JP2019169391A - Negative electrode for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Abstract

To provide a negative electrode for lithium ion secondary battery capable of high density, excellent in electrolyte permeability, charge and discharge capacity, and energy density, and to provide a lithium ion secondary battery.SOLUTION: The negative electrode for a lithium ion secondary battery includes: a negative electrode current collector; and a negative electrode active material layer held by the negative electrode current collector. The negative electrode active material layer has an inner peripheral portion A and an outer peripheral portion B. Density Dof the outer peripheral portion is smaller than density Dof the inner peripheral portion (D>D).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. It is also a promising candidate as a power source for use in hybrid vehicles and electric vehicles. In recent years, lithium ion secondary batteries have come to be used in various products as power sources for smartphones, electric vehicles, drones, power storage, etc., and accordingly, high energy density lithium ion secondary batteries are required. Has been.

  The lithium ion secondary battery is mainly composed of a positive electrode, a negative electrode, a separator, and an electrolyte, and has a configuration in which a plurality of positive electrodes, negative electrodes, and separators are laminated. The separator insulates the positive electrode from the negative electrode, and the electrolyte allows ions to move between the positive electrode and the negative electrode. Further, in order to prevent a short circuit due to lithium dendrite, the area of the negative electrode is generally designed to be larger than the area of the positive electrode. By the way, as a negative electrode of a commercially available lithium ion secondary battery, graphite is mainly used as a negative electrode active material. However, in order to further increase the capacity, the negative electrode active material is packed at a high density, or It is necessary to press the electrode with a high load (pressurization) to increase the volume density of the electrode. In connection with this, for example, attempts have been made to change the particle size distribution in order to fill the negative electrode active material with a high density (for example, Patent Document 1). It is necessary to press the load (pressurize) to increase the density of the electrode.

  However, when the electrode density is increased by high-load (pressurization) pressing on the electrode, the permeability of the electrolytic solution to the electrode may decrease. Further, even in an electrode using silicon having a larger capacity than graphite as a negative electrode active material, the permeability of the electrolytic solution to the electrode may be lowered in the same manner as in the case of an electrode having a higher density. Since the insertion / extraction reaction of lithium ions to / from the electrode material is via the electrolytic solution, a decrease in the permeability of the electrolytic solution has a problem of causing a decrease in battery characteristics during the first charge / discharge.

JP 2005-340025 A

  An object of the present invention is to provide a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery, which have been made in view of the above circumstances, can be densified, and have excellent electrolyte solution permeability. .

The present inventors have conducted extensive studies results, the negative electrode active material layer, the density D _B of the outer peripheral portion B, is made smaller than the density D _A of the inner peripheral portion A, can reduce the impregnation time of the electrolyte, the charge Since the inventors have found that a lithium ion secondary battery having excellent discharge capacity and energy density can be obtained, the present invention has been achieved.
[1] A negative electrode comprising a negative electrode current collector and a negative electrode active material layer held by the negative electrode current collector,
The negative electrode active material layer has an inner peripheral portion A and an outer peripheral portion B,
The density D _{B of the} outer peripheral portion is smaller than the density D _A of the inner peripheral portion (D _A > D _B )
The negative electrode for lithium ion secondary batteries characterized by the above-mentioned.
[2] When the area of the inner peripheral part A of the negative electrode active material layer is S _A and the area of the outer peripheral part B is S _B , S _B / S _A is 0.02 ≦ S _B / S _A ≦ 1 The negative electrode for a lithium ion secondary battery as described in [1], which is 0.0.
[3] the and density _{D A} of the inner peripheral portion A in the negative electrode active material layer, the density ratio _D A / _{D B} of the density _{D B} of the outer peripheral portion B is, 1.00 _<D A / _{D B} ≦ The negative electrode for a lithium ion secondary battery according to [1] or [2], wherein the negative electrode is 1.82.
[4] The negative electrode for a lithium ion secondary battery according to any one of [1] to [3], wherein the negative electrode active material layer includes at least a negative electrode active material and a negative electrode binder.
[5] A lithium ion secondary battery comprising the negative electrode for a lithium ion secondary battery according to any one of [1] to [4], a positive electrode, a separator, and an electrolytic solution.

  According to the present invention, it is possible to provide a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery that can be densified, have excellent electrolyte solution permeability, and have excellent charge / discharge capacity and energy density.

It is a cross-sectional schematic diagram of the lithium ion secondary battery concerning this embodiment. It is the schematic diagram which looked at the negative electrode for lithium ion secondary batteries which concerns on this embodiment from the main surface.

  Hereinafter, the present embodiment will be described in detail with appropriate reference to the drawings. In the drawings used in the following description, in order to make the characteristics of the present invention easier to understand, there are cases where the characteristic parts are enlarged for the sake of convenience, and the dimensional ratios of the respective components are different from actual ones. is there. The materials, dimensions, and the like exemplified in the following description are examples, and the present invention is not limited to them, and can be appropriately modified and implemented without departing from the scope of the invention.

[Lithium ion secondary battery]
FIG. 1 is a schematic cross-sectional view of a lithium ion secondary battery according to this embodiment. A lithium ion secondary battery 100 shown in FIG. 1 mainly includes a laminate 40, a case 50 that accommodates the laminate 40 in a sealed state, and a pair of leads 60 and 62 connected to the laminate 40.
Although not shown, the electrolyte solution is housed in the case 50 together with the laminate 40.

  The stacked body 40 is configured such that the positive electrode 20 and the negative electrode 30 are disposed to face each other with the separator 10 interposed therebetween. The positive electrode 20 is obtained by providing a positive electrode active material layer 24 on a plate-like (film-like) positive electrode current collector 22. The negative electrode 30 is obtained by providing a negative electrode active material layer 34 on a plate-like (film-like) negative electrode current collector 32.

  The positive electrode active material layer 24 and the negative electrode active material layer 34 are in contact with both sides of the separator 10. Leads 62 and 60 are connected to the end portions of the positive electrode current collector 22 and the negative electrode current collector 32, respectively, and the end portions of the leads 60 and 62 extend to the outside of the case 50. In FIG. 1, the case 50 has one laminated body 40 in the case 50, but a plurality of laminated bodies 40 may be laminated.

"Anode for lithium ion secondary battery"
“First Embodiment”
The negative electrode 30 for a lithium ion secondary battery of this embodiment includes a negative electrode current collector 32 and a negative electrode active material layer 34 provided on the negative electrode current collector 32. In FIG. 2, the schematic diagram at the time of seeing from the main surface of the negative electrode for lithium ion secondary batteries which concerns on this embodiment is shown.
Negative electrode active material layer 34, the inner peripheral portion A (Fig. 2: 34A) and the outer peripheral portion B (Fig. 2: 34B) and a, the density _{D B} of the outer peripheral portion B, the density _{D A} of the inner peripheral portion A Is smaller than (D _A > D _B ).

In the anode active material layer 34, the density D _B of the outer peripheral portion B, is made smaller than the density D _A of the inner peripheral portion A, the impregnation of the electrolytic solution of the outer peripheral portion B, earlier than the inner peripheral portion A . As a result, the impregnation property of the electrolytic solution into the inner peripheral portion A is accelerated. Since the electrolytic solution is impregnated from the outer peripheral portion B at an early stage, the electrolytic solution easily flows from the outer peripheral portion B to the inner peripheral portion A. As a result, the inner peripheral portion A is also impregnated earlier. Therefore, since the charge / discharge reaction becomes uniform, a lithium ion secondary battery excellent in charge / discharge capacity and energy density can be obtained. Furthermore, since the impregnation time of the electrolytic solution can be shortened, productivity can be improved.
On the other hand, when the density of the outer peripheral part B is the same as or larger than that of the inner peripheral part A, the impregnating property of the outer peripheral part B is lowered, so that the impregnating property to the inner peripheral part A is also delayed. Therefore, in a place where the electrolytic solution impregnation is poor in the inner peripheral portion A, the charge / discharge capacity is lowered, and it is difficult to obtain a lithium ion secondary battery having an excellent energy density.

In the negative electrode active material layer using graphite as the negative electrode active material, the density _{D A} of the inner peripheral portion A is preferably in the range of 1.0~2.2g / cm ^3, 1.3~2.1g / More preferably, it is in the range of cm ³ . Particularly in the range of 1.3 to 2.1 g / cm ³ , it is easy to obtain a lithium ion secondary battery having excellent adhesiveness between the current collector and the negative electrode active material layer and having an excellent energy density. If the density D _A of the inner peripheral portion A is smaller than 1.0 g / cm ^3, the adhesion between the current collector and the anode active material layer is weak, easily negative electrode active material layer is peeled off from the current collector. Therefore, it is difficult to obtain a lithium ion secondary battery having an excellent energy density. Moreover, when larger than 2.2 g / cm < ³ >, the density increase of a negative electrode active material layer will become difficult, and the press process of an electrode may increase. Also, when the negative electrode active material graphite is cracked, or the graphite is strongly pressed against the negative electrode current collector in the pressing process, and the current collector thickness at that location becomes thin, and the electrode is likely to break due to charge / discharge There is.
In the negative electrode active material layer using silicon as an anode active material, the density _{D A} of the inner peripheral portion A is preferably in the range of 1.0~1.7g / cm ^3, 1.1~1.6g / More preferably, it is in the range of cm ³ . Particularly in the range of 1.1 to 1.6 g / cm ³ , it is easy to obtain a lithium ion secondary battery having excellent adhesion between the current collector and the negative electrode active material layer and having an excellent energy density. If the density D _A of the inner peripheral portion A is smaller than 1.0 g / cm ^3, the adhesion between the current collector and the anode active material layer is weak, easily negative electrode active material layer is peeled off from the current collector. Therefore, it is difficult to obtain a lithium ion secondary battery having an excellent energy density. On the other hand, if it is larger than 1.7 g / cm ³ , it is difficult to increase the density of the negative electrode active material layer, and the electrode pressing process may increase. Similarly to graphite, silicon, which is the negative electrode active material, is cracked, or silicon is strongly pressed against the negative electrode current collector in the pressing process, and the thickness of the current collector at that location is reduced. It may be easy to tear. Note that since silicon has a larger capacity than graphite, the negative electrode active material layer can be made thinner than graphite. Therefore, a lithium ion secondary battery having a relatively lower density than graphite and an excellent energy density can be obtained.
Density _{D B} of the outer peripheral portion B is not particularly smaller than the density _{D A} of the inner peripheral portion A limited, is preferably in the range of 1.0 to 2.0 g / cm ^3, 1.0 to 1 More preferably, it is in the range of 6 g / cm ³ . If it is the range of 1.0-1.6 g / cm < ³ >, it will be excellent in the adhesiveness of a collector and a negative electrode active material layer, and the impregnation property of electrolyte solution is also excellent.

  The negative electrode active material layer 34 has a facing portion that faces the positive electrode active material layer 24 and a non-facing portion that does not face the positive electrode active material layer 24, and the inner peripheral portion A of the negative electrode active material layer 34 is It is a facing part that faces the positive electrode active material layer 24, and the outer peripheral part B of the negative electrode active material layer 34 is preferably a non-facing part that does not face the positive electrode active material layer 24.

  The outer peripheral portion B is preferably provided with a uniform width with respect to the inner peripheral portion A. This is because the electrolyte solution is uniformly diffused from the outer peripheral portion B to the inner peripheral portion A by setting the outer peripheral portion B to a uniform width.

The negative active in material layer 34 and the area of the inner peripheral portion A _{S A,} when the area of the outer peripheral portion B was set to _{S B, S B} / _{S A} is _{0.02 ≦ S B / S A ≦} 0.1 It is preferable that
When S _B / S _A is in the above range, the impregnation property of the electrolytic solution is excellent, so that productivity can be increased. In addition, a lithium ion secondary battery having an excellent charge / discharge capacity can be obtained. When S B _/ S _A is greater than 0.1, since the area S _B of the outer peripheral portion becomes large, the more the side reactions at the S _B, the discharge capacity tends to lower. When S B _/ S _A is less than 0.02, since the area S _B of the outer peripheral portion becomes smaller, dendrites will tends to be generated at the negative electrode surface, likewise the discharge capacity tends to lower.

Wherein the density _{D A} of the inner peripheral portion A in the negative electrode active material layer 34, the density ratio _D A / _{D B} of the density _{D B} of the outer peripheral portion _{B, 1.00 <D A / D} B ≦ 1. 82 is more preferable.
When D _A / D _B is in the above range, the impregnation property of the electrolytic solution is excellent, so that productivity can be increased. In addition, a lithium ion secondary battery having an excellent charge / discharge capacity can be obtained. When it is 1.00 or less, the impregnation property of the electrolytic solution is deteriorated, so that a long impregnation time is required, and thus the productivity is lowered.

(Negative electrode current collector)
The negative electrode current collector 32 may be a conductive plate material, and for example, a metal thin plate such as a copper foil, a stainless steel foil, or a nickel foil can be used.

(Negative electrode active material layer)
The negative electrode active material layer 34 includes a negative electrode active material and a negative electrode binder, and optionally includes a negative electrode conductive material.

(Negative electrode active material)
The negative electrode active material used for the negative electrode for lithium ion secondary batteries of this embodiment can contain a well-known negative electrode active material. Examples of the negative electrode active material include carbon materials such as metallic lithium, graphite capable of occluding and releasing lithium ions (natural graphite, artificial graphite), carbon nanotubes, non-graphitizable carbon, graphitizable carbon, and low-temperature calcined carbon. Examples thereof include metals that can be combined with lithium such as aluminum, silicon, and tin, amorphous compounds mainly composed of oxides such as tin dioxide, and particles containing lithium titanate (Li ₄ Ti ₅ O ₁₂ ). . It is preferable that the negative electrode active material used for the negative electrode for lithium ion secondary batteries of this embodiment contains silicon or graphite.

(Negative electrode conductive material)
Examples of the conductive material include carbon powders such as carbon blacks, carbon nanotubes, carbon materials, and conductive oxides such as ITO. Among these, carbon powders such as acetylene black and ethylene black are particularly preferable. In the case where sufficient conductivity can be ensured with only the negative electrode active material, the lithium ion secondary battery 100 may not include a conductive material.

(Negative electrode binder)
The binder bonds the active materials to each other and bonds the active material to the negative electrode current collector 32.
The negative electrode binder contained in the negative electrode active material layer of the present embodiment may be an organic solvent binder or an aqueous binder. For example, polyamideimide, polyimide, polyamide, polyacrylic acid, polyacrylate, alginate, styrene / butadiene rubber (SBR), carboxymethylcellulose (CMC), polyurethane and the like may be used. Multiple types can be used in combination. In particular, when silicon having a large volume expansion due to charge / discharge is used as the negative electrode active material, polyamideimide, polyimide, polyamide, and polyacrylic acid can be preferably used. On the other hand, when graphite having a smaller volume expansion than silicon and silicon compounds is used as the negative electrode active material, styrene-butadiene rubber can be suitably used. The binders listed above are not limited.

  The contents of the negative electrode active material, the conductive material, and the binder contained in the negative electrode active material layer 34 are not particularly limited. The constituent ratio of the negative electrode active material in the negative electrode active material layer 34 is preferably 65 to 98% by mass. The constituent ratio of the conductive material in the negative electrode active material layer 34 is preferably 0 to 20% by mass, and the constituent ratio of the binder in the negative electrode active material layer 34 is 2 to 35% by mass. It is preferable. In the case of an aqueous binder, the range of 2 to 20% by mass is preferable, and in the case of an organic solvent binder, it can be suitably used in the range of 5 to 30% by mass.

  By making content of a negative electrode active material and a binder into the said range, it can prevent that the quantity of a binder is too small and it becomes impossible to form a strong negative electrode active material layer. In addition, the amount of the binder that does not contribute to the electric capacity increases, and the tendency that it is difficult to obtain a sufficient volume energy density can be suppressed.

“Second Embodiment”
The negative electrode for a lithium ion secondary battery according to this embodiment is different from the negative electrode for a lithium ion secondary battery according to the first embodiment in that the negative electrode active material contains graphite.

  Examples of the negative electrode active material containing graphite include metal lithium, graphite capable of occluding and releasing lithium ions (natural graphite, artificial graphite), carbon nanotube, non-graphitizable carbon, graphitizable carbon, low-temperature calcined carbon, and the like. Examples thereof include carbon materials.

The negative electrode active material used for the negative electrode for lithium ion secondary batteries of this embodiment can contain other well-known negative electrode active materials other than the negative electrode active material containing graphite. Other negative electrode active materials include, for example, metallic lithium, metals that can occlude / release lithium ions, metals that can be alloyed with lithium such as silicon and tin, and amorphous materials mainly composed of oxides such as tin dioxide. And particles containing lithium titanate (Li ₄ Ti ₅ O ₁₂ ) and the like.

“Third Embodiment”
The negative electrode for lithium ion secondary batteries of this embodiment differs from the negative electrode for lithium ion secondary batteries according to the first embodiment in that the negative electrode active material contains silicon.

The negative electrode active material containing silicon may be any compound that can occlude and release lithium ions, and known negative electrode active materials containing silicon can be used. Examples of the negative electrode active material containing silicon include a negative electrode active material containing silicon, silicon oxide, or silicate. For example, silicon nanowires or silicon fine particles; at least one metal selected from the group consisting of tin, nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium, germanium, bismuth, antimony and chromium and silicon Alloys with boron, nitrogen, oxygen, or a compound of carbon and silicon. Specific examples of alloys or compounds of silicon include SiB ₄ , SiB ₆ , Mg ₂ Si, Ni ₂ Si, TiSi ₂ , MoSi ₂ , CoSi ₂ , NiSi ₂ , CaSi ₂ , CrSi ₂ , Cu ₅ Si, FeSi ₂ , MnSi ₂ , NbSi ₂ , TaSi ₂ , VSi ₂ , WSi ₂ , ZnSi ₂ , SiC, Si ₂ N ₂ , Si ₂ N ₂ O, SiO _X (0 <X ≦ 2) or LiSiO.

Moreover, the negative electrode active material which carry | supported or coat | covered the material with high electroconductivity on the surface of the said silicon, a silicon oxide, or a silicate can be used. For example, the negative electrode active material coated with carbon or titanium oxide on the surface of the SiO _X and the like.

  Further, a composite material in which the silicon, silicon oxide, or silicate is dispersed on a carbon substrate, or a composite material in which the silicon, silicon oxide, or silicate fine particles and artificial graphite particles are partially combined. Is mentioned.

The negative electrode active material used for the negative electrode for lithium ion secondary batteries of this embodiment can contain other well-known negative electrode active materials other than the negative electrode active material containing silicon. Other negative electrode active materials include, for example, carbon such as metallic lithium, graphite capable of occluding and releasing lithium ions (natural graphite, artificial graphite), carbon nanotubes, non-graphitizable carbon, graphitizable carbon, low-temperature calcined carbon, etc. Particles containing materials, metals that can be combined with lithium such as aluminum, silicon and tin, amorphous compounds mainly composed of oxides such as tin dioxide, lithium titanate (Li ₄ Ti ₅ O ₁₂ ), etc. Can be mentioned.

"Positive electrode"
The positive electrode 20 includes a positive electrode current collector 22 and a positive electrode active material layer 24 provided on the positive electrode current collector 22.

(Positive electrode current collector)
The positive electrode current collector 22 may be a conductive plate material, and for example, a thin metal plate of aluminum or nickel foil can be used.

(Positive electrode active material layer)
The positive electrode active material used for the positive electrode active material layer 24 includes lithium ion occlusion and release, lithium ion desorption and insertion (intercalation), or counter ions (for example, PF ₆ ⁻ ) of lithium ions and lithium ions. An electrode active material capable of reversibly proceeding doping and dedoping can be used.

For example, lithium cobalt oxide (LiCoO _2), lithium nickelate (LiNiO _2), lithium manganate (LiMnO _2), lithium manganese spinel (LiMn ₂ O _4), and the general _{formula: LiNi x Co y Mn z M} a O ₂ (x + y + z + a = 1, 0 ≦ x <1, 0 ≦ y <1, 0 ≦ z <1, 0 ≦ a <1, M is one type selected from Al, Mg, Nb, Ti, Cu, Zn, Cr Complex metal oxides represented by the above elements), lithium vanadium compounds (LiV ₂ O ₅ ), olivine-type LiMPO ₄ (where M is Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr) more shows one or more elements or VO selected), lithium titanate _{(Li 4 Ti 5 O 12)} , lithium nickel cobalt aluminate (LiNi _x Co _{Al z O 2 (0.9 <x} + y + z <1.1)) complex metal oxide such as polyacetylene, polyaniline, polypyrrole, polythiophene, and the like polyacene.

(Conductive material)
Examples of the conductive material include carbon powders such as carbon blacks, carbon nanotubes, carbon materials, metal fine powders such as copper, nickel, stainless steel, and iron, a mixture of carbon materials and metal fine powders, and conductive oxides such as ITO. . In the case where sufficient conductivity can be ensured only by the positive electrode active material, the lithium ion secondary battery 100 may not include a conductive material.

(Positive electrode binder)
The positive electrode binder contained in the positive electrode active material layer 24 of the present embodiment may be an organic solvent binder or an aqueous binder. For example, polyvinylidene fluoride (PVDF), polyimide (PI), polyamideimide (PAI), polyamide (PA), polyethylene vinyl alcohol (PVA), polyacrylate, styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC), polyurethane One of these may be used, and a plurality of types may be used in combination. In addition, it is not limited to these enumerated binders.

  The constituent ratio of the positive electrode active material in the positive electrode active material layer 24 is preferably 80 to 96% by mass. The constituent ratio of the conductive material in the positive electrode active material layer 24 is preferably 2.0 to 10% by mass, and the constituent ratio of the binder in the positive electrode active material layer 24 is 2.0 to 10% by mass. It is preferable that it is mass%.

"Separator"
The separator 10 only needs to be formed of an electrically insulating porous structure. For example, a single layer of a film made of polyethylene, polypropylene, or polyolefin, a stretched film of a laminate or a mixture of the resins, or cellulose, polyester, and Examples thereof include a fiber nonwoven fabric made of at least one constituent material selected from the group consisting of polypropylene.

"Electrolyte"
The electrolytic solution contains, for example, a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent, and may contain an additive as necessary. Examples of the non-aqueous solvent include cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC); chain carbonates such as dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). Chain carboxylic acid esters such as methyl acetate (MA), ethyl acetate (EA), methyl propionate (MP), ethyl propionate (EP); or γ-butyrolactone (GBL), γ-valerolactone (GVL); And the like, and the like. Any one of these or a mixture of two or more can be used as the non-aqueous solvent. Moreover, it is not limited to the enumerated non-aqueous solvent as long as it does not impair the characteristics when an electrolyte salt is dissolved to form a lithium ion secondary battery.

  Examples of the non-aqueous solvent include cyclic carbonates having an unsaturated bond such as vinylene carbonate (VC), fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC), 1, 3 -A flame-retardant liquid such as a sulfur-containing compound such as propane sultone (PS) or a phosphazene compound can be mixed and used as a non-aqueous solvent.

`` Electrolyte salt ''
Examples of the electrolyte include lithium salts, which dissociate in an electrolytic solution and supply lithium ions. As the lithium salt, is not particularly limited, for _{example, LiPF 6, LiBF4, LiAsF 6} , LiClO4, LiB (C6H5) 4, LiCH 3 SO 3, LiC (SO 2 CF 3) 3, LiN (CF ₃ SO ₂ ) 2 (also called LiTFSI), LiN (C ₂ F ₅ SO ₂ ) ₂ (also called LiBETI), LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiC (CF ₃ SO ₂ ) ₃ , LiN (CF ₃ SO ₂ ) (C ₂ F ₅ SO ₂ ), LiN (CF ₃ SO ₂ ) (C ₃ F ₇ SO ₂ ), LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiN (SO ₂ F) 2 (also called LiFSI), LiAlCl ₄ , LiSiF ₆ , LiCl, LiC ₄ BO ₈ (Also, sometimes called LiBOB), LiBr, and the like can be mentioned, and one selected from these, or any combination of two or more can be used. In particular, LiPF6 can be preferably used because it can obtain high ion 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 lithium ion concentration 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.

"Case"
The case 50 seals the laminated body 40 and the electrolytic solution therein. The case 50 is not particularly limited as long as it can suppress leakage of the electrolytic solution to the outside and entry of moisture and the like into the lithium ion secondary battery 100 from the outside.

  For example, as the case 50, as shown in FIG. 1, a metal laminate film in which a metal foil 52 is coated with a polymer film 54 from both sides can be used. For example, an aluminum foil can be used as the metal foil 52, and a film such as polypropylene can be used as the polymer film 54. For example, the material of the outer polymer film 54 is preferably a polymer having a high melting point, such as polyethylene terephthalate (PET) or polyamide, and the material of the inner polymer film 54 is polyethylene (PE) or polypropylene (PP). Etc. are preferred.

"Lead"
The leads 60 and 62 are made of a conductive material such as nickel or aluminum. Then, the lead 62 is welded to the positive electrode current collector 22 and the lead 60 is welded to the negative electrode current collector 32 by a known method, and the positive electrode active material layer 24 of the positive electrode 20 and the negative electrode active material layer 34 of the negative electrode 30 are interposed. The separator 10 is inserted into the case 50 together with the electrolyte, and the entrance of the case 50 is sealed.

[Method for producing lithium ion secondary battery]
Next, a method for manufacturing the lithium ion secondary battery 100 will be specifically described.

  First, a negative electrode active material, a binder, and a solvent of any one of the above embodiments are mixed to prepare a paint. You may add a electrically conductive material and a thickener as needed. As the solvent, for example, water, N-methyl-2-pyrrolidone or the like can be used. When graphite is used as the negative electrode active material, a water-based binder can be suitably used, and the composition ratio of the negative electrode active material, the conductive material, and the binder is “92 to 98% by mass: 0 to 3% by mass: It is preferable that it is 2-5 mass% ". Moreover, the composition ratio of the negative electrode active material, the conductive material, the binder, and the thickener is “92 to 98 mass%: 0 to 3 mass%: 1 to 3 mass%: 0 to 2 mass%” in mass ratio. Is preferred. These mass ratios are adjusted to 100% by mass as a whole. When silicon is used as the negative electrode active material, a solvent-based binder can be suitably used. The composition ratio of the negative electrode active material, the conductive material, and the binder is 50 to 90% by mass: 0 to 30% by mass. : 5 to 30% by mass ”. These mass ratios are adjusted to 100% by mass as a whole.

  The mixing method of these components constituting the paint is not particularly limited, and the mixing order is not particularly limited. The negative electrode active material layer 34 is applied to the negative electrode current collector 32 with the paint. There is no restriction | limiting in particular as an application | coating method, The method employ | adopted when producing an electrode normally can be used. For example, a slit die coat, a doctor blade, a comma roll coat and the like can be mentioned. Similarly, for the positive electrode, the positive electrode active material layer 24 is applied onto the positive electrode current collector 22 using a positive electrode paint.

Subsequently, the solvent in the positive electrode active material layer 24 and the negative electrode active material layer 34 applied on the positive electrode current collector 22 and the negative electrode current collector 32 is removed.
The removal method is not particularly limited. For example, the positive electrode current collector 22 and the negative electrode current collector 32 to which the paint is applied may be dried in an atmosphere of 80 ° C. to 110 ° C. Note that it is preferable to dry the positive electrode current collector and the negative electrode current collector at a temperature and time at which the positive electrode current collector and the negative electrode current collector are not oxidized.

  Then, the electrode on which the positive electrode active material layer 24 and the negative electrode active material layer 34 are formed in this way is subjected to a press treatment by a roll press device or the like as necessary.

The negative electrode active material layer 34 of the negative electrode according to the present embodiment preferably has an inner peripheral portion A and an outer peripheral portion B having different densities on the same plane. The negative electrode having different densities is not particularly limited as long as the density D _B of the outer peripheral portion B can be smaller than the density D _A of the inner peripheral portion A (D _A > D _B ). In the process, press processing is performed so as to obtain a desired density in the outer peripheral portion B. Subsequently, it can produce by performing a press process so that it may become the desired density in the inner peripheral part A using the metal mold | die etc. match | combined with the shape of the inner peripheral part A. Alternatively, it can also be produced by preparing die presses according to the shapes of the inner peripheral part A and the outer peripheral part B, and pressing them so as to obtain respective desired densities. Alternatively, the negative electrode active material layer 34 on the negative electrode current collector 32 can be applied again to only the portion corresponding to the inner peripheral portion A and pressed to form the negative electrode active material layer 34. If produced by the recoating method, the negative electrode active material layer 34 is flat and the negative electrode active material layer 34 having a different density in the same plane can be produced.

  Next, the positive electrode 20 having the positive electrode active material layer 24, the negative electrode 30 having the negative electrode active material layer 34, the separator 10 interposed between the positive electrode and the negative electrode, and the electrolytic solution are sealed in a case 50.

  For example, the laminated body 40 is produced by laminating the negative electrode 30, the separator 10, and the positive electrode 20 in this order, and laminating a plurality thereof. Leads are ultrasonically welded to the negative electrode and the positive electrode of the laminate, and the laminate 40 is put in a bag-like case 50 prepared in advance. Then, moisture is removed in a vacuum dryer at 60 ° C.

  Finally, an electrolytic solution is injected into the case 50, and the case 50 is sealed under reduced pressure. And a lithium ion secondary battery is produced by standing at room temperature or constant temperature for at least 30 minutes or more and performing an aging treatment. The aging time varies depending on the electrode size, the number of layers, the electrode density, and the like of the lithium ion secondary battery, and the aging time is the electrode size and the number of layers of the lithium ion secondary battery in Examples described later. This is a suitable time for the electrode density.

  When the negative electrode for a lithium ion secondary battery containing graphite according to the second embodiment of the present invention is used, the aging time is preferably 30 minutes or less because the effects of the present invention can be sufficiently exhibited. When the negative electrode for lithium ion secondary batteries containing silicon according to the third embodiment of the present invention is used, the effect of the present invention can be sufficiently exerted, and therefore the aging time is preferably 10 minutes or less. The reason that the aging time of the negative electrode for lithium ion secondary batteries containing silicon is shorter than that of graphite is that silicon has a larger theoretical capacity than graphite. Therefore, the basis weight of silicon contained in the negative electrode active material layer is larger than that of graphite. Less designed. Therefore, since the thickness of the negative electrode active material layer of the negative electrode for lithium ion secondary batteries containing silicon becomes small, the aging time of the electrolyte is sufficient in a shorter time than graphite.

In the first embodiment to the third negative electrode for a lithium ion rechargeable battery of the embodiment of the present invention that the density D _B of the outer portion smaller than the density D _A of the inner peripheral part, the outer peripheral portion B of the electrolyte Impregnation is faster than the inner peripheral part A. As a result, the impregnation property of the electrolytic solution into the inner peripheral portion A is accelerated. Since the electrolytic solution is impregnated from the outer peripheral portion B at an early stage, the electrolytic solution easily flows from the outer peripheral portion B to the inner peripheral portion A. As a result, the inner peripheral portion A is also impregnated earlier. Therefore, since the charge / discharge reaction becomes uniform, a lithium ion secondary battery excellent in charge / discharge capacity and energy density can be obtained. Furthermore, since the impregnation time of the electrolytic solution can be shortened, productivity is improved.
That is, when the negative electrodes for lithium ion secondary batteries according to the first to third embodiments of the present invention are used, good battery characteristics can be obtained even when the impregnation time of the electrolytic solution is short. For example, when the negative electrode for a lithium ion secondary battery containing graphite according to the second embodiment of the present invention is used, the impregnation time of 30 minutes or less, for the lithium ion secondary battery containing silicon according to the third embodiment of the present invention In the case of using the negative electrode, even in the impregnation time of 10 minutes or less, since the impregnation property of the electrolytic solution into the electrode at the first charge / discharge is excellent, a good charge / discharge capacity can be obtained. When the impregnation time is longer than the above time, an excellent charge / discharge capacity can be obtained, but the impregnation time becomes longer and the productivity is not excellent.

EXAMPLES Hereinafter, although this invention is demonstrated based on a more detailed Example, this invention is not limited to these Examples.
Example 1
[Production of negative electrode]
(Graphite negative electrode)

90% by mass of artificial graphite (manufactured by Hitachi Chemical Co., Ltd.) as a negative electrode active material, 2% by mass of acetylene black as a conductive material, 6% by mass of styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose as a thickener (CMC) was mixed with 2% by mass, and water as a solvent was further mixed and dispersed to prepare a negative electrode slurry. Then, using a comma roll coater, a negative electrode mixture layer having a thickness of 45 μm was applied to one surface of a copper foil having a thickness of 10 μm. Note that the amount of the negative electrode active material contained in the negative electrode active material layer per unit area (hereinafter referred to as the basis weight) was 4.6 mg / cm ² . After coating, the film was dried at 100 ° C., and the solvent was removed to form a negative electrode active material layer. Similarly, a negative electrode mixture layer was applied to the back surface of the copper foil so as to have the same basis weight, and then dried at 100 ° C. to form a negative electrode active material layer. And it punched into the electrode size of 4.15cm x 3.05cm using the electrode metal mold | die (electrode area 12.66cm < ² >).
In the entire area of 4.15 cm × 3.05 cm of the formed negative electrode active material layer, the central portion 4.10 cm × 3.00 cm is defined as the inner peripheral portion A, and the peripheral portion excluding the inner peripheral portion A is the outer peripheral portion B. It was. Area _{S A} of the inner peripheral portion A at this time 12.3 cm ^2, the area _{S B} of the outer peripheral portion B was set at 0.36 cm ^2. Then, on the negative electrode active material layer, a negative electrode active material layer having a thickness of 37 μm was applied again by a screen printer using the negative electrode slurry only on the inner peripheral portion A of the negative electrode active material layer. At this time, the weight per unit area in the inner peripheral part A was 8.4 mg / cm ² . Similarly, on the back surface side, the negative electrode slurry was applied only to the inner peripheral portion A, and the same was applied again with a screen printer. Then, the negative electrode active material layer is pressed on both sides of the negative electrode current collector by passing through the roll press machine until the thickness of the negative electrode active material layers on the front and rear surfaces reaches 45 μm, respectively, by means of a roll press machine. Negative electrodes having different densities in the outer peripheral portion B were produced. In addition, as a result of disassembling the below-mentioned full cell provided with the negative electrode of a present Example after first charge / discharge, and measuring the density of the inner peripheral part A and the outer peripheral part B of a negative electrode active material layer, the density DA of the inner peripheral part _A is 2.06 g / cm ^3, the density _{D B} of the outer peripheral portion B is 1.13 g / cm ^3, the density of the outer peripheral portion B was confirmed to be less than the inner periphery a.

[Preparation of positive electrode]
96% by mass of nickel cobalt lithium aluminate as a positive electrode active material, 2% by mass of ketjen black as a conductive additive, 2% by mass of PVDF as a binder, and a solvent of N-methyl-2-pyrrolidone are mixed and dispersed. Thus, a paste-like positive electrode slurry was produced.
Then, using a comma roll coater, this positive electrode slurry was applied to one surface of an aluminum foil having a thickness of 20 μm so that the basis weight of the positive electrode active material was 12 mg / cm ² . Next, the N-methyl-2-pyrrolidone solvent in the positive electrode active material was dried at 110 ° C. in a drying furnace to form a positive electrode active material layer. Similarly, the positive electrode slurry was applied to the back surface of the aluminum foil so as to have the same basis weight, and dried at 110 ° C. to form a positive electrode active material layer. And the positive electrode active material layer was press-bonded to both surfaces of the positive electrode current collector by a roll press machine, and a positive electrode having a predetermined density was produced. The obtained positive electrode was punched into an electrode size of 4.10 cm × 3.00 cm using an electrode mold (electrode area 12.3 cm ² ).

(Production of lithium ion secondary battery)
The negative electrode 7 and the positive electrode 6 are laminated via a separator (porous polyethylene sheet) so that the negative electrode active material layer and the positive electrode active material layer face each other, thereby obtaining a laminate composed of 12 layers. It was. In the negative electrode of the laminate, a negative electrode lead made of nickel is attached to the protruding end portion of the copper foil not provided with the negative electrode active material layer, while the positive electrode of the laminate has aluminum provided with no positive electrode active material layer. An aluminum positive electrode lead was attached to the protruding end of the foil by an ultrasonic welding machine. The laminated body was inserted into the outer body of the aluminum laminate film and heat sealed except for one peripheral portion to form a closed portion. And finally, electrolyte solution was inject | poured in the exterior body and it sealed with the heat seal, reducing the remaining one place with a vacuum sealing machine. After sealing, it was allowed to stand for 30 minutes in an air atmosphere at 30 ° C. (aging treatment), and a lithium ion secondary battery according to Example 1 was produced. As an electrolytic solution, a mixed solvent obtained by mixing fluoroethylene carbonate (FEC) and diethyl carbonate (DEC) at a volume ratio of 3: 7, and an electrolytic solution containing lithium hexafluorophosphate (LiPF ₆ ) at a concentration of 1 mol / L. Using.
Said

(Charge / discharge test)
Using a charge / discharge test device (made by Hokuto Denko Co., Ltd.), a lithium ion secondary battery was charged at a constant current and a constant voltage up to 4.3 V at a charge rate of 0.2 C, and fixed at 2.5 V at a discharge rate of 0.2 C. One cycle of charge / discharge for current discharge was performed. The initial charge / discharge efficiency was calculated from the following equation (1).

Initial charge / discharge efficiency [%] = (discharge capacity [Ah] in the first cycle / charge capacity [Ah] in the first cycle) × 100 (1)

(Measurement of density D _B of the density D _A and the outer peripheral portion B of the inner peripheral portion A of the anode active material layer)
About the lithium ion secondary battery produced in Example 1, after performing initial charge / discharge, the full cell was disassembled in the dry room, the negative electrode was taken out, washed lightly with dimethyl carbonate (DMC), and dried in the dry room. It was. And the thickness of the negative electrode active material layer in the inner peripheral part A and the outer peripheral part B was measured by a non-contact method with a three-dimensional length measuring machine NEXIV (manufactured by Nikon Corporation). did. And the density of the negative electrode active material layer was computed from following formula (2). The mass of active material per area of the negative electrode in the inner peripheral part A and the outer peripheral part B was the basis weight value adjusted in the preparation of the negative electrode. As a result of the measurement, the density _{D A} of the inner peripheral portion A facing the positive electrode active material layer is 2.06 g / cm ^3, the density _{D B} of the outer peripheral portion B not facing the active material layer of the positive electrode 1.13g / Cm ³ . Said density _{D A} of the inner peripheral portion A, the density ratio _D A / _{D B} of said density _{D B} of the outer peripheral portion B was 1.82.

Density [g / cm ³ ] of negative electrode active material layer = 10 × weight per unit area of negative electrode [mg / cm ² ] / thickness of negative electrode active material layer [μm] (2)

(Volume energy density)
The volume energy density of the lithium ion secondary battery produced in Example 1 is expressed by the following formula (3) from the discharge capacity obtained by the first charge / discharge measurement, the average discharge voltage, and the volume of the cell after the first charge / discharge. Calculated from

Volume energy density [Wh / L] = (discharge capacity [Ah] × average discharge voltage [V]) / cell volume after initial charge / discharge [cm ³ ] × 1000 (3)

"Examples 2 to 5 and Comparative Example 1"
The density _{D A} of the inner peripheral portion A and 2.06 g / cm ^3, to adjust the density _{D B} of the outer peripheral portion _B in the range of 1.13~2.10g / cm ^3, the density _{D B} of the outer peripheral portion _B The negative electrode of Examples 2-5 and Comparative Example 1 was obtained by changing the density ratio D _A / D _B as shown in Table 1. A lithium ion secondary battery was obtained in the same manner as Example 1 using the obtained negative electrode. And the lithium ion secondary battery which concerns on Examples 2-5 and the comparative example 1 was evaluated by the method similar to Example 1, and the result is shown in Table 1. The values of charge capacity, discharge capacity, initial efficiency, and volume energy density are shown as relative values when the result of the negative electrode of Example 1 is 100.

"Examples 6-9 and Comparative Examples 2-6"
The density _{D A} of the inner peripheral portion A and 1.82 g / cm ^3, to adjust the density _{D B} of the outer peripheral portion _B in the range of 1.13~2.03g / cm ^3, the density _{D B} of the outer peripheral portion _B The negative electrode of Examples 6-9 and Comparative Examples 2-6 was obtained by changing the density ratio D _A / D _B as shown in Table 1. A lithium ion secondary battery was obtained in the same manner as Example 1 using the obtained negative electrode. The lithium ion secondary batteries of the other examples and the comparative example other than the comparative examples 5 and 6 were evaluated by the same method as in the example 1, and the results are shown in Table 1. The lithium ion secondary batteries according to Comparative Examples 5 and 6 were evaluated in the same manner as in Example 1 with the aging time changed to 60 minutes and 120 minutes. The values of charge capacity, discharge capacity, initial efficiency, and volume energy density are shown as relative values when the result of the negative electrode of Example 6 is 100.

"Examples 10 to 16 and Comparative Example 7"
The density _{D A} of the inner peripheral portion A and 1.58 g / cm ^3, to adjust the density _{D B} of the outer peripheral portion _B in the range of 1.13~1.58g / cm ^3, the density _{D B} of the outer peripheral portion _B The negative electrode of Examples 10 to 16 and Comparative Example 7 were obtained by changing the density ratio D _A / D _B as shown in Table 1. A lithium ion secondary battery was obtained in the same manner as Example 1 using the obtained negative electrode. And the lithium ion secondary battery which concerns on Examples 10-16 and the comparative example 7 was evaluated by the method similar to Example 1, and the result is shown in Table 1. The values of charge capacity, discharge capacity, initial efficiency, and volume energy density are shown as relative values when the result of the negative electrode of Example 10 is 100.

"Examples 17 to 22 and Comparative Examples 8 to 10"
The density _{D A} of the inner peripheral portion A and 1.41 g / cm ^3, to adjust the density _{D B} of the outer peripheral portion _B in the range of 1.13~1.58g / cm ^3, the density _{D B} of the outer peripheral portion _B The negative electrode of Examples 17-22 and Comparative Examples 8-10 was obtained by changing the density ratio D _A / D _B as shown in Table 1. A lithium ion secondary battery was obtained in the same manner as Example 1 using the obtained negative electrode. And the lithium ion secondary battery which concerns on Examples 17-22 and Comparative Examples 8-10 was evaluated by the method similar to Example 1, and the result is shown in Table 1. The values of charge capacity, discharge capacity, initial efficiency, and volume energy density are shown as relative values when the result of the negative electrode of Example 17 is 100.

"Examples 23 to 26 and Comparative Examples 11 to 14"
The density _{D A} of the inner peripheral portion A and 1.35 g / cm ^3, to adjust the density _{D B} of the outer peripheral portion _B in the range of 1.13~1.60g / cm ^3, the density _{D B} of the outer peripheral portion _B The negative electrode of Examples 23-26 and Comparative Examples 11-14 was obtained by changing the density ratio D _A / D _B as shown in Table 1. A lithium ion secondary battery was obtained in the same manner as Example 1 using the obtained negative electrode. And the lithium ion secondary battery which concerns on Examples 23-26 and Comparative Examples 11-14 was evaluated by the method similar to Example 1, and the result is shown in Table 1. In addition, the values of the charge capacity, the discharge capacity, the initial efficiency, and the volume energy density are shown as relative values when the result of the negative electrode of Example 23 is 100.

Note that the negative electrode of Comparative Example except comparative example 2 and 7, to obtain a negative electrode comprising a density having a D _A and D _B are shown in Table 1 by re-applying the anode active material layer on the outer peripheral portion B. About the negative electrode of the comparative examples 2 and 7, the negative electrode used as the density shown in Table 1 was obtained by not reapplying the negative electrode active material layer to the outer peripheral part B.

"Examples 27-30"
Resize the electrode mold, and adjusting the area _{S B} of the outer peripheral portion B within the range of 0.12～1.48Cm ^2, the area _{S A} of the inner peripheral portion A, the outer peripheral portion area _{S B} of B, the outer peripheral portion By changing the ratio S _B / S _A of the area S _{B of B} and the area S _A of the inner periphery A as shown in Table 2, negative electrodes of Examples 27 to 30 were obtained in the same manner as in Example 9. Using the obtained negative electrode and the positive electrode produced in Example 1, a lithium ion secondary battery was obtained in the same manner as in Example 9. And it evaluated by the method similar to Example 9, and the result is shown in Table 2. In addition, the values of the charge capacity, the discharge capacity, the initial efficiency, and the volume energy density are shown as relative values when the result of the negative electrode of Example 9 is 100.

"Examples 31-32 and Comparative Examples 15-16"
[Production of negative electrode]
(Silicon negative electrode)

83% by mass of silicon (manufactured by Aldrich) as a negative electrode active material, 2% by mass of acetylene black as a conductive material, 15% by mass of polyamideimide as a binder, and a solvent of N-methyl-2-pyrrolidone are mixed and dispersed A negative electrode slurry was prepared. Then, using a comma roll coater, a negative electrode active material layer having a thickness of 15 μm was applied to one surface of the copper foil having a thickness of 10 μm. The amount (weight per unit area) of the negative electrode active material contained in the negative electrode active material layer per unit area was 1.38 mg / cm ² . After coating, the film was dried at 100 ° C., and the solvent was removed to form a negative electrode active material layer. Similarly, the negative electrode active material layer was applied to the back surface of the copper foil so as to have the same basis weight, and then dried at 100 ° C. to form a negative electrode active material layer. And it punched into the electrode size of 4.15cm x 3.05cm using the electrode metal mold | die (electrode area 12.66cm < ² >).
In the entire area of 4.15 cm × 3.05 cm of the formed negative electrode active material layer, the central portion 4.10 cm × 3.00 cm is defined as the inner peripheral portion A, and the peripheral portion excluding the inner peripheral portion A is the outer peripheral portion B. It was. Area _{S A} of the inner peripheral portion A at this time 12.3 cm ^2, the area _{S B} of the outer peripheral portion B was set at 0.36 cm ^2. Then, on the negative electrode active material layer, a negative electrode active material layer having a thickness of 0.7 μm was applied again by a screen printer using the negative electrode slurry only on the inner peripheral portion A of the negative electrode active material layer. At this time, the weight per unit area in the inner periphery A was 1.44 mg / cm ² . Similarly, on the back surface side, the negative electrode slurry was applied only to the inner peripheral portion A, and the same was applied again with a screen printer. Then, the negative electrode active material layer is pressed on both sides of the negative electrode current collector by passing through the roll press machine until the thickness of the negative electrode active material layers on the front and back surfaces becomes 11 μm in thickness by a roll press machine. Negative electrodes having different densities in the outer peripheral portion B were produced. Next, in order to bind the polyamideimide as a binder more firmly, heat treatment was performed at 350 ° C. for 3 hours under vacuum, and this was used as a negative electrode according to this example. In addition, as a result of disassembling the below-mentioned full cell provided with the negative electrode of a present Example after first charge / discharge, and measuring the density of the inner peripheral part A and the outer peripheral part B of a negative electrode active material layer, the density DA of the inner peripheral part _A is 1.58 g / cm ^3, the density _{D B} of the outer peripheral portion B is 1.51 g / cm ^3, the density of the outer peripheral portion B was confirmed to be less than the inner periphery a.

[Preparation of positive electrode]
It was produced in the same procedure as the positive electrode produced in Example 1 except that the positive electrode slurry was applied so that the basis weight of the positive electrode active material was 21 mg / cm ² .

(Production of lithium ion secondary battery)
Negative electrodes of Examples 31 to 32 and Comparative Examples 15 to 16 were obtained in the same manner as in Example 1 except that a negative electrode containing silicon was used as the negative electrode active material and the aging time was 10 minutes. A lithium ion secondary battery was obtained in the same manner as in Example 1 using the obtained negative electrode and the positive electrode. And it evaluated by the method similar to Example 1, and the result is shown in Table 3. In addition, the values of the charge capacity, the discharge capacity, the initial efficiency, and the volume energy density are shown as relative values when the result of the negative electrode of Example 31 is 100.

As shown in Table 1, in Examples 1 to 26, the density D _B of the outer peripheral part B is smaller than the density D _A of the inner peripheral part A, so that the impregnation is excellent, and the charge / discharge capacity is developed in an aging time of 30 minutes. The volume energy density is good. In Comparative Examples 1-14, the same as the density D _A of the density D _B is the inner peripheral portion A of the outer peripheral portion B, or high because poor impregnation property, when the aging time was 30 minutes, the discharge capacity is less likely to express The volume energy density also decreased. In Comparative Example 5, even when the aging time was 60 minutes, the capacity was hardly developed and the volume energy density was also reduced. In Comparative Example 6, even when the aging time was 120 minutes, the impregnation property was slightly poor, the discharge capacity was slightly difficult to express, and the volume energy density was also slightly decreased.

As shown in Table 2, in Examples 9,28 and 29, less than the density _{D A} of the inner peripheral portion A is the density _{D B} of the outer peripheral portion B, and the area of the inner peripheral portion A _{S A,} the outer peripheral portion B the area at the negative electrode of the ratio _S B / _{S a} of the _{S B} is _{0.02 ≦ S B / S a ≦} 0.1 is also expressed charge and discharge capacity at the aging time is 30 minutes, the volume energy density is good is there. In Example 27, less than the density _{D A} of the inner peripheral portion A is the density _{D B} of the outer peripheral portion B, and the ratio _S B / S of the area of the inner peripheral portion A _{S A,} the area of the outer peripheral portion B _{S B In the} negative electrode in which _A is S _B / S _A <0.02, the discharge capacity was slightly reduced, and the volume energy density was also slightly reduced. This is probably because the area of the outer peripheral portion B was too small, and the initial efficiency was lowered due to side reactions such as dendrite generation due to slight stacking deviation. In Example 30, in the negative electrode where S _B / S _A was S _B / S _A > 0.1, the discharge capacity was reduced and the volume energy density was also reduced. This is considered to be because the impregnation speed at the outer peripheral portion B is reduced due to the large area of the outer peripheral portion B, thereby reducing the discharge capacity. Therefore, it was found that 0.02 ≦ S _B / S _A ≦ 0.1 is preferable as the area ratio between S _A and S _B.

As shown in Table 3, in Examples 31 and 32, also in the negative electrode containing silicon, because the density D _B of the outer peripheral portion B less than the density D _A of the inner peripheral portion A, is excellent in impregnation properties, the aging time The charge / discharge capacity is developed in 10 minutes, and the volume energy density is good. In Comparative Example 15 and 16 is higher than the same or the density D _B is the density D _A of the inner peripheral portion A of the outer peripheral portion B, poor impregnation property when impregnated time was 10 minutes, charging and discharging capacity is expressed The volume energy density also decreased.

DESCRIPTION OF SYMBOLS 10 ... Separator, 20 ... Positive electrode, 22 ... Positive electrode collector, 24 ... Positive electrode active material layer, 30 ... Negative electrode,
32 ... Negative electrode current collector, 34 ... Negative electrode active material layer, 34A ... Inner circumference A, 34B ... Outer circumference B, 40 ... Laminate, 50 ... Case, 52 ... Metal foil,
54 ... polymer membrane, 60, 62 ... lead, 100 ... lithium ion secondary battery

Claims (5)

A negative electrode comprising a negative electrode current collector and a negative electrode active material layer held by the negative electrode current collector,
The negative electrode active material layer has an inner peripheral portion A and an outer peripheral portion B,
The density D _{B of the} outer peripheral portion is smaller than the density D _A of the inner peripheral portion (D _A > D _B )
The negative electrode for lithium ion secondary batteries characterized by the above-mentioned. Wherein an area of the inner peripheral portion A of the negative electrode active material layer _{S A,} the area of the outer peripheral portion B when the _{S B,} with S B / _{S A} is _{0.02 ≦ S B / S A ≦} 0.1 The negative electrode for a lithium ion secondary battery according to claim 1, wherein: Wherein the density _{D A} of the inner peripheral portion A in the negative electrode active material layer, the density ratio _D A / _{D B} of the density _{D B} of the outer peripheral portion B is, 1.00 _<D A / _{D B} ≦ 1.82 The negative electrode for a lithium ion secondary battery according to claim 1 or 2, wherein:   The said negative electrode active material layer contains a negative electrode active material and a negative electrode binder at least, The negative electrode for lithium ion secondary batteries described in any one of Claims 1-3 characterized by the above-mentioned. A negative electrode for a lithium ion secondary battery according to any one of claims 1 to 4,
A positive electrode;
A separator;
An electrolyte,
A lithium ion secondary battery comprising:

Images