Method for manufacturing a negative electrode active material, negative electrode active material and mixed negative electrode active material, and method for manufacturing a secondary battery and secondary battery

Abstract

This invention provides a method for producing reusable negative electrode active material by regenerating the used negative electrode active material contained in the negative electrode of a used secondary battery. [Solution] The method for producing a negative electrode active material is a method for producing a reusable negative electrode active material by regenerating used carbon-based negative electrode active material contained in the negative electrode of a used secondary battery, comprising the steps of: recovering used carbon-based negative electrode active material from the negative electrode; and heating the recovered used carbon-based negative electrode active material to a temperature of 700°C or higher in an inert atmosphere.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a negative electrode active material, a negative electrode active material and a mixed negative electrode active material, and a method for manufacturing a secondary battery and a secondary battery. More specifically, the present invention relates to a method for manufacturing a reusable negative electrode active material by regenerating a used active material contained in the negative electrode of a secondary battery after use, a negative electrode active material and a mixed negative electrode active material, and a method for manufacturing a secondary battery and a secondary battery.

Background Art

[0002] Regulations regarding the recycling of secondary batteries such as lithium-ion batteries are being developed mainly in the EU. Specifically, regulations are being developed that require a specified proportion or more of the materials used in secondary batteries to be reused in secondary batteries again.

[0003] Japanese Patent Application Laid-Open No. 2010-34021 describes a method for recovering an oxide-containing battery material from battery waste materials. This recovery method includes a step of immersing a substrate to which a battery material containing an oxide is attached in a solvent in which the oxide does not substantially dissolve to peel the battery material from the substrate, and a step of separating the peeled battery material from the substrate. The publication also describes firing the recovered oxide at a temperature of 600°C or higher and 1100°C or lower.

[0004] Japanese Patent No. 5141970 describes a method for recovering a positive electrode active material from the positive electrode of a lithium battery. The lithium battery targeted by this recovery method includes a positive electrode active material layer formed by applying a material containing a positive electrode active material, a conductive material, and a binder dispersed in an aqueous solvent onto the surface of a positive electrode current collector. This recovery method includes a step of immersing the positive electrode in an alkaline aqueous solution to peel the positive electrode active material layer from the positive electrode current collector, a step of adding an organic solvent to the peeled positive electrode active material layer peelings to extract the binder from the peelings, and a step of separating a supernatant portion containing the conductive material and a sediment portion containing the positive electrode active material from the extraction-treated product after the extraction step.

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] Japanese Patent Publication No. 2010-34021 [Patent Document 2] Patent No. 5141970 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] Traditionally, the recycling of materials in secondary batteries has focused primarily on the positive electrode active material. This is because the elements contained in the positive electrode active material of lithium-ion batteries, particularly cobalt, are expensive and scarce. Other forms of secondary battery material recycling include the recovery of metals used in the casing and current collector.

[0007] On the other hand, a significant increase in the use of large secondary batteries, such as those used in electric vehicles, is predicted for the future. Therefore, it is necessary to consider recycling of positive electrode active materials and non-metallic materials from the perspective of reducing environmental impact and ensuring a stable supply of resources. However, there has been very little consideration given to recycling positive electrode active materials and non-metallic materials, especially negative electrode active materials, until now.

[0008] The object of the present invention is to provide a method for producing reusable negative electrode active material by regenerating used negative electrode active material contained in the negative electrode of a used secondary battery, and a secondary battery using the obtained negative electrode active material as the negative electrode. [Means for solving the problem]

[0009] A method for producing a negative electrode active material according to one embodiment of the present invention is a method for producing a reusable negative electrode active material by regenerating used carbon-based negative electrode active material contained in the negative electrode of a used secondary battery, comprising the steps of: recovering the used carbon-based negative electrode active material from the negative electrode; and heating the recovered used carbon-based negative electrode active material to a temperature of 700°C or higher in an inert atmosphere.

[0010] An embodiment of the present invention provides a carbon-based negative electrode active material for lithium-ion secondary batteries, which is extracted from the negative electrode of a used secondary battery and contains 10 ppm by mass or more of lithium.

[0011] A negative electrode active material mixture according to the present invention comprises the above-mentioned negative electrode active material and other negative electrode active materials.

[0012] A method for manufacturing a secondary battery according to one embodiment of the present invention is a method for manufacturing a secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode uses a negative electrode active material obtained by the method for manufacturing the negative electrode active material.

[0013] A secondary battery according to one embodiment of the present invention is a secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode comprises the above-mentioned negative electrode active material or the above-mentioned mixed negative electrode active material. [Effects of the Invention]

[0014] According to the present invention, reusable negative electrode active material can be obtained from the used negative electrode active material contained in the negative electrode of a used secondary battery. Furthermore, by using the obtained negative electrode active material as the negative electrode, a secondary battery can be constructed. [Brief explanation of the drawing]

[0015] [Figure 1] Figure 1 is a flow chart of a method for producing an electrode active material according to one embodiment of the present invention. [Figure 2] Figure 2 shows the initial charge-discharge curve of the regenerated active material. [Figure 3] Figure 3 shows the initial charge-discharge curve of the regenerated active material treated with lithium carbonate. [Figure 4] Figure 4 shows the relationship between the heat treatment temperature and the performance of the negative electrode active material (initial charge / discharge efficiency and discharge capacity). [Figure 5] Figure 5 is a cross-sectional image of the negative electrode containing the regenerated active material. [Figure 6] Figure 6 is a cross-sectional photograph of a negative electrode containing new negative electrode active material (not recycled negative electrode active material).

Mode for Carrying Out the Invention

[0016] The inventors of the present invention studied a method for recovering and regenerating the used carbon-based negative electrode active material contained in a lithium-ion secondary battery after use.

[0017] First, the recovered negative electrode active material (specifically, graphite) was washed and heat-treated at a low temperature (200 °C) to verify whether the negative electrode active material could be reused. Specifically, a negative electrode was fabricated using the heat-treated negative electrode active material, and a single electrode cell with a lithium counter electrode was fabricated to measure the initial charge-discharge efficiency (Coulomb efficiency) and discharge capacity. As a result, the initial charge-discharge efficiency was as high as about 92%, but the discharge capacity was as low as 330 mAh / g.

[0018] Subsequently, the same test was carried out by changing the heat treatment temperature. Specifically, heat treatment was performed at 500 °C, 650 °C, 800 °C, and 950 °C in an inert atmosphere, and the initial charge-discharge efficiency and discharge capacity were measured in the same manner as above. As a result, although the discharge capacity recovered for the negative electrode active materials heat-treated at 500 °C and 650 °C, the initial charge-discharge efficiency decreased significantly. On the other hand, for the negative electrode active materials heat-treated at 800 °C and 950 °C, both the initial charge-discharge efficiency and the discharge capacity were improved to a practically applicable level.

[0019] The mechanism by which the initial charge-discharge efficiency and discharge capacity decrease in the used negative electrode active material, and the mechanism by which these are recovered by heat treatment at a high temperature are not necessarily clear, but are presumed to be as follows.

[0020] In a lithium-ion secondary battery, charging and discharging are performed by reversibly inserting and releasing lithium ions into and from the interlayers of the negative electrode active material (e.g., graphite). During repeated charging and discharging, due to volume changes and the like, electrical conduction is lost and lithium ions are left behind, or lithium ions are immobilized on the surface of the negative electrode active material as an SEI film and cannot return to the positive electrode. In a deteriorated negative electrode active material, it is considered that both the charge capacity and the discharge capacity are reduced because a large amount of lithium ions are trapped as an SEI film.

[0021] Lithium ions trapped as an SEI film can be released by heat treatment at a temperature of about 500°C. As a result, the discharge capacity can be restored. However, not all of the trapped lithium ions can be released, and it has been confirmed by inductively coupled plasma (ICP) emission spectrometry that lithium remains in a predetermined amount (about 500 mass ppm) even in the negative electrode active material heat-treated at 800°C.

[0022] When heat-treated at about 500°C, although the discharge capacity is restored, the initial charge capacity increases, so the initial charge-discharge efficiency deteriorates. This is considered to be due to the remaining part of the SEI film formed before regeneration. By heat treatment at a temperature of 700°C or higher, most of the SEI film is decomposed, and it is considered that both the initial charge-discharge efficiency and the discharge capacity are restored.

[0023] In addition, in the process of regenerating the positive electrode active material, a firing treatment may be performed at 600 to 1100°C, but this is different in purpose from the heat treatment of the negative electrode active material described above.

[0024] The calcination process used in the regeneration of positive electrode active materials is primarily for the purpose of removing the binder and replenishing lithium. Unlike negative electrode active materials, the lithium content of used positive electrode active materials decreases from its initial amount. To compensate for this, the regeneration of positive electrode active materials involves mixing the used positive electrode active material with a lithium salt (such as lithium carbonate) and then performing a calcination process. To ensure an efficient reaction between the positive electrode active material and lithium, it is necessary to thoroughly burn off the binder during the reaction. Therefore, the calcination process in the regeneration of positive electrode active materials is carried out in an oxygen-containing atmosphere. In some cases, a multi-stage calcination process is implemented, separating the calcination process for burning off the binder from the calcination process for reacting with lithium.

[0025] In contrast, in the regeneration of the negative electrode active material, as described above, heat treatment is performed to release trapped lithium ions and decompose the SEI film. Therefore, the addition of lithium salt is unnecessary. Furthermore, unlike when reacting with lithium, the release of lithium ions and the decomposition of the SEI film are not significantly affected even if some binder remains. In addition, since water-soluble binders such as carboxymethylcellulose or binders that disperse easily in water such as styrene-butadiene rubber are used in the negative electrode, binder separation is relatively easy. Therefore, it is not necessary to perform firing in an oxygen-containing atmosphere to burn off the binder, and it is preferable to perform heat treatment in an inert atmosphere to suppress oxidation of the negative electrode active material.

[0026] Regarding the addition of lithium salts, the inventors also conducted experiments in which lithium salts were added to used negative electrode active material and then heat-treated. This was based on the idea that if lithium ions could be inserted between the layers through heat treatment, the irreversible capacity could be reduced. However, there was almost no difference in the initial charge-discharge efficiency between cases where lithium salts were added and cases where they were not. This is thought to be because lithium ions were not inserted between the layers. Unlike positive electrode active material, negative electrode active material can exist as a stable compound in the absence of lithium ions, so it is thought that lithium ions are not inserted between the layers by heat treatment alone.

[0027] The present invention was completed based on the above findings. Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0028] [Method for manufacturing negative electrode active material] Figure 1 is a flowchart of a method for manufacturing a negative electrode active material according to one embodiment of the present invention. The method for manufacturing a negative electrode active material according to this embodiment comprises the steps of: recovering used carbon-based negative electrode active material from the negative electrode of a used secondary battery (step S1); and heating the recovered used carbon-based negative electrode active material to a temperature of 700°C or higher in an inert atmosphere (step S3).

[0029] The method for producing negative electrode active material according to this embodiment is a method for producing reusable negative electrode active material by regenerating used carbon-based negative electrode active material contained in the negative electrode of a used secondary battery. A "used secondary battery" is, but is not limited to, a secondary battery that has been recovered because its discharge capacity has decreased due to repeated charging and discharging and it is no longer suitable for reuse. The secondary battery is, but is not limited to, a lithium-ion secondary battery.

[0030] The negative electrode of a secondary battery comprises, for example, a sheet-like current collector and a negative electrode mixture formed on the current collector. Generally, metal foils such as stainless steel foil or copper foil are used for the current collector, with copper foil being particularly preferred. The negative electrode mixture is a mixture prepared by adding predetermined additives to a negative electrode active material. The negative electrode of a secondary battery is manufactured, for example, by dispersing the negative electrode mixture in a solvent to form a paste, applying and pressing it onto the current collector, and then removing the solvent by drying.

[0031] In this embodiment, the anode active material (anode material) to be recovered and recycled is a carbon-based anode active material. Examples of carbon-based anode active materials include graphite, mesocarbon microbeads (MCMB), mesophase pitch-based carbon fibers (MCF), and vapor-grown carbon fibers (VGCF). The carbon-based anode active material is preferably graphite.

[0032] The negative electrode mixture may contain a binder and conductive additives in addition to the negative electrode active material. Typically, carboxymethylcellulose (CMC) and styrene-butadiene rubber (SBR) are used as the negative electrode binder. Examples of conductive additives include carbon black and graphite powder. The method for producing the negative electrode active material according to this embodiment can be applied regardless of the type of binder or conductive additive.

[0033] First, the used negative electrode active material is recovered from the negative electrode of the used secondary battery (Step S1). Specifically, the used secondary battery is first disassembled to recover the negative electrode, and the negative electrode mixture is detached from the current collector. The negative electrode binder is usually a water-soluble binder such as CMC or a binder that disperses easily in water such as SBR. Therefore, detaching the negative electrode mixture from the current collector can be done relatively easily by immersing the negative electrode in water. In cases where a water-insoluble binder is used, the negative electrode mixture may be detached by dissolving the binder with an organic solvent or by mechanically scraping it off the current collector.

[0034] Next, the used negative electrode active material is recovered from the negative electrode mixture. As mentioned above, water-soluble binders or binders that disperse easily in water are often used as binders for the negative electrode. Therefore, for example, the negative electrode active material can be separated by immersing the negative electrode mixture in water, dissolving or dispersing the binder, and then performing decantation to remove the solvent. The recovery of the negative electrode active material may also be carried out by other appropriate methods such as centrifugation or filtration. Furthermore, it is not necessary to completely remove the binder or conductive additives, and it is not a problem if some of them remain.

[0035] Furthermore, when secondary batteries such as lithium-ion secondary batteries undergo repeated charge-discharge cycles, metal elements leached from the positive electrode active material may precipitate on the surface of the negative electrode active material, hindering its reuse. To prevent such problems, it is desirable to wash the recovered negative electrode active material with acid to remove precipitates containing metal elements (Step S2). Acid washing can be performed, for example, by immersing the negative electrode active material in an aqueous solution containing an inorganic or organic acid. If necessary, these aqueous solutions may be heated, or ultrasonic cleaning may be performed by applying ultrasound to the negative electrode active material.

[0036] The type of acid used for acid cleaning is not particularly limited, but strong acids such as hydrochloric acid, sulfuric acid, and nitric acid are preferably used. The acid concentration and treatment time can be set as appropriate. When using the cleaning solution for acid cleaning, it is desirable to adjust the acid concentration and the amount of the cleaning solution (amount ratio to the negative electrode active material) so that the pH of the treatment solution after acid cleaning is 3.0 or less, preferably 2.5 or less, and more preferably 2.0 or less.

[0037] Next, the acid-washed used negative electrode active material is heated to a temperature of 700°C or higher in an inert atmosphere (step S3). This restores both the initial charge-discharge efficiency and discharge capacity of the negative electrode active material. If the heating temperature is too low, the initial charge-discharge efficiency will not be restored. The lower limit of the heating temperature is preferably 750°C, more preferably 780°C, and even more preferably 800°C. The upper limit of the heating temperature is not particularly limited, but for example, it is 1100°C, and preferably 1050°C.

[0038] This heating should be carried out under an inert atmosphere. Heating under an oxygen-containing atmosphere may cause oxidation of the negative electrode active material, leading to a decrease in its properties. Examples of inert atmospheres include argon, nitrogen, and helium atmospheres, with an argon atmosphere being preferred.

[0039] The holding time at the heating temperature is not particularly limited, but is for example 1 to 10 hours. The lower limit of the holding time is preferably 2 hours, and more preferably 3 hours. The upper limit of the holding time is preferably 8 hours, and more preferably 5 hours.

[0040] This heating may be carried out in multiple stages. For example, it may be divided into a preheat treatment at a low temperature and a main heat treatment at a high temperature. However, in this embodiment, it is preferable to carry out the heat treatment in one stage. Even when performing a preheat treatment, it is preferable to do so in an inert atmosphere when heating to 300°C or higher.

[0041] During this heating process, it is not necessary to add lithium salt to the recovered used negative electrode active material. Unlike the positive electrode active material, simply adding lithium salt and performing heat treatment on the negative electrode active material does not introduce lithium into the negative electrode active material and does not contribute to improving its performance.

[0042] Note that Figure 1 illustrates the case where acid washing (Step S2) is performed before the heating step (Step S3) of the carbon-based active material. However, acid washing (Step S2) can also be performed after the heating step (Step S3). In the latter case, however, a drying step of the carbon-based active material is required after acid washing, so it is preferable to follow the order of steps shown in Figure 1 in order to simplify the process.

[0043] Through the above process, reusable negative electrode active material is produced. The produced negative electrode active material can be used as a material for the negative electrode of a secondary battery, just like new negative electrode active material (non-recycled negative electrode active material). When manufacturing a negative electrode, it is possible to manufacture the negative electrode using only the negative electrode active material obtained through the above process, but it is also possible to mix this negative electrode active material with, for example, new negative electrode active material or negative electrode active material other than carbon-based negative electrode active material in any proportion to create a mixed negative electrode active material, and then manufacture the negative electrode using this mixed negative electrode active material.

[0044] [Negative electrode active material] An anode active material according to one embodiment of the present invention is a carbon-based anode active material for lithium-ion secondary batteries, which is extracted from the anode of a used secondary battery and contains 10 ppm (mass ppm; the same applies hereinafter) or more of lithium. The amount of lithium contained in the anode active material can be measured by inductively coupled plasma (ICP) emission spectroscopy.

[0045] The negative electrode active material according to this embodiment is a negative electrode active material obtained by regenerating used carbon-based negative electrode active material contained in the negative electrode of a used secondary battery. The negative electrode active material according to this embodiment can be used as a material for the negative electrode of a secondary battery, just like new negative electrode active material (non-regenerated negative electrode active material).

[0046] The negative electrode active material according to this embodiment is the negative electrode active material before it is incorporated into a secondary battery. The negative electrode active material according to this embodiment may be in a powder or granular state before being mixed as a negative electrode compound, or it may be in a state where it is contained within the negative electrode before it is incorporated into a secondary battery.

[0047] The negative electrode active material according to this embodiment contains 10 ppm by mass or more of lithium. In used negative electrode active material, lithium ions are trapped between the layers. In regenerated negative electrode active material, most of the trapped lithium ions are released by the heat treatment described above, but a predetermined amount of lithium still remains. By having the negative electrode active material contain a predetermined amount of lithium, a slight but effective reduction in irreversible capacity can be obtained.

[0048] In contrast, lithium is hardly detectable in new carbon-based anode active materials (non-recycled carbon-based anode active materials) (for example, less than 3 ppm). Therefore, by examining the amount of lithium in the anode active material, it is possible to determine whether it is recycled or not. Note that the presence or absence of lithium can also be determined by laser-induced breakdown spectroscopy (LIBS).

[0049] The lower limit of the amount of lithium contained in the negative electrode active material is preferably 100 ppm, and more preferably 300 ppm. The upper limit of the amount of lithium contained in the negative electrode active material is preferably 3000 ppm, more preferably 1500 ppm, and still more preferably 1000 ppm.

[0050] The negative electrode active material according to this embodiment preferably has a porosity of 5.0 area% or more. Here, "porosity of the negative electrode active material" means the ratio of the area of ​​voids within the particles of the negative electrode active material to the cross-sectional area of ​​the particles of the negative electrode active material (including the void portion) when the cross-section of the negative electrode active material is observed.

[0051] The porosity of the negative electrode active material is measured more specifically as follows:

[0052] If the negative electrode active material is in powder or granular form, first, an observation sample is prepared by embedding the negative electrode active material in a resin or the like, and then the observation sample is cut or ground to reveal the cross-section of the negative electrode active material particles. Alternatively, instead of embedding it in a resin or the like, a negative electrode may be prepared using the negative electrode active material, and then the negative electrode may be cut or ground to reveal the cross-section of the negative electrode active material particles. The negative electrode can be manufactured, for example, by adding a predetermined additive (such as a binder or conductive additive) to the negative electrode active material to prepare a negative electrode mixture, dispersing the negative electrode mixture in a solvent to make a paste, applying and pressing it onto a current collector, and then removing the solvent by drying. The "porosity of the negative electrode active material" refers to the proportion of voids within the particles of the negative electrode active material, as described above, and therefore does not change depending on the proportion of negative electrode active material in the electrode mixture or the method of preparing the negative electrode.

[0053] If the negative electrode active material is contained within the negative electrode before being incorporated into the secondary battery, the negative electrode is cut or ground using ion milling or the like to expose the cross-section of the negative electrode active material particles.

[0054] The cross-section of the negative electrode active material particles is observed using a SEM at a magnification of 500x or higher. The observed image is binarized using image processing to identify the area of ​​the voids (dark areas) within the particles. The porosity is defined as the ratio of the area of ​​the voids within the particle to the cross-sectional area of ​​the particle (including the voids). The porosity is calculated for each of 15 or more particles, and the arithmetic mean is taken as the porosity of the negative electrode active material. Image processing can be performed using image analysis software such as WinROOF (image analysis and measurement system, manufactured by Mitani Corporation). For information on image binarization, please refer to Mitani Corporation's website "MiVLog" (https: / / www.mitani-visual.jp / mivlog / imageprocessing / bin03857.php).

[0055] Used carbon-based anode active materials, and carbon-based anode active materials recycled from used materials, have a larger porosity compared to new carbon-based anode active materials (non-recycled carbon-based anode active materials). Therefore, by evaluating the porosity of the anode active material in conjunction with the lithium content measurement results mentioned above, it is possible to determine whether the anode active material is recycled or not.

[0056] By setting the porosity of the negative electrode active material to 5.0 area% or more, the ease with which the electrolyte penetrates during injection is improved. The lower limit of the porosity of the negative electrode active material is preferably 5.5 area% and more preferably 6.0 area%. The upper limit of the porosity of the negative electrode active material is preferably 8.0 area% and more preferably 7.0 area%.

[0057] [Mixed negative electrode active material] The negative electrode active material according to this embodiment can be used alone as a negative electrode active material for the negative electrode of a secondary battery, or it can be mixed with other negative electrode active materials (for example, new negative electrode active material (negative electrode active material with a lithium content of 3 ppm or less) or negative electrode active materials other than carbon-based negative electrode active materials) in any proportion to form a mixed negative electrode active material. This mixed negative electrode active material can also be used as a negative electrode material for a secondary battery.

[0058] The above describes a method for producing a negative electrode active material and a negative electrode active material according to one embodiment of the present invention. According to this embodiment, a reusable negative electrode active material can be obtained from the used negative electrode active material contained in the negative electrode of a used secondary battery. [Examples]

[0059] The present invention will be described more specifically below with reference to examples. The present invention is not limited to these examples.

[0060] A test was conducted to recover and regenerate the used negative electrode active material from a used rechargeable battery. Specifically, the negative electrode recovered from a used lithium-ion rechargeable battery was immersed in water to remove the negative electrode mixture from the current collector, and then the binder was dissolved or dispersed in water to recover the graphite negative electrode active material by slag. After thoroughly drying to remove moisture, heat treatment was performed under an argon atmosphere at heating temperatures of 500°C, 650°C, 800°C, and 950°C. The holding time at each heating temperature was 5 hours. Hereinafter, the negative electrode active material after heat treatment may be referred to as the "regenerated active material."

[0061] Using the regenerated active material, a lithium counter electrode unipolar cell was fabricated, and the initial charge-discharge efficiency and discharge capacity were measured. The measurement conditions are as follows. The results are shown in Figure 2. Charging: 0.1C 0.01V CC-CV 0.02C Cut Discharge: 0.1C 3V Cut

[0062] Furthermore, the initial charge-discharge efficiency and discharge capacity were measured for used negative electrode active material that had been heat-treated at 500°C and 800°C by adding lithium carbonate. The results are shown in Figure 3.

[0063] Figure 4 shows the relationship between the heat treatment temperature and the performance of the negative electrode active material (initial charge / discharge efficiency and discharge capacity). As shown in Figure 4, negative electrode active materials heat-treated at 500°C and 650°C showed good discharge capacity of approximately 350 mAh / g, but low initial charge / discharge efficiency of approximately 85%. In contrast, negative electrode active materials heat-treated at 800°C and 950°C showed good initial charge / discharge efficiency and discharge capacity. Furthermore, no significant difference was observed depending on whether lithium carbonate was added or not.

[0064] Next, the lithium content of the regenerated active material, which had been heat-treated at 800°C, and the lithium content of new negative electrode active material (non-regenerated negative electrode active material) were measured for three samples each by ICP emission spectroscopy.

[0065] ICP emission spectroscopy was performed using a Thermo Fisher iCAP7600 ICP emission spectrometer (stock solution measurement). As a pretreatment, 0.1-0.2 g of the negative electrode active material (powder) was taken, placed in a decomposition container, and 7 ml of a concentrated nitric acid:concentrated hydrochloric acid = 1:1 solution was added, followed by microwave heating acid decomposition. After cooling, the residue was removed by suction filtration using a PTFE filter, and the sample was diluted to a constant volume of 50 ml. For all samples, 2 ppm of yttrium was added as an internal standard. Microwave heating acid decomposition was performed using an Anton Paar Multiwave3000 microwave processing unit, Method (INK2).

[0066] The lithium content of the regenerated active material (n=3) was 551 ppm, 565 ppm, and 574 ppm. In contrast, the lithium content of the new negative electrode active material (n=3) was 0.52 ppm, 0.58 ppm, and 0.55 ppm.

[0067] Next, the porosity of the regenerated active material after heat treatment at 800°C and the porosity of a new negative electrode active material (non-regenerated negative electrode active material) were measured. Specifically, negative electrodes were fabricated using each negative electrode active material, and the porosity was measured by observing the cross-sectional SEM image (1000x magnification) of the negative electrode. Cross-sectional photographs of the negative electrode containing the regenerated active material and the new negative electrode active material are shown in Figures 5 and 6, respectively.

[0068] The porosity of the regenerated active material was 6.31 area%. In contrast, the porosity of the new anode active material was 4.37 area%.

[0069] Next, the results of acid washing of the negative electrode active material (graphite) recovered from a lithium-ion secondary battery that underwent 72 charge-discharge cycles are shown below.

[0070] 0.5 g of recovered and dried graphite was immersed in 10 ml of aqueous solutions (treatment solutions) of various acids shown in Table 1, and allowed to stand for 1.5 hours to dissolve precipitates on the surface of the negative electrode active material. After that, the metal elements dissolved in each treatment solution were quantified by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The results are also shown in Table 1.

[0071] [Table 1]

[0072] The results in Table 1 show that Li can be removed even with dilute acids such that the pH of the treatment solution after acid washing becomes weakly acidic.

[0073] On the other hand, in order to sufficiently remove other metal elements, it was found that it is desirable to adjust the concentration of the acid and the amount of the treatment solution (amount ratio to the negative electrode active material) so that the pH of the treatment solution after acid washing is 3.0 or less, preferably 2.5 or less, and more preferably 2.0 or less. By acid washing, the amount of metal elements other than Li deposited on the surface of the recovered negative electrode active material can be reduced to, for example, 0.01% or less of the weight of the negative electrode active material.

[0074] Although embodiments of the present invention have been described above, the present invention is not limited to the embodiments described above, and various modifications are possible within the scope of the invention.

Claims

1. A method for producing reusable negative electrode active material by regenerating used carbon-based negative electrode active material contained in the negative electrode of a used secondary battery, A step of recovering the used carbon-based negative electrode active material from the negative electrode, A method for producing a negative electrode active material, comprising the step of heating the recovered used carbon-based negative electrode active material to a temperature of 700°C or higher in an inert atmosphere.

2. The method for producing a negative electrode active material according to claim 1, wherein the heating temperature is 800°C or higher.

3. A method for producing a negative electrode active material according to claim 1 or 2, further comprising the step of acid washing the recovered carbon-based negative electrode active material.

4. The method for producing a negative electrode active material according to claim 1 or 2, wherein the used secondary battery is a lithium-ion secondary battery.

5. A carbon-based negative electrode active material for lithium-ion secondary batteries, It is extracted from the negative electrode of a used rechargeable battery. A negative electrode active material containing 10 ppm or more of lithium by mass.

6. The negative electrode active material according to claim 5, wherein the amount of lithium is 3000 ppm by mass or less.

7. The negative electrode active material according to claim 5, wherein the porosity is 5.0 area% or more.

8. A mixed negative electrode active material comprising the negative electrode active material described in any one of claims 5 to 7 and another negative electrode active material.

9. The mixed negative electrode active material according to claim 8, wherein the lithium content of the other negative electrode active material is 3 ppm by mass or less.

10. A method for manufacturing a secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, A method for manufacturing a secondary battery, wherein the negative electrode uses a negative electrode active material obtained by the method for manufacturing a negative electrode active material described in claim 1 or 2.

11. A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, A secondary battery wherein the negative electrode contains the negative electrode active material described in any one of claims 5 to 7.

12. A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, A secondary battery wherein the negative electrode contains the mixed negative electrode active material described in claim 8.

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