Regenerated positive electrode active material, lithium ion secondary battery containing same, and method of producing regenerated positive electrode active material

Abstract

Provided are: a regenerated positive electrode active material obtained by replenishing lithium in a lithium-deficient positive electrode active material (a), said regenerated positive electrode active material having an oxygen 1s core spectrum, as measured using XPS, in which the ratio of the peak intensity in the region from not less than 528 eV but less than 530 eV to the peak intensity in the region from not less than 530 eV but less than 535 eV is 0.0005 or less; a lithium ion secondary battery containing the regenerated positive electrode active material; and a method of producing a regenerated positive electrode active material, said method including mixing the lithium-deficient positive electrode active material (a), a lithium compound (b) and a reducing agent (c) at proportions such that 0.13 ≤ (b) / (c) ≤ 2.80 (molar ratio) to obtain the regenerated positive electrode active material.

Description

Regenerated cathode active material, lithium ion secondary battery including the same, and method for producing the regenerated cathode active material

[0001] The present invention relates to a recycled positive electrode active material, a lithium ion secondary battery including the same, and a method for producing the recycled positive electrode active material.

[0002] Lithium-ion secondary batteries (LIBs) have a high energy density and are excellent in storage capacity and low-temperature operation, and are widely used in portable electronic devices such as mobile phones and laptops. Larger batteries are also being used in automobiles and other transportation equipment, and their use as storage devices for nighttime electricity and electricity generated by natural energy sources is also being promoted.

[0003] The positive electrode of a lithium-ion secondary battery typically has a positive electrode active material layer, which contains positive electrode active material particles capable of absorbing and releasing lithium ions during charge and discharge. The positive electrode active material layer is typically formed by preparing, coating, and drying a slurry (composition) containing the constituent components of the positive electrode active material layer, such as the positive electrode active material. The positive electrode active material is an important material that determines the battery capacity, but it is known to deteriorate over long-term operation of lithium-ion secondary batteries. This is thought to be due to the fact that lithium ions that desorb from the positive electrode active material and migrate to the negative electrode during charge do not completely return from the negative electrode to the positive electrode active material during discharge. Repeated charge and discharge cycles result in a lithium-ion-deficient positive electrode active material. With the rapid spread of lithium-ion secondary batteries, there has been growing interest in recycling technologies for their constituent materials. In particular, relithiation technology, which directly replenishes lithium in the lithium-deficient positive electrode active material of used lithium-ion secondary batteries, has attracted attention. For example, Patent Document 1 discloses a method of subjecting a lithium-deficient electrode material to a hydrothermal reaction in a solution containing lithium ions, and a specific example of this method is LiCoO ２In addition, Patent Document 2 discloses a method for electrochemical alkalization of an electrochemically active material, which comprises adding the electrochemically active material to a solution containing a reducing agent and an alkali metal salt in a solvent to produce an alkalized electrochemically active material. This method describes the use of a redox couple as the reducing agent.

[0004] U.S. Patent No. 9,287,552 Specification JP 2023-502220 A

[0005] The inventors have found through their research that regenerated positive electrode active materials obtained by relithiation may have an insufficient relithiation rate, and that when a lithium-ion secondary battery is fabricated using the regenerated positive electrode active material, the resulting lithium-ion secondary battery is prone to a decrease in discharge capacity when exposed to high temperatures. The present invention aims to provide a regenerated positive electrode active material obtained by replenishing lithium in a lithium-deficient positive electrode active material resulting from the use of a lithium-ion secondary battery. This regenerated positive electrode active material can be reused as a positive electrode active material in a lithium-ion secondary battery to produce a lithium-ion secondary battery that exhibits excellent discharge capacity even when exposed to high temperatures, and a lithium-ion secondary battery containing the regenerated positive electrode active material. Another objective of the present invention is to provide a method for producing this regenerated positive electrode active material.

[0006] The present inventors have conducted extensive research into the relithiation of lithium-deficient positive electrode active materials, and have found that an oxide film is formed on the surface of the regenerated positive electrode active material as a by-product of the relithiation process, and that when a regenerated positive electrode active material with an oxide film formed thereon is incorporated into a lithium-ion secondary battery, the secondary battery cannot achieve sufficient discharge capacity when exposed to high temperatures. The present invention was completed through further research based on these findings.

[0007] That is, the above-mentioned problems have been solved by the following means. [1] A regenerated positive electrode active material obtained by supplementing lithium to a lithium-deficient positive electrode active material (a), wherein in an oxygen 1s core spectrum measured by XPS measurement of the regenerated positive electrode active material, the ratio of the peak intensity in the region of 528 eV or more and less than 530 eV to the peak intensity in the region of 530 eV or more and less than 535 eV is 0.005 or less. [2] The regenerated positive electrode active material according to [1], wherein the lithium-deficient positive electrode active material (a) has an olivine structure. [3] The lithium-deficient positive electrode active material (a) is a regenerated positive electrode active material obtained by supplementing lithium to a lithium-deficient positive electrode active material (a), wherein the ratio of the peak intensity in the region of 528 eV or more and less than 530 eV to the peak intensity in the region of 530 eV or more and less than 535 eV is 0.005 or less. １－ xM １ P.O. ４ where x represents the amount of lithium deficiency, 0<x≦1, and M １ represents at least one element selected from Fe, Mn, and Co. [4] A lithium-ion secondary battery comprising the regenerated cathode active material according to any one of [1] to [3]. [5] A method for producing a regenerated cathode active material, comprising: mixing a lithium-deficient cathode active material (a), a lithium compound (b), and a reducing agent (c) such that the molar ratio of (b) to (c) satisfies 0.13≦(b) / (c)≦2.80, thereby replenishing lithium into (a), thereby obtaining a regenerated cathode active material. [6] A method for producing a regenerated cathode active material according to [5], wherein the amount of the lithium compound (b) blended is 10 molar equivalents or more relative to the amount of lithium deficiency in the lithium-deficient cathode active material (a). [7] The method for producing a regenerated cathode active material according to [5] or [6], wherein the lithium-deficient cathode active material (a), the lithium compound (b), and the reducing agent (c) are heated at 80 to 100°C. [8] The method for producing a regenerated cathode active material according to [7], wherein the solid particles obtained after the heating are filtered and washed. [9] The method for producing a regenerated cathode active material according to [7] or [8], wherein the solid particles obtained after the heating are fired at 400°C or higher.

[10] The method for producing a regenerated cathode active material according to any one of [5] to [9], wherein the lithium-deficient cathode active material (a) has an olivine structure.

[11] The method for producing a regenerated cathode active material according to any one of [5] to [9], wherein the lithium-deficient cathode active material (a) is Li １－ xM １ P.O. ４where x represents the amount of lithium deficiency, 0<x≦1, and M １ represents at least one element selected from Fe, Mn, and Co. The method for producing a recycled positive electrode active material according to any one of [5] to

[10] .

[0008] The regenerated positive electrode active material of the present invention suppresses the formation of an oxide film associated with relithiation, and by incorporating it into a lithium ion secondary battery, the resulting lithium ion secondary battery can exhibit excellent discharge capacity even when exposed to high temperatures. The lithium ion secondary battery of the present invention exhibits excellent discharge capacity even when exposed to high temperatures, even while using a regenerated positive electrode active material. Furthermore, according to the method for producing the regenerated positive electrode active material of the present invention, it is possible to obtain a regenerated positive electrode active material that suppresses the formation of an oxide film associated with relithiation, and by incorporating it into a lithium ion secondary battery, the resulting lithium ion secondary battery can exhibit excellent discharge capacity even when exposed to high temperatures.

[0009] FIG. 1 is a longitudinal cross-sectional view showing a schematic basic layer structure of an embodiment of a lithium ion secondary battery according to the present invention.

[0010] In the present invention, a numerical range expressed using "to" means a range including the numerical values ​​before and after "to" as the lower and upper limits. When describing a component composition in the present invention, unless otherwise specified, each component may contain one type or two or more types. In the present invention, the term "secondary battery" refers to a general device in which ions move between positive and negative electrodes via an electrolyte upon charging and discharging, storing and releasing energy at the positive and negative electrodes. That is, in the present invention, the term "secondary battery" encompasses both batteries and capacitors (e.g., lithium ion capacitors). When the ions are lithium ions, the battery is considered a lithium ion secondary battery. Secondary batteries can be broadly classified into aqueous secondary batteries and nonaqueous secondary batteries depending on the electrolyte used, and in the present invention, nonaqueous secondary batteries are preferred. In the present invention, the term "aqueous secondary battery" refers to a secondary battery using an aqueous electrolyte solution as the electrolyte. In the present invention, the term "nonaqueous secondary battery" refers to both nonaqueous electrolyte secondary batteries and all-solid-state secondary batteries. In the present invention, the term "nonaqueous electrolyte secondary battery" refers to a secondary battery using a nonaqueous electrolyte solution as the electrolyte. In the present invention, "nonaqueous electrolyte" means an electrolyte that is substantially free of water. An electrolyte that is substantially free of water means that the "nonaqueous electrolyte" may contain a small amount of water as long as the effects of the present invention are not impaired. In the present invention, the "nonaqueous electrolyte" has a water concentration of 200 ppm (by mass) or less, preferably 100 ppm or less, and more preferably 20 ppm or less. Note that it is practically difficult to make a nonaqueous electrolyte completely anhydrous, and it usually contains 1 ppm or more of water.

[0011] [Regenerated Positive Electrode Active Material] The regenerated positive electrode active material of the present invention is a regenerated positive electrode active material obtained by supplementing lithium to a lithium-deficient positive electrode active material (a). Here, the lithium-deficient positive electrode active material (a) refers to a positive electrode active material in which at least a portion of the chemical equivalent of lithium derived from the elemental composition of the compound is missing, and the amount of lithium is less than the chemical equivalent. The amount of lithium (molar amount) contained in the lithium-deficient positive electrode active material (a) is preferably 0.1 to 0.9 times, more preferably 0.2 to 0.8 times, and even more preferably 0.3 to 0.7 times, the amount of lithium (molar amount) of a positive electrode active material that is not lithium-deficient. The regenerated positive electrode active material of the present invention has a peak intensity (I ) in the region of 530 eV or more and less than 535 eV in the oxygen 1s core spectrum measured by X-ray photoelectron spectroscopy (XPS). Ａ ) the peak intensity in the region of 528 eV or more and less than 530 eV (I Ｂ ) ratio value (I Ｂ / I Ａ ) is 0.005 or less. The peak in the region of 530 eV or more and less than 535 eV is due to the presence of a positive electrode active material (e.g., LiFePO ４ ) and the peak in the region of 528 eV or more and less than 530 eV is due to an oxide film (for example, LiFePO ) formed on the surface of the positive electrode active material. ４ Fe formed on ２ O ３ Therefore, the ratio of the peak intensities of these peaks (I Ｂ / I Ａ The fact that the value of the XPS spectrum is 0.005 or less means that no oxide film is formed on the surface of the regenerated positive electrode active material, or that even if an oxide film is formed, the amount is small. The method for XPS measurement will be described in the Examples below.

[0012] When the regenerated positive electrode active material of the present invention is used as a positive electrode active material for a lithium ion secondary battery, it is possible to obtain a lithium ion secondary battery having an excellent discharge capacity retention rate.

[0013] In the regenerated positive electrode active material of the present invention, I Ｂ / I Ａis preferably 0.004 or less, more preferably 0.003 or less, even more preferably 0.002 or less, and even more preferably 0.001 or less. Ｂ / I Ａ can be 0 (0.000), and is particularly preferably 0. Ｂ / I Ａ When is 0, no peak derived from the oxide film is observed.

[0014] The lithium-deficient positive electrode active material (a), which is the raw material for the regenerated positive electrode active material of the present invention, is derived from a metal oxide commonly used as a positive electrode active material for lithium-ion secondary batteries. In lithium-ion secondary batteries, lithium-containing transition metal oxides are often used as the positive electrode active material (see, for example, JP 2023-106633 A). Among these, lithium-containing transition metal oxides include (MA) lithium-containing transition metal oxides having a layered rock salt structure, (MB) lithium-containing transition metal oxides having a spinel structure, (MC) lithium-containing transition metal phosphate compounds, (MD) lithium-containing transition metal halide phosphate compounds, and (ME) lithium-containing transition metal silicate compounds. In the present invention, (MC) lithium-containing transition metal phosphate compounds are preferred, and lithium-containing transition metal phosphate compounds having an olivine structure are particularly preferred. A specific example of a lithium-containing transition metal phosphate compound having an olivine structure is LiFePO ４ and Li ３ Fe ２ (P.O. ４ ) ３ Iron phosphate salts such as:

[0015] The lithium-deficient positive electrode active material (a) is preferably a positive electrode active material containing a lithium-containing transition metal phosphate compound, and Li １－ xM １ P.O. ４ (x represents the amount of lithium deficiency, 0<x≦1, M １ represents at least one element selected from Fe, Mn and Co. １ is preferably an element selected from Fe and Mn, and more preferably Fe. １－ xM１ P.O. ４ A positive electrode active material (a) containing no lithium deficiency (a positive electrode active material containing a chemical equivalent of lithium) corresponding to the lithium-deficient positive electrode active material (a) represented by LiM １ P.O. ４ When Li is completely lost by charging and discharging, M １ P.O. ４ The positive electrode active material is expressed as follows:

[0016] The lithium-deficient positive electrode active material (a) can be obtained by recovering the positive electrode active material from a used lithium ion secondary battery. The recovery method will be described later.

[0017] The relithiation rate of the regenerated positive electrode active material is preferably 0.80 or more, more preferably 0.90 or more, more preferably 0.93 or more, more preferably 0.95 or more, more preferably 0.97 or more, and more preferably 0.99 or more. The relithiation rate can be determined by the method described in the Examples.

[0018] The recycled positive electrode active material may be the recycled positive electrode active material alone, or may contain a trace amount of binder and / or conductive additive that was not removed by the recycling process in addition to the recycled positive electrode active material. The recycled positive electrode active material usually contains a trace amount of binder and / or conductive additive. In this case, the entire material, including the trace amount of binder and / or conductive additive, is the "recycled positive electrode active material" of the present invention. When a positive electrode active material without lithium deficiency contains a trace amount of binder, conductive additive, etc., it can be determined that this positive electrode active material is a recycled product (i.e., a recycled positive electrode active material).

[0019] The binder content in the recycled positive electrode active material is preferably 0.5% by mass or less. The binder content can be determined by thermogravimetry-differential thermal analysis (TG-DTA). An example is shown below. A dried product of the recycled positive electrode active material (which has been left at 120°C for 12 hours to remove moisture) is heated from 25°C at a rate of 5°C / min in a nitrogen atmosphere, and after reaching 600°C, the mass loss after leaving it at 600°C for 1 hour is measured. More specifically, the measurement is performed as follows. A 5 mg sample of the dried recycled positive electrode active material is placed in a sample pan for a simultaneous thermogravimetry-differential thermal analyzer (Shimadzu Corporation, TGA-50H (trade name)) and set inside the analyzer. The mass is measured under the following conditions. Gas flow rate: Nitrogen 50 ml / min Measurement conditions: After leaving the sample at 25°C for 2 hours, the sample was heated to 600°C at a rate of 5°C / min in a nitrogen atmosphere, and then left at 600°C for 1 hour. The mass loss (mass%) calculated using the following formula can be used as the binder content (mass%) of the regenerated positive electrode active material: Mass loss (mass%) = 100 x [(mass after leaving the sample at 25°C for 2 hours) - (mass after leaving the sample at 600°C for 1 hour)] / (mass after leaving the sample at 25°C for 2 hours).

[0020] The content of the conductive additive in the recycled positive electrode active material is preferably 0.5 mass% or less. The content of the conductive additive in the recycled positive electrode active material can be determined using energy dispersive X-ray spectroscopy (SEM-EDX). The regenerated positive electrode active material was applied to a conductive double-sided tape attached to a sample stage to create a sample, and a field emission scanning electron microscope (FE-SEM: Field Emission Scanning Electron Microscope) (manufactured by JEOL, JSM 7100F (trade name)) was used to perform SEM observation of the regenerated positive electrode active material at an acceleration voltage of 1.5 kV, and an energy dispersive X-ray spectrometer (EDX: Energy Dispersive X-ray Spectroscope) (manufactured by Thermo Fisher Scientific, Noran System 7 (trade name)) was used to perform elemental analysis by automatic detection. The mass ratio of the carbon content to the total element content detected by automatic detection was taken as the C content (mass%) of the regenerated positive electrode active material. Since the regenerated positive electrode active material is a metal oxide, this amount of C can be regarded as the amount of conductive additive.

[0021] The particle size of the recycled positive electrode active material is not particularly limited. For example, it can be 0.1 to 50.0 μm, preferably 0.5 to 30.0 μm, more preferably 1.0 to 20.0 μm, more preferably 2.0 to 10.0 μm, and even more preferably 2.0 to 6.0 μm. The particle size of the recycled positive electrode active material is the volume-based median diameter D50 in water obtained by dispersing the recycled positive electrode active material in water and measuring it with a laser diffraction / scattering particle size distribution measuring device (e.g., Particle LA-960V2 manufactured by HORIBA).

[0022] [Lithium-ion secondary battery] The lithium-ion secondary battery of the present invention contains the recycled positive electrode active material of the present invention as a positive electrode active material, and is otherwise similar to a conventional secondary battery. In the present invention, a lithium-ion secondary battery in which the electrode active material layer does not contain an electrolyte solution is referred to as a first embodiment, and a lithium-ion secondary battery in which the electrode active material layer contains an electrolyte solution (quasi-solid-state lithium-ion secondary battery) is referred to as a second embodiment.

[0023] A lithium ion secondary battery according to a first embodiment of the present invention includes a positive electrode, a negative electrode, and a separator disposed between the positive and negative electrodes. The positive electrode includes a positive electrode current collector and a positive electrode active material layer in contact with the positive electrode current collector, and the negative electrode includes a negative electrode current collector and a negative electrode active material layer in contact with the negative electrode current collector. In the lithium ion secondary battery according to the first embodiment of the present invention, the positive electrode active material layer is formed using the recycled positive electrode active material of the present invention. The lithium ion secondary battery according to the first embodiment of the present invention functions as a secondary battery by charging and discharging a non-aqueous electrolyte solution filled between the positive electrode and the negative electrode.

[0024] FIG. 1 is a cross-sectional view showing a schematic representation of the laminated structure of a typical lithium-ion secondary battery 10 of the first embodiment, including the working electrode when the battery is in operation. The lithium-ion secondary battery 10 has a laminated structure having, in this order, an anode current collector 1, an anode active material layer 2, a separator 3, a cathode active material layer 4, and a cathode current collector 5, as viewed from the anode side. The space between the anode active material layer 2 and the cathode active material layer 4 is filled with a non-aqueous electrolyte (not shown), and they are separated by the separator 3. The separator 3 has pores, and during normal battery use, it functions as a positive / negative electrode separator that insulates the positive and negative electrodes, allowing the electrolyte and ions to pass through the pores. This structure allows electrons (e － ) is supplied, and at the same time, lithium ions (Li ＋ ) moves and accumulates in the negative electrode. On the other hand, during discharge, the lithium ions (Li ＋ ) is returned to the positive electrode side through the electrolyte, and electrons are supplied to the operating part 6. In the illustrated example, a light bulb is used as the operating part 6, and it is lit by discharge.

[0025] Next, a basic configuration characteristic of a second embodiment of the lithium-ion secondary battery (quasi-solid-state lithium-ion secondary battery) of the present invention will be described. In a quasi-solid-state secondary battery, the electrode active material layer is formed in a slurry state containing an electrode active material and an electrolyte. Therefore, the configuration differs from the first embodiment of the lithium-ion secondary battery in that the electrode active material layer is a layer using a slurry (suspension, dispersion) in which the electrode active material is dispersed in a non-aqueous electrolyte. That is, in the first embodiment of the lithium-ion secondary battery, a coating liquid is prepared by dispersing the electrode active material in a medium that does not contain an electrolyte, and this coating liquid is applied to a current collector to form a coating film. This coating film is then dried to form a thin-film electrode active material layer. A binder is usually blended into this coating liquid, forming a hard electrode active material layer in which the electrode active material particles are firmly bound together. Since the non-aqueous electrolyte solution is present on the electrode active material layer thus formed (between the negative electrode active material layer and the positive electrode active material layer), the electrode active material layer, even if it has a portion through which the non-aqueous electrolyte solution can penetrate, is in the form of a hard solid particle layer as a whole, not a slurry layer. In contrast, in a quasi-solid-state lithium-ion secondary battery, the electrode active material layer is an electrode slurry layer formed by dispersing solid particles containing an electrode active material and a conductive additive in a non-aqueous electrolyte solution obtained by dissolving a lithium salt (electrolyte) in a non-aqueous solvent. For this electrode slurry layer to function as an electrode active material layer, strong binding between the electrode active material particles is not required, and therefore the electrode slurry layer usually does not contain a binder. Except for the fact that the electrode active material layer is an electrode slurry layer and that the electrode slurry layer is in contact with a separator, the basic layer configuration of a quasi-solid-state lithium-ion secondary battery is the same as the layer configuration shown in FIG. 1 .

[0026] The lithium ion secondary battery of the present invention includes the recycled positive electrode active material of the present invention as the positive electrode active material, and the materials and components thereof, such as the conductive additive, positive electrode current collector, negative electrode active material, negative electrode current collector, and separator, are not particularly limited. These materials and components can be appropriately applied to those used in ordinary secondary batteries. For the components and manufacturing methods typically used in these secondary batteries, reference can be made to, for example, JP 2016-201308 A, JP 2005-108835 A, JP 2012-185938 A, WO 2018 / 135395, and the like.

[0027] [Method for Producing Regenerated Cathode Active Material] The method for producing a regenerated cathode active material of the present invention (also referred to as the production method of the present invention) includes mixing a lithium-deficient cathode active material (a), a lithium compound (b), and a reducing agent (c) in a molar ratio of the lithium compound (b) to the reducing agent (c) of 0.13≦(b) / (c)≦2.80, thereby replenishing lithium into the lithium-deficient cathode active material (a) to obtain a regenerated cathode active material. This method uses the lithium compound (b) as a lithium source to replenish lithium into the lithium-deficient cathode active material (a). According to the production method for a regenerated cathode active material of the present invention, the regenerated cathode active material of the present invention can be efficiently produced under relatively mild conditions while suppressing the formation of an oxide film.

[0028] A preferred embodiment of the manufacturing method of the present invention includes mixing the lithium-deficient positive electrode active material (a) with a treatment solution containing a lithium compound (b) and a reducing agent (c) in a solvent in the above molar ratio, thereby replenishing lithium. This mixing is preferably performed under heating. In this embodiment, the relithiation reaction of the lithium-deficient positive electrode active material (a) is a so-called hydrothermal reaction when the solvent contains water. The relithiated regenerated positive electrode active material can be recovered from the reaction solution by a conventional solid-liquid separation method, such as filtration. The manufacturing method of the present invention may further include recovering the lithium-deficient positive electrode active material (a), washing the obtained regenerated positive electrode active material, calcining it, etc. The manufacturing method of the present invention will be described in more detail, focusing on the above preferred embodiment.

[0029] <Recovery of Lithium-Deficient Positive Electrode Active Material (a)> The method for recovering the lithium-deficient positive electrode active material (a) is not particularly limited as long as it can be recovered from a used lithium-ion secondary battery. For example, the lithium-deficient positive electrode active material (a) can be recovered by immersing the positive electrode removed from the lithium-ion secondary battery in a solvent, separating the current collector and the positive electrode active material using an external stimulus such as ultrasonic treatment, and removing the current collector. The lithium-deficient positive electrode active material (a) can be obtained as solid particles in the remaining suspension. For example, as shown in the examples described below, the positive electrode removed from the lithium-ion secondary battery is washed and dried, and then immersed in a solvent and ultrasonicated. The current collector is removed from the resulting suspension, and the precipitate is recovered by centrifugation and dried.

[0030] The solvent used for the immersion and ultrasonic treatment may be selected appropriately, and examples thereof include N-methylpyrrolidone (NMP), N-ethylpyrrolidone, and N,N-dimethylformamide. When the positive electrode from which the lithium-deficient positive electrode active material (a) is extracted contains a binder, the immersion and ultrasonic treatment are preferably carried out in a solvent capable of dissolving the binder. Examples of such solvents include N-methylpyrrolidone (NMP), N-ethylpyrrolidone, and N,N-dimethylformamide. The immersion is preferably carried out for a period of time sufficient to allow the components constituting the positive electrode active material layer, particularly a binder, to fully blend with the solvent when the binder is contained. The immersion time is preferably 10 minutes or more, and more preferably 20 minutes or more. Since the recovery effect remains unchanged even if the immersion time is long, the immersion time is preferably 20 minutes to 1 hour. The immersion may be carried out at room temperature or with heating.

[0031] The conditions for the ultrasonic treatment are not particularly limited as long as they can separate the positive electrode current collector from the positive electrode and obtain solid particles (solid particles mainly composed of the positive electrode active material) constituting the positive electrode active material layer in a suspended state in the solvent. For example, ultrasonic treatment can be performed for 10 to 60 minutes. Centrifugation can be performed as long as the lithium-deficient positive electrode active material (a) can be recovered as a precipitate from the suspension. Centrifugation conditions can be, for example, 2000 to 4000 rpm and 5 to 30 minutes. Drying is an operation to remove the solvent from the recovered lithium-deficient positive electrode active material (a). Drying conditions can be, for example, 80 to 200°C and 5 to 24 hours. Alternatively, drying under reduced pressure can be used.

[0032] The immersion, ultrasonic treatment, and centrifugation described above can remove most of the binder and conductive additive. However, even after the immersion, ultrasonic treatment, and centrifugation described above, a small amount of the binder and / or conductive additive usually remains. Therefore, the lithium-deficient positive electrode active material (a) that has undergone these steps usually contains a small amount of the binder and / or conductive additive.

[0033] <Relithiation> From the viewpoints of suppressing the formation of an oxide film and increasing the relithiation rate of the regenerated positive electrode active material, the molar ratio of the lithium compound (b) to the reducing agent (c) used for the relithiation of the lithium-deficient positive electrode active material (a) is preferably 0.25≦(b) / (c)≦2.50, more preferably 0.35≦(b) / (c)≦2.45, even more preferably 0.35≦(B) / (C)≦2.40, and still more preferably 0.50≦(b) / (c)≦2.30.

[0034] The amount of lithium compound (b) is not particularly limited, and from the viewpoint of suppressing the formation of an oxide film and increasing the relithiation rate of the resulting regenerated positive electrode active material, it is preferably 1 molar equivalent or more (1 mole or more of lithium atoms possessed by lithium compound (b) per mole of lithium deficiency) relative to the amount (mol) of lithium deficiency in lithium-deficient positive electrode active material (a), more preferably 2 molar equivalents or more, more preferably 5 molar equivalents or more, more preferably 10 molar equivalents or more, and even more preferably 15 molar equivalents or more. The upper limit is practically 400 molar equivalents, and is preferably 380 molar equivalents or less, more preferably 300 molar equivalents or less, and even more preferably 200 molar equivalents or less. Therefore, the blending amount of the lithium compound (b) is preferably 1 to 400 molar equivalents relative to the amount of lithium deficiency (mol), more preferably 2 to 400 molar equivalents, more preferably 5 to 400 molar equivalents, more preferably 10 to 380 molar equivalents, more preferably 15 to 300 molar equivalents, and even more preferably 15 to 200 molar equivalents. The amount of lithium deficiency in the lithium-deficient positive electrode active material (a) can be calculated from the lithium deficiency rate (%) of the lithium-deficient positive electrode active material (a). The lithium deficiency rate can be measured by the method described in the Examples.

[0035] The lithium compound (b) and the reducing agent (c) are preferably dissolved in a solvent to form a solution (treatment solution) and then reacted with the lithium-deficient positive electrode active material (a). Therefore, the solvent is preferably a solvent that can dissolve the lithium compound (b) and the reducing agent (c), and an organic solvent, an inorganic solvent, or a combination thereof can be used. The solvent is preferably water (preferably ultrapure water) or a combination of water and a water-soluble organic solvent.

[0036] When the treatment solution and the lithium-deficient positive electrode active material (a) are mixed, the concentration of the lithium-deficient positive electrode active material (a) in the mixture can be 0.01 to 1.20 mol / L, 0.03 to 1.00 mol / L, or 0.03 to 0.80 mol / L. The concentration of the lithium compound (b) in the mixture, expressed as the amount of lithium, can be 0.05 to 5.00 mol / L, or 0.10 to 4.00 mol / L. The concentration of the reducing agent (c) in the mixture can be 0.10 to 3.00 mol / L, 0.40 to 3.00 mol / L, or 0.50 to 2.00 mol / L.

[0037] The reaction temperature (temperature of the mixture) of the lithium-deficient positive electrode active material (a), the lithium compound (b), and the reducing agent (c) is not particularly limited, and is preferably 60 to 160°C, more preferably 60 to 140°C, even more preferably 60 to 120°C, and still more preferably 80 to 100°C.

[0038] The reaction time of the lithium-deficient positive electrode active material (a), the lithium compound (b), and the reducing agent (c) varies depending on the reaction temperature, but is preferably 5 to 20 hours, more preferably 7 to 18 hours, and even more preferably 10 to 15 hours.

[0039] The reaction between the lithium-deficient positive electrode active material (a), the lithium compound (b), and the reducing agent (c) is preferably carried out in a pressure-resistant container, because the pressure inside the container may become high during the reaction.

[0040] The lithium compound (b) acts as a lithium source for replenishing lithium in the lithium-deficient positive electrode active material (a). Examples of the lithium compound (b) include lithium sulfate, carbonate, bicarbonate, hydroxide, nitrate, acetate, oxalate, phosphate, halide, fluoride, oxide, etc. Preferred specific examples include lithium hydroxide (LiOH), lithium sulfate (Li ２ SO ４ ), lithium chloride (LiCl), lithium carbonate (Li ２ CO ３ ), lithium bicarbonate (LiHCO ３), lithium iodide (LiI), lithium fluoride (LiF), lithium acetate (LiCH ３ COO), lithium oxide (Li ２ The lithium compound (b) is preferably a water-soluble lithium compound, and more preferably LiCl and / or LiOH, and more preferably LiOH.

[0041] The reducing agent (c) acts as a reducing agent for the lithium-deficient positive electrode active material (a). As the reducing agent (c), a carboxylic acid compound such as citric acid, an ascorbic acid compound such as ascorbic acid and its salts, a phosphinic acid compound such as phosphinic acid and its salts, sodium hydrogen sulfite, etc. can be used, and a carboxylic acid compound and an ascorbic acid compound are preferred, citric acid and ascorbic acid are more preferred, and citric acid is even more preferred. The reducing agent (c) is preferably water-soluble.

[0042] <Other Steps> In the manufacturing method of the present invention, solid particles may be filtered and washed from the reaction solution after the relithiation treatment to remove the lithium compound (b) and reducing agent (c) not consumed during the relithiation treatment, the conductive additive, by-products formed during the relithiation treatment, and the like. In the manufacturing method of the present invention, after the relithiation treatment, calcination may be performed to control the crystalline state of the regenerated positive electrode active material, or calcination may not be performed. By performing calcination, defects in the crystals of the regenerated positive electrode active material can be reduced. By reducing the defects in the crystals, the discharge capacity retention rate when exposed to high temperatures when incorporated into a lithium-ion secondary battery can be further improved. The manufacturing method of the present invention may include either the washing or calcination described above as a post-treatment for the relithiation treatment, or may include both the washing and calcination.

[0043] The filtration may be carried out by any ordinary filtration method as long as the solid particles can be recovered from the treatment solution, and is preferably carried out by vacuum filtration.

[0044] The above-mentioned washing can be performed by washing the solid particles obtained by relithiation with a washing solution. Even if a trace amount of conductive additive remains (when recovering the lithium-deficient positive electrode active material (a)), washing can remove it due to the difference in specific gravity and particle size with the regenerated positive electrode active material. The method for removing the conductive additive is not limited, and flotation or centrifugation can be used. For example, the solid particles can be suspended in a washing solution and then centrifuged to recover the solid particles. After washing, the solid particles can be dried. Water is preferred as the washing solution, and pure water is preferable. The suspension conditions are not particularly limited. The solid particles can be mixed with the washing solution and then suspended. The rotation speed during centrifugation is not particularly limited, and is preferably 1000 to 5000 rpm, more preferably 1500 to 4000 rpm, and even more preferably 1800 to 3000 rpm. The centrifugation time is not particularly limited, and is preferably 1 to 10 minutes, more preferably 2 to 8 minutes. After washing, the solid particles are preferably further dried. This drying can be carried out, for example, by keeping it at 120° C. for 24 hours.

[0045] The calcination can be carried out after the relithiation treatment. The calcination may be carried out after the washing. The crystallinity of the regenerated positive electrode active material can be controlled by carrying out the calcination. The calcination is preferably carried out at 400°C or higher, more preferably at 500°C or higher, and even more preferably at 550°C or higher. The upper limit of the calcination temperature is practically 900°C or lower, and preferably at 650°C or lower. Therefore, the calcination temperature is preferably 400 to 900°C, more preferably 500 to 900°C, even more preferably 500 to 650°C, and even more preferably 550 to 650°C. The calcination time is preferably 1 to 5 hours, more preferably 1 to 3 hours, and even more preferably 1 to 1.5 hours. The calcination may be carried out in air or in an inert gas (e.g., argon, helium, nitrogen, etc.) atmosphere. In the present invention, it is preferable to carry out the calcination in a nitrogen atmosphere. During the calcination, a lithium compound (Li ２ CO ３It is preferable not to use oxygen sources such as impurities, because this can prevent the regenerated positive electrode active material from being oxidized due to oxygen sources contained in impurities, etc.

[0046] The present invention will be described in more detail based on examples, but the present invention should not be construed as being limited by these examples except as defined in the present invention. Furthermore, "room temperature" means 25°C. "Parts" and "%" representing compositions are based on mass unless otherwise specified.

[0047] [Preparation of Lithium-Deficient Positive Electrode Active Material (a)] A lithium ion secondary battery was fabricated, and the resulting lithium ion secondary battery was charged and discharged to cause lithium to be deficient from the positive electrode active material, and the lithium-deficient positive electrode active material (a) was recovered. The details of this process will be described below.

[0048] <Battery Fabrication (First Embodiment): Examples 1, 3 to 22, Comparative Examples 1 to 4> 1. Preparation of Non-Aqueous Electrolyte Solution 1 A non-aqueous electrolyte solution 1 was prepared by mixing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a mass ratio of EC:DMC:EMC = 3:4:3. LiPF 6 as a lithium salt was added to the non-aqueous electrolyte solution 1. ６ These were mixed to a concentration of 1M to prepare a non-aqueous electrolyte solution (non-aqueous electrolyte solution 1).

[0049] 2. Preparation of the positive electrode Lithium iron phosphate (LiFePO ) was used as the positive electrode active material. ４ ) (product code: L0386, manufactured by Tokyo Chemical Industry Co., Ltd.) 95.7 parts by mass, acetylene black (AB) (Li-100 (product name), specific surface area = 69 m) as a conductive additive, ２Positive electrode slurry P1 was prepared by mixing 4.3 parts by mass of ethylenediaminetetraacetic acid (PEA) (manufactured by Denka Co., Ltd.), polyvinylidene fluoride (PVdF) as a binder, and N-methylpyrrolidone (NMP) as a solvent (concentration of solvent-insoluble components: 75.3% by mass). The binder was 6 parts by mass per 100 parts by mass of the positive electrode active material. The positive electrode slurry P1 was applied to one side of a 12 μm thick positive electrode current collector (aluminum foil) to a thickness of 125 μm, and the dispersion medium was removed at 240 ° C. Thereafter, the mixture was pressed at a pressure of 3.2 t using a roll press to obtain a sheet-like positive electrode consisting of a positive electrode current collector and a positive electrode active material layer. The thickness of this positive electrode was approximately 80 μm.

[0050] 3. Preparation of negative electrode As the negative electrode active material, artificial graphite (UF-G5 (trade name), manufactured by Showa Denko K.K., average particle size (average particle size equivalent to sphere) 3 μm) 95.7 parts by mass, as a conductive additive, acetylene black (AB) (Li-100 (trade name), manufactured by Denka Co., Ltd.) 4.3 parts by mass, NMP, as a binder, styrene-butadiene copolymer (SBR), and as a thickener, carboxymethyl cellulose (CMC) were mixed to obtain a negative electrode slurry N1 (concentration of components insoluble in the solvent 70.6% by mass). The binder was 1 part by mass per 100 parts by mass of the negative electrode active material, and the thickener was 1 part by mass per 100 parts by mass of the negative electrode active material. The negative electrode slurry N1 was coated to a thickness of 120 μm on one side of a 12 μm thick negative electrode current collector (copper foil), and the dispersion medium was removed at 240 ° C. Thereafter, the negative electrode was pressed with a roll press at a pressure of 2 tons to obtain a sheet-like negative electrode comprising the negative electrode current collector and the negative electrode active material layer, the thickness of which was about 80 μm.

[0051] 4. Battery Fabrication The resulting positive and negative electrodes were stacked with a separator (20 μm thick) manufactured by W-SCOPE interposed between them to form a laminate consisting of a positive electrode current collector, a positive electrode active material layer, a separator, a negative electrode active material layer, and a negative electrode current collector. An aluminum tab was attached to the end of the positive electrode current collector, and a nickel tab was attached to the end of the negative electrode current collector by ultrasonic welding. A battery assembly was fabricated by housing this laminate in a laminate container. After pouring nonaqueous electrolyte 1 with the inlet open, the inlet was sealed and the case was hermetically sealed, yielding a lithium-ion secondary battery.

[0052] <Battery Production (First Embodiment): Example 23> In the production of the positive electrode in the above 2., lithium manganese phosphate (LiMnPO ) was used as the positive electrode active material instead of lithium iron phosphate. ４ A positive electrode slurry P2 was prepared in the same manner as in the above 2. Preparation of a positive electrode, except that the positive electrode slurry P2 was used. A lithium ion secondary battery was prepared in the same manner as in the above 4. Preparation of a battery, except that the positive electrode was used.

[0053] <Battery Fabrication (Second Form): Example 2> 1. Preparation of Positive Electrode Slurry P3 99.9 parts by mass of lithium iron phosphate, 0.1 parts by mass of Ketjen black (Carbon ECP (trade name), manufactured by Lion) as a conductive additive, and non-aqueous electrolyte 1 were mixed in a centrifugal planetary mixer (Thinky Corporation, Awatori Rentaro (trade name)) at 1250 rpm for 90 seconds to obtain positive electrode slurry P3. The amount of non-aqueous electrolyte 1 in positive electrode slurry P3 was 22.9 mL per 100 g of the total positive electrode active material and conductive additive in positive electrode slurry P3.

[0054] 2. Preparation of negative electrode slurry N3 98.2 parts by mass of artificial graphite (UF-G30 (trade name), manufactured by Showa Denko K.K.), 1.8 parts by mass of carbon black (LITX300 (trade name), manufactured by CABOT Corporation) as a conductive additive, and non-aqueous electrolyte 1 were mixed for 90 seconds at 1250 rpm in a centrifugal planetary mixer (Thinky Corporation, Awatori Rentaro (trade name)) to obtain negative electrode slurry N3. The amount of non-aqueous electrolyte 1 in negative electrode slurry N3 was 37.7 mL per 100 g of the total of the negative electrode active material and conductive additive in negative electrode slurry N3.

[0055] 3. Preparation of Quasi-Solid Secondary Battery A quasi-solid secondary battery was prepared with reference to JP-A-2016-500465 (Examples 10 and 11). Details are shown below. Positive electrode slurry P3 was applied to a 500 μm thick, 80 cm2 area battery. ２ The negative electrode slurry N3 was applied to an aluminum foil positive electrode current collector so as to form a positive electrode consisting of a positive electrode current collector and a positive electrode active material layer. ２ The negative electrode was formed by applying the coating to a copper foil negative electrode current collector so that the coating would be in a thickness of 100 μm. A separator (20 μm thick) manufactured by W-SCOPE was placed on the negative electrode to form a negative electrode. ２The separator was laminated on the negative electrode so that the negative electrode was positioned inside the separator, and the positive electrode was laminated on top of the separator so that the negative electrode was positioned inside the negative electrode, thereby producing a laminate of negative electrode current collector-negative electrode active material layer (slurry)-separator-positive electrode active material layer (slurry)-positive electrode current collector. The fabrication of the laminate was completed in about one minute to avoid volatilization of the non-aqueous electrolyte 1. Tabs were ultrasonically welded to the uncoated portions of the aluminum foil with the positive electrode slurry and the uncoated portions of the copper foil with the negative electrode slurry, and the laminate was wrapped in aluminum laminate and sealed with a vacuum sealer to produce a lithium ion secondary battery (quasi-solid secondary battery) for evaluation testing.

[0056] <Charge and Discharge of Lithium-Ion Secondary Battery> Each of the obtained lithium-ion secondary batteries was charged and discharged as follows to form a lithium-deficient positive electrode active material (a) in each positive electrode active material layer. The obtained lithium-ion secondary battery was charged at 0.1 C using a charge / discharge evaluation device (TOSCAT-3000 (trade name), manufactured by Toyo Systems Co., Ltd.) until the voltage reached 3.6 V, and then discharged until the voltage reached 2.0 V. This cycle was counted as one charge / discharge, and the battery was subjected to 200 charge / discharge cycles. Thereafter, the battery was further charged and discharged under the same conditions until each positive electrode active material had the lithium deficiency rate shown in Table 1.

[0057] [Recovery of Lithium-Deficient Positive Electrode Active Material (a)] The lithium-deficient positive electrode active material (a) was recovered from the positive electrode of the lithium-ion secondary battery after the charging and discharging as follows. The lithium-ion secondary battery after the charging and discharging was disassembled, and the positive electrode was removed. The removed positive electrode was washed with dimethyl carbonate (DMC) and then dried. The dried positive electrode was immersed in NMP for 30 minutes, then ultrasonicated in NMP for 20 minutes to dissolve the binder, and the current collector was separated and removed to obtain a suspension containing the lithium-deficient positive electrode active material (a). The suspension was centrifuged at 3500 rpm for 5 minutes to recover the precipitate, which was then dried at 120°C for 12 hours. In this way, the lithium-deficient positive electrode active material (a) was obtained. The conductive additive did not precipitate during the centrifugation, and the conductive additive and the lithium-deficient positive electrode active material (a) could be separated.

[0058] [Determination of Lithium Deficiency Rate of Lithium-Deficient Positive Electrode Active Material (a)] The lithium deficiency rate of the lithium-deficient positive electrode active material (a) was determined by elemental analysis as follows. The measurement was performed using an inductively coupled plasma optical emission spectrometer (ICP-OES) (Optima 7300DV (trade name), manufactured by PerkinElmer) using an absolute calibration curve method. 20 mg of the lithium-deficient positive electrode active material (a) was weighed, 60% nitric acid was added, and the mixture was microwave ashed. After ashing, the mixture was adjusted to 50 mL with ultrapure water and further diluted 100 times to prepare a measurement sample. The lithium deficiency rate was calculated from the molar ratio of Fe to Li obtained by ICP-OES measurement. However, the lithium deficiency rate was calculated from the molar ratio of Mn to Li in Example 23. When the lithium-deficient positive electrode active material (a) contains a transition metal element other than Fe and Mn, the lithium deficiency rate can be calculated in the same manner.

[0059] [Relithiation] Each of the lithium-deficient positive electrode active materials (a) obtained above was relithiated as follows.

[0060] <Relithiation: Hydrothermal Reaction> 1. Preparation of Treatment Solution Lithium compound (b) and reducing agent (c) were dissolved in ultrapure water to the contents shown in Table 1 to prepare each treatment solution used for relithiation. 2. Relithiation Each lithium-deficient positive electrode active material (a) listed in Table 1 and 40 mL of each treatment solution listed in Table 1 were mixed and sealed in a pressure-resistant container (HU-100 (trade name), manufactured by San-Ai Scientific Co., Ltd.). In this manner, the lithium-deficient positive electrode active material (a), lithium compound (b), and reducing agent (c) were mixed. The concentration of the lithium-deficient positive electrode active material (a) in the mixture is shown in Table 1. Thereafter, the mixture was maintained at the temperature and time shown in Table 1 to allow a hydrothermal reaction. The solid particles after the hydrothermal reaction were separated by vacuum filtration to obtain each regenerated positive electrode active material.

[0061] <Water washing> The regenerated positive electrode active material separated and recovered by the vacuum filtration was washed with pure water, then transferred to a centrifuge tube and suspended in pure water. The suspension was then centrifuged at 2000 rpm for 5 minutes using a centrifuge, and the precipitate was collected and dried at 120°C for 24 hours. The regenerated positive electrode active material was thus washed with water.

[0062] <Baking> The regenerated positive electrode active material after the above <Water Washing> was baked using a tabletop vacuum gas replacement furnace (KDF-75Plus (trade name), manufactured by Denken Hydenthal). The temperature was raised to 600°C at 5°C / min in a nitrogen atmosphere, held for 1 hour, and then returned to room temperature. In this way, the regenerated positive electrode active material was baked to obtain a baked regenerated positive electrode active material. The regenerated positive electrode active material of Example 21 was not baked. In the "Baking" column of Table 1, "Yes" is entered when baking was performed, and "No" is entered when baking was not performed. Hereinafter, for convenience, the baked regenerated positive electrode active material obtained by baking and the regenerated positive electrode active material that was not baked will be collectively referred to as the "regenerated positive electrode active material."

[0063] <Evaluation of Relithiation Rate> Each of the regenerated positive electrode active materials obtained as described above was analyzed by X-ray diffraction (XRD) to determine the LiFePO ４ and the peak area of ​​FePO at 2θ = 18.0 degrees ４ The peak area of ​​the above was determined and substituted into the following equation to calculate the relithiation rate: Relithiation rate = LiFePO ４ Peak area / (LiFePO ４ Peak area + FePO ４ Peak area) As the X-ray diffractometer, a MiniFlex™ 600 (manufactured by Rigaku Corporation) was used. The regenerated positive electrode active material obtained in Example 23 was analyzed by X-ray diffraction (XRD) to find that LiMnPO ４ and the peak area of ​​MnPO at 2θ = 18.0 degrees ４ The peak area of ​​the above was calculated and substituted into the following equation to calculate the relithiation rate: ４ Peak area / (LiMnPO ４ Peak area + MnPO ４Peak Area) When the lithium-deficient positive electrode active material (a) contains a transition metal element other than Fe and Mn, the relithiation rate can be determined in the same manner.

[0064] <Evaluation of Oxide Film: X-ray Photoelectron Spectroscopy (XPS) Measurement> For each recycled positive electrode active material obtained as described above, X-ray photoelectron spectroscopy (XPS) measurement was performed as follows to evaluate the state of oxide film formation on the surface of each recycled positive electrode active material, and the peak intensity ratio was calculated. 20 mg of each recycled positive electrode active material obtained as described above was packed into a powder sample holder to prepare a sample (powder film). X-ray photoelectron spectroscopy (XPS) measurement was performed using a Kratos Axis Ultra spectrometer (trade name, manufactured by Shimadzu Corporation) equipped with a focused monochromated AlKα radiation source (hυ = 1486.6 eV) as the X-ray source. Measurement was performed at a photoelectron takeoff angle of 45°, and the peak position and area were optimized by weighted least squares fitting using 80% Gaussian and 20% Lorentzian lineshapes. The peak intensity (I Ｂ ) and the peak intensity (I Ａ ) and calculate the ratio (I Ｂ / I Ａ ) was decided.

[0065] [Battery Evaluation] <Fabrication of Lithium-Ion Secondary Battery> Secondary batteries using recycled positive electrode active materials were fabricated in the same manner as in <Fabrication of Battery (First Form): Examples 1, 3 to 22, Comparative Examples 1 to 4>, <Fabrication of Battery (First Form): Example 23>, and <Fabrication of Battery (Second Form): Example 2> in [Preparation of Lithium-Deficient Positive Electrode Active Material (a)] above, except that each recycled positive electrode active material was used in <Fabrication of Battery (First Form): Examples 1, 3 to 22, Comparative Examples 1 to 4>, <Fabrication of Battery (First Form): Example 23>, and <Fabrication of Battery (Second Form): Example 2>.

[0066] <Evaluation of Output Characteristics After High-Temperature Storage> The change in discharge capacity when a lithium-ion secondary battery containing a regenerated positive electrode active material was exposed to a high temperature was investigated as follows. An output characteristics test was carried out under the following conditions for the lithium-ion secondary batteries using the regenerated positive electrode active material prepared above. Specifically, at room temperature, the batteries were charged to 3.9 V by CC-CV charging under the following conditions, and discharged to 2.0 V by CC discharge, to measure the reference discharge capacity. Thereafter, each lithium-ion secondary battery was stored at 80°C for 24 hours (high-temperature storage). Thereafter, the batteries were returned to room temperature, and charged to 3.9 V by CC-CV charging under the following conditions, and discharged to 2.0 V by CC discharge, to measure the output discharge capacity. Furthermore, the discharge capacity retention rate was calculated using the following formula and evaluated according to the following evaluation criteria.・Reference discharge capacity CC-CV charge: Current value 250mA, upper limit voltage 3.9V, final current value 25mA CC discharge: Current value 250mA, final voltage value 2.0V ・Output evaluation discharge capacity CC-CV charge: Current value 250mA, upper limit voltage 3.9V, final current value 25mA CC discharge: Current value 1000mA, final voltage value 2.0V Discharge capacity retention rate (%) = [(output evaluation discharge capacity) / (reference discharge capacity)] x 100 - Evaluation criteria - 5: 80% or more 4: 70% or more and less than 80% 3: 65% or more and less than 70% 2: 60% or more and less than 65% 1: Less than 60%

[0067]

[0068] <Notes for Table 1> "Li-deficient positive electrode active material (a)": lithium-deficient positive electrode active material (a) "Li deficiency rate": lithium deficiency rate "Li compound (b)": lithium compound (b)

[0069] The regenerated positive electrode active materials of Comparative Examples 1 to 4 were all regenerated under conditions where " did not satisfy 0.13 ≦ (b) / (c) ≦ 2.80, and in the oxygen 1s core spectrum measured by XPS measurement, the ratio of the peak intensity in the region of 528 eV or more and less than 530 eV to the peak intensity in the region of 530 eV or more and less than 535 eV exceeded 0.005. Lithium-ion secondary batteries using these regenerated positive electrode active materials had discharge capacity retention rates of less than 65%. Furthermore, the regenerated positive electrode active materials of these comparative examples all had low relithiation rates of 0.71 or less. These comparative examples show that it is difficult to obtain a regenerated positive electrode active material while sufficiently suppressing the formation of an oxide film when the molar ratio of the reducing agent (c) to the lithium compound (b) is either too high or too low compared to the above range. In Comparative Examples 1 and 3, in which the amount of reducing agent (c) was too high, the formation of an oxide film was somewhat suppressed. However, the reducing agent (c) that was not consumed during relithiation was oxidized during high-temperature storage, causing the electrode active material layer to gel, presumably resulting in a decrease in discharge capacity retention. In contrast, the regenerated positive electrode active materials of Examples 1 to 23 were regenerated under conditions in which the molar ratio of lithium compound (b) to reducing agent (c) satisfied the condition 0.13≦(b) / (c)≦2.80. In the oxygen 1s core spectrum measured by XPS, the ratio of the peak intensity in the region from 528 eV to 530 eV to the peak intensity in the region from 530 eV to 535 eV was 0.005 or less. Lithium-ion secondary batteries using these regenerated positive electrode active materials achieved discharge capacity retention rates of 65% or more. It can be seen that the regenerated positive electrode active material of the present invention suppresses the formation of an oxide film, thereby enabling the production of lithium-ion secondary batteries with excellent discharge capacity retention rates. It is also clear that the production method of the present invention makes it possible to obtain a high-quality regenerated positive electrode active material under relatively mild conditions.

[0070] While the present invention has been described in connection with embodiments thereof, we do not intend to limit our invention to any of the details of the description unless otherwise specified, and believe that the claims should be construed broadly without departing from the spirit and scope of the invention as set forth in the appended claims.

[0071] This application claims priority based on Japanese Patent Application No. 2023-219924, filed on December 26, 2023, the contents of which are incorporated herein by reference as part of the present specification.

[0072] 10 Lithium ion secondary battery 1 Negative electrode current collector 2 Negative electrode active material layer 3 Separator 4 Positive electrode active material layer 5 Positive electrode current collector 6 Operating part (light bulb)

Claims

1. A regenerated cathode active material obtained by replenishing lithium to a lithium-deficient cathode active material (a), wherein in the oxygen 1s inner shell spectrum measured by XPS measurement of the regenerated cathode active material, the ratio of the peak intensity in the region of 528 eV or more and less than 530 eV to the peak intensity in the region of 530 eV or more and less than 535 eV is 0.005 or less.

2. The regenerated cathode active material according to claim 1, wherein the lithium-deficient cathode active material (a) has an olivine-type structure.

3. The lithium-deficient cathode active material (a) is Li １－ xM １ PO ４ and is an active material represented by x represents the amount of lithium deficiency, 0 < x ≦ 1, and M １ represents at least one element selected from Fe, Mn, and Co. The regenerated cathode active material according to claim 1.

4. A lithium-ion secondary battery comprising the regenerated cathode active material according to any one of claims 1 to 3.

5. A method for producing a regenerated cathode active material, comprising mixing a lithium-deficient cathode active material (a), a lithium compound (b), and a reducing agent (c) with a molar ratio of (b) / (c) of 0.13 ≦ (b) / (c) ≦ 2.80 to replenish lithium to the (a) to obtain a regenerated cathode active material.

6. The method for producing a regenerated cathode active material according to claim 5, wherein the amount of the lithium compound (b) is 10 molar equivalents or more with respect to the lithium deficiency amount of the lithium-deficient cathode active material (a).

7. The method for producing a regenerated cathode active material according to claim 5, wherein the lithium-deficient cathode active material (a), the lithium compound (b), and the reducing agent (c) are heated at 80 to 100 °C.

8. The method for producing a regenerated cathode active material according to claim 7, wherein the solid particles obtained after the heating are collected by filtration and washed.

9. The method for producing a regenerated cathode active material according to claim 7 or 8, wherein the solid particles obtained after the heating are fired at 400 °C or higher.

10. The method for producing a regenerated cathode active material according to claim 5, wherein the lithium-deficient cathode active material (a) has an olivine-type structure.

11. The lithium-deficient cathode active material (a) is Li １－ xM １ PO ４ and is a cathode active material represented by, where x represents the lithium deficiency amount and 0 < x ≤ 1, and M １ represents at least one element selected from Fe, Mn, and Co. The method for manufacturing a recycled cathode active material according to claim 5.

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