Patent Description:
Si, a compound containing Si, or the like is an alloying material which alloys with lithium, and it is known that the alloying material can occlude a larger amount of lithium ions per unit volume in comparison to a carbon-based active material such as graphite. For example, Patent Literature <NUM> discloses a negative electrode for a lithium ion secondary battery having a negative electrode mixture layer which contains a compound containing Si and graphite as a negative electrode active material.

In a negative electrode including an alloying material, reduction of charge/discharge cycle characteristic is often problematic. This is considered be caused by a large volume change of the alloying material due to charging and discharging, which results in formation of an inter-particle gap for the negative electrode active material, and which consequently results in an increased number of particles of the negative electrode active material isolated from an electrically conductive path in the negative electrode mixture layer.

An advantage of the present disclosure lies in provision of a negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery in which the charge/discharge cycle characteristic of the lithium ion secondary battery can be improved even when an alloying material is used as a negative electrode active material.

According to one aspect of the present disclosure, there is provided a negative electrode for a lithium ion secondary battery, the negative electrode including: a negative electrode electricity collector; and a negative electrode mixture layer formed over the negative electrode electricity collector, wherein the negative electrode mixture layer includes a negative electrode active material including graphite particles A having a particle internal porosity of <NUM>% or lower, graphite particles B having a particle internal porosity of greater than <NUM>%, and an alloying material which alloys with lithium, the negative electrode mixture layer includes a first layer formed over the negative electrode electricity collector, and a second layer formed over the first layer, the graphite particles A and the alloying material are contained in larger amounts in the second layer than in the first layer, the graphite particles B are contained in a larger amount in the first layer than in the second layer, and a content of the alloying material is <NUM> mass% or lower with respect to a total amount of the negative electrode active material in the negative electrode mixture layer.

According to another aspect of the present disclosure, there is provided a lithium ion secondary battery including the negative electrode for the lithium ion secondary battery.

According to an aspect of the present disclosure, the charge/discharge cycle characteristic of the lithium ion secondary battery can be improved even when the alloying material is used as the negative electrode active material.

A negative electrode for a lithium ion secondary battery according to an embodiment of the present disclosure comprises a negative electrode electricity collector, and a negative electrode mixture layer formed over the negative electrode electricity collector, wherein the negative electrode mixture layer includes a negative electrode active material including graphite particles A having a particle internal porosity of <NUM>% or lower, graphite particles B having a particle internal porosity of greater than <NUM>%, and an alloying material which alloys with lithium, the negative electrode mixture layer includes a first layer formed over the negative electrode electricity collector, and a second layer formed over the first layer, the graphite particles A and the alloying material are contained in larger amounts in the second layer than in the first layer, the graphite particles B are contained in a larger amount in the first layer than in the second layer, and a content of the alloying material is <NUM> mass% or lower with respect to a total amount of the negative electrode active material in the negative electrode mixture layer. According to the present disclosure, although the alloying material is used as the negative electrode active material, the charge/discharge cycle characteristic of the lithium ion secondary battery can be improved. A mechanism responsible for the advantage is not sufficiently clear, but, for example, the following may be deduced.

Because the graphite particles A having the particle internal porosity of <NUM>% or lower tend to not deform, the layer including the graphite particles A and the alloying material is a layer having a small inter-particle gap for the negative electrode active material. While the negative electrode including the alloying material expands due to a large volume change of the alloying material due to charging and discharging, the battery casing in which the expanding negative electrode is housed attempts to maintain its shape. Thus, a pressure in a thickness direction is applied to the negative electrode. In consideration of this, by placing larger amounts of the graphite particles A having the particle internal porosity of <NUM>% or lower and the alloying material in the second layer which is at a surface side of the negative electrode than in the first layer, it becomes possible to effectively take advantage of the above-described pressure, so that, even when there is a change in volume of the alloying material due to charging and discharging, the formation of the inter-particle gap in the negative electrode active material can be suppressed. The surface side of the negative electrode described above is a surface opposing a separator or a positive electrode. Further, because graphite particles A having the particle internal porosity of <NUM>% or lower tend to not collapse during the manufacturing of the negative electrode, adhesiveness between the negative electrode electricity collector and the graphite particles A is low. However, because the graphite particles B having the particle internal porosity of greater than <NUM>% tend to collapse during the manufacturing of the negative electrode, adhesiveness between the negative electrode electricity collector and the graphite particles B is high. In addition, because a large change in volume is caused in the alloying material due to the charging and the discharging, adhesiveness between the negative electrode electricity collector and the alloying material tends to be easily reduced. In consideration of these, a configuration for the layer is employed in which the first layer contains lower amounts of the graphite particles A having the particle internal porosity of <NUM>% or lower and the alloying material and a higher amount of the graphite particles B having the particle internal porosity of greater than <NUM>%, so that detachment of the particles of the negative electrode active material from the negative electrode electricity collector can be suppressed. Therefore, with the use of the negative electrode according to the present disclosure, even when the charging and the discharging of the lithium ion secondary battery are repeated, formation of the inter-particle gap for the negative electrode active material and detachment of the negative electrode active material from the negative electrode electricity collector can be suppressed, and the increase in the number of particles of the negative electrode active material which are isolated from the electrically conductive path in the negative electrode mixture layer can be suppressed, and thus, the charge/discharge cycle characteristic of the lithium ion secondary battery can be improved. However, the advantage is only achievable when a content of the alloying material is <NUM> mass% or lower with respect to a total amount of the negative electrode active material in the negative electrode mixture layer.

A negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery according to an embodiment of the present disclosure will now be described in detail with reference to the drawings. In the present disclosure, a description of "numerical value (<NUM>) ~ numerical value (<NUM>)" means a value greater than or equal to the numerical value (<NUM>) and lower than or equal to the numerical value (<NUM>).

<FIG> is a cross-sectional diagram of a lithium ion secondary battery according to an embodiment of the present disclosure. A lithium ion secondary battery <NUM> shown in <FIG> comprises a rolled-type electrode element <NUM> in which a positive electrode <NUM> and a negative electrode <NUM> are rolled with a separator <NUM> therebetween, an electrolyte, insulating plates <NUM> and <NUM> respectively placed above and below the electrode element <NUM>, and a battery casing <NUM> which houses the above-described members. The battery casing <NUM> is formed from a casing body <NUM> having a circular cylindrical shape with a bottom, and a sealing element <NUM> which blocks an opening of the casing body <NUM>. Alternatively, in place of the rolled-type electrode element <NUM>, an electrode element of other forms may be employed, such as a layered-type electrode element in which the positive electrode and the negative electrode are alternately layered with the separator therebetween. As the battery casing <NUM>, there may be exemplified a metal casing of a shape such as a cylindrical shape, a polygonal shape, a coin shape, a button shape, or the like, and a resin casing (laminated-type battery) formed by laminating resin sheets.

The casing body <NUM> is, for example, a metal container having a circular cylindrical shape with a bottom. A gasket <NUM> is provided between the casing body <NUM> and the sealing element <NUM>, to secure airtightness in the battery. The casing body <NUM> has, for example, a protrusion <NUM> in which a part of a side surface portion of the casing body <NUM> protrudes to an inner side and which supports the sealing element <NUM>. The protrusion <NUM> is desirably formed in an annular shape along a circumferential direction of the casing body <NUM>, and supports the sealing element <NUM> with an upper surface thereof.

The sealing element <NUM> has a structure in which a filter <NUM>, a lower valve element <NUM>, an insulating member <NUM>, an upper valve element <NUM>, and a cap <NUM> are layered in this order from the side of the electrode element <NUM>. The members of the sealing element <NUM> have, for example, a circular disk shape or a ring shape, and members other than the insulating member <NUM> are electrically connected to each other. The lower valve element <NUM> and the upper valve element <NUM> are connected to each other at central parts thereof, and the insulating member <NUM> interposes between peripheral parts of the valve elements. When an internal pressure of the lithium ion secondary battery <NUM> is increased due to heat generation caused by internal short-circuiting or the like, for example, the lower valve element <NUM> deforms in such a manner to press the upper valve element <NUM> toward the side of the cap <NUM>, and ruptures, so that a current path between the lower valve element <NUM> and the upper valve element <NUM> is cut off. When the internal pressure further increases, the upper valve element <NUM> ruptures, and gas is discharged from an opening of the cap <NUM>.

In the lithium ion secondary battery <NUM> shown in <FIG>, a positive electrode lead <NUM> attached to the positive electrode <NUM> extends through a throughhole of the insulating plate <NUM> to the side of the sealing element <NUM>, and a negative electrode lead <NUM> attached to the negative electrode <NUM> extends through an outer side of the insulating plate <NUM> to the side of a bottom of the casing body <NUM>. The positive electrode lead <NUM> is connected by welding or the like to a lower surface of the filter <NUM> which is a bottom plate of the sealing element <NUM>, and the cap <NUM> which is a top plate of the sealing element <NUM> electrically connected to the filter <NUM> serves as a positive electrode terminal. The negative electrode lead <NUM> is connected by welding or the like to an inner surface of the bottom of the casing body <NUM>, and the casing body <NUM> serves as a negative electrode terminal.

The positive electrode <NUM>, the negative electrode <NUM>, the separator <NUM>, and the electrolyte of the lithium ion secondary battery <NUM> will now be described in detail.

The positive electrode <NUM> comprises a positive electrode electricity collector and a positive electrode mixture layer formed over the positive electrode electricity collector. For the positive electrode electricity collector, there may be employed a foil of a metal which is stable within a potential range of the positive electrode such as aluminum and an aluminum alloy, a film on a surface layer of which the metal is placed, or the like. The positive electrode mixture layer includes, for example, a positive electrode active material, a binder material, and an electrically conductive material. Desirably, the positive electrode mixture layer is formed over both surfaces of the positive electrode electricity collector. The positive electrode can be manufactured, for example, by applying a positive electrode mixture slurry including the positive electrode active material, the binder material, the electrically conductive material, or the like over the positive electrode electricity collector, drying the applied film, and rolling the dried film, to form the positive electrode mixture layer over both surfaces of the positive electrode electricity collector.

The positive electrode active material includes a lithium-containing metal composite oxide as a primary constituent. As metal elements contained in the lithium-containing metal composite oxide, there may be exemplified Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, W, Ca, Sb, Pb, Bi, and Ge. A desirable example of the lithium-containing metal composite oxide is a composite oxide containing at least one of Ni, Co, Mn, and Al.

As the electrically conductive material included in the positive electrode mixture layer, there may be exemplified carbon materials such as carbon black, acetylene black, Ketjen black, graphene, carbon nanotube, graphite, or the like. As the binder material included in the positive electrode mixture layer, there may be exemplified a fluororesin such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), or the like, polyacrylonitrile (PAN), polyimide, an acrylic resin, polyolefin, or the like. Alternatively, carboxymethyl cellulose (CMC) or a salt thereof or polyethylene oxide (PEO) or the like may be used along with the above-described resins.

The negative electrode <NUM> comprises a negative electrode electricity collector, and a negative electrode mixture layer formed over the negative electrode electricity collector. For the negative electrode electricity collector, for example, there may be employed a foil of a metal which is stable within a potential range of the negative electrode such as copper and a copper alloy, a film on a surface layer of which the metal is placed.

The negative electrode mixture layer is formed including graphite particles A having a particle internal porosity of <NUM>% or lower, graphite particles B having a particle internal porosity of greater than <NUM>%, and an alloying material which alloys with lithium. The materials will now be descried in detail.

<FIG> is a schematic diagram showing a cross section of the graphite particle. As shown in <FIG>, a graphite particle <NUM> includes a pore <NUM> which is closed and is not connected from an inside of the particle to a surface of the particle in a cross-sectional view of the graphite particle <NUM>, and a pore <NUM> connected from the inside of the particle to the surface of the particle in the cross-sectional view. The pore <NUM> will hereinafter be referred to as an internal pore <NUM>, and the pore <NUM> will hereinafter be referred to as an external pore <NUM>. In the present embodiment, the internal porosity of the graphite particle is a two-dimensional value determined from a ratio of an area of the internal pore of the graphite particle with respect to a cross-sectional area of the graphite particle.

It is sufficient that the internal porosity of the graphite particles A is <NUM>% or lower from the viewpoint of improving the charge/discharge cycle characteristic, but the internal porosity is desirably <NUM>% ~ <NUM>%, and is more desirably <NUM>% ~ <NUM>%. It is sufficient that the internal porosity of the graphite particles B is greater than <NUM>% from the viewpoint of suitable collapsing in a compression process during manufacturing of the negative electrode, but the internal porosity is desirably <NUM>% ~ <NUM>%, and is more desirably <NUM>% ~ <NUM>%. The internal porosity of the graphite particles can be determined through the following procedure.

The graphite particles A and B are manufactured, for example, in the following manner.

For example, cokes (precursors) which are a primary raw material are ground to a predetermined size, and, in a state in which the cokes are aggregated with a binder material, the aggregate is baked at a temperature of <NUM> or greater for graphitization, and the resulting graphites are then filtered to obtain the graphite particles A of a desired size. Here, the internal porosity may be adjusted to a value of <NUM>% or lower by a particle size of the precursor after the grinding, a particle size of the precursor in the aggregated state, or the like. For example, an average particle size (median size D50) of the precursor after the grinding is desirably in a range of <NUM> ~ <NUM>.

For example, the cokes (precursors) which are a primary raw material are ground to a predetermined size, the cokes are aggregated with a binder agent, and, in a state in which the aggregate is pressurized and shaped in a block shape, the aggregate is baked at a temperature of <NUM> or grater for graphitization. The block-shape formation after the graphitization is ground and filtered, to obtain the graphite particles B of a desired size. The internal porosity can be adjusted to a value of greater than <NUM>% by an amount of volatile composition added to the block-shape formation.

When a part of the binder material added to the cokes (precursors) vaporizes during the baking, the binder material may be used as the volatile composition. A pitch may be exemplified as such a binder material.

No particular limitation is imposed on the graphite particles A and B used in the present embodiment, such as natural graphite and artificial graphite, but from the viewpoint of ease of adjustment of the internal porosity, the artificial graphite is desirably employed. A plane spacing (d<NUM>) of a (<NUM>) plane determined by an X-ray wide angle diffraction for the graphite particles A and B used in the present embodiment is desirably, for example, <NUM> or greater, is more desirably <NUM> or greater, is desirably lower than <NUM>, and is more desirably <NUM> or lower. A crystallite size (Lc(<NUM>)) determined by the X-ray diffraction for the graphite particles A and B used in the present embodiment is desirably, for example, <NUM> or greater, is more desirably <NUM> or greater, is desirably <NUM> or lower, and is more desirably <NUM> or lower. No particular limitation is imposed on the average particle size for the graphite particles A and B, but the average particle size is, for example, <NUM> ~ <NUM>. The average particle size means a volume-average particle size (Dv50) in which a volume accumulated value becomes <NUM>% in a granularity distribution measured by laser diffraction scattering.

The alloying material is formed including an element which alloys with lithium, a compound containing an element which becomes a lithium alloy, or both. As elements which alloy with lithium and which can be applied for the negative electrode active material, there may be exemplified Al, Ga, In, Si, Ge, Sn, Pb, As, Sb, Bi, or the like. Of these, Si and Sn are desirable from the viewpoint of increasing the capacity, and Si is particularly desirable.

As compounds containing Si, there may be exemplified compounds containing a silicon oxide phase and Si dispersed in the silicon oxide phase, and compounds containing a lithium silicate phase and Si dispersed in the lithium silicate phase. The compounds containing the silicon oxide phase and Si dispersed in the silicon oxide phase are described as "SiO" in the following description. The compounds containing the lithium silicate phase and Si dispersed in the lithium silicate phase are described as "LSX" in the following description.

On surfaces of the particles of SiO and LSX, an electrically conductive layer formed from a material of a high electrical conductivity may be formed. An example of a desirable electrically conductive layer is a carbon coating formed from a carbon material. The carbon coating is formed from, for example, carbon black, acetylene black, Ketjen black, graphite, or a mixture of two or more of these materials. As a method of carbon-coating the surfaces of the particles of SiO and LSX, there may be exemplified a CVD method using acetylene, methane or the like, or a method in which a coal pitch, a petroleum pitch, a phenol resin, or the like is mixed with the particles of SiO and LSX, and a thermal treatment is applied. Alternatively, the carbon coating may be formed by fixing a carbon powder such as the carbon black on the surface of the particle using a binder material.

A desirable SiO has a sea-island structure in which fine Si particles are approximately uniformly dispersed in an amorphous silicon oxide phase, and is represented by a general formula, SiOx (<NUM>≤x≤<NUM>). A content of the Si particles is desirably <NUM> mass% ~ <NUM> mass% with respect to a total mass of SiO, from the viewpoint of realizing both the battery capacity and the cycle characteristic.

An average particle size of the Si particles dispersed in the silicon oxide phase is typically <NUM> or lower before charging and discharging, is desirably <NUM> or lower, and is more desirably <NUM> or lower. After the charging and discharging, the average particle size is desirably <NUM> or lower, and is more desirably <NUM> or lower. By making the Si particles finer, the volume change during the charging and discharging is reduced, and the cycle characteristic is improved. The average particle size of the Si particles is measured by observing the cross section of SiO using a scanning electron microscope (SEM) or a transmission electron microscope (TEM), and is more specifically determined as an average value of the longest sizes of <NUM> Si particles. The silicon oxide phase is formed, for example, by a collection of particles finer than the Si particles.

Desirable LSX has a sea-island structure in which fine Si particles are approximately uniformly dispersed in the lithium silicate phase represented by a general formula, Li2zSiO(<NUM>+z) (wherein <NUM><z<<NUM>). Similar to the SiO, a content of the Si particles is desirably <NUM> mass% ~ <NUM> mass% with respect to a total mass of the LSX. An average particle size of the Si particles is typically <NUM> or lower before charging and discharging, is desirably <NUM> or lower, and is more desirably <NUM> or lower. The lithium silicate phase is formed from, for example, a collection of particles finer than Si particles.

As described above, the lithium silicate phase is desirably formed from a compound represented by Li2zSiO(<NUM>+z) (wherein <NUM><z<<NUM>). That is, the lithium silicate phase does not include Li<NUM>SiO<NUM> (z=<NUM>). Li<NUM>SiO<NUM> is an unstable compound, reacts with water to show an alkaline characteristic, and thus, may alter Si to consequently cause degradation of the charge/discharge capacity. For the lithium silicate phase, desirably, Li<NUM>SiO<NUM> (z=<NUM>) or Li<NUM>Si<NUM>O<NUM> (z=<NUM>/<NUM>) is employed as a primary constituent, from the viewpoints of stability, ease of manufacture, electrical conductivity by the lithium ions, or the like.

The SiO may be manufactured by the following process.

In the above-described process, LSX may be manufactured by using lithium silicate in place of the silicon oxide.

<FIG> is a cross-sectional diagram of a negative electrode according to an embodiment of the present disclosure. In the negative electrode <NUM> shown in <FIG>, a negative electrode mixture layer <NUM> formed over a negative electrode electricity collector <NUM> is formed including a first layer <NUM> and a second layer <NUM>. The first layer <NUM> is disposed over the negative electrode electricity collector <NUM>, and the second layer <NUM> is disposed over the first layer <NUM>. Desirably, the negative electrode mixture layer <NUM> is formed over both surfaces of the negative electrode electricity collector <NUM>. The second layer <NUM> being "disposed" "over" the first layer <NUM> means that the second layer <NUM> may be disposed directly on the first layer <NUM>, or an intermediate layer may be present between the second layer <NUM> and the first layer <NUM>.

The first layer <NUM> includes the graphite particles B as the negative electrode active material. It is sufficient that the graphite particles B are contained in the first layer <NUM> in a larger amount than in the second layer <NUM> from the viewpoint of improving the charge/discharge cycle characteristic, but the amount of the graphite particles B in the first layer <NUM> is desirably in a range of <NUM> mass% ~ <NUM> mass% with respect to a total amount of the graphite particles B in the negative electrode mixture layer <NUM>. The first layer <NUM> may include the graphite particles A and the alloying material as the negative electrode active material, but, from the viewpoint of improving the charge/discharge cycle characteristic, a content of the graphite particles A in the first layer <NUM> is desirably <NUM> mass% or lower with respect to a total amount of the graphite particles A in the negative electrode mixture layer <NUM>, and a content of the alloying material in the first layer <NUM> is desirably <NUM> mass% or lower with respect to a total amount of the alloying material in the negative electrode mixture layer <NUM>.

The second layer <NUM> includes the graphite particles A and the alloying material as the negative electrode active material. It is sufficient that the graphite particles A are contained in the second layer <NUM> in a larger amount than in the first layer <NUM>, from the viewpoint of improving the charge/discharge cycle characteristic, but the amount of the graphite particles A in the second layer <NUM> is desirably in a range of <NUM> mass% ~ <NUM> mass% with respect to the total amount of the graphite particles A in the negative electrode mixture layer <NUM>. In addition, it is sufficient that the alloying material is contained in the second layer <NUM> in a larger amount than in the first layer <NUM>, from the viewpoint of improving the charge/discharge cycle characteristic, but the amount of the alloying material in the second layer <NUM> is desirably <NUM> mass% ~ <NUM> mass% with respect to the total amount of the alloying material in the negative electrode mixture layer <NUM>. However, when a ratio of the alloying material in the negative electrode mixture layer <NUM> is increased, the improvement advantage of the charge/discharge cycle characteristic is reduced, and, therefore, it is necessary that the content of the alloying material is <NUM> mass% or lower with respect to a total amount of the negative electrode active material in the negative electrode mixture layer. A lower limit value of the content of the alloying material is desirably <NUM> mass% or greater with respect to the total amount of the negative electrode active material in the negative electrode mixture layer <NUM>, from the viewpoint of a higher capacity for the lithium ion secondary battery, and is more desirably <NUM> mass% or greater.

The second layer <NUM> may include the graphite particles B as the negative electrode active material, but, from the viewpoint of improving the charge/discharge cycle characteristic, a content of the graphite particles in the second layer <NUM> is desirably <NUM> mass% or lower with respect to the total amount of the graphite particles B in the negative electrode mixture layer <NUM>.

The negative electrode active layer <NUM> desirably includes polyacrylic acid (PAA) or a salt thereof. The PAA salt is, for example, lithium salt, sodium salt, potassium salt, ammonium salt, or the like. When the PAA or the salt thereof is included, the particles of the negative electrode active material are strongly bonded to each other, and the charge/discharge cycle characteristic can thus be improved. The PAA or the salt thereof may be included in the same amount in the first layer <NUM> and in the second layer <NUM>, or may be included in a larger amount in either of the layers. In order to suppress falling-off of the alloying material from the negative electrode <NUM>, the PAA or the salt thereof is desirably contained in a larger amount in the second layer <NUM> than in the first layer <NUM>.

The negative electrode mixture layer <NUM> desirably includes styrene-butadiene rubber. When the styrene-butadiene rubber is included, the particles of the negative electrode active material are strongly bonded to each other, and the charge/discharge cycle characteristic can thus be improved. The styrene-butadiene rubber may be contained in the same amount in the first layer <NUM> and in the second layer <NUM>, or may be contained in a larger amount in either of the layers. In order to suppress detachment of the negative electrode active material from the negative electrode electricity collector <NUM>, the styrene-butadiene rubber is desirably included in a larger amount in the first layer <NUM> than in the second layer <NUM>.

The negative electrode mixture layer <NUM> desirably includes carboxymethyl cellulose (CMC) or a salt thereof. The CMC salt is, for example, lithium salt, sodium salt, potassium salt, ammonium salt, or the like. When the CMC or the salt thereof is included, the particles of the negative electrode active material are strongly bonded to each other, and the charge/discharge cycle characteristic can thus be improved. The CMC or the salt thereof may be included in the same amount in the first layer <NUM> and in the second layer <NUM>, or in a larger amount in either of the layers. In order to suppress falling-off of the alloying material from the negative electrode <NUM>, the CMC or the salt thereof is desirably contained in a larger amount in the second layer <NUM> than in the first layer <NUM>.

The negative electrode mixture layer <NUM> may include a binder material other than the PAA or the salt thereof, the styrene-butadiene rubber, and the CMC or the salt thereof. Examples of such a binder material include a fluororesin such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF) or the like, polyacrylonitrile (PAN), polyimide, an acrylic resin, polyolefin, or the like.

A content of the binder material in the negative electrode mixture layer <NUM> is, for example, desirably <NUM> mass% ~ <NUM> mass% with respect to the total amount of the negative electrode mixture layer <NUM>, and is more desirably <NUM> mass% ~ <NUM> mass%.

The negative electrode mixture layer <NUM> desirably contains fibrous carbon. By containing the fibrous carbon, a superior electrically conductive path is formed in the negative electrode mixture layer <NUM>, and the charge/discharge cycle characteristic can be improved. The fibrous carbon may be contained in the same amount in the first layer <NUM> and the second layer <NUM>, or may be contained in a larger amount in either of the layers. However, the fibrous carbon is desirably contained in a larger amount in the second layer <NUM> than in the first layer <NUM>, from the viewpoint of maintaining the electrically conductive path to the alloying material.

As the fibrous carbon, there may be exemplified carbon nanotube (CNT), carbon nanofiber, or the like. The CNT may be, in addition to a single-layer CNT, a two-layer CNT, a multi-layer CNT, or a mixture of these. The CNT may be a vapor phase grown carbon fiber. The fibrous carbon has, for example, a diameter of <NUM> ~ <NUM>, and an overall length of <NUM> ~ <NUM>. A content of the fibrous carbon in the negative electrode mixture layer <NUM> is, for example, desirably <NUM> mass% ~ <NUM> mass% with respect to the total amount of the negative electrode mixture layer <NUM>, and is more desirably <NUM> mass% ~ <NUM> mass%.

A thickness of the negative electrode mixture layer <NUM> is, for example, <NUM> ~ <NUM>, or <NUM> ~ <NUM>, on one side of the negative electrode electricity collector <NUM>. Thicknesses of the first layer <NUM> and the second layer <NUM> may be the same or different from each other.

As described above, an intermediate layer may be provided between the first layer <NUM> and the second layer <NUM>. The intermediate layer may include the graphite particles A, the graphite particles B, and the alloying material described above, or may include other negative electrode active materials or the like which are known in the prior art. In any case, the intermediate layer may be designed within a range of not adversely affecting the advantage of the present disclosure.

The negative electrode <NUM> is manufactured, for example, in the following method. A first negative electrode mixture slurry for the first layer <NUM> including the graphite particles B, and the binder material, or the like, is prepared. A second negative electrode mixture slurry for the second layer <NUM> including the graphite particles A, the alloying material, and the binder material, or the like, is prepared. The first negative electrode mixture slurry is applied over the negative electrode electricity collector <NUM>, and the applied film is dried, to form the first layer <NUM> over the negative electrode electricity collector <NUM>. Then the second negative electrode mixture slurry is applied over the first layer <NUM>, the applied film is dried, to form the second layer <NUM> over the first layer <NUM>, and then, the first layer <NUM> and the second layer <NUM> are compressed. In this manner, the negative electrode <NUM> is obtained in which the negative electrode mixture layer <NUM> including the first layer <NUM> and the second layer <NUM> is formed over the negative electrode electricity collector <NUM>.

For the separator <NUM>, a porous sheet having an ion permeability and an insulating property is used. Specific examples of the porous sheet include a microporous thin film, a woven fabric, a non-woven fabric, or the like. As the material of the separator <NUM>, desirably, an olefin-based resin such as polyethylene, polypropylene, and a copolymer including at least one of ethylene and propylene, or the like, cellulose, or the like is used. The separator <NUM> may have a single-layer structure or a layered structure. Over a surface of the separator <NUM>, a heat resistive layer or the like may be formed.

The electrolyte includes a solvent and an electrolyte salt. The electrolyte is not limited to a liquid electrolyte, and may alternatively be a solid electrolyte which uses a gel-form polymer or the like. For the electrolyte salt, for example, lithium salt such as LiFSI, LiTFSI, LiBF<NUM>, and LiPF<NUM> is employed. For the solvent, for example, there may be employed esters such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), diethyl carbonate (DEC), methyl acetate (MA), and methyl propionate (MP), ethers, nitriles, amides, or a mixture solvent of two or more of these. A non-aqueous solvent may contain a halogen substitution product in which at least a part of hydrogen of the solvent is substituted with a halogen atom such as fluorine.

As the halogen substitution product, for example, there may be exemplified fluorinated cyclic ester carbonates such as fluoroethylene carbonate (FEC), fluorinated chain ester carbonates, and fluorinated chain ester carboxylates such as fluoromethyl propionate (FMP).

The present disclosure will now be further described with reference to Examples. The present disclosure, however, is not limited to these Examples.

<NUM> parts by mass of LiNiCoAlO<NUM> serving as a positive electrode active material, <NUM> part by mass of acetylene black serving as an electrically conductive material, and <NUM> part by mass of polyvinylidene fluoride powder serving as a binder material were mixed, and a suitable amount of N-methyl-<NUM>-pyrrolidone (NMP) was added, to prepare a positive electrode mixture slurry. The slurry was applied by a doctor blade method on both surfaces of an electricity collector formed from an aluminum foil (having a thickness of <NUM>), the applied film was dried, and the dried applied film was compressed with a rolling roller, to form a positive electrode in which a positive electrode active material layer was formed over both surfaces of the positive electrode electricity collector.

Cokes were ground until an average particle size thereof (median size D50) became <NUM>, a pitch was added as a binder material to the ground cokes, and the cokes were aggregated until the average particle size (median size D50) became <NUM>. The aggregate was baked at a temperature of <NUM> for graphitization, and was filtered using a <NUM>-mesh filter, to obtain graphite particles A having an average particle size (median size D50) of <NUM>.

Cokes were ground until an average particle size (median size D50) became <NUM>, a pitch serving as a binder material was added to the ground cokes, and the cokes were aggregated. Then, an isotopic pressure was applied to form a block-shape formation having a density of <NUM>/cm<NUM> ~ <NUM>/cm<NUM>. The block-shape formation was baked at a temperature of <NUM> for graphitization, and the block-shape formation was ground and filtered using a <NUM>-mesh filter, to obtain graphite particles B having an average particle size (median size D50) of <NUM>.

<NUM> parts by mass of the graphite particles B and <NUM> parts by mass of a Si compound (SiO) were mixed. Using this mixture as a negative electrode active material, the negative electrode active material, CMC, PAA, and styrene-butadiene rubber were mixed in such a manner that a mass ratio of (negative electrode active material):(CMC):(PAA):(styrene-butadiene rubber) was <NUM>:<NUM>:<NUM>:<NUM>, and a suitable amount of water was added, to prepare a first negative electrode mixture slurry for a first layer. <NUM> parts by mass of the graphite particles A, <NUM> parts by mass of the graphite particles B, and <NUM> parts by mass of the Si compound (SiO) were mixed. Using this mixture as a negative electrode active material, the negative electrode active material, the CMC, the PAA, and the styrene-butadiene rubber were mixed in such a manner that a mass ratio of (negative electrode active material):(CMC):(PAA):(styrene-butadiene rubber) was <NUM>:<NUM>:<NUM>:<NUM>, and a suitable amount of water was added, to prepare a second negative electrode mixture slurry for a second layer.

The first negative electrode mixture slurry was applied over both surfaces of a negative electrode electricity collector formed from a copper foil, and the applied film was dried, to form a first layer over both surfaces of the negative electrode electricity collector. Then, the second negative electrode mixture slurry was applied over the first layer formed over both surfaces of the negative electrode electricity collector, and the applied film was dried, to form a second layer. The applied films were rolled using a roller, to manufacture a negative electrode in which a negative electrode mixture layer including the first layer and the second layer was formed over both surfaces of the negative electrode electricity collector. Internal porosities of the graphite particles A and the graphite particles B in the manufactured negative electrode were measured, and were respectively <NUM>% and <NUM>%. The method of measurement is identical to the method described above, and will not be repeatedly described.

To a mixture solvent in which ethylene carbonate (EC), fluorinated ethylene carbonate (FEC), and diethyl carbonate (DEC) were mixed with a volume ratio of <NUM>:<NUM>:<NUM>, <NUM> mass% of vinylene carbonate (VC) was added, and LiPF<NUM> was dissolved in a ratio of <NUM> mol/L, to prepare an electrolyte solution.

The positive electrode and the negative electrode were layered in a manner to oppose each other with the separator therebetween, and the resulting structure was rolled to manufacture an electrode element. Then, the electrode element and the electrolyte solution were housed in a battery casing body having a circular cylindrical shape with a bottom, the electrolyte solution was injected, and the opening of the battery casing body was sealed with a gasket and a sealing element, to prepare a test cell.

A test cell was manufactured in a manner similar to Example <NUM> except that, in the preparation of the first negative electrode mixture slurry, the Si compound was not used, and, in the preparation of the second negative electrode mixture slurry, a mixture in which <NUM> parts by mass of the graphite particles A, <NUM> parts by mass of the graphite particles B, and <NUM> parts by mass of the Si compound were mixed was used as the negative electrode active material.

A test cell was manufactured in a manner similar to Example <NUM> except that, in the preparation of the first negative electrode mixture slurry, the Si compound was not used, and, in the preparation of the second negative electrode mixture slurry, a mixture in which <NUM> parts by mass of the graphite particles A, and <NUM> parts by mass of the Si compound were mixed was used as the negative electrode active material.

A test cell was manufactured in a manner similar to Example <NUM> except that, in the preparation of the first negative electrode mixture slurry, the Si compound was not used, in the preparation of the second negative electrode mixture slurry, a mixture in which <NUM> parts by mass of the graphite particles A, and <NUM> parts by mass of the Si compound were mixed was used as the negative electrode active material, and, in the preparation of the second negative electrode mixture slurry, the negative electrode active material, the CMC, the PAA, the styrene-butadiene rubber, and CNT were mixed in such a manner that a mass ratio of (negative electrode active material):(CMC):(PAA):(styrene-butadiene rubber):(CNT) was <NUM>:<NUM>:<NUM>:<NUM>:<NUM>.

A test cell was manufactured in a manner similar to Example <NUM> except that, in the preparation of the first negative electrode mixture slurry, a mixture in which <NUM> parts by mass of the graphite particles B, and <NUM> parts by mass of the Si compound were mixed was used as the negative electrode active material, and, in the preparation of the second negative electrode mixture slurry, a mixture in which <NUM> parts by mass of the graphite particles B, and <NUM> parts by mass of the Si compound were mixed was used as the negative electrode active material.

Under a temperature environment of <NUM>, the test cell was charged with a constant current of <NUM>. 3C until the battery voltage reached <NUM>. 2V, and then charged with a constant voltage of <NUM>. 2V until a current value reached <NUM>/50C. Then, a constant-current discharge was performed with a constant current of <NUM>. 0C until the battery voltage reached <NUM>. The charging and discharging were performed for <NUM> cycles, and a capacity maintenance percentage at the charge/discharge cycle was calculated according to the following formula.

Results of evaluation (capacity maintenance percentage at <NUM> cycles) for the test cells of Examples <NUM> ~ <NUM> and Comparative Example <NUM> are shown in TABLE <NUM>.

The test cells of Examples <NUM> ~ <NUM> and Comparative Example <NUM> included <NUM> mass% of the Si compound in the negative electrode active material. It can be seen that the test cells of Examples <NUM> ~ <NUM> had higher values for the capacity maintenance percentage at the charge/discharge cycle than the test cell of Comparative Example <NUM>, and that the charge/discharge cycle characteristic was improved in these test cells.

The test cell of Example <NUM> differs from the test cell of Example <NUM> in that CNT was added to the second layer. Based on the evaluation result of the capacity maintenance percentage, it can be seen that the capacity maintenance percentage in Example <NUM> was improved in a greater degree than the capacity maintenance percentage in Example <NUM>.

A test cell was manufactured in a manner similar to Example <NUM> except that, in the preparation of the first negative electrode mixture slurry, a mixture in which <NUM> parts by mass of the graphite particles B, and <NUM> parts by mass of the Si compound were mixed was used as the negative electrode active material, and, in the preparation of the second negative electrode mixture slurry, a mixture in which <NUM> parts by mass of the graphite particles A, <NUM> parts by mass of the graphite particles B, and <NUM> parts by mass of the Si compound were mixed was used as the negative electrode active material.

For the test cells of Example <NUM> and Comparative Example <NUM>, the capacity maintenance percentage at <NUM> cycles was evaluated under conditions similar to the above. Results of evaluation (capacity maintenance percentage at <NUM> cycles) for the test cells of Example <NUM> and Comparative Example <NUM> are shown in TABLE <NUM>.

The test cells of Example <NUM> and Comparative Example <NUM> included <NUM> mass% of the Si compound in the negative electrode active material. It can be seen that the test cell of Example <NUM> had a higher value for the capacity maintenance percentage at the charge/discharge cycle than the test cell of Comparative Example <NUM>, and that the charge/discharge cycle characteristic was improved.

Based on these results, it can be said that the charge/discharge cycle characteristic of the lithium ion secondary battery is improved by having, when a content of the alloying material is <NUM> mass% or lower with respect to a total amount of the negative electrode active material, larger amounts of the graphite particles A having a particle internal porosity of <NUM>% or lower and of the alloying material in the second layer than in the first layer, and having a larger amount of the graphite particles B having a particle internal porosity of greater than <NUM>% in the first layer than in the second layer.

Claim 1:
A negative electrode (<NUM>) for a lithium ion secondary battery (<NUM>), the negative electrode (<NUM>) comprising:
a negative electrode electricity collector (<NUM>); and
a negative electrode mixture layer (<NUM>) formed over the negative electrode electricity collector (<NUM>), wherein
the negative electrode mixture layer (<NUM>) includes a negative electrode active material including graphite particles A having a particle internal porosity of <NUM>% or lower, graphite particles B having a particle internal porosity of greater than <NUM>%, when the particle intemal porosity is measured using scanning electron microscopy as indicated in the description, and an alloying material which alloys with lithium,
the negative electrode mixture layer (<NUM>) includes a first layer (<NUM>) formed over the negative electrode electricity collector (<NUM>), and a second layer (<NUM>) formed over the first layer (<NUM>),
the graphite particles A and the alloying material are contained in larger amounts in the second layer (<NUM>) than in the first layer (<NUM>),
the graphite particles B are contained in a larger amount in the first layer (<NUM>) than in the second layer(<NUM>), and
a content of the alloying material is <NUM> mass% or lower with respect to a total amount of the negative electrode active material in the negative electrode mixture layer (<NUM>).