JP5250948B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte secondary battery using a mixture containing lithium iron phosphate as a positive electrode active material.

  In recent years, non-aqueous electrolyte secondary batteries having high energy density and excellent cycle characteristics have attracted attention as power sources for portable devices such as mobile phones and notebook computers and electric vehicles. Among such non-aqueous electrolyte secondary batteries, lithium ion secondary batteries are currently most widely on the market. The mainstream of this battery is for small consumer use, mainly for mobile phones of 2 Ah or less.

Currently, there are many positive electrode active materials for lithium ion secondary batteries, but the most commonly known materials are lithium cobaltate (LiCoO ₂ ) and lithium nickelate having an operating voltage of around 4V. (LiNiO _2), or lithium manganate (LiMn ₂ O ₄₎ having a spinel structure such as a lithium-containing transition metal oxides having a basic structure of. Among them, lithium cobaltate is widely adopted as a positive electrode active material because of its excellent charge / discharge characteristics and energy density in a small capacity lithium secondary battery up to a battery capacity of 2 Ah.

  However, considering the future development of medium-sized and large-sized products, especially industrial applications where large demand is expected, the safety of batteries is very important. I can't be satisfied. One of the causes is that the thermal stability of these positive electrode active materials is low.

  Therefore, recently, lithium iron phosphate having an olivine structure with high thermal stability has attracted attention as a positive electrode active material. In this lithium iron phosphate, phosphorus and oxygen are covalently bonded, so that oxygen is not released even at high temperatures, and the safety of the battery can be drastically improved by using it as a positive electrode active material.

  However, since the electronic conductivity of lithium iron phosphate is low, it is known that the high rate discharge characteristics are inferior. To remedy this drawback, carbon is deposited on the surface of lithium iron phosphate, lithium iron phosphate is uniformly coated with a conductive carbon material or carbon fiber, or lithium iron phosphate and carbon fine particles are combined. The technique to make is disclosed by patent documents 1-4.

  In addition, a method for producing lithium iron phosphate includes a solid phase method as shown in Patent Document 5, a sol-gel method as shown in Non-Patent Document 1, or a hydrothermal method as shown in Non-Patent Document 2. The law is known.

Furthermore, in Patent Document 6, in order to further improve the safety of the nonaqueous electrolyte secondary battery, lithium cobaltate (LiCoO ₂ ) and lithium iron phosphate (LiFePO ₄ ) are used as the positive electrode active material, and iron phosphate is used. A technique is disclosed in which the lithium content is 1% by mass or more based on the total mass of the positive electrode active material.
Japanese Patent Laid-Open No. 2003-034534 JP 2003-292308 A Japanese Patent Laid-Open No. 2004-186075 Japanese Patent Laid-Open No. 2001-015111 JP 2000-294238 A Japanese Patent Application Laid-Open No. 2002-279989 F. Croce et. al. , Electrochem and Solid-State Letters, 5 (3) A47-A50 (2002) S. Franger et. al. Electrochem and Solid-State Letters, 5 (10) A231-A233 (2002).

  However, even when the methods disclosed in Patent Documents 1 to 4 were used, it was found that the characteristics after the cycle test deteriorate. The reason for this is that lithium iron phosphate shrinks during charging and expands during discharge, so that part of the carbon carried on the active material is desorbed during charging and discharging, so that the electron conductivity is It is thought that this is due to the decrease in.

In Patent Document 6, the data for the proportion of the lithium cobalt oxide occupying the positive electrode active material (LiCoO _2), 0 wt%, have been described only for the range of 50 wt% and 80 to 99 wt%, 0 wt Data for a range greater than% and less than 50% by weight were unknown. In Patent Document 6, it is not discussed in the relationship between high-rate discharge characteristics of the ratio and the battery of the lithium cobalt oxide (LiCoO _2).

  An object of the present invention is to provide a non-aqueous electrolyte secondary battery that is excellent in high-rate discharge characteristics after repeated charge / discharge cycles and has high safety even when lithium iron phosphate is used as a positive electrode active material. It is in.

The invention according to claim 1 is the nonaqueous electrolyte secondary battery, wherein the positive electrode active material is a mixture of lithium iron phosphate and lithium cobaltate, and the average particle size of lithium cobaltate in the mixture is 0.2 μm. Exceeding 40 μm, the ratio of the average particle diameter of the lithium iron phosphate and the lithium cobaltate (the average particle diameter of lithium cobaltate / the average particle diameter of lithium iron phosphate) is 1 or less, and Content rate is 1 mass% or more and 30 mass% or less, It is characterized by the above-mentioned. (However, it is a mixture of lithium iron phosphate and LiZr _0.005 (Ni _0.4 Co _0.3 Mn _0.3 ) _0.995 O _2, and LiZr _0.005 (Ni _0.4 Co _{0. 3} Mn _0.3 ) _0.995 O ₂ content of 1% by mass to 20% by mass of nonaqueous electrolyte secondary battery, mixture of lithium iron phosphate and LiNi _0.8 Co _0.2 O ₂ And a non-aqueous electrolyte secondary battery having a content of LiNi _0.8 Co _0.2 O ₂ of 1% by mass or more and 15% by mass or less, lithium iron phosphate and LiZr _0.005 Mg _0.005 Co ₀ a mixture of _{.99 O 2, LiZr 0.005 Mg 0.005} Co 0.99 O 2 content of from 1% by mass or more, 5% by weight or less of nonaqueous electrolyte secondary batteries, are excluded. )

When a mixture of lithium iron phosphate and lithium cobaltate is used as the positive electrode active material of the non-aqueous electrolyte secondary battery, the lithium iron phosphate, the main component of the mixture, repeatedly contracts and expands during charge and discharge. In addition, the positive electrode active material contains lithium cobalt oxide having an electronic conductivity of 10 ⁻⁶ S · cm or more, expands when lithium is released, and contracts when lithium is occluded. The electron conduction path of the active material is maintained. As a result, the non-aqueous electrolyte secondary battery using this positive electrode active material is excellent in high rate discharge characteristics even after repeated charge / discharge cycles.

  In addition, since the main component of the positive electrode active material of the nonaqueous electrolyte secondary battery is lithium iron phosphate having an olivine structure with high thermal stability, a battery with extremely high safety can be obtained.

  The present invention is characterized in that a mixture of lithium iron phosphate and lithium cobaltate is used as the positive electrode active material of the nonaqueous electrolyte secondary battery, and the content of lithium cobaltate in the mixture is 1 mass. % To 30% by mass.

The lithium iron phosphate used for the positive electrode active material of the present invention is not particularly limited as long as the basic composition having an olivine structure satisfies LiFePO ₄ . For example, lithium iron phosphate in which a part of Fe represented by the general formula LiFe _1-x M _x PO ₄ (where 0 <x ≦ 1, M = Co, Ni, Mn, etc.) is substituted with Co or the like Etc. can also be used.

Further, the lithium cobalt oxide to be mixed with the lithium iron phosphate is an electronic conductivity of 10 -6 ^{S ·} cm or more, to expand the lithium during discharge, and has the property of contracting the lithium during occlusion, the general formula LiCo _{1- x} M _x O ₂ Here, M is an element that substitutes a part of Co. In the general formula LiCo _1-x M _x O ₂ , the value of x is such that the compound has an electronic conductivity of 10 ⁻⁶ S · cm or more, expands when lithium is released, and contracts when lithium is occluded. It is limited to the range having When x = 0, LiCoO ₂ does not contain a substitution element.

  Further, in the mixture of lithium iron phosphate and lithium cobaltate as the positive electrode active material, the content of lithium cobaltate in the mixture is required to be 1% by mass or more in order to sufficiently increase the electronic conductivity of the mixture. In order to suppress expansion / contraction during charging / discharging of the positive electrode active material due to the addition of lithium cobaltate, it is necessary to be 30% by mass or less.

  In the mixture of lithium iron phosphate and lithium cobaltate, when the content of lithium cobaltate is less than 1% by mass, the electronic conductivity of the mixture is insufficient, and the non-aqueous electrolyte using this as a positive electrode active material The high rate discharge characteristics of the secondary battery are inferior. Moreover, when the content rate of lithium cobaltate is larger than 30% by mass, expansion / contraction during charge / discharge increases, so that the electron conduction path after the cycle cannot be maintained, and the capacity decreases.

  In the mixture of lithium iron phosphate and lithium cobaltate, it is preferable that the lithium cobaltate to be added is uniformly distributed in the mixture. For this purpose, these materials are preferably mixed in a dry or wet powder mixer such as a V-type mixer, an S-type mixer, a grinder, a ball mill, or a planetary ball mill.

  The average particle size of the mixture of lithium iron phosphate and lithium cobaltate is preferably 50 μm or less in order to increase the electronic conductivity of the mixture. Moreover, a uniform mixture can be obtained by setting the particle diameter of lithium iron phosphate in the range of 0.1 μm to 40 μm and the particle diameter of lithium cobaltate in the range of 0.1 μm to 40 μm.

  Furthermore, since lithium cobaltate has a role of assisting the electronic conductivity of lithium iron phosphate, it is preferable that the particle diameter is smaller than that of lithium iron phosphate, so that the average particle of lithium iron phosphate and lithium cobaltate The diameter ratio (average particle diameter of lithium cobaltate / average particle diameter of lithium iron phosphate) is preferably 1 or less.

In the present application, the “average particle diameter” means a “median diameter (50% diameter D ₅₀ )”, that is, a particle diameter corresponding to 50% of the integrated distribution curve.

  A non-aqueous electrolyte secondary battery of the present invention is composed of a positive electrode including the positive electrode active material described above, a negative electrode including a negative electrode active material, and a non-aqueous electrolyte containing an electrolyte salt in a non-aqueous solvent. Is provided with a separator and an outer package for packaging them between the positive electrode and the negative electrode.

  As the nonaqueous electrolyte, those generally proposed for use in lithium ion batteries and the like can be used. Examples of the non-aqueous solvent include cyclic carbonates such as propylene carbonate, ethylene carbonate, butylene carbonate, and chloroethylene carbonate; cyclic esters such as γ-butyrolactone and γ-valerolactone; dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and the like. Chain carbonates; chain esters such as methyl formate, methyl acetate and methyl butyrate; tetrahydrofuran or derivatives thereof; 1,3-dioxane, 1,4-dioxane, 1,2-dimethoxyethane, 1,4-dibutoxy Ethers such as ethane and methyldiglyme; Nitriles such as acetonitrile and benzonitrile; Dioxolane or derivatives thereof; Ethylene sulfide, sulfolane, sultone or derivatives thereof alone or a mixture of two or more thereof However, it is not limited to these.

Examples of the electrolyte salt include LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiPF ₆ , LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiN (SO ₂ CF _3). ) (SO ₂ C ₄ F ₉ ), LiSCN, LiBr, LiI, Li ₂ SO ₄ , Li ₂ B ₁₀ Cl ₁₀ , NaClO ₄ , NaI, NaSCN, NaBr, KClO ₄ , KSCN, etc. These ionic compounds can be used alone or in admixture of two or more. Among these ionic compounds, LiN (SO ₂ C ₂ F ₅ ) ₂ is desirable because it has excellent high-temperature stability and can suppress corrosion of the aluminum current collector and terminals during charging.

  The concentration of the electrolyte salt in the non-aqueous electrolyte is preferably 0.5 mol / l to 5 mol / l, more preferably 1 mol / l to 2.5 mol in order to reliably obtain a non-aqueous electrolyte battery having high battery characteristics. / L.

Negative electrode active materials include lithium metal and lithium alloys (lithium metal-containing alloys such as lithium-aluminum, lithium-lead, lithium-tin, lithium-aluminum-tin, lithium-gallium, and wood alloys), as well as occlusion of lithium.・ Releasable alloys, carbon materials (eg, graphite, hard carbon, low-temperature fired carbon, amorphous carbon, etc.), metal oxides, lithium metal oxides (Li ₄ Ti ₅ O ₁₂ etc.), polyphosphate compounds, etc. It is done.

Among these, graphite is preferable as a negative electrode active material because it has an operating potential very close to that of metallic lithium and can realize charge and discharge at a high operating voltage. For example, artificial graphite and natural graphite are preferable. In particular, graphite in which the surface of the negative electrode active material particles is modified with amorphous carbon or the like is desirable because it generates less gas during charging. Further, Li ₄ Ti ₅ O ₁₂ is preferable as a negative electrode active material because it can reduce self-discharge and reduce irreversible capacity in charge and discharge when a lithium salt is employed as an electrolyte salt. The negative electrode active material powder desirably has an average particle size of 100 μm or less.

  In order to obtain the positive electrode active material and the negative electrode active material powder in a predetermined shape, a pulverizer or a classifier is used. For example, a mortar, a ball mill, a sand mill, a vibrating ball mill, a planetary ball mill, a jet mill, a counter jet mill, a swirling air flow type jet mill or a sieve is used. At the time of pulverization, wet pulverization in the presence of water or an organic solvent such as ethanol can also be used. There is no particular limitation on the classification method, and a sieve, an air classifier, or the like is used as needed for both dry and wet methods.

  In addition to the respective active materials, the positive electrode and the negative electrode of the non-aqueous electrolyte secondary battery of the present invention contain a conductive agent, a binder, a thickener, a filler, and the like, and a positive electrode mixture or a negative electrode mixture To do. The conductive agent is not limited as long as it is an electron conductive material that does not adversely affect the battery performance. Usually, natural graphite (such as scaly graphite, scaly graphite, earthy graphite), artificial graphite, carbon black, acetylene black, A conductive material such as ketjen black, carbon whisker, carbon fiber, metal (copper, nickel, aluminum, silver, gold, etc.) powder, metal fiber, and conductive ceramic material can be included as one type or a mixture thereof. . Among these, as the conductive agent, acetylene black is desirable from the viewpoints of electron conductivity and coatability.

  The addition amount of the conductive agent is preferably 0.1% by mass to 50% by mass, and particularly preferably 0.5% by mass to 30% by mass with respect to the total mass of the positive electrode mixture or the negative electrode mixture. In particular, it is desirable to use acetylene black by pulverizing into ultrafine particles of 0.1 to 0.5 μm because the required carbon amount can be reduced.

  As binders, thermoplastic resins such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyethylene, and polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butadiene rubber A polymer having rubber elasticity such as (SBR) or fluoro rubber can be used as one kind or a mixture of two or more kinds. The addition amount of the binder is preferably 1 to 50% by mass, particularly preferably 2 to 30% by mass with respect to the total mass of the positive electrode or the negative electrode.

  As the thickener, polysaccharides such as carboxymethylcellulose and methylcellulose can be used as one kind or a mixture of two or more kinds. Moreover, it is desirable that the thickener having a functional group that reacts with lithium, such as a polysaccharide, be deactivated by, for example, methylation. The addition amount of the thickener is preferably 0.5 to 10% by mass, particularly preferably 1 to 2% by mass with respect to the total mass of the positive electrode or the negative electrode.

  As the filler, any material that does not adversely affect the battery performance may be used. Usually, olefin polymers such as polypropylene and polyethylene, amorphous silica, alumina, zeolite, glass, carbon and the like are used. The addition amount of the filler is preferably 30% by mass or less with respect to the total mass of the positive electrode or the negative electrode.

  The mixing method of the positive electrode mixture and the negative electrode mixture is physical mixing, and the ideal is uniform mixing. Therefore, powder mixers such as V-type mixers, S-type mixers, crackers, ball mills, and planetary ball mills can be mixed dry or wet.

  The positive electrode and the negative electrode were prepared by mixing an active material, a conductive agent and a binder in an organic solvent such as N-methylpyrrolidone and toluene, and then applying the obtained mixture onto a current collector described in detail below. It is made by drying.

  As for the application method, for example, it is desirable to apply to any thickness and any shape using means such as roller coating such as applicator roll, screen coating, doctor blade method, spin coating, bar coater, etc., but it is not limited to these. It is not something.

  The current collector may be anything as long as it is an electronic conductor that does not adversely affect the constructed battery. For example, as a current collector for positive electrode, aluminum, titanium, stainless steel, nickel, calcined carbon, conductive polymer, conductive glass, etc. are used for the purpose of improving adhesion, conductivity and oxidation resistance. A material obtained by treating the surface of copper or copper with carbon, nickel, titanium, silver or the like can be used. In addition to copper, nickel, iron, stainless steel, titanium, aluminum, calcined carbon, conductive polymer, conductive glass, Al-Cd alloy, etc., the negative electrode current collector is adhesive, conductive, anti-reduction For the purpose of the property, the thing which processed the surface of copper etc. with carbon, nickel, titanium, silver, etc. can be used.

  The surface of these materials can be oxidized. Regarding the shape of the current collector, a film shape, a sheet shape, a net shape, a punched or expanded material, a lath body, a porous body, a foamed body, a formed body of fiber groups, and the like are used in addition to the foil shape. Although there is no particular limitation on the thickness, a thickness of 1 to 500 μm is used.

  Among these current collectors, an aluminum foil having excellent oxidation resistance is used as the positive electrode, and a copper foil, nickel foil, iron foil, and the like, which are excellent in reduction resistance and conductivity, and are inexpensive as the negative electrode. It is preferable to use an alloy foil containing a part of the above. Furthermore, a foil having a rough surface surface roughness of 0.2 μmRa or more is preferable, whereby the adhesion between the positive electrode active material or the negative electrode material and the current collector is excellent.

  Therefore, it is preferable to use an electrolytic foil because it has such a rough surface. In particular, an electrolytic foil that has been subjected to a cracking treatment is most preferable. Furthermore, when the double-sided coating is applied to the foil, it is desirable that the surface roughness of the foil is the same or nearly equal.

  As a separator used for the nonaqueous electrolyte secondary battery of the present invention, it is preferable to use a porous film or a nonwoven fabric exhibiting excellent rate characteristics alone or in combination. Examples of the material constituting the separator for a non-aqueous electrolyte battery include polyolefin resins typified by polyethylene and polypropylene, polyester resins typified by polyethylene terephthalate and polybutylene terephthalate, polyvinylidene fluoride, and vinylidene fluoride-hexa. Fluoropropylene copolymer, vinylidene fluoride-perfluorovinyl ether copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-trifluoroethylene copolymer, vinylidene fluoride-fluoroethylene copolymer, fluorine Vinylidene fluoride-hexafluoroacetone copolymer, vinylidene fluoride-ethylene copolymer, vinylidene fluoride-propylene copolymer, vinylidene fluoride-trifluoropropylene copolymer, vinylidene fluoride - tetrafluoroethylene - hexafluoropropylene copolymer, vinylidene fluoride - ethylene - can be mentioned tetrafluoroethylene copolymer.

  The porosity of the nonaqueous electrolyte secondary battery separator is preferably 98% by volume or less from the viewpoint of strength. Further, the porosity is preferably 20% by volume or more from the viewpoint of charge / discharge characteristics.

  The separator for the nonaqueous electrolyte secondary battery may use a polymer gel composed of a polymer such as acrylonitrile, ethylene oxide, propylene oxide, methyl methacrylate, vinyl acetate, vinyl pyrrolidone, polyvinylidene fluoride, and an electrolyte. .

  Further, when the separator for a non-aqueous electrolyte battery is used in combination with the above-described porous film, nonwoven fabric or the like and a polymer gel, it is desirable because the liquid retention of the electrolyte is improved. That is, by forming a film in which the surface of the polyethylene microporous membrane and the microporous wall are coated with a solvophilic polymer having a thickness of several μm or less, and holding the electrolyte in the micropores of the film, Gels. Examples of the solvophilic polymer include polyvinylidene fluoride, an acrylate monomer having an ethylene oxide group or an ester group, an epoxy monomer, a polymer having a monomer having an isocyanate group, and the like crosslinked. The monomer can be subjected to a crosslinking reaction using a radical initiator in combination with heating or ultraviolet rays (UV), or using an active ray such as an electron beam (EB).

  For the purpose of controlling strength and physical properties, the solvophilic polymer can be used by blending a physical property modifier in a range that does not interfere with the formation of a crosslinked product. Examples of the physical property modifier include inorganic fillers (metal oxides such as silicon oxide, titanium oxide, aluminum oxide, magnesium oxide, zirconium oxide, zinc oxide and iron oxide, and metal carbonates such as calcium carbonate and magnesium carbonate). And polymers (polyvinylidene fluoride, vinylidene fluoride / hexafluoropropylene copolymer, polyacrylonitrile, polymethyl methacrylate, etc.) and the like. The amount of the physical property modifier added is usually 50% by mass or less, preferably 20% by mass or less, based on the crosslinkable monomer.

  In the nonaqueous electrolyte secondary battery of the present invention, the electrolyte is injected before or after laminating the separator for a nonaqueous electrolyte battery, the positive electrode, and the negative electrode, and finally sealed with an exterior material. Is preferably produced. Further, in a non-aqueous electrolyte battery in which a power generation element in which a positive electrode and a negative electrode are laminated via a separator for a non-aqueous electrolyte battery is wound, the electrolyte is injected into the power generation element before and after the winding. Is preferred. As the injection method, it is possible to inject at normal pressure, but a vacuum impregnation method and a pressure impregnation method can also be used.

  Examples of the material of the exterior body of the nonaqueous electrolyte secondary battery of the present invention include nickel-plated iron, stainless steel, aluminum, and a metal resin composite film. The configuration of the lithium secondary battery is not particularly limited, and a coin battery or button battery having a positive electrode, a negative electrode, and a single-layer or multi-layer separator, and a cylindrical battery having a positive electrode, a negative electrode, and a roll separator. Examples include square batteries, flat batteries, and the like.

  The present invention will be described below with reference to preferred embodiments.

[Examples 1-4 and Comparative Examples 1-3]
[Example 1]
[Production of positive electrode]
First, iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O), ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ), and lithium carbonate (Li ₂ CO ₃ ) in a molar ratio of 2: 2: After measuring to be 1, the components were pulverized and mixed for 2 hours in a ball mill under an inert atmosphere to obtain precursors. Next, the precursor was baked under conditions of 700 ° C. for 12 hours under a nitrogen flow (2.0 l / min) to obtain LiFePO ₄ powder. The average particle size of this powder was 9.8 μm.

The obtained LiFePO ₄ powder and LiCoO ₂ were mixed using a ball mill to produce a positive electrode active material a for a non-aqueous electrolyte secondary battery containing LiFePO ₄ and LiCoO ₂ according to the present invention. The mixing ratio of LiFePO ₄ and LiCoO ₂ in the positive electrode active material a was 95: 5 by mass ratio. Moreover, the average particle diameter of LiCoO ₂ was 8.8 μm.

  This positive electrode active material a, acetylene black as a conductive agent, and polyvinylidene fluoride (PVdF) as a binder are mixed at a mass ratio of 80: 8: 12, and N-methyl-2-pyrrolidone (NMP) is mixed. In addition, the mixture was sufficiently kneaded to produce a positive electrode paste. This positive electrode paste was applied to both sides of an aluminum foil current collector with a thickness of 20 μm, dried, and then pressed and used as a positive electrode. A positive electrode terminal was welded to the positive electrode by resistance welding.

[Production of negative electrode]
Artificial graphite as an anode material (average particle size 6 μm, interplanar spacing (d ₀₀₂ ) 0.337 nm by X-ray diffraction analysis, crystal size in the c-axis direction (Lc) 55 nm) and polyvinylidene fluoride (PVdF) as a binder ) Was mixed at a mass ratio of 94: 6, and N-methyl-2-pyrrolidone (NMP) was added and sufficiently kneaded to prepare a negative electrode paste. The negative electrode paste was applied to both surfaces of a 15 μm thick copper foil current collector, dried, and then pressed, to obtain a negative electrode. A negative electrode terminal was welded to the negative electrode by resistance welding.

[Preparation of electrolyte]
A non-aqueous electrolyte was manufactured by dissolving LiPF ₆ as a fluorine-based electrolyte salt at a concentration of 1 mol / l in a solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1: 1. The amount of water in the electrolyte was less than 30 ppm.

[Production of batteries]
Using the positive electrode, negative electrode, and electrolytic solution described above, a laminated battery was manufactured in a dry atmosphere with a dew point of −40 ° C. or lower. The positive electrode and the negative electrode were wound in an oval shape through a PP separator having a thickness of 20 μm.

  A metal resin composite film made of polyethylene terephthalate (15 μm) / aluminum foil (50 μm) / metal-adhesive polypropylene film (50 μm) is used as the outer package, and the open ends of the positive electrode terminal and the negative electrode terminal are exposed to the outside. Thus, it sealed airtight except for the part used as a liquid injection hole.

  After pouring a certain amount of non-aqueous electrolyte from the liquid injection hole, the liquid injection hole part was heat sealed in a vacuum state to produce a non-aqueous electrolyte secondary battery A according to the present invention. The obtained nonaqueous electrolyte secondary battery A had dimensions of 49.3 mm × 33.7 mm × 5.17 mm and a design capacity of 540 mAh.

[Example 2]
A positive electrode active material b for a non-aqueous electrolyte secondary battery according to the present invention was manufactured by the same method as in Example 1 except that LiFePO ₄ and LiCoO ₂ were mixed at a mass ratio of 99: 1. Further, a non-aqueous electrolyte secondary battery B was manufactured in the same manner as in Example 1 except that this positive electrode active material b was used.

[Example 3]
A positive electrode active material c for a non-aqueous electrolyte secondary battery according to the present invention was manufactured in the same manner as in Example 1 except that LiFePO ₄ and LiCoO ₂ were mixed at a mass ratio of 85:15. Further, a non-aqueous electrolyte secondary battery C was manufactured in the same manner as in Example 1 except that this positive electrode active material c was used.

[Example 4]
A positive electrode active material d for a non-aqueous electrolyte secondary battery according to the present invention was manufactured in the same manner as in Example 1 except that LiFePO ₄ and LiCoO ₂ were mixed at a mass ratio of 70:30. Furthermore, a non-aqueous electrolyte secondary battery D was manufactured in the same manner as in Example 1 except that this positive electrode active material d was used.

[Comparative Example 1]
A positive electrode active material e for a nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that LiFePO ₄ and LiCoO ₂ were mixed at a mass ratio of 65:35. Further, a non-aqueous electrolyte secondary battery E was produced in the same manner as in Example 1 except that this positive electrode active material e was used.

[Comparative Example 2]
A positive electrode active material f for a non-aqueous electrolyte secondary battery was manufactured in the same manner as in Example 1 except that LiFePO ₄ and LiCoO ₂ were mixed at a mass ratio of 99.5: 0.5. Further, a non-aqueous electrolyte secondary battery F was produced in the same manner as in Example 1 except that this positive electrode active material f was used.

[Comparative Example 3]
First, LiFePO ₄ powder was obtained in the same manner as in Example 1. Next, the powder and acetylene black were mixed using a ball mill to produce a positive electrode active material g for a non-aqueous electrolyte secondary battery containing LiFePO ₄ and acetylene black. The mixing ratio of LiFePO ₄ and acetylene black was 95: 5 by mass ratio. Further, a non-aqueous electrolyte secondary battery G was manufactured in the same manner as in Example 1 except that this positive electrode active material g was used.

  Table 1 summarizes the contents of the positive electrode active materials used in the nonaqueous electrolyte batteries A to G manufactured in Examples 1 to 4 and Comparative Examples 1 to 3.

  The high rate discharge characteristics of the batteries A to G of Examples 1 to 4 and Comparative Examples 1 to 3 were measured. The measurement conditions are summarized in Table 2, and the measurement results are summarized in Table 3.

  That is, in the first cycle, a constant current / constant voltage charge (CC / CV charge) was performed for 15 hours in total at a constant current of 54 mA (0.1 C) up to 3.9 V, and further at a constant voltage of 3.9 V, then 54 mA. Discharging was performed at a constant current up to 2.0V. In the second cycle, CC / CV charging was performed under the same conditions as in the first cycle, and then discharging was performed to 2.0 V at a constant current of 5400 mA (10 C). Furthermore, the 3rd to 50th cycles were charged and discharged under the same conditions as the 1st cycle, and the 51st cycle was the same as the 2nd cycle.

  The discharge capacity at the first cycle is the initial low rate discharge capacity (X), the discharge capacity at the second cycle is the initial high rate discharge capacity (Y), the discharge capacity at the 51st cycle is the high rate discharge capacity after 50 cycles (Z The ratio of the high rate discharge capacity to the initial low rate discharge capacity (X) is expressed as “discharge rate (%)”, and the initial discharge rate (S) and the discharge rate after 50 cycles (T) are calculated as follows: Obtained from the formula.

S = (Y / X) × 100
T = (Z / X) × 100

From Table 3, it was found that the discharge rates after 50 cycles of the batteries A to D of Examples 1 to 4 were higher than those of the batteries E to G of Comparative Examples 1 to 3. This is considered to be due to the fact that the electron conduction path is maintained in the positive electrode active material even during charge / discharge, that is, when the lithium iron phosphate contracts or expands. In the case of the battery F of Comparative Example 2, it is presumed that the content of LiCoO _{2 in} the positive electrode active material is too small, and the formation of the electron conduction path in the positive electrode active material is insufficient. .

  Furthermore, in the case of the battery E of Comparative Example 1, since the amount of lithium cobaltate added is large, conversely, expansion / contraction occurs during charging / discharging, so that the electron conduction path after the cycle cannot be maintained. It is thought to be caused.

[Examples 5 to 8 and Comparative Examples 4 to 6]
[Example 5]
A positive electrode active material h for a non-aqueous electrolyte secondary battery was manufactured in the same manner as in Example 1 except that LiFe _0.9 Co _0.1 PO ₄ was used instead of LiFePO ₄ . Further, a non-aqueous electrolyte secondary battery H was manufactured in the same manner as in Example 1 except that this positive electrode active material h was used.

[Example 6]
A positive electrode active material i for a non-aqueous electrolyte secondary battery was manufactured in the same manner as in Example 2 except that LiFe _0.9 Co _0.1 PO ₄ was used instead of LiFePO ₄ . Further, a non-aqueous electrolyte secondary battery I was manufactured in the same manner as in Example 1 except that this positive electrode active material i was used.

[Example 7]
A positive electrode active material j for a non-aqueous electrolyte secondary battery was manufactured in the same manner as in Example 3 except that LiFe _0.9 Co _0.1 PO ₄ was used instead of LiFePO ₄ . Further, a non-aqueous electrolyte secondary battery J was manufactured in the same manner as in Example 1 except that this positive electrode active material j was used.

[Example 8]
A positive electrode active material k for a non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 4 except that LiFe _0.9 Co _0.1 PO ₄ was used instead of LiFePO ₄ . Further, a non-aqueous electrolyte secondary battery K was manufactured in the same manner as in Example 1 except that this positive electrode active material k was used.

[Comparative Example 4]
A positive electrode active material 1 for a non-aqueous electrolyte secondary battery was manufactured in the same manner as in Comparative Example 1 except that LiFe _0.9 Co _0.1 PO ₄ was used instead of LiFePO ₄ . Further, a non-aqueous electrolyte secondary battery L was manufactured in the same manner as in Example 1 except that this positive electrode active material l was used.

[Comparative Example 5]
A positive electrode active material m for a non-aqueous electrolyte secondary battery was manufactured in the same manner as in Comparative Example 2, except that LiFe _0.9 Co _0.1 PO ₄ was used instead of LiFePO ₄ . Further, a non-aqueous electrolyte secondary battery M was manufactured in the same manner as in Example 1 except that this positive electrode active material m was used.

[Comparative Example 6]
A positive electrode active material n for a non-aqueous electrolyte secondary battery was manufactured in the same manner as in Comparative Example 3, except that LiFe _0.9 Co _0.1 PO ₄ was used instead of LiFePO ₄ . Further, a non-aqueous electrolyte secondary battery N was manufactured in the same manner as in Example 1 except that this positive electrode active material n was used.

  Table 4 summarizes the contents of the positive electrode active materials used in the nonaqueous electrolyte batteries H to N produced in Examples 5 to 8 and Comparative Examples 4 to 6.

  The high rate discharge characteristics of the batteries H to N of Examples 5 to 8 and Comparative Examples 4 to 6 were measured under the same conditions as the battery A of Example 1. The measurement results are summarized in Table 5.

From the results in Table 5, it was revealed that the same effect was obtained when LiFe _0.9 Co _0.1 PO ₄ was used instead of LiFePO ₄ .

[Examples 9 to 12]
[Example 9]
A positive electrode active material for a non-aqueous electrolyte secondary battery in the same manner as in Example 1 except that LiCoO ₂ having an average particle diameter of 6.8 μm is used instead of LiCoO ₂ having an average particle diameter of 8.8 μm. o was produced. Further, a non-aqueous electrolyte secondary battery O was manufactured in the same manner as in Example 1 except that this positive electrode active material o was used.

[Example 10]
A positive electrode active material for a non-aqueous electrolyte secondary battery in the same manner as in Example 1 except that LiCoO ₂ having an average particle diameter of 7.4 μm is used instead of LiCoO ₂ having an average particle diameter of 8.8 μm. p was made. Further, a non-aqueous electrolyte secondary battery P was manufactured in the same manner as in Example 1 except that this positive electrode active material p was used.

[Example 11]
A positive electrode active material for a non-aqueous electrolyte secondary battery in the same manner as in Example 1 except that LiCoO ₂ having an average particle diameter of 9.8 μm is used instead of LiCoO ₂ having an average particle diameter of 8.8 μm. q was produced. Further, a non-aqueous electrolyte secondary battery Q was manufactured in the same manner as in Example 1 except that this positive electrode active material q was used.

[ Comparative Example 7 ]
A positive electrode active material for a non-aqueous electrolyte secondary battery in the same manner as in Example 1 except that LiCoO ₂ having an average particle diameter of 12.3 μm is used instead of LiCoO ₂ having an average particle diameter of 8.8 μm. r was produced. Further, a non-aqueous electrolyte secondary battery R was manufactured in the same manner as in Example 1 except that this positive electrode active material was used.

  Table 6 summarizes the contents of the positive electrode active materials used in the nonaqueous electrolyte batteries O to R manufactured in Examples 9 to 12. Table 6 also shows the results of Example 1 for comparison.

  The high rate discharge characteristics of the batteries O to R of Examples 9 to 12 were measured under the same conditions as the battery A of Example 1. The measurement results are summarized in Table 7. Table 7 also shows the results of Example 1 for comparison.

  From the results in Table 7, the effect of the ratio of the average particle size of lithium iron phosphate and lithium cobaltate (average particle size of lithium cobaltate / average particle size of lithium iron phosphate) of 1 or less is more effective. It became clear that it was expensive.

  As a result of the above, it is a mixture of lithium iron phosphate and lithium cobaltate according to the present invention, and the positive electrode active material having a lithium cobaltate content in the mixture of 1% by mass to 30% by mass is provided. It was revealed that the high-rate discharge characteristics of the water electrolyte secondary battery are excellent.

Claims (1)

The positive electrode active material is a mixture of lithium iron phosphate and lithium cobaltate, and an average particle diameter of lithium cobaltate in the mixture is more than 0.2 μm and not more than 40 μm, and the lithium iron phosphate and the cobalt The ratio of the average particle diameter with lithium oxide (average particle diameter of lithium cobaltate / average particle diameter of lithium iron phosphate) is 1 or less, and the content is 1% by mass or more and 30% by mass or less. A non-aqueous electrolyte secondary battery. (However, it is a mixture of lithium iron phosphate and LiZr _0.005 (Ni _0.4 Co _0.3 Mn _0.3 ) _0.995 O _2, and LiZr _0.005 (Ni _0.4 Co _{0. 3} Mn _0.3 ) _0.995 O ₂ content of 1% by mass to 20% by mass of nonaqueous electrolyte secondary battery, mixture of lithium iron phosphate and LiNi _0.8 Co _0.2 O ₂ And a non-aqueous electrolyte secondary battery having a content of LiNi _0.8 Co _0.2 O ₂ of 1% by mass or more and 15% by mass or less, lithium iron phosphate and LiZr _0.005 Mg _0.005 Co ₀ a mixture of _{.99 O 2, LiZr 0.005 Mg 0.005} Co 0.99 O 2 content of from 1% by mass or more, 5% by weight or less of nonaqueous electrolyte secondary batteries, are excluded. )