to the release and evaporation of sulfur (FIG. 4). The thermogram from GPS shows one-step loss between 200 to 350° C. and ˜20 wt % residual mass (˜80 wt % sulfur loading).

[0068] FT-IR analysis was performed to understand better the chemical structure of the materials. The defluorination of the starting FG after the reaction and isolation of the product (GPS) and the presence of C—S covalent bonds were verified. The C—F band (˜1200 cm.sup.−1) decreased and a new band at ˜1570 cm.sup.−1 emerged indicating the successful defluorination and the formation of C═C bonds (aromatic ring stretching), respectively (FIG. 5). In addition to this, another peak emerged at ˜1060 cm.sup.−1 attributed to covalent C—S covalent bonds (C—S stretching).

TABLE-US-00001 TABLE 1 Atomic contents as obtained from X-ray photoelectron spectroscopy analysis for the starting graphite fluoride and for the product of Example 2 (GPS 1:8). Atomic contents % C N O F Na S FG 48.4 0 1.1 50.5 0 0 GPS 80° C. 1:8 55.4 1.1 8.9 7.5 8.7 18.4

Example 3: Synthesis of Sulfur Functionalized Graphene (at 50° C.)

[0069] The same procedure as in Example 2 was followed, but instead of heating the mixture at 80° C. for 48 h, it was heated at 50° C. for 48 h.

[0070] Thermogravimetric analysis (TGA) was performed to verify the sulfur loading in this example (FIG. 6). The GPS thermogram shows one-step loss between 200 to 350° C. and ˜25 wt % residual mass (˜75 wt % sulfur loading, which—based on XPS—could be approximately 16 at. %. It should be noted that XPS is a surface analysis technique).

Example 4: Synthesis of Sulfur Functionalized Graphene (Room Temperature)

[0071] The same procedure as in Example 2 was followed, but instead of heating the mixture at 80° C. for 48 h, was left at room temperature for 48 h.

[0072] Thermogravimetric analysis (TGA) was performed to verify the sulfur loading in this example (FIG. 7). The GPS 50° C. thermogram shows one-step loss between 200 to 350° C. and ˜29 wt % is residual mass (˜71 wt % sulfur loading).

Example 5: Synthesis of Sulfur Functionalized Graphene With Pre-Heated Graphite Fluoride (Comparative Example)

[0073] The same procedure as in Example 2 was followed, but FG was heat-treated for 48 h in absence of the product from Example 1 (the polysulfides). Firstly 250 mg of graphite fluoride was dispersed in 15 ml of NMP using a 50 ml glass spherical flask. The flask was covered and left stirring for 3 days. Then, it was sonicated for 4 hours to achieve better exfoliation. Then heating in NMP at 120° C. for 48 h took place without adding sodium polysulfide. The product of this reaction was significantly &fluorinated and contained only 15 at % of fluorine. After these 48 h of heating, the sodium polysulfide was added and the mixture was heated at 80° C. for 48 h. Thermogravimetric analysis (TGA) was performed to verify the sulfur loading in this example (FIG. 8). The GPS with preheated graphite fluoride thermogram shows one-step loss between 200 to 350° C. and ˜67 wt % residual mass (˜33 wt % sulfur loading). The low sulfur content in the case is attributed to the defluorination of graphite fluoride during the first heat-treatment at 120° C. in absence of sodium polysulfide; when i.e. the carbon are not bonded to fluorine atoms they are not electrophilic and cannot react with the polysulfide chains. This example showed clearly the important role of fluorines in fluorinated graphite for the covalent bonding of graphene's carbons with the polysulfides during the reaction (step (d) of the invention).

Example 6: Synthesis of Sulfur Functionalized Graphene (80° C., 1:4 Mass Ratio)

[0074] The same procedure as in Example 2 was followed, but instead of using 2 g of PS, 1 g was used in order to lower the FG/PS mass ratio in the reaction from 1:8 (Example 2) to 1:4.

[0075] Thermogravimetric analysis (TGA) was performed to verify the sulfur loading in this example (FIG. 9). The GPS 80° C. 1:4 mass ratio thermogram shows one-step loss between 200 to 350° C. and ˜26 wt % residual mass (˜74 wt % sulfur loading).

Example 7: Electrochemical Testing in Li—S Full-Cell Using the Product From Example 2 (80° C., 1:8 Mass Ratio)

[0076] The active material (sulfur functionalized graphene, UPS, from Example 2) was homogeneously dispersed in N-methyl-2-pyrrolidone (p.a.≥99%, Sigma-Aldrich) with binder Polyvinylidene fluoride (PVDF, Sigma-Aldrich) and conductive carbon Ketjen black (AkzoNobel) at a ratio of 90:5:5 and sonicated for 6 hours, to form a homogenous paste. Moreover, during sonication every 2 hours the slurry was mixed in a planetary mixer (Thinky ARV-310LED) to better distribute the components. The slurry was pasted on a carbon-coated aluminium foil (Cambridge Energy Solutions, thickness 15 μm) with doctor's blade technique (Erichsen, Quadruple Film Applicator, Model 360). Next, the film was dried at 80° C. in vacuum oven overnight, before electrodes with a diameter of 18 mm were cut. The obtained film was examined with scanning electron microscopy before testing, showing the sulfur particles covering the uniformly distributed graphene sheets while being attached on them (FIGS. 10A,B). For assembly of the battery device, the GPS electrode was placed in a sleeve (El-Cell insulator sleeves equipped with Whatman® glass microfiber paper separator with thickness 0.26 mm). The separator membrane was soaked with ˜100 μl of electrolyte. 1 M of LiTFSI (lithium bis-trifluoromethanesulfonimide, MTI) in a 1:1 mixture of DOL (Dioxolane, Aldrich) and TTE (1,1,2,2-Tetrafluoroethyl 2,2,3,3-Tetrafluoropropyl Ether, TCI) was used as electrolyte. All the solvents were dried with molecular sieves overnight before use. A lithium foil disc with 18 mm diameter was used as anode and both electrodes were enclosed inside the sleeve with stainless steel plungers, while the whole device was tightened and connected to the battery tester for analysis.

[0077] Before testing the device, conditioning of the electrode materials was performed as follows:

[0078] Rest the electrode until it reaches voltage equilibrium (usually ˜6 hours) and hold of potential for 3 hours at 1.5 V.

[0079] The above mentioned cells were tested via cyclic voltammetry (CV), and charge/discharge profiling. Firstly, the CV profiles were recorded in 0.1, 0.2 and 0.5 mV s.sup.−1. In all cases, the anodic peak during charge is ascribed to the oxidation of the cathode which is completed in two stages, with the broad peak at ˜2.45 V representing the conversion of Li.sub.2S.sub.1-2to Li.sub.2S.sub.n (n>2) and the formation of elemental sulfur. The reduction is also completed in two clearly separated steps: The peak at 2.25 V is ascribed at the reduction of elemental sulfur to LiS.sub.n(4≤n≤8) while the peak at 1.8 V corresponds to the subsequent reduction of LiS.sub.n to Li.sub.2S.sub.1-2 (FIG. 11A). FIG. 11B represents the electrochemical profiles of the galvanostatic charge/discharge for the sulfur functionalized grapheme cathode. The shape of the curves is well maintained for more than 75 cycles at specific current of 0.1 C (0.17 A g.sup.−1), while the capacity is practically stable indicating high electrochemical reversibility. The cathode shows good rate capability by obtaining capacities of 496 mAh g.sup.−1 at 1C (1.67 A g.sup.−1) and 298 mAh g.sup.−1 even at 2 C (3.3 A g.sup.−1). Moreover, the cathode regains its initial capacity of ˜800 mAh g.sup.−1 when the rate is hack at 0.2 C (0.33 A g.sup.−1) (FIG. 12). The stability testing of this material shows excellent results in both high and low specific currents. The initial capacity varied from 636 mAh g.sup.−1 at 1 C to 912 mAh g.sup.−1 at 0.1 C (0.17 A g.sup.−1) and the cell maintained very high stability even in low rate showing 720 mAh g .sup.−1 after 200 cycles in 0.2 C (0.33 A g.sup.−1) and 706 mA g.sup.−1 after 100 cycles in 0.1 C (FIGS. 13A,B,C). This value corresponds to 635 mAh g.sup.−1 with respect to the active material (90%), and 508 mAh g.sup.−1 with respect to the whole electrode mass (GPS, 80% sulfur) after 100 cycles at 0.1 C (0.17 A g.sup.−1). The coulombic efficiency is kept at 96.8% for 100 cycles corresponding to very low material loss during cycling. The tested material was scratched from the film after testing and was examined after washing with scanning electron microscopy showing full preservation of the structure, attributed to the strong interaction between the graphene and sulfur even after 250 charge/discharge cycles (FIGS. 14A,B).

Example 8: Electrochemical Testing in Li—S Full-Cell Using the Product From Example 6 (80° C., 1:4 Mass Ratio)

[0080] The same procedure as in Example 6 was followed, but instead of using the product of Example 2, the product of Example 6 was used. Li foil was used as anode and 1 M of LiTFSI (lithium bis-trifluoromethanesulfonimide, MTI) in a 1:1 mixture of DOL (Dioxolane, Aldrich) and TTE (1,1,2,2-Tetrafluoroethyl 2,2,3,3-Tetrafluoropropyl Ether, TCI) was used as electrolyte. Before testing the device, conditioning of the electrode materials was performed as follows: Rest the electrode until it reaches voltage equilibrium (usually ˜6 hours) and hold of potential for 3 hours at 1.5 V.

[0081] The stability testing of this material shows good results in low current density. The initial capacity was 686 mA g.sup.−1 at 0.2 C (0.33 A g.sup.−1) and the cell maintained very high stability even this rate showing 784 mAh g.sup.−1 after 100 cycles (FIG. 15). This value corresponds to 705 mAh g.sup.−1 with respect to the active material (90%), and 522 mAh g.sup.−1 with respect to the whole electrode mass (GPS, 74% sulfur) after 100 cycles at 0.2 C (0.33 A g.sup.−1). The coulombic efficiency is kept at 96% for 100 cycles corresponding to very low material loss during the cycling. These values are very close with the value of Example 7, which delivers 761 mAh g.sup.−1 after 100 cycles (FIG. 13B) corresponding to 685 mAh g.sup.−1 with respect to the active material (90% active), and 547 mAh g.sup.−1 with respect to the whole electrode mass (GPS 1:8, 80% sulfur) after 100 cycles at 0.2 C (0.33 A g.sup.−1). The similarity of the results was expected because all the parameters except the FG:PS mass ratio were kept the same. Overall, the GPS 1:8 material performs slightly better because of its higher sulfur mass loading.