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

Abstract

Disclosed is a positive electrode active material having excellent cycle stability. The positive electrode active material of the present disclosure has an O2-type structure, contains at least Li, Na, O, and at least one selected from the group consisting of Mn, Ni, and Co as constituent elements, and Na exists in a layered form.

Description

Technical Field

[0001] The present application discloses a positive electrode active material, a lithium ion secondary battery, and a method for manufacturing the positive electrode active material. Background Art

[0002] As the positive electrode active material, a substance having an O2-type structure is known. The positive electrode active material having an O2-type structure is relatively stable even at a high potential, and thus can effectively utilize charge and discharge in a high potential range. For example, a high energy density can be easily obtained. The positive electrode active material having an O2-type structure is obtained by replacing at least a part of Na of a Na compound having a P2-type structure with Li. Specifically, as disclosed in Patent Documents 1 and 2, by performing ion exchange on a Na compound having a P2-type structure using lithium halide and replacing at least a part of Na with Li, a positive electrode active material having an O2-type structure can be obtained.

[0003] Prior Art Documents

[0004] Patent Document 1: Japanese Patent Application Laid-Open No. 2021-068556

[0005] Patent Document 2: Japanese Patent Application Laid-Open No. 2021-068555 Summary of the Invention

[0006] There is room for improvement in the cycle stability of the conventional positive electrode active material having an O2-type structure.

[0007] As one means for solving the above problems, the present application discloses a positive electrode active material,

[0008] which has an O2-type structure,

[0009] contains at least Li, Na, and O, and at least one selected from Mn, Ni, and Co as constituent elements, and,

[0010] Na exists in a layered form.

[0011] The positive electrode active material of the present disclosure may also have a composition represented by Li a Na b Mn x-p Ni y-q Co z-r M p+q+r O2 (where 0 < a ≤ 1.00, 0.05 ≤ b ≤ 0.20, x + y + z = 1 and 0 ≤ p + q + r ≤ 0.15, and M is at least one element selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W).

[0012] This application discloses a lithium-ion secondary battery as one of the means to solve the above-mentioned problems, which has a positive electrode, an electrolyte layer and a negative electrode.

[0013] The positive electrode contains the positive electrode active material disclosed herein.

[0014] This application provides a method for manufacturing a positive electrode active material as one of the means to solve the above-mentioned problems, which includes the following steps:

[0015] To obtain Na compounds with a P2-type structure; and

[0016] By ion exchange, a portion of the Na in the Na compound is replaced with Li, and the Na remains in a layered state to obtain a Li compound with an O2-type structure.

[0017] In the manufacturing method disclosed herein,

[0018] The Na compound may also have Na c Mn x-p Ni y-q Co z-r M p+q+r The composition represented by O2 (where 0.70≤c≤1.00, x+y+z=1 and 0≤p+q+r≤0.15, and M is at least one element selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W) and,

[0019] In the ion exchange, at least one of the following conditions (1) to (3) can also be satisfied.

[0020] (1) The temperature in the ion exchange is above 350℃.

[0021] (2) In the ion exchange, a mixture of lithium halide (A) and lithium compound (B) other than lithium halide is used, and the mass ratio (A / B) of the lithium halide (A) to the lithium compound (B) in the mixture is 0.25 or more.

[0022] (3) In the ion exchange, at least one of LiBr and LiI is used.

[0023] The positive electrode active material disclosed herein exhibits excellent cycle stability. Attached Figure Description

[0024] Figure 1 The HAADF-STEM image of the positive electrode active material of this disclosure and the corresponding Na composition mapping diagram are illustrated.

[0025] Figure 2A This is a diagram illustrating the method used to confirm that "Na exists in a layered state".

[0026] Figure 2B This is a diagram illustrating the method used to confirm that "Na exists in a layered state".

[0027] Figure 3 This is a simplified illustration of the structure of a lithium-ion secondary battery.

[0028] Figure 4 The diagram schematically illustrates the state of lattice displacement before and after ion exchange.

[0029] Figure 5 The diagram illustrates the generation of numerous nuclei during ion exchange, which form mismatched regions as they grow.

[0030] Figure 6 The X-ray diffraction peaks of the positive electrode active materials of Examples 1 and 2 and Comparative Examples 1 and 2 are shown.

[0031] Figure 7 The cycle characteristics of the button batteries in Examples 1 and 2 and Comparative Examples 1 and 2 are shown.

[0032] Figure 8 The HAADF-STEM images of the positive electrode active material surfaces of Example 1 and Comparative Example 2 are shown, along with the corresponding Na composition mapping diagrams.

[0033] Explanation of reference numerals in the attached figures

[0034] 10 Positive electrode

[0035] 11 Positive electrode active material layer

[0036] 12 Positive current collector

[0037] 20 Electrolyte layer

[0038] 30 Negative electrode

[0039] 31 Negative Electrode Active Material Layer

[0040] 32 Negative current collector

[0041] 100 rechargeable batteries Detailed Implementation

[0042] 1. Positive electrode active material

[0043] The positive electrode active material disclosed herein has an O2-type structure and contains at least Li, Na, O, and at least one selected from Mn, Ni, and Co as constituent elements, and Na exists in a layered state.

[0044] 1.1 Crystal Structure

[0045] The positive electrode active material of the present disclosure contains at least an O2-type structure (belonging to the space group P63mc) as a crystal structure. The positive electrode active material of the present disclosure has an O2-type structure and may also have a crystal structure other than the O2-type structure. As a crystal structure other than the O2-type structure, for example, a T#2-type structure (belonging to the space group Cmca) and an O6-type structure (belonging to the space group R-3m, the c-axis length is 2.5 nm or more and 3.5 nm or less, typically 2.9 nm or more and 3.0 nm or less, different from the O3-type structure also belonging to the space group R-3m) formed when Li is inserted and detached from the O2-type structure can be cited. The positive electrode active material of the present disclosure may have an O2-type structure as the main phase or may have a crystal structure other than the O2-type structure as the main phase. In the positive electrode active material of the present disclosure, the crystal structure as the main phase changes according to its charge-discharge state.

[0046] 1.2 Composition

[0047] The positive electrode active material of the present disclosure contains at least Li, Na, and O, and at least one selected from Mn, Ni, and Co as constituent elements. In particular, when at least Li, Na, Mn, and O and at least one of Ni and Co are contained as constituent elements, and when at least Li, Na, Mn, Ni, Co, and O are contained as constituent elements, the performance of the positive electrode active material of the present disclosure is likely to be higher. However, in the positive electrode active material of the present disclosure, for example, Li is released due to charging, and the amount of Li present also approaches 0.

[0048] The positive electrode active material of the present disclosure may also have a composition represented by Li a Na b Mn x-p Ni y-q Co z-r M p+q+r O2. Among them, 0 < a ≤ 1.00, 0.05 ≤ b ≤ 0.20, x + y + z = 1, and 0 ≤ p + q + r ≤ 0.15. In addition, M is at least one element selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W. When the positive electrode active material has such a composition, it is easy to maintain the O2-type structure, and it is easy to have a sufficient amount of Na present in a layered form, and the crystal structure is more likely to be stabilized.

[0049] In the above composition, 'a' can be greater than 0, greater than 0.10, greater than 0.20, greater than 0.30, greater than 0.40, greater than 0.50, or greater than 0.60, and can also be less than 1.00, less than 0.90, less than 0.80, or less than 0.70. In the above composition, 'b' can be greater than 0.05, greater than 0.06, greater than 0.07, or greater than 0.08, and can also be less than 0.20, less than 0.15, or less than 0.10. In the above composition, 'x' can be greater than 0, greater than 0.10, greater than 0.20, greater than 0.30, greater than 0.40, or greater than 0.50, and can also be less than 1.00, less than 0.90, less than 0.80, less than 0.70, less than 0.60, or less than 0.50. In the above composition, y can be 0 or higher, 0.10 or higher, or 0.20 or higher, and can also be 1.00 or lower, 0.90 or lower, 0.80 or lower, 0.70 or lower, 0.60 or lower, 0.50 or lower, 0.40 or lower, 0.30 or lower, or 0.20 or lower. In the above composition, z can also be 0 or higher, 0.10 or higher, 0.20 or higher, or 0.30 or higher, and can also be 1.00 or lower, 0.90 or lower, 0.80 or lower, 0.70 or lower, 0.60 or lower, 0.50 or lower, 0.40 or lower, or 0.30 or lower. M mostly does not contribute to charging and discharging. In this respect, by keeping p+q+r below 0.15, it is easy to ensure high charging and discharging capacity. p+q+r can be below 0.10 or 0. The composition of O is roughly 2, but not necessarily exactly 2.0.

[0050] In the above composition, when b is 0.05 or higher, it is assumed that a Na residual phase (structure-stabilized phase) with a Na content of 50% can exist in more than one of the 10 layers. Here, the range of structural degradation caused by the suppression of dislocation introduction (when forming stacking faults, some dislocations need to traverse the particles) through the stabilized structure can be estimated by the strain field formed around the dislocation. In the case of an edge dislocation with a Burgers vector parallel to the c-axis and a length half the length of the c-axis (equivalent to one layer of a layered structure), and in the case of an oxide material, a strain field of at least 5% is formed within a range of 5 layers from the dislocation. If a structurally stabilized heterogeneous phase exists at the location where a 5% strain field is formed, a lattice mismatch of nearly 5% is formed. If a 5% lattice mismatch is formed, it is considered that a dislocation has been introduced. Therefore, if it is initially assumed that a dislocation has been introduced, the dislocation will form and multiply indefinitely. Therefore, it is believed that if a structurally stable phase exists in one of the 10 layers, the structural stabilization effect will affect the entire positive electrode active material. With b values ​​above 0.05, structural stabilization can be expected. On the other hand, if b is too large, the charge / discharge capacity may decrease. With b values ​​below 0.20, sufficient capacity is easily ensured.

[0051] In the above composition, when the valence of M is set to +n, the relationship 3.0 ≤ 4(xp) + 2(yq) + 3(zr) + n(p + q + r) ≤ 3.4 can be satisfied. This refers to the range where the total valence of the metals in the positive electrode active material is close to 3.33 (charge neutral when a is 0.67). As will be described later, the positive electrode active material with the O2 type structure is synthesized via a Na compound with the P2 type structure. In this case, the Na composition is in the range of 0.6 to 1.0, which is charge neutral, equivalent to satisfying the above relationship. Furthermore, details regarding the composition of the Na compound with the P2 type structure will be described later.

[0052] Furthermore, the composition of the positive electrode active material can be determined, for example, by inductively coupled plasma light emission analysis (ICP-AES).

[0053] 1.3 Layered Na

[0054] In the positive electrode active material disclosed herein, Na exists in a layered state. Figure 1 An example is shown: a HAADF-STEM image of the positive electrode active material according to one embodiment, and a Na composition mapping diagram corresponding to the HAADF-STEM image. For example... Figure 1 As shown in the HAADF-STEM image and Na composition mapping diagram, in the positive electrode active material of this disclosure, when the electron beam incident orientation is set to [hk0] (h and k are 0 or any positive integer) for STEM-EDXS measurement, a region where Na exists in high concentration extends in one direction, and multiple such regions exist, with a certain interval between these multiple regions. Alternatively, the positive electrode active material of this disclosure is in a state where multiple layered alkali metal sites are present in the crystal structure on a predetermined plane, with a high concentration of Na in one layered site and many Na replaced by Li in another layered site. In other words, the positive electrode active material of this disclosure exhibits heterogeneity in the portion where Na exists in high concentration and the portion where Na is replaced by Li. Due to this heterogeneity, layered Na is observed in the Na composition mapping diagram. Whether Na exists in a layered manner in the positive electrode active material is determined using the following method based on the HAADF-STEM image of the positive electrode active material and its corresponding composition mapping diagram.

[0055] (1) For the positive electrode active material, under the condition that the electron beam probe diameter is less than 0.1 nm, the electron beam incident orientation is obtained as follows: <100> , <010> , <110> The composition mapping map is obtained by dividing the 25nm × 25nm region of the HAAD-STEM image into multiple regions of 512 × 512, and obtaining the composition of each region by STEM-EDXS. The STEM-EDXS measurement uses a device structure with an X-ray detection solid angle of 1 str or more. The measurement is performed under the following conditions: the phase electron beam irradiation relative to the entire observation area is 1.5 × 10^14 e or less, the electron beam scanning speed is 0.15 frames / second or more, and the measurement time is 10 minutes or more. Furthermore, during the measurement, when extracting any 30-second interval, drift correction is performed such that the sample movement within that interval is 0.1 nm or less. The composition mapping map defines only Mn, Ni, Co, M, and Na as present elements, and the presence of these elements is expressed as atomic percent. In this composition mapping map, for each region, a spectrum related to the presence of Mn, Ni, Co, M, and Na is obtained. Here, the proportion of Na when the total amount of Mn, Ni, Co, M and Na is 100 atomic percent is defined as "Na composition".

[0056] (2) In multiple regions divided into 512×512 areas, for the region in the STEM image and <001> By summing the spectra of more than 50 consecutive regions in the vertical direction ("continuous regions"), the Na composition (atomic %) in these continuous regions is determined. Thus, in <001> Calculate the values ​​in the direction ("width direction") sequentially with <001> The Na composition (atomic %) in a vertical continuous region was plotted, resulting in a coordinate graph showing the relationship between the Na composition (atomic %) and its relative position (nm) along the width direction. As an example, the following can be obtained: Figure 2A That kind of coordinate graph.

[0057] (3) In the obtained coordinate graph, the region with a Na atom percentage of 10% or more and a width of less than 2 nm is designated as "Region A". Conversely, the region with a Na atom percentage of less than 10% and a width of more than 1 nm is designated as "Region B". For example, in... Figure 2A Given that regions A and B are defined in the coordinate graph shown, such as Figure 2B As shown.

[0058] (4) In the obtained coordinate graph, as described above, determine "Region A" and "Region B" and determine whether "Region A" and "Region B" exist continuously. For positive electrode active materials, if there are more than 3 locations where "Region A" and "Region B" exist continuously (more than 3 locations where the continuous arrangement of Region A and Region B exists within one field of view), and the HAADF-STEM image has more than 3 fields of view, it is considered that "Na exists in a layered manner" in the positive electrode active material. The so-called "locations where Region A and Region B exist continuously" and "continuous arrangement of Region A and Region B" are as follows: Figure 2B The diagram shows a combination of a region A and a region B that are adjacent to each other. Figure 2B In this context, three or more of these combinations are in a state of being adjacent (continuous) to each other, but these combinations can also be separated from each other. That is, there can be a region between one combination and another that does not belong to either region A or region B.

[0059] In the positive electrode active material disclosed herein, the width of "Region A" as analyzed and measured as described above can be 0.3 nm or more or 0.5 nm or more, and can be 1.8 nm or less or 1.5 nm or less. In addition, the width of "Region B" as measured as described above can be 1.5 nm or more or 2.0 nm or more, and can be 100 nm or less or 10 nm or less.

[0060] Conventional positive electrode active materials with an O2-type structure sometimes become unstable due to Li desorption when charged to a high potential of around 4.8V. Furthermore, the migration of transition metals to Li sites and / or the formation of stacking faults caused by crystal structure instability can lead to capacity degradation. One way to address this problem is to form a phase stable at high potentials within a portion of the crystal structure, effectively utilizing this phase as a pinning phase. According to the inventors' novel insights, in the positive electrode active material disclosed herein, Na, existing in a layered form within the crystal structure, is extremely stable and difficult to desorb during charge and discharge, thus acting as a pinning phase for the main O2-type structure. In other words, it is believed that in a positive electrode active material with an O2-type structure, when Na exists in a layered form, even if charging to a high potential reduces the amount of Li in the crystal structure, the desired crystal structure is easily maintained, capacity degradation is easily suppressed, and excellent cycle stability is easily achieved.

[0061] 1.4 Shape

[0062] The shape of the positive electrode active material of the present disclosure may be a general shape as a positive electrode active material of a battery. For example, the positive electrode active material of the present disclosure may be granular. The particles of the positive electrode active material may be solid particles, hollow particles, or particles having voids. The particles of the positive electrode active material may be primary particles or secondary particles formed by aggregation of multiple primary particles. The average particle diameter (D50) of the positive electrode active material may be, for example, 1 nm or more, 5 nm or more, or 10 nm or more, and may be 500 μm or less, 100 μm or less, 50 μm or less, or 30 μm or less. Furthermore, the average particle diameter D50 referred to in the present application means the particle diameter (median diameter) at the cumulative value of 50% in the volume-based particle size distribution determined by the laser diffraction / scattering method.

[0063] 1.5 Supplement

[0064] As described above, in the positive electrode active material having an O2-type structure, since Na exists in a layered form, the crystal structure stabilization effect becomes remarkable. On the other hand, even in a positive electrode active material having an O2-type structure and a large amount of Na, since Na is sufficiently present, the crystal structure stabilization effect can be exerted. Regarding this point, in the present application, as another mode of the positive electrode active material, a positive electrode active material having an O2-type structure and containing Li a Na b Mn x-p Ni y-q Co z-r M p+q+r O2 (where 0 < a ≤ 1.00, 0.05 ≤ b ≤ 0.20, x + y + z = 1 and 0 ≤ p + q + r ≤ 0.15, and M is at least one element selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W) represents the composition. The details of a, b, x, y, z, p, q, and r are as described above.

[0065] 2. Positive Electrode

[0066] The technology of the present disclosure also has an aspect of a positive electrode including the above positive electrode active material. That is, the positive electrode of the present disclosure includes, as a positive electrode active material, a positive electrode active material having an O2-type structure and containing at least Li, Na, and O as constituent elements, and at least one selected from Mn, Ni, and Co, and Na exists in a layered form. As Figure 3 shown, a positive electrode 10 of one embodiment may include a positive electrode active material layer 11 and a positive electrode current collector 12. In this case, the positive electrode active material layer 11 may include the above positive electrode active material having an O2-type structure.

[0067] 2.1 Positive Electrode Active Material Layer

[0068] The positive electrode active material layer 11 contains at least the positive electrode active material disclosed herein as the positive electrode active material, and may optionally contain an electrolyte, conductive additive, and binder. Furthermore, the positive electrode active material layer 11 may also contain various other additives. The respective contents of the positive electrode active material, electrolyte, conductive additive, and binder in the positive electrode active material layer 11 can be appropriately determined according to the target battery performance. For example, with the total solid component of the positive electrode active material layer 11 as 100% by mass, the content of the positive electrode active material can be 40% or more by mass, 50% or more by mass, or 60% or more by mass, and can be less than 100% by mass or less than 90% by mass. The shape of the positive electrode active material layer 11 is not particularly limited; for example, it can be a sheet-like positive electrode active material layer 11 with a generally planar surface. The thickness of the positive electrode active material layer 11 is not particularly limited; for example, it can be 0.1 μm or more, or 1 μm or more, and can be less than 2 mm or less than 1 mm.

[0069] 2.1.1 Positive electrode active material

[0070] The positive electrode active material layer 11 may contain only the positive electrode active material disclosed herein. Alternatively, the positive electrode active material layer 11 may also contain positive electrode active materials of a different type (other positive electrode active materials) in addition to the positive electrode active materials disclosed herein. From the viewpoint of further improving the technical effect of this disclosure, the content of other positive electrode active materials in the positive electrode active material layer 11 may be small. For example, with the total positive electrode active material contained in the positive electrode active material layer 11 being 100% by mass, the content of the positive electrode active material disclosed herein may be 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, 95% by mass or more, or 99% by mass or more.

[0071] The surface of the positive electrode active material can also be covered by a protective layer containing a lithium-ion conductive oxide. That is, the positive electrode active material layer 11 can also include a composite having the aforementioned positive electrode active material and a protective layer disposed on its surface. This facilitates the suppression of reactions between the positive electrode active material and sulfides (e.g., sulfide solid electrolytes described later). Examples of lithium-ion conductive oxides include Li3BO3, LiBO2, Li2CO3, LiAlO2, Li4SiO4, Li2SiO3, Li3PO4, Li2SO4, Li2TiO3, and Li4Ti5O. 12The protective layer can be made of Li₂Ti₂O₅, Li₂ZrO₃, LiNbO₃, Li₂MoO₄, or Li₂WO₄. The coverage (area ratio) of the protective layer can be, for example, 70% or more, 80% or more, or 90% or more. The thickness of the protective layer can be, for example, 0.1 nm or more, or 1 nm or more, and can be less than 100 nm or less than 20 nm.

[0072] 2.1.2 Electrolytes

[0073] The electrolyte contained in the positive electrode active material layer 11 can be a solid electrolyte, a liquid electrolyte (electrolyte), or a combination thereof.

[0074] The solid electrolyte can be any material known as a solid electrolyte for lithium-ion secondary batteries. The solid electrolyte can be an inorganic solid electrolyte or an organic polymer electrolyte. In particular, inorganic solid electrolytes exhibit excellent ion conductivity and heat resistance. Examples of inorganic solid electrolytes include, for instance, lithium lanthanum zirconate, LiPON, and Li... 1+X Al X Ge 2-X (PO4)3, Li-SiO glass, Li-Al-SO glass and other oxide solid electrolytes; Li2S-P2S5, Li2S-SiS2, LiI-Li2S-SiS2, LiI-Si2S-P2S5, Li2S-P2S5-LiI-LiBr, LiI-Li2S-P2S5, LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, Li2S-P2S5-GeS2 and other sulfide solid electrolytes. Sulfide solid electrolytes, especially those containing at least Li, S and P as constituent elements, exhibit high performance. Solid electrolytes can be amorphous or crystalline. Solid electrolytes can be granular, for example. A single solid electrolyte can be used alone, or two or more can be used in combination.

[0075] The electrolyte may contain lithium ions as carrier ions. The electrolyte can be an aqueous electrolyte or a non-aqueous electrolyte. The composition of the electrolyte should be the same as that known for the composition of electrolytes used in lithium-ion secondary batteries. For example, an electrolyte in which lithium salts are dissolved at a predetermined concentration in a carbonate-based solvent can be used. Examples of carbonate-based solvents include fluoroethylene carbonate (FEC), ethylene carbonate (EC), and dimethyl carbonate (DMC). Examples of lithium salts include LiPF6.

[0076] 2.1.3 Conductive additives

[0077] Conductive additives that may be included in the positive electrode active material layer 11 include, for example, carbon materials such as fumed carbon fiber (VGCF), acetylene black (AB), Ketjen black (KB), carbon nanotubes (CNT), and carbon nanofibers (CNF); and metallic materials such as nickel, aluminum, and stainless steel. The conductive additives may be granular or fibrous, and their size is not particularly limited. Only one conductive additive may be used alone, or two or more may be used in combination.

[0078] 2.1.4 Adhesives

[0079] The adhesives that may be included in the positive electrode active material layer 11 include, for example, butadiene rubber (BR) based adhesives, butyl rubber (IIR) based adhesives, acrylate butadiene rubber (ABR) based adhesives, styrene-butadiene rubber (SBR) based adhesives, polyvinylidene fluoride (PVdF) based adhesives, polytetrafluoroethylene (PTFE) based adhesives, and polyimide (PI) based adhesives. Only one type of adhesive may be used alone, or two or more types may be used in combination.

[0080] 2.2 Positive Current Collector

[0081] like Figure 3 As shown, the positive electrode 10 may also have a positive electrode current collector 12 in contact with the aforementioned positive electrode active material layer 11. The positive electrode current collector 12 can be any general current collector used in batteries. Furthermore, the positive electrode current collector 12 can be in the form of a foil, plate, mesh, perforated metal, or foam. The positive electrode current collector 12 can be made of metal foil or metal mesh. Metal foil, in particular, has excellent processability. The positive electrode current collector 12 can also be made of multiple foils. Examples of metals constituting the positive electrode current collector 12 include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, and stainless steel. Especially from the viewpoint of ensuring oxidation resistance, the positive electrode current collector 12 may also contain Al. For purposes such as adjusting resistance, the positive electrode current collector 12 may have a coating on its surface. Alternatively, the positive electrode current collector 12 may be a current collector formed by depositing or vapor-depositing the aforementioned metal onto a metal foil or substrate. Furthermore, when the positive current collector 12 is composed of multiple metal foils, layers may also be present between these multiple metal foils. The thickness of the positive current collector 12 is not particularly limited. For example, it can be 0.1 μm or more, or 1 μm or more, and can be 1 mm or less, or 100 μm or less.

[0082] 2.3 Other

[0083] In addition to the above-described configuration, the positive electrode 10 may also have a general configuration for a secondary battery positive electrode. For example, it may be a connector or terminal. Besides using a substance having the above-described O2-type structure as the positive electrode active material, the positive electrode 10 can be manufactured using known methods. For example, the positive electrode active material layer 11 can be easily formed by dry or wet molding of a positive electrode mixture containing the various components described above. The positive electrode active material layer 11 can be formed together with the positive electrode current collector 12, or it can be formed separately from the positive electrode current collector 12.

[0084] 3. Lithium-ion secondary batteries

[0085] like Figure 1 As shown, a lithium-ion secondary battery 100 according to one embodiment has a positive electrode 10, an electrolyte layer 20, and a negative electrode 30. Here, the positive electrode 10 contains the positive electrode active material disclosed herein. As described above, the positive electrode active material of this disclosure exhibits excellent structural stability even at high potentials. In this regard, by including the positive electrode active material of this disclosure in the positive electrode of the lithium-ion secondary battery 100, the cycle characteristics of the secondary battery 100 can be easily improved. A specific example of the configuration of the positive electrode 10 is described above.

[0086] 3.1 Electrolyte layer

[0087] The electrolyte layer 20 contains at least an electrolyte. When the lithium-ion secondary battery 100 is a solid-state battery (which may be a battery containing a solid electrolyte and partially using a liquid electrolyte, or a completely solid-state battery without a liquid electrolyte), the electrolyte layer 20 contains a solid electrolyte and may optionally contain a binder, etc. In this case, the content of the solid electrolyte and binder, etc., in the electrolyte layer 20 is not particularly limited. On the other hand, when the lithium-ion secondary battery 100 is an electrolyte battery, the electrolyte layer 20 contains an electrolyte and may also have a separator, etc., for retaining the electrolyte and preventing contact between the positive electrode active material layer 11 and the negative electrode active material layer 31. The thickness of the electrolyte layer 20 is not particularly limited; for example, it may be 0.1 μm or more, or 1 μm or more, and may be 2 mm or less, or 1 mm or less.

[0088] The electrolyte contained in the electrolyte layer 20 can be appropriately selected from the electrolytes exemplified as those that can be contained in the aforementioned positive electrode active material layer. Similarly, the adhesive that can be contained in the electrolyte layer 20 can be appropriately selected from the adhesives exemplified as those that can be contained in the aforementioned positive electrode active material layer. Only one type of electrolyte and adhesive can be used individually, or two or more types can be used in combination. The separator can be any separator commonly used in lithium-ion secondary batteries, such as separators made of resins like polyethylene (PE), polypropylene (PP), polyester, and polyamide. The separator can be a single-layer structure or a multi-layer structure. Examples of multi-layer separators include two-layer PE / PP separators, or three-layer PP / PE / PP or PE / PP / PE separators. The separator can also be made of nonwoven fabrics such as cellulose nonwoven fabric, resin nonwoven fabric, or glass fiber nonwoven fabric.

[0089] 3.2 Negative electrode

[0090] like Figure 3 As shown, the negative electrode 30 may have a negative electrode active material layer 31 and a negative electrode current collector 32.

[0091] 3.2.1 Negative Electrode Active Material Layer

[0092] The negative electrode active material layer 31 contains at least a negative electrode active material, and may optionally contain an electrolyte, conductive additives, and a binder. Furthermore, the negative electrode active material layer 31 may also contain various other additives. The respective contents of the negative electrode active material, electrolyte, conductive additives, and binder in the negative electrode active material layer 31 can be appropriately determined according to the target battery performance. For example, with the entire negative electrode active material layer 31 (solid component as a whole) as 100% by mass, the content of the negative electrode active material can be 40% or more by mass, 50% or more by mass, or 60% or more by mass, and can be less than 100% by mass or less than 90% by mass. The shape of the negative electrode active material layer 31 is not particularly limited; for example, it can be a sheet-like negative electrode active material layer with a generally planar surface. The thickness of the negative electrode active material layer 31 is not particularly limited; for example, it can be 0.1 μm or more, or 1 μm or more, and can be less than 2 mm or less than 1 mm.

[0093] As the negative electrode active material, various materials with a lower potential for absorbing and releasing lithium ions (charge / discharge potential) than the positive electrode active material disclosed above can be used. For example, silicon-based active materials such as Si, Si alloys, and silicon oxide; carbon-based active materials such as graphite and hard carbon; various oxide-based active materials such as lithium titanate; and metallic lithium and lithium alloys can be used. A single negative electrode active material can be used alone, or two or more materials can be used in combination.

[0094] The shape of the negative electrode active material can be a general shape for a battery negative electrode active material. For example, the negative electrode active material can be granular. The negative electrode active material particles can be primary particles or secondary particles formed by the aggregation of multiple primary particles. The average particle size (D50) of the negative electrode active material particles can be, for example, 1 nm or more, 5 nm or more, or 10 nm or more, and can be less than 500 μm, less than 100 μm, less than 50 μm, or less than 30 μm. Alternatively, the negative electrode active material can also be in sheet form (foil form, film form) such as lithium foil. That is, the negative electrode active material layer 31 can also be composed of sheets of negative electrode active material.

[0095] Examples of electrolytes that may be included in the negative electrode active material layer 31 include the aforementioned solid electrolytes, electrolyte solutions, or combinations thereof. Examples of conductive additives that may be included in the negative electrode active material layer 31 include the aforementioned carbon materials and the aforementioned metal materials. Examples of binders that may be included in the negative electrode active material layer 31 are appropriately selected from the binders exemplified as binders that may be included in the aforementioned positive electrode active material layer 11. Only one type of electrolyte and binder may be used individually, or two or more may be used in combination.

[0096] 3.2.2 Negative current collector

[0097] like Figure 3 As shown, the negative electrode 30 may have a negative electrode current collector 32 in contact with the aforementioned negative electrode active material layer 31. The negative electrode current collector 32 can be any general current collector used in batteries. Furthermore, the negative electrode current collector 32 can be in the form of a foil, plate, mesh, perforated metal, or foam. The negative electrode current collector 32 can be a metal foil or metal mesh, or it can be a carbon sheet. Metal foils, in particular, offer excellent processability. The negative electrode current collector 32 can be composed of multiple foils or sheets. Examples of metals constituting the negative electrode current collector 32 include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, and stainless steel. Specifically, from the viewpoint of ensuring reduction resistance and preventing alloying with lithium, the negative electrode current collector 32 may contain at least one metal selected from Cu, Ni, and stainless steel. For purposes such as adjusting resistance, the negative electrode current collector 32 may have a coating on its surface. Alternatively, the negative current collector 32 can also be a current collector formed by depositing or vapor-depositing the aforementioned metal onto a metal foil or substrate. Furthermore, if the negative current collector 32 is composed of multiple metal foils, layers may be present between these multiple metal foils. The thickness of the negative current collector 32 is not particularly limited. For example, it can be 0.1 μm or more, or 1 μm or more, and can be 1 mm or less, or 100 μm or less.

[0098] 3.3 Other matters

[0099] The lithium-ion secondary battery 100 can also be a battery in which the above-described components are housed within an outer casing. The outer casing can be any casing known as a battery casing. Furthermore, multiple batteries 100 can be arbitrarily electrically connected or arbitrarily stacked to form a battery pack. In this case, the battery pack can also be housed inside a known battery casing. The lithium-ion secondary battery 100 may also have obvious structures such as necessary terminals. Examples of shapes for the lithium-ion secondary battery 100 include button type, laminated type, cylindrical type, and square type.

[0100] The lithium-ion secondary battery 100 can be manufactured using known methods. For example, it can be manufactured as described below. However, the manufacturing method of the lithium-ion secondary battery 100 is not limited to the following methods; for example, the layers can also be formed by dry forming or the like.

[0101] (1) A slurry for the negative electrode layer is obtained by dispersing the negative electrode active material, which constitutes the negative electrode active material layer, in a solvent. There is no particular limitation on the solvent used in this case; water or various organic solvents, or N-methylpyrrolidone (NMP) can be used. Then, the negative electrode layer slurry is applied to the surface of the negative electrode current collector using a doctor blade or the like, and then dried, thereby forming a negative electrode active material layer on the surface of the negative electrode current collector, thus producing a negative electrode.

[0102] (2) A slurry for the positive electrode layer is obtained by dispersing the positive electrode active material, which constitutes the positive electrode active material layer, in a solvent. There are no particular limitations on the solvent used in this case; water or various organic solvents, or N-methylpyrrolidone (NMP) can be used. The slurry for the positive electrode layer is applied to the surface of the positive electrode current collector using a doctor blade or the like, and then dried, thereby forming a positive electrode active material layer on the surface of the positive electrode current collector, thus producing a positive electrode.

[0103] (3) The layers are stacked in such a way that the electrolyte layer (solid electrolyte layer or membrane) is sandwiched between the negative electrode and the positive electrode to obtain a laminate having a negative current collector, a negative active material layer, an electrolyte layer, a positive active material layer and a positive current collector in sequence. Other components such as terminals are installed on the laminate as needed.

[0104] (4) The laminate is housed in the battery casing. In the case of an electrolyte battery, the battery casing is filled with electrolyte, the laminate is immersed in the electrolyte, and the laminate is sealed in the battery casing, thereby producing a secondary battery. Furthermore, in the case of an electrolyte battery, the negative electrode active material layer, the separator, and the positive electrode active material layer may also contain electrolyte in the above-mentioned stage (3).

[0105] 4. Method for manufacturing positive electrode active material

[0106] The positive electrode active material disclosed herein can be manufactured, for example, by the following method. That is, the method for manufacturing the positive electrode active material disclosed herein includes the following steps:

[0107] To obtain Na compounds with a P2-type structure; and

[0108] By ion exchange, a portion of the Na in the Na compound is replaced with Li, and the Na remains in a layered state to obtain a Li compound with an O2-type structure.

[0109] 4.1 Synthesis of Na compounds with P2-type structure

[0110] Since the O2-type structure is a metastable phase, it is necessary to first temporarily synthesize a Na compound with a P2-type structure, which is similar to the O2-type structure, and then exchange at least a portion of the Na ions in this Na compound for Li, thereby obtaining the O2-type structure. Therefore, in the manufacturing method disclosed herein, a Na compound with a P2-type structure is first obtained. The Na compound with a P2-type structure can be synthesized using known methods. For example, a mixture can be obtained by mixing an ion source capable of forming a precipitate with transition metal ions in aqueous solution, a precipitate from the transition metal source, and a Na source; optionally, this mixture is shaped and pre-fired, and then formally fired, thereby synthesizing a Na compound with a P2-type structure.

[0111] Examples of ion sources capable of forming precipitates with transition metal ions include salts such as carbonates and nitrates, sodium hydroxide, and sodium oxide. Examples of transition metal sources include salts such as nitrates, sulfates, carbonates, and hydroxides. Alternatively, the ion source and the transition metal source can be prepared separately into solutions, and the precipitate can be obtained by dripping and mixing the solutions. Various sodium compounds can be used as the base; additionally, ammonia solution can be added to adjust the alkalinity. The amount of Na source mixed before firing the precipitate is determined based on the amount of Na lost during firing. Examples of Na sources include sodium carbonate, sodium oxide, sodium nitrate, and sodium hydroxide. Pre-firing is performed at a temperature below the temperature of the formal firing. Pre-firing can also be omitted. For example, the formal firing can be performed at a temperature above 700°C and below 1100°C. Preferably, it is performed at a temperature above 800°C and below 1000°C. If the firing temperature is too low, Na doping is not performed; if the firing temperature is too high, an O3-type structure is easily formed instead of a P2-type structure. There are no particular limitations on the firing atmosphere; for example, it can be an oxygen-containing atmosphere such as atmospheric atmosphere or an inert gas atmosphere.

[0112] Alternatively, after synthesizing a Na compound with a P2-type structure, the Na compound can be crushed using a mortar and pestle or a ball mill.

[0113] 4.2 Ion Exchange

[0114] In the manufacturing method disclosed herein, at least a portion of the Na in the aforementioned Na compound is replaced with Li by ion exchange, while Na remains in a layered state, resulting in a Li compound having an O2-type structure. Ion exchange includes methods using an aqueous solution containing lithium halides and methods using a mixture of lithium halides and other lithium salts (e.g., molten salts). From the viewpoint that the P2-type structure is easily damaged by water intrusion and from the viewpoint of crystallinity, the molten salt method is preferred among the two methods. That is, at least a portion of the Na in the Na compound can be replaced with Li by ion exchange by mixing the Na compound having the aforementioned P2-type structure with the molten salt and heating it to a temperature above the melting point of the molten salt.

[0115] The lithium halide constituting the molten salt is preferably at least one of lithium chloride, lithium bromide, and lithium iodide. Other lithium salts constituting the molten salt are preferably lithium nitrate. By using the molten salt, the melting point is lowered compared to using lithium halide or other lithium salts alone, enabling ion exchange at lower temperatures.

[0116] The temperature during ion exchange can be, for example, below 600°C, below 500°C, or below 400°C. If the temperature during ion exchange is too high, it is easy to form an O3-type structure instead of an O2-type structure, which is a stable phase. On the other hand, from the viewpoint of minimizing the time required for ion exchange, the temperature during ion exchange can be as high as possible.

[0117] 4.3 An example of conditions used to allow Na to remain in a layered state

[0118] According to the inventors' understanding, in the case of obtaining a Li compound with an O2 type structure from a Na compound having a P2 type structure through ion exchange, it is effective to form a structure belonging to the space group P-6m2 as an intermediate during ion exchange in order to retain Na in a layered manner. To achieve this, for example, a compound having a predetermined composition is preferably used as the Na compound having a P2 type structure. Specifically, the Na compound before ion exchange preferably has a composition consisting of Na… c Mn x-p Ni y-q Co z- r M p+q+r The composition represented by O2 (where 0.70≤c≤1.00, x+y+z=1 and 0≤p+q+r≤0.15, and M is at least one element selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W). Additionally, in the P2 type structure, when the valence of M is +n, the relationship 3.0≤4(xp)+2(yq)+3(zr)+n(p+q+r)≤3.4 can also be satisfied.

[0119] In Na compounds, when the c in the above chemical composition is 1.00 or less, the P2 type structure is easily maintained. On the other hand, although the constant proportion composition of Na is Na... 2 / 3 However, crystal structures belonging to space group P-6m2 are more easily formed when this value is exceeded. When Na is replaced by Li through ion exchange, crystal structures belonging to space group P-6m2 are stabilized when a superlattice structure with a Na:Li ratio of 1:1 is formed. If the amount of Na in the P2 type structure is low, the nucleus formation of the O2 type structure is faster compared to the formation of crystal structures belonging to space group P-6m2, making it easier to form the O2 type structure without going through the space group P-6m2 crystal structure. On the other hand, if the amount of Na in the P2 type structure is high, the nucleus formation of the O2 type structure takes longer, and the diffusion of Li and Na proceeds sufficiently, resulting in the easier formation of crystal structures belonging to space group P-6m2. From this viewpoint, c in the above chemical composition is preferably 0.70 or higher. c can also exceed 0.70, be 0.71 or higher, or be 0.72 or higher. Regarding x, y, z, p, q, and r, as described above.

[0120] Furthermore, according to the inventors' understanding, in the case of obtaining a Li compound with an O2-type structure from a Na compound having a P2-type structure through ion exchange, in order to ensure that Na remains in a layered state, in addition to using a compound having the above-mentioned chemical composition as the Na compound, it is effective to employ a method with a high nucleation rate during ion exchange, i.e., a method with a fast ion exchange reaction. Specifically, it is preferable to satisfy at least one of the following conditions (1) to (3).

[0121] (1) The temperature in ion exchange is above 350℃.

[0122] (2) In ion exchange, a mixture of lithium halide (A) and lithium compound (B) other than lithium halide (e.g., the molten salt mentioned above) is used, and the mass ratio (A / B) of lithium halide (A) to lithium compound (B) in the mixture is 0.25 or more.

[0123] (3) In ion exchange, use lithium halides with high reducing power, namely at least one of LiBr and LiI.

[0124] Figure 4 The diagram schematically illustrates the lattice shifts before and after ion exchange. Additionally, Figure 5This indicates that multiple nuclei are generated during ion exchange, and mismatch regions form at the interface as they grow. It is believed that stacking faults typically form at these locations, but if a P2*-type structure is chosen as the constituent region of the intermediate structure, Na remains in the mismatch region, thus mitigating the mismatch by adopting a P2* structure. Based on this mechanism, as in the manufacturing method of this disclosure, by using a compound with the above chemical composition as the Na compound and employing a method with a high nucleus generation rate during ion exchange (i.e., a fast ion exchange reaction), Na tends to remain in the mismatch region; that is, in the final positive electrode active material, Na tends to remain in a layered manner.

[0125] [Example]

[0126] The following embodiments illustrate the technology of this disclosure in more detail, but the technology of this disclosure is not limited to the following embodiments.

[0127] 1. Example 1

[0128] 1.1 Synthesis of Na compounds with P2-type structure

[0129] Using Mn(NO3)2·6H2O, Ni(NO3)2·6H2O, and Co(NO3)2·6H2O as raw materials, they were dissolved in pure water to achieve a molar ratio of Mn, Ni, and Co of 5:2:3. Separately, a 12% by weight Na2CO3 solution was prepared, and both solutions were added dropwise to a beaker simultaneously. The titration rate was controlled to maintain a pH above 7.0 and below 7.1. After titration, the mixed solution was stirred at 50°C and 300 rpm for 24 hours. The reaction product was washed with pure water, and the precipitate powder was separated by centrifugation. The obtained powder was dried at 120°C for 48 hours and then crushed using an agate mortar to obtain the final powder. The resulting powder was then classified as Na... 0.85 Mn 0.5 Ni 0.2 Co 0.3 Na₂CO₃ was mixed with O₂. The mixed powder was pressed into granules using a cold isostatic pressing method under a load of 2 tons. The resulting granules were pre-calcined at 600°C for 6 hours in an atmospheric atmosphere, followed by formal calcination at 900°C for 24 hours, thereby synthesizing a Na compound with a P₂ type structure.

[0130] 1.2 Ion Exchange

[0131] LiNO3 and LiCl were mixed in a mass ratio of 88:12, and the mixture was weighed such that the molar ratio of Li was 10 times that of the Na compound with a P2-type structure. This Na compound was then mixed with the LiNO3·LiCl powder and subjected to ion exchange at 350°C for 1 hour under atmospheric conditions. After ion exchange, water was added to dissolve excess salt, followed by washing with water to obtain the Li compound with an O2-type structure (positive electrode active material).

[0132] 1.3 Production of the positive electrode

[0133] In 125 mL of an n-methylpyrrolidone solution containing 5 g of PVdF as a binder, 85 g of the above-prepared positive electrode active material and 10 g of carbon black as a conductive additive were uniformly mixed to prepare a positive electrode paste. This paste was then mixed with 6 mg / cm³ of N-methylpyrrolidone solution. 2 A coating was applied to one side of an Al current collector with a thickness of 15 μm and dried to obtain a laminate. The laminate was then pressed to achieve a paste thickness of 45 μm and a paste density of 2.4 g / cm³. 3 Finally, the laminate was cut into φ16mm pieces to obtain the positive electrode.

[0134] 1.4 Fabrication of the negative electrode

[0135] The negative electrode is obtained by cutting the Li foil into φ19mm pieces.

[0136] 1.5 Fabrication of Lithium-ion Secondary Batteries

[0137] The obtained positive and negative electrodes were used to fabricate CR2032 button batteries. Furthermore, a porous PP membrane was used as the separator, and lithium hexafluorophosphate (LiPF6) was dissolved at a concentration of 1 mol / L in a mixture of EC (ethylene carbonate) and DMC (dimethyl carbonate) at a volume ratio of 3:7 as the electrolyte.

[0138] 2. Example 2

[0139] Except that the mixing ratio of LiNO3 to LiCl during ion exchange is 70:30 and the ion exchange temperature is 280°C, the positive electrode active material is obtained in the same manner as in Example 1, and a lithium-ion secondary battery is fabricated.

[0140] 3. Comparative Example 1

[0141] In addition to setting the feed composition for obtaining Na compounds to Na 0.75 Mn 0.5 Ni 0.2 Co 0.3 In addition to O2, the positive electrode active material was obtained in the same manner as in Example 1, and a lithium-ion secondary battery was fabricated.

[0142] 4. Comparative Example 2

[0143] Except for setting the ion exchange temperature to 280°C, the positive electrode active material was obtained in the same manner as in Example 1, and a lithium-ion secondary battery was fabricated.

[0144] 5. Evaluation

[0145] 5.1 Crystal phase identification by XRD

[0146] The crystal phases of the various positive electrode active materials obtained in Examples 1 and 2 and Comparative Examples 1 and 2 were identified by powder X-ray diffraction. The results are shown in... Figure 6 .like Figure 6 As shown, all positive electrode active materials have an O2-type structure (and its derivative structures).

[0147] 5.2 Composition identification by ICP-AES

[0148] ICP-AES analysis was performed on each of the Na and Li compounds (positive electrode active materials) in Examples 1 and 2 and Comparative Examples 1 and 2. The results are shown in Table 1 below. Furthermore, the compositions shown in Table 1 below have been normalized so that the sum of the transition metal compositions is 1.00. As shown in Table 1, it can be seen that in Examples 1 and 2, the Na composition of the Na compounds is 0.70 or more, while in Comparative Examples 1 and 2, the Na composition of the Na compounds is less than 0.70. In addition, it can be seen that in Examples 1 and 2, the Na composition of the Li compounds is 0.05 or more, while in Comparative Examples 1 and 2, the Na composition of the Li compounds is less than 0.05.

[0149] Table 1

[0150]

[0151] 5.3 Evaluation of Cyclic Stability

[0152] The lithium-ion secondary batteries prepared in Examples 1, 2, and Comparative Example 1 were repeatedly charged and discharged to evaluate their cycle stability. In the charge-discharge tests, the batteries were charged to 4.8V and discharged to 2.0V at 0.1C. The results are shown below. Figure 7 .like Figure 7 As shown, the lithium-ion secondary batteries of Examples 1 and 2, whose Na composition of the positive electrode active material is 0.05 or higher, exhibit less capacity degradation during charge-discharge cycles and have excellent cycle stability compared to the lithium-ion secondary batteries of Comparative Examples 1 and 2.

[0153] 5.4 Evaluation conducted by HAADF-STEM and STEM-EDXS

[0154] The Li compounds of Example 1 and Comparative Example 2 were observed using HAADF-STEM and STEM-EDXS. Figure 8 This shows the HAADF-STEM images of the surfaces of each Li compound in Example 1 and Comparative Example 2, and the corresponding Na composition mapping diagrams. For example... Figure 8 As shown, in the Li compound of Example 1, Na exists in a layered state, while in the Li compound of Comparative Example 2, Na does not exist in a layered state but is dispersed throughout the compound. It is believed that in Example 1, the layered Na in the Li compound acts as a pinning phase, stabilizing the crystal structure and ensuring excellent cycling stability.

[0155] 6. Supplement

[0156] Furthermore, while the above examples illustrate positive electrode active materials with an O2-type structure, the composition of positive electrode active materials is not limited to this. Additionally, the same effect can be achieved even when the positive electrode active material contains doping elements such as B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W.

Claims

1. A positive electrode active material, having an O2-type structure, and having a composition represented by Li a Na b Mn x-p Ni y-q Co z-r M p+q+r O2 indicates, wherein where 0 < a ≤ 1.00, 0.05 ≤ b ≤ 0.20, x + y + z = 1 and 0 ≤ p + q + r ≤ 0.15, M is at least one element selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W, and when observing the HAADF-STEM image of the positive electrode active material and the Na composition mapping corresponding to the HAADF-STEM image, Na remains in the mismatch region and Na exists in a layered form.

2. A lithium ion secondary battery having a positive electrode, an electrolyte layer, and a negative electrode, where the positive electrode contains the positive electrode active material according to claim 1.

3. A method for manufacturing the positive electrode active material according to claim 1, where when observing the HAADF-STEM image of the positive electrode active material and the Na composition mapping corresponding to the HAADF-STEM image, Na remains in the mismatch region and Na exists in a layered form, and the manufacturing method includes the following steps: obtaining a Na compound having a P2-type structure; and by ion exchange, replacing a part of Na in the Na compound with Li and allowing Na to remain in a layered form to obtain a Li compound having an O2-type structure, The Na compound has the composition of Na c Mn x-p Ni y-q Co z-r M p+q+r O2 represents the composition, wherein, where 0.70 ≤ c ≤ 1.00, x + y + z = 1 and 0 ≤ p + q + r ≤ 0.15, M is at least one element selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W, and in the ion exchange, the following condition (1) is satisfied: (1) The temperature in the ion exchange is 350 °C or higher.

4. According to the manufacturing method of claim 3, in the ion exchange, the following condition (2) is satisfied: (2) In the ion exchange, a mixture of lithium halide (A) and a lithium compound (B) other than lithium halide is used, and the mass ratio (A / B) of the lithium halide (A) to the lithium compound (B) in the mixture is 0.25 or higher.

Images