Positive electrode for secondary battery, method for manufacturing positive electrode for secondary battery, and nonaqueous electrolyte secondary battery

Abstract

A positive electrode for a secondary battery according to the present disclosure has a positive electrode current collector and a positive electrode mixture layer that is disposed on the surface of the positive electrode current collector. The positive electrode mixture layer contains a positive electrode active material and a positive electrode conductive agent. The positive electrode active material contains a lithium metal composite oxide that has a specific composition. The surface of the positive electrode active material is covered with a compound which contains at least one element that is selected from the group consisting of B, P, Mg, S, Ca, Sr, Ba, Ti, W, Zr, and Al. The positive electrode conductive agent contains carbon black and a carbon nanotube. The carbon black includes a first carbon black that has a diameter of 100 nm or less and an aspect ratio of 1.0 to 1.2 inclusive. The positive electrode mixture layer contains 50 or more first carbon blacks in a region that has an area of 10 µm2.

Description

Positive electrode for secondary battery, method for manufacturing positive electrode for secondary battery, and non-aqueous electrolyte secondary battery

[0001] The present invention relates to a positive electrode for a secondary battery, a method for manufacturing the positive electrode for a secondary battery, and a non-aqueous electrolyte secondary battery. More specifically, the present invention relates to a positive electrode for a secondary battery, a method for manufacturing the positive electrode for a secondary battery, and a non-aqueous electrolyte secondary battery including the positive electrode for a secondary battery.

[0002] Non-aqueous electrolyte secondary batteries, such as lithium ion secondary batteries, exhibit high power output and high energy density. Therefore, such non-aqueous electrolyte secondary batteries are used in small consumer applications, power storage devices, and as power sources for electric vehicles. The positive electrode and the positive electrode active material contained in the positive electrode, which are major components of non-aqueous electrolyte secondary batteries, have a significant impact on the high power output. Therefore, the positive electrode and the positive electrode active material have been extensively studied.

[0003] Patent Document 1 describes α-NaFeO ２ The present invention discloses a positive electrode active material for a non-aqueous electrolyte secondary battery, which has a structure in which an alkaline earth metal and W are present on the particle surface of a lithium transition metal composite oxide containing one or more transition metal elements selected from the group consisting of Mn, Ni, and Co.

[0004] Patent Document 2 discloses an electrode comprising a core and an active material layer provided on at least one surface of the core, the active material layer containing an active material, a binder, and a conductive agent, and the conductive agent containing agglomerates formed by entanglement of a plurality of fibrous carbon fibers and forming string-like agglomerates, and aggregates formed by aggregations of carbon materials smaller than the agglomerates. Patent Document 2 also discloses that in such an electrode, acetylene black is contained in the conductive agent, the length of the agglomerates is 30 μm or more, and the active material layer is formed within 1 mm ３ It is disclosed that 650 to 3000 agglomerates are present per cell, that the core is a positive electrode core, and that the active material layer is a positive electrode active material layer.

[0005] JP 2018-129221 A Patent No. 7168217 A

[0006] However, in non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries, there is a problem that the internal resistance of the positive electrode increases, making it difficult to maintain sufficient battery capacity. Therefore, the present disclosure provides a positive electrode for a secondary battery that can suppress the difficulty in maintaining sufficient battery capacity, and a non-aqueous electrolyte secondary battery including the positive electrode for the secondary battery. The present disclosure also provides a method for manufacturing the positive electrode for the secondary battery.

[0007] One aspect of the present invention is a positive electrode current collector, and a positive electrode mixture layer disposed on a surface of the positive electrode current collector, the positive electrode mixture layer including a positive electrode active material and a positive electrode conductive agent, the positive electrode active material having a composition formula of Li ｙ Ni ｘ M （１－ｘ） O ２ (wherein x≧0.5, 0<y≦1.2, and M contains at least one element selected from the group consisting of Co, Mn, Al, Fe, Ti, Sr, Ca, Zr, Nb, W, and B), a surface of the positive electrode active material is covered with a compound containing at least one element selected from the group consisting of B, P, Mg, S, Ca, Sr, Ba, Ti, W, Zr, and Al, the positive electrode conductive agent contains carbon black and carbon nanotubes, the carbon black contains first carbon black having a diameter of 100 nm or less and an aspect ratio of 1.0 to 1.2, and the positive electrode mixture layer has a thickness of 10 μm ２ The present invention relates to a positive electrode for a secondary battery, which contains 50 or more particles of the first carbon black within a region having an area of ​​100 μm or more.

[0008] Another aspect of the present disclosure relates to a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, wherein the positive electrode is the above-described positive electrode for a secondary battery.

[0009] Yet another aspect of the present disclosure provides a method for producing a cathode mixture slurry including a cathode active material, carbon black, and carbon nanotubes, and a cathode mixture layer forming step of forming a cathode mixture layer on a surface of a cathode current collector using the cathode mixture slurry, wherein the cathode mixture slurry producing step includes a first mixing substep of mixing the cathode active material and the carbon black to produce a first mixed powder, and a second mixing substep of mixing the first mixed powder with the carbon nanotubes to produce a second mixed powder, wherein the first mixing substep includes crushing carbon black to obtain a first carbon black having a diameter of 100 nm or less and an aspect ratio of 1.0 to 1.2, and the cathode active material is a powder having a composition formula of Li ｙ Ni ｘ M （１－ｘ） O ２ (wherein x≧0.5, 0<y≦1.2, and M contains at least one element selected from the group consisting of Co, Mn, Al, Fe, Ti, Sr, Ca, Zr, Nb, W, and B), and the surface of the positive electrode active material is covered with a compound containing at least one element selected from the group consisting of B, P, Mg, S, Ca, Sr, Ba, Ti, W, Zr, and Al.

[0010] According to the present disclosure, it is possible to provide a positive electrode for a secondary battery that can suppress difficulty in maintaining sufficient battery capacity, and a nonaqueous electrolyte secondary battery including the positive electrode for the secondary battery, and a method for manufacturing the positive electrode for the secondary battery.

[0011] 1 is a cross-sectional view schematically showing a lithium secondary battery according to a first embodiment.

[0012] In non-aqueous electrolyte secondary batteries, a composite oxide of lithium and a transition metal (lithium metal composite oxide) is used as a positive electrode active material. Such a lithium metal composite oxide includes lithium cobalt oxide (LiCoO ２) are known. However, the price of Co has been rising in recent years. In addition, since the number of countries that produce Co is limited, it is not easy to obtain a stable supply of Co. Therefore, development of Co-free composite oxides or composite oxides with a low Co content is underway. In the following, Co-free composite oxides and composite oxides with a low Co content are collectively referred to as "low Co composite oxides."

[0013] The low Co composite oxide may be, for example, a composite oxide having a composition formula of Li ｙ Ni ｘ M （１－ｘ） O ２ (wherein x≧0.5, 0<y≦1.2, and M contains at least one element selected from the group consisting of Co, Mn, Al, Fe, Ti, Sr, Ca, Zr, Nb, W, and B).

[0014] The first composite oxide tends to have an unstable crystal structure due to the extraction of lithium ions during charging, and tends to change to a structure that makes it difficult to reversibly store and release lithium ions. When a nonaqueous electrolyte secondary battery contains the first composite oxide exhibiting the above-described properties as a positive electrode active material, the battery capacity retention rate decreases. In particular, the battery capacity retention rate at high rates, in other words, the battery capacity retention rate during rapid charge and discharge, decreases significantly.

[0015] Therefore, in order to stabilize the crystal structure of the first composite oxide when lithium ions are extracted during charging, the surface of the first composite oxide may be covered with a compound containing at least one element selected from the group consisting of B, P, Mg, S, Ca, Sr, Ba, Ti, W, Zr, and Al.

[0016] By covering the surface with a compound containing the above-mentioned elements, the crystal structure of the first composite oxide is stabilized. That is, in non-aqueous electrolyte secondary batteries, the decrease in the battery capacity retention rate is suppressed to some extent. Hereinafter, a first composite oxide whose surface is covered with a compound containing the above-mentioned elements will be referred to as a "coated first composite oxide." On the other hand, in the case of a coated first composite oxide, the surface area of ​​the active portion present on the surface of the first composite oxide is reduced by covering the surface with the above-mentioned compound. Therefore, in a positive electrode containing the coated first composite oxide, the internal resistance increases due to the reduction in the area of ​​the active portion, and this increase in the internal resistance of the positive electrode can make it difficult to maintain sufficient battery capacity. This phenomenon is particularly noticeable during high-rate charging and discharging, in other words, during rapid charging and discharging.

[0017] As a result of extensive investigations by the present inventors, it has been found that in a non-aqueous electrolyte secondary battery using a positive electrode mixture layer containing a coated first composite oxide as a positive electrode active material, carbon black and carbon nanotubes are contained as a positive electrode conductive agent, the carbon black contains a first carbon black having a diameter of 100 nm or less and an aspect ratio of 1.0 to 1.2, and the positive electrode mixture layer has a thickness of 10 μm or less. ２ It has been found that by including 50 or more particles of the first carbon black within a region having an area of ​​1000 μm or more, it is possible to significantly prevent the battery capacity from becoming difficult to maintain sufficiently.

[0018] Below, embodiments of the present disclosure will be described using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values, materials, etc. may be applied as long as the effects of the present disclosure are obtained. Note that known components may be applied to components characteristic of the present disclosure. In this specification, when a "range from numerical value A to numerical value B" is mentioned, the range includes numerical value A and numerical value B.

[0019] In the following description, when lower and upper limits of numerical values ​​relating to specific physical properties, conditions, etc. are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit. When multiple materials are exemplified, one of them can be selected and used alone, or two or more can be used in combination, unless otherwise specified.

[0020] The present disclosure encompasses any combination of two or more features arbitrarily selected from the appended claims, i.e., any combination of two or more features arbitrarily selected from the appended claims can be combined unless a technical contradiction arises.

[0021] [Positive electrode for secondary battery] A positive electrode for secondary battery according to an embodiment of the present disclosure has a positive electrode current collector and a positive electrode mixture layer disposed on the surface of the positive electrode current collector, and the positive electrode mixture layer contains a positive electrode active material and a positive electrode conductive agent.

[0022] In the positive electrode for a secondary battery according to the embodiment of the present disclosure, the positive electrode active material has a composition formula of Li ｙ Ni ｘ M （１－ｘ） O ２ (wherein x≧0.5, 0<y≦1.2, and M contains at least one element selected from the group consisting of Co, Mn, Al, Fe, Ti, Sr, Ca, Zr, Nb, W, and B.) In a positive electrode for a secondary battery according to an embodiment of the present disclosure, the surface of the positive electrode active material is covered with a compound containing at least one element selected from the group consisting of B, P, Mg, S, Ca, Sr, Ba, Ti, W, Zr, and Al.

[0023] In a positive electrode for a secondary battery according to an embodiment of the present disclosure, the positive electrode conductive agent includes carbon black and carbon nanotubes. The carbon black includes a first carbon black having a diameter of 100 nm or less and an aspect ratio of 1.0 to 1.2. In a positive electrode for a secondary battery according to an embodiment of the present disclosure, the positive electrode mixture layer has a thickness of 10 μm. ２ The area includes 50 or more first carbon black particles.

[0024] In non-aqueous electrolyte secondary batteries, lithium ions are extracted from the positive electrode active material during charging. When the first composite oxide is used as the positive electrode active material, the crystal structure of the first composite oxide tends to become unstable as lithium ions are extracted, and the first composite oxide tends to change to a structure that makes it difficult to reversibly store and release lithium ions. In such cases, the battery capacity retention rate of the non-aqueous electrolyte secondary battery decreases.

[0025] Therefore, in order to stabilize the crystal structure of the first composite oxide when lithium ions are extracted during charging, the surface of the first composite oxide may be coated with, for example, a compound containing at least one element selected from the group consisting of B, P, Mg, S, Ca, Sr, Ba, Ti, W, Zr, and Al. In this case, even if lithium ions are extracted during charging, the crystal structure of the first composite oxide, more specifically, the crystal structure of the surface of the first composite oxide coated with the compound containing such an element, is stabilized. This suppresses, to some extent, the decrease in the battery capacity retention rate in nonaqueous electrolyte secondary batteries.

[0026] On the other hand, in the case of the coated first composite oxide, the surface is covered with a compound containing the above-mentioned elements, which reduces the area of ​​the active portion present on the surface of the first composite oxide. Therefore, in a positive electrode containing the coated first composite oxide, the reduction in the area of ​​the active portion causes an increase in internal resistance, and this increase in internal resistance in the positive electrode can make it difficult to maintain a sufficient battery capacity. This phenomenon is particularly noticeable during high-rate charge / discharge, in other words, during rapid charge / discharge.

[0027] When the positive electrode contains the above-described coated first composite oxide, it is possible to further add a positive electrode conductive agent such as carbon black to the positive electrode to suppress an increase in internal resistance in the positive electrode. Here, carbon black typically consists of multiple carbon fine particles with a particle diameter of about 20 nm connected together to form a structure with a length of about 2 μm. That is, carbon black typically has a high aspect ratio (the ratio of the length of the structure to the particle diameter of the carbon fine particles). Therefore, when carbon black with a structure is used as a positive electrode conductive agent, such carbon black comes into linear contact with the outer surface of the coated first composite oxide or with the positive electrode current collector. On the other hand, it is difficult to have carbon black with a structure present in the microvoids formed between adjacent coated first composite oxides. Therefore, it is difficult to attach carbon black with a structure to the surface of each coated first composite oxide in such microvoids.

[0028] However, in the positive electrode for a secondary battery according to an embodiment of the present disclosure, the positive electrode conductive agent contains carbon black and carbon nanotubes, and the carbon black contains first carbon black having a diameter of 100 nm or less and an aspect ratio of 1.0 to 1.2. Therefore, such first carbon black can be present in minute voids formed between adjacent coated first composite oxides. This allows the first carbon black to adhere to the surface of each coated first composite oxide in such minute voids. Furthermore, the positive electrode mixture layer has a thickness of 10 μm. ２The coated first composite oxide contains 50 or more particles of the first carbon black within a region having an area of ​​100 μm, i.e., a relatively large number of particles of the first carbon black within the microregion. This allows a sufficient amount of the first carbon black to be present in the microvoids formed between adjacent particles of the coated first composite oxide. This allows a sufficient amount of the first carbon black to be attached to the surface of each coated first composite oxide within the microvoids. Furthermore, in the positive electrode for a secondary battery according to the present disclosure, the positive electrode conductive agent contains carbon nanotubes. Carbon nanotubes have a nano-sized fiber diameter and a structure with a high aspect ratio (ratio of fiber length to outer diameter), so that instead of carbon black having a structure, the carbon nanotubes can be brought into linear contact with the outer surface of the coated first composite oxide or the positive electrode current collector.

[0029] Thus, in the positive electrode for a secondary battery according to an embodiment of the present disclosure, a sufficient amount of first carbon black is present in the microvoids formed between adjacent coated first composite oxides in the positive electrode mixture layer, allowing a sufficient amount of first carbon black to be attached to the surface of each coated first composite oxide within the microvoids. Furthermore, carbon nanotubes can be brought into linear contact with the outer surface of the coated first composite oxide and with the coated first composite oxide and the positive electrode current collector. Therefore, when the positive electrode contains the coated first composite oxide as the positive electrode active material, sufficient conductive paths can be formed between the coated first composite oxides and between the coated first composite oxide and the positive electrode current collector, thereby suppressing an increase in internal resistance in the positive electrode. This suppresses the difficulty in maintaining sufficient battery capacity in a nonaqueous electrolyte secondary battery due to an increase in internal resistance in the positive electrode. This significantly suppresses the difficulty in maintaining sufficient battery capacity during high-rate charge / discharge, in other words, rapid charge / discharge.

[0030] The positive electrode for a secondary battery according to an embodiment of the present disclosure will be described in more detail below.

[0031] The positive electrode current collector preferably has a strip shape (long shape) in a plan view. As the positive electrode current collector, a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet) is used. Examples of materials for the positive electrode current collector include metal materials such as Al, Al alloys, Ti, Ti alloys, and Fe alloys. The Fe alloy may be stainless steel. The thickness of the positive electrode current collector is not particularly limited, but is preferably 1 to 50 μm, more preferably 5 to 20 μm, and even more preferably 10 to 20 μm.

[0032] The positive electrode mixture layer may be disposed on both surfaces of the positive electrode current collector, or on only one surface. The surface of the positive electrode current collector is preferably a main surface of the positive electrode current collector. When the positive electrode current collector is a porous conductive substrate as described above, the positive electrode mixture layer may be disposed in a state where at least a portion of the positive electrode mixture layer is embedded in the pores of the porous substrate.

[0033] As the positive electrode active material, a material that electrochemically absorbs and releases lithium ions is used. For example, a lithium metal composite oxide can be used as the positive electrode active material. The lithium metal composite oxide may be a composite compound having a layered structure (for example, a rock salt crystal structure) containing lithium and a transition metal. As described above, the positive electrode for a secondary battery according to an embodiment of the present disclosure uses, as the positive electrode active material, a compound having a composition formula of Li ｙ Ni ｘ M （１－ｘ） O ２The positive electrode for a secondary battery according to an embodiment of the present disclosure includes a first composite oxide as a positive electrode active material. In the above formula, x≧0.5, 0<y≦1.2, and M includes at least one element selected from the group consisting of Co, Mn, Al, Fe, Ti, Sr, Ca, Zr, Nb, W, and B. M may be at least one element selected from the group consisting of Co, Mn, Al, Fe, Ti, Sr, Ca, Zr, Nb, W, and B, and among these, M preferably includes at least one element selected from the group consisting of Co, Mn, and Fe. From the viewpoint of the stability of the crystal structure, the first composite oxide may include Al as M. Specific examples of such first composite oxides include lithium-nickel-cobalt-aluminum composite oxide (Li ｙ１ Ni ｘ１ Co ｚ１ Al ｗ１ O ２ , where x1≧0.5, 0<y1≦1.2, 0<z1<0.2, and 0<w1<0.2.

[0034] Other specific examples of the first composite oxide include lithium-nickel-cobalt-manganese composite oxide (Li ｙ２ Ni ｘ２ Co ｚ２ Mn ｖ２ O ２ , where x2≧0.5, 0<y2≦1.2, 0<z2<0.2, and 0<v2<0.2.), and lithium-nickel-cobalt-manganese-aluminum composite oxide (Li ｙ３ Ni ｘ３ Co ｚ３ Mn ｖ３ Al ｗ３ O ２ In the formula, x3≧0.5, 0<y3≦1.2, 0<z3<0.2, 0<v3<0.2, and 0<w3<0.2.) The first composite oxide may contain, as M, at least one selected from the group consisting of Co and Mn in combination with at least one selected from the group consisting of Al, Fe, Ti, Sr, Ca, Zr, Nb, W, and B.

[0035] The positive electrode for a secondary battery according to the embodiment of the present disclosure may contain a lithium metal composite oxide other than the first composite oxide. Examples of such a lithium metal composite oxide include Li ａ CoO ２ , Li ａ NiO ２ , Li ａ MnO ２ , Li ａ Mn ２ O ４ , Li ａ Mn ２－ｂ M ｂ O ４ , LiMPO ４ , Li ２ MPO ４ F. In the lithium metal composite oxide, M is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B. In the lithium metal composite oxide, a and b satisfy 0<a≦1.2 and 0<b≦0.9. The value of a, which represents the molar ratio of lithium, increases or decreases with charge and discharge.

[0036] In the positive electrode for secondary battery according to the embodiment of the present disclosure, the mass ratio of the first composite oxide in the positive electrode active material may be 80 mass% or more, 90 mass% or more, or 100 mass%. That is, in the positive electrode for secondary battery according to the embodiment of the present disclosure, the entire positive electrode active material may be occupied by the first composite oxide.

[0037] As described above, the first composite oxide contains 50 atomic % or more of Ni. In this way, in lithium metal composite oxides with a high Ni content, at least one of Co, Mn, and Al, which may be contained as a metal element M other than Li and Ni, contributes to stabilizing the crystal structure. On the other hand, from the viewpoint of reducing production costs, it is desirable for the first composite oxide to have a low Co content. Therefore, when the first composite oxide has a low Co content (for example, the proportion of Co in the metal elements other than Li is 20 atomic % or less) or does not contain Co, the first composite oxide may contain at least one of Mn and Al.

[0038] The higher the Ni ratio x of the first composite oxide, the more lithium ions can be extracted from the crystal structure during charging. This increases the capacity of the nonaqueous electrolyte secondary battery. From the viewpoint of increasing the capacity of the nonaqueous electrolyte secondary battery, the Ni ratio x of the first composite oxide is 0.50 or more (x≧0.50), or may be 0.80 or more (x≧0.80), 0.85 or more (x≧0.85), or 0.90 or more (x≧0.90).

[0039] From the viewpoint of reducing production costs, the proportion of Co in the first composite oxide is preferably low. The proportion of Co in the metal elements other than Li in the first composite oxide may be 20 atomic % or less, 12 atomic % or less, or 5 atomic % or less, or may be substantially free of Co. More specifically, the first composite oxide may be Li ｙ Ni ｘ Co ｚ M １ （１－ｘ－ｚ） O ２ (However, M １ is an element M other than Co, and 0≦z≦0.20. ｙ Ni ｘ Co ｚ M １ （１－ｘ－ｚ） O ２ (However, M １ is an element M other than Co, and 0≦z≦0.12. ｙ Ni ｘCo ｚ M １ （１－ｘ－ｚ） O ２ (However, M １ is an element M other than Co, and 0≦z≦0.05.

[0040] The contents of the elements constituting the first composite oxide and the other elements constituting the lithium metal composite oxide can be measured using an inductively coupled plasma atomic emission spectroscopy (ICP-AES), an electron probe microanalyzer (EPMA), or an energy dispersive X-ray spectroscopy (EDX).

[0041] In the positive electrode for secondary batteries according to the embodiments of the present disclosure, as described above, the surface of the positive electrode active material is covered with a compound containing at least one element selected from the group consisting of B, P, Mg, S, Ca, Sr, Ba, Ti, W, Zr, and Al. In other words, the surface of the positive electrode active material is covered with at least one of a metal compound containing a metal element and a non-metal compound containing a non-metal element. That is, in the positive electrode for secondary batteries according to the embodiments of the present disclosure, the surface of the first composite oxide is covered with a compound containing the above element, forming a coated first composite oxide.

[0042] Examples of the metal compound include a compound containing Mg, a compound containing Ca, a compound containing Sr, a compound containing Ba, a compound containing Ti, a compound containing W, a compound containing Zr, and a compound containing Al. Examples of the compound containing Mg include MgO, Mg(OH) ２ , and MgCO ３ Examples of compounds containing Ca include CaO and Ca(OH). ２ , and CaCO ３ Examples of compounds containing Sr include SrO, Sr(OH) ２ , and SrCO ３Examples of compounds containing Ba include BaO and Ba(OH). ２ , and BaCO ３ Examples of compounds containing Ti include TiO ２ , Ti(OH) ４ , and Zr(CO ３ ) ２ Examples of compounds containing W include WO ３ Examples of compounds containing Zr include ZrO ２ , Zr(OH) ４ , Zr(CO ３ ) ２ , and Zr(SO ４ ) ２ ・4H ２ Examples of compounds containing Al include Al. ２ O ３ Examples include:

[0043] The non-metallic compounds include compounds containing B, compounds containing P, and compounds containing S. The compounds containing B include H ３ BO ３ , Li ３ BO ３ , and Li ２ B ４ O ７ Examples of compounds containing P include Li ３－ｘ H ｘ P.O. ４ (0≦x≦3) can be exemplified. Examples of compounds containing S include Li ２ SO ４ , Li ２ S., Li. ３ P.S. ４ Examples include:

[0044] The first composite oxide can be produced, for example, by a first step of synthesizing a metal composite hydroxide by a coprecipitation method, a second step of calcining the metal composite hydroxide to obtain a metal composite oxide, and a third step of calcining a mixture of lithium hydroxide and the metal composite oxide. After the third step, a washing step and a drying step may be carried out to obtain the first composite oxide.

[0045] In the first step, for example, a metal composite hydroxide can be synthesized by adding dropwise an alkaline solution such as sodium hydroxide to a stirred solution of a metal salt containing Ni and an arbitrary metal element (such as Co, Al, and Mn) and adjusting the pH to the alkaline side (for example, 8.5 or more and 12.5 or less). The particle size of the metal composite hydroxide tends to become smaller as the pH during synthesis increases. Furthermore, the particle size of the metal composite hydroxide tends to become larger as the amount of metal salt solution increases. Therefore, the particle size of the metal composite hydroxide can be controlled by adjusting the pH during synthesis or the amount of metal salt solution.

[0046] In the third step, the mixture is calcined at a temperature of, for example, 650°C or higher. The calcination temperature is preferably in the range of 650°C or higher and 1100°C or lower. Calcination is preferably carried out in an oxygen stream. In the third step, a lithium source (lithium hydroxide) having a stoichiometric ratio equal to or higher than that of the target product may be used. This allows the discharge capacity of the nonaqueous electrolyte secondary battery to be sufficiently increased. When the lithium source in the third step is lithium hydroxide, lithium hydroxide may be used in a stoichiometric ratio of 1 to 1.1 times that of the metal composite oxide. The calcination conditions can also be adjusted to control the particle size of the primary particles. For example, by performing calcination so as to increase the maximum temperature, the particle size of the primary particles can be increased, resulting in a first composite oxide of single particles.

[0047] In the washing step, the first composite oxide is washed with water and then dehydrated to obtain a cake-like composition. The washing and dehydration can be performed by various known methods and under various conditions. For example, they may be performed within a range that does not cause lithium to leach out of the first composite oxide and deteriorate the battery characteristics. By performing the washing step as described above, a first composite oxide with a small amount of residual alkaline components can be obtained.

[0048] In the drying step, the cake-like composition obtained in the washing step is dried to obtain a powdery composition. The drying step may be carried out under a vacuum atmosphere. Drying may be carried out, for example, at a temperature of 150°C to 400°C for 0.5 hours to 15 hours. A heat treatment may be carried out after the drying step. The heating temperature is, for example, 100°C to 450°C. This heat treatment may be carried out after the crushing treatment described below.

[0049] The powder composition may be subjected to a crushing treatment. By the crushing treatment, the first composite oxide in the form of single particles can be obtained. A jet mill or the like can be used for the crushing treatment. As the jet mill, for example, a PJM-80 (manufactured by Nippon Pneumatic Co., Ltd.) can be used.

[0050] At least one of the metal compound and the nonmetal compound described above can be attached to the surface of the first composite oxide by mixing the first composite oxide with a powder of a raw material of at least one of the metal compound and the nonmetal compound. The mixing may be performed after the third step and before the cleaning step, during the cleaning step, after the cleaning step and before the drying step, during the drying step, or after the drying step.

[0051] Among the above raw materials, the Sr raw material is Sr(OH) ２ , Sr(OH) ２ ・8H ２ O, SrO, SrCO ３ , SrSO ４ , Sr(NO ３ ) ２ , SrCl ２ , and SrAlO ４ Examples of Ca raw materials include Ca(OH) ２ , CaO, CaCO ３ , CaSO ４ , Ca(NO ３ ) ２ , CaCl ２ , and CaAlO ４ Zr raw materials include Zr(OH) ４ , ZrO ２ , Zr(CO ３ )２ , and Zr(SO ４ ) ２ ・4H ２ Examples of W raw materials include tungsten oxide (WO ３ ), lithium tungstate (Li ２ WO ４ , Li ４ WO ５ , and Li ６ W ２ O ９ As the W raw material, a solution containing W may be used. As the Al raw material, Al ２ O ３ , Al(OH) ３ , Al ２ (SO ４ ) ３ As the Al raw material, Al derived from the first composite oxide can also be used. As the P raw material, Li ３－ｘ H ｘ P.O. ４ (0≦x≦3). ３ BO ３ , Li ３ BO ３ , and Li ２ B ４ O ７ Examples of the S raw material include sulfonic acid compounds such as lithium methanesulfonate, lithium ethanesulfonate, lithium propanesulfonate, sodium methanesulfonate, magnesium methanesulfonate, and sodium fluoromethanesulfonate. Each of the above raw materials may be used after being pulverized to adjust the particle size or the water content, including that of hydrates.

[0052] The positive electrode active material preferably has a D50 particle diameter of 2 μm or more and 30 μm or less. The D50 particle diameter may be 10 μm or more. The D50 particle diameter may also be 20 μm or less, or 15 μm or less.

[0053] The D50 particle size of the positive electrode active material is the cumulative 50% particle size (median diameter) in a volume-based particle size distribution measured using a laser diffraction / scattering particle size distribution analyzer. As the laser diffraction / scattering particle size distribution analyzer, for example, the Microtrack Series MT3300 manufactured by Nikkiso Co., Ltd. is used. The measurement using the particle size distribution analyzer can be performed before the positive electrode active material is incorporated into the positive electrode mixture layer.

[0054] Carbon black is a carbon material classified as soft carbon among amorphous carbons. Examples of carbon black include acetylene black, ketjen black, and furnace black. Among these, acetylene black is preferably used.

[0055] In the positive electrode for a secondary battery according to an embodiment of the present disclosure, the carbon black includes a first carbon black having a diameter of 100 nm or less and an aspect ratio of 1.0 to 1.2. Such a sufficiently small first carbon black can penetrate into microvoids formed between adjacent positive electrode active materials in the positive electrode mixture layer. The first carbon black preferably has a diameter of 50 nm or less. This allows the first carbon black to more easily penetrate into the microvoids. As a result, a more sufficient conductive path can be formed between adjacent positive electrode active materials. The lower limit of the diameter of the first carbon black is not particularly limited, but may be, for example, 20 nm or 10 nm. The diameter of the first carbon black refers to the longest diameter of the first carbon black, and the aspect ratio of the first carbon black refers to the ratio of the longest diameter to the shortest diameter of the first carbon black (longest diameter / shortest diameter). In other words, when the first carbon black has a spherical shape, the aspect ratio is 1.0. The first carbon black may be composed of a plurality of carbon fine particles each having a diameter of about 20 nm, in which case the diameter of the first carbon black refers to the length of the longest part of the first carbon black (longest diameter).

[0056] Carbon black typically consists of multiple carbon particles with a diameter of about 20 nm, each linked together to form a structure with a length of about 2 μm. Therefore, the first carbon black can be obtained, for example, by crushing the carbon black having a structure. Crushing the carbon black having a structure can be performed using, for example, a jet mill. Examples of jet mills that can be used include the Nobilta NOB300 manufactured by Hosokawa Micron Corporation. Crushing the carbon black having a structure is preferably performed in the form of a mixture mixed with the positive electrode active material. This allows the first carbon black obtained by crushing to be sufficiently dispersed in the mixture.

[0057] In the positive electrode for a secondary battery according to the embodiment of the present disclosure, the positive electrode mixture layer has a thickness of 10 μm ２ The positive electrode active material has a surface area of ​​50 or more particles of the first carbon black. This allows a sufficient amount of the first carbon black to be present in the minute gaps formed between adjacent particles of the positive electrode active material. As a result, a more sufficient conductive path can be formed between adjacent particles of the positive electrode active material. The number of particles of the first carbon black contained in the above region can be measured, for example, by the following procedure.

[0058] Procedure (1) An arbitrary cross section of the positive electrode mixture layer is observed using an SEM or a TEM, and a 10 μm ２Ten randomly selected regions each having an area of ​​100 μm and in which carbon black can be observed are selected. The cross section is preferably a cross section in the thickness direction of the positive electrode mixture layer. (2) The number of first carbon black particles is counted for each of the ten regions. Within each region, the carbon fine particles and the positive electrode active material are binarized. Specifically, the first carbon black is represented in black, and the positive electrode active material is represented in white. The carbon fine particles are represented by black pixels having a size of, for example, 10 nm x 10 nm or less, and the positive electrode active material is represented by white pixels having a size of, for example, 10 nm x 10 nm or less. If there are no adjacent pixels (black pixels) of adjacent carbon fine particles, they are determined to be included in separate first carbon black particles. (3) The counts obtained for each of the ten regions are arithmetically averaged.

[0059] The first carbon black preferably has a standard deviation of diameter of 10% or less of the median diameter. The standard deviation of diameter may be 7% or less, or may be 5% or less of the median diameter. Such first carbon black is sufficiently crushed. This allows the first carbon black to have a relatively uniform diameter in the positive electrode mixture layer, thereby allowing the first carbon black particles to be in favorable contact with each other in the minute gaps formed between adjacent positive electrode active material particles. This further suppresses an increase in the internal resistance of the positive electrode.

[0060] Carbon nanotubes are carbon fibers with nanometer-sized fiber diameters and extremely large aspect ratios (ratios of fiber length to the outer diameter (diameter) of the fiber). In carbon fibers with large aspect ratios, contact between positive electrode active materials and between the positive electrode active material and the positive electrode current collector is linear rather than point contact. Therefore, carbon nanotubes can form excellent linear conductive paths between positive electrode active materials and between the positive electrode active material and the positive electrode current collector. Furthermore, linear contact between the carbon nanotubes and the positive electrode current collector improves current collection. Single-walled carbon nanotubes are preferably used as the carbon nanotubes. Because the fiber diameter of single-walled carbon nanotubes is smaller than that of multi-walled carbon nanotubes, the volume occupied by single-walled carbon nanotubes in the positive electrode mixture layer is smaller. Therefore, when single-walled carbon nanotubes are used, the volume occupied by carbon nanotubes in the positive electrode mixture layer can be reduced. This allows the proportion of the positive electrode active material in the positive electrode mixture layer to be increased while maintaining high conductivity of the positive electrode mixture layer, thereby increasing the capacity of the nonaqueous electrolyte secondary battery.

[0061] The average fiber length of the carbon nanotubes is preferably 2 μm or more. This allows for more sufficient formation of linear conductive paths between the positive electrode active materials and between the positive electrode active material and the positive electrode current collector. Furthermore, the positive electrode current collector can be further improved. The average fiber length of the carbon nanotubes may be 3 μm or more, or 5 μm or more. The average fiber length of the carbon nanotubes may be 40 μm or less, 30 μm or less, or 20 μm or less. By setting the average fiber length of the carbon nanotubes to 40 μm or less as described above, the carbon nanotubes can be sufficiently dispersed in the positive electrode mixture slurry. Furthermore, an excessive increase in the viscosity of the positive electrode mixture slurry can be suppressed. Furthermore, an excessive increase in the proportion of carbon nanotubes in the positive electrode mixture layer, in other words, an excessive decrease in the proportion of the positive electrode active material in the positive electrode mixture layer, can be suppressed, thereby suppressing a decrease in the capacity of the nonaqueous electrolyte secondary battery.

[0062] The diameter of the carbon nanotubes is preferably 3 nm or less, and may be 2.5 nm or less. The diameter of the carbon nanotubes may be 0.5 nm or more, 1.0 nm or more, or 1.5 nm or more. By having the carbon nanotube diameter within the above range, a sufficient number of carbon nanotubes can be present in the positive electrode mixture layer even with a relatively small content. This allows for more sufficient formation of good linear conductive paths between the positive electrode active materials and between the positive electrode active material and the positive electrode current collector. This improves the current collection performance of the positive electrode.

[0063] The carbon nanotubes present in the positive electrode may be bundles of multiple carbon nanotubes present in the positive electrode mixture layer. In such cases, the length and diameter of the carbon nanotube refer to the length and diameter of a single carbon nanotube present in the bundle of carbon nanotubes.

[0064] The average fiber length of carbon nanotubes is determined by image analysis using a scanning electron microscope (SEM). The average fiber length of carbon nanotubes is determined by measuring the lengths of 100 randomly selected carbon nanotubes and calculating the arithmetic mean. The length refers to the length of the carbon nanotubes when stretched linearly.

[0065] The diameter of carbon nanotubes can be determined by image analysis using a transmission electron microscope (TEM). The average diameter of carbon nanotubes can be measured by the following method. First, 100 carbon nanotubes are randomly selected, and the diameter (outer diameter) of each is measured at one arbitrary point. The diameter is then determined by arithmetically averaging the measured diameters.

[0066] Single-walled carbon nanotubes have an ideal one-dimensional structure with respect to electron conductivity. Therefore, electrons can be freely conducted in the axial direction of the single-walled carbon nanotube without crossing the layers. Therefore, the inclusion of single-walled carbon nanotubes in the positive electrode mixture layer can improve the conductivity. On the other hand, in multi-walled carbon nanotubes, due to the interaction between the layers, electrons move in the axial direction of the multi-walled carbon nanotube while crossing the layers. In other words, the degree of freedom of electron conduction in the axial direction is limited in multi-walled carbon nanotubes. Therefore, the inclusion of multi-walled carbon nanotubes in the positive electrode mixture layer may decrease the conductivity.

[0067] The positive electrode for a secondary battery according to an embodiment of the present disclosure may contain a positive electrode conductive agent other than carbon black and carbon nanotubes. Examples of such a positive electrode conductive agent include hard carbon, which is an amorphous carbon, and carbon fibers. Examples of carbon fibers include rod-shaped carbon materials such as VGCF, sheet-shaped carbon materials such as graphene, and carbon fibers.

[0068] Examples of the binder include resin materials. Examples of the resin material include fluororesins such as polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins such as polyethylene and polypropylene; polyamide resins such as aramid resin; polyimide resins such as polyimide and polyamideimide; vinyl resins such as polyacrylonitrile, polyvinylpyrrolidone and polyvinyl acetate; polyethersulfone, nitrile rubber, etc. That is, it is preferable to use a copolymer as the resin material. Note that the vinyl resin is a resin containing a vinyl group (CH ２ It is a resin obtained by polymerizing a monomer having the formula (=CH-).

[0069] The binder may be a resin material, and may be used alone or in combination of two or more kinds.

[0070] The binder may have a D50 particle size of 10 μm or more and 150 μm or less.

[0071] Like the D50 particle size of the positive electrode active material, the D50 particle size of the binder is the cumulative 50% particle size (median diameter) in a volume-based particle size distribution measured using a laser diffraction / scattering particle size distribution analyzer. As the laser diffraction / scattering particle size distribution analyzer, for example, the Microtrac Series MT3300 manufactured by Nikkiso Co., Ltd. is used. The measurement using the particle size distribution analyzer can be performed before the binder is incorporated into the positive electrode mixture layer.

[0072] From the viewpoint of increasing the voltage resistance, the binder preferably contains a fluororesin as a resin material, and among fluororesins, it is preferable that the binder contains polyvinylidene fluoride (PVDF).

[0073] In the positive electrode for a secondary battery according to an embodiment of the present disclosure, the positive electrode mixture layer preferably contains a positive electrode dispersant. The positive electrode dispersant allows components such as the positive electrode active material and the positive electrode conductive agent to be sufficiently dispersed in the positive electrode mixture slurry. This allows the first carbon black to be sufficiently present in minute voids formed between adjacent positive electrode active materials in the positive electrode mixture layer. As a result, an increase in internal resistance in the positive electrode can be sufficiently suppressed, thereby sufficiently suppressing the difficulty in maintaining the battery capacity of the nonaqueous electrolyte secondary battery.

[0074] The positive electrode mixture layer may contain either an organic dispersant or an inorganic dispersant as the positive electrode dispersant. On the other hand, the positive electrode mixture layer preferably contains an organic dispersant as the positive electrode dispersant, and more preferably contains at least one selected from the group consisting of nitrile rubber and cellulose compounds. By including such an organic dispersant in the positive electrode mixture layer, components such as the positive electrode active material and the positive electrode conductive agent can be more thoroughly dispersed in the positive electrode mixture slurry. For the same reasons as above, the first carbon black can be more thoroughly present in the microvoids formed between adjacent positive electrode active materials in the positive electrode mixture layer. As a result, an increase in internal resistance in the positive electrode can be more sufficiently suppressed, thereby further suppressing the difficulty in maintaining the battery capacity of the nonaqueous electrolyte secondary battery.

[0075] Examples of nitrile rubbers include nitrile rubber (NBR) and hydrogenated nitrile rubber (HNBR). Other examples of nitrile rubbers include carboxyl-modified nitrile rubber (XNBR) copolymerized with methacrylic acid and NBIR in which part of the butadiene is replaced with isoprene. Examples of cellulose compounds that can be used include cellulose derivatives such as cellulose ether. Examples of cellulose derivatives include ethyl cellulose and its modified products, carboxymethyl cellulose (CMC) and its modified products, hydroxypropyl cellulose and its modified products, and methyl cellulose. Examples of modified products of each cellulose derivative also include their salts. Examples of salts include alkali metal salts (e.g., sodium salts) and ammonium salts.

[0076] The positive electrode can be obtained, for example, by applying a slurry containing the components of the positive electrode mixture layer and a dispersion medium onto a positive electrode current collector to form a coating film, and then drying and compressing the coating film. The dispersion medium can be at least one selected from the group consisting of water and organic solvents (e.g., N-methyl-2-pyrrolidone). The components of the positive electrode mixture layer include a positive electrode active material, a positive electrode conductive agent, and a binder. The method for producing a positive electrode for a secondary battery will be described in more detail below.

[0077] [Method for Manufacturing Positive Electrode for Secondary Battery] A method for manufacturing a positive electrode for a secondary battery according to an embodiment of the present disclosure includes a positive electrode mixture slurry preparation step of preparing a positive electrode mixture slurry containing a positive electrode active material, carbon black, and carbon nanotubes, and a positive electrode mixture layer formation step of forming a positive electrode mixture layer on a surface of a positive electrode current collector using the positive electrode mixture slurry. In the method for manufacturing a positive electrode for a secondary battery according to an embodiment of the present disclosure, the positive electrode mixture slurry preparation step includes a first mixing substep of mixing the positive electrode active material and the carbon black to prepare a first mixed powder, and a second mixing substep of mixing the first mixed powder with carbon nanotubes to prepare a second mixed powder, and in the first mixing substep, the carbon black is crushed to obtain the first carbon black having a diameter of 100 nm or less and an aspect ratio of 1.0 to 1.2.

[0078] In the method for producing a positive electrode for a secondary battery according to an embodiment of the present disclosure, the positive electrode active material has a composition formula of Li ｙ Ni ｘ M （１－ｘ） O ２ (wherein x≧0.5, 0<y≦1.2, and M contains at least one element selected from the group consisting of Co, Mn, Al, Fe, Ti, Sr, Ca, Zr, Nb, W, and B), and the surface of the positive electrode active material is covered with a compound containing at least one element selected from the group consisting of B, P, Mg, Ca, Sr, Ba, Ti, W, Zr, and Al. The lithium metal composite oxide can be obtained by the co-precipitation method described above, or the like. The surface of the positive electrode active material can be covered by the method described above.

[0079] (Positive Electrode Mixture Slurry Preparation Process) The first mixing substep can be performed by mixing a powder mixture of the positive electrode active material and carbon black using a crusher. A jet mill can be used as the crusher. For example, a Nobilta NOB300 model manufactured by Hosokawa Micron Corporation can be used as the jet mill. By performing the first mixing substep, the carbon black having a structure can be suitably crushed to obtain a first carbon black having a diameter of 100 nm or less and an aspect ratio of 1.0 to 1.2. The operating conditions of the crusher can be appropriately selected taking into account the mixing ratio of the positive electrode active material and the carbon black.

[0080] In the positive electrode mixture slurry preparation step, the second mixed powder obtained in the second mixing substep is preferably mixed with an appropriate amount of a dispersion medium, which can be at least one selected from the group consisting of water and organic solvents (e.g., N-methyl-2-pyrrolidone).

[0081] (Positive electrode mixture layer forming step) The positive electrode mixture layer forming step can be performed by applying the positive electrode mixture slurry to at least one main surface of the positive electrode current collector to obtain a coating film, and then drying the coating film. In the positive electrode mixture layer forming step, the dried coating film may be compressed.

[0082] [Non-aqueous electrolyte secondary battery] A non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. In the non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure, the positive electrode is the positive electrode for the secondary battery according to the embodiment of the present disclosure described above. In the non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure, the negative electrode includes a silicon-containing material as the negative electrode active material, and the mass ratio of the silicon-containing material to the negative electrode active material may be 5 mass% or more. In the non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure, the positive electrode, the negative electrode, the separator, and the non-aqueous electrolyte are preferably housed in a battery case. The negative electrode, the separator, the non-aqueous electrolyte, and the battery case will be described below.

[0083] (Negative electrode) The negative electrode contains a carbon-based material as a negative electrode active material. In the negative electrode, the mass ratio of the carbon-based material to the negative electrode active material is 50 mass% or more. The negative electrode preferably has a negative electrode current collector and a negative electrode mixture layer disposed on the surface of the negative electrode current collector. The negative electrode mixture layer preferably contains the negative electrode active material and a binder.

[0084] The negative electrode current collector may be a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet). Examples of materials for the negative electrode current collector include metal materials such as Ni, Ni alloys, Cu, Cu alloys, and Fe alloys. The Fe alloy may be stainless steel. The thickness of the negative electrode current collector is not particularly limited, but is preferably 1 to 50 μm, and more preferably 5 to 20 μm.

[0085] The negative electrode mixture layer may be disposed on both main surfaces of the negative electrode current collector, or on only one main surface. The surface of the negative electrode current collector is preferably the main surface of the negative electrode current collector. When the negative electrode current collector is a porous conductive substrate as described above, the negative electrode mixture layer may be formed in a state where at least a portion of the layer is embedded in the pores of the porous conductive substrate.

[0086] Examples of carbonaceous materials include graphite, easily graphitizable carbon (soft carbon), and hardly graphitizable carbon (hard carbon). Of the above carbonaceous materials, graphite is preferred because it has excellent charge / discharge stability and can reduce irreversible capacity.

[0087] Graphite is a carbon material having an average interplanar spacing d002 of (002) planes measured by X-ray diffraction of, for example, 0.340 nm or less. The crystallite size Lc(002) of graphite measured by X-ray diffraction may be, for example, 5 nm or more, 5 nm or more to 300 nm or less, or 10 nm or more to 200 nm or less.

[0088] The D50 particle diameter of the graphite may be 13 μm or more and 25 μm or less. When the negative electrode further contains a silicon-containing material described below, the D50 particle diameter of the graphite is preferably larger than the D50 particle diameter of the silicon-containing material. In this case, the silicon-containing material is more easily accommodated in the voids formed between the graphite particles. This makes it easier to increase the filling rate of the negative electrode active material in the negative electrode mixture layer, making it easier to obtain a high-capacity negative electrode. Furthermore, the silicon-containing material particles present in the voids contribute to maintaining electronic contact between the graphite particles. On the other hand, even if the silicon-containing material particles present in the voids expand and contract, the negative electrode as a whole is less likely to expand and contract, and therefore is less likely to deteriorate due to charge and discharge cycles.

[0089] The negative electrode mixture layer may contain a silicon-containing material in addition to the carbon-based material. When the negative electrode contains a silicon-containing material in addition to the carbon-based material, the mass ratio of the silicon-containing material to the negative electrode active material is 5 mass% or more. When the negative electrode contains graphite as the carbon-based material, the mass ratio of the silicon-containing material to the negative electrode active material may be 6 mass% or more, or 10 mass% or more. The mass ratio of the silicon-containing material to the negative electrode active material may be 50 mass% or less, or 20 mass% or less. Furthermore, the mass ratio of graphite to the negative electrode active material may be 50 mass% or more, 70 mass% or more, 80 mass% or more, 90 mass% or more, or 95 mass% or less.

[0090] Examples of silicon-containing materials include silicon (Si), silicon alloys, silicon compounds (such as silicon oxides), and composite materials in which a silicon phase is dispersed in a lithium ion conductive phase (matrix phase). Examples of silicon oxides include SiO ＸExamples of suitable lithium ion conductive phases include particles. X is, for example, 0.5≦X<2. X may be 0.5≦X<1.6 or 0.8≦X≦1.6. The lithium ion conductive phase may be at least one selected from the group consisting of a silicon oxide phase, a silicate phase, and a carbon phase. The silicon oxide phase may be primarily composed of silicon dioxide (e.g., 95 to 100% by mass). Among the various silicon-containing materials listed above, a composite material composed of a silicate phase and a silicon phase dispersed in the silicate phase is preferred. This composite material has the advantages of high capacity and low irreversible capacity.

[0091] The silicate phase may contain, for example, at least one element selected from the group consisting of Group 1 elements of the long periodic table and Group 2 elements of the long periodic table. Examples of Group 1 elements of the long periodic table and Group 2 elements of the long periodic table include lithium (Li), potassium (K), sodium (Na), magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). Other elements may include aluminum (Al), boron (B), lanthanum (La), phosphorus (P), zirconium (Zr), or titanium (Ti). From the viewpoints of small irreversible capacity and high initial charge / discharge efficiency, a silicate phase containing lithium (hereinafter also referred to as a lithium silicate phase) is preferred as the silicate phase.

[0092] The lithium silicate phase may be an oxide phase containing lithium (Li), silicon (Si), and oxygen (O), and may contain other elements. In the lithium silicate phase, the atomic ratio of O to Si (O / Si) is, for example, greater than 2 and less than 4. O / Si is preferably greater than 2 and less than 3. In the lithium silicate phase, the atomic ratio of Li to Si (Li / Si) is, for example, greater than 0 and less than 4. The lithium silicate phase has the formula: Li ２ｚ SiO ２＋ｚThe lithium silicate phase may have a composition expressed by (0<z<2). Preferably, z satisfies the relationship 0<z<1, and more preferably z=1 / 2. In addition to Li, Si, and O, the lithium silicate phase may contain iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), molybdenum (Mo), zinc (Zn), aluminum (Al), or the like.

[0093] The D50 particle size of the composite particles composed of a silicate phase and silicon particles (silicon phase) dispersed in the silicate phase may be 1 μm to 25 μm. The D50 particle size of the composite particles may be 4 μm to 15 μm. When the D50 particle size of the composite particles is within the above numerical range, stress caused by volumetric changes in the composite particles during charge and discharge is easily alleviated, making it easier to obtain good cycle characteristics. Furthermore, the composite particles have a surface area suitable for suppressing capacity loss due to side reactions with the non-aqueous electrolyte.

[0094] The crystallite size of the silicon particles dispersed in the silicate phase is, for example, 10 nm or more. The silicon particles have a particulate phase of simple silicon (Si). When the crystallite size of the silicon particles is 10 nm or more, the surface area of ​​the silicon particles can be reduced. This makes it less likely that the silicon particles will deteriorate, which is accompanied by the generation of irreversible capacity. The crystallite size of the silicon particles can be calculated from the Scherrer formula using the half-width of the diffraction peak assigned to the Si(111) plane in the X-ray diffraction (XRD) pattern of the silicon particles.

[0095] The D50 particle diameter of the silicon particles is preferably 500 nm or less, more preferably 200 nm or less, and more preferably 50 nm or less before the first charge. After the first charge, the D50 particle diameter of the silicon particles is preferably 400 nm or less, and more preferably 100 nm or less. By miniaturizing the silicon particles, the volume change during charge and discharge can be reduced. The D50 particle diameter of the silicon particles can be determined by observing the cross section of the composite particle using SEM or TEM and arithmetically averaging the longest diameters of 100 or more silicon particles in the cross-sectional image.

[0096] From the viewpoint of increasing capacity and improving cycle characteristics, the content of silicon particles (elementary Si) in the composite particles is preferably 20% by mass to 95% by mass or less, and more preferably 35% by mass to 75% by mass. By having the silicon particle content within the above range, the diffusibility of lithium ions is improved, making it easier to obtain excellent load characteristics. Furthermore, since the surface of the silicon particles that is exposed and not covered with the lithium silicate phase is reduced, side reactions between the non-aqueous electrolyte and the silicon particles can be suppressed.

[0097] The composite particles may contain a conductive material covering at least a portion of their surface. That is, the composite particles may have a conductive layer covering at least a portion of their surface. Since the lithium silicate phase has poor electronic conductivity, the conductivity of the composite particles is likely to be low. However, by having a conductive layer on the surface of the composite particles, the conductivity of the composite particles can be sufficiently increased. The conductive layer preferably has a thickness that does not substantially affect the D50 particle size of the composite particles. From the viewpoint of ensuring conductivity and lithium diffusibility, the thickness of the conductive layer is preferably 1 nm to 200 nm, more preferably 5 nm to 100 nm.

[0098] The carbon phase can be composed of, for example, amorphous carbon with low crystallinity (i.e., amorphous carbon). The amorphous carbon may be hard carbon, soft carbon, or other. Amorphous carbon generally refers to a carbon having an average interplanar spacing d of the (002) plane measured by X-ray diffraction. ００２ This refers to carbon having a particle size greater than 0.340 nm.

[0099] As the silicon-containing material, SiO ＸAt least one type of particle selected from the group consisting of first particles containing silicon oxide represented by the formula (0.5≦X≦1.6), second particles containing a silicate phase and a silicon phase dispersed in the silicate phase, and third particles containing a carbon phase and a silicon phase dispersed in the carbon phase may be used. Specifically, the silicon-containing material may contain each of the first particles, the second particles, and the third particles independently. Furthermore, the silicon-containing material may contain two types of particles, the first particles and the second particles, two types of particles, the first particles and the third particles, or two types of particles, the second particles and the third particles. Furthermore, the silicon-containing material may contain all of the first particles, the second particles, and the third particles.

[0100] As described above, in the second particles, the silicon particles are dispersed in a silicate phase, and in the third particles, the silicon particles are dispersed in a carbon phase. Therefore, in the second particles, a sea-island structure is formed, with the silicon particles as islands and the silicate phase as the sea, and in the third particles, a sea-island structure is formed, with the silicon particles as islands and the carbon phase as the sea. Such a sea-island structure limits contact between the silicon particles and the nonaqueous electrolyte, thereby suppressing side reactions. Furthermore, the lithium ion conductive phase, which is the matrix phase, can relieve stress caused by the expansion and contraction of the silicon particles.

[0101] The content of each element contained in the silicon-containing material can be measured, for example, by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Specifically, a solution is obtained by dissolving the silicon-containing material in an acid solution while heating, and the residual carbon is removed from this solution by filtration to obtain a filtrate. The filtrate is then analyzed by ICP-AES to measure the spectral intensity of each element. Subsequently, a calibration curve is created using commercially available standard solutions of each element, and the content of each element is calculated based on this calibration curve.

[0102] Examples of binders include resin materials. Examples of resin materials include fluororesins such as polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins such as polyethylene and polypropylene; polyamide resins such as aramid resin; polyimide resins such as polyimide and polyamideimide; vinyl resins such as polyacrylonitrile, polyvinylpyrrolidone, and polyvinyl acetate; polyethersulfone; and rubber-like materials such as styrene-butadiene copolymer rubber (SBR). Examples of binders include polyacrylic acid and its derivatives, as well as salts thereof. Examples of salts of polyacrylic acid include sodium polyacrylate (PAA-Na). Binders may be used alone or in combination of two or more.

[0103] The negative electrode mixture layer preferably contains 0.5% by mass or more and 4.0% by mass or less of the binder. By containing the binder within the above numerical range, the negative electrode mixture layer can exhibit good binding properties to the negative electrode active material. The negative electrode mixture layer may contain 0.7% by mass or more of the binder, or may contain 0.8% by mass or more of the binder. The negative electrode mixture layer may contain 2.0% by mass or less of the binder, or may contain 1.5% by mass or less of the binder.

[0104] The negative electrode mixture layer may contain a negative electrode conductive agent in addition to the negative electrode active material and binder. The negative electrode conductive agent may be the same as the positive electrode conductive agent. Metal fibers, metal powders such as aluminum, etc. may also be used in the negative electrode. The negative electrode conductive agent may be used alone or in combination of two or more.

[0105] The negative electrode mixture layer may contain a thickener as needed. Examples of the thickener include cellulose derivatives such as cellulose ether. Examples of cellulose derivatives include carboxymethyl cellulose (CMC) and its modifications, methyl cellulose, and the like. Examples of modified CMC include salts of CMC. Examples of salts include alkali metal salts (e.g., sodium salts) and ammonium salts.

[0106] The negative electrode can be obtained, for example, by applying a slurry containing the components of the negative electrode mixture layer and a dispersion medium onto a negative electrode current collector to form a coating film, and then drying and compressing the coating film. The dispersion medium can be at least one selected from the group consisting of water and organic solvents (e.g., N-methyl-2-pyrrolidone). The components of the negative electrode mixture layer include a negative electrode active material, a binder, a negative electrode conductive agent, and a thickener.

[0107] (Separator) A porous sheet having ion permeability and insulating properties is used for the separator. Examples of the form of the porous sheet include a microporous film, a woven fabric, and a nonwoven fabric. The separator may be made of a polymer material. Examples of the polymer material include an olefin resin, a polyamide resin, and cellulose. Examples of the olefin resin include polyethylene, polypropylene, and a copolymer of ethylene and propylene. The separator may contain an additive as needed. Examples of the additive include an inorganic filler.

[0108] The separator may include multiple layers differing in at least one of form and composition, such as a laminate of a polyethylene microporous film and a polypropylene microporous film, or a laminate of a nonwoven fabric containing cellulose fibers and a nonwoven fabric containing thermoplastic resin fibers.

[0109] (Non-aqueous electrolyte) The non-aqueous electrolyte has ion conductivity (for example, lithium ion conductivity). The non-aqueous electrolyte may be a liquid electrolyte (electrolytic solution) or a solid electrolyte. The liquid non-aqueous electrolyte (nonaqueous electrolyte solution) contains a solvent (nonaqueous solvent) and a solute dissolved in the solvent. Examples of the solute include lithium salts. Various additives may be added to the non-aqueous electrolyte.

[0110] As the solvent, various known organic solvents can be used, such as cyclic carbonate esters, chain carbonate esters, cyclic carboxylic acid esters, chain carboxylic acid esters, chain ethers, cyclic ethers, fluorinated chain ethers, and fluorinated cyclic ethers.

[0111] Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC).

[0112] Examples of the chain carbonate ester include diethylene carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).

[0113] Examples of the cyclic carboxylic acid ester include γ-butyrolactone (GBL), γ-valerolactone (GVL), and the like.

[0114] Examples of the chain carboxylic acid ester include non-aqueous solvents such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP).

[0115] Examples of chain ethers include dimethyl ether, ethyl methyl ether, diethyl ether, ethyl propyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, and o-dimethoxybenzene. The chain ether may be a chain ether having two or more ether bonds. Examples of such chain ethers include 1,1-dimethoxymethane, 1,1-diethoxyethane, 1,2-dimethoxyethane (DME), 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol ethyl methyl ether, diethylene glycol dibutyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol ethyl methyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, and tetraethylene glycol ethyl methyl ether.

[0116] Examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, and crown ethers.

[0117] The fluorinated chain ether has a structure in which one or more hydrogen atoms of the chain ethers described above are substituted with fluorine atoms. Examples of the fluorinated chain ether include bis(2,2,2-trifluoroethyl) ether, 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether, and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether.

[0118] Fluorinated cyclic ethers are those in which one or more hydrogen atoms of the above-mentioned cyclic ethers have been substituted with fluorine atoms. Examples of fluorinated cyclic ethers include 3,3,4,4-tetrafluorotetrahydrofuran.

[0119] The above-mentioned various solvents (non-aqueous solvents) may be used alone or in combination of two or more.

[0120] Examples of the lithium salt include lithium salts of chlorine-containing acids, lithium salts of fluorine-containing acids, lithium salts of fluorine-containing acid imides, lithium halides, and lithium salts containing oxalate complexes. Examples of the lithium salts of chlorine-containing acids include LiClO ４ , LiAlCl ４ , LiB １０ Cl １０ Examples of lithium salts of fluorine-containing acids include LiPF ６ , LiPF ２ O ２ , LiBF ４ , LiSbF ６ , LiAsF ６ , LiCF ３ SO ３ , LiCF ３ CO ２Examples of lithium salts of fluorine-containing acid imides include LiN(FSO ２ ) ２ (lithium bis(fluorosulfonyl)imide, LFSI), LiN(CF ３ SO ２ ) ２ , LiN(CF ３ SO ２ ) (FSO ２ ), LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ ), LiN(C ２ F ５ SO ２ ) ２ Examples of lithium halides include LiCl, LiBr, and LiI. Examples of lithium salts containing oxalate complexes include LiB(C ２ O ４ ) ２ , LiBF ２ (C ２ O ４ ), LiPF ４ (C ２ O ４ ), LIPF ２ (C ２ O ４ ) ２ The above lithium salts may be used alone or in combination of two or more.

[0121] When the non-aqueous electrolyte is a liquid electrolyte (electrolytic solution), the non-aqueous electrolyte preferably contains fluoroethylene carbonate (FEC) as a solvent and lithium bis(fluorosulfonyl)imide (LFSI) as a lithium salt. That is, the non-aqueous electrolyte preferably contains at least one selected from the group consisting of fluoroethylene carbonate (FEC) and lithium bis(fluorosulfonyl)imide (LFSI). By including fluoroethylene carbonate (FEC), a stable solid electrolyte film is easily formed, which makes it easier to improve cycle characteristics (e.g., high-rate capacity retention) particularly during high-rate charge and discharge. Furthermore, lithium bis(fluorosulfonyl)imide has excellent ionic dissociation properties in a solvent, thereby increasing the conductivity of the non-aqueous electrolyte solution.

[0122] The concentration of the lithium salt in the liquid nonaqueous electrolyte (nonaqueous electrolytic solution) may be 1 mol / L or more and 5 mol / L or less, or 1 mol / L or more and 3 mol / L or less. By setting the lithium salt concentration within the above range, a liquid nonaqueous electrolyte (nonaqueous electrolytic solution) having excellent ionic conductivity and appropriate viscosity can be obtained.

[0123] The liquid non-aqueous electrolyte (nonaqueous electrolyte solution) may contain various known additives. Examples of such additives include 1,3-propane sultone, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, fluorobenzene, ethylene sulfite (ES), etc. Note that cyclic carbonates such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC), which are exemplified as solvents, may also function as additives.

[0124] Examples of the solid electrolyte include solid or gel polymer electrolytes and inorganic solid electrolytes. Examples of the inorganic solid electrolyte include materials known in all-solid-state lithium ion secondary batteries (e.g., oxide-based solid electrolytes, sulfide-based solid electrolytes, halogen-based solid electrolytes, etc.). Examples of the polymer electrolyte include a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt, and a matrix polymer. Examples of the matrix polymer include a polymer material that absorbs the non-aqueous solvent and gels. Examples of the polymer material include fluororesin, acrylic resin, and polyether resin.

[0125] (Battery Case) The battery case includes, for example, a rectangular case body with a bottom and an opening, and a sealing plate that seals the opening of the case body. The case body may be made of metal.

[0126] An example of the structure of a nonaqueous electrolyte secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween is housed in an outer casing together with a nonaqueous electrolyte. However, this is not limited thereto, and other forms of electrode groups may also be applied. For example, a laminated electrode group in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween may also be used. The shape of the nonaqueous electrolyte secondary battery is also not limited, and may be, for example, a cylindrical type, a prismatic type, a coin type, a button type, a laminate type, or the like.

[0127] A specific configuration of a secondary battery according to an embodiment of the present disclosure will be described below with reference to Fig. 1. Note that, hereinafter, the secondary battery according to an embodiment of the present disclosure will be simply referred to as a secondary battery according to a first embodiment.

[0128] Fig. 1 is a schematic perspective view, with a portion cut away, of a prismatic non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure. The secondary battery 1 shown in Fig. 1 includes a bottomed prismatic battery case 11, and an electrode group 10 and a non-aqueous electrolyte (not shown) housed within the battery case 11. The electrode group 10 includes a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator interposed between them to prevent direct contact. The electrode group 10 is formed by winding the negative electrode, the positive electrode, and the separator around a flat-plate-shaped winding core and then removing the winding core.

[0129] One end of a negative electrode lead 15 is attached to the negative electrode current collector of the negative electrode by welding or the like. One end of a positive electrode lead 14 is attached to the positive electrode current collector of the positive electrode by welding or the like. The other end of the negative electrode lead 15 is electrically connected to a negative electrode terminal 13 provided on the sealing plate 12. A gasket 16 is disposed between the sealing plate 12 and the negative electrode terminal 13 to insulate them from each other. The other end of the positive electrode lead 14 is connected to the sealing plate 12 and electrically connected to the battery case 11, which also serves as the positive electrode terminal. A resin frame 18 is disposed above the electrode group 10. The frame 18 separates the electrode group 10 from the sealing plate 12 and separates the negative electrode lead 15 from the battery case 11. The opening of the battery case 11 is sealed with the sealing plate 12. The sealing plate 12 has a liquid injection hole 17a formed therein. The electrolyte is injected into the battery case 11 through the injection hole 17a. The injection hole 17a is then closed with the plug 17.

[0130] (Additional Note) The above description discloses the following technology: (Technology 1) A battery comprising a positive electrode current collector and a positive electrode mixture layer disposed on a surface of the positive electrode current collector, the positive electrode mixture layer including a positive electrode active material and a positive electrode conductive agent, the positive electrode active material having a composition formula of Li ｙ Ni ｘ M （１－ｘ） O ２ (wherein x≧0.5, 0<y≦1.2, and M contains at least one element selected from the group consisting of Co, Mn, Al, Fe, Ti, Sr, Ca, Zr, Nb, W, and B), a surface of the positive electrode active material is covered with a compound containing at least one element selected from the group consisting of B, P, Mg, S, Ca, Sr, Ba, Ti, W, Zr, and Al, the positive electrode conductive agent contains carbon black and carbon nanotubes, the carbon black contains first carbon black having a diameter of 100 nm or less and an aspect ratio of 1.0 to 1.2, and the positive electrode mixture layer has a thickness of 10 μm ２A positive electrode for a secondary battery, comprising 50 or more particles of the first carbon black within a region having an area of ​​100 μm or more. (Technology 2) The positive electrode for a secondary battery according to Technology 1, wherein the carbon nanotubes have a length of 2 μm or more. (Technology 3) The positive electrode for a secondary battery according to Technology 1 or 2, wherein the carbon nanotubes have a diameter of 3 nm or less. (Technology 4) The positive electrode for a secondary battery according to any one of Technology 1 to 3, wherein the standard deviation of the diameter of the first carbon black is 10% or less of the median of the diameter. (Technology 5) The positive electrode for a secondary battery according to any one of Technology 1 to 4, wherein the first carbon black has a diameter of 50 nm or less. (Technology 6) The positive electrode for a secondary battery according to any one of Technology 1 to 5, wherein the positive electrode active material has a D50 particle size of 2 μm or more and 30 μm or less. (Technology 7) The positive electrode for a secondary battery according to any one of Technology 1 to 6, wherein the positive electrode mixture layer further contains a positive electrode dispersant. (Technology 8) The positive electrode for a secondary battery according to Technology 7, wherein the positive electrode dispersant contains at least one organic dispersant selected from the group consisting of nitrile rubbers and cellulose compounds. (Technology 9) A non-aqueous electrolyte secondary battery having a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, wherein the positive electrode is the positive electrode for a secondary battery according to any one of Technology 1 to 8. (Technology 10) The non-aqueous electrolyte secondary battery according to Technology 9, wherein the negative electrode contains a silicon-containing material as a negative electrode active material, and wherein a mass ratio of the silicon-containing material to the negative electrode active material is 5 mass% or more. (Technology 11) The non-aqueous electrolyte secondary battery according to Technology 9 or 10, wherein the non-aqueous electrolyte contains at least one selected from the group consisting of fluoroethylene carbonate (FEC) and lithium bis(fluorosulfonyl)imide (LFSI).(Technology 12) A cathode mixture slurry production method includes: a cathode mixture slurry production step of producing a cathode mixture slurry containing a cathode active material, carbon black, and carbon nanotubes; and a cathode mixture layer formation step of forming a cathode mixture layer on a surface of a cathode current collector using the cathode mixture slurry, wherein the cathode mixture slurry production step includes: a first mixing substep of mixing the cathode active material and the carbon black to produce a first mixed powder; and a second mixing substep of mixing the first mixed powder with the carbon nanotubes to produce a second mixed powder, wherein in the first mixing substep, carbon black is crushed to obtain first carbon black having a diameter of 100 nm or less and an aspect ratio of 1.0 to 1.2, and the cathode active material is a powder having a composition formula of Li. ｙ Ni ｘ M （１－ｘ） O ２ (wherein x≧0.5, 0<y≦1.2, and M contains at least one element selected from the group consisting of Co, Mn, Al, Fe, Ti, Sr, Ca, Zr, Nb, W, and B), and a surface of the positive electrode active material is covered with a compound containing at least one element selected from the group consisting of B, P, Mg, S, Ca, Sr, Ba, Ti, W, Zr, and Al.

[0131] While the present invention has been described in terms of presently preferred embodiments, such disclosure should not be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0132] Hereinafter, the present disclosure will be specifically described based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0133] [Example 1] (1) Preparation of Negative Electrode A negative electrode active material, sodium polyacrylate (PAA-Na), sodium carboxymethyl cellulose (CMC-Na), styrene-butadiene rubber (SBR), and water were mixed in a predetermined mass ratio to prepare a negative electrode mixture slurry. A mixture of graphite and a silicon-containing material was used as the negative electrode active material. In the negative electrode active material, the mixture ratio of graphite to the silicon-containing material was 95:5 by mass. That is, the mass ratio of the silicon-containing material in the negative electrode active material was 5% by mass.

[0134] The silicon-containing material used was a composite particle having a sea-island structure in which a silicon phase was dispersed in a lithium silicate phase. The composite particle was prepared by the following method.

[0135] First, silicon dioxide and lithium carbonate were mixed so that the atomic ratio Si / Li was 1.05 to obtain a first mixture, and this first mixture was fired in air at 950° C. for 10 hours to obtain a compound of the formula: Li ２ Si ２ O ５ The lithium silicate was then pulverized to have a D50 particle size of 10 μm.

[0136] Next, the obtained lithium silicate, raw silicon (3N high purity silicon powder, D50 particle size: 10 μm), and yttrium oxide (Y ２ O ３ ) were mixed in a mass ratio of 50:50:0.0005 to obtain a second mixture. This second mixture was filled into a pot (SUS, volume: 500 mL) of a planetary ball mill (Fritsch, P-5), and 24 SUS balls (diameter 20 mm) were placed in the pot, after which the lid was closed. The second mixture was then subjected to a pulverization treatment for 50 hours in an inert atmosphere at a rotation speed of 200 rpm. The powdered second mixture was then removed from the inert atmosphere, and the powdered second mixture was sintered at 800 °C for 4 hours in an inert atmosphere while applying pressure using a hot press machine, to obtain a sintered body (mother particles) of the second mixture.

[0137] Next, the sintered body of the second mixture was pulverized, and the resulting pulverized material was passed through a 40 μm mesh to obtain a sieve-size product. This sieve-size product was then mixed with coal pitch (MCP250, manufactured by JFE Chemical Corporation) to obtain a third mixture. The third mixture was fired at 800°C in an inert atmosphere to coat the surface of the sieve-size product with coal pitch carbide. This resulted in a conductive layer being formed on the surface of the sieve-size product. The conductive layer coverage was 5% by mass based on the total mass of the sieve-size product. A silicon-containing material was then obtained using a sieve, the surface of which was coated with a conductive layer and had a D50 particle size of 5 μm. This silicon-containing material contained a silicate phase and a silicon phase dispersed within the silicate phase, and was therefore the second composite material described above.

[0138] Next, the negative electrode mixture slurry was applied to each of the two main surfaces of a copper foil (negative electrode current collector) to form a coating film. Each of the coating films was dried and then rolled. In this manner, a negative electrode mixture layer was formed on each of the two main surfaces of the copper foil. In the negative electrode mixture slurry, the mixing ratio of the negative electrode active material, sodium polyacrylate (PAA-Na), sodium carboxymethyl cellulose (CMC-Na), and styrene-butadiene rubber (SBR) was set to a mass ratio of negative electrode active material:PAA-Na:CMC-Na:SBR=100:0.5:1:1.

[0139] (2) Preparation of Positive Electrode [Preparation of Positive Electrode Active Material] [Ni ０．８８ Co ０．０６ Al ０．０６ ] (OH) ２ The composite hydroxide represented by the formula (I) was calcined at 500°C for 8 hours to obtain a metal oxide (Ni ０．８８ Co ０．０６ Al ０．０６ In the synthesis of the composite hydroxide, the pH and the amount of the metal salt solution were adjusted so that the D50 of the finally obtained lithium transition metal composite oxide would be about 10 μm. The lithium transition metal composite oxide was a positive electrode active material.

[0140] Next, lithium hydroxide and the metal oxide were mixed so that the molar ratio of Li to the total amount of Ni, Co, and Al was 1.03:1 to obtain a mixture (first step). This mixture was then calcined under an oxygen stream with an oxygen concentration of 95% (10 cm ３ The mixture was fired at a temperature rise rate of 2.0°C / min from room temperature to 650°C under a flow rate of 2 mL / min per kg of mixture and 5 L / min per kg of mixture to obtain a first fired product, and then the first fired product was fired at a temperature rise rate of 0.5°C / min from 650°C to 780°C to obtain a second fired product (second step). Water was added to the second fired product to obtain a slurry with a concentration of 1500 g / L, and the slurry was stirred for 15 minutes. The stirred slurry was then filtered to obtain a cake-like composition. The cake-like composition was then mixed with powdered H ３ BO ３ was added. ３ BO ３ was added so that the molar ratio of B was 0.1 mol % relative to the total amount of Ni, Co, and Al contained in the lithium transition metal composite oxide. ３ BO ３ The cake-like composition to which the above was added was dried at 180°C for 2 hours to obtain the positive electrode active material of Example 1. That is, the positive electrode active material of Example 1 was a positive electrode active material whose surface was covered with a compound containing element B. The positive electrode active material of Example 1 had a D50 of 10 µm.

[0141] [Preparation of Positive Electrode Mixture Layer] The positive electrode active material according to Example 1 and acetylene black (AB), a conductive agent, were mixed in a predetermined mass ratio to obtain a first mixed powder (first mixing step). Then, this first mixed powder, carbon nanotubes (CNT), a conductive agent, and polyvinylidene fluoride (PVdF), a binder, were mixed in a predetermined mass ratio to obtain a second mixed powder (second mixing step). An appropriate amount of N-methyl-2-pyrrolidone (NMP) was then added to this second mixed powder to prepare a positive electrode mixture slurry. Before being mixed with the positive electrode active material, the AB had a structure in which multiple carbon fine particles having a particle diameter of about 20 nm were connected together, and the length of the structure was about 2 μm. Furthermore, the CNTs used had an average fiber length of 5.6 μm and an average fiber diameter of 2.3 nm. Next, the positive electrode mixture slurry was applied to both main surfaces of an aluminum foil serving as a positive electrode current collector to form coating films. The coating films were then dried and rolled. In this way, positive electrode mixture layers were formed on both main surfaces of the aluminum foil.

[0142] In the first mixed powder, the ratio of the positive electrode active material to AB was 99:1 by mass. The CNTs were 0.05 parts by mass per 100 parts by mass of the positive electrode active material, and the PVdF was 1 part by mass per 100 parts by mass of the positive electrode active material. In the first mixing step, mixing was performed for 25 minutes using a Nobilta (NOB300 type) manufactured by Hosokawa Micron Corporation to crush the AB having a structure. Furthermore, the positive electrode mixture layer had a density of 3.66 g / cm after drying. ３ The amount of the positive electrode mixture slurry applied was 260 g / cm ２ It was decided.

[0143] (3) Preparation of non-aqueous electrolyte solution After obtaining a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7, fluoroethylene carbonate (FEC) was added to the mixed solvent, and further, LiPF as a lithium salt was added. ６ and LiN(FSO ２ ) ２A non-aqueous electrolyte solution was prepared by adding FEC to the mixed solvent in an amount of 3 mass %. ６ The concentration of was 1.3 mol / L, and the concentration of LFSI was 1.0 mol / L.

[0144] (4) Fabrication of Secondary Battery After attaching lead tabs to each electrode (positive electrode and negative electrode), the positive electrode and negative electrode were spirally wound with a separator interposed therebetween so that the leads were positioned at the outermost periphery. In this way, a wound electrode body was obtained. Next, the wound electrode body was housed in an outer casing made of a laminate film with an aluminum foil barrier layer, and the outer casing housing the wound electrode body was then vacuum-dried. Next, the nonaqueous electrolyte solution was injected into the outer casing, and the opening of the outer casing was sealed. In this way, the secondary battery according to Example 1 was completed.

[0145] [Example 2] In the first step of preparing a positive electrode active material, lithium oxide and a metal oxide (Ni ０．８８ Co ０．０６ Al ０．０６ O) and calcium hydroxide (Ca(OH) ２ ) was mixed with the cake-like composition so that the molar ratio of the total amount of Li, Ni, Co, and Al to Ca was 1.03:1:0.0025 to obtain a mixture. ３ BO ３ was not added. Except for this, the positive electrode active material according to Example 2 was obtained in the same manner as in Example 1. That is, the positive electrode active material according to Example 2 was a positive electrode active material whose surface was covered with a compound containing Ca element. Then, using the positive electrode active material according to Example 2, a secondary battery according to Example 2 was completed in the same manner as in Example 1.

[0146] A secondary battery according to Example 3 was completed in the same manner as in Example 2, except that CNTs having an average fiber length of 0.4 μm and an average fiber diameter of 10 nm were used as the carbon nanotubes. Note that, like Example 2, the positive electrode active material according to Example 3 was a positive electrode active material whose surface was covered with a compound containing Ca element.

[0147] [Example 4] In the preparation of the positive electrode active material, powdered H ３ BO ３ Instead of the lithium methanesulfonate, powdered lithium methanesulfonate was added to the cake-like composition. The lithium methanesulfonate was added so as to be 0.5 mass % relative to the second baked product. Except for this, the positive electrode active material of Example 4 was obtained in the same manner as in Example 1. That is, the positive electrode active material of Example 4 was a positive electrode active material whose surface was covered with a compound containing S element. Then, using the positive electrode active material of Example 4, a secondary battery of Example 4 was completed in the same manner as in Example 1.

[0148] [Example 5] In the first step of preparing a positive electrode active material, lithium oxide and a metal oxide (Ni ０．８８ Co ０．０６ Al ０．０６ O) and strontium hydroxide (Sr(OH) ２ ) were mixed so that the molar ratio of the total amount of Li, Ni, Co, and Al to Sr was 1.03:1:0.0025 to obtain a mixture. Except for this, the positive electrode active material of Example 5 was obtained in the same manner as in Example 2. That is, the positive electrode active material of Example 5 was a positive electrode active material whose surface was covered with a compound containing Sr element. Then, using the positive electrode active material of Example 5, a secondary battery of Example 5 was completed in the same manner as in Example 2.

[0149] [Comparative Example 1] A secondary battery according to Comparative Example 1 was completed in the same manner as in Example 2, except that the first mixing step was not performed and the positive electrode active material, AB, CNT, and PVdF were mixed in the second mixing step. Note that the positive electrode active material according to Comparative Example 1 was a positive electrode active material whose surface was covered with a compound containing Ca element, similar to Example 2.

[0150] Comparative Example 2 In the preparation of the positive electrode active material, powdered H ３ BO ３ A secondary battery according to Comparative Example 2 was completed in the same manner as in Example 1, except that no compound containing B, Ca, S, or Sr was added. The positive electrode active material according to Comparative Example 2 was a positive electrode active material whose surface was not covered with a compound containing B, Ca, S, Sr, or the like.

[0151] A secondary battery according to Comparative Example 3 was completed in the same manner as in Example 2, except that CNTs were not mixed in the second mixing step. The positive electrode active material according to Comparative Example 3 was a positive electrode active material whose surface was covered with a compound containing Ca, as in Example 2.

[0152] (5) Evaluation (Initial Capacity) The secondary batteries according to each example (Examples 1 to 5 and Comparative Examples 1 to 3) were left in an environment of 25°C, and were subjected to constant current charging at a current of 0.5 It until the voltage reached 4.2 V, and then constant voltage charging at a constant voltage of 4.2 V until the current reached 0.02 It. Next, constant current discharging was performed at a current of 1.0 It until the voltage reached 2.5 V. The discharge capacity at the first discharge thus performed was referred to as the initial capacity C ０ was requested as follows.

[0153] (Capacity Retention Rate) After the initial discharge, the secondary batteries according to each example were left for 20 minutes and then subjected to 100 charge-discharge cycles under high load. The charge-discharge cycles under high load consisted of (1) constant-current charging at a current of 1.0 It until the voltage reached 4.2 V, followed by constant-voltage charging at a constant voltage of 4.2 V until the current reached 0.02 It, and (2) constant-current discharging at a current of 1.0 It until the voltage reached 2.5 V.

[0154] After repeating the charge-discharge cycle under high load 100 times, the discharge capacity at the 100th discharge is defined as the discharge capacity C １００ The initial capacity C ０ and discharge capacity C １００ The high-rate capacity retention rate X (%) was calculated using the following formula (1): High-rate capacity retention rate X (%) = (C １００ / C ０ ) × 100... (1)

[0155] (Number of 1st AB) After repeating 100 charge / discharge cycles under high load, the wound electrode body was removed from the secondary battery according to each example, and then each wound electrode body was disassembled to remove the positive electrode. Then, for the positive electrode mixture layer of the positive electrode according to each example, a 10 μm thick film was formed according to the procedure described in the above embodiment section. ２The number of particles of first acetylene black (first AB) present in the area having an area of ​​10 μm ２ The number of first acetylene blacks (first ABs) present in a region having an area of ​​1.0 or less is also simply referred to as the number of first ABs. The first AB refers to carbon black having a diameter of 100 nm or less and an aspect ratio of 1.0 or more and 1.2 or less.

[0156] For the secondary batteries according to each example, the calculation results of the high-rate capacity retention ratio X and the number of first ABs are shown in the following Table 1. For the secondary batteries according to each example, the diameter and length of the CNTs are also shown in the following Table 1. Furthermore, for the secondary batteries according to each example, the elements contained in the compound coating the surface of the positive electrode active material (hereinafter also simply referred to as elements in the coating compound) are also shown in the following Table 1.

[0157]

[0158] Table 1 shows that the nonaqueous electrolyte secondary battery according to Comparative Example 2 had a low high-rate capacity retention rate X of 53%, even though the number of first ABs was 159. This is thought to be because the surface of the positive electrode active material was not covered with a compound containing element B, etc., and therefore the crystalline structure of the positive electrode active material was not stabilized. Furthermore, the nonaqueous electrolyte secondary battery according to Comparative Example 1 had a relatively low high-rate capacity retention rate X of 100%, even though the surface of the positive electrode active material was covered with a compound containing element Ca. This is thought to be because the number of first ABs was only 12, which reduced the number of first ABs present in the microvoids formed between adjacent positive electrode active material particles, thereby increasing the internal resistance of the positive electrode. Furthermore, the nonaqueous electrolyte secondary battery according to Comparative Example 3 had a low high-rate capacity retention rate X of 38%, even though the number of first ABs was 181 and the surface of the positive electrode active material was covered with a compound containing Ca. This is thought to be because the absence of CNTs caused an increase in the internal resistance of the positive electrode.

[0159] In contrast, it can be seen that the high-rate capacity retention ratio X exceeded 100% in all of the nonaqueous electrolyte secondary batteries according to Examples 1 to 5. This is presumably because the surface of the positive electrode active material was covered with a compound containing the B element, the Ca element, the S element, or the Sr element, thereby stabilizing the crystal structure of the positive electrode active material, and also because the first AB was sufficiently present in minute gaps formed between adjacent positive electrode active material particles.

[0160] The positive electrode for a secondary battery and the nonaqueous electrolyte secondary battery according to the present disclosure can be used in applications where it is required to prevent the battery capacity from becoming difficult to maintain sufficiently.

[0161] 1: non-aqueous electrolyte secondary battery, 10: electrode group, 11: battery case, 12: sealing plate, 13: negative electrode terminal, 14: positive electrode lead, 15: negative electrode lead, 16: gasket, 17: sealing plug, 17a: liquid injection hole, 18: frame

Claims

1. A battery comprising a positive electrode current collector and a positive electrode mixture layer disposed on a surface of the positive electrode current collector, the positive electrode mixture layer including a positive electrode active material and a positive electrode conductive agent, the positive electrode active material having a composition formula of Li ｙ Ni ｘ M （１－ｘ） O ２ (wherein x≧0.5, 0<y≦1.2, and M contains at least one element selected from the group consisting of Co, Mn, Al, Fe, Ti, Sr, Ca, Zr, Nb, W, and B), a surface of the positive electrode active material is covered with a compound containing at least one element selected from the group consisting of B, P, Mg, S, Ca, Sr, Ba, Ti, W, Zr, and Al, the positive electrode conductive agent contains carbon black and carbon nanotubes, the carbon black contains first carbon black having a diameter of 100 nm or less and an aspect ratio of 1.0 to 1.2, and the positive electrode mixture layer has a thickness of 10 μm ２ a positive electrode for a secondary battery, the positive electrode comprising: 50 or more particles of the first carbon black within a region having an area of ​​100 mm; 2. The positive electrode for a secondary battery according to claim 1, wherein the carbon nanotubes have a length of 2 μm or more.

3. The positive electrode for a secondary battery according to claim 1 or 2, wherein the carbon nanotubes have a diameter of 3 nm or less.

4. The positive electrode for a secondary battery according to claim 1 or 2, wherein the standard deviation of the diameter of the first carbon black is 10% or less of the median diameter.

5. The positive electrode for a secondary battery according to claim 1 or 2, wherein the first carbon black has a diameter of 50 nm or less.

6. The positive electrode for a secondary battery according to claim 1 or 2, wherein the positive electrode active material has a D50 particle size of 2 μm or more and 30 μm or less.

7. The positive electrode for a secondary battery according to claim 1 or 2, wherein the positive electrode mixture layer further contains a positive electrode dispersant.

8. The positive electrode for a secondary battery according to claim 7, wherein the positive electrode dispersant contains at least one organic dispersant selected from the group consisting of nitrile rubbers and cellulose compounds.

9. A non-aqueous electrolyte secondary battery comprising: a positive electrode; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte, wherein the positive electrode is the positive electrode for a secondary battery according to claim 1 or 2.

10. The nonaqueous electrolyte secondary battery according to claim 9, wherein the negative electrode contains a silicon-containing material as a negative electrode active material, and the mass ratio of the silicon-containing material to the negative electrode active material is 5 mass % or more.

11. The nonaqueous electrolyte secondary battery according to claim 9, wherein the nonaqueous electrolyte contains at least one selected from the group consisting of fluoroethylene carbonate (FEC) and lithium bis(fluorosulfonyl)imide (LFSI).

12. A method for producing a cathode mixture slurry comprising: a cathode mixture slurry preparation step of preparing a cathode mixture slurry containing a cathode active material, carbon black, and carbon nanotubes; and a cathode mixture layer formation step of forming a cathode mixture layer on a surface of a cathode current collector using the cathode mixture slurry, wherein the cathode mixture slurry preparation step includes a first mixing substep of mixing the cathode active material and the carbon black to prepare a first mixed powder; and a second mixing substep of mixing the first mixed powder with the carbon nanotubes to prepare a second mixed powder, wherein in the first mixing substep, carbon black is crushed to obtain first carbon black having a diameter of 100 nm or less and an aspect ratio of 1.0 to 1.2, and the cathode active material is a powder having a composition formula of Li ｙ Ni ｘ M （１－ｘ） O ２ (wherein x≧0.5, 0<y≦1.2, and M contains at least one element selected from the group consisting of Co, Mn, Al, Fe, Ti, Sr, Ca, Zr, Nb, W, and B), and a surface of the positive electrode active material is covered with a compound containing at least one element selected from the group consisting of B, P, Mg, S, Ca, Sr, Ba, Ti, W, Zr, and Al.

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