Patent Publication Number: US-2017373290-A1

Title: Method for producing separator and device for producing separator

Description:
This Nonprovisional application claims priority under 35 U.S.C. §119 on Patent Application No. 2015-214892 filed in Japan on Oct. 30, 2015 and Patent Application No. 2016-062384 filed in Japan on Mar. 25, 2016, the entire contents of which are hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present invention relates to a method and a device for producing a separator. 
     BACKGROUND ART 
     In production of a separator which is used for a lithium-ion secondary battery or the like, a functional layer of various kinds is formed on a separator original sheet which serves as a base. In regard to such production of a separator, Patent Literature 1 discloses a method for producing a polyethylene microporous film which is a separator original sheet. 
     CITATION LIST 
     Patent Literature 
     [Patent Literature 1] 
     International Publication No. 2013/099539 (Publication Date: Jul. 4, 2013) 
     SUMMARY OF INVENTION 
     Technical Problem 
     In a typical production of a separator, while a separator original sheet unwound from a roll is transferred, a coating step of coating the separator original sheet with a material for a functional layer is carried out. However, the separator original sheet being transferred tends to sag at both end parts thereof, and have a flare at the both end parts, accordingly. In a case where a flare occurs at the both end parts of the separator original sheet, there is a problem that the material for the functional layer, with which the separator original sheet is coated in the coating step, has a non-uniform film thickness. 
     As a technique for preventing such an occurrence of a flare, it is possible to set a tension (unwinding tension) of the separator original sheet high. However, in a case where the tension of the separator original sheet is set high, there arise new problems that (i) the separator original sheet may break, (ii) a wrinkle (longitudinal wrinkle) at a part other than the both end parts of the separator original sheet is more likely to be formed, and (iii) the separator original sheet, which is soft, may be stretched out and deformed. 
     An aspect of the present invention has been accomplished in view of the problems, and an object of the present invention is to provide a method and a device for producing a separator, each capable of coating a separator original sheet with a material for a functional layer in a coating step so that the material has a uniform film thickness. 
     Solution to Problem 
     In order to attain the object, a method for producing a separator in accordance with an aspect of the present invention includes: a removing step of removing both end parts of a base film in a width direction of the base film, the base film being transferred; and a coating step of coating, with a material for forming a functional layer, the base film whose both end parts have been removed in the removing step. 
     In order to attain the object, a method for producing a separator in accordance with an aspect of the present invention includes: a removing step of removing both end parts of a base film in a width direction of the base film, the base film being transferred; a coating step of coating, with a material for forming a functional layer, one surface of the base film whose both end parts have been removed in the removing step; a second removing step of removing, from the base film having been coated with the material in the coating step, both end parts of the base film in the width direction of the base film; and a second coating step of coating, with a material for forming a functional layer, the other surface of the base film whose both end parts have been removed in the second removing step. 
     In order to attain the object, a device for producing a separator in accordance with an aspect of the present invention includes: a removing section for removing both end parts of a base film in a width direction of the base film, the base film being transferred; and a coating section for coating, with a material for forming a functional layer, the base film whose both end parts have been removed by the removing section. 
     With the configuration, before the base film is coated with the material by the coating section, the both end parts of the base film are removed by the removing section. This makes it possible to coat the base film with the material in a state in which flares (sags) that occurred at the both end parts of the base film have been reduced. 
     Accordingly, the configuration makes it possible to provide a device for producing a separator which device is capable of coating a base film with a material for a functional layer by a coating section so that the material has a uniform film thickness. 
     Advantageous Effects of Invention 
     An aspect of the present invention brings about an effect of providing a method and a device for producing a separator, each capable of coating a base film with a material for a functional layer in a coating step so that the material has a uniform film thickness. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram schematically illustrating a cross sectional configuration of a lithium-ion secondary battery. 
         FIG. 2  provides diagrams each schematically illustrating a state of the lithium-ion secondary battery illustrated in  FIG. 1 . 
         FIG. 3  provides diagrams each schematically illustrating a state of a lithium-ion secondary battery having another configuration. 
         FIG. 4  is a flow chart schematically showing a method for producing a separator. 
         FIG. 5  is a top surface view illustrating an example of a removing step shown in  FIG. 4 . 
         FIG. 6  is a cross-sectional view illustrating a positional relationship between (i) a razor blade included in each of cutting sections illustrated in  FIG. 5  and (ii) a separator original sheet. 
         FIG. 7  is a cross-sectional view showing an example of a coating step shown in  FIG. 4 . 
       (a) of  FIG. 8  is a cross-sectional view illustrating a configuration of a slitting apparatus for slitting a heat-resistant separator original sheet. (b) of  FIG. 8  is a perspective view illustrating states before and after the heat-resistant separator original sheet is slit. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following description will discuss an embodiment of the present invention by using, as an example, a separator film (separator) for a lithium-ion secondary battery. 
     First, the lithium-ion secondary battery will be discussed with reference to  FIGS. 1 through 3 . 
     [Configuration of Lithium Ion Secondary Battery] 
     A nonaqueous electrolyte secondary battery, typically, a lithium-ion secondary battery has a high energy density, and therefore, is currently widely used not only as batteries for use in devices such as personal computers, mobile phones, and mobile information terminals, and for use in moving bodies such as automobiles and airplanes, but also as stationary batteries contributing to stable power supply. 
       FIG. 1  is a diagram schematically illustrating a cross sectional configuration of a lithium-ion secondary battery  1 . As illustrated in  FIG. 1 , the lithium-ion secondary battery  1  includes a cathode  11 , a separator  12 , and an anode  13 . Between the cathode  11  and the anode  13 , an external device  2  is connected outside the lithium-ion secondary battery  1 . Then, while the lithium-ion secondary battery  1  is being charged, electrons move in a direction A. On the other hand, while the lithium-ion secondary battery  1  is being discharged, electrons move in a direction B. 
     (Separator) 
     The separator  12  is provided so as to be sandwiched between the cathode  11  which is a positive electrode of the lithium-ion secondary battery  1  and the anode  13  which is a negative electrode of the lithium-ion secondary battery  1 . The separator  12  separates the cathode  11  and the anode  13 , allowing lithium ions to move between the cathode  11  and the anode  13 . For example, polyolefin such as polyethylene or polypropylene is used as a material of the separator  12 . 
       FIG. 2  provides diagrams each schematically illustrating a state of the lithium-ion secondary battery  1  illustrated in  FIG. 1 . (a) of  FIG. 2  illustrates a normal state. (b) of  FIG. 2  illustrates a state in which a temperature of the lithium-ion secondary battery  1  has risen. (c) of  FIG. 2  illustrates a state in which a temperature of the lithium-ion secondary battery  1  has sharply risen. 
     As illustrated in (a) of  FIG. 2 , the separator  12  is provided with many pores P. Normally, lithium ions  3  in the lithium-ion secondary battery  1  can move back and forth through the pores P. 
     However, there are, for example, cases in which the temperature of the lithium-ion secondary battery  1  rises due to excessive charging of the lithium-ion secondary battery  1 , a high current caused by short-circuiting of the external device, or the like. In such cases, the separator  12  melts or softens and the pores P are blocked as illustrated in (b) of  FIG. 2 . As a result, the separator  12  shrinks. This stops the back-and-forth movement of the lithium ions  3 , and consequently stops the above temperature rise. 
     However, in a case where a temperature of the lithium-ion secondary battery  1  sharply rises, the separator  12  suddenly shrinks. In this case, as illustrated in (c) of  FIG. 2 , the separator  12  may be destroyed. Then, the lithium ions  3  leak out from the separator  12  which has been destroyed. As a result, the lithium ions  3  do not stop moving back and forth. Consequently, the temperature continues rising. 
     (Heat-Resistant Separator) 
       FIG. 3  provides diagrams each schematically illustrating a state of a lithium-ion secondary battery  1  having another configuration. (a) of  FIG. 3  illustrates a normal state, and (b) of  FIG. 3  illustrates a state in which a temperature of the lithium-ion secondary battery  1  has sharply risen. 
     As illustrated in (a) of  FIG. 3 , the lithium-ion secondary battery  1  can further include a heat-resistant layer  4 . The heat-resistant layer  4  can be provided to the separator  12 . (a) of  FIG. 3  illustrates a configuration in which the separator  12  is provided with the heat-resistant layer  4  serving as a functional layer. A film in which the separator  12  is provided with the heat-resistant layer  4  is hereinafter referred to as a heat-resistant separator (separator)  12   a.    
     In the configuration illustrated in (a) of  FIG. 3 , the heat-resistant layer  4  is laminated on a surface of the separator  12  which surface is on a cathode  11  side. Note that the heat-resistant layer  4  can alternatively be laminated on a surface of the separator  12  which surface is on an anode  13  side, or both surfaces of the separator  12 . Further, the heat-resistant layer  4  is provided with pores which are similar to the pores P. Normally, the lithium ions  3  move back and forth through the pores P and the pores of the heat-resistant layer  4 . The heat-resistant layer  4  contains, for example, wholly aromatic polyamide (aramid resin) as a material. 
     As illustrated in (b) of  FIG. 3 , even in a case where the temperature of the lithium-ion secondary battery  1  sharply rises and as a result, the separator  12  melts or softens, the shape of the separator  12  is maintained because the heat-resistant layer  4  supports the separator  12 . Therefore, such a sharp temperature rise results in only melting or softening of the separator  12  and consequent blocking of the pores P. This stops the back-and-forth movement of the lithium ions and consequently stops the above-described excessive discharging or excessive charging. In this way, the separator  12  can be prevented from being destroyed. 
     [Flow for Producing Separator] 
     The following description will discuss a flow for producing a separator. 
       FIG. 4  is a flow chart schematically showing a method for producing a separator. The separator is configured such that a functional layer is laminated on a separator original sheet (base film) which serves as a base. A film made of polyethylene or the like is used for the separator original sheet. Examples of the functional layer encompass a heat-resistant layer and an adhesive layer. 
     The functional layer is laminated on the separator original sheet by (i) coating the separator original sheet with coating (a material) or the like corresponding to the functional layer and then (ii) drying the separator original sheet. 
       FIG. 4  shows, as an example, a flow for producing the heat-resistant separator  12   a  in a case where the functional layer is the heat-resistant layer  4 . The flow shown in  FIG. 4  is an example of a flow in which wholly aromatic polyamide (aramid resin) employed as a material for the heat-resistant layer  4  is laminated on a polyethylene base film which is a separator original sheet (base film)  12   d.    
     The above flow includes an inspection step S 1 , a removing step S 2 , a coating step S 3 , a precipitating step S 4 , a washing step S 5 , a drying step S 6 , an inspection step S 7 , and a slitting step S 8 . 
     Note that a method for producing a separator in accordance with an aspect of the present invention can include, before the inspection step S 1 , a step of producing the separator original sheet  12   d.  Further, the inspection step S 1  can be omitted. Alternatively, the inspection step S 1  can be carried out between the removing step S 2  and the coating step S 3 . 
     The method for producing a separator in accordance with an aspect of the present invention can include a second removing step S 2 - 2  after the drying step S 6 . In this case, a flow for producing a separator includes the inspection step S 1 , the removing step S 2 , the coating step S 3 , the precipitating step S 4 , the washing step S 5 , the drying step S 6 , the second removing step S 2 - 2 , the inspection step S 7 , and the slitting step S 8 . 
     In the method for producing a separator in accordance with an aspect of the present invention, the precipitating step S 4  and the washing step S 5  can be omitted. This is because the precipitating step S 4  and the washing step S 5  can be unnecessary depending on a configuration of the functional layer and a configuration of coating used in the coating step S 3 . In this case, a flow for producing a separator includes the inspection step S 1 , the removing step S 2 , the coating step S 3 , the drying step S 6 , the inspection step S 7 , and the slitting step S 8 . 
     The method for producing a separator in accordance with an aspect of the present invention can include, after the drying step S 6 , the second removing step S 2 - 2 , a second coating step S 3 - 2 , a second precipitating step S 4 - 2 , a second washing step S 5 - 2 , and a second drying step S 6 - 2 . In this case, a flow for producing a separator includes the inspection step S 1 , the removing step S 2 , the coating step S 3 , the precipitating step S 4 , the washing step S 5 , the drying step S 6 , the second removing step S 2 - 2 , the second coating step S 3 - 2 , the second precipitating step S 4 - 2 , the second washing step S 5 - 2 , the second drying step S 6 - 2 , the inspection step S 7 , and the slitting step S 8 . In the above flow, it is possible to omit (i) the precipitating step S 4  and the washing step S 5  and (ii) the second precipitating step S 4 - 2  and the second washing step S 5 - 2 . This is because, as described above, the precipitating step S 4  and the washing step S 5  and (ii) the precipitating step S 4 - 2  and the washing step S 5 - 2  can be unnecessary depending on (i) the configuration of the functional layer and (ii) the structure of the coating used in the coating step S 3 . In this case, a flow for producing a separator includes the inspection step S 1 , the removing step S 2 , the coating step S 3 , the drying step S 6 , the second removing step S 2 - 2 , the second coating step S 3 - 2 , the second drying step S 6 - 2 , the inspection step S 7 , and the slitting step S 8 . 
     The method for producing a separator in accordance with an aspect of the present invention can further include an additional inspection step between the steps. 
     (Production Step of Producing Separator Original Sheet) 
     First, the following description will discuss production of the separator original sheet  12   d  serving as a base film, by using, as an example, a case where the separator original sheet  12   d  mainly contains polyethylene as a material. 
     The following description will discuss, as an example, a production method for producing the separator original sheet  12   d  in which production method a thermoplastic resin in which a pore forming agent is added is shaped into a film and then the pore forming agent is removed by use of an appropriate solvent. Specifically, in a case where a polyethylene resin containing an ultrahigh molecular weight polyethylene is used as a material of the separator original sheet  12   d,  steps (A) through (D) below are sequentially carried out in the production method. 
     (A) Kneading Step 
     A step of obtaining a polyethylene resin composition by kneading an ultrahigh molecular weight polyethylene and an inorganic filler such as a calcium carbonate. 
     (B) Rolling Step 
     A step of forming a film by use of the polyethylene resin composition obtained in the kneading step. 
     (C) Removal Step 
     A step of removing the inorganic filler from the film obtained in the rolling step. 
     (D) Stretching Step 
     A step of obtaining the separator original sheet  12   d  by stretching the film obtained in the removal step. 
     According to the above production method, in the removal step (C), many fine pores are provided in the film. The fine pores of the film stretched in the stretching step (D) become the above-described pores P. The separator original sheet  12   d  formed as a result is a polyethylene microporous film having a prescribed thickness and a prescribed air permeability. 
     In the kneading step (A), 100 parts by weight of the ultrahigh molecular weight polyethylene, 5 parts by weight to 200 parts by weight of a low-molecular weight polyolefin having a weight-average molecular weight of not more than 10000, and 100 parts by weight to 400 parts by weight of the inorganic filler can be kneaded. 
     Note that even in a case where the separator original sheet  12   d  contains another material, the separator original sheet  12   d  can be produced by similar production steps. The method for producing the separator original sheet  12   d  is not limited to the method in which the pore forming agent is removed, and various methods can be used for producing the separator original sheet  12   d.    
     The following description will sequentially discuss the steps S 1  through S 8  which are subsequent to the production step of producing the separator original sheet  12   d.  Note that the steps S 1  through S 8  are carried out in this order. 
     (Inspection Step S 1 ) 
     The inspection step S 1  is a step of inspecting, before a subsequent step, the separator original sheet  12   d  which serves as a base of the heat-resistant separator  12   a.    
     (Removing Step S 2 ) 
     The removing step S 2  is a step of removing, from the separator original sheet  12   d  which has been inspected in the inspection step S 1 , both end parts (edge parts) of the separator original sheet  12   d  in a width direction thereof. In other words, the removing step S 2  is a step of removing both end parts of the separator original sheet  12   d  which both end parts face each other in a direction substantially perpendicular to a transferring (longitudinal) direction of the separator original sheet  12   d.    
     As described above, the separator original sheet  12   d  tends to sag at an end part thereof while being transferred, and this causes a flare at the end part. In a case where a flare occurs at the end part of the separator original sheet  12   d,  coating for the heat-resistant layer  4 , with which the separator original sheet  12   d  is coated in the coating step S 3 , has a non-uniform film thickness. 
     Therefore, Embodiment 1 involves removing the both end parts, at each of which a flare has occurred, from the separator original sheet  12   d  in the removing step S 2 , before the coating step S 3 . 
       FIG. 5  is a top surface view illustrating an example of the removing step S 2  shown in  FIG. 4 . As illustrated in  FIG. 5 , the removing step S 2  is carried out by a removing device (removing section)  5  for removing both end parts (removal parts) e of the separator original sheet  12   d.    
     The removing device  5  includes (i) two cutting sections (first cutting section, second cutting section)  51  for removing (cutting off) the respective both end parts e of the separator original sheet  12   d  and (ii) a fixing shaft  52  for fixing the cutting sections  51 . 
     The fixing shaft  52  has a substantially linear shape extending in the width direction of the separator original sheet  12   d.  Both end parts of the fixing shaft  52  are fixed to respective fixing sections  6  that are provided on respective both sides of the removing device  5  so as to sandwich the removing device  5 . 
     As described above, in the removing device  5 , the fixing shaft  52  for fixing the cutting sections  51  has a substantially linear shape, and the two cutting sections  51  are fixed to the single fixing shaft  52 . This allows the cutting sections  51  to be more stable as compared with, for example, a case where the two cutting sections  51  are fixed to respective different members. This allows the cutting sections  51  to appropriately remove the both end parts e of the separator original sheet  12   d  at respective removal positions C. 
     The cutting sections  51  can each be configured to be fixable at a given position on the fixing shaft  52 . This makes it possible to set a width of the separator original sheet  12   d  to a given value in the removing step S 2 . 
     The cutting sections  51  each include a razor blade  512  (see  FIG. 6 ). The razor blade  512  removes corresponding one of the both end parts e of the separator original sheet  12   d  by a blade edge  512   a  that has been sharpened. This makes it possible to provide a separator original sheet (base film)  12   c  which (i) is obtained by removing, from the separator original sheet  12   d,  the both end parts e at each of which a flare has occurred and (ii) is thus flat. 
       FIG. 6  is a cross-sectional view illustrating a positional relationship between (i) the razor blade  512  included in each of the cutting sections  51  illustrated in  FIG. 5  and (ii) the separator original sheet  12   d.  As illustrated in  FIG. 6 , the razor blade  512  is provided so as to be obliquely inclined with respect to the separator original sheet  12   d  so that the blade edge  512   a  forms an angle θ with respect to the separator original sheet  12   d  that is being transferred. 
     The razor blade  512  used in Embodiment 1 is a long blade having a blade length greater than that of a slitting razor blade (short blade) which is typically used in production of a separator. Accordingly, in a case where a distance between a bottom surface of the separator original sheet  12   d  and a lower end part (cutting point) of the blade edge  512   a  is defined as a depth D, it is possible to set the depth D to be greater as compared with a conventional razor blade. 
     By setting the depth D to be thus greater, it is possible to appropriately remove, from the separator original sheet  12   d  by using the razor blade  512 , the both end parts e at each of which a flare has occurred. 
     Note that a width of a flare (i.e., a width of the flare as measured in an out-of-plane direction of the separator original sheet  12   d ) varies in accordance with factors such as a size, a transferring speed, and an unwinding tension of the separator original sheet  12   d.  Accordingly, a value of the depth D is set as appropriate in accordance with a width of a flare that has occurred at each of the both end parts e of the separator original sheet  12   d.    
     The value of the depth D is preferably not smaller than a quarter of a distance between (i) each of both end sides E of the separator original sheet  12   d,  which is being transferred, in the width direction of the separator original sheet  12   d  and (ii) corresponding one of the removal positions C at each of which the razor blade  512  is brought into contact with the separator original sheet  12   d.  In a case where the value of the depth D is not smaller than a quarter of the above distance, it is possible to more appropriately remove, from the separator original sheet  12   d  by using the razor blade  512 , the both end parts e at each of which a flare has occurred. 
     An upper limit of the depth D is preferably equal to the above distance. In a case where the value of the depth D exceeds the above distance, (i) it is difficult to secure a space sufficient for mounting the cutting sections  51 , due to a configuration of the removing device  5  and (ii) the blade edge  512   a  of the razor blade  512  is used less efficiently. It is therefore not reasonable to set the value of the depth D to be greater than the above distance. 
     A width of each of the removal parts (both end parts e) removed in the removing step S 2  is preferably not smaller than 5 mm but not greater than 200 mm. In terms of a reliable removal of a part in which a flare can occur, the width of each of the removal parts (both end parts e) removed in the removing step S 2  is preferably not smaller than 5 mm, more preferably not smaller than 8 mm, and still more preferably not smaller than 10 mm. Meanwhile, in terms of a yield of a product which is obtained after the removal parts are removed, the above width is preferably not greater than 200 mm, and more preferably not greater than 150 mm. The removal parts (both end parts e) each having a width of less than 5 mm is unsuitable because, in such a case, it is difficult to remove the both end parts e from the separator original sheet  12   d  which is thin and low in strength. 
     The removal parts that are removed in the removing step S 2  can have respective different widths. In a case where a removal part in which a flare is more likely to occur is removed by a larger portion, it is possible to prevent an occurrence of a flare without reducing an overall yield in the production of separators. 
     Note that a type of the blade used in the removing step S 2  is not particularly limited and various types of blades can be used, provided that it is possible to remove, by the blade, the both end parts e of the separator original sheet  12   d.  For example, instead of the razor blade  512 , it is possible to use a blade such as a circular blade. 
     (Coating Step S 3 ) 
     The coating step S 3  is a step of coating, with coating (a material) for the heat-resistant layer  4 , a separator original sheet  12   c  which is obtained by removing the both end parts e from the separator original sheet  12   d  in the removing step S 2 . In the coating step S 3 , it is possible to carry out the coating with respect to only one surface of the separator original sheet  12   c  or both surfaces of the separator original sheet  12   c.  In a case where the coating is carried out with respect to only one surface of the separator original sheet  12   c,  it is possible to further carry out coating with respect to the other surface of the separator original sheet  12   c  in the second coating step S 3 - 2  described later. 
     For example, in the coating step S 3 , the separator original sheet  12   c  is coated with a solution in which aramid is dissolved in NMP (N-methylpyrrolidone), as coating for the heat-resistant layer. Note that the heat-resistant layer  4  is not limited to an aramid heat-resistant layer. For example, it is possible to coat the separator original sheet  12   c  with a suspension of alumina, carboxymethyl cellulose, and water, as the coating for the heat-resistant layer. 
     According to Embodiment 1, no other step is carried out between the removing step S 2  and the coating step S 3 , and after the both end parts e at each of which a flare has occurred are removed in the removing step S 2 , the coating is carried out with respect to the separator original sheet  12   c  in the coating step S 3 , which is subsequent to the removing step S 2 . This makes it possible to carry out the coating with respect to the separator original sheet  12   c  while suitably maintaining flatness of the separator original sheet  12   c.  Accordingly, it is possible to coat the separator original sheet  12   c  with the material for the heat-resistant layer  4  so that the material has a more uniform film thickness. 
     Note, however, that another step (e.g., a winding step etc.) can be carried out between the removing step S 2  and the coating step S 3 . Even in a case where another step is carried out between the removing step S 2  and the coating step S 3 , it is possible, by carrying out the removing step S 2  of removing the both end parts e of the separator original sheet  12   d  before the coating step S 3 , to coat the separator original sheet  12   c  with the material for the heat-resistant layer  4  so that the material has a more uniform film thickness as compared with a case where the removing step S 2  is not carried out. 
       FIG. 7  is a cross-sectional view showing an example of the coating step S 3  shown in  FIG. 4 . As shown in  FIG. 7 , the coating step S 3  is carried out by a coating device (coating section)  7  for coating, with coating  74  for the heat-resistant layer, the separator original sheet  12   c  which is obtained by removing the both end parts e from the separator original sheet  12   d  in the removing step S 2 . 
     The coating device  7  has a mechanism of a bar coater method and includes a coating dropper  71 , a coating bar  72 , and a coating bar driving section  73 . According to the mechanism, the coating is carried out with respect to the separator original sheet  12   c  in the following manner. That is, a gap (clearance) is provided between a tip of the coating bar  72  and the separator original sheet  12   c,  and the separator original sheet  12   c  is transferred while coating  74  that has been dropped from the coating dropper  71  onto the separator original sheet  12   c  is being accumulated on one side of the coating bar  72  (i.e., upstream of the coating bar  72  in a transferring direction of the separator original sheet  12   c ). 
     Note that a method for coating the separator original sheet  12   c  with the coating  74  is not specifically limited as long as uniform wet coating can be performed with respect to the separator original sheet  12   c  by the method. The method can be any of various methods such as a capillary coating method, a slit die coating method, a spray coating method, a dip coating method, a roll coating method, a screen printing method, a flexo printing method, a bar coater method, a gravure coater method, or a die coater method. 
     A material for the heat-resistant layer  4  with which material the separator original sheet  12   c  is coated has a film thickness that can be controlled by adjusting a thickness of a coating wet film and a solid-content concentration in the coating solution. 
     In the coating step S 3 , the separator original sheet  12   c  obtained by removing the both end parts e from the separator original sheet  12   d  is preferably coated with the coating  74  so that surfaces of respective both edge parts of the separator original sheet  12   c  in a width direction of the separator original sheet  12   c  are left uncoated as uncoated parts (edge-uncoated coating). This can prevent the coating  74  from flowing around from the both end parts of the separator original sheet  12   c  to a back surface of the separator original sheet  12   c,  unlike in a case where an entire surface coating is carried out so that the coating  74  is applied to the separator original sheet  12   c  so as to cover up to the surfaces of the respective both edge parts of the separator original sheet  12   c.  Accordingly, it is possible to reduce a product defect which is caused when the coating  74  flows around to the back surface of the separator original sheet  12   c.    
     In this case, the uncoated parts can be removed in a step to be carried out later (e.g., the second removing step S 2 - 2 , the slitting step S 8 , etc. (described later)). This makes it possible to produce the heat-resistant separator  12   a  in which the heat-resistant layer  4  has a more uniform film thickness. 
     (Precipitating Step S 4 ) 
     The precipitating step S 4  is a step of solidifying the coating  74  with which the separator original sheet  12   c  has been coated in the coating step S 3 . In a case where the coating  74  is an aramid coating, for example, water vapor is applied to a coated surface so that aramid is solidified by humidity precipitation. This provides a heat-resistant separator original sheet  12   b  (see  FIG. 8 ) in which the heat-resistant layer  4  is formed on the separator original sheet  12   c.    
     (Washing Step S 5 ) 
     The washing step S 5  is a step of washing the heat-resistant separator original sheet  12   b  in which the coating has been solidified in the precipitating step S 4 . In a case where the heat-resistant layer  4  is an aramid heat-resistant layer, for example, water, an aqueous solution, or an alcohol-based solution is suitably used as a washing liquid. 
     (Drying Step S 6 ) 
     The drying step S 6  is a step of drying the heat-resistant separator original sheet  12   b  that has been washed in the washing step S 5 . A method for drying the heat-resistant separator original sheet  12   b  is not particularly limited, and, for example, it is possible to use various methods such as a method in which the heat-resistant separator original sheet  12   b  is brought into contact with a heated roll or a method in which hot air is blown onto the heat-resistant separator original sheet  12   b.    
     (Second Removing Step S 2 - 2 ) 
     A method for producing the heat-resistant separator  12   a  can include, after the drying step S 6 , the second removing step S 2 - 2  of further removing both end parts of the heat-resistant separator original sheet  12   b.  This makes it possible to suitably prevent a wrinkle from occurring on the heat-resistant separator original sheet  12   b  while the heat-resistant separator original sheet  12   b  is being transferred. 
     Particularly, in a case where the edge-uncoated coating has been carried out with respect to the separator original sheet  12   c  in the coating step S 3 , there is a level difference (a boundary between a coated part and an uncoated part) at the both end parts of the separator original sheet  12   c  due to the film thickness of the heat-resistant layer  4 . Accordingly, a wrinkle may be caused by the level difference. 
     In a case where the both end parts, which are uncoated parts, of the heat-resistant separator original sheet  12   b  are removed in the second removing step S 2 - 2  so that the level difference is eliminated, it is possible to prevent a wrinkle from occurring on the heat-resistant separator original sheet  12   b  while the heat-resistant separator original sheet  12   b  is being transferred. 
     That is, removal parts which are removed in the second removing step S 2 - 2  each preferably include the uncoated part and a part of the coated part. In other words, in order to reliably include the entire uncoated parts in the removal parts, it is preferable to also include, in each of the removal parts, a part of the coated part which part is adjacent to the uncoated part when the both end parts of the heat-resistant separator original sheet  12   b  are removed in the second removing step S 2 - 2 . 
     For example, in a case where the edge-uncoated coating has been carried out in the coating step S 3  so that a part which is approximately 10 mm in width as measured from each of the both end sides E of the separator original sheet  12   c  is left uncoated as an uncoated part, a part of the coated part which part has a width of not smaller than 5 mm but not greater than 80 mm is removed, in addition to the uncoated part of approximately 10 mm in width, from the separator original sheet  12   b  at each of the both end sides E of the heat-resistant separator original sheet  12   b  in the second removing step S 2 - 2 . Specific aspect (means) of the second removing step S 2 - 2  is identical to that of the removing step S 2 . 
     In terms of (i) a reliable removal of a part in which a flare can occur and (ii) a reliable elimination of a level difference caused by a film thickness of a material (functional layer) with which one surface of the base film has been coated, a width of the part of the coated part which part is removed in the removing step S 2 - 2  is preferably not smaller than 5 mm, more preferably not smaller than 8 mm, and still more preferably not smaller than 10 mm. Meanwhile, in terms of a yield of a product which is obtained after the removal parts are removed, the above width is preferably not greater than 80 mm, and more preferably not greater than 50 mm. 
     In a case where the heat-resistant separator original sheet  12   b  includes the heat-resistant layer  4  only on one surface of the heat-resistant separator original sheet  12   b,  it is possible to form a heat-resistant layer  4  also on the other surface of the resistant separator original sheet  12   b  by carrying out the second coating step S 3 - 2 , the second precipitating step S 4 - 2 , the second washing step S 5 - 2 , and the second drying step S 6 - 2 , subsequent to the second removing step S 2 - 2 . 
     That is, the method for producing a separator in accordance with an aspect of the present invention can be carried out by using, as a base film, the heat-resistant separator original sheet  12   b  including the heat-resistant layer  4  only on one surface of the heat-resistant separator original sheet  12   b.    
     (Second Coating Step S 3 - 2 ) 
     The second coating step S 3 - 2  is a step of coating the other surface of the heat-resistant separator original sheet  12   b  with coating (a material) for the heat-resistant layer  4 . For example, no other step is carried out between the second removing step S 2 - 2  and the second coating step S 3 - 2  and, after the both end parts of the heat-resistant separator original sheet  12   b  are removed in the second removing step S 2 - 2 , the coating is carried out with respect to the other surface of the heat-resistant separator original sheet  12   b  in the second coating step S 3 - 2 , which is subsequent to the second removing step S 2 - 2 . As a result, the coating is carried out while flatness of the heat-resistant separator original sheet  12   b  is suitably maintained. This makes it possible to coat the heat-resistant separator original sheet  12   b  with the material for the heat-resistant layer  4  so that the material has a more uniform film thickness. Specific aspect (means) of the second coating step S 3 - 2  is identical to that of the coating step S 3 . 
     (Second Precipitating Step S 4 - 2 ) 
     The second precipitating step S 4 - 2  is a step of solidifying the coating with which the heat-resistant separator original sheet  12   b  has been coated in the second coating step S 3 - 2 . Specific aspect (means) of the second precipitating step S 4 - 2  is identical to that of the precipitating step S 4 . 
     (Second Washing Step S 5 - 2 ) 
     The second washing step S 5 - 2  is a step of washing the heat-resistant separator original sheet  12   b  on which the coating has been solidified in the second precipitating step S 4 - 2 . Specific aspect (means) of the second washing step S 5 - 2  is identical to that of the washing step S 5 . 
     (Second Drying Step S 6 - 2 ) 
     The second drying step S 6 - 2  is a step of drying the heat-resistant separator original sheet  12   b  that has been washed in the second washing step S 5 - 2 . Specific aspect (means) of the second drying step S 6 - 2  is identical to that of the drying step S 6 . 
     (Inspection Step S 7 ) 
     The inspection step S 7  is a step of inspecting the heat-resistant separator original sheet  12   b  that has been dried in the drying step S 6  (the second drying step S 6 - 2  in a case where the flow includes the drying step S 6 - 2 ). In a case where the heat-resistant separator original sheet  12   b  is inspected, it is possible to mark a defective part as appropriate so that the defective part can be easily removed. 
     (Slitting Step S 8 ) 
     The slitting step S 8  is a step of slitting (cutting) the heat-resistant separator original sheet  12   b  inspected in the inspection step S 7  into parts each having a predetermined product width. Specifically, in the slitting step S 8 , the heat-resistant separator original sheet  12   b  is slit into parts each having a product width which is suitable for an applied product such as the lithium-ion secondary battery  1 . 
     In order to increase productivity, the heat-resistant separator original sheet  12   b  is usually produced so as to have a width that is equal to or greater than the product width. After the heat-resistant separator original sheet  12   b  is produced, the heat-resistant separator original sheet  12   b  is slit into heat-resistant separators  12   a  each having the product width. 
     In the slitting step S 8 , it is possible to remove both end parts of the heat-resistant separator original sheet  12   b,  at the same time as slitting the heat-resistant separator original sheet  12   b.  In a case where redundant films are removed from the respective both end parts of the heat-resistant separator original sheet  12   b  in the slitting step S 8 , it is possible to suitably process the heat-resistant separator original sheet  12   b  into the heat-resistant separators  12   a  each having the product width. 
     In a case where the edge-uncoated coating has been carried out in the coating step S 3  (the second coating step S 3 - 2  in a case where the flow includes the second coating step S 3 - 2 ), uncoated parts F (see  FIG. 8 ) are removed in the slitting step S 8 . For example, in a case where the edge-uncoated coating has been carried out in the coating step S 3  (or the second coating step S 3 - 2 ) so that a part which is approximately 10 mm in width as measured from each of the both end sides E of the separator original sheet  12   c  is left uncoated as an uncoated part F, a part of the coated part which part has a width of not smaller than 5 mm but not greater than 80 mm is removed, in addition to the uncoated part F of approximately 10 mm in width, from the heat-resistant separator original sheet  12   b  at each of the both end sides E of the heat-resistant separator original sheet  12   b.    
     (a) of  FIG. 8  is a cross-sectional view illustrating a configuration of a slitting apparatus  8  for slitting the heat-resistant separator original sheet  12   b.  (b) of  FIG. 8  is a perspective view illustrating states before and after the heat-resistant separator original sheet  12   b  is slit. 
     As illustrated in (a) of  FIG. 8 , the slitting apparatus  8  includes a wind-off roller  81 , rollers  82  through  85 , and a plurality of take-up rollers  86  each of which (i) has a columnar shape and (ii) is rotatably supported. The slitting apparatus  8  further includes a plurality of blades (not illustrated). 
     In the slitting apparatus  8 , a core  87  which has a cylindrical shape and on which the heat-resistant separator original sheet  12   b  is wound is fitted on the wind-off roller  81 . The heat-resistant separator original sheet  12   b  is unwound from the core  87  to a path U or a path L. The heat-resistant separator original sheet  12   b  thus unwound is transferred to the roller  84  via the roller  83  at, for example, a speed of 100 m/min. In a step of transferring the heat-resistant separator original sheet  12   b,  the heat-resistant separator original sheet  12   b  is slit substantially parallel to a transferring (longitudinal) direction. As a result, a plurality of heat-resistant separators  12   a,  into which the heat-resistant separator original sheet  12   b  are slit so as to have the product width, are produced. 
     As illustrated in (b) of  FIG. 8 , the plurality of heat-resistant separators  12   a  thus produced are wound on respective cores  88  fitted on the take-up rollers  86 . 
     Note that the method for producing the heat-resistant separator  12   a  can include a step other than the steps described above. For example, the method for producing the heat-resistant separator  12   a  can include a water removing step between the washing step S 5  and the drying step S 6  or between the second washing step S 5 - 2  and the second drying step S 6 - 2 . In the water removing step, water or the like adhered to the heat-resistant separator original sheet  12   b  is removed in order to prevent the heat-resistant separator original sheet  12   b  from insufficiently being dried in the subsequent step, i.e., the drying step S 6  or the second drying step S 6 - 2 . 
     Alternatively, the method for producing the heat-resistant separator  12   a  can include, instead of the slitting step S 8 , a winding step of winding the heat-resistant separator original sheet  12   b  that has been inspected in the inspection step S 7 . For example, a core having a cylindrical shape can be used for winding the heat-resistant separator original sheet  12   b.  The heat-resistant separator original sheet  12   b  that has been wound can be, for example, shipped as it is with a wide width, as an original sheet of the heat-resistant separator  12   a.  In this case, the slitting step S 8  is omitted. 
     [Main Points] 
     As described above, the method for producing the heat-resistant separator  12   a  in accordance with Embodiment 1 includes: the removing step S 2  of removing the both end parts e of the separator original sheet  12   d,  which is being transferred, in the width direction of the separator original sheet  12   d  to thereby obtain the separator original sheet  12   c;  and the coating step S 3  of coating the separator original sheet  12   c  with the coating for forming the heat-resistant layer  4 . 
     With the configuration, the both end parts e of the separator original sheet  12   d  are removed in the removing step S 2 , before the coating step S 3 . This makes it possible to coat the separator original sheet  12   c  with the coating in a state in which flares (sags) that occurred at each of the both end parts e of the separator original sheet  12   d  have been reduced. 
     Therefore, according to Embodiment 1, it is possible to provide a method for producing the heat-resistant separator  12   a  which method is capable of coating the separator original sheet  12   c  with the coating for the heat-resistant layer  4  in the coating step S 3  so that the coating has a uniform film thickness. 
     The present invention is not limited to the embodiments, and can be altered by a skilled person in the art within the scope of the claims. An embodiment derived from a proper combination of technical means each disclosed in a different embodiment is also encompassed in the technical scope of the present invention. 
     [Remarks] 
     A method for producing a separator in accordance with Embodiment 1 includes: a removing step of removing both end parts of a base film in a width direction of the base film, the base film being transferred; and a coating step of coating, with a material for forming a functional layer, the base film whose both end parts have been removed in the removing step. 
     With the configuration, the both end parts of the base film (separator original sheet) are removed in the removing step, before the coating step. This makes it possible to coat the base film with the material in a state in which flares (sags) that occurred at each of the both end parts of the base film have been reduced. 
     Accordingly, the configuration makes it possible to provide a method for producing a separator which method is capable of coating a base film with a material for a functional layer in a coating step so that the material has a uniform film thickness. 
     Note that, according to Embodiment 1, “a width direction of a base film” means a direction which is (i) substantially perpendicular to a transferring (longitudinal) direction of the base film and (ii) substantially parallel to a surface of the base film. 
     A method for producing a separator in accordance with Embodiment 1 preferably further includes a second removing step of removing, from the base film having been coated with the material in the coating step, both end parts of the base film in the width direction of the base film. 
     With the configuration, the both end parts of the base film that has been coated with the material is further removed in the second removing step. This makes it possible to suitably prevent a wrinkle from occurring on the base film while the base film is being transferred. 
     A method for producing a separator in accordance with Embodiment 1 includes: a removing step of removing both end parts of a base film in a width direction of the base film, the base film being transferred; a coating step of coating, with a material for forming a functional layer, one surface of the base film whose both end parts have been removed in the removing step; a second removing step of removing, from the base film having been coated with the material in the coating step, both end parts of the base film in the width direction of the base film; and a second coating step of coating, with a material for forming a functional layer, the other surface of the base film whose both end parts have been removed in the second removing step. 
     With the configuration, the both end parts of the base film (separator original sheet) are removed in the removing step, before the coating step. This makes it possible to coat one surface of the base film in the coating step in a state in which flares (sags) that occurred at each of the both end parts of the base film have been reduced. 
     With the configuration, the both end parts of the base film (separator original sheet) are removed in the second removing step, before the second coating step. This makes it possible to coat the other surface of the base film in the second coating step in a state in which flares (sags) that occurred at each of the both end parts of the base film have been reduced. Further, with the configuration, the both end parts of the base film that has been coated with the material is further removed in the second removing step. This makes it possible to suitably prevent a wrinkle from occurring on the base film while the base film is being transferred. 
     Accordingly, the configuration makes it possible to provide a method for producing a separator which method is capable of coating both surfaces of a base film with a material for a functional layer so that the material has a uniform film thickness. 
     The method for producing a separator in accordance with Embodiment 1 is preferably configured such that each of the both end parts of the base film which both end parts are removed in the second removing step includes a part of a coated part of the base film that has been coated with the material in the coating step. 
     With the configuration, it is possible to eliminate a level difference between a coated part and a non-coated part which level difference is caused by a film thickness of the material (functional layer) with which one surface of the base film has been coated. 
     Accordingly the configuration makes it possible to prevent a wrinkle caused by the level difference. 
     The method for producing a separator in accordance with Embodiment 1 is preferably configured such that the part of the coated part which part is removed in the second removing step has a width of not smaller than 5 mm but not greater than 80 mm. 
     With the configuration, it is possible to appropriately remove the both end parts of the base film which both end parts are located in an area in which a flare can occur. Further, with the configuration, in a case where the both end parts of the base film coated with the material are appropriately removed, it is possible to suitably prevent a wrinkle from occurring on the base film while the base film is being transferred. 
     The method for producing a separator in accordance with Embodiment 1 is preferably configured such that each of the both end parts of the base film which both end parts are removed in the removing step has a width of not smaller than 5 mm but not greater than 200 mm. 
     With the configuration, it is possible to appropriately remove the both end parts of the base film which both end parts are located in an area in which a flare can occur. 
     A method for producing a separator in accordance with Embodiment 1 preferably further includes a slitting step of slitting, into parts each having a predetermined product width, the base film that has been coated with the material. 
     With the configuration, in a case where the base film is slit into parts each having a predetermined product width in the slitting step, it is possible to obtain a separator in which a functional layer has a uniform film thickness. 
     The method for producing a separator in accordance with Embodiment 1 is preferably configured such that the slitting step includes removing, at the same time as slitting the base film which has been coated with the material, both end parts of the base film in the width direction of the base film. 
     With the configuration, in a case where redundant films are removed from the both end parts of the base film in the slitting step, it is possible to suitably process the base film into parts each having a product width. 
     The method for producing a separator in accordance with Embodiment 1 is preferably configured such that the base film, whose both end parts have been removed in the removing step, is coated with the material in the coating step so that surfaces of respective both edge parts of the base film in the width direction of the base film are left uncoated as uncoated parts. 
     With the configuration, by carrying out edge-uncoated coating (partial coating) in the coating step in such a manner that the base film is coated with the material so that each of the surfaces of respective both edge parts of the base film in the width direction of the base film are left uncoated as uncoated parts, it is possible to prevent the material from flowing around from the both end parts of the base film to a back surface of the base film, unlike in a case where an entire surface coating is carried out so that the material is applied to the base film so as to cover up to the surfaces of the respective both edge parts of the base film. 
     Accordingly, the configuration makes it possible to reduce a product defect which is caused when the material flows around from the both end parts of the base film to the back surface of the base film. In a case where the uncoated parts of the base film are removed in a step to be carried out later, it is possible to obtain a separator in which a functional layer has a more uniform film thickness. 
     The method for producing a separator in accordance with Embodiment 1 is preferably configured such that the removing step and the coating step are successively carried out with respect to the base film being transferred. 
     With the configuration, the removing step and the coating step are successively carried out with respect to the base film being transferred, with no other step carried out between the removing step and the coating step. Accordingly, after a flare that occurred at the both end parts has been removed in the removing step, the coating is carried out with respect to the base film in the coating step which is subsequent to the removing step. 
     With the configuration, therefore, the coating is carried out with respect to the base film while flatness of the base film is suitably maintained. This makes it possible to coat the base film with the material for the functional layer in the coating step so that the material has a more uniform film thickness. 
     A device for producing a separator in accordance with Embodiment 1 includes: a removing section for removing both end parts of a base film in a width direction of the base film, the base film being transferred; and a coating section for coating, with a material for forming a functional layer, the base film whose both end parts have been removed by the removing section. 
     With the configuration, the both end parts of the base film are removed by the removing section, before the base film is coated with the material by the coating section. This makes it possible to coat the base film with the material in a state in which flares (sags) that occurred at each of the both end parts of the base film have been reduced. 
     Accordingly, the configuration makes it possible to provide a device for producing a separator which device is capable of coating a base film with a material for a functional layer by a coating section so that the material has a uniform film thickness. 
     The device for producing a separator in accordance with Embodiment 1 is preferably configured such that the removing section includes: a first cutting section for cutting off one of the both end parts of the base film; a second cutting section for cutting off the other one of the both end parts of the base film; and a fixing shaft that extends in the width direction of the base film and fixes the first cutting section and the second cutting section. 
     With the configuration, the first cutting section and the second cutting section are fixed to the fixing shaft extending in the width direction of the base film. This allows the first cutting section and the second cutting section to be more stable as compared with, for example, a case where the first cutting section and the second cutting section are fixed to respective different fixing members. 
     Accordingly, the configuration makes it possible for the first cutting section and the second cutting section to remove the respective both end parts of the base film at appropriate positions. 
     REFERENCE SIGNS LIST 
       1 : Lithium-ion secondary battery 
       4 : Heat-resistant layer (functional layer) 
       5 : Removing device (removing section) 
       7 : Coating device (coating section) 
       12 : Separator 
       12   a:  Heat-resistant separator (separator) 
       12   b:  Heat-resistant separator original sheet (base film) 
       12   c:  Separator original sheet (base film) 
       12   d:  Separator original sheet (base film) 
       52 : Fixing shaft 
       74 : Coating (material) 
     e: Both end parts (removal part) 
     F: Uncoated part 
     S 2 : Removing step 
     S 2 - 2 : Second removing step 
     S 3 : Coating step 
     S 3 - 2 : Second coating step 
     S 8 : Slitting step