WO2025142545A1 - Method of producing regenerated positive electrode active material - Google Patents

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

The present invention relates to a method for producing recycled positive electrode active material.

Lithium-ion secondary batteries (LIBs) have a high energy density and excellent storage performance and low-temperature operation, and are widely used in portable electronic devices such as mobile phones and laptops. In addition, larger batteries are 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 progressing.

The positive electrode of a lithium ion secondary battery generally has a positive electrode active material layer, which contains positive electrode active material particles capable of absorbing or releasing lithium ions during charging and discharging. The positive electrode active material layer is usually formed by preparing a slurry (composition) containing components of the positive electrode active material layer, such as the positive electrode active material, applying the slurry, and drying the slurry.
The positive electrode active material is an important material that determines the battery capacity, but it is known that it deteriorates with long-term operation of lithium-ion secondary batteries. This is thought to be because the lithium ions that are released from the positive electrode active material and move to the negative electrode side during charging do not completely return from the negative electrode to the positive electrode active material during discharging, and the positive electrode active material becomes deficient in lithium ions after repeated charging and discharging (becoming a lithium-deficient positive electrode active material).
With the rapid spread of lithium-ion secondary batteries, there has been growing interest in recycling technologies for the constituent materials of lithium-ion secondary batteries. In particular, relithiation technology, which directly replenishes lithium to the lithium-deficient positive electrode active material in used lithium-ion secondary batteries, has attracted attention.
For example, Patent Document 1 discloses a method of hydrothermally reacting an electrode material in a lithium-deficient state in a solution containing lithium ions, and as a specific example, discloses heating an electrode material containing _LiCoO2 in about 4 M lithium hydroxide at 100 to 300° C. for 12 to 48 hours.
Also, Patent Document 2 discloses a method for electrochemical alkalization of an electrochemically active material, which comprises the step of 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, and describes the use of a redox couple as the reducing agent.

U.S. Pat. No. 9,287,552 Special Publication No. 2023-502220

The above-mentioned positive electrode active material is deteriorated not only by the loss of lithium ions due to long-term charging and discharging, but also by the collapse of the original crystal structure of the positive electrode active material (synonymous with the decrease in the regularity of the crystal structure or the decrease in crystallinity). The inventors have found that the regenerated positive electrode active material obtained by relithiating the lithium-deficient positive electrode active material may have low crystallinity. If the crystallinity of the regenerated positive electrode active material is low, there is a problem that sufficient discharge capacity cannot be obtained when it is incorporated into a lithium ion secondary battery.
Calcination is known as a method for increasing the crystallinity of a positive electrode active material. However, if a positive electrode active material with excellent crystallinity could be obtained under milder reaction conditions at a lower temperature, the positive electrode active material could be regenerated more efficiently.
The present invention aims to provide a method for producing a regenerated positive electrode active material, which includes replenishing lithium to a lithium-deficient positive electrode active material generated by the use of a lithium ion secondary battery, and which is capable of producing a regenerated positive electrode active material exhibiting excellent crystallinity under relatively mild reaction conditions.

The inventors conducted extensive research into methods for regenerating lithium-deficient positive electrode active material by replenishing it with lithium, and discovered that by using a specific reducing agent, a regenerated positive electrode active material that exhibits excellent crystallinity can be obtained under relatively mild reaction conditions. The present invention was completed through further research based on these findings.

That is, the above problems were 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 supplement lithium into the lithium-deficient positive electrode active material (a) to obtain a regenerated positive electrode active material,
The reducing agent (c) has an oxidation-reduction potential of 1.80 to 3.00 eV vs. Li ⁺ /Li;
A method for producing a regenerated positive electrode active material.
[2]
The method for producing a regenerated positive electrode active material according to [1], wherein the reducing agent (c) has an oxidation-reduction potential of 2.00 to 2.70 eV vs. Li ⁺ /Li.
[3]
The method for producing a regenerated positive electrode active material according to [1] or [2], wherein the reducing agent (c) is an ascorbic acid compound.
[4]
The method for producing a regenerated positive electrode active material according to any one of [1] to [3], wherein a mixture of the lithium-deficient positive electrode 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 positive electrode active material according to any one of [1] to [4], wherein a mixture of the lithium-deficient positive electrode 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 positive electrode 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 lithium-deficient positive electrode active material (a) is a positive electrode active material represented by Li _1- xM ¹ PO 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. The method for producing a regenerated positive electrode active material according to any one of [1] to [7].
[9]
The method for producing a regenerated positive electrode active material according to any one of [1] to [8], wherein the regenerated positive electrode active material contains a lithium-containing transition metal phosphate compound, and the half-width of the peak at 2θ = 17.1 degrees in X-ray diffraction of the regenerated positive electrode active material is 0.001 degrees or more and 0.099 degrees or less.
[10]
The method for producing a regenerated positive electrode active material according to [9], wherein the half width of the peak of the regenerated positive electrode active material is 0.005 degrees or more and 0.050 degrees or less.

The method for producing a regenerated positive electrode active material of the present invention makes it possible to obtain a regenerated positive electrode active material from a lithium-deficient positive electrode active material under relatively mild reaction conditions, while imparting excellent crystallinity.

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.
In describing the 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 device in general in which ions move between positive and negative electrodes via an electrolyte by charging and discharging, and energy is stored and released at the positive and negative electrodes. That is, the term "secondary battery" in the present invention includes 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.

[Method of manufacturing 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 to 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 having an oxidation-reduction potential of 1.80 to 3.00 eV vs. Li ⁺ /Li as the reducing agent (c).
According to the method for producing a regenerated positive electrode active material of the present invention, a regenerated positive electrode active material exhibiting excellent crystallinity can be produced under relatively mild reaction conditions. The regenerated positive electrode active material obtained by the production method of the present invention can be used as a positive electrode active material for a secondary battery.

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 by 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 production method of the present invention may further include recovering the lithium-deficient positive electrode active material (a), washing and calcining the resulting regenerated positive electrode active material, and the like.
The production method of the present invention will now be described more specifically, focusing on the above-mentioned preferred embodiment.

<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) means a positive electrode active material in a state in which at least a part of the chemical equivalent of lithium derived from the elemental composition of the compound is deficient, and the amount of lithium is less than the chemical equivalent. The amount of lithium (molar amount) of 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 the positive electrode active material in which lithium is not deficient.

The lithium-deficient positive electrode active material (a) is derived from a metal oxide that is usually 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 positive electrode active materials (see, for example, JP 2023-106633 A). Among them, (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 are mentioned. In the present invention, (MC) lithium-containing transition metal phosphate compounds are preferred, and among them, lithium-containing transition metal phosphate compounds having an olivine structure are preferred. Specific examples of lithium-containing transition metal phosphate compounds having an olivine structure include iron phosphates such as LiFePO ₄ and Li ₃ Fe ₂ (PO ₄ ) ₃ .

The lithium-deficient positive electrode active material (a) is preferably a positive electrode active material containing a lithium-containing transition metal phosphate compound, and is preferably a positive electrode active material represented by Li1 _- ^xM1PO4 (x represents the amount of lithium deficiency, 0<x≦ ₁ , and ^M1 represents at least one element selected from Fe, Mn, and Co). ^M1 is preferably an element selected from Fe and Mn, and more preferably Fe.
A positive electrode active material not deficient in lithium (a positive electrode active material containing a chemical equivalent of lithium) corresponding to the lithium-deficient positive electrode active material (a) expressed by Li _1- xM ¹ PO ₄ is a positive electrode active material expressed by LiM ¹ PO _4. When Li is completely deficient due to charging and discharging, the positive electrode active material becomes a positive electrode active material expressed by M ¹ PO ₄ .

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 method for recovering the lithium-deficient positive electrode active material (a) is not particularly limited as long as it is possible to recover the lithium-deficient positive electrode active material from a used lithium ion secondary battery.
The lithium-deficient positive electrode active material (a) can be recovered, for example, 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 by an external stimulus such as ultrasonic treatment, and removing the current collector, and then obtaining the lithium-deficient positive electrode active material (a) as solid particles in the remaining suspension. For example, as shown in the examples described later, the positive electrode removed from the lithium ion secondary battery is washed, dried, and further immersed in a solvent, and then ultrasonically treated, and the current collector is removed from the resulting suspension, and the precipitate is recovered by centrifugation and dried.

The solvent used for the immersion and ultrasonic treatment may be appropriately selected, 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, such as N-methylpyrrolidone (NMP), N-ethylpyrrolidone, N,N-dimethylformamide, etc.
The immersion is preferably performed for a time period during which the components constituting the positive electrode active material layer, particularly a binder, are sufficiently compatible with the solvent when the binder is included. The immersion time is preferably 10 minutes or more, and also preferably 20 minutes or more. Since the recovery effect does not change even if the immersion is performed for a long time, the immersion time is preferably 20 minutes to 1 hour. The immersion may be performed at room temperature or with heating.

The conditions of the ultrasonic treatment are not particularly limited as long as the positive electrode current collector can be separated from the positive electrode and the solid particles constituting the positive electrode active material layer (solid particles mainly composed of the positive electrode active material) can be obtained in a suspended state in the solvent. For example, the ultrasonic treatment can be performed for 10 to 60 minutes.
The centrifugation may be performed at 2000 to 4000 rpm for 5 to 30 minutes, as long as the lithium-deficient positive electrode active material (a) can be recovered as a precipitate from the suspension.
Drying is an operation for removing 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 may be used.

The above immersion, ultrasonic treatment, and centrifugation can remove most of the binder and conductive aid. However, even after the above immersion, ultrasonic treatment, and centrifugation, a small amount of the binder and/or conductive aid 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 aid.

<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 a too high oxidation-reduction potential is used, it becomes necessary to increase the reaction temperature or extend the reaction time during relithiation in order to perform sufficient relithiation. Under such high temperature and/or long reaction conditions, it is difficult to obtain a regenerated positive electrode active material exhibiting sufficient crystallinity, or side reactions such as iron oxide generation are likely to proceed. On the other hand, if a reducing agent with a too low oxidation-reduction potential is used, relithiation becomes difficult to proceed due to a side reaction 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 preferably 2.00 to 2.70 eV vs Li ⁺ /Li, more preferably 2.20 to 2.50 eV vs Li ⁺ /Li, and even more preferably 2.40 to 2.50 eV vs Li ⁺ /Li. By setting the oxidation-reduction potential of the reducing agent (c) within the above 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 redox potential of the reducing agent (c) may be a literature value. For example, the redox potential value described in Electrochemical Handbook, 6th Edition (edited by the Electrochemical Society, Maruzen) may be used. When the literature value is a relative value (vs SHE) to the standard hydrogen electrode (SHE), it can be converted to a value (vs Li ⁺ /Li) based on the redox potential of lithium (Li ⁺ /Li) by adding 3.05 eV to the literature value. For reducing agents whose redox potential is unknown, the redox potential can be determined by the following method.

-Method for measuring redox potential-
_A 0.25M _Li2SO4 aqueous solution is used as electrolyte a. Electrolyte b containing 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 (diameter 3 mm) 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.
Voltammograms (current-voltage curves) are obtained using an electrochemical measurement system (VMP-300 (trade name), manufactured by BioLogic). Measurements are performed at room temperature, atmospheric pressure, and at a scan rate of 120 mV/sec between 2.05 V and 4.25 V vs. Li ⁺ /Li.
Separately, a voltammogram is obtained in the same manner as above, except that the electrolyte solution a (electrolytic solution not containing the reducing agent (c)) is used instead of the electrolyte solution b.
The two voltammograms obtained above are superimposed, and the initial voltage at which the difference between the current value of electrolyte b and the current value of electrolyte a not containing reducing agent (c) becomes 0.2 mA is read during the current rise, and a value (vs Li ⁺ /Li) based on the redox potential of lithium (Li ⁺ /Li) is obtained from this redox potential based on Ag/AgCl.

The reducing agent (c) acts as a reducing agent for the lithium-deficient positive electrode active material (a). The reducing agent (c) is not particularly limited as long as it 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. 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, and an ascorbic acid compound is more preferred. From the viewpoint of achieving both excellent crystallinity and lithium conversion rate, an ascorbic acid compound is preferred, and ascorbic acid or a metal salt of ascorbic acid (preferably an alkali metal salt) is more preferred, ascorbic acid or sodium ascorbate is 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 shown below together with their redox potentials and the names of references. When the name of the reference is not given, the measured values are used. In the present invention, the redox potentials shown in the table below are used for the reducing agents shown in the table below.

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 ( _Li2SO4 ), lithium chloride ( _LiCl ), lithium carbonate ( _Li2CO3 ), lithium hydrogen carbonate ( _LiHCO3 ), lithium iodide (LiI), lithium fluoride ( _LiF ), lithium acetate ( _LiCH3COO ), _lithium oxide ( _Li2O ), etc., with LiCl, _Li2SO4 and/or LiOH being preferred, and LiOH being more preferred.
The lithium compound (b) is preferably a water-soluble lithium compound.

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 increasing the crystallinity of the regenerated positive electrode active material. By setting the molar ratio within the above range, the relithiation rate of the regenerated positive electrode active material tends to be increased. The molar ratio can also be set to 0.35≦(B)/(C)≦2.40 or 0.50≦(b)/(c)≦2.30.

The lithium compound (b) and the reducing agent (c) are preferably dissolved in a solvent to form a solution (treatment liquid) and then reacted with the lithium-deficient positive electrode active material (a).
Therefore, the solvent is preferably a solvent capable of dissolving 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.

In mixing the treatment liquid with the lithium-deficient positive electrode active material (a), the concentration of the lithium-deficient positive electrode active material (a) in the mixture can be 0.01 to 1.20 mol/L, can be 0.03 to 1.00 mol/L, can be 0.10 to 0.80 mol/L, or can be 0.10 to 0.50 mol/L.
The concentration of the lithium compound (b) in the mixture can be, in terms of the amount of lithium, 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, can be 0.05 to 0.80 mol/L, can be 0.05 to 0.60 mol/L, can be 0.06 to 0.40 mol/L, can be 0.06 to 0.20 mol/L, or can be 0.06 to 0.10 mol/L.

The reaction temperature (temperature of the mixture) between 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, it is preferable that the reaction temperature be 100°C or lower.

The reaction time between the lithium-deficient positive electrode active material (a), the lithium compound (b), and the reducing agent (c) depends 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 1 to 5 hours.

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. This is because the pressure inside the container may become high during the reaction.

From the viewpoint of increasing the crystallinity of the resulting regenerated positive electrode active material, the amount of the lithium compound (b) is preferably 1 molar equivalent or more (1 mole or more of lithium atoms of the lithium compound (b) per mole of lithium deficiency) relative to the amount of lithium deficiency (mol) of the lithium-deficient positive electrode active material (a), and 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 the lithium compound (b) is preferably 1 to 40 molar equivalents relative to the amount of lithium deficiency (mol), more preferably 1 to 30 molar equivalents, more preferably 1 to 25 molar equivalents, and more preferably 2 to 20 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.

<Other processes>
In the production method of the present invention, the lithium compound (b) and the reducing agent (c) that were not consumed during the re-lithiation treatment, the conductive assistant, by-products formed during the re-lithiation treatment, and the like may be removed by filtering and washing the solid particles from the reaction solution after the re-lithiation treatment.
In the manufacturing method of the present invention, after the relithiation treatment, the regenerated positive electrode active material may be calcined to control the crystal state of the regenerated positive electrode active material, or calcination may not be performed. By performing calcination, defects in the crystal of the regenerated positive electrode active material can be reduced. According to the manufacturing method of the present invention, a regenerated positive electrode active material with excellent crystallinity can be obtained without performing calcination.
The manufacturing method of the present invention may include either the above-mentioned washing or calcination as a post-treatment of the relithiation treatment, or may include both washing and calcination. It is preferable that the manufacturing method of the present invention includes the above-mentioned washing but does not include calcination.

The above filtration can be carried out by a normal filtration process as long as the solid particles can be recovered from the treatment liquid, and is preferably carried out by vacuum filtration.

The washing can be carried out by washing the solid particles obtained by relithiation with a washing solution. Even if a trace amount of conductive assistant 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 assistant is not limited, and flotation or centrifugation can be used.
For example, the solid particles can be suspended in a washing liquid and centrifuged to recover the solid particles. After washing, they can be dried.
The cleaning liquid is preferably water, and more preferably pure water.
The conditions for suspending the solid particles are not particularly limited. The solid particles can be mixed with the cleaning liquid and then suspended.
The rotation speed during centrifugation is not particularly limited, but is preferably 1000 to 5000 rpm, more preferably 1500 to 4000 rpm, and even more preferably 1800 to 3000 rpm. The time during centrifugation is not particularly limited, but is preferably 1 to 10 minutes, and more preferably 2 to 8 minutes.
The washed solid particles are preferably further dried, for example, at 120° C. for 24 hours.

<Regenerated cathode 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).

The crystallinity of the regenerated positive electrode active material can be determined by X-ray diffraction (XRD), using the half-width (full width at half maximum) of the LiFePO ₄ peak at 2θ=17.1 degrees as an index.
The half-width of the regenerated 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, 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 regenerated positive electrode active material can be increased.
The half width can be determined by the method described in the Examples.

The recycled positive electrode active material preferably contains a lithium-containing transition metal phosphate compound, and furthermore, the half-width of the peak at 2θ = 17.1 degrees in the X-ray diffraction is preferably 0.001 degrees or more and 0.099 degrees or less. It is more preferable that the half-width of this recycled positive electrode active material satisfies the preferred range described above.

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.

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 regeneration 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, and in this case, the whole material 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 with no 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).

The content of the binder in the regenerated positive electrode active material is preferably 0.5 mass % or less. The content of the binder can be determined by thermogravimetry-differential thermal analysis (TG-DTA). An example is shown below.
The dried product of the regenerated positive electrode active material (which has been left to stand at 120° C. for 12 hours to remove moisture) is heated in a nitrogen atmosphere from 25° C. at a rate of 5° C./min, and after reaching 600° C., the mass loss is measured after standing at 600° C. for 1 hour. More specifically, the measurement is performed as follows.
A 5 mg sample of the dried regenerated positive electrode active material was placed in a sample pan for a simultaneous thermogravimetric and differential thermal analyzer (TGA-50H (trade name), manufactured by Shimadzu Corporation) and set in the apparatus, and the mass was measured under the following conditions.

Gas flow rate: Nitrogen 50 ml/min Measurement conditions: After standing at 25°C for 2 hours, the sample was heated to 600°C at a rate of 5°C/min under a nitrogen atmosphere, and then left at 600°C for 1 hour.
The mass reduction amount (% by mass) calculated by the following formula can be regarded as the binder content (% by mass) in the regenerated positive electrode active material.

Mass loss (mass%)=100×[(mass after standing at 25° C. for 2 hours)−(mass after standing at 600° C. for 1 hour)]/(mass after standing at 25° C. for 2 hours)

The content of the conductive assistant in the regenerated positive electrode active material is preferably 0.5 mass % or less. The content of the conductive assistant in the regenerated positive electrode active material can be determined by using energy dispersive X-ray spectroscopy (SEM-EDX).
The regenerated positive electrode active material is applied to a conductive double-sided tape attached to a sample stage to prepare a sample, and the regenerated positive electrode active material is observed by SEM at an acceleration voltage of 1.5 kV using a field emission scanning electron microscope (FE-SEM: Field Emission Scanning Electron Microscope) (manufactured by JEOL, JSM 7100F (trade name)). An elemental analysis is performed by automatic detection using an energy dispersive X-ray spectroscope (EDX: Energy Dispersive X-ray Spectroscope) (manufactured by Thermo Fisher Scientific, Noran System 7 type (trade name)). The mass ratio of the carbon amount to the total amount of elements detected by automatic detection is the C amount (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.

The particle size of the regenerated positive electrode active material is not particularly limited and may be, for example, 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 regenerated positive electrode active material is the volume-based median diameter D50 in water obtained by dispersing the regenerated positive electrode active material in water and measuring it with a laser diffraction/scattering type particle size distribution measuring device (for example, Particle LA-960V2 manufactured by HORIBA).

The present invention will be described in more detail with reference to examples. However, the present invention is not to be construed as being limited to these examples except as defined in the present invention.
Additionally, "room temperature" refers to 25° C. "Parts" and "%" representing compositions are based on mass unless otherwise specified.

[Preparation of lithium-deficient positive electrode active material (a)]
A lithium ion secondary battery was produced, and the obtained lithium ion secondary battery was charged and discharged to cause lithium to be depleted from the positive electrode active material, and a lithium-deficient positive electrode active material (a) was recovered. The details will be described below.

<Battery Production: Examples 1 to 16, Comparative Examples 1 and 2>
1. Preparation of non-aqueous electrolyte 1 A non-aqueous electrolyte (non-aqueous electrolyte 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 into a mixed solvent, and adding _LiPF6 as a lithium salt to a concentration of 1 M.

2. Preparation of Positive Electrode A positive electrode slurry P1 was prepared by mixing 95.7 parts by mass of lithium iron phosphate ( _LiFePO4 ) (product code: L0386, manufactured by Tokyo Chemical Industry Co., Ltd.) as a positive electrode active material, 4.3 parts by mass of acetylene black (AB) (Li-100 (product name), specific surface area = 69 ^m2 /g, manufactured by Denka Co., Ltd.) as a conductive assistant, polyvinylidene fluoride (PVdF) as a binder, and N-methylpyrrolidone (NMP) as a solvent (concentration of components insoluble in the solvent: 75.3% by mass).
The binder was used in an amount of 6 parts by mass relative to 100 parts by mass of the positive electrode active material.
The positive electrode slurry P1 was applied to one side of a positive electrode current collector (aluminum foil) having a thickness of 12 μm to a thickness of 125 μm, and the dispersion medium was removed at 240° C. Thereafter, the positive electrode was pressed with a pressure of 3.2 t using a roll press machine to obtain a sheet-shaped positive electrode consisting of the positive electrode current collector and the positive electrode active material layer. The thickness of this positive electrode was about 80 μm.

3. Preparation of Negative Electrode As a negative electrode active material, 95.7 parts by mass of artificial graphite (UF-G5 (trade name), manufactured by Showa Denko K.K., average particle size (average particle size converted into sphere) 3 μm), 4.3 parts by mass of acetylene black (AB) (Li-100 (trade name), manufactured by Denka Co., Ltd.) as a conductive assistant, NMP, styrene-butadiene copolymer (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener were mixed to obtain a negative electrode slurry N1 (concentration of components insoluble in the solvent: 70.6% by mass).
The binder was used in an amount of 1 part by mass relative to 100 parts by mass of the negative electrode active material, and the thickener was used in an amount of 1 part by mass relative to 100 parts by mass of the negative electrode active material.
A negative electrode slurry N1 was applied to one side of a negative electrode current collector (copper foil) having a thickness of 12 μm to a thickness of 120 μm, and the dispersion medium was removed at 240 ° C. Thereafter, the negative electrode was pressed with a pressure of 2 t using a roll press machine to obtain a sheet-shaped negative electrode consisting of a negative electrode current collector and a negative electrode active material layer. The thickness of the negative electrode active material layer of this negative electrode was about 80 μm.

4. Preparation of Battery The obtained positive electrode and negative electrode were laminated via a separator (thickness 20 μm) manufactured by W-SCOPE to form a laminate consisting of a positive electrode collector-positive electrode active material layer-separator-negative electrode active material layer-negative electrode collector. An aluminum tab was attached to the end of the positive electrode collector, and a nickel tab was attached to the end of the negative electrode collector by ultrasonic welding. A battery assembly was prepared by housing this laminate in a laminate container. After injecting nonaqueous electrolyte 1 with the inlet open, the inlet was sealed to close the case, and a lithium ion secondary battery was obtained.

<Battery Production: Example 17>
A positive electrode slurry P2 was prepared in the same manner as in 2. Preparation of a positive electrode above, except that lithium manganese phosphate (LiMnPO ₄ ) was used instead of lithium iron phosphate as the positive electrode active material, and a positive electrode was obtained.
A lithium ion secondary battery was produced in the same manner as in 4. Battery production above, except that this positive electrode was used.

<Battery Production: Examples 18 to 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 assistant, 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 positive electrode slurry P3. The amount of non-aqueous electrolyte 1 in the positive electrode slurry P3 was 22.9 mL per 100 g of the total positive electrode active material and conductive assistant in the positive electrode slurry P3.

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 assistant, and non-aqueous electrolyte 1 were mixed for 90 seconds at 1250 rpm in a centrifugal planetary mixer (manufactured by Thinky Corporation, Awatori Rentaro (trade name)) to obtain negative electrode slurry N3. The amount of non-aqueous electrolyte 1 in the negative electrode slurry N3 was 37.7 mL per 100 g of the total of the negative electrode active material and conductive assistant in the negative electrode slurry N3.

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.
The positive electrode slurry P3 was applied onto an aluminum foil positive electrode current collector to a thickness of 500 μm and an area of 80 cm ² to form a positive electrode consisting of the positive electrode current collector and a positive electrode active material layer.
Similarly, the negative electrode slurry N3 was applied onto a copper foil negative electrode current collector to a thickness of 500 μm and an area of 85 cm ² to form a negative electrode consisting of the negative electrode current collector and a negative electrode active material layer.
A separator manufactured by W-SCOPE (thickness: 20 μm) was cut to have an area of 90 cm ² .
A separator was laminated on the negative electrode so that the negative electrode was inside the size of the separator, and a positive electrode was laminated on top of that so that the negative electrode was inside the size of the negative electrode, thereby producing a laminate of the negative electrode current collector-negative electrode active material layer (slurry)-separator-positive electrode active material layer (slurry)-positive electrode current collector. The production of the laminate was completed in about 1 minute to prevent the volatilization of the non-aqueous electrolyte 1.
Tabs were ultrasonically welded to the positive electrode slurry uncoated portion of the aluminum foil and to the negative electrode slurry uncoated portion of the copper foil of this laminate, 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 tests.

<Charging and discharging lithium-ion secondary batteries>
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 (product 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 charge/discharge was performed 200 times. Thereafter, charge/discharge was further repeated under the same conditions until each positive electrode active material had the lithium deficiency rate (30%) shown in Table 1.

[Recovery of lithium-deficient positive electrode active material (a)]
From the positive electrode of the lithium ion secondary battery after the above-mentioned charging and discharging, the lithium-deficient positive electrode active material (a) was recovered as follows.
The lithium ion secondary battery after the charge and discharge was disassembled, and the positive electrode was taken out. The taken-out positive electrode was washed with dimethyl carbonate (DMC) and then dried. The dried positive electrode was immersed in NMP for 30 minutes, and then ultrasonically treated in NMP for 20 minutes to dissolve the binder, and the current collector was separated and taken out to obtain a suspension containing a lithium-deficient positive electrode active material (a). The suspension was centrifuged at 3500 rpm for 5 minutes to collect the precipitate, and dried at 120 ° C. for 12 hours. In this way, a lithium-deficient positive electrode active material (a) was obtained. The conductive assistant did not precipitate in the above centrifugation, and the conductive assistant and the lithium-deficient positive electrode active material (a) could be separated.

[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 by an inductively coupled plasma optical emission spectroscope (ICP-OES) (Optima 7300DV (product name), manufactured by PerkinElmer) using an absolute calibration curve method.
20 mg of the lithium-deficient positive electrode active material (a) was weighed out, 60% nitric acid was added, and the mixture was ashed by microwave irradiation. 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, except for Example 17, where the lithium deficiency rate was calculated from the molar ratio of Mn to Li.
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 determined in the same manner.

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

<Relithiation: Hydrothermal reaction>
1. Preparation of Treatment Solution The lithium compound (b) and the reducing agent (c) were dissolved in ultrapure water so as to achieve the Li amount and concentration in the treatment solution shown in Table 1, to prepare each treatment solution to be used for relithiation.
The reducing agent (Fe(II)-EDTA) used in Comparative Example 1 was prepared in the same manner as the Fe(II)-EDTA in Example 6 of JP-T-2023-502220.
2. Relithiation In a pressure vessel (HU-100 (trade name) manufactured by San-Ai Scientific Co., Ltd.), each lithium-deficient positive electrode active material (a) shown in Table 1 and 40 mL of each treatment solution shown in Table 1 were mixed and sealed. In this manner, the lithium-deficient positive electrode active material (a), the lithium compound (b), and the 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 held at the temperature and time shown in Table 1 to cause a hydrothermal reaction. The solid particles after the hydrothermal reaction were separated by vacuum filtration to obtain each regenerated positive electrode active material.

<Washing>
The regenerated positive electrode active material separated and recovered by the vacuum filtration was washed with pure water, transferred to a centrifuge tube, and suspended in pure water. The mixture was then centrifuged at a rotation speed of 2000 rpm for 5 minutes using a centrifuge, and the precipitate was collected and dried at 120° C. for 24 hours. In this way, the regenerated positive electrode active material was washed with water.

<Evaluation of relithiation rate and crystallinity>
For each of the regenerated positive electrode active materials after the above <Water Washing>, the peak area of _LiFePO4 at 2θ = 17.1 degrees and the peak area of _FePO4 at 2θ = 18.0 degrees were obtained by X-ray diffraction (XRD), and the relithiation rate was calculated by applying the following formula.
In each of the regenerated positive electrode active materials obtained above, the peak intensity of the LiFePO4 _peak at 2θ = 17.1 degrees was approximately the same.
Furthermore, the half-width of the LiFePO4 _peak at _2θ = 17.1 degrees was determined as an index of the crystallinity of LiFePO4. The peak used in determining the half-width was normalized by performing X-ray diffraction on a positive electrode active material with no lithium deficiency under the above conditions, and taking the intensity of the LiFePO4 _peak at 2θ = 17.1 degrees obtained as 1. The smaller the half-width of this peak, the higher the crystallinity.

Relithiation rate = LiFePO4 _peak area / (LiFePO4 _peak area + _FePO4 peak area)

The X-ray diffraction apparatus used was a MiniFlex (trademark) 600 (manufactured by Rigaku Corporation).
For the regenerated positive electrode active material obtained in Example 17, the peak area of _LiMnPO4 at 2θ = 17.1 degrees and the peak area of MnPO4 at 2θ = 18.0 degrees were obtained by X-ray diffraction (XRD ₎ , and the relithiation rate was calculated by applying the results to the following formula.
Relithiation rate = LiMnPO4 _peak area / (LiMnPO4 _peak area + _MnPO4 peak area)
When the lithium-deficient positive electrode active material (a) contains a transition metal element other than Fe and Mn, the re-lithiation rate can be determined in the same manner.

<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 redox potential of Fe(II)-EDTA was determined by reference to Example 6 of JP-T-2023-502220.
The oxidation-reduction potential of hydrogen peroxide was referenced in the Electrochemistry Handbook, 6th Edition.

In the manufacturing methods of Comparative Example 1 and Comparative Example 2, in which a compound not satisfying the redox potential of 1.8 to 3.0 eV vs Li ⁺ /Li was used as the reducing agent (c), a regenerated positive electrode active material with a half-width of 0.120 degrees was obtained in the above crystallinity evaluation. In the manufacturing methods of these comparative examples, it was not possible to obtain a regenerated positive electrode active material exhibiting sufficient crystallinity. In addition, the relithiation rate was also low at 0.4 or less. It is considered that the crystallinity could not be restored at a temperature of 30 ° C. during the hydrothermal reaction. The lithium-deficient active material was recovered from the quasi-solid secondary battery, and the regenerated positive electrode active material was obtained using a compound not satisfying the redox potential of 1.80 to 3.00 eV vs Li ⁺ /Li as the reducing agent (c), and the same results as the manufacturing methods of Comparative Example 1 and Comparative Example 2 were obtained by the manufacturing method of Comparative Example 3.
In the manufacturing methods of Examples 1 to 17, in which a compound having an oxidation-reduction potential of 1.80 to 3.00 eV vs Li ⁺ /Li was used as the reducing agent (c), a regenerated positive electrode active material having a half-width of 0.090 degrees or less was obtained in the above crystallinity evaluation, and a regenerated positive electrode active material exhibiting sufficient crystallinity was obtained. In addition, the relithiation rate of the obtained regenerated positive electrode active material was also high, at 0.6 or more. The same results as those of the manufacturing methods of Examples 1 to 17 were obtained by recovering lithium-deficient active material from a quasi-solid secondary battery and obtaining a regenerated positive electrode active material using a compound having an oxidation-reduction potential of 1.80 to 3.00 eV vs ^{Li +} /Li as the reducing agent (c).
It is understood that the manufacturing method of the present invention can obtain a regenerated positive electrode active material with excellent crystallinity even under mild reaction conditions of 25 to 160° C. Furthermore, it is understood that the present invention can obtain a regenerated positive electrode active material with excellent crystallinity even at a reaction temperature of 30° C.

Although the present invention has been described in conjunction with its embodiments, we do not intend to limit our invention to any of the details of the description unless otherwise specified, and believe that the appended claims should be interpreted broadly without departing from the spirit and scope of the invention as set forth in the appended claims.

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

Claims (10)

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 supplement lithium into the lithium-deficient positive electrode active material (a) to obtain a regenerated positive electrode active material,
The reducing agent (c) has an oxidation-reduction potential of 1.80 to 3.00 eV vs. Li ⁺ /Li;
A method for producing a regenerated positive electrode active material. 2. The method for producing a regenerated positive electrode active material according to claim 1, wherein the reducing agent (c) has an oxidation-reduction potential of 2.00 to 2.70 eV vs Li ⁺ /Li. The method for producing a regenerated positive electrode active material according to claim 1, wherein the reducing agent (c) is an ascorbic acid compound. The method for producing a regenerated positive electrode active material according to claim 1, wherein the mixture of the lithium-deficient positive electrode active material (a), the lithium compound (b), and the reducing agent (c) is maintained at 25 to 160°C. The method for producing a regenerated positive electrode active material according to claim 1, wherein the mixture of the lithium-deficient positive electrode active material (a), the lithium compound (b), and the reducing agent (c) is maintained at 25 to 120°C. The method for producing a regenerated positive electrode active material according to claim 1, wherein the lithium compound (b) is lithium hydroxide. The method for producing a regenerated positive electrode active material according to claim 1, wherein the lithium-deficient positive electrode active material (a) has an olivine structure. The method for producing a regenerated positive electrode active material according to claim 1, wherein the lithium-deficient positive electrode active material (a) is a positive electrode active material represented by Li _1- xM ¹ PO 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. The method for producing a regenerated positive electrode active material according to claim 1, wherein the regenerated positive electrode active material contains a lithium-containing transition metal phosphate compound, and the half-width of the peak at 2θ = 17.1 degrees in the X-ray diffraction of the regenerated positive electrode active material is 0.001 degrees or more and 0.099 degrees or less. The method for producing a recycled positive electrode active material according to claim 9, wherein the half-width of the peak of the recycled positive electrode active material is 0.005 degrees or more and 0.050 degrees or less.