Patent Description:
PTL <NUM> discloses a negative electrode for lithium-ion secondary batteries, comprising a current collector foil, a first negative electrode mixture layer, and a second negative electrode mixture layer, wherein the first negative electrode mixture layer is arranged on the current collector foil, the second negative electrode mixture layer is arranged on the first negative electrode mixture layer, the first negative electrode mixture layer comprises graphite, a silicon oxide, and polyacrylic acid, and the second negative electrode mixture layer comprises graphite but does not comprise a silicon oxide or polyacrylic acid. PTL <NUM> discloses a negative electrode for lithium-ion secondary batteries, comprising a negative electrode current collector and a negative electrode active material layer, wherein the negative electrode active material layer has a first main surface and a second main surface, the first main surface is in contact with the negative electrode current collector, the second main surface is an opposing surface of the first main surface, the negative electrode active material layer comprises a first layer and a second layer, the second layer is laminated on the first layer, the second layer comprises the second main surface, the first layer comprises a first negative electrode active material and a first conductive material, the first negative electrode active material comprises a graphite material and a silicon material, the first conductive material comprises carbon nanotube, the first conductive material is contained at <NUM> parts by mass or more to <NUM> parts by mass or less relative to <NUM> parts by mass of the first negative electrode active material, the second layer comprises a second negative electrode active material and a second conductive material, the second negative electrode active material comprises a graphite material, the second conductive material is contained at <NUM> parts by mass or more and <NUM> parts by mass or less relative to <NUM> parts by mass of the second negative electrode active material.

Lithium-ion secondary batteries comprising conventional negative electrodes have room for improvement in terms of cycle characteristics.

The present application discloses the following plurality of aspects as means for achieving the above object.

A negative electrode for a lithium-ion secondary battery, having the following cross-sectional configuration, wherein:.

The negative electrode for a lithium-ion secondary battery according to Aspect <NUM>, wherein a porosity of the first layers in a discharging state is <NUM>% or greater.

The negative electrode for a lithium-ion secondary battery according to Aspect <NUM> or <NUM>, wherein the first layers are present in an island form on the surface of one side of the negative electrode current collector.

The negative electrode for a lithium-ion secondary battery according to Aspect <NUM> or <NUM>, wherein the first layers are present in a stripe form on the surface of one side of the negative electrode current collector.

The negative electrode for a lithium-ion secondary battery according to any of Aspects <NUM> to <NUM>, wherein.

The negative electrode for a lithium-ion secondary battery according to Aspect <NUM>, wherein the first binder has an imide skeleton or comprises polyacrylic acid.

The negative electrode for a lithium-ion secondary battery according to any of Aspects <NUM> to <NUM>, wherein
the first conductive material comprises carbon.

The negative electrode for a lithium-ion secondary battery according to any of Aspects <NUM> to <NUM>, wherein
the negative electrode current collector comprises copper.

The negative electrode for a lithium-ion secondary battery according to any of Aspects <NUM> to <NUM>, wherein
a ratio of the first regions to the surface of one side of the negative electrode current collector is less than <NUM>% by area.

The negative electrode for a lithium-ion secondary battery according to any of Aspects <NUM> to <NUM>, wherein
a ratio T<NUM>/T<NUM> of a maximum thickness T<NUM> of the first layers to a maximum thickness T<NUM> of the second layer is <NUM> or less.

A method for manufacturing a negative electrode for a lithium-ion secondary battery, comprising the following first step, second step, and third step, wherein:.

A lithium-ion secondary battery, comprising the negative electrode according to any of Aspects <NUM> to <NUM>, a positive electrode, and an electrolyte layer.

According to the negative electrode for a lithium-ion secondary battery of the present disclosure, cycle characteristics of a lithium-ion secondary battery is improved.

Hereinafter, the negative electrode for a lithium-ion secondary battery according to one embodiment will be described with reference to the drawings. However, the technique of the present disclosure is not limited to the following embodiment. <FIG> schematically show cross-sectional configurations of the negative electrode <NUM> for a lithium-ion secondary battery according to one embodiment.

As shown in <FIG>, the negative electrode <NUM> for a lithium-ion secondary battery has the following cross-sectional configuration. Specifically, the cross-sectional configuration comprises a negative electrode current collector <NUM>, a plurality of first layers <NUM>, and a second layer <NUM>. Each of the first layers <NUM> comprises a first negative electrode active material and a first conductive material. The first negative electrode active material comprises at least one of elemental silicon, a silicon alloy, and a silicon oxide. The second layer <NUM> comprises a second negative electrode active material. The second negative electrode active material comprises graphite. The second layer <NUM> does not comprise silicon. In the cross-sectional configuration, a surface <NUM> of one side of the negative electrode current collector <NUM> has a plurality of first regions 11x and a second region 11y between the first regions 11x. In the cross-sectional configuration, the first layers <NUM> are in contact with the respective first regions 11x. In the cross-sectional configuration, the second layer <NUM> is in contact with both the second region 11y and surfaces 21x of one side of the first layers <NUM>.

Any general negative electrode current collector for lithium-ion secondary batteries can be adopted as the negative electrode current collector <NUM>.

The negative electrode current collector <NUM> may be foil-like, plate-like, mesh-like, punched metal-like, or a foam. The negative electrode current collector <NUM> may be a metal foil or a metal mesh, and alternatively may be a carbon sheet. Particularly, when the negative electrode current collector <NUM> is a metal foil, the negative electrode current collector <NUM> has excellent handleability, and a first layer <NUM> and a second layer <NUM>, which will be described below, can be easily laminated on a surface <NUM> of one side of the negative electrode current collector <NUM>. The negative electrode current collector <NUM> may have a plurality of foils or sheets. The negative electrode current collector <NUM> may have on the surface thereof some coating layer for the purpose of adjusting resistance. For example, the negative electrode current collector <NUM> may have a carbon coating layer. The negative electrode current collector <NUM> may be a metal foil having a carbon coating layer. The negative electrode current collector <NUM> may also be a metal foil or a substrate plated or vapor-deposited with a metal described below. When the negative electrode current collector <NUM> has a plurality of metal foils, there may be some layer between the plurality of metal foils. The thickness of the negative electrode current collector <NUM> is not particularly limited and for example, may be <NUM> or more or <NUM> or more, and may be <NUM> or less or <NUM> or less.

Examples of a metal constituting the negative electrode current collector <NUM> include at least one selected from copper (Cu), nickel (Ni), chromium (Cr), gold (Au), platinum (Pt), silver (Ag), aluminum (Al), iron (Fe), titanium (Ti), zinc (Zn), cobalt (Co), vanadium (V), magnesium (Mg), lead (Pb), germanium (Ge), indium (In), tin (Sn), zirconium (Zr), and stainless steel. Particularly, from the viewpoint of ensuring reduction resistance and the viewpoint of making alloying with lithium difficult, the negative electrode current collector <NUM> may comprise at least one metal selected from copper, nickel, and stainless steel. Particularly, when the negative electrode current collector <NUM> comprises copper, even more excellent performance is easily ensured.

As shown in <FIG>, in the cross-sectional configuration of the negative electrode <NUM>, the surface <NUM> of one side of the negative electrode current collector <NUM> has a plurality of first regions 11x and a second region 11y between the first regions 11x. First layers <NUM> and a second layer <NUM> described below are laminated on the surface <NUM> of one side of the negative electrode current collector <NUM>.

As shown in <FIG>, in the cross-sectional configuration of the negative electrode <NUM>, each of the plurality of first regions 11x may have a length L<NUM> along the surface <NUM> of one side of the negative electrode current collector <NUM>. The length L<NUM> is not particularly limited and can be appropriately determined according to target battery performance. In one embodiment, the length L<NUM> may be <NUM> or more and <NUM> or less. One length L<NUM> may be the same as or different from another length L<NUM>.

As shown in <FIG>, in the cross-sectional configuration of the negative electrode <NUM>, an interval I<NUM> can be present between a plurality of first regions 11x. The interval I<NUM> is not particularly limited and can be appropriately determined according to target battery performance. In one embodiment, the interval I<NUM> may be <NUM> or more and <NUM> or less. One interval I<NUM> may be the same as or different from another interval I<NUM>.

A ratio L<NUM>/I<NUM> of the length L<NUM> to the interval I<NUM> described above is not particularly limited and can be appropriately determined according to target battery performance. In one embodiment, the ratio L<NUM>/I<NUM> of the length L<NUM> to the interval I<NUM> may be <NUM> or greater and <NUM> or less.

A ratio of the first regions 11x to the surface <NUM> of one side of the negative electrode current collector <NUM> is not particularly limited and can be appropriately determined according to target battery performance. Particularly, when the ratio of the first regions 11x to the surface <NUM> of one side of the negative electrode current collector <NUM> is less than <NUM>% by area, more excellent performance is easily exhibited. The ratio may be <NUM>% by area or greater and less than <NUM>% by area, or <NUM>% by area or greater and <NUM>% by area or less.

<FIG> schematically show examples of arrangement of the first layers <NUM> when only the first layers <NUM> are laminated on the surface <NUM> of one side of the negative electrode current collector <NUM>. As shown in <FIG>, in a state where only the first layers <NUM> are laminated on the surface <NUM> of one side of the negative electrode current collector <NUM>, a portion of the surface <NUM> of one side of the negative electrode current collector <NUM> is exposed. In this case, of the surface <NUM> of one side of the negative electrode current collector <NUM>, the portions on which the first layers <NUM> are laminated can correspond to the first regions 11x described above, and the exposed portion without the first layers <NUM> laminated thereon can correspond to the second region 11y described above.

The surface <NUM> of one side of the negative electrode current collector <NUM>, in addition to the first regions 11x and the second region 11y described above, may have a third region (not illustrated). The third region, for example, may be a region not in contact with any of the first layers <NUM> and the second layer <NUM>.

The form of a surface <NUM> of the other side of the negative electrode current collector <NUM> is not particularly limited. The surface <NUM> of the other side of the negative electrode current collector <NUM> may have a first layer <NUM> and a second layer <NUM> laminated thereon. Alternatively, a layer different from the first layer <NUM> and the second layer <NUM> may be laminated thereon. Alternatively, the surface may be exposed without any layer laminated thereon.

As shown in <FIG>, the cross-sectional configuration of the negative electrode <NUM> comprises a plurality of first layers <NUM>. Each of the first layers <NUM> comprises a first negative electrode active material and a first conductive material. The first negative electrode active material comprises at least one of elemental silicon, a silicon alloy, and a silicon oxide. In the cross-sectional configuration, the first layers <NUM> are in contact with the respective first regions 11x described above. In other words, in the cross-sectional configuration of the negative electrode <NUM>, the plurality of first layers <NUM> are arranged on the surface <NUM> of one side of the negative electrode current collector <NUM> at intervals along a plane direction of the surface <NUM>.

The first layers <NUM> comprise a first negative electrode active material. The first negative electrode active material comprises at least one (silicon-based negative electrode active material) of elemental silicon, a silicon alloy, and a silicon oxide. Particularly, when the first layers <NUM> comprise a silicon oxide, particularly silicon monoxide, as the first negative electrode active material, excellent performance is easily obtained. In the first layers <NUM>, one type of the first negative electrode active material may be used alone, or two or more types may be used in combination.

The first layers <NUM> may comprise another active material in addition to the above silicon-based negative electrode active material. For example, the first layers <NUM> may comprise at least one selected from carbon-based active materials such as graphite and hard carbon; various oxide-based active material such as lithium titanate; and metallic lithium and lithium alloys. The ratio of silicon-based active material to the negative electrode active material contained in the first layers <NUM> is not particularly limited, but the higher the ratio of the silicon-based active material, the more excellent performance is easily exhibited. For example, when the entire amount of the negative electrode active material contained in the first layers <NUM> is <NUM>% by mass, the silicon-based active material may be <NUM>% by mass or greater and <NUM>% by mass or less, <NUM>% by mass or greater and <NUM>% by mass or less, <NUM>% by mass or greater and <NUM>% by mass or less, <NUM>% by mass or greater and <NUM>% by mass or less, <NUM>% by mass or greater and <NUM>% by mass or less, or <NUM>% by mass or greater and <NUM>% by mass or less.

The negative electrode active material contained in the first layers <NUM> may have any shape as long as it is a general shape of negative electrode active materials of lithium-ion secondary batteries. For example, the negative electrode active material may be particulate. The negative electrode active material particles may be primary particles, or may be secondary particles of a plurality of agglomerated primary particles. The average particle diameter (D50) of the negative electrode active material particles, for example, may be <NUM> or more, <NUM> or more, or <NUM> or more, and may be <NUM> or less, <NUM> or less, <NUM> or less, or <NUM> or less. Note that the average particle diameter D50 referred herein is a particle diameter (median diameter) at an integrated value of <NUM>% in a volume-based particle size distribution determined by a laser diffraction/scattering method. The negative electrode active material particles contained in the first layers <NUM> may be larger or smaller than the negative electrode active material particles contained in the second layer <NUM> described below, or may be the same size as the negative electrode active material particles contained in the second layer <NUM> described below. In one embodiment, the negative electrode active material particles contained in the first layers <NUM> may be smaller than the negative electrode active material particles contained in the second layer <NUM> described below.

The amount of the negative electrode active material contained in the first layers <NUM> is not particularly limited. For example, the amount of the negative electrode active material contained in the first layers <NUM>, when the entirety of the first layers <NUM> is <NUM>% by mass, may be <NUM>% by mass or greater and less than <NUM>% by mass, <NUM>% by mass or greater and <NUM>% by mass or less, <NUM>% by mass or greater and <NUM>% by mass or less, or <NUM>% by mass or greater and <NUM>% by mass or less.

The first layers <NUM> comprise a first conductive material. When the first layers <NUM> do not comprise a first conductive material, cycle characteristics of a lithium-ion secondary battery deteriorate. The first conductive material may be, for example, at least one selected from carbon materials such as vapor-grown carbon fibers (VGCFs), acetylene black (AB), ketjen black (KB), carbon nanotubes (CNTs), and carbon nanofibers (CNFs); and metal materials such as nickel, titanium, aluminum, and stainless steel. Particularly, when the first conductive material comprises carbon, more excellent performance is easily exhibited. The first conductive material may be, for example, particulate or fibrous, and the size thereof is not particularly limited. One type of the first conductive material may be used alone, or two or more types may be used in combination.

The amount of the first conductive material contained in the first layers <NUM> is not particularly limited. For example, the amount of the first conductive material contained in the first layers <NUM>, when the entirety of the first layers <NUM> is <NUM>% by mass, may be greater than <NUM>% by mass and <NUM>% by mass or less, <NUM>% by mass or greater and <NUM>% by mass or less, <NUM>% by mass or greater and <NUM>% by mass or less, <NUM>% by mass or greater and <NUM>% by mass or less, or <NUM>% by mass or greater and <NUM>% by mass or less.

The first layers <NUM> may comprise a first binder. The first binder may be the same as or different from a second binder described below. Particularly, when the first layers <NUM> comprise the first binder, the second layer <NUM> comprises the second binder, and the first binder is different from the second binder, more excellent performance is easily exhibited. The first binder may be, for example, at least one selected from butadiene rubber (BR)-based binders, butylene rubber (IIR)-based binders, acrylate-butadiene rubber (ABR)-based binders, styrene-butadiene rubber (SBR)-based binders, carboxymethylcellulose (CMC)-based binders, polyvinylidene fluoride (PVdF)-based binders, polytetrafluoroethylene (PTFE)-based binders, polyimide (PI)-based binders, and polyacrylic acid (PAA)-based binders. Particularly, when the first binder has an imide skeleton (i.e., is a polyimide (PI)-based binder) or comprises polyacrylic acid (i.e., is a polyacrylic acid (PAA)-based binder), the first layers <NUM> are stronger, the volume changes of the first layers <NUM> during charging and discharging are even smaller, and cycle characteristics of the lithium-ion secondary battery are more easily improved. One type of the first binder may be used alone, or two or more types may be used in combination.

The amount of the first binder contained in the first layers <NUM> is not particularly limited. For example, the amount of the first binder contained in the first layers <NUM>, when the entirety of the first layers <NUM> is <NUM>% by mass, may be <NUM>% by mass or greater and less than <NUM>% by mass, greater than <NUM>% by mass and <NUM>% by mass or less, <NUM>% by mass or greater and <NUM>% by mass or less, <NUM>% by mass or greater and <NUM>% by mass or less, or <NUM>% by mass or greater and <NUM>% by mass or less.

The first layers <NUM> may comprise, for example, various additives such as a thickener, in addition to the above components. When a solid electrolyte is adopted as the electrolyte of a lithium-ion secondary battery, the first layers <NUM> may comprise a solid electrolyte. The solid electrolyte may be inserted inside the first layers <NUM> after the binder constituting the layers is cured. In this case, the method as described in, for example, <CIT> can be adopted.

As shown in <FIG>, in the cross-sectional configuration of the negative electrode <NUM>, the first layers <NUM> each may have a length L<NUM> along the surface of the negative electrode current collector <NUM>. The length L<NUM> is not particularly limited and can be appropriately determined according to target battery performance. In one embodiment, the length L<NUM> may be <NUM> or more and <NUM> or less. The length L<NUM> of one first layer <NUM> may be the same as or different from the length L<NUM> of another first layer <NUM>.

As shown in <FIG>, in the cross-sectional configuration of the negative electrode <NUM>, the first layers <NUM> can have a thickness in an intersecting direction to the surface of the negative electrode current collector <NUM>. The maximum thickness T<NUM> of the first layers <NUM> is not particularly limited and can be appropriately determined according to target battery performance. In one embodiment, the maximum T<NUM> may be <NUM> or more and less than <NUM>. The maximum thickness T<NUM> of one first layer <NUM> may be the same as or different from the maximum thickness T<NUM> of another first layer <NUM>.

A ratio L<NUM>/T<NUM> of the length L<NUM> to the maximum thickness T<NUM> described above is not particularly limited and can be appropriately determined according to target battery performance. In one embodiment, the ratio L<NUM>/T<NUM> of the length L<NUM> to the maximum thickness T<NUM> may be <NUM> or greater and <NUM> or less.

The first layers <NUM> may be arranged regularly on the surface <NUM> of one side of the negative electrode current collector <NUM>. <FIG> schematically show examples of arrangement of the first layers <NUM> when only the first layers <NUM> are laminated on the surface <NUM> of one side of the negative electrode current collector <NUM>. As shown in <FIG>, the first layers <NUM> may be present in an island form on the surface <NUM> of one side of the negative electrode current collector <NUM>. Alternatively, as shown in <FIG>, the first layers <NUM> may be present in a stripe form on the surface <NUM> of one side of the negative electrode current collector <NUM>. Alternatively, as shown in <FIG>, the first layers <NUM> may be present in a lattice form on the surface <NUM> of one side of the negative electrode current collector <NUM>. Alternatively, the first layers <NUM> may be arranged regularly in a form other than those above or may be arranged irregularly on the surface <NUM> of one side of the negative electrode current collector <NUM>.

The first layers <NUM> in a discharging state may have voids. By having voids in the first layers <NUM>, even when the first negative electrode active material expands during charging of the lithium-ion secondary battery, the expansion amount of the first layers <NUM> as a whole is likely small. A "discharging state" refers to a state in which lithium is not stored. The porosity of the first layers <NUM> in a discharging state may be <NUM>% or greater, <NUM>% or greater, or <NUM>% or greater. The porosity of the first layers <NUM> may be <NUM>% or greater and <NUM>% or less, <NUM>% or greater and <NUM>% or less, or <NUM>% or greater and <NUM>% or less. The porosity of the first layers <NUM> can be easily determined by observing a cross-section of the first layers <NUM> using an optical microscope or SEM and measuring the ratio of voids in the cross-section.

As shown in <FIG>, the cross-sectional configuration of the negative electrode <NUM> comprises a second layer <NUM>. The second layer <NUM> comprises a second negative electrode active material. The second negative electrode active material comprises graphite. However, the second layer <NUM> does not comprise silicon. In the cross-sectional configuration, the second layer <NUM> is in contact with both a second region 11y and surfaces 21x of one side of the first layers <NUM>. In other words, in the cross-sectional configuration of the negative electrode <NUM>, the second layer <NUM> is laminated over both the second region 11y and the surfaces 21x of one side of the first layers <NUM>.

The second layer <NUM> comprises a second negative electrode active material. The second negative electrode active material comprises graphite. However, the second layer <NUM> does not comprise silicon. Specifically, the second layer <NUM> does not comprise any of elemental silicon, a silicon alloy, and a silicon oxide. Note that, "the second layer does not comprise silicon" means that the second layer <NUM> is substantially free of silicon. In other words, the second layer <NUM> may comprise silicon at an impurity level. Specifically, the amount of silicon contained in the second layer <NUM> is less than <NUM>% by mass. The second layer <NUM> may comprise graphite and an active material other than silicon as the negative electrode active material. For example, the second layer <NUM> may comprise at least one selected from various oxide-based active materials such as lithium titanate, metallic lithium, and lithium alloys (excluding silicon alloys). The ratio of graphite in the negative electrode active material contained in the second layer <NUM> is not particularly limited, but the higher the ratio of graphite, the more excellent performance is easily exhibited. For example, when the entire amount of the negative electrode active material contained in the second layer <NUM> is <NUM>% by mass, graphite may be <NUM>% by mass or greater and <NUM>% by mass or less, <NUM>% by mass or greater and <NUM>% by mass or less, <NUM>% by mass or greater and <NUM>% by mass or less, <NUM>% by mass or greater and <NUM>% by mass or less, <NUM>% by mass or greater and <NUM>% by mass or less, or <NUM>% by mass or greater and <NUM>% by mass or less.

The negative electrode active material contained in the second layer <NUM> may have any shape as long as it is a general shape of negative electrode active materials of lithium-ion secondary batteries. For example, the negative electrode active material may be particulate. The negative electrode active material particles may be primary particles, or may be secondary particles of a plurality of agglomerated primary particles. The average particle diameter (D50) of the negative electrode active material particles, for example, may be <NUM> or more, <NUM> or more, or <NUM> or more, and may be <NUM> or less, <NUM> or less, <NUM> or less, or <NUM> or less. The negative electrode active material particles contained in the second layer <NUM> may be larger or smaller than the negative electrode active material particles contained in the first layers <NUM> described above, or may be the same size as the negative electrode active material particles contained in the first layers <NUM> described above.

The amount of the negative electrode active material contained in the second layer <NUM> is not particularly limited. For example, the amount of the negative electrode active material contained in the second layer <NUM>, when the entirety of the second layer <NUM> is <NUM>% by mass, may be <NUM>% by mass or greater and <NUM>% by mass or less, <NUM>% by mass or greater and <NUM>% by mass or less, <NUM>% by mass or greater and <NUM>% by mass or less, <NUM>% by mass or greater and <NUM>% by mass or less, or <NUM>% by mass or greater and <NUM>% by mass or less.

The second layer <NUM> may comprise a second conductive material. The second conductive material may be, for example, at least one selected from carbon materials such as vapor-grown carbon fibers (VGCFs), acetylene black (AB), ketjen black (KB), carbon nanotubes (CNTs), and carbon nanofibers (CNFs); and metal materials such as nickel, titanium, aluminum, and stainless steel. The second conductive material may be, for example, particulate or fibrous, and the size thereof is not particularly limited. One type of the second conductive material may be used alone, or two or more types may be used in combination.

The amount of the second conductive material contained in the second layer <NUM> is not particularly limited. The second layer <NUM>, as described above, comprises graphite and thereby has high conductivity. In this regard, the second layer <NUM> may not comprise a second conductive material. The amount of the second conductive material contained in the second layer <NUM>, when the entirety of the second layer <NUM> is <NUM>% by mass, may be <NUM>% by mass or greater and <NUM>% by mass or less, <NUM>% by mass or greater and <NUM>% by mass or less, <NUM>% by mass or greater and <NUM>% by mass or less, <NUM>% by mass or greater and <NUM>% by mass or less, <NUM>% by mass or greater and <NUM>% by mass or less, <NUM>% by mass or greater and <NUM>% by mass or less, or <NUM>% by mass or greater and <NUM>% by mass or less.

The second layer <NUM> may comprise a second binder. The second binder may be the same as or different from the first binder described above. Particularly, when the first binder is different from the second binder, more excellent performance is easily exhibited. The second binder may be, for example, at least one selected from butadiene rubber (BR)-based binders, butylene rubber (IIR)-based binders, acrylate-butadiene rubber (ABR)-based binders, styrene-butadiene rubber (SBR)-based binders, carboxymethylcellulose (CMC)-based binders, polyvinylidene fluoride (PVdF)-based binders, polytetrafluoroethylene (PTFE)-based binders, polyimide (PI)-based binders, and polyacrylic acid (PAA)-based binders. Particularly, when the second binder is a binder other than a polyimide (PI)-based binder or a polyacrylic acid (PAA)-based binder, irreversible capacity is smaller, resistance is smaller, and more excellent performance is easily exhibited. For example, when the second binder is at least one rubber-based binder selected from butadiene rubber (BR)-based binders, butylene rubber (IIR)-based binders, acetylene-butadiene rubber (ABR)-based binders, and styrene-butadiene rubber (SBR)-based binders, more excellent performance is easily exhibited. One type of the second binder may be used alone, or two or more types may be used in combination.

The amount of the second binder contained in the second layer <NUM> is not particularly limited. For example, the amount of the second binder contained in the second layer <NUM>, when the entirety of the second layer <NUM> is <NUM>% by mass, may be <NUM>% by mass or greater and <NUM>% by mass or less, <NUM>% by mass or greater and <NUM>% by mass or less, <NUM>% by mass or greater and <NUM>% by mass or less, <NUM>% by mass or greater and <NUM>% by mass or less, or <NUM>% by mass or greater and <NUM>% by mass or less.

The second layer <NUM> may comprise, for example, various additives such as a thickener, in addition to the above components. When a solid electrolyte is adopted as the electrolyte of a lithium-ion secondary battery, the second layer <NUM> may comprise a solid electrolyte. The solid electrolyte may be mixed with graphite when the second layer <NUM> is formed.

As shown in <FIG>, in the cross-sectional configuration of the negative electrode <NUM>, the second layer <NUM> may have a thickness in an intersecting direction to the surface of the negative electrode current collector <NUM>. The "thickness of the second layer" is a thickness from the surface in contact with the negative electrode current collector <NUM> to a surface on an opposite side. The maximum thickness T<NUM> of the second layer <NUM> is not particularly limited and can be appropriately determined according to target battery performance. In one embodiment, the maximum thickness T<NUM> may be more than <NUM> and <NUM> or less.

As shown in <FIG>, in the cross-sectional configuration of the negative electrode <NUM>, the maximum thickness T<NUM> of the second layer <NUM> is larger than the maximum thickness T<NUM> of the first layers <NUM>. Particularly, when the ratio T<NUM>/T<NUM> of the maximum thickness T<NUM> of the first layers <NUM> to the maximum thickness T<NUM> of the second layer <NUM> is <NUM> or less, the second layer <NUM> having a sufficient thickness is laminated on the surfaces 21x of one side of the first layers <NUM>, peeling of the second layer <NUM> does not easily occur, and more excellent performance is easily exhibited. The ratio T<NUM>/T<NUM> may be <NUM> or greater and <NUM> or less, or <NUM> or greater and <NUM> or less.

As shown in <FIG>, the second layer <NUM> has unevenness on the side facing the negative electrode current collector <NUM>. Specifically, the portions in contact with the surfaces 21x of one side of the first layers <NUM> can be recessed portions, and the portions in contact with the second region 11y of the surface <NUM> of one side of the negative electrode current collector <NUM> can be protruding portions. However, the shape of the opposite side to the side facing the negative electrode current collector <NUM> is not particularly limited. The second layer <NUM> may or may not have unevenness on the opposite side to the side facing the negative electrode current collector <NUM>.

The second layer <NUM> in a discharging state may have voids. The porosity of the second layer <NUM> in a discharging state may be <NUM>% or greater or <NUM>% or less, <NUM>% or greater and <NUM>% or less, <NUM>% or greater and <NUM>% or less, or <NUM>% or greater and <NUM>% or less. The porosity of the second layer <NUM> may be less than the porosity of the first layers.

The negative electrode <NUM>, in addition to the above features, may comprise general features for a negative electrode for lithium-ion secondary batteries, for example, tabs and terminals.

In a silicon-based active material during charging (when alloying with lithium), the volume expands greatly, ion conduction paths and electron conduction paths with the active material are cut, and cycle characteristics easily deteriorate. To mitigate this effect, a polyimide (PI)-based binder or a polyacrylic acid (PAA)-based binder, which has higher strength than binders generally used for graphite negative electrodes, can be used with the silicon-based active material. In addition, a conductive material such as ketjen black or acetylene black can be used with the silicon-based active material. As another mitigation method, in addition to the silicon-based active material, graphite fine particles having a small expansion coefficient can be mixed to form a composite. However, in order to obtain satisfactory cycle characteristics, a strong binder or conductive material is required near the silicon-based active material, and as a result, a strong binder or conductive material, which is originally not required, is present near the graphite. A portion of the strong binder or conductive material may react with lithium to generate irreversible capacity and decrease capacity. Further, the volume of the binder or conductive material itself may be a factor in decreasing capacity. Furthermore, adding an unnecessarily large amount of binder may inhibit the reaction between the active material and lithium, leading to an increase in resistance. To solve these issues, PTL <NUM> (<CIT>) describes that a negative electrode is separated into two layers, a layer mainly composed of a silicon-based active material and a layer mainly composed of graphite, and these layers are laminated planarly. By separating the layer mainly composed of a silicon-based active material and the layer mainly composed of graphite into two layers, it is possible, for example, to arrange a strong binder or conductive material only around the silicon-based active material, and the freedom in design is increased. However, in such a configuration, when the silicon-based active material expands, the entire layer mainly composed of the silicon-based active material expands as a whole. According to the findings of the present inventors, in a layer mainly composed of a silicon-based active material, the silicon-based active materials adjacent to each other in the plane direction exert a reaction force on each other due to expansion, and expansion in the horizontal direction parallel to the negative electrode current collector is difficult. Thus, expansion stress of the layer mainly composed of a silicon-based active material is likely to concentrate on the layer mainly composed of graphite. In other words, when the silicon-based active material expands due to charging, the layer mainly composed of graphite is likely to collapse and stress is likely to occur in the layer mainly composed of graphite. As a result, cracking and peeling occur and cycle characteristics deteriorate in the layer mainly composed of graphite, despite the small expansion amount thereof.

In contrast, in the negative electrode <NUM> for a lithium-ion secondary battery of the present disclosure, as shown in <FIG>, a second region 11y is present between a plurality of first regions 11x on the surface <NUM> of one side of the negative electrode current collector <NUM>, and an interval I<NUM> is present between one first layer <NUM> and another first layer <NUM> in the plane direction of negative electrode current collector <NUM>. Thus, when the silicon-based active material within the first layers <NUM> expands due to charging, the first layers <NUM> can expand not only in the thickness direction (normal direction) thereof but also in a direction along the plane of the current collector <NUM> (lateral direction). Specifically, the expansion of the first layers <NUM> is distributed in the normal direction and the lateral direction, stress concentrations near the contact portions between the first layers <NUM> and the second layer <NUM> are suppressed, and shape collapse and cracking in the second layer <NUM> do not easily occur. In other words, the ion conduction paths and conductive paths in the first layers <NUM> and the second layer <NUM> are easily maintained. Further, in the negative electrode <NUM> for a lithium-ion secondary battery of the present disclosure, a portion of the second layer <NUM> comprising graphite having small expansion and contraction is in direct contact with the negative electrode current collector <NUM>, and consequently paths do not easily break. As a result, when the negative electrode <NUM> of the present disclosure is applied to a lithium-ion secondary battery, cycle characteristics of the lithium-ion secondary battery can be improved.

The negative electrode for a lithium-ion secondary battery of the present disclosure can be manufactured by, for example, the following method. <FIG> shows the flow of the method for manufacturing a negative electrode <NUM> for a lithium-ion secondary battery according to one embodiment. As shown in <FIG>, the method for manufacturing a negative electrode <NUM> for a lithium-ion secondary battery comprises the following first step, second step, and third step. The first step comprises preparing a negative electrode current collector <NUM>, wherein in the cross-sectional configuration of the negative electrode current collector <NUM>, a surface <NUM> of one side of the negative electrode current collector <NUM> has a plurality of first regions 11x and a second region 11y between the first regions 11x. The second step comprises laminating a first layer <NUM> on the first regions 11x, wherein the first layer <NUM> comprises a first negative electrode active material and a first conductive material, and the first negative electrode active material comprises at least one of elemental silicon, a silicon alloy, and a silicon oxide. The third step comprises laminating a second layer <NUM> on the negative electrode current collector <NUM> and the first layer <NUM> to bring the second layer <NUM> into contact with both a surface 21x of one side of the first layer <NUM> and the second region 11y, wherein the second layer <NUM> comprises a second negative electrode active material, the second negative electrode active material comprises graphite, and the second layer <NUM> does not comprise silicon.

In the first step, a negative electrode current collector <NUM> is prepared. The configuration of the negative electrode current collector <NUM> is as described above.

In the second step, a first layer <NUM> is laminated on first regions 11x on a surface <NUM> of one side of the negative electrode current collector <NUM>. The configuration of the first layer <NUM> is as described above.

The method of laminating the first layer <NUM> is not particularly limited. For example, the first layer <NUM> can be laminated on the first regions 11x of the surface <NUM> of one side of the negative electrode current collector <NUM> by preparing a paste comprising materials constituting the first layer <NUM>, applying the paste to the first regions 11x of the surface <NUM> of one side of the negative electrode current collector <NUM>, and then drying the paste. When applying the paste, it is preferable that a portion of the surface <NUM> (second region 11y) of one side of the negative electrode current collector <NUM> be masked so that the paste is not applied to regions other than the first regions 11x of the surface <NUM> of one side of the negative electrode current collector <NUM>. The solvent constituting the paste is not particularly limited, and water and various organic solvents can be adopted. The means of applying the paste is not particularly limited, and a doctor blade may be used, or spraying may be used. Alternatively, various printing means such as inkjet printing may be used.

When a thermosetting binder such as a polyimide (PI)-based binder is adopted in the first layer <NUM>, the first layer <NUM> can be formed, for example, as follows. Specifically, a paste comprising a silicon-based active material constituting the first layer <NUM> and polyamic acid is prepared, the paste is applied to the first regions 11x of the surface <NUM> of one side of the negative electrode current collector <NUM> and then heated to imidize the polyamic acid, whereby a first layer <NUM> comprising a silicon-based active material and a polyimide-based binder can be formed on the first regions 11x.

In the third step, a second layer <NUM> is laminated on the negative electrode current collector <NUM> and the first layer <NUM>, and the second layer <NUM> is brought into contact with both a surface 21x of one side of the first layer <NUM> and the second region 11y. The configuration of the second layer <NUM> is as described above.

The method of laminating the second layer <NUM> is not particularly limited. For example, the second layer <NUM> can be brought into contact with both the surface 21x of one side of the first layer <NUM> and the second region 11y by preparing a paste comprising materials constituting the second layer <NUM>, applying the paste over both the surface 21x of one side of the first layer <NUM> and the second region 11y of the surface <NUM> of one side of the negative electrode current collector <NUM>, and then drying the paste. The solvent constituting the paste is not particularly limited, and water and various organic solvents can be adopted. The means of applying the paste is not particularly limited, and a doctor blade may be used, or spraying may be used. Alternatively, various printing means such as inkjet printing may be used. The method of forming the second layer <NUM> may be the same as or different from the method of forming the first layer <NUM>.

The method for manufacturing a negative electrode <NUM> for a lithium-ion secondary battery may comprise additional steps, in addition to the above first step, second step, and third step. For example, each layer described above may be pressed. Tabs and terminals may be attached.

The technique of the present disclosure also includes an aspect as a lithium-ion secondary battery. <FIG> schematically shows a cross-sectional configuration of the lithium-ion secondary battery <NUM> according to one embodiment. As shown in <FIG>, the lithium-ion secondary battery <NUM> comprises the above negative electrode <NUM> of the present disclosure, a positive electrode <NUM>, and an electrolyte layer <NUM>.

The configuration of the negative electrode <NUM> is as described above.

The positive electrode <NUM> may comprise, for example, a positive electrode active material layer <NUM> and a positive electrode current collector <NUM> in contact with the positive electrode active material layer <NUM>.

The positive electrode active material layer <NUM> comprises at least a positive electrode active material. In addition, the positive electrode active material layer <NUM> may optionally comprise an electrolyte, a conductive material, a binder, and various additives. The content of each component in the positive electrode active material layer <NUM> may be appropriately determined according to target battery performance. For example, when the entire solid content of the positive electrode active material layer <NUM> is <NUM>% by mass, the content of the positive electrode active material may be <NUM>% by mass or greater, <NUM>% by mass or greater, <NUM>% by mass or greater, or <NUM>% by mass or greater and may be <NUM>% by mass or less, less than <NUM>% by mass, <NUM>% by mass or less, or <NUM>% by mass or less. The shape of the positive electrode active material layer <NUM> is not particularly limited, and may be a sheet having a substantially flat surface. The thickness of the positive electrode active material layer <NUM> is not particularly limited, and for example, may be <NUM> or more, <NUM> or more, <NUM> or more, or <NUM> or more and may be <NUM> or less, <NUM> or less, <NUM> or less, or <NUM> or less.

Any known positive electrode active material for lithium-ion secondary batteries can be adopted as the positive electrode active material contained in the positive electrode active material layer <NUM>. Of known active materials, a material having a relatively high potential (charge/discharge potential) for storing and releasing lithium ions can be used as a positive electrode active material, and a material having a relatively low potential can be used as the negative electrode active material described above. The positive electrode active material may be, for example, at least one selected from various lithium-containing compounds, elemental sulfur, and sulfur compounds. The lithium-containing compound as a positive electrode active material may be a lithium-containing oxide comprising at least one element M, Li, and O. The element M may be, for example, at least one selected from Mn, Ni, Co, Al, Mg, Ca, Sc, V, Cr, Cu, Zn, Ga, Ge, Y, Zr, Sn, Sb, W, Pb, Bi, Fe, and Ti, and may be at least one selected from the group consisting of Mn, Ni, Co, Al, Fe, and Ti. More specifically, the lithium-containing oxide may be at least one selected from lithium cobaltate, lithium nickelate, lithium manganate, lithium nickel-cobalt oxide, lithium nickel-manganese oxide, lithium cobalt-manganese oxide, lithium nickel-cobalt-manganese oxide (Li<NUM>±aNixCoyMnzO<NUM>±δ (for example, <NUM> < x < <NUM>, <NUM> < y < <NUM>, <NUM> < z < <NUM>, x + y + z = <NUM>)), spinel-based lithium compounds (such as heteroelement-substituted Li-Mn spinels of compositions represented by Li<NUM>+xMn<NUM>-x-yMyO<NUM> (M is one or more selected from Al, Mg, Co, Fe, Ni, and Zn)), lithium nickel-cobalt-aluminum oxide (for example, Li<NUM>±aNipCoqAlrO<NUM>±δ (for example, p + q + r = <NUM>)), lithium titanate, and lithium metal phosphate (such as LiMPO<NUM>, where M is one or more selected from Fe, Mn, Co, and Ni). Particularly, when the positive electrode active material comprises at least a lithium-containing oxide comprising at least one of Ni, Co, and Mn, Li, and O as constituent elements, the performance of a secondary battery is easily further enhanced. Alternatively, when the positive electrode active material comprises at least a lithium-containing oxide comprising at least one of Ni, Co, and Al, Li, and O as constituent elements, the performance of a secondary battery is easily further enhanced. One type of the positive electrode active material may be used alone, or two or more types may be used in combination. The positive electrode active material may have any shape as long as it is a general shape of positive electrode active materials of secondary batteries, and, for example, may be particulate. The positive electrode active material may have voids, for example, may be porous, or may be hollow. The positive electrode active material may be primary particles, or may be secondary particles of agglomerated primary particles. The average particle diameter D50 of the positive electrode active material, for example, may be <NUM> or more, <NUM> or more, or <NUM> or more, and may be <NUM> or less, <NUM> or less, <NUM> or less, or <NUM> or less. Note that the average particle diameter D50 referred herein is a particle diameter (median diameter) at an integrated value of <NUM>% in a volume-based particle size distribution determined by a laser diffraction/scattering method.

A protective layer having ion-conducting properties may be formed on the surface of the positive electrode active material. Specifically, the positive electrode active material layer <NUM> may comprise a composite of a positive electrode active material and a protective layer, and at least a portion of the surface of the positive electrode active material of the composite may be covered by the protective layer. As a result, for example, a reaction between the positive electrode active material and another battery material (such as a sulfide solid electrolyte described below) is easily suppressed. The protective layer having ion-conducting properties can comprise various ion-conducting compounds. The ion-conducting compound may be, for example, at least one selected from ion-conducting oxides and ion-conducting halides. The ion-conducting oxide may comprise, for example, at least one element selected from B, C, Al, Si, P, S, Ti, La, Zr, Nb, Mo, Zn, and W, Li, and O. The ion-conducting oxide may be an oxynitride comprising N. More specifically, the ion-conducting oxide may be at least one selected from Li<NUM>BO<NUM>, LiBO<NUM>, Li<NUM>CO<NUM>, LiAlO<NUM>, Li<NUM>SiO<NUM>, Li<NUM>SiO<NUM>, Li<NUM>PO<NUM>, Li<NUM>SO<NUM>, Li<NUM>TiO<NUM>, Li<NUM>TisOi<NUM>, Li<NUM>Ti<NUM>O<NUM>, Li<NUM>ZrO<NUM>, LiNbO<NUM>, Li<NUM>MoO<NUM>, Li<NUM>WO<NUM>, LiPON, Li<NUM>O-LaO<NUM>, and Li<NUM>O-ZnO<NUM>. The ion-conducting oxide may have a portion of elements substituted by various doping elements. The ion-conducting halide may be, for example, at least one of various compounds exemplified as halide solid electrolytes described below. The halide having ion-conducting properties may comprise, for example, at least one element selected from the group consisting of Mg, Ca, Sr, Ba, Zn, Sn, Al, Sc, Ga, Bi, Sb, Zr, Hf, Ti, Ta, Nb, W, Y, Gd, Tb, and Sm, at least one halogen element selected from the group consisting of Cl, Br, I, and F, and Li. The halide having ion-conducting properties may comprise at least one selected from the group consisting of Ti, Al, Gd, Ca, Zr, and Y, at least one selected from the group consisting of Cl, Br, I, and F, and Li. Further, the halide having ion-conducting properties may comprise at least one selected from the group consisting of Ti and Al, at least one selected from the group consisting of Cl, Br, I, and F, and Li. Furthermore, the halide having ion-conducting properties may be a composite halide of, for example, Li, Ti, Al, and F. The coverage (area ratio) of the protective layer relative to the surface of the positive electrode active material, for example, may be <NUM>% or greater, <NUM>% or greater, or <NUM>% or greater. The thickness of the protective layer, for example, may be <NUM> or more or <NUM> or more, and may be <NUM> or less or <NUM> or less.

The electrolyte that can be contained in the positive electrode active material layer <NUM> may be a liquid electrolyte (electrolytic solution), may be a solid electrolyte, or may be a combination thereof. The liquid electrolyte (electrolytic solution) is a liquid comprising lithium ions as carrier ions. The electrolytic solution may be an aqueous electrolytic solution or a nonaqueous electrolytic solution. The composition of the electrolytic solution may be the same as one known as a composition of an electrolytic solution for lithium-ion secondary batteries. The electrolytic solution may be water or a nonaqueous solvent dissolving a lithium salt. Examples of the nonaqueous solvent include various carbonate-based solvents. Examples of the lithium salt include LiPF<NUM> and lithium amide salts. The solid electrolyte may be an inorganic solid electrolyte or an organic polymer electrolyte. Particularly, an inorganic solid electrolyte has excellent ion-conducting properties and heat resistance. Examples of the inorganic solid electrolyte include oxide solid electrolytes, sulfide solid electrolytes, and solid electrolytes having ion-bonding properties. Among inorganic solid electrolytes, sulfide solid electrolytes, particularly sulfide solid electrolytes comprising at least Li, S, and P as constituent elements, have high performance. Alternatively, among inorganic solid electrolytes, solid electrolytes having ion-bonding properties, particularly solid electrolytes comprising at least Li, Y, and a halogen (at least one of Cl, Br, I, and F) as constituent elements, have high performance. The solid electrolyte may be amorphous or crystalline. The solid electrolyte may be particulate. The average particle diameter (D50) of the solid electrolyte, for example, may be <NUM> or more and <NUM> or less. The ionic conductivity at <NUM> of the solid electrolyte, for example, may be <NUM> × <NUM>-<NUM> S/cm or more, <NUM> × <NUM>-<NUM> S/cm or more, or <NUM> × <NUM>-<NUM> S/cm or more. In the positive electrode active material layer <NUM>, one type of electrolyte may be used alone, or two or more types may be used in combination.

The conductive material and the binder that can be contained in the positive electrode active material layer <NUM> may be appropriately selected from among conductive materials and binders exemplified as ones that can be contained in the first layers <NUM> and the second layer <NUM> of the negative electrode <NUM> above. The positive electrode active material layer <NUM> may comprise various additives in addition to the above components, for example, dispersants and lubricants.

Any general positive electrode current collector for lithium-ion secondary batteries can be adopted as the positive electrode current collector <NUM>. The positive electrode current collector <NUM> may have at least one shape selected from foil-like, plate-like, mesh-like, punched metal-like, and a foam. The positive electrode current collector <NUM> may be composed of a metal foil or a metal mesh. Particularly, a metal foil has excellent handleability. The positive electrode current collector <NUM> may consist of a plurality of foils. Examples of a metal constituting the positive electrode current collector <NUM> include at least one selected from Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, V, Mg, Pb, Ge, In, Sn, Zr, and stainless steel. Particularly, from the viewpoint of ensuring oxidation resistance, the positive electrode current collector <NUM> may comprise Al. The positive electrode current collector <NUM> may have on the surface thereof some coating layer for the purpose of adjusting resistance. For example, the positive electrode current collector <NUM> may have a carbon coating layer. The positive electrode current collector <NUM> may be a metal foil or a substrate plated or vapor-deposited with a metal described above. When the positive electrode current collector <NUM> consists of a plurality of metal foils, there may be some layer between the plurality of metal foils. The thickness of the positive electrode current collector <NUM> is not particularly limited, and for example, may be <NUM> or more or <NUM> or more, and may be <NUM> or less or <NUM> or less.

An electrolyte layer <NUM> is arranged between the negative electrode <NUM> and the positive electrode <NUM>. The electrolyte layer <NUM> comprises at least an electrolyte. The electrolyte layer <NUM> may comprise at least one of a liquid electrolyte (electrolytic solution) and a solid electrolyte. When the electrolyte layer <NUM> comprises a solid electrolyte, the electrolyte layer <NUM> may further optionally comprise a binder. The contents of the electrolyte and the binder in the electrolyte layer <NUM> are not particularly limited. Alternatively, the electrolyte layer <NUM> may comprise a separator for retaining an electrolytic solution and preventing contact between the negative electrode <NUM> and the positive electrode <NUM>. The thickness of the electrolyte layer <NUM> is not particularly limited, and for example, may be <NUM> or more or <NUM> or more, and may be <NUM> or less or <NUM> or less. The electrolyte layer <NUM> may consist of one layer, or may consist of a plurality of layers. For example, the electrolyte layer <NUM> may comprise of a first electrolyte layer arranged on the negative electrode <NUM> side and a second electrolyte layer arranged on the positive electrode <NUM> side, and the first electrolyte layer may comprise a first electrolyte and the second electrolyte layer may comprise a second electrolyte. The first electrolyte and the second electrolyte may be of different types from each other. The first electrolyte and the second electrolyte may each be at least one selected from the above oxide solid electrolytes, sulfide solid electrolytes, and solid electrolytes having ion-bonding properties. For example, the first electrolyte layer may comprise at least one of a solid electrolyte having ion-bonding properties and a sulfide solid electrolyte, and the second electrolyte layer may comprise a solid electrolyte having ion-bonding properties. The electrolyte contained in the electrolyte layer <NUM> may be appropriately selected from among electrolytes (liquid electrolytes and/or solid electrolytes) exemplified as ones that can be contained in the positive electrode active material layer described above. The binder that can be contained in the electrolyte layer <NUM> may also be appropriately selected from among binders exemplified as ones that can be contained in the negative electrode <NUM> (first layers <NUM>, second layer <NUM>) and the positive electrode active material layer <NUM> described above. One type of each of the electrolyte and the binder may be used alone, or two or more types may be used in combination. The separator may be any separator as long as the separator is commonly used in lithium-ion secondary batteries. Examples thereof include those made of resins such as polyethylene (PE), polypropylene (PP), polyester, and polyamide. The separator may be of a single-layer structure, or may be of a multilayer structure. Examples of a separator having a multilayer structure can include separators of a PE/PP two-layer structure and separators of a PP/PE/PP or PE/PP/PE three-layer structure. The separator may consist of a nonwoven fabric such as cellulose nonwoven fabric, resin nonwoven fabric, or glass fiber nonwoven fabric.

The lithium-ion secondary battery <NUM>, in addition to the above configuration, may be provided with any general configuration as a lithium-ion secondary battery. For example, the above components of the lithium ion secondary battery <NUM> may be housed inside an outer package. Any known outer packaging of a battery can be adopted. In addition, a plurality of the lithium-ion secondary batteries <NUM> may be optionally electrically connected and optionally stacked to form a battery pack. In this case, the battery pack may be housed inside any known battery case. The lithium-ion secondary battery <NUM> may comprise additional obvious features such as the necessary terminals. Examples of the shape of the lithium-ion secondary battery <NUM> can include coin-type, laminate-type, cylindrical, and rectangular.

The lithium-ion secondary battery <NUM> can be manufactured by applying any known method, for example, can be manufactured as follows. However, the method for manufacturing the lithium-ion secondary battery <NUM> is not limited to the following method, and each layer may be formed by, for example, dry molding.

The technique of the present disclosure also has an aspect as a method of improving cycle characteristics of a lithium-ion secondary battery. Specifically, in the method of the present disclosure, the above negative electrode of the present disclosure is adopted in a lithium-ion secondary battery, whereby cycle characteristics of the lithium-ion secondary battery is improved. The details of the configuration of the lithium-ion secondary battery are as described above.

As described above, the lithium-ion secondary battery of the present disclosure has excellent cycle characteristics. Such a secondary battery can be suitably used in, for example, at least one type of vehicle selected from hybrid vehicles (HEVs), plug-in hybrid vehicles (PHEVs), and battery electric vehicles (BEVs). Specifically, the vehicle of the present disclosure comprises a lithium-ion secondary battery, wherein the lithium-ion secondary battery comprises a negative electrode, a positive electrode, and an electrolyte layer, wherein the negative electrode is the negative electrode of the present disclosure, i.e., has the following cross-sectional configuration: the cross-sectional configuration comprises a negative electrode current collector, a plurality of first layers, and a second layer;.

Hereinafter, the technique of the present disclosure will be further described in detail with reference to the Examples. However, the technique of the present disclosure is not limited to the following Examples.

A positive electrode active material (average particle diameter: <NUM>, LiNi<NUM>Co<NUM>Mn<NUM>O<NUM>), a conductive material (granular acetylene black), and a binder (PVdF) at a mass ratio of <NUM>:<NUM>:<NUM> were kneaded with NMP to produce a positive electrode paste. The solid content ratio of the positive electrode paste was adjusted to <NUM>% with NMP. The produced positive electrode paste was applied on a <NUM>-thick Al foil using a doctor blade and dried in a drying furnace at <NUM> for <NUM> to obtain a coated body. The single-side basis weight after drying was adjusted to <NUM>/cm<NUM>. The coated body was then pressed with a roll press machine to obtain a positive electrode comprising a positive electrode active material layer on a surface of the Al foil. The composite density of the positive electrode active material layer after pressing was <NUM>/cc.

Silicon monoxide fine particles having an average particle diameter of <NUM> were used as a negative electrode active material, polyamic acid was used as a first binder source, and acetylene black was used as a conductive material. These were mixed at a mass ratio of <NUM>:<NUM>:<NUM>, added to NMP, and kneaded, whereby a first-layer paste was produced. The solid content of the first-layer paste was adjusted to <NUM>% with NMP. A perforated Cu foil (with holes in a circular shape having an average diameter of <NUM> spaced at equal intervals over <NUM>% of the area, a thickness of <NUM>) was laid on a <NUM>-thick Cu foil, and from the top thereof the first-layer paste was applied with a blade coater. The perforated Cu foil was then removed before the coating film dried, whereby a first coated body in which a composite was scattered in an island form on about <NUM>% of the surface of the Cu foil was obtained (refer to <FIG>). The first coated body was dried at <NUM> for <NUM> and then pressed with a roll press to obtain a first pressed body. The first pressed body was then placed in a furnace and baked at <NUM> for <NUM> in a vacuum atmosphere to convert the polyamic acid into polyimide to obtain a first laminated body in which a plurality of island-like first layers were scattered on the surface of the Cu foil. The basis weight per unit area of the first layers of the first laminated body was <NUM>/cm<NUM>, the maximum thickness T<NUM> of the island-like first layers was <NUM>, and the composite density of the first layers was <NUM>/cc.

Spherical natural graphite fine particles having an average particle diameter of <NUM> was used as a negative electrode active material, carboxymethylcellulose (CMC) was used as a thickener, and styrene-butadiene rubber (SBR) was used as a second binder. These were mixed at a mass ratio of <NUM>:<NUM>:<NUM>, added to water, and kneaded, whereby a second-layer paste was prepared. The solid content of the second-layer paste was adjusted to <NUM>% with water. The second-layer paste was then applied to the above first laminated body with a blade coater to obtain a second coated body. The second coated body was dried at <NUM> for <NUM> and then pressed with a roll press to obtain a negative electrode comprising first layers and a second layer on a Cu foil. The basis weight of the second layer of the negative electrode was <NUM>/cm<NUM>, and the maximum thickness of the second layer was <NUM>. Note that the cross-section of the negative electrode was observed with an optical microscope, and as a result, it was confirmed that the thickness of the first layers did not change after formation of the second layer. In addition, it was confirmed that the second layer was in contact with both the surface of the Cu foil and the surfaces of the first layers.

The above positive electrode and negative electrode were punched out into a disk shape having a diameter of <NUM>, placed facing each other via a separator (polyethylene porous body having a porosity of <NUM>% and a thickness of <NUM>) in a disk shape having a diameter of <NUM>, and installed in a coin-type cell can. <NUM>µL of an electrolytic solution (EC:FEC:EMC:DMC = <NUM>:<NUM>:<NUM>:<NUM> (Vol ratio), <NUM> LIPF<NUM>) was added to the electrode body, the coin can was caulked and sealed, and a coin cell for evaluation was obtained.

A charge/discharge test was carried out on the coin cell under the conditions below, and the capacity, resistance, and cycle characteristics (capacity retention rate after <NUM> cycles = discharge capacity after <NUM> cycles/initial discharge capacity) of the coin cell were evaluated. Evaluation results are shown in Table <NUM> below.

Except that when forming the first layers, in place of a perforated Cu foil, a Cu foil in which stripes having a length of <NUM>, a width of <NUM>, and a distance between widths (= Cu foil width) of <NUM> were placed was stacked on a <NUM>-thick Cu foil and coated, a negative electrode was produced by the same procedure as in Example <NUM>, a coin cell was produced, and the coin cell was evaluated. Except that the first layers were in a stripe form (band-like), the negative electrode according to Example <NUM> was the same as the negative electrode according to Example <NUM>. Note that the area ratio of the first layers was <NUM>%, which was the same as in Example <NUM>. Evaluation results are shown in Table <NUM> below.

Except that the pressing pressure on the first layers was adjusted and the porosity was changed, the procedure was the same as in Example <NUM>. Evaluation results are shown in Table <NUM> below.

Except that the ratio of holes in the perforated Cu foil of Example <NUM> was adjusted and the area ratio of the first layers to the surface of the Cu foil was changed, the procedure was the same as in Example <NUM>. Evaluation results are shown in Table <NUM> below.

Except that the active material contained in the first layers was changed to Si<NUM>Fe alloy fine particles having an average particle diameter of <NUM>, the procedure was the same as in Example <NUM>. Evaluation results are shown in Table <NUM> below.

Except that the binder contained in the first layers was changed from a polyimide-based binder to a polyacrylic acid-based binder, the paste solvent was changed to water, and the heat treatment at <NUM> carried out in Example <NUM> was not carried out, the procedure was the same as in Example <NUM>. Evaluation results are shown in Table <NUM> below.

Except that a perforated Cu foil was not used during the production of the first layer and a uniform film was formed planarly on the entire surface of the Cu foil, the procedure was the same as in Example <NUM>. Evaluation results are shown in Table <NUM> below.

Except that a conductive material was not used in the first layer and the basis weight was adjusted, the procedure was the same as in Example <NUM>. Evaluation results are shown in Table <NUM> below.

The production conditions (type and amount of active material, type and amount of binder, type and amount of conductive material, basis weight of first layer, porosity of first layer, maximum thickness T<NUM> of first layer, ratio T<NUM>/T<NUM> of maximum thickness T<NUM> of first layer to maximum thickness T<NUM> of second layer, and area ratio of first layer when area of the entire Cu foil surface was <NUM>%) of the first layer and the evaluation results (capacity, resistance, and capacity retention rate after <NUM> cycles) of the coin cell for each of Examples <NUM> to <NUM> and Comparative Examples <NUM> and <NUM> are shown in Table <NUM> below.

From the results shown in Table <NUM>, the following was found.

In the above Examples, cases where a polyimide (PI)-based binder or a polyacrylic acid (PAA)-based binder was adopted in the first layers were exemplified, but the type of binder in the first layers is not limited thereto. In Examples <NUM> to <NUM> and Comparative Example <NUM>, it is considered that even if binders other than those above were used, the expansion amount is decreased by the above mechanism, and cycle characteristics of the lithium-ion secondary battery are improved in Examples <NUM> to <NUM> compared to Comparative Example <NUM>. However, in the first layers comprising the silicon-based active material, it is preferable that a strong binder be adopted in order to suppress expansion of the silicon-based active material. In this regard, it is preferable that a polyimide (PI)-based binder or a polyacrylic acid (PAA)-based binder be adopted in the first layers.

In the above Examples, first layers in an island form and first layers in a stripe form were exemplified, but the form of the first layers is not limited thereto. In the cross-sectional configuration of the negative electrode, it is considered that when there is an interval between one first layer and another first layer, the same effect can be achieved.

Although not shown in the above Examples, when a mixed layer of silicon-based active material and graphite is adopted as the negative electrode active material layer (i.e., when silicon-based active material particles and graphite particles are mixed in one layer without separating the first layers comprising the silicon-based active material and the second layer comprising graphite), capacity, resistance, and cycle characteristics cannot be achieved at the same time as described below. First, when a sufficient amount of a strong binder such as a polyimide-based binder is contained in the mixed layer, expansion of the silicon-based active material is suppressed by the binder, resulting in satisfactory cycle characteristics. However, in the mixed layer, it is difficult to preferentially arrange the binder around the silicon-based active material. In order to arrange the necessary and sufficient amount of the binder around the silicon-based active material, it is necessary to increase the absolute amount of the binder in the mixed layer, and the binder is arranged near the graphite, where the binder is not required. Thus, irreversible capacity is increased and resistance is increased. On the other hand, when a flexible binder such as a rubber-based binder is contained in the mixed layer, expansion of the silicon-based active material is not suppressed and cycle characteristics deteriorate. Further, due to the expansion of the silicon-based active material, ion conduction paths and conductive paths are cut off, and capacity and resistance deteriorate.

Claim 1:
A negative electrode for a lithium-ion secondary battery, having the following cross-sectional configuration, wherein:
the cross-sectional configuration comprises a negative electrode current collector, a plurality of first layers, and a second layer;
each of the first layers comprises a first negative electrode active material and a first conductive material;
the first negative electrode active material comprises at least one of elemental silicon, a silicon alloy, and a silicon oxide;
the second layer comprises a second negative electrode active material;
the second negative electrode active material comprises graphite;
the second layer does not comprise silicon;
in the cross-sectional configuration, a surface of one side of the negative electrode current collector has a plurality of first regions and a second region between the first regions;
in the cross-sectional configuration, the first layers are in contact with the respective first regions; and
in the cross-sectional configuration, the second layer is in contact with both the second region and surfaces of one side of the first layers.