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• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
  • H01M4/621—Binders
  • H01M4/622—Binders being polymers
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M10/00—Secondary cells; Manufacture thereof
  • H01M10/05—Accumulators with non-aqueous electrolyte
  • H01M10/052—Li-accumulators
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/04—Processes of manufacture in general
  • H01M4/0402—Methods of deposition of the material
  • H01M4/0404—Methods of deposition of the material by coating on electrode collectors
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
  • H01M4/139—Processes of manufacture
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
  • H01M4/139—Processes of manufacture
  • H01M4/1397—Processes of manufacture of electrodes based on inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/36—Selection of substances as active materials, active masses, active liquids
  • H01M4/362—Composites
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/36—Selection of substances as active materials, active masses, active liquids
  • H01M4/362—Composites
  • H01M4/364—Composites as mixtures
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/36—Selection of substances as active materials, active masses, active liquids
  • H01M4/38—Selection of substances as active materials, active masses, active liquids of elements or alloys
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/36—Selection of substances as active materials, active masses, active liquids
  • H01M4/38—Selection of substances as active materials, active masses, active liquids of elements or alloys
  • H01M4/381—Alkaline or alkaline earth metals elements
  • H01M4/382—Lithium
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/36—Selection of substances as active materials, active masses, active liquids
  • H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
  • H01M4/581—Chalcogenides or intercalation compounds thereof
  • H01M4/5815—Sulfides
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
  • H01M4/624—Electric conductive fillers
  • H01M4/625—Carbon or graphite
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/64—Carriers or collectors
  • H01M4/66—Selection of materials
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/36—Selection of substances as active materials, active masses, active liquids
  • H01M4/38—Selection of substances as active materials, active masses, active liquids of elements or alloys
  • H01M4/40—Alloys based on alkali metals
  • H01M4/405—Alloys based on lithium
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/10—Energy storage using batteries

Definitions

• This invention relates to an electrode for a lithium sulphur cell. This invention also relates to a method of producing such an electrode, and a lithium sulphur cell comprising such an electrode.
• a typical lithium-sulphur cell comprises an anode (negative electrode) formed from lithium metal or a lithium metal alloy, and a cathode (positive electrode) formed from elemental sulphur or other electroactive sulphur material.
• the sulphur or other electroactive sulphur-containing material may be mixed with an electrically conductive material, such as carbon, to improve its electrical conductivity.
• the carbon and sulphur may be ground, and then mixed with a solvent and binder to form a slurry.
• the slurry may be applied to a current collector, and then dried to remove the solvent.
• the resulting structure is calendared to form a composite structure, which is cut into the desired shape to form a cathode.
• a separator may be placed on the cathode, and electrolyte may be applied to the separator to wet the cathode and separator.
• a lithium anode may be placed on the separator to form a cell stack.
• Figure 1 shows how the discharge capacity of the cells of Example 1 vary with cycle number.
• gelatin and “gelatin-like protein” as used herein refers to any gelatin, whether extracted by traditional methods or recombinant or biosynthetic in origin, or to any molecule having at least one structural and/or functional characteristic of gelatin.
• the term encompasses both the composition of more than one polypeptide included in a gelatin product, as well as an individual polypeptide contributing to the gelatin material.
• gelatin as used in reference to the present invention encompasses both a gelatin material comprising gelatin polypeptides, as well as an individual gelatin polypeptide.
• crosslinking agent refers to a composition comprising a crosslinker.
• Crosslinker refers to a reactive chemical compound that is able to introduce covalent intra- and inter- molecular bridges in organic molecules.
• an electrode for a lithium sulphur cell comprises a matrix deposited on a current collector, wherein the matrix comprises an electrically conductive material, an electroactive sulphur material and a binder comprising a polymer that is crosslinked to form a crosslinked polymer network.
• the present invention also provides a lithium sulphur cell comprising an electrode as described above.
• the electrode is preferably the cathode of the cell.
• the anode may be an anode comprising lithium metal, for example, pure lithium metal or a lithium metal alloy.
• Other suitable anode materials include anodes suitable for use in a lithium-ion battery, for example, silicon or graphite anodes.
• the present invention provides a method of producing an electrode for a lithium sulphur cell.
• the method comprises depositing an electrode mixture comprising an electrically conductive material, an electroactive sulphur material, and a binder comprising a polymer onto a current collector, whereby the polymer is crosslinked to form a crosslinked polymer network on the current collector.
• the polymer may be crosslinked prior to being deposited on the current collector, or may be polymerised after being deposited on the current collector, for example, by exposure to a crosslinking agent.
• the electrically conductive material and electroactive sulphur material may be held in place by a crosslinked polymer network.
• This crosslinked polymer network can improve the structural integrity of the electrode, helping the charged species formed during the charge and discharge cycles of the lithium sulphur cell to remain in electrical contact with the current collector as the cell is charged and discharged.
• this can result in an improvement in the cycle life of the lithium sulphur cell.
• the binder comprises a polymer that is crosslinked to form a polymer network.
• the polymer may be any crosslinkable polymer.
• the polymer may comprise a functional group that may be crosslinked. Examples of suitable functional groups include amine, amide, carbonyl, carboxyl, ether, thioether and hydroxyl groups.
• the polymer is crosslinked via a functional group selected from at least one of amine, amide, carbonyl, carboxyl, ether, thioether, imine and hydroxyl.
• the polymer comprises amino acids residues. The crosslinking may occur via one or more of these residues.
• the polymer is a biopolymer.
• the biopolymer is selected from polypeptides and polysaccharides. Suitable polypeptides include gelatin, collagen and collagen mimetic peptides. Suitable polysaccharides include chitosan, hyaluronate and hyaluronic acid. In a preferred embodiment, the polymer is gelatin.
• the polymer is a polyacrylamide.
• the polymer may form 0.01 to 50 weight % of the matrix deposited on the current collector. Preferably, the polymer forms 0.01 to 20 weight % of the matrix deposited on the current collector. More preferably, the polymer may form 0.1 to 15 weight %, for example, 1 to 10 weight % of the matrix deposited on the current collector.
• the ratio of the weight of polymer to the weight of electrically conductive material (e.g. carbon) and electroactive sulphur material (e.g. sulphur) may be 0.01 - 10 : 30 - 60, preferably 0.1 - 5 : 35 - 55, more preferably 1 - 3 : 40 - 50.
• the weight ratio of polymer to the electrically conductive material (e.g. carbon) may be 0.01 - 15 : 5 - 35, preferably 0.1 - 10 : 8 - 25, more preferably 1 - 5 : 10 - 15.
• the weight ratio of polymer to the electroactive material (e.g. sulphur) may be 0.01 - 20 : 20 - 50, preferably 0.1 - 15 : 25 - 45, more preferably 1 - 10 : 30 - 40.
• the polymer may be crosslinked using any suitable method, for example, by heating or by exposure to irradiation, for instance, UV radiation.
• irradiation for instance, UV radiation.
• the polymer may be crosslinked by heating.
• the polymer is crosslinked by reacting a crosslinking agent with the polymer.
• a crosslinking agent may be employed.
• Suitable crosslinking agents include aldehydes. Examples of suitable aldehydes include formaldehyde, di-aldehydes, glutaraldehyde, glyceraldehyde and furfural.
• ketones are used as crosslinking agents. Suitable ketones include acetone, diacetal and other diones for example, pentanedione, for instance, a chloropentane dione.
• crosslinking agents include carbodiimide, urea, glyoxal, polyformals, imines, di-epoxy compounds and di-isocyanates. Enzymes may also be used, for example, trans-glutaminase.
• the crosslinking agent is selected from bis (2-chloroethylurea); 2- hydroxy-4,6-dichloro-1 ,3,5-triazine; reactive halogen-containing compounds disclosed in US 3,288,775; carbamoyl pyridinium compounds in which the pyridine ring carries a sulphate or an alkyl sulphate group disclosed in US 4,063,952 and US 5,529,892; divinylsulfones, and S-triazine derivatives such as 2- hydroxy-4,6-dichloro-s-triazine.
• suitable crosslinking agents include 1 -ethyl-3-[3-dimethylaminopropyl]carbodiimide hydrochloride (EDC or EDAC).
• the crosslinking agent may be added in any suitable amount.
• the weight ratio of crosslinking agent to polymer in the binder may be 0.01 - 20 : 10 - 200, preferably 0.1 - 10 : 20 - 100, more preferably 1 - 5 : 10 - 50.
• the polymer may be crosslinked by any suitable method.
• an electrode mixture comprising the electrically conductive material, electroactive sulphur material and binder may be dispersed in a solvent (e.g. water or an organic solvent) to form a slurry that is applied to a current collector.
• the slurry may be dried and the resulting structure compressed to form a matrix deposited on the current collector.
• a crosslinking agent may be applied to the matrix to crosslink the polymer in the matrix.
• the crosslinking agent may be applied as a separate solution or mixed with the electrolyte solution that is subsequently contacted with the electrode during assembly of the cell.
• the crosslinking agent may be added to the slurry before or after it is deposited on the current collector.
• the polymer may be crosslinked prior to being incorporated into the electrode mixture.
• the resulting mixture comprises the electrically conductive, material electroactive sulphur material and crosslinked polymer binder may be dispersed in a solvent (e.g. water or an organic solvent) to form a slurry that is applied to a current collector.
• the slurry may be dried and the resulting structure compressed to form a matrix deposited on the current collector.
• the crosslinking reaction may be enhanced by the addition of moisture, irradiation with, for example, UV or by increased temperature.
• the matrix includes a mixture of electroactive sulphur material and electrically conductive material. This mixture forms an electroactive layer, which is placed in contact with a current collector.
• the electroactive sulphur material may comprise elemental sulphur, sulphur- based organic compounds, sulphur-based inorganic compounds and sulphur-containing polymers.
• elemental sulphur is used.
• the electrically conductive material may be any suitable solid electrically conductive material.
• this solid electroconductive material may be formed of carbon. Examples include carbon black, carbon fibre, graphene and carbon nanotubes. Other suitable materials include metal (e.g. flakes, filings and powders) and conductive polymers. Preferably, carbon black is employed.
• the electroactive sulphur material may be present in the matrix deposited on the current collector in an amount of 60 to 90 weight %, preferably 65 to 85 weight %, more preferably 70 to 80 weight %.
• the electrically conductive material may be present in the matrix deposited on the current collector in an amount of 10 to 45 weight %, preferably 15 to 35 weight %, more preferably 20 to 25 weight %.
• the weight ratio of electroactive sulphur material to electrically conductive material may be 0.01 - 10 : 10 - 50, preferably 0.1 - 5 : 15 - 45, more preferably 1 - 5 : 20 - 35.
• the electrochemical cell of the present invention may be any suitable lithium- sulphur cell.
• the cell typically includes an anode, a cathode formed from an electrode described herein, and an electrolyte.
• the anode may be formed of lithium metal or a lithium metal alloy.
• the anode is a metal foil electrode, such as a lithium foil electrode.
• the lithium foil may be formed of lithium metal or lithium metal alloy.
• a separator may be placed between the anode and cathode. Electrolyte may be introduced into the cell to wet the cathode and separator. In one embodiment, the separator is placed on the cathode and wet with electrolyte before anode is placed over the separator. The electrolyte may be applied to the separator, for example, by coating or spraying. [0037] The electrolyte allows charge to be transferred between the anode and cathode. Preferably, the electrolyte wets the pores of the cathode as well as the pores of the separator. In one embodiment, the electrolyte used to assemble the cell comprises a crosslinking agent that can react and crosslink the polymer present in the binder. Suitable crosslinking agents are mentioned above. Where present, the crosslinking agent may be present in the electrolyte in a concentration of 0.01 - 10 weight %, preferably 0.1 - 1 weight %.
• the electrolyte may comprise an organic solvent and a salt, for example, a lithium salt.
• Suitable organic solvents include ethers, esters, amide, amine, sulfoxides, sulfamides, organophosphates and sulfones. Examples include tetrahydrofuran, 2-methyltetrahydrofuran, methylpropylpropionate,
• ethylpropylpropionate methyl acetate, 1 ,2-dimethoxyethane, 1 ,3-dioxolane, diglyme (2- methoxyethyl ether), triglyme, tetraglyme, butyrolactone, 1 ,4-dioxane, 1 ,3-dioxane, hexamethyl phosphoamide, pyridine, dimethyl sulfoxide, tributyl phosphate, trimethyl phosphate, N, N, N, N-tetraethyl sulfamide, and sulfones and their mixtures.
• Suitable electrolyte salts include lithium salts.
• Suitable lithium salts include lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium nitrate, lithium perchlorate, lithium trifluoromethanesulfonimide, lithium bis(oxalate) borate and lithium
• lithium salt is lithium
• lithium triflate trifluoromethanesulphonate
• Combinations of salts may be employed.
• lithium triflate may be used in combination with lithium nitrate.
• the lithium salt may be present in the electrolyte at a concentration of 0.1 to 5M, preferably, 0.5 to 3M.
• a separator may be placed between the anode and cathode.
• the separator may comprise any suitable porous substrate that allows ions to move between the electrodes of the cell.
• the separator should be positioned between the electrodes to prevent direct contact between the electrodes.
• the porosity of the substrate should be at least 30%, preferably at least 50%, for example, above 60%.
• Suitable separators include a mesh formed of a polymeric material. Suitable polymers include polypropylene, nylon and polyethylene. Non-woven polypropylene is particularly preferred. It is possible for a multi-layered separator to be employed.
• Figure 1 shows the discharge performance of the cell that was cycled at 0.2 C and 0.1 C rates (discharge - charge respectively) and compared to a cell where no crosslinking agent was added. It can be observed that, even though both cells showed similar initial discharge capacity values, the cell produced using the crosslinking agent remained stable over time at a sulfur utilisation of ca. 1 100 mAh g(s) "1 . On the other hand, the cell assembled using an un-crosslinked binder started fading after the third cycle.
• crosslinked binder may be attributed to the enhanced structural integrity of the cathode caused by the reaction of the polymer binder with the crosslinking agent. This crosslinking reaction is also believed to improve the coating adhesion to the current collector, reducing the risk of detachment that may cause premature fading of the cell.
• Cathodes were formed by depositing a mixture of sulphur, carbon and gelatin on a current collector. Two squares (2x2 cm) were cut from the cathodes. [0047] In a glovebox, 5 drops of formalin (37% formaldehyde in H2O with 10% methanol as stabiliser) were applied to one of the cathode samples and left overnight. A scrape test was then conducted with cotton buds and showed the formalin-treated sample to have far superior cathode integrity and adhesion to the current collector than the untreated sample.
• a TA Instruments DHR2 rheometer was used in oscillation time sweep mode to measure Storage/Loss Modulus and Phase Angle.
• a TA Instruments DHR 2 rheometer was used to measure the viscoelastic nature of the gelatin/formaldehyde solution, together with the gelatin stock solution as a baseline. Below are listed the rheometer parameters and test conditions:

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