METHOD FOR MANUFACTURING A POROUS FILM

Abstract

The present invention relates to a method for manufacturing a single-layer or multi-layer porous film, said method comprising the following steps: a) providing a flowable first base mixture for a first film layer of the film, the first base mixture comprising a solvent, a filler that is insoluble in the solvent, and a polymeric binder that is dissolved in the solvent; b) forming a film precursor film, the film precursor film comprising at least one sub-layer composed of the first base mixture; c) bringing the film precursor film into contact with a precipitant, the solvent of the first base mixture being soluble in the precipitant, the binder being insoluble in the precipitant, and the binder being precipitated to form the porous film. The invention also relates to a film manufactured using said method, an electrode material manufactured from said film, and an energy storage medium comprising said electrode material.

Claims

1. A method for preparing a single-layer or multi-layer porous film, comprising the following steps: a. providing a flowable first base mixture for a first film layer of the film, wherein the first base mixture comprises a solvent, an additive insoluble in the solvent and a polymeric binder dissolved in the solvent, b. forming a film precursor sheet, wherein the film precursor sheet comprises at least a sublayer of the first base mixture, c. contacting the film precursor sheet with a precipitant, wherein the solvent of the first base mixture is soluble in the precipitant, wherein the binder is at least partially insoluble in the precipitant, and wherein the binder is precipitated to form the porous film.

2. The method according to claim 1, further comprising: providing a flowable second base mixture for forming a second film layer, wherein the second base mixture comprises a solvent, an additive insoluble in the solvent and a polymeric binder dissolved in the solvent, and providing a flowable third base mixture for forming a separating layer, wherein the separator mixture comprises a solvent and a polymeric binder dissolved in the solvent, wherein the film precursor sheet comprises a second sublayer of the second base mixture and a third sublayer of the third base mixture, wherein the sublayers extend parallel to each other in the main extension direction of the film precursor sheet, and wherein the binder of the first, second, and third base mixtures is at least partially insoluble in the precipitant, wherein the binder is precipitated to form the porous film.

3. The method according to claim 2, wherein the third base mixture is electrically non-conductive.

4. The method according to claim 2, wherein, for forming the film precursor sheet, the third base mixture is arranged between the first base mixture and the second base mixture.

5. The method according to claim 1, wherein the polymeric binder of the first base mixture comprises or consists of polysulfones, polyimides, polystyrene, carboxymethyl cellulose, polyether ketones, polyethers, polyelectrolytes, fluorinated polymers, in particular polyvinylidene fluoride, or a mixture of at least two of them.

6. The method according to claim 1, wherein the solvent of the first base mixture comprises or consists of dimethylacetamide, dimethyl formamide, N-methyl pyrrolidone, N-ethyl pyrrolidone, sulfolane, dimethyl sulfoxide, methanol, ethanol, isopropanol, water, or a mixture of at least two of them.

7. The method according to claim 1, wherein the additive of the first base mixture is an electroactive agent.

8. The method according to claim 7, wherein: the electroactive agent comprises or consists of a lithium oxide and/or a lithium sulfide and/or a lithium fluoride and/or a lithium phosphate, or the electroactive agent comprises or consists of graphite, graphene, silicon nanoparticles, lithium titanate, tin, or a mixture thereof.

9. The method according to claim 1, wherein the first base mixture further comprises a conductive agent, wherein the conductive agent comprises or consists of conductive carbon, in particular carbon black, graphene or graphite.

10. The method according to claim 1, wherein the precipitant comprises or consists of water, at least one alcohol, such as methanol, ethanol, isopropanol, the solvent of a base mixture, or a mixture thereof.

11. The method according to claim 1, wherein the film precursor sheet comprises the unsupported extrusion of the first base mixture.

12. The method according to claim 1, wherein after step (b) and before step (c) the film precursor sheet is coated by a pre-precipitant on one or both sides, the binder of the base mixture(s) being insoluble in the pre-precipitant.

13. The method according to claim 1, further comprising: washing the film in a washing solution, wherein the washing solution comprises or consists of water or at least one alcohol, such as methanol, ethanol, isopropanol, or a mixture thereof.

14. The method according to claim 1, further comprising: drying the film in a drying apparatus with recirculating air or an inert gas.

15. The method according to claim 1, further comprising: compacting the film in a press apparatus, a roller apparatus or a calendering apparatus, or compacting the film by thermal shrinking.

16. A porous film prepared by a method according to claim 1.

17. The film according to claim 16, wherein the film has a substantially constant thickness between 50 μm and 1000 μm.

18. The film according to claim 16, wherein the film comprises a first film layer, a second film layer and a separating layer arranged between the first and the second film layer, wherein the first film layer and the second film layer have a thickness between 20 μm and 500 μm, and wherein the separating layer has a thickness between 5 μm and 50 μm.

19. The film according to claim 18, wherein the separating layer is an electrical insulator.

20. The film according to claim 18, wherein the first film layer and the second film layer are electrical insulators.

21. An electrode material, comprising a film according to claim 16 and a current dissipation layer arranged on the outer surfaces of the electrode material.

22. The electrode material according to claim 21, comprising at least two layers of a film according to claim 16.

23. An energy storage medium, comprising: an electrode material according to claim 22, an electrolyte, and two contacting elements.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0118] FIG. 1 shows a schematic process flow diagram of a second exemplary embodiment of a method according to the invention;

[0119] FIG. 2 shows a schematic representation of an energy storage medium comprising a film prepared by methods of the second exemplary embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0120] Unless otherwise indicated below, the following elements are shown in the figures: dissolving tank 1, homogenizer 2, cartridge filter 3, storage tank 4, line 5, extruder 6, line 7, precipitating liquid tank 8, precipitating bath 9, circulating pump 10, deflection roller 11, washing bath 12, drying furnace 13, winding apparatus 14, electrode material 15, contacting element 16, cell housing 17, anode layer 18, cathode layer 19, separating layer 20, film precursor sheet 21, film 22, roller 23, metal layer 24, degassing apparatus 25.

Example 1

[0121] According to a first exemplary embodiment, a method for preparing a single-layer porous film is provided. Two of these films are used to prepare an energy storage medium. The compositions of the base mixtures of the two films are indicated in table 1.

TABLE-US-00001 TABLE 1 First film Weight Second film Weight (anode foil) fraction (%) (cathode foil) fraction (%) Solvent Dimethyl- 59.7 Dimethyl- 53.2 acetamide acetamide Binder Polyvinylidene 4.2 Polyvinylidene 4.7 difluoride difluoride Electroactive Graphite 35.7 Lithium iron 40.7 agent phosphate Conductive Carbon black 0.4 Carbon black 1.4 agent

[0122] The graphite has an average particle diameter of about 15 μm, the lithium iron phosphate of about 2.0 μm, and the carbon black of about 0.05 μm.

[0123] Two base mixtures are prepared, first dry-blending the powder binder with the insoluble particulate solids, i.e., the electroactive agent and the conductive agent. Then the powder mixture thus prepared is added to the solvent. The mixture is homogenized and then filtered through an ultrasonic sieve to separate any agglomerates it may contain, which are larger than about 50 μm. The suspension is degassed under vacuum.

[0124] Each base mixture is applied to a glass plate with a doctor blade to form a film precursor sheet with a single sublayer. The thickness of the first sublayer thus prepared is about 250 μm for the anode foil and about 350 μm for the cathode foil.

[0125] The glass plate with the single-layer film precursor sheet is then immersed in a bath of precipitant at room temperature, using deionized water as the precipitant. After a residence time of about 10 min in the precipitant, the glass plate is removed from the precipitation bath. The films with dimensions of about 30×20 cm can be removed manually from the glass plate. After a washing step in a washing bath with deionized water, the films are dried in air.

[0126] The obtained films are characterized using scanning electron microscopy. An average pore size of about 1.5 μm is determined. The porosity of the prepared film is about 0.65. The thicknesses of the films are about 145 μm for the anode foil and 150 μm for the cathode foil. Sample pieces of the single-layer anode and cathode foils with a size of 70 mm×70 mm are pressed between two plane-parallel steel plates at room temperature in a lever press with a pressure of 0.3 to/cm.sup.2. The thickness of the anode foil after pressing is about 70 μm with a porosity of about 0.32, and the thickness of the cathode foil is about 100 μm with a porosity of about 0.45. The foils do not lose their good mechanical properties and flexibility.

[0127] The basis weight of the anode foil is about 100 g/m.sup.2, and the basis weight of the cathode foil is about 180 g/m.sup.2, which is two to three times the basis weight of anode and cathode coatings prepared by the classical method with solvent evaporation in a belt dryer.

[0128] An electrode material is formed from the films by placing a separating layer of a commercially available polyethylene membrane (e.g., from Celgard) between an anode foil and a cathode foil. An aluminum foil is arranged on the surface of the cathode foil and a copper foil is arranged on the surface of the anode foil as a current dissipation layer.

[0129] The electrode material is processed into a pouch cell as an energy storage medium in a form known in the art. The cells obtained have a typical cell voltage, and several charge and discharge cycles are possible.

Example 2

[0130] According to a second embodiment, a further method for preparing a single-layer porous film is provided. Two of these films are used to prepare an energy storage medium. The compositions of the base mixtures of the two films are indicated in table 2.

TABLE-US-00002 TABLE 2 First film Weight Second film Weight (anode foil) fraction (%) (cathode foil) fraction (%) Solvent Dimethyl- 53.5 Dimethyl- 53.4 acetamide acetamide Bonding Polyvinylidene 4.7 Polyvinylidene 4.6 agent difluoride difluoride Electroactive Graphite 41.4 LiNCM523 39.1 agent (Lithium Nickel0.5 Cobalt0.2 Manganese0.3 Oxide) Conductive Carbon black 0.5 Carbon black 2.8 agent

[0131] The graphite has an average particle diameter of about 1.5 μm, the lithium nickel cobalt manganese oxide of about 4.0 μm, and the carbon black of about 0.05 μm.

[0132] Two base mixtures are prepared, first dry-blending the powder binder with the insoluble particulate solids, i.e., the electroactive agent and the conductive agent. Then the powder mixture thus prepared is added to the solvent. The mixture is homogenized and then filtered through an ultrasonic sieve to separate any agglomerates it may contain, which are larger than about 50 μm. The suspension is degassed under vacuum.

[0133] Each base mixture is applied to a glass plate with a doctor blade to form a film precursor film with a single sublayer. The thickness of the first sublayer thus prepared is about 140 μm for the anode foil and about 210 μm for the cathode foil.

[0134] The glass plate with the single-layer film precursor film is then immersed in a bath of precipitant at room temperature, using deionized water as the precipitant. After a residence time of about 10 min in the precipitant, the glass plate is removed from the precipitation bath. The films with dimensions of about 30×20 cm can be removed manually from the glass plate. After a washing step in a washing bath with deionized water, the films are dried in air.

[0135] The obtained films are characterized using scanning electron microscopy. An average pore size of about 1.5 μm is determined. The porosity of the prepared films is about 0.55 for the cathode foil and 0.62 for the anode foil. The thicknesses of the films are about 100 μm for the anode foil and 100 μm for the cathode foil. Sample pieces of the single-layer anode and cathode foils with a size of 70 mm×70 mm are pressed between two plane-parallel steel plates at room temperature in a lever press with a pressure of 0.1 to/cm.sup.2. The thickness of the anode foil after pressing is about 85 μm with a porosity of about 0.56, and the thickness of the cathode foil is about 85 μm with a porosity of about 0.47. The foils do not lose their good mechanical properties and flexibility.

[0136] The basis weight of the anode foil is about 80 g/m.sup.2, and the basis weight of the cathode foil is about 139 g/m.sup.2, which is about twice the basis weight of anode and cathode coatings prepared by the classical method with solvent evaporation in a belt dryer.

[0137] An electrode material is formed from the films by placing a separating layer of a commercially available polyethylene membrane (e.g., from Celgard) between an anode foil and a cathode foil. An aluminum foil is arranged on the surface of the cathode foil and a copper foil is arranged on the surface of the anode foil as a current dissipation layer.

[0138] The electrode material is processed into a pouch cell as an energy storage medium in a form known in the art. The cells obtained have a typical cell voltage, and several charge and discharge cycles are possible.

Example 3

[0139] According to a second exemplary embodiment, a method for preparing a multi-layer porous film is provided. This film is used to prepare an energy storage medium. In an inventive step of the method, a film precursor sheet is prepared, which consists of three sublayers, each sublayer being formed from a base mixture. The compositions of the base mixtures are indicated in table 3.

TABLE-US-00003 TABLE 3 First base Second base Third base mixture mixture mixture (anode (cathode (separating layer) wt.-% layer) wt.-% layer) wt.-% Solvent Dimethyl- 59.7 Dimethyl- 53.2 Dimethyl- 92.0 acetamide acetamide acetamide Binder Polyvinylidene 4.2 Polyvinylidene 4.7 Polyvinylidene 8.0 difluoride difluoride difluoride Electroactive Graphite 35.7 Lithium iron 40.7 — 0 agent phosphate Conductive Carbon black 0.4 Carbon black 1.4 — 0 agent

[0140] The graphite has an average particle diameter of about 15 μm, the lithium iron phosphate of about 2 μm and the carbon black of about 0.05 μm.

[0141] The inventive production method according to this second exemplary embodiment is illustrated in a process flow diagram in FIG. 1.

[0142] The base mixtures are prepared by first dissolving the binder completely in the solvent. The binder solutions are prepared in the dissolving tanks 1a, 1b, 1c. In the case of the first and second base mixtures, the insoluble particulate solids, i.e., the electroactive agent and the conductive agent, are then added and the mixture is homogenized in the homogenizers 2a, 2b. All three base mixtures are then filtered through cartridge filters 3a, 3b, 3c to separate any agglomerates or other solids with a particle size of more than 30 μm that may be contained.

[0143] Then the prepared base mixtures are placed in storage tanks 4a, 4b, 4c, where they can be stored until further processing. Agitators are provided in each of the storage tanks 4a, 4b, 4c to ensure homogeneity of the base mixtures and prevent settling of particulate components. The base mixtures are fed in separate lines 5a, 5b, 5c to an extruder 6, which is designed as a multi-slot extruder with five sheet dies, each 15 cm wide. Degassing apparatuses 25a, 25b, 25c, in this case degassing membranes, are located in each of the lines 5a, 5b and 5c.

[0144] The base mixtures are introduced into the three central nozzles, with the third base mixture positioned centrally between the first base mixture and the second base mixture.

[0145] The two outermost nozzles are connected to precipitation liquid tanks 8a, 8b via lines 7a, 7b. In this exemplary embodiment, the precipitation liquid is deionized water and is brought into contact with the surfaces of the film precursor sheet 21 emerging from the extruder 6 during extrusion.

[0146] This results in a pre-precipitation of the binder even before contact with the actual precipitation bath 9.

[0147] The film precursor sheet 21 is introduced self-supportingly into the precipitation bath 9, in which the precipitation liquid is contained. In this case, the precipitation liquid is deionized water containing about 2% of the solvent dimethylacetamide. The precipitation liquid is circulated and set in motion in the precipitation bath by means of a circulating pump 10. Excess precipitation liquid leaves the precipitation bath 9 via an overflow and is pumped off and disposed of. The temperature of the precipitation bath is about 40° C. The porous three-layer film 22 formed by coagulation of the binder is led out of the precipitation bath via a deflection roller 11 and transferred to a washing bath 12 via a further deflection roller 11. The film 22 is then dried in a drying oven 13 at about 80° C. and wound onto a roll 23 by a winding apparatus 14. The continuous material thus obtained can be supplied for further use.

[0148] The obtained films are characterized using scanning electron microscopy. An average pore size of about 1.5 μm is determined. The porosity of the prepared multi-layer film is about 0.65 for the anode and cathode layers and about 0.85 for the intermediate separating layer. The overall thickness of the film is about 350 μm. Sample pieces of the three-layer film with a size of 70 mm×70 mm are pressed between two plane-parallel steel plates at room temperature in a lever press with a pressure of 0.3 to/cm.sup.2. The thickness of the film after pressing is about 180 μm, the thickness of the anode layer being about 70 μm, the thickness of the cathode foil being about 100 μm and the thickness of the separating layer being about 10 μm. The films do not lose their good mechanical properties and flexibility during pressing.

[0149] The basis weight of the multi-layer film is about 290 g/m.sup.2.

[0150] The outer film layers, i.e., the cathode layer and the anode layer, have a porosity of about 0.45 and 0.32, respectively. The average pore diameters are about 1.5 μm. The inner film layer, i.e., the separating layer, has a porosity of about 0.65. The average pore diameter is about 2.5 μm.

[0151] To form an electrode material, 15×10 cm pieces of the continuous material are vapor-deposited on both sides with a metal layer with a thickness of about 1 μm. The surface of the anode layer receives a copper coating, while the surface of the cathode layer receives an aluminum coating.

[0152] The electrode materials prepared in this way can be further processed to energy storage media. An exemplary energy storage medium is shown in FIG. 2. FIG. 2 is only a sketched illustration of an energy storage medium, and the actual proportions are not shown to scale.

[0153] The electrode material 15 is contacted at the metal layer 24 of the anode and cathode sides with contacting elements 16a, 16b, which are nickel electrodes on the anode side, while aluminum contacting elements are used on the cathode side. This arrangement is packed in an airtight manner in a cell housing 17. The electrode material 15 with the anode layer 18, the cathode layer 19 and the separating layer 20 is loaded with an electrolyte which provides free Li.sup.+ ions as a charge carrier. Wetting is achieved by the capillary action of the pores in the film.

[0154] Compared to energy storage media prepared according to the prior art, the manufacturing costs for the production of this energy storage medium according to the invention are about 20 to 25% lower. The mass-based storage density is more than 50% higher than that of conventional energy storage media, and the power-based area requirement is more than 20% lower.

[0155] The method according to the invention and the products obtained therefrom may result in the following further advantages over the prior art: [0156] The porosity of the layers is independent of the particle size of the additives used. Therefore, the use of nanoparticles as additives is possible to produce cells for high charge and discharge currents and to enable high diffusion rates of the lithium ions in the pores. [0157] The precipitation process of the binder in the precipitant creates a sponge-like elastic structure between the additives, which imparts strength and toughness to the sublayers. Cracking during manufacture and operation of the materials is greatly reduced. [0158] The separating layer can be particularly thin and highly porous, since it is present as a sublayer and as such does not have to be mechanically resilient. [0159] The reduction of the metal content results in a weight reduction of the cell. [0160] The reduced thickness of the separating layer and an increase in the porosity of the separating layer lead to a reduction in the ohmic internal cell resistance and thus to lower power loss during charging and discharging of the battery. [0161] The lower internal cell resistance allows faster charging compared to the prior art with less heating of the cell. At the same time, faster discharges (high-power cells) are also feasible. [0162] The method allows the use of materials with high softening points and temperature resistances, such as aromatic polyimides, polyamides, polyether ketones or polyether sulfones for the production of separators and electrodes. This is accompanied by increased safety during operation due to the increase in melt-down temperature.