Nonaqueous electrolyte secondary battery and method for manufacturing the same

Abstract

To provide a technique for obtaining a nonaqueous electrolyte secondary battery having excellent battery performance.SOLUTION: In one aspect of a manufacturing method of a nonaqueous electrolyte secondary battery disclosed herein, the manufacturing method includes: an assembly preparation step (S1) of preparing an assembly which includes a wound electrode body and a nonaqueous electrolyte that are housed in a battery case, while, in the wound electrode body, a ratio of the capacity of a negative electrode to the capacity of a positive electrode is 1.3 or more and less than 2.0, and a metallic lithium foil exists on the outermost peripheral portion of the negative electrode and on at least part of an area that does not face the positive electrode; an initial charging step (S2) of initially charging the assembly; and a high-temperature aging step (S3) of subjecting the assembly after initial charging to aging treatment at a temperature of 60°C or above, in a fully charged state where a negative electrode potential is 0.5 V or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery and a method for manufacturing the non-aqueous electrolyte secondary battery. [Background technology]

[0002] For example, Patent Document 1 listed below discloses a nonaqueous electrolyte secondary battery in which a lithium-containing composite oxide of a transition metal is used for the positive electrode and a carbon material is used for the negative electrode, and the positive electrode plate and the negative electrode plate are spirally wound together with a separator, and a metallic lithium foil is attached to a portion of the negative electrode plate that corresponds to the outermost periphery and does not face the positive electrode plate, and the lithium is diffused into the carbon material by a potential difference or a concentration difference. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 5-144473 Summary of the Invention [Problem to be solved by the invention]

[0004] Incidentally, according to the investigations of the present inventors, it has been found that there is still room for improvement in the above-mentioned nonaqueous electrolyte secondary batteries, for example, from the viewpoint of battery performance (for example, lithium precipitation resistance and battery durability).

[0005] The present disclosure has been made in view of the above circumstances, and its main object is to provide a technique for obtaining a non-aqueous electrolyte secondary battery with excellent battery performance. [Means for solving the problem]

[0006] To achieve this object, the present disclosure provides a method for manufacturing a non-aqueous electrolyte secondary battery including a wound electrode assembly in which a positive electrode and a negative electrode are stacked and wound with a separator interposed therebetween, and a non-aqueous electrolyte, the method comprising the following steps: an assembly preparation step of preparing an assembly in which the wound electrode assembly and the non-aqueous electrolyte are housed in a battery case, wherein in the wound electrode assembly, the ratio of the capacity of the negative electrode to the capacity of the positive electrode is 1.3 or more and less than 2.0, and a metallic lithium foil is present in at least a portion of the outermost periphery of the negative electrode that does not face the positive electrode; an initial charging step of initially charging the assembly; and a high-temperature aging step of aging the assembly after the initial charge at a temperature of 60°C or higher in a charged state in which the potential of the negative electrode is 0.5 V or higher. As will be described in detail below, a non-aqueous electrolyte secondary battery with such a configuration can be manufactured using this method.

[0007] From another aspect, the present disclosure provides a nonaqueous electrolyte secondary battery including a wound electrode assembly in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween and wound, and a nonaqueous electrolyte, wherein a highly concentrated lithium region is present in at least a part of the outermost periphery of the negative electrode that does not face the positive electrode, and the ratio of the capacity of the negative electrode to the capacity of the positive electrode is 1.3 or more and less than 2.0. A nonaqueous electrolyte secondary battery having such a configuration is manufactured by, for example, the method for manufacturing a nonaqueous electrolyte secondary battery described above, and therefore can be said to be a nonaqueous electrolyte secondary battery with excellent battery performance. [Brief explanation of the drawings]

[0008] [Figure 1] 2 is a flowchart showing steps of a method for manufacturing a nonaqueous electrolyte secondary battery according to one embodiment. [Figure 2] FIG. 10 is a schematic diagram showing the configuration of a wound electrode body prepared in an assembly preparation step according to one embodiment. [Figure 3] 1 is a schematic diagram showing the configuration of a wound electrode body included in a nonaqueous electrolyte secondary battery according to one embodiment. [Figure 4] 1 is a cross-sectional view schematically illustrating the configuration of a nonaqueous electrolyte secondary battery according to one embodiment. [Figure 5] FIG. 2 is a schematic diagram illustrating a high lithium element accumulation region in a nonaqueous electrolyte secondary battery according to one embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] Several embodiments of the technology disclosed herein will be described below with reference to the drawings. In the following drawings, components and parts that perform the same function are denoted by the same reference numerals. Furthermore, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect the actual dimensional relationships. Matters other than those specifically mentioned in this specification that are necessary for implementing the technology disclosed herein (e.g., the general configuration and manufacturing process of a battery that do not characterize the present invention) can be understood as design matters of a person skilled in the art based on conventional technology in the relevant field. The technology disclosed herein can be implemented based on the content disclosed in this specification and common technical knowledge in the relevant field. Furthermore, the following description is not intended to limit the present disclosure to the following forms. In this specification, the notation "A to B" indicating a range means "greater than A and less than B." It also encompasses the meanings of "greater than A" and "less than B."

[0010] In the present technology, the term "secondary battery" refers to a general term for a repeatedly chargeable and dischargeable power storage device, encompassing so-called storage batteries such as lithium-ion secondary batteries and lithium polymer batteries, as well as power storage elements such as electric double-layer capacitors. Furthermore, a "non-aqueous electrolyte secondary battery" refers to a secondary battery that uses a non-aqueous electrolyte as a charge carrier, and the electrolyte may be either a gel electrolyte or a non-aqueous electrolyte. A configuration that can benefit from the present technology may be, for example, a non-aqueous electrolyte solution that is liquid at room temperature (e.g., 25°C) and contains a supporting salt (electrolyte salt) that serves as a charge carrier dissolved in a non-aqueous solvent. Furthermore, the term "active material" refers to a substance that can reversibly absorb and release chemical species that serve as charge carriers in a secondary battery. Furthermore, in the present technology, "SOC" refers to the state of charge (SOC) where the upper limit of the operating voltage range for reversible charge and discharge is defined as 100% and the lower limit is defined as 0%.

[0011] Hereinafter, an embodiment will be described in which the nonaqueous electrolyte secondary battery is a flat prismatic lithium ion secondary battery, as an example. FIG. 1 is a flowchart showing the steps of a method for manufacturing a nonaqueous electrolyte secondary battery according to an embodiment. FIG. 2 is a schematic diagram showing the configuration of a wound electrode body prepared in an assembly preparation step according to an embodiment. FIG. 3 is a schematic diagram showing the configuration of a wound electrode body included in a nonaqueous electrolyte secondary battery according to an embodiment. FIG. 4 is a cross-sectional view showing the configuration of a nonaqueous electrolyte secondary battery according to an embodiment. FIG. 2 is a view corresponding to FIG. 5 of a wound electrode body prepared in an assembly preparation step. Here, in FIG. 2, the separator is omitted for clarity. The method for manufacturing a nonaqueous electrolyte secondary battery disclosed herein may further include other steps at any stage, and steps can be omitted as appropriate unless they are described as essential. Furthermore, the order of the steps can be changed as long as the effects of the technology disclosed herein are achieved.

[0012] First, the method for manufacturing a nonaqueous electrolyte secondary battery according to this embodiment is a method for manufacturing a nonaqueous electrolyte secondary battery including a wound electrode assembly in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween and wound, and a nonaqueous electrolyte. As shown in Fig. 1, the method for manufacturing such a nonaqueous electrolyte secondary battery includes an assembly preparation step of preparing an assembly in which the wound electrode assembly and the nonaqueous electrolyte are housed in a battery case; an initial charging step of initially charging the assembly; and a high-temperature aging step of aging the assembly after the initial charging at a temperature of 60°C or higher in a charged state in which the potential of the negative electrode is 0.5V or higher. Here, the assembly preparation process is characterized in that the wound electrode body used has a ratio of the capacity of the negative electrode to the capacity of the positive electrode (hereinafter also simply referred to as the "positive / negative electrode capacity ratio") of 1.3 or more and less than 2.0, and a metallic lithium foil is present in at least a part of the outermost region of the negative electrode that does not face the positive electrode (hereinafter also simply referred to as the "region of the negative electrode not facing the positive electrode").

[0013] For example, one method for improving Li (lithium) deposition resistance is to increase the positive and negative electrode capacity ratio. However, it is known that such an increase tends to reduce battery durability (which can also be referred to as the battery capacity retention rate) due to excessive formation of an SEI film. Therefore, further development of a technology that can achieve both Li deposition resistance and battery durability has been desired. As a result of extensive research, the present inventors have found that excellent Li deposition resistance can be achieved by increasing the positive and negative electrode capacity ratio to 1.3 or more and less than 2.0 in a wound electrode assembly. Furthermore, it has been found that providing a metallic lithium foil to at least a portion of the outermost periphery of the negative electrode, which does not face the positive electrode, allows Li ions to diffuse into the negative electrode and replenish Li ions during the high-temperature aging process described above, thereby achieving excellent battery durability. That is, a nonaqueous electrolyte secondary battery manufacturing method including the above-described assembly preparation step (step S1), initial charging step (step S2), and high-temperature aging step (step S3), can provide a nonaqueous electrolyte secondary battery that achieves both excellent Li deposition resistance and excellent battery durability. Each step will be described below. The structure of the assembly is similar to that of the nonaqueous electrolyte secondary battery 100 except that the wound electrode body has a metallic lithium foil, so that FIG. 4 can be referred to as appropriate in describing the assembly.

[0014] (Assembly preparation process: step S1) As described above, in this step, an assembly (battery assembly) is prepared in which the wound electrode body 20 and the non-aqueous electrolyte 80 are housed in the battery case 30. Here, the ratio of the capacity of the negative electrode 60 to the capacity of the positive electrode 50 in the wound electrode body 20 is 1.3 or more and less than 2.0. Furthermore, a metallic lithium foil 10 is present in at least a part of a region of the wound electrode body 20 that is the outermost periphery of the negative electrode 60 and does not face the positive electrode 50 (this can also be referred to as a region of the outermost periphery of the negative electrode 60 where one side of the negative electrode 60 does not face the positive electrode 50; this corresponds to the region between P1 and P2 of the negative electrode 60 in FIG. 2 ).

[0015] The positive electrode 50 used in the assembly preparation step S1 may be a known positive electrode used in lithium-ion secondary batteries. The positive electrode 50 typically includes a positive electrode current collector 52 and a positive electrode active material layer 54 supported on the positive electrode current collector 52, as shown in FIG. 3, for example. The positive electrode active material layer 54 may be provided on one side or both sides of the positive electrode current collector 52, but is preferably provided on both sides. As shown in FIG. 3, the positive electrode 50 typically includes a positive electrode active material layer-free portion 52a (i.e., a portion where the positive electrode active material layer 54 is not formed and the positive electrode current collector 52 is exposed).

[0016] In this embodiment, the outermost periphery of the wound electrode body 20 is the negative electrode 60. Therefore, as shown in Fig. 3, in the wound electrode body 20, the positive electrode 50 is located on the inner periphery side (in other words, on the winding axis WL side).

[0017] A known positive electrode current collector used in lithium ion secondary batteries may be used as the positive electrode current collector 52, and examples thereof include a sheet or foil made of a metal with good conductivity (e.g., aluminum, nickel, titanium, stainless steel, etc.). Aluminum foil is preferred as the positive electrode current collector 52.

[0018] The dimensions of the positive electrode current collector 52 are not particularly limited and may be determined appropriately depending on the battery design. When an aluminum foil is used as the positive electrode current collector 52, the thickness thereof is not particularly limited, but is, for example, 5 μm to 35 μm, and preferably 7 μm to 20 μm.

[0019] The positive electrode active material layer 54 contains a positive electrode active material. The positive electrode active material may be a known positive electrode active material used in lithium-ion secondary batteries. Specific examples of the positive electrode active material include lithium composite oxides and lithium transition metal phosphate compounds. The crystal structure of the positive electrode active material is not particularly limited, and may be a layered structure, a spinel structure, an olivine structure, or the like.

[0020] Examples of the lithium composite oxide include lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide.

[0021] In this specification, the term "lithium nickel cobalt manganese composite oxide" refers to oxides containing Li, Ni, Co, Mn, and O as constituent elements, as well as oxides containing one or more additional elements. Examples of such additional elements include transition metal elements and typical metal elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, and Sn. The additional element may also be a metalloid element such as B, C, Si, or P, or a nonmetal element such as S, F, Cl, Br, or I. This also applies to the lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide.

[0022] Examples of lithium transition metal phosphate compounds include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate.

[0023] These positive electrode active materials may be used alone or in combination of two or more. The positive electrode active material is preferably a lithium nickel cobalt manganese composite oxide.

[0024] The average particle diameter (median diameter: D50) of the positive electrode active material is not particularly limited, but is, for example, 0.05 μm to 25 μm, preferably 1 μm to 20 μm, and more preferably 3 μm to 15 μm. The average particle diameter (D50) of the positive electrode active material can be determined, for example, by a laser diffraction scattering method.

[0025] The positive electrode active material layer 54 may contain components other than the positive electrode active material, such as trilithium phosphate, a conductive material, a binder, etc. Suitable conductive materials include carbon black such as acetylene black (AB) and other carbon materials (e.g., graphite). Suitable binders include polyvinylidene fluoride (PVDF).

[0026] The content of the positive electrode active material in the positive electrode active material layer 54 (i.e., the content of the positive electrode active material relative to the total mass of the positive electrode active material layer 54) is not particularly limited, but is preferably 70% by mass or more, more preferably 80% by mass to 97% by mass, and even more preferably 85% by mass to 96% by mass. The content of trilithium phosphate in the positive electrode active material layer 54 is not particularly limited, but is preferably 1% by mass to 15% by mass, and more preferably 2% by mass to 12% by mass. The content of the conductive material in the positive electrode active material layer 54 is not particularly limited, but is preferably 1% by mass to 15% by mass, and more preferably 3% by mass to 13% by mass. The content of the binder in the positive electrode active material layer 54 is not particularly limited, but is preferably 1% by mass to 15% by mass, and more preferably 1.5% by mass to 10% by mass.

[0027] The thickness of the positive electrode active material layer 54 is not particularly limited, but is, for example, 10 μm or more and 300 μm or less, and preferably 20 μm or more and 200 μm or less.

[0028] The positive electrode 50 can be prepared by a known method. For example, a positive electrode paste containing a positive electrode active material and optional components is prepared, and the positive electrode paste is applied to the positive electrode current collector 52, dried, and optionally pressed to prepare the positive electrode 50. In this specification, the term "paste" is used to encompass forms known as "slurry" and "ink."

[0029] The negative electrode 60 used in the assembly preparation step S1 may be a known negative electrode used in lithium-ion secondary batteries. The negative electrode 60 typically includes, for example, a negative electrode current collector 62 and a negative electrode active material layer 64 supported on the negative electrode current collector 62, as shown in FIG. 3. The negative electrode active material layer 64 may be provided on one side or both sides of the negative electrode current collector 62, but is preferably provided on both sides. As shown in FIG. 3, the negative electrode 60 typically includes a negative electrode active material layer-free portion 62a (i.e., a portion where the negative electrode active material layer 64 is not formed and the negative electrode current collector 62 is exposed).

[0030] A known negative electrode current collector used in lithium ion secondary batteries may be used as the negative electrode current collector 62, and examples thereof include a sheet or foil made of a metal with good conductivity (e.g., copper, nickel, titanium, stainless steel, etc.). Copper foil is preferred as the negative electrode current collector 62.

[0031] The dimensions of the negative electrode current collector 62 are not particularly limited and may be determined appropriately depending on the battery design. When a copper foil is used as the negative electrode current collector 62, the thickness thereof is not particularly limited, but is, for example, 5 μm to 35 μm, and preferably 7 μm to 20 μm.

[0032] The negative electrode active material layer 64 contains a negative electrode active material. As the negative electrode active material, for example, a carbon material such as graphite, hard carbon, or soft carbon can be used. The graphite may be natural graphite or artificial graphite, or may be amorphous carbon-coated graphite in which graphite is coated with an amorphous carbon material.

[0033] The average particle diameter (median diameter: D50) of the negative electrode active material is not particularly limited, but is, for example, 0.1 μm to 50 μm, preferably 1 μm to 25 μm, and more preferably 5 μm to 20 μm. The average particle diameter (D50) of the negative electrode active material can be determined, for example, by a laser diffraction scattering method.

[0034] The negative electrode active material layer 64 may contain components other than the active material, such as a binder or a thickener. Examples of binders that may be used include styrene butadiene rubber (SBR) and polyvinylidene fluoride (PVDF). Examples of thickeners that may be used include carboxymethyl cellulose (CMC).

[0035] The content of the negative electrode active material in the negative electrode active material layer is preferably 90% by mass or more, more preferably 95% by mass or more and 99% by mass or less. The content of the binder in the negative electrode active material layer is preferably 0.1% by mass or more and 8% by mass or less, more preferably 0.5% by mass or more and 3% by mass or less. The content of the thickener in the negative electrode active material layer is preferably 0.3% by mass or more and 3% by mass or less, more preferably 0.5% by mass or more and 2% by mass or less.

[0036] The thickness of the negative electrode active material layer 64 is not particularly limited, but is, for example, 10 μm or more and 300 μm or less, and preferably 20 μm or more and 200 μm or less.

[0037] The negative electrode 60 can be prepared by a known method. For example, the negative electrode 60 can be prepared by preparing a negative electrode paste containing a negative electrode active material and optional components, applying the negative electrode paste to the negative electrode current collector 62, drying the paste, and pressing the paste as necessary.

[0038] The positive electrode 50 and the negative electrode 60 are typically used as a wound electrode body 20 in which the positive electrode 50 and the negative electrode 60 are stacked with a separator 70 interposed therebetween.

[0039] The separator 70 may be a porous sheet (film) made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, or polyamide. Such a porous sheet may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). A heat-resistant layer (HRL) may be provided on the surface of the separator 70.

[0040] The thickness of the separator 70 is not particularly limited, but is, for example, 5 μm or more and 50 μm or less, and preferably 10 μm or more and 30 μm or less.

[0041] The wound electrode body 20 can be fabricated according to a known method. In this embodiment, the outermost surface of the wound electrode body 20 is the negative electrode 60. Specifically, first, the positive electrode 50 and the negative electrode 60 are stacked together with a separator 70 interposed therebetween. At this time, as shown in FIG. 3 , the positive electrode 50 and the negative electrode 60 are stacked together so that the positive electrode active material layer non-forming portion 52a and the negative electrode 60 non-forming portion 62a extend in opposite directions from the widthwise ends of the two separators 70. In this embodiment, the negative electrode 60 is formed by attaching a metal lithium foil 10 to a predetermined position on the outermost periphery of the negative electrode 60, which will not face the positive electrode 50 when the wound electrode body 20 is formed.

[0042] Next, the obtained laminate is wound. The winding of this laminate can be carried out according to a known method. For example, it can be carried out by using a winding machine equipped with a known winding core to wind the laminate on the outer peripheral surface of the core. The winding conditions may be the same as known conditions.

[0043] Here, the number of windings of the wound electrode body 20 is not particularly limited as long as the effects of the technology disclosed herein are exhibited. On the other hand, the greater the number of windings of the wound electrode body 20, the more likely it is that uniform diffusion of Li ions into the inside of the negative electrode 60 becomes insufficient, and Li deposition resistance tends to decrease. That is, from this perspective, a wound electrode body with a large number of windings is suitable as a target for applying the technology disclosed herein. From this perspective, the number of windings of the wound electrode body 20 is preferably 4 times (in other words, 4 turns) or more, more preferably 10 times or more, even more preferably 20 times or more, even more preferably 25 times or more, and particularly preferably 30 times or more. The number of windings of the wound electrode body 20 may be, for example, 100 times or less, 70 times or less, or 50 times or less.

[0044] Next, in this embodiment, the wound laminate is pressed to produce a flat wound electrode body. As shown in FIG. 2, the wound electrode body 20 after this pressing process has a pair of curved portions (R portions) 20r and a flat portion 20f connecting the pair of curved portions 20r. This pressing process can be performed by pressing the laminate wound in the above-mentioned winding step using a known pressing device used in the manufacture of general flat wound electrode bodies. The pressing conditions may be the same as known conditions. However, in other embodiments, this pressing process may not be performed, and the wound electrode body may be cylindrical, etc.

[0045] 2, a metal lithium foil 10 is applied (attached) to at least a portion of the region of the negative electrode 60 at the outermost periphery of the wound electrode body 20 that does not face the positive electrode. An adhesive or the like can be used to apply the metal lithium foil 10. Examples of such adhesives include resin adhesives made of polypropylene (PP), polyethylene (PE), acrylic resins, etc.

[0046] As the metal lithium foil 10, for example, commercially available products can be used without particular limitation as long as the effects of the technology disclosed herein can be obtained. In the metal lithium foil 10, when the entire metal lithium foil is taken as 100 mass%, the content of metal lithium is, for example, 97 mass% or more, preferably 98 mass% or more, 99 mass% or more, or 99.5 mass% or more (e.g., 100 mass%). Components other than metal lithium may be unavoidably present in trace amounts due to, for example, raw materials or manufacturing processes. In this embodiment, the shape of the metal lithium foil 10 is rectangular; however, in other embodiments, the shape may be circular, elliptical, polygonal, or various other shapes. The size of the metal lithium foil 10 is preferably determined appropriately depending on the dimensions of the wound electrode assembly 20, etc. For example, the area where the metal lithium foil 10 is applied (when the metal lithium foil is applied in multiple locations, the total area of ​​the multiple locations) may be an area corresponding to 1% to 20% or 5% to 15% of the area of ​​the negative electrode 60 not facing the positive electrode, where the area is 100%. The thickness of the metal lithium foil 10 can be, for example, within a range of 0.01 mm to 5 mm (or 0.05 mm to 1 mm). An example of the dimensions of the metal lithium foil 10 is a length of 10 mm to 30 mm, a width of 10 mm to 30 mm, and a thickness of 0.01 mm to 5 mm.

[0047] The portion where the metal lithium foil 10 is applied is not particularly limited as long as it is the region of the negative electrode 60 not facing the positive electrode. The metal lithium foil 10 may be disposed on the curved portion 20r of the wound electrode assembly 20, on the flat portion 20f, or on both the curved portion 20r and the flat portion 20f. Note that, as in the present embodiment, when the metal lithium foil 10 is disposed on the flat portion 20f of the wound electrode assembly 20, the metal lithium foil 10 is more firmly fixed to the wound electrode assembly 20, which is preferable. Furthermore, in the present embodiment, the metal lithium foil 10 is disposed in only one location in the region of the negative electrode 60 not facing the positive electrode, but this is not limiting. In other embodiments, the metal lithium foil 10 may be disposed in multiple locations in the region of the negative electrode 60 not facing the positive electrode.

[0048] Furthermore, in this embodiment, in the wound electrode body 20, the ratio of the capacity of the negative electrode 60 to the capacity of the positive electrode 50 is 1.3 or more and less than 2.0. Here, if the basis weight of the negative electrode is increased to increase the positive / negative electrode capacity ratio, the energy density may decrease. For this reason, the positive / negative electrode capacity ratio is set to less than 2.0. Furthermore, if the positive / negative electrode capacity ratio is less than 1.3, Li deposition is more likely to occur, and the capacity retention rate of the battery may decrease. For this reason, the positive / negative electrode capacity ratio is set to 1.3 or more. Note that, from the viewpoint of improving Li deposition resistance, the positive / negative electrode capacity ratio is preferably 1.5 or more, and more preferably 1.8 or more.

[0049] The "positive and negative electrode capacity ratio" can be calculated by separately calculating the capacity of the positive electrode and the capacity of the negative electrode and then using the following formula: negative electrode capacity ÷ positive electrode capacity. The positive and negative electrode capacity ratio can be easily adjusted, for example, by changing the basis weight of the positive and negative electrodes or by changing the type of positive and negative electrode active material. The "basis weight" refers to the value obtained by dividing the mass of the electrode active material layer by the area of ​​the formation region (mass of the electrode active material layer / area of ​​the formation region). The positive electrode capacity can be calculated by, for example, preparing a half cell using Li metal as the counter electrode and initially charging it within a voltage range corresponding to the full cell (e.g., approximately 2 to 4.2 V) (i.e., the initial positive electrode charge capacity). The negative electrode capacity can be calculated by, for example, adding the initial negative electrode discharge capacity and the operating voltage range of the negative electrode.

[0050] Next, the battery case 30 is prepared. Specifically, as shown in FIG. 3, a main body of the battery case 30 having an opening and a lid for the battery case 30 are prepared. The opening has a dimension that allows the wound electrode assembly 20 to be inserted. The lid has a dimension that closes the opening of the main body of the battery case 30. The lid is also provided with a thin-walled safety valve 36 that is set to release the internal pressure of the battery case 30 when the internal pressure of the battery case 30 rises to a predetermined level or higher, and an injection port (not shown) for injecting the non-aqueous electrolyte. The battery case 30 is made of a lightweight metal material with good thermal conductivity, such as aluminum.

[0051] Furthermore, a nonaqueous electrolyte 80 (here, a nonaqueous electrolyte solution) is prepared. The nonaqueous electrolyte may be the same as a conventional one and is not particularly limited. The nonaqueous electrolyte typically contains a nonaqueous solvent and a supporting salt (electrolyte salt). As the nonaqueous solvent, various organic solvents such as carbonates, ethers, esters, nitriles, sulfones, and lactones that are used in electrolyte solutions of general lithium ion secondary batteries can be used without any particular limitation. Specific examples include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoromethyl difluoromethyl carbonate (F-DMC), and trifluorodimethyl carbonate (TFDMC). Such nonaqueous solvents can be used alone or in appropriate combinations of two or more.

[0052] As the supporting salt, for example, lithium salts such as LiPF6, LiBF4, and lithium bis(fluorosulfonyl)imide (LiFSI) (preferably LiPF6) can be suitably used. The concentration of the supporting salt is preferably 0.7 mol / L or more and 1.3 mol / L or less.

[0053] The nonaqueous electrolyte 80 may contain various additives other than the above-mentioned components, such as film-forming agents such as vinylene carbonate (VC) and oxalate complexes; gas generating agents such as biphenyl (BP) and cyclohexylbenzene (CHB); and thickeners, as long as the effects of the present invention are not significantly impaired.

[0054] Next, the positive electrode terminal 42 and positive electrode current collector 42a, and the negative electrode terminal 44 and negative electrode current collector 44a are attached to the lid of the battery case 30. The positive electrode current collector 42a and negative electrode current collector 44a are welded by ultrasonic welding, resistance welding, or the like to the positive electrode active material layer-free portion 52a and negative electrode active material layer-free portion 62a exposed at the ends of the wound electrode body 20, respectively. The wound electrode body 20 is then placed inside the battery case 30 body through the opening thereof.

[0055] Next, a non-aqueous electrolyte solution is poured into the battery case 30 through the filling port in the lid. After the non-aqueous electrolyte solution is poured, the filling port is sealed to obtain an assembly. The filling port can be sealed by a known method. In this manner, an assembly can be obtained.

[0056] (Initial charging process: step S2) As described above, in this step, the assembly is initially charged. In this initial charging step, the assembled battery is preferably charged to a predetermined voltage at room temperature. For example, an external power source may be connected between the positive electrode (positive electrode terminal) and the negative electrode (negative electrode terminal) of the assembly, and charging may be performed to a predetermined voltage (e.g., constant-current, constant-voltage charging). The room temperature range in the initial charging step refers to a temperature range considered to be room temperature, and may be 20°C ± 15°C. The temperature to which the assembly is exposed in this initial charging step may be selected from a temperature range of 5°C to 35°C, preferably 20°C to 30°C. The voltage between the positive and negative electrode terminals in the initial charging step (e.g., maximum voltage) may vary depending on the type of active material and nonaqueous solvent used, but may be within a voltage range that can be exhibited when the SOC of the assembly is approximately 80% or more (e.g., 90 to 105%) of the fully charged state of charge (e.g., the rated capacity of the battery). The charge rate in the initial charge process may be the same as a conventionally known charge rate that is generally employed when initially charging a conventional assembly, for example, about 0.1 to 10 C. This charge process may be performed once, or may be repeated two or more times, for example, with a discharge process in between.

[0057] (High-temperature aging process: step S3) As described above, in this step, the assembly after the initial charge is subjected to aging treatment for a predetermined time at a temperature of 60°C or higher in a charged state where the potential of the negative electrode 60 is 0.5V or higher. By aging (holding or leaving) the assembly and the negative electrode 60 in a charged state where the potential of the negative electrode 60 is 0.5V or higher in this way, Li ions eluted from the metal lithium foil 10 can be favorably diffused into the negative electrode 60. This allows the capacity retention rate of the battery 100 to be favorably maintained. In this high-temperature aging step, from the viewpoint of more favorably achieving the effects described above, the potential of the negative electrode 60 is preferably 1V or higher, and may be 2V or higher or 3V or higher. The potential of the negative electrode 60 can be, for example, 5V or lower, or 4V or lower. In this high-temperature aging step, from the viewpoint of more favorably achieving the effects described above, the temperature at which the aging treatment is performed is preferably 70°C or higher. The temperature at which the aging treatment is performed can be, for example, 90°C or lower, or 80°C or lower. The time for such high-temperature aging treatment can be, for example, a total time from the start of temperature rise of 5 to 100 hours, preferably 10 to 50 hours. As a means for heating the assembly, for example, a thermostatic bath, an infrared heater, or the like can be appropriately used.

[0058] As described above, the method for manufacturing a nonaqueous electrolyte secondary battery, which includes the assembly preparation step (step S1), the initial charging step (step S2), and the high-temperature aging step (step S3), can provide a nonaqueous electrolyte secondary battery that favorably achieves both lithium precipitation resistance and battery durability. The nonaqueous electrolyte secondary battery obtained by the method for manufacturing a nonaqueous electrolyte secondary battery according to this embodiment has the following characteristics, for example. Here, FIG. 5 is a cross-sectional view taken along line VV in FIG. 4, and is a schematic explanatory diagram for explaining a high lithium element accumulation region in a nonaqueous electrolyte secondary battery according to one embodiment. Note that the separator is omitted from FIG. 5 for clarity.

[0059] That is, the nonaqueous electrolyte secondary battery 100 obtained by the manufacturing method of the nonaqueous electrolyte secondary battery according to this embodiment is a nonaqueous electrolyte secondary battery including a wound electrode assembly 20 in which a positive electrode 50 and a negative electrode 60 are stacked and wound with a separator 70 interposed therebetween, and a nonaqueous electrolyte 80. Also, a high lithium element accumulation region 110 is present in at least a part of the outermost periphery of the negative electrode 60, which is not facing the positive electrode 50. The ratio of the capacity of the negative electrode 60 to the capacity of the positive electrode 50 is 1.3 or more and less than 2.0.

[0060] Here, the "highly lithium-element-accumulated region" can be defined as a region showing traces of the metallic lithium foil 10 after the Li ions have been eluted during the high-temperature aging process. The traces of the metallic lithium foil 10 may be scraps of the metallic lithium foil 10 or scattered pieces of the metallic lithium foil 10. Alternatively, the metallic lithium foil 10 itself may have disappeared, leaving a region where the Li element is present at a high concentration. In this specification, the "highly lithium-element-accumulated region" refers to a region of the outermost negative electrode 60 that contains a high concentration of Li element. In such a region, the ratio of Li atoms to all atoms may be 5 atomic % or more, or 10 atomic % or more. This Li atom ratio can be measured by a known method (e.g., ICP atomic emission spectroscopy, etc.).

[0061] In one embodiment, the ratio of the capacity of the negative electrode 60 to the capacity of the positive electrode 50 is 1.5 or greater, and may be 1.8 or greater.

[0062] In one embodiment, the number of windings of the wound electrode body 20 is, for example, 4 times (in other words, 4 turns) or more, and may be 10 times or more, 20 times or more, 25 times or more, or 30 times or more. The number of windings of the wound electrode body 20 may be 100 times or less, 70 times or less, or 50 times or less.

[0063] <Battery uses> Battery 100 can be used for a variety of purposes, but is preferably used, for example, as a power source (driving power source) for motors mounted on vehicles such as passenger cars and trucks. The type of vehicle is not particularly limited, but examples include plug-in hybrid electric vehicles (PHEVs), hybrid electric vehicles (HEVs), and battery electric vehicles (BEVs). Battery 100 has reduced variation in battery reaction, and is therefore preferably used to construct a battery pack.

[0064] Although one embodiment of the present disclosure has been described above, the above embodiment is merely an example. The present disclosure can be implemented in various other forms. The present disclosure can be implemented based on the contents disclosed in this specification and the common general technical knowledge in the relevant field. The technology described in the claims includes various modifications and alterations of the above-exemplified embodiment. For example, it is possible to replace part of the above-described embodiment with other modified embodiments, and it is also possible to add other modified embodiments to the above-described embodiment. Furthermore, if a technical feature is not described as essential, it can be deleted as appropriate.

[0065] For example, although the above embodiment has been described with respect to a case where a flat prismatic lithium ion secondary battery is used, the present invention is not limited to this. In other embodiments, the lithium ion secondary battery may be configured as a cylindrical lithium ion secondary battery, etc.

[0066] The nonaqueous electrolyte secondary battery disclosed herein can be constructed as a nonaqueous electrolyte secondary battery other than a lithium ion secondary battery according to known methods.

[0067] Test examples relating to the present invention will be described below, but it is not intended that the present invention be limited to those shown in these test examples.

[0068] <Preparation of Sample 1> LiNi as a positive electrode active material1 / 3 Co 1 / 3 Mn 1 / 3 O2 (LNCM), acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were mixed with N-methylpyrrolidone (NMP) in a mass ratio of LNCM:AB:PVDF = 90:5:5 to prepare a slurry for forming a positive electrode active material layer. This slurry was applied to both sides of a long aluminum foil with a basis weight of 10 mg / cm. 2 After coating and drying, the density of the positive electrode active material layer was 2.5 g / cm 3 The mixture was rolled and pressed until it reached a thickness of 100 mm, thereby preparing a positive electrode.

[0069] A natural graphite-based carbon material was prepared as the negative electrode active material. This natural graphite-based carbon material (C), styrene butadiene rubber (SBR), and carboxymethyl cellulose (CMC) were mixed with ion-exchanged water in a mass ratio of C:SBR:CMC=97:2:1 to prepare a slurry for forming a negative electrode active material layer. This slurry was applied to both sides of a long copper foil with a basis weight of 9 mg / cm. 2 After coating and drying, the density of the negative electrode active material layer was 1.2 g / cm 3 The mixture was rolled and pressed until it reached a thickness of 100 mm, thereby preparing a negative electrode.

[0070] Two separators (porous polyolefin sheets with a three-layer structure of PP / PE / PP) were prepared. The prepared positive and negative electrodes were stacked facing each other with the separator interposed therebetween and wound to prepare a wound electrode assembly. Here, the wound electrode assembly was wound 30 times. The outermost periphery of the wound electrode assembly was the negative electrode, and a rectangular metallic lithium foil was attached to a portion of the outermost negative electrode's region not facing the positive electrode. The metallic lithium foil was attached in the position shown in FIG. 2. The dimensions of the metallic lithium foil were 15 mm long, 15 mm wide, and 5 mm thick, relative to the region of the outermost negative electrode not facing the positive electrode (length: 40 mm, width: 48 mm). An electrode current collector plate was welded to the wound electrode assembly, which was then housed in a rectangular aluminum battery case and filled with a nonaqueous electrolyte. The non-aqueous electrolyte was prepared by dissolving LiPF6 as a supporting electrolyte at a concentration of 1.1 mol / L in a mixed solvent containing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of EC:EMC:DMC = 3:4:3. The battery case was then sealed to prepare an assembly.

[0071] Next, using a commercially available charger in a temperature environment of 25°C, the assembly was charged at a constant current of 0.1 C to 4.15 V, with the positive terminal as the anode and the negative terminal as the cathode, and then discharged at a constant current of 3 V. This charge / discharge cycle was repeated twice as an initial charge. The capacity at the second discharge was measured and used as the initial capacity.

[0072] The obtained lithium ion secondary battery was discharged to 2 V, and then subjected to aging treatment for 12 hours in a thermostatic chamber at 60° C. In this manner, a lithium ion secondary battery for evaluation according to Sample 1 was produced.

[0073] <Preparation of Samples 2 to 4> Lithium ion secondary batteries for evaluation according to Samples 2 to 4 were fabricated in the same manner as Sample 1, except that the positive and negative electrode capacity ratios were set as shown in Table 1. Here, the positive and negative electrode capacity ratios were adjusted by changing the basis weight of the negative electrode.

[0074] <Preparation of Samples 5 to 8> An evaluation lithium-ion secondary battery according to Samples 5 to 8 was fabricated in the same manner as Samples 1 to 4, except that a lithium metal foil was not provided on a part of the region not facing the positive electrode of the outermost negative electrode.

[0075] <Capacity Retention Rate Evaluation after Li Deposition Cycle Test> Each of the evaluation lithium-ion secondary batteries fabricated as described above was adjusted to an SOC of 50% in a temperature environment of 25°C. Then, it was placed in a temperature environment of -10°C, and charging at 20C for 10 seconds and discharging at 2C for 100 seconds were repeated for 200 cycles as one cycle for each of the evaluation lithium-ion secondary batteries. Then, it was placed in a temperature environment of 25°C, and the discharge capacity was measured in the same manner as the initial capacity. The capacity retention rate (%) was determined from the formula: (discharge capacity after charge-discharge cycles / initial capacity) × 100. The results are shown in the corresponding columns of Table 1. Here, when such a capacity retention rate is 65% or more, it is evaluated that the capacity retention rate after the Li deposition cycle test is excellent.

[0076] <Capacity Retention Rate Evaluation after 60°C Storage Test> Each of the evaluation lithium-ion secondary batteries fabricated as described above was adjusted to an SOC of 80% in a temperature environment of 25°C. Each of these evaluation lithium-ion secondary batteries was placed in a constant temperature layer at 60°C and stored for 60 days. Then, the discharge capacity after storage was measured in the same manner as the initial capacity. The capacity retention rate (%) was determined from the formula: (discharge capacity after high-temperature storage / initial capacity) × 100. The results are shown in Table 1. Here, when such a capacity retention rate is 95% or more, it is evaluated that the capacity retention rate after the 60°C storage test is excellent.

[0077]

Table 1

[0078] As shown in Table 1, the lithium-ion secondary batteries for evaluation according to Samples 1 to 3 obtained by the method for producing a nonaqueous electrolyte secondary battery disclosed herein were confirmed to have superior capacity retention rates after Li deposition cycles and after a 60°C storage test compared to Sample 4, which had a positive / negative electrode capacity ratio outside the range of 1.3 or more and less than 2.0, Samples 5 to 7, which had a positive / negative electrode capacity ratio within the range of 1.3 or more and less than 2.0 but did not have a metallic lithium foil applied to a portion of the outermost negative electrode region not facing the positive electrode, and Sample 8, which had a positive / negative electrode capacity ratio outside the range of 1.3 or more and less than 2.0 and did not have a metallic lithium foil applied to a portion of the outermost negative electrode region not facing the positive electrode. This indicates that the batteries obtained by the method for producing a nonaqueous electrolyte secondary battery disclosed herein favorably achieve both lithium deposition resistance and battery durability.

[0079] Furthermore, when comparing the evaluation lithium ion secondary batteries according to Samples 1 to 3, it was confirmed that when the positive and negative electrode capacity ratio was 1.5 or more (more preferably 1.8 or more), the capacity retention rate after Li deposition cycles was particularly excellent.

[0080] Although the embodiments of the present invention have been described above, the above embodiments are merely examples. The present invention can be implemented in various other forms. The present invention can be implemented based on the contents disclosed in this specification and the technical common sense in the relevant field. The technology described in the claims includes various modifications and alterations of the above-exemplified embodiments. [Explanation of symbols]

[0081] 10. Lithium metal foil 20 Wound electrode body 30 Battery case 36 Safety valve 42 Positive terminal 42a Positive current collector plate 44 Negative terminal 44a Negative current collector plate 50 Positive electrode (positive electrode sheet) 52 Positive electrode current collector 52a Portion where positive electrode active material layer is not formed 54 Cathode active material layer 60 Negative electrode (negative electrode sheet) 62 Negative electrode current collector 62a Part where negative electrode active material layer is not formed 64 Negative electrode active material layer 70 Separator 100 Nonaqueous electrolyte secondary battery 110 High lithium concentration region

Claims

1. a flat wound electrode body in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween and wound; a non-aqueous electrolyte; A method for manufacturing a non-aqueous electrolyte secondary battery, comprising the following steps: an assembly preparation step of preparing an assembly in which the wound electrode body and the non-aqueous electrolyte are housed in a battery case, wherein the wound electrode body has a pair of curved portions and a flat portion connecting the pair of curved portions, and is wound at least 30 times, and in the wound electrode body, the ratio of the capacity of the negative electrode to the capacity of the positive electrode is 1.3 or more and less than 2.0, and a metallic lithium foil is present in at least a part of a region of the flat portion that is the outermost periphery of the negative electrode and does not face the positive electrode; an initial charging step of initially charging the assembly; and a high-temperature aging step of subjecting the assembly after the initial charge to an aging treatment at a temperature of 60° C. or higher in a charged state in which the difference between the potential of the positive electrode and the potential of the negative electrode is at least 2 V; The method for producing a non-aqueous electrolyte secondary battery includes the steps of:

2. 2. The method for producing a nonaqueous electrolyte secondary battery according to claim 1, wherein the ratio of the capacity of said negative electrode to the capacity of said positive electrode is 1.5 or more.

3. a flat wound electrode body in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween and wound; a non-aqueous electrolyte; A non-aqueous electrolyte secondary battery comprising: the wound electrode body has a pair of curved portions and a flat portion connecting the pair of curved portions, and is wound at least 30 times; a high lithium element accumulation region is present in at least a part of the flat portion, which is the outermost periphery of the negative electrode and does not face the positive electrode; a ratio of the capacity of the negative electrode to the capacity of the positive electrode being 1.3 or more and less than 2.0;

4. 4. The nonaqueous electrolyte secondary battery according to claim 3, wherein the ratio of the capacity of the negative electrode to the capacity of the positive electrode is 1.5 or more.

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