POSITIVE ELECTRODE FOR A SECONDARY BATTERY AND SECONDARY BATTERY

Abstract

A secondary battery having excellent battery characteristics is provided. The secondary battery includes a positive electrode having a positive electrode active material layer, a negative electrode, and an electrolyte solution. The positive electrode active material layer includes a plurality of positive electrode active material particles, and each of the plurality of positive electrode active material particles has a central portion containing an olivine-type phosphate compound and a covering portion provided on a surface of the central portion. The olivine-type phosphate compound includes manganese and iron as constituent elements. When the sum of a manganese content in the olivine-type phosphate compound and a content of iron in the olivine-type phosphate compound is 100 mol parts, the manganese content in the olivine-type phosphate compound is 50 mol parts or more and 90 mol parts or less.The central part is a secondary particle formed by granulating a plurality of primary particles, wherein a first average diameter relating to the plurality of primary particles is 0.01 μm or more and 0.5 μm or less, and a second average diameter relating to the plurality of secondary particles is 1 μm or more and 20 μm or less. The cover part has a nitrile group. The positive electrode active material layer has a porosity of 20% or more and 40% or less.

Description

BACKGROUND OF THE INVENTION1. FIELD OF THE INVENTIONThe present technology relates to a positive electrode for a secondary battery and a secondary battery.2. DESCRIPTION OF THE TECHNICAL FIELDAs various electronic devices such as mobile phones are widely used, secondary batteries have been developed as power sources that are small and light and can achieve high energy density. The secondary battery includes a positive electrode as a positive electrode for a secondary battery, a negative electrode, and an electrolyte solution. Various studies have been made on the configuration of the secondary battery.Specifically, in a lithium ion secondary battery, olivine iron is used as a positive electrode active material and acrylonitrile is used as a positive electrode binder, and the particle size of the positive electrode active material, the particle size and the mixing amount of the positive electrode binder, and the porosity of the positive electrode are defined (see, e.g., International Publication No. 2011 / 122297).SUMMARY OF THE INVENTIONVarious studies have been made on the configuration of the secondary battery, but the battery characteristics of the secondary battery are still insufficient, and therefore there is room for improvement.A positive electrode for a secondary battery and a secondary battery that can achieve excellent battery characteristics is desired.A positive electrode for a secondary battery according to an embodiment of the present technology includes a positive electrode active material layer. The positive electrode active material layer includes a plurality of positive electrode active material particles, and each of the plurality of positive electrode active material particles includes a central part including an olivine type phosphate compound / olivine type phosphate compound and a cover part provided on a surface of the central part. The olivine type phosphate compound has manganese and iron as constituent elements, and when a sum of a content of manganese in the olivine type phosphate compound and a content of iron in the olivine type phosphate compound is 100 parts by mole, the content of manganese in the olivine type phosphate compound is 50 parts by mole or more and 90 parts by mole or less. The central part is a secondary particle formed by granulating a plurality of primary particles, wherein a first average diameter related to the plurality of primary particles is 0.01 μm or more and 0.5 μm or less, and a second average diameter related to the plurality of secondary particles is 1 μm or more and 20 μm or less. The cover member has a nitrile group. The positive electrode active material layer has a porosity of 20% or more and 40% or less.A secondary battery of an embodiment of the present technology includes a positive electrode, a negative electrode, and an electrolyte solution, the positive electrode having the same configuration as the configuration of the above-described positive electrode for a secondary battery of an embodiment of the present technology.The positive electrode for a secondary battery or the secondary battery of one embodiment of the present technology can have excellent battery properties because each of the plurality of positive electrode active material particles has a central part and a cover part, the central part has an olivine type phosphate compound, the manganese content in the olivine type phosphate compound is 50 mol parts or more and 90 mol parts or less, the first average diameter is 0.01 μm or more and 0.5 μm or less, the second average diameter is 1 μm or more and 20 μm or less, the cover part has a nitrile group, and the positive electrode active material layer has a porosity of 20% or more and 40% or less.The effect of the present technology is not necessarily limited to the effect described herein, and may be any effect of a variety of effects related to the present technology and described later.BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 is a sectional view illustrating a configuration of a positive electrode for a secondary battery according to an embodiment of the present technology; FIG. 2 is a sectional view illustrating a configuration of a positive electrode active material particle; FIG. 3 is a sectional view illustrating another configuration of the positive electrode for a secondary battery; FIG. 4 is a sectional view illustrating a configuration of a secondary battery according to an embodiment of the present technology; FIG. 5 is a sectional view illustrating a configuration of the battery element illustrated in FIG. 4 ; and FIG. 6 is a sectional view illustrating a configuration of a secondary battery for test.DETAILED DESCRIPTION OF THE INVENTIONHereinafter, an embodiment of the present technology will be described in detail with reference to the drawings. The order of description is as follows.1. Positive electrode for secondary battery1-1. Configuration1-2. Operation1-3. Production Method1-4. Effect and effect2. Secondary Battery2-1. Configuration2-2. Operation2-3. Production Method2-4. Effect and effect3. Examples of modifications4. Application of the Secondary Battery<1 Positive electrode for secondary battery>First, a positive electrode for a secondary battery (hereinafter, simply referred to as a "positive electrode") according to an embodiment of the present technology will be described.The positive electrode described herein is used for a secondary battery as an electrochemical device. However, the positive electrode may be used for an electrochemical device other than the secondary battery. The type of the other electrochemical device is not particularly limited, but is particularly a primary battery, a capacitor, or the like.This positive electrode includes and releases an electrode reactant during operation of the electrochemical device, in other words, during electrode reaction of the positive electrode. The kind of the electrode reactant is not particularly limited, but is particularly a light metal such as an alkali metal or an alkaline earth metal. Specific examples of alkali metals are lithium, sodium and potassium. Specific examples of alkaline earth metals are beryllium, magnesium and calcium.Next, a case where the electrode reactant is lithium will be described. Lithium is enclosed in the positive electrode and is released during the electrode reaction in ionic state.<1-1. Configuration>FIG. 1 shows a sectional configuration of a positive electrode 100 that is an example of a positive electrode according to an embodiment of the present technology. FIG. 2 shows a sectional configuration of a positive electrode active material particle (or positive electrode active material particle) 110. FIG. 3 shows another sectional configuration of the positive electrode 100.As illustrated in FIG. 1, the positive electrode 100 includes a positive electrode active material layer 100B. Here, the positive electrode 100 further includes a positive electrode current collector 100A that supports the positive electrode active material layer 100B. The positive electrode current collector 100A may be omitted.[Positive Electrode Current Collector]As illustrated in FIG. 1, the positive electrode current collector 100A is a conductive member that supports the positive electrode active material layer 100B. Here, the positive electrode current collector 100A has a pair of surfaces on which the positive electrode active material layer 100B is provided. The positive electrode current collector 100A includes a conductive material such as a metal material, specific examples of the conductive material including aluminum.[Positive Electrode Active Material Layer]Here, as illustrated in FIG. 1, the positive electrode active material layer 100B is provided on a surface of the positive electrode current collector 100A. The positive electrode active material layer 100B may be provided on both surfaces of the positive electrode current collector 100A.The positive electrode active material layer 100B includes a positive electrode active material. The positive electrode active material layer 100B may further include one or two or more of other materials such as a positive electrode binder and a positive electrode conductive agent.(Positive Electrode Active Material)Specifically, as illustrated in FIG. 2, the positive electrode active material layer 100B includes a plurality of positive electrode active material particles 110 that are a plurality of particulate positive electrode active materials that trap and release lithium. Each of the plurality of positive electrode active material particles 110 includes a central part (or central part) 110X and a cover part (or covering part) 110Y.In FIG. 2, only one of the plurality of positive electrode active material particles 110 is illustrated, and the cross-sectional shape of the positive electrode active material particle 110 is illustrated in a circular shape for convenience of illustration.(Central part)The central part 110X is a part that includes and releases lithium, and the part has one or two or more olivine type phosphate compounds. The olivine type phosphate compound is a phosphate compound having an olivine type crystal structure.The central part 110X has the olivine type phosphate compound because the crystal structure of the olivine type phosphate compound is solid and stable and the release of oxygen from the olivine type phosphate compound during the electrode reaction is suppressed. As a result, lithium is stably sealed and released in the central part 110X, and thus the electrode reaction stably proceeds. Thus, in the secondary battery including the positive electrode 100, a stable battery capacity is obtained and safety is improved.Here, since the electrode reactant is lithium as described above, the olivine type phosphate compound has lithium as a constituent element together with phosphorus and oxygen. In this case, the olivine type phosphate compound further has manganese and iron as constituent elements.The manganese content in the olivine type phosphate compound is within a predetermined range. In particular, when the sum of the manganese content in the olivine type phosphate compound and the iron content in the olivine type phosphate compound is 100 parts by mole, the manganese content in the olivine type phosphate compound is 50 to 90 parts by mole. This is because the electron conductivity of the olivine type phosphate compound improves when the content is in this range. This configuration can achieve both improvement in electron conductivity of the plurality of active material particles of the positive electrode 110 and stabilization of the operating potential and battery capacity in the secondary battery using the positive electrode 100.More specifically, the olivine type phosphate compound has one or two or more of the compounds represented by formula (1). LiMn x Fe 1-x PO 4(1) ( x satisfies 0.5 to 0.9)Specific examples of the olivine type phosphate compound include LiMn 0,5 Fe 0,5 PO 4, LiMn 0,7 Fe 0,3 PO 4 and LiMn 0,9 Fe 0,1 PO 4.Whether the central part 110X has an olivine type phosphate compound can be checked by analyzing the central part 110X with an analysis method such as X-ray diffractometry (XDR). Whether the olivine type phosphate compound has manganese and iron as components can be checked by analyzing the central part 110X using an analysis method such as inductively coupled plasma (ICP) emission spectrometry.(Average diameter)Specifically, the central part 110X is a granulated body, and more specifically, the central part is a secondary particle obtained by granulating a plurality of primary particles. Here, since the plurality of primary particles are aggregated with each other, the secondary particle, which is the central part 110X, is an aggregate of the plurality of primary particles.As described above, the positive electrode active material layer 100B includes a plurality of positive electrode active material particles 110. Thus, the positive electrode active material layer 100B includes a plurality of central parts 110X.The central part 110X is formed into a granulated body because the conductivity of the positive electrode active material layer 100B is thereby improved.In particular, the olivine type phosphate compound has substantially low electron conductivity. Therefore, in order to improve the conductivity of the olivine type phosphate compound, the olivine type phosphate compound preferably has a plurality of fine particle structures. Therefore, it is preferable to use secondary particles formed by granulating the plurality of primary particles into a granular body of a plurality of primary particles.With this configuration, the plurality of secondary particles are likely to come into contact with each other, resulting in an improvement in electron conductivity between the plurality of secondary particles. With this configuration, the plurality of primary particles are also likely to come into contact with each other, leading to an improvement in electron conductivity between the primary particles. Thus, the conductivity inside the positive electrode active material particles 110 improves, and the conductivity between the plurality of positive electrode active material particles 110 also improves, leading to an improvement in the conductivity of the positive electrode active material layer 100B.Specifically, an average diameter MD 1, which is a first average diameter of the plurality of primary particles, is 0.01 μm to 0.5 μm. This is because the plurality of primary particles are likely to come into contact with each other, and thus the electron conductivity between the plurality of primary particles is improved.In particular, the average diameter MD 1 is preferably 0.1 μm to 0.3 μm. This is because the plurality of primary particles are more likely to come into contact with each other, and thus the electron conductivity between the plurality of primary particles is further improved.An average diameter MD 2, which is a second average diameter of the plurality of secondary particles, is 1 μm to 20 μm. This is because the plurality of secondary particles are likely to come into contact with each other, and thus the electron conductivity between the plurality of secondary particles is improved.In particular, the average diameter MD 2 is 5 μm to 15 μm. This is because the plurality of secondary particles are more likely to come into contact with each other, and thus the electron conductivity between the plurality of secondary particles is further improved.The method for measuring the average diameter MD2 is as described below. Here, a case where the positive electrode active material layer 100B includes a plurality of positive electrode active material particles 110, a positive electrode binder, and a positive electrode conductive agent will be described.For measurement of the average diameter MD 2, a plurality of positive electrode active material particles 110 are analyzed using a particle size analyzer. As this particle size analyzer, a laser diffraction / scattering particle size distribution analyzer LA-960 manufactured by HORIBA, Ltd., or the like can be used.More specifically, in the measurement of the average diameter MD 2, first, the positive electrode 100 is charged into a solvent, and then the solvent is stirred to peel the positive electrode active material layer 100B from the positive electrode current collector 100A. The kind of the solvent is not particularly limited as long as the positive electrode binder can be dissolved. In this process, the binder of the positive electrode is dissolved and removed, and thus the plurality of positive electrode active material particles 110 and the positive electrode conductive agent, which are solid components, are collected.Then, the solid component is put in a solvent, and then the solid component in the solvent is centrifuged using a centrifuge. The type of the solvent is not particularly limited as long as the plurality of positive electrode active material particles 110 and the positive electrode conductive agent can be dispersed. By this operation, the plurality of positive electrode active material particles 110 are separated from the positive electrode conductive agent, and thus the plurality of positive electrode active material particles 110 are collected.Finally, the average diameter MD 2 is measured by analyzing the plurality of positive electrode active material particles 110 using the particle size analyzer.The method for measuring the average diameter MD1 is as described below.First, a portion of the positive electrode active material layer 100B is exposed by cutting the positive electrode 100 with a cutting tool such as a microtome.Subsequently, the region of the positive electrode active material layer 100B is observed using a scanning electron microscope (SEM) (observation magnification=10,000 times). As a result, the plurality of positive electrode active material particles 110 contained in the positive electrode active material layer 100B are observed, and thus a plurality of primary particles constituting each of the plurality of secondary particles, which are the plurality of central parts 110X, are observed.Subsequently, from the plurality of primary particles 50, primary particles whose entire outline (outer edge) can be observed are selected, and then the particle diameter of each of the 50 primary particles is measured. This particle diameter is the diameter of the major axis (the maximum value of the diameter in the direction of the major axis) when the primary particle has the major axis and the minor axis. Finally, an average value of 50 particle diameters is calculated to obtain the average diameter MD1.(Cover member)Since the cover part 110Y is provided on the surface of the central part 110X, it is a part that protects the surface of the central part 110X.The cover part 110Y may be provided on the entire surface of the central part 110X or only on a part of the surface of the central part 110X. When the cover part 110Y is provided only on a part of the surface of the central part 110X, a plurality of cover parts 110Y separated from each other may be provided on the surface of the central part 110X.Specifically, the cover part 110Y has a nitrile group. The number of nitrile groups is not particularly limited, and may be one, two or more.Specifically, as described later, in the step of forming the positive electrode active material layer 100B, a part of the positive electrode binder having a nitrile group adheres to the surface of the central part 110X, and thus the cover part 110Y is formed using a part of the positive electrode binder. In this case, the positive electrode binder and the central part 110X are mixed together, which does not mean that the positive electrode binder is present in the vicinity of the central part 110X. On the other hand, the cover part 110Y is formed by utilizing preferential adhesion of a part of the positive electrode binder because a part of the positive electrode binder preferentially adheres to the surface of the central part 110X due to the step of forming the positive electrode active material layer 100B.The details of the method of forming the cover part 110Y using the step of forming the positive electrode active material layer 100B will be described later.Since the cover part 110Y is formed using the positive electrode bonding agent as described above, the configuration of the cover part 110Y is the same as the configuration of the positive electrode bonding agent. Specifically, the cover part 110Y has a nitrile group, the cover part having one or two or more nitrile group-containing polymer compounds which are polymer compounds containing the nitrile group. The nitrile group-containing polymer compound may be a homopolymer obtained by polymerizing one kind of monomer, a copolymer obtained by polymerizing two or more kinds of monomers, or both.Specific examples of the homopolymer include polyacrylonitrile. Specific examples of the copolymer include an acrylonitrile-butadiene copolymer and an acrylonitrile-ethylhexyl acrylate copolymer.The cover part 110Y is provided on the surface of the central part 110X, and the cover part 110Y has a nitrile group because the surface of the central part 110X is electrochemically protected with high reactivity by using the cover part 110Y. Thus, the occurrence of side reactions on the surface of the central part 110X during the electrode reaction is suppressed, and the central part 110X has high reactivity. In this case, particularly in the secondary battery using the positive electrode 100, the occurrence of the decomposition reaction of the electrolyte solution during charging and discharging is suppressed.(Method for Checking Cover Part)The method of checking whether the cover part 110Y is present on the surface of the central part 110X is as described below.First, the positive electrode 100 is cut by ion milling to expose a portion of the positive electrode active material layer 100B.Subsequently, the region of the positive electrode active material layer 100B is subjected to negative ion analysis using time-of-flight secondary ion mass spectrometry (TOF-SIMS) to create a first mapping based on oxygen ions (O -) and a second mapping based on nitrile ions (CN -).The first mapping is the result of a two-dimensional representation of the detection amount of oxygen ions, and the presence range of the oxygen ions is displayed in color. This first mapping is used to specify the presence range of the central part 110X having the olivine type phosphate compound by indicating the presence range of the olivine type phosphate compound having oxygen as a constituent.The second mapping is the result of two-dimensional representation of a value obtained by dividing the detection amount of nitrile ions by the detection amount of all ions, and the presence range of the nitrile ions is displayed in color. This second mapping is used to specify the presence range of the cover part 110Y having the nitrile group-containing polymer compound by indicating the presence range of the nitrile group-containing polymer compound having a nitrile group.In the second mapping, a value obtained by dividing the detection amount of nitrile ions using the detection amount of all ions without using the detection amount of nitrile ions is used to eliminate the gradient of the detection amount of ions caused by the inclination of the sample.In this case, a time-of-flight secondary ion mass spectrometer "TOF-SIMS 5" of IONTOF GmbH is used as analyzer. With respect to the analysis conditions, Bi 3+ is set as a primary ion, the acceleration voltage of the ion gun is set to 25 keV, the analysis mode is set to burst alignment, the current (measurement with a pulse beam) of the irradiation ions is set to 0.03 pA, the pulse frequency is set to 10 kHz, the mass range is set to 1 amu to 80 amu, and the scanning range is set to 130 μm×130 μm. Before the measurement, the measurement site is sputtered with Ar +- ions or the like.Then, the presence range of the central part 110X is set based on the first mapping. In the first mapping, the presence range of the central part 110X is a range defined by a substantially spherical outer edge.Then, the presence range of the cover part 110Y is set based on the second mapping. In the second mapping, the presence range of the cover part 110Y is a range defined by a substantially annular or substantially arc-shaped outer edge.Of course, when the cover part 110Y is not formed, the presence range of the cover part 110Y is not specified in the second mapping, and therefore, the presence range of the cover part 110Y is not visually recognized.Finally, whether the cover part 110Y is present on the surface of the central part 110X is checked based on the first mapping and the second mapping.Specifically, the presence range of the central part 110X indicated in the first mapping and the presence range of the cover part 110Y indicated in the second mapping are compared with each other. Thus, when the presence area of the central part 110X and the presence area of the cover part 110Y are in contact with each other, it is determined that the cover part 110Y is provided on the surface of the central part 110X.On the other hand, when the presence area of the central part 110X and the presence area of the cover part 110Y are not in contact with each other as a result of comparison of the presence area of the central part 110X indicated in the first mapping and the presence area of the cover part 110Y indicated in the second mapping, it is confirmed that the cover part 110Y is not provided on the surface of the central part 110X.Here, the number of times of checking whether the cover part 110Y is provided on the surface of the central part 110X is not particularly limited, and may be performed only once or two or more times. When the number is two or more, the accuracy in checking whether the cover part 110Y is present on the surface of the central part 110X improves as compared with the case where the number is only one.However, as described above, the cover part 110Y is formed due to the characteristic step of forming the positive electrode active material layer 100B described later, so that the cover part 110Y is not formed without using the characteristic step of forming the positive electrode active material layer 100B. Thus, the accuracy in checking whether the cover part 110Y is provided on the surface of the central part 110X is ensured when the number of checks whether the cover part 110Y is provided on the surface of the central part 110X is only one.(Total formation amount ratio of cover member)Here, as described above, in the step of forming the positive electrode active material layer 100B, a part of the positive electrode binder preferentially adheres to the surface of the central part 110X, thereby forming the cover part 110Y. As a result, the amount of the cover part 110Y formed on the surface of the central part 110X is less likely to be unevenly distributed in the positive electrode active material layer 100B, and therefore is likely to be uniformly distributed in the positive electrode active material layer 100B.Specifically, since the positive electrode 100 includes the positive electrode current collector 100A, the positive electrode active material layer 100B when supported by the positive electrode current collector 100A is divided based on the position of the positive electrode current collector 100A as illustrated in FIG. 3.In this case, the positive electrode active material layer 100B is halved in a direction in which the positive electrode current collector 100A and the positive electrode active material layer 100B face each other, that is, in a thickness direction of the positive electrode active material layer 100B (up-down direction in FIG. 3 ). Thereby, the positive electrode active material layer 100B is divided into a lower positive electrode active material layer 100B 1 which is a first positive electrode active material layer and an upper positive electrode active material layer 100B 2 which is a second positive electrode active material layer. The positive electrode lower active material layer 100B 1 is a part positioned closer to the positive electrode current collector 100A than the positive electrode upper active material layer 100B 2, and the positive electrode upper active material layer 100B 2 is a part positioned farther from the positive electrode current collector 100A than the positive electrode lower active material layer 100B 1.Here, a total formation amount R 1 of the plurality of cover members 110Y within the lower positive electrode active material layer 100B 1 and a total formation amount R 2 of the plurality of cover members 110Y within the upper positive electrode active material layer 100B 2 are considered.A total formation amount ratio R, which is the ratio of the total formation amount R 2 to the total formation amount R 1, is not particularly limited, but is preferably 0.6 to 1.4.Specifically, when the total formation amount ratio R is less than 0.6, and when the total formation amount ratio R is greater than 1.4, the value of the total formation amount ratio R greatly deviates from 1.0. In this case, since the difference between the total formation amounts R 1 and R 2 becomes large, segregation of the cover part 110Y is likely to occur within the positive electrode active material layer 100B. As a result, the frequency of the plurality of cover parts 110Y within the positive electrode active material layer 100B becomes uneven, and thus the electrode reaction hardly proceeds uniformly.On the other hand, the value of the total formation amount ratio R becomes a value close to 1.0 when the total formation amount ratio R is 0.6 to 1.4. In this case, since the difference between the total formation amounts R 1 and R 2 becomes small, separation of the cover part 110Y is less likely to occur within the positive electrode active material layer 100B. As a result, the frequency of the cover part 110Y within the positive electrode active material layer 100B becomes substantially uniform, and thus the electrode reaction is likely to proceed uniformly.The method for calculating the total formation amount ratio R is as described below.First, a region of the lower positive electrode active material layer 100B 1 is analyzed using TOF-SIMS to obtain a mapping based on nitrile ions, and the total formation amount R 1 is measured based on the mapping.Subsequently, a region of the positive electrode upper active material layer 100B 2 is analyzed using TOF-SIMS to obtain a mapping based on nitrile ions, and the total formation amount R 2 is measured based on the mapping.The details of the analyzer and the analysis conditions at the time of analysis using TOF-SIMS are as described above.Finally, the total formation amount ratio R is calculated based on a calculation formula for the total formation amount ratio R (%)=(total formation amount R2 / total formation amount R1)×100.(Porosity)The positive electrode active material layer 100B has a plurality of voids. The plurality of voids are spaces in which no constituent elements such as the plurality of positive electrode active material particles 110 are present within the positive electrode active material layer 100B.A porosity P of the positive electrode active material layer 100B is 20% to 40%. This is because flexibility of the positive electrode 100 improves and thus damage to the positive electrode 100 during the electrode reaction is suppressed. Specific examples of the damage of the positive electrode 100 include the occurrence of cracks. In the secondary battery using the positive electrode 100, the impregnation property of the positive electrode active material layer 100B with the electrolyte solution is secured because the amount of the plurality of voids is secured in the positive electrode active material layer 100B.In particular, the porosity P is preferably 25% to 35%. This is because the flexibility of the positive electrode 100 is further improved, and thus damage to the positive electrode 100 during the electrode reaction is further suppressed.When the porosity P is measured, the positive electrode 100 is analyzed using a mercury intrusion method. In this case, a mercury porosimeter of AutoPore 9500 series manufactured by Micromeritics Instrument Corporation is used as the analyzer. As the analysis conditions, the surface tension of mercury is set to 485 mN / m, the contact angle is set to 130°, and the relationship between the pore size of the plurality of voids and the pressure is set to 180 / pressure = pore size.(Positive electrode binder)The positive electrode binder is a material that binds particles such as a plurality of particles of positive electrode active material to each other. As described above, the positive electrode binder is a material for forming the cover part 110Y in the step of forming the positive electrode active material layer 100B.As described above, the binder of the positive electrode comprises any one, two or more nitrile group-containing polymer compounds. Details of the nitrile group-containing polymer compounds are as described above.Here, the positive electrode active material layer 100B after the formation of the positive electrode active material layer 100B, i.e., after the formation of the cover part 110Y, has a nitrile group-containing polymer compound as a positive electrode binder. As a result, in the step of (off) forming the positive electrode active material layer 100B, a part of the positive electrode binder is consumed to form the cover part 110Y, but the rest of the positive electrode binder remains in the positive electrode active material layer 100B, and thus the rest of the positive electrode binder satisfies the original binding function.The positive electrode binder may further comprise one or two or more other materials. The other materials include one or two or more materials such as synthetic rubber and a polymer compound, wherein the nitrile group-containing polymer compound is excluded from the polymer compound described herein. Specific examples of synthetic rubber are styrene-butadiene rubber, fluororubber and ethylene-propylene-diene. Specific examples of the polymer compound include polyvinylidene fluoride, polyimide and carboxymethylcellulose.(Positive electrode conductive means)The positive electrode conductive agent is a material that improves the conductivity of the positive electrode active material layer 100B and includes one or two or more conductive materials such as a carbon material, a metal material, and a conductive polymer compound. Specific examples of the carbon material include graphite, carbon black, acetylene black and ketjen black.<1-2. Operation>In the positive electrode 100, at the time of electrode reaction, lithium in an ionic state is released from the positive electrode active material layer 100B, and lithium in an ionic state is sealed in the positive electrode active material layer 100B.<1-3. Production Method>The positive electrode 100 is manufactured using a method described below as an example.First, a plurality of primary particles comprising an olivine type phosphate compound are prepared. In this case, the average diameter MD 1 is set to be within the above range.Subsequently, the plurality of primary particles are granulated to form a plurality of secondary particles. A plurality of central parts 110X constituting a plurality of secondary particles, which are an olivine type phosphate compound, are thus obtained. The average diameter MD 2 is set to be within the above range. Granulation is carried out, for example, by spray drying. For example, a spray drying apparatus "MDP-050" manufactured by GF Corporation may be used. The granulation method is not particularly limited.Then, a dispersant (carboxymethyl cellulose) is added to a solvent (aqueous solvent) to prepare a dispersion. The type of the aqueous solvent is not particularly limited, but further specified is pure water or the like. The concentration of the dispersant is not particularly limited, but, more specifically, may be set to 1.3% with respect to 100% of the positive electrode active material.Subsequently, the plurality of central parts 110X is pretreated using the dispersion by incorporating the plurality of central parts 110X into the dispersion. In this case, the dispersion into which the plurality of central portions 110X are introduced may be stirred.Since the dispersant preferentially adheres to each of the plurality of central parts 110X by this pretreatment, a base film including the dispersant is formed on the surface of each of the plurality of central parts 110X. This base film is a film to which a part of the positive electrode binder is preferentially adhered in the subsequent process, and the thickness of the base film is extremely thin.Subsequently, the plurality of pretreated central portions 110X, the positive electrode binder having a polymer compound having a nitrile group, and the positive electrode conductive agent are mixed together to obtain a positive electrode mixture.Then, the positive electrode mixture is introduced into a solvent (aqueous solvent) to prepare a positive electrode mixture slurry.In this case, since the base film is formed on the surface of the central part 110X, a part of the positive electrode bonding agent is preferably adhered to the surface of the central part 110X. Thereby, the nitrile-containing polymer compound is formed into a film on the surface of the central part 110X, and thus the cover part 110Y is formed on the surface of the central part 110X. The capping member 110Y has the nitrile group derived from the polymer compound having a nitrile group. A plurality of positive electrode active material particles 110 including the central part 110X and the cover part 110Y are thus formed.As described above, the thickness of the base film is extremely thin, and the base film has extremely high reactivity with the polymer compound having a nitrile group. Therefore, it is difficult to directly check the presence of the base film after the formation of the cover member 110Y.However, as described in the method for checking whether the cover part 110Y is present on the surface of the central part 110X, the cover part 110Y is formed only when the base film is used. That is, the fact that the cover part 110Y is formed on the surface of the central part 110X means that the base film is formed on the surface of the central part 110X. Therefore, the presence of the base film can be indirectly confirmed on the basis of the fact that the cover part 110Y is formed on the surface of the central part 110X.Specifically, when using a positive electrode binder including a polymer compound having a nitrile group, in the step of forming the positive electrode active material layer 100B, a base film is not formed on the surface of each of the plurality of central parts 110X when the pretreatment of the plurality of central parts 110X using a dispersion including a dispersant is not performed.In such a case, there is a possibility that the nitrile group-having polymer compound may randomly approach the surface of the central part 110X, but the nitrile group-having polymer compound does not form a film while preferentially adhering to the surface of the central part 110X. For this reason, when checking whether the cover part 110Y is provided on the surface of the central part 110X, the presence area of the central part 110X and the presence area of the cover part 110Y do not contact each other, and thus it is determined that the cover part 110Y is not provided on the surface of the central part 110X.On the other hand, a base film is formed on the surface of each of the plurality of central parts 110X when a positive electrode binder including a polymer compound having a nitrile group is used in the step of forming the positive electrode active material layer 100B and the pretreatment of the plurality of central parts 110X is performed using a dispersion including a dispersant.In such a case, the nitrile group-containing polymer compound does not randomly approach the surface of the central part 110X, but the nitrile group-containing polymer compound forms a film while preferentially adhering to the surface of the central part 110X. For this reason, when checking whether the cover part 110Y is provided on the surface of the central part 110X, the presence area of the central part 110X and the presence area of the cover part 110Y are in contact with each other, and thus it is determined that the cover part 110Y is provided on the surface of the central part 110X.Subsequently, the positive electrode mixture slurry is applied to a surface of the positive electrode current collector 100A to form the positive electrode active material layer 100B.Finally, the positive electrode active material layer 100B is compression-molded using a compression device such as a roll pressing machine. In this case, the positive electrode active material layer 100B may be heated or the compression molding may be repeated a plurality of times. In addition, the compression conditions such as the pressing pressure and the pressing time are set so that the porosity P falls within the above range.The positive electrode active material layer 100B is thus formed on a surface of the positive electrode current collector 100A, thereby completing the positive electrode 100.<1-4. Effect and Effect>According to the positive electrode 100, each of the plurality of positive electrode active material particles 110 includes the central part 110X and the cover part 110, the central part 110X includes an olivine type phosphate compound, the manganese content in the olivine type phosphate compound is 50 mol parts to 90 mol parts, the central part 110X is a granulated body, the average diameter MD 1 is 0.01 μm to 0.5 μm, the average diameter MD 2 is 1 μm to 20 μm, the cover part 110Y includes a nitrile group, and the porosity P is 20% to 40%.In this case, as described above, a variety of effects described below can be obtained.First, the central portion 110X comprises an olivine-type phosphate compound. In this case, the release of oxygen from the central part 110X during the electrode reaction is suppressed. Thereby, lithium can be stably sealed in the positive electrode active material layer 100B and released, so that an electrode reaction stably proceeds.Second, the central part 110X is a granulated body, the average diameter MD 1 is 0.01 μm to 0.5 μm, and the average diameter MD 2 is 1 μm to 20 μm. In this case, the plurality of primary particles are likely to come into contact with each other, thereby improving the electron conductivity between the plurality of primary particles. The plurality of secondary particles are likely to also come into contact with each other, thereby improving the electron conductivity between the plurality of secondary particles. This improves the conductivity of the positive electrode active material layer 100B with an olivine type phosphate compound having substantially low electron conductivity.Third, the cover part 110Y has a nitrile group. This suppresses the occurrence of side reactions on the surface of the central part 110X during the electrode reaction when the central part 110X having high reactivity is used, because the surface of the central part 110X having high reactivity is electrochemically protected using the cover part 110Y.Fourth, the porosity P is 20% to 40%. In this case, the impregnation property of the positive electrode active material layer 100B with the electrolyte solution is ensured, and the flexibility of the positive electrode 100 is improved. This ensures stable electrode reaction and suppresses damage to the positive electrode 100 during the electrode reaction.Thereby, in the secondary battery having the positive electrode 100, the physical durability of the positive electrode 100 is improved while the stable progress of the electrode reaction is ensured, and the operational stability of the secondary battery is improved, whereby excellent battery characteristics can be obtained.In particular, when the average diameter MD 1 is 0.1 μm to 0.3 μm, the average diameter MD 2 is 5 μm to 15 μm, and the porosity P is 25% to 35%, the conductivity of the positive electrode active material layer 100B further improves and the flexibility of the positive electrode 100 further improves while the impregnation property of the positive electrode active material layer 100B with the electrolyte solution is further secured. Thus, a higher effect can be obtained.When the positive electrode active material layer 100B is a positive electrode binder and the positive electrode binder has a nitrile group, the cover part 110Y can be easily formed using the positive electrode binder, and thus a higher effect can be obtained. In the case where the positive electrode binder includes polyacrylonitrile, the cover part 110Y can be easily sufficiently shaped, and thus a higher effect can be obtained.In addition, when the total formation amount ratio R is 0.6 to 1.4, the distribution of the plurality of cover members 110Y within the positive electrode active material layer 100B becomes substantially uniform, and thus separation of the cover members 110Y hardly occurs. Therefore, lithium is likely to be substantially uniformly entrapped and released in the positive electrode active material layer 100B, whereby a higher effect can be obtained.<2 Secondary Battery>A secondary battery of an embodiment of the present technology to which the positive electrode 100 is applied will be described.The secondary battery described herein is a secondary battery that can obtain a battery capacity by using occlusion and release of an electrode reactant, and includes a positive electrode, a negative electrode, and an electrolyte solution. Hereinafter, as described above, a case where the electrode reactant is lithium will be described as an example. A secondary battery in which the battery capacity is obtained by using inclusion and release of lithium is a so-called lithium secondary battery (or lithium ion secondary battery). In the lithium ion secondary battery, lithium is sealed and released in an ionic state.A charging capacity of the negative electrode is preferably larger than a discharging capacity of the positive electrode. That is, an electrochemical capacity per unit area of the negative electrode is preferably larger than an electrochemical capacity per unit area of the positive electrode. This is intended to prevent lithium from being deposited on the surface of the negative electrode during the charging process.<2-1. Configuration>FIG. 4 is a sectional view of a secondary battery as an example of the secondary battery according to an embodiment of the present technology. FIG. 5 is a sectional view of a battery element 20 shown in FIG. 4.As shown in FIGS. 4 and 5, the secondary battery includes a battery case 11, a pair of insulating plates 12 and 13, the battery element 20, a terminal 25 for the positive electrode, and a terminal 26 for the negative electrode. The secondary battery described herein is a so-called cylindrical secondary battery in which the battery element 20 is accommodated in the battery container 11 having a cylindrical shape.[Battery Container]The battery case 11 is a member that accommodates the battery member 20 and the like. Since the battery case 11 has one open end part and the other closed end part, the battery case 11 has a hollow structure. The battery container 11 includes one or two or more metal materials such as iron, aluminum, an iron alloy, and an aluminum alloy. A metal material such as nickel may be applied to the surface of the battery case 11.A battery lid 14, a safety valve mechanism 15 and a PTC element 16, which is a heat-sensitive resistance element, are clamped to an open end portion of the battery case 11 with a gasket 17 interposed therebetween. The battery container 11 is thus sealed by the battery lid 14. Here, the battery lid 14 has the same material as the material for forming the battery container 11. the safety valve mechanism 15 and the PTC element 16 are provided in the battery lid 14, and the safety valve mechanism 15 is electrically connected to the battery lid 14 with the PTC element 16 interposed therebetween. The gasket 17 includes an insulating material, and asphalt or the like may be applied to the surface of the gasket 17.In the safety valve mechanism 15, when the internal pressure of the battery 11 reaches a certain value or more due to internal short-circuit, external heating, and the like, a disk plate 15A is reversed, and thus the electrical connection between the battery lid 14 and the battery element 20 is cut off. In order to prevent abnormal heat generation due to a high current, the electric resistance of the PTC element 16 increases as the temperature increases.[Insulating Plate]The insulating plates 12 and 13 are arranged to face each other with the battery element 20 interposed therebetween. Thus, the battery element 20 is disposed between the insulating plates 12 and 13.[Battery Element]The battery element 20 is a so-called power generation element including the positive electrode 21, the negative electrode 22, a separator 23, and an electrolytic solution (not shown).Since the battery element 20 is a so-called wound electrode body, the positive electrode 21 and the negative electrode 22 are wound while facing each other with the separator 23 interposed therebetween. A center pin 24 is inserted into a space 20S provided in the winding center of the battery element 20. The center pin 24 may be omitted.(Positive electrode)The positive electrode 21 has the same configuration as the configuration of the positive electrode 100.Specifically, the positive electrode 21 includes a positive electrode current collector 21A and a positive electrode active material layer 21B. The configuration of the positive electrode current collector 21A is the same as the configuration of the positive electrode current collector 100A, and the configuration of the positive electrode active material layer 21B is the same as the configuration of the positive electrode active material layer 100B. Here, the positive electrode active material layer 21B is provided on both surfaces of the positive electrode current collector 21A. However, the positive electrode active material layer 21B may be provided only on a surface of the positive electrode current collector 21A on the side where the positive electrode 21 faces the negative electrode 22.(Negative electrode)The negative electrode 22 includes a negative electrode current collector 22A and a negative electrode active material layer 22B.The negative electrode current collector 22A has a pair of surfaces on which the negative electrode active material layer 22B is provided. The negative electrode current collector 22A includes a conductive material such as a metal material, and specific examples of the conductive material include copper.The negative electrode active material layer 22B has one or two or more of the negative electrode active materials that include and release lithium. The negative electrode active material layer 22B may further include one or two or more other materials such as a negative electrode binder and a negative electrode conductive agent. The method for forming the negative electrode active material layer 22B is not particularly limited, but specifically includes one or two or more of the following methods: coating method, gas phase method, liquid phase method, thermal spraying method, firing method (sintering method), and the like.Here, the negative electrode active material layer 22B is provided on both surfaces of the negative electrode current collector 22A. However, the negative electrode active material layer 22B may be provided only on a surface of the negative electrode current collector 22A on the side where the negative electrode 22 faces the positive electrode 21.The type of the negative electrode active material is not particularly limited, and specific examples thereof include a carbon material and a metal-based material. This is because a high energy density can be obtained.Specific examples of the carbon material include graphitizable carbon, non-graphitizable carbon, and graphite. The graphite may be natural graphite or artificial graphite.The metal-based material is a material having one or two or more metal elements and semimetal elements capable of forming an alloy with lithium as constituent elements, and specific examples of the metal elements and semimetal elements include silicon and tin. The metal-based material may be a simple substance, an alloy, a compound, a mixture of two or more thereof, or a material having two or more phases thereof. However, since the simple matter may contain impurities in an arbitrary amount, the purity of the simple matter is not necessarily limited to 100%. Specific examples of the metal-based material include TiSi 2 and SiO x(0 < x≤2, or 0.2<x<1.4).The binder of the negative electrode includes one or two or more materials such as synthetic rubber and a polymer compound. Specific examples of synthetic rubber include styrene-butadiene rubber, fluororubber, and ethylene-propylene-diene. Specific examples of the polymer compound include polyvinylidene fluoride, polyimide and carboxymethylcellulose.The negative electrode conductive agent includes one or two or more conductive materials such as a carbon material, a metal material, and a conductive polymer compound, and specific examples of the carbon material include graphite, carbon black, acetylene black, and ketjen black.(Separator)The separator 23 is an insulating porous film that is disposed between the positive electrode 21 and the negative electrode 22 and allows lithium in ionic state to pass through while preventing a short circuit that would be caused by the contact of the positive electrode 21 and the negative electrode 22. The separator 23 comprises a polymeric compound such as polyethylene.(Electrolyte solution)The electrolyte solution is an electrolyte in liquid form, and both the positive electrode 21, and the negative electrode 22 and the separator 23 are impregnated with the electrolyte solution. This electrolyte solution includes a solvent and an electrolyte salt.The solvent includes one or two or more nonaqueous solvents (organic solvents), and the electrolytic solution including the nonaqueous solvent is a so-called nonaqueous electrolytic solution.The nonaqueous solvent is an ester, an ether or the like, and further specified is a carbonic acid ester-based compound, a carboxylic acid ester-based compound, and a lactone-based compound or the like. This is because the dissociative nature of the electrolyte salt and the mobility of the ions improve.The carbonic acid ester-based compound is a cyclic carbonic acid ester and a linear carbonic acid ester. Specific examples of the cyclic carbonic ester include ethylene carbonate and propylene carbonate, and specific examples of the linear carbonic ester include dimethyl carbonate, diethyl carbonate and ethylmethyl carbonate.The carboxylic acid ester-based compound is a linear carboxylic acid ester or the like. Specific examples of the linear carboxylic acid ester include ethyl acetate, ethyl propionate, propyl propionate and ethyl trimethyl acetate.The lactone-based compound is a lactone or the like. Specific examples of the lactone include γ-butyrolactone and γ-valerolactone.The ethers may be 1,2-dimethoxyethane, tetrahydrofuran, 1,3-dioxolane, 1,4-dioxane or the like.The nonaqueous solvent is an unsaturated cyclic carbonic acid ester, a fluorinated cyclic carbonic acid ester, a sulfonic acid ester, a phosphoric acid ester, an acid anhydride, a nitrile compound, an isocyanate compound, or the like. This is because the electrochemical stability of the electrolyte solution improves.Specific examples of the unsaturated cyclic carbonic acid ester include vinylene carbonate, vinyl ethylene carbonate, and methylene ethylene carbonate. Specific examples of the fluorinated cyclic carbonic acid ester include ethylene monofluorocarbonate and ethylene difluorocarbonate. Specific examples of the sulfonic acid ester include propane sultone and propene sultone. Specific examples of phosphoric acid esters include trimethyl phosphate and triethyl phosphate. Specific examples of acid anhydrides include succinic anhydride, 1,2-ethanedisulfonic anhydride and 2-sulfobenzoic anhydride. Specific examples of nitrile compounds include succinonitrile. Specific examples of isocyanate compounds include hexamethylene diisocyanate.The electrolyte salt comprises one or two or more light metal salts such as lithium salts.Specific examples of lithium salts include lithium hexafluorophosphate (LiPF 6), lithium tetrafluoroborate (LiB 4), lithium trifluoromethanesulfonate (LiCF 3 SO 3), lithium bis(fluorosulfonyl)imide (LiN(FSO 2)2), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF 3 SO 2)2), lithium tris(trifluoromethanesulfonyl)methide (LiC(CF 3 SO 2)3), lithium bis(oxalato)borate (LiB(C 2 O4)2), Lithium monofluorophosphate (Li 2 PFO 3) and lithium difluorophosphate (LiPF 2 O 2) were mentioned. This is because a high battery capacity can be obtained.The content of the electrolyte salt is not particularly limited, and is specifically 0.3 mol / kg to 3.0 mol / kg based on the solvent. This is because high ion conductivity can be obtained.[Terminal of Positive Electrode and Terminal of Negative Electrode]The positive electrode terminal 25 is connected to the positive electrode current collector 21A and includes a conductive material such as aluminum. The positive electrode terminal 25 is electrically connected to the battery lid 14 with the safety valve mechanism 15 interposed therebetween.The negative electrode terminal 26 is connected to the negative electrode current collector 22A and includes a conductive material such as nickel. The negative electrode terminal 26 is electrically connected to the battery case 11.<2-2. Operation>The secondary battery functions as follows in charging and discharging.During the charging process, in the battery element 20, lithium is released from the positive electrode 21, and the lithium is sealed in the negative electrode 22 with the electrolyte solution interposed therebetween. On the other hand, in the battery element 20, lithium is released from the negative electrode 22 and the lithium is enclosed in the positive electrode 21 with the electrolyte solution interposed therebetween. At the time of charging and discharging, lithium is trapped and released in ionic state.<2-3. Production Method>In the manufacture of the secondary battery, the positive electrode 21 and the negative electrode 22 are manufactured by an exemplary method described below, an electrolyte solution is prepared, then a secondary battery is assembled, and the stabilization treatment is performed on the assembled secondary battery.[Production of Positive Electrode]The positive electrode 21 is manufactured by forming the positive electrode active material layers 21B on both surfaces of the positive electrode current collector 21A using the same method as the manufacturing method for the positive electrode 100 described above.[Production of Negative Electrode]First, a mixture (negative electrode mixture) in which a negative electrode active material, a negative electrode binder, and a negative electrode conductive agent are mixed together is added into a solvent to prepare a paste-like negative electrode mixture slurry. The solvent may be an aqueous solvent or an organic solvent. Subsequently, the negative electrode mixture slurry is applied to both sides of the negative electrode current collector 22A to form the negative electrode active material layer 22B. Finally, the negative electrode active material layer 22B may be compression-molded using a compression device such as a roll pressing machine. In this case, the negative electrode active material layer 22B may be heated or the compression molding may be repeated a plurality of times. The negative electrode active material layer 22B is thus formed on both sides of the negative electrode current collector 22A, and the negative electrode 22 is manufactured.[Preparation of Electrolyte Solution]An electrolyte salt is added to the solvent.Thus, the electrolyte salt is dispersed or dissolved in the solvent, and an electrolyte solution is prepared.[Assembly of Secondary Battery]First, the positive electrode terminal 25 is joined to the positive electrode current collector 21A of the positive electrode 21 by a joining method such as a welding method, and the negative electrode terminal 26 is joined to the negative electrode current collector 22A of the negative electrode 22 by a joining method such as a welding method.Subsequently, the positive electrode 21 and the negative electrode 22 are laminated with the separator 23 interposed therebetween, and then the positive electrode 21, the negative electrode 22, and the separator 23 are wound to produce a wound body (not shown) having the clearance 20S. This wound body has the same configuration as the battery element 20 except that the positive electrode 21, the negative electrode 22, and the separator 23 are not impregnated with the electrolyte solution. Subsequently, the center pin 24 is inserted into the wound body space 20S.Then, the wound body and the insulating plates 12 and 13 are accommodated in the battery container 11 with the wound body sandwiched between the insulating plates 12 and 13. In this case, the positive electrode terminal 25 is connected to the safety valve mechanism 15 by a connection method such as a welding method, and the negative electrode terminal 26 is connected to the battery case 11 by a connection method such as a welding method. Then, the wound body is impregnated with the electrolyte solution by injecting the electrolyte solution into the battery can 11. Thereby, each of the positive electrode 21, the negative electrode 22, and the separator 23 is impregnated with the electrolyte solution, and thus the battery element 20 is manufactured.Finally, the battery cover 14, the safety valve mechanism 15, and the PTC element 16 are housed inside the battery case 11, and then the battery case 11 is pressed with the packing 17 interposed therebetween.Thereby, the battery lid 14, the safety valve mechanism 15, and the PTC element 16 are fixed to the battery container 11, the battery element 20 is sealed in the battery container 11, and thus the secondary battery is assembled.[Stabilization treatment on assembled secondary battery]The assembled secondary battery is charged and discharged. The charging and discharging conditions such as ambient temperature and number of charging and discharging operations (number of cycles) can be freely set. As a result, a film is formed on the surfaces of the positive electrode 21 and the negative electrode 22, respectively, thereby electrochemically stabilizing the state of the battery element 20. The secondary battery is thus completed.<2-4. Effect and Effect>According to this secondary battery, the positive electrode 21 has the same configuration as the configuration of the positive electrode 100. Thus, for the reasons described above, the physical durability of the positive electrode 21 is improved while the stable progress of the charge-discharge reaction is ensured, and the operational stability of the secondary battery is improved. Thus, excellent battery characteristics can be obtained.Particularly, when the secondary battery is a lithium secondary battery, by inclusion and release of lithium, a sufficient battery capacity can be stably obtained, whereby a higher effect can be obtained.Other measures and effects related to the secondary battery are similar to the other measures and effects related to the positive electrode 100.<3 Beispiele of Modifications>The configuration of the secondary battery described above may be changed as described below accordingly. A series of modification examples described below may be combined with each other.[Modification Example 1]In the step of forming the positive electrode active material layer 100B, when the positive electrode binder includes a nitrile group-containing polymer compound, a part of the nitrile group-containing polymer compound is consumed to form the cover part 110Y, but the rest of the nitrile group-containing polymer compound remains in the positive electrode active material layer 100B, and thus the remaining nitrile group-containing polymer compound performs a binding function.However, when the positive electrode binder includes other materials together with the nitrile group-containing polymer compound, the nitrile group-containing polymer compound does not need to remain in the positive electrode active material layer 100B because all of the nitrile group-containing polymer compound is consumed to form the cover part 110Y. Also in this case, the same effect can be obtained because the cover part 110Y is formed and other materials perform a bonding function.[Modification Example 2]The separator 23, which is a porous film, was used. Although not specifically illustrated in the drawings, a laminated separator may be used.Specifically, the laminated separator includes a porous film and a layer of a polymer compound. The porous film has a pair of surfaces, and the layer of a polymer compound is provided on one surface or both surfaces of the porous film. This is because the property of the separator to be in close contact with each of the positive electrode 21 and the negative electrode 22 improves, and thus the winding deviation of each of the positive electrode 21, the negative electrode 22, and the separator 23 is suppressed. Thus, swelling of the secondary battery is suppressed when a decomposition reaction of the electrolyte solution has occurred. The layer of polymer compound includes a polymer compound such as polyvinylidene fluoride. This is because polyvinylidene fluoride has excellent physical strength and is electrochemically stable.One or both of the porous film and the polymer layer may include a plurality of insulating particles. This is because the plurality of insulating particles promote heat dissipation at the time of heat generation of the secondary battery, which improves safety (heat resistance) of the secondary battery. The plurality of insulating particles may include one or two or more of insulating materials such as an inorganic material and a resin material. Specific examples of the inorganic material include alumina, aluminum nitride, boehmite, silica, titanium oxide, magnesium oxide, and zirconia. Specific examples of the resin material include an acrylic resin and a styrene resin.In the case of manufacturing the laminated partition member, a precursor solution comprising a polymer compound and a solvent is prepared, and then the precursor solution is applied to one surface or both surfaces of the porous film. In this case, a plurality of insulating particles may be added to the precursor solution as needed.Also in the case of using a laminated separator, lithium can migrate between the positive electrode 21 and the negative electrode 22, and thus the same effect can be obtained. In this case, particularly, as described above, safety of the secondary battery is improved, and thus a higher effect can be obtained.[Modification Example 3]An electrolyte solution was used which was an electrolyte in liquid form. Although not specifically illustrated in the drawings, an electrolyte layer that is a gel electrolyte may be used.In the battery element 20 using the electrolyte layer, the positive electrode 21 and the negative electrode 22 are wound while facing each other with the separator 23 and the electrolyte layer interposed therebetween. The electrolyte layer is disposed between the positive electrode 21 and the separator 23, and is disposed between the negative electrode 22 and the separator 23.Specifically, the electrolyte layer includes an electrolyte solution and a polymer compound, and the electrolyte solution is held by the polymer compound. This is because leakage of the electrolyte solution is prevented. The configuration of the electrolyte solution is as described above. The polymer compound includes polyvinylidene fluoride or the like. In the case of forming the electrolyte layer, a precursor solution including an electrolyte solution, a polymer compound, and a solvent is prepared, and then the precursor solution is applied to one surface or both surfaces of both the positive electrode 21 and the negative electrode 22.The same effect can be obtained also in the case of using the electrolyte layer because lithium can move between the positive electrode 21 and the negative electrode 22 with the electrolyte layer interposed therebetween. In this case, particularly, as described above, leakage of the electrolyte solution is prevented, and thus a higher effect can be obtained.<4 Application of Secondary Battery>The application (application example) of the secondary battery is not particularly limited. The secondary battery to be used as a power source may be a main power source or an auxiliary power source in electronic devices, electric vehicles, and the like. The main power source is a power supply which is preferably used regardless of whether or not there is another power source. The auxiliary power source may be a power source used in place of the main power source or a power source switched by the main power source.Specific examples of the application of the secondary battery are described as follows: electronic devices such as video camcorders, digital still cameras, cellular phones, notebook computers, stereo systems with earphones, portable radios, and portable information terminals; storage devices such as emergency power sources and memory cards; power tools such as electric power drills and electric saws; battery packs mounted on electronic devices and the like; medical electronic devices such as pacemakers and hearing aids; electric vehicles such as electric cars (including hybrid cars); and electric power storage systems such as home or industrial battery systems that store electric power for emergency cases or the like. In these applications, one secondary battery or a plurality of secondary batteries may be used.A single battery or a composite battery may be used for the battery pack. The electric vehicle is a vehicle that travels using the secondary battery as a drive power source, and may be a hybrid vehicle having a different drive source in addition to the secondary battery. In the electric power storage system for household use, household electric products and the like can be used by using the electric power stored in the secondary battery as an electric power storage source.EXAMPLESExamples of the present technology will be described.<Beispiele 1 to 21 and Comparative Examples 1 to 8>As described below, a secondary battery was produced, and then the battery properties of the secondary battery were evaluated.[Production of Secondary Battery]Here, for easy evaluation of battery characteristics, a secondary battery for a test described later was prepared. The configuration of the secondary battery for a test will be described below, and then a method for manufacturing the secondary battery for a test will be described.(Configuration of Secondary Battery)FIG. 6 shows a configuration of the secondary battery for a test in a region, and the secondary battery for a test is a so-called coin-type lithium ion secondary battery. Hereinafter, the secondary battery for a test is also referred to simply as a "secondary battery".As illustrated in FIG. 6, the secondary battery includes a test electrode 61, a counter electrode 62, a separator 63, an outer can 64, an outer container 65, a gasket 66, and an electrolyte solution (not illustrated).The test electrode 61 is housed in the outer shell 64, and the counter electrode 62 is housed in the outer can 65. The test electrode 61 and the counter electrode 62 are laminated with each other with the separator 63 interposed therebetween, and both the test electrode 61 and the counter electrode 62 and the separator 63 are impregnated with the electrolytic solution. Since the outer cup 64 and the outer container 65 are pressed together with the seal 66 interposed therebetween, the test electrode 61, the counter electrode 62, and the separator 63 are enclosed by the outer cup 64 and the outer container 65.(Method for Producing Secondary Battery)The secondary battery shown in FIG. 6 was produced by the method described later.(Preparation of test electrode)In the production of the test electrode 61, a plurality of primary particles containing an olivine type phosphate compound were first produced. In this case, as the olivine type phosphate compound, LiMn 0,5 Fe 0,5 PO 4( manganese content=50 parts by mole), LiMn 0,7 Fe 0,3 PO 4( manganese content=70 parts by mole) and LiMn 0,9 Fe 0,1 PO 4( manganese content=90 parts by mole) were used. When the average diameter MD1 (μm) was examined after the completion of the secondary battery, the results shown in Tables 1 to 3 were obtained.Subsequently, the plurality of primary particles were granulated to form a plurality of central parts as a plurality of secondary particles. Thus, a plurality of central portions comprising an olivine type phosphate compound were obtained. When the average diameter MD2 (μm) was examined after the completion of the secondary battery, the results shown in Tables 1 to 3 were obtained.Subsequently, a dispersant (carboxymethyl cellulose) was added to a solvent (pure water as an aqueous solvent), and the solvent was then stirred to prepare a dispersion (concentration = 55%).Subsequently, the plurality of central parts were added to the dispersion, and then the dispersion was stirred, whereby the plurality of central parts were pretreated using the dispersion. By this pretreatment, a base film having the dispersant was formed on the surface of each of the plurality of central portions.Subsequently, 100 parts by mass of the plurality of pretreated central parts, 3.0 parts by mass of a positive electrode binder (polyacrylonitrile (PAN)-ethylhexyl acrylate copolymer as a nitrile group-containing polymer compound), and 2.0 parts by mass of a positive electrode conductive agent (carbon black) were mixed together to obtain a positive electrode mixture.Subsequently, the positive electrode mixture was placed in a solvent (pure water as an aqueous solvent), and the solvent was stirred, thereby preparing a positive electrode mixture slurry. As a result, a part of the nitrile group-having polymer compound as a positive binder preferentially adhered to the surface of each of the plurality of central parts, thereby forming a covering part having a nitrile group. A plurality of particles of positive electrode active material having the central part and the cover part were thus obtained.Subsequently, a surface of a positive electrode current collector (aluminum foil, thickness=12 μm) was coated with the positive electrode mixture slurry using a coating device, and thereafter the positive electrode mixture slurry was dried to form a positive electrode active material layer. When the total formation amount ratio R (%) after completion of the secondary battery was checked, the results shown in Tables 1 to 3 were obtained. The total formation amount ratio R (%) can be adjusted by changing the blowing temperature and the blowing amount in drying the positive electrode mixture.Subsequently, the positive electrode active material layer was compression-molded using a pressing machine. In this case, the porosity P (%) was adjusted by changing conditions such as the pressing pressure and the pressing time. When the porosity (P) was checked after completion of the secondary battery, the results shown in Tables 1 to 3 were obtained.Finally, the positive electrode current collector on which the positive electrode active material layer was formed was stamped into a disk shape (diameter=16.5 mm). The test electrode 61 was thus prepared.For comparison, the test electrode 61 was produced by the same method except that a plurality of primary particles were used because they were not granulated. When the average diameter MD1 (μm) was examined after the completion of the secondary battery, the results shown in Table 2 were obtained.(Preparation of Counter Electrode)A lithium metal plate was stamped into a disk shape (diameter = 17 mm). The counter electrode 62 was thus obtained.(Preparation of Electrolyte Solution)An electrolyte salt (LiPF 6) was added to a solvent (ethylene carbonate as a cyclic carbonate ester and diethyl carbonate as a chain carbonate ester), and then the solvent was stirred. In this case, the mixing ratio (weight ratio) of the solvent ethylene carbonate:diethyl carbonate=30:70, and the content of the electrolyte salt in the electrolyte solution was 1 mol / kg with respect to the solvent. The electrolyte solution was thus prepared.(Structure of Secondary Battery)First, the test electrode 61 was inserted into the outer cup 64 and the counter electrode 62 was inserted into the outer container 65. Subsequently, the test electrode 61 located in the outer shell 64 and the counter electrode 62 located in the outer can 65 were laminated together with the separator 63 (microporous polyethylene film, thickness=20 μm, diameter=117.5 mm) impregnated with the electrolyte solution interposed therebetween. In this case, the positive electrode active material layer and the counter electrode 62 were faced to each other with the separator 63 interposed therebetween.Subsequently, in a state where the test electrode 61 and the counter electrode 62 were laminated with the separator 63 interposed therebetween, the outer cup 64 and the outer container 65 were pressed together with the packing 66 interposed therebetween. Thereby, the test electrode 61 and the counter electrode 62 were enclosed in the outer can 64 and the outer container 65, and thus the secondary battery was assembled.Finally, the assembled secondary battery was left to stand (life=10 hours). The secondary battery was thus completed.[Evaluation of Battery Characteristics]In the evaluation of battery characteristics, physical durability and operational stability, the results shown in Tables 1 to 3 were obtained. Here, using the method described below, the close contact property and the flexibility of the test electrode 61 were evaluated as physical durability, and the charging properties and the cycling properties of the secondary battery were evaluated as operational stability.(Close Contact Property)First, the test electrode 61 was removed by disassembling the secondary battery. Then, the test electrode 61 was washed with a washing solvent (pure water) and then dried.Subsequently, the test electrode 61 was attached to a peeling tester in a normal temperature environment (temperature=23° C.), and then a 180° peeling test was performed with the peeling tester. As a result, the positive electrode current collector was peeled from the positive electrode active material layer, and thus the peel strength (mN / mm), which is an index for evaluating the close contact property, was measured.Finally, the close contact property was determined based on the peel strength. Specifically, the peel strength was determined to be 20 mN / mm or more than "A". At a peel strength of 10 mN / m or more and less than 20 mN / m, it was determined as "B". When the peel strength was less than 10 mN / m, it was determined to be "C".(Flexibility)First, the test electrode 61 was taken out from the secondary battery, and then the test electrode 61 was washed by the same method as in the evaluation of the close contact property.Subsequently, the test electrode 61 was curved by winding the test electrode 61 around the surface of an iron metal rod (diameter = 4 mm) in a normal temperature environment, and the curved test electrode 61 was then left (leave time = 10 minutes). Subsequently, the state of the positive electrode active material layer, which is an index for evaluation of flexibility, was observed by visually checking the state of the test electrode 61.Finally, the flexibility was determined based on the state of the positive electrode active material layer. Specifically, when no abnormality occurred in the positive electrode active material layer, the rating "A" was given. When the positive electrode active material layer was not cracked but fine cracks occurred in the positive electrode active material layer, the rating "B" was given. When a severe abnormality occurred in the positive electrode active material layer, it was determined to be "C". The abnormality of the positive electrode active material layer described herein is cracking, damage, falling, and the like.(Charging Characteristics)First, the secondary battery was subjected to a charge and discharge cycle in a normal temperature environment, whereby the discharge capacity (discharge capacity in the first cycle) was measured.At the time of charging, constant current charging was performed with a current of 0.2 C until the voltage reached 3.8 V, and then constant voltage charging was performed with a voltage of 3.8 V until the current reached 0.05 C. At the time of discharging, constant current discharge was performed at a current of 0.2 C until the voltage reached 2.0 V. 0.2 C refers to a current value at which the battery capacity (theoretical capacity) can be discharged in 5 hours, and 0.05 C refers to a current value at which the battery capacity can be discharged in 20 hours.Subsequently, the secondary battery was subjected to a charge and discharge cycle again in the same environment to measure the discharge capacity (discharge capacity at the second cycle).The charging and discharging conditions were the same as in the first cycle except that the current at the time of discharge was changed from 0.2 C to 2 C. FIG. 2C refers to a current value at which the battery capacity can be discharged in 0.5 hours.Subsequently, a charge retention rate, an index for evaluating the charge characteristics, was calculated based on the following calculation formula: charge retention rate (%)=(discharge capacity at the second cycle / discharge capacity at the first cycle)×100.Finally, the charge characteristics were determined based on the charge retention rate. Specifically, the charge retention rate was determined to be 90% or more than "A". At a charge retention rate of 80% or more and less than 90%, it was determined as "B". At a charge retention rate of less than 80%, it was determined to be "C".(Cycle Characteristics)First, the secondary battery was charged and discharged in a normal temperature environment in one cycle, whereby the discharge capacity (discharge capacity in the first cycle) was measured. Subsequently, the secondary battery was repeatedly charged and discharged in the same environment until the total number of cycles reached 100 cycles to discharge capacity (discharge capacity at 100. Cycle). The charge and discharge conditions were the same as the charge and discharge conditions for the first cycle in the case of evaluating the charge characteristics.Subsequently, a cycle maintenance rate, which is an index for evaluation of cycle characteristics, was calculated based on a calculation formula of the cycle maintenance rate (%)=(discharge capacity at 100. Cycle / Discharge Capacity in the First Cycle)×100.Finally, the cycle characteristics were determined based on the cycle maintenance rate. Specifically, the cycle maintenance rate was determined to be 90% or more than "A". At a cycle maintenance rate of 80% or more and less than 90%, it was determined as "B". At a cycle maintenance rate of less than 80%, it was determined to be "C".(Comprehensive Evaluation)Here, the respective determination results of the tight contact properties and flexibility, and the respective determination results of the charging properties and cycle properties were obtained, and then the battery properties were comprehensively evaluated based on the four kinds of determination results.Specifically, when one of the four kinds of determination results was C, the comprehensive evaluation was "C". When none of the four determination results was C but one of the four determination results was B, the comprehensive evaluation was "B". When all four determination results were A, the comprehensive evaluation was determined as "A".Table 1 R=1.0% Table 1 R=1.0%Comparative Example 1LiMn 0.5 Fe 0.5 PO 4PAN0.005828Example 1LiMn 0.5 Fe 0.5 PO 4PAN0.01828Example 2LiMn 0.5 Fe 0.5 PO 4PAN0.1828Example 3LiMn 0.5 Fe 0.5 PO 4PAN0.2828Example 4LiMn 0.5 Fe 0.5 PO 4PAN0.3828Example 5LiMn 0.5 Fe 0.5 PO 4PAN0.5828Comparative Example 2LiMn 0.5 Fe 0.5 PO 4PAN0.6828Comparative Example 3LiMn 0.5 Fe 0.5 PO 4PAN0.20.828Example 6LiMn 0.5 Fe 0.5 PO 4PAN0.2128Example 7LiMn 0.5 Fe 0.5 PO 4PAN0.2528Example 8LiMn 0.5 Fe 0.5 PO 4PAN0.21528Example 9LiMn 0.5 Fe 0.5 PO 4PAN0.22028Comparative Example 4LiMn 0.5 Fe 0.3 PO 4PAN0.22528Comparative Example 5LiMn 0.5 Fe 0.5 PO 4-0.2828Table 1 (continuation)Table 1 (continuation)Comparative Example 1B. BA. AA. AC. CC. CExample 1B. BA. AA. AB. BB. BExample 2A. AA. AA. AA. AA. AExample 3A. AA. AA. AA. AA. AExample 4A. AA. AA. AA. AA. AExample 5A. AA. AB. BA. AB. BComparative Example 2B. BA. AC. CB. BC. CComparative Example 3C. CA. AA. AC. CC. CExample 6B. BA. AA. AB. BB. BExample 7A. AA. AA. AA. AA. AExample 8A. AA. AA. AA. AA. AExample 9A. AA. AB. BA. AB. BComparative Example 4A. AA. AC. CA. AC. CComparative Example 5A. AA. AA. AC. CC. CTable 2 R=1.0% Table 2 R=1.0%Comparative Example 6LiMn 0.5 Fe 0.5 PO 4PAN0.2815Example 10LiMn 0.5 Fe 0.5 PO 4PAN0.2820Example 11LiMn 0.5 Fe 0.5 PO 4PAN0.2825Example 12LiMn 0.5 Fe 0.5 PO 4PAN0.2835Example 13LiMn 0.5 Fe 0.5 PO 4PAN0.2840Comparative Example 7LiMn 0.5 Fe 0.5 PO 4PAN0.2845Example 14LiMn 0.7 Fe 0.3 PO 4PAN0.2828Example 15LiMn 0.9 Fe 0.1 PO 4PAN0.2835Comparative Example 8LiMn 0.5 Fe 0.5 PO 4PAN1-28Table 2 (continuation)Table 2 (continuation)Comparative Example 6B. BC. CC. CA. AC. CExample 10A. AB. BB. BA. AB. BExample 11A. AA. AA. AA. AA. AExample 12A. AA. AA. AA. AA. AExample 13A. AA. AA. AB. BB. BComparative Example 7A. AA. AA. AC. CC. CExample 14A. AA. AA. AA. AA. AExample 15A. AA. AA. AA. AA. AComparative Example 8B. BB. BB. BC. CC. C Table 3 MD1=0.2 μm, MD2=8 μm, P=28 μmTable 3 MD1=0.2 μm, MD2=8 μm, P=28 μmExample 16LiMn 0.5 Fe 0.5 PO 4PAN0.4Example 17LiMn 0.5 Fe 0.5 PO 4PAN0.6Example 18LiMn 0.5 Fe 0.5 PO 4PAN0.8Example 3LiMn 0.5 Fe 0.5 PO 4PAN1.0Example 19LiMn 0.5 Fe 0.5 PO 4PAN1.2Example 20LiMn 0.5 Fe 0.5 PO 4PAN1.4Example 21LiMn 0.5 Fe 0.5 PO 4PAN1.6Table 3 (continuation)Table 3 (continuation)Example 16A. AB. BB. BB. BB. BExample 17A. AA. AA. AA. AA. AExample 18A. AA. AA. AA. AA. AExample 3A. AA. AA. AA. AA. AExample 19A. AA. AA. AA. AA. AExample 20A. AA. AA. AA. AA. AExample 21B. BA. AB. BB. BB. B[Discussion]As shown in Tables 1 to 3, the comprehensive evaluation varied with respect to the four kinds of evaluation results (close contact property, flexibility, load characteristic, and cycle properties) after the configuration of the test electrode 61.Specifically, when the suitable conditions that each of the plurality of particles of the positive electrode active material had a central part and a cover part, the central part had an olivine type phosphate compound, the manganese content in the olivine type phosphate compound was 50 to 90 mol parts, the central part was a granulated body, the average diameter MD 1 was 0, 01 μm to 0.5 μm, the average diameter MD 2 was 1 μm to 20 μm, the cover part had a nitrile group, and the porosity P was 20% to 40% (Examples 1 to 21), all of the properties of close contact, flexibility, loadability, and cycle properties were improved, and thus the comprehensive evaluation was B or more.On the other hand, the close contact properties, flexibility, charging properties, and cycling properties deteriorated when the above-described suitable conditions were not satisfied (Comparative Examples 1 to 8), and thus the comprehensive evaluation was C. Particularly in this case, when the plurality of primary particles were used without granulation (Comparative Example 8), the cycling properties deteriorated remarkably.Moreover, when the suitable conditions described above were satisfied (Examples 1 to 21), a series of tendencies described below were obtained.First, when the average diameter MD 1 was 0.1 μm to 3 μm, the close contact properties, the charging properties, and the cycling properties further improved.Second, when the average diameter MD2 was 5 μm to 15 μm, the close contact properties, the charging properties and the cycling properties further improved.Third, when the porosity P was 25% to 35%, the close contact properties, the charging properties, and the cycle properties further improved.Fourth: When the covering member comprised polyacrylonitrile having a nitrile groupPolymer compound is, all of the tight contact properties, flexibility, load properties and cycle properties improved sufficiently.Fifth: When the total formation amount ratio R was 0.6 to 1.4, all of the closed contact, flexibility, charging properties, and cycle properties sufficiently improved.[Summary]From the results shown in Tables 1 and 2, it is understood that when each of the plurality of particles of the positive electrode active material had a central part and a cover part, the central part had an olivine type phosphate compound, the manganese content in the olivine type phosphate compound was 50 to 90 mol parts, the central part was a granulated body, the average diameter MD 1 was 0.01 μm to 0.5 μm, the average diameter MD 2 was 1 μm to 20 μm, the cover part had a nitrile group, and the porosity P was 20% to 40%, all of the properties of close contact, flexibility, charging properties, and cycling properties were improved, and thus excellent battery properties were obtained.Although the present technology has been described above with reference to an embodiment and examples, the configurations of the present technology are not limited to the configurations described in an embodiment and examples, and thus can be modified in various ways.Specifically, a case where the battery structure of the secondary battery is a cylindrical type and a coin type has been described. However, the battery structure of the secondary battery is not particularly limited, and thus may be a laminate sheet type, a square type, a button type, and the like.A case where the element structure of the battery element is a winding type has been described. However, the element structure of the battery element is not particularly limited, and may be a laminated type, a zigzag folded type, or the like. In the laminated type, the positive electrode and the negative electrode are laminated on each other, and in the zigzag folded type, the positive electrode and the negative electrode are zigzag folded.A case where the electrode reactant is lithium has been described, but the electrode reactant is not particularly limited. In particular, the electrode reactant as described above may also be other alkali metals such as sodium and potassium or alkaline earth metals such as beryllium, magnesium and calcium. Moreover, the electrode reactant may be another light metal such as aluminum.Since the effects described in the present specification are merely examples, the effects of the present technology are not limited to the effects described in the present specification. Therefore, other effects related to the present technology can be obtained.The present technology may also adopt the following configurations. <1> A secondary battery including:a positive electrode having a positive electrode active material layer;a negative electrode; andan electrolyte solution,whereinthe positive electrode active material layer includes a plurality of positive electrode active material particles,wherein each of the plurality of positive electrode active material particles comprises:a central portion comprising an olivine type phosphate compound; anda cover member provided on a surface of the central member,the olivine type phosphate compound has manganese and iron as components,when the sum of the manganese content in the olivine type phosphate compound and the iron content in the olivine type phosphate compound is 100 parts by mole, the manganese content in the olivine type phosphate compound is 50 parts by mole or more and 90 parts by mole or less,the central part is a secondary particle formed by granulating a plurality of primary particles, a first average diameter related to the plurality of primary particles is 0.01 μm or more and 0.5 μm or less,a second average diameter relating to the plurality of secondary particles is 1 μm or more and 20 μm or less, the cover member has a nitrile group, andthe positive electrode active material layer has a porosity of 20% or more and 40% or less.<2> The secondary battery according to <1>, whereinthe first average diameter is 0.1 μm or more and 0.3 μm or less,the second average diameter is 5 μm or more and 15 μm or less, andthe porosity is 25% or more and 35% or less.<3> The secondary battery according to <1> or <2>, whereinthe positive electrode active material layer further includes a positive electrode binder, andsaid positive electrode binder has a nitrile group.<4> The secondary battery according to <3>, wherein the positive electrode binder includes at least one of polyacrylonitrile and an acrylonitrile-ethylhexyl acrylate copolymer. <5> The secondary battery according to any one of <1> to <4>, wherein the positive electrode further includes a positive electrode current collector that supports the positive electrode active material layer, and when the positive electrode active material layer is formed into a first positive electrode active material layer disposed on a side close to the positive electrode current collector and a second positive electrode active material layer disposed on a side away from the positive electrode in a thickness direction of the positive electrode active material layer, a ratio of a total formation amount of a plurality of cover members in the second positive electrode active material layer to a total formation amount of a plurality of cover members in the first positive electrode active material layer is 0.6 or more and 1.4 or less. <6> The secondary battery according to any one of <1> to <5>, wherein the secondary battery is a lithium secondary battery. <7> A positive electrode for a secondary battery, wherein the positive electrode includes a positive electrode active material layer, wherein the positive electrode active material layer includes a plurality of positive electrode active material particles, wherein each of the plurality of positive electrode active material particles includes:a central portion comprising an olivine type phosphate compound; anda cover part provided on a surface of the central part,the olivine type phosphate compound has manganese and iron as components,when the sum of the manganese content in the olivine type phosphate compound and the iron content in the olivine type phosphate compound is 100 parts by mole, the manganese content in the olivine type phosphate compound is 50 parts by mole or more and 90 parts by mole or less,the central part is a secondary particle formed by granulating a plurality of primary particles,a first average diameter relating to the plurality of primary particles is 0.01 μm or more and 0.5 μm or less,a second average diameter relating to the plurality of secondary particles is 1 μm or more and 20 μm or less,the cover part has a nitrile group, andthe positive electrode active material layer has a porosity of 20% or more and 40% or less.Claim:References included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedWO 2011 / 122297

[0003]

Claims

A secondary battery comprising: a positive electrode having a positive electrode active material layer; a negative electrode; and an electrolyte solution, wherein the positive electrode active material layer includes a plurality of positive electrode active material particles, each of the plurality of positive electrode active material particles including: a central part including an olivine type phosphate compound; and a cover part provided on a surface of the central part, the olivine type phosphate compound including manganese and iron as components, when the sum of a content of manganese in the olivine type phosphate compound and a content of iron in the olivine type phosphate compound is 100 molar parts, the content of manganese in the olivine type phosphate compound is 50 molar parts or more and 90 molar parts or less, the central part is a secondary particle formed by granulating a plurality of primary particles, a first average diameter related to the plurality of primary particles is 0.01 μm or more and 0.5 μm or less, a second average diameter related to the plurality of secondary particles is 1 μm or more and 20 μm or less, the cover part has a nitrile group, and the positive electrode active material layer has a porosity of 20% or more and 40% or less.The secondary battery according to claim 1, wherein the first average diameter is 0.1 μm or more and 0.3 μm or less, the second average diameter is 5 μm or more and 15 μm or less, and the porosity is 25% or more and 35% or less.The secondary battery according to claim 1 or 2, wherein the positive electrode active material layer further comprises a positive electrode binder, and the positive electrode binder comprises a nitrile group.The secondary battery according to claim 3, wherein the positive electrode binder comprises at least one of polyacrylonitrile and an acrylonitrile-ethylhexyl acrylate copolymer.The secondary battery according to any one of claims 1 to 4, wherein the positive electrode further includes a positive electrode current collector that supports the positive electrode active material layer, and when the positive electrode active material layer is divided into a first positive electrode active material layer disposed on a side near the positive electrode current collector and a second positive electrode active material layer disposed on a side away from the positive electrode in a thickness direction of the positive electrode active material layer, a ratio of a total formation amount of a plurality of cover parts in the second positive electrode active material layer to a total formation amount of a plurality of cover parts in the first positive electrode active material layer is 0.6 or more and 1.4 or less.The secondary battery according to any one of claims 1 to 5, wherein the secondary battery is a lithium secondary battery.A positive electrode for a secondary battery, the positive electrode having a positive electrode active material layer, the positive electrode active material layer having a plurality of positive electrode active material particles, each of the plurality of positive electrode active material particles comprising: a central part comprising an olivine type phosphate compound; and a cover part provided on a surface of the central part, the olivine type phosphate compound having manganese and iron as constituent elements when the sum of a content of manganese in the olivine type phosphate compound and a content of iron in the olivine type phosphate compound is 100 molar parts, the content of manganese in the olivine type phosphate compound is 50 molar parts or more and 90 molar parts or less, the central part is a secondary particle, which is formed by granulating a plurality of primary particles, a first average diameter relating to the plurality of primary particles is 0.01 μm or more and 0.5 μm or less, a second average diameter relating to the plurality of secondary particles is 1 μm or more and 20 μm or less, the cover part has a nitrile group, and the positive electrode active material layer has a porosity of 20% or more and 40% or less.

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