JP5888762B2 - COMPOSITE MATERIAL AND ITS MANUFACTURING METHOD, POSITIVE ACTIVE MATERIAL, POSITIVE ELECTRODE AND NON-AQUEOUS ELECTROLYTE SECONDARY BATTERY - Google Patents

Description

  The present invention relates to a composite material that can be used as a positive electrode active material, a method for producing the same, a positive electrode active material, a positive electrode, and a nonaqueous electrolyte secondary battery.

Nonaqueous electrolyte secondary batteries are widely used as power sources for portable electronic devices. As the positive electrode active material, composite oxides such as LiCoO ₂ and LiMn ₂ O ₄ are mainly used. However, these complex oxides have a problem that oxygen is easily desorbed at about 150 ° C. in a fully charged state, and the desorbed oxygen tends to cause an oxidative exothermic reaction of the non-aqueous electrolyte. In addition, since these composite oxides have relatively high electronic conductivity (conductivity), if conductive carbon is added when preparing a slurry, the electrode for a nonaqueous electrolyte secondary battery having excellent charge / discharge characteristics Can be obtained.

In recent years, as a positive electrode active material from the viewpoint of high safety and resource amount, phosphate material LiMPO ₄ (at least one selected from M = Fe, Mn, Co), silicate material Li ₂ MSiO ₄ (M = At least one selected from Fe, Mn, and Co) is attracting attention. However, these composite oxides have low conductivity and are almost at the insulator level. For this reason, it is necessary to add conductivity to these composite oxides by adding a large amount of a carbon material.

  However, when a large amount of carbon material is added to the composite oxide, there is a problem that the energy density of the positive electrode active material is lowered.

  Moreover, the electrode for nonaqueous electrolyte secondary batteries which is excellent in charging / discharging characteristics cannot be obtained only by adding conductive carbon to a positive electrode active material material when producing a slurry.

In order to mix a carbon material with a composite oxide, it has been conventionally proposed to perform a spray drying process called a spray drying process. For example, as disclosed in Patent Document 1, carbon powder (conducting aid), a binder, and a solvent are mixed with a composite oxide composed of LiCoO ₂ and LiMn ₂ O ₄ by a ball mill, and then sprayed on them. It is disclosed that a dry treatment is performed to dry the solvent, and a composite material in which a composite oxide is held around the carbon powder is formed. In Patent Documents 2 and 3, secondary particles composed of silicon and ketjen black are formed by spray drying, and then chemical vapor deposition (CVD) is performed using toluene as a carbon source. In which the composite particles are coated with carbon.

  In Patent Documents 4 and 5, particles are formed by spray-drying active material particles such as Si and Sn, a carbon material, and a binder, and then the particles are stirred and mixed by rotation of a rotary blade. It is disclosed to prepare a paste and apply the paste to a current collector.

  Patent Documents 6 and 7 disclose that a lithium manganese composite oxide is obtained by spray drying. Patent Document 8 discloses performing a spray drying process when a conductive material made of acetylene black is dried. Patent Document 9 discloses that dextrin is a good carbon material to be mixed with an electrode active material.

JP 2003-173777 A JP 2003-303588 A JP 2005-135925 A JP 2009-289601 A JP 2010-1114030 A JP 2005-93192 A JP 2005-116316 A JP 2012-9227 A JP 2009-129587 A

  The present invention has been eagerly sought to produce a composite material that can be used as a positive electrode active material having a high discharge capacity by a method different from the methods disclosed in Patent Documents 1 to 9.

  The present invention has been made in view of such circumstances, and provides a composite material capable of increasing the discharge capacity of a battery when used as a positive electrode active material, a method for producing the same, a positive electrode active material, a positive electrode, and a nonaqueous electrolyte secondary battery. The task is to do.

(1) The composite material of the present invention is a composite material having secondary particles formed by aggregation of a plurality of primary particles,
The primary particles include a silicate core portion made of a lithium silicate compound containing lithium (Li), silicon (Si) and oxygen (O), and a carbon coating portion made of a carbon material and covering the surface of the silicate core portion. It is characterized by becoming.
(2) The method for producing a composite material of the present invention has secondary particles formed by aggregating a plurality of primary particles, and the primary particles include lithium (Li), silicon (Si), and oxygen (O). A method for producing a composite material comprising a silicate core part comprising a lithium silicate compound and a carbon coating part comprising a carbon material and covering the surface of the silicate core part,
Energy for obtaining a mixed raw material comprising the compound raw material and the carbon raw material by imparting mechanical energy to the compound raw material that is the raw material of the lithium silicate compound and the carbon raw material that is the raw material of the carbon material in the presence of a solvent. Granting process;
A granulation step of drying the solvent and granulating the mixed raw material by spray drying the mixed raw material to which the mechanical energy is applied in the presence of the solvent;
And a heat treatment step of performing a heat treatment on the granulated mixed raw material.
(3) The positive electrode active material of the present invention is characterized by comprising the above-mentioned composite material or a composite material produced by the above-described method for producing a composite material.
(4) The positive electrode of the present invention has the positive electrode active material described above.
(5) The nonaqueous electrolyte secondary battery of the present invention is characterized by comprising the positive electrode, the negative electrode, and an electrolyte.

  According to the composite material of the present invention, a plurality of primary particles composed of a silicate core portion made of a lithium silicate compound and a carbon coating portion covering the surface of the silicate core portion made of a carbon material are aggregated and composited. Become. For this reason, the composite material can increase the discharge capacity of the battery. Since the positive electrode active material, the positive electrode, and the nonaqueous electrolyte secondary battery of the present invention use the composite material, the discharge capacity is high.

It is explanatory drawing of a thermal spraying apparatus. It is a SEM (scanning electron microscope) photograph of the mixed raw material after the spray-drying treatment of Sample 1 and before the heat treatment, and the magnifications of the upper, middle, and lower photographs are 3,000 times, 5,000 times, 10,000 times. 2 is a SEM photograph of a composite material after heat treatment of Sample 1. It is a SEM cross-sectional photograph of the secondary particle of the sample 1, Comprising: The magnification of each upper and lower photograph is 20,000 times and 50,000 times. 3 is a SEM cross-sectional photograph of a composite material of Comparative Sample 1. The charging / discharging curve of the battery using the composite material of the sample 1 is shown. The charging / discharging curve of the battery using the composite material of the comparative sample 1 is shown. It is a diagram which shows the XRD analysis result of the samples 2 and 3. FIG. It is a diagram which shows the XRD analysis result of the samples 1 and 4. FIG. It is a diagram which shows the XRD analysis result of the samples 1 and 4-9. It is a SEM photograph of the composite material of samples 6 and 7, the upper stage shows sample 6 and the lower stage shows sample 7. It is a SEM photograph of the composite material of samples 5 and 9, the upper stage shows sample 5 and the lower stage shows sample 9. It is a SEM cross-sectional photograph of the secondary particle of the sample 6, Comprising: The magnification of an upper stage and a lower stage is 20,000 times and 50,000 times. It is a charging / discharging curve of the battery using the composite material of the samples 6 and 7, and the upper stage shows the sample 6 and the lower stage shows the sample 7. It is the charging / discharging curve of the battery using the composite material of the samples 5 and 9, and the upper stage shows the sample 5 and the lower stage shows the sample 9. It is a conceptual explanatory view of the composite material of the present invention.

  The composite material and its manufacturing method, positive electrode active material, positive electrode, and non-aqueous electrolyte secondary battery according to embodiments of the present invention will be described in detail.

(Composite material)
The composite material of the present invention has secondary particles formed by aggregation of a plurality of primary particles. The primary particles are composed of a silicate core portion made of a lithium silicate compound and a carbon coating portion made of a carbon material and covering the surface of the silicate particles.

  The silicate core part in the primary particles has a relatively small average diameter and is finely dispersed in the secondary particles. The silicate core portion is made of a lithium silicate compound, and preferably has a single phase with few impurities. When the composite material is used as a positive electrode active material of a battery, the silicate core portion performs a battery reaction that occludes / releases lithium ions. And the surface of a silicate core part is coat | covered with the carbon coating layer which consists of carbon materials. The carbon coating layer has conductivity (electron conductivity) and enhances conductivity between the silicate core portions. For this reason, the battery using the composite material as the positive electrode active material is excellent in charge / discharge characteristics.

The lithium silicate compound constituting the silicate core portion contains lithium (Li), silicon (Si), and oxygen (O). Lithium silicate compounds have a basic composition of Li ₂ MSiO ₄ (M is one or more selected from Fe, Mn, and Co), but some of Li, M, and Si are substituted with other elements. May be. In the case of substitution with other elements, it is preferably performed within a range that does not adversely affect the capacity. Lithium silicate compounds having a composition slightly deviating from the above composition formula due to unavoidable loss of Li, M, Si or O and oxidation of the compounds are also included. Preferred lithium silicate compounds are those represented by the general formula Li _{1 + 2y} M _{1 + z} SiO _{4 + w} (M is one or more selected from Fe, Mn, and Co, 0.25 <y <0.6, 0 <z <0.2. And 0 <w <1.2) is the basic composition.

The lithium silicate compound is monoclinic and preferably belongs to the space group P2 ₁ / n.

  Lithium silicate compounds are involved in battery reactions, but many impurities are not involved in battery reactions. For this reason, it is preferable that the silicate core part is comprised by the single phase which consists of a lithium silicate type compound. That is, the silicate core part is composed of a single phase composed of a lithium silicate compound, and preferably does not contain impurities.

  When the total amount of the composite material is 100% by mass, the mass ratio of the lithium silicate compound constituting the silicate core portion is preferably 80% by mass or more and 95% by mass or less, and more preferably 85% by mass or more and 94% by mass. It is desirable that it is less than mass%. When the mass ratio of the lithium silicate compound is too small, the portion of the composite material that contributes to the battery reaction is reduced, and the battery capacity may be reduced. When the mass ratio of the lithium silicate compound is excessive, the carbon coating portion is relatively reduced, and the conductivity of the composite material may be reduced.

  The silicate core portion has, for example, a flat shape and a spherical shape. It is preferable that a plurality of silicate core portions are continuous in parallel in the secondary particles. Thereby, a void | hole is produced between primary particles and there exists an effect that the liquid retention property of electrolyte solution improves.

  The average diameter of the silicate core part is preferably 48 nm or more and 198 nm or less. For example, it is calculated by averaging the diameters of a plurality of silicate core parts observed from the SEM image of the composite material. When the average diameter of the silicate core portion is too small, the primary particles coated with the carbon coating layer are likely to be scattered and difficult to handle. When the average diameter of the silicate core part is excessive, the ion diffusibility to the inside of the silicate core part is delayed, and there is a possibility that the rate characteristic and the discharge capacity are lowered.

  The carbon covering portion is made of a carbon material (carbon particles). The carbon particles increase the conductivity of the composite material. The carbon particles cover a part or the entire surface of the silicate core part. The carbon material constituting the carbon coating layer is a material mainly containing carbon. The carbon material includes, for example, amorphous carbon formed by thermal decomposition of a carbon raw material such as dextrin.

  When the total amount of the composite material is 100% by mass, the mass ratio of the carbon material is preferably 5% by mass to 20% by mass, and more preferably 6% by mass to 15% by mass. . If the mass ratio of the carbon material is too small, the conductivity of the composite material may be reduced. When the mass ratio of the carbon material is excessive, the mass ratio of the silicate core portion is relatively reduced, and the portion of the composite material that contributes to the battery reaction is reduced, and the battery capacity of the composite material may be reduced. There is.

  The average thickness of the carbon coating portion is preferably 2 nm or more and 20 nm or less, and more preferably 3 nm or more and 10 nm or less. The average thickness of the carbon coating portion refers to the average value of the thickness of the carbon coating layer coated with the primary particles, and is calculated, for example, by observation of the primary particles with a transmission electron microscope (TEM). When the average thickness of the carbon coating portion is too small, the entire surface of the silicate core portion may not be covered and the conductivity may be lowered. When the average diameter of the carbon coating portion is excessive, the silicate core portion contributing to the battery reaction is relatively reduced, and the battery capacity may be reduced.

  Primary particles are formed by covering the surface of the silicate core portion with the carbon coating portion. The primary particle diameter of the primary particles is preferably 50 nm or more and 200 nm or less. The primary particle diameter of the primary particles refers to the average diameter of the primary particles, and is calculated, for example, by averaging the diameters of a plurality of primary particles observed from the SEM image of the composite material. When the primary particle diameter of the primary particles is too small, the primary particles are likely to be scattered and difficult to handle. When the primary particle diameter of the primary particles is excessive, the contact area between the primary particles is reduced, and the battery capacity may be reduced.

  The secondary particle diameter of the secondary particles is preferably from 100 nm to 10 μm, and more preferably from 500 nm to 5 μm. The secondary particle diameter of the secondary particles refers to the average diameter of the secondary particles, and is calculated, for example, by averaging the diameters of a plurality of secondary particles observed from the SEM image of the composite material. If the secondary particle diameter of the secondary particles is too small, the secondary particles are likely to be scattered and difficult to handle. When the secondary particle size is excessive, when secondary particles are used as the positive electrode active material, the permeability of the electrolyte to the inside of the secondary particles decreases, and the reactivity of the battery reaction inside the secondary particles May decrease.

  It is preferable that pores are formed inside the secondary particles. The pores are gaps between primary particles existing inside the secondary particles. The pores are surrounded by a carbon coating portion and / or a silicate core portion constituting the primary particles. The pores serve as an ion conduction path through which lithium ions circulate, and the ionic conductivity of the composite material is improved. Moreover, electrolyte is hold | maintained at a pore and it becomes easy to advance the battery reaction inside a secondary particle.

  The average diameter of the pores formed in the secondary particles is preferably 20 nm or less, and more preferably 15 nm or less. The average diameter of the pores means an average value of each pore and is calculated by a gas adsorption method. If the pores are excessive, the portion that does not contribute to the battery reaction is increased, and the discharge capacity may be reduced.

The BET specific surface area of the composite material is preferably 20 m ² / g or more and 100 m ² / g or less, more preferably 40 m ² / g or more and 80 m ² / g or less. The BET specific surface area of the composite material refers to the surface area of secondary particles per gram of the composite material, and is measured by a gas adsorption method. When the BET specific surface area of the composite material is too small, the silicate particles grow large and are not finely dispersed, and the electrical conductivity of the composite material may be reduced and the discharge capacity may be reduced. If the BET specific surface area of the composite material is excessive, it becomes difficult to handle and the electrode density may be reduced.

  The pore volume per 1 g of the composite material is preferably 0.01 cc / g or more and 0.10 cc / g or less, more preferably 0.01 cc / g or more and 0.06 cc / g or less. It is desirable that it is 0.03 cc / g or more and 0.05 cc / g or less. This pore volume is measured by a gas adsorption method. When the pore volume is too small, the permeability of the electrolyte is lowered, the ion conductivity is lowered, and the rate characteristics and the battery capacity may be lowered.

(Production method of composite material)
In the manufacturing method of the composite material of this invention, an energy provision process, a granulation process, and a heat treatment process are performed. In this production method, first, in the energy application step, a compound raw material that is a raw material of a lithium silicate compound is mixed with a carbon raw material that is a raw material of a carbon material. Next, in the granulation process by spray drying such as spray drying, primary particles formed by coating the periphery of the compound raw material with a carbon raw material are formed, and a plurality of primary particles are aggregated to form secondary particles. . Then, in the heat treatment step, a lithium silicate compound is generated from the compound raw material. At this time, a single phase of the lithium silicate compound is preferably generated.

  In the heat treatment step, the carbon raw material becomes a carbon material mainly containing a carbon element by thermal decomposition, chemical reaction, or the like, and the carbon material maintains a state where the surface of the lithium silicate compound is coated. For this reason, the particle | grains by which the surface of the silicate core part which consists of a lithium silicate type compound was coat | covered with the carbon coating layer which consists of carbon materials are formed. A large number of these particles aggregate as primary particles to form secondary particles.

  According to this production method, the surface of a fine silicate core portion made of a lithium silicate compound is coated with a highly conductive carbon coating layer to form particles, and these are aggregated to form highly conductive secondary particles. can do. Therefore, the composite material produced by this production method can exhibit excellent charge / discharge characteristics when used as a positive electrode active material of a battery. In addition, a composite material can be produced by a simple process using a raw material of a lithium silicate compound and a raw material of a carbon material as starting materials.

  In the energy applying step, mechanical energy is applied to a compound raw material that is a raw material of a lithium silicate compound containing lithium (Li), silicon (Si), and oxygen (O), and a carbon raw material in the presence of a solvent. By applying mechanical energy, the compound raw material and the carbon raw material are mixed to obtain a mixed raw material. The compound raw material and the carbon raw material may be simply mixed by applying mechanical energy, or may be pulverized and mixed.

  In the energy imparting step, mechanical energy is imparted to the compound raw material and the carbon raw material while being in contact with the solvent. At this time, the carbon raw material is preferably dissolved in a solvent to cover the periphery of the compound raw material. Furthermore, the compound raw material is pulverized by applying mechanical energy, and the carbon raw material may cover the surface of the pulverized compound raw material. Thereby, a compound raw material and a carbon raw material are fully mixed.

  The energy application step may be a step of applying mechanical energy by milling. Thereby, mechanical energy can be uniformly given to the compound raw material and the carbon raw material. The milling method is not particularly limited, but various milling methods such as ball milling and bead milling can be employed. In providing mechanical energy to the compound raw material and the carbon raw material, if the compound raw material and / or the carbon raw material is relatively large, first, roughly divide the compound raw material and the carbon raw material using balls or beads having a large diameter. It is good to grind. This is because a large diameter ball or bead has a relatively large pulverization force, and therefore has a high pulverization effect and can efficiently perform the pulverization process. Then, after roughly pulverizing using a large diameter ball or bead, it is preferable to finely pulverize using a small diameter ball or bead. This is because, according to the balls or beads having a small diameter, the roughly pulverized compound raw material and carbon raw material can be finely pulverized into a uniform particle size.

The magnitude of mechanical energy and the milling time given by milling depend on the size of the compound raw material and carbon raw material and the ball or bead material / size. For example, when using beads made of zirconia having a diameter of 100 μm, one bead Is preferably 4 × 10 ⁻⁸ J or more and 16 × 10 ⁻⁸ J or less. The time for applying the mechanical energy is preferably 1 hour or more and 5 hours or less. When the magnitude of the mechanical energy and / or the milling time is too short, mixing of the compound raw materials may be insufficient. When the magnitude of mechanical energy and / or the milling time is excessive, impurities derived from balls or beads may be mixed.

  In the energy application step, for example, the compound raw material and the carbon raw material can be mixed in an inert gas atmosphere (argon gas, nitrogen gas) or in an air atmosphere.

  The mechanical energy is applied in a solvent. This is because the compound raw material and the carbon raw material are mixed and pulverized smoothly. The solvent to be used is not particularly limited, but is preferably a water-soluble liquid such as water, ethanol or acetone. The addition amount of the solvent is preferably 150 parts by mass or more and 900 parts by mass or less when the mass of the mixed raw material composed of the compound raw material and the carbon raw material is 100 parts by mass.

The compound raw material to which mechanical energy is imparted and the carbon raw material are both raw materials for the composite material. The compound raw material is a raw material capable of forming a lithium silicate-based compound containing lithium (Li), silicon (Si), and oxygen (O). The compound raw material has the general formula Li _{1 + 2y} M _{1 + z} SiO _{4 + w} (M is one or more selected from Fe, Mn, and Co, 0.25 <y <0.6, 0 <z <0.2 and 0 < It may be a raw material capable of forming a lithium silicate compound represented by w <1.2).

The compound raw material may include, for example, a lithium raw material having a lithium source, a metal raw material including one or more metal sources selected from Fe, Mn, and Co, and a silicon raw material having a silicon source. The lithium raw material may be composed of, for example, one or more selected from the group of Li ₂ CO ₃ , LiOH, and LiNO ₃ .

When M in the general formula Li _{1 + 2y} M _{1 + z} SiO _{4 + w} is Fe, the metal raw materials are Fe ₂ O ₃ , Fe ₃ O ₄ , FeOOH, Fe (NO ₃ ) ₂ , Fe (NO ₃ ) ₃ , FeSO _{4. ,} FeCl ₂ , FeCl ₃ , and FeC ₂ O ₄ . When M in the general formula Li _{1 + 2y} M _{1 + z} SiO _{4 + w} is Mn, the metal raw materials are MnO ₂ , MnO, Mn ₃ O _4, MnOOH, Mn (NO ₃ ) ₂ , Mn (NO ₃ ) ₃ , MnSO _4. , MnCl ₂ , MnCl ₃ , and MnC ₂ O ₄ . When M in the general formula Li ₂ MSiO ₄ is Co, the metal raw materials are CoO, Co ₂ O ₃ , Co ₃ O _4, CoOOH, Co (NO ₃ ) ₂ , Co (NO ₃ ) ₃ , and CoSO _4. , CoCl ₂ , and CoCl ₃ may be used.

Among metal raw materials, a soluble metal raw material that can be dissolved in a solvent can be used, and a hardly soluble metal raw material that is difficult to dissolve in a solvent can also be used. Soluble metal raw materials are, for example, Fe (NO ₃ ) ₂ , Fe (NO ₃ ) ₃ , FeSO _4, FeCl ₂ , FeCl ₃ , Mn (NO ₃ ) ₂ , Mn (NO ₃ ) ₃ , MnSO ₄ , MnCl ₂ , MnCl ₃ , Co (NO ₃ ) ₂ , Co (NO ₃ ) ₃ , CoSO ₄ , CoCl ₂ , and CoCl ₃ . Examples of the hardly soluble metal raw materials include Fe ₂ O ₃ , Fe ₃ O ₄ , FeOOH, FeC ₂ O ₄ , MnO ₂ , MnO, Mn ₃ O _4, MnOOH, MnC ₂ O ₄ , CoO, and Co ₂ O _3. , Co ₃ O _4, CoOOH, and the like.

The silicon raw material may be made of, for example, one or more selected from the group of SiO ₂ , SiO, and Si. Among silicon raw materials, a soluble silicon raw material that can be dissolved in a solvent may be used, or a hardly soluble silicon raw material that is difficult to dissolve in a solvent may be used. Examples of the hardly soluble silicon raw material include SiO ₂ , SiO, and Si.

  The silicon raw material preferably has a particle size of 50 nm to 500 nm. When the particle size of the silicon raw material is 50 nm or less, it may be difficult to handle, and when it exceeds 500 nm, it may take time to grind during the energy application step.

  The carbon raw material that is a raw material of the carbon material is preferably made of an organic substance such as dextrin, sucrose, glucose, citric acid and the like. Furthermore, it is preferable that the carbon raw material be soluble in a solvent. Thereby, a carbon raw material is fully mixed with a compound raw material. Of these, dextrin is preferred. Since dextrin is thermally conductive carbon by thermal decomposition, a composite material having good conductivity is easily obtained.

  The compound raw material may contain a hardly soluble component that is difficult to dissolve in a solvent. For example, when the compound raw material is composed of a lithium raw material, a metal raw material, and a silicon raw material, at least one of these raw materials may be particles of a hardly soluble component that does not dissolve in the solvent. In this case, the particle size of the hardly soluble component affects the performance of the composite material. Of these raw materials, many of the metal raw materials and silicon raw materials are hardly soluble. When using a hardly soluble compound raw material, the particle size of the compound raw material is important.

Before the energy application step, the average diameter of the hardly soluble compound raw material is preferably 50 nm or more and 10 μm or less. An average diameter means the average value of the particle size of a compound raw material. In particular, when the compound raw material is hard such as SiO ₂ , the average diameter of the compound raw material is preferably small, for example, 1 μm or less. When the average diameter of the compound raw material is too small, it is difficult to handle and the workability may be deteriorated. If the average diameter of the compound raw material is excessive, it may take time to grind the compound raw material during the energy application step.

  In the energy imparting step, it is preferable to impart energy to the compound raw material and the carbon raw material until the average diameter of the compound raw material is approximately the same as the average diameter of the silicate core portion. When the raw material of the compound contains a hard raw material that is difficult to dissolve in the solvent, it is preferable to impart energy to the raw material of the compound and the carbon raw material until the raw material has the same diameter as the average diameter of the silicate core portion.

  After the energy application step, the weight passing percentage D50 of the mixed raw material composed of the compound raw material and the carbon raw material is preferably 1 μm or more and 10 μm or less, and more preferably 2 μm or more and 8 μm or less. When the weight passing percentage D50 of the mixed raw material is too small, it may be difficult to handle. When the weight passage percentage D50 of the mixed raw material is excessive, the reactivity of the compound raw material in the heat treatment step is lowered, and the purity of the lithium silicate compound in the silicate core part may be lowered.

  The mixing ratio of the carbon raw material to the compound raw material is preferably 5% to 40% in terms of mass ratio. If the mixing ratio of the carbon raw materials is excessive, the amount of lithium silicate compound that contributes to the battery reaction may be reduced, and the discharge capacity may be reduced. When the mixing ratio of the carbon raw materials is too small, the amount of the carbon material in the composite material may be reduced, and the conductivity of the composite material may be reduced.

The molar ratio of lithium contained in the lithium raw material, metal contained in the metal raw material (M is one or more selected from Fe, Mn, and Co) and silicon contained in the silicon raw material is a lithium silicate compound. It is preferable that the molar ratio of each element contained in is close. When the number of moles of silicon contained in the silicon raw material is 1 mole, the lithium contained in the lithium raw material is 1.8 mol or more and 2.2 mol or less, and the metal M contained in the metal raw material is 1.0 mol or more. It is preferable that it is 1.2 mol or less. When the lithium raw material is Li ₂ CO ₃ , the metal raw material is Fe ₂ O ₃ , and the silicon raw material is SiO ₂ , Li ₂ CO ₃ is 0 when the number of moles of SiO ₂ is 1 mol. and a .9 mol to 1.1 mol, _Fe 2 _{O 3} is preferably 1.0 mol or more and 1.2 mol or less.

Of the elements in the constituent elements of the lithium silicate compound, lithium may volatilize in a slight amount during each step, so it is preferable to mix a little more than the theoretical value. From this viewpoint, when the number of moles of silicon contained in the silicon raw material is 1 mole, the lithium contained in the lithium raw material is preferably 2.0 mol or more and 2.2 mol or less. When the lithium raw material is Li ₂ CO ₃ , the metal raw material is Fe ₂ O ₃ , and the silicon raw material is SiO ₂ , Li ₂ CO ₃ is 1 when the number of moles of SiO ₂ is 1 mol. 2.0 and the mole to 1.1 mole or less, _Fe 2 _{O 3} is preferably 1.0 mol or more and 1.2 mol or less.

  In the granulation step, the compound raw material and the carbon raw material to which mechanical energy is imparted in the presence of the solvent are spray-dried to evaporate the solvent and granulate the compound raw material and the carbon raw material. The compound raw material and the carbon raw material finely dispersed in the energy application step are each granulated in the granulation step. In the granulation step, in the carbon raw material, primary particles finely dispersed are aggregated to form secondary particles.

  The spray drying is preferably performed at a temperature of 150 ° C. or higher and 220 ° C. or lower, more preferably 180 ° C. or higher and 210 ° C. or lower, and a slurry feed rate range of 10 ml / min to 100 ml / min. When the temperature at the time of spray drying is too low, the drying is insufficient, the granulation of the compound raw material and the carbon raw material becomes insufficient, and unreacted impurities may remain in the silicate core part. When the temperature at the time of spray drying is too high, the primary particles and the secondary particles are small, and there is a possibility that it takes time and effort.

  Spray drying (spray drying method) of a mixed raw material composed of a compound raw material and a carbon raw material refers to a method of spraying a slurry obtained by mixing a mixed raw material in a solvent to dry the solvent. In spray drying, the compound raw material and carbon raw material after the energy application step are sprayed together with a solvent.

  In the heat treatment step, heat treatment is performed on the mixed raw material composed of the granulated compound raw material and carbon raw material. By the heat treatment, a lithium silicate compound is produced from the compound raw material after granulation. In addition, a carbon material mainly composed of carbon is produced by pyrolysis or carbonization of the carbon raw material. A carbon material is coated on the surface of the generated lithium silicate compound. Thus, primary particles composed of a silicate core portion made of a lithium silicate compound and a carbon coating portion made of a carbon material are formed.

  The heat treatment temperature is preferably 600 ° C. or higher and 1000 ° C. or lower. If it is less than 600 ° C., the complexing reaction is insufficient and impurities may remain. When it exceeds 1000 ° C., the amount of residual carbon decreases, and the conductivity of the composite material may decrease. Further, the lithium silicate compound crystal produced from the compound raw material grows too much, so that the contact portion with the carbon raw material decreases, and the discharge capacity may be reduced.

  Furthermore, the heat treatment temperature is preferably 650 ° C. or higher and 900 ° C. or lower, and more preferably 650 ° C. or higher and 800 ° C. or lower. In this case, the silicate core portion of the primary particles can be composed of a single layer of a lithium silicate compound without leaving impurities, and the carbon coating portion can be uniformly coated on the surface of the silicate core portion. it can. For this reason, the electrical conductivity of the composite material is increased and the discharge capacity is increased.

  The heat treatment time is preferably 0.3 hours or more and 15 hours or less. Further, it is preferably 0.5 hours or more and less than 10 hours, or 1 hour or more and 3 hours or less. In this case, an appropriate amount of the carbon material remains after the heat treatment, and the surface of the silicate core portion of the primary particles can be covered with the carbon coating portion. For this reason, the electroconductivity of a composite material becomes high and discharge capacity becomes high.

  The heat treatment is preferably performed in a reducing atmosphere or an oxidation-reduction mixed atmosphere. The reason is that the metal raw material can be appropriately reduced to increase the purity of the lithium silicate compound (increase the ratio of the single phase of the silicate core portion). For example, the reducing atmosphere may be a gas atmosphere containing carbon dioxide, carbon monoxide, or hydrogen.

(Positive electrode active material)
A positive electrode active material consists of said composite material. According to such a positive electrode active material, a battery having excellent charge / discharge characteristics can be configured.

(Positive electrode)
A positive electrode consists of a positive electrode active material which consists of said composite material, and a collector. In order to fabricate a positive electrode, for example, the composite material is added to a conductive auxiliary such as acetylene black (AB), ketjen black (KB), vapor grown carbon fiber (VGCF), polyvinylidene fluoride (PolyvinylideneDiFluoride). : PVdF), polytetrafluoroethylene (PTFE), binders such as styrene-butadiene rubber (SBR), and solvents such as N-methyl-2-pyrrolidone (NMP) are added to form a paste, which is used as a current collector Apply.

  The amount of the conductive aid used is not particularly limited, but for example, it can be 5 to 20 parts by mass with respect to 100 parts by mass of the composite material. Further, the amount of the binder used is not particularly limited, but may be 5 to 20 parts by mass with respect to 100 parts by mass of the composite material, for example. Further, as another method, a method of kneading a composite material, a mixture of the above-described conductive additive and a binder with a mortar or a press to form a film, and then crimping this to a current collector with a press Can also produce a positive electrode.

  The current collector is not particularly limited, and materials conventionally used as positive electrodes for nonaqueous electrolyte secondary batteries, such as aluminum foil, aluminum mesh, and stainless steel mesh, can be used. Furthermore, a carbon nonwoven fabric, a carbon woven fabric, etc. can be used as a collector.

  The shape and thickness of the positive electrode for a nonaqueous electrolyte secondary battery are not particularly limited. For example, the positive electrode active material is filled and then compressed to have a thickness of 10 to 200 μm, more preferably It is preferable to set it as 20-100 micrometers. Therefore, the filling amount of the positive electrode active material may be appropriately determined so as to have the above-described thickness after compression, depending on the type and structure of the current collector to be used.

(Non-aqueous electrolyte secondary battery)
The nonaqueous electrolyte secondary battery includes the positive electrode described above. The nonaqueous electrolyte secondary battery can be manufactured by a known method. The positive electrode described above is used as the positive electrode material.

  The negative electrode material is composed of an element compound that can occlude and release lithium ions and can be alloyed with lithium or / and an element compound that can be alloyed with lithium. Elements capable of alloying with lithium are Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Ti, Ag, Zn, Cd, Al, Ga, In, Si, Ge. , Sn, Pb, Sb, Bi may be included. Examples of the negative electrode material include known metal-based materials such as lithium metal and graphite, silicon-based materials such as SiOx (0.5 ≦ x ≦ 1.5), alloy-based materials such as copper-tin and cobalt-tin, An oxide material such as lithium titanate is preferably used.

As an electrolytic solution, a lithium salt such as lithium perchlorate, LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ or the like in a known non-aqueous solvent such as ethylene carbonate, dimethyl carbonate, or propylene carbonate is used in an amount of 0.5 mol / L to 1.7 mol / L. A solution dissolved at a concentration of L may be used, and other known battery components may be used.

  When metal lithium is used as the negative electrode, a lithium secondary battery is obtained, and when a material other than metal lithium is used as the negative electrode, a lithium ion secondary battery is obtained. In general, many secondary batteries that perform a battery reaction with these lithium ions are non-aqueous electrolyte secondary batteries.

  The non-aqueous electrolyte secondary battery can be mounted on a vehicle. The vehicle may be an electric vehicle or a hybrid vehicle. The nonaqueous electrolyte secondary battery is preferably connected to, for example, a traveling motor mounted on a vehicle and used as a drive source. In this case, a high driving torque can be output for a long time. Further, the non-aqueous electrolyte secondary battery can be mounted on devices other than vehicles such as personal computers and portable communication devices.

As mentioned above, although embodiment of this invention was described, this invention is not limited to the said embodiment. The present invention can be implemented in various forms without departing from the gist of the present invention, with modifications and improvements that can be made by those skilled in the art.

(Sample 1)
A composite material was produced by the following method.

<Energy application process>
First, ferric oxide Fe ₂ O ₃ (high purity chemical Co., Ltd., purity 99.99%, average diameter 5 μm) 0.5 mol and lithium carbonate Li ₂ CO ₃ (Kishida Chemical Co., Ltd., purity 99.5%) ) 1.1 mol, silica SiO ₂ (manufactured by Nippon Aerosil Co., Ltd., average diameter 0.1 μm) 1.0 mol, and dextrin hydrate (manufactured by Wako Pure Chemical Industries, Ltd.) 0.6 mol were mixed. Ferric oxide is a metal raw material, lithium carbonate is a lithium raw material, silica is a silicon raw material, and these constitute the compound raw material of the present invention. Hydrate dextrin constitutes the carbon raw material of the present invention. The mixing ratio was 69% by mass for the compound raw material composed of ferric oxide, lithium carbonate and silica, and 31% by mass for the carbon raw material composed of dextrin hydrate. This mixed raw material was wet-mixed for 3.5 hours by a bead mill (ZrO ₂ beads having a diameter of 100 μm).

<Granulation process>
The obtained mixed solution was spray-dried by a thermal spray apparatus. As shown in FIG. 1, the thermal spraying device dries the gas supply unit 3, the sprayer 4 having a nozzle that sprays the mixed solution together with the gas supplied from the gas supply unit 3, and the mixed solution sprayed from the sprayer 4. A drying chamber 5, a cyclone device 6 for separating the gas from the dried powder sprayed into the drying chamber 5, and a gas discharge unit 7 for discharging the gas separated from the dried powder. The thermal spraying device is trade name MDL-050M manufactured by Fujisaki Electric Co., Ltd.

In performing the spray drying process with the thermal spraying device, the mixed solution obtained above was supplied to the sprayer 4 while supplying the gas supply unit 3 with air with an air supply rate of 0.80 m ³ / min. The primary pressure of the nozzle air in the nebulizer 4 was 0.7 MpaG, the air flow rate was 40 NL / min., And the liquid feed rate of the mixed solution was 20 mL / min. The inlet temperature of the drying chamber 5 was 200 ° C. The gas pressure inside the drying chamber 5 was 4 kPa. The exhaust gas temperature in the gas discharge part 7 was 91 degreeC.

<Heat treatment process>
The obtained powder was heat-treated under the conditions of atmosphere CO ₂ : H ₂ (70:30 mL / min.), Temperature 700 ° C., and treatment time 2 hours.

After cooling to room temperature, XRD (X-ray diffraction) measurement of the treated product was performed. As a result, a composite material composed of monoclinic crystal, Li ₂ FeSiO ₄ and carbon belonging to the space group P2 ₁ / n was obtained. FIG. 2 shows an SEM photograph of the material after the granulation step and before the heat treatment step, and FIG. 3 shows an SEM photograph of the composite material obtained after the heat treatment step. The SEM cross-sectional photograph of the composite material obtained after the heat treatment process is shown in the upper and lower stages of FIG.

  As shown in FIG. 2, a plurality of secondary particles were formed after the granulation step and before the heat treatment step. Each secondary particle had a substantially spherical shape, and a carbon coating layer having a hazy shape was observed on the surface. The secondary particle diameter of the secondary particles was 0.5 to 5 μm.

  As shown in FIG. 3, after the heat treatment step, the secondary particles formed before the heat treatment step substantially maintained their outer shape. The secondary particles had a substantially spherical shape, and a carbon coating layer having a hazy shape was observed on the surface.

  As is clear from the cross-sectional view of the secondary particles shown in FIG. 4, the secondary particles include a silicate core portion represented by a white portion and a carbon coating layer covering the surface of the silicate core portion. Was agglomerated. The average diameter of the silicate core part was 0.1 μm. The average thickness of the carbon coating part was 5 nm. The primary particle diameter of the primary particles was 0.105 μm. Further, the silicate core portion was elongated in the left-right direction on the photograph in FIG. 4 and had a flat shape. In addition, a large number of pores were formed between the primary particles. The average diameter of the pores was 4 nm. It is considered that the composite material forms such a structure for the following reason. When each raw material is brought into contact with a solvent in the energy application step, dextrin and lithium carbonate dissolve in the solvent, while silica and ferric oxide maintain a solid. When milled in this state, solid silica and ferric oxide are mixed and pulverized into a fine powder. Lithium carbonate permeates into fine powdered silica and ferric oxide, and dextrin covers it. When spray-drying treatment is performed on the mixed raw material in this state, the solvent is dried and removed, and a large number of particles are aggregated in a state where particles are formed by covering the surfaces of silica, ferric oxide and lithium carbonate with dextrin. When heat treatment is performed on the mixed raw material in this state, a primary part is formed by using a lithium silicate compound as a core part from silica, ferric oxide and lithium carbonate, and covering the surface with a carbon material as a thermal decomposition component of dextrin as a coating layer. Form particles. A large number of aggregated primary particles constitute secondary particles.

  When the carbon content of the obtained composite material was examined, the carbon mass% when the entire composite material was 100 mass% was about 10%. In the energy application step, the mass ratio of the carbon material was considerably reduced even though 31 mass% of the carbon material was mixed when the total raw material was 100 mass%. This is presumably because the carbon material in the composite material was reduced by gasification by reducing the carbon material to carbon monoxide by the Budoir reaction in the granulation process.

(Comparative sample 1)
After forming the lithium silicate compound by the molten salt method, a composite material was prepared by performing milling treatment and heat treatment together with the carbon raw material. This will be described in detail below.

<Synthesis of molten salt>
To a mixture of 0.03 mol of lithium silicate (Li ₂ SiO ₃ : manufactured by Kishida Chemical Co., Ltd., purity 99.5%) and 0.03 mol of iron (product of high purity chemical Co., Ltd., purity 99.9%) Acetone (20 mL) was added, and the mixture was mixed at 500 rpm for 60 minutes in a zirconia ball mill and dried. This was mixed with the carbonate mixture. The carbonate mixture consists of lithium carbonate (Kishida Chemical Co., Ltd., purity 99.9%), sodium carbonate (Kishida Chemical Co., Ltd., purity 99.5%), and potassium carbonate (Kishida Chemical Co., Ltd., purity 99). 0.5%) in a molar ratio of 0.435 mol: 0.315 mol: 0.25 mol. As for the mixing ratio, the carbonate mixture was 80 parts by mass with respect to 100 parts by mass of the total amount of lithium silicate and iron. 20 mL of acetone was added to the above mixture, and the mixture was mixed for 60 minutes at 500 rpm in a zirconia ball mill and dried.

  Thereafter, the obtained powder was put into a gold crucible and heated to 500 ° C. in an electric furnace in a mixed gas atmosphere of carbon dioxide (flow rate 100 mL / min.) And hydrogen (flow rate 3 mL / min.) To obtain a carbonate mixture. Was reacted for 13 hours in a molten state.

  After the reaction, when the temperature of the molten salt reached 400 ° C., the entire reactor core as a reaction system was taken out of the electric furnace and rapidly cooled to room temperature through a mixed gas.

  Next, 20 mL of water was added to the obtained reaction product and ground in a mortar, and washing and filtration were repeated using water to obtain a powder from which salt had been removed. This powder was put into a dryer at 100 ° C. and dried for about 1 hour to obtain a lithium iron silicate compound.

Then, as a result of confirming the crystal structure by powder XRD measurement, it was found that monoclinic crystal, Li ₂ FeSiO ₄ belonging to the space group P2 ₁ / n was obtained.

<Milling process>
This Li ₂ FeSiO ₄ and carbon raw material (AB: acetylene black) were milled to obtain a mixed raw material. The mass ratio of Li ₂ FeSiO ₄ and AB was Li ₂ FeSiO ₄ : AB = 5: 4. In the milling treatment, a ball milling device P-7 manufactured by Fritsch Japan was used. The processing conditions were 450 rpm for 5 hours.

<Heat treatment>
The obtained mixed raw material was heat-treated to obtain a composite material. The heat treatment conditions were 700 ° C. for 2 hours, CO ₂ / H ₂ = 100: 3 mL / min. The atmosphere. It was confirmed that the XRD pattern of this composite material was consistent with Li ₂ FeSiO ₄ immediately after synthesis.

An SEM cross-sectional photograph of the composite material of the obtained comparative sample 1 was taken and shown in the upper part of FIG. As shown in the upper part of FIG. 5, in the composite material of Comparative Sample 1, a plurality of particles indicated by a gray portion and a plurality of particles indicated by a black portion were gradually aggregated to form an aggregate. It was found that the gray part particles were made of a lithium silicate compound, and the black part particles were made of a carbon material. The average particle diameter of the particles made of the lithium silicate compound was about 200 nm. The average particle diameter of the aggregate was about 1000 nm, and there were pores inside. The BET specific surface area of this composite material was 106 m ² / g.

  When the carbon content of the obtained composite material was examined, the carbon mass ratio when the entire composite material was 100% by mass was about 40%. This carbon amount of about 40% was slightly reduced from the amount of acetylene black added in <milling> (44%). From this, it was found that most of the added amount of carbon raw material was directly incorporated into the composite material as a carbon material.

(Production and evaluation of lithium ion secondary battery)
A half cell was fabricated using the composite material of Sample 1 and Comparative Sample 1. First, an electrode having the following composition was produced. The electrode composition was, by mass ratio, lithium iron silicate material and carbon composite: acetylene black: polytetrafluoroethylene (PTFE) = 17: 5: 1. An electrode having the above electrode composition was prepared by a sheet method, and vacuum-dried at 140 ° C. for 3 hours. Further, as an electrolytic solution, a solution of 1 mol / L was prepared by dissolving LiPF ₆ in ethylene carbonate (EC): dimethylene carbonate (DMC) = 3: 7. A coin battery was manufactured using a polypropylene film (Celgard, Celgard 2400) as a separator and a glass filter, and a lithium metal foil as a negative electrode together with the above electrode and electrolyte.

These coin batteries were subjected to a charge / discharge test at 30 ° C. The test conditions were 0.05 mA / cm ² and repeated charging and discharging at a voltage of 1.5 to 4.5 V. Only the first charge was changed from 1.5V to 4.8V. The results are shown in FIGS. 6 shows a charge / discharge curve of a battery using the composite material of Sample 1, FIG. 7 shows a charge / discharge curve of a battery using the composite material of Comparative Samples 1 and 2, and FIG. It was. Table 1 shows the initial charge capacity and initial discharge capacity of these batteries.

  As shown in FIG. 6, the charge / discharge characteristics of the battery using the composite material of Sample 1 were better than the charge / discharge characteristics of the battery using the composite material of Comparative Sample 1. In particular, in Sample 1, the discharge capacity from the second time to the fifth time was high.

  The reason why the sample 1 exhibits better charge / discharge characteristics than the comparative sample 1 is considered as follows. First, in Sample 1, the surface of the silicate core portion was covered with a conductive carbon coating portion, whereby conductivity was imparted between the silicate core portions. In addition, the conductivity increased because a plurality of primary particles formed by coating the surface of the silicate core with a carbon coating layer were thinly formed. Further, the compound raw materials were finely dispersed with each other, and heat treatment was performed in a finely dispersed state. Thereby, the reactivity of each particle became high and the lithium silicate type compound with high purity was obtained.

(Sample 2)
A composite material was prepared by the following method.

<Energy application process>
First, ferric oxide Fe ₂ O ₃ (manufactured by Kojun Chemical Co., Ltd., purity 99.99%, diameter 5 μm) 25% by mass and lithium carbonate Li ₂ CO ₃ (manufactured by Kishida Chemical Co., Ltd., purity 99.5%) 25 Mass%, silica SiO ₂ (manufactured by Nippon Aerosil Co., Ltd., diameter 0.8 μm) 19 mass%, and dextrin hydrate (manufactured by Wako Pure Chemical Industries, Ltd.) 31 mass% were mixed. As for this mixing ratio, the compound raw material consisting of ferric oxide, lithium carbonate and silica was 69% by mass, and the carbon material consisting of dextrin hydrate was 31% by mass. This mixture was wet-mixed for 6 hours with a ball mill (ZrO ₂ balls having a diameter of 2000 μm). The weight passing percentage D50 of the mixture after the wet mixing was 5.8 μm.

<Granulation process>
The obtained mixed solution was spray-dried by the thermal spraying apparatus used in Sample 1. The conditions of the spray drying process were the same as those of Sample 1.

<Heat treatment process>
The obtained powder was heat-treated under the conditions of atmosphere CO ₂ : H ₂ (70 cc: 30 cc), temperature 800 ° C., and treatment time 2 hours.

After cooling to room temperature, XRD (X-ray diffraction) measurement of the treated product was performed. As a result, secondary particles composed of monoclinic crystal, Li ₂ FeSiO ₄ and carbon belonging to the space group P2 ₁ / n were obtained. The secondary particle diameter of the secondary particles was about 3 μm.

(Sample 3)
This sample 3 is the same as the sample 2 except that the diameter of silica in the compound raw material is 0.1 μm and that the milling of the energy application process is performed using a bead mill. The bead mill was equipped with ZrO ₂ beads having a diameter of 100 μm, and various materials were wet mixed for 3.5 hours. The weight passing percentage D50 of the mixture after the wet mixing and before the spray-drying treatment was 5.2 μm. After wet mixing, a granulation process and a heat treatment process were performed by spray drying.

The composite of Samples 2 and 3 was measured by powder XRD. FIG. 8 shows the XRD measurement results. The upper part of FIG. 8 shows the pattern of the composite material of Sample 2, and the lower part of FIG. 8, the peak indicated by circles ○ are derived from lithium silicate _Li 2 FeSiO _4, the peak marked with an ellipse in longitudinal, derived from _Li 2 SO _3, the peak marked with black squares _Li 5 _Fe 5 0 ₈ is derived from. Li ₂ SO ₃ and Li ₅ Fe ₅ 0 ₈ are impurities.

As shown in FIG. 8, the secondary particles of the sample 3 had a larger peak of the target lithium silicate compound Li ₂ FeSiO ₄ than the secondary particles of the sample 2, while the impurities were significantly smaller. For this reason, in sample 3 using silica having a small particle size, hard silica is finely dispersed, mixed well with iron oxide or lithium carbonate dissolved in a solvent, and heat-treated in a well-dispersed state. For this reason, it is thought that the lithium silicate type compound with high reactivity between each particle | grains and high purity produced | generated.

(Sample 4)
For this sample, a composite material was prepared in the same manner as Sample 1 except that the heat treatment temperature was 600 ° C.

The X-ray diffraction patterns of the composite materials of Samples 1 and 4 were measured and shown in FIG. The top stage of FIG. 9 is the diffraction pattern of the sample 4, the next stage is the diffraction pattern of the sample 1, and the lower five stage patterns are Fe ₃ O ₄ , Li ₂ SiO ₃ , Fe in order from the top. It is a pattern which shows the standard peak position of Si, FeO. The peak position derived from the target lithium silicate compound Li ₂ FeSiO ₄ is indicated by a circle ○ in the diffraction pattern. Note that Fe ₃ O ₄ , Li ₂ SiO ₃ , Fe, Si, and FeO are impurities that can be included in the lithium silicate compound.

  As shown in FIG. 9, the diffraction pattern of the composite material of Sample 1 had a higher peak intensity derived from lithium silicate than Sample 4. For this reason, in the composite material of the sample 1, it is thought that the crystal | crystallization of a lithium silicate type compound grows rather than the sample 4, and becomes a single phase. In sample 4, many impurities are present, and the single phase has not progressed.

(Sample 5)
For this sample, a composite material was prepared in the same manner as Sample 1 except that the heat treatment temperature was 650 ° C.

(Sample 6)
For this sample, a composite material was prepared in the same manner as Sample 1 except that the heat treatment temperature was 800 ° C.

(Sample 7)
For this sample, a composite material was prepared in the same manner as Sample 1 except that the heat treatment temperature was 900 ° C.

(Sample 8)
For this sample, a composite material was prepared in the same manner as Sample 1, except that the heat treatment temperature was 700 ° C. and the heat treatment time was 0.5 hours.

(Sample 9)
For this sample, a composite material was prepared in the same manner as Sample 1, except that the heat treatment temperature was 700 ° C. and the heat treatment time was 10 hours.

The X-ray diffraction patterns of the composite materials of Samples 1 and 4 to 8 were measured and are shown in FIG. The spectrum of sample 1 in FIG. 10 is the same as the spectrum of sample 1 in FIG. 9, and there is a peak derived from lithium silicate Li ₂ FeSiO _{4 at the} same position. As shown in FIG. 10, it was found that the higher the heat treatment temperature, the larger the peak derived from the lithium silicate compound Li ₂ FeSiO ₄ , and the crystal grew. In addition, the higher the heat treatment temperature, the less impurities. Sample 9 with the same heat treatment temperature and a treatment time of 10 hours had the same crystal structure as Sample 1 with a heat treatment time of 2 hours, and no significant growth of the lithium silicate compound was observed. Further, Sample 9 with a heat treatment time of 30 minutes was the same as Sample 1 with a heat treatment time of 2 hours, and no significant change in crystal was observed. From this, it was found that the higher the heat treatment temperature, the more the crystal grows, and the crystal growth is sufficient if the heat treatment time is 0.5 hours.

  Next, SEM photographs of Samples 1, 5 to 7 and 9 were taken. FIG. 11 shows SEM photographs of the composite materials of Samples 6 and 7, and FIG. 12 shows SEM photographs of Samples 5 and 9. In FIG. 13, the SEM cross-sectional photograph of the sample 6 was shown. As described above, the SEM photograph of Sample 1 is shown in FIG. 3, and the SEM cross-sectional photograph is shown in FIG.

  As described above, when the heat treatment temperature is 700 ° C. (sample 1), as shown in FIGS. 3 and 4, a plurality of primary particles aggregate to form secondary particles. The surface of the primary particles was covered with a carbon material (moy portion).

  As shown in FIG. 11, when the heat treatment temperature was 800 ° C. (sample 6), the crystal grains of lithium silicate grew greatly. A plurality of lithium silicate particles were aggregated and formed secondary particles together with the carbon material. The secondary particle diameter of the secondary particles was larger than when the heat treatment temperature was 700 ° C. Among the secondary particles, the carbon material did not cover the lithium silicate particles and existed between the lithium silicate particles. As shown in FIG. 13, the lithium silicate compounds are slightly continuous, but many of them formed independent spherical particles.

  As shown in FIG. 11, when the heat treatment temperature was 900 ° C. (sample 7), the crystal grains of lithium silicate grew larger. A plurality of these particles gathered to form secondary particles with a carbon material interposed between the particles. The secondary particle diameter of the secondary particles was larger than when the heat treatment temperatures were 700 ° C and 800 ° C.

  As shown in FIG. 12, when the heat treatment temperature was lowered to 650 ° C. (Sample 5), secondary particles were formed as in the case of heat treatment at 700 ° C., but the secondary particle diameter was relatively small. It was. Even in the heat treatment at 650 ° C., similarly to the heat treatment at 700 ° C., the secondary particles are coated with a carbon material on the surface of the lithium silicate particles to form primary particles, and a large number of primary particles are aggregated to form each secondary particle. Was.

  When the heat treatment temperature was 700 ° C. and the heat treatment time was as long as 10 hours (Sample 9), the secondary particle diameter of the secondary particles was larger than when the heat treatment time was 2 hours (Sample 1).

  From the above observation results, when the heat treatment temperatures are 650 ° C. and 700 ° C., as shown in FIG. 16, the surface of the silicate core portion 11 made of a lithium silicate compound is covered with the carbon covering portion 12 made of a carbon material. As a result, primary particles 1 were formed. The plurality of primary particles 1 aggregated with each other to form secondary particles 2. It was found that the composite material 20 was formed by the assembly of such secondary particles 2.

  Table 2 shows the secondary particle diameter of the secondary particles of Samples 1 to 5 and the mass ratio (mass%) of the carbon material when the entire composite material is 100 mass%.

  As shown in Table 2, as the heat treatment temperature was increased, the secondary particle size was increased and the mass ratio of the carbon material was decreased. The reason why the secondary particle size increases is that the crystal grains of lithium silicate grow as the heat treatment temperature increases. In addition, the mass ratio of the carbon material is decreased. It is estimated that the higher the heat treatment temperature, the easier the carbon (C) reacts with carbon dioxide and the gas is evaporated to evaporate. The

  Next, the BET specific surface area, the pore diameter, and the pore volume were measured for the composite materials of Samples 1 and 5-9. The BET specific surface area was measured by a gas adsorption method. The pore diameter means an average diameter of pores formed in the secondary particles, and was measured by a gas adsorption method. The pore volume refers to the volume of pores per gram of the composite material, and was measured by a gas adsorption method.

  As shown in Table 3, the BET specific surface area of the composite material tended to decrease as the heat treatment temperature increased. When the heat treatment time at 700 ° C. was changed to 0.5 hours, 2 hours, and 10 hours, the BET specific surface area was the highest in the case of 2 hours.

  The pore diameter of the secondary particles was almost constant even when the heat treatment temperature and the heat treatment time were changed, and ranged from 11.46 to 11.7 nm. The pore volume of the composite material tended to decrease as the heat treatment temperature increased. When heat treatment was performed at 700 ° C., the pore volume was highest when the heat treatment time was 2 hours.

  From the above, it was found that the composite material heat-treated at 700 ° C. for 2 hours contains many pores having a diameter of about 11 nm or less and many fine pores exist.

  Next, a lithium ion secondary battery was fabricated using the composite materials of Samples 1, 5 to 7, and 9 by the same method as the above “lithium ion secondary battery”. A charge / discharge curve of each battery was prepared. FIG. 14 shows charge / discharge curves of the batteries of Samples 6 and 7, and FIG. 15 shows charge / discharge curves of the batteries of Samples 5 and 9. Table 4 shows the initial charge capacity and initial discharge capacity of Samples 3, 5 to 7, and 9.

  As shown in FIGS. 14 and 15, Sample 1 that was heat-treated at 700 ° C. for 2 hours showed higher charge / discharge characteristics than other samples. Sample 1 also had the highest initial discharge capacity. The reason is that the silicate core part, which is the place of battery reaction, is finely dispersed, and each silicate core part is covered with a carbon coating part, so the conductivity between the silicate core parts is improved, and the entire composite material is This is probably because the conductivity was greatly improved.

  The charge and discharge characteristics were best when the heat treatment was performed at 700 ° C. for 2 hours, but when the heat treatment was performed at 700 ° C. for 10 hours (Sample 9) and when the heat treatment was performed at 650 ° C. for 2 hours (Sample 5) ) Also showed good results.

  Moreover, the charge / discharge characteristics decreased as the heat treatment temperature increased. This is because the crystal grains of the lithium silicate particles grow large, and the carbon material is dispersed in the secondary particles separately from the lithium silicate particles without being covered with the carbon material, so that the conductivity between the silicate particles is This is thought to be due to the decrease in

1: primary particles, 11: silicate core part, 12: carbon coating part, 2: secondary particles, 20: composite material, 3: gas supply part, 4: sprayer, 5: drying chamber, 6: cyclone apparatus, 7: Gas exhaust section.

Claims (21)

A composite material having secondary particles formed by aggregation of a plurality of primary particles,
The primary particles include a silicate core portion made of a lithium silicate compound containing lithium (Li), silicon (Si) and oxygen (O), and a carbon coating portion made of a carbon material and covering the surface of the silicate core portion. Become
Composite material characterized that you have formed the following average pore diameter 20nm inside of the secondary particles. The lithium silicate compound has a general formula of Li _{1 + 2y} M _{1 + z} SiO _{4 + w} (M is one or more selected from Fe, Mn, and Co, 0.25 <y <0.6, 0 <z <0.2. The composite material according to claim 1, wherein 0 <w <1.2).   The composite material according to claim 1 or 2, wherein an average diameter of the silicate core portion is 48 nm or more and 198 nm or less.   The composite material according to claim 1, wherein an average thickness of the carbon coating portion is 2 nm or more and 20 nm or less.   The composite material according to any one of claims 1 to 4, wherein a primary particle diameter of the primary particles is 50 nm or more and 200 nm or less.   The composite material according to any one of claims 1 to 5, wherein a secondary particle diameter of the secondary particles is 100 nm or more and 10 µm or less. The composite material according to any one of claims 1 to 6 , wherein a mass ratio of the carbon material is 5% by mass or more and 20% by mass or less when the total amount of the composite material is 100% by mass. A silicate core portion made of a lithium silicate compound containing lithium (Li), silicon (Si) and oxygen (O), and a carbon material having secondary particles formed by aggregating a plurality of primary particles A method for producing a composite material comprising a carbon coating portion covering the surface of the silicate core portion, comprising:
Energy for obtaining a mixed raw material comprising the compound raw material and the carbon raw material by imparting mechanical energy to the compound raw material that is the raw material of the lithium silicate compound and the carbon raw material that is the raw material of the carbon material in the presence of a solvent. Granting process;
A granulation step of drying the solvent and granulating the mixed raw material by spray drying the mixed raw material to which the mechanical energy is applied in the presence of the solvent;
And a heat treatment step of heat-treating the granulated mixed raw material at 650 ° C. or higher and 800 ° C. or lower . The method for producing a composite material according to claim 8 , wherein the mixed raw material is spray-dried in the granulation step by spray drying the mixed raw material. The method for manufacturing a composite material according to claim 8 or 9 , wherein the heat treatment performed in the heat treatment step is performed at 650 ° C or more and 700 ° C or less. The method for producing a composite material according to any one of claims 8 to 10 , wherein the heat treatment performed in the heat treatment step is performed for 0.3 hours or more and 15 hours or less. Said compound raw material, according to claim 8 comprising a lithium source with lithium source, Fe, Mn, and a metal material containing a transition metal source comprising one or more selected from among Co, and a silicon material having a silicon source The manufacturing method of the composite material of any one of -11 . The said compound raw material is a manufacturing method of the composite material of any one of Claims 8-12 containing the hardly soluble component which is hard to melt | dissolve in the said solvent. The method of producing a composite material according the silicon raw material in claim 12 or 13, which is a SiO _2. The method for producing a composite material according to any one of claims 12 to 14 , wherein a particle diameter of the silicon raw material is 50 nm or more and 500 nm or less. The method for producing a composite material according to claim 8 , wherein the carbon raw material is dextrin. The method for producing a composite material according to any one of claims 8 to 16 , wherein milling is performed in applying the mechanical energy to the compound raw material and the carbon raw material. Positive electrode active material, comprising the composite materials according to any one of claims 1-7. A positive electrode comprising the positive electrode active material according to claim 18 . A nonaqueous electrolyte secondary battery comprising the positive electrode according to claim 19 , a negative electrode, and an electrolyte.   The manufacturing method of a nonaqueous electrolyte secondary battery using the said composite material manufactured by the manufacturing method in any one of Claims 8-17.