Separator comprising a variable binder whose dimensions change with pH and method of making the same

Abstract

The present invention relates to a separator including a variable adhesive whose size changes with pH and a method for manufacturing the separator, and more particularly to a separator configured such that a variable adhesive whose size changes with pH is used in a separator coating to reduce the resistance of a battery and maintain uniform adhesion while improving air permeability, and a method for manufacturing the separator.

Description

Technical Field

[0001] This application claims the benefit of priority from Korean Patent Application No. 2020-0109204, filed on August 28, 2020, the disclosure of which is incorporated herein by reference in its entirety.

[0002] The present invention relates to a separator including a variable binder whose dimensions change with pH, ​​and a method for manufacturing the separator. More specifically, the present invention relates to a separator including: a separator substrate; and a coating on at least one surface of the separator substrate, wherein the coating includes a variable binder whose dimensions change with pH, ​​and a method for manufacturing the separator. Background Art

[0003] As a component constituting a secondary battery, a separator includes a polymer film having a porous structure and located between the positive electrode and the negative electrode. The separator is used to isolate the positive electrode and the negative electrode from each other to prevent an electrical short circuit between the two electrodes and to allow electrolytes and ions to pass through. The separator itself does not participate in the electrochemical reaction of the battery. However, the separator affects the performance and safety of the battery due to its physical properties such as electrolyte wettability, porosity, and thermal shrinkage. In this specification, the separator includes both a separator consisting solely of a separator substrate and a separator manufactured by coating at least one surface of the separator substrate with an inorganic material.

[0004] In recent years, methods have been used to improve the physical properties of separators by adding inorganic coatings to separator substrates, adding various materials to the coatings, or modifying the physical properties of the coatings. For example, inorganic particles that increase mechanical strength may be added to the coatings, or inorganic materials or hydrates that improve the flame retardancy and heat resistance of the separator substrate may be added to the coatings. To increase adhesion between the separator and the positive or negative electrode or to evenly distribute the inorganic material, a variable binder may be used in the coating.

[0005] Generally speaking, a variable adhesive can be classified into an oil-based variable adhesive used in a state of being dissolved in an organic solvent or an aqueous variable adhesive used in a state of being dissolved in an aqueous solvent such as water.

[0006] Oil-based variable adhesives offer the advantage of higher adhesion than water-based variable adhesives, but they also have the disadvantage that, if not fully dried, the coating surface remains sticky, potentially allowing external dust to adhere and increasing product defect rates. When the oil-based variable adhesive dries, the organic solvent evaporates, forming a gel. This organic solvent is trapped between the gel particles, resulting in a non-uniform coating. This can lead to decreased battery performance and safety.

[0007] With oil-based variable adhesives, an organic solvent having high volatility is used when forming a slurry for coating, thereby making it difficult to maintain a uniform slurry concentration and the chemical properties of the materials in the slurry may change.

[0008] For these reasons, water-based variable adhesive is preferred, and it can minimize the harm to human health and its concentration can be easily adjusted. Because the adhesion of water-based variable adhesive is lower than that of oil-based variable adhesive, thus water-based variable adhesive must be used in larger amount. In the coating with water-based variable adhesive added thereto, compared with the coating with oil-based variable adhesive added thereto, the content of variable adhesive based on inorganic composition is higher. As a result, the hole of separator can be blocked, and the impregnation property of separator and electrolyte can be reduced thus, or coating can serve as resistor, and the capacity and life of battery can be reduced thus. Therefore, many studies on the composition of water-based variable adhesive with high adhesion or the variable adhesive that increases adhesion have been carried out.

[0009] In Patent Document 1, a porous coating comprising inorganic particles and an acrylic acid-based copolymer variable binder mixed in a weight ratio of 70:30 to 90:10 is provided to improve the impregnation of the porous coating with an electrolyte, wherein the ratio of the variable binder polymer relative to the inorganic material is regularly adjusted, thereby reducing the resistance while ensuring adhesion to the electrode, and thus improving the life characteristics of the battery.

[0010] Patent Document 2 provides a separate adhesive layer capable of increasing adhesion without including an inorganic material. When the adhesive layer is applied to a cylindrical secondary battery, the performance of the separator itself appears to be improved. However, due to the presence of a portion unrelated to increasing battery capacity or improving battery safety by the inorganic material, the adhesive layer does not significantly contribute to improving the overall performance of the battery.

[0011] Therefore, it is necessary to develop a new adhesive that can reduce the battery resistance while providing only the adhesion required for the separator.

[0012] (Prior art literature)

[0013] (Patent Document 1) Korean Patent Application Publication No. 2017-0095024 (“Patent Document 1”)

[0014] (Patent Document 2) Korean Patent Application Publication No. 2019-0084894 (“Patent Document 2”) Summary of the Invention

[0015] Technical issues

[0016] The present invention has been made in view of the above problems, and an object of the present invention is to provide a separator configured to reduce battery resistance while providing only adhesion required for a separator coating layer, and a method of manufacturing the separator.

[0017] Technical Solution

[0018] To achieve the above object, the present invention provides a separator, comprising: a separator substrate; and a coating on at least one surface of the separator substrate, wherein the coating comprises a variable binder whose size changes with pH.

[0019] The particle size of the variable binder may decrease as the pH decreases.

[0020] The variable adhesive may be a water-based adhesive.

[0021] The variable adhesive may include an ester group (—COO).

[0022] The variable adhesive may be an ester-based water-based adhesive.

[0023] The variable binder may have a cross-linking degree of 80 mol % to 98 mol %.

[0024] The separator substrate may be a polyolefin substrate.

[0025] The coating may include an inorganic material.

[0026] The present invention provides a method for manufacturing the separator, comprising: S1) mixing a variable binder whose size changes with pH and an inorganic material in a solvent to form a coating slurry; and S2) applying the coating slurry to at least one surface of a separator substrate to form a coating layer.

[0027] In step S1 , the coating slurry may have a pH of 5 or greater.

[0028] In step S1 , the solvent may have a pH of 7 or greater.

[0029] In step S1 , the variable adhesive may have a particle size of 200 nm or more.

[0030] The method may further include S3) reducing the particle size of the variable binder of the coating as the pH decreases due to hydrogen ions generated by the reaction of the electrolyte.

[0031] In the present invention, one or more constructions that do not conflict with each other may be selected and combined from the above constructions.

[0032] Beneficial effects

[0033] As apparent from the above description, the separator according to the present invention includes a variable binder whose size changes with pH, ​​whereby the separator resistance is reduced and battery performance is improved.

[0034] In addition, since the size of the variable binder in the coating according to the present invention changes with pH, ​​the coating can be formed using a minimum amount of variable binder. As a result, the density of the inorganic material increases, and thus, the performance of the coating based on its thickness is improved.

[0035] In addition, the life of the battery is increased because the electrical resistance of the coating decreases in proportion to the battery's use. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 is a schematic diagram showing that the shape of the separator according to the present invention changes with pH.

[0037] Figure 2 is a graph showing the change in particle size of a variable binder and a non-variable binder according to the present invention as a function of pH. DETAILED DESCRIPTION

[0038] Now, the preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings so that a person having ordinary skills in the field to which the present invention belongs can easily implement the preferred embodiments of the present invention. When describing the operating principle of the preferred embodiments of the present invention in detail, however, a detailed description of known functions and configurations incorporated herein will be omitted when it may obscure the subject matter of the present invention.

[0039] In addition, the same reference numerals will be used throughout the drawings to refer to parts that perform similar functions or operations. Where a part is referred to as being connected to another part throughout the specification, not only can the part be directly connected to the other part, but the part can also be indirectly connected to the other part via another part. In addition, the inclusion of a particular element does not exclude other elements, but rather means that these elements may be further included unless otherwise mentioned.

[0040] In addition, the descriptions that embody elements by limitation or addition are applicable to all inventions unless otherwise specifically limited, and do not limit a specific invention.

[0041] Furthermore, in the description and claims of the present application, the singular form is intended to include the plural form unless otherwise mentioned.

[0042] In addition, in the invention description and claims of this application, "or" includes "and" unless otherwise stated. Therefore, "including A or B" means three cases, namely, the case including A, the case including B, and the case including A and B.

[0043] Hereinafter, the present invention will be described in more detail.

[0044] A separator according to the present invention includes a separator substrate and a coating layer on at least one surface of the separator substrate, wherein the coating layer includes a variable binder whose size changes with pH.

[0045] separator substrate

[0046] The porous substrate electrically insulates the positive and negative electrodes from each other, thereby preventing short circuits and providing a path for the movement of lithium ions. A porous membrane having high resistance to an electrolyte as an organic solvent and having a very small pore diameter can be used. There is no particular limitation on the porous substrate, as long as the separator substrate can be generally used as a material for a separator for a secondary battery. For example, the separator substrate may include a resin such as a polyolefin-based resin (polyethylene, polypropylene, or polybutylene), polyvinyl chloride, or a mixture or copolymer thereof, or may include a resin such as polyethylene terephthalate, polycycloolefin, polyethersulfone, polyamide, polyimide, polyimide amide, polyaramid, nylon, or polytetrafluoroethylene. Among them, a polyolefin-based resin is preferably used because the applicability of the slurry for the porous coating is higher and the thickness of the separator for the secondary battery is reduced, thereby increasing the percentage of the electrode active material layer in the battery and therefore increasing its capacity per unit volume. More preferably, the separator substrate according to the present invention is a polyolefin, including polyethylene or polypropylene.

[0047] The thickness of the separator substrate may be 1 μm to 100 μm, preferably 1 μm to 30 μm. The pore diameter of the separator substrate may be generally 0.01 μm to 10 μm.

[0048] coating

[0049] Although there is no particular limitation on the thickness of the coating, it is generally preferred that the coating have a sufficient thickness to exhibit a coating-added effect while the capacity of the secondary battery is high. In view of this, the thickness of the coating according to the present invention may be 1 μm to 20 μm. If the coating is thinner than 1 μm, a sufficient amount of variable binder is not included, which is not desirable. If the coating is thicker than 20 μm, the energy density is reduced, which is also not desirable.

[0050] The coating layer may be located on at least one surface of the separator substrate. In order to improve adhesion with the electrode and the effect of adding the inorganic material, it is preferred for the coating layer to be located on opposite surfaces of the separator substrate.

[0051] Inorganic materials

[0052] Inorganic materials may be included in the coating to increase the mechanical strength of the separator. The inorganic materials are not particularly limited as long as they provide a uniform thickness to the coating and do not undergo oxidation and / or reduction within the operating voltage range of the secondary battery to which the present invention is applied. In particular, in the case of using inorganic particles having ion transport ability, the ionic conductivity of the electrochemical device can be improved, thereby improving the performance of the battery. In addition, in the case where inorganic particles having a high dielectric constant are used as the inorganic particles, the dissociation degree of electrolyte salts, such as lithium salts, in the liquid electrolyte can be increased, thereby improving the ionic conductivity of the electrolyte solution.

[0053] Examples of the inorganic material may be an inorganic material having at least one of lithium ion transport ability, piezoelectricity, and flame retardancy.

[0054] An inorganic material having high lithium ion transport ability refers to an inorganic material that contains lithium element but moves lithium ions without storing lithium. Since the inorganic material having lithium ion transport ability can transport and move lithium ions due to a kind of defect present in its particle structure. As a result, the lithium ion conductivity in the battery can be improved, thereby improving the performance of the battery.

[0055] The inorganic material having lithium ion transport ability may be, for example, selected from the group consisting of lithium phosphate, lithium titanium phosphate, lithium aluminum titanium phosphate, (LiAlTiP) x O y -based glass, lithium lanthanum titanate, lithium germanium thiophosphate, lithium nitride such as Li3N (Li x N y , 0 < x < 4, 0 < y < 2), SiS2-based glass such as Li3PO4-Li2S-SiS2 (Li x Si[[ID=二十]] y S z , 0 < x < 3, 0 < y < 2, 0 < z < 4), P2S5-based glass such as LiI-Li2S-P2S5 (Li x P y S z , 0 < x < 3, 0 < y < 3, 0 < z < 7), and mixtures thereof. However, the present invention is not limited thereto.

[0056] An inorganic material having piezoelectricity refers to a material that is a non-conductor under atmospheric pressure, but when a predetermined pressure is applied thereto, due to a change in its internal structure, it has physical properties such as conductivity, is a material having a dielectric constant of 100 or more, that is, a high dielectric constant, and is configured such that when a predetermined pressure is applied thereto to cause it to be tensioned or compressed, one surface thereof becomes positively charged and the other surface becomes negatively charged, thereby generating a potential difference between its opposite surfaces.

[0057] In the case of using an inorganic material having the above characteristics, when an internal short circuit occurs between the two electrodes due to external impact caused by local crush, nails, etc., the positive electrode and the negative electrode are not in direct contact with each other due to the inorganic particles formed on the separator by coating, and due to the piezoelectricity of the inorganic particles, a potential difference is generated in the inorganic particles, whereby electrons move between the positive electrode and the negative electrode, that is, a microcurrent flows therebetween, so that the voltage of the battery slowly decreases, and thus the safety of the battery is improved.

[0058] Inorganic particles having piezoelectricity may be, for example, BaTiO3, Pb(Zr,Ti)O3 (PZT), Pb 1-x La x Zr 1- y Ti y O3(PLZT), Pb(Mg 1 / 3 Nb 2 / 3 )O3-PbTiO3 (PMN-PT), hafnium dioxide (HfO2), or a mixture thereof. However, the present invention is not limited thereto.

[0059] The inorganic material having flame retardancy may be at least one selected from the group consisting of antimony-containing compounds, metal hydroxides or metal hydrates, guanidine compounds, boron-containing compounds, and zinc stannate compounds.

[0060] The antimony-containing compound is selected from antimony trioxide (Sb2O3), antimony tetroxide (Sb2O4), and antimony pentoxide (Sb2O5). The metal hydroxide or metal hydrate is selected from magnesium hydroxide, aluminum hydroxide (Al(OH)3), aluminum oxyhydroxide (AlO(OH)), and CaO·Al2O3·6H2O. The guanidine compound is selected from guanidine nitrate, guanidine sulfamate, guanidine phosphate, and guanidine urea phosphate. The boron-containing compound is H3BO3 or HBO2. The zinc stannate compound is selected from Zn2SnO4, ZnSnO3, and ZnSn(OH)6.

[0061] Specifically, the inorganic material having flame retardancy may be at least one selected from magnesium hydroxide (Mg(OH)2), aluminum hydroxide (Al(OH)3), aluminum oxyhydroxide (AlO(OH)), and CaO·Al2O3·6H2O.

[0062] Adding a flame-retardant inorganic material can prevent overcharging, impart flame retardancy to the separator, or protect the battery from sudden temperature increases. In the flame-retardant inorganic material according to the present invention, the metal hydroxide decomposes as the temperature rises, undergoing an endothermic dehydration reaction. This endothermic reaction and the resulting water provide an additional flame-retardant effect.

[0063] In order to improve the flame retardancy of the inorganic material having flame retardancy, a flame retardant synergist may be further included. The flame retardant synergist may be a silicon-based additive, zinc oxide, tin oxide, a nickel compound, zinc borate, a melamine compound, or a mixture of two or more thereof.

[0064] Flame retardant synergists can be used in various combinations depending on the characteristics of the flame retardant inorganic material. As a material capable of improving the efficiency of the flame retardant inorganic material, flame retardant synergists other than the above-mentioned flame retardant synergists may be further included. In addition, halogens may be further added, or phosphorus or phosphorus compounds may be added. The flame retardant synergist may be provided to an extent that the efficiency of the flame retardant inorganic material can be improved. The flame retardant synergist may be added in an amount of 0.001 to 0.1 times the total weight of the flame retardant inorganic material.

[0065] In addition, the absorbent is configured to absorb water molecules generated by the flame-retardant inorganic material; however, the absorbent is limited to materials that do not degrade battery performance. Any common material can be used as the absorbent without limitation. For example, zeolite, porous silica, or porous alumina can be used. However, the present invention is not limited thereto.

[0066] Although the particle size of the inorganic material is not particularly limited, D50 may have a range of 20 nm to 10 μm, specifically, 100 nm to 1 μm, in consideration of the purpose of forming a coating layer having a uniform thickness and providing appropriate porosity.

[0067] In the particle size distribution curve of particles, D50 means the particle size of particles equivalent to 50% of the cumulative number of particles, and the average particle size of the inorganic particles is measured using a Particle Size Analyzer (product name: MASTERSIZER 3000; manufacturer: Malvern).

[0068] The inorganic material may be included in an amount of 10 to 90% by weight based on the weight of the total solid content of the coating. If the content of the inorganic material is less than 10% by weight, it is difficult to achieve the effects that can be obtained as a result of adding the inorganic material, which is not desirable. If the content of the inorganic material is greater than 90% by weight, the content of the binder is too low, and thus the adhesion between the inorganic particles may be reduced, so that the inorganic coating may separate from the separator substrate or an uncoated area may be generated during coating, which is also not desirable.

[0069] Variable adhesive

[0070] The coating according to the present invention may include a variable binder to prevent the inorganic material from separating from the coating. The inorganic material and the variable binder may be evenly distributed in the coating. Since the inorganic material is evenly distributed in the variable binder, the resistance in the secondary battery can be uniformly formed, thereby preventing short circuits at specific portions.

[0071] The variable adhesive can turn into a gel when impregnated with an electrolyte, thereby exhibiting high electrolyte impregnation. In the case where the variable adhesive polymer is a polymer with high electrolyte impregnation, the polymer can be impregnated with an electrolyte injected after assembling the battery, and the polymer with the electrolyte adsorbed therein exhibits electrolyte ion conductivity. In addition, compared with conventional hydrophobic polyolefin-based separators, the wettability to the electrolyte used for the battery is improved, and polar electrolytes for batteries that are conventionally difficult to use are also feasible. As a result, for the gel content of the variable adhesive, that is, the solubility of the variable adhesive in the solvent, greater than 60% and less than 100% is preferred. If the gel content of the variable adhesive is 60% or less, when the electrode is heated and dried, voltage fluctuations occur due to the flow of the polymer, and it cannot be used. If the gel content of the variable adhesive is 100% or more, the performance of the variable adhesive may be insufficient, and thus the bonding strength may be reduced. If possible, for the variable adhesive, having 15MPa 1 / 2 to 45MPa 1 / 2 A polymer with a solubility index of 15 MPa is preferred, more preferably 15 MPa 1 / 2 to 25MPa 1 / 2 and 30MPa 1 / 2 to 45MPa 1 / 2 If the solubility index is less than 15MPa 1 / 2 and greater than 45MPa 1 / 2 , it is difficult to achieve swelling with common liquid electrolytes used in batteries.

[0072] The variable binder may be included to account for 10 wt % or greater, specifically 50 wt % or greater, more specifically 60 wt % or greater, based on the weight of the total solid content of the coating.

[0073] Specifically, there are no restrictions on the type of variable adhesive, as long as the variable adhesive has a particle size that changes with pH and does not affect battery performance. For example, a water-based variable adhesive is preferred, considering the risks to human health, the impact on the environment, and the ease of concentration adjustment. Furthermore, the variable adhesive may include an ester group for easy deformation. In the case of an ester group, the variable adhesive may have a property such that its size changes with pH.

[0074] As an example, the variable adhesive may be configured to have a structure in which an ester group is introduced into the aqueous adhesive. The aqueous adhesive may be at least one selected from the group consisting of a polyacrylate-styrene-based polymer, a polyacrylonitrile-butadiene-based polymer, a polystyrene-butadiene-based polymer, a polyacrylonitrile-styrene-based polymer, a polyacrylic acid-based polymer, hydroxyethyl cellulose, carboxymethyl cellulose, and a polyacrylate-based polymer.

[0075] In addition, the crosslinking degree of variable adhesive can be 80mol% to 98mol%.In order to increase the crosslinking degree of variable adhesive, a crosslinking accelerator can be added to the coating slurry. Based on 100 weight portions of variable adhesive, a crosslinking accelerator can be added to account for 0.1 weight portion to 50 weight portions. Each molecule has a primary or higher polyamine (polyamine) of various functional groups, such as polyisocyanate (polyisocyanate), diethylenetriamine (DETA, diethylenetriamine), triethylenediamine (TEDA, triethylenediamine) or triethylenetetramine (TETA, triethylenetetramine), which can be used as a crosslinking accelerator. However, the present invention is not limited thereto.

[0076] The particle size of the variable adhesive when first added to the slurry can be 200nm or larger. If the particle size of the variable adhesive is less than 200nm, the amount of the variable adhesive added to the slurry increases, and thus the desired effect may not be obtained. Subsequently, even if the size of the variable adhesive changes with pH, ​​the variable adhesive thus has a minimum size, the particle size of the variable adhesive can be 110nm or larger. If the particle size of the variable adhesive is less than 110nm, the size of the variable adhesive is too small, and thus the adhesion to the inorganic particles may be reduced.

[0077] If the particle size change rate is 50% or greater, the particle size is too small, and the battery resistance may increase. For this reason, a variable binder particle size change rate of less than 50% is preferred. If the variable binder particle size change rate is too large, the coating layer may separate from the separator substrate, or the adhesion therebetween may decrease. As a result, a variable binder particle size change rate of 20% to 45% based on the variable binder particle size at pH 7 when the pH decreases or increases is preferred.

[0078] The size of the variable adhesive can be adjusted by adjusting the content of the polyacrylic acid-based dispersant or the tannic acid-based dispersant.

[0079] The variable binder may be included to account for less than 40% by weight based on the solid content of the coating slurry. If there is too much variable binder, the variable binder may act as a resistor even if the size of the variable binder is reduced. More specifically, the binder with a reduced size may be attached to an undesirable area, thereby potentially reducing battery performance.

[0080] In the separator according to the present invention, the particle size of the variable binder may decrease as the pH decreases.

[0081] Figure 1 Schematic diagram showing that the shape of the separator according to the present invention changes with pH. Figure 1 , Figure 1 The upper portion is a schematic diagram before the size of the variable adhesive 220 is reduced. Figure 1 The lower portion of FIG. 1 is a schematic diagram after the size of the variable adhesive 220 is reduced.

[0082] from Figure 1 As can be seen in FIG, the separator according to the present invention is configured such that a coating layer 200 is added to the separator substrate 100. The coating layer 200 includes an inorganic material 210 and a variable adhesive 220. The variable adhesive 220 is disposed between the inorganic material particles 210 to prevent the inorganic material 210 from separating from the coating layer 200. The variable adhesive 220 can be uniformly disposed between the inorganic material particles 210. As the pH decreases, the size of the variable adhesive 220 decreases, thereby reducing the space between the inorganic material particles 210 and increasing the bonding force between the inorganic material particles 210. In addition, the space occupied by the inorganic material 210 and the variable adhesive 220 in the coating layer 200 decreases, thereby improving air permeability while reducing resistance.

[0083] dispersants

[0084] The coating may further include a dispersant to further improve the dispersibility of the inorganic material. The dispersant is used to keep the inorganic material uniformly dispersed in the variable binder when making the coating slurry. For example, an anionic surfactant can be used to maintain uniform dispersion while increasing dispersibility.

[0085] The anionic component including at least one selected from the group consisting of carboxylate, phosphate, sulfonate, and sulfate may constitute the head of the anionic surfactant, wherein sulfonate is preferably used as the head of the anionic surfactant.

[0086] A material having nonionic surfactant properties can be used as the tail of the anionic surfactant. Although the material having nonionic surfactant properties is not particularly limited, a material containing an alkyl group can be used. The material containing an alkyl group can be a polyoxyalkylene having 5 to 200 oxyalkylene repeating units. In this case, the polyoxyalkylene can be at least one selected from the group consisting of polyethylene oxide, polypropylene oxide, and polyethylene oxide-polypropylene oxide copolymers.

[0087] A typical example of anionic surfactant is carboxyl methylcellulose (CMC).

[0088] Besides, at least one selected from the group consisting of oil-soluble polyamines, oil-soluble amine compounds, fatty acids, fatty alcohols, sorbitan fatty acid esters, tannic acid, and pyrogallic acid may be used as the dispersant.

[0089] Based on 100 parts by weight of the inorganic material, the content of the dispersant may be 0.2 parts by weight to 10 parts by weight. If the dispersant is included so as to be less than 0.2 parts by weight based on 100 parts by weight of the inorganic material, the inorganic material may be easily precipitated. If the dispersant is included so as to account for more than 10 parts by weight based on 100 parts by weight of the inorganic material, the adhesion of the coating to the separator substrate may be reduced or impurities may be generated as a result of a reaction with the electrolyte when manufacturing a secondary battery.

[0090] The unit cell according to the present invention includes a positive electrode, a negative electrode, and a separator provided between the positive electrode and the negative electrode, wherein at least one of the above-mentioned separators can be used as the separator according to the present invention.

[0091] positive electrode

[0092] For example, the positive electrode can be manufactured by applying a positive electrode mixture of a positive electrode active material composed of positive electrode active material particles, a conductive agent, and a binder to a positive electrode current collector. If necessary, a filler may be further added to the positive electrode mixture.

[0093] In general, the positive electrode collector is manufactured to have a thickness of 3 μm to 500 μm. There is no particular limitation on the positive electrode collector, as long as the positive electrode collector exhibits high conductivity and does not induce any chemical changes in the battery to which the positive electrode collector is applied. For example, the positive electrode collector may be made of stainless steel, aluminum, nickel, or titanium. Alternatively, the positive electrode collector may be made of aluminum or stainless steel whose surface is treated with carbon, nickel, titanium, or silver. Specifically, aluminum can be used. The positive electrode collector may have a microscale uneven pattern formed on its surface to increase the adhesion of the positive active material. The collector can be configured in various forms such as a film, a sheet, a foil, a mesh, a porous body, a foam, and a non-woven object.

[0094] In addition to the positive electrode active material particles, the positive electrode active material may be composed of, for example, a layered compound such as lithium nickel oxide (LiNiO2) or a compound substituted with one or more transition metals; 1+x Mn 2-x Lithium manganese oxide represented by O4 (wherein x=0 to 0.33), or lithium manganese oxide such as LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxide such as LiV3O8, V2O5, or Cu2V2O7; 1- x Ni-site lithium nickel oxide represented by MxO2 (wherein M = Co, Mn, Al, Cu, Fe, Mg, B, or Ga, x = 0.01 to 0.3); 2-x M x Lithium manganese composite oxides represented by the chemical formula LiMnMO (wherein M=Co, Ni, Fe, Cr, Zn, or Ta, and x=0.01 to 0.1) or LiMnO (wherein M=Fe, Co, Ni, Cu, or Zn); LiMnO in which a portion of the Li in the chemical formula is substituted with an alkaline earth metal ion; disulfide compounds; Fe(MoO). However, the present invention is not limited thereto.

[0095] The conductive agent is generally added so that the conductive agent occupies 0.1 wt % to 30 wt % based on the total weight of the mixture including the positive electrode active material. The conductive agent is not particularly limited as long as the conductive agent has high conductivity without inducing any chemical changes in the battery to which the conductive agent is applied. For example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermal black; conductive fibers such as carbon fibers or metal fibers; metal powders such as carbon fluoride powder, aluminum powder, or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive materials such as polyphenylene derivatives can be used as the conductive agent.

[0096] The binder included in the positive electrode is a component that assists the bonding between the active material and the conductive agent and the bonding with the current collector. The binder is generally added in an amount of 0.1% to 30% by weight based on the total weight of the mixture including the positive electrode active material. As examples of binders, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), styrene-butadiene rubber, fluororubber, and various copolymers can be used.

[0097] <Negative electrode>

[0098] The negative electrode can be manufactured by applying the negative electrode active material to the negative electrode current collector and drying it. If necessary, the above components can be selectively further included.

[0099] The negative electrode collector is generally manufactured to have a thickness of 3 μm to 500 μm. There is no particular limitation on the negative electrode collector, as long as the negative electrode collector exhibits high conductivity and does not induce any chemical changes in the battery to which the negative electrode collector is applied. For example, the negative electrode collector may be made of copper, stainless steel, aluminum, nickel, titanium, or calcined carbon. Alternatively, the negative electrode collector may be made of copper or stainless steel, or an aluminum-cadmium alloy whose surface is treated with carbon, nickel, titanium, or silver. In addition, in the same manner as the positive electrode collector, the negative electrode collector may have a microscale uneven pattern formed on its surface to increase the binding force of the negative electrode active material layer. The negative electrode collector can be configured in various forms such as a film, a sheet, a foil, a mesh, a porous body, a foam, and a non-woven object.

[0100] As the negative electrode active material, for example, carbon such as non-graphitized carbon and graphite-based carbon; x Fe2O3(0≤x≤1), Li x WO2(0≤x≤1), Sn x Me1-x Me' y O z (Me: Mn, Fe, Pb, Ge; Me': Al, B, P, Si, elements of Group 1, Group 2 or Group 3 in the periodic table, halogen; 0<x≤1; 1≤y≤3; 1≤z≤8) or the like; lithium metal; lithium alloy; silicon-based alloy; tin-based alloy; metal oxides such as SnO, SnO2, PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, and Bi2O5; conductive polymers such as polyacetylene; or Li-Co-Ni based materials.

[0101] Separator manufacturing method

[0102] The separator manufacturing method according to the present invention may include: S1) mixing a variable binder whose size changes with pH with an inorganic material to form a coating slurry; and S2) applying the coating slurry to at least one surface of a separator substrate to form a coating layer.

[0103] The separator substrate, the inorganic material particles, and the variable binder have been described above.

[0104] For the step of applying the coating slurry to at least one surface of the separator substrate, a method of impregnating the separator substrate with a slurry prepared by adding a coating composition including an inorganic material and a variable binder to a solvent or applying the slurry to the separator substrate can be used. Any common coating method known in the art to which the present invention belongs can be used as the application or coating method. For example, dip coating, die coating, roll coating, comma coating, or a combination thereof can be used.

[0105] A drying step may be further performed after coating. The drying step may be performed using an oven or a heated chamber within a temperature range set in consideration of the vapor pressure of the solvent, or the separator substrate with the coating formed thereon may be exposed to room temperature to volatilize the solvent. In this case, conditions such as a temperature range of 25°C to 100°C and a relative humidity of 40% or greater may be considered.

[0106] Any common solvent known in the art to which the present invention pertains may be used without restriction as the solvent required for manufacturing the partition according to the present invention. Preferably, acetone, tetrahydrofuran, acetonitrile, dimethylformamide, dimethyl sulfoxide, dimethylacetamide, N-methylpyrrole, or water are used. A mixture of two or more thereof may also be used. At this time, for the pH of the slurry, it is preferred to maintain uniformity so that the variable adhesive has a predetermined size before being added to the coating. As an example, in the case where the size of the variable adhesive according to the present invention decreases with pH, ​​it is preferred to use a solvent with a pH of 7 so that the size of the variable adhesive remains at a maximum. In addition, for the pH of the slurry, it is preferred to maintain uniformity so that the size of the variable adhesive included in step S1 remains uniform. For this reason, 7 or greater is preferred for the pH of the slurry. In addition, a polyacrylic acid-based dispersant or a tannic acid-based dispersant may be added to adjust the pH of the slurry. A polyacrylic acid-based dispersant or a tannic acid-based dispersant may be used to maintain the pH of the slurry uniform.

[0107] The variable binder particle size used in step S1 is preferably 200 nm or larger. If the variable binder particle size is small, it is difficult to provide a predetermined amount of variable binder between the inorganic material particles. Consequently, it is advantageous to initially have large variable binder particles. Subsequently, it is preferred that the variable binder size decreases with changes in pH, thereby improving electrolyte impregnation while maintaining only the necessary adhesion between the inorganic material particles.

[0108] In addition, the separator manufacturing method may further include a step (S3) of causing the variable binder in the coating to react with hydrogen ions generated by the reaction of the electrolyte, thereby reducing the particle size of the variable binder. As a result of the reaction, the electrolyte in the secondary battery generates hydrogen ions, thereby reducing the pH of the secondary battery. In this case, the hydrogen ions further reduce the particle size of the variable binder, thereby offsetting the reduction in battery performance caused by the use of the battery.

[0109] The variable binder may be included to account for less than 40% by weight based on the solid content of the coating slurry. If there is too much variable binder, the variable binder may act as a resistor even if the size of the variable binder is reduced. More specifically, the binder with a reduced size may be attached to an undesirable area, thereby potentially reducing battery performance.

[0110] Hereinafter, the present invention will be described with reference to the following examples. These examples are provided only to make it easier to understand the present invention and should not be construed as limiting the scope of the present invention.

[0111] <Experimental Example 1> Comparison of particle size depending on pH

[0112] <Variable Adhesive According to the Present Invention (Example)>

[0113] Methyl methacrylate butadiene styrene copolymer (styrene butadiene-co-MMA) having a weight average molecular weight (Mw) of 300,000 to 400,000 and a particle size of 230 nm was used.

[0114] <Non-variable adhesive (comparative example)>

[0115] Styrene butadiene having a weight average molecular weight (Mw) of 300,000 to 400,000 and a particle size of 230 nm was used.

[0116] Figure 2 is a graph showing the change in particle size of a variable binder and a non-variable binder according to the present invention as a function of pH.

[0117] Methyl methacrylate butadiene styrene copolymer (styrene butadiene-co-MMA) (Example) and styrene butadiene (Comparative Example) were placed in water, and their particle sizes at pH 3, 4, 5, 6, and 7 were measured using a particle size measuring instrument Mastersizer 300 at a refractive index of 1.46 and 200 rpm.

[0118] from Figure 2 As can be seen from the graph, the particle size of the binder according to the present invention decreases significantly as the pH decreases, while the particle size of styrene butadiene as a conventional binder does not change.

[0119] <Experimental Example 2> Measurement of changes in the physical properties of adhesive-specific separators

[0120] The air permeability (Gurley) and electrical resistance of each separator according to the following Examples 1 to 3 and Comparative Examples 1 to 3, and the resistance value of the unit cells using these separators were measured. The results are shown in Table 1 below.

[0121] <Example 1>

[0122] One surface of a polyolefin substrate was impregnated with a coating slurry prepared by mixing distilled water, methyl methacrylate butadiene styrene copolymer (styrene butadiene-co-MMA), and aluminum hydroxide with each other by bar coating, and dried at 80 to 90° C. to manufacture a separator.

[0123] At this time, methyl methacrylate butadiene styrene copolymer (styrenebutadiene-co-MMA) has a weight average molecular weight (Mw) of 300,000 to 400,000, a gel content of 98%, a pH of 5, and a particle size of 230 nm, and aluminum hydroxide has a D50 value of 800 nm to 1,000 nm.

[0124] In addition, aluminum hydroxide and methyl methacrylate butadiene styrene copolymer (styrene butadiene-co-MMA) were mixed at a weight ratio of 40 to 60 wt % to prepare the slurry. The final slurry had a solid content of 30 wt % when applied.

[0125] <Example 2>

[0126] A separator was manufactured in the same procedure as in Example 1, except that, compared with Example 1, methyl methacrylate butadiene styrene copolymer (styrene butadiene-co-MMA) had a pH of 7 and a particle size of 273 nm.

[0127] <Example 3>

[0128] A separator was manufactured by the same procedure as in Example 1, except that, compared with Example 1, methyl methacrylate butadiene styrene copolymer (styrene butadiene-co-MMA) had a gel content of 80%.

[0129] <Comparative Example 1>

[0130] In Comparative Example 1, a separator was manufactured in the same procedure as in Example 1, except that, compared with Example 1, methyl methacrylate butadiene styrene copolymer (styrene butadiene-co-MMA) had a pH of 3 and a particle size of 110 nm.

[0131] <Comparative Example 2>

[0132] In Comparative Example 2, a separator was manufactured in the same procedure as in Example 1, except that, compared with Example 1, methyl methacrylate butadiene styrene copolymer (styrene butadiene-co-MMA) had a gel content of 60%.

[0133] <Comparative Example 3>

[0134] In Comparative Example 3, a separator was manufactured in the same procedure as in Example 1, except that styrene butadiene was used instead of methyl methacrylate butadiene styrene copolymer (styrene butadiene-co-MMA) and the pH of the styrene butadiene was 3.

[0135] <Evaluation of Air Permeability>

[0136] Air permeability (Gurley) was measured according to ASTM D726-94. Gurley, used herein as air flow resistance, was measured using a Gurley densometer. The air permeability values ​​described herein are shown as 100cc of air passing through 1 in of each separator manufactured according to the comparative examples and the examples at a pressure of 12.2 in of water column. 2 The time (seconds) required for the cross section to be filled, that is, the air penetration time.

[0137] <Measurement of Separator Resistance Value>

[0138] The separators manufactured according to Examples 1 to 3 and Comparative Examples 1 to 3 were impregnated with a carbonate-based electrolyte having a pH of 4 to 5, and the AC resistance of each separator was measured. The results are shown in Table 1 below. In this case, the AC resistance is the resistance value of the separator measured at 1 kHz using a Hioki tester.

[0139] <Measurement of Unit Cell Resistance>

[0140] A coin cell was manufactured using a positive electrode, a negative electrode, and each separator according to Examples 1 to 3 and Comparative Examples 1 to 3. The positive electrode was manufactured by forming a positive electrode active material layer including 90 weight % of lithium cobalt oxide as a positive electrode active material, 6 weight % of PVdF as a binder, and 4 weight % of carbon black as a counter electrode, and having a thickness of 60 μm on an aluminum foil as a positive electrode current collector. The negative electrode was manufactured by forming a negative electrode active material layer including 94 weight % of graphite as a negative electrode active material, 2 weight % of CMC and 2 weight % of styrene butadiene rubber as a binder, and 2 weight % of carbon black as a conductive agent, and having a thickness of 70 μm on a copper foil as a negative electrode current collector. The resistance value of the coin cell was measured using Solartron analytical EIS under the conditions of a frequency of 300,000 to 0.1 Hz and an AC amplitude of 10 mA.

[0141] (Table 1)

[0142]

[0143] As can be seen from Table 1 above, as the particle size of the binder decreases and the gel content of the binder decreases, the air permeability of the binder increases, the resistance value of the separator increases, and the resistance value of the unit cell increases. In addition, it can be seen that under the same conditions, the air permeability and resistance value of the binder including an ester group, as well as the resistance value of the unit cell including the binder, are lower than those of the binder not including an ester group, as well as the resistance value of the unit cell including the binder.

[0144] As a result, the use of variable adhesives to improve battery performance while providing the desired adhesion is preferred.

[0145] Those skilled in the art in the art to which the present invention pertains will appreciate that, based on the above description, various applications and modifications are possible within the scope of the present invention.

[0146] [reference numerals]

[0147] 100: Partition substrate

[0148] 200: coating

[0149] 210: Inorganic materials

[0150] 220: Variable Adhesive

[0151] Industrial Applicability

[0152] As apparent from the above description, the separator according to the present invention includes a variable binder whose size changes with pH, ​​whereby the separator resistance is reduced and battery performance is improved.

[0153] In addition, since the size of the variable binder in the coating according to the present invention changes with pH, ​​the coating can be formed using a minimum amount of variable binder. As a result, the density of the inorganic material increases, and thus, the performance of the coating based on its thickness is improved.

[0154] In addition, the life of the battery is increased because the electrical resistance of the coating decreases in proportion to the battery's use.

Claims

1. A separator, comprising: separator substrate; and a coating on at least one surface of the separator substrate, wherein The coating comprises a variable binder whose dimensions change with pH, wherein the gel content of the variable adhesive is greater than 60% and less than 100%, wherein the variable adhesive is configured to have a structure in which an ester group is introduced into a water-based adhesive, the water-based adhesive being at least one selected from the group consisting of acrylate-styrene-based polymers, acrylonitrile-butadiene-based polymers, styrene-butadiene-based polymers, polyacrylonitrile-styrene-based polymers, hydroxyethyl cellulose, and carboxymethyl cellulose. 2 . The separator according to claim 1 , wherein the particle size of the variable binder decreases as the pH decreases. 3 . The separator according to claim 1 , wherein the variable binder has a crosslinking degree of 80 mol % to 98 mol %. The separator according to claim 1 , wherein the separator substrate is a polyolefin substrate. The separator of claim 1 , wherein the coating layer comprises an inorganic material.

6. A method for manufacturing the separator according to any one of claims 1 to 5, the method comprising: S1) mixing a variable binder whose size changes with pH with an inorganic material in a solvent to form a coating slurry; and S2) applying the coating slurry to at least one surface of a separator substrate to form a coating layer, wherein the gel content of the variable adhesive is greater than 60% and less than 100%, wherein the variable adhesive is configured to have a structure in which an ester group is introduced into a water-based adhesive, the water-based adhesive being at least one selected from the group consisting of acrylate-styrene-based polymers, acrylonitrile-butadiene-based polymers, styrene-butadiene-based polymers, polyacrylonitrile-styrene-based polymers, hydroxyethyl cellulose, and carboxymethyl cellulose.

7. The method according to claim 6, wherein: In step S1 , the coating slurry has a pH of 5 or greater.

8. The method according to claim 6, wherein: In step S1 , the solvent has a pH of 7 or greater.

9. The method according to claim 6, wherein: In step S1 , the variable adhesive has a particle size of 200 nm or more.

10. The method according to claim 6, further comprising S3) reducing the particle size of the variable binder of the coating as the pH decreases due to hydrogen ions generated by the reaction of the electrolyte.

Images