Positive electrode for lithium ion secondary batteries, lithium ion secondary battery, and positive electrode active material slurry

Abstract

The present invention provides a positive electrode for lithium ion secondary batteries, the positive electrode being able to have a thick and uniform thickness. A positive electrode for lithium ion secondary batteries according to the present invention comprises a current collector and a positive electrode active material layer that is provided on the current collector. The positive electrode active material layer is characterized by containing positive electrode active material particles, a latex-derived polymer binder, carboxymethyl cellulose sodium or carboxymethyl cellulose calcium, and a mannose-based water-soluble polymer.

Description

Positive electrode for lithium ion secondary battery, lithium ion secondary battery and positive electrode active material slurry

[0001] The present invention relates to a positive electrode for a lithium ion secondary battery, a lithium ion secondary battery, and a positive electrode active material slurry.

[0002] The positive electrode active material layer of a lithium-ion battery is typically produced by applying a slurry containing positive electrode active material particles, a PVdF-based binder, N-methylpyrrolidone, and a conductive additive to a current collector and drying the resulting coating. However, N-methylpyrrolidone is included in the list of restricted substances under the REACH Regulations due to concerns about its toxicity. A known method for producing a positive electrode active material layer without using N-methylpyrrolidone involves applying a slurry containing positive electrode active material particles, a water-based binder, a conductive additive, and water to a current collector and drying the resulting coating (see, for example, Patent Document 1).

[0003] JP 2017-091789 A

[0004] However, conventional cathode active material slurries using water as a dispersion medium have low viscosity, so when attempting to form a thick coating layer, the cathode active material slurry flows in the coating layer, making it impossible to form a thick and uniformly thick coating layer and cathode active material layer. Furthermore, cracks may occur in the cathode active material layer. The present invention has been made in consideration of these circumstances and provides a cathode for a lithium-ion secondary battery that can form a thick and uniformly thick cathode active material layer.

[0005] The present invention provides a positive electrode for a lithium-ion secondary battery, comprising a current collector and a positive electrode active material layer provided on the current collector, the positive electrode active material layer comprising positive electrode active material particles, a latex-derived polymer binder, sodium carboxymethylcellulose or calcium carboxymethylcellulose, and a mannose-based water-soluble polymer. The present invention also provides a lithium-ion secondary battery comprising the positive electrode of the present invention, a negative electrode, a separator, and a non-aqueous electrolyte. The present invention also provides a positive electrode active material slurry comprising positive electrode active material particles, a latex-derived polymer binder, sodium carboxymethylcellulose or calcium carboxymethylcellulose, a mannose-based water-soluble polymer, and water.

[0006] The mannose-based water-soluble polymer exhibits a high thickening effect even in small amounts, and therefore the positive electrode active material slurry of the present invention has high coating stability. The positive electrode active material layer of the positive electrode for a lithium ion secondary battery of the present invention can be formed using the positive electrode active material slurry of the present invention, and therefore can have a thick and uniform thickness. Furthermore, the occurrence of cracks in the positive electrode active material layer can be suppressed. As a result, the lithium ion secondary battery of the present invention can have a large battery capacity and long life characteristics. Furthermore, because the positive electrode active material layer contains only a small amount of the mannose-based water-soluble polymer, internal resistance can be reduced, and the lithium ion secondary battery of the present invention has excellent output characteristics.

[0007] 1A is a schematic plan view of a positive electrode for a lithium ion secondary battery according to one embodiment of the present invention, and FIG. 1B is a schematic cross-sectional view of the positive electrode taken along dashed line A-A in FIG. 1A. FIG. 1B is a schematic cross-sectional view of a lithium ion secondary battery according to one embodiment of the present invention. FIG. 1A is a schematic plan view of a negative electrode included in a lithium ion secondary battery according to one embodiment of the present invention, and FIG. 1B is a schematic cross-sectional view of the negative electrode taken along dashed line B-B in FIG. 1A. FIG. 1C is a schematic structural diagram of an electrode laminate included in a lithium ion secondary battery according to one embodiment of the present invention. FIG. 1D shows discharge curves of the lithium ion secondary batteries of Examples 1 and 2.

[0008] An embodiment of the present invention will be described below with reference to the drawings. The configurations shown in the drawings and the following description are merely illustrative, and the scope of the present invention is not limited to those shown in the drawings and the following description. A positive electrode 5 for a lithium-ion secondary battery according to this embodiment includes a current collector 3 and a positive electrode active material layer 2 provided on the current collector 3. The positive electrode active material layer 2 contains positive electrode active material particles, a latex-derived polymer binder, sodium carboxymethylcellulose or calcium carboxymethylcellulose, and a mannose-based water-soluble polymer. A lithium-ion secondary battery 30 according to this embodiment includes a positive electrode 5, a negative electrode 32, a separator 34, and a nonaqueous electrolyte 15. A positive electrode active material slurry according to this embodiment contains positive electrode active material particles, a latex-derived polymer binder, sodium carboxymethylcellulose or calcium carboxymethylcellulose, a mannose-based water-soluble polymer, and water.

[0009] The positive electrode 5 for a lithium ion secondary battery is a positive electrode 5 included in a lithium ion secondary battery 30 or a positive electrode 5 used in the production of a lithium ion secondary battery 30. The positive electrode 5 for a lithium ion secondary battery includes a positive electrode current collector 3 and a porous positive electrode active material layer 2 provided on the positive electrode current collector 3. The positive electrode current collector 3 is a sheet that serves as a substrate for providing the positive electrode active material layer 2, and is a conductor that electrically connects a positive electrode connection member 13 and the positive electrode active material layer 2. The positive electrode current collector 3 is, for example, aluminum foil. The positive electrode connection member 13 is electrically connected to an external connection terminal 18a. The positive electrode active material layer 2 may be provided on one side of the positive electrode current collector 3, or may be provided on both sides of the positive electrode current collector 3.

[0010] The positive electrode active material layer 2 is a layer containing a positive electrode active material. The thickness of the positive electrode active material layer 2 (after pressing) may be 150 μm or more and 400 μm or less. This increases the amount of positive electrode active material contained in the positive electrode 5, and the battery capacity of the lithium-ion secondary battery 30 can be increased.

[0011] The positive electrode active material layer 2 contains positive electrode active material particles, a latex-derived polymer binder, CMC (sodium carboxymethylcellulose or calcium carboxymethylcellulose), and a mannose-based water-soluble polymer. The positive electrode active material layer 2 may also contain a conductive additive. Furthermore, the positive electrode active material layer 2 may also contain a surfactant.

[0012] The positive electrode active material contained in the positive electrode active material layer 2 is a material that is directly involved in the transfer of electrons accompanying charge transfer in the positive electrode 5. The positive electrode active material is, for example, LiCoO2, LiNiO2, LiNi x Co 1-x O2 (x=0.01-0.99), LiMnO2, LiMn2O4, LiCo x Mn y Ni z O2 (x + y + z = 1) or olivine-type LiFePO4 or Li x Fe 1-y M y PO4 (where 0.05≦x≦1.2, 0≦y≦0.8, and M is at least one of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B, and Nb). The positive electrode active material layer 2 can contain these positive electrode active materials singly or in combination. Furthermore, the positive electrode active material particles (e.g., lithium iron phosphate (LiFePO4) particles) may have a conductive coating on their surfaces. This can improve the conductivity of the positive electrode active material surface where the intercalation reaction proceeds, thereby reducing the internal resistance of the positive electrode 5. The conductive coating is, for example, a carbon coating. The positive electrode active material particles may be contained in the positive electrode active material layer 2 as secondary particles formed by aggregation of primary particles. The average particle size of the primary particles of the positive electrode active material can be 30 nm to 400 nm. The average particle size can be calculated by measuring the particle sizes of 100 particles randomly selected from a cross-sectional photograph of the positive electrode active material layer 2 and calculating the arithmetic mean of these particle sizes.

[0013] The positive electrode active material layer 2 can include first lithium iron phosphate secondary particles and second lithium iron phosphate secondary particles having a particle size smaller than that of the first lithium iron phosphate secondary particles. The first lithium iron phosphate secondary particles are lithium iron phosphate secondary particles having a particle size of 10 μm or more among the plurality of lithium iron phosphate secondary particles contained in the positive electrode active material layer 2, and have an average particle size of 15 μm or more and 22 μm or less. The second lithium iron phosphate secondary particles are lithium iron phosphate secondary particles having a particle size of less than 10 μm among the plurality of lithium iron phosphate secondary particles contained in the positive electrode active material layer 2, and have an average particle size of 2 μm or more and 7 μm or less.

[0014] For example, the average particle diameter of lithium iron phosphate dibasic can be obtained by measuring the particle diameters of 100 secondary particles of lithium iron phosphate randomly selected from a cross-sectional photograph of the positive electrode active material layer 2, and arithmetically averaging the particle diameters of secondary particles having a particle diameter of 10 μm or more among the secondary particles whose particle diameters have been measured, and the average particle diameter of lithium iron phosphate dibasic can be obtained by arithmetically averaging the particle diameters of secondary particles having a particle diameter of less than 10 μm among the secondary particles whose particle diameters have been measured.

[0015] The ratio (b / a) of the mass (b) of the second lithium iron phosphate secondary particles to the mass (a) of the first lithium iron phosphate secondary particles is, for example, 1 / 9 or more and 3 / 7 or less (b:a=1:9 to 3:7). The mass of the lithium iron phosphate secondary particles can be calculated from the density and particle diameter of the lithium iron phosphate secondary particles. The density of the lithium iron phosphate secondary particles can be measured and calculated using lithium iron phosphate secondary particles (powder) equivalent to the lithium iron phosphate secondary particles contained in the positive electrode active material layer 2.

[0016] Examples of the conductive additive include furnace black, acetylene black, carbon black, fine particles of coke-based soft carbon, etc. When the positive electrode active material layer 2 contains the conductive additive, the conductivity of the positive electrode active material layer 2 can be improved, and the internal resistance of the positive electrode 5 can be reduced.

[0017] The latex-derived polymer binder is a polymer binder (aqueous polymer binder) that can be colloidally dispersed in an aqueous dispersion medium. Examples of the polymer binder include an acrylic polymer binder and styrene-butadiene rubber (SBR). Use of such a polymer binder makes it possible to use water or an aqueous solution as the dispersion medium for the positive electrode active material slurry used to form the positive electrode active material layer 2. The glass transition temperature Tg of the polymer compound that is the polymer binder is, for example, 20°C or lower, preferably 10°C or lower, and more preferably 0°C or lower. This allows the polymer binder to have flexibility. The inclusion of the polymer binder in the positive electrode active material layer 2 can prevent the positive electrode active material layer 2 from peeling off from the positive electrode current collector 3 and the positive electrode active material layer 2 from cracking.

[0018] The specific gravity of the acrylic rubber contained in the acrylic polymer binder is 1.1, the hardness range of the acrylic rubber measured according to the JIS standard is 40 to 90, and the tensile strength of the acrylic rubber is 70 to 120 kg / cm 2 The elongation of the acrylic rubber is 100 to 600%, and the volume resistance of the acrylic rubber is 10 8 ~10 10 The specific gravity of styrene butadiene rubber (SBR) is 0.94, the hardness range of SBR measured according to the JIS standard is 30 to 100, and the tensile strength of SBR is 50 to 200 kg / cm 2 The elongation of SBR is 100 to 800%, and the volume resistivity of SBR is 10 10 ~10 15 Ω / cm.

[0019] CMC (sodium carboxymethylcellulose or calcium carboxymethylcellulose) has the function of increasing the viscosity of the positive electrode active material slurry used to form the positive electrode active material layer 2. CMC may also function as a binder for the positive electrode active material layer 2. When the positive electrode active material slurry contains CMC, the viscosity of the positive electrode active material slurry can be increased, and the positive electrode active material layer 2 can have a uniform thickness. Furthermore, it becomes possible to apply the positive electrode active material slurry thickly, and it becomes possible to form a thick positive electrode active material layer 2 (for example, a thickness of 150 μm or more (after pressing)).

[0020] The mannose-based water-soluble polymer has the function of increasing the viscosity of the positive electrode active material slurry used to form the positive electrode active material layer 2. By including a mannose-based water-soluble polymer in the positive electrode active material slurry, the viscosity of the positive electrode active material slurry can be increased, allowing the positive electrode active material layer 2 to have a uniform thickness. Furthermore, it becomes possible to apply the positive electrode active material slurry thickly, making it possible to form a thick positive electrode active material layer 2 (e.g., a thickness of 150 μm or more (after pressing)). The mannose-based water-soluble polymer is a polysaccharide containing mannose as a constituent component, such as α-mannan, β-mannan, glucomannan (e.g., konjac mannan), or galactomannan (e.g., guar gum, fenugreek gum, tara gum, locust bean gum, cassia gum, etc.).

[0021] The ratio (b / a) of the mass (b) of the mannose-based water-soluble polymer contained in the positive electrode active material layer 2 to the mass (a) of the positive electrode active material particles contained in the positive electrode active material layer 2 is, for example, (0.05 / 98.3) or more and (0.25 / 98) or less. This allows the positive electrode active material slurry to have high coating stability, and the positive electrode active material layer 2 to have a thick and uniform thickness. As a result, the lithium ion secondary battery 30 can have a large battery capacity and long life characteristics. Furthermore, because the positive electrode active material layer 2 contains only a small amount of the mannose-based water-soluble polymer, internal resistance can be reduced, and the lithium ion secondary battery 30 has excellent output characteristics.

[0022] The ratio ((b+c) / a) of the total mass of the mass (b) of the mannose-based water-soluble polymer contained in the positive electrode active material layer 2 and the mass (c) of the CMC contained in the positive electrode active material layer 2 to the mass (a) of the positive electrode active material particles contained in the positive electrode active material layer 2 is, for example, (0.45 / 98.3) or more and (0.85 / 98) or less. This allows the positive electrode active material slurry to have high coating stability, and the positive electrode active material layer 2 to have a thick and uniform thickness. As a result, the lithium ion secondary battery 30 can have a large battery capacity and long life characteristics. Furthermore, because the positive electrode active material layer 2 contains only small amounts of the mannose-based water-soluble polymer and CMC, the internal resistance can be reduced, and the lithium ion secondary battery 30 has excellent output characteristics.

[0023] For example, a cathode active material layer 2 can be formed by mixing and kneading lithium iron phosphate secondary particles, a polymer binder latex (an emulsion in which a polymeric binder compound is dispersed in a colloidal state in water), CMC, a mannose-based water-soluble polymer, and water or an aqueous solution to prepare a cathode active material slurry. This slurry is then applied to a cathode current collector 3, and the applied layer is dried. A small amount of a surfactant may also be added to the cathode active material slurry. The cathode active material layer 2 may also be subjected to a pressing process. In the pressing process, pressure may be applied to the cathode active material layer 2 so that the porosity of the cathode active material layer 2 is within a range of 20% to 40%. The cathode active material slurry may have a viscosity of, for example, 4000 mPa·s or more and 15000 mPa·s or less. This allows the formation of a thick and uniform cathode active material layer 2. The viscosity of the positive electrode active material slurry may be a viscosity measured using a Brookfield viscometer at a rotation speed of 10 rpm.

[0024] For example, the positive electrode active material layer 2 can contain 95 wt % to 98.5 wt % lithium iron phosphate particles, 1.0 wt % to 1.5 wt % of a latex-derived polymer binder, 0.3 wt % to 0.7 wt % of CMC, and 0.01 wt % to 0.3 wt % of a mannose-based water-soluble polymer. The positive electrode active material slurry can be prepared by mixing the materials so that the positive electrode active material layer 2 has the above solid content composition. The solid content of the positive electrode active material slurry can be, for example, 55 wt % to 67 wt %.

[0025] The negative electrode 32 is an electrode having a porous negative electrode active material layer 36. The negative electrode active material layer 36 is, for example, a porous layer containing a negative electrode active material provided on a sheet-like negative electrode current collector 38. The negative electrode current collector 38 is electrically connected to the negative electrode connection member 14. The negative electrode connection member 14 is also electrically connected to the external connection terminal 18b. The negative electrode current collector 38 is, for example, copper foil. The negative electrode active material is a substance that directly participates in the transfer of electrons accompanying charge transfer in the negative electrode. Examples of negative electrode active materials include graphite, partially graphitized carbon, hard carbon, soft carbon, lithium titanate (LTO), and Sn alloys. The negative electrode active material layer 36 can contain these negative electrode active materials alone or in combination.

[0026] The separator 34 is sheet-shaped and is disposed between the positive electrode 5 and the negative electrode 32. The separator 34, together with the positive electrode 5 and the negative electrode 32, can constitute an electrode stack 22 as shown in FIG. 4 . The provision of the separator 34 can prevent a short-circuit current from flowing between the positive electrode 5 and the negative electrode 32. The separator 34 is not particularly limited as long as it can prevent a short-circuit current from flowing and is permeable to ions that conduct between the positive and negative electrodes. For example, the separator 34 can be a microporous polyolefin film, a cellulose sheet, or an aramid sheet. The separator 34 may also be a nonwoven fabric containing at least one of cellulose fibers, polyester fibers, polypropylene fibers, polyacrylonitrile fibers, and polyethylene terephthalate fibers.

[0027] 4, the electrode stack 22 may have a structure in which a plurality of positive electrodes 5 and a plurality of negative electrodes 32 are stacked so that the positive electrodes 5 and the negative electrodes 32 are alternately arranged. The electrode stack 22 may also have a structure in which a separator 34 is arranged between adjacent positive electrodes 5 and negative electrodes 32.

[0028] The nonaqueous electrolyte 15 may be formed using carbonates, lactones, ethers, esters, ionic liquids, or the like as a solvent, or a mixture of two or more of these solvents. Among these, a mixture of a cyclic carbonate and a chain carbonate is particularly preferred. The nonaqueous electrolyte 15 is a solution in which a lithium salt solute, such as LiCF3SO3, LiAsF6, LiClO4, LiBF4, LiPF6, LiBOB, LiN(CF3SO2)2, or LiN(CF2F5SO2), is dissolved in an organic solvent. If necessary, additives such as VC (vinylene carbonate), PS (propane sultone), VEC (vinyl ethyl carbonate), PRS (propene sultone), and flame retardants may be blended alone or in combination.

[0029] The battery case 11 is a battery exterior housing that houses the electrode stack 22 (including the positive electrode 5, the negative electrode 32, and the separator 34) and the nonaqueous electrolyte 15. The battery case 11 may be formed into a bag shape by welding a laminate film at a welding portion. In this case, the lithium-ion secondary battery 30 is a pouch battery. The battery case 11 may be a metal case or a hard resin case. The battery case 11 may also have a lid member 12.

[0030] Preparation of Positive Electrode Active Material Slurry Lithium iron phosphate powder, an acrylic polymer binder liquid (latex dispersion aqueous solution AXA391, active ingredient concentration: 40 wt %, glass transition temperature Tg: −35° C., pH: 7 to 9, B-type viscosity: 5 to 50 cP, average particle size: 180 μm, particle size distribution: peak at 170 μm (100 to 500 μm)), sodium carboxymethylcellulose (CMC, BSH6 manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd. (degree of etherification: 0.65 to 0.75, viscosity of 1% CMC aqueous solution at 25° C.: 3000 to 4000 mPa·s)), konjac mannan, guar gum, xanthan gum, and glucose (starch), as shown in the solid composition of Examples 1 to 3 and Comparative Examples 1 to 4 in Table 1, were mixed so as to obtain the solid composition shown in Table 1. Components indicated by a horizontal line in Table 1 were not mixed. Then, water was added to the mixture so as to obtain the solid content ratio shown in Table 1, and the mixture was kneaded to prepare positive electrode active material slurries for Examples 1 to 3 and Comparative Examples 1 to 4.

[0031]

[0032] Viscosity Measurement The viscosity of each of the positive electrode active material slurries of Examples 1 to 3 and Comparative Examples 1 and 2 was measured using a Brookfield viscometer (TV-22 manufactured by Toki Sangyo Co., Ltd.) (spindle rotor: H, No. 4, rotation speed: 10 rpm). The measurement results are shown in Table 1. The positive electrode active material slurries of Examples 1 to 3 were found to have a viscosity of 4000 mPa s or more and 15000 mPa s or less, which is a viscosity suitable for coating.

[0033] Evaluation of Spinnability The spinnability of each of the positive electrode active material slurries of Examples 1 to 3 and Comparative Examples 1 and 2 was evaluated. Specifically, a 100 ml beaker containing the positive electrode active material slurry was tilted, and the positive electrode active material slurry was poured from the spout (flow rate: approximately 0.5 ml / sec), and it was observed whether stringiness occurred. Furthermore, a spoon was placed into the 100 ml beaker containing the positive electrode active material slurry, and when the spoon was lifted, it was observed whether stringiness occurred in the positive electrode active material slurry attached to the spoon. The evaluation results are shown in Table 1. Stringiness was observed with the positive electrode active material slurries of Examples 1 to 3, but not with the positive electrode active material slurries of Comparative Examples 1 and 2.

[0034] Fabrication of Lithium-Ion Secondary Batteries Lithium-ion secondary batteries were fabricated using the positive electrode active material slurries of Examples 1 and 2. Specifically, the positive electrode active material slurries were applied to an aluminum foil (positive electrode current collector sheet), the coating film was dried, and then pressed to form a 221 μm-thick positive electrode active material layer (length: 142 mm, width: 84.5 mm) on one side of the positive electrode current collector sheet, thereby fabricating a positive electrode. No cracks were observed in the positive electrode active material layer.

[0035] 94 wt% (solids content) graphite powder, 5 wt% (solids content) SBR binder, and 1 wt% (solids content) sodium carboxymethyl cellulose were mixed. Water was added to this mixture and kneaded to prepare a negative electrode active material slurry. The negative electrode active material slurry was applied to a copper foil (negative electrode current collector sheet), the coating film was dried, and then pressed to form an 82.5 μm-thick negative electrode active material layer (length: 145 mm, width: 86.8 mm) on one side of the negative electrode current collector sheet, thereby producing a negative electrode.

[0036] Next, the prepared positive electrode, a separator (a three-layer separator manufactured by Toray Industries, Inc.), the prepared negative electrode, and a non-aqueous electrolyte (1.2 M LiPF electrolyte (carbonate-based electrolyte)) were placed in a battery case to prepare the lithium ion secondary batteries of Examples 1 and 2.

[0037] 5 shows the discharge curves of the lithium ion secondary batteries of Examples 1 and 2. It was found that the lithium ion secondary batteries of Examples 1 and 2 had excellent discharge characteristics.

[0038] 2: Positive electrode active material layer 3: Positive electrode current collector 5: Positive electrode 11: Battery case 12: Lid member 13: Positive electrode connecting member 14: Negative electrode connecting member 15: Non-aqueous electrolyte 18a, 18b: External connection terminals 22: Electrode laminate 25: Shrink film 30: Lithium ion secondary battery 32: Negative electrode 34: Separator 36: Negative electrode active material layer 38: Negative electrode current collector

Claims

1. A positive electrode for a lithium ion secondary battery comprising: a current collector; and a positive electrode active material layer provided on the current collector, the positive electrode active material layer comprising positive electrode active material particles, a polymer binder derived from latex, sodium carboxymethylcellulose or calcium carboxymethylcellulose, and a mannose-based water-soluble polymer.

2. The positive electrode according to claim 1, wherein the mannose-based water-soluble polymer is guar gum or glucomannan.

3. The positive electrode according to claim 1, wherein a ratio (b / a) of a mass (b) of the mannose-based water-soluble polymer contained in the positive electrode active material layer to a mass (a) of the positive electrode active material particles contained in the positive electrode active material layer is (0.05 / 98.3) or more and (0.25 / 98) or less.

4. The positive electrode according to claim 1, wherein a ratio ((b+c) / a) of a total mass of a mass (b) of the mannose-based water-soluble polymer contained in the positive electrode active material layer and a mass (c) of the sodium carboxymethylcellulose or calcium carboxymethylcellulose contained in the positive electrode active material layer to a mass (a) of the positive electrode active material particles contained in the positive electrode active material layer is (0.45 / 98.3) or more and (0.85 / 98) or less.

5. The positive electrode according to claim 1, wherein the positive electrode active material layer has a thickness of 150 μm or more and 400 μm or less.

6. A lithium ion secondary battery comprising the positive electrode according to any one of claims 1 to 5, a negative electrode, a separator, and a non-aqueous electrolyte.

7. A positive electrode active material slurry comprising positive electrode active material particles, a polymer binder derived from latex, sodium carboxymethylcellulose or calcium carboxymethylcellulose, a mannose-based water-soluble polymer, and water.

8. The positive electrode active material slurry according to claim 7, having a viscosity of 4,000 mPa·s or more and 15,000 mPa·s or less.

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