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

Abstract

An object of the present invention is to provide positive electrode active material particles for lithium ion secondary batteries that can produce lithium ion secondary batteries with excellent high-rate discharge characteristics. The present invention provides a positive electrode active material for lithium ion secondary batteries, comprising granules of positive electrode active material particles for lithium ion secondary batteries, wherein the average primary particle size is from 10 nm to 80 nm, and the percentage of primary particles having a particle size of 100 nm or greater is 5.0% or less.

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] Lithium-ion secondary batteries are primarily used in electric vehicles, and as a result, they are required to have further improved characteristics. High-rate discharge performance plays a crucial role in vehicle acceleration, requiring them to maintain high energy density even during high-rate discharge.

[0003] On the other hand, if a lithium-ion secondary battery fails, the stored energy is released in a short period of time, posing a risk of battery fire or combustion. Therefore, for lithium-ion secondary batteries, improving safety while increasing energy density is also an important issue.

[0004] The safety of lithium-ion secondary batteries is known to be significantly influenced by the positive electrode active material. Layered oxide-based positive electrode active materials, particularly those commonly used in smartphones and electric vehicles, offer high energy density, but they also present safety challenges, such as the risk of fire from the release of oxygen within the battery during overcharging.

[0005] On the other hand, olivine-based positive electrode active materials (LiMPO4) such as lithium iron phosphate (LiFePO4), which are widely used in stationary batteries, do not easily release oxygen due to the covalent bonding of oxygen and phosphorus. They are known to be relatively stable and safe positive electrode materials under high temperature conditions.

[0006] Among the olivine-based positive electrode active materials, lithium manganese iron phosphate has lower ion conductivity and electronic conductivity than lithium iron phosphate, making it difficult to achieve high-speed discharge when made into a battery. To solve this problem, research has been carried out. For example, a positive electrode active material for a lithium secondary battery having the chemical formula A x MB y O z (A is an alkali metal or alkaline earth metal, M contains at least two transition metal elements, B is a typical element that covalently bonds with oxygen O to form an anion, 0≤x≤2, 1≤y≤2, 3≤x≤7) shown in the olivine structure, with part or all of the surface covered with a carbon material (for example, refer to patent document 1); a positive electrode active material for a lithium secondary battery, which comprises carbon-covered polyanionic compound particles (for example, refer to patent document 2), etc.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent Application Laid-Open No. 2014-32803

[0010] Patent Document 2: International Publication No. 2014 / 017617. Summary of the Invention

[0011] Problems to be solved by the invention

[0012] In general, the high-rate discharge characteristics of the positive electrode materials of lithium-ion batteries are controlled by electronic conductivity and ionic conductivity, and it is often required to improve them. Among them, the electronic conductivity and ionic conductivity of lithium manganese iron phosphate are particularly low, so it is difficult to improve the energy density (Wh / kg) during high-rate discharge. In order to compensate for the low electronic conductivity and ionic conductivity, it is effective to make the primary particles of lithium manganese iron phosphate into nanoparticles with a particle size of less than 100nm, thereby reducing the solid diffusion distance of electrons and lithium ions in the primary particles during the charge and discharge reaction. In addition, it is also effective to mix lithium manganese iron phosphate with a carbon source such as glucose and calcine them at a high temperature of more than 600°C, thereby forming a carbon coating on the surface of the lithium manganese iron phosphate particles to improve electronic conductivity.

[0013] However, while nanoparticle formation shortens the diffusion distance within the solid, it also increases the surface energy of the particles. Therefore, during calcination to form the carbon coating, the particles tend to sinter together. These coarse particles, resulting from sintering, can sometimes form throughout the lithium manganese iron phosphate particles depending on the calcination temperature, but they only form locally under optimal calcination conditions.

[0014] The particles become coarser and connected due to sintering, which degrades the high-rate discharge characteristics. On the other hand, conventional evaluation methods, such as the crystallite size determined by powder X-ray diffraction and the average particle size of multiple particles measured using an electron microscope, focus on the characteristics of macroscopic sintering, and have failed to provide a solution for suppressing the microscopic sintering of lithium manganese iron phosphate primary particles.

[0015] Therefore, the materials mainly composed of lithium manganese iron phosphate obtained by the methods disclosed in Patent Documents 1 and 2 tend to produce coarse particles due to sintering of particles during calcination, resulting in insufficient high-rate discharge characteristics.

[0016] In view of the above problems, an object of the present invention is to provide a positive electrode active material for a lithium ion secondary battery that can provide a lithium ion secondary battery having excellent high-rate discharge characteristics.

[0017] Means for solving problems

[0018] In order to solve the above-mentioned problems, the present invention mainly has the following configurations.

[0019] A positive electrode active material for a lithium ion secondary battery is a granulation of positive electrode active material particles for a lithium ion secondary battery, wherein the average particle size of the primary particles is 10 nm to 80 nm, and the number ratio of primary particles with a particle size of 100 nm or more is 5.0% or less.

[0020] Effects of the Invention

[0021] 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. DETAILED DESCRIPTION

[0022] The positive electrode active material for the lithium-ion secondary battery of the present invention (hereinafter sometimes referred to as "positive electrode active material") refers to a substance that can react reversibly with lithium ions, and can be, for example, LiMn2O4 known as a spinel-based positive electrode active material, LiMO2 known as a layered oxide-based positive electrode active material (M is selected from one or more of Mn, Co, Ni, and Al), and LiMPO4 known as an olivine-based positive electrode active material (M is selected from one or more of Fe, Mn, Co, and Ni).

[0023] The average particle size of the primary particles of the positive electrode active material of the present invention is 10 nm or more and 80 nm or less, and the number ratio of particles with a particle size of 100 nm or more is 5.0% or less. The high-speed discharge characteristics of the positive electrode active material vary depending on the particle size of the primary particles. According to research by the present inventors, it was found that high high-speed discharge characteristics were obtained by setting the average particle size of the primary particles to 10 nm or more and 80 nm or less and setting the number ratio of primary particles with a particle size of 100 nm or more to 5.0% or less.

[0024] The term "granules" as used herein refers to a granular structure formed by agglomeration of a plurality of primary particles. However, if a plurality of primary particles are aggregated and the overall shape is amorphous, this is considered an agglomerate and is not included in the granules.

[0025] The average particle size of the primary particles of the positive electrode active material in the present invention is more than 10nm and less than 80nm. Generally speaking, the electronic conductivity and ion conductivity of the positive electrode active material are low. In order to improve the high-speed discharge characteristics, it is necessary to reduce the average particle size of the primary particles and shorten the solid diffusion distance of electrons and lithium ions in the particles. If the average particle size of the primary particles of the positive electrode active material is greater than 80nm, the solid diffusion distance becomes longer and the high-speed discharge characteristics are reduced. On the other hand, if the average particle size of the primary particles of the positive electrode active material is less than 10nm, the crystallinity of the surface of the primary particles is reduced, the part that cannot contribute to the charge and discharge reaction increases, and the energy density is reduced.

[0026] 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 size is measured for 200 randomly selected primary particles, and the number average is calculated, thereby determining the average particle size. 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 used as its particle size. When two or more particles are connected due to sintering, they are treated as one particle. When sintering or contact is difficult to determine, the image is binarized into white and black, and when a line that divides the connected portion is obtained, it is treated as two particles as being in contact, and when it cannot be obtained, it is treated as one particle as being sintered.

[0027] The present inventors focused on the sintering of primary particles of the microscopic positive electrode active material, and found that the high-speed charging characteristics changed according to the ratio of the number of particles with a particle size of 100 nm or more as an indicator. In the positive electrode active material of the present invention, among the positive electrode active material particles forming the granules, the ratio of the number of particles with a particle size of 100 nm or more is 5.0% or less. Particles with a particle size of 100 nm or more are particles that become coarse due to sintering, causing a decrease in high-speed discharge characteristics. Therefore, if the ratio of the number of particles with a particle size of 100 nm or more is greater than 5.0%, the high-speed charging characteristics are reduced. The ratio of the number of particles with a particle size of 100 nm or more is preferably 3.0% or less. In a state where a small amount of positive electrode active material with a particle size of 100 nm or more is mixed with positive electrode active material particles with a particle size of 80 nm or less, even if the crystallite size and average particle size are evaluated, it is difficult to obtain a result suggesting the presence of particles with a size of 100 nm or more. Therefore, in the present invention, a scanning electron microscope is used to magnify and observe the positive electrode active material at a magnification of 200,000 times, and the particle size is measured for 200 randomly selected primary particles, and the number ratio (%) of particles with 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 used as its particle size. When two or more particles are connected due to sintering, they are treated as one particle. When sintering or contact is difficult to judge, the image is binarized into white and black, and when a line is obtained to divide the connected part, it is treated as contact and as two particles. When it cannot be obtained, it is treated as sintered and as one particle.

[0028] The positive electrode active material of the present invention is preferably the above-mentioned olivine-based positive electrode active material. Olivine-based positive electrode active materials are highly safe among positive electrode active materials, but on the other hand, their electron conductivity and ion conductivity are particularly low, resulting in low high-rate discharge characteristics. The effects of the present invention improve electron conductivity and ion conductivity, making it possible to obtain a safe battery with high high-rate discharge characteristics.

[0029] The primary particles of the olivine-based positive electrode active material in the present invention preferably have a carbon coating layer on their surfaces. In other words, the carbon preferably exists in the form of a coating on the surfaces of the primary particles.

[0030] The ratio of carbon contained in the granules of the olivine-based positive active material in the present invention is preferably 2.0% by weight or more and 5.0% by weight or less. By containing more than 2.0% by weight of carbon, high conductivity is exhibited in the battery, and thus the high-speed discharge characteristics can be further improved. On the other hand, by containing less than 5.0% by weight of carbon, it is difficult to hinder the migration of lithium ions inserted and removed in the primary particles of the positive active material, and the high-speed discharge characteristics can be further improved. It should be noted that the carbon contained in the granules of the olivine-based positive active material is preferably carbon derived from a carbon coating.

[0031] Here, the weight ratio of carbon contained in the granules of the olivine-based positive electrode active material can be measured using a carbon-sulfur analyzer EMIA-810W (manufactured by Horiba, Ltd.).

[0032] The positive electrode active material of the present invention is Li among the olivine-based positive electrode active materials. α Mn a Fe b Lithium manganese iron phosphate (hereinafter sometimes referred to as "LMFP") represented by PO4 (0.9 ≤ α ≤ 1.1, 0.6 ≤ a ≤ 1.0, 0 < b ≤ 0.4, 0.9 ≤ a + b ≤ 1.1) is preferred for achieving batteries with 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.

[0033] The composition of LMFP can be estimated based on the raw material ratios used during LMFP particle synthesis. Alternatively, the composition of the resulting LMFP can be determined by atomic absorption spectrometry for lithium and ICP emission spectrometry for manganese, iron, and phosphorus. The above formulas α, a, and b are determined to the third decimal place and rounded to two decimal places.

[0034] The LMFP in the present invention is preferably a ratio of the peak intensity at 20° to the peak intensity at 29° obtained by X-ray diffraction. 20 / I 29 The ratio of the peak intensity at 35° to the peak intensity at 29° obtained by X-ray diffraction is 0.88 or more and 1.05 or less. 35 / I 29 It is preferably 1.05 or more and 1.20 or less.

[0035] The 20° peak, 29° peak, and 35° peak obtained by powder X-ray diffraction are labeled with the indices (101), (020), and (311) planes, respectively. The intensity of each peak represents the strength of the orientation on that crystal plane. In particular, the (020) plane is the plane that is most likely to grow in LMFP, and there is a tendency for the orientation to become stronger. Therefore, I 20 / I 29 and I 35 / I 29 Being within this range means that the primary particles in the positive electrode active material are not oriented in the (020) plane, but rather have undergone homogeneous crystal growth, resulting in a nearly spherical particle shape. In LMFP, where the lattice volume changes by as much as 10% during charge and discharge, having a homogeneous shape mitigates the strain within the particles that occurs during charge and discharge, further improving high-rate discharge characteristics.

[0036] Here, the X-ray diffraction peak of LMFP can be measured using an X-ray diffraction apparatus using Cu as an X-ray source.

[0037] The average diameter of the micropores contained in the granules of the positive electrode active material in the present invention is preferably 10 nm or more and 60 nm or less. The positive electrode active material expands and contracts during the charge and discharge reaction, so when repeatedly charged and discharged, the structure of the granules collapses and the cycle tolerance is reduced. The average diameter of the micropores is 60 nm or less, thereby preventing the proportion of voids in the granules from becoming too high, suppressing the granules from becoming brittle, and improving the cycle tolerance, so it is preferred. On the other hand, the average diameter of the micropores is 10 nm or more, which suppresses the delay in charge and discharge reaction caused by insufficient liquid amount of the electrolyte for exchanging the positive electrode active material and lithium ions, and can suppress the reduction in cycle tolerance caused by overvoltage generated inside the granules, so it is preferred.

[0038] Here, the average diameter of the micropores refers to the median diameter and can be measured by mercury penetration using a pore distribution analyzer, AutoPore IV 9520 (manufactured by Shimadzu Corporation). The measurement was performed under an initial pressure of 7 kPa, with mercury parameters set to a mercury contact angle of 130.0° and a mercury surface tension of 485.0 dynes / cm. However, in order to distinguish between voids between granules and micropores, the average diameter of the micropores was measured within the range of pore diameters from 1 nm to 200 nm.

[0039] In the granules of the positive electrode active material of the present invention, the total pore volume of pores having a pore diameter of 1 nm or more and 60 nm or less is preferably 0.100 cm 3 / g or more and 0.300cm 3 / g or less. In order for the positive electrode active material to contribute to the charge and discharge reaction, it is not only in contact with the electrolyte, but also requires an amount of electrolyte that can exchange lithium ions required for the charge and discharge reaction. By appropriately having fine micropores with a pore diameter of 1 nm or more and 60 nm or less in the granules, the positive electrode active material can contact the amount of electrolyte required for charge and discharge, and the charge and discharge reaction proceeds rapidly. If the sum of the pore volumes of the micropores is 0.100 cm 3 / g or more, it is possible to prevent the generation of particles that cannot contact the required amount of electrolyte and thus delay the charge and discharge reaction. As a result, the generation of overvoltage inside the granules is suppressed, and the cycle resistance can be improved, which is preferred. On the other hand, if the total pore volume is 0.300cm 3 / g or less, because the ratio of voids in the granules is prevented from becoming too high, and the granules are not brittle, the structure of the granules is less likely to collapse during repeated charge and discharge, and the cycle resistance is improved, which is preferred.

[0040] Here, the total pore volume of pores with a pore diameter of 1 nm to 60 nm can be measured by mercury intrusion porosimetry using a pore distribution analyzer AutoPore IV9520 (manufactured by Shimadzu Corporation). The measurement conditions are the same as those for the pore diameter measurement described above.

[0041] In the granules of the positive electrode active material of the present invention, the maximum value of the log differential pore volume of pores having a pore diameter of 1 nm to 60 nm is preferably 0.30 cm 3 / g or more. The log differential micropore volume is an indicator that represents the rate of change of the micropore volume with respect to the micropore diameter after logarithmic processing. The larger the maximum value, the narrower the distribution of the micropore diameter, and the more uniform the micropores. In order for the particles of the positive electrode active material to contribute to the charge and discharge reaction, it is necessary to contact with the electrolyte required for the charge and discharge reaction. Therefore, it is preferred to have fine micropores with a uniform size of 1 nm or more and 60 nm or less in the granules. If the log differential micropore volume of the fine micropores is 0.30 cm 3 When the pore size distribution is less than 1 / g, the pore size distribution is narrow, and the formation of pores with a small amount of electrolyte is less likely to occur. Therefore, the charge and discharge reaction is more likely to proceed uniformly, and overvoltage is less likely to occur in the granules, thereby improving the cycle resistance.

[0042] 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 mercury intrusion porosimetry using a pore distribution analyzer AutoPore IV9520 (manufactured by Shimadzu Corporation). The measurement conditions are the same as those for the pore diameter measurement described above.

[0043] In the granules of the positive electrode active material of the present invention, the micropore specific surface area of micropores having a micropore diameter of 1 nm or more and 60 nm or less is preferably 25 m 2 / g or above and 50m 2 / g or less. The micropore specific surface area is related to the contact area between the electrolyte and the positive electrode active material particles. If the micropore specific surface area of fine micropores with a micropore diameter of 1nm or more and 60nm or less is 25m 2 / g or more, the contact area between the positive electrode active material particles and the electrolyte increases, further suppressing the overvoltage in the granules of the positive electrode active material, while the charge and discharge reaction proceeds rapidly, thereby further improving the cycle resistance. The micropore specific surface area is more preferably 30m 2 / g or more. On the other hand, if the micropore specific surface area is 50m 2 / g or less, the formation of excessive voids in the granules of the positive electrode active material is suppressed, and the cycle resistance can be further improved. The micropore specific surface area is more preferably 40m 2 / g or less.

[0044] Here, the pore specific surface area of fine pores with a pore diameter of 1 nm to 60 nm can be measured by mercury intrusion porosimetry using a pore distribution analyzer AutoPore IV 9520 (manufactured by Shimadzu Corporation). The measurement conditions are the same as those for the pore diameter measurement described above.

[0045] As means for setting the average diameter of the micropores of the granules of the positive electrode active material in the present invention, the sum of the micropore volumes of micropores with a micropore diameter of 1 nm or more and 60 nm or less, the maximum value of the log differential micropore volume and the micropore specific surface area to the aforementioned ranges, for example, a method for producing granules of the positive electrode active material by the preferred method described later can be cited.

[0046] The specific surface area of the positive electrode active material in the present invention is preferably 30 m 2 / g or above and 45m 2 / g or less. By making the specific surface area 30m 2 / g or more, the contact area with the electrolyte in the battery becomes larger, so the high-rate discharge characteristics can be further improved. On the other hand, by making the specific surface area 45m 2 / g or less, the particle surface of the positive electrode active material is stabilized, and thus the generation of gas due to a side reaction with the electrolyte can be suppressed.

[0047] Here, the specific surface area of the positive electrode active material can be measured by the BET flow method (adsorbed gas N 2 ) using a fully automatic specific surface area measuring apparatus, Macsorb HM Model-1210 (manufactured by Mountech Co., Ltd.).

[0048] The volume resistivity of the positive electrode active material in the present invention is preferably 10 5 Ω·cm or less. Volume resistivity is 10 5 Ω·cm or less, thereby exhibiting high conductivity when manufactured into a battery, and being able to further improve high-rate discharge characteristics.

[0049] Here, the volume resistivity of the positive electrode active material is a value measured by pressing the positive electrode active material into powder. Specifically, it can be measured at 25 MPa using a powder resistance measurement system MCP-PD51 (manufactured by Mitsubishi Chemical Analytech Co., Ltd.).

[0050] The average particle size of the granules of the positive electrode active material in the present invention is preferably 1.0 μm or more and 20.0 μm or less. In general, the positive electrode active material of the lithium ion battery is prepared by using N-methylpyrrolidone as a dispersion medium to make a paste, which is then coated on an aluminum foil and dried and pressed to form a mixture layer. The thickness of the mixture layer is generally 10 μm or more and 200 μm or less, and it is preferably granulated in a manner falling within this thickness, so the average particle size is preferably 20.0 μm or less. On the other hand, if the average particle size is 1.0 μm or more, the viscosity of the aforementioned paste is appropriately suppressed, which can improve the coating properties.

[0051] The average particle size of the granules can be measured using a scanning electron microscope. Specifically, the granules are observed at 3,000x magnification using a scanning electron microscope, and the particle sizes of 100 randomly selected granules are measured and the number average is calculated. If the secondary particles are not spherical, the average of the major and minor axes that can be measured in a two-dimensional image is used as the particle size.

[0052] Next, a method for producing the positive electrode active material of the present invention will be described.

[0053] The positive electrode active material of the present invention can be obtained by, for example, preparing primary particles of LMFP, preparing a dispersion of the primary particles in a monodisperse state, granulating the LMFP primary particles from the dispersion, and calcining the particles to form a carbon coating layer.

[0054] The solid phase method and the liquid phase method are examples of methods for producing LMFP primary particles. The liquid phase method is suitable for more easily obtaining LMFP primary particles having an average particle size of 10 nm to 80 nm and a narrow particle size distribution. By producing nanoparticles by the liquid phase method, the specific surface area of the LMFP granules can be easily adjusted to 30 m 2 / g or above and 45m 2 / g or less. As the liquid phase, water or water to which an organic solvent has been added in order to miniaturize the primary particles into nanoparticles is preferred. Examples of the organic solvent include alcohol solvents such as ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, 2-propanol, 1,3-propylene glycol, and 1,4-butanediol; and dimethyl sulfoxide. Two or more of these can be used. During the synthesis process, pressure may be applied to improve the crystallinity of the particles. It should be noted that the ratio of manganese to iron contained in the LMFP primary particles can be adjusted to the desired range by adjusting the feed ratio of the raw materials.

[0055] In the liquid phase method, LMFP raw materials are added to the aforementioned liquid phase and heated to produce LMFP primary particles. Dissolving the LMFP raw materials in an organic solvent improves the uniformity of the resulting particles, so raw materials with high solubility in organic solvents are preferred. Due to their high solubility in mixed solvents of water and organic solvents, lithium hydroxide is preferably used as the lithium raw material, manganese sulfate is preferably used as the manganese raw material, ferric sulfate is preferably used as the iron raw material, and orthophosphoric acid is preferably used as the phosphoric acid raw material. These raw materials may also be hydrated.

[0056] When LMFP primary particles are obtained by a liquid phase method, the average particle size of the primary particles can be adjusted to a desired range by adjusting, for example, the mixing ratio of water to 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, or lower the synthesis temperature. Furthermore, it is preferred that a solution of a manganese raw material, an iron raw material, and a phosphoric acid raw material be added to a lithium raw material solution while stirring at high speed, and then heated to the synthesis temperature without applying pressure while maintaining the high-speed stirring. This can reduce the peak intensity ratio of the positive electrode active material containing the LMFP granules obtained by X-ray diffraction to 1. 20 / I 29 and I 35 / I 29 It can be easily adjusted to the aforementioned preferred range.

[0057] Examples of methods for granulating LMFP primary particles include fluidized bed granulation and extrusion granulation. In order to narrow the particle size distribution of the granulated product as much as possible, a spray dryer is preferably used.

[0058] As a method for forming a carbon coating on the LMFP primary particles of an LMFP granulation, it is preferred to prepare a dispersion of the LMFP primary particles, then add a sugar and dissolve it, dry and granulate it using a spray dryer, and then heat it to 600°C to 800°C under a nitrogen atmosphere for calcination. By calcining the LMFP primary particles with the sugar, a carbon coating can be formed on the surface of the primary particles. Examples of sugars include glucose, sucrose, maltose, lactose, fructose, galactose, mannose, dextrin, and cyclodextrin. Among these, when water is used as the dispersion medium during spray drying, glucose and sucrose are preferred due to their high solubility in water. The amount of sugar added can be adjusted to the desired range in terms of the proportion of carbon contained in the LMFP primary particles. In addition, by increasing the calcination temperature, the volume resistivity of the LMFP primary particles and the LMFP granulation can be reduced.

[0059] In the present invention, the LMFP primary particles in the dispersion liquid used for spray drying are preferably dispersed in a monodisperse state in order to reduce the number ratio of primary particles with a particle size of 100 nm or greater to 5.0% or less. One reason for the formation of particles with a particle size of 100 nm or greater is particle growth during calcination. While lowering the calcination temperature is effective in suppressing particle growth, this, on the other hand, leads to insufficient carbonization of the sugars, which tends to reduce conductivity. In other words, suppressing particle growth and achieving high conductivity are in a trade-off relationship. However, research by the present inventors has discovered that by maintaining a monodisperse state for the LMFP particles in the dispersion liquid used for spray drying, the contact area between particles within the granules is reduced, thereby suppressing particle growth even at high calcination temperatures.

[0060] To achieve a monodisperse dispersion of the LMFP primary particles, the LMFP primary particles are preferably subjected to liquid-phase synthesis, washed in pure water without drying, and then subjected to a disintegration step. By omitting the drying step, drying and aggregation can be suppressed. Washing with pure water also serves to adjust the pH of the dispersion. In liquid-phase synthesis, trace amounts of residual ions are present during synthesis, so repeated washing is performed until the desired pH is reached to allow pH adjustment. Compared to methods that adjust the pH by adding additives such as sodium hydroxide, this method eliminates the need to add unnecessary ions to the LMFP, thus suppressing a decrease in the battery's energy density. To improve the dispersion of the LMFP primary particles, the pH of the dispersion is preferably between 9 and 11. Examples of the disintegration apparatus used in the disintegration step include shear mixers, planetary ball mills, bead mills, ultrasonic homogenizers, and dry jet mills. From the perspective of being able to directly process the LMFP primary particles with a dispersion without drying, a shear mixer, a wet jet mill, a bead mill, and an ultrasonic homogenizer are preferred. From the perspective of being able to uniformly disintegrate the dispersion, a shear mixer and a wet jet mill are more preferred.

[0061] Here, the dispersion state of the dispersion liquid can be evaluated using a dynamic light scattering particle size distribution analyzer. If the obtained average particle size is within twice the average particle size of the primary particles measured using a scanning electron microscope, it is considered to be in a monodisperse state.

[0062] In order to make the specific surface area of the LMFP granules in the present invention to be 30 m 2 / g or above and 45m 2 / g or less, and the average particle size of the LMFP primary particles is preferably 30 nm or more and 60 nm or less.

[0063] In order to adjust the average particle size of the LMFP granules in the present invention to 1.0 μm or more and 20.0 μm or less, for example, in the above-mentioned production method, the concentration of the dispersion liquid to be spray-dried is preferably set to 20 wt % or more and 60 wt % or less.

[0064] The positive electrode for lithium ion secondary batteries can be obtained by, for example, applying a paste obtained by dispersing the aforementioned granules in a dispersion medium onto a current collector, drying, pressurizing, and forming a mixture layer. As a method for manufacturing the paste, it is preferred that the aforementioned granules, further additives such as a conductive aid, an adhesive, and N-methylpyrrolidone are mixed and refined, and dispersion media such as water and N-methylpyrrolidone are added 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, it is preferably 30% by weight or more and 80% by weight or less. The various materials of the paste can be mixed at one time, and in order to make each material uniformly dispersed in the paste, they can also be added and mixed in sequence while repeatedly refining. As a mixing device for the slurry, a planetary mixer or a thin film rotary high-speed mixer is preferred in terms of being able to mix evenly.

[0065] As the adhesive, for example, polyvinylidene fluoride, styrene butadiene rubber, etc. can be cited. They can contain two or more. The content of the adhesive in the composite layer is preferably 0.3% by weight or more and 10% by weight or less. By making the content of the adhesive more than 0.3% by weight, the bonding effect of the adhesive is utilized, and when a coating film is formed, the coating shape can be easily maintained. On the other hand, by making the content of the adhesive less than 10% by weight, the increase in resistance in the electrode can be suppressed.

[0066] As the conductive aid, for example, acetylene black, Ketjen black, carbon fiber, carbon nanotubes, etc. can be mentioned. These can contain two or more. 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 electronic resistance can be reduced. On the other hand, by setting the content of the conductive aid to 10% by weight or less, the obstruction of the migration of lithium ions can be suppressed, and the reduction of ion conductivity can be suppressed.

[0067] To increase the energy density of lithium-ion secondary batteries, the positive electrode active material is preferably contained in the mixture layer at a maximum possible ratio. The content of the positive electrode active material in the mixture layer is preferably 80 wt % or more, more preferably 90 wt % or more.

[0068] The thickness of the mixture layer is preferably 10 μm or more and 200 μm or less. By setting the thickness of the mixture layer to 10 μm or more, the proportion of the current collector in the battery is reduced, which can further improve the energy density. On the other hand, by setting the thickness of the mixture layer to 200 μm or less, the charge and discharge reactions proceed rapidly throughout the mixture layer, which can further improve the high-speed charge and discharge characteristics.

[0069] The lithium-ion secondary battery of the present invention preferably includes, in addition to the above-mentioned positive electrode, a negative electrode, a separator, and an electrolyte. Examples of battery shapes include square, wound, and laminated types, which can be appropriately selected based on the intended use. Examples of materials constituting the negative electrode include graphite, lithium titanate, and silicon oxide. Any materials may be appropriately selected for use as the separator and electrolyte.

[0070] The lithium ion secondary battery of the present invention can be obtained, for example, by laminating the aforementioned positive electrode and the negative electrode via a separator in a dry environment with a dew point of -50°C or lower, and adding an electrolyte. Example

[0071] Hereinafter, the present invention will be described in detail with reference to Examples, but the present invention is not limited to these Examples. First, the evaluation methods in the Examples will be described.

[0072] [Measurement A] Composition ratio of LMFP

[0073] 15 mg of LMFP granules used in each of the Examples and Comparative Examples were thermally decomposed using perchloric acid and nitric acid, and the volume was adjusted to 100 mL using ultrapure water. The solution was assayed for Li by atomic absorption spectrometry, and for Mn, Fe, and P by ICP emission spectrometry. The respective contents in the sample were determined and converted into atomic ratios.

[0074] [Measurement B1] Average particle size of primary particles and the number ratio of particles with a diameter of 100 nm or more, average particle size of granules

[0075] The LMFP granules used in each example and comparative example were observed at 200,000x magnification 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 number average was calculated to calculate the average particle size of the LMFP primary particles. However, if the particles were not spherical, the average of the major and minor axes that could be measured in the two-dimensional image was used as the particle size. If two or more particles were connected by sintering, they were treated as a single particle. If sintering or contact was difficult to determine, the image was binarized into white and black. If a line separating the connected portion was obtained, it was treated as two particles, indicating contact; if no line was obtained, it was treated as a single particle, indicating sintering.

[0076] Furthermore, among the 200 particles measured, the number of particles having a particle diameter of 100 nm or more was counted, and the ratio of the number of particles to the 200 particles was calculated.

[0077] Similarly, the LMFP granules used in each of the Examples and Comparative Examples were observed at 3,000x magnification 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 number average was calculated to calculate the average particle size of the granules. However, if the granules are not spherical, the average value of the major and minor axes that can be measured in a two-dimensional image is used as the particle size.

[0078] [Measurement B2] The average diameter of the pores of the granules, the sum of the pore volumes of the pores with a pore diameter of 1 nm or more and 60 nm or less, the maximum value of the log differential pore volume of the pores with a pore diameter of 1 nm or more and 60 nm or less, and the pore specific surface area of the pores with a pore diameter of 1 nm or more and 60 nm or less

[0079] 0.3 g of the LMFP granules used in each example and comparative example were collected in a 5 cc powder chamber and measured by mercury intrusion porosimetry using an AutoPore IV9520 (manufactured by Shimadzu Corporation) at an initial pressure of 7 kPa. The mercury parameters were set to a mercury contact angle of 130.0° and a mercury surface tension of 485.0 dynes / cm. However, to distinguish the voids between the granules from the micropores, the average diameter of the micropores was measured within a range of 1 nm to 200 nm. The median diameter of the micropores was used as the average diameter of the micropores.

[0080] [Measurement C] Specific surface area

[0081] The specific surface area of the LMFP granules used in each of the Examples and Comparative Examples was measured by the BET flow method (adsorption gas N 2 ) using a fully automatic specific surface area measuring apparatus, Macsorb HM Model-1210 (manufactured by Mountech Co., Ltd.).

[0082] [Measurement D] Weight ratio of carbon contained in LMFP granules

[0083] The weight ratio of carbon contained in the LMFP granules used in each of the Examples and Comparative Examples was measured using a carbon-sulfur analyzer EMIA-810W (manufactured by Horiba, Ltd.).

[0084] [Measurement E] Volume resistivity

[0085] The volume resistivity at 25 MPa was measured for 1.0 g of the positive electrode active material used in each of the Examples and Comparative Examples using a powder resistance measurement system MCP-PD51 (manufactured by Mitsubishi Chemical Analytech Co., Ltd.).

[0086] [Measurement of F] X-ray diffraction peak intensity ratio

[0087] The peak intensity ratios of the positive electrode 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°, a scan interval of 0.02°, and a scan rate of 20 seconds / deg. The peak intensity ratios were calculated using EVA (Bruker ASX) powder X-ray diffraction analysis software, with background subtraction (coefficient 1.77) performed and peak intensities read.

[0088] [Measurement G] Average particle size of LMFP primary particles in the spray drying dispersion

[0089] The average particle size of the LMFP primary particles of the spray-drying LMFP dispersion before addition of glucose used in each of the Examples and Comparative Examples was measured using a dynamic light scattering particle size distribution analyzer nanoPartica SZ-100V2 (manufactured by Horiba, Ltd.).

[0090] [Measurement H] High-rate discharge characteristics (energy density measurement)

[0091] The electrode plates prepared in each embodiment and comparative example were cut into pieces with a diameter of 15.9 mm as the positive electrode, lithium foil was cut into pieces with a diameter of 16.1 mm and a thickness of 0.2 mm as the negative electrode, "SETELA" (registered trademark) (manufactured by Toray Industries, Ltd.) was cut into pieces with a diameter of 20 mm as the isolation membrane, and a solution of ethylene carbonate: diethyl carbonate = 3:7 (volume ratio) containing 1 M LiPF6 was used as the electrolyte to prepare 2032 type coin batteries.

[0092] The resulting coin cell was charged and discharged three times at a cutoff potential of 2.5 V, a maximum charge voltage of 4.3 V, and a rate of 0.1 C. The energy density (Wh / kg) per unit positive electrode weight at 0.1 C was measured based on the third discharge. Subsequently, the cell was charged at a rate of 0.1 C and discharged at a rate of 4.0 C, and the energy density (Wh / kg) per unit positive electrode weight at 4.0 C was measured. As an evaluation of the high-rate discharge characteristics, the ratio of the energy density at 4.0 C discharge to the energy density at 0.1 C discharge was determined.

[0093] [Measurement I] Cycle resistance

[0094] A 2032-size coin cell was prepared in the same manner as for measuring H. Three cycles of charge and discharge were performed at a 0.1C rate at 25°C. Next, charge and discharge were performed once at a 1C rate at 50°C. The discharge energy density at this time was recorded as the initial energy density. Next, charge and discharge were continued at a 1C rate at 50°C. The number of cycles at which the discharge energy density fell below 80% of the initial energy density was determined and used as the cycle endurance evaluation.

[0095] In all charge and discharge tests, the battery was charged at a constant current until the maximum voltage reached 4.3 V. After reaching the maximum voltage, the battery was charged at the maximum voltage until the charge current fell below 0.01 C. The battery was discharged at a constant current until the discharge voltage fell below 2.5 V.

[0096] [Example 1]

[0097] After dissolving 60 mmol of lithium hydroxide monohydrate in 25 g of pure water, 60 g of diethylene glycol was added to prepare a lithium hydroxide / diethylene glycol aqueous solution. The resulting lithium hydroxide / diethylene glycol aqueous solution was stirred at 2000 rpm using a homodispersor (Primix Homodispersor 2.5 model). Then, an aqueous solution prepared 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 produce a lithium manganese phosphate nanoparticle precursor. The resulting precursor solution was heated to 100°C for 2 hours to obtain LMFP nanoparticles as a solid component. The resulting LMFP was not dried, but pure water was added and the solvent was removed using a centrifuge. This process was repeated to wash the dispersion until the pH was adjusted to 10.1. The resulting dispersion was adjusted to a solids concentration of 50% by weight and then dispersed using a StarBurst Mini wet jet mill (Sugino Machine Co., Ltd.) at 150 MPa and two passes.

[0098] To the resulting LMFP dispersion, glucose was added at a ratio of 0.15 g per 1.0 g of LMFP and dissolved. The LMFP dispersion was then dried and granulated using a spray dryer (MDL-050B manufactured by Fujisaki Electric Co., Ltd.) with hot air at 200°C. The resulting granules were heated at 700°C for 4 hours in a rotary kiln (tabletop rotary kiln manufactured by Takasago Industry Co., Ltd.) under a nitrogen atmosphere to obtain LMFP granules having a carbon coating layer.

[0099] After mixing acetylene black (Li-400 manufactured by Denka Co., Ltd.) and a binder (KUREHA KF Polymer L#9305 manufactured by KUREHA Co., Ltd.), the resulting LMFP granules were added and kneaded in a mortar. At this time, the mass ratio of the materials contained was 90:5:5 of granules: acetylene black: binder. Thereafter, N-methylpyrrolidone was added and adjusted to a solid content of 48% by mass to obtain a slurry-like electrode paste. N-methylpyrrolidone was added to the resulting paste until fluidity was achieved, and the mixture was treated for 30 seconds at a stirring speed of 40 m / s using a thin film rotary high-speed mixer ("FILMIX" (registered trademark) 40-L manufactured by PRIMIX Co., Ltd.).

[0100] The obtained electrode paste was applied onto 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.

[0101] [Example 2]

[0102] An electrode plate was produced in the same manner as in Example 1 except that the amount of diethylene glycol used in the synthesis of LMFP was 80 g.

[0103] [Example 3]

[0104] An electrode plate was produced in the same manner as in Example 1 except that the amount of diethylene glycol used in the synthesis of LMFP was 120 g.

[0105] [Example 4]

[0106] An electrode plate was produced in the same manner as in Example 1 except that the amount of glucose added was 0.07 g relative to 1.0 g of LMFP.

[0107] [Example 5]

[0108] An electrode plate was produced in the same manner as in Example 1 except that the amount of glucose added was 0.22 g relative to 1.0 g of LMFP.

[0109] [Example 6]

[0110] An electrode plate was produced in the same manner as in Example 1 except that the amount of glucose added was 0.11 g relative to 1.0 g of LMFP and the calcination temperature was 600°C.

[0111] [Example 7]

[0112] An electrode plate was produced in the same manner as in Example 1 except that the LMFP was dispersed using a shear mixer (Model AX5 head: emulsification mesh, manufactured by Silverson Nippon Co., Ltd.) at 5000 rpm for 5 minutes without using a wet jet mill.

[0113] [Comparative Example 1]

[0114] An electrode plate was produced in the same manner as in Example 1 except that the wet jet mill was not used for the dispersion treatment.

[0115] [Comparative Example 2]

[0116] An electrode plate was produced in the same manner as in Example 1, except that the pH of the LMFP dispersion was adjusted by adding LiOH instead of washing with pure water.

[0117] [Comparative Example 3]

[0118] 60 mmol of lithium hydroxide monohydrate, 20 mmol of phosphoric acid (85% aqueous solution), 16 mmol of manganese sulfate monohydrate, and 4 mmol of iron sulfate heptahydrate were added to 40 g of pure water. The mixture was placed in a pressure vessel, heated to 180° C., and maintained for 8 hours to obtain LMFP particles as a solid component.

[0119] The resulting LMFP was washed by adding pure water and removing the solvent using a centrifuge five times. The resulting LMFP dispersion was then dried on a hot plate to form a powder. The average particle size of the resulting LMFP primary particles was measured in the same manner as in Measurement Example B and was found to be 281 nm.

[0120] The obtained LMFP powder was pulverized using a planetary ball mill P5 (manufactured by Fritsch). The pulverization process used a 45 ml zirconia container, 18 10 mm zirconia beads, and a rotation speed of 300 rpm for 6 hours.

[0121] Water was added to the resulting LMFP to form a dispersion, and glucose was further added at a ratio of 0.15 g per 1.0 g of LMFP to dissolve the mixture. The LMFP dispersion was then dried and granulated using a spray dryer (MDL-050B manufactured by Fujisaki Electric Co., Ltd.) with hot air at 200°C. The resulting granules were heated in a rotary kiln (tabletop rotary kiln manufactured by Takasago Industry Co., Ltd.) at 700°C for 4 hours under a nitrogen atmosphere to obtain LMFP granules having a carbon coating layer.

[0122] An electrode plate was produced in the same manner as in Example 1 using the obtained LMFP granules.

[0123] [Comparative Example 4]

[0124] An electrode plate was produced in the same manner as in Comparative Example 3 except that the treatment conditions of the planetary ball mill were set to 200 rpm for 2 hours.

[0125] The evaluation results of each example and comparative example are shown in Table 1 and Table 2.

[0126] [Table 1]

[0127] .

[0128] [Table 2]

[0129] .

Claims

1. A positive electrode active material for a lithium ion secondary battery, comprising granules 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, the number ratio of particles having a particle size of 100 nm or more is 5.0% or less, and the primary particles have a carbon coating layer on their surfaces. The ratio of carbon contained in the granules is 2.0 wt% or more and 5.0 wt% or less, The positive active material particles of lithium ion secondary batteries are Li α Mn a Fe b Lithium manganese iron phosphate particles represented by PO4, Li α Mn a Fe b In PO4, 0.9≤α≤1.1, 0.6≤a≤1.0, 0<b≤0.4, 0.9≤a+b≤1.

1.

2. The positive electrode active material for lithium ion secondary batteries according to claim 1, wherein The ratio of the peak intensity at 20° to the peak intensity at 29° obtained by X-ray diffraction of lithium manganese iron phosphate particles is 20 / I 29 is 0.88 or more and 1.05 or less, and the ratio of the peak intensity at 35° to the peak intensity at 29° is 1 35 / I 29 It is 1.05 or more and 1.20 or less.

3. The positive electrode active material for lithium ion secondary batteries according to claim 1 or 2, wherein The average diameter of the pores of the granules is 10 nm to 60 nm, and the total pore volume of the pores having a diameter of 1 nm to 60 nm is 0.100 cm 3 / g or more and 0.300cm 3 / g or less, and 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 0.30 cm 3 / g or above.

4. The positive electrode active material for lithium ion secondary batteries according to claim 1 or 2, wherein The micropore specific surface area of the granules is 25 m 2 / g or above and 50m 2 / g or less.

5. The positive electrode active material for lithium ion secondary batteries according to claim 4, wherein The specific surface area of the granules is 30 m 2 / g or above and 45m 2 / g or less.

6. The positive electrode active material for lithium ion secondary batteries according to claim 1 or 2, wherein The volume resistivity of the granules is 10 5 Ω·cm or less.

7. The positive electrode active material for lithium ion secondary batteries according to claim 1 or 2, wherein The average particle size of the granules is 1.0 μm or more and 20.0 μm or less. 8 . A lithium ion secondary battery obtained by using the positive electrode active material for a lithium ion secondary battery according to claim 1 .