PEO-PVA based binder for lithium-sulfur batteries
In the formation of cathodes for lithium-sulfur battery cells, it is often desired to form the cathodes by resin-bonding particles of a sulfur-based cathode material as a porous cathode material layer on the surface(s) of a suitable metal current collector. It is found that the use of a copolymer of polyethylene oxide and polyvinyl alcohol, dissolved in water, provides a resin-particle slurry that is readily spreadable onto the current collector surface to form a uniform layer of porous cathode material particles. And upon evaporation of the water, the copolymer-bonded, sulfur-based particle coated cathodes function very well in assembled lithium-sulfur cells.
1. A method for forming a cathode for a lithium-sulfur battery cell, the method comprising:
preparing a mixture of cathode material particles comprising sulfur particles embedded in the pores of carbon particles as composite carbon/sulfur particles;
forming a slurry of the composite carbon/sulfur particles in a solution consisting of a copolymer of polyvinyl alcohol and polyethylene oxide in water;
applying the slurry to a surface of a metal current collector to form a porous wet coating layer of resin-wetted composite carbon/sulfur particles on the surface of the metal current collector; and
evaporating the water to form a porous layer of carbon/sulfur particles bonded by the copolymer to each other and to the surface of the metal current collector.
2. A method as stated in claim 1 in which graphite particles are mixed with the carbon/sulfur particles.
3. A method as stated in claim 1 in which the solution consisting of a copolymer of polyvinyl alcohol and polyethylene oxide in water consists of five percent by weight of the copolymer.
4. A method as stated in claim 1 in which the copolymer comprises two percent by weight of the carbon/sulfur particles in the dried porous layer bonded to the surface of the metal current collector.
5. A method as stated in claim 1 in which the metal current collector is an aluminum foil.
6. A method as stated in claim 1 in which the copolymer of polyvinyl alcohol and polyethylene oxide has a molecular weight of about 45,000 Daltons and consists of about 75% polyvinyl alcohol units and 25% of polyethylene oxide units.
7. A lithium-sulfur electrochemical battery cell comprising a lithium metal anode and a sulfur cathode, the sulfur cathode comprising:
sulfur particles embedded in pores of carbon particles as composite carbon/sulfur particles, the composite carbon/sulfur particles being bonded as a porous layer to a surface of a metallic current collector, the composite carbon/sulfur particles being bonded to each other and to the surface of the current collector solely by a copolymer of polyvinyl alcohol and polyethylene oxide.
8. A lithium-sulfur electrochemical battery cell as stated in claim 7 in which graphite particles are mixed with the composite carbon/sulfur particles which are bonded in the porous layer to the surface of the metal current collector.
9. A lithium-sulfur electrochemical battery cell as stated in claim 7 in which the copolymer comprises two percent by weight of the carbon/sulfur composite particles which are bonded in the porous layer to the surface of the metal current collector.
10. A lithium-sulfur electrochemical battery cell as stated in claim 7 in which the lithium metal anode and sulfur cathode are wetted with a solution of a lithium electrolyte salt dissolved in equal volume portions of 1,2-dimethyloxyethane and dioxolane.
11. A lithium-sulfur electrochemical battery cell as stated in claim 7 in which the metal current collector is an aluminum foil.
12. A lithium-sulfur electrochemical battery cell as stated in claim 7 in which the lithium electrolyte compound is lithium bis(trifluoromethane sulfonyl) imide.
13. A lithium-sulfur electrochemical battery cell as stated in claim 7 in which the copolymer of polyvinyl alcohol and polyethylene oxide has a molecular weight of about 45,000 Daltons and consists of about 75% polyvinyl alcohol units and 25% of polyethylene oxide units.
14. A method for forming a cathode for a lithium-sulfur battery cell, the cathode being formed to be used in combination with a lithium metal anode in assembled lithium-sulfur battery cell, the method comprising:
preparing a mixture of sulfur particles with porous carbon particles, the carbon particles having diameters less than one micrometer and a surface area greater than 500 m 2 /g;
temporarily melting the sulfur particles in the mixture such that the molten sulfur as absorbed into the pores of carbon particles and subsequently solidified to form composite carbon/sulfur particles as the active cathode material for the lithium-sulfur battery cell;
forming a slurry of the composite carbon/sulfur particles in a solution consisting of a copolymer of polyvinyl alcohol and polyethylene oxide in water;
applying the slurry to a surface of a metal current collector to form a porous wet coating layer of resin-wetted composite carbon/sulfur particles on the surface of the metal current collector; and
evaporating the water to form a porous layer of carbon/sulfur particles bonded by the copolymer to each other and to the surface of the metal current collector.
15. A method as stated in claim 14 in which the porous carbon particles initially have average diameters of less than one hundred nanometers.
16. A method as stated in claim 14 in which the porous carbon particles initially have average diameters of less than one hundred nanometers and a porosity of up to 800 m 2 /g.
17. A method as stated in claim 14 in which graphite particles are mixed with the carbon/sulfur particles.
18. A method as stated in claim 14 in which the copolymer comprises two percent by weight of the carbon/sulfur particles in the dried porous layer bonded to the surface of the metal current collector.
19. A method as stated in claim 14 in which the metal current collector is an aluminum foil.
20. A method as stated in claim 14 in which the copolymer of polyvinyl alcohol and polyethylene oxide has a molecular weight of about 45,000 Daltons and consists of about 75% polyvinyl alcohol units and 25% of polyethylene oxide units.