JP7478587B2 - Positive electrode active material particles for lithium ion secondary batteries and method for producing same - Google Patents

Description

The present invention relates to positive electrode active material particles for lithium ion secondary batteries that exhibit good cycle characteristics and a method for producing the same.

Secondary batteries such as lithium ion secondary batteries are used in a wide range of fields, including mobile phones, digital cameras, notebook PCs, hybrid cars, and electric cars. Lithium-based polyanion particles such as LiMn _x Fe _1-x PO ₄ are considered promising as a positive electrode material for such lithium ion secondary batteries due to their high safety and large capacity. On the other hand, lithium-based polyanion particles have low conductivity, and still require improvement in order to sufficiently improve the battery properties of the resulting lithium ion secondary batteries, so various developments have been made in the past.

For example, Patent Document 1 discloses an electrode material that is an aggregate _of electrode active materials ( _such as _LiaAxMyPO4 ) coated with a specific carbonaceous film using an ionic organic material as a carbon source, and aims to improve the discharge capacity, input/output characteristics, etc. of a battery. Patent Document 2 discloses composite particles that contain a specific _carbon material such as carbon nanotubes and a lithium-containing phosphate (such as _{LiMnxFe1 - xPO4} ) and have specific pores, and attempts to improve the rate characteristics, etc. of a battery.

Patent No. 6547891 JP 2018-139213 A

However, when the electrode active material described in Patent Document 1 is used, there is a risk that the electronic conduction path will disappear due to the expansion and contraction of the active material as the resulting secondary battery is repeatedly charged and discharged. Furthermore, even in the composite particles described in Patent Document 2, the electronic conduction path between the particles is still insufficient, and further improvements are required to fully improve the cycle characteristics.

Therefore, the object of the present invention is to provide positive electrode active material particles for lithium ion secondary batteries that can effectively improve the cycle characteristics of lithium ion secondary batteries while using lithium-based polyanion particles, and a method for producing the same.

The inventors conducted extensive research to solve the above problems and discovered that positive electrode active material particles for lithium ion secondary batteries, which are made of lithium-based polyanion particles with cellulose nanofiber-derived carbon supported on the surface and have specific particle strength and tap density, can provide excellent cycle characteristics in the resulting lithium ion secondary battery, thus completing the present invention.

That is, the present invention relates to a compound represented by the following formula (A):
Li _a Mn _b Fe _c M _z PO ₄ ... (A)
(In formula (A), M represents Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, or Gd. a, b, c, and z represent numbers that satisfy 0<a≦1.2, 0≦b≦1.2, 0≦c≦1.2, 0≦z≦0.3, and b+c≠0, and a+(valence of Mn)×b+(valence of Fe)×c+(valence of M)×z=3.)
The lithium-based polyanion particles are supported on the surface thereof by carbon derived from cellulose nanofibers,
The present invention provides positive electrode active material particles for lithium ion secondary batteries, in which the amount of particles having a particle size of 1.0 μm or less, measured when a load of integrated energy of 1.1 kJ/g is applied using a dry mixer, is 0% to 5% by volume, and the tap density is 1.0 g/ ^cm3 to 1.6 g/ ^cm3 .

The present invention also relates to a method for producing a method for producing a polymerizable compound comprising the steps of (I) to (III):
The present invention provides a method for producing positive electrode active material particles for a lithium ion secondary battery, comprising: (I) a step of calcining a granule S containing lithium-based polyanion particles and cellulose nanofibers to obtain a sintered body X; (II) a step of applying a load of an integrated energy of 0.15 kJ/g to 0.30 kJ/g to the obtained sintered body X using a dry mixer to obtain a compacted body Y; and (III) a step of calcining the obtained compacted body Y, wherein at least one of the calcinations in step (I) and step (III) is performed at a temperature of 600° C. to 750° C.

The positive electrode active material particles for lithium ion secondary batteries of the present invention can effectively improve the cycle characteristics of lithium ion secondary batteries, making them an extremely useful electrode material.

The present invention will be described in detail below.
The positive electrode active material particles for a lithium ion secondary battery of the present invention are formed by supporting carbon derived from cellulose nanofibers on the surface of lithium-based polyanion particles represented by the above formula (A),
The amount of particles having a particle size of 1.0 μm or less, measured when a load of integrated energy of 1.1 kJ/g is applied using a dry mixer, is 0% to 5% by volume, and the tap density is 1.0 g/cm ³ to 1.6 g/cm ³ .
In other words, the positive electrode active material particles for lithium ion secondary batteries of the present invention are secondary particles like a compact in which lithium-based polyanion particles having cellulose nanofiber-derived carbon firmly supported on the surface are firmly assembled as primary particles, and when a specific load of 1.1 kJ/g of integrated energy is applied by a dry mixer, the amount of particles having a particle size of 1.0 μm or less is suppressed, and the particles have a specific tap density, so that the particles have an appropriate strength that can effectively suppress excessive pulverization during the manufacturing process of lithium ion secondary batteries.

The positive electrode of a lithium-ion secondary battery is manufactured by kneading positive electrode active material particles such as lithium-based polyanion particles with a conductive assistant and a binder in a mixer or the like to prepare an electrode slurry, which is then coated and dried. However, if the positive electrode active material particles are excessively pulverized during this kneading due to impact, abrasion, or the like, the positive electrode active material will aggregate in the electrode slurry, resulting in a decrease in dispersibility with the conductive assistant, and will also absorb excessive oil from the binder, resulting in a decrease in the binding strength of the entire electrode, which may cause a decrease in the electronic conductive path. Furthermore, excessively pulverized positive electrode active material particles are susceptible to side reactions such as decomposition of the electrolyte and formation of a solid electrolyte interface during cycling, which may cause a decrease in the electronic conductive path. The present invention was developed to solve this problem.

The lithium-based polyanion particles constituting the positive electrode active material particles for a lithium ion secondary battery of the present invention are represented by the following formula (A):
Li _a Mn _b Fe _c M _z PO ₄ ... (A)
(In formula (A), M represents Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, or Gd. a, b, c, and z represent numbers that satisfy 0<a≦1.2, 0≦b≦1.2, 0≦c≦1.2, 0≦z≦0.3, and b+c≠0, and a+(valence of Mn)×b+(valence of Fe)×c+(valence of M)×z=3.)
It is expressed as:

The lithium-based polyanion particles represented by the above formula (A) are so-called olivine-type lithium transition metal phosphate compounds containing at least manganese (Mn) and iron (Fe) as transition metals, where M represents Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, or Gd.
In addition, a, b, c, and z in the above formula (A) represent numbers that satisfy 0<a≦1.2, 0≦b≦1.2, 0≦c≦1.2, 0≦z≦0.3, and b+c≠0, and also satisfy a+(valence of Mn)×b+(valence of Fe)×c+(valence of M)×z=3.
As for the lithium-based polyanion particles represented by the above formula (A), from the viewpoint of the average discharge voltage of the positive electrode active material particles for lithium ion secondary batteries, a is preferably 0.6≦a≦1.2, more preferably 0.65≦a≦1.15, and even more preferably 0.7≦a≦1.1. b is preferably 0.4≦b≦0.8, more preferably 0.5≦b≦0.8, and even more preferably 0.7≦b≦0.8. c is preferably 0.2≦c≦0.6, more preferably 0.2≦c≦0.5, and even more preferably 0.2≦c≦0.3. z is preferably 0≦z≦0.2, more preferably 0≦z≦0.15, and even more preferably 0≦z≦0.1.

_{Specifically , for example , LiMnPO4 , LiMn0.3Fe0.7PO4 , LiMn0.7Fe0.3PO4 , LiMn0.9Fe0.1PO4 , LiMn0.8Fe0.2PO4 , LiMn0.75Fe0.15Mg0.1PO4 , LiMn0.75Fe0.19Zr0.03PO4 , LiMn0.6Fe0.4PO4 , LiMn0.5Fe0.5PO4 , Li1.2Mn0.63Fe0.27PO4 , Li0.6Mn ​ ​ ​ ​ ​ ​ 0.84Fe0.36PO4 , etc. Among these , LiMn0.3Fe0.7PO4 , LiMn0.7Fe0.3PO4 , LiMn0.8Fe0.2PO4 , LiMn0.6Fe0.4PO4 , Li1.2Mn0.63Fe0.27PO4 , or Li0.6Mn0.84Fe0.36PO4 is preferred . ​ ​}

The positive electrode active material particles for lithium ion secondary batteries of the present invention are formed by supporting carbon derived from cellulose nanofibers on the surface of lithium-based polyanion particles represented by the above formula (A). The cellulose nanofibers are carbonized to carbon, which is firmly supported on the surface of the lithium-based polyanion particles. Cellulose nanofibers are a skeletal component that accounts for approximately 50% of all plant cell walls, and are lightweight, high-strength fibers that can be obtained by defibrating the plant fibers that make up such cell walls to nano-size. The fiber diameter of such cellulose nanofibers is 1 nm to 1000 nm, and they also have good dispersibility in water. In addition, since the cellulose molecular chains that make up the cellulose nanofibers form a periodic structure due to carbon, they are carbonized and firmly supported on the surface of the lithium-based polyanion particles, which provides a moderate strength that suppresses excessive pulverization, effectively suppresses the deterioration of the electronic conductive path, and allows the resulting battery to exhibit excellent cycle characteristics.

In addition, the positive electrode active material particles for lithium ion secondary batteries of the present invention may support carbon derived from a water-soluble carbon material together with carbon derived from cellulose nanofibers on the surface of the lithium-based polyanion particles represented by the above formula (A). The water-soluble carbon material is carbonized to carbon, similar to cellulose nanofibers, and this is supported on the surface of the lithium-based polyanion particles. Examples of such water-soluble carbon materials include one or more selected from sugars, polyols, polyethers, and organic acids. More specifically, examples of the water-soluble carbon materials 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 the solubility and dispersibility in the solvent and functioning effectively as a carbon material, glucose, fructose, sucrose, and dextrin are preferred, and glucose is more preferred.

The cellulose nanofiber-derived carbon supported on the surface of the lithium-based polyanion particles present in the positive electrode active material particles for lithium ion secondary batteries of the present invention is present so as to fill the gaps between the lithium-based polyanion particles represented by the above formula (A). That is, the cellulose nanofiber-derived carbon, while present on the surface of the lithium-based polyanion particles, fills the interparticle gaps of the packing structure formed by the lithium-based polyanion particles. Meanwhile, the water-soluble carbon material that can be further used as a carbon source is uniformly deposited on the surface of the lithium-based polyanion particles.
In the positive electrode active material particles for lithium ion secondary batteries of the present invention, carbon having a characteristic composite structure forms conductive paths between the lithium-based polyanion particles in a state in which the packing density of the positive electrode active material is increased, and it is therefore believed that excellent cycle characteristics are exhibited and it is also possible to effectively reduce the amount of such carbon.

The cellulose nanofibers and the water-soluble carbon material are then carbonized and are present in the lithium-ion secondary battery positive electrode active material particles of the present invention as carbon supported on the surface of the lithium-based polyanion particles. The atomic amount of carbon derived from the carbonized cellulose nanofibers supported on the surface of the lithium-ion secondary battery positive electrode active material particles is preferably 1.0% by mass to 10.0% by mass, more preferably 1.5% by mass to 7.0% by mass, and even more preferably 2.0% by mass to 5.0% by mass in the lithium-ion secondary battery positive electrode active material particles of the present invention.
The amount of carbon derived from the water-soluble carbon material in terms of atoms is preferably 0% by mass to 4.0% by mass, more preferably 0% by mass to 3.0% by mass, and even more preferably 0% by mass to 2.0% by mass.
The total of the atomic equivalent amount of carbon derived from the cellulose nanofibers and the atomic equivalent amount of carbon derived from the water-soluble carbon material is preferably 1.0 mass% to 10.0 mass%, more preferably 1.5 mass% to 7.0 mass%, and even more preferably 2.0 mass% to 5.0 mass%.

The atomic equivalent amount of carbon derived from cellulose nanofibers and the atomic equivalent amount of carbon derived from water-soluble carbon materials present in the positive electrode active material particles for lithium-ion secondary batteries can be determined by measurement using a carbon/sulfur analyzer.

The positive electrode active material particles for lithium ion secondary batteries of the present invention are secondary particles like a compact in which primary particles, which are lithium-based polyanion particles with cellulose nanofiber-derived carbon on the surface, are firmly assembled, and therefore the amount of particles with a particle diameter of 1.0 μm or less measured when a load of integrated energy of 1.1 kJ/g is applied by a dry mixer is 0% to 5% by volume. Such a load is so great that conventional positive electrode active material particles for lithium ion secondary batteries are crushed and mostly pulverized, and the amount of particles with a particle diameter of 1.0 μm or less increases excessively. In contrast, the positive electrode active material particles for lithium ion secondary batteries of the present invention are secondary particles like a compact in which primary particles, which are lithium-based polyanion particles with cellulose nanofiber-derived carbon on the surface, are firmly assembled and have appropriate strength, and therefore these particles can effectively avoid unnecessary aggregation of the positive electrode active material particles for lithium ion secondary batteries during preparation of the electrode slurry, and excessive pulverization due to impact, wear, etc. during kneading.

The amount of particles with a particle diameter of 1.0 μm or less measured in the positive electrode active material particles for lithium ion secondary batteries of the present invention when a load of an integrated energy of 1.1 kJ/g is applied using a dry mixer is 0% to 5% by volume, preferably 0% to 3%, and more preferably 0% to 1%.

The integrated energy by the dry mixer can be calculated by the following formula (1).
Accumulative energy (kJ/g)
= Load applied to powder (kW) × Processing time (s) ÷ Processing amount (g) ... Equation (1)
The amount of particles having a particle size of 1.0 μm or less measured when a load of integrated energy of 1.1 kJ/g is applied by a dry mixer is a value determined by a method similar to the rotational strength test method using a rotating drum and a driving device in JIS Z 8841-1993 (test method for strength of granulated material).
Specifically, it means the proportion (%) of particles having a particle size of 1.0 μm or less based on volume, which is measured using a particle size distribution measuring device conforming to JIS R 1629 "Method for measuring particle size distribution of fine ceramic raw materials by laser diffraction/scattering method", such as a Microtrac MT3300EX II laser diffraction/scattering particle size distribution measuring device (Microtrac Bell), after applying a compressive force and a shear force equivalent to a load of integrated energy of 1.1 kJ/g to the positive electrode active material particles using a dry mixer such as NOB-130 (manufactured by Hosokawa Micron Corporation).

The positive electrode active material particles for lithium ion secondary batteries of the present invention are secondary particles like a compact in which primary particles, which are lithium-based polyanion particles having cellulose nanofiber-derived carbon supported on their surfaces, are tightly assembled, and therefore have a tap density of 1.0 g/ ^cm3 to 1.6 g/ ^cm3 , preferably 1.2 g/ ^cm3 to 1.6 g/ ^cm3 , and more preferably 1.4 g/ ^cm3 to 1.6 g/ ^cm3 .
The tap density means the "tap bulk density" measured by the method specified in JIS R 1628 "Method for measuring bulk density of fine ceramic powders".

The positive electrode active material particles for lithium ion secondary batteries of the present invention are secondary particles formed by tightly assembling primary particles in which cellulose nanofiber-derived carbon is supported on the surface of lithium-based polyanion particles, and due to the smooth, roughly spherical surface, the particles can exhibit an appropriate level of strength. Specifically, for example, the angle of repose of the positive electrode active material particles for lithium ion secondary batteries of the present invention is preferably 30° to 45°, more preferably 30° to 40°, and even more preferably 30° to 35°.

The angle of repose is the angle of the ridgeline formed when the powder is dropped and piled up, and refers to the value (°) measured by the method specified in JIS R 9301-2-2 "Alumina powder - Part 2: Measurement method of physical properties - 2: Angle of repose". Specific measuring devices that can be used include a powder property evaluation device, such as the Powder Tester PT-X (manufactured by Hosokawa Micron Corporation).

The average particle size of the positive electrode active material particles for lithium ion secondary batteries of the present invention is preferably 5 μm to 30 μm, more preferably 9 μm to 26 μm, and even more preferably 13 μm to 22 μm, from the viewpoint of obtaining compact-like particles that are free from excessive microparticulation.

The positive electrode active material particles for a lithium ion secondary battery of the present invention are prepared by the following steps (I) to (III):
(I) A step of calcining a granule S containing lithium-based polyanion particles and cellulose nanofibers to obtain a sintered body X; (II) A step of applying a load of an integrated energy of 0.15 kJ/g to 0.30 kJ/g to the obtained sintered body X using a dry mixer to obtain a compacted body Y; and (III) A step of calcining the obtained compacted body Y, wherein at least one of the calcinations in steps (I) and (III) is performed at a temperature of 600°C to 750°C.

Step (I) is a step of calcining a granule S containing lithium-based polyanion particles and cellulose nanofibers to obtain a sintered body X.
Specifically, the granules containing lithium-based polyanion particles and cellulose nanofibers used in step (I) can be obtained by adding and mixing cellulose nanofibers to slurry water q containing lithium-based polyanion particles to obtain slurry water r, and then spray-drying the obtained slurry water r.

Here, the lithium-based polyanion particles are obtained by subjecting them to a synthesis reaction. Methods for subjecting them to such synthesis reaction are broadly divided into solid-phase methods (calcination method, melt-annealing method) and wet methods (hydrothermal method), but either method may be used, and they may be pulverized or classified after subjecting them to the synthesis reaction. Among these, from the viewpoint of obtaining lithium-based polyanion particles with small particle size and uniform particle size, those obtained by subjecting them to a hydrothermal reaction are preferred.

Specifically, the slurry water q containing the lithium-based polyanion particles may be prepared by mixing a phosphoric acid compound with a slurry containing a lithium compound.
Examples of lithium compounds include hydroxides (e.g., _LiOH.H2O , LiOH), carbonates, sulfates, and acetates. Of these, hydroxides are preferred.
Examples of the phosphoric acid compound 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, it is preferable to use phosphoric acid, and it is preferable to use it as an aqueous solution with a concentration of 70% by mass to 90% by mass.

Then, while stirring the slurry water containing the lithium compound, phosphoric acid is dropped, nitrogen is purged, and the slurry water is subjected to a hydrothermal reaction or the like to complete the reaction in the slurry, thereby obtaining the slurry water q containing the lithium-based polyanion particles represented by (A) above.
The content of the lithium-based polyanion particles in the slurry water q is preferably 10 parts by mass to 400 parts by mass, and more preferably 30 parts by mass to 210 parts by mass, based on 100 parts by mass of water.

Next, cellulose nanofibers are added to and mixed with the obtained slurry water q to obtain a slurry water r in which both the lithium-based polyanion particles and the cellulose nanofibers are sufficiently dispersed.
The content of the cellulose nanofibers in the slurry water r is preferably 1.0 to 10.0 parts by mass, more preferably 1.5 to 7.0 parts by mass, and even more preferably 2.0 to 5.0 parts by mass, calculated as the amount of carbonization residue per 100 parts by mass of the lithium-based polyanion particles in the slurry water r. The amount of cellulose nanofibers added to the slurry water q can be determined so as to achieve such a ratio.

The slurry water r thus obtained may be subjected to spray drying to obtain the granules S. The particle size of the granules S is preferably 4 μm to 30 μm, more preferably 8 μm to 26 μm, as determined by the D _{50 value in particle size distribution based on a laser diffraction/scattering method. Here, the D 50 value} in particle size distribution measurement is a value obtained by a volume-based particle size distribution based on a laser diffraction/scattering method, and the D ₅₀ value means the particle size at 50% cumulative (median diameter). Therefore, the particle size of the granules S may be adjusted by appropriately optimizing the operating conditions of the spray dryer.

In step (I) of the production method of the present invention, the granules S thus obtained are sintered to obtain a sintered body X. This carbonizes the cellulose nanofibers present in the granules S, and the carbon can be firmly supported on the surface of the lithium-based polyanion particles represented by formula (A).
The firing temperature in step (I) is preferably 400° C. to 750° C., more preferably 450° C. to 750° C., and even more preferably 550° C. to 750° C. The firing time is preferably 15 minutes to 180 minutes, and more preferably 30 minutes to 120 minutes. Furthermore, the firing atmosphere is preferably a reducing atmosphere or an inert atmosphere.

The average particle size of the resulting sintered body X is preferably 5 μm to 31 μm, and more preferably 9 μm to 27 μm.

In step (II), a load of 0.15 kJ/g to 0.30 kJ/g of cumulative energy is applied to the sintered body X obtained in step (I) using a dry mixer to obtain a compacted body Y. By applying such a load to the sintered body X that has undergone step (I), the sintered body X is once compacted and then moved to step (III) described below. In this way, the lithium-based polyanion particles, which are primary particles with carbon supported on their surfaces, are firmly assembled, and the positive electrode active material particles for lithium ion secondary batteries of the present invention can be obtained as compacted secondary particles with appropriate strength.

There are no particular limitations on the dry mixer that can be used, but for example, an MP mixer (manufactured by Nippon Coke Corporation) can be used.

The cumulative energy applied to the sintered body X is 0.15 kJ/g to 0.30 kJ/g, preferably 0.17 kJ/g to 0.30 kJ/g, and more preferably 0.20 kJ/g to 0.30 kJ/g.

The average particle size of the resulting compact Y is preferably 4 μm to 30 μm, and more preferably 8 μm to 26 μm.

Step (III) is a step of firing the compact Y obtained in step (II). This allows the compact Y, which has been compacted once, to be compacted even more firmly, and allows the compact Y to develop an appropriate strength.
The firing temperature in step (III) is preferably 200° C. to 750° C., more preferably 200° C. to 600° C., and even more preferably 200° C. to 400° C. The firing time is preferably 15 minutes to 180 minutes, and more preferably 30 minutes to 120 minutes. The firing atmosphere is preferably a reducing atmosphere or an inert atmosphere.

In the production method of the present invention, at least one of the firing in step (I) and the firing in step (III) is performed at a temperature of 600° C. to 750° C. This makes it possible to produce positive electrode active material particles for lithium ion secondary batteries as a compact having appropriate strength by repairing or restoring defects in the crystallinity of the lithium-based polyanion particles and the carbon derived from the cellulose nanofibers that may occur in part as the process proceeds while still compacting the compacted body Y more firmly.
From this viewpoint, it is preferable that the firing temperature in step (III) is lower than the firing temperature in step (I). More specifically, for example, the firing temperature in step (I) is 600°C to 750°C, and the firing temperature in step (III) is 200°C to 400°C.

The lithium ion secondary battery to which the positive electrode containing the positive electrode active material particles for lithium ion secondary batteries of the present invention can be applied is not particularly limited as long as it essentially comprises a positive electrode, a negative electrode, an electrolyte, and a separator, or a positive electrode, a negative electrode, and a solid electrolyte.

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

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 such inorganic salts, organic salts selected from LiSO3CF3, LiC(SO3CF3)2, LiN(SO3CF3)2, LiN(SO2C2F5)2 , and LiN ( SO2CF3 ) ( SO2C4F9 ) , and derivatives of such 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 , La0.51Li0.34TiO2.94 , Li1.3Al0.3Ti1.7 ( PO4 ) 3 , Li7La3Zr2O12 , 50Li4SiO4.50Li3BO3 , Li2.9PO3.3N0.46 , Li3.6Si0.6P0.4O4 , Li1.07Al0.69Ti1.46 ( PO4 ) 3 , Li1.5Al0.5Ge1.5 ( PO4 ) 3 , Li10GeP2S12 , Li3.25Ge0.25P ​ ​ ​ ​ 0.75S4 , 30Li2S.26B2S3.44LiI , 63Li2S.36SiS2.1Li3PO4 , 57Li2S.38SiS2.5Li4SiO4 , 70Li2S.30P2S5 , 50Li2S.50GeS2 , Li7P3S11 , and Li3.25P0.95S4 may be used . ​ ​ ​ ​}

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

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

[Example 1]
LiOH.H ₂ O 1272g and water 4L were mixed to obtain slurry a. Next, the obtained slurry a was stirred for 3 minutes while maintaining the temperature at 25 ° C., while 1153g of 85% phosphoric acid aqueous solution was dropped at 35mL / min, followed by adding 11.78kg of cellulose nanofiber (Wma-10002, manufactured by Sugino Machine Co., Ltd., fiber diameter 4nm to 20nm) and stirring at a speed of 400 rpm for 12 hours to obtain slurry b containing Li ₃ PO _4. The obtained slurry b was purged with nitrogen to make the dissolved oxygen concentration of slurry b 0.5mg / L, and then 1688g of MnSO _{4.5H 2} O and 834g of FeSO _{4.7H 2} O were added to the total amount of slurry b to obtain slurry c. The molar ratio of added _MnSO4 to _FeSO4 (manganese compound:iron compound) was 70:30.

Then, the obtained slurry c was put into an autoclave and subjected to a hydrothermal reaction at 170°C for 1 hour. The pressure inside the autoclave was 0.8 MPa. After the hydrothermal reaction, the generated crystals were filtered and then washed with 12 parts by mass of water per 1 part by mass of the crystals. The washed crystals were freeze-dried at -50°C for 12 hours to obtain complex d. 1000 g of the obtained complex d was taken and 1 L of water was added thereto to obtain slurry e. The obtained slurry e was dispersed for 1 minute using an ultrasonic agitator (T25, manufactured by IKA Co., Ltd.) to uniformly color the entire mixture, and then spray-dried (nozzle air flow rate 35 L/min, supply air temperature 180°C) using a spray dryer (MDL-050M, manufactured by Fujisaki Electric Co., Ltd.) to obtain granules S1.

The obtained granules S1 were sintered at 650°C for 30 minutes in an argon hydrogen atmosphere (hydrogen concentration 3%) to obtain a sintered body X1. 300 g of the obtained sintered body X1 was taken out, and a compressive force and a shear force were applied to the powder for 3 minutes at a load of 0.4 kW using an MP mixer (manufactured by Nippon Coke Co., Ltd.) (accumulated energy applied: 0.24 kJ/g) to obtain a compacted body Y1. The obtained compacted body Y1 was then sintered at 200°C for 30 minutes in an argon hydrogen atmosphere (hydrogen concentration ₃ %) to obtain secondary battery positive electrode active material particles A ( _{LiMn0.7Fe0.3PO4 ,} carbon amount = 2.0 mass%, average particle size: 15.8 μm).

[Example 2]
Positive electrode active material particles B for a secondary battery (LiMn0.7Fe0.3PO4, carbon amount = 2.0 mass%, average particle size: 16.0 μm) were obtained in the same manner as in Example 1, except that the cumulative energy loaded on the _sintered body X1 was 0.16 kJ _{/ g} (application time of the compressive force and the shear force: 2 minutes).

[Example 3]
Positive _electrode active material particles C for secondary batteries ( _{LiMn0.3Fe0.7PO4} , _carbon amount = 2.0 mass%, _average particle size: 15.5 μm) were obtained in the same manner as in Example 1, except that the amount of _MnSO4.5H2O added was 723 g, the amount of _FeSO4.7H2O added was ₁₉₄₆ g, and the _{molar ratio of MnSO4 to FeSO4 (} manganese compound:iron compound) was 30:70.

[Example 4]
Except that 600 g of composite d was separated, slurry e was obtained in the same manner as in Example 1. Next, using the obtained slurry e, secondary battery positive electrode active material particles D (LiMn0.7Fe0.3PO4, carbon amount = 2.0 mass%, average particle size: 5.2 μm) was obtained in the same manner as in Example 1, except that granules S1 were obtained by changing the spray drying conditions to a nozzle air flow rate of _{75 L} / _min and an inlet air temperature of 120°C.

[Example 5]
Except that the amount of composite d to be separated was 1,200 g, a slurry e was obtained in the same manner as in Example 1. Next, using the obtained slurry e, a secondary battery positive electrode active material particle E (LiMn0.7Fe0.3PO4, carbon amount = 2.0 mass%, average particle size: _29.7 μm ₎ was obtained in the same manner as in Example 1, except that a granule S1 was obtained by changing the spray drying conditions to a nozzle air flow rate of 20 L/min and _an inlet air temperature of 190°C.

[Example 6]
Positive electrode active material particles F for secondary batteries (LiMn0.7Fe0.3PO4, carbon content = 2.0 mass%, average particle size: 5.3 μm) were obtained in the same manner as in Example 4, except that the cumulative energy loaded on the sintered body X1 was _0.20 kJ _{/ g} (application time of compressive force and shear force: 2 minutes 30 seconds).

[Example 7]
Positive electrode active material particles G for a secondary battery (LiMn0.7Fe0.3PO4, carbon content = 2.0 mass%, average particle size: 29.6 μm) were obtained in the same manner as in Example 5, except that the cumulative energy loaded on the sintered body X1 was _0.29 kJ _{/ g} (application time of the compressive force and the shear force: 3 minutes 40 seconds).

[Example 8]
Secondary battery positive electrode active material particles H (LiMn _0.7 Fe _0.3 PO ₄ , carbon amount=1.1 mass %, average particle size: 15.6 μm) were obtained in the same manner as in Example 1, except that the amount of cellulose nanofibers added was 6481 g.

[Example 9]
Positive electrode active material particles I for secondary batteries (LiMn _0.7 Fe _0.3 PO ₄ , carbon amount=4.8 mass %, average particle size: 16.1 μm) were obtained in the same manner as in Example 1, except that the amount of cellulose nanofibers added was 28.28 kg.

[Comparative Example 1]
Positive electrode active material particles J for a secondary battery (LiMn0.7Fe0.3PO4, carbon content = 2.0 mass%, average particle size: 16.5 μm) were obtained in the same manner as in Example 1, except that the cumulative energy loaded on the sintered body X1 was _0.10 kJ _{/ g} (application time of the compressive force and shear force: 1 minute 15 seconds).

[Comparative Example 2]
Positive electrode active material particles K for a secondary battery ( _{LiMn0.7Fe0.3PO4} , carbon content = 2.0 mass%, average particle size: 13.3 μm) were obtained in the same manner as in Example 1, except that the cumulative energy loaded on the sintered body X1 was _0.40 kJ _/g (application time of compressive force and shear force: 5 minutes).

The obtained active material particles A to K were used to carry out the measurements and evaluations according to the following methods.
The results are shown in Table 1.

Average particle size
The volumetric particle size distribution was measured using a laser diffraction scattering particle size distribution measuring device Microtrac MT3300EX II (manufactured by Microtrac Bell), and the particle size at 50% of the cumulative particle size was taken as the average particle size.

<<Amount of particles with a particle size of 1 μm or less (%)>>
Using NOB-130 (manufactured by Hosokawa Micron Corporation), compressive force and shear force (load of cumulative energy 1.1 kJ/g) were applied to 300 g of positive electrode active material particles for 2 minutes at a powder load of 2.7 kW, and then the volumetric proportion of particles with a particle size of 1.0 μm or less measured using a laser diffraction scattering particle size distribution measuring device Microtrac MT3300EX II (manufactured by Microtrac-Bell) was taken as the particle amount (%) of 1 μm or less.

Tap density (g/ ^cm3 )
The tapped bulk density was measured according to the method specified in JIS R 1628 "Method for measuring bulk density of fine ceramic powders", and this was taken as the tapped density (g/cm ³ ).

《Angle of repose (°)》
The angle of repose (°) was measured using a powder property evaluation device, Powder Tester PT-X (manufactured by Hosokawa Micron Corporation), in accordance with the method specified in JIS R 9301-2-2 "Alumina powder - Part 2: Measurement of physical properties - 2: Angle of repose".

<Method of evaluating cycle characteristics>
The active material particles obtained in the examples and comparative examples were used as the positive electrode material to prepare a positive electrode for a lithium ion secondary battery. Specifically, the obtained active material particles, acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 90:5:5, and N-methyl-2-pyrrolidone was added thereto and thoroughly kneaded to prepare a positive electrode slurry. The positive electrode slurry was applied to a current collector made of aluminum foil having a thickness of 20 μm using a coater, and vacuum dried at 80° C. for 12 hours. Thereafter, the positive electrode was punched into a disk shape of φ14 mm and pressed for 2 minutes at 16 MPa using a hand press to obtain a positive electrode.
Next, a coin-type secondary battery was constructed using the above positive electrode. A lithium foil punched to φ15 mm was used for the negative electrode. The electrolyte was a mixed solvent of ethylene carbonate and ethyl methyl carbonate mixed at a volume ratio of 3:7, in which _LiPF6 was dissolved at a concentration of 1 mol/L. The separator was a known one, such as a polymer porous film of polypropylene. These battery components were assembled and housed in an atmosphere with a dew point of −50° C. or less by a conventional method, to produce a coin-type secondary battery (CR-2032).

The obtained secondary battery was used to evaluate cycle characteristics. Specifically, constant current charging at a current density of 170 mA/g and a voltage of 4.5 V and constant current discharging at a current density of 170 mA/g and a cut-off voltage of 2.0 V were performed, and the discharge capacity at a current density of 170 mA/g (1 CA) was calculated. Furthermore, a 100-cycle repeated test was performed under the same charge/discharge conditions, and the capacity retention rate (%) was calculated by the following formula (2). All charge/discharge tests were performed at 30°C.
Capacity retention rate (%)
= (discharge capacity after 100 cycles) / (discharge capacity after 1 cycle) × 100 (2)

Claims (8)

The following formula (A):
Li _a Mn _b Fe _c M _z PO ₄ ... (A)
(In formula (A), M represents Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, or Gd. a, b, c, and z represent numbers that satisfy 0<a≦1.2, 0≦b≦1.2, 0≦c≦1.2, 0≦z≦0.3, and b+c≠0, and a+(valence of Mn)×b+(valence of Fe)×c+(valence of M)×z=3.)
The lithium-based polyanion particles are supported on the surface thereof by carbon derived from cellulose nanofibers,
The amount of particles having a particle size of 1.0 μm or less, measured when a load of an integrated energy of 1.1 kJ/g is applied using a dry mixer, is 0% to 5% by volume, and the tap density is 1.0 g/cm ³ to 1.6 g/cm ³ . The positive electrode active material particles for lithium ion secondary batteries according to claim 1, wherein the angle of repose is 30° to 45°. The positive electrode active material particles for lithium ion secondary batteries according to claim 1 or 2, wherein the atomic amount of carbon derived from cellulose nanofibers supported on the surface of the positive electrode active material particles for lithium ion secondary batteries is 1.0% by mass to 10.0% by mass in the positive electrode active material particles for lithium ion secondary batteries. The positive electrode active material particles for lithium ion secondary batteries according to any one of claims 1 to 3, having an average particle size of 5 μm to 30 μm. The following steps (I) to (III):
The method for producing positive electrode active material particles for lithium ion secondary batteries according to any one of claims 1 to 4, comprising: (I) a step of calcining a granule S containing lithium-based polyanion particles and cellulose nanofibers to obtain a sintered body X; (II) a step of applying a load of an integrated energy of 0.15 kJ/g to 0.30 kJ/g to the obtained sintered body X using a dry mixer to obtain a compacted body Y; and (III) a step of calcining the obtained compacted body Y, wherein at least one of the calcination in step (I) and the calcination in step (III) is performed at a temperature of 600°C to 750°C. The method for producing positive electrode active material particles for lithium ion secondary batteries according to claim 5, wherein the firing in step (I) is performed at a temperature of 400°C to 750°C. The method for producing positive electrode active material particles for lithium ion secondary batteries according to claim 5 or 6, wherein the firing in step (III) is performed at a temperature of 200°C to 750°C. The method for producing positive electrode active material particles for lithium ion secondary batteries according to any one of claims 5 to 7, wherein the temperature in the firing step (III) is lower than the temperature in the firing step (I).