Lithium-ion secondary battery

Abstract

The present invention provides a lithium-ion secondary battery with excellent rate characteristics. The lithium-ion secondary battery has a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolytic solution. The battery is characterized in that the positive electrode has a metal foil and a positive electrode active material layer provided on the metal foil, a plurality of voids are formed in the positive electrode active material layer, and a transition metal oxide having an average particle diameter of 10 nm or more and 500 nm or less is provided on the inner wall portion of the voids in contact with the electrolytic solution.

Description

Technical Field

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

[0002] Lithium ion secondary batteries having the characteristics of being small-sized and large-capacity are not only mounted in electronic devices such as mobile phones or laptop computers, but in recent years, they are also mounted in moving bodies such as automobiles or drones, and their uses are constantly expanding.

[0003] In the above-mentioned moving bodies, it is necessary to supply power to an electric motor or the like. Therefore, for the lithium ion secondary battery mounted in the moving body, input / output characteristics (rate characteristics) higher than those of existing applications are required. Therefore, in order to improve the rate characteristics, various techniques such as improving active materials (Patent Document 1), electrode structures (Patent Document 2), and electrolytes (Patent Document 3) have been reported.

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: JP-A-2017-84628

[0007] Patent Document 2: JP-A-2011-204571

[0008] Patent Document 3: JP-A-2018-125313 Summary of the Invention

[0009] Technical Problem to be Solved by the Invention

[0010] However, the characteristics cannot be satisfied by the methods of the above-mentioned prior art, and further improvement of the rate characteristics is required.

[0011] An object of the present invention is to provide a lithium ion secondary battery having excellent rate characteristics.

[0012] Technical Solution for Solving the Technical Problem

[0013] In order to solve the above-mentioned technical problem, the present invention relates to a lithium ion secondary battery having a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte. The battery is characterized in that the positive electrode has a metal foil and a positive electrode active material layer provided on the metal foil, and a plurality of voids are formed in the positive electrode active material layer, and a transition metal oxide having an average particle diameter of 10 nm or more and 500 nm or less is provided on an inner wall portion of the voids.

[0014] It is generally known that when voids are formed in the active material layer, the permeability of the electrolyte becomes good and the diffusibility of lithium ions is improved. In addition, by supporting nanoparticles of a transition metal oxide on the inner wall portion of the voids, the wettability with respect to the electrolyte is improved due to the surface tension effect, and moreover, due to the large polarization of the transition metal oxide, the affinity with respect to the electrolyte is also improved, and the electrolyte more easily penetrates in the depth direction of the active material layer. As a result, the rate performance is improved.

[0015] In the lithium ion secondary battery according to the present invention, it is further preferable that the average diameter of the above voids is 1.0 μm or more and 10.0 μm or less.

[0016] When the above voids are too small, the permeability of the electrolyte cannot be improved, and when they are too large, the capacity per unit area of the electrode decreases and the resistance increases. When within the above range, it is suitable as the average diameter of the voids, and the rate performance can be improved while maintaining other battery characteristics.

[0017] In the lithium ion secondary battery according to the present invention, it is further preferable that the above transition metal oxide contains one or more transition metals selected from Co, Mn, and Ni.

[0018] In the lithium ion secondary battery according to the present invention, it is further preferable that at least a part of the above transition metal oxide is coated with carbon nanotubes.

[0019] Thereby, by coating the above transition metal oxide with carbon nanotubes having a high aspect ratio and low conductivity, it is possible to suppress the disconnection of the conduction path that easily occurs along with the formation of voids, and the rate performance can be further improved.

[0020] Effects of the Invention

[0021] According to the present invention, a lithium ion secondary battery having excellent rate performance is provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic cross-sectional view of a positive electrode active material layer according to an embodiment of the present invention.

[0023] Figure 2 It is a schematic cross-sectional view of a lithium ion secondary battery according to an embodiment of the present invention.

[0024] REFERENCE SIGNS LIST

[0025] 1: Positive electrode active material layer; 2: Void; 3: Transition metal oxide; 10: Positive electrode; 12: Positive electrode current collector; 14: Positive electrode active material layer; 18: Separator; 20: Negative electrode; 22: Negative electrode current collector; 24: Negative electrode active material layer; 30: Laminate; 50: Case; 60, 62: Lead wires; 100: Lithium ion secondary battery. Detailed implementation mode

[0026] Hereinafter, preferred implementation modes of the present invention will be described with reference to the accompanying drawings. However, the present invention is not limited to the following implementation modes. Moreover, among the constituent elements described below, there are elements that can be easily conceived by those skilled in the art and elements that are actually the same. Furthermore, the constituent elements described below can be appropriately combined.

[0027] 〈Lithium ion secondary battery〉

[0028] As Figure 1 shown, the lithium ion secondary battery 100 according to the present implementation mode includes: a laminate 30; an electrolyte solution containing lithium ions; a case 50 that houses them in a sealed state; a lead wire 62 having one end electrically connected to the negative electrode 20 and the other end protruding to the outside of the case; and a lead wire 60 having one end electrically connected to the positive electrode 10 and the other end protruding to the outside of the case. The laminate 30 includes a plate-shaped negative electrode 20 and a plate-shaped positive electrode 10 that face each other, and a plate-shaped separator 18 disposed adjacent to each other between the negative electrode 20 and the positive electrode 10.

[0029] The positive electrode 10 has a positive electrode current collector 12 and a positive electrode active material layer 14 formed on the positive electrode current collector 12. In addition, the negative electrode 20 has a negative electrode current collector 22 and a negative electrode active material layer 24 formed on the negative electrode current collector 22. The separator 18 is located between the negative electrode active material layer 24 and the positive electrode active material layer 14.

[0030] 〈Positive electrode〉

[0031] The positive electrode according to the present implementation mode is characterized in that it has a metal foil and a positive electrode active material layer provided on the metal foil, and a plurality of voids are formed in the positive electrode active material layer. The inner wall portion of the voids in contact with the electrolyte has transition metal oxides with an average particle diameter of 10 nm or more and 500 nm or less.

[0032] It is generally known that when voids are formed in the active material layer, the permeability of the electrolyte becomes good and the diffusibility of lithium ions is improved. In addition, by supporting nanoparticles of transition metal oxides on the inner wall portion of the voids, the wettability to the electrolyte is improved due to the surface tension effect. Moreover, due to the large polarization of the transition metal oxides, the affinity to the electrolyte is also improved, and the electrolyte more easily penetrates into the depth direction of the active material layer. As a result, the rate performance is improved.

[0033] As a method for measuring the average particle size of the above-mentioned transition metal oxide, for example, a method of observing the reflected electron image of the positive electrode cross section using a scanning electron microscope (SEM) can be cited. The difference in atomic number is easily detected in the reflected electron image, so the transition metal oxide of the inner wall of the gap can be clearly distinguished. Here, 100 transition metal oxides are observed and their average is defined as the average particle size.

[0034] As a method for making such an electrode, for example, a method using composite particles of a water-soluble compound and a transition metal oxide is advantageous, but it is not limited thereto, and any method can be used. First, the water-soluble compound and the transition metal oxide are composited by any method such as mechanochemistry. Using the composite particles, a slurry for forming a positive electrode active material is prepared with an organic solvent, and the metal foil is coated and dried. By washing the positive electrode thus obtained, the water-soluble compound is dissolved to form a void, and at the same time, the composite transition metal oxide can diffuse and adhere to the inner wall of the void.

[0035] In the positive electrode according to the present embodiment, it is further preferred that the average diameter of the voids is 1.0 μm or more and 10.0 μm or less.

[0036] As a method for measuring the average diameter of the voids, for example, a method of observing a positive electrode cross section using a SEM can be cited. Here, 100 voids are observed, and the average thereof is defined as the average diameter of the voids.

[0037] When the above-mentioned gap is too small, the permeability of the electrolyte cannot be improved. In addition, when it is too large, the capacity per unit area of ​​the electrode is reduced and the resistance is increased. When it is within the above range, it is more suitable as the average diameter of the gap, and the rate characteristics can be improved while maintaining other battery characteristics.

[0038] In addition, in the positive electrode involved in this embodiment, the greater the unit area weight of the positive electrode active material layer, the more significant the improvement effect is. Specifically, it is preferred that the coating amount per unit area (unit area weight) of the positive electrode active material layer is 20 mg / cm 2 Above 100mg / cm 2 the following.

[0039] In the positive electrode according to the present embodiment, it is further preferred that the transition metal oxide contains one or more transition metals selected from the group consisting of Co, Mn and Ni.

[0040] In the positive electrode according to the present embodiment, it is further preferred that at least a part of the transition metal oxide is covered with carbon nanotubes.

[0041] Thus, by coating the transition metal oxide with carbon nanotubes having a high aspect ratio and low conductivity, it is possible to suppress the disconnection of the conduction path that easily occurs along with the formation of voids, and the rate characteristics can be further improved.

[0042] Such a positive electrode can be obtained by adding carbon nanotubes during the process of producing composite particles from the above-mentioned water-soluble compound and transition metal oxide to produce composite particles.

[0043] The positive electrode according to this embodiment can be configured as follows as needed.

[0044] (Positive electrode current collector)

[0045] The positive electrode current collector 12 may be any conductive plate material. For example, metal thin plates (metal foils) such as aluminum or their alloys, and stainless steel can be used.

[0046] (Positive electrode active material layer)

[0047] The positive electrode active material layer 14 mainly consists of a positive electrode active material, a binder for the positive electrode, and a conductive assistant for the positive electrode.

[0048] (Positive electrode active material)

[0049] As the positive electrode active material, as long as it can reversibly occlude and release lithium ions, the detachment and insertion (intercalation) of lithium ions, or the doping and dedoping of the equilibrium anion of the lithium ions (for example, PF6 － ), there is no particular limitation, and known electrode active materials can be used. For example, lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), lithium manganese spinel (LiMn2O4), and the chemical formula: LiNi x Co y Mn z MaO2 (x + y + z + a = 1, 0 ≤ x ≤ 1, 0 ≤ y ≤ 1, 0 ≤ z ≤ 1, 0 ≤ a ≤ 1, M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, Cr), lithium vanadium compound Li a (M) b (PO4) c (where M = VO or V, and 0.9 ≤ a ≤ 3.3, 0.9 ≤ b ≤ 2.2, 0.9 ≤ c ≤ 3.3), olivine-type LiMPO4 (where M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr), lithium titanate (Li4Ti5O 12 ), LiNi x Co y Al zComposite metal oxides such as O2 (0.9 < x + y + z < 1.1).

[0050] (Binder for positive electrode)

[0051] The binder for the positive electrode binds the positive electrode active materials to each other and binds the positive electrode active material layer 14 and the current collector 12 for the positive electrode. Any binder can be used as long as it can perform the above binding. For example, fluororesins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) can be used. However, from the viewpoint of using the region where carbon mapping and oxygen mapping coincide in cross-sectional SEM-EDS in analysis, it is preferable that the binder for the positive electrode does not contain oxygen.

[0052] The content of the binder in the positive electrode active material layer 14 is not particularly limited. When added, it is preferably 0.5 to 5 parts by mass relative to 100 parts by mass of the positive electrode active material.

[0053] (Conductive additive for positive electrode)

[0054] As the conductive additive for the positive electrode, as long as it can improve the conductivity of the positive electrode active material layer 14, there is no particular limitation, and known conductive additives can be used. For example, carbon-based materials such as graphite and carbon black, metal fine powders such as copper, nickel, stainless steel, and iron, and conductive oxides such as ITO can be mentioned.

[0055] The content of the conductive additive in the positive electrode active material layer 14 is not particularly limited. When added, it is preferably 0.5 to 5 parts by mass relative to 100 parts by mass of the positive electrode active material.

[0056] 〈Negative electrode〉

[0057] (Current collector for negative electrode)

[0058] The current collector 22 for the negative electrode can be any conductive plate material. For example, thin metal plates (metal foils) such as copper can be used.

[0059] (Negative electrode active material layer)

[0060] The negative electrode active material layer 24 mainly consists of a negative electrode active material, a binder for the negative electrode, and a conductive additive for the negative electrode.

[0061] (Negative electrode active material)

[0062] As the negative electrode active material, as long as it can reversibly occlude and release lithium ions, and detach and insert (intercalate) lithium ions, there is no particular limitation, and known electrode active materials can be used. For example, carbon-based materials such as graphite and hard carbon, silicon-based materials such as silicon oxide (SiO x ), metal silicon (Si), etc., metal oxides such as lithium titanate (LTO), and metal materials such as lithium, tin, and zinc can be mentioned.

[0063] Even when a metal material is not used as the negative electrode active material, the negative electrode active material layer 24 may further contain a binder for negative electrode and a conductive assistant for negative electrode.

[0064] (Binder for negative electrode)

[0065] There is no particular limitation on the binder for negative electrode, and the same binder as the binder for positive electrode described above can be used.

[0066] (Conductive assistant for negative electrode)

[0067] There is no particular limitation on the conductive assistant for negative electrode, and the same conductive assistant as the conductive assistant for positive electrode described above can be used.

[0068] 〈Electrolyte〉

[0069] The electrolyte of the present invention mainly consists of a solvent and an electrolyte.

[0070] (Solvent)

[0071] As the above-mentioned solvent, solvents commonly used in lithium ion secondary batteries can be mixed and used in any proportion. For example, cyclic carbonate compounds such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, chain carbonate compounds such as diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), cyclic ester compounds such as γ-butyrolactone (GBL), chain ester compounds such as propyl propionate (PrP), ethyl propionate (PrE), ethyl acetate, etc. can be cited.

[0072] (Electrolyte)

[0073] As long as the electrolyte is a lithium salt that can be used as an electrolyte in a lithium ion secondary battery, there is no particular limitation. For example, inorganic acid anion salts such as LiPF6, LiBF4, lithium bis(oxalato)borate, organic acid anion salts such as LiCF3SO3, (CF3SO2)2NLi, (FSO2)2NLi, etc. can be used.

[0074] The preferred embodiments of the present invention have been described above, but the present invention is not limited to the above embodiments.

[0075] Examples

[0076] Hereinafter, the present invention will be described more specifically based on examples and comparative examples, but the present invention is not limited to the following examples.

[0077] [Example 1]

[0078] (Production of composite particles)

[0079] LiCl was used as the water-soluble compound, and Co3O4 with a particle size of 50 nm was used as the transition metal oxide. Using a planetary ball mill, 18 g of LiCl, 2 g of Co3O4, 0.1 g of single-walled carbon nanotubes (SWNTs), and 20 g of ZrO2 balls as the grinding medium were placed in a 100 cc pot, and the composite treatment was carried out at a rotational speed of 400 rpm for 3 minutes to produce composite particles.

[0080] (Fabrication of the positive electrode)

[0081] LiCoO2 was used as the positive electrode active material, carbon black was used as the conductive additive, and PVDF was used as the binder. They were mixed in the ratio of LiCoO2﹕composite particles﹕carbon black﹕PVDF = 85﹕5﹕5﹕5 (parts by mass), and the mixture was dispersed in N-methyl-2-pyrrolidone (NMP) using a hybrid mixer. Thus, a slurry for forming the positive electrode active material layer was prepared. The slurry was coated on an aluminum foil with a thickness of 20 μm at a coating amount of 10.0 mg / cm 2 and dried at 100 °C. Thus, the positive electrode active material layer was formed. Then, the positive electrode active material layer was press-molded using a roll press. Then, the electrode was washed with excessive pure water, thereby completely dissolving LiCl in the above composite particles to fabricate a positive electrode with voids.

[0082] (Fabrication of the negative electrode)

[0083] Natural graphite was used as the negative electrode active material, carbon black was used as the conductive additive, and PVDF was used as the binder. They were mixed in the ratio of natural graphite﹕carbon black﹕PVDF = 80﹕10﹕10 (parts by mass), and the mixture was dispersed in N-methyl-2-pyrrolidone (NMP) using a hybrid mixer. Thus, a slurry for forming the negative electrode active material layer was prepared. The slurry was coated on a copper foil with a thickness of 15 μm at a coating amount of 8.0 mg / cm 2 and dried at 100 °C. Thus, the negative electrode active material layer was formed. Then, the negative electrode active material layer was press-molded using a roll press to fabricate the negative electrode.

[0084] (Fabrication of the electrolyte)

[0085] Ethylene carbonate (EC) and diethyl carbonate (DEC) were used as the solvents, and lithium hexafluorophosphate (LiPF6) was used as the supporting salt. They were mixed in the ratio of EC﹕DEC = 50﹕50 (parts by volume), and LiPF6 was dissolved in the mixture at a concentration of 1.0 mol / L to fabricate the electrolyte.

[0086] (Fabrication of the lithium-ion secondary battery for evaluation)

[0087] The positive electrode and the negative electrode fabricated above are laminated in sequence with a polyethylene separator interposed therebetween. After ultrasonic welding the tab leads to this laminate, it is packaged with an aluminum laminated packaging bag. Then, the electrolyte fabricated above is injected and vacuum-sealed, thereby fabricating a lithium ion secondary battery for evaluation.

[0088] (Measurement of rate performance)

[0089] The lithium ion secondary battery for evaluation fabricated above is placed in a thermostat set at 25°C, and evaluated using a charge-discharge test apparatus manufactured by Hokuto Denko Corporation. First, it is charged at a constant current of 0.1C until the battery voltage reaches 4.2V, and then discharged at a constant current of 0.1C until the battery voltage reaches 3.0V. Here, charging at a current value of XC means the current value that can charge this battery within 1 / X hours.

[0090] Next, it is charged at a constant current of 1.0C until the battery voltage reaches 4.2V, and then discharged at a constant current of 1.0C until the battery voltage reaches 3.0V. Let the discharge capacity at this time be A (Ah). Furthermore, it is charged at a constant current of 1.0C until the battery voltage reaches 4.2V, and then discharged at a constant current of 5.0C until the battery voltage reaches 3.0V. Let the discharge capacity at this time be B (Ah). Define the 5C discharge retention rate (%) = B / A, and show the obtained value in Table 1. The higher this value, the more excellent the rate performance.

[0091] [Example 2]

[0092] Except that the particle size of the transition metal oxide is changed to the value shown in Table 1 in (Fabrication of composite particles), it is carried out in the same manner as in Example 1 to fabricate a lithium ion secondary battery for evaluation of Example 2.

[0093] [Example 3]

[0094] Except that the particle size of the transition metal oxide is changed to the value shown in Table 1 in (Fabrication of composite particles), it is carried out in the same manner as in Example 1 to fabricate a lithium ion secondary battery for evaluation of Example 3.

[0095] [Example 4]

[0096] In (Fabrication of composite particles), the processing conditions using a planetary ball mill are set to be at a rotational speed of 500 rpm for 3 minutes to increase the pulverizing force and reduce the particle size of the composite particles. Except for this, it is carried out in the same manner as in Example 1 to fabricate a lithium ion secondary battery for evaluation of Example 4.

[0097] [Example 5]

[0098] In (the production of composite particles), the processing conditions using a planetary ball mill were set to rotate at 200 rpm for 10 minutes, then the rotation speed was reduced to promote granulation of the composite particles. Otherwise, the procedure was the same as in Example 1, and a lithium-ion secondary battery for evaluation of Example 5 was fabricated.

[0099] [Example 6]

[0100] In (the production of composite particles), the processing conditions using a planetary ball mill were set to rotate at 200 rpm for 15 minutes, then the rotation speed was reduced to promote granulation of the composite particles. Otherwise, the procedure was the same as in Example 1, and a lithium-ion secondary battery for evaluation of Example 6 was fabricated.

[0101] [Example 7]

[0102] Except that the transition metal oxide used in (the production of composite particles) was changed to the substance shown in Table 1, the procedure was the same as in Example 1, and a lithium-ion secondary battery for evaluation of Example 7 was fabricated.

[0103] [Example 8]

[0104] Except that the transition metal oxide used in (the production of composite particles) was changed to the substance shown in Table 1, the procedure was the same as in Example 1, and a lithium-ion secondary battery for evaluation of Example 8 was fabricated.

[0105] [Example 9]

[0106] Except that the transition metal oxide used in (the production of composite particles) was changed to the substance shown in Table 1, the procedure was the same as in Example 1, and a lithium-ion secondary battery for evaluation of Example 9 was fabricated.

[0107] [Example 10]

[0108] Except that the transition metal oxide used in (the production of composite particles) was changed to the substance shown in Table 1, the procedure was the same as in Example 1, and a lithium-ion secondary battery for evaluation of Example 10 was fabricated.

[0109] [Example 11]

[0110] Except that the transition metal oxide used in (the production of composite particles) was changed to the substance shown in Table 1, the procedure was the same as in Example 1, and a lithium-ion secondary battery for evaluation of Example 11 was fabricated.

[0111] [Example 12]

[0112] Except that the transition metal oxide to be used in (the production of composite particles) was changed to the substance shown in Table 1, the evaluation lithium-ion secondary battery of Example 12 was produced in the same manner as in Example 1.

[0113] [Example 13]

[0114] Except that SWNT was not used in (the production of composite particles), the evaluation lithium-ion secondary battery of Example 13 was produced in the same manner as in Example 1.

[0115] [Comparative Example 1]

[0116] Except that Co3O4 was not used in (the production of composite particles), the evaluation lithium-ion secondary battery of Comparative Example 1 was produced in the same manner as in Example 1.

[0117] [Comparative Example 2]

[0118] Except that the particle size of the transition metal oxide was changed to the value shown in Table 1 in (the production of composite particles), the evaluation lithium-ion secondary battery of Comparative Example 2 was produced in the same manner as in Example 1.

[0119] [Example 14]

[0120] Except that the coating amount was set to 20.0 mg / cm 2 in (the production of the positive electrode), and the coating amount was set to 16.0 mg / cm 2 in (the production of the negative electrode), the evaluation lithium-ion secondary battery of Example 14 was produced in the same manner as in Example 1.

[0121] [Comparative Example 3]

[0122] Except that Co3O4 was not used in (the production of composite particles), the evaluation lithium-ion secondary battery of Comparative Example 3 was produced in the same manner as in Example 14.

[0123] For the evaluation lithium-ion secondary batteries produced in Examples 2 to 13 and Comparative Examples 1 to 2, (the measurement of rate performance) was carried out in the same manner as in Example 1. The results are shown in Table 1.

[0124] For the evaluation lithium-ion secondary batteries produced in Example 14 and Comparative Example 3, (the measurement of rate performance) was carried out in the same manner as in Example 1. The results are shown in Table 2.

[0125] Compared with Comparative Example 1 in which there is no transition metal on the inner wall of the void, the rate performance was improved in all of Examples 1 to 3. In addition, from the comparison with Comparative Example 2, it can be seen that the average particle size of the transition metal oxide is preferably 50 nm or more and 500 nm or less.

[0126] From the results of Examples 4 to 6, it is known that the average diameter of the voids is preferably 0.5 μm or more and 10.0 μm or less.

[0127] From the results of Examples 7 to 12, it is known that the rate performance is improved by using any one of the transition metal oxides, but it is preferable to contain one or more transition metals selected from Co, Mn, and Ni.

[0128] From the results of Example 13, it is known that it is preferable that the transition metal oxide is coated with carbon nanotubes.

[0129] From the results of Example 14 and Comparative Example 3, it is known that the greater the coating amount per unit area, the greater the improvement effect on the rate performance.

[0130]

Table 1

[0131]

[0132]

Table 2

[0133]

[0134] Industrial Applicability

[0135] According to the present invention, a lithium ion secondary battery having excellent rate performance can be provided.

Claims

1. A lithium ion secondary battery, which has a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte solution, and is characterized in that: The positive electrode has a metal foil and a positive electrode active material layer provided on the metal foil. A plurality of voids are formed in the positive electrode active material layer, and transition metal oxides having an average particle diameter of 10 nm or more and 500 nm or less are provided on the inner wall portions of the voids. The transition metal oxides contain one or more transition metals selected from Co, Mn, and Ni.

2. The lithium ion secondary battery according to claim 1, characterized in that: The average diameter of the voids is 0.5 μm or more and 10.0 μm or less.

3. The lithium ion secondary battery according to claim 1 or 2, characterized in that: At least a part of the transition metal oxides is coated with carbon nanotubes.

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