Nonaqueous electrolyte secondary battery and method of manufacturing the same

Abstract

The purpose of the present disclosure is to provide a nonaqueous electrolyte secondary battery that has excellent cycle characteristics. A nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure is provided with: an electrode body that comprises a positive electrode, a negative electrode and a separator; and a nonaqueous electrolyte. A mixture layer of the negative electrode contains a negative electrode active material, at least either one of carboxymethyl cellulose or a salt thereof, and at least either one of sorbic acid or a salt thereof; and the content of the sorbic acid and / or a salt thereof is 1,500 ppm or less relative to the mass of the mixture layer.

Description

[Technical Field]

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery and a method for manufacturing a non-aqueous electrolyte secondary battery. [Background technology]

[0002] A negative electrode for a non-aqueous electrolyte secondary battery is generally produced by applying a negative electrode mixture slurry containing a negative electrode active material and a binder in a dispersion medium to the surface of a negative electrode core, and then drying and compressing the coating (see, for example, Patent Document 1). Conventionally, in negative electrode mixture slurries containing water as a dispersion medium, at least one of carboxymethyl cellulose and a salt thereof (hereinafter, these are collectively referred to as "CMC-based compounds") has been used as a thickener.

[0003] Furthermore, Patent Documents 2 and 3 propose adding a preservative to the negative electrode mixture slurry in order to prevent deterioration of the quality of the binder in the slurry due to microorganisms, and disclose an isothiazolinone compound as the preservative. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 4-342966 [Patent Document 2] International Publication No. 2012 / 026462 [Patent Document 3] Japanese Patent Application Laid-Open No. 2013-211246 Summary of the Invention [Problem to be solved by the invention]

[0005] In non-aqueous electrolyte secondary batteries, it is important to suppress the capacity decrease that occurs during charging and discharging and thereby improve the cycle characteristics. As a result of investigations by the present inventors, it was found that the additive components in the negative electrode mixture layer have a significant effect on the cycle characteristics. [Means for solving the problem]

[0006] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure is a non-aqueous electrolyte secondary battery including an electrode assembly including a positive electrode, a negative electrode, and a separator, and a non-aqueous electrolyte, wherein the negative electrode has a negative electrode core and a negative electrode mixture layer formed on at least one surface of the negative electrode core, the negative electrode mixture layer including 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 the 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.

[0007] A method for manufacturing a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure is a method for manufacturing a non-aqueous electrolyte secondary battery including an electrode assembly including a positive electrode, a negative electrode, and a separator, and a non-aqueous electrolyte, wherein the negative electrode manufacturing process includes the steps of preparing a negative electrode mixture slurry including a negative electrode active material, at least one of carboxymethyl cellulose and a salt thereof, sorbic acid and at least one of a salt thereof, and water, and applying the negative electrode mixture slurry to at least one surface of a negative electrode core, drying and compressing the coating to form a negative electrode mixture layer. [Effects of the Invention]

[0008] According to one aspect of the present disclosure, a nonaqueous electrolyte secondary battery with excellent cycle characteristics can be provided. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a perspective view of a nonaqueous electrolyte secondary battery according to an embodiment of the present invention. [Figure 2] FIG. 2 is a cross-sectional view of an electrode assembly according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] As a result of research by the present inventors, it was found that the cycle characteristics of non-aqueous electrolyte secondary batteries can be significantly improved by adding a predetermined amount of at least one of sorbic acid and its salt to a negative electrode mixture layer containing a CMC-based compound. At least one of sorbic acid and its salt is added to a negative electrode mixture slurry containing a CMC-based compound and contained in the negative electrode mixture layer in an amount of 1500 ppm or less. Note that, if the amount added exceeds 1500 ppm, the cycle characteristics actually deteriorate. Sorbic acid or its salt added at a concentration of 1500 ppm or less improves the adhesion between the negative electrode mixture layer and the negative electrode core, which is presumably the main factor behind the improved cycle characteristics. It is believed that the carboxyl group of sorbic acid interacts with CMC to strengthen the bond between the polymers.

[0011] Furthermore, negative electrode mixture slurries containing CMC compounds lose viscosity over time, posing a problem in coating stability. This decrease in slurry viscosity is thought to be caused by enzymes produced by bacteria in the water severing the molecular chains of the CMC compounds. Sorbic acid and its salts are thought to penetrate the interior of bacteria and inhibit their growth. Therefore, adding sorbic acid or its salts to negative electrode mixture slurries inhibits the decomposition of CMC compounds, thereby preventing the decrease in slurry viscosity and improving coatability.

[0012] Suppressing the decrease in viscosity of the negative electrode mixture slurry and ensuring good coatability contributes to, for example, improving the adhesion between the negative electrode mixture layer and the negative electrode core, thereby improving the cycle characteristics of the battery. In particular, when sorbic acid or a salt thereof is added to the slurry so that the concentration in the negative electrode mixture layer is 100 ppm to 1000 ppm, the effect of improving the cycle characteristics is remarkable.

[0013] Hereinafter, an example of an embodiment of the present disclosure will be described in detail with reference to the drawings, but the present disclosure is not limited to the embodiment described below. Hereinafter, a nonaqueous electrolyte secondary battery 10, which is a laminate battery including an exterior body 11 composed of laminate sheets 11a and 11b, will be described as an example of a nonaqueous electrolyte secondary battery. However, the nonaqueous electrolyte secondary battery according to the present disclosure may be a cylindrical battery including a cylindrical battery case, a prismatic battery including a prismatic battery case, or the like, and the shape of the battery is not particularly limited.

[0014] FIG. 1 is a perspective view of a nonaqueous electrolyte secondary battery 10 according to an embodiment. The nonaqueous electrolyte secondary battery 10 includes an electrode assembly 14 and a nonaqueous electrolyte, which are housed in a housing 12 of an exterior body 11. The laminate sheets 11a and 11b are formed by laminating a metal layer and a resin layer. The laminate sheets 11a and 11b each have, for example, two resin layers sandwiching a metal layer, one of the resin layers being made of a thermocompression-bondable resin. An example of the metal layer is an aluminum layer.

[0015] The exterior body 11 has, for example, a generally rectangular shape in a plan view. The exterior body 11 has a sealing portion 13 formed by joining laminate sheets 11a and 11b together, thereby sealing the storage portion 12 in which the electrode assembly 14 is housed. The sealing portion 13 is formed in a frame shape with approximately the same width along the edge of the exterior body 11. The generally rectangular portion in a plan view surrounded by the sealing portion 13 is the storage portion 12. The storage portion 12 is provided by forming a recess in at least one of the laminate sheets 11a and 11b that can house the electrode assembly 14. In this embodiment, the recess is formed in the laminate sheet 11a.

[0016] The nonaqueous electrolyte secondary battery 10 includes a pair of electrode leads (a positive electrode lead 15 and a negative electrode lead 16) connected to the electrode assembly 14. Each electrode lead is drawn from the inside to the outside of the exterior body 11. In the example shown in FIG. 1, the electrode leads are drawn from the same end side of the exterior body 11 in approximately parallel to each other. The positive electrode lead 15 and the negative electrode lead 16 are both conductive thin plates; for example, the positive electrode lead 15 is made of a metal primarily composed of aluminum, and the negative electrode lead 16 is made of a metal primarily composed of copper or nickel.

[0017] FIG. 2 is a cross-sectional view of the electrode assembly 14. As shown in FIG. 2, the electrode assembly 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 assembly 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 electrode assembly that is pressed in the radial direction. The negative electrode 30 is formed to be slightly larger than the positive electrode 20 in order to suppress lithium deposition. The electrode assembly may also be a stacked type in which a plurality of positive electrodes and a plurality of negative electrodes are alternately stacked one by one with separators interposed therebetween.

[0018] The non-aqueous electrolyte contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent that can be used include esters, ethers, nitriles, amides, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms of these solvents have been substituted with halogen atoms such as fluorine. For example, 0.5 to 5 mass % of fluoroethylene carbonate may be added to the total mass of the non-aqueous electrolyte. Furthermore, 1 to 5 mass % of vinylene carbonate may be added to the total mass of the non-aqueous electrolyte. The non-aqueous electrolyte is not limited to a liquid electrolyte, and may also be a solid electrolyte. A lithium salt such as LiPF6 is used as the electrolyte salt.

[0019] The positive electrode 20, negative electrode 30, and separator 40 that constitute the electrode assembly 14, with the negative electrode 30 being particularly described in detail below.

[0020] [Positive electrode] The positive electrode 20 has a positive electrode core 21 and a positive electrode mixture layer 22 formed on at least one surface of the positive electrode core 21. For the positive electrode core 21, a foil of a metal that is stable within the potential range of the positive electrode 20, such as aluminum or an aluminum alloy, or a film with such a metal disposed on the surface layer, can be used. The positive electrode mixture layer 22 contains a positive electrode active material, a conductive agent, and a binder, and is preferably formed on both surfaces of the positive electrode core 21. The positive electrode 20 can be manufactured by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, a binder, etc., onto the positive electrode core 21, drying the coating, and then compressing it to form the positive electrode mixture layer 22 on both surfaces of the positive electrode core 21.

[0021] A lithium transition metal composite oxide is used as the positive electrode active material. Elements contained in the lithium transition metal composite oxide include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, and W. An example of a suitable lithium transition metal composite oxide is a composite oxide containing at least one element selected from Ni, Co, Mn, and Al. Note that inorganic compound particles such as aluminum oxide and lanthanoid-containing compounds may be adhered to the particle surface of the lithium transition metal composite oxide.

[0022] Examples of the conductive agent contained in the positive electrode mixture layer 22 include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. Examples of the binder contained in the positive electrode mixture layer 22 include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide, acrylic resin, and polyolefin. These resins may be used in combination with carboxymethyl cellulose (CMC), a salt of CMC, polyethylene oxide (PEO), or the like.

[0023] [Negative electrode] The negative electrode 30 has a negative electrode core 31 and a negative electrode mixture layer 32 formed on at least one surface of the negative electrode core 31. The negative electrode core 31 can be made of a foil of a metal such as copper or a copper alloy that is stable within the potential range of the negative electrode 30, or a film with such a metal disposed on its surface. The negative electrode mixture layer 32 contains a negative electrode active material, at least one of carboxymethyl cellulose (CMC) and its salts (CMC-based compounds), and at least one of sorbic acid and its salts, and is preferably formed on both surfaces of the negative electrode core 31. The negative electrode 30 can be manufactured by applying a negative electrode mixture slurry containing the negative electrode active material and the like onto the negative electrode core 31, drying the coating, and then compressing it to form the negative electrode mixture layer 32 on both surfaces of the negative electrode core 31.

[0024] The negative electrode active material is a carbon-based active material that reversibly absorbs and releases lithium ions. Suitable carbon-based active materials include natural graphite such as flake graphite, lump graphite, and amorphous graphite, and artificial graphite such as massive artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB). The volume-based median diameter (50% particle size) of the graphite is, for example, 18 to 22 μm. The negative electrode active material may also be a Si-containing Si-based active material, or a carbon-based active material and a Si-based active material may be used in combination. The Si-based active material has a structure in which Si particles are dispersed in an oxide phase containing Si. Examples of the oxide phase include a silicon oxide (SiO2) phase and a composite oxide phase containing Si and a metal element such as Li.

[0025] The CMC compound contained in the negative electrode mixture layer 32 functions as a thickener for the negative electrode mixture slurry and also functions as a binder that binds the particles of the negative electrode active material together and between the negative electrode active material and 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 some of the carboxyl groups are neutralized. The negative electrode mixture layer 32 (negative electrode mixture slurry) may contain a mixture of CMC and a salt of CMC, or may contain CMC or a salt of CMC alone. The weight-average molecular weight of the CMC compound is, for example, 200,000 to 500,000.

[0026] The negative electrode mixture layer 32 preferably contains a rubber-based binder as a binder in addition to the CMC-based compound. The contents of the CMC-based compound and the rubber-based binder are each preferably 0.1 to 5 mass%, more preferably 0.5 to 3 mass%, relative to the total mass of the negative electrode mixture layer 32. A suitable rubber-based binder is styrene butadiene rubber (SBR) or a modified product thereof. The modified SBR may contain at least one unit selected from an acrylonitrile unit, an acrylate unit, an acrylic acid unit, a methacrylate unit, and a methacrylic acid unit. SBR and modified products thereof are generally supplied in the form of a dispersion using water as a dispersion medium.

[0027] As described above, sorbic acid or its salts (hereinafter collectively referred to as "sorbic acid compounds") suppress a decrease in the viscosity of the negative electrode mixture slurry, maintaining good coating properties of the slurry, improving adhesion between the negative electrode substrate 31 and the negative electrode mixture layer 32, and ultimately improving the cycle characteristics of the battery. Sorbic acid is an unsaturated fatty acid represented by the molecular formula C6H8O2. Sorbic acid compounds are dissolved or dispersed in water in the negative electrode mixture slurry, and are thought to inhibit the decomposition of CMC compounds by suppressing bacterial growth.

[0028] The content of the sorbic acid-based compound is 1500 ppm or less relative to the mass of the negative electrode mixture layer 32. Even the addition of a small amount of the sorbic acid-based compound improves the cycle characteristics compared to when no sorbic acid-based compound is added, but a concentration exceeding 1500 ppm actually reduces the cycle characteristics. The content of the sorbic acid-based compound can be measured by gas chromatography.

[0029] The content of the sorbic acid 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, a decrease in the viscosity of the negative electrode mixture slurry can be effectively suppressed, and stable coating properties of the slurry can be easily ensured. Furthermore, the adhesion between the negative electrode substrate 31 and the negative electrode mixture layer 32 is further improved, resulting in a significant improvement in cycle characteristics.

[0030] The negative electrode mixture layer 32 (negative electrode mixture slurry) may contain a mixture of sorbic acid and a sorbate, or may contain sorbic acid or a sorbate alone. Suitable sorbic acid compounds include at least one selected from sorbic acid, potassium sorbate, sodium sorbate, and calcium sorbate. Among these, sorbic acid, potassium sorbate, and sodium sorbate are particularly preferred.

[0031] The negative electrode 30 is manufactured, for example, through the following two steps. (1) A step of preparing a negative electrode mixture slurry containing a negative electrode active material, at least one of carboxymethyl cellulose and a salt thereof (a CMC-based compound), at least one of sorbic acid and a salt thereof (a sorbic acid-based compound), and water. (2) A step of applying a negative electrode mixture slurry to the surface of the negative electrode substrate 31, drying the coating, and compressing it to form the negative electrode mixture layer 32. The solid content concentration of the negative electrode mixture slurry is preferably about 45 to 55 mass % from the viewpoints of handling and coating properties.

[0032] The sorbic acid compound is added in an amount of, for example, 0.1 to 20 mass%, 0.5 to 15 mass%, or 0.6 to 13 mass% relative to the mass of the CMC compound. The sorbic acid compound contained in the negative electrode mixture slurry is incorporated into the negative electrode mixture layer 32 together with the CMC compound. Therefore, the mass ratio of the CMC compound to the sorbic acid compound in the negative electrode mixture layer 32 is substantially the same as that in the negative electrode mixture slurry.

[0033] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 40. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 40 include olefin resins such as polyethylene and polypropylene, and cellulose. The separator 40 may have either a single-layer structure or a laminated structure. A heat-resistant layer or the like may be formed on the surface of the separator 40. [Example]

[0034] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0035] Example 1 [Preparation of positive electrode] LiCo as the positive electrode active material 0.979 Zr 0.001 Mg 0.01 Al 0.01 A lithium-containing metal composite oxide represented by O2 was used. The positive electrode active material, carbon black, and polyvinylidene fluoride (PVdF) were mixed in a solids mass ratio of 95:2.5:2.5 to prepare a positive electrode mixture slurry using N-methyl-2-pyrrolidone (NMP) as a dispersion medium. The positive electrode mixture slurry was applied to both sides of a long positive electrode core made of aluminum foil with a thickness of 15 μm using a doctor blade method. After drying the coating, the coating was compressed with a roller to form a positive electrode mixture layer on both sides of the positive electrode core. The positive electrode core with the formed positive electrode mixture layer was cut to a predetermined electrode size to prepare a positive electrode.

[0036] [Preparation of negative electrode] The negative electrode active materials used were graphite with a volumetric median diameter of 22 μm and a silicon-containing compound (SiO) in which silicon particles are dispersed in a silicon-containing oxide phase. The graphite and SiO were mixed in a mass ratio of 95:5. The SiO was obtained by mixing silicon metal and silicon dioxide, heat-treating the mixture under reduced pressure, and then heating it to approximately 1000°C to form a carbon coating on the particle surface using the CVD method, followed by crushing and classification.

[0037] A negative electrode active material, a sodium salt of CMC (CMC-Na), and SBR were mixed in a solids mass ratio of 97:1.5:1.0, and sorbic acid was added to a concentration of 50 ppm relative to the solids (negative electrode active material, CMC-Na, SBR). A negative electrode mixture slurry was prepared using water (ion-exchanged water) as a dispersion medium. The negative electrode mixture slurry was applied to both sides of a long negative electrode core made of copper foil using a doctor blade method. After drying the coating, the coating was compressed with a roller to form a negative electrode mixture layer on both sides of the negative electrode core. The negative electrode core with the formed negative electrode mixture layer was cut to a predetermined electrode size to produce a negative electrode.

[0038] [Preparation of non-aqueous electrolyte] A non-aqueous electrolyte solution was prepared by adding LiPF6 to a mixed solvent of ethylene carbonate (EC) and methyl ethyl carbonate (MEC) in a volume ratio of 3:7 (25°C, 1 atmosphere) to a concentration of 1 mol / L, and further adding vinylene carbonate to a concentration of 2 mass%.

[0039] [Battery construction] A positive electrode lead and a negative electrode lead were attached to the positive electrode and the negative electrode, respectively, and the positive electrode and the negative electrode were wound with a separator made of a polyethylene microporous film interposed therebetween. Polypropylene tape was attached to the outermost surface of the wound body, and the wound body was then pressed radially to produce a flat wound electrode body. Under an argon atmosphere, the electrode body and the nonaqueous electrolyte were housed in a cup-shaped housing portion of an exterior body composed of a laminate sheet having a five-layer structure of a polypropylene layer / adhesive layer / aluminum alloy layer / adhesive layer / polypropylene layer. The interior of the exterior body was then depressurized to impregnate the electrode body with the electrolyte solution, and the opening of the exterior body was sealed to produce a nonaqueous electrolyte secondary battery measuring 62 mm in height, 35 mm in width, and 3.6 mm in thickness.

[0040] <Example 2> A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that in the preparation of the negative electrode mixture slurry, the amount of sorbic acid added was changed to 100 ppm relative to the solid content.

[0041] Example 3 A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that in the preparation of the negative electrode mixture slurry, the amount of sorbic acid added was changed to 200 ppm relative to the solid content.

[0042] Example 4 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that in the preparation of the negative electrode mixture slurry, the amount of sorbic acid added was changed to 500 ppm relative to the solid content.

[0043] <Example 5> A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that in the preparation of the negative electrode mixture slurry, the amount of sorbic acid added was changed to 1000 ppm based on the solid content.

[0044] Example 6 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 3, except that in the preparation of the negative electrode mixture slurry, sorbic acid was changed to potassium sorbate.

[0045] Example 7 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 3, except that in the preparation of the negative electrode mixture slurry, sorbic acid was changed to sodium sorbate.

[0046] Example 8 A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 3, except that in the preparation of the negative electrode mixture slurry, sorbic acid was changed to calcium sorbate.

[0047] <Comparative Example 1> A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that sorbic acid was not added in the preparation of the negative electrode mixture slurry.

[0048] <Comparative Example 2> A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that in the preparation of the negative electrode mixture slurry, the amount of sorbic acid added was changed to 2000 ppm relative to the solid content.

[0049] <Comparative Example 3> A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 3, except that in the preparation of the negative electrode mixture slurry, sorbic acid was changed to 1,2-benzisothiazolin-3-one.

[0050] [Viscosity measurement of negative electrode mixture slurry] The viscosity of each negative electrode mixture slurry in the Examples and Comparative Examples was measured immediately after preparation and 48 hours after preparation using a B-type viscometer (TVC10, manufactured by Toki Sangyo Co., Ltd.) at 25°C, and the ratio of the viscosity after 48 hours to the viscosity immediately after preparation (viscosity retention rate) was calculated using the following formula. The viscosity retention rates of each negative electrode mixture slurry are shown in Table 1. Viscosity retention rate after 48 hours = (viscosity after 48 hours / viscosity immediately after preparation) x 100

[0051] [Adhesion evaluation (peel strength measurement)] For each of the negative electrodes (negative electrode mixture layer density 1.6 g / mL) of the Examples and Comparative Examples, the adhesion of the negative electrode mixture layer to the negative electrode substrate was evaluated by the method described in JP-A 2005-251481. (1) Prepare an acrylic plate (3.0 × 12 cm), double-sided tape (2 × 9 mm, Nichiban Co., Ltd., Nicetack NW-20), and a measurement electrode plate (2.5 × 16 cm) cut to the specified size from each negative electrode. (2) Attach double-sided tape to the acrylic plate, lengthwise for 8.5 cm from the edge (leaving 0.5 cm excess). (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 pulled at a speed of 100 mm / min using a tensile tester until the negative electrode mixture layer was peeled off, thereby measuring the peel strength (adhesion strength) of the negative electrode mixture layer. The peel strength measurement results are shown in Table 1.

[0052] [Capacity retention rate measurement] Each battery in the Examples and Comparative Examples was charged at 25°C at a constant current of 800 mA until the battery voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached a cut-off current of 40 mA. The battery was then discharged at a constant current of 800 mA until the battery voltage reached 2.75 V. This charge-discharge cycle was repeated 150 times, and the ratio of the discharge capacity at the 150th cycle to the discharge capacity at the first cycle (capacity retention) was calculated. The capacity retention of each battery is shown in Table 1.

[0053] [Table 1]

[0054] As can be seen from the results shown in Table 1, all of the batteries of the Examples have higher capacity retention rates after charge / discharge cycles and superior cycle characteristics compared to the batteries of the Comparative Examples. Furthermore, the negative electrodes of the Examples have higher peel strength of the negative electrode mixture layer and higher adhesion between the negative electrode core and the negative electrode mixture layer compared to the negative electrodes of the Comparative Examples. In other words, sorbic acid or a salt thereof improves the adhesion between the negative electrode core and the negative electrode mixture layer, which is thought to be the main factor in improving the cycle characteristics. On the other hand, when no sorbic acid-based compound is added, as in Comparative Examples 1 and 3, or when the sorbic acid concentration exceeds 1500 ppm, as in Comparative Example 2, the peel strength decreases and the cycle characteristics also decrease.

[0055] In particular, when the amount of sorbic acid-based compound added is 100 ppm or more and 1000 ppm or less (Examples 2 to 8), the decrease in viscosity of the slurry is suppressed and the coating properties are improved, thereby improving the adhesion between the negative electrode core and the negative electrode mixture layer and making the effect of improving cycle characteristics more pronounced. [Explanation of symbols]

[0056] 10 non-aqueous 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 separator

Claims

1. A non-aqueous electrolyte secondary battery comprising an electrode assembly including a positive electrode, a negative electrode, and a separator, and a non-aqueous electrolyte, the negative electrode has a negative electrode core and a negative electrode mixture layer formed on at least one surface of the negative electrode core, the negative electrode mixture layer contains a negative electrode active material, at least one of carboxymethyl cellulose and a salt thereof, and a sorbic acid-based compound, a total content of sorbic acid-based compounds is more than 0 ppm and 1500 ppm or less relative to the mass of the negative electrode mixture layer.

2. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the total content of the sorbic acid-based compounds is 100 ppm or more and 1000 ppm or less relative to the mass of the negative electrode mixture layer.

3. A non-aqueous electrolyte secondary battery as described in claim 1 or 2, wherein the sorbic acid-based compound includes at least one selected from potassium sorbate, sodium sorbate, and calcium sorbate.

4. A method for manufacturing a non-aqueous electrolyte secondary battery including an electrode assembly including a positive electrode, a negative electrode, and a separator, and a non-aqueous electrolyte, The manufacturing process of the negative electrode includes: preparing a negative electrode mixture slurry containing a negative electrode active material, at least one of carboxymethyl cellulose and a salt thereof, a sorbic acid-based compound, and water; a step of applying the negative electrode mixture slurry to at least one surface of a negative electrode core, drying the coating, and compressing the coating to form a negative electrode mixture layer; and preparing the negative electrode mixture slurry so that a total content of sorbic acid-based compounds is more than 0 ppm and 1500 ppm or less relative to the mass of the negative electrode mixture layer.

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