Lithium secondary battery

Abstract

The present invention provides a lithium secondary battery that has excellent stability and high energy density. The present invention relates to a lithium secondary battery comprising: a current collector that includes a resin layer and a metal layer formed on both surfaces of the resin layer; and an active material layer that is formed on at least one surface of the current collector. On the at least one surface of the current collector, a weight reduction region is provided between an active material region in which the active material layer is formed, and a non-formation region in which the active material layer is not formed, the active material layer in the weight reduction region having a smaller basis weight than that of the active material region.

Description

Lithium secondary battery

[0001] The present invention relates to a lithium secondary battery.

[0002] In recent years, technology for converting natural energy such as solar or wind power into electrical energy has been attracting attention. Accordingly, various secondary batteries have been developed as electricity storage devices that are highly safe and capable of storing a large amount of electrical energy.

[0003] Among them, lithium secondary batteries, which are charged and discharged by the movement of lithium ions between a positive electrode and a negative electrode, are known to exhibit high voltage and high energy density. A typical lithium secondary battery is a lithium ion secondary battery (LIB), whose positive and negative electrodes have active materials for retaining lithium elements. In a lithium ion secondary battery, charging and discharging are performed by the exchange of lithium ions between the positive electrode active material and the negative electrode active material.

[0004] Furthermore, current collectors have been developed with the aim of improving the safety of lithium-ion secondary batteries. For example, Patent Document 1 discloses a lithium-ion secondary battery in which, for the purpose of providing a positive / negative electrode configuration that prevents ignition in the event of abnormal heat generation during overcharge or at high temperatures while maintaining electrical resistance at the same level as conventional batteries, a current collector 2 that holds an active material layer 1 in the positive or negative electrode is made up of a resin film 3 that melts in the event of abnormal heat generation in the battery, and metal layers 4 that are vapor-deposited on both sides of the resin film 3 and exchange charge with the active material.

[0005] Japanese Patent Application Publication No. 11-102711

[0006] However, as mentioned above, current collectors using resin films are more likely to stretch than those made of metal, and when a pressing process is performed after coating the active material, there is a concern that deformation or breakage may occur due to the difference in elongation between the coated and uncoated parts. Therefore, it has been difficult to compress the current collector to a high density while preventing deformation or breakage.

[0007] The present invention has been made in view of the above circumstances, and has as its object to provide a lithium secondary battery that is excellent in stability and has a high energy density.

[0008] A lithium secondary battery according to one embodiment of the present invention comprises a current collector including a resin layer and metal layers formed on both sides of the resin layer, and an active material layer formed on at least one side of the current collector, and on at least one side of the current collector, a basis weight reduction region is provided between an active material region where the active material layer is formed and an unformed region where the active material layer is not formed, in which the basis weight of the active material layer is smaller than that of the active material region.

[0009] According to the present invention, it is possible to provide a lithium secondary battery having excellent stability and high energy density.

[0010] 1 is a schematic cross-sectional view showing an example of an electrode of the present invention. FIG. 1 is a schematic cross-sectional view showing an example of an electrode of the present invention. FIG. 1 is a schematic cross-sectional view showing an example of an electrode of the present invention. FIG. 2 is a schematic cross-sectional view showing an example of an electrode of the present invention. FIG. 3 is a schematic cross-sectional view showing an example of an electrode of the present invention. FIG. 4 is a schematic cross-sectional view showing an example of a lithium secondary battery of the present invention. FIG. 5 is a schematic cross-sectional view showing an example of a coating device used when manufacturing an electrode. FIG. 6 is a schematic cross-sectional view showing an example of a coating device used when manufacturing an electrode. FIG. 7 shows the measurement results of the basis weight ratio of Example 1 and Comparative Example 1. FIG. 8 is a graph showing the measurement results of the basis weight ratio of Example 1 and Comparative Example 1. FIG. 9 is a graph showing the measurement results of the elongation rate of Example 1 and Comparative Example 1. FIG. 10 is a graph showing the measurement results of the basis weight ratio of Example 2 and Comparative Example 2. FIG. 11 is a graph showing the measurement results of the elongation rate of Example 2 and Comparative Example 2. FIG. 12 is a photograph of the electrode of Example 1. FIG. 13 is a photograph of the electrode of Comparative Example 1.

[0011] Hereinafter, a mode for carrying out the present invention (hereinafter simply referred to as "the present embodiment") will be described in detail, but the present invention is not limited to the following embodiment. The present invention can be modified in various ways without departing from the gist of the present invention. In the drawings, the same elements are given the same reference numerals, and redundant explanations will be omitted. Furthermore, positional relationships such as up, down, left, and right are based on the positional relationships shown in the drawings unless otherwise specified. Furthermore, the dimensional ratios of the drawings are not limited to those shown in the drawings.

[0012] 1. Lithium Secondary Batteries 1.1. Electrodes In recent years, as lithium secondary batteries have become lighter and smaller, research into achieving higher energy densities has been actively conducted. While increasing the amount of active material in the battery is one way to increase energy density, this poses challenges, including the risk of internal short circuits and overcharging. Therefore, it has been shown that lithium secondary batteries using current collectors with metal foils, such as aluminum layers, formed on both sides of a resin film can be lighter than those using conventional metal current collectors, while also achieving higher energy densities.

[0013] However, the above-described current collector film tends to stretch easily, and when an active material is applied to its surface and then subjected to a pressing process, deformation and breakage are likely to occur. This problem needs improvement. After extensive research into this issue, the inventors discovered that a lithium secondary battery comprising a current collector (hereinafter simply referred to as "current collector") including a resin layer and metal layers formed on both sides of the resin layer, and an active material layer formed on at least one side of the current collector, can be achieved without deformation or breakage by providing a weight-relief region between an active material region where the active material layer is formed and an unformed region where the active material layer is not formed, on at least one side of the current collector, where the weight-relief region has a lower weight-relief region than the active material region. While the reason for this is not entirely clear, it is believed that the provision of the weight-relief region reduces the difference in elongation at the interface between the active material region and the unformed region during the pressing process, thereby reducing stress concentration and reducing deformation and breakage. However, the reasons for this are not limited to those described above.

[0014] 1 is a schematic cross-sectional view showing an example of an electrode of this embodiment. As shown in FIG. 1 , the lithium secondary battery of this embodiment includes a current collector 11 including a resin layer A and a metal layer B formed on both sides of the resin layer A, and an active material layer 12 formed on at least one side of the current collector 11. On at least one side of the current collector 11, a basis weight relaxation region 103 having a lower basis weight of the active material layer than the active material region 101 is provided between an active material region 101 where the active material layer 12 is formed and an unformed region 102 where the active material layer 12 is not formed. Thus, the active material layer 12 includes the basis weight relaxation region 103 where the basis weight varies and the active material region 101 where the basis weight remains almost unchanged.

[0015] 2 is a schematic cross-sectional view showing another example of the electrode of the present embodiment. The electrode of the lithium secondary battery of the present embodiment may include active material layers 12 on both sides of a current collector 11, as shown in FIG.

[0016] In this specification, the basis weight relaxation region 103 refers to a region between the active material region 101 where the active material layer 12 is formed and the unformed region 102 where the active material layer 12 is not formed, and which has a lower basis weight of the active material than the active material region 101. When the active material layer 12 is formed on both surfaces of the current collector 11, the region where the total basis weight of the active material on both surfaces changes is referred to as the basis weight relaxation region 103.

[0017] The material used for the current collector 11 in this embodiment is not particularly limited and may be selected in consideration of the function of each electrode. For example, the configurations described later in the description of the positive electrode and negative electrode may be used.

[0018] 1 , it is preferable that the basis weight of active material layer 12 gradually changes from the unformed region side to the active material region side in weight relaxation region 103. The gradual change in the basis weight of the active material in weight relaxation region 103 further improves the effect of the present invention of preventing deformation or breakage of current collector 11 that may occur during the pressing step, and tends to result in a lithium secondary battery that is excellent in stability and has a high energy density.

[0019] In the basis weight relaxation region 103, the region having a basis weight of 98.5% or less relative to 100% of the basis weight of the central electrode portion (active material region 101) preferably has a length of 3 mm or more and 30 mm or less. When the length of the region having a basis weight of 98.5% or less falls within the above range, the effect of the present invention of preventing deformation and breakage of the current collector is further improved, tending to result in a lithium secondary battery with excellent stability and high energy density. From the same viewpoint, the length of the region having a basis weight of 98.5% or less is more preferably 4 mm or more and 27 mm or less, and even more preferably 5 mm or more and 25 mm or less. Furthermore, from the same viewpoint as above, in the basis weight relaxation region 103, the length of the region having a basis weight of 97% or less relative to 100% of the basis weight of the central electrode is preferably 2 mm or more and 30 mm or less, more preferably 3 mm or more and 27 mm or less, and even more preferably 4 mm or more and 25 mm or less.

[0020] In the basis weight relaxation region 103, it is preferable to have a section A of 3 mm or more in which the amount of change in basis weight per mm of distance from the end is 2.5% or less relative to 100% basis weight of the central portion of the electrode (active material region 101). When the length of the section A is 3 mm or more, the effect of the present invention of preventing deformation and breakage of the current collector tends to be more effectively and reliably achieved. From the same viewpoint, the length of the section A is more preferably 4 mm or more, and even more preferably 5 mm or more.

[0021] Fig. 3 is a schematic cross-sectional view showing another example of an electrode according to the present embodiment. As shown in Fig. 3, the lithium secondary battery according to the present embodiment preferably includes a second active material layer 13 having a lower rigidity than the active material of the active material layer on the active material layer 12 in the basis weight relaxation region 103. By forming the active material layer in the basis weight relaxation region 103 into a two-layer structure having different rigidities, the existing active material layer 12 can provide the effect of improving the stability of the lithium secondary battery, while the second active material layer 13 can provide the effect of high energy density, resulting in an even better lithium secondary battery.

[0022] 3, the active material layer 12 in contact with the current collector 11 is also referred to as the "first active material layer." When the second active material layer 13 is provided as shown in FIG. 3, if the total basis weight of the first active material layer (active material layer 12) and the second active material layer 13 is less than that of the active material region 101, the active material layer 12 is said to have a basis weight relaxation region 103.

[0023] The active material of the second active material layer 13 preferably has a lower density than the active material of the first active material layer (active material layer 12). By making the density of the second active material layer 13 lower than the density of the first active material layer (active material layer 12), the effects of improving the stability and energy density of the lithium secondary battery tend to be more effective and reliable.

[0024] The difference in density between the active material compositions used in the first active material layer (active material layer 12) and the second active material layer 13 is preferably 0.03 g / cc or more and 0.3 g / cc or less. By keeping the difference in density between the active material layers within the above range, the effect of improving the stability and energy density of the lithium secondary battery according to the present invention tends to be more effective and reliable. From the same viewpoint, the difference in density between the active material compositions used in the first active material layer (active material layer 12) and the second active material layer 13 is more preferably 0.05 g / cc or more and 0.2 g / cc or less, and even more preferably 0.1 g / cc or more and 0.15 g / cc or less.

[0025] There are no particular limitations on the active material compositions used in the first active material layer (active material layer 12) and the second active material layer 13, and different active material compositions may be used depending on the needs of each electrode. For example, they may be prepared using the active materials described later in the description of the positive electrode and negative electrode.

[0026] FIG. 4 is a schematic cross-sectional view showing another example of an electrode according to this embodiment. As shown in FIG. 4 , the lithium secondary battery according to this embodiment preferably includes active material layers 12 on both sides of a current collector 11, with the active material layer 12 on one side and the active material layer 12' on the other side being misaligned. Here, the misalignment between the active material layer 12 on one side and the active material layer 12' on the other side is more preferably 0.5 mm to 3.0 mm. By misaligning the active material layers on both sides within the above range, a weight-reducing region 103, which is a region in the current collector 11 where the total weight per unit area is reduced, is formed, which tends to improve the stability and energy density of the lithium secondary battery. From the same perspective, the misalignment range is more preferably 0.7 mm to 2.5 mm, and even more preferably 1.0 mm to 2.0 mm.

[0027] 5 is a schematic cross-sectional view showing another example of an electrode according to the present embodiment. As shown in FIG. 5, in the weight reduction region 103, it is preferable to further provide an insulating material layer 14 on the active material layer 12 and the current collector 11 in the unformed region 102. By providing the insulating material layer 14, the effects of improving the stability and energy density of the lithium secondary battery according to the present invention tend to be more effective and reliable. The insulating material is not particularly limited, and is preferably, for example, an insulating material such as alumina.

[0028] The basic configuration of a lithium secondary battery according to one embodiment of the present invention will be described with reference to Fig. 6. As shown in Fig. 6, the lithium secondary battery according to one embodiment of the present invention includes a plurality of positive electrodes 10, a plurality of separators 20, and a plurality of negative electrodes 30, with the positive electrodes 10 and the negative electrodes 30 spaced apart by the separators 20. The lithium secondary battery of this embodiment also employs an electrode structure including a basis weight reduction region 103 shown in any one of Figs. 1 to 5 in at least one of the positive electrodes 10 and the negative electrodes 30. The configurations of the current collector 11 and the active material layer 12 that can be used in each of the positive electrodes 10 and the negative electrodes 30 will be described in detail below.

[0029] 1.1.1. Positive Electrode In the positive electrode 10, a film-like current collector (current collector film) including a resin layer A and a metal layer B formed on both sides of the resin layer A is preferably used as the positive electrode current collector. The resin layer A preferably includes at least one selected from the group consisting of polyethylene terephthalate (PET), polypropylene, polyamide, acrylic resin, polycarbonate, polyethylene, polyvinyl chloride, and polystyrene, and is more preferably made of polyethylene terephthalate or polypropylene. By configuring the resin layer A as described above, the effects of improving stability and energy density according to the present invention tend to be more effective and reliable.

[0030] The resin layer A is preferably in the form of a film (sheet), and its average thickness is preferably 3.0 μm or more and 10 μm or less, and more preferably 4.0 μm or more and 7.0 μm or less. By setting the thickness of the resin layer A within the above range, the effects of improving stability and energy density according to the present invention tend to be more effectively and reliably achieved.

[0031] The metal layer B included in the positive electrode current collector of this embodiment is in physical and / or electrical contact with the positive electrode active material layer and functions to donate and receive electrons to and from the positive electrode active material layer. The metal layer B of the positive electrode current collector is composed of a conductor such as a metal that does not react with lithium in a battery. The metal constituting the metal layer B of the positive electrode current collector is not particularly limited, but is preferably at least one selected from the group consisting of aluminum, titanium, stainless steel, nickel, and alloys thereof, and more preferably aluminum (Al). By configuring the metal layer B of the positive electrode current collector as described above, the effects of improving stability and energy density according to the present invention tend to be more effective and reliable.

[0032] The average thickness of each metal layer B in the positive electrode current collector is preferably 0.2 μm to 10 μm, more preferably 0.3 μm to 5.0 μm, and even more preferably 0.5 μm to 3.0 μm. By setting the average thickness of the metal layer B in the positive electrode current collector within the above range, the effects of improving stability and energy density according to the present invention tend to be more effectively and reliably achieved.

[0033] The positive electrode 10 may be a positive electrode current collector having a positive electrode active material layer formed on one or both surfaces thereof. The positive electrode active material layer may be formed by applying a positive electrode active material composition obtained by mixing the positive electrode active material with a solvent, a conductive aid, a binder, an additive, etc. as needed, followed by a pressing step.

[0034] The positive electrode active material contained in the positive electrode active material composition is not particularly limited, and examples thereof include a positive electrode active material represented by the general formula: Li ｚ Ni ｘ Co ｙ M １－ｘ－ｙ O ２＋α (wherein 0.5≦x≦1.0, 0≦y≦0.35, 0.9≦z≦1.3, −0.2≦α≦0.15, and M is one or more elements selected from the group consisting of Mn, Al, V, Mg, Mo, Nb, Ti, Zr, Fe, Cu, Cr, Zn, F, and B).

[0035] Preferably, the positive electrode active material layer is a compound represented by the general formula: Li ｚ Ni ｘ Co ｙ M １－ｘ－ｙ O ２＋α (wherein 0.7≦x≦1.0, 0≦y≦0.35, 0.9≦z≦1.3, −0.2≦α≦0.15, and M is one or more elements selected from the group consisting of Mn, Al, V, Mg, Mo, Nb, Ti, Zr, Fe, Cu, Cr, Zn, F, and B). By including the above-mentioned compound with a high nickel ratio as the positive electrode active material, the energy density of the lithium secondary battery tends to be further improved. Furthermore, as the nickel ratio increases, a redox shuttle reaction is more likely to occur, but by including an additive, which will be described in detail below, in the electrolyte, the reaction is suppressed, and the electrolyte tends to have excellent performance stability at high temperatures.

[0036] The first and second active materials in the positive electrode 10 may have different rigidities and densities when layers are formed, i.e., the same positive electrode active material or the same negative electrode active material may be used to prepare compositions with different rigidities and densities. From the viewpoint of more effectively and reliably achieving the effects of improving the stability and energy density of the lithium secondary battery according to the present invention, it is preferable to prepare different compositions using the same positive electrode active material, and LiNi ０．８ Co ０．１ Mn ０．１ O ２ It is more preferable to prepare and use the above at different concentrations.

[0037] The positive electrode active material may contain a positive electrode active material other than the above-mentioned compounds. Specifically, the other positive electrode active material in this embodiment may be a host material for lithium element (typically, lithium ions). Such other positive electrode active materials include, but are not limited to, metal oxides and metal phosphates. The metal oxides include, but are not limited to, cobalt oxide-based compounds, manganese oxide-based compounds, and nickel oxide-based compounds.

[0038] The content of the positive electrode active material is preferably 50% by mass or more and 95% by mass or less, more preferably 60% by mass or more and 85% by mass or less, and even more preferably 65% ​​by mass or more and 80% by mass or less, based on the total amount of the positive electrode active material composition. By setting the content of the positive electrode active material within the above range, the effects of improving the stability and energy density of the lithium secondary battery according to the present invention tend to be more effectively and reliably achieved.

[0039] When the positive electrode 10 has both a first active material layer (active material layer 12) and a second active material layer 13, the difference B1 between the content (% by mass) of the positive electrode active material in the first positive electrode active material composition and the content (% by mass) of the positive electrode active material in the second positive electrode active material composition is preferably 0.1% by mass or more and 5% by mass or less, more preferably 0.5% by mass or more and 3% by mass or less, and even more preferably 0.8% by mass or more and 1.5% by mass or less. Note that the content (% by mass) of the active material in the active material composition is calculated as the content of the active material relative to the total amount of the active material composition, in % by mass.

[0040] The solid content of the positive electrode active material composition is preferably 60% by mass or more and 90% by mass or less, more preferably 65% ​​by mass or more and 85% by mass or less, and even more preferably 70% by mass or more and 80% by mass or less, relative to the total amount of the positive electrode active material composition. When the solid content of the positive electrode active material composition is within the above range, the effects of the present invention tend to be more effectively and reliably achieved.

[0041] The positive electrode active material composition may contain a conductive additive. The conductive additive is not particularly limited, but may be, for example, a carbon material. Among carbon materials, carbon black is preferred, and acetylene black is more preferred, from the viewpoint of more effectively and reliably achieving the effects of the present invention.

[0042] The positive electrode active material composition may contain a binder. Examples of the binder include, but are not limited to, polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), and polyacrylic acid (PAA). Among these, PVDF is preferred from the viewpoint of more effectively and reliably achieving the effects of the present invention.

[0043] The thickness of the positive electrode active material layer is preferably 20 μm or more and 300 μm or less per surface. By setting the thickness of the positive electrode active material layer within the above range, the effect of improving the stability and energy density of the lithium secondary battery tends to be more effective and reliable. From the same viewpoint, the thickness of the positive electrode active material layer is more preferably 50 μm or more and 200 μm or less, and even more preferably 70 μm or more and 150 μm or less.

[0044] 1.1.2. Negative Electrode The negative electrode 30 preferably includes a negative electrode current collector and a negative electrode active material layer formed on one or both sides of the negative electrode current collector. The negative electrode current collector preferably includes a film-like current collector (current collector film) including a resin layer A and a metal layer B formed on both sides of the resin layer A. In the current collector film used as the negative electrode current collector, the resin layer A is preferably the same as the resin layer A used in the current collector film of the positive electrode 10 described above, and is preferably of the same thickness.

[0045] The metal layer B included in the negative electrode current collector is in physical and / or electrical contact with the negative electrode active material layer and functions to donate and receive electrons to and from the negative electrode active material layer. The metal layer B of the negative electrode current collector is not particularly limited, but is preferably copper (Cu). When the metal layer B of the negative electrode current collector is copper (Cu), the effects of improving the stability and energy density of the lithium secondary battery according to the present invention tend to be more effective and reliable.

[0046] The average thickness of each metal layer B in the negative electrode current collector is preferably 0.2 μm to 10 μm, more preferably 0.3 μm to 5.0 μm, and even more preferably 0.5 μm to 3.0 μm. By keeping the average thickness of the metal layer B in the positive electrode current collector within the above range, the effects of improving stability and energy density according to the present invention tend to be more effectively and reliably achieved.

[0047] The negative electrode 30 may be formed by forming a negative electrode active material layer on one or both surfaces of a negative electrode current collector. The negative electrode active material layer may be formed by applying a negative active material composition obtained by mixing the negative electrode active material with a solvent, a conductive aid, a binder, an additive, etc. as needed, followed by a pressing step.

[0048] The negative electrode active material is a material that causes an electrode reaction, i.e., an oxidation reaction and a reduction reaction, in the negative electrode 30. The negative electrode active material contained in the negative electrode active material composition of this embodiment is not particularly limited, and examples thereof include carbon-based active materials such as graphite, graphene, hard carbon, and carbon nanotubes; ｙ -SiO ｘ , SiC, SiO ｘ Examples of the active material include Si-based active materials such as titanium oxide-based compounds and cobalt oxide-based compounds; metal oxide-based active materials such as germanium, tin, lead, aluminum, and gallium, and metal / alloy-based active materials in which these metals are pre-doped with lithium. From the viewpoint of more effectively and reliably achieving the effects of the present invention, carbon-based active materials are preferred, and graphite is more preferred.

[0049] The content of the negative electrode active material is preferably 30% by mass or more and 80% by mass or less, more preferably 35% by mass or more and 70% by mass or less, and even more preferably 40% by mass or more and 55% by mass or less, based on the total amount of the negative electrode active material composition. By setting the content of the negative electrode active material within the above range, the effects of improving the stability and energy density of the lithium secondary battery according to the present invention tend to be more effectively and reliably achieved.

[0050] The negative electrode active material composition may contain a binder. The binder is not particularly limited, but examples thereof include the materials that can be used for the positive electrode 10 described above. Among such materials, from the viewpoint of more effectively and reliably achieving the effects of the present invention, it is preferable that the negative electrode active material composition contains carboxymethyl cellulose (CMC) or styrene-butadiene rubber (SBR), and it is more preferable that the negative electrode active material composition contains carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR).

[0051] The content of the binder is preferably 0.1% by mass or more and 10% by mass or less, more preferably 0.5% by mass or more and 5% by mass or less, and even more preferably 1.0% by mass or more and 2.5% by mass or less, relative to the total amount of the negative electrode active material composition. By keeping the content of the binder within the above range, the effects of improving the stability and energy density of the lithium secondary battery according to the present invention tend to be more effectively and reliably achieved.

[0052] The first and second active materials in the negative electrode 30 may have different rigidities or densities when formed into layers, i.e., different negative electrode active materials or the same negative electrode active material may be used to prepare compositions with different rigidities or densities. From the viewpoint of more effectively and reliably achieving the effects of the present invention, it is preferable to prepare and use different compositions using the same negative electrode active material, and it is more preferable to prepare and use graphite at different concentrations.

[0053] When the negative electrode 30 has both the first active material layer (active material layer 12) and the second active material layer 13, the difference B2 between the content (mass %) of the negative electrode active material in the first negative electrode active material composition and the content (mass %) of the negative electrode active material in the second negative electrode active material composition is preferably 0.1 mass % or more and 5 mass % or less, more preferably 0.5 mass % or more and 3 mass % or less, and even more preferably 0.8 mass % or more and 1.5 mass % or less.

[0054] The solid content of the negative electrode active material composition is preferably 30% by mass or more and 70% by mass or less, more preferably 40% by mass or more and 60% by mass or less, and even more preferably 45% by mass or more and 55% by mass or less, relative to the total amount of the negative electrode active material composition. When the solid content of the negative electrode active material composition is within the above range, the effects of the present invention tend to be more effectively and reliably achieved.

[0055] The thickness of the negative electrode active material layer according to the present invention is preferably 20 μm or more and 300 μm or less per surface. By setting the thickness of the negative electrode active material layer according to the present invention within the above range, the effect of improving the stability and energy density of the lithium secondary battery according to the present invention tends to be more effective and reliable. From the same viewpoint, the thickness of the negative electrode active material layer is more preferably 50 μm or more and 250 μm or less, and even more preferably 80 μm or more and 180 μm or less.

[0056] In addition to the resins described above, the resin layer A in the current collectors of the positive electrode 10 and the negative electrode 30 may contain other additives as appropriate depending on the desired physical properties. Examples of the additives include colorants, flame retardants, and surfactants.

[0057] The method for producing the current collector (current collector film) described above is not particularly limited as long as it is a method that can produce a current collector having the above-mentioned characteristics, and may be produced by, for example, a conventionally known method, such as a method in which a veil film (resin sheet) that will become resin layer A is used and metal layer B is formed on both sides of the veil film by vapor deposition or sputtering.

[0058] In this embodiment, the average thickness of each layer is calculated by breaking the layer in the thickness direction, observing the exposed cross section with an SEM, and taking the arithmetic mean of measurements taken three or more times, preferably five or ten or more times.

[0059] 1.2. Separator The separator of this embodiment is not particularly limited as long as it has the function of physically and / or electrically isolating the positive electrode 10 and the negative electrode 30 and the function of ensuring ionic conductivity of lithium ions. Examples of such separators include insulating porous materials, polymer electrolytes, gel electrolytes, and inorganic solid electrolytes. Typically, the separator includes at least one selected from the group consisting of insulating porous materials, polymer electrolytes, and gel electrolytes. Note that one type of separator may be used alone, or two or more types of separators may be used in combination.

[0060] When an insulating porous member is used as the separator, the pores of the member are filled with an ion-conductive substance, which allows the member to exhibit ion conductivity. Thus, in this embodiment, the pores are filled with, for example, the electrolyte solution of this embodiment or a gel electrolyte containing the electrolyte solution of this embodiment.

[0061] The material constituting the insulating porous member is not particularly limited, and examples thereof include insulating polymer materials, specifically polyethylene (PE) and polypropylene (PP). That is, the separator may be a porous polyethylene (PE) film, a porous polypropylene (PP) film, or a laminate structure thereof.

[0062] 1.3. Electrolyte A lithium secondary battery may contain an electrolyte. An electrolyte is a liquid containing a solvent and an electrolyte and has ion conductivity. The electrolyte may also be referred to as a liquid electrolyte and acts as a conductive path for lithium ions. Therefore, when a lithium secondary battery contains an electrolyte, the internal resistance can be reduced, and the energy density, capacity, and cycle characteristics can be improved.

[0063] The electrolytic solution is, for example, a solution filled in a case (pouch) of a lithium secondary battery. The electrolytic solution may be impregnated into a separator, or may be held in a polymer to form a polymer electrolyte or a gel electrolyte.

[0064] The electrolyte contained in the electrolytic solution may be a lithium salt, such as LiI, LiCl, LiBr, LiF, or LiBF. ４ , LiPF ６ , LiAsF ６ , LiSO ３ CF ３ , LiN(SO ２ F) ２ , LiN(SO ２ CF ３ ) ２ , LiN(SO ２ CF ３ CF ３ ) ２ , LiB(O ２ C ２ H ４ ) ２ , LiB(C ２ O ４ ) ２ , LiB(O ２ C ２ H ４ ) F ２ , LiB(OCOCF ３ ) ４ , LiNO ３ , and Li ２ SO ４ It may be one or a combination of two or more selected from the group consisting of:

[0065] As the solvent contained in the electrolytic solution, for example, a non-aqueous solvent containing fluorine atoms (hereinafter referred to as a "fluorinated solvent") and a non-aqueous solvent not containing fluorine atoms (hereinafter referred to as a "non-fluorinated solvent") may be added.

[0066] Examples of fluorinated solvents include 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, and 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether.

[0067] Examples of non-fluorine-containing solvents include triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,2-dimethoxyethane, dimethoxyethane, dimethoxypropane, dimethoxybutane, diethylene glycol dimethyl ether, acetonitrile, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene carbonate, propylene carbonate, chloroethylene carbonate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, trimethyl phosphate, triethyl phosphate, and 12-crown-4.

[0068] The above fluorinated solvents and / or non-fluorinated solvents may be used alone or in any combination of two or more in any ratio. The contents of the fluorinated solvent and non-fluorinated solvent are not particularly limited, and the ratio of the fluorinated solvent to the total solvent may be 0 to 100% by volume, or the ratio of the non-fluorinated solvent to the total solvent may be 0 to 100% by volume.

[0069] 2. Manufacturing Method 2.1. Electrode Manufacturing Method The method for manufacturing the electrode of the present embodiment is not particularly limited as long as it is a method that can form, between an active material region where an active material layer is formed and an unformed region where an active material layer is not formed, a basis weight relaxation region 103 in which the basis weight of the active material layer is smaller than that of the active material region.

[0070] 6A and 6B are diagrams showing an example of a coating device for forming the basis weight relaxation region 103, in which (A) is a diagram schematically showing a cross section of a coating device 60, (B) is a cross section corresponding to the view seen from the arrow S in (A), and (C) is an enlarged view of a tapered cut portion 6021 in (B). The coating device 60 has an inner deckle 602 within an outer deckle 601, and the active material composition is injected from an injection port IN and spreads and is applied along the tapered cut portion 6021 formed in the inner deckle 602, thereby forming an active material layer having the basis weight relaxation region 103 on a current collector 603 moving in a traveling direction D.

[0071] When the electrode has a second active material layer 13, for example, a coating device 70 shown in FIG. 7 can be used. In FIG. 7, (A) is a schematic diagram showing a cross section of the coating device 70, and (B) is a cross section corresponding to the view from the arrow S1 in (A) and a cross section corresponding to the view from the arrow S2 in (A). The coating device 70 has an inner deckle 702 and an inner deckle 703 within an outer deckle 701. The first active material composition is injected through an injection port IN(1), and the second active material composition is injected through an injection port IN(2). The first active material composition and the second active material composition flow out through the interior of each inner deckle. As a result, on the current collector 704 moving in the traveling direction D, a second active material layer 706 having a lower density than the first active material layer 705 is formed outside the first active material layer 705, forming a basis weight relaxation region 103. The inner deckle 702 may have a tapered cut portion.

[0072] In the manufacture of the electrode, it is preferable to perform a pressing step. When the electrode of this embodiment is subjected to the pressing step, deformation or breakage of the current collector 11 that may occur during the pressing step can be prevented, resulting in a lithium secondary battery that is excellent in both stability and energy density. Examples of types of pressing devices used in the pressing step include a roll press, a mechanical press, a hydraulic press, and a hot press. From the viewpoint of more effectively and reliably achieving the effects of the present invention, it is preferable to use a roll press device. Detailed conditions for the roll press device are not particularly limited, and roll pressing may be performed under the same conditions as those in the examples described below.

[0073] In the electrode, the ratio C of the thickness per surface of the active material layer after the pressing process to the thickness per surface of the active material layer before the pressing process is preferably 0.5 or more and 0.95 or less. By keeping the ratio of the thicknesses of the active material layer before and after the pressing process within the above range, the effects of the present invention tend to be more effectively and reliably achieved. From the same viewpoint, the ratio C is more preferably 0.6 or more and 0.9 or less, and even more preferably 0.7 or more and 0.85 or less.

[0074] 2.2. Manufacturing Method of Lithium Secondary Battery There are no particular limitations on the manufacturing method of a lithium secondary battery, as long as it is a method that can manufacture a lithium secondary battery having the above-mentioned battery configuration. For example, the following method can be mentioned.

[0075] In a method for manufacturing a lithium secondary battery, first, the positive electrode 10, separator 20, and negative electrode 30 described above are prepared. The components and reagents used for the components may be prepared by conventional methods, or commercially available products may be used. The positive electrode 10, separator 20, and negative electrode 30 prepared in this manner are stacked in this order with the positive electrode 10 and the separator 20 facing each other to obtain a laminate. The resulting laminate is enclosed in a sealed container together with an electrolyte solution to obtain a lithium secondary battery. The sealed container is not particularly limited, and examples thereof include laminate films.

[0076] 8, multiple positive electrodes 10 and negative electrodes 30 may be stacked alternately with a separator 20 sandwiched between them, which tends to further improve battery performance such as energy density. Another example of a different stacking method is to wrap the positive electrode 10 and negative electrode 30 so that they do not come into contact with each other and face opposite sides of the separator 20, and then stack the separator 20 continuously without cutting it. This type of stacking is preferable from the viewpoints of preventing short circuits and improving productivity.

[0077] The lithium secondary battery of this embodiment can be made into an anode-free lithium battery, a lithium ion battery, a lithium metal battery, a lithium sulfur battery, a lithium oxygen battery, or a lithium air battery by adding additional components or changing the materials used in each component, and among these, the anode-free lithium battery, lithium ion battery, and lithium metal battery configurations are particularly suitable.

[0078] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0079] Electrodes of the examples and comparative examples were prepared as follows.

[0080] 1. Preparation of Positive Electrode (Example 1) A solution of LiNi as a positive electrode active material in N-methyl-pyrrolidone (NMP) was ０．８ Co ０．１ Mn ０．１ O ２ A positive electrode active material composition was prepared by mixing 98.5 parts by mass of the above, 0.6 parts by mass of acetylene black as a conductive additive, and 0.9 parts by mass of polyvinylidene fluoride (PVDF) as a binder, so that the solid content was 74% by mass. A current collector film was prepared as a positive electrode current collector, which was a 6 μm polyethylene terephthalate (PET) film with a 1 μm thin aluminum film formed on both sides, and a positive electrode active material layer of 145 μm thick was formed on each side of the current collector film.

[0081] To form the active material layer, as shown in FIG. 6 , a coating device 60 having an inner deckle 602 with a tapered cut portion 6021 was used to apply the active material composition to each side of the current collector film, so that a weight reduction region 103 was formed between the active material region and the unformed region. Then, press molding was performed to form the active material layer. The current collector film on which the active material layer was formed was subjected to a pressing process, and cut out to form a positive electrode main body portion with a size (16 cm × 13 cm) and a tab lead, obtaining a positive electrode having a positive electrode active material layer with a thickness of 115 μm per side. In this pressing process, pressure was applied using a roll press device with a thrust of 5 tons at a conveying speed of 1 m / sec.

[0082] Comparative Example 1 A positive electrode was obtained in the same manner as in Example 1, except that when forming the active material layer, coating was performed using a coating device having an inner deckle with no tapered cut portion.

[0083] 2. Preparation of Negative Electrode (Example 2) A first negative electrode active material composition was prepared by mixing 97.5 parts by mass of graphite as a first negative electrode active material, 1.0 part by mass of carboxymethyl cellulose (CMC) and 1.5 parts by mass of styrene-butadiene rubber (SBR) as binders with water as a solvent, so that the solid content was 50% by mass. A second negative electrode active material composition was prepared by mixing 95.5 parts by mass of graphite as a second negative electrode active material, 2.0 parts by mass of carboxymethyl cellulose (CMC) and 2.5 parts by mass of styrene-butadiene rubber (SBR) as binders with water, so that the solid content was 50% by mass. A current collector film was prepared as a negative electrode current collector, in which a 1 μm thick copper thin film was formed on both sides of a 6 μm thick polyethylene terephthalate (PET) film. Negative electrode active material layers were formed on both sides of the current collector film so that the total thickness of the first negative electrode active material layer and the second negative electrode active material layer was 200 μm per side.

[0084] To form the active material layer, a first active material composition and a second active material composition were applied to a portion of the surface of the current collector film using a coating device having two inner deckles, as shown in Figure 7, to form a basis weight relaxation region 103 between the active material region and the unformed region, and a second active material layer 13 was formed on the first active material layer. The current collector film on which the first active material layer and the second active material layer 13 were formed was subjected to a pressing process, and cut out to form a negative electrode main body portion having a size (16 cm x 13 cm) with a tab lead, obtaining a negative electrode having a negative electrode active material layer 145 μm thick per surface. In this pressing process, pressure was applied using a roll press device with a thrust of 5 tons at a conveying speed of 1 m / sec.

[0085] Comparative Example 2 A negative electrode was obtained in the same manner as in Example 2, except that the second negative electrode active material layer was not formed when the active material layer was formed.

[0086] 3. Comparison of Basis Weight The basis weight of the active material layer of each electrode obtained above was measured and compared before and after the pressing process. This measurement was performed using a "GR-202" (model name, testing machine, manufactured by A&D). In the active material layer after pressing, the basis weight was measured every 1 mm at positions 1 to 25 mm from the end adjacent to the unformed region to the center, based on the average basis weight at distances of 30 mm, 40 mm, and 50 mm. The relative basis weight was calculated as the basis weight ratio based on the average thickness at 30 mm, 40 mm, and 50 mm. The elongation during the pressing process was calculated based on these results. The measurement results for Example 1 and Comparative Example 1 are shown in Table 1 (shown in FIG. 9) and Table 2 (shown in FIG. 11), and the measurement results for Example 2 and Comparative Example 2 are shown in Table 3 (shown in FIG. 13) and Table 4 (shown in FIG. 15). Graphs of these results are shown next to the respective tables in FIGS. 10, 12, 14, and 16.

[0087] As can be seen from the measurement results shown in FIGS. 9 to 12, in Example 1 in which the basis weight relaxation region 103 was provided, the change in elongation rate was more mitigated and deformation was less likely to occur than in Comparative Example 1 in which the basis weight relaxation region 103 was not provided.

[0088] As can be seen from the measurement results shown in Figures 13 to 16, in Example 2, which has a basis weight relaxation region 103 and further has a second active material layer 13 on the active material layer 12 (first active material layer) in the basis weight relaxation region 103, the change in elongation rate is more mitigated and the sample is less susceptible to deformation than in Comparative Example 2, which does not have this feature.

[0089] Photographs of the positive electrodes of Example 1 and Comparative Example 1 after the pressing process are shown in Fig. 17 (Example 1) and Fig. 18 (Comparative Example 1), respectively. In Fig. 17 and Fig. 18, the metallic positive electrode current collector is shown below the black background, and it was confirmed that the deformation of the positive electrode of Fig. 17 (Example 1), which is provided with basis weight relaxation region 103, is more relaxed than that of the positive electrode of Fig. 18 (Comparative Example 1), which is not provided with basis weight relaxation region 103.

[0090] <Additional Notes> The lithium secondary battery of the present embodiment may be, for example, in the following forms. [1] A lithium secondary battery comprising: a current collector including a resin layer and metal layers formed on both sides of the resin layer; and an active material layer formed on at least one side of the current collector, wherein a basis weight relaxation region is provided on at least one side of the current collector between an active material region on which the active material layer is formed and an unformed region on which the active material layer is not formed, the active material layer having a smaller basis weight than the active material region. [2] The lithium secondary battery according to [1], wherein, in the basis weight relaxation region, the basis weight of the active material layer gradually changes from the unformed region side to the active material region side. [3] The lithium secondary battery according to [1] or [2], wherein a region in the basis weight relaxation region, having a basis weight of 98.5% or less relative to 100% of the basis weight of a central portion of the electrode, has a length of 3 mm to 30 mm. [4] The lithium secondary battery according to any one of [1] to [3], further comprising a second active material layer on the active material layer in the area weight relaxation region, the second active material layer having a lower rigidity than the active material layer. [5] The lithium secondary battery according to [4], further comprising an active material layer on both sides of the current collector, the positional deviation between the active material layer on one side and the active material layer on the other side being 0.5 mm to 3.0 mm. [7] The lithium secondary battery according to any one of [1] to [6], further comprising an insulating material layer on the active material layer in the area weight relaxation region and on the current collector in the unformed region.

[0091] The lithium secondary battery of the present invention has excellent stability and high energy density, and therefore has industrial applicability as an electricity storage device for a variety of uses.

[0092] A...resin layer, B...metal layer, 101...active material region, 102...unformed region, 103...area weight reduction region, 11...current collector, 12, 12'...active material layer, 13...second active material layer, 14...insulating material layer, 10...positive electrode, 20...separator, 30...negative electrode, IN...active material composition injection port, IN(1)...first active material composition injection port, IN(2)...second active material composition injection port, D...direction of travel of current collector, S, S1, S2...direction of observation of cross section, 60, 70...coating device, 6021...tapered cut portion, 601, 701...outer deckle, 602, 702, 703...inner deckle, 603, 704...current collector, 705...first active material layer, 706...second active material layer.

Claims

1. A lithium secondary battery comprising: a current collector including a resin layer and metal layers formed on both sides of the resin layer; and an active material layer formed on at least one surface of the current collector, wherein on at least one surface of the current collector, a basis weight relaxation region is provided between an active material region where the active material layer is formed and an unformed region where the active material layer is not formed, and the basis weight of the active material layer is smaller than that of the active material region.

2. The lithium secondary battery according to claim 1, wherein in the weight-relaxing region, the weight of the active material layer gradually changes from the unformed region side to the active material region side.

3. A lithium secondary battery according to claim 1 or 2, wherein the area in the weight-reducing region having a weight of 98.5% or less relative to 100% of the weight of the central part of the electrode has a length of 3 mm or more and 30 mm or less.

4. The lithium secondary battery according to claim 1, further comprising a second active material layer having a lower rigidity than the active material layer, the second active material layer being disposed on the active material layer in the weight-reducing region.

5. The lithium secondary battery according to claim 4, wherein the second active material layer has a lower active material density than the active material layer.

6. The lithium secondary battery according to claim 1, wherein active material layers are provided on both sides of the current collector, and the positional deviation between the active material layer on one side and the active material layer on the other side is 0.5 mm or more and 3.0 mm or less.

7. The lithium secondary battery according to claim 1, further comprising an insulating material layer on the active material layer in the weight-reducing region and on the current collector in the non-formed region.

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