Electrode and secondary battery

Abstract

The present invention provides: an electrode having an insulating layer that suppresses cracking and has sufficient puncture strength; and a secondary battery.　An electrode according to an embodiment has a current collector, an electrode mixture layer provided on the current collector, and an insulating layer which is provided on the current collector and which is provided adjacent to the electrode mixture layer. The pore distribution of the insulating layer has, in a log differential pore distribution curve obtained by mercury intrusion porosimetry, a first peak (PA), which is the maximum peak of the pore volume appearing in a range in which the pore diameter is equal to or greater than 0.01 μm and less than 0.08 μm, and a second peak (PB), which is the maximum peak of the pore volume appearing in a range in which the pore diameter is equal to or greater than 0.08 μm and less than or equal to 1.0 μm.

Description

Electrode and Secondary Battery

[0001] Embodiments of the present invention relate to an electrode and a secondary battery.

[0002] In recent years, secondary batteries such as lead-acid batteries and nickel-metal hydride batteries have been used as power sources typified by those for electric vehicles, hybrid vehicles, electric motorcycles, forklifts and the like. Recently, development toward the adoption of lithium-ion secondary batteries with high energy density has been actively carried out, with developments being conducted while giving consideration to long life, safety and other factors.

[0003] For example, in a lithium-ion secondary battery (hereinafter referred to as a secondary battery), there is an electrode in which an electrode mixture layer is provided on a current collector, and an insulating layer is further provided on the current collector adjacent to the electrode mixture layer. When this electrode is used to form an electrode group, the insulating layer is disposed to face the electrode mixture layer of another electrode with a separator interposed therebetween. Even when the electrode mixture layer of the other electrode breaks through the separator, a short circuit of the electrode group can be suppressed by the insulating layer.

[0004] However, part of the insulating layer may crack and fall off. In this case, there is a risk that no insulating layer exists at the position facing the electrode mixture layer of the other electrode, which may cause a short circuit in the electrode group. Furthermore, when the electrode mixture layer of the other electrode breaks through the separator, there is a risk that the electrode mixture layer will further break through the insulating layer. In this case, the electrode mixture layer of the other electrode comes into contact with the current collector under the insulating layer, which may cause a short circuit in the electrode group.

[0005] Japanese Patent Application Laid-Open No. 2015-213073, Japanese International Publication No. 2014 / 162437

[0006] The problem to be solved by the present invention is to provide an electrode and a secondary battery having an insulating layer that suppresses cracking and has sufficient puncture strength.

[0007] To solve the above problems, the electrode of the embodiment comprises a current collector, an electrode mixture layer provided on the current collector, and an insulating layer provided on the current collector and adjacent to the electrode mixture layer. The pore distribution of the insulating layer has, in the log differential pore distribution curve obtained by the mercury intrusion method, a first peak (PA) which is the maximum peak of pore volume appearing in the range of pore diameters from 0.01 μm to less than 0.08 μm, and a second peak (PB) which is the maximum peak of pore volume appearing in the range of pore diameters from 0.08 μm to 1.0 μm.

[0008] A schematic plan view showing the electrodes of the first embodiment. A log differential pore distribution curve obtained by the mercury intrusion method for the pore distribution of the insulating layer used in the electrodes of the first embodiment. A schematic perspective view showing the secondary battery of the second embodiment. A schematic perspective view showing the electrode group used in the secondary battery of the second embodiment. A partially unfolded perspective view of the electrode group used in the secondary battery of the second embodiment, viewed from above.

[0009] The electrodes and secondary battery of the embodiment will be described below with reference to the drawings.

[0010] (First Embodiment) The electrode 2 of the first embodiment will be described with reference to Figure 1. Figure 1 is a schematic plan view showing the electrode 2 of the first embodiment. As shown in Figure 1, the electrode 2 comprises a current collector 70 and an electrode mixture layer 20 provided on the current collector 70. Furthermore, the electrode 2 comprises an insulating layer 10 provided on the current collector 70 and adjacent to the electrode mixture layer 20. The current collector 70 has an uncoated portion 70a where neither the electrode mixture layer 20 nor the insulating layer 10 is provided.

[0011] The current collector 70 of electrode 2 is rectangular in shape, for example, having a long side 12 and a short side 14 perpendicular to the long side 12 (width direction). The electrode mixture layer 20 is provided parallel to the long side 12, but it may also be provided parallel to the short side 14. The insulating layer 10 is provided adjacent to the electrode mixture layer 20. Furthermore, the electrode mixture layer 20 and the insulating layer 10 are provided on one or both sides of the current collector 70.

[0012] Here, the insulating layer 10 of this embodiment will be described. The insulating layer 10 preferably contains insulating particles (not shown) and a binder (not shown), but it does not have to contain a binder.

[0013] The insulating particles are, for example, solid particles of metal oxides, such as barium sulfate. The binder is, for example, a fluororesin, specifically polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polytetrafluoroethylene-vinylidene fluoride (PTFE-PVdF), polytetrafluoroethylene-hexafluoropropylene (PTFE-HFP), etc.

[0014] Here, the insulating layer 10 used in the electrode 2 of this embodiment will be described with reference to Figure 2. Figure 2 is a log differential pore distribution curve obtained by the mercury intrusion method for the pore distribution of the insulating layer 10 used in the electrode 2 of the first embodiment. The pore distribution curve in Figure 2 is the result for an insulating layer 10 using barium sulfate as the insulating particle and PVdF as the binder, with a mixing ratio of 92 mass%:8 mass%. As shown in the pore distribution curve in Figure 2, the insulating layer 10 of this embodiment has a first peak (PA), which is the maximum peak of pore volume appearing in the range of pore diameters from 0.01 μm to less than 0.08 μm, and a second peak (PB), which is the maximum peak of pore volume appearing in the range of pore diameters from 0.08 μm to 1.0 μm. The maximum peak of pore volume is, for example, the peak with the largest peak height for pore diameters within a predetermined range, as shown in Figure 2. Furthermore, the insulating layer 10 may have peaks other than the first peak (PA) and the second peak (PB). The insulating layer 10 may have multiple peaks in at least one of the following ranges: a pore diameter of 0.01 μm or more and less than 0.08 μm, or a pore diameter of 0.08 μm or more and 1.0 μm or less.

[0015] The log differential pore distribution curve of the insulating layer 10 obtained by the mercury intrusion method can be obtained, for example, by the following method. First, if the electrode 2 is contained within a secondary battery, it is disassembled and the electrode 2 is removed. This disassembly is carried out in a glove box under an inert gas atmosphere such as argon. The removed electrode 2 is washed with a solvent and then dried. For example, ethyl methyl carbonate is used as the solvent. The insulating layer 10 portion of the dried electrode 2 is cut to obtain multiple test pieces. The test pieces include the current collector 70, but they do not necessarily have to include the current collector 70.

[0016] Next, multiple test specimens are placed in the measuring cell of the measuring device, and mercury is introduced into the pores of the test specimens. The number of test specimens is, for example, between 16 and 32. As the measuring cell, for example, a 5cc cell for large specimens with a stem volume of 0.4cc is used. As the measuring device, for example, a Shimadzu Autopore 9520 (AA Autopore 9520 model manufactured by Shimazu Corporation) is used. For the measurement, for example, the initial pressure is set to 7kPa and the final pressure to 414MPa. 7kPa corresponds to 1.0 psia (pounds per square inch absolute pressure), which corresponds to a pore with a diameter of approximately 180 μm. Furthermore, 414 MPa corresponds to approximately 6 psia, which corresponds to a pore with a diameter of approximately 0.003 μm. The mercury contact angle is set to 130 degrees, and the mercury surface tension is set to 485 dynes / cm. By processing the obtained data, the log differential pore distribution curve of the insulating layer 10 can be obtained.

[0017] As shown in the pore distribution curve in Figure 2, the insulating layer 10 has a first peak (PA), which is the maximum peak of pore volume appearing in the range of pore diameters from 0.01 μm to less than 0.08 μm, and a second peak (PB), which is the maximum peak of pore volume appearing in the range of pore diameters from 0.08 μm to 1.0 μm. In the insulating layer 10, there are many pores A with a pore diameter of 0.01 μm to less than 0.08 μm, and many pores B that are slightly larger than pores A with a pore diameter of 0.08 μm to 1.0 μm. By having both pores A and pores B in the insulating layer 10 and appropriately maintaining the voids within the insulating layer 10, it is possible to provide an electrode 2 having an insulating layer 10 that suppresses cracking of the insulating layer 10 and has sufficient puncture strength. The reason for this will be explained in detail below.

[0018] For example, if the insulating layer 10 contains only small pores A with a pore diameter of 0.01 μm or more and less than 0.08 μm, there are few voids within the insulating layer 10, and the insulating particles are densely packed within the insulating layer 10, resulting in high density within the insulating layer 10. An insulating layer 10 with high density exhibits strong resistance to punctures and has high puncture strength due to its dense molecular structure. However, an insulating layer 10 with high density has a high elastic modulus, making it prone to cracking under external force. Therefore, an insulating layer 10 containing only pores A with a pore diameter of 0.01 μm or more and less than 0.08 μm has the advantage of high puncture strength, but the disadvantage of being prone to cracking.

[0019] On the other hand, if only pores B with a pore diameter of 0.08 μm or more and less than 1.0 μm exist within the insulating layer 10, the voids within the insulating layer 10 are greater than the voids when only pores A exist within the insulating layer 10, and the insulating particles are not densely packed within the insulating layer 10, resulting in a low density within the insulating layer 10. Here, a low density within the insulating layer 10 means that it is lower compared to the density within the insulating layer 10 having only pores A. An insulating layer 10 with low density has a low elastic modulus, which can suppress cracking under external forces. However, an insulating layer 10 with low density has a less dense molecular structure, resulting in weak puncture strength. Therefore, an insulating layer 10 having only pores B with a pore diameter of 0.08 μm or more and less than 1.0 μm has the advantage of being less prone to cracking, but it has the disadvantage of weak puncture strength.

[0020] Therefore, the insulating layer 10 must not only have pores A and pores B, but also both pores A and pores B. Small pores A with a diameter of 0.01 μm or more and less than 0.08 μm can form areas within the insulating layer 10 where insulating particles are densely packed, creating areas of high density. This high-density insulating layer 10 exhibits strong resistance to punctures and has high puncture strength due to its dense molecular structure. On the other hand, pores B with a diameter of 0.08 μm or more and less than 1.0 μm can form areas within the insulating layer 10 where insulating particles are not densely packed, creating areas with lower density than the high-density areas created by pores A. This low-density insulating layer 10 has a low elastic modulus, which suppresses cracking under external forces.

[0021] A more preferable pore diameter for pore A is 0.01 μm or more and 0.05 μm or less. Small pores A within this range allow for the easy formation of densely packed insulating particles within the insulating layer 10, creating areas of high density. A more preferable pore diameter for pore B is 0.08 μm or more and 0.4 μm or less. Pores B within this range allow for the easy formation of areas not densely packed with insulating particles within the insulating layer 10. Furthermore, since the pore diameter of pore B is close to that of pore A, if the insulating layer 10 contains a binder as described later, the binder can easily bond the insulating particles within the insulating layer 10.

[0022] Based on the above, by having both pores A and pores B in the insulating layer 10 and appropriately maintaining the voids within the insulating layer 10, it is possible to provide an electrode 2 having an insulating layer 10 that suppresses cracking of the insulating layer 10 and has sufficient puncture strength. The high-density portion provided by pores A provides an insulating layer 10 with sufficient puncture strength, and the low-density portion provided by pores B suppresses cracking of the insulating layer 10.

[0023] The insulating layer 10 of this embodiment is obtained by using insulating particles of appropriate particle size. The size of the insulating particles contained in the insulating layer 10 will be further explained. Preferably, the insulating particle diameter (D50) of the insulating particles contained in the insulating layer 10 at a cumulative 50% in the particle size distribution chart obtained by laser diffraction scattering is 0.3 μm or more and 3.0 μm or less. When D50 is 0.3 μm or more, a portion having pores B with a pore diameter of 0.08 μm or more and less than 1.0 μm can be easily formed in the insulating layer 10. Also, when D50 is 3.0 μm or less, a portion having pores A with a pore diameter of 0.01 μm or more and less than 0.08 μm can be easily formed. It is more preferable that D50 is 0.3 μm or more and 2.5 μm or less. The method for measuring the particle size distribution chart by laser diffraction scattering will be explained below. Insulating particles are peeled off from a part of the insulating layer (insulating particle-containing layer) 10, for example, using a spatula. Thus, a powder of the insulating layer 10 sample is obtained. Next, the powder of the insulating layer 10 sample is placed into a measurement cell filled with N-methylpyrrolidone until a measurable concentration is reached. Note that the capacity of the measurement cell and the measurable concentration will vary depending on the particle size distribution analyzer. The measurement cell containing N-methylpyrrolidone and the insulating layer 10 sample dissolved in it is irradiated with 40W ultrasonic waves for 5 minutes. Such ultrasonic irradiation can dissolve the aggregation of insulating particles. The ultrasonically treated measurement cell is loaded into a particle size distribution analyzer using the laser diffraction scattering method and used to measure the particle size distribution. An example of a particle size distribution analyzer is the Microtrac 3100 manufactured by Microtrac-Bell Corporation.

[0024] As mentioned above, the insulating particles are, for example, solid particles of metal oxides, but barium sulfate is more preferable. Barium sulfate has excellent heat resistance and low hygroscopicity, and in addition, in the particle size distribution chart obtained by laser diffraction scattering, the particle size at 50% cumulative size (D50) is small, at about 1.3 μm. Because the D50 of the insulating particles is about 1.3 μm, both portions with small pores A with a pore size of 0.01 μm or more and less than 0.08 μm, and portions with pores B with a pore size of 0.08 μm or more and less than 1.0 μm can be easily formed in the insulating layer 10.

[0025] Furthermore, in the particle size distribution chart, the ratio of the insulating particle diameter at 10% cumulative density (D10) to the insulating particle diameter at 90% cumulative density (D90) is in the range of 0.1 ≤ D10 / D90 ≤ 0.3. A high value of D10 means that there are many fine particles, and a high value of D90 means that there are many coarse particles. In other words, it means that the particle size distribution tends to be broad. When D10 / D90 is 0.1 or higher, the particle size of the insulating particles is not biased, making it easier to increase the density within the insulating layer 10 and making it easier to provide an insulating layer 10 with sufficient puncture strength. Also, when D10 / D90 is 0.3 or lower, it is easier to maintain an appropriate density within the insulating layer 10 and lower the elastic modulus, making it easier to provide an insulating layer 10 that suppresses cracking. As mentioned above, the D10 / D90 for barium sulfate is approximately 0.2.

[0026] Furthermore, as mentioned above, it is preferable that the insulating layer 10 includes a binder. The binder plays a role in binding the insulating particles within the insulating layer 10 and between the insulating particles and the current collector 70. The insulating layer 10 in this embodiment has both pores A and pores B, and the pores A, with a pore diameter of 0.01 μm or more and less than 0.08 μm, can form a portion within the insulating layer 10 where the insulating particles are densely packed, thus providing a high-density portion. This high-density insulating layer 10 has strong puncture strength, but by including a binder, the binder binds between the current collector 70 and the insulating particles, and between the insulating particles themselves, resulting in an insulating layer 10 with high peel strength. This suppresses the peeling of the insulating layer 10 from the current collector 70.

[0027] Furthermore, in this embodiment, the insulating layer 10 has pores B with a pore diameter of 0.08 μm or more and less than 1.0 μm, which allows for the formation of areas within the insulating layer 10 where the insulating particles are not densely packed, and thus allows for areas with a lower density than the high-density areas provided by pores A. Because the insulating layer 10 has a low elastic modulus, cracking under external forces can be suppressed. However, by including a binder in the insulating layer 10, the binder binds the insulating particles together, resulting in an insulating layer 10 with a more appropriate elastic modulus. This is because the binding of the insulating particles together by the binder increases the deformation suppression force against external forces. This also suppresses the detachment of insulating particles from the insulating layer 10.

[0028] The insulating layer 10 of this embodiment can be obtained not only by using insulating particles of an appropriate particle size, but also by adjusting the amount of binder in the insulating layer 10. The amount of binder contained in the insulating layer 10 will now be explained. Preferably, the amount of binder is 5% by mass or more and 50% by mass or less of the total amount of insulating layer 10. By having an amount of binder of 5% by mass or more of the total amount of insulating layer 10, sufficient bonding force between insulating particles can be ensured, and the detachment of insulating particles from the insulating layer 10 can be suppressed. Furthermore, by having an amount of binder of 50% by mass or less of the total amount of insulating layer 10, the amount of binder is appropriate in relation to the total amount of insulating layer 10, the material properties of the insulating layer 10 do not approach the material properties of the binder, and the puncture strength of the insulating layer 10 can be sufficiently maintained.

[0029] It is more preferable that the amount of binder be 5% by mass or more and 35% by mass or less of the total amount of insulating layer 10. In this case, the more binder is contained in the insulating layer 10, the higher the elastic modulus of the insulating layer 10 becomes, making it less prone to cracking and stronger against punctures.

[0030] The amount of binder contained in the insulating layer 10 can be measured by differential thermogravimetric analysis (TG / DTA) using a differential thermal balance. By performing TG / DTA on a sample of the insulating layer 10 and detecting the weight change with respect to temperature and atmosphere, the thermally decomposed and vaporized components can be quantified from the insulating layer 10 sample. This allows for the quantitative confirmation of the amount of binder contained in the insulating layer 10.

[0031] As mentioned above, the binder is, for example, a fluororesin, but more preferably polyvinylidene fluoride. Polyvinylidene fluoride has several advantages when electrode 2 is used in a secondary battery, such as being less reactive with the electrolyte, maintaining stable performance over a wide voltage range, being able to sufficiently bond insulating particles together, and having excellent heat resistance.

[0032] In the electrode 2 of the first embodiment described above, the insulating layer 10 has both pores A and pores B within it. Small pores A with a diameter of 0.01 μm or more and less than 0.08 μm allow for the formation of areas within the insulating layer 10 where insulating particles are densely packed, thus providing areas of high density. This high-density insulating layer 10 exhibits strong resistance to punctures and has strong puncture strength due to its dense molecular structure. Furthermore, pores B with a diameter of 0.08 μm or more and less than 1.0 μm allow for the formation of areas within the insulating layer 10 where insulating particles are not densely packed, providing areas of lower density than those provided by pores A. This low-density insulating layer 10 has a low elastic modulus, thus suppressing cracking under external forces.

[0033] Therefore, by having both pores A and pores B in the insulating layer 10 and appropriately maintaining the voids within the insulating layer 10, it is possible to provide an electrode 2 having an insulating layer 10 that suppresses cracking of the insulating layer 10 and has sufficient puncture strength. The high-density portion provided by pores A provides an insulating layer 10 with sufficient puncture strength, and the low-density portion provided by pores B suppresses cracking of the insulating layer 10.

[0034] (Second Embodiment) The secondary battery 1 of the second embodiment will be described with reference to Figure 3. Figure 3 is a schematic perspective view of the secondary battery 1 of the second embodiment. As shown in Figure 3, the secondary battery 1 comprises an outer case 3, an electrode group 5 using the electrodes 2 of the first embodiment, and an electrolyte (not shown) inside the outer case 3. The outer case 3 has an internal cavity (not shown), and an opening 9 is provided on its top surface, and a lid member 7 is placed in the opening 9. Here, the outer case 3 is cylindrical, but is not limited to these.

[0035] A gas discharge valve 21 may be provided on the surface of the lid member 7 together with the sealing plate 19. Furthermore, for example, a pair of terminals 23 are attached to the surface of the lid member 7, and the terminals 23 are electrically connected to the electrode group 5. Terminal insulators 35 may be provided between each terminal 23 and the lid member 7 to maintain insulation between them.

[0036] Here, the electrode group 5 used in the secondary battery 1 will be described with reference to Figures 4 and 5. Figure 4 is a schematic perspective view showing the electrode group 5 used in the secondary battery 1 of the second embodiment, and Figure 5 is a partially exploded perspective view of the electrode group 5 used in the secondary battery 1 of the second embodiment viewed from above. As shown in Figures 4 and 5, the electrode group 5 has the electrodes 2 of the first embodiment, and the electrodes 2 are wound around a separator 4, but are not limited to these. The electrode group 5 may also have a stacked structure in which the electrodes 2 are stacked around a separator 4.

[0037] Here, it is preferable that the other electrodes 6 have a different structure from electrode 2, and do not have an insulating layer 10. Preferably, electrode 2, which has an insulating layer 10, is the positive electrode, and the other electrodes 6 are the negative electrodes. The current collector 70 of electrode 2 is the positive electrode current collector, and the electrode mixture layer 20 of electrode 2 is the positive electrode mixture layer. This is because, when the electrode group 5 is a secondary battery 1, the width of the negative electrode mixture layer of the other electrodes 6 (in the X direction in Figure 1) may be formed wider than the width of the positive electrode mixture layer (in the X direction in Figure 1) in order to accept all the lithium ions supplied from the positive electrode mixture layer. In other words, in the electrode group 5, it is preferable that the insulating layer 10 of electrode 2, which is the positive electrode, is positioned to face the end of the negative electrode mixture layer of the other electrodes 6, which are the negative electrodes.

[0038] As shown in Figures 4 and 5, the unpainted portion 70a of the current collector 70 on electrode 2 and the unpainted portion 70b of the current collector on the other electrode 6 protrude from both ends of the electrode group 5, but are not limited to these. The unpainted portion 70a of the current collector 70 on electrode 2 and the unpainted portion 70b of the current collector on the other electrode 6 may protrude from only one end of the electrode group 5. When the electrode group 5 is used as a secondary battery, the unpainted portions (70a, 70b) of both may be connected to a conductive member.

[0039] Since the secondary battery 1 has an electrode group 5 using the electrode 2 of the first embodiment, it is possible to provide a secondary battery 1 that suppresses cracking of the insulating layer 10 and has an insulating layer 10 with sufficient puncture strength. In the electrode group 5 using electrode 2, it is possible to suppress the cracking and detachment of a part of the insulating layer 10, so that there is a sufficient insulating layer 10 in a position facing the electrode mixture layer of the other electrode 6. Furthermore, even if the electrode mixture layer of the other electrode 6 penetrates the separator, since there is an insulating layer 10 with sufficient puncture strength in a position facing the electrode mixture layer of the other electrode 6, it is possible to suppress the insulating layer 10 from penetrating and coming into contact with the current collector 70 below the insulating layer 10. As a result, even if the electrode mixture layer of the other electrode 6 penetrates the separator, a short circuit of the electrode group 5 can be suppressed by the insulating layer 10, so a secondary battery 1 with suppressed short circuits between electrodes can be provided.

[0040] According to the electrode 2 of at least one embodiment described above, the insulating layer 10 has both pores A and pores B within the insulating layer 10. The small pores A with a pore diameter of 0.01 µm or more and less than 0.08 µm enable formation of a portion where insulating particles are densely packed within the insulating layer 10, and a high-density portion can be provided. The insulating layer 10 having this high density exhibits strong resistance to puncture and has high puncture strength because the molecular structure within the insulating layer 10 is dense. Further, the pores B with a pore diameter of 0.08 µm or more and less than 1.0 µm enable formation of a portion where insulating particles are not densely packed within the insulating layer 10, and a portion having a lower density than the high-density portion provided by the pores A can be provided. The insulating layer 10 having this low density can suppress cracking against external force because the elastic modulus of the insulating layer 10 is low.

[0041] Therefore, when both pores A and pores B exist in the insulating layer 10 and the voids in the insulating layer 10 are appropriately maintained, cracking of the insulating layer 10 can be suppressed, and the electrode 2 including the insulating layer 10 having sufficient puncture strength can be provided. The high-density portion provided by the pores A gives the insulating layer 10 having sufficient puncture strength, and the low-density portion provided by the pores B can suppress cracking of the insulating layer 10.

[0042] Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and alterations can be made without departing from the gist of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the scope of the invention described in the claims and the equivalent scope thereof.

[0043] Examples of the present invention will be described below, but the present invention is not limited to the examples described below without departing from the gist of the present invention.

[0044] (Example 1) The insulating layer 10 used in the electrode 2 of Example 1 was produced by the following procedure.

[0045] <Method for producing insulating layer 10> Barium sulfate having a D50 of 1.3 μm and a D10 / D90 of 0.22 was prepared as insulating particles. Barium sulfate and polyvinylidene fluoride as a binder were prepared at a mixing ratio of 92% by mass: 8% by mass. Barium sulfate, the binder, and N-methyl-pyrrolidone (NMP) were charged into a planetary mixer. All the charged materials were stirred by the planetary mixer to obtain a barium sulfate slurry.

[0046] The barium sulfate slurry was applied to one surface of a current collector 70 made of aluminum foil, and the coating film was dried to form an insulating layer 10 on the current collector 70.

[0047] <Measurement of Log differential pore size distribution curve> The Log differential pore size distribution curve of the prepared insulating layer 10 was measured by the method described above.

[0048] <Evaluation of elastic modulus of insulating layer 10> The elastic modulus of the insulating layer 10 was evaluated by the nanoindentation method. The elastic modulus was measured by performing an indentation loading / unloading test on the insulating layer 10 using a triangular pyramid indenter (Berkovich indenter).

[0049] <Bending test of insulating layer 10> The crack resistance of the insulating layer 10 was evaluated by a bending test. In the bending test, the current collector 70 was bent at 180° with the current collector 70 on the inner side, and the contact resistance between the current collector 70 and another metal foil was measured when the R portion of the bent part was brought into contact with the another metal foil.

[0050] <Measurement of puncture strength of insulating layer 10> The puncture strength of the insulating layer 10 was evaluated by a puncture strength test. In the puncture strength test, a metal pin with a tip curvature radius of 60 µm and a tip angle of 20 degrees was used. The metal pin was vertically pierced into the insulating layer 10 placed on a flat base, and when the load applied to the insulating layer 10 was gradually increased, the short-circuit load at which the resistance between the metal pin and the current collector 70 under the insulating layer 10 became equal to or less than the kΩ order was measured.

[0051] (Example 2) Barium sulfate and polyvinylidene fluoride were prepared at a mixing ratio of 84% by mass: 16% by mass, and the other conditions were the same as those in Example 1.

[0052] (Example 3) Barium sulfate and polyvinylidene fluoride were prepared in a mixing ratio of 68% by mass:32% by mass, and the rest was the same as in Example 1.

[0053] (Example 4) Barium sulfate and polyvinylidene fluoride were prepared in a mixing ratio of 50% by mass:50% by mass, and the rest was the same as in Example 1.

[0054] (Example 5) Barium sulfate and polyvinylidene fluoride were prepared in a mixing ratio of 96% by mass:4% by mass, and the rest was the same as in Example 1.

[0055] (Comparative Example 1) Alumina with a D50 of 1.6 μm was prepared as insulating particles, and the alumina and polyvinylidene fluoride were mixed in a mixing ratio of 68% by mass:32% by mass, with the rest being the same as in Example 1.

[0056] (Comparative Example 2) In this example, insulating particles are not used, and only polyvinylidene fluoride is prepared, with the rest being the same as in Example 1.

[0057] Table 1 shows the presence or absence of the first peak (PA) and the second peak (PB), the modulus of elasticity, and the evaluation of the crack resistance of the insulating layer 10 by bending test for each example and comparative example. Here, the evaluation of the crack resistance of the insulating layer 10 was as follows: a "double circle" was given if the contact resistance between the current collector 70 and the metal foil was on the order of MΩ or higher and no cracks were observed in the insulating layer 10; a "circle" was given if the contact resistance between the current collector 70 and the metal foil was on the order of MΩ or higher but cracks were observed in the insulating layer 10; and a "no" was given if the contact resistance between the current collector 70 and the metal foil was on the order of kΩ or lower. The evaluation of the puncture strength of the insulating layer 10 was given as follows: a "double circle" was given if the short-circuit load was 0.2 N or higher; a "circle" was given if it was 0.1 N or higher and less than 0.2 N; and a "no" was given if it was less than 0.1 N. In each of Examples 1 to 5, the position of the first peak (PA) was at a pore size of 0.03 μm, and the position of the second peak (PB) was at a pore size of 0.17 μm. There was virtually no variation in peak position due to the amount of binder.

[0058]

[0059] As shown in Table 1, the example showed less cracking of the insulating layer 10 compared to the comparative example, and also exhibited sufficient puncture strength. This verified that the presence of both pores A (first peak PA) and pores B (second peak PB) in the insulating layer 10 appropriately maintains the voids within the insulating layer 10, thereby suppressing cracking of the insulating layer 10 and obtaining an insulating layer 10 with sufficient puncture strength.

[0060] In particular, in Examples 1 to 4, the insulating layer 10 had a high elastic modulus and was resistant to cracking. This confirmed that the amount of binder contained in the insulating layer 10 is preferably 5% by mass or more and 50% by mass or less of the total amount of the insulating layer 10. A binder amount of 5% by mass or more of the total amount of the insulating layer 10 ensures sufficient bonding strength between insulating particles. As shown in Example 5, when the amount of binder is about 4% by mass of the total amount of the insulating layer 10, although the elastic modulus is low, sufficient bonding strength between insulating particles cannot be ensured, and it was confirmed that cracks occurred in a part of the insulating layer 10 during the bending test, and insulating particles fell off from the insulating layer 10. Furthermore, when the amount of binder is 50% by mass or less of the total amount of the insulating layer 10, the amount of binder is appropriate relative to the total amount of the insulating layer 10, the material properties of the insulating layer 10 do not approach the material properties of the binder, and the puncture strength of the insulating layer 10 can be sufficiently maintained. As shown in Comparative Example 2, when the material properties of the insulating layer 10 and the material properties of the binder are the same, the puncture strength of the insulating layer 10 is insufficient.

[0061] Furthermore, it was verified that the amount of binder contained in the insulating layer 10 is more preferably 5% by mass or more and 35% by mass or less of the total amount of the insulating layer 10. In this case, the more binder contained in the insulating layer 10, the higher the elastic modulus of the insulating layer 10, making it less prone to cracking and stronger against punctures. However, as shown in Example 4, when the amount of binder is about 50% by mass of the total amount of the insulating layer 10, the puncture strength is weaker than in Example 3 (where the amount of binder is 32% by mass of the total amount of the insulating layer 10). This is because, as mentioned above, the amount of binder in Example 4 is greater than in Example 3 relative to the total amount of the insulating layer 10, so the material properties of the insulating layer 10 approach the material properties of the binder.

[0062] The above results are applied to electrode 2 of this embodiment. Specifically, electrode 2 has both pores A and pores B within the insulating layer 10. Small pores A with a pore diameter of 0.01 μm or more and less than 0.08 μm allow for the formation of areas within the insulating layer 10 where insulating particles are densely packed, thus providing areas of high density. This insulating layer 10 with high density exhibits strong resistance to punctures and has strong puncture strength because of its dense molecular structure. Furthermore, pores B with a pore diameter of 0.08 μm or more and less than 1.0 μm allow for the formation of areas within the insulating layer 10 where insulating particles are not densely packed, providing areas with lower density than the areas of high density provided by pores A. This insulating layer 10 with low density has a low elastic modulus, thus suppressing cracking under external forces.

[0063] Therefore, by having both pores A and pores B in the insulating layer 10 and appropriately maintaining the voids within the insulating layer 10, it is possible to provide an electrode 2 having an insulating layer 10 that suppresses cracking of the insulating layer 10 and has sufficient puncture strength. The high-density portion provided by pores A provides an insulating layer 10 with sufficient puncture strength, and the low-density portion provided by pores B suppresses cracking of the insulating layer 10.

[0064] 1...Secondary battery, 2...Electrode, 3...Outer case, 4...Separator, 5...Electrode group, 6...Other electrodes, 7...Lid member, 9...Opening, 10...Insulating layer, 12...Long side, 14...Short side, 19...Sealing plate, 20...Electrode mixture layer, 21...Gas discharge valve, 23...Terminal, 35...Terminal insulator, 70...Current collector, 70a, 70b...Unpainted parts.

Claims

1. An electrode comprising a current collector, an electrode mixture layer provided on the current collector, and an insulating layer provided on the current collector and adjacent to the electrode mixture layer, wherein the pore distribution of the insulating layer has, in the log differential pore distribution curve obtained by the mercury intrusion method, a first peak (PA) which is the maximum peak of pore volume appearing in the range of pore diameters of 0.01 μm or more and less than 0.08 μm, and a second peak (PB) which is the maximum peak of pore volume appearing in the range of pore diameters of 0.08 μm or more and 1.0 μm or less.

2. The electrode according to claim 1, wherein the insulating layer has insulating particles, and the particle diameter (D50) of the insulating particles at a cumulative 50% is 0.3 μm or more and 3.0 μm or less.

3. The electrode according to claim 2, wherein the ratio of the particle diameter of the insulating particles at a cumulative 10% (D10) to the particle diameter of the insulating particles at a cumulative 90% (D90) is in the range of 0.1 ≤ D10 / D90 ≤ 0.

3.

4. The electrode according to claim 2, wherein the insulating particles are barium sulfate.

5. The electrode according to claim 1, wherein the insulating layer has a binder, and the binder is 5% by mass or more and 50% by mass or less of the total amount of the insulating layer.

6. The electrode according to claim 5, wherein the binder is polyvinylidene fluoride.

7. The electrode according to claim 1, wherein the current collector is a positive electrode current collector and the electrode mixture layer is a positive electrode mixture layer.

8. A secondary battery comprising an electrode according to any one of claims 1 to 7 and an electrolyte.

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