Method of producing regenerated positive electrode active material

Abstract

A method of producing a regenerated positive electrode active material, said method including mixing a lithium-deficient positive electrode active material (a), a lithium compound (b), and a reducing agent (c) to replenish lithium in the lithium-deficient positive electrode active material (a) and obtain a regenerated positive electrode active material, wherein the redox potential of the reducing agent (c) is 1.80-3.00 eV vs Li+ / Li.

Description

Method for producing recycled positive electrode active material

[0001] The present invention relates to a method for producing a 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 above-mentioned positive electrode active material deteriorates not only due to the loss of lithium ions during long-term charge and discharge, but also due to the breakdown of the original crystalline structure of the positive electrode active material (equivalent to a decrease in the regularity of the crystalline structure or a decrease in crystallinity). The inventors have found that regenerated positive electrode active materials obtained by relithiating lithium-deficient positive electrode active materials can have low crystallinity. Poor crystallinity of regenerated positive electrode active materials can result in insufficient discharge capacity when incorporated into lithium-ion secondary batteries. While calcination is known as a method for improving the crystallinity of positive electrode active materials, if a positive electrode active material with excellent crystallinity could be obtained under milder reaction conditions at lower temperatures, the regeneration of the positive electrode active material could be more efficiently performed. The present invention aims to provide a method for producing a regenerated positive electrode active material, which includes replenishing lithium to lithium-deficient positive electrode active material generated during the use of a lithium-ion secondary battery, and which can produce a regenerated positive electrode active material with excellent crystallinity under relatively mild reaction conditions.

[0006] The present inventors have conducted extensive research into methods for regenerating a lithium-deficient positive electrode active material by replenishing lithium, and have found that by using a specific reducing agent, a regenerated positive electrode active material exhibiting excellent crystallinity can be obtained under relatively mild reaction conditions. Based on these findings, the present invention has been completed through further research.

[0007] That is, the above-mentioned problems have been solved by the following means: [1] A method for producing a regenerated positive electrode active material, comprising: mixing a lithium-deficient positive electrode active material (a), a lithium compound (b), and a reducing agent (c) to replenish lithium into the lithium-deficient positive electrode active material (a) to obtain a regenerated positive electrode active material, wherein the oxidation-reduction potential of the reducing agent (c) is 1.80 to 3.00 eV vs Li ＋ [2] The method for producing a regenerated positive electrode active material, wherein the oxidation-reduction potential of the reducing agent (c) is 2.00 to 2.70 eV vs. Li. ＋ / Li. [3] The method for producing a regenerated cathode active material according to [1] or [2], wherein the reducing agent (c) is an ascorbic acid compound. [4] The method for producing a regenerated cathode active material according to any one of [1] to [3], wherein a mixture of the lithium-deficient cathode active material (a), the lithium compound (b), and the reducing agent (c) is maintained at 25 to 160°C. [5] The method for producing a regenerated cathode active material according to any one of [1] to [4], wherein a mixture of the lithium-deficient cathode active material (a), the lithium compound (b), and the reducing agent (c) is maintained at 25 to 120°C. [6] The method for producing a regenerated cathode active material according to any one of [1] to [5], wherein the lithium compound (b) is lithium hydroxide. [7] The method for producing a regenerated positive electrode active material according to any one of [1] to [6], wherein the lithium-deficient positive electrode active material (a) has an olivine structure. [8] The method for producing a regenerated positive electrode active material according to any one of [1] to [6], wherein the lithium-deficient positive electrode active material (a) has an olivine structure. １－ xM １ P.O. ４ where x represents the amount of lithium deficiency, 0<x≦1, and M １represents at least one element selected from Fe, Mn, and Co. [9] The method for producing a regenerated cathode active material according to any one of [1] to [7], wherein the regenerated cathode active material contains a lithium-containing transition metal phosphate compound, and the regenerated cathode active material has an X-ray diffraction peak half-width of 0.001 degrees or more and 0.099 degrees or less at 2θ = 17.1 degrees.

[10] The method for producing a regenerated cathode active material according to [9], wherein the regenerated cathode active material has an X-ray diffraction peak half-width of 0.005 degrees or more and 0.050 degrees or less.

[0008] According to the method for producing a regenerated positive electrode active material of the present invention, a regenerated positive electrode active material can be obtained from a lithium-deficient positive electrode active material under relatively mild reaction conditions while being imparted with excellent crystallinity.

[0009] In the present invention, a numerical range expressed using "to" means a range including the numerical values ​​written before and after "to" as the lower and upper limits. When describing a component composition in the present invention, unless otherwise specified, one type of each component may be contained, or two or more types may be contained. 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, and energy is stored and released at the positive and negative electrodes. In other words, the term "secondary battery" in the present invention encompasses both batteries and capacitors (e.g., lithium ion capacitors). When the ions are lithium ions, it becomes a lithium ion secondary battery. In the present invention, the "non-aqueous 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 non-aqueous electrolyte completely anhydrous, and it usually contains 1 ppm or more of water.

[0010] [Method for Producing Regenerated Positive Electrode Active Material] The method for producing a regenerated positive electrode active material of the present invention (also referred to as the production method of the present invention) is a method for producing a regenerated positive electrode active material, which comprises mixing a lithium-deficient positive electrode active material (a), a lithium compound (b), and a reducing agent (c) to replenish lithium into the lithium-deficient positive electrode active material (a) to obtain a regenerated positive electrode active material. The production method of the present invention uses a reducing agent (c) having an oxidation-reduction potential of 1.80 to 3.00 eV vs. Li ＋ The regenerated cathode active material obtained by the manufacturing method of the present invention can be used as a cathode active material for secondary batteries.

[0011] 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 to replenish 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-mentioned preferred embodiment.

[0012] <Recovery of Lithium-Deficient Positive Electrode Active Material (a)> First, the lithium-deficient positive electrode active material (a) used as a raw material will be described. 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 deficient, resulting in a state in which 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 deficient in lithium.

[0013] The lithium-deficient positive electrode active material (a) is derived from a metal oxide that is commonly used as a positive electrode active material in 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, examples 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 among these, lithium-containing transition metal phosphate compounds having an olivine structure are 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:

[0014] 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－ xM １ 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:

[0015] 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.

[0016] 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 recovery method for the lithium-deficient positive electrode active material (a) can be achieved 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 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.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] <Relithiation> The oxidation-reduction potential of the reducing agent (c) used for relithiation is 1.80 to 3.00 eV vs Li ＋ / Li. By setting the oxidation-reduction potential of the reducing agent (c) within the above range, the crystallinity of the regenerated positive electrode active material can be sufficiently increased to a desired level. For example, if a reducing agent with too high an oxidation-reduction potential is used, it becomes necessary to increase the reaction temperature or extend the reaction time during relithiation in order to achieve sufficient relithiation. Under such high temperature and / or long reaction conditions, it becomes difficult to obtain a regenerated positive electrode active material exhibiting sufficient crystallinity, or side reactions such as the production of iron oxide tend to proceed. On the other hand, if a reducing agent with too low an oxidation-reduction potential is used, relithiation becomes difficult to proceed due to side reactions between the reducing agent and impurities such as electrolyte components contained in the recovered lithium-deficient positive electrode active material. The oxidation-reduction potential of the reducing agent (c) is 2.00 to 2.70 eV vs Li ＋ / Li is preferred, and 2.20 to 2.50 eV vs Li ＋ / Li is more preferable, and 2.40 to 2.50 eV vs Li ＋ / Li is more preferable. By setting the oxidation-reduction potential of the reducing agent (c) within the above-mentioned preferred range, the crystallinity of the regenerated positive electrode active material can be further improved. Furthermore, the relithiation rate of the regenerated positive electrode active material also tends to be further increased. The oxidation-reduction potential of the reducing agent (c) can be a literature value. For example, the oxidation-reduction potential value described in the Electrochemical Handbook, 6th Edition (edited by the Electrochemical Society, Maruzen) can be used. When the literature value is a relative value (vs. SHE) to the standard hydrogen electrode (SHE), adding 3.05 eV to the literature value can provide a value of lithium (Li ＋ / Li) based on the oxidation-reduction potential (vs Li ＋ For reducing agents whose redox potential is unknown, the redox potential can be determined by the following method.

[0021] -Method for measuring oxidation-reduction potential- Electrolyte a: 0.25M Li ２ SO ４ An aqueous solution is used. Electrolyte b, which contains 1% by mass of reducing agent (c) in electrolyte a, is prepared separately. Electrolyte b is placed in the cathode chamber of a battery in which glassy carbon (3 mm diameter) is installed as the working electrode. A platinum black mesh is placed in the anode chamber filled with electrolyte a, which is divided by a sintered glass disk. An Ag / AgCl electrode is used as the reference electrode. A voltammogram (current-voltage curve) is obtained using an electrochemical measurement system (VMP-300 (trade name), manufactured by BioLogic). Measurements are performed at room temperature, atmospheric pressure, and a scan rate of 120 mV / sec, from 2.05 V to 4.25 V vs. Li. ＋ Separately, a voltammogram is obtained in the same manner as above, except that electrolyte solution a (electrolyte solution not containing a reducing agent (c)) is used instead of electrolyte solution b. The two voltammograms obtained above are superimposed, and the initial voltage at which the difference between the current value of electrolyte solution b and the current value of electrolyte solution a not containing a reducing agent (c) becomes 0.2 mA is read during the rise of the current value. The oxidation-reduction potential of lithium (Li) is calculated from this Ag / AgCl-based oxidation-reduction potential. ＋ / Li) based on the oxidation-reduction potential (vs Li ＋ / Li).

[0022] The reducing agent (c) acts as a reducing agent for the lithium-deficient positive electrode active material (a). The reducing agent (c) has an oxidation-reduction potential of 1.80 to 3.00 eV vs. Li ＋ / Li. The reducing agent (c) is preferably an organic compound from the viewpoint of suppressing the generation of inorganic ions that may affect battery performance. Examples of the reducing agent (c) that can be used include carboxylic acid compounds such as citric acid, ascorbic acid compounds such as ascorbic acid and its salts, phosphinic acid compounds such as phosphinic acid and its salts, and sodium bisulfite. Carboxylic acid compounds and ascorbic acid compounds are preferred, and ascorbic acid compounds are more preferred. From the viewpoint of achieving both excellent crystallinity and a high lithium conversion rate, ascorbic acid compounds are preferred, ascorbic acid or metal ascorbic acid salts (preferably alkali metal salts) are more preferred, ascorbic acid or sodium ascorbate are even more preferred, and ascorbic acid is particularly preferred. The reducing agent (c) is preferably water-soluble. Preferred specific examples of the reducing agent (c) are listed below, along with their redox potentials and reference literature names. Where no literature name is listed, measured values ​​are used. In the present invention, the redox potentials shown in the table below are used for the reducing agents listed in the table below.

[0023]

[0024] 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 ２O), and LiCl, Li ２ SO ４ and / or LiOH, and more preferably LiOH. The lithium compound (b) is preferably a water-soluble lithium compound.

[0025] The molar ratio of lithium in the lithium compound (b) used for relithiation to the reducing agent (c) is preferably 0.25≦(b) / (c)≦2.80, more preferably 0.35≦(b) / (c)≦2.50, even more preferably 0.50≦(b) / (c)≦2.50, and even more preferably 1.00≦(b) / (c)≦2.50, from the viewpoint of improving the crystallinity of the regenerated positive electrode active material. By setting it within the above range, it tends to be possible to increase the relithiation rate of the regenerated positive electrode active material. The molar ratio can also be 0.35≦(B) / (C)≦2.40, or 0.50≦(b) / (c)≦2.30.

[0026] 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.

[0027] 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, 0.10 to 0.80 mol / L, or 0.10 to 0.50 mol / L. The concentration of the lithium compound (b) in the mixture, in terms of the amount of lithium, can be 0.05 to 5.00 mol / L, 0.10 to 4.00 mol / L, 0.10 to 3.00 mol / L, 0.10 to 2.00 mol / L, 0.10 to 1.00 mol / L, or 0.10 to 0.50 mol / L. The concentration of the reducing agent (c) in the mixture can be 0.05 to 1.00 mol / L, 0.05 to 0.80 mol / L, 0.05 to 0.60 mol / L, 0.06 to 0.40 mol / L, 0.06 to 0.20 mol / L, or 0.06 to 0.10 mol / L.

[0028] 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 can be 20 to 160 ° C., 25 to 160 ° C., 25 to 120 ° C., 25 to 100 ° C., 25 to 80 ° C., 25 to 59 ° C., 25 to 50 ° C., 30 to 50 ° C., or 30 to 40 ° C. From the viewpoint of increasing the crystallinity of the regenerated positive electrode active material, the reaction temperature is preferably 100 ° C. or less.

[0029] 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. The reaction time can be 1 to 7 hours, or can be 1 to 5 hours.

[0030] 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.

[0031] From the viewpoint of improving the crystallinity of the resulting regenerated cathode active material, the amount of lithium compound (b) 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 cathode active material (a), more preferably 2 molar equivalents or more. The upper limit is practically 40 molar equivalents, preferably 30 molar equivalents or less, more preferably 25 molar equivalents or less, and even more preferably 20 molar equivalents or less. Therefore, the amount of lithium compound (b) is preferably 1 to 40 molar equivalents relative to the amount (mol) of lithium deficiency, more preferably 1 to 30 molar equivalents, more preferably 1 to 25 molar equivalents, and even more preferably 2 to 20 molar equivalents. The amount of lithium deficiency in lithium-deficient cathode active material (a) can be calculated from the lithium deficiency rate (%) of lithium-deficient cathode active material (a). The lithium deficiency rate can be measured by the method described in the Examples.

[0032] <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. Calcination can reduce defects in the crystals of the regenerated positive electrode active material. According to the manufacturing method of the present invention, a regenerated positive electrode active material with excellent crystallinity can be obtained without calcination. The manufacturing method of the present invention may include either the washing or calcination as a post-treatment of the relithiation treatment, or may include both washing and calcination. Preferably, the manufacturing method of the present invention includes the washing but does not include calcination.

[0033] 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.

[0034] 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.

[0035] <Regenerated Positive Electrode Active Material> The regenerated positive electrode active material obtained by the production method of the present invention is a regenerated positive electrode active material obtained by replenishing lithium to the lithium-deficient positive electrode active material (a).

[0036] The crystallinity of the recycled positive electrode active material was confirmed by X-ray diffraction (XRD) at 2θ = 17.1 degrees. ４The half-width (full width at half maximum) of the peak can be used as an indicator to determine the full width at half maximum. The half-width of the recycled positive electrode active material is preferably 0.001 degrees or more and 0.099 degrees or less, more preferably 0.002 degrees or more and 0.095 degrees or less, even more preferably 0.003 degrees or more and 0.090 degrees or less, even more preferably 0.004 degrees or more and 0.080 degrees or less, even more preferably 0.004 degrees or more and 0.070 degrees or less, even more preferably 0.004 degrees or more and 0.060 degrees or less, and even more preferably 0.005 degrees or more and 0.050 degrees or less. By setting the half-width within the above range, the amount of lithium ions that can be inserted and removed per unit weight of the recycled positive electrode active material can be increased. The half-width can be determined by the method described in the Examples.

[0037] The regenerated cathode active material preferably contains a lithium-containing transition metal phosphate compound, and furthermore, the half-width of the peak at 2θ=17.1° in the X-ray diffraction is preferably 0.001° to 0.099°. More preferably, the half-width of this regenerated cathode active material satisfies the preferred range described above.

[0038] The relithiation rate of the regenerated positive electrode active material is preferably 0.5 or more, more preferably 0.6 or more, even more preferably 0.7 or more, still more preferably 0.8 or more, and even more preferably 0.9 or more. The relithiation rate can be determined by the method described in the Examples.

[0039] 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 product, including the trace amount of binder and / or conductive additive, is the "recycled positive electrode active material" obtained by the manufacturing method 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).

[0040] 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).

[0041] 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.

[0042] The particle size of the recycled positive electrode active material is not particularly limited. For example, it can be 0.1 to 50 μm, preferably 0.5 to 30 μm, more preferably 1 to 20 μm, more preferably 2 to 10 μm, and even more preferably 2 to 6 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).

[0043] 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.

[0044] [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.

[0045] <Battery Fabrication: Examples 1 to 16, Comparative Examples 1 and 2> 1. Preparation of Non-Aqueous Electrolyte 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).

[0046] 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, ２ Positive 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.

[0047] 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.

[0048] 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.

[0049] <Battery Fabrication: Example 17> In the fabrication 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.

[0050] <Battery Fabrication: Examples 18-21, Comparative Example 3> 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.

[0051] 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.

[0052] 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.

[0053] <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 repeatedly under the same conditions until each positive electrode active material reached the lithium deficiency rate (30%) listed in Table 1.

[0054] [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.

[0055] [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 17. 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.

[0056] [Relithiation] Each of the lithium-deficient positive electrode active materials (a) obtained above was relithiated as follows.

[0057] <Relithiation: Hydrothermal Reaction> 1. Preparation of Treatment Solution Lithium compound (b) and reducing agent (c) were dissolved in ultrapure water so that the Li amount and concentration in the treatment solution were as shown in Table 1, to prepare each treatment solution used for relithiation. The reducing agent (Fe(II)-EDTA) used in Comparative Example 1 was prepared in the same manner as Fe(II)-EDTA in Example 6 of JP-A 2023-502220. 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 lithium-deficient positive electrode active material (a) in the mixture is shown in Table 1. The mixture was then maintained at the temperature and time shown in Table 1 to allow for a hydrothermal reaction. After the hydrothermal reaction, the solid particles were separated by vacuum filtration to obtain each regenerated positive electrode active material.

[0058] <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.

[0059] <Evaluation of Relithiation Rate and Crystallinity> After the above-mentioned <Washing with Water>, each of the regenerated positive electrode active materials 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 ​​LiFePO at 2θ = 17.1 degrees was calculated and applied to the following formula to calculate the relithiation rate. ４ The peak intensities of the peaks of LiFePO were comparable. ４ As an index of the crystallinity of LiFePO at 2θ=17.1 degrees, ４ The peak used for determining the half-width was the peak of LiFePO at 2θ = 17.1 degrees obtained by performing X-ray diffraction on a positive electrode active material in which lithium was not deficient under the above conditions. ４ The intensity of the peak was normalized to 1. The smaller the half-width of this peak, the higher the crystallinity. 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 17 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.

[0060]

[0061] <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) The oxidation-reduction potential of Fe(II)-EDTA was determined with reference to Example 6 of JP-T-2023-502220. The oxidation-reduction potential of hydrogen peroxide was determined with reference to Electrochemistry Handbook, 6th Edition.

[0062] As a reducing agent (c), an oxidation-reduction potential of 1.8 to 3.0 eV vs. Li ＋ In the manufacturing methods of Comparative Examples 1 and 2, which used compounds that did not satisfy the condition / Li, a regenerated positive electrode active material with a half-width of 0.120 degrees was obtained in the crystallinity evaluation. These manufacturing methods of Comparative Examples failed to obtain a regenerated positive electrode active material exhibiting sufficient crystallinity. Furthermore, the relithiation rate was low, at 0.4 or less. It is believed that the crystallinity could not be restored at a temperature of 30°C during the hydrothermal reaction. A lithium-deficient active material was recovered from a quasi-solid secondary battery, and a reducing agent (c) was used to recover a lithium-deficient active material having an oxidation-reduction potential of 1.80 to 3.00 eV vs Li. ＋ The manufacturing method of Comparative Example 3, in which a regenerated positive electrode active material was obtained using a compound that did not satisfy the condition of / Li, also yielded results similar to those of the manufacturing methods of Comparative Examples 1 and 2. ＋ In all of the manufacturing methods of Examples 1 to 17 using compounds satisfying the above criteria, a regenerated positive electrode active material with a half-width of 0.090 degrees or less was obtained in the crystallinity evaluation, and a regenerated positive electrode active material exhibiting sufficient crystallinity was obtained. Furthermore, the relithiation rate of the obtained regenerated positive electrode active material was also high, at 0.6 or more. A lithium-deficient active material was recovered from a quasi-solid secondary battery, and a compound having an oxidation-reduction potential of 1.80 to 3.00 eV vs. Li was used as a reducing agent (c). ＋The manufacturing methods of Examples 18 to 21, in which a regenerated cathode active material was obtained using a compound satisfying the formula / Li, also yielded results similar to those of Examples 1 to 17. It can be seen that the manufacturing method of the present invention can produce a regenerated cathode active material with excellent crystallinity even under mild reaction conditions of 25 to 160°C. Furthermore, it can be seen that the present invention can produce a regenerated cathode active material with excellent crystallinity even at a reaction temperature of 30°C.

[0063] 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.

[0064] This application claims priority based on Japanese Patent Application No. 2023-219925, filed on December 26, 2023, the contents of which are incorporated herein by reference as part of the present specification.

Claims

1. A method for producing a regenerated cathode active material, comprising supplementing lithium to the lithium-deficient cathode active material (a) by mixing the lithium-deficient cathode active material (a), a lithium compound (b), and a reducing agent (c) to obtain a regenerated cathode active material, wherein the redox potential of the reducing agent (c) is 1.80 to 3.00 eV vs Li ＋ / Li. The method for producing a regenerated cathode active material.

2. The redox potential of the reducing agent (c) is 2.00 to 2.70 eV vs Li ＋ / Li, and the method for producing a regenerated positive electrode active material according to claim 1.

3. The method for producing a regenerated cathode active material according to claim 1, wherein the reducing agent (c) is an ascorbic acid compound.

4. The method for producing a regenerated cathode active material according to claim 1, wherein the mixture of the lithium-deficient cathode active material (a), the lithium compound (b), and the reducing agent (c) is maintained at 25 to 160°C.

5. The method for producing a regenerated cathode active material according to claim 1, wherein the mixture of the lithium-deficient cathode active material (a), the lithium compound (b), and the reducing agent (c) is maintained at 25 to 120°C.

6. The method for producing a regenerated cathode active material according to claim 1, wherein the lithium compound (b) is lithium hydroxide.

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

8. The lithium-deficient cathode active material (a) is Li １－ xM １ PO ４ and is a cathode active material represented by the formula, 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 producing a regenerated cathode active material according to claim 1.

9. The method for producing a regenerated cathode active material according to claim 1, wherein the regenerated cathode active material contains a lithium-containing transition metal phosphate compound, and the half-value width of the peak at 2θ = 17.1° in the X-ray diffraction of the regenerated cathode active material is 0.001° or more and 0.099° or less.

10. The method for producing a regenerated cathode active material according to claim 9, wherein the half-value width of the peak of the regenerated cathode active material is 0.005° or more and 0.050° or less.

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