ACTIVE MATERIAL OF THE POSITIVE ELECTRODE, METHOD FOR PRODUCING AN ACTIVE MATERIAL OF THE POSITIVE ELECTRODE AND BATTERY

Abstract

The invention relates to an active material for the positive electrode, a manufacturing process for an active material for the positive electrode, and a battery. The active material for the positive electrode of the present invention comprises a sodium-containing oxide. The sodium-containing oxide has a P2-type structure. The sodium-containing oxide comprises as components at least: sodium; calcium; at least one transition metal element consisting of manganese, nickel, and cobalt; and oxygen.

Description

BACKGROUND OF THE INVENTION1. Field of the InventionThis application discloses a positive electrode active material, a method of manufacturing a positive electrode active material, and a battery.2. Description of the Prior ArtJapanese Patent Application No. 2023-182443 (JP 2023-182443 A) discloses a positive electrode active material used for a sodium ion battery and having a P2-type structure. Meanwhile, J. Am. Chem. Soc. 2020, 142, 5742-5750 discloses a technology that increases the amount of Na contained in a positive electrode active material having a P2-type structure.SUMMARY OF THE INVENTIONConventional positive electrode active materials having a P2-type structure are capable of improvement in cycle characteristics.This application discloses the following aspects as a means for solving the above challenge.Aspect 1A positive electrode active material containing an Na-containing oxide, wherein the Na-containing oxide has a P2-type structure, and wherein the Na-containing oxide contains, as constituents, at least Na; Ca; at least one transition metal element of Mn, Ni, and Co; and OAspect 2The positive electrode active material according to Aspect 1, wherein an amount of Na contained in the Na-containing oxide may be 0.65 mol or more and less than 0.80 mol per mol of the Na-containing oxide, and wherein an amount of Ca contained in the Na-containing oxide may be more than 0 mol and 0.075 mol or less per mol of the Na-containing oxide.Aspect 3The positive electrode active material according to Aspect 1 or 2, wherein the Na-containing oxide may contain, as constituents, at least Na, Ca, Mn, Ni, and O.Aspect 4A method for producing a positive electrode active material, comprising:obtaining a solid mixture containing a precursor, a Na source, and a Ca source; andobtaining an Na-containing oxide having a P2-type structure by firing the solid mixture,wherein the precursor contains at least one transition metal element of Mn, Ni and Co.Aspect 5A battery comprising a positive electrode active material layer, an electrolyte layer, and a negative electrode active material layer, wherein the positive electrode active material layer comprises the positive electrode active material of any one of aspects 1 to 3.The positive electrode active material of the present invention has excellent cycle properties.BRIEF DESCRIPTION OF THE DRAWINGSFeatures, advantages and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like reference numerals denote like elements, wherein FIG. 1 shows an example of a flow of a manufacturing method of a positive electrode active material; FIG. 2 schematically shows an example of the configuration of a battery; and FIG. 3 shows a relationship between a layer pitch of Na layers in a P2-type structure and a Ca doping amount.DETAILED DESCRIPTION OF EMBODIMENTS1. Positive Electrode Active MaterialA positive electrode active material according to an embodiment includes an Na-containing oxide. The Na-containing oxide has a P2-type structure. The Na-containing oxide includes, as constituents, at least: Na; Ca; at least one transition metal element of Mn, Ni, and Co; and O.1.1 Crystal structureThe Na-containing oxide contained in the positive electrode active material has at least the P2-type structure (belonging to space group P63mc) as a crystal structure. In addition to the P2-type structure, the Na-containing oxide may have a different crystal structure from the P2-type structure. Examples of crystal structures other than the P2-type structure include various crystal structures (a P3-type structure, etc.) formed when Na is inserted into and desorbed from the P2-type structure. The Na-containing oxide may have the P2-type structure as a main phase. The crystal structure constituting the main phase of the Na-containing oxide may vary depending on a charge-discharge state.The size of crystallites of the oxide containing Na in the positive electrode active material is not particularly limited. In the Na-containing oxide, a crystallite itself may form a particle, or a particle may be formed by a plurality of crystallites. In other words, the positive electrode active material according to an embodiment may be (1) a single crystal particle independently present, (2) an aggregate (secondary particles) of a plurality of single crystal particles, (3) a polycrystalline particle including a plurality of crystallites, or (4) an aggregate (secondary particles) of a plurality of polycrystalline particles. In particular, when the Na-containing oxide forms a polycrystalline particle, in particular, when it forms a spherical polycrystalline particle, an even higher performance of the positive electrode active material is likely to be ensured. The P2-type structure is a hexagonal system in which the diffusion coefficient of Na ions is high and crystals tend to grow in a certain direction. Therefore, crystallites having the P2-type structure are typically those in which the crystal growth direction is disproportionately strong in a particular direction (e.g., plate-shaped crystallites). In this case, the end portions (end portions in the above crystal growth direction) of the crystallites of the P2-type structure are likely to become inputs and outputs for intercalation. In other words, when the Na-containing oxide forms polycrystalline particles, effects can be expected such as an effect that the reaction resistance decreases as the number of inlet and outlet ports for intercalation contained in a particle increases, an effect that the diffusion resistance decreases as the travel distance of sodium ions becomes shorter, and an effect that the amount of expansion and contraction of the particles as a whole during charging and discharging becomes smaller. For example, the diameter of crystallites constituting the Na-containing oxide may be 0.1 μm or larger and 5.0 μm or smaller, 0.5 μm or larger and 4.0 μm or smaller or 1.0 μm or larger and 3.0 μm or smaller. "crystallites" and "the diameter of crystallites" may be obtained by observing the external appearance, etc., of the Na-containing oxide under a scanning electron microscope (SEM) or a transmission electron microscope (TEM). That is, when the Na-containing oxide is observed and a closed region surrounded by a crystal boundary is observed, this region is regarded as "crystallite". The maximum Feret diameter of this crystallite is determined, and the determined diameter is considered to be "diameter of crystallite". When the Na-containing oxide is formed from a single crystal, this single crystal itself may be referred to as a crystallite, and the maximum feret diameter of this single crystal is "the diameter of the crystallite". Alternatively, the diameter of the crystallite can be obtained by EBSD or XRD. For example, the diameter of the crystallite can be obtained from a half width of a diffraction line of an XRD pattern based on the Scherer formula. When the diameter of the crystallite determined by any of these methods is within the above range, the Na-containing oxide is likely to have higher performance. The crystallites forming the Na-containing oxide may have a first surface exposed in a surface of the oxide, and this first surface may be planar.As described above, the crystallite of the Na-containing oxide having the P2structure tends to take a plate-like shape. That is, the Na-containing oxide having the P2 structure may be both a plate-shaped particle and a spherical particle because small plate-shaped crystallites are connected to each other. In other words, the Na-containing oxide as a whole may form a plate-shaped single crystal particle or a spherical polycrystalline particle. The spherical polycrystalline particle has a plurality of crystallites on its surface. When the Na-containing oxide forms a spherical polycrystalline particle, the bending angle is decreased due to the spherical shape, which seems to decrease the sodium ion conduction resistance. Thus, for example, the battery is likely to have improved rate characteristics and a higher reversible capacity. In this application, "spherical particle" means a particle having a degree of circularity of 0.80 or higher. The degree of circularity of the particle may be 0.81 or higher, 0.82 or higher, 0.83 or higher, 0.84 or higher, 0.85 or higher, 0.86 or higher, 0.87 or higher, 0.88 or higher, 0.89 or higher, or 0.90 or higher. The degree of circularity of the particle is defined by 4πS / L 2. Here, S is the orthogonal projection surface of the particle and L is the circumference of an orthogonal projection image of the particle. The degree of circularity of the particle can be obtained by observing the external appearance of the particle under a scanning electron microscope (SEM), a transmission electron microscope (TEM), or an optical microscope.1.2 Chemical CompositionThe Na-containing oxide contained in the positive electrode active material includes, as constituents, at least: Na; Ca; at least one transition metal element of Mn, Ni, and Co; and O. In particular, when the Na-containing oxide includes, as constituents, at least Na; Ca; Mn; one or both of Ni and Co; and O, in particular, when it includes, as constituents, at least Na, Ca, Mn, Ni, and O, higher performance is likely to be obtained. Or even when the Na-containing oxide includes, as constituents, at least Na, Ca, Mn, Fe, and O, higher performance is likely to be obtained.The amount of Na contained in the Na-containing oxide is not particularly limited, and may be, for example, 0.60 mol or more (0.60≤Na / O 2), 0,65 mol or more, or 0.70 mol or more and 1.40 mol or less, 1.30 mol or less, 1.20 mol or less, 1.10 mol or less, 1.00 mol or less, 0.90 mol or less, or less than 0.80 mol per mol of the Na-containing oxide. In other words, the Na-containing oxide may include, based on one mole of O, 0.30 mole or more, 0.325 mole or more, or 0.35 mole or more of Na, and 0.70 mole or less, 0.65 mole or less, 0.60 mole or less, 0.55 mole or less, 0.50 mole or less, 0.45 mole or less, or less than 0.40 mole of Na. In particular, when the amount of Na contained in the Na-containing oxide is 0.65 mol or more and less than 0.80 mol per mol of the Na-containing oxide, a high capacity is likely to be ensured.The Na-containing oxide includes at least Ca as an additive element. Na layers in the P2-type structure of the Na-containing oxide may be doped with Ca. Conventional Na-containing oxides have a problem that the Na layers in the P2-type structure break down after desorption of Na, and the amounts of Na that can be subsequently introduced and desorbed decrease, resulting in deterioration of cycle characteristics. As far as the inventor has found, increasing the amount of Na contained in the Na-containing oxide did not always result in a sufficient capacity. As a solution, in this embodiment, the Na-containing oxide includes Ca, and since this Ca functions as a support, breakdown of the Na layers is likely to be mitigated even after desorption of Na, and thus excellent cycle characteristics are likely to be ensured. Since this Ca functions as a support and weakens the breakdown of the Na layers, the amount of Na to be inserted and desorbed increases, and thus a higher capacity can be secured. The amount of Ca contained in the Na-containing oxide is not particularly limited, and can be adjusted appropriately according to the desired performance of the active material. For example, the amount of Ca contained in the Na-containing oxide per mol of the Na-containing oxide may be more than 0 mol and 0.090 mol or less (0<Ca / O 2≤0.090), more than 0 mol and 0.085 mol or less, more than 0 mol and 0.080 mol or less, more than 0 mol and 0.075 mol or less, more than 0 mol and 0.070 mol or less, more than 0 mol and 0.065 mol or less, more than 0 mol and 0.060 mol or less, more than 0 mol and 0.055 mol or less, more than 0 mol and 0.050 mol or less, more than 0 mol and 0.045 mol or less or more than 0 mol and 0.040 mol or less. In particular, when the amount of Ca contained in the Na-containing oxide is more than 0 mol and 0.075 mol or less (0<Ca / O 2≤0.075) per mol of the Na-containing oxide, a higher capacity and excellent cycle properties are likely to be ensured.The Na-containing oxide includes at least one or more of Mn, Ni, and Co as transition metal elements. In particular, when the Na-containing oxide contains, as transition metal elements, at least Mn and one or both of Ni and Co, particularly when it contains at least Mn and Ni, higher performance is likely to be obtained. The amount of the transition metal element contained in the Na-containing oxide is not particularly limited as long as the P2structure can be maintained.The Na-containing oxide may have a chemical composition expressed by Na a-2p Ca p Mnx-αNiy-βCoz-γMα+β+γO 2 (wherein 0.8≤a≤1.4, 0<p≤0.090, x+y+z=1.0, 0≤α+β+γ≤0.20, and M is at least one type selected from B, Mg, Al, K, Ti, V, Cr, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W). In this chemical composition, a is 0.8 or more and 1.4 or less, and may be 1.3 or less, 1.2 or less, 1.1 or less, or 1.0 or less. The value of p is greater than 0 and 0.090 or less, and may be 0.085 or less, 0.080 or less, 0.075 or less, 0.070 or less, 0.065 or less, 0.060 or less, 0.055 or less, 0.050 or less, 0.045 or less, or 0.040 or less. The value of x is 0 or more, 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 1.00 or less, and may be 0.90 or less, 0.80 or less, or 0.70 or less. The value of y is 0 or more, and may be 0.10 or more, 0.20 or more, or 0.30 or more, and is 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, or 0.40 or less. The value of z is 0 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, 0.20 or less, or 0.10 or less. The additive element M has a function of stabilizing the P2-type structure (e.g., a function of attenuating elution of transition metal elements) by being inserted into transition metal layers in the P2-type structure. In the chemical composition described above, particularly high capacity is likely when α+β+γ is 0 or more and 0.20 or less. The value of α+β+γ may be 0.01 or more, 0.02 or more, 0.03 or more, 0.04 or more, or 0.05 or more, and may be 0.19 or less, 0.17 or less, 0.15 or less, 0.13 or less, 0.11 or less, 0.10 or less, 0.09 or less, 0.07 or less, 0.06 or less, 0.05 or less, 0.04 or less, or 0.03 or less. Or, the Na-containing oxide may have a chemical composition expressed by Na a-p Ca p Mn x Ni y Co z O 2 (i.e., a chemical composition in which the aforementioned α + β + γ is 0). While the composition ratio of O is almost 2, it is not limited to exactly 2.0 and is variable.1.3 Otherwise,The positive electrode active material according to an embodiment may be composed of only the above-described Na-containing oxide or may be a combination of the above-described Na-containing oxide and another active material. The positive electrode active material according to an embodiment may be, for example, solid particles or hollow particles or particles having voids. While the size of the particles of the positive electrode active material is not particularly limited, a smaller size is 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 larger and 10 μm or smaller, 1.0 μm or larger and 8.0 μm or smaller or 2.0 μm or larger and 6.0 μm or smaller. The average particle diameter (D50) is a particle diameter (D50, an average diameter) at an integrated value of 50% in a volume-based particle size distribution obtained by a laser diffraction scattering method.2. Production Method of Positive Electrode Active MaterialAs shown in FIG. 1, a manufacturing method of the positive electrode active material according to an embodiment includes:S1: obtaining a solid mixture comprising a precursor, a Na source, and a Ca source; andS2: Obtaining an Na-containing oxide having the P2-type structure by firing the solid mixture.Here, the precursor includes at least one transition metal element of Mn, Ni, and Co.2.1 S1In S 1, the precursor may be a salt comprising at least one element of Mn, Ni, and Co. For example, the precursor may be at least one type of carbonate, sulfate, nitrate and acetate. Or, the precursor may be a compound other than salts. 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 forms. For example, the precursor may have a particulate form or be a spherical particle as described later. The particle diameter of the particle formed by the precursor is not particularly limited. The composition of the precursor can be determined to correspond to the composition of the Na-containing oxide which is a final product.In S 1, the precursor may be obtained as follows: Using an ion source capable of forming a precipitate with a transition metal ion in an aqueous solution and a transition metal compound including at least one transition metal element of Mn, Ni, and Co, a precipitate may be obtained as the precursor by a co-precipitation method. Thus, a spherical particle is likely to be obtained as a precursor. An "ion source capable of forming a precipitate with a transition metal ion in an aqueous solution" may be, for example, at least one kind selected from a sodium salt such as sodium carbonate or sodium nitrate, sodium hydroxide, and sodium oxide. The transition metal compound may be the aforementioned salt, hydroxide, etc. comprising at least one element of Mn, Ni and Co. Specifically, in S 1, a precipitate can be obtained as a precursor by converting the ion source and the transition metal compound into a solution, respectively, and dropping and mixing these solutions with each other. In this case, water is used as the solvent, for example. In this case, various sodium compounds may be used as the base, and an aqueous ammonia solution or the like may be added to adjust basicity. In the case of the co-precipitation method, a precipitate as a precursor can be obtained, for example, by preparing an aqueous solution of the transition metal compound and an aqueous solution of sodium carbonate and dropping and mixing these solutions together. Or, the precursor may also be obtained by a sol-gel method.In S 1, the Na source may be any compound including Na. The Na source may be, for example, a Na salt such as carbonate or sulfate or a Na compound such as sodium oxide or sodium hydroxide. In one embodiment, the Na source may be sodium carbonate.In S 1, the Ca source may be any compound including Ca. The Ca source can be, for example, a Ca salt such as carbonate or sulfate or a Ca compound such as calcium oxide or calcium hydroxide. In one embodiment, the Ca source may be calcium oxide and / or calcium hydroxide.In S 1, at least the above-described precursor, the Na source, and the Ca source are mixed together to obtain a solid mixture comprising them. The means for mixing the precursor, the Na source, and the Ca source are not particularly limited, and they may be mixed together manually using a mortar or the like, or may be mixed together mechanically using various mixing apparatuses. In S 1, the mixing ratio of the precursor, the Na source, and the Ca source may be appropriately determined according to the composition of the Na-containing oxide that is the final product. For example, the amount of the Na source to be mixed relative to the precursor may be determined in consideration of an amount of Na lost during the subsequent firing.2.2 S2In S2, the solid mixture obtained by S1 is fired to obtain an Na-containing oxide having the P2-type structure. That is, in this embodiment, the Na-containing oxide having the P2-type structure can be obtained by a so-called solid phase method. In S2, the solid mixture may optionally be shaped and optionally prebaked before being subjected to the main bake.In S2, the molding method of the solid mixture is not particularly limited. The solid mixture can be formed into pellets by well-known forming methods.In S 2, the pre-firing of the solid mixture may be performed at a temperature not higher than that of the main firing. The pre-firing can be carried out, for example, at a temperature of less than 700° C. The duration of the pre-firing is not particularly limited. Or the pre-burn may be skipped.In S 2, the main firing of the solid mixture may be performed at a temperature of, for example, 700° C. or higher and 1100° C. or lower. The temperature is preferably 800° C. or more and 1000° C. or less. If the main firing temperature is too low, the P2-type structure is not sufficiently formed, while if the main firing temperature is too high, a crystal structure other than the P2-type structure (e.g., an O3-type structure) is formed. The temperature increase condition from the pre-firing temperature to the main firing temperature is not particularly limited. The duration of the main firing is also not particularly limited, and may be, for example, 30 minutes or longer and 10 hours or shorter. The main combustion atmosphere is also not particularly limited, and may be, for example, an oxygen-containing atmosphere such as an ambient air atmosphere or an inert gas atmosphere.3. A battery batteryA battery according to an embodiment includes the above-described positive electrode active material of the invention. The positive electrode active material of the invention can be used, for example, as a positive electrode active material of a sodium ion battery. As shown in FIG. 2, a 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, the positive electrode active material layer 10 including the positive electrode active material of the invention. 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 water-containing battery. An all solid state battery refers to a battery that includes a solid electrolyte and in which the presence of liquid is allowed. The battery 100 may be a solid state battery that includes substantially no liquid. The configuration of the battery may be the same as the conventional one except that the positive electrode active material of the invention is used. Detailed description is omitted here.While an embodiment of the positive electrode active material etc. of the invention has been described above, various changes can be made to the positive electrode active material etc. of the invention within such a range that no departure from the spirit of the invention occurs. Hereinafter, the technology of the invention will be described in more detail by way of Examples, and the technology of the invention is not limited to the following Examples.1. Preparation of positive electrode active material1.1 Co-precipitation synthesis of precursorMnSO 4 ·5H 2 O and NiSO 4 ·6H 2 O were weighed at a target composition ratio and dissolved in distilled water to a concentration of 1.2 mol / L to obtain a first liquid. In a separate container, Na 2 CO 3 was dissolved in distilled water to a concentration of 1.2 mol / L to obtain a second liquid. Subsequently, into a reaction vessel to which 1000 ml of pure water had been previously added, 500 ml each of the first liquid and the second liquid was dropped at a rate of about 4 ml / min. After completion of the dropping, stirring was carried out for one hour at room temperature and a stirring speed of 150 U / min. The resulting precipitate was washed with pure water and separated into a solid and a liquid by a centrifugal separator. The resulting precipitate was dried overnight at 120°C and pulverized in a mortar, and fine particles were removed by air flow classification. Thus, a precursor comprising Mn and Ni (Mn 0,66 Ni 0,34 CO 3) was obtained.1.2 Mixing of precursor, Na source and Ca source1.2.1 Examples 1 to 4The precursor described above, NaCO 3 as the Na source and Ca(OH) 2 as the Ca source, were mixed in a mortar to obtain a solid mixture.1.2.2 Comparative Examples 1 and 2The precursor described above and NaCO 3 as a Na source were mixed in a mortar to obtain a solid mixture.1.3 Firing the solid mixtureThe firing of the solid mixture was carried out in an electric furnace using an alumina crucible in an environment of ambient air (humidity 50% or higher). Specifically, the solid mixture was formed into pellets and then subjected to a "first temperature raising step", a "pre-firing step", a "second temperature raising step", a "main firing step", and an "in-furnace cooling step" as shown in Table 1 below. Thereafter, the fired object was taken out of the electric furnace at 250° C. and pulverized in a mortar in a dry atmosphere having a dew point of -30° C. or less. Thus, an Na-containing oxide having the P2-type structure was obtained. Table 1 Table 1First Temperature Increasing Step256001155Pre-firing step6006003600Second Temperature Increasing Step6009001003Main Firing Step900900600Cooling step in the furnace90025013052. Determination of Chemical Composition of Positive Electrode Active MaterialThe chemical compositions of the positive electrode active materials of Examples 1 to 4 and Comparative Examples 1 and 2 were determined by ICP analysis. The following Table 2 shows their respective chemical compositions. Table 2 Table 2Example 1Na 0,8-2x Ca x Mn 0,66 Ni 0,34 O 2x=0,025Example 2Na 0,8-2x Ca x Mn 0,66 Ni 0,34 O 2x=0,045Example 3Na 0,8-2x Ca x Mn 0,66 Ni 0,34 O 2x=0,075Example 4Na 0,8-2x Ca x Mn 0,66 Ni 0,34 O 2x=0,085Comparative Example 1Na 0,8 Mn 0,66 Ni 0,34 O 2x=0Comparative Example 2Na 0,7 Mn 0,66 Ni 0,34 O 2x=03. Electrochemical Measurement3.1 Capacity Maintenance Rate (Cycle Characteristic)The above-described positive electrode active material, PVdF as a binder, and carbon as a conductivity auxiliary were mixed in a mass ratio of positive electrode active material:PVdF:carbon=85:5:10 and dispersed in N-methyl-2-pyrrolidone to obtain a slurry. This slurry was applied to an Al foil, and then this Al foil was pressed and vacuum-dried at 120° C. overnight to obtain a positive electrode. A coin cell was prepared using this positive electrode, a metal Na foil as a counter electrode, and a 1MNaPF 6 PC solution as an electrolyte solution. In a thermostat bath maintained at 25° C., the coin cell was charged and discharged in a voltage range of 2.0 to 4.5 V, performed at a discharge rate of 0.1 C., and the discharge capacity retention rates in the second to fifth cycles were measured with reference to the discharge capacity in the first cycle (100%). The result is shown in the following Table 3. Table 3 Table 3First Cycle100100100100100Second cycle96,294,497,599,298,2Third Cycle90,390,993,596,895,8Fourth cycle84,287,288,194,694,2Fifth cycle78,983,682,992,292,7From the results shown in Table 3, it can be seen that cycle characteristics are remarkably improved when the Na-containing oxide having the P2-type structure is doped with Ca. This is presumably because when the Na-containing oxide having the P2-type structure is doped with Ca, this Ca functions as a support, thereby preventing the Na layers from collapsing and maintaining the P2-type structure even after desorption of Na, resulting in improved cycle characteristics.3.2 Discharge Capacity in the First CycleIn a thermostat bath maintained at 25°C, the coin cell was charged and discharged in a voltage range of 2.0 to 4.5 V at a rate of 0.1 C, and the discharge capacity in the first charging and discharging cycle was measured. The result is shown in Table 4 below. Table 4 Table 4Example 1CaA0,025143,00Example 2CaA0,045148,42Example 3CaA0,075124,90Example 4CaA0,085105,80Comparative Example 1none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none0122,45Comparative Example 2none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none none0138,75From the results in Table 4, the following can be deduced. First, considering the result of the case where no Ca doping is performed (Comparative Examples 1 and 2), it is found that the positive electrode active material in which the Na content in the Na-containing oxide is 0.8 (Comparative Example 1) has a smaller capacity than the positive electrode active material in which the Na content in the Na-containing oxide is 0.7 (Comparative Example 2). One possible explanation of this is that as the content of Na in the Na-containing oxide increases, the Na-O bond becomes stronger and the layer pitch of the Na layers in the P2-type structure becomes shorter, which affects the insertion and desorption of Na. Another possible explanation is that the P2-type structure breaks down after desorption of Na, which affects the introduction of Na. On the other hand, the positive electrode active materials according to Examples 1 to 3 have higher capacitances than the positive electrode active material according to Comparative Example 1. this is presumably because, since the Na-containing oxide was doped with Ca, the Ca functioned as a column and stabilized the P2-type structure as described above, whereby the amount of Na to be desorbed increased and thus a higher capacitance was ensured. While the positive electrode active material according to Example 4 has excellent cycle characteristics as described above, the capacitance was found to be low even due to the smaller amount of Na contained in the positive electrode active material. In this regard, it can be said that when a high capacity is to be secured in addition to excellent cycle properties, it is preferable to set the amount of Na contained in the Na-containing oxide to 0.65 mol or more and less than 0.80 mol (0.65≤Na / O 2< 0,80) per mol of the Na-containing oxide, and set the amount of Ca contained in the Na-containing oxide to more than 0 mol and 0.075 mol or less (0<Ca / O 2≤0.075) per mol of the Na-containing oxide.4. Confirmation of Ca ImpurityIn each of the positive electrode active materials of Examples 1 to 4 and Comparative Example 1, the distance between the layers of the Na layers in the P2-type structure was calculated by Rietveld analysis. The result is shown in FIG. 3. As shown in FIG. 3, the distance between the layers of the Na layers in the P2structure was the largest in Comparative Example 1 in which no Ca doping was performed, and became smaller as the Ca doping amount increased. Here, it is assumed that the transition metal layers in the P2-type structure cannot be easily doped with Ca because the ionic radius of Ca 2+ is larger than the ionic radius of the transition metal element. That is, it is considered that in Examples 1 to 4, the Na layers in the P2-type structure of the Na-containing oxide were doped with Ca, and that since Ca functioned as a column, breakdown, etc. of the Na layers were mitigated to achieve the results described above.5. Conclusion: ConclusionWhile the positive electrode active materials having the specific chemical compositions have been illustrated in the above-described examples, the chemical composition of the positive electrode active material is not limited to those described above. In view of the results of the above-described examples, a positive electrode active material satisfying the following requirements (1) to (3) is considered to be a material having excellent cycle characteristics.(1) The positive electrode active material comprises an Na-containing oxide.(2) The Na-containing oxide has a P2-type structure.(3) The Na-containing oxide includes, as constituents, at least: Na; Ca; at least one transition metal element of Mn, Ni, and Co; and O.References included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedJP 2023-182443

[0002] JP 2023-182443 A

[0002] Cited Non-Patent LiteratureJ. Am. Chem. Soc. 2020, 142, 5742-5750

[0002]

Claims

A positive electrode active material comprising an Na-containing oxide, wherein the Na-containing oxide has a P2-type structure, and wherein the Na-containing oxide contains, as constituents, at least: Na; Ca; at least one transition metal element of Mn, Ni, and Co; and O.The positive electrode active material according to claim 1, wherein the amount of Na contained in the Na-containing oxide is 0.65 mol or more and less than 0.80 mol per mol of the Na-containing oxide, and wherein the amount of Ca contained in the Na-containing oxide is more than 0 mol and 0.075 mol or less per mol of the Na-containing oxide.The positive electrode active material according to claim 1, wherein the Na-containing oxide comprises, as constituents, at least Na, Ca, Mn, Ni, and O.A method for producing a positive electrode active material, comprising: obtaining a solid mixture containing a precursor, a Na source, and a Ca source; and obtaining a Na-containing oxide having a P2-type structure by firing the solid mixture, wherein the precursor contains at least one transition metal element of Mn, Ni, and Co.A battery comprising a positive electrode active material layer, an electrolyte layer, and a negative electrode active material layer, wherein the positive electrode active material layer comprises the positive electrode active material according to any one of claims 1 to 3.

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