Electrode mixture, electrode, and secondary battery

Abstract

Provided is an electrode mixture that is an electrode mixture containing a lithium-containing transition metal oxide, a conductive aid, a binder, and an organic solvent, wherein the conductive aid contains a nano-carbon material, the nano-carbon material is at least one selected from the group consisting of a multi-walled carbon nanotube, a carbon nanohorn, a carbon nanofiber, a fullerene, and graphene, the binder contains a fluorine-containing copolymer containing a vinylidene fluoride unit and a fluorinated monomer unit (excluding the vinylidene fluoride unit), and the content of the vinylidene fluoride unit in the fluorine-containing copolymer is more than 50 mol% and 99 mol% or less with respect to the total monomer units.

Description

Technical Field

[0001] This invention relates to electrode mixtures, electrodes, and secondary batteries. Background Technology

[0002] Lithium-ion secondary batteries and other non-aqueous electrolyte secondary batteries are used in small and portable electrical and electronic devices such as notebook computers, mobile phones, smartphones, tablets, and ultrabooks due to their high voltage, high energy density, low self-discharge, low memory effect, and ability to achieve ultra-lightweight design. Furthermore, they are being put into practical use as power sources for automotive applications, such as driving power supplies or stationary power supplies.

[0003] As an electrode compound used to form the electrode of such a non-aqueous electrolyte secondary battery, for example, Patent Document 1 describes a positive electrode compound slurry, characterized by comprising: a composite positive electrode active material comprising a solvent, a positive electrode active material, and a first binder covering the surface of the positive electrode active material and insoluble in the solvent; a fibrous carbon material with an average fiber diameter of 40 nm or less and an average aspect ratio of 500 or more; and a second binder soluble in the solvent, wherein the mass percentage of the first binder relative to the total mass of the composite positive electrode active material is 0.2% to 1.5% by mass, the mass percentage of the fibrous carbon material relative to the total mass of all solid components is 0.01% to 0.5% by mass, and the mass percentage of the second binder relative to the total mass of all solid components is 0.1% to 0.5% by mass, and the specific surface area of ​​the positive electrode active material is set as a(m²). 2 When b (mass%) is defined as the mass percentage of the first binder relative to the total mass of the composite positive electrode active material, b / a is 5.0 or less.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2017-182989 Summary of the Invention

[0007] The problem that the invention aims to solve

[0008] The purpose of this invention is to provide an electrode compound that can form a flexible electrode compound layer with excellent adhesion to current collectors, and can form a secondary battery with excellent battery characteristics.

[0009] Methods for solving problems

[0010] According to the present invention, an electrode binder is provided, which is an electrode binder containing a lithium-containing transition metal oxide, a conductive additive, a binder and an organic solvent, wherein the conductive additive contains a nano-carbon material, the nano-carbon material being at least one selected from the group consisting of multilayer carbon nanotubes, carbon nanotubes, carbon nanofibers, fullerenes and graphene, and the binder containing a fluorinated copolymer comprising vinylidene fluoride units and fluorinated monomer units (excluding vinylidene fluoride units), wherein the content of vinylidene fluoride units in the fluorinated copolymer is more than 50 mol% and less than 99 mol% relative to all monomer units.

[0011] The aforementioned nano-carbon material is preferably selected from at least one of the groups consisting of multilayer carbon nanotubes and graphene.

[0012] The content of vinylidene fluoride units in the above-mentioned fluorinated copolymer is preferably 57.0 mol% or more and 97.0 mol% or less relative to all monomer units.

[0013] The fluorinated monomer unit mentioned above is preferably a tetrafluoroethylene unit.

[0014] The content of the binder is preferably 0.3% to 3.0% by mass relative to the total mass of the lithium-containing transition metal oxide, the conductive additive, and the binder.

[0015] The content of the conductive additive is preferably 0.3% to 3.0% by mass relative to the total mass of the lithium-containing transition metal oxide, the conductive additive, and the binder.

[0016] The adhesive described above preferably also contains polyvinylidene fluoride.

[0017] In addition, according to the present invention, an electrode is provided, which includes a current collector and an electrode mixture layer formed of the electrode mixture disposed on one or both sides of the current collector.

[0018] In addition, according to the present invention, a secondary battery is provided having the above-described electrodes.

[0019] The effects of the invention

[0020] According to the present invention, an electrode compound can be provided that can form an electrode compound layer with excellent flexibility and adhesion to the current collector, and can form a secondary battery with excellent battery characteristics. Detailed Implementation

[0021] The specific embodiments of the present invention will be described in detail below, but the present invention is not limited to the following embodiments.

[0022] <Electrode Mixture>

[0023] The electrode compound of the present invention contains lithium-containing transition metal oxides, conductive additives, binders, and organic solvents.

[0024] <Lithium-containing transition metal oxides>

[0025] The electrode compound of the present invention contains a lithium-containing transition metal oxide. The lithium-containing transition metal oxide functions as a positive or negative electrode active material within the electrode compound layer.

[0026] The electrode mixture of the present invention can be used as a positive electrode mixture when it contains a lithium-containing transition metal oxide that functions as a positive electrode active material, and can be used as a negative electrode mixture when it contains a lithium-containing transition metal oxide that functions as a negative electrode active material. In particular, the electrode mixture of the present invention is preferably a positive electrode mixture. That is, the electrode mixture of the present invention preferably contains a lithium-containing transition metal oxide as a positive electrode active material.

[0027] Transition metals preferred for lithium-containing transition metal oxides include V, Ti, Cr, Mn, Fe, Co, Ni, and Cu.

[0028] Examples of lithium-containing transition metal oxides that function as positive electrode active materials include lithium-cobalt composite oxides such as LiCoO2, lithium-nickel composite oxides such as LiNiO2, lithium-manganese composite oxides such as LiMnO2, LiMn2O4, and Li2MnO3, and substances formed by replacing part of the transition metal atoms in the main body of these lithium-containing transition metal oxides with other metals such as Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, and Si. A specific example of such substituted substances is LiNiO2. 0.5 Mn 0.5 O2, LiNi 0.85 Co 0.10 Al 0.05 O2, LiNi 0.82 Co 0.15 Al 0.03 O2, LiNi 0.80 Co 0.15 Al 0.05 O2, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiMn 1.8 Al 0.2 O4, LiMn 1.5 Ni 0.5 O4, Li4Ti5O 12 wait.

[0029] In addition, lithium-containing transition metal oxides that function as positive electrode active materials can also be lithium-containing transition metal phosphate compounds. Specific examples of lithium-containing transition metal phosphate compounds include iron phosphates such as LiFePO4, Li3Fe2(PO4)3, and LiFeP2O7, cobalt phosphates such as LiCoPO4, and substances in which part of the transition metal atoms that form the main body of these lithium-containing transition metal phosphate compounds are replaced by other metals such as Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, Nb, and Si.

[0030] Among them, the lithium-containing transition metal oxide that functions as the positive electrode active material is preferably a lithium-nickel composite oxide, and more preferably of general formula (1):

[0031] General formula (1): Li y Ni 1-x M x O2

[0032] (In the formula, x satisfies 0.01≤x≤0.5, y satisfies 0.9≤y≤1.2, and M represents metal atoms (excluding Li and Ni).) This lithium-nickel composite oxide contains a large amount of Ni, which is beneficial for increasing the capacity of secondary batteries.

[0033] In general formula (1), x is a coefficient that satisfies 0.01≤x≤0.5. From the perspective of obtaining a secondary battery with higher capacity, it is preferred to be 0.05≤x≤0.4, and even more preferably 0.10≤x≤0.3.

[0034] In general formula (1), the metal atom of M can be V, Ti, Cr, Mn, Fe, Co, Cu, Al, Zn, Mg, Ga, Zr, Si, etc. The metal atom of M is preferably a transition metal such as V, Ti, Cr, Mn, Fe, Co, Cu, or a combination of the above transition metals with other metals such as Al, Ti, V, Cr, Mn, Fe, Co, Cu, Zn, Mg, Ga, Zr, Si.

[0035] Lithium-containing transition metal oxides, preferably LiNi, are preferred as positive electrode active materials. 0.80 Co 0.15 Al 0.05 O2, LiNi 0.82 Co 0.15 Al 0.03 O2, LiNi 0.33 Mn 0.33 Co 0.33 O2, LiNi 0.5 Mn 0.3 Co0.2 O2, LiNi 0.6 Mn 0.2 Co 0.2 O2, LiNi 0.8 Mn 0.1 Co 0.1 O2 and LiNi 0.90 Mn 0.05 Co 0.05 At least one of the groups consisting of O2, more preferably free of LiNi 0.82 Co 0.15 Al 0.03 O2, LiNi 0.6 Mn 0.2 Co 0.2 O2 and LiNi 0.8 Mn 0.1 Co 0.1 At least one of the groups consisting of O2.

[0036] It can also be used in combination with lithium-nickel composite oxides of general formula (1) and different positive electrode active materials. Specifically, examples of different positive electrode active materials include LiCoO2, LiMnO2, LiMn2O4, Li2MnO3, and LiMn2O4. 1.8 Al 0.2 O4, Li4Ti5O 12 , LiFePO4, Li3Fe2(PO4)3, LiFeP2O7, LiCoPO4, Li 1.2 Fe 0.4 Mn 0.4 O2, LiNiO2, etc.

[0037] Examples of lithium-containing transition metal oxides that function as negative electrode active materials include, for example, Li₂ with a spinel structure. 4+x Ti5O 12 (0≤x≤3), Li with orthorhombic manganese oxide structure 2+y Lithium titanate such as Ti3O7 (0≤y≤3).

[0038] As lithium-containing transition metal oxides, lithium-containing transition metal oxides with a different composition than the lithium-containing transition metal oxide can also be used. Examples of surface-attached substances include oxides such as aluminum oxide, silicon oxide, titanium oxide, zirconium oxide, magnesium oxide, calcium oxide, boron oxide, antimony oxide, and bismuth oxide; sulfates such as lithium sulfate, sodium sulfate, potassium sulfate, magnesium sulfate, calcium sulfate, and aluminum sulfate; and carbonates such as lithium carbonate, calcium carbonate, and magnesium carbonate.

[0039] These surface-attached substances can be attached to the surface of lithium-containing transition metal oxides by, for example, by dissolving or suspending them in a solvent, impregnating them into the lithium-containing transition metal oxides, and then drying them; by dissolving or suspending the surface-attached substance precursor in a solvent, impregnating it into the lithium-containing transition metal oxides, and then reacting it by heating or the like; by adding it to the lithium-containing transition metal oxide precursor while simultaneously firing it; and so on.

[0040] The amount of surface-attached material relative to the lithium-containing transition metal oxide, by mass, is preferably 0.1 ppm or more, more preferably 1 ppm or more, further preferably 10 ppm or more, preferably 20% or less, more preferably 10% or less, and further preferably 5% or less. The surface-attached material can suppress the oxidation reaction of the non-aqueous electrolyte on the surface of the lithium-containing transition metal oxide, thereby improving battery life; however, if the amount attached is too small, its effect cannot be fully realized; if the amount attached is too large, it will hinder the movement of lithium ions, and therefore the resistance may increase.

[0041] The lithium-containing transition metal oxide particles can be in the shapes of conventionally used blocky, polyhedral, spherical, ellipsoidal, plate-like, needle-like, and columnar forms. Among these, lithium-containing transition metal oxide particles formed by the aggregation of primary particles into secondary particles, with the secondary particles being spherical or ellipsoidal in shape, are preferred. In electrochemical devices, the active material in the electrode typically expands and contracts during charging and discharging, which can easily lead to stress-induced damage to the active material or disruption of conductive pathways. Therefore, compared to single-particle active materials consisting only of primary particles, materials formed by the aggregation of primary particles into secondary particles can mitigate the stress of expansion and contraction and prevent degradation, making them preferable. Furthermore, compared to plate-shaped equiaxed particles, spherical or ellipsoidal particles result in less orientation during electrode forming, thus reducing electrode expansion and contraction during charging and discharging. They also facilitate uniform mixing with the conductive agent during electrode fabrication, making them preferable as well.

[0042] The tap density of lithium-containing transition metal oxides is typically 1.3 g / cm³. 3 The above, preferably 1.5g / cm 3 The above, and more preferably, is 1.6 g / cm³. 3 The above, and the optimal value, is 1.7 g / cm³. 3The above applies. If the tap density of the lithium-containing transition metal oxide is less than the lower limit mentioned above, the amount of dispersion medium required for the formation of the electrode binder layer increases, and the amount of conductive additives or binders required also increases. Sometimes, the filling rate of the lithium-containing transition metal oxide in the electrode binder layer is limited, and the battery capacity is also limited. By using lithium-containing transition metal oxides with high tap density, a high-density electrode binder layer can be formed. Generally, the higher the tap density, the better, and there is no particular upper limit. However, if it is too high, the diffusion of lithium ions in the electrode binder layer using a non-aqueous electrolyte as the medium becomes rate-controlling, and sometimes the load characteristics are easily reduced. Therefore, it is usually 2.5 g / cm³. 3 The following, preferably 2.4 g / cm³ 3 the following.

[0043] Regarding the tap density of lithium-containing transition metal oxides, the sample was passed through a sieve with a mesh size of 300 μm and allowed to fall to a depth of 20 cm. 3 The sample is filled into a tapping cell and then subjected to 1000 oscillations with a stroke of 10 mm using a powder density meter (e.g., the Tap Denser manufactured by Seishin Enterprise). The density is then calculated from the volume and weight of the sample and is defined as the tap density.

[0044] The median diameter d50 of the lithium-containing transition metal oxide particles (the secondary particle size when primary particles agglomerate to form secondary particles) is typically 0.1 μm or more, preferably 0.5 μm or more, more preferably 1 μm or more, and most preferably 3 μm or more; typically 20 μm or less, preferably 18 μm or less, more preferably 16 μm or less, and most preferably 15 μm or less. By ensuring that the median diameter d50 of the lithium-containing transition metal oxide particles is within the above-mentioned range, the coating properties of the electrode mixture of the present invention become excellent. Here, by combining two or more lithium-containing transition metal oxides with different median diameter d50s, the electrode mixture of the present invention can form a high-density electrode mixture layer.

[0045] It should be noted that the median diameter d50 in this invention is measured using a known laser diffraction / scattering particle size distribution measuring device. Using a HORIBA LA-920 as the particle size distribution measuring device and a 0.1% (w / w) sodium hexametaphosphate aqueous solution as the dispersion medium, the refractive index was set to 1.24 after 5 minutes of ultrasonic dispersion.

[0046] When secondary particles are formed by the aggregation of primary particles, the average primary particle size of the lithium-containing transition metal oxide is typically 0.01 μm or more, preferably 0.05 μm or more, further preferably 0.08 μm or more, and most preferably 0.1 μm or more; typically 3 μm or less, preferably 2 μm or less, further preferably 1 μm or less, and most preferably 0.6 μm or less. By making the average primary particle size of the lithium-containing transition metal oxide within the above range, spherical secondary particles are easily formed, exhibiting excellent powder filling properties and crystallinity. Therefore, the electrode mixture of the present invention can form an electrode mixture layer with better flexibility and adhesion to the current collector, and can form a secondary battery with better battery characteristics. It should be noted that the primary particle size was determined by observation using a scanning electron microscope (SEM). Specifically, in a photograph at 10,000x magnification, the longest value of the slice obtained by the left and right boundary lines of any 50 primary particles relative to a straight line in the horizontal direction was calculated, and the average value was taken to determine the primary particle size.

[0047] The BET specific surface area of ​​lithium-containing transition metal oxides is typically 0.2 m². 2 / g or more, preferably 0.3m 2 / g or more, more preferably 0.4m 2 / g or above, typically 4.0m 2 / g or less, preferably 2.5m 2 / g or less, more preferably 1.5m 2 / g or less. By making the BET specific surface area of ​​the lithium-containing transition metal oxide within the above range, the electrode mixture of the present invention has excellent coatability, can form an electrode mixture layer with better flexibility and better adhesion to the current collector, and can form a secondary battery with better battery characteristics.

[0048] BET specific surface area is defined as follows: using a surface area meter (e.g., a fully automated surface area measuring device manufactured by Riken Corporation of Okura), the sample is pre-dried at 150°C for 30 minutes under nitrogen flow. Then, a nitrogen-helium mixed gas is precisely adjusted to a relative pressure of 0.3 nitrogen relative to atmospheric pressure. The measurement is performed by the nitrogen adsorption BET single-point method based on gas flow. The obtained value is defined as the BET specific surface area.

[0049] As a method for manufacturing lithium-containing transition metal oxides, common methods used in the manufacturing of inorganic compounds are employed. Various methods have been considered, particularly for producing spherical or ellipsoidal lithium-containing transition metal oxides. Examples include: dissolving or dispersing transition metal precursors such as transition metal nitrates or sulfates, along with other desired elemental precursors, in a solvent such as water; adjusting the pH under stirring to produce spherical precursors and recovering them; drying them as needed; then adding a Li source such as LiOH, Li₂CO₃, or LiNO₃ and calcining at high temperature to obtain lithium-containing transition metal oxides; or dissolving or dispersing transition metal precursors such as transition metal nitrates, sulfates, hydroxides, or oxides, along with other desired elemental precursors, in a solvent such as water. Methods include: drying and shaping a precursor in a solvent such as water using a spray dryer to form a spherical or ellipsoidal precursor; adding Li sources such as LiOH, Li2CO3, and LiNO3 to the precursor and calcining it at high temperature to obtain lithium-containing transition metal oxides; and dissolving or pulverizing transition metal raw materials such as transition metal nitrates, sulfates, hydroxides, and oxides with Li sources such as LiOH, Li2CO3, and LiNO3, as well as other raw materials of other elements as needed, in a solvent such as water; drying and shaping the precursor in a spray dryer to form a spherical or ellipsoidal precursor; and calcining it at high temperature to obtain lithium-containing transition metal oxides; etc.

[0050] It should be noted that lithium-containing transition metal oxides can be used alone, or two or more substances with different compositions or different powder properties can be used in any combination and proportion.

[0051] The content of lithium-containing transition metal oxides in the electrode binder is preferably 95.0% to 99.8% by mass, more preferably 96.0% to 99.4% by mass, and even more preferably 96.5% to 99.0% by mass, relative to the total mass of the lithium-containing transition metal oxides, conductive additives, and binders, in order to form an electrode binder layer with better flexibility and adhesion to the current collector and a secondary battery with better battery characteristics.

[0052] <Conductive additives>

[0053] The electrode compound of the present invention contains at least one nano-carbon material selected from the group consisting of multilayer carbon nanotubes, carbon nanotubes, carbon nanofibers, fullerenes, and graphene as a conductive additive. Because the electrode compound of the present invention contains a specific nano-carbon material as a conductive additive, an electrode compound layer with excellent adhesion to the current collector can be obtained without compromising flexibility when forming the electrode compound layer of the electrode in a secondary battery using the electrode compound of the present invention. Furthermore, secondary batteries possessing such an electrode compound layer exhibit high high-temperature capacity retention, low resistance increase rate, and low gas quantity change rate, resulting in excellent battery characteristics.

[0054] Carbon nanomaterials are materials formed from carbon atoms with a nanometer-sized structure. The shapes of carbon nanomaterials can be cylindrical, hollow needle-like, hollow rod-like, hollow granular, or sheet-like. Furthermore, carbon nanomaterials preferably possess electrical conductivity.

[0055] As for the nano-carbon material, there is no particular limitation as long as it is selected from at least one of the group consisting of multilayer carbon nanotubes, carbon nanotubes, carbon nanofibers, fullerenes, and graphene. However, considering the ability to form an electrode layer with superior flexibility and adhesion to the current collector, and to form a secondary battery with superior battery characteristics, at least one of the group consisting of multilayer carbon nanotubes, carbon nanotubes, and graphene is preferred; more preferably, at least one of the group consisting of multilayer carbon nanotubes and graphene is preferred; and even more preferably, multilayer carbon nanotubes are preferred. The nano-carbon material can be used alone, or two or more can be used in any combination and proportion.

[0056] Graphene is a two-dimensional material, unlike carbon nanotubes, which are known as one-dimensional materials. Furthermore, carbon nanotubes can be rolled into a cylindrical shape from graphene sheets. Among carbon nanotubes, single-walled carbon nanotubes are called single-walled carbon nanotubes (SWNTs), and multi-walled carbon nanotubes are called multi-walled carbon nanotubes (MWNTs).

[0057] The electrode mixture of the present invention preferably contains conductive additives other than nano-carbon materials. Examples of conductive additives other than nano-carbon materials include carbon black such as acetylene black and Ketjen black; natural graphite; and metal powders such as nickel and aluminum.

[0058] Carbon black is preferred as a conductive additive other than nano-carbon materials, and acetylene black is more preferred.

[0059] The content of the conductive additive in the electrode binder is preferably 0.01% by mass or more, more preferably 0.1% by mass or more, further preferably 0.3% by mass or more, particularly preferably 0.5% by mass or more, preferably 3.0% by mass or less, more preferably 2.5% by mass or less, further preferably 2.0% by mass or less, and particularly preferably 1.6% by mass or less, relative to the total mass of the lithium-containing transition metal oxide, conductive additive, and binder. This is intended to enable the formation of an electrode binder layer with superior flexibility and adhesion to the current collector, and to form a secondary battery with superior battery characteristics.

[0060] The content of nano-carbon material in the electrode binder, with the aim of forming an electrode binder layer with better flexibility and adhesion to the current collector, and forming a secondary battery with better battery characteristics, is preferably 0.01% by mass or more, more preferably 0.1% by mass or more, further preferably 0.2% by mass or more, even more preferably 0.3% by mass or more, particularly preferably 0.5% by mass or more, preferably 3.0% by mass or less, more preferably 2.5% by mass or less, further preferably 2.0% by mass or less, and particularly preferably 1.6% by mass or less, relative to the total mass of lithium-containing transition metal oxides, conductive additives and binders.

[0061] Regarding the electrode mixture of the present invention, when it contains nano-carbon material and other conductive additives as conductive additives, the ratio of nano-carbon material to other conductive additives is preferably 1 / 99 to 99 / 1, more preferably 10 / 90 to 95 / 5, and even more preferably 20 / 80 to 90 / 10, based on the mass ratio (mass of conductive additives other than nano-carbon material / mass of nano-carbon material) when it contains nano-carbon material and other conductive additives as conductive additives.

[0062] <Adhesive>

[0063] The electrode binder of the present invention contains a fluorinated copolymer comprising vinylidene fluoride units (VdF units) and fluorinated monomer units (excluding VdF units).

[0064] Examples of fluorinated monomers (excluding VdF) include tetrafluoroethylene (TFE), vinyl fluoride, trifluoroethylene, chlorotrifluoroethylene (CTFE), fluoroalkyl vinyl ethers, hexafluoropropylene (HFP), (perfluoroalkyl)ethylene, 2,3,3,3-tetrafluoropropylene, and trans-1,3,3,3-tetrafluoropropylene. Among these, from the perspective of forming an electrode mixture layer with superior flexibility and adhesion to the current collector, and forming a secondary battery with superior battery characteristics, at least one of the group consisting of TFE, CTFE, and HFP is preferred, more preferably at least one of the group consisting of TFE and HFP, and particularly preferably TFE.

[0065] Fluorinated monomer units (excluding VdF units) may or may not have polar groups.

[0066] The content of VdF units in the fluorinated copolymer is more than 50 mol% and less than 99 mol% relative to all monomer units. By keeping the VdF unit content within this range, an electrode binder layer with excellent flexibility and adhesion to the current collector can be formed, and a secondary battery with excellent battery characteristics can be formed. If the VdF unit content of the fluorinated copolymer is too low, sufficient adhesion of the electrode binder layer to the current collector cannot be obtained, or excellent battery characteristics of the secondary battery cannot be obtained.

[0067] The content of VdF units in the fluorinated copolymer is preferably 57.0 mol% or more, more preferably 60.0 mol% or more, further preferably 63.0 mol% or more, preferably 99.0 mol% or less, more preferably 97.0 mol% or less, further preferably 95.0 mol% or less, particularly preferably 90.0 mol% or less, and most preferably 85.0 mol% or less, relative to all monomer units, in order to form an electrode layer with better flexibility and better adhesion to the current collector, and a secondary battery with better battery characteristics.

[0068] The content of fluorinated monomer units (excluding VdF units) in the fluorinated copolymer is not particularly limited as long as it is less than 50 mol%. Relative to all monomer units, it is preferably 1.0 mol% or more, more preferably 3.0 mol% or more, further preferably 5.0 mol% or more, particularly preferably 10.0 mol% or more, most preferably 15.0 mol% or more, preferably 43.0 mol% or less, more preferably 40.0 mol% or less, and further preferably 37.0 mol% or less. By using a fluorinated copolymer containing fluorinated monomer units (excluding VdF units) within the above-mentioned range, the electrode binder of the present invention can form an electrode binder layer with superior flexibility and adhesion to the current collector, and can form a secondary battery with superior battery characteristics.

[0069] In this invention, the composition of the fluorinated copolymer can be, for example, determined by... 19 The determination was performed using F-NMR.

[0070] Fluorinated copolymers may further contain non-fluorinated monomer units. Examples of such non-fluorinated monomers include non-fluorinated monomers without polar groups, such as ethylene and propylene, as well as non-fluorinated monomers with polar groups (hereinafter sometimes referred to as monomers containing polar groups).

[0071] If a substance with a polar group is used as a non-fluorinated monomer, the polar group is introduced into the fluorinated copolymer, thereby obtaining better adhesion between the positive electrode layer and the current collector. As the polar group that the fluorinated copolymer may have, it is preferably selected from at least one of the group consisting of a carbonyl group, an epoxy group, a hydroxyl group, a sulfonic acid group, a sulfate group, a phosphate group, an amino group, an amide group, and an alkoxy group; more preferably, it is selected from at least one of the group consisting of a carbonyl group, an epoxy group, and a hydroxyl group; and even more preferably, it is a carbonyl group. The aforementioned hydroxyl group does not include hydroxyl groups that constitute a part of the aforementioned carbonyl group. Furthermore, the aforementioned amino group refers to a monovalent functional group obtained by removing hydrogen from ammonia, a primary amine, or a secondary amine.

[0072] The aforementioned carbonyl-containing groups refer to functional groups having a carbonyl group (-C(=O)-). Preferably, the carbonyl-containing group is one represented by the general formula -COOR (R represents a hydrogen atom, alkyl group, or hydroxyalkyl group) or a carboxylic anhydride group. The number of carbon atoms in the alkyl and hydroxyalkyl groups is preferably 1 to 16, more preferably 1 to 6, and even more preferably 1 to 3. Specifically, examples of groups represented by the general formula -COOR include -COOCH2CH2OH, -COOCH2CH(CH3)OH, -COOCH(CH3)CH2OH, -COOH, -COOCH3, and -COOC2H5. When the group represented by the general formula -COOR is -COOH, or contains -COOH, -COOH can be a carboxylic acid metal salt, ammonium carboxylic acid salt, or other carboxylic acid salt.

[0073] Alternatively, the carbonyl-containing group mentioned above can also be a group with the general formula: -X-COOR (where the main chain of X consists of 2 to 15 atoms, and the molecular weight of the atomic group represented by X is preferably 350 or less. R represents a hydrogen atom, an alkyl group, or a hydroxyalkyl group). The number of carbon atoms in the alkyl and hydroxyalkyl groups is preferably 1 to 16, more preferably 1 to 6, and even more preferably 1 to 3.

[0074] As the aforementioned amide group, the preferred group is one represented by the general formula: -CO-NRR' (where R and R' independently represent a hydrogen atom or a substituted or unsubstituted alkyl group), or a bond represented by the general formula: -CO-NR”- (where R” represents a hydrogen atom, a substituted or unsubstituted alkyl group, or a substituted or unsubstituted phenyl group).

[0075] Examples of monomers containing polar groups include: hydroxyethyl acrylate, 2-hydroxypropyl acrylate, and other (meth) hydroxyalkyl acrylates; dimethyl methylene malonate and other alkylidene malonates; vinyl carboxymethyl ether, vinyl carboxyethyl ether, and other vinyl carboxyalkyl ethers; 2-carboxyethyl acrylate, 2-carboxyethyl methacrylate, and other (meth) carboxyalkyl acrylates; acryloyloxyethyl succinate, acryloyloxypropyl succinate, methacryloyloxyethyl succinate, acryloyloxyethyl phthalate, methacryloyloxyethyl phthalate, and other (meth) acryloyloxyalkyl dicarboxylic acid esters; monomethyl maleate, monoethyl maleate, monomethyl citrate, monoethyl citrate, and other monoesters of unsaturated dicarboxylic acids; general formula (2):

[0076] [Chemistry 1]

[0077]

[0078] (where R is in the formula) 1 ~R 3 Independently represents a hydrocarbon group having 1 to 8 hydrogen atoms or carbon atoms. R 4 Y represents a single bond or a hydrocarbon group with 1 to 8 carbon atoms. 1 Monomers (2) are shown as inorganic cations and / or organic cations.

[0079] As the monomer unit containing the above-mentioned polar groups that may be contained in the fluorinated copolymer, it is preferred to be a unit based on the monomer (2) shown in general formula (2).

[0080] In general formula (2), Y 1 This refers to inorganic cations and / or organic cations. Examples of inorganic cations include H, Li, Na, K, Mg, Ca, Al, and Fe. Examples of organic cations include NH4 and NH3R. 5 NH2R 5 2. NHR 5 3. NR 5 4(R 5 A cation that independently represents an alkyl group having 1 to 4 carbon atoms, etc. As Y 1 Preferably, H, Li, Na, K, Mg, Ca, Al, and NH4 are used; more preferably, H, Li, Na, K, Mg, Al, and NH4 are used; even more preferably, H, Li, Al, and NH4 are used; and particularly preferably, H is used. It should be noted that, for convenience, the symbols and valences of specific examples of inorganic and organic cations are omitted in the description.

[0081] In general formula (2), R 1 ~R 3Independently representing a hydrocarbon group having 1 to 8 hydrogen atoms or carbon atoms. The aforementioned hydrocarbon group is a monovalent hydrocarbon group. The number of carbon atoms in the aforementioned hydrocarbon group is preferably 4 or less. Examples of such hydrocarbon groups include alkyl, alkenyl, and alkynyl groups with the aforementioned number of carbon atoms; methyl or ethyl groups are preferred. R 1 and R 2 Independently preferred are hydrogen atoms, methyl or ethyl, R 3 Preferably, it contains hydrogen atoms or methyl groups.

[0082] In general formula (2), R 4 This refers to a hydrocarbon group with a single bond or 1 to 8 carbon atoms. The aforementioned hydrocarbon group is a divalent hydrocarbon group. The number of carbon atoms in the aforementioned hydrocarbon group is preferably 4 or less. Examples of the aforementioned hydrocarbon group include alkylene groups, alkenylene groups, etc., with the aforementioned number of carbon atoms; preferably, at least one is selected from the group consisting of methylene, ethylene, ethoxylide, propylene, and isopropylene, more preferably methylene.

[0083] As monomer (2), preferably at least one is selected from the group consisting of (meth)acrylic acid and its salts, vinylacetic acid (3-butenoic acid) and its salts, 3-pentenoic acid and its salts, 4-pentenoic acid and its salts, 3-hexenoic acid and its salts, 4-heptenoic acid and its salts, and 5-hexenoic acid and its salts, more preferably at least one is selected from the group consisting of 3-butenoic acid and its salts and 4-pentenoic acid and its salts.

[0084] When the fluorinated copolymer contains the aforementioned monomer units containing polar groups, the content of the aforementioned monomer units containing polar groups in the fluorinated copolymer is preferably 0.05 mol% to 2.0 mol% relative to all monomer units, more preferably 0.10 mol% or more, further preferably 0.25 mol% or more, particularly preferably 0.40 mol% or more, and more preferably 1.5 mol% or less.

[0085] In this invention, for example, when the polar group is an acid group such as a carboxylic acid, the content of monomer units containing polar groups in the fluorinated copolymer can be determined by acid-base titration of the acid group.

[0086] Examples of fluorinated copolymers include VdF / TFE copolymers, VdF / HFP copolymers, VdF / TFE / HFP copolymers, VdF / TFE / 2,3,3,3-tetrafluoropropylene copolymers, VdF / TFE / (meth)acrylic acid copolymers, VdF / HFP / (meth)acrylic acid copolymers, VdF / CTFE copolymers, VdF / TFE / 4-pentenoic acid copolymers, VdF / TFE / 3-butenoic acid copolymers, VdF / TFE / HFP / (meth)acrylic acid copolymers, VdF / TFE / HFP / 4-pentenoic acid copolymers, VdF / TFE / HFP / 3-butenoic acid copolymers, VdF / TFE / 2-carboxyethyl acrylate copolymers, VdF / TFE / HFP / 2-carboxyethyl acrylate copolymers, VdF / TFE / acryloyloxyethyl succinate copolymers, and VdF / TFE / HFP / acryloyloxyethyl succinate copolymers.

[0087] As a fluorinated copolymer, a fluorinated copolymer consisting only of VdF units, TFE units and any non-fluorinated monomer units is preferred, based on the goal of forming an electrode layer with better flexibility and adhesion to the current collector, and a secondary battery with better battery characteristics.

[0088] When the fluorinated copolymer contains VdF units and TFE units, the molar ratio of VdF units to TFE units (VdF units / TFE units) is preferably greater than 50 / 50 and less than 99 / 1, more preferably 57 / 43 to 97 / 3, even more preferably 60 / 40 to 95 / 5, particularly preferably 63 / 37 to 90 / 10, and most preferably 63 / 37 to 85 / 15.

[0089] The weight-average molecular weight (converted to polystyrene) of the fluorinated copolymer is preferably 50,000 to 3,000,000, more preferably 80,000 or more, further preferably 100,000 or more, particularly preferably 200,000 or more, more preferably 2,400,000 or less, further preferably 2,200,000 or less, and particularly preferably 2,000,000 or less. The above-mentioned weight-average molecular weight can be determined by gel permeation chromatography (GPC) using dimethylformamide as a solvent.

[0090] The number-average molecular weight (converted from polystyrene) of the fluorinated copolymer is preferably 20,000 to 1,500,000, more preferably 40,000 or more, further preferably 70,000 or more, particularly preferably 140,000 or more, more preferably 1,400,000 or less, further preferably 1,200,000 or less, and particularly preferably 1,100,000 or less. The above-mentioned number-average molecular weight can be determined by gel permeation chromatography (GPC) using dimethylformamide as a solvent.

[0091] The melting point of the fluorinated copolymer is preferably 100°C to 170°C, more preferably 110°C to 165°C, and even more preferably 120°C to 163°C. The above melting point can be determined as follows: using a differential scanning calorimeter (DSC), the temperature is increased from 30°C to 220°C at a rate of 10°C / min, then decreased to 30°C at a rate of 10°C / min, and then increased again to 220°C at a rate of 10°C / min. The temperature at which this temperature corresponds to the maximum value in the heat of melting curve is obtained.

[0092] The elongation at break of the fluorinated copolymer is preferably 100% or more. More preferably, the elongation at break is 200% or more, and even more preferably 300% or more.

[0093] The elongation at break described above can be determined by the following method: A fluorinated copolymer solution is obtained by dissolving the fluorinated copolymer in N-methyl-2-pyrrolidone (NMP) at a concentration of 10% to 20% by mass. This solution is then poured onto a glass plate and dried at 100°C for 12 hours, followed by vacuum drying at 100°C for 12 hours to obtain a film with a thickness of 50 μm to 100 μm. The film is then cut into dumbbell shapes, and the elongation at break at 25°C is measured using Autograph.

[0094] The fluorinated copolymer preferably has a storage modulus of 1100 MPa or less at 30°C and a storage modulus of 500 MPa or less at 60°C.

[0095] The storage modulus of the fluorinated copolymer at 30°C is more preferably 800 MPa or less, and even more preferably 600 MPa or less.

[0096] The storage modulus of the fluorinated copolymer at 60°C is preferably below 350 MPa.

[0097] The storage modulus of the fluorinated copolymer at 30°C is preferably 100 MPa or more, more preferably 150 MPa or more, and even more preferably 200 MPa or more.

[0098] The storage modulus of the fluorinated copolymer at 60°C is preferably 50 MPa or more, more preferably 80 MPa or more, and even more preferably 130 MPa or more.

[0099] The storage modulus was measured as follows: for samples with a length of 30 mm, a width of 5 mm, and a thickness of 50 μm to 100 μm, the dynamic viscoelasticity was measured at 30 °C and 60 °C using a dynamic viscoelasticity device DVA220 manufactured by IT Keisokuseigyo under the following conditions: tensile mode, clamping width of 20 mm, measurement temperature from -30 °C to 160 °C, heating rate of 2 °C / min, and frequency of 1 Hz.

[0100] The test sample can be prepared as follows: For example, a fluorinated copolymer solution is obtained by dissolving a fluorinated copolymer in N-methyl-2-pyrrolidone (NMP) at a concentration of 10% to 20% by mass. The fluorinated copolymer solution is then poured onto a glass plate and dried at 100°C for 12 hours, followed by drying under vacuum at 100°C for 12 hours. The resulting film, with a thickness of 50 μm to 100 μm, is then cut into sections with a length of 30 mm and a width of 5 mm. This process can be used to prepare the test sample.

[0101] The weight gain rate of the fluorinated copolymer after impregnation in an electrolyte at 60°C for one week is preferably 250% by mass or less, more preferably 200% by mass or less. The weight gain rate of the fluorinated copolymer after impregnation in an electrolyte at 60°C for one week is further preferably 180% by mass or less, particularly preferably 160% by mass or less, and may also be 105% by mass or more.

[0102] The aforementioned weight gain rate can be determined using the following method.

[0103] An NMP solution (8% by mass) of the fluorinated copolymer was poured onto a glass petri dish and vacuum dried at 100°C for 12 hours to prepare a 200 μm thick membrane. The resulting membrane was cut into 6 mm Φ pieces and placed in a sample vial containing an electrolyte (a solution of LiPF6 dissolved at 1 M concentration in a solvent of 3 / 7 (volume ratio) of ethylene carbonate and methyl ethyl carbonate). After standing at 60°C for 1 week, the weight gain rate was calculated.

[0104] The electrode binder in the present invention preferably further contains polyvinylidene fluoride (PVdF). By using fluorinated copolymers and PVdF as binders, an electrode binder layer with better flexibility and adhesion to the current collector can be formed, and a secondary battery with better battery characteristics can be formed.

[0105] Polyvinylidene fluoride (PVdF) is a polymer containing units based on vinylidene fluoride (VdF) (hereinafter referred to as VdF units). It can be a VdF homopolymer consisting only of VdF units, or a polymer containing VdF units and units based on monomers that can copolymerize with VdF.

[0106] In the aforementioned PVdF, the monomer that can copolymerize with VdF is preferably a monomer different from tetrafluoroethylene (TFE). That is, PVdF preferably does not contain TFE units.

[0107] Among the aforementioned PVdFs, monomers capable of copolymerizing with VdFs include fluorinated monomers and non-fluorinated monomers, with fluorinated monomers being preferred. Examples of fluorinated monomers include vinyl fluoride, trifluoroethylene, chlorotrifluoroethylene (CTFE), fluoroalkyl vinyl ethers, hexafluoropropylene (HFP), (perfluoroalkyl)ethylene, 2,3,3,3-tetrafluoropropylene, and trans-1,3,3,3-tetrafluoropropylene. Examples of non-fluorinated monomers include ethylene and propylene.

[0108] In the aforementioned PVdF, the monomer capable of copolymerizing with VdF is preferably at least one fluorinated monomer selected from the group consisting of CTFE, fluoroalkyl vinyl ether, HFP and 2,3,3,3-tetrafluoropropylene, and more preferably at least one fluorinated monomer selected from the group consisting of CTFE, HFP and fluoroalkyl vinyl ether.

[0109] In the above-mentioned PVdF, the content of monomer units capable of copolymerizing with VdF is preferably 0 to 5.0 mol%, more preferably 0 to 3.0 mol%, relative to all monomer units. In the above-mentioned PVdF, the content of fluorinated monomer units capable of copolymerizing with VdF is preferably less than 5.0 mol%, more preferably less than 3.0 mol%, and even more preferably less than 1.0 mol%, relative to all monomer units.

[0110] In this invention, the composition of PVdF can be, for example, by... 19 The measurements were performed using F-NMR.

[0111] The aforementioned PVdF can have polar groups. By using fluorinated copolymers and PVdF with polar groups as binders, it is possible to form an electrode binder layer with better flexibility and adhesion to the current collector, and to form a secondary battery with better battery characteristics.

[0112] As for the aforementioned polar groups, there is no particular limitation as long as they are polar functional groups. However, from the perspective of maintaining the excellent flexibility of the formed electrode mixture layer and the excellent battery characteristics of the formed secondary battery, and further improving the adhesion of the formed electrode mixture layer to the current collector, at least one group selected from the group consisting of carbonyl groups, epoxy groups, hydroxyl groups, sulfonic acid groups, sulfate groups, phosphate groups, amino groups, amide groups, and alkoxy groups is preferred. More preferably, at least one group selected from the group consisting of carbonyl groups, epoxy groups, and hydroxyl groups is preferred. The aforementioned hydroxyl groups do not include hydroxyl groups that constitute a part of the aforementioned carbonyl groups. Furthermore, the aforementioned amino group refers to a monovalent functional group obtained by removing hydrogen from ammonia, primary amines, or secondary amines.

[0113] The aforementioned carbonyl-containing group refers to a functional group having a carbonyl group (-C(=O)-). As the aforementioned carbonyl-containing group, from the perspective of maintaining the excellent flexibility of the formed electrode mixture layer and the excellent battery characteristics of the formed secondary battery, and further improving the adhesion of the formed electrode mixture layer to the current collector, groups represented by the general formula: -COOR (R represents a hydrogen atom, alkyl group, or hydroxyalkyl group) or carboxylic anhydride group are preferred, and groups represented by the general formula: -COOR are more preferred. The number of carbon atoms in the alkyl and hydroxyalkyl groups is preferably 1 to 16, more preferably 1 to 6, and even more preferably 1 to 3. Specifically, examples of groups represented by the general formula: -COOR include -COOCH2CH2OH, -COOCH2CH(CH3)OH, -COOCH(CH3)CH2OH, -COOH, -COOCH3, -COOC2H5, etc. When the group represented by the general formula: -COOR is -COOH, or contains -COOH, -COOH can be a carboxylic acid metal salt, ammonium carboxylic acid salt, or other carboxylic acid salt.

[0114] Alternatively, the carbonyl-containing group mentioned above can also be a group with the general formula: -X-COOR (where the main chain of X consists of 2 to 15 atoms, and the molecular weight of the atomic group represented by X is preferably 350 or less. R represents a hydrogen atom, an alkyl group, or a hydroxyalkyl group). The number of carbon atoms in the alkyl and hydroxyalkyl groups is preferably 1 to 16, more preferably 1 to 6, and even more preferably 1 to 3.

[0115] As the aforementioned amide group, the preferred group is one represented by the general formula: -CO-NRR' (where R and R' independently represent a hydrogen atom or a substituted or unsubstituted alkyl group), or a bond represented by the general formula: -CO-NR”- (where R” represents a hydrogen atom, a substituted or unsubstituted alkyl group, or a substituted or unsubstituted phenyl group).

[0116] The aforementioned polar groups can be introduced into PVdF by polymerizing VdF with a monomer having the aforementioned polar groups (hereinafter referred to as a monomer containing polar groups), or by reacting PVdF with a compound having the aforementioned polar groups. From the perspective of productivity, it is preferable to polymerize VdF with the aforementioned monomer containing polar groups.

[0117] If VdF is polymerized with the aforementioned monomer containing polar groups, a PVdF containing VdF units and monomer units containing polar groups is obtained. That is, as a PVdF, from the perspective of maintaining the excellent flexibility of the formed electrode mixture layer and the excellent battery characteristics of the formed secondary battery, and further improving the adhesion of the formed electrode mixture layer to the current collector, it is preferable to contain the aforementioned monomer units containing polar groups. The content of the aforementioned monomer units containing polar groups is preferably 0.001 mol% to 5.0 mol%, more preferably 0.01 mol% to 3.0 mol%, and even more preferably 0.10 mol% to 1.5 mol% relative to all monomer units.

[0118] In this invention, for example, when the polar group is an acid group such as a carboxylic acid, the content of monomer units containing polar groups in PVdF can be determined by acid-base titration of the acid group.

[0119] Examples of monomers containing polar groups include hydroxyethyl acrylate, 2-hydroxypropyl acrylate, and other (meth)acrylate hydroxyalkyl esters; unsaturated monocarboxylic acids such as (meth)acrylic acid, crotonic acid, vinylacetic acid (3-butenoic acid), 3-pentenoic acid, 4-pentenoic acid, 3-hexenoic acid, and 4-heptenoic acid; unsaturated dicarboxylic acids such as maleic acid, maleic anhydride, citralic acid, and citralic anhydride; alkylidene malonate esters such as dimethyl methylene malonate; and vinyl carboxymethyl ether, vinyl carboxyethyl ether, etc. Alkenyl carboxyl alkyl ethers; carboxyl alkyl esters of (meth)acrylic acid such as 2-carboxyethyl acrylate and 2-carboxyethyl methacrylate; (meth)acryloyloxyalkyl dicarboxylic acid esters such as acryloyloxyethyl succinate, methacryloyloxyethyl succinate, acryloyloxyethyl phthalate, methacryloyloxyethyl phthalate; monoesters of unsaturated dicarboxylic acids such as monomethyl maleate, monoethyl maleate, monomethyl citrate, and monoethyl citrate; etc.

[0120] When PVdF is introduced into PVdF by reacting it with a compound having the aforementioned polar group, the compound having the aforementioned polar group can be a monomer containing the aforementioned polar group, or a silane-based or titanate-based coupling agent containing PVdF and a reactive group and a hydrolyzable group. As the aforementioned hydrolyzable group, an alkoxy group is preferred. When using a coupling agent, it can be added to PVdF by reacting it with PVdF dissolved or swollen in a solvent.

[0121] As PVdF, a substance obtained by partially dehydrofluorinating PVdF with an alkali and then further reacting the partially dehydrofluorinated PVdF with an oxidant can also be used. Examples of such oxidants include hydrogen peroxide, hypochlorite, palladium halide, chromium halide, alkali metal permanganate, peroxide, alkyl peroxide, and alkyl persulfate ester.

[0122] Regarding the VdF cell content of PVdF, from the perspective of being able to form an electrode compound layer with better flexibility and better adhesion to the current collector, and being able to form a secondary battery with better battery characteristics, it is preferably more than 95.0 mol%, more preferably more than 97.0 mol%, and even more preferably more than 99.0 mol% relative to all individual cells.

[0123] Furthermore, regarding the content of VdF cells in PVdF, from the perspective of forming an electrode compound layer with better flexibility and adhesion to the current collector, and forming a secondary battery with better battery characteristics, the content is preferably 95.0 mol% to 99.999 mol% relative to all individual cells, more preferably 97.0 mol% or more, further preferably 98.5 mol% or more, more preferably 99.99 mol% or less, and even more preferably 99.90 mol% or less.

[0124] Regarding the weight-average molecular weight (converted from polystyrene) of PVdF, from the perspective of forming an electrode mixture layer with superior flexibility and adhesion to the current collector, and forming a secondary battery with superior battery characteristics, the preferred weight-average molecular weight is 50,000 to 3,000,000, more preferably 80,000 or more, further preferably 100,000 or more, particularly preferably 200,000 or more, more preferably 2,400,000 or less, further preferably 2,200,000 or less, and particularly preferably 2,000,000 or less. The weight-average molecular weight can be determined by gel permeation chromatography (GPC) using N,N-dimethylformamide as a solvent. Furthermore, from the perspective of forming an electrode mixture layer with excellent flexibility and adhesion to the current collector, and forming a secondary battery with excellent battery characteristics, the weight-average molecular weight of PVdF(A) can be 1,000,000 or more, or 1,500,000 or more.

[0125] Regarding the number-average molecular weight (converted to polystyrene) of PVdF, from the perspective of forming an electrode composite layer with better flexibility and adhesion to the current collector, and forming a secondary battery with better battery characteristics, it is preferably 20,000 to 1,500,000, more preferably 40,000 or more, further preferably 70,000 or more, particularly preferably 140,000 or more, more preferably 1,400,000 or less, further preferably 1,200,000 or less, and particularly preferably 1,100,000 or less. The number-average molecular weight can be determined by gel permeation chromatography (GPC) using dimethylformamide as a solvent.

[0126] The preferred melting point of PVdF is 100℃ to 240℃. This melting point can be determined using a differential scanning calorimeter (DSC) as the temperature relative to the maximum value in the heat of fusion curve when the temperature is increased at a rate of 10℃ / min.

[0127] PVdF can be manufactured by existing known methods such as the following: for example, by appropriately mixing VdF with the above-mentioned monomers containing polar groups, polymerization initiators and other additives, and carrying out solution polymerization or suspension polymerization.

[0128] The energy storage modulus of PVdF at 30°C is preferably below 2000 MPa, and more preferably below 1800 MPa.

[0129] The energy storage modulus of PVdF at 60°C is preferably 1500 MPa or less, more preferably 1300 MPa or less.

[0130] The energy storage modulus of PVdF at 30°C is preferably 1000 MPa or more, and more preferably 1100 MPa or more.

[0131] The energy storage modulus of PVdF at 60°C is preferably 600 MPa or more, and more preferably 700 MPa or more.

[0132] The storage modulus of PVdF can be determined using the same method as that used for fluorinated copolymers.

[0133] When the binder contains PVdF in addition to the fluorinated copolymer, the mass ratio of PVdF to the fluorinated copolymer in the binder (PVdF / fluorinated copolymer) is preferably 99 / 1 to 1 / 99, more preferably 97 / 3 to 3 / 97, further preferably 95 / 5 to 5 / 95, even more preferably 90 / 10 to 10 / 90, particularly preferably 85 / 15 to 15 / 85, and most preferably 80 / 20 to 40 / 60, based on the goal of forming an electrode binder layer with better flexibility and adhesion to the current collector, and a secondary battery with better battery characteristics.

[0134] Besides PVdF and fluorinated copolymers, adhesives can also contain other polymers. Examples of other polymers include polymethyl methacrylate, polymethyl methacrylate, polyacrylonitrile, polyimide, polyamide, polyamide-imide, polycarbonate, styrene rubber, butadiene rubber, etc.

[0135] The content of the fluorinated copolymer in the binder is preferably 1% by mass or more, more preferably 3% by mass or more, further preferably 5% by mass or more, particularly preferably 10% by mass or more, most preferably 15% by mass or more, and can be 100% by mass or less, relative to the mass of the binder, so as to form an electrode binder layer with better flexibility and better adhesion to the current collector and to form a secondary battery with better battery characteristics.

[0136] The content of the binder in the electrode binder is preferably 0.01% by mass or more, more preferably 0.1% by mass or more, further preferably 0.3% by mass or more, particularly preferably 0.5% by mass or more, more preferably 3.0% by mass or less, more preferably 2.5% by mass or less, and further preferably 2.0% by mass or less, relative to the total mass of the lithium-containing transition metal oxide, conductive additive, and binder.

[0137] <Organic solvents>

[0138] The electrode mixture of the present invention further contains an organic solvent. Examples of organic solvents include nitrogen-containing organic solvents such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide, and dimethylformamide; ketone solvents such as acetone, methyl ethyl ketone, cyclohexanone, and methyl isobutyl ketone; ester solvents such as ethyl acetate and butyl acetate; ether solvents such as tetrahydrofuran and dioxane; β-alkoxypropionamides such as β-methoxy-N,N-dimethylpropionamide, β-butoxy-N,N-dimethylpropionamide, and β-hexyloxy-N,N-dimethylpropionamide; and mixed solvents thereof, as well as other low-boiling-point general organic solvents. Among them, as a solvent, from the viewpoint of excellent coatability, it is preferably selected from at least one of the group consisting of N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone and β-alkoxypropionamides, and more preferably from at least one of the group consisting of N-methyl-2-pyrrolidone and N,N-dimethylacetamide.

[0139] The content of lithium-containing transition metal oxides, binders, and carbon nanomaterials in the electrode mixture of the present invention can be determined by considering factors such as coatability on the current collector and film formation properties after drying. The total content of lithium-containing transition metal oxides, binders, and carbon nanomaterials in the electrode mixture is preferably 50% to 90% by mass, more preferably 60% to 80% by mass.

[0140] The electrode compound of the present invention can be prepared by mixing a lithium-containing transition metal oxide, a binder, nano-carbon materials, an organic solvent, and other components as needed. The order in which the components are mixed is not particularly limited. For example, the lithium-containing transition metal oxide and other components can be mixed after the binder, nano-carbon materials, and organic solvent are mixed. Alternatively, for example, a composite material of the binder and the lithium-containing transition metal oxide can be obtained by spraying a solution or dispersion of the binder onto the lithium-containing transition metal oxide and drying it, and then the resulting composite material can be mixed with nano-carbon materials, organic solvent, and other components.

[0141] For adhesives, in order to dissolve organic solvents quickly, it is preferable to use small particle sizes with an average particle size of less than 1000 μm, especially 50 μm to 350 μm.

[0142] <Electrode>

[0143] The electrode of the present invention comprises a current collector and an electrode mixture layer. The electrode mixture layer is formed using the electrode mixture of the present invention and can be disposed on one side or both sides of the current collector.

[0144] The electrode of the present invention has excellent flexibility because it has an electrode mixture layer formed using the electrode mixture of the present invention. The current collector is fully bonded to the electrode mixture layer, and it can form a secondary battery with excellent battery characteristics.

[0145] The preferred density of the electrode mixture layer is 2.0 g / cm³. 3 ~5.0g / cm 3 More preferably 2.5 g / cm³ 3 ~5.0g / cm 3 .

[0146] The density of the electrode mixture layer can be calculated from the mass and volume of the electrode mixture layer.

[0147] To achieve better battery performance, the thickness of the electrode mixture layer is preferably 20 μm or more, more preferably 45 μm or more, further preferably 55 μm or more, particularly preferably 60 μm or more, preferably 170 μm or less, and more preferably 150 μm or less. Alternatively, the thickness of the electrode mixture layer can be 85 μm or less, or less than 69 μm.

[0148] The thickness of the electrode mixture layer can be measured using a micrometer. When the electrode mixture layer is disposed on both sides of the current collector, the thickness of the electrode mixture layer in this invention is the thickness of each single side.

[0149] Examples of current collectors used in the electrodes of this invention include metal foils or metal meshes made of materials such as iron, stainless steel, copper, aluminum, nickel, and titanium, with aluminum foil being preferred.

[0150] The electrode of the present invention can be suitably manufactured by a manufacturing method that coats the electrode mixture of the present invention onto a current collector. After coating the electrode mixture, the coating film can be further dried, and the resulting dried coating film can be pressed.

[0151] The preferred amount of electrode mixture applied to the current collector is 15 mg / cm². 2 The above, and more preferably, is 17.5 mg / cm³ 2 The above is preferably 60 mg / cm³. 2 The following, or more preferably, is 50 mg / cm³ 2 The following refers to the dry weight of the electrode mixture per unit area.

[0152] Secondary batteries

[0153] In addition, according to the present invention, a secondary battery having the above-described electrodes is provided.

[0154] The secondary battery of the present invention exhibits excellent battery characteristics due to the presence of electrodes formed using the electrode mixture of the present invention, which have high high-temperature capacity retention, low resistance increase rate and low gas quantity change rate.

[0155] The secondary battery of the present invention preferably comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein one or both of the positive and negative electrodes are the aforementioned electrodes. Furthermore, the secondary battery of the present invention preferably comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the positive electrode is the aforementioned electrode.

[0156] There are no particular limitations on the non-aqueous electrolyte; one or more of the following known solvents can be used: propylene carbonate, ethylene carbonate, butyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. The electrolyte can also be any currently known substance, such as LiClO4, LiAsF6, LiPF6, LiBF4, LiCl, LiBr, CH3SO3Li, CF3SO3Li, and cesium carbonate.

[0157] The electrode of the present invention exhibits excellent flexibility, allows for thorough adhesion between the current collector and the electrode binder layer, and enables the formation of a secondary battery with excellent battery characteristics. Therefore, it is suitable for use as an electrode for wound-type secondary batteries. Furthermore, the secondary battery of the present invention can be a wound-type secondary battery.

[0158] The electrode of this invention is useful not only in non-aqueous electrolyte secondary batteries, such as the lithium-ion secondary batteries using liquid electrolytes described above, but also in polymer electrolyte lithium secondary batteries. Furthermore, it is also useful as a double-layer capacitor.

[0159] The above describes the embodiments, but it should be understood that various changes can be made to the embodiments or details without departing from the gist and scope of the claims.

[0160] Example

[0161] Next, examples are given to illustrate the embodiments of the present invention, but the present invention is not limited to the above examples.

[0162] The respective numerical values in the examples are measured by the following methods.

[0163] <Content of acrylic acid unit in PVdF>

[0164] The content of the acrylic acid unit in PVdF is measured by acid-base titration of the carboxyl group. Specifically, about 0.5 g of PVdF is dissolved in acetone at a temperature of 70 °C to 80 °C. To avoid the coagulation of PVdF, 5 ml of water is added dropwise under vigorous stirring. Titration is carried out with an aqueous NaOH solution having a concentration of 0.1 N until complete neutralization of the acidity is achieved at a neutral transfer at about -270 mV. From the measurement results, the amount of the acrylic acid unit contained in 1 g of PVdF is determined, and the content of the acrylic acid unit is calculated.

[0165] <Ratio of VdF unit to TFE unit in fluorine-containing copolymer>

[0166] Regarding the ratio of the VdF unit to the TFE unit in the fluorine-containing copolymer, using an NMR analyzer (manufactured by Agilent Technologies, VNS400 MHz), through 19 19F-NMR measurement is carried out in the state of a polymer DMF-d7 solution.

[0167] Through 19 19F-NMR measurement, the areas (A, B, C, D) of the following peaks are obtained, and the ratio of the VdF unit to the TFE unit is calculated.

[0168] A: Area of the peak from -86 ppm to -98 ppm

[0169] B: Area of the peak from -105 ppm to -118 ppm

[0170] C: Area of the peak from -119 ppm to -122 ppm

[0171] D: Area of the peak from -122 ppm to -126 ppm

[0172] Ratio of VdF unit: (4A + 2B) / (4A + 3B + 2C + 2D) × 100 [mol%]

[0173] The proportion of TFE units: (B+2C+2D) / (4A+3B+2C+2D)×100 [mol%]

[0174] <Weight-average molecular weight>

[0175] The determination was performed by gel permeation chromatography (GPC). A Tosoh AS-8010, CO-8020 column (three GMHHR-H columns connected in series) and a Shimadzu RID-10A column were used. Dimethylformamide (DMF) was flowed at a flow rate of 1.0 ml / min as the solvent, and the results were calculated from the measured data (reference: polystyrene).

[0176] Melting point

[0177] Using a differential scanning calorimetry (DSC) apparatus, the temperature was increased from 30°C to 220°C at a rate of 10°C / min, then decreased to 30°C at a rate of 10°C / min, and then increased to 220°C again at a rate of 10°C / min. The temperature corresponding to the maximum value in the heat of fusion curve at this point was taken as the melting point.

[0178] <Thickness of the positive electrode compound layer>

[0179] The positive electrodes prepared in the examples and comparative examples were punched using a Φ13mm manual punching machine to produce test pieces. The total thickness of the test pieces was measured using a micrometer with a minimum scale of 1μm, and the value obtained by subtracting the thickness of the positive electrode current collector from these measured values ​​was calculated.

[0180] <Density of the positive electrode coating layer>

[0181] The positive electrodes produced in the examples and comparative examples were punched using a Φ13mm manual punching machine to create test pieces, and the mass and area of ​​the test pieces were measured. Then, the density of the positive electrode flux layer was calculated from the mass of the test piece and the positive electrode current collector, the area of ​​the test piece, and the thickness of the positive electrode flux layer obtained by the above method.

[0182] <Adhesion between the positive electrode binder layer and the current collector>

[0183] The positive electrodes prepared in the examples and comparative examples were cut to produce test pieces measuring 1.2 cm × 7.0 cm. The positive electrode adhesive layer side of the test piece was secured to a movable clamp with double-sided tape. The tape was then adhered to the surface of the positive electrode current collector, and the tape was stretched at a speed of 100 mm / min along a 90-degree angle. The stress (N / cm) at this point was measured using an autograph. A 1 N force sensor was used in the autograph.

[0184] <Positive Flexibility>

[0185] The positive electrode mixture obtained in the examples and comparative examples was uniformly coated onto one side of the positive electrode current collector (aluminum foil with a thickness of 20 μm), allowing NMP to completely evaporate, thus producing a positive electrode before pressing. The produced positive electrode before pressing was cut to produce a 2 cm × 10 cm test piece. The test piece was pressed to produce a density-adjusted test piece with a positive electrode mixture layer density of 3.6 g / cc. The density-adjusted test piece was wound onto round rods of various sizes with diameters of 5 mm and 3 mm, and the positive electrode mixture layer was visually inspected and evaluated according to the following criteria.

[0186] 〇: No cracks were observed.

[0187] △: Cracks were observed, but no breakage was observed in the positive electrode binder layer or the positive electrode current collector.

[0188] ×: The positive electrode binder layer and the positive electrode current collector are broken.

[0189] In the examples and comparative examples, polymers having the following physical properties were used.

[0190] <pvdf>

[0191] A: VdF homopolymer

[0192] Weight average molecular weight 900,000

[0193] Melting point 171℃

[0194] B: VdF homopolymer

[0195] Weight-average molecular weight 1,800,000

[0196] Melting point 171℃

[0197] C: PVdF containing acrylic units

[0198] Acrylic acid unit content 1.0 mol%.

[0199] Weight-average molecular weight 1,100,000

[0200] Melting point 161℃

[0201] Fluorinated copolymers

[0202] a: Fluorinated copolymers containing VdF units and TFE units

[0203] VdF / TFE = 83 / 17 (mol%)

[0204] Weight-average molecular weight 1,230,000

[0205] Melting point 131℃

[0206] b: Fluorinated copolymers containing VdF units and TFE units

[0207] VdF / TFE = 63 / 37 (mol%)

[0208] Weight-average molecular weight 1,130,000

[0209] Melting point 160℃

[0210] c: Fluorinated copolymers containing VdF and TFE units

[0211] VdF / TFE = 50 / 50 (mol%)

[0212] In addition, the following positive electrode active material and conductive additive were used in the examples and comparative examples.

[0213] NMC622: LiNi 0.6 Mn 0.2 Co 0.2 O2

[0214] NMC811: LiNi 0.8 Mn 0.1 Co 0.1 O2

[0215] NCA: LiNi 0.82 Co 0.15 Al 0.03 O2

[0216] AB: Acetylene Black

[0217] CNT: Multilayer carbon nanotubes, manufactured by Cano Corporation, trade name LB136-43

[0218] Graphene: Manufactured by Sanshun Zhongke Company, trade name GNP-N

[0219] <Example 1>

[0220] (Preparation of the positive electrode mixture)

[0221] A fluorinated copolymer (a) used as a binder was dissolved in N-methyl-2-pyrrolidone (NMP) to prepare a fluorinated copolymer (a) solution with a concentration of 8% by mass. The fluorinated copolymer (a) solution, NMC622 as the positive electrode active material, acetylene black (AB) as a conductive additive, and multilayer carbon nanotubes (CNTs) were mixed using a stirrer to obtain a mixture with the composition ratios (active material / conductive additive / binder) shown in Table 1. NMP was further added to the obtained mixture and mixed to prepare a positive electrode additive with a solid content concentration of 71% by mass.

[0222] (The production of the positive electrode)

[0223] The obtained positive electrode mixture is uniformly coated onto one side of the positive electrode current collector (aluminum foil with a thickness of 20 μm). After the NMP is completely volatilized, it is pressed by applying a pressure of 10t using a roller press, thereby producing a positive electrode with a positive electrode mixture layer and a positive electrode current collector.

[0224] Table 1 shows the coating amount on each side of the positive electrode compound, the thickness of each side of the positive electrode compound layer in the positive electrode, the density of the positive electrode compound layer, and the adhesion of the positive electrode compound layer to the positive electrode current collector.

[0225] (Fabrication of NMC622 Stacked Batteries)

[0226] The fabricated positive electrode was cut into 500mm × 700mm pieces (with the positive terminal), and the strip-shaped negative electrode was cut into 502mm × 702mm pieces (with the negative terminal). Leads were soldered to each terminal. Additionally, a 20μm thick polypropylene membrane separator was cut into 504mm × 800mm pieces, and the positive and negative electrodes were wound around it using a clamping method, then placed inside packaging material. Next, 5g of electrolyte (dissolved in a solvent of ethylene carbonate and methyl ethyl carbonate at a volume ratio of 3 / 7, with LiPF6 dissolved at a concentration of 1 mol / L) was added to the packaging material and sealed to fabricate a wound-type tandem battery.

[0227] <Evaluation of Battery Characteristics>

[0228] The high-temperature storage capacity retention rate, resistance increase rate, and gas quantity change rate of the wound-type tandem battery prepared in Example 1 were determined by the following methods. The results are shown in Table 1.

[0229] [Initial Characteristic Evaluation]

[0230] For wound-type tandem batteries, under clamping and pressurization, the batteries are charged at 25°C with a constant current equivalent to 0.2C to 4.2V, then discharged at a constant current of 0.2C to 3.0V. After two cycles of this operation, the batteries are stabilized. In the third cycle, the batteries are charged at a constant current of 0.2C to 4.2V, then charged at a constant voltage of 4.2V until the current reaches 0.05C, and then discharged at a constant current of 0.2C to 3.0V. In the fourth cycle, the batteries are charged at a constant current of 0.2C to 4.2V, then charged at a constant voltage of 4.2V until the current reaches 0.05C, and then discharged at a constant current of 0.2C to 3.0V. The initial discharge capacity is then determined. Finally, the batteries are charged at a constant current of 0.2C to 4.2V, then charged at a constant voltage of 4.2V until the current reaches 0.05C, and the initial resistance is measured.

[0231] Here, 1C represents the current value at which the battery's base capacity is discharged for 1 hour, 5C represents 5 times that current value, 0.1C represents 1 / 10 of that current value, and 0.2C represents 1 / 5 of that current value.

[0232] [High-Temperature Storage Test]

[0233] After the initial characteristic evaluation, the wound-type stacked batteries were stored at 85°C for 36 hours. After the batteries were fully cooled, their volume was measured using the Archimedes method, and the rate of change in gas volume was calculated based on the volume change before and after high-temperature storage using the following formula.

[0234] Next, the capacitor was discharged at 0.5C to 3V at 25°C, and the residual capacity after high-temperature storage was determined. The high-temperature storage capacity retention rate (%) was calculated based on the following formula. Then, it was charged at a constant current of 0.2C to 4.2V, followed by a constant voltage charge at 4.2V until the current reached 0.05C, and then discharged at 0.5C to 3V. Afterward, it was charged at a constant current of 0.2C to 4.2V, followed by a constant voltage charge at 4.2V until the current reached 0.05C, and the resistance after high-temperature storage was measured. The resistance increase rate (%) was calculated based on the following formula.

[0235] High-temperature storage capacity retention rate (%) = (residual capacity) / (initial discharge capacity) × 100

[0236] Resistance increase rate (%) = (Resistance after high-temperature storage (Ω)) / (Initial resistance (Ω)) × 100

[0237] Gas volume change rate (%) = (Volume after high-temperature storage (ml)) / (Volume before high-temperature storage (ml)) × 100

[0238] <Examples 2-13 and Comparative Examples 1-4>

[0239] The binder was dissolved appropriately at a binder solution concentration of 5% to 8% by mass. The type of binder, the type of conductive additive, and the composition ratio were varied as described in the tables. Otherwise, the positive electrode mixture was prepared in the same manner as in Example 1. Furthermore, a positive electrode was fabricated using the obtained positive electrode mixture, and its evaluation was performed in the same manner as in Example 1. The results are shown in Tables 1 and 2.

[0240] <Comparative Example 5>

[0241] Aqueous dispersions of the fluorinated copolymer (c) were sprayed onto NMC622 and dried to prepare composite NMC622. Using this composite NMC622 as the positive electrode active material, a positive electrode compound was prepared in the same manner as in Example 1. Furthermore, a positive electrode was fabricated using the obtained positive electrode compound, and its evaluation was performed in the same manner as in Example 1. The evaluation results are shown in Table 2.

[0242] The entries "AB+CNT" in each table indicate the use of acetylene black and multilayer carbon nanotubes as conductive additives. Additionally, the entry "AB+graphene" indicates the use of acetylene black and graphene as conductive additives.

[0243] The "Active Material / Conductive Additive / Binder (wt%)" column in each table records the content ratio of the positive electrode active material, conductive additive, and binder in the positive electrode mixture relative to the total mass of the positive electrode active material, conductive additive, and binder. Furthermore, when the positive electrode mixture contains two conductive additives, the content ratio of each conductive additive in the positive electrode mixture is recorded separately. For example, the "96.9 / 1.2 / 0.4 / 1.5" recorded in Example 1 means that the positive electrode mixture contains 96.9% by mass of positive electrode active material, 1.2% by mass of acetylene black, 0.4% by mass of multilayer carbon nanotubes, and 1.5% by mass of binder relative to the total mass of the positive electrode active material, conductive additive, and binder.

[0244] <Example 14>

[0245] (Preparation of the positive electrode mixture)

[0246] A fluorinated copolymer (a) used as a binder was dissolved in N-methyl-2-pyrrolidone (NMP) to prepare a fluorinated copolymer (a) solution with a concentration of 8% by mass. The fluorinated copolymer (a) solution, NMC811 as the positive electrode active material, acetylene black (AB) as a conductive additive, and multilayer carbon nanotubes (CNTs) were mixed using a stirrer to obtain a mixture with the composition ratio (active material / conductive additive / binder) shown in Table 3. NMP was further added to the obtained mixture and mixed to prepare a positive electrode additive with a solid content concentration of 71% by mass.

[0247] (The production of the positive electrode)

[0248] The obtained positive electrode mixture is uniformly coated onto one side of the positive electrode current collector (aluminum foil with a thickness of 20 μm). After the NMP is completely volatilized, it is pressed by applying a pressure of 10t using a roller press, thereby producing a positive electrode with a positive electrode mixture layer and a positive electrode current collector.

[0249] Table 3 shows the coating amount on each side of the positive electrode compound, the thickness of each side of the positive electrode compound layer in the positive electrode, the density of the positive electrode compound layer, and the adhesion of the positive electrode compound layer to the positive electrode current collector.

[0250] (Fabrication of NMC811 Stacked Batteries)

[0251] The fabricated positive electrode was cut into 500mm × 700mm pieces (with the positive terminal), and the strip-shaped negative electrode was cut into 502mm × 702mm pieces (with the negative terminal). Leads were soldered to each terminal. Additionally, a 20μm thick polypropylene membrane separator was cut into 504mm × 800mm pieces, and the positive and negative electrodes were wound around it using a clamping method, then placed inside packaging material. Next, 5g of electrolyte (dissolved in a solvent of ethylene carbonate and methyl ethyl carbonate at a volume ratio of 3 / 7, with LiPF6 dissolved at a concentration of 1 mol / L) was added to the packaging material and sealed to fabricate a wound-type tandem battery.

[0252] <Evaluation of Battery Characteristics>

[0253] The high-temperature storage capacity retention rate, resistance increase rate, and gas quantity change rate of the wound-type tandem battery prepared in Example 14 were determined by the following methods. The results are shown in Table 3.

[0254] [Initial Characteristic Evaluation]

[0255] For wound-type tandem batteries, under clamping and pressurization, the batteries are charged at 25°C with a constant current equivalent to 0.2C to 4.1V, then discharged at a constant current of 0.2C to 3.0V. After two cycles of this operation, the batteries are stabilized. In the third cycle, the batteries are charged at a constant current of 0.2C to 4.1V, then charged at a constant voltage of 4.1V until the current reaches 0.05C, and then discharged at a constant current of 0.2C to 3.0V. In the fourth cycle, the batteries are charged at a constant current of 0.2C to 4.1V, then charged at a constant voltage of 4.1V until the current reaches 0.05C, and then discharged at a constant current of 0.2C to 3.0V. The initial discharge capacity is then determined. Finally, the batteries are charged at a constant current of 0.2C to 4.1V, then charged at a constant voltage of 4.1V until the current reaches 0.05C, and the initial resistance is measured.

[0256] Here, 1C represents the current value at which the battery's base capacity is discharged for 1 hour, 5C represents 5 times that current value, 0.1C represents 1 / 10 of that current value, and 0.2C represents 1 / 5 of that current value.

[0257] [High-Temperature Storage Test]

[0258] After the initial characteristic evaluation, the wound-type stacked batteries were stored at 70°C for 96 hours. After the batteries were fully cooled, their volume was measured using the Archimedes method, and the rate of change in gas volume was calculated based on the volume change before and after high-temperature storage using the following formula.

[0259] Next, the capacitor was discharged at 0.5C to 3V at 25°C, and the residual capacity after high-temperature storage was calculated. The capacity retention rate (%) after high-temperature storage was then calculated using the following formula. Then, the capacitor was charged at a constant current of 0.2C to 4.1V, and then charged at a constant voltage of 4.1V until the current reached 0.05C. Afterward, it was discharged at 0.5C to 3V. Then, it was charged at a constant current of 0.2C to 4.1V, and then charged at a constant voltage of 4.1V until the current reached 0.05C. The resistance after high-temperature storage was measured, and the resistance increase rate (%) was calculated using the following formula.

[0260] High-temperature storage capacity retention rate (%) = (residual capacity) / (initial discharge capacity) × 100

[0261] Resistance increase rate (%) = (Resistance after high-temperature storage (Ω)) / (Initial resistance (Ω)) × 100

[0262] Gas volume change rate (%) = (Volume after high-temperature storage (ml)) / (Volume before high-temperature storage (ml)) × 100

[0263] <Examples 15-26 and Comparative Examples 6-9>

[0264] The binder was dissolved appropriately at a binder solution concentration of 5% to 8% by mass. The type of binder, the type of conductive additive, and the composition ratio were varied as described in the tables. Otherwise, the positive electrode mixture was prepared in the same manner as in Example 14. Furthermore, a positive electrode was fabricated using the obtained positive electrode mixture, and its evaluation was performed in the same manner as in Example 14. The results are shown in Tables 3 and 4.

[0265] <Comparative Example 10>

[0266] Aqueous dispersions of the fluorinated copolymer (c) were sprayed onto NMC811 and dried to prepare composite NMC811. Using this composite NMC811 as the positive electrode active material, a positive electrode mixture was prepared in the same manner as in Example 14. Furthermore, a positive electrode was fabricated using the obtained positive electrode mixture, and its evaluation was performed in the same manner as in Example 14. The evaluation results are shown in Table 4.

[0267] <Example 27>

[0268] (Preparation of the positive electrode mixture)

[0269] A fluorinated copolymer (a) used as a binder was dissolved in N-methyl-2-pyrrolidone (NMP) to prepare a fluorinated copolymer (a) solution with a concentration of 8% by mass. The fluorinated copolymer (a) solution, NCA as the positive electrode active material, acetylene black (AB) as a conductive additive, and multilayer carbon nanotubes (CNTs) were mixed using a stirrer to obtain a mixture with the composition ratio (active material / conductive additive / binder) shown in Table 5. NMP was further added to the obtained mixture and mixed to prepare a positive electrode additive with a solid content concentration of 71% by mass.

[0270] (The production of the positive electrode)

[0271] The obtained positive electrode mixture is uniformly coated onto one side of the positive electrode current collector (aluminum foil with a thickness of 20 μm). After the NMP is completely volatilized, it is pressed by applying a pressure of 10t using a roller press, thereby producing a positive electrode with a positive electrode mixture layer and a positive electrode current collector.

[0272] Table 5 shows the coating amount on each side of the positive electrode compound, the thickness of each side of the positive electrode compound layer in the positive electrode, the density of the positive electrode compound layer, and the adhesion of the positive electrode compound layer to the positive electrode current collector.

[0273] (Fabrication of NCA Cascaded Batteries)

[0274] The fabricated positive electrode was cut into 500mm × 700mm pieces (with the positive terminal), and the strip-shaped negative electrode was cut into 502mm × 702mm pieces (with the negative terminal). Leads were soldered to each terminal. Additionally, a 20μm thick polypropylene membrane separator was cut into 504mm × 800mm pieces, and the positive and negative electrodes were wound around it using a clamping method, then placed inside packaging material. Next, 5g of electrolyte (dissolved in a solvent of ethylene carbonate and methyl ethyl carbonate at a volume ratio of 3 / 7, with LiPF6 dissolved at a concentration of 1 mol / L) was added to the packaging material and sealed to fabricate a wound-type tandem battery.

[0275] <Evaluation of Battery Characteristics>

[0276] The high-temperature storage capacity retention rate, resistance increase rate, and gas quantity change rate of the wound-type tandem battery prepared in Example 27 were determined by the following methods. The results are shown in Table 5.

[0277] [Initial Characteristic Evaluation]

[0278] For wound-type tandem batteries, under clamping and pressurization, the batteries are charged at 25°C with a constant current equivalent to 0.2C to 4.0V, then discharged at a constant current of 0.2C to 3.0V. After two cycles of this operation, the batteries are stabilized. In the third cycle, the batteries are charged at a constant current of 0.2C to 4.0V, then charged at a constant voltage of 4.0V until the current reaches 0.05C, and then discharged at a constant current of 0.2C to 3.0V. In the fourth cycle, the batteries are charged at a constant current of 0.2C to 4.0V, then charged at a constant voltage of 4.0V until the current reaches 0.05C, and then discharged at a constant current of 0.2C to 3.0V. The initial discharge capacity is then determined. Finally, the batteries are charged at a constant current of 0.2C to 4.0V, then charged at a constant voltage of 4.0V until the current reaches 0.05C, and the initial resistance is measured.

[0279] Here, 1C represents the current value at which the battery's base capacity is discharged for 1 hour, 5C represents 5 times that current value, 0.1C represents 1 / 10 of that current value, and 0.2C represents 1 / 5 of that current value.

[0280] [High-Temperature Storage Test]

[0281] After the initial characteristic evaluation, the wound-type stacked batteries were stored at 60°C for 720 hours. After the batteries were fully cooled, their volume was measured using the Archimedes method, and the rate of change in gas volume was calculated based on the volume change before and after high-temperature storage using the following formula.

[0282] Next, the capacitor was discharged at 0.5C to 3V at 25°C, and the residual capacity after high-temperature storage was determined. The high-temperature storage capacity retention rate (%) was calculated based on the following formula. Then, it was charged at a constant current of 0.2C to 4.0V, followed by a constant voltage of 4.0V until the current reached 0.05C, and then discharged at 0.5C to 3V. Afterward, it was charged at a constant current of 0.2C to 4.0V, followed by a constant voltage of 4.0V until the current reached 0.05C, and the resistance after high-temperature storage was measured. The resistance increase rate (%) was calculated based on the following formula.

[0283] High-temperature storage capacity retention rate (%) = (residual capacity) / (initial discharge capacity) × 100

[0284] Resistance increase rate (%) = (Resistance after high-temperature storage (Ω)) / (Initial resistance (Ω)) × 100

[0285] Gas volume change rate (%) = (Volume after high-temperature storage (ml)) / (Volume before high-temperature storage (ml)) × 100

[0286] <Examples 28-39 and Comparative Examples 11-14>

[0287] The binder was dissolved appropriately at a binder solution concentration of 5% to 8% by mass. The type of binder, the type of conductive additive, and the composition ratio were varied as described in the tables. Otherwise, the positive electrode mixture was prepared in the same manner as in Example 27. Furthermore, a positive electrode was fabricated using the obtained positive electrode mixture, and its evaluation was performed in the same manner as in Example 27. The results are shown in Tables 5 and 6.

[0288] <Comparative Example 15>

[0289] Aqueous dispersions of the fluorinated copolymer (c) were sprayed onto NCA and dried to prepare a composite NCA. Using this composite NCA as the positive electrode active material, a positive electrode compound was prepared in the same manner as in Example 27. Furthermore, a positive electrode was fabricated using the obtained positive electrode compound, and its evaluation was performed in the same manner as in Example 27. The evaluation results are shown in Table 6.

[0290] [Table 1]

[0291]

[0292] [Table 2]

[0293]

[0294] [Table 3]

[0295]

[0296] [Table 4]

[0297]

[0298] [Table 5]

[0299]

[0300] [Table 6]

[0301] < / pvdf>

Claims

1. An electrode mixture, which is an electrode mixture containing a lithium-containing transition metal oxide, a conductive aid, a binder, and an organic solvent, wherein, the conductive aid contains a nano-carbon material and carbon black, the nano-carbon material is at least one selected from the group consisting of multi-layer carbon nanotubes and graphene, the binder contains a fluorine-containing copolymer containing a vinylidene fluoride unit and a fluorinated monomer unit, wherein the fluorinated monomer unit does not include a vinylidene fluoride unit, and the content of the vinylidene fluoride unit in the fluorine-containing copolymer is 57.0 mol% or more and 97.0 mol% or less with respect to the total monomer units.

2. The electrode mixture as claimed in claim 1, wherein, the fluorinated monomer unit is a tetrafluoroethylene unit.

3. The electrode mixture as claimed in claim 1 or 2, wherein, the content of the binder with respect to the total mass of the lithium-containing transition metal oxide, the conductive aid, and the binder is 0.3 mass% to 3.0 mass%.

4. The electrode mixture as claimed in claim 1 or 2, wherein, the content of the conductive aid with respect to the total mass of the lithium-containing transition metal oxide, the conductive aid, and the binder is 0.3 mass% to 3.0 mass%.

5. The electrode mixture as claimed in claim 1 or 2, wherein, the binder further contains polyvinylidene fluoride.

6. An electrode provided with a current collector and an electrode mixture layer formed of the electrode mixture according to any one of claims 1 to 5 on one side or both sides of the current collector.

7. A secondary battery provided with the electrode according to claim 6.

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