Patent Description:
It is known that Si and Si-containing materials are capable of occluding a larger amount of lithium ions per unit volume than carbon materials, such as graphite. Therefore, using a Si-containing material or the like as a negative electrode active material increases the battery capacity. For example, PTL <NUM> discloses a nonaqueous electrolyte secondary battery produced using graphite and a Si-containing material as negative electrode active materials. PTL <NUM> also discloses a structure in which the mass ratio of the graphite to the Si-containing material in a negative electrode mixture layer (Mass of graphite/Mass of Si-containing material) increases continuously or discontinuously with the distance from the surface of the negative electrode current collector. PTL <NUM> discloses a non-aqueous electrolyte secondary battery, PTL <NUM> discloses a negative electrode and a secondary battery, PTL <NUM> discloses a non-aqueous electrolyte secondary battery and PTL <NUM> discloses a lithium secondary battery and method for manufacturing the same.

Since the volumes of Si and Si-containing materials change significantly during charging and discharging, the cycle characteristics of a nonaqueous electrolyte secondary battery produced using a Si active material composed of at least one of Si and a Si-containing material as a negative electrode active material is likely to become degraded. In particular, in the case where the nonaqueous electrolyte secondary battery includes a spirally wound electrode body, the change in the volume of the Si active material greatly affects a portion of the negative electrode which has a small radius of curvature, such as a winding start-side edge of the negative electrode from which the electrode body is wound. An object of the present disclosure is to enhance the cycle characteristics of a nonaqueous electrolyte secondary battery including a spirally wound electrode body produced using a Si active material as a negative electrode active material.

A nonaqueous electrolyte secondary battery according to the present disclosure is a nonaqueous electrolyte secondary battery including a spirally wound electrode body in which a positive electrode and a negative electrode are spirally wound with a separator interposed therebetween. The negative electrode includes a negative electrode current collector, a first negative electrode mixture layer disposed on a first surface of the negative electrode current collector, and a second negative electrode mixture layer disposed on a second surface of the negative electrode current collector. The first surface and the second surface face outward and inward of the electrode body, respectively. The first negative electrode mixture layer includes a Si active material composed of at least one of Si and a Si-containing material. A content of the Si active material in terms of Si is lower in a portion of the first negative electrode mixture layer which faces the negative electrode current collector in a thickness direction of the first negative electrode mixture layer, than in a portion of the first negative electrode mixture layer which faces a surface of the first negative electrode mixture layer in the thickness direction. In another case, the second negative electrode mixture layer includes a Si active material composed of at least one of Si and a Si-containing material. A content of the Si active material in terms of Si is lower in a portion of the second negative electrode mixture layer which faces a surface of the second negative electrode mixture layer in a thickness direction of the second negative electrode mixture layer, than in a portion of the second negative electrode mixture layer which faces the negative electrode current collector in the thickness direction.

According to an aspect of the present disclosure, the cycle characteristics of a nonaqueous electrolyte secondary battery including a spirally wound electrode body produced using a Si active material as a negative electrode active material may be enhanced.

As described above, the cycle characteristics of a nonaqueous electrolyte secondary battery including a spirally wound electrode body produced using a Si active material as a negative electrode active material is likely to become degraded. The degradation of cycle characteristics is caused due to a significant change in the volume of the Si active material which occurs during charging and discharging. Specifically, it is considered that the degradation of cycle characteristics be caused primarily because the significant change in the volume of the Si active material results in the degradation or loss of the contact between active material particles and consequently increases the amount of active material particles isolated from the electrical conduction paths present in the negative electrode mixture layer. In the case where a spirally wound electrode body is used, the change in the volume of the Si active material greatly affects a portion of the negative electrode which has a small radius of curvature, such as a winding start-side edge of the negative electrode from which the electrode body is wound.

The inventors of the present invention conducted extensive studies in order to achieve the above object and consequently found that improving the content of the Si active material in terms of Si in at least one of the first and second negative electrode mixture layers may markedly enhance the cycle characteristics of the battery. In the nonaqueous electrolyte secondary battery according to the present disclosure, the degradation of cycle characteristics may be limited while a Si active material is added to a negative electrode mixture layer with efficiency in order to increase battery capacity.

An embodiment of the present disclosure is described in detail below. Although the battery described below as an example is a cylindrical battery that includes a spirally wound electrode body <NUM> and a cylindrical battery casing <NUM> housing the spirally wound electrode body <NUM>, the shape of the battery casing is not limited to cylindrical and may be, for example, rectangular. A battery casing composed of a laminated sheet including a metal layer and a resin layer may also be used. The electrode body may be any electrode body having a spirally wound structure. Although the electrode body may be formed in a flat shape, the structure of the negative electrode according to the present disclosure may be particularly effective when the electrode body has a cylindrical, spirally wound structure.

<FIG> is a cross-sectional view of a nonaqueous electrolyte secondary battery <NUM> according to an embodiment. As illustrated in <FIG> as an example, the nonaqueous electrolyte secondary battery <NUM> includes a spirally wound electrode body <NUM>, a nonaqueous electrolyte (not illustrated in the drawing), and a battery casing <NUM> that houses the electrode body <NUM> and the nonaqueous electrolyte. The electrode body <NUM> has a spirally wound structure in which a positive electrode <NUM> and a negative electrode <NUM> are spirally wound with a separator <NUM> interposed therebetween. The battery casing <NUM> includes a closed-end, tubular packaging can <NUM> and a sealing material <NUM> with which the opening of the packaging can <NUM> is sealed. The nonaqueous electrolyte secondary battery <NUM> further includes a resin gasket <NUM> interposed between the packaging can <NUM> and the sealing material <NUM>.

The nonaqueous electrolyte includes a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent. Examples of the nonaqueous solvent include an ester, an ether, a nitrile, an amide, and mixed solvents of two or more of the above solvents. The nonaqueous solvent may include a halogen-substituted compound produced by replacing at least a part of the hydrogen atoms included in any of the above solvents with halogen atoms, such as fluorine atoms. The nonaqueous electrolyte is not limited to a liquid electrolyte and may be a solid electrolyte including a gelatinous polymer or the like. Examples of the electrolyte salt include lithium salts, such as LiPF<NUM>.

The electrode body <NUM> is constituted by a long-length positive electrode <NUM>, a long-length negative electrode <NUM>, two long-length separators <NUM>, a positive electrode tab <NUM> joined to the positive electrode <NUM>, and a negative electrode tab <NUM> joined to the negative electrode <NUM>. The negative electrode <NUM> is formed to be a size larger than the positive electrode <NUM> in order to prevent the precipitation of lithium. Specifically, the negative electrode <NUM> is formed so as to be longer than the positive electrode <NUM> in the longitudinal direction and the width direction (transverse direction). The two separators <NUM> are formed to be a size larger than at least the positive electrode <NUM> and arranged such that, for example, the positive electrode <NUM> is interposed therebetween.

Insulating plates <NUM> and <NUM> are disposed above and below the electrode body <NUM>, respectively. In the example illustrated in <FIG>, the positive electrode tab <NUM> attached to the positive electrode <NUM> extends toward the sealing material <NUM> through a through-hole formed in the insulating plate <NUM>, and the negative electrode tab <NUM> attached to the negative electrode <NUM> extends toward the bottom of the packaging can <NUM> through the outside of the insulating plate <NUM>. The positive electrode tab <NUM> is connected to the lower surface of a bottom plate <NUM> of the sealing material <NUM> by welding or the like. A cap <NUM>, which is the top plate of the sealing material <NUM> electrically connected to the bottom plate <NUM>, serves as a positive terminal. The negative electrode tab <NUM> is connected to the inner surface of the bottom of the packaging can <NUM> by welding or the like. The packaging can <NUM> serves as a negative terminal.

The packaging can <NUM> is, for example, a closed-end, cylindrical metal container. As described above, the gasket <NUM> is interposed between the packaging can <NUM> and the sealing material <NUM> to hermetically seal the internal space of the battery casing <NUM>. The packaging can <NUM> has a grooved portion <NUM> that supports the sealing material <NUM> and is formed by, for example, pressing the side surface from the outside of the packaging can <NUM>. The grooved portion <NUM> is preferably formed in a ring-like shape in the circumferential direction of the packaging can <NUM>. The sealing material <NUM> is supported by the upper surface of the grooved portion <NUM>. The upper end of the packaging can <NUM> is bent inward and caulked to the periphery of the sealing material <NUM>.

The sealing material <NUM> has a structure including the bottom plate <NUM>, a lower valve plate <NUM>, an insulating member <NUM>, an upper valve plate <NUM>, and a cap <NUM> which are stacked on top of one another in this order from closest to the electrode body <NUM>. The members constituting the sealing material <NUM> have, for example, a disk-like shape or a ring-like shape. The above members are electrically connected to one another except the insulating member <NUM>. The lower valve plate <NUM> and the upper valve plate <NUM> are connected to each other at the centers thereof. The insulating member <NUM> is interposed between the periphery of the lower valve plate <NUM> and the periphery of the upper valve plate <NUM>. In the case where the internal pressure of the battery is increased due to anomalous heat generation, rupturing occurs as a result of the lower valve plate <NUM> becoming deformed to press the upper valve plate <NUM> upward toward the cap <NUM> and, consequently, the current pathway between the lower valve plate <NUM> and the upper valve plate <NUM> becomes interrupted. If the internal pressure is further increased, the upper valve plate <NUM> becomes ruptured and the gas is exhausted through the opening of the cap <NUM>.

The positive electrode <NUM> includes a positive electrode current collector <NUM> and positive electrode mixture layers <NUM> disposed on the respective surfaces of the positive electrode current collector <NUM>. The positive electrode current collector <NUM> may be a foil made of a metal stable in the potential range of the positive electrode <NUM>, such as aluminum or an aluminum alloy, a film including a surface layer made of the above metal, or the like. The positive electrode mixture layer <NUM> includes a positive electrode active material, a conductive agent, and a binder. The positive electrode <NUM> may be produced by, for example, applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, a binder, and the like to the positive electrode current collector <NUM>, drying the resulting coating films, and then compressing the coating films to form the positive electrode mixture layers <NUM> on the respective surfaces of the positive electrode current collector <NUM>.

The positive electrode active material includes a lithium metal composite oxide as a principal constituent. Examples of the metal element included in the lithium metal composite oxide include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, and W. Examples of suitable lithium metal composite oxides include a composite oxide containing at least one element selected from Ni, Co, Mn, and Al. The lithium metal composite oxide may be provided with, for example, particles of an inorganic compound, such as aluminum oxide or a lanthanide-containing compound, adhered on the surfaces of particles thereof.

Examples of the conductive agent included in the positive electrode mixture layer <NUM> include carbon materials, such as carbon black, acetylene black, Ketjenblack, and graphite. Examples of the binder included in the positive electrode mixture layer <NUM> include fluororesins, such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), a polyimide, an acrylic resin, and a polyolefin. The above resins may be used in combination with a cellulose derivative, such as carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), or the like.

<FIG> is a cross-sectional view of the negative electrode <NUM>. As illustrated in <FIG> and <FIG> as an example, the negative electrode <NUM> includes a negative electrode current collector <NUM>, a negative electrode mixture layer <NUM> (first negative electrode mixture layer) disposed on a first surface 40a of the negative electrode current collector <NUM> which faces outward of the electrode body <NUM>, and a negative electrode mixture layer <NUM> (second negative electrode mixture layer) disposed on a second surface 40b of the negative electrode current collector <NUM> which faces inward of the electrode body <NUM>. While the positive electrode <NUM> has a layer structure including the positive electrode current collector <NUM> and the positive electrode mixture layers <NUM> that are disposed on the respective surfaces of the positive electrode current collector <NUM> and composed of a mixture having a uniform composition, in the negative electrode <NUM>, the content of a Si active material composed of at least one of Si and a Si-containing material in at least one of the negative electrode mixture layers <NUM> and <NUM> changes in a continuous or stepwise manner. In at least one of the negative electrode mixture layers <NUM> and <NUM>, the content of the Si active material varies between the surface-side portion of the mixture layer and the negative electrode current collector <NUM>-side portion of the mixture layer. The details are described below.

The negative electrode current collector <NUM> may be a foil made of a metal stable in the potential range of the negative electrode <NUM>, such as copper or a copper alloy, a film including a surface layer made of the above metal, or the like. The negative electrode mixture layer <NUM> includes a negative electrode active material and a binder. The negative electrode <NUM> may be produced by, for example, applying a negative electrode mixture slurry containing a negative electrode active material, a binder, and the like to the negative electrode current collector <NUM>, drying the resulting coating films, and then compressing the coating films to form the negative electrode mixture layer <NUM> on the first surface 40a of the negative electrode current collector <NUM> and the negative electrode mixture layer <NUM> on the second surface 40b of the negative electrode current collector <NUM>. In the production of the negative electrode <NUM>, for example, four types of negative electrode mixture slurries containing the Si active material at different proportions may be used. The details are described below.

As illustrated in <FIG> as an example, the negative electrode <NUM> constituting the spirally wound electrode body <NUM> is arranged to curve along the length thereof in the longitudinal direction. The negative electrode <NUM> is produced in the form of a flat sheet and becomes curved when it is spirally wound together with the positive electrode <NUM> and the separator <NUM> in the production of the electrode body <NUM>. The negative electrode <NUM> commonly has a radius of curvature of about <NUM> to <NUM>. The radius of curvature of the negative electrode <NUM> varies between a winding start-side of the electrode body <NUM> from which the electrode body <NUM> is wound and a winding end-side of the electrode body <NUM> to which the electrode body <NUM> is wound; the radius of curvature of the negative electrode <NUM> satisfies Winding start-side edge < Winding end-side edge. The minimum radius of curvature of the negative electrode <NUM> is, for example, <NUM> to <NUM>, or <NUM> to <NUM>.

Since the negative electrode <NUM> is produced in the form of a flat sheet, when the negative electrode <NUM> is curved, the convex-side portion of the negative electrode <NUM> is elongated and the concave-side portion of the negative electrode <NUM> is compressed. Specifically, the negative electrode mixture layer <NUM> disposed on the first surface 40a of the negative electrode current collector <NUM> which faces outward of the electrode body <NUM> is elongated, while the negative electrode mixture layer <NUM> disposed on the second surface 40b of the negative electrode current collector <NUM> which faces inward of the electrode body <NUM> is compressed. The surface-side portion of the negative electrode mixture layer <NUM> in the thickness direction of the mixture layer is elongated, while the negative electrode current collector <NUM>-side portion of the mixture layer is compressed. The negative electrode current collector <NUM>-side portion of the negative electrode mixture layer <NUM> in the thickness direction of the mixture layer is elongated, while the surface-side portion of the mixture layer is compressed. The inventors of the present invention successfully enhanced the cycle characteristics of the battery by improving the layer structure of the negative electrode <NUM> focusing on the above point.

Although one of the negative electrode mixture layers <NUM> and <NUM> does not necessarily include the Si active material, it is preferable that both of the negative electrode mixture layers <NUM> and <NUM> include the Si active material in order to, for example, increase the battery capacity. In such a case, the average content of the Si active material in terms of Si in the negative electrode mixture layer <NUM> is preferably lower than the average content of the Si active material in terms of Si in the negative electrode mixture layer <NUM>. Since the negative electrode mixture layer <NUM> is compressed as described above, the negative electrode mixture layer <NUM> has a smaller margin that allows a great change in the volume of the Si active material than the negative electrode mixture layer <NUM>. Accordingly, in order to enhance cycle characteristics, it is preferable to satisfy Content of the Si active material in terms of Si in the negative electrode mixture layer <NUM> > Content of the Si active material in terms of Si in the negative electrode mixture layer <NUM>. The content of the Si active material in terms of Si is calculated as the ratio of the mass of the Si active material included in the negative electrode mixture layer <NUM> or <NUM> in terms of Si to the mass of the negative electrode active material included in the negative electrode mixture layer. The mass of the Si active material in terms of Si is calculated by multiplying the mass of the Si active material by the mass ratio of Si to the Si active material. Examples of the method for comparing the content of the Si active material in terms of Si in the surface-side portion of the negative electrode mixture layer <NUM> or <NUM> with that in the current collector-side portion of the mixture layer include a method in which the negative electrode mixture layer <NUM> or <NUM> is divided into parts at regular intervals in the thickness direction and the contents of the Si active material in terms of Si in the respective parts are calculated and compared with one another. In the case where the negative electrode mixture layers <NUM> and <NUM> have a multilayer structure, the contents of the Si active material in terms of Si in the respective layers may be calculated to compare the content of the Si active material in terms of Si in the surface-side portion of the negative electrode mixture layer <NUM> or <NUM> with that in the current collector-side portion of the mixture layer.

In other words, the content of the Si active material in terms of Si in the negative electrode mixture layer <NUM> is set to be higher than the content of the Si active material in terms of Si in the negative electrode mixture layer <NUM>. Since the negative electrode mixture layer <NUM> is elongated as described above, the negative electrode mixture layer <NUM> has a larger margin that allows a great change in the volume of the Si active material than the negative electrode mixture layer <NUM>. Therefore, increasing the content of the Si active material in the negative electrode mixture layer <NUM> is not likely to degrade cycle characteristics.

The content of the Si active material in terms of Si in the negative electrode current collector <NUM>-side portion of the negative electrode mixture layer <NUM> in the thickness direction of the mixture layer is lower than in the surface-side portion of the mixture layer. The content of the Si active material in terms of Si in the surface-side portion of the negative electrode mixture layer <NUM> in the thickness direction of the mixture layer is lower than in the negative electrode current collector <NUM>-side portion of the mixture layer. For example, when the negative electrode mixture layer <NUM> is divided into two halves at the center in the thickness direction, the content of the Si active material in terms of Si in a portion of the mixture layer which is located on the negative electrode current collector <NUM>-side is lower than in a portion of the mixture layer which is away from the negative electrode current collector <NUM> and located on the surface-side of the mixture layer. When the negative electrode mixture layer <NUM> is divided into two halves at the center in the thickness direction, the content of the Si active material in terms of Si in a portion of the mixture layer which is located on the surface-side of the mixture layer is lower than in a portion of the mixture layer which is located on the negative electrode current collector <NUM>-side.

In this embodiment, the negative electrode mixture layers <NUM> and <NUM> have a multilayer structure. The negative electrode mixture layer <NUM> includes a first layer 41A arranged to face the surface of the mixture layer and a second layer 41B arranged closer to the negative electrode current collector <NUM> than the first layer 41A, the second layer 41B containing the Si active material at a lower proportion in terms of Si than the first layer 41A. The negative electrode mixture layer <NUM> includes a third layer 42C arranged to face the negative electrode current collector <NUM> and a fourth layer 42D arranged closer to the surface of the mixture layer than the third layer 42C, the fourth layer 42D containing the Si active material at a lower proportion in terms of Si than the third layer 42C.

It is preferable that the content of the Si active material in terms of Si decrease in the direction from the outer periphery of the negative electrode <NUM> to the inner periphery of the negative electrode <NUM>, that is, in the direction from the first layer 41A of the negative electrode mixture layer <NUM> to the fourth layer 42D of the negative electrode mixture layer <NUM>. In other words, the content of the Si active material in terms of Si is at maximum in the first layer 41A and at minimum in the fourth layer 42D. The content of the Si active material in terms of Si in the second layer 41B may be equal to that in the third layer 42C. The second layer 41B and the third layer 42C may have the same layer structure. Although the content of the Si active material in terms of Si may satisfy Second layer 41B < Third layer 42C such that the average content of the Si active material in terms of Si satisfies Negative electrode mixture layer <NUM> > Negative electrode mixture layer <NUM>, it is preferable that the content of the Si active material in terms of Si in the second layer 41B be lower than that in the third layer 42C.

That is, it is preferable that the contents of the Si active material in terms of Si in the layers constituting the negative electrode mixture layers <NUM> and <NUM> satisfy First layer 41A > Second layer 41B ≥ Third layer 42C > Fourth layer 42D. Adjusting the contents of the Si active material in terms of Si in the negative electrode mixture layers <NUM> and <NUM> to satisfy the above relationship enables the degradation of cycle characteristics to be limited while the Si active material is used with efficiency to increase battery capacity. Although each of the negative electrode mixture layers <NUM> and <NUM> is constituted by two layers containing the Si active material at different proportions in terms of Si, the negative electrode mixture layers <NUM> and <NUM> may be constituted by three or more layers.

The layers constituting the negative electrode mixture layers <NUM> and <NUM> preferably further include a carbon active material. That is, it is preferable to use the Si active material in combination with a carbon active material as negative electrode active materials. From the viewpoint of cycle characteristics, it is preferable that the content of the Si active material in each of the negative electrode mixture layers <NUM> and <NUM> be lower than the content of the carbon active material in the negative electrode mixture layer. Examples of suitable carbon active materials include natural graphite, such as flake graphite, lump graphite, or amorphous graphite; and artificial graphite, such as massive artificial graphite (MAG) or graphitized mesophase carbon microbeads (MCMB).

The Si active material is composed of at least one of Si and a Si-containing material and is preferably composed of a Si-containing material, which shows a smaller volume change than Si during charging and discharging. Examples of the Si-containing material include a material represent by SiOx (<NUM> ≤ x ≤ <NUM>). SiOx has a structure constituted by, for example, a SiO<NUM> matrix and Si microparticles dispersed therein. Alternatively, the Si-containing material may be a material (LSi) constituted by a lithium silicate (Li2ySiO(<NUM>+y) (<NUM> < y < <NUM>)) phase and Si microparticles dispersed therein. The negative electrode mixture layers <NUM> and <NUM> may include SiOx and LSi.

It is preferable that a conductive coating composed of a material having a higher electrical conductivity than the Si-containing material be formed on the surfaces of particles of the Si-containing material. Examples of the material constituting the conductive coating include at least one selected from a carbon material, a metal, and a metal compound. In particular, a carbon material, such as amorphous carbon, is preferable. A carbon coating may be formed by, for example, a CVD method in which acetylene, methane, or the like is used; or by a method in which particles of the Si-containing material are mixed with coal pitch, petroleum pitch, a phenolic resin, or the like and subsequently a heat treatment is performed. Alternatively, the conductive coating may be formed by attaching a conductive filler, such as carbon black, onto the surfaces of particles of the Si-containing material using a binder. The amount of the conductive coating is, for example, <NUM>% to <NUM>% by mass of the mass of particles of the Si-containing material.

The average contents of the Si active material in terms of Si in the negative electrode mixture layers <NUM> and <NUM> satisfy Negative electrode mixture layer <NUM> > Negative electrode mixture layer <NUM> as described above. The difference in the content of the Si active material in terms of Si between the negative electrode mixture layers <NUM> and <NUM> is preferably <NUM>% or more, is more preferably <NUM>% or more, and is further preferably <NUM>% or more. The difference in the content of the Si active material between the first layer 41A and the second layer 41B and the difference in the content of the Si active material between the third layer 42C and the fourth layer 42D are preferably <NUM>% or more, are more preferably <NUM>% or more, and are further preferably <NUM>% or more. In such a case, cycle characteristics may be further markedly improved. The content of the Si active material in terms of Si may be measured by ICP (inductively coupled plasma).

In the case where the Si active material is used in combination with the carbon active material as negative electrode active materials, the content of the Si active material (Si-containing material) on a mass basis also satisfies Negative electrode mixture layer <NUM> > Negative electrode mixture layer <NUM> and preferably satisfies First layer 41A > Second layer 41B ≥ Third layer 42C > Fourth layer 42D. The average content of the Si active material in the negative electrode mixture layer <NUM> is preferably <NUM>% to <NUM>% by mass and is more preferably <NUM>% to <NUM>% by mass. The average content of the Si active material in the negative electrode mixture layer <NUM> is lower than that in the negative electrode mixture layer <NUM>, is preferably less than <NUM>% by mass, and is more preferably <NUM>% to <NUM>% by mass. The types of the Si active materials added to the layers may be different from one another.

The proportion of the Si active material to the negative electrode active material in the negative electrode mixture layer <NUM> is preferably <NUM>% to <NUM>% by mass and is more preferably <NUM>% to <NUM>% by mass. The proportion of the Si active material to the negative electrode active material in the negative electrode mixture layer <NUM> is preferably less than <NUM>% by mass and is more preferably <NUM>% to <NUM>% by mass. In other words, the proportion of the carbon active material to the negative electrode active material in the negative electrode mixture layer <NUM> is preferably <NUM>% to <NUM>% by mass. The proportion of the carbon active material to the negative electrode active material in the negative electrode mixture layer <NUM> is preferably <NUM>% by mass or more.

Examples of the binder included in the negative electrode mixture layers <NUM> and <NUM> include a fluororesin, PAN, polyimide, an acrylic resin, and a polyolefin, as in the case for the positive electrode <NUM>. The binder included in the negative electrode mixture layers <NUM> and <NUM> is preferably a styrene-butadiene rubber (SBR) or a modified styrene-butadiene rubber. The negative electrode mixture layers <NUM> and <NUM> may further include, in addition to an SBR and the like, CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol, and the like. The types and contents of the binders included in the layers constituting the negative electrode mixture layers <NUM> and <NUM> may be different from or identical to one another.

The negative electrode mixture layers <NUM> and <NUM> may have different thicknesses and are preferably formed so as to have substantially the same thickness. The thicknesses of the negative electrode mixture layers <NUM> and <NUM> may be, for example, <NUM> to <NUM> and are preferably <NUM> to <NUM>. The layers constituting the negative electrode mixture layers <NUM> and <NUM> may have different thicknesses and may be formed so as to have substantially the same thickness. The negative electrode mixture layers <NUM> and <NUM> may further include a negative electrode active material other than the Si active material or the carbon active material. Examples of the other negative electrode active material include a metal other than Si which is capable of alloying with lithium, a compound containing such a metal, and lithium titanate.

In this embodiment, the negative electrode mixture layer <NUM> including the first layer 41A and the second layer 41B is formed on one of the surfaces of the negative electrode current collector <NUM> and the negative electrode mixture layer <NUM> including the third layer 42C and the fourth layer 42D is formed on the other surface. Accordingly, four types of negative electrode mixture slurries containing the Si active material at different proportions are used. For example, when the negative electrode mixture layer <NUM> is formed, a first negative electrode mixture slurry for the first layer 41A is applied to the negative electrode current collector <NUM> and a second negative electrode mixture slurry for the second layer 41B is applied to the resulting coating film so as to cover the coating film. In this step, the coating film formed of the first negative electrode mixture slurry may be either dried or undried.

The separator <NUM> may be a porous sheet having ionic permeability and an insulating property. Specific examples of such a porous sheet include a microporous thin-film, a woven fabric, and a nonwoven fabric. Examples of a suitable material for the separator <NUM> include olefin resins, such as polyethylene and polypropylene; and celluloses. The separator <NUM> may have either a single-layer structure or a multilayer structure. The separator <NUM> may be provided with a heat-resistant layer or the like disposed on the surface thereof.

The present disclosure is further described with reference to Examples below. The present disclosure is not limited by Examples below.

A lithium metal composite oxide represented by LiNi<NUM>Co<NUM>Al<NUM>O<NUM>, carbon black, and polyvinylidene fluoride were mixed with one another at a mass ratio of <NUM>:<NUM>:<NUM>. An appropriate amount of N-methyl-<NUM>-pyrrolidone was added to the resulting mixture. Subsequently, the mixture was kneaded. Hereby, a positive electrode mixture slurry was prepared. The positive electrode mixture slurry was applied onto both surfaces of a positive electrode current collector composed of an aluminum foil having a thickness of <NUM>. After the resulting coating films had been dried, the coating films were rolled with a roller. Then, the electrode was cut into a predetermined size. Hereby, a positive electrode including a positive electrode current collector and positive electrode mixture layers disposed on the respective surfaces thereof was prepared.

A graphite powder was mixed with a Si-containing material represented by SiOx (x = <NUM>) having a carbon coating deposited thereon at a mass ratio of <NUM>:<NUM>. Hereby, a negative electrode active material was prepared. The negative electrode active material, a sodium salt of CMC, and polyacrylic acid were mixed with one another at a mass ratio of <NUM>:<NUM>:<NUM>. To the resulting mixture, pure water was added such that the solid content was <NUM>% by mass. Then, kneading was performed for <NUM> minutes. Pure water was added to the kneaded mixture such that the solid content was <NUM>% by mass. Subsequently, a dispersion of SBR was added to the mixture in an amount equal to <NUM>% by mass of the amount of the negative electrode active material. Hereby, a first negative electrode mixture slurry was prepared.

A second negative electrode mixture slurry was prepared as in the preparation of the first negative electrode mixture slurry, except that the graphite powder was mixed with the Si-containing material at a mass ratio of <NUM>:<NUM>.

A third negative electrode mixture slurry was prepared as in the preparation of the first negative electrode mixture slurry, except that the graphite powder was mixed with the Si-containing material at a mass ratio of <NUM>:<NUM>.

A fourth negative electrode mixture slurry was prepared as in the preparation of the first negative electrode mixture slurry, except that the graphite powder was mixed with the Si-containing material at a mass ratio of <NUM>:<NUM>.

The second negative electrode mixture slurry was applied onto one of the surfaces of a negative electrode current collector composed of a copper foil having a thickness of <NUM>. The first negative electrode mixture slurry was applied to the resulting coating film so as to cover the coating film. Similarly, the third negative electrode mixture slurry was applied onto the other surface of the negative electrode current collector, and the fourth negative electrode mixture slurry was applied to the resulting coating film so as to cover the coating film. The amounts of the slurries used were the same as one another. After the coating films had been dried, the coating films were rolled with a roller. Then, the electrode was cut into a predetermined size. Hereby, a negative electrode including a first negative electrode mixture layer having a two-layer structure and a second negative electrode mixture layer having a two-layer structure was prepared. Specifically, the spirally wound electrode body described below was prepared such that the one of the surfaces of the current collector served as a first surface facing outward of the electrode body and the other surface served as a second surface facing inward of the electrode body. Table <NUM> summarizes the proportion of the amount of the Si-containing material to the total mass of the negative electrode active material in each of the layers constituting the negative electrode mixture layer. In Table <NUM>, the values in the parentheses are the contents of the Si-containing material in terms of Si in the layers.

The positive electrode and the negative electrode were wound around a core having a radius of curvature of <NUM> with a separator interposed therebetween, the separator having a thickness of <NUM> and being composed of a polyethylene microporous membrane. A tape was attached onto the outermost peripheral surface. Hereby, a cylindrical, spirally wound electrode body was prepared. An aluminum positive electrode lead was welded to a portion of the positive electrode at which the current collector was exposed. A nickel negative electrode lead was welded to a portion of the negative electrode at which the current collector was exposed.

Ethylene carbonate, dimethyl carbonate, and ethylmethyl carbonate were mixed with one another at a volume ratio of <NUM>:<NUM>:<NUM> to prepare a mixed solvent. Vinylene carbonate was dissolved in the mixed solvent at a concentration of <NUM>% by mass. Subsequently, LiPF<NUM> was dissolved in the resulting solution at a concentration of <NUM> mol/liter. Hereby, a nonaqueous electrolyte was prepared.

The electrode body was inserted into a closed-end cylindrical packaging can. The positive electrode lead was welded to a sealing material. The negative electrode lead was welded to the inner bottom of the packaging can. After the nonaqueous electrolyte had been charged into the packaging can, the opening of the packaging can was sealed with the sealing material. Hereby, a nonaqueous electrolyte secondary battery (height: <NUM>, diameter: <NUM>, design capacity: <NUM> mAh) was prepared.

The battery was charged and discharged under the following conditions at <NUM>. Then, the capacity retention factor of the battery was calculated. The evaluation results described in Table <NUM> are relative values with the capacity retention factor of the battery prepared in Comparative example <NUM> being <NUM>.

Charge: The battery was charged at a constant current of <NUM> mA until the battery voltage reached <NUM> V. The battery was further charged at a constant voltage of <NUM> V until the current reached <NUM> mA.

Discharge: The battery was discharged at a constant current of <NUM> mA until the voltage reached <NUM> V.

The battery was subjected to <NUM> cycles of the above charge and discharge. The capacity retention factor of the battery was calculated using the formula below.

A negative electrode and a nonaqueous electrolyte secondary battery were prepared as in Example <NUM>, except that the proportion of the amount of the Si-containing material to the total mass of the negative electrode active material in each of the layers was changed as described in Table <NUM>. The battery was evaluated in terms of cycle characteristics as in Example <NUM>.

A negative electrode and a nonaqueous electrolyte secondary battery were prepared as in Example <NUM>, except that the layers constituting the first and second negative electrode mixture layers were formed using a negative electrode mixture slurry prepared by mixing graphite with the Si-containing material at a mass ratio of <NUM>:<NUM>. The battery was evaluated in terms of cycle characteristics as in Example <NUM>.

As described in Table <NUM>, the batteries prepared in Examples, where the content of the Si active material in terms of Si satisfied First layer > Second layer > Third layer > Fourth layer, had a high capacity retention factor and were excellent in terms of cycle characteristics compared with the batteries prepared in Comparative examples. It was confirmed that the cycle characteristics of the battery prepared in Example <NUM>, where the differences in the content of the Si active material between the first and second layers, between the second and third layers, and between the third and fourth layers were <NUM>%, was also markedly enhanced. It was also confirmed that the capacity retention factor was significantly reduced in the case where the content of the Si active material was increased in a direction toward the negative electrode current collector as in Comparative example <NUM> and in the case where the content of the Si active material was increased in a direction opposite to the direction toward the negative electrode current collector as in Comparative example <NUM>.

<NUM> NONAQUEOUS ELECTROLYTE SECONDARY BATTERY, <NUM> POSITIVE ELECTRODE, <NUM> NEGATIVE ELECTRODE, <NUM> SEPARATOR, <NUM> ELECTRODE BODY, <NUM> BATTERY CASING, <NUM> PACKAGING CAN, <NUM> SEALING MATERIAL, <NUM>,<NUM> INSULATING PLATE, <NUM> POSITIVE ELECTRODE TAB, <NUM> NEGATIVE ELECTRODE TAB, <NUM> GROOVED PORTION, <NUM> BOTTOM PLATE, <NUM> LOWER VALVE PLATE, <NUM> INSULATING MEMBER, <NUM> UPPER VALVE PLATE, <NUM> CAP, <NUM> GASKET, <NUM> POSITIVE ELECTRODE CURRENT COLLECTOR, <NUM> POSITIVE ELECTRODE MIXTURE LAYER, <NUM> NEGATIVE ELECTRODE CURRENT COLLECTOR, 40a FIRST SURFACE, 40b SECOND SURFACE, <NUM>,<NUM> NEGATIVE ELECTRODE MIXTURE LAYER, 41A FIRST LAYER, 41B SECOND LAYER, 42C THIRD LAYER, 42D FOURTH LAYER between the third and fourth layers were <NUM>%, was also markedly enhanced. It was also confirmed that the capacity retention factor was significantly reduced in the case where the content of the Si active material was increased in a direction toward the negative electrode current collector as in Comparative example <NUM> and in the case where the content of the Si active material was increased in a direction opposite to the direction toward the negative electrode current collector as in Comparative example <NUM>.

Claim 1:
A nonaqueous electrolyte secondary battery (<NUM>) comprising a spirally wound electrode body (<NUM>) in which a positive electrode (<NUM>) and negative electrode (<NUM>) are spirally wound with a separator (<NUM>) interposed therebetween,
wherein the negative electrode (<NUM>) includes a negative electrode current collector (<NUM>), a first negative electrode mixture layer (<NUM>) disposed on a first surface (40a) of the negative electrode current collector (<NUM>), and a second negative electrode mixture layer (<NUM>) disposed on a second surface (40b) of the negative electrode current collector (<NUM>), the first surface (40a) and the second surface (40b) facing outward and inward of the electrode body (<NUM>), respectively,
wherein the first negative electrode mixture layer (<NUM>) includes a Si active material composed of at least one of Si and a Si-containing material, and the second negative electrode mixture layer (<NUM>) includes the Si active material;
characterized in that a content of the Si active material in terms of Si is lower in a portion of the first negative electrode mixture layer (<NUM>) which faces the negative electrode current collector (<NUM>) in a thickness direction of the first negative electrode mixture layer (<NUM>), than in a portion of the first negative electrode mixture layer (<NUM>) which faces a surface of the first negative electrode mixture layer (<NUM>) in the thickness direction
and in that the content of the Si active material in terms of Si is lower in a portion of the second negative electrode mixture layer (<NUM>) which faces a surface of the second negative electrode mixture layer (<NUM>) in a thickness direction of the second negative electrode mixture layer (<NUM>), than in a portion of the second negative electrode mixture layer (<NUM>) which faces the negative electrode current collector (<NUM>) in the thickness direction.