Non-aqueous electrolyte secondary battery and method for manufacturing non-aqueous electrolyte secondary battery

Abstract

Provided is a nonaqueous electrolyte secondary battery having excellent cycle characteristics. A nonaqueous electrolyte secondary battery according to an embodiment includes: an electrode body including a positive electrode, a negative electrode, and a separator; and a nonaqueous electrolyte. The negative electrode includes: a negative electrode active material; at least one of carboxymethyl cellulose and a salt thereof; and at least one of sorbic acid and a salt thereof, the content of the at least one of sorbic acid and a salt thereof being 1500 ppm or less relative to the mass of the negative electrode.

Description

TECHNICAL FIELD

[0001] The present application relates to a nonaqueous electrolyte secondary battery and a method for manufacturing a nonaqueous electrolyte secondary battery. BACKGROUND

[0002] The negative electrode of a nonaqueous electrolyte secondary battery is generally manufactured by coating a negative electrode mixture slurry containing a negative electrode active material and a binder in a dispersion medium on the surface of a negative electrode core, and drying and compressing the coated film (for example, refer to Patent Literature 1). In the past, in the negative electrode mixture slurry containing water as the dispersion medium, at least one of carboxymethyl cellulose and a salt thereof (hereinafter, collectively referred to as "CMC-based compound") has been used as a thickening agent.

[0003] In addition, in Patent Literatures 2 and 3, in order to suppress the quality reduction of the binder in the negative electrode mixture slurry due to microorganisms and the like, an antiseptic agent is added to the slurry. In Patent Literatures 2 and 3, as the antiseptic agent, an isothiazoline-based compound is disclosed.

[0004] PRIOR ART DOCUMENTS

[0005] PATENT LITERATURE

[0006] Patent Literature 1: Japanese Patent Application Laid-Open No. 4-342966

[0007] Patent Literature 2: International Publication No. 2012 / 026462

[0008] Patent Literature 3: Japanese Patent Application Laid-Open No. 2013-211246 SUMMARY

[0009] PROBLEMS TO BE SOLVED BY THE INVENTION

[0010] However, in a nonaqueous electrolyte secondary battery, it is an important issue to suppress the capacity reduction accompanying charge and discharge and to improve the cycle characteristics. As a result of the discussion by the present inventors, it was found that the additive component of the negative electrode mixture layer greatly affects the cycle characteristics.

[0011] MEANS FOR SOLVING THE PROBLEMS

[0012] The nonaqueous electrolyte secondary battery of one embodiment of the present application is characterized by including an electrode body including a positive electrode, a negative electrode, and a separator, and a nonaqueous electrolyte, and the negative electrode includes a negative electrode core and a negative electrode mixture layer formed on at least one surface of the negative electrode core, and the negative electrode mixture layer contains a negative electrode active material, at least one of carboxymethyl cellulose and a salt thereof, and at least one of sorbic acid and a salt thereof, and the content of at least one of sorbic acid and a salt thereof is 1500 ppm or less relative to the mass of the negative electrode mixture layer.

[0013] The production method of the nonaqueous electrolyte secondary battery according to one embodiment of the present application is a production method of a nonaqueous electrolyte secondary battery provided with an electrode body including a positive electrode, a negative electrode, and a separator, and a nonaqueous electrolyte, characterized in that the production process of the negative electrode includes: a process of preparing a negative electrode mixture slurry including a negative electrode active material, at least one of carboxymethyl cellulose and a salt thereof, at least one of sorbic acid and a salt thereof, and water; and a process of coating the negative electrode mixture slurry on at least one face of a negative electrode core body, drying and compressing the coated film, and forming a negative electrode mixture layer.

[0014] Effects of the Invention

[0015] According to one embodiment of the present application, a nonaqueous electrolyte secondary battery having excellent cycle characteristics can be provided. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 is a perspective view of a nonaqueous electrolyte secondary battery according to one embodiment.

[0017] Figure 2 is a cross-sectional view of an electrode body according to one embodiment. DETAILED DESCRIPTION

[0018] As a result of the research by the present inventors, it was found that the cycle characteristics of a nonaqueous electrolyte secondary battery are remarkably improved by adding at least one of sorbic acid and a salt thereof in a prescribed amount to a negative electrode mixture layer containing a CMC-based compound. At least one of sorbic acid and a salt thereof is added to a negative electrode mixture slurry containing a CMC-based compound and is contained in the negative electrode mixture layer in an amount of 1500 ppm or less. Note that when the amount of addition exceeds 1500 ppm, the cycle characteristics are instead reduced. The sorbic acid or salt thereof added at a concentration of 1500 ppm or less improves the adhesion of the negative electrode mixture layer to the negative electrode core body, and it is inferred that this is the main reason for the improvement in cycle characteristics. It is thought that the carboxyl group of sorbic acid interacts with CMC, and strengthens the bonding between polymers.

[0019] In addition, the negative electrode mixture slurry containing a CMC-based compound has a problem with stability of coating because the viscosity decreases over time. It is thought that the reason for the decrease in viscosity of the slurry is that the molecular chain of the CMC-based compound is cleaved by an enzyme produced by bacteria contained in water. It is thought that sorbic acid and a salt thereof intrude into the interior of the bacteria and have an effect of inhibiting the proliferation of the bacteria. Therefore, by adding sorbic acid or a salt thereof to the negative electrode mixture slurry, the decomposition of the CMC-based compound is inhibited, and thus the decrease in viscosity of the slurry is inhibited, and the coatability is improved.

[0020] For the purpose of suppressing the decrease in viscosity of the negative electrode mixture slurry and ensuring good coatability, for example, improving the adhesion of the negative electrode mixture layer to the negative electrode core body, contributes to the improvement in the cycle characteristics of the battery. In particular, in the case where sorbic acid or a salt thereof is added to the slurry in such a manner that the concentration in the negative electrode mixture layer becomes 100 ppm or more and 1000 ppm, the effect of improving the cycle characteristics is remarkable.

[0021] Hereinafter, an example of an embodiment of the present application will be described in detail with reference to the drawings, and the present application is not limited to the following described embodiment. Hereinafter, as a nonaqueous electrolyte secondary battery, a nonaqueous electrolyte secondary battery 10 that is a laminate battery provided with an exterior body 11 composed of laminate sheets 11a, 11b will be exemplified. However, the nonaqueous electrolyte secondary battery of the present application can be a cylindrical battery provided with a cylindrical battery case, a prismatic battery provided with a prismatic battery case, or the like, and the form of the battery is not particularly limited.

[0022] Figure 1 is a perspective view of the nonaqueous electrolyte secondary battery 10 as an example of an embodiment. The nonaqueous electrolyte secondary battery 10 is provided with an electrode body 14 and a nonaqueous electrolyte, which are housed in a housing portion 12 of an exterior body 11. The laminate sheets 11a, 11b are made of a sheet material in which a metal layer and a resin layer are laminated. The laminate sheets 11a, 11b, for example, have two layers of resin layers sandwiching a metal layer, and the other layer of the resin layer is composed of a heat sealable resin. As an example of the metal layer, an aluminum layer can be given.

[0023] The exterior body 11, for example, has a shape that is substantially rectangular in plan view. In the exterior body 11, the laminate sheets 11a, 11b are joined to each other to form a seal portion 13, thereby sealing the housing portion 12 in which the electrode body 14 is housed. The seal portion 13 is formed in a frame shape with substantially the same width along the end edge of the exterior body 11. The portion that is substantially rectangular in plan view and surrounded by the seal portion 13 is the housing portion 12. The housing portion 12 is provided by forming a recess capable of housing the electrode body 14 in at least one of the laminate sheets 11a, 11b. In the present embodiment, the recess is formed in the laminate sheet 11a.

[0024] The nonaqueous electrolyte secondary battery 10 is provided with a pair of electrode leads (a positive electrode lead 15 and a negative electrode lead 16) connected to the electrode body 14. Each of the electrode leads is drawn from the inside to the outside of the exterior body 11. In the present embodiment, the positive electrode lead 15 and the negative electrode lead 16 are drawn from the same end edge of the exterior body 11. Figure 1 In the example shown, each of the electrode leads is drawn substantially parallel to each other from the same end edge of the exterior body 11. The positive electrode lead 15 and the negative electrode lead 16 are each a thin plate of an electrically conductive material, for example, the positive electrode lead 15 is composed of a metal in which aluminum is a main component, and the negative electrode lead 16 is composed of a metal in which copper or nickel is a main component.

[0025] Figure 2 is a cross-sectional view of the electrode body 14. As shown in Figure 2As shown, the electrode body 14 has a positive electrode 20, a negative electrode 30, and a separator 40 interposed between the positive electrode 20 and the negative electrode 30. The electrode body 14 has, for example, a wound structure in which the positive electrode 20 and the negative electrode 30 are wound with the separator 40 interposed therebetween, and is a flat wound-type electrode body that is pressurized in the radial direction. In order to suppress precipitation of lithium, the negative electrode 30 is formed to be larger in size than the positive electrode 20 by one turn. Note that the electrode body can be a stacked type in which a plurality of positive electrodes and a plurality of negative electrodes are alternately stacked with separators interposed therebetween.

[0026] The nonaqueous electrolyte contains a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent. The nonaqueous solvent can be, for example, an ester, an ether, a nitrile, an amide, or a mixed solvent of two or more of these. The nonaqueous solvent can contain a halogen-substituted product in which at least a part of the hydrogen atoms of these solvents are substituted with a halogen atom such as fluorine. For example, 0.5 to 5% by mass of fluoroethylene carbonate can be added with respect to the total mass of the nonaqueous electrolyte. In addition, 1 to 5% by mass of vinylene carbonate can be added with respect to the total mass of the nonaqueous electrolyte. Note that the nonaqueous electrolyte is not limited to a liquid electrolyte, and can be a solid electrolyte. The electrolyte salt uses a lithium salt such as LiPF6.

[0027] Hereinafter, the positive electrode 20, the negative electrode 30, the separator 40, and particularly the negative electrode 30 that constitute the electrode body 14 will be described in detail.

[0028] [Positive electrode]

[0029] The positive electrode 20 has a positive electrode core 21 and a positive electrode mixture layer 22 formed on at least one face of the positive electrode core 21. The positive electrode core 21 can use a foil of a metal such as aluminum or an aluminum alloy that is stable in the potential range of the positive electrode 20, a film obtained by disposing the metal on the surface layer, or the like. The positive electrode mixture layer 22 contains a positive electrode active material, a conductive agent, and a binder, and is preferably formed on both faces of the positive electrode core 21. The positive electrode 20 can be manufactured by, for example, applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, a binder, and the like onto the positive electrode core 21, drying the coating film, and then compressing to form the positive electrode mixture layer 22 on both faces of the positive electrode core 21.

[0030] The positive electrode active material uses a lithium transition metal complex oxide. As elements contained in the lithium transition metal complex oxide, Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, W, and the like can be given. An example of a preferable lithium transition metal complex oxide is a complex oxide containing at least one selected from Ni, Co, Mn, and Al. Note that inorganic compound particles such as alumina, compounds containing lanthanoid elements, or the like can be adhered to the surface of the lithium transition metal complex oxide particles.

[0031] As the electrically conductive agent contained in the positive electrode mixture layer 22, carbon materials such as carbon black, acetylene black, ketjen black, and graphite can be exemplified. As the binder contained in the positive electrode mixture layer 22, fluorine resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide, acrylic resins, and polyolefins can be exemplified. These resins can be used in combination with carboxymethyl cellulose (CMC), a salt of CMC, polyethylene oxide (PEO), and the like.

[0032] [Negative electrode]

[0033] The negative electrode 30 has a negative electrode core 31 and a negative electrode mixture layer 32 formed on at least one face of the negative electrode core 31. The negative electrode core 31 can use a foil of copper, a copper alloy, or the like, which is stable in the potential range of the negative electrode 30, a film obtained by disposing such a metal on the surface layer, or the like. The negative electrode mixture layer 32 contains a negative electrode active material, at least one of carboxymethyl cellulose (CMC) and a salt thereof (CMC-based compound), and at least one of sorbic acid and a salt thereof, and is preferably formed on both faces of the negative electrode core 31. The negative electrode 30 can be manufactured by applying a negative electrode mixture slurry containing a negative electrode active material and the like on the negative electrode core 31, drying the coating film, and then compressing to form the negative electrode mixture layer 32 on both faces of the negative electrode core 31.

[0034] The negative electrode active material uses a carbon-based active material that reversibly occludes and releases lithium ions. Preferred carbon-based active materials are graphite such as flaky graphite, massive graphite, amorphous graphite, massive artificial graphite (MAG), graphitized mesocarbon microbeads (MCMB), and the like. The median particle diameter (50% particle diameter) of the graphite is, for example, 18 to 22 μm. Alternatively, a Si-based active material containing Si can be used, or a carbon-based active material and a Si-based active material can be used in combination. The Si-based active material has, for example, a structure in which Si particles are dispersed in an oxide phase containing Si. As the oxide phase, a silicon oxide (SiO2) phase, a composite oxide phase containing Si and a metal element such as Li can be exemplified.

[0035] The CMC-based compound contained in the negative electrode mixture layer 32 functions as a thickening agent for the negative electrode mixture slurry, and also functions as a binder that binds the particles of the negative electrode active material to each other and to the negative electrode core 31. The negative electrode mixture layer 32 preferably contains a salt of CMC. The salt of CMC is, for example, a sodium salt or an ammonium salt. The salt of CMC is generally a partially neutralized salt in which a part of the carboxyl groups is neutralized. A mixture of CMC and a salt of CMC can be contained in the negative electrode mixture layer 32 (negative electrode mixture slurry), or CMC or a salt of CMC can be contained alone. The weight average molecular weight of the CMC-based compound is, for example, 200,000 to 500,000.

[0036] The negative electrode mixture layer 32 preferably contains a rubber-based binder as a binder in addition to the CMC-based compound. The content of the CMC-based compound and the rubber-based binder is preferably 0.1 to 5% by mass and more preferably 0.5 to 3% by mass, respectively, with respect to the total mass of the negative electrode mixture layer 32. The preferable rubber-based binder is styrene-butadiene rubber (SBR) or a modified product thereof. The modified product of SBR can contain at least one selected from the group consisting of an acrylonitrile unit, an acrylate unit, an acrylic acid unit, a methacrylate unit, and a methacrylic acid unit. SBR and its modified product are generally supplied in a dispersed form with water as a dispersion medium.

[0037] As described above, sorbic acid or a salt thereof (hereinafter, they are collectively referred to as "sorbic acid-based compound") suppresses the decrease in viscosity of the negative electrode mixture slurry, maintains the coatability of the slurry well, improves the adhesion of the negative electrode core 31 to the negative electrode mixture layer 32, and further improves the cycle characteristics of the battery. Sorbic acid is an unsaturated fatty acid represented by the molecular formula C6H8O2. The sorbic acid-based compound, by being dissolved or dispersed in water in the negative electrode mixture slurry, suppresses the proliferation of bacteria and thereby suppresses the decomposition of the CMC-based compound.

[0038] The content of the sorbic acid-based compound is 1500 ppm or less with respect to the mass of the negative electrode mixture layer 32. Even a small amount of addition of the sorbic acid-based compound can achieve the effect of improving the cycle characteristics compared to the case where the sorbic acid-based compound is not added, and a concentration exceeding 1500 ppm can rather degrade the cycle characteristics. Note that the content of the sorbic acid-based compound can be measured by gas chromatography.

[0039] The content of the sorbic acid-based compound is preferably 100 ppm or more and 1000 ppm or less, more preferably 150 ppm or more and 750 ppm or less, and particularly preferably 200 ppm or more and 500 ppm or less. In this case, the decrease in viscosity of the negative electrode mixture slurry can be effectively suppressed, and the stable coatability of the slurry can be easily ensured. Furthermore, the adhesion of the negative electrode core 31 to the negative electrode mixture layer 32 is further improved, and the effect of improving the cycle characteristics is remarkable.

[0040] The negative electrode mixture layer 32 (negative electrode mixture slurry) can contain a mixture of sorbic acid and a sorbic acid salt, or can contain only sorbic acid or a sorbic acid salt. As the preferable sorbic acid-based compound, at least one selected from the group consisting of sorbic acid, potassium sorbate, sodium sorbate, and calcium sorbate can be given. Among them, sorbic acid, potassium sorbate, and sodium sorbate are particularly preferable.

[0041] The negative electrode 30 is manufactured, for example, by the following two processes.

[0042] (1) a step of preparing a negative electrode mixture slurry containing a negative electrode active material, at least one of a carboxymethyl cellulose and a salt thereof (CMC-based compound), at least one of sorbic acid and a salt thereof (sorbic acid-based compound), and water.

[0043] (2) a step of coating the negative electrode mixture slurry on the surface of the negative electrode core 31, drying and compressing the coated film to form the negative electrode mixture layer 32.

[0044] The solid content concentration of the negative electrode mixture slurry is preferably about 45 to 55% by mass from the viewpoint of handleability, coatability, and the like.

[0045] The sorbic acid-based compound is added in an amount of, for example, 0.1 to 20% by mass, 0.5 to 15% by mass, or 0.6 to 13% by mass, relative to the mass of the CMC-based compound. The sorbic acid-based compound contained in the negative electrode mixture slurry is introduced into the negative electrode mixture layer 32 together with the CMC-based compound. Therefore, the mass ratio of the CMC-based compound to the sorbic acid-based compound in the negative electrode mixture layer 32 is substantially the same as in the case of the negative electrode mixture slurry.

[0046] [Separator]

[0047] The separator 40 uses a porous sheet having ion permeability and insulating properties. As specific examples of the porous sheet, microporous membranes, woven fabrics, nonwoven fabrics, and the like can be given. As the material of the separator 40, olefin resins such as polyethylene and polypropylene, cellulose, and the like are preferred. The separator 40 can be any structure of a single-layer structure, a laminated structure, or the like. A heat-resistant layer or the like can be formed on the surface of the separator 40.

[0048] Example

[0049] Hereinafter, the present application will be further described by way of examples, but the present application is not limited to these examples.

[0050] [Example 1]

[0051] [Production of the positive electrode]

[0052] As the positive electrode active material, LiCo 0.979 Zr 0.001 Mg 0.01 Al 0.01O2 represents a lithium-containing metal complex oxide. The positive electrode active material, carbon black, and polyvinylidene fluoride (PVdF) were mixed at a solid component mass ratio of 95:2.5:2.5 to prepare a positive electrode mixture slurry with N-methyl-2-pyrrolidone (NMP) as a dispersion medium. The positive electrode mixture slurry was applied to both surfaces of a long strip-shaped positive electrode core made of an aluminum foil having a thickness of 15 μm by a doctor blade method, and the coating film was dried, and then compressed with a roll to form positive electrode mixture layers on both surfaces of the positive electrode core. The positive electrode core on which the positive electrode mixture layers were formed was cut into a prescribed electrode size to produce a positive electrode.

[0053] [Production of negative electrode]

[0054] As the negative electrode active material, graphite having a median particle diameter of 22 μm on a volume basis and a Si-containing compound (SiO) in which Si particles are dispersed in an oxide phase containing Si were used. The graphite and the SiO were mixed at a mass ratio of 95:5. The SiO can be obtained by mixing metallic silicon and silicon dioxide, performing heat treatment under reduced pressure, raising the temperature to about 1000°C, forming a carbon coating film on the particle surfaces by a CVD method, and then performing crushing and classification.

[0055] The negative electrode active material, a sodium salt of CMC (CMC-Na), and SBR were mixed at a solid component mass ratio of 97:1.5:1.0, and further, sorbic acid was added so as to have a concentration of 50 ppm with respect to the solid components (the negative electrode active material, CMC-Na, and SBR) to prepare a negative electrode mixture slurry with water (ion-exchanged water) as a dispersion medium. The negative electrode mixture slurry was applied to both surfaces of a long strip-shaped negative electrode core made of a copper foil by a doctor blade method, and the coating film was dried, and then compressed with a roll to form negative electrode mixture layers on both surfaces of the negative electrode core. The negative electrode core on which the negative electrode mixture layers were formed was cut into a prescribed electrode size to produce a negative electrode.

[0056] [Production of nonaqueous electrolyte]

[0057] Ethylene carbonate (EC) and methyl ethyl carbonate (MEC) were mixed at a volume ratio (25°C, 1 atm) of 3:7, LiPF6 was added to the obtained mixed solvent so as to have a concentration of 1 mol / L, and further, vinylene carbonate was added so as to have a concentration of 2 mass% to prepare a nonaqueous electrolyte.

[0058] [Production of battery]

[0059] A positive electrode lead and a negative electrode lead are attached to the above-described positive electrode and the above-described negative electrode, and the positive electrode and the negative electrode are wound with a separator formed of a micro-porous film made of polyethylene interposed therebetween. After a tape made of polypropylene is attached to the outermost circumferential surface of the wound body, the wound body is pressurized in the radial direction to produce a flat-shaped wound electrode body. The electrode body and the above-described nonaqueous electrolyte are housed in a cup-shaped housing portion of an exterior body made of a laminate sheet having a five-layer structure of a polypropylene layer / adhesive layer / aluminum alloy layer / adhesive layer / polypropylene layer. Then, the interior of the exterior body is depressurized to allow the electrolytic solution to permeate into the electrode body, and the opening portion of the exterior body is sealed to produce a nonaqueous electrolyte secondary battery having a height of 62 mm, a width of 35 mm, and a thickness of 3.6 mm.

[0060] <Example 2>

[0061] In the preparation of the negative electrode mixture slurry, the addition amount of sorbic acid was changed to 100 ppm with respect to the solid content, and otherwise, a nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1.

[0062] <Example 3>

[0063] In the preparation of the negative electrode mixture slurry, the addition amount of sorbic acid was changed to 200 ppm with respect to the solid content, and otherwise, a nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1.

[0064] <Example 4>

[0065] In the preparation of the negative electrode mixture slurry, the addition amount of sorbic acid was changed to 500 ppm with respect to the solid content, and otherwise, a nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1.

[0066] <Example 5>

[0067] In the preparation of the negative electrode mixture slurry, the addition amount of sorbic acid was changed to 1000 ppm with respect to the solid content, and otherwise, a nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1.

[0068] <Example 6>

[0069] In the preparation of the negative electrode mixture slurry, sorbic acid was changed to potassium sorbate, and otherwise, a nonaqueous electrolyte secondary battery was produced in the same manner as in Example 3.

[0070] <Example 7>

[0071] In the preparation of the negative electrode mixture slurry, sorbic acid was changed to sodium sorbate, and otherwise, a nonaqueous electrolyte secondary battery was produced in the same manner as in Example 3.

[0072] <Example 8>

[0073] In the preparation of the negative electrode mixture slurry, sorbic acid was changed to calcium sorbate, and otherwise, a nonaqueous electrolyte secondary battery was produced in the same manner as in Example 3.

[0074] <Comparative Example 1>

[0075] In the preparation of the negative electrode mixture slurry, sorbic acid was not added, and otherwise, a nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1.

[0076] <Comparative Example 2>

[0077] In the preparation of the negative electrode mixture slurry, the addition amount of sorbic acid was changed to 2000 ppm with respect to the solid content, and otherwise, a nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1.

[0078] <Comparative Example 3>

[0079] In the preparation of the negative electrode mixture slurry, sorbic acid was changed to 1,2-benzisothiazolin-3-one, and otherwise, a nonaqueous electrolyte secondary battery was produced in the same manner as in Example 3.

[0080] [Measurement of viscosity of negative electrode mixture slurry]

[0081] For each of the negative electrode mixture slurries of Examples and Comparative Examples, the viscosity after 48 hours from the preparation was measured at 25°C using a B-type viscometer (Toyo Seiki Industry Co., Ltd., TVC10), and the ratio of the viscosity after 48 hours to the viscosity immediately after the preparation (viscosity retention rate) was calculated by the following equation. The viscosity retention rate of each of the negative electrode mixture slurries is shown in Table 1.

[0082] Viscosity retention rate after 48 hours = (viscosity after 48 hours / viscosity immediately after the preparation) x 100

[0083] [Evaluation of adhesion (measurement of peeling strength)]

[0084] For each of the negative electrodes (density of the negative electrode mixture layer: 1.6 g / mL) of Examples and Comparative Examples, the adhesion of the negative electrode mixture layer to the negative electrode core was evaluated by the method described in Japanese Patent Application Publication No. 2005-251481.

[0085] (1) An acrylic plate (3.0 x 12 cm), double-sided tape (2 x 9 mm; NICHIBAN Co., Ltd., NICETACK NW-20), and each of the negative electrodes were cut to a predetermined size to prepare the electrode plate (2.5 x 16 cm) for measurement.

[0086] (2) The double-sided tape was attached to the acrylic plate by 8.5 cm (0.5 cm extra) from the end along the length direction.

[0087] (3) The electrode plate for measurement was attached to the double-sided tape attached to the acrylic plate, and the portion of the electrode plate for measurement to which the double-sided tape was not attached was stretched by a tensile testing machine at a rate of 100 mm / min until the negative electrode mixture layer was peeled, whereby the peeling strength (adhesion strength) of the negative electrode mixture layer was measured. The results of the measurement of the peeling strength are shown in Table 1.

[0088] [Measurement of capacity retention rate]

[0089] Each battery of the examples and comparative examples was charged at a constant current of 800 mA until the battery voltage became 4.2 V at 25°C, and then charged at a constant voltage of 4.2 V until the current became 40 mA of the terminal current. Then, the battery was discharged at a constant current of 800 mA until the battery voltage reached 2.75 V. This charge and discharge cycle was repeated 150 times, and the ratio of the discharge capacity of the 150th cycle to the discharge capacity of the 1st cycle (capacity retention rate) was calculated. The capacity retention rate of each battery is shown in Table 1.

[0090] [Table 1]

[0091]

[0092] It can be understood from the results shown in Table 1 that the batteries of the examples have higher capacity retention rates and superior cycle characteristics after charge and discharge cycles compared to the batteries of the comparative examples. In addition, the negative electrodes of the examples have higher peeling strength of the negative electrode mixture layer and higher adhesion of the negative electrode core to the negative electrode mixture layer compared to the negative electrodes of the comparative examples. That is, the sorbic acid or salt thereof improves the adhesion of the negative electrode core to the negative electrode mixture layer, and it can be considered that this is a main reason for the improvement of the cycle characteristics. On the other hand, in the case where no sorbic acid compound is added like Comparative Examples 1 and 3, and in the case where the concentration of sorbic acid exceeds 1500 ppm like Comparative Example 2, the peeling strength decreases and the cycle characteristics also decrease.

[0093] In particular, in the case where the amount of addition of the sorbic acid compound is 100 ppm or more and 1000 ppm or less (Examples 2 to 8), the decrease in the viscosity of the slurry is suppressed, the coatability is improved, and thus the adhesion of the negative electrode core to the negative electrode mixture layer is improved, and the effect of improving the cycle characteristics becomes more remarkable.

[0094] Explanation of reference numerals

[0095] 10 nonaqueous electrolyte secondary battery, 11 exterior body, 11a, 11b laminate sheet, 12 housing portion, 13 sealing portion, 14 electrode body, 15 positive electrode lead, 16 negative electrode lead, 20 positive electrode, 21 positive electrode core, 22 positive electrode mixture layer, 30 negative electrode, 31 negative electrode core, 32 negative electrode mixture layer, 40 spacer

Claims

1. A non-aqueous electrolyte secondary battery, comprising: An electrode body comprising a positive electrode, a negative electrode, and spacers, and Non-aqueous electrolytes The negative electrode has a negative electrode core and a negative electrode binder layer formed on at least one surface of the negative electrode core. The negative electrode mixture layer comprises: a negative electrode active material; at least one of carboxymethyl cellulose and its salts; and at least one of sorbic acid and its salts. The content of at least one of sorbic acid and its salt is less than 1500 ppm relative to the mass of the negative electrode mixture layer.

2. The non-aqueous electrolyte secondary battery according to claim 1, wherein, The content of at least one of sorbic acid and its salt is more than 100 ppm and less than 1000 ppm relative to the mass of the negative electrode mixture layer.

3. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein, The negative electrode mixture layer contains at least one selected from sorbic acid, potassium sorbate, sodium sorbate, and calcium sorbate.

4. A method for manufacturing a non-aqueous electrolyte secondary battery, the non-aqueous electrolyte secondary battery comprising: an electrode body including a positive electrode, a negative electrode, and a spacer; and a non-aqueous electrolyte. The manufacturing process of the negative electrode includes: The process of preparing a negative electrode slurry comprising at least one of a negative electrode active material, carboxymethyl cellulose and its salt, at least one of sorbic acid and its salt, and water; and The process of coating the negative electrode mixture slurry onto at least one surface of the negative electrode core, drying and compressing the coating to form a negative electrode mixture layer. The content of at least one of sorbic acid and its salt is less than 1500 ppm relative to the mass of the negative electrode mixture layer.

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