DE102016217403A1 - Process for the preparation of an active material composition - Google Patents

Abstract

The invention relates to a method for producing an active material composition comprising the method steps: a) providing at least one composition A comprising at least one active material, at least one polymer electrolyte and at least one binder; and b) applying shear to composition A to achieve at least partial plasticization of the binder and to obtain a pasty composition B. The composition B can be advantageously used for the production of an electrode or a freestanding active material film.

Description

State of the art

The present invention relates to a process for producing an active material composition for an electrode.

The performance, in particular the energy density, of electrochemical energy storage systems such as lithium-ion batteries (LIB) depends essentially on the choice and design of the electrodes in the cell. Two fundamentally different processes for coating the current collector with the active material (hereinafter also referred to as active material) are described in the prior art, namely by the application of an active material slurry (so-called slurry application method) and by the application of a free-standing active material foil.

The production of electrodes from freestanding active material films is known from the prior art and, for example, in US 2005/0057888 A1 . US 2015/0061176 A1 . US 2015/0062779 A1 or US 2014/234724 A1 described. The freestanding active material film is produced in a solvent-free process. In the preparation, an active material composition comprising at least one active material and at least one particulate binder and optionally at least one conductive additive is provided. By the introduction of shear forces (eg by the use of mechanical mills such as jet or ball mills) the formation of fibrils from the binder particles is achieved. These fibrils adhere to the surfaces of the active material particles and the optionally present conductive additives and thus cause a - initially weak - bond between the particles. The mass obtained by the fibrillation can be formed into a stable freestanding active material film, for example by means of an extruder and / or calender. The free-standing active material foil can then be laminated on a current collector.

DE 10 2015 106 879 A1 describes a process in which by means of extrusion granulation granulation body comprising active material particles, graphite, and optionally binder, prepared and then applied to the current collector and hot pressed.

US 4,153,661 discloses a process for preparing a particulate and PTFE composition using an intensive stirrer in the presence of water.

EP 3 007 255 discloses a method for producing a negative electrode material for a lithium ion secondary battery, wherein a mixture comprising a material capable of intercalating and releasing lithium ions, carbon nanotubes, and water in a pulverizing nozzle of a high-pressure dispersing apparatus into a paste or a slurry is processed. This can then be processed into a powder, which is applied to a current collector.

Battery cells with solid electrolytes, in particular polymer electrolytes, have the advantage that if the cell is damaged, the electrolyte can not escape. The safety of these cells is significantly greater, since no flammable liquids are included. In order to achieve a complete and uniform distribution of the solid electrolyte in the cell, it is necessary to prepare an active material composition which, in addition to active material particles, binders and optional conductive additives, also comprises the solid electrolyte. Typically, solvent-containing mixtures of the ingredients are prepared, which are then applied in a slurry process on the current collector, dried and optionally compacted. The use of solvents (usually N-methylpyrrolidone) is disadvantageous from an economic and environmental point of view. Furthermore, the layer thickness of active material layers produced in this way is limited.

It is therefore an object of the present invention to provide a method which makes it possible to provide free-standing active material layers comprising a polymer electrolyte. This object is achieved by the invention described below.

Disclosure of the invention

• a) Bereitstellen mindestens einer Zusammensetzung A, umfassend mindestens ein Aktivmaterial, mindestens einen Polymerelektrolyten sowie mindestens ein Bindemittel; und
• b) Einwirken von Scherkräften auf die Zusammensetzung A, um eine wenigsten teilweise Plastifizierung des Bindemittels zu erreichen und eine pastöse Zusammensetzung B zu erhalten.

The invention relates to a method for producing an active material composition, comprising the method steps:

• a) providing at least one composition A comprising at least one active material, at least one polymer electrolyte and at least one binder; and
• b) applying shear to composition A to achieve at least partial plasticization of the binder and to obtain a pasty composition B.

The active material may be selected from any active material known to those skilled in the art for making a positive electrode (cathode) or negative electrode (anode).

Suitable anode active materials are known to those skilled in the art. In particular, carbon or silicon based anode active materials capable of forming intercalation compounds with lithium ions are preferred. Examples are anode active materials comprising graphite or mono- or polycrystalline silicon or amorphous silicon. Furthermore, anode active materials of alkaline earth metals, in particular anodes of metallic lithium, may be mentioned.

The positive active material may comprise a composite oxide containing at least one metal selected from the group consisting of cobalt, magnesium, nickel, and lithium.

A preferred embodiment of the present invention relates to a cathode active material comprising a compound of the formula LiMO _2, wherein M is selected from Co, Ni, Mn or mixtures of these and mixtures of these with Al. In particular, LiCoO ₂ should be mentioned.

In another preferred embodiment, the cathode active material is a material comprising nickel, ie LiNi ₁ -x M ' _x O ₂ , where M' is selected from one or more of Co, Mn and Al and 0 ≤ x <1 is. Examples include lithium nickel cobalt aluminum oxide cathodes (eg, LiNi _0.8 Co _0.15 Al _0.05 O ₂ ; NCA) and lithium nickel manganese cobalt oxide cathodes (eg, LiNi _0.8 Mn _0.1 Co _0.1 O ₂ (NMC (811)), LiNi _0.33 Mn _0.33 Co _0.33 O ₂ (NMC (111)), LiNi _0.6 Mn _0.2 Co _0.2 O ₂ (NMC (622)), LiNi _0.5 Mn _0.3 Co _0.2 O ₂ (NMC (532)) or LiNi _0.4 Mn _0.3 Co _0.3 O ₂ (NMC (433)) ,

Further, as preferred positive active materials, mention may be made of overithiated oxides which are known to the person skilled in the art. Examples of these are layer oxides of the general formula n (Li ₂ MnO ₃ ): n - 1 (LiMO ₂ ) with M = Co, Ni, Mn, Cr and 0 ≦ n ≦ 1 and spinels of the general formula n (Li ₂ MnO ₃ ) : n - 1 (LiM ₂ O ₄ ) with M = Co, Ni, Mn, Cr and 0 ≤ n ≤ 1.

Furthermore, in particular spinel compounds of the formula LiM _x Mn _2-x O ₄ with M = Ni, Co, Cu, Cr, Fe (eg LiMn ₂ O ₄ , LiNi _0.5 Mn _1.5 O ₄ ), olivine compounds of the formula LiMPO ₄ with M = Mn , Ni, Co, Cu, Cr, Fe (eg LiFePO ₄ , LiMnPO ₄ ), silicate compounds of the formula Li ₂ MSiO ₄ with M = Ni, Co, Cu, Cr, Fe, Mn (eg Li ₂ FeSiO ₄ ), tavorite compounds ( eg LiVPO ₄ F), Li ₂ MnO ₃ , Li _1.17 Ni _0.17 Co _0.1 Mn _0.56 O ₂ and Li ₃ V ₂ (PO ₄ ) ₃ as suitable positive active materials.

The polymer electrolyte comprises at least one polymer and optionally a conductive salt. Polymers useful as the polymer electrolyte include polyalkylene oxide derivatives of polyethylene oxide, polypropylene oxide and the like, or polymers comprising polyalkylene oxide derivatives; Derivatives of polyvinylidene fluoride (PVDF), polyhexafluoropropylene, polycarbonates, polyphosphoric esters, polyalkylimines, polyacrylonitrile, poly (meth) acrylic esters, polyphosphazenes, polyurethanes, polyamides, polyesters, polysiloxanes, and the like, and polymers comprising derivatives thereof.

Preference is given to polymer compounds which have an oxyalkylene structure, a urethane structure or a carbonate structure in the molecule. For example, polyalkylene oxides, polyurethanes and polycarbonates are preferred in view of their good compatibility with a variety of polar solvents and their good electrochemical stability. Further, polymers having a fluorocarbon group are preferable. Polyvinylidene fluoride and polyhexafluoropropylene are preferred in view of their stability. The number of repeating units of these oxyalkylene, urethane, carbonate and / or fluorocarbon units is preferably in a range of 1 to 1,000, more preferably in a range of 5 to 100.

The polymer electrolyte preferably comprises a lithium salt in addition to the at least one polymer compound. Suitable lithium salts include, but are not limited to, LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAlCl ₄ , LiSbF ₆ , LiSCN, LiCl, LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiN (CF ₃ SO ₂ ) _2, and the like , These can be used individually or in combination with each other. The amount of lithium salt in the polymer electrolyte is preferably 1 to 10 mol / kg, more preferably 1 to 5 mol / kg. The lithium salt may have been added to the polymer electrolyte prior to carrying out the present process. However, it can also be fed to the polymer electrolyte in the course of the present process. Also, in the course of the process, an additional amount of conductive salt can be supplied. These and other embodiments of the invention will be described below.

As an additional constituent, the active material composition in one embodiment comprises at least one solid electrolyte, in particular an inorganic solid electrolyte, which is capable of conducting cations, in particular lithium ions. In accordance with the present invention, such solid inorganic lithium ion conductors include crystalline, composite and amorphous inorganic lithium ion solid conductors. Crystalline lithium ion conductors include, in particular, perovskite type lithium ion conductors, lithium lanthanum titanates, NASICON type lithium ion conductors, LISICON and thio-LISICON type lithium ion conductors, and the like Garnet-type lithium ion conductive oxides. In particular, the composite lithium-ion conductors include materials such as oxides and mesoporous oxides contain. Such solid inorganic lithium ion conductors are used in the Review article by Philippe Knauth "Inorganic solid Li ion conductors: An overview" Solid State Ionics, volume 180, issues 14-16, 25 June 2009, pages 911-916 described. According to the invention, it is also possible to include all solid lithium-ion conductors which are produced by C. Cao, et al. in "Recent advances in inorganic solid electrolytes for lithium batteries", Front. Energy Res., 2014, 2:25 to be discribed. In particular, the in EP1723080 B1 according to the invention described grenade. The solid electrolyte can be used in particular in the form of particles having an average particle diameter of ≥ 0.05 μm to ≦ 5 μm, preferably ≥ 0.1 μm to ≦ 2 μm. If the active material composition comprises a solid electrolyte, it may for example comprise from 0 to 50% by weight, preferably from 10 to 40% by weight, of the active material composition.

Furthermore, the composition A comprises at least one binder which serves, after the method has been carried out, to form adhesive bonds between the individual constituents of the active material composition. The binder is a polymeric material which is at least partially plastifiable under the action of shear forces and preferably forms fibrils. As examples, there may be mentioned polymers such as polyacrylic acid (PAA), carboxymethylcellulose (CMC), styrene-butadiene copolymer (SBR), polyvinylidene fluoride (PVDF), polytetrafluoroethene (PTFE) and ethylene-propylene-diene terpolymer (EPDM). Most preferably, the binder comprises at least PVDF. In addition, the polymer component of the polymer electrolyte can also serve as a binder of the composition. In a preferred embodiment, therefore, the binder and polymer component of the polymer electrolyte are identical.

Further, the composition may include conductive additives such as conductive black or graphite to further increase conductivity.

In a first process step a) of the present process, a mixture of the constituents active material, binder and polymer electrolyte is prepared. This process step can be carried out using any mixing method known to the person skilled in the art. Examples are free-fall mixers and compulsory mixers. In a preferred embodiment, a tumbler mixer is used. This protects the components of the composition from unnecessary shredding.

In a subsequent process step b) an at least partial plasticization of the binder, and optionally the polymer electrolyte is effected by the introduction of shear forces. In a preferred embodiment, the process step b) is carried out in a jet mill or a ball mill. Particularly preferred is the use of a jet mill. This allows for rapid plasticization of the polymer components, i. of the binder and optionally of the polymer electrolyte, without achieving excessive comminution of the active material particles. Preferably, a degree of plasticization of the binder and / or the polymer electrolyte is achieved which allows the formation of fibrils from the polymer components. These fibrils then combine a plurality of particles of composition with each other. This gives a moldable, pasty composition B.

In a preferred embodiment, the method steps a) and b) are carried out in the same mixing device, in particular in a jet mill. This allows a reduction in the number of process steps. Preferably, in this case, mixing of the components at low energy input, i. be carried out at low power of the jet mill. Subsequently, the fibrillation takes place at a power of the jet mill, which allows at least a partial plasticization of the polymer components.

As mentioned above, the polymer electrolyte may contain a conductive salt, in particular a lithium salt, selected from the abovementioned lithium salts. However, the conductive salt may also be added to the active material composition separately from the polymer electrolyte or in addition to the conductive salt contained in the polymer electrolyte. This addition can be carried out in the form of the conductive salt as a solid or in solution. For example, in one embodiment, in step a), the conductive salt is additionally added as a solid and a homogeneous composition A is produced. This is then fibrillated in the subsequent process step b).

In a further embodiment, the conductive salt is used together with the composition A in process step b), without first preparing a homogeneous mixture. The mixing thus takes place during the plasticizing or fibrillation step. The registered energy allows complete mixing and optionally comminution of the conductive salt used.

In a further preferred embodiment of the invention, the conductive salt is added in the form of a conductive salt solution in a solvent. While the addition time is in principle not restricted, the addition preferably takes place during process step a) in order to achieve a uniform distribution. But it can also take place during process step b).

Suitable solvents must be able to dissolve the conductive salt sufficiently. Examples which may be mentioned are water, acetonitrile, tetrahydrofuran, diethyl carbonate or γ-butyrolactone and also cyclic and acyclic carbonates, in particular propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethylene methyl carbonate, ethyl methyl carbonate and mixtures thereof. Preference is given to water, acetonitrile, propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethylene methyl carbonate, ethyl methyl carbonate and mixtures thereof. Particularly preferred are water, acetonitrile and tetrahydrofuran and mixtures thereof.

The resulting composition B is, if necessary, freed of solvent in a further step. In order to prevent unwanted melting of the polymer fibrils, which can lead to an occupancy of the surfaces of the active material particles, the solvent is preferably removed at temperatures which are below the melting temperature of the polymer components. Preferably, the drying is therefore at reduced pressure, e.g. at less than 1 bar, in particular at less than 0.5 bar performed. Heat energy may be supplied provided it is ensured that it does not reach a temperature above or equal to the melting temperature of the polymer component having the lowest melting temperature.

The amounts of the constituents are selected such that the respective active material makes up as large a proportion of the composition B as possible. For example, composition B comprises, based on the total weight thereof, 70 to 95% by weight of active material, 4 to 25% by weight of polymer electrolyte, 1 to 5% by weight of binder, 0 to 5% by weight of conducting salt and 0 to 5% by weight of conductive additives. More preferably, composition B comprises, based on the total weight thereof, 80 to 90% by weight of active material, 5 to 15% by weight of polymer electrolyte, 3 to 5% by weight of binder and 2 to 5% by weight of conducting salt.

The composition B obtained can then be applied to the surface of a carrier material in a further method step. In one embodiment, the substrate is the surface of a tool, eg, the surface of a treadmill. This is preferably made of plastic. The active material layer can be removed in this case at the end of the manufacturing process as a freestanding active material film. Such a method is for example off EP 1 644 136 however, is not limited to this method. In order to avoid or reduce adhesion of the active material layer to the surface of the support material, the process step is preferably carried out at a temperature which is below the glass transition temperature T _{g of} the at least one binder. The active material layer can then be detached as a free-standing active material film from the carrier material and laminated to a current collector, for example at a temperature above the glass transition temperature of the binder.

In a further embodiment, the carrier material may also be the surface of a current collector. In this case, no free-standing active material film is produced, but an electrode is obtained.

Subsequently, the active material layer, preferably at a temperature which is above the glass transition temperature T _{g of} the at least one binder, be compacted by a press, a punch or a roller. In addition, the densification step may be carried out under the action of heat to assist adhesion of the binder to the surface of the current collector and to effect permanent densification. If the carrier material is not the current collector, preferably no heat is supplied. Finally, in this step, the removal of possibly contained solvent can take place. This happens, for example, at elevated temperature and / or reduced pressure.

The present invention also relates to a material composition comprising an at least partially fibrillated polymer electrolyte and prepared by the method described above.

The material composition thus obtained may preferably be used for the production of freestanding active material films. Moreover, it is possible to use the obtained material composition for producing an active material layer deposited on a current collector.

The invention also relates to an electrode comprising at least one material composition prepared by the method described above.

Such an electrode can be used for example in electrochemical energy storage systems. In particular, lithium-ion batteries, supercapacitors and hybrid supercapacitors should be mentioned. Further fields of application for the electrodes produced according to the invention are electrolysis processes or fuel cells.

Advantages of the invention

The process according to the invention makes it possible to provide a freestanding active material layer comprising a polymer electrolyte with a solvent-free or substantially solvent-free process. In addition, the proportion of binder can be reduced because its function is at least partially taken over by the polymer component of the polymer electrolyte.

Embodiments of the invention

The following exemplary implementation of the method is intended to explain the invention in more detail.

example 1

As starting materials for the preparation of a composition according to the invention 4250 g of LiMn ₂ O ₄ having an average particle size of 10 microns, 100 g Leitruß, 250 g of PTFE (binder) and 400 g of polymer electrolyte consisting of 80 wt .-% PEO having a weight average molecular weight Mw (GPC) of 600,000 g / mol and 10 wt .-% LiBF _4, preferably in dry air or inert gas atmosphere such as argon in a free-fall mixer, eg a vortex mixer, processed to a homogeneous mixture. The mixture is processed in a jet mill (operated at 4 to 10 bar with dehumidified air whose dew point is less than -20 ° C or operated with inert gas such as argon) to a pasty mass. This can then be formed by means of a calender to an active material film, for example at 40 ° C.

Example 2

As starting materials for the preparation of a composition according to the invention 4500 g of LiMn ₂ O ₄ having an average particle size of 10 microns and 500 g of PEO having a weight average molecular weight Mw (GPC) of 600,000 g / mol, in a free-fall mixer, for example a vortex mixer to a processed homogeneous mixture. The PEO assumes the function of the polymer component of the polymer electrolyte and the binder in this example. The mixture is processed together with 20 g LiN (CF ₃ SO ₂ ) ₂ in a jet mill (operated at 4 to 10 bar with dehumidified air whose dew point is less than -20 ° C or operated with inert gas such as argon) to a pasty mass , This can then be formed by means of a calender to an active material film, for example at 50 ° C.

Example 3

Starting materials used to prepare a composition according to the invention are 4.5 kg Li _1.17 Ni _0.17 Co _0.1 Mn _0.56 O ₂ with an average particle size of 10 μm, 200 g conductive carbon black, 200 g PVDF with a weight average molecular weight Mw (GPC) of 180 000 g / mol and 300 g of PEO having a weight average molecular weight Mw (GPC) of 600,000 g / mol, in a free-fall mixer, for example a vortex mixer, processed to a homogeneous mixture. During the mixing process, a solution of 50 g of LiAlCl ₃ in 200 ml of tetrahydrofuran is sprayed into the mixture by means of a nozzle. Subsequently, the mixture is dried at 0.5 bar and 30 ° C for 30 min. The mixture thus obtained is processed in a jet mill (operated at 4 to 10 bar with dehumidified air whose dew point is less than -20 ° C or operated with inert gas such as argon) to a pasty mass. This can then be formed by means of a calender to an active material film, for example at 70 ° C.

The invention is not limited to the embodiments described herein and the aspects highlighted therein. Rather, within the scope given by the claims a variety of modifications are possible, which are within the scope of expert action.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The 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

• US 2005/0057888 A1 [0003]
• US 2015/0061176 A1 [0003]
• US 2015/0062779 A1 [0003]
• US 2014/234724 A1 [0003]
• DE 102015106879 A1 [0004]
• US 4153661 [0005]
• EP 3007255 [0006]
• EP 1723080 B1 [0020]
• EP 1644136 [0032]

Cited non-patent literature

• Review article by Philippe Knauth "Inorganic solid Li ion conductors: An overview" Solid State Ionics, Vol. 180, Issues 14-16, June 25, 2009, pages 911-916 [0020]
• C. Cao, et al. in "Recent advances in inorganic solid electrolytes for lithium batteries", Front. Energy Res., 2014, 2:25 [0020]

Claims (10)

Process for the preparation of an active material composition, comprising the process steps: a) providing at least one composition A comprising at least one active material, at least one polymer electrolyte and at least one binder; and b) applying shear to composition A to achieve at least partial plasticization of the binder and to obtain a pasty composition B. The method of claim 1, wherein the polymer electrolyte comprises at least one polymer component which can be used as a binder. Process according to claim 1 or 2, wherein in step a) or b) a conductive salt in the form of a solid or in the form of a conductive salt solution in a solvent is added. Active material composition B, produced according to one of claims 1 to 3, comprising at least one active material, at least one polymer electrolyte and at least one binder, wherein the binder and / or the polymer electrolyte is present at least partially in the form of fibrils. Use of the active material composition B according to claim 4 for the preparation of a freestanding active material film. A freestanding active material film comprising the active material composition B according to claim 4. Use of an active material composition B according to claim 4 or a free-standing active material foil according to claim 6 for the preparation of an electrode comprising at least one current collector and at least one active material layer. An electrode comprising at least one active material composition B according to claim 4 or freestanding active material foil according to claim 6. Use of an electrode according to claim 8 for the production of an electrochemical energy storage system. Electrochemical energy storage system, in particular lithium-ion battery, comprising at least one electrode according to claim 8.