Non-aqueous electrolyte secondary batteries

Abstract

Provided is a non-aqueous electrolyte secondary battery having improved charge and discharge cycle characteristics. A non-aqueous electrolyte secondary battery according to an aspect of the present disclosure comprises: an electrode body in which a belt-shaped positive electrode and a belt-shaped negative electrode are wound with a separator therebetween; an electrolyte; and an outer case which accommodates the electrode body and the electrolyte. The negative electrode has a negative electrode current collector and a negative electrode mixture layer which is formed on a surface of the negative electrode current collector and which contains a negative electrode active material and a solid electrolyte. The negative electrode mixture layer has a region in which the solid electrolyte content in the inner winding end part is higher than the solid electrolyte content in the outer winding end part, and the solid electrolyte content continuously decreases from the inner winding end part side to the outer winding end part side.

Description

[Technical Field]

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

[0002] The positive electrode and negative electrode of a nonaqueous electrolyte secondary battery each have a current collector and a mixture layer formed on the surface of the current collector. The mixture layer contains an active material capable of reversibly absorbing and releasing Li ions. Patent Documents 1 to 3 disclose techniques for incorporating an inorganic solid electrolyte having Li ion conductivity into the mixture layer in order to achieve both improved safety and performance maintenance in batteries. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-527603 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-117542 [Patent Document 3] Japanese Patent Application Laid-Open No. 2011-44252 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in non-aqueous electrolyte secondary batteries having a wound electrode assembly, the expansion and contraction of the electrode assembly due to charging and discharging can cause uneven distribution of the electrolyte within the electrode assembly, resulting in a decrease in battery capacity after repeated charging and discharging. The technology disclosed in Patent Document 1 does not consider the distribution of the electrolyte within the electrode assembly, and there is still room for improvement in the charge-discharge cycle characteristics.

[0005] An object of the present disclosure is to provide a non-aqueous electrolyte secondary battery with improved charge-discharge cycle characteristics. [Means for solving the problem]

[0006] A nonaqueous electrolyte secondary battery according to one aspect of the present disclosure includes an electrode assembly in which a strip-shaped positive electrode and a strip-shaped negative electrode are wound with a separator interposed therebetween, an electrolytic solution, and an exterior housing that accommodates the electrode assembly and the electrolytic solution. The negative electrode has a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector and containing a negative electrode active material and a solid electrolyte. The negative electrode mixture layer is characterized in that the solid electrolyte content at the inner end is higher than the solid electrolyte content at the outer end, and the negative electrode mixture layer has a region in which the solid electrolyte content continuously decreases from the inner end to the outer end. [Effects of the Invention]

[0007] According to the secondary battery of one aspect of the present disclosure, charge / discharge cycle characteristics can be improved. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is an axial cross-sectional view of a cylindrical secondary battery according to an embodiment of the present invention; [Figure 2] 2 is a perspective view of a wound electrode body included in the secondary battery shown in FIG. 1. FIG. [Figure 3] FIG. 2 is a front view showing a positive electrode and a negative electrode constituting an electrode assembly according to an embodiment in a developed state. [Figure 4] 4(a) to 4(d) are diagrams showing the change in the content of the solid electrolyte contained in the negative electrode mixture layer in the longitudinal direction of FIG. 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 an electrolyte (not shown) housed in an outer casing 15. The electrode assembly 14 has a wound structure in which a strip-shaped positive electrode 11 and a strip-shaped negative electrode 12 are wound with a separator 13 interposed therebetween. Examples of nonaqueous solvents (organic solvents) for the 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, cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC), and chain carbonates include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC). The electrolyte salt of the electrolyte solution may be LiPF, LiBF, LiCF, SO, or a mixture thereof. The amount of the electrolyte salt dissolved in the non-aqueous solvent may be, for example, 0.5 to 2.0 mol / L. For ease of explanation, the following description will be given with the sealing body 16 side as the "top" and the bottom side of the exterior body 15 as the "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 a filter 22, which is the bottom plate of the sealing body 16. In the secondary battery 10, a 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 to the bottom side of the exterior body 15 and is welded to the inner bottom surface of the exterior body 15. In the secondary battery 10, the exterior body 15 serves as the negative electrode terminal. Note that when the negative electrode lead 20 is provided at the outer end of the winding, the negative electrode lead 20 passes outside the insulating plate 18, extends to the bottom side of the exterior body 15, and is welded to the inner bottom surface of 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 electrode assembly 14. As described above, the electrode assembly 14 has a wound structure in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The positive electrode 11, the negative electrode 12, and the separator 13 are all formed in strip shapes and spirally wound around a winding core disposed along the winding axis 28, resulting in a state in which they are alternately stacked in the radial direction of the electrode assembly 14. In the radial direction, the side of the winding axis 28 is referred to as the inner peripheral side, and the opposite side is referred to as the outer peripheral side. In the electrode assembly 14, the longitudinal direction of the positive electrode 11 and the negative electrode 12 is the winding direction, and the width direction of the positive electrode 11 and the negative electrode 12 is the axial direction. The positive electrode lead 19 extends axially from the upper end of the electrode assembly 14, approximately at the center in the radial direction between the center and the outermost periphery. Furthermore, the negative electrode lead 20 extends in the axial direction from the vicinity of the winding axis 28 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 and negative electrode according to this embodiment will be described with reference to FIGS. 3 and 4. FIG. 3 is a front view of the positive electrode 11 and negative electrode 12 constituting the electrode assembly 14. FIG. 3 shows the positive electrode 11 and negative electrode 12 in a developed state. As illustrated 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 in the direction (axial direction) of the negative electrode 12 is larger than the length in the width direction of the positive electrode 12. Furthermore, the length in the longitudinal direction of the negative electrode 12 is larger than the length in the longitudinal direction of the positive electrode 11. As a result, when wound into the electrode assembly 14, at least the portion of the positive electrode 11 on which the positive electrode mixture layer 32 is formed is disposed opposite the portion of the negative electrode 12 on which 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 a positive electrode mixture layer 32 formed on the surface of the positive electrode current collector 30. The positive electrode mixture layer 32 is formed on at least one of the inner and outer circumferential sides of the positive electrode current collector 30, and is preferably formed on the entire area of ​​both sides of the positive electrode current collector 30 except for a positive electrode exposed portion 34 described below. The positive electrode current collector 30 may be, 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 preferably contains a positive electrode active material, a conductive agent, and a binder. The positive electrode mixture layer 32 can be produced, for example, 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 sides of the positive electrode current collector 30, drying the slurry, and then rolling the slurry.

[0019] The positive electrode 11 is provided with a positive electrode exposed portion 34, where the surface of the positive electrode current collector 30 is exposed. The positive electrode exposed portion 34 is a portion to which the positive electrode lead 19 is connected, and 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 exposed portion 34 is formed to be wider in the longitudinal direction than the positive electrode lead 19. The positive electrode exposed portions 34 are preferably provided on both sides of the positive electrode 11 so as to overlap in the thickness direction of the positive electrode 11. The positive electrode lead 19 is joined to the positive electrode exposed portion 34 by, for example, ultrasonic welding.

[0020] In the example shown in FIG. 3 , a positive electrode exposed portion 34 is provided in the longitudinal center of the positive electrode 11, spanning the entire width. The positive electrode exposed portion 34 may be formed at either the inner or outer end of the positive electrode 11, but from the viewpoint of current collection, it is preferably provided at a position approximately equidistant from the inner and outer ends. By connecting the positive electrode lead 19 to the positive electrode exposed portion 34 provided in such a position, when the electrode assembly 14 is wound, the positive electrode lead 19 is positioned so as to protrude upward from the end face in the width direction at approximately the center in the radial direction of the electrode assembly 14. The positive electrode exposed portion 34 is provided, for example, by intermittent application, in which the positive electrode mixture slurry is not applied to a portion of the positive electrode current collector 30.

[0021] The positive electrode active material contained in the positive electrode mixture layer 32 can be, for example, a lithium transition metal oxide containing a transition metal element such as Co, Mn, or Ni. x CoO2, Li x NiO2, Li x MnO2, Li x Co y Ni 1-y O2, Li x Co y M 1-y O z , Li x Ni 1-y M y O z , Li x Mn2O4, Li x Mn 2-y M yO4, LiMPO4, Li2MPO4F (M is at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, B; 0 < x ≤ 1.2, 0 < y ≤ 0.9, 2.0 ≤ z ≤ 2.3). These may be used alone or in combination of multiple types. In terms of achieving high capacity of the non-aqueous electrolyte secondary battery, the cathode active material is Li x NiO2, Li x Co y Ni 1-y O2, Li x Ni 1-y M y O z (M is at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, B; 0 < x ≤ 1.2, 0 < y ≤ 0.9, 2.0 ≤ z ≤ 2.3), and it is preferable to contain lithium nickel composite oxides such as etc.

[0022] Examples of the conductive agent contained in the cathode mixture layer 32 include carbon-based particles such as carbon black (CB), acetylene black (AB), ketjen black, carbon nanotube (CNT), graphene, graphite, etc. These may be used alone or in combination of two or more types.

[0023] Examples of the binder contained in the cathode mixture layer 32 include fluorine-based resins such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide-based resins, acrylic resins, polyolefin-based resins, etc. These may be used alone or in combination of two or more types. When preparing the cathode mixture slurry with an aqueous solvent, styrene-butadiene rubber (SBR), nitrile rubber (NBR), CMC or its salt, polyacrylic acid or its salt, polyvinyl alcohol, etc. may also be used.

[0024] The negative electrode 12 has a strip-shaped negative electrode current collector 40 and a negative electrode mixture layer 42 formed on the surface of the negative electrode current collector 40. The negative electrode mixture layer 42 is formed on at least one of the inner and outer circumferential sides of the negative electrode current collector 40, and is preferably formed on the entire area of ​​both sides of the negative electrode current collector 40 except for a negative electrode exposed portion 44 described below. The negative electrode current collector 40 may be, for example, a foil of a metal such as copper, or a film having such a metal disposed on its surface. The thickness of the negative electrode current collector 40 is, for example, 5 μm to 30 μm.

[0025] The anode mixture layer 42 includes an anode active material and a solid electrolyte. The anode mixture layer 42 may further include a binder. The anode mixture layer 42 can be produced, for example, by applying an anode mixture slurry containing an anode active material, a solid electrolyte, a binder, and a solvent such as water to both sides of the anode current collector 40, drying the slurry, and then rolling the slurry.

[0026] 3, a negative electrode exposed portion 44 is provided at an end of the negative electrode 12 in the longitudinal direction, over the entire length of the current collector in the width direction. The negative electrode exposed portion 44 is a portion to which the negative electrode lead 20 is connected, and 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 exposed portion 44 is formed to be wider in the longitudinal direction than the width of the negative electrode lead 20. The negative electrode exposed portions 44 are preferably provided on both surfaces of the negative electrode 12 so as to overlap in the thickness direction of the negative electrode 12.

[0027] 3, the inner winding end 42a of the negative electrode mixture layer 42 is a portion adjacent to the negative electrode exposed portion 44. On the other hand, the outer winding end 42b of the negative electrode mixture layer 42 is the same as the outer winding end of the negative electrode 12. The negative electrode mixture layer 42 exists continuously from the inner winding end 42a to the outer winding end 42b.

[0028] In this embodiment, the negative electrode lead 20 is joined to the inner peripheral surface of the negative electrode current collector 40 by, for example, ultrasonic welding. One end of the negative electrode lead 20 is disposed in the negative electrode exposed portion 44, and the other end extends downward from the lower end of the negative electrode exposed portion 44.

[0029] The position of the negative electrode lead 20 is not limited to the example shown in FIG. 3 , and the negative electrode lead 20 may be provided only at the outer end of the negative electrode 12. Alternatively, the negative electrode lead 20 may be provided at both the inner end and the outer end of the negative electrode 12. In this case, current collection performance is improved. By bringing the negative electrode exposed portion 44 at the outer end of the negative electrode 12 into contact with the inner circumferential surface of the outer package 15 (see FIG. 1 ), the outer end of the negative electrode 12 can be electrically connected to the outer package 15 without using the negative electrode lead 20 at the outer end of the negative electrode 12. The negative electrode 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.

[0030] The negative electrode active material contained in the negative electrode mixture layer 42 is not particularly limited as long as it can reversibly absorb and release lithium ions, and for example, carbon-based materials such as natural graphite and artificial graphite, metals that can be alloyed with lithium such as Si and Sn, or alloys or oxides containing these, can be used.

[0031] The negative electrode active material may contain a carbon-based material and a silicon-based material. Examples of the silicon-based material include Si, an alloy containing Si, and SiO x Examples of suitable silicon-based materials include silicon oxides such as silicon dioxide (x is 0.8 to 1.6). Silicon-based materials are negative electrode active materials that can improve battery capacity compared to carbon-based materials. The content of the silicon-based material in the negative electrode active material is preferably 3 mass% or more relative to the mass of the negative electrode active material from the viewpoints of improving battery capacity and suppressing deterioration of charge-discharge cycle characteristics. The upper limit of the silicon-based material content is, for example, 20 mass%. The average particle size (D50, volume-based median diameter) of the carbon-based material is, for example, 5 μm to 40 μm, and the D50 of the silicon-based material is, for example, 1 μm to 15 μm. D50 refers to the particle size at which the cumulative frequency of the smallest particle size in the volume-based particle size distribution is 50%, and is also called the median diameter. The particle size distributions of carbon-based and silicon-based materials can be measured using a laser diffraction particle size distribution analyzer (e.g., MT3000II, manufactured by Microtrack Bell Corporation) using water as a dispersion medium.

[0032] The solid electrolyte contained in the negative electrode mixture layer 42 is not particularly limited as long as it has Li ion conductivity, and may be an inorganic solid electrolyte or a polymer solid electrolyte. 12 (LLZ), Li 1.5 Al 0.5 Ge 1.5 P3O 12 (LAGP), Li5La3Ta2O 12 Examples of the polymer solid electrolyte include a polymer electrolyte in which an electrolyte salt such as LiPF6 is contained in polyethylene oxide (PEO).

[0033] The solid electrolyte is preferably an inorganic solid electrolyte from the viewpoint of stability, etc. The average particle size (D50, volume-based median size) of the inorganic solid electrolyte is, for example, 0.01 μm to 10 μm.

[0034] The solid electrolyte content in the negative electrode mixture layer 42 is, for example, 1% by mass to 10% by mass. Here, the solid electrolyte content is the percentage of the mass of the solid electrolyte relative to the mass of the negative electrode active material. As will be described later, the solid electrolyte content varies in the longitudinal direction of the negative electrode mixture layer 42.

[0035] Examples of the binder contained in the negative electrode mixture layer 42 include styrene butadiene rubber (SBR), nitrile butadiene rubber (NBR), carboxymethyl cellulose (CMC) or a salt thereof, polyacrylic acid (PAA) or a salt thereof (PAA-Na, PAA-K, etc., or a partially neutralized salt), polyvinyl alcohol (PVA), etc. The binder may also include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide-based resins, acrylic-based resins, polyolefin-based resins, etc. These may be used alone or in combination of two or more.

[0036] Next, with reference to (a) to (d) of FIG. 4, the change in the solid electrolyte content in the negative electrode mixture layer 42 in the longitudinal direction of FIG. 3 will be described. In FIG. 4(a), the solid electrolyte content at the inner end 42a is higher than that at the outer end 42b, and the solid electrolyte content decreases at a constant rate from the inner end 42a to the outer end 42b. When the electrode body 14 expands and contracts due to charging and discharging of the battery, the inner end 42a is subjected to greater stress than the outer end 42b, and therefore the electrolyte is less likely to penetrate therethrough than at the outer end 42b. By making the solid electrolyte content at the inner end 42a higher than that at the outer end 42b, it is possible to suppress unevenness in the reaction due to charging and discharging of the battery at the inner end 42a and the outer end 42b, thereby improving the charge and discharge cycle characteristics of the battery. The effect of the present disclosure is remarkable during high-rate charging and discharging, since non-uniformity of the electrolyte solution tends to occur between the inner winding end 42a and the outer winding end 42b.

[0037] The content of the solid electrolyte in the inner winding end 42a is preferably 1 mass % to 15 mass % relative to the mass of the negative electrode active material. This allows the battery capacity to be maintained while the battery is being subjected to charge / discharge cycles. characteristics can be improved.

[0038] Furthermore, as shown in FIG. 4( b), the slope indicating the rate of decrease in the solid electrolyte content from the inner end 42a to the outer end 42b does not have to be constant, and the slope may change along the way. In FIG. 4( c), the solid electrolyte content decreases from the inner end 42a to the outer end 42b, and the solid electrolyte content is constant between the inner end 42a and the outer end 42b. In FIG. 4( d), the solid electrolyte content decreases from the inner end 42a to the outer end 42b, and the solid electrolyte content is constant near the outer end 42b. Similarly, as long as the solid electrolyte content decreases from the inner end 42a to the outer end 42b, the solid electrolyte content may be constant near the inner end 42a. As shown in FIGS. 4( c) and 4(d), it is sufficient that at least a portion of the negative electrode mixture layer 42 has a region in which the solid electrolyte content continuously decreases from the inner end 42a to the outer end 42b. In this region, the solid electrolyte content preferably decreases linearly, but may decrease non-linearly, which allows the solid electrolyte content at the inner end 42a of the negative electrode mixture layer 42 to be higher than the solid electrolyte content at the outer end 42b.

[0039] Next, a method for forming the anode mixture layer 42 in which the solid electrolyte content varies from one end to the other of the inner end 42a and the outer end 42b will be described. To form such an anode mixture layer 42, a multilayer die coater is preferably used. By using the multilayer die coater, multiple anode mixture slurries with different solid electrolyte contents can be simultaneously applied to the anode current collector 40 while adjusting their mixing ratios. When applying the anode mixture slurry to the anode current collector 40, the anode current collector 40 moves relative to the multilayer die coater. Therefore, by applying multiple anode mixture slurries with different solid electrolyte contents to the anode current collector 40 while changing their mixing ratios at a predetermined timing, a region in which the solid electrolyte content varies from the inner end 42a to the outer end 42b can be formed at any position in the anode mixture layer 42. For example, a first anode mixture slurry containing a solid electrolyte and a second anode mixture slurry having a lower solid electrolyte content than the first anode mixture slurry are prepared. Next, using a multi-layer die coater, the first and second anode mixture slurries are applied from the inner end 42a to the outer end 42b of the anode current collector 40 while increasing the mixing ratio of the second anode mixture slurry to the first anode mixture slurry, thereby obtaining anode mixture layer 42 having the profile shown in FIG. 4(a).

[0040] Even when the negative electrode mixture layer 42 of the negative electrode 12 is divided into two or more parts by an exposed portion, as in the positive electrode 11 shown in FIG. 3 , it is sufficient that the solid electrolyte content at the inner end 42a is higher than the solid electrolyte content at the outer end 42b, and it is preferable that at least a portion of the negative electrode mixture layer 42 continuing from the inner end 42a has a region where the solid electrolyte content decreases from the inner end 42a toward the outer end 42b. [Example]

[0041] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0042] [Preparation of positive electrode] 95 parts by mass of LiNi 0.8 Co 0.15 Al 0.05 O2, 2.5 parts by mass of acetylene black (AB), and 2.5 parts by mass of polyvinylidene fluoride (PVdF) with an average molecular weight of 1.1 million were mixed, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode mixture slurry with a solids content of 70% by mass. Next, the positive electrode mixture slurry was applied to both sides of a strip-shaped positive electrode current collector made of aluminum foil, dried, rolled, and cut to the specified electrode plate size to produce a positive electrode with a positive electrode mixture layer formed on both sides of the positive electrode current collector. A positive electrode exposed portion, where no mixture layer was present and the current collector surface was exposed, was provided in the approximate center of the positive electrode's longitudinal direction, and an aluminum positive electrode lead was welded to the positive electrode exposed portion.

[0043] [Preparation of negative electrode] The negative electrode active material was graphite with an average particle size (D50) of 20 μm and SiO with a D50 of 5 μm. The solid electrolyte was Li7La3Zr2O with a D50 of 1 μm. 12 (LLZ) was used. 95 parts by mass of graphite, 5 parts by mass of SiO, 10 parts by mass of LLZ, 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. Also, 95 parts by mass of graphite, 5 parts by mass of SiO, 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 negative electrode mixture slurry and the second negative electrode mixture slurry were set in a multi-layer die coater, and coated on both sides of a strip-shaped negative electrode current collector made of copper foil from the inner end to the outer end of the winding while continuously changing the mixing ratio of the first negative electrode mixture slurry to the second negative electrode mixture slurry from 1:0 to 0:1, and then the coating was dried. The dried coating film was rolled using a roller and then cut into a predetermined electrode plate size, and a negative electrode mixture layer was formed on both sides of the negative electrode current collector. negative electrode An exposed negative electrode portion where there was no mixture layer and the current collector surface was exposed was provided at the inner end of the winding, and a nickel negative electrode lead was welded to the exposed negative electrode portion.

[0044] [Preparation of electrolyte] Ethylene carbonate (EC) and di Methyl Five parts by mass of vinylene carbonate (VC) was added to 100 parts by mass of a mixed solvent (EC:DMC=1:3 by volume) composed of vinylene carbonate (VC). LiPF was dissolved in the mixed solvent to a concentration of 1 mol / L to prepare an electrolyte.

[0045] [Secondary battery production] The positive and negative electrodes were wound with a polyethylene separator between them to prepare an electrode assembly. Insulating plates were placed on the top and bottom of the electrode assembly, and the electrode assembly was housed in a cylindrical exterior housing. The negative electrode lead was then welded to the bottom of the exterior housing, and the positive electrode lead was welded to a sealing member. After that, electrolyte was injected into the interior of the exterior housing using a reduced pressure method, and the open end of the exterior housing was sealed by crimping it to the sealing member via a gasket, thereby preparing a secondary battery. The capacity of the prepared secondary battery was 2500 mAh.

[0046] <Example 2> A secondary battery was fabricated in the same manner as in Example 1, except that in fabricating the negative electrode, the amount of LLZ contained in the first negative electrode mixture slurry was set to 6 parts by mass.

[0047] Example 3 A secondary battery was fabricated in the same manner as in Example 1, except that in fabricating the negative electrode, the amount of LLZ contained in the first negative electrode mixture slurry was set to 14 parts by mass.

[0048] Example 4 A secondary battery was fabricated in the same manner as in Example 1, except that in fabricating the negative electrode, the amount of LLZ contained in the first negative electrode mixture slurry was set to 18 parts by mass.

[0049] <Comparative Example 1> A secondary battery was produced in the same manner as in Example 1, except that in producing the negative electrode, the first negative electrode mixture slurry and the second negative electrode mixture slurry were not mixed, and only the second negative electrode mixture slurry was applied to both sides of the negative electrode current collector.

[0050] <Comparative Example 2> In producing the negative electrode, 95 parts by mass of graphite, 5 parts by mass of SiO, 5 parts by mass of LLZ, 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 third negative electrode mixture slurry. A secondary battery was produced in the same manner as in Example 1, except that only the third negative electrode mixture slurry was applied to both sides of the negative electrode current collector.

[0051] <Comparative Example 3> A secondary battery was fabricated in the same manner as in Example 1, except that in fabricating the negative electrode, the first negative electrode mixture slurry and the second negative electrode mixture slurry were applied from the inner end to the outer end of the negative electrode current collector while the mixing ratio was continuously changed from 0:1 to 1:0.

[0052] <Comparative Example 4> A secondary battery was fabricated in the same manner as in Comparative Example 2, except that in fabricating the negative electrode, the amount of LLZ contained in the third negative electrode mixture slurry was set to 3 parts by mass.

[0053] <Comparative Example 5> A secondary battery was fabricated in the same manner as in Comparative Example 2, except that in fabricating the negative electrode, the amount of LLZ contained in the third negative electrode mixture slurry was set to 7 parts by mass.

[0054] <Comparative Example 6> A secondary battery was fabricated in the same manner as in Comparative Example 2, except that in fabricating the negative electrode, the amount of LLZ contained in the third negative electrode mixture slurry was set to 9 parts by mass.

[0055] [Capacity retention rate evaluation] The nonaqueous electrolyte secondary batteries of the Examples and Comparative Examples were charged to 4.2 V at a constant current of 1 C at an ambient temperature of 25°C, and then charged at a constant voltage of 4.2 V until the current reached 0.05 C. After leaving the batteries for 20 minutes, they were discharged to 2.5 V at a constant current of 0.5 C. This cycle of charge and discharge was repeated 300 times. The capacity retention rate of the nonaqueous electrolyte secondary batteries of the Examples and Comparative Examples during the charge and discharge cycles was calculated using the following formula: Capacity retention rate = (discharge capacity at 300th cycle / discharge capacity at 1st cycle) x 100

[0056] The evaluation results of the capacity retention rates of the nonaqueous electrolyte secondary batteries of the Examples and Comparative Examples are summarized in Table 1. Table 1 also shows the solid electrolyte content at the inner end and outer end of the winding, and the solid electrolyte content in the negative electrode mixture layer (average content in the entire negative electrode mixture layer).

[0057] [Table 1]

[0058] The batteries of the Examples have improved capacity retention rates compared to the battery of Comparative Example 1, which does not contain a solid electrolyte. The batteries of the Examples also have improved capacity retention rates compared to the batteries of Comparative Examples 2 and 4 to 6, which contain a solid electrolyte uniformly throughout the negative electrode mixture layer. Furthermore, the batteries of the Examples have improved capacity retention rates compared to the battery of Comparative Example 3, which contains a high solid electrolyte content at the outer end of the negative electrode mixture layer. The results shown in Table 1 demonstrate that a specific arrangement of the solid electrolyte significantly improves the capacity retention rate. [Explanation of symbols]

[0059] 10 Secondary battery, 11 positive electrode, 12 negative electrode, 13 separator, 14 electrode body, 15 exterior body, 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 positive electrode exposed portion, 40 negative electrode current collector, 42 negative electrode mixture layer, 42a inner end of winding, 42b outer end of winding, 44 negative electrode exposed portion

Claims

1. A nonaqueous electrolyte secondary battery comprising: an electrode assembly in which a strip-shaped positive electrode and a strip-shaped negative electrode are wound with a separator interposed therebetween; an electrolytic solution; and an exterior body that accommodates the electrode assembly and the electrolytic solution, the negative electrode includes a negative electrode current collector and a negative electrode mixture layer formed on a surface of the negative electrode current collector and including a negative electrode active material and a solid electrolyte; the negative electrode mixture layer has a region in which the solid electrolyte content at an inner end is higher than the solid electrolyte content at an outer end, and the solid electrolyte content continuously decreases from the inner end to the outer end.

2. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the content of the solid electrolyte in the negative electrode mixture layer is 1% by mass or more and 10% by mass or less.

3. 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the solid electrolyte is an inorganic solid electrolyte.

4. the negative electrode active material includes a carbon-based material and a silicon-based material, 4. The nonaqueous electrolyte secondary battery according to claim 1, wherein the content of the silicon-based material in the negative electrode active material is 3 mass % or more with respect to the mass of the negative electrode active material.

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