Method for evaluating conductive material used in secondary battery

Abstract

Provided is an evaluation method for accurately evaluating the conductivity, dispersibility, and the like of a conductive material used in a secondary battery. In an evaluation method for evaluating a conductive material used in a lithium ion secondary battery, a paste containing a conductive material and simulated primary particles comprising an insulator that simulate a positive electrode active material of the lithium ion secondary battery is prepared in a paste preparation step. In the test coating film manufacturing step, the paste is applied to a simulation substrate simulating a positive electrode substrate of a lithium ion secondary battery (1) and dried, a test coating film containing simulation primary particles is manufactured, and in the conductivity evaluation step, the conductive material is evaluated by measuring the coating film resistance, which is the surface resistance of the test coating film.

Description

Technical Field

[0001] The present invention relates to an evaluation method for evaluating a conductive material used in a secondary battery. Specifically, it relates to an evaluation method for evaluating a conductive material used in a secondary battery, which can accurately evaluate the conductivity, dispersibility, etc. of the conductive material. Background Art

[0002] Secondary batteries, such as lithium-ion secondary batteries, are high-voltage batteries. Due to their large volumetric energy density [Wh / L] and gravimetric energy density [Wh / kg], they are mostly used as drive power sources for electric vehicles, including hybrid vehicles. In electric vehicles, etc., large current charge and discharge are required due to large current discharge, rapid charging, or regenerative current during high-load operation. However, regarding the binder material for a cathode active material such as lithium nickel cobalt manganese oxide (LiNi 1 / 3 Co 1 / 3Mn 1 / 3 O2), its own conductivity is not relatively high. Therefore, in order to improve the input / output performance of a lithium-ion secondary battery at high current, a highly conductive material is sometimes added to the cathode composite material layer of the cathode plate for the purpose of reducing the electrical resistance between multiple cathode active material particles and a non-aqueous electrolyte, etc. As an example of such a conductive material in this case, fibrous carbon materials, such as carbon nanotubes (CNT), and granular acetylene black (AB) and other materials with high conductivity are sometimes used. In particular, carbon nanotubes have a fibrous shape and can easily form a conductive network between cathode active materials composed of lithium transition metal oxides dispersed in the cathode composite material layer even in a small amount, and can reduce the electrical resistance between the non-aqueous electrolyte and the cathode active material.

[0003] For example, regarding the carbon nanotubes described in Patent Document 1, they are easily dispersed in a resin, show high conductivity in the resin, and can reduce the electrical resistance (coating film resistance) in the cathode composite material layer by being added thereto. Prior Art Documents Patent Documents

[0004] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2015-150515 Summary of the Invention Problems to be Solved by the Invention

[0005] In addition, the lithium transition metal oxide used in the positive electrode active material is obtained as follows: First, primary particles in the form of crystals are produced, and then these primary particles are further aggregated to form spherical secondary particles having voids, thereby effectively carrying out the reaction on the surface of the positive electrode active material. In the positive electrode active material having such a shape of secondary particles, gaps communicating with the internal voids are present on its surface, but the degree thereof varies greatly depending on conditions such as firing.

[0006] Regarding the film resistance of the positive electrode composite layer of the existing positive electrode plate, in fact, the positive electrode composite layer is formed on a substrate composed of a PET (polyethylene terephthalate) film or the like, and its surface is measured. Thus, the conductivity of the positive electrode composite layer is evaluated in the form of film resistance. However, unlike the case described in Patent Document 1 where it is dispersed in a uniform resin, in the case of the positive electrode composite layer, if the conductive material enters the inside of the particles of the positive electrode active material from this gap, the film resistance changes. For example, when the gaps between the particles of the positive electrode active material are large, a large amount of the conductive material will enter, and the conductive material present between the positive electrode active materials will decrease. In this way, it is difficult to form a conductive network based on the conductive material, and the resistance increases. For this reason, there is a problem that even if the film resistance of the positive electrode composite layer is accurately measured, the conductive material cannot be correctly evaluated.

[0007] Such a situation where the conductive material cannot be correctly evaluated is not limited to the problem in the hollow positive electrode active material of the positive electrode plate, but also occurs in the negative electrode plate. In addition, it is not limited to the problem in the lithium ion secondary battery, but also occurs in other non-aqueous electrolyte secondary batteries, alkaline secondary batteries, and other secondary batteries.

[0008] The problem to be solved by the evaluation method of the conductive material used in the secondary battery of the present invention is to accurately evaluate the conductivity, dispersibility, etc. of the conductive material of the secondary battery. Means for Solving the Problem

[0009] An evaluation method for evaluating the conductive material used in a secondary battery having an electrode plate with a composite material layer formed on a substrate and containing an active material and a conductive material according to an aspect of the present invention is characterized in that it includes the following steps: a paste preparation step of preparing a paste containing simulated primary particles made of an insulator that mimics the active material of the secondary battery and the conductive material; a test film preparation step of coating the paste prepared in the paste preparation step onto a simulated substrate that mimics the substrate of the secondary battery and drying it to prepare a test film containing the simulated primary particles; and a conductivity evaluation step of evaluating the conductive material by measuring the surface resistance of the test film, i.e., the film resistance R S and evaluating the conductive material.

[0010] In the above evaluation method, based on the mixing volume ratio R v of the active material and the conductive material, the mixing volume ratio R v of the conductive material in the paste relative to the simulated primary particles can be set.

[0011] In the above evaluation method, the mixing mass ratio R W of the conductive material relative to the simulated primary particles is set to a range including the portion with the maximum curvature of the curve graph showing the change of the mixing mass ratio R W and the film resistance of the test film.

[0012] In the above evaluation method, when the mixing mass ratio of the conductive material relative to the simulated primary particles is R w , the mixing mass ratio R W can be in the range of 1 wt% or more and 3 wt% or less.

[0013] In the above evaluation method, the average particle diameter D S (d50) of the simulated primary particles can be substantially the same particle diameter as the particles of the active material of the secondary battery. In this case, the average particle diameter D S (d50) of the simulated primary particles can be 0.1 μm or more and 50 μm or less.

[0014] In the above evaluation method, the test film can have the same thickness as the composite material layer of the secondary battery. In the above evaluation method, the active material can be secondary particles aggregated from primary particles. In the above evaluation method, the conductive material can be composed of fibrous carbon. Additionally, in the above evaluation method, the simulated primary particles can be composed of alumina.

[0015] In the above evaluation method, the above-mentioned simulated substrate can be formed of an insulator. In the above evaluation method, the above-mentioned simulated substrate can be formed of a PET film. In the above evaluation method, the average diameter D C (d50) of the above-mentioned conductive material can be 1 nm or more and 100 nm or less. In the above evaluation method, the average length L C (d50) of the above-mentioned conductive material can be 100 nm or more and 10,000 nm or less.

[0016] In the above evaluation method, the above-mentioned secondary battery can be a lithium-ion secondary battery. In addition, in the above evaluation method, the above-mentioned electrode plate can be a positive electrode plate. Effects of the Invention

[0017] According to the evaluation method of the conductive material used in the secondary battery of the present invention, the conductivity, dispersibility, etc. of the conductive material can be accurately evaluated. Description of the Drawings

[0018] Figure 1 is a three-dimensional schematic diagram obtained by magnifying the positive electrode active material. Figure 2 is a schematic cross-sectional view obtained by magnifying the positive electrode active material. Figure 3 (a) of is a schematic diagram showing the relationship between the positive electrode active material and the conductive material with almost no gap, Figure 3 (b) of is a schematic diagram showing the relationship between the positive electrode active material and the conductive material with a medium gap, Figure 3 (c) of is a schematic diagram showing the relationship between the positive electrode active material and the conductive material with many gaps. Figure 4 is a schematic cross-sectional view showing the deviation of the form of the positive electrode active material in the positive electrode composite layer. Figure 5 (a) of is a schematic diagram of the simulated positive electrode plate of the present embodiment, Figure 5 (b) of is a schematic diagram of the positive electrode plate of the lithium-ion secondary battery of the present embodiment. Figure 6 is a schematic diagram showing the relationship between the simulated primary particles and the conductive material. Figure 7 is a three-dimensional view showing an outline of the external configuration of the lithium-ion secondary battery of the present embodiment. Figure 8 is a schematic diagram showing the configuration of the wound electrode body. Figure 9 is a table showing the conditions of the experimental example. Figure 10It is a graph showing the coefficient of variation CV indicating the deviation of the positive electrode active material and the simulated primary particles, respectively. Figure 11 It is a graph showing the change in the blending mass ratio R W [wt%] of the above conductive material with respect to the simulated primary particles and the film resistance R S [Ω·cm] of the test coating film. Figure 12 It is Figure 11 a graph obtained by magnifying a part of the shown graph. Detailed implementation mode

[0019] Hereinafter, with reference to Figures 1 - 12 , as an example, the implementation mode of the evaluation method for evaluating the conductive material 322, i.e., CNT (carbon nanotube), of the positive electrode plate 3 used in the lithium ion secondary battery 1 will be described for the evaluation method of the conductive material used in the secondary battery of the present invention.

[0020] It should be noted that this implementation mode is an example, and the electrode plate is not limited to the positive electrode plate, and the secondary battery is not limited to the lithium ion secondary battery. (Summary of this implementation mode) <Background technology of this implementation mode> Figure 1 It is a three-dimensional schematic diagram obtained by magnifying the positive electrode active material 321. Figure 2 It is a schematic cross-sectional view obtained by magnifying the positive electrode active material 321. As Figure 1 , Figure 2 shown, regarding the lithium transition metal oxide used as the positive electrode active material 321, first, primary particles 321b as crystals are formed. If left as it is, they are likely to aggregate. Therefore, in order to improve the dispersibility, the primary particles 321b are further aggregated to form spherical secondary particles 321a having voids 321d. Thereby, the reaction on the surface of the positive electrode active material 321 is effectively carried out and the obtained material is used. In the positive electrode active material 321 having the shape with voids 321d of such secondary particles 321a, gaps 321c communicating with the internal voids 321d are present on its surface. The number and area degree of these gaps 321c vary greatly depending on conditions such as firing.

[0021] Figure 3 (a) of is a schematic diagram showing the relationship between the positive electrode active material 321 with almost no gaps 321c and the conductive material 322. Figure 3 (b) of is a schematic diagram showing the relationship between the positive electrode active material 321 with a medium number of gaps 321c and the conductive material 322. Figure 3(c) is a schematic diagram showing the relationship between the positive electrode active material 321 and the conductive material 322 when the number of gaps 321c is large. In the conductive material 322, the deviation of the gaps 321c is large. For example, when there are almost no gaps 321c as shown in Figure 3 (a), the conductive material 322 does not enter the cavities 321d of the positive electrode active material 321. In this case, the conductive material 322 exists in the binder 323 between the secondary particles 321a of the adjacent positive electrode active materials 321. Thus, in the positive electrode composite layer 32 in which the same amount [vol%] of the conductive material 322 is mixed, the density of the conductive material 322 per unit volume of the positive electrode composite layer 32 increases. As a result, the conductive materials 322 are likely to come into contact with each other, and a conductive network is easily formed. That is, the film resistance R of the positive electrode composite layer 32 S [Ω·cm] decreases.

[0022] On the other hand, when there are many gaps 321c as shown in Figure 3 (c), the conductive material 322 easily enters the cavities 321d of the positive electrode active material 321 through the gaps 321c. In this case, the conductive material 322 exists not only in the binder 323 between the secondary particles 321a of the positive electrode active material 321 but also in the cavities 321d. Thus, in the positive electrode composite layer 32 in which the same amount [v0l%] of the conductive material 322 is mixed, the number of the conductive materials 322 per unit volume of the positive electrode composite layer 32 also decreases. As a result, the conductive materials 322 are not likely to come into contact with each other, and a conductive network is not easily formed. That is, the film resistance R of the positive electrode composite layer 32 S [Ω·cm] increases.

[0023] In addition, when there are gaps 321c as shown in Figure 3 (b), but the number and area thereof are smaller than those shown in Figure 3 (c), it exhibits properties intermediate between those of Figure 3 (a) and Figure 3 (c). Figure 4 is a schematic cross-sectional view showing the deviation of the morphology of the positive electrode active material 321 in the positive electrode composite layer 32. Regarding the positive electrode active material 321 as an actual raw material, as shown in Figure 4 , it is not uniform, but there are differences in the number of gaps 321c as shown in Figure 3 (a) to (c). In the example of Figure 4 , there are positive electrode active materials 321 as shown in Figure 3 (a) to (c). Specifically, there are Figure 3The two positive electrode active materials 321 shown in (a) of Figure 3 the two positive electrode active materials 321 shown in (b) of Figure 3 and the four positive electrode active materials 321 shown in (c) of

[0024] In addition, regarding the actual positive electrode composite layer 32, the positive electrode composite paste of the positive electrode composite layer 32 before being coated on the positive electrode substrate 31 is subjected to pressure in the compression molding process after being coated on the positive electrode substrate 31. Therefore, there are cases where the secondary particles 321a of the positive electrode active material 321 are broken or become individual primary particles 321b. Thus, in the completed lithium ion secondary battery 1, the state of the positive electrode active material 321 is uneven. Therefore, no matter how accurately the film resistance R S [Ω·cm] of the positive electrode composite layer 32 is measured, it is impossible to evaluate the conductivity of the conductive material 322 itself and the completion state of the positive electrode composite layer 32.

[0025] <The positive electrode plate 3 and the simulated positive electrode plate 103 of the lithium ion secondary battery 1> Figure 5 Figure (a) is a schematic view of the simulated positive electrode plate 103 of the present embodiment. Figure 5 Figure (b) is a schematic view of the positive electrode plate 3 of the lithium ion secondary battery 1 of the present embodiment. The simulated positive electrode plate 103 is a test structure for accurately measuring the conductivity and dispersibility of the conductive material 322 that cannot be accurately measured using the positive electrode plate 3 of the lithium ion secondary battery 1.

[0026] <The structure of the positive electrode plate 3 of the lithium ion secondary battery 1> First, the positive electrode plate 3 of the lithium ion secondary battery 1 of the present embodiment will be described with reference to Figure 5 Figure (b). The positive electrode plate 3 of the lithium ion secondary battery 1 of the present embodiment includes a positive electrode substrate 31 made of Al foil, and a positive electrode composite layer 32 is formed on the positive electrode substrate 31. The positive electrode composite layer 32 is a layer formed by coating a positive electrode composite paste and drying and compression molding it. The positive electrode composite paste is prepared by kneading a positive electrode active material 321, a conductive material 322, and a binder 323 using a solvent.

[0027] In this positive electrode plate 3, the formation of the conductive network based on the conductive material 322 varies depending on the shape of the secondary particles 321a of the positive electrode active material 321 as described above, particularly the amount of gaps 321c. Here, the "film resistance R S [Ω·cm]" of the present embodiment will be described, and the evaluation method of the conductive material 322 will be described. The "film resistance R sWhen measuring in "[Ω·cm]", first, the positive electrode composite material layer 32 is coated on a simulated substrate 131 made of a PET (polyethylene terephthalate) film instead of the positive electrode substrate 31 made of Al foil. Then, on the surface of this positive electrode composite material layer 32, measurement points MP1 and MP2 are set at positions 1 [cm] apart. The probes of the resistance meter OM are brought into contact with these measurement points MP1 and MP2, and the surface resistance of the conductive material 322 therebetween, that is, the coating film resistance R S [Ω·cm] is measured. For the measurement, it is carried out by the four-terminal method using a resistance meter with a four-probe probe of, for example, Hioki Electric Co., Ltd. Through this "coating film resistance R S [Ω·cm]", the conductivity, dispersibility, etc. of the conductive material can be accurately evaluated.

[0028] In the past, since the positive electrode substrate 31 made of conductive Al foil was provided on the surface facing the measurement points MP1 and MP2 of the positive electrode composite material layer 32, in fact, only the resistance in the thickness direction of the positive electrode composite material layer 32 could be obtained. As a result, it was difficult to accurately evaluate the conductive material 322 itself as a material. Therefore, as described above, by using a simulated substrate 131 made of a PET (polyethylene terephthalate) film instead of the positive electrode substrate 31 made of Al foil to measure the coating film resistance R S [Ω·cm], the influence of the positive electrode substrate 31 made of Al foil can be eliminated.

[0029] However, as described above, the secondary particles 321a of the positive electrode active material 321 not only have conductivity by themselves, but also the formation of the conductive network based on the conductive material 322 will vary due to their shape, especially the amount of the gaps 321c, etc. Therefore, it is impossible to accurately observe the conductivity of the conductive material 322 itself, the state of the conductive network formed by dispersion, etc.

[0030] <Configuration of the simulated positive electrode plate 103 of the present embodiment> Next, the simulated positive electrode plate 103 of the present embodiment will be described with reference to Figure 5 (a) of. Figure 5 In the simulated positive electrode plate 103 of the present embodiment shown in (a) of, the positive electrode active material 321 is replaced with simulated primary particles 132a.

[0031] Figure 6 is a schematic diagram showing the relationship between the simulated primary particles 132a and the conductive material 132b. As Figure 6 shown, the simulated primary particles 132a are simulated Figure 3Particles made of an insulator produced from the positive electrode active material 321 of the lithium-ion secondary battery 1 in (a). Here, "simulation" means that the shape is similar to that of the secondary particles 321a of the positive electrode active material 321 and the structure imitates the positive electrode active material 321. For example, the average particle size (d50) [μm] and the like are substantially the same. That is, the average particle size DS (d50) [μm] of the simulated primary particles is substantially the same as the particle size of the secondary particles of the active material of the secondary battery (lithium-ion secondary battery in this embodiment). In this application, unless otherwise specified, "the average particle size D of the simulated primary particles 132a S [μm]" refers to the median diameter (d50) in the frequency distribution measured by the laser diffraction method, and the average diameter D of the conductive material 132b C [nm], and the average length L C [nm] refer to the values obtained by image analysis of the electron microscope photograph.

[0032] Here, "substantially" means that even if there are some differences in shape such as unevenness, the mechanical function of the simulated primary particles 132a in the test coating film 132 is equivalent to the mechanical function of the positive electrode active material 321 in the positive electrode composite material layer 32. However, its material is made of an insulator, and alumina particles are used in this embodiment, so the electrochemical functions are different.

[0033] In addition, the "primary particles" mentioned here refer to the particles in the state formed initially during particle manufacturing. In the simulated primary particles 132a, the whole becomes a solid crystalline particle. On the other hand, in the positive electrode active material 321, a plurality of primary particles aggregate to form secondary particles 321a. As a result, in Figure 3 in (b), Figure 3 in (c) of the positive electrode active material 321, there are voids 321d and gaps 321c inside the secondary particles 321a. In contrast, in the simulated primary particles 132a, there are no such internal voids and gaps.

[0034] <Conductive material 132b> Regarding the conductive material 132b, although the naming method is different, it is the same as the conductive material 322 of the positive electrode composite material layer. That is, carbon nanotubes are exemplified in this embodiment, but all of their types, lengths, diameters, masses, addition amounts, etc. are the same. In addition, the mixing volume ratio R of the conductive material 132b of the test coating paste with respect to the simulated primary particles 132a V [vol%] is set based on the mixing volume ratio R of the conductive material 322 with respect to the positive electrode active material 321 V [vol%]. That is, they are substantially the same amount. It should be noted that this mixing volume ratio R V[vol%] is replaced by the blending mass ratio R of the conductive material 132b relative to the simulated primary particle 132a W [wt%]. The reason is that due to the influence of the bulk density and porosity, it is difficult to determine the accurate volume [mm 3 , so it is replaced in advance with the mass [g] that can be simply measured for implementation.

[0035] The reason as above is described as follows. That is, the purpose of the evaluation method for evaluating the conductive material 322 used in the lithium-ion secondary battery 1 in this embodiment is to accurately evaluate the conductivity, dispersibility, etc. of the conductive material 322 itself contained in the positive electrode composite material layer 32 of the lithium-ion secondary battery 1. Therefore, it is necessary to strictly reproduce the actual state. It should be noted that usually in the engineering field, conductivity is expressed in units of [S / m], but in this embodiment, such a measurement is carried out in order to analyze the dispersion state, etc. of the actual conductive material 322 in the lithium-ion secondary battery 1.

[0036] It should be noted that in this embodiment, fibrous carbon is exemplified as the conductive materials 322 and 132b, specifically, CNT (carbon nanotube) is exemplified, but other conductive materials can also be used, such as fibrous carbon microfibers, and granular AB (acetylene black), etc. Even for granular conductive materials, they enter the void 321d or the gaps, etc. through the gaps 321c of the positive electrode active material 321 in the same way as fibrous conductive materials. And the problem of changing the formation of the conductive network based on the conductive material in the binder material 323 is the same.

[0037] Specifically, in this embodiment, the average diameter D C (d50) [nm] of the conductive material 132b is 1 [nm] or more and 100 [nm] or less. In addition, the average length L C (d50) [nm] of the conductive material is 100 [nm] or more and 10000 [nm] or less.

[0038] <simulation substrate 131> The simulated positive electrode plate 103 of the present embodiment exhibits the same external shape as the positive electrode substrate 31, but it includes a simulated substrate 131 made of an insulator instead of an Al foil. That is, the simulated electrode plate (the simulated positive electrode plate 103 in the present embodiment) including the simulated substrate 131 and the test coating film 132 has the same external shape as the electrode plate (the positive electrode plate 31 in the present embodiment) of the secondary battery (the lithium-ion secondary battery in the present embodiment). As long as it has insulation and can be coated with the test coating paste, there is no limitation on the material. For example, in this embodiment, a PET (polyethylene terephthalate) film is used, which has high insulation and is also easy to coat. The test coating film 132 is formed on the simulated substrate 131. The test coating film 132 is a layer formed by coating the test coating paste and performing drying and compression molding. The test coating paste is prepared by kneading the simulated primary particles 132a, the conductive material 132b, and the binder material 132c with a solvent. The conductive material 132b, the binder material 132c, and the solvent are the same as the conductive material 322, the binder material 323, and the solvent of the positive electrode composite paste. In other words, the test coating paste has a composition in which only the positive electrode active material 321 is replaced with the simulated primary particles 132a.

[0039] <Measurement of coating film resistance R S [Ω·cm]> In the evaluation method for evaluating the conductive material used in the lithium-ion secondary battery 1 of the present embodiment, the measurement is performed on the surface of the test coating film 132 of the completed simulated positive electrode plate 103 in the same manner as the existing measurement method. Regarding the measurement, measurement points MP1 and MP2 are set at positions 1 [cm] apart, and the probes of the resistance meter OM are brought into contact with the measurement points MP1 and MP2, and the coating film resistance R S [Ω·cm] of the conductive material therebetween is measured.

[0040] Here, the positive electrode active material 321 is replaced with the insulating simulated primary particles 132a. In addition, the positive electrode substrate 31 is also replaced with the insulating simulated substrate 131. Therefore, the simulated primary particles 132a not only have insulation by themselves, but also, due to their shape, especially due to the absence of gaps 321c, the conductive material 322 always exists at the same density. Therefore, the formation of the conductive network based on the conductive material 322 is always in the same state. Therefore, it is possible to accurately observe the conductivity of the conductive material 322 itself, the state of the conductive network formed by dispersing the conductive material 322, and the like. In addition, on the surface facing the measurement points MP1 and MP2 of the positive electrode composite material layer 32, there is a simulated substrate 131 made of an insulating resin. Therefore, for the measurement, it is possible to measure only the resistance of the conductive network formed by the conductive material 132b in the test coating film 132 without being affected by the conductive positive electrode substrate 31. As a result, it is possible to accurately evaluate the conductive material 132b itself used as a raw material.

[0041] (Configuration of this Embodiment) Here, an example of the lithium ion secondary battery 1 that is a prerequisite for the evaluation method for evaluating the conductive material 322, i.e., CNT, used in the lithium ion secondary battery 1 of this embodiment will be described. Note that it is not intended to limit the type of battery as the object.

[0042] <Configuration of Lithium Ion Secondary Battery 1> Figure 7 is a perspective view showing an outline of the external configuration of the lithium ion secondary battery 1 of this embodiment.

[0043] As Figure 7 shown, the lithium ion secondary battery 1 is a single battery that constitutes a battery module for a drive battery pack mounted on a vehicle. The lithium ion secondary battery 1 includes a plate-shaped rectangular parallelepiped battery case 11 having an opening on the upper side. An electrode body 12 is housed inside the battery case 11. A non-aqueous electrolyte 13 is filled into the battery case 11 through a liquid injection hole. The battery case 11 is made of a metal such as aluminum alloy and constitutes an electrolytic cell sealed by a lid. In addition, the lithium ion secondary battery 1 includes a positive electrode external terminal 14 and a negative electrode external terminal 15 used in the charge and discharge of electricity. The positive electrode external terminal 14 is electrically connected to the positive electrode current collector terminal 16 inside the battery case 11 via the lid. In addition, the negative electrode external terminal 15 is electrically connected to the negative electrode current collector terminal 17 inside the battery case 11 via the lid. The positive electrode current collector terminal 16 is electrically connected to the positive electrode current collecting portion 33 of the electrode body 12 (refer to Figure 8 ). In addition, the negative electrode current collector terminal 17 is electrically connected to the negative electrode current collecting portion 23 of the electrode body 12 (refer to Figure 8 ).

[0044] <Electrode body 12> Figure 8 This is a schematic diagram showing the structure of the wound electrode body 12. In the electrode body 12, the negative electrode plate 2, the positive electrode plate 3, and the separator 4 disposed therebetween are laminated. The laminated negative electrode plate 2, positive electrode plate 3, and separator 4 are wound to form a flat shape. The negative electrode plate 2 forms a negative electrode composite material layer 22 on a negative electrode substrate 21 made of copper foil as a base material. A negative electrode current collector portion 23 is provided on one end side in the width direction W (winding axis direction) orthogonal to the winding direction L. The negative electrode current collector portion 23 has a structure in which the negative electrode substrate 21 is exposed without forming the negative electrode composite material layer 22.

[0045] The positive electrode plate 3 forms a positive electrode composite material layer 32 on a positive electrode substrate 31 made of aluminum foil as a base material. As Figure 8 shown, a positive electrode current collector portion 33 is provided on the other end side (the side opposite to the negative electrode current collector portion 23) in the width direction W (winding axis direction) orthogonal to the winding direction L in which the positive electrode substrate 31 is wound. The positive electrode current collector portion 33 has a structure in which the metal of the positive electrode substrate 31 is exposed without forming the positive electrode composite material layer 32.

[0046] <Laminated structure of electrode body 12> As Figure 8 shown, the basic structure of the electrode body 12 of the lithium ion secondary battery 1 includes a negative electrode plate 2, a positive electrode plate 3, and a separator 4.

[0047] The negative electrode plate 2 has negative electrode composite material layers 22 on both sides of a negative electrode substrate 21 as a negative electrode base material. One end portion of the negative electrode substrate 21 constitutes a negative electrode current collector portion 23 where the metal is exposed. The positive electrode plate 3 has positive electrode composite material layers 32 on both sides of a positive electrode substrate 31 as a positive electrode base material. The other end portion of the positive electrode substrate 31 constitutes a positive electrode current collector portion 33 where the metal is exposed.

[0048] The negative electrode plate 2 and the positive electrode plate 3 overlap with each other with the separator 4 interposed therebetween to form a laminate. As Figure 8 shown, this laminate is wound around a winding axis in the length direction to form Figure 7 the wound electrode body 12 formed in a flat shape as shown.

[0049] <Non-aqueous electrolyte 13> Figure 7The non-aqueous electrolyte 13 of the lithium-ion secondary battery 1 according to the present embodiment shown is a composition obtained by dissolving a lithium salt in an organic solvent. As the lithium salt, LiClO4, LiPF6, LiAsF6, LiBF4, LiSO3CF3, etc. can be used. As the organic solvent, cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate, chain carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate, ether compounds such as tetrahydrofuran, 2-methyltetrahydrofuran, and dimethoxyethane, sulfur-containing compounds such as ethyl methyl sulfone and butane sultone, or phosphorus compounds such as triethyl phosphate and trioctyl phosphate, etc. can be cited. As the non-aqueous electrolyte 13, one of them or a mixture of a plurality of them can be used. It should be noted that the composition of the non-aqueous electrolyte 13 is not limited thereto.

[0050] In the present embodiment, EC (ethylene carbonate) is used as the organic solvent. In addition, in the present embodiment, additives such as LiBOB (lithium bis(oxalato)borate, LiB(C2O4)2) belonging to the film-forming material can be added.

[0051] <Constituent elements of the electrode body 12> Next, the negative electrode plate 2, the positive electrode plate 3, and the separator 4, which are the constituent elements of the electrode body 12, will be described.

[0052] <Negative electrode plate 2> As shown in Figure 8 Negative composite material layers 22 are formed on both surfaces of the negative electrode substrate 21 as the negative electrode base material to constitute the negative electrode plate 2. Regarding the negative composite material layer 22, in the initial process, a negative composite material paste is applied to the negative electrode substrate 21. Thereafter, the negative electrode plate 2 is completed through a drying process, a pressing process, and a cutting process.

[0053] Regarding the negative electrode substrate 21, in the present embodiment, it is composed of a Cu foil. The negative electrode substrate 21 constitutes the basis of the aggregate of the negative composite material layer 22 and has the function of a current collecting member for collecting electricity from the negative composite material layer 22. One end portion of the negative electrode substrate 21 constitutes a negative electrode current collecting portion 23 where the metal surface is exposed without forming the negative composite material layer 22. That is, the negative electrode active material particles are electrically connected to the negative electrode external terminal 15 via the negative electrode substrate 21, the negative electrode current collecting portion 23, and the negative electrode current collecting terminal 17.

[0054] The negative composite material layer 22 is composed of a negative electrode active material as a raw material, a binder (bonding material) constituting a secondary material therein, additives, etc. Regarding the raw material and the secondary material, an organic solvent, etc. are added and kneaded to generate a negative composite material paste. This negative composite material paste is applied to the negative electrode substrate 21. The applied negative composite material paste is dried and formed by pressing, thereby completing the negative electrode plate 2.

[0055] In this embodiment, the negative electrode active material is powdery graphite particles GP composed of graphite or the like having a layered structure, and is a material capable of occluding and releasing lithium ions Li + . <Positive electrode plate 3> As Figure 5 shown in (b) of Figure 8 , the positive electrode plate 3 is composed of a positive electrode substrate 31 as a positive electrode base material and a positive electrode composite material layer 32 coated thereon. Regarding the positive electrode composite material layer 32, in the initial process, a positive electrode composite material paste is coated on the positive electrode substrate 31, and the positive electrode plate 3 is completed through a drying process, a pressing process, and a cutting process.

[0056] The positive electrode composite material layer 32 is formed on both sides of the positive electrode substrate 31 to constitute the positive electrode plate 3. In the embodiment, the positive electrode substrate 31 is composed of an Al foil. The positive electrode substrate 31 constitutes the basis of the aggregate of the positive electrode composite material layer 32 and has the function of a current collecting member for collecting electricity from the positive electrode composite material layer 32.

[0057] First, the positive electrode base material constituting the positive electrode substrate 31 is exemplified by an Al foil, and for example, it may be composed of a conductive material (formed of a metal having good conductivity). As a material having good conductivity, for example, in addition to an Al foil, a material containing an Al alloy may also be used. The configuration of the positive electrode substrate 31 is not limited thereto.

[0058] The positive electrode composite material layer 32 is formed by coating a positive electrode composite material paste on the positive electrode substrate 31 and drying it. The positive electrode composite material layer 32 contains secondary particles 321a of a positive electrode active material 321, a conductive material 322, a binder material 323, and additives such as a dispersant.

[0059] The positive electrode active material 321 contains a lithium transition metal oxide having a layered crystal structure. The lithium transition metal oxide contains, in addition to Li, one or more specified transition metal elements. The transition metal element contained in the lithium transition metal oxide is preferably at least one of Ni, Co, and Mn. The positive electrode active material 321 of this embodiment may be exemplified by a ternary system material called so-called NCM having a lithium transition metal oxide containing all elements of Ni, Co, and Mn.

[0060] It should be noted that the positive electrode active material 321 of this embodiment is not limited to a material having a lithium transition metal oxide containing all elements of Ni, Co, and Mn. It may also have a composition containing, for example, Al in addition to them.

[0061] <Separator 4> The separator 4 is a non-woven fabric made of polypropylene or the like, which is a porous resin and has high insulation properties for holding the non-aqueous electrolyte 13 between the negative electrode plate 2 and the positive electrode plate 3. In addition, as the separator 4, porous polymer membranes such as porous polyethylene membranes, porous polyolefin membranes, and porous polyvinyl chloride membranes, or lithium-ion or ion-conductive polymer electrolyte membranes can be used alone or in combination.

[0062] <Evaluation method for evaluating the conductive material used in the lithium-ion secondary battery 1> The following is an experimental example of the evaluation method for evaluating the conductive material used in the lithium-ion secondary battery 1 of the present embodiment.

[0063] <Simulated primary particle 132a> The average particle size D (d50) [μm] of the simulated primary particle 132a is substantially the same as the particle size of the particles of the positive electrode active material 321 of the lithium-ion secondary battery 1. Specifically, in the present embodiment, the average particle size D (d50) [μm] of the simulated primary particle 132a is 0.1 [μm] or more and 50 [μm] or less.

[0064] <Measurement conditions for the alumina coating film resistance> The composition of the test coating paste 30 [g] in this experimental example is 7.41 [g] of alumina, 3.96 [g] of CNT 3% solution, and 18.63 [g] of NMP (N-methyl-2-pyrrolidone). Among them, CNT is 0.1188 [g], which is equivalent to 1.6 [wt%] of alumina.

[0065] The paste production process is carried out as follows. The kneading of the paste is carried out as follows. After stirring with a stirring and defoaming machine at 2000 [rpm] for 30 [seconds], the inside of the container is stirred with a spatula, and the presence or absence of lumps is visually confirmed. In the case of lumps, they are crushed with a spatula. Using a stirring and defoaming machine, stir at 2000 [rpm] for 5 [minutes] to produce a slurry for coating film production. As the stirring and defoaming machine, for example, a Thinky Corporation's rotation / revolution type mixer defoaming Awatori Rentaro (registered trademark) atmospheric pressure type ARE-312 can be used.

[0066] The test coating film manufacturing process is carried out as follows. The test coating film 132 has the same thickness [μm] as the thickness [μm] of the positive composite material layer 32 of the lithium ion secondary battery 1. The manufacturing of the test coating film 132 includes a coating process and a drying process, and is carried out as follows. Place 4 - 5 [ml] of the test coating film paste on the PET film serving as the simulation substrate 131, and coat it using a bar coater and a 300 [μm] applicator. After coating, heat and dry it with hot air at 120 [°C] in a dryer for 15 [minutes] to manufacture the test coating film 132. The thickness of the test coating film 132 is measured using a film thickness gauge (Digital Micro). The coating film resistance R of the test coating film 132 is measured using the four-probe method. S [Ω·cm] is measured.

[0067] Figure 9 It is a table showing the conditions of Examples 1 - 4 of the experimental examples. The conditions are the particle composition of the simulated primary particles 132a or the positive electrode active material 321, and whether its particle structure is primary particles or secondary particles. In addition, the "oil absorption amount [ml / 100g]" is measured. This oil absorption amount [ml / 100g] represents the amount of linseed oil absorbed. By using linseed oil with high permeability, the linseed oil penetrates from the gap 321c into the cavity 321d. It can be known that if the oil absorption amount [ml / 100g] is large, the conductive material 322 composed of CNT is likely to enter the cavity 321d from the gap 321c.

[0068] The conductivity evaluation process is carried out as follows. In the method shown in (a) of Figure 5 , the coating film resistance R of the test coating film 132 S [Ω·cm] is measured. In addition, in the method shown in (b) of Figure 5 , the coating film resistance R of the positive composite material layer 32 S [Ω·cm] is measured. It should be noted that during the measurement, in order to make the conditions consistent, the positive electrode substrate 31 shown in (b) of Figure 5 is replaced with the simulation substrate 131 made of PET shown in (a) of Figure 5 . In the table, they are collectively referred to as "coating film resistance R S [Ω·cm]".

[0069] "σ" represents the standard deviation, which is the standard deviation of the coating film resistance R S [Ω·cm]. "Coefficient of Variation (CV)" is a value representing deviation, which is obtained by CV = standard deviation σ ÷ average value. It is a dimensionless value without a unit used for relative evaluation of the deviation of data with different units and the relationship between data and deviation relative to the average value. Here, it is expressed as a percentage [%].

[0070] <Regarding Examples 1 to 4> · Example 1: Using the simulated primary particles 132a, the particle composition is alumina, and the particle structure is solid primary particles. Therefore, the oil absorption amount [ml / 100g] is reduced to 16 [ml / 100g]. This means that in the simulated primary particles 132a of Example 1, the voids 321d and gaps 321c of the positive electrode active material 321 are extremely few.

[0071] The film resistance R of Example 1 S [Ω·cm] The average value is 9.4 [Ω·cm]. In addition, the standard deviation σ is 0.26 [Ω·cm]. Therefore, the coefficient of variation CV is 2.8%, indicating that the deviation is small.

[0072] · Example 2: Using Figure 3 The positive electrode active material with fewer gaps 321c as shown in (b) here, which is referred to as the positive electrode active material (b). The particle composition is the positive electrode active material 321, and the particle structure is hollow secondary particles 321a. Therefore, the oil absorption amount [ml / 100g] is 41 [ml / 100g], which is approximately 2.5 times more than that of the simulated primary particles 132a in Example 1. This means that voids 321d and gaps 321c exist in the positive electrode active material (b) of Example 2.

[0073] The film resistance R of Example 2 S [Ω·cm] The average value is 9.1 [Ω·cm]. In addition, the standard deviation σ is 0.62 [Ω·cm]. Therefore, the coefficient of variation CV is 6.8%, indicating that the deviation is larger than that of Example 1.

[0074] · Example 3: Using Figure 3 The positive electrode active material with more gaps 321c as shown in (c) here, which is referred to as the positive electrode active material (c). The particle composition is the positive electrode active material 321, and the particle structure is hollow secondary particles 321a. Therefore, the oil absorption amount [ml / 100g] is 47 [ml / 100g], which is more than that of Example 2. This means that more voids 321d and gaps 321c exist compared to the positive electrode active material (b) of Example 2.

[0075] The film resistance R of Example 3 SThe average value of [[Ω·cm]] is 20.5 [[Ω·cm]]. Additionally, the standard deviation σ is 1.36 [[Ω·cm]]. Therefore, the coefficient of variation CV is 6.6%, and it can be seen that, similarly to Example 2, the deviation is larger compared to Example 1.

[0076] · Example 4: The positive electrode active material (b) with fewer gaps 321c as shown in (b) of Figure 3 is mixed with the positive electrode active material (c) with more gaps 321c as shown in (c) of Figure 3 to form what is herein called the positive electrode active material (b + c). The particle composition is the positive electrode active material 321, and the particle structure is a hollow secondary particle 321a. Therefore, the oil absorption amount [ml / 100g] is 46 [ml / 100g], showing an intermediate value between Example 2 and Example 3. This shows the intermediate properties between the positive electrode active material (b) of Example 2 and the positive electrode active material (c) of Example 3.

[0077] The film resistance R of Example 4 S [[Ω·cm]] has an average value of 14.8 [[Ω·cm]]. This shows the intermediate properties between the positive electrode active material (b) of Example 2 and the positive electrode active material (c) of Example 3. Additionally, the standard deviation σ is 5.78 [[Ω·cm]]. Therefore, the coefficient of variation CV is 39.1%, and it can be seen that the deviation is extremely large compared to Example 2 and Example 3.

[0078] <Evaluation of the coefficient of variation CV> Figure 10 is a graph showing the coefficient of variation CV representing the deviation of each of the positive electrode active material 321 and the simulated primary particle 132a. In the simulated primary particle 132a made of alumina in Example 1, due to its production process, the deviation is relatively small, and the coefficient of variation CV = 2.8 [%].

[0079] On the other hand, the coefficient of variation CV of the positive electrode active material (b) in Example 2 is CV = 6.8 [%], and the coefficient of variation CV of the positive electrode active material (c) in Example 3 is CV = 6.6 [%]. The coefficient of variation CV increases in the positive electrode active material (b) and the positive electrode active material (c) of Example 2. This is because, in the positive electrode active materials (b) and (c), deviations occur during the stage of firing the primary particles 321b to produce the secondary particles 321a. And in the positive electrode active material (b + c) of Example 4, it is a mixture of the positive electrode active material (b) and the positive electrode active material (c). Therefore, for the positive electrode active material (b) with a film resistance R S of 9.1 [[Ω·cm]] and the positive electrode active material with a film resistance R SThe influence of the positive electrode active material (c) with [[Ω·cm]] of 20.5 [[Ω·cm]] is liable to vary depending on the position, and the standard deviation increases to σ = 5.78. That is, the oil absorption amount [[ml / 100g]], the coating film resistance R S [[Ω·cm]] is the average value of Example 2 and Example 3. However, regarding the deviation, as Figure 10 shown, since a deviation occurs between the positive electrode active material (b) with a small coating film resistance R S [[Ω·cm]] and the positive electrode active material (c) with a large coating film resistance R S [[Ω·cm]], the coefficient of variation increases to CV = 39.1%.

[0080] <Optimization of the blending mass ratio R [[wt%]] of the conductive material 132b with respect to the simulated primary particle 132a> Figure 11 shows the change in the blending mass ratio R W [[wt%]] of the conductive material 132b with respect to the simulated primary particle 132a and the coating film resistance R S [[Ω·cm]] of the test coating film 132. As Figure 11 shown, when the blending mass ratio R W [[wt%]] of the conductive material 132b with respect to the simulated primary particle 132a changes from zero, initially a very high coating film resistance R S [[Ω·cm]] is shown. It can be seen that thereafter it decreases sharply near the blending mass ratio R W [[wt%]] = 1 - 2. It is considered that this is because the amount sufficient to form a conductive network based on the conductive material 132b between adjacent simulated primary particles 132a has been reached.

[0081] In addition, Figure 12 is a graph obtained by magnifying a part (0 ≤ R Figure 11 ≤ 5, 0 ≤ R W ≤ 5, 0 ≤ R S ≤ 100) of the graph shown. As Figure 12 shown, the slope of the graph has a significant change in the range of 0 ≤ R W ≤ 5, 0 ≤ R S ≤ 100. Here, the blending mass ratio R W [[wt%]] of the conductive material 132b with respect to the simulated primary particle 132a varies from 0.8 to 3 [[wt%]], and a change in the coating film resistance R S [[Ω·cm]] of the test coating film is confirmed.

[0082] In this way, it can be known that with the blending mass ratio R WA proportion of the conductive material 132b blended in a range of approximately 1 [wt%] or more and 3 [wt%] or less is sufficient. In particular, as can be seen from the Figure 12 graph shown, the sensitivity to dispersibility appears in the range around 1.6 [wt%] where the curvature of the graph is the largest. More specifically, it can be seen that when the blending mass ratio R W [wt%] is greater than 3 [wt%], the film resistance R s [Ω·cm] does not change, and the blending mass ratio R W [wt%] of the conductive material 132b is excessive instead. On the other hand, it can be seen that when the blending mass ratio R W [wt%] is less than 1 [wt%], the blending mass ratio R W [wt%] of the conductive material 132b is not sufficient to form a conductive network. As a result, it can be seen that blending the conductive material 132b in a proportion where the blending mass ratio R w [wt%] of the conductive material 132b relative to the simulated primary particles 132a is approximately 1 [wt%] or more and 3 [wt%] or less is sufficient.

[0083] (Function of this Embodiment) When measuring the conductivity of the conductive material 322 blended in the positive electrode plate 3 of the lithium-ion secondary battery 1 completed as shown in Figure 5 (b), it is affected by the conductivity of the positive electrode active material 321 itself. In addition, due to the influence of the voids 321d and gaps 321c formed in the positive electrode active material 321 shown in Figure 3 , the density in the binder material 323 changes. Furthermore, it is affected by the conductive positive electrode substrate 31. Therefore, no matter how accurately the surface resistance of the positive electrode plate 3, i.e., the film resistance R S [Ω·cm], is measured, it is impossible to correctly evaluate the conductivity, dispersibility, formation of the conductive network, etc. of the conductive material 322 itself.

[0084] Then, in the simulated positive electrode plate 103 shown in Figure 5 (a), only the positive electrode active material 321 that affects such measurement is replaced with the simulated primary particles 132a as an insulator while maintaining its mechanical structure. In addition, the positive electrode substrate 31 is also replaced with the simulated substrate 131 as an insulator while maintaining its mechanical structure.

[0085] As a result, it is possible to correctly evaluate the conductivity, dispersibility, formation of the network, etc. of the conductive material 322 itself that cannot be accurately evaluated in the completed lithium-ion secondary battery 1.

[0086] In addition, by conducting experiments using the simulated positive electrode plate 103, it is also possible to derive the appropriate addition amount of the conductive material 322 and the like. (Effects of this embodiment) (1) The evaluation method for evaluating the conductive material 322 used in the lithium-ion secondary battery 1 of this embodiment has the effect of being able to accurately evaluate the conductivity, dispersibility, etc. of the conductive material 322.

[0087] (2) This embodiment relates to an evaluation method for evaluating the conductive material 322 used in the lithium-ion secondary battery 1 in a positive electrode plate 3 having a positive electrode composite layer 32 formed on a positive electrode substrate 31 and containing a positive electrode active material 321 and a conductive material 322. Therefore, it has the effect of being able to appropriately set the addition amount of the conductive material 322 and the like when manufacturing the lithium-ion secondary battery 1 as an actual production object.

[0088] (3) The simulated positive electrode plate 103 contains simulated primary particles made of an insulator that include the positive electrode active material 321 of the simulated lithium-ion secondary battery 1. Therefore, it has the effect of being able to accurately evaluate the conductivity, dispersibility, etc. of the conductive material 322 by accurately reproducing the dispersion state of the conductive material 322.

[0089] (4) In addition, for the simulated positive electrode plate 103, a test coating film 132 containing simulated primary particles 132a is produced by coating and drying on a simulated substrate 131 that simulates the positive electrode substrate 31 of the lithium-ion secondary battery 1. Therefore, it has the effect of being able to eliminate the influence of the conductivity of the positive electrode substrate 31 and accurately evaluate the conductivity, dispersibility, etc. of the conductive material 322.

[0090] (5) The coating film resistance R S [Ω·cm] of the surface resistance of the test coating film 132 is measured. Therefore, it has the effect of being able to accurately evaluate the conductivity, dispersibility, etc. of the conductive material 322.

[0091] (6) Based on the mixing volume ratio R V [vol%] of the positive electrode active material 321 and the conductive material 322, the mixing volume ratio Rv [vol%] of the conductive material 132b relative to the simulated primary particles 132a in the test coating film paste is set. Therefore, it has the effect of being able to accurately reproduce the role of the conductive material 132b in the lithium-ion secondary battery 1 in the simulated positive electrode plate 103.

[0092] (7) The mixing mass ratio R W [wt%] of the conductive material 132b relative to the simulated primary particles 132a is set to represent the mixing mass ratio R WThe change in [wt%] and the coating resistance R of the test coating film S is within the range including the portion with the maximum curvature of the curve graph in the curve graph with [Ω·cm]. Therefore, it has the effect of being able to accurately analyze the action of the conductive material 132b.

[0093] According to the experiments of the present inventors, it has been analyzed that the mixing mass ratio R W of the conductive material 132b with respect to the simulated primary particles 132a in [wt%] is appropriately in the range of 1 [wt%] or more and 3 [wt%] or less. Therefore, it has the effect of being able to derive an appropriate addition amount of the conductive material 132b.

[0094] (8) The average particle diameter D S (d50) [μm] of the simulated primary particles 132a is substantially the same as the particle diameter of the positive electrode active material 321 of the lithium ion secondary battery 1. Therefore, it has the effect of being able to accurately reproduce the action of the conductive material 132b in the simulated positive electrode plate 103.

[0095] According to the experiments of the present inventors, it has been analyzed that it is appropriate when the average particle diameter D s (d50) [μm] of the simulated primary particles is 0.1 [μm] or more and 50 [μm] or less. (9) The thickness [μm] of the test coating film 132 and the positive electrode composite material layer 32 of the lithium ion secondary battery 1 is the same thickness [μm]. Therefore, it has the effect of being able to accurately reproduce the action of the conductive material 132b in the simulated positive electrode plate 103.

[0096] (10) This embodiment is applicable to the case where the positive electrode active material 321 is secondary particles of a lithium transition metal oxide. In such a case, the problem can be solved by this embodiment. And it has the effect of being able to be appropriately applied to the case where the conductive material 322 is composed of fibrous carbon.

[0097] (11) The simulated primary particles 132a use alumina. Alumina has high insulation, mechanical stability, and does not cause unwanted electrochemical reactions. The simulated substrate 131 is preferably formed of an insulator. In this embodiment, the simulated substrate 131 is formed of a PET film. The PET film has high insulation, mechanical stability, and does not cause unwanted electrochemical reactions. Therefore, it has the effect of being able to accurately evaluate the conductivity, dispersibility, etc. of the conductive material 322.

[0098] (12) The average diameter D C (d50) [nm] of the conductive materials 322 and 132b is 1 [nm] or more and 100 [nm] or less, and the average length L C(d50) is 100 [nm] or more and 10,000 [nm] or less. Such a material is easily affected by the shape of the positive electrode active material 321, and thus has the effect of being able to accurately evaluate the conductivity, dispersibility, etc. of the conductive material 322.

[0099] (Other examples) · The description in this embodiment is an example of the present invention and does not limit the present invention. It can be implemented as in the following other examples. In the case of these other examples, those skilled in the art can optimize the invention.

[0100] · In this embodiment, the positive electrode active material 321 having a hollow structure with a gap 321c and a cavity 321d is illustrated, but the present invention can also be implemented even when it is composed only of a solid positive electrode active material 321 without a gap 321c and a cavity 321d. In such a case, the CNT does not enter the gap 321c and the cavity 321d. However, even when the unevenness on the surface increases or the size is uneven, the conductivity and dispersibility of the conductive material can be more accurately evaluated by using the simulated primary particle 132a of the present invention.

[0101] · Similarly, the electrode plate is not limited to the positive electrode plate 3, and the present invention can also be implemented in the negative electrode plate 2. For example, as the active material of the lithium ion secondary battery 1, even when graphite is used as the negative electrode active material or when silicon is used, the verification of the conduction path can be implemented.

[0102] · In addition, the secondary battery is not limited to the lithium ion secondary battery, and the present invention can also be implemented in other non-aqueous electrolyte secondary batteries, alkaline secondary batteries, and other secondary batteries.

[0103] · In this embodiment, fibrous carbon is illustrated as the conductive materials 322 and 132b. Specifically, CNT (carbon nanotube) is illustrated, but other conductive materials such as fibrous carbon microfibers and granular AB (acetylene black) can also be used.

[0104] · The drawings are schematic drawings for explaining the evaluation method for evaluating the conductive material used in the lithium ion secondary battery of this embodiment, and their quantity, shape, size, etc. do not reflect the actual form.

[0105] · Each numerical value or numerical range such as quantity, shape, size, etc., shape, material, etc. is an example and does not limit the present invention. Of course, those skilled in the art can appropriately optimize it. · The program of the evaluation method for evaluating the conductive material used in the lithium ion secondary battery is an example, and its order can be changed, and programs can be added or deleted.

[0106] ·In addition, in the present invention, those skilled in the art can, of course, implement it by adding, deleting, or changing its composition as long as it does not deviate from the description in the claims.

Claims

1. A method for evaluating a conductive material used in a secondary battery, wherein the conductive material is evaluated in a secondary battery having an electrode plate in which a composite material layer containing an active material and a conductive material is formed on a substrate, wherein the conductive material is evaluated in the secondary battery, wherein: The evaluation method includes the following steps: a paste preparation step of preparing a paste including simulated primary particles made of an insulator simulating the active material of the secondary battery and the conductive material; a test coating film preparation step of applying the paste prepared in the paste preparation step to a simulated substrate simulating the substrate of the secondary battery and drying the simulated substrate to prepare a test coating film containing the simulated primary particles; as well as In the conductivity evaluation step, the conductive material is evaluated by measuring the surface resistance of the test coating film, that is, the coating film resistance.

2. The method for evaluating a conductive material used in a secondary battery according to claim 1, wherein: The mixing volume ratio of the conductive material to the pseudo primary particles in the paste is set based on the mixing volume ratio of the conductive material to the active material.

3. The method for evaluating a conductive material used in a secondary battery according to claim 1, wherein: The blending mass ratio of the conductive material to the pseudo primary particles is set to a range including a portion where the curvature of a graph showing a change in the blending mass ratio and the coating film resistance of the test coating film is maximum.

4. The method for evaluating a conductive material used in a secondary battery according to claim 1, wherein: The mixing mass ratio of the conductive material to the pseudo primary particles is in a range of 1 wt % to 3 wt %.

5. The method for evaluating a conductive material used in a secondary battery according to claim 1, wherein: The average particle size when d50 of the primary particles is simulated is substantially the same as the particle size of the active material particles of the secondary battery.

6. The method for evaluating a conductive material used in a secondary battery according to claim 1, wherein: The average particle size at d50 of the simulated primary particles is 0.1 μm or more and 50 μm or less.

7. The method for evaluating a conductive material used in a secondary battery according to claim 1, wherein: The test coating film and the composite material layer of the secondary battery have the same thickness.

8. The method for evaluating a conductive material used in a secondary battery according to claim 1, wherein: The active material is a secondary particle formed by aggregation of primary particles.

9. The method for evaluating a conductive material used in a secondary battery according to claim 1, wherein: The conductive material is composed of fibrous carbon.

10. The method for evaluating a conductive material used in a secondary battery according to claim 1, wherein: The simulated primary particles are composed of aluminum oxide.

11. The method for evaluating a conductive material used in a secondary battery according to claim 1, wherein: The dummy substrate is formed of an insulator.

12. The method for evaluating a conductive material used in a secondary battery according to claim 11, wherein: The dummy substrate was formed of a PET film.

13. The method for evaluating a conductive material used in a secondary battery according to claim 1, wherein: The conductive material has an average diameter at d50 of 1 nm or more and 100 nm or less.

14. The method for evaluating a conductive material used in a secondary battery according to claim 1, wherein: The conductive material has an average length at d50 of 100 nm or more and 10000 nm or less.

15. The method for evaluating a conductive material used in a secondary battery according to any one of claims 1 to 14, characterized in that: The secondary battery is a lithium ion secondary battery.

16. The method for evaluating a conductive material used in a secondary battery according to claim 15, wherein: The electrode plate is a positive electrode plate.

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