Positive electrode material for lithium ion secondary battery, and lithium ion secondary battery

Abstract

This positive electrode material for a lithium ion secondary battery includes an aggregate composition of positive electrode active material particles in which a carbonaceous film is formed on a surface of lithium metal phosphate particles represented by general formula (1) (LixAyDzPO4; where A is Co, Mn, Ni, Fe, or the like). The aggregate composition includes a first aggregate having a crystal grain size of the positive electrode active material particles of 200 nm or more and 2000 nm or less, and a second aggregate having a crystal grain size of the positive electrode active material particles of 50 nm or less. The positive electrode material has high Li ion conductivity, is excellent in load characteristics, and can suppress a decrease in capacity retention rate due to the number of cycles when a battery is cycled through charging and discharging.

Description

Positive electrode material for lithium ion secondary battery and lithium ion secondary battery

[0001] The present invention relates to a positive electrode material for a lithium ion secondary battery and a lithium ion secondary battery.

[0002] Lithium-ion secondary batteries have higher energy density and power density than lead batteries and nickel-metal hydride batteries, and are used in a variety of applications, including small electronic devices such as smartphones, home backup power sources, power tools, etc. In addition, large-capacity lithium-ion secondary batteries are being put into practical use for in-vehicle applications such as electric vehicles, and for storing renewable energy such as solar and wind power generation.

[0003] A lithium ion secondary battery typically includes a positive electrode, a negative electrode, an electrolyte, and a separator. The positive electrode active material is lithium cobalt oxide (LiCoO ２ ), lithium manganese oxide (LiMn ２ O ４ ), lithium iron phosphate (LiFePO ４ Lithium-containing metal oxides having the property of being able to reversibly insert and remove lithium ions, such as lithium iron phosphate (LiFePO), are used, and improvements are being studied from various perspectives, such as increasing the capacity of the battery, extending its life, improving safety, and reducing costs. ４ ) is a material that can be easily reduced in cost because it uses iron, which is abundant and inexpensive as a resource. Furthermore, lithium iron phosphate has excellent properties not found in oxide-based positive electrode materials such as lithium cobalt oxide, such as outstanding safety due to the strong covalent bond between phosphorus and oxygen, which prevents oxygen release at high temperatures.

[0004] On the other hand, lithium iron phosphate has poor diffusivity of Li ions and low electronic conductivity, resulting in inferior input / output characteristics compared to oxide-based positive electrode materials. This difference in characteristics becomes more pronounced as the battery operating temperature drops. Therefore, lithium iron phosphate has been considered unsuitable for automotive applications such as hybrid vehicles, which require high input / output characteristics at low temperatures.

[0005] LiMPO, which has an olivine structure, as typified by lithium iron phosphate ４(where M is a divalent metal element) has low diffusivity of Li ions and low electronic conductivity, so LiMPO ４ By miniaturizing the primary particles and coating the surface of each primary particle with a carbonaceous film, the charge-discharge characteristics can be improved.

[0006] In addition, LiMPO, which has fine primary particles, ４ Since LiMPO has a large specific surface area, it increases the viscosity of the electrode mixture slurry and requires a large amount of binder. Therefore, it is common to granulate the primary particles coated with a carbonaceous film to form secondary particles, thereby improving the handling properties of the powder and the properties of the electrode mixture slurry. ４ There are two methods: one is to control the nucleation and particle growth conditions during synthesis, and the other is to mechanically crush coarse primary particles. The former is mainly used in wet synthesis, and the latter is mainly used in solid-phase synthesis.

[0007] For example, Patent Document 1 discloses LiMPO having a particle diameter of less than 1 μm. ４ (M is a divalent metal element) powder is used as a raw material, a carbon source is added to the powder, the resulting mixture is heated and sintered in a vacuum or in an inert atmosphere, and the resulting sintered mass is crushed. ４ A method for producing a lithium-ion battery cathode material comprising a composite of titanium and carbon is disclosed.

[0008] Patent Document 2 discloses a positive electrode active material for a lithium ion battery containing fine particles of lithium iron phosphate, which exist as single particles without forming secondary aggregates, and which is characterized in that the lithium iron phosphate fine particles have an average particle diameter of 5 to 50 nm, and 90% or more of the fine particles have a particle diameter within the range of 3 to 70 nm. Furthermore, in order to improve battery characteristics, the use of an electrode in which different types of positive electrode active materials are mixed has been investigated.

[0009] For example, Patent Document 3 discloses an electrode for a secondary battery having a cathode mixture layer containing at least an olivine-based cathode active material and a layered oxide-based cathode active material as cathode active materials, in which the total weight fraction of the cathode active materials in the cathode mixture layer is 80% by weight or more and 99% by weight or less, the weight fraction of the olivine-based cathode active material is 10% by weight or more and 65% by weight or less, and the weight fraction of the layered oxide-based cathode active material is 30% by weight or more and 80% by weight or less, and the cathode mixture layer contains graphene.

[0010] JP 2009-81002 A JP 2008-159495 A International Publication No. 2020 / 066909

[0011] As in Patent Document 1 or Patent Document 2, the Li ion conductivity can be improved by miniaturizing the primary particles of the positive electrode active material, but this impairs the packing ability of the positive electrode active material in the electrode film, making it difficult to increase the density of the electrode film, resulting in a decrease in the energy density of the battery. In addition, miniaturizing the primary particles of the positive electrode active material reduces the contact area between the aluminum foil, which serves as the current collector, and the positive electrode film, resulting in a loss of adhesion of the electrode film.

[0012] In addition, during the battery reaction, an oxidation-reduction reaction occurs on the surface of the primary particles, resulting in LiFePO ４ It is known that Fe ions are eluted into the electrolyte from the surface of the primary particles of LiFePO 4 , and when these eluted Fe ions are reduced and precipitated on the surface of the carbon negative electrode, they inhibit the movement of lithium ions incorporated into the negative electrode, and the discharge capacity gradually decreases with the number of charge / discharge cycles. ４ By miniaturizing the primary particles of LiFePO, the contact area with the electrolyte increases, ４ It is believed that the rate of elution of Fe ions per unit weight increases.

[0013] Furthermore, Patent Document 3 claims that a high level of safety and high energy density can be achieved by mixing an olivine-based positive electrode active material, which has high safety, with a layered oxide-based positive electrode active material, which has high energy density. However, although the energy density and capacity retention rate have been investigated, improvements in load characteristics have not been investigated.

[0014] The present invention has been made in view of the above circumstances, and has an object to provide a positive electrode material for a lithium ion secondary battery that has high Li ion conductivity, excellent load characteristics, and is capable of suppressing a decrease in capacity retention rate that occurs with the number of charge-discharge cycles when the battery is subjected to charge-discharge cycles, and a lithium ion secondary battery that uses the positive electrode material.

[0015] As a result of extensive research into solving the above problems, the present inventors have found that the problems can be solved by the following invention.

[0016] [1] A positive electrode material for a lithium ion secondary battery, comprising an aggregate composition of positive electrode active material particles, the aggregate composition being formed by forming a carbonaceous coating on the surface of lithium metal phosphate particles represented by the following general formula (1), wherein the aggregate composition comprises a first aggregate in which the positive electrode active material particles have a crystal grain size of 200 nm or more and 2000 nm or less, and a second aggregate in which the positive electrode active material particles have a crystal grain size of 50 nm or less. ｘ A ｙ D ｚ P.O. ４ (1) However, A is at least one selected from the group consisting of Co, Mn, Ni, Fe, Cu, and Cr, D is at least one selected from the group consisting of Mg, Ca, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, Sc, and Y, and 0.9<x<1.1, 0<y≦1, 0≦z<1, 0.9<y+z<1.1.

[0017] [2] The positive electrode material for a lithium ion secondary battery according to [1], wherein the average particle size of the first aggregates and the average particle size of the second aggregates are each independently 0.5 μm or more and 50 μm or less.

[0018] [3] The density of the electrode film obtained by applying an electrode slurry obtained by mixing the positive electrode material, carbon particles, and a binder in an N-methyl-2-pyrrolidinone solvent to an aluminum foil and drying it is 1.1 g / cm ３ 2.0g / cm or more ３ The positive electrode material for a lithium ion secondary battery according to [1] or [2] below.

[0019] [4] The positive electrode material for a lithium ion secondary battery according to any one of [1] to [3], wherein a pouch cell is configured such that an electrode containing the positive electrode material is a positive electrode, an electrode containing a carbon-based negative electrode material is a negative electrode, a separator is disposed between the positive electrode and the negative electrode, and the inside is filled with an electrolyte solution, and after performing 500 cycles of 1 C full charge and 1 C full discharge at 60°C, the pouch cell is disassembled, and the amount of metal contained in the negative electrode is 1,000 mass ppm or less of the mass of lithium metal phosphate particles represented by general formula (1) contained in the first aggregate and the second aggregate in the positive electrode.

[0020] [5] A lithium ion secondary battery having at least a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode contains the positive electrode material for lithium ion secondary batteries according to any one of [1] to [4].

[0021] According to the present invention, it is possible to provide a positive electrode material for a lithium ion secondary battery that has high Li ion conductivity, excellent load characteristics, and is capable of suppressing a decrease in capacity retention rate that occurs with the number of charge-discharge cycles when the battery is subjected to charge-discharge cycling, and a lithium ion secondary battery using the positive electrode material.

[0022] The positive electrode material for a lithium ion secondary battery and the lithium ion secondary battery of the present invention will be described. Note that the present embodiment is specifically described to provide a better understanding of the gist of the invention, and does not limit the present invention unless otherwise specified.

[0023] In addition, unless otherwise specified, a numerical range described in this specification, "lower limit to upper limit," means equal to or greater than the lower limit and equal to or less than the upper limit. Furthermore, the upper and lower limit values ​​of the numerical ranges described in this specification can be arbitrarily combined. For example, when "A to B" and "C to D" are described as numerical ranges, the numerical ranges "A to D" and "C to B" are also included in the scope of the present disclosure. Similarly, when "A or more and B or less" and "C or more and D" are described as numerical ranges, the numerical ranges "A or more and D or less" and "C or more and B or less" are also included in the scope of the present disclosure.

[0024] 1. Positive Electrode Material for Lithium-Ion Secondary Battery The positive electrode material for lithium-ion secondary battery according to this embodiment contains a positive electrode material including an aggregate composition of positive electrode active material particles formed by forming a carbonaceous coating on the surface of lithium metal phosphate particles represented by general formula (1), and the aggregate composition includes first aggregates and second aggregates. Hereinafter, the positive electrode material for lithium ion secondary battery may be simply referred to as "positive electrode material". Furthermore, lithium metal phosphate represented by general formula (1) may be simply referred to as "lithium metal phosphate". Note that the "metal" in "lithium metal phosphate" refers to "A" in general formula (1). ｙ D ｚ " means.

[0025] [First Aggregate] The first aggregate is an aggregate of positive electrode active material particles formed by forming a carbonaceous coating on the surface of lithium metal phosphate particles represented by general formula (1), and the crystal grain size of the positive electrode active material particles is 200 nm to 2000 nm. That is, primary particles of the positive electrode active material particles having a crystal grain size of 200 nm to 2000 nm are aggregated to form secondary particles, which are aggregates of the positive electrode active material particles. It is generally known that hydrogen fluoride produced by the reaction between a fluorine-based electrolyte-containing electrolyte solution and water elutes metal from the positive electrode active material. Furthermore, in olivine-type phosphate compounds, during battery charge and discharge, an electrochemical reaction proceeds through a two-phase reaction between a Li-rich phase and a Li-poor phase, and lattice distortion is maximized at the phase boundary between the Li-rich phase and the Li-poor phase. Therefore, it is believed that the positive electrode active material dissolves at the phase boundary due to an oxidation-reduction reaction with hydrogen fluoride, resulting in the elution of metal. Hereinafter, the phase boundary between the Li-rich phase and the Li-poor phase may be simply referred to as the "phase boundary."

[0026] When the crystal grain size of the positive electrode active material particles in the first aggregate is 200 nm or more, the crystal grain size is sufficiently large to alleviate lattice distortion at the phase boundary, and the area of ​​the phase boundary relative to the volume of the positive electrode active material is significantly reduced, thereby reducing the amount of metal elution to a negligible level. Furthermore, the voids in the positive electrode film can be reduced, thereby improving the energy density when used in a battery. On the other hand, when the crystal grain size of the positive electrode active material particles in the first aggregate is 2000 nm or less, when a Li-rich phase and a Li-poor phase coexist during charging and discharging, the occurrence of cracks perpendicular to the crystal surface at the outer edge of the crystal grains due to crystal distortion is suppressed, thereby improving the battery life characteristics.

[0027] The crystal grain size of the positive electrode active material particles in the first aggregate is preferably 200 nm or more and 1800 nm or less, and more preferably 200 nm or more and 1700 nm or less, from the viewpoint of improving the energy density of the battery and suppressing metal elution. ４ In this case, the crystal grain size of the positive electrode active material particles in the first aggregate is more preferably 500 nm or more and 1700 nm or less, and even more preferably 600 nm or more and 1700 nm or less, from the viewpoint of improving the capacity retention rate. ０．３ Mn ０．７ P.O. ４ In this case, the crystal grain size of the positive electrode active material particles in the first aggregate is more preferably 230 nm or more and 800 nm or less, and even more preferably 250 nm or more and 600 nm or less, from the viewpoint of improving the capacity retention rate.

[0028] [Second Aggregate] The second aggregate is an aggregate of positive electrode active material particles formed by forming a carbonaceous coating on the surface of lithium metal phosphate particles represented by general formula (1), and the crystal grain size of the positive electrode active material particles is 50 nm or less. That is, primary particles of the positive electrode active material particles with a crystal grain size of 50 nm or less aggregate to form secondary particles, which are aggregates of the positive electrode active material particles. When the crystal grain size of the positive electrode active material particles in the second aggregate is 50 nm or less, the high surface energy prevents the above-mentioned two-phase separation into a Li-rich phase and a Li-poor phase within a single crystal grain, and a solid solution phase in which the Li-rich phase and the Li-poor phase coexist within the single crystal grain during the charge / discharge reaction of the battery. As a result, when the battery reaction is stopped, Li is transferred between the crystal grains, resulting in a mixed form of primary particles of the Li-rich phase and primary particles of the Li-poor phase, making it difficult for a phase boundary to form within the crystal grain, and the amount of metal elution is reduced to a negligible level. Furthermore, a large number of contact fields with the electrolyte can be secured, and high Li ion conductivity can be achieved. Hereinafter, the charge / discharge reaction of a battery may be simply referred to as the "battery reaction".

[0029] The crystal grain size of the positive electrode active material particles in the second aggregate is preferably 48 nm or less, and more preferably 45 nm or less, from the viewpoints of high Li ion conductivity and suppression of metal elution. The lower limit of the crystal grain size of the positive electrode active material particles in the second aggregate is not particularly limited, but from the viewpoint of improving crystallinity and improving battery characteristics, it can be preferably 10 nm or more, and more preferably 15 nm or more.

[0030] As described above, in a positive electrode material containing an olivine-type phosphate compound, by using heterogeneous aggregates in which the crystal grain size of the positive electrode active material particles is 50 nm or less and 200 nm to 2000 nm, metal elution associated with the battery reaction can be suppressed. For example, in a cycle test using a pouch cell having a positive electrode and a negative electrode as described below, the total mass of the positive electrode active material particles in the positive electrode material contained in the positive electrode material, i.e., the amount of metal contained in the negative electrode relative to the total mass of the positive electrode active material particles contained in the first aggregate and the second aggregate, can be kept to 1,000 ppm by mass or less, thereby suppressing battery degradation associated with metal elution from the positive electrode active material.

[0031] Furthermore, it is possible to reduce the amount of additives such as vinylene carbide, which are used when forming an insulating organic polymer coating on the surface of the negative electrode and are added for the purpose of suppressing the reduction and deposition onto the negative electrode of metals that have dissolved into the electrolyte inside the battery. As a result, battery durability is improved and Li ion conductivity is increased, thereby improving load characteristics.

[0032] Here, the amount of metal elution from the positive electrode material associated with the battery reaction can be evaluated, for example, using a pouch cell configured with an electrode containing the positive electrode material according to this embodiment as the positive electrode, an electrode containing a carbon-based negative electrode material as the negative electrode, a separator disposed between the positive electrode and the negative electrode, and the interior filled with an electrolyte. The positive electrode is obtained by applying an electrode slurry, in which the positive electrode material according to this embodiment, carbon particles, and a binder are mixed in N-methyl-2-pyrrolidinone solvent, to aluminum foil, drying, and then pressing under a uniaxial pressure of 600 kgf. The manufactured pouch cell is subjected to 500 cycles of full charge at 1 C and full discharge at 1 C at 60°C, after which the pouch cell is disassembled and the negative electrode is removed. The amount of metal (Ma) contained in the removed negative electrode is quantitatively analyzed, for example, using inductively coupled plasma (ICP) atomic emission spectroscopy. The amount of metal elution can be evaluated by determining the sum (Mc) of the masses of the lithium metal phosphate particles contained in the first and second aggregates in the positive electrode and then determining the ratio (Ma / Mc) of the metal amount (Ma) to the sum (Mc). Note that the sum (Mc) of the masses of the lithium metal phosphate particles contained in the first and second aggregates in the positive electrode usually remains unchanged before and after production of the pouch cell.

[0033] In the above "sum of masses (Mc) of lithium metal phosphate particles contained in the first aggregate and the second aggregate in the positive electrode," the first aggregate and the second aggregate refer to aggregates of positive electrode active material particles formed by forming a carbonaceous coating on the surface of lithium metal phosphate particles, as described above. On the other hand, the sum of masses (Mc) of lithium metal phosphate particles refers to the sum of masses of lithium metal phosphate particles themselves without the carbonaceous coating. Since the carbonaceous coating in the electrode is difficult to analyze, the mass of lithium metal phosphate particles excluding the analyzable carbonaceous coating is used to calculate Mc. More specifically, for example, when the chemical composition of lithium metal phosphate is LiFePO ４ In this case, the sum of the masses of the main constituent elements Li, Fe, P, and O and the mass of the impurity elements is Mc.

[0034] When metal ions eluted from the positive electrode are reduced and precipitated at the negative electrode, the insulating properties of the deposited film made of electrolyte decomposition products containing compounds containing Li, generally called Solid Electrolyte Interface (SEI), are impaired, and new SEI is generated on the surface of the negative electrode particles. ＋ On the other hand, the Li ions in the electrolyte ＋ When the ions decrease, PF becomes the counter ion. ６ － The ions remain in the electrolyte. Therefore, to maintain electrical neutrality, Li is removed from the positive electrode active material particles. ＋ The ions are released into the electrolyte to compensate, resulting in a decrease in the total amount of Li that contributes to charge and discharge in the positive electrode active material particles. This results in a decrease in battery capacity (battery deterioration). Therefore, when charge and discharge tests are repeated under the same conditions, the more metal elution the positive electrode material has, the greater the degree of battery deterioration. Therefore, the degree of influence on battery deterioration is determined by comparing the amount of metal contained in the negative electrode with the total mass of lithium metal phosphate particles in the positive electrode.

[0035] The lithium-ion secondary battery electrode of this embodiment includes a first aggregate and a second aggregate, both of which are inhibited from metal elution during battery reactions, as described above. The first aggregate has a crystal grain size of 200 nm or more and 2000 nm or less, which inhibits cracking of the positive electrode active material particles and reduces a decrease in capacity retention during charge-discharge cycling. Furthermore, the first aggregate can improve the energy density when used in a battery. Meanwhile, the second aggregate has a crystal grain size of 50 nm or less, which exhibits high Li-ion conductivity and excellent load characteristics. Therefore, the lithium-ion secondary battery electrode of this embodiment contains an aggregate composition including the first aggregate and the second aggregate, which allows the electrode to exhibit mutually excellent battery characteristics, has high Li-ion conductivity, excellent load characteristics, and can suppress a decrease in capacity retention with the number of charge-discharge cycles.

[0036] Hereinafter, items common to the first aggregate and the second aggregate contained in the positive electrode material for a lithium ion secondary battery will be described. Hereinafter, the "first aggregate and the second aggregate" may be collectively referred to simply as the "aggregate according to this embodiment."

[0037] [Composition] The first aggregate and the second aggregate contained in the electrode for a lithium ion secondary battery of this embodiment are both aggregates of positive electrode active material particles formed by forming a carbonaceous coating on the surface of lithium metal phosphate particles represented by the following general formula (1): Li ｘ A ｙ D ｚ P.O. ４ (1) However, A is at least one selected from the group consisting of Co, Mn, Ni, Fe, Cu, and Cr, D is at least one selected from the group consisting of Mg, Ca, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, Sc, and Y, and 0.9<x<1.1, 0<y≦1, 0≦z<1, 0.9<y+z<1.1.

[0038] The lithium metal phosphate particles are not particularly limited as long as they have the above-mentioned structure, but are preferably made of a transition metal lithium phosphate compound with an olivine structure. In general formula (1), A is preferably Co, Mn, Ni, and Fe, and more preferably Co, Mn, and Fe. D is preferably Mg, Ca, Sr, Ba, Ti, Zn, and Al. By using an olivine-type phosphate compound containing these elements as the lithium metal phosphate particles, a positive electrode composite layer can be obtained that can achieve a higher discharge potential and greater safety. Furthermore, since they are abundant resources, they are preferable as a material of choice.

[0039] From the viewpoint of high discharge capacity and high energy density, the lithium metal phosphate particles may be an olivine-type phosphate compound represented by the following general formula (2): Li ｘ２ Fe ｙ２ Mn １－ｙ２－ｚ２ M ｚ２ P.O. ４ (2) (wherein M is at least one selected from Mg, Ca, Co, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, Sc, and Y, and 0.9<x2<1.1, 0.05≦y2≦1.0, and 0≦z2≦0.14.)

[0040] The first aggregate and the second aggregate can be independently formed using lithium metal phosphate particles of different compositions, but can also be formed using lithium metal phosphate particles of the same composition. The mixture of the first aggregate and the second aggregate is not particularly limited, and can be, for example, a mass ratio (a1:a2) of the mass of the first aggregate (a1) to the mass of the second aggregate (a2) of 1:9 to 9:1. In other words, the a1 / a2 ratio can be 1 / 9 to 9 / 1.

[0041] The lithium metal phosphate particles are LiFePO ４In this case, the mass ratio (a1:a2) of the mass (a1) of the first aggregates to the mass (a2) of the second aggregates is preferably 1:9 to 5:5 (a1 / a2 is 1 / 9 to 5 / 5) from the viewpoint of improving the capacity retention rate, and more preferably 1:9 to 4:6 (a1 / a2 is 1 / 9 to 4 / 6) from the viewpoint of improving both the capacity retention rate and the discharge capacity ratio. ０．３ Mn ０．７ P.O. ４ In this case, from the viewpoint of improving the capacity retention rate, the mass ratio (a1:a2) of the mass (a1) of the first aggregates to the mass (a2) of the second aggregates is preferably 2:8 to 8:2 (a1 / a2 is 2 / 8 to 8 / 2), and more preferably 3:7 to 7:3 (a1 / a2 is 3 / 7 to 7 / 3).

[0042] [Particle Structure] The positive electrode material of this embodiment is composed of secondary particles, which are aggregates of the positive electrode active material particles, which are the primary particles described above. The positive electrode material of this embodiment may also contain independent primary particles. The shape of the primary particles of the positive electrode active material particles is not particularly limited, but is preferably spherical, particularly true spherical. The spherical shape of the primary particles of the positive electrode active material particles reduces the amount of solvent used when preparing the positive electrode material paste and makes it easier to apply the positive electrode material paste to a current collector. The positive electrode material paste can be prepared, for example, by mixing the positive electrode material of this embodiment with carbon particles, a binder resin (binding agent), a solvent, and the like.

[0043] The average particle size of the aggregates can be 0.5 μm or more and 50 μm or less for both the first aggregates and the second aggregates. By setting the average particle size of the aggregates to 0.5 μm or more, preferably 1.0 μm or more, and more preferably 1.5 μm or more, the need for large amounts of conductive additive and binder can be reduced when preparing a positive electrode material paste by mixing a positive electrode material, a conductive additive, a binder, and a solvent. This can increase the battery capacity per unit mass of the positive electrode mixture layer in the positive electrode of a lithium ion secondary battery. On the other hand, by setting the average particle size of the aggregates to 50 μm or less, preferably 25 μm or less, and more preferably 15 μm or less, the dispersibility and uniformity of the conductive additive and binder in the positive electrode mixture layer can be increased. This can increase the discharge capacity during high-speed charge / discharge of the lithium ion secondary battery.

[0044] The lithium metal phosphate particles are LiFePO ４ In this case, the average particle size of the first aggregates is more preferably 5 μm or more and 15 μm or less, and even more preferably 9 μm or more and 15 μm or less, from the viewpoint of improving the capacity retention rate; and the average particle size of the second aggregates is more preferably 3 μm or more and 13 μm or less, and even more preferably 5 μm or more and 10 μm or less, from the viewpoint of improving the capacity retention rate. ０．３ Mn ０．７ P.O. ４ In this case, the average particle diameters of both the first aggregates and the second aggregates are more preferably 5 μm or more and 15 μm or less, and even more preferably 7 μm or more and 13 μm or less, from the viewpoint of improving the capacity retention rate.

[0045] The average particle size of the aggregates can be determined by observing the cross-sections of the particles using a scanning electron microscope (SEM). For example, the positive electrode material according to this embodiment is embedded in a resin or the like, processed so that the cross-section of the positive electrode material can be observed, and then the cross-section of the positive electrode material is observed using an SEM. For 100 or more aggregates arbitrarily selected in the cross-sectional observation, the maximum and minimum diameters are measured, and the average of the maximum and minimum diameters is taken as the particle size of each aggregate. The average particle size can be determined by averaging the particle diameters obtained from the measured aggregates.

[0046] [Carbonaceous Coating] The positive electrode active material particles constituting the positive electrode material of this embodiment have a carbonaceous coating formed on the surface of lithium metal phosphate particles represented by general formula (1). Carbon is typically conductive, so the carbonaceous coating, carbon particles, and various carbon materials are also conductive. The carbonaceous coating can have a thickness of 1.0 nm or more, preferably 1.1 nm or more. A carbonaceous coating thickness of 1.0 nm or more can prevent the sum of electron migration resistances in the carbonaceous coating from increasing. This can prevent an increase in the internal resistance of the lithium ion secondary battery and a voltage drop at high-speed charge / discharge rates. On the other hand, the carbonaceous coating can have a thickness of 10.0 nm or less, preferably 7.0 nm or less. A carbonaceous coating thickness of 10.0 nm or less can prevent the formation of steric hindrances that hinder the diffusion of lithium ions through the carbonaceous coating, thereby reducing the lithium ion migration resistance. This can prevent an increase in the internal resistance of the battery and a voltage drop at high-speed charge / discharge rates.

[0047] The lithium metal phosphate particles are LiFePO ４ In this case, the thickness of the carbonaceous coating is more preferably 1.1 nm or more and 6.0 nm or less, and even more preferably 1.1 nm or more and 5.5 nm or less, from the viewpoint of improving the capacity retention rate. ０．３ Mn ０．７ P.O. ４ In this case, from the viewpoint of improving the capacity retention rate, the thickness of the carbonaceous coating is more preferably 1.5 nm or more and 5.0 nm or less, and even more preferably 1.7 nm or more and 4.0 nm or less.

[0048] The coverage of the carbonaceous coating on the lithium metal phosphate particles is not particularly limited, but is preferably 60% or more, and more preferably 80% or more. A coverage of 60% or more ensures a sufficient coverage effect of the carbonaceous coating. The thickness of the carbonaceous coating can be determined by preparing a thin film sample by cross-sectionally processing the positive electrode material, measuring the thickness of the carbonaceous coating on the surface of multiple primary particles using a transmission electron microscope (TEM), and averaging the measured values. The coverage of the carbonaceous coating can also be determined by observing the particles using a transmission electron microscope (TEM), an energy dispersive X-ray microanalyzer (EDX), or the like, calculating the proportion of the surface of the particles that are covered, and averaging the measured values.

[0049] The positive electrode material of this embodiment can have a carbon content of 0.7% by mass or more and 3.0% by mass or less. By setting the carbon content of the positive electrode material to 0.7% by mass or more, the number of contact points between the carbonaceous coatings can be ensured, thereby enabling the formation of sufficient conductive paths. This improves the electron conduction rate and maintains the battery capacity even when the charge / discharge rate of the lithium ion battery increases. Furthermore, by setting the carbon content to 3.0% by mass or less, the distance that lithium ions travel through the carbonaceous coating during the charge / discharge reaction (battery reaction) can be shortened, thereby suppressing a decrease in the lithium ion migration rate and maintaining the battery capacity.

[0050] From the above viewpoint, the carbon content of the positive electrode material is more preferably 0.8 mass % or more and 3.0 mass % or less, and further preferably 0.9 mass % or more and 2.9 mass % or less. Here, the carbon content can be quantitatively analyzed using a carbon analyzer and is expressed as the carbon content in the positive electrode material.

[0051] The lithium metal phosphate particles are LiFePO ４ In this case, the carbon content in the positive electrode material is more preferably 1.5% by mass or more and 2.9% by mass or less, and even more preferably 2.0% by mass or more and 2.9% by mass or less, from the viewpoint of improving the capacity retention rate. ０．３ Mn ０．７ P.O.４ In this case, from the viewpoint of improving the capacity retention rate, the carbon content in the positive electrode material is more preferably 1.0 mass % or more and 2.7 mass % or less, and even more preferably 1.0 mass % or more and 2.1 mass % or less.

[0052] [Specific Surface Area] The positive electrode material of this embodiment has a specific surface area of ​​5 m ２ / g or more 35m ２ The specific surface area of ​​the positive electrode material can be set to 5 m / g or less. ２ By setting the specific surface area of ​​the positive electrode material to 35 m / g or more, the particle size of the primary particles constituting the positive electrode material becomes fine, shortening the time required for the movement of lithium ions and electrons, thereby increasing the capacity during operation at a large current and at a low temperature. ２ / g or less, the increase in viscosity of the positive electrode material paste can be suppressed. ２ / g or more 34m ２ / g or less is more preferable, ２ / g or more 33.5m ２ It is more preferable that the SiO2 content is 1 / g or less.

[0053] The lithium metal phosphate particles are LiFePO ４ In this case, the specific surface area of ​​the positive electrode material is set to 10 m from the viewpoint of improving the capacity retention rate. ２ / g or more 33.5m ２ / g or less, and ２ / g or more 33.5m ２ / g or less. ０．３ Mn ０．７ P.O. ４ In this case, the specific surface area of ​​the positive electrode material is set to 12 m from the viewpoint of improving the capacity retention rate. ２ / g or more 30m ２ / g or less, and more preferably 16m ２ / g or more 26m ２ It is even more preferable that the SiO2 content is 1 / g or less.

[0054] [Tap Density] The positive electrode material of this embodiment has a tap density of 1.0 g / cm ３2.0g / cm or more ３ The tap density of the positive electrode material can be set to 1.0 g / cm or less. ３ By setting the tap density of the positive electrode material to 2.0 g / cm or more, the contact area between the positive electrode active material and the electrolyte solution does not become too large, and metal elution from the positive electrode active material can be suppressed. ３ From the above viewpoint, the tap density of the positive electrode material is set to 1.1 g / cm or less, whereby the contact area between the positive electrode active material and the electrolyte solution is increased, which facilitates the insertion and desorption of lithium ions into and from the positive electrode active material, thereby increasing the capacity. ３ 1.9g / cm or more ３ More preferably, it is 1.2 g / cm or less. ３ 1.8g / cm or more ３ It is even more preferable that:

[0055] The lithium metal phosphate particles are LiFePO ４ In this case, the tap density of the positive electrode material is 1.2 g / cm from the viewpoint of improving the capacity retention rate. ３ 1.7g / cm or more ３ It is even more preferable that the lithium metal phosphate particles are LiFe ０．３ Mn ０．７ P.O. ４ In this case, the tap density of the positive electrode material is 1.2 g / cm from the viewpoint of improving the capacity retention rate. ３ 1.6g / cm or more ３ It is even more preferable that the concentration is 1.3 g / cm or less. ３ 1.5g / cm or more ３ Even more preferably, the following:

[0056] [Electrode Film Density] The positive electrode material for a lithium ion secondary battery of this embodiment has an electrode film density of 1.1 g / cm when the electrode film is formed. ３ 2.0g / cm or more ３ The electrode film density can be set to 1.1 g / cm or less. ３ By setting the density to 2.0 g / cm or more, a high energy density can be obtained in the lithium ion secondary battery. ３By setting the lithium metal phosphate particles to the following, sufficient contact with the electrolyte can be achieved, and the load characteristics can be further improved. ４ In this case, the electrode film density is set to 1.2 g / cm from the viewpoint of improving the capacity retention rate. ３ 1.9g / cm or more ３ It is preferable that the density is 1.2 g / cm or less. ３ 1.8g / cm or more ３ It is more preferable that the lithium metal phosphate particles are LiFe or less. ０．３ Mn ０．７ P.O. ４ In this case, the electrode film density is set to 1.1 g / cm from the viewpoint of improving the capacity retention rate. ３ 1.8g / cm or more ３ It is preferable that the density is 1.2 g / cm or less. ３ 1.7g / cm or more ３ More preferably, it is:

[0057] Here, the electrode film density can be determined by applying an electrode slurry, which is a mixture of the positive electrode material of this embodiment, carbon particles, and a binder in an N-methyl-2-pyrrolidinone (NMP) solvent, to an aluminum foil, drying the slurry, and measuring the density of the resulting electrode film. The electrode film density can be calculated by measuring the mass (Em) of the electrode film, as well as the area and thickness of the electrode film, and dividing the mass (Em) of the electrode film by the volume (Ev) of the electrode film calculated from the area and thickness of the electrode film (Em / Ev).

[0058] 2. Method for Producing a Positive Electrode Material for Lithium-Ion Secondary Batteries The method for producing a positive electrode material for lithium-ion secondary batteries of this embodiment is not particularly limited, and may include, for example, step (A) of obtaining lithium metal phosphate particles, step (B) of adding an organic compound to the lithium metal phosphate particles obtained in step (A) to prepare a mixture, step (C) of placing the mixture in a firing sheath and firing it, and step (D) of mixing the positive electrode material obtained in step (C). The method for producing a positive electrode material for lithium-ion secondary batteries of this embodiment may further include, as desired, a crushing step for adjusting the average particle size of the aggregates (secondary particles).

[0059] [Step (A)] In step (A), the method for producing the lithium metal phosphate particles is not particularly limited, and for example, a hydrothermal synthesis method can be used to obtain lithium metal phosphate particles represented by the following general formula (1). The shape of the obtained lithium metal phosphate particles can be, for example, particulate. Li ｘ A ｙ D ｚ P.O. ４ (1) However, A is at least one selected from the group consisting of Co, Mn, Ni, Fe, Cu, and Cr, D is at least one selected from the group consisting of Mg, Ca, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, Sc, and Y, and 0.9<x<1.1, 0<y≦1, 0≦z<1, 0.9<y+z<1.1.

[0060] In the hydrothermal synthesis method, for example, a slurry mixture obtained by mixing the components of general formula (1), namely, a Li source, an A source, a P source, water, and optionally a D source, is hydrothermally synthesized. According to the hydrothermal synthesis, lithium metal phosphate particles are formed as a precipitate in water. The obtained precipitate may be a precursor of lithium metal phosphate particles. In this case, the desired lithium metal phosphate particles are obtained by calcining the precursor of the lithium metal phosphate particles. A pressure-resistant sealed container is preferably used for the hydrothermal synthesis. By pressurizing and heating in the pressure-resistant sealed container, lithium metal phosphate particles or their precursors are synthesized by a hydrothermal reaction, and by selecting the conditions, a first aggregate or a second aggregate can be obtained.

[0061] As reaction conditions for the hydrothermal synthesis to obtain the first aggregate, for example, the heating temperature can be preferably 170°C or higher and 200°C or lower, more preferably 175°C or higher and 195°C or lower. On the other hand, as reaction conditions for the hydrothermal synthesis to obtain the second aggregate, for example, the heating temperature can be preferably 120°C or higher and 170°C or lower, more preferably 150°C or higher and 165°C or lower. Furthermore, when obtaining the first aggregate, the reaction time can be preferably 2 hours or higher and 96 hours or lower, more preferably 3 hours or higher and 72 hours or lower, and even more preferably 4 hours or higher and 48 hours or lower. On the other hand, when obtaining the second aggregate, the reaction time can be preferably 2 hours or higher and 48 hours or lower, more preferably 3 hours or higher and 24 hours or lower, and even more preferably 4 hours or higher and 12 hours or lower. Furthermore, the pressure during the reaction is preferably 0.1 MPa or higher and 22 MPa or lower, more preferably 0.1 MPa or higher and 17 MPa or lower.

[0062] By setting the reaction conditions within the above ranges, it is possible to obtain positive electrode active material particles that constitute the first aggregates or the second aggregates. The molar ratio of the Li source, the A source, the D source, and the P source (Li:A:D:P) is preferably 2.5 to 4.0:0 to 1.0:0 to 1.0:0.9 to 1.15, and more preferably 2.8 to 3.5:0 to 1.0:0 to 1.0:0.95 to 1.1.

[0063] Here, examples of the Li source include hydroxides such as lithium hydroxide (LiOH); ２ CO ３ ), lithium chloride (LiCl), lithium nitrate (LiNO ３ ), lithium phosphate (Li ３ P.O. ４ ), dilithium hydrogen phosphate (Li ２ HPO ４ ) and lithium dihydrogen phosphate (LiH ２ P.O. ４ ) and other lithium inorganic acid salts; lithium acetate (LiCH ３ COO), lithium oxalate ((COOLi) ２ It is preferable to use at least one selected from the group consisting of lithium organic acid salts such as lithium phosphate (Li ) and the like; and hydrates thereof.３ P.O. ４ ) can also be used as a Li source and a P source.

[0064] Examples of the A source include chlorides, carboxylates, sulfates, etc. containing at least one element selected from the group consisting of Co, Mn, Ni, Fe, Cu, and Cr. For example, Li ｘ A ｙ D ｚ P.O. ４ When A is Fe, the Fe source may be iron (II) chloride (FeCl ２ ), iron(II) sulfate (FeSO ４ ), iron(II) acetate (Fe(CH ３ COO) ２ iron compounds or hydrates thereof, iron nitrate (III) (Fe(NO) ３ ) ３ ), iron(III) chloride (FeCl ３ ), iron(III) citrate (FeC ６ H ５ O ７ ), metallic iron, lithium iron phosphate, and the like.

[0065] Examples of the D source include chlorides, carboxylates, sulfates, etc. containing at least one element selected from the group consisting of Mg, Ca, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, Sc, and Y. For example, Li ｘ A ｙ D ｚ P.O. ４ When D is Ca, the Ca source may be calcium hydroxide (II) (Ca(OH) ２ ), calcium chloride (II) (CaCl ２ ), calcium sulfate (II) (CaSO ４ ), calcium nitrate (II) (Ca(NO ３ ) ２ ), calcium acetate (Ca(CH ３ COO) ２ ), and hydrates thereof.

[0066] The P source is phosphoric acid (H ３ P.O. ４), ammonium dihydrogen phosphate (NH ４ H ２ P.O. ４ ), diammonium hydrogen phosphate ((NH ４ ) ２ HPO ４ Among these, it is preferable to use, as the P source, at least one selected from the group consisting of phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate.

[0067] [Step (B)] In step (B), an organic compound is added to the lithium metal phosphate particles obtained in step (A) to prepare a mixture. First, the organic compound is added to the lithium metal phosphate particles, and then a solvent is added to form a slurry. The amount of the organic compound relative to the lithium metal phosphate particles, when the total mass of the organic compound is converted to elemental carbon, is preferably 0.15 to 15 parts by mass, more preferably 0.45 to 4.5 parts by mass, per 100 parts by mass of the lithium metal phosphate particles. When the amount of the organic compound is 0.15 parts by mass or more relative to 100 parts by mass of the lithium metal phosphate particles, the coverage of the carbonaceous coating formed by thermal decomposition of the organic compound on the surface of the lithium metal phosphate particles can be 80% or more. This can improve the high input characteristics and cycle characteristics of lithium-ion batteries.

[0068] On the other hand, by setting the blending amount of the organic compound to 15 parts by mass or less per 100 parts by mass of lithium metal phosphate particles, it is possible to prevent a relative decrease in the blending ratio of the lithium metal phosphate particles, which would result in a decrease in the capacity of the lithium ion battery. Furthermore, by setting the blending amount of the organic compound to 15 parts by mass or less per 100 parts by mass of lithium metal phosphate particles, it is possible to prevent a decrease in the bulk density of the positive electrode material, which would be caused by excessive support of the carbonaceous coating on the lithium metal phosphate particles. Furthermore, by preventing a decrease in the bulk density of the positive electrode material, it is possible to prevent a decrease in the electrode film density and thus a decrease in the capacity of the lithium ion battery per unit volume.

[0069] The organic compound used to prepare the mixture is not particularly limited as long as it is a compound capable of forming a carbonaceous coating on the surface of lithium metal phosphate particles, and examples thereof include polyvinyl alcohol (PVA), polyvinylpyrrolidone, cellulose, starch, gelatin, carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, polyacrylic acid, polystyrene sulfonic acid, polyacrylamide, polyvinyl acetate, phenol, phenolic resin, glucose, fructose, galactose, mannose, maltose, sucrose, lactose, glycogen, pectin, alginic acid, glucomannan, chitin, hyaluronic acid, chondroitin, agarose, polyether, and polyhydric alcohol. Examples of polyhydric alcohols include polyethylene glycol, polypropylene glycol, polyglycerin, and glycerin. These may be used alone or in combination of two or more.

[0070] Here, using a low-molecular-weight organic compound such as sucrose or lactose as the organic compound makes it easier to form a carbonaceous coating evenly on the surface of the primary particles of the lithium metal phosphate particles, but on the other hand, the carbonization degree of the carbonaceous coating obtained by pyrolysis tends to be low, making it difficult to form a carbonaceous coating that can achieve a sufficient reduction in resistance. Furthermore, using such a low-molecular-weight organic compound increases the number of micropores in the carbonaceous coating, thereby increasing the overall micropore ratio.

[0071] On the other hand, the use of polymer organic compounds such as polyvinyl alcohol and polyvinylpyrrolidone, or organic compounds having a benzene ring structure such as phenolic resin, tends to increase the degree of carbonization of the carbonaceous coating obtained by pyrolysis, thereby achieving a sufficient reduction in resistance, but it tends to make it difficult to form a carbonaceous coating evenly over the surface of the primary particles of the positive electrode material, making it difficult to achieve a sufficient reduction in resistance in lithium-ion battery positive electrode materials. Furthermore, the use of such polymer organic compounds or organic compounds having a benzene ring structure reduces the number of micropores in the carbonaceous coating, thereby lowering the overall micropore ratio.

[0072] Therefore, it is preferable to use an appropriate mixture of a low-molecular-weight organic compound, a high-molecular-weight organic compound, and an organic compound having a benzene ring structure. In particular, it is preferable to use a low-molecular-weight organic compound in powder form, because this facilitates mixing of the lithium metal phosphate particles with the organic compound and allows for the production of positive electrode active material particles in which a carbonaceous coating is evenly formed on the primary particle surfaces of the lithium metal phosphate particles. Furthermore, unlike high-molecular-weight organic compounds, low-molecular-weight organic compounds are easily dissolved in solution, eliminating the need for prior dissolution, thereby reducing the number of work steps and the cost of the dissolution process.

[0073] When adding a solvent to lithium metal phosphate particles, the solids content of the lithium metal phosphate particle-containing solvent is adjusted to preferably 10 to 60 mass %, more preferably 15 to 55 mass %, and even more preferably 25 to 50 mass %. By adjusting the solids content within the above range, the tap density of the resulting lithium ion battery positive electrode material can be adjusted to be within the above range.

[0074] Examples of the solvent include water; alcohols such as methanol, ethanol, 1-propanol, 2-propanol (isopropyl alcohol: IPA), butanol, pentanol, hexanol, octanol, and diacetone alcohol; esters such as ethyl acetate, butyl acetate, ethyl lactate, propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate, and γ-butyrolactone; ethers such as diethyl ether, ethylene glycol monomethyl ether (methyl cellosolve), ethylene glycol monoethyl ether (ethyl cellosolve), ethylene glycol monobutyl ether (butyl cellosolve), diethylene glycol monomethyl ether, and diethylene glycol monoethyl ether; ketones such as acetone, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), acetylacetone, and cyclohexanone; amides such as dimethylformamide, N,N-dimethylacetoacetamide, and N-methylpyrrolidone; and glycols such as ethylene glycol, diethylene glycol, and propylene glycol. These solvents may be used alone or in combination of two or more. Among these solvents, water is preferred. If necessary, a dispersant may be added.

[0075] The method for dispersing lithium metal phosphate particles and an organic compound in a solvent is not particularly limited, as long as the lithium metal phosphate particles are uniformly dispersed and the organic compound is dissolved or dispersed. Examples of devices used for such dispersion include media-agitation dispersers that agitate media particles at high speed, such as planetary ball mills, vibration ball mills, bead mills, paint shakers, and attritors. In particular, when obtaining a second positive electrode material, it is preferable to grind the material using a planetary ball mill, vibration ball mill, bead mill, or the like. The grinding time is not particularly limited, but can be, for example, 24 hours or more and 360 hours or less.

[0076] The lithium metal phosphate particles and the organic compound are slurried, and then the mixture is dried and granulated to prepare a mixture. The method of drying and granulation is not particularly limited, but for example, spray pyrolysis may be used to spray the slurry into the atmosphere at 110°C or higher and 200°C or lower, and then dried to produce granules of the mixture. In this spray pyrolysis method, the particle size of the droplets during spraying is preferably 0.01 μm or higher and 100 μm or lower, in order to quickly dry and produce approximately spherical granules.

[0077] [Step (C)] In step (C), the mixture obtained in step (B) is placed in a firing container, such as a firing sheath, and fired. A firing sheath made of a material with excellent thermal conductivity, such as carbon, is preferably used as the firing sheath. The firing temperature is preferably 600°C or higher and 790°C or lower, more preferably 640°C or higher and 770°C or lower. By selecting the firing conditions, a first positive electrode material or a second positive electrode material can be obtained. The firing temperature for obtaining the first aggregate can be preferably 690°C or higher and 790°C or lower, more preferably 700°C or higher and 770°C or lower. On the other hand, the firing temperature for obtaining the second aggregate can be preferably 600°C or higher and 680°C or lower, more preferably 630°C or higher and 670°C or lower.

[0078] When the firing temperature is 630°C or higher, the decomposition and reaction of the organic compound proceeds sufficiently, allowing the organic compound to be sufficiently carbonized. As a result, a low-resistance carbonaceous coating can be formed on the obtained positive electrode active material particles. On the other hand, when the firing temperature is 790°C or lower, particle growth of the lithium-ion battery positive electrode material does not proceed, allowing a sufficiently high specific surface area to be maintained. As a result, when a lithium-ion battery is formed, the discharge capacity at high-speed charge / discharge rates is large, and sufficient charge / discharge rate performance can be achieved.

[0079] The baking time is not particularly limited as long as it is a time that allows the organic compound to be sufficiently carbonized, and is, for example, from 0.1 hours to 100 hours.

[0080] The firing atmosphere is preferably nitrogen (N ２) and argon (Ar), or an inert atmosphere of an inert gas such as hydrogen (H ２ The calcination atmosphere is a reducing atmosphere containing a reducing gas such as argon. When oxidation of the mixture is to be further suppressed, the calcination atmosphere is preferably a reducing atmosphere. In the calcination of step (C), the organic compound is decomposed and reacted by the calcination to produce carbon. This carbon then adheres to the surface of the lithium metal phosphate particles to form a carbonaceous coating. As a result, the surfaces of the lithium metal phosphate particles are covered with a carbonaceous coating, forming positive electrode active material particles.

[0081] After the firing in step (C), the fired product may exhibit some agglomeration. Therefore, a crushing step may be performed to crush the fired product after firing. Crushing here refers to the process of applying mechanical energy to a fired product consisting of multiple secondary particles formed by sintering necking between secondary particles during firing, separating the secondary particles without substantially destroying the secondary particles themselves, thereby loosening the fired product. In the crushing step, for example, a pin mill, hammer mill, pulverizer, or the like may be used to crush the secondary particles to a degree that does not destroy them. After the crushing step, sieving or the like may be performed as needed to adjust the particle size distribution.

[0082] [Step (D)] In step (D), the first aggregates obtained by the firing in step (C) are mixed with the second aggregates. The mixing ratio of the first aggregates to the second aggregates is not particularly limited and may be selected depending on the properties required for the positive electrode material. For example, the mass ratio (a1:a2) of the mass of the first aggregates (a1) to the mass of the second aggregates (a2) can be 1:9 to 9:1. In other words, the a1 / a2 ratio can be 1 / 9 to 9 / 1. When the lithium metal phosphate particles are LiFePO ４ In this case, the mass ratio (a1:a2) of the mass (a1) of the first aggregates to the mass (a2) of the second aggregates is preferably 1:9 to 5:5 (a1 / a2 is 1 / 9 to 5 / 5) from the viewpoint of improving the capacity retention rate, and more preferably 1:9 to 4:6 (a1 / a2 is 1 / 9 to 4 / 6) from the viewpoint of improving both the capacity retention rate and the discharge capacity ratio. ０．３ Mn ０．７ P.O.４ In this case, from the viewpoint of improving the capacity retention rate, the mass ratio (a1:a2) of the mass (a1) of the first aggregates to the mass (a2) of the second aggregates is preferably 2:8 to 8:2 (a1 / a2 is 2 / 8 to 8 / 2), and more preferably 3:7 to 7:3 (a1 / a2 is 3 / 7 to 7 / 3).

[0083] 3. Lithium-ion secondary battery The lithium-ion secondary battery of this embodiment is a lithium-ion secondary battery having a positive electrode, a negative electrode, and an electrolyte, and the positive electrode contains the lithium-ion battery positive electrode material of this embodiment. Hereinafter, the lithium-ion secondary battery may be simply referred to as a "battery."

[0084] [Positive Electrode] In preparing the positive electrode, the positive electrode material for a lithium ion secondary battery according to the present embodiment, which contains an aggregate composition including the first aggregate and the second aggregate, a binder made of a binder resin, and a solvent are mixed to prepare a positive electrode mixture paste. At this time, a conductive additive such as carbon black, acetylene black, graphite, ketjen black, natural graphite, or artificial graphite may be added as necessary.

[0085] As the binder, that is, the binder resin, for example, polytetrafluoroethylene (PTFE) resin, polyvinylidene fluoride (PVdF) resin, fluororubber, etc. are preferably used.

[0086] The compounding ratio of the positive electrode material compound to the binder resin is not particularly limited, but for example, the binder resin is used in an amount of 1 to 30 parts by mass, preferably 3 to 20 parts by mass, per 100 parts by mass of the positive electrode material.

[0087] The solvent used in the positive electrode mixture paste may be appropriately selected in accordance with the properties of the binder resin. Examples of the solvent include water, alcohols such as methanol, ethanol, 1-propanol, 2-propanol (isopropyl alcohol: IPA), butanol, pentanol, hexanol, octanol, and diacetone alcohol; esters such as ethyl acetate, butyl acetate, ethyl lactate, propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate, and γ-butyrolactone; ethers such as diethyl ether, ethylene glycol monomethyl ether (methyl cellosolve), ethylene glycol monoethyl ether (ethyl cellosolve), ethylene glycol monobutyl ether (butyl cellosolve), diethylene glycol monomethyl ether, and diethylene glycol monoethyl ether; ketones such as acetone, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), acetylacetone, and cyclohexanone; amides such as dimethylformamide, N,N-dimethylacetoacetamide, and N-methylpyrrolidone; and glycols such as ethylene glycol, diethylene glycol, and propylene glycol. These may be used alone or in combination of two or more.

[0088] The positive electrode composite paste is then applied to one side of an aluminum foil and dried to obtain an aluminum foil having a coating film made of a mixture of the positive electrode material and binder resin formed on one side thereof. The coating film is then pressed and compressed and dried to produce an electrode (positive electrode) having a positive electrode composite layer on one side of the aluminum foil.

[0089] In this manner, a positive electrode can be produced that enables the production of a lithium ion secondary battery having high input characteristics and excellent cycle characteristics.

[0090] [Negative Electrode] The negative electrode is a sheet-like member formed by applying a negative electrode composite paste to the surface of a metal foil current collector such as copper and then drying it. This negative electrode is formed substantially in the same manner as the positive electrode, although the components constituting the negative electrode composite paste, their composition, and the material of the current collector are different. As with the positive electrode, various treatments are performed as necessary. The negative electrode composite paste is prepared by adding an appropriate solvent to a negative electrode composite, which is a mixture of a negative electrode active material and a binder, to form a paste.

[0091] The negative electrode active material can be, for example, a lithium-containing material such as metallic lithium or a lithium alloy, or an occlusion material capable of occluding and desorbing lithium ions. The occlusion material is not particularly limited, but examples include natural graphite, artificial graphite, a fired organic compound such as phenolic resin, and a powder of a carbon material such as coke. When such an occlusion material is used as the negative electrode active material, a fluorine-containing resin such as PVDF can be used as the binder, as in the positive electrode, and an organic solvent such as N-methyl-2-pyrrolidone can be used as the solvent for dispersing the negative electrode active material in the binder.

[0092] [Separator] The separator is disposed between the positive electrode and the negative electrode when a non-aqueous electrolyte solution is used, and has the function of separating the positive electrode and the negative electrode and retaining the electrolyte. The separator may be, for example, a thin membrane made of polyethylene or polypropylene having a large number of fine pores, but is not particularly limited as long as it has the above function.

[0093] [Non-aqueous electrolyte] As the non-aqueous electrolyte, for example, a non-aqueous electrolyte solution can be used. As the non-aqueous electrolyte, for example, a solution in which a lithium salt as a supporting salt is dissolved in an organic solvent can be used. Furthermore, as the non-aqueous electrolyte, a solution in which a lithium salt is dissolved in an ionic liquid can also be used. Note that the ionic liquid refers to a salt that is composed of cations and anions other than lithium ions and is liquid even at room temperature (25°C).

[0094] Examples of organic solvents that can be used include cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate; chain carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate; ether compounds such as tetrahydrofuran, 2-methyltetrahydrofuran, and dimethoxyethane; sulfur compounds such as ethyl methyl sulfone and butane sultone; and phosphorus compounds such as triethyl phosphate and trioctyl phosphate. One organic solvent selected from the above group of compounds may be used alone, or two or more organic solvents may be used in combination.

[0095] The supporting salt is LiPF ６ , LiBF ４ , LiClO ４ , LiAsF ６ , LiN(CF ３ SO ２ ) ２ , and composite salts thereof can be used. Furthermore, the non-aqueous electrolyte solution may contain a radical scavenger, a surfactant, a flame retardant, etc. The supporting salt may be used alone or in combination of two or more. Furthermore, a solid electrolyte may be used as the non-aqueous electrolyte. The solid electrolyte has the property of being able to withstand high voltage. Examples of the solid electrolyte include inorganic solid electrolytes and organic solid electrolytes.

[0096] Examples of inorganic solid electrolytes include oxide-based solid electrolytes and sulfide-based solid electrolytes. The oxide-based solid electrolyte is not particularly limited, and for example, an oxide-based solid electrolyte containing oxygen (O) and having lithium ion conductivity and electronic insulation can be suitably used. Specifically, the oxide-based solid electrolyte can be, for example, lithium phosphate (Li ３ P.O. ４ ), Li ３ P.O. ４ N Ｘ , LiBO ２ N Ｘ , LiNbO ３ , LiTaO ３ , Li ２ SiO ３, Li ４ SiO ４ -Li ３ P.O. ４ , Li ４ SiO ４ -Li ３ VO ４ , Li ２ Alumni ２ O ３ -P ２ O ５ , Li ２ O—SiO ２ , Li ２ Alumni ２ O ３ -ZnO, Li １＋Ｘ Al Ｘ Ti ２－Ｘ (P.O. ４ ) ３ (0≦X≦1), Li １＋Ｘ Al Ｘ Ge ２－Ｘ (P.O. ４ ) ３ (0≦X≦1), LiTi ２ (P.O. ４ ) ３ , Li ３Ｘ La ２／３－Ｘ TiO ３ (0≦X≦2 / 3), Li ５ La ３ Ta ２ O １２ , Li ７ La ３ Zr ２ O １２ , Li ６ BaLa ２ Ta ２ O １２ , Li ３．６ Si ０．６ P ０．４ O ４ The oxide-based solid electrolyte may be one or more selected from these.

[0097] The sulfide-based solid electrolyte is not particularly limited, and for example, a sulfide-based solid electrolyte containing sulfur (S) and having lithium ion conductivity and electronic insulation can be suitably used. Specifically, the sulfide-based solid electrolyte can be, for example, Li ２ S-P ２ S５ , Li ２ S-SiS ２ , LiI-Li ２ S-SiS ２ , LiI-Li ２ S-P ２ S ５ , LiI-Li ２ S-B ２ S ３ , Li ３ P.O. ４ -Li ２ S-Si ２ S., Li. ３ P.O. ４ -Li ２ S-SiS ２ , LiPO ４ -Li ２ S-SiS, LiI-Li ２ S-P ２ O ５ , LiI-Li ３ P.O. ４ -P ２ S ５ The sulfide-based solid electrolyte may be one or more selected from these. Note that the inorganic solid electrolyte may be other than those mentioned above, for example, Li ３ N, LiI, Li ３ N-LiI-LiOH or the like may also be used.

[0098] The organic solid electrolyte is not particularly limited as long as it is a polymer compound exhibiting ion conductivity, and examples thereof include polyethylene oxide, polypropylene oxide, and copolymers thereof. The organic solid electrolyte may also contain a supporting salt (lithium salt). Only one type of organic solid electrolyte may be used, or two or more types may be used.

[0099] [Shape and Configuration of Secondary Battery] As described above, the battery of this embodiment can be formed into various shapes, such as a cylindrical shape or a laminated shape. Regardless of the shape, when the secondary battery of this embodiment uses a nonaqueous electrolyte solution as the nonaqueous electrolyte, the positive electrode and the negative electrode are laminated via a separator to form an electrode assembly. The resulting electrode assembly is impregnated with a nonaqueous electrolyte solution, and the positive electrode current collector and the positive electrode terminal connected to the outside, and the negative electrode current collector and the negative electrode terminal connected to the outside are connected using current collecting leads or the like, and the battery can be sealed in a battery case. Note that the battery of this embodiment is not limited to a configuration using a nonaqueous electrolyte solution as the nonaqueous electrolyte. For example, it can also be a secondary battery using a solid nonaqueous electrolyte, i.e., an all-solid-state battery. When forming an all-solid-state battery, the configuration other than the positive electrode material can be appropriately changed as needed.

[0100] As described above, the battery according to this embodiment uses the positive electrode material according to this embodiment as a positive electrode material, and therefore has excellent battery capacity and input / output characteristics. Therefore, the battery according to this embodiment can be suitably used as a chargeable / dischargeable battery for mobile information terminals such as mobile phones, smartphones, tablets, and notebook computers, portable music players, digital cameras, medical devices, clean energy automobiles such as hybrid electric vehicles (HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles (PHEVs), etc.

[0101] EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples, but the present invention is not limited to the embodiments described in the Examples.

[0102] [Production of a Lithium Ion Battery Positive Electrode Material] (Example 1) An olivine-type compound LiFePO ４ was produced. Li was used as the Li source and P source. ３ P.O. ４ FeSO as an Fe source ４An aqueous solution was used, and these were mixed so that the molar ratio of Li:Fe:P was 3:1:1 to prepare 2.2 L of raw material slurry A1. Next, the raw material slurry A1 was placed in a pressure-resistant container and subjected to a heating reaction at 190°C for 48 hours to perform hydrothermal synthesis. After the reaction, the atmosphere in the heat-resistant container was cooled to room temperature, and a cake-like precipitate of the reaction product was obtained.

[0103] The precipitate was thoroughly washed multiple times with distilled water and kept at a moisture content of 40% to prevent drying, to obtain a cake-like substance. The cake-like substance was vacuum-dried at 70° C. for 2 hours to obtain LiFePO ４ A polyacrylic acid aqueous solution as a first carbon source in an amount of 4.5% by mass in terms of solid content relative to 95% by mass of the particles and a water-dispersible phenolic resin as a second carbon source in an amount of 0.5% by mass in terms of solid content were dispersed in an aqueous solvent to obtain raw material slurry α1.

[0104] The raw material slurry α1 was dried and granulated, and then nitrogen (N ２ ) atmosphere at 735° C. for 2 hours. This gave first aggregates whose particle surfaces were covered with a carbonaceous film.

[0105] Furthermore, the raw material slurry A1 placed in a pressure-resistant container was subjected to a heating reaction at 160°C for 4 hours to perform hydrothermal synthesis. After the reaction, the atmosphere in the heat-resistant container was cooled to room temperature, and a cake-like precipitate of the reaction product was obtained. This precipitate was thoroughly washed multiple times with distilled water and kept at a moisture content of 40% to prevent drying, resulting in a cake-like substance. This cake-like substance was vacuum-dried at 70°C for 2 hours to obtain LiFePO ４ A polyacrylic acid aqueous solution as a first carbon source (4.5% by mass in terms of solid content) and a water-dispersible phenolic resin as a second carbon source (0.5% by mass in terms of solid content) were dispersed in an aqueous solvent relative to 95% by mass of the particles, and the solid content was adjusted to 20% by mass. After that, the mixture was ground for 280 hours using a bead mill with a bead diameter of 0.1 mm, to obtain raw material slurry α2.

[0106] The raw material slurry α2 was dried and granulated, and then nitrogen (N ２The mixture was then baked at 650°C for 2 hours in a sintered atmosphere. This resulted in a second aggregate whose particle surfaces were coated with a carbonaceous film. The first aggregate and the second aggregate were weighed out to a blending ratio of 2:8 [mass (a1) of the first aggregate: mass (a2) of the second aggregate = 2:8] and mixed using a blender to obtain the positive electrode material composition of Example 1.

[0107] Example 2 A positive electrode material composition of Example 2 was obtained in the same manner as in Example 1, except that the first lithium ion battery positive electrode material and the second lithium ion battery positive electrode material obtained in Example 1 were weighed out so that the blending ratio (a1:a2) was 9:1 (mass of the first aggregates: mass of the second aggregates = 9:1).

[0108] Example 3 A first lithium-ion battery positive electrode material was obtained by setting the heating conditions of raw material slurry A1 in a pressure vessel during hydrothermal synthesis at 190°C for 20 hours, and the first aggregates and second aggregates were weighed out so that the blending ratio (a1:a2) of the first aggregates to the second aggregates was 5:5 (mass of the first aggregates: mass of the second aggregates = 5:5). The positive electrode material formulation of Example 3 was obtained in the same manner as in Example 1, except that the first aggregates and the second aggregates were weighed out so that the blending ratio (a1:a2) was 5:5 (mass of the first aggregates: mass of the second aggregates = 5:5).

[0109] (Example 4) A positive electrode material formulation of Example 4 was obtained in the same manner as in Example 3, except that the heating conditions of raw material slurry A1 in the pressure vessel during hydrothermal synthesis were set to 190°C and 10 hours to obtain a first lithium ion battery positive electrode material.

[0110] (Example 5) The positive electrode material composition of Example 5 was obtained in the same manner as Example 3, except that the heating conditions of raw material slurry A1 in the pressure-resistant container during hydrothermal synthesis were 190°C and 4 hours to obtain a first aggregate, and the grinding time in the bead mill was 48 hours to obtain a second aggregate.

[0111] Example 6 A positive electrode material formulation of Example 6 was obtained in the same manner as in Example 5, except that the heating conditions for raw material slurry A1 in the pressure vessel during hydrothermal synthesis were 180° C. and 8 hours to obtain a first aggregate.

[0112] (Example 7) The heating conditions for raw material slurry A1 in a pressure-resistant container during hydrothermal synthesis were 180°C for 5 hours to obtain a first aggregate, and the first aggregate and second aggregate were weighed out so that the compounding ratio (a1:a2) of the first aggregate to the second aggregate was 9:1 (mass of the first aggregate: mass of the second aggregate = 9:1). The positive electrode material composition of Example 7 was obtained in the same manner as in Example 5, except that

[0113] (Example 8) A positive electrode material composition of Example 8 was obtained in the same manner as in Example 7, except that the first aggregate and the second aggregate obtained in Example 7 were weighed out so that the compounding ratio (a1:a2) was 2:8 (mass of the first aggregate: mass of the second aggregate = 2:8).

[0114] (Example 9) Li as a Li source and a P source ３ P.O. ４ FeSO as an Fe source ４ The aqueous solution was treated with MnSO as a Mn source. ４ Using an aqueous solution, these were mixed so that the molar ratio of Li:Fe:Mn:P = 3:0.3:0.7:1 to prepare 2.2 L of raw material slurry B1. Next, the raw material slurry B1 placed in a pressure-resistant container was heated at 190 ° C for 6 hours to perform hydrothermal synthesis. After the reaction, the atmosphere in the heat-resistant container was cooled to room temperature, and a cake-like precipitate of the reaction product was obtained. This precipitate was thoroughly washed multiple times with distilled water and kept at a moisture content of 40% to prevent drying, resulting in a cake-like substance.

[0115] This cake-like substance was vacuum dried at 70°C for 2 hours to obtain LiFe ０．３ Mn ０．７ P.O. ４ A polyacrylic acid aqueous solution as a first carbon source in an amount of 4.5% by mass in terms of solid content relative to 95% by mass of the particles and a water-dispersible phenolic resin as a second carbon source in an amount of 0.5% by mass in terms of solid content were dispersed in an aqueous solvent to obtain raw material slurry β1.

[0116] The raw material slurry β1 was dried and granulated, and then nitrogen (N ２ ) atmosphere at 735° C. for 2 hours. This gave first aggregates whose particle surfaces were covered with a carbonaceous film.

[0117] Furthermore, the raw material slurry B1 placed in a pressure-resistant container was subjected to a heating reaction at 160°C for 4 hours to perform hydrothermal synthesis. After the reaction, the atmosphere in the heat-resistant container was cooled to room temperature, and a cake-like precipitate of the reaction product was obtained. This precipitate was thoroughly washed multiple times with distilled water and kept at a moisture content of 40% to prevent drying, resulting in a cake-like substance.

[0118] This cake-like substance was vacuum dried at 70°C for 2 hours to obtain LiFe ０．３ Mn ０．７ P.O. ４ A polyacrylic acid aqueous solution as a first carbon source was dispersed in an aqueous solvent at a solid content of 4.5% by mass relative to 95% by mass of the particles, and a water-dispersible phenolic resin as a second carbon source at a solid content of 0.5% by mass was dispersed in the aqueous solvent, and the solid content was adjusted to 20 wt %. After that, the raw material slurry β2 was ground for 280 hours using a bead mill with a bead diameter of 0.1 mm. After drying and granulating the raw material slurry β2, it was pulverized using a rotary kiln manufactured by Chugai Ro Engineering Co., Ltd. ２ ) atmosphere at 650° C. for 2 hours. This resulted in second aggregates whose particle surfaces were covered with a carbonaceous film.

[0119] The first aggregate and the second aggregate described above were weighed so that the compounding ratio (a1:a2) was 5:5 (mass of the first aggregate: mass of the second aggregate = 5:5), and mixed using a blender to obtain the positive electrode material compound of Example 9.

[0120] Example 10 A positive electrode material composition of Example 10 was obtained in the same manner as in Example 9, except that the heating conditions for raw material slurry B1 in the pressure vessel during hydrothermal synthesis were 180° C. and 6 hours to obtain a first aggregate.

[0121] Comparative Example 1 An olivine-type compound LiFePO ４ was produced. Li was used as the Li source and P source. ３ P.O. ４ FeSO as an Fe source ４Using an aqueous solution, these were mixed so that the molar ratio of Li:Fe:P was 3:1:1 to prepare 2.2 L of raw material slurry A1. Next, the raw material slurry A1 was placed in a pressure-resistant container and subjected to a heating reaction at 180 ° C for 1 hour to perform hydrothermal synthesis. After the reaction, the atmosphere in the heat-resistant container was cooled to room temperature, and a cake-like precipitate of the reaction product was obtained. This precipitate was thoroughly washed multiple times with distilled water and kept at a moisture content of 40% to prevent drying, resulting in a cake-like substance.

[0122] This cake-like substance was dried in vacuum at 70°C for 2 hours to obtain LiFePO ４ A glucose aqueous solution as a first carbon source (12.5% ​​by mass in terms of solid content) and a hydroxyethyl cellulose as a second carbon source (2.5% by mass in terms of solid content) were dispersed in an aqueous solvent relative to 95% by mass of the particles to obtain a raw material slurry γ1. The raw material slurry γ1 was dried and granulated, and then heated in a rotary kiln manufactured by Chugai Ro Engineering Co., Ltd. under nitrogen (N ２ ) atmosphere at 735° C. for 2 hours. As a result, a lithium ion battery positive electrode material of Comparative Example 1 in which the surfaces of the particles were covered with a carbonaceous film was obtained.

[0123] Comparative Example 2 An olivine-type compound LiFePO ４ was produced. Li was used as the Li source and P source. ３ P.O. ４ FeSO as an Fe source ４ An aqueous solution was used, and these were mixed so that the molar ratio of Li:Fe:P was 3:1:1 to prepare 2.2 L of raw material slurry A1. Next, the raw material slurry A1 was placed in a pressure-resistant container and subjected to a heating reaction at 170°C for 1 hour to perform hydrothermal synthesis. After the reaction, the atmosphere in the heat-resistant container was cooled to room temperature, and a cake-like precipitate of the reaction product was obtained.

[0124] The precipitate was thoroughly washed multiple times with distilled water and kept at a moisture content of 40% to prevent drying, to obtain a cake-like substance. The cake-like substance was vacuum-dried at 70° C. for 2 hours to obtain LiFePO ４A glucose aqueous solution as a first carbon source (2.5% by mass in terms of solid content) and a hydroxyethyl cellulose as a second carbon source (0.5% by mass in terms of solid content) were dispersed in an aqueous solvent relative to 95% by mass of the particles to obtain raw material slurry δ1. After drying and granulating the raw material slurry δ1, nitrogen (N ２ ) atmosphere at 735° C. for 2 hours. As a result, a lithium ion battery positive electrode material of Comparative Example 2 in which the surfaces of the particles were covered with a carbonaceous film was obtained.

[0125] Comparative Example 3 An olivine-type compound LiFePO ４ was produced. Li was used as the Li source and P source. ３ P.O. ４ FeSO as an Fe source ４ Using an aqueous solution, these were mixed so that the molar ratio of Li:Fe:P was 3:1:1 to prepare 2.2 L of raw material slurry A1. Next, the raw material slurry A1 was placed in a pressure-resistant container and subjected to a heating reaction at 180 ° C for 1 hour to perform hydrothermal synthesis. After the reaction, the atmosphere in the heat-resistant container was cooled to room temperature, and a cake-like precipitate of the reaction product was obtained. This precipitate was thoroughly washed multiple times with distilled water and kept at a moisture content of 40% to prevent drying, resulting in a cake-like substance.

[0126] This cake-like substance was dried in vacuum at 70°C for 2 hours to obtain LiFePO ４ A glucose aqueous solution as a first carbon source (12.5% ​​by mass in terms of solid content) and a hydroxyethyl cellulose as a second carbon source (2.5% by mass in terms of solid content) were dispersed in an aqueous solvent relative to 95% by mass of the particles to obtain a raw material slurry γ1. The raw material slurry γ1 was dried and granulated, and then heated in a rotary kiln manufactured by Chugai Ro Engineering Co., Ltd. under nitrogen (N ２ ) atmosphere at 735°C for 2 hours. This resulted in a first aggregate in which the particle surfaces were coated with a carbonaceous film. Furthermore, the raw material slurry A1 placed in a pressure-resistant container was subjected to a heating reaction at 170°C for 1 hour to perform hydrothermal synthesis. After the reaction, the atmosphere in the heat-resistant container was cooled to room temperature, and a cake-like precipitate of the reaction product was obtained.

[0127] The precipitate was thoroughly washed multiple times with distilled water and kept at a moisture content of 40% to prevent drying, to obtain a cake-like substance. The cake-like substance was vacuum-dried at 70° C. for 2 hours to obtain LiFePO ４ A glucose aqueous solution as a first carbon source in an amount of 2.5% by mass in terms of solid content relative to 95% by mass of the particles and hydroxyethyl cellulose as a second carbon source in an amount of 0.5% by mass in terms of solid content were dispersed in an aqueous solvent to obtain a raw material slurry δ1.

[0128] The raw material slurry δ1 was dried and granulated, and then nitrogen (N ２ The mixture was baked at 735°C for 2 hours in a 0.1% CO₂ atmosphere. This resulted in a second aggregate whose particle surfaces were coated with a carbonaceous coating. The first aggregate and the second aggregate were weighed out so that the blending ratio (a1:a2) was 5:5 (mass of the first aggregate: mass of the second aggregate: 5:5), and mixed using a blender to obtain the positive electrode material composition of Example 9.

[0129] [Fabrication of Lithium-Ion Secondary Battery] The positive electrode material compositions obtained in the Examples and Comparative Examples, acetylene black (AB) as a conductive additive, and polyvinylidene fluoride (PVdF) as a binder were mixed with N-methyl-2-pyrrolidinone (NMP) in a mass ratio of electrode material:AB:PVdF=90:5:5 to prepare a positive electrode material paste. The obtained paste was applied to an aluminum foil with a thickness of 30 μm and dried to obtain a dry electrode film. The density of the obtained dry electrode film was measured. Thereafter, the dry electrode film was compressed to obtain a predetermined density to prepare an electrode plate. The obtained electrode plate was cut into a 3 × 3 cm ２ The test electrode was prepared by punching out a plate with a coated surface and a tab margin, and welding the tab to it.

[0130] On the other hand, a counter electrode coated with natural graphite was used. A porous polypropylene film was used as the separator. 1 mol / L lithium hexafluorophosphate (LiPF) was used as the non-aqueous electrolyte solution. ６ ) solution was used. ６The solvent used for the solution was a 1:1 mixture of ethylene carbonate and diethyl carbonate by volume, with 2% vinylene carbonate added as an additive. Using the test electrode, counter electrode, and non-aqueous electrolyte prepared as described above, laminate-type cells were fabricated to form batteries for the examples and comparative examples.

[0131] [Evaluation of Lithium-Ion Battery Positive Electrode Materials] The properties of the lithium-ion battery positive electrode materials obtained in the Examples and Comparative Examples and the components contained therein were evaluated. The evaluation methods were as follows. The results are shown in Table 1.

[0132] (1) Carbon Content The carbon content (c) was measured using a carbon analyzer (manufactured by Horiba Ltd., model number: EMIA-220V).

[0133] (2) Specific Surface Area The specific surface area was measured by the BET method using nitrogen (N2) adsorption using a specific surface area meter (manufactured by Japan BEL Corporation, trade name: BELSORP-mini).

[0134] (3) Tap Density The tap density was measured according to JIS R 1628:1997, "Method for measuring bulk density of fine ceramic powders." A sample of a predetermined mass was collected from the positive electrode material for a lithium-ion secondary battery and placed in a 10 mL glass measuring cylinder. The sample was vibrated together with the measuring cylinder, and the volume of the sample was measured when the volume of the sample no longer changed. The value obtained by dividing the mass of the sample by the volume of the sample was defined as the tap density of the positive electrode material for a lithium-ion battery.

[0135] (4) Carbonaceous Coating Thickness (Average Value) A thin film sample was prepared by cross-sectionally processing secondary particles of a lithium-ion battery positive electrode material using a focused ion beam processing and observation device (manufactured by Hitachi High-Technologies Corporation, product name: FB2100). Images were taken using a field emission transmission electron microscope (manufactured by Hitachi High-Technologies Corporation, product name: HF2000), and the average value of the carbonaceous coating thicknesses at 10 points on each of 10 primary particles was taken as the carbonaceous coating thickness (average value).

[0136] (5) Crystal Grain Size The crystal grain size was determined by Scherrer's equation from the full width at half maximum (FWHM) of the (211) plane in powder X-ray diffraction measurement using CuKα rays.

[0137] (6) Average Particle Diameter The positive electrode material composition was embedded in resin and processed to enable cross-sectional observation of the positive electrode material composition. The cross-section of the positive electrode material composition was then observed using a scanning electron microscope (SEM). The maximum and minimum diameters of 100 aggregates randomly selected during the cross-sectional observation were measured, and the average of the maximum and minimum diameters was taken as the particle diameter of each aggregate. Furthermore, the particle diameters obtained from the measured aggregates were number-averaged to determine the average particle diameter of the aggregates.

[0138] [Evaluation of Lithium Ion Battery] The obtained lithium ion battery was evaluated by the following method. The results are shown in Tables 1 and 2.

[0139] (1) Load Characteristics (Discharge Capacity Ratio) A charge / discharge test of a lithium-ion battery was conducted three times at room temperature (25°C) with a cutoff voltage of 2.5V to 3.7V and a constant current charge / discharge rate of 0.1C (10 hours of charge followed by 10 hours of discharge), and the discharge capacity of the third test was recorded as the 0.1C discharge capacity. Furthermore, at room temperature (25°C), the battery was charged at 0.2C with a cutoff voltage of 2.5V to 3.7V (5 hours of charge) and discharged at 3C (20 minutes of discharge), and the discharge capacity was measured. The ratio of the 3C discharge capacity to the 0.1C discharge capacity was taken as the load characteristic and calculated using the following formula (2): Discharge Capacity Ratio (%) = (3C discharge capacity / 0.1C discharge capacity) × 100 (2)

[0140] (2) Direct Current Resistance (DCR) The lithium-ion battery was charged at a current of 0.1 C for 5 hours at an ambient temperature of 25°C to adjust the depth of charge (50% SOC). The DCR of the battery adjusted to 50% SOC was measured at an ambient temperature of 0°C. The following cycles were performed in this order: a first cycle of "1 C charge for 10 seconds, then a 10-minute rest, then a 1 C discharge for 10 seconds, then a 10-minute rest." a second cycle of "3 C charge for 10 seconds, then a 10-minute rest, then a 3 C discharge for 10 seconds, then a 10-minute rest." a third cycle of "5 C charge for 10 seconds, then a 10-minute rest, then a 5 C discharge for 10 seconds, then a 10-minute rest." a fourth cycle of "10 C charge for 10 seconds, then a 10-minute rest, then a 10 C discharge for 10 seconds, then a 10-minute rest." The voltage was measured 10 seconds after each charge and discharge. Each current value was plotted on the horizontal axis and the voltage after 10 seconds on the vertical axis to draw an approximate straight line, and the slope of the approximate line was taken as the DC resistance during charging (input DCR) and the DC resistance during discharging (output DCR).

[0141] (3) Cycle Characteristics The laminated cell was subjected to 500 cycles of cycling at an ambient temperature of 60°C, a cutoff voltage of 2.5V to 3.7V, and a constant current charge / discharge rate of 1C (charge for 1 hour, then discharge for 1 hour) (hereinafter referred to as the "cycling test"), and the laminated cell was fully discharged by constant current discharge at 0.1C (hereinafter referred to as the "post-test cell"). The capacity retention rate was calculated from the ratio of the initial 0.1C discharge capacity to the 0.1C discharge capacity after the cycling test using the following formula (3): Capacity retention rate (%) = (0.1C discharge capacity after cycling test / initial 0.1C discharge capacity) × 100 (3)

[0142] (4) Amount of Metal Leaching After the test, the cell used in the cycle performance evaluation was disassembled, the negative electrode layer was separated, and the negative electrode layer was dissolved in acid. The transition metal content [metal amount (Ma)] was quantified using ICP atomic emission spectroscopy. The ratio (Ma / Mc) of the total mass (Mc) of the lithium metal phosphate particles contained in the first aggregate and the second aggregate in the positive electrode of the laminate cell used in the cycle performance evaluation was defined as the amount of metal leaching.

[0143]

[0144]

[0145] As can be seen from Tables 1 and 2, in a lithium ion battery using a positive electrode formed from the positive electrode material of the example, which includes a first aggregate made of positive electrode active material particles with a crystal grain size of 200 nm to 2000 nm and a second aggregate made of positive electrode active material particles with a crystal grain size of 50 nm or less, the amount of metal elution after 500 cycles was suppressed to 1,000 mass ppm or less, and as a result, the capacity retention rate after 500 cycles was 80% or more. Furthermore, by using the positive electrode material of the example, the electrode film density in a dry state was reduced to 1.1 g / cm ３ This makes it possible to fabricate a lithium ion battery with high energy density.

[0146] On the other hand, in a lithium ion secondary battery using a positive electrode formed from a comparative positive electrode material having a crystal grain size of more than 50 nm and less than 200 nm, the amount of metal elution after 500 cycles was more than 1,000 mass ppm, and as a result, the capacity retention rate after 500 cycles was less than 80%, and the electrode film density in a dry state was 1.1 g / cm ３ From the above, it was confirmed that the lithium ion secondary battery obtained from the positive electrode material of this embodiment has excellent cycle characteristics and a high energy density.

Claims

1. A positive electrode material for a lithium ion secondary battery, comprising an aggregate composition of positive electrode active material particles, which are lithium metal phosphate particles represented by the following general formula (1) and have a carbonaceous coating formed on the surface thereof, the aggregate composition comprising: a first aggregate in which the positive electrode active material particles have a crystal grain size of 200 nm or more and 2000 nm or less; and a second aggregate in which the positive electrode active material particles have a crystal grain size of 50 nm or less. ｘ A ｙ D ｚ P.O. ４ (1) However, A is at least one selected from the group consisting of Co, Mn, Ni, Fe, Cu, and Cr, D is at least one selected from the group consisting of Mg, Ca, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, Sc, and Y, and 0.9<x<1.1, 0<y≦1, 0≦z<1, 0.9<y+z<1.

1.

2. The positive electrode material for a lithium ion secondary battery according to claim 1, wherein the average particle size of the first aggregates and the average particle size of the second aggregates are each independently 0.5 μm or more and 50 μm or less.

3. The cathode material, carbon particles, and a binder are mixed in an N-methyl-2-pyrrolidinone solvent to form an electrode slurry, which is then applied to an aluminum foil and dried to form an electrode film having a density of 1.1 g / cm. ３ 2.0g / cm or more ３ The positive electrode material for a lithium ion secondary battery according to claim 1 or 2, wherein:

4. The positive electrode material for lithium ion secondary batteries according to claim 1 or 2, wherein a pouch cell is configured such that an electrode containing the positive electrode material is a positive electrode, an electrode containing a carbon-based negative electrode material is a negative electrode, a separator is disposed between the positive electrode and the negative electrode, and the inside is filled with an electrolyte, and the pouch cell is disassembled after 500 cycles of a 1C full charge and 1C full discharge at 60°C, and the amount of metal contained in the negative electrode is 1,000 mass ppm or less of the sum of the masses of lithium metal phosphate particles represented by general formula (1) contained in the first aggregate and the second aggregate in the positive electrode.

5. A lithium ion secondary battery having at least a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode contains the positive electrode material for lithium ion secondary batteries according to claim 1 or 2.

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