Positive electrode active material, positive electrode for nonaqueous secondary batteries, nonaqueous secondary battery, slurry composition, and binder liquid

Abstract

The present invention addresses the problem of providing: a positive electrode active material which is capable of suppressing dissolution of a transition metal element into an aqueous dispersion medium, and is capable of suppressing a decrease in the initial capacity when used in a secondary battery; a positive electrode for nonaqueous secondary batteries using the positive electrode active material; a nonaqueous secondary battery; a slurry composition which contains the positive electrode active material; and a binder liquid which is used for the production of the slurry composition. The positive electrode active material contains a transition metal element, and has a bulk part and a coating part that is provided on the surface of the bulk part. The valence of the transition metal element contained in the coating part is larger than the valence of the transition metal element contained in the bulk part. The present invention solves the problem by also providing a positive electrode for nonaqueous secondary batteries, a nonaqueous secondary battery, a slurry composition, and a binder liquid.

Description

Positive electrode active material, positive electrode for non-aqueous secondary battery, non-aqueous secondary battery, slurry composition, and binder liquid

[0001] The present invention relates to a positive electrode active material used in a non-aqueous secondary battery, a positive electrode for a non-aqueous secondary battery using the same, a non-aqueous secondary battery, a slurry composition containing the positive electrode active material, and a binder liquid used in producing the slurry composition.

[0002] A non-aqueous secondary battery such as a lithium ion secondary battery generally includes battery components such as a positive electrode, a negative electrode, and a separator that separates the positive electrode from the negative electrode to prevent short circuits between the positive electrode and the negative electrode. These battery components are often formed as a layered structure.

[0003] The positive and negative electrodes of non-aqueous secondary batteries can be formed by applying a slurry composition containing an electrode active material, a binder, and a dispersion medium to a suitable substrate such as a current collector, and then volatilizing the dispersion medium. Although organic solvents are sometimes used as the dispersion medium, the use of aqueous dispersion mediums containing water as the main component is desired for ease of operation and reduction of environmental impact (Patent Document 1).

[0004] International Publication No. 2012 / 008539 (corresponding publication: U.S. Patent Application Publication No. 2013 / 0112928)

[0005] When a positive electrode active material containing a transition metal element is dispersed in an aqueous dispersion medium in a slurry composition, a portion of the transition metal element may be eluted into the dispersion medium, which may result in a decrease in the initial capacity of a nonaqueous secondary battery obtained using such a slurry composition.

[0006] The present invention has been made in view of the above-described circumstances, and has an object to provide a positive electrode active material that can suppress the elution of transition metal elements into an aqueous dispersion medium and suppress a decrease in initial capacity when used to form a nonaqueous secondary battery, a positive electrode for a nonaqueous secondary battery using the same, a nonaqueous secondary battery, a slurry composition containing the positive electrode active material, and a binder liquid used in producing the slurry composition.

[0007] The present inventors have found that by forming a coating portion on the surface of a positive electrode active material containing a transition metal element, in which the transition metal element contained in the positive electrode active material is oxidized, it is possible to suppress elution of the transition metal element from a bulk portion present inside the coating portion, and have completed the present invention.

[0008] [1] A positive electrode active material containing a transition metal element, the positive electrode active material having a bulk portion and a coating portion provided on the surface of the bulk portion, wherein the valence of the transition metal element contained in the coating portion is greater than the valence of the transition metal element contained in the bulk portion. [2] The positive electrode active material according to [1], wherein the transition metal element is iron, the bulk portion contains divalent iron, and the coating portion contains iron with a valence greater than divalent. [3] The positive electrode active material according to [1], wherein the bulk portion is LiFePO ４ and the coating portion is Fe ２ O ３ and Fe ３ O ４ [4] The cathode active material according to any one of [1] to [3], wherein the thickness of the coating portion measured by STEM-EELS line analysis is 1 nm or more and 10 nm or less. [5] The cathode active material is in a particulate form, and an EELS spectrum measured from the surface of a particle of the cathode active material to a depth of 20 nm from the center of the particle shows an average energy loss value E Ｏ and the average value E of the energy loss value Ｏ is the average value E of the energy loss value in the depth range of 10 nm to 20 nm Ｉ [6] A cathode active material according to any one of [1] to [4], which contains a transition metal element, wherein the cathode active material is particulate, and an EELS spectrum measured from the surface of a particle of the cathode active material to a depth of 20 nm from the center of the particle shows an average energy loss value E Ｐ and the average value E of the energy loss value Ｐ is the average value E of the energy loss value in the depth range of 10 nm to 20 nm Ｑ[7] A positive electrode for a non-aqueous secondary battery, comprising a current collector layer and a positive electrode mixture layer provided on the current collector layer and containing a positive electrode active material and a binder, wherein the positive electrode active material is the positive electrode active material according to any one of [1] to [6]. [8] A non-aqueous secondary battery comprising a positive electrode, a negative electrode, and an electrolyte solution, wherein the positive electrode is the positive electrode according to [7]. [9] A slurry composition comprising the positive electrode active material according to any one of [1] to [6], a binder, and water.

[10] A method for producing the slurry composition according to [9], comprising: Step 1 of dispersing a positive electrode active material containing a transition metal element in a binder liquid containing the binder, an oxidizing agent, and water; wherein the temperature of the binder liquid in Step 1 is 5°C or higher and 55°C or lower.

[11] A binder liquid comprising a binder, an oxidizing agent for a transition metal element, and water.

[12] The binder liquid according to

[11] , wherein the binder comprises a water-soluble polymer.

[13] The binder liquid according to

[11] or

[12] , wherein the oxidizing agent is a nitrogen compound.

[14] The binder liquid according to

[13] , wherein the nitrogen compound comprises one or more selected from the group consisting of nitrites, nitrates, amine compounds, and ammonium salts.

[15] The binder liquid according to

[11] , wherein the oxidizing agent is a Li salt.

[0009] The present invention can provide a positive electrode active material that can suppress the elution of transition metal elements in a positive electrode active material into an aqueous dispersion medium and suppress a decrease in initial capacity when the positive electrode active material is used to form a nonaqueous secondary battery, a positive electrode for a nonaqueous secondary battery using the positive electrode active material, a nonaqueous secondary battery, a slurry composition containing the positive electrode active material, and a binder liquid used in producing the slurry composition.

[0010] Fig. 1 is a spectrograph showing an EELS spectrum obtained by STEM-EELS line analysis of the positive electrode active material of Comparative Example 1 (without charge / discharge). Fig. 2 is a spectrograph showing an EELS spectrum obtained by STEM-EELS line analysis of the positive electrode active material of Comparative Example 1 (after aging treatment). Fig. 3 is a spectrograph showing an EELS spectrum obtained by STEM-EELS line analysis of the positive electrode active material of Example 3 (without charge / discharge). Fig. 4 is a spectrograph showing an EELS spectrum obtained by STEM-EELS line analysis of the positive electrode active material of Example 3 (after aging treatment).

[0011] The present invention will be described in detail below with reference to embodiments and examples. However, the present invention is not limited to the embodiments and examples described below, and can be implemented with any modifications within the scope of the claims of the present invention and their equivalents.

[0012] In the following description, in a polymer produced by copolymerizing a plurality of types of monomers, the ratio of a monomer unit formed by polymerizing a certain monomer in the polymer usually coincides with the ratio (feed ratio) of the certain monomer to all the monomers used in the polymerization of the polymer, unless otherwise specified.

[0013] In the following description, the structure of a molecule or a part thereof, such as a monomer unit, is not limited by its production method. For example, an aromatic vinyl monomer unit is a unit having a structure formed by polymerization of an aromatic vinyl monomer, but the aromatic vinyl monomer unit also includes units formed by other formation methods that have the same structure as the structure formed by polymerization of an aromatic vinyl monomer.

[0014] In the following description, unless otherwise specified, a substance being "water-soluble" means that when 0.5 g of the substance is dissolved in 100 g of water at 25° C., the insoluble content is less than 0.5 wt %. Furthermore, a substance being "water-insoluble" means that when 0.5 g of the substance is dissolved in 100 g of water at 25° C., the insoluble content is 90 wt % or more.

[0015] [1. Overview of the Present Invention] A cathode active material according to one embodiment of the present invention contains a transition metal element. This cathode active material has a bulk portion and a coating portion provided on the surface of the bulk portion. In the following, to distinguish between the cathode active material according to this embodiment and conventional cathode active materials that do not have a coating portion, the cathode active material according to this embodiment that has a bulk portion and a coating portion may be referred to as "cathode active material (A)," and the cathode active material that does not have a coating portion may be referred to as "cathode active material (B)."

[0016] In the positive electrode active material (A), the valence of the transition metal element contained in the coating portion is greater than the valence of the transition metal element contained in the bulk portion.

[0017] The positive electrode active material (A) according to the present embodiment has a coating portion, and when the positive electrode active material (A) is dispersed in water to prepare a slurry composition, the elution of transition metal elements from the bulk portion of the positive electrode active material (A) can be suppressed. This can suppress deterioration of the positive electrode active material (A), and therefore suppress a decrease in the initial capacity when the positive electrode active material (A) is formed into a non-aqueous secondary battery.

[0018] The positive electrode active material (A) can usually be obtained by using the positive electrode active material (B) as a raw material and oxidizing its surface to form a coating portion. Preferably, the positive electrode active material (B) can be obtained by dispersing the positive electrode active material (B) in a binder liquid containing a binder, an oxidizing agent for the transition metal element contained in the positive electrode active material (B), and water, and then oxidizing the surface of the positive electrode active material (B) using the oxidizing agent. In this case, the temperature of the binder liquid is preferably within a predetermined range. By dispersing the positive electrode active material (B) in a binder liquid containing an oxidizing agent, the surface of the positive electrode active material (B) can be oxidized to obtain the positive electrode active material (A). Alternatively, a slurry composition containing the positive electrode active material (A) can be prepared. However, the positive electrode active material (A) according to this embodiment is not limited to that obtained by the above-described production method.

[0019] The positive electrode active material (A) according to this embodiment is usually used in a positive electrode for a non-aqueous secondary battery. Hereinafter, the positive electrode for a non-aqueous secondary battery may be simply referred to as a "positive electrode," and the non-aqueous secondary battery may be simply referred to as a "secondary battery."

[0020] [2. Positive Electrode Active Material] The positive electrode active material (A) according to this embodiment contains a transition metal element and has a bulk portion and a coating portion. Furthermore, in the positive electrode active material (A), the valence of the transition metal element contained in the coating portion is greater than the valence of the transition metal element contained in the bulk portion. In this case, the coating portion may contain a transition metal element having the same valence as the transition metal element contained in the bulk portion, in combination with a transition metal element having a valence greater than the valence of the transition metal element contained in the bulk portion. The positive electrode active material (A) is usually particulate.

[0021] Examples of transition metal elements that can be contained in the positive electrode active material (A) include Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Mo.

[0022] The compound constituting the bulk portion is a compound containing the above-mentioned transition metal element, and is a compound that can function as a positive electrode active material.

[0023] A preferred example of the compound constituting the bulk portion is a composite metal oxide of lithium and a transition metal. When the composite metal oxide is dispersed in an aqueous dispersion medium, the transition metal element and the lithium element tend to be eluted together, and when the composite metal oxide is used as a secondary battery, the initial capacity tends to decrease. Therefore, by providing a coating portion on the surface of the bulk portion, as in the positive electrode active material (A), the effect of suppressing the decrease in the initial capacity can be significantly exhibited.

[0024] Examples of the composite metal oxide of lithium and a transition metal include a lithium-containing composite metal oxide having a layered structure, a lithium-containing composite metal oxide having a spinel structure, and a lithium-containing composite metal oxide having an olivine structure.

[0025] Examples of lithium-containing composite metal oxides having a layered structure include lithium-containing cobalt oxide (LiCoO ２ ), lithium-containing nickel oxide (LiNiO ２ ), Co—Ni—Mn lithium-containing composite oxide (Li(CoMnNi)O ２ ), lithium-containing composite oxides of Ni-Mn-Al, lithium-containing composite oxides of Ni-Co-Al, LiMaO ２ and Li２ MbO ３ Examples of lithium-containing composite oxides of Co—Ni—Mn include solid solutions of Li[Ni ０．５ Co ０．２ Mn ０．３ ]O ２ , Li[Ni １／３ Co １／３ Mn １／３ ]O ２ Also, LiMaO ２ and Li ２ MbO ３ As a solid solution with Li, for example, xLiMaO ２ (1-x)Li ２ MbO ３ Here, x represents a number satisfying 0<x<1, Ma represents one or more transition metals having an average oxidation state of 3+, and Mb represents one or more transition metals having an average oxidation state of 4+. Such solid solutions include Li[Ni ０．１７ Li ０．２ Co ０．０７ Mn ０．５６ ]O ２ In this specification, the term "average oxidation state" refers to the average oxidation state of the "one or more transition metals" and is calculated from the molar amount and valence of the transition metal. For example, when the "one or more transition metals" are 50 mol % Ni, ２＋ and 50 mol% Mn ４＋ When the metal is composed of the above, the average oxidation state of the "one or more transition metals" is (0.5) x (2+) + (0.5) x (4+) = 3+.

[0026] Examples of lithium-containing composite metal oxides having a spinel structure include lithium manganese oxide (LiMn ２ O ４ ) and lithium manganese oxide (LiMn ２ O ４ ) in which a part of the Mn is replaced with another transition metal. ０．５ Mn １．５ O ４ Li such as ｓ [Mn ２－ｔ Mc ｔ ]O４ Here, Mc represents one or more transition metals whose average oxidation state is 4+. Specific examples of Mc include Ni, Co, Fe, Cu, and Cr. t represents a number satisfying 0<t<1, and s represents a number satisfying 0≦s≦1. The positive electrode active material may be Li, １＋ｘ Mn ２－ｘ O ４ Lithium-excess spinel compounds represented by (0<X<2) can also be used.

[0027] Examples of lithium-containing composite metal oxides having an olivine structure include olivine-type lithium iron phosphate (LiFePO ４ ), olivine-type lithium manganese phosphate (LiMnPO ４ ), olivine-type lithium manganese iron phosphate (LiMn １－ｘ Fe ｘ P.O. ４ ; 0<x<1) and so on ｙ MdPO ４ Here, Md represents one or more transition metals having an average oxidation state of 3+, such as Mn, Fe, Co, etc., and y represents a number satisfying 0≦y≦2. Furthermore, Li ｙ M ｄ P.O. ４ In the olivine-type lithium phosphate compound represented by the formula (1), Md may be partially substituted with another metal. Examples of the metal that can be substituted include Cu, Mg, Zn, V, Ca, Sr, Ba, Ti, Al, Si, B, and Mo.

[0028] Among the above-mentioned lithium-containing composite metal oxides, the compound constituting the bulk portion is preferably olivine-type lithium iron phosphate (LiFePO ４ ) is preferable because it can improve the stability of the secondary battery against overcharging.

[0029] The coating portion is usually provided in the form of a layer on the surface of the bulk portion. The coating portion may be composed of one layer (single layer) or may be composed of a laminate of two or more layers. The presence or absence of the coating portion can usually be confirmed by observation using a scanning transmission electron microscope (STEM).

[0030] The compound constituting the coating portion is a compound containing a transition metal element similar to the transition metal element contained in the bulk portion, but with a valence greater than that of the transition metal element contained in the bulk portion. Such a compound is typically a transition metal oxide. When the bulk portion contains two or more transition metal elements, the compound constituting the coating portion may contain one of the two or more transition metal elements contained in the bulk portion, two or more transition metal elements, or all of the transition metal elements. Furthermore, when the bulk portion contains phosphorus (P), the coating portion may or may not contain phosphorus. The composition of the compound constituting the coating portion can be measured, for example, by STEM-EDX analysis.

[0031] The fact that the valence of the transition metal element contained in the coating portion is greater than the valence of the transition metal element contained in the bulk portion can usually be confirmed by an EELS spectrum obtained by STEM-EELS line analysis. STEM-EELS line analysis is an X-ray analysis method that combines a scanning transmission electron microscope and electron energy-loss spectroscopy. By performing STEM-EELS line analysis, the difference in the valence of the transition metal element contained in the positive electrode active material (A) can be measured as a difference in energy loss value (eV).

[0032] The EELS spectrum obtained by STEM-EELS line analysis can usually be measured by the following procedure. First, particles of the positive electrode active material (A) are thinned using a focused ion beam (FIB). When the positive electrode active material is contained in a positive electrode composite layer, the positive electrode composite layer is washed and then thinned using a focused ion beam. Next, the thin section is observed using a scanning transmission electron microscope (STEM) to identify the particle surface. Next, the EELS spectrum is measured from a measurement start point located near the surface of the particle identified by STEM observation and outside the surface, to a range including at least 20 nm from the surface of the particle toward the center. This measurement range is set to a range including at least 20 nm or less from the surface of the particle toward the center, and may be set to a range exceeding 20 nm from the surface of the particle toward the center. Usually, in STEM-EELS line analysis, a distribution of loss energy lost when electrons are incident and inelastically scattered is obtained for each measurement point, and the peak value of this distribution represents the energy loss value at each measurement point. Therefore, by measuring the energy loss value at each measurement point from the particle surface to a depth of at least 20 nm toward the particle center, an EESL spectrum can be obtained, with the distance from the particle surface to the measurement point (measurement distance) on the horizontal axis and the energy loss value at each measurement point on the vertical axis. When measuring the energy loss value as a peak value from the distribution of loss energy at each measurement point, it is preferable to perform Gaussian fitting on the distribution of loss energy in order to measure the energy loss value accurately. Specific processing will be described in the evaluation method of the examples.

[0033] In an EELS spectrum obtained by STEM-EELS line analysis, the larger the valence of a transition metal element, the larger its energy loss value. Therefore, due to the difference between the valence of the transition metal element contained in the coating portion and the valence of the transition metal element contained in the bulk portion, a region corresponding to the coating portion and a region corresponding to the bulk portion appear in the EELS spectrum of the positive electrode active material (A).

[0034] As an example, consider a case where the thickness of the coating portion is 10 nm or less. In this example, when the EELS spectrum of the positive electrode active material (A) is measured, a region corresponding to the coating portion appears in the EELS spectrum in a range of 0 nm or more and less than 10 nm deep from the surface of the positive electrode active material particles. Furthermore, a region corresponding to the bulk portion appears in the EELS spectrum in a range of 10 nm or more and 20 nm deep from the surface of the particles. Therefore, in the EELS spectrum, the average value of the energy loss value in the range of 10 nm or more and 20 nm deep from the surface of the positive electrode active material particles is represented by the symbol "E Ｉ ", the EELS spectrum has an average value E Ｉ The average value E of the energy loss value that is greater than Ｏ The average value E Ｏ In a region having the above, the energy loss value of the entire region is usually an average value E Ｉ At this time, the average value E Ｉ and the average value E Ｏ Difference with |E Ｉ -E Ｏ is usually 0.5 eV or more, preferably 0.7 eV or more, more preferably 1.0 eV or more, and is usually 2.0 eV or less, preferably 1.7 eV or less, more preferably 1.5 eV or less. Ｉ -E Ｏ When | is within the above range, it is possible to effectively prevent the transition metal element contained in the bulk portion from eluting into the aqueous dispersion medium.

[0035] The average value E of the energy loss value in the EELS spectrum of the positive electrode active material Ｉ is the arithmetic average value of the energy loss values ​​in the range of 10 nm to 20 nm deep from the surface of the particles of the positive electrode active material in the EELS spectrum. Ｏ The term "particles present in the range of 0 nm to less than 10 nm deep from the surface of the particles of the positive electrode active material in the EELS spectrum" means that the average value E Ｉ is the arithmetic mean value of the energy loss values ​​in the region having a larger energy loss value than

[0036] In the EELS spectrum, the average value E Ｏ The region having the average value E is in a range of 0 nm to less than 10 nm deep from the surface of the particle of the positive electrode active material, and may be a part of the range of 0 nm to less than 10 nm deep, or may be the entire range of 0 nm to less than 10 nm deep. Ｏ It is preferable that the region having the average value E Ｏ The region having the above may be a part of the range of depth from 0 nm to 5 nm, or may be the entire range of depth from 0 nm to 5 nm.

[0037] In the EELS spectrum, the point where the change in the energy loss value (eV) of the bulk portion is 0.5 eV or more is designated as point P, the region from the surface of the particle to point P is designated as the coating portion, and the region from point P to a depth of 20 nm can be identified as the bulk portion. Ideally, the energy loss value (eV) in the bulk portion can be a constant value, but, for example, the average value of the energy loss values ​​in the particle depth range of 15 nm to 20 nm can also be regarded as the energy loss value (eV) of the bulk portion.

[0038] At point P, the change in the energy loss value (eV) of the bulk portion may be 0.5 eV or more, but may also be 0.7 eV or more, or 1.0 eV or more.

[0039] In this embodiment, the average value E of the energy loss value of the bulk part in the EELS spectrum of the positive electrode active material (A) is usually Ｂ The average value of the energy loss of the coating part E Ｃ Specifically, the average value of the energy loss of the coating part E Ｃ and the average value of the energy loss value of the bulk part E Ｂ Difference with |E Ｂ -E Ｃis usually 0.5 eV or more, preferably 0.7 eV or more, more preferably 1.0 eV or more, and is usually 2.0 eV or less, preferably 1.7 eV or less, more preferably 1.5 eV or less. Ｂ -E Ｃ When | is in the above range, the transition metal element contained in the bulk portion can be effectively prevented from eluting into the aqueous dispersion medium.

[0040] The average value E of the energy loss value of the bulk part in the EELS spectrum of the positive electrode active material Ｂ is the arithmetic mean value of the energy loss values ​​in the bulk region (distance from point P to a depth of 20 nm) in the EELS spectrum identified by the above-mentioned identification method. Ｃ is the arithmetic mean value of the energy loss values ​​in the coating region (the distance from the particle surface to the depth point P) in the EELS spectrum identified by the above-mentioned identification method.

[0041] The average value E of the above-mentioned energy loss value Ｉ is the average value E of the energy loss value of the part included in the bulk part Ｂ Therefore, the average value E Ｉ The range is the average value E of the energy loss value of the bulk part Ｂ In addition, the average value E of the energy loss value may be the same as the range. Ｏ is the average value E of the energy loss value of part or all of the coated part Ｃ Therefore, the average value E Ｏ The range is the average value E of the energy loss value of the coating part. Ｃ The range may be the same as

[0042] The thickness of the coating portion is not particularly limited as long as it is sufficient to suppress elution of the transition metal element from the bulk portion. Preferably, the thickness of the coating portion measured by STEM-EELS line analysis is within a predetermined range. By performing STEM-EELS line analysis, differences in the valence of the transition metal element contained in the positive electrode active material (A) can be measured as differences in energy loss value (eV), and therefore the thickness of the coating portion containing a transition metal element with a valence different from that of the transition metal element contained in the bulk portion can be measured.

[0043] The thickness of the coating portion as determined by STEM-EELS line analysis is usually 1 nm or more, preferably 2 nm or more, more preferably 3 nm or more, and can be usually 10 nm or less, preferably 8 nm or less, more preferably 5 nm or less. This is because, when the thickness of the coating portion is in the above range, the transition metal element contained in the bulk portion can be effectively prevented from eluting into the aqueous dispersion medium.

[0044] The thickness of the coating portion determined by STEM-EELS line analysis is defined as the distance of the coating portion region (the distance from the particle surface to point P) in the EELS spectrum identified by the above-described identification method.

[0045] The average percentage (coverage) of the bulk portion surface covered by the coating portion is not particularly limited as long as it is sufficient to suppress the elution of the transition metal element of the bulk portion into the aqueous dispersion medium, but can be, for example, 20% or more, preferably 40% or more, and more preferably 50% or more. The average percentage is preferably 100%, but can be 90% or less, or even 80% or less. The average percentage is calculated as follows. In a cross-sectional observation image of the positive electrode active material (A) taken with a transmission electron microscope (TEM), the circumferential length D1 corresponding to the surface (outer surface) of the bulk portion and the length of the portion covered by the coating portion are defined as D2. The value Rc is calculated using the following formula (1). The Rc is measured for 20 or more positive electrode active materials (A), and the arithmetic average value is taken as the coverage. Rc (%) = (D2 / D1) × 100 (1)

[0046] The positive electrode active material (A) is usually particulate. The median diameter of the positive electrode active material is preferably 0.02 μm or more, more preferably 0.05 μm or more, even more preferably 0.1 μm or more, and preferably 100 μm or less, more preferably 50 μm or less, and even more preferably 30 μm or less. When the median diameter of the positive electrode active material is within the above range, it can be suitably used as a material for a lithium-ion battery electrode. The median diameter of the electrode active material can be measured by measuring the particle size of the particles using a laser diffraction particle size distribution analyzer, and determining the particle size (D50) at which the cumulative volume calculated from the smallest diameter side in the obtained particle size distribution (volume basis) becomes 50%.

[0047] The positive electrode active material (A) preferably contains iron element among the above-mentioned transition metal elements. In this case, the bulk portion preferably contains divalent iron element, and the coating portion preferably contains iron element with a valence greater than divalent. Furthermore, in this case, the bulk portion preferably contains LiFePO ４ The coating portion contains Fe ２ O ３ and Fe ３ O ４ It is more preferable that the coating portion contains at least one of Fe. ２ O ３ and Fe ３ O ４ It is preferable to include both of the above.

[0048] In addition, when the transition metal element contained in the positive electrode active material (A) is iron, the average value E Ｉ is preferably around 710 eV, specifically 709.5 eV to 710.5 eV, and the average value of the energy loss value E Ｏ is the average value E Ｉ It is preferable that the average value E Ｉ and the average value E Ｏ Difference with |E Ｉ -E Ｏ is usually 0.5 eV or more, preferably 0.7 eV or more, more preferably 1.0 eV or more, and is usually 2.0 eV or less, preferably 1.7 eV or less, more preferably 1.5 eV or less.

[0049] Also, as mentioned above, the average value E Ｉ and the average value E Ｏ is the average value E of the energy loss value of the part included in the bulk part Ｂ and the average value E of the energy loss value of part or all of the coated part Ｃ Therefore, when the transition metal element contained in the positive electrode active material (A) is iron, the average value E Ｂ is preferably the average value E Ｏ The average value E Ｂ and the average value E Ｃ Difference with |E Ｂ -E Ｃ | is the average value E Ｉ and the average value E Ｏ Difference with |E Ｉ -E Ｏ may be in the same range as |.

[0050] The positive electrode active material (A) may have at least a portion of the particle surface coated with a carbon coating layer. In this case, the positive electrode active material (A) may have a structure in which a coating portion and a bulk portion are disposed inside the carbon coating layer. Furthermore, when the particles of the positive electrode active material (A) are coated with a carbon coating layer, the surface of the particles of the positive electrode active material in the above-mentioned EELS spectrum refers not to the surface of the carbon coating layer but to the surface of the particles themselves (the surface of the active material particles).

[0051] The method for producing the positive electrode active material (A) is not particularly limited, but it can usually be produced by oxidizing the surface of the positive electrode active material (B). A preferred method for oxidizing the surface of the positive electrode active material (B) is to oxidize the surface of the positive electrode active material (B) using an oxidizing agent for the transition metal element contained in the positive electrode active material (B). In such a method, it is preferred to disperse the positive electrode active material (B) in a binder liquid containing a binder, an oxidizing agent, and water. This is because the positive electrode active material (A) can be produced and a positive electrode slurry composition containing the positive electrode active material (A) can be prepared.

[0052] The cathode active material (B) used as a raw material for the cathode active material (A) may have a carbon coating layer on its surface. Because water and an oxidizing agent can pass through the carbon coating layer, an oxidation reaction of the cathode active material occurs inside the carbon coating layer, and a cathode active material (A) having a bulk portion and a coating portion can be obtained.

[0053] As described above, the inventors have obtained a positive electrode active material (A) by oxidizing the surface of a positive electrode active material (B) to form a coating portion, and have found that this positive electrode active material (A) can suppress the elution of transition metal elements and thereby suppress the decrease in initial capacity of a secondary battery. Furthermore, the inventors have found that the positive electrode active material (A) has the unexpected effect of lowering the resistance of the positive electrode in addition to the above-mentioned effect of suppressing the decrease in initial capacity.

[0054] Although the mechanism by which the positive electrode active material (A) suppresses a decrease in the initial capacity of a secondary battery and the mechanism by which the resistance of the positive electrode is reduced are not clear, the present inventors speculate as follows, although the technical scope of the present invention is not limited to the mechanism described below.

[0055] Generally, in a positive electrode active material containing a transition metal element such as iron, the transition metal element dissolves in water, which can further dissolve elements (especially lithium) constituting the positive electrode active material, resulting in the loss of lithium in the positive electrode active material. If elements such as lithium are lost, the initial capacity can decrease. In contrast, in the positive electrode active material (A), the transition metal element is oxidized on the surface of the bulk portion, resulting in the formation of a coating portion having a higher valence than the transition metal element contained in the bulk portion. The present inventors speculate that the coating portion's ability to suppress the dissolution of the transition metal element and further the elements constituting the positive electrode active material from the bulk portion contributes to the suppression of the decrease in initial capacity.

[0056] As described above, the coating portion can be obtained by oxidizing the surface of the positive electrode active material (B). During the oxidation process of the transition metal element, the valence of the transition metal element changes, and the crystalline structure that the transition metal element can adopt also changes. As a result, a new crystalline structure exhibiting high electrical conductivity is generated, including a transition metal element having a valence higher than that of the transition metal element contained in the bulk portion. The inventors speculate that the generation of this new crystalline structure is one factor in the reduction of resistance. Furthermore, when the positive electrode active material (A) has a carbon coating layer, the coating portion improves the adhesion between the coating portion and the carbon coating layer. As a result, the inventors speculate that the suppression of peeling of the carbon coating layer improves electrical conductivity, thereby reducing the resistance of the positive electrode.

[0057] As described above, in the present invention, by oxidizing the surfaces of the particles of the positive electrode active material, it is possible to make the energy loss value (eV) of the EELS spectrum obtained by STEM-EELS line analysis different between the surface side and the center side of the particles of the positive electrode active material. Therefore, as a modified example, the following positive electrode active material can also be provided.

[0058] That is, in a modified example, the positive electrode active material contains a transition metal element, and the positive electrode active material is particulate, and an EELS spectrum measured from the surface of a particle of the positive electrode active material to a depth of 20 nm from the center of the particle shows an average value E of the energy loss value in a depth range of 0 nm or more and less than 10 nm. Ｐ and an average value E of the energy loss value Ｐ is the average value E of the energy loss value in the depth range of 10 nm to 20 nm Ｑ It is also possible to provide a positive electrode active material having a voltage higher than that of the positive electrode by 0.5 eV or more.

[0059] The positive electrode active material of the above-described modified example has an average value E Ｐ is the average value of energy loss E Ｑ When the value is greater than or equal to a predetermined value, the positive electrode active material can have a particle structure that can suppress the elution of the transition metal element into the aqueous dispersion medium.

[0060] The average value E of the energy loss value in the EELS spectrum of the modified positive electrode active material Ｐ and the average value E Ｑ The preferred range of the above-mentioned average value of the energy loss E Ｏ and the average value E Ｉ The preferred range is the same as that of

[0061] The modified positive electrode active material is usually in a particulate form. The modified positive electrode active material has an average value E Ｐ is the average value of energy loss E Ｑ Generally, the average value E of the energy loss of the positive electrode active material is not limited as long as the average value E of the energy loss of the positive electrode active material is larger than the predetermined value. Ｐ The valence of the transition metal element contained in the region having the average value of the energy loss value E Ｑ The positive electrode active material of the modified example may have a clear boundary between a coating portion and a bulk portion, or may not have such a clear boundary. The former is preferable. When the particle structure has a clear boundary between a coating portion and a bulk portion, the average value E of the energy loss value Ｐ The region having the average value E of the energy loss can be a region corresponding to the coating portion of the positive electrode active material described above. Ｑ In this case, the region showing the average value of the energy loss E Ｐ is the average value E of the above-mentioned energy loss value Ｏ In addition, the average value of the energy loss value E Ｑ is the average value E of the above-mentioned energy loss value Ｉ can take the same value as

[0062] The positive electrode active material of the modified example is the same as that described above as the positive electrode active material according to this embodiment in all respects other than those mentioned above.

[0063] 3. Binder Liquid The binder liquid according to one embodiment of the present invention contains a binder, an oxidizing agent for the transition metal element, and water.

[0064] According to this embodiment, since the binder liquid contains an oxidizing agent for the transition metal element, when a cathode active material (B) containing a transition metal element is added to prepare a cathode slurry composition, the surface of the cathode active material (B) can be oxidized to form a coating portion, and as a result, a cathode slurry containing a cathode active material in which elution of the transition metal element into an aqueous dispersion medium is suppressed can be obtained.

[0065] [3.1. Oxidizing Agent] The oxidizing agent that can be contained in the binder liquid is an oxidizing agent for transition metal elements and is a compound that can oxidize transition metal elements. Such oxidizing agents include compounds consisting of cations and anions. Examples of cations include hydrogen ions, potassium ions, sodium ions, lithium ions, and ammonium ions. Examples of anions include permanganate ions, dichromate ions, nitrate ions, nitrite ions, sulfate ions, hypochlorite ions, peroxide ions, sulfur dioxide ions, and arsenic oxide ions.

[0066] Among the oxidizing agents, nitrogen compounds are preferred. Examples of nitrogen compounds include nitrites, nitrates, amine compounds, and ammonium salts. In this embodiment, among the above, nitrites and nitrates are preferred because they can function as good oxidizing agents for transition metal elements. One type of the above nitrogen compounds may be used, or two or more types may be used.

[0067] The oxidizing agent is also preferably a Li salt, because, for example, when the nonaqueous secondary battery is a lithium secondary battery, the battery reaction is less likely to be inhibited even if the oxidizing agent remains.

[0068] More specifically, the oxidizing agent is LiNO ２ , LiNO ３ Examples include:

[0069] The content of the oxidizing agent in the binder solution is, for example, usually 1 part by weight or more, preferably 5 parts by weight or more, more preferably 10 parts by weight or more, and usually 80 parts by weight or less, preferably 70 parts by weight or less, more preferably 50 parts by weight or less, relative to 100 parts by weight of the binder. When the content of the oxidizing agent is within the above range, the surface of the positive electrode active material (B) can be effectively oxidized using the oxidizing agent.

[0070] [3.2. Binder] The binder that the binder solution may contain typically contains a polymer. This polymer can bind the components that make up the positive electrode mixture layer together in the positive electrode mixture layer, and can also bind the positive electrode mixture layer and the current collector layer together.

[0071] The polymer may contain a water-soluble polymer, a water-insoluble polymer, or both a water-soluble polymer and a water-insoluble polymer. Among these, it is preferable that the polymer contains a water-soluble polymer, because this allows the preparation of a slurry composition in which the positive electrode active material (A) is well dispersed.

[0072] (Water-Soluble Polymer) A preferred example of the water-soluble polymer is polymer X containing an acidic group-containing monomer unit and an unsaturated carboxylic acid ester monomer unit. Furthermore, in addition to the above-described acidic group-containing monomer unit and unsaturated carboxylic acid ester monomer unit, polymer X may further contain, as necessary, at least one of a hydroxyl group-containing acrylic acid ester monomer unit, a hydroxyl group-containing methacrylic acid ester monomer unit, and a (meth)acrylamide monomer unit. Furthermore, polymer X may further contain monomer units other than the above-described acidic group-containing monomer unit, unsaturated carboxylic acid ester monomer unit, hydroxyl group-containing acrylic acid ester monomer unit, hydroxyl group-containing methacrylic acid ester monomer unit, and (meth)acrylamide monomer unit (hereinafter sometimes referred to as "other monomer units"), as long as the desired effects of the present invention are achieved.

[0073] The acidic group-containing monomer unit represents a unit having a structure formed by polymerizing an acidic group-containing monomer. Examples of the acidic group-containing monomer that can form the acidic group-containing monomer unit in polymer X include a carboxylic acid group-containing monomer, a sulfonic acid group-containing monomer, and a phosphoric acid group-containing monomer.

[0074] Examples of the carboxylic acid group-containing monomer include monocarboxylic acids and derivatives thereof, dicarboxylic acids and derivatives thereof, acid anhydrides of dicarboxylic acids, and derivatives thereof.

[0075] Examples of monocarboxylic acids include acrylic acid, methacrylic acid, crotonic acid, etc. Examples of monocarboxylic acid derivatives include 2-ethylacrylic acid, isocrotonic acid, α-acetoxyacrylic acid, β-trans-aryloxyacrylic acid, α-chloro-β-E-methoxyacrylic acid, β-diaminoacrylic acid, etc.

[0076] Examples of dicarboxylic acids include maleic acid, fumaric acid, and itaconic acid. Examples of dicarboxylic acid derivatives include methylmaleic acid, dimethylmaleic acid, phenylmaleic acid, chloromaleic acid, dichloromaleic acid, and fluoromaleic acid, as well as maleic acid esters such as methylallyl maleate, diphenyl maleate, nonyl maleate, decyl maleate, dodecyl maleate, octadecyl maleate, and fluoroalkyl maleate. Examples of acid anhydrides of dicarboxylic acids include maleic anhydride, acrylic anhydride, methylmaleic anhydride, and dimethylmaleic anhydride.

[0077] Further, as the carboxylic acid group-containing monomer, an acid anhydride that generates a carboxyl group upon hydrolysis can also be used.Other examples include monoesters and diesters of α,β-ethylenically unsaturated polycarboxylic acids such as monoethyl maleate, diethyl maleate, monobutyl maleate, dibutyl maleate, monoethyl fumarate, diethyl fumarate, monobutyl fumarate, dibutyl fumarate, monocyclohexyl fumarate, dicyclohexyl fumarate, monoethyl itaconate, diethyl itaconate, monobutyl itaconate, and dibutyl itaconate.

[0078] In the carboxylic acid group of the above-mentioned carboxylic acid group-containing monomer, a hydrogen atom may be substituted with an inorganic ion or an organic ion to form an inorganic salt or an organic salt, i.e., the carboxylic acid group-containing monomer may be in the form of a carboxylate salt.

[0079] Examples of sulfonic acid group-containing monomers include vinyl sulfonic acid, methyl vinyl sulfonic acid, (meth)allyl sulfonic acid, styrene sulfonic acid, (meth)acrylic acid-2-ethyl sulfonate, 2-acrylamido-2-methylpropanesulfonic acid, and 3-allyloxy-2-hydroxypropanesulfonic acid. In this specification, the term "(meth)allyl" includes "allyl," "methallyl," and combinations thereof. Furthermore, the term "(meth)acrylic" includes "acrylic," "acrylic," "methacrylic," and combinations thereof.

[0080] In the sulfonic acid group of the sulfonic acid group-containing monomer, a hydrogen atom may be substituted with an inorganic ion or an organic ion to form an inorganic salt or an organic salt, i.e., the sulfonic acid group-containing monomer may be in the form of a sulfonate salt.

[0081] Examples of the phosphate group-containing monomer include 2-(meth)acryloyloxyethyl phosphate, methyl-2-(meth)acryloyloxyethyl phosphate, ethyl-(meth)acryloyloxyethyl phosphate, etc. In this specification, the expression "(meth)acryloyl" includes "acryloyl", "methacryloyl", and combinations thereof.

[0082] In the phosphate group of the above-mentioned phosphate group-containing monomer, a hydrogen atom may be substituted with an inorganic ion or an organic ion to form an inorganic salt or an organic salt, i.e., the phosphate group-containing monomer may be in the form of a phosphate salt.

[0083] These acidic group-containing monomers may be used alone or in any combination of two or more in any ratio. From the viewpoint of further improving the adhesiveness of the positive electrode mixture layer to be formed, it is preferable to use a carboxylic acid group-containing monomer or a sulfonic acid group-containing monomer, it is more preferable to use a carboxylic acid group-containing monomer, and it is even more preferable to use acrylic acid.

[0084] The content of the acidic group-containing monomer units in polymer X is usually 3% by weight or more, preferably 4.5% by weight or more, and more preferably 5% by weight or more, and usually 20% by weight or less, preferably 17.5% by weight or less, and more preferably 15% by weight or less, when the total weight of all monomer units (total repeating units) contained in polymer X is taken as 100% by weight. This is because the content of the acidic group-containing monomer units in the above range can improve the dispersibility of the components in the slurry composition.

[0085] An unsaturated carboxylic acid ester monomer unit is a repeating unit derived from an unsaturated carboxylic acid ester monomer. Furthermore, an unsaturated carboxylic acid ester monomer refers to a monomer containing a non-aromatic carbon-carbon unsaturated bond, which is an ester of a carboxylic acid. Therefore, an unsaturated carboxylic acid ester monomer unit typically refers to a unit having a structure formed by polymerizing an unsaturated carboxylic acid ester monomer. However, unsaturated carboxylic acid ester monomer units typically do not include those containing a hydroxyl group. Examples of unsaturated carboxylic acid ester monomers that can form unsaturated carboxylic acid ester monomer units include monomers having an ethylenically unsaturated bond, specifically 2-methoxyethyl acrylate (also referred to as "2-methoxyethyl acrylate"), ethyl acrylate, methyl acrylate, and vinyl acetate. These unsaturated carboxylic acid ester monomers may be used alone or in combination of two or more in any ratio. Among them, 2-methoxyethyl acrylate, ethyl acrylate, and methyl acrylate are preferably used as the unsaturated carboxylic acid ester monomer, and 2-methoxyethyl acrylate is particularly preferably used.

[0086] The content of the unsaturated carboxylic acid ester monomer units in polymer X is usually 5% by weight or more, preferably 6% by weight or more, more preferably 7% by weight or more, and even more preferably 25% by weight or more, and usually less than 50% by weight, preferably 45% by weight or less, and more preferably 40% by weight or less, when the total weight of all monomer units (total repeating units) contained in polymer X is taken as 100% by weight. This is because the content of the unsaturated carboxylic acid ester monomer units in the above range can improve the dispersibility of the components in the slurry composition.

[0087] It is preferable that polymer X further contains a hydroxyl group-containing acrylic ester monomer unit. This is because the adhesiveness and flexibility of the positive electrode mixture layer can be improved. The hydroxyl group-containing acrylic ester monomer unit represents a unit having a structure formed by polymerizing a hydroxyl group-containing acrylic ester monomer. Examples of hydroxyl group-containing acrylic ester monomers that can form the hydroxyl group-containing acrylic ester monomer unit in polymer X include alkanol esters of acrylic acid (also referred to as "acrylic acid hydroxyalkyl ester monomers") such as 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, and 4-hydroxybutyl acrylate; ２ =CH-COO-(C ｑ H ２ｑ O) ｐ -H (wherein p is an integer of 2 to 9, and q is an integer of 2 to 4), and esters of acrylic acid (also referred to as "acrylic acid polyalkylene glycol ester monomers"). These may be used alone or in any combination of two or more in any ratio. Among these, from the viewpoint of improving the adhesiveness and flexibility of the electrode mixture layer, it is preferable to use alkanol esters of acrylic acid (acrylic acid hydroxyalkyl ester monomers) such as 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, and 4-hydroxybutyl acrylate as the hydroxyl group-containing acrylic acid ester monomer.

[0088] The content of the hydroxyl group-containing acrylic ester monomer units in polymer X is preferably 55% by weight or more, more preferably 57.5% by weight or more, and even more preferably 60% by weight or more, and is preferably 90% by weight or less, preferably 85% by weight or less, and more preferably 80% by weight or less, when the total weight of all monomer units (total repeating units) contained in polymer X is taken as 100% by weight. This is because the content of the acrylic ester monomer units in the above range can improve the adhesion and flexibility of the positive electrode mixture layer.

[0089] It is preferable that polymer X further contains a hydroxyl group-containing methacrylic acid ester monomer unit. This is because the adhesiveness and flexibility of the positive electrode mixture layer can be improved. The hydroxyl group-containing methacrylic acid ester monomer unit represents a unit having a structure formed by polymerizing a hydroxyl group-containing methacrylic acid ester monomer. Examples of hydroxyl group-containing methacrylic acid ester monomers that can form the hydroxyl group-containing methacrylic acid ester monomer unit in polymer X include alkanol esters of methacrylic acid (also referred to as "methacrylic acid hydroxyalkyl ester monomers") such as 2-hydroxyethyl methacrylate and 2-hydroxypropyl methacrylate; ２ =C(CH ３ )-COO-(C ｓ H ２ｓ O) ｒ -H (wherein r is an integer of 2 to 9, and s is an integer of 2 to 4), and esters of methacrylic acid (also referred to as "methacrylic acid polyalkylene glycol ester monomers"). These may be used alone, or two or more may be used in any ratio. Among these, from the viewpoint of further improving the adhesiveness and flexibility of the electrode mixture layer, it is preferable to use alkanol esters of methacrylic acid (methacrylic acid hydroxyalkyl ester monomers), such as 2-hydroxyethyl methacrylate and 2-hydroxypropyl methacrylate, as the hydroxyl group-containing methacrylic acid ester monomer.

[0090] The content of the hydroxyl group-containing methacrylic acid ester monomer units in polymer X is preferably 2 wt% or more, more preferably 5 wt% or more, even more preferably 6 wt% or more, even more preferably 7 wt% or more, and preferably 30 wt% or less, more preferably 20 wt% or less, and even more preferably 15 wt% or less, when the total weight of all monomer units (total repeating units) contained in polymer X is taken as 100 wt%. If the content of the hydroxyl group-containing methacrylic acid ester monomer units in polymer X is within the above-mentioned range, the adhesiveness and flexibility of the electrode mixture layer formed using a slurry composition containing the binder composition can be further improved. The content of the hydroxyl group-containing methacrylic acid ester monomer units in polymer X can be 0 wt% or more, or can be 10 wt% or less, when the total weight of all monomer units (total repeating units) contained in polymer X is taken as 100 wt%.

[0091] The polymer X may further contain (meth)acrylamide monomer units. When the polymer X further contains (meth)acrylamide monomer units, the adhesiveness and flexibility of the resulting electrode mixture layer can be improved.

[0092] The (meth)acrylamide monomer unit refers to a unit having a structure formed by polymerizing a (meth)acrylamide monomer. The term "(meth)acrylamide monomer" encompasses acrylamide monomers, methacrylamide monomers, and combinations thereof. Examples of (meth)acrylamide monomers that can form the (meth)acrylamide monomer units in polymer X include acrylamide, methacrylamide, N-hydroxymethyl acrylamide, N-hydroxymethyl methacrylamide, N-hydroxyethyl acrylamide, and N-hydroxyethyl methacrylamide. These may be used alone, or two or more may be used in any ratio. Among these, acrylamide and N-hydroxyethyl acrylamide are preferred from the viewpoint of improving the adhesiveness and flexibility of the resulting electrode mixture layer.

[0093] The content of (meth)acrylamide monomer units in polymer X is preferably 3 wt% or more, more preferably 4.5 wt% or more, more preferably 7.5 wt% or more, and preferably 20 wt% or less, more preferably 18 wt% or less, and even more preferably 16 wt% or less, when the total weight of all monomer units (total repeating units) contained in polymer X is taken as 100 wt%. When the content of (meth)acrylamide monomer units in polymer X is equal to or greater than the above-mentioned lower limit, the adhesiveness of the resulting electrode mixture layer can be improved. Furthermore, when the content of (meth)acrylamide monomer units in polymer X is equal to or less than the above-mentioned upper limit, the adhesiveness and flexibility of the resulting electrode mixture layer can be improved, presumably because excessive hardening of polymer X can be prevented. This improves the output characteristics of the resulting secondary battery.

[0094] Polymer X is not particularly limited, and can be prepared, for example, as follows: The above-described acidic group-containing monomer, unsaturated carboxylic acid ester monomer, and optional monomers are mixed in a solvent such as water. The acidic group-containing monomer may be dissolved in an alkaline aqueous solution and further neutralized to prepare an aqueous solution, which is then mixed with the unsaturated carboxylic acid ester monomer and other optional monomers. A polymerization accelerator is added to the resulting mixture. A polymerization initiator is then added to initiate the polymerization reaction. If necessary, one or more cycles of adding a polymerization accelerator, adding a polymerization initiator, and polymerizing may then be performed. The above-described operations are preferably performed under a nitrogen atmosphere. A chain transfer agent may also be used as a polymerization aid during the polymerization reaction. After the polymerization reaction, a reaction terminator is used to terminate the polymerization reaction. The product is then cooled and placed under air, and the pH of the product is adjusted to 7.0 or more and 9.0 or less by adding an aqueous lithium hydroxide solution or the like. Known polymerization initiators, polymerization accelerators, and chain transfer agents may be used.

[0095] In addition to the above-mentioned polymer X, the water-soluble polymer may include, for example, a polymer known as a water-soluble polymer contained in the positive electrode mixture layer.

[0096] The content of the water-soluble polymer in the binder is usually 40% by weight or more, preferably 45% by weight or more, and more preferably 50% by weight or more. The content of the water-soluble polymer may be, for example, 100% by weight, but may also be 90% by weight or less, or may even be 80% by weight or less. By having the content of the water-soluble polymer in the above range, the dispersibility of the slurry composition can be improved. Furthermore, the mechanical strength of the positive electrode composite layer can be improved.

[0097] (Water-insoluble polymer) A preferred example of the water-insoluble polymer is a particulate water-insoluble polymer (hereinafter referred to as particulate polymer Y). The particulate polymer Y is a component that can function as a binder together with the polymer X in the positive electrode mixture layer.

[0098] The particulate polymer Y may or may not contain a hydrophilic group, but the former is more preferable. If a particulate polymer containing a hydrophilic group (a hydrophilic group-containing particulate polymer) is used as the particulate polymer Y, the dispersibility of the slurry composition and the adhesiveness and flexibility of the formed electrode mixture layer can be further improved.

[0099] The method for introducing a hydrophilic group into the particulate polymer Y is not particularly limited, but for example, a method can be adopted in which a hydrophilic group-containing monomer is used during polymerization of the particulate polymer Y to form a hydrophilic group-containing monomer unit in the resulting particulate polymer Y.

[0100] Examples of the hydrophilic group-containing monomer capable of forming the hydrophilic group-containing monomer unit in the particulate polymer Y include acidic group-containing monomers such as carboxylic acid group-containing monomers, sulfonic acid group-containing monomers, and phosphoric acid group-containing monomers, as well as hydroxyl group-containing monomers.

[0101] As the acidic group-containing monomer (such as a carboxylic acid group-containing monomer, a sulfonic acid group-containing monomer, or a phosphoric acid group-containing monomer), various acidic group-containing monomers that can be used in the preparation of the polymer X described above can be used.

[0102] Examples of the hydroxyl group-containing monomer include alkanol esters of ethylenically unsaturated carboxylic acids such as 2-hydroxyethyl acrylate (2-hydroxyethyl acrylate), 2-hydroxypropyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, di-2-hydroxyethyl maleate, di-4-hydroxybutyl maleate, and di-2-hydroxypropyl itaconate; ２ =CR １ -COO-(C ｑ H ２ｑ O) ｐ -H (wherein p is an integer of 2 to 9, q is an integer of 2 to 4, R １represents hydrogen or a methyl group) and (meth)acrylic acid esters; mono(meth)acrylic acid esters of dihydroxy esters of dicarboxylic acids such as 2-hydroxyethyl-2'-(meth)acryloyloxyphthalate and 2-hydroxyethyl-2'-(meth)acryloyloxysuccinate; vinyl ethers such as 2-hydroxyethyl vinyl ether and 2-hydroxypropyl vinyl ether; alkyl ethers such as (meth)allyl-2-hydroxyethyl ether, (meth)allyl-2-hydroxypropyl ether, (meth)allyl-3-hydroxypropyl ether, (meth)allyl-2-hydroxybutyl ether, (meth)allyl-3-hydroxybutyl ether, (meth)allyl-4-hydroxybutyl ether, and (meth)allyl-6-hydroxyhexyl ether; mono(meth)allyl ethers of ethylene glycol; polyoxyalkylene glycol mono(meth)allyl ethers such as diethylene glycol mono(meth)allyl ether and dipropylene glycol mono(meth)allyl ether; mono(meth)allyl ethers of halogen- and hydroxy-substituted (poly)alkylene glycols such as glycerin mono(meth)allyl ether, (meth)allyl-2-chloro-3-hydroxypropyl ether and (meth)allyl-2-hydroxy-3-chloropropyl ether; mono(meth)allyl ethers of polyhydric phenols such as eugenol and isoeugenol, and halogen-substituted products thereof; (meth)allyl thioethers of alkylene glycols such as (meth)allyl-2-hydroxyethyl thioether and (meth)allyl-2-hydroxypropyl thioether; and the like.

[0103] The above-mentioned hydrophilic group-containing monomer may be used alone or in any combination of two or more kinds in any ratio. From the viewpoint of further improving the dispersibility of the slurry composition and the adhesiveness of the electrode mixture layer to be formed, it is preferable to use an acidic group-containing monomer (such as a carboxylic acid group-containing monomer, a sulfonic acid group-containing monomer, or a phosphate group-containing monomer), and from the viewpoint of further improving the adhesiveness of the electrode mixture layer to be formed, it is more preferable to use an acidic group-containing monomer in combination with a hydroxyl group-containing monomer, and it is even more preferable to use a carboxylic acid group-containing monomer in combination with a hydroxyl group-containing monomer.

[0104] The content ratio of the hydrophilic group-containing monomer units in the particulate polymer Y is preferably 1% by weight or more and 10% by weight or less, when the total weight of all the monomer units contained in the particulate polymer Y is taken as 100% by weight. When the content ratio of the hydrophilic group-containing monomer units in the particulate polymer Y is within the above-mentioned predetermined range, the dispersibility of the slurry composition and the adhesiveness and flexibility of the formed electrode mixture layer can be further improved.

[0105] As the particulate polymer Y containing a hydrophilic group, for example, a hydrophilic group-containing acrylic polymer or a hydrophilic group-containing conjugated diene polymer is preferably used, and from the viewpoint of ensuring sufficiently high cycle characteristics of the secondary battery, it is more preferable to use a hydrophilic group-containing acrylic polymer.

[0106] The hydrophilic group-containing conjugated diene polymer is a copolymer containing conjugated diene monomer units in addition to the above-mentioned hydrophilic group-containing monomer units. The conjugated diene monomer units refer to units having a structure formed by polymerizing a conjugated diene monomer or a structure obtained by hydrogenating the same.

[0107] Specific examples of conjugated diene polymers include, but are not limited to, copolymers containing hydrophilic group-containing monomer units such as hydrophilic group-containing styrene-butadiene copolymers (hydrophilic group-containing SBR), copolymers containing aromatic vinyl monomer units and aliphatic conjugated diene monomer units, hydrophilic group-containing butadiene rubbers (hydrophilic group-containing BR) (copolymers containing hydrophilic group-containing monomer units and butadiene units), hydrophilic group-containing acrylic rubbers (hydrophilic group-containing NBR) (copolymers containing hydrophilic group-containing monomer units, acrylonitrile units, and butadiene units), and hydrogenated versions thereof. These may be used alone or in any combination of two or more in any ratio. Among these, from the viewpoint of further improving the adhesiveness and flexibility of the electrode mixture layer to be formed, it is preferable to use copolymers containing hydrophilic group-containing monomer units, aromatic vinyl monomer units, and aliphatic conjugated diene monomer units, such as hydrophilic group-containing styrene-butadiene copolymers (hydrophilic group-containing SBR).

[0108] The content ratio of the hydrophilic group-containing monomer unit in the hydrophilic group-containing conjugated diene polymer can be set to the same range as the preferred range of the content ratio of the hydrophilic group-containing monomer unit in the particulate polymer Y described above.

[0109] Examples of conjugated diene monomer units that can form the conjugated diene monomer units in the hydrophilic group-containing conjugated diene polymer include conjugated diene compounds such as 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, and 1,3-pentadiene. These may be used alone or in any combination of two or more in any ratio. Of these, 1,3-butadiene is preferred.

[0110] The content of the conjugated diene monomer units in the hydrophilic group-containing conjugated diene polymer is preferably 15% by weight or more, more preferably 20% by weight or more, and even more preferably 25% by weight or more, and is preferably 60% by weight or less, more preferably 50% by weight or less, and even more preferably 40% by weight or less, based on 100% by weight of all monomer units contained in the hydrophilic group-containing conjugated diene polymer. If the content of the conjugated diene monomer units in the hydrophilic group-containing conjugated diene polymer is within the above-mentioned range, the adhesiveness and flexibility of the formed electrode mixture layer can be further improved.

[0111] Examples of aromatic vinyl monomers that can form aromatic vinyl monomer units in the hydrophilic group-containing conjugated diene polymer include styrene, α-methylstyrene, vinyltoluene, and divinylbenzene. These may be used alone or in any combination of two or more in any ratio. Among these, styrene is preferred.

[0112] The content of aromatic vinyl monomer units in the hydrophilic group-containing conjugated diene polymer is preferably 30% by weight or more, more preferably 45% by weight or more, and even more preferably 56% by weight or more, and is preferably 80% by weight or less, more preferably 75% by weight or less, and even more preferably 71% by weight or less, based on 100% by weight of all monomer units contained in the hydrophilic group-containing conjugated diene polymer. If the content of aromatic vinyl monomer units in the hydrophilic group-containing conjugated diene polymer is within the above-mentioned range, the adhesiveness and flexibility of the formed electrode mixture layer can be further improved.

[0113] The hydrophilic group-containing acrylic polymer is a copolymer containing at least a (meth)acrylic acid ester monomer unit in addition to the above-mentioned hydrophilic group-containing monomer unit. The (meth)acrylic acid ester monomer unit refers to a unit having a structure formed by polymerizing a (meth)acrylic acid ester monomer. The term "(meth)acrylic acid ester monomer" includes an acrylic acid ester monomer, a (meth)acrylic acid ester monomer, and a combination thereof. The hydrophilic group-containing acrylic polymer preferably further contains a nitrile group-containing monomer unit or an aromatic vinyl monomer unit. The hydrophilic group-containing acrylic polymer may further contain a monomer unit other than the hydrophilic group-containing monomer unit, the (meth)acrylic acid ester monomer unit, the nitrile group-containing monomer unit, and the aromatic vinyl monomer unit.

[0114] The content ratio of the hydrophilic group-containing monomer unit in the hydrophilic group-containing acrylic polymer can be set to the same range as the preferred range of the content ratio of the hydrophilic group-containing monomer unit in the particulate polymer Y described above.

[0115] Examples of (meth)acrylic acid ester monomers that can form (meth)acrylic acid ester monomer units in the hydrophilic group-containing acrylic polymer include (meth)acrylic acid alkyl esters such as methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate, and 2-ethylhexyl acrylate. These may be used alone or in any combination of two or more in any ratio.

[0116] Here, the content of the (meth)acrylic acid ester monomer units in the hydrophilic group-containing acrylic polymer is preferably 50% by weight or more, more preferably 60% by weight or more, and even more preferably 70% by weight or more, and preferably 90% by weight or less, more preferably 85% by weight or less, and even more preferably 80% by weight or less, when the total monomer units contained in the hydrophilic group-containing acrylic polymer is taken as 100% by weight. If the content of the (meth)acrylic acid ester monomer units in the hydrophilic group-containing acrylic polymer is within the above-mentioned specified range, the adhesion and flexibility of the formed electrode mixture layer can be further improved.

[0117] Examples of nitrile group-containing monomers capable of forming nitrile group-containing monomer units in the hydrophilic group-containing acrylic polymer include α,β-ethylenically unsaturated nitrile monomers. The α,β-ethylenically unsaturated nitrile monomer is not particularly limited as long as it is an α,β-ethylenically unsaturated compound having a nitrile group, and examples include acrylonitrile; α-halogenoacrylonitriles such as α-chloroacrylonitrile and α-bromoacrylonitrile; and α-alkylacrylonitriles such as methacrylonitrile and α-ethylacrylonitrile. Among these, acrylonitrile and methacrylonitrile are preferred as nitrile group-containing monomers, with acrylonitrile being more preferred. These may be used alone or in combination of two or more.

[0118] The content of the nitrile group-containing monomer units in the hydrophilic group-containing acrylic polymer is preferably 5% by weight or more, more preferably 10% by weight or more, and even more preferably 17% by weight or more, and is preferably 40% by weight or less, more preferably 35% by weight or less, and even more preferably 26% by weight or less, when the total monomer units contained in the hydrophilic group-containing acrylic polymer is taken as 100% by weight. If the content of the nitrile group-containing monomer units in the hydrophilic group-containing acrylic polymer is within the above-mentioned range, the adhesion and flexibility of the formed electrode mixture layer can be further improved.

[0119] As the aromatic vinyl monomer capable of forming the aromatic vinyl monomer unit in the hydrophilic group-containing acrylic polymer, the aromatic vinyl monomers described above in the section "Hydrophilic Group-Containing Conjugated Diene Polymer" can be used.

[0120] The content of aromatic vinyl monomer units in the hydrophilic group-containing acrylic polymer is preferably 5% by weight or more, more preferably 10% by weight or more, and even more preferably 17% by weight or more, and is preferably 40% by weight or less, more preferably 35% by weight or less, and even more preferably 25% by weight or less, when the total amount of all monomer units contained in the hydrophilic group-containing acrylic polymer is taken as 100% by weight.

[0121] The method for producing the particulate polymer Y is not particularly limited, and any method such as solution polymerization, suspension polymerization, bulk polymerization, or emulsion polymerization may be used. Furthermore, addition polymerization such as ionic polymerization, radical polymerization, or living radical polymerization may be used as the polymerization reaction. Commonly used emulsifiers, dispersants, polymerization initiators, chain transfer agents, and the like may be used in the polymerization, and the amounts used may be the amounts commonly used.

[0122] In addition to the particulate polymer Y described above, the water-insoluble polymer may include, for example, a polymer known as a water-insoluble polymer contained in the positive electrode mixture layer.

[0123] The content of the water-insoluble polymer in the binder is usually 40% by weight or more, preferably 45% by weight or more, and more preferably 50% by weight or more. The content of the water-insoluble polymer may be, for example, 100%. However, since the binder preferably contains at least a water-soluble polymer in this embodiment, the upper limit is preferably 60% by weight or less, and more preferably 55% by weight or less.

[0124] The binder liquid contains water. Water can usually function as a solvent or dispersion medium. More specifically, water can function as a solvent for the water-soluble polymer and the oxidizing agent. Furthermore, when a water-insoluble polymer is contained, water can function as a dispersion medium for the water-insoluble polymer. Water alone is usually added to the binder liquid as a dispersion medium, but it may also be added, for example, as an aqueous solution to which an arbitrary compound has been added, or as a mixed solvent of water and a small amount of an organic solvent.

[0125] The binder liquid contains at least the above-described binder, oxidizing agent, and water. The binder liquid may further contain optional components as needed. Examples of optional components include reinforcing materials, leveling agents, viscosity modifiers, and electrolyte additives. Known components may be used as these optional components.

[0126] 3. Slurry Composition A slurry composition according to one embodiment of the present invention contains the above-described positive electrode active material (A), a binder, and water.

[0127] According to the present embodiment, the slurry composition contains the positive electrode active material (A), which can suppress elution of transition metal elements from the positive electrode active material (A) into water as a dispersion medium, and therefore can produce a positive electrode that can improve the initial capacity of a secondary battery.

[0128] The content of the positive electrode active material (A) in the slurry composition is preferably 90% by weight or more and 99% by weight or more, where the total solid content of the slurry composition is 100% by weight. When the content of the positive electrode active material (A) is in this range, the energy density of the secondary battery can be improved and the dispersibility of the slurry composition can be increased.

[0129] The content of the binder in the slurry composition is not particularly limited, but the amount of binder relative to 100 parts by weight of the positive electrode active material (A) is preferably 0.5 parts by weight or more, more preferably 1 part by weight or more, even more preferably 1.5 parts by weight or more, and preferably 15 parts by weight or less, more preferably 10 parts by weight or less, even more preferably 7 parts by weight or less. When the content of the binder in the slurry composition is within the above range, a positive electrode mixture layer having good mechanical strength can be obtained.

[0130] The slurry composition contains water. Water can usually function as a solvent for the water-soluble polymer and the oxidizing agent. When a water-insoluble polymer is contained, water can also function as a dispersion medium for the water-insoluble polymer. Usually, only water is added to the slurry composition as a dispersion medium. However, for example, water may be added as an aqueous solution to which an arbitrary compound has been added, or as a mixed solvent of water and a small amount of an organic solvent.

[0131] The slurry composition may contain any component other than those described above. For example, the slurry composition may contain any component that the binder composition may contain. Typically, the slurry composition contains the same solvent as the binder composition. Furthermore, the optional components that the slurry composition may contain may include, for example, a dispersant, a viscosity modifier (thickener), a phosphite-based antioxidant, a metal capture agent, a conductive material, etc. One type of optional component may be used alone, or two or more types may be used in combination.

[0132] In this embodiment, the slurry composition may include an oxidizing agent.

[0133] The slurry composition is usually a liquid composition. It is preferable that this slurry composition has a viscosity within a specific range. The viscosity of the slurry composition is preferably 2000 mPa·s or more, more preferably 2500 mPa·s or more, even more preferably 3000 mPa·s or more, and preferably 8000 mPa·s or less, more preferably 7000 mPa·s or less, even more preferably 6000 mPa·s or less. The viscosity of the slurry composition can be measured using a Brookfield viscometer at a temperature of 25°C and a rotation speed of 60 rpm.

[0134] [4. Method for Producing Slurry Composition] The method for producing the slurry composition according to the present embodiment is not particularly limited, but is preferably a production method that includes step 1 of dispersing a positive electrode active material containing a transition metal element in a binder liquid containing a binder, an oxidizing agent, and water, and in which the temperature of the binder liquid in step 1 is 5°C or higher and 55°C or lower.

[0135] According to the present embodiment, by dispersing the positive electrode active material in the binder liquid within a predetermined temperature range in step 1, the surface of the positive electrode active material is oxidized, and a slurry composition containing the positive electrode active material having a good coating portion can be obtained.

[0136] The temperature of the binder liquid in step 1 is usually 5°C or higher, preferably 10°C or higher, and more preferably 15°C or higher. The temperature of the binder liquid is usually 55°C or lower, preferably 40°C or lower, and more preferably 30°C or lower. When the temperature of the binder liquid is equal to or higher than the lower limit, oxidation of the surface of the positive electrode active material by the oxidizing agent can be effectively promoted. When the temperature of the binder liquid is equal to or lower than the upper limit, peeling of the formed coating portion from the bulk portion can be suppressed.

[0137] The method for dispersing the positive electrode active material in the binder liquid is not particularly limited, and examples thereof include a method of mixing the binder liquid and the positive electrode active material using a mixer, such as a ball mill, a sand mill, a bead mill, a pigment disperser, a crusher, an ultrasonic disperser, a homogenizer, a planetary mixer, or a Filmix.

[0138] In step 1, an optional component may be further dispersed in addition to the positive electrode active material and the binder liquid. In this case, the optional component may be, for example, previously dispersed in a dispersion medium such as water to prepare a mixed liquid, and the binder liquid may be added to the mixed liquid and further mixed to be dispersed.

[0139] In this embodiment, for example, when producing a slurry composition containing a positive electrode active material (A), a water-soluble polymer and a water-insoluble polymer as binders, and water, it is preferable to include a step 2 in which, in step 1, the positive electrode active material is dispersed in a first binder liquid containing a water-soluble polymer, an oxidizing agent, and water, and in step 2, a mixed liquid of the positive electrode active material and the first binder liquid is mixed with a second binder liquid containing a water-insoluble polymer. In this case, the temperature of the mixed liquid in step 2 is preferably 5° C. or higher and 55° C. or lower, similar to the temperature of the binder liquid in step 1 described above.

[0140] In this embodiment, from the viewpoint of enhancing the reaction between the positive electrode active material (B) and the oxidizing agent and the dispersibility of the positive electrode active material (A), the solid content may be lowered by further mixing water after kneading at a relatively low solid content in step 1. When step 2 is included, the solid content may be lowered by further mixing water after kneading at a relatively low solid content in steps 1 and 2.

[0141] 5. Positive electrode for non-aqueous secondary battery A positive electrode according to one embodiment of the present invention (positive electrode for non-aqueous secondary battery) has a current collector layer and a positive electrode mixture layer provided on the current collector layer and containing a positive electrode active material and a binder, and the positive electrode active material is the positive electrode active material of the present invention.

[0142] According to the present invention, since the positive electrode contains the above-mentioned positive electrode active material, it is possible to provide a positive electrode that can improve the initial capacity of a non-aqueous secondary battery.

[0143] The current collector layer is made of a material that is electrically conductive and electrochemically durable, and is preferably made of aluminum or an aluminum alloy.

[0144] The positive electrode mixture layer typically includes the positive electrode mixture layer and a binder. The positive electrode mixture layer may also include optional components such as a conductive material, as needed. The positive electrode mixture layer typically includes components other than the solvent contained in the slurry composition in the same content ratio as in the slurry composition.

[0145] The positive electrode may have a conductive adhesive layer between the current collector layer and the positive electrode composite layer, if necessary. This is because it can improve the adhesion between the current collector layer and the positive electrode composite layer. The conductive adhesive layer usually contains a conductive material and a polymer as an adhesive component. The conductive material and polymer may be any known material that can be used in battery components.

[0146] The positive electrode mixture layer can be produced by applying the above-described slurry composition to the current collector layer and drying it. The method for applying and drying the slurry composition can be the same as known methods.

[0147] The amount of the electrode mixture layer per unit area is sometimes referred to as the "basis weight." There is no limitation on the basis weight of the electrode mixture layer, but in one example, it is usually 1 mg / cm ２ or more, preferably 2 mg / cm ２ More preferably, 5 mg / cm ２ or more, usually 100 mg / cm ２ Preferably 50 mg / cm or less ２ or less, more preferably 30 mg / cm ２ The following is the result.

[0148] 6. Nonaqueous Secondary Battery A secondary battery (non-aqueous secondary battery) according to one embodiment of the present invention includes the positive electrode, negative electrode, and electrolyte solution described above. The secondary battery may further include a separator.

[0149] According to this embodiment, the positive electrode described above can improve the initial capacity. Furthermore, a nonaqueous secondary battery having the positive electrode described above can usually improve the cycle characteristics, which can contribute to extending the life of the battery.

[0150] The negative electrode may be a known negative electrode used as a negative electrode for a secondary battery. Specifically, the negative electrode may be, for example, a negative electrode including a current collector and a negative electrode composite layer formed on the current collector. The negative electrode current collector may be, for example, one formed of a metal material such as copper. The negative electrode composite layer may be a layer containing a known negative electrode active material, a conductive material, and a binder, and a known polymer may be used as the binder.

[0151] Preferred examples of the negative electrode active material include, for example, carbon powder (amorphous carbon), silica (SiO ｘ , 0<x<2), titanium composite oxide (Li ４ Ti ５ O ７ , TiO ２ , Nb ２ TiO ７ Examples of the carbon powder include tin composite oxide, lithium alloy, metallic lithium, etc., and one or more of these can be used. As the carbon powder, one or more of soft carbon (easily graphitizable carbon), hard carbon (hardly graphitizable carbon), and graphite can be used.

[0152] When the nonaqueous secondary battery is a lithium ion secondary battery, the electrolyte solution can be an electrolyte solution prepared by dissolving an electrolyte in a solvent. Here, an organic solvent capable of dissolving the electrolyte can be used as the solvent. Specifically, the solvent can be an alkyl carbonate solvent such as ethylene carbonate, propylene carbonate, or γ-butyrolactone to which a viscosity-adjusting solvent such as 2,5-dimethyltetrahydrofuran, tetrahydrofuran, diethyl carbonate, ethyl methyl carbonate, dimethyl carbonate, methyl acetate, dimethoxyethane, dioxolane, methyl propionate, or methyl formate has been added. A lithium salt can be used as the electrolyte. For example, the lithium salts described in JP 2012-204303 A can be used. Among these lithium salts, LiPF 6 is preferred as the electrolyte because it is easily soluble in organic solvents and exhibits a high degree of dissociation. ６ , LiClO ４ , C.F. ３ SO ３ Li is preferred.

[0153] Furthermore, the nonaqueous secondary battery may further include a separator as needed. Examples of separators that can be used include those described in JP 2012-204303 A. Among these, a microporous film made of a polyolefin resin (polyethylene, polypropylene, polybutene, polyvinyl chloride) is preferred because it allows the overall separator thickness to be thin, thereby increasing the proportion of electrode active material in the lithium ion secondary battery and increasing capacity per volume.

[0154] A nonaqueous secondary battery can be manufactured, for example, by stacking a positive electrode and a negative electrode with a separator interposed therebetween, rolling or folding the stack as necessary according to the battery shape, placing the stack in a battery container, injecting an electrolyte into the battery container, and sealing the battery container. To prevent internal pressure rise and overcharging / discharging, etc., a fuse, an overcurrent prevention element such as a PTC element, an expanded metal, a lead plate, etc. may be provided as necessary. The shape of the secondary battery may be any type, such as a coin type, a button type, a sheet type, a cylindrical type, a rectangular type, or a flat type.

[0155] The present invention will be described in detail below with reference to examples. However, the present invention is not limited to the examples shown below, and can be practiced with any modifications within the scope of the claims of the present invention and their equivalents.

[0156] In the following description, the "%" and "parts" that represent amounts are by weight unless otherwise specified. Furthermore, the operations described below were carried out under normal temperature and pressure conditions unless otherwise specified.

[0157] [Evaluation Method] <Amounts of Leached P and Fe Elements from Positive Electrode Active Material> For the measurement sample, the positive electrode active material and oxidizer in the same proportions by weight as those described in each Example and Comparative Example were mixed with 1,000 parts by weight of water, left to stand at room temperature for 48 hours, and then separated by ultracentrifugation at 50,000 rpm. The aqueous solution portion was removed, diluted with water to a predetermined volume, and an acid was added to prepare a solution. The amounts of eluted iron (Fe) and phosphorus (P) were quantified by inductively coupled plasma atomic emission spectrometry (ICP-AES).

[0158] <Initial Capacity Measurement> The lithium ion secondary batteries fabricated in the Examples and Comparative Examples were left standing at 25°C for 5 hours after injection of the electrolyte. Next, the lithium ion secondary batteries were subjected to an aging treatment according to the following procedure. First, the batteries were charged to a state of charge (SOC) of 17% at 25°C using a constant current method at 0.1 C, and then left standing at 45°C for 12 hours. Next, the batteries were charged to a cell voltage of 3.6 V using a constant current method at 0.2 C at 45°C, and then discharged to a cell voltage of 2.6 V. Next, the lithium ion secondary batteries were subjected to a conditioning treatment according to the following procedure. The batteries were charged to a cell voltage of 3.6 V using a constant current method at 0.2 C at 25°C, and then discharged to a cell voltage of 2.6 V. This charge-discharge cycle was repeated twice. The initial capacity of the lithium ion secondary batteries that had undergone the conditioning treatment was measured.

[0159] <Internal Resistance (Initial Resistance) Measurement> The initial resistance of the conditioned lithium-ion secondary battery was measured using the following method. First, the lithium-ion secondary battery was charged to a charge level of 50% SOC. Next, pulse discharge was performed for 10 seconds at a C rate of 0.2 C, and the voltage at 10 seconds of discharge was measured. Then, with the horizontal axis representing current and the vertical axis representing voltage, the voltage at 10 seconds of discharge was plotted against the current at 0.2 C. Next, after leaving the battery for 5 minutes, supplementary charging was performed to restore the SOC to 50%, and the battery was then left for another 5 minutes. Next, the above operation was performed for C rates of 0.5 C, 1.0 C, and 2.0 C, and the voltage at 10 seconds of discharge against the current at each C rate was plotted. The slope of the approximate line obtained from each plot using the least squares method was taken as the internal resistance (Ω) of the lithium-ion secondary battery obtained in this example.

[0160] <Cycle Characteristics> The lithium ion secondary batteries produced in the Examples and Comparative Examples were subjected to 100 cycles of charging to 3.6 V by a constant current method of 1.0 C in an atmosphere of 45° C. and discharging to 2.6 V. Then, the cycle capacity retention rate, which is expressed as the ratio of the electric capacity at the end of 100 cycles to the initial electric capacity (= [electric capacity at the end of cycles] / [initial electric capacity]×100(%)), was determined.

[0161] <Evaluation of the presence or absence of a coating portion and confirmation of elements in the coating portion> For the positive electrodes of Examples 3 and 5 and the comparative example, those that were not charged / discharged after production (positive electrodes without charging / discharging) and those that were subjected to the above-mentioned aging treatment and then removed from the secondary battery (positive electrodes after aging treatment) were analyzed, and the positive electrode active material (LiFePO coated with a carbon coating layer) was identified from the STEM observation images. ４ The presence or absence of a coating portion was confirmed for each of the positive electrodes (particles). In Comparative Example 1, no coating portion of the positive electrode active material was observed for either the positive electrode without charge / discharge or the positive electrode after aging treatment. On the other hand, in Examples 3 and 5, a positive electrode active material having a two-layer coating portion was observed for the positive electrode without charge / discharge, and a positive electrode active material having a single-layer coating portion was observed for the positive electrode after aging treatment. The STEM measurement sample was prepared by thinning the positive electrode using a focused ion beam (FIB) under conditions not exposed to the atmosphere. The method for preparing the measurement sample will be described in detail in the section on the method for measuring EELS spectra. In addition, an analytical electron microscope ARM200F manufactured by JEOL Ltd. was used as the STEM device.

[0162] STEM-EDX analysis was performed on the uncharged positive electrodes of Examples 3 and 5, confirming that iron, phosphorus, and oxygen were contained in both of the two-layer coatings of the positive electrode active material. Furthermore, it was confirmed that the layer on the bulk side of the two-layer coating contained relatively little iron and more phosphorus and oxygen, while the outer layer contained relatively little phosphorus and more iron and oxygen. The measurement device used was a combination of the above-mentioned STEM device and a JEOL JED-2300T EDX detector. The EDX mapping resolution was 256 x 256 pixels.

[0163] <Measurement of EELS spectrum and measurement of thickness of coating portion> The positive electrode active material (LiFePO coated with a carbon coating layer) in the positive electrodes of Examples 3 and 5 and Comparative Example was ４STEM-EELS line analysis was performed on the particles (positive electrode active material (X) described later), and the EELS spectrum was measured. Furthermore, the thickness of the coating portion was determined from the change in the valence of the iron element in the EELS spectrum. First, as a measurement sample, a thin-film sample was prepared by flaking the positive electrode active material in the positive electrode by the lift-out method. Specifically, the positive electrode active material in the positive electrode was flaked using a focused ion beam under conditions where it was not exposed to the atmosphere. A carbon protective film was formed on this positive electrode active material as a protective film to protect the particles from the focused ion beam, and then the flaking was performed. Next, the obtained thin-film sample was fixed to a copper mesh support, and the surface of the positive electrode active material particle was identified by observation with a scanning transmission electron microscope (STEM). Next, EELS measurement was performed within a range of 50 nm deep from a measurement starting point outside the surface near the surface of the identified particle toward the center of the particle, and an EELS spectrum of the energy loss value versus distance from the surface of the particle was obtained. In the EELS spectrum, the peak near 710 eV corresponds to the peak of divalent iron, and the peak near 712 eV corresponds to the peak of trivalent iron.

[0164] More specifically, the EELS spectrum of the energy loss value versus distance from the particle surface was obtained by the following procedure: First, EELS measurement was performed on a thin sample as described above, and the L of Fe was measured in a range of 50 nm deep from the measurement start point located near the particle surface but outside the surface, which was identified by STEM observation, toward the center of the particle. ３ The distribution (raw data) of loss energy corresponding to the peak was measured. Next, the distribution of loss energy obtained by the measurement was corrected to remove the background at each measurement point using TEM image analysis software ("Digital Micrograph" manufactured by Gatan Corporation), and Gaussian fitting was further performed on the distribution to obtain the energy loss value as the peak value of the loss energy at that measurement point. Then, the spectrum of the energy loss value versus distance from the measurement start point was obtained as the target EELS spectrum.

[0165] The EELS spectra of the positive electrode active material obtained by the above measurement are shown in FIGS. 1 to 4. FIGS. 1 and 2 are EELS spectra obtained by STEM-EELS line analysis of the positive electrode active material in the positive electrode of Comparative Example 1, which was not charged / discharged and after aging treatment. FIGS. 3 and 4 are EELS spectra obtained by STEM-EELS line analysis of the positive electrode active material in the positive electrode of Example 3, which was not charged / discharged and after aging treatment. The horizontal axis of the EELS spectra in FIGS. 1 to 4 represents the distance measured from the measurement start point, which is located near the surface of the particles of the positive electrode active material in the thin sample but outside the surface, to the center of the particle of the positive electrode active material being measured, with the surface of the particle being indicated by "s". The energy loss values ​​outside the particle surface s in the EELS spectra in FIGS. 1 to 4 correspond to a carbon protective film against a focused ion beam or a carbon coating layer covering the surface of the particles of the positive electrode active material.

[0166] As shown in FIGS. 1 and 2, in Comparative Example 1, for both the positive electrode without charge / discharge and the positive electrode after aging treatment, it was confirmed that the peak position of the measured EELS spectrum of the positive electrode active material did not change from around 710 eV, and STEM-EELS line analysis also confirmed that no coating portion was formed. Specifically, in the EELS spectrum shown in FIG. 1, it was confirmed that the energy loss value did not change from around 710 eV in the range from the surface s of the positive electrode active material particle to 10.05 nm and in the range closer to the center of the particle than 10.05 nm. Furthermore, in the EELS spectrum shown in FIG. 2, it was confirmed that the energy loss value did not change from around 710 eV in the range from the surface s of the positive electrode active material particle to 4.45 nm and in the range closer to the center of the particle than 4.45 nm. The range from the surface s of the particle to 10.05 nm in FIG. 1 and the range from the surface s of the particle to 4.45 nm in FIG. 2 are ranges that may include the ranges in which changes in energy loss values ​​occurred in Examples 3 and 5. In Comparative Example 1, no change occurred in the energy loss value within the predetermined range from the surface of the particle, which confirmed that no coating portion was formed.

[0167] On the other hand, as shown in Figure 3, in the EELS spectrum of the positive electrode active material without charge and discharge in Example 3, a peak near 711 eV was observed in the range from the surface to 7.2 nm (measured value in Figure 3: 7.15 nm), and a peak near 710 eV was observed in the range from 7.2 nm to 37.5 nm (position 50 nm deep from the measurement start point). Since a peak shift of +1 eV occurred at a point 7.2 nm from the surface based on the bulk portion, this point was designated as point P, and the distance from the surface to point P, i.e., 7.2 nm, was designated as the thickness of the coating portion.

[0168] As shown in FIG. 3, the EELS spectrum of the positive electrode active material without charge / discharge shows an average value E of the energy loss value in the range of 0 nm to 7.2 nm deep from the surface s of the positive electrode active material particles. Ｏ is the average value E of the energy loss value in the range of 10 nm to 20 nm deep from the surface s of the particle of the positive electrode active material. Ｉ It was confirmed that the difference was 0.5 eV or more larger than that in the case of

[0169] 4, in Example 3, in the EELS spectrum of the positive electrode active material after the aging treatment, a peak near 711 eV was observed in the range from the surface to 3.5 nm (measured value in FIG. 4: 3.52 nm), and a peak near 710 eV was observed in the range thereafter (a peak shift of +1 eV was observed at a point 3.5 nm from the surface, with the bulk portion as the reference). Based on this result, the thickness of the coating portion of the positive electrode active material after the aging treatment was determined to be 3.5 nm.

[0170] As shown in FIG. 4, the EELS spectrum of the positive electrode active material after the aging treatment shows that the average value E Ｏ is the average value E of the energy loss value in the range of 10 nm to 20 nm deep from the surface s of the particle of the positive electrode active material. Ｉ It was confirmed that the difference was 0.5 eV or more larger than that in the case of

[0171] Also, although not shown, in Example 5, in the EELS spectrum of the positive electrode active material without charge and discharge, a peak shift of +1 eV was observed at a point 7.0 nm from the surface based on the bulk portion, and in the EELS spectrum of the positive electrode active material after aging treatment, a peak shift of +1 eV was observed at a point 3.0 nm from the surface based on the bulk portion. In the EELS spectrum of Example 5, as in Example 3, a peak near 710 eV was confirmed in the range from the bulk portion to the point where the peak shift occurred, and a peak near 711 eV was confirmed in the range from the point where the peak shift occurred to the surface of the particle. From these results, in Example 5, the thickness of the coating portion of the positive electrode active material without charge and discharge was set to 7.0 nm, and the thickness of the coating portion of the positive electrode active material after aging treatment was set to 3.0 nm.

[0172] In Example 5, the EELS spectrum of the positive electrode active material without charge and discharge also showed an average value E of the energy loss value in the range of 0 nm to 7.0 nm deep from the surface of the particle of the positive electrode active material. Ｏ is the average value E of the energy loss value in the range of 10 nm to 20 nm deep from the surface of the particle of the positive electrode active material. Ｉ Furthermore, it was confirmed that the average value E of the energy loss in the range of 0 nm to 3.0 nm deep from the surface s of the particles of the positive electrode active material was 0.5 eV or more larger than that in the EELS spectrum of the positive electrode active material after the aging treatment. Ｏ is the average value E of the energy loss value in the range of 10 nm to 20 nm deep from the surface s of the particle of the positive electrode active material. Ｉ It was confirmed that the difference was 0.5 eV or more larger than that in the case of

[0173] The measurement device used was a combination of the above-mentioned STEM device and a GIF Quantum EELS spectrometer manufactured by GATAN. ​​The EELS line analysis resolution was 100 points / 50 nm, and the EELS dispersion was 0.25 eV / channel.

[0174] The above-mentioned STEM imaging, STEM-EDX analysis, and STEM-EELS line analysis were all performed using an electron beam with an acceleration voltage of 200 kV.

[0175] [Production Example 1: Synthesis of Water-Soluble Polymer I] 770 parts of ion-exchanged water was placed in a 10-L flask equipped with a septum and heated to 40°C. The atmosphere in the flask was purged with nitrogen gas at a flow rate of 100 mL / min. Next, 8 parts of acrylic acid as the ethylenically unsaturated carboxylic acid monomer, 8 parts of 2-methoxyethyl acrylate as the unsaturated carboxylic acid ester monomer, 9 parts of 2-hydroxyethyl methacrylate (HEMA) as the hydroxyl group-containing methacrylic acid ester monomer, and 75 parts of 2-hydroxyethyl acrylate (β-HEA) as the hydroxyl group-containing acrylic acid ester monomer were mixed and injected into the flask with a syringe. Thereafter, 3 parts of a 10.0% aqueous solution of sodium L-ascorbate as a polymerization accelerator were added with a syringe, and 10 minutes later, 38 parts of a 4.0% aqueous solution of potassium persulfate as a polymerization initiator were added to the flask with a syringe. One hour after the start of the reaction, the temperature was raised to 55°C, and the polymerization reaction was allowed to proceed. After 2 hours, 0.5 parts of a 10.0% aqueous solution of sodium bisulfite as a polymerization accelerator was added using a syringe, and 10 minutes later, 5 parts of a 4.0% aqueous solution of potassium persulfate as a polymerization initiator was added using a syringe to the flask. After 3 hours, 0.5 parts of a 10.0% aqueous solution of sodium bisulfite as a polymerization accelerator was added using a syringe, and 10 minutes later, 5 parts of a 4.0% aqueous solution of potassium persulfate as a polymerization initiator was added using a syringe to the flask. After 4 hours, 0.5 parts of a 10.0% aqueous solution of sodium bisulfite as a polymerization accelerator was added using a syringe, and 10 minutes later, 5 parts of a 4.0% aqueous solution of potassium persulfate as a polymerization initiator was added using a syringe to the flask. After 5 hours, 0.5 parts of a 10.0% aqueous solution of sodium bisulfite as a polymerization accelerator was added using a syringe, and 10 minutes later, 5 parts of a 4.0% aqueous solution of potassium persulfate as a polymerization initiator was added using a syringe to the flask. After 6 hours, a reaction terminator was added, and the flask was opened to the air to terminate the polymerization reaction. Thereafter, the pH of the product was adjusted to pH 8 using an 8% aqueous solution of lithium hydroxide, thereby obtaining a water-soluble polymer I as a binder composition.

[0176] [Production Example 2: Synthesis of Particulate Polymer I] 90 parts of ion-exchanged water and 0.5 parts of sodium lauryl sulfate as an emulsifier were added to a 1 L septum-equipped flask (reaction vessel) equipped with a stirrer. The gas phase was replaced with nitrogen gas, and the temperature was raised to 60°C. Then, 0.3 parts of ammonium persulfate (APS) as a polymerization initiator was dissolved in 20.0 parts of ion-exchanged water and added. Meanwhile, in a separate vessel (emulsion vessel), 30 parts of ion-exchanged water, 0.5 parts of sodium lauryl sulfate as an emulsifier, 78.0 parts of butyl acrylate, 2.5 parts of itaconic acid, 1.0 part of 2-hydroxyethyl acrylate, and 18.5 parts of styrene were mixed to obtain a monomer composition. This monomer composition was continuously added to the 1 L septum-equipped flask over a period of 3 hours to carry out polymerization. The reaction was carried out at 60°C during the addition. After completion of the addition, the mixture was further stirred at 80°C for 2 hours to obtain an aqueous dispersion of Particulate Polymer I. A 5.0% aqueous solution of sodium hydroxide was added to the aqueous dispersion of particulate polymer I thus obtained to adjust the pH to 8, thereby obtaining an aqueous dispersion of particulate polymer I. The obtained particulate polymer I was water-insoluble according to the definition of this specification.

[0177] Hereinafter, the procedures for the positive electrode slurry composition, the positive electrode for a lithium ion secondary battery, and the method for producing a lithium ion secondary battery in Examples 1 to 5 and Comparative Example 1 will be described. In all of Examples 1 to 5 and Comparative Example 1, the positive electrode active material was olivine-type lithium iron phosphate particles (LiFePO ) coated with a carbon coating layer (carbon film). ４ Particles (median diameter of particles including a carbon coating layer: 1 μm) were used. Hereinafter, this material will be referred to as positive electrode active material (X) in the following description.

[0178] [Example 1] <Preparation of a slurry composition for a positive electrode> 96.5 parts of the positive electrode active material (X) and 1.5 parts of acetylene black as a conductive additive were mixed in a planetary mixer (15 rpm, 15 minutes). Then, 0.85 parts (solid content equivalent) of an aqueous solution of the water-soluble polymer I as a binder and LiNO as an oxidizing agent were added. ２A mixture (binder solution) consisting of 0.3 parts of an aqueous solution (equivalent to the solid content) and ion-exchanged water was added, and the solid content concentration was adjusted to 67%, followed by mixing with a planetary mixer (40 rpm, 4 minutes). The maximum temperature indicated by the planetary mixer was 25°C. Next, 0.85 parts of an aqueous dispersion of particulate polymer I (equivalent to the solid content) was added, and the mixture was mixed with a planetary mixer (40 rpm, 4 minutes) to prepare a positive electrode slurry composition. The maximum temperature indicated by the planetary mixer was 25°C. Finally, water was added so that the viscosity of the positive electrode slurry composition was 2000 mPa s or less, thereby adjusting the viscosity of the positive electrode slurry composition. During the preparation of the positive electrode slurry composition, the viscosity of the resulting slurry composition was measured using a single cylindrical rotational viscometer (Brookfield Type B viscometer) in accordance with JIS Z8803:1991. The measurement temperature was 25° C., and the rotation speed of the viscometer was 60 rpm.

[0179] <Preparation of a Current Collector with a Conductive Adhesive Layer> A current collector with a conductive adhesive layer prepared by the following method was used as the current collector for manufacturing the positive electrode. 70 parts of conductive carbon (graphite / acetylene black = 70 / 30 (weight ratio)) and 8 parts of carboxymethyl cellulose (Daicel Corporation's "Daicel 1220") as a dispersant were added and stirred using a disperser at 3000 rpm for 60 minutes. Then, 22 parts (solids equivalent) of an aqueous dispersion of particulate polymer I was added and further stirred using a disperser at 1500 rpm for 10 minutes to prepare a conductive adhesive. The conductive adhesive was applied to aluminum foil as a current collector substrate using a casting method with a roll bar at a molding speed of 20 m / min and dried at 80°C to form a 1 μm-thick conductive adhesive layer. This resulted in a current collector with a conductive adhesive layer formed on the current collector substrate.

[0180] <Preparation of Positive Electrode for Lithium Ion Secondary Battery> The positive electrode slurry composition obtained as described above was applied to the conductive adhesive layer side of the current collector with the conductive adhesive layer using a comma coater so that the coating weight after drying was 20 mg / cm. ２The coating was applied so that the density became 2.5 g / cm 3 , and the coating was dried at 90°C for 20 minutes and then at 120°C for 20 minutes, followed by heat treatment at 60°C for 10 hours to obtain a positive electrode blank. This positive electrode blank was rolled using a roll press to obtain a positive electrode blank having a density of 2.5 g / cm 3 . ３ A sheet-shaped positive electrode was fabricated, which consisted of the positive electrode mixture layer, a conductive adhesive layer, and aluminum foil.

[0181] <Preparation of a negative electrode for a lithium ion secondary battery> 65 parts of styrene as an aromatic vinyl monomer, 35 parts of 1,3-butadiene as an aliphatic conjugated diene monomer, 2 parts of itaconic acid as an ethylenically unsaturated carboxylic acid monomer, 1 part of 2-hydroxyethyl acrylate as a hydroxyl group-containing monomer, 0.3 parts of t-dodecyl mercaptan as a molecular weight modifier, 5 parts of sodium dodecylbenzenesulfonate as an emulsifier, 150 parts of ion-exchanged water as a solvent, and 1 part of potassium persulfate as a polymerization initiator were placed in a 5 MPa pressure vessel equipped with a stirrer, and after thorough stirring, the mixture was heated to 55°C to initiate polymerization. The reaction was stopped by cooling when the monomer consumption reached 95.0%. A 5% aqueous sodium hydroxide solution was added to the aqueous dispersion containing the polymer obtained in this way, and the pH was adjusted to 8. Subsequently, unreacted monomer was removed by heated vacuum distillation. The mixture was then cooled to 30°C or below to obtain an aqueous dispersion of particulate polymer II as a binder for the negative electrode. 48.75 parts of artificial graphite and 48.75 parts of natural graphite as negative electrode active materials, and 1 part of carboxymethyl cellulose as a thickener (solid content equivalent) were added to a planetary mixer. The mixture was then diluted with ion-exchanged water to a solid content of 60%, and then kneaded for 60 minutes at a rotation speed of 45 rpm. Then, 1.5 parts of the aqueous dispersion of particulate polymer II as a negative electrode binder obtained as described above (solid content equivalent) was added, and the mixture was kneaded for 40 minutes at a rotation speed of 40 rpm. Ion-exchanged water was then added to the mixture to a viscosity of 3000±500 mPa·s (measured with a Brookfield viscometer at 25°C and 60 rpm), thereby preparing a negative electrode slurry composition. The above negative electrode slurry composition was applied to the surface of a 15 μm-thick copper foil current collector using a comma coater in a coating amount of 10.0±0.5 mg / cm. ２The copper foil coated with the negative electrode slurry composition was then transported at a speed of 400 mm / min through an oven at 80°C for 2 minutes and then through an oven at 110°C for 2 minutes to dry the negative electrode slurry composition on the copper foil, thereby obtaining a negative electrode blank having a negative electrode composite layer formed on the current collector. This negative electrode blank was rolled using a roll press to obtain a negative electrode blank having a density of 1.6 g / cm 3 A sheet-shaped negative electrode was fabricated comprising the negative electrode mixture layer and copper foil.

[0182] <Preparation of Separator> A single-layer polypropylene separator (width 65 mm, length 500 mm, thickness 25 μm; manufactured by a dry method; porosity 55%) was prepared. This separator was cut into a 3.5 cm × 4.5 cm square and used in the following lithium-ion secondary battery.

[0183] <Preparation of Secondary Battery> An aluminum packaging material was prepared as the battery packaging. The positive electrode was cut into a 2.8 cm x 3.8 cm square and placed so that the surface on the current collector side was in contact with the aluminum packaging material. Next, the square separator was placed on the surface of the positive electrode composite layer of the positive electrode. Furthermore, the negative electrode was cut into a 3.0 cm x 4.0 cm square and placed on the separator so that the surface on the negative electrode composite layer side faced the separator. Then, 1.0 M LiPF 6 was used as an electrolyte. ６ The solution (solvent was a mixed solvent of ethylene carbonate (EC) / ethyl methyl carbonate (EMC) = 3 / 7) was filled with 2 volume % vinylene carbonate as an additive. Furthermore, in order to seal the opening of the aluminum packaging material, the aluminum packaging material was heat-sealed at 150°C to close the exterior, thereby producing a laminate cell type lithium ion secondary battery.

[0184] [Example 2] In the same manner as in Example 1, the positive electrode active material (X) and the conductive additive were mixed in a planetary mixer. Next, in the same manner as in Example 1, a binder liquid was added, and the mixture was mixed in a planetary mixer (40 rpm, 5 minutes). The maximum temperature indicated by the planetary mixer was 25°C. Next, 0.85 parts (equivalent to the solid content) of an aqueous dispersion of particulate polymer I was added, and the mixture was mixed in a Filmix (peripheral speed 20 m / s, 30 seconds) to prepare a positive electrode slurry composition. The maximum temperature indicated by the Filmix was 50°C. Finally, the viscosity of the slurry composition was adjusted in the same manner as in Example 1. A lithium ion secondary battery was produced in the same manner as in Example 1, except that the positive electrode slurry composition obtained by the above procedure was used instead of the positive electrode slurry composition of Example 1.

[0185] [Example 3] 96.1 parts of the positive electrode active material (X) and 1.5 parts of acetylene black as a conductive additive were mixed in a planetary mixer (15 rpm, 15 minutes). Then, 0.9 parts (solid content) of an aqueous solution of the water-soluble polymer I as a binder composition and LiNO as an oxidizing agent were mixed in a planetary mixer (15 rpm, 15 minutes). ２ A mixture (binder solution) consisting of 0.6 parts of an aqueous solution (corresponding to the solid content) and ion-exchanged water was added, the solid content concentration was adjusted to 67%, and the mixture was mixed with a planetary mixer (40 rpm, 5 minutes). The maximum temperature indicated by the planetary mixer was 20°C. Next, 0.9 parts of an aqueous dispersion of particulate polymer I (corresponding to the solid content) was added, and the mixture was mixed with a planetary mixer (40 rpm, 5 minutes) to prepare a positive electrode slurry composition. The maximum temperature indicated by the planetary mixer was 20°C. Finally, the viscosity of the slurry composition was adjusted in the same manner as in Example 1. A lithium ion secondary battery was produced in the same manner as in Example 1, except that the positive electrode slurry composition obtained by the above procedure was used instead of the positive electrode slurry composition of Example 1.

[0186] [Example 4] 96.4 parts of the positive electrode active material (X) and 1.5 parts of acetylene black as a conductive additive were mixed in a planetary mixer (15 rpm, 15 minutes). Then, 0.85 parts (solid content) of an aqueous solution of the water-soluble polymer I as a binder composition and LiNO as an oxidizing agent were mixed in a planetary mixer (15 rpm, 15 minutes). ３A mixture (binder solution) consisting of 0.4 parts of an aqueous solution (corresponding to the solid content) and ion-exchanged water was added, the solid content concentration was adjusted to 67%, and the mixture was mixed with a planetary mixer (40 rpm, 5 minutes). The maximum temperature indicated by the planetary mixer was 20°C. Next, 0.85 parts of an aqueous dispersion of particulate polymer I (corresponding to the solid content) was added, and the mixture was mixed with a planetary mixer (40 rpm, 5 minutes) to prepare a positive electrode slurry composition. The maximum temperature indicated by the planetary mixer was 20°C. Finally, the viscosity of the slurry composition was adjusted in the same manner as in Example 1. A lithium ion secondary battery was produced in the same manner as in Example 1, except that the positive electrode slurry composition obtained by the above procedure was used instead of the positive electrode slurry composition of Example 1.

[0187] [Example 5] 96.1 parts of the positive electrode active material (X) and 1.5 parts of acetylene black as a conductive additive were mixed in a planetary mixer (15 rpm, 15 minutes). Then, 0.9 parts (solid content) of an aqueous solution of the water-soluble polymer I as a binder composition and LiNO as an oxidizing agent were mixed in a planetary mixer (15 rpm, 15 minutes). ３ A mixture (binder solution) consisting of 0.6 parts of an aqueous solution (corresponding to the solid content) and ion-exchanged water was added, and the solid content concentration was adjusted to 67%, followed by mixing (40 rpm, 5 minutes) using a planetary mixer (Hivis Dispermix 3D-2 type) equipped with a cooling jacket. The maximum temperature indicated by the planetary mixer equipped with a cooling jacket was 10°C. Next, 0.9 parts (corresponding to the solid content) of an aqueous dispersion of particulate polymer I was added, and the mixture was mixed (40 rpm, 5 minutes) using a planetary mixer equipped with a cooling jacket to prepare a positive electrode slurry composition. The maximum temperature indicated by the Filmix was 10°C. Finally, the viscosity of the slurry composition was adjusted in the same manner as in Example 1. A lithium ion secondary battery was produced in the same manner as in Example 1, except that the positive electrode slurry composition obtained by the above procedure was used instead of the positive electrode slurry composition of Example 1.

[0188] Comparative Example 1 96.5 parts of the positive electrode active material (X) and 1.5 parts of acetylene black as a conductive additive were mixed in a planetary mixer (15 rpm, 15 minutes). Then, a mixture consisting of 1.00 parts (solid content equivalent) of an aqueous solution of water-soluble polymer I as a binder composition and ion-exchanged water was added, and the solid content concentration was adjusted to 75%, followed by mixing in a planetary mixer (60 rpm, 30 minutes). The maximum temperature indicated by the planetary mixer was 35°C. Next, a mixture consisting of 0.85 parts (solid content equivalent) of an aqueous dispersion of particulate polymer I and ion-exchanged water was added, and the mixture was mixed in a Filmix (peripheral speed 30 m / s, 1 minute) to prepare a positive electrode slurry composition. The maximum temperature indicated by the Filmix was 60°C. Finally, the viscosity of the slurry composition was adjusted in the same manner as in Example 1. A lithium ion secondary battery was produced in the same manner as in Example 1, except that the positive electrode slurry composition obtained by the above procedure was used instead of the positive electrode slurry composition of Example 1.

[0189] The evaluation results of Examples 1 to 5 and Comparative Example 1 are shown in the table below. The abbreviations in the table are as follows: LiFePO ４ : Olivine-type lithium iron phosphate particles (LiFePO) coated with a carbon coating layer (carbon film) ４ Particles) (median diameter of particles including a carbon coating layer: 1 μm), that is, positive electrode active material (X). AB: acetylene black LiNO ２ : Lithium nitrite LiNO ３ : lithium nitrate Dispersion condition 1: rotation speed and mixing time of the planetary mixer when mixing the positive electrode active material with the binder liquid Dispersion temperature 1: maximum temperature of the planetary mixer when mixing the positive electrode active material with the binder liquid Dispersion condition 2: rotation speed and mixing time of the planetary mixer when mixing with the aqueous dispersion of particulate polymer I in Examples 1 and 3 to 5, and peripheral speed and mixing time of the Filmix in Example 2 and Comparative Example 1 Dispersion temperature 2: maximum temperature of the planetary mixer when mixing with the aqueous dispersion of particulate polymer I in Examples 1 and 3 to 5, and maximum temperature of the Filmix in Example 2 and Comparative Example 1

[0190]

[0191] In Examples 1 to 5, which used an oxidizing agent, it was confirmed that the elution of P and Fe elements was suppressed, the decrease in initial capacity was suppressed, and the initial resistance value was low, compared to Comparative Example 1. Furthermore, from the results of STEM observation images and STEM-EELS line analysis of Examples 3 and 5 and Comparative Example 1, it was confirmed that a coating portion was provided on the surface of the positive electrode active material by using an oxidizing agent. Since the coating portion was confirmed even after the aging treatment, it is thought that the influence of the coating portion was included in the battery evaluation after aging, and it can be understood that the effect of the coating portion is one factor in obtaining the effects of the present invention.

[0192] Although it is not entirely clear, it is presumed that the decrease in initial capacity was suppressed in the examples for the following reasons. In Comparative Example 1, when lithium iron phosphate came into contact with water, iron element was converted into iron ions (Fe ２＋ ) and then Li ３ P.O. ４ It is believed that the initial capacity decreased because lithium was dissolved in water and lost from the active material. In contrast, in Examples 1 to 5, lithium iron phosphate (LiFeO ４ ) iron ions (Fe ２＋ ) reacts with the oxidizing agent nitrite ions NO on the surface of the positive electrode active material. ２ － (or nitrate ion NO ３ － ) by oxidation to Fe ２ O ３ or Fe ３ O ４ is generated, and a coating portion is formed. By forming the coating portion, iron and further Li are released from the bulk portion. ３ P.O. ４ It is presumed that the decrease in initial capacity can be suppressed because the dissolution of

[0193] Furthermore, it was confirmed that the initial resistance was also lower in Examples 1 to 5 than in Comparative Example 1. ２ O ３ and Fe ３ O ４Since iron itself does not exhibit electrical conductivity, it is presumed that in the coating, during the process of oxidation of iron elements, a new crystalline structure exhibiting high electrical conductivity containing iron elements with a valence greater than two is generated, and that the generation of this new crystalline structure is one of the factors contributing to the reduction in resistance.

Claims

1. A positive electrode active material containing a transition metal element, comprising: a bulk portion; and a coating portion provided on a surface of the bulk portion, wherein the valence of the transition metal element contained in the coating portion is greater than the valence of the transition metal element contained in the bulk portion.

2. The positive electrode active material according to claim 1, wherein the transition metal element is iron, the bulk portion contains divalent iron, and the coating portion contains iron with a valence greater than divalent.

3. The bulk portion is LiFePO ４ The coating portion comprises Fe ２ O ３ and Fe ３ O ４ The positive electrode active material according to claim 2 , comprising at least one of the following:

4. The positive electrode active material according to claim 1, wherein the thickness of the coating portion measured by STEM-EELS line analysis is 1 nm or more and 10 nm or less.

5. The positive electrode active material is particulate, and an EELS spectrum measured from the surface of a particle of the positive electrode active material to a depth of 20 nm from the center of the particle shows an average energy loss value E Ｏ The average value E of the energy loss value Ｏ is the average value E of the energy loss value in the range of depth 10 nm to 20 nm Ｉ The positive electrode active material according to claim 1 , wherein the positive electrode active material has a capacitance greater than the capacitance of the positive electrode by 0.5 eV or more.

6. A positive electrode active material containing a transition metal element, the positive electrode active material being in a particulate form, the EELS spectrum measured from the surface of a particle of the positive electrode active material to a depth of 20 nm from the center of the particle shows an average energy loss value E Ｐ The average value E of the energy loss value Ｐ is the average value E of the energy loss value in the range of depth 10 nm to 20 nm Ｑ A positive electrode active material having a capacitance greater than 0.5 eV or more.

7. A positive electrode for a non-aqueous secondary battery, comprising a current collector layer and a positive electrode mixture layer provided on the current collector layer and containing a positive electrode active material and a binder, the positive electrode active material being the positive electrode active material according to claim 1.

8. A non-aqueous secondary battery comprising a positive electrode, a negative electrode and an electrolyte, wherein the positive electrode is the positive electrode according to claim 7.

9. A slurry composition comprising the positive electrode active material according to claim 1, a binder, and water.

10. A method for producing a slurry composition according to claim 9, comprising step 1 of dispersing a positive electrode active material containing a transition metal element in a binder liquid containing the binder, an oxidizing agent, and water, wherein the temperature of the binder liquid in step 1 is 5°C or higher and 55°C or lower.

11. A binder liquid comprising a binder, an oxidizing agent for a transition metal element, and water.

12. The binder liquid according to claim 11, wherein the binder comprises a water-soluble polymer.

13. The binder liquid according to claim 11, wherein the oxidizing agent is a nitrogen compound.

14. The binder liquid according to claim 13, wherein the nitrogen compound includes at least one selected from the group consisting of nitrites, nitrates, amine compounds, and ammonium salts.

15. The binder liquid according to claim 11, wherein the oxidizing agent is a Li salt.

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