Secondary particles from active material and method for producing secondary particles from active material

Abstract

A secondary particle of active material according to the present invention comprises a plurality of primary particles. Each of the primary particles has a P2-type structure. Each primary particle contains as components at least Na, a first dopant, a transition metal element, a second dopant, and O. The first dopant is Ca. The first dopant is contained in a Na layer of the P2-type structure. The transition metal element comprises Mn and / or Ni. The second dopant is one or more of B, Mg, and Al. The second dopant is contained in a transition metal layer of the P2-type structure. The primary particles have an average particle size of 2.0 µm or less.

Description

BACKGROUND OF THE INVENTION 1. Field of the invention

[0001] The present application discloses secondary particles made of active material and methods for producing a secondary particle made of active material. 2. Description of the state of the art

[0002] The Japanese unpublished patent application No. 2023-182443 (JP 2023-182443 A) discloses an active cathode material with a P2-type structure for use in sodium-ion batteries. SUMMARY OF THE INVENTION

[0003] Conventional active cathode material with a P2-type structure can be improved with regard to its resistance.

[0004] The present application discloses the following aspects to solve the aforementioned problem. First aspect

[0005] A secondary particle of active material comprises a plurality of primary particles. Each primary particle has a P2-type structure. Each primary particle contains at least the following elements as constituents: Na, a first dopant, a transition metal element, a second dopant, and O.

[0006] The first dopant is Ca. The first dopant is contained in a Na layer of P2-type structure. The transition metal element comprises one or both Mn and Ni. The second dopant is one or more of B, Mg, and Al. The second dopant is contained in a transition metal layer of P2-type structure. The primary particles have a mean particle size of 2.0 µm or less. Second aspect

[0007] In the secondary particle made of active material, or of the active material according to the first aspect, each of the primary particles has a chemical composition that is determined by Na a-2b Approx b Mn x Ni y A z O2 is represented 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 B, Mg and Al. Third aspect

[0008] In the secondary particle of active material according to the first or second aspect, the molar ratio of the first dopant element to O in the primary particles is greater than zero and less than or equal to 0.04, and the molar ratio of the second dopant element to O in the primary particles is greater than zero and less than or equal to 0.10. Fourth aspect

[0009] In the secondary particle made of active material according to one of the first to third aspects, the second dopant element is Mg. Fifth aspect

[0010] Method for producing a secondary particle from active material, comprising: Precursor particles are obtained through coprecipitation; Mixing the precursor particles, a sodium compound, a first dopant compound, and a second dopant compound to obtain a mixture; and Burning the mixture to obtain the secondary particle from active material, which comprises a plurality of primary particles.

[0011] The precursor particles contain Mn and / or Ni.

[0012] The first doping element compound is a Ca compound.

[0013] The second doping element compound is a compound containing one or more of the elements B, Mg and Al.

[0014] Each of the primary particles has a P2-type structure.

[0015] The primary particles have a mean particle size of 2.0 µm or less.

[0016] The secondary particle made of active material of the present invention has a low resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Features, advantages and technical and industrial significance of exemplary embodiments of the invention are described below with reference to the accompanying drawings, in which the same reference numerals denote the same elements, wherein: Fig. 1 shows an example of a process for producing a secondary particle of an active material; Fig. Figure 2 schematically shows an example of a battery configuration; and Fig. Three scanning electron microscope (SEM) images of active cathode materials from Example 1 and comparison examples 1 to 3 are shown. DETAILED DESCRIPTION OF THE EXECUTION FORMS 1. Secondary particles made of active material

[0018] A secondary particle of active material according to one embodiment contains a plurality of primary particles. Each of the primary particles has a P2-type structure. Each primary particle contains as components at least Na, a first dopant, a transition metal element, a second dopant, and O. The first dopant is Ca. The first dopant is contained in a Na layer of the P2-type structure. The transition metal element comprises Mn and / or Ni. The second dopant is one or more of B, Mg, and Al. The second dopant is contained in a transition metal layer of the P2-type structure. The primary particles have an average particle size of 2.0 µm or less. 1.1 Crystal structure

[0019] The primary particles contained in the secondary active material particles have at least one P2-type structure (belonging to the space group P63mc) as their crystal structure. In addition to the P2-type structure, the primary particles may also exhibit a crystal structure other than the P2-type. Examples of crystal structures other than the P2-type include various crystal structures (such as a P3-type structure) that are formed when Na is extracted from or inserted into the P2-type structure. The primary particles may have the P2-type structure as their main phase. The main-phase crystal structure of the primary particles may change depending on the charging or discharging state. The secondary active material particle may be an aggregate of multiple single-crystal particles with the P2-type structure or an aggregate of multiple polycrystalline particles with the P2-type structure.The P2-type structure belongs to the hexagonal crystal system and has a high sodium ion diffusion coefficient. Crystal growth of the P2-type structure tends to proceed along a specific crystallographic direction. Therefore, a crystallite with the P2-type structure may preferentially grow along a particular crystallographic direction (e.g., in the form of a plate). In this case, the end of this crystallite with the P2-type structure (the end in the crystal growth direction) can serve as the entry and exit point for intercalation. 1.2 Chemical composition

[0020] The primary particles contained in the secondary particle of the active material contain as components at least Na, a first dopant, a transition metal, a second dopant, and O. In other words, the primary particles are compound oxides containing as components at least Na, a first dopant, a transition metal, a second dopant, and O. The first dopant is Ca. The first dopant is contained in the Na layer of the P2-type structure. The transition metal includes Mn and / or Ni. In particular, if the transition metal includes both Mn and Ni, higher performance is more likely to be achieved. The second dopant is one or more of B, Mg, and Al. The second dopant is contained in the transition metal layer of the P2-type structure.The primary particles can contain a third dopant element that differs from the first and second dopants. The type of the third dopant element is not particularly restricted, as long as the P2-type structure can be maintained.

[0021] The primary particles contain Na as a constituent. The amount of Na contained in the primary particles is not particularly limited, as long as the P2-type structure is maintained. For example, the molar ratio of Na to O (Na / O) in the primary particles can be greater than zero, 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 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, if the molar ratio of Na to O in the primary particles is greater than zero and less than 0.80, especially if this molar ratio is greater than or equal to 0.60 and less than 0.80, a high capacity is more likely to be achieved.

[0022] The primary particles contain calcium as the first dopant. This first dopant, calcium, can be incorporated into the sodium layer of the P2-type structure. In conventional P2-type sodium-containing composite oxides, the sodium layer of the P2-type structure collapses after sodium is extracted. This leads to a reduction in the amount of subsequent sodium incorporation and extraction, and to a deterioration of the cycle characteristics. Furthermore, the inventors have found that even increasing the amount of sodium in a sodium-containing oxide may not achieve sufficient capacity. In contrast, in the present embodiment, calcium is incorporated into the sodium layer of the P2-type structure, and the calcium acts as a carrier. Therefore, the collapse of the sodium layer is more likely to be reduced even after sodium extraction, making it easier to achieve excellent cycle characteristics.Since calcium acts as a support and reduces the breakdown of the sodium layer, the amount of sodium incorporation and extraction is increased. Consequently, a high capacity can be achieved. The amount of the first dopant contained in the primary particles is not particularly limited and can be appropriately adjusted according to the intended performance of the active material. For example, the molar ratio of the first dopant to oxygen (first dopant / O) in the primary particles can be greater than zero and less than or equal to 0.10, greater than zero and less than or equal to 0.08, greater than zero and less than or equal to 0.06, or greater than zero and less than or equal to 0.04.In particular, if the molar ratio of the first dopant to O (first dopant / O) in the primary particles is greater than zero and less than or equal to 0.04, it is more likely that a higher capacity will be achieved along with excellent cycle properties.

[0023] The primary particles contain Mn and / or Ni as the transition metal element. In particular, if the primary particles contain both Mn and Ni as the transition metal element, higher performance is more likely to be achieved. The transition metal element in the primary particles can be either Mn or Ni, or both. The amount of the transition metal element 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) in the primary particles can be greater than or equal to 0.40 and less than or equal to 0.60, or greater than or equal to 0.45 and less than or equal to 0.55.

[0024] The primary particles contain one or more of the elements B, Mg, and Al as a second dopant. This second dopant is introduced into the transition metal layer of the P2-type structure. Because the second dopant is contained within this transition metal layer, the P2-type structure is stabilized, and the dissolution of the transition metal during sodium extraction is reduced. This increases the likelihood of achieving a high extraction capacity. Since Mg, like Mn or Ni, tends to combine with O to form an octahedral structure, it is considered suitable for doping into the transition metal layer of the P2-type structure, thus advantageously reducing the dissolution of the transition metal during sodium extraction.Therefore, if the second dopant in the primary particles is Mg, high capacity and excellent cycle characteristics are more likely to be achieved. The amount of the second dopant in the primary particles is not particularly limited and can be appropriately adjusted according to the intended performance of the active material. For example, the molar ratio of the second dopant to O (second dopant / O) in the primary particles can be greater than zero and less than or equal to 0.20, greater than zero and less than or equal to 0.18, greater than zero and less than or equal to 0.16, greater than zero and less than or equal to 0.14, greater than zero and less than or equal to 0.12, or greater than zero and less than or equal to 0.10.In particular, if the molar ratio of the second dopant to O (second dopant / O) in the primary particles is greater than zero and less than or equal to 0.10, it is more likely that a higher capacity will be achieved along with excellent cycle characteristics.

[0025] The primary particles can have a chemical composition characterized by Na a-2b Approx b Mn x Ni y A zO2 (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 the second doping element, i.e. one or more of B, Mg and Al).In this chemical composition, a is greater than zero and less than 0.80 and can 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; b is greater than zero and less than or equal to 0.08 and can 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; x is greater than or equal to 0.50 and less than or equal to 0.70 and can be 0.55 or more, 0.60 or more, or 0.65 or more; y is greater than or equal to 0.30 and less than or equal to 0.50 and can be 0.45 or less, 0.40 or less, or 0.35 or less; and z is greater than zero and less than or equal to 0.20, and can be 0.01 or more, 0.03 or more, 0.05 or more, 0.07 or more, or 0.09 or more, and can be 0.19 or less, 0.17 or less, 0.15 or less, 0.13 or less, or 0.11 or less. For example, x + y + z can be 1.The composition of O is 2, but it cannot be exactly 2.0. 1.3 Mean particle size of the primary particles

[0026] In the secondary particle of the active material according to the embodiment, the mean particle size of the primary particles is 2.0 µm or less. The mean particle size of the primary particles can 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 mean particle size of the primary particles is not particularly restricted and can 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 secondary particle of the active material according to the embodiment can be considered an aggregate of fine primary particles with a mean particle size of 2.0 µm or less.It is assumed that the formation of the secondary particle through the close aggregation of a plurality of fine primary particles facilitates the formation of ion conduction pathways and the reduction of resistance. According to a new finding by the inventors, a secondary particle, which is such an aggregate of a plurality of fine primary particles, is more likely to form when a sodium-containing composite oxide of the P2 type is obtained, which contains both the first and second dopants as components. In this case, each of the primary particles can, for example, have the shape of a plate. If the primary particles do not contain both the first and second dopants simultaneously, the primary particles are more spherical and have a mean particle size greater than 2.0 µm. The mean particle size of the primary particles from which the secondary particles of active material are composed is measured as follows.An image of the appearance of the secondary particles made of active material is acquired using a transmission electron microscope (TEM), a scanning electron microscope (SEM), etc. For each of 10 or more arbitrary primary particles included in the image, the diameter of a circle with the same area as the area of ​​that primary particle obtained from the image is determined (equivalent circle diameter). The arithmetic mean of the equivalent circle diameters of the primary particles is considered the "mean particle size of the primary particles." 1.4 Other

[0027] The number of primary particles contained in the secondary active material particle according to the embodiment is not specifically limited. The number of primary particles contained in the secondary active material particle can be 10 or more, 20 or more, 30 or more, 40 or more, or 50 or more. The overall shape of the secondary active material particle is not specifically limited. The overall shape of the secondary active material particle cannot be spherical. In the present application, the term "non-spherical secondary particle" refers to a secondary particle that, when viewed externally, has a roundness of less than 0.80. The roundness of a secondary particle is defined as 4πS / L. 2, where S represents the orthogonal projection area of ​​the secondary particle and L the perimeter of the orthogonal projection image of the secondary particle. The roundness of a secondary particle can be determined by observing the particle's appearance with an SEM, a TEM, or an optical microscope. The overall size of the secondary particle from the active material (secondary particle size) is not particularly restricted. The secondary particle size of the secondary particle from the active material can 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 size of the secondary particle from the active material is measured as follows. An image of the secondary particle's appearance is acquired using a TEM, an SEM, etc.For the secondary particles of active material contained in the image, the diameter of a circle is determined that has the same area as the area of ​​the secondary particles of active material obtained from the image (equivalent circle diameter). This equivalent circle diameter is considered the "secondary particle size of the secondary particles of active material". 2. Method for producing secondary particles from active material

[0028] As in Fig. Figure 1 shows a method for producing a secondary particle from active material according to the embodiment: S1: Obtaining precursor particles through coprecipitation; S2: Mixing the precursor particles, a sodium compound, a first dopant compound, and a second dopant compound to obtain a mixture; and S3: Burning the mixture to obtain the secondary particle from active material, which contains a plurality of primary particles.

[0029] The precursor particles contain Mn and / or Ni.

[0030] The first doping element compound is a Ca compound.

[0031] The second doping element compound is a compound containing B, Mg and / or Al.

[0032] Each of the primary particles has a P2-type structure.

[0033] The mean particle size of the primary particles is 2.0 µm or less. 2.1 S1

[0034] In S1, particles are obtained by coprecipitation. For example, a precursor precipitate is obtained by coprecipitation using an ion source capable of forming a precipitate with transition metal ions in an aqueous solution and a transition metal compound containing Mn and / or Ni. The ion source capable of forming a precipitate with transition metal ions in an aqueous solution can be, for example, at least one of sodium salts such as sodium carbonate and sodium nitrate, sodium hydroxide, and sodium oxide. The transition metal compound can be a salt or hydroxide containing Mn and / or Ni. In particular, in S1, a precursor precipitate can be obtained by preparing a solution of the ion source and a solution of the transition metal compound, and then adding and mixing these solutions dropwise. Water can be used as the solvent.Various sodium compounds can be used as bases, and an aqueous ammonia solution, for example, can be added to adjust the basicity. In S1, the precursor particles can be salts containing at least one of Mn and Ni. For example, the precursor particles can be composed of at least one of carbonates, sulfates, nitrates, and acetates. Alternatively, the precursor particles can be compounds other than salts. For example, the precursor particles can be hydroxides. The precursor particles can be hydrates. The precursor particles can be a combination of several types of compounds. The composition of the precursor particles can be determined according to the composition of the final product, namely a sodium-containing compound oxide of the P2 type. 2.2 S2

[0035] In S2, the precursor particles obtained in S1, a sodium compound, a first dopant compound, and a second dopant compound are mixed to obtain a mixture. In S2, the sodium compound can be, for example, a salt such as a carbonate or a sulfate, or a compound other than a salt such as sodium oxide or sodium hydroxide. In one embodiment, the sodium compound can be sodium carbonate. In S2, the first dopant compound is a calcium compound. For example, the calcium compound can be a salt such as a carbonate or a sulfate, or a compound other than a salt such as calcium oxide or calcium hydroxide. In one embodiment, the calcium compound can be calcium oxide and / or calcium hydroxide. In S2, the second dopant compound is a compound containing one or more of B, Mg, and Al.For example, the second dopant compound can 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 dopant compound can be an oxide and / or a hydroxide containing one or more of the elements B, Mg, and Al.

[0036] In S2, at least the precursor particles, the sodium compound, the first dopant compound, and the second dopant compound are mixed to obtain a solid mixture containing these components. The means for mixing the precursor particles, the sodium compound, the first dopant compound, and the second dopant compound are not particularly limited. These components can be mixed manually using a mortar, etc., or mechanically using various mixing devices. In S2, the mixing ratio of the precursor particles, the sodium compound, the first dopant compound, and the second dopant compound can be appropriately determined according to the composition of the final product, namely the sodium-containing composite oxide of type P2.For example, the amount of Na compound to be mixed with the particles can be determined taking into account the amount of Na loss during the subsequent firing. 2.3 S3

[0037] In step S3, the mixture obtained in step S2 is fired to obtain a secondary active material particle containing a plurality of primary particles. Each of the primary particles is a sodium-containing composite oxide with a P2-type structure. That is, in the present embodiment, the secondary active material particle can be obtained by a so-called solid-phase process. Step S3 can optionally include shaping the mixture and optionally pre-firing before the main firing. The firing conditions in S3 can be any conditions that allow a plurality of primary particles with a mean particle size of 2.0 µm or less to aggregate and form the secondary active material particle. As described above, it is more likely that the secondary active material particle obtained by firing will be an aggregate of a plurality of primary particles with a mean particle diameter of 2.0 µm or less.If the mixture contains both the first and second dopants, the secondary particle of the active material obtained by firing is more likely to be an aggregate of multiple primary particles with a mean particle diameter of 2.0 µm or less. The first and second dopants are thought to influence crystal growth and crystallinity of the P2-type structure.

[0038] In S3, the method for shaping the mixture is not particularly restricted. The mixture can be formed into pellets using known shaping agents.

[0039] In S3, pre-burning of the mixture can be carried out at a temperature lower than or equal to the temperature of the main combustion. For example, pre-burning can be performed at a temperature below 700 °C. The pre-burning time is not particularly limited. Alternatively, pre-burning can be omitted.

[0040] In S3, the main combustion of the mixture can be carried out at a temperature of, for example, 700 °C or higher and 1100 °C or lower. Preferably, the temperature is greater than or equal to 800 °C and less than or equal to 1000 °C. If the main combustion temperature is too low, the P2-type structure may not form sufficiently. If the main combustion temperature is too high, a structure other than the P2-type structure (e.g., an O3-type structure) is more likely to form. The conditions for heating from the pre-combustion temperature to the main combustion temperature are not particularly restricted. The main combustion time is also not particularly restricted and can, for example, be greater than or equal to 30 minutes and less than or equal to 10 hours. The atmosphere for the main combustion is not particularly restricted and can, for example, be an oxygen-containing atmosphere such as air or an inert gas atmosphere.The cooling conditions after the main firing are not particularly restrictive. 3rd battery

[0041] A battery according to one embodiment comprises the aforementioned secondary particle made of active material of the present invention. The secondary particle made of active material of the present invention can, for example, be used as an active cathode material for a sodium-ion battery. As described in Fig.As shown in Figure 2, a battery 100 according to one embodiment comprises a layer 10 of active cathode material, an electrolyte layer 20, and a layer 30 of active anode material. The active material layer 10 contains the secondary active material particle of the present invention. The battery 100 may include a cathode current collector 40 and an anode current collector 50. The battery 100 may be a solid-state battery or a liquid battery. The term "solid-state battery" refers to a battery that comprises a solid electrolyte and may allow the presence of a liquid. The battery 100 may be a solid-state battery that contains essentially no liquid. The configuration of the battery may be the same as that of a conventional battery, except that the secondary active material particle of the present invention is used.A detailed description of this is omitted.

[0042] As described above, one embodiment of the secondary particle made of active material, etc., of the present invention has been described. However, the secondary particle made of active material, etc., of the present invention can be modified in various other ways than in the above embodiment without departing from the concept and scope of the invention. The technology of the present invention is described in more detail below with reference to an example. However, the technology of the present invention is not limited to the following example. 1. Production of the active cathode material 1.1 Coprecipitation synthesis of precursor particles

[0043] MnSO₄·5H₂O and NiSO₄·6H₂O were weighed in a ratio corresponding to a desired composition and dissolved in distilled water to a concentration of 1.2 mol / L to obtain a first solution. In a separate container, Na₂CO₃ was dissolved in distilled water to a concentration of 1.2 mol / L to obtain a second solution. Subsequently, 500 mL of the first solution and 500 mL of the second solution were added dropwise at a rate of approximately 4 mL / min each to a reaction vessel previously filled with 1000 mL of pure water. After completion of the dropwise addition, the resulting mixture was stirred at 150 rpm for one hour at room temperature. The resulting precipitate was washed with pure water and subjected to solid-liquid separation using a centrifuge. The precipitate thus obtained was dried overnight at 120 °C to obtain particles containing Mn and Ni (Mn₂CO₃). 0,66 Ni0,34 CO3). 1.2 Mixing the precursor particles with Na compound, etc. 1.2.1 Example 1

[0044] The aforementioned precursor particles, NaCO3 as the Na compound, Ca(OH)2 as the first dopant compound and Mg(OH)2 as the second dopant compound, were mixed in a mortar to obtain a mixture. 1.2.2 Comparative Example 1

[0045] The aforementioned precursor particles and NaCO3 were mixed in a mortar to obtain a mixture. 1.2.3 Comparative Example 2

[0046] The aforementioned precursor particles, NaCO3 and Ca(OH)2, were mixed in a mortar to obtain a mixture. 1.2.4 Comparative Example 3

[0047] The aforementioned particles, NaCO3 and Mg(OH)2, were mixed in a mortar to obtain a mixture. 1.3 Burning the mixture

[0048] The mixture was fired in an aluminum oxide crucible in an electric furnace under an atmospheric humidity of 50% or higher. Specifically, the mixture was formed into pellets and then subjected to the following steps, as shown in Table 1: "first heating step," "pre-firing step," "second heating step," "main firing step," and "cooling step in the furnace." The fired material was then removed from the electric furnace at 250 °C. This yielded a sodium-containing oxide with a P2-type structure. Table 1 Step Starting temperature (°C) Final temperature (°C) Time (min) Heating or cooling rate (°C / min) First heating step 25 600 115 5 Pre-burn step 600 600 360 0 Second heating step 600 900 100 3 Main combustion step 900 900 60 0 Cooling step in the oven 900 250 130 5 2. Identification of the chemical composition of the active cathode material

[0049] The chemical compositions of the active cathode materials from Example 1 and Comparative Examples 1 to 3 were identified by ICP analysis, etc. Table 2 shows the chemical compositions of these active cathode materials. Table 2 Chemical composition Doping elements Example 1 So 0,62 Approx 0,075 (Mr 0,66 Ni 0,34 ) 0,9 Mg 0,1 O2 Ca and Mg Comparative example 1 So 0,67 Mr 0,66 Ni 0,340 O2 no Comparative example 2 So 0,62 Approx 0,075 Mr 0,66 Ni 0,34 O2 Approx Comparative example 3 That 0,67 (Mn 0,66 By 0,34 ) 0,9 Mg 0,102 Mg 3. Observation of the appearance of the active cathode material and determination of the mean primary particle size

[0050] The appearance of each of the active cathode materials from Example 1 and comparison examples 1 to 3 was observed using SEM. Fig. Figure 3 shows SEM images of the active cathode materials from Example 1 and comparison examples 1 to 3. As in Fig. As shown in Figure 3, the primary particles in the active cathode materials of Comparative Examples 1 to 3 are spherical, and these spherical primary particles are not aggregated. In contrast, the active cathode material of Example 1 is a secondary particle formed by the aggregation of fine primary particles. Table 3 shows the mean particle sizes of the primary particles in the active cathode materials of Example 1 and Comparative Examples 1 to 3. The method for measuring the mean particle size of the primary particles is as described in the embodiment. Table 3 Doping elements mean primary particle size (µm) Example 1 Ca and Mg 1,5 Comparative example 1 no 4,0 Comparative example 2 Approx 3,0 Comparative example 3 Mg 4,0 3. Electrochemical Measurement 3.1 Resistance

[0051] The aforementioned active cathode material, PVdF as a binder, and carbon as an electrically conductive additive were weighed out in a mass ratio of 85:5:10 (active cathode material:PVdF:carbon) and dispersed and mixed in N-methyl-2-pyrrolidone to produce a slurry. The suspension was applied to aluminum foil, pressed, and vacuum-dried overnight at 120 °C to obtain a cathode. A button cell was fabricated using the cathode, a sodium metal foil as the counter electrode, and a 1 M solution of NaPF6 in PC as the electrolyte. After a charge-discharge cycle of the button cell at a rate of 0.1 C in the voltage range of 2.0 V to 4.5 V in a thermostated chamber maintained at 25 °C, an impedance measurement was performed. The results are presented in Table 4. Table 4 Doping elements Impedance (Ω) Example 1 Ca and Mg 197 Comparative example 1 no 435 Comparative example 2 Approx 229 Comparative example 3 Mg 204

[0052] The results in Table 4 show that the resistance in a Ca-Mg-doped, Na-containing oxide with a P2-type structure is significantly reduced. In a Ca-Mg-doped, Na-containing oxide with a P2-type structure, the active cathode material, as described above, forms secondary particles that are aggregates of fine primary particles. This is thought to facilitate the formation of ionic conduction pathways and thereby reduce the resistance. 3.2 Discharge capacity in the first cycle

[0053] The button cell was charged and discharged at a rate of 0.1 C in the voltage range of 2.0 V to 4.5 V in a thermostated chamber maintained at 25 °C, and the discharge capacity in the first cycle was measured. The results are shown in Table 5. Table 5 Doping elements Discharge capacity (mAh / g) Example 1 Ca and Mg 164,91 Comparative example 1 no 122,45 Comparative example 2 Approx 124,90 Comparative example 3 Mg 143,27

[0054] The results in Table 5 show that the discharge capacity in a calcium- and magnesium-doped sodium-containing oxide with a P2-type structure is significantly increased. It is assumed that calcium is contained in the sodium layer of the P2-type structure. If calcium is present in the sodium layer of the P2-type structure, it acts as a column. Therefore, it is more likely that sodium layer breakdown will be reduced even after sodium extraction. Consequently, the amount of sodium incorporation and extraction is increased, presumably contributing to the high capacity. It is assumed that magnesium is present in the transition metal layer of the P2-type structure. Since magnesium is present in the transition metal layer of the P2-type structure, the P2-type structure is stabilized, and the dissolution of the transition metal layer during sodium extraction is reduced, presumably contributing to the high capacity.This means that in example 1 both Ca and Mg are doped, so they probably act synergistically and significantly increase the discharge capacity. 3.3 Capacity maintenance rate (cycle characteristics)

[0055] The button cell was charged and discharged at a rate of 0.1 C in a voltage range of 2.0 V to 4.5 V in a thermostated chamber maintained at 25 °C, and the discharge capacity retention rates from the second to the fifth cycle were measured, with the discharge capacity in the first cycle set as the reference (100%). The results are shown in Table 6. Table 6 Example 1 (doped with Ca and Mg) Comparative example 1 (not endowed) Comparative example 2 (doped with Ca) Comparative example 3 (doped with Mg) First cycle 100 100 100 100 Second cycle 99,3 96,2 99,2 99,0 Third cycle 98,3 90,3 96,8 97,8 Fourth cycle 97,3 84,2 94,6 96,5 Fifth cycle 96,2 78,9 92,2 95,1

[0056] The results in Table 6 show that the cycle properties are significantly improved in a Ca- and Mg-doped oxide containing Na and exhibiting a P2-type structure. As described above, Ca acts as a support in a Ca- and Mg-doped, Na-containing oxide with a P2-type structure. Therefore, the breakdown of the Na layer, etc., is reduced even after Na extraction, and the P2-type structure is adequately preserved. Furthermore, Mg stabilizes the P2-type structure, and the dissolution of the transition metal element, etc., during Na extraction is reduced. This is thought to contribute to the improved cycle properties. 4. Supplementary information on the doping points of Ca and Mg

[0057] As described above, the active cathode material from Example 1 was obtained by adding a Na compound, a Ca compound, and a Mg compound to precursor particles containing Mn and Ni, and then firing the resulting mixture. If the Na compound and the dopant compounds are mixed with the precursor particles after they have been obtained, and the resulting mixture is fired, Mn, Ni, and Mg, with similar ionic radii, are presumed to form the transition metal layer of the P2-type structure, while Na and Ca, with similar ionic radii, form the Na layer of the P2-type structure. If the precursor particles contain a dopant in addition to Mn and Ni before mixing with the Na compound (i.e.,(If the precursor particles containing Mn, Ni, and a dopant are obtained by coprecipitation), it is assumed that the dopant, together with Mn and Ni, forms the transition metal layer of the P2-type structure. The validity of the above considerations was confirmed by obtaining an X-ray diffraction pattern of the active cathode material and identifying the interlayer spacings of each layer by Rietveld analysis. 5. Supplementary information on doping elements other than Mg

[0058] The preceding example illustrates an active cathode material with a specific chemical composition. However, the chemical composition of the active cathode material is not limited to that described above. The inventors have confirmed that an active cathode material with excellent cycling properties, etc., can also be obtained when B or Al is used as the dopant instead of Mg (Japanese Patent Application No. 2024-129145). In other words, it is assumed that the same effects will be achieved when B or Al is used as the dopant instead of, or in combination with, Mg in Example 1. Furthermore, the molar ratio of Na and the composition ratio of the transition metal element are not limited to those described above. 6. Summary

[0059] Based on the results of the example, it is assumed that a secondary particle made of active material that meets the following conditions (1) to (9) will have low resistance, high capacity and excellent cycle properties. (1) A secondary particle made of active material comprises a plurality of primary particles. (2) Each of the primary particles has a P2-type structure. (3) Each of the primary particles contains as components at least Na, a first dopant, a transition metal element, a second dopant and O. (4) The first doping element is Ca. (5) The first doping element is contained in the Na layer of the P2-type structure. (6) The transition metal element includes Mn and / or Ni. (7) The second doping element is one or more of B, Mg and Al. (8) The second doping element is contained in the transition metal element layer of the P2-type structure. (9) The primary particles have a mean particle size of 2.0 µm or less. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] JP 2023-182443

[0002] JP 2023-182443 A

[0002] JP 2024-129145

[0058]

Claims

Secondary particles of active material comprising a plurality of primary particles, wherein: each of the primary particles has a P2-type structure; each of the primary particles contains at least the following elements as constituents: Na, a first dopant, a transition metal element, a second dopant, and O; the first dopant is Ca; the first dopant is contained in a Na layer of the P2-type structure; the transition metal element contains Mn and / or Ni; the second dopant is one or more of the elements B, Mg, and Al; the second dopant is contained in a transition metal layer of the P2-type structure; and the primary particles have an average particle size of 2.0 µm or less. Secondary particles made of active material according to claim 1, wherein each of the primary particles has a chemical composition represented by Naa-2bCabMnxNiyAzO2, wherein 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 elements B, Mg and Al. Secondary particles made of active material according to claim 1, wherein: a molar ratio of the first dopant element to O in the primary particles is greater than zero and less than or equal to 0.04; and a molar ratio of the second dopant element to O in the primary particles is greater than zero and less than or equal to 0.

10. Secondary particles made of active material according to claim 1, wherein the second doping element is Mg. A method for producing a secondary particle from active material, comprising: obtaining precursor particles by coprecipitation; mixing the precursor particles, a Na compound, a compound of the first dopant, and a compound of the second dopant to obtain a mixture; and firing the mixture to obtain the secondary particle from active material, comprising a plurality of primary particles, wherein: the precursor particles contain Mn and / or Ni; the first dopant compound is a Ca compound; the second dopant compound is a compound containing one or more elements B, Mg, and / or Al; each of the primary particles has a P2-type structure; and the primary particles have a mean particle size of 2.0 µm or less.

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