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

Abstract

To disclose cathode active material excellent in cycle stability.SOLUTION: Cathode active material according to the present disclosure has O2-type structure and includes, as constituent elements at least Li, Na, at least one of Mn, Ni and Co, and O, wherein Na exists in laminate.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Positive electrode active materials having an O2-type structure are known. Positive electrode active materials having an O2-type structure are relatively stable even at high potentials, allowing for charge / discharge in a high potential range, and for example, making it easy to obtain a high energy density. Positive electrode active materials having an O2-type structure can be obtained by substituting at least a portion of the Na in a Na compound having a P2-type structure with Li. Specifically, as disclosed in Patent Documents 1 and 2, a positive electrode active material having an O2-type structure is obtained by performing ion exchange on a Na compound having a P2-type structure using lithium halide to substitute at least a portion of the Na with Li. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-068556 [Patent Document 2] Patent Publication No. 2021-068555 Summary of the Invention [Problem to be solved by the invention]

[0004] Conventional positive electrode active materials with an O2 structure have room for improvement in terms of cycle stability. [Means for solving the problem]

[0005] As one of the means for solving the above problems, the present application provides: A positive electrode active material, It has an O2 type structure, As a constituent element, it contains at least Li, Na, at least one of Mn, Ni, and Co, and O, and in which Na exists in a layered form is disclosed.

[0006] The positive electrode active material of the present disclosure is Li a Na b Mn x-p Ni y-q Co z-r M p+q+r It may have a composition represented by 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).

[0007] As one of the means for solving the above problems, the present application provides 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 of the present disclosure is disclosed.

[0008] As one of the means for solving the above problems, the present application provides a method for manufacturing a positive electrode active material, obtaining a Na compound having a P2-type structure, and by ion exchange, while substituting part of the Na in the Na compound with Li and leaving Na in a layered form to obtain a Li compound having an O2-type structure, including the manufacturing method is disclosed.

[0009] In the manufacturing method of the present disclosure, the Na compound is Na c Mn x-p Ni y-q Co z-r M p+q+rO2 (wherein 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 In the ion exchange, at least one of the following conditions (1) to (3) may be satisfied.

[0010] (1) The temperature in the ion exchange is 350°C or higher. (2) In the ion exchange, a mixture of a lithium halide (A) and a lithium compound (B) other than a 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. (3) At least one of LiBr and LiI is used in the ion exchange. [Effects of the Invention]

[0011] The positive electrode active material of the present disclosure has excellent cycle stability. [Brief explanation of the drawings]

[0012] [Figure 1] 1 illustrates an HAADF-STEM image of a positive electrode active material of the present disclosure and a Na composition map corresponding to the HAADF-STEM image. [Figure 2A] FIG. 1 is a diagram for explaining a method for confirming that "Na exists in a layered form." [Figure 2B] FIG. 1 is a diagram for explaining a method for confirming that "Na exists in a layered form." [Figure 3] 1 shows a schematic diagram of an example of the configuration of a lithium ion secondary battery. [Figure 4] The diagram shows a schematic diagram of the lattice shift before and after ion exchange. [Figure 5]This diagram shows the schematic diagram of the formation of numerous nuclei during ion exchange, which grow to form incommensurate regions. [Figure 6] 1 shows the X-ray diffraction peaks of the positive electrode active materials according to Examples 1 and 2 and Comparative Examples 1 and 2. [Figure 7] 1 shows the cycle characteristics of each of the coin cells according to Examples 1 and 2 and Comparative Examples 1 and 2. [Figure 8] 1 shows HAADF-STEM images of the surfaces of the positive electrode active materials according to Example 1 and Comparative Example 2, and Na composition maps corresponding to the HAADF-STEM images. DETAILED DESCRIPTION OF THE INVENTION

[0013] 1.Cathode active material The positive electrode active material of the present disclosure has an O2 type structure and contains, as constituent elements, at least Li, Na, at least one of Mn, Ni, and Co, and O, with Na existing in a layered form.

[0014] 1.1 Crystal structure The cathode active material of the present disclosure includes at least an O2-type structure (belonging to the space group P63mc) as a crystalline structure. The cathode active material of the present disclosure has an O2-type structure, and may also have a crystalline structure other than the O2-type structure. Examples of crystalline structures other than the O2-type structure include a T#2-type structure (belonging to the space group Cmca) formed when Li is inserted and removed from the O2-type structure, and an O6-type structure (belonging to the space group R-3m, with a c-axis length of 2.5 nm to 3.5 nm, typically 2.9 nm to 3.0 nm, and different from the O3-type structure also belonging to the space group R-3m). The cathode active material of the present disclosure may have an O2-type structure as a main phase, or may have a crystalline structure other than the O2-type structure as a main phase. The crystalline structure of the main phase of the cathode active material of the present disclosure may change depending on the charge / discharge state.

[0015] 1.2 Composition The cathode active material of the present disclosure contains, as constituent elements, at least Li, Na, at least one of Mn, Ni, and Co, and O. In particular, when the constituent elements include at least Li, Na, Mn, at least one of Ni and Co, and O, among others, when the constituent elements include at least Li, Na, Mn, Ni, Co, and O, the performance of the cathode active material of the present disclosure is more likely to be improved. However, in the cathode active material of the present disclosure, for example, Li may be released by charging, and the abundance of Li may approach 0.

[0016] The cathode active material of the present disclosure contains Li a Na b Mn x-p Ni y-q Co z-r M p+q+r It may have a composition represented by O2, where 0 < a ≤ 1.00, 0.05 ≤ b ≤ 0.20, x + y + z = 1, and 0 ≤ p + q + r ≤ 0.15. Here, 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 cathode active material has such a composition, the O2-type structure is likely to be maintained, a sufficient amount of Na is likely to be present in a layered manner, and the crystal structure is likely to be more stabilized.

[0017] In the above composition, a may be greater than 0, 0.10 or greater, 0.20 or greater, 0.30 or greater, 0.40 or greater, 0.50 or greater, or 0.60 or greater, and may be 1.00 or less, 0.90 or less, 0.80 or less, or 0.70 or less. In the above composition, b may be 0.05 or greater, 0.06 or greater, 0.07 or greater, or 0.08 or greater, and may be 0.20 or less, 0.15 or less, or 0.10 or less. In the above composition, x may be 0 or greater, 0.10 or greater, 0.20 or greater, 0.30 or greater, 0.40 or greater, or 0.50 or greater, and may be 1.00 or less, 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, or 0.50 or less. In the above composition, y may be 0 or more, 0.10 or more, or 0.20 or more, and may be 1.00 or less, 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, 0.50 or less, 0.40 or less, 0.30 or less, or 0.20 or less. In the above composition, z may be 0 or more, 0.10 or more, 0.20 or more, or 0.30 or more, and may be 1.00 or less, 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, 0.50 or less, 0.40 or less, or 0.30 or less. M often does not contribute to charge and discharge. In this regard, a high charge and discharge capacity is easily ensured by having p + q + r be 0.15 or less. p + q + r may be 0.10 or less, or may be 0. The composition of O is approximately 2, but is not necessarily exactly 2.0 and is indefinite.

[0018] In the above composition, when b is 0.05 or greater, it is believed that a residual Na phase (structurally stabilized phase) with a 50% Na content can exist in at least one layer per 10 layers. The extent to which a stabilized structure can suppress structural degradation due to dislocation introduction (partial dislocations must cross the grain to form stacking faults) can be estimated from the strain field formed around the dislocation. In the case of an edge dislocation whose Burgers spectrum is parallel to the c-axis and has a length half the c-axis length (corresponding to one layer of the layered structure), and in the case of an oxide material, a strain field of at least 5% is formed within five layers from the dislocation. If a structurally stabilized heterophase exists at a position where a 5% strain field is formed, a lattice mismatch of nearly 5% is formed. Since the formation of a 5% lattice mismatch is thought to result in dislocation introduction, assuming dislocation introduction first, dislocations will form dislocations and multiply infinitely. Therefore, the presence of one structurally stabilized phase per 10 layers is believed to extend the structural stabilization effect to the entire positive electrode active material. In this regard, when b is 0.05 or more, a structural stabilization effect can be expected. On the other hand, if b is too large, there is a risk that the charge / discharge capacity will be small. When b is 0.20 or less, sufficient capacity is likely to be ensured.

[0019] In the above composition, when the valence of M is +n, the relationship 3.0≦4(xp)+2(yq)+3(zr)+n(p+q+r)≦3.4 may be satisfied. This indicates that the total valence of the metals in the positive electrode active material is close to 3.33 (charge neutrality when a is 0.67). As will be described later, a positive electrode active material having an O2 type structure is synthesized via a Na compound having a P2 type structure. When the Na composition at this time is in the range of 0.6 to 1.0, charge neutrality occurs, which corresponds to the case where the above relationship is satisfied. The details of the composition of the Na compound having a P2 type structure will be described later.

[0020] The composition of the positive electrode active material can be determined, for example, by inductively coupled plasma atomic emission spectroscopy (ICP-AES).

[0021] 1.3 Layered Na In the positive electrode active material of the present disclosure, Na exists in a layered structure. FIG. 1 illustrates an HAADF-STEM image of a positive electrode active material according to one embodiment, along with a corresponding Na composition map. As shown in the HAADF-STEM image and Na composition map in FIG. 1, when STEM-EDXS measurement was performed on the positive electrode active material of the present disclosure with the electron beam incident direction set to [hk0] (h and k are 0 or any positive integer), a region where Na is present in a high concentration extends in one direction, and there are multiple such regions, with a certain distance or more between these multiple regions. The positive electrode active material of the present disclosure has multiple layered alkali metal sites in its crystal structure on a specific surface, with Na present in high concentration at some layered sites, while at other layered sites, much of the Na is substituted with Li. In other words, the positive electrode active material of the present disclosure has unevenness between areas where Na is present at high concentration and areas where Na has been substituted with Li, and this unevenness is the reason why layered Na is observed in the Na composition map. Whether or not Na is present in layered form in the positive electrode active material is determined by the following method from the HAADF-STEM image of the positive electrode active material and the corresponding composition map.

[0022] (1) For the positive electrode active material, under the condition that the electron beam probe diameter is 0.1 nm or less, the electron beam incident direction is <100> , <010> , <110> A HAADF-STEM image (either HAADF or HADDF) and a corresponding composition map are obtained. The composition map is obtained by dividing a 25 nm x 25 nm region in the HAADF-STEM image into 512 x 512 areas and then obtaining the composition of each area using STEM-EDXS. STEM-EDXS measurements are performed using an instrument with an X-ray detection solid angle of 1 str or greater. Measurements are performed under conditions of an electron beam dose of 1.5 x 10^14 e or less across the entire observation area, an electron beam scanning speed of 0.15 frames / second or greater, and a measurement time of 10 minutes or longer. During measurement, drift correction is performed to ensure that sample movement within any 30-second interval is 0.1 nm or less. The composition map defines only Mn, Ni, Co, M, and Na as present elements, and their abundances are expressed in atomic percent. The composition map provides spectra for the abundances of Mn, Ni, Co, M, and Na for each area. Here, the proportion of Na when the total amount of Mn, Ni, Co, M, and Na is taken as 100 atomic %, is defined as the "Na composition."

[0023] (2) Among the multiple areas divided into 512 × 512, the STEM image <001> The above spectrum is integrated for 50 or more consecutive areas ("consecutive areas") in the direction perpendicular to the plane, and the Na composition (atomic %) in the consecutive areas is determined. <001> The Na composition (atomic %) in the continuous area perpendicular to <001> The relationship between the Na composition (atomic %) and the relative position (nm) in the width direction is plotted in a graph, as shown in Figure 2A.

[0024] (3) In the graph obtained, the area where a continuous area with a Na atomic percentage of 10% or more exists with a width of 2 nm or less is designated as "area A." Furthermore, the area where a continuous area with a Na atomic percentage of less than 10% exists with a width of 1 nm or more is designated as "area B." For example, if area A and area B are identified in the graph shown in FIG. 2A, the graph will look like FIG. 2B.

[0025] (4) In the obtained graph, "Region A" and "Region B" are identified as described above, and it is determined whether "Region A" and "Region B" exist contiguously. When three or more HAADF-STEM images are obtained for a cathode active material in which "Region A" and "Region B" exist contiguously in three or more locations (i.e., three or more locations in one visual field where Region A and Region B are contiguously arranged), the cathode active material is deemed to have "Na present in a layered structure." "A location where Region A and Region B exist contiguously" and "a contiguously arranged Region A and Region B" refer to a combination of one Region A and one Region B adjacent to each other, as shown in FIG. 2B. Although three or more such combinations are adjacent (contiguous) to each other in FIG. 2B, the combinations may be separated from each other; that is, there may be a region between one combination and another that does not correspond to Region A or Region B.

[0026] In the positive electrode active material of the present disclosure, the width of "region A" analyzed and measured as described above may be 0.3 nm or more or 0.5 nm or more, and 1.8 nm or less or 1.5 nm or less, and the width of "region B" measured as described above may be 1.5 nm or more or 2.0 nm or more, and 100 nm or less or 10 nm or less.

[0027] When conventional cathode active materials having an O2-type structure are charged to a high potential of about 4.8 V, the crystal structure may become unstable due to the desorption of Li. Furthermore, the destabilization of the crystal structure may result in the migration of transition metals to the Li site or the formation of stacking faults, which may lead to capacity degradation. One way to solve this problem is to form a phase stable at high potentials in a part of the crystal structure and utilize that phase as a pinning phase. According to the new findings of the inventors, in the cathode active material of the present disclosure, Na present in a layered form in the crystal structure is extremely stable, is unlikely to desorb during charge and discharge, and can act as a pinning phase for the bulk O2-type structure. That is, when Na is present in a layered form in a cathode active material having an O2-type structure, it is believed that the desired crystal structure is likely to be maintained even when charging to a high potential reduces the amount of Li in the crystal structure, which may lead to suppressed capacity degradation and excellent cycle stability.

[0028] 1.4 Shape The shape of the positive electrode active material of the present disclosure may be any shape commonly used for positive electrode active materials in batteries. For example, the positive electrode active material of the present disclosure may be particulate. The particles of the positive electrode active material may be solid, hollow, or void-containing. 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. Note that the average particle diameter D50 referred to in this application refers to the particle diameter (median diameter) at 50% of the cumulative value in a volume-based particle size distribution determined by a laser diffraction / scattering method.

[0029] 1.5 Supplementary Information As described above, in the cathode active material having an O2-type structure, the layered presence of Na results in a remarkable crystal structure stabilization effect. On the other hand, in a cathode active material having an O2-type structure and a large amount of Na, the crystal structure stabilization effect may be exerted by the sufficient presence of Na. In this regard, the present application discloses, as another form of the cathode active material, an O2-type structure and 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). The details of a, b, x, y, z, p, q, and r are as described above.

[0030] 2. Cathode The technology of the present disclosure also has an aspect as a cathode including the above cathode active material. That is, the cathode of the present disclosure includes, as a cathode active material, an O2-type structure, and as constituent elements, at least Li, Na, at least one of Mn, Ni, and Co, and O, and Na is present in a layered manner. As shown in FIG. 3, the cathode 10 according to an embodiment may include a cathode active material layer 11 and a cathode current collector 12. In this case, the cathode active material layer 11 may include the cathode active material having the above O2-type structure.

[0031] 2.1 Cathode Active Material Layer The positive electrode active material layer 11 contains at least the positive electrode active material of the present disclosure as the positive electrode active material, and may further contain, optionally, an electrolyte, a conductive additive, a binder, and the like. Furthermore, the positive electrode active material layer 11 may also contain various other additives. The contents of the positive electrode active material, electrolyte, conductive additive, binder, and the like in the positive electrode active material layer 11 may be appropriately determined depending on the desired battery performance. For example, the content of the positive electrode active material may be 40% by mass or more, 50% by mass or more, or 60% by mass or more, and may be 100% by mass or less, or 90% by mass or less, where the entire positive electrode active material layer 11 (total solid content) is taken as 100% by mass. The shape of the positive electrode active material layer 11 is not particularly limited, and may be, for example, a sheet-like positive electrode active material layer 11 having a substantially flat surface. The thickness of the positive electrode active material layer 11 is not particularly limited, and may be, for example, 0.1 μm or more or 1 μm or more, and 2 mm or less or 1 mm or less.

[0032] 2.1.1 Cathode active material The positive electrode active material layer 11 may contain only the positive electrode active material of the present disclosure as the positive electrode active material. Alternatively, the positive electrode active material layer 11 may contain, in addition to the positive electrode active material of the present disclosure, a different type of positive electrode active material (another positive electrode active material). From the viewpoint of further enhancing the effects of the technology of the present disclosure, the content of the other positive electrode active material in the positive electrode active material layer 11 may be small. For example, the content of the positive electrode active material of the present disclosure 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, where the total positive electrode active material contained in the positive electrode active material layer 11 is taken as 100% by mass.

[0033] The surface of the positive electrode active material may be covered with a protective layer containing a lithium ion conductive oxide. That is, the positive electrode active material layer 11 may include a composite comprising the above-mentioned positive electrode active material and a protective layer provided on the surface thereof. This makes it easier to suppress reactions between the positive electrode active material and sulfides (e.g., sulfide solid electrolytes, etc., described later). Examples of lithium ion conductive oxides include Li3BO3, LiBO2, Li2CO3, LiAlO2, Li4SiO4, Li2SiO3, Li3PO4, Li2SO4, Li2TiO3, and Li4Ti5O. 12 , Li2Ti2O5, Li2ZrO3, LiNbO3, Li2MoO4, and Li2WO4. The coverage (area ratio) of the protective layer may be, for example, 70% or more, 80% or more, or 90% or more. The thickness of the protective layer may be, for example, 0.1 nm or more, or 1 nm or more, or may be 100 nm or less, or 20 nm or less.

[0034] 2.1.2 Electrolytes The electrolyte that can be contained in the positive electrode active material layer 11 may be a solid electrolyte, a liquid electrolyte (electrolytic solution), or a combination thereof.

[0035] The solid electrolyte may be any known solid electrolyte for lithium ion secondary batteries. The solid electrolyte may be an inorganic solid electrolyte or an organic polymer electrolyte. In particular, inorganic solid electrolytes have excellent ionic conductivity and heat resistance. Examples of inorganic solid electrolytes include lithium lanthanum zirconate, LiPON, and Li 1+X Al X Ge 2-XExamples of suitable solid electrolytes include oxide solid electrolytes such as (PO4)3, Li-SiO-based glass, and Li-Al-SO-based glass; and sulfide solid electrolytes such as Li2S-P2S5, Li2S-SiS2, LiI-Li2S-SiS2, LiI-Si2S-P2S5, Li2S-P2S5-LiI-LiBr, LiI-Li2S-P2S5, LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, and Li2S-P2S5-GeS2. Sulfide solid electrolytes, especially sulfide solid electrolytes containing at least Li, S, and P as constituent elements, exhibit high performance. The solid electrolyte may be amorphous or crystalline. The solid electrolyte may be, for example, particulate. One type of solid electrolyte may be used alone, or two or more types may be used in combination.

[0036] The electrolyte may contain, for example, lithium ions as carrier ions. The electrolyte may be an aqueous electrolyte or a non-aqueous electrolyte. The composition of the electrolyte may be the same as that of known electrolytes for lithium ion secondary batteries. For example, the electrolyte may be a carbonate-based solvent in which a lithium salt is dissolved at a predetermined concentration. Examples of carbonate-based solvents include fluoroethylene carbonate (FEC), ethylene carbonate (EC), and dimethyl carbonate (DMC). Examples of lithium salts include LiPF6.

[0037] 2.1.3 Conductive additives Examples of conductive additives that can be contained in the positive electrode active material layer 11 include carbon materials such as vapor grown carbon fiber (VGCF), acetylene black (AB), ketjen black (KB), carbon nanotubes (CNT), and carbon nanofibers (CNF); and metal materials such as nickel, aluminum, and stainless steel. The conductive additive may be, for example, in the form of particles or fibers, and its size is not particularly limited. Only one type of conductive additive may be used alone, or two or more types may be used in combination.

[0038] 2.1.4 Binder Examples of binders that can be contained in the positive electrode active material layer 11 include butadiene rubber (BR)-based binders, butylene rubber (IIR)-based binders, acrylate butadiene rubber (ABR)-based binders, styrene butadiene rubber (SBR)-based binders, polyvinylidene fluoride (PVdF)-based binders, polytetrafluoroethylene (PTFE)-based binders, polyimide (PI)-based binders, etc. Only one type of binder may be used alone, or two or more types may be used in combination.

[0039] 2.2 Positive electrode current collector As shown in FIG. 3 , the positive electrode 10 may include a positive electrode current collector 12 in contact with the positive electrode active material layer 11. Any common positive electrode current collector for batteries can be used as the positive electrode current collector 12. The positive electrode current collector 12 may be in the form of a foil, plate, mesh, punched metal, foam, or the like. The positive electrode current collector 12 may be made of a metal foil or metal mesh. Metal foils are particularly easy to handle. The positive electrode current collector 12 may be made of multiple foils. Metals constituting the positive electrode current collector 12 include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, stainless steel, and the like. In particular, the positive electrode current collector 12 may contain Al to ensure oxidation resistance. The positive electrode current collector 12 may have a coating layer on its surface for purposes such as adjusting resistance. The positive electrode current collector 12 may be a metal foil or a substrate plated or vapor-deposited with the above metal. When the positive electrode current collector 12 is made of multiple metal foils, some layer may be present between the multiple metal foils. The thickness of the positive electrode current collector 12 is not particularly limited. For example, it may be 0.1 μm or more, 1 μm or more, or 1 mm or less, or 100 μm or less.

[0040] 2.3 Other In addition to the above configuration, the positive electrode 10 may also have a configuration generally used for a positive electrode of a secondary battery. For example, a tab, a terminal, etc. The positive electrode 10 can be manufactured by a known method, except that the positive electrode active material has the above-mentioned O2 type structure. For example, the positive electrode active material layer 11 can be easily formed by dry or wet molding a positive electrode mixture containing the above-mentioned various components. The positive electrode active material layer 11 may be molded together with the positive electrode current collector 12, or may be molded separately from the positive electrode current collector 12.

[0041] 3. Lithium-ion secondary batteries As shown in FIG. 1 , 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 above-described positive electrode active material of the present disclosure. As described above, the positive electrode active material of the present disclosure has excellent structural stability even at high potentials. In this regard, when the positive electrode of the lithium ion secondary battery 100 contains the positive electrode active material of the present disclosure, the cycle characteristics of the secondary battery 100 are likely to be improved. Specific configuration examples of the positive electrode 10 are as described above.

[0042] 3.1 Electrolyte layer The electrolyte layer 20 contains at least an electrolyte. When the lithium-ion secondary battery 100 is a solid-state battery (a battery containing a solid electrolyte in which a liquid electrolyte is used in part, or an all-solid-state battery containing no liquid electrolyte), the electrolyte layer 20 contains a solid electrolyte and may further contain a binder or the like. In this case, the contents of the solid electrolyte and binder or the like in the electrolyte layer 20 are not particularly limited. On the other hand, when the lithium-ion secondary battery 100 is a liquid electrolyte battery, the electrolyte layer 20 contains an electrolytic solution and may further have a separator or the like for retaining the electrolytic solution 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 and may be, for example, 0.1 μm or more or 1 μm or more, or 2 mm or less or 1 mm or less.

[0043] The electrolyte contained in the electrolyte layer 20 may be appropriately selected from the electrolytes exemplified above as those that can be contained in the positive electrode active material layer. Similarly, the binder contained in the electrolyte layer 20 may be appropriately selected from the binders exemplified above as those that can be contained in the positive electrode active material layer. Each of the electrolytes and binders may be used alone or in combination of two or more. The separator may be any separator commonly used in lithium ion secondary batteries, such as those made of resins such as polyethylene (PE), polypropylene (PP), polyester, and polyamide. The separator may have a single layer structure or a multilayer structure. Examples of multilayer separators include a two-layer structure of PE / PP, or a three-layer structure of PP / PE / PP or PE / PP / PE. The separator may be made of a nonwoven fabric such as a cellulose nonwoven fabric, a resin nonwoven fabric, or a glass fiber nonwoven fabric.

[0044] 3.2 Negative electrode As shown in FIG. 3, the negative electrode 30 may include a negative electrode active material layer 31 and a negative electrode current collector 32.

[0045] 3.2.1 Negative electrode active material layer The negative electrode active material layer 31 contains at least a negative electrode active material and may further contain, optionally, an electrolyte, a conductive additive, a binder, and the like. Furthermore, the negative electrode active material layer 31 may also contain various other additives. The contents of the negative electrode active material, electrolyte, conductive additive, binder, and the like in the negative electrode active material layer 31 may be appropriately determined depending on the desired battery performance. For example, the content of the negative electrode active material may be 40% by mass or more, 50% by mass or more, or 60% by mass or more, or 100% by mass or less, or 90% by mass or less, where the total weight of the negative electrode active material layer 31 (total solid content) is taken as 100% by mass. The shape of the negative electrode active material layer 31 is not particularly limited, and may be, for example, a sheet-like negative electrode active material layer having a substantially flat surface. The thickness of the negative electrode active material layer 31 is not particularly limited, and may be, for example, 0.1 μm or more or 1 μm or more, and 2 mm or less or 1 mm or less.

[0046] As the negative electrode active material, various materials can be used that have a lithium ion absorption / desorption potential (charge / discharge potential) that is lower than that of the positive electrode active material of the present disclosure. 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; metallic lithium and lithium alloys, etc. The negative electrode active material may be used alone or in combination of two or more.

[0047] The shape of the negative electrode active material may be any shape commonly used for negative electrode active materials in batteries. For example, the negative electrode active material may be in a particulate form. The negative electrode active material particles may be primary particles or secondary particles formed by aggregation of multiple primary particles. The average particle diameter (D50) of the negative electrode active material particles 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. Alternatively, the negative electrode active material may be in a sheet form (foil or film form) such as lithium foil. That is, the negative electrode active material layer 31 may be made of a sheet of negative electrode active material.

[0048] Examples of electrolytes that can be contained in the negative electrode active material layer 31 include the above-mentioned solid electrolytes, electrolytic solutions, and combinations thereof. Examples of conductive additives that can be contained in the negative electrode active material layer 31 include the above-mentioned carbon materials and metal materials. The binder that can be contained in the negative electrode active material layer 31 may be appropriately selected from, for example, the binders that can be contained in the above-mentioned positive electrode active material layer 11. Only one type of electrolyte or binder may be used alone, or two or more types may be used in combination.

[0049] 3.2.2 Negative electrode current collector As shown in FIG. 3 , the negative electrode 30 may include a negative electrode current collector 32 in contact with the negative electrode active material layer 31. The negative electrode current collector 32 may be any of those commonly used as negative electrode current collectors for batteries. The negative electrode current collector 32 may be in the form of a foil, plate, mesh, punched metal, foam, or the like. The negative electrode current collector 32 may be a metal foil or metal mesh, or a carbon sheet. Metal foil is particularly advantageous in terms of ease of handling. The negative electrode current collector 32 may be formed from 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. In particular, from the viewpoints of ensuring reduction resistance and being less likely to be alloyed with lithium, the negative electrode current collector 32 may contain at least one metal selected from Cu, Ni, and stainless steel. The negative electrode current collector 32 may have some kind of coating layer on its surface for the purpose of adjusting the resistance, etc. Alternatively, the negative electrode current collector 32 may be a metal foil or a substrate on which the above metal is plated or vapor-deposited. Alternatively, when the negative electrode current collector 32 is made of multiple sheets of metal foil, some kind of layer may be present between the multiple sheets of metal foil. The thickness of the negative electrode current collector 32 is not particularly limited. For example, it may be 0.1 μm or more or 1 μm or more, or may be 1 mm or less or 100 μm or less.

[0050] 3.3 Other matters The lithium ion secondary battery 100 may have the above components housed inside an exterior body. Any known exterior body for a battery can be used as the exterior body. Furthermore, a plurality of batteries 100 may be electrically connected and stacked in any manner to form an assembled battery. In this case, the assembled battery may be housed inside a known battery case. The lithium ion secondary battery 100 may also include other obvious components such as necessary terminals. The shape of the lithium ion secondary battery 100 may be, for example, a coin type, a laminate type, a cylindrical type, a prismatic type, or the like.

[0051] The lithium ion secondary battery 100 can be manufactured by applying a known method. For example, it can be manufactured as follows. However, the manufacturing method of the lithium ion secondary battery 100 is not limited to the following method, and each layer may be formed by, for example, dry molding or the like. (1) A negative electrode active material constituting the negative electrode active material layer is dispersed in a solvent to obtain a negative electrode layer slurry. The solvent used in this case is not particularly limited, and water or various organic solvents can be used, including N-methylpyrrolidone (NMP). The negative electrode layer slurry is then applied to the surface of a negative electrode current collector using a doctor blade or the like, and then dried to form a negative electrode active material layer on the surface of the negative electrode current collector, resulting in a negative electrode. (2) The positive electrode active material constituting the positive electrode active material layer is dispersed in a solvent to obtain a positive electrode layer slurry. The solvent used in this case is not particularly limited, and water or various organic solvents can be used, including N-methylpyrrolidone (NMP). The positive electrode layer slurry is applied to the surface of a positive electrode current collector using a doctor blade or the like, and then dried to form a positive electrode active material layer on the surface of the positive electrode current collector, resulting in a positive electrode. (3) The layers are stacked so that the electrolyte layer (solid electrolyte layer or separator) is sandwiched between the negative electrode and the positive electrode to obtain a laminate having, in this order, the negative electrode current collector, the negative electrode active material layer, the electrolyte layer, the positive electrode active material layer, and the positive electrode current collector. Other members such as terminals are attached to the laminate as necessary. (4) The laminate is housed in a battery case, and in the case of an electrolyte battery, the battery case is filled with an electrolyte, and the laminate is immersed in the electrolyte and sealed in the battery case to form a secondary battery. In the case of an electrolyte battery, the electrolyte may be impregnated into the negative electrode active material layer, the separator, and the positive electrode active material layer at the step (3) above.

[0052] 4. Manufacturing method of positive electrode active material The positive electrode active material of the present disclosure can be produced, for example, by the following method. Obtaining a Na compound having a P2 type structure, and By ion exchange, a part of Na in the Na compound is replaced with Li, while leaving Na in a layered state, to obtain a Li compound having an O2-type structure. Includes.

[0053] 4.1 Synthesis of Na compounds with P2 structure Because the O2-type structure is a metastable phase, it is necessary to first synthesize a Na compound having a similar P2-type structure, and then ion-exchange at least a portion of the Na in the Na compound with Li to obtain the O2-type structure. Therefore, in the manufacturing method of the present disclosure, a Na compound having a P2-type structure is first obtained. The Na compound having a P2-type structure can be synthesized by a known method. For example, a mixture can be obtained by mixing an ion source capable of forming a precipitate in aqueous solution with transition metal ions, a transition metal source, and a precipitate derived from the transition metal source, and the Na source. The mixture can then be optionally molded and pre-fired, followed by firing, to synthesize a Na compound having a P2-type structure.

[0054] Examples of ion sources capable of forming a precipitate with transition metal ions include salts such as carbonates and nitrates, as well as sodium hydroxide and sodium oxide. Examples of transition metal sources include salts such as nitrates, sulfates, and carbonates, as well as hydroxides. The ion source and the transition metal source may be prepared as solutions, and then the solutions may be added dropwise and mixed to obtain a precipitate. In this case, various sodium compounds may be used as bases, and aqueous ammonia may be added to adjust the basicity. The amount of Na source mixed before calcining the precipitate may be determined taking into account the amount of Na lost during calcination. Examples of Na sources include sodium carbonate, sodium oxide, sodium nitrate, and sodium hydroxide. Preliminary calcination is performed at a temperature equal to or lower than that of the main calcination. Preliminary calcination may be omitted. The main calcination may be performed at a temperature of 700°C to 1100°C, preferably 800°C to 1000°C. If the firing temperature is too low, Na doping will not occur, and if the firing temperature is too high, an O3 type structure will likely form instead of a P2 type structure. The firing atmosphere is not particularly limited, and may be, for example, an oxygen-containing atmosphere such as air or an inert gas atmosphere.

[0055] After synthesizing the Na compound having the P2 type structure, the Na compound may be crushed using a mortar and pestle, a ball mill, or the like.

[0056] 4.2 Ion exchange In the manufacturing method of the present disclosure, at least a portion of the Na in the above-mentioned Na compound is substituted with Li by ion exchange, while leaving the Na in a layered state, to obtain a Li compound having an O2-type structure. Ion exchange can be performed using an aqueous solution containing lithium halide, or a mixture of lithium halide and other lithium salts (e.g., molten salt). Of the two methods, the method using molten salt is preferred from the viewpoints of the P2-type structure being easily broken by the penetration of water and of crystallinity. That is, by mixing the above-mentioned Na compound having a P2-type structure with the molten salt and heating the mixture to a temperature equal to or higher than the melting point of the molten salt, at least a portion of the Na in the Na compound can be substituted with Li by ion exchange.

[0057] The lithium halide constituting the molten salt is preferably at least one of lithium chloride, lithium bromide, and lithium iodide. The other lithium salt constituting the molten salt is preferably lithium nitrate. By using the molten salt, the melting point becomes lower than when lithium halide or other lithium salt is used alone, and ion exchange can be performed at a lower temperature.

[0058] The temperature for ion exchange may be, for example, 600°C or less, 500°C or less, or 400°C or less. If the temperature for ion exchange is too high, the stable O3 structure is likely to be formed instead of the O2 structure. On the other hand, from the viewpoint of shortening the time required for ion exchange, it is preferable that the temperature for ion exchange is as high as possible.

[0059] 4.3 Example of conditions for retaining Na in a layered state According to the findings of the present inventors, when a Li compound having an O2 type structure is obtained by ion exchange from a Na compound having a P2 type structure, in order to leave Na in a layered state, it is effective to form a structure belonging to the space group P-6m2 as an intermediate during ion exchange. To achieve this, it is preferable to adopt, for example, a Na compound having a P2 type structure with a predetermined composition. Specifically, the Na compound before ion exchange is Na c Mn x-p Ni y-q Co z-r M p+q+r It is preferable that the composition be represented by the formula: 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). In addition, when the valence of M in the P2 type structure is +n, the relationship 3.0≦4(xp)+2(yq)+3(zr)+n(p+q+r)≦3.4 may be satisfied.

[0060] In Na compounds, the P2 structure is easily maintained when c in the above chemical composition is 1.00 or less. 2 / 3 However, a crystal structure belonging to the space group P-6m2 is likely to form when the Na content exceeds this value. A crystal structure belonging to the space group P-6m2 is stabilized when a superlattice-like structure with a Na to Li ratio of 1:1 is formed during ion exchange by substituting Na for Li. However, when the Na content in the P2-type structure is low, nucleation of the O2-type structure occurs more quickly than the formation of a crystal structure belonging to the space group P-6m2, and the O2-type structure is likely to form without passing through the crystal structure belonging to the space group P-6m2. On the other hand, when the Na content in the P2-type structure is high, nucleation of the O2-type structure takes longer, and sufficient diffusion of Li and Na occurs, resulting in the formation of a crystal structure belonging to the space group P-6m2. From this perspective, c in the above chemical composition is preferably 0.70 or greater. c may be greater than 0.70, 0.71 or greater, or 0.72 or greater. x, y, z, p, q, and r are as described above.

[0061] Furthermore, according to the findings of the present inventors, when a Li compound having an O2 type structure is obtained by ion exchange from a Na compound having a P2 type structure, in order to leave Na in a layered state, it is effective to use a Na compound having the above chemical composition as the Na compound, and also to adopt a method that has a high nucleation rate during ion exchange, i.e., a method that allows a fast ion exchange reaction. Specifically, the following (1) and (2) It is preferable that at least one of the following conditions is satisfied. (1) The temperature during ion exchange is 350°C or higher. (2) In the ion exchange, a mixture of a lithium halide (A) and a lithium compound (B) other than a lithium halide (e.g., the above-mentioned molten salt) 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. 。

[0062] Figure 4 shows a schematic diagram of the lattice shift before and after ion exchange. Figure 5 also shows the formation of a mismatch region at the interface when numerous nuclei are generated during ion exchange and grow. Normally, stacking faults would form at this location. However, if a compositional region in which a P2*-type structure is formed is selected as the intermediate structure, Na remains in the mismatch region, and the mismatch is thought to be alleviated by adopting a P2*-like structure. Based on the above mechanism, it is thought that by using a Na compound having the above chemical composition and employing a method that achieves a high nucleation rate during ion exchange, i.e., a method that achieves a fast ion exchange reaction, as in the manufacturing method of the present disclosure, Na is more likely to remain in the mismatch region, i.e., Na is more likely to remain in a layered form in the final positive electrode active material. [Example]

[0063] The technology of the present disclosure will be described in more detail below with reference to examples, but the technology of the present disclosure is not limited to the following examples.

[0064] 1. Example 1 1.1 Synthesis of Na compounds with P2 structure Mn(NO3)2·6H2O, Ni(NO3)2·6H2O, and Co(NO3)2·6H2O were used as raw materials and dissolved in pure water so that the molar ratio of Mn, Ni, and Co was 5:2:3. A 12 wt% Na2CO3 solution was prepared, and these two solutions were simultaneously added dropwise to a beaker. The titration rate was controlled so that the pH was between 7.0 and 7.1. After the titration, the mixed solution was stirred at 50°C and 300 rpm for 24 hours. The resulting reaction product was washed with pure water, and the precipitated powder was separated by centrifugation. The resulting powder was dried at 120°C for 48 hours and then crushed in an agate mortar to obtain a powder. Na2CO3 was added to the resulting powder, and Na2CO3 was added to the powder, and the composition ratio was adjusted to 7.0. 0.85 Mn 0.5 Ni 0.2 Co 0.3The powder mixture was mixed to form O2. The mixed powder was pressed under a load of 2 tons using cold isostatic pressing to produce pellets. The resulting pellets were pre-fired in air at 600°C for 6 hours, and then fired at 900°C for 24 hours to synthesize a sodium compound with a P2-type structure.

[0065] 1.2 Ion exchange LiNO3 and LiCl were mixed in a mass ratio of 88:12, and weighed out so that the molar ratio of Li to the Na compound with a P2-type structure was 10 times. The Na compound was mixed with the LiNO3·LiCl mixed powder, and ion exchange was carried out in an air atmosphere at 350°C for 1 hour. After the ion exchange, water was added to dissolve excess salt, and the mixture was washed with water to obtain a Li compound with an O2-type structure (positive electrode active material).

[0066] 1.3 Preparation of the positive electrode 85 g of the positive electrode active material obtained as described above and 10 g of carbon black as a conductive additive were uniformly mixed in 125 mL of n-methylpyrrolidone solution containing 5 g of PVdF as a binder, to prepare a positive electrode composite paste. This paste was applied to a 15 μm thick Al current collector in a basis weight of 6 mg / cm. 2 The laminate was then pressed to a paste thickness of 45 μm and a paste density of 2.4 g / cm. 3 Finally, this laminate was cut into a piece having a diameter of 16 mm to obtain a positive electrode.

[0067] 1.4 Preparation of the negative electrode The Li foil was cut to a diameter of 19 mm to obtain a negative electrode.

[0068] 1.5 Fabrication of lithium-ion secondary batteries A CR2032 coin cell was fabricated using the resulting positive and negative electrodes. A porous PP separator was used as the separator, and the electrolyte was a mixture of EC (ethylene carbonate) and DMC (dimethyl carbonate) in a volume ratio of 3:7, with lithium hexafluorophosphate (LiPF6) dissolved at a concentration of 1 mol / L as the supporting electrolyte.

[0069] 2. Example 2 A positive electrode active material was obtained in the same manner as in Example 1, except that the mixing ratio of LiNO3 and LiCl during ion exchange was 70:30 and the ion exchange temperature was 280°C, and a lithium ion secondary battery was fabricated.

[0070] 3. Comparative Example 1 The composition of the feed to obtain Na compounds is Na 0.75 Mn 0.5 Ni 0.2 Co 0.3 A positive electrode active material was obtained in the same manner as in Example 1 except that O2 was used, and a lithium ion secondary battery was fabricated.

[0071] 4. Comparative Example 2 A positive electrode active material was obtained in the same manner as in Example 1 except that the ion exchange temperature was set to 280°C, and a lithium ion secondary battery was fabricated.

[0072] 5. Evaluation 5.1 Identification of crystalline phases by XRD The crystalline phase of each of the positive electrode active materials obtained in Examples 1 and 2 and Comparative Examples 1 and 2 was identified by powder X-ray diffraction. The results are shown in Figure 6. As shown in Figure 6, all of the positive electrode active materials had an O2-type structure (and its derivative structure).

[0073] 5.2 Identification of composition by ICP-AES ICP-AES measurements were performed on each of the Na compounds 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. 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, the Na composition of the Na compounds in Examples 1 and 2 is 0.70 or more, while the Na composition of the Na compounds in Comparative Examples 1 and 2 is below 0.70. It is also shown that the Na composition of the Li compounds in Examples 1 and 2 is 0.05 or more, while the Na composition of the Li compounds in Comparative Examples 1 and 2 is below 0.05.

[0074] [Table 1]

[0075] 5.3 Evaluation of cycle stability The lithium ion secondary batteries produced in Examples 1 and 2 and Comparative Example 1 were repeatedly charged and discharged to evaluate their cycle stability. In the charge-discharge test, charging was performed up to 4.8 V and discharging was performed up to 2.0 V, each at 0.1 C. The results are shown in FIG. 7. As shown in FIG. 7, the lithium ion secondary batteries according to Examples 1 and 2, in which the Na composition of the positive electrode active material is 0.05 or more, show less capacity degradation during charge-discharge cycles and have superior cycle stability compared to the lithium ion secondary batteries according to Comparative Examples 1 and 2.

[0076] 5.4 Evaluation by HAADF-STEM and STEM-EDXS The Li compounds of Example 1 and Comparative Example 2 were observed by HAADF-STEM and STEM-EDXS. FIG. 8 shows HAADF-STEM images of the surfaces of the Li compounds of Example 1 and Comparative Example 2, along with Na composition maps corresponding to the HAADF-STEM images. As shown in FIG. 8, Na exists in layers in the Li compound of Example 1, whereas Na does not exist in layers in the Li compound of Comparative Example 2, but is dispersed throughout the compound. It is believed that the Na existing in layers in the Li compound of Example 1 functions as a pinning phase, stabilizing the crystal structure and thereby ensuring excellent cycle stability.

[0077] 6. Supplementary Information Although the above example shows a cathode active material having an O2-type structure with a predetermined composition, the composition of the cathode active material is not limited to this. Also, the same effect can be achieved even if the cathode 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. [Explanation of symbols]

[0078] 10 positive electrode 11 Cathode active material layer 12 Positive electrode current collector 20 Electrolyte layer 30 negative electrode 31 Negative electrode active material layer 32 Negative electrode current collector 100 Secondary battery

Claims

1. A positive electrode active material, It has an O2 type structure, Constituent elements include at least Li, Na, at least one of Mn, Ni, and Co, and O, Li ａ Na ｂ Mn ｘ Ni ｙ Co ｚ O ２ (wherein 0.50≦a≦1.00, 0.05≦b≦0.20, and x+y+z=1), and In the HAADF-STEM image and the Na composition map corresponding to the HAADF-STEM image, Na exists in a layered form. Cathode active material.

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

3. A method for producing a positive electrode active material, Obtaining a Na compound having a P2 type structure, and By ion exchange, a part of Na in the Na compound is replaced with Li, while leaving Na in a layered form, to obtain a Li compound having an O2-type structure; Including, The Na compound is Na ｃ Mn ｘ Ni ｙ Co ｚ O ２ (where 0.70≦c≦1.00, and x+y+z=1), The Li compound has a composition represented by Li a Na b Mn x Ni y Co z O 2 (wherein 0.50≦a≦1.00, 0.05≦b≦0.20, and x+y+z=1), and In the ion exchange, the following condition (1) is satisfied: Manufacturing method. (1) The temperature during the ion exchange is 350° C. or higher.

4. In the ion exchange, the following condition (2) is satisfied: The method of claim 3. (2) In the ion exchange, a mixture of a lithium halide (A) and a lithium compound (B) other than a 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.

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