JP2018160379A - Negative electrode for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery, the negative electrode being excellent in cycle characteristics.SOLUTION: The negative electrode for a lithium ion secondary battery is provided which includes a current collector and a negative electrode active material layer, and in which the negative electrode active material layer contains Li and at least one element including Si, Sn, Fe, Ni, Al, Ge, and Ti, and the concentration of Li in the negative electrode active material layer has a concentration gradient.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery.

  Lithium ion secondary batteries are widely applied as power sources for portable electronic devices because they are lighter and have a higher capacity than nickel cadmium batteries, nickel metal hydride batteries, and the like. In recent years, it has been used as a power source mounted for hybrid vehicles and electric vehicles. With the recent miniaturization and higher functionality of portable electronic devices, further increase in capacity is expected for lithium ion secondary batteries that serve as these power sources.

  Currently, carbon materials such as graphite are often used as negative electrode active materials for lithium ion secondary batteries. In recent years, a large number of alloy-based negative electrode active materials such as silicon (Si) and SiO having a larger discharge capacity than graphite have been studied. However, these alloy-based negative electrode active materials have a larger discharge capacity than graphite, and also have a large volume expansion associated with the charge / discharge reaction, and are more susceptible to particle cracking and isolation than graphite. The decrease was significant.

  Patent Documents 1 and 2 disclose that by adding a metal element to an alloy negative electrode to improve electron conductivity, a charge / discharge reaction can be uniformly performed in an electrode cross section, and cycle characteristics are improved. .

JP 2007-27102 A International Publication No. 2007/046327

  However, the method disclosed in the above prior art does not provide sufficient cycle characteristics, and further improvement is required.

  The present invention has been developed under such circumstances, and an object thereof is to provide a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery having high cycle characteristics.

  In order to achieve the above object, a negative electrode for a lithium ion secondary battery according to the present invention is a negative electrode having a current collector and a negative electrode active material layer having a negative electrode active material, wherein the negative electrode active material layer comprises Li And at least one element composed of Si, Sn, Fe, Ni, Al, Ge, Ti, and the concentration of Li in the negative electrode active material layer has a concentration gradient.

  This is because the main reaction Li in the negative electrode active material layer has a concentration gradient, so that the volume expansion associated with charge / discharge occurs along the Li concentration gradient, thereby suppressing rapid expansion of the entire active material layer. As a result, cycle characteristics are improved.

  The concentration gradient of Li in the negative electrode active material layer preferably has a concentration gradient in which the concentration of Li decreases gradually or stepwise toward the current collector side.

  Thereby, since the active material layer expands gradually or stepwise toward the current collector side along the concentration gradient of Li, the stress related to the active material layer and the current collector is reduced, thereby reducing the active material. It is considered that peeling between the layer and the current collector is suppressed, and the cycle characteristics are further improved.

  The negative electrode active material preferably contains Li and at least one metal selected from Si, Sn, Fe, and Ti, an oxide, and a mixture thereof.

  Particularly excellent cycle characteristics can be obtained with these metals, oxides and mixtures thereof.

  The negative electrode active material preferably further contains a carbon-based material.

  By including the second negative electrode active material, excellent cycle characteristics can be obtained.

  The primary particle diameter D50 having a volume cumulative ratio from the side where the primary particle diameter of the negative electrode active material is small is 50% is L1, and the volume cumulative ratio from the side where the primary particle diameter of the carbonaceous material is small is 50%. When the primary particle diameter D50 is L2, it is preferable that L2 / L1 as a ratio of L1 and L2 is 0.04 ≦ L2 / L1 ≦ 200. (Need no confirmation from the numerical value)

  As a result, the packing density of the first negative electrode active material and the second negative electrode active material is increased, and a conductive path and a Li ion transfer path are formed between the first negative electrode active material and the second negative electrode active material. Therefore, excellent cycle characteristics can be obtained.

  ADVANTAGE OF THE INVENTION According to this invention, the negative electrode for lithium ion secondary batteries and the lithium ion secondary battery which were excellent in cycling characteristics can be provided.

It is a schematic cross section of the lithium ion secondary battery according to the present embodiment.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. In addition, this invention is not limited to the following embodiment. The constituent elements described below include those that can be easily assumed by those skilled in the art and those that are substantially the same. Furthermore, the constituent elements described below can be appropriately combined.

(Lithium ion secondary battery)
FIG. 1 shows a cross-sectional configuration diagram of a lithium ion secondary battery 100 according to this embodiment. The lithium ion secondary battery 100 includes an outer body 50, a positive electrode 10 and a negative electrode 20 provided inside the outer body, and an electrode body 30 formed by being stacked via a separator 18 disposed therebetween. The separator 18 holds the non-aqueous electrolyte, which is a lithium ion transfer medium between the positive and negative electrodes during charging and discharging. Furthermore, one end is electrically connected to the negative electrode 20 and the other end protrudes outside the exterior body, and one end is electrically connected to the positive electrode 10. The other end is provided with a positive electrode lead 60 protruding outside the exterior body.

  There is no restriction | limiting in particular as a shape of a lithium ion secondary battery, For example, any, such as a cylindrical type, a square type, a coin type, a flat type, a laminate film type, may be sufficient. In the following examples, an aluminum laminated film type battery is produced and evaluated.

  The positive electrode 10 includes a positive electrode active material layer 14 containing a positive electrode active material that absorbs and releases lithium ions, a conductive additive, and a binder on at least one main surface of the positive electrode current collector 12. The negative electrode 20 includes a negative electrode active material layer 24 containing a negative electrode active material that absorbs and releases lithium ions, a conductive additive, and a binder on at least one main surface of the negative electrode current collector 22. Yes.

(Negative electrode)
The negative electrode active material layer 24 formed on the negative electrode 20 of the present embodiment contains a negative electrode active material, a binder, and a conductive additive.

  For the negative electrode active material layer 24, a paint containing a negative electrode active material, a binder, a conductive additive and a solvent is applied on the negative electrode current collector 22, and the solvent in the paint applied on the negative electrode current collector 22 is removed. Can be manufactured.

(Negative electrode active material)
The negative electrode for a lithium ion secondary battery according to this embodiment is a negative electrode having a current collector and a negative electrode active material layer having a negative electrode active material, wherein the negative electrode active material layer includes Li, Si, Sn, It contains at least one element composed of Fe, Ni, Al, Ge, and Ti, and the concentration of Li in the negative electrode active material layer has a concentration gradient.

  As a result, Li, which is the main reactant in the negative electrode active material layer, has a concentration gradient, so that volume expansion associated with charge / discharge occurs along the Li concentration gradient, thereby suppressing rapid expansion of the entire active material layer. As a result, cycle characteristics are improved.

  In the negative electrode active material layer according to the present embodiment, the Li concentration gradient in the negative electrode active material layer preferably has a concentration gradient in which the concentration of Li decreases toward the current collector.

  That is, it is preferable that the Li concentration of the negative electrode active material layer according to this embodiment has a structure that is at least equal or increases from the current collector side toward the surface side.

  The concentration of Li in the negative electrode active material layer according to the present embodiment only needs to be changed in a gradual or stepwise form, and is preferably changed gradually.

  As a result, the negative electrode active material layer expands gradually or stepwise toward the current collector side along the Li concentration gradient, so that the stress between the active material layer and the current collector decreases, thereby reducing the active material layer. It is considered that peeling between the material layer and the current collector is suppressed, and the cycle characteristics are further improved.

  The concentration of Li in the negative electrode active material layer according to this embodiment is determined by observing the cross section of the negative electrode active material layer with electron energy loss spectroscopy (EELS), or the contained elements in the depth direction from the surface of the negative electrode active material layer Can be measured by analyzing the ratio by X-ray photoelectric spectroscopy (XPS).

  The negative electrode active material according to this embodiment preferably contains Li and at least one metal selected from Si, Sn, Fe, and Ti, an oxide, and a mixture thereof.

  Particularly good cycle characteristics can be obtained with these metals, oxides and mixtures thereof.

Examples of the negative electrode active material according to the present embodiment include Si, Sn, a compound in which Fe and Li are alloyed, SiO _x (0 <x ≦ 2), SnO _y (0 <y ≦ 2), FeO ₂ , Fe ₂ O. ₃ , oxides such as Fe ₃ O ₄ , FeOOH, TiO _{2, and the} like, and a plurality of these may be used in combination.

As the negative electrode active material according to the present embodiment, a material in which Li is contained in the above compound may be used. Examples of materials containing Li include alloyed compounds such as LiSi alloy, LiSn alloy, LiFe alloy, and oxides such as LiSiO _x (0 <x ≦ 2), LiFeO ₂ , Li ₄ Ti ₅ O _12, etc. A plurality of may be mixed and used.

  In addition, the negative electrode active material according to this embodiment may be used by mixing a plurality of types of compounds exemplified above.

The negative electrode active material according to the present embodiment preferably includes silicon oxide represented by SiO _x (0 <x ≦ 2).

  The negative electrode active material according to this embodiment preferably further contains a carbon-based material.

  As the carbon material, a known carbon material that can be used for a lithium ion secondary battery can be used, and examples thereof include natural graphite, artificial graphite, and mesophase carbon microbeads (MCMB). These carbon materials may be used individually by 1 type, and may use 2 or more types together.

  The mixing amount of the carbon material according to the present embodiment is preferably in the range of 0.4 ≦ d1 / d2 ≦ 2.3 when the weight ratio d1 of the negative electrode active material and the weight ratio d2 of the carbon material are set.

  The primary particle diameter D50 having a volume accumulation rate of 50% from the side where the primary particle diameter of the negative electrode active material according to the present embodiment is small is L1, and the volume accumulation ratio from the side where the primary particle diameter of the carbonaceous material is small is 50. When the primary particle diameter D50 of% is L2, L2 / L1, which is the ratio of L1 and L2, is preferably 0.04 ≦ L2 / L1 ≦ 200, and L2 / L1 is 1 ≦ L2 / L1 More preferably, ≦ 5.

  In addition, the volume cumulative ratio from the side where the primary particle diameter of the carbon material is small with respect to L1 which is the primary particle diameter D50 where the volume cumulative ratio from the side where the primary particle diameter of the negative electrode active material according to the present embodiment is small is 50%. L2 which is D50 having a primary particle diameter of 50% is more preferably L2> L1.

  L1 which is the primary particle size D50 in which the volume cumulative ratio from the side where the primary particle size of the negative electrode active material according to the present embodiment is small is 50% is preferably in the range of 0.1 ≦ L1 ≦ 24. More preferably, it is in the range of ≦ L1 ≦ 10.

  L2 which is the primary particle diameter D50 in which the volume cumulative ratio from the side where the primary particle diameter of the carbon material according to the present embodiment is small is 50% is preferably in the range of 1 ≦ L2 ≦ 20, and 3 ≦ L2 ≦. A range of 15 is more preferable.

  Furthermore, L1 which is the primary particle diameter D50 in which the volume accumulation ratio from the side where the primary particle diameter of the negative electrode active material according to the present embodiment is small is 50%, and the volume accumulation ratio from the side where the primary particle diameter of the carbon material is small. More preferably, L2 having a primary particle diameter D50 of 50% satisfies the above range and L2> L1.

  L1 which is the primary particle diameter D50 in which the volume cumulative ratio from the side where the primary particle diameter of the negative electrode active material according to the present embodiment is small is 50%, and the volume cumulative ratio from the side where the primary particle diameter of the carbon material is small is 50. L2 which is the primary particle diameter D50 which is% can be obtained by a technique such as a laser scattering method.

  The content of the negative electrode active material in the negative electrode active material layer 24 is preferably 50 to 95% by mass based on the sum of the masses of the negative electrode active material, the conductive additive and the binder, and is 75 to 93% by mass. More preferably. If it is said range, a negative electrode with a big capacity | capacitance can be obtained.

(binder)
The binder binds the negative electrode active materials to the current collector 22 while bonding the negative electrode active materials to each other. A binder will not be specifically limited if the above-mentioned coupling | bonding is possible. For example, a fluororesin such as polyvinylidene fluoride (PVDF), cellulose, styrene / butadiene rubber, polyimide, polyamideimide, polyacrylic acid, polyacrylonitrile, polyalginic acid, or the like can be used.

  The content of the binder in the negative electrode active material layer 24 is preferably 1 to 30% by mass, and preferably 5 to 15% by mass based on the sum of the masses of the negative electrode active material, the conductive additive and the binder. Is more preferable. If it is said range, a negative electrode with a big capacity | capacitance can be obtained.

(Conductive aid)
The conductive aid is not particularly limited as long as the conductivity of the negative electrode active material layer 24 is improved, and a known conductive aid can be used. Examples thereof include carbon blacks such as acetylene black, furnace black, channel black, and thermal black, vapor grown carbon fibers (VGCF), carbon fibers such as carbon nanotubes, and carbon materials such as graphite. More than seeds can be used.

  The content of the conductive auxiliary in the negative electrode active material layer 24 is not particularly limited, but when added, it is usually 1 to 10% by mass based on the sum of the mass of the negative electrode active material, conductive auxiliary and binder. Preferably there is.

(solvent)
The solvent is not particularly limited as long as it can form the above-described negative electrode active material, conductive additive, and binder. For example, N-methyl-2-pyrrolidone, N, N-dimethylformamide, and the like can be used. .

(Negative electrode current collector)
The negative electrode current collector 22 is preferably a conductive plate material having a small thickness, and is preferably a metal foil having a thickness of 8 to 30 μm. The negative electrode current collector 22 is preferably formed of a material that does not alloy with lithium, and a particularly preferable material is copper. Examples of such copper foil include electrolytic copper foil. For example, an electrolytic copper foil is prepared by immersing a metal drum in an electrolytic solution in which copper ions are dissolved, and flowing current while rotating the copper drum, thereby depositing copper on the surface of the drum and peeling it off. It is the obtained copper foil.

  Moreover, the rolled copper foil manufactured by rolling the cast copper lump to desired thickness may be sufficient, and copper is deposited on the surface of the rolled copper foil by an electrolytic method, and the surface is roughened. There may be.

  Thus, there is no restriction | limiting in particular as an application | coating method which apply | coats the coating material containing a negative electrode active material, a binder, a conductive support agent, and a solvent mentioned above, The method employ | adopted when producing an electrode normally can be used. . Examples thereof include a slit die coating method and a doctor blade method.

(Negative electrode active material layer)
The negative electrode active material layer having a concentration gradient of Li according to the present embodiment can be prepared by using a multilayer coating method or the like in which a plurality of coating materials prepared using negative electrode active materials having different Li contents are applied. it can. The concentration of Li in the negative electrode active material layer can also be adjusted by doping Li in part of the negative electrode active material layer.

  The method for removing the solvent in the paint applied on the negative electrode current collector 22 is not particularly limited, and the negative electrode current collector 22 applied with the paint may be dried at, for example, 80 ° C. to 150 ° C.

  When preparing a negative electrode active material layer according to the present embodiment, when a plurality of coatings are applied in multiple layers, after the first coating is applied on the negative electrode current collector, the coating is dried once, and the Li content thereon After applying the second paint prepared using different negative electrode active materials, the material is dried again. A negative electrode active material can be produced by repeating these steps a plurality of times.

  In addition, when applying the second paint, the negative electrode active material layer may be prepared by applying the second paint on the first paint and then drying it in an undried state. Good.

  By applying the first paint and the second paint having different Li contents in an undried state and then drying, a mixed region is formed between the first paint and the second paint. The concentration can be changed gradually.

  Then, the negative electrode 20 on which the negative electrode active material layer 24 is formed in this way may be subsequently subjected to a press treatment using, for example, a roll press device as necessary. The linear pressure of the roll press can be set to 100 to 5000 kgf / cm, for example.

(Nonaqueous electrolyte)
The nonaqueous electrolytic solution has an electrolyte dissolved in a nonaqueous solvent, and may contain a cyclic carbonate and a chain carbonate as a nonaqueous solvent.

  The cyclic carbonate is not particularly limited as long as it can solvate the electrolyte, and a known cyclic carbonate can be used. For example, ethylene carbonate, propylene carbonate, butylene carbonate, and the like can be used.

  The chain carbonate is not particularly limited as long as the viscosity of the cyclic carbonate can be reduced, and a known chain carbonate can be used. Examples thereof include diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate. In addition, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, and the like may be mixed and used.

  The ratio of the cyclic carbonate and the chain carbonate in the non-aqueous solvent is preferably 1: 9 to 1: 1 by volume.

Examples of the electrolyte include LiPF ₆ , LiClO ₄ , LiBF ₄ , LiCF ₃ SO ₃ , LiCF ₃ , CF ₂ SO ₃ , LiC (CF ₃ SO ₂ ) ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (CF ₃ Lithium salts such as CF ₂ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiN (CF ₃ CF ₂ CO) ₂ , LiBOB can be used. In addition, these lithium salts may be used individually by 1 type, and may use 2 or more types together. In particular, LiPF ₆ is preferably included from the viewpoint of conductivity.

When LiPF ₆ is dissolved in a non-aqueous solvent, the concentration of the electrolyte in the non-aqueous electrolyte is preferably adjusted to 0.5 to 2.0 mol / L. When the concentration of the electrolyte is 0.5 mol / L or more, the conductivity of the nonaqueous electrolytic solution can be sufficiently secured, and a sufficient capacity can be easily obtained during charging and discharging. Moreover, by suppressing the electrolyte concentration to within 2.0 mol / L, it is possible to suppress an increase in the viscosity of the non-aqueous electrolyte, to sufficiently secure the mobility of lithium ions, and to obtain a sufficient capacity during charging and discharging. It becomes easy.

Even when LiPF ₆ is mixed with another electrolyte, the lithium ion concentration in the non-aqueous electrolyte is preferably adjusted to 0.5 to 2.0 mol / L, and the lithium ion concentration from LiPF ₆ is 50 mol%. More preferably, it is contained.

(Positive electrode)
The positive electrode 10 of the present embodiment has a structure in which a positive electrode active material layer 14 containing a positive electrode active material is formed on one surface or both surfaces of a positive electrode current collector 12. The positive electrode active material layer 14 is coated on the positive electrode current collector 12 with a paint containing the positive electrode active material, a binder, a conductive additive, and a solvent in the same process as the negative electrode manufacturing method. It can be produced by removing the solvent in the applied paint.

Examples of the positive electrode active material include occlusion and release of lithium ions, desorption and insertion (intercalation) of lithium ions, or doping and dedoping of lithium ions and counter anions of the lithium ions (for example, ClO ₄ ⁻ ). Is not particularly limited as long as it can reversibly proceed, and a known electrode active material can be used. For example, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganese spinel (LiMn ₂ O ₄ ), and a general formula: LiNi _x Co _y Mn _z O ₂ (x + y + z = 1) Examples include composite metal oxides, lithium vanadium compounds (LiV ₂ O ₅ ), and LiMPO ₄ (wherein M represents Co, Ni, Mn, Fe, or VO).

  In addition, oxides and sulfides capable of inserting and extracting lithium ions can be used as the positive electrode active material.

  Furthermore, each component (conductive auxiliary agent and binder) other than the positive electrode active material can be the same material as that used in the negative electrode 20.

  As the positive electrode current collector 12, various known metal foils used in current collectors for lithium ion secondary batteries can be used. For example, a metal foil such as aluminum, stainless steel, nickel, titanium, or an alloy thereof can be used, and an aluminum foil is particularly preferable.

(Separator)
As long as the separator 18 is formed of an insulating porous body, the material, the manufacturing method, and the like are not particularly limited, and a known separator used for a lithium ion secondary battery can be used. For example, as the insulating porous material, a known polyolefin resin, specifically, a crystalline homopolymer or copolymer obtained by polymerizing polyethylene, polypropylene, 1-butene, 4-methyl-1-pentene, 1-hexene, or the like. A polymer is mentioned. These homopolymers or copolymers can be used alone or in combination of two or more. Further, it may be a single layer or a multilayer.

  The outer package 50 is not particularly limited as long as it can prevent leakage of the electrolytic solution to the outside or entry of moisture or the like into the lithium ion secondary battery 100 from the outside, such as a metal can, an aluminum laminate film, or the like Can be used. The aluminum laminate film includes, for example, a structure that is configured as a three-layer structure in which polypropylene, aluminum, and nylon are laminated in this order.

  The negative electrode lead 62 and the positive electrode lead 60 may be formed of a conductive material such as aluminum or nickel.

  As mentioned above, although the example of this invention was described in detail by embodiment, this invention is not limited to the said embodiment, A various deformation | transformation is possible.

  About the produced lithium ion secondary battery, cycling characteristics were evaluated with the following method.

この容量維持率が高いほど、充放電サイクル特性が良好であることを意味する。実施例および比較例で作製したリチウムイオン二次電池は、上記の条件によって充放電を繰り返し、１００サイクル後の容量維持率を充放電サイクル特性として評価した。 (Measurement of charge / discharge cycle characteristics)
The cycle characteristic is a current value at 0.5 C when a voltage range is set to 3.0 V to 0.01 V using a secondary battery charge / discharge test apparatus and 1 C = 1600 mAh / g per weight of the negative electrode active material. The charge / discharge cycle characteristics may be evaluated under the condition of charging and discharging at a current value of 0.5C. The charge / discharge cycle characteristics are evaluated as capacity retention rate (%), and the capacity retention rate (%) is the ratio of the discharge capacity at each cycle number to the initial discharge capacity, with the discharge capacity at the first cycle being the initial discharge capacity. Yes, expressed by the following formula (1). Note that 1C is a current value at which charging / discharging is completed in exactly one hour after a constant current charge or constant current discharge is performed on a battery cell having a nominal capacity value.

A higher capacity retention rate means better charge / discharge cycle characteristics. The lithium ion secondary batteries produced in the examples and comparative examples were repeatedly charged and discharged under the above conditions, and the capacity retention rate after 100 cycles was evaluated as charge / discharge cycle characteristics.

  EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated in more detail, this invention is not limited to these Examples at all.

Example 1
(Preparation of positive electrode)
96% by weight of LiNi _0.85 Co _0.10 Al _0.05 O ₂ as a positive electrode active material, 2% by weight of carbon black as a conductive additive, 0.5% by weight of graphite, and 1 PVDF as a binder 0.5 wt% and N-methyl-2-pyrrolidone solvent were mixed and dispersed to prepare a paste-like positive electrode slurry.

The obtained positive electrode slurry was uniformly applied to both surfaces of an aluminum foil having a thickness of 20 μm using a comma roll coater so that the applied amount of the active material was 22.0 mg / cm ² . Next, a positive electrode active material layer was formed by drying the N-methyl-2-pyrrolidone solvent in the positive electrode active material in an air atmosphere at 110 ° C. in a drying furnace.

In addition, the thickness of the coating film of the positive electrode active material layer apply | coated to both surfaces of the said aluminum foil was adjusted to the substantially same film thickness. The positive electrode on which the positive electrode active material was formed was pressure-bonded to both surfaces of the positive electrode current collector by a roll press to produce a positive electrode so that the density of the positive electrode active material layer was 3.6 g / cm ³ . . The positive electrode sheet was obtained by the above.

(Preparation of negative electrode)
As the negative electrode active material, 87.9% by weight of silicon oxide having a D50 of 3 μm, 2.1% by weight of acetylene black, 10% by weight of polyamideimide resin, 3.0% by weight of LiPF6, N-methyl-2- A first coating material for forming an active material layer was prepared by mixing and dispersing a pyrrolidone solvent. Next, a second coating material for forming an active material layer was prepared in the same manner as the first coating material except that the mixing amount of LiPF6 was 1.5% by weight.

The obtained second paint was applied to one side of a 10 μm thick copper foil so that the active material application amount was 1.5 mg / cm ^2, and the first paint was applied in an undried state. The negative electrode active material layer was formed by applying the active material so that the applied amount was 1.5 mg / cm ² and drying at 100 ° C. Thereafter, a negative electrode active material layer was similarly formed on the other surface of the negative electrode current collector.

In addition, the thickness of the coating film of the negative electrode active material layer apply | coated to both surfaces of the said copper foil was adjusted to the substantially same film thickness. The negative electrode on which the negative electrode active material was formed was pressure-bonded to both surfaces of the negative electrode current collector by a roll press to produce a negative electrode so that the density of the negative electrode active material layer was 1.5 g / cm ³ . . Thus, a negative electrode sheet was obtained.

As a result of observing the cross section of the obtained negative electrode active material layer using EELS, it was confirmed that the Li concentration of the negative electrode active material layer increased asymptotically from the surface in contact with the current collector.
(Example 2)

  When forming the negative electrode active material, the negative electrode active material layer of Example 2 was formed in the same manner as in Example 1 except that the second paint was applied after the first paint was applied to obtain the negative electrode. It was.

As a result of observing the cross section of the obtained negative electrode active material layer using EELS, it was confirmed that the Li concentration of the negative electrode active material layer gradually decreased from the surface in contact with the current collector.
(Example 3)

  When forming the negative electrode active material layer, after applying the second paint, drying at 100 ° C., removing the solvent, and then applying the first paint, the same as in Example 1 The negative electrode active material layer of Example 3 was formed to obtain a negative electrode.

As a result of observing the cross section of the obtained negative electrode active material layer using EELS, the Li concentration of the negative electrode active material layer was prepared by applying the first paint, and the second paint was applied. It was confirmed that the Li concentration in the region in contact with the current collector was low, which was different from the region that was formed.
Example 4

  When forming the negative electrode active material layer, after applying the first paint, drying at 100 ° C., removing the solvent, and then applying the second paint, the same as in Example 2. The negative electrode active material layer of Example 4 was formed to obtain a negative electrode.

As a result of observing the cross section of the obtained negative electrode active material layer using EELS, the Li concentration of the negative electrode active material layer was prepared by applying the first paint, and the second paint was applied. It was confirmed that the Li concentration in the region in contact with the current collector was high, which was different from the region that was formed.
(Example 5)

The negative electrode active material layer of Example 5 was used in the same manner as in Example 1 except that a mixture of 63.1% by weight of silicon oxide and 24.8% by weight of graphite having a D50 of 5 μm was used as the negative electrode active material. And a negative electrode was obtained.
(Example 6)

The negative electrode active material layer of Example 6 was formed in the same manner as in Example 5 except that 62.4% by weight of silicon oxide and 25.5% by weight of graphite were mixed as the negative electrode active material. A negative electrode was obtained.
(Example 7)

A negative electrode active material layer of Example 7 was formed in the same manner as in Example 5 except that a mixture of 42.9% by weight of silicon oxide and 45.0% by weight of graphite was used as the negative electrode active material. A negative electrode was obtained.
(Example 8)

The negative electrode active material layer of Example 8 was formed in the same manner as in Example 5 except that 26.8% by weight of silicon oxide and 61.1% by weight of graphite were mixed as the negative electrode active material. A negative electrode was obtained.
Example 9

The negative electrode active material layer of Example 9 was formed in the same manner as in Example 5 except that 26.4% by weight of silicon oxide and 61.5% by weight of graphite were mixed as the negative electrode active material. A negative electrode was obtained.
(Example 10)

The negative electrode active material layer of Example 10 was formed in the same manner as in Example 7 except that silicon oxide having a D50 of 24.0 μm and graphite having a D50 of 1.0 μm were used as the negative electrode active material. Obtained.
(Example 11)

The negative electrode active material layer of Example 11 was formed in the same manner as in Example 7 except that silicon oxide having a D50 of 10.0 μm and graphite having a D50 of 9.0 μm were used as the negative electrode active material. Obtained.
(Example 12)

The negative electrode active material layer of Example 12 was formed in the same manner as in Example 7 except that silicon oxide having a D50 of 5.0 μm and graphite having a D50 of 6.0 μm were used as the negative electrode active material. Obtained.
(Example 13)

A negative electrode active material layer of Example 13 was formed in the same manner as in Example 7 except that silicon oxide having a D50 of 3.0 μm and graphite having a D50 of 14.4 μm were used as the negative electrode active material. Obtained.
(Example 14)

A negative electrode active material layer of Example 14 was formed in the same manner as in Example 7 except that silicon oxide having a D50 of 2.5 μm and graphite having a D50 of 12.8 μm were used as the negative electrode active material. Obtained.
(Example 15)

The negative electrode active material layer of Example 15 was formed in the same manner as in Example 7 except that silicon oxide having a D50 of 0.1 μm and graphite having a D50 of 19.9 μm were used as the negative electrode active material. Obtained.
(Comparative Example 1)

The negative electrode active material layer of Comparative Example 1 was formed in the same manner as in Example 1 except that only the coating material 1 was used so that the active material was applied in an amount of 3.0 mg / cm ^2. Got.
(Example 16)

As the negative electrode active material, tin oxide is used instead of silicon oxide, the application amount of the active material of the first paint is 5.0 mg / cm ² , and the application amount of the active material of the second paint is 5.0 mg / cm ^2. A negative electrode active material layer of Example 16 was formed in the same manner as in Example 1 except that the negative electrode was obtained.

As a result of observing the cross section of the obtained negative electrode active material layer using EELS, it was confirmed that the Li concentration of the negative electrode active material layer increased asymptotically from the surface in contact with the current collector.
(Comparative Example 2)

A negative electrode active material layer of Comparative Example 2 was formed in the same manner as in Example 1 except that only the coating material 1 was used so that the active material was applied in an amount of 10.0 mg / cm ^2. Got.
(Example 17)

As the negative electrode active material, ferric oxide is used instead of silicon oxide, the application amount of the active material of the first paint is 5.0 mg / cm ² , and the application amount of the active material of the second paint is 5.0 mg / cm ² . A negative electrode active material layer of Example 17 was formed in the same manner as in Example 1 except that the thickness was cm ² , thereby obtaining a negative electrode.

As a result of observing the cross section of the obtained negative electrode active material layer using EELS, it was confirmed that the Li concentration of the negative electrode active material layer increased asymptotically from the surface in contact with the current collector.
(Comparative Example 3)

A negative electrode active material layer of Comparative Example 3 was formed in the same manner as in Example 1 except that only the coating material 1 was used so that the active material was applied in an amount of 10.0 mg / cm ^2. Got.
(Example 18)

As the negative electrode active material, titanium dioxide is used instead of silicon oxide, the application amount of the active material of the first paint is 12.5 mg / cm ² , and the application amount of the active material of the second paint is 12.5 mg / cm ^2. A negative electrode active material layer of Example 18 was formed in the same manner as in Example 1 except that the negative electrode was obtained.

As a result of observing the cross section of the obtained negative electrode active material layer using EELS, it was confirmed that the Li concentration of the negative electrode active material layer increased asymptotically from the surface in contact with the current collector.
(Comparative Example 4)

A negative electrode active material layer of Comparative Example 4 was formed in the same manner as in Example 1 except that only the coating material 1 was used so that the active material was applied in an amount of 25.0 mg / cm ^2. Got.

(Production of evaluation lithium-ion secondary battery)
Using the positive electrode and the negative electrode prepared above, a separator made of a polyethylene microporous film is sandwiched between them and placed in an aluminum laminate pack. In this aluminum laminate pack, a 1M LiPF ₆ solution (solvent: EC) is used as an electrolytic solution. / DEC = 3/7 (volume ratio)) was injected, and then vacuum-sealed to produce a lithium ion secondary battery for evaluation.

  The cycle characteristics of the lithium ion secondary batteries produced in Examples 1 to 13 and Comparative Examples 1 to 4 were evaluated. When the discharge capacity retention rate of each comparative example was 100%, the relative value of the discharge capacity retention rate of each example with respect to the discharge capacity retention rate of each comparative example was compared as a capacity retention rate. Tables 1 to 4 show the values of cycle characteristics for each negative electrode active material used.

  The effects of the present invention are clear from the results shown in Tables 1 to 4. That is, since the Li concentration in the negative electrode active material layer has a concentration gradient, a high capacity retention rate is exhibited, and thus it was confirmed that the cycle characteristics were excellent. Moreover, when the density | concentration of Li became high toward the surface side of a negative electrode active material layer, it was confirmed that it is excellent in cycling characteristics especially.

  Further, from the results of Table 1, it was confirmed that the cycle characteristics were further improved by mixing graphite in the negative electrode active material layer, and the volume cumulative ratio from the side where the primary particle size of the negative electrode active material is small is 50%. Especially when L1 which is the primary particle diameter D50 and L2 which is the primary particle diameter D50 whose volume cumulative ratio from the side where the primary particle diameter is small is 50% are 0.04 ≦ L2 / L1 ≦ 200. It was confirmed that the characteristics were excellent.

  ADVANTAGE OF THE INVENTION According to this invention, the negative electrode for lithium ion secondary batteries and lithium ion secondary battery which are excellent in cycling characteristics can be provided.

DESCRIPTION OF SYMBOLS 10 ... Positive electrode, 12 ... Positive electrode collector, 14 ... Positive electrode active material layer, 18 ... Separator, 20 ... Negative electrode, 22 ... Negative electrode current collector, 24 ... Negative electrode active material layer, 30 ... Laminated body, 50 ... Outer body, 60 ... Positive electrode lead, 62 ... Negative electrode lead, 100 ... Lithium ion secondary battery

Claims (6)

  A negative electrode having a current collector and a negative electrode active material layer having a negative electrode active material, wherein the negative electrode active material layer is at least one of Li, Si, Sn, Fe, Ni, Al, Ge, and Ti. A negative electrode for a lithium ion secondary battery containing the above elements and having a concentration gradient of Li in the negative electrode active material layer.   2. The negative electrode for a lithium ion secondary battery according to claim 1, wherein a concentration gradient of Li in the negative electrode active material layer has a concentration gradient in which the concentration of Li decreases gradually or stepwise toward the current collector side.   3. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the negative electrode active material contains Li and at least one metal selected from Si, Sn, Fe, and Ti, an oxide, and a mixture thereof. .   The said negative electrode active material layer is a negative electrode for lithium ion secondary batteries as described in any one of Claims 1-3 containing a carbonaceous material.   The primary particle diameter D50 having a volume cumulative ratio from the side where the primary particle diameter of the negative electrode active material is small is 50% is L1, and the volume cumulative ratio from the side where the primary particle diameter of the carbonaceous material is small is 50%. The negative electrode for a lithium ion secondary battery according to claim 4, wherein L2 / L1, which is the ratio of L1 and L2, is 0.04 ≦ L2 / L1 ≦ 200 when the primary particle size D50 is L2. The lithium ion secondary battery which has a negative electrode for lithium ion secondary batteries as described in any one of Claims 1-5, a positive electrode, a separator, and a nonaqueous electrolyte.

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