Positive Electrode Active Material for Lithium-Ion Secondary Battery and Lithium-Ion Secondary Battery

Abstract

The purpose of the present invention is to provide positive electrode active substance particles for a lithium ion secondary battery, such particles being capable of producing a lithium ion secondary battery having excellent high-speed discharge properties. The present invention is a granulated body of a positive electrode active substance for a lithium ion secondary battery, wherein the primary particle average diameter is 10 to 80 nm and the number of primary particles having a diameter of 100 nm or greater is no more than 5.0%.

Description

Technical Field

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

Background Art

[0002] Since the main application of lithium ion secondary batteries has become electric vehicles, further improvement in the characteristics of lithium ion secondary batteries is required. Among them, the high-rate discharge characteristics play an important role in the part related to the acceleration of the vehicle, and it is required to maintain a high energy density even during high-rate discharge.

[0003] On the other hand, when a lithium ion secondary battery malfunctions, the stored energy is released in a short time, and there is a risk of the battery catching fire or exploding. Therefore, for lithium ion secondary batteries, improving safety is as important an issue as improving energy density.

[0004] It is well known that the positive electrode active material greatly affects the safety of lithium ion secondary batteries. In particular, a positive electrode active material called a layered oxide system, which is often used in smartphones and electric vehicles, has a high energy density, but has problems in safety, such as releasing oxygen in the battery due to overcharging and leading to the risk of ignition.

[0005] On the other hand, olivine-based positive electrode active materials (LiMPO4) such as lithium iron phosphate (LiFePO4), which are often used in stationary batteries, etc., are known to be highly safe positive electrode materials that do not easily release oxygen because oxygen is covalently bonded to phosphorus and are relatively stable even under high temperature conditions.

[0006] Among olivine-based positive electrode active materials, lithium manganese iron phosphate has lower ionic conductivity and electronic conductivity than lithium iron phosphate, so it is difficult to achieve high-rate discharge when made into a battery, and development to solve this problem is underway. For example, chemical formula A x MB yO z (A is an alkali metal or an alkaline earth metal, M contains at least two transition metal elements, B is a typical element that forms an anion by covalent bonding with oxygen O, 0 ≦ x ≦ 2, 1 ≦ y ≦ 2, and 3 ≦ x ≦ 7.) It has an olivine structure and is a positive electrode active material for a lithium secondary battery whose surface is partially or entirely covered with a carbon material (for example, see Patent Document 1), or a positive electrode active material for a lithium secondary battery containing carbon-coated polyanion compound particles (for example, see Patent Document 2) has been proposed.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] Generally, the high-rate discharge characteristics of the positive electrode material of a lithium-ion battery are rate-limited by electron conductivity and ion conductivity, and improvement thereof has always been demanded. Among them, lithium iron manganese phosphate is known to be particularly difficult to improve the energy density (Wh / kg) during high-rate discharge because of its particularly low electron conductivity and ion conductivity. In order to compensate for the low electron conductivity and ion conductivity, it is effective to reduce the particle size of the primary particles of lithium iron manganese phosphate to nanoparticles with a particle size of 100 nm or less and reduce the solid-state diffusion distance of electrons and lithium ions in the primary particles during the charge-discharge reaction. Also, by mixing lithium iron manganese phosphate with a carbon source such as glucose and firing at a high temperature of 600 °C or higher, a carbon coating layer is formed on the surface of the lithium iron manganese phosphate particles, which is also effective in increasing the electron conductivity.

[0009] However, while nanoparticle formation shortens the diffusion distance in the solid, it increases the surface energy of the particle surface. Therefore, during firing for forming the carbon coating layer, sintering between particles tends to proceed. Such sintered and coarsened particles may occur throughout the lithium iron manganese phosphate particles depending on the firing temperature conditions, but partially appear under optimized firing temperature conditions.

[0010] The particles connected and coarsened by sintering cause a decrease in high-rate discharge characteristics. On the other hand, the crystallite size obtained from powder X-ray diffraction, which is a conventional evaluation method, and the average particle size of a large number of particles using an electron microscope are characteristics of macroscopically occurring sintering, and a policy for suppressing the sintering of primary particles of microscale lithium iron manganese phosphate has not been obtained.

[0011] For this reason, the materials mainly composed of lithium iron manganese phosphate obtained by the methods disclosed in Patent Documents 1 and 2 have a problem that coarse particles are likely to be generated due to sintering between particles during firing, and the high-rate discharge characteristics are insufficient.

[0012] In view of such problems, an object of the present invention is to provide a positive electrode active material for a lithium ion secondary battery capable of obtaining a lithium ion secondary battery having excellent high-rate discharge characteristics.

Means for Solving the Problems

[0013] To solve the above problems, the present invention mainly has the following configuration. A granulated body of positive electrode active material particles for a lithium ion secondary battery, wherein the average particle size of the primary particles is 10 nm or more and 80 nm or less, and the number ratio of primary particles having a particle size of 100 nm or more is 5.0% or less.

Effects of the Invention

[0014] By using the positive electrode active material for a lithium ion secondary battery of the present invention, a lithium ion secondary battery having excellent high-rate discharge characteristics can be obtained.

Best Mode for Carrying Out the Invention

[0015] The positive electrode active material for a lithium-ion secondary battery of the present invention (hereinafter, may be simply referred to as "positive electrode active material") is a substance that can reversibly react with lithium ions. For example, LiMn2O4 called a spinel-type positive electrode active material, LiMO2 called a layered oxide-type positive electrode active material (M is one or more selected from Mn, Co, Ni, Al), LiMPO4 called an olivine-type positive electrode active material (M is one or more selected from Fe, Mn, Co, Ni), etc. can be used.

[0016] The positive electrode active material of the present invention has an average primary particle diameter of 10 nm or more and 80 nm or less, and the number ratio of particles having a particle diameter of 100 nm or more is 5.0% or less. The high-rate discharge characteristics of the positive electrode active material change depending on the particle diameter of the primary particles. However, through the studies by the present inventors, it has been found that by setting the average primary particle diameter to 10 nm or more and 80 nm or less and setting the number ratio of primary particles having a particle diameter of 100 nm or more to 5.0% or less, high high-rate discharge characteristics can be obtained.

[0017] In this specification, the "granulated body" refers to a state in which a plurality of primary particles gather to form a particle shape. However, even if a plurality of primary particles gather, if the overall shape is amorphous, it is an aggregate and is not included in the granulated body.

[0018] The average primary particle diameter of the positive electrode active material in the present invention is 10 nm or more and 80 nm or less. Generally, the positive electrode active material has low electronic conductivity and ion conductivity. In order to improve the high-rate discharge characteristics, it is necessary to reduce the average primary particle diameter and shorten the solid-state diffusion distance of electrons and lithium ions inside the particles. When the average primary particle diameter of the positive electrode active material is larger than 80 nm, the solid-state diffusion distance becomes longer, resulting in a decrease in the high-rate discharge characteristics. On the other hand, when the average primary particle diameter of the positive electrode active material is smaller than 10 nm, the crystallinity on the surface of the primary particles decreases, the portion that cannot contribute to the charge-discharge reaction increases, and the energy density decreases.

[0019] Here, the average particle size of the primary particles of the positive electrode active material can be measured using a scanning electron microscope. Specifically, the positive electrode active material is magnified and observed at a magnification of 200,000 times using a scanning electron microscope, the particle sizes of 200 randomly selected primary particles are measured, and the average particle size can be obtained by calculating the number average value. When the primary particles are not spherical, the average value of the major axis and minor axis that can be measured in the two-dimensional image is taken as the particle size. When two or more particles are connected by sintering, they are treated as one particle. When it is difficult to judge whether it is sintering or contact, the image is binarized into white and black, and when a line for dividing the connected part is obtained, it is regarded as contact and treated as two particles, and when it is not obtained, it is regarded as sintering and treated as one particle.

[0020] The inventors of the present invention focused on the sintering of the primary particles of the microscopic positive electrode active material, and found that the high-rate charging characteristics change depending on the percentage of particles having a particle size of 100 nm or more as an index. In the positive electrode active material of the present invention, among the positive electrode active material particles forming the granulated body, the percentage of particles having a particle size of 100 nm or more is 5.0% or less. Particles having a particle size of 100 nm or more are particles that have coarsened due to the progress of sintering and cause a decrease in the high-rate discharge characteristics. Therefore, when the percentage of particles having a particle size of 100 nm or more exceeds 5.0%, the high-rate charging characteristics deteriorate. The percentage of particles having a particle size of 100 nm or more is preferably 3.0% or less. Even when evaluating the crystallite size and average particle size in a state where a small amount of positive electrode active material having a particle size of 100 nm or more is mixed with positive electrode active material particles having a particle size of 80 nm or less, it is difficult to obtain a result suggesting the presence of particles having a particle size of 100 nm or more. Therefore, in the present invention, the positive electrode active material is magnified and observed at a magnification of 200,000 times using a scanning electron microscope, the particle sizes of 200 randomly selected primary particles are measured, and the percentage (%) of particles having a particle size of 100 nm or more is calculated. When the particles are not spherical, the average value of the major axis and minor axis that can be measured in the two-dimensional image is taken as the particle size. When two or more particles are connected by sintering, they are treated as one particle. When it is difficult to judge whether it is sintering or contact, the image is binarized into white and black, and when a line for dividing the connected part is obtained, it is regarded as contact and treated as two particles, and when it is not obtained, it is regarded as sintering and treated as one particle.

[0021] The positive electrode active material of the present invention is preferably the above-described olivine-based positive electrode active material. Among positive electrode active materials, the olivine-based positive electrode active material has high safety, but has particularly low electron conductivity and ion conductivity, resulting in low high-rate discharge characteristics. Due to the effect of the present invention, the electron conductivity and ion conductivity are improved, and a battery that is safe and has high high-rate discharge characteristics can be obtained.

[0022] The primary particles of the olivine-based positive electrode active material in the present invention preferably have a carbon coating layer on the surface. That is, it is preferable that carbon exists as a film on the surface of the primary particles.

[0023] The proportion of carbon contained in the granulated body of the olivine-based positive electrode active material in the present invention is preferably 2.0% by weight or more and 5.0% by weight or less. By containing 2.0% by weight or more of carbon, high conductivity can be exhibited in the battery, and the high-rate discharge characteristics can be further improved. On the other hand, by containing 5.0% by weight or less of carbon, it is difficult to inhibit the movement of lithium ions inserted and extracted into the primary particles of the positive electrode active material, and the high-rate discharge characteristics can be further improved. The carbon contained in the granulated body of the olivine-based positive electrode active material is preferably carbon derived from the carbon coating layer.

[0024] Here, the weight ratio of carbon contained in the granulated body of the olivine-based positive electrode active material can be measured using a carbon sulfur analyzer EMIA-810W (manufactured by Horiba, Ltd.).

[0025] The positive electrode active material of the present invention is among olivine-based positive electrode active materials, Li α Mn a Fe bIt is preferable to use lithium iron manganese phosphate represented by PO4 (0.9 ≤ α ≤ 1.1, 0.6 ≤ a ≤ 1.0, 0 < b ≤ 0.4, 0.9 ≤ a + b ≤ 1.1) (hereinafter sometimes referred to as "LMFP") in terms of obtaining a battery with a higher energy density. If α is less than 0.9 or greater than 1.1, the energy density decreases due to the presence of impurities other than LMFP or an increase in the number of defects in the crystal. If a + b is less than 0.9 or greater than 1.1, the energy density decreases due to the presence of impurities other than LMFP or an increase in the number of defects in the crystal.

[0026] Here, the composition of LMFP can be estimated from the raw material charging ratio during the synthesis of LMFP particles, but it can also be specified by atomic absorption analysis for lithium and ICP emission spectrometry for manganese, iron, and phosphorus using the obtained LMFP. For the above formulas α, a, and b, the measurement is carried out up to the third decimal place, and the value up to the second decimal place is adopted by rounding.

[0027] In the LMFP of the present invention, the ratio I 20 / I 29 of the peak intensity at 20° to the peak intensity at 29° obtained by X-ray diffraction is preferably 0.88 or more and 1.05 or less. Also, the ratio I 35 / I 29 of the peak intensity at 35° to the peak intensity at 29° obtained by X-ray diffraction is preferably 1.05 or more and 1.20 or less.

[0028] The 20° peak, 29° peak, and 35° peak obtained by powder X-ray diffraction can be indexed as the (101), (020), and (311) planes, respectively, and the intensity of each peak represents the strength of the orientation to the crystal plane. In particular, the (020) plane is the plane on which LMFP is most likely to grow and tends to show a strong orientation. Therefore, I 20 / I 29 and I 35 / I 29Being within the above range means that the primary particles in the positive electrode active material are not particles oriented in (020), but have grown crystallized homogeneously, and the shape of the particles is close to spherical. In LMFP where the lattice volume change of the crystal during charge and discharge is as large as around 10%, having a homogeneous shape has the effect of relaxing the strain of the crystal inside the particles generated during charge and discharge, and can further improve the high-rate discharge characteristics.

[0029] Here, the X-ray diffraction peak of LMFP can be measured using an X-ray diffractometer that uses Cu as the X-ray source.

[0030] The average pore diameter of the pores contained in the granulated body of the positive electrode active material in the present invention is preferably 10 nm or more and 60 nm or less. Since the positive electrode active material expands and contracts during the charge and discharge reaction, when repeatedly charged and discharged, the structure of the granulated body collapses and the cycle resistance decreases. The fact that the average pore diameter is 60 nm or less prevents the void ratio in the granulated body from becoming excessively high, suppresses the granulated body from becoming brittle, and can improve the cycle resistance, which is preferable. On the other hand, the fact that the average pore diameter is 10 nm or more suppresses the delay of the charge and discharge reaction due to insufficient liquid volume of the electrolytic solution that exchanges lithium ions with the positive electrode active material, and can suppress the decrease in cycle resistance due to the generation of overvoltage inside the granulated body, so it is preferable.

[0031] Here, the average pore diameter refers to the median diameter and can be measured by the mercury injection method using a pore size distribution measuring device, Autopore IV9520 type (manufactured by Shimadzu Corporation). The measurement is performed under the condition of an initial pressure of 7 kPa, and the mercury parameters are a mercury contact angle of 130.0° and a mercury surface tension of 485.0 Dynes / cm. However, in order to distinguish the voids between the granulated bodies from the pores, the measurement of the average pore diameter is performed in the range of a pore diameter of 1 nm or more and 200 nm or less.

[0032] In the granulated body of the positive electrode active material in the present invention, the total pore volume of the pores with a pore diameter of 1 nm or more and 60 nm or less is 0.100 cm 3 / g or more and 0.300 cm 3It is preferably below / g. In order for the positive electrode active material to contribute to the charge-discharge reaction, not only contact with the electrolyte is required, but also an amount of electrolyte sufficient to exchange lithium ions necessary for the charge-discharge reaction is needed. By appropriately having fine pores with a pore diameter of 1 nm or more and 60 nm or less in the granulated body, the positive electrode active material can come into contact with the amount of electrolyte necessary for charge-discharge, and the charge-discharge reaction proceeds promptly. When the total pore volume of such pores is 0.100 cm 3 / g or more, it is possible to prevent the generation of particles that cannot come into contact with the required amount of electrolyte and the charge-discharge reaction from being retarded. As a result, it is preferable because the generation of overvoltage inside the granulated body can be suppressed and the cycle resistance can be improved. On the other hand, when the total pore volume is 0.300 cm 3 / g or less, it is possible to prevent the void ratio in the granulated body from becoming excessively high and the granulated body from becoming brittle. Therefore, when repeatedly charged and discharged, the structure of the granulated body is not easily collapsed, which is preferable for improving the cycle resistance.

[0033] Here, the total pore volume of pores with a pore diameter of 1 nm or more and 60 nm or less can be measured by the mercury injection method using a pore distribution measuring device, AutoPore IV9520 type (manufactured by Shimadzu Corporation). The measurement conditions are the same as those for the measurement of the pore diameter described above.

[0034] In the granulated body of the positive electrode active material in the present invention, the maximum value of the log differential pore volume of pores with a pore diameter of 1 nm or more and 60 nm or less is preferably 0.30 cm 3 / g or more. The log differential pore volume is an index indicating the change rate of the pore volume with respect to the logarithmically handled pore diameter. The larger the maximum value, the narrower the pore diameter distribution and the more uniform pores are formed. In order for the particles of the positive electrode active material to contribute to the charge-discharge reaction, it is necessary to come into contact with the amount of electrolyte necessary for the charge-discharge reaction. For this purpose, it is preferable to have fine pores with a pore diameter of 1 nm or more and 60 nm or less in a uniform size in the granulated body. When the log differential pore volume of such fine pores is 0.30 cm 3When it is 1 nm or less, the distribution of the pore diameter size is narrow, and pores with a small amount of electrolyte solution are less likely to occur partially. Therefore, the charge and discharge reactions are likely to proceed uniformly as a whole, and overvoltage is less likely to occur inside the granulated body, so the cycle resistance is improved.

[0035] Here, the maximum value of the log differential pore volume of pores with a pore diameter of 1 nm or more and 60 nm or less can be measured by the mercury injection method using a pore size distribution measuring device, Autopore IV9520 type (manufactured by Shimadzu Corporation). The measurement conditions are the same as the measurement conditions for the pore diameter described above.

[0036] In the granulated body of the positive electrode active material in the present invention, the specific surface area of pores with a pore diameter of 1 nm or more and 60 nm or less is 25 m 2 / g or more and 50 m 2 / g or less is preferable. The specific surface area of pores correlates with the contact area between the electrolyte solution and the positive electrode active material particles. When the specific surface area of fine pores with a pore diameter of 1 nm or more and 60 nm or less is 25 m 2 / g or more, the contact area between the positive electrode active material particles and the electrolyte solution increases, and the charge and discharge reaction proceeds rapidly while more suppressing the generation of overvoltage in the granulated body of the positive electrode active material, so the cycle resistance can be further improved. The specific surface area of pores is more preferably 30 m 2 / g or more. On the other hand, when the specific surface area of pores is 50 m 2 / g or less, the formation of excessive voids in the granulated body of the positive electrode active material can be suppressed, and the cycle resistance can be further improved. The specific surface area of pores is more preferably 40 m 2 / g or less.

[0037] Here, the specific surface area of fine pores with a pore diameter of 1 nm or more and 60 nm or less can be measured by the mercury injection method using a pore size distribution measuring device, Autopore IV9520 type (manufactured by Shimadzu Corporation). The measurement conditions are the same as the measurement conditions for the pore diameter described above.

[0038] As means for making the average pore diameter of the granulated particles of the positive electrode active material, the total pore volume of pores with a pore diameter of 1 nm or more and 60 nm or less, the maximum value of the log differential pore volume, and the pore specific surface area in the present invention fall within the above ranges, for example, a method of manufacturing granulated particles of the positive electrode active material by a preferable method described later can be mentioned.

[0039] The specific surface area of the positive electrode active material in the present invention is preferably 30 m 2 / g or more and 45 m 2 / g or less. By setting the specific surface area to 30 m 2 / g or more, the contact area with the electrolyte in the battery becomes large, so the high-rate discharge characteristics can be further improved. On the other hand, by setting the specific surface area to 45 m 2 / g or less, the particle surface of the positive electrode active material is stabilized, so the generation of gas due to side reactions with the electrolyte can be suppressed.

[0040] Here, the specific surface area of the positive electrode active material can be measured by the BET flow method (adsorbing gas N2) using a fully automatic specific surface area measuring device Macsorb HM Model-1210 (manufactured by Mountech Co., Ltd.).

[0041] The volume resistivity of the positive electrode active material in the present invention is preferably 10 5 Ω·cm or less. When the volume resistivity is 10 5 Ω·cm or less, high conductivity is exhibited when the battery is formed, and the high-rate discharge characteristics can be further improved.

[0042] Here, the volume resistivity of the positive electrode active material is measured with the positive electrode active material in a pressed powder state. Specifically, it can be measured under the condition of 25 MPa using a powder resistance measurement system MCP-PD51 (manufactured by Mitsubishi Chemical Analytech Co., Ltd.).

[0043] The average particle size of the granulated particles of the positive electrode active material in the present invention is preferably 1.0 μm or more and 20.0 μm or less. As for the positive electrode active material of a lithium ion battery, it is common to form a mixture layer through coating on an aluminum foil after pasting using N-methylpyrrolidinone as a dispersion medium, followed by drying and pressing. The thickness of the mixture layer is generally 10 μm or more and 200 μm or less, and since it is preferably granulated so as to fit within this thickness, the average particle size is preferably 20.0 μm or less. On the other hand, when the average particle size is 1.0 μm or more, the viscosity of the aforementioned paste can be appropriately suppressed, and the coatability can be improved.

[0044] Here, the average particle size of the granulated particles can be measured using a scanning electron microscope. Specifically, the granulated particles are magnified and observed at a magnification of 3,000 times using a scanning electron microscope, the particle sizes of 100 randomly selected granulated particles are measured, and the number average is calculated, whereby it can be obtained. When the secondary particles are not spherical, the average value of the major axis and the minor axis that can be measured in the two-dimensional image is taken as the particle size.

[0045] Next, the manufacturing method of the positive electrode active material of the present invention will be described.

[0046] The positive electrode active material of the present invention can be obtained, for example, by manufacturing primary particles of LMFP, then making the primary particles into a dispersion in a monodispersed state, subsequently granulating the LMFP primary particles from the dispersion, and forming a carbon coating layer by firing.

[0047] Examples of the manufacturing method of LMFP primary particles include the solid phase method and the liquid phase method. The liquid phase method is suitable in that LMFP primary particles with an average particle size of 10 nm or more and 80 nm or less and a narrow particle size distribution can be obtained more simply. By manufacturing nanoparticles by the liquid phase method, the specific surface area of the LMFP granulated particles is 30 m 2 / g or more and 45 m 2It can be easily adjusted to below / g. As the liquid phase, water or water added with an organic solvent for refining primary particles to nanoparticles is preferable. Examples of the organic solvent include alcohol solvents such as ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, 2-propanol, 1,3-propanediol, and 1,4-butanediol, and dimethyl sulfoxide. Two or more of these may be used. In the process of synthesis, pressure may be applied to enhance the crystallinity of the particles. Note that the ratio of manganese to iron contained in the LMFP primary particles can be adjusted to a desired range according to the charging ratio of the raw materials.

[0048] In the liquid phase method, by adding the raw materials of LMFP to the above-mentioned liquid phase and heating, LMFP primary particles can be obtained. If the raw materials of LMFP are dissolved in an organic solvent, the uniformity of the obtained particles is improved. Therefore, it is preferable to use raw materials with high solubility in the organic solvent. In terms of having high solubility in the mixed solvent of water and the organic solvent, lithium hydroxide is preferably used as the lithium raw material, manganese sulfate is preferably used as the manganese raw material, iron sulfate is preferably used as the iron raw material, and orthophosphoric acid is preferably used as the phosphoric acid raw material. Moreover, these raw materials may be hydrates.

[0049] When obtaining LMFP primary particles by the liquid phase method, the average particle size of the primary particles can be adjusted to a desired range according to conditions such as the mixing ratio of water and the organic solvent in the liquid phase, the concentration of the synthesis solution, and the synthesis temperature. To reduce the average particle size, it is effective to reduce the proportion of water in the liquid phase, lower the concentration of the synthesis solution, and lower the synthesis temperature. Also, it is preferable to add the solutions of the manganese raw material, iron raw material, and phosphoric acid raw material while the lithium raw material solution is being vigorously stirred, and heat to the synthesis temperature without applying pressure while maintaining the vigorously stirred state. The ratio of the peak intensity I 20 / I 29 and I 35 / I 29 can be easily adjusted to the above-mentioned preferable range.

[0050] Examples of the method for granulating LMFP primary particles include, for example, a fluidized bed granulation method and an extrusion granulation method. In order to make the particle size distribution of the granulated body as narrow as possible, it is preferable to use a spray dryer.

[0051] As a method for forming a carbon coating layer on the LMFP primary particles of the LMFP granulated body, for example, after preparing a dispersion of LMFP primary particles, saccharides are added and dissolved, dried and granulated using a spray dryer, and heated and fired at 600°C to 800°C in a nitrogen atmosphere. By firing the LMFP primary particles together with the saccharides, a carbon coating layer can be formed on the surface of the primary particles. Examples of the saccharides include glucose, sucrose, maltose, lactose, fructose, galactose, mannose, dextrin, cyclodextrin, etc. Among these, when water is used as the dispersion medium during spray drying, glucose and sucrose are preferable in consideration of their high solubility in water. The proportion of carbon contained in the LMFP primary particles can be adjusted to a desired range by the addition amount of the saccharides. Also, by increasing the temperature during firing, the volume resistivity of the LMFP primary particles and the LMFP granulated body can be decreased.

[0052] In the present invention, in order to make the number ratio of primary particles having a particle size of 100 nm or more 5.0% or less, it is preferable to make the dispersion state of the LMFP primary particles in the dispersion liquid to be subjected to spray drying a monodisperse state. One of the reasons for the generation of particles having a particle size of 100 nm or more is particle growth during firing. In order to suppress particle growth, it is effective to lower the firing temperature. However, on the other hand, the carbonization of the saccharides becomes insufficient, and the conductivity tends to decrease. That is, there is a trade-off relationship between the suppression of particle growth and the expression of high conductivity. However, through the studies of the present inventors, it has been found that by making the LMFP particles in the dispersion liquid to be subjected to spray drying in a monodisperse state, the contact area between the particles inside the granulated body is reduced, so that even when the firing temperature is high, particle growth can be suppressed.

[0053] In order to make the dispersion state of LMFP primary particles a monodisperse state, after synthesizing LMFP primary particles in a liquid phase, it is preferable to wash them with pure water without drying and then go through a disintegration process. By not going through the drying process, drying aggregation can be suppressed. Washing with pure water also serves to adjust the pH of the dispersion liquid. In the case of liquid-phase synthesis, since there are trace amounts of residual ions generated by the synthesis, pH adjustment can be achieved by repeating the washing until the desired pH is reached. Compared with the method of adjusting the pH by adding additives such as sodium hydroxide, it does not require the addition of unnecessary ions to LMFP, so a decrease in the energy density in the battery can be suppressed. In order to improve the dispersion state of the primary particles of LMFP, the pH of the dispersion liquid is preferably 9 or more and 11 or less. Examples of the disintegration device used in the disintegration process include a shear mixer, a planetary ball mill, a bead mill, an ultrasonic homogenizer, a dry jet mill, etc. In terms of being able to process the LMFP primary particles without drying them and keeping them in the form of a dispersion liquid, a shear mixer, a wet jet mill, a bead mill, and an ultrasonic homogenizer are preferable, and in terms of being able to uniformly disintegrate the dispersion liquid, a shear mixer and a wet jet mill are more preferable.

[0054] Here, the dispersion state of the dispersion liquid can be evaluated by a dynamic light scattering particle size distribution measuring device. If the obtained average particle size is within twice the average particle size of the primary particles measured by a scanning electron microscope, it shall be judged to be in a monodisperse state.

[0055] In order to make the specific surface area of the LMFP granulated body in the present invention 30 m 2 / g or more and 45 m 2 / g or less, it is preferable to make the average particle size of the LMFP primary particles 30 nm or more and 60 nm or less.

[0056] In order to make the average particle size of the LMFP granulated body in the present invention 1.0 μm or more and 20.0 μm or less, for example, in the above-mentioned manufacturing method, it is preferable to make the concentration of the dispersion liquid to be subjected to spray drying 20 wt% or more and 60 wt% or less.

[0057] The positive electrode for a lithium-ion secondary battery can be obtained, for example, by applying a paste in which the above-described granulated bodies are dispersed in a dispersion medium onto a current collector, drying it, and pressing it to form a mixture layer. As a method for producing the paste, it is preferable to mix the above-described granulated bodies, and further, additives such as a conductive aid, a binder, and N-methylpyrrolidinone as required, and knead them until solid, and then add a dispersion medium such as water or N-methylpyrrolidinone to adjust the viscosity. The solid content concentration of the paste can be appropriately selected according to the coating method. From the viewpoint of making the coating film thickness uniform, 30% by weight or more and 80% by weight or less is preferable. Each material of the paste may be mixed at once, or in order to uniformly disperse each material in the paste, they may be added and mixed in order while repeating kneading. As a kneading device for the slurry, a planetary mixer or a thin-film swirling high-speed mixer is preferable in terms of being able to knead uniformly.

[0058] Examples of the binder include polyvinylidene fluoride, styrene-butadiene rubber, and the like. Two or more of these may be contained. The content of the binder in the mixture layer is preferably 0.3% by weight or more and 10% by weight or less. By setting the content of the binder to 0.3% by weight or more, the coating film shape can be easily maintained when a coating film is formed due to the binding effect of the binder. On the other hand, by setting the content of the binder to 10% by weight or less, an increase in the resistance inside the electrode can be suppressed.

[0059] Examples of the conductive aid include acetylene black, ketjen black, carbon fiber, carbon nanotube, and the like. Two or more of these may be contained. The content of the conductive aid in the mixture layer is preferably 0.3% by weight or more and 10% by weight or less. By setting the content of the conductive aid to 0.3% by weight or more, the conductivity of the positive electrode can be improved and the electron resistance can be reduced. On the other hand, by setting the content of the conductive aid to 10% by weight or less, the inhibition of the movement of lithium ions can be suppressed and the decrease in ion conductivity can be suppressed.

[0060] In order to increase the energy density of a lithium-ion secondary battery, it is preferable that the positive electrode active material be contained in the binder layer at as high a ratio as possible. The content of the positive electrode active material in the binder layer is preferably 80% by weight or more, more preferably 90% by weight or more.

[0061] The thickness of the binder layer is preferably 10 μm or more and 200 μm or less. By setting the thickness of the binder layer to 10 μm or more, the ratio of the current collector in the battery can be suppressed, and the energy density can be further improved. On the other hand, by setting the thickness of the binder layer to 200 μm or less, the charge-discharge reaction can proceed rapidly throughout the binder layer, and the high-rate charge-discharge characteristics can be further improved.

[0062] The lithium-ion secondary battery of the present invention preferably has a negative electrode, a separator, and an electrolytic solution in addition to the above positive electrode. Examples of the shape of the battery include a rectangular shape, a wound shape, and a laminate shape, and can be appropriately selected according to the purpose of use. Examples of the material constituting the negative electrode include graphite, lithium titanate, and silicon oxide. Any separator and electrolytic solution can also be appropriately selected and used.

[0063] The lithium-ion secondary battery of the present invention can be obtained, for example, by laminating the above-described positive electrode with a negative electrode through a separator in a dry environment having a dew point of -50°C or lower and adding an electrolytic solution.

Examples

[0064] Hereinafter, the present invention will be specifically described by way of examples, but the present invention is not limited to these examples only. First, the evaluation method in the examples will be described.

[0065] [Measurement A] Composition ratio of LMFP 15 mg of the LMFP granules used in each example and comparative example were decomposed by heating using perchloric acid and nitric acid, and diluted to 100 mL with ultrapure water. For this solution, Li was measured by atomic absorption spectrometry, and Mn, Fe, and P were measured by ICP emission spectrometry. The content of each element in the sample was determined and converted to an atomic ratio.

[0066] [Measurement B1] Average primary particle size, percentage of particles with a particle size of 100 nm or more, and average granule size The LMFP granules used in each example and comparative example were observed at a magnification of 200,000 times using a scanning electron microscope S-5500 (manufactured by Hitachi High-Technologies Corporation). The particle sizes of 200 randomly selected primary particles were measured, and the average particle size of the LMFP primary particles was calculated by calculating the number average value. However, when the particles were not spherical, the average value of the major axis and minor axis that could be measured in the two-dimensional image was used as the particle size. When two or more particles were connected by sintering, they were treated as one particle. When it was difficult to determine whether it was sintering or contact, the image was binarized into white and black, and if a line for dividing the connected part was obtained, it was considered contact and treated as two particles, and if not, it was considered sintering and treated as one particle.

[0067] Also, among the 200 measured particles, the number of particles with a particle size of 100 nm or more was counted, and the percentage ratio to the 200 particles was calculated.

[0068] Similarly, the LMFP granules used in each example and comparative example were observed at a magnification of 3,000 times using a scanning electron microscope S-5500 (manufactured by Hitachi High-Technologies Corporation). The particle sizes of 100 randomly selected granules were measured, and the average granule size was calculated by calculating the number average value. However, when the granules were not spherical, the average value of the major axis and minor axis that could be measured in the two-dimensional image was used as the granule size.

[0069] [Measurement B2] Average pore diameter of the mitochondria, total pore volume of pores with a pore diameter of 1 nm or more and 60 nm or less, maximum value of the log differential pore volume of pores with a pore diameter of 1 nm or more and 60 nm or less, and pore specific surface area of pores with a pore diameter of 1 nm or more and 60 nm or less 0.3 g of the LMFP granules used in each example and comparative example was placed in a 5 cc powder cell, and the pore distribution was determined by the mercury injection method using a pore distribution measuring device, Autopore IV9520 type (manufactured by Shimadzu Corporation), under the condition of an initial pressure of 7 kPa. The mercury parameters were a mercury contact angle of 130.0° and a mercury surface tension of 485.0 Dynes / cm. However, in order to distinguish the voids between the granules from the pores, the measurement of the average pore diameter was carried out in the range of a pore diameter of 1 nm or more and 200 nm or less. Also, the median value of the pore diameter was adopted as the average pore diameter.

[0070] [Measurement C] Specific surface area For the LMFP granules used in each example and comparative example, the specific surface area was measured by the BET flow method (adsorption gas N2) using a fully automatic specific surface area measuring device, Macsorb HM Model-1210 (manufactured by Mountech Co., Ltd.).

[0071] [Measurement D] Weight ratio of carbon contained in the LMFP granules For the LMFP granules used in each example and comparative example, the weight ratio of carbon contained was measured using a carbon sulfur analyzer, EMIA-810W (manufactured by Horiba, Ltd.).

[0072] [Measurement E] Volume resistivity For 1.0 g of the positive electrode active material used in each example and comparative example, the volume resistivity under 25 MPa was measured using a powder resistance measurement system, MCP-PD51 (manufactured by Mitsubishi Chemical Analytech Co., Ltd.).

[0073] [Measurement F] Peak intensity ratio of X-ray diffraction The peak intensity ratios of the X-ray diffraction of the cathode active materials used in each example and comparative example were measured using a D8 ADVANCE manufactured by Bruker·ASX. The measurement conditions were 2θ = 5° to 70°, scan interval 0.02°, and scan speed 20 seconds / deg. Each peak intensity ratio was calculated by removing the background (coefficient 1.77) using the powder X-ray diffraction analysis software EVA (manufactured by Bruker·ASX) and reading the peak intensity.

[0074] [Measurement G] Average particle size of LMFP primary particles in the dispersion liquid for spray drying Regarding the LMFP dispersion liquid for spray drying before adding glucose used in each example and comparative example, the average particle size of LMFP primary particles was measured using a dynamic light scattering particle size distribution measuring device nanoPartica SZ-100V2 (manufactured by Horiba, Ltd.).

[0075] [Measurement H] High-rate discharge characteristics (energy density measurement) The electrode plates fabricated in each example and comparative example were cut out to a diameter of 15.9 mm to serve as the positive electrode, a lithium foil cut out to a diameter of 16.1 mm and a thickness of 0.2 mm was used as the negative electrode, and a "Celgard" (registered trademark) (manufactured by Toray Industries, Inc.) separator cut out to a diameter of 20 mm was used. A solution of ethylene carbonate:diethyl carbonate = 3:7 (volume ratio) containing 1 M LiPF6 was used as the electrolyte to fabricate 2032-type coin cells.

[0076] For the obtained coin cells, the cut-off potential was set to 2.5 V, the maximum charge voltage was set to 4.3 V, charge and discharge were performed at a 0.1C rate three times, and the energy density (Wh / kg) per positive electrode weight at 0.1C was measured from the third discharge. Subsequently, charging was performed at a 0.1C rate, discharging was performed at a 4.0C rate, and the energy density (Wh / kg) per positive electrode weight at 4.0C was measured. As an evaluation of the high-rate discharge characteristics, the ratio of the energy density at 4.0C discharge to the energy density at 0.1C discharge was determined.

[0077] [Measurement I] Cycle durability A 2032 coin cell was fabricated in the same manner as the measurement H, and charge and discharge were performed 3 times at a 0.1C rate in an environment of 25°C. Subsequently, charge and discharge were performed once at a 1C rate in an environment of 50°C, and the discharge energy density at this time was defined as the initial energy density. Subsequently, charge and discharge were performed at a 1C rate while maintaining the environment at 50°C, and the number of cycles was determined when the discharge energy density became less than 80% of the initial energy density, and it was evaluated as cycle resistance.

[0078] In all charge and discharge tests, charging was carried out at a constant current until the maximum voltage of 4.3V was reached, and after reaching the maximum voltage, charging was carried out at the maximum voltage until the charging current fell below 0.01C. Discharging was carried out at a constant current until the discharge voltage fell below 2.5V.

[0079] [Example 1] After dissolving 60 mmol of lithium hydroxide monohydrate in 25 g of pure water, 60 g of diethylene glycol was added to prepare an aqueous lithium hydroxide / diethylene glycol solution. To the resulting aqueous lithium hydroxide / diethylene glycol solution being stirred at 2000 rpm using a homodisper (Homodisper 2.5 type manufactured by Primix Corporation), an aqueous solution obtained by dissolving 20 mmol of phosphoric acid (85% aqueous solution), 16 mmol of manganese sulfate monohydrate, and 4 mmol of iron sulfate heptahydrate in 10 g of pure water was added to obtain a lithium manganese phosphate nanoparticle precursor. The resulting precursor solution was heated to 100°C and held for 2 hours to obtain LMFP nanoparticles as a solid content. The obtained LMFP was washed without drying by adding pure water and repeating solvent removal by a centrifuge to adjust the pH of the dispersion to 10.1. After adjusting the solid content concentration of the obtained dispersion to 50% by weight, dispersion treatment was performed under the conditions of 150 MPa and 2 passes using a wet jet mill Starburst Mini (manufactured by Sugino Machine).

[0080] Glucose was added to the obtained LMFP dispersion at a ratio of 0.15 g per 1.0 g of LMFP and dissolved. Subsequently, the LMFP dispersion was dried and granulated with hot air at 200 °C using a spray dryer (MDL-050B manufactured by Fujisaki Electric Co., Ltd.). The obtained particles were heated at 700 °C for 4 hours in a nitrogen atmosphere using a rotary kiln (Desktop Rotary Kiln manufactured by Takasago Industries Co., Ltd.) to obtain a granulated product of LMFP having a carbon coating layer.

[0081] After mixing acetylene black (Li-400 manufactured by Denka Co., Ltd.) and a binder (KF Polymer L#9305 manufactured by Kureha Corporation), the obtained LMFP granulated product was added and kneaded in a mortar. At that time, the mass ratio of each material contained was such that granulated product:acetylene black:binder was 90:5:5. Thereafter, N-methylpyrrolidinone was added and adjusted so that the solid content became 48% by mass to obtain a slurry-like electrode paste. N-methylpyrrolidinone was added to the obtained paste until fluidity was achieved, and it was treated for 30 seconds under stirring conditions of 40 m / sec using a thin-film swirling type high-speed mixer ("Filmix" (registered trademark) 40-L type manufactured by Primix Corporation).

[0082] The obtained electrode paste was applied to an aluminum foil (thickness 18 μm) using a doctor blade (300 μm), dried at 80 °C for 30 minutes, and then pressed to produce an electrode plate.

[0083] [Example 2] An electrode plate was produced in the same manner as in Example 1 except that the amount of diethylene glycol during LMFP synthesis was changed to 80 g.

[0084] [Example 3] An electrode plate was produced in the same manner as in Example 1 except that the amount of diethylene glycol during LMFP synthesis was changed to 120 g.

[0085] [Example 4] An electrode plate was produced in the same manner as in Example 1 except that the amount of glucose to be added was changed to 0.07 g per 1.0 g of LMFP.

[0086] [Example 5] An electrode plate was prepared in the same manner as in Example 1, except that the amount of glucose added was 0.22 g with respect to 1.0 g of LMFP.

[0087] [Example 6] An electrode plate was prepared in the same manner as in Example 1, except that the amount of glucose added was 0.11 g with respect to 1.0 g of LMFP and the temperature during firing was 600 °C.

[0088] [Example 7] An electrode plate was prepared in the same manner as in Example 1, except that instead of using a wet jet mill for the dispersion treatment of LMFP, it was treated with a shear mixer (Model AX5 head: emulsifying screen, manufactured by Silver Son Nippon Co., Ltd.) at 5000 rpm for 5 minutes.

[0089] [Comparative Example 1] An electrode plate was prepared in the same manner as in Example 1, except that the dispersion treatment was not performed using a wet jet mill.

[0090] [Comparative Example 2] An electrode plate was prepared in the same manner as in Example 1, except that the pH adjustment of the LMFP dispersion was performed by adding LiOH instead of washing with pure water.

[0091] [Comparative Example 3] 60 mmol of lithium hydroxide monohydrate, 20 mmol of phosphoric acid (85% aqueous solution), 16 mmol of manganese sulfate monohydrate, and 4 mmol of ferrous sulfate heptahydrate were added to 40 g of pure water, placed in a pressure-resistant container, heated to 180 °C, and held for 8 hours to obtain LMFP particles as a solid content.

[0092] Pure water was added to the obtained LMFP, and the solvent was removed by centrifugation and repeated 5 times for washing. The obtained LMFP dispersion was dried on a hot plate to obtain a powder. When the average particle diameter of the obtained primary LMFP particles was measured in the same manner as in Measurement Example B, it was 281 nm.

[0093] The obtained LMFP powder was subjected to a grinding process using a planetary ball mill P5 (manufactured by Fritsch). The container used for the grinding process was a 45 ml zirconia container, and 18 zirconia beads with a diameter of 10 mm were used as the beads. The processing conditions were a rotation speed of 300 rpm for 6 hours.

[0094] Water was added to the obtained LMFP to form a dispersion, and further glucose was added at a ratio of 0.15 g per 1.0 g of LMFP and dissolved. Subsequently, the LMFP dispersion was dried and granulated using a spray dryer (MDL-050B manufactured by Fujisaki Electric Co., Ltd.) with hot air at 200°C. The obtained particles were heated at 700°C for 4 hours in a nitrogen atmosphere using a rotary kiln (desktop rotary kiln manufactured by Takasago Kogyo Co., Ltd.) to obtain a granulated body of LMFP having a carbon coating layer.

[0095] An electrode plate was produced in the same manner as in Example 1 using the obtained granulated body of LMFP.

[0096] [Comparative Example 4] An electrode plate was produced in the same manner as in Comparative Example 3 except that the processing conditions of the planetary ball mill were changed to 200 rpm for 2 hours.

[0097] The evaluation results of each example and comparative example are shown in Tables 1 and 2.

[0098]

Table 1

[0099]

Table 2

Claims

1. A granulated body of positive electrode active material particles for a lithium-ion secondary battery, wherein the lithium-ion secondary battery positive electrode active material particles are lithium manganese iron phosphate particles represented by Li α Mn a Fe b PO 4 (0.9 ≦ α ≦ 1.1, 0.6 ≦ a ≦ 1.0, 0 < b ≦ 0.4, 0.9 ≦ a + b ≦ 1.1), the primary particles have a carbon coating layer on the surface, the average particle size of the primary particles is 10 nm or more and 80 nm or less, and the number ratio of particles having a particle size of 100 nm or more is 5.0% or less. A positive electrode active material for a lithium-ion secondary battery.

2. The positive electrode for a lithium-ion secondary battery according to Claim 1, wherein the lithium-ion secondary battery positive electrode active material particles are olivine-type positive electrode active material particles.

3. The positive electrode active material for a lithium-ion secondary battery according to Claim 1, wherein the proportion of carbon contained in the granulated body is 2.0% by weight or more and 5.0% by weight or less.

4. The ratio I of the peak intensity at 20° to the peak intensity at 29° obtained by X-ray diffraction ２０ / I ２９ is 0.88 or more and 1.05 or less, and the ratio I of the peak intensity at 35° to the peak intensity at 29° ３５ / I ２９ is 1.05 or more and 1.20 or less, the positive electrode active material for a lithium ion secondary battery according to claim 1.

5. The sum of the pore volumes of pores having an average pore diameter of 10 nm or more and 60 nm or less and a pore diameter of 1 nm or more and 60 nm or less is 0.100 cm ３ / g or more and 0.300 cm ３ / g or less, and the maximum value of the log differential pore volume of pores having a pore diameter of 1 nm or more and 60 nm or less is 0.30 cm ３ / g or more. The positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 4.

6. The pore specific surface area of pores with a pore diameter of 1 nm or more and 60 nm or less is 25 m ２ / g or more and 50 m ２ / g or less, the positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 5.

7. The specific surface area is 30 m ２ / g or more and 45 m ２ / g or less, and the positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 6.

8. The volume resistivity is 10 ５ Ω·cm or less, and the positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 7.

9. The positive electrode active material for a lithium-ion secondary battery according to any one of Claims 1 to 8, wherein the average particle size of the granulated body is 1.0 μm or more and 20.0 μm or less.

10. A lithium-ion secondary battery using the positive electrode active material for a lithium-ion secondary battery according to any one of Claims 1 to 9.

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