Method for manufacturing an electrode foil, electrode with the electrode foil, and energy storage device comprising the electrode

Abstract

The invention relates to a dry manufacturing process without the addition of solvents for the production of an electrode foil for an energy storage device, comprising the process steps: A) Provision of starting materials, including - a fluorinated polymeric dry binder and polyvinylpyrrolidone, - an electrically conductive additive and - an electrochemically active material, B) Mixing the starting materials to form a mixture, and C) Calendering the mixture to form the electrode foil. Such a dry manufacturing process makes it possible to replace part of the fluorine-containing polymeric dry binder with polyvinylpyrrolidone.

Description

[0001] The present invention relates to a method for producing an electrode foil for an energy storage device, an electrode with the electrode foil, and an energy storage device with the electrode.

[0002] In the production of electrodes for energy storage devices, such as lithium-ion batteries or sodium-ion batteries, according to, for example, US patent application US 2013157141A1, electrode materials comprising electrode binders, for example fluorine-containing, polymeric dry binders, active material, and electrical conductivity additives are dry-mixed and subsequently calendered to form a self-supporting electrode foil. In the following, the term "lithium-ion battery" is used synonymously for all terms commonly used in the art for lithium-containing galvanic elements and cells, such as lithium cell, lithium-ion cell, lithium-ion battery cell, lithium polymer cell, lithium-ion polymer cell, lithium battery, lithium battery cell, and lithium-ion accumulator. This includes, in particular, rechargeable batteries (secondary batteries).The terms “battery” and “electrochemical cell” are also used synonymously with the term “lithium-ion battery”.

[0003] US Patent No. 1,074,1843 B2 also describes dry electrode processing using a blend of fluorine-containing polymeric electrode binders, such as polytetrafluoroethylene / polyvinylidene fluoride (PTFE / PVDF) or PTFE / polyethylene oxide (PEO). However, unlike fluorinated electrode binders, PEO is only electrochemically stable up to 3 V in the cell and cannot be used for 4 V cathode materials such as lithium cobalt oxide (LCO). No carrier solvents are used in the US patent application and the US patent, thus avoiding a complex drying process for the carrier solvent and its condensation and recovery, for example, by distillation, after the formation of the composite electrode layers.

[0004] During dry processing, the dry binders must be intensively mixed with electrically conductive additives, such as conductive carbon black, conductive graphite, or carbon nanotubes (CNTs), and electrochemically active materials to achieve homogeneous mixing. The mixture is then compressed by calendering at high pressure and, optionally, high temperatures to achieve high electrode densities, resulting in high energy densities in the electrochemical cell. Both mixing and calendering require shear forces that must be precisely tailored to the respective processes.

[0005] The fluorine-containing polymer dry binders on the anode side of energy storage devices are often not chemically stable and can be reduced, for example, by forming olefinic double bonds. In particular, the fluorine-containing polymer dry binder PTFE is also not chemically stable against lithiated graphite or lithiated silicon, which can form lithium fluoride during their decomposition. Furthermore, PTFE is increasingly undesirable from an ecological perspective, as the PTFE manufacturing process relies on toxic chemicals and PTFE degrades extremely slowly in the environment. This results in a loss of cyclable lithium in the cell, resulting in a reduction in the nominal capacity and energy of the energy storage device.

[0006] Furthermore, the production of fluorine-containing polymeric dry binders often produces perfluorinated alkyl compounds, which are persistent, difficult to degrade, hardly recyclable and therefore accumulate in the environment.

[0007] The object of the present invention is to provide a method for producing an electrode foil that is improved with respect to the above-mentioned disadvantages. A further object of the present invention is to provide an electrode foil for energy storage devices that is improved with respect to the above-mentioned disadvantages. A further object of the present invention is to at least partially replace fluorinated, polymeric dry binders.

[0008] One aspect of the present invention provides a dry manufacturing process without the addition of solvents for producing an electrode foil for an energy storage device. The dry manufacturing process comprises the following process steps: A) Providing starting materials, including - a fluorine-containing polymeric dry binder and polyvinylpyrrolidone (PVP), - an electrically conductive additive and - an electrochemically active material, B) mixing the starting materials to form a mixture, and C) Calendering the mixture to form the electrode foil.

[0009] The dry production process according to the invention allows some of the fluorine-containing polymeric dry binders to be replaced with polyvinylpyrrolidone. Thus, surprisingly, polyvinylpyrrolidone can be used as an alternative dry binder in dry production processes for electrode foils. In contrast to fluorine-containing polymeric dry binders, polyvinylpyrrolidone is a harmless polymer that is also used, for example, as an excipient in the food industry and in medicine. Furthermore, the production of polyvinylpyrrolidone is more cost-effective than the production of many fluorine-containing polymeric dry binders. Because perfluorinated alkyl compounds are not formed during production, the use of polyvinylpyrrolidone is also more sustainable.The use of polyvinylpyrrolidone, as opposed to fluorinated polymeric dry binders, results in electrode foils whose performance and capacity are comparable to dry electrode foils made with fluorinated polymeric dry binders. PVP is stable even at high voltages > 4 V.

[0010] The dry manufacturing process takes place without the addition of carrier solvents. Dry binders are used, which can be mixed without the addition of solvents. This avoids the time-consuming and energy-intensive removal of the solvent after mixing and electrode coating.

[0011] In particular, the various particles of different components for the electrode foil can be moved relative to one another using a solvent-free shear mixing process, thus mixing and fibrillating PTFE as a dry binder. At high shear stress, agglomerates of the dry binders can be reduced in size or dissolved, and fibrils of the polymeric dry binder PTFE can be formed. These fibrils can serve particularly well as binders for the other starting materials of the electrode foil. Such a high shear process can be carried out, for example, using a jet process, for example, a jet mill, a three-roll mill, a twin-screw extruder, or a combination of these processes.

[0012] Polyvinylpyrrolidone, like fluorine-containing polymeric dry binders, can withstand high shear stresses. It has been unexpectedly shown that PVP bonds the particles of the starting materials for the electrode foil with a point bond, thus acting as a dry binder.

[0013] A low-shear mixing process is specifically understood to be a mixing process in which different particles of components are mixed with each other at low velocity gradients and therefore with low shear stresses. At these low shear stresses, the particles of the different components can essentially retain their shape and are only abraded by the walls of the mixing vessel. Paddle mixers, paddle mixers, or even static mixers can be used to mix components with low shear forces.

[0014] The fluorine-containing polymeric dry binder can be selected from a group consisting of: polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), and polytetrafluoroethylene (PTFE). The fluorine-containing polymeric dry binder can preferably be polytetrafluoroethylene. Such fluorine-containing polymeric dry binders are particularly suitable for use in dry production processes without the addition of solvents. Polyvinylpyrrolidone is particularly suitable as a binder for replacing any of these fluorine-containing dry binders, especially PTFE.

[0015] Polyvinylpyrrolidone can be added as a dry binder in a proportion of 0.3% to 3% by weight, preferably in a proportion of 0.5% to 1.5% by weight, based on the total weight of the electrode foil. Such weight percentages are particularly suitable for using polyvinylpyrrolidone as an additional dry binder in a dry production process for electrode foils.

[0016] In a further embodiment of a dry production process according to the invention, polyvinylpyrrolidone is added in a proportion of 0.3% to 50% by weight, preferably in a proportion of 20% to 40% by weight, of the total weight of the dry binder. Thus, depending on the percentage by weight of the total weight of the dry binder, polyvinylpyrrolidone can replace up to half or at least up to 40% by weight of the fluorine-containing polymeric dry binder PTFE.

[0017] The total proportion of dry binder in the total weight of the electrode foil is 0.5% by weight to 5% by weight, preferably 1% by weight to 4% by weight.

[0018] Polyvinylpyrrolidone with a molecular weight of 10,000 g / mol to 500,000 g / mol, more preferably with a molecular weight of 30,000 g / mol to 400,000 g / mol, can preferably be used as a dry binder. Such molecular weights are well suited as dry binders, but are also commercially available.

[0019] The electrically conductive additive can preferably be selected from the group consisting of conductive carbon black, carbon nanotubes, graphene, graphite, expanded graphite, carbon nanofibers (CNTs), in particular gas-phase produced carbon nanofibers (VGCFs), porous carbons, and combinations thereof. The electrically conductive additive can increase the electrical conductivity of the free-standing electrode film and thus lead to improved performance properties of the free-standing electrode film and electrodes, as well as electrochemical cells with such an electrode film. Furthermore, the additive can improve the processability of the free-standing electrode film. Carbon-based electrically conductive additives can also improve the mechanical properties, for example, with regard to tear strength.

[0020] The electrically conductive additive can be added in a proportion of 0.1% by weight to 5% by weight, preferably in a proportion of 0.5% by weight to 3% by weight, of the total weight of the dry electrode foil.

[0021] An electrochemically active material is understood in particular to be a material that is capable of absorbing and releasing lithium ions if the energy storage device is a lithium-ion battery.

[0022] If the electrode film is to be used in an anode, the active material can be selected from the group consisting of carbonaceous materials, silicon, nano-silicon, silicon composites, silicon suboxides, silicon alloys, lithium, lithium alloys, aluminum alloys, indium, indium alloys, tin, tin alloys, cobalt alloys, niobium pentoxide, titanium dioxide, titanates, lithium titanate (Li4Ti5O 12), tin dioxide, and mixtures thereof. The active material is preferably selected from the group consisting of synthetic graphite, natural graphite, graphene, mesocarbon, doped carbon, hard carbon, soft carbon, fullerene, silicon-carbon composites, silicon, surface-coated silicon, silicon suboxide, silicon alloys, metallic lithium, aluminum alloys, indium, tin alloys, cobalt alloys, and mixtures thereof.

[0023] If, however, the electrode film is to be used in a cathode, the active material can be selected from the group consisting of lithium transition metal oxides such as lithium cobalt oxide (LiCoO2), lithium nickel cobalt manganese compounds (NCM or NMC), lithium nickel cobalt aluminum oxides (NCA), lithium olivines such as lithium 25 iron phosphate (LFP), lithium spinels such as lithium manganese oxide spinel (LMO), or combinations thereof. So-called over-lithiated layered oxides (OLO) can also be used.

[0024] In the case of a sodium-ion battery, an electrochemically active material is understood to be a material capable of absorbing and releasing sodium ions. Carbon, such as hard carbon or soft carbon such as graphite, can be used as anode material, as this material is capable of inter- and deintercalating sodium ions during battery operation. Materials containing sodium ions, such as phosphates and diphosphates, such as sodium iron phosphate, can be used as cathode material. Electrolytes can be sodium salts, such as sodium hexafluorophosphate (NaPF6), which are dissolved in organic solvents such as organic carbonates, such as propylene carbonate (PC), ethylene carbonate (EC), and dimethyl carbonate (DMC).

[0025] The electrochemically active material can be added in a proportion of 60 weight percent to 99 weight percent, preferably in a proportion of 80 weight percent to 98 weight percent of the total weight of the electrode foil.

[0026] In process step B), the mixture can be mixed with a pressurized gas. Preferably, the pressure can be more than 0.7 bar, more preferably 5 bar to 8 bar. In particular, in process step B), the mixture can be mixed with a pressurized gas in a jet mill.

[0027] Calendering in process step C) can be carried out at a line load of 100 N / mm to 3000 N / mm and / or a temperature of 50 °C to 280 °C, preferably at a temperature of 80 °C to 200 °C. For example, a line load of 330 N / mm can be applied to a 14-roll calender.

[0028] The electrode foil can, in particular, be a self-supporting electrode foil that can bear its own weight and can, for example, be rolled up into a roll. Such rolls can be stored for extended periods and then unrolled and processed into individual electrode foils for a single energy storage device to produce an energy storage device.

[0029] In process step C), the mixture can be calendered onto a current collector layer. Preferably, the mixture is calendered directly onto the current collector layer. This can form an electrode comprising the current collector layer and the electrode foil arranged thereon. The electrode can then either be rolled up into a roll and stored or can be incorporated into an energy storage device.

[0030] The current collector layer can, for example, be a porous current collector foil. The porous current collector foil can have openings. In process step C), the mixture can be partially pressed into the openings, and the forming electrode foil can extend into these openings. Such a porous current collector foil allows for particularly good bonding of the current collector foil to the electrode foil. The current collector foil or the porous current collector layer can be free of adhesion promoters. Due to the good bonding of the forming electrode foil to the porous current collector foil in process step C), adhesion promoters are not necessary.

[0031] The porous current collector foil can, in particular, be a perforated foil, a cut-out foil, an etched foil, a punched foil, an expanded metal, a metallized fabric, and / or a slit or stretched foil. Such porous current collector foils are particularly suitable for ensuring good bonding to the electrode foil being formed in process step C).

[0032] Alternatively, the current collector foil can also be coated with an adhesion promoter. The adhesion promoter can, in particular, comprise or consist of a polar-modified polyvinylidene fluoride (PVDF) and an electrically conductive additive.

[0033] In particular, the current collector foil coated with an adhesion promoter can be a flat current collector foil. This current collector foil nevertheless allows for a good bond between the electrode foil and the current collector foil due to the adhesion promoter.

[0034] The electrically conductive additive in the adhesion promoter can be selected from a group consisting of: conductive carbon black, conductive graphite, carbon nanofibers (CNT).

[0035] The polar-modified polyvinylidene fluoride can be modified with carboxylic acid groups (-COOH). The polar-modified PVDF can be applied to the current collector foil as described in patent application DE 10 2004 014 338 A1.

[0036] The current collector layer or current collector foil is usually made of copper in the case of anode foils and of aluminum in the case of cathode foils.

[0037] The present invention also relates to a self-supporting electrode foil which can be produced by a dry production process as already described above.

[0038] A further embodiment of the present invention provides a self-supporting electrode foil for use in electrodes of energy storage devices. The self-supporting electrode foil comprises - a fluorine-containing polymeric dry binder and polyvinylpyrrolidone, - an electrically conductive additive and - an electrochemically active material.

[0039] Such a self-supporting electrode foil is, on the one hand, more environmentally friendly than an electrode foil that contains exclusively fluorine-containing, polymeric dry binders, but, on the other hand, has the same capacities as an electrode foil that contains exclusively fluorine-containing, polymeric dry binders or contains fluorine-containing polymeric dry binders to a greater extent.

[0040] The self-supporting electrode foil has the same components and weight percentages of the dry binders, the electrically conductive additives and the electrochemically active materials that have already been described with regard to the manufacturing process.

[0041] Another embodiment of the present invention provides an electrode for an energy storage device. The electrode comprises: - a self-supporting electrode foil as described above, and - a current collector foil, wherein the electrode foil is arranged on the current collector foil.

[0042] The electrode foil can preferably be calendered onto the current collector foil. It is particularly preferred if the self-supporting electrode foil is arranged, in particular calendered, on both main surfaces of the current collector foil. This makes it possible to provide an electrode with a particularly high capacity.

[0043] The present invention also relates to an energy storage device comprising an electrode as described above. The energy storage device further comprises a counter electrode and a separator located between the electrode and the counter electrode. Furthermore, the energy storage device also comprises an electrolyte.

[0044] The energy storage device can in particular be a lithium-ion battery, a sodium-ion battery or a solid-state battery.

[0045] In a lithium-ion battery, an electrode according to the invention can be an anode and / or a cathode. The separator prevents an electrical short circuit between the electrode and the counter electrode and can be a porous, electrically insulating layer made of polyethylene or propylene, glass fibers, a polyolefin membrane, or another porous, electrically insulating material. In particular, an electrolyte solution containing an aprotic solvent such as ethylene carbonate (EC), propylene carbonate (PV), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), or 1,2-dimethoxyethane and a lithium conducting salt, for example lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), or lithium bis(oxalato)borate (LiBOB), which is dissolved in the aprotic solvent, can be used as the electrolyte. The electrolyte can also be a solid lithium-ion conductor, as described, for example, in EP 2767512.

[0046] An electrode according to the invention can also be used in a solid-state battery. In such an energy storage device, metallic lithium can be used as the anode, and an electrode according to the invention can be used as the cathode. A ceramic permeable to lithium ions, such as lithium orthosilicate, or glass can be used as the solid-state electrolyte.

[0047] The present invention also relates to the use of polyvinylpyrrolidone as a dry binder in a self-supporting electrode foil for energy storage devices.

[0048] In the following, the invention will be explained in more detail using exemplary embodiments. Reference example 1:

[0049] To produce a dry electrode as a cathode, 2% by weight of PTFE as a dry binder, 1.5% by weight of Super C65 as an electrically conductive additive (carbon powder) and 96.5% by weight of NMC622 (LiNi 0,6Mn 0,2 Co 0,2 O2) and then calendered either onto an aluminum foil with an adhesion promoter or onto a perforated aluminum foil without an adhesion promoter.

[0050] The conventional cathode is then incorporated into an energy storage cell, a lithium-ion battery, and tested. The electrode / separator assembly is packaged in an aluminum composite foil (113 µm, Show, Japan) and activated with liquid electrolyte (1 M LiPF6 in EC / DMC 3:7, by volume) and made ready for use. The result is a Li-ion cell with a nominal capacity of 150 mAh. Example 1:

[0051] In process step A), a mixture of 1.3 weight percent PTFE, 0.7 weight percent PVP (K-30, BASF), 1.5 weight percent Super C 65 (Imerys), and 96.5 weight percent NMC622 is provided for the production of a cathode foil. The mixture is dry fibrillated in a jet mill in process step B) and then calendered onto a cathode current collector foil, which can be either an aluminum foil with an adhesion promoter or a perforated aluminum foil without an adhesion promoter, in process step C).

[0052] The cathode according to the invention is then incorporated into an energy storage cell, a lithium-ion battery, as described in Reference Example 1, and tested. The Li-ion cell has the same nominal capacity as in Reference Example 1.

[0053] The invention is not limited by the description based on the exemplary embodiments. Rather, the invention encompasses any novel feature and any combination of features, including, in particular, any combination of features in the patent claims, even if this feature or combination itself is not explicitly stated in the patent claims or exemplary embodiments. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] US 2013157141A1

[0002] US 10741843 B2

[0003] DE 10 2004 014 338 A1

[0035] EP 2767512

[0045]

Claims

[1] Dry manufacturing process without the addition of solvents for producing an electrode foil for an energy storage device, comprising the process steps: A) Providing starting materials, including - a fluorine-containing polymeric dry binder and polyvinylpyrrolidone, - an electrically conductive additive and - an electrochemically active material, B) mixing the starting materials to form a mixture, and C) Calendering the mixture to form the electrode foil. [2] Dry manufacturing process according to the preceding claim, wherein the fluorine-containing polymeric dry binder is selected from a group consisting of: polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP) and polytetrafluoroethylene (PTFE), preferably wherein the fluorine-containing polymeric dry binder comprises or consists of PTFE. [3] Dry manufacturing process according to one of the preceding claims, wherein polyvinylpyrrolidone is added in a proportion of 0.3 wt% to 3 wt%, preferably in a proportion of 0.5 wt% to 1.5 wt% of the total weight of the electrode foil. [4] Dry production process according to one of the preceding claims, wherein polyvinylpyrrolidone is added in a proportion of 0.1 wt% to 50 wt%, preferably in a proportion of 20 wt% to 40 wt% of the total weight of the dry binders. [5] Dry production process according to one of the preceding claims, wherein the polyvinylpyrrolidone has a molecular weight of 10,000 g / mol to 500,000 g / mol, preferably a molecular weight of 30,000 g / mol to 400,000 g / mol. [6] Dry production process according to one of the preceding claims, wherein in process step B) the starting materials are mixed with a pressurized gas, preferably wherein the pressure is more than 0.7 bar, more preferably 5 bar to 8 bar. [7] Dry production process according to one of the preceding claims, wherein the calendering in process step C) is carried out at a line load of 100 N / mm to 3000 N / mm and / or a temperature of 50 °C to 280 °C, preferably a temperature of 80 °C to 200 °C. [8] Dry manufacturing process according to one of the preceding claims, wherein in process step C) the mixture is calendered onto a current collector layer, and wherein an electrode comprising the current collector layer and the electrode foil arranged thereon is formed. [9] Self-supporting electrode foil producible by a dry production process according to one of the preceding claims. [10] Self-supporting electrode foil for use in electrodes of energy storage devices, comprising - a fluorine-containing polymeric dry binder and polyvinylpyrrolidone, - an electrically conductive additive and - an electrochemically active material, [11] Electrode for an energy storage device, comprising: - a self-supporting electrode foil according to the preceding claim and a current collector foil, wherein the electrode foil is arranged on the current collector foil, preferably calendered, further preferably wherein the self-supporting electrode foil is arranged on both main surfaces of the current collector foil. [12] Use of polyvinylpyrrolidone as a dry binder in a self-supporting electrode foil for energy storage.