Lithium-ion battery

Abstract

The present invention discloses a lithium secondary battery (10) which is provided with: a positive electrode (11) that contains a positive electrode active material which is capable of absorbing and desorbing lithium ions; a negative electrode (12) that is provided with a negative electrode collector; a separator (13) that is arranged between the positive electrode (11) and the negative electrode (12); and a nonaqueous electrolyte that has lithium ion conductivity. At the negative electrode, lithium metal is precipitated during charging, while lithium metal is dissolved during discharging. The oxygen content in the negative electrode collector is 50 ppm or less; and the negative electrode collector has a break strength of 300 MPa or less and an elongation at break of 4% or more.

Description

[Technical Field]

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

[0002] Non-aqueous electrolyte secondary batteries are used in applications such as ICT (Information and Communication Technology) for personal computers and smartphones, automotive applications, and energy storage. In these applications, there is a demand for even higher capacity in non-aqueous electrolyte secondary batteries. Lithium-ion batteries are known as high-capacity non-aqueous electrolyte secondary batteries. Higher capacity lithium-ion batteries can be achieved by using a combination of alloy active materials, such as graphite and silicon compounds, as the negative electrode active material. However, the capacity of lithium-ion batteries is reaching its limits.

[0003] As a high-capacity non-aqueous electrolyte secondary battery that surpasses lithium-ion batteries, lithium secondary batteries (lithium metal secondary batteries) are promising. In lithium secondary batteries, lithium metal is deposited on the negative electrode during charging, and this lithium metal dissolves into the non-aqueous electrolyte during discharge.

[0004] Patent Document 1 describes a non-aqueous electrolyte secondary battery comprising a positive electrode having a positive electrode active material made of a lithium-containing transition metal oxide, a negative electrode having a negative electrode current collector on which lithium metal is deposited during charging, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, wherein the molar ratio of the total amount of lithium in the positive electrode and the negative electrode to the amount of transition metal contained in the positive electrode is 1.1 or less, and in the discharge state, there is a space layer between the negative electrode and the separator, and the positive electrode capacity per unit area of ​​the positive electrode is α (mAh / cm²). 2 A non-aqueous electrolyte secondary battery has been proposed in which the mean value and the average thickness X (μm) of the space layer satisfy 0.05 ≤ α / X ≤ 0.2. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] International Publication No. 2019 / 087709 Brochure [Overview of the project] [Problems that the invention aims to solve]

[0006] During charging and discharging, the negative electrode current collector may break, leading to a decrease in cycle performance. [Means for solving the problem]

[0007] One aspect of this disclosure relates to a lithium secondary battery comprising a positive electrode containing a positive electrode active material capable of intercalating and releasing lithium ions, a negative electrode comprising a negative electrode current collector, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte having lithium ion conductivity, wherein in the negative electrode, lithium metal is deposited during charging and the lithium metal dissolves during discharge, the oxygen content in the negative electrode current collector is 50 ppm or less, and the negative electrode current collector has a breaking strength of 300 MPa or less and a breaking elongation of 4% or more. [Effects of the Invention]

[0008] According to this disclosure, it is possible to suppress the deterioration of the cycle characteristics of lithium secondary batteries. While novel features of the present invention are described in the appended claims, the present invention, both in terms of its structure and content, will be better understood by the following detailed description in conjunction with the drawings, in conjunction with other objects and features of the present invention. [Brief explanation of the drawing]

[0009] [Figure 1] This is a schematic longitudinal cross-sectional view showing a lithium secondary battery according to one embodiment of the present disclosure. [Figure 2] This is an enlarged cross-sectional view of region II in Figure 1. [Figure 3] This is an enlarged cross-sectional view of region III in Figure 1. [Modes for carrying out the invention]

[0010] Embodiments of this disclosure relate to a lithium secondary battery (lithium metal secondary battery) that uses lithium metal as the negative electrode active material. Specifically, the lithium secondary battery according to the embodiment of this disclosure comprises a positive electrode containing a positive electrode active material capable of intercalating and releasing lithium ions, a negative electrode equipped with a negative electrode current collector, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte having lithium ion conductivity. In the negative electrode, lithium metal is deposited during charging and dissolves during discharge.

[0011] In lithium secondary batteries, for example, 70% or more of the rated capacity is due to 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, 70-100% (e.g., 80-100% or 90-100%) of the movement of electrons (or current, in other words) in the negative electrode during charging and discharging is due to the deposition and dissolution of lithium metal. In other words, the negative electrode of the lithium secondary battery according to this disclosure differs from a negative electrode where the movement of electrons in the negative electrode during charging and discharging is mainly due to the intercalation and release of lithium ions by the negative electrode active material (such as graphite). For example, the negative electrode of the lithium secondary battery according to this disclosure does not need to contain a negative electrode active material (such as graphite) that intercalates and releases lithium ions.

[0012] In batteries that deposit lithium metal at the negative electrode during charging, the open-circuit voltage (OCV) of the negative electrode at full charge is, for example, 70mV or less relative to the lithium metal (lithium dissolution potential). Full charge refers to the state of charge (SOC) of the battery, where the rated capacity of the battery is C, for example, 0.98 × C or higher. The open-circuit voltage (OCV) of the negative electrode at full charge can be measured by disassembling the fully charged battery under an argon atmosphere, removing the negative electrode, and assembling a cell with 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.

[0013] In a lithium secondary battery, during charging, lithium metal is deposited on the negative electrode, so the amount of expansion of the negative electrode tends to increase. Here, "expansion of the negative electrode" means that the total volume of the volume of the negative electrode and the volume of the deposited lithium metal increases. In particular, when lithium metal is deposited in a dendrite shape, the amount of expansion becomes even larger. As a result, stress is likely to occur in the negative electrode.

[0014] The inventors of the present invention have intensively studied the factors causing the fracture of the negative electrode current collector during charge and discharge. As a result, it has been newly found that the negative electrode current collector becomes brittle with repeated charge and discharge, and the brittleness of the negative electrode current collector together with the stress generated in the negative electrode is the factor causing the fracture of the negative electrode current collector. From the SEM observation of the fracture part of the negative electrode current collector, the following new findings were obtained. In the fracture part of the negative electrode current collector, almost no deformation in the ductile direction of the crystal grains when the negative electrode current collector extends and breaks, and no tapered deformation of the negative electrode current collector was observed, and the fracture part showed brittle fracture. A large number of grain boundary cracks were observed along the grain boundaries on the surface of the negative electrode current collector. It is considered that brittle fracture occurs based on the grain boundary cracks.

[0015] Furthermore, it was suggested that some substance generated in the negative electrode affects the embrittlement (grain boundary cracking) of the negative electrode current collector. Therefore, the inventors of the present invention focused on the amount of oxygen in the negative electrode current collector (for example, copper foil) and intensively studied it, and newly found that the embrittlement of the negative electrode current collector can be suppressed when the amount of oxygen is small.

[0016] In the lithium secondary battery according to the present disclosure, (i) the oxygen content in the negative electrode current collector is 50 ppm or less, (ii) the negative electrode current collector has a breaking strength (tensile strength) of 300 MPa or less, and (iii) the negative electrode current collector has an elongation at break of 4% or more. When the above (i) is satisfied, the embrittlement of the negative electrode current collector is suppressed. When the above (i), (ii), and (iii) are satisfied, the negative electrode current collector has appropriate strength and flexibility, and a negative electrode current collector excellent in resistance to stress generated in the negative electrode can be obtained. As a result, the occurrence of fracture of the negative electrode current collector during charge and discharge and the accompanying deterioration of cycle characteristics are suppressed.

[0017] If the oxygen content in the negative electrode current collector is greater than 50 ppm, the negative electrode current collector tends to become brittle. If the tensile strength of the negative electrode current collector is greater than 300 MPa, the positive electrode is more likely to buckle, which can result in it piercing the separator and causing a short circuit. If the elongation at break of the negative electrode current collector is less than 4%, it may not be able to withstand the stress caused by the expansion of the dendrites, and the negative electrode current collector may fracture due to ductile failure.

[0018] In the lithium secondary battery relating to this disclosure, the "oxygen content in the negative electrode current collector" refers to the oxygen content in the base material excluding the oxide film covering the surface of the negative electrode current collector. Furthermore, if the negative electrode current collector is rolled foil, the "breaking strength" and "breaking elongation" refer to the breaking strength and breaking elongation in the rolling direction, respectively.

[0019] The oxygen content in the negative electrode current collector is 50 ppm or less, may be 30 ppm or less, or may be 15 ppm or less. Note that the oxygen content (ppm) in the negative electrode current collector is expressed as a mass ratio.

[0020] The breaking strength of the negative electrode current collector is 300 MPa or less, but may be 250 MPa or less, or 200 MPa or less. From the viewpoint of improving the reliability of battery manufacturing, the breaking strength of the negative electrode current collector may be 50 MPa or more. If the breaking strength of the negative electrode current collector is less than 50 MPa, it is prone to breaking during roll transport, and handling will be significantly reduced. The range of the breaking strength may be any combination of the above upper and lower limits.

[0021] The break elongation of the negative electrode current collector is 4% or more, may be 5% or more, or may be 10% or more. From the viewpoint of improving the reliability of battery manufacturing, the break elongation of the negative electrode current collector may be 50% or less. If the break elongation of the negative electrode current collector exceeds 50%, it is prone to deformation during roll transport, and handling is significantly reduced. The range of the break elongation may be any combination of the above upper and lower limits.

[0022] The oxygen content in the negative electrode current collector can be determined by the following method. The initial battery is disassembled and the negative electrode current collector is removed to obtain the sample. The sample may be the negative electrode current collector used in the battery manufacturing process. The sample is washed with nitric acid (1+1) for 10 seconds to remove the oxide film on the sample surface. This washing is repeated until the sample volume is reduced by 10% by mass or more. Next, the sample is washed with distilled water, alcohol, and acetone in that order. Then, the sample is dried with hot air and immediately analyzed 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.

[0023] The breaking strength (tensile strength) and breaking elongation of the negative electrode current collector are determined in accordance with JIS Z 2241 (Tensile Testing Method for Metallic Materials). However, since the negative electrode current collector is a low-strength, thin foil, measuring the breaking elongation requires skilled techniques, and it is desirable to have this measurement performed by an experienced institution.

[0024] From the viewpoint of improving cycle characteristics, the ratio of the separator thickness Y to the negative electrode current collector thickness X, Y / X, may be 2.5 or greater, 3 or greater, or 4 or greater. From the viewpoint of battery capacity, Y / X is, for example, 5 or less. The range of Y / X may be any combination of the above upper and lower limits.

[0025] Note that the separator thickness Y refers to the thickness of the separator before the electrode group is assembled (before the electrode group is housed in the battery case). If the separator is composed of multiple thin films layered together, Y is the sum of the thicknesses of those multiple materials. If the separator has different thicknesses in different locations, Y is the maximum thickness. If the separator has multiple regions with different thicknesses, the region where the separator has thickness Y (maximum value) will, for example, cover 20% to 80% of the total area of ​​the separator facing the negative electrode. The thickness X of the negative electrode current collector is, for example, 5 μm or more and 30 μm or less.

[0026] For a negative electrode current collector that satisfies conditions (i) to (iii) above, if Y / X is 2.5 or higher, the cycle characteristics are significantly improved. When Y / X is 2.5 or higher (when the separator has a thickness of 2.5 times or more the thickness of the negative electrode current collector), the separator buffers the expansion of the negative electrode during charging (dendritic expansion), and the stress generated in the negative electrode (negative electrode current collector) is relieved. The combined effect of suppressing embrittlement of the negative electrode current collector and improving resistance to stress generated in the negative electrode when the negative electrode current collector satisfies conditions (i) to (iii) above, and the stress relief effect in the negative electrode (negative electrode current collector) when Y / X is 2.5 or higher, results in a remarkable improvement in cycle characteristics. If the negative electrode current collector does not satisfy conditions (i) to (iii) above, the impact of fracture in the negative electrode current collector becomes greater, and the effect of stress relief in the negative electrode when Y / X is 2.5 or higher is less likely to be achieved.

[0027] The thickness X of the negative electrode current collector can be determined by measuring the thickness of 10 arbitrary points on the negative electrode current collector using a scanning electron microscope (SEM) and calculating the average value. The thickness Y of the separator can be determined in the same way. If the separator has multiple regions with different thicknesses, the thickness can be measured at 10 arbitrary points in the region with the maximum thickness and the average value can be calculated.

[0028] (Negative electrode) The negative electrode is equipped with a negative electrode current collector. In lithium secondary batteries, 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 current collector during charging, becoming lithium metal, and depositing on the surface of the negative electrode current collector. The lithium metal deposited on the surface of the negative electrode current collector dissolves as lithium ions in the non-aqueous electrolyte 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.

[0029] The negative electrode current collector is usually a foil (sheet) containing a metal (or alloy) material. From the viewpoint of having high conductivity, easily ensuring high capacity and high charge / discharge efficiency, and easily obtaining the above-mentioned negative electrode current collector, rolled copper foil may be used as the negative electrode current collector, or rolled copper foil that has been heat-treated in an inert atmosphere may be used. The fracture strength, fracture elongation, and grain size can be adjusted by heat treatment. Generally, heat treatment causes recrystallization, resulting in coarser grain size, decreased strength, and an improved fracture elongation. The rolled 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 rolled copper foil may be 99.9% by mass or more, or 99.96% by mass or more. Examples of materials used for rolled copper foil include oxygen-free copper (JIS H 3100, alloy number C1020).

[0030] From the viewpoint of compensating for the irreversible capacity of the positive electrode, a lithium metal sheet may be pre-placed on the surface of the negative electrode current collector before the initial charge. The lithium metal sheet is formed, for example, by attaching lithium metal to the surface of the negative electrode current collector and then electrodepositing or vapor-depositing it. Alternatively, a negative electrode composite layer may be formed on the surface of the negative electrode current collector. In this case, the negative electrode composite layer is formed thinly enough so that lithium metal can be deposited on the negative electrode during charging. The negative electrode composite layer is formed by applying a negative electrode composite slurry containing a negative electrode active material such as graphite to the surface of the negative electrode current collector. The thickness of the lithium metal sheet (or negative electrode composite layer) is not particularly limited, and is, for example, 3 to 300 μm. The lithium metal sheet (or negative electrode composite layer) may be formed on one surface of the negative electrode current collector, or on both sides of the negative electrode current collector.

[0031] The surface of the negative electrode current collector may be smooth. This makes it easier for lithium metal originating from the positive electrode to deposit evenly on the negative electrode current collector during charging. Smoothness means that the maximum height roughness Rz of the negative electrode current collector is 20 μm or less. The maximum height roughness Rz of the negative electrode current collector may be 10 μm or less. The maximum height roughness Rz is measured in accordance with JIS B 0601:2013.

[0032] (positive electrode) The positive electrode comprises, for example, a positive electrode current collector and a positive electrode composite layer supported on 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.

[0033] 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.

[0034] 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 one or more transition metal elements. The transition metal elements may be Co, Ni, and / or Mn. Lithium-containing transition metal oxides may optionally contain one or more main group elements. Examples of main group elements include Mg, Al, Ca, Zn, Ga, Ge, Sn, Sb, Pb, and Bi. The main group elements may also be Al, etc.

[0035] Conductive materials include, for example, carbon materials. Examples of carbon materials include carbon black, acetylene black, Ketjenblack, carbon nanotubes, and graphite.

[0036] Examples of binders include fluororesins, polyacrylonitrile, polyimide resins, acrylic resins, polyolefin resins, and rubbery polymers. Examples of fluororesins include polytetrafluoroethylene and polyvinylidene fluoride.

[0037] Foil, film, or the like can be used for the positive electrode current collector. The surface of the positive electrode current collector may be coated with a carbon material. Examples of materials for the positive electrode current collector include metallic materials containing Al, Ti, Fe, etc. The metallic material may be Al, Al alloy, Ti, Ti alloy, Fe alloy, etc. The Fe alloy may be stainless steel (SUS). The thickness of the positive electrode current collector is not particularly limited, for example, 5 μm or more and 30 μm or less.

[0038] (Separator) A porous sheet having ion permeability and insulating properties is used as the separator. Examples of porous sheets include thin films, woven fabrics, and nonwoven fabrics with microporous properties. The material of the separator is not particularly limited, but it may be a polymer material. Examples of polymer materials include olefin resins, polyamide resins, and cellulose. Examples of olefin resins include polyethylene, polypropylene, and copolymers of ethylene and propylene. The separator may contain additives as needed. Examples of additives include inorganic fillers. The separator may be composed of multiple layers with different forms and / or compositions.

[0039] (Non-aqueous electrolytes) A non-aqueous electrolyte having lithium ion conductivity includes, for example, a non-aqueous solvent and a lithium salt that dissolves in the non-aqueous solvent. The non-aqueous electrolyte may be in liquid or gel form. A 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.

[0040] The gel-like non-aqueous electrolyte comprises a lithium salt and a matrix polymer, or a lithium salt, a non-aqueous solvent, and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs the non-aqueous solvent and gels is used. Examples of polymer materials include fluororesins, acrylic resins, and polyether resins.

[0041] As the lithium salt or anion, known ones used in the non-aqueous electrolyte of a lithium secondary battery can be used. Specifically, BF4 - , ClO4 - , PF6 - , CF3SO3 - , CF3CO2 - , anions of imides, anions of oxalate complexes, etc. may be mentioned. As the anion of imides, N(SO2CF3)2 - , N(C m F 2m+1 SO2) x (C n F 2n+1 SO2) 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. may be mentioned. The anion of the oxalate complex may contain boron and / or phosphorus. As the anion of the oxalate complex, bisoxalate borate anion, BF2(C2O4) - , PF4(C2O4) - , PF2(C2O4)2 - etc. may be mentioned. The non-aqueous electrolyte may contain these anions alone or two or more of them.

[0042] From the viewpoint of suppressing the dendritic precipitation of lithium metal, it is preferable that the non-aqueous electrolyte contains at least an anion of an oxalate complex. Due to the interaction between the anion of the oxalate complex and lithium, lithium metal is likely to precipitate uniformly in fine particle form. Therefore, it becomes easy to suppress the local precipitation of lithium metal. The anion of the oxalate complex may be combined with other anions. The other anion may be PF6 - and / or an anion of imides.

[0043] Examples of non-aqueous solvents include ester compounds, ether compounds, nitrile compounds, and amide compounds. These compounds include halogen-substituted compounds. Examples of halogen-substituted compounds include fluorides. The non-aqueous electrolyte may contain one of these non-aqueous solvents or two or more of them.

[0044] In particular, from the viewpoint of suppressing embrittlement of the negative electrode current collector, the non-aqueous solvent may contain an ether compound as its main component. Here, "main component" means that the ether compound content in the non-aqueous solvent is 50% by mass or more, and may be 80% by mass or more. Furthermore, the ether compound content in the non-aqueous solvent may be 95% by mass or less, or 100% by mass or less. The range of the ether compound content in the non-aqueous solvent may be any combination of the above upper and lower limits.

[0045] Ether compounds exhibit excellent stability (especially resistance to reduction), suppressing the formation of decomposition products on the surface of the negative electrode current collector, and are presumed to have little impact on the negative electrode current collector. When a non-aqueous electrolyte mainly composed of ether compounds is used for a negative electrode current collector that satisfies the above conditions (i) to (iii), a significant improvement in cycle characteristics due to the suppression of embrittlement of the negative electrode current collector can be obtained. If the negative electrode current collector does not satisfy the above conditions (i) to (iii), the impact of fracture occurrence in the negative electrode current collector becomes greater, and the effect of ether compounds is less likely to be exhibited.

[0046] Examples of ether compounds 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, diethylene glycol dimethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether. From the viewpoint of suppressing embrittlement of the negative electrode current collector, 1,2-dimethoxyethane and 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether are particularly preferred.

[0047] Examples of ester compounds include carbonate esters and carboxylic acid esters. Examples of cyclic carbonate esters include ethylene carbonate and propylene carbonate. 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.

[0048] The concentration of lithium salt in the non-aqueous electrolyte is, for example, between 0.5 mol / L and 3.5 mol / L. The concentration of anion in the non-aqueous electrolyte may also be between 0.5 mol / L and 3.5 mol / L. Furthermore, the concentration of anion of the oxalate complex in the non-aqueous electrolyte may be between 0.05 mol / L and 1 mol / L.

[0049] The non-aqueous electrolyte may contain additives. The additives may form a film on the negative electrode. The formation of a film derived from the additive on the negative electrode makes it easier to suppress dendrite formation. Examples of such additives include vinylene carbonate, fluoroethylene carbonate (FEC), vinyl ethyl carbonate (VEC), and the like.

[0050] (Lithium-ion secondary battery) The configuration of the lithium secondary battery relating to this disclosure will be described below with reference to the drawings, using a cylindrical battery equipped with a wound electrode group as an example. However, this disclosure is not limited to the following configuration.

[0051] Figure 1 is a schematic longitudinal cross-sectional view showing an example of a lithium secondary battery according to the embodiment of this disclosure. Figure 2 is an enlarged view of the portion enclosed by area II in Figure 1 (a portion including the positive electrode). Figure 3 is an enlarged view of the portion enclosed by area III in Figure 1 (a portion including the negative electrode). Note that each figure is schematic, and the dimensions (e.g., thickness) of each component may differ from those of the actual product.

[0052] The lithium secondary 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 electrode group 14 is formed by winding a strip-shaped positive electrode 11 and a strip-shaped negative electrode 12 with a separator 13 interposed between the positive electrode 11 and the negative electrode 12.

[0053] The negative electrode 12 is composed of a negative electrode current collector. The negative electrode 12 (negative electrode current collector) has a thickness X and faces a separator 13 which has a thickness Y. In Figure 3, thickness Y refers to the thickness of the separator 13 before the electrode group 14 is housed in the case body 15. In this embodiment, the negative electrode 12 is composed only of a negative electrode current collector. However, the negative electrode may be constructed by pre-arranging a lithium metal sheet on the surface of the negative electrode current collector before the initial charge, or by supporting a negative electrode composite material layer on the surface of the negative electrode current collector.

[0054] The negative electrode 12 is electrically connected to the case body 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 case body 15.

[0055] The positive electrode 11 comprises a positive electrode current collector 30 and a positive electrode composite layer 31, and 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 positive electrode lead 19 extending from the positive electrode 11 extends to the filter 22 through a through hole (not shown) formed in the insulating plate 17. The other end of the positive electrode lead 19 is welded to the side of the filter 22 facing the electrode group 14.

[0056] The battery case consists of 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, thereby ensuring 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.

[0057] The case body 15 has a stepped portion 21 formed, for example, by partially pressing the side wall of the case body 15 from the outside. The stepped portion 21 may be formed in an annular shape on the side wall of the case body 15 along the circumferential direction of the case body 15. In this case, the sealing body 16 is supported on the opening side of the stepped portion 21.

[0058] 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. In the sealing body 16, these components are stacked in this order. The sealing body 16 is fitted into the opening of the case body 15 such that the cap 26 is located on the outside of the case body 15 and the filter 22 is located on the inside of the case body 15. Each of the above components constituting the sealing body 16 is, for example, disc-shaped or ring-shaped. 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. In other words, each component except the insulating member 24 is electrically connected to each other.

[0059] The lower valve body 23 has a ventilation hole (not shown) formed therein. 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 from an opening (not shown) formed in the cap 26.

[0060] Although the illustrated example describes a cylindrical lithium secondary battery, this embodiment is applicable to other types of batteries as well. Depending on the application, the shape of the lithium secondary battery can be appropriately selected from various shapes other than cylindrical, such as coin-shaped, prismatic, sheet-shaped, and flat-shaped batteries. Furthermore, although the illustrated example shows a wound electrode group composed of a positive electrode and a negative electrode wound around each other with a separator, the shape of the electrode group is not particularly limited, and a stacked electrode group composed of a positive electrode and a negative electrode stacked with a separator may also be used. In addition, known components other than the electrode group and non-aqueous electrolyte of the lithium secondary battery can be used without particular restriction.

[0061] [Examples] The lithium secondary batteries related to this disclosure will be described in detail below based on examples and comparative examples. This disclosure is not limited to the following examples.

[0062] Examples 1-5 and Comparative Examples 1-4 (1) Preparation of the positive electrode A lithium-containing transition metal oxide (NCA; positive electrode active material) containing Li, Ni, Co, and Al, acetylene black (AB; conductive material), and polyvinylidene fluoride (PVdF; binder) were mixed in a mass ratio of NCA:AB:PVdF = 95:2.5:2.5. An appropriate amount of N-methyl-2-pyrrolidone (NMP) was then added and the mixture was stirred to prepare a positive electrode mixture slurry. Next, the obtained positive electrode mixture slurry was applied to both sides of an Al foil that functions as a positive electrode current collector, dried, and the coating of the positive electrode mixture was rolled using a roller. Finally, the resulting laminate of the positive electrode current collector and the positive electrode mixture was cut to a predetermined electrode size to create a positive electrode with positive electrode mixture layers on both sides of the positive electrode current collector.

[0063] (2) Fabrication of the negative electrode A rectangular negative electrode current collector (thickness X: 10 μm) was prepared with oxygen content, tensile strength, and elongation at break as shown in Table 1. A lithium alloy foil (thickness 25 μm) was pressed onto the negative electrode current collector in an inert gas atmosphere. The negative electrode was thus fabricated.

[0064] In Table 1, rolled copper foil was used for the negative electrode current collector in E1 to E5. In E4 and E5, oxygen-free copper foil (JIS H 3100, alloy number C1020) was used, and the foil was heat-treated in an inert atmosphere.

[0065] In Table 1, electrolytic copper foil was used as the negative electrode current collector for C4, tough pitch copper (JIS H 3100, alloy number C1100) foil was used for C1 and C2, and phosphorus deoxidized copper (JIS H 3100, alloy number C1201) foil was used for C3.

[0066] (3) Preparation of non-aqueous electrolytes Liquid non-aqueous electrolytes were prepared by dissolving LiPF6 at a concentration of 1 mol / L and LiBF2(C2O4) at a concentration of 0.1 mol / L in a non-aqueous solvent. In Table 1, 1,2-dimethoxyethane and 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether were used as the non-aqueous solvents for the ether-based non-aqueous electrolytes. Dimethyl carbonate was used as the non-aqueous solvent for the carbonate-based non-aqueous electrolytes.

[0067] (4) Making a battery An aluminum tab was attached to the positive electrode obtained above. A nickel tab was attached to the negative electrode obtained above. In an inert gas atmosphere, the positive and negative electrodes were interposed with a separator and wound in a spiral shape to create a wound electrode group. A polyethylene thin film was used as the separator, and the thickness Y of the separator was set to the value shown in Table 1. The obtained electrode group was housed in a bag-shaped outer casing made of a laminate sheet with an aluminum layer, the non-aqueous electrolyte was injected into the outer casing containing the electrode group, and then the outer casing was sealed to create a lithium secondary battery. In Table 1, E1 to E5 are the batteries of Examples 1 to 5, and C1 to C4 are the batteries of Comparative Examples 1 to 4.

[0068] [evaluation] Each obtained battery underwent a charge-discharge cycle test at 25°C. The charge-discharge cycles were performed under the following conditions: a 20-minute pause was taken between charge and discharge cycles.

[0069] (charging) Constant current charging was performed at 10mA until the voltage reached 4.1V, and then constant voltage charging was performed at 4.1V until the current reached 1mA.

[0070] (discharge) A constant current discharge of 10mA was performed until the voltage reached 3V.

[0071] The batteries were repeatedly charged and discharged up to 100 cycles, and the discharge capacity at 100 cycles was measured. The evaluation results are shown in Table 1. In Table 1, the discharge capacity at 100 cycles for each battery is shown as a relative value, with the discharge capacity of E1 at 100 cycles set to 100.

[0072] [Table 1]

[0073] In E1-E5, the discharge capacity at 100 cycles was higher and the cycle characteristics were improved compared to C1-C4. After disassembling each battery after 100 cycles, SEM observation of the negative electrode current collector confirmed that fracture in the negative electrode current collector was suppressed in E1-E5. On the other hand, fracture occurred in the negative electrode current collector in C1-C4, and in all of C1-C4, there was almost no elongation deformation at the fracture site, confirming that it was brittle fracture initiated by grain boundary cracking.

[0074] Comparing E1 to E3, which have an oxygen content of 50 ppm or less in the negative electrode current collector, E3, which uses an ether-based non-aqueous electrolyte, showed even better cycle characteristics than E1 to E2, which use a carbonate-based non-aqueous electrolyte. On the other hand, when comparing C1 and C2, which have an oxygen content greater than 50 ppm in the negative electrode current collector, the cycle characteristics of C2, which uses an ether-based non-aqueous electrolyte, were worse than those of C1, which uses a carbonate-based non-aqueous electrolyte. From the above, it has been shown that when an ether-based non-aqueous electrolyte is used for a negative electrode current collector that satisfies conditions (i) to (iii) above, a significant improvement in cycle characteristics can be obtained.

[0075] Comparing E3 to E5, cycle characteristics were further improved in E4 to E5, where Y / X is 2.5 or higher, compared to E3, where Y / X is less than 2.5. Note that E3 to E5 have the same negative electrode current collector specifications: oxygen content of 50 ppm or less, tensile strength of 300 MPa or less, tensile elongation of 4% or more, and non-aqueous electrolyte. On the other hand, comparing C3 to C4, C4, with a Y / X ratio of 2.5 or higher, had the same discharge capacity at 100 cycles as C3, with a Y / X ratio of less than 2.5. For C3 and C4, the negative electrode current collector had an oxygen content of over 50 ppm, a breaking strength of over 300 MPa, an elongation at break of less than 4%, and the non-aqueous electrolyte was the same. From the above, it was shown that for a negative electrode current collector that satisfies (i) to (iii) above, a significant improvement in cycle characteristics can be obtained when Y / X is 2.5 or greater. [Industrial applicability]

[0076] The lithium secondary battery of this disclosure can be used in electronic devices such as mobile phones, smartphones, and tablet devices, electric vehicles including hybrid and plug-in hybrid vehicles, and home battery storage systems combined with solar cells. 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. [Explanation of Symbols]

[0077] 10 Lithium-ion rechargeable batteries 11 Positive electrode 12 Negative electrode 13 Separator 14 electrode group 15 Case body 16 Sealing body 17, 18 Insulating board 19 Positive lead 20 Negative lead 21 Step section 22 filters 23 Lower valve body 24 Insulating material 25 Upper valve body 26 caps 27 Gasket 30 Positive electrode current collector 31. Positive electrode composite layer

Claims

1. A positive electrode comprising a positive electrode active material capable of intercalating and releasing lithium ions, A negative electrode equipped with a negative electrode current collector, A separator is placed between the positive electrode and the negative electrode, A non-aqueous electrolyte having lithium ion conductivity, In the aforementioned negative electrode, lithium metal is deposited during charging, and the lithium metal dissolves during discharge. The oxygen content in the negative electrode current collector is 50 ppm or less. The negative electrode current collector has a breaking strength of 300 MPa or less and a breaking elongation of 4% or more. A lithium secondary battery in which the ratio of the thickness Y of the separator to the thickness X of the negative electrode current collector, Y / X, is 4.5 or greater.

2. The non-aqueous electrolyte comprises a non-aqueous solvent and a lithium salt that dissolves in the non-aqueous solvent. The lithium secondary battery according to claim 1, wherein the non-aqueous solvent contains 80% by mass or more of an ether compound.

3. The lithium secondary battery according to claim 2, wherein the ether compound comprises 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether.

4. The lithium secondary battery according to claim 3, further comprising 1,2-dimethoxyethane as the ether compound.

5. The oxygen content in the negative electrode current collector is 30 ppm or less. The lithium secondary battery according to any one of claims 1 to 4, wherein the negative electrode current collector has a breaking strength of 200 MPa or less and a breaking elongation of 10% or more.

6. The oxygen content in the negative electrode current collector is 15 ppm or less. The lithium secondary battery according to any one of claims 1 to 4, wherein the negative electrode current collector has a breaking strength of 50 MPa or more and 150 MPa or less, and a breaking elongation of 15% or more and 50% or less.

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