Lithium ion secondary battery

Abstract

The present invention aims to reduce the internal resistance of a lithium ion secondary battery. The lithium ion secondary battery has a positive electrode including a positive electrode composite material including positive electrode active material particles and a conductive material, a negative electrode including a negative electrode composite material, and an electrolyte, the positive electrode composite material including primary particles as the positive electrode active material particles, first agglomerated particles that are hollow agglomerates of a plurality of primary particles and have a hollow portion with a diameter of less than 1 μm, and second agglomerated particles that are hollow agglomerates of a plurality of primary particles and have a hollow portion with a diameter of 1 μm or more, the ratio of the volume occupied by the first particles to the total volume of the positive electrode active material particles being 5% or more and 70% or less when the primary particles and the first agglomerated particles are considered as first particles, the porosity of the positive electrode composite material being 20% or more and 60% or less, and the aspect ratio of the conductive material being 1:10 or more.

Description

TECHNICAL FIELD

[0001] The present application relates to a lithium ion secondary battery. BACKGROUND

[0002] In an electric automobile, or a hybrid automobile having a motor and an engine as a drive source of a vehicle, a lithium ion secondary battery is used as a power source.

[0003] In such a lithium ion secondary battery, an active material capable of reversibly intercalating and deintercalating lithium ions (Li ions) is provided in a positive electrode and a negative electrode. The positive electrode active material particles contained in the positive electrode as a positive electrode active material include primary particles as the smallest unit of particles, and agglomerated particles formed by agglomeration of the primary particles. In the case where the proportion of the primary particles in the positive electrode is large, the specific surface area involved in the battery reaction increases, and thus an improvement in battery characteristics is expected.

[0004] For example, in Patent Literature 1, it is described that, in the case where the specific surface area of the active material is large, the battery characteristics such as discharge capacity are improved, on the other hand, decomposition of the electrolyte or decomposition of a by-product of the positive electrode active material proceeds, and gas is easily generated. Also, in order to suppress the generation of gas, in a positive electrode composite material in which primary particles and secondary particles as agglomerated particles are mixed, the average particle diameter of the primary particles is controlled and the proportion of the primary particles existing alone is increased. Specifically, the average particle diameter of the primary particles is made 1.5 μm to 15 μm, and the proportion "A / (A+B)" of the number of primary particles A with respect to the sum of the number of primary particles A and the number of secondary particles B is made 0.8 or more.

[0005] PRIOR ART DOCUMENTS

[0006] PATENT LITERATURE

[0007] Patent Literature 1: Japanese Patent Application Laid-Open No. 2000-133246 SUMMARY

[0008] PROBLEMS TO BE SOLVED BY THE INVENTION

[0009] However, in the case where the proportion of the number of primary particles is 0.8 or more, although there is a possibility that decomposition of the electrolyte can be suppressed, the density of the positive electrode composite material containing the positive electrode active material increases, and the passage through which the electrolyte flows becomes narrow. If the passage through which the electrolyte flows becomes narrow, the internal resistance of the battery increases.

[0010] MEANS FOR SOLVING THE PROBLEMS

[0011] The lithium ion secondary battery of one aspect of the present application has a positive electrode including a positive electrode composite material including positive electrode active material particles and a conductive material, a negative electrode including a negative electrode composite material, and an electrolyte, the positive electrode composite material including primary particles as the positive electrode active material particles, first agglomerated particles that are hollow agglomerates of a plurality of the primary particles and have a hollow portion with a diameter of less than 1 μm, and second agglomerated particles that are hollow agglomerates of a plurality of the primary particles and have a hollow portion with a diameter of 1 μm or more, the total volume of the primary particles and the first agglomerated particles being 5% or more and 70% or less of the total volume of the positive electrode active material particles, the positive electrode composite material having a porosity of 20% or more and 60% or less, and the conductive material having an aspect ratio of 1:10 or more.

[0012] In the lithium ion secondary battery, the volume ratio of the first particles can be 20% or more and 50% or less.

[0013] In the lithium ion secondary battery, the porosity can be 30% or more and 50% or less.

[0014] In the lithium ion secondary battery, the aspect ratio of the conductive material can be 1:30 or more.

[0015] In the lithium ion secondary battery, the content of the conductive material can be 0.1% by weight or more and 5% by weight or less with respect to the weight of the positive electrode composite material.

[0016] In the lithium ion secondary battery, the average diameter of the conductive material can be 1 nm or more and 100 nm or less.

[0017] Effects of the Invention

[0018] According to the present application, the internal resistance of a lithium ion secondary battery can be reduced. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is a view showing an outline of an electrode body of a lithium secondary battery as a nonaqueous secondary battery in one embodiment in which a nonaqueous secondary battery is embodied.

[0020] Figure 2 is a view schematically showing the distribution of positive electrode active material particles and a conductive material in this embodiment.

[0021] Figure 3 is a view showing the relationship between the ratio of first particles and the porosity and the battery resistance in this embodiment.

[0022] Figure 4FIG. 1 is a graph showing the relationship between the proportion of the first particles and the porosity and the battery resistance.

[0023] Figure 5 FIG. 2 is a graph showing the relationship between the proportion of the first particles and the porosity and the direct current resistance when the porosity is 30%.

[0024] Figure 6 FIG. 3 is a graph showing the relationship between the proportion of the first particles and the porosity and the reaction resistance when the porosity is 30%.

[0025] Figure 7 FIG. 4 is a graph showing the relationship between the proportion of the first particles and the porosity and the diffusion resistance when the porosity is 30%.

[0026] Figure 8 FIG. 5 is a graph showing the relationship between the proportion of the first particles and the porosity and the total resistance when the porosity is 30%.

[0027] Figure 9 FIG. 6 is a graph showing the relationship between the proportion of the first particles and the porosity and the direct current resistance when the porosity is 50%.

[0028] Figure 10 FIG. 7 is a graph showing the relationship between the proportion of the first particles and the porosity and the reaction resistance when the porosity is 50%.

[0029] Figure 11 FIG. 8 is a graph showing the relationship between the proportion of the first particles and the porosity and the diffusion resistance when the porosity is 50%.

[0030] Figure 12 FIG. 9 is a graph showing the relationship between the proportion of the first particles and the porosity and the total resistance when the porosity is 50%.

[0031] Figure 13 FIG. 10 is a graph showing the relationship between the porosity and the direct current resistance when the proportion of the first particles is 20%.

[0032] Figure 14 FIG. 11 is a graph showing the relationship between the porosity and the reaction resistance when the proportion of the first particles is 20%.

[0033] Figure 15 FIG. 12 is a graph showing the relationship between the porosity and the diffusion resistance when the proportion of the first particles is 20%.

[0034] Figure 16 FIG. 13 is a graph showing the relationship between the porosity and the total resistance when the proportion of the first particles is 20%.

[0035] Figure 17 FIG. 14 is a graph showing the relationship between the porosity and the direct current resistance when the proportion of the first particles is 50%.

[0036] Figure 18 FIG. 15 is a graph showing the relationship between the porosity and the reaction resistance when the proportion of the first particles is 50%.

[0037] Figure 19 This is a graph showing the relationship between porosity and diffusion resistance when the proportion of the first particle is 50%.

[0038] Figure 20 This is a graph showing the relationship between porosity and total resistance when the proportion of the first particle is 50%.

[0039] Figure 21 A table evaluating the individual and total resistances of lithium-ion secondary batteries containing conductive materials with an aspect ratio of 1:10 or higher in the positive electrode.

[0040] Figure 22 This is a table that evaluates the individual resistances and total resistance of lithium-ion secondary batteries containing granular conductive materials in the positive electrode. Detailed Implementation

[0041] The following describes one embodiment of the present invention.

[0042] <Composition of Lithium-ion Secondary Batteries>

[0043] like Figure 1 As shown, the lithium-ion secondary battery 10 includes a casing (not shown), an electrode body 11, and a non-aqueous electrolyte. The electrode body 11 is a wound body formed by winding multiple sheets. The electrode body 11 is formed by laminating a positive electrode sheet 15 (as a positive electrode plate) and a negative electrode sheet 16 (as a negative electrode plate) with a separator 17 in between, and then winding the laminated body. The positive electrode sheet 15 has an elongated shape and includes a positive current collector 18 and positive electrode composite material layers 19 disposed on both sides of the positive current collector 18. The positive electrode composite material layer 19 is a layer formed by coating a positive electrode composite material paste onto the positive current collector 18 and then drying it. The negative electrode sheet 16 has an elongated shape and includes a sheet-like negative current collector 20 and negative electrode composite material layers 21 disposed on both sides of the negative current collector 20. The negative electrode composite material layer 21 is a layer formed by coating a negative electrode composite material paste and then drying it. Before winding, the laminated body is formed by laminating the positive electrode 15, separator 17, negative electrode 16, and separator 17 in the same order, with the length direction of the positive electrode 15 and negative electrode 16 aligned. The laminated body is wound with the positive electrode 15 as the innermost layer. The length direction of the positive electrode 15 and negative electrode 16 is defined as the "length direction Y", and the direction orthogonal to the "length direction Y" is defined as the "width direction X".

[0044] As for the electrode body 11, the laminate is wound in the length direction Y, and the wound laminate is pressed from the peripheral surface thereof, whereby a flat shape is formed. An uncoated portion 15A in which the positive electrode current collector 18 is exposed without the positive electrode composite material layer 19 being formed is provided at one end portion in the width direction X of the positive electrode sheet 15. Also, an uncoated portion 16A in which the negative electrode current collector 20 is exposed without the negative electrode composite material layer 21 being formed is provided at one end portion in the width direction X of the negative electrode sheet 16. In the lithium ion secondary battery 10, a connection portion composed of a metal material is joined at the uncoated portions 15A, 16A, and the connection portion is electrically connected to an external terminal located on the outer peripheral surface of the case, whereby the battery is configured to be able to take out electric power.

[0045] Next, the positive electrode will be described. A metal foil such as an aluminum foil is used for the positive electrode current collector 18. The positive electrode composite material layer 19 contains a positive electrode active material, a conductive material, a binding material (binder), and the like. The positive electrode active material can use one or more of various materials known to be usable as a positive electrode active material of a lithium ion secondary battery. As appropriate examples, layered systems, spinel systems, and the like, lithium complex metal oxides (for example, LiNi02, LiCo02, LiFe02, LiMn204, LiNi04, LiCrMn04, LiFeP04) can be given. As the binding material, polyvinylidene fluoride (PVDF), styrene butadiene copolymer (SBR), polytetrafluoroethylene (PTFE), and the like can be given. The proportion of the positive electrode active material in the entire positive electrode composite material is preferably 60% by weight or more (typically 60% by weight or more and 99% by weight or less). Alternatively, the proportion of the positive electrode active material in the entire positive electrode composite material can be 70% by weight or more and 99% by weight or less. 0.5 Mn 1.5 O4, LiCrMnO4, LiFePO4). As the binding material, polyvinylidene fluoride (PVDF), styrene butadiene copolymer (SBR), polytetrafluoroethylene (PTFE), and the like can be given. The proportion of the positive electrode active material in the entire positive electrode composite material is preferably 60% by weight or more (typically 60% by weight or more and 99% by weight or less). Alternatively, the proportion of the positive electrode active material in the entire positive electrode composite material can be 70% by weight or more and 99% by weight or less.

[0046] Next, the materials of the negative electrode will be described. The negative electrode current collector 20 is formed of a metal foil of copper, nickel, or the like. The negative electrode composite material layer 21 contains a negative electrode active material, a conductive material, a binding material, and the like. The negative electrode active material can use one or two or more of various materials known to be usable as a negative electrode active material of a lithium ion secondary battery. For example, carbon materials such as graphite (black lead), hard graphitizable carbon (hard carbon), easy graphitizable carbon (soft carbon), and carbon nanotubes can be given. Among these, a graphite-based material (particularly, natural graphite) can be preferably used for the reason that the conductivity is excellent and a high energy density is obtained. As the binding material, the same binding material as in the positive electrode can be used. In addition, a tackifying material, a dispersant, and the like can also be appropriately used. For example, as the tackifying material, carboxymethyl cellulose (CMC), methyl cellulose (MC) can be used.

[0047] The proportion of the negative electrode active material in the overall negative electrode composite layer is preferably 50% by weight or more. The proportion of the negative electrode active material can be 90% by weight or more and 99% by weight or less. When a binder is used, the proportion of the binder in the overall negative electrode composite layer 21 is preferably 0.5% by weight or more and 10% by weight or less, or it can be 0.5% by weight or more and 5% by weight or less. When a tackifying material is used, the proportion of the tackifying material in the overall negative electrode composite layer 21 is preferably 0.5% by weight or more and 10% by weight or less, or it can be 0.5% by weight or more and 5% by weight or less.

[0048] The separator 17 has a porous layer formed of resin. The porous layer can be, for example, a single-layer structure made of porous polyethylene, porous polyolefin, or porous polyvinyl chloride, or a laminated structure made of multiple materials. Additionally, the porous layer may contain fillers for purposes such as improving strength. An adhesive layer may be sandwiched between the separator 17 and the negative electrode 16.

[0049] A non-aqueous electrolyte is a composition containing a supporting salt in a liquid non-aqueous solvent. Here, as the non-aqueous solvent, one or more materials selected from propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) can be used. Furthermore, as the supporting salt, one or more lithium compounds (lithium salts) selected from LiPF6, LiBF4, LiClO4, LiAsF6, LiCF3SO3, LiC4F9SO3, LiN(CF3SO2)2, LiC(CF3SO2)3, and LiI can be used.

[0050] <Positive Electrode Composite Materials>

[0051] Reference Figure 2 and Figure 3 A detailed description of the positive electrode composite material is provided.

[0052] Figure 2A cross section of the positive electrode active material particle 30 is schematically shown. The positive electrode active material particle 30 includes primary particles 31, first agglomerated particles 32, and second agglomerated particles 33. The primary particles 31 are the smallest unit of the particles, and are particles having boundaries between the particles that cannot be further subdivided. The first agglomerated particles 32 and the second agglomerated particles 33 are hollow agglomerates formed by agglomeration of the primary particles 31. The first agglomerated particles 32 and the second agglomerated particles 33 are generated by agglomeration of the primary particles 31 in the process of manufacturing the positive electrode active material particle 30. Also, the primary particles 31 and the first agglomerated particles 32 are generated by breakage or deformation of the second agglomerated particles 33 in the process of manufacturing the positive electrode composite material. In this way, by breakage or deformation of the second agglomerated particles 33, a portion of the agglomerated primary particles 31 falls off, and the primary particles 31 that constitute the agglomerated particles become separated from each other.

[0053] The particle diameters of the first agglomerated particles 32 and the second agglomerated particles 33 are larger than those of the primary particles 31. The first agglomerated particles 32 and the second agglomerated particles 33 have shell portions 35. The shell portions 35 have hollow portions 36 on the inner sides thereof. The shell portions 35 can have through-holes 39 that pass through the shell portions 35. The through-holes 39 can be one or a plurality of through-holes.

[0054] The diameters of the hollow portions 36 differ between the first agglomerated particles 32 and the second agglomerated particles 33. In other words, the inner diameters of the shell portions 35 differ between the first agglomerated particles 32 and the second agglomerated particles 33. The first agglomerated particles 32 have diameters of less than 1 μm The diameter of the hollow portion 36 is the largest length among the relative lengths of the primary particles 31 that constitute the first agglomerated particles 32 and that divide the hollow portion 36, and does not include the through-holes 39. The second agglomerated particles 33 have diameters of 1 μm or more That is, in the first agglomerated particles 32, by breakage of the shell portions 35 and falling off of the primary particles 31, the inner diameter of the hollow portion 36 is reduced.

[0055] The first agglomerated particles 32 and the second agglomerated particles 33 are separated by the diameters of the hollow portions 36, and the average particle diameter of the second agglomerated particles 33 is larger than the average particle diameter of the first agglomerated particles 32. Specifically, the average particle diameter of the first agglomerated particles 32 is 0.1 μm or more and 10 μm or less. The average particle diameter of the second agglomerated particles 33 is 2 μm or more and 10 μm or less. The average particle diameter of the positive electrode active material particle 30 can be the 50% integral value measured by laser diffraction particle size distribution measurement using MIE scattering theory.

[0056] The specific surface area of the second agglomerated particles 33 is smaller than that of the first agglomerated particles 32, and on the other hand, since the primary particles 31 are densely agglomerated, the direct current resistance can be reduced. The inventors found that the proportion of the second agglomerated particles 33 in the positive electrode active material particles 30 has an influence on the internal resistance of the lithium ion secondary battery 10. Hereinafter, the primary particles 31 and the first agglomerated particles 32 will be distinguished as first particles 37, and the second agglomerated particles 33 will be distinguished as second particles 38.

[0057] The positive electrode composite material that constitutes the positive electrode composite layer 19 satisfies the following conditions 1 to 3. Furthermore, the positive electrode composite material is more preferably at least one of conditions 4 and 5.

[0058] (Proportion of first particles)

[0059] The proportion of the volume occupied by the first particles 37 with respect to the total volume of the positive electrode active material particles is 5% or more and 70% or less (condition 1). In addition, the proportion of the volume occupied by the first particles 37 is preferably 20% or more and 50% or less. The proportion of the first particles 37 is a value calculated from the positive electrode sheet 15 in a state in which the lithium ion secondary battery 10 can be shipped. When the positive electrode sheet 15 is produced, a part of the second agglomerated particles 33 is broken and becomes the first particles 37 by pressing the positive electrode composite layer 19. Or a part of the second agglomerated particles is deformed or broken and becomes the first agglomerated particles 32. When the positive electrode composite paste is produced, the positive electrode active material particles 30 mixed with the conductive material or the dispersion medium and the like are mostly in the state of the second agglomerated particles 33. In addition to the selection of the material of the positive electrode active material, the proportion of the first particles 37 can be adjusted by controlling the pressure at the time of pressing. The proportion of the first particles 37 can be measured, for example, by a method using a scanning electron microscope. In this method, similarly to the measurement of the void ratio, an ion beam is irradiated to the positive electrode composite layer 19 to expose a cross section. In addition, the entire cross section of the positive electrode composite layer 19 is photographed using a scanning electron microscope to obtain a cross-sectional image. Furthermore, the first particles 37 and the second particles 38 are distinguished in the cross-sectional image, and the occupied area of the first particles 37 and the occupied area of the second particles 38 are found. Note that the proportion of the occupied area of the first particles 37 and the occupied area of the second particles 38 in the cross section is approximately the same as the proportion of the occupied volume of the first particles 37 and the occupied volume of the second particles 38 per unit volume of the positive electrode composite material. The proportion of the occupied area of the first particles 37 with respect to the sum of these occupied areas is found. Furthermore, the positive electrode composite layer 19 is irradiated with an ion beam to expose a new cross section, and the calculation of the proportion of the occupied area of the first particles 37 using the cross-sectional image is repeated 10 times, and the average of the proportion of the occupied area of the first particles 37 is taken as the proportion (%) of the first particles 37.

[0060] Further, in the step of kneading the positive electrode active material, the conductive material, and the binder material with the dispersion medium to produce the positive electrode composite paste, it is preferable that the positive electrode active material particles 30 maintain the state of the second particles 38. This is because, when the content of the first particles 37 in the positive electrode active material particles 30 is 5% or more from the stage of producing the positive electrode composite paste, the cohesion acts between the first particles 37 in the positive electrode composite paste, and the viscosity of the paste becomes too large. In the case where the viscosity of the paste is increased by the cohesion of the first particles 37 as such, the amount of solvent needs to be increased, and the manufacturing cost increases. Further, in the first particles 37, the volume per unit weight increases, and thus the cost related to the handling increases.

[0061] (void ratio)

[0062] The void ratio in the positive electrode composite layer 19 is 20% or more and 60% or less (Condition 2). Further, the void ratio is preferably 30% or more and 50% or less. The void ratio indicates the proportion of the volume of the voids in which the positive electrode active material particles, the conductive material, and the binder material are not filled in the positive electrode composite layer 19. The volume of the voids includes the volume of the hollow portions 36 and the through-holes 39 of the first agglomerated particles 32 and the second agglomerated particles 33. The void ratio can be adjusted by the pressure applied to the positive electrode composite layer 19 in the pressing step at the time of producing the positive electrode sheet 15. The method of measuring the void ratio is not particularly limited. The void ratio can be calculated, for example, by subtracting the "positive electrode composite volume" from the "unit space volume". The positive electrode composite volume can be calculated from the basis weight, the thickness, the composition ratio, and the true density of each material of the positive electrode composite. The true density of each material can be measured, for example, by the method according to JIS K 0061:2001 "Method of testing density and specific gravity of chemical products".

[0063] The thinner the shell portion 35 of the second particles 38, the more easily the second particles 38 break, and the more easily the second particles 38 become the first particles 37. The thickness of the shell portion 35 can be estimated from the oil absorption amount. The oil absorption amount of the second particles 38 is preferably 20 ml / 100 g or more. In order to make the proportion of the first particles 37 5% or more and 70% or less and to make the void ratio in the above range after the pressing at the time of producing the positive electrode sheet, it is preferable that the oil absorption amount be 20 ml / 100 g or more and 60 ml / 100 g or less. The oil absorption amount referred to here is the amount of refined linseed oil absorbed by the second particles 38 under certain conditions, and can be measured by the method based on JIS K 5101-13-1 "Pigment test methods Part 13: Oil absorption 1st section: refined linseed oil method". That is, in the second particles 38, oil is absorbed into the hollow portion 36, and it can be said that the more the oil absorption amount, the larger the hollow portion 36 and the thinner the shell portion 35.

[0064] (conductive material)

[0065] The conductive material 40 has an elongated shape. The conductive material 40 is composed of a carbon-based material. The conductive material 40 can be, for example, one or two or more selected from among various carbon blacks (for example, acetylene black, ketjen black), coke, activated carbon, graphite, carbon fibers (PAN-based carbon fibers, pitch-based carbon fibers), carbon nanotubes, and the like.

[0066] The aspect ratio of the conductive material 40 is 1:10 or more (condition 3). Furthermore, the aspect ratio of the conductive material 40 is preferably 1:30 or more. The aspect ratio is the ratio of the short side to the long side of the conductive material 40. In the case of a cylindrical conductive material 40 such as a carbon nanotube, the aspect ratio is the ratio of the diameter of the cylinder to the height in the length direction of the cylinder. By making the aspect ratio of the conductive material 40 1:10 or more, for example, 1:50, 1:100, or the like, the conductive material 40 can be interposed in the minute gaps between the positive electrode active material particles 30 and come into contact with a plurality of the positive electrode active material particles 30, and it is possible to construct a conductive network between the positive electrode active material particles 30. In the case where the aspect ratio of the conductive material 40 is less than 1:10, for example, 1:5 or the like, it is not easy to construct a conductive network between a plurality of the positive electrode active material particles 30.

[0067] The average (average diameter) of the diameter of the conductive material 40 is preferably 100 nm or less on the basis of condition 3 for the aspect ratio (condition 4). This is because the conductive material 40 is preferably interposed in the gaps between the positive electrode active material particles 30 and the conductive material 40, and a conductive network that bridges a plurality of the positive electrode active material particles 30 is constructed. If the average diameter of the conductive material 40 exceeds 100 nm, it is difficult to construct a conductive network.

[0068] Furthermore, the average diameter (average diameter) of the conductive material 40 is preferably 1 nm or more. If the average diameter of the conductive material 40 is less than 1 nm, the cohesion of the conductive material 40 strongly acts, and the conductive material 40 will cohere to each other and it will be difficult to disperse. Furthermore, the average diameter of the conductive material 40 is preferably 5 nm or more and 50 nm or less. The method of measuring the average diameter of the conductive material 40 is not particularly limited, and for example, the average diameter can be calculated by selecting a certain number of conductive materials 40, for example, 20, from an image obtained using a transmission electron microscope, measuring the outer diameter, and averaging.

[0069] Furthermore, the average length of the conductive material 40 is preferably 100 nm to 10,000 nm (10 μm) on the basis of condition 3 for the aspect ratio. If the average length of the conductive material 40 is less than 100 nm, it is not easy to form a conductive network between the positive electrode active material particles, and if it is greater than 10,000 nm, it is difficult to disperse, and there are problems in terms of manufacturing.

[0070] Regarding the proportion of the conductive material 40, it is preferable that it be 0.1% by weight or more and 5% by weight or less with respect to the weight of the positive electrode composite material (Condition 5). In a case where the proportion of the conductive material 40 is less than 0.1% by weight, the conductivity of the positive electrode composite material decreases, and the internal resistance increases. If the proportion of the conductive material 40 is more than 5% by weight, the proportion of the positive electrode active material decreases, and the battery capacity decreases. Or the path of the electrolyte solution becomes narrow due to the decrease in the void ratio, and thus the diffusion resistance in the electrolyte solution increases. Or the proportion of the binder material decreases, and the adhesion of the positive electrode composite material layer 19 to the positive electrode current collector 18 decreases.

[0071] In the present embodiment, a carbon nanotube is used as the conductive material 40. The carbon nanotube is a fibrous conductive material. The carbon nanotube is composed of a six-membered ring network (graphene sheet) made of carbon and has a single-layer or multi-layer structure. The carbon nanotube has a cylindrical shape, is high in strength, and is stable to heat. In addition, the carbon nanotube is excellent in conductivity, thermal conductivity, and heat resistance. In the present embodiment, the carbon nanotube can be any shape such as a single layer or a multi-layer, an open end, or a closed end. By adding the conductive material 40 composed of the carbon nanotube, even a material such as a binder that does not normally conduct electricity can be provided with conductivity. In addition, the carbon nanotube is not easily broken even when a bending stress is applied, and is thus flexible, and thus can contact in a manner that deforms according to the shape of the gap and winds around a plurality of particles in the gap of the positive electrode active material particles 30.

[0072] <Internal resistance of battery>

[0073] The internal resistance of the lithium ion secondary battery 10 will be described. In the lithium ion secondary battery 10, each component of the internal resistance can be measured by an alternating current impedance method. The alternating current impedance method is a method of observing an impedance spectrum by applying a voltage or a current to an electrode of the lithium ion secondary battery 10 by periodically changing a frequency with a small amplitude. The alternating current can be a sine wave, or a rectangular wave alternating current, a triangular wave alternating current, or a sawtooth wave alternating current. The analysis result of the lithium ion secondary battery 10 based on the alternating current impedance method is output, for example, in the form of Nyquist plots. The Nyquist plot is a graph in which an imaginary value Zi and a real value Zr of the resistance when a voltage or a current is applied by periodically changing a frequency are expressed in two dimensions. With the Nyquist plot, it is possible to obtain information related to the direct current resistance, the reaction resistance, and the diffusion resistance of the lithium ion secondary battery 10.

[0074] The direct current resistance is also referred to as an electron movement resistance, and is represented by the real value Zr. The direct current resistance indicates the resistance when an electron moves in the electrolyte solution, the electrode composite material, and the current collector. If the proportion of the voids in the positive electrode composite material layer 19 increases, the direct current resistance increases. In addition, by appropriately constructing a conductive network with the conductive material 40, the direct current resistance decreases.

[0075] The reaction resistance is, for example, a resistance measured at an intermediate frequency of from 100 Hz to 0.1 Hz. The reaction resistance is a resistance at the time of electron donation and acceptance reaction on the surface of the active material. The reaction resistance is reduced by increasing the surface area of the positive electrode active material particles 30. If the proportion of the first particles 37 is increased, the surface area of the positive electrode active material particles 30 is increased.

[0076] The diffusion resistance is, for example, a resistance measured at a low frequency of less than 0.1 Hz. The diffusion resistance is a resistance at the time of ion diffusion in the electrolyte. In the case of using a non-aqueous electrolyte having fluidity, if the density of the positive electrode active material particles 30 is reduced, a movement path of the non-aqueous electrolyte between the positive electrode active material particles 30 is ensured. Therefore, if the density of the positive electrode composite layer 19 is reduced, the diffusion resistance is reduced.

[0077] Referring to Figure 3 and Figure 4 The relationship between the proportion of the first particles 37 and the positive electrode composite layer 19 and each of the resistance components will be described. Figure 3 The relationship between the ranges Z1 to Z5 divided based on the threshold values of each of the resistance components and the proportion of the first particles 37 and the void ratio is shown. That is, upper limit values are set for the direct current resistance, the reaction resistance, and the diffusion resistance, and regions Z1 to Z4 exceeding the upper limit values with changes in the proportion of the first particles 37 and the void ratio are limited. In the positive electrode of the lithium ion secondary battery 10, carbon nanotubes are used as the conductive material 40. In the region Z1 in which the void ratio of the positive electrode composite layer 19 is 0% or more and less than 20%, the movement path of ions is reduced or blocked. As a result, the diffusion resistance becomes excessively large. In the region Z2 in which the void ratio of the positive electrode composite layer 19 is greater than 60% and 100% or less, the diffusion resistance is reduced, and on the other hand, the conductive network is cut by the void, so the direct current resistance becomes excessively large.

[0078] In addition, in the region Z3 in which the proportion of the first particles 37 of the positive electrode composite layer 19 is excessively small, that is, the proportion of the first particles 37 is less than 5%, which is a region between the regions Z1 and Z2, the reaction resistance becomes excessively large due to the reduction in the specific surface area. In the region Z4 in which the proportion of the first particles 37 of the positive electrode composite layer 19 is greater than 70%, the direct current resistance of the positive electrode composite layer 19 becomes excessively large because the primary particles 31 are densely agglomerated and the second particles 38 capable of reducing the direct current resistance are few.

[0079] Thus, since the direct current resistance, the reaction resistance, and the diffusion resistance are in a trade-off relationship, even if one resistance component is reduced, another resistance component can increase. Therefore, in order to improve the battery characteristics, it is preferable to increase the reaction speed of the rate-determining stage of the battery reaction as much as possible, and to reduce each resistance component well balanced. In order to reduce each resistance component, it is necessary to control the void ratio and the proportion of the first particles 37. In the region Z5 in which each resistance component is low and the total resistance is low, the void ratio is 20% or more and 60% or less, and the proportion of the first particles 37 is 5% or more and 70% or less.

[0080] Figure 4 is a graph showing the relationship between the proportion of the first particles 37 and each resistance component for a lithium ion secondary battery 10 including acetylene black as the conductive material 40. Figure 4 The graph of Figure 3 The characteristics of the lithium ion secondary battery 10 were measured under the same conditions as those of the graph of Figure 4 When the conductive material is acetylene black, if the proportion of the conductive material is not 5% by weight to 20% by weight, the conductivity cannot be ensured. If acetylene black is added at this proportion, the void ratio decreases, and the path of the electrolyte solution decreases, so the diffusion resistance of the electrolyte solution increases. Therefore, in the region Z11 in which the void ratio is less than 40%, the diffusion resistance becomes excessively large. Figure 3 In the region Z11 of Figure 4 In the region Z12 of Figure 3 In the region Z12 of

[0081] Since the ranges of the regions Z11 and Z12 are large, it is not possible to confirm the appropriate range such as the regions Z3 to Z5 shown in Figure 3 by making the conductive material 40 elongated with an aspect ratio of 1:10 or more, it is possible to produce the region Z5 in which the total resistance is low, or it is possible to expand the region Z5 in which the total resistance is low.

[0082] As described above, according to the above-described embodiment, the following effects can be obtained.

[0083] (1) In the above embodiment, the proportion of the total volume of the first particles 37 with respect to the total volume of the positive electrode active material particles 30 is 5% or more and 70% or less, and thus the specific surface area of the positive electrode active material particles 30 can be increased as compared with the case where the positive electrode active material particles 30 are all the second particles 38. Thus, the reaction resistance of the lithium ion secondary battery 10 can be reduced. In addition, as compared with the case where the positive electrode active material particles 30 are all the first particles 37, the passage of the electrolyte solution can be ensured. In addition, since the porosity is 20% or more and 60% or less, the direct current resistance and the diffusion resistance of the lithium ion secondary battery 10 can be reduced. Furthermore, since the aspect ratio of the conductive material 40 is 1:10 and the shape thereof is elongated, the conductive material 40 can be positioned in a narrow gap between the positive electrode active material particles 30. Thus, a mesh-like conductive network can be constructed using the conductive material 40, and thus the direct current resistance can be reduced. By reducing the direct current resistance, the reaction resistance, and the diffusion resistance as described above, the total resistance of the lithium ion secondary battery 10 can be reduced.

[0084] (2) When the proportion of the volume of the first particles 37 is 20% or more and 50% or less, the reaction resistance can be further reduced.

[0085] (3) When the porosity is 30% or more and 50% or less, the direct current resistance and the diffusion resistance can be further reduced.

[0086] (4) When the aspect ratio of the conductive material 40 is 1:30 or more, a dense conductive network can be constructed, and thus the direct current resistance of the lithium ion secondary battery 10 can be further reduced.

[0087] (5) The content of the conductive material 40 with respect to the weight of the positive electrode composite is preferably 0.1% by weight or more and 5% by weight or less. In this way, a dense conductive network can be constructed, and thus the direct current resistance can be further reduced.

[0088] (6) In the lithium ion secondary battery 10, when the average diameter of the conductive material 40 is 1 nm or more and 100 nm or less, a dense conductive network can be constructed, and thus the direct current resistance can be further reduced.

[0089] (Other Embodiments)

[0090] The above embodiment can be implemented by being changed as follows. The above embodiment and the following modified examples can be implemented in combination with each other within a range in which they do not contradict in technology.

[0091] • The electrode body 11 is not limited to the electrode structure in which the positive electrode sheet 15 and the negative electrode sheet 16 are wound with the separator 17 interposed therebetween, and can be appropriately changed depending on the shape of the lithium ion secondary battery 10 and the purpose of use. For example, it can be an electrode structure of a non-wound type in which the positive electrode sheet 15 and the negative electrode sheet 16 are stacked with the separator 17 interposed therebetween.

[0092] The lithium-ion secondary battery 10 can also be used in applications other than as a drive source for electric vehicles and as a drive source for hybrid vehicles. For example, the lithium-ion secondary battery 10 can also be mounted in vehicles such as gasoline vehicles and diesel vehicles. In addition, the lithium-ion secondary battery 10 can also be used as a power source for mobile bodies such as trains, ships, and aircraft, robots, information processing devices, and the like.

[0093] [Examples]

[0094] <Experiment of the ratio of the first particles and the porosity>

[0095] Next, examples and comparative examples of the lithium-ion secondary battery 10 will be described. Note that these examples and comparative examples do not limit the present application.

[0096] The lithium-ion secondary batteries 10 of examples and comparative examples in which the porosity was fixed and the ratio of the first particles was changed, and the lithium-ion secondary batteries 10 of examples and comparative examples in which the ratio of the first particles was fixed and the porosity was changed, were prepared, and the DC resistance, reaction resistance, diffusion resistance, and total resistance were evaluated for each example and each comparative example.

[0097] <Porosity 30%>

[0098] In Examples 1 to 5 and Comparative Examples 1 and 2, the porosity of the positive electrode composite material was fixed and the ratio of the first particles was changed as shown in Table 1 below.

[0099] [Table 1]

[0100]

[0101] (Example 1)

[0102] As the positive electrode active material, lithium nickel cobalt manganese oxide (LiNi 1 / 3 Co 1 / 3Mn1 / 3O2). The positive electrode active material particles were substantially all second particles. The positive electrode active material paste was obtained by mixing 98% by weight of the positive electrode active material, 1% by weight of the conductive material, and 1% by weight of the binder material, adding NMP (N-methyl-2-pyrrolidone) as a dispersion medium, and kneading to achieve a prescribed viscosity. The positive electrode active material particles contained in the positive electrode composite paste were mostly maintained in the state of the second particles. The conductive material used was carbon nanotubes. The average diameter of the carbon nanotubes was 10 nm, the average length was 1000 nm, and the aspect ratio was 1:100. The positive electrode composite paste was applied to both sides of a positive electrode current collector composed of an aluminum foil, and dried. The positive electrode sheet after drying was calendered by roll pressing. By adjusting the pressure applied by the roll pressing and controlling the gap between the rolls during roll pressing, the second particles were crushed, and the proportion of the first particles was made to be 70% and the void ratio was made to be 30%.

[0103] In addition, a natural graphite powder as a negative electrode active material, styrene butadiene rubber (SBR), and carboxymethyl cellulose (CMC) were dispersed in water, and kneaded. The negative electrode mixture was applied to both sides of a long strip-shaped copper foil, and dried, thereby producing a negative electrode sheet. The negative electrode composite layer after drying was subjected to pressing.

[0104] The positive electrode sheet and the negative electrode sheet were layered with a separator, and a nonaqueous electrolyte was used to produce a laminated lithium ion secondary battery 10.

[0105] (Example 2)

[0106] A positive electrode sheet was produced in the same manner as in Example 1, except that the proportion of the first particles was made to be 50%.

[0107] (Example 3)

[0108] A positive electrode sheet was produced in the same manner as in Example 1, except that the proportion of the first particles was made to be 20%.

[0109] (Example 4)

[0110] A positive electrode sheet was produced in the same manner as in Example 1, except that the proportion of the first particles was made to be 10%.

[0111] (Example 5)

[0112] A positive electrode sheet was produced in the same manner as in Example 1, except that the proportion of the first particles was made to be 5%.

[0113] (Comparative Example 1)

[0114] A positive electrode sheet was produced in the same manner as in Example 1, except that the proportion of the first particles was made to be 100%.

[0115] (Comparative Example 2)

[0116] The positive electrode sheet was produced in the same manner as in Example 1, except that the proportion of the first particles was 0%.

[0117] <porosity 50%>

[0118] In Examples 6 to 10 and Comparative Examples 3 and 4, the proportion of the first particles was changed as shown in Table 2, while the porosity was 50%.

[0119] [Table 2]

[0120]

[0121] (Example 6)

[0122] The positive electrode sheet was produced in the same manner as in Example 1, except that the porosity was 50% and the proportion of the first particles was 70%.

[0123] (Example 7)

[0124] The positive electrode sheet was produced in the same manner as in Example 1, except that the porosity was 50% and the proportion of the first particles was 50%.

[0125] (Example 8)

[0126] The positive electrode sheet was produced in the same manner as in Example 1, except that the porosity was 50% and the proportion of the first particles was 20%.

[0127] (Example 9)

[0128] The positive electrode sheet was produced in the same manner as in Example 1, except that the porosity was 50% and the proportion of the first particles was 10%.

[0129] (Example 10)

[0130] The positive electrode sheet was produced in the same manner as in Example 1, except that the porosity was 50% and the proportion of the first particles was 5%.

[0131] (Comparative Example 3)

[0132] The positive electrode sheet was produced in the same manner as in Example 1, except that the porosity was 50% and the proportion of the first particles was 100%.

[0133] (Comparative Example 4)

[0134] The positive electrode sheet was produced in the same manner as in Example 1, except that the porosity was 50% and the proportion of the first particles was 0%.

[0135] (Proportion of first particles 20%)

[0136] As shown in Table 3, in Examples 11 to 16 and Comparative Examples 5 to 7, the proportion of the first particles was changed to 20% while the porosity was changed.

[0137] [Table 3]

[0138]

[0139] (Example 11)

[0140] The positive electrode sheet was produced in the same manner as in Example 1 except that the proportion of the first particles was changed to 20% and the porosity was changed to 20%.

[0141] (Example 12)

[0142] The positive electrode sheet was produced in the same manner as in Example 1 except that the proportion of the first particles was changed to 20% and the porosity was changed to 30%.

[0143] (Example 13)

[0144] The positive electrode sheet was produced in the same manner as in Example 1 except that the proportion of the first particles was changed to 20% and the porosity was changed to 39%.

[0145] (Example 14)

[0146] The positive electrode sheet was produced in the same manner as in Example 1 except that the proportion of the first particles was changed to 20% and the porosity was changed to 48%.

[0147] (Example 15)

[0148] The positive electrode sheet was produced in the same manner as in Example 1 except that the proportion of the first particles was changed to 20% and the porosity was changed to 52%.

[0149] (Example 16)

[0150] The positive electrode sheet was produced in the same manner as in Example 1 except that the proportion of the first particles was changed to 20% and the porosity was changed to 56%.

[0151] (Comparative Example 5)

[0152] The positive electrode sheet was produced in the same manner as in Example 1 except that the proportion of the first particles was changed to 20% and the porosity was changed to 13%.

[0153] (Comparative Example 6)

[0154] The positive electrode sheet was produced in the same manner as in Example 1 except that the proportion of the first particles was changed to 20% and the porosity was changed to 61%.

[0155] (Comparative Example 7)

[0156] An electrode sheet was produced in the same manner as in Example 1, except that the proportion of the first particles was made 50% and the porosity was made 67%.

[0157] (Proportion of first particles 50%)

[0158] As shown in Table 4, in Examples 17 to 22 and Comparative Examples 8 to 10, the proportion of the first particles was made 50% and the porosity was changed.

[0159] [Table 4]

[0160]

[0161] (Example 17)

[0162] An electrode sheet was produced in the same manner as in Example 1, except that the proportion of the first particles was made 50% and the porosity was made 20%.

[0163] (Example 18)

[0164] An electrode sheet was produced in the same manner as in Example 1, except that the proportion of the first particles was made 50% and the porosity was made 30%.

[0165] (Example 19)

[0166] An electrode sheet was produced in the same manner as in Example 1, except that the proportion of the first particles was made 50% and the porosity was made 39%.

[0167] (Example 20)

[0168] An electrode sheet was produced in the same manner as in Example 1, except that the proportion of the first particles was made 50% and the porosity was made 48%.

[0169] (Example 21)

[0170] An electrode sheet was produced in the same manner as in Example 1, except that the proportion of the first particles was made 50% and the porosity was made 52%.

[0171] (Example 22)

[0172] An electrode sheet was produced in the same manner as in Example 1, except that the proportion of the first particles was made 50% and the porosity was made 56%.

[0173] (Comparative Example 8)

[0174] An electrode sheet was produced in the same manner as in Example 1, except that the proportion of the first particles was made 50% and the porosity was made 13%.

[0175] (Comparative Example 9)

[0176] An anode sheet was produced in the same manner as in Example 1, except that the proportion of the first particles was made 50% and the porosity was made 61%.

[0177] (Comparative Example 10)

[0178] An anode sheet was produced in the same manner as in Example 1, except that the proportion of the first particles was made 50% and the porosity was made 67%.

[0179] <Assessment 1>

[0180] For each of the above examples and comparative examples, direct current resistance, reaction resistance, and diffusion resistance were evaluated by complex impedance measurement. The measurement section was equipped with an alternating voltage generator for generating an alternating voltage, a voltage application section, and an impedance measurement section. The frequency was changed in stages in the range of 0.001 Hz to 100,000 Hz at the maximum, and an alternating voltage was applied. Thereafter, output was obtained in the form of a Nyquist plot, and direct current resistance, reaction resistance, and diffusion resistance were obtained. The values of each resistance component are shown in Tables 1 to 4.

[0181] Figures 5-7 A graph obtained by plotting the values of direct current resistance, reaction resistance, and diffusion resistance of Examples 1 to 5 and Comparative Examples 1 and 2 is shown in FIG. 1. When the proportion of the first particles was higher than 70%, the direct current resistance sharply rose. In the range of less than 20%, the reaction resistance rose as the proportion of the first particles decreased. In detail, in the case where the proportion of the first particles was less than 5%, the reaction resistance sharply rose, and in the case where it was 5% or more and less than 20%, the reaction resistance slowly rose. As for the diffusion resistance, it slowly rose when the proportion of the first particles was more than 50% and less than 70%, and sharply rose when it was 70% or more.

[0182] Figure 8 A graph obtained by plotting the values of total resistance of Examples 1 to 5 and Comparative Examples 1 and 2 is shown in FIG. 2. In the case where the proportion of the first particles was less than 5%, and in the case where it was more than 70% and 100% or less, the total resistance was more than 490 mΩ. In the case where the proportion of the first particles was 5% or more and less than 20%, and in the case where it was more than 50% and 70% or less, the total resistance was 466 mΩ or less, and slightly decreased. In the case where the proportion of the first particles was 20% or more and 50% or less, the total resistance was 456 mΩ or 457 mΩ, and was the lowest.

[0183] Figures 9-11The graph obtained by plotting the values of the direct current resistance, the reaction resistance and the diffusion resistance of Examples 6 to 10 and Comparative Examples 3 and 4 is shown in FIG. 2. The direct current resistance sharply rises when the proportion of the first particles is 70% or more. In the range of less than 20%, the reaction resistance rises as the proportion of the first particles decreases. In detail, the reaction resistance rises when the proportion of the first particles is less than 5%, and slightly decreases when the proportion of the first particles is 5% or more and less than 20%. The diffusion resistance slowly rises when the proportion of the first particles is 50% or more. In addition, the ratio of the rise of the diffusion resistance with respect to the proportion of the first particles, i.e., the slope, increases when the proportion of the first particles is 70% or more, and the diffusion resistance further rises.

[0184] Figure 12 The graph obtained by plotting the values of the total resistance of Examples 6 to 10 and Comparative Examples 3 and 4 is shown in FIG. 3. The total resistance is greater than 490 mΩ when the proportion of the first particles is less than 5%, and when the proportion of the first particles is greater than 70% and 100% or less. The total resistance is 461 mΩ or less and slightly decreases when the proportion of the first particles is 5% or more and less than 20%, and when the proportion of the first particles is greater than 50% and 70% or less. The total resistance is 444 mΩ to 451 mΩ and is the lowest when the proportion of the first particles is 20% or more and 50% or less.

[0185] Figures 13-15 The graph obtained by plotting the values of the direct current resistance, the reaction resistance and the diffusion resistance of Examples 11 to 16 and Comparative Examples 5 to 7 is shown in FIG. 4.

[0186] The direct current resistance rises when the void ratio is greater than 50%, and sharply rises when the void ratio exceeds 70%. The reaction resistance is fixed at 25 mΩ. The diffusion resistance sharply rises when the void ratio is less than 30%.

[0187] Figure 16 The graph obtained by plotting the values of the total resistance of Examples 11 to 16 and Comparative Examples 5 to 7 is shown in FIG. 5. The total resistance is the lowest when the void ratio is in the range of 30% or more and 50% or less. With respect to the total resistance, the total resistance sharply rises when the void ratio is less than 20%, and when the void ratio is greater than 60%. The total resistance slightly rises when the void ratio is 20% or more and less than 30%, and when the void ratio is greater than 50% and 60% or less.

[0188] Figures 17-19 The graph obtained by plotting the values of the direct current resistance, the reaction resistance and the diffusion resistance of Examples 17 to 22 and Comparative Examples 8 to 10 is shown in FIG. 6.

[0189] As for the direct current resistance, it slowly rises when the porosity is greater than 50% and is 60% or less, and sharply rises when it is greater than 60%. The reaction resistance is fixed at 26 mΩ. In the range of 30% or less, the diffusion resistance rises as the porosity decreases. The diffusion resistance slowly rises when the porosity is 20% or more and is less than 30%, and sharply rises when it is less than 20%.

[0190] Figure 20 A graph obtained by plotting the values of the total resistance of Examples 17 to 22 and Comparative Examples 8 to 10 is shown in FIG. 6. The total resistance is the lowest in the range of 30% or more and 50% or less of the porosity. As for the total resistance, it sharply rises when the porosity is less than 20%, and when the porosity is greater than 60%. In the range of 20% or more and less than 30% of the porosity, and in the range of greater than 50% and 60% or less of the porosity, the total resistance slowly rises.

[0191]

[0192] <About the Conductive Material>

[0193] For Examples in which carbon nanotubes are used as the conductive material, and Comparative Examples 17 to 29 in which acetylene black is used, the porosity is changed to compare each resistance component and the total resistance. Note that the total resistance when the aspect ratio of the conductive material is changed is significantly affected by the porosity of the positive electrode composite material layer 19, and the effect of the proportion of the first particles is relatively small, so the porosity is changed here.

[0194] (Examples 25 to 33)

[0195] In Examples 25 to 33, the porosity is adjusted at intervals of 5% between 20% and 60%, and the proportion of the first particles is made 20%, and the lithium ion secondary battery 10 is produced in the same manner as in Example 1 except for this.

[0196] (Comparative Examples 11 to 16)

[0197] In Comparative Examples 11 to 14, the porosity is made 0%, 5%, 10%, and 15%. In Comparative Examples 15 and 16, the porosity is made 65% and 70%. In these Comparative Examples, carbon nanotubes are used as the conductive material. The lithium ion secondary battery 10 is produced in the same manner as in Example 1 except for this.

[0198] (Comparative Examples 17 to 29)

[0199] ​In Comparative Examples 17 to 29, acetylene black was used as the conductive material. The aspect ratio of the acetylene black was 1:10 or less. Further, the void ratio was adjusted in 5% increments between 0% and 60%. Note that in the positive electrode composite material paste containing carbon nanotubes as the conductive material, the void ratio at the stage of coating onto the positive electrode current collector and drying was 70%, and thus a positive electrode sheet having a void ratio greater than 70% could not be produced. On the other hand, in the positive electrode composite material paste containing acetylene black as the conductive material, the void ratio at the stage of coating onto the positive electrode current collector and drying was 60%. Thus, a comparative example having a void ratio of 65% and a comparative example having a void ratio of 70% could not be produced.

[0200] <Assessment 2>

[0201] A case where the direct current resistance was 175 mΩ or more was noted as "X", a case where the direct current resistance was 155 mΩ or more and less than 175 mΩ was noted as "Δ", and a case where the direct current resistance was less than 155 mΩ was noted as "O".

[0202] A case where the reaction resistance was 50 mΩ or more was noted as "X", a case where the reaction resistance was 30 mΩ or more and less than 50 mΩ was noted as "Δ", and a case where the reaction resistance was less than 30 mΩ was noted as "O".

[0203] A case where the diffusion resistance was 310 mΩ or more was noted as "X", a case where the diffusion resistance was 290 mΩ or more and less than 310 mΩ was noted as "Δ", and a case where the diffusion resistance was less than 290 mΩ was noted as "O".

[0204] A case where the total resistance was 485 mΩ or more was noted as "X", a case where the total resistance was 465 mΩ or more and less than 485 mΩ was noted as "O", and a case where the total resistance was less than 465 mΩ was noted as "◎".

[0205] Reference Figure 21 First, Examples 25 to 33 and Comparative Examples 11 to 16, which contain carbon nanotubes as the conductive material, will be described. With respect to the direct current resistance, Examples 25 to 31 and Comparative Examples 11 to 14 were "O", Examples 32 and 33 were "Δ", and Comparative Examples 15 and 16 were "X".

[0206] With respect to the reaction resistance, all of the examples and comparative examples were "O".

[0207] With respect to the diffusion resistance, Examples 27 to 33 and Comparative Examples 15 and 16 were "O", and Comparative Examples 11 to 14 were "X".

[0208] With respect to the total resistance, Examples 27 to 31 were "◎", Examples 25, 26, 32, and 33 were "O", and Comparative Examples 11 to 16 were "X".

[0209] Reference Figure 22Comparative Examples 17 to 29 in which acetylene black was used as the conductive material were described. As for the direct current resistance, Comparative Examples 17 to 25 were "O", and Comparative Examples 26 to 29 in which the void ratio was 45% or more were "X". That is, in the case where the conductive material was carbon nanotubes, the direct current resistance was "X" when the void ratio was 65% or more, whereas in the case where the conductive material was acetylene black, the direct current resistance was "X" when the void ratio was 45% or more, and the range of the void ratio in which a low direct current resistance was obtained was narrowed.

[0210] As for the reaction resistance, all of the examples and comparative examples were "O".

[0211] As for the diffusion resistance, Comparative Examples 17 to 25 in which the void ratio was 40% or less were "X", and Comparative Examples 26 to 29 in which the void ratio was 45% or more were "O". That is, in the case where carbon nanotubes were used as the conductive material, it was "O" when the void ratio was 30% or more and 60% or less, and it was "Δ" when the void ratio was less than 30%; but when the conductive material was made granular, it was "X" when the void ratio was 30% or more and 40% or less. That is, in the case where the conductive material was carbon nanotubes, the diffusion resistance was "X" when the void ratio was 15% or less; whereas in the case where the conductive material was acetylene black, the diffusion resistance was "X" when the void ratio was 40% or less, and the range of the void ratio in which a low diffusion resistance was obtained was narrowed.

[0212] As for the total resistance, Comparative Examples 17 to 29 in which acetylene black was used were "X".

Claims

1. A lithium ion secondary battery comprising: a positive electrode including a positive electrode composite material including positive electrode active material particles and a conductive material; a negative electrode including a negative electrode composite material; and an electrolyte, wherein the positive electrode composite material includes primary particles as the positive electrode active material particles, first agglomerated particles that are hollow agglomerates of a plurality of the primary particles and have a hollow portion with a diameter of less than 1 μm, and second agglomerated particles that are hollow agglomerates of a plurality of the primary particles and have a hollow portion with a diameter of 1 μm or more, wherein, when the primary particles and the first agglomerated particles are regarded as first particles, the proportion of the volume of the first particles to the total volume of the positive electrode active material particles is 5% or more and 70% or less, wherein the porosity of the positive electrode composite material is 20% or more and 60% or less, wherein the aspect ratio of the conductive material is 1:10 or more, wherein the aspect ratio is the ratio of the short side to the long side of the conductive material, wherein the proportion of the volume of the first particles is 20% or more and 50% or less, wherein the porosity is 30% or more and 50% or less, wherein the aspect ratio of the conductive material is 1:30 or more, wherein the content of the conductive material is 0.1% by weight or more and 5% by weight or less with respect to the weight of the positive electrode composite material, and wherein the average diameter of the conductive material is 1 nm or more and 100 nm or less. ​ ​ ​ ​ ​ ​ ​ ​ 2. The lithium-ion secondary battery according to claim 1, wherein ​ 3. The lithium-ion secondary battery according to claim 1 or 2, wherein ​ 4. The lithium-ion secondary battery according to claim 1 or 2, wherein ​ 5. The lithium-ion secondary battery according to claim 1 or 2, wherein ​ 6. The lithium-ion secondary battery according to claim 1 or 2, wherein ​

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