Negative electrode plate manufacturing method

Abstract

Conventional manufacturing methods have presented a problem in that it is difficult to increase the specific surface area of ​​the negative electrode active material layer of the negative electrode plate using simple equipment. [Solution] The present invention provides a method for manufacturing a negative electrode plate, comprising: a coating step (S1) of coating a negative electrode foil with a slurry obtained by mixing a negative electrode active material having graphite and hard carbon with a binder and a solvent; a pressing step (S2) of pressing the slurry on the negative electrode foil to form a negative electrode active material layer; and a hard carbon removal step (S3) of applying vibration to the pressed slurry to remove the hard carbon exposed on the surface of the negative electrode active material layer.

Description

Technical Field

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[0001] The present invention relates to a method for manufacturing a negative electrode plate used in a secondary battery such as a lithium-ion battery, for example.

Background Art

[0002] In a lithium-ion battery, it is known that the battery performance such as input / output characteristics improves as the opposing area of the electrode plate increases. Therefore, techniques for increasing the specific surface area of the electrode plate are described in Patent Documents 1 to 3.

[0003] Patent Document 1 discloses a method for manufacturing a positive electrode of a non-aqueous secondary battery having an electrode mixture layer on the surface of a current collector foil, the method including: a film-forming step of forming a positive electrode having an electrode mixture layer by applying an electrode mixture obtained by mixing an electrode active material, a binder, and a solvent onto a positive electrode current collector foil; and a shaping step of shaping the electrode mixture layer of the positive electrode by passing the positive electrode conveyed by a pair of press rolls including a first roll and a second roll rotating opposite to the first roll through a gap between the first roll and the second roll to contact the surface of the positive electrode while applying pressure thereto, wherein in the shaping step, an active material having a particle size Dm that is not removed from the surface of the positive electrode and an active material having a particle size Dp smaller than the particle size Dm to be removed from the surface of the positive electrode are contacted with the surface of the positive electrode while applying pressure thereto by the press roll having irregularities corresponding to the particle size Dp to be removed, so as to remove the active material having the particle size Dp.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] <00However, the technology described in Patent Document 1 requires replacing the press roll with one having irregularities corresponding to the particle size Dp to be removed, which presents a problem as it requires a lot of effort to update the equipment. Furthermore, the technology described in Patent Document 1 is applied to the manufacture of positive electrode plates and cannot be directly applied to the manufacture of negative electrode plates.

[0006] This invention has been made in view of the above circumstances, and aims to increase the specific surface area of ​​the negative electrode active material layer of a negative electrode plate using simple equipment. [Means for solving the problem]

[0007] One embodiment of the method for manufacturing a negative electrode plate according to the present invention includes a coating step of coating a negative electrode foil with a slurry obtained by mixing a negative electrode active material having graphite and hard carbon, a binder and a solvent; a pressing step of pressing the slurry on the negative electrode foil to form a negative electrode active material layer; and a hard carbon removal step of applying vibration to the pressed slurry to remove the hard carbon exposed on the surface of the negative electrode active material layer. [Effects of the Invention]

[0008] According to the method for manufacturing a negative electrode plate of the present invention, the specific surface area of ​​the negative electrode active material layer of the negative electrode plate can be increased with simple equipment modifications. [Brief explanation of the drawing]

[0009] [Figure 1] This graph illustrates the relationship between the specific surface area of ​​the electrode plates and the reaction resistance in a secondary battery. [Figure 2] This diagram illustrates the process for manufacturing a negative electrode plate according to Embodiment 1. [Figure 3] This is a schematic diagram of a manufacturing facility illustrating an example of equipment used in the manufacturing method of the negative electrode plate according to Embodiment 1. [Figure 4] This diagram illustrates the density changes of natural graphite and hard carbon before and after the pressing process. [Figure 5]This diagram illustrates the difference in peel strength between natural graphite and hard carbon. [Figure 6] This table illustrates the specific surface area of ​​the negative electrode in the negative electrode plate manufacturing method according to Embodiment 1. [Figure 7] This graph illustrates the specific surface area of ​​the negative electrode in the negative electrode plate manufacturing method according to Embodiment 1. [Modes for carrying out the invention]

[0010] For clarity of explanation, the following descriptions and drawings have been omitted and simplified as appropriate. In each drawing, the same elements are denoted by the same reference numeral, and redundant explanations have been omitted where necessary.

[0011] Embodiment 1 First, we will explain the relationship between the specific surface area of ​​the negative electrode plate in a lithium-ion battery and the battery performance. Figure 1 shows a graph illustrating the relationship between the specific surface area of ​​the electrode plate and the reaction resistance in a secondary battery. In the graph shown in Figure 1, the vertical axis shows the reaction resistance of the lithium-ion battery, and the horizontal axis shows the specific surface area of ​​the negative electrode plate (electrode plate BET). There are various methods for measuring specific surface area, such as the BET method, liquid adsorption method, immersion heat method, and permeation method, but in the following explanation, we will use the specific surface area measured using the BET method. Electrode plate BET is the specific surface area of ​​the negative electrode active material layer measured using the BET method.

[0012] As shown in Figure 1, it can be seen that in lithium-ion batteries, the reaction resistance tends to decrease as the electrode plate BET increases. Therefore, in the negative electrode plate manufacturing method according to Embodiment 1 described below, the specific surface area of ​​the negative electrode active material layer is increased by peeling off the hard carbon contained in the negative electrode active material layer. Note that the negative electrode active material layer is the one in which carbon material is the main active material, and in the following description, the negative electrode active material layer will be simply referred to as the active material layer.

[0013] Figure 2 shows a diagram illustrating the flow of the manufacturing method for the negative electrode plate according to Embodiment 1. As shown in Figure 2, in the manufacturing method for the negative electrode plate according to Embodiment 1, first, an active material layer 10, which is the negative electrode active material layer, is coated onto the negative electrode foil 11 (step S1). More specifically, in the coating process, a slurry obtained by kneading a binder and a solvent with the negative electrode active material containing natural graphite and hard carbon is coated onto the negative electrode foil 11. In the active material layer 10 formed in the slurry coated in this coating process, both the natural graphite and hard carbon are in an uncompressed state.

[0014] Furthermore, the specific surface area of ​​natural graphite used as the negative electrode active material is larger than that of hard carbon. In addition, the particle size of natural graphite is more than seven times larger than that of hard carbon. Natural graphite is easily crushed under pressure, while hard carbon is not easily crushed. This difference in crushability is used to create irregularities on the surface of the active material layer 10.

[0015] Next, in the negative electrode plate manufacturing method according to Embodiment 1, a pressing step is performed in which the coated slurry is pressed to form the active material layer 10, which will become the negative electrode active material layer (Step S2). In the pressing step, the natural graphite is crushed by the pressure applied by the press machine. On the other hand, the hard carbon is not crushed like the natural graphite, and a portion of it becomes exposed on the surface of the active material layer 10.

[0016] Therefore, Figure 4 shows a diagram illustrating the density changes of natural graphite and hard carbon before and after the pressing process. In Figure 4, the change in density before and after pressing (Δdensity) is shown on the vertical axis. As shown in Figure 4, it can be seen that the density of natural graphite changes significantly, while the density change of hard carbon is almost zero (i.e., it does not collapse).

[0017] Subsequently, in the method for manufacturing a negative electrode plate according to Embodiment 1, a hard carbon removal step is performed (step S3) in which vibration is applied to the pressed slurry to remove the hard carbon exposed on the surface of the negative electrode active material layer. In this hard carbon removal step, the difference in the peeling strength between natural graphite and hard carbon is utilized to peel the hard carbon from the active material layer 10. Therefore, FIG. 5 shows a diagram for explaining the difference in the peeling strength between natural graphite and hard carbon.

[0018] In FIG. 5, the magnitude of the peeling strength between natural graphite and hard carbon is taken on the vertical axis. The peeling strength of natural graphite in FIG. 5 is that of natural graphite after the pressing step of step S2. As shown in FIG. 5, natural graphite does not peel off unless a force about 5 to 6 times that of hard carbon is applied. Therefore, in the hard carbon removal step, by applying vibration such as ultrasonic waves to the active material layer 10 after pressing, the hard carbon exposed on the surface of the active material layer 10 is peeled off and removed from the active material layer 10. Then, a depression is formed in the portion from which the hard carbon has been removed, making it possible to increase the electrode plate BET of the negative electrode plate.

[0019] An example of a negative electrode plate manufacturing apparatus 20 for realizing the above manufacturing method will be schematically described. FIG. 3 shows a schematic diagram of manufacturing equipment for explaining an example of the equipment used in the method for manufacturing a negative electrode plate according to Embodiment 1. The negative electrode plate manufacturing apparatus 20 shown in FIG. 3 shows only the portions that perform the coating step (step S1), the pressing step (step S2), and the hard carbon removal step (step S3) among the negative electrode plate manufacturing apparatus, and although not shown, other mechanisms for performing other steps are incorporated in the entire negative electrode plate manufacturing apparatus 20. Note that the example shown in FIG. 3 schematically shows the negative electrode plate manufacturing apparatus 20, and the actual mechanism is simply illustrated.

[0020] As shown in FIG. 3, the negative electrode plate manufacturing apparatus 20 includes an unwinding roll 21, a slurry discharge device 22, a first pressing roll 23, a second pressing roll 24, a driven roller 25, an ultrasonic oscillator 26, and a driven roller 27.

[0021] In the negative electrode plate manufacturing apparatus 20, the negative electrode foil 11 wound around the unwinding roll 21 is unwound, and a coating process is performed in which the slurry that will become the negative electrode foil 11 is coated onto the negative electrode foil 11 by the slurry discharge device 22. The negative electrode plate manufacturing apparatus 20 then performs a pressing process in which the slurry-coated negative electrode foil 11 is passed through a pressing mechanism that sandwiches the negative electrode foil 11 between a first press roll 23 and a second press roll 24, thereby pressing the active material layer 10 and compressing the natural graphite within the active material layer 10.

[0022] Subsequently, the negative electrode plate manufacturing apparatus 20 changes the transport direction of the negative electrode foil 11 using the driven roller 25. Behind the driven roller 25, the surface of the negative electrode foil 11 coated on the negative electrode foil 11 faces downwards from the horizontal. Then, with the surface of the active material layer 10 facing downwards from the horizontal, the negative electrode plate manufacturing apparatus 20 applies vibration to the active material layer 10 using the ultrasonic oscillator 26. Even a vibration of about 1 second is sufficient to achieve an effect. By applying vibration to the active material layer 10 with its surface facing downwards from the horizontal, the hard carbon, which has weak peel strength, naturally peels off and falls from the active material layer 10. The areas where the hard carbon on the surface of the active material layer 10 has peeled off become concave, and the areas where the surface of the active material layer 10 is natural graphite and did not peel off become convex, thus forming a negative electrode plate with many irregularities. Subsequently, the negative electrode plate manufacturing apparatus 20 further changes the direction of travel of the negative electrode foil 11 using the driven roller 27 to carry out other processes for manufacturing the negative electrode plate, such as the slitting process.

[0023] Here, the relationship between the amount of hard carbon mixed in the negative electrode active material and the size of the electrode plate BET in the negative electrode plate manufacturing method according to Embodiment 1 will be explained with reference to Figures 6 and 7. Figure 6 is a table illustrating the specific surface area of ​​the negative electrode in the negative electrode plate manufacturing method according to Embodiment 1. Figure 7 is a graph illustrating the specific surface area of ​​the negative electrode in the negative electrode plate manufacturing method according to Embodiment 1. Figures 6 and 7 show the measured electrode plate BET when the amount of hard carbon mixed in is increased, with the negative electrode active material with 0% hard carbon mixed in being used as a reference.

[0024] As shown in Figures 6 and 7, even after the coating process, the BET increases as the amount of hard carbon mixed increases. However, the increase in the BET of the electrode plate based on the difference in the amount of hard carbon mixed becomes even greater after going through the pressing process and the hard carbon removal process.

[0025] As described above, according to the negative electrode plate manufacturing method of Embodiment 1, it is possible to increase the size of the electrode plate BET and improve battery performance simply by adding an ultrasonic oscillator 26 to the process. Furthermore, according to the negative electrode plate manufacturing method of Embodiment 1, the size of the electrode plate BET can be adjusted by adjusting at least one of the amount of hard carbon mixed and the particle size of the hard carbon, making it possible to easily adjust the size of the electrode plate BET.

[0026] It should be noted that the present invention is not limited to the embodiments described above, and can be modified as appropriate without departing from the spirit of the invention. [Explanation of symbols]

[0027] 10 Active material layer 11 Negative electrode foil 20 Negative plate manufacturing equipment 21 unwinding roll 22. Slurry Discharge Device 23 First press roll 24. Second press roll 25 Driven roller 26. Ultrasonic Oscillator 27 Driven roller

Claims

1. A coating process in which a slurry is obtained by mixing a negative electrode active material having graphite and hard carbon, a binder and a solvent, and coating the negative electrode foil with it. A pressing step in which the slurry on the negative electrode foil is pressed to form the negative electrode active material layer, A hard carbon removal step involves applying vibration to the pressed slurry to remove the hard carbon exposed on the surface of the negative electrode active material layer, A method for manufacturing a negative electrode plate having the following characteristics.

2. The method for manufacturing a negative electrode plate according to claim 1, wherein, in the hard carbon removal step, vibration is applied to the negative electrode active material layer in such a state that the surface of the negative electrode active material layer faces downwards from the horizontal direction.

3. The method for manufacturing a negative electrode plate according to claim 1, wherein the specific surface area of ​​the graphite is greater than the specific surface area of ​​the hard carbon.

4. The method for manufacturing a negative electrode plate according to claim 1, wherein the particle size of the graphite is seven times or larger than the particle size of the hard carbon.

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