Method for producing regenerated carbon material

Abstract

To provide a method for easily obtaining a regenerated carbon material by removing more Li from a carbon-based negative electrode active material containing Li.SOLUTION: The method for producing a regenerated carbon material of the present disclosure includes a first heat-treatment step of heat-treating a carbon-based negative electrode active material containing Li together with ethylene carbonate at a temperature lower than the boiling point of ethylene carbonate to obtain a heat-treated product containing lithium ethylene dicarbonate, and a second heat-treatment step of heat-treating the heat-treated product at a temperature equal to or higher than the boiling point of ethylene carbonate to convert lithium ethylene dicarbonate to lithium carbonate.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a method for producing recycled carbon materials. [Background technology]

[0002] Lithium-ion secondary batteries are widely used in various fields, such as as power sources for driving vehicles and portable power sources. In recent years, from the perspective of the SDGs, there has been a demand for promoting the recycling of used lithium-ion secondary batteries. As an example of this recycling, used lithium-ion secondary batteries are collected by separating them into their respective materials (see, for example, Patent Documents 1 and 2).

[0003] On the other hand, as described in Patent Documents 1 and 2, carbon-based negative electrode active materials such as graphite are known as negative electrode active materials for lithium ion secondary batteries. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2019-169309 [Patent Document 2] Special Publication No. 2022-547698 Summary of the Invention [Problem to be solved by the invention]

[0005] In the above-mentioned conventional technologies, processes such as calcination and acid leaching are performed to recycle used lithium-ion secondary batteries. However, the carbon-based negative electrode active material of used lithium-ion secondary batteries contains lithium (Li). The Li contained in this carbon-based negative electrode active material cannot be completely separated from the carbon-based negative electrode active material by calcination or acid leaching, and most of the Li remains in the carbon-based negative electrode active material. Therefore, the presence of lithium limits the applications of recovered carbon-based negative electrode active materials when used as recycled carbon materials. For these reasons, there is a need for the development of a technology that can easily remove Li from carbon-based negative electrode active materials.

[0006] Therefore, an object of the present disclosure is to provide a method that can easily remove a larger amount of Li from a carbon-based negative electrode active material that contains Li to obtain a recycled carbon material. [Means for solving the problem]

[0007] The method for producing a recycled carbon material of the present disclosure includes a first heat treatment step in which a carbon-based negative electrode active material containing Li is heat-treated together with ethylene carbonate at a temperature below the boiling point of ethylene carbonate to obtain a heat-treated product containing lithium ethylene dicarbonate, and a second heat treatment step in which the heat-treated product is heat-treated at a temperature equal to or higher than the boiling point of ethylene carbonate to convert the lithium ethylene dicarbonate into lithium carbonate.

[0008] According to this configuration, it is easy to remove a larger amount of Li from the carbon-based negative electrode active material containing Li to obtain a recycled carbon material. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a flowchart showing the steps of the method for producing recycled carbon materials according to the present disclosure. [Figure 2] FIG. 2 is a longitudinal cross-sectional view schematically illustrating the internal structure of an example of a lithium ion secondary battery including a carbon-based negative electrode active material containing Li used in the method for producing a recycled carbon material according to the present disclosure. [Figure 3] FIG. 3 is a schematic exploded view showing the configuration of the electrode body of the lithium ion secondary battery shown in FIG. [Figure 4] FIG. 4 is a schematic cross-sectional view of the negative electrode of the lithium ion secondary battery shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. Matters not mentioned in this specification but necessary for implementing the present disclosure can be understood as design matters for a person skilled in the art based on the prior art in the relevant field. The present invention can be implemented based on the contents disclosed in this specification and the common general technical knowledge in the relevant field. In the following drawings, components and parts that perform the same function are denoted by the same reference numerals. Dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect actual dimensional relationships. In this specification, a numerical range expressed as "A to B" includes A and B.

[0011] In this specification, the term "secondary battery" refers to an electricity storage device that can be repeatedly charged and discharged. In addition, in this specification, the term "lithium ion secondary battery" refers to a secondary battery that uses lithium ions as charge carriers and achieves charging and discharging by the transfer of charge associated with the lithium ions between the positive and negative electrodes.

[0012] 1, the method for producing a recycled carbon material according to the present disclosure includes, as essential steps, a first heat-treatment step S101 in which a carbon-based negative electrode active material containing Li is heat-treated together with ethylene carbonate (hereinafter sometimes abbreviated as "EC") at a temperature below the boiling point of ethylene carbonate to obtain a heat-treated product containing lithium ethylene dicarbonate, and a second heat-treatment step S102 in which the heat-treated product is heat-treated at a temperature equal to or higher than the boiling point of ethylene carbonate to convert the lithium ethylene dicarbonate into lithium carbonate. Each step will be described in detail below.

[0013] <First heat treatment step> First, the first heat treatment step S101 will be described. The carbon-based negative electrode active material containing Li is typically a carbon-based negative electrode active material recovered from used lithium-ion secondary batteries. That is, the carbon-based negative electrode active material containing Li is typically the negative electrode active material of used lithium-ion secondary batteries. Because the amount of active material used is large and there is a particularly high demand for recycling, the carbon-based negative electrode active material containing Li is preferably the negative electrode active material of lithium-ion secondary batteries used as a power source for driving vehicles. However, the carbon-based negative electrode active material containing Li is not limited to this. For example, the carbon-based negative electrode active material containing Li may be the negative electrode active material of a lithium-ion secondary battery that was not shipped due to a manufacturing defect.

[0014] When a lithium-ion secondary battery is charged, lithium ions are absorbed into the carbon-based negative electrode active material. Furthermore, when the lithium-ion secondary battery is repeatedly charged and discharged, lithium ions may precipitate as metallic lithium on the surface of the carbon-based negative electrode active material. Alternatively, when recovering the carbon-based negative electrode active material from the lithium-ion secondary battery, lithium ions contained in the electrolyte may adhere to the surface of the carbon-based negative electrode active material. Thus, the carbon-based negative electrode active material recovered from the lithium-ion secondary battery may contain Li in this way. That is, the Li contained in the carbon-based negative electrode active material may be Li absorbed into the carbon-based negative electrode active material, Li precipitated on the surface of the carbon-based negative electrode active material, Li adhered to the surface of the carbon-based negative electrode active material, or the like.

[0015] Examples of carbon-based negative electrode active materials include graphite, hard carbon, and soft carbon. Among these, graphite is preferred. The graphite may be natural graphite or artificial graphite, or may be amorphous carbon-coated graphite in which graphite is coated with an amorphous carbon material.

[0016] The carbon-based negative electrode active material typically has a median diameter (D50) of, for example, 0.1 μm or more and 50 μm or less, preferably 1 μm or more and 25 μm or less, and more preferably 5 μm or more and 20 μm or less, but is not limited thereto. The median diameter (D50) of the carbon-based negative electrode active material can be determined, for example, by a laser diffraction scattering method.

[0017] In the first heat treatment step S101, the Li-containing carbon-based negative electrode active material is heat-treated together with EC at a temperature below the boiling point of EC, which is 244°C.

[0018] This heat treatment causes a reaction between Li contained in the carbon-based negative electrode active material and EC, as shown in formula (I) below, to obtain a heat-treated product containing lithium ethylene dicarbonate ((CH2OCO2Li)2). 2(CH2O)2CO+2Li→(CH2OCO2Li)2+C2H4...(I)

[0019] From the viewpoint of efficiently carrying out the reaction represented by the above formula (I), the heat treatment temperature in the first heat treatment step S101 is preferably 90° C. or higher, more preferably 100° C. or higher, and even more preferably 110° C. or higher. Furthermore, from the viewpoint of preventing EC from volatilizing before the reaction between Li contained in the carbon-based negative electrode active material and EC has sufficiently progressed, the heat treatment temperature in the first heat treatment step S101 is preferably 220° C. or lower, more preferably 170° C. or lower, and even more preferably 140° C. or lower.

[0020] The heat treatment time in the first heat treatment step S101 may be appropriately determined depending on the heat treatment temperature, and is, for example, 30 minutes to 72 hours, preferably 40 minutes to 24 hours, and more preferably 50 minutes to 6 hours.

[0021] The heat treatment in the first heat treatment step S101 is preferably carried out at 90°C to 220°C for 30 minutes to 72 hours, more preferably at 100°C to 170°C for 40 minutes to 24 hours, and even more preferably at 110°C to 140°C for 50 minutes to 6 hours.

[0022] Note that, if water is present in the atmosphere, it may cause side reactions such as a reaction between Li and water. Therefore, the first heat treatment step S101 is preferably performed in a dry atmosphere (e.g., a dry air atmosphere, an inert gas atmosphere, etc.). The first heat treatment step S101 can be performed using a known heating device.

[0023] <Second heat treatment step S102> In the second heat treatment step S102, the heat-treated product obtained in the first heat treatment step is heat-treated at a temperature equal to or higher than the boiling point of ethylene carbonate. The boiling point of EC is 244°C, so the heat treatment temperature in the second heat treatment step S102 is 244°C or higher.

[0024] This heat treatment causes a reaction between the lithium ethylene dicarbonate contained in the heat-treated product obtained in the first heat treatment step and the Li contained in the carbon-based negative electrode active material, as shown in the following formula (II), to convert the lithium ethylene dicarbonate into lithium carbonate. (CH2OCO2Li)2+2Li→2Li2CO3+C2H4...(II)

[0025] From the viewpoint of efficiently carrying out the reaction represented by the above formula (II), the heat treatment temperature in the second heat treatment step S102 is preferably 250° C. or higher, more preferably 255° C. or higher, and even more preferably 260° C. or higher. Moreover, the heat treatment temperature in the second heat treatment step S102 is preferably 350° C. or lower, more preferably 320° C. or lower, and even more preferably 290° C. or lower.

[0026] The heat treatment time in the second heat treatment step S102 may be appropriately determined depending on the heat treatment temperature, and is, for example, 30 minutes to 72 hours, preferably 40 minutes to 24 hours, and more preferably 50 minutes to 6 hours.

[0027] The heat treatment in the second heat treatment step S102 is preferably carried out at 250°C to 350°C for 30 minutes to 72 hours, more preferably at 255°C to 320°C for 40 minutes to 24 hours, and even more preferably at 260°C to 290°C for 50 minutes to 6 hours.

[0028] Note that, if water is present in the atmosphere, it may cause side reactions such as a reaction between Li and water. Therefore, the second heat treatment step S102 is preferably performed in a dry atmosphere (e.g., a dry air atmosphere or an inert gas atmosphere). The second heat treatment step S102 can be performed using a known heating device.

[0029] Specific methods for carrying out the first heat treatment step S101 and the second heat treatment step S102 will be described below with reference to first to third embodiments as examples.

[0030] First, an example of the structure of a lithium-ion secondary battery is shown in Figures 2 and 3. Figure 2 is a longitudinal cross-sectional view schematically showing the internal structure of an example of a lithium-ion secondary battery including a carbon-based negative electrode active material containing Li used in the method for producing a recycled carbon material according to the present disclosure. Figure 3 is an exploded view schematically showing an electrode body of the lithium-ion secondary battery shown in Figure 2. Figure 4 is a schematic cross-sectional view taken along the thickness direction of the negative electrode of the lithium-ion secondary battery shown in Figure 2.

[0031] As shown in Fig. 2, the lithium-ion secondary battery 100 is a sealed battery in which a flat electrode assembly 20 and a non-aqueous electrolyte (not shown) are housed inside a battery case 30. As shown in Fig. 2, the battery case 30 is composed of an exterior body 32 that houses the electrode assembly 20, and a lid body 34 that seals the opening of the exterior body 32. The exterior body 32 and the lid body 34 are welded and sealed by laser welding or the like. The battery case 30 is made of a material such as aluminum, an aluminum alloy, or resin.

[0032] In the illustrated example, the battery case 30 is rectangular. However, the shape of the battery case 30 is not limited thereto and may be, for example, cylindrical. Alternatively, the battery case 30 may be a laminate case having a gas barrier layer such as an aluminum layer and a sealant layer containing a thermoplastic resin.

[0033] The battery case 30 includes a positive electrode terminal 42 and a negative electrode terminal 44 for external connection. The battery case 30 also includes a safety valve 36 that is configured to release internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. The battery case 30 also includes an injection port (not shown) for injecting a non-aqueous electrolyte. The positive electrode terminal 42 is electrically connected to a positive electrode current collector plate 42a. The negative electrode terminal 44 is electrically connected to a negative electrode current collector plate 44a.

[0034] 2 and 3, the electrode assembly 20 has a configuration in which a long positive electrode sheet 50 and a long negative electrode sheet 60 are overlapped with two long separator sheets 70 interposed therebetween and wound in the longitudinal direction. Thus, in this embodiment, the electrode assembly 20 is a wound electrode assembly. However, the electrode assembly 20 is not limited to this, and may be a stacked electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

[0035] 3 and 4, in the negative electrode sheet 60, a negative electrode active material layer 64 is formed along the longitudinal direction on one or both sides (both sides in this case) of a negative electrode current collector 62. The negative electrode sheet 60 has a negative electrode active material layer-free portion 62a, which is a portion where the negative electrode active material layer 64 is not formed and the negative electrode current collector 62 is exposed. A negative electrode current collector plate 44a is joined to the negative electrode active material layer-free portion 62a.

[0036] An example of the negative electrode current collector 62 constituting the negative electrode sheet 60 is copper foil. The negative electrode active material layer 64 contains a carbon-based negative electrode active material containing Li. The negative electrode active material layer 64 may contain components other than the carbon-based negative electrode active material, such as a binder (e.g., styrene butadiene rubber (SBR)) or a thickener (e.g., carboxymethyl cellulose (CMC)).

[0037] The graphite content in the negative electrode active material layer 64 is preferably 90% by mass or more, and more preferably 95% by mass to 99% by mass. The binder content in the negative electrode active material layer 64 is preferably 0.1% by mass to 8% by mass or less, and more preferably 0.5% by mass to 3% by mass. The thickener content in the negative electrode active material layer 64 is preferably 0.3% by mass to 3% by mass or less, and more preferably 0.5% by mass to 2% by mass.

[0038] In the positive electrode sheet 50, a positive electrode active material layer 54 is formed along the longitudinal direction on one or both sides (both sides in this case) of a positive electrode current collector 52. The positive electrode sheet 50 has a positive electrode active material layer-free portion 52a, which is a portion where the positive electrode active material layer 54 is not formed and the positive electrode current collector 52 is exposed. A positive electrode current collector plate 42a is joined to the positive electrode active material layer-free portion 52a.

[0039] An example of the positive electrode current collector 52 constituting the positive electrode sheet 50 is aluminum foil. The positive electrode active material layer 54 contains a positive electrode active material. Examples of the positive electrode active material include lithium composite metal oxides and lithium transition metal phosphate compounds. The positive electrode active material layer 54 may contain a conductive material (e.g., carbon black, carbon nanotubes, etc.), a binder (e.g., polyvinylidene fluoride (PVDF), etc.), etc.

[0040] An example of the separator 70 is a porous resin sheet made of polyethylene, polypropylene, or the like. The porous sheet may have a single-layer structure or a multi-layer structure. A heat-resistant layer (HRL) may be provided on the surface of the separator 70.

[0041] The nonaqueous electrolyte typically contains a nonaqueous solvent and a supporting salt (in other words, an electrolyte salt). Examples of the nonaqueous solvent include carbonates (e.g., EC, ethyl methyl carbonate, dimethyl carbonate, etc.), esters, and ethers. Examples of the supporting salt include lithium salts such as LiPF6. The concentration of the supporting salt is not particularly limited, but is preferably 0.7 mol / L to 1.3 mol / L. The nonaqueous electrolyte may contain various additives such as a gas generating agent and a film forming agent. Although a nonaqueous electrolyte is used as the electrolyte in this embodiment, the electrolyte may be a solid electrolyte.

[0042] The lithium ion secondary battery 100 is preferably for in-vehicle use (i.e., for use as a driving power source for vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs)), but is not limited thereto. The lithium ion secondary battery 100 may also be used as a power source for electronic devices, etc.

[0043] (First embodiment) In the first embodiment, a used lithium ion secondary battery 100 is subjected to a heat treatment in a first heat treatment step S101 and a heat treatment in a second heat treatment step S102.

[0044] When the non-aqueous electrolyte of the lithium-ion secondary battery 100 contains EC, the carbon-based negative electrode active material and EC coexist in the lithium-ion secondary battery 100. In this case, the battery can be subjected to heat treatment as is. Generally, as a non-aqueous solvent for a non-aqueous electrolyte, EC, which is a cyclic carbonate, is often used in combination with a chain carbonate such as dimethyl carbonate (DMC) or ethyl methyl carbonate (EMC). Since the boiling point of DMC is 90°C and the boiling point of EMC is 107°C, even when EC is used in combination with DMC and EMC in a non-aqueous electrolyte, DMC and EMC volatilize preferentially over EC, thereby efficiently reacting EC with Li contained in the carbon-based negative electrode active material.

[0045] When the non-aqueous electrolyte of the lithium ion secondary battery 100 does not contain EC, the non-aqueous electrolyte is extracted from the lithium ion secondary battery 100, and EC is injected into the battery case 30. This allows the carbon-based negative electrode active material and EC to coexist in the lithium ion secondary battery 100.

[0046] The lithium-ion secondary battery 100 in which the carbon-based negative electrode active material and EC coexist is subjected to a heat treatment. Specifically, for example, the lithium-ion secondary battery 100 is placed in a heating device such as a thermostatic dryer, heater, or electric furnace, and heated at a temperature below the boiling point of EC so that lithium ethylene dicarbonate is produced by the reaction represented by the above formula (I). This allows the first heat treatment step S101 to be performed.

[0047] Thereafter, the temperature setting of the heating device is changed to a temperature equal to or higher than the boiling point of EC, thereby performing the heat treatment in the second heat treatment step S102. After performing the first heat treatment step S101, the heat-treated object may be removed from the heating device and transferred to another heating device, where the heat treatment in the second heat treatment step S102 may be performed.

[0048] (Second embodiment) In the second embodiment, a negative electrode 60 recovered from a used lithium ion secondary battery 100 is subjected to a heat treatment in a first heat treatment step S101 and a heat treatment in a second heat treatment step S102.

[0049] Therefore, in the second embodiment, first, the negative electrode 60 is removed from the used lithium-ion secondary battery 100. This can be done according to a known method. Specifically, for example, the battery case 30 of the lithium-ion secondary battery 100 is opened to expose the electrode assembly 20. The battery case 30 can be opened, for example, by cutting a portion of the exterior body 32 slightly below the lid body 34 using cutting means such as a tool with a cutting blade (e.g., an electric saw), an electric cutting tool (e.g., a grinder, a router), a water cutter, or a laser cutter.

[0050] Before dismantling the used lithium ion secondary battery 100, the used lithium ion secondary battery 100 may be discharged. This can reduce the amount of Li contained in the carbon-based negative electrode active material and also make dismantling safer.

[0051] Next, the exposed electrode assembly 20 is removed, and the negative electrode 60 is taken out from the electrode assembly 20. For example, first, the positive electrode active material layer-free portion 52a and the negative electrode active material layer-free portion 62a of the electrode assembly 20 are detached from the positive electrode current collector plate 42a and the negative electrode current collector plate 44a, respectively. Next, for example, if the electrode assembly 20 is a wound electrode assembly as in the illustrated example, the wound electrode assembly 20 is unwound and separated into the positive electrode 50, the negative electrode 60, and the separator 70. If the electrode assembly 20 is a stacked electrode assembly, it is separated into the positive electrode 50, the separator 70, and the negative electrode 60.

[0052] Here, when the non-aqueous electrolyte contains EC and the non-aqueous electrolyte is attached to the negative electrode 60, the carbon-based negative electrode active material and the EC coexist in the negative electrode 60.

[0053] When the non-aqueous electrolyte does not contain EC, the carbon-based negative electrode active material contained in the negative electrode active material layer 64 of the negative electrode 60 is brought into contact with EC. Specifically, for example, the negative electrode 60 is immersed in EC and then pulled out. Alternatively, for example, EC is applied to the negative electrode active material layer 64 by spraying or the like. In this way, the carbon-based negative electrode active material contained in the negative electrode active material layer 64 and EC coexist.

[0054] The negative electrode 60 containing the carbon-based negative electrode active material and EC is placed in a heating device such as a thermostatic dryer, heater, or electric furnace, and heated at a temperature below the boiling point of EC so as to produce lithium ethylene dicarbonate by the reaction represented by the above formula (I), thereby performing the first heat treatment step S101.

[0055] Thereafter, the temperature setting of the heating device is changed to a temperature equal to or higher than the boiling point of EC, thereby performing the heat treatment in the second heat treatment step S102. After performing the first heat treatment step S101, the heat-treated object may be removed from the heating device and transferred to another heating device, where the heat treatment in the second heat treatment step S102 may be performed.

[0056] (Third embodiment) In the third embodiment, a carbon-based negative electrode active material recovered from a used lithium-ion secondary battery 100 is subjected to a heat treatment in a first heat treatment step S101 and a heat treatment in a second heat treatment step S102.

[0057] Therefore, first, the negative electrode 60 is removed from the lithium-ion secondary battery 100 using the same procedure as in the second embodiment. The carbon-based negative electrode active material is recovered from the negative electrode active material layer 64 of the removed negative electrode 60. This can be done using a known method. Specifically, for example, the negative electrode active material layer 64 is peeled off from the negative electrode 60, and the binder and thickener contained in the negative electrode active material layer 64 are removed using a solvent or the like. For example, if CMC is used as the thickener, the negative electrode active material layer 64 is washed with water to dissolve and remove the CMC. If SBR is used as the binder, the negative electrode active material layer 64 is washed with an organic solvent having a solubility parameter of 7.7 to 9.5 (e.g., toluene, benzene, etc.), to dissolve and remove the SBR in the organic solvent.

[0058] The recovered carbon-based negative electrode active material is brought into contact with EC. Specifically, for example, the carbon-based negative electrode active material is immersed in EC and then pulled out. Alternatively, for example, EC is applied to the carbon-based negative electrode active material by spraying or the like. In this way, the carbon-based negative electrode active material and EC are allowed to coexist.

[0059] The carbon-based negative electrode active material coexisting with EC is placed in a heating device such as an electric furnace, a hot air dryer, or a high-temperature dryer, and heated at a temperature below the boiling point of EC so that lithium ethylene dicarbonate is produced by the reaction represented by the above formula (I). This allows the first heat treatment step S101 to be performed.

[0060] Thereafter, the temperature setting of the heating device is changed to a temperature equal to or higher than the boiling point of EC, thereby performing the heat treatment in the second heat treatment step S102. After performing the first heat treatment step S101, the heat-treated object may be removed from the heating device and transferred to another heating device, where the heat treatment in the second heat treatment step S102 may be performed.

[0061] In the first and second embodiments, since EC and the carbon-based negative electrode active material already coexist, it is advantageous for the non-aqueous electrolyte of the lithium-ion secondary battery 100 to contain EC. In the method for producing a recycled carbon material according to the present disclosure, it is particularly preferred that the carbon-based negative electrode active material is used together with an electrolyte solution containing EC, and that the first heat treatment step and the second heat treatment step are performed in a dry atmosphere.

[0062] As in the first to third embodiments described above, the Li contained in the carbon-based negative electrode active material is converted to lithium carbonate by performing the heat treatments in the first heat treatment step S101 and the second heat treatment step S102. Therefore, the method for producing a recycled carbon material according to the present disclosure may further include a step of removing lithium carbonate from the heat-treated product obtained in the second heat treatment step S102. Here, the lithium carbonate can be easily removed from the carbon-based negative electrode active material by washing with water.

[0063] Therefore, the method for producing a recycled carbon material according to the present disclosure preferably includes a step of washing the heat-treated product obtained in the second heat treatment step S102 with water to remove lithium carbonate from the carbon-based negative electrode active material (lithium carbonate removal step).

[0064] The lithium carbonate removal step can be carried out according to a known method. After washing with water, the product may be dried according to a known method.

[0065] When the lithium carbonate removal step is performed, a step of recovering washing water generated by the water washing and volatilizing the water from the washing water to recover lithium carbonate may be further performed. This step can be performed according to a known method. Lithium carbonate can be used for various purposes, and can be used, for example, as a lithium source for lithium composite oxides used as positive electrode active materials in lithium ion secondary batteries.

[0066] When the first heat treatment step S101 and the second heat treatment step S102 are performed on the lithium-ion secondary battery 100 or the negative electrode 60 as in the first or second embodiment, a carbon-based negative electrode active material from which a larger amount of Li has been removed can be recovered by, for example, the method described in the second or third embodiment, to obtain a recycled carbon material. The recycled carbon material can be used not only as a negative electrode active material but also for various other applications.

[0067] As described above, the method for producing a recycled carbon material according to the present disclosure makes it easy to remove a larger amount of Li from a carbon-based negative electrode active material containing Li to obtain a recycled carbon material. The method for producing a recycled carbon material according to the present disclosure is useful in material recycling.

[0068] Test examples relating to the present disclosure will be described in detail below, but it is not intended that the present invention be limited to those shown in these examples.

[0069] [Preparation of Evaluation Battery] A used lithium-ion secondary battery was prepared. This used lithium-ion secondary battery had a configuration in which a wound electrode body and a nonaqueous electrolyte were housed in an aluminum battery case. The positive electrode included aluminum foil and a positive electrode active material layer. The positive electrode active material layer contained lithium nickel cobalt manganese composite oxide, acetylene black, and polyvinylidene fluoride in a mass ratio of 92:5:3. The negative electrode included copper foil and a negative electrode active material layer. The negative electrode active material layer contained graphite, a carbon-based negative electrode active material, styrene butadiene rubber, and carboxymethyl cellulose in a mass ratio of 99:0.5:0.5. The nonaqueous electrolyte contained ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate as a nonaqueous solvent in a volume ratio of 3:3:4, and LiPF6 as a supporting electrolyte at a concentration of approximately 1.1 mol / L.

[0070] After discharging the used lithium-ion secondary battery, the battery case was opened under an argon gas atmosphere. The wound electrode assembly was unwound and the negative electrode was removed. The negative electrode active material layer of the negative electrode was impregnated with a non-aqueous electrolyte. Therefore, EC contained in the non-aqueous electrolyte adhered to the graphite.

[0071] The negative electrode was placed in a constant temperature dryer under an argon gas atmosphere while the negative electrode active material layer was still impregnated with the non-aqueous electrolyte. Heat treatment was performed on the negative electrode under the conditions shown in Table 1. After the heat treatment, the negative electrode was washed with water and dried to remove the water. However, in Comparative Example 1, the negative electrode was washed with water and dried without being subjected to heat treatment. A portion of the negative electrode active material layer was scraped off, and the residue obtained by burning the carbon was prepared as a measurement sample. The Li content (mass%) in the negative electrode active material layer was measured using this measurement sample and a commercially available IPC emission spectrometer. The results are shown in Table 1.

[0072] [Table 1]

[0073] As the results in Table 1 show, by subjecting a carbon-based negative electrode active material containing Li to a two-stage heat treatment together with EC, first at a temperature below the boiling point of EC and then at a temperature above the boiling point of EC, it is clear that the Li contained in the carbon-based negative electrode active material can be easily and efficiently removed.

[0074] Although specific examples of the present disclosure have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific examples exemplified above.

[0075] That is, the method for producing a recycled carbon material according to the present disclosure includes the following items [1] to

[10] . [1] a first heat treatment step in which a carbon-based negative electrode active material containing Li is heat-treated together with ethylene carbonate at a temperature lower than the boiling point of ethylene carbonate to obtain a heat-treated product containing lithium ethylene dicarbonate; and a second heat treatment step of heat treating the heat-treated product at a temperature equal to or higher than the boiling point of ethylene carbonate to convert lithium ethylene dicarbonate into lithium carbonate; A method for producing a recycled carbon material comprising the steps of: [2] The method according to item [1], wherein the heat treatment in the first heat treatment step is carried out at 90°C to 220°C for 30 minutes to 72 hours. [3] The method according to item [1], wherein the heat treatment in the first heat treatment step is carried out at 110°C to 140°C for 50 minutes to 6 hours. [4] The method according to any one of items [1] to [3], wherein the heat treatment in the second heat treatment step is carried out at 250° C. to 350° C. for 30 minutes to 72 hours. [5] The method according to any one of items [1] to [3], wherein the heat treatment in the second heat treatment step is carried out at 260°C to 290°C for 50 minutes to 6 hours. [6] The method according to any one of items [1] to [5], further comprising the step of washing the heat-treated product obtained in the second heat treatment step with water to remove lithium carbonate from the carbon-based negative electrode active material. [7] The method according to item [6], further comprising recovering washing water generated by the water washing and volatilizing water from the washing water to recover lithium carbonate. [8] The method according to any one of items [1] to [7], wherein the carbon-based negative electrode active material is used together with a non-aqueous electrolyte solution containing ethylene carbonate, and the first heat treatment step and the second heat treatment step are performed in a dry atmosphere. [9] The method according to any one of items [1] to [8], wherein the carbon-based negative electrode active material is graphite.

[10] The method according to any one of items [1] to [9], wherein the carbon-based negative electrode active material is a negative electrode active material of a used lithium ion secondary battery. [Explanation of symbols]

[0076] 20 Electrode body 30 Battery case 32 Exterior body 34 Lid 36 Safety valve 42 Positive terminal 42a Positive current collector plate 44 Negative terminal 44a Negative current collector plate 50 positive electrode 52 Positive electrode current collector 52a Portion where positive electrode active material layer is not formed 54 Cathode active material layer 60 negative electrode 62 Negative electrode current collector 62a Part where negative electrode active material layer is not formed 64 Negative electrode active material layer 70 Separator 100 Lithium-ion secondary battery

Claims

1. a first heat treatment step of heat-treating a carbon-based negative electrode active material containing Li together with ethylene carbonate at a temperature lower than the boiling point of ethylene carbonate to obtain a heat-treated product containing lithium ethylene dicarbonate; and a second heat treatment step of heat treating the heat-treated product at a temperature equal to or higher than the boiling point of ethylene carbonate to convert lithium ethylene dicarbonate into lithium carbonate; A method for producing a recycled carbon material comprising the steps of:

2. The method according to claim 1, wherein the heat treatment in the first heat treatment step is carried out at 90°C to 220°C for 30 minutes to 72 hours.

3. The method according to claim 1, wherein the heat treatment in the first heat treatment step is carried out at 110°C to 140°C for 50 minutes to 6 hours.

4. The method according to claim 1, wherein the heat treatment in the second heat treatment step is carried out at 250°C to 350°C for 30 minutes to 72 hours.

5. The method according to claim 1, wherein the heat treatment in the second heat treatment step is carried out at 260°C to 290°C for 50 minutes to 6 hours.

6. The method according to claim 1 , further comprising the step of washing the heat-treated product obtained in the second heat treatment step with water to remove lithium carbonate from the carbon-based negative electrode active material.

7. 7. The method according to claim 6, further comprising the steps of recovering washing water generated by the water washing and volatilizing water from the washing water to recover lithium carbonate.

8. The manufacturing method according to claim 1 , wherein the carbon-based negative electrode active material is used together with a non-aqueous electrolyte solution containing ethylene carbonate, and the first heat treatment step and the second heat treatment step are performed in a dry atmosphere.

9. The method according to claim 1 , wherein the carbon-based negative electrode active material is graphite.

10. The method according to claim 1 , wherein the carbon-based negative electrode active material is a negative electrode active material of a used lithium ion secondary battery.

Images