Active material secondary particles and method for producing active material secondary particles

Abstract

This reduces the resistance of the positive electrode active material having a P2 type structure. [Solution] The active material secondary particles of the present disclosure include a plurality of primary particles, each of which has a P2 type structure, and each of which contains at least Na, a first doping element, a transition metal element, a second doping element, and O as constituent elements, wherein the first doping element is Ca, the first doping element is included in the Na layer in the P2 type structure, the transition metal element includes one or both of Mn and Ni, the second doping element is one or more of B, Mg, and Al, the second doping element is included in the transition metal element layer in the P2 type structure, and the average particle diameter of the primary particles is 2.0 μm or less.

Description

[Technical Field]

[0001] This application discloses active material secondary particles and a method for producing active material secondary particles. [Background technology]

[0002] Patent Document 1 discloses a positive electrode active material used in sodium-ion batteries, which has a P2-type structure. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2023-182443 [Overview of the project] [Problems that the invention aims to solve]

[0004] Conventional positive electrode active materials with a P2-type structure have room for improvement in terms of resistance. [Means for solving the problem]

[0005] This application discloses several embodiments as means for solving the above-mentioned problems. <Aspect 1> Active material secondary particles comprising a plurality of primary particles, Each of the primary particles has a P2-type structure, Each of the aforementioned primary particles has as a constituent element, Na and First doped element and, Transition metal elements, Second doped element and, O and It includes at least, The first doped element is Ca, The first doping element is contained in the Na layer in the P2-type structure, The transition metal element includes one or both of Mn and Ni. The second doping element is one or more of B, Mg, and Al. The second doped element is included in the layer of transition metal elements in the P2-type structure, The average particle diameter of the primary particles is 2.0 μm or less. Active material secondary particles. <Aspect 2> Active material secondary particles of embodiment 1, Each of the aforementioned primary particles Na a-2b Ca b Mn x Ni y A z O2 Here, 0 <a<0.80 0 <b≦0.08 0.50 ≤ x ≤ 0.70 0.30 ≤ y ≤ 0.50 0 <z≦0.20 A is one or more of B, Mg, and Al. Having the chemical composition shown, Active material secondary particles. <Aspect 3> Active material secondary particles according to embodiment 1 or 2, The molar ratio of the first doping element to the O contained in the primary particle (first doping element / O) is greater than 0 and less than or equal to 0.04. The molar ratio of the second doping element to the O contained in the primary particle (second doping element / O) is greater than 0 and less than or equal to 0.10. Active material secondary particles. <Aspect 4> Active material secondary particles according to any of embodiments 1 to 3, The second doping element is Mg. Active material secondary particles. <Aspect 5> A method for producing secondary particles of an active material, Precursor particles are obtained by coprecipitation. The aforementioned precursor particles, a Na compound, a first doped element compound, and a second doped element compound are mixed to obtain a mixture, and Firing the mixture to obtain active material secondary particles containing a plurality of primary particles. and includes the precursor particles contain one or both of Mn and Ni, the first doping element compound is a Ca compound, the second doping element compound is a compound containing one or more of B, Mg, and Al, each of the primary particles has a P2-type structure, the average particle diameter of the primary particles is 2.0 μm or less. Method for producing active material secondary particles.

Advantages of the Invention

[0006] The active material secondary particles of the present disclosure have low resistance.

Brief Description of the Drawings

[0007] [Figure 1] An example of the process flow of the method for producing active material secondary particles is shown. [Figure 2] An example of the configuration of a battery is schematically shown. [Figure 3] SEM images of the positive electrode active materials of Example 1 and Comparative Examples 1 to 3.

Modes for Carrying Out the Invention

[0008] 1. Active Material Secondary Particles The active material secondary particles according to one embodiment include a plurality of primary particles. Each of the primary particles has a P2-type structure. Each of the primary particles contains at least Na, a first doping element, a transition metal element, a second doping element, and O as constituent elements. The first doping element is Ca. The first doping element is included in the Na layer in the P2-type structure. The transition metal element includes one or both of Mn and Ni. The second doping element is one or more of B, Mg, and Al. The second doping element is included in the transition metal element layer in the P2-type structure. The average particle diameter of the primary particles is 2.0 μm or less.

[0009] 1.1 Crystal structure The primary particles contained in the active material secondary particles have at least a P2-type structure (belonging to space group P63mc) as a crystalline structure. The primary particles may have a P2-type structure as well as other crystalline structures. Examples of crystalline structures other than the P2-type structure include various crystalline structures (such as the P3-type structure) formed when Na is deintercalated from the P2-type structure. The primary particles may have a P2-type structure as their main phase. The crystalline structure of the main phase of the primary particles may change depending on the charge-discharge state. The active material secondary particles may be aggregates of multiple single-crystal particles having a P2-type structure, or aggregates of multiple polycrystalline particles having a P2-type structure. The P2-type structure is hexagonal, has a large diffusion coefficient for Na ions, and is prone to crystal growth in a specific direction. Therefore, crystallites having a P2-type structure may have a crystal growth direction biased in a specific direction (for example, plate-like). In this case, the ends of the P2-type crystallites (the ends in the crystal growth direction mentioned above) can become the inlet and outlet for intercalation.

[0010] 1.2 Chemical composition The primary particles contained in the active material secondary particles contain at least Na, a first doping element, a transition metal element, a second doping element, and O as constituent elements. In other words, the primary particles are a composite oxide containing at least Na, a first doping element, a transition metal element, a second doping element, and O as constituent elements. The first doping element is Ca. The first doping element is contained in the Na layer in the P2 type structure described above. The transition metal element contains either or both Mn and Ni. In particular, higher performance is more likely to be obtained when the transition metal element contains both Mn and Ni. The second doping element is one or more of B, Mg, and Al. The second doping element is contained in the transition metal layer in the P2 type structure described above. The primary particles may also contain a third doping element different from the first and second doping elements as constituent elements. The type of the third doping element is not particularly limited as long as the P2 type structure is maintained.

[0011] Primary particles contain Na as a constituent element. The amount of Na contained in primary particles is not particularly limited as long as the P2 type structure is maintained. For example, the molar ratio of Na to O contained in primary particles (Na / O) 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, or 0.60 or more, and may be 1.40 or less, 1.30 or less, 1.20 or less, 1.10 or less, 1.00 or less, 0.90 or less, or less than 0.80. In particular, when the molar ratio of Na to O contained in primary particles is greater than 0 and less than 0.80, especially when it is 0.60 or more and less than 0.80, a high volume is easily secured.

[0012] The primary particles contain Ca as the first doping element. The first doping element, Ca, can be doped into the Na layer of the P2-type structure described above. Conventional P2-type Na-containing composite oxides have the problem that after Na detachment, the Na layer of the P2-type structure collapses, reducing the amount of subsequent Na de-insertion and de-insertion, and thus degrading the cycle characteristics. Furthermore, as far as the inventors have confirmed, even if the amount of Na contained in the Na-containing oxide is increased, a sufficient capacity may not be obtained. In contrast, in this embodiment, by including Ca in the Na layer of the P2-type structure, the Ca functions as a pillar, making it easier to suppress the collapse of the Na layer even after Na detachment, and as a result, excellent cycle characteristics are easily ensured. In addition, because the collapse of the Na layer is suppressed by the Ca functioning as a pillar, the amount of Na de-insertion and de-insertion increases, and as a result, a high capacity can be ensured. The amount of the first doping element contained in the primary particles is not particularly limited and can be adjusted as appropriate according to the performance of the target active material. For example, the molar ratio of the first doped element to the oxygen contained in the primary particles (first doped element / O) may be greater than 0 and less than or equal to 0.10, greater than 0 and less than or equal to 0.08, greater than 0 and less than or equal to 0.06, or greater than 0 and less than or equal to 0.04. In particular, when the molar ratio of the first doped element to the oxygen contained in the primary particles (first doped element / O) is greater than 0 and less than or equal to 0.04, it is easier to secure a higher capacity along with excellent cycle characteristics.

[0013] The primary particles contain, as transition metal elements, one or both of Mn and Ni. In particular, when the primary particles contain both Mn and Ni as transition metal elements, higher performance is more likely to be obtained. The transition metal element contained in the primary particles may be only one or both of Mn and Ni. The amount of the transition metal element contained in the primary particles is not particularly limited as long as the P2-type structure is maintained. For example, the molar ratio of the transition metal element to O (transition metal element / O) contained in the primary particles may be 0.40 or more and 0.60 or less, or 0.45 or more and 0.55 or less.

[0014] The primary particles contain, as the second doping element, one or more of B, Mg, and Al. The second doping element is doped into the layer of the transition metal element having the above-described P2-type structure. By including the second doping element in the layer of the transition metal element having the P2-type structure, the P2-type structure is stabilized, elution of the transition metal element during desorption of Na, etc. is suppressed, and as a result, a high capacity is easily ensured. In particular, since Mg is likely to combine with O to form an octahedral structure like Mn, Ni, etc., it is considered that it can be appropriately doped by the layer of the transition metal element having the P2-type structure, and elution of the transition metal element during desorption of Na can be more appropriately suppressed. Therefore, when the second doping element contained in the primary particles is Mg, excellent cycle characteristics are easily ensured together with a high capacity. The amount of the second doping element contained in the primary particles is not particularly limited and may be appropriately adjusted according to the intended active material performance. For example, the molar ratio of the second doping element to O (second doping element / O) contained in the primary particles may be more than 0 and 0.20 or less, more than 0 and 0.18 or less, more than 0 and 0.16 or less, more than 0 and 0.14 or less, more than 0 and 0.12 or less, or more than 0 and 0.10 or less. In particular, when the molar ratio of the second doping element to O (second doping element / O) contained in the primary particles is more than 0 and 0.10 or less, a higher capacity is easily ensured together with excellent cycle characteristics.

[0015] The primary particles are Na a-2b Ca b Mn x Ni y A zIt may have a chemical composition represented by O2 (where 0 < a < 0.80, 0 < b ≤ 0.08, 0.50 ≤ x ≤ 0.70, 0.30 ≤ y ≤ 0.50, 0 < z ≤ 0.20, and A is one or more of the second doping elements, namely B, Mg, and Al). In this chemical composition, a is greater than 0 and less than 0.80, 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. Also, b is greater than 0 and less than or equal to 0.08, 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, or 0.07 or more. Also, x is 0.50 or more and 0.70 or less, and may be 0.55 or more, 0.60 or more, or 0.65 or more. Also, y is 0.30 or more and 0.50 or less, and may be 0.45 or less, 0.40 or less, or 0.35 or less. Also, z is greater than 0 and less than or equal to 0.20, and may be 0.01 or more, 0.03 or more, 0.05 or more, 0.07 or more, or 0.09 or more, and may be 0.19 or less, 0.17 or less, 0.15 or less, 0.13 or less, or 0.11 or less. x + y + z may be, for example, 1. The composition of O is 2, but it is not necessarily exactly 2.0.

[0016] 1.3 Average particle diameter of primary particles In the active material secondary particles according to one embodiment, the average particle diameter of the primary particles is 2.0 μm or less. The average particle diameter of the primary particles may be 1.9 μm or less, 1.8 μm or less, 1.7 μm or less, 1.6 μm or less, or 1.5 μm or less. The lower limit of the average particle diameter of the primary particles is not particularly limited and may be greater than 0 μm, 0.1 μm or more, 0.2 μm or more, 0.3 μm or more, 0.4 μm or more, 0.5 μm or more, 0.6 μm or more, 0.7 μm or more, 0.8 μm or more, 0.9 μm or more, 1.0 μm or more, 1.1 μm or more, 1.2 μm or more, 1.3 μm or more, 1.4 μm or more, or 1.5 μm or more. The active material secondary particles according to one embodiment can be described as aggregates of fine primary particles with an average particle diameter of 2.0 μm or less. Thus, it is believed that when multiple fine primary particles come into close proximity to each other to form secondary particles, ion conduction paths are more easily secured, and resistance is more easily reduced. According to the inventor's new findings, when obtaining a P2-type Na-containing composite oxide containing both the first and second doping elements described above as constituent elements, secondary particles, which are aggregates of multiple fine primary particles, are easily obtained. In this case, each primary particle may be, for example, plate-shaped. If the primary particles do not contain one or both of the first and second doping elements, the primary particles tend to be spherical, and their average particle diameter tends to exceed 2.0 μm. The average particle diameter of the primary particles constituting the active material secondary particles is measured as follows: an external image of the active material secondary particles is obtained using TEM or SEM, and for any 10 or more primary particles included in the external image, the diameter of a circle with an area equivalent to the area obtained from the external image (circular equivalent diameter) is determined, and the arithmetic mean of the circular equivalent diameters of each primary particle is considered as the "average particle diameter of the primary particles".

[0017] 1.4 Others The number of primary particles contained in the active material secondary particles according to one embodiment is not particularly limited. This number may be 10 or more, 20 or more, 30 or more, 40 or more, or 50 or more. The overall shape of the active material secondary particles is also not particularly limited. The active material secondary particles may be non-spherical as a whole. In this application, "non-spherical secondary particles" refers to secondary particles whose circularity, when observed visually, is less than 0.80. The circularity of the secondary particles is 4πS / L. 2 It is defined as follows: Here, S is the orthographic area of ​​the secondary particle, and L is the perimeter of the orthographic image of the secondary particle. The circularity of the secondary particle can be determined by observing the appearance of the particle using a scanning electron microscope (SEM), transmission electron microscope (TEM), or optical microscope. The overall size of the active material secondary particle (secondary particle diameter) is not particularly limited. The secondary particle diameter of the active material secondary particle may be greater than 1.5 μm and less than or equal to 100 μm, greater than 2.0 μm and less than or equal to 100 μm, greater than 2.0 μm and less than or equal to 50 μm, or greater than 2.0 μm and less than or equal to 20 μm. The secondary particle diameter of the active material secondary particle is measured as follows: That is, an image of the appearance of the active material secondary particle is obtained using a TEM or SEM, and for the active material secondary particle included in the appearance image, the diameter of a circle with an area equivalent to the area obtained from the appearance image (equivalent circle diameter) is determined, and this equivalent circle diameter is considered to be the "secondary particle diameter of the active material secondary particle".

[0018] 2. Method for producing secondary particles of active material As shown in Figure 1, a method for producing a positive electrode active material according to one embodiment is: S1: Obtaining precursor particles by coprecipitation method. S2: Mixing the precursor particles, the Na compound, the first doped element compound, and the second doped element compound to obtain a mixture, and S3: The mixture is calcined to obtain active material secondary particles containing a plurality of primary particles. This includes, The aforementioned precursor particles include one or both of Mn and Ni. The first doped element compound is a Ca compound, The second doped element compound is a compound containing one or more of B, Mg, and Al. Each of the aforementioned primary particles has a P2-type structure, The average particle diameter of the primary particles is 2.0 μm or less.

[0019] 2.1 S1 In S1, precursor particles are obtained by coprecipitation. For example, a precipitate as a precursor is obtained by coprecipitation using an ion source capable of forming a precipitate with transition metal ions in aqueous solution and a transition metal compound containing one or both of Mn and Ni. The "ion source capable of forming a precipitate with transition metal ions in aqueous solution" may be 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 a salt or hydroxide containing one or both of Mn and Ni. Specifically, in S1, the precipitate as a precursor may be obtained by preparing solutions of the ion source and the transition metal compound separately, and then adding and mixing each solution dropwise. In this case, water may be used as the solvent. Various sodium compounds may be used as a base, and aqueous ammonia may be added to adjust the basicity. In S1, the precursor particles may be a salt containing at least one of the elements Mn and Ni. For example, the precursor particles may be at least one of carbonates, sulfates, nitrates, and acetates. Alternatively, the precursor particles may be compounds other than salts. For example, the precursor particles may be hydroxides. The precursor particles may be hydrates. The precursor particles may be a combination of multiple types of compounds. The composition of the precursor particles can be appropriately determined to correspond to the composition of the final product, a P2-type Na-containing complex oxide.

[0020] 2.2 S2 In S2, the precursor particles obtained in S1 are mixed with the Na compound, the first doped element compound, and the second doped element compound to obtain a mixture. In S2, the Na compound may be a salt such as a carbonate or sulfate, or a compound other than a salt such as sodium oxide or sodium hydroxide. In one embodiment, the Na compound may be sodium carbonate. In S2, the first doped element compound is a Ca compound. The Ca compound may be a salt such as a carbonate or sulfate, or a compound other than a salt such as calcium oxide or calcium hydroxide. In one embodiment, the Ca compound may be one or both of calcium oxide and calcium hydroxide. In S2, the second doped element compound is a compound containing one or more of B, Mg, and Al. The second doped element compound may be a salt such as a carbonate or sulfate containing one or more of B, Mg, and Al, or a compound other than a salt such as an oxide or hydroxide containing one or more of B, Mg, and Al. In one embodiment, the second doped element compound may be one or both of an oxide and a hydroxide containing one or more of B, Mg, and Al.

[0021] In step S2, a solid mixture containing at least the aforementioned precursor particles, a Na compound, a first doped element compound, and a second doped element compound is obtained by mixing them. The means of mixing the precursor particles, the Na compound, the first doped element compound, and the second doped element compound are not particularly limited; they may be mixed manually using a mortar and pestle, or mechanically using various mixing devices. In step S2, the mixing ratio of the precursor particles, the Na compound, the first doped element compound, and the second doped element compound can be appropriately determined according to the composition of the final product, a P2-type Na-containing composite oxide. For example, the amount of Na compound mixed with the precursor particles can be determined taking into account the amount of Na lost during subsequent calcination.

[0022] 2.3 S3 In step S3, the mixture obtained in step S2 is calcined to obtain active material secondary particles containing multiple primary particles. Each primary particle is a Na-containing composite oxide having a P2-type structure. In other words, in this embodiment, the active material secondary particles can be obtained by a so-called solid-phase method. Step S3 may also involve arbitrarily shaping the mixture, arbitrarily pre-calcining it, and then performing the main calcination. The calcination conditions in step S3 should be such that multiple primary particles with an average particle diameter of 2.0 μm or less aggregate to form active material secondary particles. As described above, because the mixture contains both the first and second doping elements, the active material secondary particles after calcination tend to be aggregates of multiple primary particles with an average particle diameter of 2.0 μm or less. It is thought that the first and second doping elements affect the crystal growth and crystallinity of the P2-type structure.

[0023] In S3, the method for molding the mixture is not particularly limited. The mixture may be molded into pellets by known molding means.

[0024] In step S3, pre-firing of the mixture may be performed at a temperature lower than or equal to the main firing temperature. For example, pre-firing can be performed at a temperature below 700°C. The pre-firing time is not particularly limited. Alternatively, pre-firing may be omitted.

[0025] In S3, the final firing of the mixture may be carried out at a temperature of, for example, 700°C to 1100°C. Preferably, it is 800°C to 1000°C. If the final firing temperature is too low, the P2 type structure will not be sufficiently formed, and if the final firing temperature is too high, crystalline structures other than the P2 type structure (e.g., O3 type structure) are likely to be formed. The heating conditions from the pre-firing temperature to the final firing temperature are not particularly limited. The final firing time is also not particularly limited and may be, for example, 30 minutes to 10 hours. The final firing atmosphere is also not particularly limited and may be, for example, an oxygen-containing atmosphere such as air or an inert gas atmosphere. The cooling conditions after final firing are also not particularly limited.

[0026] 3.Battery A battery according to one embodiment has the active material secondary particles of the present disclosure described above. The active material secondary particles of the present disclosure can be used, for example, as the positive electrode active material of a sodium-ion battery. As shown in Figure 2, a battery 100 according to one embodiment has a positive electrode active material layer 10, an electrolyte layer 20, and a negative electrode active material layer 30, wherein the positive electrode active material layer 10 contains the active material secondary particles 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-system battery. A solid-state battery is a battery that includes a solid electrolyte and may allow the presence of liquid. The battery 100 may be an all-solid-state battery that substantially does not contain liquid. The configuration of the battery may be the same as conventional batteries, except that the active material secondary particles of the present disclosure are used. A detailed explanation is omitted here. [Examples]

[0027] As described above, one embodiment of the active material secondary particles etc. of the present disclosure has been explained, but the active material secondary particles etc. of the present disclosure can be modified in various ways other than the above embodiment without departing from the gist of the disclosure. The technology of the present disclosure will be explained in more detail below with reference to examples, but the technology of the present disclosure is not limited to the following examples.

[0028] 1. Preparation of positive electrode active material 1.1 Coprecipitation synthesis of precursor particles MnSO4·5H2O and NiSO4·6H2O were weighed to the desired composition ratio and dissolved in distilled water to a concentration of 1.2 mol / L to obtain the first solution. In a separate container, Na2CO3 was dissolved in distilled water to a concentration of 1.2 mol / L to obtain the second solution. Next, 500 mL each of the first and second solutions were added dropwise to a reaction vessel pre-filled with 1000 mL of pure water at a rate of approximately 4 mL / min. After the dropwise addition was complete, the mixture was stirred at 150 rpm for 1 hour at room temperature. The precipitate was washed with pure water and solid-liquid separation was performed using a centrifuge. The obtained precipitate was dried overnight at 120°C to obtain precursor particles (Mn) containing Mn and Ni. 0.66 Ni 0.34 CO3 was obtained.

[0029] 1.2 Mixing of precursor particles with Na compounds, etc. 1.2.1 Example 1 The above precursor particles, along with NaCO3 as a Na compound, Ca(OH)2 as a first doping element compound, and Mg(OH)2 as a second doping element compound, were mixed in a mortar to obtain a mixture.

[0030] 1.2.2 Comparative Example 1 The above precursor particles and NaCO3 were mixed in a mortar to obtain a mixture.

[0031] 1.2.3 Comparative Example 2 The above precursor particles, along with NaCO3 and Ca(OH)2, were mixed in a mortar to obtain a mixture.

[0032] 1.2.4 Comparative Example 3 The above precursor particles, along with NaCO3 and Mg(OH)2, were mixed in a mortar to obtain a mixture.

[0033] 1.3 Firing of the mixture The mixture was calcined in an electric furnace using an alumina crucible under atmospheric conditions (humidity of 50% or higher). Specifically, the mixture was formed into pellets, and then the following steps were performed as shown in Table 1: "first heating step," "pre-calcination step," "second heating step," "main calcination step," and "furnace cooling step." After that, the calcined product was removed from the electric furnace at 250°C to obtain a Na-containing oxide having a P2-type structure.

[0034] [Table 1]

[0035] 2. Identification of the chemical composition of the positive electrode active material. The chemical composition of each positive electrode active material in Example 1 and Comparative Examples 1-3 was determined by ICP analysis, etc. The respective chemical compositions are shown in Table 2 below.

[0036] [Table 2]

[0037] 3. Visual inspection of the positive electrode active material and determination of the average primary particle size. The appearance of each positive electrode active material of Example 1 and Comparative Examples 1-3 was observed using a scanning electron microscope (SEM). Figure 3 shows the SEM images of each positive electrode active material of Example 1 and Comparative Examples 1-3. As shown in Figure 3, the positive electrode active materials of Comparative Examples 1-3 have spherical primary particles, and these spherical primary particles are not aggregated with each other. On the other hand, the positive electrode active material of Example 1 has secondary particles formed by the aggregation of fine primary particles. Table 3 below shows the average particle diameter of the primary particles of each positive electrode active material of Example 1 and Comparative Examples 1-3. The method for measuring the average particle diameter of the primary particles is as described in the embodiment.

[0038] [Table 3]

[0039] 3. Electrochemical measurements 3.1 Resistors The above-mentioned positive electrode active material, PVdF as a binder, and carbon as a conductive additive were weighed in a mass ratio of positive electrode active material:PVdF:carbon = 85:5:10, and dispersed and mixed in N-methyl-2-pyrrolidone to obtain a slurry. After coating the slurry onto an Al foil and pressing it, the positive electrode was obtained by vacuum drying overnight at 120°C. A coin cell was fabricated using this positive electrode, a metallic Na foil as a counter electrode, and a 1M NaPF6PC solution as the electrolyte. After one charge-discharge cycle of the coin cell in a constant temperature bath maintained at 25°C at a voltage range of 2.0-4.5V and a rate of 0.1C, impedance measurements were performed. The results are shown in Table 4 below.

[0040] [Table 4]

[0041] The results shown in Table 4 indicate that when Ca and Mg are doped into Na-containing oxides having a P2-type structure, the resistance is significantly reduced. As mentioned above, when Ca and Mg are doped into Na-containing oxides having a P2-type structure, the positive electrode active material becomes secondary particles, which are aggregates of fine primary particles. This is thought to facilitate the establishment of ion conduction paths, thereby reducing resistance.

[0042] 3.2 Discharge capacity in the first cycle Coin cells were charged and discharged in a constant temperature bath maintained at 25°C at a voltage range of 2.0-4.5V and a rate of 0.1C, and the discharge capacity of the first charge / discharge cycle was measured. The results are shown in Table 5 below.

[0043] [Table 5]

[0044] The results shown in Table 5 indicate that when Ca and Mg are doped into a Na-containing oxide having a P2-type structure, the discharge capacity increases significantly. Here, Ca is thought to be contained within the Na layer of the P2-type structure. When Ca is contained within the Na layer of the P2-type structure, it functions as a pillar, making it easier to suppress the collapse of the Na layer even after Na desorption, increasing the amount of Na removed and inserted, and resulting in a higher capacity. Mg is thought to be contained within the transition metal element layer of the P2-type structure. The inclusion of Mg in the transition metal element layer of the P2-type structure stabilizes the P2-type structure, suppressing the dissolution of transition metal elements during Na desorption, and resulting in a higher capacity. In other words, in Example 1, it is thought that the doping of both Ca and Mg resulted in a synergistic effect, significantly improving the discharge capacity.

[0045] 3.3 Capacity retention rate (cycle characteristics) Coin cells were charged and discharged in a constant temperature bath maintained at 25°C at a voltage range of 2.0-4.5V and a rate of 0.1C. The discharge capacity of the first cycle was used as the baseline (100%), and the retention rate of the discharge capacity from the second to fifth cycles was measured. The results are shown in Table 6 below.

[0046] [Table 6]

[0047] The results shown in Table 6 indicate that when Ca and Mg are doped into Na-containing oxides having a P2-type structure, the cycle characteristics are significantly improved. When Ca and Mg are doped into Na-containing oxides having a P2-type structure, as described above, Ca functions as a pillar, suppressing the collapse of the Na layer even after Na detachment, and thus maintaining the P2-type structure appropriately. Furthermore, as described above, Mg stabilizes the P2-type structure, suppressing the dissolution of transition metal elements during Na detachment. As a result, the cycle characteristics are thought to be improved.

[0048] 4. Supplementary information on Ca and Mg doping sites As described above, the positive electrode active material according to Example 1 was obtained by adding Na compound, Ca compound, and Mg compound to precursor particles containing Mn and Ni and then calcining them. In this way, when Na compound and doped element compound are mixed with the precursor particles after obtaining them and then calcined, it can be said that Mn, Ni, and Mg, which have similar ion sizes, constitute a P2-type transition metal layer, and Na and Ca, which have similar ion sizes, constitute a P2-type Na layer. If the precursor particles before being mixed with the Na compound contain doped elements along with Mn and Ni (i.e., when precursor particles containing Mn, Ni, and doped elements are obtained by coprecipitation), then it can be said that the doped elements, along with Mn and Ni, constitute a P2-type transition metal layer. The validity of the above has been confirmed by obtaining the X-ray diffraction pattern of the positive electrode active material and determining the interlayer distance of each layer by performing Rietveld analysis.

[0049] 5. Supplementary information on doping elements other than Mg In the above examples, a positive electrode active material having a specific chemical composition was illustrated, but the chemical composition of the positive electrode active material is not limited to that described above. The inventors have confirmed that a positive electrode active material with excellent cycle characteristics can be obtained even when B or Al is doped instead of Mg as the doping element (Japanese Patent Application No. 2024-129145). In other words, it is believed that similar effects can be obtained in Example 1 above when B or Al is doped instead of Mg, or together with Mg. Furthermore, the molar ratio of Na and the composition ratio of transition metal elements are not limited to those described above.

[0050] 6. Summary Based on the results of the examples, it is believed that the active material secondary particles that satisfy the following requirements (1) to (9) have low resistance, as well as high capacity and excellent cycle characteristics.

[0051] (1) The active material secondary particles contain multiple primary particles. (2) Each of the primary particles has a P2-type structure. (3) Each of the primary particles comprises at least Na, a first doping element, a transition metal element, a second doping element, and O as constituent elements. (4) The first doping element is Ca. (5) The first doping element is contained in the Na layer in the P2-type structure. (6) The transition metal element includes either or both of Mn and Ni. (7) The second doping element is one or more of B, Mg, and Al. (8) The second doped element is included in the transition metal element layer in the P2-type structure. (9) The average particle diameter of the primary particles is 2.0 μm or less. [Explanation of Symbols]

[0052] 10 Cathode active material layer 20 Electrolyte layer 30 Negative Active Material Layer 40 Positive current collector 50 Negative current collector 100 batteries

Claims

1. Active material secondary particles comprising a plurality of primary particles, Each of the primary particles has a P2-type structure, Each of the aforementioned primary particles is a constituent element Na and, First doped element and, Transition metal elements, Second doped element and, O and It includes at least, The first doping element is Ca, The first doping element is contained in the Na layer in the P2 type structure, The transition metal element includes one or both of Mn and Ni. The second doping element is one or more of B, Mg, and Al. The second doped element is included in the layer of transition metal elements in the P2-type structure, The average particle diameter of the primary particles is 2.0 μm or less. Active material secondary particles.

2. The active material secondary particles according to claim 1, Each of the aforementioned primary particles, Na ａ－２ｂ Ca ｂ Mn ｘ Ni ｙ A ｚ O ２ Here, 0 < a < 0.80 0 < b ≤ 0.08 0.50 ≤ x ≤ 0.70 0.30 ≤ y ≤ 0.50 0 < z ≤ 0.20 A is one or more of B, Mg, and Al. Having the chemical composition shown, Active material secondary particles.

3. The active material secondary particles according to claim 1, The molar ratio of the first doped element to the O contained in the primary particle (first doped element / O) is greater than 0 and less than or equal to 0.

04. The molar ratio of the second doping element to the oxygen contained in the primary particle (second doping element / O) is greater than 0 and less than or equal to 0.

10. Active material secondary particles.

4. The active material secondary particles according to claim 1, The second doping element is Mg. Active material secondary particles.

5. A method for producing secondary particles of an active material, Precursor particles are obtained by coprecipitation. A mixture is obtained by mixing the precursor particles, the Na compound, the first doped element compound, and the second doped element compound, and The mixture is calcined to obtain active material secondary particles containing a plurality of primary particles. Includes, The precursor particles include one or both of Mn and Ni. The first doped element compound is a Ca compound, The second doped element compound is a compound containing one or more of B, Mg, and Al. Each of the primary particles has a P2-type structure, The average particle diameter of the primary particles is 2.0 μm or less. A method for producing secondary particles of active material.

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