Method for producing positive electrode active material

Abstract

To provide a method for increasing the capacity of an O2-type positive electrode active material.SOLUTION: A method for manufacturing a positive electrode active material disclosed herein includes calcining a precursor containing Na, followed by cooling to obtain a Na-containing transition metal oxide having a P2 type structure, and substituting at least a portion of the Na in the Na-containing transition metal oxide with Li by ion exchange to obtain the positive electrode active material having an O2 type structure, and after calcining the precursor, the cooling rate from 250°C to the cooling end temperature is 20°C / min or more.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present application discloses a method for producing a positive electrode active material. [Background technology]

[0002] Positive electrode active materials having an O2-type structure are 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 Na in a sodium-containing transition metal oxide having a P2-type structure with Li. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-186937 Summary of the Invention [Problem to be solved by the invention]

[0004] Conventional positive electrode active materials with an O2-type structure have room for improvement in terms of capacity. [Means for solving the problem]

[0005] The present application discloses the following aspects as means for solving the above problems. <Aspect 1> A method for producing a positive electrode active material, Obtaining a sodium-containing transition metal oxide having a P2 type structure, and and substituting at least a portion of the Na in the Na-containing transition metal oxide with Li by ion exchange to obtain a positive electrode active material having an O2-type structure, The water content of the Na-containing transition metal oxide is 1000 ppm or less. A method for producing a positive electrode active material. <Aspect 2> A method for producing a positive electrode active material, After firing a precursor containing Na and a transition metal element and then cooling it, a Na-containing transition metal oxide having a P2-type structure is obtained, and at least a part of Na in the Na-containing transition metal oxide is replaced with Li by ion exchange to obtain a positive electrode active material having an O2-type structure, after firing the precursor, the cooling rate from 250 °C to the end temperature of cooling is 20 °C / min or more, A method for producing a positive electrode active material. <Aspect 3> The firing of the precursor is carried out in a heating furnace, and the cooling from 250 °C to the end temperature of cooling is carried out outside the heating furnace, The production method of Aspect 2. <Aspect 4> The chemical composition of the positive electrode active material is Li a Na b Mn x-p Ni y-q Co z-r M p+q+r O2 (where 0 < a ≤ 1.00, 0 ≤ b ≤ 0.20, x + y + z = 1, and 0 ≤ p + q + r ≤ 0.07, 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). The production method according to any one of Aspects 1 to 3.

Advantages of the Invention

[0006] The positive electrode active material of the present disclosure has a high capacity.

Brief Description of the Drawings

[0007] [Figure 1] An example of the process of the method for producing a positive electrode active material according to the first embodiment is shown. [Figure 2] An example of the process of the method for producing a positive electrode active material according to the second embodiment is shown. [Figure 3] The firing conditions of the precursor in Example 1 are shown. [Figure 4]This shows the relationship between the water content of the P2-type sodium-containing transition metal oxide and the discharge capacity of the coin cell. DETAILED DESCRIPTION OF THE INVENTION

[0008] 1. Method for producing positive electrode active material (first embodiment) 1, the method for producing a positive electrode active material according to the first embodiment includes obtaining a sodium-containing transition metal oxide having a P2-type structure (step S1), and substituting at least a portion of the sodium in the sodium-containing transition metal oxide with lithium by ion exchange to obtain a positive electrode active material having an O2-type structure (step S2). Here, the water content of the sodium-containing transition metal oxide is 1000 ppm or less.

[0009] 1.1 Process S1 A positive electrode active material having an O2-type structure is produced by obtaining a Na-containing transition metal oxide having a P2-type structure similar to the O2-type structure, and then ion-exchanging at least a portion of the Na in the Na-containing transition metal oxide with Li. The inventors have found that H2O molecules are easily trapped between the layers of a P2-type structure. If H2O molecules are trapped between the layers of a P2-type structure, the P2-type structure may collapse, restricting the Na conduction path. In other words, even if a Na-containing transition metal oxide with a high water content is used to ion-exchange the Na in the oxide with Li, the ion exchange may not proceed easily, and the desired O2-type structure may not be obtained. The inventors have found that the above-mentioned problems can be solved by obtaining a Na-containing transition metal oxide having a P2-type structure with a water content of 1000 ppm or less in step S1 and then using this to perform step S2.

[0010] In step S1, the Na-containing transition metal oxide having a P2 type structure can be obtained, for example, by obtaining a precursor containing Na and a transition metal element, optionally shaping the precursor, optionally pre-firing the precursor, and then performing main firing.

[0011] In step S1, the precursor may be obtained by, for example, mixing a transition metal source and a Na source. The transition metal source may be, for example, a transition metal salt such as a carbonate, sulfate, nitrate, or acetate, or a transition metal compound such as a transition metal hydroxide. The transition metal element may be at least one of Mn, Ni, and Co. The transition metal source may be Me(CO3) x (Me is at least one transition metal element selected from Mn, Ni, and Co, and x is the valence of Me), or a salt represented by Me(SO4) x or a salt represented by Me(NO3) x or a salt represented by Me(CH3COO) x or a salt represented by Me(OH) x The Na source may be, for example, a Na salt such as a carbonate or a sulfate, or a Na compound such as sodium oxide or sodium hydroxide. The amount of the Na source to be mixed with the transition metal source may be determined taking into account the amount of Na lost during subsequent calcination. In step S1, the surfaces of particles made of the transition metal source may be coated with the Na source to obtain coated particles as precursors. Here, the coated particles may be obtained by coating at least a portion of the surfaces of particles made of the transition metal source with the Na source. The coated particles may be obtained by coating 40 area % or more, 50 area % or more, 60 area % or more, or 70 area % of the surfaces of particles made of the transition metal source with the Na source.

[0012] In step S1, the precursor may be obtained by mixing, for example, a transition metal source, a Na source, and an M source containing the element M. Here, 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. The element M can further stabilize the P2 type structure or the O2 type structure. The M source may be, for example, a salt such as a nitrate, sulfate, carbonate, or acetate, or a compound other than a salt such as a hydroxide. The amount of the M source in the precursor may be appropriately determined depending on the target composition of the Na-containing transition metal oxide after firing.

[0013] In step S1, the precursor may be obtained by, for example, obtaining a precipitate using an ion source capable of forming a precipitate in an aqueous solution with transition metal ions and a transition metal compound, and then mixing the precipitate with a sodium source. Examples of ion sources capable of forming a precipitate with transition metal ions include sodium salts such as sodium carbonate and sodium nitrate, as well as sodium hydroxide and sodium oxide. Examples of transition metal compounds include salts and hydroxides such as nitrates, sulfates, carbonates, and acetates. In step S1, the ion source and the transition metal compound may be prepared as solutions, and then the solutions may be added dropwise and mixed to obtain a precipitate. In this case, various sodium compounds may be used as bases, and aqueous ammonia or the like may be added to adjust the basicity. More specifically, in step S1, the precipitate may contain at least one transition metal element selected from Mn, Ni, and Co. The precipitate may be obtained by a solution method such as a coprecipitation method or a sol-gel method. Specifically, in the case of the coprecipitation method, for example, Me(SO4) xAn aqueous solution of Na2CO3 and an aqueous solution of Na2CO3 are prepared, and the respective aqueous solutions are added dropwise and mixed to obtain a precipitate. The precipitate may be collected and then mixed with a Na source. The amount of Na source to be mixed with the precipitate may be determined taking into account the amount of Na lost during subsequent calcination. Alternatively, the surfaces of particles made of the precipitate may be coated with a Na salt to obtain coated particles as precursors. Here, the coated particles may be obtained by coating at least a portion of the surfaces of particles made of the above precipitate with a Na salt. The coated particles may be obtained by coating 40 area% or more, 50 area% or more, 60 area% or more, or 70 area% of the surfaces of particles made of the above precipitate with a Na salt.

[0014] In step S1, the precursor obtained as described above is pre-baked at a temperature equal to or lower than that of the main baking. For example, pre-baking can be performed at a temperature lower than 700°C. The pre-baking time is not particularly limited. Alternatively, pre-baking may be omitted.

[0015] In step S1, the precursor may be calcined at a temperature of, for example, 700°C or higher and 1100°C or lower. The temperature is preferably 800°C or higher and 1000°C or lower. If the calcination temperature is too low, Na doping will not occur, and if the calcination temperature is too high, an O3 structure will likely form rather than a P2 structure. The temperature rise conditions from the pre-calcination temperature to the calcination temperature are not particularly limited. The calcination time is also not particularly limited, and may be, for example, 30 minutes to 10 hours. The calcination atmosphere is also not particularly limited, and may be, for example, an oxygen-containing atmosphere such as air or an inert gas atmosphere.

[0016] In step S1, the Na-containing transition metal oxide having a P2 structure is obtained by cooling after the main firing. Here, as will be described later as a second embodiment, the amount of water contained in the Na-containing transition metal oxide can be reduced by controlling the cooling rate after the main firing. Alternatively, the amount of water contained in the Na-containing transition metal oxide can be reduced by cooling in an atmosphere with a low water content.

[0017] In step S1, after the main firing, the Na-containing transition metal oxide having a P2 type structure may be doped with the element M. That is, after synthesizing a Na-containing transition metal oxide having a P2 type structure that does not contain the element M, the oxide may be doped with the element M. The doping with the element M may be performed by, for example, ion exchange.

[0018] The Na-containing transition metal oxide obtained by step S1 may contain, for example, as constituent elements, at least one element selected from Mn, Ni, and Co, Na, and O. In particular, when the constituent elements contain at least Na, Mn, at least one of Ni and Co, and O, and especially when the constituent elements contain at least Na, Mn, Ni, Co, and O, the performance of the positive electrode active material is likely to be further improved. More specifically, the Na-containing transition metal oxide obtained by step S1 contains Na c Mn x-p Ni y-q Co z-r M p+q+rIt may have a chemical composition represented by O2 (where 0 < c ≤ 1.00, x + y + z = 1, and 0 ≤ p + q + r ≤ 0.07, and the element M is at least one selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W). When the Na-containing transition metal oxide has such a chemical composition, the P2-type structure is likely to be maintained. In the above chemical composition, c may be greater than 0, 0.10 or more, 0.20 or more, 0.30 or more, 0.40 or more, 0.50 or more, or 0.60 or more, and may also be 1.00 or less, 0.90 or less, 0.80 or less, or 0.70 or less. Also, x may be 0 or more, 0.10 or more, 0.20 or more, 0.30 or more, 0.40 or more, or 0.50 or more, and may also be 1.00 or less, 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, or 0.50 or less. Also, y may be 0 or more, 0.10 or more, or 0.20 or more, and may also be 1.00 or less, 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 may be 0 or more, 0.10 or more, 0.20 or more, or 0.30 or more, and may also be 1.00 or less, 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, 0.50 or less, 0.40 or less, or 0.30 or less. Also, p + q + r may be 0 or more, greater than 0, 0.01 or more, 0.02 or more, or 0.03 or more, and may also be 0.07 or less, 0.06 or less, 0.05 or less, or 0.04 or less. The composition of O is approximately 2, but is not necessarily exactly 2.0 and is indeterminate.

[0019] As described above, in step S1, the water content of the Na-containing transition metal oxide can be reduced by controlling, for example, the cooling atmosphere or the cooling rate. It is important that the water content of the Na-containing transition metal oxide obtained in step S1 is 1000 ppm or less. The water content may be 900 ppm or less, 850 ppm or less, 800 ppm or less, 750 ppm or less, 700 ppm or less, 650 ppm or less, 600 ppm or less, or 550 ppm or less. There is no particular lower limit for the water content, and it may be 0 ppm or more, 10 ppm or more, 50 ppm or more, or 100 ppm or more. The water content is the mass ratio of water to the total mass of the Na-containing transition metal oxide. In other words, 1000 ppm or less means 0.1 mass% or less. The "water content of the Na-containing transition metal oxide" referred to herein refers to the water content at 200°C as measured by Karl Fischer titration.

[0020] 1.2 Process S2 In step S2, at least a portion of the Na in the Na-containing transition metal oxide obtained in step S1 is substituted with Li by ion exchange to obtain a positive electrode active material having an O2-type structure. In step S2, at least a portion of the Na in the Na-containing transition metal oxide can be substituted with Li by, for example, ion exchange using a lithium salt. For example, a Na-containing transition metal oxide having a P2-type structure and a lithium salt are mixed, and then heated to a temperature equal to or higher than the melting point of the lithium salt to melt the lithium salt, thereby substituting at least a portion of the Na with Li by ion exchange. The lithium salt may be, for example, a lithium halide. The lithium halide may be at least one of lithium chloride, lithium bromide, and lithium iodide. Alternatively, the lithium salt may be lithium nitrate. Alternatively, the lithium salt may be a mixed salt of a lithium halide and lithium nitrate.

[0021] In step S2, the element M may be doped during the ion exchange. For example, a salt containing the element M may be heated and melted, and then brought into contact with the Na-containing transition metal oxide to dope the oxide with the element M. Examples of the salt containing the element M include halides of the element M. In step S2, the Na-containing transition metal oxide may be brought into contact with a salt containing Li and the element M, thereby ion-exchanging at least a portion of the Na in the Na-containing transition metal oxide particles with Li and doping the oxide with the element M. When a salt containing Li and the element M (a mixed salt of a lithium salt and a salt of the element M, or a composite salt of Li and the element M) is used, the melting point of the salt may be lower than when a lithium salt or a salt of the element M is used alone. In particular, when a salt containing at least one of Al and Ga and Li is used as the element M, the melting point is likely to be significantly lower. That is, the temperature required for melting is lowered, and the ion exchange of Li and the doping of the element M can be performed at a low temperature. There are no particular limitations on the mixing ratio of the lithium salt and the salt of element M. Specific examples of the salt containing Li and element M include salts containing Li, element M, and a halogen (mixed salts of lithium halide and a halide of element M, or composite halide of Li and element M).

[0022] The temperature in step S2 (for example, the heating temperature when a lithium salt is brought into contact with the Na-containing transition metal oxide particles and then heated and melted to perform ion exchange) may be, for example, 600°C or lower, 500°C or lower, 400°C or lower, 350°C or lower, 300°C or lower, 280°C or lower, 250°C or lower, 230°C or lower, 200°C or lower, 170°C or lower, or 150°C or lower, and may be room temperature or higher or 100°C or higher. If the temperature is too high, a stable O3-type structure is likely to be formed instead of an O2-type structure. When melting the lithium salt, as described above, it is sufficient to heat to a temperature higher than the melting point of the lithium salt. The time in step S2 (for example, the heating time when a lithium salt is brought into contact with the Na-containing transition metal oxide and then heated and melted to perform ion exchange) may be adjusted so that most of the Na in the Na-containing transition metal oxide particles is replaced with Li. From the viewpoint of ensuring sufficient time for the lithium salt to melt, the time in step S2 may be, for example, 10 minutes or more or 60 minutes or more, and 12 hours or less or 6 hours or less. The atmosphere in step S2 is not particularly limited and may be, for example, an oxygen-containing atmosphere such as air atmosphere or an inert gas atmosphere. After ion exchange, the Li-containing transition metal oxide having an O2-type structure may be subjected to some post-treatment such as washing.

[0023] 2. Method for producing positive electrode active material (second embodiment) 2, the method for producing a positive electrode active material according to the second embodiment includes steps of calcining a precursor containing Na and a transition metal element, followed by cooling to obtain a Na-containing transition metal oxide having a P2-type structure (step S11), and substituting at least a portion of Na in the Na-containing transition metal oxide with Li by ion exchange to obtain a positive electrode active material having an O2-type structure (step S12). Here, after calcining the precursor, the cooling rate from 250°C to the cooling end temperature is 20°C / min or more.

[0024] 2.1 Process S11 The conditions for preparing the precursor and the conditions for firing the precursor in step S11 are the same as those in step S1 of the first embodiment, and therefore a detailed description thereof will be omitted here. Step S11 is characterized in that, when cooling the fired precursor, the precursor is rapidly cooled at a cooling rate of 20°C / min or more from at least 250°C to the cooling end temperature. For example, after firing the precursor at a firing temperature of 700°C to 1100°C, cooling is initiated from the firing temperature, and cooling is controlled so that the cooling rate is 20°C / min or more from at least 250°C to the cooling end temperature.

[0025] In step S11, after the precursor is calcined, the cooling rate from the calcination temperature to 250°C is not particularly limited. For example, the cooling rate from the calcination temperature to 250°C may be 20°C / min or more, or may be less than 20°C / min. The cooling atmosphere from the calcination temperature to 250°C is also not particularly limited. For example, cooling may be performed in the same atmosphere as the calcination atmosphere.

[0026] In step S11, the cooling rate from 250°C to the cooling end temperature is 20°C / min or more. The "cooling end temperature" refers to any temperature below 100°C. The cooling end temperature is not necessarily the temperature at which cooling is completely completed (cooling is stopped), but may be any temperature below 100°C. For example, when the precursor is fired in a heating furnace, the cooling end temperature may be the same as the temperature outside the heating furnace. In step S11, for example, the cooling rate from 250°C to 100°C may be 20°C / min or more, the cooling rate from 250°C to 50°C may be 20°C / min or more, and the cooling rate from 250°C to 25°C may be 20°C / min or more. The cooling rate may be 21°C / min or more or 22°C / min or more. The upper limit of the cooling rate is not particularly limited and may be, for example, 100°C / min or less, 80°C / min or less, 60°C / min or less, or 40°C / min or less.

[0027] In step S11, the precursor may be calcined in a heating furnace, and cooling may be performed outside the heating furnace at least from 250°C to the cooling end temperature. For example, in step S11, the precursor may be calcined in a heating furnace to obtain a P2-type Na-containing transition metal oxide, and the Na-containing transition metal oxide may be removed from the heating furnace at any temperature equal to or higher than 250°C and cooled. In this way, by cooling outside the heating furnace (e.g., by cooling in the air) at least from 250°C to the cooling end temperature, the cooling rate from 250°C to the cooling end temperature can be set to 20°C / min or more. Note that the heating furnace may be a known heating furnace such as a muffle furnace or an electric furnace.

[0028] In step S11, when the calcined precursor is cooled, rapid cooling at a cooling rate of 20°C / min or more from at least 250°C to the cooling end temperature makes it difficult for moisture to penetrate between the layers of the P2 type structure, thereby reducing the moisture content contained in the Na-containing transition metal oxide obtained after cooling. For example, the moisture content contained in the Na-containing transition metal oxide may be 1000 ppm or less, 900 ppm or less, 850 ppm or less, 800 ppm or less, 750 ppm or less, 700 ppm or less, 650 ppm or less, 600 ppm or less, or 550 ppm or less. There is no particular lower limit to the moisture content, and it may be 0 ppm or more, 10 ppm or more, 50 ppm or more, or 100 ppm or more. According to the inventors' estimation, in a certain temperature range from 250°C to the cooling end temperature, moisture is likely to penetrate between the layers of the P2 type structure due to atomic vibration, molecular motion, etc. When cooling a Na-containing transition metal oxide having a P2 type structure, it is thought that by shortening the time during which the temperature range in which moisture easily penetrates is reached (i.e., by cooling quickly), the amount of moisture that penetrates between the layers of the P2 type structure can be reduced.

[0029] 2.2 Process S12 Step S12 is similar to step S2 in the first embodiment, and therefore a detailed description thereof will be omitted here.

[0030] 3.Cathode active material As described above, the method for producing a cathode active material according to the first or second embodiment can produce a cathode active material having an O2-type structure (O2-type Li-containing transition metal oxide). The Na-containing transition metal oxide used in steps S2 and S12 has a low water content, properly maintains a P2-type crystal structure, and properly maintains the Na conduction path. By using such a Na-containing transition metal oxide, Na can be efficiently ion-exchanged with Li in steps S2 and S12.

[0031] 3.1 Crystal structure The positive electrode active material has at least an O2-type structure (belonging to the space group P63mc). The positive electrode active material has the O2-type structure, but may also have a crystal structure other than the O2-type structure. Examples of crystal structures other than the O2-type structure include a T#2-type structure (belonging to the space group Cmca) formed when Li is inserted and removed from the O2-type structure, and an O6-type structure (belonging to the space group R-3m, with a c-axis length of 2.5 nm to 3.5 nm, typically 2.9 nm to 3.0 nm, and different from the O3-type structure also belonging to the space group R-3m). The positive electrode active material may have an O2-type structure as the main phase, or may have a crystal structure other than the O2-type structure as the main phase. However, a positive electrode active material having an O2-type structure as the main phase is particularly preferred. The crystal structure of the main phase of the positive electrode active material may change depending on the charge / discharge state.

[0032] 3.2 Chemical composition The positive electrode active material may contain, as constituent elements, at least one element selected from Mn, Ni, and Co, Li, and O. In particular, when the constituent elements include at least Mn, at least one of Ni and Co, Li, and O, and especially when the constituent elements include at least Li, Mn, Ni, Co, and O, higher performance is likely to be ensured. However, in the positive electrode active material, for example, Li may be almost completely released upon charging, and the molar concentration of Li may approach 0. Furthermore, the positive electrode active material may contain Na as a constituent element due to the above-mentioned manufacturing process. Furthermore, the positive electrode active material may contain the above-mentioned element M. Furthermore, the positive electrode active material may contain other impurity elements. The chemical composition of the positive electrode active material is Li a Na b Mn 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 ≤ 0.07, 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). It may be represented by this. In this chemical composition, a may be greater than 0, 0.10 or more, 0.20 or more, 0.30 or more, 0.40 or more, 0.50 or more, or 0.60 or more, and may also be 1.00 or less, 0.90 or less, 0.80 or less, or 0.70 or less. Also, b may be 0 or more or greater than 0, and may also be 0.15 or less, 0.10 or less, 0.05 or less, 0.04 or less, 0.03 or less, 0.02 or less, or 0.01 or less. Also, x may be 0 or more, 0.10 or more, 0.20 or more, 0.30 or more, 0.40 or more, or 0.50 or more, and may also be 1.00 or less, 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, or 0.50 or less. Also, y may be 0 or more, 0.10 or more, or 0.20 or more, and may also be 1.00 or less, 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 may be 0 or more, 0.10 or more, 0.20 or more, or 0.30 or more, and may also be 1.00 or less, 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, 0.50 or less, 0.40 or less, or 0.30 or less. The element M has a small contribution to charge and discharge. In this regard, in the above chemical composition, when p + q + r is 0.07 or less, it is easy to ensure a high charge and discharge capacity. p + q + r may be 0.06 or less, 0.05 or less, or 0.04 or less. On the other hand, when the element M is included, the O2-type structure is likely to be stabilized. In this regard, in the above chemical composition, p + q + r is 0 or more, and may also be greater than 0, 0.01 or more, 0.02 or more, or 0.03 or more. The composition of O is approximately 2, but it is not necessarily exactly 2.0 and is indefinite. Also, in the above chemical composition of the positive electrode active material, when the valence of the element M is +n, the relationship 3.0 ≤ 4(x - p) + 2(y - q) + 3(z - r) + n(p + q + r) ≤ 3.5 may be satisfied.This indicates that the total valence of the metals in the positive electrode active material is in a range close to 3.33 (charge neutrality when a is 0.67). As described above, a positive electrode active material having an O2-type structure is synthesized via a sodium-containing transition metal oxide having a P2-type structure, and charge neutrality occurs when the Na composition at this time is in the range of 0.5 to 1.0, which corresponds to the case where the above relationship is satisfied.

[0033] 3.3 Shape The positive electrode active material may be in the form of particles. The positive electrode active material particles may be solid particles, hollow particles, or particles having voids. The positive electrode active material particles may be primary particles or secondary particles formed by aggregation of a plurality of primary particles. The average particle diameter (D50) of the positive electrode active material particles may be, for example, 1 nm or more, 5 nm or more, or 10 nm or more, and may be 500 μm or less, 100 μm or less, 50 μm or less, or 30 μm or less. The average particle diameter D50 referred to in this application is the particle diameter (median diameter) at 50% cumulative value in a volume-based particle size distribution determined by a laser diffraction / scattering method.

[0034] 4. Lithium-ion battery manufacturing method The positive electrode active material produced as described above is used, for example, as a positive electrode active material for a lithium ion battery. The lithium ion battery can be produced, for example, as follows, but is not limited thereto, and each layer may be formed by dry molding or the like. (1) A cathode active material constituting the cathode active material layer of a lithium-ion battery is dispersed in a solvent to obtain a cathode layer slurry. The solvent used in this case is not particularly limited, and water or various organic solvents can be used. The cathode layer slurry is then applied to the surface of a cathode current collector using a doctor blade or the like, and then dried to form a cathode active material layer on the surface of the cathode current collector, resulting in a cathode. (2) A negative electrode layer slurry is obtained by dispersing the negative electrode active material and other components that constitute the negative electrode active material layer of a lithium-ion battery in a solvent. The solvent used in this case is not particularly limited, and water or various organic solvents can be used. The negative electrode layer slurry is then applied to the surface of a 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, resulting in a negative electrode. (3) The layers are stacked so that the electrolyte layer (solid electrolyte layer or separator) is sandwiched between the negative electrode and the positive electrode to obtain a laminate having, in this order, the negative electrode current collector, the negative electrode active material layer, the electrolyte layer, the positive electrode active material layer, and the positive electrode current collector. Other members such as terminals are attached to the laminate as necessary. (4) The laminate is housed in a battery case, and in the case of an electrolyte battery, the battery case is filled with an electrolyte, and the laminate is immersed in the electrolyte and sealed in the battery case to form a secondary battery. In the case of an electrolyte battery, the electrolyte may be impregnated into the negative electrode active material layer, the separator, and the positive electrode active material layer at the step (3) above. [Example]

[0035] As described above, one embodiment of the method for producing a positive electrode active material of the present disclosure has been described, but the method for producing a positive electrode active material of the present disclosure can be modified in various ways other than the above embodiment without departing from the gist thereof. Hereinafter, the technology of the present disclosure will be described in more detail with reference to examples, but the technology of the present disclosure is not limited to the following examples.

[0036] 1. Preparation of precursor (mixture of transition metal source and Na source) 1.1 Coprecipitation synthesis of transition metal sources 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. Next, 500 mL of each of the first and second solutions was added dropwise at a rate of approximately 4 mL / min to a reaction vessel containing 1000 mL of pure water. After the addition, the mixture was stirred at 150 rpm for 1 hour at room temperature. The precipitate was washed with pure water and separated into solid and liquid using a centrifuge. The resulting precipitate was dried overnight at 120°C, crushed in a mortar, and then air-classified to remove fine particles, yielding mixed salt particles containing Mn, Ni, and Co (transition metal source).

[0037] 1.2 Mixing of transition metal source and sodium source (Na coating) After weighing out Na2CO3 and distilled water so that the concentration was 1150 g / L, the mixture was stirred using a stirrer until completely dissolved to prepare an aqueous Na2CO3 solution. The above mixed salt particles were mixed into the aqueous Na2CO3 solution to prepare a slurry. The Na2CO3 and the above mixed salt particles were dried and then dissolved in Na 0.7 Mn 0.5 Ni 0.2 Co 0.3 The resulting slurry was dried by spray drying. Specifically, a spray drying apparatus DL410 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 m 3 The surface of the above mixed salt was coated with Na2CO3 under the conditions of 0.3 MPa spray air pressure and 0.5 MPa spraying speed.

[0038] 2. Calcination of precursor particles 2.1 Example 1 The precursor particles were fired in an electric furnace using an alumina crucible under an atmospheric air atmosphere (humidity 50% or higher). Specifically, the precursor particles were subjected to the "first heating step," "pre-firing step," "second heating step," "main firing step," "in-furnace cooling step," and "out-furnace cooling step" as shown in Table 1 below and FIG. 3. The "in-furnace cooling step" refers to a cooling step in an electric furnace, and the "out-furnace cooling step" refers to a step of cooling in the air outside the electric furnace. Thereafter, the precursor particles were pulverized in a mortar at a dew point of -30°C or lower to obtain a sodium-containing transition metal oxide (Na 0.7 Mn 0.5 Ni 0.2 Co 0.3 The resulting Na-containing transition metal oxide was stored in a dry room (dew point -30°C or less).

[0039] [Table 1]

[0040] 2.2 Example 2 The procedure was the same as in Example 1, except that the obtained Na-containing transition metal oxide was stored in an atmosphere with a humidity of 50%.

[0041] 2.3 Comparative Example 1 The procedure was the same as in Example 1, except that the end temperature in the in-furnace cooling process (temperature at which the material was removed from the furnace) was 200°C, the in-furnace cooling time was 140 minutes, the start temperature in the out-furnace cooling process was 200°C, and the cooling rate in the out-furnace cooling process was 17.5°C / min.

[0042] 2.4 Comparative Example 2 The procedure was the same as in Example 1, except that the end temperature in the in-furnace cooling process (temperature at which the material was removed from the furnace) was 150°C, the in-furnace cooling time was 150 minutes, the start temperature in the out-furnace cooling process was 150°C, and the cooling rate in the out-furnace cooling process was 12.5°C / min.

[0043] 2.5 Comparative Example 3 The procedure was the same as in Example 1, except that the end temperature in the in-furnace cooling process (temperature at which the material was removed from the furnace) was 100°C, the in-furnace cooling time was 160 minutes, the start temperature in the out-furnace cooling process was 100°C, and the cooling rate in the out-furnace cooling process was 7.5°C / min.

[0044] 3. Identifying the moisture content The amount of water contained in each of the above Na-containing transition metal oxide powders at 200° C. was measured by Karl Fischer titration.

[0045] 4. Ion Exchange LiNO3 and LiCl were weighed out to a molar ratio of 50:50 and mixed with the above-mentioned Na-containing transition metal oxide in a molar ratio 10 times the minimum Li amount required for ion exchange to obtain a mixture. Subsequently, the mixture was calcined at 280 °C for 1 hour in an alumina crucible under an air atmosphere (humidity 50% or higher). The salt remaining after calcination was washed with pure water and subjected to solid-liquid separation by vacuum filtration. The obtained precipitate was dried overnight at 120 °C to obtain a Li-containing transition metal oxide (Li) with an O2 structure as the positive electrode active material. a Mn 0.5 Ni 0.2 Co 0.3 O2) was obtained. The Li composition ratio a was about 0.6.

[0046] 5. Coin Cell Fabrication The positive electrode active material, acetylene black (AB) as a conductive material, and PVdF as a binder were weighed out to a mass ratio of positive electrode active material:AB:PVdF = 85:10:5, and dispersed and mixed in N-methyl-2-pyrrolidone to obtain a positive electrode slurry. The positive electrode slurry was applied to an Al foil and vacuum dried overnight at 120 °C to obtain a positive electrode. A coin cell (CR2032) was fabricated using the obtained positive electrode, an electrolyte (TDDK-217, manufactured by Daikin Corporation), and a metallic Li foil as a negative electrode.

[0047] 6. Coin Cell Evaluation Charge and discharge were carried out in a thermostatic chamber maintained at 25°C in a voltage range of 2-4.8V at a 0.1C rate (1C=220mA / g), and the discharge capacity was measured.

[0048] 7. Evaluation Results Table 2 below and FIG. 4 show the amount of water contained in each of the Na-containing transition metal oxides of Examples 1 and 2 and Comparative Examples 1 to 3, and the discharge capacity of the coin cells.

[0049] [Table 2]

[0050] As shown in Table 2 and Figure 4, when coin cells were fabricated using positive electrode active materials (Examples 1 and 2) manufactured by a method that satisfied the following condition 1 or 2, the discharge capacity of the coin cells was found to be improved compared to coin cells fabricated using positive electrode active materials (Comparative Examples 1 to 3) manufactured by a method that did not satisfy either condition 1 or 2. Condition 1: The water content of the P2-type sodium-containing transition metal oxide before ion exchange is 1000 ppm or less. Condition 2: After calcining the precursor to obtain a P2-type sodium-containing transition metal oxide, the cooling rate from 250°C to the cooling end temperature is 20°C / min or more.

[0051] In the above examples, a precursor containing Na and a transition metal is obtained through coprecipitation and spray drying, but the conditions for preparing the precursor are not limited to these. Furthermore, in the above examples, a precursor having a specific chemical composition and a P2-type Na-containing transition metal oxide are prepared, and an O2-type positive electrode active material is produced using the precursor. However, the chemical composition of the O2-type positive electrode active material is not limited to these. Furthermore, various conditions can be changed as long as at least one of the above conditions 1 and 2 is satisfied.

Claims

[Claim 1] A method for producing a positive electrode active material, A precursor containing Na and a transition metal element is calcined and then cooled to obtain a Na-containing transition metal oxide having a P2 structure; and and substituting at least a portion of the Na in the Na-containing transition metal oxide with Li by ion exchange to obtain a positive electrode active material having an O2-type structure, after calcining the precursor, a cooling rate from 250°C to a cooling end temperature is 20°C / min or more, and the calcination of the precursor is carried out in a heating furnace, and the cooling from at least 250°C to the cooling end temperature is carried out outside the heating furnace; The positive electrode active material has a chemical composition represented by Li a Na b Mn x-p Ni y-q Co z-r M p+q+r O 2 (wherein 0<a≦1.00, 0≦b≦0.20, 0.30≦x≦0.60, 0.10≦y≦0.50, 0.10≦z≦0.50, x+y+z=1, and 0≦p+q+r≦0.07, 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). A method for producing a positive electrode active material.

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