Binder composition for secondary batteries and electrode mixture for secondary batteries containing the same

Abstract

The present invention relates to a binder composition for a secondary battery, which contains emulsion polymer particles A having a core-shell structure including a core part and a shell part occupying part or all of the surface of the core part, and emulsion polymer particles B having no core-shell structure, and satisfies the following [Relational Expression 1]. [Relational Expression 1] 50 < N(A) / N(B) < 400 (In the formula, N(A) is the number of particles of the emulsion polymer particles A having the core-shell structure, and N(B) is the number of particles of the emulsion polymer particles B having no core-shell structure.)

Description

[Technical Field]

[0001] The present invention relates to a binder composition for secondary batteries, an electrode mixture for secondary batteries containing the same, and a secondary battery. [Background technology]

[0002] In recent years, with the technological development and increasing demand for portable devices such as portable computers, mobile phones, and cameras, the demand for secondary batteries as an energy source has surged. Among such secondary batteries, lithium-ion batteries, which exhibit high energy density and operating potential, long cycle life, and low self-discharge rate, have been the subject of extensive research and have been commercialized and are widely used.

[0003] Generally, in lithium-ion batteries, charging and discharging occur through repeated insertion and removal of lithium ions from the positive electrode to the negative electrode. This repeated insertion and removal of lithium ions loosens the bonds between the electrode active material or conductive material, increasing the contact resistance between particles. As a result, the ohmic resistance of the electrode increases, and the battery performance deteriorates. Therefore, the binder needs to act as a buffer against the expansion and contraction of the electrode active material caused by the insertion and removal of lithium ions in the electrode, and it is preferable that the binder be an elastic polymer.

[0004] Furthermore, the binder is required to have sufficient adhesive strength to maintain the bonding force between the electrode active material and the current collector during the drying process of the electrode plates. In particular, when natural graphite, which has a theoretical discharge capacity of 372 mAh / g, is used in combination with materials such as silicon, tin, or silicon-tin alloys, which have high discharge capacities, in order to increase the discharge capacity, the volume expansion of the materials increases significantly as charging and discharging progresses. This causes the negative electrode material to detach, and as a result, the battery capacity decreases rapidly with repeated cycles.

[0005] Therefore, there is a strong demand in this industry for research on binders and electrode materials that can prevent separation between electrode active materials or between electrode active materials and current collectors during electrode manufacturing, and that can control the volume expansion of electrode active materials that occurs during repeated charge and discharge cycles through superior physical properties, thereby improving the structural stability of electrodes and the performance of batteries even after hundreds of cycles.

[0006] Furthermore, since binders can act as resistors themselves, they must not significantly affect the resistance and ionic conductivity characteristics of the battery. In the case of medium and large batteries, high output is often required, making resistance one of the important physical properties of the battery, and there is a strong demand for research on binders with low resistance. [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] The object of the present invention is to provide a binder for secondary batteries that reduces contamination during pressing of secondary battery electrodes, thereby improving electrode productivity, ensures high adhesion even during high-speed coating, and ultimately contributes to extending the lifespan of secondary batteries.

[0008] Furthermore, an object of the present invention is to provide a binder for secondary batteries that can solve the problem of electrode surface defects caused by roll contamination (a phenomenon in which electrode active materials, conductive materials, etc. are adsorbed onto the roll and contaminate the roll surface) that occurs during the pressing process, by inducing sufficient distribution of the binder on the electrode surface of the secondary battery. [Means for solving the problem]

[0009] A binder composition for secondary batteries according to one embodiment of the present invention comprises emulsion polymer particles A having a core-shell structure including a core portion and a shell portion occupying part or all of the surface of the core portion, and emulsion polymer particles B not having a core-shell structure, and satisfies the following [Relational Formula 1].

[0010] [Relationship 1] 50 < N(A) / N(B) < 400

[0011] In the above [Relational Expression 1], N(A) is the number of particles of the emulsion polymerized particles A having the above core-shell structure, and N(B) is the number of particles of the emulsion polymerized particles B not having the above core-shell structure.

[0012] In the above [Relational Expression 1], N(A) / N(B) is determined by the following [Relational Expression 2].

[0013] [Relational Expression 2] N(A) / N(B) = {D(B) / D(A)}

[0019] , , , ,

[0018] *{X(A) / X(B)}

[0014] In the above [Relational Expression 2], D(A) is the average particle diameter (D50) of the particles A, D(B) is the average particle diameter (D50) of the particles B, and X(A) / X(B) is the mixing ratio of A to B.

[0015] As a more preferable example, the emulsion polymerized particles A having the above core-shell structure have an average particle diameter (D50) of 100 nm or less.

[0016] As a more preferable example, all the polymer particles contained in the above binder composition for secondary batteries have an average particle diameter (D50) of 400 nm or less.

[0017] As a more preferable example, the glass transition temperature (Tg) of the emulsion polymerized particles A having the above core-shell structure is less than 0°C.

[0018] As a more preferable example, the glass transition temperature (Tg) of the emulsion polymerized particles B not having the above core-shell structure is less than 0°C.

[0019] As a more preferred example, the core portion of the emulsified polymer particle A having the above core-shell structure includes one or more monomers selected from the group consisting of (a) conjugated diene monomers or conjugated diene polymers, (b) acrylate monomers, vinyl monomers, (meth)acrylamide monomers, and nitrile monomers, and one or more monomers or polymers selected from (c) unsaturated carboxylic acid monomers.

[0020] As a more preferred example, the core portion of the emulsion polymer particle A having the above core-shell structure contains a monomer or polymer polymer in which, with respect to the total weight of the core portion, the monomer or polymer of group (a) is 30 to 70% by weight, the monomer of group (b) is 20 to 50% by weight, and the unsaturated carboxylic acid monomer of group (c) is 0.5 to 20% by weight.

[0021] As a more preferred example, the core portion of the emulsion polymer particle A having the above core-shell structure contains styrene-butadiene latex.

[0022] As a more preferred example, the shell portion of the emulsified polymer particle A having the above core-shell structure comprises one or more monomers selected from the group consisting of (a) (meth)acrylic acid ester monomers, (b) acrylate monomers, vinyl monomers, (meth)acrylamide monomers, and nitrile monomers, and one or more monomers or polymers selected from (c) unsaturated carboxylic acid monomers.

[0023] As a more preferred example, the shell portion of the emulsion polymer particle A having the core-shell structure described above contains a monomer polymer in which, based on the total weight of the shell portion, the monomers of group (a) are present in an amount of 50 to 90% by weight, the monomers of group (b) are present in an amount of 0.5 to 30% by weight, and the unsaturated carboxylic acid monomers of group (c) are present in an amount of 0.5 to 20% by weight.

[0024] An electrode mixture for a secondary battery according to one embodiment of the present invention comprises the above-mentioned binder composition for a secondary battery.

[0025] A secondary battery according to one embodiment of the present invention includes the above-mentioned electrode mixture for secondary batteries. [Effects of the Invention]

[0026] According to the present invention, it is possible to provide a binder for secondary batteries that reduces contamination during pressing of secondary battery electrodes, improves the productivity of electrodes, ensures high adhesion even during high-speed coating, and ultimately contributes to extending the lifespan of secondary batteries.

[0027] Furthermore, according to the present invention, by inducing sufficient distribution of the binder on the surface of the secondary battery electrodes, it is possible to provide a binder for secondary batteries that can solve the problem of electrode surface defects caused by roll contamination (a phenomenon in which electrode active materials, conductive materials, etc. are adsorbed onto the roll and contaminate the roll surface) that occurs during the pressing process. [Modes for carrying out the invention]

[0028] The term "including" as used herein should be understood as open-ended terms that may include other embodiments.

[0029] As used herein, "preferred" and "preferred" refer to embodiments of the present invention in which a predetermined effect is obtained under a predetermined environment. However, other preferred embodiments may exist under the same or different environments. Furthermore, although one or more preferred embodiments are given, this does not mean that other embodiments are not useful, nor does it mean that other embodiments are excluded from the scope of the present invention.

[0030] A binder composition for secondary batteries according to one embodiment of the present invention comprises emulsion polymer particles A having a core-shell structure including a core portion and a shell portion that occupies part or all of the surface of the core portion.

[0031] In this invention, a core-shell structure means that the composition, structure, or properties of the core and shell of a particle are different.

[0032] For example, the weight ratio of the core and shell parts may be 95:5 to 5:95, more specifically 90:10 to 10:90, more specifically 90:10 to 30:70, and even more specifically 90:10 to 50:50. The above weight ratio is the ratio for which the binder according to the present invention exhibits optimal adhesive strength, and if it falls outside the above range, the desired effect in the present invention cannot be achieved, which is undesirable.

[0033] In the present invention, emulsion polymer particles refer to polymer particles polymerized by emulsion polymerization in the presence of an emulsifier and a polymerization initiator, and these can be produced by well-known emulsion polymerization methods.

[0034] For example, the polymerization temperature and polymerization time can be set as appropriate depending on the circumstances. For instance, the polymerization temperature may be approximately 10°C to approximately 95°C, and the polymerization time may be approximately 0.5 hours to approximately 20 hours.

[0035] As an example, emulsifiers that can be used in the above emulsion polymerization include, but are not limited to, one or more emulsifiers selected from the group consisting of anionic emulsifiers, cationic emulsifiers, and nonionic emulsifiers. Such emulsifiers are substances that simultaneously have hydrophilic and hydrophobic groups, and form micelle structures during the emulsion polymerization process, allowing polymerization of each monomer to occur within the micelle structure.

[0036] As an example, polymerization initiators that can be used in the above emulsion polymerization include inorganic or organic peroxides. For example, water-soluble initiators such as potassium persulfate, sodium persulfate, and ammonium persulfate, and oil-soluble initiators such as cumene hydroperoxide and benzoyl peroxide can be used, but are not limited to these.

[0037] As an example, the polymerization initiator may further contain an activator to promote the initiation of the peroxide reaction. Such activators include, but are not limited to, one or more selected from the group consisting of sodium formaldehyde sulfoxylate, sodium ethylenediaminetetraacetate, ferrous sulfate, and dextrose.

[0038] In one embodiment of the present invention, the binder composition for secondary batteries contains polymer particles B, where polymer particles B are defined as all polymer particles in the binder composition for secondary batteries except for the emulsified polymer particles A having the core-shell structure described above. That is, it contains polymer particles B that do not have a core-shell structure.

[0039] The binder composition for secondary batteries of the present invention can achieve the best effect as intended by including emulsion polymer particles A having a core-shell structure and polymer particles B not having a core-shell structure.

[0040] As a more preferred example, the polymer particles B may be emulsion polymer particles.

[0041] When N(A) is the number of emulsion polymer particles A having the core-shell structure described above, and N(B) is the number of emulsion polymer particles B not having the core-shell structure, the binder composition for secondary batteries satisfies the following [Relational Formula 1].

[0042] [Relationship 1] 50 <N(A) / N(B)<400

[0043] More preferably, the lower limit of N(A) / N(B) may be 100.

[0044] Furthermore, more preferably, the upper limit of N(A) / N(B) may be 300.

[0045] As can be confirmed from the experimental examples described later, the present invention can achieve the best effect intended within the above numerical range.

[0046] As a more preferred example, the emulsion polymer particles A having the above core-shell structure may have an average particle size (D50) of preferably 100 nm or less, more preferably 90 nm or less, and even more preferably 80 nm or less. As can be seen from the experimental examples described later, the present invention can achieve the best effect intended within the above numerical range.

[0047] As a more preferred example, all polymer particles contained in the above secondary battery binder composition may have an average particle size (D50) of preferably 400 nm or less, more preferably 300 nm or less. As can be seen from the experimental examples described later, the present invention can achieve the best effect intended within the above numerical range.

[0048] In the present invention, when the emulsion polymer particles A and / or polymer particles B having the core-shell structure described above are emulsion polymers, the average particle size (D50) of polymer particles B can be adjusted by the amount of emulsifier. Generally, the particle size tends to decrease as the amount of emulsifier increases and increase as the amount of emulsifier decreases. The desired average particle size can be achieved by adjusting the amount of emulsifier used, taking into consideration the desired particle size, reaction time, reaction stability, etc.

[0049] As a more preferred example, the glass transition temperature (Tg) of the emulsion polymer particles A having the core-shell structure described above may be less than 0°C, less than -5°C, less than -10°C, or less than -15°C. As can be seen from the experimental examples described later, the present invention can achieve the best effect intended within the above numerical range.

[0050] As a more preferred example, the glass transition temperature (Tg) of the emulsion polymer particles B that do not have the core-shell structure described above may be less than 0°C, less than -5°C, or less than -10°C. As can be seen from the experimental examples described later, the present invention can achieve the best effect intended within the above numerical range.

[0051] As a more preferred example, the gel content of the emulsion polymer particles A having the core-shell structure and / or the emulsion polymer particles B not having the core-shell structure is preferably 95% or more. More preferably, the gel content of the emulsion polymer particles A having the core-shell structure and / or the emulsion polymer particles B not having the core-shell structure may be 95-99%, 95-98%, or 96-97.5%. If the gel content of the emulsion polymer particles A having the core-shell structure and / or the emulsion polymer particles B not having the core-shell structure is less than 95%, swelling in the electrolyte may increase, which may reduce the battery life.

[0052] The gel content mentioned above is calculated using the following [Mathematical Formula 1].

[0053] [Mathematical formula 1] Gel content (%) = M b / M a *100

[0054] In the above [Mathematical Formula 1], M a This is the weight measured after drying a sample of emulsion polymer particles at 80°C for 24 hours, and M b This refers to the weight of the mesh and the weight of the emulsion polymer particles remaining on the mesh after sieving a sample of emulsion polymer particles whose weight was measured in 50g of THF (tetrahydrofuran) for 24 hours or more, and then sieving it through a 200-mesh sieve.

[0055] The gel content of such emulsion polymer particles indicates the degree of crosslinking of the copolymer and is calculated as shown in [Mathematical Equation 1] above, and is expressed as the insoluble fraction in the electrolyte.

[0056] As a more preferred example, the core portion of the emulsion polymer particle A having the above core-shell structure may contain one or more monomers selected from the group consisting of (a) conjugated diene monomers or conjugated diene polymers, (b) acrylate monomers, vinyl monomers, (meth)acrylamide monomers, and nitrile monomers, and (c) unsaturated carboxylic acid monomers, or polymers.

[0057] Examples of the above (group A) conjugated diene monomers include, but are not limited to, one or more monomers selected from the group consisting of 1,3-butadiene, isoprene, chloroprene, and pyrerlidene.

[0058] Examples of the above-mentioned conjugated diene polymers include, but are not limited to, polymers of two or more monomers selected from the group consisting of 1,3-butadiene, isoprene, chloroprene, and pyrelidene; styrene-butadiene copolymers; acrylonitrile-butadiene copolymers; styrene-isoprene copolymers; acrylate-butadiene rubber; acrylonitrile-butadiene-styrene rubber; ethylene-propylene-diene polymers; polymers of which these polymers are partially hydrogenated, epoxidized, or brominated; and mixtures thereof.

[0059] In the above (group B), the acrylate monomers include, but are not limited to, one or more monomers selected from the group consisting of methacryloyloxyethyl ethylene urea, β-carboxyethyl acrylate, aliphatic monoacrylate, dipropylene diacrylate, ditrimethylolpropane tetraacrylate, dipentaerythritol hexaacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, and glycidyl methacrylate.

[0060] Examples of the vinyl monomers mentioned above include, but are not limited to, one or more monomers selected from the group consisting of styrene, α-methylstyrene, β-methylstyrene, pt-butylstyrene, and divinylbenzene.

[0061] Examples of the (meth)acrylamide monomers include, but are not limited to, one or more monomers selected from the group consisting of acrylamide, n-methylolacrylamide, n-butoxymethylacrylamide, methacrylamide, n-methylolmethacrylamide, and n-butoxymethylmethacrylamide.

[0062] Examples of the nitrile monomers mentioned above include alkenyl cyanides, specifically one or more monomers selected from the group consisting of acrylonitrile, methacrylonitrile, and allyl cyanides, but are not limited to these.

[0063] Examples of the above (group C) unsaturated carboxylic acid monomers include, but are not limited to, one or more monomers selected from the group consisting of maleic acid, fumaric acid, methacrylic acid, acrylic acid, glutaconic acid, itaconic acid, tetrahydrophthalic acid, crotonic acid, isocrotonic acid, and nadic acid.

[0064] More preferably, the core portion of the emulsion polymer particle A having the above core-shell structure may contain polymers of (a) conjugated diene monomers or conjugated diene polymers, (b) vinyl monomers, and (c) unsaturated carboxylic acid monomers.

[0065] As a more preferred example, the core portion of the emulsion polymer particle A having the above core-shell structure may contain a polymer of monomers or polymers in which, based on the total weight of the core portion, the monomer or polymer of group (a) is 30 to 70% by weight, the monomer of group (b) is 20 to 50% by weight, and the unsaturated carboxylic acid monomer of group (c) is 0.5 to 20% by weight.

[0066] As a more preferred example, the core portion of the emulsion polymer particle A having the core-shell structure described above may be styrene-butadiene latex. The binder composition for secondary batteries of the present invention can exhibit the best effects intended in the present invention by including emulsion polymer particles having a core-shell structure.

[0067] As a more preferred example, the shell portion of the emulsified polymer particle A having the above core-shell structure may contain polymers of one or more monomers selected from the group consisting of (a) (meth)acrylic acid ester monomers, (b) acrylic monomers, vinyl monomers, (meth)acrylamide monomers, and nitrile monomers, and one or more monomers or polymers selected from (c) unsaturated carboxylic acid monomers.

[0068] Examples of the above (group A) (meth)acrylic acid ester monomers include, but are not limited to, one or more monomers selected from the group consisting of allyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, n-amyl acrylate, isoamyl acrylate, n-ethylhexyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, n-amyl methacrylate, isoamyl methacrylate, n-hexyl methacrylate, n-ethylhexyl methacrylate, 2-ethylhexyl methacrylate, lauryl acrylate, ceryl acrylate, stearyl acrylate, lauryl methacrylate, cetyl methacrylate, and stearyl methacrylate.

[0069] The monomers of the above-mentioned (group B) and (group C) are as exemplified in the core portion of the emulsion polymer particle A having the above-mentioned core-shell structure, but are not limited to these.

[0070] More preferably, the shell portion of the emulsion polymer particles A having the above core-shell structure may contain polymers of all of the following: (a) (meth)acrylic acid ester monomers, (b) vinyl monomers, and (c) unsaturated carboxylic acid monomers.

[0071] As a more preferred example, the shell portion of the emulsion polymer particle A having the core-shell structure described above may include a monomer polymer in which the monomers of group (a) are present in an amount of 50 to 90% by weight, the monomers of group (b) are present in an amount of 0.5 to 30% by weight, and the unsaturated carboxylic acid monomers of group (c) are present in an amount of 0.5 to 20% by weight, relative to the total weight of the shell portion.

[0072] In the present invention, antioxidants and preservatives may be further added to the above-mentioned binder composition for secondary batteries. In particular, when the binder for secondary batteries of the present invention is used, antioxidants may be suitably used to reduce degradation.

[0073] Furthermore, the above-mentioned binder composition for secondary batteries may further contain one or more substances selected from the group consisting of viscosity modifiers and fillers. The viscosity modifiers and fillers will be described in detail below.

[0074] An electrode mixture for a secondary battery according to one embodiment of the present invention comprises the above-mentioned binder composition for secondary batteries and an electrode active material capable of intercalating and releasing lithium.

[0075] The above-mentioned electrode mixture for secondary batteries may further contain a conductive material. The conductive material will be described in detail below.

[0076] A secondary battery according to one embodiment of the present invention includes an electrode for a secondary battery containing the above-mentioned electrode mixture for secondary batteries. The electrode for the secondary battery can be manufactured by applying the electrode mixture onto a current collector, followed by drying and rolling. The electrode for the secondary battery may be a positive electrode or a negative electrode.

[0077] The above-mentioned positive electrode is manufactured, for example, by applying a mixture of a positive electrode active material, a conductive material, a binder, etc. onto a positive electrode current collector and then drying it. The negative electrode is manufactured by applying a mixture of a negative electrode active material, a conductive material, a binder, etc. onto a negative electrode current collector and then drying it. In some cases, the negative electrode may not contain a conductive material.

[0078] The above-mentioned positive electrode active material contains two or more transition metals as a lithium transition metal oxide. For example, layered compounds such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), etc. substituted with one or more transition metals; lithium manganese oxide substituted with one or more transition metals; chemical formula LiNi 1-y M y O2 (where M = Co, Mn, Al, Cu, Fe, Mg, B, Cr, Zn or Ga, contains one or more of the above elements, and 0.01 ≦ y ≦ 0.7) represented by lithium nickel-based oxide; Li 1+z Ni 1 / 3 Co 1 / 3 Mn 1 / 3 O2, Li 1+z Ni 0.4 Mn 0.4 Co 0.2 O2 and so on like Li 1+z Ni b Mn c Co 1-(b+c+d) M d O (2-e) A e (where -0.5 ≦ z ≦ 0.5, 0.1 ≦ b ≦ 0.8, 0.1 ≦ c ≦ 0.8, 0 ≦ d ≦ 0.2, 0 ≦ e 0.2, b + c + d < 1, M = Al, Mg, Cr, Ti, Si, or Y, and A = F, P or Cl) represented by lithium nickel cobalt manganese composite oxide; chemical formula Li 1+x M 1-y M’ y PO 4-z X zExamples include, but are not limited to, olivine-based lithium metal phosphates, represented by (where M = transition metal, preferably Fe, Mn, Co, or Ni; M' = Al, Mg, or Ti; X = F, S, or N, with -0.5 ≤ x ≤ +0.5, 0 ≤ y ≤ 0.5, and 0 ≤ z ≤ 0.1).

[0079] Examples of negative electrode active materials include carbon and graphite materials such as natural graphite, artificial graphite, expanded graphite, carbon fiber, non-graphitizable carbon, carbon black, carbon nanotubes, fullerenes, and activated carbon; metals such as Al, Si, Sn, Ag, Bi, Mg, Zn, In, Ge, Pb, Pd, Pt, and Ti that can be alloyed with lithium, and compounds containing such elements; composites of metals and their compounds with carbon and graphite materials; and lithium-containing nitrides. Among these, carbon-based active materials, silicon-based active materials, tin-based active materials, or silicon-carbon-based active materials are more preferred, and these may be used alone or in combination of two or more, but are not limited to these.

[0080] The conductive material described above is a component for further improving the conductivity of the electrode active material and may be added in an amount ranging from 0.01% to 30% by weight relative to the total weight of the electrode mixture. Such conductive materials are not particularly limited as long as they have conductivity that does not cause chemical changes in the battery, but examples include graphite such as natural graphite and artificial graphite; carbon black such as carbon black, acetylene black, Kechen black, channel black, furnace black, lamp black, and thermal black; carbon derivatives such as carbon nanotubes and fullerenes, conductive fibers such as carbon fibers and metal fibers; metal powders such as carbon fluoride, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0081] In the above-mentioned electrodes, the current collector is the part where electron movement occurs due to the electrochemical reaction of the active material, and depending on the type of electrode, there are positive electrode current collectors and negative electrode current collectors.

[0082] The positive electrode current collector described above is generally formed with a thickness of 3 to 500 μm. Such a positive electrode current collector is not particularly limited as long as it has high conductivity that does not cause chemical changes in the battery, and examples include stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel whose surfaces have been treated with carbon, nickel, titanium, silver, etc.

[0083] The above-mentioned negative electrode current collector is generally formed with a thickness of 3 to 500 μm. Such a negative electrode current collector is not particularly limited as long as it has conductivity that does not cause chemical changes in the battery, and examples include copper, stainless steel, aluminum, nickel, titanium, plastic carbon, copper or stainless steel with surface treatment with carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloy.

[0084] These current collectors can also have fine irregularities formed on their surface to strengthen the bonding force of the electrode active material, and can be used in various forms such as films, sheets, foils, nets, porous materials, foams, and nonwoven fabrics.

[0085] The mixture of electrode active material, conductive material, binder, etc. (electrode mixture) may further contain one or more substances selected from the group consisting of viscosity modifiers and fillers.

[0086] The viscosity modifier described above is a component that adjusts the viscosity of the electrode mixture so that the mixing step of the electrode mixture and the coating step on the current collector can be easily carried out, and may be added up to 30% by weight of the total weight of the electrode mixture. Examples of such viscosity modifiers include, but are not limited to, carboxymethylcellulose and polyacrylic acid.

[0087] The above-mentioned filler is an auxiliary component that suppresses the expansion of the electrodes and is not particularly limited as long as it is a fibrous material that does not cause chemical changes in the battery. Examples include olefin polymers such as polyethylene and polypropylene; and fibrous materials such as glass fibers and carbon fibers.

[0088] The secondary battery of the present invention may further include, in addition to electrodes, a separator and a lithium salt-containing non-aqueous electrolyte.

[0089] As the separator described above, an insulating thin film with high ion permeability and mechanical strength is used, interposed between the positive and negative electrodes. The pore diameter of the separator is generally 0.01 to 10 μm, and the thickness is generally 5 to 300 mm. Examples of such separators include sheets or nonwoven fabrics made of olefin polymers such as polypropylene with chemical resistance and hydrophobicity, glass fibers, or polyethylene. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte can also serve as the separator.

[0090] The above lithium salts are readily soluble in the above non-aqueous electrolyte, for example, LiCi, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 Examples include, but are not limited to, LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, (CF3SO2)2NLi, lithium chloroborane, lithium lower aliphatic carboxylate, lithium 4-phenylborate, and imides.

[0091] Examples of the non-aqueous electrolytes include, but are not limited to, aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydroxyfuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

[0092] As another example, organic solid electrolytes, inorganic solid electrolytes, etc., may also be used.

[0093] Examples of the above-mentioned organic solid electrolytes include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate ester polymers, polyagitation lysine, polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride, and polymers containing ionic dissociation groups.

[0094] Examples of the inorganic solid electrolytes used include lithium nitrides, halides, and sulfates such as Li3N, LiI, Li5NI2, Li3N-LiI-LiOH, LiSiO4, LiSiO4-LiI-LiOH, Li2SiS3, Li4SiO4, Li4SiO4-LiI-LiOH, and Li3PO4-Li2S-SiS2.

[0095] The lithium secondary battery according to the present invention can be used as a battery cell for powering small devices, and can also be used as a single cell in medium- and large-sized battery modules consisting of a large number of battery cells.

[0096] The present invention will be described in detail below with reference to examples and experimental examples, but the present invention is not limited to these examples.

[0097] <Examples of manufacturing emulsion polymer particles> Manufacturing Example 1: Production of emulsion polymer particles A1 having a core-shell structure First polymerization: Formation of the core For core formation, 50 g of 1,3-butadiene, 35 g of styrene, 10 g of methyl methacrylate, and 5 g of a mixture of acrylic acid and itaconic acid in a 5:5 ratio were used as monomers.

[0098] As a solvent, approximately 400 parts by weight of water was used for a total of 100 parts by weight of the above monomer components.

[0099] Water, the above monomers, and approximately 3 parts by weight of sodium lauryl sulfate per 100 parts by weight of the monomer components were added to a nitrogen-purged polymerization reactor. After raising the temperature to approximately 75°C, 0.005 moles of potassium persulfate were added as a polymerization initiator to start emulsion polymerization.

[0100] The polymerization reaction was carried out for approximately 7 hours while maintaining the temperature at about 75°C to obtain an emulsion-type binder.

[0101] Second polymerization: Formation of the shell As additional monomers for shell formation, 69.9 g of n-butyl acrylate, 20.0 g of styrene, 10 g of methacrylic acid, and 0.1 g of allyl methacrylate were used, and 933.3 g (233.33 g based on solid content) of the emulsion polymer particles obtained in Production Example 1 above was used.

[0102] Water, the monomers, the latex particle cores from the first polymerization, and approximately 0.2 parts by weight of sodium polyoxyethylene lauryl ether sulfate per 100 parts by weight of the monomer components were added to a nitrogen-purged polymerization reactor. While maintaining the temperature at approximately 80°C, 0.33 parts by weight of ammonium per 100 parts by weight of the monomer components were added as a polymerization initiator to start emulsion polymerization.

[0103] The reaction was carried out for approximately 5 hours while maintaining the temperature at approximately 80°C to obtain an emulsion-type binder containing emulsifying polymer particles having a core-shell structure. (Average particle size: 70 nm)

[0104] The pH was adjusted to 7 using a sodium hydroxide solution.

[0105] Manufacturing Example 2: Production of emulsion polymer particles A2 having a core-shell structure Emulsified polymer particles A2 having a core-shell structure were produced in the same manner as in Production Example 1, except that during the second polymerization (shell formation), 69.9 g of n-butyl acrylate, 25.0 g of styrene, 5 g of methacrylic acid, and 0.1 g of allyl methacrylate were used as monomers for shell formation. (Average particle size: 71 nm)

[0106] Manufacturing Example 3: Production of emulsion polymer particles A3 having a core-shell structure In Production Example 1, the emulsifier content was reduced during the first polymerization step, and the average particle size (D50) was changed as shown in Table 1 below to produce emulsified polymer particles A3 having a core-shell structure. (Average particle size: 102 nm)

[0107] Manufacturing Example 4: Production of emulsion polymer particles A4 having a core-shell structure In addition to using 34 g of 1,3-butadiene, 51 g of styrene, 10 g of methyl methacrylate, and 5 g of a mixture of acrylic acid and itaconic acid in a 5:5 ratio as monomers for the first polymerization (core portion) in Production Example 1, emulsion polymer particles A4 having a core-shell structure were produced in the same manner as in Production Example 1. (Average particle size: 73 nm)

[0108] Manufacturing Example 5: Production of Emulsified Polymer Particles B1 As monomers, 56.8 g of 1,3-butadiene, 38.5 g of styrene, 3 g of methyl methacrylate, and 1.7 g of a mixture of acrylic acid and itaconic acid in a 5:5 ratio were used.

[0109] As a solvent, approximately 150 parts by weight of water was used for a total of 100 parts by weight of the above monomer components.

[0110] Water, the above monomers, and approximately 1 part by weight of sodium lauryl sulfate per 100 parts by weight of the monomer components were added to a nitrogen-purged polymerization reactor. After raising the temperature to approximately 75°C, 0.005 moles of potassium persulfate were added as a polymerization initiator to start emulsion polymerization.

[0111] The polymerization reaction was carried out for approximately 10 hours while maintaining the temperature at approximately 75°C. After the reaction was complete, approximately 0.5 parts by weight of sodium lauryl sulfate was added as a stabilizing emulsifier to obtain an emulsion-type binder. (Average particle size: 261 nm)

[0112] The pH was adjusted to 7 using a sodium hydroxide solution.

[0113] Manufacturing Example 6: Production of Emulsified Polymer Particles B2 In Production Example 5, the amount of 1,3-butadiene added was reduced, the styrene content was increased, and the glass transition temperature (Tg) was changed as shown in Table 1 below to produce emulsion polymer particles B2. (Average particle size: 262 nm)

[0114] Manufacturing Example 7: Production of Emulsified Polymer Particles B3 In Production Example 5, the amount of sodium lauryl sulfate was increased to produce emulsion polymer particles B3 with an average particle size of 184 nm.

[0115] Experimental Example 1: Evaluation of average particle size, glass transition temperature (Tg), and gel content of emulsion polymer particles. The average particle size (D50), glass transition temperature (Tg), and gel content (%) of the emulsion polymer particles produced in the above production examples 1 to 7 were measured by the method described below, and the results are shown in Table 1.

[0116] Measurement of average particle size (D50) Samples diluted to 200 ppm or less were prepared, and the average particle size (D50) was measured at room temperature (23°C) using Nicomp380 by dynamic laser light scatteering, following the intensity Gaussian distribution.

[0117] Measurement of glass transition temperature (Tg) Measurements were taken using a differential scanning calorimetry (DSC).

[0118] Measurement of gel content After drying a sample of emulsion polymer particles at 80°C for 24 hours, take approximately 0.5g, accurately measure its weight, and then use this as the M a Next, the sample of emulsion polymer particles whose weight had been measured was immersed in 50g of THF (tetrahydrofuran) for 24 hours. Then, the emulsion polymer particle sample contained in the THF was sieved with a 200-mesh mesh of known weight, and the weight of the mesh and the emulsion polymer remaining on the mesh were measured. The weight of the emulsion polymer remaining on the mesh was calculated by subtracting the weight of the 200-mesh mesh from this value. b M was measured according to the method described above. a and M b The gel content was calculated by substituting the values ​​into the following [Mathematical Formula 1]. At this time, the gel content of each emulsion polymer particle was measured for each sample (at least three samples), and the average value was calculated.

[0119] [Mathematical formula 1] Gel content = M b / M a *100

[0120] [Table 1]

[0121] Examples 1-3 and Comparative Examples 1-6 Manufacturing of mixed binders The emulsion polymer particles produced as described above were mixed in the particle number ratios shown in Table 2 below to produce the mixed binders according to Examples 1-3 and Comparative Examples 1-6.

[0122] Manufacturing of electrode slurry and electrodes Using water as a dispersion medium, artificial graphite (95.5g), acetylene black (1g), the mixed binder produced in Examples 1-3 and Comparative Examples 1-6 (2.5g), and carboxymethylcellulose (1g) as a thickener were mixed based on 100g of total solids to produce a slurry for the negative electrode, so that the total solids content was 48% by weight. This slurry was then coated onto copper foil to a thickness of 200μm, vacuum dried, and pressed to produce the negative electrode.

[0123] Experimental Example 2: Evaluation of the adhesive strength of the mixed binder and the color difference meter readings after pressing. The adhesive strength (gf / cm) and the L value of the colorimeter after pressing of the mixed binders produced in Examples 1-3 and Comparative Examples 1-6 were measured using the method described below, and the results are shown in Table 2.

[0124] Determination of N(A) / N(B) Samples diluted to 200 ppm or less were prepared, and the average particle size (D50) was measured at room temperature (23°C) using Nicomp380 by dynamic laser light scatteering according to the intensity Gaussian distribution. Then, N(A) / N(B) was determined based on the following [Relationship Equation 2].

[0125] [Relationship 2] N(A) / N(B)={D(B) / (D)A} 3 *{X(A) / X(B)}

[0126] In the above [Relationship 2], D(A) is the average particle size (D50) of particle A, D(B) is the average particle size (D50) of particle B, and X(A) / X(B) is particle B particle This is the mixing ratio of A. In this specification, the mixing ratio of particles A and particles B is based on the composition ratio of binder compositions prepared by adding each monomer and emulsion polymer particle in grams, as shown in the examples, and those skilled in the art will recognize this as a ratio based on mass.

[0127] Measurement of adhesive strength (gf / cm) The negative electrode plate was cut to a uniform size, placed on a glass slide, the current collector was removed, and the 180-degree peel strength was measured and the average value was taken.

[0128] Measurement using a colorimeter This experiment was conducted to evaluate the changes in the numerical values ​​of the colorimeter due to contamination caused by roll adhesion during the rolling process in each electrode manufactured in the examples and comparative examples.

[0129] Specifically, the press rolls can become contaminated during the process of rolling the coated negative electrode (i.e., after applying the negative electrode slurry to the current collector and then drying it) using a roll press.

[0130] At this time, when a colorimeter was applied to the surface of the press roll to take measurements, the value was 80 when the press roll was in a very clean state, but the value decreased as it became contaminated, and in the case of actual contamination similar to that of the electrode surface, the value could drop as low as 40. In connection with this, after each rolling process in Examples 1 to 3 and Comparative Examples 1 to 6, colorimeter measurements were taken of the press roll surface.

[0131] [Table 2]

[0132] As can be seen from Table 2, in Comparative Examples 1, 3, and 6, where the N(A) / N(B) value falls outside the range of greater than 50 and less than 400, it was confirmed that the L value of the colorimeter after pressing decreased significantly to 60 or less. In particular, it was confirmed that defects occurred on the electrode surface in Comparative Example 1, and electrode detachment occurred in Comparative Example 6.

[0133] Furthermore, in particular, Comparative Example 3, in which the average particle size (D50) of emulsion polymer particles A exceeds 100 nm, was found to have the lowest L value of 46 on the colorimeter after pressing among Comparative Examples 1, 3, and 6, whose N(A) / N(B) values ​​fall outside the range of more than 50 and less than 400.

[0134] Furthermore, in Comparative Example 4, where the glass transition temperature (Tg) of emulsion polymer particles A was 0°C or higher, the adhesive strength decreased significantly to 41 gf / cm, and the L value measured by the colorimeter after pressing was 69, confirming a slight decrease.

[0135] Furthermore, in Comparative Example 5, where the glass transition temperature (Tg) of emulsion polymer particles B was 0°C or higher, the adhesive strength decreased significantly to 35 gf / cm, and the L value of the colorimeter after pressing was 58, confirming a significant decrease. In addition, electrode detachment was confirmed.

[0136] In contrast, in Examples 1, 2, and 3, where the N(A) / N(B) value was in the range of greater than 50 and less than 400, the average particle size (D50) of emulsion polymer particle A was 100 nm or less, the glass transition temperature (Tg) of emulsion polymer particle A was less than 0°C, and the glass transition temperature (Tg) of emulsion polymer particle B was less than 0°C, it was confirmed that the adhesive strength was high, the L value of the colorimeter after pressing was excellent at 78, and the material had excellent physical properties without any defects in the electrodes.

Claims

1. The emulsion polymer particles A have a core-shell structure including a core portion and a shell portion that occupies part or all of the surface of the core portion, and the emulsion polymer particles B do not have a core-shell structure. The emulsion polymer particles A having the above core-shell structure have an average particle size (D50) of 100 nm or less. The glass transition temperature (Tg) of the emulsion polymer particles A having the above core-shell structure is less than 0°C. The glass transition temperature (Tg) of emulsion polymer particles B that do not have the above core-shell structure is less than 0°C. The following [Relationship 1] is satisfied, The core portion of the emulsion polymer particle A having the above core-shell structure contains styrene-butadiene latex. The shell portion of the emulsion polymer particle A having the above core-shell structure is (Group A) (meth)acrylic acid ester monomers, (Group B) One or more monomers selected from the group consisting of acrylate monomers, vinyl monomers, (meth)acrylamide monomers, and nitrile monomers, and (Group C) Unsaturated carboxylic acid monomers, It comprises one or more monomers or polymers selected from among, The emulsion polymer particles B, which do not have the above-mentioned core-shell structure, contain styrene-butadiene latex, in a binder composition for secondary battery electrodes. [Relationship 1] 50<N(A) / N(B)<400 (In the formula, N(A) is the number of emulsion polymer particles A having the core-shell structure described above, N(B) is the number of emulsion polymer particles B not having the core-shell structure described above, and N(A) / N(B) is determined by the following [Relational Formula 2].) [Relationship Equation 2] N(A) / N(B)={D(B) / D(A)} ３ *{X(A) / X(B)} (In the formula, D(A) is the average particle size (D50) of particle A, D(B) is the average particle size (D50) of particle B, and X(A) / X(B) is the mixing ratio (by mass) of particle A to particle B.)

2. The binder composition for secondary battery electrodes according to claim 1, wherein all polymer particles contained in the above-mentioned binder composition for secondary battery electrodes have an average particle size (D50) of 400 nm or less.

3. The core portion of the emulsion polymer particle A having the above core-shell structure is (Group A) Conjugated diene monomers or conjugated diene polymers, (Group B) One or more monomers selected from the group consisting of acrylate monomers, vinyl monomers, (meth)acrylamide monomers, and nitrile monomers, and (Group C) Unsaturated carboxylic acid monomers, The binder composition for secondary battery electrodes according to claim 1, comprising one or more monomers or polymer polymers selected from among the following.

4. The binder composition for secondary battery electrodes according to claim 3, wherein the core portion of the emulsion polymer particle A having the above-described core-shell structure comprises a monomer or polymer polymer containing, with respect to the total weight of the core portion, 30 to 70% by weight of the above-described monomer or polymer of group (a), 20 to 50% by weight of the above-described monomer of group (b), and 0.5 to 20% by weight of the above-described unsaturated carboxylic acid monomer of group (c).

5. The binder composition for secondary battery electrodes according to claim 1, wherein the shell portion of the emulsion polymer particles A having the core-shell structure described above comprises a monomer polymer containing, with respect to the total weight of the shell portion, 50 to 90% by weight of the monomer of group (a), 0.5 to 30% by weight of the monomer of group (b), and 0.5 to 20% by weight of the unsaturated carboxylic acid monomer of group (c).

6. A secondary battery electrode mixture comprising the binder composition for secondary battery electrodes described in claim 1.

7. A secondary battery comprising the electrode mixture for secondary batteries described in claim 6.

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