Method for regenerating positive electrode active material

Abstract

To suppress reduction in the specific capacity of a regenerated positive electrode active material by suppressing granulation of the positive electrode active material during hydrothermal treatment in the regeneration treatment of the positive electrode active material of a used lithium ion secondary battery.SOLUTION: A method for regenerating a positive electrode active material includes pretreating, including pulverizing, a positive electrode mixture consisting of a positive electrode active material containing a composite oxide of lithium and a transition metal, a conductive material, and a binder containing a thermoplastic fluororesin, hydrothermally treating the pretreated positive electrode mixture in an aqueous solution containing lithium ions while suppressing a reaction between the positive electrode active material and hydrogen fluoride generated by thermal decomposition of the binder, washing the positive electrode mixture after the hydrothermal treatment, firing the washed positive electrode mixture, and cooling the fired positive electrode mixture.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a method for regenerating a positive electrode active material of a used lithium ion secondary battery. [Background technology]

[0002] A lithium-ion secondary battery generally comprises a positive electrode, whose positive electrode active material is a composite oxide of lithium and a transition metal, such as lithium cobalt oxide; a negative electrode, whose negative electrode active material is a carbon-based material; an electrolyte filled between the negative electrode and the positive electrode; and a housing enclosing the positive electrode, negative electrode, and electrolyte. Each of the positive and negative electrodes is formed by coating an electrode mixture onto a metal foil, such as aluminum foil or copper foil, which serves as a current collector. The electrode mixture is composed of an active material, a conductive material, and a binder. Examples of binders include thermoplastic resins made of fluororesins, such as polyvinylidene fluoride (PVDF).

[0003] From the perspective of resource conservation, there has been active development of recycling technologies for used lithium-ion secondary batteries. In recent years, a recycling process for used lithium-ion secondary batteries called direct recycling has been proposed. In direct recycling, used lithium-ion secondary batteries are disassembled to extract the individual components, and the positive electrode active material, one of the components, is regenerated and reused without being returned to the starting material.

[0004] When a lithium-ion secondary battery is repeatedly charged and discharged, the crystalline structure of the positive electrode active material partially collapses, and the original Li + The specific capacity decreases when a transition metal element enters a site where the positive electrode active material is present. In direct recycling, the function of the positive electrode active material is restored by repairing the crystal structure. A recycling method for used lithium-ion secondary batteries, including the regeneration of such positive electrode active material, is disclosed in Patent Document 1.

[0005] The method for recycling the positive electrode active material of used lithium-ion secondary batteries disclosed in Patent Document 1 includes: (i) a step of mechanically crushing the positive electrode in an alkaline medium to separate the positive electrode mixture from the current collector and crush the positive electrode mixture into particles; (ii) a step of collecting a solid derived from the crushed positive electrode mixture and rinsing it with a liquid to remove the alkaline medium and electrolyte from the collected solid; (iii) a step of drying and crushing the rinsed solid; (iv) a step of hydrothermally treating the crushed solid in a concentrated aqueous lithium hydroxide solution; (v) a step of separating the solid after the hydrothermal treatment from the aqueous lithium hydroxide solution; (vi) a step of rinsing the solid separated from the aqueous lithium hydroxide solution with an alkaline liquid; (vii) a step of drying and crushing the rinsed solid, followed by calcining; and (viii) a step of cooling the calcined solid in an inert atmosphere to obtain a regenerated positive electrode active material. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] U.S. Patent No. 9,825,341 Summary of the Invention [Problem to be solved by the invention]

[0007] The inventors of the present application have confirmed that in the method for recycling the positive electrode active material of used lithium-ion secondary batteries described in Patent Document 1, the positive electrode active material is unintentionally pulverized during the hydrothermal treatment, and as a result, the specific capacity of the recycled positive electrode active material is significantly reduced compared to the positive electrode active material of the lithium-ion secondary battery before use. [Means for solving the problem]

[0008] The inventors of the present application conducted extensive research into the cause of the decrease in the specific capacity of the regenerated positive electrode active material and found that during the hydrothermal treatment of the positive electrode mixture, the fluorinated resin used as a binder is thermally decomposed under high temperature and alkaline conditions, causing a dehydrofluorination reaction, and the hydrogen fluoride produced by the dehydrofluorination reaction corrodes the positive electrode active material, causing the positive electrode active material to become finer.

[0009] The present disclosure has been made in consideration of the above circumstances, and its purpose is to suppress a decrease in the specific capacity of the regenerated positive electrode active material by suppressing granulation of the positive electrode active material during hydrothermal treatment in a method for recycling the positive electrode active material of used lithium-ion secondary batteries.

[0010] In order to solve the above problem, a method for regenerating a positive electrode composite of a used lithium ion secondary battery according to one embodiment of the present disclosure includes: a positive electrode mixture including the positive electrode active material containing a composite oxide of lithium and a transition metal, a conductive material, and a binder containing a thermoplastic fluororesin, and performing a pretreatment including pulverization; hydrothermally treating the pretreated cathode mixture in an aqueous solution containing lithium ions while suppressing a reaction between hydrogen fluoride generated by thermal decomposition of the binder and the cathode active material; washing the positive electrode mixture after the hydrothermal treatment; Firing the washed positive electrode mixture; and and cooling the fired positive electrode mixture. [Effects of the Invention]

[0011] According to the present disclosure, in a method for recycling a positive electrode active material of a used lithium-ion secondary battery, by suppressing the granulation of the positive electrode active material during hydrothermal treatment, it is possible to suppress a decrease in the specific capacity of the regenerated positive electrode active material. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a flow diagram of a process for regenerating a positive electrode active material of a used lithium ion secondary battery according to the present disclosure. [Figure 2] FIG. 2 shows SEM photographs of the particles of the positive electrode mixture before and after hydrothermal treatment. DETAILED DESCRIPTION OF THE INVENTION

[0013] The method for regenerating a positive electrode active material according to the present disclosure is part of a recycling process for used lithium-ion secondary batteries, and in particular, regenerates the crystal structure of the positive electrode active material in order to reuse the positive electrode active material.

[0014] <<General Configuration of Lithium-ion Secondary Battery>> First, the general configuration of a lithium ion secondary battery will be described. However, the lithium ion secondary battery to which the method of the present disclosure is applied is not limited to the configuration of the present disclosure, and includes lithium ion secondary batteries with a wide range of general structures.

[0015] A lithium-ion secondary battery battery pack is composed of a battery module, which is a group of many battery cells. Each battery cell includes a positive electrode, a negative electrode, a separator, an electrolyte, and a casing that houses them. An electrode assembly, which is a stack or wound assembly of a positive electrode, a separator, and a negative electrode, is housed in the casing in a state impregnated with electrolyte. The negative electrode is a metal foil such as copper foil carrying a negative electrode active material. Examples of negative electrode active materials include carbon-based materials such as graphite, cokes, and carbon black. The separator is a film or nonwoven fabric made of a material such as polyolefin resin, fluororesin, or nitrogen-containing aromatic polymer. The electrolyte is composed of a lithium salt such as lithium hexafluorophosphate and an organic solvent such as ethylene carbonate.

[0016] The positive electrode is formed by supporting a positive electrode mixture containing a positive electrode active material, a conductive material, and a binder on a positive electrode current collector.

[0017] The positive electrode active material of the positive electrode composite is a composite oxide of lithium and a transition metal, such as lithium cobalt oxide (LiCoO), lithium nickel oxide (LiNiO), lithium manganese oxide (LiMnO), lithium iron phosphate (LiFePO), lithium manganese phosphate (LiMnPO), lithium iron (LiFeO), sodium iron (NaFeO), or a solid solution compound or mixture of one or more of these.

[0018] The conductive material for the positive electrode composite may be a metal-based conductive material such as metal particles or a carbon-based conductive material such as graphite powder, carbon black, acetylene black, or a fibrous carbon material.

[0019] Thermoplastic fluororesins are used as binders for the positive electrode composite. Examples of such fluororesins include polyvinylidene fluoride (PVDF), polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymers, hexafluoropropylene-vinylidene fluoride copolymers, and tetrafluoroethylene-perfluorovinyl ether copolymers. The binder may also be a thermoplastic resin made by mixing two or more types of fluororesin and polyolefin resins such as polyethylene and polypropylene.

[0020] The positive electrode current collector is made of a metal foil such as aluminum foil, nickel foil, stainless steel foil, etc. Examples of methods for supporting the positive electrode composite on the positive electrode current collector include a pressure molding method and a method in which a paste is formed using an organic solvent or the like, and the paste is applied to the positive electrode current collector, dried, and then pressed to adhere the paste to the positive electrode current collector.

[0021] <<Regeneration process for the positive electrode active material of used lithium-ion secondary batteries>> Here, the regeneration process for the positive electrode active material of a used lithium-ion secondary battery will be described with reference to Fig. 1. Fig. 1 is a flow chart of the regeneration process for the positive electrode active material of a used lithium-ion secondary battery according to the present disclosure. As shown in Fig. 1, the regeneration process for the positive electrode active material of a used lithium-ion secondary battery includes the following steps [i] to [vii].

[0022] [i] Recovery process of used positive electrodes After a battery pack of used lithium-ion secondary batteries is sufficiently discharged, the battery pack is disassembled into battery modules, the battery modules are disassembled into battery cells, and the battery cells are disassembled. For example, the battery cell casing can be disassembled or destroyed to remove the used positive electrode from the casing. Here, the positive electrode includes a current collector and a positive electrode composite supported on the current collector, and the positive electrode composite includes a positive electrode active material, a conductive material, and a binder.

[0023] [ii] Pretreatment process The recovered positive electrode is subjected to pretreatment. In the pretreatment, the positive electrode composite is separated from the positive electrode current collector, and the separated positive electrode composite is pulverized to prepare the positive electrode composite in a powder form of a size suitable for hydrothermal treatment. In the pretreatment, for example, the positive electrode is mechanically pulverized in an alkaline medium, thereby separating the current collector from the positive electrode composite and pulverizing the positive electrode composite. The powder-like positive electrode composite suspended in the alkaline medium is recovered by filtration or centrifugation.

[0024] [iii] Hydrothermal treatment process The pretreated positive electrode composite is subjected to hydrothermal treatment. In the hydrothermal treatment, the pretreated positive electrode composite is introduced into an autoclave together with a solution containing lithium ions and maintained under high pressure and high temperature. Here, a lithium hydroxide solution or a lithium carbonate solution is used as the solution containing lithium ions. As a non-limiting example, the positive electrode composite in the autoclave is maintained at an ambient temperature of 200 to 250°C for 12 to 14 hours. By performing the hydrothermal treatment on the positive electrode composite, the crystal structure of the positive electrode active material contained in the positive electrode composite is restored, and the stoichiometric lithium content of the positive electrode active material is restored.

[0025] [iv] Cleaning process The positive electrode mixture after the hydrothermal treatment is naturally cooled, removed from the autoclave, and washed with weakly alkaline water or deionized water to remove excess lithium hydroxide adhering to the positive electrode mixture. The washed positive electrode mixture is then dried.

[0026] [v] Firing process The washed positive electrode mixture is then fired. The washed positive electrode mixture is then maintained in an oxygen or air atmosphere at a predetermined firing temperature for a predetermined firing time. As a non-limiting example, the positive electrode mixture is maintained at an atmospheric temperature of 800°C for 9 hours. The positive electrode active material in the positive electrode mixture is fired to increase its crystal size and convert it into an isotope that is more stable at high temperatures. Furthermore, the binder components in the positive electrode mixture are thermally decomposed during the firing process, and no binder remains in the fired positive electrode mixture.

[0027] [vi] Cooling process The sintered positive electrode mixture is cooled. As a non-limiting example, the sintered positive electrode mixture is naturally cooled from the sintering temperature to room temperature. The cooling may be performed in a carbon dioxide atmosphere. By cooling the sintered positive electrode mixture, the crystals of the positive electrode active material in the positive electrode mixture are locked into isotopes that are stable at high temperatures.

[0028] [vii] Particle size adjustment process The rapidly cooled positive electrode mixture is further crushed to a size suitable for use as a positive electrode active material to be newly applied to a current collector, thereby obtaining a recycled positive electrode active material.

[0029] In the above-described process for regenerating a positive electrode active material, the method for regenerating a positive electrode active material according to the present disclosure is characterized by suppressing the reaction between the hydrogen fluoride generated from the binder and the positive electrode active material during the hydrothermal treatment step. By suppressing the reaction between hydrogen fluoride and the positive electrode active material during the hydrothermal treatment, corrosion of the positive electrode active material by hydrogen fluoride is suppressed, and the particle size reduction of the positive electrode active material is suppressed. This prevents the specific capacity of the regenerated positive electrode active material from decreasing significantly compared to the positive electrode active material of a used lithium-ion secondary battery. To suppress the reaction between hydrogen fluoride and the positive electrode active material during the hydrothermal treatment, one or more of the following measures (a) to (c) are taken:

[0030] {a} The binder is removed from the positive electrode mixture to be subjected to hydrothermal treatment (I). In the pretreatment step [ii] above, the binder is removed from the positive electrode mixture. At this stage, at least 50%, preferably at least 70%, of the binder contained in the positive electrode mixture is removed. Solvent washing is used as a method for removing the binder from the positive electrode mixture. A solvent that dissolves the binder but does not dissolve the positive electrode active material is used as the solvent used for washing. For example, when the binder contains PVDF, acetone, N-methyl-2-pyrrolidone (NMP), dimethylacetamide (DMAC), or a mixture thereof may be used as such a solvent. The binder is removed from the positive electrode mixture by washing the positive electrode mixture with the solvent. In addition to the binder, the alkaline medium and electrolyte adhering to the positive electrode mixture may also be removed from the positive electrode mixture by washing. Washing the positive electrode mixture may be repeated until the binder is sufficiently removed from the positive electrode mixture. The washed positive electrode mixture is filtered or centrifuged to remove the solvent, and the positive electrode mixture, which has a reduced binder content compared to before the pretreatment, is then subjected to hydrothermal treatment.

[0031] Removing the binder from the positive electrode mixture to be subjected to hydrothermal treatment (II). In the pretreatment step [ii] above, the binder is removed from the positive electrode mixture. At this stage, at least 50%, preferably at least 70%, of the binder contained in the positive electrode mixture is removed. Sintering is used as a method for removing the binder from the positive electrode mixture. For example, the positive electrode mixture is held at a temperature equal to or higher than the thermal decomposition temperature of the binder for a predetermined period of time, whereby the binder is gasified by thermal decomposition and separated from the positive electrode mixture. As a non-limiting example, when the binder contains PVDF, the positive electrode mixture is held at an ambient temperature of approximately 800°C, which is higher than the thermal decomposition temperature of PVDF (approximately 360°C), for nine hours, naturally cooled, and then subjected to hydrothermal treatment.

[0032] {c} Neutralize the hydrogen fluoride generated during hydrothermal treatment. In the hydrothermal treatment step (iii) described above, the amount of lithium ions in the hydrothermal treatment lithium hydroxide aqueous solution is in excess of the theoretical amount required to regenerate the positive electrode active material. The theoretical amount refers to an amount that is neither too much nor too little, theoretically required to regenerate the positive electrode active material. However, because the binder content of the positive electrode composite varies, it is difficult to specify a specific value for the theoretical amount (or concentration) of lithium ions required to regenerate the positive electrode active material. As a non-limiting numerical example, when a 4M lithium hydroxide aqueous solution is used, the weight ratio of this lithium hydroxide aqueous solution to the positive electrode composite may be 5:1 to 10:1. By performing the hydrothermal treatment under conditions in which the amount of lithium ions is in excess of the theoretical amount required to regenerate the positive electrode active material, the hydrogen fluoride generated by the binder during the hydrothermal treatment immediately reacts with lithium hydroxide before acting on the positive electrode active material. The reaction between lithium hydroxide and hydrogen fluoride produces lithium fluoride and water. In this way, the hydrogen fluoride generated in the hydrothermal treatment and derived from the binder is immediately neutralized, thereby suppressing the reaction between the hydrogen fluoride and the positive electrode active material, and suppressing the corrosion of the positive electrode active material by the hydrogen fluoride.

[0033] A comparative experiment was conducted to confirm the effect of differences in treatment conditions (particularly the amount of lithium ions) on the degree of particle refinement after hydrothermal treatment of a positive electrode composite. In this experiment, a 4M lithium hydroxide aqueous solution was used, and the positive electrode composite was subjected to hydrothermal treatment with 1, 5, and 10 times the mass ratio of the positive electrode composite to the lithium hydroxide aqueous solution. Figure 2 shows (a) a comparative example (SEM photograph of particles of an untreated positive electrode composite (i.e., before hydrothermal treatment); (b) Example 1 (SEM photograph of particles of a positive electrode composite after hydrothermal treatment with a positive electrode composite to lithium hydroxide aqueous solution mass ratio of 1:1); (c) Example 2 (SEM photograph of particles of a positive electrode composite after hydrothermal treatment with a positive electrode composite to lithium hydroxide aqueous solution mass ratio of 1:5); and (d) Example 3 (SEM photograph of particles of a positive electrode composite after hydrothermal treatment with a positive electrode composite to lithium hydroxide aqueous solution mass ratio of 1:10). As shown in Example 1 of Figure 2, the particles after hydrothermal treatment at a mass ratio of 1:1 were significantly finer than the untreated particles. On the other hand, as shown in Examples 2 and 3, the particles after hydrothermal treatment at mass ratios of 1:5 and 1:10 had particle sizes equivalent to those of the untreated particles of Comparative Example 1, and particle fineness was suppressed. This experiment confirmed that particle fineness due to hydrothermal treatment was suppressed by performing hydrothermal treatment under conditions in which lithium ions (lithium hydroxide in the above experiment) were in excess of the theoretical amount required to regenerate the positive electrode active material.

[0034] [Summary] A method for regenerating a positive electrode active material according to a first aspect of the present disclosure is a method for regenerating a positive electrode active material of a used lithium ion secondary battery, comprising: A positive electrode mixture including a positive electrode active material containing a composite oxide of lithium and a transition metal, a conductive material, and a binder containing a thermoplastic fluororesin is subjected to a pretreatment including pulverization; hydrothermal treatment of the pretreated positive electrode mixture in an aqueous solution containing lithium ions while suppressing a reaction between hydrogen fluoride generated by thermal decomposition of the binder and the positive electrode active material; Washing the positive electrode mixture after the hydrothermal treatment; Firing the washed positive electrode mixture; and and cooling the fired positive electrode mixture.

[0035] According to the method of the first aspect, the reaction between the hydrogen fluoride generated from the binder and the positive electrode active material during the hydrothermal treatment is suppressed, thereby suppressing the corrosion of the positive electrode active material by the hydrogen fluoride and suppressing the grain refinement of the positive electrode active material, thereby preventing a significant decrease in the specific capacity of the regenerated positive electrode active material compared to the positive electrode active material of the used lithium-ion secondary battery.

[0036] The method for regenerating a positive electrode active material according to the second aspect of the present disclosure is the same as the method for regenerating a positive electrode active material according to the first aspect, except that the binder is removed from the positive electrode composite in a pretreatment step to suppress the generation of hydrogen fluoride during the hydrothermal treatment, thereby suppressing the reaction between hydrogen fluoride and the positive electrode active material during the hydrothermal treatment.

[0037] According to the method relating to the second item, the binder, i.e., the source of hydrogen fluoride generation during hydrothermal treatment, is removed from the positive electrode mixture by pretreatment, thereby effectively suppressing the reaction between hydrogen fluoride and the positive electrode active material during heat treatment.

[0038] A method for regenerating a positive electrode active material according to a third aspect of the present disclosure is the method for regenerating a positive electrode active material according to the second aspect, in which the binder is removed from the positive electrode mixture by washing the positive electrode mixture with a solvent that does not dissolve the positive electrode active material but dissolves the binder.

[0039] A method for regenerating a positive electrode active material according to a fourth aspect of the present disclosure is the method for regenerating a positive electrode active material according to the second aspect, in which the binder is removed from the positive electrode composite by firing the positive electrode composite at a temperature equal to or higher than the thermal decomposition temperature of the binder.

[0040] According to the methods for regenerating a positive electrode active material according to the third and fourth aspects, the binder can be removed from the positive electrode mixture by pretreatment.

[0041] A method for regenerating a positive electrode active material according to a fifth aspect of the present disclosure is the method for regenerating a positive electrode active material according to the first or second aspect, in which the amount of lithium ions in the aqueous solution during the hydrothermal treatment is in excess of the amount required to regenerate the positive electrode active material, and hydrogen fluoride generated during the hydrothermal treatment is reacted with the lithium ions, thereby suppressing the reaction between hydrogen fluoride and the positive electrode active material during the hydrothermal treatment.

[0042] According to the method of the fifth item, hydrogen fluoride generated during the hydrothermal treatment reacts with lithium ions in the aqueous solution to form lithium fluoride and water, thereby making it possible to suppress the reaction between hydrogen fluoride and the positive electrode active material during the hydrothermal treatment.

Claims

1. A method for regenerating a positive electrode active material of a used lithium ion secondary battery, comprising: a positive electrode mixture including the positive electrode active material containing a composite oxide of lithium and a transition metal, a conductive material, and a binder containing a thermoplastic fluororesin, and performing a pretreatment including pulverization; hydrothermally treating the pretreated cathode mixture in an aqueous solution containing lithium ions while suppressing a reaction between hydrogen fluoride generated by thermal decomposition of the binder and the cathode active material; washing the positive electrode mixture after the hydrothermal treatment; Firing the washed positive electrode mixture; and Cooling the fired positive electrode mixture. A method for regenerating positive electrode active material.

2. the binder is removed from the positive electrode mixture in the pretreatment to suppress generation of hydrogen fluoride during the hydrothermal treatment, thereby suppressing a reaction between the hydrogen fluoride and the positive electrode active material during the hydrothermal treatment; The method for regenerating a positive electrode active material according to claim 1 .

3. removing the binder from the positive electrode mixture by washing the positive electrode mixture with a solvent in which the positive electrode active material is insoluble but the binder is soluble; The method for regenerating a positive electrode active material according to claim 2 .

4. The positive electrode mixture is fired at a temperature equal to or higher than the thermal decomposition temperature of the binder, thereby removing the binder from the positive electrode mixture. The method for regenerating a positive electrode active material according to claim 2 .

5. the amount of lithium ions in the aqueous solution is in excess of the amount required to regenerate the positive electrode active material during the hydrothermal treatment, and the hydrogen fluoride generated during the hydrothermal treatment reacts with the lithium ions, thereby suppressing the reaction between the hydrogen fluoride and the positive electrode active material during the hydrothermal treatment. The method for regenerating a positive electrode active material according to claim 1 or 2.

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