Non-aqueous electrolyte secondary battery

Abstract

A non-aqueous electrolyte secondary battery according to one embodiment, wherein a mixture layer of an electrode has a first layer formed on a core body, and second and third layers formed on the first layer. The first, second, and third layers contain a binding agent, and the third layer is formed on at least a part of an end of the electrode. The content of the binding agent in the third layer is higher than the content of the binding agent in the first layer, and is greater than 0.8 mass% and less than 2.0 mass%.

Description

Technical Field

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[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery.

Background Art

[0002] In recent years, non-aqueous electrolyte secondary batteries such as lithium-ion batteries have been applied to in-vehicle use, power storage use, etc. The required performance of non-aqueous electrolyte secondary batteries applied to these uses includes high capacity and good charge-discharge cycle characteristics. Since the electrodes, which are the main components of the battery, greatly affect these battery performances, many studies have been conducted on electrode structures. For example, in Patent Document 1, in order to prevent the lack of the mixture layer during the production of the electrode and increase the capacity, the concentration of the binder in the positive electrode mixture at the cutting site, which is the end of the electrode, is relatively higher than the concentration of the binder in the positive electrode mixture at the non-cutting site, and a method of applying the binder solution along the cutting pattern is disclosed.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] According to the method of Patent Document 1, it is possible to suppress the lack of the mixture layer during the production of the electrode. However, when an electrode body is formed using the electrode of Patent Document 1, the electrolyte extruded from the inside of the electrode body due to the expansion of the electrode body during charge and discharge becomes difficult to return to the inside of the electrode body, and as a result, the electrode reaction becomes non-uniform, and it has been found that the capacity reduction during charge and discharge increases. In this case, since the portion where the concentration of the binder becomes high is formed at the end of the electrode, it is considered that the return of the electrolyte is inhibited.

[0005] The objective of this disclosure is to provide a non-aqueous electrolyte secondary battery that can suppress the defect of the electrode mixture layer and has excellent cycle characteristics. [Means for solving the problem]

[0006] The non-aqueous electrolyte secondary battery according to this disclosure comprises a core body, an electrode having a composite layer formed on the core body, and a non-aqueous electrolyte, wherein the composite layer has a first layer formed on the core body, and a second and third layer formed on the first layer, the first, second and third layers contain a binder, the third layer is formed on at least a part of the end of the electrode, the binder content in the third layer is higher than the binder content in the first layer and is greater than 0.8% by mass and less than 2.0% by mass, and the binder content in the second layer is less than or equal to the binder content in the first layer. [Effects of the Invention]

[0007] The non-aqueous electrolyte secondary battery described herein can achieve good cycle characteristics while suppressing the loss of the electrode mixture layer. [Brief explanation of the drawing]

[0008] [Figure 1] This is a cross-sectional view of a non-aqueous electrolyte secondary battery, which is an example of an embodiment. [Figure 2] This is a plan view of a positive electrode, which is an example of an embodiment. [Figure 3] This is a cross-sectional view along line AA in Figure 2. [Modes for carrying out the invention]

[0009] When the electrode mixture layer is missing, it not only contributes to a decrease in capacity, but the missing mixture layer can also become conductive foreign matter, potentially causing minute short circuits. Therefore, preventing the loss of the mixture layer is an important issue. Since the loss of the mixture layer is more likely to occur at the ends of the electrode, especially during the cutting process in electrode manufacturing, a method has been proposed to prevent this by increasing the amount of binder in the mixture layer at the ends of the electrode. However, as mentioned above, simply increasing the amount of binder in the mixture layer inhibits the return of the electrolyte to the inside of the electrode body, leading to uneven electrode reactions and a greater decrease in capacity during charging and discharging.

[0010] The inventors of the present invention have diligently studied to solve the above problems and have found that by forming an electrode mixture layer with a layer structure including a first layer formed on the core body and second and third layers formed on the first layer, and by forming the third layer, which has a higher binder content than the first layer, on at least a part of the end of the electrode, it is possible to improve cycle characteristics while suppressing the loss of the mixture layer. In this case, the loss of the mixture layer is highly suppressed by the third layer with a large amount of binder. Furthermore, at the end of the electrode, since the first layer with a small amount of binder is present below the third layer, the electrolyte penetrates through the first layer, and the electrolyte pushed out from inside the electrode body is more easily returned. Therefore, it is possible to achieve both suppression of the loss of the mixture layer and improvement of cycle characteristics.

[0011] In particular, by making the binder content in the second layer lower than that in the first layer, the penetration of the electrolyte into the composite layer is further improved, and the improvement in cycle characteristics becomes even more pronounced.

[0012] Hereinafter, an example of an embodiment of the non-aqueous electrolyte secondary battery according to this disclosure will be described in detail with reference to the drawings. It should be noted that selective combination of the multiple embodiments and modifications described below is included in this disclosure.

[0013] In the following, a cylindrical battery in which a wound electrode body 14 is housed in a bottomed cylindrical outer casing 16 is given as an example. However, the battery casing is not limited to a cylindrical outer casing, and may be, for example, a rectangular outer casing (rectangular battery) or a coin-shaped outer casing (coin-type battery), or an outer casing made of a laminate sheet including a metal layer and a resin layer (pouch-type battery). Furthermore, the electrode body may be a laminated electrode body in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators in between.

[0014] Figure 1 is a schematic diagram showing a cross-section of a non-aqueous electrolyte secondary battery 10, which is an example of an embodiment. As shown in Figure 1, the non-aqueous electrolyte secondary battery 10 comprises a wound electrode body 14, a non-aqueous electrolyte, and an outer casing 16 that houses the electrode body 14 and the non-aqueous electrolyte. The electrode body 14 has a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are wound in a spiral shape via the separator 13. The outer casing 16 is a bottomed cylindrical metal container with one side open in the axial direction, and the opening of the outer casing 16 is sealed by a sealing body 17. In the following, for convenience of explanation, the side of the battery with the sealing body 17 will be referred to as the top, and the bottom side of the outer casing 16 as the bottom.

[0015] Non-aqueous electrolytes comprise a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles, amides, and mixtures of two or more of these. The non-aqueous solvent may also contain halogen-substituted solvents, in which at least some of the hydrogen atoms in the solvent are replaced with halogen elements such as fluorine. Examples of non-aqueous solvents include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and mixtures thereof. Examples of lithium salts, such as LiPF6, are used as electrolyte salts.

[0016] The positive electrode 11, negative electrode 12, and separator 13 constituting the electrode body 14 are all elongated strip-shaped bodies that are alternately stacked in the radial direction of the electrode body 14 by being wound in a spiral shape. The negative electrode 12 is formed to be slightly larger in dimensions than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer in the longitudinal direction and the width direction (short direction) than the positive electrode 11. The separator 13 is formed to be at least slightly larger in dimensions than the positive electrode 11, and two separators are arranged so as to sandwich the positive electrode 11. The electrode body 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

[0017] Insulating plates 18 and 19 are positioned above and below the electrode body 14, respectively. In the example shown in Figure 1, the positive electrode lead 20 extends through a through-hole in the insulating plate 18 towards the sealing body 17, and the negative electrode lead 21 extends outside the insulating plate 19 towards the bottom of the outer can 16. The positive electrode lead 20 is connected to the lower surface of the internal terminal plate 23 of the sealing body 17 by welding or the like, and the cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, becomes the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, and the outer can 16 becomes the negative electrode terminal.

[0018] As described above, the outer casing 16 is a bottomed cylindrical metal container with one side open in the axial direction. A gasket 28 is provided between the outer casing 16 and the sealing body 17 to ensure airtightness inside the battery and insulation between the outer casing 16 and the sealing body 17. The outer casing 16 has a grooved portion 22 formed on its side, which protrudes inward to support the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the outer casing 16, and its upper surface supports the sealing body 17. The sealing body 17 is fixed to the upper part of the outer casing 16 by the grooved portion 22 and the open end of the outer casing 16 which is crimped to the sealing body 17.

[0019] The sealing body 17 has a structure in which an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are laminated in this order from the side of the electrode body 14. Each member constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each member except the insulating member 25 is electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected at the central portions of each, and the insulating member 25 is interposed between the peripheral portions of each. When an abnormality occurs in the battery and the internal pressure rises, the lower valve body 24 is deformed and broken so as to push up the upper valve body 26 toward the cap 27 side, thereby cutting off the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further rises, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0020] Hereinafter, the positive electrode 11, the negative electrode 12, and the separator 13 constituting the nonaqueous electrolyte secondary battery 10 will be described in detail, particularly the positive electrode 11.

[0021] [Electrode Structure] An electrode which is an example of an embodiment includes a core body and a mixture layer formed on the core body. The mixture layer has a first layer formed on the core body, and a second layer and a third layer formed on the first layer. The first layer, the second layer, and the third layer contain a binder. The third layer is formed on at least a part of the end portion of the electrode. The content ratio of the binder in the third layer is higher than the content ratio of the binder in the first layer, and the content ratio of the binder in the second layer is not more than the content ratio of the binder in the first layer. And the third layer is formed on at least a part of the end portion of the first layer. According to such an electrode structure, while suppressing the dropout of the mixture layer, good cycle characteristics can be realized. The content ratio of the binder is calculated as the ratio of the mass of the binder to the mass of the mixture layer.

[0022] The above electrode structure can also be applied to the negative electrode 12, but it is particularly effective in the positive electrode 11. Hereinafter, it will be described assuming that the mixture layer of the positive electrode 11 has the above structure. Note that the above electrode structure may be applied to both the positive electrode 11 and the negative electrode 12.

[0023] [Positive Electrode] Figure 2 is a plan view showing a part of the positive electrode 11, and Figure 3 is a cross-sectional view taken along line AA in Figure 2. As shown in Figures 2 and 3, the positive electrode 11 comprises a positive electrode core 30 and a positive electrode mixture layer 31 formed on the positive electrode core 30. As described above, the positive electrode 11 is a long, strip-shaped body with a constant width along its longitudinal direction. The positive electrode core 30 can be made of a metal foil that is stable in the potential range of the positive electrode 11, such as aluminum or an aluminum alloy, or a film with the metal arranged on its surface. The positive electrode mixture layer 31 contains a positive electrode active material, a conductive agent, and a binder, and is preferably provided on both sides of the positive electrode core 30, excluding the core exposed portion where the positive electrode lead is connected. The thickness of the positive electrode mixture layer 31 is, for example, 50 μm to 150 μm on one side of the positive electrode core.

[0024] The positive electrode active material is mainly composed of a lithium transition metal composite oxide. Elements other than Li that may be included 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, W, Si, and P. A suitable example of a lithium transition metal composite oxide is one containing at least one of Ni, Co, and Mn. Specific examples include lithium transition metal composite oxides containing Ni, Co, and Mn, and lithium transition metal composite oxides containing Ni, Co, and Al.

[0025] Examples of conductive agents included in the positive electrode mixture layer 31 include carbon materials such as carbon black, acetylene black, Ketjen black, and graphite. Examples of binders included in the positive electrode mixture layer 31 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 cellulose derivatives such as carboxymethylcellulose (CMC) or its salts, polyethylene oxide (PEO), etc.

[0026] The positive electrode mixture layer 31 has multiple layers with different binder content, and includes a first layer 32 formed on the positive electrode core 30, and second and third layers 33 and 34 formed on the first layer 32. The first layer 32, second layer 33, and third layer 34 all contain positive electrode active material, conductive agent, and binder, but the binder content is highest in the third layer 34. The binder content in the first layer 32 and the second layer 33 may be the same, but it is preferable that the binder content is lowest in the second layer 33. In other words, the binder content is second layer 33 ≤ first layer 32 < third layer 34, and preferably second layer 33 < first layer 32 < third layer 34.

[0027] The first layer 32 is a lower layer of the positive electrode mixture layer 31, which is formed directly on the surface of the positive electrode core 30, and is formed over the entire surface of the positive electrode core 30, except for the core exposed portion to which the positive electrode lead is connected. The second layer 33 and the third layer 34 are upper layers formed on the first layer 32. Preferably, the second layer 33 and the third layer 34 are formed so as not to substantially overlap. In this embodiment, they are formed in a stripe pattern along the longitudinal direction of the positive electrode 11, and the second layer 33 and the third layer 34 form the outermost surface of the positive electrode mixture layer 31. The positive electrode 11 may also be provided with other layers, such as a protective layer covering the surface of the positive electrode mixture layer 31, to the extent that it does not impair the purpose of this disclosure.

[0028] The second layer 33 is formed in the widthwise center of the positive electrode 11. The second layer 33 is wider than the third layer 34 and broadly covers the surface of the first layer 32. The third layer 34 is formed on at least a portion of the end of the first layer 32. In this embodiment, the third layer 34 is formed at both ends in the widthwise direction along the longitudinal direction of the positive electrode 11.

[0029] As described above, the upper layer of the positive electrode mixture layer 31 is formed in a stripe-like pattern in which the second layer 33 is sandwiched between third layers 34 located at both ends in the width direction of the positive electrode 11. By placing the third layer 34, which has a high binder content, at both ends in the width direction of the positive electrode 11, the loss of the positive electrode mixture layer 31 during the manufacturing process of the positive electrode 11 can be effectively suppressed. Furthermore, the effect of suppressing the loss of the positive electrode mixture layer 31 is achieved if the third layer 34 is formed on at least a part of the end of the positive electrode 11. On the other hand, if the third layer 34 is present at the end of the positive electrode 11, it will hinder the return of the electrolyte that has been pushed out from inside the electrode body 14 by charging and discharging. However, since there is a first layer 32 at the end of the positive electrode 11 with a lower binder content than the third layer 34, this problem can be addressed. In other words, the first layer 32 acts as a passage for the electrolyte, enabling the smooth return of the electrolyte.

[0030] The third layer 34 is formed from both ends of the first layer 32 in the width direction to a predetermined width. The width of the third layer 34 is not particularly limited, but is preferably 1 to 8 mm, and more preferably 2 to 6 mm. If the width of the third layer 34 is within this range, the effect of suppressing defect in the positive electrode mixture layer 31 and the effect of improving cycle characteristics become more pronounced. The width of the second layer 33 is greater than the width of the third layer 34, for example, 50 to 60 mm. It is preferable that each third layer 34 formed at both ends of the first layer 32 in the width direction is formed to be substantially the same width. The first layer 32 is formed over the entire width of the positive electrode core 30, and the width of the first layer 32 is the sum of the width of the second layer 33 and the width of the third layer 34.

[0031] The binder content in the third layer 34 is higher than that in the first layer 32 and the second layer 33, and is greater than 0.8% by mass and less than 2.0% by mass. If the binder content in the third layer 34 is 0.8% by mass or less, the additive layer is more likely to be lost at the edges of the positive electrode additive layer 31, especially at the cut portion. On the other hand, if the binder content exceeds 2.0% by mass, the capacity decrease associated with charging and discharging becomes larger. The binder content in the third layer 34 is more preferably 0.9 to 1.8% by mass, and particularly preferably 1.0 to 1.5% by mass. In this case, the effect of suppressing the loss of the positive electrode additive layer 31 and the effect of improving the cycle characteristics become more pronounced.

[0032] As described above, the binder content in the second layer 33 is less than or equal to the binder content in the first layer 32, and preferably lower than the binder content in the first layer 32. The binder content in the first layer 32 is preferably 0.5 to 1.0 mass%. The binder content in the second layer 33 is preferably 0.4 to 0.8 mass%. Since the second layer 33 is formed over a large area on the first layer 32, lowering the binder content in the second layer 33 improves the permeability of the electrolyte from the surface of the positive electrode mixture layer 31, making the improvement in cycle characteristics more pronounced.

[0033] The binder content in each layer can be adjusted, for example, by changing the amount of positive electrode active material and binder added while keeping the conductive agent content constant. An example of the conductive agent content in each layer is 0.5 to 1.5% by mass.

[0034] The ratio (T) of the thickness of the second layer 33 and the third layer 34 to the thickness of the first layer 32 is, for example, 20:80 to 80:20, and preferably 30:70 to 70:30. The thicknesses of the second layer 33 and the third layer 34 are substantially the same. Therefore, the ratio (T) is, in other words, the ratio of the thickness of the upper layer to the thickness of the lower layer of the positive electrode mixture layer 31. If the ratio of the thicknesses of the upper and lower layers of the positive electrode mixture layer 31 is within this range, the defect suppression effect and the improvement effect of the cycle characteristics of the positive electrode mixture layer 31 become more pronounced. The thicknesses of the upper and lower layers may be substantially the same.

[0035] The positive electrode 11 can be manufactured by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder to the surface of a wide, elongated core, drying the coating, compressing it to form a positive electrode mixture layer 31 on both sides of the elongated core, and then cutting it to a predetermined size. Two or more slurries with different binder content in the solids can be used for the positive electrode mixture slurry. The wide, elongated core is cut to become the positive electrode core 30 and has a width corresponding to the width of multiple positive electrodes 11. After forming positive electrode mixture layers 31 corresponding to multiple positive electrodes 11 on both sides of the elongated core, the elongated core is cut along the longitudinal direction and to a predetermined length at the center of the width direction of the third layer 34 to obtain a positive electrode 11 having the above configuration.

[0036] [Negative electrode] The negative electrode 12 comprises a negative electrode core and a negative electrode mixture layer provided on the surface of the negative electrode core. The negative electrode 12 is a long, strip-shaped body and is wider than the positive electrode 11. The negative electrode core can be made of a metal foil that is stable in the potential range of the negative electrode 12, such as copper, or a film with the metal arranged on its surface. The negative electrode mixture layer contains a negative electrode active material and a binder, and is preferably provided on both sides of the negative electrode core. The negative electrode 12 can be manufactured, for example, by applying a negative electrode mixture slurry containing a negative electrode active material and a binder to the surface of the negative electrode core, drying the coating, and then compressing it to form the negative electrode mixture layer on both sides of the negative electrode core. The negative electrode mixture layer may contain the same conductive agent as in the case of the positive electrode 11.

[0037] The negative electrode mixture layer contains, as a negative electrode active material, a carbon material that reversibly intercepts and releases lithium ions, for example. A preferred example of the carbon material is graphite, such as natural graphite like flake graphite, lump graphite, or clay graphite, or artificial graphite such as lump graphite (MAG) or graphitized mesophase carbon microbeads (MCMB). Alternatively, an active material containing at least one of an element that alloys with Li, such as Si or Sn, and a compound containing such an element may be used as the negative electrode active material. A preferred example of such an active material is a silicon material in which Si fine particles are dispersed in a silicon oxide phase or a silicate phase such as lithium silicate. For example, a carbon material such as graphite and a silicon material may be used in combination as the negative electrode active material.

[0038] The binder included in the negative electrode mixture layer can be fluororesin, PAN, polyimide, acrylic resin, polyolefin, etc., as in the case of the positive electrode 11, but styrene-butadiene rubber (SBR) is preferred. Furthermore, the negative electrode mixture layer preferably contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), etc. In particular, a combination of SBR and CMC or a salt thereof, or PAA or a salt thereof is preferred.

[0039] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator 13 include polyethylene, polypropylene, polyolefins such as copolymers of ethylene and α-olefins, cellulose, polystyrene, polyester, polyphenylene sulfide, polyether ether ketone, and fluororesin. The separator 13 may have either a single-layer structure or a laminated structure. The surface of the separator 13 may have a heat-resistant layer containing inorganic particles, or a heat-resistant layer composed of a highly heat-resistant resin such as aramid resin, polyimide, or polyamide-imide. [Examples]

[0040] The present disclosure will be further illustrated by the following examples, but the present disclosure is not limited to these examples.

[0041] <Example 1> [Preparation of cathode mixture slurry] Lithium cobalt oxide, acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 98.2:1:0.8, and an appropriate amount of N-methyl-2-pyrrolidone was added to prepare cathode slurry A. Lithium cobalt oxide, acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 98:1:1, and an appropriate amount of N-methyl-2-pyrrolidone was added to prepare cathode slurry B.

[0042] [Fabrication of the positive electrode] A positive electrode slurry A was applied to one side of a strip-shaped positive electrode core made of aluminum foil. After the coating was dried, a 56 mm wide layer of positive electrode slurry A and a 4 mm wide layer of positive electrode slurry B were applied to the coating along the longitudinal direction of the positive electrode core, and the coating was dried. This formed a two-layer coating consisting of a lower layer made of positive electrode slurry A and an upper layer made of positive electrode slurry A and B. At this time, the amount of each slurry applied was adjusted so that the ratio of the thickness of the upper layer to the thickness of the lower layer was 50:50. A two-layer coating was formed on the other side of the positive electrode core in the same manner. The coating was compressed using a roller so that the total thickness of the coating was 160 μm, and the positive electrode core with the coating formed on it was cut to a predetermined size to produce a positive electrode with positive electrode slurry layers formed on both sides of the positive electrode core.

[0043] A first layer of the additive mixture is formed by a positive electrode additive slurry A applied directly to the surface of the positive electrode core, a second layer of the additive mixture is formed by a positive electrode additive slurry A applied on top of the first layer, and a third layer of the additive mixture is formed by a positive electrode additive slurry B applied on top of the first layer. The binder content in the first, second, and third layers is 0.8% by mass, 0.8% by mass, and 1.0% by mass, respectively. The core, with the additive mixture layers formed on both sides, is cut along the longitudinal direction at the center of the third layer in the width direction. This results in a positive electrode with a 2 mm wide third layer formed at both ends in the width direction along the longitudinal direction of the positive electrode.

[0044] [Fabrication of the negative electrode] A negative electrode slurry was prepared by mixing graphite, sodium salt of carboxymethylcellulose, and a dispersion of styrene-butadiene copolymer in a solid content mass ratio of 98:1:1, and adding an appropriate amount of water. This negative electrode slurry was applied to a strip-shaped negative electrode core made of copper foil, and after the coating film was dried, the coating film was compressed using a roller, and the negative electrode core was cut to a predetermined size to produce a negative electrode in which negative electrode slurry layers were formed on both sides of the negative electrode core.

[0045] [Preparation of non-aqueous electrolyte solution] Ethylene carbonate and diethyl carbonate were mixed in a 1:1 volume ratio, and then fluoroethylene carbonate was added to a concentration of 2% by mass. LiPF6 was added to this mixed solvent to a concentration of 1 mol / L to obtain a non-aqueous electrolyte.

[0046] [Fabrication of non-aqueous electrolyte secondary batteries] A wound electrode body was fabricated by spirally winding the positive electrode, which had aluminum leads welded to it, and the negative electrode, which had nickel leads welded to it, via a separator (a composite porous film of polyethylene and polypropylene, 20 μm thick). This electrode body was housed in a bottomed cylindrical outer casing with a diameter of 18 mm and a height of 65 mm. After injecting 5.2 mL of non-aqueous electrolyte into the outer casing, the opening of the outer casing was sealed with a sealing body via a gasket to obtain a test battery (non-aqueous electrolyte secondary battery).

[0047] [Evaluation of defects in the cathode mixture layer] The widthwise ends of the positive electrode were visually inspected to check for any defects in the positive electrode mixture layer.

[0048] [Evaluation of cycle characteristics] Under a temperature of 25°C, the fabricated test battery was charged with a constant current of 0.7It until the battery voltage reached 4.2V, and then charged with a constant voltage until the current reached 0.05It at 4.2V. Subsequently, it was discharged with a constant current of 0.7It until the battery voltage reached 2.5V. This charge-discharge cycle was performed 100 times, and the capacity retention rate was calculated using the following formula. The evaluation results are shown in Table 1 (the same applies to the examples and comparative examples described later). The evaluation results shown in Table 1 are relative values ​​with the capacity retention rate of the test battery in Comparative Example 1 set to 100. Capacity retention rate (%) = (Discharge capacity at 100th cycle / Discharge capacity at 1st cycle) × 100

[0049] <Example 2> Lithium cobalt oxide, acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 98.4:1:0.6, and an appropriate amount of N-methyl-2-pyrrolidone was added to prepare cathode slurry C. The cathode and test battery were prepared in the same manner as in Example 1, except that cathode slurry C was used as the cathode slurry to form the second layer.

[0050] <Example 3> Lithium cobalt oxide, acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 97.5:1:1.5, and an appropriate amount of N-methyl-2-pyrrolidone was added to prepare cathode slurry D. A cathode and a test battery were prepared in the same manner as in Example 1, except that cathode slurry B was used to form the first layer and cathode slurry D was used to form the third layer.

[0051] <Example 4> The positive electrode and test battery were fabricated in the same manner as in Example 2, except that the ratio of the thickness of the upper layer to the thickness of the lower layer was set to 20:80.

[0052] <Example 5> The positive electrode and test battery were fabricated in the same manner as in Example 2, except that the ratio of the thickness of the upper layer to the thickness of the lower layer was set to 30:70.

[0053] <Example 6> The positive electrode and test battery were fabricated in the same manner as in Example 2, except that the ratio of the thickness of the upper layer to the thickness of the lower layer was set to 70:30.

[0054] <Example 7> The positive electrode and test battery were fabricated in the same manner as in Example 2, except that the ratio of the thickness of the upper layer to the thickness of the lower layer was set to 80:20.

[0055] <Example 8> The positive electrode and test battery were manufactured in the same manner as in Example 2, except that the width of the third layer formed at each of the widthwise ends of the positive electrode was set to 6 mm.

[0056] <Example 9> The positive electrode and test battery were manufactured in the same manner as in Example 2, except that the width of the third layer formed at each of the widthwise ends of the positive electrode was set to 8 mm.

[0057] <Comparative Example 1> A positive electrode and a test battery were prepared in the same manner as in Example 1, except that positive electrode mixture slurry B was used as the slurry for forming the first and second layers.

[0058] <Comparative Example 2> The positive electrode and test battery were prepared in the same manner as in Example 1, except that positive electrode mixture slurry B was used as the slurry to form the second layer.

[0059] <Comparative Example 3> Lithium cobalt oxide, acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 98.5:1:0.5, and an appropriate amount of N-methyl-2-pyrrolidone was added to prepare cathode mixture slurry E. The cathode mixture slurry was used as the slurry to form the first layer. -C Using this method, and using cathode mixture slurry E as the slurry to form the second layer, Cathode mixture slurry A was used as the slurry to form the third layer. Except for the above, the positive electrode and test battery were prepared in the same manner as in Example 1.

[0060] <Comparative Example 4> Lithium cobalt oxide, acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 97:1:2, and an appropriate amount of N-methyl-2-pyrrolidone was added to prepare cathode slurry F. The cathode and test battery were prepared in the same manner as in Example 3, except that cathode slurry F was used as the slurry to form the third layer.

[0061] [Table 1]

[0062] As shown in Table 1, the test battery of the example has a higher capacity retention rate after charge-discharge cycles and superior cycle characteristics compared to the test batteries of Comparative Examples 1, 2, and 4. In the test batteries of Comparative Examples 1 and 4, the binder content at both ends in the width direction of the positive electrode is too high, which is thought to have inhibited the return of the electrolyte that was pushed out from inside the electrode body due to the expansion of the negative electrode during charging and discharging, resulting in an uneven electrode reaction and a large decrease in capacity. In the test battery of Comparative Example 2, the binder content in the upper layer (second layer) of the positive electrode mixture layer is too high, which is thought to have made it difficult for the electrolyte to penetrate into the inside of the positive electrode mixture layer, resulting in an uneven electrode reaction and a large decrease in capacity. In contrast, in the test battery of the example, the pushed-out electrolyte easily returns to the inside of the electrode body and the electrolyte easily penetrates into the inside of the positive electrode mixture layer, so the unevenness of the electrode reaction is suppressed and good cycle characteristics are obtained.

[0063] Furthermore, while the positive electrode mixture layer was missing in the test battery of Comparative Example 3, no such absence was observed in the test battery of the Example. In Comparative Example 3, it is thought that the amount of binder in the mixture layer was insufficient at the end of the positive electrode, resulting in the absence of the mixture layer. In contrast, the test battery of the Example can achieve good cycle characteristics while suppressing the absence of the positive electrode mixture layer. In particular, the improvement in cycle characteristics was more pronounced when the binder content in the second layer was lower than that in the first layer, when the thickness ratio of the upper and lower layers was 30:70 to 70:30, and when the width of the third layer was 2 to 6 mm. [Explanation of Symbols]

[0064] 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Outer casing, 17 Sealing body, 18,19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved section, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating material, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Positive electrode core, 31 Positive electrode mixture layer, 32 First layer, 33 Second layer, 34 Third layer

Claims

1. A non-aqueous electrolyte secondary battery comprising a core body, an electrode having a composite layer formed on the core body, and a non-aqueous electrolyte, The aforementioned mixture layer comprises a first layer formed on the core body, and a second and third layer formed on the first layer. The first layer, the second layer, and the third layer each contain a binder. The third layer is formed on at least a portion of the end of the electrode, The binder content in the third layer is higher than the binder content in the first layer, and is 1.0 to 1.5% by mass. A non-aqueous electrolyte secondary battery, wherein the content of the binder in the second layer is less than or equal to the content of the binder in the first layer, and is 0.4 to 0.8% by mass.

2. The electrode is formed in a strip shape, The non-aqueous electrolyte secondary battery according to claim 1, wherein the third layer is formed at both ends in the width direction along the longitudinal direction of the electrode.

3. The non-aqueous electrolyte secondary battery according to claim 2, wherein the width of the third layer is 2 to 6 mm.

4. The content of the binder in the second layer is lower than the content of the binder in the first layer. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein the content of the binder in the first layer is 0.5 to 1.0% by mass.

5. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the ratio of the thickness of the second and third layers to the thickness of the first layer is 30:70 to 70:30.

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