Lithium-ion secondary battery

Abstract

The present invention provides a lithium-ion secondary battery capable of suppressing generation of gas derived from a separator more than in the past.　The present invention has a positive electrode active material, a negative electrode active material, a separator, and an electrolyte, wherein the positive electrode active material has an operating potential of 4.3 V (vs. Li / Li+) or greater, and the separator is composed of fibers having a diameter of 10 µm or greater, with polyethylene terephthalate and / or polybutylene terephthalate as a main component.

Description

Lithium-ion rechargeable battery

[0001] This invention relates to a lithium-ion secondary battery.

[0002] In recent years, in order to maintain a high operating voltage while suppressing the decomposition of the electrolyte during charging and discharging, the inventors have developed a core-shell type positive electrode active material for lithium-ion secondary batteries, in which lithium nickel manganese oxide (hereinafter also referred to as LNMO), which has a high operating voltage, is used as the core part and lithium iron phosphate (hereinafter also referred to as LFP) is used as the shell layer (for example, Patent Document 1). In the lithium-ion secondary battery of Patent Document 1, since the LNMO core part is covered with LFP as the shell layer, the core part is not directly exposed to the electrolyte, and the decomposition of the electrolyte can be suppressed significantly compared to when LNMO is used as the positive electrode active material.

[0003] International Publication No. 2023 / 149363

[0004] Incidentally, in each embodiment of Patent Document 1, a polypropylene separator, which is commonly used as a separator in general lithium-ion secondary batteries, is used. However, the inventors have discovered that when a positive electrode using a positive electrode active material with a high operating potential, as in Patent Document 1, is used, a part of the separator undergoes oxidative decomposition, generating a gas derived from the separator. Therefore, the secondary battery of Patent Document 1 had room for further improvement.

[0005] Therefore, the present invention aims to provide a lithium-ion secondary battery that can suppress the generation of separator-derived gases compared to conventional batteries.

[0006] One aspect of the present invention for solving the above-mentioned problems comprises a positive electrode active material, a negative electrode active material, a separator, and an electrolyte, wherein the positive electrode active material has an operating potential of 4.3V (vs. Li / Li ＋ The lithium-ion secondary battery is characterized by the above, wherein the separator is a fiber with a diameter of 10 μm or more, and whose main component is polyethylene terephthalate and / or polybutylene terephthalate.

[0007] According to this pattern, the operating potential is 4.3V (vs. Li / Li＋ ) Even if a positive electrode active material with the above properties is used, the separator is formed of fibers having a diameter of 10 µm or more, and the main components of the separator are polyethylene terephthalate and / or polybutylene terephthalate, so the separator is less prone to oxidative decomposition, and generation of gas originating from the separator can be suppressed compared to conventional separators.

[0008] In a preferred embodiment, 90% or more of the entire separator is composed of polyethylene terephthalate.

[0009] In a preferred embodiment, the separator has a basis weight of 35 g / m ２ or less and is formed of a nonwoven fabric.

[0010] In a preferred embodiment, the separator is composed of a plurality of fibers, and when 10 fibers are randomly extracted, at least one of the 10 fibers has a length 10 times or more the length in the longitudinal direction of the separator when the separator is developed.

[0011] In a preferred embodiment, the separator is formed of a spunbonded nonwoven fabric.

[0012] In a preferred embodiment, the positive electrode active material is a compound represented by the following formula (1). Li １＋ｘ M ｙ Mn ２－ｘ－ｙ O ４ ...(1) (In the formula (1), x and y respectively satisfy 0≤x≤0.2 and 0<y≤0.8, and M is at least one selected from the group consisting of Al, Mg, Zn, Ni, Co, Fe, Ti, Cu, and Cr)

[0013] In a preferred embodiment, the positive electrode active material is coated with a lithium-containing oxide containing lithium.

[0014] In a preferred embodiment, the negative electrode active material has an operating potential of 0.5 V (vs. Li / Li ＋ ) or more and 2.5 V (vs. Li / Li ＋ ) or less.

[0015] In a preferred embodiment, the negative electrode active material contains titanium.

[0016] As long as the above aspects are included within the technical scope of the present invention, they may be mutually dependent between respective aspects, may cite partial configurations, or may substitute partial configurations.

[0017] According to the present invention, the generation of gas derived from a separator can be suppressed compared to conventional techniques.

[0018] FIG. 1 is a conceptual explanatory view showing the secondary battery according to the first embodiment of the present invention, wherein (a) is a perspective view of the secondary battery, and (b) is a cross-sectional view taken along line A-A of (a). FIG. 2 is an explanatory view of the electrode portion in FIG. 1, wherein (a) is a cross-sectional view of the positive electrode portion, and (b) is a cross-sectional view of the negative electrode portion.

[0019] Hereinafter, embodiments of the present invention will be described in detail.

[0020] The secondary battery 1 according to the first embodiment of the present invention is a lithium ion secondary battery using lithium ions as carriers, and as shown in FIG. 1, includes a battery stack 2, electrode lead-out members 3a and 3b, an electrolytic solution 5, and an outer package 6.

[0021] <Battery Stack 2> As shown in FIG. 1(b), the battery stack 2 includes a plurality of positive electrode portions 10, a plurality of negative electrode portions 11, and a plurality of separators 12. In the battery stack 2 of the present embodiment, as in ... / separator 12 / positive electrode portion 10 / separator 12 / negative electrode portion 11 / separator 12 / ..., each of the electrode portions 10 and 11 and the separators 12 are alternately arranged, and the separators 12 are respectively disposed on the outermost sides in the stacking direction.

[0022] (Positive Electrode Portion 10) As shown in FIG. 2(a), the positive electrode portion 10 is formed by laminating a positive electrode composite active material layer 21 on at least one main surface of a positive electrode current collector 20, and is an intercalation electrode that allows insertion and desorption of lithium ions. The positive electrode composite active material layer 21 is a layered body including a positive electrode composite active material 22, a conductive aid, and a binder.

[0023] As shown in the enlarged view of FIG. 2(a), the positive electrode composite active material 22 is composed of a plurality of first core-shell particles 23. Each of the first core-shell particles 23 is formed by coating the surface of a first core portion 25 (positive electrode active material) with a first shell layer 26.

[0024] The first core portion 25 is a positive electrode active material composed of a lithium-ion conductive oxide, and is capable of inserting and deinserting lithium ions. The first core portion 25 has an average potential of lithium deinsertion and insertion relative to the deposition potential of Li (vs. Li / Li ＋ (Also indicated as) Preferably, the operating potential is 4.3V or higher, and more preferably 4.5V or higher and 5.0V or lower. That is, the first core portion 25, on its own, preferably has an operating potential of 4.3V or higher, and more preferably 4.5V or higher and 5.0V or lower relative to lithium metal. Potential of lithium ion insertion / desorption reaction (hereinafter also referred to as voltage) (vs. Li / Li ＋ This can be determined, for example, by measuring the charge-discharge characteristics of a half-cell using the first core portion 25 as the operating electrode and lithium metal as the counter electrode, and reading the voltage values ​​at the start and end of the plateau. If there are two or more plateaus, it is sufficient that the plateau with the lowest voltage value is above the lower threshold, and the plateau with the highest voltage value is below the upper threshold.

[0025] The first core portion 25 is not particularly limited as long as it has ionic conductivity, but a spinel-type lithium manganese oxide represented by the following formula (2) is preferred. １＋ｘ M ｙ Mn ２－ｘ－ｙ O ４ ... (2) In equation (2) above, x and y satisfy 0 ≤ x ≤ 0.2 and 0 < y ≤ 0.8, respectively, and M is at least one selected from the group consisting of Al, Mg, Zn, Ni, Co, Fe, Ti, Cu, and Cr.

[0026] Among the above formula (2), lithium nickel manganese oxide (hereinafter also referred to as LNMO) in which M is Ni is preferred.

[0027] The first shell layer 26 is a coating composed of a lithium-ion conductive oxide and is made of an intercalation material. The first shell layer 26 is a lithium-containing oxide containing lithium as an element, specifically, lithium manganese phosphate (LiMnPO4), whose main component has an olivine-type crystal structure. ４(Hereafter also referred to as LMP). The thickness of the first shell layer 26 is preferably thinner than the particle size of the first core portion 25. The first shell layer 26 covers at least a portion of the surface of the first core portion 25, preferably 95% or more, and more preferably completely.

[0028] (Negative electrode section 11) As shown in Figure 2(b), the negative electrode section 11 is an intercalation electrode in which lithium ions can be inserted and removed, with a negative electrode composite active material layer 31 laminated on at least one main surface of the negative electrode current collector 30. The negative electrode composite active material layer 31 comprises a negative electrode composite active material 32, a conductive additive, and a binder.

[0029] The negative electrode composite active material 32 is composed of a plurality of second core-shell particles 33, as shown in the enlarged view in Figure 2(b). The second core-shell particles 33 are formed by coating the surface of the second core portion 35 (negative electrode active material) with a second shell layer 36.

[0030] The second core portion 35 is a negative electrode active material composed of a lithium-ion conductive oxide, and is capable of inserting and removing lithium ions. The average potential of lithium removal and insertion in the second core portion 35 is (vs. Li / Li) relative to the deposition potential of Li. ＋ (Also indicated as ), preferably 0.5V to 2.5V. That is, the second core portion 35 preferably has an operating potential of 0.5V to 2.5V relative to lithium metal.

[0031] The second core portion 35 is a titanium-containing oxide containing titanium, and it is preferable to use lithium titanate (hereinafter also referred to as LTO) from the viewpoint that lithium deposition is less likely to occur and safety is improved. Among lithium titanates, lithium titanate with a spinel structure is particularly preferred for the second core portion 35 because the expansion and contraction of the negative electrode composite active material 32 in the lithium ion insertion and deinsertion reaction is small.

[0032] The second shell layer 36 is a coating composed of a lithium-ion conductive oxide and is made of an intercalation material. The second shell layer 36 is a lithium-containing oxide containing phosphorus as an element, and specifically, its main component is lithium iron phosphate (LiFePO4), which has an olivine-type crystal structure. ４ It is composed of (hereinafter also referred to as LFP). The thickness of the second shell layer 36 is preferably thinner than the particle size of the second core portion 35. The second shell layer 36 covers at least a part of the surface of the second core portion 35, preferably covering 10% or more, more preferably covering 20% ​​or more, even more preferably covering 95% or more, and particularly preferably covering completely.

[0033] The current collectors 20 and 30 are not particularly limited, but are preferably made of aluminum or an aluminum alloy because they are stable in the positive electrode reaction atmosphere and the negative electrode reaction atmosphere. The current collectors 20 and 30 can also be made of a metal other than aluminum (copper, SUS, nickel, titanium, and their alloys) coated with a metal that does not react at the potential of the positive electrode portion 10 and the negative electrode portion 11.

[0034] The conductive additive used in the active material layers 21 and 31 is not particularly limited, but carbon materials are preferred. The carbon material is preferably at least one selected from natural graphite, artificial graphite, vapor-grown carbon fibers, carbon nanotubes, acetylene black, Ketjen black, and furnace black. The amount of conductive additive contained in the active material layers 21 and 31 is preferably 1 part by weight or more and 30 parts by weight or less per 100 parts by weight of the active material 22 and 32. Within this range, the conductivity of the active material layers 21 and 31 is ensured, adhesion to the binder is maintained, and sufficient adhesion to the current collectors 20 and 30 can be obtained.

[0035] The binder used in the active material layers 21 and 31 is not particularly limited, but for either of the active material layers 21 or 31, at least one selected from the group consisting of polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber, polyimide, and derivatives thereof can be used. Preferably, the amount of binder contained in the active material layers 21 and 31 is 1 part by weight or more and 30 parts by weight or less per 100 parts by weight of the active material 22 and 32. Within this range, the adhesion between the active material 22 and 32 and the conductive additive can be maintained, and sufficient adhesion to the current collectors 20 and 30 can be obtained.

[0036] <Electrode Extraction Members 3a, 3b> The positive electrode extraction member 3a is a positive electrode terminal that extends inside and outside the outer casing 6 and is at least electrically connected to one of the positive electrode portions 10 that make up the battery stack 2, as shown in Figure 1(b). The negative electrode extraction member 3b is a negative electrode terminal that extends inside and outside the outer casing 6 and is at least electrically connected to one of the negative electrode portions 11 that make up the battery stack 2, as well as within the outer casing 6. The electrode extraction members 3a and 3b are conductive plate-like bodies, and are not particularly limited as long as they are conductive; for example, metals such as aluminum or their alloys can be used.

[0037] <Electrolyte 5> The electrolyte 5 is a non-aqueous electrolyte having lithium ion conductivity to which one or more additives are added. Preferably, the electrolyte 5 contains 0.1% to 20% of the additives relative to the weight of the electrolyte 5, and more preferably 1% to 15%.

[0038] (Non-aqueous electrolyte) A non-aqueous electrolyte is a solution in which a solute is dissolved in a non-aqueous solvent.

[0039] The non-aqueous solvent is not particularly limited, but for example, cyclic aprotic solvents and / or linear aprotic solvents can be used. Examples of cyclic aprotic solvents include cyclic carbonates, cyclic esters, cyclic sulfones, and cyclic ethers. Examples of linear aprotic solvents include linear carbonates, linear carboxylic acid esters, linear ethers, and acetonitrile.

[0040] The solute is not particularly limited, but for example, LiBF ４ LiPF ６ LiAsF ６ LiCF ３ SO ３ , LiN (SO ２ CF ３ ) ２ Fluorine-containing lithium can be used.

[0041] (Additives) Additives are not particularly limited, but for example, cyclic siloxanes having a cyclic siloxane bond as the main chain can be used, such as 2,4,6,8-tetravinyl-2,4,6,8-tetramethylcyclotetrasiloxane (4VC4S), octamethylcyclotetrasiloxane, octadecamethylcyclononasiloxane, and 1,3,5-tris(3,3,3-trifluoropropyl)-1,3,5-trimethylcyclotrisiloxane.

[0042] <Separator 12> The separator 12 is installed between the positive electrode portion 10 and the negative electrode portion 11, is insulating, and is capable of being impregnated with the electrolyte 5. The separator 12 is composed of fibers with a diameter of 10 μm or more, and its main component is polyethylene terephthalate or polybutylene terephthalate, preferably 90% or more of the whole is composed of polyethylene terephthalate or polybutylene terephthalate. The separator 12 is preferably composed of a spunbond nonwoven fabric formed by the spunbond method. The separator 12 has a basis weight of 40 g / m². ２ The following nonwoven fabrics are preferred, with a density of 35 g / m². ２ It is more preferable that the nonwoven fabric has a basis weight of less than 5 g / m². The separator 12 has a basis weight of 5 g / m². ２ Preferably, it is a nonwoven fabric of the above quality, 10 g / m² ２It is more preferable that the nonwoven fabric is as described above. The separator 12 is composed of multiple fibers, and when 10 fibers are randomly selected, it is preferable that at least one of the 10 fibers is a continuous long fiber that is 10 times or more the length of the separator 12 in the longitudinal direction when the separator 12 is unfolded. It is preferable that the continuous long fibers of the separator 12 are aligned in a certain direction.

[0043] <Outer casing 6> As shown in Figure 1(b), the outer casing 6 has an internal space 40 and is a sealing member that houses and seals the battery stack 2 and electrolyte 5 in the internal space 40. It is chemically stable to the electrolyte 5 and has water vapor barrier properties. As shown in Figure 1, the outer casing 6 is composed of a first outer film 41 and a second outer film 42, and can be sealed by sandwiching the battery stack 2 and electrolyte 5 between the outer films 41 and 42. The outer films 41 and 42 are made of laminate films containing a laminate resin.

[0044] The secondary battery 1 of this embodiment has a first core portion 25 (positive electrode active material), a second core portion 35 (negative electrode active material), a separator 12, and an electrolyte 5, and the first core portion 25 has an operating potential of 4.3V (vs. Li / Li ＋ The separator 12 is made of fibers with a diameter of 10 μm or more, and its main component is polyethylene terephthalate and / or polybutylene terephthalate. With this configuration, the operating potential is 4.3 V (vs. Li / Li ＋ Even when the first core portion 25 described above is used as the positive electrode active material, since it is a fiber with a diameter of 10 μm or more and its main component is polyethylene terephthalate and / or polybutylene terephthalate, the separator 12 is less susceptible to oxidative decomposition, and the generation of gas originating from the separator 12 can be suppressed compared to conventional methods.

[0045] In the embodiment described above, the secondary battery 1 was a laminated type secondary battery in which the battery stack 2 was sealed with outer films 41 and 42, but the present invention is not limited thereto. Other types of secondary batteries may also be used. For example, a cylindrical secondary battery in which a wound battery stack 2 is sealed in a cylindrical housing may also be used.

[0046] In the embodiments described above, the positive electrode composite active material 22 was composed of first core-shell particles 23 having a core-shell structure, but the present invention is not limited thereto. The positive electrode composite active material 22 may be composed of general positive electrode active material particles. For example, it may be composed of positive electrode active material particles that do not have a core-shell structure, such as LNMO. That is, the positive electrode composite active material 22 may be composed only of the first core portion 25.

[0047] In the embodiments described above, the negative electrode composite active material 32 was composed of second core-shell particles 33 having a core-shell structure, but the present invention is not limited thereto. The negative electrode composite active material 32 may be composed of general negative electrode active material particles. For example, it may be composed of negative electrode active material particles that do not have a core-shell structure, such as LTO. That is, the negative electrode composite active material 32 may be composed only of the second core portion 35.

[0048] In the embodiments described above, the components can be freely substituted or added between each embodiment, as long as they fall within the technical scope of the present invention.

[0049] The present invention will be specifically described below with reference to examples. However, the present invention is not limited to the following examples and can be implemented with appropriate modifications without altering its essence.

[0050] (Example 1) (a) Preparation of positive electrode composite active material First, a predetermined amount of ethanol, which is used as a solvent, was mixed with lithium manganese phosphate (LMP) powder having an olivine-type crystal structure, and the mixture was treated with a bead mill for 3 hours using beads with a diameter of 0.2 mm. After removing the beads from the mixture, a portion of the ethanol was removed to obtain a slurry containing 16.4 wt% of first precursor particles (LMP fine powder).

[0051] As the first core portion of the positive electrode, a spinel-type lithium nickel manganese oxide (LiNi) with a median diameter of 8.8 μm is used. ０．５ Mn １．５ O ４(hereinafter also referred to as LNMO) was used. 30 g of LNMO was placed in a grinding mill, and while rotating at a clearance of 0.8 mm, rotor load power of 1.5 kW, and 2600 rpm, an ethanol-dispersed slurry of first precursor particles was added in two portions so that the amount of first precursor particles added was 1.2 wt%. Thereafter, the rotor rotation speed was maintained in the range of 2600 rpm to 3000 rpm and the material was treated at room temperature in an air atmosphere for 10 minutes, and then heat-treated at 350°C for 1 hour to obtain a cathode composite active material containing first core-shell particles in which the surface of LNMO (first core portion) was coated with LMP (first shell layer).

[0052] (b) Preparation of the positive electrode A slurry was prepared by dispersing a mixture containing 90 parts by weight, 6 parts by weight, and 4 parts by weight of the obtained positive electrode composite active material, acetylene black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder, respectively, in N-methyl-2-pyrrolidone (NMP). The binder used was an N-methyl-2-pyrrolidone (NMP) solution with a solid content of 5 wt%, and further NMP was added to adjust the viscosity to facilitate coating.

[0053] The slurry was coated onto 15 μm aluminum foil and then dried in an oven at 120°C. After performing this operation on both sides of the aluminum foil, the positive electrode portion was fabricated by further vacuum drying at 170°C.

[0054] (c) Preparation of negative electrode composite active material First, lithium iron phosphate (LFP) powder having an olivine-type crystal structure was mixed with a predetermined amount of ethanol as a solvent, and the mixture was ball-milled for 3 hours using zirconia balls with a ball diameter of 0.5 mm. After removing the zirconia balls from the mixture, a portion of the ethanol was removed to obtain a slurry containing 16.4 wt% of second precursor particles (LFP fine powder).

[0055] As the second core of the negative electrode, a spinel-type lithium titanate (Li) with a median diameter of 8.5 μm is used. ４ Ti ５ O １２(hereinafter also referred to as LTO) was used. 30 g of LTO was placed in a grinding mill, and while rotating at a clearance of 0.8 mm, rotor load power of 1.5 kW, and 2600 rpm, an ethanol-dispersed slurry of second precursor particles was added in two portions so that the amount of second precursor particles added was 3.6 wt%. After that, the rotor rotation speed was maintained in the range of 2600 rpm to 3000 rpm and the material was treated at room temperature in an air atmosphere for 10 minutes, and then heat-treated at 350°C for 1 hour to obtain a negative electrode composite active material containing second core-shell particles in which the surface of LTO (second core portion) was coated with LFP (second shell layer).

[0056] (d) Preparation of the negative electrode A slurry was prepared by dispersing a mixture containing 92 parts by weight, 3 parts by weight, and 5 parts by weight of the obtained negative electrode composite active material, acetylene black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder, respectively, in N-methyl-2-pyrrolidone (NMP) at solid content concentrations. The binder used was an N-methyl-2-pyrrolidone (NMP) solution with a solid content concentration of 5 wt%, and further NMP was added to adjust the viscosity to facilitate coating.

[0057] The slurry was coated onto 15 μm aluminum foil and then dried in an oven at 120°C. After performing this operation on both sides of the aluminum foil, the negative electrode portion was fabricated by further vacuum drying at 170°C.

[0058] (e) Fabrication of lithium-ion secondary battery The positive electrode and negative electrode fabricated in (b) and (d) above, and a basis weight of 15 g / m ２ A battery was fabricated using a spunbond nonwoven fabric made of polyethylene terephthalate (hereinafter also referred to as PET) with a fiber diameter of 20 μm as a separator, following the procedure below. First, the positive electrode and negative electrode sections were dried under reduced pressure at 80°C for 12 hours. Next, a battery stack was formed by laminating 15 positive electrodes and 16 negative electrodes in the order of negative electrode / separator / positive electrode section. The outermost layer of both battery stacks was made of separator. Next, aluminum tabs were vibration-welded to the positive and negative electrode sections at both ends.

[0059] Two aluminum laminate films were prepared to serve as the outer casing. After forming recesses for the battery section and gas collection section by pressing, the battery stack was placed inside. The outer periphery, leaving space for electrolyte injection, was heat-sealed at 180°C for 7 seconds. From the unsealed area, ethylene carbonate, propylene carbonate, and ethyl methyl carbonate were mixed by volume in a solvent of ethylene carbonate / propylene carbonate / ethyl methyl carbonate = 15 / 15 / 70, and LiPF was added. ６ A non-aqueous electrolyte solution was prepared by dissolving the substance at a concentration of 1 mol / L. 5 wt% of 2,4,6,8-tetravinyl-2,4,6,8-tetramethylcyclotetrasiloxane (4VC4S) was added to the non-aqueous electrolyte solution as an additive. The unsealed areas were then heat-sealed at 180°C for 7 seconds under reduced pressure.

[0060] (f) Aging of the lithium-ion secondary battery The obtained battery was charged with a constant current value equivalent to 0.2C until the battery voltage reached the termination voltage of 3.4V, and then the charging was stopped. After that, it was left to stand in a 60°C environment for 24 hours, and then discharged with a constant current value equivalent to 0.2C, and the discharge was stopped when the battery voltage reached 2.5V. After the discharge was stopped, the gas accumulated in the gas collection section was extracted as aging gas, and the section was resealed. A lithium-ion secondary battery for evaluation was fabricated by the above procedure.

[0061] (Example 2) In the above (e) preparation of lithium-ion secondary battery, the basis weight was 20 g / m ２ Example 2 was constructed in the same manner as Example 1, except that a PET spunbond nonwoven fabric with a fiber diameter of 20 μm was used as the separator.

[0062] (Example 3) In the above (e) preparation of lithium-ion secondary battery, the basis weight was 25 g / m ２ Example 3 was constructed in the same manner as Example 1, except that a PET spunbond nonwoven fabric with a fiber diameter of 20 μm was used as the separator.

[0063] (Example 4) In the above (e) preparation of lithium-ion secondary battery, the basis weight was 35 g / m ２The procedure was the same as in Example 1, except that a PET spunbond nonwoven fabric with a fiber diameter of 20 μm was used as the separator, and this was designated as Example 4.

[0064] (Comparative Example 1) The process for producing the lithium-ion secondary battery described above (e) was the same as in Example 1, except that a polypropylene microporous film (PP microporous) was used as the separator. This was designated as Comparative Example 1.

[0065] (Comparative Example 2) In the above (e) lithium-ion secondary battery fabrication, the basis weight is 15 g / m². ２ Comparative Example 2 was constructed in the same manner as in Example 1, except that a nonwoven fabric formed from two components, PET fibers and polypropylene fibers (PP fibers), was used as the separator.

[0066] (Comparative Example 3) In the above (e) lithium-ion secondary battery fabrication, the basis weight is 20 g / m² ２ Comparative Example 3 was constructed in the same manner as in Example 1, except that a nonwoven fabric formed from two components—PET fibers with a diameter of 20 μm and PET fine fibers with a diameter of 5 μm—was used as the separator.

[0067] (Evaluation of Cycle Characteristics of Lithium-ion Secondary Batteries) The lithium-ion secondary batteries prepared in each of Examples 1 to 4 and Comparative Examples 1 to 3 were connected to a charge / discharge device (HJ1005SD8, manufactured by Hokuto Denko Co., Ltd.) and cycle operation was performed. Under a 45°C environment, constant current charging was performed at a current value equivalent to 1.0C until the battery voltage reached the termination voltage of 3.4V, and then charging was stopped. Subsequently, constant current discharging was performed at a current value equivalent to 1.0C, and discharging was stopped when the battery voltage reached 2.5V. This was considered one cycle, and the charge / discharge cycle was repeated 100 times.

[0068] (Gas Generation Measurement) The gas generation amount of lithium-ion secondary batteries before and after aging and before and after cycle characteristic evaluation in each of Examples 1 to 4 and Comparative Examples 1 to 3 was evaluated using the Archimedes method, i.e., the buoyancy of the lithium-ion secondary battery. The evaluation was performed as follows.

[0069] First, the weight of the lithium-ion secondary battery was measured using an electronic balance. Next, the weight in water was measured using a hydrometer (Alpha Mirage Co., Ltd., model number: MDS-3000), and the buoyancy was calculated by taking the difference between these weights. This buoyancy was calculated using the density of water (1.0 g / cm³). ３ The volume of the lithium-ion secondary battery was calculated by dividing by ( ). The amount of gas generated was calculated by comparing the volume before aging with the volume after aging, or the volume after aging with the volume after 100 cycles of cycle characteristic evaluation.

[0070] Table 1 shows the evaluation results for each of Examples 1 to 5 and Comparative Examples 1 to 3. Note that the capacity retention rate after 100 cycles in Table 1 was evaluated using the discharge capacity at the 100th cycle, with the initial discharge capacity set to 100. Furthermore, the gas generation amount after 100 cycles in Table 1 is normalized to match the gas generation amount in Comparative Example 1, which is 1.

[0071]

[0072] In Examples 1-4, which used PET spunbond nonwoven fabric, the volume retention rate increased and gas generation decreased compared to Comparative Example 1, which used PP microporous film. In Examples 1-4, which used PET spunbond nonwoven fabric, the volume retention rate decreased as the basis weight increased. Also, at a basis weight of 35 g / m², ２ In Examples 1-3, the amount of gas generated increased as the basis weight increased. ２ In Example 4, the basis weight was 35 g / m². ２ Although the amount of gas generated was suppressed compared to Examples 1-3, the capacity retention rate decreased significantly.

[0073] From these findings, it was found that, from the perspective of suppressing gas generation, it is effective to use PET spunbond nonwoven fabric composed of long fibers with a diameter of 10 μm or more as a separator. Among these, from the perspective of achieving both improved volume retention and suppression of gas generation, a basis weight of 35 g / m² is suitable. ２ It was found that being less than a certain value is valid.

[0074] In Comparative Example 2, which used a two-component fiber made of PET and PP fibers, the volume retention rate improved compared to Comparative Example 1, which used a microporous film made of PP, but the amount of gas generated increased significantly. Furthermore, in Comparative Example 2, which used a two-component fiber made of PET and PP fibers, the volume retention rate decreased and the amount of gas generated increased significantly compared to Example 1, which used a spunbond nonwoven fabric made of PET.

[0075] From these findings, it was determined that, from the perspective of achieving both improved capacity retention and reduced gas generation, it is effective to have PET fibers make up the majority of the separator.

[0076] In Comparative Example 3, which included PET fine fibers, the volume retention rate increased and the amount of gas generated increased significantly compared to Comparative Example 1, which used a PP microporous film. Furthermore, in Comparative Example 3, which included PET fine fibers, the volume retention rate decreased and the amount of gas generated increased significantly compared to Example 2, which used a PET spunbond nonwoven fabric.

[0077] These findings suggest that even when using PET fibers, the use of PET microfibers increases the amount of gas generated, as gas originating from the PET microfibers is produced. In other words, it was found that using PET fibers of 10 μm or larger as the separator is effective in suppressing gas generation.

[0078] From the above results, the following (1) to (3) were found: (1) It was found that gas generation is suppressed by using a PET spunbond nonwoven fabric containing long fibers with a diameter of 10 μm or more as a separator. (2) A separator with a basis weight of 35 g / m ２ It was found that using PET spunbond nonwoven fabric with a diameter of less than 10 μm improves the volume retention rate and suppresses gas generation. (3) It was found that using PET fibers with a diameter of 10 μm or more as the separator improves the volume retention rate and suppresses gas generation.

[0079] 1 Secondary battery 5 Electrolyte 12 Separator 25 First core (positive electrode active material) 26 First shell layer 35 Second core (negative electrode active material)

Claims

1. The device comprises a positive electrode active material, a negative electrode active material, a separator, and an electrolyte, wherein the positive electrode active material has an operating potential of 4.3V (vs. Li / Li ＋ A lithium-ion secondary battery wherein the separator is a fiber with a diameter of 10 μm or more, and whose main component is polyethylene terephthalate and / or polybutylene terephthalate.

2. The lithium-ion secondary battery according to claim 1, wherein the separator is composed of polyethylene terephthalate for 90% or more of the total volume.

3. The separator has a basis weight of 35 g / m². ２ A lithium-ion secondary battery according to claim 1 or 2, comprising a nonwoven fabric of less than 100%.

4. The lithium-ion secondary battery according to claim 1 or 2, wherein the separator is composed of a plurality of fibers, and when 10 fibers are randomly selected, at least one of the 10 fibers is 10 times or more the length of the separator in the longitudinal direction when the separator is unfolded.

5. The lithium-ion secondary battery according to claim 1 or 2, wherein the separator is made of spunbond nonwoven fabric.

6. The lithium-ion secondary battery according to claim 1 or 2, wherein the positive electrode active material is a compound represented by the following formula (1). １＋ｘ M ｙ Mn ２－ｘ－ｙ O ４ ... (1) (In equation (1) above, x and y satisfy 0 ≤ x ≤ 0.2 and 0 < y ≤ 0.8 respectively, and M is at least one selected from the group consisting of Al, Mg, Zn, Ni, Co, Fe, Ti, Cu, and Cr.) 7. The lithium-ion secondary battery according to claim 1 or 2, wherein the positive electrode active material is coated with a lithium-containing oxide containing lithium.

8. The negative electrode active material has an operating potential of 0.5 V (vs. Li / Li ＋ ) or more 2.5V (vs.Li / Li ＋ A lithium-ion secondary battery according to claim 1 or 2, wherein the following conditions apply.

9. The lithium-ion secondary battery according to claim 1 or 2, wherein the negative electrode active material contains titanium.

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