Method for reproducing negative electrode active material, negative electrode material, and secondary battery

Abstract

The present disclosure relates to a method for reproducing a negative electrode active material having improved cycle characteristics from a secondary battery. The technique disclosed herein is a method for reproducing a negative electrode active material, comprising: a preparation step (S10) in which a secondary battery (1) comprising a positive electrode (30), a negative electrode (40), an electrolyte solution, and a phosphorus-containing compound is prepared; a charging step (S20) in which the secondary battery (1) is charged in an environment at a temperature equal to or higher than the decomposition temperature of the phosphorus-containing compound; and a recovery step (S30) for recovering the negative electrode active material from the negative electrode (40) for the secondary battery (1) after the charging step (S20).

Description

Technical Field

[0001] The present disclosure relates to a method for reproducing a negative electrode active material, a negative electrode material, and a secondary battery. Background Art

[0002] Secondary batteries are suitable for use in various applications, including driving power supplies installed in vehicles such as hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV), and electric vehicles (BEV), and their demand is rapidly expanding. In response to this demand, the demand for technologies related to the reproduction of negative electrode active materials by secondary batteries is also rising. As a technology related to the reproduction of negative electrode active materials, for example, the following technology is disclosed in Japanese Patent Publication No. 2006-228509: the negative electrode plate is taken out from the secondary battery, the negative electrode plate is washed with a liquid containing water, and the negative electrode plate is separated into a negative electrode composite material containing a negative electrode active material and a binder and a current collector substrate, and the above-mentioned negative electrode composite material is mixed in a solvent that can dissolve or disperse the binder to prepare a negative electrode paste, which is applied to the negative electrode collector.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: Japanese Patent Publication No. 2006-228509 Summary of the invention

[0006] Problems to be solved by the invention

[0007] In recent years, there is a need to improve the cycle characteristics of negative electrode active materials. The cycle characteristics are not considered in the negative electrode active materials recovered by the prior art disclosed in Japanese Patent Publication No. 2006-228509. Therefore, the requirements for the above-mentioned cycle characteristics cannot be fully met.

[0008] The technology disclosed herein is completed in view of the above actual situation, and relates to a method for reproducing a negative electrode active material with improved cycle characteristics from a secondary battery. In addition, as another purpose, it relates to a negative electrode active material with improved cycle characteristics and a secondary battery having the negative electrode active material.

[0009] Means for solving problems

[0010] The technology disclosed herein is a method for reproducing negative electrode active materials, comprising: a preparation process, wherein a secondary battery including a positive electrode, a negative electrode, an electrolyte and a phosphorus-containing compound is prepared; a charging process, wherein the secondary battery is charged in a temperature environment above the decomposition temperature of the phosphorus-containing compound; and a recovery process, wherein, for the secondary battery after the charging process, the negative electrode active material is recovered from the negative electrode.

[0011] According to this configuration, the phosphorus-containing compound contained in the secondary battery is decomposed by charging the secondary battery under predetermined conditions. Thus, the negative electrode active material having the decomposition product of the phosphorus-containing compound disposed on the surface can be recovered. Therefore, the negative electrode active material with improved cycle characteristics can be reproduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 This is a cross-sectional view schematically showing the internal structure of a secondary battery according to one embodiment.

[0013] Figure 2 It is a perspective view schematically showing an electrode body of a secondary battery according to one embodiment.

[0014] Figure 3 A flowchart for explaining a reproduction method according to one embodiment.

[0015] Figure 4 This is a flowchart for explaining a sub-process of a preparation process according to one embodiment.

[0016] Figure 5 This is a flow chart for explaining a sub-process of a recovery process according to one embodiment.

[0017] Figure 6 It is a schematic diagram showing a negative electrode active material (recovered negative electrode active material) obtained by the negative electrode active material reproduction method according to one embodiment.

[0018] Figure 7 This is a surface SEM-EDS image of the recovered negative electrode active material according to one embodiment.

[0019] Figure 8 It is a graph showing the relationship between the number of cycles and the capacity maintenance rate. DETAILED DESCRIPTION

[0020] Hereinafter, the embodiments of the technology disclosed herein will be described with reference to the accompanying drawings. In addition, matters other than those specifically mentioned in this specification, i.e., matters necessary for the implementation of the technology disclosed herein, can be understood as design matters for those skilled in the art based on the prior art in the field. The technology disclosed herein can be implemented based on the contents disclosed in this specification and the technical common sense in the field.

[0021] Furthermore, in the following drawings, the same figure marks are marked on the components and parts that play the same role, and repeated descriptions are sometimes omitted or simplified. In addition, in this specification, the notation "A to B" indicating the range and the meaning of "above A and below B" both include the meanings of "preferably larger than A" and "preferably smaller than B". Furthermore, in this specification, the so-called "secondary battery" refers to a storage device that can be repeatedly charged and discharged, and is a term that includes storage elements such as so-called storage batteries and double-layer capacitors. In addition, in this specification, the so-called "lithium-ion secondary battery" refers to a secondary battery that uses lithium ions as charge carriers and uses the movement of charges accompanied by lithium ions between positive and negative electrodes to achieve charging and discharging.

[0022] 1. Prepare the Object

[0023] In the method for reproducing the negative electrode active material according to the present embodiment, a predetermined secondary battery is prepared to reproduce the negative electrode active material. As an example of the preparation object here, a used secondary battery can be cited. The secondary battery is specifically described below. Figure 1 It is a longitudinal cross-sectional view schematically showing the internal structure of a secondary battery. Figure 2 To illustrate Figure 1 A three-dimensional diagram of an electrode body of a secondary battery is shown. Figure 1 As shown, the secondary battery 1 includes a housing 10, an electrode body 20, an electrolyte (not shown), and a phosphorus-containing compound (not shown).

[0024] (1) Housing

[0025] The outer shell 10 is a box-shaped container. The electrode body 20 and the electrolyte are accommodated inside the outer shell 10. For example, a metal material (aluminum (Al) etc.) having a certain strength is used in the outer shell 10. An injection hole 16 may be provided in the outer shell 10. The injection hole 16 is a hole for injecting the electrolyte. After the electrolyte is injected, the injection hole 16 is sealed with a sealing member 18. In addition, a positive terminal 12 and a negative terminal 14 are installed in the outer shell 10. The positive terminal 12 and the negative terminal 14 are connected to the electrode body 20 inside the outer shell 10. Specifically, the positive terminal 12 is connected to the positive plate 30 (refer to Figure 2 ) is connected. 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 plate 40 of the electrode body 20. Copper (Cu) or the like is used for the negative electrode terminal 14.

[0026] (2) Electrode body

[0027] The electrode body 20 is a power generation element of the secondary battery 1. Figure 2 As shown, the electrode body 20 includes a positive electrode plate 30, a negative electrode plate 40 and a separator 50. Figure 2The electrode body 20 shown is a wound electrode body. The wound electrode body is made by stacking a positive electrode plate 30, a negative electrode plate 40 and a separator 50 to form a long strip-shaped laminate, and winding the laminate. However, there is no particular limitation on the structure of the electrode body 20, and it can be other structures known in the past (laminated electrode body, etc.). In addition, the positive electrode plate 30 is an example of the "positive electrode" disclosed herein, and the negative electrode plate 40 is an example of the "negative electrode" disclosed herein.

[0028] The positive electrode plate 30 includes: a positive electrode core 32 which is a metal foil having conductivity; 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. In addition, the positive electrode active material layer 34 is a composite 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 absorb and release charge carriers. Examples of the positive electrode active material include lithium nickel composite oxides (such as LiNiO2, etc.), lithium cobalt composite oxides (such as LiCoO2, etc.), lithium nickel cobalt manganese composite oxides (such as LiNi 1 / 3 Co 1 / 3Mn 1 / 3 O2, etc.), lithium nickel cobalt aluminum composite oxides (such as LiNi 0.8 Co 0.15 Al 0.5 O2, etc.), lithium manganese composite oxides (such as LiMn2O4, etc.), lithium nickel manganese composite oxides (such as LiNi 0.5 Mn 1.5 Lithium transition metal composite oxides such as LiO4, etc.; lithium transition metal phosphate compounds (such as LiFePO4, etc.). In addition, as conductive materials, carbon materials such as acetylene black and graphite can be listed. In addition, as binders, resin materials such as polyvinylidene fluoride (PVdF) can be listed.

[0029] On the other hand, the negative electrode plate 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. For the negative electrode core 42, copper (Cu) or the like is used. In addition, the negative electrode active material layer 44 is a composite layer containing a negative electrode active material, a binder, a thickener, etc. The negative electrode active material is a particulate material that can reversibly absorb and release charge carriers. As an example of the negative electrode active material to be prepared, carbon materials such as graphite, hard carbon, and soft carbon can be listed. The graphite may be natural graphite or artificial graphite. In addition, as the negative electrode active material, lithium titanate (LTO), silicon carbide, a composite containing carbon and silicon (Si-C composite), silicon oxide (SiO X ) etc. As the binder, resin materials such as styrene butadiene rubber (SBR) can be cited. As the tackifier, resin materials such as carboxymethyl cellulose (CMC) can be cited.

[0030] In addition, the separator 50 is an insulating sheet between the positive electrode plate 30 and the negative electrode plate 40. For the separator 50, for example, a resin material such as polyethylene (PE), polypropylene (PP), polyester, cellulose, polyamide, etc. is used. In addition, a heat-resistant layer containing an inorganic filler can be formed on the surface of the separator 50. Examples of the inorganic filler include inorganic oxides such as aluminum oxide, magnesium oxide, silicon oxide, and titanium oxide, nitrides such as aluminum nitride and silicon nitride, metal hydroxides such as calcium hydroxide, magnesium hydroxide, and aluminum hydroxide, mica, talc, boehmite, zeolite, apatite, and clay minerals such as kaolin.

[0031] (3) Electrolyte

[0032] The electrolyte is present between the positive plate 30 and the negative plate 40. Thus, the charge carrier can be moved between the positive plate 30 and the negative plate 40. As an example of the electrolyte, non-aqueous electrolyte, gel electrolyte, etc. can be listed. In addition, the electrolyte can use the electrolyte that can be used in the secondary battery without particular restrictions. The electrolyte typically contains a solvent and a supporting salt. As a solvent, various non-aqueous solvents used in such secondary batteries are used, such as carbonate-based non-aqueous solvents such as ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). As a supporting salt, according to the type of secondary battery, for example, lithium salts, sodium salts, magnesium salts, etc. (for example, lithium salts in lithium ion secondary batteries) can be used. As a lithium salt used as a supporting salt, for example, fluorine-containing lithium salts such as LiPF6 and LiBF4, LiClO4, etc. can be listed. There is no particular limitation on the concentration of the supporting salt, preferably 0.7 mol / L or more and 1.3 mol / L or less. The electrolyte solution may contain components other than the above-mentioned solvent and supporting salt, for example, various additives such as a gas generating agent, a film forming agent, a dispersant, and a thickening agent, as long as the effect of the present technology is not significantly impaired.

[0033] (4) Phosphorus-containing compounds

[0034] The secondary battery 1 as a preparation object involved in the present embodiment is characterized in that it contains a phosphorus-containing compound. Specifically, as will be described later, by making the secondary battery 1 containing the phosphorus-containing compound as the preparation object, it is possible to reproduce the negative electrode active material with improved cycle characteristics. The phosphorus-containing compound can be contained as an electrolyte, for example, or it can be contained independently of the electrolyte. In the case of containing a phosphorus-containing compound as an electrolyte, for example, the phosphorus-containing compound can also serve as a supporting salt. For example, phosphorus-containing compounds can include fluorophosphates. The fluorophosphate is preferably an alkali metal salt (fluorophosphate of an alkali metal). As the alkali metal, Li, Na and K are preferred, and Li is more preferred. As alkali metals, as fluorophosphates of alkali metals containing Li, Li2PO3F ((monofluorophosphate), LiDFP (difluorophosphate), LiPF6 (hexafluorophosphate), LiPFO (difluorobis(oxalate)phosphate), LiTFOP (tetrafluoro(oxalate)phosphate)) can be cited. In some preferred embodiments, the secondary battery 1 preferably includes LiPF6 as a phosphorus-containing compound. LiPF6 has the property of being easily decomposed in a relatively high temperature environment among phosphorus-containing compounds. Therefore, by including LiPF6 as a phosphorus-containing compound, a negative electrode active material with improved cycle characteristics can be more appropriately reproduced.

[0035] The content of the phosphorus-containing compound is not particularly limited, but is preferably 0.01 wt% or more, more preferably 0.1 wt% or more, when the total electrolyte is 100 wt%. Thus, in the charging step S20 described later, the phosphorus-containing oxide can be appropriately attached to the surface of the negative electrode active material. On the other hand, the amount of the phosphorus-containing compound added is not particularly limited, but is preferably 20 wt% or less, more preferably 16 wt% or less, when the total electrolyte is 100 wt%.

[0036] As an example of the preparation object in the reproduction method involved in this embodiment, the secondary battery 1 is described above. However, the reproduction method disclosed herein is not limited to the method using the secondary battery 1 of the above-mentioned structure as the preparation object. That is, the preparation object of the reproduction method disclosed herein may include a positive electrode, a negative electrode, an electrolyte, and a phosphorus-containing compound, and is not particularly limited to a specific structure.

[0037] 2. Reproduction method of negative electrode active material

[0038] Next, a method for reproducing the negative electrode active material according to this embodiment will be described. Figure 3 A flowchart for explaining a reproduction method according to one embodiment. Figure 4 This is a flowchart for explaining the sub-steps of the preparation step S10 according to one embodiment. Figure 5 This is a flowchart for explaining the sub-processes of the recovery process S30 according to one embodiment.

[0039] like Figure 3 As shown, the negative electrode active material reproduction method according to the present embodiment includes a preparation step S10, a charging step S20, and a recovery step S30. In addition, the reproduction method disclosed herein may further include other steps at any stage, and the other processes may be the same as before. Each step is described below.

[0040] (1) Preparation step S10

[0041] In the preparation step, a secondary battery including a positive electrode, a negative electrode, an electrolyte and a phosphorus-containing compound is prepared. Although not particularly limited, the preparation step S10 may be as follows: Figure 4 As shown in FIG. 1 , the sub-processes include a battery preparation process S11 and an adding process S12 .

[0042] (1-1) Battery Preparation Step S11

[0043] In the battery preparation step S11, a secondary battery for reproducing the negative electrode active material is prepared. The secondary battery prepared here includes a positive electrode, a negative electrode and an electrolyte. The secondary battery may or may not include a phosphorus-containing compound at the time of the battery preparation step S11. In addition, the details of the secondary battery have been described, so repeated description is omitted.

[0044] (1-2) Adding step S12

[0045] In some preferred embodiments, the preparation step S10 may include an adding step S12 as a sub-step. In the adding step S12, a phosphorus-containing compound is added to the secondary battery 1 prepared in the battery preparation step S11.

[0046] When performing the adding step S12, the phosphorus-containing compound to be added can be the same phosphorus-containing compound as the phosphorus-containing compound prepared above. The adding step S12 can be performed on a secondary battery that does not contain a phosphorus-containing compound or on a secondary battery 1 that already contains a phosphorus-containing compound.

[0047] In other words, the preparation step S10 includes: adding a phosphorus-containing compound to a secondary battery that does not contain a phosphorus-containing compound; or further adding a phosphorus-containing compound to a secondary battery that contains a phosphorus-containing compound.

[0048] There is no particular limitation on the method for adding the phosphorus-containing compound in the adding step S12. For example, the phosphorus-containing compound can be added from the injection hole 16. In addition, it is not limited thereto, and it is preferred that the secondary battery be discharged to the discharge termination voltage before the adding step S12. Thus, the adding step S12 can be performed more safely. In addition, the adding step S12 is not necessary and can be omitted. For example, when the secondary battery contains the phosphorus-containing compound at the time of preparation in the battery preparation step S11, the adding step S12 can be omitted.

[0049] (2) Charging step S20

[0050] In the charging step S20, the secondary battery prepared in the preparation step S10 is charged. In the charging step S20, it is characterized in that the charging is performed in a temperature environment above the decomposition temperature of the phosphorus-containing compound contained in the secondary battery 1. As a result, the phosphorus-containing compound is decomposed, and the decomposition product of the phosphorus-containing compound (typically, phosphorus-containing oxide) is attached to the surface of the negative electrode active material to be prepared.

[0051] The temperature when performing the charging step S20 is preferably 60°C or higher, more preferably 80°C or higher. Thus, the phosphorus-containing compound contained in the secondary battery 1 is suitably decomposed. Therefore, the decomposition product of the phosphorus-containing compound (typically, phosphorus-containing oxide) is suitably attached to the surface of the negative electrode active material to be prepared. The temperature when performing the charging step S20 is not particularly limited, for example, it is 100°C or lower, preferably 90°C or lower.

[0052] In the charging step S20, from the viewpoint of decomposing the phosphorus-containing compound and ensuring the charge amount of the decomposition product of the phosphorus-containing compound (typically, phosphorus-containing oxide) for attaching the decomposition product of the phosphorus-containing compound to the surface of the negative electrode active material to be prepared, it is preferred to charge the secondary battery 1 to a state of charge (SOC: State of Charge) of 50% or more. In addition, it is more preferred to charge until the SOC of the secondary battery 1 becomes 80% or more, and it is further preferred to charge until it becomes 100% (rated voltage). In addition, charging may be performed once, for example, it may be repeated twice or more with discharging in between.

[0053] The charging rate of the charging step S20 is preferably 0.1C or less, and more preferably 0.05C or less. Thus, the decomposition product of the phosphorus-containing compound (typically, phosphorus-containing oxide) is suitably attached to the surface of the negative electrode active material to be prepared. The charging rate of the charging step S20 is not particularly limited, and is, for example, 0.001C or more, preferably 0.01C or more.

[0054] (3) Recovery process S30

[0055] In the recovery step S30, the negative electrode active material is recovered from the secondary battery after the charging step S20. The recovery method of the negative electrode active material involved in the recovery step S30 can use a conventionally known technique, and therefore is not particularly limited. Figure 5 As shown, as sub-processes, a discharge process S31, a calcination process S33, a screening process S35, an acid treatment process S37, and a magnetic screening process S39 may be included.

[0056] (3-1) Discharging Step S31

[0057] In the discharge step S31, the secondary battery is discharged until the discharge termination voltage. Thus, the function as a battery can be stopped. For the method of the discharge step S31, the technology used in the previous recycling technology can be used without particular restrictions. Since it is not a feature of the technology disclosed herein, a detailed description is omitted. However, the discharge step S31 is not necessary and can be omitted. For example, the function as a battery can also be stopped by implementing the roasting step S33 described later.

[0058] (3-2) Calcination Step S33

[0059] In the roasting step S33, the secondary battery is roasted at a predetermined temperature. Thus, the liquid components (electrolyte, etc.) in the recycling object can be removed and the resin components (binder, separator, etc.) can be carbonized. In addition, by implementing the roasting step S33, the function as a battery can be stopped. For the method of the roasting step S33, the technology used in the previous recycling technology can be used without special restrictions. Since it is not a feature of the technology disclosed herein, a detailed description is omitted.

[0060] (3-3) Screening step S35

[0061] In the screening step S35, each component included in the secondary battery 1 is screened. The screening step S35 can be performed by a conventionally known method, and for example, screening using a sieve or visual inspection can be performed.

[0062] Furthermore, in the screening step S35, the secondary battery 1 may be crushed as needed. Thus, the crushed secondary battery 1 can be obtained, and the efficiency of screening each component can be improved. For example, when the secondary battery 1 is a recycling object, the outer shell 10 and the electrode body 20 can be crushed. Thus, the outer shell 10, the negative electrode core 42 and the positive electrode plate 30 can be easily removed from the secondary battery 1.

[0063] Without limitation to this, the crushed product of the secondary battery 1 obtained by the above-mentioned crushing process is screened using a sieve. In this case, typically, on the sieve, as coarse particles, mainly metal components (Al, Cu, etc.) from the outer shell 10, the positive electrode core 32, and the negative electrode core 42 remain, and under the sieve, as fine particles, black lumps are obtained from which the metal components (Al, Cu, etc.) are substantially removed. The black lumps typically contain negative electrode active materials and positive electrode active materials (such as Ni, Co, etc.).

[0064] In addition, in the screening step S35, the screened black agglomerates can be further screened using a flotation separation method. In this way, components other than the negative electrode active material (such as Ni, Co, etc.) in the black agglomerates can be substantially removed. The flotation separation method can use well-known techniques such as multi-oil flotation method, water surface flotation method, and foam flotation method without particular limitation.

[0065] (3-4) Acid Treatment Step S37

[0066] In the acid treatment step S37, the black lumps screened in the screening step S35 are mixed with an acidic solution. As a result, the metal elements (Al, Cu, Ni, Co, etc.) in the black lumps are dissolved in the acidic solution, while the negative electrode active material is not dissolved in the acidic solution and remains as a residue. In addition, depending on the composition of the recovery object, the composition of the acidic solution, etc., some of the metal elements (Fe, etc.) in the black lumps are sometimes not dissolved in the acidic solution and remain as a residue. That is, the residue obtained in the acid treatment step S37 may contain, for example, Fe, etc. in addition to the negative electrode active material. In this case, for example, the metal elements in the residue can be removed by the magnetic screening step S39 described later. The steps of the acid treatment step S37 can adopt the steps known in the past without particular restrictions. As an example, the pH of the acid solution used in the acid treatment step S37 is preferably -1.5 to 1.5 (more preferably -0.5 to 0.5). As a result, the metal components in the black lumps can be suitably dissolved. In addition, 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. In addition, the present invention is not limited thereto, and in the acid treatment step S37, a reducing agent such as hydrogen peroxide can be added in addition to the acid solution. Thus, the metal elements in the black mass are suitably dissolved in the acid solution, and the treatment time of the acid treatment step S37 can be shortened.

[0067] The acidic solution and the residue after the acid treatment step S37 are subjected to solid-liquid separation by filtering or the like, thereby obtaining a residue containing the negative electrode active material. The solid-liquid separation method can be adopted by conventionally known means without particular limitation. Furthermore, the acidic solution after the solid-liquid separation is not limited thereto, and for example, the metal elements (Ni, Co, etc.) in the acidic solution can be recovered by conventionally known treatment methods.

[0068] (3-5) Magnetic screening step S39

[0069] In the magnetic screening step S39, the residue obtained in the acid treatment step S37 is screened by magnetic force. Thus, the negative electrode active material and the magnetized material (metal component) as non-magnetized materials in the residue can be screened. That is, the negative electrode active material involved in the present embodiment can be obtained by using the magnetic screening step S39. The magnetic screening step S39 can adopt the conventionally known steps without particular limitation.

[0070] The above is an explanation of the reproduction method of the negative electrode active material involved in the present embodiment. As described above, it includes a preparation step S10, a charging step S20, and a recovery step S30. According to the reproduction method involved in the present embodiment, a negative electrode active material having a decomposition product of a phosphorus-containing compound (typically, a phosphorus-containing oxide) attached to the surface of the negative electrode active material (hereinafter referred to as "active material substrate") as the preparation object can be obtained. Therefore, according to the reproduction method of the negative electrode active material involved in the present embodiment, a negative electrode active material with excellent cycle characteristics can be obtained.

[0071] 3. Recycling of negative electrode active materials

[0072] As for the negative electrode active material obtained by the negative electrode active material reproduction method according to the present embodiment (hereinafter also referred to as "recovered negative electrode active material"), as described above, phosphorus-containing oxide is attached to the surface of the negative electrode active material (hereinafter referred to as "active material substrate") to be prepared. That is, it is different from the negative electrode active material contained in the secondary battery 1 prepared in the preparation step S10. The features of the recovered negative electrode active material 80 are described in detail below. Figure 6 Schematic diagram showing negative electrode active material (recovered negative electrode active material 80) obtained by a negative electrode active material reproduction method according to an embodiment. Figure 6 As shown, the recovered negative electrode active material 80 includes an active material substrate 82 and a phosphorus-containing oxide 84. The recovered negative electrode active material 80 is an example of a "negative electrode active material including a carbon material and a phosphorus-containing oxide disposed on the surface of the carbon material".

[0073] The active material substrate 82 corresponds to the negative electrode active material to be prepared. That is, the active material substrate 82 is a particulate material that can reversibly absorb and release charge carriers. The active material substrate typically uses a carbon material such as graphite, hard carbon, and soft carbon. The graphite may be natural graphite or artificial graphite.

[0074] The average particle size (median diameter: D50) of the active material base material 82 is not particularly limited, and is, for example, 0.1 μm to 50 μm, preferably 1 μm to 25 μm. The average particle size (D50) of the active material base material 82 can be determined, for example, by laser diffraction scattering.

[0075] As for the phosphorus-containing oxide 84, the phosphorus-containing compound in the secondary battery 1 is decomposed through the charging step S20, and is arranged on the surface of the active material substrate 82 (the surface of the negative electrode active material to be prepared) as the phosphorus-containing oxide 84. The phosphorus-containing oxide 84 has the function of a film called SEI film (Sol id Electrolyte Interface). In detail, by arranging the phosphorus-containing oxide 84 on the surface of the active material substrate 82, the recovered negative electrode active material 80 is stabilized and the decomposition of the electrolyte is suppressed. Therefore, according to the reproduction method involved in this embodiment, a negative electrode active material with high cycle characteristics can be obtained.

[0076] The phosphorus-containing oxide 84 is disposed on the surface of the active material substrate 82, and can be measured by, for example, a scanning electron microscope (SEM: scanning electron microscope) and energy dispersive X-ray spectroscopy (EDS: energy dispersive X-ray spectroscopy) (SEM-EDS). That is, the existence state of the phosphorus-containing oxide 84 in the recovered negative electrode active material 80 can be grasped by SEM-EDS. Figure 7 This is a surface SEM-EDS image of the recovered negative electrode active material according to one embodiment. Figure 7 (A) shows the surface SEM image of the recovered negative electrode active material. Figure 7 (B) shows the mapping of oxygen (O), Figure 7 (C) shows the mapping of phosphorus (P). Figure 7 The light-colored part (bright part) of (B) indicates the location of the O atom. Figure 7 The light-colored part (bright part) of (C) indicates the location of the P atom. Figure 7 As shown in (A) to (C), it can be seen that P atoms and O atoms exist on the surface of the recovered negative electrode active material. Thus, it can be confirmed that the phosphorus-containing oxide is arranged on the surface of the active material substrate.

[0077] Elements other than P and O may be disposed on the surface of the recovered negative electrode active material 80 within a range that does not inhibit the effects of the present disclosure. For example, elements such as silicon (Si) and sulfur (S) may be disposed.

[0078] In the recovered negative electrode active material 80, the average value of the mass ratio of the phosphorus (P) element relative to the total mass of the element detected by the element mapping obtained by SEM-EDS is preferably 0.2 mass% or more, more preferably 0.4 mass% or more, further preferably 0.6 mass% or more, and further preferably 1.0 mass% or more. The higher the average value of the mass ratio of the phosphorus (P) element, the more suitable it is to configure the phosphorus-containing oxide 84 on the surface of the active material substrate 82. In other words, the higher the average value of the mass ratio of the phosphorus (P) element, the more preferably the cycle characteristics of the recovered negative electrode active material 80 are improved. Furthermore, in the recovered negative electrode active material 80, the upper limit of the average value of the mass ratio of the phosphorus (P) element relative to the total mass of the element detected by the element mapping obtained by SEM-EDS is not particularly limited, for example, it can be 10 mass% or less, preferably 5 mass% or less.

[0079] It should be noted that the "average value of the mass ratio of the phosphorus (P) element relative to the total mass of the element detected by the element mapping obtained by SEM-EDS" in this specification can be determined by the following steps. First, obtain an SEM image of the surface of the recovered negative electrode active material 80. At this time, the SEM image (video) is obtained by adjusting the field of view so that one particle of the recovered negative electrode active material 80 enters the photographic area as a whole. Next, by performing EDS analysis on the above-mentioned SEM image, the mass ratio (mass %) of each element when the total mass of the detected elements is set to 100 mass % is obtained for one particle of the negative electrode active material. At this time, the mass ratio (mass %) of the phosphorus (P) element can also be obtained. It can be calculated by performing this analysis on more than 10 particles (n≥10) of the recovered negative electrode active material 80 and calculating the average value.

[0080] As another aspect of the technology disclosed herein, a negative electrode active material (recovered negative electrode active material) of the above-mentioned embodiment is provided. Specifically, a negative electrode active material is provided, characterized in that it includes: a carbon material, and a phosphorus-containing oxide configured on the surface of the carbon material, and in the negative electrode active material, the average value of the mass ratio of the phosphorus element relative to the total mass of the element detected by element mapping obtained by SEM-EDS is 0.2 mass% or more. Thus, a negative electrode active material with improved cycle characteristics is provided.

[0081] As another aspect of the technology disclosed herein, a secondary battery is provided. The secondary battery disclosed herein includes: a negative electrode containing a negative electrode active material, a positive electrode, and an electrolyte. The above-mentioned negative electrode contains the negative electrode active material (recovered negative electrode active material) of the above-mentioned embodiment as the negative electrode active material. Specifically, the above-mentioned negative electrode contains a negative electrode active material having a carbon material and a phosphorus oxide configured on the surface of the carbon material. Moreover, the average value of the mass ratio of the phosphorus element relative to the total mass of the element detected by the element mapping obtained by SEM-EDS of the above-mentioned negative electrode active material is 0.2% by mass or more. Thus, a secondary battery with improved cycle characteristics is provided. Furthermore, the above-mentioned negative electrode may contain other negative electrode active materials in addition to the negative electrode active material (recovered negative electrode active material) of the above-mentioned embodiment within the range that does not significantly hinder the effect of the present disclosure. In addition, the above-mentioned electrolyte may contain a phosphorus-containing compound or may not contain it. Furthermore, for the other configurations of the above-mentioned secondary battery, it may be the same as the secondary battery described as the preparation object above, and since it has been described, repeated descriptions are omitted.

[0082] The shape of the secondary battery disclosed herein is not limited to a square shape, and may be a coin shape, a button shape, a cylindrical shape, etc. In addition, the secondary battery may be configured as a secondary battery having a laminated outer case.

[0083] An embodiment of the technology disclosed herein has been described above. The technology disclosed herein is not limited to the above-described embodiment, and includes other embodiments in which various configurations are modified.

[0084] <Applications of secondary batteries>

[0085] The secondary battery including the above-mentioned negative electrode active material can be used in various applications. For example, due to its high capacity and excellent thermal stability, it can be used as a power source (driving power source) for motors installed in vehicles such as cars and trucks. There is no particular limitation on the type of vehicle, for example, plug-in hybrid electric vehicles (PHEV), hybrid electric vehicles (HEV), electric vehicles (BEV), etc. can be listed. The secondary battery can also be used as a battery pack 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 restraining mechanism.

[0086] Hereinafter, examples related to the technology disclosed herein will be described, but the technology disclosed herein is not intended to be limited to the contents shown in these examples.

[0087] [Preparation of secondary battery]

[0088] (Example 1)

[0089] First, a secondary battery is prepared in which an electrode body including a positive electrode plate, a negative electrode plate, and a separator and an electrolyte solution are housed in a casing.

[0090] As the positive electrode plate, a positive electrode core body made of aluminum is provided with a LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 A positive electrode plate having a positive electrode active material layer with O2 (lithium nickel cobalt manganese composite oxide) as the positive electrode active material.

[0091] As the negative electrode plate, a negative electrode plate was prepared in which a negative electrode active material layer containing natural graphite as a negative electrode active material was provided on the surface of a negative electrode core body made of copper.

[0092] As the above-mentioned electrolyte, an electrolyte prepared by dissolving LiPF6 as a supporting salt in 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 in a manner to achieve a concentration of 1 mol / L is prepared. That is, in Example 1, a secondary battery containing a phosphorus-containing compound (LiPF6) as a supporting salt in the electrolyte is prepared.

[0093] [Charge]

[0094] The secondary battery prepared as described above was charged by constant current constant voltage (CCCV) at a charge rate of 0.1 C in a temperature environment of 60° C. until the SOC at the end of charging reached 100%.

[0095] [Recovery of negative electrode active materials]

[0096] The charged secondary battery is discharged to the discharge termination voltage. Then, the secondary battery is disassembled and the negative electrode plate (negative electrode) is taken out. The negative electrode active material layer is peeled off from the negative electrode plate by washing with water. The negative electrode active material is recovered from the negative electrode active material layer and dried at 100° C. to remove moisture. In this way, the powdered negative electrode active material is recovered.

[0097] (Example 2~Example 4)

[0098] In Examples 2 to 4, secondary batteries having the same configuration as in Example 1 were prepared. In the charging process, constant current constant voltage (CCCV) charging was performed at a charging rate of 0.1 C under a temperature environment of 80° C. until the SOC (charge end SOC) shown in Table 1 was reached. Except for this point, the negative electrode active materials involved in Examples 2 to 4 were recovered in the same manner as in Example 1.

[0099] (Example 5)

[0100] In Example 5, LiBF6 is used as a supporting salt instead of LiPF6 to dissolve the electrolyte in a manner of 1 mol / L. In addition, 1 wt% of lithium difluorobis(oxalato)phosphate (LiPFO) is added to the secondary battery relative to the entire electrolyte in the same manner as in Example 1. In addition, a secondary battery having the same structure as in Example 1 is prepared. In Example 5, constant current constant voltage (CCCV) charging is performed at a charging rate of 0.1C under a temperature environment of 80°C until the SOC at the end of charging becomes 100%. In addition to this, the negative electrode active material is recovered in the same manner as in Example 1.

[0101] (Example 6)

[0102] In Example 6, a secondary battery having the same configuration as in Example 1 was prepared. In Example 6, the charging step was not performed. The negative electrode active material was recovered in the same manner as in Example 1 except for this point.

[0103] [Evaluation of recovered negative electrode active material]

[0104] For the recovered negative electrode active materials involved in Examples 1 to 6 (hereinafter also referred to as "recovered negative electrode active materials"), the mass ratio of the phosphorus element in the recovered negative electrode active material was evaluated. Specifically, first, a scanning electron microscope (JSM-IT800) manufactured by JEOL Ltd. was used to obtain an SEM image of the recovered negative electrode active material (recovered negative electrode active material). At this time, one particle of the recovered negative electrode active material entered the field of view of the imaging area and was photographed. For this one particle, an energy dispersive X-ray analysis (EDS) device manufactured by Oxford Instruments was used to determine the mass ratio (mass %) of the phosphorus element when the total mass of the elements detected by EDS analysis was set to 100 mass %. The measurement was performed on 10 particles (n=10) for each example, and the average value was calculated as the mass ratio of the phosphorus element involved in each example. The results are shown in Table 1.

[0105] [Preparation of coin battery cell for evaluation]

[0106] In order to evaluate the performance of the recovered negative electrode active material, a coin battery cell as a monopolar battery cell in which a negative electrode as a working electrode and metallic lithium as a counter electrode are opposite to each other is prepared. First, a negative electrode is prepared using the recovered negative electrode active material. Specifically, the recovered negative electrode active material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener are mixed with ion exchange water as a solvent in a solid content mass ratio of recovered negative electrode active material: SBR: CMC = 98:1:1 to prepare a negative electrode paste. The negative electrode paste is applied to one side of a long sheet of copper foil (10 μm thick) as a negative electrode core (negative electrode collector), and after drying, it is pressed with a roller press to prepare a sheet-like negative electrode.

[0107] As the separator, a 24 μm thick porous polyolefin sheet having a three-layer structure of PP / PE / PP was used. In addition, a separator having a ceramic layer (4 μm thick) of alumina (Al2O3), boehmite, etc. coated on one side was used.

[0108] As an electrolyte solution, an electrolyte solution prepared by dissolving LiPF6 as a supporting salt at a concentration of 1.0 mol / L in 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.

[0109] The negative electrode, separator, and metal lithium are stacked in this order and impregnated with an electrolyte to produce a coin cell. At this time, the ceramic layer of the separator is stacked so that it faces the negative electrode.

[0110] [Evaluation of capacity retention rate (cycle characteristics)]

[0111] Using the evaluation coin battery cells obtained above, the capacity retention rate of the recovered negative electrode active material involved in Examples 1 to 6 was evaluated. In addition, the higher the value of the capacity retention rate, the better the cycle characteristics can be considered. Specifically, each evaluation coin battery cell was placed in an environment of 25°C, charged to 3mV at a current rate of 1C, and then charged at a constant voltage of 3mV until the current value became 1 / 10 of the current rate, and paused for 10 minutes. Next, it was discharged to 1.6V at a current rate of 1C, and then discharged at a constant voltage of 1.6V until the current value became 1 / 10 of the current rate, and paused for 10 minutes. The above-mentioned charge and discharge was regarded as 1 cycle, and the charge and discharge were repeated for 10 cycles. At this time, from the 1st cycle to the 10th cycle, the discharge capacity was measured in each cycle. The discharge capacity of the 1st cycle and the discharge capacity of the 10th cycle were measured, and the capacity retention rate was calculated according to the following formula (1). The results are shown in Table 1.

[0112] Capacity retention rate (%) = ((discharge capacity at the 10th cycle) / (discharge capacity at the 1st cycle)) × 100 Formula (1)

[0113] in addition, Figure 8 This is a graph showing the relationship between the number of cycles and the capacity retention rate. For Example 1, Example 2, and Example 6, the capacity retention rate compared with the first cycle was calculated for each cycle for the above capacity retention rate test and plotted on a graph with the number of cycles as the horizontal axis and the capacity retention rate (%) as the vertical axis. The results are shown in Figure 8 .

[0114] [Table 1]

[0115] Table 1

[0116]

[0117] As shown in the results of Table 1, in Example 1 in which the charging process was performed on the secondary battery containing the phosphorus-containing compound, the P mass ratio of the recovered negative electrode active material was increased compared with Example 6 in which the charging process was not performed. It can be seen that in Example 1, by performing the charging process, a negative electrode active material having a phosphorus-containing oxide attached to the surface of the active material substrate was obtained. Furthermore, Figure 8 As shown in the results, in the coin cell using the negative electrode active material of Example 6, a decrease in the capacity retention rate was found in each cycle. On the other hand, in the coin cell of Example 1, the decrease in the capacity retention rate per cycle was greatly suppressed compared with Example 6. It can be seen that the cycle characteristics of the negative electrode active material are improved by attaching the phosphorus-containing oxide to the surface of the active material substrate.

[0118] From the results of Examples 1 and 2, in Example 2 where the charging process was performed in a temperature environment of 80° C., the P mass ratio of the recovered negative electrode active material was greater, and a better capacity retention rate was obtained.

[0119] In addition, from the results of Examples 2 to 4, the higher the charge end SOC is, the greater the P mass ratio on the surface of the negative electrode active material is, and the capacity retention rate is also improved. From the results of Example 3, it can be seen that even when the charge end SOC is 50%, the phosphate can be appropriately attached to the surface of the negative electrode active material, which can improve the capacity retention rate.

[0120] From the results of Example 5, for a secondary battery not containing a phosphorus-containing compound, when a phosphorus-containing compound (LiPFO) is added, the P mass ratio of the recovered negative electrode active material increases, and good results are obtained regarding the capacity retention rate.

[0121] As described above, specific aspects of the technology disclosed herein include aspects described in the following items.

[0122] Item 1: A method for reproducing negative electrode active materials, comprising: a preparation step, wherein a secondary battery comprising a positive electrode, a negative electrode, an electrolyte and a phosphorus-containing compound is prepared; a charging step, wherein the secondary battery is charged in a temperature environment above the decomposition temperature of the phosphorus-containing compound; and a recovery step, wherein, for the secondary battery after the charging step, the negative electrode active material is recovered from the negative electrode.

[0123] Item 2: The method for reproducing a negative electrode active material according to Item 1, wherein the preparation step comprises: adding the phosphorus-containing compound to a secondary battery that does not contain the phosphorus-containing compound, or further adding the phosphorus-containing compound to the secondary battery that contains the phosphorus-containing compound.

[0124] Item 3: The method for reproducing a negative electrode active material according to Item 1 or 2, wherein the charging step is performed at a charging rate of 0.1 C or less.

[0125] Item 4: The method for reproducing a negative electrode active material according to any one of Items 1 to 3, wherein the charging step is performed until the state of charge (SOC) of the secondary battery reaches 50% or more.

[0126] Item 5: The method for reproducing a negative electrode active material according to any one of Items 1 to 4, wherein the charging step is performed under a temperature environment of 60° C. or higher.

[0127] Item 6: The method for reproducing a negative electrode active material according to any one of Items 1 to 5, wherein the secondary battery comprises LiPF6 as the phosphorus-containing compound.

[0128] Item 7: A negative electrode active material comprising: a carbon material, and a phosphorus-containing oxide arranged on the surface of the carbon material, wherein the average value of the mass ratio of phosphorus element relative to the total mass of the element detected by element mapping obtained using SEM-EDS is greater than 0.2 mass %.

[0129] Item 8: A secondary battery comprising: a negative electrode comprising the negative electrode active material according to Item 7, a positive electrode, and an electrolyte.

Claims

1. A method for reproducing negative electrode active material, comprising: a preparation step, wherein a secondary battery including a positive electrode, a negative electrode, an electrolyte and a phosphorus-containing compound is prepared; a charging step, wherein the secondary battery is charged in a temperature environment above the decomposition temperature of the phosphorus-containing compound; and A recovery step of recovering a negative electrode active material from the negative electrode of the secondary battery after the charging step.

2. The method for reproducing the negative electrode active material according to claim 1, wherein: The preparation step includes adding the phosphorus-containing compound to a secondary battery that does not contain the phosphorus-containing compound, or further adding the phosphorus-containing compound to the secondary battery that contains the phosphorus-containing compound.

3. The method for reproducing the negative electrode active material according to claim 1 or 2, wherein: The charging step is performed at a charging rate of 0.1C or less.

4. The method for reproducing a negative electrode active material according to claim 1 or 2, wherein: The charging step is performed until the state of charge (SOC) of the secondary battery reaches 50% or more.

5. The method for reproducing the negative electrode active material according to claim 1 or 2, wherein: The charging step is performed in a temperature environment of 60° C. or higher.

6. The method for reproducing the negative electrode active material according to claim 1 or 2, wherein: The secondary battery includes LiPF6 as the phosphorus-containing compound.

7. A negative electrode active material, comprising: The carbon material and the phosphorus-containing oxide disposed on the surface of the carbon material have an average value of 0.2 mass % or more of the mass ratio of phosphorus element relative to the total mass of elements detected by element mapping using SEM-EDS in the negative electrode active material.

8. A secondary battery comprising: A negative electrode, a positive electrode, and an electrolyte comprising the negative electrode active material according to claim 7.

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