Latex and production method of the same, and electrochemical device

Abstract

To provide a latex and a method for producing the same, and an electrochemical device using the latex.SOLUTION: A latex is a dispersion liquid containing a binder and a solvent. The binder contains particles having a core-shell structure, the core of the core-shell structure is a cavity, and the shell of the core-shell structure is a binder polymer. The solvent contains water. The latex production method includes a step of subjecting a mixture of an oil phase and an aqueous phase to shear processing, subjecting it to polymerization reaction at a pressure of 0.1 to 1 MPa, and obtaining a latex after decompression. The oil phase contains alkane and a raw material of the binder polymer, the boiling point of the alkane is less than 100°C, and the raw material of the binder polymer contains at least a monomer. The aqueous phase contains water and an emulsifier.EFFECT: The latex can achieve an adhesive force level in a higher mass used amount of an existing binder with a lower mass used amount.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a latex, a method for producing the same, and an electrochemical device. [Background technology]

[0002] Improving mass energy density is a key challenge in the development of lithium / sodium batteries. Reducing the mass fraction of the binder in the electrode sheets of lithium / sodium batteries is a simple and effective approach. However, reducing the mass fraction of the binder generally means a reduction in the adhesive area, which reduces the peel strength and cohesion of the electrode sheets of lithium / sodium batteries and makes the integrity of the conductive network more susceptible to damage. This ultimately manifests as an increase in the impedance and a decrease in the cycle life of lithium / sodium batteries.

[0003] To approach the lower limit of binder usage, researchers are working on developing various high-performance binders. Typical methods include introducing polar functional groups, increasing molecular weight, and optimizing the ratio of each monomer. However, existing strategies cannot significantly reduce binder usage because they only improve adhesive strength to a limited extent. Summary of the Invention [Problem to be solved by the invention]

[0004] In order to solve the drawback of existing technologies that cannot reduce the mass occupancy of the binder in a battery electrode sheet while simultaneously maintaining the cycle stability of the battery, a latex, a method for producing the same, and an electrochemical device have been provided. When the latex provided in the present invention is used as a binder material for an electrode sheet of an electrochemical device, a higher level of adhesive strength can be achieved with a relatively small amount of latex used, thereby achieving both the energy density and cycle stability of the electrochemical device. [Means for solving the problem]

[0005] In a first aspect, the present invention provides a latex. The latex is a dispersion containing a binder and a solvent. The binder contains particles having a core-shell structure, the core of the core-shell structure being a cavity, and the shell of the core-shell structure being a binder polymer. The solvent includes water.

[0006] In a second aspect, the present invention provides a method for producing the above-mentioned latex, the method comprising the steps of: The method includes a step of shearing a mixture of an oil phase and an aqueous phase, carrying out a polymerization reaction under a pressure of 0.1 to 1 MPa, and obtaining a latex after reducing the pressure. Here, the oil phase contains an alkane and a raw material for the binder polymer, the boiling point of the alkane being less than 100°C, and the raw material for the binder polymer containing at least a monomer. The aqueous phase comprises water and an emulsifier.

[0007] In a third aspect, the present invention provides an electrochemical device, wherein the above-mentioned latex is used in the manufacturing process of an electrode sheet of the electrochemical device.

[0008] On a basis consistent with practice in the art, the preferred conditions set forth above may be combined in any way to obtain more preferred embodiments of the present invention.

[0009] All reagents and materials used in the present invention are commercially available. [Effects of the Invention]

[0010] The positive advancement effects of the present invention are as follows: The latex provided by the present invention can achieve the same adhesive strength level as existing binders at higher mass amounts with a lower mass fraction of latex. When the latex is used as an adhesive material to prepare an electrode sheet for an electrochemical device (especially a lithium / sodium battery), the mass fraction of the binder can be reduced while ensuring that the electrochemical device has a higher mass energy density and the battery has excellent cycling stability. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is an SEM image of the latex in Example 1. [Figure 2] FIG. 2 is a TEM image of the latex in Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention will be further described below through examples, but the present invention is not limited to the scope of the examples described. In the following examples, experimental methods for which no specific conditions are noted are selected according to conventional methods and conditions or product instructions.

[0013] latex

[0014] The latex provided in the first aspect of the present invention is a dispersion comprising a binder and a solvent, the binder comprising particles having a core-shell structure, the core of the core-shell structure being a cavity, and the shell of the core-shell structure being a binder polymer, and the solvent comprising water.

[0015] In some optional embodiments, the latex is a water dispersion.

[0016] In the present invention, the true density of the binder is 0.3 to 0.95 g / cm 3 Preferably, it is 0.3 to 0.9 g / cm 3 For example, 0.32 g / cm 3 , 0.36g / cm 3 , 0.44g / cm 3 , 0.74g / cm 3 , or 0.91 g / cm 3 The true density of the binder polymer itself is generally 1.0 to 1.5 g / cm 3Obviously, the true density value of the binder of the present invention reflects the fact that most of the binder is particles having a core-shell structure in which the core is a cavity, and is not a solid particle. The true density of the binder can be measured using a conventional measurement method in the field to obtain the mass and volume of the binder, and then calculated according to a formula. The formula for calculating the true density of the binder is: "True density = m バインダー / V バインダー " where m バインダー is the mass of the binder, and V バインダー is the volume of the binder.

[0017] In the present invention, the porosity of the binder is 50% or more. Porosity refers to the ratio of the volume of cavities in the binder to the total volume of the binder. In an ideal situation, all particles in the binder have a core-shell structure. In this case, an example of calculation based on a single core / single shell is as follows: "P (porosity) = V キャビティー / V 粒子 " In the formula, P refers to the porosity of a single particle (the proportion of the cavity volume), and V キャビティー is the volume of the cavity in the particle, and V 粒子 is the total volume of the particle.

[0018] In the present invention, the porosity of the binder is determined by V from an image taken by TEM. キャビティー and V 粒子 can be obtained by substituting it into the above formula.

[0019] In some optional embodiments, the particle size of the binder is 0.1 to 10 μm, for example, 0.3 μm.

[0020] In some optional embodiments, the shell thickness of the core-shell structure is 0.01 to 1 μm, selected from 15 to 50 nm, for example, 20 nm, 30 nm, 40 nm, or 45 nm.

[0021] In some optional embodiments, the mole percentage of particles having the core-shell structure among all particles in the binder is greater than 99%.

[0022] In some optional embodiments, the solids content of the latex is ≦60%, e.g., 9.2%, 9.9%, or 16.9%, where solids percentage is the weight percentage of the binder in the latex.

[0023] Latex manufacturing method

[0024] A method for producing a latex provided in a second aspect of the present invention includes the following steps: shearing a mixture of an oil phase and an aqueous phase, carrying out a polymerization reaction under a pressure of 0.1 to 1 MPa, and obtaining a latex after reducing the pressure. Here, the oil phase contains an alkane and raw materials for the binder polymer. The boiling point of the alkane is less than 100°C. The raw materials for the binder polymer include at least a monomer. The aqueous phase contains water and an emulsifier. If the pressure of the polymerization reaction is less than 0.1 MPa, the alkane is easily released, which is unfavorable for the release of the alkane and the formation of a core-shell structure in the later decompression process. If the boiling point of the alkane exceeds 100°C, the alkane will be difficult to escape during the decompression process, and the volatilization of the alkane will be disadvantageous for obtaining a core-shell structure.

[0025] In a preferred embodiment, the boiling point of the alkane is 80°C or less.

[0026] In a specific embodiment, the boiling point of the alkane is 27.5°C.

[0027] In some optional embodiments, the mass ratio of the oil phase to the water phase is 1:(1.5-9), optionally 1:(3.3-6.3), such as 1:1.7, 1:2.9, 1:3.4, 1:4, 1:8.3, or 1:8.5. The mass ratio of the oil phase to the water phase mainly affects the stability of the latex, and only when the mass ratio of the oil phase to the water phase is within an appropriate range can it be ensured that the resulting latex has relatively good stability.

[0028] In some optional embodiments, the weight ratio of the alkane to the monomer is 1:(0.4-6), for example, 2:1, 1.5:1, 1:2, or 1:5.

[0029] In the present invention, the mass ratio of the emulsifier to water may be (0.01 to 5):(150 to 1000), for example, 0.01:1000.

[0030] In the present invention, the shearing step can be performed using a high-shear liquid-liquid dispersing device, which preferably includes one of a high-speed shearer, a high-pressure homogenizer, a cell disrupter, and a super-gravity field generator, such as a high-speed shearer or an ultrasonic cell disrupter.

[0031] In some optional embodiments, the shear rate of the high-speed shear machine is 2000 to 5000 rpm.

[0032] In some optional embodiments, the ultrasonic cell disrupter has a power output of 200 to 500 W.

[0033] In the present invention, the time for the shearing treatment may be 5 to 15 minutes.

[0034] In the present invention, the temperature of the shear treatment may be equal to or lower than the ambient temperature. The ambient temperature refers to the temperature of the environment when the shear treatment is performed, and is preferably 25° C. The reason for setting the temperature of the shear treatment in the present invention to be equal to or lower than the ambient temperature is to prevent a large amount of low-boiling-point alkanes from vaporizing due to high temperatures.

[0035] In the present invention, the temperature of the polymerization reaction may be 50 to 85°C, for example, 75°C.

[0036] In the present invention, the time for the polymerization reaction may be 6 to 12 hours, for example, 10 hours.

[0037] In the present invention, the alkane is a C5 to C 10 The short chain alkanes may include one or more of the following:

[0038] In some embodiments, the alkane is isopentane.

[0039] In some optional embodiments, the monomers include one or more of acrylic acid esters, methacrylic acid esters, hydroxyethyl acrylate, styrene, acrylic acid, butadiene, methacrylic acid, acrylonitrile, methacrylonitrile, acrylamide, and derivatives thereof.

[0040] In the present invention, the raw materials for the binder polymer may further contain an initiator. The initiator may be an azo-based initiator and / or a peroxide-based initiator having a water solubility of 0.5 g / kg water or less at 25°C. The azo-based initiator preferably includes azobisisobutyronitrile and / or azobisisoheptonitrile. The peroxide-based initiator preferably includes one or more of benzoyl peroxide, ammonium persulfate, and potassium persulfate.

[0041] In some embodiments, the initiator is azobisisobutyronitrile.

[0042] In the present invention, the binder polymer raw material may further include a crosslinking agent, which may be selected according to a conventional method in the art. Typically, the crosslinking agent may include one or more of divinylbenzene, ethylene glycol dimethacrylate, allyl methacrylate, ethylene glycol diacrylate, butylene glycol diacrylate, butylene glycol dimethacrylate, neopentyl glycol dimethacrylate, hexanediol dimethacrylate, triethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate, trimethylolpropane, and pentaerythritol tetramethacrylate.

[0043] In some embodiments, the crosslinker is divinylbenzene.

[0044] In some optional embodiments, the emulsifier comprises one or more of sodium dodecyl sulfate, hexadecyltrimethylammonium bromide, nonylphenol polyoxyethylene ether, OP surfactants, Span surfactants, Tween surfactants, and sodium dodecylbenzenesulfonate.

[0045] In some embodiments, the emulsifier is sodium dodecyl sulfate.

[0046] In some optional embodiments, the aqueous phase further comprises one or more of a dispersant, a metal salt, and a polymerization inhibitor.

[0047] The dispersant can improve the stability of large latex particles. The dispersant may be a conventional dispersant used in the art. Generally, the dispersant is selected from one or more of polyvinyl alcohol, magnesium hydroxide, calcium carbonate, calcium phosphate, silica sol, polyvinylpyrrolidone, styrene-maleic anhydride copolymer, and carboxymethyl cellulose. For example, polyvinyl alcohol.

[0048] Here, the metal salt can reduce the diffusion of highly water-soluble monomers into the aqueous phase, capture free radicals in the aqueous phase, and improve the synthesis efficiency of the core-shell structure. The metal salt can be any metal salt commonly used in the art. Generally, it is an ionic compound, such as sodium chloride.

[0049] Here, the polymerization inhibitor may include one or more of nitrite and potassium dichromate.

[0050] In some optional embodiments, the oil phase comprises an initiator, an alkane, a monomer, and a crosslinker, and the weight ratio of the initiator, the alkane, the monomer, and the crosslinker may be (0.5-3):(20-200):100:(2-10).

[0051] In some optional embodiments, the aqueous phase contains a dispersant, an emulsifier, a metal salt, a polymerization inhibitor, and water, and the mass ratio of the dispersant, the emulsifier, the metal salt, the polymerization inhibitor, and the water may be (1 to 50):(0.01 to 5):1:(0.01 to 2):(150 to 1000).

[0052] Electrochemical Equipment

[0053] In the electrochemical device according to the third aspect of the present invention, the above-mentioned latex is used in the process of manufacturing the electrode sheet of the electrochemical device.

[0054] In the present invention, the electrode sheet is preferably a negative electrode sheet.

[0055] In the present invention, the electrochemical device may be a sodium ion battery or a lithium ion battery, preferably a lithium ion battery.

[0056] In some optional embodiments, the lithium ion battery includes the above-described negative electrode sheet, positive electrode sheet, separator, and electrolyte.

[0057] Negative electrode sheet

[0058] In some optional embodiments, the negative electrode sheet can be manufactured by a method conventional in the art. For example, the following method can be used: A negative electrode active material, a conductive agent, the latex, and a thickener are mixed in a certain mass ratio, and then a solvent is added and mixed uniformly to obtain a negative electrode slurry; the negative electrode slurry is then uniformly coated on a negative electrode current collector; and the negative electrode sheet is manufactured through processes such as drying, rolling, and cutting.

[0059] The mass ratio of the negative electrode active material, the conductive agent, the latex, and the thickener may be (96.4 to 98.1):0.5:(0.9 to 2.6):0.5, where the mass ratio is calculated based on the mass of the solid phase in the latex.

[0060] The negative electrode active material may be a negative electrode active material commonly used in the field for preparing a negative electrode sheet, and may be selected from one or more of artificial graphite, natural graphite, soft carbon, hard carbon, mesophase carbon microbeads, silicon monoxide, and silicon carbide.

[0061] The addition of the thickener can increase the viscosity of the system of each component in the negative electrode slurry, and may be a thickener commonly used in the art for producing negative electrode sheets, such as sodium carboxymethyl cellulose (CMC).

[0062] There are no particular limitations on the conductive agent, and it may be any material that is conductive and does not cause chemical changes in the battery. For example, the following may be used: graphite, such as natural graphite or artificial graphite; carbon-based materials, such as carbon black (Super P), acetylene black, ketjen black, channel black, furnace black, lamp black, thermal carbon black, or carbon fiber; metal powder or metal fiber, such as copper, nickel, aluminum, or silver; conductive whiskers, such as zinc oxide whiskers or potassium titanate whiskers; conductive metal oxides, such as titanium dioxide; or conductive polymers, such as polyphenylene derivatives.

[0063] The negative electrode current collector may be a current collector commonly used in the art for negative electrodes, and may be a conventional current collector or a composite current collector. The negative electrode current collector may be made of any electrically conductive material that does not undergo chemical change. For example, the following materials may be used: copper, stainless steel, aluminum, nickel, titanium, sintered carbon, or an aluminum-cadmium alloy; or copper, stainless steel, or an aluminum-cadmium alloy that has been surface-treated with carbon, nickel, titanium, or silver. Furthermore, the surface of the negative electrode current collector may be finely embossed to enhance adhesion of the negative electrode active material. The negative electrode current collector may be in various forms, such as a film, sheet, foil, mesh, or porous body.

[0064] In some optional embodiments, the thickness of the negative electrode current collector may be 5 to 10 μm.

[0065] In some specific embodiments, the negative electrode current collector is a copper foil having a thickness of 10 μm.

[0066] Positive electrode sheet

[0067] In some optional embodiments, the positive electrode sheet can be manufactured by a conventional method in the art. For example, the following method can be adopted. The positive electrode active material, conductive agent, and binder are mixed in a specific mass ratio, and then a solvent is added and mixed uniformly to obtain a positive electrode slurry. The positive electrode slurry is then uniformly coated on the positive electrode current collector, followed by drying, cold pressing, and cutting to obtain a positive electrode sheet.

[0068] The positive electrode active material is a positive electrode active material commonly used in the art for manufacturing a positive electrode sheet, such as LiFePO4 or LiNi 0.5 Co 0.2 Mn 0.3 O2 ternary positive electrode material.

[0069] The conductive agent is not particularly limited, and may be any material that is conductive and does not cause chemical changes in the battery. For example, the following may be used: graphite, such as natural graphite or artificial graphite; carbon-based materials, such as carbon black (Super P), acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, or carbon fiber; metal powder or metal fiber, such as copper, nickel, aluminum, or silver; conductive whiskers, such as zinc oxide whiskers or potassium titanate whiskers; conductive metal oxides, such as titanium dioxide; or conductive polymers, such as polyphenylene derivatives.

[0070] The binder enhances adhesion between positive electrode active materials and between the positive electrode active material and the positive electrode current collector, and is not particularly limited in type. Specific examples of binders include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof, and a mixture of any one or more of these can be used.

[0071] Any material that does not undergo chemical changes and has high conductivity can be used for the positive electrode current collector without any restrictions. For example, the following materials are typically used: stainless steel, aluminum, nickel, titanium, or calcined carbon, or aluminum or stainless steel materials that have been surface-treated with carbon, nickel, titanium, silver, or the like. The surface of the positive electrode current collector can be finely embossed to improve adhesion of the positive electrode active material. The positive electrode current collector can be used in various forms, such as a film, sheet, foil, mesh, or porous body.

[0072] In some optional embodiments, the positive electrode current collector may have a thickness of 8 to 16 μm.

[0073] In some specific embodiments, the positive electrode current collector is an aluminum foil having a thickness of 16 μm.

[0074] Separator

[0075] In some optional embodiments, the separator is selected from a polypropylene film or a polyethylene film.

[0076] Here, the separator may have an air permeability of 180 to 380 s / 100 mL.

[0077] Here, the porosity of the separator may be 30 to 50%.

[0078] Here, the thickness of the separator may be 12 μm.

[0079] In one specific embodiment, the separator is a polypropylene film, the thickness of the separator is 12 μm, the air permeability of the separator is 230 s / 100 mL, and the porosity of the separator is 40%.

[0080] electrolyte

[0081] In some embodiments, the electrolyte may be an electrolyte commonly used in lithium-ion batteries, generally comprising a non-aqueous solvent, a lithium salt, and an additive.

[0082] Here, the non-aqueous solvent may be a non-aqueous solvent conventional in the art, preferably an ester-based solvent, more preferably a carbonate-based solvent, and may be one or more selected from the group consisting of vinylene carbonate, dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC).

[0083] Here, the additive may be one or more selected from the group consisting of vinylene carbonate (VC), fluoroethylene carbonate (FEC), vinylethylene carbonate (VEC), ethylene sulfate (DTD), vinylene sulfate, 1,3-propane sultone (PS), 1-propene 1,3-sultone, and 1,4-butane sultone.

[0084] Here, the lithium salt may be any lithium salt commonly used in the art, such as LiPF6.

[0085] In the present invention, the lithium ion battery may be manufactured by a conventional manufacturing method in the art. The method may include winding the positive electrode sheet, the negative electrode sheet, and the separator to obtain a battery cell, packaging the battery cell in a case, and injecting the electrolyte. Alternatively, the method may include stacking the negative electrode sheet, the separator, the positive electrode sheet, and the separator in this order to obtain a battery cell, packaging the battery cell in a case, and injecting the electrolyte.

[0086] On a basis consistent with practice in the art, the preferred conditions set forth above may be combined in any manner to obtain preferred embodiments of the present invention.

[0087] Example 1 Preparation of latex Step (1): An oil phase is formed by mixing 0.5 parts by mass of azobisisobutyronitrile, 200 parts by mass of isopentane, 100 parts by mass of monomers (including 50 parts by mass of styrene, 40 parts by mass of hydroxyethyl acrylate, and 10 parts by mass of methyl methacrylate), and 2 parts by mass of divinylbenzene. Step (2): 10 parts by weight of polyvinyl alcohol, 0.01 parts by weight of sodium dodecyl sulfate, 0.01 parts by weight of sodium nitrite, and 1 part by weight of sodium chloride are dispersed or dissolved in 1000 parts by weight of deionized water to form an aqueous phase. Step (3): The oil and water phases were mixed and the resulting mixture was sheared at 5000 rpm for 10 minutes using a high-speed shearer to obtain an oil / water dispersion, where the mass ratio of the oil phase to the water phase was 1:3.3. Step (4): The oil / water dispersion is transferred to a high-pressure cooker, the pressure inside the cooker is controlled to 0.6 MPa, the temperature is raised to 70°C, and after polymerization for 8 hours, the pressure is reduced to obtain a latex. Here, the latex obtained in Example 1 was measured using an electron scanning microscope (SU-8010, Hitachi, Ltd., Japan) to obtain an SEM image (shown in Figure 1). The latex obtained in Example 1 was also tested using a transmission electron microscope (HT-7700, Hitachi, Ltd., Japan) to obtain a TEM image (shown in Figure 2). Figure 2 shows that the latex prepared in Example 1 contains particles with a core-shell structure. Here, the core of the core-shell structure is a cavity. The sample preparation method for the above SEM and TEM tests is as follows: 0.01 mL of latex is taken, diluted 5 to 10 times, and then dried to obtain latex particles.

[0088] Example 2 Preparation of latex Based on Example 1, only the amount of isopentane used in step (1) of Example 1 was changed to 150 parts by mass, with the other conditions remaining unchanged.

[0089] Example 3 Preparation of latex Based on Example 1, the amount of divinylbenzene used in step (1) of Example 1 was changed to 10 parts by mass, with the other conditions remaining unchanged.

[0090] Example 4 Preparation of latex Based on Example 3, only the amount of isopentane used in step (1) of Example 1 was changed to 50 parts by mass, with the other conditions remaining unchanged.

[0091] Example 5 Preparation of latex Based on Example 1, the amount of isopentane used in step (1) of Example 1 was changed to 20 parts by mass, with the other conditions remaining unchanged.

[0092] Example 6 Preparation of latex Based on Example 1, only the amount of isopentane used in step (1) of Example 1 was changed to 16.7 parts by mass, while the other conditions were not changed.

[0093] Example 7 Preparation of latex Based on Example 1, only the amount of isopentane used in step (1) of Example 1 was changed to 250 parts by mass, with the other conditions remaining unchanged.

[0094] Example 8 Preparation of latex Based on Example 1, the amount of sodium dodecyl sulfate used in step (2) of Example 1 was changed to 5 parts by mass, with the other conditions remaining unchanged.

[0095] Example 9 Preparation of latex Based on Example 1, the amount of sodium dodecyl sulfate used in step (2) of Example 1 was changed to 6 parts by mass, with the other conditions remaining unchanged.

[0096] Example 10 Preparation of latex Based on Example 1, but without changing other conditions, the amount of polyvinyl alcohol used in step (2) of Example 1 is changed to 5 parts by mass, the amount of sodium dodecyl sulfate used to 0.005 parts by mass, the amount of sodium nitrite used to 0.005 parts by mass, the amount of sodium chloride used to 0.5 parts by mass, and the amount of deionized water used to 500 parts by mass.

[0097] By adjusting the parameters of the above process, latexes with different properties of Examples 1 to 10 are obtained.

[0098] Comparative Example 1 The latex used is BM-451B latex from Japan's ZEON.

[0099] Comparative Example 2 The latex used is BM-451B latex from Japan's ZEON.

[0100] Table 1 shows the relationship between the true density, porosity, particle size, shell thickness of the core-shell structure, and amount of each component used in the latex of the binder in the latexes of Examples 1 to 10, and the true density, porosity, particle size, etc. of the binder in the latexes of Comparative Examples 1 and 2.

[0101] Here, the true density of the binder can be calculated by the following measurement method. Constant volume V ラテックス Take the latex and let its mass be m ラテックス Next, the mixture is dried to evaporate the solvent, and the mass of the solvent m 溶媒 and volume V 溶媒 The true density of the binder can be calculated using the following formula: true density=m バインダー / V バインダー =(m ラテックス -m 溶媒 ) / (V ラテックス -V 溶媒 ). The specific procedure for measuring the true density is a conventional technical means in the field, and therefore will not be described in detail here.

[0102] Here, the size of the porosity of the binder can be predicted by the following calculation method. The specific calculation method is as follows. Take a single core / single shell calculation as an example: P=V アルカン =ρ バインダーポリマー / (ρ バインダーポリマー +ρ アルカン ×m バインダーポリマー / m アルカン where P is the porosity of a single particle (volume fraction of the cavity), and V アルカン is the volume fraction of alkanes, and ρ バインダーポリマー is the density of the binder polymer, and m バインダーポリマー is the mass of the monomer, and m アルカン indicates the mass of the alkane. The following assumptions are made for this equation to hold: (1) The core-shell structure is perfect and symmetrical, and (2) The phase separation between the alkane and the binder polymer is complete.

[0103] [Table 1]

[0104] Note: In Table 1, " / " indicates that the condition parameter is not involved in the specific experiment.

[0105] Effect Example 1

[0106] (1) Manufacturing of negative electrode sheets The manufacturing method of the negative electrode sheet employs the following steps. For the negative electrode sheets of Examples 1 to 10 and Comparative Examples 1 and 2, the negative electrode active materials graphite, conductive carbon black, and latex, and the thickener sodium carboxymethylcellulose (CMC) were mixed according to the amounts used in Table 2, and then deionized water was added as a solvent and stirred to uniformly mix, yielding a negative electrode slurry. The negative electrode slurry was uniformly coated on a 10 μm copper foil current collector. The negative electrode sheets of Examples 1 to 10 and Comparative Examples 1 and 2 were manufactured through processes such as drying, roll pressing, and cutting. The roll pressing pressure was 5 T.

[0107] (2) Electrode sheet peel strength test The electrode sheet to be tested is cut into a long, thin strip (length x width = 10 cm x 3 cm) and attached to a long, thin quartz glass plate (the area of the long, thin quartz glass plate is slightly larger than the electrode sheet) with VHB tape attached. A special small roller is used to repeatedly press the electrode sheet three times to ensure sufficient adhesion between the electrode sheet and VHB. Using a universal tensile tester, the standard method for measuring 180° peel force is selected to measure the peel force value of the electrode sheet. Each sample is measured three times and the average value is calculated.

[0108] The results of the above tests are shown in Table 2.

[0109] Effect Example 2

[0110] Lithium-ion battery manufacturing

[0111] (1) Preparation of positive electrode sheet: The positive electrode active material, lithium iron phosphate, conductive carbon black (Super P), and binder, polyvinylidene fluoride (PVDF), are uniformly mixed in a mass ratio of 97:1:2 (total 100 parts by mass), and N-methylpyrrolidone (NMP) is added. The mixture is stirred to disperse uniformly, yielding a positive electrode slurry for lithium-ion batteries. The positive electrode slurry is then uniformly applied to a 16 μm thick aluminum foil. After drying, roll pressing, and cutting, a positive electrode sheet is obtained. Here, the surface density of the positive electrode sheet is 19.88 mg / cm 2The positive electrode sheet is cut to a size of 45 mm x 58 mm. The mass of the positive electrode active material is calculated by multiplying the surface density of the positive electrode sheet by the area and the mass ratio of the positive electrode active material, i.e., 19.88 mg / cm. 2 × 45 mm × 58 mm × 97%. The positive electrode sheet is the same in each example and comparative example.

[0112] (2) Manufacture of negative electrode sheet: The manufacture of the negative electrode sheet is the same as in (1) of Example 1. Here, the size of the negative electrode sheet is cut to 50 mm × 60 mm. The mass of the negative electrode active material is the surface density of the negative electrode sheet × the area × the mass ratio of the negative electrode active material. When a negative electrode sheet is manufactured using the latex of Comparative Example 1, the surface density of the negative electrode sheet is 9.0 mg / cm 2 The mass of the negative electrode active material used in Examples 1 to 10 and Comparative Example 2 is the same as that in Comparative Example 1. When the mass of the negative electrode active material exceeds the mass of the positive electrode active material, the initial discharge capacity is determined by the amount of positive electrode active material used. If the amounts of positive electrode active material used are consistent, the initial discharge capacities should also be consistent, and any discrepancy is considered a measurement error.

[0113] (3) Preparation of electrolyte: Ethylene carbonate (EC) and propylene carbonate (PC) are mixed uniformly in a volume ratio of 1:1, and then an appropriate amount of LiPF6 is dissolved in the mixed solution to prepare an electrolyte with a concentration of 1 mol / L.

[0114] (4) Preparation of separator: The separator is a polypropylene film. The thickness of the separator is 12 μm, the air permeability of the separator is 230 s / 100 mL, and the porosity of the separator is 40%.

[0115] (5) Fabrication of a lithium-ion battery: The fabricated positive electrode sheet, separator, negative electrode sheet fabricated from the latex of Examples 1 to 10 and Comparative Example 1, and electrolyte were assembled in this order (here, 8 layers of positive electrode sheets and 9 layers of negative electrode sheets). The positive electrode sheet, separator, negative electrode sheet fabricated from the latex of Examples 1 to 10 and Comparative Example 1, and separator were alternately combined, and a soft-pack battery with a capacity of 1 Ah was obtained after a chemical conversion treatment.

[0116] The negative electrode sheets and lithium ion batteries prepared from the latexes of Examples 1 to 10 and Comparative Example 1 were subjected to the following tests. Test method for capacity retention rate (%) of lithium-ion batteries after 1000 cycles: In a constant temperature environment of 25°C, the battery was charged at a constant current and constant voltage at a rate of 1C up to 4.25V (the cut-off current at constant voltage was 0.05C), and then discharged at a constant current of 0.33C down to 2.8V. This procedure constitutes one cycle. This cycle was repeated 1000 times, and the ratio of the discharge capacity at the 1000th cycle to the discharge capacity at the first cycle (initial discharge capacity) was calculated as the capacity retention rate. The test results are shown in Table 2.

[0117] The results of the above tests are shown in Table 2.

[0118] [Table 2]

[0119] Note: In Table 2, " / " indicates that the parameter in question is not involved in the specific experiment.

[0120] The true density of the binder in the latex prepared in Examples 1 to 10 of the present invention is 1 g / cm 3 The molar percentage of particles having the core-shell structure exceeds 99% and can be basically considered as 100%. On the other hand, the BM-451B latex of Japan Zeon, which was used in Comparative Examples 1 and 2, has a true density of 1.1 g / cm 3 and is a solid particle.

[0121] When preparing negative electrode sheets using the latexes of Examples 1 to 10 of the present invention, the amount of latex used by mass was 0.9 to 2.6%, while the amount of latex used by mass in Comparative Example 1 was 3%. The peel strength of the electrode sheets prepared using the latexes of Examples 1 to 10 of the present invention was equivalent to that of the electrode sheet prepared in Comparative Example 1. A comparison of Comparative Examples 1 and 2 reveals that the existing latex of Japan's ZEON BM-451B cannot be used properly unless it is used at a ratio of at least 3%. Therefore, if the latex ratio is less than 3% (for example, the latex ratio in Comparative Example 2 is 2.6%), the material falls off and the material cannot be used to prepare negative electrode sheets.

[0122] When preparing a negative electrode sheet using the latexes of Examples 1 to 10, if the amount of latex used by mass is 0.9 to 2.6%, the capacity retention rate (1000 cycles) of the resulting lithium-ion battery can reach 90% or more, even 91%. On the other hand, when the amount of latex used by mass in Comparative Example 1 is 3%, the capacity retention rate (1000 cycles) of the resulting lithium-ion battery is 90.4%. This shows that the latexes of the examples of the present invention can achieve equivalent capacity retention rates of lithium-ion batteries with smaller amounts used. Furthermore, when preparing a negative electrode sheet using the latexes of Examples 1 to 10, the initial discharge capacity and discharge capacity after 1000 cycles of the resulting lithium-ion battery are equivalent to those of Comparative Example 1.

[0123] In Examples 5 and 6, the porosity of the binder in the obtained latex is lower than that in the other examples, and therefore, when applied to the preparation of a negative electrode sheet, a higher mass amount of latex is required.

[0124] From the above, it can be seen that the latex of the present invention can achieve the same adhesive strength level as that of a conventional binder at a higher mass amount with a lower mass amount of latex. When the above latex is used as a binder to prepare an electrode sheet for a lithium ion battery, the mass fraction of the binder can be reduced while providing the lithium ion battery with the same energy density. At the same time, excellent cycle stability of the battery is ensured, and there is no decrease in cycle life during use.

[0125] Although the above describes specific embodiments of the present invention, those skilled in the art should understand that these are merely examples. The protection scope of the present invention is limited by the appended claims. Those skilled in the art may make various changes and modifications to these embodiments without departing from the principles and essence of the present invention, and all such changes and modifications shall fall within the protection scope of the present invention. [Industrial Applicability]

[0126] The latex of the present invention can be used in an electrode sheet of a battery.

Claims

1. A latex that is a dispersion containing a binder and a solvent, the binder comprises particles having a core-shell structure, the core of the core-shell structure being a cavity, and the shell of the core-shell structure being a binder polymer; The latex is characterized in that the solvent includes water.

2. The latex according to claim 1, which satisfies one or more of the following conditions (a) to (c): (a) The true density of the binder is 0.3 to 0.95 g / cm ３ is. (b) The porosity of the binder is 50% or more. (c) The particle size of the binder is 0.1 to 10 μm.

3. The latex according to claim 1, which satisfies one or two of the following conditions (a) to (c): (a) The thickness of the shell of the core-shell structure is 0.01 to 1 μm. (b) The form of the core-shell structure includes one or more of a single core, multiple cores, a single shell, and multiple shells. (c) The molar percentage of particles having the core-shell structure among all particles in the binder is more than 99%.

4. 2. The latex of claim 1, wherein the solids content (the mass percentage of the binder in the latex) of the latex is ≤60%.

5. A method for producing the latex according to any one of claims 1 to 4, comprising the steps of: The method for producing the latex includes the steps of shearing a mixture of an oil phase and an aqueous phase, carrying out a polymerization reaction under a pressure of 0.1 to 1 MPa, and obtaining the latex after reducing the pressure; the oil phase contains an alkane and a raw material for the binder polymer, the boiling point of the alkane being less than 100°C, and the raw material for the binder polymer containing at least a monomer; The method for producing the emulsion, wherein the aqueous phase comprises water and an emulsifier.

6. The method according to claim 5, wherein the method satisfies one or more of the following conditions (a) to (h): (a) The boiling point of the alkane is less than 80°C. (b) The mass ratio of the oil phase to the aqueous phase is 1:(1.5 to 9). (c) The mass ratio of the alkane to the monomer is 1:(0.4 to 6). (d) The mass ratio of the emulsifier to the water is (0.01 to 5):(150 to 1000). (e) The shearing time is 5 to 15 minutes. (f) The temperature of the shearing treatment is lower than the ambient temperature. (g) The temperature of the polymerization reaction is 50 to 85°C. (h) The polymerization reaction time is 6 to 12 hours.

7. The method according to claim 5, wherein the method satisfies one or more of the following conditions (a) to (d): (a) the alkane is C ５ ~C １０ It contains one or more of the short-chain alkanes. (b) The monomer includes one or more of acrylic acid ester, methacrylic acid ester, hydroxyethyl acrylate, styrene, acrylic acid, butadiene, methacrylic acid, acrylonitrile, methacrylonitrile, acrylamide, and derivatives thereof. (c) The raw materials for the binder polymer further include an initiator, and the initiator is selected from an azo-based initiator and a peroxide-based initiator. (d) The raw materials of the binder polymer further include a crosslinking agent, and the crosslinking agent includes one or more of divinylbenzene, ethylene glycol dimethacrylate, allyl methacrylate, ethylene glycol diacrylate, butylene glycol diacrylate, butylene glycol dimethacrylate, neopentyl glycol dimethacrylate, hexanediol dimethacrylate, triethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate, trimethylolpropane, and pentaerythritol tetramethacrylate.

8. The method according to claim 5, wherein the method satisfies one or both of the following conditions (a) and (b): (a) The emulsifier includes one or more of sodium dodecyl sulfate, hexadecyltrimethylammonium bromide, nonylphenol polyoxyethylene ether, OP surfactants, Span surfactants, Tween surfactants, and sodium dodecylbenzenesulfonate. (b) The aqueous phase further comprises one or more of a dispersant, a metal salt, and a polymerization inhibitor, the dispersant comprising one or more of polyvinyl alcohol, magnesium hydroxide, calcium carbonate, calcium phosphate, silica sol, polyvinylpyrrolidone, styrene-maleic anhydride copolymer, and carboxymethyl cellulose, the metal salt being an ionic compound, and the polymerization inhibitor comprising one or more of a nitrite and potassium dichromate.

9. The method according to claim 5, wherein the method satisfies one or both of the following conditions (a) and (b): (a) The oil phase contains an initiator, an alkane, a monomer, and a crosslinking agent, and the mass ratio of the initiator, the alkane, the monomer, and the crosslinking agent is (0.5 to 3):(20 to 200):100:(2 to 10). (b) The aqueous phase contains a dispersant, an emulsifier, a metal salt, a polymerization inhibitor, and water, and the mass ratio of the dispersant, the emulsifier, the metal salt, the polymerization inhibitor, and the water is (1 to 50):(0.01 to 5):1:(0.01 to 2):(150 to 1000).

10. An electrochemical device, characterized in that the latex according to any one of claims 1 to 4 is used in the process of producing an electrode sheet for the electrochemical device.

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