Abstract:
Disclosed is a highly safe battery separator, in particular a separator for a lithium ion secondary battery, which reduces internal resistance, achieves good ionic conductivity, prevents passing of electrode active materials, and also prevents electrical short circuit by controlling deposition of lithium metal (dendrite). Also disclosed is a means for stably producing the battery separator with high productivity. Specifically disclosed are: a battery separator which is composed of a porous polyolefin sheet that is formed from a group of polyolefin nanofilaments that have an average filament diameter of less than 1 μm and a filament size distribution of 0.2 or less; and a means for producing the battery separator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national phase application of PCT International Patent Application PCT/JP2011/059896 filed Apr. 22, 2011, which claims benefit of JP 2010105128, filed Apr. 30, 2010, each of which is herein incorporated by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a battery separator comprising a porous sheet composed of polyolefin filaments whose average diameter is less than 1 μm, and relates more specifically to a highly safe lithium-ion secondary battery which has low internal resistance and good ionic conductivity, prevents passing of electrode active material, and also prevents electrical short circuit by controlling lithium metal depositions (dendrites). 
     BACKGROUND OF THE INVENTION 
     Evolutionary improvement of batteries is the most important element in recent advancements of electric automobiles and mobile electrical devices. A battery separator is a basic structural element of a battery, and improving performance of the separator is an important factor in achieving higher battery performance and making batteries smaller and safer. A porous film comprising a polyolefin film has previously been used as a battery separator, but the film method had a limitation of increasing the number of pores. Therefore, attempts have been made to create a non-woven cloth separator using microfibers (JP2004-15980A and the like). However, the use of a sea-islands structure type fiber method to manufacture a fine filament made the process complicated. The filament obtained in this manner was not sufficiently fine, and the steps needed to cut the filament and convert the short fibers into a non-woven cloth were complex, creating problems in terms of both quality and cost. 
     The electro-spinning method (henceforth referred to as the ES method) is a typical technique used to create fibers used in non-woven cloth as fine in a nanoscale as possible (You, Y., et. al “Journal of Applied Polymer Science, Vol. 95, p. 193-200, 2005”). However, the ES method, which involves dissolving a polymer in a solvent and applying high voltage, is unacceptable in the case that there is no safe solvent, such as in the case of polyolefin. In addition, a web created using the ES method does not contain continuous filaments and also contains short fibers since the filaments are cut during spinning. One of the problems is the generation of many molten resin lumps that are larger than the fiber diameter due to the shrinkage of cut sections during spinning. In addition, a molten type ES method that was conducted without using a solvent was tried (JP2009-299212A and the like), but the average fiber diameter did not reach a nanoscale. In addition, many defects greater than the fiber diameter were thought to be generated, resulting in problems similar to those observed in the solvent method. Furthermore, both a laser and a high voltage electrode were used in the manufacturing process which results in a complex process and causes both quality and cost problems when the process is used to manufacture a battery separator. Moreover, a melt-blown method is also one of the manufacturing processes used to create a super fine filament non-woven cloth, but the filament diameter reaches a nanoscale only with difficulty and the generation of molten resin lumps causes similar problems. 
     The present invention is obtained applying the technology which was a filament drawing technology using infrared radiation for heating under reduced pressure and an application of which was previously filed by the inventors (International Publication Number WO2008/084797A1). The present invention further improves the technology so that the technology can be effectively applied to a battery separator comprising a porous polyolefin sheet. 
     CROSS REFERENCE TO RELATED APPLICATIONS 
     
         
         Patent Document 1: JP2004-115980 (Claims 1-3) 
         Patent Document 2: JP2009-299212 (Claim 1, FIGS. 1 and 4) 
         Patent Document 3: International Publication No. WO2008/084797A1 (Pages 1-2, FIGS. 1-3) 
       
    
     Non-patent Document 
     
         
         Non-patent Document 1: You, Y., et. al “Journal of Applied Polymer Science, Vol. 95, pp. 193-200, 2005 (USA) 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     The purpose of the present invention is that resides in a highly safe battery separator with low internal resistance and good ionic conductivity that prevents passing of electrode active material and also prevents shorting by controlling lithium metal depositions (dendrites). More specifically, the present invention presents a battery separator for lithium ion secondary batteries and also presents a means to manufacture the battery separator consistently with good productivity. 
     Means to Solve the Problems 
     The present invention relates to a battery separator. A battery is a device that directly delivers direct current electrical power that has been generated by the electrical potential difference generated by a chemical reaction and so on. Among many different types of batteries, the present invention relates to a chemical battery and particularly relates to a secondary battery that can be repeatedly charged and used. Recently extremely high performance, downsizing, higher output power and improved safety of the secondary batteries are highly desired. The present invention was developed to respond to these demands. Among all the secondary batteries, lithium ion batteries have attracted much attention recently due to their high output power and excellent storage capacity. The present invention relates to a battery separator designed to enhance the performance of lithium ion batteries within the class of chemical batteries. Such lithium batteries encompass large batteries designed to be used in automobiles as well as small button batteries. 
     The present invention relates to a battery separator. A battery separator segregates the positive electrode and the negative electrode in a battery, prevents electrical short circuit between electrodes, and retain the electrolyte solution to insure ionic conductivity between the positive and negative electrodes. Therefore, a battery separator is required to be electrically insulating to prevent short circuits, to retain electrolyte solution well to achieve a good ionic conductivity, and to be stable in electrolyte solution. A battery separator is also required to have the physical strength needed in an assembly process and to prevent passing of active materials presented in a battery in the form of micron size particles. Furthermore a battery separator needs to enhance the safety function that prevents short circuits by controlling the deposition of metals such as lithium metal (dendrite). 
     The battery separator of the present invention comprises a porous sheet composed of polyolefin filaments. A polyolefin is a polymer formed by the polymerization of an olefin monomer containing a carbon-to-carbon double bond in the molecule. Polypropylene, high density polyethylene, and polymethylpentene (PMP) are particularly utilized in the present invention, and modified resins thereof are also used. Polyolefin exhibit good chemical resistance, are not decomposed in electrolysis solutions and can be used as stable battery separators. In addition, polyolefin have good oxidation resistance and contribute to longer operating life. 
     The polyolefin porous sheet of the present invention is characterized by the constituting filaments (filament grouping) comprising nanofilaments with an average filament diameter of less than 1 μm. A filament is a type of fiber that is basically of continuous length and is usually longer than ordinarily staple fibers that are about 50 mm long. The filaments are at least 100 mm long. The aspect ratio (length/diameter) is very large, particularly in the present invention, since the filament diameter is small, and the filament is basically continuous. The filament in the present invention is characterized as a nanofilament with an average filament diameter of less than 1 μm, but an average filament diameter of 0.7 μm or less is more desirable, and an average filament diameter of 0.5 μm or less is most preferred. The filament diameter (the average filament diameter) is obtained by counting one hundred filaments using an electron microscope at a magnification of from several thousand to twenty-thousand and calculating a mathematical average. Since the filament of the present invention has a small diameter, the number of filaments in a set area increases. The number of apertures constituting a porous sheet is extremely large, and the aperture diameter is micronized. A high performance battery separator is created by increasing the number of apertures and reducing the aperture size. 
     Since the filament of the present invention is a continuous filament, it is characterized by very few resin pieces referred to as lumps or shots. The small resin lumps or shots are thought to be generated by shrinking of both sides of cutting pieces when filaments are cut during spinning or drawing. The porous sheet of the present invention comprises drawn polyolefin filaments, and the filament diameter is on a nanoscale. However, the filament rarely breaks during drawing and does not generate lumps or shots since it is basically a continuous filament. Therefore, a high performance battery separator can be obtained. 
     Drawn nanofilaments constituting a porous sheet in the present invention are characterized by a very consistent filament diameter. The filament diameter distribution is obtained using SEM (Scanning Electron Microscopy) photographs and software for measuring the diameters of one hundred filaments. In addition, the standard deviation is calculated from the measurements and is used as a measure of the filament diameter distribution. Furthermore, the average value of the filament diameter is obtained using the measurement method and is used as the average filament diameter for the present invention. The standard deviation of the filament diameter distribution for ordinary spun-bonded non-woven fabric and melt-blown non-woven fabric is more than 0.5, but the standard deviation in a nanofilament porous sheet of the present invention was 0.2 or lower and was preferably 0.1 or lower in spite of the fact that the filament diameter was smaller. A consistent filament diameter results in uniform pore size of the filament, lots of number of filaments per weight of a porous sheet, and inexistence of extremely thick or extremely thin filament. This gives the porous sheet excellent chemical stability, such as chemical resistance and the like. 
     The nanofilaments constituting the porous sheet of the present invention are characterized by super high multiple drawing, and the multiple is at least 100,000, preferably 200,000 and most preferably at least 500,000. Such super high multiple drawing forms filaments with nanosized diameter. The high quality battery separator described above is obtained using these filaments. In addition, the extensive drawing induces high strength low elongation as described below and high degree of molecular orientation which improves crystallinity, resulting in the development of thermal stability. 
     As described above, the nanofilaments constituting the porous sheet of the present invention are subjected to super high multiple drawing and are characterized by high strength and low elongation. The nanofilaments have extremely small filament diameter, and a single filament is difficult to handle. Therefore, the filament strength is difficult to measure. However, the high degree of thermal stability indicated by DSC measurement is an indication of a high degree of molecular orientation. The high strength displayed by the porous sheet is an advantage when incorporating a battery separator into a battery. The low elongation results in good dimensional stability when dynamic external force is applied and makes the porous sheet easier to incorporate into a battery. Thus, the porous sheet is highly compatible with automatic assembly devices. 
     A non-woven cloth is ordinarily a porous sheet formed by some kinds of confounding fibers. The number of polyolefin filaments per unit weight is extremely large in the present invention since the filament diameter is very small. Therefore, filaments tangle without a confounding step when polyolefin filaments are accumulated similarly to melt-blown non-woven fabrics manufacturing process, so a sheet can be formed using a simple pressing process. Means such as thermal embossing, needle punching, water-jet machining, and adhesive bonding may naturally be used. 
     The polyolefin porous sheet of the present invention is an application of an earlier invention (Patent Document 3) to polyolefin filaments, an earlier invention which was developed by one of the inventors of the present invention and was obtained by using super high multiple filaments drawing means involving a carbon dioxide gas laser beam and the pressure difference between orifices. Superdrawning by a multiple of several 10,000 to several 100,000 of the original polyolefin filaments with a filament diameter of from several tens of micrometers to several hundreds of micrometers yields super fine filaments with a diameter of from several micrometers to several tens of nanometers. The drawn filaments are accumulated to form a porous sheet. The original polyolefin filaments of the present invention are filaments manufactured and then wound on a reel and the like. The filaments obtained by cooling or coagulating filaments fused or dissolved in the spinning step can be used in the subsequent spinning step as the original filaments of the present invention. The original filaments are preferably used individually, but from several to several tens of the filament may also be used. 
     In the present invention, the original filaments transferred from a filament let-off step are drawn. Many types of filament transfer methods such as nip rollers, a combination of several stages of driving rollers and the like may be used to transfer the original filaments at a constant transfer rate. 
     In the present invention, many polyolefin original filaments are transferred at P 1  pressure using a transfer method and are introduced into a drawing chamber under P 2  pressure (P 1 &gt;P 2 ) through orifices. The many filaments (filament grouping or clump) that passed through the orifices are heated using carbon dioxide gas laser beam irradiation, and the traction generated by the gas flow caused by the pressure difference between P 1  and P 2  is used to draw the filaments. Now, a preferred mode of operation is to have the pressure P 1  be atmospheric pressure and the pressure P 2  be a reduced pressure inside the drawing chamber when the original filaments are discharged since the pressure difference is conveniently achieved. When P 1  is added pressure and P 2  is reduced pressure, the pressure difference between P 1  and P 2  can be agreeable without creating an intense degree of pressure reduction for P 2 . Now, the drawing chamber is sometimes divided into a drawing chamber in the narrow sense in which original polyolefin filaments are drawn using a laser beam and a filaments accumulation chamber in a narrow sense in which the drawn filaments are stacked at the orifices discharge outlet, but the drawing chamber in the narrow sense and the filament accumulation chamber in the narrow sense are connected into one unit and maintained at the same pressure as each other to constitute a drawing chamber in the broad sense. 
     Now, ordinary room temperature air is used to generate P 1  and P 2 . However, heated air is sometimes used when attempting to preheat original filaments or attempting to heat treat drawn filaments. 
     The original filaments supply chamber and the drawing chamber in the present invention are connected by orifices. Inside the orifice, high speed gas flow is created in the narrow gap between the original filament and the inside diameter of the orifice and is generated by the pressure difference P 1 &gt;P 2 . The internal orifice diameter D of the orifice and the diameter d of the filament must not be too different in order to create a high speed gas flow. Based on experimental results, D&gt;d and D&lt;30 d, preferably D&lt;10 d, more preferably D&lt;5 d and most preferably D&lt;2 d. 
     The internal orifice diameter, D, mentioned above refers to the diameter of the orifice at the exit end. If cross-section of the orifice is not round, the diameter D refers to the narrowest diameter. Similarly, the filament diameter, d, also refers to the smallest diameter when the cross-section is not round. The smallest cross-section was determined by measuring at ten locations, and an arithmetic average was calculated. In addition, a tapered orifice shape that narrows to the exit without uniform orifice diameter is preferred. As far as the orifice exit is concerned, the exit is at the bottom in a vertically positioned orifice since an original filament ordinarily moves from top to bottom. When an original filament moves from bottom to top, the exit is located on top of the orifice. Similarly, the exit is on a side of an orifice when the orifice is horizontally positioned and an original filament moves in the horizontal direction. 
     An internal orifice structure with low resistance is desirable since a high speed gas flows inside the orifice as described above. In the present invention, the orifices may be shaped independently, but many openings may be created in a board to obtain a multi opening orifice. A round cross-section of the orifice interior is preferred. When multiple numbers of filaments are generated, or when elliptical or tape shaped filaments are prepared, orifices with elliptical or rectangular cross-sections may be used. In addition, an orifice with a large entrance to easily introduce an original filament and a narrow exit to reduce the run resistance on the filament is preferred since such a shape raises the flow rate from the orifice exit. 
     The orifice in the present invention plays a different role than a conventional air introduction pipe located before drawing. The conventional air introduction pipe plays a role in focusing a laser beam on a set position on a filament and in transferring an original filament to the set position with lower resistance. The present invention differs in that it adds a high speed gas flow generated by the pressure difference between the pressure P 1  of the original filament supply chamber and the pressure P 2  of the drawing chamber. Now, in an ordinary spun-bond non-woven cloth manufacturing process, tension is applied to a fused filament using an air-sucker and the like. However, an air-sucker in a spun-bond non-woven cloth manufacturing process and the orifice in the present invention produce totally different action mechanisms and effects. In a spun-bond process, a fused filament is moved with a high speed fluid inside an air-sucker, and most of the attenuation of the filament diameter is completed inside the air-sucker. In contrast, solid filaments are transferred using orifices in the present invention, the filaments are not attenuated inside the orifices, the filaments are irradiated with a laser beam after they exit the orifices, and the drawing are initiated for the first time at this point. In addition, a high speed fluid is generated in a spun-bond process by sending high pressure air into the air-sucker, and the process of the present invention differs in that the high speed fluid is generated inside the orifices by the pressure difference between the chambers before and after the orifices. In addition, the effects differ greatly. In a spun-bond process, the smallest filament diameter obtained is at most about 10 μm. In contrast, nanofilaments with filament diameter less than 1 μm are obtained in the present invention. 
     The air velocity v at the orifice exit in the orifice used in the present invention is expressed by the equation shown below. (Graham&#39;s theorem)
 
 v={ 2( P 1 −P 2)/ρ} 1/2  
 
     Here, ρ is the air density. 
     The air velocity at an orifice exit and its speed of sound conversion are shown in Table 1 where P 1  represents atmospheric pressure and P 2  is a negative pressure. In the present invention, the air velocity at the orifice exit is preferably at least 340 m/sec (the speed of sound at 15° C.) and at least 365 m/sec is more preferred. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 P2 
                 The air velocity 
                 Mach number 
               
               
                 kPa 
                 at the orifice exit (m/sec) 
                 at r.t. 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 85 
                 152 
                 0.449 
               
               
                 80 
                 182 
                 0.523 
               
               
                 75 
                 204 
                 0.600 
               
               
                 70 
                 223 
                 0.658 
               
               
                 65 
                 241 
                 0.710 
               
               
                 60 
                 258 
                 0.759 
               
               
                 50 
                 289 
                 0.834 
               
               
                 30 
                 342 
                 0.987 
               
               
                 20 
                 365 
                 1.06 
               
               
                 6 
                 396 
                 1.15 
               
               
                   
               
             
          
         
       
     
     The original filaments discharged from orifices are heated at the orifice exit with a carbon dioxide laser beam, and they are drawn by the tension generated by the high speed fluid from the orifices. The center of carbon dioxide laser beam should be focused at 30 mm or less from the orifice tip with 10 mm or less preferred and 5 mm or less most preferred. If the focused center separates so far away from the orifice tip, the original filaments swing and move continuously, so they do not remain in a fixed position. Therefore, a carbon dioxide laser beam cannot capture the filaments consistently. In addition, the high speed air from orifices exit loses its speed rapidly once it moves away from the orifices, the tension applied on filaments weaken with distance from the orifices and the consistency also decreases. 
     The present invention is characterized by heating and drawing original filaments using a carbon dioxide gas laser beam. The carbon dioxide gas laser beam of the present invention has a wavelength about 10.6 μm. The laser can focus its irradiation (beam) tightly and also concentrate on a specific wavelength without wasting much energy. The carbon dioxide gas laser of the present invention has a power density of at least 50 W/cm 2 , preferably at least 100 W/cm 2  and most preferably at least 180 W/cm 2 . By concentrating high power density energy on a narrow drawing zone, the super high multiple drawing of the present invention is made possible. Now, the laser beam of the present invention may also be used by changing the shape using a beam expander and the like. 
     The original filaments of the present invention are heated to a temperature appropriate for drawing using a carbon dioxide gas laser beam, and this heated zone which is located along the filament axis within 4 mm up and down (8 mm length) of the filament center is preferred, within 3 mm up and down is more preferred, and within 2 mm up and down is most preferred. The diameter of the beam is measured along the filament axis in motion. In the present invention, original filaments are composed of multiple numbers filament, the diameter of beam is measured along the original filament axis. The present invention makes possible a high degree of attenuation by rapidly drawing in a narrow area, and yields drawn filaments attenuated into the nanofilament size zone. In addition, the frequency of draw fracture was minimized even though the filaments were drawn to a super high multiple. Now, when the original filament irradiated with the carbon dioxide gas laser beam is a multi-filament, the filament center described above identifies the center of a filament bundle in a multi-filament. 
     Many original polyolefin filaments discharged from orifice are drawn upon irradiation with a laser beam. At this point, the laser beam needs to uniformly hit the many original filaments. In this regard, the entire drawing chamber is finely rotated so that the entire original filaments are uniformly drawn. The drawing is preferably initiated at its ideal rotated position. Now, the entire drawing chamber is not only rotated but is also finely moved in the horizontal direction (direction X), the direction of beam irradiation (direction Y) and the direction of height (direction Z) to seek an ideal position. 
     A running conveyor is used as the accumulation device for the drawn filaments of the present invention. A conveyor is used to accumulate and stack the drawn filaments to provide an accumulation of super fine filaments or a porous sheet that can be rolled up. A porous sheet comprising nanofilaments can be manufactured using this process. A net-like forwarding unit is ordinarily used as the conveyor in the present invention, but a belt or a cylinder may also be used for the accumulation. 
     A winding device for filaments, sheets and the like may also be used as the accumulation device for the drawn filaments of the present invention. A winder equipped with a tubular paper tube or an aluminum tube corresponding to the width of the descending, drawn filaments and rotated about its axis is used to directly accumulate, capture and wind the drawn filaments onto such a tubular material. 
     When a winder is used as the accumulation device, the installation of a collection guide that bows from the wall along the winding axis is desirable. The collection guide has a width corresponding to the width of the many drawn filaments descending onto the outside of the rotating tube. The corresponding width refers to a width wider than the descending filaments. A width that is about 50 mm wider on both sides is preferred, and a width that is about 100 mm wider on both sides is most preferred. When drawn filaments moving from orifices along with high speed air are wound axially, the high speed air is reflected along the winding axis and is diffused to the surrounding space. The filaments accumulated along the winding axis are sometimes disturbed. In such a case, the high speed air is curved in the direction of the rotating axis by the collection guiding wall to prevent wandering of the drawn filaments. The distance along the axial winding to the collection guide wall is 500 mm or less, and 200 mm or less is preferred and 100 mm or less is most preferred. 
     The drawn filaments accumulated on a conveyor are preferably heat-set to form a sheet. When heat-set, the porous sheet of the present invention can be used as a battery separator possessing dimensional stability and thermal stability. Thus the porous sheet is preferably wound on a sheet winding device installed inside a drawing chamber. The heat-setting is executed by passing the porous sheet through a space in which hot air is circulated or by passing the porous sheet over a roller heated using induction heating and the like. The heat setting temperature for a porous sheet comprising polyolefin nanofilaments of the present invention is preferably in a temperature range that is higher than 30° C. below the polyolefin melting point and lower than 5° C. below the melting point and more preferably in a temperature range that is higher than 20° C. below the melting point and lower than 10° C. below the melting point. 
     The draw ratio λ in the present invention is represented by the equation below using the original filament diameter, do, and the filament diameter after drawing, d. In this case, the calculations were made on the assumption that the filament density was constant. The filament diameter was measured using a scanning electron microscope (SEM). The original filament was photographed at a magnification of one hundred, and the drawn filaments were photographed at a magnification of one thousand or greater. The average value of one hundred measurements was used.
 
λ=( do/d ) 2  
 
     The drawn filament of the present invention undergoes molecular orientation during drawing and becomes thermally stable. The drawn filament of the present invention has a very small filament diameter, and then the molecular orientation of the filament is difficult to measure. Thermal analysis results indicated that the drawn filament of the present invention is not only thin but also has molecular orientation. The differential scanning calorimeter (DSC) measurements of the original filaments and drawn filaments were made using THEM PLUS2 DSC8230 manufactured by Rigaku K. K. at a temperature increase rate of 10° C./min. 
     Advantages of the Invention 
     The battery separator of the present invention is a porous sheet comprising nanofilaments and has a high porosity ratio and low internal resistance to yield good conductivity. Furthermore, the battery separator of the present invention prevents passing of active electrode substances that cause deterioration of the electrodes and controls lithium metal deposition (dendrites) to prevent short circuits and enhance safety, which becomes the separator particularly suitable for secondary lithium batteries. Since the separator of the present invention comprises polyolefin filaments, it is chemically stable and has chemical resistance that prevents separator decomposition in electrolysis solutions. Since it is a polyolefin the separator, it also has oxidation resistant and can extend the battery life. In addition, the filament diameter is consistent and the porous sheet has very uniform pore size, to yield high performance and stability in a separator. Furthermore, the polyolefin nanofilament constituting the battery separator of the present invention is highly drawn and exhibits extensive molecular orientation. So the battery separator gains the exceptional strength, which improves its physical strength and makes handling of the separator easier when incorporating it into a battery. In addition, since the porous sheet used as a battery separator in the present invention is highly heat treated, it is characterized by excellent dimensional stability and safety. 
     The ES process used previously to produce nanofibers included a step in which a polymer is dissolved in a solvent, and the solvent needed to be removed from the product. This method made the production process complex and made the production cost increasing. In addition, the product that was obtained experienced quality problems, such as the formation of small resin lumps, and a broad filament diameter distribution. In addition, the fibers obtained were short fibers, and their lengths were from several millimeters to at most several tens of millimeters. 
     The present invention yields polyolefin nanofilaments with improved molecular orientation that are readily obtained using simple means without requiring a special high precision, highly sophisticated device. The present invention is characterized by the fact that drawn filaments can be wound directly on a winding device to obtain a porous sheet. Therefore, the present invention presents a means to manufacture a high performance battery separator consistently and with good productivity. 
     Furthermore, the present invention allows nanofilaments to be manufactured in a sealed chamber in a closed system and prevents dispersion of the nanofilaments obtained into the atmosphere as is the case with melt-blown nonwoven fabrics manufacturing process or ES process conducted in an open system. Therefore, the present invention can help maintain a very safe work environment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional drawing of an example of a battery with the battery separator of the present invention. 
         FIG. 2  is a cross-sectional drawing of an apparatus, illustrating an example in which original filaments supply chamber in the present invention is a chamber at P 1  pressure and a drawing chamber is a chamber at P 2  pressure. 
         FIG. 3  is an oblique perspective drawing of an example in which polyolefin nanofilaments are obtained and the filaments are accumulated directly on a winding device. 
         FIG. 4  is a histogram of the filament diameter measurements of filaments drawn in the present invention. 
         FIG. 5  is a scanning electron microscope image (20,000 magnification) of filaments obtained when the laser power was 20 W. 
         FIG. 6  is a scanning electron microscope image (1,000 magnification) of filaments obtained when the laser power was 20 W. 
         FIG. 7  is a scanning electron microscope image (20,000 magnification) of filaments obtained when the laser power was 25 W. 
         FIG. 8  shows endothermic curves obtained using differential scanning calorimetry measurements of nanofilaments constituting the porous sheet of the present invention. 
         FIG. 9  is an outside appearance photograph of a single layer laminate cell with a porous sheet. 
         FIG. 10  shows discharge curves of battery A in discharge rate test, using a porous sheet of the present invention. 
         FIG. 11  shows discharge curves of battery B in discharge rate test, using a porous sheet of a comparative example. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Examples of the embodiments of the present invention are described below using the figures.  FIG. 1  is the cross-sectional view of a battery used in a performance test for the battery separator of the present invention. Battery  1  contains a positive electrode  3  and a negative electrode  4  facing each other inside a can  2 . A battery separator  5  that is a porous sheet comprising polyolefin nanofilaments of the present invention is sandwiched between the positive electrode  3  and negative electrode  4 , and the surrounding space is immersed in an electrolyte solution  6 . The positive electrode  3 , negative electrode  4 , separator  5 , electrolyte solution  6  and the like inside the can  2  are covered with a lid  8  through a packing material  7 . Since the can  2  and the lid  8  are conductive, the can  2  plays as a positive electrode terminal and the lid  8  plays as a negative electrode terminal. So, electrical energy can be discharged. 
       FIG. 2  is a cross-sectional view showing an example of a equipment used to manufacture a porous sheet comprising polyolefin nanofilaments used in the battery separator of the present invention. The original filaments supply chamber  11  is a chamber at P 1  pressure, and the drawing chamber  12  is a chamber at P 2  pressure. The original filaments supply chamber  11  is connected to a compressor (or a vacuum pump) through a valve  13  and a pipe  14 . Pressure P 1  is controlled by reference to a pressure gauge  15 . The drawing chamber  12  is connected to a vacuum pump (or a compressor) through a valve  16  and a pipe  17 . Pressure P 2  is controlled by reference to a pressure gauge  18 . The polyolefin original filaments  19   a ,  19   b ,  19   c  are wound on reels  20   a ,  20   b  and  20   c  are let-off at a constant speed from nip rollers  22   a ,  23   a ,  22   b ,  23   b ,  22   c ,  23   c  through combs  21   a ,  21   b ,  21   c  to orifices  24   a ,  24   b ,  24   c.    
     The space after the exit from the orifices  24   a ,  24   b  and  24   c  in  FIG. 2  is the drawing chamber  12  at P 2  pressure. The polyolefin original filaments  19   a ,  19   b ,  19   c , exiting the orifices  24  are introduced into the drawing chamber  12  along with high speed air generated by the pressure difference, P 1 -P 2 , between the original filaments supply chamber  11  and the drawing chamber  12 . The discharged polyolefin original filaments  19   a ,  19   b ,  19   c  are irradiated immediately under the orifice with a laser beam  26  generated by a carbon dioxide gas laser beam emitting device  25 . The laser beam  26  is used to irradiate the moving original filaments  19   a ,  19   b  and  19   c . A laser beam power meter  27  is installed at the location reached by the laser beam  26 , and the laser power is preferably adjusted to a constant level. Tension is applied to the lower section of the original filaments  19   a ,  19   b ,  19   c  heated with the laser beam  26  by the high speed air generated by the pressure difference P 1 -P 2  to draw the filaments. The drawn filaments  28   a ,  28   b ,  28   c  descend and are accumulated on a conveyor  29  to form a porous sheet  30  comprising numerous polyolefin nanofilaments. 
     In  FIG. 2 , the polyolefin porous sheet  30  on the conveyor  29  is preferably stabilized by suction from a negative pressure suction chamber  31  at the back of the conveyor  29 . The polyolefin porous sheet  30  is preferably heat treated with at least one of the heat treatment means described below. The heat treatment means  1 ) involves a heat treatment using a radiated heat from an infrared lamp  32  to heat treat a polyolefin porous sheet  30 . The heat treatment means  2 ) involves heating of a polyolefin porous sheet  30  with a hot air ejected from a hot air nozzle  33 . The porous sheet  30  discharged from a conveyor  29  is preferably compressed using a rubber roller  34  on the conveyor  29  to form a sheet. The heat treatment means  3 ) involves treating a polyolefin porous sheet  30  discharged from the conveyor  29  using a heated roller  35  and compressing using a rubber roller  36  to form a sheet. The heat treated polyolefin porous sheet  37  is wound on a winding roller  38 . 
       FIG. 3  shows an oblique perspective drawing of an example of an equipment that uses a winding device as a filament accumulation device in the present invention and contains a collection guide inside a drawing chamber. Numerous polyolefin original filaments  41   a ,  41   b ,  41   c  . . . pass through a filament supply device (not shown in the drawing) and are led into a drawing chamber  43  under pressure, P 2 , (maintained under negative pressure in this drawing using a valve V) through orifices  42   a ,  42   b ,  42   c  . . . . The laser beam  45  emitted from a carbon dioxide gas laser emitting device  44  irradiates many original filaments  41   a ,  41   b ,  41   c  . . . immediately under the orifices  42 . The laser beam  45  passes through a window comprising Zn—Se before it is introduced into the drawing chamber  43 , but the window is not shown in the drawing. The original filaments  41   a ,  41   b ,  41   c  . . . are drawn using the heating caused by the laser beam  45  and the tension applied to the lower section of the filament by the high speed air generated by the pressure difference between P 1  and P 2 . The drawn filaments  47   a ,  47   b ,  47   c  . . . descend and are wound directly on a winding device  48 . The winding device  48  comprises a winding tube  50  attached to a winding trestle  49 , and a motor (not shown in the drawing) drives the winding tube  50  to rotate. The drawn filaments  47  are wound directly on the winding tube  50  where they accumulates and form a porous sheet  51  comprising numerous polyolefin nanofilaments. The drawing chamber  43  is characterized by the presence of a collection guide  52  that curves along the winding tube  50 . The collection guide  52  stabilizes the polyolefin porous sheet  51 , winds the sheet around the winding tube  50  and yields a polyolefin porous sheet  51  with good texture. 
     Now, in  FIG. 3 , the drawing chamber  43  is mounted on fine position adjustment platforms  53 ,  54  and  55 , and the position is finely adjusted to optimally collect the polyolefin original filaments  41   a ,  41   b ,  41   c  exiting from orifices  42   a ,  42   b ,  42   c  in the irradiation zone of the laser beam  45 . The lower most fine position adjustment platform  55  adjusts up and down (Z axis), the middle fine position adjustment platform  54  adjusts horizontally (X and Y axis) and the topmost fine position adjustment platform  53  is a turntable that finely adjusts the position by rotating. 
     Example 1 
     163.2 μm polypropylene filaments were prepared as original filaments. The weight average molecular weight of the polymer was 349,000, and the number average molecular weight was 89,300. The isotacticity was 92.4%. In the equipment shown in  FIG. 3 , the laser beam diameter of the laser generated by the carbon dioxide gas laser emitting device was 3.5 mm. The experiment was conducted using an internal orifice diameter of 0.3 mm, seven orifices and filament supply rate of 0.1 m/min. The drawing ratios calculated using the filament diameter when the laser power was changed from 10 W to 25 W at a degree of vacuum in the drawing chamber of 10 kPa are shown in Table 2. According to the data in Table 2, the average filament diameter was already in the nanofilament zone when the laser power was 10 W, and the standard deviation for the filament diameter distribution was as low as 0.12. Thus the calculated drawing ratio exceeded 200,000. The standard deviation further decreased to 0.0455 for an average filament diameter of 220 nm at a laser power of 20 W, and the drawing ratio reached 550,000. The average filament diameter was 200 nm or smaller when the laser power was 25 W, and the drawing ratio reached 750,000. A histogram of the filament diameter distribution at laser power settings of 10 W, 15 W, 20 W and 25 W is shown in  FIG. 4 . In addition, scanning electron microscope images (SEM images) of the filaments obtained at a laser power setting of 20 W are shown in  FIG. 5  (20,000 magnification) and  FIG. 6  (1,000 magnification). The SEM images demonstrated that the filaments were continuous filaments with no lumps and the filament diameter was uniform. In addition, the SEM images (20,000 magnification) of the filaments obtained using a laser power setting of 25 W is shown in  FIG. 7 . 
     
       
         
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Filament Diameter 
                   
               
             
          
           
               
                   
                   
                   
                   
                 Standard 
                 Drawing 
               
               
                 Sample 
                 max. μm 
                 min. μm 
                 av. μm 
                 deviation 
                 Ratio 
               
               
                   
               
             
          
           
               
                 original 
                   
                   
                 163.2 
                   
                 1 
               
               
                 Power 10 W 
                 0.910 
                 0.142 
                 0.364 
                 0.1216 
                 201,018 
               
               
                 Power 15 W 
                 0.645 
                 0.231 
                 0.339 
                 0.0624 
                 231,763 
               
               
                 Power 20 W 
                 0.439 
                 0.155 
                 0.220 
                 0.0455 
                 550,296 
               
               
                 Power 25 W 
                 0.338 
                 0.101 
                 0.188 
                 0.0467 
                 753,576 
               
               
                   
               
               
                 Filament supply rate: 0.1 m/min 
               
               
                 P2: 10 kPa 
               
             
          
         
       
     
     Example 2 
     Differential scanning calorimetry (DSC) measurements were conducted on the filaments obtained in Example 1, described above, and the results are shown in Table 3 and  FIG. 8 . The  FIG. 8  showed that the endothermic peak was suddenly sharpened upon drawing, indicating increase in crystallinity. The table 3 also showed increase in melting point, the heat of fusion (ΔHm) and the crystallinity indices (Xc) as average filament diameter (day) decreased. These results suggested the enhanced molecular orientation in the filaments. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
                 Tm 
                 ΔHm 
                 Xc 
                 dav 
               
               
                   
                 Sample 
                 (° C.) 
                 (J/g) 
                 (%) 
                 (nm) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Original 
                 159.9 
                 −95.84 
                 45.9 
                 163.2 
               
               
                   
                 Filament 
               
               
                   
                 Power 10 W 
                 158.7/163.1 
                 −104.07 
                 49.8 
                 0.364 
               
               
                   
                 Power 15 W 
                 161.6 
                 −106.08 
                 50.8 
                 0.339 
               
               
                   
                 Power 20 W 
                 160.7 
                 −110.64 
                 52.9 
                 0.220 
               
               
                   
                 Power 25 W 
                 160.0 
                 −108.8 
                 51.1 
                 0.188 
               
               
                   
                   
               
               
                   
                 Filament supply rate: 0.1 m/min 
               
               
                   
                 P2: 10 kPa 
               
             
          
         
       
     
     Example 3 
     The equipment in Example 1 was used with the same original filaments. The filaments were directly wound for ten minutes on a winding tube in the lower section ( FIG. 3 ) using a drawing chamber with a degree of vacuum of 20 kPa, filament supply rate of 0.1 m/min and laser power of 20 W. The wound sheet was cut from the winding tube to obtain porous sheets 18 cm wide, 17 cm long, with a weight of 4.16 g/m 2  and average filament diameter of 0.409 μm. A single layer laminated cell with a battery separator obtained using this sheet was prepared ( FIG. 9 ), and various properties of the battery were measured. The results are shown in Tables 4 and 5. Now, the comparative example was a monoaxially drawn porous polypropylene film (trade name “CELGARD,” 25 μm thick). The positive electrode of the battery was a molded product containing 89 wt % LiCoO 2 , 6 wt % conductive carbon black and 5 wt % binder [poly(vinylidene fluoride)]. The negative electrode was molded using 90 wt % mesocarbon graphitized micro-beads with 10 wt % poly (vinylidene fluoride) used as the binder. The electrolyte solution was obtained by dissolving 1 mole/liter of LiPF 6  in a 1:1 mixed solvent (volumetric ratio) of ethylene carbonate and dimethyl carbonate. Using such a battery, an eight hour constant current, constant voltage charging test was conducted at a charging current of 1.8 mA and charging voltage of 4.2 V. The alternate current impedance was subsequently measured at each frequency, and the results are shown in Table 4. The examples had lower electrical resistance at each frequently when compared to comparative examples. In addition, an eight hour constant current, constant voltage charging test was conducted at a charging current of 1.8 mA and charging voltage of 4.2 V. The battery was subsequently discharged at a discharge current of 1.8 mA to 2.7 V to measure the initial capacity of the battery. Then, only the discharge current was changed to 18 mA, and the charge-discharge cycle was repeated. The results (Table 5) clearly showed superiority of the examples to the comparative examples in the capacity and capacity retention at increased discharge current. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 Frequency 
                   
                 Comparative 
               
               
                 (Hz) 
                 Example 
                 Example 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 20000 
                 0.61 Ω 
                 1.11 Ω 
               
               
                 1000 
                 1.15 Ω 
                 1.63 Ω 
               
               
                 1 
                 4.03 Ω 
                 4.90 Ω 
               
               
                 0 
                 4.40 Ω 
                 5.28 Ω 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 5 
               
             
             
               
                   
                   
               
               
                   
                 Dischage Current 
                   
               
             
          
           
               
                   
                 1.8 mA 
                 18 mA 
               
               
                   
                   
               
             
          
           
               
                   
                 Example 
                 Capacity/mAh 
                 8.75 
                 8.21 
               
               
                   
                   
                 Capacity 
                 100 
                 93.8 
               
               
                   
                   
                 Retention (%) 
               
               
                   
                 Comparative 
                 Capacity/mAh 
                 8.66 
                 8.03 
               
               
                   
                 Example 
                 Capacity 
                 100 
                 91.8 
               
               
                   
                   
                 Retention (%) 
               
               
                   
                   
               
             
          
         
       
     
     Example 4 
     The separator of the present invention was evaluated using a model 2032 coin battery and tests were conducted in the manner described below. The positive electrode was prepared as described below. NMP was added to a mixture of 90 wt. % of LiCoO 2  used as the cathode active material, 5 wt. % of acetylene black used as the conductive additive and 5 wt. % of PDVF used as the binder, and a slurry was prepared upon mixing. The slurry was dropped onto an aluminum current collector, and a film was formed using a film applicator equipped with a micrometer and an automatic coating device. After dried in a 110° C. oven in a nitrogen atmosphere, the film was stamped out and pressed to form disk shape positive electrode with a diameter of 15 mm. The positive electrode contained about 26 mg of the active material. The negative electrode was prepared as described below. NMP was added to a mixture of 94 wt. % of a artificial graphite used as the anode active material, 1 wt. % of acetylene black as the conductive additive and 5 wt. % PVDF as the binder, and a slurry was prepared upon mixing. The slurry was dropped onto a copper current collector, and a film was formed using a film applicator equipped with a micrometer and an automatic coating device. After dried in a 110° C. oven in a nitrogen atmosphere, the film was stamped out and pressed to form disk shape negative electrode with a diameter of 15 mm. The negative electrode contained about 13 mg of the active material. The positive electrode, negative electrode, an electrolyte solution obtained by dissolving 1 mole/liter of LiPF 6  in a mixed solvent of ethylene carbonate and dimethyl carbonate in a volumetric ratio of three to seven and the separator stamped out into a disk were used to prepare a coin battery. A polypropylene nanofilament sheet of the present invention with filament diameter of about 0.4 μm and weight of 6 g/m 2  was used to prepare Battery A. As a comparative example, a battery with the separator of a polypropylene porous film (trade name “CELGARD,” PP, 25 μm) was prepared. This battery was referred as Battery B. The coin batteries prepared in the manner described above were placed inside a 25° C. constant temperature vessel and charge-discharge tests were conducted. Now, the designed battery capacity was about 4 mAh. The first eight hours of charging were conducted at 0.5 C constant current and 4.2V constant voltage, and the battery was subsequently discharged to 3V at 0.25 C constant current. The battery was rested for 10 minutes after discharged. Then, the batteries were subjected to discharge rate test. The discharge current was increased for each cycles from 0.25 C, 0.5 C, 10 up to 2 C while charging process was conducted at a constant current of 0.5 C and constant voltage of 4.2V for four hours throughout the discharge rate test. 10 minutes rest was also included after discharging in each cycle. The curves of the discharge rate test for Batteries A and B are shown in  FIGS. 10 and 11 , respectively. The discharge capacity retention was normalized using the discharge capacity at 0.250 as the standard, and the results are summarized in Table 6. The result (Table 6) clearly indicated that Battery A had a higher discharge capacity retention than Battery B at a high C rate. Therefore, the use of the nanofilament sheet of the present invention as the separator improved the battery properties at a high C rate, compared to the batteries prepared using conventional porous films. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                 0.25 C 
                 0.5 C 
                 1.0 C 
                 2.0 C 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Battery A 
                 100 
                 99.0 
                 98.0 
                 96.0 
               
               
                   
                 Battery B 
                 100 
                 98.6 
                 97.1 
                 94.9 
               
               
                   
                   
               
             
          
         
       
     
     INDUSTRIAL APPLICABILITY 
     The present invention relates to a battery separator constituting a porous sheet comprising polyolefin nanofilaments. 
     DESCRIPTION OF THE SYMBOLS 
     
         
           1 : Battery 
           2 : Can 
           3 : Positive electrode 
           4 : Negative electrode 
           5 : Battery separator 
           6 : Electrolyte solution 
           7 : Packing 
           8 : Lid 
           11 : Original filament supply chamber 
           12 : Drawing chamber 
           13 : Valve 
           14 : Pipe 
           15 : Pressure gauge 
           16 : Valve 
           17 : Pipe 
           18 : Pressure gauge 
           19 : Polyolefin original filament 
           20 : Reel 
           21 : Comb 
           22 ,  23 : Feeding nip rollers 
           24 : Orifice 
           25 : Carbon dioxide gas laser emitting device 
           26 : Laser beam 
           27 : Power meter 
           28 : Drawn filament 
           29 : Conveyor 
           30 : Polyolefin porous sheet 
           31 : Negative pressure suction chamber 
           32 : Infrared lamp 
           33 : Hot air nozzle 
           34 : Rubber roller 
           35 : Heated roller 
           36 : Rubber roller 
           37 : Polyolefin nanofilament porous sheet 
           38 : Winding roller 
           41 : Polyolefin original filament 
           42 : Orifice 
           43 : Drawing chamber 
           44 : Carbon dioxide laser emitting device 
           45 : Laser beam 
           47 : Drawn filament 
           48 : Winding device 
           49 : Winding trestle 
           50 : Winding pipe 
           51 : Porous sheet 
           52 : Collection guide 
           53 ,  54 ,  55 : Fine position adjusting platform