JP7765232B2 - Positive electrode active material composite particles for lithium ion secondary batteries - Google Patents

Description

The present invention relates to positive electrode active material composite particles for lithium ion secondary batteries that improve the cycle characteristics of lithium ion secondary batteries.

Secondary batteries such as lithium ion secondary batteries are used in a wide range of fields, including mobile phones, digital cameras, notebook PCs, hybrid vehicles, electric vehicles, etc. As the positive electrode material for these lithium ion secondary batteries, particles having an olivine structure, such as so-called lithium manganese phosphate or lithium iron phosphate, such as LiMn _x Fe _1-x PO _4, are considered promising due to their high safety and large capacity, and various developments utilizing such particles are being carried out.

For example, Patent Document 1 discloses a positive electrode for a lithium ion secondary battery in which a positive electrode active material layer is arranged, the positive electrode active material layer being formed of a first layer containing an active material such as lithium manganese oxide and a second layer containing an active material such as _LiFePO4 , and attempts to improve the discharge rate characteristics.
Furthermore, Patent Document 2 discloses an electrode active material having a core formed from a lithium-containing transition metal oxide such as LiMn ₂ O ₄ and a shell containing lithium metal oxide particles such as lithium manganese iron phosphate and a polymer, thereby improving safety and stability.

JP 2018-147790 A Special table 2015-503196 publication

However, positive electrode active materials with a layered structure such as those in Patent Document 1 have not yet been able to sufficiently improve the cycle characteristics of the resulting lithium-ion secondary batteries. Furthermore, even positive electrode active materials with a core-shell structure such as those in Patent Document 2 still have room for improvement in terms of improving cycle characteristics.

Therefore, an object of the present invention is to provide positive electrode active material composite particles for lithium ion secondary batteries that can effectively improve the cycle characteristics of lithium ion secondary batteries.

Therefore, the present inventors have conducted extensive research to solve the above problems and have found that by forming composite particles from lithium manganese iron phosphate nanoparticles and nanoparticles represented by a specific formula, _such as _LiMn2O4 , in a specific mass ratio, it is possible to obtain positive electrode active material composite particles for lithium ion secondary batteries that can exhibit excellent cycle characteristics in lithium ion secondary batteries.

That is, the present invention provides a compound of the following formula (a):
Li _f Mn _g Fe _h M ¹ _x PO ₄ ...(a)
(In formula (a), ^M1 represents one or more elements selected from Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, and Gd. f, g, h, and x represent numbers that satisfy 0<f≦1.2, 0.3≦g≦1.2, 0.2≦h≦1.2, 0≦x≦0.3, and 7/13≦g/h≦17/3, and that satisfy f+(valence of Mn)×g+(valence of Fe)×h+(valence of ^M1 )×x=3.)
Nanoparticles A represented by the following formula (b):
LiM ² _a Mn _b O ₄ ...(b)
(In formula (b), ^M2 represents one or more elements selected from Ni, Co, Al, Mg, Ti, V, Cr, Fe, Zr, Ga, Cu, and Si. a and b represent numbers that satisfy 0≦a≦0.1, 0<b≦2, and (valence of ^M2 )×a+(valence of Mn)×b=7.)
and the average particle size is 100 nm to 200 nm.
A composite particle formed by
The present invention provides positive electrode active material composite particles for lithium ion secondary batteries, in which the mass ratio of nanoparticles A to nanoparticles B (A:B) is 3:97 to 45:55.

The positive electrode active material composite particles for lithium ion secondary batteries of the present invention make it possible to realize lithium ion secondary batteries with effectively improved cycle characteristics.

The present invention will be described in detail below.
The positive electrode active material composite particle for a lithium ion secondary battery of the present invention has the following formula (a):
Li _f Mn _g Fe _h M ¹ _x PO ₄ ...(a)
(In formula (a), ^M1 represents one or more elements selected from Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, and Gd. f, g, h, and x represent numbers that satisfy 0<f≦1.2, 0.3≦g≦1.2, 0.2≦h≦1.2, 0≦x≦0.3, and 7/13≦g/h≦17/3, and that satisfy f+(valence of Mn)×g+(valence of Fe)×h+(valence of ^M1 )×x=3.)
Nanoparticles A represented by the following formula (b):
LiM ² _a Mn _b O ₄ ...(b)
(In formula (b), ^M2 represents one or more elements selected from Ni, Co, Al, Mg, Ti, V, Cr, Fe, Zr, Ga, Cu, and Si. a and b represent numbers that satisfy 0≦a≦0.1, 0<b≦2, and (valence of ^M2 )×a+(valence of Mn)×b=7.)
and the average particle size is 100 nm to 200 nm.
A composite particle formed by
The mass ratio (A:B) of nanoparticles A to nanoparticles B is 3:97 to 45:55.

In this way, the positive electrode active material composite particles for lithium ion secondary batteries of the present invention are composite particles formed by maintaining a limited mass ratio between nanoparticles A and B, which are extremely fine particles both represented by a specific formula. This allows nanoparticles A and B to aggregate while tightly entangled, preventing the transition metal component of particle B from leaching into the electrolyte, and effectively improving the cycle characteristics of the resulting lithium ion secondary battery.

The nanoparticles A forming the positive electrode active material composite particles for a lithium ion secondary battery of the present invention are represented by the following formula (a):
Li _f Mn _g Fe _h M ¹ _x PO ₄ ...(a)
(In formula (a), ^M1 represents one or more elements selected from Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, and Gd. f, g, h, and x represent numbers that satisfy 0<f≦1.2, 0.3≦g≦1.2, 0.2≦h≦1.2, 0≦x≦0.3, and 7/13≦g/h≦17/3, and that satisfy f+(valence of Mn)×g+(valence of Fe)×h+(valence of ^M1 )×x=3.)
and the average particle size is 50 nm to 200 nm.

Nanoparticles A represented by the above formula (a) are an olivine-type lithium transition metal phosphate compound containing at least both manganese (Mn) and iron (Fe) as transition metals, and are extremely fine nanoscale particles. The positive electrode active material composite particles for lithium ion secondary batteries of the present invention are formed by closely entangling and combining such nanoparticles A with nanoparticles B (described below) while maintaining a specific mass ratio, thereby effectively improving cycle characteristics.

From the viewpoint of average discharge voltage, the value of f in the nanoparticles A is preferably 0.6≦f≦1.2, more preferably 0.65≦f≦1.15, and even more preferably 0.7≦f≦1.1. The value of g is preferably 0.4≦g≦0.8, more preferably 0.5≦g≦0.8, and even more preferably 0.7≦g≦0.8. The value of h is preferably 0.2≦h≦0.6, more preferably 0.2≦h≦0.5, and even more preferably 0.2≦h≦0.3. The value of x is preferably 0≦x≦0.2, more preferably 0≦x≦0.15, and even more preferably 0≦x≦0.1. Furthermore, g/h is the molar ratio of Mn to Fe constituting the so-called nanoparticles A, and is preferably 2/3≦g/h≦4, more preferably 9/11≦g/h≦3, and even more preferably 1≦g/h≦7/3.

_{Specific examples include LiMn0.4Fe0.6PO4 , LiMn0.7Fe0.3PO4 , LiMn0.8Fe0.2PO4 , LiMn0.75Fe0.15Mg0.1PO4 , LiMn0.75Fe0.19Zr0.03PO4 , LiMn0.6Fe0.4PO4 , LiMn0.5Fe0.5PO4 , Li1.2Mn0.63Fe0.27PO4 , and Li0.6Mn0.84Fe0.36PO4 . ​ ​ ​ ​ ​ ​ ​ ​ ​ Of these , LiMn0.6Fe0.4PO4 , LiMn0.4Fe0.6PO4 , LiMn0.8Fe0.2PO4 , LiMn0.7Fe0.3PO4 , Li1.2Mn0.63Fe0.27PO4 , or Li0.6Mn0.84Fe0.36PO4 is preferred . ​ ​}

The average particle size of the nanoparticles A represented by the above formula (a) is 50 nm to 200 nm, preferably 60 nm to 190 nm, more preferably 70 nm to 180 nm, even more preferably 80 nm to 170 nm, and still more preferably 90 nm to 160 nm, from the viewpoint of effectively promoting the insertion and desorption of lithium ions and from the viewpoint of handling.
Here, the "average particle size" of nanoparticles A means the average particle size of 100 particles observed under a SEM (JSM-7001F, manufactured by JEOL Ltd.).

Nanoparticles A may be particles having cellulose nanofiber-derived carbon and/or water-soluble carbon material-derived carbon supported on their surfaces, in order to ensure excellent discharge capacity and further improve cycle characteristics.
Cellulose nanofibers are the skeletal component that accounts for approximately 50% of all plant cell walls. They are lightweight, high-strength fibers that can be obtained by defibrating the plant fibers that make up these cell walls to nano-size. The fiber diameter of these cellulose nanofibers is 1 nm to 1,000 nm, and they also have good dispersibility in water. Furthermore, the cellulose molecular chains that make up the cellulose nanofibers form a periodic structure of carbon. Therefore, when these cellulose nanofibers are carbonized to form carbon, which is firmly supported on the surface of the nanoparticles A, they form dense composite particles together with the ultrafine nanoparticles B, effectively suppressing the deterioration of the electronic conductive path and effectively increasing the degree of compaction, ensuring the resulting battery exhibits excellent cycle characteristics.

When carbon derived from cellulose nanofibers is supported on the surface of nanoparticles A, the atomic equivalent amount of carbon derived from the carbonized cellulose nanofibers, i.e., the amount of carbon derived from the cellulose nanofibers, is preferably more than 0% by mass and not more than 5.0% by mass, more preferably 0.1% by mass to 4.0% by mass, and even more preferably 0.2% by mass to 3.0% by mass, based on 100% by mass of the total amount of nanoparticles A including the amount of carbon supported.

Like cellulose nanofibers, water-soluble carbon materials are carbonized to form carbon, and when this is supported on the surface of nanoparticles A, like cellulose nanofibers, it is possible to effectively suppress the deterioration of the electronic conductive path and effectively increase the degree of compaction, thereby ensuring that the resulting battery exhibits excellent cycle characteristics.
Examples of such water-soluble carbonaceous materials include one or more selected from sugars, polyols, polyethers, and organic acids. More specific examples include monosaccharides such as glucose, fructose, galactose, and mannose; disaccharides such as maltose, sucrose, and cellobiose; polysaccharides such as starch and dextrin; polyols and polyethers such as ethylene glycol, propylene glycol, diethylene glycol, polyethylene glycol, butanediol, propanediol, polyvinyl alcohol, and glycerin; and organic acids such as citric acid, tartaric acid, and ascorbic acid. Among these, from the viewpoint of increasing solubility and dispersibility in a solvent and effectively functioning as a carbonaceous material, glucose, fructose, sucrose, and dextrin are preferred, and glucose is more preferred.

When carbon derived from a water-soluble carbon material is supported on the surface of nanoparticles A, the atomic equivalent amount of carbon derived from the water-soluble carbon material, i.e., the amount of carbon derived from the water-soluble carbon material supported, is preferably more than 0% by mass and not more than 4.0% by mass, more preferably more than 0% by mass and not more than 3.0% by mass, and even more preferably more than 0% by mass and not more than 2.0% by mass, based on 100% by mass of the total amount of nanoparticles A including the amount of carbon supported.

The cellulose nanofiber-derived carbon and the water-soluble carbon material-derived carbon may be supported in such a manner that only the cellulose nanofiber-derived carbon, only the water-soluble carbon material-derived carbon, or both the cellulose nanofiber-derived carbon and the water-soluble carbon material-derived carbon are supported. Of these, from the viewpoint of efficiently covering the surface of the nanoparticles A, it is preferable to support the cellulose nanofiber-derived carbon.

When the surface of nanoparticle A is supported with carbon derived from cellulose nanofibers and carbon derived from a water-soluble carbon material, the sum of the atomic equivalent amounts of carbon derived from cellulose nanofibers and carbon derived from the water-soluble carbon material, i.e., the total amount of carbon derived from cellulose nanofibers and carbon derived from the water-soluble carbon material, is preferably 0.1% to 5.0% by mass, more preferably 0.1% to 4.0% by mass, even more preferably 0.2% to 3.0% by mass, and even more preferably 0.5% to 2.0% by mass, based on 100% by mass of the total amount of nanoparticle A including the amount of supported carbon.

The atomic equivalent amount (supported amount) of carbon derived from cellulose nanofibers present in nanoparticles A and the atomic equivalent amount (supported amount) of carbon derived from the water-soluble carbon material are values determined by measurement using a carbon/sulfur analyzer.
Furthermore, when nanoparticles A carry carbon derived from cellulose nanofibers and/or carbon derived from a water-soluble carbon material on their surfaces, the amount of nanoparticles A includes the amount of these carbons carried.

The nanoparticles A can be obtained, for example, by the following production method. Specifically, the nanoparticles A include the following steps (Ia) to (IIa):
(Ia) adding a metal compound including a lithium compound, a manganese compound, and an iron compound, a phosphate compound, and optionally cellulose nanofibers and/or a water-soluble carbon material, and water to obtain slurry water i, and then subjecting the resulting slurry water to a hydrothermal reaction to obtain slurry water ii; and (IIa) filtering the obtained slurry water ii, washing it with water, and drying it to obtain nanoparticles A.

The above-mentioned step (Ia) is a step in which a metal compound including a lithium compound, a manganese compound, and an iron compound, a phosphate compound, and optionally cellulose nanofibers and/or a water-soluble carbon material, and water are added to obtain slurry water I, and then the obtained slurry water II is obtained by subjecting the mixture to a hydrothermal reaction.
Examples of the lithium compound to be used include hydroxides (for example, LiOH.H ₂ O, LiOH), carbonates, acetates, and nitrates, with hydroxides being preferred.
Examples of manganese compounds include manganese acetate, manganese nitrate, and manganese oxide. These compounds may be used alone or in combination of two or more. Among these, manganese oxide is preferred from the viewpoint of improving battery characteristics.
In addition to these lithium compounds and manganese compounds, metal (M ¹ ) compounds other than manganese compounds and iron compounds may also be used as the metal compounds.

Examples of phosphoric acid compounds include orthophosphoric acid (H ₃ PO ₄ , phosphoric acid), metaphosphoric acid, pyrophosphoric acid, triphosphoric acid, tetraphosphoric acid, ammonium phosphate, ammonium hydrogen phosphate, etc. Among these, phosphoric acid is preferably used, and is preferably used as an aqueous solution with a concentration of 70% by mass to 90% by mass.
When carbon derived from cellulose nanofibers and/or carbon derived from a water-soluble carbon material is to be supported on the surface of nanoparticles A, the above-mentioned cellulose nanofibers and/or water-soluble carbon material may be further used.

The slurry water i can be prepared according to a conventional method by determining the amounts of lithium compounds, metal compounds including manganese compounds and iron compounds, and phosphate compounds, etc., depending on the composition of the desired nanoparticles A.

The resulting slurry water i is then subjected to a hydrothermal reaction to obtain slurry water ii.
The amount of water used in the hydrothermal reaction is preferably 10 mol to 50 mol, more preferably 12.5 mol to 45 mol, per mol of phosphate ions contained in the slurry water i, from the viewpoints of the solubility of the metal compound, ease of stirring, synthesis efficiency, and the like.

The hydrothermal reaction should be carried out at a temperature of 100°C or higher, preferably 130°C to 200°C. The hydrothermal reaction is preferably carried out in a pressure-resistant vessel. When the reaction is carried out at 130°C to 200°C, the pressure is preferably 0.3 MPa to 1.6 MPa, and when the reaction is carried out at 140°C to 160°C, the pressure is preferably 0.3 MPa to 0.6 MPa. The hydrothermal reaction time is preferably 0.1 to 48 hours, more preferably 0.2 to 24 hours.

The above-mentioned step (IIa) is a step in which the obtained slurry water II is filtered, washed with water, and dried to obtain nanoparticles A. Freeze drying and vacuum drying are used as drying methods.

The nanoparticles B forming the positive electrode active material composite particles for a lithium ion secondary battery of the present invention are represented by the following formula (b):
LiM ² _a Mn _b O ₄ ...(b)
(In formula (b), ^M2 represents one or more elements selected from Ni, Co, Al, Mg, Ti, V, Cr, Fe, Zr, Ga, Cu, and Si. a and b represent numbers that satisfy 0≦a≦0.1, 0<b≦2, and (valence of ^M2 )×a+(valence of Mn)×b=7.)
and the average particle size is 100 nm to 200 nm.

Nanoparticles B represented by the above formula (b) are particles having a spinel structure. By forming the composite particles of the present invention together with the above nanoparticles A, while maintaining a specific mass ratio, the cycle characteristics can be dramatically improved.

Specific examples of nanoparticles _B represented by formula (b _{) that can be used include LiMn2O4, LiNi0.5Mn1.5O4 , LiCoMnO4 , LiCrMnO4 , LiFeMnO4 ,} LiAlMnO4 _, and _{LiCu0.5Mn1.5O4 .} Of these, _LiMn2O4 is preferred.

From the viewpoint of ensuring excellent cycle characteristics and ease of handling, the average particle size of the nanoparticles B is 100 nm to 200 nm, preferably 110 nm to 190 nm, more preferably 120 nm to 180 nm, even more preferably 130 nm to 170 nm, and still more preferably 135 nm to 165 nm.
Here, the "average particle size" of nanoparticles B means the average particle size of 100 particles observed under a SEM (JSM-7001F, manufactured by JEOL Ltd.).

The nanoparticles B can be obtained, for example, by the following production method. Specifically, the nanoparticles B include the following steps (Ib) to (IIb):
(Ib) a step of mixing a lithium compound and a manganese compound and firing the mixture at a temperature of 250°C to 500°C to obtain preliminary particles b; and (IIb) a step of firing the obtained preliminary particles b at a temperature of 500°C to 750°C in a reducing atmosphere or an inert atmosphere to obtain nanoparticles B.

The above (Ib) is a process in which a lithium compound and a manganese compound are mixed and fired at a temperature of 250°C to 500°C to obtain preliminary particles b.

As the lithium compound, the same compounds as those used for the nanoparticles A can be used, but hydroxides and nitrates are preferred.
As the manganese compound, the same one as used for the nanoparticles A can be used.
In addition to these lithium compounds and manganese compounds, metal (M ² ) compounds other than these compounds may also be used.

The amounts of these lithium compounds, manganese compounds, etc. used can be determined appropriately depending on the desired composition of nanoparticles B, and can be mixed according to conventional methods.

Next, preliminary particles b are obtained by firing at a temperature between 250°C and 500°C. In this way, by undergoing a two-stage firing process in which firing is performed at a temperature lower than the firing temperature in step (IIb) and then at a higher temperature in the subsequent step (IIb), it is possible to obtain extremely fine nanoparticles B with a high degree of crystallinity and the desired average particle size.

The firing temperature is 250°C to 500°C, preferably 260°C to 490°C, and more preferably 270°C to 480°C. The firing time is preferably 0.2 hours to 24 hours, more preferably 0.5 hours to 15 hours, and even more preferably 1 hour to 12 hours. The firing atmosphere is preferably air.

The above (IIb) is a step of calcining the preliminary particles b obtained in step (Ib) at a temperature of 500° C. to 750° C. in a reducing atmosphere or an inert atmosphere to obtain nanoparticles B.
The firing temperature is 500° C. to 750° C., preferably 520° C. to 730° C., and more preferably 540° C. to 710° C. The firing time is preferably 12 hours to 36 hours, more preferably 14 hours to 30 hours, and even more preferably 16 hours to 24 hours.

The positive electrode active material composite particles for a lithium ion secondary battery of the present invention are composite particles formed from nanoparticles A and nanoparticles B.
In the positive electrode active material composite particles for a lithium ion secondary battery of the present invention, the mass ratio (A:B) of nanoparticles A to nanoparticles B is 3:97 to 45:55, preferably 5:95 to 44:56, more preferably 7:93 to 42:58, and even more preferably 9:91 to 40:60, from the viewpoint of complexing the nanoparticles A and B while densely entangled, and effectively enhancing cycle characteristics.

The positive electrode active material composite particles for lithium ion secondary batteries of the present invention may contain, in addition to nanoparticles A and nanoparticles B, other particles such as lithium manganese phosphate or lithium iron phosphate, or may be composite particles formed from nanoparticles A, nanoparticles B, and other particles.

Specific examples of such lithium manganese phosphate or _lithium iron phosphate include _{LiMnPO4 , LiMn0.9Mg0.1PO4 , LiMn0.97Zr0.03PO4} , _{Li1.2Mn0.9PO4 , Li0.6Mn1.2PO4 , LiFePO4 , LiFe0.9Mg0.1PO4 , LiFe0.97Zr0.03PO4 , Li1.2Fe0.9PO4 , Li0.6Fe1.2PO4 , etc. Of these , LiMnPO4} and _{LiFePO4 are preferred} .

The total amount of nanoparticles A and nanoparticles B in 100% by mass of the positive electrode active material composite particles for a lithium ion secondary battery of the present invention is preferably 95% by mass to 100% by mass, more preferably 96% by mass to 100% by mass, and even more preferably 97% by mass to 100% by mass.
The other particles may be contained as the remainder.

The positive electrode active material composite particles for a lithium ion secondary battery of the present invention have an average particle size of preferably 8 μm to 50 μm, more preferably 9 μm to 40 μm, and even more preferably 9.5 μm to 30 μm, from the viewpoint of effectively improving cycle characteristics by being composite particles in which nanoparticles A and nanoparticles B are densely combined.
Here, the "average particle size" of the positive electrode active material composite particles for a lithium ion secondary battery of the present invention means the _D50 value (particle size at 50% of the cumulative diameter (median diameter)) obtained from the volume-based particle size distribution based on the laser diffraction/scattering method.

The positive electrode active material composite particles for lithium ion secondary batteries of the present invention are a fired product of granules containing the above-mentioned nanoparticles A and nanoparticles B in a mass ratio (A:B) of nanoparticles A to nanoparticles B of 3:97 to 45:55. Such positive electrode active material composite particles for lithium ion secondary batteries can be obtained, for example, by the following production method. Specifically, the following steps (Ix) to (IIx) are performed:
(Ix) adding nanoparticles A, nanoparticles B, and optionally the other particles, and water to obtain a slurry water i', which is then spray-dried to obtain granules X; and (IIx) firing the obtained granules X in a reducing atmosphere or an inert atmosphere to obtain composite particles.

The above-mentioned (Ix) is a step in which nanoparticles A, nanoparticles B, and optionally the other particles, and water are added to obtain a slurry water i', which is then spray-dried to obtain granules X. The amounts of the nanoparticles A and nanoparticles B may be appropriately adjusted so as to satisfy the above-mentioned mass ratio (A:B).
The solid content concentration of the slurry water i' is preferably 5% by mass to 30% by mass, and more preferably 5% by mass to 25% by mass.

In spray drying, the operating conditions may be set appropriately depending on the apparatus used.
For example, as treatment conditions for a micromist dryer (MDL-050M manufactured by Fujisaki Electric Co., Ltd.) equipped with a four-fluid nozzle, the hot air temperature is preferably 110° C. to 300° C., more preferably 150° C. to 250° C. The volume ratio of the supply amount of hot air to the supply amount of slurry water (supply amount of hot air/supply amount of slurry water) is preferably 500 to 10,000, more preferably 1,000 to 9,000.

The step (IIx) is a step of firing the granules X obtained in the step (Ix) in a reducing atmosphere or an inert atmosphere to obtain the composite particles of the present invention.
The firing temperature is preferably 500° C. to 750° C., more preferably 550° C. to 700° C., and the firing time is preferably 0.3 to 3 hours, more preferably 0.5 to 2 hours.

The positive electrode active material composite particles for lithium ion secondary batteries of the present invention can be used as a positive electrode material to construct a lithium ion secondary battery having, as its essential components, a positive electrode, a negative electrode, an electrolyte, and a separator, or a positive electrode, a negative electrode, and a solid electrolyte. Specifically, for example, the positive electrode active material composite particles for lithium ion secondary batteries of the present invention are kneaded with acetylene black, ketjen black, polyvinylidene fluoride, N-methyl-2-pyrrolidone, or the like to prepare a positive electrode slurry, which is then coated on a current collector and press-molded to form a positive electrode.
The positive electrode active material composite particles for a lithium ion secondary battery of the present invention are formed from the above-mentioned ultrafine nanoparticles A and ultrafine nanoparticles B, and these nanoparticles A and nanoparticles B are densely combined while maintaining a limited mass ratio, so that it is possible to effectively increase the electrode density and obtain a highly useful positive electrode that can effectively improve the cycle characteristics.

The negative electrode is not particularly limited in terms of its material composition, and any known material composition can be used, as long as it can absorb lithium ions during charging and release them during discharging. For example, lithium metal, graphite, silicon-based materials (Si, SiOx), lithium titanate, or carbon materials such as amorphous carbon can be used. It is preferable to use an electrode made of an intercalating material that can electrochemically absorb and release lithium ions, particularly a carbon material. Furthermore, two or more of the above negative electrode materials may be used in combination; for example, a combination of graphite and silicon-based materials can be used.

The electrolyte solution is a supporting salt dissolved in an organic solvent. There are no particular limitations on the organic solvent, as long as it is an organic solvent typically used in electrolytes for lithium-ion secondary batteries. Examples of organic solvents that can be used include carbonates, halogenated hydrocarbons, ethers, ketones, nitriles, lactones, and oxolane compounds.

The supporting salt is not particularly limited in type, but is preferably at least one of inorganic salts selected from _LiPF6 , _LiBF4 , _{LiClO4 , and LiAsF6, derivatives of these inorganic salts, organic salts selected from LiSO3CF3, LiC(SO3CF3)2 , LiN ( SO3CF3 ) 2 , LiN (SO2C2F5)2 , and LiN} ( _{SO2CF3 )} ( _{SO2C4F9 )} , and derivatives of _these organic salts.

The separator serves to electrically insulate the positive and negative electrodes and retain the electrolyte. For example, a porous synthetic resin membrane, particularly a porous membrane made of a polyolefin polymer (polyethylene, polypropylene), may be used.

The solid electrolyte electrically insulates the positive electrode and the negative electrode and exhibits high lithium ion conductivity. For example, La _0.51 Li _0.34 TiO _2.94 , Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ , Li ₇ La ₃ Zr ₂ O ₁₂ , 50Li ₄ SiO ₄・50Li ₃ BO ₃ , Li _2.9 PO _3.3 N _0.46 , Li _3.6 Si _0.6 P _0.4 O ₄ , Li _1.07 Al _0.69 Ti _1.46 (PO ₄ ) ₃ , Li _1.5 Al _0.5 Ge _1.5 (PO ₄ ) ₃ , Li ₁₀ GeP ₂ S ₁₂ , _{Li3.25Ge 0.25} P _0.75 S ₄ , 30Li ₂ S・26B ₂ S ₃・44LiI, 63Li ₂ S・36SiS ₂・1Li ₃ PO ₄ , 57Li ₂ S・38SiS ₂・5Li ₄ SiO ₄ , 70Li ₂ S・30P ₂ S ₅ , 50Li ₂ S·50GeS ₂ , Li ₇ P ₃ S ₁₁ , and Li _3.25 P _0.95 S ₄ may be used.

The shape of a lithium-ion secondary battery having the above configuration is not particularly limited, and may be a variety of shapes such as coin-shaped, cylindrical, or rectangular, or may be an irregular shape enclosed in a laminated outer casing.

The present invention will be explained in detail below based on examples, but the present invention is not limited to these examples.

[Production Example 1: Production of Nanoparticles A-1]
1,272 g of LiOH.H ₂ O and 4 L of water were mixed to obtain slurry water i _1. Next, while maintaining the temperature at 25 ° C., the obtained slurry water i ₁ was stirred for 3 minutes while 1,153 g of 85% aqueous phosphoric acid solution was added dropwise at 35 mL / min, followed by the addition of 6,481 g of cellulose nanofibers (Wma-10002, manufactured by Sugino Machine Co., Ltd., fiber diameter 4-20 nm) and stirring at a speed of 400 rpm for 12 hours to obtain slurry water i ₂ containing Li ₃ PO _4. The obtained slurry water i ₂ was purged with nitrogen to adjust the dissolved oxygen concentration of the slurry water i ₂ to 0.5 mg / L, and then 964 g of MnSO _{4.5H 2} O and 1,668 g of FeSO _{4.7H 2} O were added to the total amount of slurry water i ₂ ' to obtain slurry water i ₃ . The molar ratio of the added MnSO ₄ to FeSO ₄ (manganese compound:iron compound) was 40:60.
The resulting slurry water _i3 was then placed in an autoclave and subjected to a hydrothermal reaction at 200°C for 5 hours. The pressure inside the autoclave was 1.6 MPa. After the hydrothermal reaction, the resulting crystals were filtered and then washed with 12 parts by mass of water per part by mass of the crystals. The washed crystals were freeze-dried at -50°C for 12 hours to obtain nanoparticles A _- 1 ( _{LiMn0.4Fe0.6PO4 ,} carbon loading = 1.1% by mass, average particle size: 188 nm).

[Production Example 2: Production of Nanoparticles A-2]
Nanoparticles A-2 ( _{LiMn0.6Fe0.4PO4} , carbon loading = 1.0 mass%, average _particle size: 122 nm) were obtained in the same manner as in Production Example 1, except that 5,892 g of cellulose nanofibers, _{1,446 g} of _MnSO4.5H2O , _and 1,112 g of _FeSO4.7H2O were added, and the hydrothermal reaction was carried out at 180°C for 3 hours.

[Production Example 3: Production of nanoparticles A-3]
Nanoparticles A-3 ( _{LiMn0.8Fe0.2PO4} , carbon loading = 1.1 mass%, average particle size: 63 nm) were obtained in the same manner as in Production Example 1, except that 6481 g of cellulose nanofibers, _{1928 g} of _MnSO4.5H2O , _and 556 g _{of FeSO4.7H2O} were added, and the hydrothermal reaction was carried out at 140°C for 1 hour.

[Production Example 4: Production of nanoparticles A-4]
Nanoparticles A-4 ( _{LiMn0.6Fe0.4PO4} , carbon loading = 0.8 mass%, average particle size: 44 nm) were obtained in the same manner as in Production Example 1, except that 4713 g of cellulose nanofibers, _{1446 g} of _MnSO4.5H2O , _and 1112 g _{of FeSO4.7H2O} were added, and the hydrothermal reaction was carried out at 140°C for 0.4 hours.

[Production Example 5: Production of Nanoparticles A-5]
Nanoparticles A-5 ( _{LiMn0.3Fe0.7PO4} , carbon loading = 1.2 mass%, average particle size: 124 nm) were obtained in the same manner as in Production Example 1, except that ₇₀₇₀ g of cellulose nanofibers, ₇₂₃ g of _MnSO4.5H2O , and ₁₉₄₆ g _{of FeSO4.7H2O} were added, and the hydrothermal reaction was carried out at 180°C for 3 hours.

[Production Example 6: Production of Nanoparticles A-6]
Nanoparticles A-6 ( _{LiMn0.9Fe0.1PO4} , carbon loading = 1.0 mass%, average particle size: 123 nm) were obtained in the same manner as in Production Example 1, except that 5,892 g of cellulose nanofibers, _{2,169 g} of _MnSO4.5H2O , _and 278 g _{of FeSO4.7H2O} were added, and the hydrothermal reaction was carried out at 180°C for 3 hours.

[Production Example 7: Production of nanoparticles B-1]
348 g of manganese dioxide and 84 g of lithium hydroxide monohydrate were mixed and crushed in a ball mill so that the molar ratio of Mn:Li was 2:1, and then the mixture was calcined at 470°C for 12 hours in an air atmosphere and crushed. Then, the mixture was calcined at 700°C for 20 hours in an inert atmosphere to obtain particles B _- 1 ( _LiMnO , average particle size 150 nm).

[Examples 1 and 3, Comparative Examples 3 and 4]
According to the formulation shown in Table 2, nanoparticles A and nanoparticles B were each dispensed so that the total amount was 1,500 g, and 1 L of water was added to each dispensed nanoparticles A and B to obtain slurry water i' ^1. The obtained slurry water i' ¹ was dispersed for 1 minute using an ultrasonic agitator (T25, manufactured by IKA Corporation) to uniformly color the entire mixture, and then spray-dried using a spray dryer (MDL-050M, manufactured by Fujisaki Electric Co., Ltd.) (nozzle air flow rate 35 L/min, inlet air temperature 190°C) to obtain granules X ¹ .
The resulting granules ^X1 were fired at 700° C. for 1 hour in an argon-hydrogen atmosphere (hydrogen concentration 3%) to obtain positive electrode active material composite particles.

[Example 2, Comparative Example 1]
According to the formulation shown in Table 2, nanoparticles A and nanoparticles B were each dispensed so that the total amount was 1,500 g, and 1 L of water was added to each dispensed nanoparticles A and B to obtain slurry water i' ^2. The obtained slurry water i' ² was dispersed for 1 minute using an ultrasonic agitator (T25, manufactured by IKA Corporation) to uniformly color the entire mixture, and then spray-dried using a spray dryer (MDL-050M, manufactured by Fujisaki Electric Co., Ltd.) (nozzle air flow rate 30 L/min, inlet air temperature 190°C) to obtain granules X ² .
The resulting granules ^X2 were fired at 700° C. for 1 hour in an argon-hydrogen atmosphere (hydrogen concentration 3%) to obtain positive electrode active material composite particles.

[Example 4]
According to the formulation shown in Table 2, nanoparticles A and nanoparticles B were each dispensed so that the total amount was 1,500 g, and 1 L of water was added to each dispensed nanoparticles A and B to obtain slurry water i' ^3. The obtained slurry water i' ³ was dispersed for 1 minute using an ultrasonic agitator (T25, manufactured by IKA Corporation) to uniformly color the entire mixture, and then spray-dried using a spray dryer (MDL-050M, manufactured by Fujisaki Electric Co., Ltd.) (nozzle air flow rate 43 L/min, inlet air temperature 190°C) to obtain granules X ³ .
The obtained granules ^X3 were fired at 700° C. for 1 hour in an argon-hydrogen atmosphere (hydrogen concentration 3%) to obtain positive electrode active material composite particles.

[Example 5, Comparative Example 2]
According to the formulation shown in Table 2, nanoparticles A and nanoparticles B were each dispensed so that the total amount was 1,500 g, and 1 L of water was added to each dispensed nanoparticles A and B to obtain slurry water i' ^4. The obtained slurry water i' ⁴ was dispersed for 1 minute using an ultrasonic agitator (T25, manufactured by IKA Corporation) to uniformly color the entire mixture, and then spray-dried using a spray dryer (MDL-050M, manufactured by Fujisaki Electric Co., Ltd.) (nozzle air flow rate 45 L/min, inlet air temperature 190°C) to obtain granules X ⁴ .
The resulting granules ^X4 were fired at 700° C. for 1 hour in an argon-hydrogen atmosphere (hydrogen concentration 3%) to obtain positive electrode active material composite particles.

<Evaluation of battery characteristics (cycle characteristics)>
Each of the obtained positive electrode active material composite particles was used as a positive electrode material to prepare a positive electrode for a lithium ion secondary battery. Specifically, each of the obtained positive electrode active material composite particles, acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 90:5:5, and N-methyl-2-pyrrolidone was added to the mixture and thoroughly kneaded to prepare a positive electrode slurry. The positive electrode slurry was applied to a current collector made of aluminum foil with a thickness of 20 μm using a coating machine and vacuum dried at 80°C for 12 hours. The resulting mixture was then punched into a φ14 mm disk and pressed using a hand press at 16 MPa for 2 minutes to form a positive electrode.

Next, a coin-type secondary battery was constructed using the above positive electrode. A lithium foil punched to a diameter of 15 mm was used as the negative electrode. The electrolyte used was a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 3:7, with _LiPF6 dissolved at a concentration of 1 mol/L. A polymer porous film was used as the separator. These battery components were assembled and housed in an atmosphere with a dew point of -50°C or lower using standard methods to obtain a coin-type secondary battery (CR-2032).

Using the obtained coin-type secondary battery, charging and discharging was repeated 100 times at 1 C in an environment at an ambient temperature of 30°C using a discharge capacity measuring device (HJ-1001SD8, manufactured by Hokuto Denko Corporation), and the discharge capacity (mAh/g) was determined, and the value of the cycle characteristics (capacity retention rate (%)) was calculated using the following formula (x).
Cycle characteristics
= (discharge capacity after 100 cycles)/(discharge capacity after 1 cycle)×100
...(x)
The results are shown in Table 2.

Claims (4)

The following formula (a):
Li _f Mn _g Fe _h M ¹ _x PO ₄ ...(a)
(In formula (a), ^M1 represents one or more elements selected from Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, and Gd. f, g, h, and x represent numbers that satisfy 0<f≦1.2, 0.3≦g≦1.2, 0.2≦h≦1.2, 0≦x≦0.3, and 7/13≦g/h≦17/3, and that satisfy f+(valence of Mn)×g+(valence of Fe)×h+(valence of ^M1 )×x=3.)
Nanoparticles A represented by the following formula (b):
LiM ² _a Mn _b O ₄ ...(b)
(In formula (b), ^M2 represents one or more elements selected from Ni, Co, Al, Mg, Ti, V, Cr, Fe, Zr, Ga, Cu, and Si. a and b represent numbers that satisfy 0≦a≦0.1, 0<b≦2, and (valence of ^M2 )×a+(valence of Mn)×b=7.)
and the average particle size is 100 nm to 200 nm.
A composite particle formed by
Positive electrode active material composite particles for lithium ion secondary batteries, in which the mass ratio (A:B) of nanoparticles A to nanoparticles B is 3:97 to 45:55. 2. The positive electrode active material composite particles for a lithium ion secondary battery according to claim 1 , wherein the average particle size is 8 μm to 50 μm. 3. The positive electrode active material composite particle for a lithium ion secondary battery according to claim 1 or 2, wherein the nanoparticles A have surfaces supported with carbon derived from cellulose nanofibers and/or carbon derived from a water-soluble carbon material, and the amount of carbon supported is 0.1% by mass to 5.0% by mass relative to 100% by mass of the total amount of the nanoparticles A. The positive electrode active material composite particles for a lithium ion secondary battery according to any one of claims 1 to 3, which are a fired product of granules containing nanoparticles A and nanoparticles B in a mass ratio (A:B) of nanoparticles A to nanoparticles B of 3:97 to 45:55.