JP2011049102A - Active material, electrode containing the same, lithium secondary battery provided therewith and method for manufacture of the active material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an active material capable of producing a large discharge capacity with a high rate characteristic. <P>SOLUTION: A method for manufacturing the active material includes: a hydrothermal synthesis step of heating under pressure a mixture that contains a lithium source, a vanadium source, a phosphoric acid source, water, and an ascorbic acid, the ratio of the mole number of lithium atoms to the mole number of vanadium atoms and the ratio of the mole number of phosphorus atoms to the mole number of vanadium atoms are 0.95-1.2, and the ratio of the mole number of the ascorbic acid to the mole number of the vanadium atoms is 0.05-0.6; and a firing step of heating a material obtained at the hydrothermal synthesis step to obtain LiVOPO<SB>4</SB>having a β-type crystal structure. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an active material, an electrode including the active material, a lithium secondary battery including the electrode, and an active material manufacturing method.

In the crystal represented by the structural formula LiVOPO ₄ , it is known that Li is reversibly inserted and released. In Patent Document 1, β-type crystal structure (orthorhombic) LiVOPO ₄ and α-type crystal structure (triclinic) LiVOPO ₄ are prepared by a solid phase method, and these are used as the electrode activity of a non-aqueous electrolyte secondary battery. It is disclosed to be used as a substance. Then, the non-aqueous discharge capacity of electrolyte secondary battery, compared with the LiVOPO ₄ of α-type crystal structure (triclinic), towards the LiVOPO ₄ of β-type crystal structure is large is described.

Non-Patent Document 1 discloses a method for producing LiVOPO ₄ having a β-type crystal structure by heating VOPO ₄ and Li ₂ CO ₃ in the presence of carbon and reducing Li ₂ CO ₃ with carbon (carbothermal reduction). Law (CTR Law)) is disclosed.

JP 2004-303527 A

J. et al. Baker et al. , J .; Electrochem. Soc. 151, A796 (2004)

However, the active material containing LiVOPO ₄ having a β-type crystal structure obtained by the methods described in Patent Document 1 and Non-Patent Document 1 has high rate characteristics and does not provide a large discharge capacity.

  Therefore, an object of the present invention is to provide an active material having high rate characteristics and a large discharge capacity.

As a result of intensive studies to achieve the above object, the inventors of the present invention include a lithium source, a vanadium source, a phosphate source, water, and ascorbic acid, and the amount of lithium atoms relative to the number of moles of vanadium atoms. The ratio of the number of moles, and the ratio of the number of moles of phosphorus atoms to the number of moles of vanadium atoms is 0.95 to 1.2, and the ratio of the number of moles of ascorbic acid to the number of moles of vanadium atoms is 0.05 to 0.6. The mixture is heated under pressure, and the material heated under pressure is fired, so that the average primary particle diameter is extremely small and the shape of the secondary particles approximates a sphere, and β-type It has been found that LiVOPO ₄ having a high crystal structure ratio can be obtained, and the present invention has been completed.

That is, the method for producing an active material of the present invention includes a lithium source, a vanadium source, a phosphoric acid source, water, and ascorbic acid, the ratio of the number of moles of lithium atoms to the number of moles of vanadium atoms, and vanadium. Water in which a mixture in which the ratio of the number of moles of phosphorus atoms to the number of moles of atoms is 0.95 to 1.2 and the ratio of the number of moles of ascorbic acid to vanadium atoms is 0.05 to 0.6 is heated under pressure. A thermal synthesis step, and a firing step of heating the material obtained in the hydrothermal synthesis step under pressure to obtain LiVOPO ₄ having a β-type crystal structure.

The active material obtained by the production method according to the present invention has an aggregate structure in which the average primary particle size is small and the shape of secondary particles is very close to a sphere, and the ratio of LiVOPO _{4 having} a β-type crystal structure is high. A lithium ion secondary battery using such an active material has high rate characteristics and a large discharge capacity. The reason for this is not clear, but the active material obtained by the production method according to the present invention has a large discharge capacity and a large discharge capacity due to the main component of LiVOPO ₄ having a β-type crystal structure. The particle size is very small and the shape of the secondary particles has an agglomerated structure that is very close to a sphere, so that Li ions easily diffuse isotropically, and even when the discharge current density is high, a large discharge capacity. It is presumed that it can be obtained.

The active material according to the present invention has an aggregate structure in which the average primary particle diameter is 100 to 350 nm, and the ratio of the length of the minor axis to the length of the major axis of the secondary particle is 0.80 to 1. And LiVOPO ₄ having a β-type crystal structure as a main component.

It contains β-type crystal structure LiVOPO ₄ as a main component, the average primary particle diameter of the active material is within the above range, and the ratio of the length of the minor axis to the length of the major axis of the secondary particle is By having a value within the above range, that is, a shape approximating a sphere, a high discharge characteristic and a large discharge capacity can be obtained. Such an active material is easily manufactured by the above-described method.

  Here, the active material according to the present invention preferably has an average secondary particle diameter of 1500 nm to 8000 nm. When the average secondary particle diameter of the active material is a value within the above range, it is easy to obtain a large discharge capacity with high rate characteristics.

  Moreover, it is preferable that the electrode which concerns on this invention is equipped with a collector and the active material layer provided on the collector containing the said active material. Thereby, an electrode having high rate characteristics and a large discharge capacity can be obtained.

  Moreover, it is preferable that the lithium secondary battery which concerns on this invention is equipped with the said electrode. Thereby, it is easy to obtain a lithium ion secondary battery having high rate characteristics and a large discharge capacity.

  According to the present invention, it is possible to provide an active material having high rate characteristics and a large discharge capacity.

FIG. 1 is a schematic cross-sectional view of an active material according to this embodiment. FIG. 2 is a schematic cross-sectional view of a lithium ion secondary battery including an active material layer containing an active material according to the present embodiment. FIG. 3 is an electron micrograph of the active material obtained in Example 1 when the set magnification at the time of observation is 30000 times. FIG. 4 is an electron micrograph of the active material obtained in Example 1 when the set magnification during observation is 5000 times.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In addition, the dimension ratio of each drawing does not necessarily correspond with an actual dimension ratio.

<Method for producing active material>
A preferred embodiment of a method for producing an active material according to the present invention will be described.
[Hydrothermal synthesis process]
The hydrothermal synthesis process according to the present embodiment includes a lithium source, a vanadium source, a phosphoric acid source, water, and ascorbic acid, the ratio of the number of moles of lithium atoms to the number of moles of vanadium atoms, and vanadium. A mixture having a ratio of the number of moles of phosphorus atoms to the number of moles of atoms of 0.95 to 1.2, and a ratio of moles of ascorbic acid to the number of moles of vanadium atoms is heated under pressure. It is a process to do.

(blend)
Examples of the lithium source include lithium compounds such as LiNO ₃ , Li ₂ CO ₃ , LiOH, LiCl, Li ₂ SO _4, and CH ₃ COOLi. Among these, LiNO ₃ and Li ₂ CO ₃ are preferable.
Examples of the vanadium source include vanadium compounds such as V ₂ O ₅ and NH ₄ VO ₃ .
Examples of the phosphoric acid source include PO ₄ -containing compounds such as H ₃ PO ₄ , NH ₄ H ₂ PO ₄ , (NH ₄ ) ₂ HPO _4, and Li ₃ PO ₄ . Among these, H ₃ PO ₄ and (NH ₄ ) ₂ HPO ₄ are preferable.

  In the lithium source, phosphate source, and vanadium source, the ratio of the number of moles of lithium atoms to the number of moles of vanadium atoms is 0.95 to 1.2, and the ratio of the number of moles of phosphorus atoms to the number of moles of vanadium atoms is 0.00. It mix | blends so that it may become 95-1.2. When the blending ratio of at least one of lithium atoms and phosphorus atoms is less than 0.95, the discharge capacity of the obtained active material tends to decrease, and the rate characteristics tend to decrease. When the blending ratio of at least one of lithium atoms and phosphorus atoms is more than 1.2, the discharge capacity of the obtained active material tends to decrease.

Ascorbic acid is blended so that the ratio of the number of moles of ascorbic acid to the number of moles of vanadium atoms is 0.05 to 0.6. By incorporating ascorbic acid, an active material mainly containing LiVOPO ₄ having a β-type crystal structure can be obtained, and the average primary particle size and the average secondary particle size tend to be reduced. When ascorbic acid is blended at a ratio of 0.05 to 0.6 with respect to the number of moles of vanadium atoms, the shape of the active material becomes very close to a sphere, with high rate characteristics and a large discharge capacity. Can do. Such knowledge has not been obtained so far, and such an effect is a remarkable effect as compared with the prior art.

  By the way, when an active material-containing layer of an electrode is produced using the obtained active material, a conductive material such as a carbon material is usually brought into contact with the surface of the active material in order to increase conductivity. As this method, the active material and the conductive material may be mixed after the production of the active material to form the active material-containing layer. For example, the carbon material is used as the conductive material in the mixture as a raw material for hydrothermal synthesis. It can also be added to cause carbon to adhere to the active material.

  Examples of the conductive material in the case where a conductive material that is a carbon material is added to the mixture include activated carbon, graphite, soft carbon, and hard carbon. Among these, it is preferable to use activated carbon that can easily disperse carbon particles in a mixture during hydrothermal synthesis. However, the conductive material is not necessarily mixed in the mixture at the time of hydrothermal synthesis, and it is preferable that at least a part of the conductive material is mixed in the mixture at the time of hydrothermal synthesis. Thereby, the binder at the time of forming an active material content layer can be reduced, and a capacity density can be increased.

  The content of the conductive material such as carbon particles in the mixture in the hydrothermal synthesis step is the ratio C2 / mol of the number of moles C2 of carbon atoms constituting the carbon particles and the number of moles M of vanadium atoms contained in the vanadium compound, for example. It is preferable to prepare such that M satisfies 0.04 ≦ C2 / M ≦ 4. When there is too little content (molar number C2) of an electrically conductive material, there exists a tendency for the electronic conductivity and capacity density of the electrode active material comprised with an active material and an electrically conductive material to fall. When there is too much content of a electrically conductive material, the weight of the active material which occupies for an electrode active material will reduce relatively, and there exists a tendency for the capacity density of an electrode active material to reduce. By setting the content of the conductive material within the above range, these tendencies can be suppressed.

  The amount of water in the mixture is not particularly limited as long as hydrothermal synthesis is possible, but the ratio of substances other than water in the mixture is preferably 35% by mass or less.

The order in which the raw materials are charged when adjusting the mixture is not particularly limited. For example, the raw materials of the above mixture may be mixed together, and the vanadium compound is first added to the mixture of water and PO ₄ -containing compound, then ascorbic acid is added, and then the lithium compound is added. May be added. It is preferable that the mixture is sufficiently mixed and the additive components are sufficiently dispersed.

In the hydrothermal synthesis step, first, the above-mentioned mixture (lithium compound, vanadium compound, PO ₄ -containing compound, water, ascorbic acid, etc.) is placed in a reaction vessel (for example, an autoclave) having a function of heating and pressurizing the inside. throw into. In addition, you may adjust a mixture within reaction container.

Next, the reaction vessel is sealed, and the mixture is heated while being pressurized, thereby causing the hydrothermal reaction of the mixture to proceed. Thereby, the substance containing the precursor of LiVOPO _{4 having} a β-type crystal structure is hydrothermally synthesized.

A substance containing a precursor of LiVOPO ₄ having a β-type crystal structure obtained by hydrothermal synthesis usually precipitates as a solid in the liquid after hydrothermal synthesis. The precursor of LiVOPO _{4 having} a β-type crystal structure contained in this substance is considered to be in a hydrated state. Then, the liquid after hydrothermal synthesis is filtered, for example, to collect solids, and the collected solids are washed with water, acetone or the like and then dried to obtain this precursor with high purity. it can.

In the hydrothermal synthesis step, the pressure applied to the mixture is preferably 0.1 to 30 MPa. When the pressure applied to the mixture is too low, the crystallinity of LiVOPO ₄ having a β-type crystal structure finally obtained tends to decrease, and the volume density of the active material tends to decrease. If the pressure applied to the mixture is too high, the reaction vessel is required to have high pressure resistance, and the active material production cost tends to increase. By setting the pressure applied to the mixture within the above range, these tendencies can be suppressed.

The temperature of the mixture in the hydrothermal synthesis step is preferably 200 to 300 ° C., and more preferably 210 to 250 ° C. from the viewpoint of improving the discharge capacity and rate characteristics of the obtained active material. When the temperature of the mixture is too low, the crystallinity of LiVOPO ₄ having a β-type crystal structure finally obtained tends to decrease, and the capacity density of the active material tends to decrease. When the temperature of the mixture is too high, the reaction vessel is required to have high heat resistance, and the production cost of the active material tends to increase. By setting the temperature of the mixture within the above range, these tendencies can be suppressed.

[Baking process]
The firing step according to this embodiment is a step in which a material obtained by hydrothermal synthesis, that is, a precursor of LiVOPO ₄ having a β-type crystal structure is heated to obtain LiVOPO ₄ having a β-type crystal structure. In this step, it is considered that the impurities remaining in the precursor are removed, and the precursor of LiVOPO _{4 having} a β-type crystal structure is dehydrated to cause crystallization.

Here, in the firing step, it is preferable to heat the above-described precursor to 400 ° C to 600 ° C. If the heating temperature is too low, the crystallinity of the finally obtained β-type crystal structure LiVOPO ₄ tends to decrease, and the capacity density of the active material tends to decrease. On the other hand, if the heating temperature is too high, grain growth of the active material proceeds and the particle size (primary particle size and / or secondary particle size) increases, resulting in slow diffusion of lithium in the active material, and capacity density of the active material. Tend to decrease. By setting the heating temperature within the above range, these tendencies can be suppressed. The heating time is not particularly limited, but is preferably 3 to 6 hours.

  Although the atmosphere of a baking process is not specifically limited, In order to make removal of ascorbic acid easy, it is preferable that it is an air atmosphere. On the other hand, it can also be performed in an inert atmosphere such as argon gas or nitrogen gas.

According to the method for producing an active material including the hydrothermal synthesis step and the firing step described above, the ratio of the number of moles of lithium atoms to the number of moles of vanadium atoms and the ratio of moles of phosphorus atoms to the number of moles of vanadium atoms are The mixture obtained by heating under pressure a mixture of 0.95 to 1.2 and having a molar ratio of ascorbic acid to vanadium atoms of 0.05 to 0.6 is fired. Thus, LiVOPO ₄ having an agglomerated structure in which the average primary particle diameter is extremely small and the shape of the secondary particles approximates a sphere and the ratio of the β-type crystal structure is high can be obtained. And the lithium ion secondary battery using such an active material can obtain a high discharge capacity with a high rate characteristic.

<Active material>
Next, the active material according to the present embodiment will be described. FIG. 1 is a schematic cross-sectional view of an active material 2 according to this embodiment. The active material 2 of the present embodiment is one in which the primary particles 1 aggregate to form secondary particles.

  The active material 2 has an average primary particle size of 100 to 350 nm. Here, the “average primary particle diameter of the active material” defined in the present invention is a value of D50 in which the cumulative ratio is 50% in the number-based particle size distribution measured for the primary particles 1 of the active material 2. It is. The number-based particle size distribution of the primary particles 1 of the active material 2 is, for example, a projected area equivalent circle diameter is measured from the projected area of the primary particles 1 of the active material 2 based on an image observed with a high-resolution scanning electron microscope, It can be calculated from the cumulative rate. The projected area equivalent circle diameter assumes a sphere having the same projected area as the projected area of the particles (primary particles 1 of the active material 2), and the diameter of the sphere (equivalent circle diameter) is the particle diameter (active material 2). Particle diameter of primary particles). The “average secondary particle diameter of the active material” to be described later is the same as the average primary particle diameter described above with respect to the active material 2 that is an aggregated particle (corresponding to the secondary particles of the active material of the present invention). This is the value of D50 with a cumulative rate of 50% in the measured particle size distribution based on the number.

  The ratio of the length of the short axis to the length of the long axis of the active material 2 is 0.80-1. Here, “the length of the long axis of the active material” of the secondary particles defined in the present invention means the longest length in an image observed with a high-resolution scanning electron microscope. “Axis length” means the length of the vertical bisector of the major axis. When the ratio of the length of the short axis to the length of the long axis is 1, the shape of the active material is a sphere. When this ratio is 0.80 to 1, the shape of the secondary particles of the obtained active material is a sphere or a shape very close to a sphere. Among them, it is easy to produce a product having this ratio of 0.81 to 0.93.

The active material 2 contains LiVOPO ₄ having a β-type crystal structure as a main component. Here, “mainly composed of a β-type crystal structure of LiVOPO ₄ ” means that, in the active material 2, the β-type crystal structure of LiVOPO ₄ is replaced with a β-type crystal structure of LiVOPO ₄ and an α-type crystal structure of LiVOPO ₄ . It means that 80 mass% or more is included with respect to the sum total. Here, the amount of LiVOPO ₄ having a β-type crystal structure, LiVOPO _{4 having an} α-type crystal structure, or the like in the particle can be measured by, for example, an X-ray diffraction method. Usually, LiVOPO ₄ of β-type crystal structure peaks appear in 2 [Theta] = 27.0 degrees, LiVOPO ₄ of α-type crystal structure peaks appear in 2 [Theta] = 27.2 degrees. Note that the active material may contain a small amount of unreacted raw material components in addition to LiVOPO ₄ having a β-type crystal structure and LiVOPO _{4 having an} α-type crystal structure.

Such an active material is easily manufactured by the above manufacturing method. This active material has high rate characteristics and can provide a large discharge capacity. Although the reason for this is not clear, the discharge capacity is increased by using LiVOPO ₄ having a β-type crystal structure having a large discharge capacity as a main component, and the average primary particle diameter is very small, and the shape of the secondary particles is extremely spherical. This is presumed to be because Li ions easily diffuse isotropically by having an agglomerated structure close to, and a large discharge capacity can be obtained even when the discharge current density is high. As described above, since the active material 2 has an agglomerated structure, that is, a porous structure, the impregnation ability of the electrolytic solution is high.

  The average particle size (average secondary particle size) of the active material 2 is preferably 1500 nm to 8000 nm. Such an active material has high rate characteristics and easily obtains a large discharge capacity.

<Lithium ion secondary battery>
Next, a lithium ion secondary battery using the above active material as a positive electrode active material will be briefly described with reference to FIG.

  The lithium ion secondary battery 100 mainly includes a laminate 30, a case 50 that accommodates the laminate 30 in a sealed state, and a pair of leads 60 and 62 connected to the laminate 30.

  The laminated body 30 is configured such that a pair of the positive electrode 10 and the negative electrode 20 are opposed to each other with the separator 18 interposed therebetween. The positive electrode 10 is a product in which a positive electrode active material layer 14 is provided on a positive electrode current collector 12. The negative electrode 20 is a product in which a negative electrode active material layer 24 is provided on a negative electrode current collector 22. The positive electrode active material layer 14 and the negative electrode active material layer 24 are in contact with both sides of the separator 18. Leads 60 and 62 are connected to the end portions of the positive electrode current collector 12 and the negative electrode current collector 22, respectively, and the end portions of the leads 60 and 62 extend to the outside of the case 50.

(Positive electrode)
As illustrated in FIG. 2, the positive electrode 10 includes a plate-like (film-like) positive electrode current collector 12 and a positive electrode active material layer 14 formed on the positive electrode current collector 12.

  The positive electrode current collector 12 may be a conductive plate material, and for example, a thin metal plate of aluminum, copper, or nickel foil can be used. The positive electrode active material layer 14 mainly includes the above-described active material 2 and a binder. The positive electrode active material layer 14 may contain a conductive additive.

  The binder binds the active materials to each other and binds the active material to the positive electrode current collector 12.

  The material of the binder is not limited as long as the above-described bonding is possible. For example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoro Ethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride A fluororesin such as (PVF) is used.

  In addition to the above, as the binder, for example, vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP) -TFE fluorine rubber), vinylidene fluoride-pentafluoropropylene fluorine rubber (VDF-PFP fluorine rubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene fluorine rubber (VDF-PFP-TFE fluorine rubber) , Vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene fluorine rubber (VDF-PFMVE-TFE fluorine rubber), vinylidene fluoride-chlorotrifluoroethylene fluorine It may be used vinylidene fluoride-based fluorine rubbers such as beam (VDF-CTFE-based fluorine rubber).

  In addition to the above, for example, polyethylene, polypropylene, polyethylene terephthalate, aromatic polyamide, cellulose, styrene / butadiene rubber, isoprene rubber, butadiene rubber, ethylene / propylene rubber, and the like may be used as the binder. Also, thermoplastic elastomeric polymers such as styrene / butadiene / styrene block copolymers, hydrogenated products thereof, styrene / ethylene / butadiene / styrene copolymers, styrene / isoprene / styrene block copolymers, and hydrogenated products thereof. May be used. Further, syndiotactic 1,2-polybutadiene, ethylene / vinyl acetate copolymer, propylene / α-olefin (carbon number 2 to 12) copolymer may be used.

  Alternatively, an electron conductive conductive polymer or an ion conductive conductive polymer may be used as the binder. Examples of the electron conductive conductive polymer include polyacetylene. In this case, since the binder also exhibits the function of the conductive auxiliary agent particles, it is not necessary to add the conductive auxiliary agent.

As the ion-conductive conductive polymer, for example, those having ion conductivity such as lithium ion can be used. For example, polymer compounds (polyether-based polymer compounds such as polyethylene oxide and polypropylene oxide) A crosslinked polymer of a polyether compound, polyepichlorohydrin, polyphosphazene, polysiloxane, polyvinylpyrrolidone, polyvinylidene carbonate, polyacrylonitrile, etc.) monomers, LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAsF ₆ , LiCl, LiBr, Examples include Li (CF ₃ SO ₂ ) ₂ N, LiN (C ₂ F ₅ SO ₂ ) ₂ lithium salt, or a composite of an alkali metal salt mainly composed of lithium. Examples of the polymerization initiator used for the combination include a photopolymerization initiator or a thermal polymerization initiator that is compatible with the above-described monomer.

  It is preferable that the content rate of the binder contained in the positive electrode active material layer 14 is 0.5-6 mass% on the basis of the mass of an active material layer. When the binder content is less than 0.5% by mass, the amount of the binder is too small and a tendency to fail to form a strong active material layer increases. Moreover, when the content rate of a binder exceeds 6 mass%, the quantity of the binder which does not contribute to an electric capacity will increase, and the tendency for it to become difficult to obtain sufficient volume energy density becomes large. In this case, particularly, when the electronic conductivity of the binder is low, the electric resistance of the active material layer is increased, and a tendency that a sufficient electric capacity cannot be obtained increases.

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

(Production method of positive electrode)
A slurry is prepared by adding the above-mentioned active material and binder, and a necessary amount of a conductive additive to a solvent. As the solvent, for example, N-methyl-2-pyrrolidone, N, N-dimethylformamide and the like can be used. Then, a slurry containing an active material, a binder, and the like may be applied to the surface of the positive electrode current collector 12 and dried.

(Negative electrode)
The negative electrode 20 includes a plate-shaped negative electrode current collector 22 and a negative electrode active material layer 2 formed on the negative electrode current collector 22.
4 is provided. The negative electrode current collector 22, the binder, and the conductive additive can each be the same as the positive electrode. Moreover, a negative electrode active material is not specifically limited, A well-known negative electrode active material for batteries can be used. Examples of the negative electrode active material include graphite, non-graphitizable carbon, graphitizable carbon, and low-temperature calcined carbon that can occlude / release (intercalate / deintercalate, or dope / dedope) lithium ions. Carbon materials, metals that can be combined with lithium such as Al, Si and Sn, amorphous compounds mainly composed of oxides such as SiO ₂ and SnO ₂ , lithium titanate (Li ₄ Ti ₅ O ₁₂ ), etc. The particle | grains containing are mentioned.

(Electrolyte)
The electrolyte solution is contained in the positive electrode active material layer 14, the negative electrode active material layer 24, and the separator 18. The electrolyte solution is not particularly limited. For example, in the present embodiment, an electrolyte solution containing a lithium salt (electrolyte aqueous solution, electrolyte solution using an organic solvent) can be used. However, the electrolyte aqueous solution is preferably an electrolyte solution (non-aqueous electrolyte solution) using an organic solvent because the electrochemical decomposition voltage is low, and the withstand voltage during charging is limited to a low level. As the electrolyte solution, a lithium salt dissolved in a non-aqueous solvent (organic solvent) is preferably used. Examples of the lithium salt include LiPF ₆ , LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiCF ₃ , LiCF ₂ SO ₃ , LiC (CF ₃ SO ₂ ) ₃ , LiN (CF ₃ SO ₂ ) ₂ , A salt such as LiN (CF ₃ CF ₂ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiN (CF ₃ CF ₂ CO) ₂ , LiBOB or the like can be used. In addition, these salts may be used individually by 1 type, and may use 2 or more types together.

  Moreover, as an organic solvent, propylene carbonate, ethylene carbonate, diethyl carbonate, etc. are mentioned preferably, for example. These may be used alone or in combination of two or more at any ratio.

  In the present embodiment, the electrolyte solution may be a gel electrolyte obtained by adding a gelling agent in addition to liquid. Further, instead of the electrolyte solution, a solid electrolyte (a solid polymer electrolyte or an electrolyte made of an ion conductive inorganic material) may be contained.

  The separator 18 is an electrically insulating porous body, for example, a single layer of a film made of polyethylene, polypropylene or polyolefin, a stretched film of a laminate or a mixture of the above resins, or a group consisting of cellulose, polyester and polypropylene. Examples thereof include a nonwoven fabric made of at least one selected constituent material.

  The case 50 seals the laminated body 30 and the electrolytic solution therein. The case 50 is not particularly limited as long as it can prevent leakage of the electrolytic solution to the outside and entry of moisture and the like into the electrochemical device 100 from the outside. For example, as the case 50, as shown in FIG. 1, a metal laminate film in which a metal foil 52 is coated with a polymer film 54 from both sides can be used. For example, an aluminum foil can be used as the metal foil 52 and a film such as polypropylene can be used as the polymer film 54. For example, the material of the outer polymer film 54 is preferably a polymer having a high melting point such as polyethylene terephthalate (PET) or polyamide, and the material of the inner polymer film 54 is preferably polyethylene or polypropylene.

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

  Then, the leads 60 and 62 are welded to the positive electrode current collector 12 and the negative electrode current collector 22 by a known method, respectively, and a separator is provided between the positive electrode active material layer 14 of the positive electrode 10 and the negative electrode active material layer 24 of the negative electrode 20. 18 may be inserted into the case 50 together with the electrolytic solution with the 18 interposed therebetween, and the entrance of the case 50 may be sealed.

  As mentioned above, although the manufacturing method of active material particle, the active material obtained by it, the electrode containing the said active material, and suitable one Embodiment of the lithium ion secondary battery provided with the said electrode were demonstrated in detail, this invention is The present invention is not limited to the above embodiment.

  For example, the active material can also be used as an electrode material for electrochemical elements other than lithium ion secondary batteries. As such an electrochemical element, a secondary battery other than a lithium ion secondary battery, such as a metallic lithium secondary battery (which uses an electrode containing the composite particles of the present invention as a cathode and metallic lithium as an anode). 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
<Hydrothermal synthesis process>
In a 500 ml Meyer flask, 4.63 g (0.04 mol) of H ₃ PO ₄ (Nacalai Tesque, purity 85%) and 180 g of distilled water (Nacalai Tesque, HPLC) were added, and magnetic. Stir with a stirrer. Subsequently, 3.67 g (0.02 mol) of V ₂ O ₅ (manufactured by Nacalai Tesque, purity 99%) was added, and stirring was continued for about 2.5 hours.
Next, 1.77 g (0.01 mol) of ascorbic acid was added into the above mixture. After ascorbic acid was added, stirring was continued for about 60 minutes.
Subsequently, 1.70 g (0.04 mol) of LiOH.H ₂ O (manufactured by Nacalai Tesque, 99% purity) was added over about 10 minutes. After adding 20 g of distilled water to the obtained pasty substance, 210.91 g of the substance in the flask was transferred into a cylindrical glass container of a 0.5 L autoclave. When the pH of the substance in the container was measured, the pH was 5. The container was sealed and held at 250 ° C. for 12 hours to perform hydrothermal synthesis.

After the heater switch was turned off, the mixture was allowed to cool for about 7 hours to obtain a suspension containing a brown precipitate. The pH of this substance was measured and found to be 6. After removing the supernatant, about 200 ml of distilled water was added, and the precipitate in the container was washed while stirring. Thereafter, suction filtration was performed. After washing with water, about 200 ml of acetone was added, and the precipitate was washed in the same manner as the washing with water. The filtered material was transferred to a petri dish and dried in the air to obtain 6.51 g of a brown solid. The yield was 96.7% at LiVOPO ₄ terms.

<Baking process>
1.00 g of a brown solid obtained in the hydrothermal synthesis process is placed in an alumina crucible, heated from room temperature to 450 ° C. over 60 minutes in an air atmosphere, and heat treated at 450 ° C. for 4 hours to obtain a powder. It was.

<Measurement of β ratio>
The proportion of beta-type crystal structure to the sum of the LiVOPO ₄ of LiVOPO ₄ and α-type crystal structure of the beta-type crystal structure in the active material of Example 1 (beta ratio) was determined from the result of powder X-ray diffraction (XRD) . The β ratio in the active material of Example 1 was 97%.

<Measurement of average primary particle size and average secondary particle size>
The projected area circle equivalent diameter obtained from the projected area (100 each) of the active material based on the image observed with the high-resolution scanning electron microscope for the particle size distribution of the primary particles and secondary particles of the active material of Example 1 It calculated by the accumulation rate of each. Based on the obtained particle size distribution based on the number of active materials, the average primary particle size (D50) and the average secondary particle size (D50) of the active material were calculated. The average primary particle diameter (D50) of the active material was 160 nm, and the average secondary particle diameter (D50) was 2200 nm. In addition, the value of D10 having a cumulative ratio of 10% in the number-based particle size distribution measured for the secondary particles of the active material obtained in Example 1 is 1150 nm, and the value of D90 having a cumulative ratio of 90%. The value was 2730 nm.

<Measurement of minor axis length / major axis length of secondary particles>
From the image observed with a high-resolution scanning electron microscope, the minor axis length and the major axis length of 100 secondary particles of the active material were measured, and the average value of the ratio of the minor axis length to the major axis length was calculated. The value of short axis length / major axis length of the active material of Example 1 was 0.93.

<Measurement of discharge capacity>
A mixture of the active material of Example 1, polyvinylidene fluoride (PVDF) as a binder, and acetylene black was dispersed in N-methyl-2-pyrrolidone (NMP) as a solvent to prepare a slurry. The slurry was prepared so that the weight ratio of the active material, acetylene black, and PVDF was 84: 8: 8 in the slurry. This slurry was applied onto an aluminum foil as a current collector, dried, and then rolled to obtain an electrode (positive electrode) on which an active material layer containing the active material of Example 1 was formed.

Next, the obtained electrode and the Li foil as the counter electrode were laminated with a separator made of a polyethylene microporous film interposed therebetween to obtain a laminate (element body). This laminate was put in an aluminum laminate pack, and 1M LiPF ₆ solution was injected as an electrolyte into the aluminum laminate pack, followed by vacuum sealing to produce an evaluation cell of Example 1.

  Using the evaluation cell of Example 1, the discharge capacity (unit: mAh / unit) when the discharge rate is 0.01 C (current value at which discharge is completed in 100 hours when constant current discharge is performed at 25 ° C.) g) was measured. The discharge capacity at 0.01 C was 153 mAh / g. Moreover, the discharge capacity (unit: mAh / g) was measured when the discharge rate was 0.1 C (current value at which discharge was completed in 10 hours when constant current discharge was performed at 25 ° C.). The discharge capacity at 0.1 C was 148 mAh / g.

<Evaluation of rate characteristics>
The percentage of the discharge capacity at 0.1 C relative to the discharge capacity at 0.01 C was calculated and evaluated as a rate characteristic. The rate characteristic of the evaluation cell of Example 1 was 96.7%.

(Examples 2-15, Comparative Examples 1-11)
In the hydrothermal synthesis step, the ratio of the number of moles of lithium atoms to the number of moles of vanadium atoms in the mixture, the ratio of the number of moles of phosphorus atoms to the number of moles of vanadium atoms, the amount of ascorbic acid added to the mixture, The active materials of Examples 2 to 15 and Comparative Examples 1 to 11 were the same as Example 1 except that the type, hydrothermal synthesis temperature, and firing temperature in the firing step were changed as shown in Tables 1 and 2 below. Got. The ratio (β ratio) of the β-type crystal structure to the total of LiVOPO ₄ having a β-type crystal structure and LiVOPO ₄ having an α-type crystal structure in the obtained active material, the average primary particle diameter (D50) of the active material, and the average secondary Tables 3 and 4 show the particle diameter (D50), the ratio of the minor axis length to the major axis length of the secondary particles, and the discharge capacity and rate characteristics of the evaluation cells using these active materials. The ratios of the secondary particles of Examples 2 to 15 to D10 and D90 to D50 were substantially the same values as in Example 1.

As shown in Table 3, the active materials obtained under the conditions of Examples 1 to 15 had an average primary particle size of 120 nm to 340 nm. Further, the ratio of the minor axis length to the major axis length of the secondary particles was 0.81 to 0.99, and the secondary particles had an agglomerated structure very close to a sphere. Further, this active material contained LiVOPO ₄ having a β-type crystal structure as a main component. The cells using the active materials of Examples 1 to 15 had high rate characteristics and a large discharge capacity.

  DESCRIPTION OF SYMBOLS 1 ... Primary particle, 2 ... Active material (secondary particle) 10, 20 ... Electrode, 12 ... Positive electrode collector, 14 ... Positive electrode active material layer, 18 ... Separator, 22 ... Negative electrode collector, 24 ... Negative electrode active Material layer 30 ... Laminated body 50 ... Case 52 ... Metal foil 54 ... Polymer film 60,62 ... Lead 100 ... Lithium ion secondary battery

Claims (5)

A lithium source, a vanadium source, a phosphate source, water, and ascorbic acid, the ratio of the number of moles of lithium atoms to the number of moles of vanadium atoms, and the number of moles of phosphorus atoms relative to the number of moles of vanadium atoms A hydrothermal synthesis step of heating a mixture having a ratio of 0.95 to 1.2 and a molar ratio of ascorbic acid to a molar number of vanadium atoms of 0.05 to 0.6 under pressure;
A firing step of heating the material obtained in the hydrothermal synthesis step to obtain LiVOPO ₄ having a β-type crystal structure;
A method for producing an active material comprising: LiVOPO _{4 having an} average primary particle diameter of 100 to 350 nm and an aggregated structure in which the ratio of the length of the minor axis to the length of the major axis of the secondary particle is 0.80 to 1, and having a β-type crystal structure An active material containing as a main component.   The active material of Claim 2 whose average secondary particle diameter is 1500 nm-8000 nm.   An electrode comprising: a current collector; and an active material layer including the active material according to claim 3 and provided on the current collector.   A lithium secondary battery comprising the electrode according to claim 4.