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

Abstract

The present application provides a lithium ion secondary battery having excellent charge-discharge characteristics and cycle characteristics, and a positive electrode material for a lithium ion secondary battery and a positive electrode for a lithium ion secondary battery capable of obtaining the battery. A positive electrode material for a lithium ion secondary battery, characterized by containing carbon, and having, in peaks of carbon present at 2200 to 3400 cm ‑1 -1 existing in the Raman scattering measurement, a peak 1 having a peak top at 2200 to 2380 cm ‑1 -2, a peak 2 having a peak top at 2400 to 2550 cm ‑1 -3, a peak 3 having a peak top at 2600 to 2750 cm ‑1 -4, a peak 4 having a peak top at 2850 to 2950 cm ‑1 -5, and a peak 5 having a peak top at 3100 to 3250 cm ‑1 -6, when the peaks are separated by peak separation of the five peaks composed of the Voigt function, the average of the proportions of the Gaussian function in the peak 3 and the peak 4 is 90% or more and less than 100%.

Description

Technical Field

[0001] This invention relates to a positive electrode material for lithium-ion secondary batteries, a positive electrode for lithium-ion secondary batteries, and a lithium-ion secondary battery. Background Technology

[0002] Compared to lead-acid and nickel-metal hydride batteries, lithium-ion rechargeable batteries have higher energy and power densities, making them suitable for various applications, including home backup power supplies for small electronic devices such as smartphones, and power tools. Furthermore, the practical application of high-capacity lithium-ion rechargeable batteries is being promoted as they are used to store renewable energy from solar and wind power generation.

[0003] For example, in Patent Document 1, in order to obtain a positive electrode material for lithium-ion batteries that inhibits the insertion and extraction of lithium ions into active material particles and improves electronic conductivity, a positive electrode active material-graphene composite particle is disclosed. This positive electrode active material-graphene composite particle is a composite particle-shaped positive electrode material for lithium-ion batteries obtained by combining positive electrode active material particles and a matrix containing graphene. The positive electrode active material particles are dispersed and distributed in the matrix. The carbon element ratio (%) on the surface of the material, as measured by X-ray photoelectron measurement, is 5% or more and 50% or less, and the carbon element ratio (%) in the entire material is 2% or more and 20% or less. The value obtained by dividing the carbon element ratio (%) on the surface of the material by the carbon element ratio (%) in the entire material is 1.5 or more and 7 or less.

[0004] In Patent Document 2, in order to obtain an energy storage device with a large capacity per unit weight or per unit volume, a method for manufacturing a positive electrode active material for an energy storage device is disclosed. The method is characterized by mixing raw materials that become positive electrode active materials to make a mixture, performing a first calcination, pulverizing the mixture, adding graphene oxide to the pulverized mixture for a second calcination, thereby forming a reaction product and reducing the graphene oxide, and coating the surface of the reaction product with graphene.

[0005] Furthermore, in Patent Document 3, in order to obtain an electrode material that uses an electrode active material with a carbonaceous coating formed on its surface and can suppress voltage drop during high-speed discharge at low temperatures, an electrode material is disclosed. This electrode material is a particle-shaped electrode material formed by forming a carbonaceous coating on the surface of electrode active material particles with an olivine-type crystal structure. The average ratio of the discharge capacity of a single particle of this electrode material at -10°C to the discharge capacity of a single particle at -10°C is 0.50 or more, and the XRD (CuKα ray source) peak of the (002) plane caused by the graphene layer of the carbonaceous coating is 2θ = 25.7° or less.

[0006] Patent Document 1: Japanese Patent No. 6237617

[0007] Patent Document 2: Japanese Patent Application Publication No. 2012-099467

[0008] Patent Document 3: Japanese Patent No. 5743011 Summary of the Invention

[0009] The problem the invention aims to solve

[0010] In olivine-based cathode materials, olivine-type phosphates are usually used as cathode active materials. There are many structures with carbon coatings on the primary particles and the outermost layer of the granules of the active material. The carbon coating plays an important role in the internal insertion and extraction of lithium ions and electronic conductivity.

[0011] Here, as in Patent Documents 1 and 2, when the carbon source is calcined as is, the carbon in the carbon coating adopts a highly crystalline graphene structure, which easily hinders the insertion and extraction of lithium ions. Moreover, with high crystallinity, the flexibility of carbon decreases relative to the volume change of the olivine-type phosphate, i.e., the positive electrode active material, during charge and discharge, becoming the main reason for the carbon coating peeling off from the positive electrode active material. In Patent Document 3, the crystallinity of graphene is defined by its curvature, but the crystallinity is high enough to be detected by X-ray diffraction (XRD), requiring further improvement. Furthermore, in regions where grapheneization is not performed and lithium ions can easily be inserted and extracted, electronic conductivity is low, and resistance tends to increase. Moreover, in these regions, the carbon coating is soft, thus becoming the main reason for the carbon coating peeling off from the positive electrode active material by shear force during the fabrication of the positive electrode forming paste.

[0012] The present invention was made in view of this actual situation, and its object is to provide a lithium-ion secondary battery with excellent charge-discharge characteristics and cycle characteristics, as well as a positive electrode material for a lithium-ion secondary battery and a positive electrode for a lithium-ion secondary battery that can obtain the battery.

[0013] Solution for solving the problem

[0014] In order to solve the above-mentioned problems, the inventors conducted in-depth research and found that the flexibility of carbon can be adjusted by controlling the proportion of the Gaussian function of a specific peak obtained by Raman scattering measurement of carbon contained in the positive electrode material of lithium-ion secondary batteries. Therefore, the charge-discharge characteristics and cycle characteristics of lithium-ion secondary batteries can be improved.

[0015] This invention was made based on this insight.

[0016] That is, the present invention provides the following [1] to [7].

[0017] [1] A positive electrode material for lithium-ion secondary batteries, characterized in that it contains carbon and is present in the range of 2200–3400 cm⁻¹ as measured by Raman scattering. -1 The peak of the carbon in the above-mentioned carbon is between 2200 and 2380 cm⁻¹. -1 Peak 1, with a apex, is located between 2400 and 2550 cm. -1 Peak 2 has a apex at 2600–2750 cm. -1 Peak 3, located at 2850–2950 cm, has a apex. -1 Peak 4 exists at the apex and at 3100–3250 cm. -1 When performing peak separation on the five peaks consisting of the Vogt function, the average proportion of the Gaussian function in peaks 3 and 4 is greater than 90% and less than 100%.

[0018] [2] The positive electrode material for lithium-ion secondary batteries according to [1] above is characterized in that, in the peak separation, the coefficient of determination of the peak of the measured Raman scattering of carbon is 0.998 or higher.

[0019] [3] The positive electrode material for lithium-ion secondary batteries according to [1] or [2] above is characterized in that, in the peak separation, the peak 4 has the greatest intensity at its apex, and the half-width of peak 3 is 150 cm. -1 Above and 330cm -1 Hereinafter, the half-width of peak 4 is 280 cm. -1 Above and 360cm -1 the following.

[0020] [4] The positive electrode material for lithium-ion secondary batteries according to any one of [1] to [3] above is characterized in that it contains an active material and has a carbon content of 0.5% by mass or more and 7% by mass or less, wherein the active material is coated with a carbonaceous film containing the carbon and contains primary particles or granules of olivine structure.

[0021] [5] The positive electrode material for lithium-ion secondary batteries according to any one of [1] to [4] above is characterized in that the microcrystal size analyzed by X-ray diffraction is 50 nm or more and 250 nm or less.

[0022] [6] A positive electrode for a lithium-ion secondary battery, comprising an electrode current collector and a positive electrode mixture layer formed on the electrode current collector, wherein the positive electrode mixture layer contains any one of the above [1] to [5] positive electrode materials for lithium-ion secondary batteries.

[0023] [7] A lithium-ion secondary battery having a positive electrode, a negative electrode and a non-aqueous electrolyte, wherein the lithium-ion secondary battery is characterized in that the positive electrode is the positive electrode for lithium-ion secondary batteries described above [6].

[0024] The effects of the invention

[0025] According to the present invention, a lithium-ion secondary battery with excellent charge-discharge characteristics and cycle characteristics can be provided, as well as a positive electrode material for a lithium-ion secondary battery and a positive electrode for a lithium-ion secondary battery that can be obtained from the battery. Attached Figure Description

[0026] Figure 1 This is the Raman spectrum of carbon in the positive electrode material for the lithium-ion secondary battery in Example 1.

[0027] Figure 2 This is the Raman spectrum of carbon in the positive electrode material for the lithium-ion secondary battery in Comparative Example 1. Detailed Implementation

[0028] <Positive electrode materials for lithium-ion secondary batteries>

[0029] The positive electrode material for lithium-ion secondary batteries in this embodiment (hereinafter, also simply referred to as "positive electrode material") contains carbon, and its presence at 2200–3400 cm⁻¹ is measured by Raman scattering. -1 The peak of the carbon in the above-mentioned carbon is between 2200 and 2380 cm⁻¹. -1 Peak 1, with a apex, is located between 2400 and 2550 cm. -1 Peak 2 has a apex at 2600–2750 cm. -1 Peak 3, located at 2850–2950 cm, has a apex. -1 Peak 4 exists at the apex and at 3100–3250 cm. -1 When separating the five peaks consisting of the Vogt function, the average proportion of Gaussian functions in peaks 3 and 4 is above 90% and less than 100%.

[0030] [Proportion of Gaussian function]

[0031] Regarding the carbon contained in the cathode material, Raman scattering is used for spectroscopic measurement, thereby obtaining the Raman spectrum. In this invention, the Raman spectrum contains carbon present in the 2200–3400 cm⁻¹ region. -1 The peaks are separated into peaks 1 to 5 as described above. Note the convolution state of the Gaussian and Lorentz distributions of the Vogt function in peaks 3 and 4.

[0032] Peaks 1-5 are considered to represent the 2D band peaks of carbon observed in particulate carbon and graphene, respectively. Peak 3 is observed as the largest of the multiple peaks in particulate carbon. In graphene, a single layer shows a single peak, but in multilayer graphene, the double resonance process between layers increases, resulting in peak 4 becoming the largest of the multiple peaks. Carbon in the cathode material exhibits a structure both coated on particles and a particulate structure between particles; therefore, peaks 3 and 4 are the most intense 2D band peaks in the cathode material. Furthermore, peaks 1, 2, and 5 not only have low intensity but are also buried in the shoulders of peaks 3 and 4; therefore, the peak shapes are dominated by the shapes of peaks 3 and 4 in the fitting process. Thus, by calculating the proportions of the Gaussian functions of each peak in peaks 3 and 4 and obtaining their average values, the crystallinity of carbon in the carbon material of the cathode active material can be calculated.

[0033] Here, in the Raman scattering peaks, peaks representing crystalline solids are represented by a Lorentz distribution, while peaks representing amorphous and liquid substances are represented by a Gaussian distribution. Therefore, the peaks are considered as Vogt functions, which are considered the convolution of a Gaussian function and a Lorentz function. Thus, by calculating the proportion of the Gaussian function of the peak, the crystallinity of the represented peak can be calculated. Furthermore, the Vogt function simplifies calculations, allowing for the calculation of a possible Vogt function.

[0034] The average proportion of the Gaussian function refers to the average proportion of the Gaussian function of peak 3 and the proportion of the Gaussian function of peak 4, and is also called the Gaussian rate.

[0035] In theory, the Gaussian ratio of each peak obtained through peak separation is calculated through curve fitting. The suspected Vogt function is represented by the following formula.

[0036] V(x) = M × G(x) + (1 - M) × L(x)

[0037] [Formula 1]

[0038]

[0039] [Formula 2]

[0040]

[0041] V(x): A possible Vogt function

[0042] M: Proportion of the Gaussian function

[0043] G(x): Gaussian function

[0044] L(x): Lorentz function

[0045] A: Peak intensity

[0046] ω: Peak half-width

[0047] x: wavenumber

[0048] x0: Peak wavenumber

[0049] Typically, the proportions of the Gaussian functions of each peak are calculated using numerical calculation software installed in the Raman spectroscopy measurement device (Raman microscope, etc.) based on the Raman spectroscopy measurement data.

[0050] When the average proportion of Gaussian functions in peaks 3 and 4 is less than 90%, the crystallinity is high, so carbon hinders the insertion and extraction of lithium ions. If it is 100%, it becomes completely amorphous and cannot maintain conductivity.

[0051] From the viewpoint of further improving the charge-discharge characteristics and cycle characteristics of lithium-ion secondary batteries, the average value of the proportion of Gaussian functions in peaks 3 and 4 is preferably 93% or more, more preferably 95% or more, and even more preferably 97% or more. Furthermore, from the same viewpoint, the average value of the proportion of Gaussian functions in peaks 3 and 4 is preferably 99.9% or less, more preferably 99.7% or less.

[0052] [Coefficient of Determination]

[0053] In the peak separation of the Raman spectrum obtained by measuring the Raman scattering of carbon in the cathode material, the coefficient of determination of the peak of the measured Raman scattering of carbon is preferably 0.998 or higher.

[0054] The coefficient of determination (COD) of the Raman scattering peaks of carbon and the fitting function indicates the thickness of the graphene layer and the amount of particulate carbon. When the graphene layer is one layer, the Raman scattering peak becomes a single peak; when there are five peaks, the function cannot be fitted, thus the COD decreases. Furthermore, if the graphene layer becomes thicker than 10 layers, the peaks converge to two, so even if it becomes too thick, the COD will decrease. Additionally, if the amount of particulate carbon increases, the shape of the Raman scattering peaks changes. A COD greater than 0.998 indicates a graphene layer thickness between 2 and 10 layers, suitable for conductivity and lithium-ion migration, with low free carbon content.

[0055] From the viewpoint of further improving the charge-discharge characteristics and cycle characteristics of lithium-ion secondary batteries, the coefficient of determination is more preferably 0.9985 or higher, more preferably 0.9990 or higher, and even more preferably 0.9995 or higher.

[0056] The coefficient of determination is obtained during peak fitting, but it is usually calculated using numerical calculation software set in the device, based on Raman spectroscopy data measured using a Raman spectroscopy measuring device (Raman microscope, etc.).

[0057] [Half-value width]

[0058] In the peak separation of the Raman spectrum obtained by Raman scattering measurement of carbon in the cathode material, peak 4 has the highest peak intensity. In addition, peak 3 has a half-width of 150 cm⁻¹. -1 Above and 330cm -1 Below, the half-width of peak 4 is 280 cm. -1 Above and 360cm -1 the following.

[0059] If the peak of peak 4 among peaks 1 to 5 has the highest intensity, it indicates that there is more carbon forming the graphene layer than particulate carbon, and that the proportion of carbon that contributes to electronic conductivity is high.

[0060] Furthermore, the half-width of peak 3 is 150 cm. -1 Above and 330cm -1 Below, the half-width of peak 4 is 280 cm. -1 Above and 360cm -1 This area thus becomes a region where the carbonization intensity is suitable for electronic conductivity and lithium ion passage.

[0061] The half-width of peak 3 is more preferably 160 cm. -1 Above and 325cm -1 The following is a further preferred size: 170cm -1 Above and 320cm -1 The following is a further preferred size: 180cm -1 Above and below 315cm -1 .

[0062] The half-width of peak 4 is more preferably 282 cm. -1 Above and 355cm -1 The following is a further preferred size: 284cm -1 Above and 350cm -1 The following is a further preferred size: 285cm -1 Above and 345cm -1 the following.

[0063] [Carbon-coated active substances]

[0064] The cathode material involved in this embodiment is preferably a carbonaceous coated active material containing carbon having the Raman properties described above. Specifically, the active material preferably contains primary particles with an olivine structure or granules thereof, and the active material is coated with a carbonaceous film containing carbon having the Raman properties described above.

[0065] Regarding carbon possessing the Raman properties described above, its crystallinity is not very high, making it difficult to hinder lithium-ion insertion and extraction. Furthermore, it possesses the flexibility to prevent the carbon coating from being peeled off from the active material. Therefore, it is believed that by including an active material coated with a carbon coating containing carbon possessing the described Raman properties in the cathode material, high electronic conductivity can be achieved, thereby improving the charge-discharge characteristics and cycle performance of lithium-ion secondary batteries.

[0066] (Active substances)

[0067] The positive electrode material in this embodiment preferably contains an active material (positive electrode active material) of the general formula Li. x A y D z PO4 represents olivine-type phosphate compounds.

[0068] In the general formula, A is selected from at least one of the groups including Co, Mn, Ni, Fe, Cu and Cr, D is selected from at least one of the groups including Mg, Ca, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, Sc and Y, and x, y and z are 0.9 < x < 1.1, 0 < y ≤ 1.0, 0 ≤ z < 1.0, and 0.9 < y + z < 1.1.

[0069] In the general formula, A and D can each be two or more independent types. For example, they can be derived from Li. x A 1 y1A 2 y2D 1 z1D 2 z2D 3 z3D 4 The expression is like z4PO4. In this case, the total of y1 and y2 only needs to be within the range of y, that is, greater than 0 and less than 1.0, and the total of z1, z2, z3 and z4 only needs to be within the range of z, that is, greater than 0 and less than 1.0.

[0070] There are no particular limitations on olivine-type phosphate compounds as long as they have the above-described structure, but transition metal lithium phosphate compounds containing an olivine structure are preferred.

[0071] In the general formula Li x A y D z In PO4, A is preferably Co, Mn, Ni, and Fe, more preferably Co, Mn, and Fe. Furthermore, D is preferably Mg, Ca, Sr, Ba, Ti, Zn, and Al. By incorporating these elements into an olivine-type phosphate compound, a positive electrode flux layer capable of achieving high discharge potential and high safety can be formed. Moreover, these materials are preferred due to their abundant resources.

[0072] From the perspective of high discharge capacity and high energy density, olivine-type phosphate compounds can be derived from the general formula LiFe. x2 Mn 1-x2-y2 M y2 PO4 indicates.

[0073] In the general formula LiFe x2 Mn 1-x2-y2 M y2 In PO4, M is selected from at least one of Mg, Ca, Co, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, Sc and Y, and 0.05≤x2≤1.0 and 0≤y2≤0.14.

[0074] The olivine-type phosphate compound in this embodiment is preferably in the shape of primary particles and their granules (secondary particles as an aggregate of primary particles).

[0075] The shape of the primary particles of the olivine-type phosphate compound is not particularly limited, but spherical shape is preferred, and especially spherical shape is preferred. By making the primary particles spherical, the amount of solvent required when preparing the positive electrode forming paste using the positive electrode material of this embodiment can be reduced, and the positive electrode forming paste can be easily applied to the current collector. In addition, the positive electrode forming paste can be prepared, for example, by mixing the positive electrode material of this embodiment, the adhesive resin (binder), and the solvent.

[0076] The primary particles of olivine-type phosphate compounds and their granules are collectively referred to as active material particles.

[0077] (Carbon coating)

[0078] In this embodiment, the carbon containing the described Raman properties is preferably contained in the cathode material as a carbonaceous coating that coats the active material particles.

[0079] The carbon coating is a pyrolytic carbon coating obtained by carbonizing organic matter that will become the raw material for the carbon coating.

[0080] As organic compounds, there are no particular limitations on any compound that can form a carbonaceous coating on the surface of active material particles. Examples 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 polyols. Examples of polyols include polyethylene glycol, polypropylene glycol, polyglycerol, and glycerol. Only one type can be used, or two or more can be mixed.

[0081] (Carbon content)

[0082] In this embodiment, the carbon content of the cathode material is preferably 0.5% by mass or more and 7% by mass or less. When the cathode material contains carbon-coated active material particles, the carbon content in the cathode material is determined as the mass of the carbon coating relative to the total mass of the carbon coating and the active material particles.

[0083] By using a carbon content of 0.5% by mass or higher, the discharge capacity of lithium-ion secondary batteries at high charge-discharge rates increases, enabling optimal charge-discharge rate performance. By using a carbon content of 7% by mass or lower, excessive capacity loss per unit mass of the positive electrode material in lithium-ion secondary batteries can be suppressed.

[0084] From the viewpoint of improving the charge-discharge characteristics and cycle characteristics of lithium-ion secondary batteries, the carbon content in the cathode material is preferably 0.5% by mass or more and 7% by mass or less, more preferably 0.5% by mass or more and 7% by mass or less, and even more preferably 0.5% by mass or more and 7% by mass or less.

[0085] In addition, the aforementioned carbon content can be measured using a carbon analyzer (e.g., manufactured by HORIBA, Ltd., model: EMIA-220V).

[0086] (Crystal size)

[0087] Regarding the cathode material in this embodiment (preferably carbon-coated active material particles), the microcrystal size analyzed by X-ray diffraction is preferably 50 nm or more and 250 nm or less.

[0088] If the crystallite size of the cathode material is 50 nm or larger, the amount of carbon required to fully coat the surface of the active material particles (as the central particles) with a carbonaceous coating can be suppressed, and the amount of binder used in forming the carbonaceous coating can also be suppressed. Therefore, the mass of the active material in the cathode can be increased, thereby improving the battery capacity. Furthermore, the peeling of the carbonaceous coating from the active material particles due to insufficient adhesion is less likely to occur.

[0089] On the other hand, if the microcrystal size of the cathode material is below 250nm, the internal resistance of the active material can be suppressed, and the discharge capacity at high charge and discharge rates can be improved when the battery is formed.

[0090] The microcrystal size of the cathode material is more preferably 50 nm or more and 220 nm or less, even more preferably 60 nm or more and 170 nm or less, even more preferably 60 nm or more and 140 nm or less, and even more preferably 70 nm or more and 117 nm or less.

[0091] In addition, the crystallite size of the cathode material can be measured using an X-ray diffraction device (e.g., RINT2000, manufactured by RIGAKU), and the half-width of the diffraction peaks on the (020) plane of the obtained powder X-ray diffraction pattern and the diffraction angle (2θ) can be calculated using the Scherrer formula.

[0092] [Specific surface area]

[0093] The specific surface area of ​​the cathode material (preferably carbon-coated active material particles) is preferably 5-25 m². 2 / g.

[0094] The specific surface area of ​​the positive electrode material is 5m². 2 A concentration of 1 g or higher can suppress the coarsening of the cathode material and accelerate the diffusion rate of lithium ions within its particles. This, in turn, improves the battery characteristics of lithium-ion secondary batteries.

[0095] The specific surface area of ​​the positive electrode material is 25m². 2 With a density of less than / g, the cathode density within the cathode containing cathode material can be increased, thus enabling the provision of lithium-ion secondary batteries with high energy density.

[0096] The specific surface area mentioned above can be measured using a specific surface area meter (e.g., manufactured by Bayer Corporation, Japan, trade name: BELSORP-mini) via the BET method based on nitrogen (N2) adsorption.

[0097] [Average particle size of primary particles]

[0098] The average particle size of the primary particles of the active material particles coated with a carbonaceous film (carbonaceous coated active material particles) is preferably 50 nm or more, more preferably 70 nm or more, even more preferably 100 nm or more, and preferably 500 nm or less, more preferably 450 nm or less, and even more preferably 400 nm or less. If the average particle size of the primary particles is 50 nm or more, the increase in carbon content caused by the increase in the specific surface area of ​​the cathode material can be suppressed, thereby suppressing the decrease in the charge and discharge capacity of the lithium-ion secondary battery. On the other hand, if it is 500 nm or less, the migration time of lithium ions or electrons migrating within the cathode material can be shortened. This suppresses the deterioration of output characteristics caused by the increase in the internal resistance of the lithium-ion secondary battery.

[0099] Here, the so-called average particle size of primary particles refers to the number-average particle size. The average particle size of the aforementioned primary particles can be obtained by averaging the particle sizes of more than 200 particles observed and measured by scanning electron microscopy (SEM).

[0100] [Average particle size of secondary particles]

[0101] The average particle size of the secondary particles of the carbonaceous coated active material particles is preferably 0.5 μm or more, more preferably 1.0 μm or more, even more preferably 1.5 μm or more, and preferably 20 μm or less, more preferably 18 μm or less, and even more preferably 15 μm or less. If the average particle size of the secondary particles is 0.5 μm or more, the need for large amounts of conductive additives and binders can be suppressed when preparing a positive electrode material paste for lithium-ion secondary batteries by mixing positive electrode materials, conductive additives, binder resins (binders), and solvents. This improves the battery capacity per unit mass of lithium-ion secondary battery in the positive electrode composite layer of the positive electrode. On the other hand, if the particle size is 20 μm or less, the dispersion and uniformity of the conductive additives or binders in the positive electrode composite layer of the positive electrode of the lithium-ion secondary battery can be improved. As a result, the discharge capacity of the lithium-ion secondary battery under high-speed charge and discharge increases.

[0102] Here, the average particle size of secondary particles refers to the volume average particle size. The average particle size of these secondary particles can be measured using a laser diffraction scattering particle size distribution measurement device, etc.

[0103] [Thickness of carbon coating]

[0104] The thickness (average value) of the carbonaceous coating covering the active material particles is preferably 1.0 nm or more, more preferably 1.4 nm or more, and preferably 10.0 nm or less, more preferably 7.0 nm or less. If the thickness of the carbonaceous coating is 1.0 nm or more, the sum of electron migration resistance in the carbonaceous coating can be suppressed from increasing. This suppresses the increase in the internal resistance of the lithium-ion secondary battery and prevents voltage drop at high charge / discharge rates. On the other hand, if the thickness is 10.0 nm or less, the formation of steric hindrance hindering the diffusion of lithium ions in the carbonaceous coating can be suppressed, thereby reducing the lithium-ion migration resistance. As a result, the increase in the battery's internal resistance can be suppressed, and voltage drop at high charge / discharge rates can be prevented.

[0105] [Coating rate of carbon coating]

[0106] The coating rate of the carbonaceous film relative to the active material particles is preferably 60% or more, more preferably 80% or more. With a coating rate of 60% or more, the coating effect of the carbonaceous film can be fully achieved.

[0107] In addition, the coverage of carbonaceous coatings can be determined by observing particles using a transmission electron microscope (TEM) or an energy dispersive X-ray microanalyzer (EDX), calculating the proportion of the part covering the particle surface, and then obtaining the coverage from the average value.

[0108] [Density of carbon coating]

[0109] The density of the carbonaceous coating, calculated based on the amount of carbon constituting it, is preferably 0.3 g / cm³. 3 The above, more preferably 0.4 g / cm³ 3 The above, and preferably 2.0 g / cm³ 3 The preferred value is 1.8 g / cm³. 3 The density of a carbonaceous coating, calculated based on the amount of carbon that makes up the coating, refers to the mass per unit volume of the carbonaceous coating, assuming that the coating is composed solely of carbon.

[0110] If the density of the carbon coating is 0.3 g / cm³ 3 The above indicates that the carbon coating exhibits sufficient electronic conductivity. On the other hand, if it is 2.0 g / cm³... 3 Since the amount of graphite crystallites with a layered structure in the carbonaceous coating is small, no steric hindrance caused by the graphite crystallites occurs during lithium-ion diffusion within the carbonaceous coating. Therefore, the lithium-ion migration resistance does not increase. Consequently, the internal resistance of the lithium-ion secondary battery does not rise, and there is no voltage drop at high charge / discharge rates.

[0111] (Manufacturing method of positive electrode material for lithium-ion secondary batteries)

[0112] Regarding the method for manufacturing the positive electrode material for lithium-ion secondary batteries according to this embodiment, there are no particular limitations as long as the method can include carbon having the Raman properties described.

[0113] A method for manufacturing a positive electrode material includes, for example, a step (A) of obtaining active material particles; a step (B) of adding an organic compound to the active material particles obtained in step (A) to prepare a mixture; and a step (C) of placing the mixture in a calcining sagger for calcination.

[0114] By adjusting the amount of organic compound added in step (B) and the calcination conditions of the mixture in step (C), carbon with the Raman properties described above can be easily produced. Details will be provided later.

[0115] [Process (A)]

[0116] Active material particles can be manufactured using conventional methods such as solid-phase, liquid-phase, and gas-phase methods. Li obtained by this method... x A y D z PO4, for example, can be exemplified by particulate matter (hereinafter, sometimes referred to as "Li"). x A yD z PO4 particles".).

[0117] Li x A y D z PO4 particles can be obtained, for example, through hydrothermal synthesis of a slurry mixture of a mixed Li source, an A source, a P source, water, and, if necessary, a D source. According to the hydrothermal synthesis, Li... x A y D z PO4 forms a precipitate in water. The precipitate obtained can be Li. x A y D z A precursor to PO4. In this case, through the reaction of Li... x A y D z The target Li is obtained by calcining the PO4 precursor. x A y D z PO4 particles.

[0118] It is preferable to use a pressure-resistant, sealed container in this hydrothermal synthesis.

[0119] As reaction conditions for hydrothermal synthesis, the heating temperature is preferably 110°C or higher and 200°C or lower, more preferably 115°C or higher and 195°C or lower, and even more preferably 120°C or higher and 190°C or lower. By setting the heating temperature within the above range, the specific surface area of ​​the active material particles can be set within the above range.

[0120] Furthermore, the reaction time is preferably 30 minutes or more and 120 hours or less, more preferably 1 hour or more and 24 hours or less, and even more preferably 5 hours or more and 15 hours or less.

[0121] Furthermore, the pressure during the reaction is preferably 0.1 MPa or more and 22 MPa or less, more preferably 0.1 MPa or more and 17 MPa or less.

[0122] The molar ratio (Li:A:D:P) of the Li source, A source, D source and P source is preferably 2.5-4.0:0-1.0:0-1.0:0.9-1.15, more preferably 2.8-3.5:0-1.0:0-1.0:0.95-1.1.

[0123] Here, as the Li source, it is preferred to use at least one of the following: lithium hydroxides such as lithium hydroxide (LiOH); lithium carbonate (Li2CO3), lithium chloride (LiCl), lithium nitrate (LiNO3), lithium phosphate (Li3PO4), lithium hydrogen phosphate (Li2HPO4), and lithium dihydrogen phosphate (LiH2PO4); lithium organic acid salts such as lithium acetate (LiCH3COO) and lithium oxalate ((COOLi)2); and their hydrates.

[0124] In addition, lithium phosphate (Li3PO4) can also be used as a Li source and a P source.

[0125] Examples of sources A include chlorides, carboxylates, sulfates, etc., containing at least one from the group consisting of Co, Mn, Ni, Fe, Cu, and Cr. For example, in Li x1 A y1 D z1 When A in PO4 is Fe, iron compounds or their hydrates, such as ferric chloride (II) (FeCl2), ferric sulfate (II) (FeSO4), ferric acetate (II) (Fe(CH3COO)2), ferric nitrate (III) (Fe(NO3)3), ferric chloride (III) (FeCl3), ferric citrate (III) (FeC6H5O7), and lithium iron phosphate, can be used as sources of Fe.

[0126] Examples of sources of D include chlorides, carboxylates, sulfates, etc., containing at least one of the following: Mg, Ca, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, Sc, and Y. For example, in Li... x1 A y1 D z1 When D in PO4 is Ca, examples of Ca sources include calcium hydroxide (II) (Ca(OH)2), calcium chloride (II) (CaCl2), calcium sulfate (II) (CaSO4), calcium nitrate (II) (Ca(NO3)2), calcium acetate (II) (Ca(CH3COO)2), and their hydrates.

[0127] Examples of phosphoric acid (P) sources include phosphoric acid (H3PO4), ammonium dihydrogen phosphate (NH4H2PO4), and diammonium hydrogen phosphate ((NH4)2HPO4). Among these, at least one selected from the group consisting of phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate is preferred as the P source.

[0128] [Process (B)]

[0129] In step (B), an organic compound is added to the active material particles obtained in step (A) to prepare a mixture.

[0130] First, an organic compound is added to the above-mentioned active substance particles, and then a solvent is added.

[0131] Regarding the amount of organic compound relative to the active material particles, when the total mass of the organic compound is converted into carbon elements, it is preferably 0.15 parts by mass or more and 15 parts by mass or less, more preferably 0.45 parts by mass or more and 4.5 parts by mass or less, relative to 100 parts by mass of the active material particles.

[0132] If the amount of organic compound relative to the active material particles is 0.15 parts by mass or more, the coating rate on the surface of the active material particles with a carbonaceous coating produced by heat treatment of the organic compound can be set to 80% or more. This improves the charge-discharge characteristics and cycle characteristics of the lithium-ion secondary battery. Conversely, if the amount of organic compound relative to the active material particles is 15 parts by mass or less, the decrease in the capacity of the lithium-ion secondary battery due to a relative decrease in the active material particle ratio can be suppressed. Furthermore, if the amount of organic compound relative to the active material particles is 15 parts by mass or less, the increase in the bulk density of the active material particles due to excessive loading of the carbonaceous coating relative to the active material particles can be suppressed. In addition, by suppressing the increase in the bulk density of the active material particles, the decrease in electrode density can be suppressed, and the decrease in the capacity of the lithium-ion secondary battery per unit volume can be suppressed.

[0133] The above-mentioned compounds can be used as organic compounds for preparing mixtures.

[0134] Here, by using low-molecular-weight organic compounds such as sucrose and lactose as the aforementioned organic compounds, a carbonaceous coating can be easily formed uniformly on the surface of the primary particles of the cathode material. However, on the other hand, there is a tendency for the carbonization degree of the carbonaceous coating obtained through pyrolysis to decrease, making it difficult to form a carbonaceous coating that can achieve sufficient resistance reduction. Furthermore, by using such low-molecular-weight organic compounds, the amount of micropores in the carbonaceous coating increases, and the overall micropore ratio increases. On the other hand, by using high-molecular-weight organic compounds such as polyvinyl alcohol and polyvinylpyrrolidone, or organic compounds with benzene ring structures such as phenolic resins, there is a tendency for the carbonization degree of the carbonaceous coating obtained through pyrolysis to increase, which can achieve sufficient resistance reduction. However, on the other hand, there is a tendency for it to be difficult to form a carbonaceous coating uniformly on the surface of the primary particles of the cathode material, resulting in problems such as difficulty in achieving sufficient resistance reduction of the cathode material. Furthermore, by using such high-molecular-weight organic compounds or organic compounds with benzene ring structures, the amount of micropores in the carbonaceous coating decreases, and the overall micropore ratio decreases.

[0135] Therefore, it is preferable to use a suitable mixture of low-molecular-weight organic compounds and high-molecular-weight organic compounds, or organic compounds with benzene ring structures.

[0136] In particular, it is preferable to use low-molecular-weight organic compounds in powder form because it is easy to mix the active material particles and the organic compound, and it is possible to obtain a cathode material with a carbonaceous coating uniformly formed on the primary particle surface of the active material particles. Furthermore, unlike high-molecular-weight organic compounds, low-molecular-weight organic compounds are easily soluble in solution, eliminating the need for pre-dissolution work, thus reducing the number of processing steps and the cost of dissolution work.

[0137] When adding solvent to the active material particles, the solid content is preferably adjusted to 10-60% by mass, more preferably to 15-55% by mass, and even more preferably to 25-50% by mass. By setting the solid content within the above range, the tap density of the obtained cathode material can be set within the above range.

[0138] Examples of solvents mentioned above include: water; alcohols such as methanol, ethanol, 1-propanol, 2-propanol (isopropanol: 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-dimethylacetamide, and N-methylpyrrolidone; and diols such as ethylene glycol, diethylene glycol, and propylene glycol. One or more of these solvents may be used. Water is the preferred solvent among these.

[0139] Additionally, dispersants can be added as needed.

[0140] As a method for dispersing active material particles and organic compounds in a solvent, there are no particular limitations as long as the active material particles are uniformly dispersed and the organic compounds are dissolved or dispersed. Examples of devices used for such dispersion include ball mills, vibratory ball mills, bead mills, paint mixers, and grinding mills—media stirring type dispersion devices that use high-speed stirring of media particles.

[0141] The mixture can be sprayed and dried in a high-temperature atmosphere, such as above 110°C and below 200°C, to generate granules of the mixture.

[0142] In this spray pyrolysis method, in order to quickly dry and generate approximately spherical granules, the droplet size during spraying is preferably 0.01 μm or more and 100 μm or less.

[0143] [Process (C)]

[0144] In step (C), the mixture obtained in step (B) is placed in a calcining sagger and calcined.

[0145] The preferred method for calcining the mixture is to (1) heat the mixture to produce granulated powder (granulation process), and then (2) rapidly increase the heating temperature and control the calcination time (rapid heating process). By calcining the mixture in this process, the grapheneization reaction can be promoted at the interface between the active material particles and the carbon source, making it easy to produce active material particles coated with a carbonaceous film containing low-crystallinity graphene. The carbonaceous coated active material produced in this way maintains appropriate electronic conductivity and lithium-ion permeability, and the carbon is also highly flexible, which can improve the charge-discharge characteristics and cycle characteristics of lithium-ion secondary batteries.

[0146] As a calcining sagger, a calcining sagger containing a material with excellent thermal conductivity, such as carbon, is preferred.

[0147] (1) Granulation process

[0148] In the granulation process, the mixture is heated to produce granulated powder.

[0149] For example, a spray dryer can be used to dry the mixture at a temperature of 40–80°C at the dryer outlet, followed by granulation. The heating temperature during the granulation process is preferably 50–70°C.

[0150] (2) Rapid heating process

[0151] In the rapid heating process, the heating temperature of the granulated powder obtained in the granulation process is rapidly increased, and the calcination time is controlled. In order to promote the graphene formation reaction at the interface between the active material particles and the carbon source, it is preferable to rapidly heat the granulated powder to the carbonization temperature region and maintain it at this temperature for a specific time.

[0152] The rapid heating process should ideally be repeated at least twice.

[0153] For example, in the case of repeating two rapid heating processes, in the first rapid heating process, it is preferable to raise the heating temperature of the granulated powder to 200°C or higher and 450°C or lower at a heating rate of 3°C / min or higher and 15°C / min or lower, and maintain it for calcination for 10 minutes or higher and 120 minutes or lower.

[0154] The initial heating rate is more preferably 3°C / min or more and 13°C / min or less, and even more preferably 4°C / min or more and 9°C / min or less.

[0155] The temperature after the first heating is more preferably above 230°C and below 420°C, and even more preferably above 250°C and below 380°C.

[0156] The first calcination time is more preferably 10 minutes or more and 80 minutes or less, and even more preferably 20 minutes or more and 50 minutes or less.

[0157] In the second rapid heating process, it is preferable to raise the heating temperature of the granulated powder to above 630°C and below 770°C at a heating rate of 10°C / min and below 25°C / min, and maintain it for calcination for a time of 10 minutes and below 120 minutes.

[0158] The second heating rate is more preferably 12°C / min or more and 22°C / min or less, and even more preferably 13°C / min or more and 18°C / min or less.

[0159] The temperature after the second heating is more preferably above 650°C and below 750°C, and even more preferably above 650°C and below 740°C.

[0160] The second calcination time is more preferably 10 minutes or more and 70 minutes or less, and even more preferably 25 minutes or more and 50 minutes or less.

[0161] The maximum calcination temperature is preferably above 630℃ and below 770℃.

[0162] If the maximum calcination temperature is above 630°C, the organic compounds undergo sufficient decomposition and reaction, resulting in complete carbonization. This leads to the formation of a low-resistivity carbonaceous coating on the obtained cathode material. Conversely, if the maximum calcination temperature is below 770°C, high specific surface area can be maintained without the need for cathode material particle growth. This results in increased discharge capacity at high charge / discharge rates when forming a lithium-ion secondary battery, thereby achieving optimal charge / discharge rate performance.

[0163] The maximum calcination temperature is preferably above 680°C and below 770°C.

[0164] When the rapid heating process is repeated more than twice, the total calcination time is only the time required for the organic compound to be fully carbonized, and there are no special restrictions, such as more than 0.2 hours and less than 100 hours.

[0165] The calcination atmosphere is preferably an inert atmosphere containing inert gases such as nitrogen (N2) and argon (Ar), or a reducing atmosphere containing reducing gases such as hydrogen (H2). Furthermore, a superheated steam atmosphere can be used to promote the carbonization reaction. A reducing atmosphere is more preferred when further suppressing the oxidation of the mixture.

[0166] Through calcination in step (C), organic compounds decompose and react to generate carbon. This carbon then adheres to the surface of the active material particles, forming a carbonaceous coating. Thus, the surface of the active material particles is covered by this carbonaceous coating.

[0167] In this embodiment, in step (C), a thermally conductive auxiliary material with a thermal conductivity higher than that of the active material particles can be added to the mixture, and then the mixture is calcined. This results in a more uniform temperature distribution within the calcination sagger. Consequently, it is possible to suppress incomplete carbonization of organic compounds or reduction of active material particles by carbon due to uneven temperature within the calcination sagger.

[0168] There are no particular limitations on the thermally conductive auxiliary material, as long as it has a thermal conductivity higher than that of the active material particles. Preferably, it is a material that is difficult to react with the active material particles. This is because reacting the thermally conductive auxiliary material with the active material particles may impair the battery activity of the active material particles obtained after calcination, or it may be impossible to recover the thermally conductive auxiliary material for reuse after calcination.

[0169] Examples of thermally conductive auxiliary materials include carbonaceous materials, alumina ceramics, magnesia ceramics, zirconia ceramics, silica ceramics, calcium oxide ceramics, and aluminum nitride. One type of these thermally conductive auxiliary material can be used, or two or more can be used in combination.

[0170] The preferred thermally conductive aid is a carbonaceous material, such as graphite, acetylene black (AB), vapor-grown carbon fiber (VGCF), carbon nanotubes (CNTs), and graphene. One or more of these thermally conductive aids can be used. Among these carbonaceous materials, graphite is more preferred as a thermally conductive aid.

[0171] There is no particular limitation on the size of the thermally conductive auxiliary material. However, from the perspective of thermal conductivity efficiency, in order to ensure a sufficiently uniform temperature distribution within the calcination crucible and to reduce the amount of thermally conductive auxiliary material added, the average length of the thermally conductive auxiliary material in the long direction is preferably 1 mm or more and 100 mm or less, more preferably 5 mm or more and 30 mm or less. Furthermore, if the average length of the thermally conductive auxiliary material in the long direction is 1 mm or more and 100 mm or less, it is easier to separate the thermally conductive auxiliary material from the positive electrode material using a sieve.

[0172] Furthermore, materials with a higher specific gravity than the cathode material are preferred because they are easier to separate using methods such as air classifiers.

[0173] Regarding the amount of thermally conductive auxiliary material added, although it is also affected by the size of the thermally conductive auxiliary material, when the above mixture is set to 100% by volume, it is preferably 1% by volume or more and 50% by volume or less, more preferably 5% by volume or more and 30% by volume or less. If the amount of thermally conductive auxiliary material added is 1% by volume or more, the temperature distribution inside the calcining sagger can be made sufficiently uniform. On the other hand, if the amount of thermally conductive auxiliary material added is 50% by volume or less, the amount of active material particles and organic compounds calcined inside the calcining sagger can be prevented from decreasing.

[0174] After calcination, the mixture of thermally conductive auxiliary material and positive electrode material is passed through a sieve or similar device to separate the thermally conductive auxiliary material from the positive electrode material.

[0175] <Positive electrode for lithium-ion secondary batteries>

[0176] The positive electrode for a lithium-ion secondary battery according to this embodiment includes an electrode current collector and a positive electrode additive layer formed on the electrode current collector. In this positive electrode for a lithium-ion secondary battery, the positive electrode additive layer contains the positive electrode material of this embodiment.

[0177] The positive electrode for a lithium-ion secondary battery of this embodiment includes the positive electrode material for a lithium-ion secondary battery of this embodiment. Therefore, the lithium-ion secondary battery using the positive electrode for a lithium-ion secondary battery of this embodiment has excellent charge-discharge characteristics and cycle characteristics.

[0178] Hereinafter, the positive electrode of a lithium-ion secondary battery will sometimes be referred to simply as the "positive electrode".

[0179] When manufacturing the positive electrode, the above-mentioned positive electrode material, a binder containing a binder resin, and a solvent are mixed to prepare a coating or paste for positive electrode formation. At this time, conductive additives such as carbon black, acetylene black, graphite, Ketjen black, natural graphite, and artificial graphite can be added as needed.

[0180] As a binder or adhesive resin, polytetrafluoroethylene (PTFE) resin, polyvinylidene fluoride (PVdF) resin, fluororubber, etc. are preferred.

[0181] There is no particular limitation on the mixing ratio of the positive electrode material and the binder resin. For example, the binder resin is set to 1 to 30 parts by mass relative to 100 parts by mass of the positive electrode material, preferably 3 to 20 parts by mass.

[0182] The solvent used in coatings or pastes for positive electrode formation can be selected appropriately based on the properties of the adhesive resin.

[0183] Examples include: water; alcohols such as methanol, ethanol, 1-propanol, 2-propanol (isopropanol: 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-dimethylacetamide, and N-methylpyrrolidone; and diols such as ethylene glycol, diethylene glycol, and propylene glycol. Only one type of these can be used, or two or more can be mixed.

[0184] Next, a coating or paste for forming the positive electrode is applied to one main surface of the electrode current collector to form a coating film. This coating film is then dried to obtain an electrode current collector with a coating film formed on one main surface, consisting of a mixture containing the aforementioned positive electrode material and binder. The coating film is then press-fitted and dried to create a positive electrode with a positive electrode binder layer on one main surface of the electrode current collector.

[0185] More specifically, for example, the coating is applied to one side of an aluminum foil. The coating is then dried to obtain an aluminum foil with a coating made of a mixture containing a positive electrode material and a binder formed on one side. The coating is then pressed and dried to create a current collector (positive electrode) with a positive electrode binder layer on one side of the aluminum foil.

[0186] In this way, it is possible to manufacture the positive electrode of a lithium-ion secondary battery that can achieve high input characteristics and excellent cycle characteristics.

[0187] <Lithium-ion secondary batteries>

[0188] The lithium-ion secondary battery of this embodiment has a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode of this lithium-ion secondary battery is the positive electrode for lithium-ion secondary batteries of this embodiment.

[0189] The lithium-ion secondary battery of this embodiment is not limited to the structure described above; for example, it may also include a separator.

[0190] 〔negative electrode〕

[0191] Examples of negative electrodes include metallic Li, natural graphite, hard carbon and other carbon materials, Li alloys, and Li4Ti5O. 12 Si(Li) 4.4 Negative electrode materials such as Si)

[0192] [Non-aqueous electrolytes]

[0193] Non-aqueous electrolytes can be exemplified by the following: a mixture of ethylene carbonate (ethylene carbonate; EC) and ethyl methyl carbonate (ethyl methyl carbonate; EMC) in a volume ratio of 1:1, and lithium hexafluorophosphate (LiPF6) dissolved in the resulting mixed solvent to a concentration, for example, 1 mol / dm³. 3 .

[0194] [Diaphragm]

[0195] In this embodiment, the positive and negative electrodes can be placed opposite each other with a membrane in between. Porous propylene can be used as the membrane, for example.

[0196] Furthermore, solid electrolytes can be used to replace non-aqueous electrolytes and membranes.

[0197] In the lithium-ion secondary battery of this embodiment, the positive electrode has a positive electrode flux layer containing the positive electrode material for lithium-ion secondary batteries of this embodiment. Therefore, the migration of surrounding Li ions in any of the battery components is excellent, resulting in excellent high input characteristics and cycle characteristics. Therefore, it is suitable for batteries used to drive electric vehicles or batteries used to drive hybrid electric vehicles.

[0198] Example

[0199] The present invention will now be specifically described with reference to embodiments and comparative examples. However, the present invention is not limited to the methods described in the embodiments.

[0200] <Manufacturing of Cathode Materials for Lithium-ion Secondary Batteries>

[0201] [Example 1]

[0202] 1. Manufacturing of active substances

[0203] LiOH was used as the Li source, NH4H2PO4 as the P source, and FeSO4·7H2O as the Fe source. They were mixed in pure water in a mass ratio of Li:Fe:P = 3:1:1 to prepare a homogeneous slurry mixture of 200 ml.

[0204] Next, the mixture was placed in a 500 mL pressure-resistant sealed container and subjected to hydrothermal synthesis at 170 °C for 12 hours. After the reaction, the container was cooled to room temperature (25 °C), resulting in a precipitated cake-like product. The precipitate was thoroughly washed several times with distilled water, maintaining a moisture content of 30% to prevent drying, thus forming a cake-like substance.

[0205] The results of X-ray diffraction measurements of a small amount of the obtained cake-like material, followed by vacuum drying at 70°C for 2 hours to obtain powder, confirmed the formation of a single-phase LiFePO4.

[0206] 2. Preparation of mixtures

[0207] 20g of the obtained LiFePO4 (active substance) and 0.73g of sucrose as a carbon source were mixed in water to make a total of 100g. 150g of zirconia beads were bead-milled together to obtain a slurry (mixture) with a dispersed particle size (d50) of 100nm.

[0208] 3. Calcination of the mixture

[0209] (Granulation process)

[0210] The mixture was dried using a spray dryer at a drying outlet temperature of 60°C and then granulated.

[0211] (Rapid heating process)

[0212] Using a tubular furnace, the granulated powder was heated to 300°C at a heating rate of 5°C / min and held for 30 minutes (first heating). Then, the granulated powder was heated to 700°C at a heating rate of 15°C / min and held for 30 minutes (second heating), thereby obtaining the cathode material of Example 1 containing carbonaceous coated active material.

[0213] [Example 2]

[0214] In the rapid heating process of Example 1, the highest calcination temperature (second calcination temperature) in the tubular furnace was set to 680°C. Otherwise, the cathode material of Example 2 containing carbonaceous coated active material was obtained in the same manner as in Example 1.

[0215] [Example 3]

[0216] In the rapid heating process of Example 1, the calcination conditions of the tubular furnace were changed in the following manner. Otherwise, the cathode material of Example 3 containing carbonaceous coated active material was obtained in the same manner as in Example 1.

[0217] The granulated powder was heated to 300°C at a heating rate of 10°C / min and held for 60 minutes. Then, the heating temperature was increased to 750°C at a heating rate of 15°C / min and held for 20 minutes.

[0218] [Example 4]

[0219] In the preparation of the mixture in Example 1, the amount of sucrose was changed to 0.3g, and the mixture was otherwise obtained in the same manner as in Example 1.

[0220] Moreover, in Example 3, the above-mentioned mixture was used instead of the mixture used in Example 3. After the mixture in the granulation process was granulated and dried, 2g of polyvinyl alcohol powder was added to the granulated powder and kneaded. Otherwise, the positive electrode material of Example 4 containing carbonaceous coated active material was obtained in the same manner as in Example 3.

[0221] [Comparative Example 1]

[0222] In the manufacture of the cathode material of Example 4, the amount of polyvinyl alcohol powder added was set to 3g. Otherwise, the cathode material of Comparative Example 1 containing carbonaceous coated active material was obtained in the same manner as in Example 4.

[0223] [Comparative Example 2]

[0224] In the rapid heating process of Comparative Example 1, the highest calcination temperature (second calcination temperature) in the tubular furnace was set to 680°C. Otherwise, the cathode material of Comparative Example 2 containing carbonaceous coated active material was obtained in the same manner as Comparative Example 1.

[0225] [Comparative Example 3]

[0226] In the rapid heating process of Example 1, the calcination conditions of the tubular furnace were changed in the following manner. Otherwise, the cathode material of Comparative Example 3 containing carbonaceous coated active material was obtained in the same manner as in Example 1.

[0227] The granulated powder was heated to 750°C at a heating rate of 10°C / min and held for 120 minutes.

[0228] [The manufacture of lithium-ion secondary batteries]

[0229] The positive electrode material obtained in the examples and comparative examples, acetylene black (AB) as a conductive additive, and polyvinylidene fluoride (PVdF) as a binder were mixed in N-methyl-2-pyrrolidone (NMP) at a weight ratio of positive electrode material:AB:PVdF = 90:5:5 to prepare a positive electrode material paste. The obtained paste was coated onto an aluminum foil with a thickness of 30 μm and dried, and then pressed together at a specified density to form an electrode plate.

[0230] The obtained electrode plate was punched into 3×3cm pieces. 2 The experimental electrode was made by welding the plate to the edge of the plate (coated surface).

[0231] On the other hand, a coated electrode coated with natural graphite was also used in the counter electrode. A porous polypropylene membrane was used as the separator. Furthermore, a 1 mol / L lithium hexafluorophosphate (LiPF6) solution was used as the non-aqueous electrolyte solution. Additionally, a solvent obtained by mixing ethylene carbonate and diethyl carbonate in a 1:1 volume ratio and adding 2% vinylene carbonate as an additive was used as the solvent in this LiPF6 solution.

[0232] Using the test electrode, counter electrode, and non-aqueous electrolyte prepared in the manner described above, a laminated battery was fabricated to produce the batteries of the examples and comparative examples.

[0233] [Evaluation of cathode materials]

[0234] The physical properties of the cathode materials obtained in the examples and comparative examples, as well as the components contained therein, were evaluated. The evaluation method is as follows. The results are shown in Table 1.

[0235] (1) Carbon content

[0236] The carbon content (mass %) of the carbonaceous active material was measured using a carbon analyzer (manufactured by HORIBA, Ltd., carbon and sulfur analysis device EMIA-810W).

[0237] (2) Microcrystalline grain size

[0238] Regarding the microcrystal size of the active material, the half-width of the diffraction peaks on the (020) plane of the powder X-ray diffraction pattern, measured by X-ray diffraction measurement (RIGAKU manufactured, X-ray diffraction device: RINT2000), and the diffraction angle (2θ) were calculated using the Scherrer formula.

[0239] (3) Raman properties of carbon

[0240] Raman spectra of carbon contained in the cathode material were measured using a Raman microscope (manufactured by HORIBA, Ltd., XploRA PLUS Raman microscope).

[0241] The measurement wavelength used was 538 nm, within the range of 1500–3500 cm⁻¹. -1 Measurements were taken between 2000 and 3500 cm. -1 Peak separation was performed, and peak fitting was conducted for five Vogt functions. Peak fitting was performed using numerical calculation software, and parameters were set accordingly. The range for the five peaks was set to 2200–2380 cm⁻¹. -1 Peak 1, with a apex, is located between 2400 and 2550 cm. -1 Peak 2 has a apex at 2600–2750 cm. -1Peak 3, located at 2850–2950 cm, has a apex. -1 Peak 4 exists at the apex and at 3100–3250 cm. -1 Peak 5 has a apex. The Gaussian ratio, peak intensity, peak half-width, and coefficient of determination of each peak are calculated using numerical calculation software. Based on the obtained Gaussian ratios of each peak, the average proportion of the Gaussian function in peaks 3 and 4 is calculated and shown in the "Gaussian Ratio" column of Table 1.

[0242] exist Figure 1 The image shows the Raman spectrum of carbon in the cathode material of Example 1. Figure 2 The Raman spectrum of carbon in the cathode material of Comparative Example 1 is shown. Additionally, Figure 1 and Figure 2 In the diagram, the dotted lines (···) represent fitted curves, the middle dashed lines (---) represent measured values, the small dashed lines (---) represent peak 1, the single-point dashed lines (-·-·-) represent peak 2, the double-point dashed lines (-··-··-) represent peak 3, the solid lines (-) represent peak 4, and the large dashed lines (---) (dashed lines longer than the middle dashed lines) represent peak 5.

[0243] <Evaluation of Lithium-ion Secondary Batteries>

[0244] The discharge capacity and capacity retention based on cycle tests were measured using lithium-ion secondary batteries obtained in the examples and comparative examples. The cutoff voltage was set to 2.5–3.7 V (vs. carbon anode). The results are shown in Table 1.

[0245] (1) Discharge capacity

[0246] At an ambient temperature of 25°C, the charging current was set to 1C and the discharging current was set to 10C. The discharge capacity was measured by constant current charging and discharging.

[0247] The allowable range is above 80mAh / g.

[0248] (2) Capacity maintenance rate

[0249] At an ambient temperature of 25°C, the charging current and discharging current were both set to 2C. The discharge capacity was measured using constant current charge-discharge, and the measured value was set as the initial discharge capacity. Then, the ambient temperature was set to 45°C, the charging current was set to 2C, and the discharging current was set to 2C. 600 constant current charge-discharge cycles were performed. Then, at an ambient temperature of 25°C, the charging current and discharging current were again set to 2C, and the discharge capacity was measured again using constant current charge-discharge to obtain the discharge capacity after the cycles.

[0250] The capacity retention rate based on cyclic testing was calculated using the following formula.

[0251] Cyclic test capacity retention = Discharge capacity after cycles / Initial discharge capacity

[0252] The allowable range is above 70%.

[0253] [Table 1]

[0254]

[0255] (Summary of Results)

[0256] As shown in Table 1, the batteries manufactured using the cathode material of Comparative Example 3, where the average proportion of Gaussian functions in peaks 3 and 4 is 0%, and the batteries using the cathode material of Comparative Example 1, which has a proportion slightly lower than 70%, exhibit poor charge-discharge and cycle characteristics. Furthermore, the batteries manufactured using the cathode material of Comparative Example 2, where the average proportion of Gaussian functions in peaks 3 and 4 is 100%, show high capacity retention but low discharge capacity.

[0257] In contrast, it can be seen that the battery obtained by using the cathode material of the embodiment in which the average proportion of the Gaussian function in peaks 3 and 4 is more than 70% and less than 100% has a large discharge capacity and capacity retention rate, and excellent charge-discharge characteristics and cycle characteristics.

[0258] Industrial availability

[0259] The positive electrode material for lithium-ion secondary batteries of the present invention is useful as the positive electrode of lithium-ion secondary batteries.

Claims

1. A positive electrode material for lithium-ion secondary batteries, characterized in that, Active material comprising carbon and primary particles or granules thereof coated with a carbonaceous film containing said carbon and containing an olivine structure, and present in the range of 2200–3400 cm⁻¹ as measured by Raman scattering. -1 The peak of the carbon in the above-mentioned carbon is between 2200 and 2380 cm⁻¹. -1 Peak 1, with a apex, is located between 2400 and 2550 cm. -1 Peak 2 has a apex at 2600–2750 cm. -1 Peak 3, located at 2850–2950 cm, has a apex. -1 Peak 4 exists at the apex and at 3100–3250 cm. -1 When performing peak separation on the five peaks consisting of the Vogt function, peak 5, which has a apex, the average proportion of the Gaussian function in peaks 3 and 4 is above 90% and less than 100%. The carbon content relative to the total mass of the carbon coating and active material particles is more than 0.5% by mass and less than 7% by mass; The average particle size of the primary particles of the active material particles coated by the carbonaceous film is greater than 50 nm and less than 500 nm. The active substance is composed of the general formula Li x A y D z PO4 indicates that A is selected from at least one of the groups including Co, Mn, Ni, Fe, Cu and Cr, D is selected from at least one of the groups including Mg, Ca, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, Sc and Y, and x, y and z are 0.9 < x < 1.1, 0 < y ≤ 1.0, 0 ≤ z < 1.0, and 0.9 < y + z < 1.

1.

2. The positive electrode material for lithium-ion secondary batteries according to claim 1, characterized in that, In the peak separation, the coefficient of determination of the peak of the measured Raman scattering of carbon is greater than 0.

998.

3. The positive electrode material for lithium-ion secondary batteries according to claim 1 or 2, characterized in that, In the peak separation, peak 4 has the highest intensity at its apex, and peak 3 has a half-width of 150 cm. -1 Above and 330cm -1 Hereinafter, the half-width of peak 4 is 280 cm. -1 Above and 360cm -1 the following.

4. The positive electrode material for lithium-ion secondary batteries according to claim 1 or 2, characterized in that, The microcrystal size, as determined by X-ray diffraction analysis, is greater than 50 nm and less than 250 nm.

5. A positive electrode for a lithium-ion secondary battery, comprising an electrode current collector and a positive electrode flux layer formed on the electrode current collector, characterized in that... The positive electrode mixture layer contains the positive electrode material for lithium-ion secondary batteries as described in any one of claims 1 to 4.

6. A lithium-ion secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, characterized in that... The positive electrode comprises the positive electrode for a lithium-ion secondary battery as described in claim 5.

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