CONTROLLING BINDER MOBILITY DURING THE SHRINKAGE PHASE OF A DRYING PROCESS IN ELECTRODE FABRICATION USING AN ELECTRIC FIELD-INDUCED DIELECTROPHORETIC FORCE

Abstract

A method for producing a battery electrode includes applying an electrode material to a surface of a current collector, the electrode material comprising a binder material having binder particles, initiating a drying process to dry the electrode material, and applying a non-uniform electric field to the electrode material during the drying process. The non-uniform electric field defines a strength gradient to generate dielectrophoretic forces on the binder particles, thereby agitating the binder particles.

Description

REGIONThe present disclosure relates to battery manufacture and, more particularly, to drying an electrode for use in such a battery.GENERAL STATE OF THE ARTThe statements in this section merely provide background information related to the present disclosure and may not constitute the prior art.A typical process for making battery electrodes consists of mixing an electrode material, coating the electrode material on a surface of a current collector, drying the electrode material, and calendering (compressing) the dried electrode. Each step of the electrode fabrication process affects the subsequent fabrication steps and overall performance of the battery.Referring to FIG. 1, a typical drying process for manufacturing an electrode battery that includes multiple drying phases is shown. Generally, the mechanism by which a solvent and binder particles move is different during each phase and may affect the performance of the battery, as set forth in more detail below.Before drying, a slurry of an electrode material 100 containing electrode particles 110, particles of a binder material 120, and solvent 130 is applied to a current collector 140 in a phase before drying (a). At the beginning of drying, the evaporation speed of the solvent 130 is higher in a shrinkage phase (b) and the drying process is faster. In this phase, the solvent 130 moves smoothly to an upper surface of the electrode material 100 where evaporation takes place. Small particles, such as binder particles 120, may be entrained with the solvent 130 towards the surface and remain. As a result, the concentration of the small binder particles 120 may be higher toward the upper surface of the electrode material 100 and lower toward the current collector 140 without intervention. Due to the removal of the solvent 130, the electrode material 100 shrinks and begins to consolidate.Following the shrinkage phase (b), the thickness of the electrode material 100 remains constant, wherein the solvent 130 and the binder particles 120 remain in a capillary phase (c) and a compacting phase (d) in the network of the porous microstructure. In the "capillary phase", the binder particles 120 and the electrode particles 110 begin to bond to a network of pores. The solvent 130 and the binder particles 120 move through the pore network between the electrode particles 110 under capillary force. The net convection of the solvent 130 continues to occur upward by capillary action, with more binder particles 120 being entrained toward the upper surface of the electrode material 100. However, the slower effect allows diffusion so that some binder particles 120 migrate into a more balanced distribution. In the compacting phase (d), the porous microstructure of the electrode material 100 dries and solidifies and isolated regions of the solvent 130 are formed. In the localized vaporization regions, the binder particles 120 begin to crystallize and lose their mobility. Finally, the solvent 130 in the dry electrode (e) is completely evaporated, and the electrode particles 110 are bonded to each other by the binder particles 130 to form a porous structure.In each phase, the mobility of the binder particles 120 in the direction of the surface of the electrode material 100 depends on the binder material, the size of the particles, the thickness of the electrode material, and the temperature and the air flow of drying and the mechanism of drying. Typically, a higher drying rate will result in a higher binder concentration toward the top surface because greater upward transport forces are induced for binders and there is not sufficient time for the binder to diffuse and balance. However, a higher drying speed is desirable for high throughput production due to the shortened drying time. It is therefore desirable to control and reduce binder migration while still maintaining a high drying rate.These problems associated with controlling binder distribution in an electrode material for battery manufacture are addressed by the present disclosure.SUMMARYThis section provides a general summary of the disclosure and is not a comprehensive disclosure of its full scope or all of its features.In one form, the present disclosure provides a method of producing a battery electrode. The method includes applying an electrode material to a surface of a current collector, the electrode material comprising a binder material having binder particles; initiating a drying process to dry the electrode material; and applying a non-uniform electric field to the electrode material during the drying process. The non-uniform electric field defines a strength gradient for creating dielectrophoretic forces on the binder particles, thereby agitating the binder particles.In variations of this method, which may be implemented individually or in any combination: the drying process is performed in a temperature range of 70° C. to 120° C.; the electrode material is monolithic; the electric field is an alternating current electric field having a frequency between 0.01 Hz and 10 kHz; the electric field has a strength profile in a range between 0.1 kV cm -1 and 200 kV cm -1 the electrode is a cathode, the binder material is polyvinylidene fluoride (PVDF) and is a solvent N-methyl-2-pyrrolidone (NMP); and the electrode is an anode, the binder material particles are water soluble and a solvent is water.The present disclosure further provides another method of producing a battery electrode. The method includes applying an electrode material to a surface of a current collector; applying a non-uniform electric field to the electrode material; and drying the electrode material in a drying process. The electrode material includes a binder material including binder particles. The non-uniform electric field defines a strength gradient for creating dielectrophoretic forces on the binder particles such that the particles of binder material are concentrated toward the current collector.In variations of this method, which may be implemented alone or in any combination: the electrode is a cathode, the binder material is polyvinylidene fluoride (PVDF), and is a solvent N-methyl-2-pyrrolidone (NMP); the electrode is an anode, the binder material particles are water soluble and is a solvent water; the drying process is performed in a temperature range of 70° C. to 120° C.; the electric field has a strength profile in a range between 0.1 kV cm -1 and 200 kV cm -1 ; and the electrode material is monolithic.In another form, the present disclosure provides a method of producing a battery electrode. The method includes applying an electrode material to a surface, applying a non-uniform electric field to the electrode material, and drying the electrode material in a drying process. The electrode material includes a binder material. The non-uniform electric field has a higher strength during a first period and a lower strength during a second period. The non-uniform electric field creates dielectrophoretic forces on the binder particles, thereby agitating the binder particles.In variations of this method, which may be implemented alone or in any combination: the higher thickness has a range between 0.1 kV cm -1 and 200 kV cm -1 ; the lower thickness has a range between 0.1 kV cm -1 and 200 kV cm -1 ; if the first time period is prior to the beginning of the drying process; if the electrode is a cathode, the binder material is polyvinylidene fluoride (PVDF) and is a solvent N-methyl-2-pyrrolidone (NMP); if the drying process is performed in a temperature range of 70° C. to 120° C.; and the electrode material is monolithic.Further areas of applicability will become apparent from the description provided herein. It is to be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.DRAWINGSFor a thorough understanding of the disclosure, various forms thereof will now be described by way of example with reference to the accompanying drawings, in which: FIG. 1 is a schematic diagram illustrating stages of a battery electrode drying process according to the related art; FIG. 2 is a schematic diagram illustrating a particle in a non-uniform electric field according to the teachings of the present disclosure; and FIG. 3 is a flow diagram illustrating a novel method of producing a battery electrode in accordance with the teachings of the present disclosure.The drawings described herein are for illustrative purposes only and are not intended to limit the scope of the present disclosure in any way.DETAILED DESCRIPTIONThe following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.To counteract the uneven mobility of binder particles as set forth above, the inventors have discovered that applying a non-uniform electric field to the electrode material can control the movement and thus the final distribution of binder particles within the electrode material. The non-uniform electric field has a voltage strength gradient to generate dielectrophoretic forces acting on the binder particles 120, as will be described in more detail below.Referring to FIG. 2, the general use of dielectrophoretic forces with a non-uniform electric field 210 applied to a particle 220, such as a particle of binder material 120, is illustrated. The non-uniform electric field 210 polarizes the particle 220, inducing a force along the field lines 240 at the poles 230. The orientation of the resulting dipole (separation of opposing electrical charges) depends on the relative polarizability of the particle 220 and a surrounding medium, such as a solvent. Because the electric field 210 is non-uniform, the pole 230 exposed to the greatest electric field predominates over the others and the particle 220 moves. The poles 230 and the corresponding strength gradient on the particle 220 are oriented to move the particle 220 towards the current collector. In the negative dielectrophoresis arrangement shown in (a), the non-uniform electric field 210 has a greater strength in the direction of the current collector 140, and the particle is attracted toward the current collector 140. Alternatively, in the positive dielectrophoresis arrangement shown in (b), the strength gradient is configured to repel the particle 220 away from the upper surface of the electrode material towards the current collector 140.Referring to FIG. 3, a novel method of producing a battery electrode according to the teachings of the present disclosure using the dielectrophoretic forces described above is shown. Initially, an electrode material 100 is applied to a surface of a current collector 140. The electrode material 100 is in the form of a homogeneous slurry and is deposited as a single monolithic layer. The slurry components generally include, but are not necessarily limited to, electrode particles 110, a binder material 120, and a solvent 130. In the single monolithic layer, the binder material is uniformly distributed at the beginning of the application. Additional layers of varying composition are not required.The electrode particles 110 include active particles and may optionally include electrically conductive additive particles. The active particles form the bulk of the electrode and interact with ions in the battery fluid, such as lithium ions in a lithium battery. In one form of the present disclosure, the active particles are a carbon material, such as graphite. In other forms, the active particles include lithium titanate (Li4Ti5O12), lithium nickel manganese cobalt oxide, and graphene. The conductive additive particles aid in the conductivity of the electrode and the movement of ions to the active particles. In one form, the conductive additive particles include carbon black. It should be appreciated that other materials for the electrode particles 110 having the described characteristics and the functions set forth herein may be implemented while remaining within the scope of the present disclosure.The binder material 120 includes binder particles and may be any of a variety of polymer materials that serve to hold the electrode particles 110 together. For example, the binder material 120 may be, but is not limited to, fluoro acrylic polymer, polytetrafluoroethylene (PTFE), polyvinylidene fluoride, polyacrylates, aliphatic polymers, aromatic polymers, oligo- and polysaccharides, chitosan, alginate, pectin, amylose, starch, gums, lignin, and proteins. It should be appreciated that other materials used to bind the electrode particles 110 may be utilized while remaining within the scope of the present disclosure.The solvent 130 generally serves to suspend or disperse the electrode particles 110 and binder particles 120, and may be any of a variety of materials including, but not limited to, H2O (water), N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethylacetamide (DMAC), and dimethyl sulfoxide (DMSO). It should be understood that other solvents may be reacted while remaining within the scope of the present disclosure.After being applied to the current collector, the electrode material 100 is dried to remove the solvent 130, thereby leaving the binder material 120 and the electrode particles 110 forming a porous nanostructure. In one form, the electrode material 100 is dried at a temperature of 70° C. to 120° C.During the drying process, the non-uniform electric field is applied to the electrode material 100. In one form, the electric field is an AC field having a frequency between 0.01 Hz and 10 kHz. In one form, the electric field has a strength of 0.1 kV cm -1 to 200 kV cm -1.In various forms, the non-uniform electric field 210 may be applied during the pre-drying phase (a), during the shrinking phase (b) of the drying process, during the capillary phase (c), during the compacting phase (d) of the drying process, or any combination thereof. In one form, the dielectrophoretic forces 210 cause the binder particles 220 to move into a predetermined distribution. For example, the predetermined distribution may include binder particles 120 concentrated toward the surface of the current collector 140. The particles 220 are moved to the predetermined distribution when the non-uniform field 210 is applied in the phase before drying (a) before the drying process has begun, or when the dielectric force 210 is stronger than the forces 130 provided by the movement of the solvent. In another form, the dielectrophoretic forces 210 balance the movement of the solvent 130 during the drying process, keeping the binder particles 120 in place. In this form, the approximate distribution of binder particles 120 is maintained, either in the original uniform distribution of the phase prior to drying (a) or in the predetermined distribution.One approach to moving the binder particles 120 into a predetermined distribution is to apply different voltage strength gradients in different phases of the drying process. In one form, the non-uniform electric field 210 has a higher strength during a first period and a lower strength during a second period. For example, the higher strength force may be applied before drying (a) to concentrate the binder particles 120 toward the surface of the current collector 140, and the lower strength force may be applied during the shrinkage phase (b). Alternatively, the higher strength force may be applied during the shrinking phase (b) and the lower strength force may be applied during the compacting phase (d).Unless expressly stated otherwise herein, all numerical values indicating mechanical / thermal properties, percentages of compositions, dimensions and / or tolerances, or other characteristics are to be understood as modified by the word "about" or "about" when describing the scope of the present disclosure. This modification is desirable for various reasons including industrial practice, material, manufacture and assembly tolerances, and testability.As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A OR B OR C), using a non-exclusive logical OR, and should not be construed to mean "at least one of A, at least one of B, and at least one of C.".The description of the disclosure is merely exemplary in nature and, thus, variations that do not depart from the gist of the disclosure are intended to be within the scope of the disclosure. Such variations are not to be regarded as a departure from the spirit and scope of the disclosure.According to the present invention, a method of producing a battery electrode includes: applying an electrode material to a surface of a current collector, the electrode material comprising a binder material having binder particles; applying a non-uniform electric field to the electrode material, the non-uniform electric field defining a strength gradient for generating dielectrophoretic forces on the binder particles such that particles of the binder material are concentrated toward the current collector; and drying the electrode material in a drying process.In one aspect of the invention, the electrode is a cathode, the binder material is polyvinylidene fluoride (PVDF), and is a solvent N-methyl-2-pyrrolidone (NMP).In one aspect of the invention, the electrode is an anode, the binder particles are water soluble, and a solvent is water.In one aspect of the invention, the drying process is carried out in a temperature range of 70° C. to 120° C.In one aspect of the invention, the electric field has a strength profile in a range between 0.1 kV cm -1 and 200 kV cm -1.In one aspect of the invention, the electrode material is monolithic.According to the present invention, a method of producing a battery electrode includes: applying an electrode material to a surface, the electrode material comprising a binder material having binder particles; applying a non-uniform electric field to the electrode material, the non-uniform electric field having a higher strength during a first period and a lower strength during a second period, the non-uniform electric field generating dielectrophoretic forces on the binder particles, thereby agitating the binder particles; and drying the electrode material in a drying process.In one aspect of the invention, the higher thickness has a range between 0.1 kV cm -1 and 200 kV cm -1.In one aspect of the invention, the lower thickness has a range between 0.1 kV cm -1 and 200 kV cm -1.In one aspect of the invention, the first period of time is before the beginning of the drying process.In one aspect of the invention, the electrode is a cathode, the binder material is polyvinylidene fluoride (PVDF), and is a solvent N-methyl-2-pyrrolidone (NMP).In one aspect of the invention, the drying process is carried out in a temperature range of 70° C. to 120° C.In one aspect of the invention, the electrode material is monolithic.

Claims

A method of producing a battery electrode, the method comprising: applying an electrode material to a surface of a current collector, the electrode material comprising a binder material having binder particles; initiating a drying process to dry the electrode material; and applying a non-uniform electric field to the electrode material during the drying process, wherein the non-uniform electric field defines a strength gradient to generate dielectrophoretic forces on the binder particles, thereby moving the binder particles.The method of claim 1, wherein the drying process is performed in a temperature range of 70°C-120°C.The method of claim 1, wherein the electrode material is monolithic.The method of claim 1, wherein the electric field is an AC electric field having a frequency between 0.01 Hz and 10 kHz.The method of claim 1, wherein the electric field has a strength profile in a range between 0.1 kV cm -1 and 200 kV cm -1.The method of claim 1, wherein the electrode is a cathode, the binder material is polyvinylidene fluoride (PVDF), and a solvent is N-methyl-2-pyrrolidone (NMP).The method of claim 1, wherein the electrode is an anode, the binder particles are water soluble, and a solvent is water.The method of claim 1, wherein the non-uniform electric field defines the strength gradient for generating dielectrophoretic forces on the binder particles such that the particles of binder material are concentrated toward the current collector.The method of claim 1, wherein the non-uniform electric field has a higher strength during a first time period and a lower strength during a second time period.The method of claim 9, wherein the higher strength has a range between 0.1 kV cm -1 and 200 kV cm -1.The method of claim 9, wherein the lesser thickness has a range between 0.1 kV cm -1 and 200 kV cm -1.The method of claim 9, wherein the first time period is before the beginning of the drying process.The method of claim 7, wherein the electrode material is monolithic.

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