JP2020149927A - Positive electrode for nonaqueous electrolyte secondary battery and lithium ion secondary battery - Google Patents

Abstract

To provide a positive electrode for a nonaqueous electrolyte secondary battery, which achieves a superior load characteristic when being arranged in a lithium ion secondary battery.SOLUTION: A positive electrode for a nonaqueous electrolyte secondary battery comprises at least (A) LiM1P(1-x)O2(2-x) and (B) LiM2P(1-y)O2(2-y) as a positive electrode active material. ("x" in the positive electrode active material (A) satisfies 0.03≤x≤0.3, "y" in the positive electrode active material (B) satisfies 0≤y≤0.02, and M1 and M2 each include one or more kinds of metal elements selected from a group consisting of Fe, Co, Ni, Mn, Al, Ti and Zr.)SELECTED DRAWING: None

Description

The present invention relates to a positive electrode for a non-aqueous electrolyte secondary battery and a lithium ion secondary battery including the positive electrode for the non-aqueous electrolyte secondary battery.

Lithium-ion secondary batteries are used as large-scale stationary power sources for power storage, power sources for electric vehicles, etc., and in recent years, research on miniaturization and thinning of batteries has been progressing. A lithium ion secondary battery generally includes both electrodes (positive electrode and negative electrode) having an electrode active material layer formed on the surface of a metal foil, and a separator arranged between the two electrodes. The separator plays a role of preventing a short circuit between both electrodes and holding an electrolytic solution.

As the positive electrode active material used for the positive electrode of the lithium ion secondary battery, various compounds such as lithium cobalt oxide, lithium nickel oxide, and lithium iron compound are used. It is known that the positive electrode active material affects the capacity, cycle characteristics, etc. of the lithium ion secondary battery depending on the type, and research is being actively promoted (for example, Patent Documents 1 and 2).

Japanese Unexamined Patent Publication No. 2014-032961 Japanese Unexamined Patent Publication No. 2017-069185

In recent years, lithium ion secondary batteries have been strongly demanded to improve their performance such as quick chargeability and fast discharge performance, and lithium ions having high capacity even when the charge / discharge rate is increased, that is, excellent load characteristics. The development of secondary batteries is expected.
Therefore, an object of the present invention is to provide a positive electrode for a non-aqueous electrolyte secondary battery and a lithium ion secondary battery provided with the positive electrode, which are used for manufacturing a lithium ion secondary battery having better load characteristics than conventional ones. To do.

As a result of diligent studies, the present inventors have found that a positive electrode for a non-aqueous electrolyte secondary battery in which specific lithium metal oxides having different oxygen contents are used in combination, and a lithium ion secondary battery provided with the positive electrode can solve the above problems. , And completed the following invention. The gist of the present invention is the following [1] to [7].
[1] The positive electrode active material contains at ^least (A) LiM ¹ P _(1-x) O _{2 (2-x)} and (B) LiM ² P _(1-y) O _{2 (2-y)} . Positive electrode for non-aqueous electrolyte secondary batteries.
(In the positive electrode active material (A), x satisfies 0.03 ≦ x ≦ 0.3, and in the positive electrode active material (B), y satisfies 0 ≦ y ≦ 0.02, M ¹ and M. ² is composed of one or more metal elements selected from the group consisting of Fe, Co, Ni, Mn, Al, Ti, and Zr, respectively).
[2] The positive electrode for a non-aqueous electrolyte secondary battery according to the above [1], wherein each of M ¹ and M ² contains at least Fe.
[3] The non-aqueous electrolyte secondary battery according to the above [1] or [2], which comprises a positive electrode active material layer and a positive electrode current collector, and the positive electrode active material layer contains the above (A) and (B). Positive electrode.
[4] When the contents of the positive electrode active material (A) and the positive electrode active material (B) are DA and DB, respectively, the relationship of 0.03 ≦ DA / (DA + DB) ≦ 0.5 is satisfied. The positive electrode for a non-aqueous electrolyte secondary battery according to [3].
[5] The positive electrode for a non-aqueous electrolyte secondary battery according to the above [3] or [4], wherein the positive electrode active material layer further contains a positive electrode binder and a conductive auxiliary agent.
[6] When the average particle diameters of the positive electrode active material (A) and the positive electrode active material (B) are PA and PB, respectively, the relationship of 0.1 ≦ PA / PB ≦ 0.7 is satisfied. The positive electrode for a non-aqueous electrolyte secondary battery according to any one of 1] to [5].
[7] A lithium ion secondary battery comprising the positive electrode according to any one of the above [1] to [6], a negative electrode, a separator arranged between the positive electrode and the negative electrode, and an electrolytic solution.

According to the present invention, it is possible to provide a positive electrode for a non-aqueous electrolyte secondary battery used in a lithium ion secondary battery having excellent load characteristics, and a lithium ion secondary battery using the positive electrode.

It is the schematic sectional drawing which shows one Embodiment of the lithium ion secondary battery of this invention.

<Positive electrode for non-aqueous electrolyte secondary battery>
The positive electrode for a non-aqueous electrolyte secondary battery of the present invention has (A) LiM ¹ P _(1-x) O _{2 (2-x)} and (B) LiM ² P _(1-y) O as positive electrode active materials. Includes at least _{2 (2-y)} .
(In the positive electrode active material (A), x satisfies 0.03 ≦ x ≦ 0.3, and in the positive electrode active material (B), y satisfies 0 ≦ y ≦ 0.02, M ¹ and M. ² is composed of one or more metal elements selected from the group consisting of Fe, Co, Ni, Mn, Al, Ti, and Zr, respectively).
In the present specification, the positive electrode for a non-aqueous electrolyte secondary battery may be simply referred to as a positive electrode.

The positive electrode for a non-aqueous electrolyte secondary battery of the present invention contains at least the positive electrode active material (A) and the positive electrode active material (B), thereby improving the load characteristics of the lithium ion secondary battery including the positive electrode.

The _{x in} LiM ¹ P _(1-x) O _{2 (2-x} ), which is the positive electrode active material (A), satisfies 0.03 ≦ x ≦ 0.3. When x is a positive electrode active material outside such a range, a lithium ion secondary battery having excellent load characteristics cannot be obtained even if the positive electrode active material and the positive electrode active material (B) described later are combined. Further, when a positive electrode active material in which x exceeds 0.3 is used, the electrode density becomes low, and it becomes difficult to obtain a high-capacity lithium ion secondary battery. The x in the positive electrode active material (A) is preferably 0.05 ≦ x ≦ 0.28, more preferably 0.1 ≦ x ≦ 0.25, and further preferably 0.15 ≦ x ≦ 0. 25.

M ¹ in the positive electrode active material (A) is composed of one or more metal elements selected from the group consisting of Fe, Co, Ni, Mn, Al, Ti, and Zr. Among them, M ¹ preferably contains at least Fe from the viewpoint of improving the load characteristics, and more preferably contains only Fe.

The _{y in} LiM ² P _(1-y) O _{2 (2-y} ), which is the positive electrode active material (B), satisfies 0 ≦ y ≦ 0.02. When y is a positive electrode active material outside such a range, even if the positive electrode active material and the above-mentioned positive electrode active material (A) are combined, a lithium ion secondary battery having excellent load characteristics cannot be obtained. The y in the positive electrode active material (B) is preferably 0.001 ≦ y ≦ 0.018, and more preferably 0.005 ≦ y ≦ 0.015.

M ² in the positive electrode active material (B) is composed of one or more metal elements selected from the group consisting of Fe, Co, Ni, Mn, Al, Ti, and Zr. Among them, M ² preferably contains at least Fe from the viewpoint of improving the load characteristics, and more preferably contains only Fe.
Further, when M ¹ in the positive electrode active material (A) contains at least Fe, it is preferable that M ² in the positive electrode active material (B) also contains at least Fe. That is, from the viewpoint of improving the load characteristics, it is preferable that each of M ¹ and M ² contains at least Fe, and it is more preferable that each of M ¹ and M ² is composed of only Fe.

The average particle size of the positive electrode active material (A) is preferably 0.01 to 50 μm, more preferably 0.05 to 10 μm, from the viewpoint of improving the load characteristics of the lithium ion secondary battery and improving the electrode density. It is more preferably 0.08 to 5 μm, and further preferably 0.1 to 2.4 μm.
The average particle size of the positive electrode active material (B) is preferably 0.01 to 50 μm, more preferably 0.5 to 30 μm, from the viewpoint of improving the load characteristics of the lithium ion secondary battery and improving the electrode density. It is more preferably 1 to 20 μm, still more preferably 2.5 to 10 μm.
The average particle size (PB (μm)) of the positive electrode active material (B) is preferably larger than the average particle size (PA (μm)) of the positive electrode active material (A), and the ratio of PA to PB (PA / PB). ) Is preferably 0.05 or more, more preferably 0.1 or more, still more preferably 0.12 or more, and preferably 0.8 or less, more preferably 0.7 or less.
The average particle size of each positive electrode active material means the particle size (D50) when the volume integration is 50% in the particle size distribution of the positive electrode active material obtained by the laser diffraction / scattering method.

When the content (mass) of the positive electrode active material (A) is DA and the content (mass) of the positive electrode active material (B) is DB, the ratio of DA to the total of DA and DB (DA / (DA + DB)). Is preferably 0.01 or more, more preferably 0.03 or more, still more preferably 0.1 or more, and preferably 0.1 or more, from the viewpoint of increasing the electrode density to obtain a high-capacity lithium ion secondary battery. Is 0.99 or less, more preferably 0.7 or less, still more preferably 0.5 or less.
Further, from the viewpoint of lowering the volume expansion coefficient of the lithium ion secondary battery, improving the shape stability, and improving the cycle characteristics, the DA / (DA + DB) is preferably 0.7 or less, more preferably 0. It is preferably 5.5 or less.
The positive electrode for a non-aqueous electrolyte secondary battery of the present invention may contain other positive electrode active materials other than the positive electrode active materials (A) and (B) as long as the effects of the present invention are not impaired. When other positive electrode active materials are contained, the content thereof is preferably 20% by mass or less, more preferably 5% by mass or less, and further preferably 0% by mass based on the total amount of the positive electrode active materials.

The method for preparing the positive electrode active materials (A) and (B) is not particularly limited, and for example, a method for firing a mixture containing a metal raw material, a phosphorus raw material, and a lithium raw material, a metal raw material, and a mixture containing a phosphorus / lithium raw material can be used. Examples include a method of firing.
The metal raw material contains one or more metal elements selected from the group consisting of Fe, Co, Ni, Mn, Al, Ti, and Zr, and preferably contains at least Fe as the metal element. The metal raw material can be used in various forms such as metal oxides, metal chlorides, metal sulfates, and metal phosphates. Among them, metal oxides and metal phosphates are preferable, and specifically. a metal oxide such as _Fe 2 _{O 3,} and metal phosphates, such as _{Fe 3 (PO 4) 2 ·} 8H 2 O. One type of metal raw material may be used alone, or a plurality of types may be used in combination.
Examples of the phosphorus raw material include H ₃ PO ₄ , (NH ₄ ) H ₂ PO ₄ , and (NH ₄ ) ₂ HPO ₄ . Of these, H ₃ PO ₄ is preferable.
Examples of the lithium raw material include LiOH, LiOH · nH ₂ O (n is 1 for example), Li ₂ CO _{3 and the} like. Of these, Li ₂ CO ₃ is preferable.
Further, instead of using the phosphorus raw material and the lithium raw material separately, a phosphorus / lithium raw material in which both the phosphorus element and the lithium element are contained in the same compound may be used. Examples of the phosphorus / lithium raw material include LiH ₂ PO ₄ , Li ₃ PO _{4, and the} like, with Li ₃ PO ₄ being preferable.

When a mixture containing a metal raw material, a phosphorus raw material, and a lithium raw material is used, the molar ratio of each of these components is not particularly limited, but the x and y values of the positive electrode active materials (A) and (B) are set to desired values. It is advisable to adjust the molar ratio so as to do so.
When a mixture containing a metal raw material and a phosphorus / lithium raw material is used, the molar ratio of each of these components is not particularly limited, but the x and y values of the positive electrode active materials (A) and (B) are set to desired values. It is advisable to adjust the molar ratio so as to do so.

The mixture may optionally contain a conductive carbon material. Examples of the conductive carbon material include carbon blacks such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black, carbon fibers, natural graphite, artificial graphite, and the like. Among them, carbon black. Classes are preferred, and acetylene black is more preferred. These can be used alone or in combination of two or more.
When the conductive carbon material is blended, the molar ratio of the metal raw material to the conductive carbon material is not particularly limited, but for example, the metal raw material: the conductive carbon material = 1: 0.5 to 5 may be used.

The firing method of the mixture is not particularly limited. The firing temperature is preferably 300 to 1000 ° C, more preferably 400 ° C to 800 ° C. The firing time is preferably 1 to 20 hours, more preferably 3 to 15 hours. The firing may be performed only once or may be performed a plurality of times. Further, the mixture to be fired may be in the form of powder or a molded product.
Firing can be performed in a nitrogen atmosphere or in a nitrogen / oxygen mixed gas atmosphere. By adjusting the firing atmosphere, the x and y values of the positive electrode active materials (A) and (B) can be adjusted to desired values.
After firing the mixture, it is advisable to perform mechanochemical treatment, ball mill treatment, or the like so that the obtained fired product has a desired particle size.

The positive electrode for a non-aqueous electrolyte secondary battery of the present invention includes a positive electrode active material layer and a positive electrode current collector, and preferably, a positive electrode active material layer is laminated on the positive electrode current collector. The positive electrode active material layer preferably contains the above-mentioned positive electrode active materials (A) and (B), and further contains a binder for the positive electrode and a conductive auxiliary agent.

Examples of the binder for the positive electrode include a fluorine-containing resin such as polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), and polytetrafluoroethylene (PTFE), and polymethyl acrylate (PMA). , Acrylic resin such as polyvinylidene methacrylate (PMMA), polyvinylidene acetate, polyimide (PI), polyamide (PA), polyvinylidene chloride (PVC), polyether nitrile (PEN), polyethylene (PE), polypropylene (PP), Examples thereof include polyacrylonitrile (PAN), acrylonitrile-butadiene rubber, styrene butadiene rubber, poly (meth) acrylic acid, carboxymethyl cellulose, hydroxyethyl cellulose, polyvinyl alcohol and the like. These binders may be used alone or in combination of two or more. Further, carboxymethyl cellulose and the like may be used in the form of a salt such as a sodium salt. Among these, a fluorine-containing resin is preferable, and polyvinylidene fluoride (PVDF) is preferably used among the fluorine-containing resins.
The content of the positive electrode binder in the positive electrode active material layer is preferably 0.1 to 15% by mass, more preferably 0.5 to 10% by mass, and further preferably 1 based on the total amount of the positive electrode active material layer. ~ 8% by mass.

As the conductive auxiliary agent, a material having higher conductivity than the positive electrode active material is used, and specific examples thereof include carbon materials such as Ketjen black, acetylene black, carbon nanotubes, and rod-shaped carbon.
The content of the conductive auxiliary agent is preferably 0.1 to 15% by mass, more preferably 0.5 to 10% by mass, and further preferably 1 to 8% by mass based on the total amount of the positive electrode active material layer. is there.

The thickness of the positive electrode active material layer is not particularly limited, but is preferably 10 to 200 μm, and more preferably 50 to 150 μm.

Examples of the positive electrode current collector in the positive electrode for a non-aqueous electrolyte secondary battery of the present invention include conductive metals such as copper, aluminum, titanium, nickel and stainless steel, preferably aluminum or copper, more preferably. Aluminum is used. The positive electrode current collector is generally made of a metal foil, and its thickness is not particularly limited, but is preferably 1 to 50 μm.

The positive electrode for a non-aqueous electrolyte secondary battery of the present invention of the present invention can be formed by applying a composition for a positive electrode active material layer onto a positive electrode current collector and drying it. The composition for the positive electrode active material layer contains the positive electrode active materials (A) and (B), a binder for the positive electrode, and a conductive auxiliary agent. The composition for the positive electrode active material layer preferably further contains a solvent. The composition for the positive electrode active material layer is generally a slurry. The content of each component in the composition for the positive electrode active material layer may be adjusted so that the content of each component excluding the solvent becomes the content described in the above-mentioned positive electrode active material layer.

<Lithium-ion secondary battery>
The positive electrode for a non-aqueous electrolyte secondary battery of the present invention can be used for a non-aqueous electrolyte secondary battery, and is particularly preferably used for a lithium ion secondary battery.
The lithium ion secondary battery of the present invention includes a positive electrode composed of the positive electrode for a non-aqueous electrolyte secondary battery described above, a negative electrode, a separator arranged between the positive electrode and the negative electrode, and an electrolytic solution.
FIG. 1 is a schematic cross-sectional view showing an embodiment of the lithium ion secondary battery of the present invention. The lithium ion secondary battery 10 includes a positive electrode 12, a negative electrode 11 arranged so as to face the positive electrode 12, and a separator 13 arranged between the positive electrode 12 and the negative electrode 11.
The positive electrode 12 is a positive electrode for a non-aqueous electrolyte secondary battery of the present invention, and includes a positive electrode current collector 12a and a positive electrode active material layer 12b laminated on the positive electrode current collector 12a. The positive electrode active material layer 12b contains the above-mentioned specific positive electrode active materials (A) and (B), and therefore, the lithium ion secondary battery 10 is excellent in load characteristics.
Similarly, the negative electrode 11 also includes a negative electrode current collector 11a and a negative electrode active material layer 11b laminated on the negative electrode current collector 11a.
An insulating layer (not shown) may be provided between the negative electrode active material layer 11b and the separator 13, or between the positive electrode active material layer 12b and the separator 13. By providing the insulating layer, a short circuit between the positive electrode 12 and the negative electrode 11 can be effectively prevented.
Further, the electrolytic solution is usually filled in a battery cell in which the above-mentioned negative electrode, positive electrode, and separator are housed.

As the positive electrode in the lithium ion secondary battery of the present invention, the above-mentioned positive electrode for a non-aqueous electrolyte secondary battery can be used.
The negative electrode active material layer in the negative electrode in the lithium ion secondary battery of the present invention typically includes a negative electrode active material and a binder for the negative electrode.

Examples of the negative electrode active material used for the negative electrode active material layer include carbon materials such as graphite and hard carbon, a composite of tin compound, silicon and carbon, and lithium, and among these, carbon materials are preferable. , Graphite is more preferred.
The negative electrode active material is not particularly limited, but its average particle size is preferably 0.5 to 50 μm, more preferably 1 to 30 μm. The average particle size of the negative electrode active material means the particle size (D50) when the volume integration is 50% in the particle size distribution of the positive electrode active material obtained by the laser diffraction / scattering method.
The content of the negative electrode active material in the negative electrode active material layer is preferably 50 to 99% by mass, more preferably 60 to 98.5% by mass, based on the total amount of the negative electrode active material layer.

As the negative electrode binder contained in the negative electrode active material layer, the same type of binder as that used in the above-mentioned positive electrode binder can be used.
The content of the binder for the negative electrode in the negative electrode active material layer is preferably 1.0 to 40% by mass, more preferably 1.5 to 25% by mass, based on the total amount of the negative electrode active material layer.

The thickness of the negative electrode active material layer is not particularly limited, but is preferably 10 to 200 μm, and more preferably 50 to 150 μm.

Examples of the material constituting the negative electrode current collector include conductive metals such as copper, aluminum, titanium, nickel, and stainless steel. Among these, aluminum or copper is preferable, and copper is more preferable. The negative electrode current collector is generally made of a metal foil, and its thickness is not particularly limited, but is preferably 1 to 50 μm.

The negative electrode of the present invention of the present invention can be formed by applying a composition for a negative electrode active material layer on a negative electrode current collector and drying it. The composition for a negative electrode material contains a negative electrode active material and a binder for a negative electrode. The composition for the negative electrode active material layer preferably further contains a solvent. The composition for the negative electrode active material layer is generally a slurry. The content of each component in the composition for the negative electrode active material layer may be adjusted so that the content of each component excluding the solvent becomes the content described in the above-mentioned negative electrode active material layer.

The lithium ion secondary battery of the present invention includes a separator arranged between the negative electrode and the positive electrode. The separator effectively prevents short circuits between the positive and negative electrodes.
Examples of the separator include a porous polymer film, a non-woven fabric, and glass fiber, and among these, a porous polymer film is preferable. Examples of the porous polymer film include an olefin-based porous film such as a polyethylene porous film.

The lithium ion secondary battery of the present invention may be provided with an insulating layer on the negative electrode active material layer or the positive electrode active material layer. The insulating layer effectively prevents short circuits between the positive and negative electrodes. The insulating layer is preferably a layer having a porous structure containing insulating fine particles and a binder for an insulating layer, and the insulating fine particles are bound by a binder for an insulating layer.
The insulating fine particles are not particularly limited as long as they are insulating, and may be either organic particles or inorganic particles. Specific organic particles include, for example, crosslinked polymethyl methacrylate, crosslinked styrene-acrylic acid copolymer, crosslinked acrylonitrile resin, polyamide resin, polyimide resin, poly (lithium 2-acrylamide-2-methylpropanesulfonate), and the like. Examples thereof include particles composed of organic compounds such as polyacetal resin, epoxy resin, polyester resin, phenol resin, and melamine resin. Inorganic particles include silicon dioxide, silicon nitride, alumina, boehmite, titania, zirconia, boron nitride, zinc oxide, tin dioxide, niobium oxide (Nb ₂ O ₅ ), tantalum oxide (Ta ₂ O ₅ ), potassium fluoride, and foot. Examples thereof include particles composed of inorganic compounds such as lithium pentoxide, clay, zeolite, and calcium carbonate. Further, the inorganic particles may be particles composed of known composite oxides such as niobium-tantalum composite oxide and magnesium-tantalum composite oxide. One type of insulating fine particles may be used alone, or a plurality of types may be used in combination.
The average particle size of the insulating fine particles is not particularly limited as long as it is smaller than the thickness of the insulating layer, and is, for example, 0.001 to 1 μm, preferably 0.05 to 0.8 μm, and more preferably 0.1 to 0.6 μm. is there.
The content of the insulating fine particles contained in the insulating layer is preferably 15 to 95% by mass, more preferably 40 to 90% by mass, and further preferably 60 to 85% by mass based on the total amount of the insulating layer. When the content of the insulating fine particles is within the above range, the insulating layer can form a uniform porous structure and is provided with appropriate insulating properties.

(Electrolytic solution)
The lithium ion secondary battery of the present invention includes an electrolytic solution. Examples of the electrolytic solution include an organic solvent and an electrolytic solution containing an electrolyte salt. Examples of the organic solvent include ethylene carbonate, propylene carbonate, dimethyl carbonate, γ-butyrolactone, sulfolane, dimethyl sulfoxide, acetonitrile, dimethylformamide, dimethylacetamide, 1,2-dimethoxyethane, 1,2-diethoxyethane, and tetrohydra. Polar solvents such as furan, 2-methyltetraethane, dioxolane, and methylacetamide, or mixtures of two or more of these solvents can be mentioned. Electrolyte salts include LiClO ₄ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiSbF ₆ , LiCF ₃ CO ₂ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ CF ₂ CF ₃ ) ₂ , LiN (COCF ₃ ). Examples include lithium-containing salts such as ₂ and LiN (COCF ₂ CF ₃ ) ₂ , lithium bisoxalate boronate (LiB (C ₂ O ₄ ) ₂ ), and lithium organic acid salt-boron trifluoride complex, LiBH. Complexes such as _{4 and the} like Complexes such as hydrides can be mentioned. These salts or complexes may be used alone or in admixture of two or more.

The lithium ion secondary battery may have a multi-layer structure in which a plurality of negative electrodes and a plurality of positive electrodes are laminated. In this case, the negative electrode and the positive electrode may be provided alternately along the stacking direction. Further, the separator may be arranged between each negative electrode and each positive electrode, and when an insulating layer is provided, it may be provided between the negative electrode and the separator or between the positive electrode and the separator.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

The obtained lithium ion secondary batteries in each Example and Comparative Example were evaluated by the following evaluation methods.

(Discharge capacity ratio)
The lithium ion secondary batteries manufactured in each Example and Comparative Example are placed in a constant temperature bath at 25 ° C., the charging rate is 0.2C, the discharging rate is 0.2C, and after charging and discharging, the charging rate is 1C and the discharging rate. Was set to 1C, and charging / discharging was performed, and 100 × (discharge capacity of 1C) / (discharge capacity of 0.2C) was defined as the discharge capacity ratio (%). It can be judged that the higher the discharge capacity ratio, the better the load characteristics.

(Float test volume expansion coefficient)
The lithium ion secondary batteries produced in each Example and Comparative Example were placed in a constant temperature bath at 55 ° C., CV charging was performed at 4.3 V with a charging rate of 0.2 C, and 100 × (lithium ion two after one week). The volume expansion rate (%) was defined as (volume of secondary battery) / (volume of lithium ion secondary battery before test). The volume of the secondary battery was measured by using the Archimedes method using pure water.

(Density evaluation of positive electrode active material layer)
The density of the positive electrode active material layer was measured as follows. First, a plurality of measurement samples in which the positive electrode is punched out to a predetermined size (for example, diameter 16 mm) are prepared. Weigh the mass of each measurement sample with a precision balance and measure the mass. By subtracting the mass of the positive electrode current collector measured in advance from the measurement result, the mass of the positive electrode active material layer in the measurement sample can be calculated. In addition, the thickness of the positive electrode active material layer is measured by a known method such as observing a measurement sample that has been cross-sectioned with an SEM. The density of the positive electrode active material layer can be calculated from the average value of each measured value based on the following formula (1).
Density of positive electrode active material layer (g / cc) = Mass of positive electrode active material layer (g) / [(Thickness of positive electrode active material layer (cm) x area of punched positive electrode (cm ² )] ... (1)

[Example 1]
(Manufacturing of positive electrode active material (A))
Ferric oxide (Fe ₂ O ₃ ), lithium carbonate (Li ₂ CO ₃ ) and acetylene black are mixed at a molar ratio of 1: 1: 2, water is added to form a slurry, and after sufficient mixing, phosphorus is added. The acid was added in a molar ratio of 0.8 and stirred to obtain a slurry-like mixture. The obtained mixture was dried at 150 ° C. for 20 hours under a nitrogen atmosphere and further calcined at 460 ° C. for 10 hours to obtain a precursor. The obtained precursor was treated (pulverized) with a ball mill for 30 minutes and then calcined in nitrogen gas at 650 ° C. for 10 hours to obtain a positive electrode active material (A). The positive electrode active material (A) was LiFeP _0.8 O _3.6 , which was x = 0.2, as a result of XRD analysis (X-ray diffraction analysis) and ICP emission analysis.

(Manufacturing of positive electrode active material (B))
Ferrous salt hydrate _Fe 3-phosphate and _{(PO 4) 2 · 8H 2} O, lithium phosphate _Li 3 PO _4, the molar ratio of 1: 1, dispersed in water, after preparation slurried, wet It was placed in a bead mill device and wet-mixed. The water in the resulting slurry was then evaporated and dried to give a mixture. 10 g of the obtained raw material mixture was press-molded at 44 MPa with a hand press machine to obtain a pressure-molded body. The obtained pressure-molded body was fired at 600 ° C. for 5 hours in a nitrogen atmosphere, and after firing, cooled in a nitrogen atmosphere. Then, the obtained calcined product was pulverized and then classified to obtain a composite (B1). When the obtained complex (B1) was subjected to XRD analysis and ICP emission analysis, it was confirmed that single-phase LiFeP _0.99 O _3.98 was produced. The complex (B1) was put into a Nobilta (model: NOB-130) manufactured by Hosokawa Micron Corporation and subjected to mechanochemical treatment for 12 hours to obtain a positive electrode active material (B). The positive electrode active material (B) was LiFeP _0.99 O _3.98 , which was y = 0.01, as a result of XRD analysis (X-ray diffraction analysis) and ICP emission analysis.

(Preparation of positive electrode)
When the mixing amounts of the positive electrode active material (A) and the positive electrode active material (B) produced as described above are DA and DB, the mixing ratio [DA / (DA + DB)] is 0.3. To obtain a mixture of positive electrode active material. Next, 90 parts by mass of the mixture of the positive electrode active material, 5 parts by mass of PVDF (manufactured by Kureha Co., Ltd. # 7200) as a binder, 5 parts by mass of acetylene black as a conductive auxiliary agent, and NMP (N-methyl) as a solvent. Pyrrolidone) was mixed to obtain a slurry-like composition for a positive electrode active material layer adjusted to a solid content of 45% by mass. This composition for the positive electrode active material layer was applied to an aluminum foil which is a positive electrode current collector, pre-dried, and then vacuum dried at 120 ° C. to obtain a laminated body in which the positive electrode active material layer was laminated on the aluminum foil. .. The laminate was pressure-pressed at 30 MPa and further punched into a 50 mm square (excluding the tab portion) of the electrode size to prepare a positive electrode.

(Preparation of negative electrode)
A slurry-like negative electrode having a solid content of 50% by mass by mixing 98 parts by mass of graphite as a negative electrode active material, 1 part by mass of carboxymethyl cellulose as a binder, 1 part by mass of styrene butadiene rubber as a binder, and water as a solvent. A composition for an active material layer was obtained. This composition for the negative electrode active material layer was applied to a copper foil as a negative electrode current collector and vacuum dried at 100 ° C. to obtain a laminated body in which the negative electrode active material layer was laminated on the copper foil. The laminate was pressure-pressed at 2 kN and further punched into an electrode size of 52 mm square (excluding the tab portion) to prepare a negative electrode.

(Preparation of electrolyte)
LiPF ₆ as an electrolyte was dissolved in a solvent obtained by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3: 7 so as to be 1 mol / liter to prepare a non-aqueous electrolyte solution 1.

(Separator)
As the separator, a polyethylene porous film (melting point 128 ° C.) was used.

(Manufacturing of lithium-ion secondary batteries)
Two negative electrodes, one positive electrode, and two separators prepared as described above were laminated in the order of negative electrode, separator, positive electrode, separator, and negative electrode to obtain a laminated structure. Terminal tabs are electrically connected to each of the positive electrode current collector exposed part and the negative electrode current collector exposed part, and the laminated structure is sandwiched between aluminum laminate films so that the terminal tabs protrude to the outside, and the three sides. Was sealed by laminating. A lithium ion secondary battery (laminate cell) was manufactured by injecting the non-aqueous electrolytic solution 1 from one side left unsealed and vacuum-sealing. Various evaluations were performed on the lithium ion secondary battery, and the results are shown in Table 1.

[Examples 2 to 13]
Table 1 shows the mechanochemical treatment time, ball mill crushing time, and mixing ratio [DA / (DA + DB)] of the positive electrode active materials (A) and (B) when producing the positive electrode active materials (A) and (B). A lithium ion secondary battery was manufactured in the same manner as in Example 1 except for the modification. Various evaluations were performed on the lithium ion secondary battery, and the results are shown in Table 1.

[Comparative Example 1]
In the production of the positive electrode active material (A) in Example 1, the positive electrode active material (A) was obtained in the same manner as in Example 1 except that phosphoric acid was added in a molar ratio of 0.6. The positive electrode active material (A) was LiFeP _0.6 O _3.2 , which was x = 0.4, as a result of XRD analysis (X-ray diffraction analysis) and ICP emission analysis. Using the positive electrode active material, a lithium ion secondary battery was obtained in the same manner as in Example 1, and various evaluations were performed.

It was found that the lithium ion secondary batteries of each example provided with a positive electrode using a specific positive electrode active material have a high discharge capacity ratio, and therefore have excellent load characteristics. On the other hand, it was found that the lithium ion secondary battery of the comparative example provided with the positive electrode in which the specific positive electrode active material was not used in combination was inferior in load characteristics because the discharge capacity ratio was low.

10 Lithium-ion secondary battery 11 Negative electrode 11a Negative electrode current collector 11b Negative electrode active material layer 12 Positive electrode 12a Positive electrode current collector 12b Positive electrode active material layer 13 Separator

Claims (7)

A non-aqueous electrolyte containing at least (A) LiM ¹ P _(1-x) O _{2 (2-x)} and (B) LiM ² P _(1-y) O _{2 (2-y)} as the positive electrode active material. Positive electrode for secondary batteries.
(In the positive electrode active material (A), x satisfies 0.03 ≦ x ≦ 0.3, and in the positive electrode active material (B), y satisfies 0 ≦ y ≦ 0.02, M ¹ and M. ² is composed of one or more metal elements selected from the group consisting of Fe, Co, Ni, Mn, Al, Ti, and Zr, respectively). The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein each of M ¹ and M ² contains at least Fe. The non-aqueous electrolyte secondary battery according to claim 1 or 2, further comprising a positive electrode active material layer and a positive electrode current collector, and the positive electrode active material layer containing the positive electrode active material (A) and the positive electrode active material (B). Positive electrode. According to claim 3, when the contents of the positive electrode active material (A) and the positive electrode active material (B) are DA and DB, respectively, the relationship of 0.03 ≤ DA / (DA + DB) ≤ 0.5 is satisfied. The positive electrode for a non-aqueous electrolyte secondary battery described. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 3 or 4, wherein the positive electrode active material layer further contains a positive electrode binder and a conductive auxiliary agent. Claims 1 to 5 satisfy the relationship of 0.1 ≤ PA / PB ≤ 0.7 when the average particle diameters of the positive electrode active material (A) and the positive electrode active material (B) are PA and PB, respectively. The positive electrode for a non-aqueous electrolyte secondary battery according to any one of. A lithium ion secondary battery comprising the positive electrode according to any one of claims 1 to 6, a negative electrode, a separator arranged between the positive electrode and the negative electrode, and an electrolytic solution.