JP2008243529A - Positive electrode for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a positive electrode exerting an excellent load factor characteristic, and a lithium ion secondary battery using the positive electrode. <P>SOLUTION: This lithium ion secondary battery has an electrode group formed by rolling a positive electrode plate and a negative electrode plate to face each other through a separator. The positive electrode plate has aluminum foil as a positive electrode collector. A positive electrode mix containing a positive electrode active material is generally uniformly applied to both surfaces of the aluminum foil. For the positive electrode active material, lithium iron phosphate represented by chemical formula Li<SB>1+x</SB>Fe<SB>1-x</SB>PO<SB>4</SB>(0<x<1), and having an olivine crystal structure is used. In the positive electrode mix, a conductive material containing three or more kinds of carbon materials different in powder properties, and a binder of polyvinylidene fluoride are mixed in addition to the positive electrode active material. Electron conduction is mediated between the lithium iron phosphate and the aluminum foil by the three or more kinds of carbon materials different in powder properties. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

The present invention relates to a positive electrode for a lithium ion secondary battery and a lithium ion secondary battery, and in particular, a positive electrode active material represented by a chemical formula Li _{1 + x} Fe _1-x PO ₄ (0 <x <1) and having an olivine crystal structure; The present invention also relates to a positive electrode for a lithium ion secondary battery in which a mixture containing a conductive material is applied to a current collector substantially evenly, and a lithium ion secondary battery using the positive electrode.

Lithium ion secondary batteries, which are representative of non-aqueous electrolyte secondary batteries, are widely used as power sources for portable devices such as notebook computers, taking advantage of their high energy density, and are also practical as power sources for electric vehicles. It has become. In a general lithium ion secondary battery, a lithium-containing metal oxide capable of reversibly occluding and releasing lithium ions as a positive electrode active material and a carbon material as a negative electrode active material are used. A nonaqueous electrolytic solution in which a lithium salt such as lithium fluorophosphate (LiPF ₆ ) is dissolved is used.

  Conventionally, lithium cobalt oxide having a layered crystal structure or a spinel crystal structure and having a high capacity and a long life has been mainly used as the lithium-containing metal oxide of the positive electrode active material. However, lithium cobaltate has a small amount of cobalt as a raw material, and increases the cost of the lithium ion secondary battery. For this reason, in recent years, for the purpose of improving safety and reducing the cost, research on using lithium iron phosphate having an olivine crystal structure as a positive electrode active material has been actively conducted.

  Since lithium iron phosphate having an olivine crystal structure operates at a relatively high potential, it has the advantage of increasing battery capacity. On the other hand, since a polyanion is used as a basic skeleton, a localized electronic structure is formed, and thus there is a problem that the electronic conductivity is lower than that of a lithium-containing metal oxide having a layered crystal structure or a spinel crystal structure. is there. For this reason, at the time of high load, the charge / discharge characteristics are greatly deteriorated. In order to overcome these problems and obtain sufficient performance as a battery, various techniques have been disclosed. For example, a technique for making the particle size of lithium iron phosphate ultrafine (see Patent Document 1) and a technique for substituting a part of the iron element constituting lithium iron phosphate with a different element (see Patent Document 2) are disclosed. ing.

JP 2004-79276 A JP 2005-50556 A

  However, the technique of Patent Document 1 has a problem in that the particle size of lithium iron phosphate is made ultrafine, so that it is difficult to handle during electrode fabrication. In the technique of Patent Document 2, management of reaction conditions and the like when replacing an iron element with a different element becomes complicated, and there is a problem in cost. On the other hand, when an electrode is produced by adding a large amount of a conductive material in order to compensate for the low electronic conductivity of lithium iron phosphate, the proportion of the positive electrode active material is relatively reduced, which is disadvantageous in terms of energy density. Become.

  In view of the above-described case, an object of the present invention is to provide a positive electrode capable of exhibiting excellent load factor characteristics and a lithium ion secondary battery using the positive electrode.

In order to solve the above problems, a first aspect of the present invention includes a positive electrode active material represented by a chemical formula Li _{1 + x} Fe _1-x PO ₄ (0 <x <1) and having an olivine crystal structure, and a conductive material. In the positive electrode for a lithium ion secondary battery in which the containing mixture is applied to the current collector substantially uniformly, the conductive material includes three or more types of carbon materials having different powder properties.

In the first aspect, since the conductive material included in the mixture applied to the current collector includes three or more types of carbon materials having different powder properties, the chemical formula Li _{1 + x} Fe _1-x PO ₄ (0 <x <1) Since the electron conduction between the positive electrode active material represented by <1) and the current collector and the current collector is mediated by three or more kinds of carbon materials having different powder properties, the load factor characteristics are excellent as a positive electrode. Can be demonstrated.

In a first aspect, the positive electrode active material having an average particle diameter _{D 50} of 10μm or less, and, BET specific surface area may be more than ^{5 m} 2 / g. Moreover, it is preferable that the BET specific surface areas of the carbon materials are different. At this time, the BET specific surface area of the carbon material is at least 1 type to 2 times or less, the other type is 3 times to 5 times, and the other type is 50 times the BET specific surface area of the positive electrode active material. It is good also as 100 times or less. Moreover, you may enlarge content in a mixture, so that the BET specific surface area of a carbon material is small. At this time, the content of the carbon material in the mixture is 80% or more of the carbon material having the smallest BET specific surface area and 20% of the carbon material having the second BET specific surface area of the total carbon content in the mixture. % Or less and the carbon material having the largest BET specific surface area can be less than 10%. Further, the average particle diameter D ₅₀ of the carbon material of the smallest particle size may be more than twice the average particle diameter D ₅₀ of the positive electrode active material of the carbon material.

  Moreover, in order to solve the said subject, the 2nd aspect of this invention is a lithium ion secondary battery using the positive electrode of the 1st aspect.

According to the present invention, since the conductive material included in the mixture applied to the current collector includes three or more types of carbon materials having different powder properties, the chemical formula Li _{1 + x} Fe _1-x PO ₄ (0 <x <1) Since the electron conduction between the positive electrode active material represented by <1) and the current collector and the current collector is mediated by three or more kinds of carbon materials having different powder properties, the load factor characteristics are excellent as a positive electrode. The effect that can be exhibited can be acquired.

  Embodiments of a cylindrical lithium ion secondary battery to which the present invention is applied will be described below with reference to the drawings.

(Constitution)
As shown in FIG. 1, the cylindrical lithium ion secondary battery 20 of this embodiment includes an electrode group wound in a cross-sectional spiral shape so that a positive electrode plate (positive electrode) and a negative electrode plate face each other with a separator W5 interposed therebetween. 6. The electrode group 6 is housed in a bottomed cylindrical battery container 7 made of steel plated with nickel.

  A cylindrical shaft core 1 made of polypropylene resin is used at the winding center of the electrode group 6. On the upper side of the electrode group 6, an annular positive electrode current collecting ring 4 for collecting the electric potential from the positive electrode plate is disposed on an almost extension line of the shaft core 1. The positive electrode current collecting ring 4 is fixed to the upper end portion of the shaft core 1. The edge part of the positive electrode lead piece 2 led out from the positive electrode plate is joined by ultrasonic welding to the peripheral edge of the flange part integrally protruding from the periphery of the positive electrode current collecting ring 4. Above the positive electrode current collecting ring 4, a disc-shaped battery lid 12 is provided that incorporates a safety valve and serves as a positive electrode external terminal. One end of a positive electrode lead 9 is joined to the upper part of the positive electrode current collecting ring 4, and the other end of the positive electrode lead 9 is joined to the lower surface of the battery lid 12.

  On the other hand, an annular negative electrode current collecting ring 5 for collecting electric potential from the negative electrode plate is disposed below the electrode group 6. The outer peripheral surface of the lower end portion of the shaft core 1 is fixed to the inner peripheral surface of the negative electrode current collecting ring 5. The end of the negative electrode lead piece 3 led out from the negative electrode plate is joined to the outer peripheral edge of the negative electrode current collecting ring 5 by welding. The lower part of the negative electrode current collection ring 5 is joined to the inner bottom part of the battery container 7 which also serves as a negative electrode external terminal via a negative electrode lead 8 by welding.

  The battery lid 12 is caulked and fixed to the upper part of the battery container 7 via the gasket 10. The gasket 10 is made of an insulating and heat resistant material such as polypropylene resin. For this reason, the inside of the lithium ion secondary battery 20 is sealed. In addition, a non-aqueous electrolyte is injected into the battery container 7. As the non-aqueous electrolyte, a solution obtained by dissolving lithium hexafluorophosphate at a concentration of 1 mol / L in a mixed solvent such as ethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate is used.

  In the electrode group 6, the positive electrode plate and the negative electrode plate are wound around the shaft core 1 through a microporous separator W5 made of polyethylene or the like so that the both electrode plates do not directly contact each other. The separator W5 is set to have a thickness in the range of about 20 to 50 μm and a width (length in the longitudinal direction of the shaft core 1) larger than the width of the positive electrode plate and the negative electrode plate. The positive electrode lead piece 2 and the negative electrode lead piece 3 are arranged on both end surfaces of the electrode group 6 opposite to each other. Insulation coating is applied to the entire circumference of the collar peripheral surface of the electrode group 6 and the positive electrode current collecting ring 4 in order to prevent electrical contact between the electrode group 6 and the battery container 7. The number of windings is adjusted so that the maximum diameter portion of the electrode group 6 becomes an insulating coating existing portion, and the maximum diameter is set slightly smaller than the inner diameter of the battery container 7.

  The positive electrode plate constituting the electrode group 6 has an aluminum foil W1 as a positive electrode current collector. In this example, the thickness of the aluminum foil W1 is set to 20 μm. A positive electrode mixture W2 containing a positive electrode active material is applied to both surfaces of the aluminum foil W1 substantially evenly. In addition to the positive electrode active material, the positive electrode mixture W2 includes a binder (binder) such as a conductive material containing three or more types of carbon materials having different powder properties and polyvinylidene fluoride (hereinafter abbreviated as PVDF). It is blended.

As the positive electrode active material, lithium iron phosphate powder represented by the chemical formula Li _{1 + x} Fe _1-x PO ₄ (0 <x <1) and having an olivine crystal structure is used. Lithium iron phosphate powder has an average particle diameter _{D 50} of 10μm or less, and, BET specific surface area is set to more than ^{5 m} 2 / g. The average particle size is a numerical value obtained as a particle size having a cumulative volume of 50% from the particle size distribution measured with a laser diffraction particle size distribution measuring device. The BET specific surface area is a numerical value measured by the BET method using adsorption of nitrogen gas or the like.

Three or more types of carbon materials used for the conductive material have a BET specific surface area and an average particle diameter within a predetermined range as relative values to the positive electrode active material lithium iron phosphate. That is, each carbon material is set to have a different BET specific surface area. The BET specific surface area of carbon materials is 1 to 2 times, 1 type is 3 to 5 times, and another type is 50 to 100 times the BET specific surface area of lithium iron phosphate. It is set as follows. The carbon material having a smaller BET specific surface area has a larger content in the positive electrode mixture W2. The content of each carbon material in the positive electrode mixture W2 is 80% or more of the carbon material having the smallest BET specific surface area relative to the total carbon content (content of the entire conductive material) in the positive electrode mixture W2, and the BET ratio The carbon material having the second smallest surface area is set to 20% or less, and the carbon material having the largest BET specific surface area is set to less than 10%. Further, the particle size of each carbon material, the average particle diameter D ₅₀ of the carbon material of the smallest particle size is set to more than twice the average particle diameter D ₅₀ of the lithium iron phosphate of the carbon material.

  When the positive electrode mixture W2 is applied to the aluminum foil W1, the viscosity is adjusted with a dispersion solvent such as N-methylpyrrolidone (hereinafter abbreviated as NMP). The positive electrode plate is formed into a thin film with a density adjusted by a roll press after drying. An uncoated portion (plain portion) of the positive electrode mixture W2 is formed on the side edge on one side in the longitudinal direction of the aluminum foil W1. The uncoated part is cut out in a comb shape, and the positive electrode lead piece 2 is formed in the notch remaining part. As for the density of the positive electrode mixture W2, since the respective materials are set to the above-described powder properties, the optimum values differ depending on the respective settings. Therefore, it is necessary to individually examine and find the optimum value.

  On the other hand, the negative electrode plate has a copper foil W3 as a negative electrode current collector. In this example, the thickness of the copper foil W3 is set to 10 μm. On both surfaces of the copper foil W3, a negative electrode mixture W4 containing a powder of carbon material such as graphite capable of reversibly occluding and releasing lithium ions as a negative electrode active material is applied substantially evenly. In addition to the negative electrode active material, the negative electrode mixture W4 contains a conductive material such as acetylene black and a binder such as PVDF. When the negative electrode mixture W4 is applied to the copper foil W3, the viscosity is adjusted with a dispersion solvent such as NMP. The negative electrode plate is formed into a thin film with a density adjusted by a roll press after drying. An uncoated portion of the negative electrode mixture W4 is formed on the side edge on one side in the longitudinal direction of the copper foil W3 as in the case of the positive electrode plate, and the negative electrode lead piece 3 is formed. The length of the negative electrode plate is such that when the positive electrode plate and the negative electrode plate are wound, the positive electrode plate does not protrude from the negative electrode plate in the winding direction at the innermost winding and outermost winding. It is set longer than the length. Further, the width of the negative electrode mixture W4 coating portion is such that the positive electrode mixture W2 coating portion does not protrude from the negative electrode mixture W4 coating portion in the longitudinal direction of the shaft core 1. It is set longer than the width of.

(Battery assembly)
In the assembly of the lithium ion secondary battery 20, first, the produced positive and negative electrode plates are wound around the shaft core 1 through a separator to produce the electrode group 6. A separator W5 is fixed to the shaft core 1 with a tape or the like. At the beginning of winding, only the separator W5 is wound about 2-3 times, and then the positive electrode plate and the negative electrode plate are appropriately opposed to the positive electrode mixture W2 application surface and the negative electrode mixture W4 application surface, and the positive electrode lead piece 2 and the negative electrode lead piece 3 are wound so that they are positioned in opposite directions. After all of the positive electrode plate and the negative electrode plate are wound, the separator W5 is wound about 2-3 times at the end of winding so that the electrode is not exposed at the outermost periphery.

  A resin insulating plate having the same diameter as that of the electrode group 6 is disposed at both ends of the produced electrode group 6, and the positive electrode lead piece 2 and the negative electrode lead piece 3 respectively led to the both end surfaces of the electrode group 6 are collected into the positive electrode. After welding to the current ring 4 and the negative current collector ring 5, the electrode group 6 is inserted into the battery container 7 with the negative current collector ring 5 facing the bottom. A welding rod is inserted into the hollow portion formed on the shaft core 1, and the negative electrode lead 8 previously welded to the negative electrode current collecting ring 5 is joined to the inner bottom portion of the battery container 7 by resistance welding. Grooving is performed slightly on the bottom side from the opening of the battery container 7, and the other end of the positive electrode lead 9 whose one end is previously welded to the positive electrode current collecting ring 4 is joined to the lower surface of the battery lid 12 by welding. In order to remove moisture adhering to the inside of the battery such as the electrode group 6 and the battery container 7, vacuum drying is performed at 60 ° C. for 20 hours with the gasket 10 mounted on the upper side of the grooved portion. After vacuum drying, a non-aqueous electrolyte is poured into the battery container 7. The non-aqueous electrolyte is injected in a glove box substituted with argon in order to prevent reattachment of moisture. After injecting the non-aqueous electrolyte, the battery lid 12 is caulked and fixed to the top of the battery container 7 to complete the assembly of the lithium ion secondary battery 20.

(Action etc.)
Next, the operation and the like of the cylindrical lithium ion secondary battery 20 of the present embodiment will be described.

In the lithium ion secondary battery 20 of the present embodiment, lithium iron phosphate having the olivine crystal structure represented by the chemical formula Li _{1 + x} Fe _1-x PO ₄ (0 <x <1) is used as the positive electrode active material, The positive electrode mixture W2 applied to the aluminum foil W1 of the positive electrode current collector is blended with three types of carbon materials having different specific surface areas and particle sizes as conductive materials. Lithium iron phosphate having an olivine crystal structure is known to have a lower electronic conductivity than a compound having a layered crystal structure or a spinel crystal structure because a localized electronic structure is formed. In the lithium ion secondary battery 20, since the positive electrode mixture W2 includes three types of carbon materials, electronic conduction between the lithium iron phosphate and the aluminum foil W1 is mediated by these carbon materials. Thereby, since the low electronic conductivity of lithium iron phosphate is relieved, battery performance, in particular, load factor characteristics can be improved.

  Moreover, in this embodiment, each carbon material used as the conductive material has a different specific surface area, and the specific surface area of each carbon material is within a predetermined range with respect to the specific surface area of lithium iron phosphate as the positive electrode active material. Is set. That is, among the three types of carbon materials, one specific surface area is 1 to 2 times the specific surface area of lithium iron phosphate, the other is 3 to 5 times, and the other is 50 times or more. It is set to 100 times or less. Furthermore, in this embodiment, the compounding quantity in the positive electrode mixture W2 of each carbon material is set so large that a specific surface area is small. That is, the blending amount of each carbon material is 80% or more of the carbon material having the smallest specific surface area and 20% or less of the carbon material having the second smallest specific surface area with respect to the total carbon content in the positive electrode mixture W2. The carbon material having the largest specific surface area is set to less than 10%. Since the blending amount of each carbon material in the positive electrode mixture W2 is determined by the specific surface area set in a predetermined range in relation to lithium iron phosphate, in the positive electrode plate coated with the positive electrode mixture W2, the positive electrode mixture W2 The arrangement of the lithium iron phosphate and each carbon material is optimized, and a good electronic conduction path is formed. Thereby, since the high capacity | capacitance performance which lithium iron phosphate has can be exhibited, the fall of the discharge performance at the time of high load can be suppressed.

Furthermore, in the present embodiment, the average particle diameter D ₅₀ of the carbon material of the smallest particle size is set to more than twice the average particle diameter D ₅₀ of the lithium iron phosphate of the carbon material. For this reason, since each carbon material has an average particle size larger than that of lithium iron phosphate, it is possible to reliably mediate electronic conduction of lithium iron phosphate having high capacity performance.

  In addition, in this embodiment, the example which each determines content of the three types of carbon material contained in the positive electrode mixture W2 with respect to the total carbon content in the positive electrode mixture W2 is shown, and it mentions about total carbon content However, the total carbon content is not particularly limited, and may be set to a content that is usually used in a lithium ion secondary battery. When the content of the conductive material is increased to compensate for the low electronic conductivity of lithium iron phosphate used for the positive electrode active material, the proportion of the positive electrode active material is relatively decreased, so the volume energy density is decreased. It will be. On the other hand, if the content of the conductive material is too small, it becomes difficult to obtain sufficient electronic conductivity even if three or more types of carbon materials are used as shown in the present embodiment. For this reason, it is preferable to set the content of the conductive material in a range of about 0.01% by weight to 50% by weight. As described above, considering that the electron conductivity is reduced in the olivine crystal structure, it is more preferable to set the content of the conductive material to about 5% by weight or more.

  In the present embodiment, graphite is exemplified as the negative electrode active material. However, the present invention is not limited to this, and a carbon material normally used for a lithium ion secondary battery can be used. Examples of the negative electrode active material other than the present embodiment include amorphous carbon, various graphite materials, coke and the like, and the particle shape also includes scaly, spherical, fibrous, massive, etc. It is not limited. Moreover, although acetylene black was illustrated as a electrically conductive material of a negative electrode plate, this invention is not limited to this. For example, it is not necessary to mix a conductive material in the negative electrode plate, and in the case of mixing, a material usually used for a lithium ion secondary battery may be used.

  Furthermore, in this embodiment, the non-aqueous electrolyte was prepared by dissolving lithium hexafluorophosphate at 1 mol / liter in a mixed organic solvent such as ethylene carbonate and dimethyl carbonate, but the present invention is particularly limited. Any of those commonly used in lithium ion secondary batteries can be used. As the non-aqueous electrolyte other than the present embodiment, a non-aqueous electrolyte in which a general lithium salt is used as an electrolyte and dissolved in an organic solvent can be used, and the lithium salt and the organic solvent are not particularly limited. .

  Furthermore, in the present embodiment, the cylindrical lithium ion secondary battery 20 in which the electrode group 6 produced by winding the positive and negative electrode plates is housed in the bottomed cylindrical battery container 7 is illustrated, but the present invention is a battery shape. However, the present invention is not limited to the structure, and can be applied to, for example, a rectangular or other polygonal battery or a stacked battery in which positive and negative electrode plates are stacked. Moreover, as a battery structure to which the present invention can be applied, for example, a battery having a structure in which positive and negative external terminals penetrate through a battery lid and are pressed through a shaft core in a battery container can be exemplified.

  Next, examples of the lithium ion secondary battery 20 manufactured according to the present embodiment will be described. In addition, it describes together about the lithium ion secondary battery of the comparative example produced for the comparison.

Example 1
In Example 1, lithium iron phosphate having an average particle diameter D ₅₀ of 3 μm and a BET specific surface area of 8 m ² / g was used as the positive electrode active material. The following three types of carbon materials were used as the conductive material. That is, graphite having an average particle diameter D ₅₀ of 12 μm and a BET specific surface area of 8 m ² / g as the carbon material 1, and acetylene black, carbon having an average particle diameter D ₅₀ of 9 μm and a BET specific surface area of 35 m ² / g as the carbon material 2. As the material 3, carbon black having an average particle diameter D ₅₀ of 10 μm and a BET specific surface area of 650 m ² / g was used. PVDF was used for the binder. A positive electrode plate was prepared using a positive electrode mixture W2 in which a positive electrode active material, three types of carbon materials, and a binder were mixed so that the solid content ratio was 80/8 / 1.5 / 0.5 / 10 wt%. On the other hand, graphite was used as the negative electrode active material, acetylene black was used as the conductive material, and PVDF was used as the binder. A negative electrode plate was prepared using a negative electrode mixture W4 in which a negative electrode active material, a conductive material, and a binder were mixed so that the solid content ratio was 90/5/5 wt%.

As for the three types of carbon materials used for the conductive material of the positive electrode plate, the BET specific surface area is 1 time for the carbon material 1, 4375 times for the carbon material 2, and 81.25 for the carbon material 3. Doubled. The content of each carbon material in the positive electrode mixture is set to be larger as the carbon material has a smaller BET specific surface area, and is set smaller in the order of carbon material 1> carbon material 2> carbon material 3. Further, regarding the carbon materials, when the content relative to the total carbon content in the positive electrode mixture W2 is compared, the carbon material 1 having the smallest BET specific surface area is 80%, and the carbon material 2 having the second smallest BET specific surface area is 15%. %, The carbon material 3 having the largest BET specific surface area is 5%. Further, the particle size of each carbon material, and the smallest particle size mean particle diameter D ₅₀ of the carbon material 2 is 9μm of the carbon material, and 3 times the average particle diameter D ₅₀ of the lithium iron phosphate ₍₃ [mu] m) Become.

(Comparative Example 1 and Comparative Example 2)
In Comparative Example 1 and Comparative Example 2, it is necessary to obtain an appropriate slurry property at the time of producing the positive electrode plate, including the carbon material 1 having the largest content among the three types of carbon materials used in Example 1. Two types including the carbon material 1 were used in combination. That is, in Comparative Example 1, a positive electrode in which carbon material 1 and carbon material 2 are used in combination, and a positive electrode active material, two types of carbon materials, and a binder are mixed so that the solid content ratio is 80/8/2/10 wt%. A positive electrode plate was produced using the mixture. In Comparative Example 2, a positive electrode in which carbon material 1 and carbon material 3 are used in combination, and a positive electrode active material, two types of carbon materials, and a binder are mixed so that the solid content ratio is 80/8/2/10 wt%. A positive electrode plate was produced using the mixture. In order to surely confirm the effect in the evaluation test, in the positive electrode plate, the positive electrode mixture amount per unit area, the positive electrode mixture density, and the electrode thickness after pressing, which are parameters other than the mixture composition, are as in Example 1. Set the same. The negative electrode plate was produced in the same manner as in Example 1.

(Battery evaluation test)
About the produced lithium ion secondary battery of each Example and a comparative example, the discharge time rate was changed and the discharge capacity was measured and the load factor characteristic was evaluated. That is, each battery was charged to 3.7 V at room temperature (25 ° C.), and the discharge capacity of each battery was measured when discharged at 2.5 hours at a 5-hour rate, 2-hour rate, and 1-hour rate with a final voltage.

As shown in FIG. 2, in the lithium ion secondary batteries of Comparative Example 1 and Comparative Example 2 using two types of carbon materials as the conductive material of the positive electrode plate, the time rate decreases (the discharge current value increases). The discharge capacity is greatly reduced. On the other hand, in the lithium ion secondary battery 20 of Example 1 using three kinds of carbon materials as the conductive material of the positive electrode plate, it is clear that the decrease in discharge capacity is suppressed even when the time rate is reduced. became. Therefore, when the lithium iron phosphate having the olivine crystal structure represented by the chemical formula Li _{1 + x} Fe _1-x PO ₄ (0 <x <1) is used as the positive electrode active material, three types of carbon materials are blended as the conductive material. Thus, it was confirmed that the load factor characteristics can be improved, and in particular, the battery performance at the time of high load can be secured.

  The present invention contributes to the manufacture and sale of lithium ion secondary batteries in order to provide a positive electrode capable of exhibiting excellent load factor characteristics and a lithium ion secondary battery using the positive electrode. Have availability.

It is sectional drawing which shows the cylindrical lithium ion secondary battery of embodiment to which this invention is applied. It is a graph which shows the high rate discharge characteristic when the discharge capacity with respect to a discharge time rate is measured about the lithium ion secondary battery of an Example and a comparative example.

Explanation of symbols

6 Electrode group 20 Cylindrical lithium ion secondary battery (lithium ion secondary battery)
W1 Aluminum foil (current collector)
W2 cathode mix (mixture)

Claims (8)

Lithium ion in which a mixture containing a positive electrode active material represented by the chemical formula Li _{1 + x} Fe _1-x PO ₄ (0 <x <1) and having an olivine crystal structure and a conductive material is applied to the current collector almost evenly. The positive electrode for secondary batteries WHEREIN: The said electrically conductive material contains the 3 or more types of carbon material from which powder properties differ, The positive electrode characterized by the above-mentioned. 2. The positive electrode according to claim 1, wherein the positive electrode active material has an average particle diameter D ₅₀ of 10 μm or less and a BET specific surface area of 5 m ² / g or more.   The positive electrode according to claim 1, wherein the carbon materials have different BET specific surface areas.   The carbon material has a BET specific surface area of at least one type of 1 to 2 times, another type of 3 to 5 times the BET specific surface area of the positive electrode active material, and another one. The positive electrode according to claim 3, wherein the type is 50 times or more and 100 times or less.   5. The positive electrode according to claim 3, wherein the carbon material has a larger content in the mixture as a BET specific surface area is smaller.   The content of the carbon material in the mixture is 80% or more of the carbon material having the smallest BET specific surface area with respect to the total carbon content in the mixture, and the carbon material having the second smallest BET specific surface area. The positive electrode according to claim 5, wherein the carbon material having a BET specific surface area of less than 10% is less than 10%. Carbon material of the smallest particle size of the carbon material, one having an average particle diameter D ₅₀ of the claims 1 to 6, characterized in that at least 2 times the average particle diameter D ₅₀ of the positive electrode active material 2. The positive electrode according to item 1.   The lithium ion secondary battery using the positive electrode of any one of Claim 1 thru | or 7.

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