Lithium secondary battery

Abstract

This lithium secondary battery comprises: an electrode group including a positive electrode, a negative electrode, and a first separator disposed between the positive electrode and the negative electrode; and a nonaqueous electrolyte. In the negative electrode, lithium metal is deposited during charging, and lithium metal is dissolved in the nonaqueous electrolyte during discharging. At least a part of a surface of the negative electrode is covered with a protective layer that contains a heat-resistant resin.

Description

Lithium-ion battery Cross-reference of related applications

[0001] This disclosure claims priority with respect to Japanese Patent Application No. 2024-170737, filed with the Japan Patent Office on 30 September 2024, and the entirety of the said patent application is incorporated herein by reference.

[0002] This disclosure relates to lithium secondary batteries.

[0003] Lithium-ion batteries (lithium metal secondary batteries) are used in a variety of applications as high-capacity secondary batteries. In lithium-ion 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-ion batteries.

[0004] Patent Document 1 describes a lithium secondary battery comprising a positive electrode, a negative electrode, and a polymer layer disposed on the surface of the negative electrode, wherein 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, and the negative electrode current collector has a first region that overlaps with the positive electrode and a second region that does not overlap with the positive electrode, the negative electrode active material layer is selectively provided in the first region of the negative electrode current collector, and the polymer layer includes a copolymer of polyvinylidene fluoride (PVDF) and hexafluoropropylene (HFP) and an ionic liquid, and covers the negative electrode current collector via the negative electrode active material layer in the first region and covers the negative electrode current collector without the negative electrode active material layer in the second region.

[0005] Japanese Patent Publication No. 2020-95931

[0006] In lithium-ion secondary batteries, a problem arises when lithium metal precipitates in a dendrite-like manner during charging. This problem leads to a decrease in charge-discharge cycle characteristics. Currently, there is a need to improve the charge-discharge cycle characteristics of lithium-ion secondary batteries. One of the objectives of this disclosure is to provide a lithium-ion secondary battery with good charge-discharge cycle characteristics.

[0007] One aspect of the present disclosure relates to a lithium secondary battery including a positive electrode, a negative electrode, and an electrode group including a first separator 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. At least a part of the surface of the negative electrode is covered with a protective layer, and the protective layer contains a heat-resistant resin.

[0008] 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 described in the appended claims. The present invention will be better understood from the following detailed description taken in conjunction with the drawings, in terms of both its configuration and content, together with other objects and features of the present invention.

[0009] It is a cross-sectional view schematically showing an example of the lithium secondary battery according to the present disclosure. It is a diagram schematically showing a cross-section of an electrode group. It is a diagram schematically showing a cross-section of another electrode group.

[0010] Hereinafter, embodiments according to the present disclosure will be described with 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 can be obtained. In this specification, the description "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "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 numerical value regarding a specific physical property or condition are exemplified, any combination of any of the exemplified lower limits and any of the exemplified upper limits can be used as long as the lower limit is not more than the upper limit. In the following description, when examples of components and examples of methods are listed, unless otherwise specified, only one of the listed examples may be used, or a plurality of the listed examples may be used in combination.

[0011] (Lithium Secondary Battery) The lithium secondary battery according to this embodiment may be referred to as "lithium secondary battery (B)" hereinafter. The lithium secondary battery (B) includes an electrode group including a positive electrode, a negative electrode, and a first separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. The electrode group may be an electrode group in which the positive electrode and the negative electrode are wound with a separator interposed therebetween. In the negative electrode, lithium metal is deposited during charging and dissolved in the non-aqueous electrolyte during discharging.

[0012] In a lithium secondary battery, for example, 70% or more of the rated capacity is manifested by the deposition and dissolution of lithium metal. The movement of electrons in the negative electrode during charging and discharging is mainly due to the deposition and dissolution of lithium metal in the negative electrode. Specifically, 70 to 100% (for example, 80 to 100% or 90 to 100%) of the movement of electrons (current from another perspective) in the negative electrode during charging and discharging is due to 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 movement of electrons in the negative electrode during charging and discharging is mainly due to the insertion and extraction 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 inserts and extracts lithium ions.

[0013] Generally, in a lithium secondary battery, lithium metal is likely to deposit in a dendrite shape on the negative electrode during charging. When lithium metal deposits in a dendrite shape, it causes an internal short circuit and an increase in lithium metal that does not contribute to charging and discharging, and the charge-discharge cycle characteristics (hereinafter may be simply referred to as "cycle characteristics") of the lithium secondary battery deteriorate.

[0014] (Protection Layer) At least a part of the surface of the negative electrode of the lithium secondary battery (B) is covered with a protection layer, and the protection layer includes a heat-resistant resin. It is preferable that 80% or more or 90% or more of the surface of the negative electrode is covered with the protection layer, and it is more preferable that 99% to 100% of the surface of the negative electrode is covered with the protection layer.

[0015] As a result of their investigation, the inventors of the present invention have newly discovered that the cycle characteristics can be significantly improved by covering at least a portion of the surface of the negative electrode with a protective layer containing a heat-resistant resin. This disclosure is based on this new finding.

[0016] Because heat-resistant resins have a rigid molecular structure, they have a significant effect in inhibiting the growth of dendrite-like lithium metal. Furthermore, heat-resistant resins with such molecular structures exhibit excellent adhesion to lithium metal. Therefore, by covering at least a portion of the negative electrode surface with a protective layer containing heat-resistant resin, the deposition of dendrite-like lithium metal is significantly suppressed, internal short circuits are reduced, and the amount of lithium metal isolated from the conductive network and not contributing to charging and discharging is decreased. As a result, cycle characteristics are improved.

[0017] Furthermore, lithium metal is deposited on the negative electrode during charging. Therefore, lithium metal or a lithium alloy is usually present on the surface of the negative electrode, which is covered by the protective layer. In other words, the protective layer is usually in contact with metallic lithium, or lithium carbonate, oxide, or hydroxide.

[0018] Because heat-resistant resins have high heat resistance, the protective layer containing the heat-resistant resin can maintain its covering of the negative electrode surface without shrinking even if the first separator shrinks due to heat. Therefore, it also serves to ensure the safety of the battery when the battery becomes hot enough for the first separator to shrink due to heat.

[0019] The protective layer may be formed on the surface of the negative electrode, on the surface of the first separator, or a sheet independent of the negative electrode and the first separator may be used as the protective layer. After forming the protective layer on a sheet-like substrate, the protective layer may be transferred from the substrate to the surface of the negative electrode or the surface of the first separator.

[0020] To highly suppress the growth of dendrite-like lithium metal, it is preferable that the protective layer be bonded to the surface of the negative electrode. Furthermore, when the battery temperature becomes high enough for the first separator to shrink due to heat, the safety of the battery is enhanced by the adhesion of the protective layer to the surface of the negative electrode.

[0021] (First Separator) The first separator can be a porous sheet having ion permeability and insulating properties. Examples of porous sheets include microporous membranes, woven fabrics, and nonwoven fabrics. The material of the porous sheet is not particularly limited, and polymer materials may be used. Typical examples of polymer materials include olefin resins. Examples of olefin resins include polyethylene, polypropylene, and copolymers of ethylene and propylene. The first separator may optionally contain additives (such as inorganic fillers). The first separator may consist of multiple layers with different forms and / or compositions. As the first separator, a separator used in known lithium secondary batteries may be used as is.

[0022] The first separator has a first main surface located on the negative electrode side and a second main surface located on the positive electrode side. At least a portion of the first main surface may be bonded to the surface of the negative electrode by a protective layer. In other words, the first separator may be bonded to the negative electrode by a protective layer. In this case, the adhesion between the first main surface of the first separator and the surface of the negative electrode is significantly improved, and the dendritic deposition of lithium metal is further significantly suppressed. If the surface of the negative electrode is the surface of lithium metal or a lithium alloy, the adhesion between the first main surface of the first separator and the surface of the negative electrode is further significantly improved.

[0023] The first separator may be used integrated with the protective layer, or it may be used separately from the protective layer. By forming the protective layer on the surface of the first separator, a first separator integrated with the protective layer may be formed.

[0024] When the first separator is bonded to the negative electrode by a protective layer, the first separator integrated with the protective layer may be stacked with the negative electrode via the protective layer, and the laminate of the first separator, protective layer, and negative electrode may be pressurized using a flat hot press, a heated rolling press, or the like.

[0025] The first separator, which is integrated with the protective layer, may be formed, for example, by applying a coating solution containing the components of the protective layer to the first separator and then drying it. The method of applying the coating solution is not limited, and known methods may be used. For example, the coating may be applied using a spray method, a method using rollers or dies (gravure coating method, die coating method, etc.), or a printing method (screen printing method, inkjet method, etc.).

[0026] The solvents used in the coating solution can include acetone, dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, and tripropylene glycol.

[0027] <Heat-resistant resin> A heat-resistant resin refers to a resin with a high heat distortion temperature. Ideally, the heat distortion temperature of a heat-resistant resin should be 260°C or higher. Here, the heat distortion temperature is the deflection temperature determined under a load of 1.82 MPa, in accordance with ASTM-D648.

[0028] Examples of heat-resistant resins that can be used include polyamide, polyamide-imide, polyimide, cellulose, cellulose acetate, polyvinylidene fluoride, polysulfone, polyethersulfone, polyetherimide, nylon 6T, polyetheretherketone, amorphous polyarylate, polyphenylene sulfide, polypyromelite imide, and polycarbonate. These may be used individually or in combination of two or more. Among these, aromatic polyamide, aromatic polyamide-imide, and aromatic polyimide are preferred, and aramid (total aromatic polyamide) is more preferred. Total aromatic polyamide may be meta-aromatic polyamide or para-aromatic polyamide. The heat distortion temperature of the heat-resistant resins exemplified here is 260°C or higher.

[0029] In particular, aramid has a rigid molecular structure consisting of a benzene ring and an amide bond, which further inhibits the growth of dendritic lithium metal and gives it excellent resistance to reducing atmospheres caused by lithium metal. In addition, aramid has high polarity and excellent adhesion to lithium metal.

[0030] Meta-aromatic polyamides are aromatic polyamides with a zigzag molecular skeleton. For example, meta-aromatic polyamides are polymers in which the meta position or a position equivalent to the meta position of the aromatic skeleton in the monomer is linked by amide bonds. Examples of atomic groups having meta or equivalent bond positions include 1,3-phenylene, 3,4'-biphenyl, 1,6-naphthalene, 1,7-naphthalene, and 2,7-naphthalene.

[0031] Meta-aromatic polyamides, for example, have an amino group (-NH) at the meta position or a position equivalent to the meta position. ２ It is obtained by condensation polymerization of an aromatic diamine (meth-aromatic diamine) to which a "-COCl" bond is attached (meth-aromatic dicarboxylic acid dichloride) to which a "-COCl" bond is attached at the meta position or a position equivalent to the meta position (meth-aromatic dicarboxylic acid dichloride). The meta-aromatic polyamide is preferably polymetaphenylene isophthalamide. Polymetaphenylene isophthalamide is a polyamide substantially composed of repeating units in which the meta position or a position equivalent to the meta position of the aromatic skeleton in the monomer is attached by an amide bond.

[0032] Examples of meta-aromatic polyamides include polymetaphenylene isophthalamide, poly(methabenzamide), poly(3,4'-benzanilide isophthalamide), poly(metaphenylene-3,4'-biphenylenedicarboxylic acid amide), and poly(metaphenylene-2,7-naphthalenedicarboxylic acid amide).

[0033] Examples of meta-aromatic diamines include 1,3-phenylenediamine, 1,6-naphthalenediamine, 1,7-naphthalenediamine, 2,7-naphthalenediamine, and 3,4'-biphenyldiamine.

[0034] Examples of meta-aromatic dicarboxylic acid dichlorides include isophthalic acid dichloride, 1,6-naphthalenedicarboxylic acid dichloride, 1,7-naphthalenedicarboxylic acid dichloride, and 3,4'-biphenyldicarboxylic acid dichloride.

[0035] Para-aromatic polyamides are polymers in which the para or equivalent positions of the aromatic skeleton in the monomer are bonded by amide bonds. Examples of atomic groups having para or equivalent bond positions include 4,4'-biphenylene, 1,5-naphthalene, and 2,6-naphthalene.

[0036] Para-aromatic polyamides are obtained, for example, by condensation polymerization of an aromatic diamine (para-aromatic diamine) in which an amino group (-NH2) is bonded at the para position or a position equivalent to the para position, and an aromatic dicarboxylic acid dichloride (para-aromatic dicarboxylic acid dichloride) in which "-COCl" is bonded at the para position or a position equivalent to the para position. Polypara-aromatic polyamides are preferably polypara-phenylene terephthalamides. Polypara-phenylene terephthalamide is a polyamide substantially composed of repeating units in which the para position or a position equivalent to the para position of the aromatic skeleton in the monomer is bonded by amide bonds.

[0037] Examples of para-aromatic polyamides include poly(paraphenylene terephthalamide), poly(parabenzamide), poly(4,4'-benzanilide terephthalamide), poly(paraphenylene-4,4'-biphenylenedicarboxylic acid amide), poly(paraphenylene-2,6-naphthalenedicarboxylic acid amide), and poly(2-chloro-paraphenylene terephthalamide). Furthermore, examples of aromatic polyamides also include copolymers of paraphenylenediamine and 2,6-dichloroparaphenylenediamine with terephthalic acid dichloride.

[0038] Examples of para-aromatic diamines include 1,4-phenylenediamine, 1,5-naphthalenediamine, 2,6-naphthalenediamine, and 4,4'-biphenyldiamine.

[0039] Examples of meta-aromatic dicarboxylic acid dichlorides include terephthalic acid dichloride, 1,5-naphthalenedicarboxylic acid dichloride, 2,6-naphthalenedicarboxylic acid dichloride, and 4,4'-biphenyldicarboxylic acid dichloride.

[0040] Examples of aromatic polyimides include polymers formed by condensation polymerization of monomers having an aromatic skeleton and containing imide bonds in the repeating units. Aromatic polyimides can be obtained, for example, by condensation polymerization of aromatic compounds that are tetracarboxylic dianhydrides and aromatic diamines. Examples of aromatic compounds that are tetracarboxylic dianhydrides include pyromellitic dianhydride, 3,3',4,4'-diphenylsulfone tetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 2,2'-bis(3,4-dicarboxyphenyl)hexafluoropropane, and 3,3',4,4'-biphenyl tetracarboxylic dianhydride. Examples of aromatic diamines include 4,4'-diaminodiphenyl ether, paraphenylenediamine, 3,3'-methylenedianiline, 3,3'-diaminobenzophenone, and 3,3'-diaminobenzosulfone.

[0041] Examples of aromatic polyamide-imides include polymers formed by condensation polymerization of monomers having an aromatic skeleton, and which contain amide and imide bonds. Aromatic polyamide-imides can be obtained, for example, by condensation polymerization of an aromatic compound (A), which is a tricarboxylic acid anhydride, with an aromatic diisocyanate. Furthermore, aromatic polyamide-imides can also be obtained by condensation polymerization of an aromatic compound (B), in which a carboxylic acid anhydride group (two carboxyl groups formed by dehydration condensation) and "-COCl" are bonded to an aromatic ring, with an aromatic diamine.

[0042] Examples of aromatic compounds (A) include trimellitic anhydride. Examples of aromatic diisocyanates include 4,4'-diphenylmethane diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, and m-xylene diisocyanate. Examples of aromatic compounds (B) include trimellitic anhydride chloride. Examples of aromatic diamines include the aromatic diamines mentioned above.

[0043] <Filler> The protective layer may contain a filler. The filler helps maintain high mechanical strength of the protective film. Therefore, even if the electrodes expand and the internal pressure of the electrode group increases when the charge-discharge cycle is repeated, the protective film will not be crushed.

[0044] The filler content in the protective layer may be, for example, 10% by mass or more, or 20% by mass or more, or 99% by mass or less, or 90% by mass or less.

[0045] The filler may be inorganic or organic particles. Inorganic particles are preferred. Examples of inorganic particles include oxides, oxide hydrates, hydroxides, nitrides, carbides, and sulfides, and these may contain metallic elements.

[0046] Examples of oxides and oxide hydrates include aluminum oxide (alumina) and boehmite (Al ２ O ３ ・H ２ Examples of nitrides include 0 or AlOOH, magnesium oxide, titanium oxide, zirconium oxide, silicon oxide, yttrium oxide, zinc oxide, etc. Examples of nitrides include silicon nitride, aluminum nitride, boron nitride, titanium nitride, etc. Examples of carbides include silicon carbide, boron carbide, etc. Examples of sulfides include barium sulfate, etc. Examples of hydroxides include aluminum hydroxide, magnesium hydroxide, etc. In addition, inorganic particles include porous aluminosilicates such as zeolites, layered silicates such as talc, and barium titanate (BaTiO). ３ ), strontium titanate (SrTiO ３ ) etc. are also acceptable.

[0047] The average primary particle size of inorganic particles may be 0.1 μm or more, or 1.0 μm or more, or 5.0 μm or less, or 3.0 μm or less. The average primary particle size is the median diameter (D) at which the cumulative volume in the volume-based particle size distribution becomes 50%. ５０ ) is the median diameter (D ５０ This can be measured using a laser diffraction / scattering particle size distribution analyzer.

[0048] <Porous Structure> From the viewpoint of ensuring good lithium-ion conductivity of the protective layer, it is preferable that the protective layer be porous. By using a porous protective layer, the contact area between the protective layer and the non-aqueous electrolyte increases, the retention of the non-aqueous electrolyte in the protective layer improves, and lithium-ion conductivity improves. As a result, the resistance of the battery decreases, and good cycle characteristics can be obtained. In particular, it is preferable that the protective layer be a porous structure obtained by the para-aromatic polyamide described above forming fibril-like bodies and linking multiple fibril-like bodies together.

[0049] A porous protective layer can be formed, for example, by the NIPS method (non-solvent-induced phase separation method), but the method for producing a porous protective layer is not particularly limited. In the NIPS method, for example, a coating solution is prepared by dissolving a resin in the solvent (good solvent) of the coating solution described above, the coating solution is applied to a first separator to form a coating film, and then, before the coating film dries, the coating film is immersed in a poor solvent to make the coating film porous and form a porous protective layer.

[0050] Poor solvents that can be used include water, methanol, ethanol, isopropyl alcohol, or mixtures thereof. Furthermore, by mixing the above poor solvents with acetone, dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, tripropylene glycol, etc., the porosity of the protective layer (coating film) can be altered.

[0051] The porosity of the protective layer is preferably, for example, 5% to 50%. By setting the porosity of the protective layer to 5% or more, it is possible to lower the internal resistance of the battery while ensuring a high effect in suppressing the dendritic deposition of lithium metal. Furthermore, by setting the porosity of the protective layer to 40% or less, a protective film with sufficient mechanical strength can be formed. The porosity of the protective layer is more preferably, for example, 10% or more, and more preferably 50% or less.

[0052] The porosity of the protective layer can be measured by the following method. First, expose the cross-section of the protective layer and photograph an arbitrary cross-section with a scanning electron microscope (SEM) to obtain a cross-sectional image. Next, perform binarization processing on each of the obtained cross-sectional images so that they are divided into a void region and a non-void region. The area of the region to be binarized is, for example, 100 μm ２ or more. Next, measure the ratio of the area of the void region to the area of the binarized region as the porosity. Perform the same measurement five times and obtain the porosity of the protective layer by calculating the arithmetic mean of the obtained porosities.

[0053] <Lithium Salt> From the viewpoint of ensuring good lithium ion conductivity of the protective layer, the protective layer may contain a lithium salt. In particular, when the protective layer is not porous, it is preferable that the protective layer contains a lithium salt. The type of lithium salt to be included in the protective layer is not particularly limited and may be the same as the lithium salt dissolved in the non-aqueous electrolyte or a lithium salt different from the lithium salt dissolved in the non-aqueous electrolyte.

[0054] The content rate of the lithium salt in the protective layer is, for example, 1 mass% to 15 mass%, and may also be 2 mass% to 10 mass%. The lithium salt can be included in the protective layer, for example, by dissolving it in a coating liquid that is a raw material of the protective layer containing the components of the protective layer.

[0055] <Thickness of the Protective Layer> The thickness of the protective layer may be, for example, in the range of 0.2 μm to 10 μm (for example, in the range of 1 μm to 8 μm, 2 μm to 4 μm, or 4 μm to 10 μm). The thickness of the protective layer may be 0.2 μm or more, 0.3 μm or more, 0.5 μm or more, 1 μm or more, 2 μm or more, 3 μm or more, or 4 μm or more. The thickness of the protective layer may be 10 μm or less, 8 μm or less, 7 μm or less, or 5 μm or less. When the thickness of the protective layer is 0.2 μm or more, it is advantageous in terms of the effect of suppressing the dendritic deposition of lithium metal and the effect of enhancing the safety of the battery in case of abnormality. When the thickness of the protective layer is 10 μm or less, it is advantageous in terms of being able to lower the internal resistance of the battery.

[0056] (Second Separator) The lithium secondary battery (B) may further comprise a second separator positioned between the positive electrode and the first separator. In this case, the first separator and the second separator are stacked and positioned between the positive electrode and the negative electrode. The first separator and the second separator usually have the same shape. However, as long as they are positioned between the positive electrode and the negative electrode, the shape of the first separator may differ from the shape of the second separator. The first separator and the second separator may or may not be bonded together. By not bonding the first separator and the second separator, for example, stress generated inside the electrode group due to the expansion of the negative electrode can be alleviated, and damage to the components constituting the electrode group can be suppressed.

[0057] The thickness of the second separator may be greater than the thickness of the first separator. In other words, the first separator may be made relatively thinner to serve to reinforce the protective layer. In this case, it is preferable to adhere at least a portion of the first main surface of the first separator to the surface of the negative electrode with the protective layer. As a result, the first separator, the protective layer, and the negative electrode are integrated, which greatly increases the mechanical strength of the protective layer and enhances the effect of suppressing the growth of dendrite-like lithium metal.

[0058] On the other hand, the second separator, which is thicker than the first separator, relieves the stress generated inside the electrode group during charging and discharging, and also functions as a reservoir that holds a sufficient amount of non-aqueous electrolyte. In lithium secondary batteries, lithium metal is deposited on the negative electrode during charging, which tends to increase the amount of expansion of the negative electrode. If the amount of expansion of the negative electrode (especially the expansion in the thickness direction of the negative electrode) is large, it becomes easier for the circulation of the non-aqueous electrolyte to decrease and for the negative electrode current collector to break due to increased stress, thus degrading the cycle characteristics of the lithium secondary battery. In contrast, by using a thick second separator, it becomes easier to avoid the decrease in the circulation of the non-aqueous electrolyte and the breakage of the negative electrode current collector.

[0059] The second separator can be made of a material such as the porous sheet described as the first separator. The second separator has a third main surface located on the first separator side and a fourth main surface located on the positive electrode side. At least one of the third and fourth main surfaces of the second separator may have any of the aforementioned protective layers integrated into it. For example, any of the aforementioned protective layers may be integrated into the fourth main surface located on the positive electrode side of the second separator.

[0060] It is preferable that the porosity V2 of the second separator is greater than the porosity V1 of the first separator. By making the porosity V2 of the second separator greater than the porosity V1 of the first separator, the expansion of the negative electrode is more easily absorbed by the second separator.

[0061] The ratio of porosity V2 (%) to porosity V1 (%), V2 / V1, may be 1.1 or greater, or 1.2 or greater, or 2.0 or less, or 1.7 or less. The porosity V2 may be 60% or greater, or 70% or greater, or 90% or less, or 80% or less. Setting the porosity V2 to 60% or greater makes it easier for the second separator to absorb the expansion of the negative electrode. Setting the porosity V2 to 90% or less helps maintain the strength of the second separator. Furthermore, reducing the porosity of the first separator adjacent to the negative electrode makes it easier to suppress the dendritic deposition of lithium metal on the negative electrode.

[0062] The porosity of each separator can be measured by the following method. First, 4 cm square samples are taken at four locations in the center of the separator's width. Next, the mass and thickness of each separator are measured. Then, the apparent density of the separator is calculated from the mass, area, and thickness of the separator. The porosity of the separator is determined by the following formula. The true density of the separator is determined based on the material of the separator.

[0063] Porosity (%) = 100 × {1 - (Apparent density of the substrate layer) / (True density of the substrate layer)}

[0064] <Thickness of the first and second separators> Including the case in which the lithium secondary battery (B) does not have a second separator, the total thickness of the first and second separators may be, for example, 30 μm to 60 μm or 40 μm to 55 μm. Therefore, in the case in which the lithium secondary battery (B) does not have a second separator, the thickness of the first separator alone may be 30 μm to 60 μm or 40 μm to 55 μm.

[0065] When the lithium secondary battery (B) comprises a first separator and a second separator, the ratio of the thickness T2 of the second separator to the thickness T1 of the first separator (T2 / T1) may be, for example, 1.1 or more, 1.3 or more, or 1.5 or more, or 3.0 or less, or 2.0 or less. By setting the ratio Ts2 / Ts1 to 1.5 or more, the expansion of the negative electrode is more easily absorbed by the second separator. In addition, the mechanical strength of the protective layer is increased, and a lithium secondary battery (B) with excellent circulation of the non-aqueous electrolyte can be obtained.

[0066] The thickness of each separator is determined by taking the arithmetic mean of the thicknesses at 15 points. The thickness of the separator can be measured according to the method compliant with JIS (Japanese Industrial Standards) K6250. Specifically, the thickness can be measured using a test piece thickness gauge (SDA-12 type) manufactured by Polymer Instruments Co., Ltd., which complies with JIS K6250. The measurement is performed using a measuring probe with a diameter of 5 mm and a pressure of 22 kPa. The thickness is measured at 15 intersection points of five lines that divide the separator into six equal parts in the longitudinal direction and three lines that divide it into four equal parts in the width direction. The arithmetic mean of the 15 measured thicknesses is then taken as thickness T1 or T2.

[0067] (Manufacturing Method for Lithium Secondary Battery (B)) The manufacturing method is not limited as long as it can produce lithium secondary battery (B). The matters described for lithium secondary battery (B) can be applied to the following manufacturing methods, so redundant explanations will be omitted.

[0068] The first method includes the step of forming a laminate of a first separator, a protective layer, and a negative electrode by bonding the first separator to both sides of the negative electrode (or negative electrode current collector) via a protective layer. The bonding of the first separator, protective layer, and negative electrode may be performed by stacking them and hot pressing. The heating temperature during hot pressing should be selected appropriately according to the material of the protective layer. For example, if the protective layer contains aromatic polyamide, the heating temperature during hot pressing may be in the range of 60 to 120°C. Subsequently, an electrode group is formed using the negative electrode integrated with the first separator via the protective layer.

[0069] The second method includes the step of forming a protective layer on a substrate sheet instead of the first separator, and then transferring the protective layer to the surface of the negative electrode or the surface of the first separator. If the protective layer is transferred to the surface of the negative electrode, the electrode group is then constructed using a separately prepared first separator. If the protective layer is transferred to the surface of the first separator, the electrode group is constructed using the first separator integrated with the resulting protective layer.

[0070] The third method is a method in which, in addition to the first and second methods, a second separator is stacked on top of the first separator to form an electrode group.

[0071] (Examples of Components) Examples of other components of the lithium secondary battery (B) are described below in detail. Note that the components described below are illustrative, 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.

[0072] (Negative electrode) The negative electrode includes a negative electrode current collector. In lithium secondary batteries, lithium metal is deposited on the negative electrode during charging. The deposited lithium metal dissolves as lithium ions in the non-aqueous electrolyte during discharge.

[0073] The negative electrode current collector may consist only of a base sheet in which lithium is not the main component (content: 50% by mass or more). Alternatively, the negative electrode may include a base sheet and a lithium-containing metal layer laminated on both sides of the base sheet. A conductive sheet such as metal foil can be used for the base sheet. Examples of metal foil materials include copper, copper alloys, and stainless steel. Copper and copper alloys are preferred because they have high conductivity. Stainless steel is preferred because it is difficult to cut.

[0074] The lithium-containing metal layer laminated on the base sheet is either a lithium metal layer or a lithium alloy layer. The lithium alloy layer contains trace amounts (less than 10 atomic percent) of elements other than lithium. Examples of elements other than lithium in lithium alloys include aluminum, magnesium, indium, and zinc. Forming a lithium-containing metal layer suppresses the decrease in discharge capacity associated with repeated charging and discharging. Furthermore, forming a lithium-containing metal layer suppresses the dendritic deposition of lithium metal. The method for forming the lithium-containing metal layer is not particularly limited and may be formed by known methods. For example, a lithium-containing metal layer may be formed by pressing a lithium metal foil or lithium alloy foil onto the base sheet. The lithium-containing metal layer is distinguishable from the lithium metal deposited during charging (which is generally porous) by its dense structure.

[0075] The surface of the negative electrode current collector made of metal may be smooth. Using a negative electrode current collector with a smooth surface makes it easier for lithium metal to deposit evenly on the negative electrode current collector during charging. A smooth surface of a negative electrode current collector made of metal means that the maximum height roughness Rz of the negative electrode current collector is 20 μm or less. The maximum height roughness Rz of a negative electrode current collector made of metal may be 10 μm or less. The maximum height roughness Rz is measured in accordance with JIS (Japanese Industrial Standards) B 0601:2013.

[0076] The thickness of the base sheet may be 3 μm or more, or 5 μm or more, or 20 μm or less, or 15 μm or less. If 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.

[0077] The negative electrode current collector (base 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 at the negative electrode. As a result, rupture of the negative electrode current collector during charging and discharging and the resulting deterioration of cycle characteristics are suppressed.

[0078] The term "austenitic stainless steel" above refers to stainless steel with an austenitic content of 50% or more. Austenitic content refers to the proportion (mass ratio) of the austenite phase in stainless steel. When the content of the austenite phase, ferrite phase, and martensite phase in stainless steel is O, F, and M, respectively, the austenitic content is calculated as {O / (O+F+M)} × 100. The austenitic structure is a face-centered cubic lattice structure (FCC structure), while the ferrite and martensite structures are body-centered cubic lattice structures (BCC structure).

[0079] The austenite content may be 70% or more, 90% or more, or 100%.

[0080] The austenitity can be determined by the following method. First, a sample of the negative electrode current collector (stainless steel foil) (for example, size: 25 mm square) 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 part) is, for example, 15 mm square.

[0081] The following are the preferred XRD measurement conditions. <Analytical Instrument> 2D Micro-X-ray Diffractometer (Rigaku Corporation, RINT-RAPID II) <Analytical Conditions> Tube: Co Monochromatorization: Monochromator used (CoKα) Tube Output: 40kV-30mA Detector: Imaging plate (2D) (Reflection method) Collimator: Φ300μm ω angle: 25°~35° (2° / sec) Φ angle: 360° rotation (1° / sec) Measurement time (exposure): 30 minutes

[0082] The diffraction peaks observed in the obtained XRD pattern are fitted using the least squares method with a standard database, and then quantitative analysis is performed 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 attached to the analytical instrument. Through this analysis, the ratio (mass ratio) of the austenite phase to the sum of the austenite, ferrite, and martensite phases is determined as the austenite fraction. Several measurement points are arbitrarily selected from the above sample, the austenite fraction in each measurement point is determined, and their average value is calculated.

[0083] Austenitic stainless steel may contain components other than Fe, such as C, Si, Mn, P, S, Ni, Cr, Mn, Mo, Cu, N, etc. This stainless steel may be low-carbon, ultra-low-carbon, or nitrogen-added stainless steel, and may also be a duplex stainless steel containing austenite.

[0084] 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.

[0085] 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., the surface resin layer). As described above, the negative electrode current collector may include a lithium-containing metal layer laminated on the base sheet.

[0086] Resin films are lightweight and easily increase the energy density of secondary batteries. Resin films are less likely to break during roll transport and are easy to handle. Resin films are less prone to embrittlement even at low negative electrode potentials. Resin films are excellent as current collector materials because they have high resistance to stress during electrode expansion and contraction and are less likely to break. In particular, lithium secondary batteries tend to experience significant expansion of the negative electrode due to the deposition of lithium metal during charging. The amount of expansion is even greater when lithium metal is deposited in a dendrite-like manner. This makes it easy for stress to be generated in the negative electrode.

[0087] The main surface of the resin film may be smooth, roughened, or subjected to plasma treatment or corona treatment. If the main surface of the resin film is smooth, the maximum height roughness Rz of the main surface is 2.5 μm or less. If the main surface of the resin film is roughened, the maximum height roughness Rz of the main surface may exceed 2.5 μm and 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 of the resin film other than the edge face, and is the two surface with the largest area. In this specification, "surface" usually means "main surface".

[0088] From the viewpoint of increasing the energy density of secondary batteries, it is desirable for the resin film to be as thin as possible while ensuring mechanical strength. An example of a preferred range for the thickness of the resin film is 1.5 μm to 30 μm or less. 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 taking the arithmetic mean of these measurements.

[0089] The surface resin layer of the resin film may contain a nitrogen-containing resin. 90% or more 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 3% by mass or more. The nitrogen atom content in the surface resin layer may be 2.5% by mass or more.

[0090] Nitrogen-containing resins may contain nitrogen-hydrogen bonds (bonds between nitrogen atoms and hydrogen atoms). The infrared absorption peak characteristic of nitrogen-hydrogen bonds is 1655 cm⁻¹. －１ (1640-1670cm －１ ) Nearby, 1530 cm －１ (1515-1545cm －１ They exist in the vicinity of the ) peaks. Based on these peaks, the presence or absence of nitrogen-hydrogen bonds can be determined. Specifically, first, the active material of the negative electrode is scraped off or wet-removed, then the transition metal layer is dissolved with an aqueous nitric acid solution to expose the surface resin layer. Next, 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 the chemical shift of X-ray electron spectroscopy (XPS).

[0091] (FTIR-ATR method) Measurement device: Varian 670FTIR (manufactured by Varian) Measurement mode: Attenuated total internal reflection Light source: Special ceramics Detector: DLaTGS (Deuterated L-alanine-doped triglycine sulfate) Resolution: 4 cm －１ Total number of shots: 256 IRE: Ge Incident angle: 60 degrees Attachment: 1-reflection ATR attachment (Seagull)

[0092] The nitrogen-containing resin may be a polymer having at least one selected from the group consisting of urea bonds, melamine structures, triazine rings, amino groups, amide bonds, aramid bonds, imide bonds, urethane bonds, carbodiimide bonds, uretdione 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 because they are highly flexible, highly resistant to stress when electrodes expand and contract, and less prone to breakage.

[0093] Polyurethane resins can be synthesized by reacting a polyol with a bifunctional or more polyisocyanate (especially a diisocyanate). By arbitrarily selecting the polyol and polyisocyanate, polyurethane resins with a variety of physical properties can be synthesized.

[0094] The nitrogen-containing resin may be a polymer having at least one selected from the group consisting of aliphatic isocyanate groups, aromatic isocyanate groups, allophanate groups, and biuret groups. In particular, thermosetting polyurethane resins use polyisocyanate as a raw material, so there is a high probability that they will have residual unreacted isocyanate groups. It is thought that the isocyanate groups are reduced at the negative electrode to produce coating components that form a stable film on the negative electrode. Furthermore, because thermosetting resins form a strong three-dimensional film, they have a great inhibitory effect on copper damage. UV-curable resins have a similar effect. The isocyanate groups are present at 2250 cm⁻¹. －１ (2270-2240cm －１ ) Since it has an infrared absorption peak due to antisymmetric stretching vibration in its vicinity, its presence or absence can be confirmed based on the infrared absorption peak.

[0095] The surface resin layer may contain fillers. By adding fillers (such as silica or alumina) to the surface resin layer, irregularities are formed on the surface resin layer. As a result, an anchoring effect occurs, making it possible to improve the adhesion between the surface resin layer and the base resin layer. The filler used can be one that can roughen the surface of the surface resin layer and does not easily cause a decrease in battery performance due to side reactions with non-aqueous electrolytes. The filler can be made of particles such as resin, metal oxide, ceramic, or metal.

[0096] 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.

[0097] The thickness of the surface resin layer is, for example, 5 μm or less, and in a preferred example, it is 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 on the cross-section of a predetermined member (negative electrode or negative electrode current collector) using a scanning electron microscope (SEM) and taking the arithmetic mean of these measurements.

[0098] Most resin films are not conductive. The transition metal layer plays a role in imparting good conductivity to the negative electrode current collector. The transition metal layer can be any layer that contains a transition metal and has electronic conductivity. Preferably, the transition metal layer contains a transition metal in a metallic state that has electronic conductivity due to free electrons.

[0099] The transition metal layer preferably contains copper, nickel, chromium, titanium, iron, silver, gold, tin, etc., as this facilitates ensuring corrosion resistance and conductivity. It is desirable that the transition metal layer contains at least one selected from the group consisting of copper, copper alloys, stainless steel, nickel, and nickel alloys. It is particularly desirable that the transition metal layer contains copper or copper alloys, which have excellent conductivity.

[0100] Transition metals have the effect of embrittlement of resin films. In particular, when the transition metal layer contains copper, embrittlement of the resin film is likely to occur. As a new finding that has not yet been reported, it has also been found that lithium metal significantly accelerates the embrittlement of the resin film by transition metals (e.g., copper damage). In other words, in lithium secondary batteries in which lithium metal is deposited at the negative electrode, degradation of the resin film by the transition metal layer can occur significantly. In contrast, by including nitrogen-containing resin in at least the surface resin layer of the resin film, embrittlement of the resin film can be significantly suppressed.

[0101] Furthermore, a nickel-chromium alloy layer can be considered as a metal layer to shield against the migration of copper ions to the resin film. However, in lithium secondary batteries, no shielding effect is obtained from the nickel-chromium alloy layer. This is presumed to be because when easily ionized metallic lithium is in contact with a transition metal such as copper, the ionization of the transition metal is accelerated, and the rupture of the polymer is accelerated by the transition metal ions. On the other hand, a surface resin layer containing nitrogen-containing resin shows a remarkable effect in suppressing polymer rupture.

[0102] The most significant factor in the embrittlement of resin films is the rupture of carbon-carbon bonds. When a resin film contains nitrogen-containing resin, the nitrogen-containing resin contains bonds between carbon atoms and nitrogen atoms. These carbon-carbon-nitrogen bonds are considered less susceptible to the influence of transition metals or their ions. It is possible that the transition metals are stabilized by the nitrogen atoms, suppressing the rupture of carbon-carbon bonds.

[0103] The transition metal layer may be formed by depositing it onto the surface of the surface resin layer using a liquid-phase or gas-phase method. For the liquid-phase method, electrodeposition methods such as electrolytic or electroless plating can be used. For the gas-phase method, vapor deposition, sputtering, or atomic layer deposition (ALD) can be used. A base layer may be formed by sputtering, and then the transition metal layer may be thickened on top of it using electrolytic plating. In other words, it is possible to use a combination of multiple methods. The transition metal layer may also be formed by lamination. However, the method of forming the transition metal layer is not particularly limited.

[0104] The thickness of the transition metal layer is, for example, 5 μm or less, and may be 3 μm or less. 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 performed as follows: First, a scanning electron microscope (SEM) is used to measure the thickness of 10 arbitrary points in the cross-section of a predetermined member (negative electrode or negative electrode current collector) of the transition metal layer. Next, the thickness of the transition metal layer (average thickness) is determined by taking the arithmetic mean of the obtained measurements. The transition metal layer may be composed of multiple layers made of different metals.

[0105] The base resin layer is the main part of the negative electrode current collector and is usually thicker than the surface resin layer and thicker than the transition metal layer. At least 51% by mass of the base resin layer is composed of resin or organic material. To improve adhesion with the transition metal layer and lithium layer, the base resin layer may contain inorganic materials such as inorganic particles. The base resin layer may be a stretched film, a non-porous film (a film without holes), or a film having multiple regularly arranged holes. The base resin layer may be insulating, conductive, or non-conductive. The form and physical properties of the base resin layer are not particularly limited.

[0106] The base resin layer is formed, for example, by molding raw materials (general-purpose plastics, general-purpose engineering plastics, etc.) into a sheet. Examples of raw materials include polyester resin, olefin resin, polyphenylene sulfide resin, acrylic resin, polycarbonate resin, polyetheretherketone 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 base resin layer may be used individually or in combination of two or more types.

[0107] The resin contained in the base resin layer is preferably a resin having an aromatic ring, a resin that does not contain fluorine atoms, or an olefin resin. When the resin has an aromatic ring (e.g., a benzene ring) in its molecule, the affinity between the base resin layer and the lithium metal layer increases, improving the adhesive strength between the two.

[0108] As polyester resins, aromatic polyesters are preferred, such as unoriented polyethylene terephthalate, biaxially oriented polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate. As acrylic resins, polymethyl methacrylate can be used. As polyimide resins, aromatic polyimides are preferred. As polyamide resins, aromatic polyamides (aramid resins) are preferred. As olefin resins, unoriented 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, and ionomers are preferred.

[0109] The extrusion method for the base 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, copolymer, or terpolymer. The arrangement of the constituent units of the resin is not limited and may be a random copolymer or a block copolymer. Two or more types of base resin layers may be combined. For example, two or more base 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 methods such as rapid cooling. The base resin layer may be an alloy resin composed of multiple resins as described above.

[0110] To ensure adhesion with other layers (e.g., vapor-deposited films), the surface of the base resin layer may be subjected to corona treatment or plasma treatment. To improve adhesion with the surface resin layer by creating irregularities on the base resin layer, fillers (fillers made of ceramics, resins, metals, etc.) may be added to the base resin layer to form irregularities on the surface of the base resin layer.

[0111] (Positive Electrode) The positive electrode comprises a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector. The positive electrode mixture layer contains 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 known methods. For example, first, a positive electrode mixture slurry containing the positive electrode active material and additives is prepared. Next, a coating film is formed by applying the positive electrode mixture slurry to the positive electrode current collector and then drying it. Next, the positive electrode is obtained by rolling the laminate consisting of the positive electrode current collector and the coating film. The formed positive electrode can be cut to a predetermined size as needed.

[0112] The thickness of the positive electrode mixture layer may be 50 μm or more, or 100 μm or more, or 300 μm or less, or 250 μm or less.

[0113] The positive electrode active material can be a substance capable of reversibly intercalating 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 because they have low manufacturing costs and a high average discharge voltage.

[0114] Examples of transition metal elements included 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 may be at least one selected from the group consisting of Co, Ni, and Mn. Lithium-containing transition metal oxides may also contain one or more main group elements. Examples of main group elements include Mg, Al, Ca, Zn, Ga, Ge, Sn, Sb, Pb, Bi, and B.

[0115] Conductive materials can include carbon materials. Examples of carbon materials include carbon black (acetylene black, Ketjenblack, etc.), carbon nanotubes, and graphite.

[0116] Examples of binders include fluororesins, polyacrylonitrile, polyimide resins, acrylic resins, polyolefin resins, and rubbery polymers. Examples of fluororesins include polytetrafluoroethylene and polyvinylidene fluoride.

[0117] Cellulose derivatives can be used as thickeners. Examples of cellulose derivatives include carboxymethylcellulose (CMC) and its modified forms, and methylcellulose. Examples of modified forms of CMC include salts of CMC. Examples of salts include alkali metal salts (e.g., sodium salts) and ammonium salts.

[0118] A conductive sheet can be used as the positive electrode current collector. Examples of conductive sheets include metal foil. The surface of the positive electrode current collector may also be coated with a carbon material.

[0119] Examples of materials for the positive electrode current collector (conductive sheet) include metallic materials containing Al, Ti, Fe, etc. The metallic material may be Al, Al alloy, Ti, Ti alloy, 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.

[0120] (Separator) The separator described above can be used as the separator.

[0121] The lithium secondary battery (B) may or may not include a spacer positioned between the positive electrode and the second separator. The spacer may be formed on the positive electrode side main surface of the second separator or on the positive electrode. Typically, the lithium secondary battery (B) does not include a spacer (not a separator) positioned between the positive electrode and the negative electrode.

[0122] The spacer may include linear and / or dot-shaped portions. The linear portions may be arranged in a mesh-like (e.g., honeycomb) pattern. Alternatively, the spacer may include multiple linear portions arranged in a stripe pattern.

[0123] (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 in liquid or gel form. A liquid non-aqueous electrolyte (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.

[0124] The gel-like non-aqueous electrolyte may contain a lithium salt and a matrix polymer, or it may contain a lithium salt, a non-aqueous solvent, and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs the non-aqueous solvent and gels is used. Examples of polymer materials include fluororesins, acrylic resins, and polyether resins.

[0125] 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 solely of ether. By using a non-aqueous solvent containing ether, the deposition of lithium metal in a dendrite-like manner at the negative electrode during charging can be suppressed, and as a result, the cycle characteristics can be further improved.

[0126] The ether may be a linear ether, a fluorinated linear ether, a cyclic ether, a fluorinated cyclic ether, etc. That is, the ether may be a fluoroether containing a fluoro group (-F), or a hydrofluoroether. A hydrofluoroether contains a carbon atom to which hydrogen and fluorine atoms 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. The hydrofluoroether 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 only of hydrofluoroether.

[0127] The fluorination rate of the hydrofluoroether is preferably 60% or higher, and more preferably 65% ​​or higher. The fluorination rate may also be 95% or lower, 90% or lower, or 80% or lower. The fluorination rate of a single hydrofluoroether is defined by the following formula: Fluorination rate (%) = 100 × (Number of fluorine atoms in the hydrofluoroether) / (Total number of fluorine and hydrogen atoms in the hydrofluoroether)

[0128] The hydrofluoroether used as 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.

[0129] Other known solvents besides those listed above may be used as non-aqueous solvents. Such non-aqueous solvents may include cyclic carbonate esters, linear carbonate esters, cyclic carboxylic acid esters, and linear carboxylic acid esters. Examples of cyclic carbonate esters include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC). Examples of linear carbonate esters 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 linear 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 linear ethers include 1,2-dimethoxyethane, diethyl ether, ethyl vinyl ether, methylphenyl ether, benzyl ethyl ether, diphenyl ether, dibenzyl ether, 1,2-diethoxyethane, and diethylene glycol dimethyl ether. Non-aqueous solvents may be used individually or in combination of two or more.

[0130] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO2). 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 salts of fluorine-containing acidimides (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.), oxalate complex-containing lithium salts (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 These include (etc.). Lithium salts may be used individually or in combination of two or more types.

[0131] The concentration of lithium salt in the non-aqueous electrolyte may be 0.5 mol / L or higher, 1.0 mol / L or higher, or 1.5 mol / L or higher, or 3.5 mol / L or lower, 2.0 mol / L or lower, or 1.5 mol / L or lower. By setting the lithium salt concentration within the above range, a non-aqueous electrolyte with excellent ionic conductivity and appropriate viscosity can be obtained.

[0132] Non-aqueous electrolytes may contain additives (e.g., known additives). Examples of additives include 1,3-propanesalton, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, and fluorobenzene.

[0133] (Outer casing) The outer casing houses the non-aqueous electrolyte and the electrode group. The outer casing is not particularly limited, and known outer casings can be used. The outer casing may include a bottomed cylindrical battery case and a sealing body and gasket that seal the opening of the battery case.

[0134] In the following, an example of the lithium secondary battery (B) of this embodiment will be specifically described with reference to the drawings. The components of the lithium secondary battery example described below can be the components described above. Furthermore, the components of the example described below can be modified based on the above description. In addition, the matters described below may be applied to the above embodiment. Furthermore, in the lithium secondary battery described below, components that are not essential to the lithium secondary battery (B) according to this disclosure may be omitted.

[0135] (Embodiment 1) Figure 1 is a schematic longitudinal cross-sectional view showing an example of a lithium secondary battery according to Embodiment 1. The cylindrical lithium secondary battery 10 shown in Figure 1 includes a cylindrical battery case and an electrode group 14 and a non-aqueous electrolyte (not shown) housed within the battery case. The electrode group 14 includes a positive electrode 11, a negative electrode 12, and a separator 13. The electrode group 14 is a wound-type electrode group formed by winding the positive electrode 11, the negative electrode 12, and the separator 13. The separator 13 is positioned between the positive electrode 11 and the negative electrode 12.

[0136] The battery case includes a case body 15, which is a bottomed cylindrical metal container, and a sealing body 16 that seals the opening of the case body 15. A gasket 27 is placed between the case body 15 and the sealing body 16. The gasket 27 ensures that the battery case is airtight. Inside the case body 15, insulating plates 17 and 18 are placed at both ends of the electrode group 14 in the direction of the winding axis, respectively. The case body 15 has a stepped portion 21.

[0137] 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. The insulating member 24 is positioned 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 components of the sealing body 16, except for the insulating member 24, are electrically connected.

[0138] The lower valve body 23 has a ventilation hole. Therefore, if the internal pressure of the battery case rises due to abnormal heat generation or the like, the upper valve body 25 bulges towards the cap 26 and separates from the lower valve body 23. This disconnects the electrical connection between the lower valve body 23 and the upper valve body 25. If the internal pressure rises further, the upper valve body 25 ruptures, and gas is released through the opening formed in the cap 26.

[0139] The positive electrode 11 is electrically connected to the cap 26, which functions as a positive terminal, via the positive lead 19. The negative electrode 12 is electrically connected to the case body 15, which functions as a negative terminal, via the negative lead 20.

[0140] A schematic cross-sectional structure of an example of an electrode group is shown in Figure 2. The separator 13 consists only of a first separator 131. A protective layer 132 is provided on the first main surface 131s located on the negative electrode 12 side of the first separator 131. The protective layer 132 is bonded to the first main surface 131s. The negative electrode 12 comprises a negative electrode current collector 121 and a lithium-containing metal layer 122. At least a portion of the protective layer 132 is bonded to the lithium-containing metal layer 122.

[0141] Figure 3 schematically shows the cross-sectional structure of another example of an electrode group. The separator 13 in Figure 3 is composed of a first separator 131 and a second separator 132. The first separator 131 is thinner than the second separator 132, and the second separator 132 is formed to be sufficiently thicker than the first separator 131. Except for this point, the electrode group in Figure 3 has the same configuration as the electrode group in Figure 2. Note that the second separator 132 may have a protective layer similar to the protective layer 132 on the positive electrode 11 side.

[0142] (Note) The above description discloses the following technologies. (Technology 1) A lithium secondary battery comprising: an electrode group comprising a positive electrode, a negative electrode, and a first separator disposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte, wherein, in the negative electrode, lithium metal is deposited during charging, and the lithium metal dissolves in the non-aqueous electrolyte during discharge, and at least a portion of the surface of the negative electrode is covered with a protective layer, the protective layer comprising a heat-resistant resin. (Technology 2) The lithium secondary battery according to Technology 1, wherein the first separator has a first main surface disposed on the negative electrode side and a second main surface disposed on the positive electrode side, and at least a portion of the first main surface is bonded to the surface of the negative electrode by the protective layer. (Technology 3) The lithium secondary battery according to Technology 1 or 2, wherein the protective layer is porous. (Technology 4) The lithium secondary battery according to Technology 3, wherein the porosity of the protective layer is 5% or more and 50% or less. (Technical 5) The lithium secondary battery according to any one of Technical 1 to 4, wherein the protective layer comprises a filler. (Technical 6) The lithium secondary battery according to any one of Technical 1 to 5, wherein the protective layer comprises a lithium salt. (Technical 7) The lithium secondary battery according to any one of Technical 1 to 6, further comprising a second separator disposed between the positive electrode and the first separator, wherein the thickness of the second separator is greater than the thickness of the first separator.

[0143] The lithium secondary battery relating to this disclosure will be described in detail below with reference to examples. However, this disclosure is not limited to the following examples. In these examples, several lithium secondary batteries with different configurations were fabricated and evaluated.

[0144] (Battery A1) Battery A1 was manufactured using the following procedure.

[0145] (1) Preparation of the positive electrode A positive electrode slurry was prepared by mixing lithium-containing transition metal oxide (positive electrode active material), acetylene black (AB, conductive material), and polyvinylidene fluoride (PVDF, binder) in a mass ratio of positive electrode active material:AB:PVDF = 95:2.5:2.5, and then adding an appropriate amount of N-methyl-2-pyrrolidone (NMP) and stirring. A lithium-containing transition metal oxide containing Li, Ni, Co, and Al was used as the positive electrode active material.

[0146] Next, the positive electrode mixture slurry was applied to both sides of the positive electrode current collector (aluminum foil), dried, and then rolled using a roller to create the coating of the positive electrode mixture. 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 was fabricated, comprising a positive electrode current collector and positive electrode mixture layers formed on both sides of the positive electrode current collector. Next, an aluminum tab was attached to the fabricated positive electrode.

[0147] (2) Fabrication of the negative electrode A negative electrode containing a copper foil and a lithium-containing metal layer was fabricated by pressing a lithium alloy foil (thickness: 25 μm) onto each of the two sides of a copper foil (thickness: 10 μm). Next, a nickel tab was attached to the negative electrode.

[0148] (3) Preparation of non-aqueous electrolytes LiPF ６ The concentration becomes 1 mol / L and LiBF ２ (C ２ O ４ A non-aqueous electrolyte was prepared by dissolving the following substances in a non-aqueous solvent to a concentration of 0.1 mol / L. Dimethyl carbonate (carbonate ester) was used as the non-aqueous solvent.

[0149] (4) Preparation of protective layer and separator A 12 μm thick polyethylene microporous membrane (porosity 50%) was prepared. On one side of the microporous membrane, a fully aromatic polyamide (aramid) and a lithium salt LiN(FSO) were laid. ２ ) ２A protective layer containing (LiFSI) (3 μm thick, 0% porosity) was formed. Next, the lithium-containing metal layer of the negative electrode and the microporous film were laminated with the protective layer in between, and the negative electrode and protective layer were bonded by heating and rolling press at 70°C, after which the microporous film was peeled off.

[0150] On the other hand, a microporous polyethylene membrane (thickness 42 μm, porosity V 175%) was prepared as the first separator.

[0151] (5) Battery Fabrication Next, a wound electrode group was fabricated by winding the negative electrode and the positive electrode with a first separator in an inert gas atmosphere. Then, the electrode group and the non-aqueous electrolyte were housed in an outer casing, and the outer casing was sealed to fabricate battery A1 (lithium secondary battery). A bag-shaped outer casing made of a laminate sheet containing an aluminum layer was used for the outer casing.

[0152] [Evaluation] (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 pause was taken between charging and discharging. The charge-discharge cycle was repeated 100 times, and the discharge capacity at 100 cycles was measured. The ratio of the discharge capacity at 100 cycles to the initial discharge capacity was then calculated as the capacity retention rate. (Charging) 10mA / cm until the voltage reached 4.1V ２ Constant current charging is performed, and then the current is 1 mA / cm². ２ Constant voltage charging was performed at 4.1V until the voltage reached 3V. (Discharge) 10mA / cm until the voltage reached 3V. ２ Constant current discharge was performed.

[0153] (Batteries A2, A3) Batteries A2 and A3 were manufactured and evaluated using the same method and conditions as battery A1, except that the porosity of the protective layer was changed as shown in Table 1 and lithium salt (LiFSI) was not included in the protective layer.

[0154] (Batteries A4-A6) Batteries A4-A6 were manufactured and evaluated using the same method and conditions as battery A1, except that the filler shown in Table 1 was added to the protective layer at a content of 50% by mass, the porosity of the protective layer was changed as shown in Table 1, and lithium salt (LiFSI) was not included in the protective layer. The average primary particle size (D) of the filler was used. ５０ The particle sizes used were 1 μm for alumina, 1.5 μm for magnesium hydroxide, and 1.5 μm for boehmite.

[0155] (Battery A7) Battery A7 was manufactured and evaluated using the same method and conditions as Battery A1, except that the same alumina used in Battery A4 was added as a filler to the protective layer at a content of 50% by mass.

[0156] (Battery A8) Instead of a 12 μm thick polyethylene microporous membrane, a protective layer (3 μm thick, 45% porosity) containing 50% by mass of fully aromatic polyamide (aramid) and the same alumina used in Battery A4 was formed on one side of a 42 μm thick polyethylene microporous membrane (75% porosity). Next, the lithium-containing metal layer of the negative electrode and the microporous membrane were laminated with the protective layer in between, and the negative electrode and protective layer were bonded by heating and rolling at 70°C. After that, Battery A8 was manufactured and evaluated using the same method and conditions as Battery A1, except that the microporous membrane was used as the first separator without being peeled off. In other words, in Battery A8, the first separator and the protective layer are integrated.

[0157] (Battery A9) Instead of a 12 μm thick polyethylene microporous membrane, a 42 μm thick polyethylene microporous membrane (porosity 75%) was used, and a protective layer (3 μm thick, 45% porosity) containing polysulfone and 50% by mass of the same alumina used in Battery A4 was formed on one side. Then, Battery A9 was fabricated and evaluated using the same method and conditions as Battery A8. In other words, in Battery A9, the first separator and the protective layer are integrated.

[0158] (Battery A10) Instead of a 12 μm thick polyethylene microporous membrane, a 42 μm thick polyethylene microporous membrane (porosity 75%) was used, and a protective layer (3 μm thick, 45% porosity) containing cellulose acetate and the same alumina used in Battery A4 at a content of 50% by mass was formed on one side. Then, Battery A10 was fabricated and evaluated using the same method and conditions as Battery A8. In other words, in Battery A10, the first separator and the protective layer are integrated.

[0159] (Battery A11) Instead of a 12 μm thick polyethylene microporous membrane, a 42 μm thick polyethylene microporous membrane (porosity 75%) was used, and a protective layer (3 μm thick, 45% porosity) containing polyimide and the same alumina used in Battery A4 at a content of 50% by mass was formed on one side. Then, Battery A10 was fabricated and evaluated using the same method and conditions as Battery A8. In other words, in Battery A10, the first separator and the protective layer are integrated.

[0160] (Battery A12) A protective layer (3 μm thick, 45% porosity) containing 50% by mass of fully aromatic polyamide (aramid) and the same alumina used in Battery A4 was formed on one side of a 12 μm thick polyethylene microporous membrane (50% porosity). Next, the lithium-containing metal layer of the negative electrode and the microporous membrane were laminated with the protective layer in between, and the negative electrode and protective layer were bonded by heating and rolling press at 70°C, after which the microporous membrane was peeled off. Meanwhile, a 12 μm thick microporous membrane (50% porosity) was prepared as the first separator, and a 30 μm thick microporous membrane (75% porosity) was prepared as the second separator. Then, Battery A12 was manufactured and evaluated using the same method and conditions as Battery A1, except that the first and second separators were stacked.

[0161] (Battery A13) Battery A13 was manufactured and evaluated using the same method and conditions as Battery A12, except that the lithium-containing metal layer of the negative electrode and the microporous film were laminated with a protective layer in between, the negative electrode and the protective layer were bonded by heating and rolling press at 70°C, and then the microporous film was used as the first separator without being peeled off. In other words, Battery A13 has the same configuration as Battery A12, except that the first separator and the protective layer are integrated.

[0162] (Battery C1) Battery C1 was fabricated and evaluated using the same method and conditions as battery A1, except that polystyrene was used instead of all aromatic polyamide (aramid) in the protective layer.

[0163] Battery C2 was fabricated and evaluated using the same method and conditions as Battery A1, except that polyvinyl chloride was used instead of all aromatic polyamide (aramid) in the protective layer, the same alumina used in Battery A4 was added to the protective layer at a content of 50% by mass, the porosity of the protective layer was changed to 30%, and lithium salt (LiFSI) was not included in the protective layer.

[0164] Battery C3 was fabricated and evaluated using the same method and conditions as Battery A1, except that polyacrylonitrile was used instead of all aromatic polyamide (aramid) in the protective layer, the same alumina used in Battery A4 was added to the protective layer at a content of 50% by mass, the porosity of the protective layer was changed to 40%, and lithium salt (LiFSI) was not included in the protective layer.

[0165] Table 1 shows some of the manufacturing conditions and evaluation results for each battery. In Table 1, the capacity retention rate of each battery after 100 cycles is shown as a relative value with the capacity retention rate of battery A1 after 100 cycles set to 100.

[0166]

[0167] Batteries A1 to A10 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 A10 had better cycle characteristics compared to batteries C1 to C3. Furthermore, cycle characteristics were particularly improved when the protective layer and the first separator were bonded together and the protective layer contained a filler.

[0168] This disclosure can be used in lithium secondary batteries.

[0169] Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be interpreted restrictively. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be interpreted as encompassing all modifications and alterations without departing from the true spirit and scope of the invention.

[0170] 10: Lithium secondary battery 11: Positive electrode 12: Negative electrode 121: Negative electrode current collector 122: Lithium-containing metal layer 13: Separator 131: First separator 131s: First main surface 132: Protective layer 132: Second separator 14: Electrode group 15: Case body 16: Sealing body 17, 18: Insulating plate 19: Positive electrode lead 20: Negative electrode lead 21: Step 22: Filter 23: Lower valve body 24: Insulating member 25: Upper valve body 26: Cap 27: Gasket

Claims

1. A lithium secondary battery comprising: an electrode group having a positive electrode, a negative electrode, and a first separator disposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte, wherein lithium metal is deposited on the negative electrode during charging, and the lithium metal dissolves in the non-aqueous electrolyte during discharge, and at least a portion of the surface of the negative electrode is covered with a protective layer, the protective layer comprising a heat-resistant resin.

2. The lithium secondary battery according to claim 1, wherein the first separator has a first main surface disposed on the negative electrode side and a second main surface disposed on the positive electrode side, and at least a portion of the first main surface is bonded to the surface of the negative electrode by the protective layer.

3. The lithium secondary battery according to claim 1, wherein the protective layer is porous.

4. The lithium secondary battery according to claim 1, wherein the porosity of the protective layer is 5% or more and 50% or less.

5. The lithium secondary battery according to claim 1, wherein the protective layer includes a filler.

6. The lithium secondary battery according to claim 1, wherein the protective layer comprises a lithium salt.

7. The lithium secondary battery according to claim 1, further comprising a second separator disposed between the positive electrode and the first separator, wherein the thickness of the second separator is greater than the thickness of the first separator.

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