Positive electrode active material, positive electrode, secondary battery, and manufacturing method for positive electrode active material

Abstract

This positive electrode active material for a secondary battery is characterized by having a crystal structure belonging to space group R-3m, and by being represented by the compositional formula (Li1-yNay)aNi1-b-cMnbCocXdOe, where in the formula, X includes at least one element selected from among the representative elements and the transition metal elements other than Li, Na, Ni, Mn, and Co, a≤1.15, 0<y≤0.2, 0≤1-b-c≤1, 0≤b≤1, 0≤c≤1, 0≤d≤1, and e is a value satisfying electrical neutrality. The positive electrode active material is further characterized in that in an X-ray diffraction pattern obtained via X-ray diffraction when the secondary battery is initially charged, there is a peak due to a compound differing from the compound represented by the compositional formula in a range in which the diffraction angle 2θ is 11° or greater and less than 15.7°.

Description

Positive electrode active material, positive electrode, secondary battery, and method for manufacturing positive electrode active material

[0001] This disclosure relates to a positive electrode active material, a positive electrode, a secondary battery, and a method for manufacturing a positive electrode active material.

[0002] In secondary batteries such as lithium-ion secondary batteries, the positive electrode active material significantly affects battery performance, including input / output characteristics, capacity, and durability. Therefore, much research has been conducted on positive electrode active materials. Generally, lithium transition metal composite oxides containing transition metal elements such as Ni and Mn are used as positive electrode active materials. For example, Patent Documents 1 to 3 disclose methods for improving charge-discharge cycle characteristics by using lithium transition metal composite oxides having a layered rock salt structure.

[0003] Japanese Patent Publication No. 2022-521211, Japanese Patent Publication No. 2017-174558, International Publication No. 2012 / 039413

[0004] In recent years, secondary batteries have been used, for example, as power sources for vehicle propulsion, and there is a demand for suppressing the deterioration of charge-discharge cycle characteristics while also achieving even higher capacity.

[0005] Therefore, the purpose of this disclosure is to provide a positive electrode active material that can achieve both increased capacity and suppression of deterioration in charge-discharge cycle characteristics of a secondary battery, a positive electrode and a secondary battery equipped with the positive electrode active material, and a method for manufacturing the positive electrode active material.

[0006] A positive electrode active material for a secondary battery, according to one aspect of this disclosure, has a crystal structure belonging to space group R-3m and has the composition formula (Li １－ｙ Na ｙ ) ａ Ni １－ｂ－ｃ Mn ｂ Co ｃ X ｄ O ｅrepresented by the formula, wherein X includes at least one element selected from transition metal elements and main group elements other than Li, Na, Ni, Mn and Co, a≤1.15, 0<y≤0.2, 0≤1-b-c≤1, 0≤b≤1, 0≤c≤1, 0≤d≤1, and e is a value that satisfies electrical neutrality; and when the secondary battery is charged for the first time, in an X-ray diffraction pattern obtained by X-ray diffraction, there is a peak derived from a compound different from the compound represented by the composition formula in a range where a diffraction angle 2θ is not less than 11° and less than 15.7°.

[0007] The method for producing the positive electrode active material, which is one aspect of the present disclosure, comprises a composition formula Na ｅ Ni １－ｆ－ｇ－ｈ Mn ｆ Co ｇ X ｈ O ｉ (wherein X includes at least one element selected from metal elements other than Li, Na, Ni, Mn and Co, e≤1.15, 0<1-f-g-h≤1, 0≤f<1, 0≤g<1, 0≤h<1, and i is a value that satisfies electrical neutrality), and a step of reacting the Na composite oxide with a lithium compound to exchange part of Na in the Na composite oxide with Li.

[0008] A positive electrode, which is one aspect of the present disclosure, includes the above positive electrode active material.

[0009] A secondary battery, which is one aspect of the present disclosure, includes the above positive electrode.

[0010] According to the present disclosure, it is possible to provide a positive electrode active material capable of achieving both increased capacity of a secondary battery and suppression of deterioration of charge-discharge cycle characteristics, a positive electrode including the positive electrode active material, a secondary battery, and a method for producing the positive electrode active material.

[0011] It is a cross-sectional view showing an example of the secondary battery according to the embodiment. It is a figure showing an X-ray diffraction pattern of the positive electrode active material produced in Example 1. It is a figure showing an X-ray diffraction pattern of the positive electrode active material produced in Example 2. It is a figure showing an X-ray diffraction pattern of the positive electrode active material produced in Example 3. It is a figure showing an X-ray diffraction pattern of the positive electrode active material produced in Example 4. It is a figure showing an X-ray diffraction pattern of the positive electrode active material produced in Comparative Example 1.

[0012] Hereinafter, with reference to the drawings, an example of an embodiment of the positive electrode active material, a positive electrode having said positive electrode active material, a secondary battery, and a method for manufacturing said positive electrode active material will be described. Note that configurations obtained by selectively combining the components of the multiple embodiments and modified examples described below are included within the scope of this disclosure.

[0013] Figure 1 is a cross-sectional view showing an example of a secondary battery according to the embodiment. The secondary battery 10 shown in Figure 1 comprises a wound electrode body 14 in which a positive electrode 11 and a negative electrode 12 are wound around a separator 13, an electrolyte, insulating plates 18 and 19 arranged above and below the electrode body 14, a battery case 15, a positive electrode lead 20, and a negative electrode lead 21. In addition to the wound electrode body 14, other forms of electrode bodies may be used, such as a laminated electrode body in which the positive and negative electrodes are alternately stacked with a separator. The battery case 15 is composed of an outer can 16 having an opening and housing the electrode body 14, etc., and a sealing body 17 that closes the opening of the outer can 16. Examples of the battery case 15 include metal cases such as cylindrical, square, coin-shaped, and button-shaped cases, and resin cases (so-called pouch type) formed by laminating resin sheets.

[0014] The electrolyte, for example, has lithium ion conductivity. The electrolyte may be a liquid electrolyte (electrolyte solution) or a solid electrolyte.

[0015] A liquid electrolyte (electrolyte solution) comprises a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles, amides, and mixtures of two or more of these. Examples of non-aqueous solvents include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixtures thereof. The non-aqueous solvent may also contain halogen-substituted solvents (e.g., fluoroethylene carbonate) in which at least some of the hydrogen atoms in the solvent are replaced with halogen atoms such as fluorine. Examples of electrolyte salts include LiPF4. ６ Lithium salts such as these are used.

[0016] As the solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc., can be used. As the inorganic solid electrolyte, materials known for all-solid-state lithium-ion secondary batteries, etc. (for example, oxide-based solid electrolytes, sulfide-based solid electrolytes, halogen-based solid electrolytes, etc.) can be used. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. Examples of polymer materials include fluororesins, acrylic resins, polyether resins, etc. Although the electrolytes exemplified above are non-aqueous electrolytes, the electrolyte is not limited to non-aqueous electrolytes and may also be an aqueous electrolyte.

[0017] The outer casing 16 is, for example, a metal container in the shape of a bottomed cylinder. A gasket 28 is provided between the outer casing 16 and the sealing body 17 to further ensure airtightness inside the battery. The outer casing 16 has, for example, a protruding portion 22 that supports the sealing body 17, which is a part of the side surface that protrudes inward. The protruding portion 22 is preferably formed in an annular shape along the circumferential direction of the outer casing 16, and its upper surface supports the sealing body 17.

[0018] The sealing body 17 has a structure in which an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked in order from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each component except the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected to each other at their respective centers, with the insulating member 25 interposed between their respective peripheral edges. When the internal pressure of the secondary battery 10 rises due to heat generation caused by an internal short circuit or the like, for example, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 towards the cap 27, and the current path between the lower valve body 24 and the upper valve body 26 is interrupted. If the internal pressure rises further, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0019] In the secondary battery 10 shown in Figure 1, one end of the positive electrode lead 20 is attached to the positive electrode 11. The positive electrode lead 20 attached to the positive electrode 11 extends through a through-hole in the insulating plate 18 towards the sealing body 17 and is connected by welding or other means to the lower surface of the internal terminal plate 23, which is the bottom plate of the sealing body 17. As a result, the cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, becomes the positive electrode terminal. In the secondary battery 10 shown in Figure 1, one end of the negative electrode lead 21 is attached to the negative electrode 12. The negative electrode lead 21 attached to the negative electrode 12 extends through the outside of the insulating plate 19 towards the bottom of the outer casing 16 and is connected by welding or other means to the inner surface of the bottom of the outer casing 16. As a result, the outer casing 16 becomes the negative electrode terminal.

[0020] The positive electrode 11, negative electrode 12, and separator 13 will be described below.

[0021] [Positive Electrode] The positive electrode 11 includes, for example, a positive electrode core and a positive electrode mixture layer disposed on the positive electrode core. The positive electrode mixture layer may be disposed on only one side of the positive electrode core or on both sides of the positive electrode core. The positive electrode core can be made of a metal foil that is stable in the potential range of the positive electrode 11, such as aluminum, aluminum alloy, stainless steel, or titanium, or a film with the metal disposed on its surface. The positive electrode mixture layer contains a positive electrode active material and may also contain a conductive agent, a binder, etc. The positive electrode 11 can be manufactured, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder onto the positive electrode core, drying the coating, and then compressing it to form a positive electrode mixture layer on both sides of the positive electrode core.

[0022] Examples of conductive agents included in the positive electrode mixture layer include carbon black such as acetylene black and Ketjenblack, graphite, carbon nanotubes (CNTs), carbon nanofibers, graphene, metal fibers, metal powders, and conductive whiskers. One type of conductive agent may be used alone, or multiple types may be used in combination. The content of the conductive agent is not particularly limited, but is, for example, 0.1% by mass or more and 5% by mass or less relative to the mass of the positive electrode mixture layer.

[0023] Examples of binders included in the positive electrode mixture layer include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide resins, acrylic resins, polyolefin resins, styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), carboxymethylcellulose or its salts, polyacrylic acid (PAA) or its salts, and polyvinyl alcohol (PVA). One type of binder may be used alone, or multiple types may be used in combination. The binder content is not particularly limited, but is, for example, 0.1% by mass or more and 5% by mass or less relative to the mass of the positive electrode mixture layer.

[0024] The positive electrode active material has a crystalline structure belonging to space group R-3m, and its compositional formula is (Li １－ｙ Na ｙ ) ａ Ni １－ｂ－ｃ Mn ｂ Co ｃ X ｄ O ｅ The compound contains a lithium sodium transition metal composite oxide represented by (hereinafter sometimes referred to as Li-Na composite oxide). In the composition formula, X contains at least one element selected from transition metal elements other than Li, Na, Ni, Mn, and Co, and typical elements, with a ≤ 1.15, 0 < y ≤ 0.2, 0 ≤ 1 - b - c ≤ 1, 0 ≤ b ≤ 1, 0 ≤ c ≤ 1, 0 ≤ d ≤ 1, and e is a value that satisfies electrical neutrality. The content of Li, Na, Ni, Mn, Co, and X in the Li-Na composite oxide can be measured using an ICP emission spectrometer (for example, a CIROS-120 manufactured by SPECTRO).

[0025] Composition formula (Li １－ｙ Na ｙ ) ａ Ni １－ｂ－ｃ Mn ｂ Co ｃ X ｄ O ｅIn this case, the molar ratio (y) of Na should be greater than 0 and 0.2 or less (0 < y ≤ 0.2), but the lower limit is preferably 0.02 or more, more preferably 0.05 or more, and more preferably 0.07 or more. The upper limit of the molar ratio (y) of Na is preferably 0.15 or less, more preferably 0.14 or less, and more preferably 0.13 or less. In terms of further increasing the capacity of the secondary battery or suppressing the deterioration of the charge-discharge cycle characteristics, a suitable range for the molar ratio (y) of Na is, for example, 0.02 ≤ y ≤ 0.2, 0.05 ≤ y ≤ 0.2, 0.07 ≤ y ≤ 0.2, 0 < y ≤ 0.15, 0 < y ≤ 0.14, 0 < y ≤ 0.13, 0.02 ≤ y ≤ 0.15, 0.05 ≤ y ≤ 0.14, or 0.07 ≤ y ≤ 0.13.

[0026] Composition formula (Li １－ｙ Na ｙ ) ａ Ni １－ｂ－ｃ Mn ｂ Co ｃ X ｄ O ｅ In this case, the total molar ratio (a) of Li and Na should be 1.15 or less (a ≤ 1.15), but the lower limit is preferably 0.80 or more, more preferably 0.85 or more, more preferably 0.90 or more, and more preferably 0.95 or more. The upper limit of the total molar ratio (a) of Li and Na is preferably 1.10 or less, more preferably 1.05 or less. In terms of achieving higher capacity of secondary batteries or suppressing the deterioration of charge-discharge cycle characteristics, a preferred range for the total molar ratio (a) of Li and Na is, for example, 0.80 ≤ a ≤ 1.15, 0.80 ≤ a ≤ 1.10, 0.80 ≤ a ≤ 1.05, 0.85 ≤ a ≤ 1.15, 0.85 ≤ a ≤ 1.10, 0.85 ≤ a ≤ 1.05, 0.90 ≤ a ≤ 1.15, 0.90 ≤ a ≤ 1.10, 0.90 ≤ a ≤ 1.05, 0.95 ≤ a ≤ 1.15, 0.95 ≤ a ≤ 1.10, or 0.95 ≤ a ≤ 1.05.

[0027] Composition formula (Li １－ｙ Na ｙ ) ａ Ni １－ｂ－ｃ Mn ｂ Co ｃ X ｄ O ｅIn this case, the molar ratio of Ni (1-b-c) should be between 0 and 1 (0 ≤ 1-b-c ≤ 1), but the upper limit is preferably 0.8 or less, and more preferably 0.7 or less. The lower limit of the molar ratio of Ni (1-b-c) is preferably 0.3 or more, and more preferably 0.4 or more. In terms of further increasing the capacity of the secondary battery or suppressing the deterioration of the charge-discharge cycle characteristics, a suitable range for the molar ratio of Ni (1-b-c) is, for example, 0 ≤ 1-b-c ≤ 0.8, 0 ≤ 1-b-c ≤ 0.7, 0.3 ≤ 1-b-c ≤ 1, 0.4 ≤ 1-b-c ≤ 1, 0.30 ≤ 1-b-c ≤ 0.80, or 0.4 ≤ 1-b-c ≤ 0.7.

[0028] Composition formula (Li １－ｙ Na ｙ ) ａ Ni １－ｂ－ｃ Mn ｂ Co ｃ X ｄ O ｅ In this case, the molar ratio (b) of Mn should be between 0 and 1 (0 ≤ b ≤ 1), but the upper limit is preferably 0.75 or less, and more preferably 0.65 or less. The lower limit of the molar ratio (b) of Mn is preferably 0.20 or more, and more preferably 0.25 or more. In terms of further increasing the capacity of the secondary battery or suppressing the deterioration of the charge-discharge cycle characteristics, a suitable range for the molar ratio (b) of Mn is, for example, 0 ≤ b ≤ 0.75, 0 ≤ b ≤ 0.65, 0.20 ≤ b ≤ 1, 0.25 ≤ b ≤ 1, 0.20 ≤ a ≤ 0.75, or 0.25 ≤ a ≤ 0.65.

[0029] Composition formula (Li １－ｙ Na ｙ ) ａ Ni １－ｂ－ｃ Mn ｂ Co ｃ X ｄ O ｅIn this case, the molar ratio of Co (c) should be between 0 and 1 (0 ≤ c ≤ 1), but the upper limit is preferably 0.75 or less, and more preferably 0.65 or less. The lower limit of the molar ratio of Co (c) is preferably greater than 0, more preferably 0.20 or more, and more preferably 0.25 or more. In terms of further increasing the capacity of the secondary battery or suppressing the deterioration of the charge-discharge cycle characteristics, a suitable range for the molar ratio of Co (c) is, for example, 0 ≤ c ≤ 0.75, 0 ≤ c ≤ 0.65, 0 < c ≤ 1, 0.20 ≤ c ≤ 1, 0.25 ≤ c ≤ 1, 0 < c ≤ 0.75, 0.20 ≤ c ≤ 0.75, or 0.25 ≤ c ≤ 0.65.

[0030] Composition formula (Li １－ｙ Na ｙ ) ａ Ni １－ｂ－ｃ Mn ｂ Co ｃ X ｄ O ｅ In this, X may be at least one element selected from transition metal elements other than Li, Na, Ni, Mn, and Co, and main group elements, for example, at least one element selected from Mg, Ca, Sr, Ba, Sn, Ti, Si, V, Cr, Fe, Cu, Zn, Bi, Sb, B, Ga, In, P, Zr, Hf, Nb, Ta, Mo, W, and Al. The molar ratio (d) of X may be 0 or more and 1 or less (0 ≤ d ≤ 1), but is preferably 0 or more and 0.10 or less (0 ≤ d ≤ 0.10), more preferably 0 or more and 0.08 or less (0 ≤ d ≤ 0.08), and more preferably 0 or more and 0.05 or less (0 ≤ d ≤ 0.05), in terms of further increasing the capacity of the secondary battery or suppressing the deterioration of charge-discharge cycle characteristics.

[0031] Composition formula (Li １－ｙ Na ｙ ) ａ Ni １－ｂ－ｃ Mn ｂ Co ｃ X ｄ O ｅIn the above, X is preferably at least one selected from Al and Ti. For example, when Al is contained in the positive electrode active material, an example of a suitable range for the molar ratio (d) of Al is 0.005 ≤ b ≤ 0.025. When Ti is contained in the positive electrode active material, an example of a suitable range for the molar ratio (d) of Ti is 0.005 ≤ b ≤ 0.1.

[0032] Composition formula (Li １－ｙ Na ｙ ) ａ Ni １－ｂ－ｃ Mn ｂ Co ｃ X ｄ O ｅ In the above, the molar ratio (e) of O is a value that satisfies electrical neutrality. In other words, the molar ratio (e) of O is a value that satisfies the valence of O in the positive electrode active material. The molar ratio (e) of O is not particularly limited as long as it satisfies the valence in the positive electrode active material, but from the viewpoint of further achieving higher capacity of a secondary battery or suppressing deterioration of charge-discharge cycle characteristics, when the total of the molar ratios of Ni, Mn, Co, and X is taken as 1, the molar ratio (e) of O is preferably 1.8 or more and 2.3 or less (1.8 ≤ e ≤ 2.3), more preferably 1.85 or more and 2.25 or less (1.85 ≤ e ≤ 2.25). The oxygen content of the positive electrode active material can be measured using an oxygen / nitrogen analyzer (for example, EMGA-920 manufactured by Horiba, Ltd.).

[0033] The Li-Na composite oxide is, for example, a secondary particle formed by aggregation of a plurality of primary particles. An example of the volume-based median diameter (D50) of the Li-Na composite oxide is 1 µm or more and 30 µm or less, or 3 µm or more and 20 µm or less. The D50 of the composite oxide is the particle diameter at which the cumulative volume value reaches 50% in the particle size distribution measured by laser diffraction scattering method. The BET specific surface area of the Li-Na composite oxide is, for example, 0.1 m ２ / g or more and 10 m ２ / g or less, or 0.5 m ２ / g or more and 5 m ２ / g or less. The BET specific surface area of the composite oxide is measured according to the BET method (nitrogen adsorption method) described in JIS R1626. When D50 and the BET specific surface area are within the above ranges, this may lead to higher capacity of the secondary battery.

[0034] The positive electrode active material is mainly composed of the aforementioned Li-Na composite oxide. Here, the main component refers to the component with the highest mass ratio among the constituent components of the positive electrode active material. A composite oxide other than the Li-Na composite oxide may be used in combination as a positive electrode active material in the positive electrode mixture layer, but the content of the Li-Na composite oxide is preferably 50% by mass or more, and may be substantially 100% by mass.

[0035] In the X-ray diffraction pattern obtained by X-ray diffraction when the secondary battery is initially charged, the aforementioned Li-Na composite oxide has a diffraction angle 2θ in the range of 11° or more and less than 15.7°, in the composition formula (Li １－ｙ Na ｙ ) ａ Ni １－ｂ－ｃ Mn ｂ Co ｃ X ｄ O ｅ it has a peak derived from a compound different from the compound represented by . In the composition formula, X includes at least one element selected from transition metal elements other than Li, Na, Ni, Mn, and Co, and main group elements, a≤1.15, 0<y≤0.2, 0≤1-b-c≤1, 0≤b≤1, 0≤c≤1, 0≤d≤1, and e is a value that satisfies electrical neutrality.

[0036] In this specification, the phrase "when the secondary battery is initially charged" refers to a case where initial charging is performed on a secondary battery constituted by a positive electrode containing the positive electrode active material made of the aforementioned Li-Na composite oxide, a negative electrode made of lithium metal foil, and an electrolyte, and specific conditions are as described in the examples below. In addition, the charging voltage during initial charging is not particularly limited as long as it is a voltage at which the peak appears, but for example, it is 3.8 V or higher, may be 4.0 V or higher, or may be 4.1 V or higher.

[0037] Figure 2 shows an X-ray diffraction pattern of the Li-Na composite oxide of the present embodiment during charge and discharge. Figure 2 is an X-ray diffraction pattern of the Li-Na composite oxide synthesized in Example 1 described below before charging and discharging, and during initial charging and discharging.

[0038] As shown in Figure 2, the Li-Na composite oxide of Example 1 has a peak in the range of 2θ between 18.1° and 18.8° in its X-ray diffraction pattern before charging and discharging. Hereinafter, the diffraction peak appearing in the range of 2θ between 18.1° and 18.8° will be referred to as the first peak. Furthermore, in this specification, "diffraction peak appearing in the range of (A)° or more and (B)° or less" means a diffraction peak having its peak top in the range of (A)° or more and (B)° or less.

[0039] The first peak is the empirical formula (Li １－ｙ Na ｙ ) ａ Ni １－ｂ－ｃ Mn ｂ Co ｃ X ｄ O ｅ This is the diffraction peak of the (003) plane of the crystalline phase (hereinafter referred to as the first crystalline phase) of the Li-Na composite oxide represented by the formula (wherein X includes at least one element selected from transition metal elements other than Li, Na, Ni, Mn, and Co, and main group elements, with a ≤ 1.15, 0 < y ≤ 0.2, 0 ≤ 1 - b - c ≤ 1, 0 ≤ b ≤ 1, 0 ≤ c ≤ 1, 0 ≤ d ≤ 1, and e is a value that satisfies electrical neutrality). Furthermore, the Li-Na composite oxide of Example 1 does not have any peaks in the range of 2θ between 11° and 15.7° in the state before charging and discharging.

[0040] As shown in Figure 2, in the X-ray diffraction pattern of the Li-Na composite oxide of Example 1, when the charging voltage is 4.23V or higher, a peak appears in the range of 2θ from 11° to less than 15.7°. This peak is presumed to be the diffraction peak of the (003) plane of the Na composite oxide that contains almost no Li. More specifically, this peak has a crystal structure belonging to the space group R-3m and has the compositional formula NaNi １－α－β－γ Mn α Co β X γ O δIt is presumed that this is the diffraction peak of the (003) plane of the crystalline phase (hereinafter referred to as the second crystalline phase) of the Na composite oxide represented by the formula. In the formula, X includes at least one element selected from transition metal elements other than Li, Na, Ni, Mn, and Co, and main group elements, and 0 < 1 - α - β - γ < 1, 0 ≤ α < 1, 0 ≤ β < 1, 0 ≤ γ < 1, and δ is a value that satisfies electrical neutrality. Hereinafter, the diffraction peak of the (003) plane of the second crystalline phase that appears in the range of 2θ from 11° to less than 15.7° will be referred to as the second peak.

[0041] As shown in Figure 2, in the X-ray diffraction pattern of the Li-Na composite oxide of Example 1, the second peak does not appear in the pre-charging state. In other words, the second crystalline phase is not present in the positive electrode active material in the pre-charging state, and the second crystalline phase appears when the charging voltage increases. Although not shown, when the non-aqueous electrolyte secondary battery is charged to 4.5V and then discharged for the first time, the second peak disappears. Therefore, it can be inferred that the crystalline structure of the Li-Na composite oxide of Example 1 remains the same as the crystalline structure before charging, even after charging and discharging.

[0042] Furthermore, as a result of the inventor's investigation, the composition formula (Li １－ｙ Na ｙ ) ａ Ni １－ｂ－ｃ Mn ｂ Co ｃ X ｄ O ｅIt has become clear that achieving both increased capacity and suppression of deterioration in charge-discharge cycle characteristics of a secondary battery is possible only when a Li-Na composite oxide is used as the positive electrode active material, which is represented by the formula (wherein X includes at least one element selected from transition metal elements other than Li, Na, Ni, Mn, and Co, and a ≤ 1.15, 0 < y ≤ 0.2, 0 ≤ 1 - b - c ≤ 1, 0 ≤ b ≤ 1, 0 ≤ c ≤ 1, 0 ≤ d ≤ 1, and e is a value that satisfies electrical neutrality), and which has a peak in the range of diffraction angle 2θ between 11° and 15.7° in the X-ray diffraction pattern when the secondary battery is first charged. In other words, even if the X-ray diffraction pattern when the secondary battery is first charged has a peak in the range of diffraction angle 2θ between 11° and 15.7°, if the composition deviates from the above composition formula, such as not containing Na, the effects of this disclosure cannot be obtained. Furthermore, even if the composition is represented by the above compositional formula, if the X-ray diffraction pattern when the secondary battery is first charged does not have a peak in the diffraction angle 2θ range of 11° or more and less than 15.7°, the effects of this disclosure cannot be obtained. Also, even if the composition is represented by the above compositional formula, if the X-ray diffraction pattern before the secondary battery is first charged, i.e., at the time the positive electrode active material is synthesized, has a peak in the diffraction angle 2θ range of 11° or more and less than 15.7°, the effects of this disclosure cannot be obtained.

[0043] Here, the X-ray diffraction pattern of the positive electrode active material is obtained using a desktop X-ray diffractometer (manufactured by Rigaku Corporation, product name "MiniFlex 600"). The diffracted X-rays are detected by a high-speed one-dimensional detector (D / teX Ultra 2). The measurement conditions using the above X-ray diffractometer are as follows: X-ray source: CuKα ray Tube voltage: 40kV Tube current: 15mA Divergent slit (DS): 0.625° Scattering slit (SS): 13mm (open) Receiving slit (RS): 8mm Scan axis: 2θ / θ Scanning method: Continuous 2θ scan range: 10-80° Scan speed: 10° / min Step width: 0.02°

[0044] A positive electrode active material, which is an example of an embodiment, can be manufactured by the following method. Note that the manufacturing method described here is just one example, and the method for manufacturing the positive electrode active material is not limited to this method.

[0045] Li-Na composite oxides are produced by (1) a process of synthesizing Na composite oxide by mixing and calcining sodium and nickel raw materials, and (2) a process of reacting Na composite oxide with a lithium compound to replace some of the Na in the Na composite oxide with Li. In step (1), manganese raw materials, cobalt raw materials, and raw materials containing element X may be added, and the composition formula is Na ｅ Ni １－ｆ－ｇ－ｈ Mn ｆ Co ｇ X ｈ O ｉ A Na composite oxide is synthesized, represented by the formula (wherein X contains at least one element selected from metal elements other than Li, Na, Ni, Mn, and Co, with e ≤ 1.15, 0 < 1 - f - g - h ≤ 1, 0 ≤ f < 1, 0 ≤ g < 1, 0 ≤ h < 1, and i is a value that satisfies electrical neutrality).

[0046] For the sodium raw material, at least one selected from the group consisting of, for example, metallic sodium and sodium compounds is used. The sodium compound is not particularly limited as long as it contains Na, for example, CH ３ COONa, CH ３ COONa 3H ２ Acetates of O, etc., NaNO ３ Nitrates such as Na ２ SO ４ Sulfates such as Na ２ CO ３ Carbonates such as NaHCO3 ３ bicarbonates such as NaOH, hydroxides such as Na ２ O, Na ２ O ２ Examples include oxides such as Na. ２ CO ３ NaHCO ３ , NaOH, NaNO ３ It is preferable.

[0047] For the nickel raw material, at least one selected from the group consisting of metallic nickel and nickel compounds is used. The nickel compound is not particularly limited as long as it contains Ni, for example, oxides such as NiO, Ni(OH) ２ Hydroxides such as NiOOH, NiNO ３ Nitrates such as NiCO2 ３ Ni ４ CO ３ (OH) ６ (H ２ O) ４ Carbonates such as NiSO ４ Examples include sulfates such as Ni(OH) ２ It is preferable.

[0048] For the manganese raw material, at least one selected from the group consisting of, for example, metallic manganese and manganese compounds is used. The manganese compound is not particularly limited as long as it contains Mn, for example, MnO, Mn ２ O ３ Mn ３ O ４ MnO ２ Oxides such as Mn(OH) ２ Hydroxides such as MnOOH, MnCO ３ Carbonates such as Mn(NO) ３ ) ２ Nitrates such as MnSO4 ４ Examples include sulfates such as Mn(OH) ２ It is preferable.

[0049] For the cobalt raw material, at least one selected from the group consisting of, for example, metallic cobalt and cobalt compounds is used. The cobalt compound is not particularly limited as long as it contains Co, for example, CoO, Co ２ O ３ Co ３ O ５ CoO ２ Oxides such as Co(OH) ２ , hydroxides such as CoOOH, CoCO ３ Carbonates such as CO (NO ３ ) ２ Nitrates such as COSO ４Examples include sulfates such as Co(OH) ２ Preferred

[0050] The raw material containing element X is selected from the group consisting of element X and compounds of element X, and is at least one of these. The compound containing element X is not particularly limited as long as it contains X, and examples include oxides, hydroxides, carbonates, nitrates, sulfates, etc. Furthermore, as the raw material for the Na complex oxide, compounds containing Ni and Mn, compounds containing Ni and X, compounds containing Mn and X, compounds containing Co and X, or compounds containing Ni, Mn, Co and X may be used.

[0051] The mixing ratio of the raw materials for the Na composite oxide can be set as appropriate and is not particularly limited, but for example, when the molar ratio of Na in the mixture of raw materials for the Na composite oxide is e, the molar ratio of Ni is 1-f-g-h, the molar ratio of Mn is f, the molar ratio of Co is g, and the molar ratio of element X is h, it is preferable to set the ratio so that 0.90 ≤ e ≤ 1.15, 0.4 ≤ 1-f-g-h ≤ 0.82, 0.25 ≤ f ≤ 0.65, 0.25 ≤ g ≤ 0.65, and 0.005 ≤ h ≤ 0.05. Furthermore, the method of mixing the raw materials is not particularly limited as long as it can uniformly mix the raw materials, and mixing using a known mixer such as a mixer can be given as an example.

[0052] The mixture of the above raw materials is fired in an air or oxygen stream using a firing furnace. The firing temperature is preferably 600°C or higher and 900°C or lower, and more preferably 750°C or higher and 850°C or lower. The heating rate is preferably gradual, for example, 0.3°C / min or higher and 5.0°C / min or lower, or 0.5°C / min or higher and 3.0°C / min or lower. The firing time is preferably 20 hours or more when the firing temperature is 750°C or higher and 850°C or lower. Here, firing time means the time from when the temperature of the firing furnace reaches the firing temperature until the firing is completed and cooling begins. The fired product is rapidly cooled in the air, for example, by being removed from the firing furnace.

[0053] In step (2), a portion of the Na in the Na composite oxide is replaced with Li. That is, it is necessary to replace the Li so that a predetermined amount of Na remains. A suitable method for replacing Na with Li is to add a molten salt of a lithium compound (hereinafter referred to as "lithium molten salt") to the Na composite oxide and heat it. For example, at least one selected from the group consisting of lithium hydroxide, lithium carbonate, lithium bicarbonate, lithium nitrate, lithium sulfate, lithium chloride, lithium iodide, and lithium bromide can be used as the lithium molten salt, but it is preferable to use at least one selected from the group consisting of lithium hydroxide, lithium carbonate, and lithium bicarbonate, and it is more preferable to use lithium hydroxide. Note that lithium hydroxide may be anhydrous or hydrated. By using at least one selected from the group consisting of lithium hydroxide, lithium carbonate, and lithium bicarbonate in the lithium molten salt, it becomes easy to leave a predetermined amount of Na. When lithium hydroxide is used as the lithium molten salt, the molar ratio of lithium hydroxide to the total moles of the lithium molten salt is preferably 5 mol% or more, more preferably 25 mol% or more, even more preferably 50 mol% or more, and still more preferably 75 mol% or more. Alternatively, the lithium molten salt may consist substantially of lithium hydroxide alone.

[0054] In step (2), the mixing ratio of the Na composite oxide and the lithium molten salt can be set as appropriate. When only lithium hydroxide is used as the lithium molten salt, the mixing ratio of the Na composite oxide and the lithium molten salt is preferably such that the molar ratio of Li in the lithium molten salt to Na in the Na composite oxide (Li / Na) is 0.9 or higher, more preferably 1.0 or higher, and even more preferably 1.5 or higher. Furthermore, (Li / Na) is preferably 15 or less, preferably 10 or less, and even more preferably 8 or less. Therefore, when only lithium hydroxide is used as the lithium molten salt, the mixing ratio of the Na composite oxide and the lithium molten salt is preferably such that (Li / Na) is 0.9 or higher and 15 or less, more preferably 1.0 or higher and 10 or less, and even more preferably 1.5 or higher and 8 or less.

[0055] The heating temperature in the Li exchange process is preferably 200°C or higher and 400°C or lower, and more preferably 250°C or higher and 350°C or lower. If the heating temperature exceeds 400°C, the reaction may proceed too rapidly, potentially resulting in a non-uniform reaction. On the other hand, if the heating temperature falls below 200°C, the reaction may not proceed sufficiently, and excess Na may remain. The heat treatment time is set to, for example, 3 to 10 hours after the temperature is raised at a heating rate of 3.0°C / min or higher and 8.0°C / min or lower to reach the target heat treatment temperature. After the heat treatment, the mixture is cooled. The cooling method is not particularly limited and can be, for example, natural cooling (cooling in the furnace).

[0056] After cooling, the obtained product is thoroughly washed with water, ethanol, or methanol, and then dried to obtain a Li-Na composite oxide. The drying atmosphere after washing can be air or vacuum, and is not particularly limited. Alternatively, heating or washing may be performed again after washing.

[0057] [Negative Electrode] The negative electrode 12 may be made of, for example, a metallic Li foil. Alternatively, the negative electrode 12 may have, for example, a negative electrode core and a negative electrode mixture layer disposed on the negative electrode core. The negative electrode mixture layer may be disposed on only one side of the negative electrode core or on both sides of the negative electrode core. The negative electrode core can be made of a metal foil that is stable in the potential range of the negative electrode 12, such as copper or a copper alloy, or a film with the metal disposed on its surface. The thickness of the negative electrode core is, for example, 5 μm or more and 30 μm or less. The negative electrode mixture layer includes, for example, a negative electrode active material and a binder. The thickness of the negative electrode mixture layer is, for example, 10 μm or more and 150 μm or less on one side of the negative electrode core. The negative electrode 12 can be manufactured, for example, by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, etc., to the surface of the negative electrode core, drying the coating, and then rolling it to form a negative electrode mixture layer on both sides of the negative electrode core.

[0058] The negative electrode active material contained in the negative electrode mixture layer is not particularly limited as long as it can reversibly intercept and release lithium ions, and generally carbon materials such as graphite are used. The graphite may be any of the following: natural graphite such as flake graphite, lump graphite, or clay graphite; lump artificial graphite; or artificial graphite such as graphitized mesophase carbon microbeads. In addition, metals that alloy with Li such as Si and Sn, metal compounds containing Si and Sn, or lithium titanium composite oxides may be used as the negative electrode active material. Furthermore, materials with a carbon coating may be used. Also, for example, SiO ｘ Si-containing compounds represented by (0.5 ≤ x ≤ 1.6), or Li ２ｙ SiO （２＋ｙ） A Si-containing compound in which fine Si particles are dispersed in a lithium silicate phase represented by (0 < y < 2) may be used in combination with graphite.

[0059] The binder included in the negative electrode mixture layer may be the same as that exemplified in the positive electrode 11.

[0060] [Separator] The separator 13 is made of a porous sheet having ion permeability and insulating properties. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator 13 include polyethylene, polyolefins such as polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. In addition, a heat-resistant resin layer, such as aramid resin, may be formed on the surface of the separator 13.

[0061] A filler layer containing an inorganic filler may be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. Examples of inorganic fillers include oxides containing metal elements such as Ti, Al, Si, and Mg, and phosphoric acid compounds. The filler layer can be formed by coating the surface of the positive electrode 11, the negative electrode 12, or the separator 13 with a slurry containing the filler.

[0062] The present disclosure will be further illustrated by the following examples, but the present disclosure is not limited to these examples.

[0063] <Example 1> [Preparation of positive electrode active material] A hydroxide containing Ni and Mn in a molar ratio of 12:7, cobalt hydroxide, and sodium carbonate were mixed in a molar ratio of Ni:Mn:Co:Na = 12:7:1:21. The resulting mixture was heated at a heating rate of 1°C / min, calcined in air at 800°C for 24 hours, and then rapidly cooled in air to obtain a Na composite oxide. Next, a lithium molten salt consisting only of lithium hydroxide and the Na composite oxide were mixed in a molar ratio of Li:Na = 1:1. The resulting mixture was heated at a heating rate of 5°C / min and heated in air at 280°C for 5 hours. After that, the product was cooled at a cooling rate of 2°C / min and washed with a sufficient amount of water. Then, the filtrate was removed from the solution by suction filtration, and the residue remaining on the filter paper was heat-treated in a vacuum at 160°C for 4 hours to obtain a Li-Na composite oxide. Analysis of the composition of the Li-Na composite oxide obtained using an ICP emission spectrometer (CIROS-120, manufactured by SPECTRO) and an oxygen / nitrogen analyzer (EMGA-920, manufactured by Horiba, Ltd.) confirmed that it satisfies the composition defined in claim 1 of this disclosure.

[0064] [Preparation of the positive electrode] The above-mentioned Li-Na composite oxide was used as the positive electrode active material. The positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed in a solid content mass ratio of 92:5:3, and N-methyl-2-pyrrolidone (NMP) was used as the dispersion medium to prepare a positive electrode mixture slurry. This positive electrode slurry was applied to a positive electrode core made of aluminum foil, and after the coating film was dried, the coating film was rolled with a rolling roller to obtain a positive electrode in which a positive electrode mixture layer was formed on the positive electrode core.

[0065] [Preparation of Non-Aqueous Electrolyte] A mixed solvent prepared by mixing fluoroethylene carbonate (FEC) and methyl propionate (FMP) in a volume ratio of 1:3 is prepared, to which lithium hexafluoride phosphate (LiPF) is added. ６ A non-aqueous electrolyte was prepared by dissolving the solution to a concentration of 1 mole / liter.

[0066] [Preparation of Test Cell] A lithium metal foil was used as the negative electrode, and the positive and negative electrodes were arranged facing each other via a separator to form an electrode body. This electrode body and the non-aqueous electrolyte were placed in a coin-shaped outer casing, and the opening of the outer casing was sealed with a gasket and a sealing body to produce a test cell (non-aqueous electrolyte secondary battery).

[0067] [X-ray diffraction measurement] For the above test cells, (1) a test cell in the state before charging and (2) a test cell in the state after constant current charging at a constant current of 0.2C under conditions of 25°C were prepared. For (2), multiple test cells with different battery voltages were prepared. Each test cell was then disassembled, and X-ray diffraction measurements were performed on the positive electrode containing the composite oxide. Figure 2 shows the X-ray diffraction patterns of the Li-Na composite oxide synthesized in Example 1 before charging and discharging, and during the first charge.

[0068] [Evaluation of Charge / Discharge Capacity] The above test cell was charged with a constant current of 0.2C at 25°C until the battery voltage reached 4.5V, and then charged with a constant voltage of 4.5V until the current value was 0.02C to determine the charge capacity. After a 20-minute rest, the cell was discharged with a constant current of 0.2C until the battery voltage reached 2.5V to determine the discharge capacity.

[0069] [Evaluation of Charge / Discharge Cycle Characteristics] The above test cell was charged at a constant current of 0.2C at 25°C until the battery voltage reached 4.5V, then charged at a constant voltage of 4.5V until the current value was 0.02C, and finally discharged at a constant current of 0.2C until the battery voltage reached 2.5V. This charge / discharge cycle was considered one cycle, and 25 cycles were performed. The ratio of the discharge capacity of the first cycle to the discharge capacity of the 25th cycle was calculated (capacity retention rate).

[0070] <Example 2> In the preparation of the positive electrode active material, a test cell was prepared and evaluated in the same manner as in Example 1, except that a hydroxide containing Ni and Mn in a molar ratio of 15:8, cobalt hydroxide, aluminum hydroxide, and sodium carbonate were mixed in a molar ratio of Ni:Mn:Co:Al:Na = 60:32:5:3:105. In the preparation of the positive electrode active material, a test cell was prepared and evaluated in the same manner as in Example 1. The composition of the prepared Li-Na composite oxide was analyzed in the same manner as in Example 1, and it was confirmed that it satisfies the composition defined in claim 1 of this disclosure. Figure 3 shows the X-ray diffraction patterns of the Li-Na composite oxide synthesized in Example 2 before charging and discharging, and during the first charge.

[0071] <Example 3> In preparing the positive electrode active material, a test cell was prepared and evaluated in the same manner as in Example 1, except that a hydroxide containing Ni and Mn in a molar ratio of 30:17, cobalt hydroxide, aluminum hydroxide, and sodium carbonate were mixed in a molar ratio of Ni:Mn:Co:Al:Na = 60:34:5:1:105. In the same manner as in Example 1, the composition of the prepared Li-Na composite oxide was analyzed and confirmed to satisfy the composition specified in claim 1 of this disclosure. Figure 4 shows the X-ray diffraction patterns of the Li-Na composite oxide synthesized in Example 3 before charging and discharging, and during the first charge.

[0072] <Example 4> In the preparation of the positive electrode active material, a test cell was prepared and evaluated in the same manner as in Example 1, except that a hydroxide containing Ni and Mn in a molar ratio of 25:22, cobalt hydroxide, aluminum hydroxide, and sodium carbonate were mixed in a molar ratio of Ni:Mn:Co:Al:Na = 50:44:5:1:105. In the preparation of the positive electrode active material, a test cell was prepared and evaluated in the same manner as in Example 1. The composition of the prepared Li-Na composite oxide was analyzed in the same manner as in Example 1, and it was confirmed that it satisfies the composition defined in claim 1 of this disclosure. Figure 5 shows the X-ray diffraction patterns of the Li-Na composite oxide synthesized in Example 4 before charging and discharging, and during the first charge.

[0073] <Comparative Example 1> In the preparation of the positive electrode active material, a hydroxide containing Ni and Mn in a molar ratio of 12:7, cobalt hydroxide, and lithium hydroxide were mixed in a molar ratio of Ni:Mn:Co:Li = 12:7:1:21. The resulting mixture was heated at a heating rate of 5°C / min, calcined in air at 900°C for 10 hours, and then cooled at a cooling rate of 10°C / min to obtain a composite oxide that does not contain Na. A test cell was prepared and evaluated in the same manner as in Example 1, except that this composite oxide was used as the positive electrode active material. Figure 6 shows the X-ray diffraction patterns of the composite oxide synthesized in Comparative Example 1 before charging and discharging, and during the first charge.

[0074] <Comparative Example 2> In the preparation of the positive electrode active material, a test cell was prepared and evaluated in the same manner as in Comparative Example 1, except that a hydroxide containing Ni and Mn in a molar ratio of 25:22, cobalt hydroxide, aluminum hydroxide, and lithium hydroxide were mixed in a molar ratio of Ni:Mn:Co:Al:Li = 50:44:5:1:105. The prepared composite oxide is a composite oxide that does not contain Na.

[0075] As shown in Figures 2 to 6, in the X-ray diffraction patterns of the composite oxides of Examples 1 to 4 and Comparative Example 1, a first peak attributable to the (003) plane of the composite oxide was observed in the range of 2θ between 18.1° and 18.8° in the pre-charge / discharge state. Although not shown, a similar first peak was observed in the composite oxide of Comparative Example 2. Furthermore, as shown in Figures 2 to 5, in the composite oxides of Examples 1 to 4, when the charging voltage was 4.23V or higher, a second peak attributable to the (003) plane of the Na composite oxide was observed in the range of 2θ between 11° and less than 15.7° in the composite oxides of Examples 1 to 4. On the other hand, no second peak was observed in the composite oxides of Comparative Examples 1 and 2 during charging.

[0076] Table 1 shows the results of the charging and discharging capacities of the test cells for Examples 1 to 4 and Comparative Examples 1 and 2.

[0077]

[0078] As shown in Table 1, the test cells of Examples 1 to 5 showed higher capacities compared to the test cells of Comparative Examples 1 and 2. Furthermore, the test cells of Examples 1 to 5 showed high capacity retention rates of 90% or more in charge-discharge cycle tests, confirming the effect of suppressing the deterioration of charge-discharge cycle characteristics. In other words, by using a positive electrode active material that contains a predetermined amount of Na and, when the secondary battery is first charged, has a peak in the X-ray diffraction pattern obtained by X-ray diffraction with a diffraction angle 2θ in the range of 11° or more and less than 15.7°, it can be said that both high capacity and suppression of deterioration of charge-discharge cycle characteristics of secondary batteries can be achieved.

[0079] [Note] This disclosure is further described by the following embodiments. Configuration 1: A positive electrode active material used in a secondary battery, having a crystal structure belonging to space group R-3m, and composition formula (Li １－ｙ Na ｙ ) ａ Ni １－ｂ－ｃ Mn ｂ Co ｃ X ｄ O ｅRepresented by the formula, where X includes at least one element selected from transition metal elements other than Li, Na, Ni, Mn, and Co and main group elements, a ≤ 1.15, 0 < y ≤ 0.2, 0 ≤ 1 - b - c ≤ 1, 0 ≤ b ≤ 1, 0 ≤ c ≤ 1, 0 ≤ d ≤ 1, and e is a value that satisfies electrical neutrality, and the positive electrode active material wherein, when the secondary battery is charged for the first time, the X-ray diffraction pattern obtained by X-ray diffraction has a peak originating from a compound different from the compound represented by the above compositional formula in the range of diffraction angle 2θ of 11° or more and less than 15.7°. １－α－β－γ Mn α Co β X γ O δ The peak is attributed to the compound represented by the formula, where X is at least one element selected from transition metal elements other than Li, Na, Ni, Mn, and Co and main group elements, and 0 < 1 - α - β - γ < 1, 0 ≤ α < 1, 0 ≤ β < 1, 0 ≤ γ < 1, and δ is a value that satisfies electrical neutrality, the positive electrode active material according to configuration 1. Configuration 3: The composition formula (Li １－ｙ Na ｙ ) ａ Ni １－ｂ－ｃ Mn ｂ Co ｃ X ｄ O ｅ In this configuration, the molar ratio of Co (c) is 0 < c ≤ 1, wherein the positive electrode active material is as described in configuration 1 or 2. Configuration 4: The composition formula (Li １－ｙ Na ｙ ) ａ Ni １－ｂ－ｃ Mn ｂ Co ｃ X ｄ O ｅ In this configuration, the molar ratio of Na (y) is 0.02 ≤ y ≤ 0.15, wherein the positive electrode active material is one of the configurations 1 to 3. Configuration 5: The composition formula (Li １－ｙ Na ｙ ) ａ Ni １－ｂ－ｃ Mn ｂ Co ｃ X ｄ O ｅIn this configuration, the molar ratio of Ni (1-b-c) is 0.3 ≤ 1-b-c ≤ 0.8, wherein the positive electrode active material is one of the configurations 1 to 4. Configuration 6: The composition formula (Li １－ｙ Na ｙ ) ａ Ni １－ｂ－ｃ Mn ｂ Co ｃ X ｄ O ｅ In this configuration, X is at least one selected from Al and Ti, wherein the positive electrode active material is as described in any one of configurations 1 to 5. Configuration 7: The composition formula (Li １－ｙ Na ｙ ) ａ Ni １－ｂ－ｃ Mn ｂ Co ｃ X ｄ O ｅ In this configuration, the molar ratio of O (e) is 0.85 ≤ e ≤ 1, wherein the positive electrode active material is as described in any one of configurations 1 to 6. Configuration 8: A method for producing the positive electrode active material as described in any one of configurations 1 to 7, wherein the compositional formula is Na ｅ Ni １－ｆ－ｇ－ｈ Mn ｆ Co ｇ X ｈ O ｉ A method for producing a positive electrode active material, comprising the steps of: synthesizing a Na composite oxide represented by the formula (wherein X contains at least one element selected from metal elements other than Li, Na, Ni, Mn, and Co, e ≤ 1.15, 0 < 1 - f - g - h ≤ 1, 0 ≤ f < 1, 0 ≤ g < 1, 0 ≤ h < 1, i is a value that satisfies electrical neutrality); and reacting the Na composite oxide with a lithium compound to replace a portion of the Na in the Na composite oxide with Li. Configuration 9: A positive electrode comprising the positive electrode active material described in any one of Configurations 1 to 7. Configuration 10: A secondary battery comprising the positive electrode described in Configuration 9.

[0080] 10 Secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 15 Battery case, 16 Outer can, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Protruding part, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket.

Claims

1. A positive electrode active material used in secondary batteries, having a crystal structure belonging to space group R-3m, and having a compositional formula (Li １－ｙ Na ｙ ) ａ Ni １－ｂ－ｃ Mn ｂ Co ｃ X ｄ O ｅ A positive electrode active material expressed as follows, where X includes at least one element selected from transition metal elements other than Li, Na, Ni, Mn, and Co and main group elements, a ≤ 1.15, 0 < y ≤ 0.2, 0 ≤ 1 - b - c ≤ 1, 0 ≤ b ≤ 1, 0 ≤ c ≤ 1, 0 ≤ d ≤ 1, and e is a value that satisfies electrical neutrality, wherein when the secondary battery is charged for the first time, the X-ray diffraction pattern obtained by X-ray diffraction has a peak originating from a compound different from the compound represented by the composition formula in the range of diffraction angle 2θ of 11° or more and less than 15.7°.

2. The said peak belongs to the space group R-3m, and is a peak attributed to a compound represented by the compositional formula NaNi １－α－β－γ Mn α Co β X γ O δ δ, wherein X comprises at least one element selected from transition metal elements other than Li, Na, Ni, Mn, and Co and main group elements, 0 < 1-α-β-γ < 1, 0≦α<1, 0≦β<1, 0≦γ<1, and δ is a value that satisfies electrical neutrality. The positive electrode active material according to claim 1.

3. The aforementioned composition formula (Li １－ｙ Na ｙ ) ａ Ni １－ｂ－ｃ Mn ｂ Co ｃ X ｄ O ｅ The positive electrode active material according to claim 1, wherein the molar ratio (c) of Co is 0 < c ≤ 1.

4. The aforementioned composition formula (Li １－ｙ Na ｙ ) ａ Ni １－ｂ－ｃ Mn ｂ Co ｃ X ｄ O ｅ The positive electrode active material according to claim 1, wherein the molar ratio (y) of Na is 0.02 ≤ y ≤ 0.

15.

5. The aforementioned composition formula (Li １－ｙ Na ｙ ) ａ Ni １－ｂ－ｃ Mn ｂ Co ｃ X ｄ O ｅ The positive electrode active material according to claim 1, wherein the molar ratio of Ni (1-b-c) is 0.3 ≤ 1-b-c ≤ 0.

8.

6. The aforementioned composition formula (Li １－ｙ Na ｙ ) ａ Ni １－ｂ－ｃ Mn ｂ Co ｃ X ｄ O ｅ The positive electrode active material according to claim 1, wherein X is at least one selected from Al and Ti.

7. The aforementioned composition formula (Li １－ｙ Na ｙ ) ａ Ni １－ｂ－ｃ Mn ｂ Co ｃ X ｄ O ｅ The positive electrode active material according to claim 1, wherein the molar ratio (e) of O is 0.85 ≤ e ≤ 1.

8. A method for producing a positive electrode active material according to any one of claims 1 to 7, wherein the composition formula is Na ｅ Ni １－ｆ－ｇ－ｈ Mn ｆ Co ｇ X ｈ O ｉ A method for producing a positive electrode active material, comprising: a step of synthesizing a Na composite oxide represented by the formula (wherein X comprises at least one element selected from metal elements other than Li, Na, Ni, Mn, and Co, e ≤ 1.15, 0 < 1 - f - g - h ≤ 1, 0 ≤ f < 1, 0 ≤ g < 1, 0 ≤ h < 1, i is a value that satisfies electrical neutrality); and a step of reacting the Na composite oxide with a lithium compound to replace a portion of the Na in the Na composite oxide with Li.

9. A positive electrode comprising the positive electrode active material described in any one of claims 1 to 7.

10. A secondary battery comprising the positive electrode described in claim 9.

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