Method for Reproducing Negative Electrode Active Material

Abstract

To provide a reproducing method for obtaining a negative electrode active material having improved rapid charge / discharge characteristics from a used lithium ion secondary battery.SOLUTION: A method for reproducing a negative electrode active material from a used lithium ion secondary battery includes a preparation step S10 for preparing a used lithium ion secondary battery 1 including a positive electrode 30, a negative electrode 40 including a negative electrode active material containing a carbon material, and an electrolyte, a charging step S20 for charging the lithium ion secondary battery 1, a discharging step S30 for discharging the lithium ion secondary battery 1 after the charging step S20, the discharging being performed at a discharge rate higher than the charge rate of the charging step S20, and a recovery step S40 for recovering the negative electrode active material containing the carbon material from the negative electrode 40 for the lithium ion secondary battery 1 after the discharging step S30.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present disclosure relates to a method for reproducing a negative electrode active material.

Background Art

[0002] Secondary batteries are suitably used for various applications including driving power sources mounted on vehicles such as hybrid vehicles (HEV), plug-in hybrid vehicles (PHEV), and battery electric vehicles (BEV), and the demand for them is rapidly expanding. In response to such demand, the demand for technologies related to the reproduction of negative electrode active materials from used lithium-ion secondary batteries is also increasing. As a technology related to the reproduction of negative electrode active materials, for example, Patent Document 1 discloses that a negative electrode is taken out from a lithium-ion secondary battery, the negative electrode is washed with a liquid containing water, and the negative electrode is separated into a negative electrode composite material containing a negative electrode active material and a binder and a current collector substrate, and then the negative electrode composite material is mixed with a solvent capable of dissolving or dispersing the binder to prepare a negative electrode paste, and the paste is applied onto a negative electrode current collector.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Regarding the negative electrode active material, not only technologies related to the reproduction of the negative electrode active material but also performance improvement of the reproduced negative electrode active material, for example, improvement of rapid charge and discharge characteristics, are required. The rapid charge and discharge characteristics are not considered in the negative electrode active material recovered by the conventional technology as disclosed in Patent Document 1. Therefore, it is not possible to sufficiently meet the above-mentioned requirements for rapid charge and discharge characteristics.

[0005] The technology disclosed herein has been made in view of the above circumstances, and relates to a reproduction method for obtaining a negative electrode active material with improved rapid charge-discharge characteristics from a used lithium-ion secondary battery.

Means for Solving the Problems

[0006] The technology disclosed herein is a method for reproducing a negative electrode active material from a used lithium-ion secondary battery, including a preparation step of preparing a used lithium-ion secondary battery including a positive electrode, a negative electrode including a negative electrode active material containing a carbon material, and an electrolytic solution; a charging step of charging the lithium-ion secondary battery; a discharging step of discharging the lithium-ion secondary battery after the charging step at a discharging rate higher than the charging rate of the charging step; and a recovery step of recovering the negative electrode active material containing the carbon material from the negative electrode with respect to the lithium-ion secondary battery after the discharging step.

[0007] According to such a configuration, by discharging the lithium-ion secondary battery at a discharging rate higher than the charging rate, the surface layer of the negative electrode active material of the negative electrode becomes amorphous, and the rapid charge-discharge characteristics of the negative electrode active material are improved. Therefore, a negative electrode active material with improved rapid charge-discharge characteristics can be reproduced.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Best Mode for Carrying Out the Invention

[0009] Hereinafter, embodiments of the technology disclosed herein will be described with reference to the drawings. In addition, matters other than those specifically mentioned in this specification and matters necessary for the implementation of the technology disclosed herein can be grasped as design matters of those skilled in the art based on the prior art in the relevant field. The technology disclosed herein can be implemented based on the content disclosed in this specification and common general knowledge in the relevant field.

[0010] In the following drawings, members and parts having the same function are denoted by the same reference numerals, and redundant descriptions may be omitted or simplified. In addition, the notation "A to B" indicating a range in this specification includes the meaning of "not less than A and not more than B", and also includes the meaning of "preferably greater than A" and "preferably less than B". In this specification, the "secondary battery" refers to a power storage device capable of repeated charge and discharge, and is a term including so-called storage batteries and power storage elements such as electric double layer capacitors. In addition, in this specification, the "lithium ion secondary battery" refers to a secondary battery that uses lithium ions as charge carriers and realizes charge and discharge by the movement of charges associated with lithium ions between the positive and negative electrodes.

[0011] 1. Preparation Target The method for reproducing the negative electrode active material according to this embodiment prepares a predetermined lithium ion secondary battery and reproduces the negative electrode active material. As an example of the preparation target here, a used lithium ion secondary battery can be mentioned. Hereinafter, this lithium ion secondary battery will be specifically described. FIG. 1 is a longitudinal sectional view schematically showing the internal structure of a lithium ion secondary battery. FIG. 2 is a perspective view schematically showing the electrode body of the lithium ion secondary battery shown in FIG. 1. As shown in FIG. 1, the lithium ion secondary battery 1 includes a case 10, an electrode body 20, and an electrolytic solution (not shown).

[0012] (1) Case The case 10 is a box-shaped container. Inside this case 10, an electrode body 20 and an electrolytic solution are accommodated. For the case 10, a metal material (such as aluminum (Al)) having a certain strength is used, for example. The case 10 may be provided with a liquid injection hole 16. The liquid injection hole 16 is a hole for injecting the electrolytic solution. After injecting the electrolytic solution, the liquid injection hole 16 is sealed by a sealing member 18. Further, a positive electrode terminal 12 and a negative electrode terminal 14 are attached to the case 10. The positive electrode terminal 12 and the negative electrode terminal 14 are connected to the electrode body 20 inside the case 10. Specifically, the positive electrode terminal 12 is connected to the positive electrode 30 (see FIG. 2) of the electrode body 20. Aluminum (Al) or the like is used for the positive electrode terminal 12. On the other hand, the negative electrode terminal 14 is connected to the negative electrode 40 of the electrode body 20. Copper (Cu) or the like is used for the negative electrode terminal 14.

[0013] (2) Electrode body The electrode body 20 is a power generation element of the lithium-ion secondary battery 1. As shown in FIG. 2, the electrode body 20 includes a positive electrode 30, a negative electrode 40, and a separator 50. Note that the electrode body 20 shown in FIG. 2 is a wound electrode body. This wound electrode body is produced by laminating the positive electrode 30, the negative electrode 40, and the separator 50 to form a long strip-shaped laminate and winding the laminate. However, the structure of the electrode body 20 is not particularly limited, and other conventionally known structures (such as a laminated electrode body) may be used.

[0014] The positive electrode 30 includes a positive electrode core 32 which is a conductive metal foil, and a positive electrode active material layer 34 provided on the surface of the positive electrode core 32. Aluminum (Al) or the like is used for the positive electrode core 32. Further, the positive electrode active material layer 34 is a composite material layer containing a positive electrode active material, a conductive material, a binder, and the like. The positive electrode active material is a particulate material that can reversibly occlude and release charge carriers. Examples of the positive electrode active material include lithium nickel-based composite oxides (e.g., LiNiO2, etc.), lithium cobalt-based composite oxides (e.g., LiCoO2, etc.), lithium nickel cobalt manganese-based composite oxides (e.g., LiNi 1 / 3 Co 1 / 3 Mn 1 / 3Oxygen (O2, etc.), lithium nickel cobalt aluminum composite oxides (e.g., LiNi 0.8 Co 0.15 Al 0.5 O2, etc.), lithium manganese composite oxides (e.g., LiMn2O4, etc.), lithium nickel manganese composite oxides (e.g., LiNi 0.5 Mn 1.5 O4, etc.), and other lithium transition metal composite oxides; lithium transition metal phosphate compounds (e.g., LiFePO4, etc.). Examples of the conductive material include carbon materials such as acetylene black and graphite. Examples of the binder include resin materials such as polyvinylidene fluoride (PVdF).

[0015] The negative electrode 40 includes a negative electrode core 42 which is a conductive metal foil, and a negative electrode active material layer 44 provided on the surface of the negative electrode core 42. Copper (Cu), etc. is used for the negative electrode core 42. The negative electrode active material layer 44 is a composite material layer containing a negative electrode active material, a binder, a thickener, etc. The negative electrode active material is a particulate material capable of reversibly occluding and releasing charge carriers. The negative electrode active material to be prepared must include a carbon material as an essential component. Examples of the carbon material include graphite, hard carbon, soft carbon, etc., and among them, graphite is preferably used. The graphite may be natural graphite or artificial graphite. In addition, as the negative electrode active material, as long as the effects of the present technology are not significantly impaired, in addition to the above carbon material, lithium titanate (LTO), silicon carbide, a composite containing carbon and silicon (Si-C composite), silicon oxide (SiO X ) may be included. Although not particularly limited, from the viewpoint of reproduction efficiency, when the total amount of the negative electrode active material is 100% by mass, the proportion of the carbon material is preferably 50% by mass or more. Examples of the binder include resin materials such as styrene butadiene rubber (SBR). Examples of the thickener include resin materials such as carboxymethyl cellulose (CMC).

[0016] In some preferred embodiments, the positive-to-negative electrode capacity ratio of the lithium-ion secondary battery 1 is preferably 1.0 or more. Thereby, a negative electrode active material with excellent rapid charge and discharge performance can be reproduced more preferably. The positive-to-negative electrode capacity ratio is preferably 1.0 or more, more preferably 1.05 or more, still more preferably 1.2 or more, and even more preferably 1.5 or more. When the positive-to-negative electrode capacity ratio is closer to 1.0, the utilization rate of the negative electrode active material increases. Therefore, the surface of the negative electrode active material can be suitably amorphized, and a negative electrode active material with excellent rapid charge and discharge performance can be obtained. Further, when the positive-to-negative electrode capacity ratio is less than 1.0 (less than 1.0), metallic lithium may precipitate on the negative electrode, which may cause a short circuit. Therefore, the positive-to-negative electrode capacity ratio is preferably 1.0 or more. The upper limit of the positive-to-negative electrode capacity ratio is not particularly limited, but is preferably 2.2 or less, more preferably 2.0 or less, still more preferably 1.9 or less, and even more preferably 1.7 or less. In this specification, the "positive-to-negative electrode capacity ratio" can be obtained by separately determining the positive electrode capacity and the negative electrode capacity and using the following formula: positive-to-negative electrode capacity ratio = negative electrode capacity / positive electrode capacity. Here, the positive-to-negative electrode capacity ratio can be easily adjusted by, for example, changing the basis weight of the positive and negative electrodes or changing the types of the positive and negative electrode active materials. The "basis weight" refers to the value obtained by dividing the mass of the electrode active material layer by the area of the formation region (mass of the electrode active material layer / area of the formation region). The positive electrode capacity can be determined, for example, as the capacity (i.e., the initial positive electrode charge capacity) when a half cell with Li metal as the counter electrode is fabricated and initial charging is performed in a voltage range corresponding to the full cell (for example, about 2 to 4.2 V). The negative electrode capacity can be determined, for example, by summing the initial negative electrode discharge capacity and the use voltage range of the negative electrode.

[0017] Separator 50 is an insulating sheet interposed between the positive electrode 30 and the negative electrode 40. For this separator 50, resin materials such as polyethylene (PE), polypropylene (PP), polyester, cellulose, polyamide, etc. are used. Further, a heat-resistant layer containing an inorganic filler may be formed on the surface of the separator 50. Examples of such inorganic fillers include inorganic oxides such as aluminum oxide, magnesium oxide, silicon oxide, titanium oxide, nitrides such as aluminum nitride, silicon nitride, metal hydroxides such as calcium hydroxide, magnesium hydroxide, aluminum hydroxide, and clay minerals such as mica, talc, boehmite, zeolite, apatite, kaolin, etc.

[0018] (3) Electrolyte The electrolyte exists between the positive electrode 30 and the negative electrode 40. Thereby, charge carriers can be moved between the positive electrode 30 and the negative electrode 40. Examples of the electrolyte include non-aqueous electrolytes, gel-like electrolytes, and the like. Note that the electrolyte can be used without particular limitation as long as it is an electrolyte that can be used in a conventional lithium-ion secondary battery. The electrolyte typically contains a solvent and a supporting salt. As the solvent, various non-aqueous solvents used in this type of lithium-ion secondary battery, for example, carbonate-based non-aqueous solvents such as ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) are used. As the supporting salt, for example, lithium salts such as LiPF6, LiBF4, and lithium bis(fluorosulfonyl)imide (LiFSI) can be preferably used. The concentration of the supporting salt is not particularly limited, but is preferably about 0.7 mol / L or more and 1.3 mol / L or less. Note that the electrolyte may contain components other than the above-described solvent and supporting salt as long as the effects of the present technology are not significantly impaired, and may include various additives such as a gas generating agent, a film forming agent, a dispersant, and a thickening agent. Examples of the additive used in the non-aqueous electrolyte 80 include positive and negative electrode film forming agents such as vinylene carbonate (VC), fluoroethylene carbonate (FEC), and 1,3-propane sultone (PS); overcharge preventives such as biphenyl (BP), cyclohexylbenzene (CHB), t-butylbenzene, and t-amylbenzene.

[0019] As described above, as an example of the preparation target in the reproduction method according to the present embodiment, the lithium-ion secondary battery 1 has been described. However, the reproduction method disclosed herein is not limited to only the method for preparing the lithium-ion secondary battery 1 having the above-described configuration. That is, the lithium-ion secondary battery as the preparation target of the reproduction method disclosed herein only needs to include a positive electrode, a negative electrode, and an electrolyte, and is not particularly limited to a specific structure.

[0020] 2. Method for Reproducing Negative Electrode Active Material Hereinafter, a method for reproducing a negative electrode active material according to this embodiment will be described. FIG. 3 is a flowchart for explaining the reproduction method according to one embodiment. FIG. 4 is a flowchart for explaining sub-steps of the recovery step S40 according to one embodiment.

[0021] As shown in FIG. 3, the method for reproducing a negative electrode active material according to the first embodiment includes a preparation step S10, a charging step S20, a discharging step S30, and a recovery step S40. Further, the reproduction method disclosed herein may further include other steps at any stage, and the other processes may be the same as those in the prior art. Hereinafter, each step will be described.

[0022] (1) Preparation step S10 In the preparation step S10, a lithium-ion secondary battery for reproducing a negative electrode active material is prepared. The lithium-ion secondary battery prepared here includes a positive electrode, a negative electrode including a carbon material as a negative electrode active material, and an electrolytic solution. Note that the details of the lithium-ion secondary battery have already been described, so redundant descriptions will be omitted.

[0023] (2) Charging step S20 In the charging step S20, the lithium-ion secondary battery prepared in the preparation step S10 is charged. The charging step S20 is characterized in that the lithium-ion secondary battery is charged at a charging rate relatively lower than the discharging rate of the discharging step S30.

[0024] In some preferred embodiments, it is preferable to perform the charging step S20 at a charging rate of 0.5C or less. Thereby, a negative electrode active material excellent in rapid charge and discharge performance can be reproduced. The charging rate of the charging step S20 is preferably 0.5C or less, more preferably 0.3C or less, and still more preferably 0.2C or less. The charging rate of the charging step S20 is not particularly limited, but is, for example, 0.01C or more, and preferably 0.1C or more.

[0025] In some preferred embodiments, in the charging step S20, it is preferable to charge the lithium-ion secondary battery 1 until the state of charge (SOC) reaches 50% or more. Thereby, a negative electrode active material excellent in rapid charge and discharge performance can be reproduced. Further, it is more preferable to charge until the SOC of the lithium-ion secondary battery 1 reaches 80% or more, and it is even more preferable to charge until it reaches 100% (rated voltage). The higher the SOC in the charging step S20, the more widely lithium ions diffuse into the layer of the negative electrode active material, so that the surface of the negative electrode active material can be more suitably amorphized. That is, a negative electrode active material excellent in rapid charge and discharge performance can be suitably reproduced. Further, the charging may be performed once, or for example, it can be repeated two or more times with discharging in between.

[0026] The temperature at the time of performing the charging step S20 is not particularly limited, but is, for example, 100° C. or lower, preferably 80° C. or lower. Further, the temperature at the time of performing the charging step S20 is not particularly limited, but is, for example, 0° C. or higher, preferably 40° C. or higher, and more preferably 50° C. or higher.

[0027] (3) Discharging step S30 In the discharging step S30, the lithium-ion secondary battery that has undergone the charging step S20 is discharged. Here, the discharging step S30 is characterized in that the lithium-ion secondary battery is discharged at a discharge rate higher than the charging rate of the charging step S20.

[0028] FIG. 5 is a partially enlarged view schematically showing the surface layer of the negative electrode active material 46 and the state of lithium ions 60 according to an embodiment. FIG. 5(A) shows the state before performing the charging step S20. FIGS. 5(B) and 5(C) show the state during the charging step S20. FIG. 5(D) shows the state during the discharging step S30. In FIGS. 5(B) to 5(D), for convenience of explanation, the behavior of the lithium ions 60 is illustrated by white arrows. In FIG. 5, the negative electrode active material 46 is a carbon material. Before performing the charging step S20, lithium ions 60 are typically stabilized in a state (solvation) in which solvent molecules 62 constituting the electrolyte are coordinated in the electrolyte (see FIG. 5(A)). Here, as shown in FIG. 5(B), when the charging step S20 is performed on the lithium ion secondary battery 1, the lithium ions 60 in the electrolyte are inserted into the interlayer of the surface layer of the negative electrode active material 46. The inserted lithium ions 60, as shown in FIG. 5(C), are desolvated (the solvent molecules 62 are removed) and diffused into the layer of the negative electrode active material 46 as the charging step S20 progresses. As a result, the interlayer of the negative electrode active material 46 expands. Then, by performing the discharging step S30 with a discharging rate relatively higher than the charging rate of the charging step S20, the lithium ions 60 inserted into the interlayer of the negative electrode active material 46 are desorbed to the outside of the negative electrode active material 46 again as shown in FIG. 5(D). By the charging step S20 and the discharging step S30, the surface layer of the negative electrode active material 46 in which the insertion and desorption of the lithium ions 60 have occurred among the negative electrode active materials becomes amorphous (see FIG. 5(D)). By the surface layer of the negative electrode active material 46 becoming amorphous, the rapid charge-discharge performance of the negative electrode active material 46 is improved. And, by the recovery step S40 described later, the negative electrode active material 46 with improved rapid charge-discharge performance can be recovered.

[0029] In some preferred embodiments, it is preferable to perform the discharging step S30 at a discharging rate of 0.5C or more. The discharging rate of the discharging step S30 is preferably 0.5C or more, more preferably 0.8C or more, and even more preferably 1.0C or more. Thereby, a negative electrode active material excellent in rapid charge-discharge performance can be reproduced. The discharging rate of the discharging step S30 is not particularly limited, but is 20C or less, preferably 10C or less.

[0030] In some preferred embodiments, in the discharging step S30, it is preferable to discharge until the state of charge (SOC) of the lithium-ion secondary battery 1 becomes 30% or less. Further, it is more preferable to discharge until the SOC of the lithium-ion secondary battery 1 becomes 20% or less, more preferably 10% or less, still more preferably 5% or less, and even more preferably until it becomes 0%. Further, the discharging may be performed once, or for example, it may be repeated two or more times with charging in between.

[0031] The difference between the discharging rate and the charging rate (discharging rate - charging rate) is preferably 0.4C or more, preferably 0.7C or more, preferably 0.8C or more, preferably 0.9C or more, and more preferably 3.9C or more. The larger the difference between the discharging rate and the charging rate, the more suitably the surface of the negative electrode active material can be made amorphous. Therefore, a negative electrode active material excellent in rapid charge and discharge performance can be reproduced. The upper limit of the difference between the discharging rate and the charging rate (discharging rate - charging rate) is not particularly limited, but can be, for example, 10C or less.

[0032] (4) Recovery step S40 In the recovery step S40, the negative electrode active material is recovered from the lithium-ion secondary battery that has undergone the charging step S20 and the discharging step S30. Since a conventionally known technique can be used for the method of recovering the negative electrode active material according to the recovery step S40, it is not particularly limited. The recovery step S40 may include, for example, as shown in FIG. 4, as sub-steps, a roasting step S41, a sorting step S43, an acid treatment step S45, and a magnetic separation step S47. This will be specifically described below.

[0033] (4-1) Roasting step S41 In the baking step S41, the lithium-ion secondary battery is baked at a predetermined temperature. By doing so, the liquid components (such as electrolytes) in the object to be recovered can be removed, and the resin components (such as binders and separators) can be carbonized. Also, by performing the baking step S41, the functions of the battery can be stopped. Regarding the method of the baking step S41, the techniques used in conventional recovery technologies can be used without particular limitation, and since it does not characterize the techniques disclosed herein, detailed description thereof is omitted.

[0034] (4-2) Sorting step S43 In the sorting step S43, each member included in the lithium-ion secondary battery 1 is sorted. As the method of the sorting step S43, conventionally known methods can be used, for example, sorting can be performed by sieving, visual inspection, etc.

[0035] In addition, in the sorting step S43, if necessary, a crushing treatment may be performed on the lithium-ion secondary battery 1. By doing so, the crushed material of the lithium-ion secondary battery 1 can be obtained, and the efficiency in sorting each member can be improved. For example, when the lithium-ion secondary battery 1 is the object to be recovered, it is advisable to crush the case 10 and the electrode body 20. This makes it easier to remove the case 10, the negative electrode core 42, and the positive electrode 30 from the lithium-ion secondary battery 1.

[0036] Although not limited thereto, the crushed material of the lithium-ion secondary battery 1 obtained by the above-described crushing treatment is sorted using a sieve. In this case, typically, on the sieve, as coarse particles, mainly metal components (such as Al and Cu) derived from the case 10, the positive electrode core 32, and the negative electrode core 42 remain, and as fine particles, a black mass from which the metal components (such as Al and Cu) are removed is obtained under the sieve. The black mass typically contains a negative electrode active material and a positive electrode active material (such as Ni and Co).

[0037] In addition, in the sorting step S43, the selected black mass can be further sorted by a flotation method. Thereby, components other than the negative electrode active material in the black mass (for example, Ni, Co, etc.) can be roughly removed. As the flotation method, known techniques such as the multi-oil flotation method, the surface flotation method, and the froth flotation method can be used without particular limitation.

[0038] (4-3) Acid treatment step S45 In the acid treatment step S45, the black mass obtained in the sorting step S43 is mixed with an acidic solution. Thereby, metal elements (such as Al, Cu, Ni, Co, etc.) in the black mass dissolve in the acidic solution, while the negative electrode active material does not dissolve in the acidic solution and remains as a residue. Note that depending on the composition of the component to be recovered and the composition of the acidic solution, some metal elements (such as Fe) in the black mass may not dissolve in the acidic solution and remain as a residue. That is, the residue obtained in the acid treatment step S45 may contain, for example, Fe in addition to the negative electrode active material. In this case, the metal elements in the residue can be removed, for example, by the magnetic separation step S47 described later. The procedure of the acid treatment step S45 can adopt a conventionally known procedure without particular limitation. As an example, the pH of the acid solution used in the acid treatment step S45 is preferably -1.5 to 1.5 (more preferably -0.5 to 0.5). Thereby, the metal components in the black mass can be preferably dissolved. Specific examples of the acid solution include inorganic acids such as sulfuric acid, nitric acid, hydrochloric acid, and phosphoric acid, and organic acids such as citric acid, ascorbic acid, oxalic acid, and acetic acid. Further, although not limited thereto, in the acid treatment step S45, a reducing agent such as hydrogen peroxide can be added to the acidic solution. Thereby, the metal elements in the black mass can be preferably dissolved in the acidic solution, and the treatment time of the acid treatment step S45 can be shortened.

[0039] By subjecting the acidic solution after the acid treatment step S45 and the residue to solid-liquid separation by means such as filtration, a residue containing the negative electrode active material can be obtained. As the method of solid-liquid separation, conventionally known means can be adopted without particular limitation. Note that the acidic solution after solid-liquid separation, although not limited thereto, for example, metal elements (such as Ni and Co) in the acidic solution can be recovered by a conventionally known treatment method.

[0040] (4-4) Magnetic separation step S47 In the magnetic separation step S47, the residue obtained in the acid treatment step S45 is subjected to separation by magnetic force. Thereby, the negative electrode active material as the non-magnetized material in the residue and the magnetized material (metal component) can be separated. That is, the negative electrode active material according to the present embodiment can be obtained by the magnetic separation step S47. The magnetic separation step S47 can adopt a conventionally known procedure without particular limitation.

[0041] As described above, the method for reproducing the negative electrode active material according to the present embodiment has been described. According to the reproduction method according to the present embodiment, which includes the preparation step S10, the charging step S20, the discharging step S30, and the recovery step S40, a negative electrode active material in which the surface of the negative electrode active material to be prepared (hereinafter referred to as "active material base material") is amorphized can be obtained. Therefore, according to the method for reproducing the negative electrode active material according to the present embodiment, a negative electrode active material excellent in rapid charge-discharge characteristics can be obtained.

[0042] 3. Recovered negative electrode active material (negative electrode active material obtained in the recovery step S40) The negative electrode active material obtained by the method for reproducing the negative electrode active material according to the present embodiment (hereinafter also referred to as "recovered negative electrode active material") is, as described above, a material in which the surface of the negative electrode active material to be prepared is amorphized by the charging step S20 and the discharging step S30. That is, it has characteristics different from those of the negative electrode active material contained in the lithium ion secondary battery 1 prepared in the preparation step S10. Hereinafter, the characteristics of the recovered negative electrode active material will be described.

[0043] The recovered negative electrode active material corresponds to the carbon material among the negative electrode active materials to be prepared. In other words, the recovered negative electrode active material is a carbon material whose surface has been made amorphous by the charging step S20 and the discharging step S30. The recovered negative electrode active material is a particulate material that can reversibly store and release charge carriers. Typically, a carbon material such as graphite, hard carbon, or soft carbon is used as the active material substrate. The graphite may be natural graphite or artificial graphite.

[0044] As an example of an index relating to the surface properties of the recovered negative electrode active material according to this embodiment, the G band intensity (I G ) versus D band intensity (I D ) ratio (I D / I G ) can be mentioned. D / I G The ratio is average information showing the crystallinity at a depth of nano-order from the surface of the negative electrode active material. G ) versus D band intensity (I D ) ratio (I D / I G The "Raman spectrum" can be measured using a known Raman spectrometer as follows. Specifically, the Raman spectrum of the negative electrode active material is measured using a laser with a wavelength of 532 nm. -1 The peak intensity near the D band intensity (I D ), 1590cm -1 The peak intensity near the G band intensity (I G ), respectively, and I D / I G It can be found by calculating the value of

[0045] In some preferred embodiments, the G band intensity (I G ) versus D band intensity (I D ) ratio (I D / I G) is preferably 0.38 or more. The G-band intensity (I G ) is an index typically indicating a regular graphite structure, and the D-band intensity (I D ) is an index typically indicating an irregular graphite structure. That is, the larger the ratio of I D / I G of the recovered negative electrode active material, the more suitably it can be said that the surface of the recovered negative electrode active material is amorphized. Therefore, the larger the ratio of I D / I G of the recovered negative electrode active material, the more improved the lithium ion acceptance capacity. In other words, the discharge capacity of the recovered negative electrode active material is improved and the rapid charge and discharge performance is good. The ratio of I D / I G of the recovered negative electrode active material is preferably 0.38 or more, more preferably 0.40 or more, still more preferably 0.42 or more, and even more preferably 0.47 or more. The upper limit of the ratio of I D / I G of the recovered negative electrode active material is not particularly limited, but can be, for example, 1.0 or less.

[0046] Further, as another example of the index regarding the surface properties of the recovered negative electrode active material according to the present embodiment, the interlayer distance d(002) of the recovered negative electrode active material based on the X-ray diffraction method (XRD: X-ray diffraction) can be mentioned. The interlayer distance d(002) of the negative electrode active material is average information indicating the crystallinity at a depth on the micron order from the surface of the negative electrode active material. Note that the "interlayer distance d(002) of the negative electrode active material" in this specification can be measured by a commercially available X-ray diffractometer using a CuKα ray source according to a known method.

[0047] In some preferred embodiments, it is preferable that the interlayer distance d(002) of the recovered negative electrode active material based on X-ray diffraction method (XRD) is 3.350 Å or more and 3.369 Å or less. In this example, the surface of the negative electrode active material is amorphized through the above-described charging step S20 and discharging step S30. Therefore, when the negative electrode active material is recovered, the interlayer distance d(002) becomes smaller. The smaller the interlayer distance d(002) of the recovered negative electrode active material is, the more suitably the surface of the recovered negative electrode active material is amorphized, and the easier it is for lithium ions to be inserted into or desorbed from the negative electrode active material. Thus, the rapid charge and discharge performance is good. The interlayer distance d(002) of the recovered negative electrode active material is preferably 3.369 Å or less, more preferably 3.361 Å or less, and still more preferably 3.359 Å or less. The interlayer distance d(002) of the recovered negative electrode active material is preferably 3.350 Å or more.

[0048] The average particle size (median diameter: D50) of the recovered negative electrode active material is not particularly limited. For example, it is 0.1 μm or more and 50 μm or less, preferably 1 μm or more and 25 μm or less. The average particle size (D50) of the recovered negative electrode active material can be obtained by, for example, the laser diffraction scattering method.

[0049] The above describes one embodiment of the technology disclosed herein. Note that the technology disclosed herein is not limited to the above-described embodiment, and includes other embodiments with various configurations changed.

[0050] <Use of the negative electrode active material> The negative electrode active material (recovered negative electrode active material) obtained by the reproduction method according to the present embodiment can be used, for example, as the negative electrode active material of a secondary battery, although not limited thereto. The secondary battery including the negative electrode active material obtained by the reproduction method according to the present embodiment can be used for various applications. For example, it can be suitably used as a power source (driving power source) for a motor mounted on a vehicle such as a passenger car or a truck. The type of the vehicle is not particularly limited, and examples thereof include a plug-in hybrid electric vehicle (PHEV), a hybrid electric vehicle (HEV), and a battery electric vehicle (BEV). The secondary battery can also be suitably used as an assembled battery in which a plurality of secondary batteries are arranged in a predetermined arrangement direction and a load is applied from the arrangement direction by a restraint mechanism. The shape of the secondary battery is not limited to a rectangular shape, and may be a coin type, a button type, a cylindrical type, or the like. Further, it can be configured as a secondary battery including a laminate case.

[0051] Hereinafter, examples related to the technology disclosed herein will be described, but it is not intended to limit the technology disclosed herein to those shown in such examples.

[0052] <Examination 1: Examination of charging conditions and discharging conditions> [Preparation of lithium ion secondary battery] (Example 1) First, a used lithium ion secondary battery in which an electrode body including a positive electrode, a negative electrode, and a separator and an electrolytic solution are housed inside a case was prepared. As the positive electrode, a positive electrode active material layer containing LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (lithium nickel cobalt manganese composite oxide) as a positive electrode active material was provided on the surface of an aluminum positive electrode core. As the negative electrode, a negative electrode active material layer containing natural graphite as a negative electrode active material was provided on the surface of a copper negative electrode core. As the above-mentioned electrolyte, a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of EC:DMC:EMC = 30:30:40 was prepared, and LiPF6 was dissolved as a supporting salt to a concentration of 1 mol / L.

[0053] [Charging process] For the lithium-ion secondary battery prepared above, under a temperature environment of 25°C, constant current constant voltage (CCCV) charging was performed at a charging rate of 0.1C until the state of charge (SOC) at the end of charging reached 100%.

[0054] [Discharging process] For the lithium-ion secondary battery that underwent the charging process above, under a temperature environment of 25°C, constant current constant voltage (CCCV) discharging was performed at a discharging rate of 1C until the state of charge (SOC) at the end of discharging reached 10%.

[0055] [Recovery of negative electrode active material] The lithium-ion secondary battery that underwent the discharging process above was disassembled, and the negative electrode was taken out. The negative electrode active material layer was peeled off from the negative electrode by washing with water. The negative electrode active material was recovered from the negative electrode active material layer and dried at 100°C to remove moisture. In this way, the powdery negative electrode active material was recovered.

[0056] (Examples 2 to 4) In Examples 2 to 4, lithium-ion secondary batteries having the same configuration as in Example 1 were prepared, and the charging process was performed at the charging rates shown in Table 1. Except for this, the charging process and the discharging process were carried out under the same conditions as in Example 1, and the negative electrode active materials according to Examples 2 to 4 were recovered.

[0057] (Examples 5 to 6) In Examples 5 and 6, lithium-ion secondary batteries having the same configuration as in Example 1 were prepared, and the charging process was performed until the SOC at the end of charging reached the values described in Table 1. Except for this, the charging process and the discharging process were carried out under the same conditions as in Example 1, and the negative electrode active materials according to Examples 5 and 6 were recovered.

[0058] (Examples 7 to 10) In Examples 7 to 10, a lithium ion secondary battery having the same configuration as that of Example 1 was prepared, and a discharging process was performed at the discharging rates shown in Table 1. The charging process and the discharging process were carried out under the same conditions as those in Example 1 except for this, and the negative electrode active materials according to Examples 7 to 10 were recovered.

[0059] (Examples 11 to 14) In Examples 11 to 14, a lithium ion secondary battery having the same configuration as that of Example 1 was prepared, and a discharging process was performed until the SOC at the end of discharging reached the numerical values described in Table 1. The charging process and the discharging process were carried out under the same conditions as those in Example 1 except for this, and the negative electrode active materials according to Examples 11 to 14 were recovered.

[0060] (Example 15) In Example 15, a lithium ion secondary battery having the same configuration as that of Example 1 was prepared. In Example 15, the charging process and the discharging process were not carried out, and the negative electrode active material was recovered as it was. Other than this, it was the same as Example 1.

[0061] [Evaluation of the interlayer distance d(002) of the recovered negative electrode active material] The recovered negative electrode active materials according to Examples 1 to 15 (hereinafter also referred to as "recovered negative electrode active materials") were evaluated. First, the interlayer distance d(002) of the recovered negative electrode active material by X-ray diffraction method (XRD) was evaluated. Specifically, XRD measurement was performed on the recovered negative electrode active material particles under the following conditions. The results are shown in Table 1. Measuring device: Smart Lab (manufactured by Rigaku Corporation) Measurement method: Wide-angle method X-ray source: CuKα ray Measurement range: 5 to 90° Voltage: 45 kV Current: 200 mA

[0062] [Evaluation of the Raman peak intensity ratio (I D / I G ) of the recovered negative electrode active material] Next, with respect to the recovered negative electrode active materials according to Examples 1 to 15, the Raman peak intensity ratio (I D / I GThe evaluation of ( ) was carried out. Specifically, using a commercially available microscopic Raman analyzer (manufactured by Thermo Fisher Scientific), Raman spectra were obtained under the following conditions. The peak intensity of the obtained Raman spectrum at 1470 cm -1 was designated as I D , and the peak intensity at 1570 cm -1 was designated as I G . Then, the Raman peak intensity ratio (I D / I G ) was calculated. The above Raman spectrum measurement was performed 10 times (n = 10), and the average value was calculated. The results are shown in Table 1. Objective lens: 50x Laser wavelength: 532 nm Laser output: 1 mV

[0063] [Fabrication of Evaluation Coin Cell] To evaluate the performance of the recovered anode active material, here, a coin cell was fabricated as a single-pole cell with the anode as the working electrode and metallic lithium as the counter electrode facing each other. First, an anode was fabricated using the recovered anode active material. Specifically, the recovered anode active material, styrene-butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener were mixed with ion-exchanged water as a solvent so that the mass ratio of the solid content was recovered anode active material: SBR: CMC = 98:1:1 to prepare an anode paste. Such an anode paste was applied to one side of a long sheet-shaped copper foil (thickness 10 μm) as an anode core (anode current collector), and after drying, it was pressed with a roll press to fabricate a sheet-shaped anode. The sheet-shaped anode was punched into the size of a coin cell, and the weight (g) of the punched anode was measured. From the weight of such an anode, the weight of the anode core was subtracted, and the result was multiplied by 0.98 (the ratio of the recovered anode active material to the entire anode active material layer) to obtain the weight (g) of the anode active material.

[0064] As the separator, a porous polyolefin sheet with a three-layer structure of PP / PE / PP and a thickness of 24 μm was used. In addition, a separator with a ceramic layer (thickness 4 μm) having alumina (Al2O3), boehmite, etc. applied to one side was used.

[0065] As the electrolyte, a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 30:30:40 was prepared, and LiPF6 as a supporting salt was dissolved therein at a concentration of 1.0 mol / L.

[0066] The negative electrode, separator, and metallic lithium were laminated in this order, impregnated with the electrolyte, and a coin cell was fabricated. At this time, they were laminated so that the ceramic layer of the separator faced the negative electrode.

[0067] [Evaluation of discharge specific capacity] Using the evaluation coin cells obtained above, the discharge specific capacities of the recovered negative electrode active materials according to Examples 1 to 15 were evaluated. Specifically, each evaluation coin cell was placed in an environment at 25°C and charged at a constant current up to 3 mV with respect to the lithium counter electrode at a current rate of 0.2C, and then charged at a constant voltage of 3 mV until the current rate reached 1 / 10C. Thereafter, each evaluation coin cell was discharged at a constant current up to 1.6 V with respect to the lithium electrode at a current value of 0.2C, and then discharged at a constant voltage of 1.6 V until the current rate reached 1 / 10C. In this way, the discharge capacity (mAh) at the time of 0.2C discharge was determined. From this discharge capacity and the weight (g) of the negative electrode active material, the discharge specific capacity (mAh / g) of the negative electrode active material was calculated. The results are shown in Table 1.

[0068] [Evaluation of rate performance] Next, using the above-described evaluation coin cell, the rate performance was evaluated as an index of the rapid charge and discharge performance of the recovered negative electrode active material according to Examples 1 to 15. Specifically, after the discharge specific capacity measurement, each evaluation coin cell was placed in an environment at 25°C and charged at a constant current up to 3 mV with respect to the lithium counter electrode at a current rate of 1C, and then charged at a constant voltage of 3 mV until the current rate reached 1 / 10C. Thereafter, each evaluation coin cell was discharged at a constant current up to 1.6 V with respect to the lithium electrode at a current value of 1C, and then discharged at a constant voltage of 1.6 V until the current rate reached 1 / 10C. In this way, the discharge capacity (mAh) at 1C discharge was determined. Using the discharge capacity (mAh) at 1C discharge and the discharge capacity (mAh) at 0.2C discharge measured during the evaluation of the above-described discharge specific capacity, the rate performance (%) was determined by the following formula (1). Note that the higher the value of the rate performance (%), the better the charge and discharge performance of the recovered negative electrode active material. The results are shown in Table 1. Rate performance (%) = (Discharge capacity (mAh) at 1C discharge / Discharge capacity (mAh) at 0.2C discharge) × 100 ··· Formula (1)

[0069]

Table 1

[0070] As shown in the results of Table 1, in Example 1, compared with Example 15 in which the negative electrode active material was recovered without performing the charging process and the discharging process, good results were obtained for I D / I G and the interlayer distance d(002), and good results were also obtained for both the discharge specific capacity and the rate performance. Further, from the results of Examples 2 to 4 and Examples 7 to 10, when the discharge rate was relatively higher than the charge rate and the charge rate and the discharge rate were made different, good results were obtained for I D / I G and the interlayer distance d(002), and it was also found that good results were obtained for both the discharge specific capacity and the rate performance. Also, for Examples 5 and 6 in which the charge termination SOC was different from that of Example 1, and Examples 11 to 14 in which the discharge termination SOC was different, I D / I GGood results were obtained for the interlayer distance d(002), and good results were also obtained for both the discharge specific capacity and the rate performance.

[0071] <Examination Example 2: Examination of the positive and negative electrode capacity ratio in a lithium-ion secondary battery to be prepared> (Examples 16 to 21) Here, the positive and negative electrode capacity ratio in the lithium-ion secondary battery to be prepared was examined. Specifically, in Examples 16 to 21, lithium-ion secondary batteries having the same configuration as in Example 1 were prepared except that the positive and negative electrode capacity ratio was as shown in Table 2. Then, for the lithium-ion secondary batteries according to Examples 16 to 21, a charging process and a discharging process were carried out under the same conditions as in Example 1, and the negative electrode active material according to Examples 16 to 21 was recovered (hereinafter referred to as "recovered negative electrode active material"). The recovered negative electrode active materials according to Examples 16 to 21 were evaluated in the same manner as in Examination Example 1. The results are shown in Table 2. Thereafter, coin cells for evaluation according to Examples 16 to 21 were produced and evaluated in the same manner as in Examination Example 1. The results are shown in Table 2.

[0072]

Table 2

[0073] As shown in the results of Table 2, for any of Examples 16 to 21 where the positive and negative electrode capacity ratio was 1.0 or more, better results were obtained for both the discharge specific capacity and the rate performance compared to Example 15 where the negative electrode active material was recovered without performing the charging process and the discharging process.

[0074] As described above, specific aspects of the technology disclosed herein include those described in the following items. Item 1: A method for reproducing a negative electrode active material from a used lithium-ion secondary battery, a preparation step of preparing a used lithium-ion secondary battery including a positive electrode, a negative electrode including a negative electrode active material containing a carbon material, and an electrolytic solution, a charging step of charging the lithium-ion secondary battery, A step of discharging the lithium-ion secondary battery after the charging step, the discharging step being a discharging step at a discharging rate higher than the charging rate of the charging step; A recovery step of recovering the negative electrode active material containing the carbon material from the negative electrode with respect to the lithium-ion secondary battery after the discharging step, A method for reproducing a negative electrode active material. Item 2: The positive and negative electrode capacity ratio (negative electrode capacity / positive electrode capacity) of the lithium-ion secondary battery prepared in the preparation step is 1.0 or more. The method for reproducing a negative electrode active material according to Item 1. Item 3: The charging step is performed at a charging rate of 0.5C or less, and the method for reproducing a negative electrode active material according to Item 1 or 2. Item 4: The charging step is performed until the state of charge (SOC) of the lithium-ion secondary battery reaches 50% or more, and the method for reproducing a negative electrode active material according to any one of Items 1 to 3. Item 5: The discharging step is performed at a discharging rate of 0.5C or more, and the method for reproducing a negative electrode active material according to any one of Items 1 to 4. Item 6: The discharging step is performed until the SOC of the lithium-ion secondary battery reaches 30% or less, and the method for reproducing a negative electrode active material according to any one of Items 1 to 5. Item 7: The ratio (ID / IG) of the D band intensity (ID) to the G band intensity (IG) of the negative electrode active material recovered in the recovery step determined by Raman spectroscopy is 0.38 or more, and the method for reproducing a negative electrode active material according to any one of Items 1 to 6. Item 8: The interlayer distance d(002) of the negative electrode active material recovered in the recovery step based on X-ray diffraction method is 3.350 Å or more and 3.369 Å or less, and the method for reproducing a negative electrode active material according to Item 7.

Explanation of symbols

[0075] 1 Lithium-ion secondary battery 10 Case 12 Positive electrode terminal 14 Negative electrode terminal 16 Liquid injection hole 18 Sealing member 20 Electrode body 30 Positive electrode 32 Positive electrode core 34 Positive electrode active material layer 40 Negative electrode 42 Negative electrode core 44 Negative electrode active material layer 46 Negative electrode active material 50 Separator 60 Lithium ion 62 Solvent molecule

Claims

1. A method for reproducing a negative electrode active material from a used lithium-ion secondary battery, comprising: a preparation step of preparing a used lithium-ion secondary battery including a positive electrode, a negative electrode including a carbon material as a negative electrode active material, and an electrolytic solution; a charging step of charging the lithium-ion secondary battery; a discharging step of discharging the lithium-ion secondary battery after the charging step, the discharging step being performed at a discharging rate higher than the charging rate of the charging step; a recovery step of recovering the negative electrode active material including the carbon material from the negative electrode with respect to the lithium-ion secondary battery after the discharging step. A method for reproducing a negative electrode active material.

2. The positive-negative electrode capacity ratio (negative electrode capacity / positive electrode capacity) of the lithium-ion secondary battery prepared in the preparation step is 1.0 or more. The method for reproducing a negative electrode active material according to Claim 1.

3. The charging step is performed at a charging rate of 0.5 C or less.

4. The charging step is performed until the state of charge (SOC) of the lithium-ion secondary battery reaches 50% or more.

5. The discharging step is performed at a discharging rate of 0.5 C or more.

6. The discharging step is performed until the SOC of the lithium-ion secondary battery reaches 30% or less.

7. The ratio (I Ｇ / I Ｄ ) of the D-band intensity (I Ｄ ) to the G-band intensity (I Ｇ ) of the negative electrode active material recovered in the recovery step obtained by Raman spectroscopy is 0.38 or more. The method for reproducing the negative electrode active material according to claim 1 or 2.

8. The interlayer distance d(002) of the negative electrode active material recovered in the recovery step based on the X-ray diffraction method is 3.350 Å or more and 3.369 Å or less.

Images