Method for producing positive electrode active material

Abstract

To provide a method for producing a positive electrode active material in which deterioration of resistance characteristics is suppressed.SOLUTION: A technology disclosed herein relates to a method of manufacturing a positive electrode active material after firing, the method comprising: a preparation step of preparing a process end material including a positive electrode mixture including a positive electrode active material of a secondary battery and a binder containing fluorine; and a firing step of firing the positive electrode mixture in a container. The firing step is performed in a state in which magnesium hydroxide is present in the container. Thus, the positive electrode active material in which deterioration of resistance characteristics is suppressed can be provided.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The technology disclosed herein relates to a method for producing a positive electrode active material. [Background technology]

[0002] In recent years, secondary batteries have been suitably used as portable power sources for personal computers, mobile terminals, and the like, and as power sources for driving vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).

[0003] Positive electrodes used in secondary batteries generally contain positive electrode active materials. In recent years, there has been an increasing demand for recovery technologies that recover metals (e.g., lithium, nickel, cobalt, etc.) contained in positive electrode active materials from process waste materials of positive electrode plates and reuse them as materials for secondary batteries. Furthermore, in the course of studying such recovery technologies, development has been underway for technologies that recover and regenerate positive electrode active materials directly from positive electrode plates as positive electrode active materials without returning them to the metal level. As an example of such a technology, for example, Patent Document 1 discloses a method for recovering active metals for lithium secondary batteries, which includes preparing a preliminary positive electrode active material mixture containing a lithium composite oxide and a binder, removing the binder from the preliminary positive electrode active material mixture by heat treatment in a fluidized bed reactor to form a positive electrode active material mixture, and collecting a lithium precursor from the positive electrode active material mixture. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2023-516430 Summary of the Invention [Problem to be solved by the invention]

[0005] Incidentally, conventional secondary batteries sometimes use a binder containing fluorine. Fluorine-containing binders are typically difficult to decompose due to heat. According to the inventors' investigations, the technology described in Patent Document 1 fails to completely remove the fluorine contained in the binder, and the resistance characteristics of the resulting positive electrode active material are found to be inferior to those of a new positive electrode active material.

[0006] The present disclosure has been made in view of the above circumstances, and aims to provide a method for producing a positive electrode active material that suppresses deterioration of resistance characteristics. [Means for solving the problem]

[0007] To address the above-mentioned problems, there is provided a method for producing a positive electrode active material (hereinafter also simply referred to as "production method") having the following configuration.

[0008] The technology disclosed herein relates to a method for producing a fired positive electrode active material, the method comprising: a preparation step of preparing a process waste material including a positive electrode composite material containing a positive electrode active material of a secondary battery and a binder containing fluorine; and a firing step of firing the positive electrode composite material in a container, the firing step being carried out in a state in which magnesium hydroxide is present in the container.

[0009] In the manufacturing method configured as described above, the cathode composite is added to a container during the firing step, and the cathode composite is fired in the presence of magnesium hydroxide. With this configuration, the magnesium hydroxide promotes decomposition of the binder in the cathode composite, thereby obtaining a cathode active material from which fluorine contained in the binder has been suitably removed after firing. Therefore, a cathode active material with reduced deterioration in resistance characteristics can be obtained. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic cross-sectional view of a positive electrode plate according to this embodiment taken along the thickness and width directions. [Figure 2] FIG. 2 is a flowchart illustrating the manufacturing method according to this embodiment. [Figure 3]FIG. 3 is a flowchart illustrating a manufacturing method according to a modified example. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the technology disclosed herein will be described with reference to the drawings. It should be noted that matters other than those specifically mentioned in this specification that are necessary for implementing the technology disclosed herein can be understood as design matters for a person skilled in the art based on the prior art in the relevant field. The technology disclosed herein can be implemented based on the contents disclosed in this specification and common technical knowledge in the relevant field.

[0012] In this specification, the term "secondary battery" refers to any electricity storage device that can be repeatedly charged and discharged by the movement of charge carriers between a positive electrode and a negative electrode via an electrolyte. In addition, the expression "A to B" indicating a range in this specification includes the meaning of "A or more and B or less," as well as "preferably larger than A" and "preferably smaller than B."

[0013] The method for producing a fired cathode active material according to this embodiment (hereinafter also referred to as the "method for producing a cathode active material") is carried out by using process waste comprising a cathode composite containing a cathode active material of a secondary battery and a binder containing fluorine. Note that the "process waste comprising a cathode composite" in this specification encompasses process waste in which charge carriers have never entered or left the cathode active material contained in the cathode composite. In other words, the "process waste comprising a cathode composite" also encompasses cathode composite waste generated during the production of a secondary battery, cathode composites that have become non-standard products, and cathode composites removed from battery assemblies that have become non-standard products before charging and discharging.

[0014] 1. Process waste material containing cathode composite An example of a process waste material comprising a positive electrode composite is a positive electrode plate of a lithium-ion secondary battery that uses lithium ions as a charge carrier. Hereinafter, a process waste material comprising a positive electrode composite will be described in detail using a positive electrode plate of a lithium-ion secondary battery as an example. However, it is not intended to limit the technology disclosed herein to the embodiment described above. FIG. 1 is a schematic cross-sectional view of a positive electrode plate 10 according to this embodiment, taken along the thickness and width directions. The positive electrode plate 10 is an example of a "process waste material comprising a positive electrode composite" in the technology disclosed herein.

[0015] 1, a positive electrode plate 10 includes a positive electrode current collector 12 and a positive electrode composite layer 14 supported by the positive electrode current collector 12. In the illustrated example, the positive electrode composite layer 14 is provided on both sides of the positive electrode current collector 12. However, the positive electrode composite layer 14 may be provided on one side of the positive electrode current collector 12.

[0016] 1, a positive electrode composite layer-free portion 12a where no positive electrode composite layer 14 is provided may be provided at one end in the width direction of positive electrode plate 10. Positive electrode composite layer-free portion 12a is a portion where positive electrode current collector 12 is exposed, and functions as a current collecting portion.

[0017] The positive electrode current collector 12 may be a known positive electrode current collector used in conventional secondary batteries. For example, an aluminum sheet or foil may be used in view of good conductivity. The dimensions of the positive electrode current collector 12 are not particularly limited and can be changed as appropriate depending on the capacity and size of the secondary battery. When aluminum foil is used as the positive electrode current collector 12, its thickness is not particularly limited, but may be, for example, 5 μm or more and 35 μm or less.

[0018] The positive electrode composite layer 14 is a layer made of a positive electrode composite containing a positive electrode active material, a binder, and the like. The positive electrode active material is a material capable of reversibly absorbing and releasing charge carriers (e.g., lithium, etc.). When lithium ions are used as charge carriers, examples of such positive electrode active materials include lithium transition metal composite oxides such as lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium manganese cobalt composite oxide, lithium nickel cobalt composite oxide, and lithium nickel cobalt manganese composite oxide. Other examples of positive electrode active materials include lithium transition metal phosphate compounds such as lithium iron phosphate, lithium manganese phosphate, and lithium manganese iron lithium phosphate.

[0019] In this specification, the term "lithium nickel cobalt manganese composite oxide" refers to oxides containing Li, Ni, Co, Mn, and O as constituent elements, as well as oxides containing one or more additional elements. Examples of such additional elements include transition metal elements and typical metal elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, and Sn. The additional element may also be a metalloid element such as B, C, Si, or P, or a nonmetal element such as S, F, Cl, Br, or I. This also applies to the lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese-based composite oxide, lithium nickel manganese composite oxide, lithium manganese cobalt composite oxide, and lithium nickel cobalt composite oxide described above.

[0020] The positive electrode active material may be in the form of, for example, primary particles or secondary particles. In this specification, the term "primary particle" refers to the smallest particle unit constituting the positive electrode active material, specifically the smallest unit determined from the geometric shape of its appearance. In this specification, an aggregate of such primary particles is referred to as a "secondary particle." The secondary particle may be, for example, an aggregate of 2 to 100 primary particles.

[0021] The D50 particle size (average particle size) of the positive electrode active material is not particularly limited, but is typically 0.05 μm to 25 μm in primary particle units, for example, 1 μm to 20 μm, and preferably 3 μm to 15 μm. In this specification, the term "D50 particle size" refers to a particle size corresponding to a cumulative frequency of 50% by volume from the side of fine particles with smaller particle sizes in a volume-based particle size distribution based on a laser diffraction / scattering method. The D50 particle size can be obtained from a known laser diffraction / scattering particle size distribution.

[0022] The binder may be a polymer that is soluble or dispersible in the solvent used. The technology disclosed herein includes a fluorine-containing binder. Examples of fluorine-containing binders include polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE). However, a fluorine-free binder may be used in combination with a fluorine-containing binder. Examples of such binders include styrene butadiene rubber (SBR) and carboxymethyl cellulose (CMC).

[0023] Positive electrode mixture layer 14 may contain optional components as needed in addition to the positive electrode active material and binder, such as a conductive material.

[0024] Examples of conductive materials that can be used include solid carbon such as carbon black (acetylene black, furnace black, ketjen black, thermal black, etc.), coke, activated carbon, graphite (natural graphite and modified graphite, artificial graphite), carbon fiber (PAN-based carbon fiber, pitch-based carbon fiber), fullerene, and graphene.

[0025] The positive electrode plate 10 used in a lithium-ion secondary battery has been described above as an example of a process waste material including a positive electrode composite to be prepared in the manufacturing method according to this embodiment. However, the manufacturing method disclosed herein is not limited to a method in which the positive electrode plate 10 used in a lithium-ion secondary battery having the above-described configuration is prepared.

[0026] 2. Manufacturing method of positive electrode active material A method for producing a positive electrode active material according to this embodiment will be described below. FIG. 2 is a flowchart illustrating the production method according to this embodiment. As shown in FIG. 2, the method for producing a positive electrode active material according to this embodiment includes a preparation step S10 and a firing step S40. The production method disclosed herein may further include other steps at any stage, and the remaining production process may be the same as conventional. Each step will be described below.

[0027] (1) Preparation process S10 In the preparation step S10, a process waste material is prepared, which includes a cathode composite material containing a cathode active material of a secondary battery and a binder containing fluorine. The technology disclosed herein is characterized by preparing a process waste material containing a cathode composite material of a secondary battery and a binder containing fluorine, and other configurations may be the same as those of conventional technology. Details of the process waste material have already been described, so repeated description will be omitted.

[0028] The process waste may be prepared in a state similar to the positive electrode plate described above, or may be prepared by obtaining a positive electrode composite only (for example, a state in which the positive electrode current collector has been removed from the positive electrode plate). In the present embodiment, as an example, the process waste is prepared in a state in which only the positive electrode composite (positive electrode composite layer) is present. The state of the process waste in the preparation step S10 is not particularly limited, and the process waste may be in a cut state depending on the scale of the manufacturing equipment, etc.

[0029] (2) Firing process S40 In the firing step S40, the positive electrode composite is fired (heated) in a container. This decomposes and removes the binder contained in the positive electrode composite. The technology disclosed herein is characterized in that the firing step S40 is performed in a state in which magnesium hydroxide (Mg(OH)2) is present in the container. Note that, in this specification, "a state in which magnesium hydroxide is present in a container" refers to a state in which magnesium hydroxide is placed in the same space as the positive electrode composite is placed in the container. That is, in the firing step S40 of the manufacturing method disclosed herein, the positive electrode composite and magnesium hydroxide are fired in the same space in the container. Note that, the positive electrode composite and magnesium hydroxide may be separately placed in the container before the firing step S40 (before firing), or the positive electrode composite and magnesium hydroxide may be mixed and placed in the container.

[0030] Conventionally, fluorine-containing binders have the property of being resistant to thermal decomposition. Therefore, when the binder contains fluorine, simply firing the positive electrode composite material tends to make the binder difficult to decompose, and the fluorine content in the positive electrode composite material is difficult to remove. If a large amount of fluorine remains in the fired product (positive electrode active material) after firing, the resistance performance of the manufactured positive electrode active material may be reduced. In the technology disclosed herein, the firing step S40 is performed in the presence of magnesium hydroxide in the container. This configuration promotes the thermal decomposition of the binder. As a result, fluorine can be suitably removed from the positive electrode composite material through the firing step S40, resulting in a positive electrode active material with reduced resistance performance degradation. While the details of the mechanism promoting the thermal decomposition of the binder are unclear, it is speculated as follows. First, magnesium hydroxide reacts with the alkyl group in the binder, carbonates the alkaline earth metal, and decarbonates the binder. It is speculated that the decarbonated binder then forms a highly reactive intermediate, which facilitates the decomposition of the fluoride ions in the binder.

[0031] The average particle size of magnesium hydroxide is not particularly limited, but from the viewpoint of ensuring a uniform reaction with the binder, it is, for example, 50 μm or less, preferably 10 μm or less, and more preferably 5 μm or less. On the other hand, the lower limit of the average particle size of magnesium hydroxide is not particularly limited, but it can be, for example, 0.1 μm or more. The "average particle size of magnesium hydroxide" can be measured using a laser particle size distribution analyzer.

[0032] When the entire (total amount) of the positive electrode composite material subjected to the firing step S40 is taken as 100 wt%, it is preferable that magnesium hydroxide be present at a ratio of 0.01 wt% or more (more preferably 0.03 wt% or more). The higher the ratio of magnesium hydroxide present in the firing step S40, the more effectively the binder contained in the positive electrode composite material can be decomposed and fluorine can be removed. On the other hand, from the viewpoint of ensuring a migration path for charge carriers in the positive electrode active material and obtaining favorable resistance performance, it is preferable that magnesium hydroxide be present at a ratio of 3 wt% or less (more preferably 1 wt% or less, and even more preferably 0.7 wt% or less) when the entire (total amount) of the positive electrode composite material subjected to the firing step S40 is taken as 100 wt%. Therefore, in some preferred embodiments, magnesium hydroxide is present at a ratio of 0.01 wt% to 3 wt%, and more preferably 0.03 wt% to 1 wt%. Therefore, in some preferred embodiments, in the positive-electrode composite firing step S40, when the total weight of the positive-electrode composite is taken as 100 wt%, magnesium hydroxide is preferably present in an amount of 0.01 wt% to 3 wt%, and more preferably 0.03 wt% to 1 wt%. This allows for the fluorine in the positive-electrode composite to be suitably removed while ensuring sufficient migration paths for charge carriers in the positive-electrode active material during firing. This allows for the production of a positive-electrode active material with better suppressed deterioration of resistance characteristics.

[0033] The firing temperature in the firing step S40 (in other words, the maximum temperature in the firing treatment, more specifically, the temperature inside the container) is preferably 500°C or higher (more preferably 600°C or higher) from the viewpoint of effectively removing fluorine contained in the binder in the positive electrode composite. Furthermore, the higher the firing temperature in the firing step S40, the more accelerated the decomposition of carbon in the binder contained in the positive electrode composite, thereby enabling more effective removal of fluorine. On the other hand, from the viewpoint of crystal growth of the positive electrode active material, the firing temperature in the firing step S40 (in other words, the maximum temperature in the firing treatment, more specifically, the temperature inside the container) is preferably 900°C or lower (more preferably 800°C or lower). Therefore, in some preferred embodiments, the firing step S40 is preferably performed in a temperature range of 500°C to 900°C, and more preferably in a temperature range of 600°C to 800°C.

[0034] In the firing step S40, firing is preferably performed in a mixed (mixed) state of the positive electrode composite and magnesium hydroxide. This allows for efficient promotion of decomposition of the binder contained in the positive electrode composite by the magnesium hydroxide. However, the positive electrode composite and magnesium hydroxide do not need to be mixed completely uniformly. The positive electrode composite and magnesium hydroxide may be mixed by wet mixing or dry mixing, and any conventionally known method can be used without limitation. For example, the mixing can be performed using a known stirring or mixing device such as a mortar, shaker mixer, Lödige mixer, Julia mixer, V-type mixer, or ball mill.

[0035] The vessel used in the firing step S40 can be appropriately selected from vessels used in conventional firing processes depending on the scale of the facility. Examples of such vessels include a batch firing furnace, a roller hearth kiln, and a rotary kiln.

[0036] The firing atmosphere in the firing step S40 is preferably an oxygen-containing atmosphere, for example, an oxygen atmosphere or an air atmosphere. The oxygen concentration of the oxygen-containing atmosphere is preferably 10 vol% or more, more preferably 18 to 100 vol%.

[0037] The firing time in the firing step S40 is not particularly limited, as it differs depending on the amount of positive electrode composite material, the container used, etc. The firing time in the firing step S40 is, for example, preferably 1 hour to 12 hours, and more preferably 2 hours to 8 hours.

[0038] The manufacturing method of the positive electrode active material according to this embodiment has been described above. As described above, the manufacturing method according to this embodiment includes a preparation step S10 and a firing step S40. In the manufacturing method according to this embodiment, the fluorine contained in the positive electrode mixture is suitably removed in the firing step S40. Therefore, the obtained fired body can be used as the positive electrode active material as is. The positive electrode active material obtained by the manufacturing method disclosed herein can be handled in the same way as conventional positive electrode active materials. Furthermore, the obtained positive electrode active material may be mixed with new positive electrode active material.

[0039] 3. Other Embodiments Furthermore, the technology disclosed herein is not limited to the above-described embodiment, but includes other embodiments with various modifications. Other embodiments of the technology disclosed herein will be described below.

[0040] (1) What to prepare For example, in the above-described embodiment, a cathode composite (cathode composite layer) was prepared as the process waste in the preparation step S10. However, this is not limited thereto, and a cathode plate including the above-described cathode composite layer and cathode current collector may be prepared as the process waste. A cathode plate that is a non-standard cathode plate may also be prepared. Furthermore, at secondary battery manufacturing sites, after secondary battery assemblies are assembled, defects may occur before charging and discharging, rendering the product unusable. The process waste prepared by the manufacturing method disclosed herein may be a cathode plate removed from such a secondary battery assembly. In other words, the process waste prepared by the manufacturing method disclosed herein is not particularly limited to a specific embodiment. Note that the configuration of the secondary battery assembly other than the above-described positive electrode plate does not characterize the technology disclosed herein, and therefore will not be described here.

[0041] The process waste prepared in the preparation step S10 may be in a state after the electrolyte has been attached (for example, in a state after being removed from a secondary battery). When preparing process waste after the electrolyte has been attached, the electrolyte may remain attached, or the electrolyte attached to the positive electrode plate may be washed away. From the viewpoint of work efficiency, it is preferable that the positive electrode plate prepared in the preparation step S10 is in a state before the electrolyte has been attached.

[0042] In some preferred embodiments, the cathode active material contained in the cathode mixture of process waste material prepared in the preparation step S10 preferably contains a layered lithium composite oxide. Typically, Mg ions (Mg 2+ ) radius (0.72Å) of the Li ion (Li + ) radius (0.76 Å). When the cathode active material contained in the process waste is a layered lithium composite oxide, Mg ions derived from magnesium hydroxide enter the Li sites of the Li layer of the cathode active material during the firing step S40. This stabilizes the Li layer of the cathode active material, making it possible to more effectively improve durability. Examples of the layered lithium composite oxide include lithium nickel composite oxide, lithium cobalt composite oxide, and lithium nickel manganese cobalt composite oxide.

[0043] (2-1) Regarding other processes, In the manufacturing method according to the embodiment described above, the firing step S40 is performed after the preparation step S10. However, the technology disclosed herein is not limited to this. A modified example of the technology disclosed herein will be described below. FIG. 3 is a flowchart illustrating a manufacturing method according to the modified example. As shown in FIG. 3, in addition to the embodiment described above, this method further includes a peeling step S20, a cleaning step S30, and a crushing step S50. In other words, in the manufacturing method disclosed herein, any steps can be added, deleted, or changed as needed.

[0044] (2-2) Peeling step S20 In some preferred embodiments, a peeling step S20 can be performed. In the peeling step S20, the positive electrode composite is peeled off (in other words, the positive electrode current collector is removed) from the process waste prepared in the preparation step S10. A conventionally known method can be used for the peeling step S20, and there are no particular limitations. For example, the peeling step S20 can be performed by immersing the process waste in an alkaline solution. As a result, the positive electrode current collector (aluminum) of the positive electrode plate dissolves in the alkaline solution. On the other hand, the positive electrode composite is insoluble in the alkaline solution and remains as a solid in the alkaline solution. The positive electrode composite is then recovered from the alkaline solution by solid-liquid separation (filtration, centrifugation, decantation, etc.). This allows the positive electrode composite to be peeled off from the process waste.

[0045] In the peeling step S20, the positive electrode current collector may be completely dissolved, but it is not necessary to completely dissolve the positive electrode current collector as long as the positive electrode composite can be peeled off from the positive electrode plate. When obtaining the positive electrode composite without completely dissolving the positive electrode current collector, it is preferable to immerse the positive electrode plate in the alkaline solution while stirring. This makes it easier to physically peel off the positive electrode composite and the positive electrode current collector from the positive electrode plate.

[0046] Specific examples of the alkaline solution used in the peeling step S20 include a lithium hydroxide (LiOH) aqueous solution, a sodium hydroxide (NaOH) aqueous solution, and a potassium hydroxide (KOH) aqueous solution. Among these, a lithium hydroxide aqueous solution and a sodium hydroxide aqueous solution can be preferably used as the alkaline solution. These aqueous solutions exhibit strong alkalinity, and when such an alkaline solution is used in the peeling step S20, it is possible to selectively dissolve the positive electrode current collector while suppressing damage to the positive electrode active material in the positive electrode plate.

[0047] The pH of the alkaline solution used in the peeling step S20 is preferably 12 or higher (more preferably 13 or higher). If the pH of the alkaline solution is too low, the positive electrode current collector will not dissolve in the alkaline solution, making it difficult to peel the positive electrode composite, and there is a risk of damaging the positive electrode active material in the positive electrode composite. On the other hand, if the pH of the alkaline solution is too high, the dissolution reaction of the positive electrode current collector will be significant, which may result in structural destruction of the positive electrode active material. Therefore, the pH of the alkaline solution used in the peeling step S20 is preferably 14 or lower (more preferably 13.5 or lower).

[0048] The temperature of the alkaline solution in the peeling step S20 is not particularly limited, but is preferably 20°C or higher (more preferably 30°C or higher, and even more preferably 40°C or higher). This promotes dissolution of the positive electrode current collector in the alkaline solution, thereby shortening the time required for the peeling step S20. On the other hand, the upper limit of the temperature of the alkaline solution in the peeling step S20 is not particularly limited, and may be, for example, 80°C or lower, 60°C or lower, or 50°C or lower.

[0049] (2-3) Cleaning process S30 In some preferred embodiments, it is preferable to perform the washing step S30 before the firing step S40. In the washing step S30, the positive electrode mixture is washed with water or an alkaline solvent. This makes it possible to more effectively remove the binder contained in the positive electrode mixture.

[0050] The solvent used in the washing step S30 can be water or an alkaline solvent. In some preferred embodiments, it is preferable to use an alkaline solvent as the solvent. This makes it possible to more effectively remove the binder contained in the positive electrode mixture while further suppressing damage to the positive electrode active material in the positive electrode mixture. Examples of water that can be used as the solvent include distilled water, ion-exchanged water, pure water, and ultrapure water. When an alkaline solvent is used as the solvent, the alkaline solvent is not particularly limited, and examples include an aqueous sodium hydroxide (NaOH) solution, an aqueous lithium hydroxide (LiOH) solution, and an aqueous potassium hydroxide (KOH) solution.

[0051] The pH of the solvent (water or alkaline solvent) used in the washing step S30 is preferably 6.5 or higher (more preferably 9 or higher, and even more preferably 10 or higher). If the pH of the solvent is too low, there is a risk of damaging the positive electrode active material in the positive electrode mixture. On the other hand, if the pH of the solvent is too high, there is a possibility of the structure of the positive electrode active material being destroyed. Therefore, the pH of the alkaline solution used in the washing step S30 is preferably 14 or lower (more preferably 13.5 or lower).

[0052] The number of washings in the washing step S30 may be one or multiple times (two or more times), and is not particularly limited. The temperature of the solvent (water or alkaline solvent) used in the washing step S30 is not particularly limited, and may be, for example, 20° C. or higher and 80° C. or lower. However, the washing step S30 is not essential.

[0053] (2-4) Crushing process S50 In some preferred embodiments, a crushing step S50 can be performed to crush the sintered body obtained in the sintering step S40. The sintered body obtained in the sintering step S40 can be in the form of particle agglomerates (lumps) in which multiple secondary particles of a positive electrode active material further aggregate. Here, performing the crushing step S50 makes it easier to handle as a positive electrode active material. In this specification, "crushing" refers to the operation of applying mechanical energy to particle agglomerates to loosen the bonds between the aggregated particles. Therefore, this is a different operation from "pulverization," which further reduces the particle size of particles by applying mechanical energy.

[0054] The crushing step S50 can be carried out by a conventionally known method, such as a rotary dry sieve, a ball mill, a rotary tap ball mill, a vibrating ball mill, a planetary ball mill, a rotary cutter mill, or a sand mill.

[0055] (3) Other When magnesium hydroxide and the positive electrode composite are mixed, the timing of mixing is not particularly limited. For example, mixing may be performed immediately before the firing step S40, or magnesium hydroxide may be mixed when the peeling step S20 or the cleaning step S30 is performed. Magnesium hydroxide is typically almost insoluble in water and alkaline solvents. Therefore, even when magnesium hydroxide and the positive electrode composite are mixed at the above-mentioned timing, the firing step S40 can be performed with magnesium hydroxide present in the container.

[0056] [Test example] Test examples relating to the technology disclosed herein will be described below. Note that the contents of the test examples described below are not intended to limit the technology disclosed herein.

[0057] <Production of positive electrode active material> (Example 1) In this test, a new positive electrode plate (i.e., one that had never undergone charge carrier extraction) was prepared as a process waste, in which a positive electrode composite layer was disposed on the surface of the positive electrode current collector, and the following process was carried out on the positive electrode plate. The positive electrode current collector used in this test was an aluminum foil with a thickness of 15 μm. The positive electrode composite constituting the positive electrode composite layer used in this test was a layered lithium composite oxide, LiNi, as the positive electrode active material. 1 / 3 Co 1 / 3 Mn 1 / 3 The positive electrode active material used was a mixture of O2 (lithium nickel cobalt manganese composite oxide), polyvinylidene fluoride (PVDF) as a binder, and acetylene black (AB) as a conductive material in a mass ratio of 85:10:5 (positive electrode active material:AB:PVDF).

[0058] (peeling process) Next, the prepared positive electrode plate was immersed in a sodium hydroxide (NaOH) aqueous solution (concentration: 30 wt%) and left to stand at 40°C for 120 minutes. This resulted in the positive electrode composite being peeled off from the positive electrode plate (in other words, the Al foil was dissolved), and the positive electrode composite was obtained. The weight of the obtained positive electrode composite was measured by weighing, and then the positive electrode composite was subjected to the subsequent firing process.

[0059] (Firing process) The positive electrode composite and magnesium hydroxide (reagent, Hayashi Pure Chemical Industries, Ltd.) (average particle size 5 μm) were mixed in a mortar so that the magnesium hydroxide content was 0.01 wt% when the entire positive electrode composite obtained above was taken as 100 wt%. The mixture of the positive electrode composite and magnesium hydroxide was then placed in a container (sagger) and fired under atmospheric conditions at a firing temperature (maximum temperature) of 500°C for a firing time of 3 hours. This resulted in a fired body, which was the positive electrode active material of Example 1 (hereinafter also referred to as "recovered positive electrode active material").

[0060] (Measurement of F content in recovered positive electrode active material) Here, the F (fluorine) content (wt%) in the recovered positive electrode active material was measured and calculated. The F content was measured by ion chromatography analysis. The results are shown in the "F content (wt%)" column in Table 1.

[0061] (Example 2) In Example 2, the positive electrode composite obtained in the peeling step was washed with water. The positive electrode composite was washed once. The washed positive electrode composite was then filtered and vacuum dried at 100°C before being subjected to the firing step. The rest of the procedure was the same as in Example 1.

[0062] (Example 3) Example 3 was the same as Example 2, except that an aqueous solution of sodium hydroxide with a concentration of 0.3 wt % was used instead of water.

[0063] (Examples 4 to 8) In Examples 4 to 8, the firing step was carried out after adjusting the proportion of magnesium hydroxide present as shown in the "Proportion (wt%)" column in Table 1. Other than this, the same procedures as in Example 3 were carried out.

[0064] (Examples 9 to 11) In Examples 9 to 11, the firing step was carried out at the firing temperatures shown in Table 1. Other than this, the same procedures as in Example 3 were carried out.

[0065] (Example 12) In Example 12, the firing step was carried out in the absence of magnesium hydroxide. In other words, only the positive electrode composite obtained in the peeling step was placed in a container (sagger), and firing was carried out under atmospheric conditions at a firing temperature of 500°C for a firing time of 3 hours. Other than this, the same procedure was followed as in Example 1.

[0066] (Example 13) In Example 13, the firing process was carried out in the same manner as in Example 3 except that calcium hydroxide (Ca(OH)2) was used instead of magnesium hydroxide in an amount of 0.01 wt% when the positive electrode composite material to be fired was taken as 100 wt%.

[0067] <Preparation of secondary battery for evaluation> Using the recovered positive electrode active materials obtained in Examples 1 to 13 above, secondary batteries for evaluation were fabricated in the following manner.

[0068] (Examples 1 to 13) The recovered positive electrode active materials obtained in Examples 1 to 13, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were mixed in N-methylpyrrolidone (NMP) in a mass ratio of recovered positive electrode active material:AB:PVDF = 85:10:5 to prepare a paste for forming a positive electrode composite layer. This paste was applied to a 15 μm-thick positive electrode current collector (Al foil) and dried to produce a positive electrode plate.

[0069] A paste for forming a negative electrode composite layer was prepared by mixing natural graphite as the negative electrode active material, styrene butadiene rubber (SBR) as the binder, and carboxymethyl cellulose (CMC) as the thickener in a mass ratio of natural graphite:SBR:CMC = 98:1:1 in ion-exchanged water. This paste was applied to a 10 μm-thick Cu foil and dried to prepare a negative electrode plate.

[0070] In addition, a porous polyolefin sheet having a thickness of 20 μm and a three-layer structure of PP / PE / PP was prepared as a separator sheet.

[0071] The positive electrode plate, negative electrode plate, and separator sheet were stacked together, and electrode terminals were attached and housed in a laminate case. Subsequently, a non-aqueous electrolyte was poured into the laminate case, and the laminate case was airtightly sealed. The non-aqueous electrolyte used was a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 3:4:3, with LiPF6 dissolved as a supporting electrolyte at a concentration of 1.0 mol / L. In this manner, secondary batteries for evaluation according to Examples 1 to 13 were obtained.

[0072] (Reference example) Furthermore, evaluation secondary batteries according to Reference Examples were fabricated to compare the recovered positive electrode active materials according to Examples 1 to 13 with new positive electrode active materials. Specifically, positive electrode active materials similar to those of Examples 1 to 13 were prepared except that they were new. Evaluation secondary batteries according to Reference Examples were fabricated under the same conditions as described above except that such positive electrode active materials were used.

[0073] <Activation> Each of the evaluation secondary batteries prepared above was placed in an environment of 25°C. Activation (initial charging) was performed using a constant current-constant voltage method, where each evaluation secondary battery was charged at a constant current of 1 / 3C up to 4.2V, and then charged at a constant voltage until the current reached 1 / 50C, and fully charged. Thereafter, each evaluation secondary battery was discharged at a constant current of 1 / 3C down to 3.0V.

[0074] <Initial resistance measurement> After each evaluation secondary battery was subjected to an activation treatment, the state of charge (SOC) was adjusted to 50% and placed in an environment at 25°C. Discharge was performed for 10 seconds at a current value of 100 mA, and the voltage drop ΔV was determined. The voltage drop ΔV was divided by the discharge current value (100 mA) to calculate the battery resistance, which was taken as the initial resistance. Then, assuming that the initial resistance of the evaluation secondary battery of the reference example was 1, the ratio of the initial resistance of each of the other evaluation secondary batteries was determined. The results are shown in the "Initial Resistance" column in Table 1. Note that a smaller "Initial Resistance" value in Table 1 indicates better initial resistance characteristics.

[0075] <Evaluation of resistance increase rate during durability> Each evaluation secondary battery was placed in a 60°C environment and subjected to constant current charging at a charge rate of 1C up to 4.1 V, followed by constant current discharging at a discharge rate of 1C down to 3.0 V. This charge / discharge cycle was repeated 200 times. Next, the battery resistance after 200 cycles was measured using the same method as for measuring the initial resistance, and the durability resistance increase rate was calculated using the following formula: durability resistance increase rate = battery resistance after 200 cycles / initial resistance. The results are shown in the "durability resistance increase rate" in Table 1. A smaller value for "durability resistance increase rate" in Table 1 indicates better durability resistance characteristics.

[0076] [Table 1]

[0077] From the results in Table 1, it was found that when the positive electrode composite was fired in the presence of magnesium hydroxide, the deterioration of the initial resistance and the resistance characteristics during endurance was suppressed. In other words, by firing the positive electrode composite in the presence of magnesium hydroxide, a positive electrode active material was obtained in which both the initial resistance and the rate of increase in resistance during endurance were reduced. On the other hand, in Example 13, in which calcium hydroxide was used in the firing process instead of magnesium hydroxide, good results were not obtained in terms of resistance performance. This is because the calcium contained in calcium hydroxide reacts with sulfate residues (sulfate ions, SO4 2- ) to generate CaSO4, which is thought to have affected the resistance performance.

[0078] The technology disclosed herein has been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific examples exemplified above. In other words, the technology disclosed herein encompasses the aspects described in the following items.

[0079] <Item 1> a preparation step of preparing a process waste material including a positive electrode composite material containing a positive electrode active material of a secondary battery and a binder containing fluorine; a firing step of firing the positive electrode mixture in a container, The firing step is carried out in the presence of magnesium hydroxide in the container. A method for producing the above-mentioned fired positive electrode active material.

[0080] <Item 2> Item 2. The manufacturing method according to item 1, further comprising a washing step of washing the positive electrode mixture with water or an alkaline solvent before the firing step.

[0081] <Item 3> 3. The manufacturing method according to item 1 or 2, wherein in the firing step, the magnesium hydroxide is present in a proportion of 0.01 wt % to 3 wt % when the entire positive electrode mixture is taken as 100 wt %.

[0082] <Item 4> 4. The method according to any one of items 1 to 3, wherein the maximum temperature during the firing step is 500°C or higher and 900°C or lower.

[0083] <Item 5> 5. The method according to any one of items 1 to 4, wherein the positive electrode active material contains a lithium composite oxide having a layered structure. [Explanation of symbols]

[0084] 10 Positive electrode plate 12 Positive electrode current collector 12a Part where positive electrode composite layer is not formed 14 Positive electrode mixture layer

Claims

1. a preparation step of preparing a process waste material including a positive electrode composite material containing a positive electrode active material of a secondary battery and a binder containing fluorine; a firing step of firing the positive electrode mixture in a container, The calcination step is carried out in the presence of magnesium hydroxide in the container. A method for producing the fired positive electrode active material.

2. The manufacturing method according to claim 1 , further comprising a washing step of washing the positive electrode mixture with water or an alkaline solvent before the firing step.

3. 3. The manufacturing method according to claim 1, wherein in the firing step, the magnesium hydroxide is present in a proportion of 0.01 wt % to 3 wt % when the entire positive electrode mixture is taken as 100 wt %.

4. The method according to claim 1 or 2, wherein the maximum temperature during the firing step is 500°C or higher and 900°C or lower.

5. The method according to claim 1 or 2, wherein the positive electrode active material contains a layered lithium composite oxide.

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