Positive electrode active material, positive electrode, nonaqueous electrolyte secondary battery, and method for producing positive electrode active material

Abstract

A positive electrode active material according to one aspect of the present disclosure is used in a nonaqueous electrolyte secondary battery (10) and is characterized by having a crystal structure that belongs to a space group R-3m, being represented by a composition formula LixNayNi1-a-bMnaXbOc, where X is at least one selected from the group consisting of transition metal elements other than Li, Na, Ni, and Mn and representative elements, 0.80≤x≤1.15, 0<y≤0.20, 0.80<x+y≤1.20, 0<1-a-b≤1, 0≤a<1, 0≤b<1, and c is a value that satisfies electrical neutrality, and, when the nonaqueous electrolyte secondary battery has been subjected initial charge, having a peak derived from a compound that differs from the compound represented by the composition formula in the range of a diffraction angle 2θ of 15.7-18.0° in an X-ray diffraction pattern which is obtained by X-ray diffraction.

Description

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

[0001] The present disclosure relates to a positive electrode active material, a positive electrode, a non-aqueous electrolyte secondary battery, and a method for producing a positive electrode active material.

[0002] In non-aqueous electrolyte secondary batteries such as lithium-ion secondary batteries, the positive electrode active material has a significant impact on battery performance, such as input / output characteristics, capacity, and durability, and therefore much research has been conducted on the positive electrode active material. Generally, lithium transition metal composite oxides containing transition metal elements such as Ni and Mn are used as positive electrode active materials. The types and amounts of elements contained in the lithium transition metal composite oxide, as well as the crystalline structure of the composite oxide, significantly affect battery performance, and even slight changes in these physical properties can make it difficult to achieve the desired performance. For example, Patent Documents 1 to 3 disclose improving the crystalline structure of a positive electrode active material with a specific composition in order to improve battery performance, such as increasing capacity.

[0003] Japanese Patent No. 7258373 Japanese Patent Application Laid-Open No. 2017-174558 International Publication No. 2012 / 039413

[0004] In recent years, non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries have been used as power sources for driving vehicles, and there is a demand for further increases in capacity and improvements in initial charge-discharge efficiency. The positive electrode active materials of Patent Documents 1 to 3 still have significant room for improvement in terms of increasing capacity and improving initial charge-discharge efficiency.

[0005] A positive electrode active material according to one embodiment of the present disclosure is a positive electrode active material for use in a nonaqueous electrolyte secondary battery, having a crystal structure belonging to the space group R-3m and a composition formula Li ｘ Na ｙ Ni １－ａ－ｂ Mn ａ X ｂ O ｃwherein X is at least one selected from the group consisting of transition metal elements and typical elements other than Li, Na, Ni, and Mn, and 0.80≦x≦1.15, 0<y≦0.20, 0.80<x+y≦1.20, 0<1−a−b≦1, 0≦a<1, 0≦b<1, and c are values ​​that satisfy electrical neutrality, and when the nonaqueous electrolyte secondary battery is initially charged, an X-ray diffraction pattern obtained by X-ray diffraction has a peak at a diffraction angle 2θ in the range of 15.7° or more and 18.0° or less that is attributable to a compound other than the compound represented by the above composition formula.

[0006] A method for producing a positive electrode active material according to one embodiment of the present disclosure is a method for producing a positive electrode active material used in a non-aqueous electrolyte secondary battery, the method comprising the steps of: ｅ Ni １－ｆ－ｇ Mn ｆ X ｇ O ｈ (wherein X is at least one element selected from metal elements other than Li, Na, Ni, and Mn, and e≦1.15, 0<1−f−g≦1, 0≦f<1, 0≦g<1, and h is a value that satisfies electrical neutrality), and a step of reacting the Na composite oxide with a lithium compound to exchange part of the Na in the Na composite oxide for Li, wherein the lithium compound contains at least one selected from the group consisting of lithium hydroxide, lithium carbonate, and lithium hydrogencarbonate.

[0007] A positive electrode according to one aspect of the present disclosure is characterized by including the above positive electrode active material.

[0008] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure includes the above positive electrode, a negative electrode, and a non-aqueous electrolyte.

[0009] The positive electrode active material according to the present disclosure can achieve a high capacity and an improved initial charge-discharge efficiency of a non-aqueous electrolyte secondary battery.

[0010] FIG. 1 is an axial cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment. It shows an X-ray diffraction pattern of a positive electrode active material prepared in Example 1, where (a) is a general view from 10° to 80° and (b) is an enlarged view from 10° to 30°. It shows an X-ray diffraction pattern of a positive electrode active material prepared in Example 2, where (a) is a general view from 10° to 80° and (b) is an enlarged view from 10° to 30°. It shows an X-ray diffraction pattern of a positive electrode active material prepared in Example 3, where (a) is a general view from 10° to 80° and (b) is an enlarged view from 10° to 30°. It shows an X-ray diffraction pattern of a positive electrode active material prepared in Example 4, where (a) is a general view from 10° to 80° and (b) is an enlarged view from 10° to 30°. 1 is a diagram showing an X-ray diffraction pattern of a positive electrode active material prepared in Example 5, (a) a general view from 10° to 80°, and (b) an enlarged view from 10° to 30°. 2 is a diagram showing an X-ray diffraction pattern of a positive electrode active material prepared in Comparative Example 1, (a) a general view from 10° to 80°, and (b) an enlarged view from 10° to 30°. 3 is a diagram showing an X-ray diffraction pattern of a positive electrode active material prepared in Comparative Example 2, (a) a general view from 10° to 80°, and (b) an enlarged view from 10° to 30°. 4 is a diagram showing charge-discharge curves of test cells of Examples and Comparative Examples. 5 is a diagram showing an X-ray diffraction pattern of a positive electrode active material prepared in Example 6, (a) a general view from 10° to 80°, and (b) an enlarged view from 10° to 30°. 1A and 1B are diagrams showing X-ray diffraction patterns of the positive electrode active material prepared in Example 7, in which (a) is a general view from 10° to 80° and (b) is an enlarged view from 10° to 30°, respectively. 1B and 1C are diagrams showing X-ray diffraction patterns of the positive electrode active material prepared in Example 8, in which (a) is a general view from 10° to 80° and (b) is an enlarged view from 10° to 30°.

[0011] As a result of the investigations by the present inventors, it has been found that the compound has a crystal structure belonging to the space group R-3m and has the composition formula Li ｘ Na ｙ Ni １－ａ－ｂ Mn ａ X ｂ O ｃ(wherein X is at least one selected from the group consisting of transition metal elements and typical elements other than Li, Na, Ni, and Mn, and 0.80≦x≦1.15, 0<y≦0.20, 0.80<x+y≦1.20, 0<1−a−b≦1, 0≦a<1, 0≦b<1, and c is a value satisfying electrical neutrality), and the X-ray diffraction pattern obtained when the non-aqueous electrolyte secondary battery is initially charged has a peak attributable to a compound other than the compound represented by the above composition formula within a diffraction angle 2θ range of 15.7° or more and 18.0° or less. It has been found that by using such a positive electrode active material in a non-aqueous electrolyte secondary battery, it is possible to achieve a high battery capacity and an improvement in initial charge-discharge efficiency.

[0012] Here, the peak appearing in the X-ray diffraction pattern at a diffraction angle 2θ of 15.7° or more and 18.0° or less is presumed to be a peak attributable to a sodium transition metal composite oxide (hereinafter referred to as "Na composite oxide"). As will be described in detail later, the inventors' investigations have revealed that in non-aqueous electrolyte secondary batteries using a positive electrode active material having this peak, the charge-discharge reaction proceeds in a state in which a crystalline phase of the lithium sodium transition metal composite oxide (hereinafter referred to as "Li—Na composite oxide") represented by the above composition formula and a crystalline phase of a Li-poor Li—Na composite oxide from which Li has been desorbed coexist. As a result, as shown in FIG. 9 in the Examples section described later, a plateau region appears in the initial charge-discharge curve, where there is almost no change in voltage with respect to a certain change in capacity. As a result, it is possible to achieve high capacity and improved initial charge-discharge efficiency in non-aqueous electrolyte secondary batteries.

[0013] Hereinafter, with reference to the drawings, an example of an embodiment of a positive electrode active material according to the present disclosure and a nonaqueous electrolyte secondary battery using the positive electrode active material will be described in detail. Note that configurations obtained by selectively combining the respective components of the multiple embodiments and modifications described below are included within the scope of the present disclosure.

[0014] In the following, a cylindrical battery in which a wound electrode assembly 14 is housed in a cylindrical, bottomed exterior body 16 is exemplified as a nonaqueous electrolyte secondary battery; however, the exterior body of the battery is not limited to a cylindrical exterior body. The nonaqueous electrolyte secondary battery according to the present disclosure may be, for example, a prismatic battery with a prismatic exterior body, a coin-type battery with a coin-type exterior body, or a pouch-type battery with an exterior body composed of a laminate sheet including a metal layer and a resin layer. Furthermore, the electrode assembly is not limited to a wound type, but may also be a stacked type electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween. Furthermore, the design of the nonaqueous electrolyte secondary battery according to the present disclosure is not limited to the design of the exemplified nonaqueous electrolyte secondary battery, and known nonaqueous electrolyte secondary battery designs may also be applied.

[0015] FIG. 1 is an axial cross-sectional view of a cylindrical nonaqueous electrolyte secondary battery 10 according to an embodiment. As shown in FIG. 1 , the nonaqueous electrolyte secondary battery 10 includes a wound electrode assembly 14, a nonaqueous electrolyte, and an exterior body 16 that accommodates the electrode assembly 14 and the nonaqueous electrolyte. The electrode assembly 14 includes a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The exterior body 16 is a cylindrical metal container with a bottom that is open on one axial side, and the opening of the exterior body 16 is closed by a sealing body 17. Hereinafter, for convenience of explanation, the sealing body 17 side of the battery will be referred to as the top, and the bottom side of the exterior body 16 will be referred to as the bottom.

[0016] The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode assembly 14 are all rectangular, elongated bodies that are spirally wound in the longitudinal direction and stacked alternately in the radial direction of the electrode assembly 14. The separator 13 isolates the positive electrode 11 and the negative electrode 12 from each other. Two separators 13 are arranged, for example, to sandwich the positive electrode 11. The electrode assembly 14 includes a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like. In the electrode assembly 14, the longitudinal direction of the positive electrode 11 and the negative electrode 12 is the winding direction, and the lateral direction of the positive electrode 11 and the negative electrode 12 is the axial direction. That is, the lateral end faces of the positive electrode 11 and the negative electrode 12 form the axial end faces of the electrode assembly 14.

[0017] Insulating plates 18 and 19 are disposed above and below the electrode body 14. In the example shown in Fig. 1 , the positive electrode lead 20 passes through a through hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes outside the insulating plate 19 and extends toward the bottom side of the exterior body 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 23 of the sealing body 17 by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the exterior body 16 by welding or the like, and the exterior body 16 serves as the negative electrode terminal.

[0018] A gasket 28 is provided between the exterior body 16 and the sealing body 17 to ensure airtightness inside the battery. The exterior body 16 has a grooved portion 22 formed on its side surface that protrudes inward and supports the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the exterior body 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the exterior body 16 by the grooved portion 22 and the open end of the exterior body 16 that is crimped to the sealing body 17.

[0019] The sealing body 17 has a structure in which, in order from the electrode body 14 side, 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. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0020] The positive electrode 11, the negative electrode 12, the separator 13, and the nonaqueous electrolyte that constitute the nonaqueous electrolyte secondary battery 10 will be described in detail below, with the positive electrode 11 being particularly described below.

[0021] The positive electrode 11 includes a positive electrode core and a positive electrode mixture layer disposed on the positive electrode core. The positive electrode core can be a foil of a metal such as aluminum, an aluminum alloy, stainless steel, or titanium that is stable within the potential range of the positive electrode 11, or a film having such a metal disposed on its surface.

[0022] The positive electrode mixture layer preferably contains a positive electrode active material, a conductive agent, and a binder, and is provided on both sides of the positive electrode core. The positive electrode 11 can be produced, for example, by applying a positive electrode mixture slurry containing the positive electrode active material, the conductive agent, and the binder onto the positive electrode core, drying the coating, and then compressing it to form positive electrode mixture layers on both sides of the positive electrode core.

[0023] Examples of the conductive agent contained in the positive electrode mixture layer include carbon black such as acetylene black and ketjen black, graphite, carbon nanotubes (CNT), carbon nanofibers, graphene, metal fibers, metal powder, 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 mass% or more and 5 mass% or less with respect to the mass of the positive electrode mixture layer.

[0024] Examples of binders contained in the positive electrode mixture layer include fluorine-containing resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF); olefin resins such as polyethylene, polypropylene, ethylene-propylene-isoprene copolymer, and ethylene-propylene-butadiene copolymer; and acrylic resins such as polyacrylonitrile (PAN), polyimide, polyamide, and ethylene-acrylic acid copolymer. These resins may also be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), or the like. One type of binder may be used alone, or multiple types may be used in combination. The content of the binder 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.

[0025] The positive electrode active material has a crystal structure belonging to the space group R-3m and has the composition formula Li ｘ Na ｙ Ni １－ａ－ｂ Mn ａX ｂ O ｃ The positive electrode active material includes a lithium-sodium transition metal composite oxide (Li—Na composite oxide) represented by the formula: In this composition formula, X represents at least one element selected from the group consisting of transition metal elements and typical elements other than Li, Na, Ni, and Mn, and 0.80≦x≦1.15, 0<y≦0.20, 0.80<x+y≦1.20, 0<1−a−b≦1, 0≦a<1, 0≦b<1, and c represents a value satisfying electrical neutrality. The composite oxide constituting the positive electrode active material contains Li, Na, and Ni as essential elements, and preferably further contains Mn. The contents of Li, Na, Ni, Mn, and X contained in the positive electrode active material can be measured using an ICP optical emission spectrometer (e.g., CIROS-120 manufactured by SPECTRO).

[0026] Composition formula Li ｘ Na ｙ Ni １－ａ－ｂ Mn ａ X ｂ O ｃ In the formula, the molar ratio of Na (y) may be greater than 0 and less than or equal to 0.2 (0<y≦0.2), but is preferably greater than or equal to 0.02, more preferably greater than or equal to 0.05, and even more preferably greater than or equal to 0.07. When the molar ratio of Na (y) is 0.02≦y≦0.2, or 0.05≦y≦0.2, or 0.07≦y≦0.2, a peak appears prominently in the X-ray diffraction pattern during the initial charge / discharge in the diffraction angle 2θ range of 15.7° or greater and 18.0° or less. As a result, a plateau region appears in the initial charge / discharge curve, and the improvement in charge / discharge capacity and initial charge / discharge efficiency becomes more pronounced.

[0027] Also, the composition formula Li ｘ Na ｙ Ni １－ａ－ｂ Mn ａ X ｂ O ｃIn the formula, the molar ratio (y) of Na is preferably 0.15 or less, more preferably 0.14 or less, and even more preferably 0.13 or less. When the molar ratio (y) of Na is 0<y≦0.15, 0<y≦0.14, or 0<y≦0.13, the layered structure of the composite oxide is thought to be stable, and the improvement in charge / discharge capacity and initial charge / discharge efficiency is more significant. Note that when the molar ratio (y) of Na exceeds 0.2, Na ions may be extracted by the positive electrode during charge and the extracted Na ions may be occluded by the negative electrode. This may result in a reaction with the non-aqueous electrolyte during charge / discharge, generating by-products that may reduce the charge / discharge capacity and charge / discharge efficiency of the battery. Therefore, an example of a suitable range for the molar ratio (y) of Na is 0.02≦y≦0.15, 0.05≦y≦0.14, or 0.06≦y≦0.13.

[0028] Composition formula Li ｘ Na ｙ Ni １－ａ－ｂ Mn ａ X ｂ O ｃ In the formula, the molar ratio (x) of Li may be 0.80 or more and 1.15 or less (0.80≦x≦1.15), preferably 0.82 or more, more preferably 0.84 or more. The upper limit of the molar ratio (x) of Li is preferably 1.00, more preferably 0.95. An example of a suitable range of the molar ratio (x) of Li is 0.80≦x≦1.00, 0.80≦x≦0.95, 0.82≦x≦1.15, 0.82≦x≦1.00, 0.82≦x≦0.95, 0.84≦x≦1.15, 0.84≦x≦1.00, or 0.84≦x≦0.95. If the molar ratio (x) of Li is within this range, the effect of improving the charge / discharge capacity and the initial charge / discharge efficiency becomes more significant.

[0029] Composition formula Li ｘ Na ｙ Ni １－ａ－ｂ Mn ａ X ｂ O ｃIn the formula, the total molar ratio of Li and Na (x+y) may be more than 0.80 and not more than 1.20 (0.80<x+y≦1.20), but is preferably 0.85 or more, more preferably 0.90 or more, and even more preferably 0.95 or more. The upper limit of the total molar ratio of Li and Na (x+y) is preferably 1.10, more preferably 1.05. An example of a suitable range for the total molar ratio (x + y) of Li and Na is 0.80 < x + y ≦ 1.10, 0.80 < x + y ≦ 1.05, 0.85 ≦ x + y ≦ 1.20, 0.85 ≦ x + y ≦ 1.10, 0.85 ≦ x + y ≦ 1.05, 0.90 ≦ x + y ≦ 1.20, 0.90 ≦ x + y ≦ 1.10, 0.90 ≦ x + y ≦ 1.05, 0.95 ≦ x + y ≦ 1.20, 0.95 ≦ x + y ≦ 1.10, or 0.95 ≦ x + y ≦ 1.05. If the total molar ratio (x + y) of Li and Na is within this range, the effect of improving the charge / discharge capacity and initial charge / discharge efficiency becomes more significant.

[0030] Composition formula Li ｘ Na ｙ Ni １－ａ－ｂ Mn ａ X ｂ O ｃ In the formula, the molar ratio of Ni (1-a-b) may be 1 or less (0<1-a-b≦1), preferably 0.82 or less, more preferably 0.70 or less. The molar ratio of Ni (1-a-b) is preferably 0.30 or more, more preferably 0.40 or more. Therefore, an example of a suitable range for the molar ratio of Ni (1-a-b) is 0.30≦1-a-b≦0.82, or 0.40≦1-a-b≦0.70. In this case, the improvement effects of charge / discharge capacity and initial charge / discharge efficiency become more significant.

[0031] As described above, the positive electrode active material preferably contains Mn. ｘ Na ｙ Ni １－ａ－ｂ Mn ａ X ｂ O ｃIn the formula (I), the molar ratio (a) of Mn is preferably 0.75 or less, more preferably 0.65 or less. The molar ratio (a) of Mn is preferably 0.20 or more, more preferably 0.25 or more. An example of a suitable range for the molar ratio (a) of Mn is 0.20≦a≦0.75 or 0.25≦a≦0.65. In this case, the improvement effects of charge / discharge capacity and initial charge / discharge efficiency become more pronounced.

[0032] Composition formula Li ｘ Na ｙ Ni １－ａ－ｂ Mn ａ X ｂ O ｃ In the formula, X may be 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, Co, and Al. When an appropriate amount of X is added, the effect of improving the charge / discharge capacity and the initial charge / discharge efficiency becomes more significant. The molar ratio (b) of X is preferably 0.10 or less (0≦b≦0.10), more preferably 0.08 or less (0≦b≦0.08), and even more preferably 0.05 or less (0≦b≦0.05).

[0033] Composition formula Li ｘ Na ｙ Ni １－ａ－ｂ Mn ａ X ｂ O ｃ In the formula (1), X is preferably at least one selected from Al, Co, and Zr, and among these, Al or Co is preferred. For example, when Al is contained in the positive electrode active material, an example of a suitable range for the molar ratio (b) of Al is 0.005≦b≦0.025. When Co is contained in the positive electrode active material, an example of a suitable range for the molar ratio (b) of Co is 0.005≦b≦0.1.

[0034] Composition formula Li ｘ Na ｙ Ni １－ａ－ｂ Mn ａ X ｂ O ｃIn the formula, the molar ratio (c) of O is a value that satisfies electrical neutrality. In other words, the molar ratio (c) of O is a value that satisfies the valence of O in the positive electrode active material. The molar ratio (c) of O is not particularly limited as long as it satisfies the valence in the positive electrode active material, and may be, for example, c = x + y + 1, or c < x + y + 1, or c > x + y + 1. In particular, in the positive electrode active material immediately after production, when c < x + y + 1, the layered rock salt structure has a structure in which oxygen is deficient. Furthermore, in the positive electrode active material immediately after production, when c > x + y + 1, the layered rock salt structure has a structure in which oxygen is in excess. In a structure in which oxygen is deficient, the oxygen deficiency improves the electronic conductivity of the positive electrode active material, but as the oxygen deficiency increases, it becomes impossible to maintain a crystal structure belonging to the space group R-3m, which is thought to cause a decrease in charge / discharge capacity and cycle characteristics. In addition, in the structure where oxygen is in excess, the presence of oxygen between lattices improves the electronic conductivity of the positive electrode active material, but when the amount of oxygen in excess increases, the valence of Ni and Mn in the positive electrode active material increases, and it is thought that the charge capacity will be greatly reduced.Therefore, for example, when the total molar ratio of Ni, Mn and X is 1, the molar ratio (c) of O is preferably 1.8 or more and 2.3 or less, more preferably 1.85 or more and 2.25 or less.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.).

[0035] The Li-Na composite oxide is, for example, a secondary particle formed by aggregation of a plurality of primary particles. The volume-based median diameter (D50) of the Li-Na composite oxide is, for example, 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 size at which the volume integrated value is 50% in the particle size distribution measured by the 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, 10m ２ / g or less, or 0.5m ２ / g or more, 5m ２ The BET specific surface area of ​​the composite oxide is measured in accordance with the BET method (nitrogen adsorption method) described in JIS R1626. If the D50 and BET specific surface area are within the ranges, it is easy to increase the capacity.

[0036] The positive electrode active material contains the Li—Na composite oxide as a main component. Here, the term "main component" refers to the component that has the highest mass ratio among the components constituting the positive electrode active material. The mixture layer of the positive electrode 11 may contain a composite oxide other than the Li—Na composite oxide as the positive electrode active material, but the content of the Li—Na composite oxide is preferably 50 mass% or more, and may be substantially 100 mass%.

[0037] The Li—Na composite oxide has a composition formula Li in a diffraction angle 2θ range of 15.7° or more and 18.0° or less in an X-ray diffraction pattern obtained by X-ray diffraction when the nonaqueous electrolyte secondary battery is initially charged. ｘ Na ｙ Ni １－ａ－ｂ Mn ａ X ｂ O ｃ In the formula, X represents at least one element selected from the group consisting of transition metal elements and typical elements other than Li, Na, Ni, and Mn, and 0.80≦x≦1.15, 0<y≦0.20, 0.80<x+y≦1.20, 0<1−a−b≦1, 0≦a<1, 0≦b<1, and c represents a value that satisfies electrical neutrality.

[0038] In this specification, "when a nonaqueous electrolyte secondary battery is initially charged" refers to the case where a nonaqueous electrolyte secondary battery comprising a positive electrode containing a positive electrode active material made of the Li-Na composite oxide, a negative electrode made of lithium metal foil, and a nonaqueous electrolyte is initially charged, and specific conditions are as described in the Examples section below. The charging voltage during the initial charge is not particularly limited as long as it is a voltage at which the peak appears, but may be, for example, 3.8 V or higher, 4.0 V or higher, or 4.2 V or higher.

[0039] Figure 2 shows X-ray diffraction patterns during charge and discharge of the Li-Na composite oxide of this embodiment. Figure 2 shows X-ray diffraction patterns of the Li-Na composite oxide synthesized in Example 1 (described later) before charge and discharge and during the initial charge and discharge. Figure 2(a) is an overall view from 10° to 80°, and Figure 2(b) is an enlarged view from 10° to 30°.

[0040] As shown in Figure 2, the Li-Na composite oxide of Example 1 has a peak in the 2θ range of 18.1° or more and 18.8° or less in the X-ray diffraction pattern before charge and discharge. Hereinafter, the diffraction peak appearing in the 2θ range of 18.1° or more and 18.8° or less will be referred to as the first peak. Furthermore, in this specification, "a diffraction peak appearing in the range of numerical value (A)° or more and numerical value (B)° or less" means a diffraction peak having a peak top in the range of numerical value (A)° or more and numerical value (B)° or less.

[0041] The first peak is represented by the composition formula Li ｘ Na ｙ Ni １－ａ－ｂ Mn ａ X ｂ O ｃ (wherein X is at least one selected from the group consisting of transition metal elements and typical elements other than Li, Na, Ni, and Mn, and 0.80≦x≦1.15, 0<y≦0.20, 0.80<x+y≦1.20, 0<1−a−b≦1, 0≦a<1, 0≦b<1, and c is a value satisfying electrical neutrality). Furthermore, the Li—Na composite oxide of Example 1 does not have a peak within a 2θ range of 15.7° or more and 18.0° or less before charge and discharge.

[0042] As shown in Figure 2, in the X-ray diffraction pattern of the Li-Na composite oxide of Example 1, when the charging voltage was set to 3.8 V or higher, a peak appeared in the 2θ range of 15.7° or higher and 18.0° or lower. This peak is presumed to be the diffraction peak of the (003) plane of the Na composite oxide containing almost no Li. More specifically, this peak is due to the crystal structure belonging to the space group R-3m and the composition formula NaNi １－α－β Mn α X β O γIt is presumed to be a diffraction peak of the (003) plane of a crystalline phase of a Na composite oxide (hereinafter referred to as the second crystalline phase) represented by the formula: In the formula, X is at least one selected from the group consisting of transition metal elements and typical elements other than Na, Ni, and Mn, and 0<1-α-β≦1, 0≦α<1, 0≦β<1, and γ 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 of 15.7° or more and 18.0° or less will be referred to as the second peak.

[0043] As shown in FIG. 2, in the X-ray diffraction pattern of the Li—Na composite oxide of Example 1, the intensity of the second peak increases as the charge voltage is further increased from 3.8 V. Furthermore, as described above, the second peak does not appear in the X-ray diffraction pattern before charge. From this, it can be said that the second crystalline phase is not present in the positive electrode active material before charge, that the second crystalline phase appears as the charge voltage is increased, and that the proportion of the second crystalline phase present in the positive electrode active material increases as the charge voltage is further increased. Furthermore, when the nonaqueous electrolyte secondary battery is charged to 4.5 V and then initially discharged, the intensity of the second peak decreases. From this, it can be said that the proportion of the second crystalline phase present in the positive electrode active material decreases as the battery is discharged. Furthermore, when the battery is discharged at a lower voltage, the second peak no longer appears. Therefore, it is presumed that the crystal structure of the Li—Na composite oxide of Example 1 remains the same as the crystal structure before charge, even after charge and discharge.

[0044] 2, in the X-ray diffraction pattern of the Li—Na composite oxide of Example 1, when the charging voltage is set to 4.32 V or higher, in addition to the first and second peaks, a peak appears in the 2θ range of 19.0° or higher and 21.0° or lower. Hereinafter, the diffraction peak appearing in the 2θ range of 19.0° or higher and 21.0° or lower is referred to as the third peak.

[0045] The third peak is presumably a diffraction peak of the (003) plane of a Li-poor Li—Na composite oxide from which Li has been desorbed during charging. In other words, in the Li—Na composite oxide of Example 1, the charge-discharge reaction proceeds in a state in which the first crystalline phase and a crystalline phase of a Li-poor Li—Na composite oxide represented by a composition formula different from that of the first crystalline phase (hereinafter referred to as the third crystalline phase) coexist.

[0046] 2 , as the charge voltage increases, the intensity of the first peak attributed to the first crystalline phase decreases, while the intensity of the third peak attributed to the third crystalline phase increases. In other words, during charge and discharge, the abundance ratio of the first crystalline phase and the third crystalline phase changes as charge and discharge proceeds. As a result, a plateau region in which there is almost no change in voltage appears while the reaction proceeds while the abundance ratio of the first crystalline phase and the third crystalline phase changes, thereby achieving a high capacity and an improved initial charge and discharge efficiency of the nonaqueous electrolyte secondary battery.

[0047] Although the detailed mechanism is not clear, the inventors have found that the occurrence of the plateau region occurs when the composition formula Li ｘ Na ｙ Ni １－ａ－ｂ Mn ａ X ｂ O ｃ (wherein X is at least one selected from the group consisting of transition metal elements and typical elements other than Li, Na, Ni, and Mn, and 0.80≦x≦1.15, 0<y≦0.20, 0.80<x+y≦1.20, 0<1−a−b≦1, 0≦a<1, 0≦b<1, and c is a value satisfying electrical neutrality), and it has been revealed that this occurs only when a Li—Na composite oxide having a peak in the diffraction angle 2θ range of 15.7° or more and 18.0° or less in the X-ray diffraction pattern obtained when the nonaqueous electrolyte secondary battery is initially charged is used as the positive electrode active material. In other words, even if the X-ray diffraction pattern obtained when the nonaqueous electrolyte secondary battery is initially charged has a peak in the diffraction angle 2θ range of 15.7° or more and 18.0° or less, if the composition deviates from the above composition formula, for example, if the battery does not contain Na, the effects of the present disclosure cannot be obtained. Furthermore, even if a composition represented by the above composition formula is used, the effects of the present disclosure cannot be obtained if the X-ray diffraction pattern obtained when the nonaqueous electrolyte secondary battery is initially charged does not have a peak in the diffraction angle 2θ range of 15.7° or more and 18.0° or less. Furthermore, even if a composition represented by the above composition formula is used, the effects of the present disclosure cannot be obtained if the X-ray diffraction pattern obtained before the nonaqueous electrolyte secondary battery is initially charged, i.e., at the time the positive electrode active material is synthesized, has a peak in the diffraction angle 2θ range of 15.7° or more and 18.0° or less.

[0048] Here, the X-ray diffraction pattern of the complex oxide is obtained using a desktop X-ray diffractometer (manufactured by Rigaku Corporation, trade name "MiniFlex600"). Diffracted X-rays are detected by a high-speed one-dimensional detector (D / teX Ultra 2). The measurement conditions using the X-ray diffractometer are as follows: X-ray source: CuKα ray; Tube voltage: 40 kV; Tube current: 15 mA; Divergence slit (DS): 0.625°; Scattering slit (SS): 13 mm (open); Receiving slit (RS): 8 mm; Scan axis: 2θ / θ; Scan method: Continuous; 2θ scan range: 10-80°; Scan speed: 10° / min; Step width: 0.02°

[0049] The positive electrode active material, which is one example of the embodiment, can be produced by the following method: Note that the production method described here is only an example, and the method for producing the positive electrode active material is not limited to this method.

[0050] The Li-Na composite oxide is produced through the steps of (1) mixing and baking a sodium raw material and a nickel raw material to synthesize a Na composite oxide, and (2) reacting the Na composite oxide with a lithium compound to exchange a portion of the Na in the Na composite oxide for Li. In step (1), it is preferable to further add a manganese raw material, and a raw material containing element X may also be added, and the Li-Na composite oxide is produced through the steps of (1) mixing and baking a sodium raw material and a nickel raw material to synthesize a Na composite oxide. ｅ Ni １－ｆ－ｇ Mn ｆ X ｇ O ｈ In the formula, X is at least one element selected from metal elements other than Li, Na, Ni, and Mn, and e≦1.15, 0<1−f−g≦1, 0≦f<1, 0≦g<1, and h is a value that satisfies electrical neutrality.

[0051] As the sodium source, at least one selected from the group consisting of metallic sodium and sodium compounds is used. The sodium compound is not particularly limited as long as it contains Na, and examples thereof include CH ３ COONa, CH ３ COONa 3H ２ Acetates such as O, NaNO３ Nitrates, Na etc. ２ SO ４ Sulfates such as Na ２ CO ３ Carbonates such as NaHCO ３ Hydrogen carbonates such as NaOH, hydroxides such as Na ２ O, Na ２ O ２ Among them, Na ２ CO ３ , NaHCO ３ , NaOH, NaNO ３ is preferred.

[0052] The nickel raw material is at least one selected from the group consisting of metallic nickel and nickel compounds. The nickel compound is not particularly limited as long as it contains Ni, and examples thereof include oxides such as NiO, Ni(OH), and the like. ２ , hydroxides such as NiOOH, NiNO ３ Nitrates such as NiCO ３ , Ni ４ CO ３ (OH) ６ (H ２ O) ４ Carbonates such as NiSO ４ Among them, Ni(OH) ２ is preferred.

[0053] The manganese raw material is at least one selected from the group consisting of metallic manganese and manganese compounds. The manganese compound is not particularly limited as long as it contains Mn, and examples thereof include 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 MnSO ４ Among them, Mn(OH) ２ is preferred.

[0054] The raw material containing element X is at least one selected from the group consisting of element X and compounds of element X. The compound containing element X is not particularly limited as long as it contains X, and examples include oxides, hydroxides, carbonates, nitrates, and sulfates. Note that the raw material for the Na composite oxide may be a compound containing Ni and Mn, a compound containing Ni and X, a compound containing Mn and X, or a compound containing Ni, Mn, and X.

[0055] The mixing ratio of the raw materials for the Na composite oxide may be set appropriately and is not particularly limited, but for example, when the molar ratio of Na in the mixture obtained by mixing the raw materials for the Na composite oxide is e, the molar ratio of Ni is 1-f-g, the molar ratio of Mn is f, and the molar ratio of element X is g, it is preferable to set them so that 0.90≦e≦1.15, 0.4≦1-f-g≦0.82, 0.25≦f≦0.65, and 0.005≦g≦0.05. Furthermore, the method for mixing the raw materials is not particularly limited as long as it can mix the raw materials uniformly, and examples include mixing using a known mixer such as a mixer.

[0056] The mixture of raw materials is fired in a firing furnace in the atmosphere or in an oxygen stream. The firing temperature is preferably 700°C or higher and 900°C or lower, and more preferably 750°C or higher and 850°C or lower. The temperature rise rate is preferably slow, 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. When the firing temperature is 750°C or higher and 850°C or lower, the firing time is preferably 20 hours or longer. Here, the firing time refers to the time from when the temperature of the firing furnace reaches the firing temperature to when firing ends and cooling begins. The fired product is, for example, rapidly cooled in the atmosphere by being removed from the firing furnace.

[0057] In step (2), a portion of the Na in the Na composite oxide is exchanged with Li. That is, the Li exchange must be performed so that a predetermined amount of Na remains. A suitable method for exchanging Na for Li includes adding a molten salt of a lithium compound (hereinafter referred to as "lithium molten salt") to the Na composite oxide and heating it. The lithium molten salt may be, 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. Among these, 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. Lithium hydroxide may be anhydrous or hydrated. Using at least one selected from the group consisting of lithium hydroxide, lithium carbonate, and lithium bicarbonate as the lithium molten salt makes it easier to leave a predetermined amount of Na. When lithium hydroxide is used in 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 even more preferably 75 mol % or more. Furthermore, the lithium molten salt may essentially consist of only lithium hydroxide.

[0058] In step (2), the mixing ratio of the Na composite oxide and the lithium molten salt can be set appropriately. 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 such that the molar ratio (Li / Na) of Li in the lithium molten salt to Na in the Na composite oxide is preferably 0.9 or more, more preferably 1.0 or more, and even more preferably 1.5 or more. 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 0.9 or more and 15 or less, more preferably 1.0 or more and 10 or less, and even more preferably 1.5 or more and 8 or less.

[0059] The heating temperature in the Li exchange step is preferably 200°C or higher and 400°C or lower, more preferably 250°C or higher and 350°C or lower. If the heating temperature exceeds 400°C, the reaction may proceed rapidly, resulting in a non-uniform reaction. On the other hand, if the heating temperature is lower than 200°C, the reaction may not proceed sufficiently, and excess Na may remain. The heat treatment time is set to, for example, 3 hours or higher and 10 hours or lower after the temperature is increased at a 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 may be, for example, natural cooling (cooling in a furnace).

[0060] After cooling, the resulting product is thoroughly washed with water, ethanol, methanol, or the like, and then dried to obtain a Li—Na composite oxide. The atmosphere for drying after washing is not particularly limited and may be air or vacuum. Furthermore, after washing, another heating treatment or another washing treatment may be performed.

[0061] [Negative Electrode] The negative electrode 12 may have, for example, a negative electrode core and a negative electrode mixture layer formed on the surface of the negative electrode core, or a metal Li foil may be used as the negative electrode 12. Alternatively, the negative electrode 12 may have a negative electrode core, and lithium metal may be deposited on the surface of the negative electrode core upon charging. When the negative electrode 12 has a negative electrode mixture layer, the negative electrode mixture layer is preferably formed on both sides of the negative electrode core. The negative electrode core may be a foil of a metal stable within the potential range of the negative electrode 12, such as copper or a copper alloy, or a film with such a metal disposed on the 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 produced, for example, by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, etc. to the surface of a 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.

[0062] The negative electrode active material contained in the negative electrode mixture layer is not particularly limited as long as it can reversibly absorb and release lithium ions, and generally, carbon materials such as graphite are used. Graphite may be any of natural graphite such as flake graphite, lump graphite, and amorphous graphite, or artificial graphite such as lump artificial graphite and graphitized mesophase carbon microbeads. Furthermore, metals that alloy with Li, such as Si and Sn, metal compounds containing Si, Sn, and lithium-titanium composite oxides may also be used as the negative electrode active material. Furthermore, those provided with a carbon coating may also be used. For example, SiO ｘ (0.5≦x≦1.6) or Li ２ｙ SiO （２＋ｙ） A Si-containing compound in which fine particles of Si are dispersed in a lithium silicate phase represented by (0<y<2) may be used in combination with graphite.

[0063] Examples of binders contained in the negative electrode mixture layer include styrene butadiene rubber (SBR), nitrile butadiene rubber (NBR), carboxymethyl cellulose (CMC) or a salt thereof, polyacrylic acid (PAA) or a salt thereof (PAA-Na, PAA-K, etc., or a partially neutralized salt), polyvinyl alcohol (PVA), etc. These may be used alone or in combination of two or more.

[0064] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include polyolefins such as polyethylene and polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. Furthermore, a highly heat-resistant resin layer such as an aramid resin may be formed on the surface of the separator 13.

[0065] 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 and phosphate compounds containing metal elements such as Ti, Al, Si, and Mg. The filler layer can be formed by applying a slurry containing the filler to the surface of the positive electrode 11, the negative electrode 12, or the separator 13.

[0066] [Non-aqueous electrolyte] The non-aqueous electrolyte has ion conductivity (for example, lithium ion conductivity). The non-aqueous electrolyte may be a liquid electrolyte (electrolytic solution) or a solid electrolyte.

[0067] The liquid electrolyte (electrolytic solution) contains, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent that can be used include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms in these solvents are substituted with halogen atoms such as fluorine. Examples of the halogen-substituted compound include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP).

[0068] Examples of the esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone (GBL) and γ-valerolactone (GVL); and chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP).

[0069] Examples of the 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, cyclic ethers such as crown ethers, 1,2-dimethoxyethane ethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, and methyl phenyl ether. and chain ethers such as 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.

[0070] The electrolyte salt is preferably a lithium salt. Examples of the lithium salt include LiClO ４ , LiBF ４ , LiPF ６ , LiAlCl ４ , LiSbF ６ , LiSCN, LiCF ３ SO ３ , LiCF ３ CO ２ , LiAsF ６ , LiB １０ Cl １０ , lower aliphatic lithium carboxylates, LiCl, LiBr, LiI, phosphates, borates, and imide salts. Examples of phosphates include lithium difluorophosphate (LiPO ２ F ２Examples of the borate salt include lithium bis(oxalato)borate (LiBOB) and lithium difluoro(oxalato)borate (LiDFFOB). Examples of the imide salt include lithium bisfluorosulfonylimide (LiN(FSO ２ ) ２ ), lithium bistrifluoromethanesulfonyl imide (LiN(CF ３ SO ２ ) ２ ), lithium trifluoromethanesulfonate nonafluorobutanesulfonate imide (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), lithium bispentafluoroethanesulfonyl imide (LiN(C ２ F ５ SO ２ ) ２ Among these, LiPF is preferred from the viewpoint of ionic conductivity, electrochemical stability, etc. ６ The concentration of the lithium salt may be, for example, 4 mol or less, or 3 mol or less, preferably 1.8 mol or less, and more preferably 0.8 mol or more and 1.8 mol or less, per 1 L of the non-aqueous solvent.

[0071] The non-aqueous electrolyte may contain an additive such as an unsaturated carbonate ester, an acid anhydride, a phenol compound, a benzene compound, a nitrile compound, an isocyanate compound, a sultone compound, a sulfate compound, a borate ester compound, a phosphate ester compound, or a phosphite ester compound.

[0072] Examples of unsaturated cyclic carbonates include vinylene carbonate, 4-methylvinylene carbonate, 4,5-dimethylvinylene carbonate, 4-ethylvinylene carbonate, 4,5-diethylvinylene carbonate, 4-propylvinylene carbonate, 4,5-dipropylvinylene carbonate, 4-phenylvinylene carbonate, 4,5-diphenylvinylene carbonate, vinylethylene carbonate, and divinylethylene carbonate. One type of unsaturated cyclic carbonate may be used alone, or two or more types may be used in combination. Some of the hydrogen atoms in the unsaturated cyclic carbonate may be substituted with fluorine atoms. The acid anhydride may be an anhydride formed by intermolecular condensation of multiple carboxylic acid molecules, but is preferably an acid anhydride of a polycarboxylic acid. Examples of polycarboxylic acid anhydrides include succinic anhydride, maleic anhydride, and phthalic anhydride.

[0073] Examples of phenolic compounds include phenol, hydroxytoluene, etc. Examples of benzene compounds include fluorobenzene, hexafluorobenzene, cyclohexylbenzene (CHB), etc.

[0074] Examples of nitrile compounds include adiponitrile, pimelonitrile, propionitrile, and succinonitrile. Examples of isocyanate compounds include methyl isocyanate (MIC), diphenylmethane diisocyanate (MDI), hexamethylene diisocyanate (HDI), toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), and bisisocyanatomethylcyclohexane (BIMCH). Examples of sultone compounds include propane sultone and propene sultone. Examples of sulfate compounds include ethylene sulfate, ethylene sulfite, dimethyl sulfate, and lithium fluorosulfate. Examples of borate ester compounds include trimethyl borate and tris(trimethylsilyl)borate. Examples of phosphate ester compounds include trimethyl phosphate and tris(trimethylsilyl)phosphate. Examples of phosphite ester compounds include trimethyl phosphite and tris(trimethylsilyl)phosphite.

[0075] 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, a material known in all-solid-state lithium ion secondary batteries, etc. (for example, an oxide-based solid electrolyte, a sulfide-based solid electrolyte, a halogen-based solid electrolyte, 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 the non-aqueous solvent and gels is used. As the polymer material, for example, a fluororesin, an acrylic resin, a polyether resin, etc. can be used.

[0076] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0077] Example 1 Preparation of Positive Electrode Active Material A hydroxide containing Ni and Mn in a 1:1 molar ratio was mixed with sodium carbonate in a molar ratio of Ni:Mn:Na = 0.5:0.5:1.05. 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 was mixed with a Na composite oxide in a molar ratio of Li:Na = 2:1. The resulting mixture was heated at a heating rate of 5°C / min and then heated in air at 280°C for 5 hours. The product was then cooled at a cooling rate of 2°C / min and washed with a sufficient amount of water. The solution was then suction filtered to remove the filtrate, and the residue remaining on the filter paper was heat-treated in vacuum at 160°C for 4 hours to obtain a Li-Na composite oxide. The composition of the obtained Li-Na composite oxide was analyzed using an ICP emission spectrometer (SPECTRO CIROS-120) and an oxygen / nitrogen analyzer (HORIBA EMGA-920). The composition formula was Li ０．８４３ Na ０．１２２ Ni ０．５０３ Mn ０．４９７ O １．９３ It was confirmed that the resulting product was a Li—Na composite oxide represented by the formula:

[0078] [Fabrication of Positive Electrode] The 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 solids mass ratio of 92:5:3, and N-methyl-2-pyrrolidone (NMP) was used as a 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 the coating film was dried. The coating film was then rolled with a rolling roller to obtain a positive electrode having a positive electrode mixture layer formed on the positive electrode core.

[0079] [Preparation of Non-Aqueous Electrolyte] A mixed solvent of fluoroethylene carbonate (FEC) and methyl propionate (FMP) in a volume ratio of 1:3 was dissolved in lithium hexafluorophosphate (LiPF ６ ) was dissolved in the solution to a concentration of 1 mol / liter to prepare a non-aqueous electrolyte.

[0080] [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 with a separator interposed therebetween to prepare an electrode assembly. This electrode assembly and the nonaqueous electrolyte solution were placed in a coin-shaped outer can, and the opening of the outer can was sealed with a gasket and a sealing member to prepare a test cell (nonaqueous electrolyte secondary battery).

[0081] [X-ray Diffraction Measurement] The test cells were prepared in the following states: (1) before charging; (2) after constant-current charging at a constant current of 0.2 C at 25°C; and (3) after constant-current charging at a constant current of 0.2 C at 25°C until the battery voltage reached 4.5 V, followed by constant-voltage charging at a voltage of 4.5 V until the current value reached 0.02 C, and then constant-current discharging at a constant current of 0.2 C. Note that for (2) and (3), 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 and during the initial charge and discharge.

[0082] [Evaluation of charge / discharge capacity and initial charge / discharge efficiency] The test cell was charged at a constant current of 0.2 C at 25°C until the battery voltage reached 4.5 V, and then charged at a constant voltage of 4.5 V until the current value reached 0.02 C, and the charge capacity was determined. After a 20-minute rest, the test cell was discharged at a constant current of 0.2 C until the battery voltage reached 2.5 V, and the discharge capacity was determined. The initial charge / discharge efficiency of the test cell was then calculated using the following formula: Initial charge / discharge efficiency (%) = Discharge capacity / Charge capacity × 100

[0083] Example 2 A test cell was prepared and evaluated in the same manner as in Example 1, except that the lithium molten salt and the mixing ratio of the lithium molten salt and the Na composite oxide were changed in the preparation of the positive electrode active material. More specifically, in the preparation of the positive electrode active material, the lithium molten salt was prepared by mixing lithium hydroxide, lithium nitrate, and lithium chloride in a molar ratio of 25:66:9. The lithium molten salt and the Na composite oxide were mixed in a molar ratio of Li:Na=5:1. The composition formula of the prepared Li-Na composite oxide was Li ０．８８２ Na０．０８３３ Ni ０．５０６ Mn ０．４９４ O １．８３７ 3 shows the X-ray diffraction patterns of the Li—Na composite oxide synthesized in Example 2 before charge / discharge and during the first charge / discharge.

[0084] Example 3 A test cell was prepared and evaluated in the same manner as in Example 1, except that the lithium molten salt was changed, and the mixing ratio of the lithium molten salt and the Na composite oxide and the heating temperature were changed in the preparation of the positive electrode active material. More specifically, in the preparation of the positive electrode active material, a lithium molten salt prepared by mixing lithium hydroxide, lithium nitrate, and lithium chloride in a molar ratio of 50:44:6 was used. The lithium molten salt and the Na composite oxide were mixed in a molar ratio of Li:Na=5:1, and the resulting mixture was heated at a temperature increase rate of 5°C / min and heated in air at 350°C for 5 hours. The composition formula of the prepared Li-Na composite oxide was Li ０．９１４ Na ０．０７３５ Ni ０．５０４ Mn ０．４９６ O １．９２９ 4 shows the X-ray diffraction patterns of the Li—Na composite oxide synthesized in Example 3 before charge / discharge and during the first charge / discharge.

[0085] Example 4 A test cell was prepared and evaluated in the same manner as in Example 1, except that the lithium molten salt was changed, and the mixing ratio of the lithium molten salt and the Na composite oxide and the heating temperature were changed in the preparation of the positive electrode active material. More specifically, in the preparation of the positive electrode active material, a lithium molten salt prepared by mixing lithium hydroxide, lithium nitrate, and lithium chloride in a molar ratio of 75:22:3 was used. In addition, the above lithium molten salt and Na composite oxide were mixed in a molar ratio of Li:Na=5:1, and the resulting mixture was heated at a temperature increase rate of 5°C / min and heated in air at 350°C for 5 hours. The composition formula of the prepared Li-Na composite oxide was Li ０．８９７ Na ０．０８６１ Ni ０．５０５ Mn ０．４９５ O １．８５９5 shows the X-ray diffraction patterns of the Li—Na composite oxide synthesized in Example 4 before charge / discharge and during the first charge / discharge.

[0086] Example 5 A test cell was prepared and evaluated in the same manner as in Example 1, except that the lithium molten salt was changed, and the mixing ratio of the lithium molten salt and the Na composite oxide and the heating temperature were changed in the preparation of the positive electrode active material. More specifically, in the preparation of the positive electrode active material, a lithium molten salt prepared by mixing lithium hydroxide, lithium nitrate, and lithium chloride in a molar ratio of 25:66:9 was used. In addition, the above lithium molten salt and Na composite oxide were mixed in a molar ratio of Li:Na=5:1, and the resulting mixture was heated at a temperature increase rate of 5°C / min and heated in air at 350°C for 5 hours. The composition formula of the prepared Li-Na composite oxide was Li ０．９１５ Na ０．０６５２ Ni ０．５０６ Mn ０．４９４ O １．８６１ 6 shows the X-ray diffraction patterns of the Li—Na composite oxide synthesized in Example 5 before charge / discharge and during the first charge / discharge.

[0087] Comparative Example 1 In the preparation of a positive electrode active material, a hydroxide containing Ni and Mn in a molar ratio of 1:1 and lithium hydroxide were mixed in a molar ratio of Ni:Mn:Li=0.5:0.5:1.05, and the resulting mixture was heated at a temperature increase rate of 5° C. / min, baked at 900° C. for 10 hours in air, and then cooled at a temperature decrease rate of 10° C. / min to obtain a compound having the composition formula Li １．０ Ni ０．５ Mn ０．５ O ２ A composite oxide represented by the formula (1) was obtained. A test cell was fabricated and evaluated in the same manner as in Example 1, except that this composite oxide was used as the positive electrode active material. Fig. 7 shows the X-ray diffraction patterns of the composite oxide synthesized in Comparative Example 1 before charge / discharge and during the initial charge / discharge.

[0088] Comparative Example 2 A test cell was prepared and evaluated in the same manner as in Example 1, except that the lithium molten salt and the mixing ratio of the lithium molten salt and the Na composite oxide were changed in the preparation of the positive electrode active material. More specifically, in the preparation of the positive electrode active material, only lithium bromide was used as the lithium molten salt. Furthermore, the molten salt made of lithium bromide and the Na composite oxide were mixed at a molar ratio of Li:Na=5:1. The composition formula of the prepared composite oxide was Li ０．９３８ Na ０．０２２９ Ni ０．５ Mn ０．５ O ２ 8 shows the X-ray diffraction patterns of the composite oxide synthesized in Comparative Example 2 before charge / discharge and during the first charge / discharge.

[0089] 2 to 8, the composite oxides of Examples 1 to 5 and Comparative Examples 1 and 2 have a first peak attributable to the (003) plane of the composite oxide in the X-ray diffraction pattern before charge / discharge in the range of 2θ of 18.1° or more and 18.8° or less. Here, the first peaks of the composite oxides of Examples 1 to 5 and Comparative Example 2 appear at lower angles than the first peak of the composite oxide of Comparative Example 1. This is because Na has a larger lattice constant than Li, and therefore the lattice constants of the composite oxides of Examples 1 to 5 and Comparative Example 2 containing Na are larger than the lattice constant of the composite oxide of Comparative Example 1.

[0090] 2 to 8, in the composite oxides of Examples 1 to 5, when the charging voltage was 3.8 V or higher, a second peak attributable to the (003) plane of the Na composite oxide appeared within a 2θ range of 15.7° or higher and 18.0° or lower. Furthermore, in the composite oxides of Examples 1 to 5, when the charging voltage was 4.32 V or higher, a third peak attributable to the (003) plane of the Li-poor Li—Na composite oxide appeared within a 2θ range of 19.0° or higher and 21.0° or lower. That is, at some voltages, the first peak and the second peak appeared simultaneously, at some voltages, the first peak, the second peak, and the third peak appeared simultaneously, or at some voltages, the second peak and the third peak appeared simultaneously. On the other hand, in the composite oxide of Comparative Example 1, when the charging voltage was 3.8 V or higher, the second peak did not appear, and even when the charging voltage was 4.32 V or higher, the third peak did not appear. Furthermore, in the composite oxide of Comparative Example 2, the second peak does not appear when the charging voltage is 3.8 V or higher, and the third peak appears when the charging voltage is 4.32 V or higher, but the first and third peaks do not appear simultaneously.

[0091] Next, Table 1 shows the charge capacity, discharge capacity, and initial charge / discharge efficiency of the test cells of Examples 1 to 5 and Comparative Examples 1 and 2. Table 1 also shows the composition of the composite oxide, the type of lithium molten salt, and whether or not a second peak occurred during the initial charge. FIG. 9 shows the initial charge / discharge curves of the test cells of Examples 1 to 3 and Comparative Example 1.

[0092]

[0093] As shown in Table 1, the test cells of Examples 1 to 5 had higher capacities and improved initial charge-discharge efficiencies compared to the test cells of Comparative Examples 1 and 2. In other words, by using a positive electrode active material containing a predetermined amount of Na and having a peak in the X-ray diffraction pattern obtained by X-ray diffraction, where the diffraction angle 2θ is in the range of 15.7° or more and 18.0° or less, when the nonaqueous electrolyte secondary battery is initially charged, high battery capacity and improved initial charge-discharge efficiencies can be achieved. Furthermore, the positive electrode active material of Comparative Example 2, which uses lithium bromide as the lithium molten salt, contains almost no Na, and therefore does not exhibit a second peak during charging, failing to achieve high battery capacity.

[0094] Furthermore, as shown in FIG. 9 , the test cells of Examples 1 to 3 exhibit a plateau region where there is almost no change in voltage when the charge voltage and discharge voltage are near 4.3 V. This is presumably because, as described above, the first crystalline phase and the third crystalline phase coexist at a predetermined voltage, and charge and discharge proceed while changing their abundance ratios. On the other hand, the test cell of Comparative Example 1 does not exhibit a plateau region when the charge voltage and discharge voltage are near 4.3 V. This is consistent with the fact that the test cell of Comparative Example 1 did not have a third peak attributed to the (003) plane of the third crystalline phase in the X-ray diffraction pattern. Furthermore, although not shown, it was confirmed that the test cell of Comparative Example 2, like the test cell of Comparative Example 1, did not exhibit a plateau region when the charge voltage and discharge voltage were near 4.3 V. That is, the test cell of Comparative Example 2 has a peak attributable to the (003) plane of the third crystalline phase in the X-ray diffraction pattern, but no plateau region appears at charge and discharge voltages of around 4.3 V. This is presumably because charge and discharge proceeded without the first crystalline phase and the third crystalline phase coexisting. Therefore, it is presumed that the second crystalline phase that appears during charge and discharge acts as a medium for allowing the first crystalline phase and the third crystalline phase to coexist, and causes a plateau region to appear in which charge and discharge proceeds while changing the abundance ratio of each.

[0095] As shown in Table 1, the test cells of Examples 4 and 5, like those of Examples 1 to 3, have higher capacities and improved initial charge-discharge efficiencies compared to the test cells of Comparative Examples 1 and 2. This is due to the use of a positive electrode active material containing a predetermined amount of Na, and exhibiting a peak in the X-ray diffraction pattern obtained by X-ray diffraction when the nonaqueous electrolyte secondary battery is initially charged, with a diffraction angle 2θ in the range of 15.7° or more and 18.0° or less. Furthermore, it was confirmed that the test cells of Examples 4 and 5 exhibit a plateau region in the initial charge-discharge curve, with almost no voltage change, at charge and discharge voltages of around 4.3 V, similar to the test cells of Examples 1 to 3.

[0096] Example 6: A hydroxide containing Ni and Mn in a molar ratio of 60:40 was mixed with sodium carbonate in a molar ratio of Ni:Mn:Na = 0.60:0.40:1.05. The resulting mixture was heated at a heating rate of 1°C / min and calcined at 600°C for 20 hours in an oxygen atmosphere with a flow rate of 4 L / min, followed by slow cooling in a furnace to obtain a Na composite oxide. Next, a lithium molten salt consisting only of lithium hydroxide was mixed with a Na composite oxide in a molar ratio of Li:Na = 5:1. The resulting mixture was heated at a heating rate of 5°C / min and then heated in air at 280°C for 5 hours. The product was then cooled at a cooling rate of 2°C / min and washed with a sufficient amount of water. The solution was then suction filtered to remove the filtrate, and the residue remaining on the filter paper was heat-treated in vacuum at 160°C for 4 hours to obtain a Li-Na composite oxide. The composition formula of the prepared Li-Na composite oxide was Li ０．８９７ Na ０．０９４４ Ni ０．５９６ Mn ０．４０４ O ２ In the same manner as in Example 1, a positive electrode was prepared, a test cell was prepared, and the charge / discharge capacity and initial charge / discharge efficiency were evaluated, and X-ray diffraction measurements were performed. FIG. 10 shows the X-ray diffraction patterns of the Li—Na composite oxide synthesized in Example 6 before and during charge / discharge. As shown in FIG. 10, the Li—Na composite oxide of Example 6 exhibits a second peak within the 2θ range of 15.7° or more and 18.0° or less when the charge voltage is 4.0 V or more.

[0097] Example 7 A test cell was prepared and evaluated in the same manner as in Example 6, except that in the preparation of the positive electrode active material, the hydroxide containing Ni and Mn was changed and the mixing ratio of Ni, Mn, and Na was changed. More specifically, in the preparation of the Na composite oxide, a hydroxide containing Ni and Mn in a molar ratio of 70:30 was used, and the Na composite oxide was prepared by mixing them in a molar ratio of Ni:Mn:Na=0.70:0.30:1.05. The composition formula of the prepared Li-Na composite oxide was Li ０．９１９ Na ０．０７１４ Ni ０．７０３ Mn ０．２９７ O ２ 11 shows the X-ray diffraction patterns of the Li—Na composite oxide synthesized in Example 7 before charge / discharge and during the initial charge / discharge. As shown in FIG. 11, the Li—Na composite oxide of Example 7 exhibits a second peak within the 2θ range of 15.7° to 18.0° when the charge voltage is 3.8 V or higher.

[0098] Example 8 A test cell was prepared and evaluated in the same manner as in Example 6, except that in the preparation of the positive electrode active material, the hydroxide containing Ni and Mn was changed and the mixing ratio of Ni, Mn, and Na was changed. More specifically, in the preparation of the Na composite oxide, a hydroxide containing Ni and Mn in a molar ratio of 82:18 was used, and the Na composite oxide was prepared by mixing them in a molar ratio of Ni:Mn:Na=0.82:0.18:1.05. The composition formula of the prepared Li-Na composite oxide was Li ０．８４８ Na ０．０８７２ Ni ０．８２０ Mn ０．１８０ O ２ 12 shows the X-ray diffraction patterns of the Li—Na composite oxide synthesized in Example 8 before charge / discharge and during the initial charge / discharge. As shown in FIG. 12, the Li—Na composite oxide of Example 7 exhibits a second peak within the 2θ range of 15.7° to 18.0° when the charge voltage is 4 V or higher.

[0099] Comparative Example 3 A test cell was prepared and evaluated in the same manner as in Comparative Example 1, except that in the preparation of the positive electrode active material, Ni, Mn, and Li were mixed in a molar ratio of Ni:Mn:Li = 0.60:0.40:1.05, and the firing atmosphere was an oxygen atmosphere with a flow rate of 4 L / min. The composition formula of the prepared composite oxide was Li １．０ Ni ０．５９６ Mn ０．４０４ O ２ It was.

[0100] Comparative Example 4 A test cell was prepared and evaluated in the same manner as in Comparative Example 1, except that in the preparation of the positive electrode active material, Ni, Mn, and Li were mixed in a molar ratio of Ni:Mn:Li = 0.70:0.30:1.05, and the firing atmosphere was an atmosphere with an oxygen flow rate of 4 L / min. The composition formula of the prepared composite oxide was Li １．０ Ni ０．７０３ Mn ０．２９７ O ２ It was.

[0101] Comparative Example 5 A test cell was prepared and evaluated in the same manner as in Comparative Example 1, except that in the preparation of the positive electrode active material, Ni, Mn, and Li were mixed in a molar ratio of Ni:Mn:Li = 0.82:0.18:1.05, the firing temperature was set to 830°C, and the firing atmosphere was set to an oxygen flow rate of 4 L / min. The composition formula of the prepared composite oxide was Li １．０ Ni ０．８２０ Mn ０．１８０ O ２ It was.

[0102] The charge capacities, discharge capacities, and initial charge / discharge efficiencies of the test cells of Examples 6, 7, and 8 and Comparative Examples 3, 4, and 5 are shown in Tables 2, 3, and 4. Tables 2, 3, and 4 also show the composition of the composite oxide, the type of lithium molten salt, and whether or not a second peak occurred during the initial charge.

[0103]

[0104]

[0105]

[0106] As shown in Tables 2, 3, and 4, the test cells of Examples 6, 7, and 8 have higher capacities and improved initial charge-discharge efficiencies compared to the test cells of Comparative Examples 3, 4, and 5. In other words, the effects of the present disclosure are exerted even when the Ni content of the composite oxide is increased.

[0107] The present disclosure is further described by the following embodiments. Configuration 1: A positive electrode active material for use in a non-aqueous electrolyte secondary battery, having a crystal structure belonging to the space group R-3m and having the composition formula Li ｘ Na ｙ Ni １－ａ－ｂ Mn ａ X ｂ O ｃ wherein X is at least one element selected from the group consisting of transition metal elements and typical elements other than Li, Na, Ni, and Mn, and 0.80≦x≦1.15, 0<y≦0.20, 0.80<x+y≦1.20, 0<1−a−b≦1, 0≦a<1, 0≦b<1, and c are values ​​satisfying electrical neutrality; and when the nonaqueous electrolyte secondary battery is initially charged, an X-ray diffraction pattern obtained by X-ray diffraction has a peak at a diffraction angle 2θ in the range of 15.7° or more and 18.0° or less, which is attributable to a compound other than the compound represented by the composition formula. Configuration 2: The positive electrode active material according to Configuration 1, wherein the charge voltage during the initial charge is 3.8 V or more. Configuration 3: The positive electrode active material according to Configuration 1, wherein the peak belongs to space group R-3m and has the composition formula NaNi １－α－β Mn α X β O γ In the formula, X is at least one selected from the group consisting of transition metal elements and typical elements other than Na, Ni, and Mn, and γ is a value satisfying electrical neutrality, where 0<1-α-β≦1, 0≦α<1, 0≦β<1, and γ is a value satisfying electrical neutrality. ｘ Na ｙ Ni １－ａ－ｂ Mn ａ X ｂ O ｃ The positive electrode active material according to any one of the first to third aspects, wherein the molar ratio (y) of Na is 0.02≦y≦0.15. ｘ Naｙ Ni １－ａ－ｂ Mn ａ X ｂ O ｃ The positive electrode active material according to any one of the first to fourth aspects, wherein the molar ratio (y) of Na is 0.06≦y≦0.13. ｘ Na ｙ Ni １－ａ－ｂ Mn ａ X ｂ O ｃ The positive electrode active material according to any one of the first to fifth aspects, wherein the molar ratio (1-a-b) of Ni is 0.3≦1-b-c≦0.82. ｘ Na ｙ Ni １－ａ－ｂ Mn ａ X ｂ O ｃ In the formula (I), X is at least one 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, Co, and Al. ｘ Na ｙ Ni １－ａ－ｂ Mn ａ X ｂ O ｃ In the positive electrode active material of any one of the first to seventh embodiments, X is at least one selected from Al and Co. Aspect 9: A method for producing a positive electrode active material for use in a non-aqueous electrolyte secondary battery, comprising: ｅ Ni １－ｆ－ｇ Mn ｆ X ｇ O ｈ(wherein X is at least one element selected from metal elements other than Li, Na, Ni, and Mn, and e≦1.15, 0<1−f−g≦1, 0≦f<1, 0≦g<1, and h is a value satisfying electrical neutrality), and reacting the Na composite oxide with a lithium compound to exchange a portion of Na in the Na composite oxide for Li, wherein the lithium compound comprises at least one selected from the group consisting of lithium hydroxide, lithium carbonate, and lithium hydrogen carbonate. Aspect 10: A positive electrode comprising the positive electrode active material according to any one of Aspects 1 to 8. Aspect 11: A non-aqueous electrolyte secondary battery comprising the positive electrode according to Aspect 10, a negative electrode, and a non-aqueous electrolyte.

[0108] REFERENCE SIGNS LIST 10 nonaqueous electrolyte secondary battery, 11 positive electrode, 12 negative electrode, 13 separator, 14 electrode body, 16 outer can, 17 sealing body, 18, 19 insulating plate, 20 positive electrode lead, 21 negative electrode lead, 22 grooved portion, 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 non-aqueous electrolyte secondary batteries, It has a crystal structure belonging to the space group R-3m, Composition formula Li ｘ Na ｙ Ni １－ａ－ｂ Mn ａ X ｂ O ｃ It is represented as, In the formula, X is at least one element selected from the group consisting of transition metal elements other than Li, Na, Ni, and Mn, and main group elements, where 0.80 ≤ x ≤ 1.15, 0 < y ≤ 0.20, 0.80 < x + y ≤ 1.20, 0 < 1 - a - b ≤ 1, 0 ≤ a < 1, 0 ≤ b < 1, and c is a value that satisfies electrical neutrality. A positive electrode active material wherein, when the non-aqueous electrolyte secondary battery is charged for the first time, the X-ray diffraction pattern obtained by X-ray diffraction has peaks originating from a compound different from the compound represented by the composition formula, in the diffraction angle 2θ range of 15.7° to 18.0°.

2. The positive electrode active material according to claim 1, wherein the charging voltage during the initial charge is 3.8V or higher.

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

4. The aforementioned composition formula Li ｘ Na ｙ Ni １－ａ－ｂ Mn ａ X ｂ O ｃ 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 ｘ Na ｙ Ni １－ａ－ｂ Mn ａ X ｂ O ｃ The positive electrode active material according to claim 1, wherein the molar ratio (y) of Na is 0.06 ≤ y ≤ 0.

13.

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

82.

7. The aforementioned composition formula Li ｘ Na ｙ Ni １－ａ－ｂ Mn ａ X ｂ O ｃ The positive electrode active material according to claim 1, wherein X is at least one 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, Co, and Al.

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

9. A method for producing a positive electrode active material used in a non-aqueous electrolyte secondary battery, Composition formula Na ｅ Ni １－ｆ－ｇ Mn ｆ X ｇ O ｈ The process involves synthesizing a Na composite oxide represented by the formula (wherein X is at least one element selected from metal elements other than Li, Na, Ni, and Mn, e ≤ 1.15, 0 < 1 - f - g ≤ 1, 0 ≤ f < 1, 0 ≤ g < 1, and h is a value that satisfies electrical neutrality), 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, Includes, A method for producing a positive electrode active material, wherein the lithium compound comprises at least one selected from the group consisting of lithium hydroxide, lithium carbonate, and lithium bicarbonate.

10. A positive electrode comprising the positive electrode active material described in any one of claims 1 to 8.

11. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte as described in claim 10.