Positive electrode active material, lithium-ion battery, and method for manufacturing a positive electrode active material

Abstract

To disclose a positive electrode active material having an O2-type structure and capable of contributing to reduction of resistance.SOLUTION: The positive electrode active material according to the present disclosure has an O2-type structure and has a carbon amount of 500 ppm at a maximum. The method for manufacturing the positive electrode active material of the present disclosure includes: ion-exchanging at least a part of Na in a Na-containing oxide with a P2-type structure with Li and obtaining a Li-containing oxide with an O2-type structure; and heating the Li-containing oxide and reducing the carbon amount of the Li-containing oxide.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] This application discloses a positive electrode active material, a lithium-ion battery, and a method for manufacturing the positive electrode active material. [Background technology]

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

[0003] [Patent Document 1] Japanese Patent Publication No. 2011-170994 [Overview of the project] [Problems that the invention aims to solve]

[0004] Conventional positive electrode active materials with an O2-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> A positive electrode active material having an O2 type structure, Having a carbon content of 500 ppm or less, Cathode active material. <Aspect 2> A positive electrode active material according to embodiment 1, Having a sulfur content of 300 ppm or less, Cathode active material. <Aspect 3> It is a lithium-ion battery, It has a positive electrode active material layer, an electrolyte layer, and a negative electrode active material layer. The positive electrode active material layer contains the positive electrode active material of Embodiment 1 or 2. Lithium ion battery. <Embodiment 4> A method for manufacturing a positive electrode active material, comprising: Ion-exchanging at least a part of Na of a Na-containing oxide having a P2-type structure with Li to obtain a Li-containing oxide having an O2-type structure, and heating the Li-containing oxide to reduce the carbon content of the Li-containing oxide. A method for manufacturing a positive electrode active material, comprising the above steps. <Embodiment 5> A method for manufacturing the positive electrode active material of Embodiment 4, wherein the atmosphere during heating of the Li-containing oxide is an inert gas atmosphere or an oxygen-containing atmosphere, and the temperature during heating of the Li-containing oxide is 200°C or higher and 300°C or lower. A method for manufacturing a positive electrode active material. <Embodiment 6> A method for manufacturing the positive electrode active material of Embodiment 4 or 5, comprising: washing the Li-containing oxide with water to reduce the sulfur content of the Li-containing oxide. A method for manufacturing a positive electrode active material, comprising the above step.

Advantages of the Invention

[0006] The positive electrode active material of the present disclosure has an O2-type structure and low resistance.

Brief Description of the Drawings

[0007] [Figure 1] An example of the process of a method for manufacturing a positive electrode active material having an O2-type structure is shown. [Figure 2] A mechanism for removing carbon by heating is schematically shown. [Figure 3] An example of the configuration of a lithium ion battery is schematically shown.

Modes for Carrying Out the Invention

[0008] The following describes one embodiment of the positive electrode active material, lithium-ion battery, and method for manufacturing the positive electrode active material according to this disclosure. However, the positive electrode active material, lithium-ion battery, and method for manufacturing the positive electrode active material according to this disclosure are not limited to the embodiment described below.

[0009] 1.Cathode active material A positive electrode active material according to one embodiment has an O2-type structure and a carbon content of 500 ppm or less.

[0010] 1.1 Crystal structure In one embodiment, the positive electrode active material has at least an O2-type structure (belonging to space group P63mc) as its crystal structure. In addition to having an O2-type structure, the positive electrode active material may also have a crystal structure other than the O2-type structure. Examples of crystal structures other than the O2-type structure include the T#2-type structure (belonging to space group Cmca) and the O6-type structure (belonging to space group R-3m, with a c-axis length of 2.5 nm to 3.5 nm, typically 2.9 nm to 3.0 nm, and different from the O3-type structure, which also belongs to space group R-3m), which are formed when Li is removed or inserted from the O2-type structure. In one embodiment, the positive electrode active material may have an O2-type structure as its main phase. In one embodiment, the positive electrode active material may have an O2-type structure as well as one or both of the T#2-type structure and the O6-type structure. In one embodiment, the crystal structure of the main phase of the positive electrode active material may change depending on its charge and discharge state.

[0011] 1.2 Chemical composition The chemical composition of the positive electrode active material according to one embodiment is not particularly limited as long as the above-described O2-type structure is maintained. The positive electrode active material having an O2-type structure may contain, as constituent elements, at least one element from among Mn, Ni, and Co, Li, and O. The positive electrode active material is particularly likely to yield higher performance when it contains, as constituent elements, at least Li, Mn, one or both of Ni and Co, and O, and especially when it contains, at least Li, Mn, Ni, Co, and O. The positive electrode active material according to one embodiment is Lia So b Mr x-p Ni y-q Co z-r M p+q+rO2 (where 0 < a < 1.00, 0 ≤ b ≤ 0.20, x + y + z = 1, and 0 ≤ p + q + r < 0.17, and the element M is at least one selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W) may have a chemical composition represented thereby. When the positive electrode active material has such a chemical composition, the O2-type structure is likely to be maintained. In the above chemical composition, a is greater than 0, and may be 0.10 or more, 0.20 or more, 0.30 or more, 0.40 or more, 0.50 or more, or 0.60 or more, and less than 1.00, and may be 0.90 or less, 0.80 or less, or 0.70 or less. In the above chemical composition, b is 0 or more, and may be 0.01 or more, 0.02 or more, or 0.03 or more, and 0.20 or less, and may be 0.15 or less, or 0.10 or less. Also, x is 0 or more, and may be 0.10 or more, 0.20 or more, 0.30 or more, or 0.40 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, or 0.40 or less. Also, y is 0 or more, and may be 0.10 or more, or 0.20 or more, and 1.00 or less, and may be 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, 0.50 or less, 0.40 or less, 0.30 or less, or 0.20 or less. Also, z is 0 or more, and may be 0.10 or more, 0.20 or more, 0.30 or more, or 0.40 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, or 0.40 or less. The element M has a small contribution to charge and discharge. In this regard, in the above chemical composition, when p + q + r is less than 0.17, it is easy to ensure a high charge and discharge capacity. p + q + r may be 0.16 or less, 0.15 or less, 0.14 or less, 0.13 or less, 0.12 or less, 0.11 or less, or 0.10 or less. On the other hand, when the element M is included, the O2-type structure is likely to be stabilized. In the above chemical composition, p + q + r is 0 or more, and may be 0.01 or more, 0.02 or more, 0.03 or more, 0.04 or more, 0.05 or more, 0.06 or more, 0.07 or more, 0.08 or more, 0.09 or more, or 0.10 or more.The composition of O is approximately 2, but it is not necessarily exactly 2.0; it is indeterminate.

[0012] 1.3 Carbon content In conventional positive electrode active materials having an O2-type structure, lithium carbonate is inevitably generated due to the manufacturing process. Specifically, positive electrode active materials having an O2-type structure can be obtained by substituting Na with Li (ion exchange) in a Na-containing oxide having a P2-type structure. Here, Li or Li compounds that do not constitute the O2-type structure inevitably remain on the surface of the positive electrode active material after ion exchange. Furthermore, the O2-type structure is a metastable phase, and Li or Li compounds can also be generated when some of the O2-type structure collapses. Such Li or Li compounds react with, for example, carbon dioxide from the atmosphere to form lithium carbonate. When a positive electrode active material having an O2-type structure contains a large amount of lithium carbonate, the resistance of the positive electrode active material increases.

[0013] In contrast, the positive electrode active material according to one embodiment has a carbon content of 500 ppm or less by mass. That is, the positive electrode active material according to one embodiment has a significantly lower amount of lithium carbonate, an impurity, compared to conventional positive electrode active materials. Therefore, the positive electrode active material according to one embodiment has lower resistance compared to conventional positive electrode active materials. The carbon content of the positive electrode active material according to one embodiment may be 400 ppm or less, 300 ppm or less, or 200 ppm or less. The lower limit of the carbon content is not particularly limited. The carbon content may be 0 ppm or more, 10 ppm or more, 50 ppm or more, 100 ppm or more, or 150 ppm or more. Such a positive electrode active material with a low carbon content can be produced by heating (additional calcination) the Li-containing oxide obtained after ion exchange, as will be described later.

[0014] The carbon content of the positive electrode active material can be measured, for example, by combustion-infrared absorption spectroscopy. A carbon-sulfur analyzer (CS844 model) from LECO is used as the measuring instrument.

[0015] 1.4 Sulfur content In one embodiment, the positive electrode active material may have a sulfur content of 300 ppm or less by mass, in addition to the above-mentioned carbon content. Reducing the sulfur content of the positive electrode active material tends to further reduce the resistance of the positive electrode active material. The lower limit of the sulfur content is not particularly limited. The sulfur content may be 0 ppm or more, 10 ppm or more, 50 ppm or more, 100 ppm or more, 150 ppm or more, or 200 ppm or more. Such a positive electrode active material with a low sulfur content can be produced by washing the Li-containing oxide obtained after ion exchange with water, as will be described later.

[0016] The sulfur content of the positive electrode active material can be measured, for example, by combustion-ion chromatography. The measurement equipment used is a sample combustion device (AQF-2100S) from Mitsubishi Chemical Analytec and an ion chromatograph (Compact IC Flex) from Metrohm.

[0017] 1.5 Carbon / Sulfur Ratio In one embodiment, the mass ratio of carbon to sulfur (C / S) in the positive electrode active material may be, for example, 0.70 to 3.50, or 0.74 to 3.46. When the mass ratio of carbon to sulfur in the positive electrode active material is within this range, it is easier to obtain a positive electrode active material with a better balance of performance.

[0018] 1.6 Shape One embodiment of the positive electrode active material can be obtained by substituting Na with Li in a Na-containing oxide having a P2-type structure. Here, the P2-type structure is hexagonal, has a large diffusion coefficient for Na ions, and is prone to crystal growth in a specific direction. In particular, when at least one of Mn, Ni, and Co is included as a transition metal element constituting the P2-type structure, it is prone to plate-like crystal growth in a specific direction. Therefore, Na-containing oxides having a P2-type structure usually become plate-like particles with a large aspect ratio, where the crystal growth direction is biased in a specific direction. The positive electrode active material according to one embodiment may be obtained from such plate-like Na-containing oxide particles, or, as will be described later, may be obtained from spherical Na-containing oxide particles. That is, the shape of the positive electrode active material may be plate-like particles or spherical particles. When the positive electrode active material is spherical particles, the reaction resistance decreases due to the reduction in crystallite size, and the diffusion resistance inside the particles tends to decrease. Furthermore, when applied to a battery, it is thought that the degree of bending is reduced by spheroidization, and the lithium-ion conduction resistance decreases. This improves, for example, the rate characteristics and makes it easier to increase the reversible capacity. In this application, "spherical particle" means a particle with a circularity of 0.80 or higher. The 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 circularity of the particle is 4πS / L 2 The circularity of a particle is defined as follows: Here, S is the orthographic area of ​​the particle, and L is the perimeter of the orthographic image of the particle. The circularity of a particle can be determined by observing its appearance using a scanning electron microscope (SEM), transmission electron microscope (TEM), or optical microscope.

[0019] The positive electrode active material according to one embodiment may be, for example, solid particles, hollow particles, or particles with voids. The size of the positive electrode active material particles is not particularly limited, but a smaller size is considered advantageous. For example, the average particle diameter (D50) of the positive electrode active material particles may be 0.1 μm or more and 10 μm or less, 1.0 μm or more and 8.0 μm or less, or 2.0 μm or more and 6.0 μm or less. The average particle diameter (D50) is the particle diameter (D50, median diameter) at 50% of the cumulative value in the volume-based particle size distribution obtained by laser diffraction and scattering.

[0020] 2. Method for manufacturing positive electrode active material As shown in Figure 1, a method for producing a positive electrode active material according to one embodiment is: S1: To obtain a Li-containing oxide having an O2-type structure by ion-exchanging at least a portion of the Na in a Na-containing oxide having a P2-type structure with Li, and S2: Heating the Li-containing oxide to reduce the carbon content of the Li-containing oxide. It may include [something].

[0021] 2.1 S1 In S1, at least a portion of the Na in a Na-containing oxide having a P2-type structure is ion-exchanged for Li to obtain a Li-containing oxide having an O2-type structure.

[0022] 2.1.1 Preparation of Na-containing oxides having a P2-type structure Na-containing oxides having a P2-type structure include, for example, S1-1: Obtain a precursor (for example, a precursor containing at least one element among Mn, Ni, and Co). S1-2: The surface of the precursor is coated with a Na source to obtain a composite, and S1-3: The composite is fired. It can be obtained through this process.

[0023] In S1-1, for example, a precursor containing at least one element from among Mn, Ni, and Co is obtained. The precursor may contain at least Mn and one or both of Ni and Co, or it may contain at least Mn, Ni, and Co. The precursor may be a salt containing at least one element from among Mn, Ni, and Co. For example, the precursor may be at least one of carbonates, sulfates, nitrates, and acetates. Alternatively, the precursor may be a compound other than a salt. For example, the precursor may be a hydroxide. The precursor may be a hydrate. The precursor may be a combination of multiple types of compounds. The precursor may be in various shapes. For example, the precursor may be particulate, or it may be spherical particles as described later. The particle size of the particles made up of the precursor is not particularly limited. In S1-1, a precipitate as the precursor may be 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 at least one element from among Mn, Ni, and Co. This makes it easier to obtain spherical particles as the precursor. The "ion source capable of forming a precipitate with transition metal ions in an aqueous solution" may be, for example, at least one selected from sodium salts such as sodium carbonate and sodium nitrate, sodium hydroxide, and sodium oxide. The transition metal compound may be the above-mentioned salts or hydroxides containing at least one element among Mn, Ni, and Co. Specifically, in S1-1, the ion source and the transition metal compound may be prepared as separate solutions, and the precipitate as a precursor may be obtained by adding and mixing each solution dropwise. In this case, water may be used as the solvent. Various sodium compounds may be used as the base, and aqueous ammonia solution may be added to adjust the basicity. In the case of the coprecipitation method, for example, an aqueous solution of the transition metal compound and an aqueous solution of sodium carbonate may be prepared, and the precipitate as a precursor may be obtained by adding and mixing each aqueous solution dropwise. Alternatively, the precursor can also be obtained by the sol-gel method. Spherical particles as the precursor are particularly easy to obtain by the coprecipitation method. In S1-1, the precursor may contain element M.Element M is at least one selected from B, Mg, Al, K, Ca, Ti, V, Cr, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W. These elements M have functions such as stabilizing P2-type structures and O2-type structures. The method for obtaining a precursor containing element M is not particularly limited. When obtaining a precursor by coprecipitation in S1-1, for example, an aqueous solution of a transition metal compound containing at least one of Mn, Ni, and Co, an aqueous solution of sodium carbonate, and an aqueous solution of a compound of element M are prepared, and each aqueous solution is added dropwise and mixed to obtain a precursor containing element M along with at least one of Mn, Ni, and Co. Alternatively, in the manufacturing method of this disclosure, element M may not be added in S1-1, and element M may be doped when Na-doping calcination is performed in S1-2 and S1-3 described later.

[0024] In S1-2, the surface of the precursor obtained in S1-1 is coated with a Na source to obtain a composite. The Na source may be a salt containing Na, such as a carbonate or nitrate, or a compound other than a salt, such as sodium oxide or sodium hydroxide. In S1-2, the amount of Na source coated on the surface of the precursor should be determined taking into account the amount of Na lost during subsequent calcination. In S1-2, the coverage rate of the Na source on the surface of the precursor is not particularly limited. For example, in S1-2, the composite may be obtained by coating 40% or more, 50% or more, 60% or more, or 70% or more of the surface of the precursor with the Na source. Here, if the precursor obtained in S1-1 is spherical particles, and the composite obtained in S1-2 is obtained by coating 40% or more of the surface of the precursor with the Na source, then in S1-3 described later, the Na-containing oxide having a P2-type structure is likely to become spherical particles. If the coverage of the Na source is low, when the composite is calcined, P2-type crystals tend to grow on the surface of the composite, and the Na-containing oxide tends to become plate-like. If the coverage of the Na source is high, when the composite is calcined, the crystallites of the P2-type crystals tend to be small, and the Na-containing oxide tends to become spherical particles corresponding to the shape of the precursor. In S1-2, the method of coating the surface of the precursor with the Na source is not particularly limited. As mentioned above, when coating 40% or more of the surface of the precursor with the Na source, various methods can be used. For example, the rolling flow coating method and the spray drying method can be used. That is, a coating solution in which the Na source is dissolved is prepared, and the coating solution is brought into contact with the surface of the precursor, or dried at the same time as, or after, contact. By adjusting the coating conditions (temperature, time, number of times, etc.), 40% or more of the surface of the precursor can be coated with the Na source. In S1-2, the precursor may be coated with the M source along with the Na source. For example, in S1-2, a composite may be obtained by mixing the precursor obtained in S1-1 with a Na source and an M source containing at least one element M selected from B, Mg, Al, K, Ca, Ti, V, Cr, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W.The M source may be, for example, a salt containing element M, such as a carbonate or sulfate, or a compound other than a salt, such as an oxide or hydroxide. The amount of the M source relative to the precursor should be determined according to the chemical composition of the Na-containing oxide after calcination.

[0025] In S1-3, the composite obtained in S1-2 is calcined to obtain a Na-containing oxide having a P2-type structure. S1-3 includes, for example, S1-3A, S1-3B, and S1-3C as follows.

[0026] In S1-3A, the composite is pre-fired at a temperature of 300°C or higher but less than 700°C for a period of 2 to 10 hours. In S1-3A, the composite may be arbitrarily molded before pre-fired. Pre-fired is performed at a temperature lower than that of the main firing. If the pre-fired in S1-3A is insufficient, the formation of the P2 phase in the final Na-containing oxide may be insufficient. In S1-3A, by setting the pre-fired temperature to 300°C or higher but less than 700°C and the pre-fired time to 2 to 10 hours, sufficient pre-fired treatment can be performed on the composite, increasing thermal uniformity and making it easier to obtain a suitable Na-containing oxide via S1-3B and S1-3C described later. The pre-firing temperature may be between 400°C and 700°C, 450°C and 700°C, 500°C and 700°C, 550°C and 700°C, or 550°C and 650°C. The pre-firing time may be between 2 hours and 8 hours, 3 hours and 8 hours, 4 hours and 8 hours, 5 hours and 8 hours, or 5 hours and 7 hours. The pre-firing atmosphere is not particularly limited and may be, for example, an oxygen-containing atmosphere.

[0027] In S1-3B, following the pre-sintering described above, the composite is subjected to main firing at a temperature of 700°C to 1100°C for a period of 30 minutes to 10 hours. In S1-3B, the main firing temperature of the composite is 700°C to 1100°C, preferably 800°C to 1000°C. If the main firing temperature is too low, the P2 phase will not be formed, and if the main firing temperature is too high, the O3 phase or the like is likely to be formed instead of the P2 phase. The heating conditions from the pre-sintering temperature to the main firing temperature are not particularly limited. The main firing time is not particularly limited and may be, for example, 30 minutes to 48 hours. However, the shape of the Na-containing oxide can be controlled by the main firing time. As described above, in the method of this disclosure, if the coverage rate of the Na source in the composite is 40 area % or more, when the composite is fired, small P2-type crystals are likely to be formed on its surface. In the method of this disclosure, P2-type crystals are grown along the surface of the particles by linking one P2-type crystallite to another, so that the shape of the Na-containing oxide corresponds to the shape of the precursor. For example, if the precursor is a spherical particle, the Na-containing oxide may also be a spherical particle. If the firing time is too short, the formation of the P2 phase will be insufficient. On the other hand, if the firing time is too long, the P2 phase will grow excessively, resulting in plate-like particles instead of spherical ones. As far as the inventors have confirmed, spherical particles of Na-containing oxide are more easily obtained when the firing time is 30 minutes or more and 3 hours or less. The Na-containing oxide obtained after firing may have multiple crystallites on its surface and may have a structure in which the crystallites are linked to each other.

[0028] In step S1-3C, following the main firing described above, the composite is rapidly cooled (cooled at a cooling rate of 20°C / min or more) from a temperature T1 of 200°C or higher to a temperature T2 of 100°C or lower. The above pre-firing and main firing are performed, for example, in a heating furnace. In step S3-3, for example, after the main firing of the composite is performed in a heating furnace, it is cooled in the heating furnace to an arbitrary temperature T1 of 200°C or higher, and after reaching that temperature T1, the fired product is removed from the heating furnace and rapidly cooled outside the furnace to an arbitrary temperature T2 of 100°C or lower. Temperature T1 is any temperature of 200°C or higher, and may also be any temperature of 250°C or higher. Temperature T2 is any temperature of 100°C or lower, and may also be any temperature of 50°C or lower, and may also be the cooling completion temperature. In the predetermined temperature range between temperature T1 and temperature T2, moisture easily penetrates between the layers of the P2 type structure due to atomic vibrations, molecular motion, etc. When cooling the composite (Na-containing oxide having a P2-type structure) after the main firing, it is thought that reducing the time spent in the temperature range where moisture easily penetrates (i.e., rapid cooling) will reduce the amount of moisture penetrating into the interlayers of the P2-type structure. In this regard, in step S3-3, when cooling the composite after the main firing, if the cooling is performed in a dry atmosphere outside the furnace from an arbitrary temperature T1 of 200°C or higher to an arbitrary temperature T2 of 100°C or lower, the cooling rate from temperature T1 to temperature T2 becomes high (e.g., 20°C / min or higher), making it difficult for moisture to penetrate into the interlayers of the P2-type structure and suppressing the collapse of the P2-type structure. As a result, Na can be efficiently ion-exchanged with Li in S4.

[0029] As described above, Na-containing oxides having a P2-type structure and a predetermined chemical composition can be produced by S1-1 to S1-3. Na-containing oxides, for example, contain at least one transition metal element from among Mn, Ni, and Co, as well as Na and O as constituent elements. In particular, when the constituent elements contain at least Na, Mn, at least one from among Ni and Co, and O, the performance of the positive electrode active material tends to be even higher when the constituent elements contain at least Na, Mn, Ni, Co, and O. Na-containing oxides are Nac Mn x-p Ni y-q Co z-r M p+q+r It may have a chemical composition represented by O2. Here, 0 < c < 1.00, x + y + z = 1, and 0 ≤ p + q + r < 0.17. Also, M is at least one element selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W. When the Na-containing oxide has such a chemical composition, the P2-type structure is more likely to be maintained. In the above chemical composition, c is greater than 0, and may be 0.10 or more, 0.20 or more, 0.30 or more, 0.40 or more, 0.50 or more, or 0.60 or more, and less than 1.00, and may be 0.90 or less, 0.80 or less, or 0.70 or less. x is 0 or more, and may be 0.10 or more, 0.20 or more, 0.30 or more, or 0.40 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, or 0.40 or less. Also, y is 0 or more, and may be 0.10 or more or 0.20 or more, and 1.00 or less, and may be 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, 0.50 or less, 0.40 or less, 0.30 or less, or 0.20 or less. Also, z is 0 or more, and may be 0.10 or more, 0.20 or more, 0.30 or more, or 0.40 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, or 0.40 or less. The element M has a small contribution to charge and discharge. In this regard, in the above chemical composition, when p + q + r is less than 0.17, it is easy to ensure a high charge and discharge capacity. p + q + r may be 0.16 or less, 0.15 or less, 0.14 or less, 0.13 or less, 0.12 or less, 0.11 or less, or 0.10 or less. On the other hand, when the element M is included, the P2-type structure and the O2-type structure are likely to be stabilized. In the above chemical composition, p + q + r is 0 or more, and may be 0.01 or more, 0.02 or more, 0.03 or more, 0.04 or more, 0.05 or more, 0.06 or more, 0.07 or more, 0.08 or more, 0.09 or more, or 0.10 or more. The composition of O is approximately 2, but is not necessarily exactly 2.0 and is indefinite.

[0030] 2.1.2 Ion exchange In S1, a Li-containing oxide having an O2-type structure is obtained by ion-exchanging at least a portion of the Na in the Na-containing oxide obtained as described above for Li. For ion exchange, there are, for example, a method using an aqueous solution containing lithium halide and a method using a mixture of lithium halide and other lithium salts (e.g., a molten salt). From the viewpoint that the P2-type structure is easily broken by the intrusion of water and from the viewpoint of crystallinity, the method using a molten salt is preferred among the two methods above. That is, by mixing the Na-containing oxide having the P2-type structure described above with the molten salt and heating it to a temperature above the melting point of the molten salt, at least a portion of the Na in the Na-containing oxide can be replaced with Li by ion exchange. The lithium halide constituting the molten salt is preferably at least one of lithium chloride, lithium bromide, and lithium iodide. The other lithium salt constituting the molten salt is preferably lithium nitrate. By using a molten salt, the melting point becomes lower than when lithium halide or other lithium salts are used alone, and ion exchange becomes possible at lower temperatures. The temperature during ion exchange may be, for example, above the melting point of the molten salt and below 600°C, 500°C, 400°C, or 300°C. If the temperature during ion exchange is too high, the stable O3-type structure is more likely to form than the O2-type structure. On the other hand, from the viewpoint of minimizing the time required for ion exchange, it is desirable for the temperature during ion exchange to be as high as possible.

[0031] 2.2 S2 In step S2, the Li-containing oxide obtained in step S1 is heated to reduce the amount of carbon in the Li-containing oxide. Specifically, the Li-containing oxide having an O2-type structure obtained after ion exchange is subjected to additional calcination to remove at least a portion of the carbon (lithium carbonate) present on the surface of the Li-containing oxide.

[0032] Li-containing oxides having an O2-type structure adopt a Li-deficient crystal structure. That is, in positive electrode active materials having an O2-type structure, the Li / O composition ratio of Li to O is less than 0.5 (for example, "a" in the above composition formula is less than 1.0), and there is room to insert Li into the crystal structure. When such a Li-deficient Li-containing oxide is heated, as shown in Figure 2, the lithium carbonate on the surface of the Li-containing oxide combines with Li and CO x They separate, and Li is incorporated into the crystal structure, while CO x It is believed that the carbon (lithium carbonate) is removed from the outside of the Li-containing oxide. In other words, by subjecting the Li-containing oxide having an O2-type structure obtained after ion exchange to additional calcination, at least a portion of the carbon (lithium carbonate) present on the surface of the Li-containing oxide is removed, and the carbon content of the final positive electrode active material becomes 500 ppm or less. Such actions and effects are unique to the heating of Li-containing oxides having an O2-type structure.

[0033] In S2, the atmosphere used when heating the Li-containing oxide should be one that maintains the O2-type structure of the Li-containing oxide and reduces the amount of carbon in the Li-containing oxide. For example, the heating atmosphere may be an inert gas atmosphere such as an Ar atmosphere, or an oxygen-containing atmosphere such as an air atmosphere.

[0034] In S2, the temperature at which the Li-containing oxide is heated should be such that the O2-type structure of the Li-containing oxide is maintained and the amount of carbon in the Li-containing oxide is reduced. If the temperature is too low, the carbon removal reaction described above will not occur easily. On the other hand, if the temperature is too high, the metastable O2-type structure will easily change to the stable O3-type structure. From the viewpoint of appropriately removing carbon while maintaining the O2-type structure, the temperature in S2 may be, for example, 200°C to 300°C. This temperature may also be 220°C or 240°C or higher, or 280°C or 260°C or lower.

[0035] In step S2, the heating time of the Li-containing oxide should be such that the O2-type structure of the Li-containing oxide is maintained and the amount of carbon in the Li-containing oxide is reduced. The time in step S2 (holding time at the above heating temperature) may be, for example, 30 minutes to 10 hours, 2 hours to 8 hours, or 4 hours to 6 hours.

[0036] 2.3 Other processes According to the method comprising S1 and S2 described above, a positive electrode active material having an O2-type structure and a carbon content of 500 ppm or less can be produced. Furthermore, the method for producing a positive electrode active material according to one embodiment may include washing the Li-containing oxide obtained by S1 with water to reduce the sulfur content of the Li-containing oxide. The washing of the Li-containing oxide may be performed before or after S2. In particular, it is considered possible to reduce the water content, along with the carbon and sulfur content, in the positive electrode active material by washing the Li-containing oxide with water. Washing the Li-containing oxide with water results in a final positive electrode active material with a sulfur content of 300 ppm. However, while washing the Li-containing oxide with water is effective in reducing the sulfur content of the final positive electrode active material, it has virtually no effect on reducing the carbon content. To reduce the carbon content of the final positive electrode active material, additional calcination, such as described above, is necessary.

[0037] 3. Lithium-ion batteries Figure 3 schematically shows the configuration of a lithium-ion battery according to one embodiment. As shown in Figure 3, the lithium-ion 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. Here, the positive electrode active material layer 10 contains the positive electrode active material according to the above embodiment. As shown in Figure 3, the lithium-ion battery 100 may include a positive electrode current collector 40 and a negative electrode current collector 50.

[0038] 3.1 Cathode active material layer The positive electrode active material layer 10 contains a positive electrode active material having the above-described O2-type structure. The positive electrode active material layer 10 may optionally contain other positive electrode active materials, electrolytes, conductive additives, binders, etc. The positive electrode active material layer 10 may also contain various other additives. The respective content of the positive electrode active material, electrolyte, conductive additive, binder, etc. in the positive electrode active material layer 10 can be appropriately determined according to the desired battery performance. For example, if the total solid content in the positive electrode active material layer 10 is taken as 100% by mass, the content of the positive electrode active material may be 40% by mass or more and 100% by mass or less.

[0039] An ion-conductive protective layer may be formed on the surface of the positive electrode active material. That is, the positive electrode active material layer 10 may include a composite of the positive electrode active material and the protective layer, and in the composite, at least a portion of the surface of the positive electrode active material may be covered by the protective layer. The ion-conductive protective layer may include various ion-conductive compounds. The ion-conductive compound may be at least one selected from, for example, ion-conductive oxides and ion-conductive halides. The coverage rate (area ratio) of the protective layer on the surface of the positive electrode active material may be, for example, 70% or more, 80% or more, or 90% or more. The thickness of the protective layer may be, for example, 0.1 nm or more or 1 nm or more, 100 nm or less, or 20 nm or less.

[0040] The positive electrode active material contained in the positive electrode active material layer 10 may consist only of the positive electrode active material having the above-described O2-type structure, or it may contain other positive electrode active materials (other positive electrode active materials) together with the said positive electrode active material. From the viewpoint of further enhancing the effects of the technology of this disclosure, the proportion of other positive electrode active materials in the total positive electrode active material may be small. For example, with the total positive electrode active material being 100% by mass, the content of the positive electrode active material having the above-described O2-type structure may be 50% by mass or more and 100% by mass or less, 60% by mass or more and 100% by mass or less, 70% by mass or more and 100% by mass or less, 80% by mass or more and 100% by mass or less, 90% by mass or more and 100% by mass or less, 95% by mass or more and 100% by mass or less, or 99% by mass or more and 100% by mass or less.

[0041] The electrolyte contained in the positive electrode active material layer 10 may be a solid electrolyte, a liquid electrolyte, or a combination thereof. In particular, when the positive electrode active material layer 10 contains a solid electrolyte, and especially when it contains a sulfide solid electrolyte, high performance is easily ensured. The sulfide solid electrolyte may be a glass-based sulfide solid electrolyte (sulfide glass), a glass-ceramic-based sulfide solid electrolyte, or a crystalline sulfide solid electrolyte. The sulfide glass is amorphous. The sulfide glass may have a glass transition temperature (Tg). Furthermore, when the sulfide solid electrolyte has a crystalline phase, examples of crystalline phases include Thio-LISICON type crystalline phase, LGPS type crystalline phase, and argyrodite type crystalline phase. The sulfide solid electrolyte may contain, for example, Li, P, and S elements. The sulfide solid electrolyte may further contain element X (where X is at least one of As, Sb, Si, Ge, Sn, B, Al, Ga, or In). The sulfide solid electrolyte may also further contain at least one of element O and a halogen element. Furthermore, the sulfide solid electrolyte may contain element S as the main component of the anionic element. Examples of sulfide solid electrolytes include Li2S-P2S5, Li2S-P2S5-LiI, Li2S-P2S5-GeS2, Li2S-P2S5-Li2O, Li2S-P2S5-Li2O-LiI, Li2S-P2S5-LiI-LiBr, Li2S-SiS2-P2S5-LiI, Li2S-P2S5-ZmSn (where m and n are positive numbers, and Z is one of Ge, Zn, or Ga), Li2S-SiS2-Li3PO4, and Li2S-SiS2-Li x MO y(However, x and y are positive numbers. M may be at least one selected from P and optionally any one of Si, Ge, B, Al, Ga, and In.) It may be at least one selected. Alternatively, the sulfide solid electrolyte is not particularly limited. For example, it may have at least one chemical composition selected from xLi2S·(100 - x)P2S5 (70 ≤ x ≤ 80), yLiI·zLiBr·(100 - y - z)(xLi2S·(1 - x)P2S5) (0.7 ≤ x ≤ 0.8, 0 ≤ y ≤ 30, 0 ≤ z ≤ 30), etc. Alternatively, the sulfide solid electrolyte has the general formula: Li 4-x Ge 1-x P x S4 (0 < x < 1). In the above general formula, at least a part of Ge may be substituted with at least one of Sb, Si, Sn, B, Al, Ga, In, Ti, Zr, V, and Nb. In the above general formula, a part of P may be substituted with at least one of Sb, Si, Sn, B, Al, Ga, In, Ti, Zr, V, and Nb. In the above general formula, a part of Li may be substituted with at least one of Na, K, Mg, Ca, and Zn. In the above general formula, a part of S may be substituted with a halogen (at least one of F, Cl, Br, and I). Alternatively, the sulfide solid electrolyte has the formula Li 7-a PS 6-a X a (X is at least one of Cl, Br, and I, and a is a number from 0 or more to 2 or less). a may be 0, or may be greater than 0. In the latter case, a may be 0.1 or more, may be 0.5 or more, or may be 1 or more. Also, a may be 1.8 or less, may be 1.5 or less. The sulfide solid electrolyte may be in particulate form. The average particle diameter (D50) of the sulfide solid electrolyte may be, for example, 10 nm or more and 100 μm or less.

[0042] Examples of conductive additives that may be included in the positive electrode active material layer 10 include carbon materials such as vapor-processed carbon fiber (VGCF), acetylene black (AB), Ketjenblack (KB), carbon nanotubes (CNT), and carbon nanofibers (CNF); and metallic materials such as nickel, titanium, aluminum, and stainless steel. The conductive additive may be in the form of parts or fibers, and its size is not particularly limited. One type of conductive additive may be used alone, or two or more types may be used in combination.

[0043] Examples of binders that may be included in the positive electrode active material layer 10 include butadiene rubber (BR) binders, butylene rubber (IIR) binders, acrylate butadiene rubber (ABR) binders, styrene butadiene rubber (SBR) binders, polyvinylidene fluoride (PVdF) binders, polytetrafluoroethylene (PTFE) binders, and polyimide (PI) binders. A single binder may be used alone, or two or more binders may be used in combination.

[0044] The positive electrode active material layer 10 may contain various additives in addition to the above-mentioned components. For example, dispersants and lubricants.

[0045] 3.2 Electrolyte layer The electrolyte layer 20 is positioned between the positive electrode active material layer 10 and the negative electrode active material layer 30. The electrolyte layer 20 contains at least an electrolyte. The electrolyte layer 20 may contain at least one of a solid electrolyte and a liquid electrolyte, and may also optionally contain a binder or the like. In particular, when the electrolyte layer 20 contains a solid electrolyte, especially when it contains a sulfide solid electrolyte, higher performance is more easily ensured. The content of electrolyte and binder or the like in the electrolyte layer 20 is not particularly limited. Alternatively, the electrolyte layer 20 may have a separator or the like to hold the electrolyte and prevent contact between the positive electrode active material layer 10 and the negative electrode active material layer 30. The thickness of the electrolyte layer 20 is not particularly limited; for example, it may be 0.1 μm or more or 1 μm or more, or 2 mm or less or 1 mm or less.

[0046] The electrolyte included in the electrolyte layer 20 may be appropriately selected from the examples of electrolytes that can be included in the positive electrode active material layer 10 (solid electrolytes and / or liquid electrolytes). Similarly, the binder included in the electrolyte layer 20 may be appropriately selected from the examples of binders that can be included in the positive electrode active material layer. The electrolyte and binder may be used individually or in combination of two or more types. The separator may be any separator commonly used in lithium-ion batteries, such as those made of polyethylene (PE), polypropylene (PP), polyester, and polyamide resins. The separator may have a single-layer structure or a multi-layer structure. Examples of multi-layer separators include a PE / PP two-layer separator, or a PP / PE / PP or PE / PP / PE three-layer separator. The separator may also be made of a nonwoven fabric such as cellulose nonwoven fabric, resin nonwoven fabric, or glass fiber nonwoven fabric.

[0047] 3.3 Negative electrode active material layer The negative electrode active material layer 30 contains at least negative electrode active material. The negative electrode active material layer 30 may also optionally contain an electrolyte, a conductive additive, a binder, and various other additives. The content of each component in the negative electrode active material layer 30 can be appropriately determined according to the desired battery performance. For example, taking the total solid content of the negative electrode active material layer 30 as 100% by mass, the content of the negative electrode active material may be 40% by mass or more, 50% by mass or more, 60% by mass or more, or 70% by mass or more, or it may be 100% by mass or less, less than 100% by mass, 95% by mass or less, or 90% by mass or less. Alternatively, taking the entire negative electrode active material layer 30 as 100% by volume, the negative electrode active material and optionally the electrolyte, conductive additive, and binder may together account for 85% by volume or more, 90% by volume or more, or 95% by volume or more, with the remainder being void or other components. The shape of the negative electrode active material layer 30 is not particularly limited and may, for example, be a sheet having a substantially flat surface. The thickness of the negative electrode active material layer 30 is not particularly limited and may be, for example, 0.1 μm or more, 1 μm or more, 10 μm or more, or 30 μm or more, or 2 mm or less, 1 mm or less, 500 μm or less, or 100 μm or less.

[0048] The negative electrode active material contained in the negative electrode active material layer 30 can be any known negative electrode active material for lithium-ion batteries. Among the known active materials, various materials can be used that have a lower potential for intercalating and releasing lithium ions (charge / discharge potential) compared to the positive electrode active material. For example, silicon-based active materials such as Si, Si alloys, and silicon oxide; carbon-based active materials such as graphite and hard carbon; various oxide-based active materials such as lithium titanate; metallic lithium and lithium alloys can be used. In particular, the performance of the lithium-ion battery 100 tends to improve when the negative electrode active material layer 30 contains Si as the negative electrode active material. The negative electrode active material may be used alone or in combination of two or more types. The shape of the negative electrode active material may be any shape that is common for negative electrode active materials in lithium-ion batteries. For example, the negative electrode active material may be particulate. The negative electrode active material particles may be primary particles or secondary particles formed by the aggregation of multiple primary particles. The average particle size (D50) of the negative electrode active material particles may be, for example, 1 nm or more, 5 nm or more, or 10 nm or more, or it may be 500 μm or less, 100 μm or less, 50 μm or less, or 30 μm or less. Alternatively, the negative electrode active material may be in the form of a sheet (foil or film) such as lithium foil. That is, the negative electrode active material layer 30 may consist of a sheet of negative electrode active material.

[0049] Examples of electrolytes that may be included in the negative electrode active material layer 30 include the solid electrolyte, electrolyte solution, or a combination thereof as described above. Conductive additives that may be included in the negative electrode active material layer 30 may be appropriately selected from among the examples of conductive additives that may be included in the positive electrode active material layer described above. Binders that may be included in the negative electrode active material layer 30 may be appropriately selected from among the examples of binders that may be included in the positive electrode active material layer described above. Electrolytes, conductive additives, and binders may each be used individually or in combination of two or more types.

[0050] 3.4 Other Configurations As shown in Figure 3, the lithium-ion battery 100 may include a positive electrode current collector 40 that contacts the positive electrode active material layer 10, and a negative electrode current collector 50 that contacts the negative electrode active material layer 30. The configuration of the current collectors themselves is publicly known. In addition to the above configuration, the lithium-ion battery 100 may have other configurations common to batteries, such as tabs and terminals. The lithium-ion battery 100 may have each of the above configurations housed inside an outer casing. Any known battery casing can be used. Furthermore, multiple batteries 100 may be electrically connected and stacked as desired to form a battery pack. In this case, the battery pack may be housed inside a known battery case. Examples of shapes for the lithium-ion battery 100 include coin type, laminate type, cylindrical type, and prismatic type. The lithium-ion battery 100 may also be a secondary battery.

[0051] Lithium-ion batteries 100 can be manufactured by applying known methods, except for the use of the specific positive electrode active material described above. For example, they can be manufactured as follows. (1) A slurry for the positive electrode layer is obtained by dispersing the positive electrode active material and other materials constituting the positive electrode active material layer in a solvent. The solvent used in this case is not particularly limited, and water or various organic solvents can be used. The slurry for the positive electrode layer is applied to the surface of the positive electrode current collector using a doctor blade or the like, and then dried to form a positive electrode active material layer on the surface of the positive electrode current collector, which serves as the positive electrode. (2) The negative electrode active material and other materials constituting the negative electrode active material layer are dispersed in a solvent to obtain a slurry for the negative electrode layer. The solvent used in this case is not particularly limited, and water or various organic solvents can be used. The slurry for the negative electrode layer is applied to the surface of the negative electrode current collector using a doctor blade or the like, and then dried to form a negative electrode active material layer on the surface of the negative electrode current collector, which serves as the negative electrode. (3) The layers are stacked so that an electrolyte layer (solid electrolyte layer or separator) is sandwiched between the negative electrode and the positive electrode, and a laminate is obtained having the negative electrode current collector, negative electrode active material layer, electrolyte layer, positive electrode active material layer and positive electrode current collector in this order. Other members such as terminals are attached to the laminate as needed. (4) The laminate is placed in a battery case, and in the case of an electrolyte battery, the electrolyte is filled into the battery case, the laminate is immersed in the electrolyte, and the laminate is sealed inside the battery case to form a battery. In the case of an electrolyte battery, the negative electrode active material layer, separator and positive electrode active material layer may be made to contain the electrolyte at the stage of (3) above. [Examples]

[0052] The technology of this disclosure will be described in more detail below with reference to examples, but the technology of this disclosure is not limited to the following examples.

[0053] 1. Preparation of positive electrode active material 1.1 Example 1 1.1.1 Preparation of Precursor Particles (1) MnSO4·5H2O, NiSO4·6H2O, and CoSO4·7H2O 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. (2) 1000 mL of pure water was placed in a reaction vessel (with baffles), and 500 mL of the first solution and 500 mL of the second solution were added dropwise, each at a rate of approximately 4 mL / min. (3) After the dropwise addition was complete, the mixture was stirred at room temperature at a stirring speed of 100 rpm for 1 hour to obtain the product. (4) The product was washed with pure water, and solid-liquid separation was performed using a centrifuge to recover the precipitate. (5) The obtained precipitate was dried overnight at 120°C, ground in a mortar, and then separated into coarse particles and fine particles by air classification. Here, both the coarse particles and fine particles are composite salts containing Mn, Ni, and Co. In Example 1, the above coarse particles were used as precursor particles.

[0054] 1.1.2 Fabrication of composite particles (1) After weighing Na2CO3 and distilled water to a concentration of 1150 g / L, an aqueous solution of Na2CO3 was prepared by stirring with a stirrer until it was completely dissolved. (2) The above precursor particles were mixed in an aqueous solution of Na2CO3 to form a slurry. The target composition after calcination, as described later, is Na2CO3. 0.7 Mn 0.4 Ni 0.2 Co 0.4 The mixture was prepared to produce O2. (3) The obtained slurry was dried by spray drying. Specifically, a DL410 spray dryer was used with a slurry delivery rate of 30 mL / min, an inlet temperature of 200°C, and a circulating air volume of 0.8 m³. 3 Composite particles were obtained by coating more than 70% of the surface area of ​​the above precursor particles with Na2CO3 under conditions of / min and a spray air pressure of 0.3 MPa.

[0055] 1.1.3 Firing of composite particles The composite particles were placed in an alumina crucible and calcined under an atmospheric environment to obtain a Na-containing oxide having a P2-type structure. The calcination conditions were as follows (1) to (7). (1) Place an alumina crucible containing the above-mentioned composite particles in a heating furnace in an atmospheric environment. (2) Heat the inside of the furnace from room temperature (25°C) to 600°C in 115 minutes. (3) Maintain the temperature inside the heating furnace at 600°C for 360 minutes to perform pre-firing. (4) After pre-firing, the temperature inside the heating furnace is raised from 600°C to 800°C over 60 minutes. (5) Maintain the temperature inside the furnace at 800°C for 60 minutes to perform the final firing. (6) After the main firing, the temperature inside the furnace is lowered from 800°C to 250°C over 80 minutes. (7) Remove the alumina crucible from the heating furnace at 250°C and allow it to cool in a dry atmosphere.

[0056] By grinding the calcined material after cooling in a dry atmosphere using a mortar and pestle, Na-containing oxide particles having a P2-type structure were obtained.

[0057] 1.1.4 Ion exchange (1) LiNO3 and LiCl were weighed in a molar ratio of 50:50 and mixed with the above Na-containing oxide particles in a molar ratio that was 10 times the minimum amount of Li required for ion exchange to obtain a mixture. (2) Using an alumina crucible, ion exchange was performed at 280°C for 1 hour under an atmospheric atmosphere to obtain a product containing Li-containing oxide particles. (3) The salt remaining in the product was washed with pure water, and solid-liquid separation was performed by vacuum filtration to obtain a precipitate. (4) The obtained precipitate was dried overnight at 120°C to obtain Li-containing oxide particles having an O2-type structure.

[0058] 1.1.5 Additional firing The obtained Li-containing oxide particles were placed in an alumina crucible and fired in an Ar atmosphere at 250°C for 5 hours to obtain the positive electrode active material according to Example 1. XRD analysis confirmed that the positive electrode active material according to Example 1 had an O2-type structure.

[0059] 1.2 Example 2 1.2.1 Preparation of Precursor Particles Precursor particles were prepared in the same manner as in Example 1.

[0060] 1.2.2 Fabrication of composite particles The target composition after firing is Na 0.7 Mn 0.5 Ni 0.2 Co 0.3 Composite particles were prepared in the same manner as in Example 1, except that Na2CO3 was mixed with the above-mentioned precursor particles to obtain a slurry so that the result was O2.

[0061] 1.2.3 Firing of composite particles The composite particles were placed in an alumina crucible and calcined under an atmospheric environment to obtain a Na-containing oxide having a P2-type structure. The calcination conditions were as follows (1) to (7). (1) Place an alumina crucible containing the above-mentioned composite particles in a heating furnace in an atmospheric environment. (2) Heat the inside of the furnace from room temperature (25°C) to 600°C in 115 minutes. (3) Maintain the temperature inside the heating furnace at 600°C for 360 minutes to perform pre-firing. (4) After pre-firing, the temperature inside the heating furnace is raised from 600°C to 900°C in 100 minutes. (5) Maintain the temperature inside the furnace at 900°C for 60 minutes to perform the final firing. (6) After the main firing, the temperature inside the furnace is lowered from 900°C to 250°C over 120 minutes. (7) Remove the alumina crucible from the heating furnace at 250°C and allow it to cool in a dry atmosphere.

[0062] 1.2.4 Ion exchange and additional calcination Similar to Example 1, after grinding in a mortar, ion exchange and additional calcination were performed to obtain the positive electrode active material according to Example 2. XRD confirmed that the positive electrode active material according to Example 2 had an O2 type structure.

[0063] 1.3 Example 3 The procedure up to ion exchange was the same as in Example 2. After ion exchange, the Li-containing oxide particles obtained were placed in an alumina crucible and calcined at 250°C for 5 hours in an atmospheric environment (air atmosphere) to obtain the positive electrode active material according to Example 3. Confirmation by XRD showed that the positive electrode active material according to Example 3 had an O2 type structure.

[0064] 1.4 Comparative Example 1 Except for not performing additional calcination, the procedure was the same as in Examples 2 and 3. XRD analysis confirmed that the positive electrode active material in Comparative Example 1 had an O2-type structure.

[0065] 1.5 Comparative Examples 2 and 3 As a lithium-free cathode active material, a cathode active material having an O3-type structure was prepared. The cathode active material in Comparative Example 2 is an NCA-based cathode active material (LiNi 0.85 Co 0.10 Al 0.05 O2) and the positive electrode active material in Comparative Example 3 is an NCM-based positive electrode active material (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3The positive electrode active material in Comparative Examples 2 and 3 was obtained by mixing a Li source and a transition metal source and firing them at a high temperature.

[0066] 2. Determination of impurities contained in the positive electrode active material. For each of the positive electrode active materials in Examples 1-3 and Comparative Examples 1-3, the carbon content, sulfur content, chlorine content, and nitrogen content were quantified using the following method. (1) The amount of carbon in the sample was measured by combustion-infrared absorption spectroscopy. A carbon-sulfur analyzer (CS844 model) from LECO was used as the measuring instrument. (2) The amount of sulfur and chlorine in the sample was measured by combustion-ion chromatography. The measurement equipment used was a sample combustion device (AQF-2100S) from Mitsubishi Chemical Analytec and an ion chromatograph (Compact IC Flex) from Metrohm. (3) The amount of nitrogen in the sample was measured by the inert gas melting method. The measuring device used was the EMGA-920Type-Ar from Horiba, Ltd.

[0067] 3. Creating evaluation cells Evaluation cells were fabricated using each of the above-mentioned positive electrode active materials. The procedure for fabricating the evaluation cells is as follows. (1) A positive electrode composite was obtained by weighing and mixing positive electrode active material, sulfide solid electrolyte (Li2S-P2S5-LiI-LiBr), PVDF, and VGCF in the mass ratio of positive electrode active material:sulfide solid electrolyte:PVDF:VGCF = 81.1:15.9:0.6:2.4. The obtained positive electrode composite was dispersed in a solvent (butyl butyrate) to obtain a positive electrode slurry. The obtained positive electrode slurry was coated onto a positive electrode current collector (Al foil) and dried. Then, a positive electrode active material layer was formed on the surface of the positive electrode current collector by pressing with a linear pressure of 100kN at the press temperature shown in Table 1 below using a roll press. (2) A negative electrode composite was obtained by weighing and mixing negative electrode active material (lithium titanate), sulfide solid electrolyte (Li2S-P2S5-LiI-LiBr), PVDF, and VGCF in a mass ratio of negative electrode active material:sulfide solid electrolyte:PVDF:VGCF = 72.1:22.7:3.5:1.7. The obtained negative electrode composite was dispersed in a solvent (butyl butyrate) to obtain a negative electrode slurry. The obtained negative electrode slurry was coated onto a negative electrode current collector (Cu foil) and dried. Then, a negative electrode active material layer was formed on the surface of the negative electrode current collector by pressing with a roll press at room temperature (25°C) and a linear pressure of 60kN. (3) A sulfide solid electrolyte (Li2S-P2S5-LiI-LiBr) and acrylate butadiene rubber (ABR) were weighed and mixed in a mass ratio of sulfide solid electrolyte:ABR = 99.4:0.6 to obtain an electrolyte mixture. (4) An electrolyte mixture was sandwiched between the positive electrode active material layer and the negative electrode active material layer to obtain a laminate having a positive electrode current collector, a positive electrode active material layer, a solid electrolyte layer, a negative electrode active material layer, and a negative electrode current collector in this order. The obtained laminate was pressed to obtain an evaluation cell (solid battery).

[0068] 4. Evaluation of charge and discharge characteristics For each evaluation cell of Examples 1-3 and Comparative Example 1, a charge-discharge test was performed in a constant temperature bath maintained at 25°C, with a voltage range of 1.8-4.6V and a current of 0.1C (1C = 220mA / g) for two cycles. Subsequently, in a constant temperature bath maintained at 25°C, the DCIR resistance was measured by applying a current equivalent to 3C for 10 seconds at a state of 50% SOC (2.35V vs. LTO). The measurement results are shown in Table 1 below.

[0069] 5. Evaluation Results Table 1 below shows, for each of Examples 1-3 and Comparative Examples 1-3, the crystal structure and chemical composition of the positive electrode active material, the atmosphere during additional calcination after ion exchange, the amount of impurities contained in the positive electrode active material (carbon, sulfur, chlorine, nitrogen), the results of identification of the impurities by XPS, the position of the (002) plane peak top by XRD, and the results of resistance measurement of the evaluation cell.

[0070] [Table 1]

[0071] As is clear from the results shown in Table 1, the positive electrode active material in Comparative Example 1 had a high carbon content of 900 ppm, resulting in a high resistance in the evaluation cell. In contrast, the positive electrode active materials in Examples 1 to 3 had a low carbon content of 500 ppm or less, resulting in a low resistance in the evaluation cell. It is thought that the carbon content of the positive electrode active materials in Examples 1 to 3 was reduced because some of the lithium carbonate impurity was decomposed by additional calcination of the Li-containing oxide after ion exchange (Figure 2). Comparing Comparative Example 1 with Examples 1 to 3, the peak top of the 002 plane in the X-ray diffraction pattern is shifted, which is thought to be due to the incorporation of lithium produced by the decomposition of lithium carbonate impurity into the O2-type structure, changing the c-axis length. Such decomposition and removal of impurities by additional calcination is unique to positive electrode active materials having an O2-type structure, which is a Li-deficient structure.

[0072] Furthermore, the positive electrode active material in Comparative Example 2 had high levels of both carbon and sulfur. The positive electrode active material in Comparative Example 3 had a low carbon content but a high sulfur content. The positive electrode active materials in Comparative Examples 2 and 3 have a Li-free O3 type structure, and even if additional calcination were performed, it would be difficult to reduce the carbon content.

[0073] 6. Supplement In the above examples, the case in which precursor particles are obtained by coprecipitation is illustrated, but precursor particles can also be obtained by other methods. Also, in the above examples, the case in which composite particles are obtained by mixing precursor particles with a Na2CO3 solution is illustrated, but composite particles can also be obtained by other methods. Furthermore, in the above examples, Na-containing oxides having a P2-type structure and Li-containing oxides having an O2-type structure are illustrated with examples having a predetermined chemical composition, but the chemical composition of Na-containing oxides and Li-containing oxides is not limited to these. For example, Na-containing oxides and Li-containing oxides may be doped with an element M other than Mn, Ni, and Co. The element M is as described in the examples. [Explanation of symbols]

[0074] 100 Lithium-ion batteries 10 Cathode active material layer 20 Electrolyte layer 30 Negative electrode active material layer 40 Positive electrode current collector 50 Negative electrode current collector

Claims

1. A method for producing a positive electrode active material, To obtain a Li-containing oxide having an O2-type structure by ion-exchanging at least a portion of the Na in a Na-containing oxide having a P2-type structure with Li, and Heating the Li-containing oxide to reduce the carbon content of the Li-containing oxide, A method for producing a positive electrode active material, including the active material.

2. A method for producing a positive electrode active material according to claim 1, The atmosphere used when heating the Li-containing oxide is an inert gas atmosphere or an oxygen-containing atmosphere. The temperature at which the Li-containing oxide is heated is 200°C or higher and 300°C or lower. A method for manufacturing a positive electrode active material.

3. A method for producing a positive electrode active material according to claim 1 or 2, Washing the Li-containing oxide with water to reduce the amount of sulfur in the Li-containing oxide, A method for producing a positive electrode active material, including the active material.

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