Separation membrane for electrochemical elements and electrochemical elements equipped therewith

Abstract

The present invention relates to a separator for an electrochemical device, which comprises a porous polymer substrate and an organic / inorganic composite porous coating layer formed on at least one surface of the polymer substrate, the porous coating layer comprising a particulate binder resin and inorganic particles, and wherein a ratio of [binder particle content at an upper part of the porous coating layer before impregnation with an electrolyte solution / binder particle content at a central part of the porous coating layer before impregnation with an electrolyte solution]-[binder particle content at an upper part of the porous coating layer after impregnation with an electrolyte solution / binder particle content at a central part of the porous coating layer after impregnation with an electrolyte solution] is 0.32 or less, and an electrochemical device including the separator.

Description

[Technical Field]

[0001] This invention claims the benefits as of the filing date of Patent Application No. 10-2022-0107939, filed with the Korean Intellectual Property Office on August 26, 2022, and all of its contents are included in this invention.

[0002] The present invention relates to a separation membrane for an electrochemical element and an electrochemical element equipped therewith, and more specifically, to a separation membrane for an electrochemical element that maintains adhesion to electrodes even when impregnated with an electrolyte, and an electrochemical element equipped therewith. [Background technology]

[0003] Among the components of an electrochemical element, the separation membrane is a porous polymer substrate located between the positive and negative electrodes. Its role is to isolate the positive and negative electrodes, prevent electrical short circuits between the two electrodes, and allow electrolytes and ions to pass through. Although the separation membrane itself does not participate in the electrochemical reaction, its physical properties, such as wettability to the electrolyte, degree of porosity, and thermal shrinkage rate, affect the performance and safety of the electrochemical element.

[0004] As a result, various methods have been attempted to enhance the physical properties of the separation membrane by adding a coating layer to a porous polymer substrate and then adding various substances to the coating layer to change its properties. For example, inorganic substances may be added to the coating layer to improve the mechanical strength of the separation membrane, or inorganic substances or hydrates may be added to the coating layer to improve the flame retardancy and heat resistance of the polymer substrate.

[0005] The separation membrane can be bonded to the electrode via a lamination process, and a binder resin can be added to the coating layer composition of the separation membrane to ensure adhesion between the electrode and the separation membrane.

[0006] On one hand, when the polymer component contained in the organic / inorganic composite porous coating layer coated on the porous polymer substrate is swollen or dissolved by the impregnated electrolyte solution, the adhesion of the porous coating layer to the polymer substrate decreases, and the function of suppressing the thermal shrinkage of the separation membrane may deteriorate. Also, the adhesion of the separation membrane to the electrode becomes weak, and problems may occur from the perspective of the quality of the battery cell, such as separation membrane breakage and poor appearance of the battery cell.

Summary of the Invention

Problems to be Solved by the Invention

[0007] The technical problem to be solved by the present invention is to provide a separation membrane for an electrochemical device including an organic / inorganic composite porous coating layer capable of maintaining the adhesion between the electrode and the separation membrane even in a state where the electrolyte solution is injected.

[0008] Also, the technical problem to be solved by the present invention is to provide an electrochemical device including the above-described separation membrane.

[0009] However, the problems to be solved by the present invention are not limited to the above-described problems, and other problems not mentioned will be clearly understood by those skilled in the art from the following description.

Means for Solving the Problems

[0010] According to an embodiment of the present invention for achieving the above-described problems, there is provided a separation membrane for an electrochemical device including a porous polymer substrate and an organic / inorganic composite porous coating layer formed on at least one surface of the polymer substrate, the porous coating layer including particulate binder resin and inorganic particles, and ([content of binder particles in the upper part of the porous coating layer before electrolyte impregnation / content of binder particles in the central part of the porous coating layer before electrolyte impregnation]) - ([content of binder particles in the upper part of the porous coating layer after electrolyte impregnation / content of binder particles in the central part of the porous coating layer after electrolyte impregnation]) is 0.32 or less.

[0011] The particulate binder resin can be a mixture of acrylic polymer binder particles and vinylidene fluoride polymer binder particles.

[0012] In this case, the weight ratio of the acrylic polymer binder particles to the vinylidene fluoride polymer binder particles can be 7:3 to 3:7.

[0013] Furthermore, the acrylic polymer binder particles can be a first copolymer binder particle containing styrene and acrylate in a weight ratio of 5:5 to 7:3.

[0014] In this case, the average particle size of the first copolymer binder particles can be between 350 nm and 700 nm.

[0015] Furthermore, the vinylidene fluoride polymer binder particles can be second copolymer binder particles in which vinylidene fluoride polymer particles containing 10% by weight or less of hexafluoropropylene (HFP) and acrylate are crosslinked in a weight ratio of 5:5 to 9:1.

[0016] In this case, the average particle size of the second copolymer binder particles can be between 150 nm and 250 nm.

[0017] Furthermore, the particulate binder resin may further contain a third copolymer polymer particle containing methyl methacrylate and 2-ethylhexyl methacrylate in a weight ratio of 2:8 to 5:5.

[0018] In this case, based on the total content of the particulate binder resin, the acrylic polymer binder particles may be 4 to 95% by weight, the vinylidene fluoride polymer binder particles may be 4 to 95% by weight, and the third copolymer polymer particles may be 1% to 10% by weight.

[0019] On the other hand, according to another embodiment of the present invention, an electrochemical element is provided which comprises a positive electrode, a negative electrode, and a separation membrane interposed between the positive electrode and the negative electrode, wherein the separation membrane is the separation membrane for electrochemical elements described above. [Effects of the Invention]

[0020] According to one embodiment of the present invention, by adjusting the difference between the ratio of binder particle content in the upper and central parts of the porous coating layer before electrolyte impregnation and the ratio of binder particle content in the upper and central parts of the porous coating layer after electrolyte impregnation to be small, the adhesion between the separation membrane and the electrode can be maintained even during electrolyte impregnation.

[0021] Furthermore, even when the separation membrane is impregnated with the electrolyte, the particulate polymer binder, which exhibits less swelling, is maintained in a higher concentration above the center of the porous coating layer. Therefore, the adhesion between the separation membrane and the electrode can be maintained even during electrolyte impregnation.

[0022] Therefore, it is possible to prevent a decrease in the performance of the electrochemical element. [Brief explanation of the drawing]

[0023] [Figure 1] Figure 1(A) shows the distribution of the binder before electrolyte impregnation according to the thickness of the porous coating layer of the separation membrane according to one embodiment of the present invention, and Figure 1(B) shows the distribution of the binder after electrolyte impregnation according to the thickness of the porous coating layer of the separation membrane according to one embodiment of the present invention. [Figure 2] Figure 2(A) shows the distribution of the binder before electrolyte impregnation according to the thickness of the porous coating layer of a separation membrane according to one comparative example of the present invention, and Figure 2(B) shows the distribution of the binder after electrolyte impregnation according to the thickness of the porous coating layer of a separation membrane according to one comparative example of the present invention. [Modes for carrying out the invention]

[0024] Embodiments of the present invention will be described in detail below.

[0025] The embodiments of the present invention described below are provided to provide a clearer explanation of the invention to those who have ordinary skill in the art, and the scope of the invention is not limited to the embodiments described below, and the embodiments described below can be modified into various other forms.

[0026] The terms used herein are used to describe specific embodiments and are not intended to limit the invention. A singular term used herein may include multiple forms unless explicitly stated otherwise. Furthermore, the terms “comprise” and / or “comprising” used herein specify the presence of a particular shape, step, figure, action, member, element, and / or group thereof, but do not exclude the presence or addition of one or more other shapes, steps, figures, actions, members, elements, and / or groups thereof. Additionally, the term “connected” used herein encompasses not only the direct connection of members but also indirect connection through the interposition of other members.

[0027] Furthermore, when a component is described as being "on top of" another component in this specification, this includes not only cases where one component is in contact with another component, but also cases where there are other components between the two components. The terms "and / or" as used herein include any one or all combinations of the items listed. In addition, terms of degree such as "approximately" and "substantially" as used herein are used to mean a range of numerical values ​​or degrees or close to them, in light of inherent manufacturing and material tolerances, and are used to prevent infringers from unfairly exploiting disclosures that refer to precise or absolute numerical values ​​provided to aid the understanding of this application.

[0028] A separation membrane for an electrochemical element according to one aspect of the present invention comprises a porous polymer substrate and an organic / inorganic composite porous coating layer formed on at least one surface of the polymer substrate, wherein the porous coating layer contains particulate binder resin and inorganic particles, and is characterized in that the ratio of [content of binder particles in the upper part of the porous coating layer before electrolyte impregnation / content of binder particles in the central part of the porous coating layer before electrolyte impregnation] - [content of binder particles in the upper part of the porous coating layer after electrolyte impregnation / content of binder particles in the central part of the porous coating layer after electrolyte impregnation] (hereinafter referred to as formula A) is 0.32 or less.

[0029] Here, the upper part of the porous coating layer refers to the portion from the surface of the porous coating layer to 1 / 3 of the total thickness of the porous coating layer, the central part of the porous coating layer refers to the portion from the lower surface of the upper part of the porous coating layer to 1 / 3 of the total thickness of the porous coating layer, and the lower part of the porous coating layer refers to the portion excluding the upper part and the central part.

[0030] In this specification, the thickness of the polymer substrate and / or the porous coating layer can be measured using a contact thickness gauge. For example, the VL-50S-B from Mitutoyo can be used as the contact thickness gauge.

[0031] For excellent adhesion between the electrode and the porous coating layer, it is advantageous for the binder resin content to be relatively higher in the upper part of the porous coating layer than in the central part. This condition satisfies the following equation.

[0032] [Binder particle content in the upper part of the porous coating layer / Binder particle content in the central part of the porous coating layer] (hereinafter referred to as formula B) > 1.0

[0033] Furthermore, in order to prevent problems such as a decrease in battery performance, the adhesive force between the electrodes and the porous coating layer must be maintained even when the electrolyte is injected. That is, even when the electrolyte is injected, the content of the binder resin located in the upper part of the porous coating layer must be greater than that in the central part, and in this case as well, the aforementioned formula B must be satisfied.

[0034] On the other hand, generally, after electrolyte injection (i.e., in the wet state), the binder components within the porous coating layer swell or dissolve due to the injected electrolyte, resulting in a decrease in the adhesion strength of the porous coating layer to the electrode compared to before electrolyte injection (in the dry state). This is thought to be because the binder resin that was present on the upper part of the porous coating layer migrates to the central part of the porous coating layer due to electrolyte impregnation, and the relative content of binder resin on the upper part of the porous coating layer decreases.

[0035] However, even in the wet state, in order to minimize the decrease in adhesion strength of the porous coating layer to the electrode, it is preferable that the ratio of binder particles in the upper part of the porous coating layer to the binder particles in the central part is maintained at a similar level to that in the dry state, and as mentioned above, it is preferable that formula A is 0.32 or less. If formula A exceeds 0.32, the adhesion strength of the porous coating layer to the electrode may be completely lost in the wet state.

[0036] On the other hand, the polymer substrate according to the present invention contains pores that electrically insulate the negative and positive electrodes to prevent short circuits while allowing lithium ions to pass through, and has high resistance to the electrolyte, which is an organic solvent, and a porous membrane with fine pore diameter can be used. The polymer substrate can be used without special limitations as long as it is normally usable as a separation membrane material for secondary batteries, and can include resins such as polyethylene, polypropylene, polyolefins including polybutene, polyvinyl chloride, polyethylene terephthalate, polycycloolefin, polyethersulfone, polyamide, polyimide, polyimidamide, polyaramid, nylon, polytetrafluoroethylene, and mixtures thereof, or copolymers thereof. Among these, polyolefin resins have excellent coatability for coating layer slurries, and can reduce the thickness of the separation membrane for secondary batteries, thereby increasing the proportion of the electrode active material layer in the battery and increasing the capacity per unit volume.

[0037] The thickness of the polymer substrate can be 1 μm or more and 100 μm or less, more specifically, 1 μm or more and 30 μm or less, and the pore diameter of the polymer substrate can generally be 0.01 μm or more and 10 μm or less.

[0038] In the present invention, the porosity of the polymer substrate can be 30 volume% or more. Specifically, the porosity of the polymer substrate can be 30 volume% to 70 volume%, 32 volume% to 68 volume%, 34 volume% to 66 volume%, 36 volume% to 64 volume%, 38 volume% to 62 volume%, 40 volume% to 60 volume%, 42 volume% to 58 volume%, 44 volume% to 56 volume%, 46 volume% to 54 volume%, or 48 volume% to 52 volume%. By adjusting the porosity of the polymer substrate within the above range, ion movement can be maintained in the separation membrane and an increase in the resistance of the separation membrane can be prevented. Specifically, if the porosity is 70 volume% or less, mechanical properties that can withstand the pressing process for bonding with electrodes can be ensured, and the surface opening ratio does not become too high, making it suitable for ensuring adhesive strength. On the other hand, if the porosity is 30% by volume or higher, it is advantageous in terms of ion permeability.

[0039] In this specification, "porosity" refers to the ratio of the volume occupied by pores to the total volume, and is expressed in % by volume. It can be used interchangeably with terms such as void ratio and porosity. Porosity and pore size can be measured using the BET6 method by nitrogen gas adsorption flow using scanning electron microscope (SEM) images, a Mercuryporosimeter, a capillary flow porometer, or a porosimetry analyzer (Bell Japan Inc., Belsorp-II mini). In this case, using a capillary flow porometer may be advantageous.

[0040] A porous coating layer can be formed on the polymer substrate by applying and drying an organic / inorganic composite porous coating layer forming slurry. Before applying the slurry, a surface treatment such as plasma treatment or corona discharge can be performed to improve impregnation with the electrolyte.

[0041] Furthermore, the porous coating layer includes inorganic particles for improving the mechanical properties and insulating properties of the porous polymer substrate, and a binder resin for improving the adhesion between the electrode and the polymer substrate. The binder resin provides adhesion between the electrode and the polymer substrate, while simultaneously bonding adjacent inorganic particles and maintaining the bond. The thickness of the porous coating layer can be 0.1 μm or more and 5 μm or less.

[0042] The binder resin according to the present invention may include particulate binder resin, and such particulate binder resin may be a mixture of acrylic polymer binder particles and vinylidene fluoride polymer binder particles.

[0043] In this case, the acrylic polymer binder particles are a polymer containing carboxylic acid esters as repeating units, and specifically, they can be methacrylic acid esters. Specific examples of such methacrylic acid esters include, for example, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, i-propyl methacrylate, n-butyl methacrylate, i-butyl methacrylate, n-amyl methacrylate, i-amyl methacrylate, hexyl methacrylate, cyclohexyl methacrylate, 2-ethylhexyl methacrylate, n-octyl methacrylate, nonyl methacrylate, decyl methacrylate, hydroxymethyl methacrylate, hydroxyethyl methacrylate, ethylene glycol methacrylate, ethylene glycol dimethacrylate, propylene glycol dimethacrylate, trimethylolpropane trimethacrylate, pentaerythritol tetramethacrylate, dipentaerythritol hexamethacrylate, allyl methacrylate, and ethylene dimethacrylate, and one or more of these can be selected. Among these, it is preferable that one or more are selected from methyl methacrylate, ethyl methacrylate, and 2-ethylhexyl methacrylate.

[0044] The acrylic polymer binder particles may have a glass transition temperature (Tg) of 60°C or less, more specifically, 50°C or less, and more specifically, 40°C or less. When the glass transition temperature (Tg) of the acrylic polymer binder particles is 60°C or less, it can be advantageously used to improve the adhesion strength of the porous coating layer.

[0045] Furthermore, the vinylidene fluoride polymer binder particles are insoluble in the electrolyte and can be a homopolymer of vinylidene fluoride, a copolymer of vinylidene fluoride with a different polymerizable monomer, or a mixture of two or more of these.

[0046] Polymerizable monomers different from vinylidene fluoride include, but are not limited to, one or more selected from the group consisting of tetrafluoroethylene, hexafluoropropylene, tritrifluoroethylene, chlorofluoroethylene, 1,2-difluoroethylene, perfluoro(methyl vinyl) ether, perfluoro(ethyl vinyl) ether, perfluoro(propyl vinyl) ether, perfluoro(1,3-dioxole), perfluoro(2,2-dimethyl-1,3-dioxole), trichloroethylene, and vinyl fluoride. In particular, the vinylidene fluoride-based polymer binder particles can be copolymers of vinylidene fluoride and hexafluoropropylene. The content of polymerizable monomers different from vinylidene fluoride can be 1% to 20% by weight of the copolymer, but is not limited to this.

[0047] In the present invention, the content of hollow monomers in the vinylidene fluoride polymer binder particles can be measured by 1H-NMR using a Varian 500 MHz NMR spectrum. For detailed measurement procedures, please refer to Journal of Materials Chemistry, 2012, 22, 341 or AMT-3412-0k. For confirmation of the NMR spectrum, appropriate instruments such as a Bruker Avance III HD 700 MHz NMR spectrum or a Varian 500 MHz NMR spectrum can be used.

[0048] Here, the weight ratio of the acrylic polymer binder particles to the vinylidene fluoride polymer binder particles can be 7:3 to 3:7. If the content of the acrylic polymer binder particles is greater than the range, or if the content of the vinylidene fluoride polymer binder particles is less than the range, wet state adhesion will not be formed, which is undesirable. If the content of the acrylic polymer binder particles is less than the range, or if the content of the vinylidene fluoride polymer binder particles is greater than the range, dry state adhesion may be difficult to form, which is also undesirable. Specifically, since the acrylic polymer binder particles have high solubility in the electrolyte, they are maintained on top of the porous coating layer before being impregnated in the electrolyte. The acrylic polymer binder particles maintained in this way can maintain dry adhesion during the lamination process between the electrode and the separation membrane, preventing the separation membrane from bending. When the separation membrane is impregnated with the electrolyte, the acrylic polymer binder particles dissolve in the electrolyte, and the vinylidene fluoride polymer binder particles, which have low solubility in the electrolyte, are maintained on top of the porous coating layer. The vinylidene fluoride polymer binder particles maintained in this way maintain their wet adhesion even when impregnated with the electrolyte, thereby preventing electrolyte impregnation into the separation membrane and thus preventing the deposition of lithium dendrites.

[0049] Furthermore, the acrylic polymer binder particles may be first copolymer binder particles containing styrene and acrylate in a weight ratio of 5:5 to 7:3. Specifically, they may be first copolymer binder particles containing styrene repeating units and acrylate repeating units in a weight ratio of 5:5 to 7:3. In this case, if the styrene content is greater than the range or the acrylate content is less than the range, the glass transition temperature (Tg) will be high, making it difficult to form dry state adhesive strength under lamination process conditions (50 to 80°C), which is undesirable. Conversely, if the styrene content is less than the range or the acrylate content is greater than the range, the glass transition temperature (Tg) will be low, causing the binder layer structure to collapse at room temperature, making it difficult to form dry state adhesive strength, which is also undesirable.

[0050] In this specification, the glass transition temperature can be measured using a differential scanning calorimetry (DSC). Specifically, the glass transition temperature can be measured using a differential scanning calorimetry at a heating rate of 10°C / min (-50°C to 250°C). For example, the glass transition temperature can be measured using a DSC250 (TA Corporation).

[0051] At this time, the average particle size (D) of the first copolymer binder particles 50 The particle size can be between 350 nm and 700 nm, specifically between 350 nm and 500 nm. In this case, if the average particle size of the first copolymer binder particles is smaller than the above numerical range, soaking occurs between the inorganic particles, making it difficult to form a binder layer structure, which is undesirable. If it exceeds the above numerical range, it is difficult to form and fix the porous coating layer, causing problems in forming a uniform surface, which is also undesirable.

[0052] Furthermore, the vinylidene fluoride polymer binder particles can be second copolymer binder particles in which vinylidene fluoride polymer particles having 10% by weight or less of hexafluoropropylene (HFP) and acrylate are crosslinked in a weight ratio of 5:5 to 9:1. In this case, if the content of vinylidene fluoride polymer particles is greater than the range or the content of acrylate is less than the range, it is difficult to form wet state adhesion, which is undesirable. If the content of vinylidene fluoride polymer particles is less than the range or the content of acrylate is greater than the range, it may be difficult to manufacture a slurry for forming a porous coating layer, which is also undesirable.

[0053] At this time, the average particle size (D) of the second copolymer binder particles 50 The particle size can be between 150 nm and 250 nm, specifically between 150 nm and 200 nm. In this case, if the average particle size of the second copolymer binder particles is smaller than the above numerical range, soaking occurs between the inorganic particles, making it difficult to form a binder layer structure, which is undesirable. If it exceeds the above numerical range, the porous coating layer may be formed unevenly, which is also undesirable.

[0054] In particular, when the average particle size of the first copolymer binder particles and the second copolymer binder particles is fully satisfied, it is even more preferable because the effect of uniform adhesion between the positive and negative electrodes in both the dry and wet states can be achieved.

[0055] In this specification, "D 50"Particle size" refers to the particle size at the 50% point of the cumulative particle number distribution by particle size. This particle size can be measured using the laser diffraction method. Specifically, after dispersing the powder to be measured in a dispersion medium, it is introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac S3500), and the particle size distribution is calculated by measuring the difference in diffraction patterns due to particle size as the particles pass through the laser beam. By calculating the particle diameter at the point where the cumulative particle number distribution by particle size in the measuring device reaches 50%, D 50 It can measure particle size.

[0056] On the other hand, the particulate binder resin contained in the porous coating layer may further contain a third copolymer polymer particle containing methyl methacrylate and 2-ethylhexyl methacrylate in a weight ratio of 2:8 to 5:5. Specifically, it may further contain a third copolymer polymer particle in which methyl methacrylate and 2-ethylhexyl methacrylate are crosslinked in a weight ratio of 2:8 to 5:5. That is, it may further contain a third copolymer polymer particle containing repeating units of methyl methacrylate and repeating units of 2-ethylhexyl methacrylate in a weight ratio of 2:8 to 5:5. In this case, if the content of methyl methacrylate is greater than the range or the content of 2-ethylhexyl methacrylate is less than the range, the glass transition temperature (Tg) may increase and the peel strength may decrease, which is undesirable. Furthermore, if the content of methyl methacrylate is less than the range or the content of 2-ethylhexyl methacrylate is greater than the range, the particle morphology of the binder may collapse, posing a risk of soaking of the polymer substrate, i.e., the fabric, which is also undesirable.

[0057] Here, based on the total content of the particulate binder resin, the acrylic polymer binder particles may be 4% to 95% by weight, the vinylidene fluoride polymer binder particles may be 4% to 95% by weight, and the third copolymer polymer particles may be 1% to 10% by weight. If the content falls outside this range, the resistance of the separation membrane may increase, which can lead to side effects such as difficulty in forming adhesive strength.

[0058] The inorganic particles contained in the porous coating layer according to the present invention are not particularly limited, as long as they form a uniform thickness in the porous coating layer and do not cause oxidation and / or reduction reactions within the operating voltage range of the electrochemical element to which they are applied. In one embodiment, the uniformity of the thickness can have a tolerance of + / - 2 μm. In particular, when inorganic particles with ion transfer capability are used, the ionic conductivity within the electrochemical element can be increased to improve performance. Furthermore, when inorganic particles with a high dielectric constant are used as inorganic particles, they can contribute to increasing the degree of dissociation of electrolyte salts in the liquid electrolyte, such as lithium salts, thereby improving the ionic conductivity of the electrolyte.

[0059] As an example of the inorganic particles mentioned above, an inorganic material having at least one of the following properties can be cited: lithium ion transport capability, piezoelectricity, and flame retardancy.

[0060] The inorganic particles with good lithium ion transport capability refer to inorganic particles that contain lithium but have the function of transporting lithium ions without storing lithium. These inorganic particles with lithium ion transport capability can transport and move lithium ions due to a type of defect present within their particle structure. Therefore, the lithium ion conductivity in the battery is improved, thereby improving battery performance.

[0061] Examples of inorganic particles having lithium ion transport capability include lithium phosphate, lithium titanium phosphate, lithium aluminum titanium phosphate, (LiAlTiP)x O y It can be one or more selected from the group consisting of 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 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), LLZO-based, or a mixture thereof, but is not limited thereto.

[0062] The inorganic particles having piezoelectricity are insulators under normal pressure, but when a certain pressure is applied, they mean substances having the property of conducting electricity due to internal structure changes. They not only exhibit high dielectric constant characteristics with a dielectric constant of 100 or more, but also when a certain pressure is applied and they are stretched or compressed, charges are generated and one side is positively charged and the opposite side is negatively charged, respectively, so that a potential difference is generated between both side surfaces.

[0063] When using inorganic particles having the above characteristics, when an internal short circuit between the positive electrode and the negative electrode occurs due to local pressing (Local crush), external shock such as a nail, etc., not only do the inorganic particles coated on the separator prevent the direct contact between the positive electrode and the negative electrode, but also due to the piezoelectricity of the inorganic particles, a potential difference is generated inside the particles. As a result, electron transfer between the positive electrode and the negative electrode, that is, a flow of fine current is made, thereby achieving a slow decrease in the battery voltage and improving safety thereby.

[0064] Examples of the inorganic particles having piezoelectricity include BaTiO3, Pb(Zr, Ti)O3 (PZT), Pb 1-x La xZr 1-y Ti y O3(PLZT)(0 <x<1、0<y<1)、Pb(Mg 1 / 3 Nb 2 / 3 This includes, but is not limited to, O3-PbTiO3(PMN-PT)hafnia (HfO2), or mixtures thereof.

[0065] The flame-retardant inorganic particles not only prevent overcharging of the electrochemical element, but can also add flame-retardant properties to the separation membrane or prevent a sudden rise in temperature inside the battery. The flame-retardant inorganic particles can be one or more selected from the group consisting of antimony-containing compounds, metal hydroxides or metal hydrates, guanidine-based compounds, boron-containing compounds, and zinc stannate compounds.

[0066] Specifically, 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 alumina (Al2O3), magnesium hydroxide, aluminum hydroxide (Al(OH)3), aluminum oxyhydroxyl (AlO(OH)), and CaOAl2O36H2O; the guanidine-based compound is selected from the group consisting of nitrogenized guanidine, sulfamic acid guanidine, phosphate guanidine, and phosphate guanylate elements; the boron-containing compound is H3BO3 or HBO2; and the zinc stannate compound can be selected from Zn2SnO4, ZnSnO3, and ZnSn(OH)6.

[0067] Average particle size of inorganic particles (D 50The specific surface area can be 50 nm to 5000 nm, preferably 200 nm to 1000 nm, and more preferably 300 nm to 700 nm. If the average particle size of the inorganic particles is less than 50 nm, as the specific surface area increases, more binder resin is required for bonding between the inorganic particles, which is disadvantageous in terms of electrical resistance. Also, the more binder resin present, the lower the peel strength and binder peeling may be. If the average particle size of the inorganic particles exceeds 5000 nm, the uniformity of the coating layer surface will be low. In addition, if the size of the protruding particles is large after coating, the separation film and the electrode may be damaged during lamination, causing a short circuit.

[0068] The aspect ratio of the inorganic particles is 1 to 2, preferably 1.2 to 2, and the BET specific surface area of ​​the inorganic particles is 5 m². 2 / g or more 25m 2 It can be less than or equal to / g. Within the above range, the movement of the binder resin through the voids between inorganic particles is easy, and in particular, the smaller the aspect ratio as a monodisperse and the smaller the BET specific surface area, the better the movement of the binder resin within the coating layer.

[0069] In this specification, BET may be defined as the Brunauer-Ennett-Teller model (BET) used to calculate the BET surface area from the measured N2 adsorption isotherm, which is measured to 1 bar at -196°C using a BET-specific surface area analyzer (BEL, Microtrac).

[0070] Furthermore, the porous coating layer may further contain a dispersant to further improve the dispersibility of inorganic particles. The dispersant functions to maintain a uniform dispersion of inorganic particles within the binder resin during the production of the slurry for forming the porous coating layer. Examples of substances that can be used as the dispersant include one or more selected from oil-soluble polyamines, oil-soluble amine compounds, fatty acids, fatty alcohols, sorbitan fatty acid esters, tannic acid, and pyrogallic acid. When a dispersant is included, the porous coating layer may contain the dispersant in an amount of 5% by weight or less.

[0071] On the other hand, in the separation membrane for electrochemical elements according to the present invention, the adhesive strength of the separation membrane in a state impregnated with electrolyte, i.e., in a wet state, can be 1.0 gf / 25 mm or more, more specifically, it can be 10 gf / 25 mm or more, and more specifically, it can be 15 gf / 25 mm or more and 20 gf / 25 mm or less. If the adhesive strength in the wet state is less than 1.0 gf / 25 mm, the stiffness of the cell will be low, which may cause assembly problems or problems with the separation membrane bending during the manufacture of the electrode assembly. If it exceeds 20 gf / 25 mm, electrolyte impregnation into the separation membrane will be hindered, which may cause lithium dendrites to precipitate.

[0072] In this specification, the wet adhesive strength can be measured by cutting an aluminum foil coated with an NCM-based cathode mixture and the separation membrane to a width of 25 mm, placing them in a pouch, pouring in a carbonate-based electrolyte, and preparing a sample for electrode adhesion strength measurement by pressurizing the pouch at 5 kgf, 70°C, and 4 minutes, and then performing a 90° peel test at 200 mm / min using an Instron UTM device.

[0073] Furthermore, in the separation membrane for the electrochemical element, the adhesive strength of the separation membrane before impregnation with the electrolyte, i.e., in the dry state, can be 10 gf / 25 mm or more, more specifically, it can be 10 gf / 25 mm or more and less than 100 gf / 25 mm, and more specifically, it can be 30 gf / 25 mm or more and less than 100 gf / 25 mm. If the dry state adhesive strength exceeds 100 gf / 25 mm, electrolyte impregnation into the separation membrane is hindered, which can lead to the deposition of lithium dendrites.

[0074] In this specification, the dry adhesion strength can be measured by cutting a negative electrode coated with a carbon-based negative electrode mixture onto a copper foil and the separation membrane to a width of 25 mm and arranging them so that they overlap, preparing a sample for electrode adhesion strength measurement by pressurizing it using a hot press at 60°C, 6.5 MPa, and 1 second, and then performing a 180° peel test at 300 mm / min using an Instron UTM.

[0075] On the other hand, an electrochemical element according to another aspect of the present invention comprises a positive electrode, a negative electrode, and a separation membrane interposed between the positive electrode and the negative electrode, wherein the separation membrane is the separation membrane for electrochemical elements according to the present invention described above.

[0076] In the present invention, the positive electrode comprises a positive electrode current collector and a positive electrode active material layer on at least one surface of the current collector, comprising a positive electrode active material, a conductive material, and a polymer binder. The positive electrode active material is a layered compound such as lithium manganese composite oxide (LiMn2O4, LiMnO2, etc.), lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; chemical formula Li 1+x Mn 2-x Lithium manganese oxides such as O4 (where x is 0 to 0.33), LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, LiV3O4, V2O5, Cu2V2O7; chemical formula LiNi 1-x M xNi-site type lithium nickel oxide represented by O2 (where M = Co, Mn, Al, Cu, Fe, Mg, B, or Ga and x = 0.01 - 0.3); chemical formula LiMn 1-x M x Lithium manganese composite oxide represented by O2 (where M = Co, Ni, Fe, Cr, Zn, or Ta and x = 0.01 - 0.1) or Li2Mn3MO8 (where M = Fe, Co, Ni, Cu, or Zn); LiMn2O4 in which part of Li in the chemical formula is substituted by alkaline earth metal ions; disulfide compound; It can contain one or more mixtures of Fe2(MoO4)3.

[0077] In the present invention, the negative electrode includes a negative electrode current collector and a negative electrode active material layer containing a negative electrode active material, a conductive material, and a polymer binder on at least one surface of the current collector. The negative electrode uses, as the negative electrode active material, carbon such as lithium metal oxide, graphitized carbon, graphite-based carbon; LixFe2O3 (0 ≤ x ≤ 1), Li x WO2 (0 ≤ x ≤ 1), Si, SiO x (0 < x < 2), silicon-based materials such as SiC, Si alloy; Sn x Me 1-x Me’ y O z (Me: Mn, Fe, Pb, Ge; Me’: Al, B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, halogen; 0 < x ≤ 1; 1 ≤ y ≤ 3; 1 ≤ z ≤ 8), etc. metal composite oxides; lithium metal; lithium 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; Li-Co-Ni-based materials; It can contain one or more mixtures selected from titanium oxides.

[0078] In the present invention, the current collector is not particularly limited as long as it has high conductivity without inducing a chemical change in the electrochemical element. For example, stainless steel, copper, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc., can be used.

[0079] In the present invention, the conductive material may be one selected from the group consisting of graphite, carbon black, carbon fiber or metal fiber, metal powder, conductive whisker, conductive metal oxide, carbon nanotube, activated carbon, and polyphenylene derivative, or a mixture of two or more conductive materials selected from these. The carbon nanotube has a graphite sheet with a nano-sized diameter in a cylindrical shape and an sp2 bond structure, and its properties as a conductor or semiconductor are expressed by the angle and structure in which the graphite sheet is wound. Carbon nanotubes can be classified into single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), and multi-walled carbon nanotubes (MWCNTs) according to the bonds forming the wall, and these carbon nanotubes can be appropriately selected depending on the application of the dispersion. More specifically, it may be one or a mixture of two or more conductive materials selected from the group consisting of natural graphite, artificial graphite, super-p, acetylene black, Ketjen black, channel black, Phanese black, lamp black, summer black, denka black, aluminum powder, nickel powder, zinc oxide, potassium titanate, and titanium oxide.

[0080] In the present invention, the polymer binder can be a polymer binder that is commonly used in electrodes in the industry. Non-limiting examples of such polymer binders include polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-cotrichloroethylene, polymethyl methacrylate, polyethylhexyl acrylate, polybutyl acrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinyl acetate, polyethylene-co-vinyl acetate, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, and cellulose acetate propionate. Examples include, but are not limited to, acetatepropionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethylcellulose, cyanoethylsucrose, pullulan, and carboxymethyl cellulose.

[0081] The electrode assembly according to the present invention comprises a positive electrode, a negative electrode, and a separation membrane interposed between the positive electrode and the negative electrode, and an electrochemical element can be manufactured by placing the electrode assembly in a suitable case and injecting an electrolyte.

[0082] In the present invention, the electrolyte is A + B - A salt with a structure like this, + Li + kaNa + , K + It contains alkali metal cations or ions consisting of combinations thereof, B - PF6 - BF4 - Cl - , Br - , I - ClO4 - AsF6 - CH3CO2 - CF3SO3 - , N(CF3SO2)2 - , C(CF2SO2)3 - Salts containing anions such as those listed above, or ions consisting of combinations thereof, may be dissolved or dissociated in organic solvents consisting of propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethyl methyl carbonate (EMC), gamma-butyrolactone (γ-butyrolactone), or mixtures thereof, but are not limited to these.

[0083] Furthermore, the present invention provides a battery module including the electrode assembly as a unit battery, a battery pack including the battery module, and a device including the battery pack as a power source. Specific examples of the device include, but are not limited to, a power tool powered by a battery-powered motor; electric vehicles including electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs); electric motorcycles including electric bicycles (E-bikes) and electric scooters; electric golf carts; and power storage systems.

[0084] The present invention will be described in detail below with reference to examples. However, the examples of the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the examples detailed below. The examples of the present invention are provided to give a more complete explanation of the present invention to a person of average knowledge in the industry.

[0085] <Example 1>

[0086] Acrylic polymer binder particles [LGC, AD-S11, D50:400nm, copolymer of styrene and butyl acrylate (Tg 40℃)] 10 parts by weight, vinylidene fluoride polymer binder particles [Arkema, LBG4330LX, D 50 10 parts by weight of a copolymer obtained by polymerizing VDF and HFP in a weight ratio of 95:5 at 200 nm, and a copolymer obtained by crosslinking an ethyl acrylate and methyl methacrylate copolymer (Tg 20℃) in a weight ratio of 7:3, and inorganic particles (Al2O3, D 50 80 parts by weight of 500nm, Sumitomo, AES11) were added to water and dispersed to prepare a slurry for forming a porous coating layer.

[0087] Next, a porous polymer substrate made of polyethylene material (porosity 40% by volume, thickness 9 μm) was prepared, and the slurry was applied to both sides of the surface of the polymer substrate using a bar coating method with a doctor blade. The substrate was then dried with hot air at 50°C using a heat gun to form a porous coating layer with a thickness of 12 μm based on the thickness of one side.

[0088] <Example 2>

[0089] A porous coating layer was formed in the same manner as in Example 1, except that the slurry for forming the porous coating layer was prepared by changing the amount of acrylic polymer binder particles to 14 parts by weight and vinylidene fluoride polymer binder particles to 6 parts by weight.

[0090] <Example 3>

[0091] A porous coating layer was formed in the same manner as in Example 1, except that a slurry for forming a porous coating layer was prepared by changing the composition of the slurry for forming the porous coating layer to 8 parts by weight of acrylic polymer binder particles, 8 parts by weight of vinylidene fluoride polymer binder particles, and an additional 4 parts by weight of polymer binder particles containing methyl methacrylate and 2-ethylhexyl methacrylate in a 5:5 weight ratio.

[0092] <Comparative Example 1>

[0093] A porous coating layer was formed in the same manner as in Example 1, except that the slurry for forming the porous coating layer was prepared by changing the amount of acrylic polymer binder particles to 0 parts by weight and vinylidene fluoride polymer binder particles to 20 parts by weight.

[0094] <Comparative Example 2>

[0095] A porous coating layer was formed in the same manner as in Example 1, except that a slurry for forming a porous coating layer was prepared by changing the amount of acrylic polymer binder particles to 20 parts by weight and vinylidene fluoride polymer binder particles to 0 parts by weight.

[0096] <Experimental Example 1> Binder distribution analysis

[0097] After measuring the cross-section of the porous coating layer of the separation membranes produced in the examples and comparative examples using SEM, mapping is performed via EDX analysis. More specifically, after staining the separation membrane with RuO4, a cross-sectional sample is prepared by ion milling, then measured using SEM in BSE mode, and mapping analysis is performed using EDX. By dividing the cross-section of the porous coating layer into upper, central, and lower sections, and then calculating the area via integration, the binder distribution results depending on the cross-sectional position of the porous coating layer can be confirmed. Table 1 below numerically shows the binder distribution results depending on the cross-sectional position of the porous coating layer in the examples and comparative examples.

[0098] [Table 1]

[0099] Figures 1(A) and 1(B) show the binder distribution according to the thickness of the porous coating layer of the separation membrane according to Example 1, while Figures 2(A) and 2(B) show the binder distribution according to the thickness of the porous coating layer of the separation membrane according to Comparative Example 1.

[0100] As shown in Figures 1 and 2, in Example 1, it can be confirmed that the binder content in the upper part of the porous coating layer remains even higher when comparing before and after electrolyte impregnation. In Comparative Example 1, the binder content was higher in the upper part of the porous coating layer before electrolyte impregnation, but after electrolyte impregnation, it can be confirmed that the binder content in the middle part of the porous coating layer became higher than the upper part. As a result, it is expected that in Comparative Example 1, the adhesion strength between the porous coating layer and the electrode will decrease after electrolyte impregnation, but in Example 1, the adhesion strength between the porous coating layer and the electrode will be sufficiently maintained even after electrolyte impregnation.

[0101] <Experimental Example 2> Dry adhesive force measurement

[0102] The negative electrode, which has a carbon-based negative electrode mixture coated on a copper foil, and the separation membrane produced in the above examples and comparative examples are each cut to a width of 25 mm and arranged to overlap.

[0103] This was then pressurized using a hot press at 60°C, 6.5 MPa, and for 1 second to prepare a sample for measuring electrode adhesion strength.

[0104] To measure the adhesion between the negative electrode and the separation membrane, a 180° peel test was performed at a rate of 300 mm / min using Instron's UTM equipment.

[0105] The dry adhesive strengths measured in this manner are listed in Table 2 below.

[0106] <Experimental Example 3> Wet adhesive force measurement

[0107] The positive electrode, which had an NCM-based positive electrode mixture coated on aluminum foil, and the separation membrane produced in the above examples and comparative examples were each cut to a width of 25 mm, placed in pouches, and a carbonate-based electrolyte was poured in.

[0108] The aforementioned pouch was pressurized under the conditions of 5 kgf, 70°C, and 4 minutes to prepare a sample for measuring electrode adhesion strength.

[0109] To measure the adhesion strength between the positive electrode and the separation membrane, a 90° peel test was performed at a rate of 20 mm / min using Instron's UTM equipment.

[0110] The wet adhesive strengths measured in this manner are listed in Table 2 below.

[0111] [Table 2]

[0112] Referring to Table 2 above, in the case of the examples, the dry adhesive strength was measured to be approximately 40 gf / 25 mm, and the wet adhesive strength was measured to be approximately 15-17 gf / 25 mm, confirming that appropriate and excellent adhesive strength was formed in both the dry and wet states. However, in the case of the comparative example, it can be confirmed that while some dry adhesive strength was formed, the wet adhesive strength was completely lost.

[0113] This specification discloses preferred embodiments of the present invention, and while specific terms and other terms are used, these are merely general terms used to facilitate the explanation of the technical content of the invention and aid in its understanding, and are not intended to limit the scope of the invention. It is obvious to a person with ordinary skill in the art to which the present invention belongs that other modifications and variations based on the technical idea of ​​the present invention are also possible, in addition to the embodiments disclosed herein. For example, a person with ordinary skill in the art will understand that the separation membrane for an electrochemical element according to the embodiment and the electrochemical element comprising the same can be modified in various ways. Therefore, the scope of the invention should not be determined by the embodiments described, but by the technical idea described in the claims.

Claims

1. The invention comprises a porous polymer substrate and an organic / inorganic composite porous coating layer formed on at least one surface of the polymer substrate. The porous coating layer has a single-layer structure containing particulate binder resin and inorganic particles. A separation membrane for electrochemical elements, wherein the ratio of [binder particle content in the upper part of the porous coating layer before electrolyte impregnation / binder particle content in the central part of the porous coating layer before electrolyte impregnation] - [binder particle content in the upper part of the porous coating layer after electrolyte impregnation / binder particle content in the central part of the porous coating layer after electrolyte impregnation] is 0.32 or less.

2. The separation membrane for an electrochemical element according to claim 1, wherein the particulate binder resin is a mixture of acrylic polymer binder particles and vinylidene fluoride polymer binder particles.

3. The separation membrane for an electrochemical element according to claim 2, wherein the weight ratio of the acrylic polymer binder particles to the vinylidene fluoride polymer binder particles is 7:3 to 3:

7.

4. The separation membrane for an electrochemical element according to claim 2, wherein the acrylic polymer binder particles are first copolymer binder particles containing styrene and acrylate in a weight ratio of 5:5 to 7:

3.

5. The separation membrane for an electrochemical element according to claim 4, wherein the average particle size of the first copolymer binder particles is 350 nm to 700 nm.

6. The separation membrane for an electrochemical element according to claim 2, wherein the vinylidene fluoride polymer binder particles are second copolymer binder particles obtained by crosslinking vinylidene fluoride polymer particles having hexafluoropropylene (HFP) content of 10% by weight or less with acrylate in a weight ratio of 5:5 to 9:

1.

7. The separation membrane for an electrochemical element according to claim 6, wherein the average particle size of the second copolymer binder particles is 150 nm to 250 nm.

8. The separation membrane for an electrochemical element according to claim 2, further comprising a third copolymer polymer particle in which the particulate binder resin contains methyl methacrylate and 2-ethylhexyl methacrylate in a weight ratio of 2:8 to 5:

5.

9. The separation membrane for an electrochemical element according to claim 8, wherein, based on the total content of the particulate binder resin, the acrylic polymer binder particles are 4% by weight or more and 95% by weight or less, the vinylidene fluoride polymer binder particles are 4% by weight or more and 95% by weight or less, and the third copolymer polymer particles are 1% by weight or more and 10% by weight or less.

10. An electrochemical element comprising a positive electrode, a negative electrode, and a separation membrane interposed between the positive electrode and the negative electrode, wherein the separation membrane is a separation membrane for an electrochemical element as described in any one of claims 1 to 9.

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