Secondary batteries

Abstract

To provide a secondary battery with high energy density that does not easily decrease even after repeated charging and discharging. [Solution] The above problem is solved by a secondary battery comprising at least a positive electrode, a negative electrode, and an electrolyte, wherein the ratio of the capacity per unit area of ​​the negative electrode to the capacity per unit area of ​​the positive electrode (AC ratio) is 0.2 or less, and the negative electrode current collector is an enlarged conductor.

Description

Technical Field

[0001] The present invention relates to a secondary battery. More specifically, in a secondary battery including at least a positive electrode, a negative electrode, and an electrolyte as components, the present invention relates to a secondary battery characterized in that the ratio (AC ratio) of the capacity per unit area of the negative electrode to the capacity per unit area of the positive electrode is 0.2 or less, and the negative electrode current collector is a conductor with an enlarged surface area.

Background Art

[0002] With the expansion of markets for portable electronic devices, electric vehicles, and the like, batteries used in these devices are required to have a high energy density. To date, secondary batteries have been developed that utilize electrochemical reactions associated with the transfer of charges using alkali metal ions such as lithium as charge carriers. In particular, lithium-ion secondary batteries are widely used because of their high energy density.

[0003] In lithium-ion secondary batteries, lithium-containing transition metal oxides are used as the positive electrode active material, and graphite is used as the negative electrode active material. Charging and discharging are performed using the insertion and desorption reactions of lithium ions with respect to these positive and negative electrode active materials. However, the development of new technologies for achieving an even higher energy density is required.

[0004] In response to such requirements, increasing the capacity density of the negative electrode and reducing the irreversible capacity have been studied. For example, Patent Document 1 discloses a secondary battery including a negative electrode having a lithium storage layer made of Si or a Si compound that can react with a large number of electrons and has a theoretical capacity more than twice that of graphite. Patent Document 2 discloses a secondary battery including a negative electrode in which a metal foil mainly made of lithium is attached to an electrode layer mainly made of a material capable of storing and releasing lithium to compensate for the irreversible capacity.

[0005] Generally, lithium-ion batteries are manufactured by using lithium-containing transition metal oxides as the positive electrode active material and graphite as the negative electrode active material, with electrode layers containing these materials facing each other. In this case, the ratio of the design capacity of the negative electrode per unit area to the design capacity of the positive electrode, i.e., the AC ratio, is often set to 1 or more. If it is less than 1, the amount of lithium that can be intercalated in the form of ions during charging exceeds the amount of lithium that can be intercalated, causing metallic lithium to precipitate and leading to a short circuit. On the other hand, if the AC ratio is too large, the proportion of irreversible capacity proportional to the capacity of the negative electrode increases, reducing the effective capacity. Therefore, the AC ratio is selected in the range of 1 to 1.2. Meanwhile, Patent Document 3 by the present inventors discloses a secondary battery comprising at least a positive electrode, a negative electrode, and an electrolyte as components, in which the ratio of the capacity per unit area of ​​the negative electrode to the capacity per unit area of ​​the positive electrode (AC ratio) is 0.3 or less, resulting in a short charge-discharge cycle life but ultra-high energy secondary battery. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2004-87251 [Patent Document 2] Japanese Patent Publication No. 2000-182602 [Patent Document 3] Japanese Patent Publication No. 2024-58225 [Overview of the project] [Problems that the invention aims to solve]

[0007] While Patent Document 1 uses high-capacity silicon as the active material, its irreversible capacity is greater than that of graphite, resulting in virtually no increase in actual capacity. Patent Document 2 compensates for the capacity lost as irreversible capacity by placing lithium on the negative electrode, but its effect is small, and the capacity is hardly improved. For example, in aircraft such as drones, the required work may not be possible due to insufficient flight time. Furthermore, in Patent Document 2, because there is no negative electrode active material layer, and therefore no mass of the negative electrode active material layer or electrolyte impregnated into it, the energy density is greatly improved, but it drops sharply after repeated charge-discharge cycles. Thus, a secondary battery that operates stably at high energy density has yet to be found.

[0008] The present invention was made to solve the above problems, and its objective is to provide a secondary battery with high energy density and minimal capacity degradation even after repeated charge-discharge cycles. [Means for solving the problem]

[0009] The present invention provides a secondary battery. The secondary battery of the present invention comprises at least a positive electrode, a negative electrode, and an electrolyte, has an AC ratio of 0.2 or less, and is characterized in that the negative electrode current collector is an enlarged conductor.

[0010] In the secondary battery according to the present invention, the positive electrode active material constituting the positive electrode is preferably a lithium-containing compound, and more preferably a lithium-containing transition metal oxide, from the viewpoint of the effects of the invention. [Effects of the Invention]

[0011] According to the present invention, in a secondary battery comprising at least a positive electrode, a negative electrode, and an electrolyte, by having an AC ratio of 0.2 or less, irreversible capacity due to the negative electrode is reduced, and a secondary battery with high energy density and small changes in discharge voltage can be obtained. Furthermore, by having a metal with an enlarged surface area as the negative electrode current collector, a secondary battery with little capacity degradation even after repeated charging and discharging can be obtained.

[0012] By implementing this invention, for example, applying it to a drone battery will enable longer flight times compared to before.

[0013] It should be noted that the above summary of the invention does not list all the necessary features of the present invention. Furthermore, subcombinations of these features may also constitute an invention. [Brief explanation of the drawing]

[0014] [Figure 1] This is a cross-sectional view showing an example of a secondary battery according to the present invention. [Figure 2] The charge-discharge curves for up to 10 cycles are shown for a coin-shaped cell fabricated with a non-expanding copper foil as the counter electrode at an AC ratio of 0. [Figure 3] This shows the charge-discharge curves up to 10 cycles of a coin-shaped cell fabricated using the same positive electrode but with an enlarged copper foil as the counter electrode. [Modes for carrying out the invention]

[0015] The embodiments for carrying out the present invention will be described below with reference to the drawings. However, the technical scope of the present invention should be determined based on the scope description in the claims and is not limited to the following embodiments only.

[0016] [Secondary battery] The secondary battery according to the present invention is characterized by having at least a positive electrode, a negative electrode, and an electrolyte, and having an AC ratio of 0.2 or less. The form of the secondary battery is not particularly limited, but the example in Figure 1 is a coin-type secondary battery. In this example, it has a positive electrode 4, a negative electrode 6, and an electrolyte 10, and is composed of a positive electrode case 2, a negative electrode case 3, a gasket 9, a separator 5, and a metal spring 8.

[0017] (Structural elements) The positive electrode case 2 is provided as an exterior member of the positive electrode 4 and serves as a positive electrode current collector. At the center of the bottom of the positive electrode case 2, an electrode layer composed of a positive electrode active material capable of electrochemically undergoing oxidation-reduction and a conductive material is disposed. On the positive electrode 4, a separator 5 made of a porous sheet or film such as a microporous membrane, non-woven fabric, or woven fabric is laminated. Further, a negative electrode 6 is disposed on the separator. On this negative electrode 6, a negative electrode current collector 15 made of an enlarged conductor is laminated. Further, a metal spring 8 is placed on the negative electrode current collector 7. Here, a coin-type secondary battery will be described, but it goes without saying that the battery shape is not particularly limited and can also be applied to cylindrical, rectangular, sheet-type, etc. Also, the exterior method is not particularly limited, and a metal case, a molded resin, an aluminum laminate film, etc. may be used.

[0018] In FIG. 1, the negative electrode case 3 is fixed to the positive electrode case 2 against the biasing force of the metal spring 8. In the internal space formed by fixing the positive electrode case 2 and the negative electrode case 3, the positive electrode 4, the separator 5, the negative electrode 6, and a negative electrode current collector 7 made of an enlarged conductor are laminated and placed, and the electrolyte 10 is filled. The gap between the positive electrode case 2 and the negative electrode case 3 forming the internal space is sealed via a gasket 9.

[0019] In the present invention, the AC ratio is 0.2 or less, but when it is 0, the negative electrode 6 is omitted. Usually, in a secondary battery, the positive electrode and the negative electrode are used facing each other. Therefore, the capacity ratio per unit area to be opposed affects the characteristics of the battery. In the present invention, the capacities of the entire positive electrode and negative electrode are not particularly limited. Also, in the present invention, the capacity per unit area for obtaining the AC ratio is the design capacity, and it is calculated from the amount of the electrode active material and the theoretical capacity of each electrode layer.

[0020] (Positive electrode and negative electrode) In the present invention, the electrode active material constituting the positive electrode is not particularly limited, and compounds that undergo electrochemically reversible oxidation-reduction reactions can be used, but lithium-containing compounds are particularly preferred. Examples of such compounds include lithium sulfide, lithified disulfide compounds, quinone compounds, and lithium transition metal oxides, but lithium transition metal oxides are particularly preferred due to their ease of handling and stability.

[0021] Examples of lithium transition metal oxides include lithium manganese composite oxide, lithium cobalt oxide, lithium nickelate, lithium manganese spinel, and LiNi. x Co y Mn z Examples of lithium-containing metal oxides include O2 (x+y+z=1, 0≦x≦1, 0≦y≦1, 0≦z≦1) and olivine-type LiMPO4 (where M is one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr).

[0022] In the present invention, the positive electrode 4 can be manufactured by conventionally known methods. Specifically, it can be manufactured by mixing a positive electrode active material such as lithium metal oxide with a conductive material and a binder, adding a solvent to produce a slurry, coating the slurry onto an electrode foil that will serve as a current collector using conventionally known methods, and drying it.

[0023] In the present invention, the negative electrode 6 can be manufactured by a conventionally known method.

[0024] The electrode active material constituting the negative electrode is not particularly limited, and compounds that undergo electrochemically reversible oxidation-reduction reactions can be used. Such compounds include, for example, graphite such as artificial graphite, natural graphite, and coated natural graphite, carbon materials such as soft carbon and hard carbon, and Li4Ti5O 12Examples include lithium transition metal composite oxides and silicon compounds such as Si and SiO. In the present invention, the negative electrode can be formed in the same way as in conventional methods, for example, by mixing the electrode active material with a conductive material (such as graphite) and a binder, adding a solvent to prepare a slurry, and then applying the slurry to the electrode foil which will serve as the current collector in a conventionally known manner and drying it.

[0025] In this invention, when the AC ratio is 0, the process of coating and drying the negative electrode is omitted, so a negative electrode current collector with low strength, such as a thin film, can be used. In this invention, the material constituting the negative electrode current collector is not particularly limited as long as it is a conductor, but copper foil and titanium foil are preferred from the viewpoint of the effects of the invention. In this invention, an enlarged conductor is a conductor in which the ratio of the actual surface area to the apparent area is large, and examples include electrode foil made of a conductor that has been etched or roughened, and a net made of wires made of a conductor. In this invention, the actual surface area of ​​the enlarged conductor is measured with a scanning probe microscope or a laser displacement type three-dimensional shape measuring machine.

[0026] In the present invention, the conductive materials used to manufacture the positive and negative electrodes are not particularly limited, and examples include carbonaceous fine particles such as carbon black, Ketjenblack, and acetylene black; carbonaceous fibers such as vapor-grown carbon fibers, carbon nanotubes, and carbon nanohorns; and carbonaceous sheets such as graphene. Two or more of these conductive materials can be used in combination as needed. The solvent is also not particularly limited, and examples include aproton solvents such as N-methylpyrrolidone, dimethyl sulfoxide, dimethylformamide, propylene carbonate, diethyl carbonate, dimethyl carbonate, γ-butyrolactone, acetonitrile, tetrahydrofuran, nitrobenzene, and acetone, as well as methanol, ethanol, and water. The binder is not particularly limited as long as it binds the electrode active material and conductive material, and examples include various resins such as polyethylene, polyvinylidene fluoride, polyhexafluoropropylene, polytetrafluoroethylene, polyethylene oxide, carboxymethylcellulose, styrene-butadiene rubber, polyacrylic acid, and polyimide resin.

[0027] In this invention, when the AC ratio is 0, the electrode consisting of an enlarged conductor that becomes the negative electrode is used as is.

[0028] According to the inventors' research, when using negative electrode active materials such as graphite or silicon, during charging, lithium reacts with the negative electrode active material up to the AC ratio, and beyond that, it is deposited on the electrode in the form of metallic Li. Next, during discharge, the metallic Li, which has a lower potential, is ionized first, and then the lithium that has reacted with the active material dissociates into ions. The reaction potential is lowest for metallic Li, and becomes about 0.1 to 0.3 V higher when reacting with the active material. Also, the reaction rate is highest for metallic Li, and becomes lower when reacting with the active material. In other words, in secondary batteries with an AC ratio of 1 or less, a high voltage and high-power discharge are possible in the region above that AC ratio.

[0029] Incidentally, a lower AC ratio results in a shorter charge-discharge cycle life. Figure 2 shows the charge-discharge curve up to 10 cycles for a coin-type cell made with a non-expanded copper foil as the counter electrode and an AC ratio of 0. As is clear from the figure, the capacity decreases by about 10% with each repeated charge-discharge. On the other hand, Figure 3 shows the charge-discharge curve up to 10 cycles for a coin-type cell made with the same positive electrode but with an expanded copper foil as the counter electrode. It can be seen that the capacity decrease associated with charge-discharge cycles is significantly reduced by using an expanded copper foil. The reason for this is not fully understood, but it is thought that when the negative electrode current collector is smooth, the Li metal generated during charging is deposited perpendicular to the electrode surface, whereas with an expanded negative electrode current collector, the metal is oriented in various directions relative to the electrode surface, thus reducing the proportion of Li lost in a single charge-discharge cycle.

[0030] (electrolyte) In the present invention, the electrolyte 10 is interposed between the positive electrode 4 and the negative electrode 6 to transport charge carriers between the two electrodes, and is not particularly limited as long as it has ion conductivity, and can be used at room temperature. -6Liquid, gel-like, and solid electrolytes having an ionic conductivity of S / cm or higher can be used. In this invention, due to their ease of reaction, liquid and gel-like electrolytes are preferably used. In the case of a liquid electrolyte, it is an organic solvent containing an electrolyte salt. Examples of electrolyte salts include LiPF6, LiClO4, LiBF4, LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiC(CF3SO3)3, LiC(C2F5SO2)3, etc. Examples of organic solvents include ethylene carbonate, fluoroethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, γ-butyrolactone, tetrahydrofluoric acid, and methyl ethyl carbonate. Examples include lanes, dioxolane, dimethyl sulfone, ethyl methyl sulfone, diethyl sulfone, propyl methyl sulfone, isopropyl methyl sulfone, propyl ethyl sulfone, isopropyl ethyl sulfone, dipropyl sulfone, diisopropyl sulfone, sulfolane, pentamethylene sulfone, hexamethylene sulfone, 3-methylsulfolane, 2,4-dimethylsulfolane, N,N-dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, and mixed solvents thereof.

[0031] In this invention, the concentration of the electrolyte salt is not particularly limited, but can be arbitrarily selected within the range of 0.1 mol / L to 2.5 mol / L, and a common concentration such as 1 mol / L may also be used. Furthermore, electrolyte additives such as vinylene carbonate, hydrofluoroether, and biphenyl may be added to the electrolyte.

[0032] The electrolyte 10 may be a gel electrolyte made by impregnating a polymer compound with a solvent to form a gel, an ionic liquid, a symmetric glycol diether such as Glyme, or a chain sulfone. Examples of polymer compounds include vinylidene fluoride polymers such as polyvinylidene fluoride, vinylidene fluoride-tetrafluoroethylene copolymer, and vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene ternary copolymer; acrylonitrile polymers such as acrylonitrile-methyl methacrylate copolymer; polyethylene oxide, ethylene oxide-propylene oxide copolymer, and polymers of these acrylates and methacrylates containing an electrolyte.

[0033] [Manufacturing method for secondary batteries] The method for manufacturing a secondary battery according to the present invention is a method for manufacturing the secondary battery 1 described above, and is a method for manufacturing a secondary battery having at least a positive electrode 4, a negative electrode 6, and an electrolyte 10. The secondary battery 1 manufactured by this method has a ratio of 0.2 or less between the capacity per unit area of ​​the negative electrode 6 and the capacity per unit area of ​​the positive electrode 4, and is characterized by having a small irreversible capacity, a high discharge voltage, and small fluctuations.

[0034] A typical lithium-ion secondary battery manufacturing method consists of an electrode formation process, an electrode and separator lamination process, an electrolyte injection and impregnation process, an electrode extraction process, and an outer packaging process. In this invention, a typical lithium-ion secondary battery manufacturing method can be used, except that the AC ratio is set under certain conditions.

[0035] It should be noted that the technical scope of the present invention is not limited to the embodiments described above, and various modifications can be made without departing from the spirit of the invention. The present invention will be further described in detail below with reference to examples and comparative examples. [Examples]

[0036] [Example 1] (Manufacturing of secondary batteries) N-methylpyrrolidone (NMP) solutions containing 9.4 g of lithium cobalt oxide, 0.4 g of acetylene black, and 0.3 g of polyvinylidene fluoride (PVDF) were weighed out, and NMP was added and kneaded to form a mixture. Further NMP was added to this mixture to adjust the viscosity and prepare a positive electrode slurry for coating. This positive electrode slurry was coated onto an aluminum foil with a coating thickness of 100 μm, a width of 130 mm, and a thickness of 12 μm using a knife roll coater, and dried at 120°C to obtain a single-sided coated electrode. Subsequently, the same positive electrode slurry was used to coat the back side to a thickness of 100 μm to obtain a double-sided coated foil. Next, the obtained double-sided coated electrode was roll-pressed to form a 50 μm thick positive electrode active material layer on both sides of the aluminum foil current collector, thereby obtaining a positive electrode with lithium cobalt oxide as the positive electrode active material.

[0037] The positive electrode prepared as described above was cut into a circle with a diameter of 13 mm, and placed opposite a copper mesh with a thread diameter of 0.05 mm, a mesh opening of 0.05 mm, and a thickness of 0.1 mm via a separator. This was then placed in a coin cell component, and an ethylene carbonate / diethyl carbonate mixed solvent (3 / 7) containing 1 M LiPF6, which served as the electrolyte, was added. The entire assembly was then crimped together with a retaining spring to create a coin-type battery. The AC ratio of this secondary battery was 0.

[0038] (Checking the operation of the rechargeable battery) The fabricated secondary battery was charged with a constant current of 1.0 mA until the voltage reached 4.5 V, and then discharged with a constant current of 1.0 mA to 2.5 V. As a result, it was confirmed that this cell is a secondary battery with a discharge capacity of 3.8 mAh. Subsequently, the charge and discharge cycle was repeated 10 times in the range of 2.5 to 4.5 V, and even after 10 cycles, the capacity remained above 3 mAh. The energy density of this cell, calculated from its energy and mass excluding the coin cell components, was above 450 Wh / kg even after 10 cycles.

[0039] [Comparative Example 1] (Manufacturing of secondary batteries) A positive electrode, prepared as described above, was coated using the method of Example 1 and cut out in the same manner as in Example 1. This positive electrode was then cut into a circle with a diameter of 13 mm and placed in a coin cell component via a separator, using a 10 μm thick copper foil instead of the copper mesh used in Example 1. An electrolyte solution was added to produce a coin-type battery. The AC ratio of this secondary battery was 0.

[0040] (Checking the operation of the rechargeable battery) The fabricated secondary battery was charged with a constant current of 1.0 mA until the voltage reached 4.5 V, and then discharged with a constant current of 1.0 mA to 2.5 V. As a result, it was confirmed that this cell is a secondary battery with a discharge capacity of 3.9 Ah. The energy density, calculated from the energy of this cell and the mass of the coin cell material, was 450 Wh / kg in the first cycle. Subsequently, when the charge and discharge cycle was repeated 10 times in the range of 2.5 to 4.5 V, the capacity remained at 1.5 Ah even after 10 cycles, indicating a short charge-discharge cycle life where the capacity decreases with both charging and discharging.

[0041] [Example 2] (Manufacturing of secondary batteries) A positive electrode, prepared as described above, was coated using the method of Example 1 and cut out in the same manner as in Example 1. This positive electrode was then cut into a circle with a diameter of 13 mm and placed in a coin cell component via a separator, using embossed copper foil with a thickness of 35 μm instead of the copper mesh used in Example 1. An electrolyte solution was added to produce a coin-type battery. The AC ratio of this secondary battery was 0.

[0042] (Checking the operation of the rechargeable battery) The fabricated secondary battery was charged with a constant current of 1.0 mA until the voltage reached 4.5 V, and then discharged with a constant current of 1.0 mA to 2.5 V. As a result, it was confirmed that this cell was a secondary battery with a discharge capacity of 3.7 mAh. Subsequently, the charge and discharge cycle was repeated 10 times in the range of 2.5 to 4.5 V, and the capacity remained above 3.0 Ah even after 10 cycles. The energy density calculated from the energy and mass of this cell was 450 Wh / kg after 10 cycles.

[0043] [Example 3] (Manufacturing of secondary batteries) A positive electrode, prepared as described above, was coated using the method of Example 1 and cut out in the same manner as in Example 1. This positive electrode was then cut into a circle with a diameter of 13 mm and placed in a coin cell component via a separator, using the same method as in Example 1 except that a 15 μm thick perforated copper foil was used instead of the copper mesh of Example 1. An electrolyte solution was then added to produce a coin-type battery. The AC ratio of this secondary battery was 0.

[0044] (Checking the operation of the rechargeable battery) The fabricated secondary battery was charged with a constant current of 1.0 mA until the voltage reached 4.5 V, and then discharged with a constant current of 1.0 mA to 2.5 V. As a result, it was confirmed that this cell was a secondary battery with a discharge capacity of 3.8 mAh. Subsequently, the charge and discharge cycle was repeated 10 times in the range of 2.5 to 4.5 V, and the capacity remained above 3.5 mAh even after 10 cycles. The energy density calculated from the energy and mass of this cell was 450 Wh / kg after 10 cycles.

[0045] [Example 4] (Manufacturing of secondary batteries) Except for using lithium nickel-cobalt-manganate (Ni:Co:Mn=1:1:1, LiNMC) instead of lithium cobalt oxide in Example 1, a slurry was prepared and coated in the same manner as in Example 1. Electrodes cut out in the same manner as in Example 1 were placed in a coin cell component using the copper mesh of Example 1 via a separator, and an electrolyte was added to produce a coin-type battery. The AC ratio of this secondary battery was 0.

[0046] (Checking the operation of the rechargeable battery) The fabricated secondary battery was charged with a constant current of 1.0 mA until the voltage reached 4.5 V, and then discharged with a constant current of 1.0 mA to 2.5 V. As a result, it was confirmed that this cell was a secondary battery with a discharge capacity of 4.1 mAh. Subsequently, the charge and discharge cycle was repeated 10 times in the range of 2.5 to 4.5 V, and the capacity remained above 3.6 mAh even after 10 cycles. The energy density calculated from the energy and mass of this cell was 450 Wh / kg after 10 cycles. [Explanation of Symbols]

[0047] 1 Secondary battery 2 Positive electrode case 3. Negative electrode case 4 Positive electrode 5 Separators 6 negative electrode 7 Negative electrode current collector 8. Metal spring 9 Gasket 10 Electrolytes

Claims

1. A secondary battery comprising at least a positive electrode, a negative electrode, and an electrolyte, characterized in that the ratio (AC ratio) of the capacity per unit area of ​​the negative electrode to the capacity per unit area of ​​the positive electrode is 0.2 or less, and the negative electrode current collector is an enlarged conductor.

2. The secondary battery according to claim 1, wherein the positive electrode active material constituting the positive electrode contains a lithium-containing compound.

3. The secondary battery according to claim 1 or 2, wherein the enlarged conductor is made of net, roughened, or embossed copper or titanium.

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