Method for processing lithium secondary battery
The method of discharging lithium secondary batteries under reduced pressure and subsequent peeling effectively addresses the separation challenge in recycling lithium secondary batteries with lithium metal negative electrodes, enhancing recycling efficiency.
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- NISSAN MOTOR CO LTD
- Filing Date
- 2024-12-03
- Publication Date
- 2026-06-11
Smart Images
Figure JP2024042744_11062026_PF_FP_ABST
Abstract
Description
Method for treating a lithium secondary battery
[0001] The present invention relates to a method for treating a lithium secondary battery.
[0002] In recent years, research and development on lithium secondary batteries using oxide-based or sulfide-based solid electrolytes as electrolytes have been actively conducted. A solid electrolyte is a material mainly composed of an ion conductor capable of ion conduction in a solid. Therefore, in a lithium secondary battery using a solid electrolyte such as an all-solid-state battery, there is an advantage that various problems caused by a flammable organic electrolyte solution do not occur in principle as in a conventional liquid secondary battery using a non-aqueous electrolyte solution. Generally, when using a high-potential and large-capacity positive electrode material and a large-capacity negative electrode material, a significant improvement in the output density and energy density of the battery can be achieved.
[0003] On the other hand, conventionally, recycling of used batteries whose service life has expired has been carried out. For example, in a liquid secondary battery, a method is known in which either the lead portion of the positive electrode layer or the negative electrode layer is cut, pressurized air is blown to drop the cut electrode layer, and the positive electrode layer and the negative electrode layer are peeled off and separated. However, in a lithium secondary battery using a solid electrolyte, the solid interfaces of the positive electrode, the solid electrolyte layer, and the negative electrode are firmly bonded, and it is difficult to separate and recover each layer during recycling.
[0004] In response to this problem, Japanese Patent Application Laid-Open No. 2016-157608 discloses a method for treating an all-solid-state battery having a negative electrode current collector containing copper and a negative electrode active material layer containing a sulfide solid electrolyte. By performing a heat treatment or a treatment for increasing the potential of the negative electrode active material, copper sulfide is generated between the negative electrode active material layer and the negative electrode current collector, and the negative electrode active material layer and the negative electrode current collector are separated through the copper sulfide. A method for treating an all-solid-state battery is disclosed. According to Japanese Patent Application Laid-Open No. 2016-157608, the negative electrode active material layer and the negative electrode current collector can be efficiently separated during recycling of the all-solid-state battery by the above method.
[0005] However, in order to use the recycling method described in Japanese Patent Application Laid-Open No. 2016-157608, it is necessary for the negative electrode active material layer to contain a sulfide solid electrolyte. Therefore, the above method cannot be applied to a lithium secondary battery having a negative electrode (lithium metal negative electrode) in which the negative electrode active material layer is a lithium metal layer.
[0006] Therefore, an object of the present invention is to provide a means capable of easily separating a negative electrode during recycling of a lithium secondary battery having a lithium metal negative electrode and using a solid electrolyte.
[0007] The inventors of the present invention have intensively studied to solve the above problems. As a result, it has been found that in the recycling of a lithium secondary battery, the above problems can be solved by performing discharge in a state where the restraint pressure is reduced and then peeling and separating the negative electrode, and the present invention has been completed.
[0008] That is, one embodiment of the present invention is a power generation element having at least one single cell layer including a positive electrode, a negative electrode having a negative electrode current collector and having a lithium metal layer as a negative electrode active material layer, and a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte, and a lithium secondary battery treatment method including a pressing member that applies a restraint pressure in the stacking direction of the power generation element, the discharge step of discharging the lithium secondary battery in a state where a restraint pressure lower than the minimum restraint pressure during battery use is applied in the stacking direction of the power generation element, and the separation step of separating the negative electrode from the single cell layer.
[0009] FIG. 1 is a cross-sectional view schematically showing the overall structure of a laminated (internally parallel connection type) all-solid-state lithium secondary battery (laminated secondary battery) which is an example of the lithium secondary battery in this embodiment. FIG. 2 is a side view of the laminated secondary battery. FIG. 3 is a diagram schematically showing a cross-section of a single cell layer of the lithium secondary battery in each step in the treatment method of the lithium secondary battery of this embodiment.
[0010] Hereinafter, embodiments of the present invention will be described. However, the technical scope of the present invention should be determined based on the description of the claims and is not limited to only the following embodiments.
[0011] One embodiment of the present invention is a method for processing a lithium secondary battery comprising a power generation element having at least one single cell layer having a positive electrode, a negative electrode having a negative electrode current collector and a lithium metal layer as a negative electrode active material layer, and a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte, and a pressurizing member that applies a restraining pressure in the stacking direction of the power generation element, the method comprising a discharge step of discharging the lithium secondary battery while applying a restraining pressure lower than the minimum restraining pressure during battery use in the stacking direction of the power generation element, and a separation step of separating the negative electrode from the single cell layer. According to the processing method of this embodiment, the negative electrode can be easily separated when recycling a lithium secondary battery having a lithium metal negative electrode and using a solid electrolyte.
[0012] The lithium secondary battery in this embodiment will be described below with reference to the attached drawings. In the description of the drawings, the same elements are denoted by the same reference numeral, and redundant explanations are omitted. Also, the dimensional ratios in the drawings are exaggerated for illustrative purposes and may differ from actual ratios.
[0013] Figure 1 is a schematic cross-sectional view showing the overall structure of a stacked (internal parallel connection type) all-solid-state lithium secondary battery (hereinafter also simply referred to as "stacked secondary battery"), which is an example of a lithium secondary battery in this embodiment. Figure 1 shows a cross-section of a stacked secondary battery during charging in a lithium deposition type all-solid-state battery. The stacked secondary battery 10a shown in Figure 1 has a structure in which a roughly rectangular power generation element 21, where the charge and discharge reaction actually proceeds, is sealed inside a laminate film 29 which is the battery casing. Here, the power generation element 21 has a structure in which a negative electrode, a solid electrolyte layer 17, and a positive electrode are stacked. The negative electrode has a structure in which a negative electrode current collector 11' and a negative electrode active material layer 13 made of lithium metal deposited on the surface of the negative electrode current collector 11' are stacked. A negative electrode intermediate layer 14 is arranged adjacent to the surface of the negative electrode active material layer 13 that faces the solid electrolyte layer 17. The positive electrode has a structure in which a positive electrode active material layer 15 is arranged on the surface of the positive electrode current collector 11''. The negative electrode, solid electrolyte layer, and positive electrode are stacked in this order, with the negative electrode intermediate layer 14 and the positive electrode active material layer 15 facing each other via a solid electrolyte layer 17. As a result, adjacent negative electrodes, solid electrolyte layers, and positive electrodes constitute one single cell layer 19. Therefore, the stacked secondary battery 10a shown in Figure 1 can also be said to have a configuration in which multiple single cell layers 19 are stacked and electrically connected in parallel. The negative electrode current collector 11' and the positive electrode current collector 11'' are fitted with a negative electrode current collector plate 25 and a positive electrode current collector plate 27, respectively, which are electrically connected to the respective electrodes (negative electrode and positive electrode), and have a structure in which they are led out to the outside of the laminate film 29 by being sandwiched between the edges of the laminate film 29. The stacked secondary battery 10a is subjected to restraining pressure in the stacking direction of the power generation element 21 by a pressurizing member (not shown). Therefore, the volume of the power generation element 21 is kept constant.
[0014] In the above explanation, the lithium secondary battery described was a so-called lithium deposition type, where lithium metal is deposited on the surface of the negative electrode current collector during charging, and was described as a stacked (internal parallel connection type) all-solid-state lithium secondary battery. However, the type of lithium secondary battery to which the present invention can be applied is not particularly limited, and it can also be applied to lithium secondary batteries that are not of the lithium deposition type, as well as bipolar lithium secondary batteries.
[0015] Figure 2 is a side view of a stacked secondary battery. As shown in Figure 2, the stacked secondary battery 100 has a plurality of power generation elements 21 sealed in laminate films 29 as shown in Figure 1, two metal plates 200 that sandwich the power generation elements 21 sealed in the laminate films 29, and bolts 300 and nuts 400 as fastening members. These fastening members (bolts 300 and nuts 400) have the function of fixing the metal plates 200 in a state where they are sandwiching the power generation elements 21 sealed in the laminate films 29. As a result, the metal plates 200 and the fastening members (bolts 300 and nuts 400) function as pressurizing members that pressurize (restrain) the power generation elements 21 in the stacking direction. The pressurizing members are not particularly limited as long as they are members that can pressurize the power generation elements 21 in the stacking direction. Typically, a combination of a plate made of a rigid material such as the metal plate 200 and the fastening members described above is used as the pressurizing members. Furthermore, regarding the fastening members, not only bolts 300 and nuts 400 may be used, but also tension plates that fix the ends of the metal plates 200 so as to restrain the power generation element 21 in its stacking direction.
[0016] The lower limit of the load applied to the power generation element 21 when the battery is in use (restraining pressure in the stacking direction of the power generation element) is, for example, 1 MPa or more, preferably 3 MPa or more. That is, the minimum restraining pressure during use is, for example, 1 MPa or more, preferably 3 MPa or more. The upper limit of the restraining pressure in the stacking direction of the power generation element is, for example, 100 MPa or less, preferably 70 MPa or less, more preferably 40 MPa or less, and even more preferably 10 MPa or less.
[0017] In FIG. 2, an example is shown in which the power generation elements 21 sealed in a plurality of laminate films 29 are laminated and a restraint pressure is applied in the lamination direction of the power generation elements 21 by a pressing member. However, the laminated secondary battery 100 may be configured such that a restraint pressure is applied in the lamination direction of the power generation element 21 to the power generation element 21 sealed in one laminate film 29 by a pressing member.
[0018] Hereinafter, the main constituent members of the above-described laminated secondary battery 10a will be described.
[0019] [Current collector] The current collector (negative electrode current collector, positive electrode current collector) has a function of mediating the movement of electrons from the electrode active material layer (negative electrode active material layer, positive electrode active material layer). There is no particular limitation on the material constituting the current collector. As the constituent material of the current collector, for example, metals such as aluminum, nickel, iron, stainless steel, titanium, and copper, or conductive resins can be employed. There is also no particular limitation on the thickness of the current collector, but as an example, it is 10 to 100 μm.
[0020] [Positive electrode active material layer] The positive electrode active material layer contains a positive electrode active material and may contain a solid electrolyte, a binder, and a conductive assistant as necessary.
[0021] The positive electrode active material has a function of releasing ions such as lithium ions during charging and occluding ions such as lithium ions during discharging. As the positive electrode active material, for example, metal oxides are used. Specifically, LiCoO ２ , LiMnO ２ , LiNiO ２ , LiVO ２ , Li(Ni - Mn - Co)O ２ and other layered rock salt type active materials, LiMn ２ O ４ , LiNi ０．５ Mn １．５ O ４ and other spinel type active materials, LiFePO ４ , LiMnPO ４ and other olivine type active materials, Li ２ FeSiO ４ , Li ２ MnSiO ４ and other Si-containing active materials such as metal oxides are exemplified.
[0022] The average particle size of the positive electrode active material is not particularly limited, but is preferably 1 to 100 μm, more preferably 1 to 20 μm. In this specification, the average particle size of the particles is the median diameter (D50) measured by a laser diffraction / scattering particle size distribution analyzer.
[0023] The positive electrode active material layer may further contain a solid electrolyte. Examples of solid electrolytes include sulfide solid electrolytes and oxide solid electrolytes. In this specification, the term "solid electrolyte" refers to a material mainly composed of an ion conductor capable of conducting ions in a solid state, and in particular, a lithium ion conductivity of 1 × 10⁻¹⁶ at room temperature (25°C) －５ This refers to a material with a lithium ion conductivity of S / cm or higher, and this lithium ion conductivity is preferably 1 × 10⁻⁶. －４ The value is S / cm or higher. Here, the ionic conductivity can be measured by the AC impedance method.
[0024] The solid electrolyte contained in the positive electrode active material layer is not particularly limited, but from the viewpoint of exhibiting excellent lithium ion conductivity and being able to better follow the volume change of the positive electrode active material due to charging and discharging, it is preferably a sulfide solid electrolyte containing the element S. The solid electrolyte contained in the positive electrode active material layer more preferably contains the elements Li, M, and S, and the M element is a sulfide solid electrolyte containing at least one element selected from the group consisting of P, Si, Ge, Sn, Ti, Zr, Nb, Al, Sb, Br, Cl, and I, and even more preferably a sulfide solid electrolyte containing the elements S, Li, and P.
[0025] Sulfide solid electrolytes are Li ３ PS ４ It may have a skeleton, Li ４ P ２ S ７ It may have a skeleton, Li ４ P ２ S ６ It may have a skeleton. ３ PS ４ Examples of sulfide solid electrolytes with a skeleton include LiI-Li ３ PS ４ , LiI-LiBr-Li３ PS ４ Li ３ PS ４ This can be cited. Also, Li ４ P ２ S ７ Examples of sulfide solid electrolytes having a framework include Li-P-S system solid electrolytes called LPS. Also, as sulfide solid electrolytes, for example, Li （４－ｘ） Ge （１－ｘ） P ｘ S ４ You may also use LGPS, etc., which is represented as (where x satisfies 0 < x < 1). More specifically, for example, LPS(Li ２ S-P ２ S ５ ), Li ７ P ３ S １１ Li ３．２ P ０．９６ S, Li ３．２５ Ge ０．２５ P ０．７５ S ４ Li １０ GeP ２ S １２ , or Li ６ PS ５ Examples include X (where X is Cl, Br, or I). ２ S-P ２ S ５ The description of " is Li ２ S and P ２ S ５ This refers to a sulfide solid electrolyte made using a raw material composition containing the above, and the same applies to other descriptions. In particular, the sulfide solid electrolyte is preferably LPS (Li) because it has high ionic conductivity and a low bulk modulus, and can therefore follow the volume change of the electrode active material accompanying charging and discharging. ２ S-P ２ S ５ ), Li ６ PS ５ X (where X is Cl, Br, or I), Li ７ P ３ S １１ Li ３．２ P ０．９６ S and Li ３ PS ４It is selected from the group consisting of the following.
[0026] The solid electrolyte can take the form of particulate matter, such as a perfect sphere or an ellipsoid. When the solid electrolyte is particulate, its average particle size (D50) is not particularly limited, but is preferably 0.01 μm or more and 40 μm or less, more preferably 0.1 μm or more and 20 μm or less, and even more preferably 0.1 μm or more and 10 μm or less.
[0027] The positive electrode active material layer may further contain at least one of a conductive additive and a binder.
[0028] Examples of conductive additives include carbon fibers (specifically, vapor-grown carbon fibers (VGCF), polyacrylonitrile-based carbon fibers, pitch-based carbon fibers, rayon-based carbon fibers, activated carbon fibers, etc.), carbon nanotubes (CNTs), and carbon black (specifically, acetylene black, Ketjen black®, furnace black, channel black, thermal lamp black, etc.). The content of the conductive additive in the positive electrode active material layer is not particularly limited, but is for example 1 to 10% by mass.
[0029] The binder used in the positive electrode active material layer is not particularly limited, and known binders can be used as appropriate. Examples of binders include polyvinylidene fluoride (PVDF), compounds in which hydrogen atoms of PVDF are substituted with other halogen elements, polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), and carboxymethylcellulose (CMC). The binder content in the positive electrode active material layer is not particularly limited, but is, for example, 0.1 to 10% by mass.
[0030] The thickness of the positive electrode active material layer is not particularly limited, but is, for example, 0.1 to 1000 μm, and preferably 10 to 200 μm.
[0031] [Solid Electrolyte Layer] The solid electrolyte layer is interposed between the positive electrode active material layer and the negative electrode active material layer and contains a solid electrolyte (usually as the main component). There are no particular restrictions on the specific form of the solid electrolyte contained in the solid electrolyte layer, and the solid electrolyte and its preferred form exemplified in the section on the positive electrode active material layer can be used in the same way. The solid electrolyte contained in the solid electrolyte layer may be the same as or different from the solid electrolyte contained in the positive electrode active material layer.
[0032] The solid electrolyte content in the solid electrolyte layer is preferably in the range of 10 to 100% by mass, more preferably in the range of 50 to 100% by mass, and even more preferably in the range of 90 to 100% by mass, relative to the mass of the solid electrolyte layer.
[0033] The solid electrolyte layer may further contain a binder in addition to the solid electrolyte. The binder is not particularly limited, and known binders can be used as appropriate, but for example, the binder described above for the positive electrode active material layer can be similarly employed. The binder content in the solid electrolyte layer is not particularly limited, but for example, it is 1 to 10% by mass.
[0034] The solid electrolyte layer may be in the form of a solid electrolyte sheet in which the solid electrolyte is disposed in the voids of a support having voids. This allows for both thinning of the solid electrolyte layer and maintenance of mechanical strength, and the solid electrolyte layer can exist as a self-supporting film. When the solid electrolyte layer of a lithium secondary battery is such a self-supporting film, it is preferable because the strength of the solid electrolyte layer is high, making it easier to peel off during the separation process.
[0035] The support having voids (also simply referred to as "support") is not particularly limited, but examples include fibrous structures such as nonwoven fabrics and woven fabrics, and integral porous bodies such as foams in which multiple spaces are formed inside an integral structure. Among these, the support is preferably a fibrous structure, and more preferably a nonwoven fabric.
[0036] The material of the support is not particularly limited, but an insulator (with an electronic conductivity of 10 at 25°C) is preferable. －６Preferably, the ratio is S / m or less, and specifically, examples include organic materials such as polyester (e.g., polyethylene terephthalate), polypropylene, polyethylene, cellulose, aramid, and polytetrafluoroethylene, as well as inorganic materials such as glass and alumina.
[0037] The means for placing a solid electrolyte in the voids of a voided support are not particularly limited, but examples include a method of preparing a slurry by mixing the solid electrolyte and, if necessary, a binder with a solvent, impregnating the support with the resulting slurry, and then removing the solvent.
[0038] The thickness of the solid electrolyte layer is not particularly limited, but is, for example, 0.1 to 1000 μm, and preferably 10 to 100 μm.
[0039] [Negative Electrode Intermediate Layer] In this embodiment, if the lithium secondary battery is a lithium deposition type all-solid-state battery, it is preferable that the lithium secondary battery has a negative electrode intermediate layer adjacent to the surface of the solid electrolyte layer facing the negative electrode current collector. The negative electrode intermediate layer is not particularly limited, but it is preferable that it contains at least one material selected from the group consisting of a metallic material that can be alloyed with lithium and a carbon material that can absorb lithium ions, and a binder. It is preferable that the negative electrode intermediate layer as a whole is conductive. The volume resistivity of the negative electrode intermediate layer is not particularly limited, but is preferably 10 ２ The resistivity is Ω·cm or less, and more preferably 10Ω·cm or less. In this specification, the volume resistivity of the negative electrode intermediate layer is the value measured using an electrode resistance measurement system (manufactured by HIOKI E.E. CORPORATION, product name: RM2610).
[0040] Specific examples of metallic materials that can be alloyed with lithium include, for example, indium (In), aluminum (Al), silicon (Si), tin (Sn), magnesium (Mg), gold (Au), silver (Ag), zinc (Zn), and alloys containing at least one of these.
[0041] Specific examples of carbon materials capable of storing lithium ions include carbon black (specifically, acetylene black, Ketjenblack®, furnace black, channel black, thermal lamp black, etc.), carbon nanotubes (CNTs), graphite, and hard carbon.
[0042] In one embodiment, the negative electrode intermediate layer includes at least one metal particle containing a lithium-alloyable metal material as described above, and at least one carbon particle containing a lithium-ion-adsorbable carbon material as described above. When both carbon particles and metal particles are used, the mass ratio of carbon particles to metal particles (carbon particles:metal particles) is preferably 10:1 to 1:1, and more preferably 5:1 to 2:1.
[0043] The binder is not particularly limited, but for example, the binder described above for the positive electrode active material layer can be used in a similar manner. The binder content in the negative electrode intermediate layer is also not particularly limited.
[0044] The thickness of the negative electrode intermediate layer is not particularly limited, but is, for example, 1 to 20 μm, and preferably 2 to 15 μm.
[0045] [Negative Electrode Active Material Layer] The negative electrode active material layer contains the negative electrode active material. In this embodiment, the negative electrode active material of the lithium secondary battery is lithium metal. In this embodiment, the negative electrode active material layer is a lithium metal layer substantially composed of lithium metal. Substantially composed of lithium metal means that the inclusion of impurities of about 2-3% by mass or less is permissible. In a preferred embodiment, the negative electrode active material layer consists solely of lithium metal. The thickness of the negative electrode active material layer (lithium metal layer) is not particularly limited, but is, for example, 0.1 to 1000 μm.
[0046] In this embodiment, the lithium secondary battery may be a so-called lithium deposition type, in which lithium metal as the negative electrode active material is deposited on the negative electrode current collector during the charging process. Therefore, in this embodiment, the thickness of the negative electrode active material layer increases as the charging process progresses, and decreases as the discharging process progresses. The negative electrode active material layer does not need to be present during complete discharge, but in some cases, a negative electrode active material layer consisting of a certain amount of lithium metal may be present during complete discharge. Furthermore, the thickness of the negative electrode active material layer (lithium metal layer) during complete charge is not particularly limited, but is usually 0.1 to 1000 μm.
[0047] [Current Collector Plates] The materials constituting the current collector plates (25, 27) are not particularly limited, and known highly conductive materials conventionally used as current collector plates for secondary batteries can be used. Although not shown in the figures, the current collectors (11'', 11') and the current collector plates (27, 25) may be electrically connected via positive lead and negative lead.
[0048] [Battery Enclosure] As the battery enclosure, a bag-shaped case made of an aluminum-containing laminate film 29 that can cover the power generation element, as shown in Figure 1, is preferably used. For the laminate film, for example, a three-layer laminate film made by laminating PP, aluminum, and nylon in that order can be used, but is not limited to these.
[0049] It should be noted that the lithium secondary battery processed by this processing method does not have to be all-solid type. That is, the solid electrolyte layer may further contain a conventionally known liquid electrolyte (electrolyte). There are no particular restrictions on the amount of liquid electrolyte (electrolyte) that can be contained in the solid electrolyte layer, but it is preferable that the amount is such that the shape of the solid electrolyte layer formed by the solid electrolyte is maintained and no leakage of the liquid electrolyte (electrolyte) occurs.
[0050] <Method for Processing a Lithium Secondary Battery> A method for processing a lithium secondary battery according to one embodiment of the present invention comprises a discharge step in which the lithium secondary battery is discharged while a restraining pressure lower than the minimum restraining pressure during battery use is applied in the stacking direction of the power generation elements, and a separation step in which the negative electrode is separated from the single cell layer. Hereinafter, each step in the processing method of this embodiment will be explained using the single cell layer shown in Figure 3 as an example. Figure 3 is a schematic diagram showing the cross-section of the single cell layer of a lithium secondary battery in each step of the processing method of a lithium secondary battery according to this embodiment. Figure 3(a) schematically shows the cross-section of the single cell layer when the battery is in use, Figure 3(b) schematically shows the cross-section of the single cell layer in the discharge step, and Figure 3(c) schematically shows the cross-section of the single cell layer in the separation step. Note that the single cell layer 119 shown in Figures 3(a) to (c) does not include a negative electrode intermediate layer, and when the battery is in use, the solid electrolyte layer 117 and the negative electrode active material layer 113 are in contact. The single cell layer 119 shown in Figures 3(a) to 3(c) comprises a positive electrode 122 having a positive electrode current collector 111'' and a positive electrode active material layer 115, a negative electrode 121 having a negative electrode current collector 111' and a negative electrode active material layer 113, and a solid electrolyte layer 117 interposed between the positive electrode 122 and the negative electrode 121. The positive electrode active material layer 115 contains a positive electrode active material 105 and a solid electrolyte 107.
[0051] [Discharge Process] The discharge process is a process in which a lithium secondary battery is discharged while a restraining pressure lower than the minimum restraining pressure during battery use is applied in the stacking direction of the power generation elements.
[0052] A lithium secondary battery can be, for example, a used battery. The specific form of a lithium secondary battery is as described above. The minimum confinement pressure during battery use refers to the confinement pressure if it does not change over time, and if it does change over time, it refers to the confinement pressure at which it becomes the lowest. Furthermore, "during battery use" means when a lithium secondary battery is used in a way that it performs its function as a battery (charging or discharging).
[0053] In lithium secondary batteries using a solid electrolyte, as shown by the arrow in Figure 3(a), applying a restraining pressure in the stacking direction of the power generation elements maintains contact at the interface between the solid electrolyte layer 117 (or the negative electrode intermediate layer if one is present) and the lithium metal layer, which is the negative electrode active material layer 113, thereby enabling discharge. That is, during discharge, the lithium metal of the negative electrode active material layer 113 dissolves and moves towards the positive electrode side, but because a restraining pressure is applied in the stacking direction of the power generation elements, the dissolved lithium metal creeps and follows the interface with the solid electrolyte layer 117 (or the negative electrode intermediate layer if one is present). Therefore, contact at the interface between the negative electrode active material layer (lithium metal layer) 113 and the solid electrolyte layer 117 (or the negative electrode intermediate layer if one is present) is maintained, and discharge can proceed. In contrast, in the discharge step of the processing method for the lithium secondary battery of this embodiment shown in Figure 3(b), the restraining pressure applied to the power generation elements is made lower than the minimum restraining pressure during battery use. In this way, when lithium metal dissolves due to discharge and moves to the positive electrode side as indicated by the arrow in Figure 3(b), the negative electrode active material layer (lithium metal layer) 113 will not be able to adequately follow the interface with the solid electrolyte layer 117 (or the negative electrode intermediate layer if one is present), and a void 108 will be created in the interface region of the negative electrode active material layer (lithium metal layer) 113 with the solid electrolyte layer 117 (or the negative electrode intermediate layer if one is present) due to the dissolution of lithium metal. The creation of this void 108 at the interface reduces the bonding force at the interface, making it possible to easily separate the negative electrode.
[0054] The confinement pressure of the battery during the discharge process is not particularly limited as long as it is less than the minimum confinement pressure during battery use, but for example, it may be less than or equal to half of the minimum confinement pressure during battery use. It is preferable that the confinement pressure of the battery during the discharge process be as low as possible, and it is even more preferable that no confinement pressure is applied by the pressurizing member (i.e., the confinement pressure of the battery is zero).
[0055] The discharge conditions in the discharge process are not particularly limited. The discharge conditions may be the same as or different from the discharge conditions during battery use. Discharge may be constant current discharge, constant voltage discharge, or constant current and constant voltage discharge. The current density during discharge is also not particularly limited, but for example, 0.1 to 10 mA / cm² is acceptable.２ The discharge termination voltage is not particularly limited. In this process, the power generation element is constrained at a constraining pressure lower than the minimum constraining pressure when the battery is in use. As the discharge progresses, lithium metal dissolves from the surface of the lithium metal layer, creating voids at the interface, and the discharge stops because contact at the interface can no longer be maintained. The state at which the discharge stops can be considered the termination point of this process.
[0056] In the discharge process, the lithium secondary battery may be discharged with the pressurizing member still attached, while controlling the confinement pressure to be lower than the minimum confinement pressure during battery use, or the discharge may be performed with the pressurizing member removed. It is more preferable to discharge with the pressurizing member removed in the discharge process, as this creates more voids at the interface and further reduces the bonding force at the interface.
[0057] In the lithium secondary battery shown in Figure 1, it is preferable that the power generation element 21 is vacuum-sealed inside a battery casing, such as an aluminum-containing laminate film 29, which can cover the power generation element 21. By reducing the pressure inside the battery casing in this way, the power generation element is constrained by atmospheric pressure in the stacking direction of the battery. In contrast, in a preferred embodiment of the processing method according to this embodiment, the discharge process is performed in a state where atmospheric pressure is not applied, for example, by vacuum-opening the battery casing. In other words, in the discharge process, it is more preferable to discharge in a state where atmospheric pressure is not applied due to vacuum sealing, in addition to the absence of constraining pressure from the pressurizing member. This creates more voids at the interface and further reduces the bonding force at the interface. Therefore, the negative electrode can be separated more easily.
[0058] When the discharge process is performed with the battery casing vacuum-opened, the discharge process may be carried out in the atmosphere or in an inert gas atmosphere. Examples of inert gases include nitrogen and noble gases (helium, argon, xenon, neon), with argon being preferred. In this specification, an inert gas atmosphere refers to an environment filled with an inert gas in which the total content of impurities is less than 0.1 volume percent.
[0059] The temperature of the lithium secondary battery during the discharge process is not particularly limited and can be in the range of, for example, -20°C to 40°C, but it is preferable to perform the discharge process at a temperature lower than room temperature (25°C). The lower the temperature during the discharge process, the less likely creep deformation of the lithium metal is to occur, making it easier for voids to form at the interface. As a result, more voids can be formed at the interface, the bonding force at the interface can be further reduced, and the negative electrode can be separated more easily. The lower limit of the temperature of the lithium secondary battery during the discharge process is not particularly limited, but it is preferable to be 0°C or higher from the viewpoint of stable discharge. In one embodiment, the temperature of the lithium secondary battery during the discharge process is, for example, 0 to 20°C, preferably 5 to 15°C, and more preferably 5 to 10°C.
[0060] [Charging Process] In the processing method for lithium secondary batteries of this embodiment, it is preferable to further include a charging process in which the lithium secondary battery is charged before the discharge process described above. In lithium secondary batteries in which the negative electrode active material layer is a lithium metal layer, as the charging process progresses, more lithium metal is deposited on the negative electrode current collector as a lithium metal layer. As described above, a void is created at the interface during the discharge process, so if contact at the interface is no longer maintained as the discharge progresses, the discharge may stop. Therefore, since the dissolution of the lithium metal layer during the discharge process occurs only in the region near the surface of the metallic lithium layer, the lithium metal layer deposited during the charging process is maintained almost as is. As a result, more lithium metal can be recovered when the negative electrode is separated in the subsequent separation process.
[0061] Furthermore, it is preferable that the above charging process is carried out with a constraining pressure applied in the stacking direction of the power generation elements of the lithium secondary battery by a pressurizing member, and that this constraining pressure is preferably greater than the constraining pressure of the battery in the discharge process. This allows more lithium metal to be deposited on the negative electrode current collector before the discharge process, and more lithium metal to be recovered. From a similar viewpoint, it is preferable that the above charging process is carried out with a constraining pressure applied in the stacking direction of the power generation elements that is the same as or greater than the minimum constraining pressure of the lithium secondary battery when the battery is in use.
[0062] The charging conditions during the charging process are not particularly limited. The charging conditions may be the same as or different from the charging conditions during battery use. Charging may be constant current charging, constant voltage charging, or constant current and constant voltage charging. The current density during charging is also not particularly limited, but for example, 0.1 to 10 mA / cm² is acceptable. ２ The charging termination voltage in the charging process is not particularly limited, but it is preferable to charge to a voltage higher than the charging limit voltage when the battery is in use. Here, the charging limit voltage refers to the threshold voltage at which charging of the battery continues and it becomes fully charged. By charging to a voltage higher than the charging limit voltage when the battery is in use, that is, by charging beyond the fully charged state to an overcharged state, more lithium metal can be deposited on the negative electrode current collector before the discharge process. Therefore, when the negative electrode is separated in the subsequent separation process, more lithium metal can be recovered. Specifically, the charging termination voltage in the charging process can be appropriately adjusted depending on the type of positive electrode active material and negative electrode active material used, but for example, it is 3.6 to 4.5V.
[0063] The temperature of the lithium secondary battery during the charging process is not particularly limited, and is, for example, in the range of 0 to 100°C, preferably in the range of 20 to 80°C.
[0064] Furthermore, the charging process and the discharging process may be repeated alternately before performing the separation process described later. This allows more lithium metal to be deposited on the negative electrode current collector and more lithium metal to be recovered. For example, for a used battery, the charging and discharging cycle can be repeated 2 to 10 times, preferably 2 to 3 times, before performing the separation process described later. When the discharge process is performed multiple times, the conditions such as the confinement pressure, current density, and temperature of the lithium secondary battery in each discharge process may be the same or different. Similarly, when the charging process is performed multiple times, the conditions such as the confinement pressure, current density, and temperature of the lithium secondary battery in each charging process may be the same or different.
[0065] [Heat Treatment Step] The processing method for the lithium secondary battery of this embodiment may further include a heat treatment step of heating the power generation element after the discharge step described above and before the separation step described later. Performing the heat treatment step may generate gas at the interface between the solid electrolyte layer or negative electrode intermediate layer and the lithium metal layer. This is preferable because it can further reduce the adhesive strength of the interface, making it easier to peel off in the separation step described later.
[0066] The heating temperature in the heat treatment process is not particularly limited, but is, for example, 100 to 200°C, preferably more than 100°C and 200°C or less, and more preferably 120 to 180°C. The heating time is also not particularly limited, but is, for example, 1 minute to 1 hour.
[0067] The heat treatment process may be carried out with or without confinement pressure applied to the lithium secondary battery. When performed with confinement pressure, the confinement pressure is preferably the same as or lower than the confinement pressure of the battery during the discharge process. Furthermore, the heat treatment process may be carried out with the power generation element sealed under reduced pressure inside the battery casing, or with the vacuum opened.
[0068] [Gas Generation Process] The processing method for lithium secondary batteries in this embodiment may further include a gas generation process after the discharge process described above and before the separation process described later, in which the power generation element is brought into contact with a gas that reacts with the solid electrolyte to generate a product gas. Performing this process generates a product gas at the interface between the solid electrolyte layer or negative electrode intermediate layer and the lithium metal layer, further reducing the adhesion force of the interface, which is preferable because it makes it easier to peel off in the separation process described later.
[0069] The gas that reacts with the solid electrolyte is not particularly limited, but examples include water vapor contained in the atmosphere. Water vapor in the atmosphere reacts with sulfide solid electrolytes, for example, to generate product gases such as hydrogen sulfide at the interface with the lithium metal layer. Similarly, vapors such as methanol, ethanol, and acetone can also react with the solid electrolyte to generate gases. The gas that reacts with the solid electrolyte can be introduced, for example, during or after the discharge process by removing the pressurizing member and vacuum-opening the battery casing.
[0070] Furthermore, both the heating process and the gas generation process may be performed after the discharge process. If both are performed, there are no particular restrictions on their order. These processes may also be performed simultaneously, such as heating the lithium secondary battery during the gas generation process.
[0071] [Separation Process] The separation process is a process of separating the negative electrode 121 from the single cell layer 119, as shown in Figure 3(c). Here, "separating the negative electrode" refers to a state in which the negative electrode (the lithium metal layer, which is the negative electrode active material layer, and the negative electrode current collector) is peeled off from the interface. Due to the discharge process described above, the adhesive force at the interface between the solid electrolyte layer 117 or the negative electrode intermediate layer and the negative electrode active material layer (lithium metal layer) 113 is reduced, so the negative electrode 121 can be easily peeled off and separated at the interface between the solid electrolyte layer 117 or the negative electrode intermediate layer and the negative electrode active material layer (lithium metal layer) 113.
[0072] The method for separating the negative electrode is not particularly limited, as long as it can separate the negative electrode from the interface. For example, the power generation element can be removed from the battery casing, the positive electrode current collector plate and the negative electrode current collector plate can be grasped, and the negative electrode can be detached.
[0073] The following embodiments are also included in the scope of the present invention: Item 1: A method for processing a lithium secondary battery comprising a power generation element having at least one single cell layer having a positive electrode, a negative electrode having a negative electrode current collector and a lithium metal layer as a negative electrode active material layer, and a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte, and a pressurizing member that applies a restraining pressure in the stacking direction of the power generation element, comprising: a discharge step of discharging the lithium secondary battery while applying a restraining pressure lower than the minimum restraining pressure when the battery is in use in the stacking direction of the power generation element; and a separation step of separating the negative electrode from the single cell layer; Item 2: The method for processing a lithium secondary battery according to Item 1, wherein in the discharge step, the lithium secondary battery is discharged with the pressurizing member removed; Item 3: The method for processing a lithium secondary battery according to Item 2, wherein in the discharge step, the discharge is performed without atmospheric pressure due to vacuum sealing; Item 4: The method for processing a lithium secondary battery according to any one of Items 1 to 3, wherein the discharge step is performed at a temperature lower than room temperature; Item 5: A method for processing a lithium secondary battery according to any one of items 1 to 4, further comprising a charging step of charging the lithium secondary battery before the discharge step; Item 6: A method for processing a lithium secondary battery according to item 5, wherein the confinement pressure in the charging step is higher than the confinement pressure in the discharge step; Item 7: A method for processing a lithium secondary battery according to item 5 or 6, wherein the charging step and the discharge step are repeatedly performed alternately before the separation step; Item 8: A method for processing a lithium secondary battery according to any one of items 5 to 7, wherein in the charging step, the battery is charged to a voltage higher than the charging limit voltage when in use; Item 9: A method for processing a lithium secondary battery according to any one of items 1 to 8, further comprising a heating step of heating the power generation element after the discharge step and before the separation step; Item 10: A method for processing a lithium secondary battery according to any one of items 1 to 9, further comprising a gas generation step of contacting the power generation element with a gas that reacts with the solid electrolyte to generate a product gas after the discharge step and before the separation step; Item 11: The method for processing a lithium secondary battery according to any one of items 1 to 10, wherein the solid electrolyte layer is a self-supporting membrane.
[0074] 10a, 100 Stacked secondary battery, 11', 111' Negative electrode current collector, 11'', 111'' Positive electrode current collector, 13, 113 Negative electrode active material layer, 14 Negative electrode intermediate layer, 15, 115 Positive electrode active material layer, 17, 117 Solid electrolyte layer, 19, 119 Single cell layer, 21 Power generation element, 25 Negative electrode current collector plate, 27 Positive electrode current collector plate, 29 Laminate film, 105 Positive electrode active material, 107 Solid electrolyte, 108 Void, 121 Negative electrode, 122 Positive electrode, 200 Metal plate, 300 Bolt, 400 Nut.
Claims
1. A method for processing a lithium secondary battery comprising a power generation element having at least one single cell layer having a positive electrode, a negative electrode having a negative electrode current collector and a lithium metal layer as a negative electrode active material layer, and a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte, and a pressurizing member that applies a restraining pressure in the stacking direction of the power generation element, the method comprising: a discharge step of discharging the lithium secondary battery while applying a restraining pressure lower than the minimum restraining pressure during battery use in the stacking direction of the power generation element; and a separation step of separating the negative electrode from the single cell layer.
2. The method for processing a lithium secondary battery according to claim 1, wherein in the discharge step, the lithium secondary battery is discharged with the pressurizing member removed.
3. The method for processing a lithium secondary battery according to claim 2, wherein the discharge step is performed in a state where atmospheric pressure is not applied due to vacuum sealing.
4. The method for processing a lithium secondary battery according to claim 1, wherein the discharge step is performed at a temperature lower than room temperature.
5. The method for processing a lithium secondary battery according to claim 1, further comprising a charging step of charging the lithium secondary battery before the discharge step.
6. The method for processing a lithium secondary battery according to claim 5, wherein the restraining pressure in the charging step is higher than the restraining pressure in the discharging step.
7. The method for processing a lithium secondary battery according to claim 5, wherein the charging step and the discharging step are repeatedly performed alternately before the separation step.
8. The method for processing a lithium secondary battery according to claim 5, wherein in the charging step, the battery is charged to a voltage higher than the charging limit voltage when the battery is in use.
9. A method for processing a lithium secondary battery according to claim 1, further comprising a heat treatment step of heating the power generation element after the discharge step and before the separation step.
10. A method for processing a lithium secondary battery according to claim 1, further comprising a gas generation step of contacting the power generation element with a gas that reacts with the solid electrolyte to generate a product gas, after the discharge step and before the separation step.
11. The method for processing a lithium secondary battery according to claim 1, wherein the solid electrolyte layer is a self-supporting membrane.