Positive electrode active material and lithium ion secondary battery

Abstract

To increase the weight energy density of a positive electrode active material having an O2-type structure.SOLUTION: The positive electrode active material of the present disclosure includes Li-containing oxide particles having an O2-type structure. The Li-containing oxide particles include at least one of Mn, Ni, and Co and includes Li and O as constituent elements. The Li-containing oxide particles have an average particle diameter of at least 2.0 μm. The Li-containing oxide particles have an average aspect ratio of at least 1.0 and 3.0 at largest.SELECTED DRAWING: Figure 5

Description

[Technical Field]

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

[0002] Positive electrode active materials having an O2-type structure are known. As disclosed in Patent Document 1, a positive electrode active material having an O2-type structure is obtained by ion-exchanging at least a portion of Na in a sodium-containing oxide having a P2-type structure with Li. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-170994 Summary of the Invention [Problem to be solved by the invention]

[0004] Conventional positive electrode active materials have room for improvement in terms of weight energy density. [Means for solving the problem]

[0005] The present application discloses the following aspects as means for solving the above problems. <Aspect 1> A positive electrode active material comprising Li-containing oxide particles, The Li-containing oxide particles have an O2 type structure, The Li-containing oxide particles contain, as constituent elements, at least one element selected from Mn, Ni, and Co, Li, and O, the Li-containing oxide particles have an average particle size of 2.0 μm or more, The Li-containing oxide particles have an average aspect ratio of 1.0 or more and 3.0 or less. Cathode active material. <Aspect 2> The positive electrode active material of Aspect 1, The Li-containing oxide particles contain, as constituent elements, 0.33 mol or more of Li with respect to 1 mol of O. Positive electrode active material. <Aspect 3> The positive electrode active material according to Aspect 1 or 2, wherein the Li-containing oxide particles are Li a Na b Mn x-p Ni y-q Co z-r M p+q+r O2 (where 0 < a ≤ 1.00, 0 ≤ b ≤ 0.20, x + y + z = 1, and 0 ≤ p + q + r < 0.17, and the element M is at least one selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W).) having a chemical composition represented by Positive electrode active material. <Aspect 4> The positive electrode active material according to any one of Aspects 1 to 3, wherein the Li-containing oxide particles are Li a Na b Mn x-p Ni y-q Co z-r M p+q+r O2 (where 0 < a ≤ 1.00, 0 ≤ b ≤ 0.20, 0 < x < 1.00, 0 < y < 0.50, 0 < z < 1.00, x + y + z = 1, and 0 ≤ p + q + r < 0.17, and the element M is at least one selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W).) having a chemical composition represented by Positive electrode active material. <Aspect 5> The positive electrode active material according to any one of Aspects 1 to 4, wherein the Li-containing oxide particles are Li a Na b Mn x-p Ni y-q Co z-r M p+q+rO2 (where 0.66 < a ≤ 1.00, 0 ≤ b ≤ 0.20, 0.30 < x < 0.60, 0.10 < y < 0.40, 0.10 < z < 0.50, x + y + z = 1, and 0 ≤ p + q + r < 0.17, and the element M is at least one selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W.) having a chemical composition represented by a positive electrode active material. <Aspect 6> A lithium ion secondary battery comprising a positive electrode active material layer, an electrolyte layer, and a negative electrode active material layer, where the positive electrode active material layer contains the positive electrode active material of any one of Aspects 1 to 5. a lithium ion secondary battery.

Advantages of the Invention

[0006] The positive electrode active material of the present disclosure has excellent weight energy density.

Brief Description of the Drawings

[0007] [Figure 1] An example of the process for manufacturing a positive electrode active material having an O2-type structure is shown. [Figure 2] An example of the configuration of a lithium ion secondary battery is schematically shown. [Figure 3] The X-ray diffraction patterns of the positive electrode active materials according to Examples 1 and 2 are shown. [Figure 4] The X-ray diffraction patterns of the positive electrode active materials according to Comparative Examples 1 and 2 are shown. [Figure 5] The cross-sectional SEM image of the positive electrode active material according to Example 1 is shown. [Figure 6] The cross-sectional SEM image of the positive electrode active material according to Comparative Example 1 is shown.

Modes for Carrying Out the Invention

[0008] 1. Positive electrode active material 1.1 First form The positive electrode active material according to the first embodiment includes Li-containing oxide particles. The Li-containing oxide particles have an O2-type structure. The Li-containing oxide particles include, as constituent elements, at least one element selected from Mn, Ni, and Co, Li, and O. The Li-containing oxide particles have an average particle diameter of 2.0 μm or more. The Li-containing oxide particles have an average aspect ratio of 1.0 or more and 3.0 or less.

[0009] 1.1.1 Crystal structure The Li-containing oxide particles according to the first embodiment have at least an O2-type structure (belonging to the space group P63mc) as a crystal structure. The Li-containing oxide particles according to an embodiment have the O2-type structure and may have a crystal structure other than the O2-type structure. Examples of crystal structures other than the O2-type structure include a T#2-type structure (belonging to the space group Cmca) formed when Li is deintercalated 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 Li-containing oxide particles according to an embodiment may have an O2-type structure as a main phase, or may have a crystal structure other than the O2-type structure (O6 structure or T#2 structure) as a main phase. However, the crystal structure of the Li-containing oxide particles as a main phase may change depending on the charge / discharge state.

[0010] The Li-containing oxide particles according to the first embodiment may be single crystals consisting of a single crystallite or polycrystalline having multiple crystallites. The end faces of the crystallites of the Li-containing oxide particles are considered to be the entrance and exit points for intercalation. That is, when the crystallites of the Li-containing oxide particles are small, the number of entrances and exits for intercalation increases, thereby reducing reaction resistance; the distance that lithium ions travel decreases, thereby reducing diffusion resistance; and the absolute amount of expansion and contraction during charge and discharge decreases, making cracking less likely. For example, the diameter of the crystallites constituting the Li-containing oxide particles may be 0.1 μm to 5.0 μm, 0.5 μm to 4.0 μm, or 1.0 μm to 3.0 μm. The "crystallite" and "crystallite diameter" can be determined by observing the Li-containing oxide particles with a scanning electron microscope (SEM) or a transmission electron microscope (TEM). That is, when a single closed region surrounded by a grain boundary is observed in the Li-containing oxide particles, the region is considered to be a "crystallite." The maximum Feret diameter of the crystallite is determined and considered to be the "crystallite diameter." When the Li-containing oxide particle is composed of a single crystal, the particle itself can be considered a single crystallite, and the maximum Feret diameter of the particle is the "crystallite diameter." Alternatively, the crystallite diameter can be determined by EBSD or XRD. For example, the crystallite diameter can be determined based on the Scherrer equation from the half-width of the diffraction line in the XRD pattern. When the crystallite diameter determined by any of these methods is within the above-mentioned range, the Li-containing oxide particle is likely to exhibit higher performance. The crystallite constituting the Li-containing oxide particle may have a first surface exposed on the surface of the oxide, and the first surface may be planar.

[0011] 1.1.2 Chemical composition The Li-containing oxide particles according to the first embodiment contain, as constituent elements, at least one element selected from Mn, Ni, and Co, Li, and O. The Li-containing oxide particles are particularly likely to exhibit higher performance when they contain, as constituent elements, at least Li, Mn, one or both of Ni and Co, and O, particularly when they contain, as constituent elements, at least Li, Mn, Ni, Co, and O. Alternatively, the Li-containing oxide particles are also likely to exhibit higher performance when they contain, as constituent elements, at least Li, Mn, Fe, and O. Furthermore, the Li-containing oxide particles may contain, as a constituent element, 0.33 mol or more of Li per 1 mol of O. The upper limit of the amount of Li relative to O is not particularly limited. The Li-containing oxide particles may contain, as a constituent element, 0.33 mol or more and 0.50 mol or less, more than 0.33 mol and 0.50 mol or less, or 0.35 mol or more and 0.45 mol or less of Li per 1 mol of O. Thus, Li-containing oxide particles containing 0.33 mol or more of Li per 1 mol of O tend to have an excellent weight energy density as a positive electrode active material.

[0012] The Li-containing oxide particles according to the first embodiment contain Li a Na b Mn x-p Ni y-q Co z-r M p+q+rIt may have a chemical composition represented by O2 (where 0 < a ≤ 1.00, 0 ≤ b ≤ 0.20, x + y + z = 1, and 0 ≤ p + q + r < 0.17, and the element M is at least one selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W). When the Li-containing oxide particles have such a chemical composition, the O2-type structure is likely to be maintained. In the above chemical composition, a may be greater than 0, 0.10 or more, 0.20 or more, 0.30 or more, 0.40 or more, 0.50 or more, 0.60 or more, 0.62 or more, 0.64 or more, 0.66 or more, more than 0.66, 0.67 or more, 0.68 or more, or 0.70 or more, and may be 1.00 or less, or 0.90 or less. b may be 0 or more, 0.01 or more, 0.02 or more, or 0.03 or more, and may be 0.20 or less, 0.15 or less, or 0.10 or less. Also, x may be 0 or more, greater than 0, 0.10 or more, 0.20 or more, 0.30 or more, 0.40 or more, or 0.50 or more, and may be 1.00 or less, less than 1.00, 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, 0.50 or less, or less than 0.50. Also, y may be 0 or more, greater than 0, 0.10 or more, or 0.20 or more, and may be 1.00 or less, less than 1.00, 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. Also, z may be 0 or more, greater than 0, 0.10 or more, 0.20 or more, or 0.30 or more, and may be 1.00 or less, less than 1.00, 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. The element M has a small contribution to charge and discharge. In this regard, in the above chemical composition, when p + q + r is less than 0.17, it is easy to ensure a high charge and discharge capacity. p + q + r may be 0.16 or less, 0.15 or less, 0.14 or less, 0.13 or less, 0.12 or less, 0.11 or less, or 0.10 or less. On the other hand, when the element M is included, the P2-type structure is likely to be stabilized.In the above chemical composition, p + q + r is 0 or more, and may be 0.01 or more, 0.02 or more, 0.03 or more, 0.04 or more, 0.05 or more, 0.06 or more, 0.07 or more, 0.08 or more, 0.09 or more, or 0.10 or more. The composition of O is approximately 2, but is not necessarily exactly 2.0 and is indeterminate.

[0013] The Li-containing oxide particles according to the first form are Li a Na b Mn x-p Ni y-q Co z-r M p+q+r O2 (where 0 < a ≤ 1.00, 0 ≤ b ≤ 0.20, 0 < x < 1.00, 0 < y < 0.50, 0 < z < 1.00, x + y + z = 1, and 0 ≤ p + q + r < 0.17, and the element M is at least one selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W.) may have a chemical composition represented by. Further, the Li-containing oxide particles according to the first form are Li a Na b Mn x-p Ni y-q Co z-r M p+q+r O2 (where 0.66 < a ≤ 1.00, 0 ≤ b ≤ 0.20, 0.30 < x < 0.60, 0.10 < y < 0.40, 0.10 < z < 0.50, x + y + z = 1, and 0 ≤ p + q + r < 0.17, and the element M is at least one selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W.) may have a chemical composition represented by. Conventionally, it has been difficult to produce Li-containing oxide particles having an O2-type structure and such a chemical composition. However, in the present embodiment, as the production conditions of Li-containing oxide particles having an O2-type structure, by adopting specific conditions described later, Li-containing oxide particles having an O2-type structure and such a chemical composition can be obtained. Li-containing oxide particles having such a chemical composition tend to have excellent weight energy density as a positive electrode active material.

[0014] 1.1.3 Shape The Li-containing oxide particles according to the first embodiment may be solid particles, hollow particles, or particles having voids. The Li-containing oxide particles have the following average aspect ratio and average particle size, and thus have an excellent weight energy density as a positive electrode active material.

[0015] 1.1.3.1 Aspect Ratio A positive electrode active material having an O2-type structure can be obtained by ion-exchanging at least a portion of the Na in a Na-containing oxide having a P2-type structure with Li. Here, the P2-type structure is a hexagonal crystal system with a large diffusion coefficient of Na ions, which facilitates crystal growth in a specific direction. In particular, when at least one of Mn, Ni, and Co is included as a transition metal element constituting the P2-type structure, plate-like crystal growth in a specific direction is facilitated. Therefore, Na-containing oxide particles having a P2-type structure are usually plate-like particles with a large aspect ratio, with the crystal growth direction biased in a specific direction. Li-containing oxide particles having an O2-type structure obtained by ion-exchanging these particles also have a large aspect ratio. Specifically, Li-containing oxide particles having an O2-type structure are usually plate-like particles with an average aspect ratio significantly greater than 3.0. Here, the edges of the plate-like particles serve as intercalation entrances and exits. As far as the inventors have confirmed, plate-like particles with a large aspect ratio tend to have a smaller proportion of the portion of the particle that contributes to intercalation, and are therefore likely to have a lower weight energy density.

[0016] In contrast, the Li-containing oxide particles according to the first embodiment have a reduced bias in the crystal growth direction and an aspect ratio of a certain level or less. Specifically, the Li-containing oxide particles according to the first embodiment have an average aspect ratio of 1.0 to 3.0. When the Li-containing oxide particles having an O2-type structure have an average aspect ratio of 3.0 or less, the weight energy density and other properties of the positive electrode active material are likely to be increased. The average aspect ratio of the Li-containing oxide particles according to the first embodiment may be 1.0 to 2.9, 1.0 to 2.8, 1.0 to 2.7, 1.0 to 2.6, 1.0 to 2.5, or 1.0 to 2.4.

[0017] The "average aspect ratio" of the Li-containing oxide particles is measured as follows. Specifically, a cross section of the Li-containing oxide particles (a cross section of the positive electrode active material layer when the Li-containing oxide particles are contained in the positive electrode active material layer described later) is observed using a scanning electron microscope (SEM) or a transmission electron microscope (TEM) to identify the shape of the Li-containing oxide particles. The maximum Feret diameter of the shape is identified and regarded as the "major axis." Furthermore, the largest diameter of the shape perpendicular to the "major axis" is regarded as the "minor axis." The ratio of the "major axis" to the "minor axis" (major axis / minor axis) is regarded as the "aspect ratio" of the Li-containing oxide particles. The "aspect ratio" is determined for each Li-containing oxide particle, and the number average value is regarded as the "average aspect ratio."

[0018] 1.1.3.2 Average particle size As described above, in the prior art, Li-containing oxide particles having an O2-type structure are plate-like particles with a large aspect ratio, with the crystal growth direction biased in a specific direction. In the prior art, attempts to suppress the growth of the P2 phase result in an extremely small average particle size of the Na-containing oxide particles, which in turn results in an extremely small average particle size of the Li-containing oxide having an O2-type structure. This leads to concerns about excessive particle aggregation and the inability to obtain a sufficient amount of P2 phase or O2 phase. As a result, it is difficult for the Li-containing oxide particles according to the prior art to achieve a sufficient weight energy density. In contrast, the Li-containing oxide particles according to the first embodiment can solve these problems by having a certain size or larger while still having the above-described average aspect ratio. Specifically, the Li-containing oxide particles according to the first embodiment have an average particle size of 2.0 μm or larger. The average particle size of the Li-containing oxide particles may be 2.0 μm or more and 5.0 μm or less, 2.0 μm or more and 4.0 μm or less, or 2.0 μm or more and 3.0 μm or less.

[0019] The "average particle size" of the Li-containing oxide particles is the particle size at 50% cumulative value in the volume-based particle size distribution determined by a laser diffraction / scattering method (D50, median size).

[0020] 1.1.4 Other As described above, the positive electrode active material according to the first embodiment has excellent weight energy density by including Li-containing oxide particles having the above specific average particle diameter and average aspect ratio. The positive electrode active material according to the first embodiment may consist only of the above Li-containing oxide particles, or may include, together with the above Li-containing oxide particles, other positive electrode active materials (other positive electrode active materials). From the viewpoint of further enhancing the above effects, the proportion of other positive electrode active materials in the entire positive electrode active material may be small. For example, assuming that the total amount of the positive electrode active material is 100% by mass, the content of the above Li-containing oxide particles may be 50% by mass or more and 100% by mass or less, 60% by mass or more and 100% by mass or less, 70% by mass or more and 100% by mass or less, 80% by mass or more and 100% by mass or less, 90% by mass or more and 100% by mass or less, 95% by mass or more and 100% by mass or less, or 99% by mass or more and 100% by mass or less.

[0021] 1.2 Second Embodiment According to the findings of the present inventors, when the Li-containing oxide has a specific chemical composition, the average discharge potential tends to be high. From this viewpoint, the positive electrode active material of the present disclosure can also be specified as follows. That is, the positive electrode active material according to the second embodiment includes Li-containing oxide particles. The Li-containing oxide particles contain Li a Na b Mn x-p Ni y-q Co z-r M p+q+r O2 (where 0.66 < a ≤ 1.00, 0 ≤ b ≤ 0.20, x + y + z = 1, and 0 ≤ p + q + r < 0.17, and the element M is at least one selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W).) has a chemical composition represented by

[0022] 1.2.1 Crystal Structure The Li-containing oxide particles according to the second embodiment may have at least an O2-type structure (belonging to the space group P63mc) as a crystal structure, similar to the Li-containing oxide particles according to the first embodiment. Alternatively, the Li-containing oxide particles according to the second embodiment may have one or both of an O6-type structure and a T#2-type structure as a crystal structure. The Li-containing oxide particles according to one embodiment may have an O2-type structure as a main phase, or may have a crystal structure other than the O2-type structure (an O6 structure or a T#2-type structure) as a main phase. The crystal structure of the Li-containing oxide particles as a main phase may change depending on the charge / discharge state.

[0023] The Li-containing oxide particles according to the second embodiment, like the Li-containing oxide particles according to the first embodiment, may be single crystals consisting of one crystallite or polycrystalline having multiple crystallites. As described above, the end faces of the crystallites of the Li-containing oxide particles are considered to be the entrance and exit points for intercalation. That is, when the crystallites of the Li-containing oxide particles are small, the effect of increasing the number of entrances and exits for intercalation and reducing the reaction resistance, the effect of shortening the migration distance of lithium ions and reducing the diffusion resistance, and the effect of reducing the absolute amount of expansion and contraction during charge and discharge and making cracks less likely to occur can be expected. For example, the diameter of the crystallites constituting the Li-containing oxide particles may be 0.1 μm to 5.0 μm, 0.5 μm to 4.0 μm, or 1.0 μm to 3.0 μm. The crystallites constituting the Li-containing oxide particles may have a first surface exposed on the surface of the oxide, and the first surface may be planar.

[0024] 1.2.2 Chemical composition The Li-containing oxide particles according to the second embodiment contain Li a Na b Mn x-p Ni y-q Co z-r M p+q+rO2 (where 0.66 < a ≤ 1.00, 0 ≤ b ≤ 0.20, x + y + z = 1, and 0 ≤ p + q + r < 0.17, and the element M is at least one selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W). The Li-containing oxide particles according to the second form have a chemical composition represented by Li a Na b Mn x-p Ni y-q Co z-r M p+q+rIt may have a chemical composition represented by O2 (where 0.66 < a ≤ 1.00, 0 ≤ b ≤ 0.20, 0.30 < x < 0.60, 0.10 < y < 0.40, 0.10 < z < 0.50, x + y + z = 1, and 0 ≤ p + q + r < 0.17, and the element M is at least one selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W). Conventionally, for example, it has been difficult to produce Li-containing oxide particles having an O2-type structure and such a chemical composition. However, in this embodiment, by adopting specific conditions as the manufacturing conditions of the Li-containing oxide particles, Li-containing oxide particles having such a chemical composition can be obtained. Li-containing oxide particles having such a chemical composition have a high average discharge potential as a positive electrode active material. a may be more than 0.66, may be 0.67 or more, 0.68 or more, 0.69 or more, 0.70 or more, or more than 0.70, and may be 1.00 or less, 0.90 or less, or less than 0.90. b may be 0 or more, 0.01 or more, 0.02 or more, or 0.03 or more, and may be 0.20 or less, 0.15 or less, or 0.10 or less. Also, x may be more than 0, 0.10 or more, 0.20 or more, 0.30 or more, more than 0.30, 0.40 or more, or 0.50 or more, and may be less than 1.00, 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, less than 0.60, 0.50 or less, or less than 0.50. Also, y may be more than 0, 0.10 or more, more than 0.10, or 0.20 or more, and may be less than 0.50, 0.45 or less, or 0.40 or less. Also, z may be more than 0, 0.10 or more, more than 0.10, 0.20 or more, or 0.30 or more, and may be less than 1.00, 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, 0.50 or less, less than 0.50, 0.40 or less, or 0.30 or less. p + q + r is 0 or more, may be 0.01 or more, 0.02 or more, 0.03 or more, 0.04 or more, 0.05 or more, 0.06 or more, 0.07 or more, 0.08 or more, 0.09 or more, or 0.10 or more, and may be 0.17 or less, 0.16 or less, 0.15 or less, 0.14 or less, 0.13 or less, 0.12 or less, 0.11 or less, or 0.10 or less.The composition of O is approximately 2, but is not necessarily exactly 2.0 and is variable.

[0025] 1.2.3 Shape The Li-containing oxide particles according to the second embodiment may be solid particles, hollow particles, or particles having voids. The Li-containing oxide particles according to the second embodiment may have the following average aspect ratio and average particle size.

[0026] 1.2.3.1 Aspect Ratio The Li-containing oxide particles according to the second embodiment may have a crystal growth direction with reduced bias and an aspect ratio of a certain level or less. Specifically, the Li-containing oxide particles according to the second embodiment may have an average aspect ratio of 1.0 to 3.0. When the average aspect ratio of the Li-containing oxide particles is 3.0 or less, the weight energy density as a positive electrode active material is likely to be increased, as described above. The average aspect ratio of the Li-containing oxide particles according to the second embodiment may be 1.0 to 2.9, 1.0 to 2.8, 1.0 to 2.7, 1.0 to 2.6, 1.0 to 2.5, or 1.0 to 2.4. The method for measuring the "average aspect ratio" is as described above.

[0027] 1.2.3.2 Average particle size The Li-containing oxide particles according to the second embodiment may have an average particle diameter of 2.0 μm or more. The average particle diameter of the Li-containing oxide particles according to the second embodiment may be 2.0 μm or more and 5.0 μm or less, 2.0 μm or more and 4.0 μm or less, or 2.0 μm or more and 3.0 μm or less. The method for measuring the "average particle diameter" of the Li-containing oxide particles is as described above.

[0028] 1.2.4 Other As described above, the positive electrode active material according to the second embodiment has a high average discharge voltage due to the inclusion of Li-containing oxide particles having the specific chemical composition. The positive electrode active material according to the second embodiment may consist solely of the Li-containing oxide particles, or may contain the Li-containing oxide particles together with other positive electrode active materials (other positive electrode active materials). From the viewpoint of further enhancing the above effect, the proportion of the other positive electrode active materials in the overall positive electrode active material may be small. For example, the content of the Li-containing oxide particles may be 50% by mass to 100% by mass, 60% by mass to 100% by mass, 70% by mass to 100% by mass, 80% by mass to 100% by mass, 90% by mass to 100% by mass, 95% by mass to 100% by mass, or 99% by mass to 100% by mass, where the overall positive electrode active material is taken as 100% by mass.

[0029] 2. Manufacturing method of positive electrode active material The Li-containing oxide particles according to the first and second embodiments can be produced, for example, by the following method. As shown in FIG. 1, a method for producing Li-containing oxide particles having an O2-type structure according to one embodiment includes the following steps: S1: Obtaining a precursor containing at least one element of Mn, Ni, and Co; S2: Coating the surface of the precursor with a Na source to obtain a composite; S3: Calcining the composite to obtain a Na-containing oxide having a P2 type structure; and S4: ion-exchanging at least a portion of the Na in the Na-containing oxide with Li to obtain a Li-containing oxide having an O2-type structure. S3-1: Pre-firing the composite at a temperature of 300°C or higher and lower than 700°C for 2 hours or higher and 10 hours or lower; S3-2: Following the preliminary firing, the composite is subjected to main firing at a temperature of 700°C or higher and 1100°C or lower for 30 minutes or higher and 48 hours or lower; and S3-3: Following the main sintering, the composite is allowed to cool in the atmosphere from a temperature T1 of 200°C or higher to a temperature T2 of 100°C or lower.

[0030] 2.1 S1 In step S1, a precursor containing at least one element selected from Mn, Ni, and Co is obtained. The precursor may contain at least Mn and one or both of Ni and Co, or may contain at least Mn, Ni, and Co. The precursor may be a salt containing at least one element selected from Mn, Ni, and Co. For example, the precursor may be at least one of carbonate, sulfate, nitrate, and acetate. Alternatively, the precursor may be a compound other than a salt. For example, the precursor may be a hydroxide. The precursor may be a hydrate. The precursor may be a combination of multiple compounds. The precursor may have various shapes. For example, the precursor may be particulate, or may be spherical particles as described below. The particle size of the precursor particles is not particularly limited. The composition of the precursor may be appropriately determined so as to correspond to the composition of the Li-containing oxide, which is the final product.

[0031] In S1, the precursor precipitate may be obtained by coprecipitation using an ion source capable of forming a precipitate in aqueous solution with transition metal ions and a transition metal compound containing at least one element selected from Mn, Ni, and Co. This facilitates the production of spherical particles as the precursor. The "ion source capable of forming a precipitate in aqueous solution with transition metal ions" may be, for example, at least one selected from sodium salts such as sodium carbonate and sodium nitrate, sodium hydroxide, and sodium oxide. The transition metal compound may be the above salt or hydroxide containing at least one element selected from Mn, Ni, and Co. Specifically, in S1, the ion source and the transition metal compound may be prepared as separate solutions, and the respective solutions may be added dropwise and mixed to obtain the precursor precipitate. In this case, water, for example, is used as the solvent. Various sodium compounds may be used as bases, and aqueous ammonia or the like may be added to adjust the basicity. In the case of the coprecipitation method, for example, an aqueous solution of the transition metal compound and an aqueous solution of sodium carbonate are prepared, and the respective aqueous solutions are added dropwise and mixed to obtain the precursor precipitate. Alternatively, the precursor can be obtained by a sol-gel method, and in particular, by a coprecipitation method, spherical particles can be easily obtained as the precursor.

[0032] In step S1, the precursor may contain element M. The element M is at least one element selected from B, Mg, Al, K, Ca, Ti, V, Cr, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W. The element M, for example, functions to stabilize the P2 structure. The method for obtaining a precursor containing element M is not particularly limited. When obtaining a precursor by coprecipitation in step S1, for example, an aqueous solution of a transition metal compound containing at least one of Mn, Ni, and Co, an aqueous solution of sodium carbonate, and an aqueous solution of a compound of element M are prepared, and then these aqueous solutions are added dropwise and mixed to obtain a precursor containing element M together with at least one of Mn, Ni, and Co. Alternatively, in the manufacturing method of the present disclosure, element M may not be added in step S1, and element M may be doped during the Na-doping calcination in steps S2 and S3 described below. Alternatively, element M may be doped during ion exchange in step S4 described below.

[0033] 2.2 S2 In S2, the surface of the precursor obtained in S1 is coated with a Na source to obtain a composite. The Na source may be a salt containing Na, such as a carbonate or a nitrate, or a compound other than a salt, such as sodium oxide or sodium hydroxide. In S2, the amount of Na source coated on the surface of the precursor may be determined taking into account the amount of Na lost during the subsequent calcination.

[0034] In S2, the coverage of the Na source on the surface of the precursor is not particularly limited. For example, in S2, the composite may be obtained by covering 40 area% or more, 50 area% or more, 60 area% or more, or 70 area% or more of the surface of the precursor with the Na source. Alternatively, in S2, the composite may be obtained by covering less than 40 area%, 35 area% or less, or 30 area% or less of the surface of the precursor with the Na source. If the coverage of the Na source is low, P2 type crystals are likely to grow on the surface of the composite when the composite is fired. On the other hand, if the coverage of the Na source is high, the crystallites of the P2 type crystals are likely to become small when the composite is fired, and the growth of the P2 type crystals is likely to be suppressed.

[0035] In S2, the method for coating the surface of the precursor with the Na source is not particularly limited. For example, the precursor and the Na source can be mixed dry or wet to coat the surface of the precursor with the Na source. Alternatively, the Na source may be coated on the surface of the precursor by a tumbling fluidized coating method or a spray drying method. That is, a coating solution in which the Na source is dissolved is prepared, and the coating solution is brought into contact with the surface of the precursor, and then dried at the same time as or after the contact. The coverage rate of the Na source on the surface of the precursor can be controlled by adjusting the coating conditions (temperature, time, number of times, etc.).

[0036] In S2, the precursor may be coated with an M source together with a Na source. For example, in S2, the precursor obtained in S1 may be mixed with a Na source and an M source containing at least one element M selected from B, Mg, Al, K, Ca, Ti, V, Cr, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W to obtain a composite. The M source may be, for example, a salt containing the element M, such as a carbonate or a sulfate, or a compound other than a salt, such as an oxide or a hydroxide. The amount of the M source relative to the precursor may be determined depending on the chemical composition of the Na-containing oxide after firing.

[0037] 2.3 S3 In S3, the composite obtained in S2 is calcined to obtain a Na-containing oxide having a P2 type structure. S3 includes the above S3-1, S3-2, and S3-3.

[0038] 2.3.1 S3-1 In S3-1, the composite is pre-fired at a temperature of 300°C or higher but lower than 700°C for 2 hours to 10 hours. In S3-1, the composite may be optionally molded and then pre-fired. The pre-fired temperature is lower than that of the main firing. By performing the pre-fired temperature sufficiently, the P2 phase can be appropriately generated in the main firing, while suppressing the generation of crystalline phases other than the P2 phase. That is, in S3-1, the pre-fired temperature is 300°C or higher but lower than 700°C, and the pre-fired time is 2 hours to 10 hours, so that the composite can be sufficiently pre-fired. This allows the P2 phase to be efficiently and appropriately generated in the Na-containing oxide particles obtained through S3-2 and S3-3 described below, while suppressing the generation of phases other than the P2 phase. As a result, the Li-containing oxide particles obtained through S4 have an O2-type structure and a desired average particle size and average aspect ratio. The pre-firing temperature may be 400°C or higher and lower than 700°C, 450°C or higher and lower than 700°C, 500°C or higher and lower than 700°C, 550°C or higher and lower than 700°C, or 550°C or higher and lower than 650°C. The pre-firing time may be 2 hours or higher and lower than 8 hours, 3 hours or higher and lower than 8 hours, 4 hours or higher and lower than 8 hours, 5 hours or higher and lower than 8 hours, or 5 hours or higher and lower than 7 hours. The pre-firing atmosphere is not particularly limited, and may be, for example, an oxygen-containing atmosphere.

[0039] 2.3.2 S3-2 In S3-2, following the pre-firing, the composite is subjected to main firing at a temperature of 700°C to 1100°C for 30 minutes to 48 hours. In S3-2, the main firing temperature of the composite is 700°C to 1100°C, preferably 800°C to 1000°C. If the main firing temperature is too low, the P2 phase will not form, while if the main firing temperature is too high, an O3 phase or the like will likely form instead of the P2 phase. The temperature rise conditions from the pre-firing temperature to the main firing temperature are not particularly limited. As described above, the main firing time is 30 minutes to 48 hours. However, the shape of the Na-containing oxide can be controlled by the main firing time. As described above, in the method of the present disclosure, if the coverage of the Na source in the composite is 40 area % or more, small P2-type crystals are likely to form on the surface of the composite when the composite is fired. In the method of the present disclosure, abnormal growth of P2 type crystals is suppressed by growing the P2 type crystals along the surface of the composite. As a result, the shape of the Na-containing oxide particles has a predetermined average aspect ratio and average particle size. If the sintering time is too short, the P2 phase is not sufficiently generated. On the other hand, if the sintering time is too long, the P2 type crystals grow excessively, and the predetermined average aspect ratio and average particle size cannot be achieved. As far as the inventors have confirmed, when the sintering time is 30 minutes or more and 3 hours or less, the Na-containing oxide particles are likely to have the predetermined average aspect ratio and average particle size.

[0040] 2.3.3 S3-3 In step S3-3, following the main firing, the composite is rapidly cooled (cooled at a cooling rate of 20°C / min or more) from a temperature T1 of 200°C or higher to a temperature T2 of 100°C or lower. The preliminary firing and main firing are performed, for example, in a heating furnace. In step S3-3, for example, the composite is main fired in a heating furnace, then cooled to a temperature T1 of 200°C or higher in the heating furnace. After reaching temperature T1, the fired product is removed from the heating furnace and rapidly cooled outside the furnace to a temperature T2 of 100°C or lower. Temperature T1 may be any temperature of 200°C or higher, or any temperature of 250°C or higher. Temperature T2 may be any temperature of 100°C or lower, or any temperature of 50°C or lower, or may be the cooling end temperature. In the predetermined temperature range between temperature T1 and temperature T2, moisture is likely to penetrate between the layers of the P2-type structure due to atomic vibration, molecular motion, etc. When cooling the sintered composite (Na-containing oxide having a P2-type structure), it is believed that the amount of moisture that penetrates between the layers of the P2-type structure can be reduced by shortening the time spent in this temperature range where moisture easily penetrates (i.e., by rapid cooling). In this regard, in step S3-3, when cooling the sintered composite, allowing it to cool in a dry atmosphere outside the furnace from an arbitrary temperature T1 of 200°C or higher to an arbitrary temperature T2 of 100°C or lower results in a high cooling rate (e.g., 20°C / min or higher) from temperature T1 to temperature T2, which makes it difficult for moisture to penetrate between the layers of the P2-type structure and prevents the collapse of the P2-type structure. As a result, Na-containing oxide particles having a P2-type structure and a predetermined chemical composition can be obtained.

[0041] By going through the above steps S1 to S3, desired Na-containing oxide particles can be obtained. The Na-containing oxide particles can be obtained, for example, by (1) having a P2 type structure, (2) Containing at least one element selected from Mn, Ni, and Co, Na, and O as constituent elements, (3) having an average particle size of 2.0 μm or more, and (4) Having an average aspect ratio of 1.0 or more and 3.0 or less.

[0042] Furthermore, according to the findings of the present inventors, the c-axis length of the P2 type structure of the Na-containing oxide particles obtained through the specific process described above is likely to be smaller than that of conventional particles. For example, the P2 type structure in the Na-containing oxide particles may have a c-axis length of 11.10 Å or less. The P2 type structure may have a c-axis length of 11.05 Å or more and 11.10 Å or less. The lattice constants (a-axis length, b-axis length, and c-axis length) of the P2 type structure can be determined from the X-ray diffraction pattern of the Na-containing oxide particles using the least squares method. Here, the "X-ray diffraction pattern of the Na-containing oxide particles" refers to one obtained under the following conditions. That is, an X-ray diffraction pattern is obtained for the Na-containing oxide particles using an X-ray diffractometer (Rigaku, fully automatic multipurpose X-ray diffractometer SmartLab) with a CuKα radiation source, a tube voltage of 45 kV, a tube current of 200 mA, a step width of 0.02°, and a scan speed of 1° / min, performing 2θ / θ scanning.

[0043] The Na-containing oxide particles obtained through the specific process described above may be single crystals consisting of one crystallite, or polycrystals having multiple crystallites. The diameter of the crystallite constituting the Na-containing oxide particles may be, for example, 0.1 μm to 5.0 μm, 0.5 μm to 4.0 μm, or 1.0 μm to 3.0 μm. The terms "crystallite" and "diameter of crystallite" are as described above. The crystallite constituting the Na-containing oxide particles may have a first surface exposed on the surface of the oxide, and the first surface may be planar.

[0044] The Na-containing oxide particles contain, as constituent elements, at least one element selected from Mn, Ni, and Co, as well as Na and O. The Na-containing oxide particles are particularly likely to exhibit higher performance when they contain, as constituent elements, at least Na, Mn, one or both of Ni and Co, and O, particularly when they contain, as constituent elements, at least Na, Mn, Ni, Co, and O. Alternatively, the Na-containing oxide particles are also likely to exhibit higher performance when they contain, as constituent elements, at least Na, Mn, Fe, and O. Furthermore, the Na-containing oxide particles obtained through the specific process described above may contain, as a constituent element, more than 0.35 mol of Na per mol of O. The upper limit of the amount of Na relative to O is not particularly limited. The Na-containing oxide particles may contain, as a constituent element, more than 0.35 mol and not more than 0.50 mol, or 0.38 mol to 0.45 mol of Na per mol of O. In this way, by performing S4 described later using Na-containing oxide particles containing more than 0.35 mol of Na per 1 mol of O, Li-containing oxide particles containing a large amount of Li can be obtained, and the Li-containing oxide particles tend to have an excellent weight energy density as a positive electrode active material.

[0045] The Na-containing oxide particles are c Mn x-p Ni y-q Co z-r M p+q+rIt may have a chemical composition represented by O2 (where 0 < c ≤ 1.00, x + y + z = 1, and 0 ≤ p + q + r < 0.17, and the element M is at least one selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W). When the Na-containing oxide particles have such a chemical composition, the P2-type structure is likely to be maintained. In the above chemical composition, c is greater than 0, and may be 0.10 or more, 0.20 or more, 0.30 or more, 0.40 or more, 0.50 or more, 0.60 or more, or 0.70 or more, and is 1.00 or less, and may be 0.90 or less. Also, x is 0 or more, and may be greater than 0, 0.10 or more, 0.20 or more, 0.30 or more, 0.40 or more, or 0.50 or more, and is 1.00 or less, and may be less than 1.00, 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, 0.50 or less, or less than 0.50. Also, y is 0 or more, and may be greater than 0, 0.10 or more, or 0.20 or more, and is 1.00 or less, and may be less than 1.00, 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. Also, z is 0 or more, and may be greater than 0, 0.10 or more, 0.20 or more, or 0.30 or more, and is 1.00 or less, and may be less than 1.00, 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. The element M has a small contribution to charge and discharge. In this regard, in the above chemical composition, when p + q + r is less than 0.17, it is easy to ensure a high charge and discharge capacity. p + q + r may be 0.16 or less, 0.15 or less, 0.14 or less, 0.13 or less, 0.12 or less, 0.11 or less, or 0.10 or less. On the other hand, when the element M is included, the P2-type structure is likely to be stabilized. In the above chemical composition, p + q + r is 0 or more, and may be 0.01 or more, 0.02 or more, 0.03 or more, 0.04 or more, 0.05 or more, 0.06 or more, 0.07 or more, 0.08 or more, 0.09 or more, or 0.10 or more. The composition of O is approximately 2, but is not necessarily exactly 2.0 and is indefinite.

[0046] The Na-containing oxide particles are Na c Mn x-p Ni y-q Co z-r M p+q+r O2 (where 0 < c ≤ 1.00, 0 < x < 1.00, 0 < y < 0.50, 0 < z < 1.00, x + y + z = 1, and 0 ≤ p + q + r < 0.17, and the element M is at least one selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W.) may have a chemical composition represented by this formula. Also, the Na-containing oxide particles are Na c Mn x-p Ni y-q Co z-r M p+q+r O2 (where 0.70 < c ≤ 1.00, 0 < x < 1.00, 0 < y < 0.50, 0 < z < 1.00, x + y + z = 1, and 0 ≤ p + q + r < 0.17, and the element M is at least one selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W.) may have a chemical composition represented by this formula. Also, the Na-containing oxide particles are Na c Mn x-p Ni y-q Co z-r M p+q+rIt may have a chemical composition represented by O2 (where 0.70 < c ≤ 1.00, 0.30 < x < 0.60, 0.10 < y < 0.40, 0.10 < z < 0.50, x + y + z = 1, and 0 ≤ p + q + r < 0.17, and the element M is at least one selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W). Conventionally, it has been difficult to produce Na-containing oxide particles having a P2-type structure and such a chemical composition. However, in the present embodiment, by adopting the above specific conditions as the production conditions of the Na-containing oxide particles having a P2-type structure, it is possible to obtain Na-containing oxide particles having a P2-type structure and such a chemical composition. When Li-containing oxide particles having an O2-type structure are produced using Na-containing oxide particles having such a chemical composition, the Li-containing oxide particles have excellent weight energy density and the like as a positive electrode active material.

[0047] The Na-containing oxide particles may be solid particles, hollow particles, or particles having voids. The Na-containing oxide particles according to the first embodiment have an excellent weight energy density as a positive electrode active material by having the following average aspect ratio and average particle diameter.

[0048] The Na-containing oxide particles have a suppressed bias in the crystal growth direction and have an aspect ratio of a certain value or less. Specifically, the Na-containing oxide particles may have an average aspect ratio of 1.0 or more and 3.0 or less. The average aspect ratio of the Na-containing oxide particles may be 1.0 or more and 2.9 or less, 1.0 or more and 2.8 or less, 1.0 or more and 2.7 or less, 1.0 or more and 2.6 or less, 1.0 or more and 2.5 or less, or 1.0 or more and 2.4 or less. Further, the Na-containing oxide particles may have a size of a certain value or more while having such an average aspect ratio. Specifically, the Na-containing oxide particles may have an average particle diameter of 2.0 μm or more. The average particle diameter of the Na-containing oxide particles may be 2.0 μm or more and 5.0 μm or less, 2.0 μm or more and 4.0 μm or less, or 2.0 μm or more and 3.0 μm or less.

[0049] 2.4 S4 In S4, at least a portion of the Na in the Na-containing oxide particles obtained in S3 is ion-exchanged with Li to obtain Li-containing oxide particles having an O2-type structure. The ion exchange in S4 can be performed, for example, using an aqueous solution containing lithium halide or a mixture of lithium halide and other lithium salts (e.g., molten salt). From the viewpoints of the P2-type structure being easily broken by water penetration and of crystallinity, the method using molten salt is preferred. That is, by mixing the Na-containing oxide particles having the 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-containing oxide particles can be ion-exchanged with Li.

[0050] In S4, 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.

[0051] In S4, the temperature for ion exchange may be, for example, equal to or higher than the melting point of the molten salt and equal to or lower than 600°C, 500°C, 400°C, or 300°C. If the temperature for ion exchange is too high, the stable O3 structure is likely to be formed rather than 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.

[0052] By the above-described method, the Li-containing oxide particles according to the first embodiment and the Li-containing oxide particles according to the second embodiment can be produced.

[0053] 3. Lithium-ion secondary batteries The positive electrode active material according to the embodiment includes the specific Li-containing oxide described above. The positive electrode active material according to the embodiment can be used, for example, as a positive electrode active material for a lithium-ion secondary battery. FIG. 2 schematically shows the configuration of a lithium-ion secondary battery according to an embodiment. As shown in FIG. 2, a lithium-ion secondary battery 100 according to an embodiment includes a positive electrode active material layer 10, an electrolyte layer 20, and a negative electrode active material layer 30. Here, the positive electrode active material layer 10 includes the positive electrode active material according to the embodiment described above.

[0054] 3.1 Cathode active material layer The positive electrode active material layer 10 includes at least the positive electrode active material according to the above embodiment and may further include, optionally, an electrolyte, a conductive additive, a binder, and the like. Furthermore, the positive electrode active material layer 10 may also include various other additives. The respective contents of the positive electrode active material, electrolyte, conductive additive, binder, and the like in the positive electrode active material layer 10 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, or 100% by mass or less, or 90% by mass or less, where the entire positive electrode active material layer 10 (total solid content) is taken as 100% by mass. The shape of the positive electrode active material layer 10 is not particularly limited, and may be, for example, a sheet-like positive electrode active material layer 10 having a substantially flat surface. The thickness of the positive electrode active material layer 10 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.

[0055] 3.1.1 Cathode active material Regarding the positive electrode active material, it is as described above. That is, the positive electrode active material contains the Li-containing oxide particles according to the above-described embodiment. As described above, the positive electrode active material may consist only of the above Li-containing oxide particles, or may contain, together with the above Li-containing oxide particles, other positive electrode active materials (other positive electrode active materials). From the viewpoint of further enhancing the effects of the technology of the present disclosure, the proportion of other positive electrode active materials in the entire positive electrode active material may be small. For example, assuming the total of the positive electrode active material is 100% by mass, the content of the above Li-containing oxide particles may be 50% by mass or more and 100% by mass or less, 60% by mass or more and 100% by mass or less, 70% by mass or more and 100% by mass or less, 80% by mass or more and 100% by mass or less, 90% by mass or more and 100% by mass or less, 95% by mass or more and 100% by mass or less, or 99% by mass or more and 100% by mass or less.

[0056] Any of the other positive electrode active materials known as positive electrode active materials for lithium ion secondary batteries can be adopted. The other positive electrode active materials may be, for example, at least one selected from various lithium compounds other than the above Li-containing oxides, elemental sulfur, sulfur compounds, and the like. The lithium compound as the other positive electrode active material may be a Li-containing oxide containing at least one element M, Li, and O. Element M may be, for example, at least one selected from Mn, Ni, Co, Al, Mg, Ca, Sc, V, Cr, Cu, Zn, Ga, Ge, Y, Zr, Sn, Sb, W, Pb, Bi, Fe, and Ti, or may be at least one selected from the group consisting of Mn, Ni, Co, Al, Fe, and Ti. More specifically, the Li-containing oxide as the other positive electrode active material is lithium cobalt oxide, lithium nickel oxide, lithium manganate, lithium nickel cobalt oxide, lithium nickel manganate, lithium cobalt manganate, lithium nickel cobalt manganate (Li 1±α Ni x Co y Mn z O 2±δ (for example, 0 < x < 1, 0 < y < 1, 0 < z < 1, x + y + z = 1)), spinel-type lithium compounds (Li 1+x Mn 2-x-y My Li-Mn spinel substituted with different elements having a composition represented by the formula O4 (M is one or more selected from Al, Mg, Co, Fe, Ni and Zn), lithium nickel cobalt aluminate (for example, Li 1±α Ni p Co q Al r O 2±δ The other positive electrode active material may be at least one selected from the group consisting of lithium phosphate (e.g., LiMPO4, where M is one or more selected from Fe, Mn, Co, and Ni), lithium titanate, and lithium metal phosphate (e.g., LiMPO4, where M is one or more selected from Fe, Mn, Co, and Ni). In particular, when the other positive electrode active material contains, as constituent elements, a Li-containing oxide containing at least one of Ni, Co, and Mn, Li, and O, the performance of the secondary battery is likely to be further improved. Alternatively, when the other positive electrode active material contains, as constituent elements, a Li-containing oxide containing at least one of Ni, Co, and Al, Li, and O, the performance of the secondary battery is likely to be further improved. Only one type of other positive electrode active material may be used alone, or two or more types may be used in combination. The shape of the other positive electrode active material may be any shape commonly used for positive electrode active materials in secondary batteries. The other positive electrode active material may be, for example, particulate. The other positive electrode active material may be solid or may have voids, for example, porous or hollow. The other positive electrode active material may be primary particles or secondary particles formed by aggregation of a plurality of primary particles. The average particle diameter D50 of the other 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.

[0057] 3.1.2 Protective layer An ion-conductive protective layer may be formed on the surface of the positive electrode active material. That is, the positive electrode active material layer 10 may include a composite of the positive electrode active material and the protective layer, and at least a portion of the surface of the positive electrode active material in the composite may be covered with the protective layer. This, for example, makes it easier to suppress reactions between the positive electrode active material and other battery materials (such as the sulfide solid electrolyte described below). The ion-conductive protective layer may include various ion-conductive compounds. The ion-conductive compound may be, for example, at least one selected from ion-conductive oxides and ion-conductive halides.

[0058] The ion-conductive oxide may contain, for example, at least one element selected from B, C, Al, Si, P, S, Ti, La, Zr, Nb, Mo, Zn, and W, Li, and O. The ion-conductive oxide may also be an oxynitride containing N. More specifically, the ion-conductive oxide may be Li3BO3, LiBO2, Li2CO3, LiAlO2, Li4SiO4, Li2SiO3, Li3PO4, Li2SO4, Li2TiO3, or Li4Ti5O. 12 , Li2Ti2O5, Li2ZrO3, LiNbO3, Li2MoO4, Li2WO4, LiPON, Li2O-LaO2, Li2O-ZnO2, etc. The ion-conductive oxide may be one in which some elements are substituted with various doping elements.

[0059] The ion-conductive halide may be, for example, at least one of the various compounds exemplified as halide solid electrolytes described below. The ion-conductive halide may contain, for example, at least one element selected from the group consisting of Mg, Ca, Sr, Ba, Zn, Sn, Al, Sc, Ga, Bi, Sb, Zr, Hf, Ti, Ta, Nb, W, Y, Gd, Tb, and Sm, at least one halogen element selected from the group consisting of Cl, Br, I, and F, and Li. The ion-conductive halide may contain at least one element selected from the group consisting of Ti, Al, Gd, Ca, Zr, and Y, at least one element selected from the group consisting of Cl, Br, I, and F, and Li. The ion-conductive halide may also contain at least one element selected from the group consisting of Ti and Al, at least one element selected from the group consisting of Cl, Br, I, and F, and Li. The ion-conductive halide may also be, for example, a complex halide of Li, Ti, Al, and F.

[0060] The coverage (area ratio) of the protective layer with respect to the surface of the positive electrode active material 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, and may be 100 nm or less or 20 nm or less.

[0061] 3.1.2 Electrolytes The positive electrode active material layer 10 may contain an electrolyte. The electrolyte that may be contained in the positive electrode active material layer 10 may be a solid electrolyte, a liquid electrolyte, or a combination thereof.

[0062] 3.1.2.1 Solid electrolyte As the solid electrolyte, any known solid electrolyte for lithium ion secondary batteries may be used. 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 oxide solid electrolytes, sulfide solid electrolytes, and ionically bonded inorganic solid electrolytes. Among inorganic solid electrolytes, sulfide solid electrolytes, and sulfide solid electrolytes containing at least Li, S, and P as constituent elements, exhibit high performance. Alternatively, among inorganic solid electrolytes, ionically bonded solid electrolytes, and solid electrolytes containing at least Li, Y, and a halogen (at least one of Cl, Br, I, and F) as constituent elements exhibit high performance. The solid electrolyte may be amorphous or crystalline. The solid electrolyte may be particulate. The average particle diameter (D50) of the solid electrolyte may be, for example, 10 nm or more and 10 μm or less. The ionic conductivity of the solid electrolyte at 25° C. may be, for example, 1×10 -5 S / cm or more, 1×10 -4 S / cm or more, or 1×10 -3 The solid electrolyte may be used singly or in combination of two or more kinds.

[0063] Oxide solid electrolytes include lithium lanthanum zirconate, LiPON, and Li 1+X Al X Ge 2-X It may be one or more selected from (PO4)3, Li-SiO-based glass, Li-Al-SO-based glass, etc. Furthermore, when an oxide solid electrolyte is combined with a liquid electrolyte, ionic conductivity can be improved.

[0064] The sulfide solid electrolyte may be a glass-based sulfide solid electrolyte (sulfide glass), a glass-ceramic-based sulfide solid electrolyte, or a crystalline sulfide solid electrolyte. The sulfide glass is amorphous. The sulfide glass may have a glass transition temperature (Tg). When the sulfide solid electrolyte has a crystalline phase, examples of the crystalline phase include a Thio-LISICON-type crystalline phase, an LGPS-type crystalline phase, and an Argyrodite-type crystalline phase. The sulfide solid electrolyte may be in a particulate form. The average particle diameter (D50) of the sulfide solid electrolyte may be, for example, 10 nm or more and 100 μm or less.

[0065] The sulfide solid electrolyte may contain, for example, Li, X (X is at least one of P, As, Sb, Si, Ge, Sn, B, Al, Ga, and In), and S. The sulfide solid electrolyte may further contain at least one of O and a halogen element. The sulfide solid electrolyte may also contain S as a main anion element.

[0066] Examples of sulfide solid electrolytes include Li2S-P2S5, Li2S-P2S5-LiI, Li2S-P2S5-GeS2, Li2S-P2S5-Li2O, Li2S-P2S5-Li2O-LiI, Li2S-P2S5-LiI-LiBr, Li2S-SiS2, Li2S-SiS2-LiI, Li2S-SiS2-LiBr, Li2S-SiS2-LiCl, Li2S-SiS2-B2S3-LiI, Li2S-SiS2-P2S5-LiI, Li2S-B2S3, Li2S-P2S5-ZmSn (where m and n are positive numbers, and Z is Ge, Zn, or Ga), Li2S-GeS2, Li2S-SiS2-Li3PO4, and Li2S-SiS2-Li x MO y (where x and y are positive numbers, and M is any one of P, Si, Ge, B, Al, Ga, and In).

[0067] The composition of the sulfide solid electrolyte is not particularly limited. For example, xLi2S·(100 - x)P2S5 (70 ≤ x ≤ 80), yLiI·zLiBr·(100 - y - z)(xLi2S·(1 - x)P2S5) (0.7 ≤ x ≤ 0.8, 0 ≤ y ≤ 30, 0 ≤ z ≤ 30), etc. may be mentioned. Alternatively, the sulfide solid electrolyte may have a composition represented by the general formula: Li 4-x Ge 1-x P x S4 (0 < x < 1). In the above general formula, at least a part of Ge may be substituted with at least one of Sb, Si, Sn, B, Al, Ga, In, Ti, Zr, V, and Nb. In the above general formula, at least a part of P may be substituted with at least one of Sb, Si, Sn, B, Al, Ga, In, Ti, Zr, V, and Nb. In the above general formula, a part of Li may be substituted with at least one of Na, K, Mg, Ca, and Zn. In the above general formula, a part of S may be substituted with a halogen (at least one of F, Cl, Br, and I). Alternatively, the sulfide solid electrolyte may be Li 7-a PS 6-a X a (X is at least one of Cl, Br, and I, and a is a number of 0 or more and 2 or less). a may be 0, or may be greater than 0. In the latter case, a may be 0.1 or more, may be 0.5 or more, or may be 1 or more. Also, a may be 1.8 or less, may be 1.5 or less.

[0068] The ionically bonded solid electrolyte may contain, for example, at least one element selected from the group consisting of Mg, Ca, Sr, Ba, Zn, Sn, Al, Sc, Ga, Bi, Sb, Zr, Hf, Ti, Ta, Nb, W, Y, Gd, Tb, and Sm. These elements can generate cations in water. The ionically bonded solid electrolyte material may further contain, for example, at least one halogen element selected from the group consisting of Cl, Br, I, and F. These elements can generate anions in water. The ionically bonded solid electrolyte may contain at least one element selected from the group consisting of Gd, Ca, Zr, and Y, at least one element selected from the group consisting of Cl, Br, I, and F, and Li. The ionically bonded solid electrolyte may also contain Li and Y, and at least one element selected from the group consisting of Cl, Br, I, and F. More specifically, the ionic solid electrolyte may contain Li, Y, Cl, and Br, or may contain Li, Ca, Y, Gd, Cl, and Br, or may contain Li, Zr, Y, and Cl. Even more specifically, the ionic solid electrolyte may be Li3YBr2Cl4, Li 2.8 Ca 0.1 Y 0.5 Gd 0.5 Br2Cl4 and Li 2.5 Y 0.5 Zr 0.5 It may be at least one of Cl6.

[0069] The ionically bonded solid electrolyte may be a halide solid electrolyte. The halide solid electrolyte has excellent ion conductivity. Examples of the halide solid electrolyte include a halide solid electrolyte represented by the formula (A): Li α M β X γ (A) It may have a composition represented by the following. Here, α, β, and γ are each independently a value greater than 0, M is at least one selected from the group consisting of metal elements other than Li and metalloid elements, and X is at least one selected from the group consisting of Cl, Br, and I. Note that the "metalloid element" may be at least one selected from the group consisting of B, Si, Ge, As, Sb, and Te. Also, the "metal element" may include (i) all elements contained in Groups 1 to 12 of the periodic table (excluding hydrogen) and (ii) all elements contained in Groups 13 to 16 of the periodic table (excluding B, Si, Ge, As, Sb, Te, C, N, P, O, S, and Se). The metal element can form an inorganic compound with a halide ion and become a cation.

[0070] In formula (A), M may contain Y (i.e., yttrium). The halide solid electrolyte containing Y is Li a Me b Y c It may have a composition represented by X6 (where a + mb + 3c = 6, c > 0, Me is at least one selected from the group consisting of metal elements and metalloid elements other than Li and Y, and m is the valence of Me). Me may be, for example, at least one selected from the group consisting of Mg, Ca, Sr, Ba, Zn, Sc, Al, Ga, Bi, Zr, Hf, Ti, Sn, Ta, and Nb.

[0071] The halide solid electrolyte may have a composition represented by formula (A1): Li 6-3d Y d X6. In formula (A1), X is one or more elements selected from the group consisting of Cl, Br, and I. d may satisfy 0 < d < 2, or d = 1. The halide solid electrolyte may have a composition represented by formula (A2): Li 3-3δ Y 1+δ Cl6. In formula (A2), 0 < δ ≦ 0.15 may hold. The halide solid electrolyte may have a composition represented by formula (A3): Li 3-3δ Y 1+δIt may have a composition represented by Br6. In formula (A3), 0 < δ ≤ 0.25 may be satisfied. The halide solid electrolyte has the formula (A4): Li 3-3δ+a Y 1+δ-a Me a Cl 6-x-y Br x I y and may have a composition represented by. In formula (A4), Me may be at least one selected from the group consisting of Mg, Ca, Sr, Ba, and Zn. In formula (A4), for example, -1 < δ < 2, 0 < a < 3, 0 < (3 - 3δ + a), 0 < (1 + δ - a), 0 ≤ x ≤ 6, 0 ≤ y ≤ 6, and (x + y) ≤ 6 are satisfied. The halide solid electrolyte has the formula (A5): Li 3-3δ Y 1+δ-a Me a Cl 6-x-y Br x I y and may have a composition represented by. In formula (A5), Me may be at least one selected from the group consisting of Al, Sc, Ga, and Bi. In formula (A5), -1 < δ < 1, 0 < a < 2, 0 < (1 + δ - a), 0 ≤ x ≤ 6, 0 ≤ y ≤ 6, and (x + y) ≤ 6 may be satisfied. The halide solid electrolyte has the formula (A6): Li 3-3δ-a Y 1+δ-a Me a Cl 6-x-y Br x I y and may have a composition represented by. In formula (A6), Me may be at least one selected from the group consisting of Zr, Hf, and Ti. In formula (A6), -1 < δ < 1, 0 < a < 1.5, 0 < (3 - 3δ - a), 0 < (1 + δ - a), 0 ≤ x ≤ 6, 0 ≤ y ≤ 6, and (x + y) ≤ 6 may be satisfied. The halide solid electrolyte has the formula (A7): Li 3-3δ-2a Y 1+δ-a Me a Cl 6-x-y Br x I yIn formula (A7), Me may be at least one selected from the group consisting of Ta and Nb. In formula (A7), -1<δ<1, 0 <a<1.2、0<(3-3δ-2a)、0<(1+δ-a)、0≦x≦6、0≦y≦6、かつ、(x+y)≦6であってもよい。

[0072] The ionic solid electrolyte may be a complex hydride solid electrolyte. The complex hydride solid electrolyte may be composed of Li ions and complex ions containing H. The complex ions containing H may have, for example, an element M containing at least one of a nonmetal element, a semimetal element, and a metal element, and H bonded to the element M. In addition, the complex ions containing H may have the element M as a central element and H surrounding the element M bonded to each other via a covalent bond. In addition, the complex ions containing H may be composed of (M m H n ) α- In this case, m is any positive number, and n and α can be any positive number depending on m and the valence of element M. Element M may be any nonmetallic element or metallic element capable of forming a complex ion. For example, element M may contain at least one of B, C, and N as a nonmetallic element, or may contain B. Furthermore, for example, element M may contain at least one of Al, Ni, and Fe as a metallic element. In particular, when the complex ion contains B or when it contains C and B, higher ionic conductivity is likely to be ensured. Specific examples of complex ions containing H include (CBH 10 ) - , (CB 11 H 12 ) - , (B 10 H 10 ) 2- , (B 12 H 12 ) 2- , (BH4) - , (NH2) - , (AlH4) - , and combinations thereof. In particular, (CB9H 10 ) - , (CB 11H 12 ) - In other words, the complex hydride solid electrolyte may contain Li, C, B, and H.

[0073] 3.1.2.2 Liquid electrolyte The liquid electrolyte (electrolytic solution) is a liquid containing lithium ions as carrier ions. The electrolytic solution may be an aqueous electrolytic solution or a non-aqueous electrolytic solution. The composition of the electrolytic solution may be the same as that of the known electrolytic solution for lithium ion secondary batteries. The electrolytic solution may be one in which a lithium salt is dissolved in water or a non-aqueous solvent. Examples of non-aqueous solvents include various carbonate-based solvents. Examples of lithium salts include lithium amide salts and LiPF6.

[0074] 3.1.3 Conductive additives Examples of conductive additives that can be contained in the positive electrode active material layer 10 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, titanium, 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.

[0075] 3.1.4 Binder Examples of binders that can be contained in the positive electrode active material layer 10 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.

[0076] 3.1.5 Other The positive electrode active material layer 10 may contain various additives in addition to the above components, such as a dispersant and a lubricant.

[0077] 3.2 Electrolyte layer The electrolyte layer 20 is disposed between the positive electrode active material layer 10 and the negative electrode active material layer 30. The electrolyte layer 20 contains at least an electrolyte. The electrolyte layer 20 may contain at least one of a solid electrolyte and an electrolytic solution, and may further contain a binder or the like. The contents of the electrolyte and binder or the like in the electrolyte layer 20 are not particularly limited. Alternatively, the electrolyte layer 20 may include a separator or the like that holds the electrolytic solution and prevents contact between the positive electrode active material layer 10 and the negative electrode active material layer 30. 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.

[0078] The electrolyte layer 20 may consist of one layer or multiple layers. For example, the electrolyte layer 20 may include a first layer disposed on the positive electrode active material layer 10 side and a second layer disposed on the negative electrode active material layer 30 side, and the first layer may contain the first electrolyte and the second layer may contain the second electrolyte. The first electrolyte and the second electrolyte may be different from each other. The first electrolyte and the second electrolyte may each be at least one selected from the oxide solid electrolyte, sulfide solid electrolyte, and ionic solid electrolyte described above. For example, the first layer may contain the ionic solid electrolyte, and the second layer may contain at least one of the ionic solid electrolyte and the sulfide solid electrolyte.

[0079] The electrolyte contained in the electrolyte layer 20 may be appropriately selected from the exemplified electrolytes (solid electrolytes and / or liquid electrolytes) that can be contained in the positive electrode active material layer 10 described above. The binder that can be contained in the electrolyte layer 20 may also be appropriately selected from the exemplified binders that can be contained in the positive electrode active material layer described above. 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.

[0080] 3.3 Negative electrode active material layer The negative electrode active material layer 30 contains at least a negative electrode active material. The negative electrode active material layer 30 may also optionally contain an electrolyte, a conductive additive, a binder, various additives, and the like. The content of each component in the negative electrode active material layer 30 may be appropriately determined depending on the desired battery performance. For example, the total solid content of the negative electrode active material layer 30 is taken as 100% by mass, and the content of the negative electrode active material may be 40% by mass or more, 50% by mass or more, 60% by mass or more, or 70% by mass or more, or 100% by mass or less, less than 100% by mass, 95% by mass or less, or 90% by mass or less. Alternatively, the total content of the negative electrode active material layer 30 may be 85% by volume or more, 90% by volume or more, or 95% by volume or more, and the remainder may be voids or other components. The shape of the negative electrode active material layer 30 is not particularly limited and may be, for example, a substantially flat sheet. The thickness of the negative electrode active material layer 30 is not particularly limited and may be, for example, 0.1 μm or more, 1 μm or more, 10 μm or more, or 30 μm or more, or 2 mm or less, 1 mm or less, 500 μm or less, or 100 μm or less.

[0081] The negative electrode active material may be any of those known as negative electrode active materials for lithium ion secondary batteries. Among known active materials, various materials may be used that have a lithium ion absorption / desorption potential (charge / discharge potential) lower than that of the positive electrode active material. Examples of such materials include 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, lithium alloys, and the like. In particular, when the negative electrode active material layer 30 contains Si as the negative electrode active material, the performance of the lithium ion secondary battery 100 is likely to be improved. The negative electrode active material may be used alone or in combination of two or more types. The shape of the negative electrode active material may be any shape commonly used for negative electrode active materials in secondary batteries. For example, the negative electrode active material may be particulate. The negative electrode active material particles may be primary particles or secondary particles formed by agglomeration 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 the form of a sheet (foil or film) such as lithium foil. That is, the negative electrode active material layer 30 may be made of a sheet of negative electrode active material.

[0082] Examples of electrolytes that can be contained in the negative electrode active material layer 30 include the above-mentioned solid electrolytes, electrolytic solutions, and combinations thereof. The conductive additives that can be contained in the negative electrode active material layer 30 may be appropriately selected from, for example, the conductive additives that can be contained in the above-mentioned positive electrode active material layer. The binders that can be contained in the negative electrode active material layer 30 may be appropriately selected from, for example, the binders that can be contained in the above-mentioned positive electrode active material layer. Each of the electrolytes, conductive additives, and binders may be used alone or in combination of two or more.

[0083] 3.4 Positive electrode current collector As shown in FIG. 2, the lithium-ion secondary battery 100 may include a positive electrode current collector 40 in contact with the positive electrode active material layer 10. The positive electrode current collector 40 may be any of those commonly used as positive electrode current collectors for secondary batteries. The positive electrode current collector 40 may have at least one shape selected from foil, plate, mesh, punched metal, and foam. The positive electrode current collector 40 may be made of metal foil or metal mesh. Metal foil is particularly advantageous in terms of ease of handling. The positive electrode current collector 40 may be made of multiple foils. Examples of metals constituting the positive electrode current collector 40 include at least one selected from Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, V, Mg, Pb, Ge, In, Sn, Zr, and stainless steel. In particular, the positive electrode current collector 40 may contain Al from the viewpoint of ensuring oxidation resistance. The positive electrode current collector 40 may have some kind of coating layer on its surface for the purpose of adjusting the resistance, etc. For example, the positive electrode current collector 40 may have a carbon coating layer. Alternatively, the positive electrode current collector 40 may be a metal foil or a substrate on which the above metal is plated or vapor-deposited. Furthermore, when the positive electrode current collector 40 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 positive electrode current collector 40 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.

[0084] 3.5 Negative electrode current collector As shown in FIG. 2, the lithium-ion secondary battery 100 may include a negative electrode current collector 50 in contact with the negative electrode active material layer 30. The negative electrode current collector 50 may be any of those commonly used as negative electrode current collectors for secondary batteries. The negative electrode current collector 50 may be in the form of a foil, plate, mesh, punched metal, foam, or the like. The negative electrode current collector 50 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 50 may be composed of multiple foils or sheets. Examples of metals constituting the negative electrode current collector 50 include at least one selected from Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, V, Mg, Pb, Ge, In, Sn, Zr, and stainless steel. In particular, from the viewpoint of ensuring reduction resistance and being less likely to alloy with lithium, the negative electrode current collector 50 may contain at least one metal selected from Cu, Ni, and stainless steel. The negative electrode current collector 50 may have some kind of coating layer on its surface for the purpose of adjusting resistance, etc. For example, the negative electrode current collector 50 may have a carbon coating layer. The negative electrode current collector 50 may be an aluminum foil having a carbon coating layer. The negative electrode current collector 50 may also be a metal foil or a substrate plated or vapor-deposited with the above metal. Furthermore, when the negative electrode current collector 50 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 50 is not particularly limited. For example, it may be 0.1 μm or more or 1 μm or more, or 1 mm or less or 100 μm or less.

[0085] 3.6 Other Configurations In addition to the above configuration, the lithium ion secondary battery 100 may also include general configurations for secondary batteries. For example, tabs, terminals, etc. The lithium ion secondary battery 100 may have the above configurations housed inside an exterior body. Any known exterior body for a battery can be used as the exterior body. Furthermore, a plurality of secondary batteries 100 may be electrically connected and stacked in any desired manner to form an assembled battery. In this case, the assembled battery may be housed inside a known battery case. 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, etc.

[0086] The lithium ion secondary battery 100 can be manufactured by applying a known method, except for using the above-mentioned specific positive electrode active material. 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) The positive electrode active material and other components that constitute the positive electrode active material layer are 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. 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. (2) The negative electrode active material and other components that constitute the negative electrode active material layer are 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. 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 to form a negative electrode active material layer on the surface of the negative electrode current collector, resulting in a negative 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.

[0087] 4. Methods for increasing the gravimetric energy density of lithium-ion secondary batteries The technology of the present disclosure also has an aspect as a method for increasing the weight energy density of a lithium ion secondary battery. That is, the method for increasing the weight energy density of a lithium ion secondary battery of the present disclosure is characterized by using the above-described positive electrode active material of the present disclosure in a positive electrode active material layer of the lithium ion secondary battery.

[0088] 5. Vehicles equipped with lithium-ion secondary batteries As described above, the positive electrode active material of the present disclosure has an excellent weight energy density and is suitable as a positive electrode active material for lithium-ion secondary batteries. A lithium-ion secondary battery having such a high weight energy density can be suitably used in, for example, at least one type of vehicle selected from a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), and an electric vehicle (BEV). That is, the technology of the present disclosure also has an aspect of a vehicle having a lithium-ion secondary battery, the lithium-ion secondary battery having a positive electrode active material layer, an electrolyte layer, and a negative electrode active material layer, and the positive electrode active material layer containing the positive electrode active material of the present disclosure. [Example]

[0089] As described above, one embodiment of the positive electrode active material and the lithium ion secondary battery, etc., has been described, but the technology of the present disclosure can be modified in various ways other than the above embodiment without departing from the gist thereof. Hereinafter, the technology of the present disclosure will be described in more detail with reference to examples, but the technology of the present disclosure is not limited to the following examples.

[0090] 1. Preparation of positive electrode active material 1.1 Example 1 1.1.1 Precursor preparation (1) MnSO4·5H2O, NiSO4·6H2O, and CoSO4·7H2O were weighed to achieve the desired composition ratio and dissolved in distilled water to a concentration of 1.2 mol / L to obtain solution 1. In a separate container, Na2CO3 was dissolved in distilled water to a concentration of 1.2 mol / L to obtain solution 2. (2) 1000 mL of pure water was placed in a reaction vessel (with a baffle plate), and 500 mL of the first solution and 500 mL of the second solution were added dropwise thereto at a rate of about 4 mL / min. (3) After the dropwise addition was completed, the mixture was stirred at room temperature for 1 hour at a stirring speed of 150 rpm to obtain a product. (4) The product was washed with pure water, and the solid was separated into liquid using a centrifuge to recover the precipitate. (5) The obtained precipitate was dried overnight at 120°C, crushed in a mortar, and then fine particles were removed by air classification to obtain precursor particles. The precursor particles were a composite salt containing Mn, Ni, and Co. The molar ratio of Mn, Ni, and Co in the precursor particles was Mn:Ni:Co=4:3:3.

[0091] 1.1.2 Preparation of the complex The precursor particles and Na2CO3 were mixed 0.9 Mn 0.4 Ni 0.3 Co 0.3 The precursor and Na2CO3 were weighed out to obtain a composite.

[0092] 1.1.3 Firing of the composite The composite was placed in an alumina crucible and fired in an air atmosphere to obtain a Na-containing oxide having a P2 structure under the firing conditions (1) to (7) below. (1) The alumina crucible containing the composite is placed in a heating furnace in an air atmosphere. (2) The temperature inside the heating furnace is raised from room temperature (25°C) to 600°C in 115 minutes. (3) The temperature in the heating furnace is maintained at 600°C for 360 minutes to perform pre-baking. (4) After the preliminary firing, the temperature inside the heating furnace is increased from 600°C to 900°C in 100 minutes. (5) The temperature in the heating furnace is kept at 900°C for 60 minutes to carry out the main firing. (6) After the main firing, the temperature in the heating furnace is lowered from 900°C to 250°C over 120 minutes. (7) At 250°C, the alumina crucible is removed from the heating furnace and allowed to cool in a dry atmosphere outside the furnace until it reaches 25°C in 10 minutes.

[0093] The fired product after cooling was crushed in a mortar in a dry atmosphere to obtain Na-containing oxide particles having a P2 type structure.

[0094] 1.1.4 Ion exchange (1) LiNO3 and LiCl were weighed out to a molar ratio of 50:50, and mixed with the above P2 type particles in a molar ratio that was 10 times the minimum Li amount required for ion exchange to obtain a mixture. (2) Using an alumina crucible, ion exchange was carried out at 280°C for 1 hour in an air atmosphere to obtain a product containing a Li-containing oxide. (3) The salt remaining in the product was washed with pure water, and solid-liquid separation was carried out by vacuum filtration to obtain a precipitate. (4) The obtained precipitate was dried overnight at 120° C. to obtain the positive electrode active material according to Example 1.

[0095] 1.2 Example 2 The procedure was the same as in Example 1, except that the composition of the precursor particles and the charging ratio of the precursor particles to Na2CO3 were changed. In Example 2, the molar ratio of Mn, Ni, and Co in the precursor particles was set to Mn:Ni:Co=4:2:4. In addition, the precursor particles and Na2CO3 were mixed in a ratio of 0.01 to 0.01. 0.8 Mn 0.4 Ni 0.2 Co 0.4 The mixture was weighed to obtain the O2 feed composition.

[0096] 1.3 Comparative Example 1 The procedure was the same as in Example 1, except that the composition of the precursor particles and the charging ratio of the precursor particles to Na2CO3 were changed. In Comparative Example 1, the molar ratio of Mn, Ni, and Co in the precursor particles was set to Mn:Ni:Co=5:2:3. In addition, the precursor particles and Na2CO3 were mixed in a mixture of Na 0.7 Mn 0.5 Ni 0.2 Co 0.3 The mixture was weighed to obtain the O2 feed composition.

[0097] 1.4 Comparative Example 2 The procedure was the same as in Example 1, except that the composition of the precursor particles and the charging ratio of the precursor particles to Na2CO3 were changed. In Comparative Example 2, the molar ratio of Mn, Ni, and Co in the precursor particles was set to Mn:Ni:Co=5:3:2. In addition, the precursor particles and Na2CO3 were mixed in a mixture of Na 0.8 Mn 0.5 Ni 0.3 Co 0.2 The mixture was weighed to obtain the O2 feed composition.

[0098] 2. Evaluation of positive electrode active material 2.1 Elemental analysis For each of Examples 1 and 2 and Comparative Examples 1 and 2, elemental analysis was performed on the P2 type particles before ion exchange to determine the molar ratio of Na to 2 moles of O contained in the P2 type particles (the molar ratio of Na to 1 mole of the transition metal Me). Furthermore, elemental analysis was performed on each of the positive electrode active materials of Examples 1 and 2 and Comparative Examples 1 and 2 to determine their chemical compositions. The results are shown in Table 1 below.

[0099] 2.2 Identification of crystal structure by X-ray diffraction measurement X-ray diffraction measurements were performed using CuKα as a radiation source on each of the positive electrode active materials of Examples 1 and 2 and Comparative Examples 1 and 2 to obtain X-ray diffraction patterns. FIG. 3 shows the X-ray diffraction patterns for each of Examples 1 and 2. FIG. 4 shows the X-ray diffraction patterns for each of Comparative Examples 1 and 2. As shown in FIGS. 3 and 4, it can be seen that all of the positive electrode active materials of Examples 1 and 2 and Comparative Examples 1 and 2 have a P2 structure belonging to the space group P63mc. The crystalline phases contained in each of the positive electrode active materials of Examples 1 and 2 and Comparative Examples 1 and 2 are shown in Table 1 below.

[0100] 2.3 Measurement of average particle size The average particle diameter (D50) was measured for each of the positive electrode active materials of Examples 1 and 2 and Comparative Examples 1 and 2. The results are shown in Table 1 below.

[0101] 2.4 Measurement of average aspect ratio by SEM observation The positive electrode active materials of Examples 1 and 2 and Comparative Examples 1 and 2 were each formed into pellets and subjected to CP processing. Then, cross-sections were observed using an FE-SEM to measure the average aspect ratio. The results are shown in Table 1 below. For reference, FIG. 5 shows an SEM image of the cross-section of the positive electrode active material of Example 1 formed into a pellet. FIG. 6 shows an SEM image of the cross-section of the positive electrode active material of Comparative Example 1 formed into a pellet.

[0102] 3. Preparation of evaluation cells Coin cells were fabricated using the positive electrode active materials of Examples 1 and 2 and Comparative Examples 1 and 2. The coin cell fabrication procedure was as follows. (1) The positive electrode active material, acetylene black (AB) as a conductive additive, and polyvinylidene fluoride (PVdF) as a binder were weighed out in a mass ratio of positive electrode active material:AB:PVdF = 85:10:5, and dispersed and mixed in N-methyl-2-pyrrolidone to obtain a positive electrode composite slurry. The positive electrode composite slurry was coated onto aluminum foil and vacuum dried overnight at 120°C to obtain a positive electrode, which is a laminate of a positive electrode active material layer and a positive electrode current collector. (2) An electrolyte solution was obtained by dissolving LiPF6 at a concentration of 1 M in a mixed solvent of trifluoropropylene carbonate (TFPC) and trifluoroethyl methyl carbonate (TFEMC) in a ratio of TFPC:TFEMC = 30 vol%:70 vol%. (3) Metallic lithium foil was prepared as the negative electrode. (4) A coin cell (CR2032) was fabricated as a liquid battery using a positive electrode, an electrolyte, and a negative electrode.

[0103] 4.Charge-discharge characteristic evaluation (1) Each of the coin cells of Examples 1 and 2 and Comparative Examples 1 and 2 was charged and discharged at 0.1 C (1 C = 220 mA / g) in a voltage range of 2.0 to 4.8 V in a thermostatic chamber maintained at 25° C., and the initial discharge capacity, average discharge potential, and weight energy density were measured. The results are shown in Table 2 below.

[0104] 5. Evaluation Results For each of Examples 1 and 2 and Comparative Examples 1 and 2, the chemical composition, average particle diameter (D50), and average aspect ratio of the positive electrode active material, as well as the initial discharge capacity, average discharge potential, and weight energy density of the evaluation cells are shown.

[0105] [Table 1]

[0106] [Table 2]

[0107] As is clear from the results shown in Tables 1 and 2, the positive electrode active materials according to Examples 1 and 2, which have a relatively large D50 of 2.0 μm or more and a relatively small average aspect ratio of 3.0 or less, have superior weight energy density to the positive electrode active materials according to Comparative Example 1, which have a relatively small D50 of less than 2.0 μm and a relatively large average aspect ratio of more than 3.0, and Comparative Example 2, which have a relatively large average aspect ratio of more than 3.0. Furthermore, the positive electrode active materials according to Examples 1 and 2 have higher initial discharge capacities and average discharge potentials than those of Comparative Examples 1 and 2.

[0108] Furthermore, as is clear from the results shown in Tables 1 and 2, the positive electrode active materials according to Examples 1 and 2 and Comparative Example 2, in which the chemical composition ratio of Li to O2 is 0.66 or more, have a higher average discharge potential than the positive electrode active material according to Comparative Example 1, in which the chemical composition ratio of Li to O2 is 0.61. In particular, the positive electrode active materials according to Examples 1 and 2, in which the chemical composition ratio of Li to O2 is greater than 0.66, have a higher average discharge potential and superior weight energy density than the positive electrode active materials according to Comparative Examples 1 and 2, in which the chemical composition ratio of Li to O2 is less than 0.66.

[0109] 6. Additional Experiments Positive electrode active materials according to Comparative Examples 3 and 4 were prepared by changing the composition of the precursor particles, and were evaluated in the same manner as above. In Comparative Example 3, the molar ratio of Mn, Ni, and Co in the precursor particles was set to Mn:Ni:Co=5:1:4. In Comparative Example 4, the molar ratio of Mn, Ni, and Co in the precursor particles was set to Mn:Ni:Co=6:1:3. The chemical composition of the positive electrode active material according to Comparative Example 3 was Li 0.57 Mn 0.52 Ni 0.08 Co 0.39 The chemical composition of the positive electrode active material according to Comparative Example 4 was Li 0.57 Mn 0.62 Ni 0.08 Co 0.30 The results of the initial discharge capacity, average discharge potential, and weight energy density for Comparative Examples 3 and 4 are shown in Table 3 below.

[0110] [Table 3]

[0111] 7. Supplementary Information In the above examples, the precursor is obtained by coprecipitation, but the precursor can also be obtained by other methods. In the above examples, the precursor and the Na source are mixed in a mortar to obtain a composite, but the composite can also be obtained by other methods. In the above examples, the Na-containing oxide having a P2-type structure and the Li-containing oxide having an O2-type structure are exemplified as having a predetermined chemical composition, but the chemical compositions of the Na-containing oxide and the Li-containing oxide are not limited to these. The Li-containing oxide may be doped with an element M other than Mn, Ni, and Co. The element M is as described in the embodiments.

[0112] 8. Summary As described above, in a positive electrode active material containing a Li-containing oxide, if the Li-containing oxide satisfies the following requirements (1-1) to (1-4), the energy density of the positive electrode active material increases. (1-1) The Li-containing oxide particles have an O2 type structure. (1-2) The Li-containing oxide particles contain at least one element selected from Mn, Ni, and Co, Li, and O as constituent elements. (1-3) The Li-containing oxide particles have an average particle size of 2.0 μm or more. (1-4) The Li-containing oxide particles have an average aspect ratio of 1.0 or more and 3.0 or less.

[0113] Furthermore, in a positive electrode active material containing a Li-containing oxide, if the Li-containing oxide satisfies the following requirements (2-1) and (2-2), the average discharge potential of the positive electrode active material becomes high. (2-1) The Li-containing oxide particles have an O2 type structure. (2-2) The Li-containing oxide particles are Li a Na b Mn x-p Niy-q Co z-r M p+q+r O2 (where 0.66 < a ≤ 1.00, 0 ≤ b ≤ 0.20, x + y + z = 1, and 0 ≤ p + q + r < 0.17, and the element M is at least one selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W). It has a chemical composition represented by this formula.

Explanation of symbols

[0114] 100 Lithium-ion secondary battery 10 Positive electrode active material layer 20 Electrolyte layer 30 Negative electrode active material layer 40 Positive electrode current collector 50 Negative electrode current collector

Claims

1. A positive electrode active material including Li-containing oxide particles, The Li-containing oxide particles have an O2 type structure, The Li-containing oxide particles have a chemical composition represented by Li a Na b Mn x-p Ni y-q Co z-r M p+q+r O 2 (wherein 0.66<a≦1.00, 0≦b≦0.20, 0.30<x<0.60, 0.10<y<0.40, 0.10<z<0.50, x+y+z=1, and 0≦p+q+r<0.17, and the element M is at least one selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W), The Li-containing oxide particles have an average particle size of 2.0 μm or more, The Li-containing oxide particles have an average aspect ratio of 1.0 or more and 3.0 or less. Cathode active material.

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

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