Current collector for secondary battery and secondary battery

Abstract

A current collector for a secondary battery has a resin layer and a metal foil covering both sides of the resin layer. A plurality of holes are formed in the metal foil.

Description

TECHNICAL FIELD

[0001] The present application relates to a current collector of a secondary battery and a secondary battery. BACKGROUND

[0002] A secondary battery having an electrode sheet in which an active material layer in which ions are occluded is formed on a current collector has been known in the past. For example, Japanese Patent Application Publication No. 2007-26913 discloses a current collector for a lithium ion battery, which has a laminated metal foil in which roughened faces of two metal foils are opposed to each other and a gap is provided between the two metal foils or a resin is sandwiched. An active material layer in which lithium ions are occluded is formed on the metal foil. According to Japanese Patent Application Publication No. 2007-26913, it is described that since the gap between the metal foils or the resin moderates expansion of the active material layer, peeling of the active material from the current collector accompanying charge and discharge is suppressed. Thus, a lithium ion battery having excellent cycle characteristics can be obtained.

[0003] In addition, Japanese Patent Application Publication No. 2008-311171 discloses a lithium ion secondary battery having an electrode in which an active material composite layer is formed on a current collector in which through holes are formed. The periphery of these through holes becomes a protruding portion that protrudes compared to other portions of the current collector. According to Japanese Patent Application Publication No. 2008-311171, it is described that the retention ability of the active material composite layer formed on the surface of the current collector can be improved by the protruding portion around the hole, and peeling of the active material composite layer from the current collector is prevented.

[0004] PRIOR ART DOCUMENTS

[0005] PATENT DOCUMENTS

[0006] Patent Document 1: Japanese Patent Application Publication No. 2007-26913

[0007] Patent Document 2: Japanese Patent Application Publication No. 2008-311171 SUMMARY

[0008] A current collector of a secondary battery in which peeling of an active material layer is less likely to occur and in which the secondary battery is easily closed at the time of internal short circuit is provided. In addition, a secondary battery provided with such a current collector is provided.

[0009] The current collector of a secondary battery provided herein has a resin layer and a metal foil that covers both faces of the resin layer, and a plurality of holes are formed in the metal foil. According to the current collector of a secondary battery described above, peeling of an active material layer from the current collector is less likely to occur. In addition, according to the current collector of a secondary battery described above, the secondary battery is more easily closed at the time of internal short circuit.

[0010] In the above current collector, the metal foil can have protrusions formed around at least a portion of the plurality of holes and protruding toward the resin layer side. The protrusions can preferably have a protrusion height of 0.01 μm to 3 μm. According to the above current collector for a secondary battery, the cycle characteristics of the secondary battery can be improved, and the peeling strength between the current collector and the active material layer can be improved.

[0011] In the above current collector, the holes can have a diameter of 0.001 μm to 100 μm. According to the above current collector for a secondary battery, the cycle characteristics of the secondary battery can be improved, and the peeling strength between the current collector and the active material layer can be improved. In addition, in the above current collector, the opening ratio of the plurality of holes in the metal foil can be 10% to 80%. According to the above current collector for a secondary battery, the cycle characteristics of the secondary battery and the peeling strength between the current collector and the active material layer can be further improved.

[0012] In the above current collector, the resin layer can have a melting point of 255°C or lower. According to the above current collector for a secondary battery, the secondary battery can be more easily closed at the time of internal short circuit.

[0013] The secondary battery can have at least one of a positive electrode sheet having a positive electrode active material layer formed on the above any current collector and a negative electrode sheet having a negative electrode active material layer formed on the above any current collector. According to the above secondary battery, the secondary battery can have characteristics improved by the current collector. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 is a cross-sectional view of a secondary battery.

[0015] Figure 2 is a cross-sectional view of a negative electrode sheet. DETAILED DESCRIPTION

[0016] Hereinafter, one embodiment of a secondary battery will be described. Note that the embodiment described here is by no means intended to particularly limit the application. Moreover, the drawings are schematic and by no means intended to be literally reflected in the actual implementation. In the following description, like parts are marked with like reference numerals, and repeated description of these parts is omitted or simplified as appropriate.

[0017] [Structure of Secondary Battery]

[0018] Figure 1 is a cross-sectional view of a secondary battery 100. As Figure 1 indicated in the drawing, the secondary battery 100 includes a battery case 10, an electrode body 20, and an electrode terminal 50. The battery case 10 houses the electrode body 20 and an electrolyte solution. As Figure 1 indicated in the drawing, the battery case 10 includes a case main body 11 and a lid 12. The case main body 11 is a substantially rectangular parallelepiped-shaped container.

[0019] The electrode body 20 is obtained by overlapping the negative electrode sheet 30 and the positive electrode sheet 40 in the form of a sheet, with the separator sheets 21, 22 interposed therebetween, and forming a substantially rectangular parallelepiped shape. The electrode body 20 is housed in the case main body 11. The first separator sheet 21, the positive electrode sheet 40, the second separator sheet 22, and the negative electrode sheet 30 are overlapped in this order and wound, and are housed in the case main body 11. Note that, in the present embodiment, the electrode body 20 is a wound electrode body in which the positive electrode sheet 40, the negative electrode sheet 30, and the separator sheets 21, 22 are wound, but can also be a laminated electrode body in which the positive electrode sheet, the negative electrode sheet, and the separator sheet are laminated.

[0020] The negative electrode sheet 30 is a member in which a negative electrode active material layer 32 containing a negative electrode active material is formed on both faces of a current collector 31 in the form of a foil having a predetermined width and thickness. The negative electrode active material is, for example, a material that can occlude lithium ions at the time of charging and release the occluded lithium ions at the time of discharging, such as natural graphite, in a lithium ion secondary battery. As the negative electrode active material, various substances other than natural graphite have been proposed, and there is no particular limitation. The configuration of the current collector 31 of the negative electrode will be described later.

[0021] The positive electrode sheet 40 is a member in which a positive electrode active material layer 42 containing a positive electrode active material is formed on both faces of a current collector 41 in the form of a foil having a predetermined width and thickness. The positive electrode active material is, for example, a material that can release lithium ions at the time of charging and occlude lithium ions at the time of discharging, such as a lithium transition metal composite material, in a lithium ion secondary battery. As the positive electrode active material, various substances other than the lithium transition metal composite material have been proposed, and there is no particular limitation. The configuration of the current collector 41 of the positive electrode will be described later. The negative electrode sheet 30 and the positive electrode sheet 40 are connected to an electrode terminal 50 provided outside the battery case 10, respectively.

[0022] [Configuration of current collector]

[0023] Hereinafter, the configuration of the current collector 31 of the negative electrode will be described. Figure 2 is a schematic view of a cross section of the negative electrode sheet 30. As Figure 2 indicated, the current collector 31 of the negative electrode has resin layers 33 and metal foils 34, 35 that cover both faces of the resin layers 33. A plurality of holes 34a, 35a are formed in the metal foils 34, 35, respectively. The metal foils 34, 35 are, for example, copper foils. Among them, the material of the metal foils 34, 35 is not particularly limited as long as it can be used as a negative electrode current collector foil. The thickness of the metal foils 34, 35 can be preferably 2 μm to 10 μm. The plurality of holes 34a of the metal foil 34 penetrate the metal foil 34 in the thickness direction. The diameter of the holes 34a can be preferably 0.001 μm to 100 μm. The holes 34a can be formed by punching processing when they are relatively large. The holes 34a can be formed by laser-based hole opening processing, etching based on a chemical solution, or the like when they are relatively small.

[0024] Here, a plurality of holes 34a are substantially uniformly dispersed in the metal foil 34. These holes 34a may, for example, be formed only in the portion of the metal foil 34 where the negative electrode active material layer 32 is formed. For the portion of the metal foil 34 where the plurality of holes 34a are formed, the plurality of holes 34a are preferably substantially uniformly dispersed. The aperture ratio of the plurality of holes 34a in the portion of the metal foil 34 where the plurality of holes 34a are formed can be 10% to 80%. The aperture ratio is the proportion of the holes 34a on the surface of the metal foil 34 in the portion of the metal foil 34 where the plurality of holes 34a are formed.

[0025] The metal foil 34 has protrusions 34b formed around a plurality of holes 34a and protruding toward the resin layer 33. The protrusions 34b may be formed around at least a portion of the holes 34a, or not around all of them. The protrusions 34b may be blanking burrs formed during the machining of the holes 34a. The protrusion height of the protrusions 34b may be 0.01 μm to 3 μm. The protrusions 34b protrude into the resin layer 33.

[0026] Metal foil 35 is constructed in the same manner as metal foil 34. The protrusion 35b of metal foil 35 also protrudes towards the resin layer 33. The diameter and aperture ratio of the hole 35a may not be the same as those of the hole 34a in metal foil 34. The protrusion height of the protrusion 35b may also not be the same as that of the protrusion 34b in metal foil 34.

[0027] A resin layer 33 is sandwiched between two metal foils 34 and 35. Here, the resin layer 33 is made of polyethylene (PE). The resin layer 33 is not limited as long as it has a sufficiently low melting point. The melting point of the resin layer 33 can appropriately be below 255°C. The melting point of PE varies depending on its molecular weight, and is approximately 95°C to 135°C. The thickness of the resin layer 33 can be, for example, 10 μm to 30 μm.

[0028] The current collector 31 is manufactured, for example, by sandwiching a resin layer 33 between metal foils 34 and 35, which are respectively formed with holes 34a and 35a, and then compressing (hot pressing) them. A negative electrode active material layer 32 is formed on the outer surface of the metal foils 34 and 35 (the back side of the side in contact with the resin layer 33).

[0029] The current collector 41 of the positive electrode sheet 40 can be configured similarly to the current collector 31 of the negative electrode sheet 30 except for the material of the metal foil. The material of the metal foil of the positive electrode is, for example, aluminum. Among them, the secondary battery 100 can have at least one of the positive electrode sheet 40 in which the positive electrode active material layer 42 is formed on the current collector 41 and the negative electrode sheet 30 in which the negative electrode active material layer 32 is formed on the current collector 31. Therefore, the positive electrode sheet 40 or the negative electrode sheet 30 can typically form an active material layer on a metal foil similarly to the conventional positive electrode sheet or negative electrode sheet.

[0030] [Results of Evaluation Test of Current Collector Foil]

[0031] Tables 1 and 2 are tables showing the results of the evaluation test of the current collector.

[0032] [Table 1]

[0033] Table 1

[0034]

[0035] [Table 2]

[0036] Table 2

[0037]

[0038] Here, 25 samples were prepared to perform the evaluation test. As shown in Tables 1 and 2, Samples 1 to 25 are evaluation samples produced by changing the presence or absence of the resin layer and the material, the presence or absence of the hole of the metal foil, the height of the protruding portion (the height of the protruding portion "0" indicates that the protruding portion is not provided), the diameter of the hole, and the opening rate of the hole (the diameter of the hole "0" and the opening rate of the hole "0" indicate that the hole is not provided).

[0039] Among the samples 1 to 25 produced for the evaluation test, the thickness of the metal foil (copper foil) was 5 μm. In addition, the thickness of the resin layer was 20 μm. Among the samples 1 to 25, the sample in which the diameter of the hole of the metal foil was greater than 100 μm was formed by blanking processing. The sample in which the diameter of the hole of the metal foil was greater than 5 μm and 100 μm or less was formed by laser processing. The sample in which the diameter of the hole of the metal foil was 5 μm or less was formed by etching using a chemical solution. The protruding portion was formed by rapidly heating the metal foil using an electromagnetic induction heating method to sag a part of the metal foil. The protruding height of the protruding portion was controlled by the heating time. The melting point of PE used here was 120°C. The hot pressing was performed at a temperature of 100°C, a pressure of 5 MPa, and a time of 30 seconds.

[0040] The capacity retention rate was calculated by comparing the capacity before and after 200 cycles of charge and discharge at a voltage of 3.3 V to 4.2 V in an environment of 60°C. The capacity retention rate was a value obtained by dividing the capacity after 200 cycles by the initial capacity. The initial and the capacity after 200 cycles were capacities after discharging the secondary battery in a fully charged state at a prescribed condition. When the secondary battery was brought to a fully charged state, constant current charging was performed at a current value of 1 / 3 C until 4.2 V, and then constant voltage charging was performed until the current value reached 1 / 50 C. Discharging after the secondary battery was brought to a fully charged state was performed at a current value of 1 / 3 C until the voltage reached 3 V. The temperature at the time of capacity measurement was 25°C. The capacity at that time was used in the calculation of the capacity retention rate.

[0041] In the safety evaluation, a nail made of iron having a diameter of 3 mm was inserted through the center of the battery at a speed of 10 mm / s with respect to a secondary battery brought to a fully charged state by performing constant current charging at a current value of 1 / 3 C until the current value reached 1 / 10 C after the upper limit voltage of charging was set to 4.2 V. The temperature of the outer surface of the battery at that time was measured using a thermocouple, and a case where the maximum temperature was less than 200°C was recorded as "O", and a case where the maximum temperature was 200°C or more was recorded as "X". The test temperature was 25°C. In the evaluation of the material of the resin layer shown in Table 2, the same temperature measurement as described above was performed, and a case where the maximum temperature was less than 150°C was recorded as "O", and a case where the maximum temperature was 150°C or more was recorded as "X". The safety evaluation was a test for evaluating the fusing property of the current collector after internal short-circuiting by inserting a nail into the secondary battery.

[0042] In the measurement of the peeling strength, a general adhesive tape having a width of 24 mm and a length of 300 mm was attached to the surface of the active material layer and fixed to a test plate, and the force with which the active material layer could be peeled at an angle of 90° with respect to the test plate was measured as the peeling strength. The measured peeling strength was standardized with respect to the peeling strength of Sample 1.

[0043] [Effects of the holes of the resin layer and the metal foil]

[0044] In Table 1, Samples 1 and 2 are compared with Sample 11. As is apparent from Table 1, in Samples 1 and 2 in which no resin layer is provided, the result of the safety test is "X". In Sample 11 in which a resin layer is provided, the result of the safety test is "0". This indicates that in the current collector provided with a resin layer, the metal foil is fused before the temperature of the outer surface of the secondary battery reaches 200°C when an internal short circuit occurs. When the temperature of the metal foil rises due to the internal short circuit, the resin layer of the current collector is fused by the heat. The metal foil provided with many holes is easily broken, and thus when the resin layer is fused, it is partially broken by losing support. Thus, in Sample 11, the path of the current is reduced, resulting in local overheating. By the fuse chain of this locally overheated portion, the current collector is broken at a lower temperature than the melting point of the metal foil (here, the melting point of copper is about 1084°C). Therefore, according to the current collector of Sample 11, the secondary battery is more easily closed when an internal short circuit occurs.

[0045] In addition, as is apparent from Table 1, the peel strength of Sample 11 is greater than that of Samples 1 and 2. This is considered to be because in Sample 11, the active material layer enters the many holes of the metal foil and is caught in the holes.

[0046] Further, as is apparent from Table 1, the capacity retention rate of Sample 11 is higher than that of Samples 1 and 2. Therefore, it is considered that Sample 11 has better cycle characteristics than Samples 1 and 2. This is considered to be because the active material layer becomes less likely to peel from the current collector by the influence of expansion and contraction of the active material layer due to charge and discharge being moderated by the resin layer being present between the two metal foils.

[0047] [Effects of the protrusions]

[0048] In Table 1, Samples 1 to 4 are compared with Samples 13 to 15. In the case of a low height of the protrusions (Samples 1 to 3, including the case where the protrusions are not present) and the case of a high height of the protrusions (Sample 4), the capacity retention rate is low, and the normalized peeling strength is small, as compared with the case where the height of the protrusions is appropriate (Samples 13 to 15). By providing the protrusions with a height of more than a certain degree, the cycle characteristics are improved, and the peeling strength becomes large. Among them, when the height of the protrusions is higher than the appropriate height, the cycle characteristics are again deteriorated, and the peeling strength is reduced. From Table 1, it is known that, when the protruding height of the protrusions is 0.01 μm to 3 μm, at least as compared with ranges other than this range, the cycle characteristics are improved, and the peeling strength is increased. It is presumed that, when the protruding height of the protrusions is appropriate, the cycle characteristics are improved because the adhesion of the resin layer to the metal foil is appropriate, and the metal foil and the resin layer can move during charge and discharge, and are not easily peeled. It is presumed that, when the protruding height of the protrusions is too high, the cycle characteristics are deteriorated because the adhesion of the resin layer to the metal foil is too strong, and the metal foil and the resin layer are not easily moved during charge and discharge, and the current collector is easily broken. It is presumed that, when the protruding height of the protrusions is appropriate, the peeling strength is improved because the metal foil and the resin layer are moderately moved during the peeling test, and absorb a part of the peeling force. It is presumed that, when the protruding height of the protrusions is too high, the peeling strength is deteriorated because the metal foil and the resin layer are not easily moved during the peeling test. It is presumed that, when the protruding height of the protrusions is low, no effect is seen because the protruding height is too low to function as the protrusions. In addition, it is expected that, when the protruding height of the protrusions is appropriate, the result of the safety test is also improved. The present inventors have confirmed that, when the protruding height of the protrusions is higher than 3 μm, the resin layer and the metal foil are easily moved together, and the metal layer becomes less likely to be broken.

[0049] [Effect of the hole diameter]

[0050] In Table 1, Samples 5 and 6 are compared with Samples 2 and 16 to 19. In the case of a small diameter of the holes of the metal foil (Sample 5) and the case of a large diameter of the holes of the metal foil (Sample 6), the capacity retention rate is low, and the normalized peeling strength is small, as compared with the case where the diameter of the holes is appropriate (Samples 2 and 16 to 19). By making the diameter of the holes larger than a certain degree, the cycle characteristics are improved, and the peeling strength becomes large. Among them, when the size of the holes is larger than the appropriate size, the cycle characteristics are again deteriorated, and the peeling strength is reduced. From Table 1, it is known that, when the diameter of the holes of the metal foil is 0.001 μm to 100 μm, at least as compared with ranges other than this range, the cycle characteristics are improved, and the peeling strength is increased. When the diameter of the holes of the metal foil is 50 μm to 100 μm, the cycle characteristics and the peeling strength show more favorable values. It is presumed that, when the size of the holes is appropriate, the cycle characteristics are improved because the resin layer appropriately enters the holes, and the metal foil and the resin layer can move during charge and discharge, and are not easily peeled. It is presumed that, when the size of the holes is appropriate, the peeling strength is increased because the active material layer enters the holes, and is securely caught in the holes.

[0051] [Effect of the opening ratio]

[0052] In Table 1, Samples 7 to 10 are compared with Samples 8 and 20 to 23. In the case of a small opening ratio of the holes in the metal foil (Samples 7 and 8) and in the case of a large opening ratio of the holes in the metal foil (Samples 9 and 10), the capacity retention rate is low and the normalized peeling strength is small as compared with the case of an appropriate opening ratio of the holes (Samples 8 and 20 to 23). By making the opening ratio of the holes larger than a certain degree, the cycle characteristics are improved and the peeling strength becomes large. Among them, when the opening ratio is higher than the appropriate opening ratio, the cycle characteristics are deteriorated and the peeling strength is reduced. As is apparent from Table 1, at least when the opening ratio of the holes in the metal foil is 10% to 80%, the cycle characteristics are improved and the peeling strength becomes large as compared with the case other than this range. It is presumed that the cycle characteristics are improved when the opening ratio is appropriate because the resin layer appropriately enters the holes and the metal foil and the resin layer can move at the time of charge and discharge and are not easily peeled. It is considered that the peeling strength becomes large when the opening ratio is appropriate because many active material layers enter the holes and the strength of the metal foil also maintains an appropriate strength. In addition, it is confirmed that the result of the safety test is also improved when the opening ratio of the holes is 10% to 80%. The present inventors confirmed that the secondary battery is qualified in a more severe safety test (determination threshold: 150°C) shown in Table 2 when the opening ratio of the holes is 10% to 80%.

[0053] [Effects of the melting point of the resin layer]

[0054] In Table 2, Sample 24 is compared with Samples 2 and 25. As is apparent from Table 2, in the case of a high melting point of the resin layer (Sample 24), the temperature of the battery at the time of internal short circuit becomes high as compared with the case of an appropriate melting point of the resin layer (Samples 2 and 25). As is apparent from Table 1, at least when the melting point of the resin layer is 255°C or lower, the secondary battery is qualified in a more severe safety test in which the determination threshold is low (here, the determination threshold is 150°C). It is known that when the melting point of the resin layer is 265°C, the secondary battery is not qualified in this test, and therefore if the resin layer uses a material having a melting point of 255°C or lower, the safety of the more severe safety test level can be ensured. In this way, if the resin layer uses a material having a melting point of 255°C or lower, the secondary battery becomes more easily closed at the time of internal short circuit.

[0055] The above describes one embodiment of the current collector of the secondary battery and the secondary battery according to the present application. However, the above embodiment is merely one example and can be implemented in other ways. The above embodiment does not limit the present application except for the cases specifically mentioned.

Claims

1. A current collector for a secondary battery, comprising a resin layer and a metal foil having a plurality of through holes and covering both sides of the resin layer. The metal foil has protrusions formed around at least a portion of the plurality of through holes and protruding toward the resin layer. The protrusion extends into the resin layer. A portion of the resin layer extends into the interior of the plurality of through-holes.

2. The current collector of the secondary battery according to claim 1, wherein, A portion of the resin layer penetrates into the interior of the plurality of through holes until it reaches a side closer to the outer surface of the metal foil than the protrusion, thus blocking the plurality of through holes.

3. The current collector of the secondary battery according to claim 1 or 2, wherein, The protrusion height of the protrusion is 0.01μm to 3μm.

4. The current collector of the secondary battery according to claim 1 or 2, wherein, The diameter of the through hole is 0.001μm to 100μm.

5. The current collector of the secondary battery according to claim 1 or 2, wherein, The aperture ratio of the plurality of through holes in the portion of the metal foil that forms the plurality of through holes is 10% to 80%.

6. The current collector of the secondary battery according to claim 1 or 2, wherein, The melting point of the resin layer is below 255°C.

7. A secondary battery comprising at least one of a positive electrode sheet having a positive active material layer formed on a current collector according to any one of claims 1 to 6 and a negative electrode sheet having a negative active material layer formed on a current collector according to any one of claims 1 to 6.

Images