Secondary battery

Abstract

This secondary battery comprises: a wound electrode group having a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode; a nonaqueous electrolyte; a core member disposed in the hollow of the electrode group; and a buffer member disposed between the electrode group and the core member. The buffer member can expand and contract in the radial direction of the electrode group.

Description

secondary battery Cross-reference of related applications

[0001] This disclosure claims priority with respect to Japanese Patent Application No. 2024-167860, filed with the Japan Patent Office on 26 September 2024, and the entirety of the said patent application is incorporated herein by reference.

[0002] This disclosure relates to secondary batteries.

[0003] Patent Document 1 proposes a "lithium secondary battery comprising a wound electrode group having a positive electrode, a negative electrode equipped with a negative electrode current collector, a separator disposed between the positive electrode and the negative electrode, a core member disposed in the hollow of the electrode group, and a non-aqueous electrolyte having lithium ion conductivity, wherein lithium metal is deposited in the negative electrode during charging and the lithium metal dissolves during discharge, and the negative electrode current collector is austenitic stainless steel foil or oxygen-free copper foil."

[0004] International Publication No. 2023 / 190870

[0005] Conventionally, in lithium secondary batteries equipped with wound electrode groups having a hollow structure, the electrodes deform near the hollow portion of the electrode group during charging and discharging, leading to electrode buckling, electrode fracture, and other problems.

[0006] One aspect of the present disclosure relates to a secondary battery comprising: a wound electrode group having a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode; a lithium-ion conductive non-aqueous electrolyte; a core member disposed in the hollow of the electrode group; and a buffer member disposed between the electrode group and the core member, wherein the buffer member is expandable and contractible in the radial direction of the electrode group.

[0007] According to this disclosure, the cycle characteristics of secondary batteries can be improved. Novel features of the present invention are described in the appended claims, but the present invention, both in terms of structure and content, and in conjunction with other objects and features of the present invention, will be better understood by the following detailed description in conjunction with the drawings.

[0008] This is a schematic longitudinal cross-sectional view showing a lithium secondary battery according to one embodiment of the present disclosure. This is a schematic diagram showing an example of the configuration of a buffer member. This is a schematic diagram showing another example of the configuration of a buffer member.

[0009] The embodiments of this disclosure will be described below with examples, but this disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be given as examples, but other numerical values ​​and materials may be applied as long as the effects of this disclosure are 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 "greater than or equal to numerical value A and less than or equal to numerical value B". In the following description, when lower and upper limits of numerical values ​​relating to specific physical properties or conditions are given as examples, either of the given lower limits and either of the given upper limits can be arbitrarily combined, as long as the lower limit is not greater than or equal to the upper limit. When multiple materials are given as examples, one of them may be selected and used alone, or two or more may be used in combination.

[0010] A secondary battery according to one embodiment of the present disclosure comprises a wound electrode group and a non-aqueous electrolyte having lithium-ion conductivity. The wound electrode group has a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode. A core member is disposed in the hollow of the electrode group. Hereinafter, a wound electrode group with a core member disposed in the hollow will also be referred to as a "core member electrode group".

[0011] Secondary batteries include lithium-ion secondary batteries that use a material that reversibly intercepts and releases lithium ions as the negative electrode active material, lithium secondary batteries in which lithium metal is deposited at the negative electrode during charging and dissolves during discharge (lithium metal secondary batteries), solid batteries containing a gel electrolyte, and all-solid-state secondary batteries in which the non-aqueous electrolyte is solid.

[0012] The electrode group of a secondary battery repeatedly expands and contracts with charging and discharging. This generates stress within the electrode group. In particular, lithium secondary batteries (lithium metal secondary batteries), in which lithium metal is deposited on the negative electrode during charging and dissolved in a non-aqueous electrolyte during discharging, experience large volume changes in the electrode group during charging and discharging. Therefore, large stresses are easily generated within the electrode group, making electrode fracture and other problems likely. In addition, needle-shaped byproducts called dendrites are formed on the lithium metal of the negative electrode as the charge-discharge cycle progresses. Dendrites tend to increase with the cycle and, along with charging, contribute to the generation of internal stress. Dendrites consist of lithium metal in the interior and a lithium compound on the surface.

[0013] In contrast, in the secondary battery according to this disclosure, by arranging a core member in the hollow of the electrode group, deformation of the electrodes near the hollow portion of the electrode group is suppressed, and electrode breakage is suppressed. Since the electrode group is normally in contact with the inner surface of the battery case during charging and discharging, by using a core member, both the outer and inner circumferences of the electrode group are fixed by the battery case and the core member. As a result, the shape of the electrode group is stabilized, and surface pressure is more easily and stably applied to the electrodes throughout the electrode group. Furthermore, since a buffer member that can expand and contract in the radial direction of the electrode group is arranged between the electrode group and the core member, sudden surface pressure is not applied to the electrodes during charging and discharging, and electrode deformation and breakage are suppressed even more effectively.

[0014] Furthermore, in lithium secondary batteries, a consistently good surface pressure is applied to the electrodes, significantly suppressing the formation of needle-shaped dendrites. Therefore, partial isolation of lithium metal, expansion of electrode groups, and increase in negative electrode resistance due to dendrite formation are suppressed. Consequently, the improvement in cycle characteristics is particularly pronounced in lithium secondary batteries.

[0015] From the perspective of improving cycle characteristics, it is preferable that the outer diameter D1 of the core member and the outer diameter D2 of the electrode group during discharge satisfy the relationship D1 / D2 ≤ 1 / 4. When D1 / D2 is 1 / 4 or less, the portion that does not contribute to the charge-discharge reaction can be reduced. Also, in this case, the pressure applied from the core member toward the inner peripheral surface of the electrode group and the pressure applied from the battery can toward the outer peripheral surface of the electrode group are likely to be of appropriate magnitudes respectively.

[0016] The inner diameter of the battery can may be such that the outer peripheral surface of the electrode group and the inner peripheral surface of the battery can contact each other during initial discharge (for example, during the first discharge after purchasing the battery or during discharge after several charge-discharge cycles since manufacturing the battery). In this case, a proper pressure is constantly applied from the battery can to the electrode group from the outer peripheral side from charge to discharge, suppressing deformation of the electrodes. In this case, the outer diameter D2 of the electrode group is approximately the same dimension as the inner diameter of the battery can.

[0017] In a lithium secondary battery, for example, 70% or more of the rated capacity is exhibited by the deposition and dissolution of lithium metal. The movement of electrons in the negative electrode during charge and discharge is mainly due to the deposition and dissolution of lithium metal in the negative electrode. Specifically, 70 - 100% (for example, 80 - 100% or 90 - 100%) of the movement of electrons (current from another perspective) in the negative electrode during charge and discharge is due to the deposition and dissolution of lithium metal. That is, the negative electrode according to the present disclosure is different from a negative electrode in which the movement of electrons in the negative electrode during charge and discharge is mainly due to the insertion and release of lithium ions by a negative electrode active material (such as graphite).

[0018] In a battery that deposits lithium metal on the negative electrode during charging, the open circuit potential (OCV: Open Circuit Voltage) of the negative electrode at full charge is, for example, 70 mV or less with respect to the lithium metal (lithium dissolution and deposition potential). Full charge means that when the rated capacity of the battery is C, the battery is charged until it reaches a state of charge (SOC: State of Charge) of, for example, 0.98×C or more. The open circuit potential (OCV) of the negative electrode at full charge can be measured by disassembling the battery in a full charge state under an argon atmosphere to take out the negative electrode, and assembling a cell with the lithium metal as the counter electrode. The non-aqueous electrolyte of the cell may have the same composition as the non-aqueous electrolyte in the disassembled battery.

[0019] The positive electrode, negative electrode, and separator are each, for example, in the form of a long sheet. The electrode group with a core member is produced, for example, by winding the positive electrode and the negative electrode around the core member through the separator. For example, the core member may be arranged at a predetermined position in the laminate of the positive electrode, negative electrode, and separator, fixed, and then wound. The fixing method is not particularly limited, and examples include methods using tape, adhesive, thermal welding, ultrasonic welding, etc. In this case, the core member is fixed to the end on the winding start side of the electrode group, and the core member is fixed and arranged in the hollow of the electrode group. In this case, the shape of the entire electrode group is further stabilized, and buckling and bending of the electrodes near the hollow part of the electrode group are significantly suppressed. Also, it is easy for a uniform surface pressure to be applied to the entire electrode group. In a lithium secondary battery, the effect of suppressing the generation of lithium metal dendrites can be stably obtained, and the cycle characteristics are further improved.

[0020] Alternatively, the electrode group with a core member may be produced by winding the positive electrode and the negative electrode around the periphery of a predetermined winding core through the separator, removing the winding core to obtain an electrode group with a hollow, and inserting the core member into the hollow of the electrode group. In this case, the core member is not fixed to the end on the winding start side of the electrode group, but the effect of stabilizing the shape of the electrode group can be obtained by the arrangement of the core member.

[0021] (Core member) Considerable pressure is repeatedly applied to the core member inserted into the hollow of the electrode group during charge and discharge. Therefore, the material of the core member is required to have considerable mechanical strength and durability.

[0022] The Young's modulus of the core material may be, for example, 100 GPa or more, or 150 GPa or more. If the Young's modulus of the core material is 100 GPa or more, the core material will have sufficient strength, and the effect of avoiding electrode buckling will be greatly enhanced.

[0023] The shape of the core member should be selected to match the hollow shape of the electrode group. If the electrode group is cylindrical, the core member may also be cylindrical. The outer diameter of the core member may be such that, for example, it contacts the circumferential surface of the electrode group of the secondary battery in the discharged state after the initial charge and discharge. In this case, a moderate pressure is constantly applied from the core member to the electrode group from the inner circumference side, from charging to discharging, so that electrode buckling is effectively suppressed.

[0024] The core member should preferably be cylindrical with a hollow core. A cylindrical core member can expand and contract moderately and can act like a spring. Furthermore, a cylindrical core member with a hollow core member has sufficient space to accommodate the non-aqueous electrolyte, making it less likely for the electrolyte to run out.

[0025] The tensile strength of the core material may be 200 MPa or more, or 400 MPa or more. Alternatively, the tensile strength of the core material may be 1200 MPa or less, and preferably 850 MPa or less. The elongation of the core material is preferably 30% or less, and more preferably 10% or less. The elongation of the core material may be 1% or more, or 2% or more.

[0026] The tensile strength and elongation of the core material are determined in accordance with JIS Z 2241 (Tensile Test Method for Metallic Materials).

[0027] The core material may be a metallic material, or a metallic material with a plated surface. The metallic material may contain one type of metal, or an alloy containing two or more types of metals. Examples of metallic materials include aluminum, magnesium, titanium, copper, iron, steel, carbon steel, nickel, duralumin, cupronickel, bronze, and brass. Among these, nickel-plated carbon steel and stainless steel are preferred from the viewpoint of strength and durability, and among stainless steels, austenitic stainless steel is particularly preferred. In this case, the radial thickness (sidewall thickness) of the core material may be, for example, 100 μm or more and 500 μm or less.

[0028] The core material may be a resin material. For example, the core material may be made of polyester resin (e.g., polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, etc.), olefin resin (e.g., polyethylene, polypropylene, etc.), polyphenylene sulfide resin, acrylic resin (e.g., polymethyl methacrylate, etc.), polycarbonate resin, polyether ether ketone resin, polyether sulfone resin, polyamide resin (e.g., aramid resin), polyimide resin, etc. These resin materials may be used individually or in combination of two or more. Among these, polypropylene, polyethylene, and polyethylene terephthalate are desirable from the viewpoint of electrolyte resistance, etc. In this case, the radial thickness (side wall thickness) of the core material may be, for example, 0.1 mm or more and 2 mm or less.

[0029] The maximum outer diameter of the core member may be 1.5 mm or more and 7 mm or less, and preferably 1.5 mm or more and 6 mm or less (or 4 mm or less). If the outer shape of the cross-section of the core member is circular, the maximum outer diameter of the core member is the diameter of the circle of that outer shape. If the outer shape of the cross-section of the core member is elliptical, the maximum outer diameter of the core member is the major axis of the ellipse of that outer shape.

[0030] The core member may have a slit formed along its longitudinal direction (winding axis direction). The slit enhances the spring function of the core member. That is, when the electrode group expands, the core member presses against the electrode group from the inside, and the slit narrows, thereby relieving the stress caused by the expansion. The core member may also have through holes in its side walls. The through holes facilitate the flow of the non-aqueous electrolyte through the hollow of the electrode group, thereby enhancing the effect of suppressing liquid depletion.

[0031] (Cushioning Member) The cushioning member is positioned between the electrode group and the core member and must have the function of expanding and contracting in the radial direction of the electrode group to relieve the stress generated inside the electrode group. Therefore, the cushioning member is required to have considerable elasticity.

[0032] The tensile strength Sbu of the cushioning member is usually smaller than the tensile strength Sce of the core member. The ratio of the tensile strength Sbu of the cushioning member to the tensile strength Sc of the core member (Tbu / Tce) is, for example, 1 / 3 or less, and preferably in the range of 1 / 500 to 1 / 5.

[0033] The shape of the buffer member can be selected to match the shape of the hollow electrode group and the core member. The buffer member may be cylindrical (tubular) and surround the core member. The core member may be inserted into the hollow of a cylindrical buffer member to integrate the core member and the buffer member. The core member and the buffer member may be bonded to each other. The core member and the buffer member may be integrally molded using a method such as insert molding.

[0034] The tensile strength of the buffer member is preferably 200 MPa or less, but may be 100 MPa or less. In this case, the buffer member is sufficiently expandable and contractible in the radial direction of the electrode group, ensuring that surface pressure is stably applied to the electrodes throughout the entire electrode group, and preventing sudden surface pressure from being applied to the electrodes throughout the entire electrode group during charging and discharging. The tensile strength of the buffer member may be, for example, 5 MPa or more, 7 MPa or more, or 10 MPa or more.

[0035] The elongation of the cushioning member may be, for example, 5% or more, 10% or more, or 20% or more. An elongation of 20% or more is preferred. The elongation of the cushioning member may be, for example, 1000% or less, or 700% or less.

[0036] The material of the cushioning member may be a metal material or a resin material. The cushioning member material may be used alone or in combination of two or more materials. The metal material and resin material may be any material selected from the materials exemplified as the core member material. However, the material of the cushioning member is preferably more flexible and elastic than the material of the core member.

[0037] Preferred resin materials that can be used as cushioning members include, for example, elastomers with rubber elasticity, fluororesins, and polyolefin resins. As for elastomers, fluororubber and silicone rubber are preferred from the viewpoint of viscoelasticity and durability.

[0038] The cushioning member preferably has a first portion with a strength of 200 MPa or less and an elongation of 20% or more. Such a cushioning member has sufficient elasticity and can significantly exert the effect of expanding and contracting in the radial direction of the electrode group to relieve stress generated inside the electrode group. The first portion preferably occupies 70% or more of the volume of the cushioning member, and may occupy 80% to 100%.

[0039] The first part may be a part of the buffer member, or the entire buffer member may be the first part. When the buffer member can be divided into an outer peripheral portion located on the electrode group side in the radial direction and an inner peripheral portion located on the core member side, it is preferable that at least the outer peripheral portion is the first part. With such a configuration, the tensile strength of the material gradually decreases and the elasticity improves from the core member through the inner peripheral portion to the outer peripheral portion. Such a combination of core member and buffer member has excellent radial flexibility of the electrode group. Therefore, surface pressure is gently applied to the electrodes during charging and discharging, and deformation and fracture of the electrodes are more effectively suppressed.

[0040] From the viewpoint of improving the radial stretchability of the electrode group of the buffer member, the first portion may have voids. For example, the first portion may be made of foamed resin (or porous resin). Foamed resin has excellent radial stretchability of the electrode group.

[0041] The porosity of the first portion is, for example, 20% or more, but may also be 25% or more, or 30% or more. If the porosity of the first portion is too high, the mechanical strength of the cushioning member will gradually decrease, so it is preferable that the porosity of the first portion be 50% or less.

[0042] The porosity of the first portion can be measured by the following method. First, the cross section including the first portion of the buffer material is exposed, and an arbitrary cross section is photographed with a scanning electron microscope (SEM) to obtain a cross-sectional image. Next, each of the obtained cross-sectional images is binarized so that it is divided into a region of voids and a region of non-voids. The area of ​​the region to be binarized is, for example, 0.5 mm². ２ This concludes the explanation. Next, the ratio of the area of ​​the void region to the area of ​​the binarized region is measured as the porosity. The same measurement is performed five times, and the porosity of the first portion is determined by taking the arithmetic mean of the obtained porosities.

[0043] If the buffer member can be divided into an outer circumferential portion located on the electrode group side in the radial direction and an inner circumferential portion located on the core member side, then only the outer circumferential portion may have a gap, while the inner circumferential portion does not. In this case, the outer circumferential portion is the first portion described above, and the inner circumferential portion may be a portion other than the first portion. In such a configuration, the buffer member has excellent elasticity in the radial direction of the electrode group.

[0044] The cushioning member may have a single layer structure, a two-layer structure consisting of an inner circumferential portion and an outer circumferential portion, or a three-layer or more structure with one or more additional layers between the inner circumferential portion and the outer circumferential portion.

[0045] When the buffer member can be divided into an outer circumferential portion located on the electrode group side in the radial direction and an inner circumferential portion located on the core member side, both the outer and inner circumferential portions may have voids. In this case, the outer circumferential portion is the first portion described above, and the inner circumferential portion may be a portion other than the first portion. Furthermore, it is preferable that the void ratio of the outer circumferential portion is higher than that of the inner circumferential portion. Even in such a configuration, the buffer member has excellent elasticity in the radial direction of the electrode group.

[0046] The ratio (T1 / T2) of the radial thickness of the outer circumferential portion (sidewall thickness) T1 to the radial thickness T2 of the inner circumferential portion is, for example, 2 / 4 to 5 / 4, and may also be 3 / 5 to 4 / 4.

[0047] The ratio of the radial thickness Tbu of the buffer member to the radial thickness Tce of the core member (side wall thickness) (Tbu / Tce) is, for example, 2 or more, and may be 8 or more.

[0048] (Negative electrode current collector) The negative electrode current collector may be, for example, copper foil, copper alloy foil, nickel foil, nickel alloy foil, austenitic stainless steel foil, oxygen-free copper foil, etc. When the negative electrode current collector is austenitic stainless steel foil, embrittlement of the negative electrode current collector is suppressed, and the negative electrode current collector has appropriate strength and flexibility and excellent resistance to stress generated at the negative electrode. When copper foil with a low oxygen content is used, embrittlement of the negative electrode current collector is also suppressed. The thickness of the negative electrode current collector is not particularly limited, for example, 5 μm or more and 300 μm or less.

[0049] Austenitic stainless steel is stainless steel having an austenitic content of 50% or more. The austenitic content may be 70% or more, 90% or more, or 100%.

[0050] Austeniticity refers to the proportion (mass ratio) of the austenite phase in stainless steel. When the content of the austenite, ferrite, and martensite phases in stainless steel is x, y, and z, respectively, the austeniticity is calculated as {x / (x + y + z)} × 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).

[0051] 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.

[0052] 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. The austenitic stainless steel is not limited to those exemplified above, and may also be stainless steel with an austenitic content of 50% or more, arbitrarily produced by a melting method. Furthermore, the foil of the austenitic stainless steel may be foil that has been softened by annealing.

[0053] The austenitity can be determined by the following method: Prepare a stainless steel foil sample (for example, size: 25 mm square), and perform X-ray diffraction (XRD) measurement on the sample using a two-dimensional detection function to obtain an XRD pattern (vertical axis: X-ray diffraction intensity, horizontal axis: diffraction angle 2θ). The size of the measurement area (micro-area) is, for example, 15 mm square.

[0054] The following are the preferred XRD measurement conditions. <Analytical Instrument> 2D Micro-area 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) Cometator: Φ300μm ω angle: 25° to 35° (2° / sec) Φ angle: 360° rotation (1° / sec) Measurement time (exposure): 30 minutes

[0055] The obtained XRD pattern is fitted using the least squares method with a standard database, and then quantitative analysis is performed using 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.

[0056] Furthermore, the austenite content can also be estimated using Schaeffler's microstructure diagram, which shows the relationship between ferrite-stabilizing elements and austenite-stabilizing elements and the microstructure. This diagram shows the microstructure ratio with ferrite-stabilizing elements and austenite-stabilizing elements on opposite axes. The vertical axis of this diagram represents Ni equivalent, and the horizontal axis represents Cr equivalent. Cr equivalent is the degree of ferrite-stabilizing elements converted to chromium content, and can be expressed by the formula Cr equivalent = %Cr + %Mo + 1.5 × %Si + 0.5 × %Nb. Ni equivalent is the degree of austenite-stabilizing elements converted to nickel content, and can be expressed by the formula Ni equivalent = %Ni + 30 × %C + 0.5 × %Mn.

[0057] In accordance with JIS G 0321, the system can perform component analysis of stainless steel, including quantitative analysis of austenite-stabilizing elements (Ni, Mn, C, etc.) and ferrite-stabilizing elements (Cr, Mo, Si, Nb).

[0058] Oxygen-free copper foil is copper foil with an oxygen content of 50 ppm or less. The oxygen content may be 30 ppm or less, or 15 ppm or less. Note that the oxygen content refers to the oxygen content in the base material excluding the oxide film covering the surface of the copper foil.

[0059] Oxygen-free copper foil may contain trace amounts of components other than copper (e.g., Ni, Cr, Fe, Zn, Sn, Ag, Pb, Bi, Cd, Hg, O, P, S, Se, Te, H, etc.). The Cu content in the copper foil may be 99.9% by mass or more, or 99.96% by mass or more. The copper foil may be rolled copper foil. Examples of oxygen-free copper include JIS H 3100, alloy number C1020.

[0060] The oxygen content in copper foil can be determined by the following method: Wash the copper foil sample with nitric acid (1+1) for 10 seconds to remove the oxide film on the sample surface. Repeat the above washing until the sample volume is reduced by 10% by mass or more. Next, wash the sample with distilled water, alcohol, and acetone in that order. Then, dry the sample with hot air and immediately perform analysis by inert gas fusion-infrared absorption spectroscopy to determine the oxygen content in the sample. An oxygen-nitrogen simultaneous analyzer (LECO TC-336) can be used as the analyzer.

[0061] (Separator) A porous substrate having ion permeability and insulating properties is used for the separator. The separator may be composed of one substrate or multiple substrates. The substrate may be in the form of a sheet. The separator may be a laminate of multiple sheet substrates, or it may be composed of a laminate of substrates and protrusions (spacers). A space may be formed between the electrode and the substrate by the protrusions (spacers). In a lithium secondary battery, lithium metal deposited during charging may be contained in this space. The protrusions may be arranged, for example, in multiple lines along the longitudinal direction of the separator, in a honeycomb pattern, or dispersed in a dot pattern.

[0062] Examples of substrates include microporous membranes, woven fabrics, nonwoven fabrics, and heat-resistant layers. Resin materials are used for microporous membranes, such as olefin resins, polyamide resins, polyimide resins, polyester resins, and cellulose resins. Examples of olefin resins include polyethylene, polypropylene, and copolymers of ethylene and propylene. Examples of polyester resins include polyethylene terephthalate. Examples of fibrous materials that make up nonwoven fabrics include glass fibers, olefin resins, polyamide resins, polyimide resins, polyester resins, and cellulose resins. The heat-resistant layer is, for example, a mixed layer of inorganic materials (e.g., aluminum oxide, aluminum hydroxide) and resin materials.

[0063] The thickness of the separator is not particularly limited, but is, for example, 10 μm or more, preferably 20 μm or more, and may be 30 μm or more. The thickness of the separator may be, for example, 10 μm or more and 80 μm or less, 20 μm or more and 70 μm or less, or 30 μm or more and 70 μm or less. If the separator is a laminate of multiple substrates, the thickness of the separator is the total thickness of the laminate.

[0064] The separator may include at least one substrate selected from the group consisting of a microporous sheet (a sheet-like microporous membrane) and a nonwoven fabric sheet. The separator may include a microporous sheet and a heat-resistant layer disposed on at least one surface of the microporous sheet. In the electrode group, the heat-resistant layer may be disposed between at least one of the positive electrode and the negative electrode and the microporous sheet. The heat-resistant layer may be disposed to cover all of at least one surface of the microporous sheet, or it may be disposed in a line on at least one surface of the microporous sheet. A space may be provided between at least one of the positive electrode and the negative electrode and the microporous sheet by the line-shaped heat-resistant layer.

[0065] The heat-resistant layer includes, for example, inorganic particles and a resin material supporting the inorganic particles. Examples of resin materials include fluororesins such as polyvinylidene fluoride (PVdF) and polytetrafluoroethylene, fluororubbers such as vinylidene fluoride-tetrafluoroethylene copolymers, and aramid resins. Examples of inorganic particles include insulating metal oxides. Examples of metal oxides include aluminum oxide (alumina, boehmite, gibbsite), magnesium oxide, titanium oxide (titania), zirconium oxide, silicon oxide (silica), magnesium hydroxide, and aluminum hydroxide.

[0066] The average particle size of inorganic particles is not particularly limited, but is preferably 10 μm or less, and more preferably 0.1 μm or more and 2.0 μm or less. The particle size of inorganic particles is determined by taking an electron microscope image of the cross-section of the separator, performing image processing such as binarization to identify the particles, and determining the diameter of the equivalent circle having the same area as the particle. The average particle size is determined, for example, by determining the particle sizes of 100 or more particles and averaging them.

[0067] The heat-resistant layer may be formed, for example, by applying a treatment solution containing a resin material and inorganic particles to the surface of a sheet-like substrate, a positive electrode, or a negative electrode, and then drying it. For example, N-methyl-2-pyrrolidone (NMP) can be used as the solvent or dispersion medium for the treatment solution. The content of inorganic particles in the heat-resistant layer (treatment solution) is, for example, 70 parts by mass or more and 100 parts by mass or less per 100 parts by mass of resin material. In this case, the strength and heat resistance of the heat-resistant layer are easily ensured.

[0068] The separator may comprise a base material and a protrusion (spacer). In this case, a spacer is interposed between at least one of the positive and negative electrodes and the base material. This creates a space between at least one of the positive and negative electrodes and the base material. The formation of this space absorbs the volume increase due to the deposition of lithium metal in the negative electrode, and relieves the stress generated in the negative electrode. In addition, the non-aqueous electrolyte remains in this space, suppressing depletion of the electrolyte and reducing variations in the reaction (uneven reaction) within the electrode group. The protrusion may include, for example, a resin material, or a resin material and inorganic particles. The resin material and inorganic particles can be those exemplified in the heat-resistant layer described above.

[0069] The space in question only needs to exist in the discharge state. Here, the discharge state is the state after a large amount of lithium metal has dissolved from the negative electrode, and may be, for example, a state of charge (SOC) of 0.1 × C or less. However, the space does not need to be completely filled with lithium metal in the charge state; for example, the space may exist even in a fully charged state.

[0070] The spacer is positioned on at least one selected from the group consisting of the surface of the positive electrode, the surface of the negative electrode, and the surface of the substrate. Preferably, the spacer is positioned on the surface of the positive electrode or the positive electrode side surface of the substrate. In this case, surface pressure from the substrate is easily applied to the negative electrode, making it difficult for dendrite-shaped lithium metal to deposit, which is advantageous for improving the capacity retention rate during charge-discharge cycles.

[0071] The height of the spacer can be appropriately designed to match the thickness of the substrate and the distance between the electrode plates. When the separator is equipped with a spacer, the negative electrode has a first region facing the spacer and a second region not facing the spacer. The space between the electrode plates is formed corresponding to the second region. The ratio of the area of ​​the first region to the total area of ​​the first and second regions is not particularly limited, but considering the balance between cycle characteristics and internal resistance, it may be, for example, 5% or more and 30% or less, or 5% or more and 20% or less.

[0072] Embodiments of the battery according to this disclosure will be further described below with reference to the drawings. In each drawing, the shape or features of each component do not necessarily reflect the actual dimensions and are not necessarily shown at the same scale. The same reference numerals are used for the same components in each drawing.

[0073] Figure 1 is a schematic longitudinal cross-sectional view showing a cross-section parallel to the winding axis of a lithium secondary battery according to one embodiment of the present disclosure. Figure 2 is a schematic diagram showing the configuration of a buffer member. Figure 3 is a schematic diagram showing another configuration of the buffer member.

[0074] The battery 10 comprises a cylindrical battery case, a wound electrode group 14 housed within the battery case, and a non-aqueous electrolyte (not shown). The battery case consists of a bottomed cylindrical battery can 15 and a sealing body 16 that seals the opening of the battery can 15. The battery can 15 has an annular stepped portion 21 formed by partially pressing the side wall from the outside near the opening. The sealing body 16 is supported by the opening-side surface of the stepped portion 21. A gasket 27 is placed between the battery can 15 and the sealing body 16, thereby ensuring the airtightness of the battery can. Inside the battery can 15, insulating plates 17 and 18 are placed at both ends of the electrode group 14 in the winding axis direction, respectively.

[0075] The sealing body 16 comprises a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26. The cap 26 is located on the outside of the battery can 15, and the filter 22 is located on the inside of the battery can 15. The lower valve body 23 and the upper valve body 25 are connected to each other at their respective centers, with the insulating member 24 interposed between their respective peripheries. The filter 22 and the lower valve body 23 are connected to each other at their respective peripheries. The upper valve body 25 and the cap 26 are connected to each other at their respective peripheries. A ventilation hole is formed in the lower valve body 23. When the internal pressure of the battery can 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 discharged from the opening formed in the cap 26.

[0076] The electrode group 14 consists of a positive electrode 11, a negative electrode (negative electrode current collector) 12, and a separator 13. The positive electrode 11, the negative electrode 12, and the separator 13 interposed between them are all in the form of long sheets (or strips), and are wound so that their respective width directions are parallel to the winding axis. A core member 29 is positioned in the hollow of the electrode group 14. In the illustrated example, the core member 29 is cylindrical, but its shape is not limited to this, and may be, for example, columnar. A buffer member 30 is positioned between the core member 29 and the electrode group 14. Figure 2 is an enlarged view of the buffer member 30 fixed to the core member 30 in Figure 1, and Figure 3 is an enlarged view of another buffer member 30. In Figure 3, the buffer member 30 has a two-layer structure consisting of an inner circumferential portion 31 and an outer circumferential portion 32. That is, the buffer member can be divided into an outer circumferential portion 32 positioned on the electrode group 14 side in the radial direction, and an inner circumferential portion 31 positioned on the core member 29 side. For example, the outer circumferential portion 32 may have voids, while the inner circumferential portion may not. Alternatively, both the outer circumferential portion 32 and the inner circumferential portion 31 may have voids, and the void ratio of the outer circumferential portion 32 may be higher than that of the inner circumferential portion 31.

[0077] The positive electrode 11 comprises a positive electrode current collector and a positive electrode composite layer. The positive electrode 11 is electrically connected to a cap 26, which also serves as a positive electrode terminal, via a positive electrode lead 19. One end of the positive electrode lead 19 is connected, for example, near the center of the positive electrode 11 in the longitudinal direction. The other end of the positive electrode lead 19, extending from the positive electrode 11, is welded to the inner surface of the filter 22 through a through hole formed in the insulating plate 17.

[0078] The negative electrode 12 is electrically connected to the battery can 15, which also serves as the negative electrode terminal, via a negative electrode lead 20. One end of the negative electrode lead 20 is connected, for example, to the longitudinal end of the negative electrode 12, and the other end is welded to the inner bottom surface of the battery can 15.

[0079] The negative electrode, positive electrode, and non-aqueous electrolyte of a lithium secondary battery will be described in detail below. (Negative Electrode) The negative electrode is equipped with a negative electrode current collector. In a lithium secondary battery, lithium metal is deposited on the surface of the negative electrode during charging. More specifically, lithium ions contained in the non-aqueous electrolyte accept electrons on the negative electrode during charging to become lithium metal, which is deposited on the surface of the negative electrode. The lithium metal deposited on the surface of the negative electrode dissolves into the non-aqueous electrolyte as lithium ions during discharge. The lithium ions contained in the non-aqueous electrolyte may originate from lithium salts added to the non-aqueous electrolyte, or they may be supplied from the positive electrode active material during charging, or both.

[0080] The negative electrode may comprise a negative electrode current collector and a sheet-like lithium metal or lithium alloy in close contact with the surface of the negative electrode current collector. That is, the negative electrode current collector may be provided with a pre-existing underlayer containing lithium metal (a layer of lithium metal or lithium alloy (hereinafter also referred to as the "lithium metal underlayer")). The lithium alloy may contain elements other than lithium, such as aluminum, magnesium, indium, zinc, copper, and silver. By providing a lithium metal underlayer and allowing lithium metal to deposit on it during charging, dendrite-like deposition can be suppressed even more effectively. The thickness of the lithium metal underlayer is not particularly limited, but may be, for example, in the range of 5 μm to 25 μm.

[0081] The negative electrode may include a lithium ion storage layer supported on the negative electrode current collector (a layer that exhibits capacity through the absorption and release of lithium ions by the negative electrode active material (graphite, etc.)). In this case, the open-circuit potential of the negative electrode when fully charged may be 70 mV or less relative to the lithium metal (lithium dissolution and release potential). When the open-circuit potential of the negative electrode when fully charged is 70 mV or less relative to the lithium metal, lithium metal is present on the surface of the lithium ion storage layer when fully charged. That is, the negative electrode exhibits capacity through the deposition and dissolution of lithium metal.

[0082] The lithium-ion storage layer is formed by creating layers of a negative electrode composite material containing a negative electrode active material. In addition to the negative electrode active material, the negative electrode composite material may also contain binders, thickeners, conductive agents, etc.

[0083] Examples of negative electrode active materials include carbonaceous materials, Si-containing materials, Sn-containing materials, etc. The negative electrode may contain one type of negative electrode active material, or a combination of two or more types. Examples of carbonaceous materials include graphite, easily graphitizable carbon (soft carbon), and difficult-to-graphitize carbon (hard carbon).

[0084] Conductive materials include, for example, carbon materials. Examples of carbon materials include carbon black, acetylene black, Ketjenblack, carbon nanotubes, and graphite.

[0085] Examples of binders include fluororesins, polyacrylonitrile, polyimide resins, acrylic resins, polyolefin resins, and rubbery polymers. Examples of fluororesins include polytetrafluoroethylene and polyvinylidene fluoride.

[0086] (Positive electrode) The positive electrode comprises, for example, a positive electrode current collector and a positive electrode composite layer supported by the positive electrode current collector. The positive electrode composite layer includes, for example, a positive electrode active material, a conductive material, and a binder. The positive electrode composite layer may be formed on only one side of the positive electrode current collector or on both sides. The positive electrode can be obtained, for example, by applying a positive electrode composite slurry containing the positive electrode active material, a conductive material, and a binder to both sides of the positive electrode current collector, drying the coating, and then rolling it. For example, known materials can be used as the positive electrode active material, binder, conductive material, etc.

[0087] The positive electrode active material is a material that intercepts and releases lithium ions. Examples of positive electrode active materials include lithium-containing transition metal oxides, transition metal fluorides, polyanions, fluorinated polyanions, and transition metal sulfides. Among these, lithium-containing transition metal oxides are preferred because they have low manufacturing costs and a high average discharge voltage.

[0088] Lithium-containing transition metal oxides are composite oxides that contain lithium and a metal other than lithium, Me, wherein the metal Me contains at least one transition metal. Among lithium-containing transition metal oxides, composite oxides having a layered rock salt type (layered rock salt type) crystal structure are preferred for obtaining high capacity.

[0089] Metallic Me may contain transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, and W. Lithium-containing transition metal oxides may contain one transition metal element or two or more. It is desirable that metallic Me contains at least one transition metal element selected from the group consisting of Co, Ni, and Mn, and it is desirable that it contains at least Ni as a transition metal.

[0090] Lithium-containing transition metal oxides may contain one or more main group elements as needed. Examples of main group elements include Mg, Al, Ca, Zn, Ga, Ge, Sn, Sb, Pb, and Bi. The main group element may also be Al. That is, metallic Me may contain Al as an optional component.

[0091] Lithium-containing transition metal oxides include, for example, those with the general formula (1): Li ａ Ni ｂ M １－ｂ O ２ It is represented by the following: In general formula (1), 0.9 ≤ a ≤ 1.2 and 0.65 ≤ b ≤ 1 are satisfied, and M is at least one element selected from the group consisting of Co, Mn, Al, Ti, Fe, Nb, B, Mg, Ca, Sr, Zr, and W.

[0092] The molar ratio of the total amount of Li contained in the positive and negative electrodes, mLi / mMe, to the amount of metallic Me (mMe) contained in the lithium-containing transition metal oxide is, for example, 1.2 or less, and may also be 1.1 or less.

[0093] Examples of materials for the positive electrode current collector include metallic materials containing Al, Ti, Fe, etc. The metallic material may also be Al, Al alloy, Ti, Ti alloy, Fe alloy (stainless steel (SUS), etc.).

[0094] The thickness of the positive electrode current collector is not particularly limited, but is, for example, 5 μm or more and 300 μm or less.

[0095] (Non-aqueous electrolyte) A non-aqueous electrolyte having lithium ion conductivity contains, for example, a non-aqueous solvent, lithium ions dissolved in the non-aqueous solvent, and anions. The non-aqueous electrolyte may be liquid or gel-like. The liquid non-aqueous electrolyte is 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.

[0096] As the lithium salt or anion, known materials used in the non-aqueous electrolyte of a lithium secondary battery can be used. Specifically, BF ４ － , ClO ４ － , PF ６ － , CF ３ SO ３ － , CF ３ CO ２ － , anions of imides, anions of oxalate complexes, etc. are mentioned. As the anions of imides, N(SO ２ CF ３ ), N(C ２ － ), N(C ｍ F ２ｍ＋１ SO ２ ), N(C ｘ ), N(C ｎ F ２ｎ＋１ SO ２ ), N(C ｙ － (m and n are each independently an integer of 0 or 1 or more, x and y are each independently 0, 1 or 2, and x + y = 2 is satisfied.) etc. are mentioned. The anion of the oxalate complex may contain boron and / or phosphorus. The non-aqueous electrolyte may contain these anions alone or in combination of two or more.

[0097] From the viewpoint of suppressing the dendritic deposition of lithium metal, the non-aqueous electrolyte preferably contains at least an oxalate complex anion, and more preferably an oxalate complex anion containing fluorine. The interaction between the fluorine-containing oxalate complex anion and lithium makes it easier for lithium metal to precipitate uniformly in fine particulate form. Therefore, it is easier to suppress localized deposition of lithium metal. The fluorine-containing oxalate complex anion may be combined with other anions. Other anions include PF ６ － and / or imide anions. Examples of oxalate complex anions include bisoxalate borate anions and difluorooxalate borate anions (BF ２ (C ２ O ４ ) － ), PF ４ (C ２ O ４ ) － , PF ２ (C ２ O ４ ) ２ － Examples include the above, and it is desirable to use at least a difluorooxalate borate anion.

[0098] Examples of non-aqueous solvents include esters, ethers, nitriles, amides, or halogen-substituted compounds thereof. The non-aqueous electrolyte may contain one of these non-aqueous solvents or two or more of them. Examples of halogen-substituted compounds include fluorides.

[0099] Examples of esters include carbonate esters and carboxylic acid esters. Examples of cyclic carbonate esters include ethylene carbonate, propylene carbonate, and fluoroethylene carbonate (FEC). Examples of linear carbonate esters include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate. Examples of cyclic carboxylic acid esters include γ-butyrolactone and γ-valerolactone. Examples of linear carboxylic acid esters include ethyl acetate, methyl propionate, and methyl fluoropropionate.

[0100] Examples of ethers include cyclic ethers and linear ethers. 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.

[0101] The concentration of lithium salt in the non-aqueous electrolyte is, for example, 0.5 mol / L or more and 3.5 mol / L or less. The concentration of anion in the non-aqueous electrolyte may be 0.5 mol / L or more and 3.5 mol / L or less. The concentration of anion of the oxalate complex in the non-aqueous electrolyte may be 0.05 mol / L or more and 1 mol / L or less.

[0102] (Note) The above description discloses the following technologies: (Technology 1) A secondary battery comprising: a wound electrode group having a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode; a non-aqueous electrolyte; a core member disposed in the hollow of the electrode group; and a buffer member disposed between the electrode group and the core member, wherein the buffer member is expandable and contractible in the radial direction of the electrode group. (Technology 2) The secondary battery according to Technology 1, wherein the buffer member has a first portion having a tensile strength of 200 MPa or less and an elongation of 20% or more. (Technology 3) The secondary battery according to Technology 2, wherein the first portion has voids. (Technology 4) The secondary battery according to Technology 3, wherein the void ratio of the first portion is 20% or more. (Technology 5) The buffer member has an outer peripheral portion arranged on the electrode group side in the radial direction and an inner peripheral portion arranged on the core member side, the outer peripheral portion has air gaps, and the inner peripheral portion does not have air gaps, as described in Technology 1 or 2. (Technology 6) The buffer member has an outer peripheral portion arranged on the electrode group side in the radial direction and an inner peripheral portion arranged on the core member side, the outer peripheral portion and the inner peripheral portion have air gaps, and the porosity of the outer peripheral portion is higher than the porosity of the inner peripheral portion, as described in Technology 1 or 2. (Technology 7) The ratio of the thickness Tbu of the buffer member in the radial direction to the thickness Tce of the core member in the radial direction (Tbu / Tce) is 2 or more, as described in any one of Technology 1 to 6. (Technology 8) The thickness of the separator is 30 μm or more, as described in any one of Technology 1 to 7. (Technology 9) A secondary battery according to any one of Technologies 1 to 8, wherein lithium metal is deposited in the negative electrode by charging, and the lithium metal dissolves in the non-aqueous electrolyte by discharging.

[0103] [Examples] The lithium secondary battery according to the present disclosure will be described in more detail below based on examples and comparative examples. However, the present disclosure is not limited to the following examples.

[0104] Examples 1-13, Comparative Examples 1-3 (Preparation of the negative electrode) A strip-shaped negative electrode current collector (10 μm thick) was prepared. In a dry atmosphere with a dew point of -30 degrees Celsius or lower, lithium metal foil (30 μm thick) was pressed onto both sides of the negative electrode current collector to form a lithium metal layer (underlayment). The negative electrode 12 was thus manufactured. Ni negative electrode leads 20 were attached to predetermined positions on the negative electrode 12. The lithium metal layer had a thickness of 55 μm during charging and a thickness of 30 μm during discharging. That is, the thickness of the lithium metal deposited on the negative electrode during charging was 25 μm.

[0105] The negative electrode current collector used austenitic stainless steel foil, oxygen-free copper foil, or electrolytic copper foil. SUS304 foil was used for the austenitic stainless steel foil.

[0106] (Preparation of the positive electrode) 95 parts by mass of positive electrode active material were mixed with 2.5 parts by mass of acetylene black and 2.5 parts by mass of polyvinylidene fluoride, and then an appropriate amount of N-methyl-2-pyrrolidone was added and stirred to prepare a positive electrode mixture slurry. The positive electrode active material used was a rock salt type lithium-containing transition metal oxide having a layered structure and containing Li, Ni, Co, and Al (the molar ratio of Li to the total of Ni, Co, and Al is 1.0).

[0107] A positive electrode composite slurry was applied to both sides of a strip-shaped Al foil (positive electrode current collector), dried, and the coating was rolled to obtain a laminate in which positive electrode composite layers were formed on both sides of the positive electrode current collector. The laminate was cut to a predetermined electrode size to obtain a strip-shaped positive electrode 11. An Al positive electrode lead 19 was attached to a predetermined position on the positive electrode 11. The amount of positive electrode composite layer (positive electrode active material) was appropriately adjusted so that the thickness of the lithium metal deposited on the negative electrode current collector during charging was 25 μm.

[0108] (Preparation of separators) Microporous polyethylene membranes of the thicknesses listed in Tables 1 and 2 were prepared as separators.

[0109] (Preparation of core member) A core member made of SUS304 (side wall thickness 0.2 mm) was prepared. The core member was cylindrical in shape. The outer diameter D1 of the core member was 2 mm. The outer diameter D2 of the electrode group was equivalent to the inner diameter of the battery can (9 mm). The outer surface of the electrode group and the inner surface of the battery can were adjusted to make contact during the initial discharge.

[0110] (Cushioning Member) A core member was inserted into the hollow of a cylindrical (tubular) cushioning member with the configuration described in Tables 1 and 2, integrating the core member and the cushioning member. The inner diameter of the cylindrical cushioning member was set to 2 mm, and the outer surface of the core member was brought into close contact with the inner surface of the cushioning member.

[0111] (Fabrication of electrode group with core member) The ends of a pair of separators were fixed with tape to a buffer member fixed to the core member, and the negative electrode was sandwiched between the pair of separators. The positive and negative electrodes were wound around the separator to form an electrode group. The positive electrode lead and the negative electrode lead were exposed from one end face of the columnar wound electrode group.

[0112] (Preparation of non-aqueous electrolyte) LiPF is added to a mixed solvent of 1,2-dimethoxyethane and 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether. ６ and LiBF ２ (C ２ O ４ Non-aqueous electrolytes were prepared by dissolving each of the following: LiPF in the non-aqueous electrolyte. ６ The concentration was set to 1 mol / L. LiBF in non-aqueous electrolytes ２ (C ２ O ４ The concentration of ) was set to 0.1 mol / L.

[0113] (Battery Assembly) The electrode group was inserted into a bottomed cylindrical battery case, a non-aqueous electrolyte was injected, and the opening of the battery case was sealed with a sealing body. At this time, the positive electrode lead was connected to the sealing body and the negative electrode lead was connected to the battery case. A gasket was placed between the sealing body and the battery case. In this way, the lithium secondary battery was completed. In Tables 1 and 2, A1 to A13 are lithium secondary batteries of Examples 1 to 13, respectively. In Tables 1 and 2, C1 to C3 are batteries of Comparative Examples 1 to 3.

[0114]

[0115]

[0116] [Evaluation] Each obtained battery underwent a charge-discharge cycle test at 25°C. The charge-discharge cycle was performed under the following conditions. A 20-minute pause was taken between charge and discharge cycles.

[0117] (Charging) Constant current charging was performed at 10mA until the battery voltage reached 4.1V, and then constant voltage charging was performed at 4.1V until the current value reached 1mA.

[0118] (Discharge) Constant current discharge was performed at 10mA until the battery voltage reached 3.0V.

[0119] The charge-discharge cycle was repeated up to 100 times, and the cycle capacity retention rate (%) was calculated as the ratio of the discharge capacity C2 at cycle 100 to the discharge capacity C1 at cycle 1: (C2 / C1) × 100.

[0120] The evaluation results are shown in Tables 1 and 2. The cycle capacity retention rates in Tables 1 and 2 are expressed as relative values ​​(exponents) with the cycle capacity retention rate of battery A1 in Example 1 set to 100.

[0121] Tables 1 and 2 show that batteries A1 to A13 exhibit superior cycle characteristics compared to batteries B1 to B5. In particular, good results are obtained when the thickness ratio (Tbu / Tce) is 3.0 or higher, or when the void ratio between the inner and outer circumferences is varied.

[0122] The secondary battery of this disclosure can be used in electronic devices such as mobile phones, smartphones, and tablet devices, as well as electric vehicles, hybrid vehicles, plug-in hybrid vehicles, and home energy storage systems.

[0123] 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.

[0124] 10 Lithium secondary battery 11 Positive electrode 12 Negative electrode 13 Separator 13A First substrate 13B Second substrate 13C Linear protrusion 14 Electrode group 15 Battery can 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 28 Space 29 Core member 100, 600 Positive electrode composite 200, 400 Negative electrode composite 300, 500, 700 Positive and negative electrode laminate

Claims

1. A secondary battery comprising: a wound electrode group having a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode; a non-aqueous electrolyte; a core member disposed in the hollow of the electrode group; and a buffer member disposed between the electrode group and the core member, wherein the buffer member is expandable and contractible in the radial direction of the electrode group.

2. The secondary battery according to claim 1, wherein the buffer member has a tensile strength of 200 MPa or less and a first portion with an elongation of 20% or more.

3. The secondary battery according to claim 2, wherein the first part has a void.

4. The secondary battery according to claim 3, wherein the void ratio of the first portion is 20% or more.

5. The buffer member has an outer peripheral portion arranged on the electrode group side in the radial direction and an inner peripheral portion arranged on the core member side, wherein the outer peripheral portion has a gap and the inner peripheral portion does not have a gap, as described in claim 1 or 2.

6. The buffer member has an outer peripheral portion disposed on the electrode group side in the radial direction and an inner peripheral portion disposed on the core member side, the outer peripheral portion and the inner peripheral portion have air gaps, and the porosity of the outer peripheral portion is higher than the porosity of the inner peripheral portion, the secondary battery according to claim 1 or 2.

7. The secondary battery according to claim 1 or 2, wherein the ratio of the thickness Tbu of the buffer member in the radial direction to the thickness Tce of the core member in the radial direction (Tbu / Tce) is 2 or more.

8. The secondary battery according to claim 1 or 2, wherein the thickness of the separator is 30 μm or more.

9. The secondary battery according to claim 1 or 2, wherein lithium metal is deposited in the negative electrode by charging, and the lithium metal dissolves in the non-aqueous electrolyte by discharging.

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