JP7429001B2 - Positive electrode for lithium ion secondary batteries and lithium ion secondary batteries - Google Patents

Description

The present invention relates to a positive electrode for a lithium ion secondary battery and a lithium ion secondary battery having the positive electrode.

Polyanionic compounds with excellent thermal stability are attracting attention as positive electrode active materials for lithium ion secondary batteries. As polyanionic positive electrode active materials, lithium iron phosphate having an olivine structure, lithium manganese iron phosphate or lithium manganese phosphate in which part or all of the iron in lithium iron phosphate is replaced with manganese are being considered.

However, polyanionic positive electrode active materials have a problem in that they do not have sufficient electrical conductivity. Therefore, it has been proposed to mix and use a positive electrode active material, which is a composite oxide containing lithium and a transition metal such as nickel, cobalt, and manganese, with a polyanionic positive electrode active material (Patent Document 1).

Japanese Patent Application Publication No. 2018-142420

Patent Document 1 describes that the average particle diameter of secondary particles of a "first electrode active material" which is a polyanionic positive electrode active material is compared with that of a "second electrode active material" which is a composite oxide of lithium and other transition metals. By setting the average particle diameter of the secondary particles of the "substance" to 0.01 to 0.30 times, the secondary particles of the first electrode active material are formed by the secondary particles of the second electrode active material. By entering the gaps, the electrode density is promoted, and the gaps between the secondary particles of the second electrode active material are filled with the first electrode active material, which reduces the heat of the lithium ion secondary battery electrode. It is stated that the conductivity is improved compared to the case of the second electrode active material alone.

However, when a lithium ion secondary battery having a positive electrode using a polyanionic positive electrode active material is exposed to high temperatures, metal elements such as manganese and iron are eluted from the polyanionic positive electrode active material such as lithium manganese iron phosphate. , the stability of the battery at high temperatures may be impaired.

An object of the present invention is to provide a positive electrode for a lithium ion secondary battery that has high storage stability, especially at high temperatures.

As a result of intensive studies for the above purpose, the present inventors have developed a positive electrode for lithium ion secondary batteries that has a polyanionic positive electrode active material and a lithium composite oxide positive electrode active material. It has been found that when the median diameter of the secondary particles is made larger than the median diameter of the secondary particles of the lithium composite oxide positive electrode active material, the discharge capacity is less likely to decrease even when stored at high temperatures. The present invention includes, but is not limited to, the following:
[1] The following formula (I) or (II):
LiNi _a Co _b Mn _c M ¹ _x O ₂ ...(I)
(In formula (I), ^M1 is Mg, Ti, Nb, Fe, Cr, Si, Al, Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb , Bi, and Ge, and a, b, c, and x are 0.3≦a<1, 0<b≦0.7, 0<c≦0 .7, 0≦x≦0.3, and is a number that satisfies 3a + 3b + 3c + (M valence of ¹ ) x x = 3)
LiNi _d Co _e Al _f M ² _y O ₂ ...(II)
(In formula (II), ^M2 is Mg, Ti, Nb, Fe, Cr, Si, Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb, Bi , and Ge, and d, e, f, and y are 0.4≦d<1, 0<e≦0.6, 0<f≦0.3 , 0≦y≦0.3, and 3d+3e+3f+(valence of ^M2 )×y=3)
Lithium composite oxide particles (A) represented by
The following formula (III):
LiFe _m M _n M ³ _o PO ₄ ... (III)
(In formula (III), ^M3 is Co, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, or Gd, and m, n, and o are 0≦ It is a number that satisfies m≦1, 0≦n≦1, 0≦o≦0.3, and m+n≠0, and 2m+2n+(valence of ^M3 )×o=2)
Lithium-based polyanion particles (B) represented by
A positive electrode for a lithium secondary battery, wherein the median diameter of the secondary particles of the particles (A) is smaller than the median diameter of the secondary particles of the particles (B).
[2] The lithium ion secondary battery according to [1], wherein the median diameter of the secondary particles of the particles (A) is less than 0.80 times the median diameter of the secondary particles of the particles (B). Positive electrode.
[3] The positive electrode for a lithium ion secondary battery according to [1] or [2], wherein the powder pH of the particles (A) is higher than the powder pH of the particles (B).
[4] The positive electrode for a lithium ion secondary battery according to any one of [1] to [3], wherein the particles (A) are a compound represented by the formula (I).
[5] The positive electrode for a lithium ion secondary battery according to [4], wherein x in the formula (I) is 0.
[6] Any one of [1] to [5], wherein the particle (B) is a compound satisfying 0<m≦1, 0<n≦1, and o=0 in the formula (III). A positive electrode for a lithium ion secondary battery as described in .
[7] A lithium ion secondary battery having the positive electrode for a lithium ion secondary battery according to any one of [1] to [6].
[8] The lithium ion secondary battery according to [7], which has a fluorine-containing lithium salt as the electrolyte.

By using the positive electrode of the present invention, a lithium ion secondary battery whose discharge capacity is less likely to decrease even when exposed to high temperatures can be manufactured.

The positive electrode for a lithium ion secondary battery of the present invention comprises particles (A) that are a composite oxide of lithium and other transition metals represented by a specific formula, and particles (A) that are a composite oxide of lithium and other transition metals represented by a specific formula. It is manufactured using polyanion particles (B) having a transition metal and phosphoric acid.

<Particles (A)>
The particles (A) used in the positive electrode of the present invention have the following formula (I) or (II):
LiNi _a Co _b Mn _c M ¹ _x O ₂ ...(I)
(In formula (I), ^M1 is Mg, Ti, Nb, Fe, Cr, Si, Al, Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb , Bi, and Ge, and a, b, c, and x are 0.3≦a<1, 0<b≦0.7, 0<c≦0 .7, 0≦x≦0.3, and is a number that satisfies 3a + 3b + 3c + (M valence of ¹ ) x x = 3)
LiNi _d Co _e Al _f M ² _y O ₂ ...(II)
(In formula (II), ^M2 is Mg, Ti, Nb, Fe, Cr, Si, Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb, Bi , and Ge, and d, e, f, and y are 0.4≦d<1, 0<e≦0.6, 0<f≦0.3 , 0≦y≦0.3, and 3d+3e+3f+(valence of ^M2 )×y=3)
These are lithium composite oxide particles having a layered rock salt structure represented by:

In order to obtain the lithium composite oxide particles (A) represented by the above formula (I), a mixed powder containing a lithium compound, a nickel compound, a cobalt compound, and a manganese compound is fired. Specifically, first, a nickel compound, a cobalt compound, and a manganese compound are dissolved in water to obtain a desired composition of a lithium composite oxide to obtain an aqueous solution a. Examples of such nickel compounds, cobalt compounds, and manganese compounds include sulfates, nitrates, carbonates, acetates, oxalates, oxides, hydroxides, and halides of these metal elements. Specific examples include nickel sulfate, cobalt sulfate, manganese sulfate, nickel acetate, cobalt acetate, manganese acetate, etc., but are not limited to these. If necessary, Mg, Ti, Nb, Fe, Cr, Si, Al, Mg, Ti, Nb, Fe, Cr, Si, ^Al , One or more elements selected from Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb, Bi, and Ge may be mixed.

Next, an alkaline solution is added to the aqueous solution a to form an aqueous solution b, and while stirring, dissolved metal components are co-precipitated by a neutralization reaction to produce a metal composite hydroxide. The alkaline solution used here is preferably added dropwise in an amount sufficient to maintain the pH of the aqueous solution b at 10 to 14. As such an alkaline solution, for example, an aqueous solution of sodium hydroxide, potassium hydroxide, sodium carbonate, ammonia, etc. can be used, and among them, it is preferable to use sodium hydroxide, sodium carbonate, or a mixed solution thereof.

The temperature of the aqueous solution b in the above neutralization reaction is preferably 30°C or higher, preferably 30 to 60°C. Further, the stirring time of the aqueous solution b is preferably 30 to 120 minutes, more preferably 30 to 60 minutes.

After stirring, the metal composite hydroxide can be recovered by filtering the aqueous solution b. The recovered metal composite hydroxide is preferably washed with water and then dried.
Next, the recovered metal composite hydroxide and the lithium compound are dry-mixed so as to have the desired composition of the lithium composite oxide, and the resulting mixed powder is fired in an oxygen atmosphere. Examples of the lithium compound used here include lithium hydroxide or its hydrate, lithium peroxide, lithium nitrate, and lithium carbonate. From the viewpoint of increasing the powder pH of particles (A), which will be described later, it is preferable to add a large amount of lithium hydroxide. In the dry mixing of the metal composite hydroxide and the lithium compound, an ordinary dry mixer or mixing granulator such as a ball mill or a V-blender can be used, and it is more preferable to use a planetary ball mill that can rotate around its axis.

It is preferable that the mixed powder is fired in two stages (preliminary firing and main firing). By performing the two-stage firing, the main firing is performed after removing thermally decomposed components such as water molecules and carbon dioxide from the hydroxides and carbonates in the mixed powder during the preliminary firing, which allows efficient lithium production. Composite oxide particles (A) can be obtained. The conditions for pre-firing are not particularly limited, but the heating temperature is preferably 1 to 20° C./min from room temperature. Further, the firing atmosphere is preferably an air atmosphere or an oxygen atmosphere. The firing temperature is preferably 700 to 1000°C, more preferably 650 to 750°C. Further, the firing time is preferably 3 to 20 hours, more preferably 4 to 6 hours.

It is preferable to crush the obtained pre-fired product in a mortar or the like, mix it with an appropriate amount of binder, granulate it, and then perform main firing. The fired product obtained after the main firing is secondary particles of lithium composite oxide particles (A).

The conditions for the main firing are not particularly limited, but the heating temperature is preferably 1 to 20° C./min starting from room temperature again. Further, the firing atmosphere is preferably an air atmosphere or an oxygen atmosphere. The firing temperature is preferably 700 to 1200°C from the viewpoint of controlling the average particle diameter of the primary particles constituting the secondary particles of the lithium composite oxide particles (A) obtained after main firing to a desired value, The temperature is more preferably 700 to 1000°C, and even more preferably 750 to 900°C. The firing time is preferably 3 to 20 hours, more preferably 8 to 10 hours.

For such two-stage firing, it is preferable to use an electric furnace, rotary kiln, tubular furnace, pusher furnace, etc. in which the oxygen concentration in the gas atmosphere is adjusted to 20% by mass or more.
In order to obtain the lithium composite oxide particles (A) represented by the above formula (II), a mixed powder containing a lithium compound, a nickel compound, a cobalt compound, and an aluminum compound is fired. Specifically, first, a nickel compound, a cobalt compound, and an aluminum compound are dissolved in water to obtain a desired composition of a lithium composite oxide to obtain an aqueous solution a'. Examples of such nickel compounds, cobalt compounds, and aluminum compounds include sulfates, nitrates, carbonates, acetates, oxalates, oxides, hydroxides, and halides of these metal elements. Specific examples include nickel sulfate, cobalt sulfate, aluminum sulfate, nickel acetate, cobalt acetate, aluminum acetate, etc., but are not limited to these. If necessary, Mg, Ti, Nb, Fe, Cr, Si, ^Al , Mg, Ti, Nb, Fe, Cr, Si, Al, One or more elements selected from Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb, Bi, and Ge may be mixed.

Next, an alkaline solution is added to the aqueous solution a' to form an aqueous solution b', and while stirring, the dissolved metal components are co-precipitated by a neutralization reaction to produce a metal composite hydroxide. The alkaline solution used here is preferably added dropwise in an amount sufficient to maintain the pH of the aqueous solution b' at 10 to 14. As such an alkaline solution, for example, an aqueous solution of sodium hydroxide, potassium hydroxide, sodium carbonate, ammonia, etc. can be used, and among them, it is preferable to use sodium hydroxide, sodium carbonate, or a mixed solution thereof.

The temperature of the aqueous solution b' in the above neutralization reaction is preferably 40°C or higher, preferably 40 to 60°C. Further, the stirring time of the aqueous solution b' is preferably 30 to 120 minutes, more preferably 30 to 60 minutes. In order to obtain a metal composite hydroxide with a high bulk density in order to obtain a useful positive electrode active material, an oxidizing agent such as sodium hypochlorite or hydrogen peroxide is further added to the aqueous solution b' after the neutralization reaction. You may.

After stirring, the metal composite hydroxide can be recovered by filtering the aqueous solution b'. The recovered metal composite hydroxide is preferably calcined to form a metal composite oxide from the viewpoint of stabilizing the quality of the obtained lithium composite oxide and from the viewpoint of reacting uniformly and sufficiently with lithium.

The firing conditions for obtaining a metal composite oxide from a metal composite hydroxide are not particularly limited, and for example, the firing may be performed in an air atmosphere, preferably at 500 to 1100°C, more preferably at 60 to 900°C.

Next, the metal composite oxide obtained by the above firing and the lithium compound are dry-mixed so as to have a desired composition of the lithium composite oxide, and the resulting mixed powder is fired in an oxygen atmosphere. Examples of the lithium compound used here include lithium hydroxide or its hydrate, lithium peroxide, lithium nitrate, and lithium carbonate. From the viewpoint of increasing the powder pH of particles (A), which will be described later, it is preferable to add a large amount of lithium hydroxide. In the dry mixing of the metal composite oxide and the lithium compound, an ordinary dry mixer or mixing granulator, such as a ball mill or a V-blender, can be used. For firing, an electric furnace, a rotary kiln, a tubular furnace, a pusher furnace, etc. in which the oxygen concentration in the gas atmosphere is adjusted to 20% by mass or more can be used.

The firing conditions for the above mixed powder are determined from the viewpoints of avoiding the crystals of the obtained lithium composite oxide from becoming underdeveloped and becoming structurally unstable, and from the viewpoint of preventing the layered structure of the lithium composite oxide from collapsing and allowing insertion of lithium ions. - From the viewpoint of avoiding difficulty in desorption, the firing temperature is preferably 650 to 850°C, more preferably 700 to 800°C. Further, the firing time is preferably 5 to 20 hours, more preferably 6 to 10 hours.

It is preferable that the mixed powder is fired in two stages (preliminary firing and main firing). By performing the two-stage firing, the main firing is performed after removing thermally decomposed components such as water molecules and carbon dioxide from the hydroxides and carbonates in the mixed powder during the preliminary firing, which allows efficient lithium production. Composite oxide particles (A) can be obtained. As for the conditions for the preliminary firing, it is preferable to perform the firing at a firing temperature of 400 to 600° C. for one hour or more. After crushing the fired product obtained by preliminary firing in a mortar or the like, an appropriate amount of binder is mixed and granulated, and the product is subjected to main firing. The main firing conditions are preferably a firing temperature of 650 to 850°C for 5 hours or more.

After washing the fired product obtained in the main firing with water, it is filtered and dried to obtain secondary particles of lithium composite oxide particles (A). The slurry concentration when washing the fired product obtained in the main firing with water is preferably 200 to 4000 g/L, and 500 to 4000 g/L, from the viewpoint of suppressing desorption of lithium from the obtained lithium composite oxide particles (A). -2000g/L is more preferable.

The electrical conductivity of the water used for washing with water should be less than 10 μS/cm in order to avoid precipitation of lithium carbonate on the lithium composite oxide particles (A) due to the water containing a large amount of carbon dioxide gas. is preferable, and 1 μS/cm or less is more preferable.

Drying is preferably carried out in two stages. The first step of drying is performed at 90° C. until the moisture content (moisture content measured at a vaporization temperature of 300° C.) in the lithium composite oxide secondary particles becomes 1% by mass or less. After that, it is preferable to perform the second stage of drying at 120° C. or higher.

The average particle diameter of the primary particles of the lithium composite oxide represented by the above formulas (I) and (II) is preferably 500 nm or less, more preferably 300 nm or less. The lower limit of the average particle diameter of the primary particles is not particularly limited, but from the viewpoint of handling properties, it is preferably 50 nm or more.

The median diameter of the secondary particles of the lithium composite oxide particles (A) represented by the above formulas (I) and (II) is preferably 25 μm or less, more preferably 20 μm or less. The lower limit of the median diameter of the secondary particles is not particularly limited, but from the viewpoint of handling properties, it is preferably 1 μm or more, and more preferably 5 μm or more.

In the positive electrode material of the present invention, the median diameter of the secondary particles of particles (A) represented by formula (I) and formula (II) is greater than the median diameter of secondary particles of particles (B) described below. It's small. Preferably, the median diameter of the secondary particles of particles (A) is less than 0.80 times the median diameter of the secondary particles of particles (B). Since the median diameter of the secondary particles of particles (A) is smaller than the median diameter of the secondary particles of particles (B), the gaps between particles (B) can be efficiently filled with particles (A), and the Elution of metal elements such as manganese and iron from the particles (B) can be efficiently prevented, and the high-temperature storage characteristics of the battery can be improved.

In addition, in this specification, the average particle diameter of a primary particle means the average value of the particle diameter (length of a major axis) of several dozen particles measured by observation with the electron microscope of SEM or TEM. The median diameter of secondary particles means the volume-based median diameter (D50) measured using a laser diffraction particle size distribution meter, and can be measured by the following method:
Using ethanol as a dispersion medium, particles were dispersed in ethanol using ultrasound to prepare a measurement sample, and the particle size distribution was measured using a laser diffraction/scattering particle size distribution analyzer (MT3300EXII manufactured by MicrotracBEL Co., Ltd.). Calculate the median diameter (D50).

The median diameter of the secondary particles of particles (A) and (B) tends to be maintained at almost the same level even in the positive electrode, so the median diameter of the secondary particles of particles (A) and (B) in the positive electrode The diameter can also be measured, for example, in the following way:
The positive electrode is observed using a scanning electron microscope JCM-6000 (manufactured by JEOL JEOL Ltd.) at a magnification of x1000 and 3000, and measured using the [length measurement] function of the observation software (JCM-6000). Discrimination of positive electrode particles can be performed by elemental analysis using an attached EDX device.

The powder pH of the particles (A) represented by the above formulas (I) and (II) is preferably in the range of 10 to 14, more preferably in the range of 10 to 13. The powder pH of the particles (A) is preferably higher than that of the particles (B) described below. By increasing the powder pH of the particles (A), it is possible to efficiently prevent the elution of metal elements such as manganese and iron from the particles (B) at high temperatures, further improving the high-temperature storage characteristics of the battery. Can be done. The mechanism is not clear, but the inventors speculate as follows: When a lithium-ion secondary battery is exposed to high temperatures of 60°C or higher, a small amount of water mixed into the battery , supporting salts such as LiPF ₆ commonly used in electrolytes can hydrolyze to produce free HF. When free HF attacks the particles (B), manganese and iron in the particles (B) may be eluted, which may impair the high temperature storage characteristics of the battery. By allowing particles (A), which have a smaller median diameter and a higher powder pH than particles (B), to exist in the gaps between particles (B), the attack of free HF (acid) is weakened, and the metal from particles (B) is It is thought that the elution of elements can be prevented.

The powder pH of the particles (A) can be increased by adding a large amount of alkaline component during the production of the particles (A) so that the alkali remains in the particles (A).
In addition, in this specification, powder pH is measured by the following procedure:
Add 1 g of each particle (powder) to 50 mL of pure water, stir for 5 minutes at room temperature with a stirrer, and measure the pH of the suspension with a pH meter after standing for 1 minute.

<Particles (B)>
The particles (B) used in the positive electrode of the present invention have the following formula (III):
LiFe _m M _n M ³ _o PO ₄ ...(III)
(In formula (III), ^M3 is Co, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, or Gd, and m, n, and o are 0≦ It is a number that satisfies m≦1, 0≦n≦1, 0≦o≦0.3, and m+n≠0, and 2m+2n+(valence of ^M3 )×o=2)
These are polyanion particles having an olivine structure and containing lithium, at least one of iron and manganese, and phosphoric acid. Carbon may be supported on the particle surface.

The lithium-based polyanion particles (B) represented by the above formula (III) can be obtained by the following procedure:
(i) Mixing a lithium compound and a phosphoric acid compound, and then mixing a metal salt containing at least an iron compound and/or a manganese compound to obtain a mixed solution A.
(ii) The obtained mixture A is subjected to a hydrothermal reaction to obtain a composite B.

When carbon is supported on the lithium-based polyanion particles (B), the following steps can be further performed:
(iii) mixing the obtained complex B, carbon source, and water to obtain a mixed liquid C;
(iv) drying the obtained mixture C to obtain granules D;
(v) The obtained granules D are fired in a reducing atmosphere or an inert atmosphere.

Examples of the lithium compound that can be used in step (i) include lithium hydroxide (for example, LiOH, LiOH.H ₂ O), lithium carbonate, lithium sulfate, and lithium acetate, with lithium hydroxide being preferred. The lithium compound is preferably in the form of an aqueous solution having a concentration of 5 to 50 parts by weight, more preferably 7 to 45 parts by weight, based on 100 parts by weight of water. Before mixing the phosphoric acid compound with the lithium compound, it is preferable to stir the aqueous solution of the lithium compound for 1 to 15 minutes, more preferably for 3 to 10 minutes. Further, the temperature of the aqueous solution is preferably 20 to 90°C, more preferably 20 to 70°C.

Examples of the phosphoric acid compound to be mixed with the lithium compound include orthophosphoric acid (H ₃ PO ₄ , phosphoric acid), metaphosphoric acid, pyrophosphoric acid, triphosphoric acid, tetraphosphoric acid, ammonium phosphate, ammonium hydrogen phosphate, and the like. Among them, it is preferable to use phosphoric acid, and it is preferable to use it as an aqueous solution with a concentration of 70 to 90% by mass. The mixture of the lithium compound and the phosphoric acid compound preferably contains 2.7 to 3.3 moles of lithium, more preferably 2.8 to 3.1 moles, per mole of phosphoric acid.

After mixing the phosphoric acid compound, the reaction in the mixture may be completed by purging with nitrogen to generate a precursor of lithium-based polyanion particles (trilithium phosphate, Li ₃ PO ₄ ). The pressure when purging nitrogen is preferably 0.1 to 0.2 MPa, more preferably 0.1 to 0.15 MPa. Further, the temperature of the mixture after mixing the phosphoric acid compound is preferably 20 to 80°C, more preferably 20 to 60°C. The reaction time is preferably 5 to 60 minutes, more preferably 15 to 45 minutes. When purging with nitrogen, it is preferable to stir at a stirring speed of 200 to 700 rpm, more preferably 250 to 600 rpm, from the viewpoint of promoting the reaction well.

After mixing the lithium compound and the phosphoric acid compound, a metal salt containing at least an iron compound and/or a manganese compound is added to obtain a mixed solution A. The total amount of these metal salts added is preferably 0.990 to 1.010 mol, more preferably 0.995 to 1.005 mol, per 1.000 mol of phosphate ion. When adding a metal (M ³ ) salt other than an iron compound and a manganese compound, the total amount of the iron compound, manganese compound, and metal (M ³ ) salt added is 1.000% of the phosphoric acid in the obtained mixed solution A. The amount is preferably 0.990 to 1.010 mol, more preferably 0.995 to 1.005 mol. The order of addition of the iron compound, manganese compound, and metal (M ³ ) salt is not particularly limited. Furthermore, in addition to adding these metal salts, an antioxidant may be added as necessary. As the antioxidant, sodium sulfite (Na ₂ SO ₃ ), sodium hydrosulfite (Na ₂ S ₂ O ₄ ), ammonia water, etc. can be used. The amount of antioxidant added is preferably 0.01 to 1.00 mol, more preferably 0.01 to 1.00 mol in total of the metal salt, since the generation of active material is suppressed if added in excess. 0.03 to 0.50 mol.

Examples of iron compounds that can be used include iron acetate, iron nitrate, iron sulfate, and the like. These may be used alone or in combination of two or more. Among these, iron sulfate is preferred from the viewpoint of improving the battery characteristics of the lithium-based polyanion particles (B).

Manganese compounds that can be used include manganese acetate, manganese nitrate, manganese sulfate, and the like. These may be used alone or in combination of two or more. Among these, manganese sulfate is preferred from the viewpoint of improving the battery characteristics of the lithium-based polyanion particles (B).

It is preferable that the particles (B) contain both iron and manganese. That is, in the above formula (iii), it is preferable that 0<m≦1 and 0<n≦1. Lithium manganese iron phosphate (LMFP), which contains both iron and manganese, exhibits safety equivalent to lithium iron phosphate (LFP), which contains iron and does not contain manganese, but is 0.4 to 0.5 V lower than LFP. Since it has a high operating potential, it is thought to contribute to improving battery safety and increasing energy density. Therefore, it is preferable to use both an iron compound and a manganese compound as the metal compound when producing the particles (B). When both an iron compound and a manganese compound are used, the molar ratio of the manganese compound and the iron compound (manganese compound: iron compound) is preferably 99:1 to 1:99, more preferably 90:10 to 10: It is 90.

As the metal (M ³ ) salt, sulfates of metals other than iron and manganese, halogen compounds, organic acid salts, hydrates thereof, etc. can be used. These may be used alone or in combination of two or more. Among these, sulfates are preferred from the viewpoint of improving battery characteristics.

Next, the mixed solution (A) obtained in step (i) is subjected to a hydrothermal reaction to obtain composite B (step (ii)). The amount of water used in the hydrothermal reaction is determined based on the solubility of the metal salt, ease of stirring, synthesis efficiency, etc., per mole of phosphate ion contained in mixture A. , preferably 10 to 50 mol, more preferably 10 to 45 mol, even more preferably 10 to 30 mol, and still more preferably 12.5 to 25 mol.

The temperature during the hydrothermal reaction may be 100°C or higher, preferably 130 to 180°C. The hydrothermal reaction is preferably carried out in a pressure-resistant container, and when the reaction is carried out at 130 to 180°C, the pressure is preferably 0.3 to 0.9 MPa, and when the reaction is carried out at 140 to 160°C, the pressure is 0. .3 to 0.6 MPa is preferred. The reaction time is preferably 0.1 to 48 hours, more preferably 0.2 to 24 hours.

The obtained composite B is a lithium-based polyanion particle (B) represented by formula (III). Polyanion particles (B) can be isolated by filtering complex B, washing with water, and drying. Examples of drying means include freeze drying, vacuum drying, and the like.

When carbon is supported on lithium-based polyanion particles, a carbon source is mixed with a mixture containing composite B and water to create mixed liquid C (step (iii)). Examples of the carbon source include cellulose nanofibers and water-soluble carbon materials. Cellulose nanofibers (hereinafter also referred to as "CNF") are fine cellulose fibers that can be obtained by defibrating plant fibers to nano-sized fibers with a fiber diameter of about 1 to 100 nm. The water-soluble carbon material means a carbon material that dissolves in 100 g of water at 25° C. in terms of carbon atom equivalent to 0.4 g or more, preferably 1.0 g or more. Examples of the water-soluble carbon material include one or more selected from saccharides, polyols, polyethers, and organic acids. Specifically, for example, monosaccharides such as glucose, fructose, galactose, and mannose; disaccharides such as maltose, sucrose, and cellobiose; polysaccharides such as starch and dextrin; ethylene glycol, propylene glycol, diethylene glycol, polyethylene glycol, butanediol , propanediol, polyvinyl alcohol, glycerin, and other polyols and polyethers; citric acid, tartaric acid, ascorbic acid, and other organic acids. Among these, glucose, fructose, sucrose, and dextrin are preferred, and glucose is more preferred, in view of their high solubility in solvents.

The content of the complex B in the liquid mixture C is preferably 10 to 400 parts by mass, more preferably 30 to 210 parts by mass, based on 100 parts by mass of water. The content of the carbon source in the mixed liquid C is preferably 0.03 to 320 parts by mass, more preferably 0.2 to 140 parts by mass, and even more preferably 0.2 to 140 parts by mass, based on 100 parts by mass of water. The amount is 64 parts by mass, more preferably 0.2 to 28 parts by mass.

From the viewpoint of uniformly dispersing the composite B and the carbon source, the mixed liquid C is preferably treated using a disperser. Examples of such dispersing machines include disintegrators, beating machines, low-pressure homogenizers, high-pressure homogenizers, grinders, cutter mills, ball mills, jet mills, short-screw extruders, twin-screw extruders, ultrasonic stirrers, household juicer mixers, etc. can be mentioned. Among these, an ultrasonic stirrer is preferred from the viewpoint of dispersion efficiency. The degree of dispersion uniformity of the mixed liquid C can be quantitatively evaluated, for example, by the light transmittance using a UV/visible light spectrometer or the viscosity, and the uniformity of white turbidity can be confirmed by visual inspection. It is also possible to perform a simple evaluation by doing this. The time for processing with a disperser is preferably 0.5 to 6 minutes, more preferably 2 to 5 minutes.

When CNF is used as a carbon source, it is preferable to wet-classify the mixed liquid C from the viewpoint of effectively removing CNF in agglomerated state. For wet classification, a sieve or a commercially available wet classifier can be used. The opening of the sieve may vary depending on the fiber length of the CNF used, but from the viewpoint of work efficiency, it is preferably 140 to 160 μm.

The mixed liquid C obtained in step (iii) is dried to obtain granules D (step (iv)). For drying, methods such as spray drying and medium fluidized drying can be used. By adjusting the drying conditions, the median diameter of the granules D can be adjusted.

Next, the granules D obtained in step (iv) are fired in a reducing atmosphere or an inert atmosphere (step (v)). Thereby, the carbon source present in the granules D can be carbonized, and lithium-based polyanion particles (B) having carbon supported on their surfaces can be obtained. The firing conditions are preferably 400°C or higher, more preferably 400 to 800°C, preferably 10 minutes to 3 hours, and more preferably 0.5 to 1 hour in a reducing atmosphere or inert atmosphere. .

The median diameter of the secondary particles of particles (B) is larger than the median diameter of the secondary particles of particles (A). Preferably, the median diameter of the secondary particles of particles (A) is less than 0.80 times the median diameter of the secondary particles of particles (B). Since the median diameter of the secondary particles of particles (B) is larger than the median diameter of the secondary particles of particles (A), the gaps in particles (B) can be more efficiently filled by particles (A), and the Elution of metal elements such as manganese and iron from the particles (B) can be efficiently prevented, and the high-temperature storage characteristics of the battery can be improved.

In addition, when referring to the median diameter of the secondary particles of the particles (B), when carbon is not supported, it refers to the median diameter of the secondary particles of the particles (B) that do not support carbon; If so, it refers to the median diameter of the secondary particles of particles (B) including supported carbon.

Although the median diameter of the secondary particles of particles (B) is not limited to this, for example, the amount of air sent to the nozzle during drying such as spray drying during granulation may be adjusted, or the temperature may be optimized. It can be adjusted (increased) by

The powder pH of particles (B) is preferably lower than that of particles (A). By increasing the powder pH of the particles (A), it is possible to efficiently prevent the elution of metal elements such as manganese and iron from the particles (B) at high temperatures, further improving the high-temperature storage characteristics of the battery. Can be done.

<Cathode material>
The obtained particles (A) and particles (B) are mixed at a predetermined ratio to obtain a material for a positive electrode of a lithium ion secondary battery. The mixing method is not particularly limited, and any device that can uniformly mix particles (A) and particles (B) may be used. Examples include ball mills, sand mills, planetary mixers, high-speed shear mills, blade-type kneaders, high-speed mixers, and the like. The mixing ratio (mass ratio) of particles (A) and particles (B) is preferably 96:4 to 60:40, more preferably 96:4 to 70: particles (A):particles (B). 30, more preferably 96:4 to 80:20.

<Positive electrode>
A positive electrode for a lithium ion secondary battery can be manufactured using the obtained positive electrode material. The positive electrode is made by adding a solvent such as N-methyl-2-pyrrolidone to the above-mentioned positive electrode material, a conductive agent such as carbon black or carbon nanotubes, and a binder such as polyvinylidene fluoride, and thoroughly kneading the mixture. After obtaining a positive electrode slurry, it can be applied on a current collector such as aluminum foil, compressed with a roller press or the like, and dried.

The material constituting the current collector is not particularly limited, but metal is preferable, and examples thereof include aluminum, nickel, iron, stainless steel, titanium, copper, and other alloys. In addition, a cladding material of nickel and aluminum, a cladding material of copper and aluminum, or a plating material of a combination of these metals can be preferably used. Alternatively, it may be a foil whose metal surface is coated with aluminum. From the viewpoint of conductivity and working potential, aluminum, stainless steel, and copper are preferable.

The size of the current collector is not particularly limited, and may be determined depending on the intended use. The thickness of the current collector is also not particularly limited. Usually, it is about 1 to 100 μm.

The binder is not particularly limited. For example, in addition to polyvinylidene fluoride, polyethylene, polypropylene, polyethylene terephthalate (PET), polyethernitrile, polyacrylonitrile, polyimide, polyamide, cellulose, carboxymethylcellulose (CMC) and its salts, ethylene-vinyl acetate copolymer, polychloride Vinyl, styrene-butadiene rubber (SBR), isoprene rubber, butadiene rubber, ethylene-propylene rubber, ethylene-propylene-diene copolymer, styrene-butadiene-styrene block copolymer and its hydrogenated products, styrene-isoprene-styrene Thermoplastic polymers such as block copolymers and their hydrogenated products, polytetrafluoroethylene (PTFE), tetrafluoroethylene/hexafluoropropylene copolymer (FEP), tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer ( Fluororesins such as PFA), ethylene/tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene/chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF), vinylidene fluoride Ride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-TFE-based fluororubber), vinylidene fluoride-pentafluoropropylene-based Fluororubber (VDF-PFP fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene fluororubber (VDF-PFP-TFE fluororubber), vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene fluororubber Examples include vinylidene fluoride-based fluororubbers such as rubber (VDF-PFMVE-TFE-based fluororubber), vinylidene fluoride-chlorotrifluoroethylene-based fluororubber (VDF-CTFE-based fluororubber), and epoxy resins. Among these, polyvinylidene fluoride is most preferred. These binders may be used alone or in combination of two or more.

The solvent used to prepare the positive electrode slurry is not particularly limited, but examples include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, methylformamide, cyclohexane, hexane, and the like.

The composition of the positive electrode slurry is not particularly limited, but for example, the binder is 0.1 to 15% by mass, preferably 1 to 10% by mass, more preferably 2% by mass, based on the total solid content (components other than the solvent) of the slurry. -8% by mass, particularly preferably 3-8% by mass, and the positive electrode material is preferably 70-99.5% by mass, preferably 75-99% by mass, from the viewpoint of obtaining sufficient battery capacity. The range is preferably from 80 to 98% by weight, particularly preferably from 90 to 97.5% by weight. It is considered that sufficient binding strength can be developed within the above range. The content of the conductive aid is preferably in the range of 0 to 10% by mass, more preferably in the range of 3 to 7% by mass.

<Lithium ion secondary battery>
The lithium ion secondary battery of the present invention includes at least the above-described positive electrode, negative electrode, electrolyte, and separator. The material composition of the negative electrode is not particularly limited as long as it occludes lithium ions during charging and releases lithium ions during discharging. Examples include lithium metal, graphite, carbon materials such as amorphous carbon, and the like. It is preferable to use a carbon material that is an intercalating material that can electrochemically absorb and release lithium ions.

The electrolytic solution is one in which a supporting salt is dissolved in an organic solvent. The organic solvent is not particularly limited as long as it is an organic solvent commonly used in electrolytes of lithium ion secondary batteries. For example, carbonates, halogenated hydrocarbons, ethers, ketones, nitriles, lactones, oxolane compounds, etc. can be used.

The type of supporting salt is not particularly limited, but includes inorganic salts selected from LiPF ₆ , LiBF ₄ , LiClO ₄ , and LiAsF ₆ , derivatives of the inorganic salts, LiSO ₃ CF ₃ , LiC(SO ₃ CF ₃ ) ₂ and LiN. (SO ₃ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ and LiN (SO ₂ CF ₃ ) (SO ₂ C ₄ F ₉ ), and at least one of the organic salts. It is preferable. Among these, salts containing elemental fluorine are preferable, and LiPF ₆ is particularly preferable, since the effect of enhancing high temperature stability by the positive electrode of the present invention is noticeable.

The separator serves to electrically insulate the positive electrode and the negative electrode and to retain the electrolyte. Examples include porous synthetic resin membranes, particularly porous membranes of polyolefin polymers (polyethylene, polypropyropyrene).

The shape of the lithium ion secondary battery having the above configuration is not particularly limited, and may be various shapes such as a coin shape, a cylindrical shape, a square shape, or an irregular shape enclosed in a laminate exterior body.

Hereinafter, the present invention will be specifically explained based on Examples, but the present invention is not limited to these Examples.
<Lithium composite oxide particles (A)>
As particles (A), commercially available particles (LiNi _0.5 Mn _0.3 Co _0.2 O ₂ , secondary particle median diameter (D50): 10.5 μm, powder pH: pH 10.9) were used. .

<Lithium-based polyanion particles (B1)>
As particles (B1), commercially available particles (LiMn _0.7 Fe _0.3 PO ₄ containing 1.5% by mass of carbon, median diameter of secondary particles (D50): 14.8 μm, powder pH: pH 10 .4) was used.

<Lithium-based polyanion particles (B2)>
As particles (B2), commercially available particles (LiMn _0.7 Fe _0.3 PO ₄ containing 1.5% by mass of carbon, median diameter of secondary particles (D50): 8.0 μm, powder pH: pH 10 .4) was used.

<Example 1>
The above particles (A) and particles (B1) were mixed at a mass ratio of 80:20 to obtain the positive electrode material of Example 1.

<Comparative example 1>
The above particles (A) and particles (B2) were mixed at a mass ratio of 80:20 to obtain a positive electrode material of Comparative Example 1.

<Comparative example 2>
A positive electrode material for Comparative Example 2 was prepared using only the particles (B2).
<Manufacture of lithium ion secondary batteries>
A layer containing 97.5% by mass of each of the positive electrode materials of Example 1 and Comparative Examples 1 and 2, 0.5% by mass of carbon nanotubes, and 2% by mass of polyvinylidene fluoride was placed on a 16 μm thick aluminum foil. Comparative Example 1 was coated on both sides with a coating amount of 18 mg/cm ² on one side, and Comparative Example 2 was coated on both sides with a coating amount of 21 mg/cm ² on one side, pressed and vacuum dried to form a double-sided coated positive electrode with a size of 30 mm x 40 mm. One piece of each was obtained.

A layer containing 95% by mass of graphite, 2.5% by mass of styrene-butadiene rubber, and 2.5% by mass of carboxymethyl cellulose was coated on both sides of 8 μm thick copper foil at a coating amount of 9.8 mg/cm ² on one side, After pressing and vacuum drying, two double-sided coated negative electrodes of 32 mm x 42 mm in size were obtained.

One of each of the obtained positive electrodes, two negative electrodes, and two polypropylene separators with a size of 35 mm x 45 mm with a thickness of 28 μm were laminated in the order of negative electrode - separator - positive electrode - separator - negative electrode, and then placed in an aluminum laminate exterior body. 0.6 mL of an ethylene carbonate+diethyl carbonate solution (3:7) containing 1M LiPF ₆ was injected into the cell, and the cell was sealed to obtain a lithium ion secondary battery.

<60℃ storage test>
(1) The capacity of each obtained lithium ion secondary battery at 25° C. and 13 mA was confirmed under the following conditions. Let the discharge capacity at this time be (1):
Charging 13mA-4.2V/CCCV (6.5h)
Charging pause 10 minutes Discharging 13mA-2.5V/CC
Discharge pause 10 minutes (2) Then, it was stored at 60° C. for 7 days at 100% SOC. The conditions are as follows:
25℃ charging 13mA-4.2V/CCCV (6.5h)
25℃ charging pause 10 minutes 60℃ storage for 7 days 25℃ discharge 13mA-2.5V/CC
Discharge pause at 25°C for 10 minutes (3) Then, in the same manner as in (1), the capacity at 25°C and 13 mA was confirmed under the following conditions. Let the discharge capacity at this time be (3):
Charging 13mA-4.2V/CCCV (6.5h)
Charging pause 10 minutes Discharging 13mA-2.5V/CC
Discharge pause: 10 minutes (4) The discharge capacity maintenance rate after holding at high temperature (60°C) was calculated using the following formula:
Discharge capacity maintenance rate (%) = (discharge capacity of (3)/discharge capacity of (1)) x 100.

The results of the discharge capacity retention rate of each lithium ion secondary battery are as follows:
Battery using the positive electrode material of Example 1: 95%
Battery using positive electrode material of Comparative Example 1: 85%
Battery using positive electrode material of Comparative Example 2: 80%
It can be seen that the discharge capacity of the lithium ion secondary battery using the positive electrode of the present invention is less likely to decrease than that of the comparative example even after being maintained at a high temperature for a certain period of time. With the positive electrode of the present invention, a lithium ion secondary battery with higher stability at high temperatures can be formed.

Claims (7)

The following formula (I) or (II):
LiNi _a Co _b Mn _c M ¹ _x O ₂ ...(I)
(In formula (I), M ¹ is Mg, Ti, Nb, Fe, Cr, Si, Al, Ga, V, Zn,
One or more elements selected from Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb, Bi, and Ge, and a, b, c, x are 0. 3≦a<1, 0<b≦0.7, 0<c≦0.7, 0≦x≦0.3, and is a number that satisfies 3a+3b+3c+(valence of M ¹ )×x=3)
LiNi _d Co _e Al _f M ² _y O ₂ ...(II)
(In formula (II), ^M2 is Mg, Ti, Nb, Fe, Cr, Si, Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb, Bi , and Ge, and d, e, f, and y are 0.4≦d<1, 0<e≦0.6, 0<f≦0.3 , 0≦y≦0.3, and 3d+3e+3f+(valence of ^M2 )×y=3)
Lithium composite oxide particles (A) represented by
The following formula (III):
LiFe _m M _n M ³ _o PO ₄ ...(III)
(In formula (III), ^M3 is Co, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, or Gd, and m, n, and o are 0≦ It is a number that satisfies m≦1, 0≦n≦1, 0≦o≦0.3, and m+n≠0, and 2m+2n+(valence of ^M3 )×o=2)
Lithium-based polyanion particles (B) represented by
The median diameter of the secondary particles of the particles (A) is smaller than the median diameter of the secondary particles of the particles (B).
A positive electrode for a lithium secondary battery , wherein the powder pH of the particles (A) is higher than the powder pH of the particles (B) . The positive electrode for a lithium ion secondary battery according to claim 1, wherein the median diameter of the secondary particles of the particles (A) is less than 0.80 times the median diameter of the secondary particles of the particles (B). The positive electrode for a lithium ion secondary battery according to claim 1 or 2 , wherein the particles (A) are a compound represented by the formula (I). The positive electrode for a lithium ion secondary battery according to claim 3 , wherein x in the formula (I) is 0. The lithium according to any one of claims 1 to 4 , wherein the particles (B) are a compound satisfying 0<m≦1, 0<n≦1, and o=0 in the formula (III). Positive electrode for ion secondary batteries. A lithium ion secondary battery comprising the positive electrode for a lithium ion secondary battery according to any one of claims 1 to 5 . The lithium ion secondary battery according to claim 6 , comprising a fluorine-containing lithium salt as an electrolyte.