Method for manufacturing secondary battery

Abstract

To reduce internal resistance of a secondary battery.SOLUTION: A secondary battery includes a positive electrode plate 50 formed by coating a positive electrode mixture 62 on a current collector foil 51. A manufacturing process of the positive electrode plate 50 includes a step of pressing a positive electrode mixture layer 32P formed by the coating. During the coating of the current collector foil 51, a viscosity in a high shear rate condition when the positive electrode mixture 62 is discharged is defined as a viscosity η1 at a high shear rate, and a viscosity in a low shear rate condition after the positive electrode mixture 62 is applied onto the current collector foil 51 is defined as a viscosity η2 at a low shear rate. Further, a viscosity recovery rate obtained by dividing a difference value between the viscosity η2 at a low shear rate and the viscosity η1 at a high shear rate by a recovery time from the viscosity η1 at a high shear rate to the viscosity η2 at a low shear rate is adjusted to be 750 mPa s / s or more. In addition, a crush rate of the positive electrode mixture layer 32P by the pressing and a porosity of the pressed positive electrode mixture layer 32P are controlled to be 19% or more, and to be 33% or more and 57% or less, respectively.SELECTED DRAWING: Figure 7

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a secondary battery.

Background Art

[0002] Conventionally, for example, in a secondary battery such as a lithium-ion secondary battery, there is a type that forms an electrode plate by coating an electrode mixture containing an electrode active material on a base material serving as a current collector. Incidentally, in such a secondary battery, the base material is referred to as a current collector foil due to its foil-like outer shape and function. Also, in a secondary battery, the porosity of the electrode mixture layer laminated on the current collector foil greatly affects the battery performance. Specifically, in a positive electrode plate containing a conductive material in the positive electrode mixture layer, when the porosity is small, the conductive material easily forms a conductive path. On the other hand, when the porosity is large, ions serving as carriers easily diffuse in the electrolytic solution impregnated in the positive electrode mixture layer. And, for example, in Patent Document 1, based on the porosity of this positive electrode mixture layer, by appropriately controlling the electron transfer resistance due to the formation of the conductive path and the diffusion resistance due to the diffusion of ions, a configuration for reducing the internal resistance of the secondary battery is disclosed.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, for applications that require high-level battery performance, such as electric vehicles, further performance improvement is being sought every day. For this reason, even the above-mentioned conventional technology is also required to be further improved in order to meet the evolving required standards.

Means for Solving the Problems

[0005] Each aspect of a method for manufacturing a secondary battery that solves the above problems will be described. Aspect 1 is a method for manufacturing a secondary battery including a step of pressing a positive electrode composite layer formed by applying a positive electrode composite material containing a positive electrode active material onto a current collector foil, where the viscosity of the positive electrode composite material in a high shear rate state when the positive electrode composite material is discharged onto the current collector foil is defined as the high shear rate viscosity, the viscosity of the positive electrode composite material in a low shear rate state after the positive electrode composite material adheres to the current collector foil is defined as the low shear rate viscosity, and a value obtained by dividing the difference value between the low shear rate viscosity and the high shear rate viscosity by the recovery time from the high shear rate viscosity to the low shear rate viscosity is defined as the viscosity recovery rate. The positive electrode composite material is adjusted so that the viscosity recovery rate is 750 mPa·s / s or more, and the compression rate of the positive electrode composite layer by the pressing is 19% or more, and the porosity of the pressed positive electrode composite layer is controlled to be 33% or more and 57% or less.

[0006] That is, by setting a high viscosity recovery rate of the positive electrode composite material, when this positive electrode composite material is applied to the current collector foil, the positive electrode active material and the conductive material are likely to be three-dimensionally deposited on the coating surface. As a result, it becomes possible to increase the thickness of the positive electrode composite layer before pressing. And thereby, in the pressing step after the coating, it becomes possible to more greatly compress the positive electrode composite layer.

[0007] Furthermore, it is possible to achieve both a high compression rate and ensuring an appropriate porosity in the positive electrode composite layer after pressing. That is, based on the high compression rate of the positive electrode composite layer, the conductive material contained in the positive electrode composite material is likely to form a conductive path between the positive electrode active materials and between the current collector foil, thereby reducing the electron transfer resistance between them. Also, by ensuring a state where the electrolyte can easily flow through the gaps formed between the positive electrode active materials, the diffusion resistance among the internal resistances of the secondary battery can be reduced. And according to the above configuration, the internal resistance of the secondary battery can be effectively reduced.

[0008] Aspect 2 is the method for manufacturing a secondary battery according to Aspect 1, wherein the positive electrode composite material contains a conductive fibrous carbon material, and when the thickness region of the positive electrode composite material layer is partitioned into a current collector foil side and a surface side, the thickness region on the current collector foil side contains 0.1 wt% or more and 0.52 wt% or less of the conductive fibrous carbon material.

[0009] According to the above configuration, the conductive fibrous carbon material in the positive electrode composite material functions as a conductive material. Also, when the positive electrode composite material is in a low shear rate state, this conductive fibrous carbon material tends to aggregate. And thereby, by increasing the viscosity recovery rate of the positive electrode composite material, it is possible to achieve both a high compaction rate by pressing and ensuring an appropriate porosity in the positive electrode composite material layer after pressing.

[0010] Also, due to its aggregation, the conductive fibrous carbon material tends to form relatively large lumps in the positive electrode composite material layer. And thereby, it becomes difficult for the aggregated conductive fibrous carbon material to move in the positive electrode composite material layer, so that even after drying the positive electrode composite material layer, the conductive fibrous carbon material tends to remain in the thickness region on the current collector foil side. As a result, a conductive path is more easily formed between the positive electrode active material and the current collector foil through the conductive fibrous carbon material as a conductive material. And thereby, the internal resistance of the secondary battery can be reduced more effectively.

[0011] Furthermore, when increasing the total amount of the conductive material contained in the positive electrode composite material in order to increase the content of the conductive material in the thickness region on the current collector foil side, the ratio of the positive electrode active material contained in the positive electrode composite material layer decreases. That is, the substantial battery reaction components decrease. However, by using the conductive fibrous carbon material, it is possible to maintain an appropriate amount of the conductive material contained in the positive electrode composite material while setting the content of the conductive material in the thickness region on the current collector foil side within the preferable range shown in the above configuration. And thereby, the internal resistance of the secondary battery can be reduced more effectively.

[0012] Aspect 3 is the method for manufacturing a secondary battery according to Aspect 2, wherein carbon nanotubes are used as the conductive fibrous carbon material. According to the above configuration, the internal resistance of the secondary battery can be reduced more effectively.

Effects of the Invention

[0013] According to the present invention, the internal resistance of the secondary battery can be reduced.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

Figure 15

Embodiments for Carrying Out the Invention

[0015] Hereinafter, an embodiment of a method for manufacturing a secondary battery will be described with reference to the drawings. (Lithium Ion Secondary Battery) As shown in FIG. 1, the secondary battery 1 includes an electrode body 10 in which a positive electrode 3, a negative electrode 4, and a separator 5 are integrated, and a case 20 that houses the electrode body 10. And the secondary battery 1 of this embodiment has a configuration as a lithium ion secondary battery in which the electrode body 10 in the case 20 is impregnated with a non-aqueous electrolyte (not shown).

[0016] Specifically, in the secondary battery 1 of this embodiment, the positive electrode 3, the negative electrode 4, and the separator 5 have a sheet-like outer shape and are laminated. And by winding the laminate of the positive electrode 3, the negative electrode 4, and the separator 5, an electrode body 10 is formed in which the positive and negative electrodes and the separator 5 are alternately arranged in the radial direction with the separator 5 sandwiched between the positive electrode 3 and the negative electrode 4.

[0017] Also, the case 20 of this embodiment includes a flat substantially rectangular box-shaped case body 21 and a lid member 22 that closes the open end 21x of the case body 21. And the electrode body 10 of this embodiment has a flat outer shape corresponding to the box shape of the case 20.

[0018] (Electrode Sheet and Electrode Body) More specifically, as shown in FIG. 2, in the secondary battery 1 of this embodiment, the positive electrode 3 and the negative electrode 4 each have a configuration as an electrode sheet 35 including a current collector 31 having a sheet-like outer shape and an electrode composite layer 32 laminated on the current collector 31.

[0019] Specifically, for the electrode sheet 35P for the positive electrode 3, a composite electrode material containing a lithium transition metal oxide serving as a positive electrode active material, that is, a composite paste 37P as a positive electrode composite material, is applied onto a base material 36P made of aluminum or the like that constitutes the positive electrode current collector 31P. For the electrode sheet 35N for the negative electrode 4, a slurry-like electrode composite material containing a carbon-based material serving as a negative electrode active material, that is, a composite paste 37N as a negative electrode composite material, is applied onto a base material 36N made of copper or the like that constitutes the negative electrode current collector 31N. Furthermore, these composite pastes 37P and 37N each contain a binder. In the secondary battery 1 of the present embodiment, when these composite pastes 37P and 37N dry, corresponding positive electrode composite material layers 32P and negative electrode composite material layers 32N are formed on the positive and negative electrode sheets 35P and 35N, respectively.

[0020] Furthermore, in the secondary battery 1 of the present embodiment, these positive and negative electrode sheets 35P and 35N are each shaped into a strip. And the electrode body 10 of the present embodiment has a configuration as a wound body in which the positive and negative electrode sheets 35P and 35N laminated with the separator 5 interposed therebetween are wound around a winding shaft 10x extending in the width direction (the left-right direction in FIG. 2) of the strip shape.

[0021] In FIG. 2, the separator 5 and each electrode sheet 35 are wound in such a manner that the electrode sheet 35P constituting the positive electrode 3 is wound inside. However, this figure is an example showing the structure of the electrode body 10, and there may be a case where the separator 5 and each electrode sheet 35 are wound in such a manner that the electrode sheet 35N constituting the negative electrode 4 is wound inside. And thereby, it is determined whether the electrode sheet 35 disposed on the outermost shell of the electrode body 10 is the electrode sheet 35P constituting the positive electrode 3 or the electrode sheet 35N constituting the negative electrode 4.

[0022] Also, as shown in FIGS. 1 to 3, the lid member 22 of the case 20 is provided with a positive electrode terminal 38P and a negative electrode terminal 38N that protrude outside the case 20. Further, each electrode sheet 35 is formed with an uncoated portion 39 where the electrode composite layer 32 is not formed on the current collector 31 thereof. And in the secondary battery 1 of the present embodiment, by using these uncoated portions 39, the electrode sheet 35P constituting the positive electrode 3 is electrically connected to the positive electrode terminal 38P, and the electrode sheet 35N constituting the negative electrode 4 is electrically connected to the negative electrode terminal 38N.

[0023] Specifically, the electrode body 10 of the present embodiment is housed in the case 20 with its winding axis 10x along the longitudinal direction (the left - right direction in FIG. 1) of the lid member 22 having a long and substantially rectangular plate shape. Further, in this state, the uncoated portion 39P of the electrode sheet 35P constituting the positive electrode 3 is connected to the positive electrode terminal 38P via a connection member 40P. And similarly, the uncoated portion 39N of the electrode sheet 35N constituting the negative electrode 4 is connected to the negative electrode terminal 38N via a connection member 40N.

[0024] Furthermore, an electrolytic solution 45 is injected into the case 20. That is, for the electrolytic solution 45 of the secondary battery 1 having a configuration as a lithium - ion secondary battery, a solution in which a lithium salt serving as a supporting salt is dissolved in an organic solvent is used. And the secondary battery 1 of the present embodiment is configured such that the electrode body 10 sealed in the case 20 is impregnated with the electrolytic solution 45.

[0025] (Pressing process of the electrode composite layer, and compaction rate) As shown in FIG. 4, in the secondary battery 1, for the positive electrode composite layer 32P laminated on the base material 36P that becomes the positive electrode current collector 31P by coating the composite paste 37P, after the composite paste 37P as the positive electrode composite dries, a pressing process of the positive electrode composite layer 32P is performed.

[0026] Specifically, in the pressing process of the electrode sheet 35P that becomes the positive electrode plate 50, the positive electrode mixture layer 32P is compressed in the thickness direction (the vertical direction in FIG. 4) integrally with the base material 36P called the current collector foil 51 due to its shape and function. As a result, in the positive electrode plate 50 of the present embodiment, the thickness d of the positive electrode mixture layer 32P after the pressing process is thinner than the thickness d' before the pressing process (d < d'). Further, in the secondary battery 1, for this positive electrode plate 50, the crushing rate α before and after the pressing process is controlled. That is, the calculation formula for this crushing rate α is "α = (d' - d) / d'", and in percentage notation (percent display), it can also be obtained by the following formula: "α = 100 - (d / d') × 100". And the secondary battery 1 of the present embodiment is controlled so that this crushing rate α becomes 19% or more in the manufacturing process of the positive electrode plate 50.

[0027] (Crushing Rate, Porosity, and Discharge Resistance of Secondary Battery) More specifically, as shown in FIG. 5, the stronger the pressing pressure of the positive electrode plate 50 in the pressing process (on the left side in FIG. 5), the smaller the porosity ε of the positive electrode mixture layer 32P. And the crushing rate α of the positive electrode mixture layer 32P also becomes higher (on the upper side in the figure).

[0028] That is, the porosity ε of the positive electrode mixture layer 32P indicates the ratio of the volume of voids not filled with the positive electrode active material, conductive material, and binder in the positive electrode mixture layer 32P. This porosity ε can be calculated, for example, by obtaining the "unit space volume" occupied by the positive electrode mixture layer 32P and the "actual volume of the positive electrode mixture" forming this positive electrode mixture layer 32P from the thickness d of the positive electrode mixture layer 32P, the composition ratio of the positive electrode mixture, the true density of each material, etc. And in FIG. 5, for the positive electrode plate 50 of the secondary battery 1, two types of samples with different relationships between the porosity ε and the crushing rate α of the positive electrode mixture layer 32P are shown.

[0029] Specifically, in FIG. 5, when comparing "Sample 1" indicated by the "hollow circle mark" with "Sample 2" indicated by the "filled circle mark", "Sample 2" has a specification with more gaps in the positive electrode composite layer 32P that is more easily crushed by the pressing process than "Sample 1". And thus, even when the porosity ε is the same, "Sample 2" has a higher crushing rate α than "Sample 1".

[0030] Furthermore, as shown in FIG. 6, when manufacturing the secondary battery 1 using these samples, if the porosity ε of the positive electrode composite layer 32P is the same, regardless of the value, "Sample 2" has a lower discharge resistance than "Sample 1".

[0031] Specifically, the "0.1s discharge resistance" shown on the vertical axis of FIG. 6 is a resistance value calculated based on Ohm's law by measuring the voltage drop amount 0.1 seconds after the start of discharge when the secondary battery manufactured using each sample is discharged at a constant current. Incidentally, the results shown in this FIG. 6 are values obtained under the measurement conditions of a temperature of 25°C and an SOC of "56%" at the start of discharge. And as shown in the figure, in both "Sample 1" and "Sample 2", the smaller the porosity ε of the positive electrode composite layer 32P, the smaller the value of the "0.1s discharge resistance".

[0032] That is, the "0.1 s discharge resistance" is an index indicating the magnitude of the plate resistance immediately after the start of discharge, which is dominated by electron transfer between the positive electrode active material particles constituting the positive electrode composite material layer 32P and between the current collector foil 51, that is, the magnitude of the internal resistance of the secondary battery 1. Further, this "0.1 s discharge resistance" decreases in value as the conductive material contained in the positive electrode composite material forms a conductive path between the positive electrode active material particles and between the current collector foil 51. And the test results shown in FIG. 6 indicate that even when the porosity ε of the positive electrode composite material layer 32P is the same, the higher the compaction rate α, the smaller the value of the "0.1 s discharge resistance". That is, it shows that by ensuring a higher compaction rate α of the positive electrode composite material layer 32P, the formation of the conductive path by the conductive material is promoted. And the effect of promoting the formation of the conductive path by the execution of this pressing process is considered to be due to the fact that the voids in the positive electrode composite material layer 32P are crushed, making it easier for the conductive material to come into contact with the positive electrode active material particles and the current collector foil 51.

[0033] Also, as time elapses from the start of discharge, the internal resistance of the secondary battery 1 shifts from a state where the so-called "electron transfer resistance" due to the formation of the conductive path as described above is dominant to a state where the so-called "diffusion resistance" due to the diffusion of lithium ions serving as carriers is dominant. And in this state, a structure of the positive electrode composite material layer 32P in which the electrolyte 45 can easily flow through the gaps formed between the positive electrode active material particles, that is, a larger porosity ε of the positive electrode composite material layer 32P results in a smaller value of the "diffusion resistance".

[0034] Based on this point, in the secondary battery 1 of the present embodiment, a positive electrode plate having a positive electrode composite material layer 32P with a specification having many gaps in the positive electrode composite material layer 32P that is easily compacted by the pressing process, such as "Sample 2" shown in FIGS. 5 and 6, is adopted. And thereby, while maintaining an appropriate porosity ε and ensuring a higher compaction rate α, the internal resistance is efficiently reduced.

[0035] (Coating of the positive electrode composite material and change in shear rate) As shown in FIG. 7, in the secondary battery 1 of the present embodiment, when manufacturing the electrode sheet 35P that serves as the positive electrode plate 50, the base material 36P as the current collector foil 51 is continuously conveyed in a state where its strip-shaped foil shape is wound around the roll 61 of the conveying device 60. Further, the coater 63 for the positive electrode mixture 62 with respect to this current collector foil 51 includes a coating die 65 disposed at a position outside the radial direction of the roll 61 and sandwiching the current collector foil 51 between the roll 61 and the coating die 65. Furthermore, this coating die 65 has a discharge port 65x of the positive electrode mixture 62 at a position facing the coating surface 51s of the current collector foil 51. And in the secondary battery 1 of the present embodiment, thereby, the positive electrode mixture 62 discharged from the discharge port 65x of this coating die 65 is continuously coated on the current collector foil 51.

[0036] That is, when coating the positive electrode mixture 62, this positive electrode mixture 62 is fed into the flow path 67 of the coater 63 continuous with the discharge port 65x of the coating die 65 in the state of the mixture paste 37P having fluidity. Further, at this time, the positive electrode mixture 62 moves in the flow path 67, and in the vicinity of the discharge port 65x, its shear rate D becomes high, that is, a high shear rate state. And the positive electrode mixture 62 is discharged from the discharge port 65x of the coating die 65 toward the coating surface 51s of the current collector foil 51 in this high shear rate state.

[0037] Also, the positive electrode mixture 62 discharged from this coating die 65 becomes in a state where its shear rate D is low, that is, a low shear rate state, in the process of forming the positive electrode mixture layer 32P by liquid landing on the coating surface 51s of the current collector foil 51, in the process called "liquid landing and film formation". That is, when the viscosity η of the positive electrode mixture 62 in the high shear rate state is defined as the viscosity η1 at high shear rate, and the viscosity η of the positive electrode mixture 62 in the low shear rate state is defined as the viscosity η2 at low shear rate, the viscosity η2 at low shear rate is larger than the viscosity η1 at high shear rate (η1 < η2). That is, the fluidity of the positive electrode mixture 62 is lower in the low shear rate state than in the high shear rate state. And thereby, in the electrode sheet 35N constituting the positive electrode plate 50, the thickness d of the positive electrode mixture layer 32P laminated on the current collector foil 51 is maintained.

[0038] (Viscosity recovery rate) Here, as shown in FIG. 8, in the secondary battery 1 of the present embodiment, for the positive electrode composite material 62 applied to the current collector foil 51 as described above, the "viscosity recovery rate", which is an index indicating the ease of recovery from the high shear rate state to the low shear rate state, is calculated. Specifically, this viscosity recovery rate Z is the slope when the viscosity η of the positive electrode composite material 62 recovers from the high shear rate viscosity η1 to the low shear rate viscosity η2 during coating on the current collector foil 51. That is, this viscosity recovery rate Z is calculated by dividing the difference value between the low shear rate viscosity η2 and the high shear rate viscosity η1 by the recovery time t from the high shear rate viscosity η1 to the low shear rate viscosity η2 using the following formula: "Z = (η2 - η1) / t".

[0039] Note that FIG. 8 is an example of measuring the high shear rate viscosity η1 and the low shear rate viscosity η2 using a rotational rheometer (manufactured by Anton Paar, MCR302). The high shear rate state is "1000 s-1", and the low shear state is "0.1 s-1". And in the figure, the recovery time t is 10 seconds.

[0040] That is, as shown in FIG. 9, when the positive electrode composite material 62 is in the high shear rate state, the positive electrode active material 70 and the conductive material 71 contained in the positive electrode composite material 62 are in a dispersed state with each other. And thereby, the fluidity of the positive electrode composite material 62 increases. That is, the viscosity η of the positive electrode composite material 62 becomes the high shear rate viscosity η1.

[0041] On the other hand, as shown in FIG. 10, when the positive electrode composite material 62 is in the low shear rate state, the positive electrode active material 70 and the conductive material 71 contained in the positive electrode composite material 62 are in an aggregated state again. And thereby, the fluidity of the positive electrode composite material 62 decreases. That is, the viscosity η of the positive electrode composite material 62 becomes the low shear rate viscosity η2.

[0042] That is, as shown in FIG. 11, when the positive electrode composite material 62 with a high viscosity recovery rate Z is used, after the liquid is applied to the current collector foil 51, it is considered that the positive electrode active material 70 and the conductive material 71 in the positive electrode composite material 62 re-aggregate rapidly. Further, due to the aggregation of these positive electrode active material 70 and conductive material 71, gaps are likely to be formed in the positive electrode composite material layer 32P before pressing laminated on the current collector foil 51. That is, with respect to the coating surface 51s of the current collector foil 51, the aggregated positive electrode active material 70 and conductive material 71 are three-dimensionally deposited, so that the thickness d´ (see FIG. 4) of the positive electrode composite material layer 32P before pressing can be increased. And thereby, this positive electrode plate 50 becomes a specification in which the positive electrode composite material layer 32P is easily crushed by the pressing process.

[0043] As a result, as shown in FIG. 12, even after pressing, a higher crushing rate α can be set while leaving gaps in the positive electrode composite material layer 32P. And thereby, while reducing the electron transfer resistance by promoting the formation of the conductive path based on the high crushing rate α, the diffusion resistance can be reduced by ensuring the appropriate porosity ε.

[0044] On the other hand, as shown in FIG. 13, when the positive electrode composite material 62 with a low viscosity recovery rate Z is used, even after the liquid is applied to the current collector foil 51, a state where the positive electrode active material 70 and the conductive material 71 in the positive electrode composite material 62 do not aggregate with each other and the mutual bonding force is small is likely to be maintained. And thereby, since the positive electrode active material 70 and the conductive material 71 can easily move in the positive electrode composite material 62, it becomes difficult to form gaps in the positive electrode composite material layer 32P before pressing laminated on the current collector foil 51. In other words, since the positive electrode active material 70 and the conductive material 71 are likely to be filled in the positive electrode composite material layer 32P without gaps, this positive electrode composite material layer 32P becomes difficult to be crushed by the pressing process.

[0045] That is to say, as shown in FIG. 14, since there are few gaps crushed by the execution of the pressing process in the positive electrode composite material layer 32P, when the crushing rate α is increased, the high crushing rate α is likely to directly lead to a decrease in the porosity ε. As a result, there arises a problem that it is difficult to achieve both the maintenance of the appropriate porosity ε and the ensuring of the high crushing rate α. In addition to this, there is a problem that it is difficult to obtain the effect of promoting the formation of the conductive path by pressing because the arrangement of the positive electrode active material 70 and the conductive material 71 with a small mutual bonding force is maintained without aggregating with each other after pressing.

[0046] In view of this point, in the secondary battery 1 of the present embodiment, the viscosity restoration rate Z of the positive electrode composite material 62 applied to the current collector foil 51 in the state of the composite paste 37P is set high (see FIGS. 7 and 8).

[0047] Specifically described, as shown in FIGS. 9 to 12, in the secondary battery 1 of the present embodiment, the positive electrode composite material 62 contains carbon nanotubes CNT as a conductive fibrous carbon material. Specifically, the content of the carbon nanotubes CNT contained in the positive electrode composite material layer 32P is 0.6 wt% or more and 1.2 wt% or less. Further, the carbon nanotubes CNT used in the secondary battery 1 of the present embodiment has a fiber diameter of, for example, 6 nm or more and 17 nm or less. Furthermore, this carbon nanotubes CNT has a fiber length of, for example, 0.4 μm or more and 2.0 μm or less. And the positive electrode composite material 62 used in the secondary battery 1 of the present embodiment is configured such that, when in a low shear rate state, the carbon nanotubes CNT having the function as the conductive material 71 are likely to aggregate with each other. That is to say, it is configured to promote the formation of the conductive path by this carbon nanotubes CNT and to ensure an appropriate porosity ε in the positive electrode composite material layer 32P.

[0048] In the secondary battery 1 of the present embodiment, the content of the dispersant contained in the positive electrode composite material 62 is adjusted within the range of 0.02 wt% or more and 0.19 wt% or less. That is, the higher the content of the dispersant, the lower the viscosity recovery rate Z of the positive electrode composite material 62 having the structure as the composite paste 37P, and the lower the content of the dispersant, the higher the viscosity recovery rate Z of the positive electrode composite material 62. In the secondary battery 1 of the present embodiment, thereby, the viscosity recovery rate Z of the positive electrode composite material 62 is appropriately controlled.

[0049] Incidentally, as the dispersant, for example, polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinyl pyrrolidone (PVP) can be used. And carboxymethyl cellulose, polyacrylate, polymethacrylate, polyoxyethylene alkyl ether, polyalkylene polyamine, benzimidazole, etc. can be used.

[0050] (Preferred range) More specifically, in the secondary battery 1 of the present embodiment, it is preferable that the viscosity recovery rate Z of the positive electrode composite material 62 is set to 750 mPa·s / s or more and 3800 mPa·s / s or less. Also, regarding the crushing rate α of the positive electrode composite material layer 32P by performing the pressing process, it is preferably 19% or more. Further, regarding the porosity ε of the positive electrode composite material layer 32P after pressing, it is preferably 33% or more and 57% or less. And when manufacturing the positive electrode plate 50, by controlling so as to be within these preferred ranges, the internal resistance of the secondary battery 1 can be reduced.

[0051] (Suppression of segregation of conductive material) Further, it is known that the conductive material 71 contained in the positive electrode composite material layer 32P is likely to segregate during the drying process of the positive electrode composite material layer 32P after the positive electrode composite material 62 is applied to the current collector foil 51. That is, when the positive electrode composite material layer 32P laminated on the current collector foil 51 is dried, when the solvent evaporates from the surface 32s of the positive electrode composite material layer 32P, the conductive material 71 contained in the positive electrode composite material layer 32P is likely to move together with this solvent. That is, so-called migration is likely to occur. And the segregation of the conductive material 71 caused thereby may inhibit the formation of a conductive path between the positive electrode active material 70 and the current collector foil 51 via the conductive material 71.

[0052] However, in the secondary battery 1 of the present embodiment, as described above, by setting the viscosity recovery rate Z of the positive electrode composite material 62 to be high, migration of the conductive material 71 is less likely to occur even in the drying process of the positive electrode composite material layer 32P. That is, when the positive electrode composite material 62 having a high viscosity recovery rate Z is used, the positive electrode active material 70 and the conductive material 71 contained in the positive electrode composite material layer 32P laminated on the current collector foil 51 are likely to aggregate (see FIGS. 11 and 12). In particular, the carbon nanotube CNT as a conductive fibrous carbon material tends to form relatively large lumps due to its aggregation. And thereby, the lump of the aggregated conductive material 71 becomes difficult to move in the positive electrode composite material layer 32P, so that segregation of the conductive material 71 due to drying as described above, that is, a state in which the conductive material 71 contained in the positive electrode composite material layer 32P is unevenly distributed on the surface 32s side can be avoided.

[0053] Specifically, as shown in FIG. 4, in the secondary battery 1 of the present embodiment, the thickness region δ of the positive electrode composite material layer 32P laminated on the current collector foil 51 is partitioned into a thickness region δ1 on the current collector foil 51 side and a thickness region δ2 on the surface 32s side. And the positive electrode plate 50 used in the secondary battery 1 of the present embodiment has a configuration containing 0.1 wt% or more and 0.52 wt% or less of carbon nanotube CNT with respect to the thickness region δ1 on the current collector foil 51 side in the positive electrode composite material layer 32P.

[0054] Specifically, the segregation state of the conductive material 71 contained in the positive electrode composite material layer 32P can be analyzed using, for example, an electron probe microanalyzer (EPMA), energy-dispersive X-ray spectroscopy (SEM-EDX), or the like. That is, EPMA is an analytical instrument that observes the structure and morphology of the surface of a solid sample by performing local elemental analysis by irradiating the surface of the solid sample with a finely focused electron beam in a vacuum. And EDX is a method of performing elemental analysis and composition analysis by detecting and spectroscopically analyzing X-rays generated by electron beam irradiation.

[0055] As a measurement procedure, first, the cross-section of the positive electrode plate 50 is prepared by ion milling or the like. Further, mapping of fluorine (F) and carbon (C) is performed by analyzing the cross-section of the positive electrode composite material layer 32P using the above measurement method. In the secondary battery 1 of the present embodiment, at this time, by bisecting the positive electrode composite material layer 32P in the thickness direction, the thickness region δ is partitioned into a thickness region δ1 on the current collector foil 51 side and a thickness region δ2 on the surface 32s side. Then, based on the analysis results, the segregation state can be observed by comparing the integrated values of the detection intensities of carbon (C) for the thickness region δ1 on the current collector foil 51 side and the thickness region δ2 on the surface 32s side.

[0056] That is, when the carbon nanotubes CNT contained in the positive electrode composite material layer 32P are uniform throughout the thickness region δ, the ratio of the detection intensities for the thickness region δ1 on the current collector foil 51 side and the thickness region δ2 on the surface 32s side becomes "1". Since the binder also contains carbon atoms (C), for example, when PVdF is used as the binder, the detection intensity of carbon (C) derived from the conductive material 71 is corrected based on the detection intensity of fluorine (F) contained in this PVdF. And thereby, the content of the carbon nanotubes CNT contained in the thickness region δ1 on the current collector foil 51 side can be obtained by multiplying the ratio of the detection intensities by the content of the carbon nanotubes CNT contained in the entire positive electrode composite material layer 32P.

[0057] (Function) In the secondary battery 1 of the present embodiment, by setting the viscosity recovery rate Z of the positive electrode composite material 62 to be high, when the positive electrode composite material 62 coated on the current collector foil 51 forms the positive electrode composite material layer 32P, the conductive material 71 contained in the positive electrode composite material layer 32P is likely to aggregate. Further, since the agglomerated mass of the conductive material 71 is difficult to move in the positive electrode composite material layer 32P, even after the drying process, the conductive material 71 in the positive electrode composite material layer 32P is difficult to segregate into the thickness region δ2 on the surface 32s side. That is, more of the conductive material 71 tends to remain in the thickness region δ1 on the current collector foil 51 side. And thereby, the formation of the conduction path between the positive electrode active material 70 and the current collector foil 51 through the conductive material 71 is promoted, and the internal resistance of the secondary battery 1 is reduced.

[0058] Next, the effects of the present embodiment will be described. (1) The secondary battery 1 includes a positive electrode plate 50 formed by coating a current collector foil 51 with a positive electrode composite material 62 containing a positive electrode active material 70. The manufacturing process of the positive electrode plate 50 includes a step of pressing the positive electrode composite material layer 32P formed by coating the positive electrode composite material 62. Also, when coating the current collector foil 51, the viscosity η in the high shear rate state when the positive electrode composite material 62 is discharged is defined as the high shear rate viscosity η1, and the viscosity η in the low shear rate state after the positive electrode composite material 62 adheres to the current collector foil 51 is defined as the low shear rate viscosity η2. Further, the difference value between the low shear rate viscosity η2 and the high shear rate viscosity η1 is divided by the recovery time t from the high shear rate viscosity η1 to the low shear rate viscosity η2 to obtain the viscosity recovery rate Z, and the positive electrode composite material 62 is adjusted so that the viscosity recovery rate Z is 750 mPa·s / s or more. Then, the positive electrode plate 50 is formed such that the crushing rate α of the positive electrode composite material layer 32P by the pressing is 19% or more, and the porosity ε of the pressed positive electrode composite material layer 32P is 33% or more and 57% or less.

[0059] That is, by setting a high viscosity recovery rate Z of the positive electrode composite material 62, when this positive electrode composite material 62 is applied to the current collector foil 51, the positive electrode active material 70 and the conductive material 71 are likely to be three-dimensionally deposited on the coating surface 51s. As a result, for the positive electrode composite material layer 32P before pressing, its thickness d´ can be increased. And thereby, in the pressing process after the coating, the positive electrode composite material layer 32P can be crushed more greatly.

[0060] Furthermore, it is possible to achieve both a high crushing rate α and ensuring an appropriate porosity ε in the positive electrode composite material layer 32P after pressing. That is, based on the high crushing rate α of the positive electrode composite material layer 32P, the conductive material 71 contained in the positive electrode composite material 62 is likely to form a conductive path between the positive electrode active materials 70 and between the current collector foil 51, so that the electron transfer resistance between them can be reduced. Also, by ensuring a state where the electrolytic solution 45 can easily flow through the gaps formed between the positive electrode active materials 70, the diffusion resistance among the internal resistances of the secondary battery 1 can be reduced. And according to the above configuration, the internal resistance of the secondary battery 1 can be effectively reduced.

[0061] (2) The positive electrode composite material 62 contains carbon nanotubes CNT as a conductive fibrous carbon material. Further, the thickness region δ of the positive electrode composite material layer 32P is partitioned into a thickness region δ1 on the current collector foil 51 side and a thickness region δ2 on the surface 32s side. And the positive electrode plate 50 contains 0.1 wt% or more and 0.52 wt% or less of carbon nanotubes CNT in the thickness region δ1 on the current collector foil 51 side in the positive electrode composite material layer 32P.

[0062] According to the above configuration, the carbon nanotubes CNT in the positive electrode composite material 62 function as the conductive material 71. Also, when the positive electrode composite material 62 is in a low shear rate state, this carbon nanotubes CNT is likely to aggregate. And thereby, by increasing the viscosity recovery rate Z of the positive electrode composite material 62, it is possible to achieve both a high crushing rate α by pressing and ensuring an appropriate porosity ε in the positive electrode composite material layer 32P after pressing.

[0063] In addition, due to the aggregation of carbon nanotubes CNTs, in the positive electrode composite layer 32P, there is a tendency to easily form relatively large lumps. And thereby, it becomes difficult for the aggregated carbon nanotubes CNTs to move in the positive electrode composite layer 32P, so that even after the drying of the positive electrode composite layer 32P, the carbon nanotubes CNTs tend to remain in the thickness region δ1 on the current collector foil 51 side. As a result, a conductive path is likely to be formed between the positive electrode active material 70 and the current collector foil 51 via the carbon nanotubes CNTs as the conductive material 71. And thereby, the internal resistance of the secondary battery 1 can be reduced more effectively.

[0064] Furthermore, when the total amount of the conductive material 71 added to the positive electrode composite 62 is increased in order to increase the content of the conductive material 71 in the thickness region δ1 on the current collector foil 51 side, the ratio of the positive electrode active material 70 contained in the positive electrode composite layer 32P decreases. That is to say, the substantial battery reaction components decrease. However, by using carbon nanotubes CNTs as the conductive material 71, the content of the conductive material 71 in the thickness region δ1 on the current collector foil 51 side can be set within the preferable range shown in the above configuration while maintaining an appropriate amount of the conductive material 71 contained in the positive electrode composite 62. And according to the above configuration, the internal resistance of the secondary battery 1 can be reduced more effectively.

[0065] Note that the above embodiment can be implemented with the following modifications. The above embodiment and the following modification examples can be implemented in combination with each other as long as they do not technically conflict.

[0066] ·In the above embodiment, it is preferable that the viscosity recovery rate Z of the positive electrode composite 62 is set to be 750 mPa·s / s or more and 3800 mPa·s / s or less. However, it is not limited thereto, and the viscosity recovery rate Z may be greater than 3800 mPa·s / s. It is only necessary to ensure the viscosity η2 at a low shear rate that does not hinder the coating on the current collector foil 51.

[0067] · In the above embodiment, carbon nanotube CNT as a conductive fibrous carbon material is used as the conductive material 71. Further, the content of the carbon nanotube CNT contained in the positive electrode composite layer 32P was set to 0.6 wt% or more and 1.2 wt% or less. And this carbon nanotube CNT has a fiber diameter of 6 nm or more and 17 nm or less, and a fiber length of 0.4 μm or more and 2.0 μm or less.

[0068] However, not limited to this, the content of the carbon nanotube CNT contained in the positive electrode composite layer 32P may be arbitrarily changed. However, it is preferable to contain 0.1 wt% or more and 0.52 wt% or less of carbon nanotube CNT in the thickness region δ1 on the current collector foil 51 side. And also regarding the fiber diameter and fiber length of the carbon nanotube CNT, they may be arbitrarily changed respectively.

[0069] · Further, for example, any fibrous carbon material having conductivity such as carbon nanofiber (CNF) etc., as long as it forms a conductive path with the positive electrode active material 70 located in the vicinity in the positive electrode composite 62, the conductive fibrous carbon material used for the conductive material 71 may be arbitrarily changed. And a configuration using a conductive material 71 other than the conductive fibrous carbon material may also be adopted.

[0070] · In the above embodiment, when partitioning the thickness region δ of the positive electrode composite layer 32P into the thickness region δ1 on the current collector foil 51 side and the thickness region δ2 on the surface 32s side, the positive electrode composite layer 32P was bisected in the thickness direction. However, it does not necessarily have to be a precise bisection. It is sufficient if it can be confirmed that the carbon nanotube CNT serving as the conductive material 71 remains in the vicinity of the current collector foil 51 even after the positive electrode composite layer 32P is dried.

[0071] · Further, the dispersant added to the positive electrode composite 62 is arbitrary. And also regarding its content, it may be arbitrarily changed. ·In the above-described embodiment, the electrode body 10 was configured as a wound body. However, it is not necessarily required that the positive and negative electrode sheets 35P and 35N laminated with the separator 5 therebetween be wound. That is, the shape of the positive electrode plate is arbitrary. And the secondary battery 1 to which this positive electrode plate is applied does not necessarily have to be a lithium-ion secondary battery, and it may be applied to other non-aqueous electrolyte secondary batteries.

[0072] ·Regarding the terminal shapes of the positive electrode terminal 38P and the negative electrode terminal 38N, they are not limited to the shapes shown in FIG. 1 and may be arbitrarily changed. And regarding the shape of the case 20 that forms the outer shape of the secondary battery 1, it is not necessarily limited to a flat rectangular box shape, and it may be arbitrarily changed, for example, to a cylindrical shape or the like.

[0073] (Addendum) Next, the technical idea that can be grasped from the above-described embodiment and modification examples will be described. (A) The content of the conductive fibrous carbon material contained in the positive electrode composite layer is 0.6 wt% or more and 1.2 wt% or less.

[0074] (B) The conductive fibrous carbon material has a fiber diameter of 6 nm or more and 17 nm or less and a fiber length of 0.4 μm or more and 2.0 μm or less. According to each of the above configurations, the conductive fibrous carbon material can function effectively as a conductive material. Further, when the positive electrode composite is in a low shear rate state, the conductive fibrous carbon material is likely to aggregate. And thereby, it is possible to promote the formation of a conductive path by this conductive fibrous carbon material and ensure an appropriate porosity in the positive electrode composite layer.

[0075] (C) The content of the dispersant contained in the positive electrode composite is 0.02 wt% or more and 0.19 wt% or less. Thereby, the viscosity recovery rate of the positive electrode composite can be appropriately controlled.

[0076] (B) A method for manufacturing a secondary battery, comprising a positive electrode plate formed by coating a positive electrode composite material containing a positive electrode active material on a current collector foil, and having a step of pressing a positive electrode composite material layer formed by the coating. When the positive electrode composite material is discharged onto the current collector foil, the viscosity of the positive electrode composite material in a high shear rate state is defined as the high shear rate viscosity, and when the positive electrode composite material adheres to the current collector foil, the viscosity of the positive electrode composite material in a low shear rate state is defined as the low shear rate viscosity. A value obtained by dividing the difference value between the low shear rate viscosity and the high shear rate viscosity by the recovery time from the high shear rate viscosity to the low shear rate viscosity is defined as the viscosity recovery rate, and the positive electrode composite material is adjusted so that the viscosity recovery rate is 750 mPa·s / s or more.

Example

[0077] Hereinafter, examples and the like for more specifically describing the configuration and effects of the present invention will be described. However, the present invention is not limited to these examples. FIG. 15 is a table showing a list of the results of a performance test conducted on the secondary battery 1. That is, in this performance test, for the positive electrode plate 50 of the secondary battery 1, the difference in "discharge resistance" was measured when the compaction rate α and the porosity ε of the positive electrode composite material layer 32P were different. In addition, together with this, for each sample of "Comparative Example 1" to "Comparative Example 5" and "Example 1" to "Example 4" used in the performance test, the viscosity recovery rate Z of the positive electrode composite material 62 coated on the current collector foil 51 was measured. Further, for the carbon nanotube CNT contained in the positive electrode composite material layer 32P as the conductive material 71, the content in the thickness region δ1 on the current collector foil 51 side was measured. For the sake of convenience of explanation, hereinafter, the content of the carbon nanotube CNT in the thickness region δ1 on the current collector foil 51 side will be simply abbreviated as "amount of conductive material on foil side". Then, the influence of the viscosity recovery rate Z on the compaction rate α and the porosity ε was confirmed, and the relationship between the "amount of conductive material on foil side" and the "discharge resistance", that is, the influence on the internal resistance of the secondary battery 1 was confirmed.

[0078] In addition, the specifications of the positive electrode plate 50 used in the performance test are substantially the same as those of the positive electrode plate 50 shown in the above embodiment. That is, in the positive electrode mixture 62, a binder and a dispersant are included, and as described above, as the conductive material 71, carbon nanotube CNT as a conductive fibrous carbon material is included. Incidentally, the content of the positive electrode active material 70 contained in the positive electrode mixture layer 32P is 97.3 wt% to 98.6 wt%. Further, the content of carbon nanotube CNT contained in the positive electrode mixture layer 32P is 0.6 wt% or more and 1.2 wt% or less, the fiber diameter is 6 nm or more and 17 nm or less, and the fiber length is 0.4 μm or more and 2.0 μm or less. And the content of the dispersant contained in the positive electrode mixture 62 was set to 0.02 wt% or more and 0.19 wt% or less.

[0079] Furthermore, the compaction rate α and porosity ε of the positive electrode mixture layer 32P, and the measurement method of the "amount of conductive material on the foil side" are the same as the methods detailed in the above embodiment, respectively. And regarding the viscosity recovery rate Z of the positive electrode mixture 62 as well, the measurement was carried out by the same method as the method detailed in the above embodiment.

[0080] Also, regarding the "discharge resistance" of the secondary battery 1, the measurement was carried out by the same method as the method detailed in the above embodiment. Furthermore, in this performance test, in addition to the measurement of the "0.1 s discharge resistance" which is an index of the "electron transfer resistance" among the internal resistances of the secondary battery 1, the measurement of the "0.1 s to 10 s discharge resistance" was carried out as an index of the "diffusion resistance". That is, this "0.1 s to 10 s discharge resistance", similar to the "0.1 s discharge resistance", for each sample, when the secondary battery 1 is discharged at a constant current, the voltage drop amount from 0.1 second after the start of discharge to 10 seconds after is measured, and it is calculated based on Ohm's law. Incidentally, the test conditions are a temperature of 25°C and an SOC at the start of discharge of "56%". And from the measurement results of these "0.1 s discharge resistance" and "0.1 s to 10 s discharge resistance", for each sample of "Comparative Example 1" to "Comparative Example 5" and "Example 1" to "Example 4", the reduction effect of the internal resistance was confirmed respectively.

[0081] Specifically, for the "0.1 s discharge resistance", if the measured value is less than 0.16 Ω, a "circle mark" is described in the table; if the measured value is 0.16 Ω or more and less than 0.20 Ω, a "triangle mark" is described; and if the measured value is 0.20 Ω or more, a "cross mark" is described. For the "0.1 s to 10 s discharge resistance", if the measured value is less than 0.36 Ω, a "circle mark" is described in the table; if the measured value is 0.36 Ω or more and less than 0.38 Ω, a "triangle mark" is described; and if the measured value is 0.38 Ω or more, a "cross mark" is described.

[0082] To elaborate, for "Comparative Example 1" to "Comparative Example 4", the compaction rate α of the positive electrode composite material layer 32P is "10%", "19%", "27%", and "40%" respectively, in the given order. Also, for these "Comparative Example 1" to "Comparative Example 4", the viscosity recovery rate Z of the positive electrode composite material 62 forming the positive electrode composite material layer 32P is "370". Note that the unit of the viscosity recovery rate Z is "mPa·s / s". Hereinafter, for the sake of convenience in explanation, the descriptions in the text are omitted. Furthermore, the porosity ε is "57%", "49%", "42%", and "30%" respectively, in the given order. And thus, it can be confirmed that when the viscosity recovery rate Z of the positive electrode composite material 62 is the same, the higher the compaction rate α of the positive electrode composite material layer 32P, the lower the porosity ε.

[0083] Also, for these "Comparative Example 1" to "Comparative Example 4", it was confirmed that as the compaction rate α of the positive electrode composite material layer 32P increases, the "0.1 s discharge resistance" tends to decrease. That is, as the compaction rate α increases, the formation of the conduction path between the conductive material 71 and the positive electrode active material 70 and between the current collector foil 51 is promoted. As a result, among the internal resistances of the secondary battery 1, it is considered that especially the "electron transfer resistance" is likely to decrease. And the relationship between the compaction rate α and the "0.1 s discharge resistance" shown in the table of Fig. 15 supports this.

[0084] Furthermore, for these "Comparative Example 1" to "Comparative Example 4", it was confirmed that as the porosity ε of the positive electrode composite layer 32P increased, its "discharge resistance from 0.1 s to 10 s" tended to decrease. That is, as the porosity ε increased, the electrolyte 45 flowed more easily through the gaps formed between the positive electrode active materials 70. As a result, it is considered that the "diffusion resistance" decreased because the lithium ions serving as carriers diffused more easily. And the relationship between the porosity ε and the "discharge resistance from 0.1 s to 10 s" shown in the table of FIG. 15 supports this result.

[0085] Also, in "Comparative Example 1" to "Comparative Example 4" where the viscosity recovery rate Z of these positive electrode composites 62 is the same value, "370", the "amount of conductive material on the foil side" is also the same value, "0.07 wt%". Furthermore, in "Comparative Example 5" where the viscosity recovery rate Z is "3800", the "amount of conductive material on the foil side" is "0.52 wt%". And for this "Comparative Example 5", the compaction rate α of the positive electrode composite layer 32P is "51%", and the porosity ε is "30%".

[0086] Furthermore, in "Example 1", the viscosity recovery rate Z of the positive electrode composite 62 is "750", and the "amount of conductive material on the foil side" is "0.1 wt%". Also, in "Example 2" and "Example 3", the viscosity recovery rate Z is "1950" for both, and the "amount of conductive material on the foil side" is "0.23 wt%". And in "Example 4", the viscosity recovery rate Z is the same value as in "Comparative Example 5", "3800", and the value of the "amount of conductive material on the foil side" is also the same value as in "Comparative Example 5", "0.52 wt%".

[0087] Also, "Example 1" to "Example 4" each have a different combination of the compaction rate α and the porosity ε of the positive electrode composite layer 32P. Specifically, the compaction rate α of the positive electrode composite layer 32P is "19%", "32%", "47%", "36%" in order. And the porosity ε is "57%", "49%", "33%", "39%" in order.

[0088] Furthermore, from the comparison between combinations with the same viscosity recovery rate Z of the positive electrode composite material 62, that is, the comparison between "Comparative Example 1" to "Comparative Example 4", the comparison between "Comparative Example 5" and "Example 4", and the comparison between "Example 2" and "Example 3", the relationship between the "amount of conductive material on the foil side" and the viscosity recovery rate Z is considered. As a result, it can be confirmed that regardless of the compaction rate α and porosity ε of the positive electrode composite material layer 32P, the "amount of conductive material on the foil side" becomes a value corresponding to the viscosity recovery rate Z of the positive electrode composite material 62. In addition to this, it was confirmed that the higher the viscosity recovery rate Z, the more the "amount of conductive material on the foil side" tended to increase.

[0089] That is, when the viscosity recovery rate Z of the positive electrode composite material 62 is high, in the state where the positive electrode composite material 62 is applied to the current collector foil 51, the carbon nanotubes CNT included as the conductive material 71 in the positive electrode composite material 62 tend to aggregate. Therefore, the higher the viscosity recovery rate Z, the easier it is for the aggregated carbon nanotubes CNT to form relatively large lumps. As a result, even in the drying process of the positive electrode composite material layer 32P, the migration of the carbon nanotubes CNT is less likely to occur. That is, it is considered that more carbon nanotubes CNT tend to remain in the thickness region δ1 on the current collector foil 51 side. And the above comparison results support this speculation.

[0090] Furthermore, when comparing "Comparative Example 4" and "Comparative Example 5" with the same porosity ε of the positive electrode composite material layer 32P, "Comparative Example 5" with a larger "amount of conductive material on the foil side" showed better results for the "discharge resistance from 0.1s to 10s" than "Comparative Example 4" with a smaller "amount of conductive material on the foil side". That is, it is considered that when more carbon nanotubes CNT remain in the thickness region δ1 on the current collector foil 51 side, a conductive path is more easily formed between the positive electrode active material 70 and the current collector foil 51 through the carbon nanotubes CNT as the conductive material 71. And the above comparison results support this speculation.

[0091] In addition, in "Example 1" to "Example 4", both the compaction rate α and the porosity ε of the positive electrode composite material layer 32P are within their preferred ranges. That is, the compaction rate α of the positive electrode composite material layer 32P by pressing is 19% or more, and the porosity ε of the pressed positive electrode composite material layer 32P is 33% or more and 57% or less. Furthermore, in these "Example 1" to "Example 4", the viscosity recovery rate Z of the positive electrode composite material 62 is also a value within its preferred range, that is, "750" or more. As a result, the "amount of foil-side conductive material" can also be set to a value within its preferred range, that is, 0.1 wt% or more and 0.52 wt% or more, without adding an excessive amount of conductive material 71 into the positive electrode composite material layer 32P. From the above test results, it was confirmed that it is appropriate to control the compaction rate α and the porosity ε of these positive electrode composite material layers 32P, and the viscosity recovery rate Z of the positive electrode composite material 62 within their respective preferred ranges.

Explanation of Signs

[0092] 1... Secondary battery 32P... Positive electrode composite material layer 50... Positive electrode plate 51... Current collector foil 62... Positive electrode composite material 70... Positive electrode active material η... Viscosity η1... Viscosity at high shear rate η2... Viscosity at low shear rate t... Recovery time Z... Viscosity recovery rate α... Compaction rate ε... Porosity

Claims

1. A method for manufacturing a secondary battery, comprising a positive electrode plate formed by coating a current collector foil with a positive electrode composite material containing a positive electrode active material, and having a step of pressing a positive electrode composite material layer formed by the coating, wherein the viscosity of the positive electrode composite material in a high shear rate state when the positive electrode composite material is discharged onto the current collector foil is defined as the high shear rate viscosity, the viscosity of the positive electrode composite material in a low shear rate state after the positive electrode composite material adheres to the current collector foil is defined as the low shear rate viscosity, a value obtained by dividing a difference value between the low shear rate viscosity and the high shear rate viscosity by a recovery time from the high shear rate viscosity to the low shear rate viscosity is defined as a viscosity recovery rate, the positive electrode composite material is adjusted so that the viscosity recovery rate is 750 mPa·s / s or more, the crushing rate of the positive electrode composite material layer by the pressing is 19% or more, and the porosity of the pressed positive electrode composite material layer is 33% or more and 57% or less A method for manufacturing a secondary battery, which is controlled to be as described above.

2. The positive electrode composite material contains a conductive fibrous carbon material, and when the thickness region of the positive electrode composite material layer is partitioned into a current collector foil side and a surface side, in the thickness region on the current collector foil side, contains 0.1 wt% or more and 0.52 wt% or less of the conductive fibrous carbon material The method for manufacturing a secondary battery according to claim 1.

3. Carbon nanotubes are used as the conductive fibrous carbon material The method for manufacturing a secondary battery according to claim 2.

Images