JP7766639B2 - Nonaqueous electrolyte secondary battery and method for manufacturing the same - Google Patents

Description

This disclosure relates to a non-aqueous electrolyte secondary battery and a method for manufacturing the non-aqueous electrolyte secondary battery.

For example, Patent Document 1 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, with the positive and negative electrode plates wound in a spiral shape together with a separator. The nonaqueous electrolyte secondary battery is characterized in that a metallic lithium foil is attached to a portion of the negative electrode plate that corresponds to the outermost periphery but does not face the positive electrode plate, and the lithium is diffused into the carbon material due to a potential difference or concentration difference.

Japanese Patent Application Publication No. 5-144473

However, according to the inventors' research, it has been found that there is still room for improvement in the non-aqueous electrolyte secondary batteries described above, for example, in terms of battery performance (e.g., lithium precipitation resistance and battery durability).

This disclosure was made in light of these circumstances, and its main purpose is to provide technology for obtaining non-aqueous electrolyte secondary batteries with excellent battery performance.

To achieve this objective, 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 the ratio of the capacity of the negative electrode to the capacity of the positive electrode in the wound electrode assembly is 1.3 or more and less than 2.0, and metal 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, this method for manufacturing a non-aqueous electrolyte secondary battery with such a configuration enables the production of a non-aqueous electrolyte secondary battery with excellent battery performance.

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 and wound with a separator interposed therebetween, and a nonaqueous electrolyte, wherein a high lithium element accumulation region is present in at least a portion 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 can therefore be said to be a nonaqueous electrolyte secondary battery with excellent battery performance.

2 is a flowchart showing steps of a method for manufacturing a nonaqueous electrolyte secondary battery according to one embodiment. FIG. 10 is a schematic diagram showing the configuration of a wound electrode body prepared in an assembly preparation step according to one embodiment. 1 is a schematic diagram showing the configuration of a wound electrode body included in a nonaqueous electrolyte secondary battery according to one embodiment. 1 is a cross-sectional view schematically illustrating the configuration of a nonaqueous electrolyte secondary battery according to one embodiment. FIG. 2 is a schematic diagram illustrating a high lithium element accumulation region in a nonaqueous electrolyte secondary battery according to one embodiment.

Below, several embodiments of the technology disclosed herein are described 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 not specifically mentioned in this specification but necessary for implementing the technology disclosed herein (for example, the general structure and manufacturing process of a battery that does 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."

In this technology, the term "secondary battery" refers to a general energy storage device that can be repeatedly charged and discharged, and includes so-called storage batteries such as lithium-ion secondary batteries and lithium polymer batteries, as well as energy storage elements such as electric double-layer capacitors. A "nonaqueous electrolyte secondary battery" refers to a secondary battery that uses a nonaqueous electrolyte as a charge carrier to achieve charging and discharging. The electrolyte may be either a gel electrolyte or a nonaqueous electrolyte. A configuration that can benefit from the benefits of this technology may be, for example, a nonaqueous electrolyte solution that is liquid at room temperature (e.g., 25°C) and contains a supporting salt (electrolyte salt) that acts as a charge carrier dissolved in a nonaqueous solvent. An "active material" refers to a substance that can reversibly absorb and release chemical species that act as charge carriers in a secondary battery. In this technology, the term "SOC" refers to the state of charge (SOC), which indicates the state of charge at which the upper limit of the operating voltage range for reversible charging and discharging is obtained, defined as 100%, and the state of charge at which the lower limit of the voltage range for reversible charging and discharging is obtained, defined as 0%.

As an example, an embodiment in which the nonaqueous electrolyte secondary battery is a flat prismatic lithium ion secondary battery will be described below. FIG. 1 is a flowchart showing the steps of a method for manufacturing a nonaqueous electrolyte secondary battery according to one embodiment. FIG. 2 is a schematic diagram showing the configuration of a wound electrode assembly prepared in an assembly preparation step according to one embodiment. FIG. 3 is a schematic diagram showing the configuration of a wound electrode assembly included in a nonaqueous electrolyte secondary battery according to one embodiment. FIG. 4 is a cross-sectional view showing the configuration of a nonaqueous electrolyte secondary battery according to one embodiment. FIG. 2 is a corresponding view of the wound electrode assembly prepared in the assembly preparation step according to one embodiment. For clarity, the separator is omitted from FIG. 2. The method for manufacturing a nonaqueous electrolyte secondary battery disclosed herein may include additional steps at any stage, and steps that are not described as essential may be omitted as appropriate. The order of the steps may be reversed as long as the effects of the technology disclosed herein are achieved.

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 and wound with a separator interposed therebetween, and a nonaqueous electrolyte. As shown in FIG. 1, the method for manufacturing such a nonaqueous electrolyte secondary battery includes an assembly preparation step in which an assembly in which a wound electrode assembly and a nonaqueous electrolyte are housed in a battery case is prepared; an initial charging step in which the assembly is initially charged; and a high-temperature aging step in which the assembly after the initial charge is aged 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 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") 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 region of the negative electrode that does not face the positive electrode (hereinafter also simply referred to as the "negative electrode non-facing region")

For example, one method for improving Li (lithium) deposition resistance is to increase the positive/negative electrode capacity ratio. However, it is known that such a method tends to reduce battery durability (which can also be referred to as the battery's capacity retention rate) due to excessive formation of an SEI film. Therefore, further development of technology that can achieve both Li deposition resistance and battery durability is needed. After extensive research, the inventors have found that excellent Li deposition resistance can be achieved by increasing the positive/negative electrode capacity ratio to 1.3 or more and less than 2.0 in a wound electrode assembly. Furthermore, by applying a metallic lithium foil to at least a portion of the outermost periphery of the negative electrode that does not face the positive electrode, Li ions can be diffused and replenished within the negative electrode during high-temperature aging as described above, thereby achieving excellent battery durability. That is, the manufacturing method for a nonaqueous electrolyte secondary battery, which includes the above-described assembly preparation step (step S1), initial charging step (step S2), and high-temperature aging step (step S3), can produce a nonaqueous electrolyte secondary battery that combines excellent Li deposition resistance with excellent battery durability. Each step is described below. Note that the structure of the assembly is similar to that of nonaqueous electrolyte secondary battery 100, except that the wound electrode body includes metallic lithium foil, and therefore, FIG. 4 can be referenced as appropriate when describing the assembly.

(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 ).

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. 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), as shown in FIG. 3.

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

The positive electrode current collector 52 may be a known positive electrode current collector used in lithium-ion secondary batteries, such as 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.

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

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, spinel structure, olivine structure, or the like.

Examples of lithium composite oxides include lithium nickel composite oxides, lithium cobalt composite oxides, lithium manganese composite oxides, lithium nickel manganese composite oxides, lithium nickel cobalt manganese composite oxides, lithium nickel cobalt aluminum composite oxides, and lithium iron nickel manganese composite oxides.

In this specification, the term "lithium nickel cobalt manganese composite oxide" encompasses 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 non-metal 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.

Examples of lithium transition metal phosphate compounds include lithium iron phosphate (LiFePO ₄ ), lithium manganese phosphate (LiMnPO ₄ ), and lithium manganese iron phosphate.

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

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

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).

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 or more and 97% by mass or less, and even more preferably 85% by mass or more and 96% by mass or less. The content of trilithium phosphate in the positive electrode active material layer 54 is not particularly limited, but is preferably 1% by mass or more and 15% by mass or less, and more preferably 2% by mass or more and 12% by mass or less. The content of the conductive material in the positive electrode active material layer 54 is not particularly limited, but is preferably 1% by mass or more and 15% by mass or less, and more preferably 3% by mass or more and 13% by mass or less. The content of the binder in the positive electrode active material layer 54 is not particularly limited, but is preferably 1% by mass or more and 15% by mass or less, and more preferably 1.5% by mass or more and 10% by mass or less.

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.

The positive electrode 50 can be prepared by known methods. 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. Note that in this specification, the term "paste" is used to encompass forms known as "slurry" and "ink."

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 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, for example. 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. 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), as shown in FIG. 3.

The negative electrode current collector 62 may be a known negative electrode current collector used in lithium-ion secondary batteries, such as 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.

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

The negative electrode active material layer 64 contains a negative electrode active material. Examples of the negative electrode active material that can be used include carbon materials such as graphite, hard carbon, and soft carbon. Graphite may be natural graphite or artificial graphite, or it may be amorphous carbon-coated graphite, in which graphite is coated with an amorphous carbon material.

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

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).

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.

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.

The negative electrode 60 can be prepared by a known method. For example, a negative electrode paste containing a negative electrode active material and optional components is prepared, and the negative electrode paste is applied to the negative electrode current collector 62, dried, and pressed as necessary to prepare the negative electrode 60.

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

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.

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.

The wound electrode assembly 20 can be fabricated using known methods. In this embodiment, the outermost surface of the wound electrode assembly 20 forms the negative electrode 60. Specifically, the positive electrode 50 and the negative electrode 60 are first stacked together with a separator 70 interposed therebetween. 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-free portion 52a and the negative electrode 60 are stacked together so that they extend in opposite directions from the widthwise ends of the two separators 70. In this embodiment, the negative electrode 60 is fabricated by attaching a metallic 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 assembly 20 is completed.

The resulting laminate is then wound. This winding of the 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 around the outer surface of the core. The winding conditions may be the same as known conditions.

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 achieved. 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 interior of the negative electrode 60 will be insufficient, and Li deposition resistance will tend to decrease. In other words, from this perspective, wound electrode bodies with a large number of windings are suitable as targets 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.

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 winding process 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.

Next, as shown in FIG. 2, a metallic 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 assembly 20 that is not facing the positive electrode. An adhesive, for example, can be used to apply the metallic lithium foil 10. One example of such an adhesive is a resin adhesive made from polypropylene (PP), polyethylene (PE), acrylic resin, or the like.

As the metal lithium foil 10, for example, commercially available products can be used without particular restrictions 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% by mass, the content of metal lithium is, for example, 97% by mass or more, preferably 98% by mass or more, 99% by mass or more, or 99.5% by mass or more (e.g., 100% by mass). Components other than metal lithium may be unavoidably present in trace amounts due to, for example, raw materials or manufacturing processes. While the shape of the metal lithium foil 10 is rectangular in this embodiment, in other embodiments, it may be circular, elliptical, polygonal, or various other shapes. The size of the metal lithium foil 10 is preferably determined appropriately based on the dimensions of the wound electrode assembly 20, etc. For example, the area where the metal lithium foil 10 is applied (if the metal lithium foil is applied in multiple locations, the total area of those multiple locations) may be an area equivalent to 1% to 20% or 5% to 15% of the area of the negative electrode 60 not facing the positive electrode, where 100% is the area. The thickness of the metal lithium foil 10 can be, for example, within the range of 0.01 mm to 5 mm (or 0.05 mm to 1 mm). Examples of dimensions of the metal lithium foil 10 include 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.

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 this embodiment, the metal lithium foil 10 is preferably disposed on the flat portion 20f of the wound electrode assembly 20, because the metal lithium foil 10 is more firmly fixed to the wound electrode assembly 20. Furthermore, in this 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 a limitation. 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.

In addition, in this embodiment, 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. Here, increasing the basis weight of the negative electrode to increase the positive/negative electrode capacity ratio can reduce the energy density. 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 becomes more likely to occur, and the capacity retention rate of the battery can decrease. For this reason, the positive/negative electrode capacity ratio is set to 1.3 or more. From the perspective of achieving better Li deposition resistance, the positive/negative electrode capacity ratio is preferably 1.5 or more, and more preferably 1.8 or more.

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, 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.

Next, the battery case 30 is prepared. Specifically, as shown in FIG. 3, a battery case 30 body having an opening and a lid for the battery case 30 are prepared. The opening is sized to allow the wound electrode assembly 20 to be inserted. The lid is sized to close the opening in the battery case 30 body. The lid is also provided with a thin-walled safety valve 36 that is designed to release the internal pressure of the battery case 30 if the internal pressure rises above a predetermined level, 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.

A nonaqueous electrolyte 80 (here, a nonaqueous electrolyte solution) is also prepared. The nonaqueous electrolyte may be the same as conventional electrolytes and is not particularly limited. The nonaqueous electrolyte typically contains a nonaqueous solvent and a supporting salt (electrolyte salt). The nonaqueous solvent may be any organic solvent commonly used in electrolytes for lithium-ion secondary batteries, including carbonates, ethers, esters, nitriles, sulfones, and lactones. 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). These nonaqueous solvents may be used alone or in combination of two or more.

As the supporting salt, for example, a lithium salt such as LiPF ₆ , LiBF ₄ , or lithium bis(fluorosulfonyl)imide (LiFSI) (preferably LiPF ₆ ) 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.

In addition, the non-aqueous electrolyte 80 may contain various additives other than those mentioned above, 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.

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 exposed positive electrode active material layer-free portion 52a and negative electrode active material layer-free portion 62a at the ends of the wound electrode body 20, respectively. The wound electrode body 20 is then inserted into the battery case 30 through the opening in the body.

Next, non-aqueous electrolyte is poured into the filling port in the lid of the battery case 30. After pouring the non-aqueous electrolyte, the filling port is sealed to obtain an assembly. The filling port can be sealed using known methods. In this manner, the assembly can be obtained.

(Initial charging process: step S2)
As described above, in this step, the assembly is initially charged. In this initial charging, it is preferable to charge the constructed assembly 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 (e.g., constant current/constant voltage charging) may be performed to a predetermined voltage. Here, 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 process may be selected from a temperature range of, for example, 5°C to 35°C, preferably 20°C to 30°C. Furthermore, the voltage between the positive and negative electrode terminals in the initial charging process (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 charging process may be the same as a conventionally known charge rate that can generally be adopted when initially charging a conventional assembly, for example, about 0.1 to 10 C. This charging process may be performed once, or may be repeated two or more times, for example, with a discharge process in between.

(High-temperature aging process: step S3)
As described above, in this step, the assembly after the initial charge is subjected to an 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. In this way, by performing an aging treatment (holding or resting) at a high temperature of 60°C or higher in a charged state where the potential of the assembly and the negative electrode 60 is 0.5V or higher, Li ions eluted from the metallic lithium foil 10 can be favorably diffused into the interior of 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.

As described above, the method for manufacturing a nonaqueous electrolyte secondary battery, which includes the assembly preparation step (step S1), initial charging step (step S2), and high-temperature aging step (step S3), can produce a nonaqueous electrolyte secondary battery that favorably achieves both lithium precipitation resistance and battery durability. Furthermore, 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, Figure 5 is a cross-sectional view taken along line V-V in Figure 4, and is a schematic diagram for explaining the high lithium element accumulation region in a nonaqueous electrolyte secondary battery according to one embodiment. Note that the separator is omitted from Figure 5 for clarity.

That is, the nonaqueous electrolyte secondary battery 100 obtained by the method for manufacturing a nonaqueous electrolyte secondary battery according to this embodiment is a nonaqueous electrolyte secondary battery comprising 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. Furthermore, a high lithium element concentration region 110 is present in at least a portion of the outermost periphery of the negative electrode 60 that does not face 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.

Here, the "high lithium element accumulation 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 may be 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 "high lithium element accumulation 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.).

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.

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

<Battery uses>
Battery 100 can be used for a variety of purposes, but can be suitably used, for example, as a power source (driving power source) for a motor mounted on a vehicle such as a passenger car or truck. 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 can therefore be suitably used to construct a battery pack.

The above describes one embodiment of the present disclosure, but the above embodiment is merely one example. The present disclosure can be implemented in various other forms. The present disclosure can be implemented based on the content disclosed in this specification and the common general technical knowledge in the relevant field. The technology described in the claims includes various modifications and variations 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 may be deleted as appropriate.

For example, in the above embodiment, a flat prismatic lithium ion secondary battery is described, but this is not limiting. In other embodiments, the lithium ion secondary battery may be configured as a cylindrical lithium ion secondary battery, etc.

The nonaqueous electrolyte secondary battery disclosed herein can be constructed as a nonaqueous electrolyte secondary battery other than a lithium ion secondary battery using known methods.

Test examples related to the present invention are described below, but it is not intended that the present invention be limited to those shown in these test examples.

<Preparation of Sample 1>
A positive electrode active material layer was prepared by mixing LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (LNCM) as the positive electrode active material, acetylene black (AB) as the conductive material, and polyvinylidene fluoride (PVDF) as the binder with N-methylpyrrolidone (NMP) in a mass ratio of LNCM:AB:PVDF = 90:5:5. This slurry was applied to both sides of a long piece of aluminum foil so that the basis weight was 10 mg/cm ² and dried, and then rolled and pressed until the density of the positive electrode active material layer reached 2.5 g/cm ³ , thereby producing a positive electrode.

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 so that the basis weight was 9 mg/ ^cm2 and dried. After that, the negative electrode was fabricated by rolling and pressing until the density of the negative electrode active material layer reached 1.2 g/ ^cm3 .

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 produce a wound electrode assembly. 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 that did not face the positive electrode. The metallic lithium foil was attached in the position shown in Figure 2. The dimensions of the metallic lithium foil were 15 mm long, 15 mm wide, and 5 mm thick, relative to the outermost negative electrode that did not face 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 nonaqueous 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.

Next, in a temperature environment of 25°C, the assembly was charged at a constant current of 0.1 C to 4.15 V using a commercially available charger, 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 after the second discharge was measured and used as the initial capacity.

The resulting lithium-ion secondary battery was discharged to 2 V and then aged for 12 hours in a constant temperature chamber at 60°C. In this way, a lithium-ion secondary battery for evaluation relating to Sample 1 was produced.

<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.

<Preparation of Samples 5 to 8>
Lithium ion secondary batteries for evaluation according to Samples 5 to 8 were fabricated in the same manner as Samples 1 to 4, respectively, except that no metallic lithium foil was provided on a part of the region of the outermost negative electrode that did not face the positive electrode.

<Evaluation of capacity retention rate after Li deposition cycle test>
Each lithium-ion secondary battery for evaluation prepared as described above was adjusted to an SOC of 50% in a temperature environment of 25°C. Thereafter, the battery was placed in a temperature environment of -10°C, and each lithium-ion secondary battery for evaluation was subjected to 200 cycles of charge and discharge, each cycle consisting of charging at 20C for 10 seconds and discharging at 2C for 100 seconds. Thereafter, the battery was placed in a temperature environment of 25°C, and the discharge capacity was measured in the same manner as for the initial capacity. The capacity retention rate (%) was calculated using the formula: (discharge capacity after charge and discharge cycles/initial capacity) x 100. The results are shown in the corresponding column in Table 1. Here, a capacity retention rate of 65% or higher was evaluated as being excellent in capacity retention rate after the Li deposition cycle test.

<Evaluation of capacity retention rate after 60°C storage test>
Each lithium-ion secondary battery for evaluation prepared as described above was adjusted to an SOC of 80% in a temperature environment of 25°C. Each lithium-ion secondary battery for evaluation was placed in a thermostatic chamber at 60°C and stored for 60 days. Thereafter, the discharge capacity after storage was measured using the same method as for the initial capacity. The capacity retention rate (%) was calculated using the formula: (discharge capacity after high-temperature storage/initial capacity) x 100. The results are shown in Table 1. Here, a capacity retention rate of 95% or higher was evaluated as excellent in capacity retention rate after a 60°C storage test.

As shown in Table 1, the lithium-ion secondary batteries for evaluation, Samples 1 to 3, obtained by the manufacturing method for 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 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 metallic lithium foil applied to a portion of the outermost negative electrode region not facing the positive electrode. This demonstrates that the batteries obtained by the manufacturing method for a nonaqueous electrolyte secondary battery disclosed herein favorably achieve both lithium deposition resistance and battery durability.

Furthermore, when comparing the evaluation lithium-ion secondary batteries of Samples 1 to 3, it was confirmed that when the positive/negative electrode capacity ratio was 1.5 or higher (more preferably 1.8 or higher), the capacity retention rate after the Li deposition cycle was particularly excellent.

The above describes an embodiment of the present invention, but the above embodiment is merely an example. The present invention can be embodied in various other forms. The present invention can be implemented based on the contents disclosed in this specification and common technical knowledge in the relevant field. The technology described in the claims includes various modifications and variations of the above-described embodiment.

10 Metallic lithium foil 20 Wound electrode body 30 Battery case 36 Safety valve 42 Positive electrode terminal 42a Positive electrode current collector plate 44 Negative electrode terminal 44a Negative electrode current collector plate 50 Positive electrode (positive electrode sheet)
52 Positive electrode current collector 52a Positive electrode active material layer non-forming portion 54 Positive electrode active material layer 60 Negative electrode (negative electrode sheet)
62 Negative electrode current collector 62a Negative electrode active material layer non-forming portion 64 Negative electrode active material layer 70 Separator 100 Non-aqueous electrolyte secondary battery 110 Highly accumulated lithium element region

Claims (4)

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 aging the assembly after the initial charging at a temperature of 60° C. or higher in a charged state where 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: The method for manufacturing a nonaqueous electrolyte secondary battery according to claim 1, wherein the ratio of the capacity of the negative electrode to the capacity of the positive electrode is 1.5 or greater. 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. 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.