Active material for secondary battery electrode and secondary battery using the same

Abstract

The object of the present invention is to provide an active material for a secondary battery electrode having excellent rate characteristics and cycle tolerance. The present invention is an active material for a secondary battery electrode, which has an olivine-type crystal structure, has a carbon layer on the surface, and the ratio of the average thickness of the carbon layer present on the surface perpendicular to the crystal axis b-axis to the average thickness of the carbon layer present on the surface not perpendicular to the aforementioned b-axis is 0.30 or more and 0.80 or less.

Description

Technical Field

[0001] The present invention relates to an active material for secondary battery electrodes and a secondary battery using the same. Background Art

[0002] In recent years, due to the increasing awareness of environmental issues, especially global warming, reducing the use of fossil fuels has become an important issue. In the fields of power supply and transportation, where the use of fossil fuels is high, research is being conducted on renewable energy sources and electrification of power sources. In these fields, there is an increasing demand for power storage devices such as secondary batteries for renewable energy to balance power and for electrification of power to store power sources.

[0003] One type of secondary battery, namely, a lithium-ion secondary battery, has a high energy density and excellent output characteristics. However, if a malfunction occurs, the stored energy is released in a short time, which may cause the battery to catch fire or burn. Therefore, for lithium-ion secondary batteries, improving safety while improving output characteristics is an important issue.

[0004] It is widely known that the cause of significant influence on the safety of lithium-ion secondary batteries is the positive electrode active material. Layered rock salt-based transition metal lithium oxide positive electrode active materials are widely used in large batteries for electric vehicles, especially positive electrode active materials that show high energy density. However, due to the change of crystal structure accompanied by the release of oxygen caused by charging and discharging, deterioration is promoted, and there is a risk of smoke and fire depending on the usage conditions, so there are safety issues.

[0005] On the other hand, the positive electrode active material with an olivine crystal structure represented by lithium iron phosphate is a safe positive electrode material that is relatively stable even under high temperature conditions because oxygen and phosphorus are covalently bonded and oxygen is not easily released, but its electronic conductivity and ion conductivity are known to be lower than those of layered rock salt-based transition metal lithium oxide positive electrode active materials. Therefore, as a technology to improve the electronic conductivity and ion conductivity of positive electrode active materials with an olivine crystal structure, the provision of a conductive carbon covering layer is being studied.

[0006] Regarding the above-mentioned technology, for example, the following have been proposed so far: a method for manufacturing an electrode material having a core-shell structure in which an active material core is covered with polyaniline, by heat-treating an electrode material precursor having a core-shell structure in which an active material core is covered with carbon at 300 to 900° C. in a reducing atmosphere (for example, see Patent Document 1); a lithium iron phosphate positive electrode material, which is a lithium iron phosphate positive electrode material having primary particles of lithium iron phosphate with a conductive carbon covering layer, characterized in that the conductive carbon covering layer has a thick layer portion with a thickness of more than 2 nm, and a thin layer portion with a thickness of less than 2 nm (for example, see Patent Document 2); an electrode material comprising An agglomerate obtained by agglomerating carbonaceous coated electrode active material particles having a carbonaceous coating formed on the surface of the electrode active material particles with a coverage rate of more than 80%, the carbonaceous coated electrode active material particles comprising first carbonaceous coated electrode active material particles having a carbonaceous coating with a film thickness of more than 0.1 nm and less than 3.0 nm and an average film thickness of more than 1.0 nm and less than 2.0 nm, and second carbonaceous coated electrode active material particles having a carbonaceous coating with a film thickness of more than 1.0 nm and less than 10.0 nm and an average film thickness of more than 2.0 nm and less than 7.0 nm (for example, refer to Patent Document 3), etc.

[0007] However, in an electrode active material having an olivine-type crystal structure, it is known that carrier ions show a high diffusion rate only in one dimension with respect to the direction of the crystal b axis inside the material (see, for example, Non-Patent Document 1).

[0008] Prior art literature

[0009] Patent Literature

[0010] Patent Document 1: Japanese Patent Application Publication No. 2010-40357

[0011] Patent Document 2: Japanese Patent Application Publication No. 2012-216473

[0012] Patent document 3: Japanese Patent Application Publication No. 2014-146513.

[0013] Non-patent literature

[0014] Non-patent literature 1: Gardiner, GR; Islam, MS Anti-Site Defects and Ion Migration in the LiFe0.5Mn0.5PO4 Mixed-Metal Cathode Material. Chem. Mater. 2010, 22 (3), 1242-1248. Summary of the invention

[0015] Problems to be solved by the invention

[0016] By forming a carbon layer on the surface of the secondary battery electrode active material having an olivine-type crystal structure, the electronic conductivity and ion conductivity can be improved, and the rate characteristics can be improved. In addition, in the electrode using the secondary battery electrode active material, the area of ​​direct contact between the electrolyte and the secondary battery electrode active material is reduced, thereby inhibiting the dissolution of the secondary battery electrode active material in the electrolyte, and the cycle tolerance can be improved. From the viewpoint of improving electronic conductivity and cycle tolerance, the thickness of the carbon layer is preferably thick.

[0017] On the other hand, from the perspective of promoting the entry and exit of carrier ions such as lithium in the secondary battery electrode active material inside and outside the secondary battery electrode active material, the thickness of the carbon layer is preferably thin. In the aforementioned patent documents 1 to 3, electrode active material particles with different thicknesses in the carbon layer are disclosed, but the thickness of the carbon layer varies randomly relative to the crystal axis, and the carrier ions in the aforementioned electrode active material with an olivine-type crystal structure only show a high diffusion rate in one dimension relative to the direction of the crystal axis b axis, and the effect of improving the rate characteristics and cycle tolerance is insufficient.

[0018] In view of the above problems, an object of the present invention is to provide an active material for a secondary battery electrode having excellent rate characteristics and cycle resistance.

[0019] Means for solving problems

[0020] In order to solve the above-mentioned problems, the present invention mainly has the following configurations.

[0021] An active material for secondary battery electrodes having an olivine crystal structure and a carbon layer on the surface, wherein the ratio of the average thickness of the carbon layer on a surface perpendicular to the crystal axis b-axis to the average thickness of the carbon layer on a surface not perpendicular to the aforementioned b-axis is greater than 0.30 and less than 0.80.

[0022] Effects of the Invention

[0023] By using the secondary battery electrode active material of the present invention, a secondary battery having excellent rate characteristics and cycle resistance can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 These are a transmission electron microscope image (a) of the secondary battery electrode active material of the present invention produced in Example 2 and a Fourier transform image (b) thereof.

[0025] Figure 2 The transmission electron microscope image (a) and the Fourier transform image (b) of the conventional active material for secondary battery electrodes prepared in Comparative Example 2 are shown. DETAILED DESCRIPTION

[0026] The active material for secondary battery electrodes having an olivine-type crystal structure of the present invention (hereinafter sometimes referred to as "olivine-type active material") is characterized in that it has a carbon layer on the surface, and the average thickness of the carbon layer existing on the surface perpendicular to the crystal axis b-axis relative to the average thickness of the carbon layer existing on the surface not perpendicular to the aforementioned b-axis is greater than 0.30 and less than 0.80. As mentioned above, a carbon layer is formed on the surface of the active material for secondary battery electrodes having an olivine-type crystal structure, thereby improving rate characteristics and cycle tolerance. Further, in the present invention, according to the crystallographic characteristics of the olivine-type active material, the carrier ions of the electrodes such as lithium and sodium (in this specification, the carrier ions refer to the ions that enter and exit the active material during charge and discharge) inside the olivine-type active material show a high diffusion coefficient only in one dimension relative to the direction of the crystal axis b-axis, and the thickness of the carbon layer is thinned in the direction of the crystal axis b-axis to promote the entry and exit of the carrier ions inside and outside the olivine-type active material and improve the rate characteristics. On the other hand, the thickness of the carbon layer is thickened in other directions to improve the cycle tolerance, so the numerical range of the ratio of the average thickness of the carbon layer existing on the surface perpendicular to the crystal axis b-axis to the average thickness of the carbon layer existing on the surface not perpendicular to the aforementioned b-axis is limited. The following is an explanation of the olivine-type active material of the present invention.

[0027] In the present invention, the olivine-type active material is not particularly limited as long as it has an olivine-type crystal structure, and preferably has a chemical composition represented by the general formula ABXO4 (A and B each independently represent one or more metal elements, and X represents any one or more elements other than metal elements). A is preferably an alkali metal, B is preferably a transition metal, and X is preferably silicon, phosphorus, etc. As the chemical composition represented by the general formula ABXO4, for example, LiFePO4, LiMnPO4, LiFe 1-x Mn x PO4 (0<x<1), LiCoPO4, LiNiPO4, NaFePO4, NaMnPO4, NaCoPO4, NaNiPO4, LiFeSiO4, NaFeSiO4, etc. It may contain two or more of these substances. Among these, lithium generally tends to show a high diffusion coefficient, so as the A site, it is preferred to use lithium, which can further improve the rate characteristics. Among the olivine-type active materials, lithium iron manganese phosphate tends to have low electronic conductivity and lithium ion conductivity, and can more significantly exert the effect of utilizing the carbon layer in the present invention. For example, as described later, the energy density ratio in Example 5 and Comparative Example 4 using lithium iron phosphate is increased by 0.06. In Example 1 and Comparative Example 1 using lithium iron manganese phosphate, the energy density ratio is increased by 0.16, which more significantly exerts the effect of utilizing the carbon layer in the present invention.

[0028] The olivine-type active material may have a structure in which the chemical composition is different inside the primary or higher-order particles (eg, a core-shell structure). Furthermore, the olivine-type active material may contain a doping element.

[0029] The olivine-type active material of the present invention has a carbon layer on the surface. The carbon layer only needs to cover at least a portion of the surface of the olivine-type active material, preferably covering more than 80% of the surface area. In addition, the carbon layer may have functional groups such as carbonyl and hydroxyl groups on its surface. The content of the carbon layer in the olivine-type active material is preferably more than 1% by weight from the viewpoint of further improving the electron conductivity and further improving the rate characteristics. On the other hand, from the viewpoint of further improving the cycle tolerance by suppressing the side reaction of the carbon layer and the olivine-type active material, it is preferably less than 6% by weight.

[0030] Here, the content of the carbon layer in the olivine-type active material can be measured using, for example, a carbon-sulfur simultaneous quantitative analyzer EMIA-920V (manufactured by Horiba, Ltd.) The content of the carbon layer can be adjusted to a desired range by adjusting, for example, the amount of the carbon source added in the method for producing the olivine-type active material described later.

[0031] The ratio of the average thickness of the carbon layer on the surface perpendicular to the crystal axis b axis of the olivine type active material of the present invention to the average thickness of the carbon layer on the surface not perpendicular to the b axis (average thickness of the carbon layer on the vertical surface / average thickness of the carbon layer on the non-vertical surface) is 0.30 or more and 0.80 or less. When the value of the ratio is less than 0.30, the carbon layer on the surface perpendicular to the crystal axis b axis is too thin, or the carbon layer on the surface not perpendicular to the crystal axis b axis is too thick. In the former case, it is difficult to obtain the effect of improving the electron conductivity generated by the carbon layer, and in the latter case, the weight ratio of the carbon layer in the olivine type active material is too large, and the rate characteristics of the secondary battery are reduced in either case. On the other hand, when the value of the ratio is greater than 0.80, the carbon layer on the surface perpendicular to the crystal axis b axis is too thick, or the carbon layer on the surface not perpendicular to the crystal axis b axis is too thin. In the former case, it is difficult to obtain the effect of promoting the entry and exit of carrier ions in the b-axis direction inside and outside the olivine type active material, and the rate characteristics of the secondary battery are reduced. In the latter case, it is difficult to obtain the cycle resistance improvement effect by the carbon layer, and the cycle resistance of the secondary battery is reduced.

[0032] The average thickness of the carbon layer on the surface perpendicular to the crystal axis b axis is preferably 0.6 nm or more, preferably 2.0 nm or less, from the viewpoint of further improving the rate characteristics of the secondary battery. In the olivine-type active material of the present invention, when the surface of the olivine-type active material particle is regarded as a polyhedron, the thickness of the carbon layer in the same plane is preferably uniform, and the electric field applied to the surface of the olivine-type active material in the electrode reaction of the charge and discharge of the secondary battery is uniform, which can further improve the cycle tolerance. In particular, the thickness of the carbon layer on the surface perpendicular to the crystal axis b axis is preferably small, and the standard deviation of the thickness of the carbon layer on the surface perpendicular to the crystal axis b axis is preferably 0.3 nm or less.

[0033] Here, the average thickness of the carbon layer on the surface of the olivine-type active material on different crystal planes can be measured using a transmission electron microscope. Specifically, under the conditions of an acceleration voltage of 300 kV and a magnification of 2,000,000 times, the multi-wave interference image of the olivine-type active material is measured. The thickness of the carbon layer is measured for more than 20 measurement points evenly selected on the periphery of the particles of the olivine-type active material. In addition, the obtained lattice image is subjected to Fourier transform, the distance from the origin to the bright spot is measured, and the corresponding d value is calculated, thereby obtaining information on the crystal orientation. At each measurement point where the thickness of the carbon layer is measured, the crystal orientation perpendicular to the surface of the particle is calculated. When the angle between the crystal orientation perpendicular to the surface of the particle and the crystal axis b-axis is less than 20 degrees, the measurement point is deemed to exist on a plane perpendicular to the crystal axis b-axis. When the angle formed is greater than 20 degrees, it is deemed not to exist on a plane perpendicular to the crystal axis b-axis. By performing this process on all measurement points, the average thickness and standard deviation of the carbon layer existing on the surface perpendicular to the crystal axis b-axis and the carbon layer existing on the surface not perpendicular to the crystal axis b-axis can be calculated. The value of the above ratio can be calculated based on the obtained average thickness.

[0034] As a method for making the ratio and standard deviation of the average thickness of the carbon layer fall within the above range, for example, there can be mentioned a method of obtaining an olivine-type active material by a production method described later.

[0035] The crystallite diameter of the olivine type active material of the present invention is preferably less than 60nm. Generally speaking, the olivine type active material has low electronic conductivity and ionic conductivity, so the voltage drop under the condition of large charge and discharge current tends to increase. By making the crystallite diameter less than 60nm, the diffusion distance of electrons and carrier ions in the crystal grains is shortened, so the rate characteristics can be further improved. The specific surface area of ​​the active material particles with a small crystallite diameter becomes larger, so the effect produced by the average thickness ratio of the carbon layer in the present invention can be further significantly exerted.

[0036] Here, the crystallite diameter of the olivine type active material can be measured by powder X-ray diffraction for a powder sample of the olivine type active material under the condition that the diffraction angle 2θ is greater than 10 degrees and less than 70 degrees, and the obtained diffraction pattern is subjected to Rietveld analysis to calculate. The olivine type active material of the present invention can be directly measured, and in the case of the secondary battery described later, the olivine type active material obtained by grinding the electrode mixture peeled from the secondary battery electrode can also be measured. The crystallite diameter of the olivine type active material can be adjusted to the desired range by, for example, adjusting the mixing ratio of water and an organic solvent used as a solvent, the total amount of the solvent relative to the raw material, the synthesis temperature, the calcination temperature, etc. in the method for manufacturing the olivine type active material described later.

[0037] In the olivine-type active material of the present invention, the crystallinity and particle shape of lithium manganese iron phosphate can be determined by the 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° 35 / I 29 To evaluate. If I 20 / I 29 When the ratio is greater than 0.88 and less than 1.05, it means that the lithium manganese iron phosphate is not extremely oriented in the b-axis direction, and the shape of the particles is closer to a sphere than a plate. When the particles are close to a spherical shape, the strain of the lattice caused by the insertion and removal of lithium ions during charge and discharge is alleviated, and the rate characteristics and cycle tolerance can be further improved. In addition, if I 35 / I 29 When the ratio is 1.05 or more and 1.20 or less, the orientation of the crystal of lithium manganese iron phosphate is further reduced, forming a more homogeneous crystal orientation, and the particle shape is closer to a sphere. Therefore, the strain of the crystal lattice caused by the insertion and removal of carrier ions during charge and discharge is alleviated, and the rate characteristics and cycle tolerance can be further improved. 20 / I 29 and I 35 / I 29 The method of achieving the above range includes, for example, a method of obtaining lithium manganese iron phosphate by a production method described later.

[0038] Next, the method for manufacturing the olivine-type active material of the present invention is described. The olivine-type active material of the present invention can be obtained by any method such as a solid phase method and a liquid phase method. From the viewpoint of easily adjusting the crystallite diameter to the aforementioned preferred range, the liquid phase method is preferably used. In the liquid phase, in addition to water, in order to refine the crystallites to nanoparticles, it is also suitable to use an organic solvent. As the type of solvent, for example, alcohol solvents such as ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, 2-propanol, 1,3-propylene glycol, 1,4-butanediol, dimethyl sulfoxide, etc. can be cited. These can use more than two kinds. During the synthesis process, in order to improve the crystallinity of the particles, pressure can also be applied. In addition, the chemical composition of the olivine-type active material can be adjusted to the desired range by the feed ratio of the raw materials. In addition, in the synthesis of lithium manganese iron phosphate using a liquid phase method, it is preferred that a solution containing a portion of the raw materials is stirred at high speed, and then a solution containing the remaining raw materials is added, and the solution is heated to the synthesis temperature without pressurizing while maintaining the high-speed stirring state, so that the peak intensity ratio of the lithium manganese iron phosphate obtained by X-ray diffraction can be easily increased to 1. 20 / I 29 with I 35 / I 29 Adjust to the above-mentioned preferred range.

[0039] When the primary particles of the olivine-type active material are obtained by the liquid phase method, the crystallite diameter can be adjusted to a desired range by, for example, the mixing ratio of water and organic solvent in the solvent, the concentration of the synthesis solution, the synthesis temperature, the feed ratio of the raw materials, etc. Typically, in order to increase the crystallite diameter, it is effective to increase the proportion of water in the solvent, increase the concentration of the synthesis solution, increase the synthesis temperature, etc.

[0040] As a carbon coating method for forming a carbon layer on an olivine-type active material obtained by a liquid phase method, it is preferably a method of calcining in an inert gas atmosphere after mixing a powder containing primary particles and / or secondary particles of an olivine-type active material, a slurry containing them, and a carbon source. As a carbon source, for example, sugars such as glucose, sucrose, trehalose, maltose, dextrin hydrates, cyclodextrins, organic acids such as citric acid, malic acid, succinic acid, fumaric acid, maleic acid, polyaniline, polyacrylonitrile, polyvinyl alcohol, polyvinyl pyrrolidone, organic polymers, coal tar, asphalt, asphalt, etc. can be cited. They can use more than two kinds. Among these, when only sugars and organic polymers are used as carbon sources, the standard deviation of the thickness of the carbon layer can be easily adjusted to the aforementioned preferred range.

[0041] As a method for mixing olivine-type active substances and carbon sources, it is preferred to dissolve or disperse the carbon source and olivine-type active substances in a medium such as water, ethanol, acetonitrile, N-methylpyrrolidone, dimethyl sulfoxide, etc., and use a mixing device such as a disperser, a jet mill, a high shear mixer, an ultrasonic homogenizer, etc. to mix and disperse them.

[0042] In order to make the average thickness ratio of the carbon layer relative to the crystal axis b-axis direction within the aforementioned range, it is preferred to instantaneously dry the slurry containing the olivine-type active material and the carbon source before calcination to obtain a precursor of the olivine-type active material in which the olivine-type active material and the carbon source are tightly mixed. Specifically, it is preferred to use a spray dryer for drying.

[0043] As the inert gas used during calcination, for example, nitrogen, argon, etc. can be mentioned. In order to remove the gas generated by the mixture of the olivine-type active material and the carbon source during calcination to the outside of the system, it is preferred to circulate the inert gas. The calcination temperature is preferably 500° C. or more and 1000° C. or less, and the calcination time is preferably 30 minutes or more and 24 hours or less.

[0044] In order to make the average thickness ratio of the carbon layer relative to the crystal axis b-axis direction within the aforementioned range, it is preferred to combine a plurality of compounds with different melting points as carbon sources, and pre-calcine at a temperature between the melting point of the compound with a low melting point and the melting point of the compound with a high melting point for more than 1 hour and less than 24 hours before the aforementioned calcination, and then calcine under the aforementioned conditions. By adjusting the mixing ratio of carbon sources with different melting points, the average thickness ratio of the carbon layer relative to the crystal axis b-axis direction can be adjusted. In this case, it is preferred that the amount of the compound with the highest melting point added is more than 0.50 times and less than 5.0 times the total amount of the compounds with a lower melting point than the compound.

[0045] When the primary particle size of the olivine type active material is less than 1 μm, the active material is preferably treated as a secondary particle obtained by agglomerating the primary particles, and the secondary particle size is preferably 3 μm or more from the viewpoint of the handleability of the paste in the manufacturing method of the secondary battery electrode described later. On the other hand, the secondary particle size of the olivine type active material is preferably 40 μm or less based on the relationship with the thickness of the mixture layer described later.

[0046] The secondary particle size of the olivine-type active material refers to the arithmetic mean of the particle size of the secondary particles, which can be measured using a scanning electron microscope. Specifically, a scanning electron microscope is used to observe the electrode at a magnification of 3,000 times, and the secondary particle size is measured for 100 randomly selected secondary particles. By calculating its number mean, the secondary particle size of the olivine-type active material can be found. When only less than 100 secondary particles are observed in an observation field of view, observations are performed at other parts of the sample until the cumulative number of observed secondary particles reaches 100. In addition, the secondary particle size of the secondary particles of the olivine-type active material used as a raw material when manufacturing the electrode can also be measured using a scanning electron microscope.

[0047] As a method for producing secondary particles of olivine-type active material, from the viewpoint of narrowing the particle size distribution of the secondary particles as much as possible, it is preferred to use a spray dryer to dry and granulate a dispersion containing primary particles of olivine-type active material. The secondary particle size of the olivine-type active material can be easily adjusted to a desired range by, for example, changing the weight concentration of the aqueous dispersion of the olivine-type active material as a raw material in the aforementioned method for producing the olivine-type active material.

[0048] The olivine-type active material particles of the present invention are suitable for secondary battery electrodes. The secondary battery electrode of the present invention has a mixture layer containing the olivine-type active material of the present invention and additives such as an adhesive and a conductive aid on a current collector such as an aluminum foil, a copper foil, a stainless steel foil, or a platinum foil. In addition, together with the olivine-type active material of the present invention, other active materials such as an active material having a layered oxide-type crystal structure and an active material having a spinel-type crystal structure may also be contained.

[0049] Examples of the adhesive include polyvinylidene fluoride and styrene butadiene rubber, and two or more of these may be contained.

[0050] The content of the adhesive in the electrode mixture 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, utilizing the bonding effect of the adhesive, the coating shape can be easily maintained when forming a coating. On the other hand, by making the content of the adhesive less than 10% by weight, the increase of the resistance in the electrode can be suppressed.

[0051] Examples of the conductive additive include acetylene black, Ketjen black, carbon fiber, carbon nanotube, graphene, reduced graphene oxide, etc. Two or more of these may be contained.

[0052] The content of the conductive aid in the composite layer is preferably 0.3% by weight or more and 10% by weight or less. By making the content of the conductive aid 0.3% by weight or more, the conductivity of the electrode can be improved and the electronic resistance can be reduced. On the other hand, by making the content of the conductive aid 10% by weight or less, the hindrance of the carrier ion migration of the battery caused by the conductive aid can be suppressed, and the ion conductivity can be further improved.

[0053] In order to increase the energy density of the secondary battery, the electrode mixture layer preferably contains olivine-type active materials at as high a ratio as possible. The total content of olivine-type active materials and other active materials in the electrode mixture layer is preferably 80 wt % or more, more preferably 85 wt % or more.

[0054] The thickness of the electrode mixture layer is preferably 10 μm or more and 200 μm or less. By making the thickness of the mixture layer 10 μm or more, the proportion of the current collector in the electrode can be suppressed, further improving the energy density. On the other hand, by making the thickness of the mixture layer 200 μm or less, the charge and discharge reaction can be carried out rapidly in the entire mixture layer, improving the high-speed charge and discharge characteristics.

[0055] The secondary battery electrode can be obtained by, for example, applying a paste obtained by dispersing the aforementioned olivine-type active material secondary particles in a dispersion medium on a current collector, drying, and pressurizing to form a mixture layer. As a method for manufacturing a paste, it is preferred that the aforementioned olivine-type active material secondary particles, further additives such as conductive aids, adhesives, and dispersion media such as 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, it can also be added and mixed in sequence while repeatedly refining. As a mixing device for the slurry, in terms of being able to mix evenly, a planetary mixer and a thin film rotary high-speed mixer are preferably used.

[0056] The secondary battery of the present invention preferably has a counter electrode, a separator, and an electrolyte in addition to the above-mentioned electrodes. As the shape of the battery, for example, a coin type, a square type, a winding type, a laminated type, etc. can be cited, which can be appropriately selected according to the purpose of use. As the material constituting the counter electrode, for example, graphite, lithium titanate, silicon oxide, lithium cobalt oxide, etc. can be cited. For the separator and the electrolyte, any material can also be appropriately selected and used.

[0057] The secondary battery of the present invention can be obtained, for example, by laminating the above-mentioned secondary battery electrode with a counter electrode via a separator in a dry environment with a dew point of -50°C or less, and adding an electrolyte solution. Example

[0058] Hereinafter, the present invention will be specifically described by way of examples, but the present invention is not limited to these examples. First, the evaluation method in the examples will be described.

[0059] [Measurement A] Average thickness and standard deviation of carbon layer

[0060] The powder samples including the olivine-type active material secondary particles obtained in each of the Examples and Comparative Examples were subjected to multi-wave interference imaging using a transmission electron microscope at an accelerating voltage of 300 kV and a magnification of 2,000,000 times. Figure 1 In (a), the thickness of the carbon layer is measured at 21 selected points selected approximately equally on the periphery of the observed primary particle. In addition, at each measurement point where the thickness of the carbon layer is measured, a two-dimensional Fourier transform is performed on the obtained lattice image, and the obtained Figure 1 In (b), measure the distance from the origin to the bright spot and calculate the corresponding d value ( Figure 1 (b) 0.630 nm and 0.534 nm), the crystal orientation was assigned by applying the nearest plane spacing and the crystal orientation perpendicular to the surface of the particle was calculated.

[0061] Figure 1 In (b), 0.630 nm corresponds to (010), 0.534 nm corresponds to (100), and the crystal axis b is parallel to (010).

[0062] It should be noted that the index shown in brackets as the crystal orientation in this specification indicates a plane in ordinary crystallography, but in this specification, it indicates the direction of the normal line of the plane. The direction of the normal line of the plane can be expressed by the primary translation vectors k1, k2, k3 (arrows omitted) corresponding to the reciprocal lattice of the lattice as shown in the following formula.

[0063] [Number 1]

[0064]

[0065]

[0066]

[0067] (Wherein, x1, x2, x3 (arrows omitted) represent primary translation vectors parallel to the a-axis, b-axis, and c-axis of the lattice, respectively).

[0068] When the angle between the crystal orientation perpendicular to the surface of the particle and the crystal axis b-axis is less than 20 degrees, the measurement point is deemed to exist on the plane perpendicular to the crystal axis b-axis. When the angle is greater than 20 degrees, it is deemed not to exist on the plane perpendicular to the crystal axis b-axis. Figure 1 In (b), the angle between (010) and the b-axis is 0 degrees, and the angle between (100) and the b-axis is 90 degrees.

[0069] If there is only one point on the observed primary particle where the angle with the crystal axis b is 20 degrees or less, the b axis is considered to be perpendicular to the observation surface of the microscope image and another primary particle is observed.

[0070] In this way, the average thickness X of the carbon layer existing on the surface perpendicular to the crystal axis b-axis, the average thickness Y of the carbon layer existing on the surface not perpendicular to the crystal axis b-axis, and the standard deviation of the thickness of the carbon layer existing on the surface perpendicular to the crystal axis b-axis are calculated respectively.

[0071] [Measurement B] Crystallite diameter and X-ray diffraction peak intensity ratio

[0072] For the powder sample containing the secondary particles of the olivine-type active material obtained in each example and comparative example, powder X-ray diffraction measurement was performed under the condition that the diffraction angle 2θ was 10 degrees or more and 70 degrees or less using an X-ray diffraction device D8 ADVANCE manufactured by Bruker AXS Co., Ltd. Based on the obtained diffraction pattern, Rietveld analysis was performed using the analysis software TOPAS manufactured by Bruker AXS Co., Ltd., thereby calculating the crystallite diameter of the olivine-type active material.

[0073] The analysis was performed assuming an olivine-type crystal structure as the starting structure. The GOF (= (R wp / R exp ) 2 ) value is less than 4.0, the active material is considered to have an olivine-type crystal structure. The peak intensity ratios were calculated by using DIFFRAC.EVA, a powder X-ray diffraction analysis software manufactured by Bruker AXS Co., Ltd., after background removal (coefficient 1.77) and reading the peak intensity. The values ​​obtained by dividing the peak intensity at 20° and 35° by the peak intensity at 29° were recorded as I 20 / I 29 ,I 35 / I 29 .

[0074] [Measurement C] Rate characteristics of secondary batteries

[0075] The electrode plate obtained by each embodiment and comparative example was cut into a diameter of 15.9 mm as the positive electrode, and the lithium foil was cut into a diameter of 16.1 mm and a thickness of 0.2 mm as the negative electrode. "Setiera" (registered trademark) was used as the isolation membrane, and as the electrolyte, ethylene carbonate containing LiPF6 at a concentration of 1M: diethyl carbonate = 3:7 (volume ratio) was used to produce a 2032 type coin battery.

[0076] For the 2032-type coin battery produced, the cutoff voltage was set to 2.5V, the maximum charge voltage was set to 4.3V, and the charge and discharge were set to 0.1C rate for 2 times, and then the charge and discharge were set to 3C rate for 2 times. For each charge and discharge rate, the discharge energy was measured according to the second discharge, and the ratio of the energy at 3C rate divided by the energy at 0.1C rate was calculated to evaluate the rate characteristics.

[0077] [Measurement D] Cycle durability of secondary batteries

[0078] A laminated battery with a capacity of 1 Ah was prepared using the electrode plates obtained by each embodiment and comparative example, a commercially available carbon-based negative electrode (negative electrode active material: artificial graphite MAG manufactured by Hitachi Chemical Co., Ltd.) as a negative electrode, "セティーラ" (registered trademark) as a separator, and 1M LiPF6-containing ethylene carbonate: diethyl carbonate = 3:7 (volume ratio) as an electrolyte. The number of layers is 7 layers for the positive electrode (size: 70 mm × 40 mm) and 8 layers for the negative electrode (size: 74 mm × 44 mm), and the capacity ratio (NP ratio) of the relative positive and negative electrodes is set to 1.05.

[0079] The laminated battery was subjected to a cycle test in which charge and discharge were repeated at a rate of 1C at 55°C after three cycles of charge and discharge at a rate of 0.1C at 25°C. The energy density in the first discharge test at 55°C was taken as 100%, and the number of cycles until the energy density fell below 80% was measured to evaluate the cycle tolerance.

[0080] [Example 1]

[0081] (Step 1: Preparation of secondary particles of olivine-type active material)

[0082] After dissolving 60 mmoles of lithium hydroxide monohydrate in 16 g of pure water, 104 g of diethylene glycol was added to prepare a lithium hydroxide / diethylene glycol aqueous solution. The obtained lithium hydroxide / diethylene glycol aqueous solution was stirred at 2000 rpm using a homogenizer (homogenizer 2.5 manufactured by PRIMIX), and then an aqueous solution obtained by dissolving 20 mmoles of phosphoric acid (85% aqueous solution), 16 mmoles of manganese (II) sulfate 1 hydrate, and 4 mmoles of ferrous sulfate (II) 7 hydrate in 10 g of pure water was added to obtain an olivine-type structured lithium manganese iron phosphate nanoparticle precursor. The obtained precursor solution was heated to 110 ° C and kept for 2 hours to obtain lithium manganese iron phosphate as a solid component. Pure water was added to the obtained particles, and the solvent was repeatedly removed by a centrifuge, thereby washing. The synthesis was repeated until the washed lithium manganese iron phosphate reached 10 g.

[0083] In the obtained lithium manganese iron phosphate 10g, glucose 1.0g (melting point 146°C) and polyvinyl alcohol 0.75g (melting point 300°C) as carbon sources, and pure water 40g were added and mixed, and granulated under the conditions of nozzle diameter 400μm, drying temperature 150°C, and atomization pressure 0.2MPa using a spray drying device (ADL-311-A manufactured by Yamato Scientific). The obtained granules were pre-calcined at 175°C for 1 hour using a calcining furnace, and then calcined at 700°C for 1 hour under a nitrogen atmosphere to obtain a powder sample containing secondary particles of lithium manganese iron phosphate.

[0084] (Step 2: Preparation of electrode plates)

[0085] After mixing acetylene black (Li-400 manufactured by Denka Corporation) and a binder (Kureha KF Polymer L#9305 manufactured by Kureha Corporation), add the lithium iron manganese phosphate secondary particles obtained by the above method and knead them in a mortar. At this time, the weight ratio of the materials included is set to 90:5:5 of lithium iron manganese phosphate secondary particles: acetylene black: binder. Thereafter, N-methylpyrrolidone is added and adjusted so that the solid content reaches 48% by weight to obtain a slurry-like electrode paste. N-methylpyrrolidone is added to the obtained paste until fluidity appears, and a thin film rotary high-speed mixer ("Filmicus" (registered trademark) 40-L manufactured by PRIMIX Corporation) is used to treat it for 30 seconds under stirring conditions of 40 m / s.

[0086] 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.

[0087] [Example 2]

[0088] Lithium manganese iron phosphate secondary particles and an electrode plate were prepared in the same manner as in Example 1, except that the amount of glucose used in Step 1 was 1.50 g and the amount of polyvinyl alcohol used was 1.10 g. Figure 1 (a) shows a transmission electron microscope image of the obtained lithium manganese iron phosphate secondary particles. Figure 1 (b) shows its Fourier transform image. Figure 1 In (a), reference numeral 1 indicates the thickness of the carbon layer at each measurement point.

[0089] [Example 3]

[0090] In step 1, instead of glucose, a reduced pressure heavy oil stock solution with a viscosity of 600 mPa·sec at 20°C (using a B-type viscometer at a rotation speed of 6 rpm) was used. Otherwise, lithium manganese iron phosphate secondary particles and electrode plates were prepared in the same manner as in Example 1.

[0091] [Example 4]

[0092] Lithium manganese iron phosphate secondary particles and an electrode plate were prepared in the same manner as in Example 1 except that the amount of diethylene glycol added in step 1 was changed to 60 g.

[0093] [Example 5]

[0094] Olivine-type active material secondary particles and an electrode plate were prepared in the same manner as in Example 1 except that the amount of iron (II) sulfate heptahydrate added in step 1 was 20 mmol and manganese (II) sulfate monohydrate was not added.

[0095] [Example 6]

[0096] 200 g of dimethyl sulfoxide and 390 mmol of lithium hydroxide monohydrate were added to 150 g of pure water. To the obtained solution, 120 mmol of phosphoric acid was further added using an 85 wt % phosphoric acid aqueous solution, and 84 mmol of manganese (II) sulfate monohydrate and 36 mmol of iron (II) sulfate 7 hydrate were added. The obtained solution was transferred to an autoclave and heated for 4 hours in a manner maintaining 150° C. in the container. After heating, the supernatant of the solution was discarded to obtain lithium iron manganese phosphate as a precipitate. After dispersing the obtained lithium iron manganese phosphate in pure water, the supernatant was removed by centrifugation, and the above operation was repeated 5 times, thereby washing. For 10 g of the obtained lithium iron manganese phosphate, a powder sample containing secondary particles of lithium iron manganese phosphate was obtained in the same manner as in Example 1. Thereafter, secondary particles of olivine-type active material and electrode plates were prepared in the same manner as in Example 1.

[0097] [Comparative Example 1]

[0098] Lithium manganese iron phosphate secondary particles and an electrode plate were prepared in the same manner as in Example 1 except that the amount of glucose added in step 1 was 0.25 g and the amount of polyvinyl alcohol added was 1.5 g.

[0099] [Comparative Example 2]

[0100] Granulated bodies were obtained in the same manner as in Example 1 except that the amount of glucose added in step 1 was 1.75 g and polyvinyl alcohol was not added. The obtained granulated bodies were calcined in a calcining furnace at 700° C. for 1 hour in a nitrogen atmosphere to obtain a powder sample containing secondary particles of lithium manganese iron phosphate. Thereafter, an electrode plate was prepared in the same manner as in step 2 of Example 1. Figure 2 (a) shows a transmission electron microscope image of the obtained lithium manganese iron phosphate nanoparticle granules, and (b) shows a Fourier transform image thereof (b).

[0101] [Comparative Example 3]

[0102] 200 g of dimethyl sulfoxide and 390 mmol of lithium hydroxide monohydrate were added to 150 g of pure water. 120 mmol of phosphoric acid was further added to the obtained solution using an 85 wt% phosphoric acid aqueous solution, and 84 mmol of manganese (II) sulfate monohydrate and 36 mmol of iron (II) sulfate 7 hydrate were added. The obtained solution was transferred to an autoclave and heated for 4 hours in a manner that the container was maintained at 150°C. After heating, the supernatant of the solution was discarded to obtain lithium manganese iron phosphate as a precipitate. After dispersing the obtained lithium manganese iron phosphate in pure water, the supernatant was removed by centrifugation, and the above operation was repeated 5 times to wash. For 10 g of the obtained lithium manganese iron phosphate, 10 g of pure water and 0.75 g of glucose were added and stirred to obtain a lithium manganese iron phosphate dispersion, and then granulated using a spray drying device (ADL-311-A manufactured by Yamato Scientific) at a nozzle diameter of 400 μm, a drying temperature of 150°C, and an atomization pressure of 0.2 MPa. The obtained granulated particles were calcined in a calcining furnace at 700° C. for 1 hour in a nitrogen atmosphere to obtain a powder sample containing lithium manganese iron phosphate secondary particles. Thereafter, an electrode plate was prepared in the same manner as in step 2 of Example 1.

[0103] [Comparative Example 4]

[0104] Olivine-type active material secondary particles and an electrode plate were prepared in the same manner as in Comparative Example 3 except that manganese (II) sulfate monohydrate was not added and the added amount of iron (II) sulfate heptahydrate was 120 mmol.

[0105] [Comparative Example 5]

[0106] 200 g of dimethyl sulfoxide and 390 mmol of lithium hydroxide monohydrate were added to 150 g of pure water. To the resulting solution, 120 mmol of phosphoric acid was further added using an 85 wt % aqueous phosphoric acid solution, and 84 mmol of manganese (II) sulfate monohydrate and 36 mmol of ferrous (II) sulfate 7 hydrate were added. The resulting solution was transferred to an autoclave and heated for 4 hours in a manner that maintained 150° C. in the container. After heating, the supernatant of the solution was discarded to obtain lithium manganese iron phosphate as a precipitate. After the obtained lithium manganese iron phosphate was dispersed in pure water, the supernatant was removed by centrifugation, and the above operation was repeated 5 times to thereby wash.

[0107] To 10 g of the obtained lithium manganese iron phosphate, a reduced pressure heavy oil stock solution having a viscosity of 600 mPa·sec at 20° C. (using a B-type viscometer at a rotation speed of 6 rpm) was added in an amount of 4.0 wt % relative to the weight of the aforementioned lithium manganese iron phosphate, and the mixture was further precisely mixed using a jet mill (manufactured by Suginomashin), and the mixture was calcined at 700° C. for 3 hours.

[0108] [Comparative Example 6]

[0109] Granulation was obtained in the same manner as in Example 1 except that the amount of glucose added in step 1 was 2.50 g and polyvinyl alcohol was not added. The obtained granulation was calcined in a calcining furnace at 700° C. for 1 hour in a nitrogen atmosphere to obtain a powder sample containing secondary particles of lithium manganese iron phosphate. The powder sample and the powder sample obtained in the same manner as in Comparative Example 2 were mixed in a mortar in a weight ratio of 1:1 to obtain an olivine-type active material powder. Thereafter, an electrode plate was prepared in the same manner as in step 2 of Example 1.

[0110] Table 1 shows the evaluation results of each example and comparative example.

[0111]

[0112] Description of Reference Numerals

[0113] 1. Thickness of carbon layer at each measuring point

[0114] 2 origin

[0115] 3 Highlights

[0116] 4 Distance from origin to bright spot

Claims

1. An active material for a secondary battery electrode, having an olivine crystal structure, having a carbon layer on the surface, wherein the ratio of the average thickness of the carbon layer on the surface perpendicular to the crystal axis b-axis to the average thickness of the carbon layer on the surface not perpendicular to the aforementioned b-axis is 0.30 or more and 0.80 or less, and the crystallite diameter is 60 nm or less, When the angle between the crystal orientation perpendicular to the surface of the particle and the crystal axis b-axis is less than 20 degrees, the measurement point is deemed to exist on the plane perpendicular to the crystal axis b-axis. When the angle is greater than 20 degrees, it is deemed not to exist on the plane perpendicular to the crystal axis b-axis.

2. The secondary battery electrode active material according to claim 1, wherein The average thickness of the carbon layer existing on the plane perpendicular to the crystal axis b-axis is 2.0 nm or less.

3. The active material for secondary battery electrodes according to claim 1 or 2, wherein The standard deviation of the thickness of the carbon layer existing on the plane perpendicular to the crystal axis b-axis is 0.3 nm or less.

4. The active material for secondary battery electrodes according to claim 1 or 2, wherein At least a portion of the A site of the olivine crystal structure represented by the general formula ABXO4 is occupied by lithium. In ABXO4, A and B each independently represent one or more metal elements, and X represents any one or more elements other than metal elements. 5 . The active material for secondary battery electrodes according to claim 1 , which is lithium manganese iron phosphate.

6. The active material for secondary battery electrodes according to claim 5, wherein The ratio of the peak intensity at 20° to the peak intensity at 29° obtained by X-ray diffraction 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 35 / I 29 It is 1.05 or more and 1.20 or less. 7 . A secondary battery obtained by using the active material for a secondary battery electrode according to claim 1 .

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