Negative electrode manufacturing method

Abstract

To manufacture a negative electrode that can sufficiently reduce the internal resistance of a battery, regardless of a negative electrode active material used.SOLUTION: A method includes the steps of applying a negative electrode slurry containing a negative electrode active material to the surface of a negative electrode current collector, and removing the surface of the layer containing the negative electrode slurry applied to the surface of the negative electrode current collector.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a method for manufacturing a negative electrode.

Background Art

[0002] Secondary batteries such as lithium-ion secondary batteries include an electrolyte layer containing an electrolytic solution or a solid electrolyte between a positive electrode and a negative electrode. The positive electrode and the negative electrode are each produced by applying a slurry containing an active material to the surface of a current collector. In Patent Document 1, it is pointed out as a problem that when a slurry containing a negative electrode active material is passed between a pair of rolls while being pressurized, and a layer of the slurry is formed on the surface of one of the rolls and transferred to a strip-shaped current collector, the particles of the negative electrode active material are crushed by the pressure. As disclosed in Patent Document 1, if the negative electrode active material on the surface of the negative electrode is crushed, the internal resistance of the battery increases.

[0003] According to Patent Document 1, by using a negative electrode active material in which the average particle diameter [μm] and the tap density [g / cm 3 have a predetermined relationship, the above problem was solved and the internal resistance of the battery could be reduced.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, the method disclosed in Patent Document 1 limits the average particle diameter and the tap density of the negative electrode active material to be used, and cannot be said to be a general-purpose method applicable to all negative electrode active materials. Furthermore, the method disclosed in Patent Document 1 has a problem that a part of the negative electrode active material is crushed by pressure and the internal resistance of the battery cannot be sufficiently reduced.

[0006] Therefore, in view of the above-described circumstances, an object of one embodiment of the present disclosure is to provide a method for manufacturing a negative electrode that can sufficiently reduce the internal resistance of a battery regardless of the negative electrode active material used.

Means for Solving the Problems

[0007] The present disclosure that has achieved the above object includes the following. <1> A method for manufacturing a negative electrode, comprising a step of applying a negative electrode slurry containing a negative electrode active material to the surface of a negative electrode current collector, and a step of removing the surface layer of a negative electrode composite layer containing the negative electrode slurry applied to the surface of the negative electrode current collector. <2> The method for manufacturing a negative electrode according to <1>, wherein the negative electrode slurry and the negative electrode current collector are sandwiched between a pair of rolls, the pair of rolls are rotationally driven in opposite directions around an axis, and the negative electrode slurry is applied to the surface of the negative electrode current collector. <3> The method for manufacturing a negative electrode according to <1> or <2>, wherein a brush roll is brought into contact with the surface of the negative electrode composite layer to remove the surface layer of the negative electrode composite layer. <4> The method for manufacturing a negative electrode according to any one of <1> to <3>, wherein, as the surface layer of the negative electrode composite layer, a depth of 20 μm to 30 μm from the surface is removed. <5> The method for manufacturing a negative electrode according to any one of <1> to <4>, wherein the negative electrode active material is graphite having an average particle diameter of 5 μm to 25 μm. <6> A secondary battery, comprising a negative electrode manufactured by the method for manufacturing a negative electrode according to any one of <1> to <5>, a positive electrode, and an electrolyte layer disposed between the negative electrode and the positive electrode.

Advantages of the Invention

[0008] According to the method for manufacturing a negative electrode of one embodiment of the present disclosure, a negative electrode that retains the shape of the negative electrode active material can be manufactured. Further, by using the negative electrode manufactured by the method for manufacturing a negative electrode of the present disclosure, the internal resistance in a secondary battery can be kept low.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2A

Figure 2B

Figure 3

Embodiments for Carrying Out the Invention

[0010] Hereinafter, embodiments of the present disclosure will be described. The description is illustrative of the embodiments and does not limit the scope of the present disclosure.

[0011] In this specification, a numerical range indicated using "~" indicates a range including the numerical values described before and after "~" as the minimum value and the maximum value, respectively. In the numerical ranges described stepwise in this specification, the upper limit value or the lower limit value described in one numerical range may be replaced with the upper limit value or the lower limit value of another numerically described range. Also, in the numerical ranges described in this specification, the upper limit value or the lower limit value of that numerical range may be replaced with the value shown in the examples.

[0012] In this specification, the term "step" includes not only an independent step but also a step that cannot be clearly distinguished from other steps as long as the intended purpose of that step is achieved.

[0013] In this specification, when an embodiment is described with reference to the drawings, the configuration of the embodiment is not limited to the configuration shown in the drawings. Also, the sizes of the members in each drawing are conceptual, and the relative relationships of the sizes between the members are not limited thereto.

[0014] In this specification, each component may contain a plurality of corresponding substances. When referring to the amount of each component in the composition in this embodiment, if there are a plurality of substances corresponding to each component in the composition, unless otherwise specified, it means the total amount of the plurality of substances present in the composition.

[0015] The method for manufacturing a negative electrode according to the present disclosure includes a step of coating a negative electrode slurry containing a negative electrode active material on the surface of a negative electrode current collector (hereinafter, also referred to as a coating step), and a step of removing the surface layer of a negative electrode composite layer containing the negative electrode slurry coated on the surface of the negative electrode current collector (hereinafter, also referred to as a surface layer removal step).

[0016] According to the method for manufacturing a negative electrode of the present disclosure, even if pressure, frictional force, or the like is applied to the surface of the negative electrode composite layer in the step of coating the negative electrode slurry on the surface of the negative electrode current collector or a subsequent step, it is possible to prevent a decrease in the porosity in the surface layer of the negative electrode composite layer caused by the crushed or pulverized negative electrode active material. Here, the surface layer in the negative electrode composite layer can be set to a depth of 30 μm from the surface of the negative electrode composite layer, and may be set to a depth of 25 μm from the surface, may be set to a depth of 20 μm from the surface, may be set to a depth of 15 μm from the surface, or may be set to a depth of 10 μm from the surface.

[0017] Further, a decrease in the porosity in the surface layer means that the porosity in the surface layer within the above range is lower than the porosity in the deeper part than the surface layer. Preventing a decrease in the porosity in the surface layer means, in other words, that the porosity in the surface layer within the above range is equivalent to the porosity in the deeper part than the surface layer. That is, according to the method for manufacturing a negative electrode of the present disclosure, a negative electrode can be manufactured in which the porosity in the surface layer within the above range is equivalent to the porosity in the deeper part than the surface layer. Specifically, according to the method for manufacturing a negative electrode of the present disclosure, the porosity in the surface layer within the above range is 90% or more, preferably 93% or more, more preferably 95% or more, and still more preferably 97% or more of the porosity in the deeper part than the surface layer. By using the negative electrode manufactured by the method for manufacturing a negative electrode of the present disclosure, a secondary battery with a low internal resistance can be produced.

[0018] In the present disclosure, for the porosity of the negative electrode composite material layer, for example, a measurement system using X-rays can be utilized. Specifically, a tomographic image of the negative electrode composite material layer can be obtained using X-ray laminography, and the porosity distribution in the depth direction of the obtained tomographic image can be obtained by image analysis of the tomographic image. As the specific method of image analysis, the method described in the examples below can be adopted.

[0019] <Negative electrode active material> In the present disclosure, the negative electrode active material is not particularly limited, and conventionally known materials can be appropriately used. Examples of the negative electrode active material include carbon materials. Examples of the carbon materials include cokes such as petroleum coke, pitch coke, and coal coke; carbon blacks such as carbides of organic compounds, carbon fibers, and acetylene black; and graphites such as artificial graphite and natural graphite. In addition to these, as the negative electrode active material, conductive polymers, lithium titanate, silicon, silicon compounds, etc. can also be used. As the negative electrode active material, the above-described materials may be used alone or in combination of a plurality of the above-described materials.

[0020] In particular, as the negative electrode active material, it is preferable to use graphite such as artificial graphite and natural graphite. Graphite such as artificial graphite and natural graphite is a material that is easily crushed or broken by pressure or frictional force, and thus can be said to be a particularly suitable material in the method for manufacturing the negative electrode of the present disclosure.

[0021] Further, the average particle diameter of the negative electrode active material is not particularly limited, and for example, it can be 1 μm to 100 μm, it can be 5 μm to 80 μm, it can be 10 μm to 50 μm, it can be 10 μm to 30 μm, it can be 10 μm to 25 μm, and it can be 10 μm to 20 μm. In particular, in the method for manufacturing a negative electrode of the present disclosure, the surface layer containing the negative electrode active material crushed or pulverized by pressure or frictional force is removed. From this viewpoint, the negative electrode active material is preferably graphite (either one or both of artificial graphite and natural graphite) having an average particle diameter of 5 μm to 25 μm, more preferably graphite (either one or both of artificial graphite and natural graphite) having an average particle diameter of 10 μm to 25 μm, and still more preferably graphite (either one or both of artificial graphite and natural graphite) having an average particle diameter of 10 μm to 20 μm. The average particle diameter is a value measured by a conventional method using a laser diffraction particle size distribution measuring device.

[0022] <Negative electrode slurry> The negative electrode slurry according to the present disclosure contains the negative electrode active material. Further, the negative electrode slurry contains a binder, a solvent, and other components in addition to the negative electrode active material.

[0023] The binder is not particularly limited, and a binder conventionally used when producing a negative electrode slurry can be used. For example, examples of the binder include butadiene rubber (BR), butylene rubber (IIR), acrylate butadiene rubber (ABR), styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP), and the like.

[0024] The solvent is not particularly limited, and a solvent conventionally used when producing a negative electrode slurry can be used. Although not particularly limited as the solvent, for example, 1, 2, 3, 4-tetrahydronaphthalene, butyl acetate, butyl butyrate, mesitylene, tetralin, heptane, and N-methyl-2-pyrrolidone (NMP) can be mentioned.

[0025] Examples of other components include thickeners and conductive aids. Examples of thickeners include carboxymethyl cellulose, sodium salts of carboxymethyl cellulose, and the like. Examples of conductive aids include carbon black (such as acetylene black, thermal black, furnace black, etc.), oxides exhibiting conductivity, nitrides exhibiting conductivity, and the like.

[0026] The negative electrode slurry can be prepared, for example, by stirring and kneading components such as the above-described negative electrode active material using a stirrer, ball mill, super sand mill, pressure kneader, or the like, and further adjusting the viscosity as necessary.

[0027] <Negative electrode current collector> The negative electrode current collector is not particularly limited, and a negative electrode current collector conventionally used when manufacturing a negative electrode can be used. The material of the negative electrode current collector is not particularly limited, and examples thereof include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, and stainless steel. The thickness of the negative electrode current collector is not particularly limited, and can be, for example, in the range of 0.1 μm to 1 mm. Further, as the negative electrode current collector, a strip-shaped one such as a foil shape, a perforated foil shape, or a mesh shape can be used.

[0028] Hereinafter, an example of the method for manufacturing the negative electrode of the present disclosure will be described with reference to the drawings. <Coating step> In the coating step, a negative electrode slurry containing a negative electrode active material is coated on the surface of the negative electrode current collector. The method for coating the negative electrode slurry on the negative electrode current collector is not particularly limited. As one embodiment, a roll coating method capable of coating the negative electrode slurry on a strip-shaped negative electrode current collector can be mentioned. In the roll coating method, the negative electrode slurry can be coated on the negative electrode current collector with a predetermined film thickness using a roll coater device combining a plurality of rolls. An example of the roll coater device is shown in FIG. 1.

[0029] The roll coater device 1 shown in Fig. 1 includes a coating section 40 having a first roll 10, a second roll 20, and a third roll 30, and a surface layer removing section 60 having a brush roll 50 and a support roll 51 arranged downstream of the coating section 40 in the running direction of the negative electrode current collector 110. Further, the roll coater device 1 includes a partition section 90 that fills the negative electrode slurry 130 and supplies the negative electrode slurry 130 between the first roll 10 and the second roll 20. Although not shown, the roll coater device 1 includes a rotational drive device connected to the first roll 10, the second roll 20, the third roll 30, and the brush roll 50 respectively, a control device that controls the operation of these rotational drive devices, a supply device that supplies the belt-shaped negative electrode current collector 110, and the like.

[0030] In the roll coater device 1, the first roll 10, the second roll 20, the third roll 30, and the brush roll 50 are arranged such that their axial directions are parallel. In the roll coater device 1, the first roll 10 and the second roll 20 are arranged so as to be closest to each other with a distance GA at the first facing position A. In the roll coater device 1, the second roll 20 and the third roll 30 are arranged so as to be closest to each other with a distance GB at the second facing position B. The brush roll 50 faces the support roll 51 with a distance GC at the third facing position C. The partition section 90 is placed on the upper parts of the first roll 10 and the second roll 20.

[0031] The roll coater device 1 configured as described above fills the space partitioned by the partition portion 90 with the negative electrode slurry 130, and winds the belt-shaped negative electrode current collector 110 around the peripheral surface 31 passing through the second opposing position B on the third roll 30. In order to apply the negative electrode slurry to the negative electrode current collector 110 in the coating section 40, in this state, the first roll 10, the second roll 20, and the third roll 30 are rotated about the axis in the direction indicated by the arrow in FIG. 1. That is, the first roll 10 and the third roll 30 are rotated clockwise, and the second roll 20 is rotated counterclockwise. When the first roll 10 and the second roll rotate in this way, the negative electrode slurry 130 filled in the partition portion 90 is supplied from the inside of the partition portion 90 to the gap GA at the first opposing position A where the peripheral surface 11 of the first roll 10 and the peripheral surface 21 of the second roll 20 face each other.

[0032] At this time, the negative electrode slurry supplied to the gap GA is rolled while being pressed between the peripheral surface 11 of the first roll 10 and the peripheral surface 21 of the second roll 20, and the negative electrode active materials contained in the negative electrode slurry are bound by the action of the binder. In the roll coater device 1, by making the rotation speed of the second roll 20 faster than the rotation speed of the first roll 10, the rolled negative electrode slurry 130 supplied to the gap GA adheres to the peripheral surface 21 of the second roll 20 as the film-forming sheet 131. As an example, by setting the ratio of the rotation speed of the first roll 10 to the rotation speed of the second roll 20 to, for example, 1:1.25 to 1:2.5 (rotation speed of the first roll 10:rotation speed of the second roll 20), the film-forming sheet 131 can be formed on the peripheral surface 21 of the second roll 20.

[0033] As described above, in the roll coater device 1, the film-forming sheet 131 made of the negative electrode slurry 130 and the negative electrode current collector 110 are sandwiched between the second roll 20 and the third roll 30 (between a pair of rolls), and the second roll 20 and the third roll 30 are rotationally driven in opposite directions about the axis, so that the film-forming sheet 131 is transferred onto the surface of the negative electrode current collector 110, and the negative electrode slurry 130 can be applied. That is, according to the roll coater device 1, in the coating section 40, the negative electrode composite material layer 120 can be formed on one main surface of the negative electrode current collector 110.

[0034] <Surface removal process> In the surface removal process, the surface layer of the negative electrode composite material layer coated on the surface of the negative electrode current collector is removed. In the roll coater device 1, subsequently to the above, the negative electrode current collector 110 having the negative electrode composite material layer 120 formed on one main surface is conveyed to the surface removal unit 60, and the surface layer of the negative electrode composite material layer 120 is removed. By removing the surface layer, the crushed active material particles present on the surface can be removed. In the surface removal unit 60 of the roll coater device 1, the surface layer of the negative electrode composite material layer 120 can be removed by bringing the brush roll 50 into contact with the surface of the negative electrode composite material layer 120. In the roll coater device 1, the distance GC between the brush roll 50 and the support roll 51 is adjusted so that the surface layer of the negative electrode composite material layer 120 can be removed with a predetermined thickness.

[0035] The thickness of the surface layer to be removed from the negative electrode composite material layer 120 is not particularly limited, but it can be set to a thickness corresponding to 1 to 5 grains of the negative electrode active material constituting the negative electrode composite material layer 120, preferably a thickness corresponding to 1 to 4 grains of the negative electrode active material, more preferably a thickness corresponding to 1 to 3 grains of the negative electrode active material, and particularly preferably a thickness corresponding to 1 to 2 grains of the negative electrode active material. From this perspective, the thickness of the surface layer to be removed from the negative electrode composite material layer 120 can be, for example, 10 μm to 50 μm, preferably 15 μm to 40 μm, and more preferably 20 μm to 30 μm.

[0036] <Other processes> Before or after removing the surface layer of the negative electrode composite material layer 120, a process (drying process) for drying the negative electrode composite material layer 120 can be provided as necessary. From the perspective of workability, after removing the surface layer of the negative electrode composite material layer 120 (after the surface removal process), it is preferable to manufacture the negative electrode through a process (drying process) of drying the negative electrode composite material layer 120. To dry the negative electrode composite material layer 120, for example, a method of heat treatment at a temperature of 60°C to 200°C for 10 minutes to 24 hours can be mentioned. Note that when the amount of the solvent contained in the negative electrode slurry is sufficiently low, there is no need to perform the drying process.

[0037] Further, if necessary, a negative electrode having a desired shape can be manufactured by undergoing a cutting process.

[0038] <Other Embodiments> Note that the method of applying the negative electrode slurry to the negative electrode current collector is not limited to the roll coating method described above. As other embodiments, known methods such as the metal mask printing method, electrostatic coating method, dip coating method, spray coating method, doctor blade method, gravure coating method, and screen printing method may be applied. After applying the negative electrode slurry to the negative electrode current collector by these methods, it is preferable to perform a rolling process using a flat press, calender roll, etc. as necessary. After applying the negative electrode slurry to the negative electrode current collector by these methods, or after performing the rolling process described above, the surface layer of the negative electrode composite material layer is removed.

[0039] The method of removing the surface of the negative electrode composite material layer containing the negative electrode slurry applied on the negative electrode current collector is not limited to the form of using a brush roll as described above. For example, after attaching an adhesive sheet to the surface of the negative electrode composite material layer, the surface of the negative electrode composite material layer can be removed by peeling off the adhesive sheet. Also, after applying the negative electrode slurry to the negative electrode current collector, the surface of the negative electrode composite material layer can be removed by irradiating with a laser. Furthermore, after applying the negative electrode slurry to the negative electrode current collector, the surface of the negative electrode composite material layer may be removed by polishing.

[0040] Also, the step of removing the surface of the negative electrode composite material layer may be before or after drying the negative electrode slurry applied on the negative electrode current collector. However, from the viewpoint of ease of processing, it is preferable to remove the surface of the negative electrode composite material layer before drying the negative electrode slurry applied on the negative electrode current collector.

[0041] <Secondary Battery> The method for manufacturing the negative electrode of the present disclosure can be applied to the manufacture of secondary batteries. The secondary battery can include a negative electrode manufactured as described above, a positive electrode, and an electrolyte layer disposed between the negative electrode and the positive electrode. In the secondary battery of the present disclosure, the positive electrode and the electrolyte layer are not particularly limited, and conventionally known positive electrodes and electrolyte layers can be applied. Further, in the secondary battery of the present disclosure, the electrolyte layer may contain a liquid electrolyte without containing a solid electrolyte, or may contain a solid electrolyte without containing a liquid electrolyte, or may contain both a liquid electrolyte and a solid electrolyte. When the electrolyte layer contains a liquid electrolyte, it is preferable that the electrolyte layer has a separator for holding the liquid electrolyte and preventing contact between the positive electrode and the negative electrode. Further, when the electrolyte layer contains a solid electrolyte, the electrolyte layer may optionally contain a binder or the like in addition to the solid electrolyte.

[0042] Note that the positive electrode can be obtained by forming a positive electrode material layer on the surface of a current collector in the same manner as the negative electrode described above. As the liquid electrolyte, for example, a so-called organic electrolyte solution in which a lithium salt such as LiClO4, LiPF6, LiAsF6, LiBF4, LiSO3CF3, etc., which are electrolytes, is dissolved in a non-aqueous solvent such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, cyclopentanone, sulfolane, 3-methylsulfolane, 2,4-dimethylsulfolane, 3-methyl-1,3-oxazolidin-2-one, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, butyl methyl carbonate, ethyl propyl carbonate, butyl ethyl carbonate, dipropyl carbonate, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, methyl acetate, ethyl acetate, etc., alone or as a mixture of two or more components can be used.

[0043] The structure of the secondary battery of the present disclosure is not particularly limited, and typically, a positive electrode, a negative electrode, and an optional separator are wound into a flat spiral to form a wound electrode assembly, or these are stacked as flat plates to form a stacked electrode assembly, and the electrode assembly is then sealed in an exterior case. The secondary battery of the present disclosure is not particularly limited, and is used as a paper battery, a button battery, a coin battery, a stacked battery, a cylindrical battery, a prismatic battery, etc. [Example]

[0044] The present disclosure will be described in more detail below using examples, but the technical scope of the present disclosure is not limited to the following examples.

[0045] [Example 1] <Preparation of negative electrode> In this example, artificial graphite (average particle size: 15.0 μm, tap density: 1.09 g / cm ) was used as the negative electrode active material. 3 ) was used. The negative electrode slurry was prepared by mixing the artificial graphite, carbon nanotubes, styrene butadiene rubber, and sodium carboxymethyl cellulose in this order in a mass ratio of 96.95:0.05:2.4:0.6. The solid content (NV) of the wet granules in the prepared negative electrode slurry was 75%.

[0046] In this example, a negative electrode was fabricated using a roll coater 1 shown in FIG. 1. Copper foil was used as the negative electrode current collector, and the above-described negative electrode slurry was applied to one main surface of the copper foil. In this example, the gap GA between the first roll 10 and the second roll 20 was set to 210 μm, and the gap GB between the second roll 20 and the third roll 30 was set to 180 μm. The rotation speeds of the first roll 10, the second roll, and the third roll were set to 1.4 rpm, 2.0 rpm, and 2.6 rpm, respectively.

[0047] In particular, in this embodiment, after applying the negative electrode slurry to one main surface of the negative electrode current collector, a portion having a depth of about 20 μm from the surface layer was removed. Specifically, in this embodiment, in the aspect of removing the crushed particles on the surface layer by the surface layer removal portion 60, instead, the copper foil was cut out while the negative electrode slurry was in an undried state, and the surface was rubbed using a waste on the workbench to remove the particles on the crushed surface layer.

[0048] <Fabrication of Test Cell> In this embodiment, a test cell was fabricated using the negative electrode fabricated as described above. That is, the fabricated negative electrode was adjusted in film thickness so as to have a composite material density of 1.4 g / cm 3 using a roll press. The counter positive electrode was prepared by mixing a paste of an active material obtained by mixing single particles and secondary particles of a ternary system at a ratio of 1:1, acetylene black, and polyvinylidene fluoride (PVdF), and applying the paste onto an aluminum foil so as to have a basis weight of 34.0 mg / cm 2 and drying it, and then adjusting the film thickness so as to have a composite material density of 3.3 g / cm 3 using a roll press. The fabricated positive electrode and negative electrode were cut out into rectangles, laminated with a separator interposed therebetween so that their composite material layers faced each other, and housed together with an electrolytic solution in a laminate film (outer packaging member).

[0049] <Evaluation> Regarding the negative electrode fabricated in this embodiment, the cross-section was observed and the porosity distribution in the depth direction was measured. In addition, the battery performance of the test cell using the fabricated negative electrode was evaluated. Specifically, it was carried out as follows. <Cross-Section Observation> A cross-section was generated by bending a sample in which a positive electrode, a separator, and a negative electrode were laminated, and a cross-sectional SEM image was obtained for the generated cross-section using a scanning electron microscope (SEM). <Measurement of Porosity Distribution> Tomographic images were obtained using X-ray laminography (tilt angle of the rotation axis: 30 degrees, exposure time: 0.1 seconds). Image analysis was performed on the obtained tomographic images as follows. First, in the image analysis, the cumulative relative frequency (the value obtained by accumulating the number of pixels from luminance value 0 / total number of pixels) was derived from the luminance value histogram of all the obtained tomographic images. Then, the luminance value at which the cumulative relative frequency was closest to the average porosity obtained by the following calculation was determined as the threshold for binarization. Next, all the tomographic images were binarized, and the porosity in each image was calculated. Average porosity = (1 - density of composite material / true density of composite material) Note that the density of the composite material was calculated from the basis weight (weight) and the film thickness measurement results of the above sample. The true density of the composite material was calculated from the mixing ratio and the true density of each material in the composite material. <Battery performance evaluation (resistance measurement)> Using the charge-discharge device TOSCAT manufactured by Toyo System Co., Ltd., after adjusting the SOC to a charged state of SOC 50%, the charging resistance 10 seconds after charging at 1C was measured.

[0050] [Comparative Example 1] In Comparative Example 1, the negative electrode was fabricated in the same manner as in Example 1 except that the surface layer of the negative electrode composite layer was not removed. Also, in Comparative Example 1, a test cell was fabricated in the same manner as in Example 1 using the fabricated negative electrode. For Comparative Example 1 as well, in the same manner as in Example 1, the cross-section of the fabricated negative electrode was observed, the porosity distribution in the depth direction was measured, and the battery performance of the test cell was evaluated.

[0051] [Results] The results of observing the cross-sections of the negative electrode fabricated in Example 1 and the negative electrode fabricated in Comparative Example 1 are shown in FIGS. 2A and 2B. FIG. 2A is a photograph of the cross-section of the negative electrode fabricated in Example 1, and FIG. 2B is a photograph of the cross-section of the negative electrode fabricated in Comparative Example 1. Also, the results of measuring the porosity distribution in the depth direction of the negative electrode fabricated in Example 1 and the negative electrode fabricated in Comparative Example 1 are shown in FIG. 3.

[0052] As shown in FIGS. 2A and 2B and FIG. 3, the negative electrode produced in Comparative Example 1 had a low porosity in the surface layer due to the crushing of the negative electrode active material in the surface layer, whereas the negative electrode produced in Example 1 had a low porosity in the surface layer due to the non-crushed negative electrode active material in the surface layer, and the porosity in the surface layer did not decrease significantly.

[0053] The resistance values of the fabricated test cells were measured and the results are shown in Table 1. [Table 1]

[0054] As shown in Table 1, the test cell of Example 1 had a lower resistance value than the test cell of Comparative Example 1, and was found to exhibit excellent battery performance. [Explanation of symbols]

[0055] REFERENCE SIGNS LIST 1... roll coater device, 10... first roll, 20... second roll, 30... third roll, 40... coating section, 50... brush roll, 60... surface layer removal section, 110... negative electrode current collector, 120... negative electrode composite layer, 130... negative electrode slurry

Claims

1. A step of coating a negative electrode slurry containing a negative electrode active material on the surface of a negative electrode current collector; A step of removing the surface layer of a negative electrode composite layer containing the negative electrode slurry coated on the surface of the negative electrode current collector; A method for manufacturing a negative electrode, comprising the above steps.

2. The method for manufacturing a negative electrode according to claim 1, wherein the negative electrode slurry and the negative electrode current collector are sandwiched between a pair of rolls, and the pair of rolls are rotationally driven in opposite directions around an axis to coat the negative electrode slurry on the surface of the negative electrode current collector.

3. The method for manufacturing a negative electrode according to claim 1, wherein a brush roll is brought into contact with the surface of the negative electrode composite layer to remove the surface layer of the negative electrode composite layer.

4. The method for manufacturing a negative electrode according to claim 1, wherein a depth of 20 μm to 30 μm is removed from the surface as the surface layer of the negative electrode composite layer.

5. The method for manufacturing a negative electrode according to claim 1, wherein the negative electrode active material is graphite having an average particle diameter of 5 μm to 25 μm.

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