Non-aqueous electrolyte secondary battery

Abstract

The purpose of the present disclosure is to provide a nonaqueous electrolyte secondary battery configured to suppress the occurrence of internal short-circuiting in the vicinity of the inner side end of winding. This nonaqueous electrolyte secondary battery according to one embodiment includes: an electrode body in which a positive electrode and a negative electrode are wound with a separator therebetween; a nonaqueous electrolyte; and an exterior body for housing the electrode body and the nonaqueous electrolyte. A negative electrode (12) includes a first negative electrode active material, and a second negative electrode active material, the second negative electrode active material exhibiting a greater expansion rate than the first negative electrode active material during charging. When the proportion of mass of the second negative electrode active material with respect to the total mass of the first negative electrode active material and the second negative electrode material is defined as the second negative electrode active material ratio, the second negative electrode active material ratio on the side of the inner winding end (12a) is smaller than the second negative electrode active material ratio on the side of the outer winding end (12b).

Description

[Technical Field]

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

[0002] Conventionally, nonaqueous electrolyte secondary batteries have been widely used, in which a wound electrode assembly, in which strip-shaped positive and negative electrodes are wound with a separator interposed therebetween, is housed in a metal case. In recent years, in order to increase the capacity of batteries, the use of silicon (Si)-based materials, which can occlude more lithium ions per unit mass than carbon-based materials such as graphite, as the negative electrode active material contained in the negative electrode mixture layer has been considered. Patent Document 1 discloses a negative electrode containing a Si-based material in a predetermined ratio in the mixture layer. Patent Document 2 also discloses a negative electrode in which the content of the Si-based material increases toward the surface in the thickness direction of the negative electrode mixture layer. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-212228 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-178913 Summary of the Invention [Problem to be solved by the invention]

[0004] In a wound electrode assembly, when the electrode plates expand due to battery charging, higher pressure is applied near the inner end of the winding than near the outer end, which can cause a portion of the separator to break and lead to a micro-short circuit. In particular, Si-based materials undergo a larger volume change due to lithium ion absorption than carbon-based materials, so the risk of micro-short circuits increases in negative electrodes containing Si-based materials. Patent Documents 1 and 2 do not consider how to prevent internal short circuits near the inner end of the electrode assembly, and there is still room for further study.

[0005] Therefore, an object of the present disclosure is to provide a negative electrode that suppresses the occurrence of internal short circuits near the inner end of a winding. [Means for solving the problem]

[0006] A non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure includes an electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, a non-aqueous electrolyte, and an exterior housing that houses the electrode assembly and the non-aqueous electrolyte, wherein the negative electrode includes a first negative electrode active material and a second negative electrode active material that has a larger expansion coefficient during charging than the first negative electrode active material, and wherein, when the ratio of the mass of the second negative electrode active material to the total mass of the first negative electrode active material and the second negative electrode active material is defined as the second negative electrode active material ratio, the second negative electrode active material ratio at an inner end of the winding is smaller than the second negative electrode active material ratio at an outer end of the winding. [Effects of the Invention]

[0007] According to the nonaqueous electrolyte secondary battery according to the present disclosure, the occurrence of internal short circuits near the inner end of the electrode assembly can be suppressed, and the reliability of the battery can be improved. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is an axial cross-sectional view of a cylindrical secondary battery as an example of an embodiment. [Figure 2] FIG. 2 is a perspective view of a wound electrode body included in the secondary battery shown in FIG. [Figure 3] FIG. 3 is a front view showing the positive electrode and negative electrode constituting the electrode assembly as an example of the embodiment in a developed state. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, an example of an embodiment of a cylindrical secondary battery according to the present disclosure will be described in detail with reference to the drawings. In the following description, specific shapes, materials, numerical values, directions, etc. are examples for facilitating understanding of the present invention and can be appropriately changed according to the specifications of the cylindrical secondary battery. Furthermore, when the following description includes multiple embodiments and modified examples, it is assumed from the beginning that the characteristic portions of those embodiments and modified examples can be appropriately combined and used.

[0010] FIG. 1 is an axial cross-sectional view of a cylindrical secondary battery 10 according to an embodiment. The secondary battery 10 shown in FIG. 1 includes an electrode assembly 14 and a nonaqueous electrolyte (not shown) housed in an exterior case 15. The electrode assembly 14 has a wound structure in which a positive electrode 11 and a negative electrode 12 are wound with a separator 13 interposed therebetween. Examples of nonaqueous solvents (organic solvents) for the nonaqueous electrolyte include carbonates, lactones, ethers, ketones, and esters, and two or more of these solvents can be mixed. When two or more solvents are mixed, a mixed solvent containing a cyclic carbonate and a chain carbonate is preferably used. For example, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and the like can be used as the cyclic carbonate, and dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and the like can be used as the chain carbonate. Examples of electrolyte salts that can be used for the non-aqueous electrolyte include LiPF, LiBF, LiCF, SO, and mixtures thereof. The amount of electrolyte salt dissolved in the non-aqueous solvent can be, for example, 0.5 to 2.0 mol / L. For ease of explanation, the following description will refer to the sealing body 16 side as "top" and the bottom side of the exterior body 15 as "bottom."

[0011] The open end of the exterior body 15 is sealed with the sealing body 16, thereby sealing the interior of the secondary battery 10. Insulating plates 17 and 18 are provided above and below the electrode body 14. The positive electrode lead 19 extends upward through a through hole in the insulating plate 17 and is welded to the underside of the filter 22, which is the bottom plate of the sealing body 16. In the secondary battery 10, the cap 26, which is the top plate of the sealing body 16 and is electrically connected to the filter 22, serves as the positive electrode terminal. On the other hand, the negative electrode lead 20 extends through a through hole in the insulating plate 18 toward the bottom side of the exterior body 15 and is welded to the inner surface of the bottom of the exterior body 15. In the secondary battery 10, the exterior body 15 serves as the negative electrode terminal. Furthermore, the negative electrode 12 is located on the outermost peripheral surface of the electrode body 14 and is in contact with the exterior body 15.

[0012] Exterior body 15 is, for example, a cylindrical metal exterior can with a bottom. A gasket 27 is provided between exterior body 15 and sealing body 16, ensuring the airtightness of the interior of secondary battery 10. Exterior body 15 has a grooved portion 21 that supports sealing body 16, formed, for example, by pressing the side surface from the outside. Grooved portion 21 is preferably formed in an annular shape along the circumferential direction of exterior body 15, and supports sealing body 16 on its upper surface.

[0013] The sealing body 16 includes a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26, which are stacked in this order from the electrode body 14 side. Each component of the sealing body 16 has, for example, a disk or ring shape, and all components except for the insulating member 24 are electrically connected to each other. The lower valve body 23 and the upper valve body 25 are connected to each other at their respective centers, with the insulating member 24 interposed between their respective peripheral edges. If the internal pressure of the battery increases due to abnormal heat generation, for example, the lower valve body 23 may break, causing the upper valve body 25 to bulge toward the cap 26 and separate from the lower valve body 25, thereby cutting off the electrical connection between them. If the internal pressure continues to increase, the upper valve body 25 may break, and gas may be released from the opening 26a of the cap 26.

[0014] Next, the electrode assembly 14 will be described with reference to FIG. 2. FIG. 2 is a perspective view of the wound electrode assembly 14 included in the secondary battery 10 shown in FIG. 1. The electrode assembly 14 is composed of a strip-shaped positive electrode 11, a strip-shaped negative electrode 12, two strip-shaped separators 13, a positive electrode lead 19 joined to the positive electrode 11, and a negative electrode lead 20 joined to the negative electrode 12. The positive electrode 11, the negative electrode 12, and the separators 13 are spirally wound around a winding axis, thereby being alternately stacked in the radial direction of the electrode assembly 14. The two separators 13 are formed to be slightly larger than the positive electrode 11 and are arranged to sandwich the positive electrode 11. The negative electrode 12 is exposed at the outermost periphery of the electrode assembly 14. Here, in the radial direction of the electrode assembly 14, the side facing the winding axis is referred to as the inner circumferential side, and the opposite side is referred to as the outer circumferential side. In the electrode body 14, the longitudinal direction of the positive electrode 11 and the negative electrode 12 is the winding direction, and the strip width direction of the positive electrode 11 and the negative electrode 12 is the axial direction. The positive electrode lead 19 extends in the axial direction from approximately the center in the radial direction between the center and the outermost periphery at the upper end of the electrode body 14. The negative electrode lead 20 extends in the axial direction from the vicinity of the winding axis at the lower end of the electrode body 14.

[0015] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. The material for the separator 13 is preferably an olefin resin such as polyethylene or polypropylene. The thickness of the separator 13 is, for example, 10 μm to 50 μm. Separators 13 tend to be thinner as the capacity and output of batteries increase. The separator 13 has a melting point of, for example, about 130°C to 180°C.

[0016] Next, the positive electrode 11 and negative electrode 12 that constitute the electrode assembly 14 will be described with reference to FIG. 3. FIG. 3 is a front view showing the positive electrode 11 and negative electrode 12 in a developed state. As shown in FIG. 3, in the electrode assembly 14, the negative electrode 12 is formed larger than the positive electrode 11 to prevent lithium deposition on the negative electrode 12. Specifically, the length of the negative electrode 12 in the strip width direction (axial direction) is larger than the length of the positive electrode 11 in the strip width direction. Furthermore, the length of the negative electrode 12 in the longitudinal direction is larger than the length of the positive electrode 11 in the longitudinal direction. As a result, when wound into the electrode assembly 14, at least the portion of the positive electrode 11 where the positive electrode mixture layer 32 is formed is disposed opposite the portion of the negative electrode 12 where the negative electrode mixture layer 42 is formed, with the separator 13 interposed therebetween.

[0017] The positive electrode 11 has a strip-shaped positive electrode current collector 30 and positive electrode mixture layers 32 formed on both sides of the positive electrode current collector 30. The positive electrode current collector 30 may be made of, for example, a foil of a metal such as aluminum, or a film having such a metal disposed on its surface. The thickness of the positive electrode current collector 30 is, for example, 10 μm to 30 μm.

[0018] The positive electrode mixture layer 32 is preferably formed on the entire surface of both surfaces of the positive electrode current collector 30 except for a positive electrode current collector exposed portion 34, which will be described later. The positive electrode mixture layer 32 preferably contains a positive electrode active material, a conductive agent, and a binder. The positive electrode mixture layer 32 is produced by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, a binder, and a solvent such as N-methyl-2-pyrrolidone (NMP) to both surfaces of the positive electrode current collector 30, and then drying and compressing the positive electrode mixture layer 32.

[0019] 3, a positive electrode current collector exposed portion 34 is provided in the longitudinal center of the positive electrode 11 over the entire length in the strip width direction. This improves current collection. The positive electrode current collector exposed portion 34 is a portion of the surface of the positive electrode current collector 30 that is not covered with the positive electrode mixture layer 32. The positive electrode current collector exposed portion 34 is provided, for example, by intermittent application of the positive electrode mixture slurry to a portion of the positive electrode current collector 30.

[0020] One end of the positive electrode lead 19 is connected to the exposed portion 34 of the positive electrode current collector by ultrasonic welding or the like. From the viewpoint of workability of the connection operation of the positive electrode lead 19, it is preferable that the exposed portion 34 of the positive electrode current collector is provided on both surfaces of the positive electrode 11 so as to overlap in the thickness direction of the positive electrode 11. When the other end of the positive electrode lead 19 is wound as the electrode body 14, it extends upward from the end surface in the bandwidth direction at an intermediate position in the radial direction of the electrode body 14. Note that the arrangement position of the positive electrode lead 19 is not particularly limited to the example shown in FIG. 3, and may be, for example, an inner winding end portion or an outer winding end portion.

[0021] Examples of the positive electrode active material contained in the positive electrode mixture layer 32 include lithium-containing transition metal oxides containing transition metal elements such as Co, Mn, and Ni. The lithium-containing transition metal oxide is not particularly limited, but is preferably a composite oxide represented by the general formula Li 1+x MO2 (where -0.2 < x ≦ 0.2 and M includes at least one of Ni, Co, Mn, and Al).

[0022] Examples of the conductive agent contained in the positive electrode mixture layer 32 include carbon materials such as carbon black (CB), acetylene black (AB), ketjen black, and graphite.

[0023] Examples of the binder contained in the positive electrode mixture layer 32 include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide (PI), acrylic resins, and polyolefin resins. These may be used alone or in combination of two or more. In addition, these resins may be used in combination with carboxymethyl cellulose (CMC) or its salt, polyethylene oxide (PEO), or the like.

[0024] The negative electrode 12 includes a strip-shaped negative electrode current collector 40 and negative electrode mixture layers 42 formed on both surfaces of the negative electrode current collector 40. For the negative electrode current collector 40, for example, a metal foil such as copper or a film having the metal disposed on the surface layer is used. The thickness of the negative electrode current collector 40 is, for example, 5 μm to 30 μm.

[0025] The negative electrode mixture layer 42 is preferably formed on the entire surface of each of the negative electrode current collectors 40, excluding a negative electrode current collector exposed portion 44, which will be described later. The negative electrode mixture layer 42 preferably contains a negative electrode active material and a binder. The negative electrode mixture layer 42 is produced by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, and a solvent such as water to each of the two surfaces of the negative electrode current collector 40, and then drying and compressing the negative electrode mixture layer 42.

[0026] 3, a negative electrode current collector exposed portion 44 is provided over the entire length of the current collector in the strip width direction at the inner end 12a and outer end 12b of the negative electrode 12 in the longitudinal direction. The negative electrode current collector exposed portion 44 is a portion of the surface of the negative electrode current collector 40 that is not covered with the negative electrode mixture layer 42. The negative electrode current collector exposed portion 44 is provided, for example, by intermittent application of the negative electrode mixture slurry to a portion of the negative electrode current collector 40.

[0027] One end of the negative electrode lead 20 is connected to the negative electrode current collector exposed portion 44 at the inner winding end 12a by ultrasonic welding or the like. From the viewpoint of workability in connecting the negative electrode lead 20, the negative electrode current collector exposed portion 44 is preferably provided on both sides of the negative electrode 12 so as to overlap in the thickness direction of the negative electrode 12. When wound into the electrode body 14, the other end of the negative electrode lead 20 extends downward from the end face in the strip width direction near the center of the reel of the electrode body 14. Note that the position of the negative electrode lead 20 is not particularly limited to the example shown in FIG. 3 , and the negative electrode current collector exposed portion 44 can be provided in accordance with the position of the negative electrode lead 20. For example, the negative electrode lead 20 may be provided at the outer winding end 12b of the negative electrode 12.

[0028] The negative electrode current collector exposed portion 44 of the outer winding end portion 12b is located on the outermost peripheral surface of the electrode body 14 and is in contact with the outer casing 15. This ensures a current path to the negative electrode terminal in addition to the negative electrode lead 20, thereby improving the output characteristics of the battery. It is more preferable that the negative electrode current collector 40 is exposed over the entire outermost peripheral surface of the electrode body 14. This increases the contact area between the negative electrode current collector exposed portion 44 and the outer casing 15, further improving the output characteristics of the battery. When the negative electrode current collector 40 is exposed over the entire outermost peripheral surface of the electrode body 14, the longitudinal length of the negative electrode current collector exposed portion 44 may be greater than the length of the outermost peripheral surface of the electrode body 14.

[0029] The negative electrode mixture layer 42 includes a first negative electrode active material and a second negative electrode active material that has a larger expansion coefficient during charging than the first negative electrode active material. The first negative electrode active material may be, for example, a carbon-based material such as natural graphite or artificial graphite. The second negative electrode active material may be, for example, a metal that alloys with lithium, such as Si or Sn, or an alloy or oxide containing these. The second negative electrode active material is preferably a Si-based material. Examples of Si-based materials include Si, alloys containing Si, and SiO x (x is 0.8 to 1.6) can be exemplified. The second negative electrode active material can occlude more lithium ions than the first negative electrode active material, and therefore, by using the second negative electrode active material as the negative electrode active material, it is possible to increase the capacity of the battery.

[0030] If the ratio of the mass of the second negative electrode active material to the total mass of the first and second negative electrode active materials is defined as the second negative electrode active material ratio, the second negative electrode active material ratio at the inner end 12a of the electrode body 14 is smaller than the second negative electrode active material ratio at the outer end 12b. This prevents excessive pressure from being applied to the vicinity of the inner end during charging, thereby suppressing the occurrence of internal short circuits. The second negative electrode active material ratio may decrease at a constant rate from the outer end 12b to the inner end 12a, or the rate of decrease may vary from the outer end 12b to the inner end 12a. When forming a negative electrode mixture layer using a multi-layer die coater as described below, it is preferable for the second negative electrode active material ratio to decrease at a constant rate from the outer end 12b to the inner end 12a for ease of production.

[0031] The mass ratio of the second negative electrode active material to the total mass of the first negative electrode active material and the second negative electrode active material in the entire negative electrode mixture layer 42 is preferably 2 mass % to 20 mass %, and more preferably 5 mass % to 15 mass %. Within this range, the battery can have a higher capacity and can have a higher reliability.

[0032] As in the case of the positive electrode 11, the binder contained in the negative electrode mixture layer 42 may be a fluororesin such as PTFE or PVdF, PAN, PI, an acrylic resin, or a polyolefin resin, but is preferably styrene-butadiene rubber (SBR). The negative electrode mixture layer 42 may also contain, as a thickener, CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like. These may be used alone or in combination of two or more. The content of the binder in the negative electrode mixture layer 42 may be, for example, 0.5% by mass to 10% by mass.

[0033] The negative electrode mixture layer 42 can be formed, for example, by using a multi-layer die coater. By using a multi-layer die coater, multiple negative electrode mixture slurries with different second negative electrode active material ratios can be simultaneously applied to the negative electrode current collector 40 while adjusting the application amount ratio between them. When the negative electrode mixture slurry is applied to the negative electrode current collector 40, the negative electrode current collector 40 moves relative to the multi-layer die coater. Therefore, by applying multiple negative electrode mixture slurries with different second negative electrode active material ratios to the negative electrode current collector 40 while changing the application amount ratio between them at a predetermined timing, a region in which the second negative electrode active material ratio changes from the inner winding end 12a side to the outer winding end 12b side can be formed at any position in the negative electrode mixture layer 42.

[0034] For example, a first anode mixture slurry and a second anode mixture slurry having a lower second anode active material ratio than the first anode mixture slurry are prepared. Next, the first and second anode mixture slurries are applied from the inner winding end 12a to the outer winding end 12b using a multi-layer die coater while increasing the application amount ratio of the first anode mixture slurry to the second anode mixture slurry, thereby obtaining an anode mixture layer 42 in which the second anode active material ratio decreases at a constant rate from the outer winding end 12b to the inner winding end 12a.

[0035] The application amounts of the first and second negative electrode mixture slurries may be adjusted so that the charge capacity per unit area of ​​the negative electrode mixture layer 42 is constant in the longitudinal direction of the negative electrode 12. For example, in the above example, the proportion of the second negative electrode active material is smaller at the inner end 12a side than at the outer end 12b side, so by increasing the application amount at the inner end 12a side compared to the outer end 12b side, the charge capacity per unit area of ​​the negative electrode mixture layer 42 can be made constant in the longitudinal direction of the negative electrode 12. [Example]

[0036] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0037] Example 1 [Preparation of positive electrode] 100 parts by mass of LiNi 0.88 Co 0.09 Al 0.03 O2, 1 part by mass of acetylene black (AB), and 0.9 parts by mass of polyvinylidene fluoride (PVdF) were mixed, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied to both sides of a strip-shaped positive electrode current collector made of aluminum foil with a thickness of 15 μm. After drying the coating, the dried coating was compressed using a roll press to a total thickness of 0.144 mm. Further, the positive electrode mixture slurry was cut to a width of 62.6 mm and a length of 861 mm. A positive electrode was fabricated in which a positive electrode mixture layer was formed on both sides of the positive electrode current collector while having a positive electrode current collector exposed in the approximate center of the longitudinal direction. Then, one end of an aluminum positive electrode lead was welded to the exposed positive electrode current collector.

[0038] [Preparation of negative electrode] A mixture A containing 89 parts by mass of graphite and 11 parts by mass of SiO was used as the negative electrode active material. 100 parts by mass of mixture A, 1 part by mass of carboxymethyl cellulose (CMC), and 1 part by mass of styrene butadiene rubber (SBR) were mixed, and an appropriate amount of water was added to prepare a first negative electrode mixture slurry. Furthermore, a mixture B containing 93 parts by mass of graphite and 7 parts by mass of SiO was used as the negative electrode active material. 100 parts by mass of mixture B, 1 part by mass of CMC, and 1 part by mass of SBR were mixed, and an appropriate amount of water was added to prepare a second negative electrode mixture slurry. Next, the first and second negative electrode mixture slurries were placed in a multilayer die coater and applied to both sides of a strip-shaped negative electrode current collector made of 8 μm-thick copper foil, from the inner end to the outer end, while continuously changing the coating amount ratio of the first negative electrode mixture slurry to the second negative electrode mixture slurry from 0:1 to 1:0, so that the charge capacity per unit area was constant in the longitudinal direction. The coating was then dried. The dried coating was compressed using a roll press to a total thickness of 0.160 mm and then cut to a width of 64.2 mm and a length of 959 mm. A negative electrode was produced in which a negative electrode mixture layer was formed on both sides of the negative electrode current collector, with exposed portions of the negative electrode current collector at the inner and outer ends of the winding. One end of a nickel / copper negative electrode lead was then welded to the exposed portion of the negative electrode current collector at the inner end.

[0039] [Preparation of electrode body] The positive and negative electrodes were wound with a polyethylene separator between them to prepare an electrode assembly. The outermost surface of the electrode assembly was covered with the exposed portion of the negative electrode current collector. Polypropylene tape measuring 12 mm wide, 30 μm thick, and 50 mm long was attached to the upper and lower ends of the outermost surface of the electrode assembly.

[0040] [Preparation of electrolyte] Five parts by mass of vinylene carbonate (VC) was added to 100 parts by mass of a mixed solvent consisting of ethylene carbonate (EC) and dimethyl methyl carbonate (DMC) (volume ratio of EC:DMC = 1:3). LiPF was dissolved in the mixed solvent to a concentration of 1.5 mol / L to prepare an electrolyte solution.

[0041] [Fabrication of cylindrical secondary batteries] Insulating plates were placed above and below one electrode assembly, and the electrode assembly was housed in an outer casing. The negative electrode lead was then welded to the bottom of the outer casing, and the positive electrode lead was welded to a sealing member. After that, an electrolyte was injected into the outer casing under reduced pressure, and the open end of the outer casing was crimped to the sealing member via a gasket to produce a cylindrical secondary battery. The capacity of the produced battery was 4600 mAh.

[0042] <Example 2> A battery was produced in the same manner as in Example 1, except that in producing the negative electrode, a mixture of 90 parts by mass of graphite and 10 parts by mass of SiO was used as mixture A, and a mixture of 92 parts by mass of graphite and 8 parts by mass of SiO was used as mixture B.

[0043] Example 3 A battery was produced in the same manner as in Example 1, except that in producing the negative electrode, a mixture of 88 parts by mass of graphite and 12 parts by mass of SiO was used as mixture A, and a mixture of 92 parts by mass of graphite and 8 parts by mass of SiO was used as mixture B.

[0044] <Comparative Example> A battery was fabricated in the same manner as in Example 1, except that in fabricating the negative electrode, a mixture of 91 parts by mass of graphite and 9 parts by mass of SiO was used as mixture A, and only the first negative electrode mixture slurry was applied to both sides of the negative electrode current collector.

[0045] [Evaluation of deformation of the negative electrode near the inner end of the winding] The batteries of the examples and comparative examples were charged at a constant current of 0.3 C (1380 mA) in a 45°C environment until the battery voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.02 C (92 mA). Subsequently, they were discharged at a constant current of 0.5 C (2300 mA) until the battery voltage reached 2.5 V, constituting one cycle. After repeating this charge-discharge cycle 1000 times, they were charged at a constant current of 0.3 C until the battery voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.02 C. An X-ray CT (Computed Tomography) device was used to observe the cross section of the electrode body of each battery near its inner end to confirm whether or not the negative electrode had deformed.

[0046] The evaluation results of the Examples and Comparative Examples are shown in Table 1. Table 1 also shows the proportions of SiO in the first and second negative electrode mixture slurries.

[0047] [Table 1]

[0048] In Examples 1 to 3, no deformation was observed in the negative electrode after 1000 cycles, and the risk of micro-short circuits occurring near the inner end of the winding was reduced compared to the comparative example. [Explanation of symbols]

[0049] 10 secondary battery, 11 positive electrode, 12 negative electrode, 12a inner end of winding, 12b outer end of winding, 13 separator, 14 electrode body, 15 outer case, 16 sealing body, 17, 18 insulating plate, 19 positive electrode lead, 20 negative electrode lead, 21 grooved portion, 22 filter, 23 lower valve body, 24 insulating member, 25 upper valve body, 26 cap, 26a opening, 27 gasket, 28 winding shaft, 30 positive electrode current collector, 32 positive electrode mixture layer, 34 exposed portion of positive electrode current collector, 40 negative electrode current collector, 42 negative electrode mixture layer, 44 exposed portion of negative electrode current collector

Claims

1. A non-aqueous electrolyte secondary battery comprising: an electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween; a non-aqueous electrolyte; and an exterior housing that accommodates the electrode assembly and the non-aqueous electrolyte, the negative electrode includes a first negative electrode active material and a second negative electrode active material having a larger expansion coefficient during charging than the first negative electrode active material, when the ratio of the mass of the second negative electrode active material to the total mass of the first negative electrode active material and the second negative electrode active material is defined as a second negative electrode active material ratio, the second negative electrode active material ratio at the inner end of the winding is smaller than the second negative electrode active material ratio at the outer end of the winding, the second negative electrode active material ratio decreases at a constant rate from the outer winding end side to the inner winding end side, or decreases with a varying rate from the outer winding end side to the inner winding end side.

2. the first negative electrode active material is a carbon-based material, 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the second negative electrode active material is a silicon-based material.

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