JP5807599B2 - Active material and lithium ion secondary battery - Google Patents

Description

  The present invention relates to an active material and a lithium ion secondary battery.

  In recent years, various electric vehicles are expected to be widely used for solving environmental and energy problems. As an in-vehicle power source such as a motor driving power source that holds the key to practical application of these electric vehicles, lithium ion secondary batteries have been intensively developed. However, in order to make a battery widely used as an in-vehicle power source, it is necessary to make the battery high performance and make it cheaper. In addition, it is necessary to make the one-mile travel distance of an electric vehicle closer to a gasoline engine vehicle, and a higher energy battery is desired.

In order to increase the energy density of the battery, it is necessary to increase the amount of electricity stored per unit mass of the positive electrode and the negative electrode. A so-called solid solution positive electrode has been studied as a positive electrode material (positive electrode active material) that may meet this demand. Among them, the solid solution of the electrochemically inactive layered Li ₂ MnO ₃ and the electrochemically active layered LiAO ₂ (A is a transition metal such as Co or Ni) exceeds 200 mAh / g. It is expected as a candidate for a high-capacity positive electrode material that can exhibit a large electric capacity (see Patent Document 1 below).

JP-A-9-55211 JP 2007-242581 A JP 2011-105594 A

However, as shown in Patent Document 2, in the solid solution system positive electrode using Li ₂ MnO ₃ , the irreversible capacity is high, so the initial charge / discharge efficiency of the positive electrode active material is low, and therefore the negative electrode facing in the battery design is There was a problem such as a decrease in battery capacity due to excessive use.

Further, a positive electrode active material excellent in rate characteristics as in Patent Document 3 is required.

  The present invention has been made in view of the above-described problems of the prior art, and an object thereof is to provide an active material and a lithium ion secondary battery that have high initial charge / discharge efficiency and excellent rate characteristics.

In order to achieve the above object, an active material according to the present invention has a layered structure, a compound having a composition represented by the following formula (1), and a columnar shape having an average aspect ratio of 16 or more and 62 or less. A mixed oxide of Li and V is mixed.
_{Li y Ni a Co b Mn c} M d O x ··· (1)
[In the above formula (1), the element M is at least one element selected from the group consisting of Al, Si, Zr, Ti, Fe, Mg, Nb, Ba and V, and 1.9 ≦ (a + b + c + d + y) ≦ 2.1, 1.0 <y ≦ 1.3, 0 <a ≦ 0.3, 0 <b ≦ 0.25, 0.3 ≦ c ≦ 0.7, 0 ≦ d ≦ 0.1, 9 ≦ x ≦ 2.1. ]

By mixing a compound having a composition represented by the formula (1) (hereinafter referred to as “compound”) and a composite oxide of Li and V, rate characteristics are improved. This is considered to be because the presence of the columnar Li and V composite oxide has a three-dimensional connection as a whole of the active material, and thus assists the conductive path between the active materials.
Moreover, the first charge / discharge efficiency is improved by mixing the compound and the composite oxide of Li and V. This is considered because the complex oxide of Li and V suppresses the reaction between the compound and the electrolytic solution.

  In the active material according to the present invention, it is preferable that a composite oxide of Li and V is present on the surface of the compound.

  Thereby, an irreversible capacity | capacitance falls and a first time charge / discharge efficiency becomes high. It is considered that the reaction between the electrolytic solution and the compound is further suppressed by the presence of the Li and V composite oxide on the surface of the compound.

  A lithium ion secondary battery according to the present invention includes a positive electrode current collector, a positive electrode active material layer including a positive electrode active material, a negative electrode current collector, and a negative electrode active material layer including a negative electrode active material, And a separator positioned between the positive electrode active material layer and the negative electrode active material layer, a negative electrode, a positive electrode, and an electrolyte in contact with the separator, wherein the positive electrode active material includes the active material described above It is characterized by that.

  The lithium ion secondary battery of the present invention comprising the active material mixed with the compound of the present invention and a composite oxide of Li and V in the positive electrode active material layer has high initial charge / discharge efficiency and excellent rate characteristics. .

  According to the present invention, an active material and a lithium ion secondary battery having high initial charge / discharge efficiency and excellent rate characteristics can be provided.

It is a schematic cross section of a lithium ion secondary battery. It is a TEM image of the positive electrode active material which concerns on one Embodiment of this invention. It is a SEM image of the positive electrode active material which concerns on one Embodiment of this invention. It is a mimetic diagram of the particle concerning one embodiment of the present invention. It is a mimetic diagram of the particle concerning one embodiment of the present invention. It is a mimetic diagram of the particle concerning one embodiment of the present invention.

  Hereinafter, an active material and a lithium ion secondary battery according to an embodiment of the present invention will be described. In addition, this invention is not limited to the following embodiment.

(Active material)
An active material according to an embodiment of the present invention has a layered structure, a compound having a composition represented by the following formula (1), and columnar Li and V having an average aspect ratio of 16 or more and 62 or less. The composite oxide is mixed.
_{Li y Ni a Co b Mn c} M d O x ··· (1)
[In the above formula (1), the element M is at least one element selected from the group consisting of Al, Si, Zr, Ti, Fe, Mg, Nb, Ba and V, and 1.9 ≦ (a + b + c + d + y) ≦ 2.1, 1.0 <y ≦ 1.3, 0 <a ≦ 0.3, 0 <b ≦ 0.25, 0.3 ≦ c ≦ 0.7, 0 ≦ d ≦ 0.1, 9 ≦ x ≦ 2.1. ]

  Here, the columnar Li and V composite oxides are particles having shapes as shown in FIGS. 4A to 4C, for example. As shown in FIG. 4A, the composite oxide a has a substantially constant width from end to end, or as shown in FIG. 4B, one end of the particle is thin and the end is thick needle-like composite oxide b. This is a dendritic complex oxide c in which a plurality of columnar or needle-like particles are bound. Here, as an example of a columnar shape, particles that are on a straight line are illustrated, but the particles also include particles that are bent in the middle.

These aspect ratios can be obtained from image analysis by observing a cross section of the active material with a scanning electron microscope (SEM). The aspect ratio is calculated by the length L of the composite oxide particles / the width W of the particles. Here, the length of the particle refers to the longest distance portion of the particle when the particle is observed with an SEM, and the width of the particle refers to the shortest distance portion of the particle when observed with the SEM.
In FIG. 4A, the particle length of the composite oxide a is L1, and the particle width is W1. In FIG. 4B, the particle length of the composite oxide b is L2, and the particle width is W2. W2 refers to the maximum width portion of the particle. The particle length and width are excluded because there are particles having the shape of FIG. 4C, but the aspect ratio cannot be calculated. Although not shown, when the columnar composite of Li and V is bent, the length L of the particle indicates a linear distance from one end of the particle to the other end.

  The aspect ratio is obtained by randomly obtaining the aspect ratio of 30 or more composite oxide particles for one active material, and similarly calculating the average value of the aspect ratio of 10 or more active materials. That is, it is an average value of at least 300 composite oxide particles.

  By mixing the complex oxides of Li and V in the above form, rate characteristics are improved. This is presumably because the Li and V composite oxide having a columnar three-dimensional form has an action of assisting a conductive path between the active materials.

  In addition, when the average aspect ratio is less than 16 or the composite oxide is in a state of thinly covering the compound represented by the formula (1), an effect of assisting the conductive path between the active materials is exhibited. It is thought not to. Further, if the average aspect ratio is larger than 62, the density of the active material in the electrode is lowered, and the battery capacity per volume is considered to be reduced.

The average value of the aspect ratio is more preferably 16 or more and 42 or less.
If the average value of the aspect ratio is less than 16, the effect of improving the rate characteristics is small, and if the average value of the aspect ratio is within 42, the influence of the decrease in electrode density tends to be small.

Further, the average aspect ratio is more preferably 16 or more and 28 or less. If the average value of the aspect ratio is within 28, it is considered that the influence of the electrode density reduction is very small.

Further, the columnar Li and V composite oxide is preferably present on the surface of the compound. This further improves the rate characteristics.
Furthermore, it is considered that the presence of a Li and V composite oxide on the surface of the compound and in the vicinity of the surface suppresses the reaction between the active material and the electrolytic solution and improves the initial charge / discharge efficiency.

The composite oxides of Li and V are Li ₃ VO ₄ and LiV ₂ O _{5. By} mixing these and the compound represented by the formula (1), the initial charge / discharge efficiency is high and the rate characteristics are excellent. An active material is obtained.
In addition to the above, Li and V composite oxides may include LiVO ₂ , LiVO ₃ , LiV ₂ O ₄ and other composite oxides.

(Method for producing active material)
By mixing the compound and V ₂ O ₅ and heating at 300 ° C. or more and 900 ° C. or less, Li and V react to produce Li and V composite oxide, and the compound and Li and V composite oxidation An active material in which things are mixed is obtained. By using this active material as the positive electrode active material, a lithium ion secondary battery having high initial charge / discharge efficiency and excellent rate characteristics can be obtained.

When the heat treatment temperature is less than 300 ° C., unreacted V ₂ O ₅ remains, and when the heat treatment temperature is higher than 900 ° C., the active material grows and battery characteristics deteriorate.

Further, V ₂ O ₅ and Li ₂ CO ₃ or LiNO ₃ are mixed in advance and heat-treated at 200 ° C. to 1000 ° C. to produce a Li and V composite compound, and the Li and V composite oxide and the compound May be mixed and heat-treated at 300 ° C. or higher and 900 ° C. or lower.

Further, a positive electrode active material coated with a composite compound of Li and V by mixing a positive electrode active material with a mixture of V ₂ O ₅ and Li ₂ CO ₃ or LiNO ₃ in advance and heat-treating at 400 ° C. to 900 ° C. or lower. May be produced.

  The mixing method is not particularly limited, and a ball mill, a bead mill, or the like is used.

  The composite oxide of Li and V is preferably present on the surface of the compound particle. Presence of a complex oxide of Li and V on the surface and in the vicinity of the surface is considered to suppress the reaction with the electrolytic solution, the irreversible capacity decreases, and the initial charge / discharge efficiency increases.

(Lithium ion secondary battery)
As shown in FIG. 1, a lithium ion secondary battery 100 according to the present embodiment is disposed adjacent to each other between a plate-like negative electrode 20 and a plate-like positive electrode 10 facing each other, and the negative electrode 20 and the positive electrode 10. A plate-shaped separator 18, an electrolyte containing lithium ions, a case 50 for containing these in a sealed state, and one end electrically connected to the negative electrode 20 and the other The negative electrode lead 62 whose one end protrudes outside the case, and the positive electrode lead 60 whose one end is electrically connected to the positive electrode 10 and whose other end protrudes outside the case.

  The negative electrode 20 includes a negative electrode current collector 22 and a negative electrode active material layer 24 formed on the negative electrode current collector 22. The positive electrode 10 includes a positive electrode current collector 12 and a positive electrode active material layer 14 formed on the positive electrode current collector 12. The separator 18 is located between the negative electrode active material layer 24 and the positive electrode active material layer 14.

The positive electrode active material contained in the positive electrode active material layer 14 has a layered structure, a compound having a composition represented by the following formula (1), and columnar Li and V having an average aspect ratio of 16 or more and 62 or less. A mixed oxide of the above is used.
_{Li y Ni a Co b Mn c} M d O z ··· (1)
[In the above formula (1), the element M is at least one element selected from the group consisting of Al, Si, Zr, Ti, Fe, Mg, Nb, Ba and V, and 1.9 ≦ (a + b + c + d + y) ≦ 2.1, 1.0 <y ≦ 1.3, 0 <a ≦ 0.3, 0 <b ≦ 0.25, 0.3 ≦ c ≦ 0.7, 0 ≦ d ≦ 0.1, 9 ≦ x ≦ 2.1. ]

As the negative electrode active material used for the negative electrode of the lithium ion secondary battery, any material that can deposit or occlude lithium ions may be selected. For example, titanium-based materials such as lithium titanate having a spinel crystal structure typified by Li [Li _1/3 Ti _5/3 ] O ₄ , alloy-based materials such as Si, Sb, and Sn-based lithium metal, lithium alloys (Lithium metal-containing alloys such as lithium-silicon, lithium-aluminum, lithium-lead, lithium-tin, lithium-aluminum-tin, lithium-gallium, and wood alloy), lithium composite oxide (lithium-titanium), silicon oxide In addition, an alloy capable of inserting and extracting lithium, a carbon material (for example, graphite, hard carbon, low-temperature fired carbon, amorphous carbon, etc.) can be used.

  The positive electrode active material layer 14 and the negative electrode active material layer 24 may contain a conductive additive, a binder, a thickener, a filler, and the like as other components in addition to the above-described active material.

  The conductive auxiliary agent is not limited as long as it is an electron conductive material that does not adversely affect the battery performance. Usually, natural graphite (scale-like graphite, scale-like graphite, earth-like graphite, etc.), artificial graphite, carbon black, acetylene black , Ketjen black, carbon whisker, carbon fiber, metal (copper, nickel, aluminum, silver, gold, etc.) powder, metal fiber, conductive material such as conductive ceramic material. These conductive assistants may be used alone or a mixture thereof may be used.

  In particular, acetylene black is preferable as the conductive assistant from the viewpoints of electron conductivity and coatability. The addition amount of the conductive assistant is preferably 0.1% by weight to 50% by weight, and more preferably 0.5% by weight to 30% by weight with respect to the total weight of the positive electrode active material layer or the negative electrode active material layer. In particular, acetylene black is preferably used after being pulverized into ultrafine particles of 0.1 to 0.5 μm because the required carbon amount can be reduced. These mixing methods are 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.

  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 fluororubber can be used as one kind or a mixture of two or more kinds. The amount of the binder added is preferably 1 to 50% by weight and more preferably 2 to 30% by weight with respect to the total weight of the positive electrode active material layer or the negative electrode active material layer.

  As the thickener, polysaccharides such as carboxymethylcellulose and methylcellulose can be used as one kind or a mixture of two or more kinds.

  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 weight or less with respect to the total weight of the positive electrode active material layer or the negative electrode active material layer.

  The positive electrode active material layer 14 or the negative electrode active material layer 24 was obtained after kneading main constituent components and other materials into a mixture and mixing them in an organic solvent such as N-methyl-2-pyrrolidone and toluene. The liquid mixture is preferably applied on the current collector or pressure-bonded and heat-treated at a temperature of about 50 ° C. to 250 ° C. for about 2 hours. As for the application method, for example, it is preferable 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. Is not to be done.

  As the positive electrode current collector 12 and the negative electrode current collector 22, iron, copper, stainless steel, nickel, and aluminum can be used. Moreover, a sheet | seat, a foam, a mesh, a porous body, an expanded lattice, etc. can be used as the shape. Further, the current collector can be used with a hole formed in an arbitrary shape.

  As the electrolyte, those generally proposed for use in lithium batteries and the like can be used. Examples of the solvent used for the electrolyte include propylene carbonate, ethylene carbonate, butylene carbonate, chloroethylene carbonate, vinylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

  Furthermore, the electrolyte can be used in combination with a solid electrolyte.

Examples of the electrolyte salt used for the electrolyte include LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiPF _{6, and the} like. These ionic compounds can be used alone or in admixture of two or more.

  A normal temperature molten salt or an ionic liquid may be used as the electrolyte.

  As a separator for a lithium ion secondary battery, it is preferable to use a porous film or a nonwoven fabric exhibiting excellent high rate discharge performance 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.

  As mentioned above, although one suitable embodiment of the present invention was described in detail, the present invention is not limited to the above-mentioned embodiment.

  For example, the shape of the lithium ion secondary battery is not limited to that shown in FIG. For example, the shape of the lithium ion secondary battery may be a square, an ellipse, a coin, a button, a sheet, or the like.

  The active material of this embodiment can also be used as an electrode material for electrochemical devices other than lithium ion secondary batteries. As such an electrochemical element, other than lithium ion secondary batteries such as a metal lithium secondary battery (an electrode containing an active material obtained by the present invention is used as a positive electrode and metal lithium is used as a negative electrode). Examples include secondary batteries and electrochemical capacitors such as lithium capacitors. These electrochemical elements can be used for power sources such as self-propelled micromachines and IC cards, and distributed power sources arranged on or in a printed circuit board.

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

(Example 1)
[Production of compounds]
Distillation of lithium nitrate, cobalt nitrate hexahydrate, nickel nitrate hexahydrate, manganese nitrate hexahydrate adjusted to produce Li _1.2 Ni _0.17 Co _0.08 Mn _0.55 O ₂ A powder (precursor) mixed in a solution of glucose, nitric acid and polyvinyl alcohol mixed with water and evaporated to dryness is pulverized in a mortar for about 10 minutes, and then calcined in the atmosphere at 900 ° C. for 10 hours to obtain a compound (lithium Compound) was obtained.

As a result of composition analysis by inductively coupled plasma method (ICP method), it was confirmed that the composition of the lithium compound (positive electrode active material) was Li _1.2 Ni _0.17 Co _0.08 Mn _0.55 O ₂ . .

[Production of active material]
The compound and V ₂ O ₅ were mixed at a weight ratio of 90:10 for about 10 minutes using a mortar, and then baked in the atmosphere at 400 ° C. for 3 hours. A mixed active material was obtained. The active material of Example 1 was analyzed by a powder X-ray diffraction method. The coating of Example 1 was Li ₃ VO ₄ .

Moreover, V element was mapped to the active material of Example 1 by STEM-EDS. The result is shown in FIG. From FIG. 2, it was found that the V element coats the surface of the compound. Its thickness is about 100 nm. It was also confirmed that the inside of the compound, that is, the surface of the primary particles was coated.
Moreover, the surface of the active material of Example 1 was observed by SEM. The observation results are shown in FIG. It was confirmed that columnar particles were present on the particle surface as shown in FIG.
The aspect ratio was determined from the SEM image. The aspect ratio of Example 1 was 2.5 as the minimum value and 38 as the maximum value. The average aspect ratio was 16. The method for calculating the average aspect ratio is shown below. In one active material particle, 30 composite oxide particles were selected at random and the aspect ratio was determined. Similarly, the aspect ratio of 20 active material particles was determined, and the average value thereof was determined.

[Production of positive electrode]
A positive electrode paint was prepared by mixing the active material of Example 1, a conductive additive, and a solvent containing a binder. The positive electrode coating material was applied to an aluminum foil (thickness 20 μm) as a positive electrode current collector by a doctor blade method, dried at 100 ° C., and rolled. This obtained the positive electrode comprised from the layer of a lithium compound (positive electrode active material) and a positive electrode electrical power collector. As the conductive assistant, carbon black (manufactured by Denki Kagaku Kogyo Co., Ltd., DAB50) and graphite (manufactured by Timcal Co., Ltd., KS-6) were used. As a solvent containing a binder, N-methyl-2-pyrrolidinone (KF7305, manufactured by Kureha Chemical Industry Co., Ltd.) in which PVDF was dissolved was used.

[Production of negative electrode]
A negative electrode paint was prepared in the same manner as the positive electrode paint except that natural graphite was used in place of the active material of Example 1 and only carbon black was used as the conductive additive. The negative electrode coating material was applied to a copper foil (thickness 16 μm) as a negative electrode current collector by a doctor blade method, dried at 100 ° C., and rolled. This obtained the negative electrode comprised from a negative electrode active material layer and a negative electrode electrical power collector.

[Production of lithium ion secondary battery]
The positive electrode, negative electrode, and separator (polyolefin microporous membrane) produced above were cut into predetermined dimensions. The positive electrode and the negative electrode were provided with portions to which no electrode paint was applied in order to weld the external lead terminals. A positive electrode, a negative electrode, and a separator were laminated in this order. When laminating, a small amount of hot melt adhesive (ethylene-methacrylic acid copolymer, EMAA) was applied and fixed so that the positive electrode, the negative electrode, and the separator did not shift. An aluminum foil (width 4 mm, length 40 mm, thickness 100 μm) and nickel foil (width 4 mm, length 40 mm, thickness 100 μm) were ultrasonically welded to the positive electrode and the negative electrode, respectively, as external lead terminals. Polypropylene (PP) grafted with maleic anhydride was wrapped around this external lead terminal and thermally bonded. This is to improve the sealing performance between the external terminal and the exterior body. An aluminum laminate material composed of a PET layer, an Al layer, and a PP layer was used as a battery outer package enclosing a battery element in which a positive electrode, a negative electrode, and a separator were stacked. The thickness of the PET layer was 12 μm. The thickness of the Al layer was 40 μm. The thickness of the PP layer was 50 μm. PET is polyethylene terephthalate and PP is polypropylene. In producing the battery outer package, the PP layer was disposed inside the outer package. A battery element was placed in the outer package, an appropriate amount of electrolyte was added, and the outer package was vacuum-sealed. Thereby, a lithium ion secondary battery using the active material of Example 1 was produced. As the electrolyte solution, were used as the LiPF ₆ in a mixed solvent of ethylene carbon sulfonate (EC) and dimethyl carbonate (DMC) was dissolved at a concentration 1M (1mol / L). The volume ratio of EC to DMC in the mixed solvent was EC: DMC = 30: 70.

[Measurement of electrical characteristics]
The battery of Example 1 was charged at a constant current of up to 4.6 V at a current value of 30 mA / g, and then discharged at a constant current of up to 2.0 V at a current value of 30 mA / g. The initial charge / discharge efficiency of Example 1 was 97.7%. The initial charge / discharge efficiency is the ratio of the discharge capacity at 0.1 C and the charge capacity at 0.1 C in the first cycle. The rate characteristic (ratio of 1 C discharge capacity to 0.1 C discharge capacity) was 92.8%.

(Examples 2-6, Comparative Examples 1-4)
Examples 2 to 6 were produced in the same manner as in Example 1 except that the heat treatment temperature after mixing the compound and V ₂ O ₅ was changed. Comparative Examples 1 to 4 were performed in the same manner as in Example 1 except that the material mixed with the compound was changed.

In Example 2, the compound and V ₂ O ₅ were mixed at a weight ratio of 90:10 for about 10 minutes using a mortar, and then baked in the atmosphere at 750 ° C. for 5 hours, thereby combining Li and V. An active material coated with an oxide was obtained. The active material coated with the Li and V composite oxide of Example 2 was analyzed by powder X-ray diffraction. The coating of Example 2 seems to be a mixture of Li ₃ VO ₄ and LiV ₂ O ₅ .
The aspect ratio was determined from the SEM image. The aspect ratio was a minimum value of 2.1 and a maximum value of 73. The average value was 42.

In Example 3, the compound and V ₂ O ₅ were mixed at a ratio of 90:10 in the same manner as in Example 1, then heated to 370 ° C. and held for 10 hours. The sample of Example 3 was analyzed by powder X-ray diffraction method. V ₂ O ₅ and Li ₃ VO ₄ .
The aspect ratio was a minimum value of 3.2 and a maximum value of 74. The average value was 62.

In Example 4, the compound and V ₂ O ₅ were mixed at a ratio of 92: 8 in the same manner as in Example 1, then heated to 400 ° C. and held for 5 hours. The sample of Example 4 was analyzed by powder X-ray diffraction method. Li ₃ VO ₄ .
The aspect ratio was a minimum value of 2.2 and a maximum value of 45. The average value was 27.

In Comparative Example 1, the compound and ZrO ₂ were prepared in the same manner as in Example 1 at a weight ratio of 90:10. The sample of Comparative Example 1 was analyzed by powder X-ray diffraction method. It remained ZrO ₂ without change.
The aspect ratio was a minimum value of 1 and a maximum value of 1.3. The average value was 1.1.

In Comparative Example 2, the compound and SnO ₂ were prepared in the same manner as in Example 1 at a weight ratio of 90:10. The sample of Comparative Example 2 was analyzed by powder X-ray diffraction method. It remained SnO ₂ without change.
The aspect ratio was a minimum value of 1 and a maximum value of 1.2. The average value was 1.1.

In Comparative Example 3, the compound and V ₂ O ₅ were mixed at a weight ratio of 99: 1 by the same method as in Example 1, heated to 350 ° C. in the heat treatment, and cooled without being held. The sample of Comparative Example 3 was analyzed by a powder X-ray diffraction method. V ₂ O ₅ and Li ₃ VO ₄ .
The aspect ratio was a minimum value of 1 and a maximum value of 1.5. The average value was 1.2.

In Example 5, after mixing a compound of Li _1.2 Ni _0.17 Co _0.07 Mn _0.56 O ₂ and V ₂ O ₅ in a weight ratio of 90:10 in the same manner as in Example 1. The temperature was raised to 400 ° C. and held for 3 hours. The sample of Example 5 was analyzed by powder X-ray diffraction method. Li ₃ VO ₄ .
The aspect ratio was a minimum value of 2.3 and a maximum value of 42. The average value was 22.

In Example 6, after mixing the compound of Li _1.2 Ni _0.1 Co _0.14 Mn _0.56 O ₂ and V ₂ O ₅ in a weight ratio of 92: 8 in the same manner as in Example 1. The temperature was raised to 400 ° C. and held for 5 hours. The sample of Example 6 was analyzed by powder X-ray diffraction method. Li ₃ VO ₄ .
The aspect ratio was a minimum value of 2.4 and a maximum value of 48. The average value was 28.

In Comparative Example 4, a compound of Li _1.2 Co _0.3 Mn _0.5 O ₂ and V ₂ O ₅ were mixed at a weight ratio of 98: 2 in the same manner as in Example 1, and then up to 400 ° C. The temperature was raised and held for 5 hours. The sample of Comparative Example 5 was analyzed by a powder X-ray diffraction method. Li ₃ VO ₄ .
The aspect ratio was a minimum value of 2.5 and a maximum value of 45. The average value was 24.

In the same manner as in Example 1, the initial charge / discharge efficiency and rate characteristics of the samples prepared in Examples 2-6 and Comparative Examples 1-4 were determined. The results are shown in Table 1.
As in Comparative Examples 1 to 3, a metal oxide having a mean aspect ratio of less than 16 and a composite oxide were mixed in Li _1.2 Ni _0.17 Co _0.08 Mn _0.55 O ₂ , respectively. Although the sample used as the active material was evaluated, neither initial charge / discharge efficiency nor rate characteristics were improved.
On the other hand, the first charge / discharge efficiency was improved when a compound oxide and a compound of Li and V having an average aspect ratio of 16 or more and 62 or less were mixed as in Examples 1 to 4.
The above effects are also obtained for the compounds as in Examples 5-6. In Comparative Example 4, the initial charge / discharge efficiency was improved, but the rate characteristics were not improved much. If the amount of the transition metal in the compound is too small, the rate characteristic improving effect may be reduced.

DESCRIPTION OF SYMBOLS 10 ... Positive electrode, 20 ... Negative electrode, 12 ... Positive electrode collector, 14 ... Positive electrode active material layer, 18 ... Separator, 22 ... Negative electrode collector, 24 ... Negative electrode Active material layer, 30 ... power generation element, 50 ... case, 60, 62 ... lead, 100 ... lithium ion secondary battery, L1 ... length of columnar particles, W1 ... columnar Width of particle, L2 ... length of needle-like particle, W2 ... width of needle-like particle

Claims (4)

A compound having a layered structure and having a composition represented by the following formula (1):
An active material, wherein a columnar Li and V composite oxide having an average aspect ratio of 16 to 62 is mixed.
_{Li y Ni a Co b Mn c} M d O x ··· (1)
[In the above formula (1), the element M is at least one element selected from the group consisting of Al, Si, Zr, Ti, Fe, Mg, Nb, Ba and V, and 1.9 ≦ (a + b + c + d + y) ≦ 2.1, 1.0 <y ≦ 1.3, 0 <a ≦ 0.3, 0 <b ≦ 0.25, 0.3 ≦ c ≦ 0.7, 0 ≦ d ≦ 0.1, 9 ≦ x ≦ 2.1. ]   The active material according to claim 1, wherein the complex oxide of Li and V is present on the surface of a compound having a composition represented by the formula (1). The active material according to claim 1, wherein the complex oxide of Li and V is Li ₃ VO ₄ or LiV ₂ O ₅ . A positive electrode having a positive electrode current collector and a positive electrode active material layer containing a positive electrode active material;
A negative electrode having a negative electrode current collector and a negative electrode active material layer containing a negative electrode active material;
A separator positioned between the positive electrode active material layer and the negative electrode active material layer;
An electrolyte in contact with the negative electrode, the positive electrode, and the separator,
The positive electrode active material includes the active material according to claim 1,
Lithium ion secondary battery.