Positive electrode for secondary battery, and secondary battery

Abstract

The present invention comprises a positive electrode, a negative electrode, a separator positioned between the positive electrode and the negative electrode, and an electrolyte, wherein the positive electrode has a positive electrode current collector and a positive electrode mixture layer that is positioned on a surface of the positive electrode current collector, the positive electrode mixture layer includes a positive electrode active material and a positive electrode conductive agent, and the positive electrode conductive agent contains single-walled carbon nanotubes. The positive electrode active material contains a lithium metal composite oxide, and the atomic ratio of Ni among non-Li metal elements contained in the lithium metal composite oxide is 80% or more. The surface of the positive electrode active material is covered by a compound that contains at least one element selected from the group consisting of B, F, P, S, Cl, Mg, Sr, Ca, Ti, W, Zr, rare earths, and Al.

Description

Positive electrode for secondary battery and secondary battery

[0001] The present disclosure relates to a positive electrode for a secondary battery and a secondary battery.

[0002] Non-aqueous electrolyte secondary batteries, such as lithium ion secondary batteries, have high power output and high energy density, and are therefore used as power sources for small consumer devices, power storage devices, and electric vehicles.

[0003] In recent years, non-aqueous electrolyte secondary batteries have been required to have even higher durability and higher energy density, and one known method for achieving this is to use a positive electrode in which a large amount of positive electrode active material with a large charge / discharge capacity is loaded per unit area and compressed under high pressure. In addition, increasing the conductivity of the positive electrode mixture layer has also been proposed as a method for improving the performance of non-aqueous electrolyte secondary batteries.

[0004] Patent Document 1 describes a dispersion liquid used in the manufacture of an electrode, which contains "bundle-type carbon nanotubes, a dispersion medium, and partially hydrogenated nitrile rubber having a residual double bond (RDB) value calculated by the following formula of 0.5 to 40% by weight, and the dispersed particle size of the carbon nanotubes has a particle size distribution D ５０ The RDB value is expressed by RDB (wt%) = BD weight / (BD weight + HBD weight) x 100, where BD means a structural unit derived from a conjugated diene, and HBD means a structural unit derived from a hydrogenated conjugated diene.

[0005] Japanese Patent Application Laid-Open No. 2020-19705

[0006] A composite oxide of lithium and a transition metal (lithium transition metal composite oxide) is used as a positive electrode active material for nonaqueous electrolyte secondary batteries. Lithium transition metal composite oxides have a relatively high resistance, and positive electrodes containing lithium transition metal composite oxides as a positive electrode active material tend to have high resistance. For this reason, the cycle retention rate is likely to decrease, especially under high loads.

[0007] Cobalt (Co)-containing composite oxides are known as lithium transition metal composite oxides. However, the price of Co has risen sharply in recent years, and supply stability is low due to the limited number of countries producing Co. Therefore, development of composite oxides that do not contain Co or have a low Co content is underway. However, these low Co-containing composite oxides often have higher resistance and tend to have a lower cycle retention rate under high load.

[0008] Furthermore, the crystal structure of these low Co-containing composite oxides is easily destabilized as lithium ions are extracted during charging, and the structure is easily changed to one that makes it difficult to reversibly store and release lithium ions, resulting in a significant decrease in cycle retention rate under high load.

[0009] In view of the above, one aspect of the present disclosure relates to a positive electrode for a secondary battery, comprising: a positive electrode current collector; and a positive electrode mixture layer disposed on a surface of the positive electrode current collector, wherein the positive electrode mixture layer includes a positive electrode active material and a positive electrode conductive agent, the positive electrode conductive agent includes single-walled carbon nanotubes, the positive electrode active material includes a lithium metal composite oxide, an atomic ratio of Ni to metal elements other than Li contained in the lithium metal composite oxide is 80% or more, and a surface of the positive electrode active material is covered with a compound including at least one element selected from the group consisting of B, F, P, S, Cl, Mg, Sr, Ca, Ti, W, Zr, rare earth elements, and Al.

[0010] Another aspect of the present disclosure relates to a secondary battery including the above-described positive electrode for a secondary battery, a negative electrode, a separator disposed between the positive electrode for a secondary battery and the negative electrode, and an electrolyte.

[0011] According to the present disclosure, a secondary battery can be realized that achieves both high capacity and high capacity retention rate during high-rate charge-discharge cycles.

[0012] The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0013] 1 is a schematic perspective view of a secondary battery according to an embodiment of the present disclosure, with a portion cut away;

[0014] The following describes embodiments of the present disclosure 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 ​​and materials may be applied as long as the effects of the present disclosure are obtained. In this specification, the term "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less." In the following description, when lower and upper limits are exemplified for numerical values ​​of specific physical properties or conditions, 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 the materials may be selected and used alone, or two or more materials may be used in combination.

[0015] The present disclosure also encompasses combinations of two or more features arbitrarily selected from the appended claims, i.e., two or more features arbitrarily selected from the appended claims can be combined unless a technical contradiction arises.

[0016] A positive electrode for a secondary battery according to an embodiment of the present disclosure (hereinafter simply referred to as "the positive electrode according to this embodiment") includes a positive electrode current collector and a positive electrode mixture layer disposed on at least the surface of the positive electrode current collector. The positive electrode mixture layer includes a positive electrode active material and a positive electrode conductive agent.

[0017] The positive electrode conductive agent includes carbon nanotubes. The carbon nanotubes include single-walled carbon nanotubes. Hereinafter, "single-walled carbon nanotubes" may be referred to as "single-walled CNTs." Carbon nanotubes other than single-walled CNTs may be referred to as "multi-walled CNTs."

[0018] Carbon nanotubes are generally classified into single-walled, double-walled, and multi-walled carbon nanotubes, where multi-walled CNTs include carbon nanotubes with two or more walls.

[0019] The positive electrode conductive agent may further contain at least one conductive agent selected from the group consisting of multi-walled CNTs and carbon black, other than the single-walled CNTs.

[0020] According to the positive electrode of this embodiment, by including single-walled CNTs in the positive electrode conductive agent, a secondary battery using this positive electrode can reduce the resistance of the positive electrode while realizing high capacity, and even when the density of the positive electrode mixture layer is increased, a high capacity retention rate (hereinafter referred to as "high-rate cycle retention rate") can be achieved under high-load charge-discharge cycle conditions. Therefore, by using the positive electrode of this embodiment, a secondary battery that achieves both high capacity and high high-rate cycle retention rate can be realized.

[0021] Carbon nanotubes have nanometer-sized fiber diameters and extremely large aspect ratios (ratio of fiber length to outer diameter). Therefore, the contact between the carbon nanotubes between the active materials and between the active materials and the current collector is linear rather than point contact. This allows the highly conductive carbon nanotubes to form linear conductive paths between the active materials and between the active materials and the current collector, and also form linear contacts with the current collector, improving current collection. Furthermore, even when the positive electrode mixture layer is rolled (compressed) after application, gaps are secured between the fibers that can hold the electrolyte, maintaining high liquid circulation.

[0022] By simply adding a small amount of single-walled CNTs to the positive electrode mixture layer, the above-mentioned line contact can be achieved, and the resistance of the positive electrode can be reduced while ensuring gaps that can hold the electrolyte within the positive electrode mixture layer. In addition, compared to multi-walled CNTs, single-walled CNTs are more likely to exist at the primary particle interface of the active material, making them less likely to obstruct the flow of electrolyte between secondary particles of the active material. Furthermore, even when single-walled CNTs exist at the secondary particle interface of the active material, their small occupancy volume makes them less likely to obstruct the flow of electrolyte. Furthermore, because a small amount of single-walled CNTs can be added to achieve a sufficient reduction in positive electrode resistance, it is easy to increase the proportion of positive electrode active material in the positive electrode mixture layer, thereby achieving a high-capacity secondary battery.

[0023] The positive electrode active material contains a lithium metal composite oxide. The atomic ratio of Ni to the metal elements other than Li contained in the lithium metal composite oxide is 80% or more. A lithium metal composite oxide with a high Ni ratio can release many lithium ions during charging, making it easy to obtain a high-capacity secondary battery. However, the high resistance makes it easy to increase the resistance of the positive electrode. In addition, the release of a large amount of lithium during charging can destabilize the crystalline structure of the active material surface, which may change to a structure that makes it difficult to reversibly store and release lithium. However, the secondary battery according to this embodiment can reduce the resistance of the positive electrode, thereby achieving both high capacity and a high high-rate cycle retention rate.

[0024] The surface of the positive electrode active material is covered with a compound (hereinafter referred to as "compound A") containing at least one element selected from the group consisting of B, F, P, S, Cl, Mg, Sr, Ca, Ti, W, Zr, rare earth elements, and Al. Compound A covers at least a portion of the surface of the lithium metal composite oxide, thereby stabilizing the crystalline structure of the surface of the lithium metal composite oxide even when a large amount of lithium is extracted during charging. As a result, compound A contributes to suppressing a decrease in the high-rate cycle retention rate.

[0025] The positive electrode active material (lithium metal composite oxide) particles may exist in the form of secondary particles formed by aggregation of a plurality of primary particles. When the active material particles exist in the form of secondary particles, compound A covers at least the surfaces of the secondary particles. Compound A may also cover the surfaces of the primary particles.

[0026] The average particle diameter of the positive electrode active material may be in the range of 2 to 30 μm. The average particle diameter means the median diameter D50 on a volume basis. When the positive electrode active material exists in the form of secondary particles formed by aggregation of multiple primary particles, the average particle diameter is the average particle diameter of the secondary particles.

[0027] The positive electrode active material may include a first positive electrode active material having an average particle size in the range of 1 μm or more and less than 5 μm, and a second positive electrode active material having an average particle size in the range of 5 μm or more and 30 μm or less. By using a mixture of two types of positive electrode active materials having different average particle sizes, a secondary battery with a higher capacity can be easily obtained and the resistance of the positive electrode can also be reduced.

[0028] The first positive electrode active material having a small particle size can be arranged in the positive electrode mixture layer so as to fill the gaps between the second positive electrode active material having a particle size larger than that of the first positive electrode active material. This allows for a high-capacity positive electrode. Additionally, the formation of a conductive path between adjacent second positive electrode active materials via the first positive electrode active material reduces the resistance of the positive electrode. Furthermore, the presence of single-walled CNTs in the conductive path between the second positive electrode active material and the first positive electrode active material forms a network of numerous branched conductive paths, further reducing the resistance of the positive electrode. As a result, both high capacity and high high-rate cycle retention can be achieved at a higher level.

[0029] When a positive electrode active material includes multiple types of positive electrode active materials (e.g., a first positive electrode active material and a second positive electrode active material) with different average particle sizes, the particle size distribution of the positive electrode active material particles may exhibit multiple peaks corresponding to the multiple types of positive electrode active materials. For example, when the positive electrode active material includes a first positive electrode active material and a second positive electrode active material, a first peak attributed to the first positive electrode active material and a second peak attributed to the second positive electrode active material may appear. These peaks may overlap and be observed as a single peak overall, but when the peaks are sufficiently separated from each other, the positive electrode active material may have a particle size distribution with two peaks in the particle size range of 1 to 30 μm (bimodal particle size distribution).

[0030] The average particle size and particle size distribution of the positive electrode active material particles can be determined by analyzing a cross-sectional image of the positive electrode mixture layer obtained using a scanning electron microscope, as described below. The grain boundaries of the positive electrode active material particles are identified in the cross-sectional image, and the diameter of a circle (equivalent circle) having an area equal to the area occupied by each positive electrode active material particle in the cross section is taken as the particle size of the positive electrode active material. The particle size distribution can be determined by determining the particle size of each active material particle appearing in the cross section of the positive electrode mixture layer. Furthermore, the volume of a sphere having a diameter equal to the diameter of the equivalent circle is taken as the volume of the active material particle, and the volume-based particle size distribution can be determined to determine the average particle size.

[0031] To achieve high capacity, the density of the positive electrode mixture layer is, for example, 3.65 g / cm ３ In order to achieve a high capacity, the coating amount of the positive electrode mixture layer on the positive electrode current collector is set to 250 g / cm or more. ２ The secondary battery according to this embodiment can achieve a high high-rate cycle retention ratio even when the coating amount of the positive electrode mixture layer is increased in this way. The coating amount refers to the value obtained by dividing the mass of the positive electrode mixture layer by the supporting area of ​​the positive electrode current collector on which the positive electrode mixture layer is supported.

[0032] The positive electrode conductive agent may contain carbon nanotubes other than single-walled CNTs (i.e., multi-walled CNTs). When the positive electrode conductive agent contains carbon nanotubes, the proportion of single-walled CNTs in the total carbon nanotubes may be 50 mass % or more, 70 mass % or more, 80 mass % or more, or 90 mass % or more.

[0033] In addition to single-walled CNTs, the positive electrode conductive agent may also contain multi-walled CNTs and particulate conductive carbon materials such as carbon black. By mixing carbon nanotubes, which are fibrous conductive materials, and carbon black, which is a particulate conductive material, in the positive electrode mixture layer, multiple conductive paths are formed between adjacent positive electrode active materials and between the positive electrode active material and the positive electrode current collector, thereby reducing the resistance of the positive electrode. Examples of carbon black include furnace black and acetylene black. The positive electrode conductive agent may also contain sheet-like carbon materials such as graphene and graphite, rod-like carbon materials such as vapor-grown carbon fiber (VGCF), and carbon fibers other than carbon nanotubes.

[0034] The content of the single-walled CNTs in the entire positive electrode conductive agent may be, for example, 0.001% by mass or more, and may be 0.005% by mass or more. According to the secondary battery of this embodiment, the addition of such a small amount of single-walled CNTs significantly reduces the positive electrode resistance. Therefore, the increase in manufacturing costs due to the use of carbon nanotubes is suppressed, and the secondary battery can be manufactured with minimal increase in manufacturing costs. The content of the single-walled CNTs in the entire positive electrode conductive agent may be 0.001% by mass or more and 100% by mass or less, 0.001% by mass or more and 50% by mass or less, or 0.001% by mass or more and 30% by mass or less. In terms of facilitating the preparation of a positive electrode slurry in which carbon nanotubes are dispersed together with the positive electrode active material and of facilitating the suppression of an increase in the viscosity of the positive electrode slurry, the content of the single-walled CNTs in the entire positive electrode conductive agent may be 0.15% by mass or less, 0.1% by mass or less, or 0.07% by mass or less.

[0035] The content of single-walled CNTs in the entire positive electrode mixture layer may be, for example, 0.0001 mass% or more, or 0.0005 mass% or more, or 0.06 mass% or less, 0.05 mass% or less, 0.04 mass% or less, 0.02 mass% or less, or 0.015 mass% or less.

[0036] The carbon nanotube content ratio relative to the entire positive electrode mixture layer can be determined from a sample obtained by extracting only the positive electrode active material layer (positive electrode mixture layer) from a discharged secondary battery. Specifically, the discharged secondary battery is first disassembled to extract the positive electrode. Next, the positive electrode is washed with an organic solvent and further vacuum-dried, and then the mixture layer is peeled off to obtain a sample. The pulverized sample is dispersed in a dispersion medium such as water or alcohol and centrifuged to separate the carbon nanotubes. Furthermore, by performing thermal analysis such as TG-DTA on the sample, the ratio of binder components and conductive components other than the positive electrode active material can be calculated. Microscopic Raman spectroscopy can be performed on a cross-section of the positive electrode mixture layer to identify carbon species such as carbon nanotubes and carbon black, and their proportions can be calculated from thermal analysis such as TG-DTA of the peeled sample.

[0037] The BET specific surface area of ​​the carbon nanotubes contained in the positive electrode mixture layer is 200 m ２ In this case, the content ratio of the single-walled CNTs in the carbon nanotubes is sufficiently high, and both reduced resistance and high capacity in the positive electrode can be achieved. ２ / g or more, 200m ２ / g or more, or 400m ２ The upper limit of the BET specific surface area of ​​the single-walled CNT is not particularly limited, but is preferably 2000 m ２ The BET specific surface area of ​​the carbon nanotubes can be measured based on the BET method (nitrogen adsorption method) described in JIS R1626 for the carbon nanotubes separated from the positive electrode mixture layer.

[0038] Among the carbon nanotubes contained in the positive electrode mixture layer, the single-walled CNTs may have an average fiber diameter of 3 nm or less, preferably 1.7 nm or less. When the average fiber diameter of the single-walled CNTs is within the above range, the single-walled CNTs can form a dense conductive network in the positive electrode mixture layer, thereby reducing the positive electrode resistance. The average fiber diameter of the single-walled CNTs may be 0.1 nm or more and 3 nm or less, 0.3 nm or more and 3 nm or less, 0.1 nm or more and 1.7 nm or less, or 0.3 nm or more and 1.7 nm or less.

[0039] The average fiber length and average fiber diameter of carbon nanotubes are determined for a positive electrode removed from a secondary battery by the following method. First, the discharged secondary battery is disassembled to remove the positive electrode. Next, the positive electrode is washed with an organic solvent and further vacuum-dried, and then only the positive electrode mixture layer is peeled off to obtain a sample. Next, the cross section of the positive electrode mixture layer is exposed. For example, a method for exposing the cross section is to cut out a portion of the positive electrode and process it using an ion milling device (e.g., IM4000PLUS, manufactured by Hitachi High-Technologies Corporation) to obtain a cross section of the positive electrode mixture layer.

[0040] Next, a cross-sectional image of the exposed positive electrode mixture layer is taken using a scanning electron microscope (SEM) or a transmission electron microscope (TEM). The magnification when taking a cross-sectional image with an SEM is, for example, 800x, and the magnification when taking a cross-sectional image with a TEM is, for example, 100,000x. The obtained image is subjected to image processing to identify the fiber diameter of the carbon nanotubes appearing in the cross-sectional image, thereby obtaining the fiber diameter distribution in the cross section. The above-mentioned cross-sectional image can also be used to identify the fiber length and number of layers of the carbon nanotubes. By identifying the fiber diameter and fiber length of the carbon nanotubes and the number of layers of the carbon nanotubes from the cross-sectional image, it is possible to calculate the mass of each carbon nanotube appearing in the cross-sectional image and determine the proportion of single-walled CNTs in the total carbon nanotubes.

[0041] A plurality of carbon nanotubes (for example, about 100 to 1000) are arbitrarily selected, and the fiber length and fiber diameter are measured, and the average values ​​are taken as the average fiber length and average fiber diameter, respectively. The fiber length refers to the length when the nanotube is in a straight line. The fiber diameter refers to the length in the direction perpendicular to the fiber length direction, and means the outer diameter of the carbon nanotube. It is known that carbon nanotubes may not be single fibers but may form a bundle of multiple nanotubes, and the average length of each carbon nanotube in this bundle state may be taken as the fiber length of each carbon nanotube. Furthermore, in the case of carbon nanotubes with a single wall, the Raman shift in the Raman spectrum is 150 to 300 cm. －１ The fiber diameter can be estimated by analyzing the RBM (Radial Breathing Mode) that appears in the range.

[0042] Generally, there is a correlation between the BET specific surface area of ​​carbon nanotubes and the fiber diameter and fiber length. Specifically, if the fiber diameter is 10 nm and the fiber length is 1 μm, the BET specific surface area is 200 m ２ / g or more and 250m ２ Therefore, the BET specific surface area of ​​the carbon nanotubes in the positive electrode taken out from the secondary battery can also be calculated using the fiber diameter and fiber length.

[0043] The positive electrode mixture layer may contain a binder. The binder may contain a nitrile-based rubber, which provides good dispersibility of carbon nanotubes when preparing a positive electrode slurry. Examples of nitrile-based rubbers include copolymers of monomers containing acrylonitrile and diene (e.g., butadiene). Examples of nitrile-based rubbers include acrylonitrile-based rubbers such as nitrile-butadiene rubber (NBR) and hydrogenated nitrile-butadiene rubber (HNBR). Of these, hydrogenated nitrile-butadiene rubber (HNBR) is most preferred.

[0044] The binder may contain polyvinylpyrrolidones (PVP) to improve the dispersibility of carbon nanotubes when preparing the positive electrode slurry. The polyvinylpyrrolidones are at least one selected from the group consisting of polyvinylpyrrolidone and polyvinylpyrrolidone derivatives. Examples of polyvinylpyrrolidone derivatives include polymers in which the hydrogen atoms of polyvinylpyrrolidone are substituted with other substituents, such as alkylated polyvinylpyrrolidone. As the polyvinylpyrrolidones, polyvinylpyrrolidone alone may be used, or a copolymer of vinylpyrrolidone and other monomolecules may be used. Examples of other monomolecules include styrene-based and vinyl acetate-based monomolecules.

[0045] Examples of the positive electrode and its components according to this embodiment will be described in more detail below. Note that known components may be applied to components that are not characteristic of the present disclosure.

[0046] (Positive Electrode) The positive electrode typically includes a positive electrode current collector and a positive electrode active material layer (positive electrode mixture layer) formed on the surface of the positive electrode current collector. The positive electrode mixture layer can be formed by applying a positive electrode slurry, in which the positive electrode mixture is dispersed in a dispersion medium, to the surface of the positive electrode current collector and drying the applied layer. The dried coating film may be rolled as necessary. The positive electrode mixture includes a positive electrode active material as an essential component and single-walled CNTs as a conductive agent. The positive electrode mixture may include optional components such as a binder and a conductive agent other than single-walled CNTs.

[0047] (Positive electrode active material) A lithium metal composite oxide can be used as the positive electrode active material. The lithium metal composite oxide may be a composite oxide having a layered structure (e.g., a rock salt crystal structure) containing lithium and a transition metal. Examples of the lithium metal composite oxide include Li ａ CoO ２ , Li ａ NiO ２ , Li ａ MnO ２ , Li ａ Co ｂ Ni １－ｂ O ２ , Li ａ Co ｂ M１－ｂ O ｃ , Li ａ Ni １－ｂ M ｂ O ｃ , Li ａ Mn ２ O ４ , Li ａ Mn ２－ｂ M ｂ O ４、 LiGPO ４、 Li ２ GPO ４ Examples of the lithium-ion battery include F. Here, 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, Zr, Nb, W, and B. G includes at least a transition element (for example, at least one selected from the group consisting of Mn, Fe, Co, and Ni). Here, 0≦a≦1.2, 0≦b≦0.9, and 2.0≦c≦2.3. The value a, which indicates the molar ratio of lithium, increases or decreases with charge and discharge.

[0048] More specifically, the lithium metal composite oxide is, for example, Li ａ Ni ｘ M １－ｘ O ２ (where 0<a≦1.2, 0.8≦x<1, and M includes at least one selected from the group consisting of Co, Al, Mn, Fe, Ti, Sr, Na, Mg, Ca, Sc, Y, Cu, Zn, Cr, Zr, Nb, W, and B). In particular, M preferably includes at least one selected from the group consisting of Co, Mn, and Fe. From the viewpoint of the stability of the crystal structure, M may include Al. Specific examples of such composite oxides include lithium-nickel-cobalt-aluminum composite oxide (LiNi ０．９ Co ０．０５ Al ０．０５ O ２ etc.)

[0049] Here, from the viewpoint of obtaining high capacity, it is desirable that the proportion of Ni in the metal elements other than Li contained in the lithium metal composite oxide is 80 atomic % or more. The proportion of Ni in the metal elements other than Li may be 85 atomic % or more (x≧0.85), or may be 90 atomic % or more (x≧0.9). The proportion of Ni in the metal elements other than Li is desirably, for example, 95 atomic % or less (x≦0.95). When limiting the range, these upper and lower limits can be combined arbitrarily.

[0050] In lithium-nickel composite oxides, Co, Mn, and / or Al may be contained as metal elements M other than Li and Ni, and contribute to stabilizing the crystal structure of composite oxides with a high Ni content. However, from the viewpoint of reducing production costs, a lower Co content is more desirable. Lithium-nickel composite oxides with a low Co content (for example, where the proportion of Co among metal elements other than Li is 5 atomic % or less) or that do not contain Co may contain Mn and Al.

[0051] In the lithium-nickel composite oxide, the higher the Ni ratio x, the more lithium ions can be extracted from the lithium-nickel composite oxide during charging, thereby increasing the capacity. From the viewpoint of obtaining a high capacity, the Ni ratio x in the lithium-containing composite oxide may be 0.85 or more (x≧0.85) or may be 0.9 or more (x≧0.9).

[0052] In terms of reducing production costs, it is preferable that the cobalt ratio in the lithium metal composite oxide is low. It is desirable that the lithium metal composite oxide does not contain Co, or that the ratio of Co to the metal elements other than Li contained in the lithium metal composite oxide is 5 atomic % or less. More specifically, the lithium-containing composite oxide is the above-mentioned lithium-nickel composite oxide, and ａ Ni ｘ Co ｙ M １ １－ｘ―ｙ O ２ (However, M １ is an element M excluding Co, and 0≦y≦0.05.

[0053] The lithium metal composite oxide tends to have a higher resistance as the Ni ratio increases and a lower Co ratio as the Co ratio decreases, resulting in a higher positive electrode resistance. However, by incorporating single-walled CNTs into the conductive agent, the positive electrode resistance can be significantly reduced, achieving both a high capacity and a high cycle retention rate under high load conditions.

[0054] The contents of the 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), an energy dispersive X-ray spectroscopy (EDX), or the like.

[0055] A metal compound may be present on the surface of the lithium metal composite oxide. The metal compound contains, for example, at least one metal element selected from the group consisting of Sr, Ca, W, Zr, rare earth elements, and Al. Examples of the Sr-containing compound include SrO, Sr(OH), and the like. ２ , and SrCO ３ Examples of compounds containing Ca include CaO and Ca(OH). ２ , and CaCO ３ Examples of compounds containing W include WO ３ Examples of the compound containing Al include Al ２ O 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 rare earth-containing compounds include oxides, hydroxides, carbonates, sulfates, nitrates, and phosphates of rare earth elements. The metal compound may contain a plurality of types of these metal elements, and examples thereof include SrAlO ４, CaAlO ４ The metal compound may further contain Li, and an example thereof is lithium tungstate.

[0056] A nonmetallic compound may be present on the surface of the lithium metal composite oxide. The nonmetallic compound contains, for example, at least one nonmetallic element selected from the group consisting of B, F, P, S, and Cl. Examples of the P-containing compound include Li, ３－ｘ H ｘ P.O. ４ (0≦x≦3) can be exemplified. ３ BO ３ , Li ３ BO ３ , Li ２ B ４ O ７ Examples of compounds containing S include LiCH ３ SO ３ , Li ２ SO ４ , Li ２ S., Li. ３ P.S. ４ Examples of compounds containing F include LiF, Li ２ SiF ６ , LiCF ３ SO ３ Examples of compounds containing Cl include LiCl, LiClO ４ Examples include:

[0057] The above-mentioned metal compound and non-metal compound cover at least a portion of the surface of the lithium metal composite oxide, thereby stabilizing the crystal structure of the lithium metal composite oxide even when a large amount of lithium is extracted during charging, and suppressing the change of the crystal structure to a structure that makes it difficult to reversibly store and release lithium.

[0058] On the other hand, if the above metal compounds and nonmetal compounds cover at least a portion of the surface of the lithium metal composite oxide, the area of ​​the active portion of the lithium metal composite oxide surface may decrease, resulting in increased resistance. However, if the positive electrode mixture layer contains single-walled CNTs, electrons are efficiently supplied to the lithium metal composite oxide surface, making it possible to reduce the resistance per unit area on the lithium metal composite oxide surface. Therefore, if the surface of the lithium metal composite oxide is covered with at least one of a metal compound and a nonmetal compound, and the positive electrode mixture layer contains single-walled CNTs, the stabilization of the crystal structure of the lithium metal composite oxide and the reduction in resistance per unit area on the lithium metal composite oxide surface can be expected to significantly improve the high-rate cycle retention rate of the secondary battery.

[0059] The lithium metal 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 heat-treating the metal composite hydroxide to obtain a metal composite oxide, and a third step of mixing the metal composite oxide with lithium hydroxide and firing the mixture. After the third step, a washing step and a drying step may be performed to obtain the lithium metal composite oxide.

[0060] In the first step, for example, a metal composite hydroxide can be synthesized by dropping an alkaline solution such as sodium hydroxide into a stirred solution of a metal salt containing Ni and an arbitrary metal element (Co, Al, Mn, etc.) and adjusting the pH to the alkaline side (e.g., 8.5 or more and 12.5 or less). The particle size of the metal composite hydroxide tends to decrease as the pH during synthesis increases. The particle size of the metal composite hydroxide can also be controlled by adjusting the amount of metal salt solution added; for example, the particle size tends to increase as the amount of solution increases. The first positive electrode active material and the second positive electrode active material can be produced separately by controlling the particle size of the metal composite hydroxide, which is their respective precursor.

[0061] In the third step, the mixture is fired at a temperature of, for example, 650°C or higher. The firing temperature is preferably in the range of 650°C or higher and 1100°C or lower. In the third step, which is preferably performed in an oxygen stream, an excess amount of lithium source (lithium hydroxide) may be used relative to the stoichiometric ratio of the target product from the viewpoint of discharge capacity. Preferably, lithium hydroxide may be used in a stoichiometric ratio of 1 to 1.1 times that of the metal composite oxide. Furthermore, by adjusting the firing conditions, the particle size of the primary particles can be adjusted, and single particles can also be produced. For example, the particle size of the single particles can be increased by increasing the maximum temperature.

[0062] In the washing step, the lithium metal composite oxide is washed with water and dehydrated to obtain a cake-like composition. The washing and dehydration can be performed by known methods and conditions. The washing and dehydration should be performed within a range that does not cause lithium to leach out of the lithium metal composite oxide and deteriorate the battery characteristics. Since the positive electrode active material according to this embodiment is washed with water, the amount of residual alkaline components is small.

[0063] 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 performed under a vacuum atmosphere. Drying conditions are, for example, 150°C to 400°C for 0.5 hours to 15 hours. A heat treatment may be performed after the drying step. The heating temperature is, for example, 100°C to 450°C. The heat treatment may be performed after the crushing step described below.

[0064] The powder composition obtained in the drying step can be crushed to obtain single particles. A jet mill or the like can be used for crushing. For crushing using a jet mill, for example, a PJM-80 (manufactured by Nippon Pneumatic Co., Ltd.) is used.

[0065] The metal compound and / or nonmetal compound can be attached to the surface of the lithium metal composite oxide by mixing the lithium metal composite oxide with a powder of a raw material of the metal compound and / or nonmetal compound, for example, during the above-mentioned lithium metal composite oxide production process (e.g., the third process), after the production process, during the cleaning process, after the cleaning process, during the drying process, or during the drying process. ２ , Sr(OH) ２ ・8H ２ O, SrO, SrCO ３ , SrSO ４ , Sr(NO ３ ) ２ , SrCl ２ , SrAlO ４ Examples of Ca raw materials include Ca(OH) ２ , CaO, CaCO ３ , CaSO ４ , Ca(NO ３ ) ２ , CaCl ２ , CaAlO ４ The Zr raw material is Zr(OH) ４ , ZrO ２ , Zr(CO ３ ) ２ , Zr(SO ４ ) ２ ・4H ２ Examples of rare earth raw materials include oxides, hydroxides, and carbonates of rare earth elements. Examples of W raw materials include tungsten oxide (WO ３ ), lithium tungstate (Li ２ WO ４ , Li ４ WO ５ , 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 ) ３The P raw material may be Al derived from a lithium transition metal composite oxide. ３－ｘ H ｘ P.O. ４ (0≦x≦3) and the like. ３ BO ３ , Li ３ BO ３ , Li ２ B ４ O ７ These compounds may be used after being pulverized to appropriately change the particle size or by adjusting the water content, including that of hydrates.

[0066] (Carbon Nanotubes) Carbon nanotubes are carbon fibers with nanometer-sized fiber diameters and extremely large aspect ratios (ratio of fiber length to outer diameter). Carbon fibers with large aspect ratios form linear contacts between active materials and between the active materials and the current collector, rather than point contacts. Carbon fibers with excellent electrical conductivity are interposed between active material particles, forming linear contacts with the particles. This allows the highly conductive carbon nanotubes to form linear conductive paths between the active materials and between the active materials and the current collector, and also form linear contacts with the current collector, improving current collection. In particular, single-walled CNTs occupy a small volume within the positive electrode mixture layer. This allows for a high proportion of positive electrode active material in the mixture layer while maintaining high conductivity of the positive electrode mixture layer, facilitating high capacity.

[0067] The average fiber length of the carbon nanotubes is, for example, 0.4 μm or more. In this case, even when the volume of the negative electrode active material changes significantly due to charge and discharge, linear contact with the carbon nanotube fibers is maintained in accordance with the volume change, and electrical connection with the positive electrode active material can be maintained. When the average fiber length of the carbon nanotubes is 0.4 μm or more, current collection defects are significantly suppressed. The average fiber length may be 0.8 μm or more, and preferably 1.0 μm or more.

[0068] On the other hand, as the average fiber length of the carbon nanotubes increases, they tend to aggregate more easily, which can lead to poor dispersion and increased viscosity of the slurry during slurry preparation due to aggregation of the carbon nanotubes. Furthermore, the longer the average fiber length, the greater the carbon nanotube content in the mixture layer when securing the required number of carbon nanotubes, making it difficult to achieve high capacity. To achieve high capacity, to facilitate preparation of a slurry in which the carbon nanotubes are dispersed together with the active material, and to suppress an increase in the viscosity of the slurry, the average fiber length of the carbon nanotubes may be 8 μm or less. The average fiber length is preferably 4 μm or less.

[0069] The average fiber length of the carbon nanotubes may be 0.4 μm to 40 μm, 0.6 μm to 35 μm, or 1.0 μm to 30 μm.

[0070] The average fiber diameter of the carbon nanotubes may be 0.5 nm or more, 0.7 nm or more, or 1 nm or more. On the other hand, the larger the average fiber diameter of the carbon nanotubes, the fewer carbon nanotubes contained in the mixture layer when the carbon nanotube content is the same, making it difficult to suppress current collection defects. To suppress current collection defects, the carbon nanotube content may be increased, but the higher the carbon nanotube content in the slurry, the more likely the carbon nanotubes are to aggregate in the slurry, making it difficult to uniformly disperse them, and the more likely the viscosity of the slurry is to increase. In order to easily prepare a slurry in which carbon nanotubes are dispersed together with the active material and to suppress an increase in the viscosity of the slurry, the average fiber diameter of the carbon nanotubes may be 20 nm or less. By setting the average fiber diameter to 20 nm or less, current collection defects are significantly suppressed and a positive electrode that is easy to manufacture is obtained.

[0071] Among the carbon nanotubes used as cathode conductive agents, single-walled CNTs have an ideal one-dimensional structure for electron conductivity, allowing electrons to conduct freely along the axis of the single-walled CNT. Therefore, electrons conduct in single-walled CNTs without crossing layers, and as a result, using single-walled CNTs as cathode conductive agents can increase conductivity. On the other hand, multi-walled CNTs have interactions between layers, so electrons must cross layers, which can result in reduced conductivity.

[0072] As the positive electrode conductive agent, a conductive carbon material other than carbon nanotubes may be mixed with carbon nanotubes. Examples of conductive carbon materials other than carbon nanotubes include at least one selected from the group consisting of amorphous carbon and carbon fiber. Amorphous carbon includes hard carbon and soft carbon. Examples of soft carbon include carbon black, which is a particulate conductive carbon material, such as acetylene black, ketjen black, and furnace black. Examples of carbon fiber include rod-shaped carbon materials such as VGCF, sheet-shaped carbon materials such as graphene, and carbon fiber. A combination of these materials may also be used as the positive electrode conductive agent. However, if a large amount of conductive carbon materials other than carbon nanotubes is included in the positive electrode mixture layer, the proportion of the positive electrode material in the positive electrode mixture layer decreases. Therefore, the mass of the conductive agent other than the carbon nanotubes contained in the positive electrode active material layer may be 100 times or less (for example, in a range of 0 to 80 times, 0 to 50 times, or 0 to 10 times) the mass of the carbon nanotubes contained in the positive electrode mixture layer.

[0073] Examples of carbon nanotubes include carbon nanofibers. Various types of carbon nanotubes are commercially available, so commercially available ones may be used. Alternatively, carbon nanotubes may be synthesized by known synthesis methods.

[0074] The binder and conductive agent may be the same as those exemplified for the negative electrode. Hydrogenated nitrile butadiene rubber (HNBR) may be used as the binder because it improves the dispersibility of carbon nanotubes in the positive electrode slurry.

[0075] The shape and thickness of the positive electrode current collector can be selected from the shape and range corresponding to those of the negative electrode current collector. Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0076] [Secondary Battery] A secondary battery according to an embodiment of the present disclosure includes the positive electrode (secondary battery positive electrode) according to the embodiment described above, a negative electrode, a separator, and an electrolyte. The separator is disposed between the positive electrode and the negative electrode. The secondary battery may be a non-aqueous electrolyte secondary battery. The positive electrode, negative electrode, electrolyte, and separator may be disposed in, for example, an exterior body (battery case).

[0077] The shape of the secondary battery is not limited, and may be cylindrical, rectangular, coin-shaped, button-shaped, pouch-shaped, etc. The battery case is selected according to the shape of the secondary battery.

[0078] (Negative Electrode) The negative electrode typically includes a negative electrode current collector and a negative electrode active material layer (negative electrode mixture layer) disposed on the surface of the negative electrode current collector. The negative electrode mixture layer can be formed by applying a negative electrode slurry, in which the negative electrode mixture is dispersed in a dispersion medium, to the surface of the negative electrode current collector and drying the applied layer. The dried coating may be rolled as necessary. The negative electrode mixture includes a negative electrode active material as an essential component and, if necessary, includes other components in addition to the negative electrode active material. Examples of other components include a binder, a conductive agent, a thickener, etc. These other components may include components used in known secondary batteries. The negative electrode active material may include a silicon-containing material.

[0079] (Negative Electrode Active Material) Although metallic lithium, lithium alloys, etc. may be used as the negative electrode active material, materials capable of electrochemically absorbing and releasing lithium ions are preferably used. Examples of such materials include carbon materials and alloy-based materials. Examples of carbon materials include graphite, easily graphitized carbon (soft carbon), and hardly graphitized carbon (hard carbon). Examples of alloy-based materials include those containing at least one metal capable of forming an alloy with lithium, such as silicon, tin, silicon alloys, tin alloys, and silicon compounds. Silicon oxides and tin oxides formed by bonding these with oxygen may also be used. The negative electrode active material preferably contains a material containing silicon element (hereinafter sometimes referred to as a "Si-containing material"), and more preferably contains graphite and a Si-containing material.

[0080] Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles. The graphite may be any known graphite used as a negative electrode active material. A carbon material other than graphite may be included in the negative electrode active material. One type of carbon material may be used alone, or two or more types may be used in combination. Among carbon materials, graphite is preferred because it has excellent charge / discharge stability and a small irreversible capacity.

[0081] Graphite refers to a material with a developed graphite crystal structure, and generally has an average interplanar spacing d of (002) planes measured by X-ray diffraction. ００２ The carbon material has a particle size of 0.340 nm or less.

[0082] Examples of the Si-containing material include simple 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). Examples of silicon oxides include SiO ＸExamples of the lithium ion conductive phase include particles. X is, for example, 0.5≦X<2, and 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 main component (e.g., 95 to 100% by mass) of the silicon oxide phase may be silicon dioxide. Among these, a composite material composed of a silicate phase and a silicon phase dispersed in the silicate phase is preferred because of its high capacity and low irreversible capacity.

[0083] The silicate phase may contain, for example, at least one element selected from the group consisting of Group 1 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), and titanium (Ti). Among these, a silicate phase containing lithium (hereinafter also referred to as a lithium silicate phase) is preferred due to its small irreversible capacity and high initial charge / discharge efficiency.

[0084] The lithium silicate phase may be an oxide phase containing lithium (Li), silicon (Si), and oxygen (O), and may contain other elements. The atomic ratio of O to Si in the lithium silicate phase, O / Si, is, for example, greater than 2 and less than 4. Preferably, O / Si is greater than 2 and less than 3. The atomic ratio of Li to Si in the lithium silicate phase, Li / Si, is, for example, greater than 0 and less than 4. The lithium silicate phase has the formula: Li ２ｚ SiO ２＋ｚ (0<z<2). Preferably, z satisfies the relationship 0<z<1, and more preferably z=1 / 2. Examples of elements other than Li, Si, and O that can be contained in the lithium silicate phase include iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), molybdenum (Mo), zinc (Zn), and aluminum (Al).

[0085] The average particle size (volume-based median diameter D50) of composite particles composed of a silicate phase and silicon particles (silicon phase) dispersed in the silicate phase may be in the range of 1 μm to 25 μm (e.g., 4 μm to 15 μm). This range facilitates mitigating stress caused by volumetric changes in the composite particles during charge and discharge, making it easier to achieve good cycle characteristics. Furthermore, the surface area of ​​the composite particles is also appropriate, suppressing capacity loss due to side reactions with the nonaqueous electrolyte.

[0086] 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 kept small, making it less likely for the silicon particles to deteriorate, which would otherwise lead to the generation of irreversible capacity. The crystallite size of the silicon particles is calculated using the Scherrer equation from the half-width of the diffraction peak assigned to the Si(111) plane in the X-ray diffraction (XRD) pattern of the silicon particles.

[0087] The average particle size of the silicon particles may be preferably 500 nm or less (more preferably 200 nm or less, and even more preferably 50 nm or less) before the first charge. After the first charge, the average particle size of the silicon particles is preferably 400 nm or less (more preferably 100 nm or less). By miniaturizing the silicon particles, the volume change during charge and discharge is reduced, and the structural stability of the second particles is further improved. The average particle size of the silicon particles is determined by observing the cross section of the composite particle using SEM or TEM and averaging the longest diameters of 100 or more silicon particles in the cross-sectional image.

[0088] From the viewpoint of increasing capacity and improving cycle characteristics, the content of silicon particles (elementary Si) in the composite particles is preferably in the range of 20% by mass to 95% by mass (e.g., in the range of 35% by mass to 75% by mass). This range also improves lithium ion diffusibility, making it easier to obtain excellent load characteristics. Furthermore, the exposed surface of the silicon particles that is not covered with the lithium silicate phase is reduced, suppressing side reactions between the nonaqueous electrolyte and the silicon particles.

[0089] The composite particles may include a conductive material that coats at least a portion of their surfaces. Because the lithium silicate phase has poor electronic conductivity, the conductivity of the composite particles tends to be low. By coating the surfaces with a conductive material, the conductivity can be dramatically increased. The conductive layer is preferably thin enough that it does not substantially affect the average particle size of the composite particles. For example, from the viewpoint of ensuring conductivity and lithium ion diffusibility, the thickness of the conductive layer may be 1 nm to 200 nm (e.g., in the range of 5 nm to 100 nm).

[0090] The carbon phase may be composed of, for example, amorphous carbon with low crystallinity. The amorphous carbon may be, for example, 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 a carbon material having a particle size exceeding 0.34 nm.

[0091] Examples of Si-containing materials include SiO Ｘ At 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 lithium silicate phase, and third particles containing a carbon phase and a silicon phase dispersed in the carbon phase may be used. The Si-containing material may contain multiple types of particles selected from the group consisting of the first particles, the second particles, and the third particles. For example, the Si-containing material may be composed of two types of particles selected from these, or may contain all three types of particles. Specifically, the Si-containing material may include the first particles and the second particles, the first particles and the third particles, or the second particles and the third particles. Alternatively, the Si-containing material may include all of the first, second, and third particles. The Si-containing material is preferably used as a negative electrode active material in combination with graphite.

[0092] The second and third particles each have a so-called sea-island structure. The silicon particles (islands) in the second and third particles are dispersed in a matrix (sea) of silicate and carbon phases, respectively, and are covered with an ion-conducting phase (silicate and carbon phases). The sea-island structure limits contact between the silicon particles and the electrolyte, thereby suppressing side reactions. Furthermore, stress generated by the expansion and contraction of the silicon particles is alleviated by the matrix of the lithium ion-conducting phase.

[0093] The composition and component contents of the second and third particles can be analyzed by the method described in WO 2018 / 179969.

[0094] The content of each element contained in the Si-containing material may be measured, for example, by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Specifically, the Si-containing material is dissolved in a heated acid solution, and carbon remaining in the solution is removed by filtration. The resulting 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.

[0095] The average particle diameter of the graphite (graphite particles) contained as the active material in the negative electrode may be 13 μm or more and 25 μm or less. The average particle diameter of the graphite is preferably larger than the average particle diameter of the Si-containing material. According to this configuration, voids are formed between the relatively large graphite particles, and the Si-containing material particles are easily accommodated in the voids. Therefore, it is easy to increase the filling rate of the active material in the negative electrode, making it easier to obtain a negative electrode with a higher capacity. Furthermore, the Si-containing material particles present in the voids contribute to maintaining electronic contact between the graphite particles. On the other hand, even if the Si-containing material particles present in the voids expand or contract, the expansion or contraction of the entire negative electrode is unlikely to occur, and therefore deterioration due to charge / discharge cycles is unlikely to occur.

[0096] When the negative electrode active material contains graphite and a Si-containing material, the content of the Si-containing material relative to the total of the graphite and the Si-containing material may be 5 to 50 mass %. In this case, a higher capacity can be achieved compared to when the negative electrode active material is graphite alone. The content of the Si-containing material relative to the total of the graphite and the Si-containing material is preferably 6 to 20 mass %.

[0097] The graphite content in the negative electrode active material may be in the range of 50 to 99 mass %. Note that when particles of the Si-containing material contain graphite on the surface and / or inside, the graphite is not included in the above graphite content. The graphite content is the content of graphite not contained in the Si-containing material.

[0098] The negative electrode active material may contain an active material other than graphite and a Si-containing material, such as a carbonaceous material other than graphite, such as easily graphitizable carbon (soft carbon) or hardly graphitizable carbon (hard carbon).

[0099] As the negative 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 the material for the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, and copper alloy. The thickness of the negative electrode current collector is not particularly limited, but may be, for example, 1 to 50 μm, or 5 to 30 μm.

[0100] Examples of the dispersion medium used to prepare the negative electrode slurry include water, alcohol, ether, N-methyl-2-pyrrolidone (NMP), a mixed solvent of these, etc. The ratio of the components in the negative electrode mixture can be adjusted by changing the mixing ratio of the materials for the negative electrode mixture.

[0101] Examples of binders include fluororesin, polyolefin resin, polyamide resin, polyimide resin, vinyl resin, styrene-butadiene copolymer rubber (SBR), polyacrylic acid and its derivatives and salts. Examples of conductive agents include conductive carbon materials, carbon fluoride, organic conductive materials, etc. Examples of thickeners include carboxymethyl cellulose (CMC), polyvinyl alcohol, etc. These components may be used alone or in combination of two or more materials.

[0102] (Electrolyte) The electrolyte may be an electrolytic solution containing a solvent and a solute dissolved in the solvent. The solute is an electrolyte salt that ionically dissociates in the electrolytic solution. The solute may include, for example, a lithium salt. Components of the electrolytic solution other than the solvent and the solute are additives. The electrolytic solution may include various additives.

[0103] A non-aqueous solvent is used as the solvent. Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), and vinylene carbonate (VC). Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of chain carboxylic acid esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP). One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0104] Other examples of the non-aqueous solvent include cyclic ethers, chain ethers, nitriles such as acetonitrile, and amides such as dimethylformamide.

[0105] 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.

[0106] Examples of chain ethers include 1,2-dimethoxyethane, dimethyl ether, diethyl 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, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0107] These solvents may be fluorinated solvents in which some of the hydrogen atoms are substituted with fluorine atoms. As the fluorinated solvent, fluoroethylene carbonate (FEC) may be used because it is easy to form a stable solid electrolyte film and to improve the high-rate cycle retention rate.

[0108] Examples of the lithium salt include lithium salts of chlorine-containing acids (LiClO ４ , LiAlCl ４ , LiB １０ Cl １０ etc.), lithium salts of fluorine-containing acids (LiPF ６ , LiPF ２ O ２ , LiBF ４ , LiSbF ６ , LiAsF ６ , LiCF ３ SO ３ , LiCF ３ CO ２ etc.), lithium salt of fluorine-containing acid imide (LiN(FSO ２ ) ２ , LiN(CF ３ SO ２ ) ２ , LiN(CF ３ SO ２ ) (C ４ F ９ SO２ ), LiN(C ２ F ５ SO ２ ) ２ Lithium salts such as LiCl, LiBr, and LiI can be used alone or in combination of two or more. Among these, LiN(FSO ２ ) ２ (LFSI) may also be used.

[0109] The concentration of the lithium salt in the electrolyte solution may be 1 mol / L or more and 2 mol / L or less, or 1 mol / L or more and 1.5 mol / L or less. By controlling the lithium salt concentration within the above range, an electrolyte solution having excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0110] The electrolyte may contain other known additives, such as 1,3-propane sultone, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, and fluorobenzene.

[0111] (Separator) A separator is interposed between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulation. The separator can be made of a microporous thin film, woven fabric, nonwoven fabric, or the like. Polyolefins such as polypropylene and polyethylene are preferred as the separator material. Aramid fibers or the like may also be used to increase mechanical strength.

[0112] 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.

[0113] 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.

[0114] 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.

[0115] (Additional Notes) The above embodiments disclose the following technologies. (Technology 1) A positive electrode for a secondary battery, comprising: a positive electrode current collector; and a positive electrode mixture layer disposed on a surface of the positive electrode current collector, wherein the positive electrode mixture layer contains a positive electrode active material and a positive electrode conductive agent, wherein the positive electrode conductive agent contains single-walled carbon nanotubes, and the positive electrode active material contains a lithium metal composite oxide, wherein an atomic ratio of Ni to metal elements other than Li contained in the lithium metal composite oxide is 80% or more, 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, F, P, S, Cl, Mg, Sr, Ca, Ti, W, Zr, rare earth elements, and Al. (Technology 2) A positive electrode for a secondary battery according to Technology 1, wherein the positive electrode conductive agent further contains at least one element selected from the group consisting of carbon nanotubes other than the single-walled carbon nanotubes and carbon black. (Technology 3) The positive electrode for a secondary battery according to Technology 1 or 2, wherein the content ratio of the single-walled carbon nanotubes in the entire positive electrode mixture layer is 0.0005% by mass or more and 0.05% by mass or less. (Technology 4) The positive electrode for a secondary battery according to any one of Technology 1 to 3, wherein the average particle diameter of the positive electrode active material is in the range of 2 to 30 μm. (Technology 5) The positive electrode for a secondary battery according to any one of Technology 1 to 4, wherein the positive electrode active material includes a first positive electrode active material having an average particle diameter in the range of 1 μm or more and less than 5 μm, and a second positive electrode active material having an average particle diameter in the range of 5 μm or more and 30 μm or less. (Technology 6) The positive electrode for a secondary battery according to any one of Technology 1 to 5, wherein at least one element selected from the group consisting of B, S, Sr, and Ca is present on the surface of the positive electrode active material. (Technology 7) The positive electrode for a secondary battery according to any one of Technologies 1 to 6, wherein the positive electrode mixture layer further contains a binder, and the binder contains a nitrile-based rubber. (Technology 8) The positive electrode for a secondary battery according to any one of Technologies 1 to 7, wherein the positive electrode mixture layer further contains a binder, and the binder contains a polyvinylpyrrolidone. (Technology 9) The positive electrode active material is a compound represented by the formula Li ａ Ni ｘ M １－ｘ O ２(wherein 0<a≦1.2, 0.8≦x≦1, and M includes at least one selected from the group consisting of Co, Al, Mn, Fe, Ti, Sr, Na, Mg, Ca, Sc, Y, Cu, Zn, Cr, Zr, Nb, W, and B). (Technology 10) The positive electrode for a secondary battery according to Technology 9, wherein x≧0.85 in the lithium metal composite oxide. (Technology 11) The positive electrode for a secondary battery according to Technology 9 or 10, wherein the lithium metal composite oxide does not contain Co, or the atomic ratio of Co to metal elements other than Li contained in the lithium metal composite oxide is 5% or less. (Technology 12) A secondary battery comprising the positive electrode for a secondary battery according to any one of Technology 1 to 11, a negative electrode, a separator arranged between the positive electrode for a secondary battery and the negative electrode, and an electrolyte. (Technology 13) The secondary battery according to Technology 12, wherein the electrolyte contains at least one selected from the group consisting of fluoroethylene carbonate (FEC) and lithium bis(fluorosulfonyl)imide (LFSI).

[0116] 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.

[0117] 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 slurry. A mixture of graphite and a Si-containing material was used as the negative electrode active material. The mixture ratio of the graphite and the Si-containing material in the negative electrode active material was graphite:Si-containing material = 92:8 by mass. As the Si-containing material, composite particles (second particles) having a sea-island structure in which a silicon phase was dispersed in a lithium silicate phase were prepared. The average particle size of the composite particles was 5 μm, and the ratio of the lithium silicate phase to the silicon phase was 50:50 by mass.

[0118] Next, the negative electrode slurry was applied to the surface of a copper foil (negative electrode current collector) to form a coating film. The coating film was dried and then rolled. In this way, negative electrode active material layers were formed on both sides of the copper foil.

[0119] (2) Preparation of Positive Electrode [Synthesis of First Positive Electrode Active Material] [Ni ０．８８ Co ０．０６ Mn ０．０６ ](OH) ２ The composite hydroxide represented by the formula (I) was calcined at 500°C for 8 hours, and the metal oxide (Ni ０．８８ Co ０．０６ Mn ０．０６ O ２ 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 4 μm.

[0120] Next, lithium oxide 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 flow with an oxygen concentration of 95% (10 cm sintering furnace). ３ The mixture was fired at a flow rate of 2 mL / min per kg of mixture and 5 L / min per kg of mixture at a temperature increase rate of 2.0°C / min from room temperature to 650°C, and then at a temperature increase rate of 0.5°C / min from 650°C to 780°C to obtain a fired product (second step). Water was added to this fired product so that the slurry concentration was 1500 g / L, and the mixture was stirred for 15 minutes and filtered to obtain a cake-like composition. This cake-like composition was mixed with powdered H ３ BO ３ At this time, the amount of H added was adjusted so that the molar ratio of B was 0.1 mol % relative to the total amount of Ni, Co, and Mn contained in the lithium transition metal composite oxide. ３ BO ３ After the adding step, a drying step was carried out under conditions of 180° C. for 2 hours in a vacuum atmosphere, thereby obtaining a first positive electrode active material of Example 1 having a D50 of 4 μm (third step).

[0121] [Synthesis of Second Positive Electrode Active Material] A second positive electrode active material was obtained in the same manner as in the synthesis of the first positive electrode active material, except that 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.

[0122] The positive electrode active material was a mixture of the first positive electrode active material and the second positive electrode active material in a mass ratio of 2:8. The positive electrode active material, carbon nanotubes as a conductive agent, and hydrogenated nitrile butadiene rubber (HNBR) were mixed in a predetermined mass ratio to prepare a positive electrode slurry. The carbon nanotubes were single-walled CNTs (average fiber length 300 μm, average fiber diameter 1.5 nm, BET surface area 700 m). ２ Next, the positive electrode slurry was applied to the surface of an aluminum foil serving as a positive electrode current collector, and the coating was dried and then rolled to form positive electrode mixture layers on both sides of the aluminum foil.

[0123] The mass ratio of the positive electrode active material, single-walled CNT, and HNBR in the positive electrode slurry was positive electrode active material:single-walled CNT:HNBR=99.998:0.001:0.001. The positive electrode mixture layer had a density of 3.66 g / cm after drying. ３ The coating amount of the positive electrode mixture layer was 260 g / cm ２ It was decided.

[0124] (3) Preparation of Electrolyte Solution Fluoroethylene carbonate (FEC) was added to a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7 so that the amount of FEC was 3 mass % relative to the mixed solvent. LiPF6 and lithium bis(fluorosulfonyl)imide LiN(FSO ２ ) ２ The electrolyte solution was prepared by adding LiPF6 and LFSI to the non-aqueous electrolyte solution at a concentration of 1.3 mol / L. The concentration of LFSI in the non-aqueous electrolyte solution was 1.0 mol / L.

[0125] (4) Fabrication of Secondary Battery A lead tab was attached to each electrode. Next, the positive electrode and negative electrode were spirally wound with a separator interposed therebetween so that the lead was located at the outermost periphery. In this manner, an electrode group was fabricated. Next, the electrode group was inserted into an outer casing made of a laminate film with an aluminum foil barrier layer, and vacuum dried. Next, a nonaqueous electrolyte solution was injected into the outer casing, and the opening of the outer casing was sealed. In this manner, a secondary battery A1 according to Example 1 was completed.

[0126] (5) Evaluation (Initial Capacity) The completed battery was placed in a 25°C environment and 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. Subsequently, 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 was defined as the initial capacity C ０ was requested as follows.

[0127] (Capacity Retention Rate) After the initial discharge, the battery was left for 20 minutes and then subjected to 100 charge-discharge cycles under the following high load: (Charge) The battery was charged at a constant current of 1.0 It until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.02 It; (Discharge) The battery was discharged at a constant current of 1.0 It until the voltage reached 2.5 V.

[0128] The discharge capacity after repeating the above charge-discharge cycle 100 times is C １００ The high rate capacity retention rate X (%) was evaluated using the following formula: (High rate capacity retention rate X (%)) = 100 × C １００ / C ０

[0129] Example 2 In the synthesis of the first and second positive electrode active materials, lithium oxide and a metal oxide (Ni ０．８８ Co ０．０６ Mn ０．０６ O ２ ) and calcium hydroxide (Ca(OH) ２ ) were mixed so that the molar ratio of Li, the total amount of Ni, Co, and Mn, to Ca was 1.03:1:0.0025 to obtain a mixture (first step).３ BO ３ was not added. Except for this, the first and second positive electrode active materials were synthesized in the same manner as in Example 1. Using the synthesized first and second positive electrode active materials, a secondary battery A2 was completed in the same manner as in Example 1 and evaluated in the same manner.

[0130] Example 3 As carbon nanotubes, the single-walled CNTs used in Example 1 and multi-walled CNTs (average fiber length 1 μm, BET surface area 250 m) were used. ２ / g). The ratio of the positive electrode active material, single-walled CNT, multi-walled CNT, and HNBR in the positive electrode slurry was set to a mass ratio of positive electrode active material:single-walled CNT:multi-walled CNT:HNBR=99.798:0.001:0.20:0.001. Except for the above, a secondary battery A3 according to Example 3 was completed in the same manner as in Example 2 and evaluated in the same manner.

[0131] Example 4 The conductive agent used was the same as in Example 1, but containing acetylene black (AB). The mass ratio of the positive electrode active material, single-walled CNT, AB, and HNBR in the positive electrode slurry was positive electrode active material:single-walled CNT:AB:HNBR=98.998:0.001:1:0.001. A secondary battery A4 according to Example 4 was completed in the same manner as in Example 2 and evaluated in the same manner.

[0132] Example 5 A secondary battery A5 according to Example 5 was completed in the same manner as in Example 2, except that the mass ratio of the positive electrode active material, single-walled CNT, and HNBR in the positive electrode slurry was set to positive electrode active material:single-walled CNT:HNBR=99.9:0.05:0.05.

[0133] Example 6 In the synthesis of the first and second positive electrode active materials, powdered H ３ BO ３Instead, powdered lithium methanesulfonate was added to the cake-like composition. The amount of lithium methanesulfonate added was adjusted so that the lithium methanesulfonate content was 0.5 mass% relative to the lithium transition metal composite oxide obtained by firing. Except for this, first and second positive electrode active materials were synthesized in the same manner as in Example 1. Using the synthesized first and second positive electrode active materials, a secondary battery A6 was completed in the same manner as in Example 1 and evaluated in the same manner.

[0134] Example 7 In the synthesis of the first and second positive electrode active materials, lithium oxide and a metal oxide (Ni ０．８８ Co ０．０６ Mn ０．０６ O ２ ) and strontium hydroxide (Sr(OH) ２ ) were mixed so that the molar ratio of Li, Ni, Co, and Mn to Sr was 1.03:1:0.0025 to obtain a mixture (first step). Except for this, the first and second positive electrode active materials were synthesized in the same manner as in Example 2. Using the synthesized first and second positive electrode active materials, a secondary battery A7 was completed in the same manner as in Example 2 and evaluated in the same manner.

[0135] Example 8: [Ni] synthesized by coprecipitation method ０．８８ Co ０．０６ Mn ０．０６ ](OH) ２ The composite hydroxide represented by the formula (I) was calcined at 500°C for 8 hours, and the metal oxide (Ni ０．８８ Co ０．０６ Mn ０．０６ O ２ 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 8 μm.

[0136] Other than this, a cathode active material having a D50 of 8 μm was obtained in the same manner as in the synthesis of the first cathode active material in Example 2. A secondary battery A8 was completed using only this cathode active material in the same manner as in Example 2 and was similarly evaluated.

[0137] Comparative Example 1 A positive electrode active material, carbon nanotubes as a conductive agent, and hydrogenated nitrile butadiene rubber (HNBR) were mixed in a predetermined mass ratio to prepare a positive electrode slurry. The carbon nanotubes were multi-walled CNTs (average fiber length 1 μm, BET surface area 250 m). ２ / g). The mass ratio of the positive electrode active material, multi-walled CNT, and HNBR in the positive electrode slurry was positive electrode active material:multi-walled CNT:HNBR=98.99:1:0.01. Except for this, a secondary battery B1 according to Comparative Example 1 was completed in the same manner as in Example 2 and evaluated in the same manner.

[0138] Comparative Example 2 In the synthesis of the first and second positive electrode active materials, powdered H ３ BO ３ was not added to the cake-like composition. Except for this, the first and second positive electrode active materials were synthesized in the same manner as in Example 1. Using the synthesized first and second positive electrode active materials, a secondary battery B2 according to Comparative Example 2 was completed in the same manner as in Example 1 and evaluated in the same manner.

[0139] Table 1 shows the evaluation results for Batteries A1 to A8 and B1 to B2. In Table 1, the high-rate capacity retention ratio X is shown as a relative value, with the result for Battery B1 of Comparative Example 1 set to 100. As shown in Table 1, Batteries A1 to A8, in which the positive electrode contains single-walled carbon nanotubes and the surface of the positive electrode active material is covered with a compound containing an additive element, were able to increase the capacity retention ratio at a high rate.

[0140]

[0141] The positive electrode for a secondary battery according to the present disclosure can provide a secondary battery having high capacity and excellent high-rate cycle retention. The positive electrode for a secondary battery and the secondary battery according to the present disclosure are useful for small consumer applications, power storage devices, and main power sources for electric vehicles.

[0142] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to 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.

[0143] 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 positive electrode for a secondary battery, comprising a positive electrode current collector and a positive electrode mixture layer disposed on the 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 conductive agent including single-walled carbon nanotubes, the positive electrode active material including a lithium metal composite oxide, an atomic ratio of Ni in metal elements other than Li contained in the lithium metal composite oxide being 80% or more, and a surface of the positive electrode active material being covered with a compound containing at least one element selected from the group consisting of B, F, P, S, Cl, Mg, Sr, Ca, Ti, W, Zr, rare earths, and Al.

2. The positive electrode for a secondary battery according to claim 1, wherein the positive electrode conductive agent further includes at least one selected from the group consisting of carbon nanotubes other than the single-walled carbon nanotubes and carbon black.

3. The positive electrode for a secondary battery according to claim 1, wherein a content ratio of the single-walled carbon nanotubes in the entire positive electrode mixture layer is 0.0005% by mass or more and 0.05% by mass or less.

4. The positive electrode for a secondary battery according to claim 1, wherein an average particle diameter of the positive electrode active material is in a range of 2 to 30 μm.

5. The positive electrode for a secondary battery according to claim 1, wherein the positive electrode active material includes a first positive electrode active material having an average particle diameter in a range of 1 μm or more and less than 5 μm and a second positive electrode active material having an average particle diameter in a range of 5 μm or more and 30 μm or less.

6. The positive electrode for a secondary battery according to claim 1, wherein at least one element selected from the group consisting of B, S, Sr, and Ca is present on a surface of the positive electrode active material.

7. The positive electrode for a secondary battery according to claim 1, wherein the positive electrode mixture layer further includes a binder, and the binder includes a nitrile rubber.

8. The positive electrode for a secondary battery according to claim 1, wherein the positive electrode mixture layer further includes a binder, and the binder includes polyvinylpyrrolidones.

9. The positive electrode active material is represented by the formula Li ａ Ni ｘ M １－ｘ O ２ (where 0 < a ≤ 1.2, 0.8 ≤ x ≤ 1, and M contains at least one selected from the group consisting of Co, Al, Mn, Fe, Ti, Sr, Na, Mg, Ca, Sc, Y, Cu, Zn, Cr, Zr, Nb, W, and B). The positive electrode for a secondary battery according to claim 1, which contains a lithium metal composite oxide represented by this formula.

10. In the lithium metal composite oxide, x ≧ 0.

85. The positive electrode for a secondary battery according to claim 9.

11. The positive electrode for a secondary battery according to claim 9, wherein the lithium metal composite oxide does not contain Co, or an atomic ratio of Co in metal elements other than Li contained in the lithium metal composite oxide is 5% or less.

12. A secondary battery, comprising the positive electrode for a secondary battery according to any one of claims 1 to 11, a negative electrode, a separator disposed between the positive electrode for a secondary battery and the negative electrode, and an electrolyte.

13. The secondary battery according to claim 12, wherein the electrolyte contains at least one selected from the group consisting of fluoroethylene carbonate (FEC) and lithium bis(fluorosulfonyl)imide (LFSI).

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