Non-aqueous electrolyte secondary battery

Abstract

A negative electrode (12) is characterized by comprising: a negative electrode current collector (40) and a negative electrode mixture layer (42) disposed on the surface of the negative electrode current collector (40); wherein the negative electrode mixture layer (42) contains a carbon material and a silicon-containing material as negative electrode active materials, and has a region in which, from a winding inner end (42X) side to a winding outer end (42Y) side, the content percentage of the silicon-containing material with respect to the total mass of the negative electrode active materials continuously increases, and the thickness of the negative electrode mixture layer (42) continuously decreases.

Description

Nonaqueous electrolyte secondary battery

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

[0002] Non-aqueous electrolyte secondary batteries are widely used as high-energy-density secondary batteries. The negative electrode of a non-aqueous electrolyte secondary battery consists of a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector. Generally, the negative electrode active material contained in the negative electrode mixture layer is a carbon material such as graphite or a silicon-containing material containing Si. Patent Document 1 discloses a negative electrode in which the content of silicon-containing material relative to the total mass of the negative electrode active material differs in the thickness direction of the negative electrode mixture layer.

[0003] International Publication No. 2019 / 230298

[0004] In recent years, driven by the increasing popularity of electric vehicles, there has been a growing demand for higher capacity non-aqueous electrolyte secondary batteries. Silicon-containing materials can absorb more lithium ions per unit mass compared to carbon materials such as graphite. Therefore, increasing the content of silicon-containing materials in the negative electrode mixture layer can increase the capacity of non-aqueous electrolyte secondary batteries.

[0005] On the other hand, silicon-containing materials exhibit greater volume changes (expansion and contraction) due to lithium ion absorption compared to carbon materials. Therefore, increasing the silicon-containing material content throughout the negative electrode mixture layer can lead to an excessive increase in surface pressure on the inner side of the electrode body, where the radius of curvature is small. This can worsen the circulation of the non-aqueous electrolyte on the inner side of the electrode body, resulting in a deterioration of cycle characteristics.

[0006] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure is a non-aqueous electrolyte secondary battery comprising an electrode body in which a positive electrode and a negative electrode are wound longitudinally with a separator between them, wherein the negative electrode comprises a negative electrode current collector and a negative electrode mixture layer disposed on the surface of the negative electrode current collector, and the negative electrode mixture layer comprises a carbon material and a silicon-containing material as negative electrode active material, and has a region in which the content of the silicon-containing material relative to the total mass of the negative electrode active material continuously increases from the inner end to the outer end, and the thickness of the negative electrode mixture layer continuously decreases.

[0007] According to a non-aqueous electrolyte secondary battery, which is one aspect of this disclosure, it is possible to achieve high capacity while improving cycle characteristics.

[0008] This is an axial cross-sectional view of a non-aqueous electrolyte secondary battery, which is one example of an embodiment. This is a perspective view of the electrode body of a non-aqueous electrolyte secondary battery, which is one example of an embodiment. This is a cross-sectional view of a negative electrode, which is one example of an embodiment. This is a cross-sectional view of a negative electrode, which is another example of an embodiment. This is a cross-sectional view of a negative electrode, which is another example of an embodiment. This is a cross-sectional view of a negative electrode, which is another example of an embodiment. This is a cross-sectional view of the negative electrode of Comparative Example 1.

[0009] In the following, 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. In the following description, specific shapes, materials, numerical values, directions, etc., are examples to facilitate understanding of the present invention and can be appropriately modified according to the specifications of the non-aqueous electrolyte secondary battery. Furthermore, if the following description includes multiple embodiments and modifications, it is intended from the outset that their characteristic parts may be used in appropriate combinations.

[0010] Figure 1 is an axial cross-sectional view 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 (not shown), and an outer casing 16 that houses the electrode body 14 and the non-aqueous electrolyte.

[0011] As will be described in more detail later, the electrode body 14 has a positive electrode 11, a negative electrode 12, and a separator 13, and has a structure in which the positive electrode 11 and the negative electrode 12 are wound in a spiral shape via the separator 13. 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.

[0012] Non-aqueous electrolytes are lithium ion conductive. A non-aqueous electrolyte comprises 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. Examples of non-aqueous solvents include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixtures thereof. Non-aqueous solvents may also contain halogen-substituted compounds (e.g., fluoroethylene carbonate) in which at least some of the hydrogen atoms in these solvents are replaced with halogen atoms such as fluorine. Examples of electrolyte salts include LiPF4. ６ Lithium salts such as these are used.

[0013] The outer casing 16 is a bottomed cylindrical metal container with one end open in the axial direction, and the opening of the outer casing 16 is sealed by a sealing body 17. For the sake of explanation, the side of the non-aqueous electrolyte secondary battery 10 with the sealing body 17 will be referred to as "upper," and the bottom side of the outer casing 16 will be referred to as "lower."

[0014] 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 through a through-hole in the insulating plate 19 towards the bottom of the outer casing 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 bottom inner surface of the outer casing 16 by welding or the like, and the outer casing 16 becomes the negative electrode terminal.

[0015] A gasket 28 is provided between the outer casing 16 and the sealing body 17 to ensure airtightness inside the battery. The outer casing 16 has a grooved portion 22 formed on its side surface, 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.

[0016] 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 stacked in order from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each component except the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, with the insulating member 25 interposed between their respective peripheries. When the internal pressure of the battery rises due to abnormal heat generation, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 towards the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure rises further, the upper valve body 26 breaks, and gas is discharged from the vent hole in the cap 27.

[0017] Next, the electrode body 14 will be described in detail with further reference to Figure 2. Figure 2 is a perspective view of the electrode body 14, showing a portion of the outer end unfolded.

[0018] As shown in Figures 1 and 2, the electrode body 14 has a structure in which a strip-shaped positive electrode 11 and a strip-shaped negative electrode 12 are wound along the longitudinal direction via a separator 13. The positive electrode 11, the negative electrode 12, and the separator 13 are wound in a spiral shape and are alternately stacked in the radial direction of the electrode body 14. In the electrode body 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.

[0019] The positive electrode 11 comprises a positive electrode current collector 30 and a positive electrode mixture layer 32 disposed on the positive electrode current collector 30. The positive electrode current collector 30 can be made of a metal foil that is stable within the potential range of the positive electrode 11, such as aluminum or an aluminum alloy, or a film with the metal disposed on its surface. The positive electrode mixture layer 32 contains a positive electrode active material, a conductive agent, and a binder. The positive electrode 11 can be manufactured, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder onto the positive electrode current collector 30, drying the coating, and then compressing it to form the positive electrode mixture layer 32 on both sides of the positive electrode current collector 30.

[0020] The positive electrode composite layer 32 contains particulate lithium-containing composite oxide as the positive electrode active material. The lithium-containing composite oxide is a composite oxide containing metal elements such as Co, Mn, Ni, and Al in addition to Li. The metal elements constituting the lithium-containing composite oxide are, for example, at least one selected from Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Y, Zr, Sn, Sb, W, Pb, and Bi. Among these, it is preferable to contain at least one selected from Co, Ni, Al, and Mn. Examples of suitable composite oxides include lithium-containing composite oxides containing Ni, Co, and Mn, and lithium-containing composite oxides containing Ni, Co, and Al.

[0021] Examples of conductive agents included in the positive electrode mixture layer 32 include carbon black such as acetylene black and Ketjenblack, graphite, carbon nanotubes (CNTs), carbon nanofibers, and graphene. Examples of binders included in the positive electrode mixture layer 32 include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resins, and polyolefins. In addition, these resins may be used in combination with carboxymethylcellulose (CMC) or its salts, polyethylene oxide (PEO), etc.

[0022] As will be described in more detail later, the negative electrode 12 has a negative electrode current collector 40 and a negative electrode mixture layer 42 disposed on the negative electrode current collector 40. The negative electrode current collector 40 can be made of a metal foil that is stable in the potential range of the negative electrode 12, such as copper or a copper alloy, or a film with the metal disposed on its surface. The negative electrode mixture layer 42 contains a negative electrode active material and a binder. An exposed current collector portion is formed on the inner end side of the negative electrode 12 where the negative electrode current collector 40 is exposed, and the negative electrode lead 21 is connected to this exposed current collector portion.

[0023] The separator 13 is made of a porous sheet having ion permeability and insulating properties. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator 13 include polyethylene, polyolefins such as polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. Furthermore, a heat-resistant resin layer, such as aramid resin, may be formed on the surface of the separator 13.

[0024] In this embodiment, the negative electrode compound layer 42 is not formed on the outermost periphery of the electrode body 14, and the negative electrode current collector 40 is exposed, with an exposed current collector portion that contacts the inner surface of the outer casing 16. By providing a negative electrode lead 21 and having the exposed current collector portion contact the inner surface of the outer casing 16, the internal resistance of the battery can be further reduced. A separator 13 may also be placed on the outermost periphery of the electrode body 14.

[0025] Next, with reference to Figure 3, the negative electrode 12 that constitutes the electrode body 14 will be described in detail. Figure 3 is a schematic diagram showing a cross-section of the negative electrode 12.

[0026] As shown in Figure 3, the negative electrode 12 has a negative electrode current collector 40 and a negative electrode mixture layer 42 disposed on the surface of the negative electrode current collector 40. Preferably, the negative electrode mixture layer 42 is formed on both sides of the negative electrode current collector 40. As described above, the negative electrode current collector 40 can be made of a metal foil that is stable in the potential range of the negative electrode 12, such as copper or a copper alloy, or a film with the metal disposed on its surface. The thickness of the negative electrode current collector 40 is, for example, 5 μm or more and 30 μm or less.

[0027] The negative electrode mixture layer 42 contains a negative electrode active material and a binder. The negative electrode 12 can be manufactured, for example, by applying a negative electrode mixture slurry containing the negative electrode active material and binder to the surface of the negative electrode current collector 40, drying the coating, and then rolling it to form the negative electrode mixture layer 42 on the surface of the negative electrode current collector 40.

[0028] The negative electrode mixture layer 42 contains a carbon material and a silicon-containing material as the negative electrode active material. The carbon material included in the negative electrode mixture layer 42 is, for example, at least one selected from the group consisting of natural graphite, artificial graphite, soft carbon, and hard carbon. In particular, it is preferable to use artificial graphite such as bulk artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB), natural graphite such as flake graphite, bulk graphite, and earthy graphite, or a mixture thereof as the carbon material. The volume-based D50 of the carbon material is, for example, 1 μm or more and 30 μm or less, preferably 5 μm or more and 25 μm or less.

[0029] The silicon-containing material included in the negative electrode mixture layer 42 can be any material containing silicon, and examples include silicon alloys, silicon compounds, and silicon-containing composite materials. Among these, silicon-containing composite materials are preferred. The D50 of composite materials is generally smaller than that of carbon materials. For example, the D50 of composite materials is 1 μm or more and 15 μm or less. One type of silicon-containing material may be used alone, or two or more types may be used in combination. Silicon-containing materials can absorb more lithium ions per unit mass compared to carbon materials such as graphite. Therefore, by using a silicon-containing material as the negative electrode active material, it is possible to achieve a higher battery capacity.

[0030] A suitable silicon-containing material is a composite particle comprising an ionic conductive phase and a silicon phase dispersed within the ionic conductive phase. The silicon phase is formed by the dispersion of silicon elements in fine particulate form. The composite particle may also have a conductive layer covering a portion of the surface of the ionic conductive phase. The conductive layer is composed of a material with higher conductivity than the ionic conductive phase and forms a good conductive path in the negative electrode mixture layer 42. The conductive layer, for example, contains conductive carbon and covers an area of ​​30% to 90% of the surface of the ionic conductive phase. The coverage rate of the conductive layer can be calculated, for example, using X-ray photoelectron spectroscopy (XPS).

[0031] The ionic conductive phase is a continuous phase composed of an aggregate of particles finer than those in the silicon phase described above. The ionic conductive phase is, for example, at least one selected from the group consisting of silicate phase, carbon phase, silicide phase, and silicon oxide phase.

[0032] The silicate phase preferably contains at least one element selected from lithium, sodium, potassium, rubidium, cesium, francium, beryllium, magnesium, calcium, strontium, barium, and radium, for example, due to its high lithium ion conductivity. Among these, a silicate phase containing lithium (hereinafter sometimes referred to as the lithium silicate phase) is preferred due to its high lithium ion conductivity.

[0033] The lithium silicate phase is, for example, given by formula: Li ２ｚ SiO ２＋ｚ This is expressed as (0 < z < 2). From the viewpoint of stability, ease of fabrication, lithium-ion conductivity, etc., it is preferable that z satisfies the relationship 0 < z < 1, and more preferably z = 1 / 2.

[0034] Another example of a suitable silicon-containing material is one having a sea-island structure in which a fine silicon phase is dispersed substantially uniformly within an amorphous silicon oxide phase, and the overall general formula is SiO ｘ This is a composite particle represented by (0 < x ≤ 2). The main component of silicon oxide may be silicon dioxide. The ratio of oxygen to silicon (x) is, for example, 0.5 ≤ x < 2.0, and preferably 0.8 ≤ x ≤ 1.5.

[0035] Another example of a suitable silicon-containing material is a composite particle having a sea-island structure in which fine silicon phase is dispersed substantially uniformly within a carbon phase. The carbon phase is preferably an amorphous carbon phase. The carbon phase may contain crystalline phase components, but it is preferable that the amorphous phase components are more abundant. The amorphous carbon phase is composed of a carbon material in which, for example, the average interplanar spacing of (002) planes, as measured by X-ray diffraction, exceeds 0.34 nm. The amorphous carbon phase is composed of, for example, amorphous carbon particles. When the silicon-containing material is a composite particle having a sea-island structure in which fine silicon phase is dispersed substantially uniformly within a carbon phase, the total mass of silicon elements contained in the silicon-containing material may be equal to the mass of the silicon phase.

[0036] The ratio of the mass of the silicon phase to the total mass of the silicon-containing material is, for example, 30% by mass or more. In this case, the discharge capacity increases, and the battery tends to have high output. Alternatively, the ratio of the mass of the silicon phase to the total mass of the silicon-containing material is, for example, 60% by mass or less. In this case, the volume change of the negative electrode mixture layer 42 during charging and discharging can be reduced, the expansion and contraction of the electrode body 14 during charging and discharging can be suppressed, and the battery tends to have excellent durability. Therefore, the ratio of the mass of the silicon phase to the total mass of the silicon-containing material is, for example, 30% by mass or more and 60% by mass or less.

[0037] The crystallite size of the silicon phase constituting the above composite particles is, for example, between 10 nm and 30 nm. The crystallite size of the silicon phase is calculated from the full width at half maximum of the analytical peaks attributed to the Si(111) plane in the X-ray diffraction pattern of the silicon phase using Scherrer's formula.

[0038] As shown in Figure 3, the negative electrode mixture layer 42 of this embodiment includes a first negative electrode mixture layer 44 and a second negative electrode mixture layer 46, each having a different content of silicon-containing material relative to the total mass of the negative electrode active material. Specifically, the content of silicon-containing material in the first negative electrode mixture layer 44 is smaller than the content of silicon-containing material in the second negative electrode mixture layer 46.

[0039] The negative electrode mixture layer 42 has a first region 42A in the vicinity of the inner end 42X of the winding of the negative electrode mixture layer 42, where only the first negative electrode mixture layer 44 is disposed on the negative electrode current collector 40. Further, the negative electrode mixture layer 42 has a second region 42B in the vicinity of the outer end 42Y of the winding of the negative electrode mixture layer 42, where only the second negative electrode mixture layer 46 is disposed on the negative electrode current collector 40. Further, the negative electrode mixture layer 42 has a third region 42C between the first region 42A and the second region 42B in the longitudinal direction of the negative electrode 12, where the second negative electrode mixture layer 46 is disposed on the first negative electrode mixture layer 44.

[0040] Here, in the third region 42C, from the inner end 42X side to the outer end 42Y side of the winding of the negative electrode mixture layer 42, the ratio of the thickness of the second negative electrode mixture layer 46 to the thickness of the first negative electrode mixture layer 44 continuously increases, and the total thickness of the negative electrode mixture layer 42 continuously decreases. As described above, the content rate of the silicon-containing material in the first negative electrode mixture layer 44 is smaller than the content rate of the silicon-containing material in the second negative electrode mixture layer 46. Therefore, in the third region 42C, from the inner end 42X side to the outer end 42Y side of the winding of the negative electrode mixture layer 42, the content rate of the silicon-containing material continuously increases, and the total thickness of the negative electrode mixture layer 42 continuously decreases.

[0041] Although the silicon-containing material can occlude more lithium ions per unit mass than carbon materials such as graphite, the volume change during charge and discharge is larger than that of carbon materials. As a result of the study by the present inventors, in the wound electrode body 14, when the content rate of the silicon-containing material is increased over the entire longitudinal direction, when the battery is charged and discharged, the pressure load due to the expansion and contraction of the negative electrode mixture layer 42 becomes larger on the inner end 42X side of the negative electrode mixture layer 42, and the surface pressure on the inner side of the winding of the electrode body 14 may excessively increase. As a result, the non-aqueous electrolyte is likely to be extruded on the inner side of the winding of the electrode body 14, the liquid circulation of the non-aqueous electrolyte deteriorates, and reaction unevenness of the charge and discharge reaction is likely to occur in the electrode body 14. As a result, for example, the cycle characteristics tend to deteriorate.

[0042] As in this embodiment, by providing a region where the content rate of the silicon-containing material continuously increases from the inner end 42X side to the outer end 42Y side of the negative electrode mixture layer 42 and the total thickness of the negative electrode mixture layer 42 continuously decreases, the pressure load due to the expansion and contraction of the negative electrode mixture layer 42 on the inner end 42X side of the negative electrode mixture layer 42 can be reduced. As a result, the non-aqueous electrolyte is less likely to be extruded on the inner side of the electrode body 14, the liquid circulation of the non-aqueous electrolyte is improved, and the cycle characteristics are improved.

[0043] Further, by increasing the content rate of the silicon-containing material as it goes toward the outer end 42Y side of the negative electrode mixture layer 42, the amount of the silicon-containing material in the entire negative electrode mixture layer 42 can be ensured, and the high capacity of the non-aqueous electrolyte secondary battery 10 can be realized. That is, according to the configuration of this embodiment, while realizing the high capacity of the non-aqueous electrolyte secondary battery 10, the cycle characteristics can be improved. Further, by continuously reducing the total thickness of the negative electrode mixture layer 42 from the outer end 42Y side of the negative electrode mixture layer 42, the number of windings of the electrode body 14 can be increased, the energy density is increased, and the high capacity of the non-aqueous electrolyte secondary battery 10 can be realized. Also, as described above, the silicon-containing material has a larger lithium ion occlusion amount per unit mass than the carbon material. Therefore, by continuously reducing the total thickness of the negative electrode mixture layer 42 from the outer end 42Y side of the negative electrode mixture layer 42, the uniformity of the in-plane current density distribution is achieved, and the reaction unevenness of the charge-discharge reaction is less likely to occur in the electrode body 14. As a result, the cycle characteristics can be improved.

[0044] In this embodiment, the thicknesses of the negative electrode mixture layer 42 in the first region 42A and the second region 42B are substantially uniform, respectively. The ratio (T42B / T42A) of the thickness (T42B) of the negative electrode mixture layer 42 (second negative electrode mixture layer 46) in the second region 42B to the thickness (T42A) of the negative electrode mixture layer 42 (first negative electrode mixture layer 44) in the first region 42A is, for example, 0.1 or more and 0.8 or less, and may be 0.2 or more and 0.6 or less. The ratio (T42B / T42A) can be set based on the discharge capacities of the first negative electrode mixture layer 44 and the second negative electrode mixture layer 46, and is preferably set so that the in-plane discharge capacity distribution is uniform.

[0045] The thickness of the negative electrode mixture layer 42 (first negative electrode mixture layer 44) in the first region 42A is, for example, 100 μm or more and 300 μm or less on one side of the negative electrode current collector 40. The thickness of the negative electrode mixture layer 42 (second negative electrode mixture layer 46) in the second region 42B is, for example, 30 μm or more and 250 μm or less on one side of the negative electrode current collector 40.

[0046] The length of the first region 42A in the longitudinal direction of the negative electrode 12 may be, for example, 1% or more and 20% or less of the length of the negative electrode 12, or 3% or more and 15% or less. The length of the first region 42A in the longitudinal direction of the negative electrode 12 may be, for example, 0.1m or more and 1m or less. The length of the second region 42B in the longitudinal direction of the negative electrode 12 may be, for example, 1% or more and 20% or less of the length of the negative electrode 12, or 3% or more and 15% or less, similar to the length of the first region 42A. The length of the second region 42B in the longitudinal direction of the negative electrode 12 may be, for example, 0.1m or more and 1m or less. The lengths of the first region 42A and the second region 42B in the longitudinal direction of the negative electrode 12 may be the same or different from each other.

[0047] In this embodiment, the content of silicon-containing material relative to the total mass of the negative electrode active material in the first region 42A and the second region 42B is substantially uniform. The content of silicon-containing material in the negative electrode mixture layer 42 (first negative electrode mixture layer 44) at the inner end 42X of the negative electrode mixture layer 42 is, for example, 95% or less, preferably 90% or less, and more preferably 85% or less, of the content of silicon-containing material in the negative electrode mixture layer 42 (second negative electrode mixture layer 46) at the outer end 42Y of the negative electrode mixture layer 42. In this case, the liquid flow of the non-aqueous electrolyte on the inner side of the electrode body 14 can be further improved.

[0048] The silicon-containing material content of the negative electrode mixture layer 42 (first negative electrode mixture layer 44) in the first region 42A is, for example, 0% by mass or more and 10% by mass or less. In other words, the first negative electrode mixture layer 44 may contain only carbon material and substantially no silicon-containing material as the negative electrode active material. When the first negative electrode mixture layer 44 contains only carbon material as the negative electrode active material, the circulation of the non-aqueous electrolyte on the inside of the electrode body 14 can be further improved. Furthermore, the silicon-containing material content of the negative electrode mixture layer 42 (second negative electrode mixture layer 46) in the second region 42B is, for example, 5% by mass or more and 100% by mass or less, and may also be 10% by mass or more and 80% by mass or less. In other words, the second negative electrode mixture layer 46 may contain only silicon-containing material and substantially no carbon material as the negative electrode active material.

[0049] The negative electrode mixture layer 42 contains a binder in addition to the negative electrode active material. The binder content is preferably 0.5% by mass or more and 10% by mass or less, and more preferably 1% by mass or more and 8% by mass or less, relative to the mass of the negative electrode mixture layer 42. The binder content may be the same in the first negative electrode mixture layer 44 and the second negative electrode mixture layer 46, or it may be different from that of the first negative electrode mixture layer 44.

[0050] The binder contained in the negative electrode mixture layer 42 may be fluororesin, PAN, polyimide, acrylic resin, polyolefin, etc., as in the case of the positive electrode 11, but styrene-butadiene rubber (SBR) is preferred. The negative electrode mixture layer 42 may further contain CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), etc. It is preferable to use SBR and CMC or a salt thereof in combination as the binder in the negative electrode mixture layer 42.

[0051] The negative electrode mixture layer 42 may contain materials other than the negative electrode active material and binder, such as conductive agents and thickeners. Examples of conductive agents included in the negative electrode mixture layer 42 include carbon black such as acetylene black and Ketjen black, graphite, carbon nanotubes (CNTs), carbon nanofibers, and graphene. The conductive agent may be included in only one of the first negative electrode mixture layer 44 or the second negative electrode mixture layer 46. The content of the conductive agent is, for example, 0.1% by mass or more and 5% by mass or less, relative to the mass of the negative electrode mixture layer 42. The content of the conductive agent may be the same in the first negative electrode mixture layer 44 and the second negative electrode mixture layer 46, or they may be different.

[0052] Next, an example of a method for manufacturing the negative electrode 12 having the negative electrode mixture layer 42 of this embodiment will be described. However, the method for manufacturing the negative electrode 12 is not limited to the method described below.

[0053] The method for manufacturing the negative electrode 12 comprises a preparation step of preparing a first negative electrode mixture slurry and a second negative electrode mixture slurry, each having a different content of silicon-containing material relative to the total mass of the negative electrode active material, and a coating step of applying the first negative electrode mixture slurry and the second negative electrode mixture slurry to the surface of the negative electrode current collector 40.

[0054] The content of silicon-containing material relative to the total mass of the anode active material in the first anode mixture slurry is, for example, 0% by mass or more and 15% by mass or less. Similarly, the content of silicon-containing material relative to the total mass of the anode active material in the second anode mixture slurry is, for example, 5% by mass or more and 100% by mass or less.

[0055] In the coating process, for example, using a coating device having two discharge ports, the first negative electrode mixture slurry and the second negative electrode mixture slurry are simultaneously applied to the surface of the negative electrode current collector 40, in the order of first negative electrode mixture slurry and second negative electrode mixture slurry from the surface side of the negative electrode current collector 40. Specifically, at the inner end of the winding of the electrode plate, only the first negative electrode mixture slurry is applied to the surface of the negative electrode current collector 40. Then, as you move from the inner side of the winding of the electrode plate to the outer side, the discharge amount of the first negative electrode mixture slurry is decreased, and the discharge amount of the second negative electrode mixture slurry is continuously increased. By changing the discharge amounts of the first negative electrode mixture slurry and the second negative electrode mixture slurry, the thickness ratio of the first negative electrode mixture layer 44 and the second negative electrode mixture layer 46 can be changed. Furthermore, at this time, the total discharge amount of the first negative electrode mixture slurry and the second negative electrode mixture slurry is continuously reduced as you move from the inside to the outside of the electrode plate winding. This allows the thickness of the negative electrode mixture layer 42 to be continuously reduced as you move from the inside to the outside of the electrode plate winding. Then, at the outside end of the electrode plate winding, only the second negative electrode mixture slurry is applied to the surface of the negative electrode current collector 40. Finally, the negative electrode 12 of this embodiment can be manufactured by drying the coating film produced in the coating process and rolling it.

[0056] Next, a modified example of the negative electrode 12 will be described with reference to Figures 4 to 6. Figures 4 to 6 schematically show cross-sections of the modified negative electrode 12.

[0057] The negative electrode 12 shown in Figure 4 differs from the negative electrode 12 shown in Figure 3 in that the negative electrode mixture layer 42 does not have a first region 42A composed only of the first negative electrode mixture layer 44, and a second region 42B composed only of the second negative electrode mixture layer 46. In other words, in the negative electrode 12 shown in Figure 4, the silicon-containing material content of the negative electrode mixture layer 42 increases continuously from the inner end 42X to the outer end 42Y of the negative electrode mixture layer 42, and the total thickness of the negative electrode mixture layer 42 decreases continuously.

[0058] The negative electrode 12 shown in Figure 5 differs from the negative electrode 12 shown in Figure 4 in that the first negative electrode 14 is arranged on top of the second negative electrode 14. Even in this case, the pressure load due to the expansion and contraction of the negative electrode 14 on the winding end 42X side of the negative electrode 14 can be reduced. As a result, the non-aqueous electrolyte is less likely to be pushed out on the winding side of the electrode body 14, improving the circulation of the non-aqueous electrolyte and improving the cycle characteristics.

[0059] Furthermore, as shown in Figures 3 and 4, by placing a second negative electrode mixture layer 46 with a high silicon content on the surface of the first negative electrode mixture layer 44, lithium deposition on the surface of the negative electrode 12 due to a decrease in negative electrode potential can be suppressed, thereby further improving cycle characteristics. In addition, silicon-containing materials are less prone to crushing than carbon materials during the rolling process when manufacturing the negative electrode 12. Therefore, by placing the second negative electrode mixture layer 46 on the surface of the first negative electrode mixture layer 44, voids are more easily formed on the surface of the negative electrode 12. As a result, non-aqueous electrolytes can more easily penetrate into the interior of the negative electrode 12, making it easier to achieve higher capacity and improved cycle characteristics.

[0060] The negative electrode 12 shown in Figure 6 differs from the negative electrode 12 shown in Figures 3 to 5 in that both the first negative electrode 44 and the second negative electrode 46 are arranged at the inner end 42X and the outer end 42Y of the negative electrode 42. In the negative electrode 12 shown in Figure 6, the silicon-containing material content increases continuously from the inner end 42X to the outer end 42Y of the negative electrode 42, and the total thickness of the negative electrode 42 decreases continuously. Even in this case, the pressure load due to the expansion and contraction of the negative electrode 42 at the inner end 42X of the negative electrode 42 can be reduced. As a result, the non-aqueous electrolyte is less likely to be pushed out on the inner side of the electrode body 14, improving the flow of the non-aqueous electrolyte and improving the cycle characteristics.

[0061] In the negative electrode 12 shown in Figures 3 to 6, the negative electrode mixture layer 42 all have a first negative electrode mixture layer 44 and a second negative electrode mixture layer 46, but the negative electrode mixture layer 42 may also have a single-layer structure. A negative electrode 12 having a single-layer structure can be manufactured, for example, by applying a mixture of the first negative electrode mixture slurry and the second negative electrode mixture slurry, mixed while changing the mixing ratio, to the surface of the negative electrode current collector 40.

[0062] The present disclosure will be further illustrated by the following examples, but the present disclosure is not limited to these examples.

[0063] <Example 1> [Fabrication of the positive electrode] As the positive electrode active material, LiNi ０．８ Mn ０．１ Co ０．１ O ２ A lithium transition metal composite oxide represented by [formula] was used. The above positive electrode active material, acetylene black, carbon nanotubes, and polyvinylidene fluoride were mixed in a mass ratio of 100:0.75:0.4:0.9 to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied to both sides of an aluminum foil, which was to be used as the positive electrode current collector, and the coating was dried. Then, the coating was rolled using a roller, cut to a predetermined electrode size, and a positive electrode was fabricated in which positive electrode mixture layers were formed on both sides of the positive electrode current collector.

[0064] [Preparation of the first negative electrode mixture slurry] Graphite, a carbon material, was used as the negative electrode active material. The above negative electrode active material, styrene-butadiene copolymer, and carboxymethylcellulose were mixed in a mass ratio of 100:1:1.5, and an appropriate amount of water was added to prepare the first negative electrode mixture slurry that forms the first negative electrode mixture layer.

[0065] [Preparation of the second negative electrode mixture slurry] As the negative electrode active material, a mixture of silicon-containing material SiO and carbon material graphite in a mass ratio of 10:90 was used. The above negative electrode active material, carbon nanotubes, styrene-butadiene copolymer, and carboxymethylcellulose were mixed in a mass ratio of 100:0.5:1:1.5, and an appropriate amount of water was added to prepare the second negative electrode mixture slurry that forms the second negative electrode mixture layer.

[0066] [Preparation of Negative Electrode] Using a coating device having two discharge ports, the first negative electrode mixture slurry and the second negative electrode mixture slurry were simultaneously applied to both sides of the negative electrode current collector so as to be applied in this order from the surface side of the copper foil serving as the negative electrode current collector, and the coating film was dried. At this time, as shown in FIG. 3, a first region where only the first negative electrode mixture slurry was applied and a second region where only the second negative electrode mixture slurry was applied were provided on both end sides in the longitudinal direction of the negative electrode. Then, after rolling the coating film using a roller, it was cut into a predetermined electrode size to produce a negative electrode in which a negative electrode mixture layer was formed on both sides of the negative electrode current collector.

[0067] Note that the thicknesses of the negative electrode mixture layers at the inner winding end (first region) and the outer winding end (second region) of the negative electrode were set to 250 μm and 120 μm, respectively. Also, the lengths of the first region and the second region in the longitudinal direction of the negative electrode were each set to about 6% of the total length of the negative electrode. Further, as shown in FIG. 3, in the third region sandwiched between the first region and the second region, the negative electrode mixture layer was produced such that the content rate of the silicon-containing material continuously increased and the thickness of the negative electrode mixture layer continuously decreased from the inner winding end side to the outer winding end side.

[0068] [Preparation of Non-aqueous Electrolyte (Electrolytic Solution)] An electrolytic solution was prepared by adding LiPF ６ (lithium salt). The concentration of LiPF ６ in the electrolytic solution was set to 1.0 mol / L. As the non-aqueous solvent, a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of EC:EMC = 3:7 was used.

[0069] [Fabrication of Test Cell (Non-Aqueous Electrolyte Secondary Battery)] An aluminum lead was attached to a portion of the positive electrode and a nickel lead to a portion of the negative electrode. The positive and negative electrodes were then wound in a spiral shape via a polyolefin separator to create a wound electrode body. Insulating plates were placed above and below the electrode body, and the electrode body was housed in a bottomed cylindrical outer casing. The negative electrode lead was welded to the bottom of the outer casing, and the positive electrode lead was welded to a sealing body. Then, electrolyte was injected into the outer casing, and the opening of the outer casing was sealed with a sealing body via a gasket to create a non-aqueous electrolyte secondary battery as a test cell.

[0070] [Evaluation of Cycle Characteristics] The fabricated test cell was charged at a constant current of 0.3C in a temperature environment of 25°C until the battery voltage reached 4.2V. Then, it was charged again at a constant voltage of 4.2V until the current value was 0.02C. After that, it was discharged at a constant current of 0.5C until the battery voltage reached 2.5V, and the discharge capacity at this time was defined as the initial discharge capacity. This charge-discharge cycle was considered one cycle, and 100 cycles were performed. The discharge capacity at the 100th cycle was determined, and the capacity retention rate was calculated using the following formula: Capacity retention rate (%) = Discharge capacity at the 100th cycle / Initial discharge capacity × 100

[0071] <Example 2> In the preparation of the negative electrode, as shown in Figure 4, a negative electrode mixture layer was prepared in the same manner as in Example 1, except that the second negative electrode mixture layer was placed on the surface of the first negative electrode mixture layer without having a first and second region. The test cell was then evaluated.

[0072] <Example 3> In the preparation of the negative electrode, as shown in Figure 5, a negative electrode mixture layer was prepared in the same manner as in Example 1, except that the first negative electrode mixture layer was placed on the surface of the second negative electrode mixture layer without having a first and second region. A test cell was then prepared and evaluated.

[0073] <Comparative Example 1> In the preparation of the negative electrode, as shown in Figure 7, a negative electrode mixture layer was prepared in the same manner as in Example 1, except that the second negative electrode mixture layer was arranged on the first negative electrode mixture layer over the entire longitudinal direction of the negative electrode. The negative electrode mixture layer of Comparative Example 1 had a substantially uniform thickness over the entire longitudinal direction of the negative electrode, and the thickness of the negative electrode mixture layer was set to 185 μm. In addition, the ratio of the thickness of the first negative electrode mixture layer to the thickness of the second negative electrode mixture layer was set to 1:1.

[0074] Table 1 shows the results of the volume retention rate of the test cells in Examples 1 to 3 and Comparative Example 1.

[0075]

[0076] As shown in Table 1, the capacity retention rate of the test cells in Examples 1 to 3 is improved compared to that of Comparative Example 1. This is presumed to be because the surface pressure on the inner side of the electrode body decreased, improving the flow of the non-aqueous electrolyte and reducing the likelihood of uneven reaction during the charge-discharge reaction within the electrode body. Furthermore, the test cell in Example 1 shows improved capacity retention compared to the test cells in Examples 2 and 3. Although the detailed mechanism is not clear, it is presumed that the surface pressure on the inner side of the electrode body was further reduced by placing a larger amount of the first negative electrode mixture layer with a lower silicon content on the inner end side of the winding.

[0077] This disclosure is further illustrated by the following embodiments. Configuration 1: A non-aqueous electrolyte secondary battery comprising an electrode body in which a positive electrode and a negative electrode are wound longitudinally with a separator between them, wherein the negative electrode comprises a negative electrode current collector and a negative electrode mixture layer disposed on the surface of the negative electrode current collector, the negative electrode mixture layer comprising a carbon material and a silicon-containing material as negative electrode active material, and having a region in which the content of the silicon-containing material relative to the total mass of the negative electrode active material continuously increases from the inner end to the outer end, and the thickness of the negative electrode mixture layer continuously decreases. Configuration 2: The non-aqueous electrolyte secondary battery according to Configuration 1, wherein the content of the silicon-containing material relative to the total mass of the negative electrode active material at the inner end of the negative electrode mixture layer is 95% or less of the content of the silicon-containing material relative to the total mass of the negative electrode active material at the outer end of the negative electrode mixture layer. Configuration 3: The non-aqueous electrolyte secondary battery according to Configuration 1 or 2, wherein the negative electrode mixture layer comprises a first negative electrode mixture layer and a second negative electrode mixture layer disposed on the surface of the negative electrode current collector or on the surface of the first negative electrode mixture layer, wherein the content of the silicon-containing material relative to the total mass of the negative electrode active material is greater than the content of the silicon-containing material relative to the total mass of the negative electrode active material in the first negative electrode mixture layer, and the ratio of the thickness of the second negative electrode mixture layer to the thickness of the first negative electrode mixture layer continuously increases from the inner end to the outer end, and the thickness of the negative electrode mixture layer continuously decreases in a region. Configuration 4: The non-aqueous electrolyte secondary battery according to Configuration 3, wherein the second negative electrode mixture layer is disposed on the surface of the first negative electrode mixture layer. Configuration 5: The non-aqueous electrolyte secondary battery according to Configuration 3, wherein the second negative electrode mixture layer is disposed between the negative electrode current collector and the first negative electrode mixture layer. Configuration 6: A non-aqueous electrolyte secondary battery according to any one of Configurations 3 to 5, wherein only the first negative electrode mixture layer is arranged at the inner end of the negative electrode mixture layer, and only the second negative electrode mixture layer is arranged at the outer end of the negative electrode mixture layer. Configuration 7: A non-aqueous electrolyte secondary battery according to any one of Configurations 3 to 5, wherein the first negative electrode mixture layer and the second negative electrode mixture layer are arranged at the inner and outer ends of the negative electrode mixture layer, respectively.

[0078] 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 member, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Positive electrode current collector, 32 Positive electrode mixture layer, 40 Negative electrode current collector, 42 Negative electrode mixture layer, 42A First region, 42B Second region, 42C Third region, 42X Inner end of winding, 42Y Outer end of winding, 44 First negative electrode mixture layer, 46 Second negative electrode mixture layer.

Claims

1. A non-aqueous electrolyte secondary battery comprising an electrode body in which a positive electrode and a negative electrode are wound longitudinally with a separator between them, wherein the negative electrode comprises a negative electrode current collector and a negative electrode mixture layer disposed on the surface of the negative electrode current collector, the negative electrode mixture layer comprises a carbon material and a silicon-containing material as negative electrode active material, and has a region in which the content of the silicon-containing material relative to the total mass of the negative electrode active material continuously increases from the inner end to the outer end, and the thickness of the negative electrode mixture layer continuously decreases.

2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the content of the silicon-containing material relative to the total mass of the negative electrode active material at the inner end of the negative electrode mixture layer is 95% or less of the content of the silicon-containing material relative to the total mass of the negative electrode active material at the outer end of the negative electrode mixture layer.

3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the negative electrode mixture layer comprises: a first negative electrode mixture layer; and a second negative electrode mixture layer disposed on the surface of the negative electrode current collector or on the surface of the first negative electrode mixture layer, wherein the content of the silicon-containing material relative to the total mass of the negative electrode active material is greater than the content of the silicon-containing material relative to the total mass of the negative electrode active material in the first negative electrode mixture layer, and the negative electrode mixture layer has a region from the inner end to the outer end of the winding in which the ratio of the thickness of the second negative electrode mixture layer to the thickness of the first negative electrode mixture layer continuously increases and the thickness of the negative electrode mixture layer continuously decreases.

4. The non-aqueous electrolyte secondary battery according to claim 3, wherein the second negative electrode mixture layer is disposed on the surface of the first negative electrode mixture layer.

5. The non-aqueous electrolyte secondary battery according to claim 3, wherein the second negative electrode mixture layer is disposed between the negative electrode current collector and the first negative electrode mixture layer.

6. The non-aqueous electrolyte secondary battery according to claim 3, wherein only the first negative electrode mixture layer is arranged at the inner end of the negative electrode mixture layer winding, and only the second negative electrode mixture layer is arranged at the outer end of the negative electrode mixture layer winding.

7. The non-aqueous electrolyte secondary battery according to claim 3, wherein the first negative electrode mixture layer and the second negative electrode mixture layer are arranged at the inner and outer ends of the negative electrode mixture layer, respectively.

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