Lithium secondary battery

Abstract

The disclosed lithium secondary battery comprises: an electrode group in which a positive electrode, a negative electrode, and a separator (13) are wound such that the separator (13) is disposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte. In the negative electrode, lithium metal is precipitated during charging, and the lithium metal is dissolved in the non-aqueous electrolyte during discharging. The separator (13) includes a base material layer (13a) having a first main surface (13a1) disposed on the negative electrode side and a second main surface (13a2) disposed on the positive electrode side, and an adhesive layer (13b) formed on the first main surface (13a1) of the base material layer. At least a portion of the adhesive layer (13b) is adhered to the negative electrode. In a 90% charged state, the average value (Tn (m)) of the increase in the thickness of the single surface of the negative electrode due to charging and the average thickness (Ts (m)) of the separator (13) satisfy 1.2 lt. Ts / Tn.

Description

Technical Field

[0001] This disclosure relates to lithium secondary batteries. Background Technology

[0002] Lithium-ion batteries (lithium metal batteries) are high-capacity rechargeable batteries used for various applications. During charging, lithium metal is deposited at the negative electrode. The deposited lithium metal dissolves in the non-aqueous electrolyte during discharge. Various solutions have been proposed for lithium-ion batteries.

[0003] Claim 1 of Patent Document 1 (Japanese Patent Application Publication No. 2020-95931) describes "a lithium secondary battery, characterized in that it comprises a positive electrode, a negative electrode and a polymer layer disposed on the surface of the negative electrode, the negative electrode comprising a negative electrode current collector and a negative electrode active material layer, the planar dimension of the negative electrode current collector being larger than that of the positive electrode, thereby the negative electrode current collector having a first region overlapping with the positive electrode and a second region not overlapping with the positive electrode, the negative electrode active material layer being selectively disposed in the first region of the negative electrode current collector, the polymer layer comprising a copolymer of polyvinylidene fluoride (PVDF) and hexafluoropropylene (HFP) and an ionic liquid, the negative electrode current collector being covered in the first region with the negative electrode active material layer in between, and the negative electrode current collector being covered in the second region without the negative electrode active material layer in between."

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2020-95931 Summary of the Invention

[0007] The problem the invention aims to solve

[0008] Lithium-ion batteries suffer from a problem when lithium metal precipitates in a dendritic form during charging. This, in turn, leads to significant expansion of the electrode assembly during charging. These issues result in degraded charge-discharge cycle characteristics. Currently, there is a demand for improved charge-discharge cycle characteristics of lithium-ion batteries. One objective of this disclosure is to provide a lithium-ion battery with excellent charge-discharge cycle characteristics.

[0009] Solution for solving the problem

[0010] One aspect of this disclosure relates to a lithium secondary battery comprising:

[0011] An electrode assembly consisting of a positive electrode, a negative electrode, and a separator wound together such that the separator is positioned between the positive and negative electrodes; and

[0012] Non-aqueous electrolytes

[0013] In the negative electrode, lithium metal is deposited during charging, and the lithium metal dissolves in the non-aqueous electrolyte during discharging.

[0014] The separator includes a base material layer and an adhesive layer. The base material layer has a first main surface disposed on the negative electrode side and a second main surface disposed on the positive electrode side. The adhesive layer is formed on the first main surface of the base material layer.

[0015] At least a part of the adhesive layer is adhered to the negative electrode.

[0016] At a 90% charged state, the average value Tn (μm) of the increase in thickness caused by charging on one side of the negative electrode and the average thickness Ts (μm) of the separator satisfy the relationship of 1.2 < Ts / Tn.

[0017] Effects of the Invention

[0018] According to the present disclosure, a lithium secondary battery with good charge-discharge cycle characteristics can be obtained.

[0019] The novel features of the present invention are described in the claims. However, the present invention relates to both the structure and the content, and together with other objects and features of the present invention, it can be better understood through the following detailed description with reference to the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic diagram for explaining a method of measuring the thickness of the negative electrode.

[0021] Figure 2 It is a cross-sectional view schematically showing an example of the lithium secondary battery related to the present disclosure.

[0022] Figure 3 It is a cross-sectional view schematically showing an example of the separator. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0023] Hereinafter, examples will be given to describe the embodiments related to the present disclosure. However, the present disclosure is not limited to the examples described below. In the following description, specific numerical values and / or materials may be exemplified, but as long as the effects of the present disclosure can be obtained, other numerical values and / or materials can also be applied. In this specification, the description of "numerical value A to numerical value B" includes numerical value A and numerical value B, and can be replaced with "numerical value A or more and numerical value B or less". In the following description, when the lower limit and the upper limit of a specific physical property and / or condition, etc. are exemplified, as long as the lower limit does not exceed the upper limit, any one of the exemplified lower limits can be arbitrarily combined with any one of the exemplified upper limits. In the following description, when examples of components and examples of methods are listed, unless otherwise specified, only one of the listed examples can be used, or multiple of the listed examples can be used together.

[0024] (Lithium secondary battery)

[0025] Hereinafter, the lithium secondary battery according to the present embodiment may sometimes be referred to as "lithium secondary battery (B)". The lithium secondary battery (B) includes: an electrode group formed by winding a positive electrode, a negative electrode, and a separator in such a manner that the separator is disposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte. In the negative electrode, lithium metal is deposited during charging, and lithium metal is dissolved in the non-aqueous electrolyte during discharging. The separator includes a base material layer and an adhesive layer. The base material layer has a first main surface disposed on the negative electrode side and a second main surface disposed on the positive electrode side. The adhesive layer is formed on the first main surface of the base material layer. At least a part of the adhesive layer is adhered to the negative electrode. In a 90% state of charge (SOC), the average value Tn (μm) of the thickness increase value caused by charging on one side of the negative electrode and the average thickness Ts (μm) of the separator satisfy the relationship of 1.2 < Ts / Tn.

[0026] In a lithium secondary battery, for example, 70% or more of the rated capacity is exhibited by the deposition and dissolution of lithium metal. The migration of electrons in the negative electrode during charging and discharging mainly depends on the deposition and dissolution of lithium metal in the negative electrode. Specifically, 70 to 100% (for example, 80 to 100%, 90 to 100%) of the migration (in other views, the current) of electrons in the negative electrode during charging and discharging depends on the deposition and dissolution of lithium metal. That is, the negative electrode of the lithium secondary battery according to the present disclosure is different from a negative electrode in which the migration of electrons in the negative electrode during charging and discharging mainly depends on the absorption and release of lithium ions by a negative electrode active material (such as graphite). For example, the negative electrode of the lithium secondary battery according to the present disclosure may not include a negative electrode active material (such as graphite) that absorbs and releases lithium ions.

[0027] Generally, in a lithium secondary battery, lithium metal is deposited on the negative electrode during charging, so the amount of expansion of the negative electrode tends to be large. In particular, when lithium metal is deposited in a dendritic shape, the amount of expansion of the negative electrode further increases. If the amount of expansion of the negative electrode (especially the expansion amount in the thickness direction of the negative electrode) increases, it is likely to cause a decrease in the cyclicity of the non-aqueous electrolyte, cutting of the negative electrode current collector foil due to an increase in stress of the negative electrode, and the like. As a result, the charge-discharge cycle characteristics (hereinafter, sometimes simply referred to as "cycle characteristics") of the lithium secondary battery are reduced. Furthermore, the dendritic deposition of lithium metal sometimes leads to an increase in lithium metal that does not contribute to charge and discharge. In this regard, the dendritic deposition of lithium metal also causes a decrease in the cycle characteristics of the lithium secondary battery.

[0028] As a result of the research, the inventors of the present application newly found that by using a separator having an adhesive layer and a specified thickness, the cycle characteristics can be significantly improved. The present disclosure is based on this new insight. By bonding the negative electrode and the separator with the adhesive layer, dendritic precipitation of lithium metal can be suppressed. Moreover, by satisfying the relationship of 1.2 < Ts / Tn, expansion of the electrode group during charging can be suppressed. The significant improvement in the cycle characteristics is not obtained by simply adding the above two effects, and is considered to be obtained by the synergistic effect of bonding the negative electrode and the separator based on the adhesive layer and satisfying the relationship of 1.2 < Ts / Tn.

[0029] (Separator)

[0030] As described above, the separator includes a base material layer and an adhesive layer, and at least a part of the adhesive layer is bonded to the negative electrode. The method of bonding the adhesive layer of the separator to the negative electrode is not particularly limited. For example, methods such as planar hot pressing and hot rolling with a heating roll can be used.

[0031] The base material layer is not particularly limited, and a separator used in a known lithium secondary battery can be used. The base material layer can use a porous sheet having ion permeability and insulation. Examples of the porous sheet include microporous films, woven fabrics, non-woven fabrics, etc. The material of the base material layer is not particularly limited, and a polymer material can be used. Examples of the polymer material include olefin resins, polyamide resins, cellulose, etc. Examples of the olefin resin include polyethylene, polypropylene, and copolymers of ethylene and propylene, etc. The base material layer can contain additives (inorganic fillers, etc.) as needed. The base material layer can be composed of multiple layers having different morphologies and / or compositions.

[0032] The adhesive layer is formed of a material that can bond to the negative electrode. For the material of the adhesive layer, stability within the lithium secondary battery (B) is required. The adhesive layer can contain a polyvinylidene fluoride-based polymer synthesized by polymerizing a monomer containing polyvinylidene fluoride. The polyvinylidene fluoride-based polymer is preferred in terms of high adhesiveness to the negative electrode and high stability within the lithium secondary battery (B).

[0033] The polyvinylidene fluoride-based polymer contains a structural unit derived from polyvinylidene fluoride. Among all the structural units of the polyvinylidene fluoride-based polymer, the proportion of VDF units (structural units derived from polyvinylidene fluoride) is 50 mol% or more and 100 mol% or less. This proportion can be 75 mol% or more or 90 mol% or more, and can be 99.5 mol% or less or 95 mol% or less. Examples of the monomer copolymerized with polyvinylidene fluoride include tetrafluoroethylene, hexafluoropropylene, etc. The content rate of the polyvinylidene fluoride-based polymer in the adhesive layer can be 3 mass% or more or 50 mass% or more, and can be 100 mass% or less or 80 mass% or less.

[0034] Vinylidene fluoride (PVDF) based polymers can be polyvinylidene fluoride (PVDF). Alternatively, PVDF based polymers can also be copolymers synthesized by polymerizing monomers containing PVDF and hexafluoropropylene. That is, the adhesive layer can contain a copolymer synthesized by polymerizing monomers containing PVDF and hexafluoropropylene. Hexafluoropropylene has a larger molecular structure than PVDF, therefore the copolymer synthesized by polymerizing monomers containing PVDF and hexafluoropropylene has a lower density than PVDF based polymers. By using monomers containing hexafluoropropylene, polymer crystallization is suppressed, resulting in a flexible adhesive layer. Furthermore, the retention of non-aqueous electrolytes in the adhesive layer is improved, and lithium-ion conductivity is improved. Battery resistance decreases, further improving cycle characteristics.

[0035] Vinylidene fluoride polymers can be copolymers of vinylidene fluoride and hexafluoropropylene (HFP). In all the structural units of vinylidene fluoride polymers, the proportion of HFP units (structural units derived from hexafluoropropylene) can be in the range of 0 to 50 mol% (e.g., 0.5 to 25 mol%, 2 to 10 mol%).

[0036] The adhesive layer can contain inorganic particles. By including inorganic particles in the adhesive layer, the strength and durability of the adhesive layer are improved.

[0037] Inorganic particles can preferably be insulating, but conductive inorganic particles can also be used. Inorganic particles can be insulating materials such as metal oxides, metal hydroxides, metal nitrides, metal carbides, and metal sulfides. Examples of materials for inorganic particles include alumina, magnesium oxide, titanium oxide, zirconium oxide, silicon oxide, zinc oxide, magnesium hydroxide, silicon nitride, aluminum nitride, titanium nitride, silicon carbide, aluminum hydroxide, lithium phosphate, lithium fluoride, carbon black (acetylene black, etc.), amorphous silica, and crystalline silica. Solid electrolyte particles such as LLZO (lithium lanthanum zirconium oxide) and LATP (lithium aluminum titanium phosphate) can also be used.

[0038] The average primary particle size of inorganic particles can be greater than or equal to 0.1 µm or 1.0 µm, or less than or equal to 5.0 µm or 3.0 µm. The average primary particle size is the median particle size (D0) that accumulates to 50% of the total volume in the particle size distribution on a volume basis. 50 Median particle size (D) 50 The particle size distribution can be measured using a laser diffraction / scattering particle size distribution measuring device.

[0039] The content of inorganic particles in the adhesive layer can be above 10% by mass or above 20% by mass, or below 99% by mass or below 90% by mass.

[0040] The adhesive layer can be porous. By using a porous adhesive layer, the contact area between the adhesive layer and the non-aqueous electrolyte is increased, thereby further improving the retention of the non-aqueous electrolyte in the adhesive layer and improving lithium-ion conductivity. As a result, the battery resistance decreases, and good cycle characteristics are achieved.

[0041] The thickness of the adhesive layer can be greater than 0.1µm or greater than 0.3µm, or less than 5µm or less than 2µm. It should be noted that the adhesive layer can be formed on both sides of the substrate layer. That is, the adhesive layer can be formed on both the first main surface and the second main surface.

[0042] The average thickness Ts (µm) of the separator was obtained by arithmetically averaging the thicknesses at 15 locations. The thickness of the separator was measured according to JIS (Japanese Industrial Standard) K6250. Specifically, a test piece thickness gauge (SDA-12 type) manufactured by Polymer Gauge Co., Ltd., based on JIS K6250, was used to measure the thickness. A 5mm diameter probe was used, and the measurement was performed under a pressure of 22kPa. The thickness was measured at 15 intersection points where five lines dividing the separator into six equal parts along its length and three lines dividing it into four equal parts along its width. The arithmetic mean of the 15 measured thicknesses was then taken as the average thickness Ts.

[0043] As described above, the ratio Ts / Tn, which is the average increase in thickness of one side of the negative electrode caused by charging (Tn) to the average thickness Ts (µm) of the separator, is greater than 1.2 (e.g., greater than 1.20). The value of Ts / Tn can be greater than or equal to 1.25 or 1.50, and can be less than or equal to 5.0, 3.5, or 2.25. By making the value of Ts / Tn greater than 1.2, the expansion of the electrode assembly during charging can be suppressed. By setting the value of Ts / Tn to less than 3.5, both the reduction in volumetric capacity density and the improvement of battery performance can be achieved. By setting the value of Ts / Tn to less than 2.25, the reduction in volumetric capacity density can be particularly suppressed.

[0044] The average value Tn (µm) can be determined using the following method. First, prepare two batteries manufactured under identical conditions. Then, set one battery to a state of discharge (SOC = 0%) and the other to a state of charge (SOC = 90%). It should be noted that a state of charge (SOC) of 90% means that the battery is charged to 90% of its rated capacity. A state of charge (SOC) of 0% means that the battery has discharged 100% of its rated capacity from a fully charged state. It should also be noted that in this specification, the rated capacity of the battery refers to the 1-hour rate capacity.

[0045] Next, the negative electrode was removed from both the discharged and charged batteries. The thickness T(0) (µm) of the negative electrode in the discharged state and the thickness T(90) (µm) of the negative electrode in the charged state (SOC = 90%) were measured at three points on the negative electrode. Specifically, firstly, as... Figure 1 As shown, the portion 12a of the negative electrode 12 that faces the positive electrode through a separator is divided into four equal regions 12a1, 12a2, 12a3, and 12a4 along the length of the negative electrode 12. Portion 12a is where lithium metal is deposited during charging. Next, positions p1, p2, and p3 are defined as the central positions in the width direction of the three boundaries between the four regions 12a1 to 12a4. Then, thicknesses T(0) and T(90) are measured at positions p1, p2, and p3. The two sides of the negative electrode at positions p1, p2, and p3 face the positive electrode through the separator. Therefore, lithium metal is deposited on both sides of positions p1, p2, and p3 during charging. The average value Tn of the increase in thickness on one side of the negative electrode due to charging is calculated using the following formula.

[0046] Tn = {(total thickness T(90) at 3 points) - (total thickness T(0) at 3 points)} / 6

[0047] The porosity of the substrate layer can be 40% or more, or 50% or more, or 90% or less, or 80% or less. The porosity of the substrate layer can be 40% or more and 90% or less (e.g., 50% or more and 80% or less). The porosity of the substrate layer can be determined by the following method: First, collect 4 cm square samples at four points in the central portion of the spacer in the width direction. Next, peel off the adhesive layer of each sample using adhesive tape, separating only the substrate layer. Next, measure the mass of each substrate layer, and measure the thickness of the substrate layer using the method described above (the method for measuring the thickness of the spacer). Next, calculate the apparent density of the substrate layer from the mass, area, and thickness of the substrate layer. The porosity of the substrate layer is calculated using the following formula. The true density of the substrate layer is determined based on the material of the substrate layer.

[0048] Porosity (%) = 100 × {1 - (apparent density of the substrate layer) / (true density of the substrate layer)}

[0049] (Method for making the separator)

[0050] An example of the method (M) for manufacturing the separator used in this embodiment will be described. However, the separator for the lithium secondary battery (B) can also be manufactured by methods other than the manufacturing method (M) described below. Regarding the separator, the above-mentioned matters are applicable to the manufacturing method (M), so repeated descriptions are omitted. The matters described in the manufacturing method (M) can also be applied to the separator for the lithium secondary battery (B).

[0051] In manufacturing method (M), the separator is manufactured by forming an adhesive layer on a substrate layer. The method for forming the adhesive layer is not particularly limited and can be any known method. One example is forming the adhesive layer by applying a coating containing adhesive layer components to the substrate layer and then allowing it to dry. The method for applying the coating is not limited and can be any known method. For example, it can be applied using spraying, roller coating, die-casting (gravure coating, mold coating, etc.), or printing (screen printing, inkjet printing, etc.).

[0052] Solvents for the coating solution can include acetone, dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, tripropylene glycol, etc. When forming a porous adhesive layer, a non-solvent-induced phase separation (NIPS) method can be used. One example method involves first preparing a coating solution by dissolving the resin in the aforementioned solvent (a good solvent), and then applying this coating solution to a substrate layer to form a coating film. Next, before the coating film dries, immersing it in a poor solvent can make the coating film porous. Poor solvents can include water, methanol, ethanol, isopropanol, or mixtures thereof. Furthermore, by mixing acetone, dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, tripropylene glycol, etc., into the aforementioned poor solvents, the porosity of the adhesive layer can be altered.

[0053] Non-aqueous electrolytes can be used, which contain a non-aqueous solvent and a lithium salt. The non-aqueous solvent may contain ether. The ether content in the non-aqueous solvent may be 80% by mass or more, or 90% by mass or more. The non-aqueous solvent may consist of ether only. By using a non-aqueous solvent containing ether, the dendritic precipitation of lithium metal in the negative electrode during charging can be suppressed, thereby further improving cycle characteristics.

[0054] The ether used in the non-aqueous solvent may be any ether described later. The ether may be a fluoroether containing a fluorine group (-F) or a hydrofluoroether. Hydrofluoroethers contain carbon atoms bonded to hydrogen and fluorine atoms. By using hydrofluoroethers, the reduction resistance of the non-aqueous electrolyte is improved, and decomposition of the non-aqueous electrolyte at the negative electrode surface is less likely. The content of hydrofluoroether in the non-aqueous solvent may be 80% by mass or more, or 90% by mass or more. The non-aqueous solvent may consist solely of hydrofluoroethers.

[0055] The fluorination rate of hydrofluoroethers is preferably 60% or more, more preferably 65% ​​or more. This fluorination rate can be 95% or less, 90% or less, or 80% or less. The fluorination rate of one hydrofluoroether is defined by the following formula.

[0056] Fluorination rate (%) = 100 × (number of fluorine atoms in the hydrofluoroether) / (total number of fluorine and hydrogen atoms in the hydrofluoroether)

[0057] The hydrofluoroether used in non-aqueous solvents may be at least one selected from the group consisting of 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0058] (Manufacturing method of lithium secondary battery (B))

[0059] The manufacturing method is not limited as long as lithium secondary batteries (B) can be manufactured. One example manufacturing method includes steps (i), (ii), and (iii). The matters described regarding lithium secondary batteries (B) are applicable to the following manufacturing method, therefore repeated descriptions are omitted.

[0060] Step (i) is a process of forming a laminate of the spacer and the negative electrode by bonding the spacer to both sides of the negative electrode (negative electrode current collector) using an adhesive layer. The bonding of the spacer and the negative electrode can be achieved by overlapping and hot-pressing the two together. The heating temperature during hot pressing can be selected appropriately based on the material of the adhesive layer. For example, if the adhesive layer contains a vinylidene fluoride-based polymer, the heating temperature during hot pressing can be in the range of 60~120°C.

[0061] Step (ii) is a process of forming a wound electrode assembly by winding the aforementioned laminated body with the positive electrode. A separator is placed between the positive and negative electrodes. Step (iii) is a process of sealing the electrode assembly and the non-aqueous electrolyte into an outer casing. The method of performing steps (ii) and (iii) is not limited, and known methods can be used. In this way, a lithium secondary battery (B) is manufactured. However, the lithium secondary battery (B) can also be manufactured by methods other than this manufacturing method.

[0062] (Examples of constituent elements)

[0063] The following provides specific examples of other components of the lithium secondary battery (B). It should be noted that the components described below are illustrative, and the components of the lithium secondary battery (B) in this embodiment are not limited to the examples shown below. Components other than the characteristic portions of this embodiment may also use known components.

[0064] (negative electrode)

[0065] The negative electrode contains the negative current collector. During charging, lithium metal is deposited on the negative current collector. The deposited lithium metal dissolves as lithium ions in the non-aqueous electrolyte during discharge.

[0066] The negative current collector may consist solely of a substrate sheet in which lithium is not a major component (content: 50% by mass or more). Alternatively, the negative current collector may also comprise a substrate sheet and lithium-containing metal layers laminated on both sides of the substrate sheet. The substrate sheet may be a conductive sheet such as a metal foil. Examples of materials for the metal foil include copper, copper alloys, and stainless steel. Copper and copper alloys are preferred for their high conductivity. Stainless steel is preferred for its ease of cutting.

[0067] The lithium-containing metal layer laminated on the substrate is a lithium metal layer or a lithium alloy layer. Elements other than lithium contained in the lithium alloy layer are present in trace amounts (less than 10 atomic percent). Examples of elements other than lithium contained in lithium alloys include aluminum, magnesium, indium, and zinc. By forming a lithium-containing metal layer, the decrease in discharge capacity associated with repeated charging and discharging can be suppressed. Furthermore, by forming a lithium-containing metal layer, the dendritic precipitation of lithium metal can be suppressed. The method for forming the lithium-containing metal layer is not particularly limited and can be formed using known methods. For example, a lithium-containing metal layer can be formed by pressing a lithium metal foil or a lithium alloy foil onto the substrate. The density of the lithium-containing metal layer distinguishes it from the lithium metal deposited during charging (which is typically porous).

[0068] The surface of a negative electrode current collector made of metal can be smooth. By using a negative electrode current collector with a smooth surface, lithium metal can be easily and uniformly deposited on the negative electrode current collector during charging. Surface smoothness of a negative electrode current collector made of metal refers to a maximum vertical roughness Rz of 20 µm or less. The maximum vertical roughness Rz of a negative electrode current collector made of metal can be 10 µm or less. The maximum vertical roughness Rz is measured according to JIS (Japanese Industrial Standard) B 0601:2013.

[0069] The thickness of the substrate can be greater than 3µm or greater than 5µm, or less than 20µm or less than 15µm. When the negative electrode current collector includes a lithium-containing metal layer, the thickness of the lithium-containing metal layer can be in the range of 5µm to 25µm.

[0070] The negative electrode current collector (e.g., the substrate sheet) may contain austenitic stainless steel. In this case, embrittlement of the negative electrode current collector is suppressed, and the negative electrode current collector has moderate strength and flexibility, resulting in a negative electrode current collector with excellent resistance to stress generated at the negative electrode. As a result, fracture of the negative electrode current collector during charging and discharging and the subsequent reduction in cycle characteristics are suppressed.

[0071] It should be noted that the "austenitic stainless steel" mentioned above refers to stainless steel with an austenite content of 50% or more. The austenite content refers to the proportion (by mass) of the austenite phase in stainless steel. When the contents of the austenite, ferrite, and martensite phases in stainless steel are set as O, F, and M, respectively, the austenite content is calculated by {O / (O+F+M)}×100. The austenite structure is a face-centered cubic (FCC) crystal structure, while the ferrite and martensite structures are body-centered cubic (BCC) crystal structures.

[0072] The austenite content can be above 70%, above 90%, or even 100%.

[0073] The austenite ratio can be determined using the following method. First, prepare a sample of the negative current collector (stainless steel foil) (e.g., size: 25 mm square). Perform X-ray diffraction (XRD) on the sample using a two-dimensional detection function to obtain the XRD pattern (vertical axis: X-ray diffraction intensity, horizontal axis: diffraction angle 2θ). The size of the measurement area (micro-area) is, for example, 15 mm square.

[0074] The following shows the desired XRD measurement conditions.

[0075] <Analytical Device>

[0076] Two-dimensional micro-area X-ray diffraction apparatus (RINT-RAPID II, manufactured by Rigaku Corporation)

[0077] <Analysis Conditions>

[0078] X-ray tube: Co

[0079] Monochromaticization: using a monochromator (CoKα)

[0080] X-ray tube output: 40kV-30mA

[0081] Detector: Imaging panel (two-dimensional)

[0082] (Reflection method)

[0083] Collimator: Φ300µm

[0084] ω angle: 25°~35° (2° / sec)

[0085] Φ angle: 360° rotation (1° / sec)

[0086] Measurement time (exposure): 30 minutes

[0087] For the diffraction peaks observed in the obtained XRD pattern, a standard database was used for least-squares fitting, followed by quantitative analysis based on Rietveld analysis. The XRD pattern can have at least one diffraction peak corresponding to one of the austenite, ferrite, and martensite phases. This analysis can be performed using software attached to the analytical apparatus. Through this analysis, the proportion (mass ratio) of the austenite phase relative to the total of the austenite, ferrite, and martensite phases is determined as the austenite ratio. Several measurement regions were arbitrarily selected in the above sample, and the austenite ratio in each measurement region was calculated, and their average value was determined.

[0088] Austenitic stainless steels can contain elements other than Fe, such as C, Si, Mn, P, S, Ni, Cr, Mn, Mo, Cu, and N. These stainless steels can be low-carbon, ultra-low-carbon, or nitrogen-added stainless steels, or duplex stainless steels containing austenite.

[0089] Examples of austenitic stainless steels include SUS301, SUS302, SUS303, SUS304, SUS305, SUS309, SUS310, SUS312, SUS315, SUS316L, SUS317, SUS321, and SUS347. Among these, SUS304 and SUS316L are preferred.

[0090] The negative current collector may include a resin film and a transition metal layer laminated on the resin film. That is, the substrate may include a resin film and a transition metal layer laminated on the resin film. The negative current collector (e.g., the substrate) may be composed of a resin film and a transition metal layer. The resin film may include a substrate resin layer and a surface resin layer formed on the substrate resin layer. The surface resin layer is present on the surface of the resin film. The transition metal layer may be in contact with the resin film (e.g., the surface resin layer). As described above, the negative current collector may include a lithium-containing metal layer laminated on the substrate.

[0091] Resin films are lightweight, which facilitates the improvement of energy density in secondary batteries. They are also less prone to breakage during roller transport and are easy to handle. Furthermore, resin films are not easily embrittled even at low negative electrode potentials. Resin films are excellent current collector materials due to their high resistance to stress during electrode expansion and contraction and their resistance to breakage. In lithium-ion secondary batteries, lithium metal is deposited at the negative electrode during charging, leading to significant expansion of the negative electrode. This expansion is further amplified when lithium metal is deposited in a dendritic form, thus easily generating stress at the negative electrode.

[0092] The main surface of the resin membrane can be smooth, roughened, or treated with plasma, corona, etc. When the main surface of the resin membrane is smooth, the maximum vertical roughness Rz is less than 2.5 µm. When the main surface of the resin membrane is roughened, the maximum vertical roughness Rz can be greater than 2.5 µm and greater than 8 µm. The maximum vertical roughness Rz is measured according to JIS B 0601:2013. The main surface of the resin membrane refers to the surface other than the end faces, and consists of the two surfaces with the largest area. In this specification, "surface" usually refers to the "main surface".

[0093] From the perspective of improving the energy density of secondary batteries, a thin resin film is desirable, provided that mechanical strength can be ensured. An example of a preferred range for resin film thickness is 1.5 µm or more and 30 µm or less. The thickness of the resin film can be determined by measuring the thickness at any 10 points on a cross-section of the resin film using a scanning electron microscope (SEM) and then arithmetically averaging them.

[0094] The surface resin layer of the resin film may contain nitrogen-containing resin. At least 90% by mass of the surface resin layer may be nitrogen-containing resin. The nitrogen-containing resin may be a polymer having nitrogen atoms in its main chain and / or side chains. The nitrogen atom content in the nitrogen-containing resin may be at least 3% by mass. The nitrogen atom content in the surface resin layer may be at least 2.5% by mass.

[0095] Nitrogen-containing resins can contain nitrogen-hydrogen bonds (bonds between nitrogen and hydrogen atoms). The characteristic infrared absorption peak of nitrogen-hydrogen bonds is located at 1655 cm⁻¹. -1 (1640~1670cm -1 Near 1530cm -1 (1515~1545cm -1 The presence or absence of nitrogen-hydrogen bonds can be determined based on peaks near the anode. Specifically, firstly, after removing or wet-processing the active material of the negative electrode, the transition metal layer is dissolved with an aqueous nitric acid solution, exposing the surface resin layer. Then, the presence or absence of nitrogen-hydrogen bonds can be determined by analyzing the exposed surface resin layer using infrared absorption spectroscopy. A specific example of the analysis is shown below. Alternatively, the presence or absence of nitrogen-hydrogen bonds can also be determined by chemical shifts in X-ray electron spectroscopy (XPS).

[0096] (FTIR-ATR method)

[0097] Measurement apparatus: Varian 670FTIR (manufactured by Varian)

[0098] Measurement mode: Attenuated total internal reflection

[0099] Light source: special ceramic

[0100] Detector: DLaTGS (deuterated L-alanine-doped triglycine sulfate)

[0101] Resolution: 4cm -1

[0102] Total number of times: 256

[0103] IRE: Ge

[0104] Angle of incidence: 60 degrees

[0105] Attachment: Attachment for 1-time reflection ATR (seagull)

[0106] Nitrogen-containing resins can be polymers having at least one group selected from the group consisting of urea bonds, melamine structures, triazine rings, amino groups, amide bonds, aromatic polyamide bonds, imide bonds, urethane bonds, carbodiimide bonds, urea diketone structures, isocyanurate rings, nitrile groups, and amide groups. Examples of such polymers include polyurethane resins, polyurea resins, melamine resins, polyamide resins, aramid resins, and polyimide resins. Among these, polyurethane resins are excellent as current collector materials due to their high flexibility, high resistance to stress during electrode expansion and contraction, and resistance to breakage.

[0107] Polyurethane resins can be synthesized by reacting polyols with polyisocyanates (especially diisocyanates) that have two or more functional groups. By arbitrarily selecting the polyols and polyisocyanates, polyurethane resins with various physical properties can be synthesized.

[0108] Nitrogen-containing resins can be polymers having at least one group selected from the group consisting of aliphatic isocyanate groups, aromatic isocyanate groups, urea-formate groups, and biuret groups. Thermosetting polyurethane resins, in particular, use polyisocyanates as raw materials, thus having a high probability of residual unreacted isocyanate groups. It is believed that the isocyanate groups can be reduced at the negative electrode to generate a coating component for forming a stable coating at the negative electrode. Furthermore, thermosetting resins form a strong three-dimensional coating, thus exhibiting a significant inhibitory effect on copper damage. UV-curable resins also have the same effect. Isocyanate groups at 2250 cm⁻¹... -1 (2270~2240cm -1 The vicinity of the infrared absorption peak is based on antisymmetric stretching vibration, so its presence or absence can be confirmed based on the infrared absorption peak.

[0109] The surface resin layer may contain fillers. By adding fillers (silica, alumina, etc.) to the surface resin layer, an uneven surface is created. This results in an anchoring effect, improving the adhesion between the surface resin layer and the substrate resin layer. The fillers used can roughen the surface of the surface resin layer and are less prone to degradation of battery characteristics due to side reactions with non-aqueous electrolytes. Fillers can be particles of resin, metal oxides, ceramics, metals, etc.

[0110] The surface resin layer can be a coating formed by applying a nitrogen-containing resin to the surface of a substrate resin layer. In this case, the nitrogen-containing resin can be a thermosetting resin or a UV-curable resin. The nitrogen-containing resin can be diluted with a solvent and applied to the surface of the substrate resin layer. The cured product of the curable resin can have a three-dimensional network of molecular chains.

[0111] The thickness of the surface resin layer is, for example, less than 5 µm, and preferably in the range of 0.05 µm to 1.5 µm. The thickness of the surface resin layer can be determined by measuring the thickness of any 10 points in a cross-section of a specified component (negative electrode or negative electrode current collector) using a scanning electron microscope (SEM) and then arithmetically averaging the measurements.

[0112] Most resin films are non-conductive. The transition metal layer plays a crucial role in imparting good conductivity to the negative electrode current collector. The transition metal layer simply needs to be a layer containing a transition metal and possessing electronic conductivity. Ideally, the transition metal layer should contain a transition metal in a metallic state with electronic conductivity based on free electrons.

[0113] For ease of ensuring corrosion resistance and conductivity, the transition metal layer preferably comprises copper, nickel, chromium, titanium, iron, silver, gold, tin, etc. The transition metal layer preferably comprises at least one material selected from the group consisting of copper, copper alloys, stainless steel, nickel, and nickel alloys. The transition metal layer particularly preferably comprises copper or copper alloys with excellent conductivity.

[0114] Transition metals can cause resin film embrittlement. This embrittlement is particularly pronounced when the transition metal layer contains copper. As a previously unreported insight, it has also been determined that lithium metal significantly accelerates resin film embrittlement caused by transition metals (e.g., copper damage). That is, in lithium secondary batteries where lithium metal is deposited at the negative electrode, significant degradation of the resin film due to the transition metal layer is possible. In contrast, by including at least the surface resin layer of the resin film in a nitrogen-containing resin, resin film embrittlement is significantly suppressed.

[0115] It should be noted that a nickel-chromium alloy layer is considered as the metal layer to shield the transfer of copper ions to the resin film. However, in lithium-ion batteries, the shielding effect of the nickel-chromium alloy layer is completely lost. This is presumably because, when easily ionized lithium metal comes into contact with transition metals such as copper, the ionization of the transition metal is accelerated, and the breakage of polymers is accelerated due to the transition metal ions. On the other hand, a surface resin layer containing nitrogen-containing resin can significantly suppress polymer breakage.

[0116] The breaking of carbon-carbon bonds has the greatest impact on the embrittlement of resin films. In the case of resin films containing nitrogen-containing resins, carbon-nitrogen bonds exist within the nitrogen-containing resin. It is believed that these carbon-nitrogen bonds are not easily affected by transition metals or their ions. There is a possibility that transition metals are stabilized by nitrogen atoms, thus suppressing the breaking of carbon-carbon bonds.

[0117] The transition metal layer can be formed by depositing it on the surface of the surface resin layer using liquid-phase or vapor-phase methods. Liquid-phase methods include electrodeposition techniques such as electrolytic plating / chemical plating. Vapor-phase methods include evaporation, sputtering, and atomic layer deposition (ALD). Alternatively, a base layer can be formed by sputtering, and the transition metal layer can be thickened on top using electrolytic plating. In other words, multiple methods can be used in combination. The transition metal layer can also be formed by lamination. However, the method for forming the transition metal layer is not particularly limited.

[0118] The thickness of the transition metal layer is, for example, less than 5 µm, or less than 3 µm. A preferred range for the thickness of the transition metal layer is 0.05 µm to 1.5 µm, and it can be 0.1 µm to 1.5 µm. An example of measuring the thickness of the transition metal layer is as follows: First, using a scanning electron microscope (SEM), the thickness of the transition metal layer is measured at any 10 points in a cross-section of a specified component (negative electrode or negative electrode current collector). Then, the thickness of the transition metal layer (average thickness) is calculated by arithmetic averaging the obtained measurements. The transition metal layer can consist of multiple layers composed of different metals.

[0119] The substrate resin layer is the main component of the negative electrode current collector, and is typically thicker than the surface resin layer and the transition metal layer. At least 51% by mass of the substrate resin layer is composed of resin or organic matter. To improve adhesion to the transition metal layer and lithium layer, the substrate resin layer may contain inorganic materials such as inorganic particles. The substrate resin layer can be a stretched film, a non-porous film, or a film with multiple regularly arranged pores. The substrate resin layer can be insulating, conductive, or non-conductive. There are no particular limitations on the morphology and physical properties of the substrate resin layer.

[0120] The substrate resin layer is formed, for example, by molding raw materials (general-purpose plastics, general-purpose engineering plastics, etc.) into sheets. Examples of raw materials include polyester resins, olefin resins, polyphenylene sulfide resins, acrylic resins, polycarbonate resins, polyetheretherketone resins, polysulfone resins, polyphenylene sulfone resins, polyethersulfone resins, polyamide resins, polyimide resins, polyetherimide resins, polybenzimidazole resins, liquid crystal polymer resins, polyacetal resins, polyvinyl chloride resins, polyarylate resins, silicone resins, nylon resins, polyvinylidene chloride resins, ethylene-vinyl alcohol copolymers, polyvinyl alcohol resins, polystyrene resins, epoxy resins, polyurethane resins, phenolic resins, melamine resins, urea resins, unsaturated polyester resins, etc. The resins contained in the substrate resin layer can be used alone or in combination of two or more.

[0121] The resin contained in the substrate resin layer is preferably a resin with aromatic rings, a resin without fluorine atoms, or an olefin resin. When the resin has aromatic rings (e.g., benzene rings) in its molecule, the affinity between the substrate resin layer and the lithium metal layer increases, and the adhesion between the two is improved.

[0122] As a polyester resin, aromatic polyesters are preferred, especially unstretched polyethylene terephthalate, biaxially stretched polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate. As an acrylic resin, polymethyl methacrylate (PMMA) can be used. As a polyimide resin, aromatic polyimide is preferred. As a polyamide resin, aromatic polyamide (aramid resin) is preferred. As an olefin resin, unstretched polypropylene, biaxially stretched polypropylene, low-density polyethylene, medium-density polyethylene, high-density polyethylene, linear low-density polyethylene, metallocene polyethylene, ethylene-vinyl acetate copolymer, ethylene-methyl acrylate copolymer, ethylene-ethyl acrylate copolymer, ethylene-methyl methacrylate copolymer, ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer, and ionomers are preferred.

[0123] The extrusion method for the substrate resin layer can be either T-die extrusion or blow extrusion, and can involve non-stretching, uniaxial stretching, sequential biaxial stretching, or simultaneous biaxial stretching. The resin can be a homopolymer, a copolymer, or a terpolymer. The arrangement of the structural units constituting the resin is not limited; it can be a random copolymer or a block copolymer. Two or more substrate resin layers can also be combined. For example, two or more substrate resin layers can be stacked. The crystalline resin can be in a crystalline state, an amorphous state, or a mixture of both. Amorphous resin can be formed by methods such as rapid cooling. The substrate resin layer can be an alloy resin composed of multiple resins as described above.

[0124] To ensure adhesion to other layers (such as vapor-deposited films), the surface of the substrate resin layer can be subjected to corona treatment or plasma treatment. To improve adhesion to the surface resin layer by creating irregularities in the substrate resin layer, fillers (including fillers made of ceramics, resins, metals, etc.) can be added to the substrate resin layer to form irregularities on its surface.

[0125] (positive electrode)

[0126] The positive electrode comprises a positive current collector and a positive electrode additive layer disposed on the positive current collector. The positive electrode additive layer contains a positive electrode active material, such as a positive electrode active material and additives (conductive material, binder, thickener, etc.). The positive electrode additive layer is formed on both sides of the positive current collector. The positive electrode can be formed by known methods. For example, firstly, a positive electrode additive slurry containing a positive electrode active material and additives is prepared. Next, the positive electrode additive slurry is coated onto the positive current collector and dried, thereby forming a coating film. Then, the laminate consisting of the positive current collector and the coating film is calendered, thereby obtaining the positive electrode. It should be noted that the formed positive electrode is cut to a specified size as needed.

[0127] The thickness of the positive electrode compound layer can be greater than 50µm or greater than 100µm, or less than 300µm or less than 250µm.

[0128] The positive electrode active material can be a substance capable of reversibly absorbing and releasing lithium ions. Examples of positive electrode active materials include lithium-containing transition metal oxides, transition metal fluorides, polyanions, fluorinated polyanions, and transition metal sulfides. Lithium-containing transition metal oxides are preferred for their low manufacturing cost and high average discharge voltage.

[0129] Examples of transition metal elements contained in lithium-containing transition metal oxides include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, and W. Lithium-containing transition metal oxides may contain only one transition metal element or two or more. The transition metal element can be at least one element selected from the group consisting of Co, Ni, and Mn. Lithium-containing transition metal oxides may contain more than one typical element. Examples of typical elements include Mg, Al, Ca, Zn, Ga, Ge, Sn, Sb, Pb, Bi, and B.

[0130] Conductive materials can include carbon materials. Examples of carbon materials include carbon black (acetylene black, Ketjen black, etc.), carbon nanotubes, and graphite.

[0131] Examples of adhesives include fluoropolymers, polyacrylonitrile, polyimide resins, acrylic resins, polyolefin resins, and rubber-like polymers. Examples of fluoropolymers include polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF).

[0132] Thickeners can be cellulose derivatives, etc. Examples of cellulose derivatives include carboxymethyl cellulose (CMC) and its modified forms, methyl cellulose, etc. Examples of modified CMCs also include salts of CMC. As salts, alkali metal salts (e.g., sodium salts), ammonium salts, etc., can be cited.

[0133] A conductive sheet can be used as the positive current collector. Metal foil or similar materials can be used as the conductive sheet. Carbon material can be coated onto the surface of the positive current collector.

[0134] Examples of materials for the positive current collector (conductive sheet) include metallic materials containing Al, Ti, Fe, etc. The metallic materials can be Al, Al alloys, Ti, Ti alloys, Fe alloys (e.g., stainless steel), etc. The thickness of the positive current collector is not particularly limited and can range from 5 to 300 µm.

[0135] (Separator)

[0136] The separators described above can be used.

[0137] The lithium secondary battery (B) may or may not include a spacer disposed between the positive electrode and the separator. The spacer may be formed on the main surface of the separator on the positive electrode side, or it may be formed on the positive electrode. Typically, the lithium secondary battery (B) does not include a spacer disposed between the positive electrode and the negative electrode (a spacer that is not a separator).

[0138] The spacer may include linear portions and / or dotted portions. The linear portions may also be mesh portions configured in a mesh-like (e.g., honeycomb) pattern. Alternatively, the spacer may also include multiple linear portions configured in a stripe pattern.

[0139] (Non-aqueous electrolyte)

[0140] Non-aqueous electrolytes can be non-aqueous electrolytes with lithium-ion conductivity. Non-aqueous electrolytes can be in liquid or gel form. Liquid non-aqueous electrolytes are prepared by dissolving a lithium salt in a non-aqueous solvent. By dissolving the lithium salt in the non-aqueous solvent, lithium ions and anions are generated.

[0141] Gel-like non-aqueous electrolytes can contain lithium salts and matrix polymers, or they can contain lithium salts, non-aqueous solvents, and matrix polymers. For example, a polymeric material that gels by absorbing a non-aqueous solvent can be used. Examples of polymeric materials include fluoropolymers, acrylic resins, and polyether resins.

[0142] As a non-aqueous solvent, known solvents can be used. Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, chain carboxylic acid esters, chain ethers, fluorinated chain ethers, cyclic ethers, and fluorinated cyclic ethers. Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC). Examples of chain carbonates include diethyl carbonate (DEC), methyl ethyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of chain carboxylic acid esters include ethyl acetate, methyl propionate, and fluoromethyl propionate. Examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, and 2-methyltetrahydrofuran. Examples of chain ethers include 1,2-dimethoxyethane, diethyl ether, ethyl vinyl ether, methyl phenyl ether, benzyl ethyl ether, diphenyl ether, dibenzyl ether, 1,2-diethoxyethane, diethylene glycol dimethyl ether, etc. Non-aqueous solvents can be used alone or in combination of two or more.

[0143] Examples of lithium salts include lithium salts containing chloric acid (LiClO4, LiAlCl4, LiB). 10 Cl 10 Lithium salts include those containing fluorine acids (LiPF6, LiPF2O2, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts containing fluorinated imides (LiN(FSO2)2, LiN(CF3SO2)2, LiN(CF3SO2)(FSO2), LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, etc.), lithium halides (LiCl, LiBr, LiI, etc.), and lithium salts containing oxalate complexes (LiB(C2O4)2, LiBF2(C2O4), LiPF4(C2O4), LiPF2(C2O4)2, etc.). Lithium salts can be used alone or in combination of two or more.

[0144] The concentration of lithium salt in the non-aqueous electrolyte can be above 0.5 mol / L, above 1.0 mol / L, or above 1.5 mol / L, or below 3.5 mol / L, below 2.0 mol / L, or below 1.5 mol / L. By setting the concentration of lithium salt within the above ranges, a non-aqueous electrolyte with excellent ionic conductivity and moderate viscosity can be obtained.

[0145] Non-aqueous electrolytes may contain additives (such as known additives). Examples of additives include 1,3-propanesulfonyl lactone, methyl benzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, fluorobenzene, etc.

[0146] (outer body)

[0147] The outer casing houses the non-aqueous electrolyte and electrode assembly. There are no particular limitations on the outer casing; any known outer casing can be used. The outer casing may include a bottom cylindrical battery housing, a sealing body that seals the opening of the battery housing, and a gasket.

[0148] Hereinafter, an example of a lithium secondary battery (B) according to this embodiment will be specifically described with reference to the accompanying drawings. The constituent elements of the lithium secondary battery of the example described below can be applied using the constituent elements described above. Furthermore, the constituent elements of the example described below can be modified based on the above description. Additionally, the matters described below can also be applied to the above embodiment. Furthermore, in the lithium secondary battery described below, constituent elements not required for the lithium secondary battery (B) of this disclosure can be omitted.

[0149] (Implementation Method 1)

[0150] Figure 2 This is a longitudinal cross-sectional view schematically illustrating an example of a lithium secondary battery according to Embodiment 1. Figure 2 The cylindrical lithium secondary battery 10 shown includes: a cylindrical battery casing, and an electrode assembly 14 and a non-aqueous electrolyte (not shown) housed within the battery casing. The electrode assembly 14 includes a positive electrode 11, a negative electrode 12, and a separator 13.

[0151] The battery casing includes a casing body 15, which is a bottomed cylindrical metal container, and a sealing body 16 that seals the opening of the casing body 15. A gasket 27 is disposed between the casing body 15 and the sealing body 16. The gasket 27 ensures the airtightness of the battery casing. Inside the casing body 15, insulating plates 17 and 18 are respectively disposed at both ends of the electrode assembly 14 in the winding axis direction. The casing body 15 has a stepped portion 21.

[0152] The sealing body 16 includes a perforated metal plate 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cover 26. The lower valve body 23 and the upper valve body 25 are connected at their respective central portions. The insulating member 24 is disposed between the peripheral portions of the lower valve body 23 and the upper valve body 25. The perforated metal plate 22 and the lower valve body 23 are connected at their respective peripheral portions. The upper valve body 25 and the cover 26 are connected at their respective peripheral portions. All components constituting the sealing body 16, except for the insulating member 24, are electrically connected.

[0153] A vent hole is formed on the lower valve body 23. Therefore, when the internal pressure of the battery casing rises due to abnormal heating, the upper valve body 25 expands towards the cover 26, separating from the lower valve body 23. As a result, the electrical connection between the lower valve body 23 and the upper valve body 25 is cut off. When the internal pressure rises further, the upper valve body 25 breaks, and gas is discharged from the opening formed in the cover 26.

[0154] The positive electrode 11 is electrically connected to the lid 26 that functions as a positive terminal via the positive electrode lead 19. The negative electrode 12 is electrically connected to the housing main body 15 that functions as a negative terminal via the negative electrode lead 20.

[0155] Figure 3 It is a cross-sectional view schematically showing an example of the separator 13. The separator 13 includes a base material layer 13a and an adhesive layer 13b formed on the base material layer 13a. The base material layer 13a has a first main surface 13a1 and a second main surface 13a2. The first main surface 13a1 is disposed on the side of the negative electrode 12, and the second main surface 13a2 is disposed on the side of the positive electrode 11. That is, the first main surface 13a1 faces the negative electrode 12, and the second main surface 13a2 faces the positive electrode 11. The adhesive layer 13b is laminated on the first main surface 13a1. As described above, in the electrode group 14, at least a part of the adhesive layer 13b is adhered to the negative electrode 12.

[0156] (Supplementary Note)

[0157] Through the above description, the following technology is disclosed.

[0158] (Technology 1)

[0159] A lithium secondary battery, comprising:

[0160] An electrode group formed by winding a positive electrode, a negative electrode, and a separator in such a manner that the separator is disposed between the positive electrode and the negative electrode; and

[0161] A non-aqueous electrolyte,

[0162] In the negative electrode, lithium metal is deposited during charging, and the lithium metal dissolves in the non-aqueous electrolyte during discharging,

[0163] The separator includes a base material layer and an adhesive layer, the base material layer has a first main surface disposed on the side of the negative electrode and a second main surface disposed on the side of the positive electrode, and the adhesive layer is formed on the first main surface of the base material layer,

[0164] At least a part of the adhesive layer is adhered to the negative electrode,

[0165] In a 90% charged state, the average value Tn (μm) of the increase in thickness caused by charging on one side of the negative electrode and the average thickness Ts (μm) of the separator satisfy the relationship of 1.2 < Ts / Tn.

[0166] (Technology 2)

[0167] The lithium secondary battery according to Technology 1, wherein the adhesive layer contains a polyvinylidene fluoride-based polymer synthesized by polymerizing a monomer containing polyvinylidene fluoride.

[0168] (Technology 3)

[0169] According to the lithium secondary battery of technology 1 or 2, the adhesive layer contains a copolymer synthesized by polymerizing monomers comprising vinylidene fluoride and hexafluoropropylene.

[0170] (Technology 4)

[0171] In any one of the technologies 1 to 3, the adhesive layer of the lithium secondary battery contains inorganic particles.

[0172] (Technology 5)

[0173] In the lithium secondary battery according to any one of techniques 1 to 4, the adhesive layer is porous.

[0174] (Technology 6)

[0175] The lithium secondary battery according to any one of techniques 1 to 5, wherein the non-aqueous electrolyte contains a non-aqueous solvent and a lithium salt.

[0176] The non-aqueous solvent contains ether.

[0177] The content of the ether in the non-aqueous solvent is 80% or more by mass.

[0178] (Technology 7)

[0179] In the lithium secondary battery according to any one of techniques 1 to 6, the porosity of the substrate layer is 40% or more and 90% or less.

[0180] (Technology 8)

[0181] In the lithium secondary battery according to any one of techniques 1 to 7, the negative electrode includes a negative current collector.

[0182] The negative electrode current collector contains austenitic stainless steel.

[0183] (Technology 9)

[0184] In the lithium secondary battery according to any one of techniques 1 to 7, the negative electrode includes a negative current collector.

[0185] The negative electrode current collector comprises a resin film and a transition metal layer stacked on the resin film.

[0186] The resin film comprises a substrate resin layer and a surface resin layer formed on the substrate resin layer.

[0187] The surface resin layer contains nitrogen-containing resin.

[0188] (Technology 10)

[0189] According to the lithium secondary battery of technology 9, the nitrogen-containing resin contains nitrogen-hydrogen bonds.

[0190] (Technology 11)

[0191] According to the lithium secondary battery of technique 9 or 10, the nitrogen-containing resin is a polymer having at least one of the following groups: urea bond, melamine structure, triazine ring, amino group, amide bond, aromatic polyamide bond, imide bond, urethane bond, carbodiimide bond, urea diketone structure, isocyanurate ring, nitrile group, and amide group.

[0192] (Technology 12)

[0193] According to any one of the technologies 9 to 11, the lithium secondary battery wherein the nitrogen-containing resin is a polymer having at least one selected from the group consisting of aliphatic isocyanate groups, aromatic isocyanate groups, urea-formate groups and biuret groups.

[0194] Example

[0195] The following examples will provide a detailed description of the lithium secondary battery involved in this disclosure. However, this disclosure is not limited to the following examples. In these examples, multiple lithium secondary batteries with different configurations are fabricated and evaluated.

[0196] (Battery A1)

[0197] Make battery A1 by following these steps.

[0198] (1) Production of the positive electrode

[0199] A lithium-containing transition metal oxide (positive electrode active material), acetylene black (AB, conductive material), and polyvinylidene fluoride (PVDF, binder material) were mixed at a mass ratio of positive electrode active material:AB:PVDF = 95:2.5:2.5. Then, an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added and the mixture was stirred to prepare the positive electrode slurry. The positive electrode active material used was a lithium-containing transition metal oxide containing Li, Ni, Co, and Al.

[0200] Next, the positive electrode slurry is coated onto both sides of the positive current collector (aluminum foil) and dried. The coating of the positive electrode slurry is then calendered using rollers. Finally, the resulting laminate of the positive current collector and the positive electrode slurry is cut to the specified electrode size. In this way, a positive electrode comprising a positive current collector and positive electrode slurry layers formed on both sides of the positive current collector is fabricated. Then, aluminum tabs are attached to the fabricated positive electrode.

[0201] (2) Fabrication of the negative current collector (negative electrode)

[0202] A negative electrode current collector consisting of copper foil and a lithium alloy foil layer is fabricated by pressing lithium alloy foil (thickness: 10µm) onto both sides of a copper foil. Next, nickel tabs are attached to the negative electrode current collector.

[0203] (3) Preparation of non-aqueous electrolytes

[0204] A non-aqueous electrolyte was prepared by dissolving LiPF6 at a concentration of 1 mol / L and LiBF2(C2O4) at a concentration of 0.1 mol / L in a non-aqueous solvent. Dimethyl carbonate (carbonate) was used as the non-aqueous solvent.

[0205] (4) Preparation of separators

[0206] As a separator, a separator consisting of a substrate layer and an adhesive layer formed only on one side (first main side) of the substrate layer is prepared. The substrate layer is a microporous film of polyethylene. The adhesive layer is an adhesive layer made of polyvinylidene fluoride (PVDF) (thickness: approximately 1 µm). The average thickness Ts of the separator is measured using the method described above.

[0207] (5) Battery manufacturing

[0208] The adhesive layer is bonded to the negative current collector by overlapping spacers on both sides of the negative current collector with the adhesive layer in contact with the negative current collector and then being heated and rolled. In this way, a laminate with spacers stacked on both sides of the negative electrode (negative current collector) is formed.

[0209] Next, in an inert gas atmosphere, the aforementioned laminate (negative electrode current collector and separator) is wound with the positive electrode to create a wound electrode assembly. Then, the electrode assembly and the aforementioned non-aqueous electrolyte are housed in an outer casing, which is then sealed to create battery A1 (lithium secondary battery). The outer casing is a bag-shaped casing formed from a laminate containing an aluminum layer.

[0210] (Determination of average value Tn)

[0211] For battery A1, the average value Tn (µm) of the increase in thickness of one side of the negative electrode caused by charging was measured at 90% charge using the method described above.

[0212] (Charge-discharge cycle test)

[0213] For battery A1, a charge-discharge cycle test was conducted at 25°C. The charge and discharge were performed under the following conditions: a 20-minute rest period between charging and discharging. The charge-discharge cycle was repeated 100 times, and the discharge capacity after 100 cycles was measured.

[0214] (Charge)

[0215] At 10mA / cm2 Perform constant current charging until the voltage reaches 4.1V, then perform constant voltage charging at 4.1V until the current reaches 1mA / cm. 2 .

[0216] (Discharge)

[0217] At 10mA / cm 2 Perform constant current discharge until the voltage reaches 3V.

[0218] (Batteries A2~A11 and batteries C1~C3)

[0219] The average value Tn (µm), separator, non-aqueous solvent of non-aqueous electrolyte, and substrate of negative electrode current collector are modified as shown in Table 1. Otherwise, batteries A2 to A9 and batteries C1 to C3 are manufactured using the same methods and conditions as battery A1. As mentioned above, the average value Tn (µm) is the average value of the increase in thickness on one side of the negative electrode caused by 90% charging. The thickness of the adhesive layer included in the separator is the same as the thickness of the adhesive layer included in the separator of battery A1. The average value Tn varies depending on the thickness of the positive electrode adhesive layer. In the separators of batteries A2 to A11 and C1, an adhesive layer (VDF-HFP copolymer) is formed on the negative electrode side of the substrate layer, and the adhesive layer is bonded to the negative electrode. No adhesive layer is formed on the main surface of the substrate layer of the separator on the positive electrode side. The separators of batteries C2 and C3 do not have an adhesive layer and consist only of the substrate layer. In Table 1, P(VDF-HFP) refers to a copolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP). Inorganic particles (Al2O3 particles) are added to the adhesive layer of batteries A7 and A9.

[0220] The negative electrode current collector of battery A10 is configured the same as that of battery A1, except that austenitic stainless steel foil (SUS foil) is used instead of copper foil. The negative electrode current collector of battery A11 is configured the same as that of battery A1, except that a specified laminate (a laminate having a resin film and a transition metal layer laminated on the resin film) is used instead of copper foil. As the resin film, a polyethylene terephthalate (PET) film coated with polyurethane resin (nitrogen-containing resin) is used. A copper layer is used as the transition metal layer. Specifically, copper is first deposited onto the resin film by sputtering, and then deposited by wet plating. Both the negative electrode current collectors of battery A10 and battery A11 have a lithium-containing metal layer, similar to those of battery A1.

[0221] Table 1 lists the non-aqueous solvents used in the non-aqueous electrolytes. In Table 1, "carbonate" indicates the use of dimethyl carbonate, and "ether" indicates the use of 1,2-dimethoxyethane (DME). Each battery was evaluated in the same manner as battery A1.

[0222] Table 1 shows a portion of the manufacturing conditions and evaluation results for each battery. In Table 1, the discharge capacity of each battery at 100 cycles is expressed as a relative value when the discharge capacity of battery A1 at 100 cycles is set to 100.

[0223] [Table 1]

[0224]

[0225] Batteries A1 to A11 are lithium secondary batteries (B) according to this disclosure. Batteries C1 to C3 are comparative examples. As shown in Table 1, batteries A1 to A11 exhibit better cycle characteristics compared to batteries C1 to C3. As shown in Table 1, the cycle characteristics are improved by including inorganic particles in the adhesive layer. Furthermore, the cycle characteristics are improved by including ether in a non-aqueous solvent. Additionally, the cycle characteristics are improved by using stainless steel foil and resin film as the substrate for the negative electrode current collector.

[0226] Industrial availability

[0227] This disclosure can be used in lithium secondary batteries.

[0228] The present invention has been described with reference to preferred embodiments, but such disclosure should not be interpreted as limiting. Various modifications and alterations will be apparent to those skilled in the art from the foregoing disclosure. Therefore, the appended claims should be construed as encompassing all modifications and alterations without departing from the true spirit and scope of the invention.

[0229] Explanation of reference numerals in the attached figures

[0230] 10: Lithium-ion secondary batteries

[0231] 11: Positive electrode

[0232] 12: Negative electrode

[0233] 13: Separator

[0234] 13a: Substrate layer

[0235] 13a1: First Main Face

[0236] 13a2: Second Main Face

[0237] 13b: Adhesive layer

[0238] 14: Electrode assembly

Claims

1. A lithium secondary battery, comprising: An electrode group formed by winding a positive electrode, a negative electrode, and a separator in such a manner that the separator is disposed between the positive electrode and the negative electrode; and A non-aqueous electrolyte, In the negative electrode, lithium metal is deposited during charging, and the lithium metal dissolves in the non-aqueous electrolyte during discharging, The separator includes a base material layer and an adhesive layer. The base material layer has a first main surface disposed on the negative electrode side and a second main surface disposed on the positive electrode side, and the adhesive layer is formed on the first main surface of the base material layer, At least a part of the adhesive layer is adhered to the negative electrode, In a 90% charged state, the average value Tn (μm) of the increase in thickness caused by charging on one side of the negative electrode and the average thickness Ts (μm) of the separator satisfy the relationship of 1.2 < Ts / Tn.

2. The lithium secondary battery according to claim 1, wherein, The adhesive layer contains a vinylidene fluoride-based polymer synthesized by polymerizing a monomer containing vinylidene fluoride.

3. The lithium secondary battery according to claim 1, wherein, The adhesive layer contains a copolymer synthesized by polymerizing a monomer containing vinylidene fluoride and hexafluoropropylene.

4. The lithium secondary battery according to any one of claims 1 to 3, wherein, The adhesive layer contains inorganic particles.

5. The lithium secondary battery according to any one of claims 1 to 3, wherein, The adhesive layer is porous.

6. The lithium secondary battery according to any one of claims 1 to 3, wherein, The non-aqueous electrolyte contains a non-aqueous solvent and a lithium salt, The non-aqueous solvent contains an ether, The content rate of the ether in the non-aqueous solvent is 80 mass% or more.

7. The lithium secondary battery according to any one of claims 1 to 3, wherein, The porosity of the base material layer is 40% or more and 90% or less.

8. The lithium secondary battery according to any one of claims 1 to 3, wherein, The negative electrode includes a negative electrode current collector containing austenitic stainless steel.

9. The lithium secondary battery according to any one of claims 1 to 3, wherein, The negative electrode includes a negative electrode current collector, The negative electrode current collector includes a resin film and a transition metal layer laminated on the resin film, The resin film includes a base resin layer and a surface resin layer formed on the base resin layer, The surface resin layer contains a nitrogen-containing resin.

10. The lithium secondary battery according to claim 9, wherein, The nitrogen-containing resin contains a nitrogen-hydrogen bond.

11. The lithium secondary battery according to claim 9, wherein, The nitrogen-containing resin is a polymer having at least one selected from the group consisting of a urea bond, a melamine structure, a triazine ring, an amino group, an amide bond, an aromatic polyamide bond, an imide bond, a urethane bond, a carbodiimide bond, a uretdione structure, an isocyanurate ring, a nitrile group, and an amide group.

12. The lithium secondary battery according to claim 9, wherein, The nitrogen-containing resin is a polymer having at least one selected from the group consisting of an aliphatic isocyanate group, an aromatic isocyanate group, a urethane group, and a biuret group.

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