Lithium secondary battery

Abstract

A lithium secondary battery (10) according to the present disclosure is provided with: a positive electrode (11) which contains a positive electrode active material that is capable of absorbing and desorbing lithium ions; a negative electrode (12) which comprises a negative electrode collector; a separator (13) which is arranged between the positive electrode (11) and the negative electrode (12); and a non-aqueous electrolyte which has lithium ion conductivity. At the negative electrode, lithium metal is precipitated during charging, while the lithium metal is dissolved during discharging. The negative electrode collector contains austenitic stainless steel.

Description

Lithium secondary battery

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

[0002] Non-aqueous electrolyte secondary batteries are used in applications such as ICT (Information Communication Technology) applications such as personal computers and smartphones, in-vehicle applications, and power storage. In these applications, non-aqueous electrolyte secondary batteries are required to have even higher capacities. Lithium-ion batteries are known as high-capacity non-aqueous electrolyte secondary batteries. Higher capacities in lithium-ion batteries can be achieved by using, for example, graphite and an alloy active material such as a silicon compound as the negative electrode active material. However, the capacity of lithium-ion batteries is reaching its limit.

[0003] Lithium secondary batteries (lithium metal secondary batteries) are promising non-aqueous electrolyte secondary batteries with a higher capacity than lithium ion batteries. In lithium secondary batteries, lithium metal precipitates on the negative electrode during charging and dissolves in the non-aqueous electrolyte during discharge.

[0004] Patent Document 1 discloses 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, in which the molar ratio of the total amount of lithium contained in the positive electrode and the negative electrode to the amount of transition metal contained in the positive electrode is 1.1 or less, a space layer is present between the negative electrode and the separator in a discharged state, and the positive electrode capacity α (mAh / cm) per unit area of ​​the positive electrode is 0.05. ２ A non-aqueous electrolyte secondary battery has been proposed in which the average value of the thickness of the space layer, α / X (μm), satisfies 0.05≦α / X≦0.2.

[0005] International Publication No. 2019 / 087709

[0006] During charging and discharging, the negative electrode current collector is broken, resulting in a decrease in cycle characteristics.

[0007] One aspect of the present disclosure relates to a lithium secondary battery including: a positive electrode including a positive electrode active material capable of absorbing and releasing lithium ions; a negative electrode including 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 lithium metal precipitates in the negative electrode during charging and dissolves during discharging, and the negative electrode current collector includes austenitic stainless steel.

[0008] According to the present disclosure, it is possible to suppress the deterioration of the cycle characteristics of a lithium secondary battery. The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0009] 1 is a longitudinal sectional view schematically illustrating a lithium secondary battery according to an embodiment of the present disclosure, and FIG. 2 is an enlarged sectional view of a region II in FIG. 1. FIG. 3 is an enlarged sectional view of a region III in FIG.

[0010] The present disclosure relates to a lithium secondary battery (lithium metal secondary battery) that uses lithium metal as a negative electrode active material. Specifically, the lithium secondary battery according to the present disclosure includes a positive electrode including a positive electrode active material capable of absorbing and releasing lithium ions, a negative electrode including 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 precipitates during charging and dissolves during discharging.

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

[0012] In a battery in which lithium metal is deposited on the negative electrode during charging, the open circuit potential (OCV) of the negative electrode at full charge is, for example, 70 mV or less relative to lithium metal (lithium dissolution and deposition potential). A fully charged state refers to a state in which the battery is charged to a state of charge (SOC) of, for example, 0.98×C or more, where C is the rated capacity of the battery. The open circuit potential (OCV) of the negative electrode at full charge can be measured by disassembling a fully charged battery under an argon atmosphere, removing the negative electrode, and assembling a cell using lithium metal as a counter electrode. The nonaqueous electrolyte of the cell may have the same composition as the nonaqueous electrolyte in the disassembled battery.

[0013] During charging, lithium secondary batteries tend to expand significantly due to the deposition of lithium metal on the negative electrode. Here, "expansion of the negative electrode" refers to an increase in the total volume of the negative electrode and the volume of the deposited lithium metal. In particular, when lithium metal is deposited in a dendritic form, the expansion is even greater. This can easily cause stress in the negative electrode.

[0014] The present inventors have conducted extensive research into the causes of fractures in negative electrode current collectors during charge and discharge. As a result, they have newly discovered that the negative electrode current collector becomes brittle with repeated charge and discharge, and that the brittleness of the negative electrode current collector, together with stress generated in the negative electrode, is the cause of fractures in the negative electrode current collector. SEM observation revealed for the first time that the fractured portion of the negative electrode current collector exhibited brittle fracture, with almost no ductile deformation of the crystal grains or tapering deformation of the negative electrode current collector, which occurs when the negative electrode current collector elongates and breaks.

[0015] Furthermore, it was suggested that some substance generated in the negative electrode may be affecting the embrittlement of the negative electrode current collector. Therefore, the inventors conducted further intensive research focusing on the crystalline structure of stainless steel (ferrite, austenite, martensite with different slip planes, etc.), and discovered that austenite has a significant effect of suppressing the embrittlement of the negative electrode current collector.

[0016] In the lithium secondary battery according to the present disclosure, the negative electrode current collector contains austenitic stainless steel. In this case, embrittlement of the negative electrode current collector is suppressed, and the negative electrode current collector has appropriate strength and flexibility, resulting in a negative electrode current collector with excellent resistance to stress generated in the negative electrode. As a result, the occurrence of fracture of the negative electrode current collector during charge and discharge and the resulting deterioration of cycle performance are suppressed.

[0017] The above-mentioned "austenitic stainless steel" refers to stainless steel with an austenite fraction of 50% or more. The austenite fraction refers to the proportion (mass ratio) of the austenite phase in the stainless steel. When the contents of the austenite phase, ferrite phase, and martensite phase in the stainless steel are x, y, and z, respectively, the austenite fraction is calculated by {x / (x+y+z)}×100. The austenite structure has a face-centered cubic lattice structure (FCC structure), and the ferrite structure and martensite structure have a body-centered cubic lattice structure (BCC structure).

[0018] The austenite ratio may be 70% or more, 90% or more, or even 100%.

[0019] The austenite fraction can be determined by the following method. A sample (e.g., 25 mm square) of a negative electrode current collector (stainless steel foil) is prepared, and X-ray diffraction (XRD) measurement using a two-dimensional detection function is performed on the sample to obtain an XRD pattern (vertical axis: X-ray diffraction intensity, horizontal axis: diffraction angle 2θ). The size of the measurement area (microscopic portion) is, for example, 15 mm square.

[0020] Desirable XRD measurement conditions are as follows: <Analytical device> Two-dimensional micro X-ray diffractometer (RINT-RAPID II, manufactured by Rigaku Corporation) <Analysis conditions> Tube: Co Monochromatization: Use of a monochromator (CoKα) Tube output: 40 kV-30 mA Detector: Imaging plate (two-dimensional) (reflection method) Collimator: Φ300 μm ω angle: 25° to 35° (2° / sec) Φ angle: 360° rotation (1° / sec) Measurement time (exposure): 30 minutes

[0021] The obtained XRD pattern is fitted by the least squares method using a standard database, and then quantitatively analyzed by Rietveld analysis. The XRD pattern may have diffraction peaks corresponding to at least one of the austenite, ferrite, and martensite phases. This analysis can be performed using software provided with the analysis device. This analysis determines the austenite fraction, which is the ratio (mass ratio) of the austenite phase to the total of the austenite, ferrite, and martensite phases. Several measurement areas are randomly selected from the sample, and the austenite fraction in each measurement area is determined and the average value is calculated.

[0022] The negative electrode current collector preferably has a breaking strength of 850 MPa or less and a breaking elongation of 3% or more. In this case, a negative electrode current collector having good strength and flexibility and excellent resistance to stress generated in the negative electrode is easily obtained. For example, an austenitic stainless steel foil may be heat-treated to obtain breaking strength and breaking elongation within the above ranges. Generally, heat treatment tends to coarsen the crystal grain size due to recrystallization, reducing strength and improving breaking elongation.

[0023] The breaking strength of the negative electrode current collector may be 700 MPa or less, or may be 650 MPa or less. From the viewpoint of improving reliability in battery production, the breaking strength of the negative electrode current collector may be 400 MPa or more. The breaking strength may be within a range that combines the above upper and lower limits.

[0024] The breaking elongation of the negative electrode current collector may be 5% or more, or may be 10% or more. From the viewpoint of improving reliability in battery production, the breaking elongation of the negative electrode current collector may be 60% or less. The breaking elongation may be within any range that combines the above upper and lower limits.

[0025] The breaking strength (tensile strength) and breaking elongation are determined in accordance with JIS Z 2241 (Method of tensile testing for metallic materials). However, since the negative electrode current collector is a thin foil with low strength, measurement of breaking elongation requires skilled techniques, and it is desirable to have the measurement performed by an institution with a proven track record.

[0026] From the viewpoint of improving cycle characteristics, the ratio Y / X, which is the ratio of the separator thickness Y to the negative electrode current collector thickness X, may be 2.5 or more, 3 or more, or 4 or more. 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.

[0027] The thickness Y of the separator refers to the thickness of the separator before the electrode group is formed (before the electrode group is housed in the battery case). When the separator is formed by stacking multiple thin film-like materials, Y is the total thickness of the multiple materials. When the separator has thicknesses that vary depending on the location, Y is the maximum value of the thicknesses. When the separator has multiple regions with different thicknesses, the region of the separator having thickness Y (maximum value) has an area that is, for example, 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.

[0028] When the Y / X ratio is 2.5 or greater for a negative electrode current collector containing austenitic stainless steel, cycle characteristics are significantly improved. When the Y / X ratio is 2.5 or greater (when the separator is 2.5 times or greater than the thickness of the negative electrode current collector), the separator buffers expansion of the negative electrode (dendrite swelling) during charging, alleviating stress generated in the negative electrode (negative electrode current collector). The austenitic stainless steel's effects of suppressing embrittlement of the negative electrode current collector and improving resistance to stress generated in the negative electrode, combined with the effect of alleviating stress generated in the negative electrode (negative electrode current collector) when the Y / X ratio is 2.5 or greater, significantly improve cycle characteristics. When the austenite fraction is less than 50%, the negative electrode current collector is more susceptible to fracture, and the effect of alleviating stress generated in the negative electrode when the Y / X ratio is 2.5 or greater is less likely to be achieved.

[0029] The thickness X of the negative electrode current collector is determined by measuring the thickness at any 10 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 a similar manner. When the separator has multiple regions with different thicknesses, the thickness can be measured at any 10 points in the thickest region and the average value can be calculated.

[0030] (Negative Electrode) The negative electrode includes 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 receive electrons on the negative electrode current collector during charging, becoming lithium metal, which is then deposited 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 upon discharge. The lithium ions contained in the non-aqueous electrolyte may be derived from a lithium salt added to the non-aqueous electrolyte, may be supplied from the positive electrode active material during charging, or may be both.

[0031] The negative electrode current collector is typically a foil (sheet) of austenitic stainless steel. Austenitic stainless steel may contain, in addition to Fe, elements such as C, Si, Mn, P, S, Ni, Cr, Mn, Mo, Cu, and N. The stainless steel may be low-carbon, ultra-low-carbon, or nitrogen-added stainless steel, or may be austenite-containing duplex stainless steel.

[0032] Examples of austenitic stainless steel include SUS301, SUS302, SUS303, SUS304, SUS305, SUS309, SUS310, SUS312, SUS315, SUS316L, SUS317, SUS321, and SUS347. Of these, SUS304 and SUS316L are preferred.

[0033] The austenite fraction can be measured by XRD, but can also be estimated using the Schaeffler structural diagram, which shows the relationship between ferrite-stabilizing elements and austenite-stabilizing elements and the structure. This structural diagram plots the structure ratio with ferrite-stabilizing elements and austenite-stabilizing elements on the two axes. The vertical axis of this structural diagram represents the Ni equivalent, and the horizontal axis represents the Cr equivalent. The Cr equivalent is a value obtained by converting the level of ferrite-stabilizing elements into the amount of chromium, and can be expressed by the formula: Cr equivalent = %Cr + %Mo + 1.5 × %Si + 0.5 × %Nb. The Ni equivalent is a value obtained by converting the level of austenite-stabilizing elements into the amount of nickel, and can be expressed by the formula: Ni equivalent = %Ni + 30 × %C + 0.5 × %Mn.

[0034] The composition of stainless steel can be analyzed in accordance with JIS G 0321, and quantitative analysis can be performed on austenite stabilizing elements (Ni, Mn, C, etc.) and ferrite stabilizing elements (Cr, Mo, Si, Nb).

[0035] In order to compensate for the irreversible capacity of the positive electrode, a lithium metal sheet may be disposed 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 performing electrodeposition or vapor deposition. 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 thin enough that lithium metal can be deposited at 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 (negative electrode composite layer) may be formed on one surface of the negative electrode current collector or on both surfaces of the negative electrode current collector.

[0036] The surface of the negative electrode current collector may be smooth. This makes it easier for lithium metal from the positive electrode to deposit evenly on the negative electrode current collector during charging. "Smooth" 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.

[0037] (Positive Electrode) The positive electrode includes, 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 may be formed on both sides. The positive electrode is obtained, for example, by applying a positive electrode composite slurry including the positive electrode active material, the conductive material, and the binder to both sides of the positive electrode current collector, drying the coating, and then rolling.

[0038] The positive electrode active material is a material that absorbs and releases lithium ions. Examples of the positive electrode active material 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 of their low production cost and high average discharge voltage.

[0039] Examples of transition metal elements contained in the lithium-containing transition metal oxide include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, and W. The lithium-containing transition metal oxide may contain one or more transition metal elements. The transition metal element may be Co, Ni, and / or Mn. The lithium-containing transition metal oxide may contain one or more typical elements as needed. Examples of typical elements include Mg, Al, Ca, Zn, Ga, Ge, Sn, Sb, Pb, and Bi. The typical element may be Al, etc.

[0040] The conductive material is, for example, a carbon material, such as carbon black, acetylene black, ketjen black, carbon nanotubes, and graphite.

[0041] Examples of the binder include fluororesin, polyacrylonitrile, polyimide resin, acrylic resin, polyolefin resin, rubber polymer, etc. Examples of the fluororesin include polytetrafluoroethylene, polyvinylidene fluoride, etc.

[0042] The positive electrode current collector may be a foil, a film, or the like. The surface of the positive electrode current collector may be coated with a carbon material. Examples of the material for the positive electrode current collector include metal materials containing Al, Ti, Fe, and the like. The metal material may be Al, an Al alloy, Ti, a Ti alloy, an Fe alloy, or the like. The Fe alloy may be stainless steel (SUS). The thickness of the positive electrode current collector is not particularly limited and is, for example, 5 μm or more and 30 μm or less.

[0043] (Separator) A porous sheet having ion permeability and insulating properties is used for the separator. Examples of porous sheets include thin films, woven fabrics, and nonwoven fabrics having micropores. The material of the separator is not particularly limited, but 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 shapes and / or compositions.

[0044] (Non-aqueous electrolyte) A non-aqueous electrolyte having lithium ion conductivity includes, for example, a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. 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. The dissolution of the lithium salt in the non-aqueous solvent generates lithium ions and anions.

[0045] The gel-like non-aqueous electrolyte contains a lithium salt and a matrix polymer, or a lithium salt, a non-aqueous solvent, and a matrix polymer. The matrix polymer is, for example, a polymer material that absorbs the non-aqueous solvent and gels. Examples of the polymer material include fluororesin, acrylic resin, and polyether resin.

[0046] As the lithium salt or anion, any known material used in non-aqueous electrolytes for lithium secondary batteries can be used. Specifically, BF ４ － , ClO ４ － , P.F. ６ － , C.F. ３ SO ３ － , C.F. ３ CO ２ － , anions of imides, anions of oxalate complexes, etc. Examples of the anions of imides include N(SO ２ CF ３ )２ － , N(C ｍ F ２ｍ＋１ SO ２ ) ｘ (C ｎ F ２ｎ＋１ SO ２ ) ｙ － (m and n are each independently an integer of 0 or 1 or more, and x and y are each independently 0, 1, or 2, satisfying the relationship x+y=2.) The anion of the oxalate complex may contain boron and / or phosphorus. Examples of the anion of the oxalate complex include bisoxalate borate anion, BF ２ (C ２ O ４ ) － , P.F. ４ (C ２ O ４ ) － , P.F. ２ (C ２ O ４ ) ２ － The non-aqueous electrolyte may contain one of these anions alone or two or more of them.

[0047] From the viewpoint of suppressing the deposition of lithium metal in a dendritic form, the nonaqueous electrolyte preferably contains at least an anion of an oxalate complex. The interaction between the anion of the oxalate complex and lithium facilitates the uniform deposition of lithium metal in the form of fine particles. This facilitates the suppression of localized deposition of lithium metal. The anion of the oxalate complex may be combined with another anion. The other anion may be PF ６ － and / or an anion of an imide.

[0048] 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 or more of these non-aqueous solvents.

[0049] In particular, from the viewpoint of suppressing embrittlement of the negative electrode current collector, the nonaqueous solvent may contain an ether compound as a main component. Here, the term "main component" means that the content of the ether compound in the nonaqueous solvent is 50% by mass or more, and may be 80% by mass or more. The content of the ether compound in the nonaqueous solvent may be 95% by mass or less, or may be 100% by mass or less. The range of the content of the ether compound in the nonaqueous solvent may be any combination of the above upper and lower limits.

[0050] It is presumed that ether compounds have excellent stability (particularly reduction resistance), suppress the generation of decomposition products on the surface of the negative electrode current collector, and have little effect on the negative electrode current collector. When a nonaqueous electrolyte containing an ether compound as a main component is used for a negative electrode current collector containing austenitic stainless steel, the effect of improving cycle characteristics by suppressing embrittlement of the negative electrode current collector is significantly achieved. When the austenite fraction is less than 50%, the impact of fracture of the negative electrode current collector becomes significant, and the effect of the ether compound is difficult to achieve.

[0051] Examples of the ether compound include cyclic ethers and chain ethers. Examples of the cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, and 2-methyltetrahydrofuran. Examples of the chain ethers include 1,2-dimethoxyethane, diethyl ether, ethyl vinyl ether, methyl phenyl ether, benzyl ethyl ether, diphenyl ether, dibenzyl ether, 1,2-diethoxyethane, diethylene glycol dimethyl ether, 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 preferred.

[0052] Examples of ester compounds include carbonate esters and carboxylic acid esters. Examples of cyclic carbonate esters include ethylene carbonate and propylene carbonate. Examples of chain 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 chain carboxylic acid esters include ethyl acetate, methyl propionate, and methyl fluoropropionate.

[0053] The concentration of the 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 the anion in the non-aqueous electrolyte may be 0.5 mol / L or more and 3.5 mol / L or less. Furthermore, the concentration of the anion of the oxalate complex in the non-aqueous electrolyte may be 0.05 mol / L or more and 1 mol / L or less.

[0054] The non-aqueous electrolyte may contain an additive. The additive may form a coating on the negative electrode. The formation of a coating derived from the additive on the negative electrode makes it easier to suppress the formation of dendrites. Examples of such additives include vinylene carbonate, fluoroethylene carbonate (FEC), and vinyl ethyl carbonate (VEC).

[0055] (Lithium secondary battery) Hereinafter, the configuration of a lithium secondary battery according to the present disclosure will be described with reference to the drawings, taking as an example a cylindrical battery including a wound electrode group, although the present disclosure is not limited to the following configuration.

[0056] Fig. 1 is a longitudinal cross-sectional view schematically illustrating an example of a lithium secondary battery according to an embodiment of the present disclosure. Fig. 2 is an enlarged view of a portion surrounded by region II in Fig. 1 (a portion including a positive electrode). Fig. 3 is an enlarged view of a portion surrounded by region III in Fig. 1 (a portion including a negative electrode). Note that each figure is a schematic illustration, and the ratio of dimensions (e.g., thickness) of each component may differ from the actual ratio.

[0057] The lithium secondary battery 10 includes a cylindrical battery case, a wound electrode group 14 housed in the battery case, and a non-aqueous electrolyte (not shown). The electrode group 14 is constructed 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.

[0058] 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 having a thickness Y. Note that the thickness Y in FIG. 3 refers to the thickness of the separator 13 before the electrode group 14 is housed in the case body 15. Note that in this embodiment, the negative electrode 12 is composed only of a negative electrode current collector, but the negative electrode may be constructed by previously placing a lithium metal sheet on the surface of the negative electrode current collector before the initial charge, or by supporting a negative electrode composite layer on the surface of the negative electrode current collector.

[0059] The negative electrode 12 is electrically connected to the case body 15, which also serves as a negative electrode terminal, via a negative electrode lead 20. One end of the negative electrode lead 20 is connected to, for example, an end of the negative electrode 12 in the longitudinal direction, and the other end is welded to the inner bottom surface of the case body 15.

[0060] The positive electrode 11 includes 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 extends from the positive electrode 11 through a through-hole (not shown) formed in the insulating plate 17 and reaches the filter 22. The other end of the positive electrode lead 19 is welded to the surface of the filter 22 on the electrode group 14 side.

[0061] The battery case is composed of a case body 15, which is a cylindrical metal container with a bottom, and a sealing body 16 that seals the opening of the case body 15. A gasket 27 is disposed between the case body 15 and the sealing body 16, thereby ensuring the airtightness of the battery case. Within the case body 15, insulating plates 17 and 18 are disposed at both ends of the electrode group 14 in the winding axis direction.

[0062] Case body 15 has a step 21 formed, for example, by pressing a portion of the side wall of case body 15 from the outside. Step 21 may be formed in an annular shape along the circumferential direction of case body 15 on the side wall of case body 15. In this case, sealing body 16 is supported by the surface of step 21 on the opening side.

[0063] The sealing body 16 includes a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26. These components are stacked in this order in the sealing body 16. The sealing body 16 is attached to the opening of the case body 15 so that the cap 26 is located outside the case body 15 and the filter 22 is located inside the case body 15. The above-mentioned components constituting the sealing body 16 are, for example, disk-shaped or ring-shaped. The lower valve body 23 and the upper valve body 25 are connected to each other at their respective centers, and an insulating member 24 is interposed between their respective peripheral edges. The filter 22 and the lower valve body 23 are connected to each other at their respective peripheral edges. The upper valve body 25 and the cap 26 are connected to each other at their respective peripheral edges. In other words, all components except the insulating member 24 are electrically connected to each other.

[0064] A vent hole (not shown) is formed in the lower valve body 23. Therefore, if the internal pressure of the battery case increases due to abnormal heat generation or the like, the upper valve body 25 bulges toward the cap 26 and separates from the lower valve body 23. This cuts off the electrical connection between the lower valve body 23 and the upper valve body 25. If the internal pressure increases further, the upper valve body 25 breaks, and gas is discharged from an opening (not shown) formed in the cap 26.

[0065] In the illustrated example, a cylindrical lithium secondary battery is described, but this embodiment is not limited to this case and can be applied to other types. The shape of the lithium secondary battery can be appropriately selected from various shapes such as a cylindrical shape, a coin shape, a square shape, a sheet shape, and a flat shape, depending on the application. In addition, in the illustrated example, a wound electrode group configured by winding a positive electrode and a negative electrode with a separator interposed therebetween is shown, but the shape of the electrode group is not particularly limited, and it may be a stacked electrode group configured by stacking a positive electrode and a negative electrode with a separator interposed therebetween. In addition, known configurations other than the electrode group and non-aqueous electrolyte of the lithium secondary battery can be used without particular limitation.

[0066] [Examples] The lithium secondary battery according to the present disclosure will be specifically described below based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0067] Examples 1 to 6 and Comparative Examples 1 to 4 (1) Preparation of 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, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added and stirred to prepare a positive electrode composite slurry. Next, the obtained positive electrode composite slurry was applied to both sides of an Al foil that served as a positive electrode current collector, dried, and the coating of the positive electrode composite was rolled using a roller. Finally, the obtained laminate of the positive electrode current collector and the positive electrode composite was cut to a predetermined electrode size, and a positive electrode having a positive electrode composite layer on both sides of the positive electrode current collector was prepared.

[0068] (2) Preparation of Negative Electrode A rectangular stainless steel foil (thickness X: 10 μm) having the austenite fraction, breaking strength, and breaking elongation shown in Table 1 was prepared as a negative electrode current collector. A lithium alloy foil (thickness 25 μm) was pressure-bonded to the stainless steel foil in an inert gas atmosphere. In this manner, a negative electrode was prepared.

[0069] In Table 1, the following stainless steel foils were used for E1 to E6: E1: SUS304 modified 2 (Ni content: 5% by mass), E2: SUS304 modified 1 (Ni content: 6.5% by mass), E3: SUS304, E4: SUS316, E5: SUS316L, and E6: SUS316L.

[0070] In Table 1, the following stainless steel foils were used for C1 to C4: C1: SUS304 modified 3 (Ni content: 3.5% by mass) C2: SUS304 modified 4 (Ni content: 2% by mass) C3: SUS444 C4: SUS444 For E2, E1, C1, and C2, in that order, stainless steels based on SUS304 were used, with the Ni content reduced and the ferrite content increased (the austenite percentage reduced).

[0071] (3) Preparation of non-aqueous electrolyte: LiPF ６ 1 mol / L and LiBF ２ (C ２ O ４ ) were dissolved in each solution to a concentration of 0.1 mol / L to prepare liquid non-aqueous electrolytes. In Table 1, for the ether-based non-aqueous electrolytes, 1,2-dimethoxyethane and 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether were used as the non-aqueous solvent. For the carbonate-based non-aqueous electrolytes, dimethyl carbonate was used as the non-aqueous solvent.

[0072] (4) Battery Fabrication An Al tab was attached to the positive electrode obtained above. A Ni tab was attached to the negative electrode obtained above. A separator was interposed between the positive electrode and the negative electrode, and the positive electrode and the negative electrode were spirally wound in an inert gas atmosphere to prepare a wound electrode assembly. 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 assembly was housed in a bag-shaped exterior body formed of a laminate sheet with an Al layer. The nonaqueous electrolyte was then injected into the exterior body housing the electrode assembly, and the exterior body was sealed to prepare a lithium secondary battery. In Table 1, E1 to E6 are batteries of Examples 1 to 6, and C1 to C4 are batteries of Comparative Examples 1 to 4.

[0073] [Evaluation] A charge-discharge cycle test was carried out on each of the obtained batteries in an environment of 25° C. The charge-discharge was carried out under the following conditions: A 20-minute break was left between charge and discharge.

[0074] (Charging) The battery was charged at a constant current of 10 mA until the voltage reached 4.1 V, and then charged at a constant voltage of 4.1 V until the current reached 1 mA.

[0075] (Discharge) A constant current discharge of 10 mA was carried out until the voltage reached 3V.

[0076] The charge / discharge cycle was repeated 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 of each battery is shown as a relative value when the discharge capacity at 100 cycles of E1 is set to 100.

[0077]

[0078] In E1 to E6, which had an austenite fraction of 50% or more, the discharge capacity at 100 cycles was higher and cycle characteristics were improved compared to C1 to C4, which had an austenite fraction of less than 50%. After 100 cycles, each battery was disassembled and the negative electrode current collector was observed under SEM. It was confirmed that fractures in the negative electrode current collector were suppressed in E1 to E6 and no cracks occurred in the surface layer. Meanwhile, in C1 to C4, fractures occurred in the negative electrode current collector, and almost no elongation deformation was observed in the fractured area, confirming that the fracture was due to brittle fracture.

[0079] Comparing E3 and E4, which have an austenite fraction of 50% or more, E4, which used an ether-based nonaqueous electrolyte, showed improved cycle characteristics compared to E3, which used a carbonate-based nonaqueous electrolyte. On the other hand, comparing C2 and C3, which have an austenite fraction of less than 50%, C3, which used an ether-based nonaqueous electrolyte, showed no improvement in cycle characteristics compared to C2, which used a carbonate-based nonaqueous electrolyte. These findings demonstrate that the use of an ether-based nonaqueous electrolyte with a negative electrode current collector having an austenite fraction of 50% or more significantly improves cycle characteristics.

[0080] Comparing E4 to E6, which have an austenite ratio of 100%, E5 to E6, which have a Y / X ratio of 2.5 or more, showed further improved cycle characteristics compared to E4, which has a Y / X ratio of less than 2.5. On the other hand, comparing C2 and C4, which have an austenite ratio of less than 50%, C4, which has a Y / X ratio of 2.5 or more, showed no improvement in cycle characteristics compared to C2, which has a Y / X ratio of less than 2.5. These findings demonstrate that when Y / X is 2.5 or more for a negative electrode current collector with an austenite ratio of 50% or more, a significant improvement in cycle characteristics can be obtained.

[0081] The lithium secondary battery of the present disclosure can be used in electronic devices such as mobile phones, smartphones, and tablet devices, electric vehicles including hybrids and plug-in hybrids, and home storage batteries combined with solar cells. While the present invention has been described with reference to presently preferred embodiments, such disclosure should not be construed as limiting. Various modifications and alterations will undoubtedly become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. Accordingly, the appended claims should be construed to include all modifications and alterations without departing from the true spirit and scope of the present invention.

[0082] REFERENCE SIGNS LIST 10 Lithium secondary battery 11 Positive electrode 12 Negative electrode 13 Separator 14 Electrode group 15 Case body 16 Sealing body 17, 18 Insulating plate 19 Positive electrode lead 20 Negative electrode lead 21 Step 22 Filter 23 Lower valve body 24 Insulating member 25 Upper valve body 26 Cap 27 Gasket 30 Positive electrode current collector 31 Positive electrode mixture layer

Claims

1. a positive electrode including a positive electrode active material capable of occluding and releasing lithium ions; a negative electrode including 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 is dissolved during discharging, and the negative electrode current collector includes austenitic stainless steel, a lithium secondary battery.

2. The lithium secondary battery according to claim 1, wherein the negative electrode current collector has a breaking strength of 850 MPa or less and an elongation at break of 3% or more.

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

4. The lithium secondary battery according to claim 1 or 2, wherein a ratio of a thickness Y of the separator to a thickness X of the negative electrode current collector: Y / X is 2.5 or more.