Lithium secondary battery

Abstract

Disclosed is a lithium secondary battery comprising: 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 nonaqueous electrolyte. At the negative electrode, lithium metal is deposited during charging, and lithium metal dissolves in the nonaqueous electrolyte during discharging. The separator (13) includes: a substrate 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 substrate layer. At least a portion of the adhesive layer (13b) is adhered to the negative electrode. Tn (μm), which is the average increase in thickness due to charging on one side of the negative electrode at a 90% charge state, and Ts (μm), which is the average thickness of the separator (13), satisfy the relationship 1.2＜Ts / Tn.

Description

Lithium secondary battery

[0001] The present disclosure relates to lithium secondary batteries.

[0002] Lithium secondary batteries (lithium metal secondary batteries) are used in a variety of applications as high-capacity secondary batteries. In lithium secondary batteries, lithium metal is deposited on the negative electrode during charging. The deposited lithium metal dissolves in the non-aqueous electrolyte during discharge. Various proposals have been made regarding lithium secondary batteries.

[0003] Claim 1 of Patent Document 1 (JP 2020-95931 A) describes a lithium secondary battery comprising: a positive electrode, a negative electrode, and a polymer layer disposed on the surface of the negative electrode; the negative electrode includes a negative electrode current collector and a negative electrode active material layer; the negative electrode current collector has a larger planar size than the positive electrode, such that the negative electrode current collector has a first region overlapping with the positive electrode and a second region not overlapping with the positive electrode; the negative electrode active material layer is selectively provided in the first region of the negative electrode current collector; the polymer layer includes a copolymer of polyvinylidene fluoride (PVDF) and hexafluoropropylene (HFP) and an ionic liquid; and the polymer layer covers the negative electrode current collector on the first region via the negative electrode active material layer, and covers the negative electrode current collector on the second region without the negative electrode active material layer interposed therebetween.

[0004] Japanese Patent Application Laid-Open No. 2020-95931

[0005] A problem with lithium secondary batteries is the deposition of lithium metal in the form of dendrites during charging. Another problem with lithium secondary batteries is the significant expansion of the electrode group during charging. These problems result in a deterioration in charge-discharge cycle characteristics. Currently, there is a demand for improving the charge-discharge cycle characteristics of lithium secondary batteries. One of the objectives of the present disclosure is to provide a lithium secondary battery with good charge-discharge cycle characteristics.

[0006] One aspect of the present disclosure relates to a lithium secondary battery including: an electrode group in which a positive electrode, a negative electrode, and a separator are wound such that the separator is disposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte, wherein lithium metal precipitates in the negative electrode during charging and dissolves in the non-aqueous electrolyte during discharge, the separator including a base material layer having a first main surface disposed on the negative electrode side and a second main surface disposed on the positive electrode side, and an adhesive layer formed on the first main surface of the base material layer, at least a portion of the adhesive layer being adhered to the negative electrode, and wherein an average value Tn (μm) of an increase in thickness due to charging on one side of the negative electrode at a 90% charged state and an average thickness Ts (μm) of the separator satisfy the relationship 1.2<Ts / Tn.

[0007] According to the present disclosure, a lithium secondary battery having good charge-discharge cycle characteristics can be obtained. The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0008] Fig. 1 is a schematic diagram for explaining a method for measuring the thickness of a negative electrode. Fig. 2 is a cross-sectional view schematically showing an example of a lithium secondary battery according to the present disclosure. Fig. 3 is a cross-sectional view schematically showing an example of a separator.

[0009] The following describes embodiments of the present disclosure using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and materials may be applied as long as the effects of the present disclosure are obtained. In this specification, the term "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or greater and numerical value B or less." In the following description, when lower and upper limits for specific physical properties or conditions are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not greater than the upper limit. In the following description, when examples of components or methods are listed, only one of the listed examples may be used, or multiple of the listed examples may be used in combination, unless otherwise specified.

[0010] (Lithium Secondary Battery) The lithium secondary battery according to this embodiment may be referred to as a "lithium secondary battery (B)" below. The lithium secondary battery (B) includes an electrode group in which a positive electrode, a negative electrode, and a separator are wound such that the separator is disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. In the negative electrode, lithium metal precipitates during charging and dissolves into the non-aqueous electrolyte during discharge. The separator includes a substrate layer having a first main surface disposed on the negative electrode side and a second main surface disposed on the positive electrode side, and an adhesive layer formed on the first main surface of the substrate layer. At least a portion of the adhesive layer is adhered to the negative electrode. At a 90% state of charge (SOC), the average thickness increase Tn (μm) due to charging on one side of the negative electrode and the average thickness Ts (μm) of the separator satisfy the relationship 1.2<Ts / Tn.

[0011] In lithium secondary batteries, for example, 70% or more of the rated capacity is achieved by the deposition and dissolution of lithium metal. The movement of electrons at the negative electrode during charging and discharging is mainly due to the deposition and dissolution of lithium metal at the negative electrode. Specifically, 70 to 100% (e.g., 80 to 100% or 90 to 100%) of the movement of electrons (current, from another perspective) at the negative electrode during charging and discharging is due to the deposition and dissolution of lithium metal. In other words, the negative electrode of the lithium secondary battery according to the present disclosure differs from a negative electrode in which the movement of electrons at the negative electrode during charging and discharging is mainly due to 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 does not need to include a negative electrode active material (such as graphite) that absorbs and releases lithium ions.

[0012] Generally, lithium secondary batteries tend to expand significantly due to the deposition of lithium metal on the negative electrode during charging. In particular, the amount of expansion of the negative electrode increases when lithium metal deposits in a dendritic form. The amount of expansion of the negative electrode (particularly the amount of expansion in the thickness direction of the negative electrode) increases, which can lead to a decrease in the circulability of the non-aqueous electrolyte and to breakage of the negative electrode current collector foil due to increased stress on the negative electrode. As a result, the charge-discharge cycle characteristics (hereinafter sometimes simply referred to as "cycle characteristics") of the lithium secondary battery decrease. Furthermore, the dendritic deposition of lithium metal can result in an increase in lithium metal that does not contribute to charge and discharge. In this respect, the dendritic deposition of lithium metal also leads to a decrease in the cycle characteristics of the lithium secondary battery.

[0013] As a result of further investigation, the inventors of the present application have newly discovered that cycle characteristics can be significantly improved by using a separator having an adhesive layer and a predetermined thickness. The present disclosure is based on this new finding. By bonding the negative electrode and separator with an adhesive layer, it is possible to suppress dendritic precipitation of lithium metal. Furthermore, by satisfying the relationship 1.2<Ts / Tn, it is possible to suppress expansion of the electrode group during charging. The significant improvement in cycle characteristics is not due to a simple addition of the above two effects, but is thought to be due to the synergistic effect of the adhesion between the negative electrode and separator by the adhesive layer and the satisfaction of the relationship 1.2<Ts / Tn.

[0014] (Separator) As described above, the separator includes a substrate layer and an adhesive layer, and at least a portion of the adhesive layer is adhered to the negative electrode. The method for adhering the adhesive layer of the separator to the negative electrode is not particularly limited. For example, a method such as flat hot pressing or heated roll pressing may be used.

[0015] The substrate layer is not particularly limited, and separators used in known lithium secondary batteries may be used. A porous sheet having ion permeability and insulating properties may be used for the substrate layer. Examples of porous sheets include microporous membranes, woven fabrics, and nonwoven fabrics. The material of the substrate layer is not particularly limited, and a polymeric material may be used. Examples of polymeric materials include olefin resins, polyamide resins, and cellulose. Examples of olefin resins include polyethylene, polypropylene, and copolymers of ethylene and propylene. The substrate layer may contain additives (such as inorganic fillers) as necessary. The substrate layer may be composed of multiple layers with different shapes and / or compositions.

[0016] The adhesive layer is formed of a material that can adhere to the negative electrode. The material of the adhesive layer is required to be stable within the lithium secondary battery (B). The adhesive layer may contain a vinylidene fluoride polymer synthesized by polymerizing a monomer containing vinylidene fluoride. The vinylidene fluoride polymer is preferred because of its high adhesion to the negative electrode and its high stability within the lithium secondary battery (B).

[0017] The vinylidene fluoride polymer contains structural units derived from vinylidene fluoride. The proportion of VDF units (structural units derived from vinylidene fluoride) in all structural units of the vinylidene fluoride polymer is 50 mol% or more and 100 mol% or less. This proportion may be 75 mol% or more, or 90 mol% or more, and may be 99.5 mol% or less, or 95 mol% or less. Examples of monomers copolymerizable with vinylidene fluoride include tetrafluoroethylene and hexafluoropropylene. The content of the vinylidene fluoride polymer in the adhesive layer may be 3 mass% or more, or 50 mass% or more, and may be 100 mass% or less, or 80 mass% or less.

[0018] The vinylidene fluoride polymer may be polyvinylidene fluoride. Alternatively, the vinylidene fluoride polymer may be a copolymer synthesized by polymerizing a monomer containing vinylidene fluoride and hexafluoropropylene. That is, the adhesive layer may contain a copolymer synthesized by polymerizing a monomer containing vinylidene fluoride and hexafluoropropylene. Because hexafluoropropylene has a bulkier molecular structure than vinylidene fluoride, a copolymer synthesized by polymerizing a monomer containing vinylidene fluoride and hexafluoropropylene has a lower density than a vinylidene fluoride polymer. By using a monomer containing hexafluoropropylene, crystallization of the polymer is suppressed, resulting in a flexible adhesive layer. Furthermore, the adhesive layer's ability to retain non-aqueous electrolyte is improved, thereby improving lithium ion conductivity. The battery resistance is reduced, further improving cycle characteristics.

[0019] The vinylidene fluoride polymer may be a copolymer of vinylidene fluoride and hexafluoropropylene (HFP). The proportion of HFP units (structural units derived from hexafluoropropylene) in all structural units of the vinylidene fluoride polymer may be in the range of 0 to 50 mol % (e.g., in the range of 0.5 to 25 mol %, or in the range of 2 to 10 mol %).

[0020] The adhesive layer may contain inorganic particles, which improves the strength and durability of the adhesive layer.

[0021] The inorganic particles are preferably insulating inorganic particles, but conductive inorganic particles may also be used. The inorganic particles may be insulating inorganic particles such as metal oxides, metal hydroxides, metal nitrides, metal carbides, and metal sulfides. Examples of inorganic particle materials include aluminum oxide, 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 (such as acetylene black), amorphous silica, and crystalline silica. Solid electrolyte particles such as LLZO (lithium lanthanum zirconium oxide) and LATP (lithium aluminum titanium phosphate) may also be used.

[0022] The average primary particle size of the inorganic particles may be 0.1 μm or more, or 1.0 μm or more, and may be 5.0 μm or less, or 3.0 μm or less. The average primary particle size is the median diameter (D ５０ ) The median diameter (D ５０ ) can be measured using a laser diffraction / scattering particle size distribution measuring device.

[0023] The content of inorganic particles in the adhesive layer may be 10% by mass or more, or 20% by mass or more, and may be 99% by mass or less, or 90% by mass or less.

[0024] The adhesive layer may 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 resistance of the battery is reduced and good cycle characteristics can be obtained.

[0025] The thickness of the adhesive layer may be 0.1 μm or more, or 0.3 μm or more, or may be 5 μm or less, or 2 μm or less. The adhesive layer may be formed on both sides of the base layer. That is, the adhesive layer may be formed on both the first main surface and the second main surface.

[0026] The average thickness Ts (μm) of the separator is determined by taking the arithmetic mean of the thickness at 15 locations. The thickness of the separator can be measured by a method conforming to JIS (Japanese Industrial Standards) K6250. Specifically, the thickness can be measured using a test piece thickness gauge (SDA-12 model) manufactured by Kobunshi Keiki Co., Ltd., conforming to JIS K6250. Measurements are performed using a probe with a diameter of 5 mm at a pressure of 22 kPa. The thickness is measured at 15 intersections of five lines dividing the separator into six equal parts in the longitudinal direction and three lines dividing the separator into four equal parts in the width direction. The arithmetic mean of the 15 measured thicknesses is then taken as the average thickness Ts.

[0027] As described above, the ratio Ts / Tn of the average thickness increase Tn (μm) on one side of the negative electrode due to charging 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 may be 1.25 or greater, or 1.50 or greater, and may be 5.0 or less, 3.5 or less, or 2.25 or less. By increasing the value of Ts / Tn to greater than 1.2, expansion of the electrode group during charging can be suppressed. By setting the value of Ts / Tn to 3.5 or less, it is possible to suppress both the reduction in volumetric capacity density and the improvement of battery performance. By setting the value of Ts / Tn to 2.25 or less, it is possible to particularly suppress the reduction in volumetric capacity density.

[0028] The average value Tn (μm) can be measured by the following method. First, two batteries manufactured under the same conditions are prepared. One battery is set to a discharged state (SOC = 0%), and the other battery is set to a charged state (SOC = 90%). A state of charge (SOC) of 90% means that 90% of the rated capacity of the battery is charged. A state of charge (SOC) of 0% means that 100% of the rated capacity of the battery is discharged from a fully charged battery. In this specification, the rated capacity of a battery means the 1-hour rate capacity.

[0029] Next, the negative electrode was removed from each of the discharged and charged batteries, and 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, as shown in FIG. 1, the portion 12a of the negative electrode 12 facing the positive electrode across the separator was divided into four equal parts in the longitudinal direction of the negative electrode 12: region 12a1, region 12a2, region 12a3, and region 12a4. The portion 12a is where lithium metal precipitates during charging. Next, the central positions in the width direction at the three boundaries between the four regions 12a1 to 12a4 were designated positions p1, p2, and p3. The thickness T(0) and thickness T(90) were then measured at positions p1, p2, and p3. Both surfaces of the negative electrode at positions p1, p2, and p3 face the positive electrode with the separator sandwiched between them. Therefore, lithium metal is deposited on both surfaces at positions p1, p2, and p3 during charging. The average thickness increase Tn on one surface of the negative electrode due to charging can be calculated using the following formula: Tn = {(total of thicknesses T(90) at three locations) - (total of thicknesses T(0) at three locations)} / 6

[0030] The porosity of the substrate layer may be 40% or more, or 50% or more, or may be 90% or less, or 80% or less. The porosity of the substrate layer may be 40% or more and 90% or less (for example, 50% or more and 80% or less). The porosity of the substrate layer can be measured by the following method. First, 4 cm square samples are taken at four locations in the center of the separator in the width direction. Next, the adhesive layer of each sample is peeled off using adhesive tape to separate only the substrate layer. Next, the mass of each substrate layer is measured, and the thickness of the substrate layer is measured using the measurement method described above (method for measuring separator thickness). Next, the apparent density of the substrate layer is calculated 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. Porosity (%) = 100 × {1 - (apparent density of substrate layer) / (true density of substrate layer)}

[0031] (Method for manufacturing separator) An example of a method for manufacturing a separator (M) used in this embodiment will be described. However, the separator of the lithium secondary battery (B) may be manufactured by a method other than the manufacturing method (M) described below. The matters described above regarding the separator are applicable to the manufacturing method (M), so redundant explanations will be omitted. The matters described regarding the manufacturing method (M) may also be applied to the separator of the lithium secondary battery (B).

[0032] In the manufacturing method (M), a separator is manufactured by forming an adhesive layer on a substrate layer. The method for forming the adhesive layer is not particularly limited, and it may be formed by a known method. In one example of the manufacturing method, the adhesive layer is formed by applying a coating liquid containing the components of the adhesive layer to the substrate layer and then drying it. The method for applying the coating liquid is not limited, and known methods may be used. For example, application may be performed using a spray method, a method using a roller or die (e.g., gravure coating method, die coating method), a printing method (e.g., screen printing method, inkjet method), or the like.

[0033] Examples of solvents that can be used for the coating liquid include acetone, dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, and tripropylene glycol. To form a porous adhesive layer, a non-solvent-induced phase separation (NIPS) method may be used. In one example, a coating liquid is prepared by dissolving a resin in the above-mentioned solvent (good solvent), and the coating liquid is then applied to a substrate layer to form a coating film. Next, before the coating film dries, the coating film can be made porous by immersing it in a poor solvent. Examples of poor solvents that can be used include water, methanol, ethanol, isopropyl alcohol, and mixtures thereof. The porosity of the adhesive layer can be altered by mixing the above-mentioned poor solvent with acetone, dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, and tripropylene glycol.

[0034] The nonaqueous electrolyte may be a nonaqueous electrolyte containing a nonaqueous solvent and a lithium salt. The nonaqueous solvent may contain an ether. The content of the ether in the nonaqueous solvent may be 80% by mass or more, or 90% by mass or more. The nonaqueous solvent may be composed solely of an ether. By using a nonaqueous solvent containing an ether, dendritic precipitation of lithium metal at the negative electrode during charging can be suppressed, and as a result, cycle characteristics can be further improved.

[0035] The ether used in the non-aqueous solvent may be an ether described below. The ether may be a fluoroether containing a fluoro group (—F), or may be a hydrofluoroether. The hydrofluoroether contains a carbon atom to which a hydrogen atom and a fluorine atom are bonded. By using a hydrofluoroether, the reduction resistance of the non-aqueous electrolyte is improved, and decomposition of the non-aqueous electrolyte on the negative electrode surface becomes less likely to occur. The content of the hydrofluoroether in the non-aqueous solvent may be 80% by mass or more, or 90% by mass or more. The non-aqueous solvent may be composed solely of hydrofluoroether.

[0036] The fluorination rate of the hydrofluoroether is preferably 60% or more, more preferably 65% ​​or more. The fluorination rate may be 95% or less, 90% or less, or 80% or less. The fluorination rate of one hydrofluoroether is defined by the following formula: Fluorination rate (%) = 100 × (number of fluorine atoms contained in the hydrofluoroether) / (total number of fluorine atoms and hydrogen atoms contained in the hydrofluoroether).

[0037] The hydrofluoroether used in the non-aqueous solvent 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.

[0038] (Method for producing lithium secondary battery (B)) The production method is not limited as long as the lithium secondary battery (B) can be produced. An example of the production method includes steps (i), (ii), and (iii). The matters described for the lithium secondary battery (B) are applicable to the production method described below, and therefore, redundant description will be omitted.

[0039] Step (i) is a step of forming a laminate of the separator and the negative electrode by bonding a separator to both sides of the negative electrode (negative electrode current collector) via an adhesive layer. The separator and the negative electrode may be bonded by overlapping them and hot pressing them. The heating temperature during hot pressing may be selected appropriately depending on the material of the adhesive layer. For example, when the adhesive layer contains a vinylidene fluoride polymer, the heating temperature during hot pressing may be in the range of 60 to 120°C.

[0040] Step (ii) is a step of forming a wound electrode group by winding the laminate and a positive electrode. A separator is disposed between the positive electrode and the negative electrode. Step (iii) is a step of encapsulating the electrode group and the non-aqueous electrolyte in an outer casing. The method for performing steps (ii) and (iii) is not limited, and known methods may be used. In this manner, a lithium secondary battery (B) is produced. However, the lithium secondary battery (B) may also be produced by a method other than this method.

[0041] (Examples of Components) Examples of other components of the lithium secondary battery (B) will be specifically described below. Note that the components described below are merely examples, and the components of the lithium secondary battery (B) of this embodiment are not limited to the following examples. Known components may be used for components other than those characteristic of this embodiment.

[0042] (Negative Electrode) The negative electrode includes a negative electrode current collector. In a lithium secondary battery, lithium metal is deposited on the negative electrode current collector during charging. The deposited lithium metal dissolves in the non-aqueous electrolyte as lithium ions during discharging.

[0043] The negative electrode current collector may be composed solely of a substrate sheet in which lithium is not the main component (content: 50% by mass or more). Alternatively, the negative electrode current collector may include 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, stainless steel, etc. Copper and copper alloys are preferred because they have high conductivity. Stainless steel is preferred because it is resistant to cutting.

[0044] The lithium-containing metal layer laminated on the substrate sheet is a lithium metal layer or a lithium alloy layer. The lithium alloy layer contains trace amounts of elements other than lithium (10 atomic % or less). Examples of elements other than lithium contained in the lithium alloy include aluminum, magnesium, indium, and zinc. Forming a lithium-containing metal layer can suppress the decrease in discharge capacity due to repeated charge and discharge. Furthermore, forming a lithium-containing metal layer can suppress the dendritic deposition of lithium metal. The method for forming the lithium-containing metal layer is not particularly limited, and it may be formed by a known method. For example, the lithium-containing metal layer may be formed by pressing a lithium metal foil or a lithium alloy foil onto the substrate sheet. The lithium-containing metal layer is dense, which makes it distinct from lithium metal (generally porous) that precipitates during charging.

[0045] The surface of the metal negative electrode current collector may be smooth. By using a negative electrode current collector with a smooth surface, lithium metal is more easily deposited evenly on the negative electrode current collector during charging. The smooth surface of the metal negative electrode current collector means that the maximum height roughness Rz of the negative electrode current collector is 20 μm or less. The maximum height roughness Rz of the metal negative electrode current collector may be 10 μm or less. The maximum height roughness Rz is measured in accordance with JIS (Japanese Industrial Standards) B 0601:2013.

[0046] The thickness of the substrate sheet may be 3 μm or more, or 5 μm or more, and may be 20 μm or less, or 15 μm or less. When the negative electrode current collector includes a lithium-containing metal layer, the thickness of the lithium-containing metal layer may be in the range of 5 μm to 25 μm.

[0047] The negative electrode current collector (e.g., a 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 appropriate strength and flexibility, resulting in a negative electrode current collector with excellent resistance to stress generated in the negative electrode. As a result, fracture of the negative electrode current collector during charge and discharge and the resulting deterioration of cycle characteristics are suppressed.

[0048] The above-mentioned "austenitic stainless steel" refers to stainless steel with an austenite fraction of 50% or more. The austenite fraction refers to the proportion (mass ratio) of the austenite phase in the stainless steel. When the contents of the austenite phase, ferrite phase, and martensite phase in the stainless steel are O, F, and M, respectively, the austenite fraction is calculated by {O / (O+F+M)}×100. The austenite structure has a face-centered cubic lattice structure (FCC structure), and the ferrite structure and martensite structure have a body-centered cubic lattice structure (BCC structure).

[0049] The austenite ratio may be 70% or more, 90% or more, or even 100%.

[0050] The austenite fraction can be determined by the following method. First, a sample (e.g., 25 mm square) of a negative electrode current collector (stainless steel foil) is prepared, and X-ray diffraction (XRD) measurement using a two-dimensional detection function is performed on the sample to obtain an XRD pattern (vertical axis: X-ray diffraction intensity, horizontal axis: diffraction angle 2θ). The size of the measurement area (microscopic portion) is, for example, 15 mm square.

[0051] Desirable XRD measurement conditions are as follows: <Analytical device> Two-dimensional micro X-ray diffractometer (RINT-RAPID II, manufactured by Rigaku Corporation) <Analysis conditions> Tube: Co Monochromatization: Use of a monochromator (CoKα) Tube output: 40 kV-30 mA Detector: Imaging plate (two-dimensional) (reflection method) Collimator: Φ300 μm ω angle: 25° to 35° (2° / sec) Φ angle: 360° rotation (1° / sec) Measurement time (exposure): 30 minutes

[0052] The diffraction peaks observed in the obtained XRD pattern are fitted by the least-squares method using a standard database, and then quantitatively analyzed by Rietveld analysis. The XRD pattern may have diffraction peaks corresponding to at least one of the austenite, ferrite, and martensite phases. This analysis can be performed using software provided with the analysis device. This analysis determines the austenite fraction, which is the ratio (mass ratio) of the austenite phase to the total of the austenite, ferrite, and martensite phases. Several measurement areas are randomly selected from the sample, and the austenite fraction in each measurement area is determined and the average value is calculated.

[0053] Austenitic stainless steel may contain, as components other than Fe, for example, C, Si, Mn, P, S, Ni, Cr, Mn, Mo, Cu, and N. The stainless steel may be low-carbon, ultra-low-carbon, or nitrogen-added stainless steel, or may be a duplex stainless steel containing austenite.

[0054] Examples of austenitic stainless steel include SUS301, SUS302, SUS303, SUS304, SUS305, SUS309, SUS310, SUS312, SUS315, SUS316L, SUS317, SUS321, and SUS347. Of these, SUS304 and SUS316L are preferred.

[0055] The negative electrode current collector may include a resin film and a transition metal layer laminated on the resin film. That is, the base sheet may include a resin film and a transition metal layer laminated on the resin film. The negative electrode current collector (e.g., base sheet) may be composed of a resin film and a transition metal layer. The resin film may include a base resin layer and a surface resin layer formed on the base 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., surface resin layer). As described above, the negative electrode current collector may include a lithium-containing metal layer laminated on the base sheet.

[0056] Resin films are lightweight and can easily increase the energy density of secondary batteries. Resin films are resistant to tearing when transported in a roll and are easy to handle. Resin films are resistant to embrittlement even when the negative electrode potential is low. Resin films are excellent as current collector materials because they are highly resistant to stress caused by electrode expansion and contraction and are resistant to breakage. In particular, lithium secondary batteries tend to experience significant expansion of the negative electrode during charging because lithium metal precipitates on the negative electrode. In particular, when lithium metal precipitates in the form of dendrites, the amount of expansion becomes even greater. This tends to cause stress on the negative electrode.

[0057] The main surface of the resin film may be smooth or roughened, or may be subjected to plasma treatment, corona treatment, or the like. When the main surface of the resin film is smooth, the maximum height roughness Rz of the main surface is 2.5 μm or less. When the main surface of the resin film is roughened, the maximum height roughness Rz of the main surface may be greater than 2.5 μm and may be 8 μm or more. The maximum height roughness Rz is measured in accordance with JIS B 0601:2013. The main surface of the resin film refers to the surface other than the end surfaces of the resin film, and refers to the two surfaces having the largest areas. In this specification, "surface" usually means "main surface."

[0058] From the viewpoint of increasing the energy density of the secondary battery, it is desirable that the resin film be thin as long as mechanical strength is ensured. An example of a preferred range of the resin film thickness is 1.5 μm to 30 μm. The thickness of the resin film can be determined by measuring the thickness at any 10 points on the cross section of the resin film using a scanning electron microscope (SEM) and calculating the arithmetic average.

[0059] The surface resin layer of the resin film may contain a nitrogen-containing resin. 90% by mass or more of the surface resin layer may be a nitrogen-containing resin. The nitrogen-containing resin may be a polymer having nitrogen atoms in the main chain and / or side chain. The content of nitrogen atoms contained in the nitrogen-containing resin may be 3% by mass or more. The content of nitrogen atoms contained in the surface resin layer may be 2.5% by mass or more.

[0060] The nitrogen-containing resin may contain a nitrogen-hydrogen bond (a bond between a nitrogen atom and a hydrogen atom). The infrared absorption peak specific to the nitrogen-hydrogen bond is at 1655 cm －１ (1640-1670cm －１ ) Near 1530 cm －１ (1515-1545cm －１ ) and other regions. The presence or absence of a nitrogen-hydrogen bond can be identified based on these peaks. Specifically, first, the active material of the negative electrode is scraped off or wet-removed, and then the transition metal layer is dissolved in a nitric acid solution to expose the surface resin layer. Next, the exposed surface resin layer is analyzed by infrared absorption spectroscopy to identify the presence or absence of a nitrogen-hydrogen bond. A specific example of the analysis is shown below. Alternatively, the presence or absence of a nitrogen-hydrogen bond can also be identified by the chemical shift of X-ray photoelectron spectroscopy (XPS).

[0061] (FTIR-ATR method) Measurement device: Varian 670FTIR (manufactured by Varian) Measurement mode: attenuated total reflection Light source: special ceramics Detector: DLaTGS (deuterated L-alanine-doped triglycine sulfate) Resolution: 4 cm －１ Number of integrations: 256 IRE: Ge Incident angle: 60 degrees Attachment: Single reflection ATR attachment (Seagull)

[0062] The nitrogen-containing resin may be 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 aramid bond, an imide bond, a urethane bond, a carbodiimide bond, a uretdione structure, an isocyanurate ring, a nitrile group, and an amide group. 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 because of their high flexibility, high resistance to stress caused by the expansion and contraction of electrodes, and resistance to breakage.

[0063] Polyurethane resins can be synthesized by reacting a polyol with a di- or higher functional polyisocyanate (particularly a diisocyanate). By appropriately selecting the polyol and polyisocyanate, polyurethane resins with various physical properties can be synthesized.

[0064] The nitrogen-containing resin may be a polymer having at least one selected from the group consisting of an aliphatic isocyanate group, an aromatic isocyanate group, an allophanate group, and a biuret group. In particular, thermosetting polyurethane resins use polyisocyanate as a raw material, so there is a high probability of residual unreacted isocyanate groups. It is believed that the isocyanate groups are reduced at the negative electrode to produce coating components for forming a stable coating on the negative electrode. Furthermore, thermosetting resins form a strong coating with a three-dimensional structure, which significantly suppresses copper damage. UV-curable resins also have a similar effect. The isocyanate groups have a peak at 2250 cm －１ (2270~2240cm －１ ) has an infrared absorption peak due to antisymmetric stretching vibration in the vicinity thereof, so that its presence or absence can be confirmed based on the infrared absorption peak.

[0065] The surface resin layer may contain a filler. Adding a filler (such as silica or alumina) to the surface resin layer creates irregularities in the surface resin layer. As a result, an anchor effect occurs, making it possible to enhance adhesion between the surface resin layer and the base resin layer. The filler can be a filler that can roughen the surface of the surface resin layer and is less likely to cause a decrease in battery characteristics due to a side reaction with the non-aqueous electrolyte. The filler can be particles of resin, metal oxide, ceramic, metal, or the like.

[0066] The surface resin layer may be a coating layer formed by applying a nitrogen-containing resin to the surface of the base resin layer. In this case, the nitrogen-containing resin may be a thermosetting resin or a UV-curable resin. The nitrogen-containing resin may be diluted with a solvent and applied to the surface of the base resin layer. The cured product of the curable resin may have a three-dimensional network of molecular chains.

[0067] The thickness of the surface resin layer is, for example, 5 μm or less, and in a preferred example, 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 the surface resin layer at any 10 points on the cross section of a predetermined member (negative electrode or negative electrode current collector) using a scanning electron microscope (SEM) and calculating the arithmetic average of the measured thicknesses.

[0068] Most resin films are not electrically conductive. The transition metal layer serves to impart good electrical conductivity to the negative electrode current collector. The transition metal layer may be any layer that contains a transition metal and has electronic conductivity. The transition metal layer desirably contains a transition metal in a metallic state that has electronic conductivity due to free electrons.

[0069] The transition metal layer preferably contains copper, nickel, chromium, titanium, iron, silver, gold, tin, etc., as this makes it easier to ensure corrosion resistance and electrical conductivity. The transition metal layer desirably contains at least one selected from the group consisting of copper, a copper alloy, stainless steel, nickel, and a nickel alloy. The transition metal layer particularly desirably contains copper or a copper alloy, which has excellent electrical conductivity.

[0070] Transition metals have the effect of embrittling resin films. In particular, when the transition metal layer contains copper, embrittlement of the resin film is likely to occur. A new, yet unreported finding has been that lithium metal significantly accelerates the embrittlement of the resin film (e.g., copper damage) caused by transition metals. That is, in lithium secondary batteries in which lithium metal is deposited on the negative electrode, significant deterioration of the resin film due to the transition metal layer can occur. In contrast, by incorporating a nitrogen-containing resin into at least the surface resin layer of the resin film, embrittlement of the resin film is significantly suppressed.

[0071] A nickel-chromium alloy layer is considered as a metal layer that blocks the migration of copper ions into the resin film. However, in lithium secondary batteries, the nickel-chromium alloy layer does not provide any shielding effect. This is presumably because when easily ionized metallic lithium comes into contact with a transition metal such as copper, the ionization of the transition metal is accelerated, and the transition metal ions accelerate the rupture of the polymer. On the other hand, a surface resin layer containing a nitrogen-containing resin provides a significant effect of suppressing the rupture of the polymer.

[0072] The most significant factor in embrittlement of a resin film is the rupture of carbon-carbon bonds. When a resin film contains a nitrogen-containing resin, the nitrogen-containing resin contains carbon-nitrogen bond. The carbon-nitrogen bond is thought to be less susceptible to the influence of transition metals or their ions. It is possible that the transition metal is stabilized by the nitrogen atoms, suppressing the rupture of carbon-carbon bonds.

[0073] The transition metal layer may be formed by depositing it on the surface of the surface resin layer using a liquid phase method or a gas phase method. As the liquid phase method, an electrodeposition method such as electrolytic or electroless plating can be used. As the gas phase method, an evaporation method, a sputtering method, an atomic layer deposition method (ALD), etc. can be used. A base layer may be formed by a sputtering method, and then a thick transition metal layer may be formed thereon by electrolytic plating. That is, a combination of multiple methods can be used. The transition metal layer may also be formed by a lamination method. However, the method for forming the transition metal layer is not particularly limited.

[0074] The thickness of the transition metal layer is, for example, 5 μm or less, and may be 3 μm or less. An example of a preferred range for the thickness of the transition metal layer is 0.05 μm to 1.5 μm, and may 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 on the cross section of a predetermined member (negative electrode or negative electrode current collector). Next, the thickness (average thickness) of the transition metal layer is determined by arithmetically averaging the obtained measurements. The transition metal layer may be composed of multiple layers made of different metals.

[0075] The substrate resin layer is the main part of the negative electrode current collector and is usually thicker than the surface resin layer and the transition metal layer. At least 51% by mass of the substrate resin layer is composed of a resin or an organic material. To improve adhesion with the transition metal layer or the lithium layer, the substrate resin layer may contain an inorganic material such as inorganic particles. The substrate resin layer may be a stretched film, a nonporous film (a film without holes), or a film having a plurality of regularly arranged holes. The substrate resin layer may be insulating, conductive, or non-conductive. The shape and physical properties of the substrate resin layer are not particularly limited.

[0076] The substrate resin layer is formed, for example, by molding a raw material (such as a general-purpose plastic or a general-purpose engineering plastic) into a sheet. Examples of the raw material include polyester resin, olefin resin, polyphenylene sulfide resin, acrylic resin, polycarbonate resin, polyether ether ketone resin, polysulfone resin, polyphenylsulfone resin, polyethersulfone resin, polyamide resin, polyimide resin, polyetherimide resin, polybenzimidazole resin, liquid crystal polymer resin, polyacetal resin, polyvinyl chloride resin, polyarylate resin, silicone resin, nylon resin, polyvinylidene chloride resin, ethylene-vinyl alcohol copolymer, polyvinyl alcohol resin, polystyrene resin, epoxy resin, polyurethane resin, phenolic resin, melamine resin, urea resin, and unsaturated polyester resin. The resin contained in the substrate resin layer may be used alone or in combination of two or more.

[0077] The resin contained in the substrate resin layer is preferably a resin having an aromatic ring, a resin not containing a fluorine atom, or an olefin resin. When the resin has an aromatic ring (e.g., a benzene ring) in its molecule, the affinity between the substrate resin layer and the lithium metal layer is increased, and the adhesive strength between them is improved.

[0078] As the polyester resin, aromatic polyester is preferred, and unstretched polyethylene terephthalate, biaxially oriented polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, etc. are preferred. As the acrylic resin, polymethyl methacrylate, etc. can be used. As the polyimide resin, aromatic polyimide is preferred. As the polyamide resin, aromatic polyamide (aramid resin) is preferred. As the olefin resin, unstretched polypropylene, biaxially oriented 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, ionomer, etc. are preferred.

[0079] The extrusion method for the substrate resin layer may be a T-die method or an inflation method, and may be unstretched, uniaxially stretched, sequentially biaxially stretched, or simultaneously biaxially stretched. The resin may be a homopolymer, a copolymer, or a terpolymer. The arrangement of the structural units constituting the resin is not limited, and may be a random copolymer or a block copolymer. Two or more types of substrate resin layers may be combined. For example, two or more substrate resin layers may be laminated. The crystalline resin may be in a crystalline state, an amorphous state, or a mixture of both. The amorphous resin can be formed by a method such as rapid cooling. The substrate resin layer may be an alloy resin consisting of two or more of the resins listed above.

[0080] To ensure adhesion to other layers (e.g., vapor-deposited films), the surface of the base resin layer may be subjected to corona treatment or plasma treatment. To provide irregularities in the base resin layer to improve adhesion to the surface resin layer, a filler (a filler made of ceramic, resin, metal, or the like) may be added to the base resin layer to form irregularities on the surface of the base resin layer.

[0081] (Positive Electrode) The positive electrode includes a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector. The positive electrode mixture layer includes a positive electrode active material, for example, a positive electrode active material and additives (conductive material, binder, thickener, etc.). The positive electrode mixture layer is formed on both sides of the positive electrode current collector. The positive electrode can be formed by a known method. For example, first, a positive electrode mixture slurry containing a positive electrode active material and additives is prepared. Next, the positive electrode mixture slurry is applied to the positive electrode current collector and then dried to form a coating film. Next, a laminate consisting of the positive electrode current collector and the coating film is rolled to obtain a positive electrode. The formed positive electrode is cut to a predetermined size as necessary.

[0082] The thickness of the positive electrode mixture layer may be 50 μm or more, or 100 μm or more, and may be 300 μm or less, or 250 μm or less.

[0083] The positive electrode active material can be a material capable of reversibly absorbing and releasing lithium ions. Examples of the positive electrode active material include lithium-containing transition metal oxides, transition metal fluorides, polyanions, fluorinated polyanions, transition metal sulfides, etc. Lithium-containing transition metal oxides are preferred because of their low production cost and high average discharge voltage.

[0084] Examples of transition metal elements contained in the lithium-containing transition metal oxide include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, W, etc. The lithium-containing transition metal oxide may contain only one transition metal element or may contain two or more transition metal elements. The transition metal element may be at least one element selected from the group consisting of Co, Ni, and Mn. The lithium-containing transition metal oxide may contain one or more typical elements. Examples of typical elements include Mg, Al, Ca, Zn, Ga, Ge, Sn, Sb, Pb, Bi, B, etc.

[0085] The conductive material may be a carbon material, etc. Examples of the carbon material include carbon black (acetylene black, ketjen black, etc.), carbon nanotubes, and graphite.

[0086] Examples of the binder include fluororesin, polyacrylonitrile, polyimide resin, acrylic resin, polyolefin resin, rubber polymer, etc. Examples of the fluororesin include polytetrafluoroethylene, polyvinylidene fluoride, etc.

[0087] The thickener may be a cellulose derivative. Examples of the cellulose derivative include carboxymethyl cellulose (CMC) and its modified forms, methyl cellulose, etc. Examples of modified forms of CMC include salts of CMC. Examples of salts include alkali metal salts (e.g., sodium salts) and ammonium salts.

[0088] The positive electrode current collector may be a conductive sheet, such as a metal foil, and the surface of the positive electrode current collector may be coated with a carbon material.

[0089] Examples of materials for the positive electrode current collector (conductive sheet) include metal materials containing Al, Ti, Fe, etc. The metal material may be Al, an Al alloy, Ti, a Ti alloy, an Fe alloy (e.g., stainless steel), etc. The thickness of the positive electrode current collector is not particularly limited and may be in the range of 5 to 300 μm.

[0090] (Separator) The separator may be any of the separators described above.

[0091] 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 may be formed on the positive electrode. Typically, the lithium secondary battery (B) does not include a spacer (a spacer other than a separator) disposed between the positive electrode and the negative electrode.

[0092] The spacer may include linear portions and / or dot-shaped portions. The linear portions may be mesh-shaped portions arranged in a mesh (e.g., honeycomb) pattern. Alternatively, the spacer may include a plurality of linear portions arranged in a stripe pattern.

[0093] (Non-aqueous electrolyte) A non-aqueous electrolyte having lithium ion conductivity can be used as the non-aqueous electrolyte. The non-aqueous electrolyte may be liquid or gel-like. A liquid non-aqueous electrolyte can be prepared by dissolving a lithium salt in a non-aqueous solvent. When the lithium salt dissolves in the non-aqueous solvent, lithium ions and anions are generated.

[0094] The gel-like non-aqueous electrolyte may contain a lithium salt and a matrix polymer, or may contain a lithium salt, a non-aqueous solvent, and a matrix polymer. The matrix polymer may be, for example, a polymer material that absorbs the non-aqueous solvent and gels. Examples of the polymer material include fluororesin, acrylic resin, and polyether resin.

[0095] Known solvents can be used as the nonaqueous solvent. Examples of nonaqueous solvents that can be used 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), ethyl methyl 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 methyl fluoropropionate. Examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, and 2-methyltetrahydrofuran. Examples of the chain ether 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. The non-aqueous solvent may be used alone or in combination of two or more.

[0096] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO ４ , LiAlCl ４ , LiB １０ Cl １０ etc.), lithium salts of fluorine-containing acids (LiPF ６ , LiPF ２ O ２ , LiBF ４ , LiSbF ６ , LiAsF ６ , LiCF ３ SO ３ , LiCF ３ CO ２ etc.), lithium salt of fluorine-containing acid imide (LiN(FSO ２ ) ２ , LiN(CF ３ SO ２ ) ２ , LiN(CF ３ SO ２ ) (FSO ２ ), LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ ), LiN(C ２ F ５ SO ２ ) ２ etc.), lithium halides (LiCl, LiBr, LiI etc.), lithium salts containing oxalate complexes (LiB(C ２ O ４ ) ２ , LiBF ２ (C ２ O ４ ), LiPF ４ (C ２ O ４ ), LiPF ２ (C ２ O ４ ) ２ The lithium salts may be used alone or in combination of two or more.

[0097] The concentration of the lithium salt in the non-aqueous electrolyte may be 0.5 mol / L or more, 1.0 mol / L or more, or 1.5 mol / L or more, and may be 3.5 mol / L or less, 2.0 mol / L or less, or 1.5 mol / L or less. By setting the lithium salt concentration within the above range, a non-aqueous electrolyte having excellent ionic conductivity and appropriate viscosity can be obtained.

[0098] The non-aqueous electrolyte may contain an additive (for example, a known additive). Examples of the additive include 1,3-propane sultone, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, and fluorobenzene.

[0099] (Exterior Body) The exterior body accommodates the nonaqueous electrolyte and the electrode group. The exterior body is not particularly limited, and a known exterior body can be used. The exterior body may include a cylindrical battery case with a bottom, and a sealing body and a gasket that seal the opening of the battery case.

[0100] An example of the lithium secondary battery (B) of this embodiment will be specifically described below with reference to the drawings. The components described above can be applied to the components of the lithium secondary battery of the example described below. Furthermore, the components of the example described below can be modified based on the above description. Furthermore, the matters described below may be applied to the above embodiment. Furthermore, in the lithium secondary battery described below, components that are not essential for the lithium secondary battery (B) according to the present disclosure may be omitted.

[0101] (Embodiment 1) Fig. 2 is a longitudinal cross-sectional view schematically illustrating an example of a lithium secondary battery according to Embodiment 1. The cylindrical lithium secondary battery 10 shown in Fig. 2 includes a cylindrical battery case, and an electrode group 14 and a non-aqueous electrolyte (not shown) housed in the battery case. The electrode group 14 includes a positive electrode 11, a negative electrode 12, and a separator 13.

[0102] The battery case includes a case body 15, which is a cylindrical metal container with a bottom, and a sealing body 16 that seals the opening of the case body 15. A gasket 27 is disposed between the case body 15 and the sealing body 16. The gasket 27 ensures the airtightness of the battery case. Within the case body 15, insulating plates 17 and 18 are disposed at both ends of the electrode group 14 in the winding axis direction. The case body 15 has a step portion 21.

[0103] The sealing body 16 includes a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26. The lower valve body 23 and the upper valve body 25 are connected at their respective centers. An insulating member 24 is disposed between the peripheral edge of the lower valve body 23 and the peripheral edge of the upper valve body 25. The filter 22 and the lower valve body 23 are connected at their respective peripheral edges. The upper valve body 25 and the cap 26 are connected at their respective peripheral edges. All of the components constituting the sealing body 16, except for the insulating member 24, are electrically connected.

[0104] A vent hole is formed in the lower valve body 23. Therefore, if the internal pressure of the battery case increases due to abnormal heat generation or the like, the upper valve body 25 bulges toward the cap 26 and separates from the lower valve body 23. This cuts off the electrical connection between the lower valve body 23 and the upper valve body 25. If the internal pressure increases further, the upper valve body 25 breaks, and gas is discharged from an opening formed in the cap 26.

[0105] The positive electrode 11 is electrically connected to a cap 26, which functions as a positive electrode terminal, via a positive electrode lead 19. The negative electrode 12 is electrically connected to a case body 15, which functions as a negative electrode terminal, via a negative electrode lead 20.

[0106] A cross-sectional view of an example of the separator 13 is shown schematically in FIG. 3. 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 negative electrode 12 side, and the second main surface 13a2 is disposed on the positive electrode 11 side. 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 portion of the adhesive layer 13b is adhered to the negative electrode 12.

[0107] (Additional Notes) The above description discloses the following technology: (Technology 1) A lithium secondary battery comprising: an electrode group in which a positive electrode, a negative electrode, and a separator are wound such that the separator is disposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte, in which lithium metal precipitates in the negative electrode during charging and dissolves in the non-aqueous electrolyte during discharge, the separator comprising a substrate layer having a first main surface disposed on the negative electrode side and a second main surface disposed on the positive electrode side, and an adhesive layer formed on the first main surface of the substrate layer, at least a portion of the adhesive layer being adhered to the negative electrode, and in which an average value Tn (μm) of an increase in thickness due to charging on one side of the negative electrode at a 90% charged state and an average thickness Ts (μm) of the separator satisfy the relationship 1.2<Ts / Tn. (Technology 2) The lithium secondary battery according to Technology 1, wherein the adhesive layer contains a vinylidene fluoride-based polymer synthesized by polymerizing a monomer containing vinylidene fluoride. (Technology 3) The lithium secondary battery according to Technology 1 or 2, wherein the adhesive layer contains a copolymer synthesized by polymerizing a monomer containing vinylidene fluoride and hexafluoropropylene. (Technology 4) The lithium secondary battery according to any one of Technology 1 to 3, wherein the adhesive layer contains inorganic particles. (Technology 5) The lithium secondary battery according to any one of Technology 1 to 4, wherein the adhesive layer is porous. (Technology 6) The lithium secondary battery according to any one of Technology 1 to 5, wherein the non-aqueous electrolyte contains a non-aqueous solvent and a lithium salt, the non-aqueous solvent contains an ether, and a content of the ether in the non-aqueous solvent is 80 mass% or more. (Technology 7) The lithium secondary battery according to any one of Technology 1 to 6, wherein the porosity of the base layer is 40% or more and 90% or less. (Technology 8) The lithium secondary battery according to any one of Technologies 1 to 7, wherein the negative electrode includes a negative electrode current collector, and the negative electrode current collector contains austenitic stainless steel.(Technology 9) The lithium secondary battery according to any one of Techniques 1 to 7, 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, and the surface resin layer contains a nitrogen-containing resin. (Technology 10) The lithium secondary battery according to Technique 9, wherein the nitrogen-containing resin contains a nitrogen-hydrogen bond. (Technology 11) The lithium secondary battery according to Technique 9 or 10, 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 aramid bond, an imide bond, a urethane bond, a carbodiimide bond, a uretdione structure, an isocyanurate ring, a nitrile group, and an amide group. (Technology 12) The lithium secondary battery according to any one of Techniques 9 to 11, 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, an allophanate group, and a biuret group.

[0108] The lithium secondary battery according to the present disclosure will be specifically described below using examples. However, the present disclosure is not limited to the following examples. In these examples, a plurality of lithium secondary batteries with different configurations were fabricated and evaluated.

[0109] (Battery A1) Battery A1 was fabricated according to the following procedure.

[0110] (1) Preparation of Positive Electrode A lithium-containing transition metal oxide (positive electrode active material), acetylene black (AB, conductive material), and polyvinylidene fluoride (PVDF, binder) were mixed in a mass ratio of positive electrode active material:AB:PVDF=95:2.5:2.5, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added and stirred to prepare a positive electrode mixture slurry. The positive electrode active material used was a lithium-containing transition metal oxide containing Li, Ni, Co, and Al.

[0111] Next, the positive electrode mixture slurry was applied to both sides of a positive electrode current collector (aluminum foil), dried, and the coating of the positive electrode mixture was rolled using a roller. Finally, the resulting laminate of the positive electrode current collector and the positive electrode mixture was cut to a predetermined electrode size. In this way, a positive electrode including a positive electrode current collector and a positive electrode mixture layer formed on both sides of the positive electrode current collector was produced. Next, an aluminum tab was attached to the produced positive electrode.

[0112] (2) Preparation of negative electrode current collector (negative electrode) A negative electrode current collector including a copper foil (thickness: 10 μm) and a lithium-containing metal layer was prepared by pressing a lithium alloy foil (thickness: 25 μm) onto each side of a copper foil (thickness: 10 μm). Next, a nickel tab was attached to the negative electrode current collector.

[0113] (3) Preparation of non-aqueous electrolyte LiPF ６ The concentration of LiBF becomes 1 mol / L. ２ (C ２ O ４ The non-aqueous electrolyte was prepared by dissolving the above components in a non-aqueous solvent so that the concentration of the component (I) was 0.1 mol / L. Dimethyl carbonate (carbonate ester) was used as the non-aqueous solvent.

[0114] (4) Preparation of Separator A separator was prepared, consisting of a substrate layer and an adhesive layer formed on only one side (first main surface) of the substrate layer. A polyethylene microporous membrane was used for the substrate layer. An adhesive layer (thickness: approximately 1 μm) made of polyvinylidene fluoride (PVDF) was used for the adhesive layer. The average thickness Ts of the separator was measured using the method described above.

[0115] (5) Battery Fabrication A separator was placed on both sides of the negative electrode current collector so that the adhesive layer was in contact with the negative electrode current collector, and the adhesive layer and the negative electrode current collector were bonded together by hot rolling and roll pressing. In this way, a laminate was formed in which a separator was laminated on both sides of the negative electrode (negative electrode current collector).

[0116] Next, the laminate (negative electrode current collector and separator) and the positive electrode were wound in an inert gas atmosphere to prepare a wound electrode assembly. The electrode assembly and the nonaqueous electrolyte were then housed in an outer package, which was then sealed to prepare a battery A1 (lithium secondary battery). The outer package was a bag-shaped outer package made of a laminate sheet including an aluminum layer.

[0117] (Measurement of Average Value Tn) For Battery A1, the average value Tn (μm) of the increase in thickness due to charging on one side of the negative electrode at a 90% charged state was measured by the method described above.

[0118] (Charge-Discharge Cycle Test) Battery A1 was subjected to a charge-discharge cycle test in an environment of 25°C. Charging and discharging were performed under the following conditions. A 20-minute break was allowed between charging and discharging. 100 charge-discharge cycles were repeated, and the discharge capacity after 100 cycles was measured. (Charging) 10 mA / cm ２ After that, the current was increased to 1 mA / cm ２ The battery was charged at a constant voltage of 4.1 V until the voltage reached 3 V. (Discharge) ２ A constant current discharge was performed.

[0119] (Batteries A2 to A11 and Batteries C1 to C3) Batteries A2 to A9 and Batteries C1 to C3 were fabricated using the same method and conditions as Battery A1, except that the average value Tn (μm), separator, nonaqueous solvent for the nonaqueous electrolyte, and substrate for the negative electrode current collector were changed as shown in Table 1. As described above, the average value Tn (μm) is the average increase in thickness on one side of the negative electrode upon 90% charge. The thickness of one adhesive layer included in the separator was the same as the thickness of the adhesive layer included in the separator of Battery A1. The average value Tn was changed depending on the thickness of the positive electrode mixture layer. In the separators of Batteries A2 to A11 and C1, an adhesive layer (VDF-HFP copolymer) was formed on the negative electrode side of the substrate layer, and the adhesive layer was adhered to the negative electrode. No adhesive layer was formed on the main surface of the separator substrate layer facing the positive electrode. The separators of Batteries C2 and C3 did not have an adhesive layer and consisted only of a substrate layer. In Table 1, P(VDF-HFP) means a copolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP). The adhesive layers of batteries A7 and A9 contained inorganic particles (Al ２ O ３ particles) was added.

[0120] The negative electrode current collector of Battery A10 had the same configuration as that of Battery A1, except that an austenitic stainless steel foil (SUS foil) was used instead of copper foil. The negative electrode current collector of Battery A11 had the same configuration as that of Battery A1, except that a predetermined laminate (a laminate having a resin film and a transition metal layer laminated on the resin film) was used instead of copper foil. The resin film was a polyethylene terephthalate film (PET film) coated with polyurethane resin (nitrogen-containing resin). The transition metal layer was a copper layer. Specifically, copper was first attached to the resin film by sputtering, and then copper was deposited by wet plating. The negative electrode current collectors of Battery A10 and Battery A11 each had a lithium-containing metal layer, similar to the negative electrode current collector of Battery A1.

[0121] The nonaqueous solvent in Table 1 indicates the nonaqueous solvent used in the nonaqueous electrolyte. In Table 1, "carbonate" indicates that dimethyl carbonate was used, and "ether" indicates that 1,2-dimethoxyethane (DME) was used. Each of the fabricated batteries was evaluated in the same manner as Battery A1.

[0122] Some of the manufacturing conditions and evaluation results for each battery are shown in Table 1. In Table 1, the discharge capacity of each battery at 100 cycles is shown as a relative value when the discharge capacity of Battery A1 at 100 cycles is set to 100.

[0123]

[0124] Batteries A1 to A11 are lithium secondary batteries (B) according to the present disclosure. Batteries C1 to C3 are comparative examples. As shown in Table 1, Batteries A1 to A11 had better cycle characteristics than Batteries C1 to C3. As shown in Table 1, the cycle characteristics were improved by including inorganic particles in the adhesive layer. Furthermore, the cycle characteristics were improved by including an ether in the nonaqueous solvent. Furthermore, the cycle characteristics were improved by using a stainless steel foil or a resin film as the substrate of the negative electrode current collector.

[0125] The present disclosure can be used for lithium secondary batteries. Although the present invention has been described with reference to presently preferred embodiments, such disclosure should not be interpreted as limiting. Various modifications and alterations will undoubtedly become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. Therefore, the appended claims should be construed to cover all modifications and alterations without departing from the true spirit and scope of the present invention.

[0126] 10: Lithium secondary battery 11: Positive electrode 12: Negative electrode 13: Separator 13a: Base material layer 13a1: First main surface 13a2: Second main surface 13b: Adhesive layer 14: Electrode group

Claims

1. A lithium secondary battery comprising: an electrode group in which a positive electrode, a negative electrode, and a separator are wound such that the separator is disposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte, in which lithium metal precipitates in the negative electrode during charging and dissolves into the non-aqueous electrolyte during discharging, the separator comprises a base material layer having a first main surface disposed on the negative electrode side and a second main surface disposed on the positive electrode side, and an adhesive layer formed on the first main surface of the base material layer, at least a portion of the adhesive layer is adhered to the negative electrode, and an average value Tn (μm) of an increase in thickness due to charging on one side of the negative electrode at a 90% charged state and an average thickness Ts (μm) of the separator satisfy the relationship 1.2<Ts / Tn.

2. The lithium secondary battery according to claim 1, wherein the adhesive layer contains a vinylidene fluoride 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 monomers including 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, and a content 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 substrate 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, and the negative electrode current collector contains 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, and 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 aramid 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, an allophanate group, and a biuret group.

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