JP7074006B2 - Composite particles - Google Patents

Description

The present invention relates to Li ₅ FeO ₄ used as a positive electrode material for a lithium ion secondary battery or the like.

Since the lithium ion secondary battery is small and has a large capacity, it is used as a battery for various devices such as mobile phones and notebook personal computers. The lithium ion secondary battery includes a positive electrode, a negative electrode and an electrolytic solution as main components. The positive electrode has a current collector and a positive electrode active material layer formed on the surface of the current collector and containing a positive electrode active material.

It is known that various lithium metal composite oxides are used as a positive electrode active material of a lithium ion secondary battery, and Li ₅ FeO ₄ is known as one of them. It is also known that Li ₅ FeO ₄ has a small reversible capacity and generates gas during charging, and is used as a lithium ion feeder for supplementing the irreversible capacity of the negative electrode and as an additive for the positive electrode. It is also known to be used in the above applications.

For example, Patent Document 1 specifically describes a lithium ion secondary battery including a positive electrode using LiMn ₂ O ₄ and Li ₅ FeO ₄ at a mass ratio of 95: 5, and Li ₅ FeO ₄ . It is stated that the addition was able to compensate for the initial capacity loss of the negative electrode active material.

Patent Document 2 describes a lithium ion secondary battery having only Li ₅ FeO ₄ as a positive electrode active material, and a lithium ion secondary battery having Li ₅ FeO ₄ as a positive electrode active material and a metal oxide containing no Li. However, it is specifically described.

Patent Document 3 specifically describes a lithium ion secondary battery including a positive electrode using LiCoO ₂ and Li ₅ FeO ₄ as positive electrode active materials at a mass ratio of 91: 9 to 97: 3. It is described that the initial charge capacity could be improved by adding ₅ FeO ₄ .

Patent Document 4 describes that Li ₅ FeO ₄ is added to the positive electrode active material layer as a positive electrode additive that generates gas during the initial charge, and the gas is generated during the initial charge to empty the positive electrode active material layer. It is stated that holes can be formed.

Japanese Unexamined Patent Publication No. 2007-287446 Japanese Unexamined Patent Publication No. 2012-99316 Japanese Unexamined Patent Publication No. 2014-157653 International Publication No. 2014/118834

In recent years, the industrial world has been demanding lithium-ion secondary batteries having excellent battery characteristics and excellent materials for manufacturing lithium-ion secondary batteries, and new technologies for realizing them have been demanded. There is.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a new Li ₅ FeO ₄ related material.

When the present inventor investigated Li ₅ FeO ₄ , it was found that the battery equipped with Li ₅ FeO ₄ does not sufficiently develop the capacity. The present inventor presumed that such a phenomenon was due to the low conductivity, that is, the high resistance property of Li ₅ FeO ₄ . Then, for the purpose of increasing the conductivity of Li ₅ FeO ₄ , a carbon-coated Li ₅ FeO ₄ in which the surface of the Li ₅ FeO ₄ was coated with a carbon film was manufactured and tested. As a result, the carbon-coated Li ₅ FeO ₄ was tested. However, it was found that the volume may not always be sufficiently expressed.

The present inventor tried to produce a carbon-coated Li ₅ FeO ₄ under various conditions in order to improve the capacity of the battery, and aimed to produce a composite particle containing Li ₅ FeO ₄ and carbon. The present inventor has completed the present invention through further diligent research.

The composite particles of the present invention are
It is a composite particle containing Li ₅ FeO ₄ and carbon, and has a BET specific surface area of 2 m ² / g or less and a carbon mass percentage (Wc) within the range of 1 ≦ Wc ≦ 6.

The composite particle of the present invention develops a sufficient capacity as a positive electrode material.

It is an SEM image of the composite particle of the reference example 1 in the evaluation example 5. It is an X-ray diffraction chart of the composite particle of Example 1 and the composite particle of Comparative Example 1 in Evaluation Example 6. It is a Raman spectroscopic spectrum of the composite particle of Example 3, the composite particle of Comparative Example 2, the composite particle of Reference Example 1, and graphite in Evaluation Example 7.

Hereinafter, embodiments for carrying out the present invention will be described. Unless otherwise specified, the numerical range "a to b" described in the present specification includes the lower limit a and the upper limit b in the range. Then, a numerical range can be configured by arbitrarily combining these upper limit values and lower limit values, as well as the numerical values listed in the examples. Further, a numerical value arbitrarily selected from these numerical values can be used as a new upper limit or lower limit numerical value.

The composite particle of the present invention is a composite particle containing Li ₅ FeO ₄ and carbon, has a BET specific surface area of 2 m ² / g or less, and has a carbon mass percentage (Wc) within the range of 1 ≦ Wc ≦ 6. Therefore, a sufficient capacity (lithium ion emission amount) as a positive electrode material is developed.

The composite particle of the present invention is a composite particle containing Li ₅ FeO ₄ and carbon. From a different point of view, the composite particle of the present invention can also be said to be a particle in which a phase of Li ₅ FeO ₄ and a phase of carbon are composited. Hereinafter, if necessary, the phase of Li _{5 FeO 4 contained in the composite particle of the present invention is referred to as Li 5 FeO 4 phase ,} and the carbon phase is referred to as carbon phase.

The Li ₅ FeO ₄ phase and the carbon phase in the composite particle of the present invention may have any size or shape, and the composite particle of the present invention is used as a positive electrode material for a lithium ion secondary battery. If so, it is preferable that the carbon phase is present on the surface of the particulate Li ₅ FeO ₄ phase. Although the Li ₅ FeO ₄ phase is mainly involved in the battery reaction, it is relatively inferior in conductivity, and the carbon phase is a part that supplements the conductivity of the Li ₅ FeO ₄ phase by its own excellent conductivity. Because.

The carbon phase present on the surface of the particulate Li ₅ FeO ₄ phase is preferably in the form of particles having a smaller diameter than that of the Li ₅ FeO ₄ , and may be scattered on the entire surface of the Li ₅ FeO ₄ . It may be present only on a part of the surface of the Li ₅ FeO ₄ .

In any case, the mass% (Wc) of carbon in the composite particle of the present invention is within the range of 1 ≦ Wc ≦ 6. Preferred ranges for the mass of the carbon include 1.5 ≦ Wc ≦ 5.0, 2.0 ≦ Wc ≦ 3.5, and 2.2 ≦ Wc ≦ 3.0. be able to.

The BET specific surface area of the composite particle of the present invention may be 2 m ² / g or less, and suitable ranges of the BET specific surface area are 1.8 m ² / g or less, 1.5 m ² / g or less, and 1.2 m. The ranges of ² / g or less, 1.0 m ² / g or less, and 1.0 m ² / g or less can be mentioned. There is no lower limit to the BET specific surface area of the composite particles of the present invention, but it is realistic to set it to 0.1 m ² / g or more.

If the BET specific surface area is too small, the capacity may be inferior. On the other hand, if the BET specific surface area is excessive, it may react with moisture or carbon dioxide in the atmosphere to deteriorate Li ₅ FeO ₄ , or may adversely affect the physical characteristics of the slurry during the production of the positive electrode. Therefore, in order to stabilize the slurry, specifically, to suppress gelation of the slurry, it can be said that it is preferable that the BET specific surface area of the composite particles of the present invention is small. Since the BET specific surface area of the composite particle of the present invention is 2 m ² / g or less, it can be said that it is sufficiently small. That is, according to the composite particles of the present invention, gelation of the slurry during the production of the positive electrode can be suppressed.

Further, since Li ₅ FeO ₄ is a strong alkali, it can be a factor of gelation of the slurry. Therefore, in order to suppress gelation of the slurry, it is considered preferable to separate the Li ₅ FeO ₄ phase in the composite particles of the present invention from the solvent constituting the slurry.
In the composite particle of the present invention, as described above, a carbon phase is present on the surface of the particulate Li ₅ FeO ₄ phase. It can be said that the solvent is separated from the Li ₅ FeO ₄ phase by the carbon phase. From this, it can be said that according to the composite particles of the present invention, gelation of the slurry during the production of the positive electrode can be suppressed.

The composite particles of the present invention are preferably in a powder state. As the powder resistance value of the composite particle of the present invention, 10 ^-1 to 5 × 10 ³ Ωcm, 1 to 1 × 10 ³ Ωcm, and 10 to 5 × 10 ² Ωcm can be exemplified.
The powder resistance value in the present specification means the volume resistance value when a load of 4 kN is applied to the measurement sample.

The method for producing the composite particle of the present invention will be described. Hereinafter, the production method will be referred to as a method for producing composite particles of the present invention, if necessary.
The method for producing composite particles of the present invention comprises a step of heating a Li source and an Fe source in the presence of a carbon source to synthesize Li ₅ FeO ₄ . If necessary, the process is referred to as a synthesis process.

As a method for synthesizing Li ₅ FeO ₄ , a method of reacting a Li source and an Fe source under heating conditions to synthesize them is known. In the synthesis step of the method for producing composite particles of the present invention, Li ₅ FeO ₄ is synthesized in a reaction system in which a carbon source is present in addition to the Li source and the Fe source.

Examples of the Li source include lithium alone, lithium oxide, lithium hydroxide, lithium carbonate, lithium hydrogen carbonate, and lithium fluoride. Examples of the Fe source include iron alone, iron oxide, iron hydroxide, iron oxyhydroxide, iron sulfate, iron nitrate, and iron chloride. When the composition of the Li source and the Fe source contains less oxygen than the theoretical amount, oxygen may be introduced into the reaction system.
From the viewpoint of the purity of the product, it is preferable to react lithium simple substance with iron oxide, react lithium oxide with simple substance iron, or react lithium oxide with iron oxide.

In the method for producing composite particles of the present invention, a mixed raw material containing a Li source, an Fe source and a carbon source is heated to the synthesis temperature of Li ₅ FeO ₄ . By doing so, it is considered that the synthesis of Li ₅ FeO ₄ using the Li source and the Fe source as raw materials proceeds in the vicinity of the carbon source. When a simple substance such as graphite is used as the carbon source, the carbon source does not affect the synthesis of Li ₅ FeO ₄ using the Li source and the Fe source as raw materials, so that the carbon phase composed of the simple carbon is Li ₅ FeO ₄ . It is considered that composite particles attached to the surface of the particles can be obtained.

When an organic substance such as a hydrocarbon is used as the carbon source, it is necessary to heat the mixed material to the carbonization temperature of the organic substance to obtain a carbon phase. However, since the carbonization temperature of the organic substance is often higher than the synthesis temperature of Li ₅ FeO ₄ , Li ₅ FeO ₄ may be adversely affected by heat when the organic substance is carbonized. Further, as will be described later, under high temperature, Li ₅ FeO ₄ may react with carbon contained in the carbon source and be reduced and decomposed.

Therefore, it is preferable to use simple carbon as the carbon source in the method for producing composite particles of the present invention. Considering cost, handleability, actual use in the positive electrode of the lithium ion secondary battery, and the like, it is preferable to select graphite as the carbon simple substance. Graphite is used as a conductive auxiliary agent in the positive electrode of a lithium ion secondary battery.

By the way, in the method for producing composite particles of the present invention, since the mixed raw material used in the synthesis step contains a carbon source, it can be said that the carbon of the carbon source exists in the vicinity of Li ₅ FeO ₄ produced in the synthesis step.

Here, when the synthesis step is carried out at a temperature exceeding 650 ° C., the Li ₅ FeO ₄ produced in the synthesis step may react with carbon and be reduced and decomposed. As the reductive decomposition of Li ₅ FeO ₄ progresses, it becomes difficult to produce the composite particles of the present invention in good yield.
In addition, Fe crystals are generated by this reduction decomposition. When the Fe crystals are brought into the positive electrode active material layer of the lithium ion secondary battery, Fe crystals may precipitate on the surface of the negative electrode when the lithium ion secondary battery is charged. When Fe crystals are deposited on the surface of the negative electrode, dendrites may grow around the Fe crystals as nuclei.
In order to avoid these problems, it is not preferable to synthesize Li ₅ FeO ₄ at a temperature exceeding 650 ° C.

The synthesis temperature of Li ₅ FeO ₄ is considered to be different depending on the type of Li source. When lithium carbonate is used as the Li source, the temperature required for the synthesis of Li ₅ FeO ₄ is relatively high and exceeds 650 ° C., but when lithium hydroxide or lithium oxide is used as the Li source, the temperature is less than 650 ° C. Li ₅ FeO ₄ can be synthesized at a low temperature.
Therefore, in order to produce the composite particles of the present invention in good yield and suppress the formation of Fe crystals, it can be said that it is preferable to select lithium hydroxide or lithium oxide as the Li source.

Further, when lithium oxide or lithium carbonate is used as the Li source, the Li source may stick to the reactor of the heating device that heats the mixed raw material in the synthesis step. When the Li source is fixed, the yield of the composite particle deteriorates.
By adding an organic additive that can function as a lubricant such as a sucrose fatty acid ester or a fatty acid to the mixed raw material, the problem of sticking of the Li source can be suppressed. However, in this case, due to the function of the organic additive, the particle size of the composite particle may be reduced, and the BET specific surface area of the composite particle may exceed the upper limit of the BET specific surface area of the composite particle of the present invention described above.

When lithium hydroxide is used as the Li source, the lithium hydroxide itself can function as a lubricant, so that the problem of sticking of the Li source is unlikely to occur, and it is not necessary to add an organic additive. As a result, it is suppressed that the particle size of the composite particles becomes too small, and the BET specific surface area of the composite particles also falls within the range specified in the present invention. Therefore, it can be said that it is particularly preferable to select lithium hydroxide as the Li source of the composite particles of the present invention. As the lithium hydroxide, a hydrate may be used or an anhydrous one may be used.

In the synthesis process, as a device for heating the mixed raw material of the Li source, the Fe source and the carbon source, for example, a hot wall type, a cold wall type, a horizontal type, a vertical type and the like, a flow layer reactor, a rotary furnace and a tunnel A known heating device such as a furnace, a batch firing furnace, or a rotary kiln may be used.

In addition, it is desirable that the synthesis step is carried out by putting the above-mentioned mixed raw materials in a fluid state. By doing so, the Li source, the Fe source and the carbon source can be uniformly dispersed, the Li ₅ FeO ₄ can be efficiently produced, and the carbon phase is uniformly formed on the surface of the Li ₅ FeO ₄ phase. The resulting composite particles can be produced. Further, in this case, it is possible to suppress the binding of the composite particles. There are various methods for making the mixed raw material in a fluidized state, such as using a fluidized bed, but it is preferable to stir the mixed raw material. For example, if a rotary furnace having a baffle plate inside is used, the mixed raw materials staying in the baffle plate are agitated by falling from a predetermined height as the rotary furnace rotates.

The heating temperature in the synthesis step may be appropriately determined based on the synthesis temperature of Li ₅ FeO ₄ depending on the Li source, but is preferably 650 ° C. or lower in consideration of the above-mentioned decomposition of Li ₅ FeO ₄ . More preferable heating temperature ranges include 600 ° C. or lower, 550 ° C. or lower, 500 ° C. or lower, 490 ° C. or lower, and 470 ° C. or lower.
The lower limit of the heating temperature in the synthesis step is not particularly limited, but may be 400 ° C. or higher. It has been confirmed by the present inventor that Li ₅ FeO ₄ can be synthesized by heating at 400 ° C. using lithium oxide as the Li source and iron alone as the Fe source.
The synthesis step may be carried out in an atmosphere of an inert gas such as argon, helium or nitrogen, or may be carried out in an atmosphere of an oxidizing gas.

The Li source and Fe source contained in the mixed raw material are preferably pulverized in advance. If necessary, the carbon source may be pulverized. The mixed raw material may be simply added Li source, Fe source and carbon source, or may be uniformly mixed manually or by using a mixing device. For example, mixed raw materials include ball mills, planetary ball mills, hybridization system (NHS) and Miralo of Nara Machinery Co., Ltd., Mechanofusion and Nobilta of Hosokawa Micron Co., Ltd., Theta Composer of Tokuju Co., Ltd., etc. Depending on the device, it can be crushed, mixed or integrated.

The above-mentioned organic additives may be added to the mixed raw materials in the synthesis step, if necessary. The amount of the organic additive added may be appropriately adjusted according to the BET specific surface area of the composite particles. Examples of the organic additive include sucrose fatty acid ester, fatty acid, fatty acid salt, fatty acid amide, N, N'-ethylenebis fatty acid amide, fumaric acid alkyl ester alkali metal salt and cured oil.
By adding the organic additive, it is possible to suppress the coarsening of the particles of Li ₅ FeO ₄ , and it is possible to suppress the BET specific surface area of the composite particles from becoming too small. Further, as described above, there is also an effect of suppressing the adhesion of the Li source to the reactor of the heating device.
The amount of the organic additive added is in the range of 0.1 to 10% by mass, in the range of 0.5 to 5% by mass, and in the range of 1 to 3% by mass with respect to the total of the Li source and the Fe source. Each range of, can be mentioned.

The composite particles of the present invention can function as a positive electrode active material, as a lithium ion feeder, or as various additives. In particular, the composite particle of the present invention is a positive electrode material for a lithium ion secondary battery, and functions suitably as a lithium ion feeder at the time of initial charging in the positive electrode of a lithium ion secondary battery. Hereinafter, the positive electrode for a lithium ion secondary battery provided with the composite particles of the present invention is referred to as "the positive electrode of the present invention", and the lithium ion secondary battery provided with the positive electrode of the present invention is referred to as "the lithium ion secondary battery of the present invention". In some cases . As will be described later, the composite particles in the positive electrode of the present invention are not only the composite particles of the present invention but also "composite particles containing Li ₅ FeO ₄ and carbon and having a BET specific surface area of less than 1 m ² / g". In some cases. Similarly, the lithium ion secondary battery provided with the positive electrode of the present invention may also contain "composite particles containing Li ₅ FeO ₄ and carbon and having a BET specific surface area of less than 1 m ² / g" as composite particles. ..

In the positive electrode of the present invention, the composite particles of the present invention are preferably added to the positive electrode active material layer in which the positive electrode active material is present. One aspect of the positive electrode of the present invention comprises a positive electrode active material layer containing the composite particles of the present invention and a current collector. The positive electrode active material layer is formed on the current collector. When the composite particle of the present invention is a lithium ion feeder, the blending amount of the composite particle of the present invention in the positive electrode active material layer may be determined according to the irreversible capacity of the negative electrode.

A current collector is a chemically inert electron conductor that keeps current flowing through the electrodes during the discharge or charging of a lithium-ion secondary battery. The material of the current collector is not particularly limited as long as it is a metal that can withstand a voltage suitable for the active material to be used. The collector material is at least one selected from silver, copper, gold, aluminum, tungsten, cobalt, zinc, nickel, iron, platinum, tin, indium, titanium, ruthenium, tantalum, chromium, molybdenum, and stainless steel. Metallic materials such as can be exemplified. The current collector may be covered with a known protective layer. A current collector whose surface is treated by a known method may be used as the current collector.

When the potential of the positive electrode is 4 V or more based on lithium, it is preferable to use aluminum as the current collector for the positive electrode.

Specifically, it is preferable to use a current collector for the positive electrode made of aluminum or an aluminum alloy. Here, aluminum refers to pure aluminum, and aluminum having a purity of 99.0% or higher is referred to as pure aluminum. An alloy obtained by adding various elements to pure aluminum is called an aluminum alloy. Examples of the aluminum alloy include Al—Cu, Al—Mn, Al—Fe, Al—Si, Al—Mg, Al—Mg—Si, and Al—Zn—Mg.

Further, as aluminum or an aluminum alloy, specifically, for example, A1000 series alloys such as JIS A1085 and A1N30 (pure aluminum series), A3000 series alloys such as JIS A3003 and A3004 (Al—Mn series), JIS A8079, A8021 and the like. A8000 series alloy (Al—Fe series) can be mentioned.

The current collector can take the form of a foil, a sheet, a film, a linear shape, a rod shape, a mesh, or the like. Therefore, as the current collector, for example, a metal foil such as a copper foil, a nickel foil, an aluminum foil, or a stainless steel foil can be preferably used. When the current collector is in the form of a foil, a sheet, or a film, the thickness thereof is preferably in the range of 1 μm to 100 μm.

The positive electrode active material layer preferably contains a known positive electrode active material in addition to the composite particles of the present invention.
As the positive electrode active material, the general formula of the layered rock salt structure: Li _a Ni _b Co _c Mn _{d De Of} (0.2 ≦ a ≦ 2, b + c + d + e = 1, 0 ≦ _e <1, D is W, Mo, Re, Pd, Ba, Cr, B, Sb, Sr, Pb, Ga, Al, Nb, Mg, Ta, Ti, La, Zr, Cu, Ca, Ir, Hf, Rh, Fe, Ge, Zn, Ru, Li _a Ni, a lithium composite metal oxide represented by 1.7 ≦ f ≦ 3), at least one element selected from Sc, Sn, In, Y, Bi, S, Si, Na, K, P and V. _b Co _c Al _d De Of ( _0.2≤a≤2 , b + c + d + _e = 1, 0≤e <1, D is W, Mo, Re, Pd, Ba, Cr, B, Sb, Sr, Pb, Ga, Nb, Mg, Ta, Ti, La, Zr, Cu, Ca, Ir, Hf, Rh, Fe, Ge, Zn, Ru, Sc, Sn, In, Y, Bi, S, Si, Na, K, Examples thereof include Li ₂ MnO ₃ , a lithium composite metal oxide represented by at least one element selected from P and V, 1.7 ≦ f ≦ 3). Further, as the positive electrode active material, a metal oxide having a spinel structure such as LiMn ₂ O ₄ , a solid solution composed of a mixture of the metal oxide having a spinel structure and a layered compound, LiMPO ₄ , LiMVO ₄ or Li ₂ MSiO ₄ (in the formula). M is selected from at least one of Co, Ni, Mn, and Fe) and the like. Further, examples of the positive electrode active material include a tabolite compound represented by LiMPO ₄ F (M is a transition metal) such as LiFePO ₄ F, and a borate compound represented by LiMBO ₃ (M is a transition metal) such as LiFeBO ₃ . be able to. Any metal oxide used as the positive electrode active material may have the above composition formula as the basic composition, and those in which the metal element contained in the basic composition is replaced with another metal element can also be used. Further, as the positive electrode active material, a substance that does not contain a charge carrier (for example, lithium ions that contribute to charge / discharge) may be used. For example, sulfur alone, compounds that combine sulfur and carbon, metal sulfides such as TiS ₂ , oxides such as _V2O5 and _MnO2 , polyaniline and anthraquinone, and compounds containing these aromatics in their chemical structures, conjugated _two . Conjugated materials such as acetic acid-based organic substances and other known materials can also be used. Further, a compound having a stable radical such as nitroxide, nitronylnitroxide, galbinoxyl, phenoxyl and the like may be adopted as the positive electrode active material. When a positive electrode active material that does not contain a charge carrier such as lithium is used, it is necessary to add a charge carrier to the positive electrode and / or the negative electrode in advance by a known method. The charge carrier may be added in an ionic state or may be added in a nonionic state such as a metal. For example, when the charge carrier is lithium, a lithium foil may be attached to the positive electrode and / or the negative electrode to integrate them.

As a positive electrode active material, a general formula of a layered rock salt structure: Li _a Ni _b Co _c Mn _{d De Of} (0.2 ≦ a ≦ 2, b + c + d + _e = 1, 0 ≦) is excellent in terms of high capacity and durability. e <1, D are W, Mo, Re, Pd, Ba, Cr, B, Sb, Sr, Pb, Ga, Al, Nb, Mg, Ta, Ti, La, Zr, Cu, Ca, Ir, Hf, It is represented by at least one element selected from Rh, Fe, Ge, Zn, Ru, Sc, Sn, In, Y, Bi, S, Si, Na, K, P and V, 1.7 ≦ f ≦ 3). Lithium composite metal oxide or Li _a Ni _b Co _c Al _{d De Of} (0.2 ≦ a ≦ 2, b + c + d + e = 1, 0 ≦ _e <1, D is W, Mo, Re, Pd, Ba, Cr, B, Sb, Sr, Pb, Ga, Nb, Mg, Ta, Ti, La, Zr, Cu, Ca, Ir, Hf, Rh, Fe, Ge, Zn, Ru, Sc, Sn, In, It is preferable to use a lithium composite metal oxide represented by at least one element selected from Y, Bi, S, Si, Na, K, P and V, 1.7 ≦ f ≦ 3).

As a positive electrode active material, Li _x Mn _{2-y Ay} O ₄ (A is Ca, Mg, S, Si, Na, K, Al, P, Ga) having a spinel structure is excellent in terms of high capacity and durability. , At least one element selected from Ge, and at least one metal element selected from transition metal elements such as Ni. 0 <x≤2.2, 0≤y≤1) can be exemplified. Examples of the range of the value of x include 0.5 ≦ x ≦ 1.8, 0.7 ≦ x ≦ 1.5, and 0.9 ≦ x ≦ 1.2, and the range of the value of y is 0. Examples thereof include ≦ y ≦ 0.8 and 0 ≦ y ≦ 0.6. Specific examples of the compound having a spinel structure include LiMn ₂ O ₄ and LiMn _1.5 Ni _0.5 O ₄ .

Specific examples of the positive electrode active material include LiFePO ₄ , Li ₂ FeSiO ₄ , LiCoPO ₄ , Li ₂ CoPO ₄ , Li ₂ MnPO ₄ , Li ₂ MnSiO ₄ , and Li ₂ CoPO ₄ F. As another specific positive electrode active material, Li ₂ MnO ₃ -LiCoO ₂ can be exemplified.

The positive electrode active material layer preferably contains a binder and a conductive auxiliary agent. As the binder and the conductive auxiliary agent contained in the positive electrode active material layer, those described in the negative electrode described later may be appropriately and appropriately adopted.

Specifically, the lithium ion secondary battery of the present invention includes the positive electrode, the negative electrode, the electrolytic solution, and the separator of the present invention. The negative electrode includes a current collector and a negative electrode active material layer formed on the current collector. The negative electrode active material layer contains a negative electrode active material, and may contain additives such as a binder and a conductive auxiliary agent, if necessary. The current collector of the negative electrode may be appropriately selected from those described in the positive electrode of the present invention.

As the negative electrode active material, a material capable of occluding and releasing charge carriers can be used. Therefore, there is no particular limitation as long as it is a simple substance, an alloy or a compound capable of occluding and releasing a charge carrier such as lithium ion. For example, Li, Group 14 elements such as carbon, silicon, germanium, and tin, Group 13 elements such as aluminum and indium, Group 12 elements such as zinc and cadmium, Group 15 elements such as antimony and bismuth, and magnesium as negative electrode active materials. , Alkaline earth metals such as calcium, and Group 11 elements such as silver and gold may be used alone. Specific examples of the alloy or compound include tin-based materials such as Ag—Sn alloys, Cu—Sn alloys, and Co—Sn alloys, carbon-based materials such as various graphites, and SiO _x that disproportionates to silicon alone and silicon dioxide. Examples thereof include silicon-based materials such as 0.3 ≦ x ≦ 1.6), silicon alone, or a composite of a silicon-based material and a carbon-based material. Further, the negative electrode active material is an oxide such as Nb ₂ O ₅ , TIO ₂ , Li ₄ Ti ₅ O ₁₂ , WO ₂ , MoO ₂ , Fe ₂ O ₃ , or Li _3-x M _x N (M =). A nitride represented by Co, Ni, Cu) may be adopted. One or more of these can be used as the negative electrode active material.

From the viewpoint of the possibility of increasing the capacity, graphite, Si-containing material, and Sn-containing material can be mentioned as preferable negative electrode active materials. In particular, when a Si-containing material in which the existence of an irreversible capacity is an important issue is used as the negative electrode active material, the effect of Li ₅ FeO ₄ contained in the composite particles of the present invention as a lithium ion feeder is remarkably exhibited. Will be done.

As a specific example of the Si-containing material, Si alone or SiO _x (0.3 ≦ x ≦ 1.6) disproportionated into two phases of a Si phase and a silicon oxide phase can be exemplified. The Si phase in SiO _x can occlude and release lithium ions, and its volume changes with the charging and discharging of the secondary battery. The silicon oxide phase has less volume change due to charging and discharging than the Si phase. That is, SiO _x as the negative electrode active material realizes a high capacity by the Si phase and suppresses the volume change of the entire negative electrode active material by having the silicon oxide phase. If x is less than the lower limit, the ratio of Si becomes excessive, so that the volume change during charging / discharging becomes too large, and the cycle characteristics of the secondary battery deteriorate. On the other hand, when x exceeds the upper limit value, the Si ratio becomes too small and the energy density decreases. The range of x is more preferably 0.5 ≦ x ≦ 1.5, and even more preferably 0.7 ≦ x ≦ 1.2.

In the above-mentioned SiO _x , it is considered that an alloying reaction between lithium and silicon in the Si phase occurs during charging and discharging of the lithium ion secondary battery. It is believed that this alloying reaction contributes to the charging and discharging of the lithium ion secondary battery. Similarly, it is considered that the Sn-containing material described later can be charged and discharged by the alloying reaction of tin and lithium.

Specific examples of the Sn-containing material include Sn simple substance, tin alloys such as Cu—Sn and Co—Sn, amorphous tin oxide, and tin silicon oxide. As the amorphous tin oxide, SnB _0.4 P _0.6 O _3.1 can be exemplified, and as the tin silicon oxide, SnSiO ₃ can be exemplified.

The Si-containing material and the Sn-containing material are preferably combined with a carbon material to form a negative electrode active material. Due to the composite, the structure of silicon and / or tin is particularly stable, and the durability of the negative electrode is improved. The above compounding may be performed by a known method. As the carbon material used for the composite, graphite, hard carbon, soft carbon or the like may be adopted. The graphite may be natural graphite or artificial graphite.

As a specific example of the Si-containing material, a silicon material disclosed in International Publication No. 2014/08608 or the like (hereinafter, simply referred to as “silicon material”) can be mentioned.

The silicon material has a structure in which a plurality of plate-shaped silicon bodies are laminated in the thickness direction. The silicon material is, for example, subjected to a step of reacting CaSi ₂ with an acid to synthesize a layered silicon compound containing polysilane as a main component, and a step of heating the layered silicon compound at 300 ° C. or higher to release hydrogen. It is manufactured.

The ideal reaction formula when hydrogen chloride is used as an acid shows the method for producing a silicon material as follows.
3CaSi ₂ + 6HCl → Si ₆ H ₆ + 3CaCl ₂
Si 6H ₆ → 6Si _{+ 3H 2 ↑}

However, in the upper reaction for synthesizing Si ₆ H ₆ which is polysilane, water is usually used as a reaction solvent from the viewpoint of removing by-products and impurities. Since Si ₆ H ₆ can react with water, the layered silicon compound is rarely produced as containing only Si ₆ H ₆ in the step of synthesizing the layered silicon compound including the reaction in the upper stage, and is layered. Silicon compounds are represented by Si ₆ H _s (OH) _t X _u (X is an element or group derived from an anion of an acid, s + t + u = 6, 0 <s <6, 0 <t <6, 0 <u <6). Manufactured as a compound. In the above chemical formula, unavoidable impurities such as Ca that may remain are not considered. The silicon material obtained by heating the layered silicon compound also contains an element derived from an anion of oxygen or acid.

As described above, the silicon material has a structure in which a plurality of plate-shaped silicon bodies are laminated in the thickness direction. In order to efficiently occlude and release a charge carrier such as lithium ion, the plate-shaped silicon body preferably has a thickness in the range of 10 nm to 100 nm, and more preferably in the range of 20 nm to 50 nm. The length of the plate-shaped silicon body in the longitudinal direction is preferably in the range of 0.1 μm to 50 μm. Further, the plate-shaped silicon body preferably has (length in the longitudinal direction) / (thickness) in the range of 2 to 1000. The laminated structure of the plate-shaped silicon body can be confirmed by observation with a scanning electron microscope or the like. Further, this laminated structure is considered to be a remnant of the Si layer in the raw material CaSi ₂ .

The silicon material preferably contains amorphous silicon and / or silicon crystallites. In particular, in the plate-shaped silicon body, it is preferable that amorphous silicon is used as a matrix and silicon crystallites are scattered in the matrix. The size of the silicon crystallite is preferably in the range of 0.5 nm to 300 nm, more preferably in the range of 1 nm to 100 nm, further preferably in the range of 1 nm to 50 nm, and particularly preferably in the range of 1 nm to 10 nm. The size of the silicon crystallite is calculated from the Scherrer's equation using the half width of the diffraction peak on the Si (111) plane of the obtained X-ray diffraction chart by performing X-ray diffraction measurement on the silicon material. ..

The abundance and size of the plate-shaped silicon body, amorphous silicon and silicon crystallites contained in the silicon material mainly depend on the heating temperature and the heating time. The heating temperature is preferably in the range of 350 ° C to 950 ° C, more preferably in the range of 400 ° C to 900 ° C.

The silicon material may be coated with carbon. The carbon-coated silicon material has excellent conductivity.

The average particle size of the silicon material is preferably in the range of 2 to 7 μm, more preferably in the range of 2.5 to 6.5 μm. If a silicon material having an average particle size too small is used, it may be difficult to manufacture a negative electrode from the viewpoint of cohesiveness and wettability. Specifically, in the slurry prepared at the time of manufacturing the negative electrode, the silicon material having an average particle size too small may aggregate. On the other hand, a lithium ion secondary battery provided with a negative electrode using a silicon material having an average particle size too large may not be able to perform suitable charging / discharging. In a silicon material having an average particle size too large, it is presumed that the cause is that lithium ions cannot sufficiently diffuse into the inside of the silicon material. The average particle size in the present specification means D50 when the sample is measured by a general laser diffraction type particle size distribution measuring device.

Examples of the binder include fluororesins such as polyvinylidene fluoride, polytetrafluoroethylene and fluororubber, thermoplastic resins such as polypropylene and polyethylene, imide resins such as polyimide and polyamideimide, alkoxysilyl group-containing resins and carboxymethyl celluloses. , A known material such as styrene-butadiene rubber may be adopted.

Further, a crosslinked polymer obtained by cross-linking a carboxyl group-containing polymer such as polyacrylic acid or polymethacrylic acid with a polyamine such as diamine disclosed in International Publication No. 2016/06382 may be used as a binder.

Examples of the diamine used in the crosslinked polymer include alkylene diamines such as ethylenediamine, propylenediamine and hexamethylenediamine, 1,4-diaminocyclohexane, 1,3-diaminocyclohexane, isophoronediamine and bis (4-aminocyclohexyl) methane. Saturated carbocyclic diamine, m-phenylenediamine, p-phenylenediamine, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl ether, bis (4-aminophenyl) sulfone, benzidine, o-tridine, 2,4- Examples thereof include aromatic diamines such as tolylene diamine, 2,6-tolylene diamine, xylylene diamine and naphthalenediamine.

The blending ratio of the binder in the active material layer is preferably 0.5 to 20% by mass, more preferably 1 to 15% by mass, further preferably 2 to 10% by mass, and particularly preferably 3 to 5% by mass. This is because if the amount of the binder is too small, the moldability of the electrode is lowered, and if the amount of the binder is too large, the energy density of the electrode is lowered.

Conductivity aids are added to increase the conductivity of the electrodes. Therefore, the conductive auxiliary agent may be arbitrarily added when the conductivity of the electrode is insufficient, and may not be added when the conductivity of the electrode is sufficiently excellent. The conductive auxiliary agent may be a chemically inert electron high conductor, and examples thereof include carbon black, which are carbonaceous fine particles, graphite, Vapor Grown Carbon Fiber, and various metal particles. To. Examples of carbon black include acetylene black, Ketjen black (registered trademark), furnace black, and channel black. These conductive auxiliary agents can be added to the active material layer alone or in combination of two or more. The blending ratio of the conductive auxiliary agent in the active material layer is preferably 0.5 to 20% by mass, more preferably 1 to 15% by mass, further preferably 2 to 10% by mass, and particularly preferably 2 to 5% by mass. This is because if the amount of the conductive auxiliary agent is too small, an efficient conductive path cannot be formed, and if the amount of the conductive auxiliary agent is too large, the formability of the active material layer is deteriorated and the energy density of the electrode is lowered.

In order to form the active material layer on the surface of the current collector, a conventionally known method such as a roll coating method, a die coating method, a dip coating method, a doctor blade method, a spray coating method, or a curtain coating method is used to collect electricity. The active substance may be applied to the surface of the body. Specifically, the components of the active material layer and the solvent are mixed to form a slurry, and then the slurry is applied to the surface of the current collector and then dried. Examples of the solvent include N-methyl-2-pyrrolidone, methanol, methyl isobutyl ketone, and water. In order to increase the electrode density, the dried one may be compressed.

In the composition for producing a positive electrode active material layer in the form of a slurry comprising the composite particles of the present invention and a solvent for producing the positive electrode active material layer, the amount of the solid content is in the range of 30 to 90% by mass. It is preferably in the range of 50 to 75% by mass, more preferably. Here, the solid content means a component other than the solvent contained in the composition for producing the positive electrode active material layer.

The blending amount of the composite particles of the present invention in the composition for producing a positive electrode active material layer may be appropriately determined according to the use of the composite particles of the present invention.
When the composite particles of the present invention are used as a lithium ion feeder for supplementing lithium ions corresponding to the irreversible capacity of the negative electrode, as the blending amount of the composite particles of the present invention in the composition for producing a positive electrode active material layer. Can be exemplified in the range of 1 to 10% by mass, the range of 3 to 9% by mass, and the range of 5 to 8% by mass with respect to the solid content.

The blending amount of the positive electrode active material in the composition for producing the positive electrode active material layer is in the range of 80 to 95% by mass, in the range of 83 to 93% by mass, and 85 to 90% by mass with respect to the solid content. The range can be exemplified. The blending amount of the binder in the composition for producing the positive electrode active material layer is in the range of 0.5 to 10% by mass, in the range of 1 to 5% by mass, and 2 to 4% by mass with respect to the solid content. The range can be exemplified. The blending amount of the conductive auxiliary agent in the composition for producing the positive electrode active material layer is in the range of 0.5 to 5% by mass, in the range of 1 to 4% by mass, and 1 to 3% by mass with respect to the solid content. The range can be exemplified.

The separator separates the positive electrode and the negative electrode, and allows lithium ions to pass through while preventing a short circuit due to contact between the two electrodes. As the separator, a known one may be adopted, and synthetic resins such as polytetrafluoroethylene, polypropylene, polyethylene, polyimide, polyamide, polyaramid (Aromatic polyamide), polyester and polyacrylonitrile, polysaccharides such as cellulose and amylose, and fibroin , Natural polymers such as keratin, lignin, and sverin, porous bodies using one or more electrically insulating materials such as ceramics, non-woven fabrics, and woven fabrics. Further, the separator may have a multi-layer structure.

The electrolytic solution contains a non-aqueous solvent and an electrolyte dissolved in the non-aqueous solvent.

As the non-aqueous solvent, cyclic carbonates, cyclic esters, chain carbonates, chain esters, ethers and the like can be used. Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate, and examples of the cyclic ester include gamma-butyrolactone, 2-methyl-gamma-butyrolactone, acetyl-gamma-butyrolactone, and gamma-valerolactone. Examples of the chain carbonate include dimethyl carbonate, diethyl carbonate, dibutyl carbonate, dipropyl carbonate, and ethylmethyl carbonate, and examples of the chain ester include propionic acid alkyl ester, malonic acid dialkyl ester, and acetate alkyl ester. Examples of ethers include tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, and 1,2-dibutoxyethane. As the non-aqueous solvent, a compound in which some or all of the chemical structure of the specific solvent is replaced with fluorine may be adopted.

Examples of the electrolyte include lithium salts such as LiClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , and LiN (CF ₃ SO ₂ ) ₂ .

As the electrolytic solution, 0.5 mol / mol / mol of lithium salts such as LiClO ₄ , LiPF ₆ , LiBF ₄ , and LiCF ₃ SO ₃ is added to a non-aqueous solvent such as fluoroethylene carbonate, ethylene carbonate, dimethyl carbonate, ethylmethyl carbonate, and diethyl carbonate. An example is an example of a solution dissolved at a concentration of about 1.7 mol / L from L.

A specific method for manufacturing the lithium ion secondary battery of the present invention will be described.
For example, a separator is sandwiched between a positive electrode and a negative electrode to form an electrode body. The electrode body may be either a laminated type in which a positive electrode, a separator and a negative electrode are stacked, or a wound type in which a laminated body of a positive electrode, a separator and a negative electrode is wound. After connecting the positive electrode collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal leading to the outside using a current collector lead or the like, an electrolytic solution is added to the electrode body to form a lithium ion secondary battery. good.

The shape of the lithium ion secondary battery of the present invention is not particularly limited, and various shapes such as a cylindrical type, a square type, a coin type, and a laminated type can be adopted.

The lithium ion secondary battery of the present invention may be mounted on a vehicle. The vehicle may be a vehicle that uses electric energy from a lithium ion secondary battery for all or part of its power source, and may be, for example, an electric vehicle or a hybrid vehicle. When a lithium ion secondary battery is mounted on a vehicle, it is advisable to connect a plurality of lithium ion secondary batteries in series to form an assembled battery. In addition to vehicles, devices equipped with lithium-ion secondary batteries include various battery-powered home appliances such as personal computers and portable communication devices, office equipment, and industrial equipment. Further, the lithium ion secondary battery of the present invention includes wind power generation, solar power generation, hydraulic power generation and other power storage devices and power smoothing devices, power supply sources for power and / or auxiliary equipment such as ships, aircraft, and aircraft. Power supply for spacecraft and / or auxiliary equipment, auxiliary power for vehicles that do not use electricity as a power source, power for mobile home robots, power for system backup, power for non-disruptive power supply, It may be used as a power storage device for temporarily storing the electric power required for charging in a charging station for an electric vehicle or the like.

Although the embodiment of the present invention has been described above, the present invention is not limited to the above embodiment. As long as it does not deviate from the gist of the present invention, it can be carried out in various forms with modifications, improvements, etc. that can be made by those skilled in the art.

Hereinafter, the present invention will be described in more detail by showing various specific examples. The present invention is not limited to these specific examples.

(Example 1)
(Synthesis process)
A mixed raw material containing 71.3 parts by mass of LiOH / H ₂ O as a Li source, 27.1 parts by mass of Fe ₂ O ₃ as an Fe source, and 1.6 parts by mass of graphite as a carbon source was used in a planetary ball mill. I put it in. The planetary ball mill was operated at a rotation speed of 800 rpm and a revolution speed of 800 rpm, and the mixed raw materials were pulverized and mixed until the diameter of the obtained particles was 10 μm or less.

The pulverized and mixed mixed raw materials were placed in a reactor and heated in an electric furnace at 460 ° C. for 4 hours in an argon atmosphere to synthesize the composite particles of Example 1.
The composite raw material in Example 1 contained a Li source and an Fe source in an amount such that Li and Fe had an atomic ratio of 5: 1. As for graphite, an amount of 3 parts by mass when Li ₅ FeO ₄ produced in the synthesis step was 100 parts by mass was blended.

90 parts by mass of the composite particles of Example 1, 5 parts by mass of acetylene black as a conductive auxiliary agent, 5 parts by mass of polyvinylidene fluoride as a binder, and an appropriate amount of N-methyl-2-pyrrolidone were mixed. It was made into a slurry. An aluminum foil was prepared as a current collector, a slurry was applied thereto, and the mixture was dried to obtain a positive electrode of Example 1.

A lithium foil was prepared and used as the negative electrode. As a separator, a glass filter (Hoechst Celanese Co., Ltd.) and a single-layer polypropylene celgard 2400 (Polypore Co., Ltd.) were prepared. Further, an electrolytic solution prepared by dissolving LiPF ₆ at a concentration of 1 mol / L in a solvent in which 3 parts by volume of ethylene carbonate, 3 parts by volume of ethylmethyl carbonate and 4 parts by volume of dimethyl carbonate was mixed. Two types of separators were sandwiched between the negative electrode and the positive electrode of Example 1 in the order of the negative electrode, the glass filter, celgard2400, and the positive electrode of Example 1 to form an electrode body. This electrode body was housed in a coin-type battery case CR2032 (Hosen Co., Ltd.), and an electrolytic solution was further injected to obtain a sealed coin-type battery. This was used as the lithium ion secondary battery of Example 1.

(Example 2)
Except for the use of a mixed raw material containing 70.6 parts by mass of LiOH / H ₂ O as a Li source, 26.8 parts by mass of Fe ₂ O ₃ as an Fe source, and 2.6 parts by mass of graphite as a carbon source. Produced the composite particles of Example 2 in the same manner as in Example 1. Further, using the composite particles of Example 2, the positive electrode of Example 2 and the lithium ion secondary battery of Example 2 were manufactured in the same manner as in Example 1.
The composite raw material in Example 2 contained a Li source and an Fe source in an amount such that Li and Fe had an atomic ratio of 5: 1. As for graphite, an amount of 5 parts by mass when Li ₅ FeO ₄ produced in the synthesis step was 100 parts by mass was blended.

(Example 3)
Contains 3.805 g (158.9 mmol) of LiOH as a Li source, 2.537 g (15.9 mmol) of Fe ₂ O ₃ as a Fe source, and 0.257 g (21.5 mmol in carbon equivalent) of graphite as a carbon source. The composite particles of Example 3 were produced in the same manner as in Example 1 except that the mixed raw materials used were used.

( Test Example 1)
(Synthesis process)
Li ₂ O as a Li source and Fe ₂ O ₃ as an Fe source were weighed so as to have a molar ratio of 5: 1 and charged into a planetary ball mill. The planetary ball mill was operated at a rotation speed of 800 rpm and a revolution speed of 800 rpm, and the mixed raw materials were pulverized and mixed until the diameter of the obtained particles was 10 μm or less.
The pulverized and mixed raw materials were placed in a rotary kiln type reactor and heated at 600 ° C. for 60 minutes in an argon atmosphere to synthesize particulate Li 5 FeO 4. Subsequently, the surface of Li ₅ FeO ₄ was carbon-coated by performing thermal CVD under the conditions of 700 ° C. and 40 minutes under the aeration of a hexane-argon mixed gas to obtain composite particles of Test Example 1. During the synthesis of Li ₅ FeO ₄ and the carbon coating, the rotary kiln type reactor was put into a rotating state.
Using the composite particles of Test Example 1, the positive electrode of Test Example 1 and the lithium ion secondary battery of Test Example 1 were manufactured in the same manner as in Example 1. In this specification and drawings, Comparative Example 1 shall be read as Test Example 1. Further, Comparative Example 2 shall be read as Comparative Example.

(Comparative Example 2)
Lithium stearate, which is an organic additive, was added to the mixed raw material in an amount corresponding to 2% by mass with respect to the total amount of Li ₂ O and Fe ₂ O ₃ , and thermal CVD was performed for 20 minutes. Except for the above, the composite particles of Comparative Example 2 were produced in the same manner as in Comparative Example 1.

(Reference example 1)
4.138 g (56.0 mmol) of Li ₂ CO ₃ as a Li source, 1.789 g (11.2 mmol) of Fe ₂ O ₃ as an Fe source, and 0.538 g (44.8 mmol in terms of carbon) of graphite as a carbon source. A mixed raw material containing lithium stearate as an organic additive in an amount corresponding to 2% by mass based on the total amount of Li ₂ CO ₃ and Fe ₂ O ₃ was heated at 720 ° C. for 26 hours in an argon atmosphere. Except for this, the composite particles of Reference Example 1 were produced in the same manner as in Example 1.

(Evaluation example 1)
The specific surface area of each of the composite particles of Examples 1 to 3, Comparative Example 1 and Comparative Example 2 was measured by the BET method and used as the BET specific surface area. Table 1 shows the values of the BET specific surface area of each composite particle.

(Evaluation example 2)
Using a carbon / sulfur analyzer, elemental analysis targeting carbon was performed on each of the composite particles of Examples 1 to 3, Comparative Example 1 and Comparative Example 2. The results are shown in Table 1 together with the results of Evaluation Example 1.

As shown in Table 1, the composite particles of Examples 1 to 3 have the provision that "the BET specific surface area is 2 m ² / g or less and the carbon mass percentage (Wc) is within the range of 1 ≦ Wc ≦ 6." Although it was satisfied, the composite particles of Comparative Example 1 and Comparative Example 2 did not satisfy the above-mentioned regulation.

Focusing on the BET specific surface area, the BET specific surface area of the composite particles of Comparative Example 2 in which the organic additive was added to the mixed raw material was 3.9 m ² / g, which was a relatively large value. This is due to the effect of the organic additive.
On the other hand, since the composite particles of Examples 1 to 3 which are the composite particles of the present invention are produced from a mixed raw material containing no organic additive, their BET specific surface area is 1 m ² / g. It is a relatively small value of less than.
From the above results, when the composite particles of the present invention are specified based on the presence or absence of the use of organic additives, the composite particles of the present invention contain Li ₅ FeO ₄ and carbon, and have a BET specific surface area of 1 m ² / g. It can be regarded as a composite particle that is less than.

(Evaluation example 3)
For the composite particles of Examples 1 to 3 and Comparative Example 1, the volume resistance value was measured under a load of 4 kN using a powder resistance measurement system (Mitsubishi Analytech Co., Ltd.). The results are shown in Table 2 together with the results of Evaluation Example 1 and Evaluation Example 2.

As shown in Table 2, the composite particles of Examples 1 to 3 satisfying the provision that "the BET specific surface area is 2 m ² / g or less and the carbon mass percentage (Wc) is within the range of 1 ≦ Wc ≦ 6" The powder resistance was smaller and the conductivity was excellent as compared with the composite particles of Comparative Example 1 which did not satisfy the above-mentioned regulation.

(Evaluation example 4)
Each of the lithium ion secondary batteries of Example 1, Example 2 and Comparative Example 1 was charged at a current of 0.1 mA until the voltage reached 4.4 V. The observed charge capacity is shown in Table 3 together with the results of Evaluation Example 1 to Evaluation Example 3.

As described above, the composite particles of Example 1 and the composite particles of Example 2 satisfy the provisions of the present invention, and the composite particles of Comparative Example 1 do not satisfy the provisions.
Further, the charging capacity of the lithium ion secondary battery of Example 1 using the composite particles of Example 1 and the charging capacity of the lithium ion secondary battery of Example 2 using the composite particles of Example 2 are comparative examples. It was far superior to the charge capacity of the lithium ion secondary battery of Comparative Example 1 using the composite particles of 1.
From this result, it can be said that the composite particles satisfying the regulation that "the BET specific surface area is 2 m ² / g or less and the carbon mass percentage (Wc) is within the range of 1 ≦ Wc ≦ 6" are excellent in charge capacity.

Focusing on the BET specific surface area, the lithium ion secondary battery of Comparative Example 1 using the composite particles of Comparative Example 1 having a specific surface area of 0.75 m ² / g has a specific surface area of 0.95 m ² / g in Example 1. Compared with the lithium ion secondary battery of Example 1 using the composite particles of Example 1 and the lithium ion secondary battery of Example 2 using the composite particles of Example 2 having a specific surface area of 0.82 m ² / g. Inferior in charging capacity. From this, it can be said that the BET specific surface area of the composite particle of the present invention is preferably more than 0.75 m ² / g, and more preferably 0.8 m ² / g or more.

Further, when the composite particles of the present invention are specified based on the presence or absence of the organic additive as described above, the composite particles of the present invention contain Li ₅ FeO ₄ and carbon, and BET, in consideration of the charge capacity. Composite particles with a specific surface area of more than 0.75 m ² / g and less than 1 m ² / g, or containing Li ₅ FeO ₄ and carbon, with a BET specific surface area of 0.8 m ² / g or more and 1 m ² / g. It can also be regarded as a composite particle that is less than.

Focusing on the mass percentage Wc of carbon, the composite particle of Example 1 in which the Wc was 2.6% by mass is a powder as compared with the composite particle of Example 2 in which the Wc was 5.3% by mass. Although the resistance is a little high, it is superior in charge capacity. From this, it can be said that the Wc in the composite particle of the present invention is preferably 5% by mass or less, 4% by mass or less, or 3% by mass or less.

(Evaluation example 5)
The appearance of the composite particles of Reference Example 1 was observed using a scanning electron microscope (SEM). The SEM image of the composite particle of Reference Example 1 is shown in FIG.

As shown in FIG. 1, the composite particle of Reference Example 1 has a relatively large particle and a large number of fine particles existing on the surface of the particle. From the composition of the mixed raw material, it is presumed that the relatively large particles in FIG. 1 are Li ₅ FeO ₄ and the fine particles are carbon. That is, it can be said that the composite particle of Reference Example 1 has a relatively large particulate Li ₅ FeO ₄ phase and a large number of fine particulate carbon phases present on the surface thereof.
The composite particles of Reference Example 1 are composite particles produced by synthesizing Li ₅ FeO ₄ in the presence of a carbon source, similarly to the composite particles of Examples 1 to 3. Therefore, the composite particles of Examples 1 to 3 have a relatively large particulate Li ₅ FeO ₄ phase and a large number of fine particulate carbon phases present on the surface thereof, similarly to the composite particles of Reference Example 1. Is presumed to have.

(Evaluation example 6)
The composite particles of Example 1 and the composite particles of Comparative Example 1 were subjected to X-ray diffraction analysis using synchrotron radiation for analysis. The analysis conditions were as follows. The obtained X-ray diffraction chart is shown in FIG.
<Analysis conditions>
Measurement beam line: Aichi Synchrotron BL5S2
Measurement energy: 12.4 keV
Camera length: 340mm
Exposure time: 300s
Capillary: diameter 0.3 mm

As shown in FIG. 2, it was confirmed that Li ₅ FeO ₄ was produced in the composite particles of Example 1 and Comparative Example 1. Moreover, the diffraction peak derived from the Fe crystal was clearly detected from the composite particles of Comparative Example 1. On the other hand, the peak was not detected in the composite particles of Example 1. This result confirms that Fe crystals are present in the composite particles of Comparative Example 1 and Fe crystals are not present in the composite particles of Example 1.
As described above, it is considered that reduction decomposition of Li ₅ FeO ₄ occurs when exposed to a temperature exceeding 650 ° C. in the presence of carbon. Since the heating temperature in the synthesis step of Comparative Example 1 was 700 ° C., it is presumed that Fe crystals, which are the reduction decomposition products of Li ₅ FeO ₄ , were produced. On the other hand, since the heating temperature in the synthesis step of Example 1 was 460 ° C., it is presumed that the reductive decomposition of Li ₅ FeO ₄ was suppressed and the formation of Fe crystals was not observed.

(Evaluation example 7)
The composite particles of Example 3, the composite particles of Comparative Example 2, the composite particles of Reference Example 1, and graphite were analyzed with a Raman spectrophotometer. The obtained Raman spectroscopic spectrum is shown in FIG.

As shown in FIG. 3, in 1200 to 1700 m ^-1 of the Raman spectroscopic spectrum of graphite, a peak derived from carbon called G-Band derived from graphite and a peak derived from carbon derived from dangling bond are referred to as D-Band. A carbon-derived peak was detected.

A sharp G-Band with a half width of 40 cm ^-1 or less was detected in the Raman spectroscopic spectra of each composite particle of Example 3 and Reference Example 1. However, no sharp G-Band was detected in the Raman spectroscopic spectrum of the composite particle of Comparative Example 2, and only a broad peak with a half width of 88 cm ^-1 was detected.

Focusing on D-Band, in the composite particle of Example 3 using lithium hydroxide as the Li source, clear D-Band was detected in the Raman spectroscopic spectrum. On the other hand, in the composite particle of Reference Example 1 in which lithium carbonate was used as the Li source, the D-Band detected in the Raman spectroscopic spectrum was very small. The G / D ratio obtained by dividing the peak area of G-Band by the peak area of D-Band was 3.6 for the composite particle of Reference Example 1 and 0.8 for the composite particle of Example 3. there were. Based on this result, each range of 0.5 to 2, 0.5 to 1.5, and 0.5 to 1.0 can be mentioned as a preferable range of the G / D ratio in the composite particle of the present invention. can.

Claims (6)

A composite particle containing Li ₅ FeO ₄ and carbon, wherein the BET specific surface area is 2 m ² / g or less and the mass percentage (Wc) of carbon is within the range of 1 ≦ Wc ≦ 6. The composite particle according to claim 1, wherein the BET specific surface area is 0.5 m ² / g or more. The composite particle according to claim 1 or 2, wherein the peak derived from the Fe crystal is not detected in the X-ray diffraction chart of the composite particle. The composite particle according to any one of claims 1 to 3, wherein G-Band is detected in a Raman spectroscopic spectrum obtained by analyzing the composite particle with a Raman spectrophotometer. A positive electrode comprising the composite particles according to any one of claims 1 to 4 , or composite particles containing Li ₅ FeO ₄ and carbon and having a BET specific surface area of less than 1 m ² / g . A lithium ion secondary battery comprising the positive electrode according to claim 5 .

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