Patent Publication Number: US-2022238958-A1

Title: Separator for secondary battery, manufacturing method therefor, and lithium secondary battery comprising same

Description:
TECHNICAL FIELD 
     A separator for a lithium secondary battery, a manufacturing method therefor, and a lithium secondary battery comprising the same are related. 
     BACKGROUND ART 
     A positive electrode and a negative electrode including a material capable of intercalating and deintercalating lithium ions are impregnated into an electrolyte, and a separator is disposed between the positive electrode and the negative electrode. Among them, the separator is used as a movement path of the electrolyte ions while preventing direct contact (internal short-circuit) between the positive electrode and the negative electrode. 
     In the case of manufacturing a lithium secondary battery, if the electrode and the separator are not properly bound, the positive and negative electrodes shrink and expand repeatedly during charging and discharging of the lithium battery, which may cause deformation of the battery shape and problems in battery performance and stability by a non-uniform reaction during a battery operation. 
     When a lithium secondary battery is externally shorted, a large current flows to generate heat, thereby increasing a battery temperature and starting a thermal runaway, which may lead to an operation of a safety valve or ignition due to evaporation of an electrolyte or heat-generation. In order to prevent this, a porous body including a heat-melting resin such as polyolefin is used in the separator. When the temperature inside the battery rises above a certain temperature, the separator melts to block the openings, thereby a shutdown function of stopping a reaction of a battery and suppressing heat-generation may be applied. 
     However, in the case of a large secondary battery for power storage or a vehicle, heat dissipation is poor, and when overheating occurs inside the secondary battery, a temperature of a secondary battery increases to 400 to 500° C. in a short time. If this condition persists, a meltdown or melt cracking of the separator may occur, causing a short circuit current to flow due to a contact between the electrodes, and an exothermic state causes thermal runaway. 
     DISCLOSURE 
     Technical Problem 
     An embodiment provides a separator for secondary battery according to an embodiment having improved adhesion between an electrode and a separator and reduced interfacial resistance, a method of manufacturing the separator, and a lithium secondary battery including the separator. 
     Technical Solution 
     An embodiment provides a separator for a secondary battery including a porous substrate; and a coating layer including a plurality of ring patterns on at least one surface of the porous substrate, wherein the ring pattern includes a plurality of polymer particulates, the ring pattern is spaced apart from each other at regular intervals, a particle diameter of the ring pattern is 10 μm to 200 μm, and a ring of the ring pattern has a width (thickness of the ring) of 0.2 μm to 1.5 μm. 
     The ring pattern may be spaced apart from each other at an interval of 10 μm to 1000 μm. 
     The ring pattern may have a particle diameter of 50 μm to 150 μm. 
     The ring of the ring pattern may have a width (thickness of the ring) of 0.5 μm to 1.2 μm. 
     The polymer particulates may include at least one selected from fluorine-based polymer particulates, (meth)acrylic polymer particulates, and a mixture thereof 
     A particle diameter of the polymer particulates may be 100 nm to 600 nm. 
     The fluorine-based polymer particulates may include at least one selected from polyvinylidene fluoride (PVdF), polyvinylidene fluoride-co-hexafluoropropylene, and polyvinylidene fluoride-co-trichloroethylene. 
     The (meth)acrylic polymer particulates may include at least one selected from poly(meth)acrylate, polybutyl(meth)acrylate, polypentyl(meth)acrylate, polyhexyl(meth)acrylate, and polyacrylonitrile. 
     The ring pattern may have a loading amount of 0.5 g/m 2  to 1.5 g/m 2 . 
     The coating layer may have a thickness of 0.1 μm to 5 μm. 
     An area of the ring pattern may be 40% to 80% of the total area of the porous substrate, and an area occupied by the rings of the ring pattern may be 5% to 30% of the total area of the porous substrate. 
     In another embodiment, a method of manufacturing the separator for a secondary battery includes mixing at least one of fluorine-based polymer particulates, (meth)acrylic polymer particulates, and a mixture thereof with water to prepare a coating layer composition, and coating the coating layer composition on at least one surface of a porous substrate by an inkjet coating method and drying it. 
     The coating layer composition may have a viscosity of 0.1 cps to 10 cps. 
     The coating layer composition may include fluorine-based polymer particulates and (meth)acrylic polymer particulates in a weight ratio of 90:10 to 50:50. 
     The fluorine-based polymer particulates may include polymer particulates having a particle diameter of 100 nm to 300 nm. 
     The (meth)acrylic polymer may include polymer particulates having a particle diameter of 250 nm to 600 nm. 
     The inkjet coating method may include spraying and coating the coating layer composition with a nozzle number density of 50 (dpi) to 1000 (dpi) of an inkjet head, at a frequency of 1.0 KHz to 10 KHz at a temperature of 20° C. to 55° C., and at a speed of 10 mm/sec to 500 mm/sec on a porous substrate. 
     Another embodiment provides a lithium secondary battery including a positive electrode; a negative electrode; the separator for a secondary battery between the positive electrode and the negative electrode; and an electrolyte. 
     Advantageous Effects 
     By improving the adhesion between the electrode and the separator and reducing the interfacial resistance, cycle-life characteristics of the battery can be improved. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view showing a coating layer of a separator according to an embodiment. 
         FIG. 2  is a schematic view of a ring pattern included in a coating layer of a separator according to an embodiment. 
         FIG. 3  is an exploded perspective view of a lithium secondary battery according to an embodiment. 
         FIGS. 4 and 5  are SEM photographs of the separator according to Example 1 measured at different magnifications, respectively. 
         FIGS. 6 and 7  are cross-sectional SEM photographs of separators according to Examples 1 and 2, respectively. 
       
         
           
             
                 
               
                 
                     
                 
                 
                   &lt;Description of Symbols&gt; 
                 
                 
                     
                 
               
              
                 
                     
                 
              
             
             
                 
                 
                 
              
                 
                     
                   1: ring pattern 
                   3: ring 
                 
                 
                     
                   5: coating layer 
                   7: porous substrate 
                 
                 
                     
                   100: lithium secondary battery 
                   112: negative electrode 
                 
                 
                     
                   113: separator 
                   114: positive electrode 
                 
                 
                     
                   120: battery case 
                   140: sealing member 
                 
                 
                     
                     
                 
              
             
           
         
       
     
    
    
     MODE FOR INVENTION 
     Hereinafter, the present invention will be described in more detail. Configurations shown in the embodiments and drawings described herein are only one of the most preferred embodiments of the present invention, and do not represent all of the technical spirit of the present invention, there are various equivalents that may be substituted for them or exemplary variations at the time of the present application. 
     In the present specification, when a definition is not otherwise provided, “(meth)acrylic” and “(meth)acrylate” may mean acrylic or methacrylic, and acrylate or methacrylate, respectively. 
     In addition, in the present specification when a definition is not otherwise provided, the particle diameter may be an average particle size (D50) at a volume ratio of 50% in a cumulative size-distribution curve. The average particle size (D50) may be measured by a well-known method for a person of an ordinary skill in the art. For example, it may be measured by a particle size analyzer, or from TEM (transmission electron microscope) or SEM (scanning electron microscope) photographs. Alternatively, a dynamic light-scattering measurement device is used to perform a data analysis, and the number of particles is counted for each particle size range and from this, the D50 value may be easily obtained through a calculation. 
     An embodiment provides a separator including a porous substrate and a coating layer including a plurality of ring patterns on at least one surface of the porous substrate. 
     Lithium secondary batteries are generally manufactured by interposing a separator between a positive electrode and a negative electrode, winding them, and then (heat) compressing at a predetermined temperature and pressure. By the compression process, as the adhesion (resistance) at the interface between the positive electrode and the separator and/or at the interface between the negative electrode and the separator increases, discharge capacity may continuously decrease as charge/discharge cycle of the lithium secondary battery progresses, high-rate charge/discharge characteristics may be reduced, and a problem in terms of battery safety may occur. In order to solve these problems, a separator having a coating layer formed by coating a mixture of a plurality of binder polymers and inorganic materials on at least one surface of a porous substrate is being manufactured. However, even in this case, in order to increase bondability between the separator and the electrode, a significant amount of the binder polymer is exposed to the surface of the separator, so that a side reaction between the binder polymer and the electrolyte may occur, causing the separator to swell, and as a result, ionic conductivity and the battery cycle-life may be deteriorated. 
     In order to solve the above problem, provided is a separator for a secondary battery including the coating layer of the separator includes a plurality of ring patterns spaced apart from each other at regular intervals, wherein the ring pattern includes a plurality of polymer particulates, a particle diameter of the ring pattern is 10 μm to 200 μm, and a ring of the ring pattern have a width (thickness of the ring) of 0.2 μm to 1.5 μm. 
     Hereinafter, the separator is described with reference to  FIGS. 1 and 2 . 
       FIG. 1  is a schematic view showing the coating layer of the separator, and  FIG. 2  is a schematic view of a ring pattern included in a coating layer of a separator. 
     Referring to  FIG. 1 , a plurality of ring patterns included in the coating layer 5 according to an embodiment may be spaced apart from each other at regular intervals. The plurality of ring patterns spaced apart from each other at regular intervals may have a spacing distance corresponding to greater than or equal to 50% and specifically, greater than or equal to 60%, or greater than or equal to 80% based on 100% of the total number of the ring patterns within a range of any spacing distance (D) (e.g., an average space distance) ±20% (e.g., ±10%). 
     The plurality of ring patterns may have a spacing distance (D) at regular intervals of 10 μm to 1000 μm, specifically, greater than or equal to 10 μm, greater than or equal to 20 μm, greater than or equal to 30 μm, greater than or equal to 40 μm, or greater than or equal to 50 μm and less than or equal to 1000 μm, less than or equal to 800 μm, less than or equal to 600 μm, less than or equal to 400 μm, less than or equal to 300 μm, or less than or equal to 270 μm. On the other hand, the spacing distance (D) may be a distance between centers of the adjacent ring patterns 1. When the spacing distance (D) is within the ranges, an adhesion effect between the coating layer and the electrode may be increased. Accordingly, a cell swelling phenomenon that occurs, when the adhesion between the electrode and the separator is insufficient, may be effectively suppressed, and ion conductivity may be improved, improving cycle-life characteristics of a battery. 
     On the other hand, in the separator for a secondary battery according to an embodiment, the coating layer is formed in an inkjet coating method which is described later, and the plurality of ring patterns included in the coating layer may be regularly aligned. 
     Referring to  FIG. 2 , in the plurality of ring patterns 1, a plurality of polymer particulates is present in a ring  3 , the polymer particulates are substantially not present or present in a small loading amount of less than equal to 50 wt % inside of the ring pattern 1 excluding the ring  3 , compared with in the ring  3 . Accordingly, compared with a case of applying a ring pattern filled with a polymer material or evenly coating the polymer material on the whole surface of a porous substrate, a pore-clogging phenomenon may be minimized. Accordingly, the separator swelling phenomenon due to a side reaction with an electrolyte solution may be improved, and ion conductivity may be improved. 
     On the other hand, in order to maintain an adhesive force between the electrode and the separator, it is important to find an optimum numerical range of the particle diameter of the ring pattern  1  and a width of the ring  3  (thickness (d) of the ring). In this regard, the ring pattern may have a particle diameter of 10 μm to 200 μm, for example, a particle diameter of 50 μm to 150 μm, or 100 μm to 150 μm. The ring of the ring pattern has a width of 0.2 μm to 1.5 μm, for example, 0.4 μm to 1.3 μm, or 0.5 μm to 1.1 μm. When the ring pattern has a particle diameter and a width of the ring (a thickness of the ring) within the ranges, cycle-life characteristics of a battery may be improved by improving the adhesion between electrode and separator and minimizing interfacial resistance. 
     On the other hand, the particle diameter range of the ring pattern 1 and the thickness range of the ring of the ring pattern 3 specifically belong to greater than or equal to 50% and specifically, greater than or equal to 60% or greater than or equal to 80% of the ring pattern 1 based on 100% of the total number of the ring pattern 1. 
     The plurality of polymer particulates present in the ring 3 of the ring pattern 1 may include at least one selected from fluorine-based polymer particulates, (meth)acrylic polymer particulates, and a mixture thereof, and the polymer particulates may have a particle diameter of 100 nm to 600 nm. 
     The fluorine-based polymer particulates may include at least one selected from polyvinylidene fluoride (PVdF), polyvinylidene fluoride-co-hexafluoropropylene, and polyvinylidene fluoride-co-trichloroethylene. In addition, the particle diameter of the fluorine-based polymer particulates may be 100 nm to 300 nm, for example 100 nm to 250 nm, 100 nm to 200 nm, or 150 nm to 200 nm. Accordingly, the adhesion and oxidation resistance between separator and electrode plate may be improved, and when the fluorine-based polymer particulates have a particle diameter within the range, battery performance may be improved by minimizing movement resistance of lithium ions and also, minimizing the thickness of the coating layer. 
     The (meth)acrylic polymer particulates may include at least one selected from poly(meth)acrylate, polybutyl(meth)acrylate, polypentyl(meth)acrylate, polyhexyl(meth)acrylate, and polyacrylonitrile. 
     A particle diameter of the (meth)acrylic polymer particulates may be greater than or equal to 250 nm, greater than or equal to 300 nm, or greater than or equal to 350 nm, and less than or equal to 600 nm, less than or equal to 550 nm, less than or equal to 500 nm, or less than or equal to 450 nm. When the particle diameter of the (meth)acrylic polymer particulates falls within the above ranges, movement resistance of lithium ions may be minimized to ensure a performance of the lithium secondary battery. 
     In the coating layer, a loading amount of the ring pattern may be 0.5 g/m 2  to 1.5 g/m 2 , for example 0.7 g/m 2  to 1.3 g/m 2 , or 1.0 g/m 2  to 1.2 g/m 2 . When the ring pattern has a loading amount within the range, cycle-life characteristics of a battery may be improved by improving the adhesion and ion conductivity of the separator but reducing the interfacial resistance between electrode and separator. 
     The coating layer may be formed as a thin film with a thickness (total thickness)of 0.1 μm to 5 μm, for example, 0.5 μm to 3.5 μm, 0.5 μm to 3.0 μm, 0.5 μm to 1.5 μm, or 0.5 μm to 1.2 μm. 
     In the coating layer, an area of the ring pattern (including an inner space of the ring pattern) may be 40% to 80% of a total area of the porous substrate, for example, 40% to 75%. On the other hand, the area of the ring pattern indicates an area sum of the area of the ring of the ring pattern and an area of an inner region of the ring pattern except for the ring. 
     In the coating layer, an area occupied by the ring of the ring pattern (not including the inner space of the ring pattern) may be 5% to 30% of the total area of the porous substrate, for example, 10% to 20%. In the coating layer, when the area of the ring pattern and the area occupied by the ring of the ring pattern are respectively within the ranges, the adhesion between the separator and electrode may be maintained at an appropriate level, and the interfacial resistance may be reduced, thereby improving cycle-life characteristics of a battery. 
     Hereinafter, a method of manufacturing a separator according to an embodiment is described. 
     Another embodiment provides a method of manufacturing a separator for a secondary battery that includes mixing at least one of fluorine-based polymer particulates, (meth)acrylic polymer particulates, and a mixture thereof with water to prepare a coating layer composition, coating the coating layer composition on at least one surface of a porous substrate by an inkjet coating method and drying it to manufacture the separator. 
     The fluorine-based polymer particulates have polymer particulates having a particle diameter of 100 nm to 300 nm, for example, 100 nm to 250 nm, 100 nm to 200 nm, or 150 nm to 200 nm, and the (meth)acrylic polymer particulates may include polymer particulates having a particle diameter of greater than or equal to 250 nm, greater than or equal to 300 nm, or greater than or equal to 350 nm and less than or equal to 600 nm, less than or equal to 550 nm, or less than or equal to 500 nm. 
     The coating layer composition may include the fluorine-based polymer particulates and the (meth)acrylic polymer particulates in a weight ratio of 90:10 to 50:50, 90:10 to 55:45, 90:10 to 60:40, 90:10 to 65:35, or 90:10 to 70:30. Within the ranges, interfacial resistance between the electrode and the separator is reduced, adhesion of the separator is improved, and accordingly, cycle-life characteristics of a battery may be improved. 
     The coating layer composition may have viscosity in a range of greater than or equal to 0.1 cps, greater than or equal to 0.5 cps, greater than or equal to 1 cps, or greater than or equal to 2 cps, and less than or equal to 50 cps, less than or equal to 45 cps, less than or equal to 40 cps, less than or equal to 35 cps, less than or equal to 30 cps, less than or equal to 25 cps, less than or equal to 20 cps, less than or equal to 15 cps, less than or equal to 10 cps, or less than or equal to 7 cps. When the coating layer composition has viscosity within the ranges, a ring pattern may be well formed to have a particle diameter and a width of a ring with within the ranges according to an embodiment of the present invention. When the coating layer composition has too low or high a viscosity, a coating layer may be difficult to form by applying the inkjet coating method, and also, the particle diameter and the ring width of the ring pattern according to an embodiment may be difficult to realize. 
     An Inkjet coater used for manufacturing the separator according to an embodiment is a piezoelectric-type or thermal bubble-type coater and may be used to spray the coating layer composition from a nozzle. The inkjet coating method is a coating method of applying a voltage through frequency control to push an ink (the coating layer composition). 
     Specifically, the inkjet coating method may be to spray and coat the coating layer composition on a porous substrate by setting the number of nozzles of an Inkjet head to have a density of 50 (dpi) to 1000 (dpi) at 20° C. to 55° C. at a frequency of 1.0 KHz to 10 KHz and a coating speed of 10 mm/sec to 500 mm/sec. When the inkjet coating method has the process conditions within the ranges, a coating layer in which ring patterns having a particle diameter and a ring width (thickness) are spaced apart at regular intervals on the porous substrate may be well formed. For example, when the density of the number of nozzles is increased, the particle diameter of the ring pattern, the ring width, and the spacing distance of the ring pattern may all decrease, and the number of the ring patterns in the coating layer may increase. On the contrary, when the density of the number of nozzles is decreased, the opposite result may be obtained. 
     A lithium secondary battery according to an embodiment includes a positive electrode; a negative electrode; the separator between the positive electrode and the negative electrode; and an electrolyte solution. 
     The separator  113  separates a positive electrode  114  and a negative electrode  112  and provides a transporting passage for lithium ions. Detailed descriptions thereof are as described above. The entire thickness of the separator may be determined by a target capacity of the battery. A thickness of the separator may be, for example, 5 μm to 30 μm. 
     The negative electrode  112  includes a current collector and a negative active material layer formed on the current collector and, and the negative active material layer includes a negative active material. 
     The negative active material may include a material that reversibly intercalates/deintercalates lithium ions, a lithium metal, a lithium metal alloy, a material capable of doping and dedoping lithium, or a transition metal oxide. 
     The material that reversibly intercalates/deintercalates lithium ions may be a carbon material which is any generally-used carbon-based negative active material in a lithium ion secondary battery and examples thereof may be crystalline carbon, amorphous carbon, or a combination thereof. Examples of the crystalline carbon may be graphite such as amorphous, sheet-shape, flake, spherical shape or fiber-shaped natural graphite or artificial graphite. Examples of the amorphous carbon may be soft carbon (fired carbon at a low temperature) or hard carbon, a mesophase pitch carbonized product, fired coke, and the like. 
     The lithium metal alloy may be an alloy of lithium and a metal selected from Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, or Sn. 
     The material capable of doping and dedoping lithium may be Si, SiO x  (0&lt;x&lt;2), a Si—C composite, a Si—Q alloy (wherein Q is selected from an alkali metal, an alkaline-earth metal, Group 13 to 16 elements, a transition element, a rare earth element, and a combination thereof, and not Si), Sn, SnO 2 , a Sn—C composite, Sn—R (wherein R is selected from an alkali metal, an alkaline-earth metal, Group 13 to 16 elements, a transition element, a rare earth element, and a combination thereof, and not Sn). The elements Q and R may be selected from Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ti, Ge, P, As, Sb, Bi, S, Se, Te, Po, or a combination thereof. 
     The transition elements oxide may include a vanadium oxide, a lithium vanadium oxide, and the like. 
     The negative active material layer also includes a binder and may further optionally include a conductive material. 
     The binder improves binding properties of negative active material particles with one another and with a current collector and examples thereof may be polyvinylalcohol, carboxylmethylcellulose, hydroxypropylcellulose, polyvinyl chloride, carboxylated polyvinylchloride, polyvinylfluoride, an ethylene oxide-containing polymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, a styrene-butadiene rubber, an acrylated styrene-butadiene rubber, an epoxy resin, nylon, and the like, but is not limited thereto. 
     The conductive material improves electrical conductivity of an electrode, and any electrically conductive material may be used as a conductive material, unless it causes a chemical change and examples thereof may be a carbon-based material such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, a carbon fiber and the like; a metal-based material such as a metal powder, a metal fiber, and the like of copper, nickel, aluminum, silver, and the like; a conductive polymer such as a polyphenylene derivative and the like; or a mixture thereof. 
     The current collector may be a copper foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam, a copper foam, a polymer substrate coated with a conductive metal, or a combination thereof. 
     The positive electrode  114  includes a current collector and a positive active material layer formed on the current collector. 
     The positive active material may be a compound (lithiated intercalation compound) capable of intercalating and deintercallating lithium. Specifically, at least one lithium metal composite oxide of lithium and a metal of cobalt, manganese, nickel, or a combination thereof may be used, and specific examples thereof may be a compound represented by one of the following chemical formulas. Li a A 1-b R b D 2  (wherein, in the above chemical formulas, 0.90≤a≤1.8 and 0≤b≤0.5); Li a E 1-b R b O 2-c D c  (wherein, in the above chemical formula, 0.90≤a≤1.8, 0≤b≤0.5 and 0≤c≤0.05); LiE 2-b R b O 4-c D c  (wherein, in the above chemical formula, 0≤b≤0.5, 0≤c≤0.05); Li a Ni 1-b-c Co b-c R c D α  (wherein, in the above chemical formula, 0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05 and 0 21  α≤2); Li a Ni 1-b-c Co b R c O 2-α Z a  (wherein, in the above chemical formula, 0.90≤a≤1.8, 0 23  b≤0.5, 0≤c≤0.05 and 0&lt;a&lt;2); Li a Ni 1-b-c Co b R c O 2-a Z 2 , (wherein, in the above chemical formula, 0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05 and 0&lt;a&lt;2); Li a Ni 1-b-c Mn b R c D a  (wherein, in the above chemical formula, 0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05 and 0&lt;α≤2); Li a Ni 1-b-c Mn b R c O 2-α Z α  (wherein, in the above chemical formula, 0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05 and 0&lt;α&lt;2); Li a Ni 1-b-c Mn b R c O 2-α Z 2 , (wherein, in the above chemical formula, 0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05 and 0&lt;α&lt;2); Li a Ni b R c G d O 2  (wherein, in the above chemical formula, 0.90≤a≤1.8, 0≤b≤0.9, 0≤c≤0.5 and 0.001 ≤d≤0.1); Li a Ni b Co c Mn d GeO 2  (wherein, in the above chemical formula, 0.90≤a≤1.8, 0 ≤b≤0.9, 0≤c≤0.5, 0≤d≤0.5 and 0.001≤e≤0.1); Li a NiG b O 2  (wherein, in the above chemical formula, 0.90≤a≤1.8 and 0.001≤b≤0.1); Li a CoG b O 2  (wherein, in the above chemical formula, 0.90≤a≤1.8 and 0.001≤b≤0.1); Li a MnG b O 2  (wherein, in the above chemical formula, 0.90≤a≤1.8 and 0.001≤b≤0.1); Li a Mn 2 G b O 4  (wherein, in the above chemical formula, 0.90≤a≤1.8 and 0.001≤b≤0.1); QO 2 ; QS 2 ; LiQS 2 ; V 2 O 5 ; LiV 2 O 5 ; LiTO 2 ; LiNiVO 4 ; Li (3-f) J 2 (PO 4 ) 3 (0≤f≤2); and LiFePO 4 . 
     In the above chemical formulas, A is Ni, Co, Mn, or a combination thereof; R is Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, a rare earth element, or a combination thereof; D is O, F, S, P, or a combination thereof; E is Co, Mn, or a combination thereof; Z is F, S, P, or a combination thereof G is Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, or a combination thereof Q is Ti, Mo, Mn, or a combination thereof T is Cr, V, Fe, Sc, Y, or a combination thereof and J is V, Cr, Mn, Co, Ni, Cu, or a combination thereof. 
     The positive active material may include the positive active material with the coating layer, or a compound of the active material and the active material coated with the coating layer. The coating layer may include a coating element compound of an oxide or hydroxide of a coating element, oxyhydroxide of a coating element, oxycarbonate of a coating element, or hydroxycarbonate of a coating element. The compound for the coating layer may be either amorphous or crystalline. The coating element included in the coating layer may be Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr, or a mixture thereof. The coating process may include any conventional processes as long as it does not causes any side effects on the properties of the positive active material (e.g., ink jet coating, dipping), which is well known to persons having ordinary skill in this art, so a detailed description thereof is omitted. 
     The positive active material layer may also include a binder and a conductive material. 
     The binder improves binding properties of positive active material particles with one another and with a current collector and examples thereof may be polyvinyl alcohol, carboxylmethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinylchloride, carboxylated polyvinylchloride, polyvinylfluoride, an ethylene oxide-containing polymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, a styrene-butadiene rubber, an acrylated styrene-butadiene rubber, an epoxy resin, nylon, and the like, but are not limited thereto. 
     The conductive material improves conductivity of an electrode and any electrically conductive material may be used as a conductive material, unless it causes a chemical change, and examples thereof may be one or more of natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, a carbon fiber, a metal powder, a metal fiber, and the like of copper, nickel, aluminum, silver, and the like, or a polyphenylene derivative and the like. 
     The current collector may use Al but is not limited thereto. 
     The negative and positive electrodes may be fabricated by a method including mixing the active material, a conductive material, and a binder into an active material composition, and coating the composition on a current collector. The electrode manufacturing method is well known, and thus is not described in detail in the present specification. The electrolyte includes a non-aqueous organic solvent and a lithium salt. 
     The non-aqueous organic solvent serves as a medium for transmitting ions taking part in the electrochemical reaction of a battery. 
     The non-aqueous organic solvent may be selected from a carbonate-based, ester-based, ether-based, ketone-based, alcohol-based, or aprotic solvent. The carbonate-based solvent includes dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), methylethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and the like, and the ester-based solvent includes methyl acetate, ethyl acetate, n-propyl acetate, 1,1-dimethylethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, decanolide, valerolactone, mevalonolactone, caprolactone, and the like. The ether-based solvent includes dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, and the like, and the ketone-based solvent includes cyclohexanone, and the like. In addition, the alcohol-based solvent may be ethanol, isopropyl alcohol, and the like. The aprotic solvent may include nitriles such as R-CN (wherein R is a C2 to C20 linear, branched, or cyclic hydrocarbon group, and R may include a double bond, an aromatic ring, or an ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, sulfolanes, and the like. 
     The non-aqueous organic solvent may be used alone or in combination of one or more, and a mixing ratio in the case of mixing one or more may be appropriately adjusted according to desired battery performance, which is widely understood by those skilled in the art. 
     The carbonate-based solvent may include a mixture of cyclic carbonate and chain-type carbonate. The cyclic carbonate and the linear carbonate are mixed together in the volume ratio of about 1:1 to about 1:9. Within this range, performance of electrolyte may be improved. 
     The non-aqueous organic electrolyte may be further prepared by mixing a carbonate-based solvent with an aromatic hydrocarbon-based solvent. The carbonate-based and the aromatic hydrocarbon-based solvents may be mixed together in a volume ratio ranging from about 1:1 to about 30:1. 
     The aromatic hydrocarbon-based organic solvent may be an aromatic hydrocarbon-based compound represented by Chemical Formula 1. 
     
       
         
         
             
             
         
       
     
     In Chemical Formula 1, R 1  to R 6  are independently hydrogen, a halogen, a C1 to C10 alkyl group, a C1 to C10 haloalkyl group, or a combination thereof. 
     The aromatic hydrocarbon-based organic solvent may include benzene, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4-difluorobenzene, 1,2,3-trifluorobenzene, 1,2,4-trifluorobenzene, chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene, iodobenzene, 1,2-diiodobenzene, 1,3-diiodobenzene, 1,4-diiodobenzene, 1,2,3-triiodobenzene, 1,2,4-triiodobenzene, toluene, fluorotoluene, 1,2-difluorotoluene, 1,3-difluorotoluene, 1,4-difluorotoluene, 1,2,3-trifluorotoluene, 1,2,4-trifluorotoluene, chlorotoluene, 1,2-dichlorotoluene, 1,3-dichlorotoluene, 1,4-dichlorotoluene, 1,2,3-trichlorotoluene, 1,2,4-trichlorotoluene, iodotoluene, 1,2-diiodotoluene, 1,3-diiodotoluene, 1,4-diiodotoluene, 1,2,3-triiodotoluene, 1,2,4-triiodotoluene, xylene, or a combination thereof. 
     The non-aqueous electrolyte may further include an additive of vinylene carbonate or an ethylene carbonate-based compound of Chemical Formula 2 in order to improve cycle-life of a battery. 
     
       
         
         
             
             
         
       
     
     In Chemical Formula 2, R 7  and R 8  are independently hydrogen, a halogen group, a cyano group (CN), a nitro group (NO 2 ), or a C1 to C5 fluoroalkyl group, provided that at least one of R 7  and R 8  is a halogen group, a cyano group (CN), a nitro group (NO 2 ), or a C1 to C5 fluoroalkyl group. 
     Examples of the ethylene-based carbonate-based compound may be difluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, bromoethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, cyanoethylene carbonate, fluoroethylene carbonate, and the like. When the vinylene carbonate or the ethylene carbonate-based compound is further used, the amount thereof may be adjusted appropriately in order to improve cycle-life. 
     The lithium salt dissolved in a non-aqueous organic solvent supplies a battery with lithium ions, basically operates the lithium secondary battery, and improves transportation of the lithium ions between a positive electrode and a negative electrode. Examples of the lithium salt include at least one supporting salt selected from LiPF 6 , LiBF 4 , LiSbF 6 , LiAsF 6 , LiC 4 F 9 SO 3 , LiClO 4 , LiAlO 2 , LiAlCl 4 , LiN(C x F 2x−1 SO 2 )(C y F 2y+1 SO 2 ) (wherein, x and y are natural numbers), LiCl, LiI, LiB(C 2 O 4 ) 2  (lithium bis(oxalato) borate: LiBOB), or a combination thereof. A concentration of the lithium salt may range from 0.1 M to 2.0 M. When the lithium salt is included at the above concentration range, an electrolyte may have excellent performance and lithium ion mobility due to optimal electrolyte conductivity and viscosity. 
     The lithium secondary battery may be classified into a lithium ion battery, a lithium ion physical gel polymer battery, and a lithium ion chemical gel polymer battery according to types of separators and electrolyte solutions thereof. It may be classified into cylindrical, prismatic, coin-type, or pouch according to a shape, and may be classified into a bulk type and a thin film type according to a size. Structures and manufacturing methods for these batteries pertaining to this disclosure are well known in the art. 
       FIG. 3  is an exploded perspective view of a lithium secondary battery according to an embodiment. Referring to  FIG. 3 , the lithium secondary battery  100  is a cylindrical battery that includes a negative electrode  112 , a positive electrode  114 , a separator  113  disposed between the negative electrode  112  and the positive electrode  114 , an electrolyte (not shown) impregnating the negative electrode  112 , positive electrode  114 , and separator  113 , a battery case  120 , and a sealing member  140  sealing the battery case  120 . The lithium secondary battery  100  is fabricated by sequentially laminating a negative electrode  112 , a separator  113 , and a positive electrode  114 , spirally winding them, and housing the spiral-wound product in a battery case  120 . 
     Hereinafter, Preparation Examples and Examples are described in detail to describe the present invention in detail. However, Preparation Examples and Examples according to the present invention may be modified in various other forms, and the scope of the present invention should not be construed as being limited to the preparation examples and examples described below. Preparation Examples and Examples of the present invention are provided to explain in detail the present invention to those skilled in the art. 
     Preparation Examples 
     Preparation Example 1 
     A polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP) (XPH-883, manufactured by Solvey, weight average molecular weight of 450,000 g/mol) latex was mixed to be 25 wt % with distilled water and then, stirred at 25° C. for 30 minutes to prepare a solution including fluorine-based polymer particulates having a particle diameter of 200 nm. The prepared solution was mixed with an acrylic polymer particulates having a particle diameter of 350 nm (compound composition: a mixture of polystyrene, 2-ethylhexyl acrylate and butyl acrylate, BM900B, manufactured by ZEON) to have a weight ratio of 90:10 between the PVdF-HFP copolymer latex and acrylic polymer particulates. Distilled water was added thereto to have a solid content of 5 wt % in the mixed solution and then, stirred at 25° C. for 30 minutes to prepare a coating layer composition. The coating layer composition had a viscosity (at 25° C.) of 4 cps. 
     Preparation Example 2 
     A polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP) (a weight average molecular weight: 450,000 g/mol, XPH-883 manufactured by Solvey) latex was mixed to be 25 wt % with distilled water and then, stirred at 25° C. for 30 minutes to prepare a solution including fluorine-based polymer particulates having a particle diameter of 200 nm, and distilled water was added thereto to have a solid content of 5 wt % and then, stirred at 25° C. for 30 minutes to prepare a coating layer composition. The coating layer composition had viscosity (at 25° C.) of 2 cps. 
     Preparation Example 3 
     Distilled water was added to acrylic polymer particulates having a particle diameter of 350 nm (a compound: a mixture of polystyrene, 2-ethylhexyl acrylate, and butyl acrylate, BM900B manufactured by ZEON) to have a solid content of 5 wt % and then, stirred at 25° C. for 30 minutes to prepare a coating layer composition. The coating layer composition had a viscosity (at 25° C.) of 6 cps. 
     Comparative Preparation Example 1 
     A polyvinylidene fuloride-hexafluoropropylene copolymer (PVdF-HFP) (a weight average molecular weight of 450,000 g/mol, XPH-883 manufactured by Solvey) latex was mixed to be 25 wt % with distilled water and then, stirred at 25° C. for 30 minutes to prepare a solution including fluorine-based polymer particulates having a particle diameter of 200 nm. The prepared solution was mixed with an acrylic polymer particulates having a particle diameter of 350 nm (compound: a mixture of polystyrene, 2-ethylhexyl acrylate, and butyl acrylate, BM900B manufactured by ZEON) to have a weight ratio of 90:10 between the PVdF-HFP copolymer latex and acrylic polymer particulates. Distilled water was added to the mixed solution to have a solid content of 20 wt % and then, stirred at 25° C. for 30 minutes to prepare a coating layer composition. The coating layer composition had a viscosity (at 25° C.) ranging from 50 cps to 100 cps. 
     EXAMPLES 
     Example 1 
     (Manufacture of Separator) 
     The coating layer composition prepared in Preparation Example 1 was coated on both surfaces of a porous substrate (SK 612HS, thickness: 12 μm, permeability: 115 sec/100 cc) to form a coating layer including a plurality of ring patterns spaced at regular intervals in an inkjet coating method. The inkjet coating was conducted by using a digital coater (Techno Smart Corp.), in which the number of nozzles of an Inkjet head was set to have a density of 360 (dpi), at 25° C., a frequency of 2.0 KHz, and 83 mm/sec. Subsequently, the coating layer was dried at a temperature of 80° C. and a wind speed of 15 m/sec for 3 minutes, manufacturing a separator with the coating layer. 
     (Manufacture of Coin Cell) 
     A positive active material, a carbon conductive agent (Denka Black, Denka 
     Korea), and polyvinylidene fluoride (PVDF) were mixed in a weight ratio of 92:4:4 and then, with N-methyl pyrrolidone (NMP) to prepare slurry. The slurry was bar-coated on a 15 μm-thick aluminum current collector, dried at room temperature and once more at 120° C. under vacuum, and compressed and punched to manufacture a 45 μm-thick positive electrode plate. 
     A coin cell was manufactured by a conventional method using the manufactured positive electrode plate, the lithium metal counter electrode, the separator prepared by the above-described process, and the electrolyte. The electrolyte was prepared by dissolving 1.5 M LiPF 6  in a mixed solvent of EC (ethylene carbonate), DEC (diethyl carbonate), and EMC (ethylmethyl carbonate) (a volume ratio of 2:4:4). 
     Example 2 
     A coin cell was manufactured according to the same method as Example 1 except that the coating layer composition of Preparation Example 1 was used, and the number of nozzles of the Inkjet head was set to have a density of 120 dpi. 
     Example 3 and Example 4 
     A separator and a coin cell were manufactured according to the same methods respectively as Examples 1 and 2 except that a 16 μm-thick separator (a separator formed by coating an inorganic layer including inorganic particles with a total thickness of 4 μm on both surfaces of a 12 μm-thick polyethylene porous substrate) was used instead of the porous substrate. 
     Comparative Example 1 
     A separator and a coin cell were manufactured according to the same methods respectively as Examples 1 and 2 except that the coating was performed in a dip coating method. 
     Comparative Example 2 
     A separator and a coin cell were manufactured according to the same methods as Example 3 except that the dip coating method was used. 
     Comparative Example 3 
     A separator and a coin cell were manufactured according to the same method as 
     Example 1 except that the coating layer composition of Comparison Preparation Example 1 was used instead of the coating layer composition of Preparation Example 1. 
     Reference Example 1 
     A coating layer composition of Preparation Example 1 was spray-coated under a liquid pressure of 0.2 bar and an air pressure of 0.5 bar with pulses of 60% to from a coating layer on both surfaces of a porous substrate, while the porous substrate (a thickness: 12 μm, air permeability: 115 sec/100 cc, 612HS manufactured by SK) was moved at 20 m/min. Subsequently, the coated porous substrate was dried at 80° C. at a wind speed of 15 m/sec for 0.03 hours to manufacture a separator having the coating layer and a coin cell was manufactured in the same manner as in Example 1. 
     Table 1 below shows, regarding the separators according to Examples 1 to 4, Comparative Examples 1 to 3, and Reference Example 1, a particle diameter of a ring pattern, a width of a ring (a thickness of the ring) of the ring pattern, an area ratio of the ring pattern based on an area of a porous substrate, and an area ratio of a ring portion of the ring pattern based on the area of the porous substrate, a loading amount of a coating layer composition, a total thickness of a coating layer, and a coating method. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 Area ratio of 
                   
                   
                   
               
               
                   
                   
                   
                   
                 Area ratio 
                 the ring 
                   
                   
                   
               
               
                   
                   
                   
                   
                 of ring 
                 portion of the 
                 Loading 
                   
                   
               
               
                   
                   
                   
                   
                 pattern 
                 ring pattern 
                 amount 
                 Total 
                   
               
               
                   
                 Particle 
                   
                 Spacing 
                 (based on 
                 (based on 
                 of 
                 thickness 
                   
               
               
                   
                 diameter 
                 Width 
                 distance of 
                 area of 
                 area of 
                 coating 
                 of 
                   
               
               
                   
                 of ring 
                 (thickness) 
                 ring 
                 porous 
                 porous 
                 layer 
                 coating 
                   
               
               
                   
                 pattern 
                 of ring 
                 pattern 
                 substrate) 
                 substrate) 
                 composition 
                 layer 
                 Coating 
               
               
                   
                 (μm) 
                 (μm) 
                 (μm) 
                 (%) 
                 (%) 
                 (g/m 2 ) 
                 (μm) 
                 method 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Ex. 1 
                 120 
                 1.0 
                 75 
                 70 
                 30 
                 1.0 
                 1.0 
                 Inkjet 
               
               
                 Ex. 2 
                 60 
                 0.7 
                 210 
                 40 
                 29 
                 0.5 
                 0.7 
                 Inkjet 
               
               
                 Ex. 3 
                 110 
                 1.0 
                 70 
                 60 
                 25 
                 1.1 
                 3.0 
                 Inkjet 
               
               
                 Ex. 4 
                 70 
                 0.6 
                 205 
                 35 
                 23 
                 0.7 
                 2.7 
                 Inkjet 
               
               
                 Comp. Ex. 1 
                 — 
                 — 
                 — 
                 100 
                 — 
                 1.3 
                 1.3 
                 Dip 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 coating 
               
               
                 Comp. Ex. 2 
                 — 
                 — 
                 — 
                 100 
                 — 
                 1.2 
                 1.3 
                 Dip 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 coating 
               
               
                 Ref. Ex. 1 
                 190 
                 30 
                 irregular 
                 — 
                 — 
                 1.2 
                 2.0 
                 Spray 
               
               
                   
               
            
           
         
       
     
     On the other hand, in Comparative Example 3, a separator including a coating layer including a ring pattern was not formed in the inkjet coating method due to high viscosity of the coating layer composition. 
     Evaluation Example 1 
     Measurement of Air Permeability of Separator 
     The separators according to Examples 1 to 4, Comparative Examples 1 and 2, and Reference Example 1 were respectively cut into a size of 50 mm×50 mm to prepare each specimen. The air permeability was obtained by measuring time (sec) that it took for 100 cc of air to completely pass the specimens respectively, and the results are shown in Table 2. 
     Evaluation Example 2 
     Evaluation of Ion Conductivity Characteristics of Separator 
     Ion conductivity of the separators according to Example 1 to Example 4, 
     Comparative Example 1, Comparative Example 2, and Reference Example 1 was measured by using an electric impedance spectroscopy measurement instrument, a VSP model manufactured by Bio-Logic SAS. Herein, an amplitude of 1000 mV at an open circuit potential and a frequency from 10000 MHz to 1 Hz was scanned and the results are shown in Table 2. 
     Evaluation Example 3 
     Evaluation of Bending Strength Characteristics (Adhesion) of Separator 
     Electrode adhesion of the separators according to Example 1 to Example 4, 
     Comparative Example 1, Comparative Example 2, and Reference Example 1 was measured. The electrode adhesion was evaluated by measuring adhesion (bending strength) between an active material of a substrate and each separator was measured in a 3-point bending (Instron) method. Pouch cells (a charge (1 C/36 min) state for sale) after the formation were pressed at 5 mm/min by using a jig, and a MAX value (N, MPa) from a zero-point up to bending of 3 mm was measured. The bending strength was obtained as an average by measuring five samples and averaging three values except for a maximum value and a minimum value. The results are shown in Table 2. 
     Evaluation Example 4 
     Measurement of Film Resistance of Coin Cell 
     The coin cells according to Example 1 to Example 4, Comparative Example 1, 
     Comparative Example 2, and Reference Example 1 were allowed to stand at room temperature for one day, and resistance of the separators was respectively measured by using an impedance-measuring method. The results are shown in Table 2. 
     Evaluation Example 5 
     Evaluation of High Temperature Cycle-life Characteristics 
     The coin cell according to Example 1 to Example 4, Comparative Example 1, 
     Comparative Example 2, and Reference Example 1 were constant current-charged at a current of a 0.1 C rate up to a voltage of 4.3 V (vs. Li) at 45° C. and then, cut off at a current of a 0.05 C rate, while 4.3 V was maintained in a constant voltage mode. Subsequently, the coin cells were discharged at a constant current of a 0.1 C rate to a voltage of 3.0 V (vs. Li) (the 1st cycle). Then, the coin cells were constant current-charged at a current of a 1.0 C rate to a voltage of 4.3 V (vs. Li) at 45° C. and cut off at a current of a 0.05 C rate, while 4.3 V was maintained in a constant voltage mode. Next, the coin cells were discharged at a constant current of a 1.0 C rate and discharged to a voltage of 3.0 V (vs. Li), whose cycle was repeated up to the 500 cycles. In all the charge and discharge cycles, a pause of 10 minutes was set by every charge/discharge cycle. The cycle life results reflecting the charge and discharge experiment results are shown in Table 2. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Air 
                   
                   
                   
                   
               
               
                   
                 permeability 
                 Ion 
                 Bending 
                 Film 
                 High temperature 
               
               
                   
                 (sec/100 
                 conductivity 
                 strength 
                 resistance 
                 cycle-life (45° C., 
               
               
                   
                 cc) 
                 (S/mm) 
                 (N) 
                 (Ω) 
                 500 cycle) (%) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Example 1 
                 115 
                 0.021 
                 420 
                 0.43 
                 93 
               
               
                 Example 2 
                 111 
                 0.022 
                 410 
                 0.40 
                 94 
               
               
                 Example 3 
                 135 
                 0.018 
                 450 
                 0.53 
                 91 
               
               
                 Example 4 
                 130 
                 0.017 
                 440 
                 0.56 
                 90 
               
               
                 Comparative 
                 140 
                 0.0087 
                 440 
                 0.81 
                 83 
               
               
                 Example 1 
               
               
                 Comparative 
                 175 
                 0.0057 
                 435 
                 0.84 
                 81 
               
               
                 Example 2 
               
               
                 Reference 
                 120 
                 0.016 
                 445 
                 0.65 
                 88 
               
               
                 Example 1 
               
               
                   
               
            
           
         
       
     
     Referring to Table 2, the separators according to Examples 1 to 4 exhibited improved air permeability from 111 sec/100 cc to 135 sec/100 cc, compared with the comparative examples. In addition, the separators of Examples 1 to 4 exhibited similar bending strength ranging from 410 N to 450 N and thus similar adhesion between electrode and separator to those of the comparative examples, greatly improved ion conductivity of 0.017 S/mm to 0.022 S/mm, and greatly improved film resistance, compared to those of the comparative examples. Accordingly, the coin cells according to Examples 1 to 4 exhibited greatly improved high temperature cycle-life characteristics of 90% or more at the 500 cycles, compared with those of the comparative examples. 
     Evaluation Example 6 
     Optical Microscope Image Measurement and Scanning Electron Microscope (SEM) Image Measurement of Separator 
     SEM photographs taken with respect to the separator of Example 1 at different magnifications are shown in  FIGS. 4 and 5 . 
     In addition, SEM photographs showing cross-sections of the separators according to Examples 1 and 2 are provided in  FIGS. 6 and 7 . 
     Referring to  FIGS. 4 and 5 , the ring pattern according to an embodiment was spaced apart at regular intervals and had a particle diameter of 10 μm to 200 μm and a width (a thickness of the ring) of 0.2 μm to 1.5 μm, and most of polymer particulates were present in the ring portion of the ring pattern. 
     In addition, referring to  FIGS. 6 and 7 , in each separator of Examples 1 and 2, a very thin coating layer having each thickness of 1.06 μm and 661 nm was formed. 
     Although preferred embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and improvements by those skilled in the art using the basic concept of the present invention as defined in the appended claims are also within the scope of the invention.