JP7135673B2 - Composite particle manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing Li ₅ FeO ₄ used as a positive electrode material for lithium ion secondary batteries.

Lithium ion secondary batteries are small and have a large capacity, so they are used as batteries for various devices such as mobile phones and laptop computers. A lithium-ion secondary battery has 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.

Various lithium metal composite oxides are known to be used as positive electrode active materials for lithium ion secondary batteries, and one of them is Li ₅ FeO ₄ . It is also known that Li ₅ FeO ₄ has a small reversible capacity and generates gas during charging. It is also known to be used for the purpose of

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

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

Patent Document 3 specifically describes a lithium ion secondary battery having a positive electrode using LiCoO ₂ and Li ₅ FeO ₄ at a mass ratio of 91:9 to 97:3 as positive electrode active materials. It is described that the addition of ₅ FeO ₄ was able to improve the initial charge capacity.

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. It is stated that pores can be formed.

JP 2007-287446 A JP 2012-99316 A JP 2014-157653 A WO2014/118834

In recent years, the industrial world has been demanding lithium-ion secondary batteries with excellent battery characteristics and excellent materials for manufacturing lithium-ion secondary batteries, and new technologies are required to achieve this. there is

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

When the present inventors investigated Li ₅ FeO ₄ , they discovered a phenomenon in which a battery comprising Li ₅ FeO ₄ does not develop a sufficient capacity. The inventors presumed that this phenomenon is due to the low electrical conductivity of Li ₅ FeO ₄ , that is, the property of high resistance. Then, for the purpose of increasing the conductivity of Li ₅ FeO ₄ , carbon-coated Li ₅ FeO ₄ was produced by coating the surface of Li ₅ FeO ₄ with a carbon film, and when this was tested, carbon-coated Li ₅ FeO ₄ was obtained. However, the inventors have found that there are cases where the capacity is not necessarily expressed sufficiently.

The present inventor attempted to produce carbon-coated Li ₅ FeO ₄ under various conditions in order to improve the capacity of the battery, aiming to produce composite particles containing Li ₅ FeO ₄ and carbon. The present inventor completed the present invention through further intensive research.

The method for producing composite particles of the present invention comprises:
A method for producing composite particles containing Li ₅ FeO ₄ and carbon, including the step of heating a Li source, an Fe source, and graphite in the presence of an oxidizing gas.

According to the production method of the present invention, composite particles that exhibit sufficient capacity as a positive electrode material can be produced.

2 is an X-ray diffraction chart of the composite particles of Example 1, the composite particles of Example 2, and the carbon-coated Li ₅ FeO ₄ of Comparative Example 1 in Evaluation Example 1. FIG. 4 is an X-ray diffraction chart of composite particles of Examples 3 to 5 in Evaluation Example 4. FIG. 4 is an X-ray diffraction chart of the composite particles of Example 6 and Example 7 in Evaluation Example 4. FIG. 4 is an X-ray diffraction chart of the composite particles of Example 6 and Example 7 in Evaluation Example 4. FIG. 3 is an X-ray diffraction chart of the composite particles of Example 3, the composite particles of Example 7, and the composite particles of Example 8 in Evaluation Example 4. FIG. 3 is an X-ray diffraction chart of the composite particles of Example 3, the composite particles of Example 7, and the composite particles of Example 8 in Evaluation Example 4. FIG.

EMBODIMENT OF THE INVENTION Below, the form for implementing this invention is demonstrated. Unless otherwise specified, the numerical range "a to b" described in this specification includes the lower limit a and the upper limit b. Further, a numerical range can be configured by arbitrarily combining these upper and lower limits and the numerical values listed in the examples. Furthermore, a numerical value arbitrarily selected from within these numerical ranges can be used as the new upper and lower numerical values.

The method for producing composite particles of the present invention is a method for producing composite particles containing Li ₅ FeO ₄ and carbon by a step of heating a Li source, an Fe source and graphite in the presence of an oxidizing gas.

The composite particles produced by the method for producing composite particles of the present invention are composite particles containing Li ₅ FeO ₄ and carbon. From a different point of view, the composite particles produced by the production method of the present invention can be said to be particles in which the phase of Li ₅ FeO ₄ and the phase of carbon are combined. Hereinafter, the method for producing composite particles of the present invention will be simply referred to as the production method of the present invention as necessary, and in the production method of the present invention, "heating Li source, Fe source, and graphite in the presence of an oxidizing gas. process" is referred to as a synthetic process.
Further, as necessary, the composite particles produced by the production method of the present invention are referred to as the composite particles of the present invention, and the Li ₅ FeO ₄ phase contained in the composite particles of the present invention is referred to as the Li ₅ FeO ₄ phase. , the carbon phase is called the carbon phase.
The Li ₅ FeO ₄ phase is a portion that is mainly involved in the battery reaction, but has relatively poor conductivity, and the carbon phase is a portion that plays a role of supplementing the conductivity of the Li ₅ FeO ₄ phase with its own excellent conductivity. It can be said that there is.

By the way, the carbon _- coated Li ₅ FeO ₄ manufactured by the present inventor as described above is produced by heating particles of Li ₅ FeO ₄ together with a carbon source made of an organic substance to a temperature _equal to or higher than the carbonization temperature of the carbon source. is coated with a carbon film by a so-called thermal CVD method. The production method of the present invention is essentially different from the method of producing carbon-coated Li ₅ FeO ₄ by the thermal CVD method.

As a method of synthesizing Li ₅ FeO ₄ , a method of synthesizing by reacting a Li source and an Fe source under heating conditions is known. Examples of the Li source include elemental lithium, lithium oxide, lithium hydroxide, lithium carbonate, lithium hydrogen carbonate, and lithium fluoride. Examples of the Fe source include elemental iron, iron oxide, iron hydroxide, iron oxyhydroxide, and sulfuric acid. Iron, iron nitrate and iron chloride are exemplified.
From the point of purity of the product, it is common to react lithium elemental substance and iron oxide, lithium oxide and iron elemental substance, or lithium oxide and iron oxide to react.

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 graphite is present in addition to a Li source and an Fe source, and composite particles having a Li ₅ FeO ₄ phase and a carbon phase. to manufacture.
Since graphite becomes the carbon phase as it is, in the synthesis step of the production method of the present invention, the Li ₅ FeO ₄ phase is synthesized in contact with the carbon phase, and the carbon phase adheres to the surface of the Li ₅ FeO ₄ particles. It is believed that particles are obtained. That is, in the synthesis step of the production method of the present invention, it is not necessary to carbonize the carbon source such as an organic substance, and the Li source, Fe source and graphite may be heated at the synthesis temperature of Li ₅ FeO ₄ in the synthesis step.

The carbonization temperature of organic substances such as hydrocarbons is often a high temperature exceeding the synthesis temperature of Li ₅ FeO ₄ . Therefore, when the carbon-coated Li ₅ FeO ₄ is produced using the thermal CVD method, the Li ₅ FeO ₄ may be adversely affected by heat during the carbonization of the organic matter. According to the production method of the present invention, since it is not necessary to expose Li ₅ FeO ₄ to high temperatures for carbonization of the carbon source, the above-described adverse effects of heat on Li ₅ FeO ₄ can be suppressed. Further, since graphite has a track record of being used as a conductive aid in the positive electrode of lithium ion secondary batteries, it can be said that graphite is particularly preferable as the carbon phase of the composite particles used in the positive electrode of lithium ion secondary batteries.
Hereinafter, the raw material of the composite particles containing the Li source, the Fe source, and the graphite and subjected to the synthesis step will be referred to as a mixed raw material, if necessary.

By the way, in the synthesis step in the production method of the present invention, Li ₅ FeO ₄ is synthesized in the presence of graphite. Therefore, it can be said that during the synthesis process, Li ₅ FeO ₄ produced in the synthesis process is heated while being composited with graphite. In the production method of the present invention, this may cause adverse effects.

That is, depending on the conditions of the synthesis process, Li ₅ FeO ₄ produced in the synthesis process may react with carbon and be reductively decomposed. If the reductive decomposition of the Li ₅ FeO ₄ progresses, it becomes difficult to produce the composite particles of the present invention with high yield.
In addition, Fe crystals are generated with this reductive decomposition. If the Fe crystals are brought into the positive electrode active material layer of the lithium ion secondary battery, Fe may precipitate on the surface of the negative electrode during charging of the lithium ion secondary battery. If Fe precipitates on the surface of the negative electrode, dendrites may grow with the Fe as nuclei.

The reductive decomposition of Li ₅ FeO ₄ described above can be suppressed by conducting the synthesis process in an oxidizing atmosphere. In the production method of the present invention, the synthesis step is performed in the presence of an oxidizing gas, that is, in an oxidizing atmosphere, so that the composite particles of the present invention can be produced while suppressing reductive decomposition of Li ₅ FeO ₄ .

Also, it is considered that the reductive decomposition reaction of Li ₅ FeO ₄ proceeds at temperatures exceeding 650°C. Therefore, in order to suppress reductive decomposition of Li ₅ FeO ₄ , it is preferable to carry out the synthesis process at 650° C. or lower.

Here, the synthesis temperature of Li ₅ FeO ₄ differs depending on the type of Li source. Specifically, the Li ₅ FeO ₄ synthesis temperature is about 500° C. when lithium oxide is used as the Li source, and the Li ₅ FeO ₄ synthesis temperature is about 460° C. when lithium hydroxide is used as the Li source. The synthesis temperature of Li ₅ FeO ₄ when lithium carbonate is used as is about 730°C. That is, when lithium carbonate is used as the Li source, the synthesis temperature of Li ₅ FeO ₄ exceeds 650° C., but when lithium hydroxide or lithium oxide is used as the Li source, the temperature is relatively low, less than 650° C. can synthesize Li ₅ FeO ₄ .
Therefore, in order to suppress the reductive decomposition reaction of Li ₅ FeO ₄ itself, to produce the composite particles of the present invention with good yield, and to suppress the generation of Fe crystals, lithium hydroxide or lithium oxide is used as the Li source. It can be said that it is preferable to select

It is considered that the synthesis reaction of Li ₅ FeO ₄ using lithium hydroxide as the Li source and ferric oxide as the Fe source proceeds in two stages. Among them, the reaction in the first stage is considered to be the reaction (1) below, and the reaction in the second stage is considered to be the reaction (2) below.
10LiOH+ _Fe2O3- >2LiFeO2 ₊ 8LiOH _{+ H2O} ...(1)
2LiFeO ₂ +8LiOH→2Li ₅ FeO ₄ +4H ₂ O (2)

H ₂ O, which is a kind of oxidizing gas, is produced in the above-described first-stage and second-stage reactions. Therefore, it can be said that an oxidizing gas exists in the reaction system after the start of the reaction in the first stage. Therefore, even if the heating temperature exceeds 650° C., that is, the temperature at which reductive decomposition of Li ₅ FeO ₄ can occur, the oxidizing gas suppresses reductive decomposition of Li ₅ FeO ₄ .
In other words, when lithium hydroxide is used as the Li source, the reaction system can be brought into an oxidizing atmosphere by the oxidizing gas produced as a by-product during the synthesis of Li ₅ FeO ₄ without supplying an oxidizing gas separately to the reaction system. can. Therefore, even if the synthesis step is performed at a high temperature exceeding 650° C. without supplying an oxidizing gas separately, the composite particles of the present invention can be produced while suppressing reductive decomposition of Li ₅ FeO ₄ . Of course, an oxidizing gas may be separately supplied to the reaction system when lithium hydroxide is used as the Li source.
Examples of the oxidizing gas supplied to the reaction system include water (steam) and carbon dioxide.

It is considered that ferric oxide, which is the Fe source, is also reduced by being heated together with carbon. However, the reduction reaction of the ferric oxide is also suppressed in the presence of an oxidizing gas such as _H2O .
In addition, the reduction reaction of the ferric oxide can occur before the start of the Li ₅ FeO ₄ synthesis reaction in the first stage, but since the product of the reduction reaction is oxidized by the oxidizing gas, it is oxidized again. produces iron. Therefore, by performing the synthesis step in the presence of an oxidizing gas, there is also the advantage that the synthesis of Li ₅ FeO ₄ and the production of composite particles can be efficiently performed while suppressing the loss of raw materials.

On the other hand, the synthesis reaction of Li ₅ FeO ₄ using lithium carbonate as the Li source and ferric oxide as the Fe source produces CO ₂ gas. Since this CO ₂ gas also functions as an oxidizing gas, even when lithium carbonate is used as the Li source, the oxidizing gas produced as a by-product in the synthesis reaction of Li ₅ FeO ₄ causes ferric oxide and ferric oxide as Fe sources. A reduction reaction of Li ₅ FeO ₄ can be suppressed.

Note that when lithium carbonate is used as the Li source, the synthesis temperature of Li ₅ FeO ₄ exceeds 650°C. Therefore, when lithium carbonate is used as the Li source and ferric oxide is used as the Fe source, it is premised that a reduction reaction of ferric oxide and Li ₅ FeO ₄ occurs in the synthesis process. Therefore, in this case, it is preferable to separately supply an oxidizing gas to the reaction system. In particular, when the synthesis reaction of Li ₅ FeO ₄ is completed, the supply of oxidizing gas by the reaction is also completed. Therefore, when lithium carbonate is used as the Li source, it is preferable to separately supply an oxidizing gas before the completion of the synthesis reaction of Li ₅ FeO ₄ .

By the way, when lithium oxide, lithium carbonate, or the like is used as the Li source, the Li source may stick to the reactor of the heating device that heats the mixed raw material containing the Li source, the Fe source, and the graphite in the synthesis process. There is If the Li source sticks, the yield of the composite particles deteriorates.
By adding an organic additive that can function as a lubricant, such as sucrose fatty acid ester or fatty acid, to the mixed raw material, the problem of sticking of the Li source can be suppressed. However, for example, if the amount of the organic additive added is large, a large amount of contaminants derived from the organic additive will be present in the composite particles. In this case, there is a possibility that the purity of the composite particles will decrease and the function of the composite particles as a positive electrode active material or the like will be impaired.

When lithium hydroxide is used as the Li source, since the lithium hydroxide itself can function as a lubricant, the problem of sticking of the Li source is less likely to occur, and the addition of an organic additive becomes unnecessary. This fact also makes it possible to say that lithium hydroxide is preferably used as a Li source. As the lithium hydroxide, a hydrate may be used, or an anhydrous one may be used.

In the synthesis process, the apparatus for heating the mixed raw material of the Li source, the Fe source, and the graphite includes, for example, a hot wall type, cold wall type, horizontal type, vertical type, fluidized bed reactor, rotary furnace, and tunnel furnace. , a batch type kiln, a rotary kiln, or the like may be used.

It should be noted that the synthesis step is desirably carried out with the mixed raw material in a fluid state. By doing so, the Li source, the Fe source and the graphite can be uniformly dispersed, the Li ₅ FeO ₄ can be produced efficiently, and the carbon phase is uniformly formed on the surface of the Li ₅ FeO ₄ phase. composite particles can be produced. Furthermore, in this case, binding between the composite particles can be suppressed. Various methods such as using a fluidized bed are available for making the mixed raw material into a fluid state, but it is preferable to stir the mixed raw material. For example, if a rotary kiln having a baffle inside is used, the mixed raw material remaining on the baffle is stirred by dropping from a predetermined height as the rotary kiln 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 considering the decomposition of Li ₅ FeO ₄ described above, it is preferably 650° C. or less. More preferable heating temperature ranges include ranges of 600° C. or lower, 550° C. or lower, 500° C. or lower, 490° C. or lower, and 470° C. or lower.

Although the lower limit of the heating temperature in the synthesis step is not particularly limited, it can be 390° C. or higher. A particularly preferable range of the heating temperature is 390°C to 640°C. The present inventor has already confirmed that Li ₅ FeO ₄ can be synthesized by heating at 400° C. using lithium oxide as the Li source and elemental iron as the Fe source.

The heating temperature in the synthesis step is preferably over 4 hours, more preferably 6 hours or more, still more preferably 8 hours or more, and particularly preferably 10 hours or more. Although there is no particular upper limit to the heating time, the heating time is preferably 50 hours or less in consideration of the production efficiency of the composite particles.

Preferred heating temperature and heating time in the synthesis step are exemplified for each Li source.
When lithium hydroxide is used as the Li source, the heating temperature is preferably 440° C. or higher, more preferably 450° C. or higher, and particularly preferably 460° C. or higher. When lithium hydroxide is used as the Li source and the heating temperature is set to 460° C. or higher, preferable heating time ranges are 2 hours or longer, 3 hours or longer, and 4 hours or longer.
When lithium carbonate is used as the Li source, the heating temperature is preferably 720° C. or higher, more preferably 730° C. or higher, and particularly preferably 740° C. or higher. When lithium carbonate is used as the Li source and the heating temperature is 720° C. or higher, the preferable heating time is 15 hours or longer, 20 hours or longer, 24 hours or longer, and 26 hours or longer.
When lithium carbonate is used as the Li source and the heating temperature is set to 730° C. or higher, preferable heating time ranges are 8 hours or longer, 10 hours or longer, and 13 hours or longer.
When lithium carbonate is used as the Li source and the heating temperature is 740° C. or higher, the preferred heating time is 6 hours or longer, 8 hours or longer, 10 hours or longer, 12 hours or longer, and 13 hours or longer. .

The mixed raw material, which is the raw material for the synthesis step, may be mixed with the Li source and the Fe source in amounts based on the stoichiometric ratio of the synthesis reaction of Li ₅ FeO ₄ . The amount of graphite is not particularly limited, but a preferable range is 0.5% by mass to 4.5% by mass when Li ₅ FeO ₄ produced in the synthesis step is taken as 100% by mass. Each range of an amount of 0.5% to 3.5% by mass and an amount of 0.5% to 3.5% by mass can be mentioned.
Alternatively, the preferable range of the amount of graphite in the mixed raw material is less than 1 mol, 0.9 mol or less, 0.8 mol or less, 0.6 mol or less, or 0.5 mol or less in terms of carbon per 1 mol of lithium contained in the Li source. Each range is included. Especially when the carbon source is Li ₂ CO ₃ , the preferable range of the amount of graphite in the mixed raw material is less than 2 mol, 1.8 mol or less, and 1.6 mol or less in terms of carbon per 1 mol of Li ₂ CO ₃ . , 1.2 mol or less, and 1.0 mol or less.

The Li source and Fe source contained in the mixed raw material are preferably pulverized in advance. If necessary, graphite may be pulverized. The mixed raw material may be a mixture of the Li source, the Fe source and the graphite, or may be a mixture of the Li source, the Fe source, and the graphite. is preferred. For example, mixed raw materials include ball mills, planetary ball mills, Nara Machinery Co., Ltd. Hybridization System (NHS) and Miralo (MIRALO), Hosokawa Micron Co., Ltd. Mechanofusion and Nobilta, Tokuju Kosakusho Co., Ltd. Theta Composer, etc. The equipment can grind, mix or coalesce.

If necessary, the above-described organic additives may be added to the raw material mixture in the synthesis step. Examples of organic additives include sucrose fatty acid esters, fatty acids, fatty acid salts, fatty acid amides, N,N'-ethylenebis fatty acid amides, fumaric acid alkyl ester alkali metal salts and hardened oils.
Addition of the organic additive can suppress the particles of Li ₅ FeO ₄ from becoming coarse, and can suppress the BET specific surface area of the composite particles from becoming too small. In addition, as described above, it also has the effect of suppressing 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, 0.5 to 5% by mass, and 1 to 3% by mass with respect to the total Li source and Fe source. , can be mentioned.

The composite particles of the present invention are described below.
The Li ₅ FeO ₄ phase and the carbon phase in the composite particles of the present invention may have any size and any shape, but the composite particles of the present invention are used as positive electrode materials for lithium ion secondary batteries. , it is preferable that the carbon phase exists on the surface of the particulate Li ₅ FeO ₄ phase. This is because, as described above, the Li ₅ FeO ₄ phase is mainly involved in the battery reaction, and the carbon phase plays a role of supplementing the electrical conductivity of the Li ₅ FeO ₄ phase.

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

The mass % (Wc) of carbon in the composite particles of the present invention is not particularly limited, but is preferably within the range of 1≦Wc≦10. A more preferable range of the mass of the carbon is within the range of 2.0<Wc≦8.0, within the range of 2.5≦Wc≦6.0, and within the range of 3.0≦Wc≦5.0. can be mentioned.

The BET specific surface area of the composite particles of the present invention is not particularly limited, but is 0.1 to 15 m ^{2 /g, 0.1 to 12 m 2 /g, 0.1 to 10 m 2 /g, 0.1 to 7 m 2 / g .} Each range of can be mentioned.
Incidentally, 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 excessively large, Li ₅ FeO ₄ may react with moisture and carbon dioxide in the atmosphere, degrading Li 5 FeO 4 , or adversely affecting the physical properties of the slurry during the production of the positive electrode.

The composite particles of the present invention are preferably in powder form. Examples of the powder resistance value of the composite particles of the present invention include 10 ⁻¹ to 5×10 ³ Ωcm, 1 to 1×10 ³ Ωcm, and 10 to 5×10 ² Ωcm.
In addition, the powder resistance value in this specification means the volume resistance value when a load of 4 kN is applied to the measurement sample.

In the production method of the present invention described above, the synthesis step is performed in the presence of an oxidizing gas, so that the reduction of Li ₅ FeO ₄ and ferric oxide is suppressed, and it is believed that the formation of Fe crystals is suppressed. . Therefore, in the production method of the present invention, it is preferable to produce composite particles having no Fe crystals. It can be confirmed by X-ray diffraction that the composite particles do not have Fe crystals. Specifically, if no peak derived from Fe crystals is detected in the X-ray diffraction chart of the composite particles, it can be said that the composite particles do not have Fe crystals. Therefore, it can be said that the composite particles of the present invention obtained by the production method of the present invention are particularly preferably those in which peaks derived from Fe crystals are not detected in the X-ray diffraction chart of the composite particles.

The composite particles of the present invention can function as a positive electrode active material, as a lithium ion supplier, or as various additives. In particular, the composite particles of the present invention are a positive electrode material for lithium ion secondary batteries, and function favorably as a lithium ion supplier during the initial charge in the positive electrode of the lithium ion secondary battery. Hereinafter, a positive electrode for a lithium ion secondary battery comprising the composite particles of the present invention will be referred to as "positive electrode of the present invention", and a lithium ion secondary battery comprising the positive electrode of the present invention will be referred to as "lithium ion secondary battery of the present invention". It says.

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. A positive electrode active material layer is formed on the current collector. When the composite particles of the present invention are lithium ion donors, the amount of the composite particles of the present invention to be blended in the positive electrode active material layer may be determined according to the irreversible capacity of the negative electrode.

A current collector refers to a chemically inert electronic conductor for continuing current flow to an electrode during discharge or charge 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 used. The material of the current collector 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. can be exemplified by metal materials such as The current collector may be covered with a known protective layer. A current collector whose surface has been 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 employ aluminum as the current collector for the positive electrode.

Specifically, it is preferable to use aluminum or an aluminum alloy as the positive electrode current collector. Here, aluminum refers to pure aluminum, and aluminum with a purity of 99.0% or more is called pure aluminum. An alloy obtained by adding various elements to pure aluminum is called an aluminum alloy. Aluminum alloys include Al--Cu, Al--Mn, Al--Fe, Al--Si, Al--Mg, Al--Mg--Si and Al--Zn--Mg.

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

The current collector can take the form of foil, sheet, film, wire, rod, mesh, and the like. Therefore, metal foils such as copper foil, nickel foil, aluminum foil, and stainless steel foil can be preferably used as the current collector. When the current collector is in the form of foil, sheet or film, the thickness 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, a general formula of layered rock salt structure: _{LiaNibCocMndDeOf} ( _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, At least one element selected from Sc, Sn, In, Y, Bi, S, Si, Na, K, P, and V, a lithium composite metal oxide represented by 1.7 ≤ f ≤ 3), 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, Y, Bi, S, Si, Na, K, At least one element selected from P and V, a lithium composite metal oxide represented by 1.7≦f≦3), and Li ₂ MnO ₃ can be mentioned. Further, as the positive electrode active material, a spinel-structured metal oxide such as LiMn ₂ O ₄ , a solid solution composed of a mixture of a spinel-structured metal oxide and a layered compound, LiMPO ₄ , LiMVO ₄ or Li ₂ MSiO ₄ (in the formula is selected from at least one of Co, Ni, Mn and Fe). Furthermore, examples of the positive electrode active material include taborite compounds represented by LiMPO ₄ F (M is a transition metal) such as LiFePO ₄ F, and borate compounds represented by LiMBO ₃ (M is a transition metal) such as LiFeBO ₃ . be able to. Any metal oxide used as a positive electrode active material may have the above compositional formula as a basic composition, and those in which the metal elements contained in the basic composition are replaced with other metal elements can also be used. Further, as the positive electrode active material, a material that does not contain charge carriers (for example, lithium ions that contribute to charging and discharging) may be used. Examples include elemental sulfur, compounds of sulfur and carbon, metal _sulfides such as _TiS2 , oxides such as _V2O5 and _MnO2 , polyaniline and anthraquinone, and compounds containing these aromatics in their chemical structures. Conjugated materials such as acetic acid-based organic substances and other known materials can also be used. Furthermore, compounds having stable radicals such as nitroxide, nitronyl nitroxide, galvinoxyl, and phenoxyl may be employed as positive electrode active materials. When using a positive electrode active material that does not contain a charge carrier such as lithium, it is necessary to add the 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 in a non-ionic 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.

From the viewpoint of high capacity and excellent durability, as a positive electrode active material, a layered rock salt structure with a general formula: Li _a Ni _b Co _{c Mnd 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, at least one element selected from Rh, Fe, Ge, Zn, Ru, Sc, Sn, In, Y, Bi, S, Si, Na, K, P and V; Lithium composite metal oxide, or Li _a Nib Coc 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 adopt a lithium composite metal oxide represented by at least one element selected from Y, Bi, S, Si, Na, K, P, and V and 1.7≦f≦3).

Li _x Mn _2-y A _y O ₄ (A is Ca, Mg, S, Si, Na, K, Al, P, Ga , Ge, and at least one metal element selected from transition metal elements such as Ni, 0<x≦2.2, 0≦y≦1). Examples of the range of values of x are 0.5≦x≦1.8, 0.7≦x≦1.5, and 0.9≦x≦1.2, and the range of values of y is 0. ≤y≤0.8 and 0≤y≤0.6 can be exemplified. Specific spinel structure compounds include LiMn ₂ O ₄ and LiMn _1.5 Ni _0.5 O ₄ .

_LiFePO4 , _{Li2FeSiO4 , LiCoPO4} , _Li2CoPO4 , _Li2MnPO4 , _Li2MnSiO4 , _{and Li2CoPO4F can} be _exemplified as specific _positive electrode active materials. Another specific positive electrode active material is Li ₂ MnO ₃ —LiCoO ₂ .

The positive electrode active material layer preferably contains a binder and a conductive aid. As the binder and the conductive aid contained in the positive electrode active material layer, those described below for the negative electrode may be appropriately employed.

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

Materials that can occlude and release charge carriers can be used as the negative electrode active material. Therefore, there is no particular limitation as long as it is a simple substance, alloy or compound that can occlude and release charge carriers such as lithium ions. For example, as the negative electrode active material, Li, carbon, silicon, germanium, group 14 elements such as tin, aluminum, group 13 elements such as indium, zinc, group 12 elements such as cadmium, antimony, group 15 elements such as bismuth, magnesium , alkaline earth metals such as calcium, and Group 11 elements such as silver and gold may be used singly. Specific examples of alloys or compounds include tin-based materials such as Ag—Sn alloys, Cu—Sn alloys, and Co—Sn alloys, carbon-based materials such as various types of graphite, and SiO _x ( 0.3≦x≦1.6), silicon alone, or a composite of a silicon-based material and a carbon-based material. In addition, as a negative electrode active material, oxides 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= Nitrides represented by (Co, Ni, Cu) may be employed. One or more of these materials can be used as the negative electrode active material.

Graphite, Si-containing materials, and Sn-containing materials can be cited as preferable negative electrode active materials from the viewpoint of the possibility of increasing the capacity. In particular, when a Si-containing material in which the presence of irreversible capacity is an important issue is employed 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 supplier is remarkably exhibited. be done.

Specific examples of the Si-containing material include simple Si and SiO _x (0.3≦x≦1.6) disproportionated into two phases of a Si phase and a silicon oxide phase. The Si phase in SiO _x can occlude and release lithium ions, and its volume changes as the secondary battery is charged and discharged. The silicon oxide phase undergoes less volume change during charging and discharging than the Si phase. In other words, SiO _x as the negative electrode active material realizes a high capacity due to the Si phase, and suppresses the volume change of the entire negative electrode active material by having the silicon oxide phase. Note that if x is less than the lower limit, the ratio of Si becomes excessive, so that the volume change during charging and discharging becomes too large, and the cycle characteristics of the secondary battery deteriorate. On the other hand, if x exceeds the upper limit, the Si ratio becomes too small and the energy density decreases. The range of x is more preferably 0.5≤x≤1.5, more preferably 0.7≤x≤1.2.

In addition, in the SiO _x described above, it is believed that an alloying reaction occurs between lithium and silicon in the Si phase during charging and discharging of the lithium ion secondary battery. It is believed that this alloying reaction contributes to charging and discharging of the lithium ion secondary battery. The Sn-containing material, which will be described later, is also considered to be chargeable and dischargeable through an alloying reaction between tin and lithium.

Specific examples of Sn-containing materials include single Sn, tin alloys such as Cu—Sn and Co—Sn, amorphous tin oxides, and tin silicon oxides. SnB _0.4 P _0.6 O _3.1 can be exemplified as an amorphous tin oxide, and SnSiO ₃ can be exemplified as a tin silicon oxide.

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 compositing, the structure of silicon and/or tin in particular is stabilized, and the durability of the negative electrode is improved. The above compounding may be performed by a known method. Graphite, hard carbon, soft carbon, or the like may be used as the carbon material used for compositing. Graphite may be natural graphite or artificial graphite.

Specific examples of Si-containing materials include silicon materials (hereinafter simply referred to as “silicon materials”) disclosed in International Publication No. 2014/080608 and the like.

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

The method for producing a silicon material is represented by an ideal reaction formula using hydrogen chloride as an acid as follows.
_{3CaSi2 +} 6HCl→ _Si6H6 + _3CaCl2
Si ₆ H ₆ → 6Si+3H ₂ ↑

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

As described above, the silicon material has a structure in which a plurality of plate-like silicon bodies are laminated in the thickness direction. In order to efficiently absorb and release charge carriers such as lithium ions, the thickness of the plate-shaped silicon body is preferably in the range of 10 nm to 100 nm, more preferably in the range of 20 nm to 50 nm. The longitudinal length of the plate-shaped silicon body is preferably within the range of 0.1 μm to 50 μm. In addition, it is preferable that (longitudinal length)/(thickness) of the plate-like silicon body is within the range of 2 to 1,000. The lamination structure of the plate-shaped silicon bodies can be confirmed by observation with a scanning electron microscope or the like. It is also believed that this layered structure is a remnant of the Si layer in the CaSi ₂ raw material.

Preferably, the silicon material includes amorphous silicon and/or silicon crystallites. In particular, in the plate-like silicon body, a state in which amorphous silicon is used as a matrix and silicon crystallites are scattered in the matrix is preferable. The size of the silicon crystallites is preferably in the range of 0.5 nm to 300 nm, more preferably in the range of 1 nm to 100 nm, even more 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 Scherrer's formula using the half width of the diffraction peak of the Si (111) plane of the X-ray diffraction chart obtained by performing X-ray diffraction measurement on the silicon material. .

The amount and size of the plate-shaped silicon bodies, amorphous silicon and silicon crystallites contained in the silicon material are mainly affected by the heating temperature and heating time. The heating temperature is preferably within the range of 350°C to 950°C, more preferably within the range of 400°C to 900°C.

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

The average particle size of the silicon material is preferably in the range of 2-7 μm, more preferably in the range of 2.5-6.5 μm. If a silicon material with too small an average particle size is used, it may become difficult to manufacture the negative electrode from the viewpoint of cohesiveness and wettability. Specifically, a silicon material having too small an average particle size may aggregate in the slurry prepared when manufacturing the negative electrode. On the other hand, a lithium ion secondary battery having a negative electrode using a silicon material having an excessively large average particle size may not be able to be charged and discharged appropriately. The reason for this is presumed to be that lithium ions cannot sufficiently diffuse into the silicon material with an excessively large average particle size. In addition, the average particle size in this specification means D50 when a sample is measured with a general laser diffraction type particle size distribution analyzer.

Binders include fluorine-containing resins 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 cellulose. , styrene-butadiene rubber, etc. may be employed.

Also, a crosslinked polymer obtained by crosslinking a carboxyl group-containing polymer such as polyacrylic acid or polymethacrylic acid with a polyamine such as diamine disclosed in International Publication No. 2016/063882 may be used as a binder.

Diamines used in the crosslinked polymer include alkylenediamines such as ethylenediamine, propylenediamine and hexamethylenediamine, 1,4-diaminocyclohexane, 1,3-diaminocyclohexane, isophoronediamine, bis(4-aminocyclohexyl)methane and the like. saturated carbocyclic diamine, m-phenylenediamine, p-phenylenediamine, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl ether, bis(4-aminophenyl)sulfone, benzidine, o-tolidine, 2,4- Aromatic diamines such as tolylenediamine, 2,6-tolylenediamine, xylylenediamine and naphthalenediamine are included.

The content of the binder in the active material layer is preferably 0.5 to 20% by mass, more preferably 1 to 15% by mass, still more 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 formability of the electrode will deteriorate, and if the amount of the binder is too large, the energy density of the electrode will be low.

A conductive aid is added to increase the conductivity of the electrode. Therefore, the conductive aid may be added arbitrarily when the conductivity of the electrode is insufficient, and may not be added when the conductivity of the electrode is sufficiently excellent. The conductive aid may be a chemically inert high electron conductor, and examples include carbon black, graphite, vapor grown carbon fiber, and various metal particles, which are carbonaceous fine particles. be. Examples of carbon black include acetylene black, Ketjenblack (registered trademark), furnace black, and channel black. These conductive aids can be added to the active material layer singly or in combination of two or more. The mixing ratio of the conductive aid in the active material layer is preferably 0.5 to 20% by mass, more preferably 1 to 15% by mass, even more preferably 2 to 10% by mass, and particularly preferably 2 to 5% by mass. This is because if the amount of the conductive aid is too small, an efficient conductive path cannot be formed, and if the amount of the conductive aid 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 an 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, a curtain coating method, etc. is used to form a current collector. The active material 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 the slurry is applied to the surface of the current collector and then dried. Examples of solvents include N-methyl-2-pyrrolidone, methanol, methyl isobutyl ketone, and water. In order to increase the electrode density, it may be compressed after drying.

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

The amount of the composite particles of the present invention to be blended in the composition for producing a positive electrode active material layer may be appropriately determined according to the application of the composite particles of the present invention.
When the composite particles of the present invention are used as a lithium ion supply agent for supplementing lithium ions corresponding to the irreversible capacity of the negative electrode, the amount of the composite particles of the present invention in the composition for producing a positive electrode active material layer is is within the range of 1 to 10% by mass, within the range of 3 to 9% by mass, and within the range of 5 to 8% by mass relative to the solid content.

In addition, the amount of the positive electrode active material in the composition for producing the positive electrode active material layer is 80 to 95% by mass, 83 to 93% by mass, and 85 to 90% by mass based on the solid content. Within the range can be exemplified. The amount of the binder in the composition for producing the positive electrode active material layer is 0.5 to 10% by mass, 1 to 5% by mass, and 2 to 4% by mass based on the solid content. Within the range can be exemplified. The amount of the conductive aid in the composition for producing the positive electrode active material layer is 0.5 to 5% by mass, 1 to 4% by mass, and 1 to 3% by mass based on the solid content. Within the range can be exemplified.

The separator separates the positive electrode from the negative electrode and allows lithium ions to pass through while preventing short circuits 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, polyacrylonitrile, polysaccharides such as cellulose and amylose, and fibroin. , natural polymers such as keratin, lignin and suberin, and porous bodies, non-woven fabrics, and woven fabrics using one or a plurality of electrically insulating materials such as ceramics. Also, the separator may have a multilayer structure.

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

Cyclic carbonates, cyclic esters, chain carbonates, chain esters, ethers and the like can be used as non-aqueous solvents. Examples of cyclic carbonates include ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate, and examples of cyclic esters include gamma-butyrolactone, 2-methyl-gamma-butyrolactone, acetyl-gamma-butyrolactone, and gamma-valerolactone. Examples of chain carbonates include dimethyl carbonate, diethyl carbonate, dibutyl carbonate, dipropyl carbonate and ethyl methyl carbonate, and examples of chain esters include alkyl propionate, dialkyl malonate and alkyl acetate. 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 hydrogen atoms in the chemical structure of the above specific solvent are substituted with fluorine may be employed.

Examples of electrolytes 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, a lithium salt such as LiClO ₄ , LiPF ₆ , LiBF ₄ or LiCF ₃ SO ₃ is added to a non-aqueous solvent such as fluoroethylene carbonate, ethylene carbonate, dimethyl carbonate, ethylmethyl carbonate or diethyl carbonate at 0.5 mol/mol. A solution dissolved at a concentration of about L to 1.7 mol/L can be exemplified.

A specific method for manufacturing the lithium ion secondary battery of the present invention will be described.
For example, an electrode body is formed by sandwiching a separator between a positive electrode and a negative electrode. The electrode body may be of either a laminated type in which a positive electrode, a separator and a negative electrode are laminated, or a wound type in which a laminated body of a positive electrode, a separator and a negative electrode is wound. After connecting the current collector of the positive electrode and the current collector of the negative electrode to the positive electrode terminal and the negative electrode terminal leading to the outside using a current collecting lead or the like, an electrolyte is added to the electrode body to make 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 cylindrical, square, coin-shaped, and laminate-shaped can be employed.

The lithium ion secondary battery of the present invention may be mounted on a vehicle. The vehicle may be any vehicle that uses electrical energy from a lithium-ion secondary battery as a power source in whole or in part, 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 preferable to connect a plurality of lithium ion secondary batteries in series to form an assembled battery. Devices equipped with lithium ion secondary batteries include, in addition to vehicles, personal computers, mobile communication devices, various home electric appliances driven by batteries, office equipment, industrial equipment, and the like. Furthermore, the lithium ion secondary battery of the present invention is used for wind power generation, solar power generation, hydraulic power generation, and other power storage devices and power smoothing devices for power systems, power sources for ships and/or auxiliary equipment, aircraft, power source for spacecraft and/or auxiliary equipment, auxiliary power source for vehicles that do not use electricity as a power source, power source for mobile home robots, power source for system backup, power source for uninterruptible power supply, It may be used as a power storage device that temporarily stores electric power required for charging in a charging station for an electric vehicle.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments. Various modifications, improvements, etc. that can be made by those skilled in the art can be implemented without departing from the scope of the present invention.

Various specific examples are shown below to describe the present invention more specifically. However, the present invention is not limited to these specific examples.

(Example 1)
(Synthesis process)
A mixed raw material prepared by weighing 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 was put into a planetary ball mill. The planetary ball mill was operated at an autorotation speed of 800 rpm and a revolution speed of 800 rpm to pulverize and mix the mixed raw material until the resulting particles had a diameter of 10 μm or less.

The mixed raw material pulverized and mixed was placed in a reactor and heated at 460° C. for 12 hours in an argon atmosphere in an electric furnace to synthesize composite particles of Example 1.
The mixed raw material in Example 1 was mixed with the Li source and the Fe source in such an amount that the atomic ratio of Li to Fe was 5:1. Graphite was blended in an amount that would be 3% by mass when Li ₅ FeO ₄ produced in the synthesis step is taken as 100% by mass.

(Example 2)
(Synthesis process)
62.7 parts by mass of _Li2CO3 as a Li source _, 27.1 parts by mass of _Fe2O3 as an Fe source, 8.1 parts by mass of graphite, and _2.0 parts by mass of lithium stearate as an organic additive. A mixed raw material was obtained by weighing parts by mass. Moreover, the heating temperature and the heating time in the synthesis process were 730° C. and 13 hours. Composite particles of Example 2 were obtained in the same manner as in Example 1 except for the above.
The mixed raw material in Example 2 was mixed with the Li source, the Fe source, and the graphite in such an amount that the atomic ratio of Li, Fe, and C was 5:1:2.

(Comparative 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 were charged into a planetary ball mill. Furthermore, lithium stearate, which is an organic additive, was weighed in an amount corresponding to 2% by mass with respect to the total amount of Li ₂ O and Fe ₂ O ₃ and charged into a planetary ball mill. The planetary ball mill was operated at an autorotation speed of 800 rpm and a revolution speed of 800 rpm to pulverize and mix the mixed raw material until the resulting particles had a diameter of 10 μm or less.

The pulverized and mixed raw material was placed in a rotary kiln type reactor, and then the mixed raw material was heated at 600° C. for 60 minutes in an argon atmosphere to synthesize Li ₅ FeO ₄ . Subsequently, thermal CVD was performed at 700° C. for 40 minutes under a hexane-argon mixed gas flow to form a carbon film on the surface of Li ₅ FeO ₄ to obtain the carbon-coated Li ₅ FeO of Comparative Example 1. ₄ was produced. During the synthesis of Li ₅ FeO ₄ and carbon-coated Li ₅ FeO ₄ , the rotary kiln type reactor was kept rotating.

(Evaluation example 1)
The composite particles of Example 1, the composite particles of Example 2, and the carbon-coated Li ₅ FeO ₄ of Comparative Example 1 were analyzed by X-ray diffraction analysis using synchrotron radiation. The analysis conditions were as follows. The obtained X-ray diffraction chart is shown in FIG.
<Analysis conditions>
Measurement beamline: Aichi Synchrotron BL5S2
Measured energy: 12.4 keV
Camera length: 340mm
Exposure time: 300s
Capillary: diameter 0.3 mm

As shown in FIG. 1, it was confirmed that Li ₅ FeO ₄ was generated in the composite particles of Example 1, the composite particles of Example 2, and the carbon-coated Li ₅ FeO ₄ of Comparative Example 1. Moreover, from the carbon-coated Li ₅ FeO ₄ of Comparative Example 1, diffraction peaks derived from Fe crystals were clearly detected. In contrast, the peak was not detected from the composite particles of Example 1 and the composite particles of Example 2. This result confirms that Fe crystals are present in the carbon-coated Li ₅ FeO ₄ of Comparative Example 1, and Fe crystals are not present in the composite particles of Example 1 and Example 2. The reason why the composite particles of Example 1 and the composite particles of Example 2 do not have Fe crystals is considered as follows.

As previously mentioned, exposure to temperatures above 650° C. in the presence of carbon is believed to cause reductive decomposition of Li ₅ FeO ₄ . Since the heating temperature in the synthesis step of Comparative Example 1 was 700° C., it is presumed that Fe crystals, which are products of reductive decomposition of Li ₅ FeO ₄ , were generated.

In addition, in the synthesis process of Comparative Example 1, the synthesis reaction of Li ₅ FeO ₄ using lithium oxide as the Li source occurs, but no oxidizing gas is produced as a by-product in the reaction. Furthermore, in the synthesis process of Comparative Example 1, no oxidizing gas is supplied to the reaction system. That is, it is considered that the synthesis step in the production method of Comparative Example 1 was performed in the absence of an oxidizing gas, and the reductive decomposition of Li ₅ FeO ₄ was not suppressed.

On the other hand, since the heating temperature in the synthesis process of Example 1 was 460° C., it is presumed that the reductive decomposition of Li ₅ FeO ₄ was suppressed and no Fe crystals were formed. In addition, in the synthesis step in the production method of Example 1, a synthesis reaction of Li ₅ FeO ₄ occurs, in which lithium hydroxide is used as the Li source and H ₂ O, which is an oxidizing gas, is produced as a by-product. Therefore, even if the synthesis step in the production method of Example 1 is performed in the presence of an oxidizing gas, and reductive decomposition of Li ₅ FeO ₄ occurs, it is considered that the reductive decomposition is suppressed.

Further, since the heating temperature in the synthesis step of Example 2 is 730° C., reductive decomposition of Li ₅ FeO ₄ may occur in the synthesis step of Example 2. However, in the synthesis step in the production method of Example 2, a synthesis reaction of Li ₅ FeO ₄ occurs in which lithium carbonate is used as the Li source and CO ₂ , which is an oxidizing gas, is produced as a by-product. Therefore, it is considered that the synthesis step in the production method of Example 2 is also performed in the presence of an oxidizing gas, and as a result, reductive decomposition of Li ₅ FeO ₄ is suppressed. Therefore, it is presumed that Fe crystals were not observed in the X-ray diffraction chart of the composite particles of Example 2.

(Evaluation example 2)
Using a carbon/sulfur analyzer, the composite particles of Examples 1 and 2 and the Li ₅ FeO ₄ of Comparative Example 1 were subjected to elemental analysis targeting carbon. The results are shown in Table 1 together with the results of Evaluation Example 3 below.

(Evaluation example 3)
For the composite particles of Examples 1 and 2 and the carbon-coated Li ₅ FeO ₄ of Comparative Example 1, the volume after applying a load of 4 kN using a powder resistivity measurement system (Mitsubishi Analyticch Co., Ltd.) Resistance values were measured. The results are shown in Table 1 together with the results of Evaluation Example 2.

As shown in Table 1, the composite particles of Examples 1 and 2 obtained by reacting the Li source and the Fe source in the presence of graphite were carbon-coated with Li ₅ FeO ₄ by a thermal CVD method. Compared with the obtained carbon-coated Li ₅ FeO ₄ of Comparative Example 1, it was comparable in terms of carbon content and conductivity.

(Example 3)
(Synthesis process)
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 were blended. The mixed raw material was put into a planetary ball mill. Further, lithium stearate, which is an organic additive, was added to the planetary ball mill in an amount corresponding to 2% by mass with respect to the total amount of Li ₂ CO ₃ , Fe ₂ O ₃ , graphite and lithium stearate. A planetary ball mill was operated at a rotation speed of 800 rpm and a revolution speed of 800 rpm to produce a composite of the mixed raw materials.
The composite of this mixed raw material was placed in a rotary kiln reactor and heated at 720° C. for 26 hours in an argon atmosphere to obtain particulate composite particles of Example 3.
The mixed raw material in Example 3 was mixed with the Li source and the Fe source in such an amount that the atomic ratio of Li to Fe was 5:1. Graphite was blended in an amount of 0.4 mol in terms of carbon with respect to 1 mol of lithium contained in the Li source.

(Example 4)
Composite particles of Example 4 were obtained in the same manner as in Example 3, except that the heating temperature and heating time in the synthesis step were 730° C. for 13 hours.

(Example 5)
In the same manner as in Example 3, except that the heating temperature and heating time in the synthesis step were first 720 ° C. for 4 hours, then 740 ° C. for 4 hours, and 720 ° C. for 4.5 hours in total, for a total of 12.5 hours. Composite particles of Example 5 were obtained.

(Example 6)
(Synthesis process)
4.055 g (54.9 mmol) of Li ₂ CO ₃ as a Li source, 1.753 g (11.0 mmol) of Fe ₂ O ₃ as an Fe source, 0.658 g of graphite (54.9 mmol in terms of carbon), and Lithium stearate as an organic additive was weighed in an amount corresponding to 2% by mass with respect to the total amount of Li ₂ O, Fe ₂ O ₃ , graphite and lithium stearate to prepare a mixed raw material. Also, the heating temperature and the heating time in the synthesis step were 720° C. and 4 hours. Composite particles of Example 6 were obtained in the same manner as in Example 3 except for this.
The mixed raw material in Example 6 was mixed with the Li source and the Fe source in such an amount that the atomic ratio of Li to Fe was 5:1. Graphite was blended in an amount of 0.5 mol in terms of carbon with respect to 1 mol of lithium contained in the Li source.

(Example 7)
(Synthesis process)
3.680 g (49.8 mmol) of Li ₂ CO ₃ as the Li source, 1.591 g (10.0 mmol) of Fe ₂ O ₃ as the Fe source, 1.195 g of graphite (99.6 mmol in terms of carbon), and Lithium stearate as an organic additive was weighed in an amount corresponding to 2% by mass with respect to the total amount of Li ₂ O, Fe ₂ O ₃ , graphite and lithium stearate to prepare a mixed raw material. Also, the heating temperature and the heating time in the synthesis step were 720° C. and 4 hours. Composite particles of Example 7 were obtained in the same manner as in Example 3 except for this.
The mixed raw material in Example 7 was mixed with the Li source and the Fe source in such an amount that the atomic ratio of Li to Fe was 5:1. Graphite was blended in an amount of 1 mol in terms of carbon with respect to 1 mol of lithium contained in the Li source.

(Example 8)
Composite particles of Example 8 were obtained in the same manner as in Example 6, except that the heating time in the synthesis step was set to 20 hours.
As in Example 6, the mixed raw material in Example 8 was mixed with the Li source and the Fe source in such an amount that the atomic ratio of Li to Fe was 5:1. Similarly to Example 6, graphite was blended in an amount of 0.5 mol in terms of carbon with respect to 1 mol of lithium contained in the Li source.

(Evaluation example 4)
The composite particles of Examples 3 to 8 were analyzed by powder X-ray diffraction analysis using a Cu tube as a radiation source. The obtained X-ray diffraction charts are shown in FIGS. 2 is an X-ray diffraction chart of the composite particles of Examples 3 to 5, and FIGS. 3 and 4 are X-ray diffraction charts of the composite particles of Examples 6 and 8. 5 and 6 are X-ray diffraction charts of the composite particles of Examples 3, 7 and 8. FIG.

As shown in FIGS. 2 to 6, it was confirmed that Li ₅ FeO ₄ was generated in each of the composite particles of Examples 3 to 8.

As shown in FIG. 2, when comparing the X-ray diffraction charts of the composite particles of Examples 3 to 5, the X-ray diffraction chart of the composite particles of Example 3 and the X-ray diffraction chart of the composite particles of Example 4 are shown. While no lithium carbonate, which is the Li source, was confirmed in Example 5, lithium carbonate, which is the Li source, was slightly confirmed in the X-ray diffraction chart of the composite particles of Example 5. This is considered to be due to the heating temperature and heating time in the synthesis process.

Among the production methods of Examples 3 to 8, in the production method of Example 4, the heating temperature in the synthesis step was relatively high at 730 ° C. Therefore, even if the heating time was relatively short at 13 hours, the synthesis reaction was completed. It is considered to have progressed sufficiently.
Moreover, in the production method of Example 3, the heating temperature in the synthesis step was relatively low at 720° C., but the heating time was relatively long at 26 hours.

On the other hand, as in the production method of Example 5, in the synthesis step, although the heating was performed at a relatively high temperature of 740°C, the time was as short as 4 hours, and the heating was performed at a relatively low temperature of 720°C. If the heating time is as short as 8.5 hours, the reaction for synthesizing Li ₅ FeO ₄ does not proceed sufficiently, and the Li source, which is the raw material of Li ₅ FeO ₄ , remains.

From this result, it can be said that when lithium carbonate is used as the Li source, the heating temperature is preferably 730° C. or higher, and more preferably 740° C. or higher.

When lithium carbonate is used as the Li source and the heating temperature is 720° C. or higher, the heating time is preferably 15 hours or longer, more preferably 20 hours or longer, and 24 hours or longer. is more preferable, and 26 hours or longer is particularly preferable.
When lithium carbonate is used as the Li source and the heating temperature is 730° C. or higher, the heating time is preferably 8 hours or longer, more preferably 10 hours or longer, and preferably 13 hours or longer. It can be said that it is particularly preferable.
When lithium carbonate is used as the Li source and the heating temperature is 740° C. or higher, the heating time is preferably 6 hours or longer, more preferably 8 hours or longer, and preferably 10 hours or longer. More preferably, it is more preferably 12 hours or more, and particularly preferably 13 hours or more.

As shown in FIG. 4, when comparing the X-ray diffraction charts of the composite particles of Examples 6 and 7, Fe crystals were not confirmed in the X-ray diffraction chart of the composite particles of Example 6. , Fe crystals were confirmed in the X-ray diffraction chart of the composite particles of Example 7. This is considered to be due to the amount of graphite contained in the mixed raw material.

That is, in the production method of Example 6, the amount of 0.5 mol in terms of carbon for 1 mol of lithium contained in the Li source (or the amount of 1 mol in terms of carbon to 1 mol of Li ₂ CO ₃ ) graphite was added. On the other hand, in the production method of Example 7, 1 mol of lithium contained in the Li source is 1 mol in terms of carbon (or 2 mol in terms of carbon is used for 1 mol of Li ₂ CO ₃ amount) of graphite was added.
As a result, the production method of Example 7 contained more graphite than the production method of Example 6, so that the reduction reaction rate of Li ₅ FeO ₄ was increased, and as a result, Fe crystals were generated. Conceivable.

From this result, the preferable range of the amount of graphite in the mixed raw material is less than 1 mol, 0.9 mol or less, 0.8 mol or less, 0.6 mol or less, and 0.5 mol in terms of carbon per 1 mol of lithium contained in the Li source. Each of the following ranges can be mentioned.
Further, when the carbon source is Li ₂ CO ₃ , the preferred range of the amount of graphite in the mixed raw material is less than 2 mol, 1.8 mol or less, 1.6 mol or less in terms of carbon per 1 mol of Li ₂ CO ₃ , 1.2 mol or less, and 1.0 mol or less.

By the way, as shown in FIG. 4, it is considered that the synthesis reaction of Li ₅ FeO ₄ does not proceed sufficiently in the composite particles of Example 6. Therefore, the present inventor produced the composite particles of Example 8 by the production method of Example 8 in which the heating time in the synthesis step was 20 hours, and conducted an X-ray diffraction chart of the composite particles of Example 8. The X-ray diffraction chart of the composite particles of Example 3 and the X-ray diffraction chart of the composite particles of Example 7 were compared.
As a result, as shown in FIG. 6, it was confirmed that the synthesis reaction of Li ₅ FeO ₄ proceeded sufficiently in the composite particles of Example 8, and the composite particles of Example 8 contained no Fe crystals. Not confirmed.

(Example 9)
(Synthesis process)
A mixed raw material prepared by weighing 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 was put into a planetary ball mill. The planetary ball mill was operated at an autorotation speed of 800 rpm and a revolution speed of 800 rpm to pulverize and mix the mixed raw material until the resulting particles had a diameter of 10 μm or less.

The mixed raw material pulverized and mixed was placed in a reactor and heated at 460° C. for 4 hours in an argon atmosphere in an electric furnace to synthesize composite particles of Example 9.
The mixed raw material in Example 9 was mixed with the Li source and the Fe source in such an amount that the atomic ratio of Li to Fe was 5:1. Graphite was blended in an amount that would be 3% by mass when Li ₅ FeO ₄ produced in the synthesis step is taken as 100% by mass.

90 parts by mass of the composite particles of Example 9, 5 parts by mass of acetylene black as a conductive agent, 5 parts by mass of polyvinylidene fluoride as a binder, and an appropriate amount of N-methyl-2-pyrrolidone are mixed, Slurry. A positive electrode of Example 9 was obtained by preparing an aluminum foil as a current collector, applying the slurry to this, and drying it.

A lithium foil was prepared and used as the negative electrode. A glass filter (Hoechst Celanese) and celgard 2400 (Polypore Co., Ltd.), which is a single-layer polypropylene, were prepared as separators. Further, an electrolytic solution was prepared by dissolving LiPF ₆ at a concentration of 1 mol/L in a mixed solvent of 3 parts by volume of ethylene carbonate, 3 parts by volume of ethyl methyl carbonate and 4 parts by volume of dimethyl carbonate. Two types of separators were sandwiched between the negative electrode and the positive electrode of Example 8 in the order of negative electrode, glass filter, celgard2400, and positive electrode of Example 9 to form an electrode body. This electrode body was housed in a coin-shaped battery case CR2032 (Hosen Co., Ltd.), and an electrolytic solution was injected to obtain a sealed coin-shaped battery. This was used as a lithium ion secondary battery of Example 9.

(Example 10)
Except for using a mixed raw material containing 70.6 parts by mass of LiOH·H ₂ O as the Li source, 26.8 parts by mass of Fe ₂ O ₃ as the Fe source, and 2.6 parts by mass of graphite. Composite particles of Example 10 were produced in the same manner as in Example 9. Also, using the composite particles of Example 10, a positive electrode of Example 10 and a lithium ion secondary battery of Example 10 were produced in the same manner as in Example 9.
The mixed raw material in Example 10 was mixed with the Li source and the Fe source in such an amount that the atomic ratio of Li to Fe was 5:1. Graphite was blended in an amount of 5% by mass when Li ₅ FeO ₄ produced in the synthesis step was taken as 100% by mass.

(Comparative example 2)
(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 were put into a planetary ball mill. The planetary ball mill was operated at an autorotation speed of 800 rpm and a revolution speed of 800 rpm to pulverize and mix the mixed raw material until the resulting particles had a diameter of 10 μm or less.
The pulverized and mixed raw material was placed in a rotary kiln 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 at 700° C. for 40 minutes under a hexane-argon mixed gas flow to obtain the carbon-coated Li ₅ FeO ₄ of Comparative Example 2. got During the synthesis of Li ₅ FeO ₄ and the carbon coating, the rotary kiln type reactor was rotated.
Using the carbon-coated Li ₅ FeO ₄ of Comparative Example 2, a negative electrode of Comparative Example 2 and a lithium ion secondary battery of Comparative Example 2 were produced in the same manner as in Example 9.

(Evaluation example 5)
Each of the lithium ion secondary batteries of Examples 9, 10 and Comparative Example 2 was charged at a current of 0.1 mA until the voltage reached 4.4V. Table 2 shows the observed charge capacities.

Both the lithium ion secondary battery of Example 9 using the composite particles of Example 9 and the lithium ion secondary battery of Example 10 using the composite particles of Example 10 are comparative examples by thermal CVD. Compared with the lithium ion secondary battery of Comparative Example 2 using the carbon-coated Li ₅ FeO ₄ of No. 2, the charge capacity was far superior.
From these results, it can be said that a lithium-ion secondary battery having excellent charge capacity can be obtained according to the manufacturing method of the present invention.

In addition, since the lithium ion secondary battery of Example 9 is superior to the lithium ion secondary battery of Example 10 in charge capacity, when LiOH is used as the Li source, considering the battery characteristics, the mixed raw material It can be said that the amount of graphite to be blended is preferably less than 5% by mass when Li ₅ FeO ₄ produced in the synthesis step is taken as 100% by mass. A preferable range of the amount of the graphite is 4.5% by mass or less, 4% by mass or less, or 3.5% by mass when Li ₅ FeO ₄ produced in the synthesis step is 100% by mass. %. There is no particular lower limit to the amount of graphite, but it is thought that it should be at least 0.5% by mass when Li ₅ FeO ₄ produced in the synthesis step is 100% by mass.

Claims (1)

A step of heating the Li source, the Fe source, and the graphite in the presence of an oxidizing gas ;
A method for producing composite particles containing Li ₅ FeO ₄ and carbon , using water and/or carbon dioxide as the oxidizing gas, and comprising the following (1) or (2) .
(1) the Li source is LiOH or lithium oxide, and the heating is performed at 390° C. to 640° C.;
(2) the Li source is Li ₂ CO ₃ and the heating is performed at 720°C to 740°C;

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