Binder for battery, composition for forming electrode, electrode for battery, non-aqueous electrolyte secondary battery, and manufacturing method thereof

Abstract

To reduce the internal resistance of a non-aqueous electrolyte secondary battery compared to before.SOLUTION: A battery binder includes a copolymer represented by the following general formula (1). (In the formula, R1 is a component represented by the general formula (2). R2 is a component represented by either general formula (3) or (4). R3 is a component containing at least a thiocarbonylthio group, and R4 is a component containing at least one of a nitrile group, an aromatic ring and a carbonyl group.)SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a battery binder, an electrode-forming composition, a battery electrode, a non-aqueous electrolyte secondary battery, and methods for producing these. [Background technology]

[0002] The binder contained in the non-aqueous electrolyte secondary battery preferably has as low an internal resistance as possible from the viewpoint of improving the operation of the non-aqueous electrolyte secondary battery at low temperatures.

[0003] Therefore, as described in Patent Document 1, binders capable of suppressing internal resistance lower than that of SBR, a binder conventionally used as a binder for lithium ion secondary batteries, which are non-aqueous electrolyte secondary batteries, have been considered. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-042408 Summary of the Invention [Problem to be solved by the invention]

[0005] However, as the range of applications of non-aqueous electrolyte secondary batteries, including lithium ion secondary batteries, expands, further reduction in internal resistance is desired.

[0006] The present invention was completed as a result of extensive research by the present inventors to solve the above-mentioned problems, and as a result of the inventors conceiving the idea that by using a binder containing a copolymer having a predetermined structure, the internal resistance of a non-aqueous electrolyte secondary battery can be reduced more than when a conventional binder is used. [Means for solving the problem]

[0007] That is, the binder for a battery according to the present invention is characterized by containing a copolymer represented by the following general formula (Chemical Formula 1). [ka] (In the formula, R 1 is a component represented by the following general formula (Chemical Formula 2): 2 is a component represented by either of the following general formulas (Chemical Formula 3) and (Chemical Formula 4). 3 is a component containing at least a thiocarbonylthio group. 4 is a component containing at least one of a nitrile group, an aromatic ring, and a carbonyl group.

[0008] [ka] (In the formula, x is an integer of 14 or more and 20 or less, y is an integer of 14 or more and 20 or less, z is 1, and q is 8 or 9.)

[0009] [ka] (In the formula, l and m are numbers whose sum is 1 and the value of l / (l+m) is 0.95 or more and less than 1.00. n is an integer of 150 or more and 160 or less.)

[0010] [ka] (In the formula, p is an integer of 150 or more and 160 or less.)

[0011] The battery binder thus configured can reduce the internal resistance of the battery more than ever before.

[0012] The R3 is preferably represented by the following structural formula (Chemical Formula 5). [ka]

[0013] R 4 is preferably one represented by any one of the following structural formulas (Chemical Formula 6), (Chemical Formula 7) and (Chemical Formula 8). [ka] [ka] [ka]

[0014] The present invention provides a method for producing the above-mentioned battery binder, comprising: 1 and R 3 and R 4 After preparing a polymerization reagent consisting of the above, the polymerization reagent and R 2 The present invention also includes a method for producing a binder for a battery, which comprises a polymerization step of reacting a polymerizable compound with a monomer that forms a binder in water.

[0015] In the polymerization step, the polymerization reagent and R 2 It is preferred to emulsion polymerize the copolymer with a monomer that forms the following:

[0016] The present invention includes an electrode-forming composition containing the battery binder as described above, a battery electrode formed from the electrode-forming composition, and a nonaqueous electrolyte secondary battery equipped with the battery electrode. [Effects of the Invention]

[0017] According to the present invention, the internal resistance of a non-aqueous electrolyte secondary battery can be reduced more than before. As a result, the operability of the non-aqueous electrolyte secondary battery at low temperatures, such as -20°C or lower, can be improved compared to conventional batteries. [Brief explanation of the drawings]

[0018] [Figure 1] 1 shows the results of AFM measurement of a copolymer constituting a binder for a battery according to this embodiment. [Figure 2] 1 is an SEM image of a surface of a negative electrode according to an example of the present invention. [Figure 3] 1A and 1B are cross-sectional views illustrating the structure of nonaqueous electrolyte secondary batteries according to examples and comparative examples of the present invention. [Figure 4] 1 is a Nyquist plot of electrochemical impedance of a nonaqueous electrolyte secondary battery according to an example of the present invention. [Figure 5] 1 is an SEM image of the surface of a negative electrode according to a comparative example of the present invention. [Figure 6] 1 is an SEM image of the surface of a negative electrode according to a comparative example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] A specific configuration of a secondary battery according to one embodiment of the present invention will be described below. <1. Basic structure of non-aqueous electrolyte secondary battery> The lithium ion secondary battery according to this embodiment includes a positive electrode and a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. The ultimate charge voltage (oxidation-reduction potential) of this lithium ion secondary battery is, for example, 4.0 V (vs. Li / Li+) to 5.0 V, in particular 4.2 V to 5.0 V. The shape of the lithium ion secondary battery is not particularly limited, and may be, for example, cylindrical, prismatic, laminate, or button-shaped.

[0020] (1-1. Positive electrode) The positive electrode includes a positive electrode current collector and a positive electrode mixture layer formed on the positive electrode current collector. The positive electrode current collector may be any conductive material, for example, a plate or foil, and is preferably made of aluminum, stainless steel, nickel-coated steel, or the like. The positive electrode mixture layer contains at least a positive electrode active material, and may further contain a conductive agent and a positive electrode binder that binds the positive electrode active material and the conductive agent onto the positive electrode current collector.

[0021] The positive electrode active material is, for example, a lithium-containing transition metal oxide or solid solution oxide, and is not particularly limited as long as it is a material that can electrochemically absorb and release lithium ions. Examples of lithium-containing transition metal oxides include Li 1.0 Ni 0.88 Co 0.1 Al 0.01 Mg 0.01 O2, but also LiCoO2 and other Li-Co based composite oxides, LiNi x Co y Mn z Examples of solid solution oxides include Li·Ni·Co·Mn-based composite oxides such as LiO2, Li·Ni-based composite oxides such as LiNiO2, and Li·Mn-based composite oxides such as LiMn2O4. a Mn x Co y Ni z O2 (1.150≦a≦1.430, 0.45≦x≦0.6, 0.10≦y≦0.15, 0.20≦z≦0.28), LiMn 1.5 Ni 0.5 Examples include O4, etc. The content (content ratio) of the positive electrode active material is not particularly limited, and may be any content that is applicable to the positive electrode mixture layer of a non-aqueous electrolyte secondary battery. These compounds may be used alone or in combination.

[0022] The conductive agent is not particularly limited as long as it is capable of increasing the conductivity of the positive electrode. Specific examples of the conductive agent include one or more selected from the group consisting of carbon black, natural graphite, artificial graphite, and fibrous carbon. Examples of the carbon black include furnace black, channel black, thermal black, ketjen black, and acetylene black. Examples of the fibrous carbon include carbon nanotubes, graphene, and carbon nanofibers. The content of the conductive agent is not particularly limited as long as it is a content that can be applied to the positive electrode mixture layer of a non-aqueous electrolyte secondary battery.

[0023] Examples of the positive electrode binder include fluorine-containing resins such as polyvinylidene fluoride, ethylene-containing resins such as styrene-butadiene rubber, ethylene-propylene-diene terpolymer, acrylonitrile-butadiene rubber, fluororubber, polyvinyl acetate, polymethylmethacrylate, polyethylene, polyvinyl alcohol, carboxymethyl cellulose or carboxymethyl cellulose derivatives (such as salts of carboxymethyl cellulose), and nitrocellulose. The positive electrode binder is not particularly limited as long as it can bind the positive electrode active material and the conductive agent to the positive electrode current collector.

[0024] (1-2. Negative electrode) The negative electrode includes a negative electrode current collector and a negative electrode mixture layer formed on the negative electrode current collector. The negative electrode current collector may be any conductive material, and is preferably, for example, in the form of a plate or foil, and is made of copper, stainless steel, nickel-plated steel, or the like.

[0025] The negative electrode mixture layer contains at least a negative electrode active material, and may further contain a conductive agent and a negative electrode binder that binds the positive electrode active material and the conductive agent onto the positive electrode current collector.

[0026] The negative electrode active material is not particularly limited as long as it can electrochemically absorb and release lithium ions. Examples of the negative electrode active material include graphite active materials (artificial graphite, natural graphite, a mixture of artificial graphite and natural graphite, natural graphite coated with artificial graphite, etc.), Si-based active materials or Sn-based active materials (for example, a mixture of fine particles of silicon (Si) or tin (Sn) or an oxide thereof with a graphite active material, fine particles of silicon or tin, and an alloy based on silicon or tin), metallic lithium, and Li4Ti5O 12 Examples of the negative electrode active material include titanium oxide compounds such as those listed above, and lithium nitrides. One of the above-listed materials may be used alone, or two or more may be used in combination. Silicon oxide is represented by SiOx (0≦x≦2).

[0027] The conductive agent is not particularly limited as long as it is capable of increasing the conductivity of the negative electrode, and for example, the same agents as those described in the section on the positive electrode can be used.

[0028] The negative electrode binder is a feature of this embodiment, and will be described in detail later.

[0029] (1-3. Separator) The separator is not particularly limited, and any separator suitable for use in lithium ion secondary batteries may be used. As the separator, a porous membrane or a nonwoven fabric, which exhibits excellent high-rate discharge performance, is preferably used alone or in combination. Resins constituting the separator include, for example, polyolefin resins typified by polyethylene, polypropylene, etc., polyester resins typified by polyethylene terephthalate, polybutylene terephthalate, etc., polyvinylidene difluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-perfluorovinyl ether copolymer, vinylidene difluoride-tetrafluoroethylene copolymer, vinylidene difluoride-trifluoroethylene copolymer, vinylidene fluoride-fluoroethylene copolymer, vinylidene di ... copolymer), vinylidene fluoride-hexafluoroacetone copolymer, vinylidene fluoride-ethylene copolymer, vinylidene difluoride-propylene copolymer, vinylidene fluoride-trifluoropropylene copolymerExamples of the separator include vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-ethylene-tetrafluoroethylene copolymer, etc. The porosity of the separator is not particularly limited, and the porosity of the separators of conventional lithium ion secondary batteries can be applied as desired.

[0030] The separator surface may have a heat-resistant layer containing inorganic particles to improve heat resistance, or a layer containing an adhesive to bond with the electrodes and fix the battery element. Examples of inorganic particles include Al2O3, AlOOH, Mg(OH)2, and SiO2. Examples of adhesives include vinylidene fluoride-hexafluoropropylene copolymers, acid-modified vinylidene fluoride polymers, and styrene-(meth)acrylic acid ester copolymers.

[0031] (1-4.Non-aqueous electrolyte) The nonaqueous electrolyte may be the same as any nonaqueous electrolyte conventionally used in lithium ion secondary batteries, and has a composition in which an electrolyte salt is contained in a nonaqueous solvent that is a solvent for the electrolyte. Examples of the non-aqueous solvent include cyclic carbonates such as propylene carbonate, ethylene carbonate, butylene carbonate, chloroethylene carbonate, fluoroethylene carbonate, and vinylene carbonate; cyclic esters such as γ-butyrolactone and γ-valerolactone; chain carbonates such as dimethyl carbonate, diethyl carbonate, and ethylmethyl carbonate; methylformate, methylacetate, methylbutyrate, ethylpropionate, and propylpropionate. propionate, tetrahydrofuran or its derivatives, 1,3-dioxane, 1,4-dioxane, 1,2-dimethoxyethane, 1,4-dibutoxyethane (1,4-dibutoxyethane, or methyldiglyme, ethers such as ethylene glycol monopropyl ether and propylene glycol monopropyl ether, nitriles such as acetonitrile and benzonitrile, dioxolane or its derivatives, ethylene sulfide, sulfolane, sultone or its derivatives, etc., can be used alone or in combination of two or more. When two or more of the nonaqueous solvents are used in combination, the mixing ratio of the nonaqueous solvents can be the same as that used in conventional lithium ion secondary batteries.

[0032] Examples of the electrolyte salt include LiClO4, LiBF4, LiAsF6, LiPF6, and LIPF6-x (C n F 2n+1 )x[However, 1 <x<6、n=1or2]、LiSCN、LiBr、LiI、Li2SO4、Li2B 10 Cl 10, inorganic ion salts containing one of lithium (Li), sodium (Na) or potassium (K) such as NaClO4, NaI, NaSCN, NaBr, KClO4, KSCN, LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(CF3SO2)(C4F9SO2), LiC(CF3SO2)3, LiC(C2F5SO2)3, (CH3)4NBF4, (CH3)4NBr, (C2H5)4NClO4, (C2H5)4NI, (C3H7)4NBr, (n-C4H9)4NClO4, (n-C4H9)4NI, (C2H5)4N-maleate, (C2H5)4N-benzoate, (C2H5)4N-phtalate, lithium stearyl sulfonic acid Examples of suitable ionic salts include organic ionic salts such as lithium, octyl sulfonic acid lithium, and dodecyl benzenesulfonic acid lithium. These ionic compounds can be used alone or in combination of two or more. The concentration of the electrolyte salt may be the same as that of nonaqueous electrolytes used in conventional lithium-ion secondary batteries and is not particularly limited. In this embodiment, it is preferable to use a nonaqueous electrolyte containing the above-mentioned lithium compound (electrolyte salt) at a concentration of approximately 0.8 mol / L to 1.5 mol / L.

[0033] Various additives may be added to the non-aqueous electrolyte. Examples of such additives include negative electrode additives, positive electrode additives, ester-based additives, carbonate-based additives, sulfate-based additives, phosphate-based additives, borate-based additives, acid anhydride-based additives, and electrolyte-based additives. Any one of these additives may be added to the non-aqueous electrolyte, or multiple additives may be added to the non-aqueous electrolyte.

[0034] 2. Characteristic Configuration of the Non-Aqueous Electrolyte Secondary Battery According to the Present Embodiment The characteristic configuration of the nonaqueous electrolyte secondary battery according to this embodiment will be described below. In this embodiment, a water-dispersible battery binder is used as the negative electrode binder. This battery binder is characterized by including a copolymer represented by the following general formula (Chemical Formula 1). The battery binder is preferably made of a copolymer represented by the following general formula (Chemical Formula 1). [ka] R in formula (1) 1 is a component represented by the following general formula (Chemical Formula 2).

[0035] [ka]

[0036] In the formula (Chemical Formula 2), x is an integer of 14 or more and 20 or less. In the formula (Chemical Formula 2), x is preferably 16 or more and 20 or less, and more preferably 18 or more and 20 or less.

[0037] In the formula (Chemical Formula 2), y is an integer of 14 or more and 20 or less. In the formula (Chemical Formula 2), y is preferably 15 or more and 19 or less, and more preferably 16 or more and 18 or less.

[0038] In the formula (Chemical Formula 2), z is 1.

[0039] In the formula (Chemical Formula 2), q is 8 or 9.

[0040] R in formula (1) 2 is a component represented by either of the following general formulas (Chemical Formula 3) and (Chemical Formula 4). [ka] The sum of l and m in formula (Chemical Formula 3) is 1, and the value of l / (l+m) is 0.95 or more and less than 1.00. The value of l / (l+m) is 0.95 or more and less than 1.00 when the constituent elements of formula (Chemical Formula 3) are repeated, for example, n=100 times, and the following occurs:l The structure shown in () appears 95 times or more and 99 times or less. m This refers to the case where the structure shown in the parentheses occurs at a frequency of 1 to 5 times. n in the formula (Chemical Formula 3) is the degree of polymerization, which is determined when determining the aforementioned l / (l+m). The value of this degree of polymerization n is an integer of 150 to 160. In the structure represented by formula (Chemical Formula 3), as the ratio of l (i.e., the value of l / (l+m)) decreases, the glass transition temperature of the resulting copolymer tends to decrease. Therefore, it is preferable that the value of l / (l+m) obtained when a sample of the copolymer obtained by the polymerization reaction is analyzed by proton NMR or the like is in a range such that the glass transition temperature is equal to or higher than the reaction temperature of the polymerization reaction (i.e., 0.95 or more and less than 1.00).

[0041] [ka] The value of the degree of polymerization p in the formula (Chemical Formula 4) is an integer of 150 or more and 160 or less.

[0042] R in formula (1) 3 is a component containing at least a thiocarbonylthio group. Examples of the component containing at least a thiocarbonylthio group include dithiobenzoates, trithiocarbonates, dithiocarbamates, and xanthates. More specifically, R in formula (1) 3 However, examples of the structure include, but are not limited to, those having the structure shown in Chemical Formula 5 below. [ka]

[0043] R in formula (1) 4 is a component containing at least one of a nitrile group, an aromatic ring, and a carbonyl group. More specifically, R in formula (1) 4Examples of the compound represented by the formula (6), (7) or (8) below include, but are not limited to: [ka] [ka] [ka]

[0044] 3. Method for manufacturing a non-aqueous electrolyte secondary battery according to this embodiment (3-1. Manufacturing method of battery binder) The above-mentioned battery binder can be produced by the following steps.

[0045] First, R in the general formula (Chemical Formula 1) 1 and R 3 and R 4 A polymerization reagent containing the following is prepared. The polymerization reagent is, for example, R 1 , R 3 and R 4 The monomers forming each of the above are mixed in an appropriate solvent such as dioxane, a polymerization initiator is added to polymerize the monomers, unreacted monomers and the polymerization initiator are removed by dialysis or the like, and the resulting mixture is freeze-dried to obtain the above. This polymerization reagent functions as a so-called chain transfer agent (macro CTA reagent), and when obtaining a copolymer represented by general formula (Chemical Formula 1), R 2 The degree of polymerization can be controlled.

[0046] The polymerization reagent prepared as described above and R 2 and the monomers that form R 2The copolymer represented by the general formula (Chemical Formula 1) can be prepared by mixing the polymerization reagent and the R 2 The mixing ratio with the monomer that forms the copolymer (molar number of monomer / molar number of polymerization reagent) is preferably, for example, 150 to 250. The reaction temperature is preferably about 70°C to 90°C, and more preferably 80°C. The reaction time is preferably 2 to 6 hours, more preferably 3 to 5 hours, and particularly preferably 4 hours. The binder for a battery may be any material containing the copolymer prepared in this manner, and in this embodiment, this copolymer is the binder for a battery.

[0047] R in formula (1) 2 Since the monomer forming the R 2 It is preferred to use reversible addition-fragmentation emulsion polymerization in which monomers forming the following are copolymerized:

[0048] (3-2. Method for manufacturing non-aqueous electrolyte secondary battery) Next, a method for producing a lithium ion secondary battery using the binder described above will be described. The positive electrode is fabricated as follows. First, a positive electrode active material, a conductive agent, and a positive electrode binder are mixed in a desired ratio and dispersed in a positive electrode slurry solvent to form a positive electrode slurry (also referred to as a battery electrode-forming composition, more specifically, a positive electrode-forming composition). Next, this positive electrode slurry is applied to a positive electrode current collector and dried to form a positive electrode mixture layer. The application method is not particularly limited. Examples of application methods include a knife coater method, a gravure coater method, a reverse roll coater, and a slit die coater. The following application steps are also performed in the same manner. Next, the positive electrode mixture layer is pressed using a press to a desired density. This completes the fabrication of the positive electrode.

[0049] The negative electrode is also fabricated in the same manner as the positive electrode. First, a mixture of materials constituting the negative electrode mixture layer is dispersed in a solvent for negative electrode slurry to prepare a negative electrode slurry (a battery electrode-forming composition, more specifically, also referred to as a negative electrode-forming composition). Next, the negative electrode slurry is applied to a negative electrode current collector and dried to form a negative electrode mixture layer. Next, the negative electrode mixture layer is pressed to a desired density using a press. This completes the fabrication of the negative electrode.

[0050] In this case, the amount of binder added to the negative electrode slurry is preferably an amount that results in a binder content of 0.1% by mass or more and 5% by mass or less, more preferably an amount that results in a binder content of 0.5% by mass or more and 3% by mass or less, and particularly preferably an amount that results in a binder content of 1.0% by mass or more and 2.0% by mass or less, relative to the total mass of the negative electrode mixture layer.

[0051] Next, the separator is sandwiched between the positive electrode and the negative electrode to produce an electrode structure. The electrode structure is then processed into a desired shape (e.g., cylindrical, rectangular, laminated, button-shaped, etc.) and inserted into a container of that shape. Next, a nonaqueous electrolyte is injected into the container, impregnating the pores in the separator and the gaps in the positive and negative electrodes with the electrolyte. This completes the production of a lithium-ion secondary battery.

[0052] <4. Effects of this embodiment> The above-described battery binder makes it possible to significantly reduce the internal resistance at low temperatures of a non-aqueous electrolyte secondary battery equipped with a negative electrode containing this battery binder compared to conventional batteries. The mechanism by which such an effect can be achieved is thought to be as follows. It has been confirmed that the binder according to this embodiment aggregates in an aqueous solution to form a worm-like (string-like) structure. When a negative electrode mixture layer is formed using a negative electrode slurry containing this worm-like binder, the worm-like binder forms a mesh-like network inside and on the surface of the negative electrode mixture layer. As a result, the binder exerts binding force without covering the surface of the negative electrode mixture layer, and the internal resistance can be kept low. While achieving these effects, the viscosity of the negative electrode slurry and the coating method can be the same as in the past, and therefore there is no impact on the productivity of non-aqueous electrolyte secondary batteries.

[0053] Since the binder forms a mesh-like network, the content of the binder in the negative electrode can be kept as low as possible.

[0054] R 2 However, since the structure is the above-mentioned (Chemical Formula 3) or (Chemical Formula 4), it is possible to prevent the material from dissolving in the electrolyte or swelling.

[0055] <5. Other embodiments> The present invention is not limited to the above-described embodiment. The negative electrode binder described in the above-described embodiment is a battery binder that can be used as a negative electrode binder in a negative electrode or as a positive electrode binder in a positive electrode. The battery binder according to the present invention may be used as both a negative electrode binder and a positive electrode binder, or may be used for only one of them. The negative electrode binder and the positive electrode binder may be exactly the same type, or different types may be used in combination. The battery binder according to the present invention can also be used in solid secondary batteries using a solid electrolyte and all-solid secondary batteries. Furthermore, the present invention is not limited to these embodiments, and it goes without saying that various modifications are possible without departing from the spirit of the present invention. [Example]

[0056] The present invention will be described in more detail below based on specific examples. However, the following examples are merely examples of the present invention, and the present invention is not limited to the following examples.

[0057] <Synthesis of polymerization reagent (macro CTA1)> First, a macro CTA1 (Chain Transfer Agent 1) composition, which is a polymerization reagent, was prepared. In a sample bottle, a monomer, poly(ethylene glycol) methyl ether acrylate (hereinafter referred to as PEGA) with a number average molecular weight of 480 (4.53 × 10 -3 mol, 2.174 g) and acrylic acid (hereafter referred to as AA) (4.53 × 10 -3 mol, 0.326 g) was weighed out, followed by the addition of 2-(dodecylthiocarbonothioylthio)-2-methylpropanoic acid (hereinafter referred to as TTCA) (1.81 × 10 -4 mol, 66 mg) was added (Mixture 1). In a sample bottle different from the one containing (Mixture 1), the initiator 4,4'-azobis(4-cyanovaleric acid) (hereinafter referred to as V-501) (2.26 × 10 -5mol, 6.4 mg) was weighed out and added to the sample bottle containing (Mixture 1) (Mixture 2). A portion of the solvent, 1,4-dioxane (2.43 g), was added to the sample bottle containing (Mixture 2), stirred, and then transferred to a 50 ml recovery flask (3). The remaining 1,4-dioxane or ethanol (5.00 g) was added to the recovery flask (3), stirred, and then transferred to a 50 ml recovery flask (4). A three-way stopcock was attached to (4), and after freeze-degassing, the mixture was returned to room temperature and placed in a 70 °C oil bath to initiate polymerization. After 4 hours, the mixture was removed from the oil bath and immersed in tap water. The three-way stopcock was then opened to terminate the polymerization (5). For purification, an excess amount of diethyl ether in a beaker was cooled to 0 °C, and (5) was added dropwise to cause reprecipitation. After the addition, the supernatant was removed, and the precipitated polymer was dissolved in methanol and transferred to a sample bottle. After drying under reduced pressure, P(PEGA-co-AA) macro-CTA1 was obtained. The obtained P(PEGA-co-AA) macro-CTA1 was stored in a freezer.

[0058] The macro-CTA1 obtained as described above was analyzed by proton NMR, and it was confirmed to have the following structure (Chemical Formula 9). The proton NMR was measured using a JEPL Model JNM-ECX500 (500 MHz). Specifically, after evaporating water from a solution containing macro-CTA1, copolymer A was dissolved in deuterated chloroform, and the integral ratio at each chemical shift was calculated using tetramethylsilane as the standard. [ka] (In the formula, x=20, y=17, z=1, and q=8 to 9.)

[0059] <Synthesis of polymerization reagent (macro CTA2)> Macro-CTA2, which is P(PEGA-co-AA) with x = 23, y = 25, z = 1, and q = 8–9, was obtained by synthesizing it using the same procedure as the synthesis of macro-CTA1 described above, except that the polymerization time was changed to 5 hours.

[0060] <Synthesis of binders for batteries> (Synthesis of Copolymer A) In a sample bottle, add P(PEGA-co-AA) macroCTA1 (5.566 × 10 -5 mol, 0.600 g) was weighed out, and then a portion of the water solvent (9.780 g) was added, followed by stirring and dissolution (Solution 6). The pH of Solution 6 was measured and found to be 3.09. 20 μL of 1 mol / L hydrochloric acid was added to the sample bottle containing Solution 6 to adjust the pH to 2.72, and then 10 μL of 1 mol / L aqueous sodium hydroxide solution was added to adjust the pH to 2.89 (Mixed Solution 7). Sodium bicarbonate (5.964 × 10 -4 mol, 50.1 mg, adjusted to a final concentration of 0.038 mol / L) and the remaining water (4.640 g) was added (Solution 8). In a new sample bottle, V-501 (1.427 × 10 -5 mol, 4 mg) was weighed out, transferred to the sample bottle containing solution 8, and stirred (mixed solution 9). Mixed solution 9 was added to the previous mixed solution 7, and stirred (mixed solution 10). Styrene (hereinafter referred to as St) (1.117 × 10 -5 mol, 1.163 g) was weighed out, and mixed solution 10 was added thereto (mixed solution 11). A three-way stopcock was attached to the eggplant-shaped flask containing mixed solution 11, and after freeze-degassing, the flask was returned to room temperature and placed in an 80°C oil bath to initiate polymerization under nitrogen. After 4 hours, the flask was removed from the oil bath and immersed in tap water. The three-way stopcock was then opened to let air in and terminate the polymerization, yielding a water-dispersible copolymer A. The polymerization conversion rate was 78%.

[0061] A sample of a predetermined amount of copolymer A obtained as described above was analyzed by proton NMR, and it was confirmed that the structure was as shown in Chemical Formula 10 below. Proton NMR was measured using a JEOL Model JNM-ECX500 (500 MHz). Specifically, water was evaporated from a solution containing copolymer A, and then copolymer A was dissolved in deuterated chloroform. The integral ratio at each chemical shift was calculated using tetramethylsilane as the reference. The polymerization conversion of St was calculated from the integral value using the signal characteristic of P(PEGA-co-AA) as the reference, and the value of the degree of polymerization p (l = 1, m = 0) was determined.

[0062] [ka] (In the formula, x = 20, y = 17, z = 1, q = 8 to 9, and p = 156.)

[0063] Copolymer A was diluted to 0.05 wt% with water and 20 μL was dropped onto a mica surface measuring approximately 1 cm x 1 cm. The morphology was observed using an atomic force microscope (AFM, Shimadzu SPM-9700) with an Olympus OMCLAC240TS silicon probe in dynamic mode. As shown in Figure 1, worm-like aggregates were confirmed.

[0064] (Synthesis of copolymer A with different composition only) Various copolymers (A'1 to A'11) shown in Table 1 were synthesized using the same procedure as for copolymer A described above, except for changing the type of macro-CTA used, the molar ratio of St to macro-CTA, and the polymerization time. The results of AFM observation of these copolymers are shown in Table 1. The values ​​of l, m, and p in the table were calculated from the molar ratio of St to macro-CTA and the polymerization conversion rate of St, as described above. p indicates the degree of polymerization.

[0065] [Table 1]

[0066] (Synthesis of Copolymer B) In a new sample vial, add P(PEGA-co-AA) macroCTA1 (2.40 × 10 -5 mol, 0.238 g) was weighed out, followed by the addition of a portion of the water solvent (2.41 g), and then stirred to dissolve (Solution 6'). The pH of Solution 6' was measured and found to be 3.19. 5 μL of 1 mol / L hydrochloric acid was added to this to adjust the pH to 2.50, and then 2.5 μL of 1 mol / L aqueous sodium hydroxide solution was added to adjust the pH to 2.85 (Mixed Solution 7'). Sodium bicarbonate (1.51 × 10 -4 mol, 0.0127 g, adjusted to a final concentration of 0.038 M) was weighed out, and the remaining water (1.20 g) was added (Solution 8'). In a new sample bottle, V-501 (2.50 × 10 -6 mol, 0.7 mg) was weighed out and transferred to the sample bottle containing solution 8' and stirred (mixture 9'). Mixture 9' was added to the previous mixture 7' and stirred (mixture 10'). The monomer St (2.97 × 10 -3 mol, 0.297 g) and 2-ethylhexyl acrylate (hereafter referred to as HEA) (8.14 × 10 -5 mol, 0.0150 g) was weighed out, and (Mixed Solution 10') was added to it (Mixed Solution 11'). A three-way stopcock was attached to the Schlenk flask containing Mixed Solution 11', and after freeze-degassing, it was returned to room temperature and placed in an 80°C oil bath to initiate polymerization under nitrogen. After 4 hours, it was removed from the oil bath and immersed in tap water, after which the three-way stopcock was opened to let air in and terminate the polymerization. The polymerization conversion rate was 79%.

[0067] Copolymer B obtained as described above was analyzed by proton NMR, and it was confirmed that it had the following structure (Chemical Formula 11). Proton NMR was performed under the same conditions as for copolymer A.

[0068] [ka] (In the formula, x = 20, y = 17, z = 1, q = 8 to 9, l = 0.95, m = 0.05, and n = 155.)

[0069] The morphology of this copolymer B was observed using AFM in exactly the same manner as for copolymer A, and it was confirmed that the copolymer was a worm-like aggregate.

[0070] (Synthesis of copolymer B with different composition only) The macro CTA used was designated macro CTA1, and various copolymers (B'1 to B'2) shown in Table 2 were synthesized using the same procedure as for copolymer B described above, except that the molar ratio of St and HEA and the polymerization time were changed when the macro CTA was designated 1. The results of AFM observation of these copolymers are shown in Table 2. The values ​​of l, m, and n in the table are values ​​calculated from the molar ratio of St and HEA to the macro CTA and the polymerization conversion rate, as described above.

[0071] [Table 2]

[0072] From the synthesis examples of copolymer A, copolymers A'1 to A'11, copolymer B, and copolymers B'1 and B'2 described above, it was found that when macro CTA1 was used, worm-shaped copolymers could be synthesized. Furthermore, even when the copolymer composition was changed, it was found that worm-like copolymers could be synthesized by adjusting the molar ratio of macro-CTA1 to St (or St + HEA), reaction time, etc.

[0073] In order to form a worm-like copolymer, it is considered preferable that the macro-CTA, which is a polymerization reagent, functions as an emulsifier. The ability of macro CTAs to function as emulsifiers is influenced by their hydrophilicity. One indicator of hydrophilicity is the HLB value, defined by the Griffin method (HLB value = 20 × sum of formula weights of hydrophilic moieties / molecular weight).

[0074] In the aforementioned experiment, macro-CTA1, which was able to form a worm-like copolymer, had an HLB value of 16.7 by the Griffin method, while macro-CTA2, which failed to form a worm-like copolymer, had an HLB value of 16.5 by the Griffin method. Furthermore, substances with too high an HLB value by the Griffin method (e.g., polyacrylic acid has an HLB value of 17.0) are known not to function as emulsifiers. Therefore, it is preferable that the HLB value of the macro-CTA be greater than 16.5 and less than 17.0. To satisfy this preferable HLB value range, x and y in (Chemical Formula 9), which shows the structure of the macro-CTA, should each be an integer between 14 and 20.

[0075] In order to form a worm-like copolymer, it is also important to maintain an appropriate balance in the volume fraction between the shell layer (hydrophilic layer) and the core layer (hydrophobic layer) in the micelles formed by the macro-CTA during emulsion polymerization to form the copolymer. If the values ​​of x and y in (Chemical Formula 9), which shows the structure of the macro-CTA, are too small, the volume fraction of the shell layer relative to the core layer becomes too small. On the other hand, if the values ​​of x and y in (Chemical Formula 9) are too large, the volume fraction of the shell layer relative to the core layer becomes too large. Therefore, to achieve an appropriate volume balance between the shell layer and the core layer, it is preferable that the values ​​of x and y in (Chemical Formula 9) be 14 or more and 20 or less.

[0076] (Synthesis of Copolymer C) 0.45g of sodium dodecylbenzenesulfonate, 18.75g of styrene and 18.75g of 2-ethylhexyl acrylate, the raw materials for the hydrophobic copolymer, and 100.36g of ion-exchanged water were added and stirred, and the system was replaced with nitrogen. The temperature was raised to 60°C, and an aqueous initiator solution of 408mg of ammonium persulfate dissolved in 5.0g of ion-exchanged water was added. The polymerization reaction was carried out with stirring for 12 hours after the addition of the initiator, yielding a milky white water-dispersed copolymer C consisting of styrene and 2-ethylhexyl acrylate.

[0077] Example 1 (Preparation of positive electrode) A positive electrode mixture slurry was prepared by dispersing lithium cobalt oxide, carbon black, and polyvinylidene fluoride (PVDF) in N-methylpyrrolidone so that the mass ratio in a dry state (solid content) was 97.5:1.2:1.3. The positive electrode mixture slurry was then applied to one side of an aluminum foil current collector with a thickness of 12 μm and then dried. The dried coating layer was rolled to increase the density of the positive electrode mixture layer formed on the aluminum foil current collector to 4.15 g / cm. 3 The coating amount is 19.5 mg / cm 2 The positive electrode was fabricated so that

[0078] (Preparation of negative electrode) A negative electrode mixture slurry was prepared by dissolving or dispersing graphite, copolymer A, and sodium carboxymethylcellulose in an aqueous solvent in a dry (solids) mass ratio of 97:2:1. This negative electrode mixture slurry was then applied to one side of a 10 μm-thick copper foil current collector and dried. The dried coating layer was rolled to a density of 1.65 g / cm. 3 The total coating weight of the two negative electrode mixture layers was 11 mg / cm 2 During the process of producing this negative electrode, no cracking or peeling of the negative electrode mixture layer was observed. An SEM image of the negative electrode surface before rolling is shown in Figure 2. Observation was performed using a JEOL JSM-7800F field emission electron microscope with a top detector at an accelerating voltage of 0.3 kV. As can be seen from Figure 2, it was confirmed that copolymer A maintained the morphology of worm-like aggregates even within the electrode, forming a network structure at least on the surface of the negative electrode mixture layer.

[0079] (Fabrication of non-aqueous electrolyte secondary battery) Aluminum and nickel lead wires were welded to the single-sided positive and negative electrodes, respectively. The positive and negative electrodes were then placed with their composite layers facing each other, sandwiching a porous polyethylene separator between them (Figure 3). The positive electrode, separator, and negative electrode were then housed in an aluminum laminate film with the lead wires extended to the outside. An electrolyte was then poured into the battery, which was then vacuum-sealed to produce a pre-charged secondary battery. The electrolyte used was a 1M LiPF6 solution in a solvent mixture of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate in a volume ratio of 2:4:4.

[0080] (Low temperature output evaluation) After the electrolyte was poured into the battery, it was left at 25°C for 12 hours, then sandwiched between two Bakelite plates and subjected to constant current charging at 25°C at 1 / 5 CA (1 CA is a 1-hour discharge rate) to 4.4 V, followed by constant voltage charging at 4.4 V to 1 / 20 CA. This was followed by constant current discharging at 1 / 5 CA to 3.0 V. This cycle was repeated three times: constant current charging at 1 / 5 CA to 4.4 V, constant voltage charging at 4.4 V to 1 / 20 CA, and constant current discharging at 1 / 2 CA to 3.0 V. This cycle was then repeated three times: constant current charging at 1 / 5 CA to 4.4 V, constant voltage charging at 4.4 V to 1 / 20 CA, and constant current discharging at 1 / 2 CA to 3.0 V. After this, the secondary battery was transferred to a -20°C thermostatic chamber and left there for 3 hours, then cooled to the set temperature. It was then subjected to constant current discharging at 1 / 2 CA to 3.0 V. The discharge capacity at this time was divided by the discharge capacity of the third 1 / 2 CA discharge performed at 25°C and expressed as a percentage to calculate the discharge capacity retention rate at -20°C relative to 25°C (low-temperature discharge capacity retention rate). The low-temperature discharge capacity retention rate of the nonaqueous electrolyte secondary battery according to Example 1 was 74%.

[0081] (Internal resistance evaluation) The same battery used in the low-temperature output evaluation was similarly charged at a constant current of 1 / 5 CA to 4.4 V, and then at a constant voltage of 4.4 V to 1 / 20 CA. The electrochemical impedance of the charged battery was measured using a Solartron impedance analyzer. The amplitude was 10 mV, and the frequency ranged from 1 MHz to 0.1 Hz. A Nyquist plot was created for the obtained data (Figure 4). The unit was Ωcm, normalized by the area of ​​the positive electrode. 2 The Z' axis value at the end of the distorted arc was read, and the internal resistance of this battery was determined to be 15.8 Ω cm 2 To measure the internal resistance of a battery, it is necessary to consider the time dependence of multiple resistance components, such as circuit resistance, solution resistance due to the electrolyte in the battery, charge transfer resistance between the positive and negative electrodes, and diffusion resistance, but in this example, because only the negative electrode binder was changed, the difference in resistance shown in the Nyquist plot was considered to be due to the negative electrode binder, and the various resistances described above were not separated, but were simply considered to be the internal resistance of the battery for evaluation.

[0082] <Comparative Example 1> A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that copolymer C was used instead of copolymer A. SEM observation (Fig. 5) of the surface of the negative electrode before surface rolling, obtained under the same conditions as in Fig. 2, confirmed that copolymer C had aggregated into particles and covered part of the surface of the negative electrode mixture layer. For this Comparative Example 1, a low-temperature output evaluation was carried out under the same procedures and conditions as in Example 1. The low-temperature discharge capacity retention rate was 68% and the internal resistance was 17.9 Ωcm 2 It was.

[0083] <Comparative Example 2> A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that copolymer A was replaced with a commercially available modified styrene butadiene rubber (hereinafter also referred to as modified SBR). SEM observation of the surface of the negative electrode before surface rolling (Figure 6), obtained under the same conditions as Figure 2, confirmed that the modified SBR had aggregated into particles and covered part of the surface of the negative electrode mixture layer. For this Comparative Example 2, a low-temperature output evaluation was carried out under the same procedures and conditions as in Example 1. The low-temperature discharge capacity retention rate was 48% and the internal resistance was 34.0 Ωcm. 2 It was.

[0084] (Discussion on the results of Examples and Comparative Examples) The experimental results for Example 1, Comparative Example 1, and Comparative Example 2 described above are summarized in Table 3 below.

[0085] [Table 3]

[0086] As shown in Table 3, it was confirmed that in Example 1, which is a nonaqueous electrolyte secondary battery produced using the battery binder according to the present invention, the internal resistance of the battery was kept low compared to Comparative Examples 1 and 2, which are conventional examples. Moreover, it was found that Example 1 had a significantly higher low-temperature discharge capacity retention rate than Comparative Examples 1 and 2. The reason why the low-temperature discharge capacity retention rate was dramatically higher than in Comparative Examples 1 and 2 is thought to be that the internal resistance of the nonaqueous electrolyte secondary battery was reduced.

[0087] The reason why the internal resistance can be reduced to a lower level than conventional binders is that the copolymer contained in the binder for batteries according to the present invention forms worm-like aggregates, as shown in FIG. 1, which bind the negative electrode mixture layer in a mesh-like manner, as shown in FIG. 2.

[0088] As can be clearly seen by comparing Fig. 2 relating to Example 1 with Fig. 5 and Fig. 6 relating to the Comparative Examples, the battery binder shown in Example 1 can bind the negative electrode mixture layer by covering it in a mesh-like pattern as shown in Fig. 2, and therefore can prevent the surface of the negative electrode mixture layer from being continuously covered by aggregates of the battery binder as much as possible, compared to the conventional battery binders (Figs. 5 and 6) shown in Comparative Examples 1 and 2. As a result, the entire negative electrode mixture layer can be made to contribute uniformly to charge and discharge, which is thought to keep the internal resistance of the entire battery low.

[0089] According to the present invention, as shown in Example 1, the internal resistance of a non-aqueous electrolyte secondary battery can be reduced compared to conventional batteries, which has the advantage of enabling faster charging than conventional batteries. This is expected to make a significant contribution to expanding the fields of application of non-aqueous electrolyte secondary batteries.

[0090] It can be fully inferred from the mechanism explained above that the above-mentioned effects of the present invention can be achieved not only when the battery binder according to the present invention is used in a negative electrode, but also when the battery binder is contained in a positive electrode slurry and used in a positive electrode.

Claims

1. A binder for a battery comprising a copolymer represented by the following general formula (Chemical Formula 1): 【Chemical 1】 (In the formula, R １ is a component represented by the following general formula (Chemical Formula 2): ２ is a component represented by either of the following general formulas (Chemical Formula 3) and (Chemical Formula 4). ３ is a component containing at least a thiocarbonylthio group. ４ is a component containing at least one of a nitrile group, an aromatic ring, and a carbonyl group. 【Chemistry 2】 (In the formula, x is an integer of 14 or more and 20 or less, y is an integer of 14 or more and 20 or less, z is 1, and q is 8 or 9.) 【Chemistry 3】 (In the formula, l and m are numbers whose sum is 1 and the value of l / (l+m) is 0.95 or more and less than 1.

00. n is an integer of 150 or more and 160 or less.) 【Chemistry 4】 (In the formula, p is an integer of 150 or more and 160 or less.)

2. The battery binder according to claim 1 , which contains worm-like aggregates formed by the copolymer.

3. The R ３ The binder for a battery according to claim 1 or 2, wherein is represented by the following structural formula (Chemical Formula 5): 【Chemistry 5】

4. The R ４ The binder for a battery according to any one of claims 1 to 3, wherein is represented by any one of the following structural formulas (Chemical Formula 6), (Chemical Formula 7), or (Chemical Formula 8). 【Chemistry 6】 【Chemistry 7】 【Chemistry 8】

5. A composition for forming a battery electrode, comprising the binder according to any one of claims 1 to 4.

6. The composition for forming a battery electrode according to claim 5 , further comprising a water-soluble polymer.

7. A battery electrode formed using the electrode-forming composition according to claim 5 or 6.

8. A non-aqueous electrolyte secondary battery comprising the battery electrode according to claim 7.

9. A method for producing a binder for a battery containing a copolymer represented by the following general formula (Chemical Formula 1): R in the general formula (Chemical Formula 1) １ and R ３ and R ４ After preparing a polymerization reagent containing The polymerization reagent and R in the general formula (Chemical Formula 1) ２ A method for producing a binder for a battery, comprising a polymerization step of reacting a binder containing a binder with a monomer that forms a binder in water. 【Chemical 1】 (In the formula, R １ is a component represented by the following general formula (Chemical Formula 2): ２ is a component represented by either of the following general formulas (Chemical Formula 3) and (Chemical Formula 4). ３ is a component containing at least a thiocarbonylthio group. ４ is a component containing at least one of a nitrile group, an aromatic ring, and a carbonyl group. 【Chemistry 2】 (In the formula, x is an integer of 14 or more and 20 or less, y is an integer of 14 or more and 20 or less, z is 1, and q is 8 or 9.) 【Chemistry 3】 (In the formula, l and m are numbers whose sum is 1 and the value of l / (l+m) is 0.95 or more and less than 1.

00. n is an integer of 150 or more and 160 or less.) 【Chemistry 4】 (In the formula, p is an integer of 150 or more and 160 or less.)

10. In the polymerization step, the polymerization reagent and R ２ The method for producing a binder for a battery according to claim 9 , wherein the binder is emulsion-polymerized with a monomer that forms the formula:

11. 11. The method for producing a binder for a battery according to claim 9 or 10, wherein the polymerization reagent is represented by the following general formula (Chemical Formula 9): 【Chemistry 9】 (In the formula, x = 20, y = 17, z = 1, and q = 8 to 9.)

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