JP2010218830A - Active material, electrode containing the active material, lithium-ion secondary battery including the electrode, and method for manufacturing the active material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an active material that achieves a sufficient discharge capacity at high discharge current density, an electrode including the active material, a lithium-ion secondary battery including the electrode, and a method for manufacturing the active material. <P>SOLUTION: The active material 5 contains active material particles 1 mainly composed of LiVOPO<SB>4</SB>having a β-type crystal structure, and a plurality of hemispherical carbon particles 2 that are supported on the surfaces of the active material particles 1 and respectively have a height of 5 to 20 nm. The active material has an average primary particle size of 50 to 1,000 nm. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an active material, an electrode including the active material, a lithium ion 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 cannot obtain a sufficient discharge capacity at a high discharge current density.

  Accordingly, an object of the present invention is to provide an active material capable of obtaining a sufficient discharge capacity at a high discharge current density, an electrode including the active material, a lithium ion secondary battery including the electrode, and a method for manufacturing the active material. .

The active material according to the present invention includes active material particles mainly composed of LiVOPO ₄ having a β-type crystal structure, and a plurality of hemispherical carbon particles supported on the surface of the active material particles and having a height of 5 to 20 nm. The average primary particle diameter is 50 to 1000 nm.

According to the present invention, since the active material has the above structure and particle size, a sufficient discharge capacity can be obtained at a high discharge current density. The reason for this is not clear, but the following can be considered. First, the active material according to the present invention has the above structure and particle size, so that the particle size of the active material particles is equal to or less than 50 to 1000 nm, has a large specific surface area, and is in contact with the electrolytic solution. The area will increase. Accordingly, it is considered that lithium ions are easily diffused into the crystal lattice of LiVOPO ₄ and lithium ions are easily inserted and desorbed.
Secondly, the carbon particles of the active material have a height of 5 to 20 nm and are hemispherical, so that, for example, the active material particles and the electrolytic solution are compared with the case where the carbon material forms a film. The contact area is increased and ion conductivity is ensured. Also, compared to the case where, for example, spherical carbon particles are supported, the contact area between the active material particles and the carbon particles is increased and the electron conductivity is ensured. Will be. Thereby, it is thought that ion conductivity and electronic conductivity can be made compatible.

  Moreover, the electrode which concerns on this invention is equipped with a collector and the active material layer provided on the collector containing the active material mentioned above. Thereby, an electrode having a large discharge capacity can be obtained.

  The lithium ion secondary battery of this invention is equipped with the electrode mentioned above. Thereby, a lithium ion secondary battery having a large discharge capacity can be obtained.

Here, the method for producing the active material according to the present invention includes heating a mixture containing a lithium source, a vanadium source, a phosphoric acid source, carbon black, and water having a pH of 7 or less under pressure to form a β-type crystal. comprises a hydrothermal synthesis step of obtaining a precursor of LiVOPO ₄ structure, and a firing step to obtain a LiVOPO ₄ of the precursor of the LiVOPO ₄ of beta-type crystal structure was heated at five hundred and thirty to six hundred and seventy ° C. beta-type crystal structure.

  According to the manufacturing method of the active material which concerns on this invention, the quality material which concerns on this invention which has the structure mentioned above can be obtained. As a result, an active material having a sufficient discharge capacity at a high discharge current density, an electrode including the active material, and a lithium ion secondary battery including the electrode can be obtained.

  ADVANTAGE OF THE INVENTION According to this invention, the active material from which sufficient discharge capacity is obtained in high discharge current density, the electrode containing this, the lithium ion secondary battery containing the said electrode, and the manufacturing method of an active material can be provided.

FIG. 1 is a schematic cross section of an active material according to the present 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 a TEM image of the active material in Example 3.

  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.

<Active material>
The active material according to the present embodiment will be described. FIG. 1 is a schematic cross-sectional view of an active material 5 according to this embodiment. The active material 5 of the present embodiment includes active material particles 1 and a plurality of hemispherical carbon particles 2.

The active material 5 uses active material particles mainly composed of LiVOPO ₄ having a β-type crystal structure as a base (core), and has hemispherical carbon particles having a height of 5 to 20 nm on the surface of the base. Further, the active material 5 has an average primary particle size _{R 5} is 50-1000 nm.
Here, the “average primary particle diameter R _{5 of the} active material” defined in the present invention is a value of d50 having a cumulative rate of 50% in the number-based particle size distribution measured for the active material 5. . The number-based particle size distribution of the active material 5 can be calculated from, for example, the projected area circle equivalent diameter measured from the projected area of the active material 5 based on the image observed with a high-resolution scanning electron microscope and the cumulative ratio thereof. it can. The projected area equivalent circle diameter assumes a sphere having the same projected area as the projected area of the particles (active material 5), and the diameter (equivalent circle diameter) of the sphere is the particle diameter (particle diameter of the active material 5). It is expressed as
In addition, in the number-based particle size distribution of the active material 5 calculated from the numerical value of the projected area equivalent circle diameter, the primary particle diameter d10 having a cumulative ratio of 10% is 10 to 50 nm, and the cumulative ratio is 50%. The primary particle diameter d50 is preferably 50 to 1000 nm, and the primary particle diameter d90 having a cumulative ratio of 90% is preferably 1000 to 10,000 nm.
When the average primary particle diameter R ₅ (d50) exceeds 1000 nm, the discharge capacity tends to deteriorate. On the other hand, the average primary particle diameter R ₅ is less than 50 nm, there tends to be difficult to carry the carbon particles.

The phrase “having β-type crystal structure LiVOPO ₄ as a main component” means that the amount of β-type crystal structure LiVOPO ₄ in the active material particles 1 is 90% or more, preferably 95% or more on a mass basis. . The component other than the β-type crystal structure LiVOPO ₄ mainly includes the α-type crystal structure LiVOPO _4, but may contain a small amount of unreacted raw material components in addition to LiVOPO ₄ . 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 α-type crystal structure LiVOPO ₄ is preferably 10% by mass or less with respect to the β-type crystal structure LiVOPO ₄ .

  The hemispherical carbon particle 2 has a height h of 5 to 20 nm and is supported on the surface of the active material particle 1 so that a convex surface is formed on the side opposite to the active material particle 1 side as shown in FIG. Has been. The “height of the hemispherical carbon particle 2” defined in the present invention means the height from the surface of the active material particle 1 to the apex of the convex surface of the hemispherical carbon particle 2. When the height h exceeds 20 nm, the discharge capacity tends to decrease because the ion conductivity decreases depending on the height. On the other hand, if the height h is less than 5 nm, the coverage is increased and the ionic conductivity is lowered, or the discharge capacity tends to be lowered. The carbon particles 2 are not densely arranged so that a plurality of hemispherical carbon particles 2 form a film on the surface of the active material particles 1, for example, but as shown in FIG. And are scattered independently.

  The height and existence state of the hemispherical carbon particles 2 can be observed with a TEM or the like. Moreover, it is preferable that the ratio (surface coverage) which the hemispherical carbon particle 2 coat | covers the surface of the active material particle 1 is 50 to 90%. The surface coverage can be measured by TEM observation. The hemispherical carbon particles 2 are preferably formed in a single layer on the surface of the active material particle 1, but another hemispherical carbon particle 2 overlaps the surface of the hemispherical carbon particle 2. It doesn't matter. However, from the viewpoints of ionic conductivity and electronic conductivity, the hemispherical carbon particles 2 are preferably laminated in two or less layers. The hemispherical carbon particles 2 are derived from, for example, carbon black.

Since such an active material 5 has the above structure, a sufficient discharge capacity can be obtained at a high discharge current density. The reason for this is not clear, but the following can be considered. First, the active material 5 is composed of active material particles 1 mainly composed of LiVOPO ₄ having a β-type crystal structure, and a plurality of hemispheres supported on the surface of the active material particles 1 and having a height h of 5 to 20 nm. Carbon particles 2 and the average primary particle diameter R ₅ is 50 to 1000 nm, the particle diameter R ₁ of the active material particles 1 is equal to or less than 50 to 1000 nm, and the specific surface area is large. The contact area with the electrolytic solution will increase. Accordingly, it is considered that lithium ions are easily diffused into the crystal lattice of LiVOPO ₄ and lithium ions are easily inserted and desorbed. Secondly, the carbon particles 2 of the active material 5 have a height of 5 to 20 nm and are hemispherical, so that the active material particles 1 and the electrolysis are compared with a case where, for example, the carbon material forms a film. The contact area with the liquid is increased and ion conductivity is ensured, and the contact area between the active material particles 1 and the carbon particles 2 is increased as compared with the case where, for example, spherical carbon particles are supported. Electron conductivity will be ensured. Thereby, it is thought that ion conductivity and electronic conductivity can be made compatible.

<Method for producing active material>
Then, the manufacturing method of the active material 5 is demonstrated. The manufacturing method of the active material 5 which concerns on this embodiment heats the mixture whose pH is 7 or less containing a lithium source, a vanadium source, a phosphoric acid source, carbon black, and water, and has a β-type crystal structure. comprises a hydrothermal synthesis step of obtaining a precursor of LiVOPO _4, and calcined to obtain a LiVOPO ₄ of beta-type precursor of LiVOPO ₄ crystal structure is heated to five hundred thirty to six hundred and seventy ° C. beta-type crystal structure.

[Hydrothermal synthesis process]
(material)
The raw material used in the hydrothermal synthesis process is a mixture containing at least a lithium source, a vanadium source, a phosphoric acid source, carbon black, and water.
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.

The mixing ratio of the lithium source, the phosphate source, and the vanadium source in the raw materials used in the hydrothermal synthesis step is set to a composition represented by the composition formula: LiVOPO ₄ , that is, Li atoms: V atoms: P atoms: O Adjustment may be made so that atoms = 1: 1: 1: 5 (molar ratio).

  The carbon black is added to the mixture as the raw material in order to support the hemispherical carbon particles 2 on the surface of the active material 1. Carbon black is not particularly limited. For example, carbon black having a particle diameter of 30 to 100 nm that is generally commercially available may be used. The substantially spherical carbon black supplied as a raw material through the hydrothermal synthesis step and the firing step described later is hemispherically supported on the surface of the active material 1 because of its hardness. Carbon black can be easily dispersed in the aqueous solution as the raw material during hydrothermal synthesis.

  The content of carbon black in the mixture as a raw material for hydrothermal synthesis is such that the ratio C1 / M between the number of moles C1 of carbon atoms constituting the carbon black and the number of moles M of vanadium atoms contained in the vanadium compound is 0, for example. It is preferable to prepare such that .04 ≦ C1 / M ≦ 4. When the content of carbon atoms (number of moles C1) is too small, the electronic conductivity and capacity density of the active material 5 tend to decrease. When there is too much content of a carbon atom, there exists a tendency for the weight of the active material particle 1 which occupies for the active material 5 to reduce relatively, and the capacity density of an active material to reduce. By setting the carbon atom content within the above range, these tendencies can be suppressed.

In order to obtain a precursor of LiVOPO _{4 having} a β-type crystal structure by hydrothermal synthesis, for example, the pH of the above mixture is set to 7 or less. Although pH can be adjusted with the kind of compound used as a lithium source, a phosphoric acid source, and a vanadium source, it can also adjust using pH adjusters, such as hydrochloric acid and aqueous ammonia. In addition to setting the pH to 7 or less, a precursor of LiVOPO _{4 having} a β-type crystal structure can also be obtained by mixing a peroxide such as H ₂ O ₂ and making the raw material an oxidizing atmosphere. it can. If the pH exceeds _7, a precursor of LiVOPO ₄ having an α-type crystal structure tends to be easily formed.

(Hydrothermal synthesis)
In the hydrothermal synthesis step, first, the precursor raw materials (for example, lithium compound, vanadium compound, PO ₄ -containing compound, carbon black) are placed in a reaction vessel (for example, an autoclave) having a function of heating and pressurizing the inside. , And water) to prepare an aqueous solution in which these are dispersed (hereinafter referred to as “raw material mixture”). When preparing the raw material mixture, the raw materials of the precursors may be mixed together and stirred for a predetermined time and refluxed. For example, first, a vanadium compound, a PO ₄ -containing compound, and water were mixed. After refluxing, a lithium compound and carbon black may be added thereto. By this reflux, a complex of the vanadium compound and the PO ₄ -containing compound can be formed.

Next, the reaction vessel is sealed, and the raw material mixture is heated while being pressurized, so that the hydrothermal reaction of the mixture proceeds. 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 is usually
It is a paste-like substance with low fluidity. LiVOPO with β-type crystal structure contained in this substance
The precursor of ₄ is considered to be in a hydrated state.

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

The temperature of the raw material mixture in the hydrothermal synthesis step is preferably 120 to 200 ° C. If the temperature of the mixture is too low, the crystallinity of the active material particles mainly composed of LiVOPO ₄ having a β-type crystal structure finally obtained tends to be lowered, and the capacity density of the active material tends to be reduced. 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]
Then, the baking process which heats the obtained precursor to 530-670 degreeC is performed. Thereby, the crystallization of the active material particles 1 and the support of the hemispherical carbon particles 2 are completed, and the above-described active material 5 is obtained. In this step, it is considered that impurities remaining in the mixture after the hydrothermal synthesis step are removed, and the precursor of LiVOPO _{4 having} a β-type crystal structure is dehydrated to cause crystallization. In addition, it seems that the precursor obtained in the hydrothermal synthesis step contains a carbon component derived from carbon black. By firing the precursor in the above temperature range, the surface of the active material particles 1 is hemispherical. Of carbon particles 2 are supported. If the heating temperature is lower than the lower limit of the above range, the active material particles may not be sufficiently grown, or the carbon surface tends to be covered with the active material particles. On the other hand, when the heating temperature is higher than the upper limit of the above range, the active material particles 1 tend to decrease the proportion of β phase.

Here, in the firing step, it is preferable to heat the above precursor to 530 to 670 ° C. for 0.5 to 10 hours. If the heating time is too short, the crystallinity of the active material particles mainly composed of LiVOPO ₄ having a β-type crystal structure as a main component is lowered, and the capacity density of the active material tends to be reduced. On the other hand, if the heating time is too long, the particle growth of the active material particles proceeds and the particle size increases. As a result, the diffusion of lithium in the active material becomes slow, and the capacity density of the active material tends to decrease. By setting the heating time within the above range, these tendencies can be suppressed.

  The atmosphere of the firing step is not particularly limited, but it is preferably performed in an inert atmosphere such as argon gas or nitrogen gas in order to prevent the carbon black from being oxidized. Moreover, it is good also as reducing atmosphere by making the inside of a furnace into inert atmosphere, and also covering the circumference | surroundings of a furnace with the container containing activated carbon.

According to the method for producing an active material according to the present embodiment, β-LiVOPO ₄ is synthesized by hydrothermal synthesis. Therefore, the average primary particle diameter of β-LiVOPO ₄ that is an active material particle is equal to or less than 50 to 1000 nm. The active material 5 can be easily manufactured, and the hemispherical carbon particles having a predetermined particle size range can be supported on the surface of the active material particles. In addition, the particle size distribution of the active material particles 1 can be sharpened.

  By the way, in the active material-containing layer of the electrode, in order to increase conductivity, a conductive material such as a carbon material is usually further brought into contact with the surface of the active material. As this method, after the active material is manufactured, the active material and the conductive material may be mixed to form the active material-containing layer. For example, in the raw material for hydrothermal synthesis, a conductive material that is a carbon material other than carbon black is used. It is also possible to add carbon to the active material particles by adding a material.

  Examples of the conductive material in the case where a conductive material that is a carbon material is added to the raw material for hydrothermal synthesis 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 (in an aqueous solution) as a raw material during hydrothermal synthesis. However, the conductive material does not necessarily have to be mixed in the mixture as a raw material at the time of hydrothermal synthesis, and it is preferable that at least a part of the conductive material is mixed into the mixture as a raw material 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 as the raw material for hydrothermal synthesis is the ratio C2 of the number of moles C2 of carbon atoms constituting the carbon particles to 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 by the active material 5 and an electrically conductive material to fall. When the content of the conductive material is too large, not only does the weight of the active material particles in the electrode active material relatively decrease and the capacity density of the electrode active material tends to decrease, but also on the surface of the active material particles 1. There is a tendency that the hemispherical carbon particles 2 are not easily supported in a desired state. By setting the content of the conductive material within the above range, these tendencies can be suppressed.

<Lithium ion secondary battery>
Next, the electrode and the lithium ion secondary battery according to this embodiment 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 electrodes 10 and 20 are arranged to face 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.

  As the positive electrode current collector 12 of the positive electrode 10, for example, an aluminum foil or the like can be used. The positive electrode active material layer 14 is a layer including the above-described active material 5, a binder, and a conductive material added as necessary. Examples of the conductive material added as necessary include carbon blacks, carbon materials, and conductive oxides such as ITO.

  The binder is not particularly limited as long as it can bind the active material 5 and the conductive material to the current collector, and a known binder can be used. Examples thereof include fluororesins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and vinylidene fluoride-hexafluoropropylene copolymer.

  Such a positive electrode is obtained by using a known method, for example, an electrode active material containing the active material 5 described above, or an active material 5, a binder, and a conductive material. A slurry added to a solvent such as methyl-2-pyrrolidone or N, N-dimethylformamide is applied to the surface of the positive electrode current collector 12 and dried.

  As the negative electrode current collector 22, a copper foil or the like can be used. Moreover, as the negative electrode active material layer 24, the thing containing a negative electrode active material, a electrically conductive material, and a binder can be used. It does not specifically limit as a electrically conductive material, A well-known electrically conductive material can be used. Examples thereof include carbon blacks, carbon materials, metal powders such as copper, nickel, stainless steel, and iron, mixtures of carbon materials and metal powders, and conductive oxides such as ITO. As the binder used for the negative electrode, known binders can be used without any particular limitation. For example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer ( Fluorine resins such as ECTFE and polyvinyl fluoride (PVF). This binder not only binds constituent materials such as active material particles and conductive materials added as necessary, but also contributes to binding between the constituent materials and the current collector. . In addition to the above, as the binder, 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. 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 and the like may be used. Further, a conductive polymer may 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.

  The manufacturing method of the negative electrode 20 should just adjust slurry and apply | coat to a collector like the manufacturing method of the positive electrode 10. FIG.

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 , CF 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, dimethyl carbonate, methyl ethyl 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 may also be formed of an electrically insulating porous structure, 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 cellulose. And a fiber nonwoven fabric made of at least one constituent material selected from the group consisting of polyester and polypropylene.

  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 suppress leakage of the electrolytic solution to the outside and entry of moisture and the like into the lithium ion secondary battery 100 from the outside. For example, as the case 50, as shown in FIG. 2, 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 synthetic resin 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.

  As described above, the preferred embodiment of the active material, the electrode including the active material, the battery including the electrode, and the method for producing the active material has been described in detail. However, the invention is not limited to the embodiment. .

  For example, the active material particles 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>
34.59 g (0.35 mol) of H ₃ PO ₄ (molecular weight: 98.00, manufactured by Nacalai Tesque, special grade, purity: 85% by weight), 750 g of H ₂ O (manufactured by Nacalai Tesque, HPLC (high performance liquid chromatography) ) 27.56 g (0.15 mol) of V ₂ O ₅ (molecular weight: 181.88, manufactured by Nacalai Tesque, special grade, purity: 99 wt%), 11.2 g (0.15 mol) of Li ₂ CO ₃ (molecular weight: 73.89, manufactured by Nacalai Tesque, special grade, purity: 99% by weight) and 1.52 g (0.13 mol) of carbon black (CB) (molecular weight: 12, average particle size 50 nm, electrochemical Kogyo Co., Ltd.) was introduced in this order into a 1.5 L autoclave vessel to prepare a mixture having a pH of 3.5. The amount of these raw materials corresponds to the amount of stoichiometrically producing about 50 g (0.3 mol) of LiVOPO ₄ (molecular weight: 168.85).

  The container was sealed, and the mixture was stirred at room temperature for about 30 minutes, and then the pressure in the container was 0.5 MPa, the temperature was 160 ° C., and a hydrothermal synthesis reaction was performed for 16 hours. The pH of the mixture after the hydrothermal synthesis reaction was 2.7.

  The paste-like mixture after the hydrothermal synthesis reaction was opened in a vat and evaporated to dryness at 90 ° C. for about 21 hours. Thereafter, the sample was turned over and further evaporated to dryness at 90 ° C. for about 5 hours. The mixture (63.16 g) after evaporation to dryness was pulverized with a small pulverizer (SK-M500, manufactured by Kyoritsu Riko Co., Ltd.) to obtain a black-green powder (active material precursor).

<Baking process>
5.00 g of the precursor was placed in an alumina crucible and heated from room temperature to 600 ° C. at a rate of temperature increase of 100 ° C./min. After holding at 600 ° C. for 4 hours, it was cooled to room temperature at 10 ° C./min. In addition, at the time of temperature increase and temperature decrease, nitrogen gas was flowed at 5 L / min from 200 ° C., and the inside of the furnace was made a nitrogen atmosphere. By this firing step, 40.43 g of a dull green particle group (active material of Example 1) was obtained.

<Measurement of crystal structure>
The results of powder X-ray diffraction (XRD), the active material of Example 1, it was confirmed that the β-type crystal structure of LiVOPO _4.

<Measurement of number-based particle size distribution and average primary particle size>
The number-based particle size distribution of the active material of Example 1 was calculated by the cumulative ratio of the projected area equivalent circle diameter obtained from the projected area of the active material based on the image observed with a high-resolution scanning electron microscope. Based on the obtained particle size distribution based on the number of active materials, the average primary particle diameter (d50) of the active materials was calculated. The average primary particle diameter (d50) of the active material was 500 nm.

<Observation of carbon particle shape and measurement of size>
The active material of the example was observed by TEM. The shape of the carbon particles supported on the surface of the active material particles was observed and the size was measured. As an example, a TEM image of the active material of Example 3 is shown in FIG. Hemispherical CB particles were supported on the surface of the active material particles. The average height h from the active material particle surface to the highest position of the convex surface of the hemispherical CB particles was about 9 nm.

<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 laminator 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 0.1 C (current value at which discharge is completed in 10 hours when constant current discharge is performed at 25 ° C.) g) was measured. The discharge capacity at 0.1 C was 120 mAh / g.

(Example 2)
An active material was prepared in the same manner as in Example 1 except that hydrochloric acid was added to adjust the pH before hydrothermal synthesis to 1.8.

Example 3
An active material was prepared in the same manner as in Example 1 except that hydrochloric acid was added to adjust the pH before hydrothermal synthesis to 3.6.

Example 4
An active material was prepared in the same manner as in Example 1 except that hydrochloric acid was added to adjust the pH before hydrothermal synthesis to 2.5.

(Example 5)
An active material was prepared in the same manner as in Example 1 except that hydrochloric acid was added to adjust the pH before hydrothermal synthesis to 6.5.

(Example 6)
An active material was prepared in the same manner as in Example 1 except that hydrochloric acid was added to adjust the pH before hydrothermal synthesis to 2.5 and the firing temperature to 550 ° C.

(Example 7)
An active material was produced in the same manner as in Example 1 except that hydrochloric acid was added to adjust the pH before hydrothermal synthesis to 2.5 and the firing temperature to 650 ° C.

(Comparative Example 1)
An active material was produced in the same manner as in Example 4 except that the carbon material was graphite.

(Comparative Example 2)
An active material was produced in the same manner as in Example 4 except that the firing temperature was 500 ° C.

(Comparative Example 3)
An active material was prepared in the same manner as in Example 1 except that ammonia water was added to adjust the pH before hydrothermal synthesis to 7.8.

(Comparative Example 4)
An active material was prepared in the same manner as in Example 1 except that the pH before hydrothermal synthesis was changed to 9.2 by adding aqueous ammonia.

(Comparative Example 5)
An active material was produced in the same manner as in Example 1 except that ammonia water was added to adjust the pH before hydrothermal synthesis to 8.1.

(Comparative Example 6)
An active material was prepared by the following solid phase method without hydrothermal synthesis.
LiNO ₃ , V ₂ O ₅ , and H ₃ PO ₄ were dissolved in water at a molar ratio of 2: 1: 2 and stirred at 80 ° C. This solution was evaporated to dryness, dried at 110 ° C. overnight, pulverized, and calcined in air at 700 ° C. for 14 hours. From the X-ray diffraction pattern of the obtained powder, the active material particles were β-type (orthorhombic). The obtained powder was mixed with 3% by mass of carbon black, mixed and pulverized with a ball mill to prepare an active material.

(Comparative Example 7)
Example 1 except that carbon black was not added to the raw material used for hydrothermal synthesis, carbon black was added to the precursor of LiVOPO _{4 having} a β-type crystal structure after the hydrothermal synthesis process for 16 hours, and these were dry mixed. An active material was prepared in the same manner as described above.

  Table 1 shows the experimental conditions and measurement results of Examples 1 to 7 and Comparative Examples 1 to 6.

In Examples 1 to 7, an active material having an average primary particle size of 260 to 890 nm, in which hemispherical carbon particles having a height of 6 to 18 nm are supported on LiVOPO ₄ having a β-type crystal structure, is obtained. The discharge capacity at 0.1 C showed a high value of 93 to 130 mAh / g.
In Comparative Example 1, flake-like (about 20 μm) carbon particles were scattered on the surface of LiVOPO ₄ having a β-type crystal structure, and in Comparative Example 6, carbon particles having a small particle size (about 50 nm) were scattered. In Comparative Example 2, LiVOPO ₄ having a β-type crystal structure became fine particles and covered the surface of the carbon particles. In Comparative Examples 3 to 5, an active material carrying hemispherical carbon particles was obtained, but the active material particles were LiVOPO ₄ having an α-type crystal structure. In Comparative Example 7, since carbon black was added after hydrothermal synthesis, the carbon black did not become hemispherical and was supported on the surface of LiVOPO ₄ having a β-type crystal structure in the same shape (spherical) as before addition. In all the examples, the discharge capacity at 0.1 C was lower than those in Comparative Examples 1 to 7.

  According to the active material of the present invention and the electrode including the active material, it is possible to provide a lithium ion secondary battery having a sufficient discharge capacity at a high discharge current density. In addition, according to the method for producing an active material of the present invention, an active material capable of obtaining a sufficient discharge capacity at a high discharge current density can be provided.

1 ... the active material particles, 2 ... carbon particles, 5 ... active material, h ... a height of the carbon particles, the particle size of the R 1 _... active material particles, the average primary particle diameter of R 5 _... active material, 10, 20 ... electrode , 12 ... Positive electrode current collector, 14 ... Positive electrode active material layer, 18 ... Separator, 22 ... Negative electrode current collector, 24 ... Negative electrode active material layer, 30 ... Laminate, 50 ... Case, 52 ... Metal foil, 54 ... High Molecular film, 60, 62 ... lead, 100 ... lithium ion secondary battery.

Claims (4)

active material particles mainly composed of LiVOPO ₄ having a β-type crystal structure;
A plurality of hemispherical carbon particles supported on the surface of the active material particles and having a height of 5 to 20 nm,
An active material having an average primary particle size of 50 to 1000 nm.   An electrode comprising: a current collector; and an active material layer including the active material according to claim 1 and provided on the current collector.   A lithium ion secondary battery comprising the electrode according to claim 2. A hydrothermal synthesis step of obtaining a precursor of LiVOPO _{4 having} a β-type crystal structure by heating a mixture containing Li source, V source, P source, carbon black, and water and having a pH of 7 or less under pressure;
A firing step of heating the precursor of LiVOPO _{4 having} a β-type crystal structure to 530 to 670 ° C. to obtain LiVOPO ₄ having a β-type crystal structure;
A method for producing an active material comprising: