Method for manufacturing electrode sheet

Abstract

A method for manufacturing an electrode sheet having an electrode layer on the second surface of a collector foil includes a roll pressing step. The uncompressed electrode sheet comprises a collector foil and an uncompressed electrode layer formed on the collector foil and composed of a stack of composite particles comprising active material particles bonded to a plurality of binder particles. In the roll pressing step, the uncompressed electrode sheet, at room temperature, is passed through a gap between a first roller and a second roller, compressed in the thickness direction, and heated in the gap to bond the active material particles to each other and to the collector foil using a binder resin. The roll pressing step is performed under conditions where the temperature of a first peripheral surface of the first roller in contact with the collector foil is within a temperature range of +5°C to +25°C, the melting start temperature of the binder resin, and the temperature of a second peripheral surface of the second roller in contact with the outer surface of the electrode layer of the uncompressed electrode layer is lower than the temperature of the first peripheral surface and within a temperature range of +5°C or less, the melting start temperature.

Description

Technical Field

[0001] The invention relates to a method for manufacturing an electrode sheet. Background Art

[0002] Conventionally, electrode sheets having an electrode composite material layer provided on one or both surfaces of a collector foil are known. Patent Document 1 (see the claims of Patent Document 1, etc.) is a prior art method for manufacturing such an electrode sheet.

[0003] In patent document 1, first, a mixed powder consisting of composite particles containing no solvent is prepared, an active material powder and a binder powder are mixed, and a plurality of binder particles are bound to the surface of the active material particles. Then, an uncompressed electrode sheet having an uncompressed electrode layer is formed, and the uncompressed electrode layer is formed by utilizing electrostatic force to make the mixed powder fly and accumulate on the surface of the collector foil. Then, in a rolling process, the uncompressed electrode sheet is passed through the gap between a pair of opposing and rotating hot rollers (the first roller and the second roller) (for rolling). Thus, while the binder resin contained in the electrode layer is softened or melted, the electrode layer and the collector foil are compressed, thereby manufacturing an electrode sheet in which the electrode layer having active material particles and binder resin is bonded to the surface of the collector foil.

[0004] Patent Document 1: Japanese Patent Application Laid-Open No. 2020-68113 Summary of the Invention

[0005] However, sometimes a portion of the electrode layer peels off, leaving the binder resin and active material particles attached to the roller surface, or sometimes a portion of the electrode layer peels off from the collector foil and floats up, even though the binder resin and active material particles are not attached to the roller surface. The present invention was developed in response to this problem and aims to provide an improved method for manufacturing an electrode sheet.

[0006] (1) One form of the present invention for solving the above-mentioned problems is a method for manufacturing an electrode sheet, wherein the electrode sheet has an electrode layer on the above-mentioned second surface of a collector foil having a first surface and a second surface on the side opposite to the above-mentioned first surface, and the manufacturing method includes a rolling process, in which the following operation is performed on an uncompressed electrode sheet whose sheet temperature is room temperature, wherein the above-mentioned uncompressed electrode sheet has the above-mentioned collector foil and the uncompressed electrode layer, the above-mentioned uncompressed electrode layer is formed by stacking composite particles, and the composite particles are formed by combining active material particles with a plurality of adhesive particles smaller in diameter than the above-mentioned active material particles and composed of an adhesive resin, and the above-mentioned operation refers to causing the above-mentioned uncompressed electrode sheet to be rolled from a first roller having a first outer peripheral surface in contact with the above-mentioned first surface of the collector foil and a roller having an outer surface of the electrode layer in contact with the above-mentioned uncompressed electrode layer. The uncompressed electrode sheet passes through the roller gap of the second roller on the second peripheral surface, is compressed in the thickness direction, and is heated in the above-mentioned roller gap, and the above-mentioned active material particles are bonded to each other by the above-mentioned binder resin, and the above-mentioned active material particles are bonded to the above-mentioned second surface of the collector foil, and the above-mentioned rolling process is carried out under the following conditions: the first peripheral surface temperature (TR1) of the above-mentioned first peripheral surface of the above-mentioned first roller is set to a temperature range of the melting start temperature (Ti) of the above-mentioned binder resin + 5°C to the above-mentioned melting start temperature (Ti) + 25°C (Ti+5°C≤TR1≤Ti+25°C); the second peripheral surface temperature (TR2) of the above-mentioned second peripheral surface of the above-mentioned second roller is set to a temperature lower than the above-mentioned first peripheral surface temperature (TR1) and within the temperature range of the above-mentioned melting start temperature (Ti) + 5°C or less than the above-mentioned binder resin (TR2<TR1, TR2≤Ti+5°C).

[0007] The binder resin of the present application has the following characteristics: The temperature characteristics of the loss tangent (tan δ) obtained by dynamic viscoelasticity measurement show that the loss tangent (tan δ) that increases linearly and slowly with increasing temperature in the temperature range below the melting point (Tm) of the binder resin changes to a characteristic of rapidly increasing linearly as the temperature approaches the melting point (Tm). The temperature at which this rapid increase begins is referred to as the "melting onset temperature" (Ti) of the binder resin.

[0008] That is, for the binder resin, the relationship between temperature and dynamic viscoelasticity is measured within a temperature range from a temperature exceeding the glass transition point in the rubbery state to a temperature exceeding the melting point (Tm) (for example, in the range of Tm-50°C to Tm+20°C), and the temperature characteristics of the loss modulus (G"), storage modulus (G'), and their ratio, namely the loss tangent (tanδ(=G" / G')) are obtained. Therefore, in the temperature range closer to the melting point (Tm) than the glass transition point, the loss modulus (G") and storage modulus (G') of the binder resin gradually decrease as the temperature rises. However, when the temperature approaches the melting point (Tm) even further, the loss modulus (G") and storage modulus (G') begin to decrease sharply as the temperature rises. However, when the temperature reaches the melting point (Tm) and the binder resin becomes molten, the rapid decrease in the loss modulus (G") and the storage modulus (G') becomes convergent, and the loss modulus (G") and the storage modulus (G') gradually decrease as the temperature rises. Since the loss modulus (G") and the storage modulus (G') change in this way, the loss tangent (tanδ (=G" / G')) of the binder resin, which is their ratio, gradually and roughly linearly increases with increasing temperature in a temperature range closer to the melting point (Tm) than the glass transition point. As the temperature rises, both the loss modulus (G") and the storage modulus (G') decrease. It is speculated that this is because the higher the temperature, the greater the loss ratio of the vibration energy applied to soften the binder resin. However, when the temperature approaches the melting point (Tm) further, a region appears in which the loss tangent (tanδ) rises sharply (in the form of a ramp function) as the temperature rises. This is presumably because the loss modulus (G") and storage modulus (G') decrease sharply as part of the binder resin begins to melt, but the rate of decrease in the storage modulus (G') is large, and the loss tangent (tan δ), or loss, increases significantly with increasing temperature. However, when the temperature reaches the melting point (Tm) and the binder resin becomes molten, the loss tangent (tan δ) stabilizes at a high and approximately constant value. Therefore, the temperature at which the binder resin begins to melt and the loss tangent (tan δ) begins to increase sharply with increasing temperature is referred to as the "melting start temperature" (Ti).

[0009] In addition, the resin that can obtain the temperature characteristics of the above-mentioned loss tangent (tanδ) is a crystalline resin having a glass transition point and a melting point. For example, fluororesins such as PVDF (polyvinylidene chloride) and PTFE (polytetrafluoroethylene), PE (polyethylene), PP (polypropylene), PA (polyamide), POM (polyacetal, polyoxymethylene), PET (polyethylene terephthalate), PBT (polybutylene terephthalate), PPS (polyphenylene sulfide), PEEK (polyetheretherketone), etc. can be listed.

[0010] In the above-described manufacturing method, during the rolling step, rolling is performed under the condition that the first peripheral surface temperature (TR1) of the first peripheral surface of the first roller, whose first peripheral surface contacts the first surface of the collector foil, is at least 5°C above the melting start temperature (Ti) of the binder resin (Ti + 5°C ≤ TR1). Consequently, the temperature (TC) of the collector foil in contact with the first peripheral surface of the first roller is also substantially at least 5°C above the melting start temperature (Ti) of the binder resin (Ti + 5°C ≤ TC ≈ TR1). Consequently, in the uncompressed electrode layer, the binder particles of the composite particles located near the second surface of the collector foil melt, allowing the active material particles to reliably adhere to the second surface of the collector foil via the melted binder resin.

[0011] Furthermore, the temperature (TR1) of the first outer peripheral surface of the first roller is a temperature equal to or lower than the melting start temperature (Ti) + 25°C (TR1 ≤ Ti + 25°C). Therefore, the temperature (TC) of the collector foil in contact with the first outer peripheral surface of the first roller is also approximately equal to or lower than the melting start temperature (Ti) of the binder resin + 25°C (TC ≈ TR1 ≤ Ti + 25°C). Therefore, in the uncompressed electrode layer, the viscosity of the binder resin formed by melting the binder particles of the composite particles located near the second surface of the collector foil does not decrease, thereby preventing the adhesive force from being excessively reduced. This prevents the binder resin and active material particles temporarily attached to the second surface of the collector foil, that is, the electrode layer during compression, from peeling off from the second surface of the collector foil and attaching to the second outer peripheral surface of the second roller.

[0012] Furthermore, in the above-described manufacturing method, during the rolling step, rolling is performed so that the second outer peripheral surface temperature (TR2) of the second outer peripheral surface of the second roller, which contacts the outer surface of the uncompressed electrode layer, is lower than the first outer peripheral surface temperature (TR1) of the first outer peripheral surface of the first roller, which contacts the first surface of the collector foil. Therefore, the temperature of the portion of the uncompressed electrode layer on the outer surface side of the electrode layer is lower than that on the collector foil side, and the binder resin is less likely to melt. Therefore, the uncompressed electrode layer is less likely to adhere to the second roller and peel off.

[0013] Furthermore, the roll pressing is performed under the condition that the second outer peripheral surface temperature TR2 is set within a temperature range of the melting start temperature (Ti) of the binder resin + 5° C. or less (TR2 < TR1, TR2 ≤ Ti + 5° C.).

[0014] As a result, the temperature (TE) of the outer surface of the uncompressed electrode layer in contact with the second outer peripheral surface of the second roller is also a temperature below the binder resin's melting start temperature (Ti) + 5°C (TE ≈ TR2 ≤ Ti + 5°C). Furthermore, since the uncompressed electrode sheet, which has not been preheated and has a room temperature, is passed through the roller gap and compressed in the thickness direction, and the uncompressed electrode sheet is heated in the roller gap, the heating period is short. Therefore, the temperature (TE) of the outer surface of the uncompressed electrode layer in contact with the second outer peripheral surface of the second roller does not reach the binder resin's melting start temperature (Ti) (TE < Ti), or even if it does, the temperature is extremely short, and at most TE ≤ Ti + 5°C. Therefore, the binder resin is not fully melted on the outer surface of the electrode layer. This prevents the binder particles of the composite particles located near the outer surface of the uncompressed electrode layer from melting, causing the melted binder resin to adhere to the second outer peripheral surface of the second roller, or the active material particles from adhering to the second outer peripheral surface of the second roller via the binder resin.

[0015] In addition, "normal temperature" means a temperature within a temperature range of +5°C to +35°C.

[0016] (2) The method for manufacturing an electrode sheet according to (1) is preferably: the rolling process is carried out under the condition that the second peripheral surface temperature (TR2) is set to be at least 10°C lower than the first peripheral surface temperature (TR1) and within a temperature range below the melting start temperature (Ti) of the adhesive resin + 5°C (TR2≤TR1-10°C, TR2≤Ti+5°C).

[0017] In this manufacturing method, the roll pressing step is performed while the second outer peripheral surface temperature (TR2) is set to be at least 10°C lower than the first outer peripheral surface temperature (TR1) and within a temperature range of the binder resin's melting start temperature (Ti) + 5°C or less. By setting the second outer peripheral surface temperature (TR2) sufficiently lower than the first outer peripheral surface temperature (TR1), the binder resin and active material particles forming the electrode layer can be reliably bonded to the collector foil, while reliably preventing adhesion to the second outer peripheral surface of the second roller.

[0018] (3) In the method for producing an electrode sheet according to (1) or (2), preferably, the active material particles are graphite particles, and the binder resin is PVDF.

[0019] In this production method, the active material particles used in the electrode layer are graphite particles and the binder resin is PVDF, so the electrode sheet can be appropriately produced. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a top view of the negative electrode sheet according to the embodiment.

[0021] Figure 2 This is a cross-sectional view of a negative electrode sheet according to an embodiment.

[0022] Figure 3 This is a graph showing the temperature characteristics of the dynamic viscoelasticity (storage modulus, loss tangent), as well as the melting start temperature and melting point of the binder resin.

[0023] Figure 4 This is a flowchart showing the process of manufacturing the negative electrode sheet according to the embodiment.

[0024] Figure 5 It is an explanatory diagram showing the production of the negative electrode sheet according to the embodiment.

[0025] Figure 6 It is an explanatory diagram showing how the negative electrode sheet is compressed in the roll pressing step according to the embodiment.

[0026] Figure 7 This is a graph showing the relationship between the first outer peripheral surface temperature and the second outer peripheral surface temperature in the roll pressing step, and the quality of the negative electrode sheet of each investigation example.

[0027] Description of Reference Numerals

[0028] 1…Negative electrode sheet (electrode sheet); 2…Collector foil; 2A…First surface (of collector foil); 2B…Second surface (of collector foil); 3…Negative electrode layer (electrode layer); 3U…Outer surface of the negative electrode layer (of the negative electrode layer); 6…Active material particles; 7…Binder resin; 11…Uncompressed negative electrode sheet (uncompressed electrode sheet); 13…Uncompressed negative electrode layer (uncompressed electrode layer); 13U…Outer surface of the negative electrode layer (outer surface of the electrode layer) (of the uncompressed negative electrode layer); 15…Composite particles; 17…Binder particles; DL…Lengthwise direction (of the negative electrode sheet, uncompressed negative electrode sheet) ; 110…1st roller; 110G…1st peripheral surface (of the 1st roller); 120…2nd roller; 120G…2nd peripheral surface (of the 2nd roller); GP…roller gap; TR1…1st peripheral surface temperature; TR2…2nd peripheral surface temperature; T…temperature; Ti…melting start temperature (of the binder resin); Tm…melting point (of the binder resin); TS…sheet temperature (of the negative electrode sheet, uncompressed negative electrode sheet); TC…collector foil temperature (of the collector foil); TE…outer surface temperature (of the negative electrode layer, the outer surface of the negative electrode layer of the uncompressed negative electrode layer); S2…rolling process. DETAILED DESCRIPTION

[0029] (Implementation Method)

[0030] Below, while referring to Figures 1 to 6The accompanying drawings illustrate an embodiment of the present disclosure. This embodiment applies the present disclosure to the manufacture of a negative electrode sheet of a lithium-ion secondary battery. That is, in this embodiment, as a method for manufacturing an electrode sheet, a method for manufacturing a negative electrode sheet 1 is exemplified. In this embodiment, a strip-shaped negative electrode sheet 1 (an example of an electrode sheet of the present disclosure) having a strip-shaped collector foil 2 and a strip-shaped negative electrode layer 3 (an example of an electrode layer of the present disclosure) formed on a surface 2B on one side of the collector foil 2 is manufactured (refer to Figure 1 、 Figure 2 ).

[0031] First, the negative electrode sheet 1 of the present embodiment will be described. As described above, the negative electrode sheet 1 having a strip shape that is longer in the longitudinal direction DL includes a collector foil 2 having a strip shape that is longer in the longitudinal direction DL and a negative electrode layer 3 having a strip shape that is longer in the longitudinal direction DL. The collector foil 2 has a pair of surfaces, namely a first surface 2A and a second surface 2B. On the other hand, the negative electrode layer 3 is formed in the center of the width direction DW on the second surface 2B of the collector foil 2. Therefore, the negative electrode sheet 1 has an overlapping portion 1S located in the center of the width direction DW where the collector foil 2 and the negative electrode layer 3 overlap, and also has a collector portion 1C located on both sides of the width direction DW ( Figure 1 The upper and lower sides of the collector foil 2 are exposed portions 2E.

[0032] In the negative electrode sheet 1, the collector foil 2 is composed of a copper foil having a thickness of 8 μm. In addition, the negative electrode layer 3 is composed of active material particles 6 as graphite particles and a binder resin 7 that binds the active material particles 6 to each other and to the collector foil 2. Examples of graphite particles as active material particles 6 include spherical graphite and flaky graphite. In the present embodiment, spherical graphite is used. In addition, examples of the binder resin 7 include fluorine-based resins such as PTFE and PVDF. In the present embodiment, PVDF is used as the binder resin 7. The melting start temperature Ti of the binder resin 7 (PVDF) used in the present embodiment is 155°C (Ti=155°C) (see Figure 3 ).

[0033] In addition, the value of the melting start temperature Ti of the adhesive resin 7 (PVDF) is obtained as follows. That is, using the dynamic viscoelasticity measuring device DVA-220 manufactured by IT Measurement Control Co., Ltd., the dynamic viscoelasticity (storage modulus G', loss modulus G") and loss tangent tanδ (=G" / G') are measured under the condition that 0.05g of the adhesive resin 7 is deformed at 10Hz. And, as the temperature T increases, the adhesive resin begins to melt, and the storage modulus G' and loss modulus G" decrease sharply, while the loss tangent tanδ begins to increase sharply. The temperature T is taken as the "melting start temperature" Ti. Specifically, at Figure 3 In the temperature characteristics of the loss tangent tanδ of the binder resin 7 shown, the loss tangent tanδ, which increases linearly and gradually with increasing temperature T within a temperature range including the melting point Tm (164°C) of the binder resin 7 (e.g., a temperature range of 100 to 164°C), changes to a linear and abrupt increase with increasing temperature T as the temperature T approaches the melting point Tm. The temperature T at which this abrupt increase starts is referred to as the melting start temperature Ti.

[0034] More specifically, in Figure 3 In the graph, the loss tangent tanδ measurement points in the temperature range of 100-150°C, indicated by "●," yield a regression line LA showing that tanδ increases linearly and slightly with increasing temperature T. Furthermore, the regression line LB showing that tanδ increases linearly and sharply with increasing temperature T is derived from the measurement points in the temperature range of 156-164°C. The temperature T corresponding to these intersection points, CRP, is defined as the melting start temperature Ti. In the case of the adhesive 7 of this embodiment, the melting start temperature Ti is understood to be 155°C (Ti = 155°C).

[0035] The melting point Tm is the temperature at which the storage modulus G' and the loss modulus G" rapidly decrease with increasing temperature T and switch to a gradual decrease with increasing temperature. The melting point Tm is Tm = 164°C.

[0036] Next, a method for manufacturing the negative electrode sheet 1 of this embodiment will be described (see Figures 4 to 6 First, in a stacking step S1, an uncompressed negative electrode sheet 11 is formed. The uncompressed negative electrode sheet 11 includes composite particles 15 stacked on the second surface 2B of a strip-shaped current collector foil 2 having dimensions of 200 mm in length, 100 mm in width, and 8 μm in thickness, forming an uncompressed negative electrode layer 13 having a width of 55 mm and a thickness of 180 μm. This uncompressed negative electrode sheet 11 is compressed to form a negative electrode sheet 1.

[0037] The composite particles 15 used in the stacking step S1 are obtained by mixing powdered active material particles 6 and powdered binder particles 17 at a weight ratio of 97.5:2.5. More specifically, the active material particles 6 and binder particles 17 are mixed at the above ratio using a high-speed mixer (manufactured by EARTHTECHNICA), thereby producing composite particles 15 in which a plurality of binder particles 17 having a smaller diameter than the active material particles 6 are attached to the active material particles 6.

[0038] In the stacking step S1, the Figure 5The deposition device DP shown by the dotted line in the figure forms an uncompressed negative electrode layer 13 formed by the accumulation of composite particles 15 on the second surface 2B of the collector foil 2. Examples of the deposition method used by the deposition device DP include a method in which the composite particles 15 are caused to fly toward the collector foil 2 by electrostatic force, and the composite particles 15 are accumulated on the collector foil 2 to form the uncompressed negative electrode layer 13.

[0039] More specifically, the following method can be used: the composite particles 15 are mixed with magnetic carrier particles (not shown) to obtain composite carrier particles (not shown) in which the composite particles 15 are electrostatically adsorbed to the magnetic carrier particles. The obtained composite carrier particles are magnetically adsorbed to the surface of a magnetic roller (not shown) and moved toward a film forming area (not shown). A DC voltage is applied between a support roller (not shown) around which the collector foil 2 is wound and the magnetic roller, thereby applying an electrostatic force Fs to the composite particles 15 adsorbed to the magnetic carrier particles, causing the composite particles 15 to fly from the magnetic roller toward the collector foil 2 and accumulate.

[0040] Next, in the rolling step S2, the uncompressed negative electrode sheet 11 is compressed in the thickness direction DT using the sheet compression device 100 to form the negative electrode sheet 1 (see Figure 5 、 Figure 6 ). First, the sheet compression device 100 is described. The sheet compression device 100 includes a first roller 110 and a second roller 120. The second roller 120 is configured such that its axis 120AX is parallel to the axis 110AX of the first roller 110, and the first roller 110 and the second roller 120 are opposed to each other with a roller gap GP therebetween. Figure 5 As indicated by the middle arrow, the uncompressed negative electrode sheet 11 rotates in the forward direction (clockwise for the first roller 110 and counterclockwise for the second roller 120 ), and the uncompressed negative electrode sheet 11 sandwiched in the roller gap GP is compressed in the thickness direction DT and conveyed in the conveyance direction DH.

[0041] In the present embodiment, a SA602 small table type roller press manufactured by TESTER SANGYO Co., Ltd. of Japan is used, and the first roller 110 and the second roller 120 are both Φ100×165 mm.

[0042] Among them, Figure 5 、 Figure 6 The temperature of the first outer peripheral surface 110G of the first roller 110 located at the bottom, that is, the first outer peripheral surface temperature TR1 is TR1 = 170°C. Figure 5 、 Figure 6The temperature of the second outer peripheral surface 120G of the second roller 120 located above the first roller 110, i.e., the second outer peripheral surface temperature TR2, was TR2 = 140°C. Furthermore, a sensor probe for measuring surface temperature (sensor probe IK-500, manufactured by CUSTOM) was attached to a digital thermometer (digital thermometer CT-1310, manufactured by CUSTOM), and the tip of the sensor probe was brought into contact with the outer peripheral surfaces 110G and 120G of each roller to measure the first outer peripheral surface temperature TR1 and the second outer peripheral surface temperature TR2.

[0043] Furthermore, in this embodiment, prior to rolling in the roller gap GP, the uncompressed negative electrode sheet 11 is contacted with one of the rollers at a large wrap angle exceeding 30 degrees, as described in Patent Document 1, and preheating is not performed before pressing. Specifically, the sheet temperature TS of the uncompressed negative electrode sheet 11 before passing through the roller gap GP is room temperature, i.e., within the range of 5 to 35°C. Without preheating, the uncompressed negative electrode sheet 11 having room temperature TS passes through the roller gap GP between the first roller 110 and the second roller 120. However, in roller pressing, the uncompressed negative electrode sheet 11 can be passed through the roller gap GP at a wrap angle of 10 degrees or less relative to the first roller 110 or the second roller 120.

[0044] More specifically, if Figure 5 、 Figure 6 As shown, the uncompressed negative electrode sheet 11 is thrown toward the roller gap GP in such a manner that the collector foil 2 is located at the lower side in the figure and the uncompressed negative electrode layer 13 is located at the upper side, so that the first outer peripheral surface 110G of the first roller 110 contacts the first surface 2A of the collector foil 2, and the second outer peripheral surface 120G of the second roller 120 contacts the negative electrode layer outer surface 13U of the uncompressed negative electrode layer 13.

[0045] Furthermore, the roller gap GP is set to a line pressure of 600 kN / m, and the first roller 110 and the second roller 120 are rotated at a circumferential speed of 1 m / min to perform roller pressing, while the uncompressed negative electrode sheet 11 is conveyed at a speed of 1 m / min. In this manner, the uncompressed negative electrode sheet 11 is compressed in the thickness direction DT within the roller gap GP. Furthermore, the uncompressed negative electrode sheet 11 is heated by the first roller 110 and the second roller 120, and the active material particles 6 are bonded to each other and to the second surface 2B of the collector foil 2 via the binder resin 7. In other words, the negative electrode layer 3 is bonded to the second surface 2B of the collector foil 2. Thus, strip-shaped negative electrode sheets 1 that have been heated and compressed in the thickness direction DT are continuously manufactured in this manner.

[0046] Next, in the winding step S3 , the strip-shaped negative electrode sheet 1 is wound onto a winding reel (not shown).

[0047] In this embodiment, there is no undesirable situation in which a portion of the negative electrode layer 3 is peeled off and the binder resin 7 and the active material particles 6 are attached to the second outer peripheral surface 120G of the second roller 120, nor is there a undesirable situation in which a portion of the negative electrode layer 3 is peeled off from the collector foil 2 and floats up although not attached to the second outer peripheral surface 120G, thereby obtaining a good strip-shaped negative electrode sheet 1.

[0048] In the rolling step S2 of this embodiment, as described above, the first outer peripheral surface temperature TR1 of the first roller 110 is TR1 = 170°C. That is, the first outer peripheral surface temperature TR1 is 15°C higher than the melting start temperature Ti (=155°C) of the binder resin 7 (PVDF) (TR1 = Ti + 15°C). On the other hand, the second outer peripheral surface temperature TR2 of the second roller 120 is TR2 = 140°C. That is, the second outer peripheral surface temperature TR2 is lower than the first outer peripheral surface temperature TR1 (TR2 < TR1) and 15°C lower than the melting start temperature Ti (=155°C) of the binder resin 7 (TR2 = Ti - 15°C).

[0049] Thus, in this embodiment, the second outer peripheral surface temperature TR2 of the second outer peripheral surface 120G is 15°C lower than the melting start temperature Ti of the binder resin 7, that is, a temperature at which the binder particles 17 (binder resin 7) do not melt. Consequently, in the roller gap GP, the temperature TE of the negative electrode layer outer surface 13U (negative electrode layer outer surface 3U of the negative electrode layer 3) of the uncompressed negative electrode layer 13 that contacts the second outer peripheral surface 120G of the second roller 120 is also approximately the same as the second outer peripheral surface temperature TR2, that is, a temperature lower than the melting start temperature Ti of the binder resin 7 (TE≈TR2<Ti).

[0050] Therefore, under the pressure of roller pressing, the active material particles 6 can be bonded to each other via the binder particles 17 (binder resin 7), and the negative electrode layer 3 can be formed. However, the adhesive force generated by the binder particles 17 is relatively weak, which inhibits the binder particles 17 (binder resin 7) from adhering to the second outer peripheral surface 120G of the second roller 120, and the active material particles 6 from adhering to the second outer peripheral surface 120G of the second roller 120 via the binder particles 17.

[0051] Meanwhile, rolling is performed under conditions where the first peripheral surface temperature TR1 of the first peripheral surface 110G of the first roller 110, which contacts the first surface 2A of the collector foil 2, is set to 15°C higher than the binder resin's melting start temperature Ti, i.e., a temperature at which the binder particles 17 (binder resin 7) are suitably melted. Consequently, the temperature TC of the collector foil 2 in contact with the first peripheral surface 110G of the first roller 110 is also approximately the same as the first peripheral surface temperature TR1, i.e., higher than the binder resin 7's melting start temperature Ti (TC≈TR1>Ti). Consequently, in the uncompressed negative electrode layer 13, the binder particles 17 of the composite particles 15 located near the second surface 2B of the collector foil 2 melt, allowing the active material particles 6 to reliably adhere to the second surface 2B of the collector foil 2 via the melted binder resin 7.

[0052] Therefore, even if the second outer peripheral surface temperature TR2 is too low and the binder particles 17 do not melt, the active material particles 6 can be bonded to each other via the binder particles 17 (binder resin 7) by applying pressure, thereby forming a layered (thin film) negative electrode layer 3. On the other hand, since the first outer peripheral surface temperature TR1 is not too high, it is believed that the viscosity of the melted binder resin 7 is too low, which reduces the adhesive force and prevents the negative electrode layer 3 from being partially peeled off from the collector foil 2 and adhering to the second outer peripheral surface 120G of the second roller 120.

[0053] (Survey example)

[0054] Furthermore, the results of confirming the quality of the negative electrode sheets 1 of each investigation example in which the negative electrode sheets 1 were formed in the same manner as in the present embodiment but the first peripheral surface temperature TR1 and the second peripheral surface temperature TR2 in the roll pressing step S2 were changed are shown in FIG. Figure 7 .exist Figure 7 The above embodiment corresponds to a survey example in which TR1 = 170°C and TR2 = 140°C.

[0055] In this Figure 7 In the example shown by the circle, there is no separation of the negative electrode layer 3 from the collector foil 2, no adhesion of the active material particles 6 and the like to the second outer peripheral surface 120G of the second roller 120, and no floating caused by separation of the negative electrode layer 3 from the collector foil 2. The example shown by the circle is considered to be caused when the formed negative electrode layer 3 is properly adhered to the collector foil 2 and is difficult to adhere to the second roller 120.

[0056] On the other hand, the example marked with a triangle indicates that the negative electrode layer 3 peeled off from the collector foil 2 and floated. This example marked with a triangle can be considered to be due to the first peripheral surface temperature TR1 being too low, and the binder particles 17 not melting. Although roller pressing can bond the active material particles 6 to each other via the binder particles 17 (binder resin 7), forming the negative electrode layer 3, the adhesion between the negative electrode layer 3 and the collector foil 2 is not strong. Furthermore, the second peripheral surface temperature TR2 is also lower than the melting start temperature Ti, so the binder particles 17 are not melted on the second roller 120 side, and the adhesion force of the negative electrode layer 3 to the second roller 120 is weak. Therefore, although the negative electrode layer 3 does not partially peel off and adhere to the second roller 120, it does peel off from the collector foil 2 and float.

[0057] On the other hand, the example marked with an "×" indicates that a portion of the negative electrode layer 3 was peeled off from the current collector foil 2 and was lost, while the active material particles 6 and the like adhered to the second outer peripheral surface 120G of the second roller 120. This example marked with an "×" is considered to be caused when the formed negative electrode layer 3 adheres more easily to the second roller 120 than the current collector foil 2.

[0058] According to the Figure 7 The results show that: first, regarding the second peripheral surface temperature TR2, it is better to perform rolling under the condition that the second peripheral surface temperature TR2 is set to be lower than the first peripheral surface temperature TR1 and within the temperature range below the melting start temperature Ti+5°C of the adhesive resin 7 (TR2<TR1 and TR2≤Ti+5°C, in this embodiment, TR2<TR1 and TR2≤160°C).

[0059] Thus, in the roll gap GP, the temperature TE of the negative electrode layer outer surface 13U of the uncompressed negative electrode layer 13 (negative electrode layer outer surface 3U of the negative electrode layer 3) in contact with the second outer peripheral surface 120G of the second roller 120 is also below the melting start temperature Ti+5°C of the binder resin 7 (TE≈TR2≤Ti+5°C). Furthermore, since the uncompressed negative electrode sheet 11, whose sheet temperature TS is at room temperature, is passed through the roll gap GP without preheating and compressed in the thickness direction DT, and the uncompressed negative electrode sheet 11 is heated in the roll gap GP, the heating period is short. Consequently, the temperature TE of the negative electrode layer outer surface 13U of the uncompressed negative electrode layer 13 in contact with the second outer peripheral surface 120G of the second roller 120 does not reach the melting start temperature Ti of the binder resin 7 (TE<Ti), or even if it does, it does so only very briefly, and at most, TE≤Ti+5°C, preventing the binder resin 7 from fully melting. Therefore, the following situation is suppressed, namely: the binder particles 17 of the composite particles 15 located near the negative electrode layer outer surface 13U of the uncompressed negative electrode layer 13 are melted, the melted binder resin 7 is attached to the second outer peripheral surface 120G of the second roller 120, or the active material particles 6 are attached to the second outer peripheral surface 120G of the second roller 120 via the binder resin.

[0060] In addition, according to the Figure 7 The results show that: regarding the first peripheral surface temperature TR1, it is better to perform rolling under the condition that the first peripheral surface temperature TR1 is set within the temperature range of the melting start temperature Ti+5℃ to the melting start temperature Ti+25℃ of the adhesive resin 7 (Ti+5℃≤TR1≤Ti+25℃, in this embodiment, etc., 160℃≤TR1≤180℃).

[0061] During the rolling process, if the rolling is performed with the first outer peripheral surface temperature TR1 set to a temperature not lower than the melting start temperature Ti+5°C of the binder resin 7 (Ti+5°C≤TR1, 160°C≤TR1 in the present embodiment, etc.), the temperature TC of the collector foil 2 in contact with the first outer peripheral surface 110G of the first roller 110 also reaches a temperature not lower than the melting start temperature Ti+5°C of the binder resin 7 (Ti+5°C≤TC≈TR1, 160°C or higher in the present embodiment, etc.). Therefore, in the uncompressed negative electrode layer 13, the binder particles 17 of the composite particles 15 located near the second surface 2B of the collector foil 2 melt, allowing the active material particles 6 to reliably adhere to the second surface 2B of the collector foil 2 via the melted binder resin 7.

[0062] Furthermore, since the first outer peripheral surface temperature TR1 is a temperature equal to or lower than the melting start temperature Ti+25°C (TR1≤Ti+25°C, in this embodiment, etc., TR1≤180°C), the temperature TC of the collector foil 2 in contact with the first outer peripheral surface 110G of the first roller 110 can also be a temperature equal to or lower than the melting start temperature Ti+25°C of the binder resin 7 (TC≈TR1≤Ti+25°C, in this embodiment, etc., TC≈TR1≤180°C). Therefore, in the uncompressed negative electrode layer 13, although the viscosity of the binder resin 7 formed by melting the binder particles 17 of the composite particles 15 located near the second surface 2B of the collector foil 2 decreases, resulting in a decrease in binding strength, the binding strength does not decrease excessively. This prevents the binder resin 7 and active material particles 6 temporarily attached to the second surface 2B of the collector foil 2, that is, the negative electrode layer 3 immediately after compression, from being separated from the second surface 2B of the collector foil 2 and attached to the second outer peripheral surface 120G of the second roller 120.

[0063] Furthermore, it is preferable to carry out the rolling process S2 under the condition that the second peripheral surface temperature TR2 is set to a temperature range of 10°C or more lower than the first peripheral surface temperature TR1 and below the melting start temperature Ti+5°C of the adhesive resin 7 (TR2≤TR1-10°C and TR2≤Ti+5°C, in this embodiment, etc., TR2≤TR1-10°C and TR2≤160°C).

[0064] By setting the second outer peripheral surface temperature TR2 sufficiently lower than the first outer peripheral surface temperature TR1 in this manner, it is possible to reliably suppress adhesion of the binder resin 7 and the active material particles 6 to the second outer peripheral surface 120G of the second roller 120 .

[0065] In particular, in the present embodiment and each investigated example, graphite particles were used as the active material particles 6 and PVDF was used as the binder resin 7. Therefore, the negative electrode sheet 1 could be manufactured appropriately.

[0066] As mentioned above, the present invention has been described based on the embodiments and the various investigation examples. However, the present invention is not limited to the above-described embodiments and the like, and can be applied with appropriate modifications without departing from the gist of the present invention.

[0067] For example, when forming the uncompressed negative electrode layer 13 of the uncompressed negative electrode sheet 11, in the above embodiment, the following stacking device DP is used, namely: the composite particles 15 are temporarily adsorbed on the magnetic carrier particles, and after the composite particles 15 are transported to the film forming area by a magnetic roller, the electrostatic force is used to make the composite particles 15 fly toward the collector foil 2, so that the composite particles 15 are deposited on the collector foil 2 to form the uncompressed negative electrode layer 13.

[0068] However, it is sufficient to form an uncompressed negative electrode sheet 11 having an uncompressed negative electrode layer 13 formed by depositing composite particles 15 on the collector foil 2. For example, a depositing device DP as follows may be used, namely: a gravure roller (not shown) having a concave and convex shape on its outer peripheral surface is used, the composite particles 15 are filled in the concave portion of the gravure roller, and the composite particles 15 in the concave portion are continuously moved to the second surface 2B of the collector foil 2 by electrostatic force, and the composite particles 15 are deposited on the collector foil 2 to form an uncompressed negative electrode layer 13.

[0069] Alternatively, a deposition apparatus DP may be used that scatters and deposits the composite particles 15 on the second surface 2B of the current collector foil 2 without using electrostatic force to form the uncompressed negative electrode layer 13 .

Claims

1. A method for manufacturing an electrode sheet (1), wherein the electrode sheet (1) comprises a collector foil (2) having a first surface (2A) and a second surface (2B) opposite to the first surface (2A), and an electrode layer (3) on the second surface (2B), wherein: The manufacturing method includes a rolling step (S2), In the above-mentioned rolling step (S2), the following operation is performed on the uncompressed electrode sheet (11) whose sheet temperature (TS) is room temperature: The uncompressed electrode sheet (11) comprises the collector foil (2) and an uncompressed electrode layer (13). The uncompressed electrode layer (13) is formed on the second surface (2B) of the collector foil (2). The uncompressed electrode layer (13) is formed by stacking composite particles (15), wherein the composite particles (15) are formed by bonding active material particles (6) with a plurality of binder particles (17) having a smaller diameter than the active material particles (6) and consisting of a binder resin (7). The above operation refers to making the above uncompressed electrode sheet (11) pass through the roller gap (GP) between the first roller (110) having the first peripheral surface (110G) in contact with the above first surface (2A) of the above collector foil (2) and the second roller (120) having the second peripheral surface (120G) in contact with the electrode layer outer surface (13U) of the above uncompressed electrode layer (13), compressing it in the thickness direction (DT), and heating the above uncompressed electrode sheet (11) in the above roller gap (GP), making the above active material particles (6) bonded to each other by using the above binder resin (7), and making the above active material particles (6) bonded to the above second surface (2B) of the above collector foil (2), The above-mentioned rolling process (S2) is carried out under the following conditions: The first peripheral surface temperature TR1 of the first peripheral surface (110G) of the first roller (110) is set within a temperature range of the melting start temperature Ti+5°C to the melting start temperature Ti+25°C of the binder resin (7), that is, Ti+5°C≤TR1≤Ti+25°C; The second peripheral surface temperature TR2 of the second peripheral surface (120G) of the second roller (120) is set to be lower than the first peripheral surface temperature TR1 and within the temperature range below the melting start temperature Ti+5°C of the adhesive resin (7), that is, TR2<TR1, TR2≤Ti+5°C.

2. The method for manufacturing an electrode sheet (1) according to claim 1, wherein: The above-mentioned rolling process (S2) is carried out under the following conditions: The second peripheral surface temperature TR2 is set to be 10°C lower than the first peripheral surface temperature TR1 and within a temperature range below the melting start temperature Ti+5°C of the binder resin (7), i.e., TR2≤TR1-10°C, TR2≤Ti+5°C.

3. The method for manufacturing an electrode sheet (1) according to claim 1 or 2, wherein: The active material particles (6) are graphite particles. The binder resin (7) is PVDF.

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