JP2019160517A - Positive electrode for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode for a lithium ion secondary battery having excellent output characteristics and a lithium ion secondary battery using the same. A positive electrode having an active material layer containing an active material, a conductive auxiliary agent, and a binder on a current collector, wherein the thickness of the active material layer is bisected in the thickness direction. In addition, the ratio P1/P2 of the porosity P1 in the region R1 on the current collector side of the positive electrode and the porosity P2 in the other region R2 is within the range of 0.33≦P1/P2≦0.84. A positive electrode for a lithium ion secondary battery, wherein each of the region R1 and the region R2 has two or more kinds of conductive auxiliary agents. [Selection diagram] Figure 1

Description

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

  In recent years, lithium ion secondary batteries have been developed for fields requiring high output characteristics, such as electric vehicles, hybrid vehicles, and power tools. For higher output, control the porosity inside the electrode, improve the liquid retention and lithium ion diffusibility, improve the dispersibility of the conductive auxiliary agent, the interface between the nonaqueous electrolyte and the active material and the current collection. It is required to improve the electron conductivity by suppressing an increase in internal resistance at the interface between the body and the active material.

  In Patent Document 1 (Japanese Patent No. 5815617), the curvature of the inside of the electrode is calculated based on the three-dimensional image analysis of the electrode, and the relationship with the battery characteristics is evaluated by grasping the structure of the gap. It has been reported.

  Patent Document 2 (Japanese Patent Laid-Open No. 9-320569) reports that the capacity density is improved by changing the composition or structure of the positive electrode active material layer in a direction perpendicular to the current collector.

Patent No. 5815617 JP-A-9-320569

Patent Document 1 (Patent No. 5815617) specifies the effect of improving the porosity and output characteristics when the electrode density is changed, but does not specify anything about the state of the active material and the conductive additive inside the electrode. In addition, there is room for improvement in the output characteristics of the battery.

  In Patent Document 2 (Japanese Patent Laid-Open No. 9-320569), there is no specific description about the voids inside the electrodes, the distribution state of the conductive assistant, and the physical property values of the conductive assistant, and there is room for improvement in the output characteristics.

The output characteristics in Patent Documents 1 and 2 are not sufficient, and there is room for improvement.

  The present invention has been made in view of the above prior art, and an object thereof is to provide a positive electrode for a lithium ion secondary battery excellent in output characteristics and a lithium ion secondary battery using the same.

  In order to solve the above problems, a positive electrode according to the present invention is a positive electrode having an active material layer containing an active material, a conductive additive, and a binder on a current collector, and the thickness of the active material layer When the thickness is equally divided into two in the thickness direction, the ratio P1 / P2 of the porosity P1 in the region R1 on the current collector side of the positive electrode and the porosity P2 in the other region R2 is 0.33 ≦ P1 / It is within the range of P2 ≦ 0.84, and each of the region R1 and the region R2 has two or more kinds of conductive assistants.

  Although this effect is not necessarily clear, by adopting this configuration, the region R2 closer to the positive electrode surface maintains a higher porosity than the region R1, so that the nonaqueous electrolyte permeates from the region R2 to the region R1. Easily improves the liquid retention inside the positive electrode, and by two or more kinds of conductive assistants, it improves the absorption of nonaqueous electrolyte, improves the electronic conductivity by forming a structure structure between conductive assistants, and the nonaqueous electrolyte. Improved interfacial reaction. These effects improve the output characteristics.

  Furthermore, in the positive electrode according to the present invention, each of the region R1 and the region R2 has a conductive auxiliary agent a having a ratio DBP / BET of 1 or more between the oil absorption rate (DBP) and the specific surface area (BET) of the conductive auxiliary agent, It is preferable to include one or more conductive aids b that are 1 or less.

  According to this, the absorption of the nonaqueous electrolyte in the pores is further improved by the oil absorption rate of the conductive auxiliary agent a having DBP / BET of 1 or more, and the electronic conductivity is improved by the structure structure made of the conductive auxiliary agent. More improved. In addition, since the contact with the nonaqueous electrolyte is obtained by the conductive additive b having DBP / BET of 1 or less, the reaction at the interface is further improved. The output characteristics are further improved by these effects.

  Furthermore, in the positive electrode according to the present invention, when the conductive assistants a and b in the region R1 are a1 and b1, the ratio Wa1 / Wb1 between the mass Wa1 of a1 and the mass Wb1 of b1 is 0.02 ≦ Wa1 /. It is preferable that Wb1 ≦ 0.50.

  According to this, in the space | gap of area | region R1 (the side near a collector foil), absorption of the nonaqueous electrolyte by the conductive support agent a1 improves more, and formation of the structure structure which consists of the conductive support agent a becomes more appropriate. . This further improves the output characteristics.

  Furthermore, in the positive electrode according to the present invention, when the conductive assistants a and b in the region R2 are a2 and b2, the ratio Wa2 / Wb2 between the mass Wa2 of a2 and the mass Wb2 of b2 is 0.02 ≦ Wa2 / It is preferable that Wb2 ≦ 0.07.

  According to this, in the region R2 (surface side), the contact between the conductive additive b2 and the nonaqueous electrolyte becomes more appropriate, and the reaction at the interface is further improved. This further improves the output characteristics.

  Furthermore, in the positive electrode according to the present invention, the ratio of the conductive assistants a1 and b1 in the region R1 is 0.02 ≦ Wa1 / Wb1 ≦ 0.50, and the conductive assistants a2 and b2 in the region R2 The ratio is preferably 0.02 ≦ Wa2 / Wb2 ≦ 0.07.

  According to this, the absorption improvement of the nonaqueous electrolyte and the formation of the structure structure by the conductive auxiliary agent a1 in the region R1 can be further obtained, and the interface reaction with the nonaqueous electrolyte by the conductive auxiliary agent b2 in the region R2 can be further improved. It is done. These effects further improve the output characteristics.

  A lithium ion secondary battery according to the present invention includes the above-described positive electrode for a lithium ion secondary battery, a negative electrode having a negative electrode active material, a separator interposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte. Prepare.

  Thereby, the lithium ion secondary battery which can achieve the effect excellent in output characteristics can be obtained.

  ADVANTAGE OF THE INVENTION According to this invention, the positive electrode for lithium ion secondary batteries excellent in output characteristics and a lithium ion secondary battery using the same 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>
The lithium ion secondary battery according to this embodiment will be briefly described with reference to FIG.

  The lithium ion secondary battery 100 mainly includes a power generation element 30, an exterior body 50 that houses the power generation element 30 in a sealed state, and a pair of leads 60 and 62 connected to the power generation element 30.

  The power generation element 30 is configured such that the positive electrode 20 and the negative electrode 10 are disposed to face each other with the separator 18 interposed therebetween, and is accommodated in the outer package 50 in a state in which a nonaqueous electrolyte (not shown) is immersed.

  The positive electrode 20 is a product in which a positive electrode active material layer 24 is provided on a positive electrode current collector 22. As the positive electrode current collector 22, for example, an aluminum foil or the like can be used.

  The negative electrode 10 is a product in which a negative electrode active material layer 14 is provided on a negative electrode current collector 12. As the negative electrode current collector 12 of the negative electrode 10, a copper foil or the like can be used.

  The positive electrode active material layer 24 and the negative electrode active material layer 14 are in contact with both sides of the separator 18.

  The exterior body 50 seals the power generation element 30 and the nonaqueous electrolyte therein. The outer package 50 is not particularly limited as long as it can prevent leakage of the nonaqueous electrolyte to the outside and entry of moisture and the like into the lithium ion secondary battery 100 from the outside. For example, a metal laminate film is used. Available.

  The leads 60 and 62 are made of a conductive material such as aluminum.

  The separator 18 is made of a single layer of a film made of polyethylene, polypropylene or polyolefin, a stretched film of a laminate or a mixture of the above resins, or at least one constituent material selected from the group consisting of cellulose, polyester and polypropylene. A fiber nonwoven fabric can be used.

  Leads 60 and 62 are connected to the ends of the positive electrode current collector 22 and the negative electrode current collector 12, respectively, and the ends of the leads 60 and 62 extend to the outside of the exterior body 50.

<Positive electrode active material layer>
The positive electrode active material layer 24 contains at least the positive electrode active material according to the present embodiment and a conductive additive. The positive electrode active material layer 24 may include a binder that binds the positive electrode active material and the conductive additive.

  Examples of the conductive assistant include carbon materials such as carbon blacks, metal powders such as copper, nickel, stainless steel, and iron, mixtures of carbon materials and metal powders, and conductive oxides such as ITO.

  The binder is not particularly limited as long as the positive electrode active material and the conductive additive can be bound to the positive electrode current collector 22, and a known binder can be used. Examples thereof include fluororesins such as polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), and vinylidene fluoride-hexafluoropropylene copolymer.

  The ratio of the positive electrode active material, the conductive additive, and the binder in the positive electrode active material layer 24 is not particularly limited, but the electrode density tends to decrease when the ratio of the positive electrode active material is small, and the ratio of the positive electrode active material is 80% by mass or more. Is preferred.

<Positive electrode active material>
The positive electrode active material according to the present embodiment is a transition metal composite oxide represented by the following general formula (1).
Li _a Ni _x Co _y M _z O ₂ (1)
(M is at least one element selected from Mg, Ca, Sr, Ba, Ti, Al, Cr, Mn, Fe, Ge, W, Cu, and Zn, and 0.95 ≦ a ≦ 1.3, 0 ≦ x <1.0, 0 <y ≦ 1, 0 ≦ z <0.65, 0.95 ≦ x + y + z ≦ 1.10.

  The transition metal composite oxide that forms the positive electrode active material according to the present embodiment is represented by the above general formula (1), and M is Mg, Ca, Sr, Ba, Ti, Al, Cr, Mn, Fe, Ge. , W, Cu, Zn contains at least one transition metal element. When lithium ions are inserted into and extracted from the positive electrode active material by charge and discharge, the transition metal element becomes a transition metal ion by an oxidation-reduction reaction, whereby the charge neutrality in the positive electrode active material is maintained.

  The positive electrode active material according to the present embodiment may be composed of the above-described one kind, or a plurality of kinds may be mixed.

  The particle size of the positive electrode active material according to this embodiment is preferably 0.01 μm to 1.4 μm.

<Method for producing positive electrode>
The positive electrode 20 is applied to a surface of the positive electrode current collector 22 by applying a known method, for example, a positive electrode active material, a conductive additive, and a binder added to an organic solvent or an aqueous solvent according to the type of the positive electrode current collector 22. Can be manufactured.

  Examples of the organic solvent include N-methyl-2-pyrrolidone, N, N-dimethylformamide, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, and toluene.

  The aqueous solvent may be water or a mixed solution with an organic solvent (lower alcohol, lower ketone, etc.) that can be uniformly mixed with water.

<Method for producing positive electrode active material>
The positive electrode active material of this embodiment has a general formula (1) Li _a Ni _x Co _y M _z O ₂
(M is at least one element selected from Mg, Ca, Sr, Ba, Ti, Al, Cr, Mn, Fe, Ge, W, Cu, and Zn, and 0.95 ≦ a ≦ 1.3, 0 ≦ x <1.0, 0 <y ≦ 1, 0 ≦ z <0.65, 0.95 ≦ x + y + z ≦ 1.10). .

  The production step can be produced by various production methods such as known solid phase synthesis, hydrothermal synthesis, carbothermal reduction method, coprecipitation method, sol-gel process, and the like, and is not limited to a specific method.

  The positive electrode active material of this embodiment may be coated with a conductive additive, an oxide, or the like.

  The coating step is not particularly limited as long as the surface of the positive electrode active material can be uniformly coated. A known surface coating method, that is, a sputtering method, a CVD (Chemical Vapor Deposition) method, or a dip coating method is used. A general-purpose coating method such as a soaking method as described above or a dry processing method using a dry particle design apparatus can be used.

<Negative electrode active material layer>
The negative electrode active material layer 14 may include a negative electrode active material, a conductive additive, and a binder.

  A conductive support agent is not specifically limited, A carbon material, a metal powder, etc. can be used.

  As the binder, a fluororesin such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP) or the like can be used.

<Negative electrode active material>
The negative electrode active material of the present invention is an amorphous material mainly composed of carbon materials such as graphite and non-graphitizable carbon, metals that can be combined with lithium such as Al, Si, and Sn, and oxides such as SiO ₂ and SnO _2. Particles containing high quality compounds, lithium titanate (Li ₄ Ti ₅ O ₁₂ ) and the like.

<Method for producing negative electrode>
The manufacturing method of the negative electrode 10 may be applied to the negative electrode current collector 12 by adjusting the slurry in the same manner as the manufacturing method of the positive electrode 20.

<Nonaqueous electrolyte>
The nonaqueous electrolyte sealed in the outer package 50 is not particularly limited. For example, in the present embodiment, a nonaqueous electrolyte containing a lithium salt in an organic solvent can be used. Examples of the lithium _salt, LiPF _6, LiClO 4, salts of LiBF ₄ or the like can be used. In addition, these salts may be used individually by 1 type, and may use 2 or more types together.

  Preferable examples of the organic solvent include propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, and methyl ethyl carbonate. These may be used alone or in combination of two or more at any ratio.

  The positive electrode active material of this embodiment can also be used as an electrode material for electrochemical elements other than lithium ion secondary batteries. As such an electrochemical element, two types of batteries other than lithium ion secondary batteries such as a metal lithium secondary battery (one using the positive electrode active material of the present embodiment as the positive electrode and using metal lithium as the negative electrode) are used. Examples thereof include secondary batteries and electrochemical capacitors such as lithium capacitors.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example.

Example 1
<Production of region R1>
LiNi _0.8 Co _0.15 Al _0.05 O ₂ was used as the positive electrode active material. Two types of conductive assistants were used, a1 and b1. a1 used Denka Black FX-35 (made by Denka Co., Ltd.), and b1 used Super-P (made by IMERYS). As the binder, polyvinylidene fluoride (PVDF, KF 7305 manufactured by Kureha Chemical) was used.

  The positive electrode active material, the conductive additive (a1 + b1), and the binder were weighed at a weight ratio of 95.48: 2.02: 2.5. At this time, the weight ratio of a1 and b1 was set to 0.02: 2.

  The positive electrode active material and the conductive additive (a1 + b1) were mixed, and the mixture was treated for 1 minute × 5 times with a stirring defoamer to obtain a mixture of the positive electrode active material and the conductive additive.

  A binder was added to the resulting mixture in small portions in five portions, and dispersed in N-methyl-2-pyrrolidone (NMP) as a solvent to prepare a slurry. This slurry was applied onto an aluminum foil as a current collector, dried, and then rolled at a linear pressure of 1500 kg / cm. Thus, a region R1 on the current collector side of the positive electrode was produced.

<Preparation of region R2>
LiNi _0.8 Co _0.15 Al _0.05 O ₂ was used as the positive electrode active material. Two types of conductive assistants were used, a2 and b2. Carbon ECP200 (made by Ketjen Black International Co., Ltd.) was used for a2, and carbon ECP600JP (made by Ketjen Black International Co., Ltd.) was used for b2. As the binder, polyvinylidene fluoride (PVDF, KF 7305 manufactured by Kureha Chemical) was used.

  The positive electrode active material, the conductive additive (a2 + b2), and the binder were weighed in a weight ratio of 96.93: 0.57: 2.5. At this time, the weight ratio of a2 and b2 was set to 0.07: 0.5.

  The positive electrode active material and the conductive additive (a2 + b2) were mixed, and the mixture was treated for 1 minute × 5 times with a stirring deaerator to obtain a mixture of the positive electrode active material and the conductive additive.

  A binder was added to the resulting mixture in small portions in five portions, and dispersed in N-methyl-2-pyrrolidone (NMP) as a solvent to prepare a slurry. The slurry was applied on the positive electrode in which the region R1 was produced, dried, and then rolled at a linear pressure of 200 kg / cm. Thus, a region R2 close to the positive electrode surface was produced.

  In this way, a positive electrode having regions R1 and R2 was obtained.

  Next, artificial graphite (MAGE manufactured by Hitachi, Ltd.) and N-methylpyrrolidone (NMP) 5 wt% solution of polyvinylidene fluoride (PVdF) as a negative electrode were mixed so that the ratio of artificial graphite: polyvinylidene fluoride = 93: 7 was obtained. A slurry paint was prepared. The negative electrode was produced by apply | coating a coating material to the copper foil which is a collector, and drying and rolling.

A positive electrode and a negative electrode were laminated with a separator made of a polyethylene microporous film interposed therebetween to obtain a laminate. This laminate was placed in an aluminum laminate pack. As the non-aqueous electrolyte, ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 3: 7, and LiPF ₆ was dissolved as a supporting salt to a concentration of 1 mol / L.

  After injecting the non-aqueous electrolyte into an aluminum laminate pack containing a power generation element, vacuum sealing was performed to produce an evaluation cell of Example 1.

<Measurement of battery characteristics>
The evaluation cell of Example 1 was charged and discharged at 25 ° C. In the charging, the upper limit charging voltage was set to 4.5 V (VS. Li / Li +), the charging rate was 0.2 C, the positive electrode voltage reached the upper limit charging voltage, and the charging current was attenuated to 1/20 C. . For the discharge, the lower limit discharge voltage was 2.8 V (VS. Li / Li +), the discharge rate was 0.2 C, and this was the 0.2 C discharge capacity. 0.2 C is a current value at which the discharge is terminated by a constant current discharge for 5 hours.
Next, after charging with the charge rate changed to 0.5C, the battery was discharged at a discharge rate of 2C, which was defined as a 2C discharge capacity. For evaluation of the rate characteristics, 2C discharge capacity / 0.2C discharge capacity was calculated. The results are shown in Table 1.

(Example 2)
An evaluation cell was prepared in the same manner as in Example 1 except that rolling treatment was performed at a linear pressure of 700 kg / cm when the region R2 in the positive electrode was manufactured, and battery characteristics were measured. The results are shown in Table 1.

(Example 3)
An evaluation cell was prepared in the same manner as in Example 1 except that rolling treatment was performed at a linear pressure of 1200 kg / cm when the region R2 in the positive electrode was manufactured, and battery characteristics were measured. The results are shown in Table 1.

Example 4
An evaluation cell was prepared in the same manner as in Example 1 except that carbon ECP600JD was used as the conductive auxiliary agent b1 in the region R1, and Denka Black FX-35 was used as the conductive auxiliary agent a2 in the region R2. Battery characteristics were measured. The results are shown in Table 2.

(Example 5)
Evaluation was performed in the same manner as in Example 1 except that carbon ECP200 was used for conductive auxiliary agent b1 in region R1, and Denka Black FX-35 was used for conductive auxiliary agent a2 and carbon ECP200 was used for b2 in region R2. A cell was prepared and battery characteristics were measured. The results are shown in Table 2.

(Example 6)
In region R1, Toka Black # 5500 (made by Tokai Carbon) was used as conductive additive b1, and in Region R2, Denka Black FX-35 was used as conductive aid a2, and Toka Black # 5500 (made by Tokai Carbon) was used in b2. An evaluation cell was prepared in the same manner as in Example 1 except that was used, and the battery characteristics were measured. The results are shown in Table 2.

(Example 7)
In region R1, Vulcan XC-72 (manufactured by CABOT) was used for conductive auxiliary agent b1, and in region R2, Denka Black FX-35 was used for conductive auxiliary agent a2, and Vulcan XC-72 (made by CABOT) was used for b2. An evaluation cell was prepared in the same manner as in Example 1 except that the battery characteristics were measured. The results are shown in Table 2.

(Example 8)
In the region R1, Niteron # 300 (manufactured by Nippon Kayaku Co., Ltd.) was used as the conductive additive b1, and in the region R2, Denka Black FX-35 was used as the conductive additive a2, and An evaluation cell was prepared in the same manner as in Example 1 except that the product was used, and the battery characteristics were measured. The results are shown in Table 2.

Example 9
In the region R1, the input amount of the conductive auxiliary agent a1 was increased, and the ratio of the positive electrode active material, the conductive auxiliary agent (a1 + b1), and the binder was weighed at a weight ratio of 95.46: 2.04: 2.5. At this time, the weight ratio of a1 and b1 was 0.04: 2. Other than that was carried out similarly to Example 4, and produced the evaluation cell, and measured the battery characteristic. The results are shown in Table 3.

(Example 10)
In the region R1, the input amount of the conductive auxiliary agent a1 was increased, and the ratio of the positive electrode active material, the conductive auxiliary agent (a1 + b1), and the binder was weighed at a weight ratio of 94.5: 3: 2.5. At this time, the weight ratio of a1 and b1 was 1: 2. Otherwise, an evaluation cell was prepared in the same manner as in Example 9, and the battery characteristics were measured. The results are shown in Table 3.

(Example 11)
In the region R1, the input amount of the conductive auxiliary agent a1 was increased, and the ratio of the positive electrode active material, the conductive auxiliary agent (a1 + b1), and the binder was weighed at a weight ratio of 94.41: 3.09: 2.5. At this time, the weight ratio of a1 and b1 was 1.09: 2. Other than that was carried out similarly to Example 10, and produced the evaluation cell, and measured the battery characteristic. The results are shown in Table 3.

(Example 12)
In the region R2, the input amount of the conductive auxiliary agent b2 was increased, and the ratio of the positive electrode active material, the conductive auxiliary agent (a2 + b2), and the binder was weighed at a weight ratio of 96.63: 0.87: 2.5. At this time, the weight ratio of a2 and b2 was 0.07: 0.8. Other than that was carried out similarly to Example 4, and produced the evaluation cell, and measured the battery characteristic. The results are shown in Table 3.

(Example 13)
In the region R2, the amount of the conductive auxiliary agent b2 was increased, and the ratio of the positive electrode active material, the conductive auxiliary agent (a2 + b2), and the binder was weighed at a weight ratio of 96.43: 1.07: 2.5. At this time, the weight ratio of a2 and b2 was set to 0.07: 1. Other than that was carried out similarly to Example 12, and produced the evaluation cell, and measured the battery characteristic. The results are shown in Table 3.

(Example 14)
In the region R2, the amount of the conductive auxiliary agent b2 was increased, and the ratio of the positive electrode active material, the conductive auxiliary agent (a2 + b2), and the binder was weighed at a weight ratio of 94.43: 3.07: 2.5. At this time, the weight ratio of a2 and b2 was set to 0.07: 3. Other than that was carried out similarly to Example 13, and produced the evaluation cell, and measured the battery characteristic. The results are shown in Table 3.

(Example 15)
In the region R1, the input amount of the conductive auxiliary agent a1 was increased, and the ratio of the positive electrode active material, the conductive auxiliary agent (a2 + b2), and the binder was weighed at a weight ratio of 95.46: 2.04: 2.5. At this time, the weight ratio of a1 and b1 was 0.04: 2. In the region R2, the amount of the conductive auxiliary agent b2 was increased, and the ratio of the positive electrode active material, the conductive auxiliary agent (a2 + b2), and the binder was weighed at a weight ratio of 94.43: 3.07: 2.5. At this time, the weight ratio of a2 and b2 was set to 0.07: 3. Other than that was carried out similarly to Example 4, and produced the evaluation cell, and measured the battery characteristic. The results are shown in Table 3.

(Example 16)
In the region R1, the input amount of the conductive auxiliary agent a1 was increased, and the ratio of the positive electrode active material, the conductive auxiliary agent (a2 + b2), and the binder was weighed at a weight ratio of 94.5: 3: 2.5. At this time, the weight ratio of a1 and b1 was 1: 2. In the region R2, the amount of the conductive auxiliary agent b2 was increased, and the ratio of the positive electrode active material, the conductive auxiliary agent (a2 + b2), and the binder was weighed at a weight ratio of 96.43: 1.07: 2.5. At this time, the weight ratio of a2 and b2 was set to 0.07: 1. Other than that was carried out similarly to Example 4, and produced the evaluation cell, and measured the battery characteristic. The results are shown in Table 3.

(Comparative Example 1)
An evaluation cell was prepared in the same manner as in Example 1 except that the rolling treatment was performed at a linear pressure of 150 kg / cm when the region R2 in the positive electrode was manufactured, and the battery characteristics were measured. The results are shown in Table 1.

(Comparative Example 2)
An evaluation cell was prepared in the same manner as in Example 1 except that rolling treatment was performed at a linear pressure of 1300 kg / cm when the region R2 in the positive electrode was manufactured, and battery characteristics were measured. The results are shown in Table 1.

(Comparative Example 3)
In the positive electrode, one kind of c was used as the conductive additive in the region R1. One kind of carbon ECP600JD was used as the conductive additive in the region R2. When the region R2 was produced, rolling treatment was performed at a linear pressure of 150 kg / cm. Otherwise, an evaluation cell was prepared in the same manner as in Example 1, and the battery characteristics were measured. The results are shown in Table 1.

(Comparative Example 4)
In the positive electrode, one type of carbon ECP600JD was used as the conductive additive in the region R1. As the conductive auxiliary agent in the region R2, one type of Super-P was used. During the production of the region R2, a rolling process was performed at a linear pressure of 1300 kg / cm. Otherwise, an evaluation cell was prepared in the same manner as in Example 1, and the battery characteristics were measured. The results are shown in Table 1.

  From the results of Examples 1 to 16, the porosity ratio P1 / P2 is in the range of 0.33 ≦ P1 / P2 ≦ 0.84, and the region R1 and the region R2 each have two or more kinds of conductive assistants. It was confirmed that the high output characteristics were exhibited by having

  Furthermore, the ratio DBP / BET between the oil absorption rate (DBP) and the specific surface area (BET) of the conductive assistant includes one or more conductive assistants a and 1 or less conductive assistants b. It was confirmed that higher output characteristics can be obtained.

  Furthermore, the ratio between the conductive assistants a1 and b1 is 0.02 ≦ Wa1 / Wb1 ≦ 0.5, and the ratio Wa2 / Wb2 between the conductive assistants a2 and b2 is 0.02 ≦ Wa2 / Wb2. It was confirmed that higher output characteristics can be obtained when ≦ 0.07.

  On the other hand, Comparative Examples 1-4 did not reach the characteristics of this example.

  As mentioned above, although the specific example of this invention was demonstrated in detail, these are only illustrations and do not limit a claim. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.

  By using the positive electrode active material for a lithium ion secondary battery of the present invention, higher output characteristics can be obtained. Therefore, the present invention is a useful technique in the field of secondary batteries.

DESCRIPTION OF SYMBOLS 10 ... Negative electrode, 12 ... Negative electrode collector, 14 ... Negative electrode active material layer, 30 ... Power generation element, 18 ... Separator, 20 ... Positive electrode, 22 ... Positive electrode current collector Body, 24 ... positive electrode active material layer, 50 ... exterior body, 60, 62 ... lead, 100 ... lithium ion secondary battery

Claims (5)

  A positive electrode having an active material layer containing an active material, a conductive additive, and a binder on a current collector, wherein the active material layer is divided into two in the thickness direction, and the active material layer The ratio P1 / P2 of the porosity P1 in the region R1 on the current collector side of the material layer and the porosity P2 in the other region R2 is in the range of 0.33 ≦ P1 / P2 ≦ 0.84, And the said area | region R1 and the said area | region R2 each have two or more types of conductive support agents, The positive electrode for lithium ion secondary batteries characterized by the above-mentioned.   The region R1 and the region R2 include a conductive additive a having a ratio DBP / BET of 1 or more of oil absorption rate (DBP) and specific surface area (BET), and a conductive auxiliary agent b having DBP / BET of 1 or less. The positive electrode for a lithium ion secondary battery according to claim 1, comprising one or more types.   When the conductive assistant a and the conductive assistant b in the region R1 are a1 and b1, the ratio Wa1 / Wb1 between the mass Wa1 of the a1 and the mass Wb1 of the b1 is 0.02 ≦ Wa1 / Wb1. The positive electrode for a lithium ion secondary battery according to claim 2, wherein ≦ 0.50.   When the conductive auxiliary agent a and the conductive auxiliary agent b in the region R2 are a2 and b2, the ratio of the mass Wa2 of the a2 and the mass Wb2 of the b2 is Wa2 / Wb2, 0.02 ≦ Wa2 / The positive electrode for a lithium ion secondary battery according to claim 2, wherein Wb2 ≦ 0.07.   A positive electrode for a lithium ion secondary battery according to any one of claims 1 to 4, a negative electrode having a negative electrode active material, a separator interposed between the positive electrode for a lithium ion secondary battery and the negative electrode; A lithium ion secondary battery comprising a nonaqueous electrolyte.

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