CATHODES FOR LI-S BATTERIES

Abstract

The present invention concerns a process for the preparation of a porous carbon structure comprising the steps: a) providing a template comprising voids, b) filling of at least part of the voids with a precursor for the formation of the porous carbon structure, c) carbonizing the precursor for the formation of the porous carbon structure and d) removing at least part of the template. In preferred embodiments the precursor for the formation of the porous carbon structure is a formaldehyde-phenol resin, especially a cross-linked resorcinol-formaldehyde resin. The template further preferably comprises a block copolymer and an amphiphilic molecule, wherein the block copolymer comprises polymeric units of at least one lipophilic monomer and polymeric units of at least one hydrophilic monomer. Further preferred is a process wherein the template comprises a bimodal mixture of particles of silicon dioxide.

Claims

1. Process for the preparation of a porous carbon structure comprising the following steps: a) providing a template comprising voids, b) filling at least part of the voids with a precursor for the formation of the porous carbon structure, c) carbonizing the precursor for the formation of the porous carbon structure, and d) removing at least part of the template.

2. Process according to claim 1 further comprising the following steps: providing a solution comprising a solvent, a block copolymer and an amphiphilic molecule, producing a body by evaporation of the solvent, removing the amphiphilic molecule to produce the template comprising voids, and after carbonizing the precursor for the formation of the porous carbon structure, removing the block copolymer by heating, wherein the block copolymer comprises polymeric units of at least one lipophilic monomer and polymeric units of at least one hydrophilic monomer.

3. Process according to claim 2 wherein the amphiphilic molecule is removed by soaking the body in a solvent, optionally an ethanol/water or methanol/water mixture.

4. Process according to claim 2 wherein the hydrophilic monomer comprises at least one functional group selected from the group consisting of a nitrogen atom with one lone electron pair, an oxygen atom with two lone electron pairs and an fluorine atom and the amphiphilic molecule comprises at least one functional group selected from the group consisting of N—H and —O—H or the amphiphilic molecule comprises at least one functional group selected from the group consisting of a nitrogen atom with one lone electron pair, an oxygen atom with two lone electron pairs and a fluorine atom and the hydrophilic monomer comprises at least one functional group selected from the group consisting of N—H and —O—H.

5. Process according to claim 2 wherein the block copolymer is a polystyrene—poly(4-vinylpyridine) block copolymer.

6. Process according to claim 2 wherein the amphiphilic molecule is selected from the group consisting of 3-pentadecylphenol and 2-(4′-hydroxyphenylazo)benzoic acid.

7. Process according claim 1 wherein the template comprising the precursor for the formation of the porous carbon structure is heated to more than 600° C. to carbonize the precursor for the formation of the porous carbon structure and to remove the template.

8. Process according to claim 1 wherein the template comprises a mixture of inorganic oxide particles with a bimodal particle size distribution, which are at least partially removed in step d) by etching with a suitable acid or base and wherein the template comprises voids between the particles.

9. Process according to claim 8 wherein the inorganic oxide particles with a bimodal particle size distribution are silicon dioxide, titanium dioxide, aluminium oxide, vanadium(V) oxide or zinc oxide.

10. Process according to claim 8 wherein the template comprises particles with a size of 100 nm or less and particles with a size of 500 nm or more.

11. Process according to claim 1 wherein the precursor for the formation of the porous carbon structure is a cross-linked formaldehyde-phenol resin, which is formed by filling at least part of the voids of the template with a starting material for the preparation of the cross-linked formaldehyde-phenol resin and crosslinking the starting material for the preparation of the cross-linked formaldehyde-phenol resin.

12. Process according to claim 11 wherein the template is heated to 60 to 110° C. to form the cross-linked formaldehyde-phenol resin from the starting material for the preparation of the cross-linked formaldehyde-phenol resin.

13. A porous carbon structure prepared according to the process of claim 1.

14. A method for preparing a porous carbon structure, the method comprising providing a template comprising voids, wherein the template comprises a block copolymer and an amphiphilic molecule and wherein the block copolymer comprises polymeric units of at least one lipophilic monomer and polymeric units of at least one hydrophilic monomer.

15. A method for preparing a battery, the method comprising providing inorganic oxide particles within a porous carbon structure for the preparation of the battery.

16. Process according to claim 7 wherein the template comprising the precursor for the formation of the porous carbon structure is heated to about 900° C. to carbonize the precursor for the formation of the porous carbon structure and to remove the template.

17. Process according to claim 8 wherein the suitable acid or base comprises hydrofluoric acid (HF).

18. Process according to claim 9 wherein the inorganic oxide particles with a bimodal particle distribution are silicon dioxide particles.

19. Process according to claim 10 wherein the template comprises inorganic oxide particles with a size of 60 nm or less and particles with a size of 1 μm or more.

20. The method of claim 15, wherein the inorganic oxide particles are silica particles.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0062] FIG. 1 describes the process of Example 1.

[0063] FIG. 2a shows an SEM image of the product of the porous carbon obtained in Example 1, FIG. 2b shows the cyclovoltametric measurement of the cathode produced in Example 1 and FIG. 2c shows its charge/discharge measurements.

[0064] FIG. 3 shows the scheme of fabrication of porous carbon using block copolymer via phase inversion route as described in Example 2.

[0065] FIG. 4 shows the SEM image of (a) block copolymer template, (b) after filling the template with resorcinol-formaldehyde resin and (c) porous carbon after pyrolsis of the template obtained in Example 2. FIG. 4d shows the cyclovoltametric measurement of the cathode produced in Example 2 and FIG. 4e shows its charge/discharge measurements.

[0066] FIG. 5 depicts the process of the present invention according to Example 3, wherein silicon dioxide particles are comprised in the template.

[0067] FIG. 6 depicts (a) the cyclovoltametric measurements and (b) charge/discharge measurements.

[0068] FIG. 7 shows SEM images of the final carbon structure according to Example 3 at different magnifications at the stage of (a, c) resin-silica after carbonization and (b, d) after etching of silica with HF.

EXAMPLES

Measurement Methods:

[0069] Cyclovoltametric measurements: The cyclic voltammetry measurements of the lithium-sulfur batteries with different cathodes have been performed in between 1 V-3 V at a scanning rate of 0.05 mV/s. The details of the method is described in Agrawal, M.; Choudhury, S.; Gruber, K.; Simon, F.; Fischer, D.; Albrecht, V.; Gabel, M.; Koller, S.; Stamm, M.; Ionov, L, Porous carbon materials for Li—S batteries based on resorcinol-formaldehyde resin with inverse opal structure, Journal of Power Sources 2014; 261, 363-370.

[0070] Charge/discharge measurements: The galvanostatic charge/discharge measurement was done following the method described in Agrawal, M.; Choudhury, S.; Gruber, K.; Simon, F.; Fischer, D.; Albrecht, V.; Göbel, M.; Koller, S.; Stamm, M.; Ionov, L, Porous carbon materials for Li—S batteries based on resorcinol-formaldehyde resin with inverse opal structure, Journal of Power Sources 2014; 261, 363-370.

[0071] XPS: X-ray photoelectron spectroscopy of all carbon samples were done using the protocol described in Agrawal, M.; Choudhury, S.; Gruber, K.; Simon, F.; Fischer, D.; Albrecht, V.; Göbel, M.; Koller, S.; Stamm, M.; Ionov, L, Porous carbon materials for Li—S batteries based on resorcinol-formaldehyde resin with inverse opal structure, Journal of Power Sources 2014; 261, 363-370. Raman spectroscopy: Raman spectroscopy of the carbon samples were done in the way described in Agrawal, M.; Choudhury, S.; Gruber, K.; Simon, F.; Fischer, D.; Albrecht, V.; Göbel, M.; Koller, S.; Stamm, M.; Ionov, L, Porous carbon materials for Li—S batteries based on resorcinol-formaldehyde resin with inverse opal structure, Journal of Power Sources 2014; 261, 363-370.

[0072] The XPS and Raman spectra of the carbon material are indicative of the conducting nature (extent of graphitic nature). In all three of the examples the carbon precursor were kept the same to keep the nature of the carbon, so the XPS and Raman spectra are very similar in all cases.

Example 1

Polystyrene-poly(4-vinylpyridine) block copolymer and of 3-pentadecylphenol

[0073] 44.3 mg of polystyrene—poly(4-vinylpyridine) block copolymer (obtained from Polymer Source Inc.) and 30,7 mg of 3-pentadecylphenol where dissolved in 5 ml chloroform . The solution was placed in a small bottle in a closed chamber together with six small bottles filled with chloroform for 11 days. During this period the block copolymer attains the stable gyroid morphology. Afterwards, the solvent was slowly evaporated for 5 more days and a 100 μm thick film resulted. The obtained block copolymer 3-pentadecylphenol complex film was inserted into 10 ml of ethanol for a time of 24 hours to obtain the template. FTIR spectroscopy was used to confirm the supramolecular association of the nitrogen atom of the pyridine group with the hydroxyl group of 3-pentadecylphenol. Subsequently, the template was immersed in a solution of 5 ml resorcinol-formaldehyde resin (prepared by mixing resorcinol and formalin solution in 1:1.8 molar ratio catalyzed by 0.1 ml of 1% Na.sub.2CO.sub.3 solution, all chemicals obtained from Sigma Aldrich, Germany), which was used as starting material for impregnation. Then the sample is heated, resulting in the formation of cross-linked resorcinol-formaldehyde resin inside the template. Finally, the resin-filled template is pyrolyzed at 800° C. for a time of 2 hours in the flow of argon to obtain a highly porous gyroid carbon replica. Pyrolysis of the samples not only converts the cross-linked resin into carbon but also removes the block copolymer phase leaving behind the interconnected porous carbon network with high surface area, which is 885 m.sup.2/g. Porous carbon material was pre-mixed with sulfur powder in a mortar in 1:2 w/w ratio in a mortar followed by mixing in a ball mill for even intensive mixing and subsequently heat treated at 155° C. for 5 h in an oven operated under argon. Heat treatment at 155° C. was done to facilitate the pore coverage by sulfur in to the entire available surface area of carbon.

[0074] A cathode was produced via the following route. Cathode slurry of desired viscosity was prepared by adding carbon-sulfur composite material (82 wt %), blended with, Super P® Li (10 wt %) conducting additive (carbon black) in a solution of PVdF in N-Methyl-2-pyrrolidone (8 wt % with respect to 82 wt % of carbon-sulfur composite). A thin layer of the as prepared slurry was coated on nickel foil (60-80 μm wet thickness) and disc shaped cathodes were cut out from the whole piece after proper drying in a vacuum oven for 2 days. Cyclovoltametric measurements of this cathode performed as described above show two reduction peaks at 2.4 V and 2 V. The result of the measurements is shown in FIG. 2b. A lithium sulfur battery was assembled in the following way. Resulting electrodes had a mass load of 0.40 mg cm.sup.−2 in dry state and a sulfur content of 55 wt %. A Swagelok® T-cell was used as testing device. Carbon-sulfur composite electrode served as working electrode, lithium metal served as counter and reference electrode respectively. 1 M solution of lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) in a mixture of 1,2-Dimethoxyethane/1,3-Dioxolane (1/1, v/v)) was used as electrolyte, which was used to soak the nonwoven polypropylene separator during cell assembly. The charging curve of the battery is represented by a typical single peak at 2.4V. It was found that cathode demonstrate moderate capacity in the first charge/discharge cycle, which is about 1600 mAh/g.sub.s. The charge/discharge curve for the first 50 cycles is depicted in FIG. 2c. The capacity rapidly decreases during cycling and reaches the value of around 300 mAh/g.sub.s after 50 cycles. The rapid capacity degradation may be due to the well-known problems occurring in lithium sulfur batteries, like the dissolution of long chain polysulfides in the polar electrolyte solvent and the subsequent migration to and direct reduction at the lithium anode during cycling (polysulfide shuttle).

Example 2

Polystyrene-poly(4-vinylpyridine) block copolymer and of 2-(4′-hydroxyphenylazo) benzoic acid

[0075] Example 2 is performed as Example 1, however instead of 30.7 mg of 3-pentadecylphenol, 202.8 mg of 2-(4′-hydroxyphenylazo)benzoic acid is used. A 5 ml 20 wt. percent solution of the block copolymer and 2-(4′-hydroxyphenylazo)benzoic acid in 1:1 molar ratio of 2-(4′-hydroxyphenylazo)benzoic acid to 4-vinylpyridine unit is prepared in 56:24 w/w ratio of N,N-dimethylformamide/tetrahydrofuran solvent mixture and casted on a substrate to form 100 μm thick films. After 30 seconds the film formed is immersed in a water/methanol mixture, resulting in the phase inversion. The phase inverted membrane was washed to get rid of 2-(4′-hydroxyphenylazo)benzoic acid and to get template comprising void spaces. This template is a block copolymer mat consisting of cylindrical micelles where polystyrene forms the core and poly(4-vinylpyridine) part pointing outward (FIG. 3). Subsequently, the nanoporous carbon was prepared with the help of the template by following the same procedure described in claim 1.

[0076] The obtained carbon has very high conductivity 222.4 mS/cm. The carbon was mixed with elemental sulfur in molten state to get carbon-sulfur composite as cathode powder in the same way as described in claim 1. The cyclic voltammetry curves look like typical ones characteristic to Li—S batteries (FIG. 4d). It was found that cathode demonstrate moderate capacity in first charge-discharge cycle, which is in the range 600-800 mAh/g.sub.s. The capacity however rapidly decreases during cycling and reaches the value of around 100 mAh/g.sub.s after 50 cycles that is due to polysulfide shuttle (FIG. 4e).

Example 3

Bimodal Silicon Dioxide Particles

[0077] Example 3 is also performed as Example 1 and 2 but differs only in the template. In Example 3, inorganic template was used instead of using polymeric template but the carbon precursor is the same in all three claims which is resorcinol-formaldehyde resin. 40 nm SiO.sub.2 particles were prepared in the following way. 455 mg of L-Arginin dissolved in 345 ml of water then 25 ml of cyclohexane was added. Then it was heated to 60° C. After this 27.5 ml of Tetraethyl orthosilicate (TEOS) was added and stirred for 48 hours yielding 70% of SiO.sub.2. Then cyclohexane was separated by separating funnel and solution was concentrated up to ˜conc. 450 mg/ml in a rotary evaporator.

[0078] A mixture of 4 ml of dispersion of as synthesized 40 nm of silica particles that corresponds to 1.8 g and 200 mg of 1 μm amine functionalized silica microparticles (obtained from Kisker Biotech GmbH & Co.) were ultrasonicated for 1 hour until a nice dispersion of particles were achieved. 0.2 g of resorcinol was added to the dispersion and stirred for complete dissolution followed by addition of sodium carbonate solution (20 mg in 0.5 ml of water) which acts as catalyst. Afterwards, 0.25 ml of formaldehyde was added. The total mixture formed a gel immediately upon addition of formaldehyde. In order to obtain dispersion again ˜1 ml of water was added and kept stirring over the weekend, drying the residual water in a water bath. Then the gelled material was kept in a close vessel and crosslinked at 80° C. for 4 days to avoid any changes in the nanostructure arising out of the flow of polymer. Finally, resulting dark brown colored material were carbonized at 800° C. at a heating rate of 4° C./min for 2 h in an argon atmosphere. Thereafter, a nanoporous carbon was prepared by etching silica particles by dilute hydrofluoric acid. The removal of silica particles was done by dipping the pyrolyzed carbon-silica mixture in 400 ml of 1 wt % dilute hydrofluoric acid. It was followed by washing with water several times and drying in a vacuum oven.

[0079] The obtained carbon has poor conductivity relative to two other systems studied so far of 13 mS/cm. The poor conductivity was due to leftover 20 wt % of silica particles which was washed only from the periphery and stayed inside the system like loose spheres. During etching out of silica particles it was controlled considering the reaction of silica with hydrofluoric acid (SiO.sub.2+6HF.fwdarw.H.sub.2SiF.sub.6+2H.sub.2O) so that all the 40 nm sized silica particles could be washed away and the 1 pm silica particles could be washed partially from the periphery and residing in the carbon matrix creating adsorption sites for polysulfides. The washing with hydrofluoric acid leads to the formation of bimodal porous carbon with polysulfide reservoir inside. These silica particles have advantages to retain the cycle stability by creating adsorption sites on the surface of such inorganic particle surfaces. The carbon was mixed with elemental sulfur in molten state to get carbon-sulfur composite as cathode powder in the same way as described in claim 1 and electrochemistry was also performed keeping the same parameter.

[0080] The cyclic voltammetry curves look like typical ones intrinsic to Li—S batteries (FIG. 6a). Interestingly, from the charge-discharge experiment (FIG. 6b) it was found that the initial specific capacity started a bit low (800 mAh/g.sub.s in the second discharge cycle) which is nearly 50% of the theoretical specific capacity of sulfur electrode. Very interesting that contrary to other two cathode systems this one demonstrate quite good cycle stability reached to 600 mAh/g.sub.s up to 50 charging/discharging cycles. The stable cycle performance was due to the polysulfide adsorption on the surfaces of silica particles reside as loose spheres and polysulfides remain entrapped within cathode and do not diffuse to anode and avoid passivation of lithium anode by formation of Li.sub.2S and Li.sub.2S.sub.2 as well. This effect altogether helps in minimizing the active material loss per charging/discharging cycles.