Lithium secondary battery

Abstract

Disclosed is a lithium secondary battery comprising: 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. At the negative electrode, lithium metal is deposited during charging and dissolves into the nonaqueous electrolyte during discharging. The first separator has at least a first substrate layer. The first substrate layer contains polyolefin. The ratio Mw / Mn of the weight average molecular weight Mw to the number average molecular weight Mn of the polyolefin is 12-125. The thickness T1 of the first separator is 25-80 μm.

Description

Lithium-ion battery Cross-reference of related applications

[0001] This disclosure claims priority rights to Japanese Patent Application No. 2025-025219, filed with the Japan Patent Office on 19 February 2025, 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 proposes 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, the negative electrode current collector is covered in the first region via the negative electrode active material layer, and the negative electrode current collector is covered in the second region without the negative electrode active material layer.

[0005] Patent Document 2 states that "the material is made of high-density polyethylene having a weight-average molecular weight of 400,000 to 2,000,000 and a weight-average molecular weight / number-average molecular weight (Mw / Mn) ratio of 25 or less, has a thickness of 20 to 50 μm, a porosity of 50 to 80%, and an electrical resistance of 0.5 to 1.5 Ωcm in an organic electrolyte." ２ They are proposing a polyethylene microporous membrane for lithium battery separators, which is available in sheets of one.

[0006] Patent Document 3 states, "(1) Weight-average molecular weight is 7 × 10 ５We propose a lithium battery separator characterized by being a microporous membrane comprising a polyethylene composition containing 1% by weight or more of the above-mentioned ultra-high molecular weight polyethylene, with a weight-average molecular weight / number-average molecular weight of 10 to 300, a thickness of 0.1 to 25 μm, a porosity of 40 to 95%, an average through-hole diameter of 0.001 to 0.1 μm, and a breaking strength of 0.5 kg or more at a width of 10 mm.

[0007] Patent document 4 states that "the mass-average molecular weight is 7 × 10 ５ The present invention proposes a polyethylene microporous membrane made of a polyethylene resin having a proportion of ultra-high molecular weight polyethylene of 1% by mass or more and a ratio of mass-average molecular weight to number-average molecular weight of 5 to 300, characterized in that it comprises (a) a coarse structural layer formed on at least one surface with an average pore diameter greater than 0.04 μm, and (b) a dense structural layer with an average pore diameter of 0.04 μm or less, and the area ratio of the coarse structural layer to the dense structural layer in the cross-section of the membrane is 0.1 to 0.8.

[0008] JP 2020-95931, JP 02-94356, JP 03-105851, JP 2007-106992

[0009] 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.

[0010] One aspect of the present disclosure relates to a 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 in the negative electrode during charging, and the lithium metal dissolves in the non-aqueous electrolyte during discharge, the first separator has at least a first substrate layer, the first substrate layer contains a polyolefin, the ratio of the weight-average molecular weight Mw to the number-average molecular weight Mn of the polyolefin (Mw / Mn) is 12 or more and 125 or less, and the thickness T1 of the first substrate layer is 25 μm or more and 80 μm or less.

[0011] According to the present disclosure, a lithium secondary battery with good charge-discharge cycle characteristics can be obtained. The novel features of the present invention are described in the appended claims. However, the present invention will be better understood from the following detailed description in combination with the drawings, with reference to both the configuration and the content, as well as other objects and features of the present invention.

[0012] It is a cross-sectional view schematically showing an example of a lithium secondary battery according to the present disclosure. It is a view schematically showing a cross-section of an example of an electrode group.

[0013] 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 made 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.

[0014] The present disclosure includes combinations of matters described in two or more claims arbitrarily selected from a plurality of claims described in the appended claims. That is, as long as no technical contradiction occurs, matters described in two or more claims arbitrarily selected from a plurality of claims described in the appended claims can be combined.

[0015] In the present disclosure, the fully discharged state of a lithium metal secondary battery means a state in which, when the rated capacity of the battery is C, it is discharged until the state of charge (SOC: State of Charge) becomes 0.05×C or less. For example, it means a state of discharging to the lower limit voltage at a constant current of 0.05C. The lower limit voltage is, for example, 2.5V or less.

[0016] (Lithium Secondary Battery) The lithium secondary battery according to this embodiment may hereinafter be referred to as "lithium secondary battery (B)". 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 the first separator interposed therebetween. In the negative electrode, lithium metal is deposited during charging and dissolved into the non-aqueous electrolyte during discharging.

[0017] 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.

[0018] Generally, in a lithium secondary battery, lithium metal is likely to be deposited in a dendrite shape on the negative electrode during charging. When lithium metal is deposited in a dendrite shape, the negative electrode expands, and the liquid circulation property of the non-aqueous electrolyte in the electrode group may decrease, or the electrode current collector may crack or break. Further, when lithium metal is deposited in a dendrite shape, it causes an internal short circuit and an increase in lithium metal that does not contribute to charging and discharging. Therefore, when lithium metal is deposited in a dendrite shape, the charge-discharge cycle characteristics (hereinafter may be simply referred to as "cycle characteristics") of the lithium secondary battery deteriorate.

[0019] (First Separator) The first separator is a porous sheet having ion permeability and insulation. The first separator has at least a first base material layer.

[0020] The first base layer may be a microporous film, a woven fabric, a nonwoven fabric, etc. Among these, a microporous film is preferred. A microporous film is a film having fine pores. The microporous film may also be a resin sheet manufactured by uniaxial stretching or biaxial stretching. Fine pores can be formed by stretching and other processes. Alternatively, a resin sheet may be formed from a film raw material that has been pre-impregnated with a pore-forming agent, and then the pore-forming agent may be removed to form fine pores.

[0021] The first separator may consist only of the first base material layer. The first separator may also be a laminate of the first base material layer and a layer other than the first base material layer. For example, the first separator may be a laminate of the first base material layer and a porous heat-resistant layer that has higher heat resistance than the first base material layer.

[0022] The first base layer contains polyolefin. Polyolefin is inexpensive, has high insulating properties, high flexibility, and excellent chemical resistance.

[0023] The first base layer may contain materials other than polyolefin. Examples of materials other than polyolefin include fillers such as inorganic powders and organic additives such as paraffin. However, the first base layer mainly contains polyolefin. The polyolefin content in the first base layer is, for example, 80% by mass or more, and preferably 95% by mass or more.

[0024] The ratio of the weight-average molecular weight Mw to the number-average molecular weight Mn (Mw / Mn ratio) of the polyolefin constituting the first base layer is between 12 and 125, and the thickness T1 of the first base layer is between 25 μm and 80 μm.

[0025] By using polyolefins with an Mw / Mn ratio of 12 or higher, the stretchability of the raw material sheet of the first base layer containing such polyolefin is increased, making it easier to form pores and increasing flexibility even when the first base layer is formed thickly. As a result, the porosity of the first base layer is increased, and the first separator has a structure suitable for relieving stress caused by the expansion and contraction of the negative electrode. The first separator also exhibits excellent Li ion conductivity due to the increased porosity of the first base layer.

[0026] In addition, by setting the thickness T1 of the first substrate layer to a predetermined thickness, the stress relaxation effect is improved, the ability to retain the non-aqueous electrolyte (liquid retention) is enhanced, the liquid circulation of the non-aqueous electrolyte is improved, and the reaction resistance is reduced. As a result, it becomes possible to significantly improve the cycle characteristics of the lithium secondary battery.

[0027] The weight-average molecular weight (Mw), number-average molecular weight (Mn), and Mw / Mn ratio of polyolefins can be measured by high-temperature gel permeation chromatography (GPC). The measurement can be performed under the following conditions: Apparatus: HLC-8321GPC / HT (manufactured by Tosoh Corporation) Column: Shodex GPC HT-806M x 2 Oven temperature: 135°C Sample concentration: 2 g / L Eluent: o-dichlorobenzene (containing 0.5 g / L dibutylhydroxytoluene (BHT)) Detector: RI Standard substance: Polystyrene Injection volume: 300 μL Pretreatment: Filtration through a metal filter with a pore size of 1 μm

[0028] The polyolefin constituting the first base layer can be polyethylene, polypropylene, or a copolymer of ethylene and propylene. In particular, it is preferable that the polyolefin constituting the first base layer contains at least polyethylene. Because polyethylene has a low softening temperature, it can exhibit a so-called shutdown mechanism that cuts off the current when the battery temperature rises in an abnormal situation. In this context, the shutdown mechanism is a mechanism in which the pores of the first base layer are blocked, restricting the movement of ions between the positive and negative electrodes.

[0029] If the first base layer is composed of multiple layers, at least one layer may contain only polyethylene as a polyolefin, provided that each layer satisfies the above Mw / Mn ratio.

[0030] The polyethylene content in the polyolefin constituting the first base material layer is, for example, 90% by mass or more, may be 95% by mass or more, or 100% of the polyolefin may be polyethylene.

[0031] The first substrate layer may consist of multiple layers. In this case, each layer may differ from the other layers in at least one of its form and composition. However, each layer must satisfy the above-mentioned Mw / Mn ratio.

[0032] When the first separator is composed only of the first base material layer, the thickness of the first base material layer is the same as the thickness T1 of the first separator.

[0033] The ratio of the weight-average molecular weight Mw to the number-average molecular weight Mn (Mw / Mn ratio) of the polyolefin constituting the first base layer may be 12 or more, but is preferably 25 or more, more preferably greater than 25, even more preferably 26 or more, and may be 30 or more. Also, the Mw / Mn ratio may be 125 or less, but may be 100 or less, 80 or less, or 60 or less. For example, the Mw / Mn ratio may be 26 to 125, preferably 30 to 125, may be 26 to 100, may be 30 to 100, may be 26 to 80, or may be 30 to 80. When the above Mw / Mn ratio is satisfied, it becomes easier to increase the porosity of the first base layer, which increases the effect of mitigating stress caused by the expansion and contraction of the negative electrode by the first separator.

[0034] When the Mw / Mn ratio is less than 12, the stretchability of the raw material sheet for the first substrate layer containing such polyolefin becomes insufficient. As a result, it becomes difficult to obtain a first substrate layer with a high porosity and sufficient thickness. When the Mw / Mn ratio is less than 12, it is not possible to obtain a first substrate layer of the desired thickness when trying to achieve a certain size of porosity, and only a thin substrate layer can be formed.

[0035] On the other hand, when the Mw / Mn ratio exceeds 125, the amount of low molecular weight components increases, reducing the elastic function and the ability to suppress dendrites. Therefore, it becomes difficult to obtain a first substrate layer with high porosity and sufficient thickness.

[0036] The weight-average molecular weight Mw of the polyolefin constituting the first base layer may be, for example, 400,000 to 1,000,000.

[0037] The thickness T1 of the first substrate layer may be 25 μm or more, but is preferably 30 μm or more, and more preferably 40 μm or more. Also, the thickness T1 of the first substrate layer may be 80 μm or less, but is preferably 60 μm or more, and even more preferably 50 μm or less. The thickness T1 of the first substrate layer is preferably 30 μm to 80 μm, may be 30 μm to 60 μm, may be 40 μm to 80 μm, or may be 40 μm to 60 μm. When the above T1 is satisfied, the stress relaxation effect and liquid retention of the first substrate layer are sufficiently achieved, the electrode group can have a sufficiently high volumetric energy density, and the resistance between the positive electrode and the negative electrode can be sufficiently reduced.

[0038] The porosity of the first substrate layer may be, for example, 50% or more, 55% or more, 60% or more, preferably 65% ​​or more, and may also be 70% or more. In this case, the first substrate layer can be said to have a very high porosity despite its large thickness T1. Therefore, the stress relaxation effect by the first separator is significantly improved, the liquid retention capacity of the non-aqueous electrolyte is also sufficiently increased, the liquid circulation of the non-aqueous electrolyte is further improved, and the cycle characteristics of the lithium secondary battery can be significantly improved. The porosity of the first substrate layer may be, for example, 50% to 90%, 60% to 90%, or 65% to 90%.

[0039] The porosity of the first substrate layer can be measured by the following method. First, a 4 cm square sample of the first substrate layer is taken at four locations in the central part of the width direction of the first separator. Next, the mass, area, and thickness of each sample are measured. Then, the apparent density of the first substrate layer is calculated from the mass, area, and thickness of the samples. The porosity of the first substrate layer is obtained by the following formula. The true density of the first substrate layer is determined based on the constituent materials of the first substrate layer.

[0040] Porosity (%) = 100 × {1 - (Apparent density of the first substrate layer) / (True density of the first substrate layer)}

[0041] The first base layer may contain paraffin. The inclusion of paraffin in the first base layer promotes the permeability of the non-aqueous electrolyte to the first separator, which can further improve the cycle characteristics. The type of paraffin is not particularly limited, but may be at least one selected from the group consisting of normal paraffin, isoparaffin, and cycloparaffin. Among these, paraffin with 15 to 45 carbon atoms is preferred, and it is preferable that it is composed of aliphatic hydrocarbons that do not have unsaturated bonds.

[0042] The paraffin content in the first substrate layer is, for example, 0.05% by mass or more and 3% by mass or less, and preferably 0.1% by mass or more and 0.8% by mass or less.

[0043] The molecular weight distribution of paraffins can be analyzed and quantified by gas chromatography-mass spectrometry (GC / MS). Measurements can be performed under the following conditions: Instrument: Agilent Technologies 8890B, 5977C Column: Agilent J&B HP-5MS UI (length 30m, inner diameter 0.25mm, film thickness 0.25μm) Column temperature: 50°C (1 min) → (heating rate 10°C / min) → 320°C (12 min) Injection method: Splitless Injection volume: 1 μL Injection port temperature: 280°C Transfer line temperature: 300°C Carrier gas: He Measurement mode: SCAN Scan range: m / z = 20-800

[0044] (Second Separator) The electrode group may further include a second separator disposed between the negative electrode and the first separator. The second separator is a porous sheet having ion permeability and insulating properties. The second separator has at least a second substrate layer. The second separator may consist only of the second substrate layer. The second separator may be a laminate of the second substrate layer and a layer other than the second substrate layer. For example, the second separator may be a laminate of the second substrate layer and a porous heat-resistant layer having higher heat resistance than the second substrate layer.

[0045] The second base material layer can be a microporous membrane, woven fabric, nonwoven fabric, etc., similar to the second base material layer. Among these, a microporous membrane is preferred.

[0046] The second base layer may contain polyolefin. The second base layer may also contain materials other than polyolefin. Examples of materials other than polyolefin include fillers such as inorganic powders and organic additives such as paraffin. The polyolefin content in the second base layer is, for example, 80% by mass or more, and preferably 95% by mass or more.

[0047] The second separator may have at least one of the features described for the first separator. The second substrate layer may have at least one of the features described for the first substrate layer.

[0048] The first separator and the second separator are stacked and placed between the positive and negative electrodes. The first and second separators typically have the same shape. However, as long as they are placed between the positive and negative electrodes, the shape of the second separator may differ from that of the first separator.

[0049] An adhesive layer may be placed between the negative electrode and the second separator. At least a portion of the adhesive layer may be bonded to the negative electrode. In other words, at least a portion of the second separator may be bonded to the negative electrode via the adhesive layer. In this case, the second separator is integrated with the negative electrode together with the adhesive layer. This enhances the effect of suppressing the growth of dendrite-like lithium metal.

[0050] The first separator and the second separator may be bonded together, but it is preferable that they are not 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 more easily relieved, and damage to the components constituting the electrode group can be suppressed. In particular, when the second separator is bonded to the negative electrode, the first separator, which has a high degree of freedom of movement, can move to an appropriate position, making it easier to obtain a stress buffering effect.

[0051] The ratio of the thickness T1 of the first substrate layer to the thickness T2 of the second substrate layer (T1 / T2 ratio) is preferably greater than 1, and may be between 1.1 and 5.0, or between 2.0 and 4.2. By making the second separator adjacent to the negative electrode relatively thinner and the first separator, which has a high degree of freedom of movement, relatively thicker, it is possible to ensure sufficient volumetric energy density of the electrode group while increasing the stress relaxation effect of the first separator.

[0052] The first separator, which is thicker than the second separator, 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 liquid 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 first separator, it becomes easier to avoid the decrease in liquid circulation of the non-aqueous electrolyte and the breakage of the negative electrode current collector.

[0053] The thickness T2 of the second substrate layer is, for example, 5 μm to 20 μm, and may be 10 μm to 20 μm, or 10 μm to 15 μm.

[0054] The total thickness (T1 + T2) of the first substrate layer and the second substrate layer is, for example, 30 μm to 85 μm, but may also be 35 μm to 85 μm, 40 μm to 80 μm, or 40 μm to 75 μm.

[0055] The thickness of each substrate layer is determined by taking the arithmetic mean of the thicknesses at 15 points. The thickness of the substrate layers 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 substrate layer 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.

[0056] It is preferable that the porosity V1 of the first substrate layer is greater than the porosity V2 of the second substrate layer. By making the porosity V1 of the first substrate layer greater than the porosity V2 of the second substrate layer, the expansion of the negative electrode is more easily absorbed by the first separator.

[0057] The ratio of the porosity V1 (%) to the porosity V2 (%) (V1 / V2 ratio) may be 1.1 or greater, or 1.2 or greater. The V1 / V2 ratio may be 2.0 or less, or 1.7 or less. By relatively reducing the porosity of the second separator adjacent to the negative electrode, it becomes easier to suppress the deposition of lithium metal in a dendrite-like manner on the negative electrode.

[0058] (Porous Heat-Resistant Layer) The porous heat-resistant layer is a layer containing a heat-resistant resin. A heat-resistant resin refers to, for example, a resin with a high heat distortion temperature. The heat distortion temperature of the heat-resistant resin is preferably 260°C or higher. Here, the heat distortion temperature is the load deflection temperature determined at a load of 1.82 MPa in accordance with ASTM-D648.

[0059] 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.

[0060] (Adhesive layer) The adhesive layer has the function of bonding the negative electrode and at least a portion of the second separator. Preferably, 80% or more or 90% or more of the surface of the negative electrode is covered with the adhesive layer, and more preferably, 99% to 100% of the surface of the negative electrode is covered with the adhesive layer.

[0061] By covering at least a portion of the negative electrode surface with an adhesive layer, the cycle characteristics can be significantly improved. The adhesive layer works in cooperation with the second separator to inhibit the growth of dendrite-like lithium metal. This suppresses internal short circuits and reduces the amount of lithium metal that is isolated from the conductive network and does not contribute to charging and discharging. As a result, the cycle characteristics are improved.

[0062] Since lithium metal is deposited on the negative electrode during charging, lithium metal or a lithium alloy is usually present on the surface of the negative electrode. Therefore, the adhesive layer is usually in contact with metallic lithium or lithium compounds (carbonates, oxides, or hydroxides).

[0063] The adhesive layer can maintain its covering of the negative electrode surface without shrinking even if the second separator shrinks due to heat. Therefore, if the battery becomes hot enough for the second separator to shrink due to heat, the adhesive layer also serves to ensure the safety of the battery.

[0064] The adhesive layer may be formed by applying a coating solution containing the components of the adhesive layer to the surface of the negative electrode or the surface of the second separator, and then drying the coating film. The method of applying the coating solution is not limited, and known methods may be used. For example, it 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.).

[0065] The solvents used in the coating solution can include acetone, dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, and tripropylene glycol.

[0066] The adhesive layer may contain a vinylidene fluoride polymer synthesized by polymerizing a monomer containing vinylidene fluoride. The vinylidene fluoride polymer is preferred because it exhibits high adhesion to the negative electrode and high stability within the lithium secondary battery (B).

[0067] The content of vinylidene fluoride polymer in the adhesive layer may be 3% by mass or more, 50% by mass or more, 100% by mass or less, or 80% by mass or less.

[0068] The vinylidene fluoride polymer contains constituent units derived from vinylidene fluoride. The proportion of constituent units derived from vinylidene fluoride (VDF units) to the total constituent 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 copolymerized with vinylidene fluoride include tetrafluoroethylene and hexafluoropropylene. The vinylidene fluoride polymer may also be polyvinylidene fluoride.

[0069] The vinylidene fluoride polymer may be a copolymer synthesized by polymerizing monomers containing vinylidene fluoride and hexafluoropropylene. That is, the adhesive layer may contain a copolymer synthesized by polymerizing monomers containing vinylidene fluoride and hexafluoropropylene. Since hexafluoropropylene has a bulkier molecular structure than vinylidene fluoride, the copolymer synthesized by polymerizing monomers containing vinylidene fluoride and hexafluoropropylene has a lower density than polyvinylidene fluoride. By using monomers containing hexafluoropropylene, the crystallization of the polymer is suppressed, and a flexible adhesive layer is obtained. In addition, the retention of non-aqueous electrolytes in the adhesive layer is improved, and lithium ion conductivity is improved. As a result, the resistance of the battery is reduced, and the cycle characteristics can be further improved. The proportion of hexafluoropropylene-derived structural units (HFP units) in the total structural units of the vinylidene fluoride polymer may be in the range of 0 to 50 mol% (for example, in the range of 0.5 to 25 mol% or 2 to 10 mol%).

[0070] The adhesive layer may contain inorganic particles. The inclusion of inorganic particles in the adhesive layer facilitates the formation of a porous structure, improving the strength and durability of the adhesive layer. Therefore, even when repeated charge-discharge cycles cause the electrodes to expand and the internal pressure of the electrode group to increase, the adhesive layer will not be crushed.

[0071] Examples of the inorganic particles include oxides, oxide hydrates, hydroxides, nitrides, carbides, sulfides, etc. Examples of the oxides and oxide hydrates include aluminum oxide (alumina), boehmite (Al ２ O ３ ·H ２ O or AlOOH), magnesium oxide, titanium oxide, zirconium oxide, silicon oxide, yttrium oxide, zinc oxide, etc. Examples of the nitrides include silicon nitride, aluminum nitride, boron nitride, titanium nitride, etc. Examples of the carbides include silicon carbide, boron carbide, etc. Examples of the sulfides include barium sulfate, etc. Examples of the hydroxides include aluminum hydroxide, magnesium hydroxide, etc. Further, the inorganic particles may be porous aluminosilicates such as zeolite, layered silicates such as talc, barium titanate (BaTiO ３ ), strontium titanate (SrTiO ３ ), etc.

[0072] 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 ) at which the cumulative volume becomes 50% in the particle size distribution based on volume. The median diameter (D ５０ ) can be measured using a laser diffraction / scattering type particle size distribution measuring device.

[0073] The content rate of the inorganic particles in the adhesive layer may be 10% by mass or more, may be 20% by mass or more, may be 99% by mass or less, or may be 90% by mass or less.

[0074] The adhesive layer preferably has a porous structure. By using the porous adhesive layer, the contact area between the adhesive layer and the non-aqueous electrolyte increases, whereby the retention of the non-aqueous electrolyte in the adhesive layer is further improved and the lithium ion conductivity is improved. As a result, the resistance of the battery is lowered and good cycle characteristics can be obtained.

[0075] The thickness of the adhesive 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 adhesive 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 adhesive layer is 0.1 μm or more, the effect of suppressing the dendritic deposition of lithium metal is enhanced. When the thickness of the protective layer is 10 μm or less, the internal resistance of the battery can be lowered.

[0076] A porous adhesive layer can be formed, for example, by the NIPS method (non-solvent-induced phase separation method), but the method for producing a porous adhesive 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 second separator to form a coating film, and then, before the coating film dries, the coating film is immersed in a poor solvent. This makes the coating film porous, and a porous adhesive layer is formed.

[0077] 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.

[0078] (Lithium-ion secondary battery (B)) Examples of other components of lithium-ion secondary battery (B) are described below in detail. Note that the components described below are illustrative, and the components of lithium-ion secondary battery (B) in this embodiment are not limited to the following examples. Known components may be used for components other than those characteristic of this embodiment.

[0079] (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.

[0080] The negative electrode current collector is usually composed of a conductive sheet. The conductive sheet may be composed of a conductive material other than lithium metal and lithium alloys. The conductive material may be a metallic material such as a metal or alloy. The metallic material may be a material that does not react with lithium (a material that does not form any alloys or intermetallic compounds with lithium). Examples of such metallic materials are copper, nickel, iron, and alloys containing these metallic elements. As for alloys, copper alloys and stainless steel may also be used. From the viewpoint of easily ensuring high capacity and high charge / discharge efficiency by having high conductivity, the metallic material may include at least one of copper and copper alloys. The conductive sheet may contain one of these conductive materials or two or more.

[0081] Conductive sheets can be foils, films, etc. Conductive sheets may be porous. From the viewpoint of easily ensuring high conductivity, conductive sheets may be metal foils, or metal foils containing copper. Such metal foils may be copper foils or copper alloy foils.

[0082] Furthermore, since it is easier to ensure a high volumetric energy density, the negative electrode may consist only of a negative electrode current collector in the fully discharged state of the lithium metal secondary battery. Also, from the viewpoint of easily ensuring high charge and discharge efficiency, in the fully discharged state, the negative electrode may consist of a negative electrode current collector and a negative electrode active material layer arranged on the surface of the negative electrode current collector. When assembling the battery, only the negative electrode current collector may be used as the negative electrode, or a negative electrode consisting of a negative electrode active material layer and a negative electrode current collector may be used.

[0083] Examples of negative electrode active materials included in the negative electrode active material layer include metallic lithium, lithium alloys, and materials that reversibly intercalate and release lithium ions. The negative electrode active material may be one used in lithium-ion batteries. Examples of lithium alloys include lithium-aluminum alloys. Examples of materials that reversibly intercalate and release lithium ions include carbon materials and alloying materials. Examples of carbon materials include graphite materials, soft carbon, hard carbon, and amorphous carbon. Examples of alloying materials include silicon-containing materials and tin-containing materials. Examples of alloying materials include elemental silicon, silicon alloys, silicon compounds, elemental tin, tin alloys, and tin compounds. Examples of silicon compounds and tin compounds include oxides and nitrides, respectively. The negative electrode active material layer may contain one type of negative electrode active material, or a combination of two or more types.

[0084] The negative electrode active material layer may be formed by depositing the negative electrode active material onto the surface of the negative electrode current collector using a gas-phase method such as electrodeposition or vapor deposition. Alternatively, it may be formed by coating the surface of the negative electrode current collector with a negative electrode mixture containing the negative electrode active material and a binder.

[0085] (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, 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.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] Conductive materials can include carbon materials. Examples of carbon materials include carbon black (acetylene black, Ketjenblack, etc.), carbon nanotubes, and graphite.

[0090] Examples of binders include fluororesins, polyimide resins, acrylic resins, polyolefin resins, and rubbery polymers. Examples of fluororesins include polytetrafluoroethylene and vinylidene fluoride polymers such as polyvinylidene fluoride.

[0091] 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.

[0092] 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.

[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 in liquid or gel form. A liquid non-aqueous electrolyte (electrolyte) can be prepared by dissolving a lithium salt in a non-aqueous solvent.

[0094] The non-aqueous solvent may contain an ether compound. The content of the ether compound in the non-aqueous solvent may be, for example, 50% by volume or more, but may also be 80% by volume or more, or 90% by volume or more. The non-aqueous solvent may consist only of the ether compound. The ether compound has a high affinity for paraffin, which may be contained in the first separator. Therefore, by using a non-aqueous solvent containing an ether compound, the cycle characteristics can be further significantly improved. In addition, the ether compound has the effect of suppressing the dendritic deposition of lithium metal at the negative electrode during charging.

[0095] The ether compound may be a linear ether, a fluorinated linear ether, a cyclic ether, or a fluorinated cyclic ether. That is, the ether compound 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 volume or more, or 90% by volume or more. The non-aqueous solvent may consist solely of hydrofluoroether.

[0096] The fluorination rate of the hydrofluoroether is preferably 60% or more, and more preferably 65% ​​or more. The fluorination rate may also be 95% or less, 90% or less, or 80% or less. The fluorination rate of a single hydrofluoroether is defined by the following formula.

[0097] Fluorination rate (%) = 100 × (number of fluorine atoms in the hydrofluoroether) / (total number of fluorine and hydrogen atoms in the hydrofluoroether)

[0098] 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.

[0099] 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.

[0100] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO2). ４ LiAlCl ４ LiB １０ Cl １０ (e.g.), lithium salts of fluorine-containing acids (LiPF) ６ LiPF ２ O ２ LiBF ４ LiSbF ６ LiAsF６ 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.

[0101] 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, and may be 3.5 mol / L or lower, 2.0 mol / L or lower, or 1.5 mol / L or lower.

[0102] (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.

[0103] 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.

[0104] (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.

[0105] 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.

[0106] 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.

[0107] 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.

[0108] 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.

[0109] Figure 2 schematically shows the cross-sectional structure of an example of an electrode group. A separator 13 is placed between the positive electrode 11 and the negative electrode 12. The separator 13 is composed of a first separator 132 and a second separator 131. The first separator 132 is thinner than the second separator 131, and the second separator 131 is formed to be sufficiently thicker than the first separator 132. The negative electrode 12 comprises a negative electrode current collector 121 and a lithium-containing metal layer 122. The second separator 131 is bonded to the lithium-containing metal layer 122 by an adhesive layer 132.

[0110] (Note) The above description discloses the following technologies. (Technology 1) A 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; the first separator has at least a first substrate layer; the first substrate layer comprises a polyolefin; the ratio of the weight-average molecular weight Mw to the number-average molecular weight Mn of the polyolefin: Mw / Mn is 12 or more and 125 or less; and the thickness T1 of the first substrate layer is 25 μm or more and 80 μm or less. (Technology 2) The secondary battery according to Technology 1, wherein the Mw / Mn ratio is 30 or more and 125 or less. (Technology 3) The secondary battery according to Technology 1 or 2, wherein the thickness T1 of the first substrate layer is 30 μm or more and 80 μm or less. (Technology 4) The secondary battery according to any one of Technology 1 to 3, wherein the porosity of the first substrate layer is 65% or more and 90% or less. (Technology 5) The secondary battery according to any one of Technology 1 to 4, wherein the polyolefin comprises at least polyethylene. (Technology 6) The secondary battery according to any one of Technology 1 to 5, wherein the first substrate layer comprises paraffin, and the paraffin content in the first substrate layer is 1% by mass or more and 3% by mass or less. (Technology 7) The secondary battery according to any one of Technology 1 to 6, wherein the electrode group further comprises a second separator disposed between the negative electrode and the first separator, and an adhesive layer disposed between the negative electrode and the second separator, and at least a part of the adhesive layer is bonded to the negative electrode. (Technical 8) The secondary battery according to Technical 7, wherein the second separator has at least a second substrate layer, the second substrate layer contains a polyolefin, and the ratio of the thickness T1 of the first substrate layer to the thickness T2 of the second substrate layer, T1 / T2, is 2.0 or more and 4.2 or less. (Technical 9) The secondary battery according to any one of Technical 1 to 8, wherein the non-aqueous solvent of the non-aqueous electrolyte contains an ether compound, and the content of the ether compound contained in the non-aqueous solvent is 50% by volume or more.

[0111] 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.

[0112] (Battery A1) Battery A1 was manufactured using the following procedure.

[0113] (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.

[0114] 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.

[0115] (2) Fabrication of the negative electrode A negative electrode containing copper foil and lithium alloy foil (i.e., lithium-containing metal layer) was fabricated by pressing lithium alloy foil (thickness: 25 μm) onto each of the two sides of copper foil (thickness: 10 μm). Next, a nickel tab was attached to the negative electrode.

[0116] (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.

[0117] (4) Preparation of the separator A microporous membrane made of polyethylene (PE) with a thickness of 25 μm (first substrate layer) (porosity 50%, paraffin content 0.2 mass%) was prepared as the first separator. The Mw / Mn ratio of the polyethylene constituting the microporous membrane was 12.

[0118] (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.

[0119] [Evaluation 1] (Charge-Discharge Cycle Test) Battery A1 underwent a charge-discharge cycle test in an environment of 25°C. 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. The charge-discharge conditions are shown below.

[0120] (Charging) 10mA / cm until the voltage reaches 4.1V ２ Constant current charging is performed, and then the current is 1 mA / cm². ２ The battery was charged at a constant voltage of 4.1V until it reached this point.

[0121] (Discharge) 10mA / cm until the voltage reaches 3V ２ Constant current discharge was performed.

[0122] [Evaluation 2] (Calculation of volumetric energy density) For battery A1, the ratio of the initial discharge capacity to the volume of the electrode group was calculated and used as the volumetric energy density. (Battery A2) A microporous film made of polypropylene (PP) with a thickness of 25 μm (first substrate layer) (porosity 50%, paraffin content 0.2 mass%) was prepared as the first separator. The Mw / Mn ratio of the polypropylene constituting the microporous film was 12. On the other hand, a microporous film made of polyethylene (porosity 50%, paraffin content 0.3 mass%) with a thickness of 12 μm was prepared as the second separator (second substrate layer). An adhesive layer (thickness 3 μm, inorganic particle content 80%) containing polyvinylidene fluoride and boehmite (inorganic particles) was formed on one side of the second substrate layer. Next, the lithium-containing metal layer of the negative electrode and the second substrate layer were laminated with the adhesive layer in between, and the second substrate layer was bonded to both sides of the negative electrode by heating, rolling, and pressing at 70°C. A wound electrode group was fabricated by winding a negative electrode and a positive electrode, each having a second substrate layer on both sides, via a first separator in an inert gas atmosphere. Battery A2 was fabricated and evaluated in the same manner as battery A1, except as described above.

[0123] (Battery A3) Battery A3 was fabricated and evaluated in the same manner as Battery A2, except that it used the same first separator as Battery A1.

[0124] (Batteries A4-9) Batteries A4-A9 were manufactured and evaluated in the same manner as battery A3, except that the Mw / Mn ratio, thickness T1, porosity, and paraffin content of the first substrate layer (microporous membrane), which is the first separator, were changed as shown in Table 1.

[0125] (Batteries A10-19) Batteries A4-A9 were fabricated and evaluated in the same manner as battery A3, except that the Mw / Mn ratio, thickness T1, porosity, and paraffin content of the first substrate layer (microporous membrane), which is the first separator, were changed as shown in Table 1, and a portion of the dimethyl carbonate, a non-aqueous solvent in the electrolyte, was replaced with 1,2-dimethoxyethane, an ether compound, to contain the ether compound in the electrolyte at the content shown in Table 1.

[0126] (Batteries B1-B3, B5) Batteries B1-B2 and B5 were fabricated and evaluated in the same manner as battery A3, except that the Mw / Mn ratio, thickness T1, porosity, and paraffin content of the first substrate layer (microporous membrane), which is the first separator, were changed as shown in Table 1.

[0127] (Battery B4) A 100 μm thick polypropylene microporous membrane (first substrate layer) (porosity 50%, paraffin content 0.2 mass%) was prepared as the first separator. The Mw / Mn ratio of the polypropylene constituting the microporous membrane was 20. Except as described above, battery B4 was fabricated and evaluated in the same manner as battery A2.

[0128] Table 1 shows some of the components 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. Also, the volumetric energy density of each battery is shown as a relative value with the volumetric energy density of battery A1 set to 100.

[0129]

[0130] Batteries A1 to A19 are lithium secondary batteries (B) according to this disclosure. Batteries C1 to C5 are comparative examples. As shown in Table 1, batteries A1 to A19 had better cycle characteristics compared to batteries C1 to C5.

[0131] In battery C1, the Mw / Mn ratio of the first separator is small and the porosity is insufficient, resulting in a small stress relaxation effect and insufficient electrolyte circulation, which is thought to have led to a decrease in cycle characteristics.

[0132] In battery C2, the porosity and thickness T1 of the first separator appeared to be sufficient, but the Mw / Mn ratio was small, resulting in insufficient separator flexibility. This prevented sufficient stress relaxation, leading to a decrease in cycle characteristics.

[0133] In battery C3, the thickness T1 of the first separator was insufficient, resulting in a small stress relaxation effect and insufficient electrolyte circulation, which is thought to have led to a decrease in cycle characteristics.

[0134] In battery C4, the thickness T1 of the first separator is excessively thick, which is thought to have resulted in an excessively large distance between the positive and negative electrodes, leading to increased reaction resistance and a decrease in cycle characteristics. Furthermore, the excessive thickness T1 of the first separator and the large volume of the electrode group are thought to have reduced the initial discharge capacity (volume energy density) relative to the volume of the electrode group.

[0135] In battery C5, it is thought that the high concentration of low molecular weight components in the polyolefin reduced the elastic function of the first separator, thereby impairing its ability to suppress dendrite formation.

[0136] This disclosure can be used in lithium secondary batteries (i.e., lithium metal secondary batteries) in which, for example, 70% or more of the rated capacity is obtained by the deposition and dissolution of lithium metal.

[0137] 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.

[0138] 10: Lithium secondary battery, 11: Positive electrode, 12: Negative electrode, 121: Negative electrode current collector, 122: Lithium-containing metal layer, 13: Separator, 131: Second separator, 132: Adhesive layer, 132: First separator, 14: Electrode group, 15: Case body, 16: Sealing body, 17, 18: Insulating plate, 19: Positive electrode lead, 20: Negative electrode lead, 21: Step section, 22: Filter, 23: Lower valve body, 24: Insulating member, 25: Upper valve body, 26: Cap, 27: Gasket

Claims

1. A 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 lithium metal is deposited in the negative electrode during charging, and the lithium metal dissolves in the non-aqueous electrolyte during discharge; the first separator has at least a first substrate layer; the first substrate layer comprises a polyolefin; the ratio of the weight-average molecular weight Mw to the number-average molecular weight Mn of the polyolefin (Mw / Mn) is 12 or more and 125 or less; and the thickness T1 of the first substrate layer is 25 μm or more and 80 μm or less.

2. The secondary battery according to claim 1, wherein the Mw / Mn ratio is 30 or more and 125 or less.

3. The secondary battery according to claim 1, wherein the thickness T1 of the first substrate layer is 30 μm or more and 80 μm or less.

4. The secondary battery according to claim 1, wherein the porosity of the first substrate layer is 65% or more and 90% or less.

5. The secondary battery according to claim 1, wherein the polyolefin comprises at least polyethylene.

6. The secondary battery according to claim 1, wherein the first substrate layer contains paraffin, and the paraffin content in the first substrate layer is 0.05% by mass or more and 3% by mass or less.

7. The secondary battery according to claim 1, wherein the electrode group further includes a second separator disposed between the negative electrode and the first separator, and an adhesive layer disposed between the negative electrode and the second separator, and at least a portion of the second separator is bonded to the negative electrode by the adhesive layer.

8. The secondary battery according to claim 7, wherein the second separator has at least a second substrate layer, the second substrate layer comprises a polyolefin, and the ratio T1 / T2 of the thickness T1 of the first substrate layer to the thickness T2 of the second substrate layer is 2.0 or more and 4.2 or less.

9. The secondary battery according to claim 1, wherein the non-aqueous solvent of the non-aqueous electrolyte contains an ether compound, and the content of the ether compound in the non-aqueous solvent is 50% by volume or more.

Images