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

Abstract

To disclose a technique capable of improving the cycle characteristics of a battery when the battery is configured using a positive electrode active material having an O2-type structure.SOLUTION: The positive electrode active material of the present disclosure includes a plurality of crystallites having an O2-type structure. 90% or more of all the crystallites have open pores, and the number of the open pores per unit length on a surface of the crystallite is 0.15 pores / μm or more and 0.50 pores / μm or less.SELECTED DRAWING: Figure 1A

Description

[Technical Field]

[0001] The present application discloses a cathode active material, a battery, and a method for making the cathode active material. [Background technology]

[0002] Positive electrode active materials having an O2-type structure are known. As disclosed in Patent Documents 1 and 2, electrode active materials having an O2-type structure are 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 Publication No. 2022-097885 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-186937 Summary of the Invention [Problem to be solved by the invention]

[0004] Batteries using positive electrode active materials with an O2 structure have room for improvement in terms of cycle characteristics. [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 a plurality of crystallites having an O2-type structure, 90% or more of the total number of said crystallites have open pores; the number of the open pores per unit length on the surface of the crystallite is 0.15 / μm or more and 0.50 / μm or less; Cathode active material. <Aspect 2> The positive electrode active material of Aspect 1, It is a spherical polycrystalline particle, Cathode active material. <Aspect 3> A battery having a positive electrode active material layer, an electrolyte layer, and a negative electrode active material layer, The positive electrode active material layer includes the positive electrode active material of embodiment 1 or 2. battery. <Aspect 4> A method for producing a positive electrode active material, Obtaining a Na-containing oxide having a P2 type structure, and bringing the Na-containing oxide into contact with an ion exchange material to ion-exchange at least a portion of the Na contained in the Na-containing oxide with Li, thereby obtaining a Li-containing oxide having an O2-type structure; Including, The ion exchange material contains 0.5 mol% or more and 1.0 mol% or less of lithium hydride. A method for producing a positive electrode active material. <Aspect 5> A method for producing a positive electrode active material according to aspect 4, comprising: The ion exchange material is a lithium salt comprising one or both of lithium nitrate and lithium halide; the lithium hydride; Including, A method for producing a positive electrode active material. [Effects of the Invention]

[0006] When a battery is constructed using the positive electrode active material of the present disclosure, the cycle characteristics of the battery are likely to be improved. [Brief explanation of the drawings]

[0007] [Figure 1A] An example of an open pore present in the crystallite of the positive electrode active material is shown. [Figure 1B] An example of an open pore present in the crystallite of the positive electrode active material is shown. [Figure 2] 1 shows an example of a flow of a method for producing a positive electrode active material. [Figure 3] 1 shows a schematic diagram of an example of a battery configuration. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, one embodiment of the cathode active material, battery, and method for manufacturing a cathode active material according to the present disclosure will be described, but the cathode active material, battery, and method for manufacturing a cathode active material according to the present disclosure are not limited to the embodiment described below.

[0009] 1.Cathode active material A positive electrode active material according to one embodiment includes a plurality of crystallites having an O2 structure, wherein 90% or more of the total number of the crystallites have open pores, and the number of the open pores per unit length on the surface of the crystallites is 0.15 to 0.50 per μm.

[0010] 1.1 Crystal structure A positive electrode active material according to one embodiment includes a plurality of crystallites having an O2-type structure. The plurality of crystallites may have a crystal structure other than the O2-type structure, in addition to the O2-type structure (belonging to the space group P63mc). Examples of crystal structures other than the O2-type structure include a T#2-type structure (belonging to the space group Cmca) 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 plurality of crystallites may have an O2-type structure as the main phase, or may have a crystal structure other than the O2-type structure as the main phase.

[0011] 1.2 Crystallite morphology In a positive electrode active material according to one embodiment, a single crystallite may form a single particle, or multiple crystallites may form a single particle. In other words, the "positive electrode active material including multiple crystallites" used herein may be (1) a single particle in which multiple single crystal particles exist independently of each other, (2) an aggregate (secondary particle) of multiple single crystal particles, (3) a polycrystalline particle including multiple crystallites, or (4) an aggregate (secondary particle) of multiple polycrystalline particles. In particular, when the positive electrode active material is a polycrystalline particle, particularly when the positive electrode active material is a spherical polycrystalline particle (described later), high performance as a positive electrode active material is likely to be ensured.

[0012] When the positive electrode active material is a polycrystalline particle (grain boundaries exist in the particles of the positive electrode active material), it is likely to exhibit high performance as a positive electrode active material. As described below, a positive electrode active material according to one embodiment 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, has a large diffusion coefficient of Na ions, and is prone to crystal growth in a specific direction. Therefore, crystallites having a P2-type structure usually have a crystal growth direction biased in a specific direction (e.g., plate-like). When Na in a P2-type crystallite whose crystal growth direction is biased in this way is ion-exchanged with Li to obtain an O2-type crystallite, the ends of the O2-type crystallite (the ends in the crystal growth direction) tend to serve as inlet and outlet ports for intercalation. In other words, when the positive electrode active material is polycrystalline particles, the effects of increasing the number of entrances and exits for intercalation and thereby reducing reaction resistance, shortening the distance that lithium ions travel and thereby reducing diffusion resistance, and reducing the amount of expansion and contraction of the particles as a whole during charge and discharge can be expected.

[0013] As described above, the crystallites of a Na-containing oxide having a P2 structure tend to be plate-shaped. That is, a Na-containing oxide having a P2 structure can be formed into plate-shaped particles, or small plate-shaped crystallites can be connected to each other to form spherical particles. In other words, a single positive electrode active material particle may be a plate-shaped single crystal particle as a whole, or a spherical polycrystalline particle. Spherical polycrystalline particles have multiple crystallites on their surfaces. When the positive electrode active material is a spherical polycrystalline particle, the reduction in crystallite size tends to reduce the reaction resistance and the diffusion resistance inside the particle. Furthermore, when applied to a battery, the spheroidization is thought to reduce the degree of curvature and reduce the lithium ion conduction resistance. This, for example, tends to improve rate characteristics and increase the reversible capacity. In this application, "spherical particles" refers to particles with a circularity of 0.80 or more. The circularity of the particles may be 0.81 or more, 0.82 or more, 0.83 or more, 0.84 or more, 0.85 or more, 0.86 or more, 0.87 or more, 0.88 or more, 0.89 or more, or 0.90 or more. The circularity of the particles is 4πS / L 2 where S is the orthogonal projected area of ​​the particle, and L is the perimeter of the orthogonal projected image of the particle. The circularity of a particle can be determined by observing the appearance of the particle using a scanning electron microscope (SEM), transmission electron microscope (TEM), or optical microscope.

[0014] The size of the crystallites constituting the positive electrode active material may be large or small, but smaller crystallite sizes tend to exhibit the above-mentioned advantageous effects. For example, when the diameter of the crystallites constituting the positive electrode active material is 5 μm or less, 3 μm or less, or 1 μm or less, higher performance is likely to be obtained. The "crystallite" and "crystallite diameter" can be determined by observing the surface of the positive electrode active material using a scanning electron microscope (SEM) or a transmission electron microscope (TEM). That is, when observing the surface of the positive electrode active material, if a single closed region surrounded by a grain boundary is observed, that region is considered to be a "crystallite." The maximum Feret diameter of the crystallite is determined and considered to be the "crystallite diameter." If a 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."

[0015] 1.3 Open pore As used herein, the term "open pore" refers to a three-dimensional lattice defect observed when observing a crystallite using a TEM, which is a region free of atoms and extends to the surface of the crystallite (having an opening on the surface of the crystallite). The "region free of atoms" refers to a region where the signal is less than S / N when an HAADF-STEM image is acquired. As shown in Figures 1A and 1B, the open pore may extend along the (001) plane of the O2 structure. The open pore may also have a maximum width of 0.9 nm to 10 nm in the direction perpendicular to the (001) plane.

[0016] The O2-type structure exhibits large expansion and contraction during charging and discharging. Therefore, when a battery employs a cathode active material having an O2-type structure, the cathode active material expands and contracts during charging and discharging, eliminating contact between the cathode active material and other battery materials (e.g., electrolytes, conductive additives, etc.), and ionic conduction paths and conductive paths are likely to be eliminated. In contrast, when crystallites having an O2-type structure have open pores, even if the O2-type structure expands and contracts during charging and discharging, crack propagation is likely to occur starting from the open pores, causing preferential cracking within the crystallite, which is thought to suppress expansion and contraction of the crystallite as a whole. As a result, contact between the cathode active material and other battery materials is likely to be maintained, and ionic conduction paths and conductive paths are likely to be maintained. On the other hand, if the number of open pores in the crystallites with the O2-type structure is too large, numerous cracks will occur when the O2-type structure expands and contracts during charging and discharging, and isolated crystallites will appear in areas that were originally single crystallites, which will no longer contribute to charging and discharging, and this is thought to lead to a decrease in capacity.In addition, as the cracks propagate, the crystal structure will change to a rock salt structure, which is thought to lead to an increase in resistance.

[0017] In one embodiment of the positive electrode active material, 90% or more of the total number of crystallites having the above-mentioned O2-type structure have open pores. That is, the proportion of open pores ([number of crystallites having open pores] / [total number of crystallites]×100(%)) is 90% or more. The proportion of open pores may be 90% or more and 100% or less, or 95% or more and 100% or less.

[0018] The "rate of open pores" is measured as follows. First, the positive electrode active material is embedded in a resin, and then sliced ​​by dry mechanical polishing and ion polishing. The slice is observed using a TEM. The TEM used is an ARM-200F manufactured by JEOL Ltd. The acceleration voltage is 200 kV. For observation, a OneView camera manufactured by Gatan is used with a resolution of 4,096 pixels x 4,096 pixels. The incident direction of the electron beam with respect to the particle to be observed is <100> Or <1-10>. The observation field is 400nm x 400nm, and the observation magnification is 100,000 times. For each crystallite, one or more images are taken so that the entire outer periphery of the crystallite can be recorded. Twenty crystallites contained in the positive electrode active material are randomly selected and observed using a TEM under the above-mentioned conditions to determine whether or not each selected crystallite has open pores. If 18 or more of the selected 20 crystallites have open pores, it can be said that the presence rate of the above-mentioned open pores is 90% or more.

[0019] In one embodiment of the positive electrode active material, the number of open pores per unit length on the surface of the crystallites having the O2 structure is 0.15 to 0.50 per μm, which allows cracks to be generated appropriately during charging and discharging, thereby achieving the above-mentioned advantageous effects. The number of open pores per unit area on the surface of the crystallites having the O2 structure may be 0.16 to 0.40 per μm, or 0.17 to 0.30 per μm.

[0020] The "number of open pores per unit length on the surface of a crystallite having an O2 type structure (pieces / μm)" is measured as follows: In other words, in the same manner as in the "abundance rate of open pores" described above, 20 crystallites constituting the positive electrode active material are randomly selected, the electron beam incident direction is determined for each crystallite as described above, the number of open pores in that incident direction is counted, and the counted number is divided by the perimeter length (μm) of the surface of the crystallite to determine the "number of open pores per unit length on the surface of a crystallite having an O2 type structure (pieces / μm)."

[0021] 1.4 Chemical composition The chemical composition of the crystallites having an O2 type structure is not particularly limited as long as the O2 type structure can be maintained. The crystallites having an O2 type structure may, for example, contain at least one element selected from Mn, Ni, and Co, Li, and O as constituent elements. The crystallites having an O2 type structure are particularly likely to achieve higher performance when they contain at least Li, Mn, one or both of Ni and Co, and O as constituent elements, and particularly when they contain at least Li, Mn, Ni, Co, and O as constituent elements. The crystallites having an O2 type structure are likely to achieve higher performance when they contain at least Li, Mn, Ni, Co, and O as constituent elements. 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 crystallites having the O2-type structure have such a chemical composition, the O2-type structure is likely to be maintained. In the above chemical composition, a 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, or 0.60 or more, and is at most 1.00, and may be 0.90 or less, 0.80 or less, or 0.70 or less. In the above chemical composition, b is 0 or more, and may be 0.01 or more, 0.02 or more, or 0.03 or more, and is at most 0.20, and may be 0.15 or less, or 0.10 or less. Also, x is 0 or more, and may beThe composition of O is approximately 2, but is not necessarily exactly 2.0 and is variable.

[0022] 1.5 Other The positive electrode active material according to one embodiment may be, for example, solid particles, hollow particles, or particles having voids. The particle size of the positive electrode active material is not particularly limited, but smaller sizes are considered to be more advantageous. For example, the average particle diameter (D50) of the positive electrode active material particles may be 0.1 μm or more and 10 μm or less, 1.0 μm or more and 8.0 μm or less, or 2.0 μm or more and 6.0 μm or less. The average particle diameter (D50) is the particle diameter (D50, median diameter) at 50% cumulative in a volume-based particle size distribution determined by a laser diffraction / scattering method.

[0023] 2. Manufacturing method of positive electrode active material A cathode active material according to an embodiment can be produced by, for example, the following method. That is, as shown in FIG. 2, a method for producing a cathode active material according to an embodiment may include obtaining a Na-containing oxide having a P2-type structure (S1), and contacting the Na-containing oxide with an ion exchange material to ion-exchange at least a portion of the Na contained in the Na-containing oxide with Li to obtain a Li-containing oxide having an O2-type structure (S2). In this case, when the ion exchange material contains 0.5 mol % to 1.0 mol % of lithium hydride, a large amount of dislocations can be densely packed during ion exchange, which can appropriately introduce crystal defects and appropriately form the above-mentioned open pores in crystallites having an O2-type structure.

[0024] 2.1 S1 In S1, the Na-containing oxide having a P2 type structure is, for example, S11: Obtaining a precursor (e.g., a precursor containing at least one element of Mn, Ni, and Co); S12: Coating the surface of the precursor with a Na source to obtain a composite; and S13: Calcining the composite Here, the step S13 can be produced through the following steps: S13-1: 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; S13-2: 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 S13-3: Following the main sintering, the composite may be rapidly cooled from a temperature T1 of 200°C or higher to a temperature T2 of 100°C or lower.

[0025] 2.1.1 Precursor preparation 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 of 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 types of 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 particles made of the precursor is not particularly limited.

[0026] In S11, the precursor precipitate may be obtained by coprecipitation using an ion source capable of forming a precipitate in an 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 an 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 S11, 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.

[0027] In step S11, 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 has the function of stabilizing, for example, the P2-type structure or the O2-type structure. The method for obtaining a precursor containing element M is not particularly limited. When obtaining a precursor by coprecipitation in step S11, 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 the respective aqueous solutions are added dropwise and mixed to obtain a precursor containing element M together with at least one element of Mn, Ni, and Co. Alternatively, in the manufacturing method of the present disclosure, element M may not be added in step S11, and element M may be doped during the Na-doping calcination in steps S2 and S3 described below.

[0028] 2.1.2 Preparation of the complex In S12, the surface of the precursor obtained in S11 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 S12, the amount of Na source coated on the surface of the precursor may be determined taking into account the amount of Na lost during subsequent calcination. In S12, the coverage rate of the Na source on the surface of the precursor is not particularly limited. In S12, 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 may be mixed in a mortar or a mixer, or the precursor may be brought into contact with a solution containing the Na source using a tumbling fluidized coating method, a spray drying method, or the like, and then dried.

[0029] In S12, the precursor may be coated with an M source together with the Na source. For example, in S12, the precursor obtained in S11 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.

[0030] 2.1.3 Firing of the composite In S13, the composite obtained in S12 is calcined to obtain a Na-containing oxide having a P2-type structure. S13 may include the above-mentioned S13-1, S13-2, and S13-3. By adjusting the conditions in S13-1, S13-2, and S13-3, the crystallinity and shape (plate-like particles or spherical particles) of the P2-type Na-containing oxide obtained in S13 can be adjusted.

[0031] In S13-1, the composite is pre-fired at a temperature of 300°C or higher but lower than 700°C for 2 hours or higher but 10 hours or lower. In S13-1, the composite may be optionally shaped and then pre-fired. The pre-fired temperature is lower than that of the main firing. If the pre-fired temperature in S13-1 is insufficient, the P2 phase may not be sufficiently formed in the finally obtained Na-containing oxide. In S13-1, by setting the pre-fired temperature to 300°C or higher but lower than 700°C and the pre-fired time to 2 hours or higher but 10 hours or lower, the composite can be sufficiently pre-fired, improving heat uniformity and making it easier for the Na-containing oxide obtained via S13-2 and S13-3 described below to be suitable. 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.

[0032] In S13-2, 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 S13-2, the main firing temperature of the composite may be 800°C to 1000°C. If the main firing temperature is too low, the P2 phase will not be formed, and 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. In S13-2, the shape of the Na-containing oxide can be controlled by the main firing time. If the main firing time is too short, the P2 phase will not be formed sufficiently. On the other hand, if the main firing time is too long, the P2 phase will grow excessively, and the particles will likely become coarse and plate-like.

[0033] In step S13-3, 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 S13-3, 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 (a Na-containing oxide having a P2 structure), it is believed that the amount of moisture that penetrates between the layers of the P2 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, when cooling the sintered composite in step S13-3, for example, by allowing it to cool in a dry atmosphere outside the furnace from a temperature T1 of 200°C or higher to a temperature T2 of 100°C or lower, the cooling rate from temperature T1 to temperature T2 is high (e.g., 20°C / min or higher), making it difficult for moisture to penetrate between the layers of the P2 structure, thereby preventing the collapse of the P2 structure. As a result, Na can be efficiently ion-exchanged with Li in S2.

[0034] By S13, it is possible to produce a Na-containing oxide having a P2 type structure and a predetermined chemical composition. The Na-containing oxide contains, as constituent elements, at least one transition metal element selected from Mn, Ni, and Co, Na, and O. In particular, when the constituent elements contain at least Na, Mn, at least one of Ni and Co, and O, and especially when the constituent elements contain at least Na, Mn, Ni, Co, and O, the performance of the positive electrode active material is likely to be further improved. The Na-containing oxide contains Na c Mnx-p Ni y-q Co z-r M p+q+r It may have a chemical composition represented by O2. Here, 0 < c < 1.00, x + y + z = 1, and 0 ≤ p + q + r < 0.17. Also, M is at least one element selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W. When the Na-containing oxide has such a chemical composition, the P2-type structure is more 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, or 0.60 or more, and less than 1.00 and may be 0.90 or less, 0.80 or less, or 0.70 or less. x is 0 or more and may be 0.10 or more, 0.20 or more, 0.30 or more, 0.40 or more, or 0.50 or more, and 1.00 or less and may be 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, or 0.50 or less. Also, y is 0 or more and may be 0.10 or more or 0.20 or more, and 1.00 or less and may be 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 0.10 or more, 0.20 or more, or 0.30 or more, and 1.00 or less and may be 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 little 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 and the O2-type structure are 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.

[0035] 2.2 S2 In step S2, the Na-containing oxide obtained in step S1 is brought into contact with an ion exchange material to ion-exchange at least a portion of the Na contained in the Na-containing oxide with Li, thereby obtaining a Li-containing oxide having an O2-type structure. A Li compound is used as the ion exchange material. Specifically, a lithium salt such as lithium nitrate or lithium halide is preferably used as the ion exchange material. However, if only a lithium salt such as lithium nitrate or lithium halide is used as the ion exchange material, crystal defects cannot be introduced during ion exchange, and it is difficult to form the above-mentioned open pores in the crystallites having the O2-type structure. In contrast, in this embodiment, by using an ion exchange material containing lithium hydride, crystal defects can be introduced during ion exchange due to the reducing power of the lithium hydride, and the above-mentioned open pores can be formed in the crystallites having the O2-type structure. However, if a large amount of lithium hydride is used as the ion exchange material, excessive crystal defects are introduced during ion exchange, resulting in the formation of excessive open pores in the crystallites having the O2-type structure. According to the inventors' findings, when the ratio of the number of moles of lithium hydride to the total number of moles (100 mol%) of compounds constituting the ion exchange material is 0.5 mol% or more and 1.0 mol% or less, crystal defects can be appropriately introduced during ion exchange, and the above-mentioned open pores can be appropriately formed in crystallites having an O2 structure. The ion exchange material may contain a lithium salt containing one or both of lithium nitrate and lithium halide, and the above-mentioned lithium hydride. More specifically, the ion exchange material may contain 99 mol% or more and less than 100 mol% of a lithium salt containing one or both of lithium nitrate and lithium halide, and 0.5 mol% or more and 1.0 mol% or less of lithium hydride. In this case, the lithium halide constituting the ion exchange material is preferably at least one of lithium chloride, lithium bromide, and lithium iodide, and more preferably lithium chloride. The lithium salt is preferably a combination of lithium nitrate and lithium halide.The combination of lithium nitrate and lithium halide lowers the melting point compared to when lithium nitrate or lithium halide is used alone, enabling ion exchange at lower temperatures. Butyllithium is an example of a compound that can be expected to have the same effect as lithium hydride. In other words, butyllithium may be used as the ion exchange material. In this case, the proportion of butyllithium in the ion exchange material can be adjusted appropriately.

[0036] When the ion exchange material is brought into contact with the Na-containing oxide, the ion exchange material may be in a molten state or a solid state. A molten state is particularly preferable. That is, by mixing the Na-containing oxide having the P2 structure with the ion exchange material and heating the mixture to a temperature equal to or higher than the melting point of the ion exchange material, at least a portion of the Na in the Na-containing oxide can be replaced with Li by ion exchange. The temperature for ion exchange may be, for example, equal to or higher than the melting point of the ion exchange material and equal to or lower than 600°C, 500°C, 400°C, or 300°C. If the temperature for ion exchange is too high, a stable O3 structure is likely to be formed instead of an O2 structure. On the other hand, from the viewpoint of shortening the time required for ion exchange, the temperature for ion exchange should be as high as possible.

[0037] 3.Battery As shown in FIG. 3 , a battery 100 according to one embodiment includes a positive electrode active material layer 10, an electrolyte layer 20, and a negative electrode active material layer 30. The positive electrode active material layer 10 includes the positive electrode active material of the present disclosure. The battery 100 may include a positive electrode current collector 40 and a negative electrode current collector 50. The battery 100 may be a solid-state battery or a liquid-based battery. A solid-state battery refers to a battery that includes a solid electrolyte and can tolerate the presence of a liquid. The battery 100 may also be an all-solid-state battery that is substantially free of liquid. The battery configuration may be the same as a conventional battery, except that the positive electrode active material of the present disclosure is used. A detailed description will be omitted here. [Example]

[0038] As described above, one embodiment of the positive electrode active material 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.

[0039] 1. Preparation of positive electrode active material 1.1 Example 1 1.1.1 Preparation of precursor particles Mn(NO3)2·6H2O, Ni(NO3)2·6H2O, and Co(NO3)2·6H2O were dissolved in pure water in a molar ratio of 5:2:3 to obtain a transition metal-containing solution. A 12 wt% Na2CO3 solution was also prepared. These two solutions were simultaneously titrated into a beaker. The titration rate was controlled to maintain a pH between 7.0 and 7.1. After the titration, the mixture was stirred at 50°C and 300 rpm for 24 hours. The resulting reaction product was washed with pure water, and the precipitated powder was separated by centrifugation. The resulting powder was dried at 120°C for 48 hours and then crushed in an agate mortar to obtain precursor particles.

[0040] 1.1.2 Preparation of the complex The obtained precursor particles were mixed with Na2CO3 in a composition ratio of Na 0.75 Mn 0.5 Ni 0.2 Co 0.3 The powder mixture was then cold isostatically pressed under a load of 2 tons to produce a pellet (composite).

[0041] 1.1.3 Firing of the composite The obtained pellets were pre-fired in an air atmosphere at 600°C for 6 hours, then fired at 900°C for 1 hour, cooled to 250°C, and then allowed to cool (rapid cooling) in a dry atmosphere to obtain Na-containing oxide particles with a P2 type structure. The Na-containing oxide particles with a P2 type structure were spherical particles with multiple crystallites with a P2 type structure present on the particle surface.

[0042] 1.1.4 Ion exchange An ion exchange material was obtained by mixing LiNO3, LiCl, and LiH in a molar ratio of 87.0:12.0:1.0. The ion exchange material was weighed so that the molar ratio of Li contained in the ion exchange material was 10 times that of the Na-containing oxide particles. The Na-containing oxide particles and the ion exchange material were mixed, and ion exchange was performed under conditions of 280°C in an air atmosphere for 1 hour. After ion exchange, water was added to dissolve the salt, and further washing with water was performed to obtain positive electrode active material particles according to Example 1. The positive electrode active material particles were spherical particles with multiple crystallites having an O2 structure present on the particle surface.

[0043] 1.2 Example 2 Positive electrode active material particles were obtained in the same manner as in Example 1, except that a mixture of LiNO3, LiCl, and LiH in a molar ratio of 87.5:12.0:0.5 was used as the ion exchange material.

[0044] 1.3 Example 3 Positive electrode active material particles were obtained in the same manner as in Example 1, except that the time for main baking of the composite was changed to 24 hours.

[0045] 1.4 Comparative Example 1 Positive electrode active material particles were obtained in the same manner as in Example 1, except that a mixture of LiNO3 and LiCl in a molar ratio of 88.0:12.0 was used as the ion exchange material.

[0046] 1.5 Comparative Example 2 Positive electrode active material particles were obtained in the same manner as in Example 1, except that a mixture of LiNO3, LiCl, and LiH in a molar ratio of 83.0:12.0:5.0 was used as the ion exchange material.

[0047] 1.6 Comparative Example 3 Positive electrode active material particles were obtained in the same manner as in Example 1, except that a mixture of LiNO3, LiCl, and LiH in a molar ratio of 87.8:12.0:0.2 was used as the ion exchange material.

[0048] 1.7 Comparative Example 4 Positive electrode active material particles were obtained in the same manner as in Comparative Example 1, except that the time for main baking of the composite was changed to 24 hours.

[0049] 2. Observation of positive electrode active material particles The crystallites of each positive electrode active material particle were observed by TEM to measure the proportion of open pores in the crystallites and the number of open pores per unit length (number / μm) on the surface of the crystallites. The TEM observation conditions were as described in the above embodiment.

[0050] 3. Battery Fabrication The positive electrode active material, sulfide-based solid electrolyte, polyvinylidene fluoride (PVDF) as a binder, and vapor-grown carbon fiber (VGCF) were weighed in a mass ratio of 62.5:30.8:0.5:6.2 and dispersed and mixed in butyl butyrate to form a slurry. The slurry was dried at 165°C to prepare a positive electrode composite.

[0051] Li4Ti5O as a negative electrode active material 12 The sulfide-based solid electrolyte, PVDF, and VGCF were weighed in a weight ratio of 72.1:22.7:3.5:1.7, and dispersed and mixed in butyl butyrate to form a slurry. The slurry was dried at 165°C to prepare a negative electrode composite.

[0052] A sulfide-based solid electrolyte was used as the separator layer, and the positive electrode composite and negative electrode composite were pressed together to produce an all-solid-state battery.

[0053] 4. Battery evaluation The coin cells fabricated as described above were subjected to a cycle test (voltage range: 2.0 V - 4.8 V) in a constant temperature bath at 25°C. The discharge capacity at the first cycle (initial capacity (mAh / g)) and the capacity retention rate after 100 cycles ([discharge capacity at 100th cycle] / [discharge capacity at 100th cycle] × 100 (%)) were measured. In addition, the resistance was measured at 3.66 V before and after the cycle test, and the resistance increase rate (%) was calculated.

[0054] 5. Evaluation Results Table 1 below shows the results of observation of open pores for each positive electrode active material, and the evaluation results of the initial capacity and cycle characteristics (capacity retention rate, resistance increase rate) for each coin cell. [Table 1]

[0055] 6. Summary From the above results, it can be said that the cycle characteristics of a battery can be improved by constructing the battery using a positive electrode active material that satisfies the following (1) to (3). (1) The positive electrode active material contains a plurality of crystallites having an O2 type structure. (2) 90% or more of the total number of said crystallites have open pores. (3) The number of the open pores per unit length on the surface of the crystallite is 0.15 / μm or more and 0.50 / μm or less. [Explanation of symbols]

[0056] 100 batteries 10 Cathode 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 comprising a plurality of crystallites having an O2-type structure, 90% or more of the total number of said crystallites have open pores; the number of the open pores per unit length on the surface of the crystallite is 0.15 / μm or more and 0.50 / μm or less; Cathode active material.

2. The positive electrode active material according to claim 1 , It is a spherical polycrystalline particle, Cathode active material.

3. A battery having 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 or 2. battery.

4. A method for producing a positive electrode active material, Obtaining a Na-containing oxide having a P2 type structure; and bringing the Na-containing oxide into contact with an ion exchange material to ion-exchange at least a portion of the Na contained in the Na-containing oxide with Li, thereby obtaining a Li-containing oxide having an O2-type structure; Including, The ion exchange material contains 0.5 mol% or more and 1.0 mol% or less of lithium hydride. A method for producing a positive electrode active material.

5. The method for producing a positive electrode active material according to claim 4, The ion exchange material is a lithium salt comprising one or both of lithium nitrate and lithium halide; the lithium hydride; Including, A method for producing a positive electrode active material.

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