TECHNICAL FIELD 5
The present application claims priority to Korean Patent Application No. 10-2020-0135562 filed on October 19, 2020 in the Republic of Korea. The present disclosure relates to a separator and a lithium secondary battery including the same.
BACKGROUND ART 10
Due to the high growth of electric vehicle market, lithium secondary batteries including a high-energy cell has been increasingly in demand. Therefore, batteries using a high-loading electrode (5 mAh/cm2 or more, based on electrode area) have been developed.
Such a lithium secondary battery includes a separator interposed between a positive electrode and a negative electrode in order to prevent a physical contact and electrical short-circuit 15 between the positive electrode and the negative electrode. Such a separator frequently uses a polyolefin-based porous polymer substrate, and a separator provided with a porous coating layer, containing inorganic particles and a binder polymer, on at least one surface of the porous polymer substrate is used frequently in order to prevent the porous polymer substrate from heat shrinking and to enhance the adhesion to an electrode. 20
However, when applying commercially available separators to batteries using high-loading electrodes, it is difficult to ensure high-rate discharge characteristics and output/life characteristics.
DISCLOSURE 25
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Technical Problem
The present disclosure is designed to solve the problems of the related art, and therefore the present disclosure is directed to improving lithium ion and electron transport rate by coating the surface of a porous coating layer with a conductive material. In this manner, it is possible to improve high-rate discharge characteristics and output characteristics. 5
Technical Solution
In one aspect of the present disclosure, there is provided a separator for a lithium secondary battery according to any one of the following embodiments.
According to the first embodiment, there is provided a separator for a lithium secondary 10 battery, including:
a porous polymer substrate;
a porous coating layer formed merely on the first surface of the porous polymer substrate or formed on both the first surface and the second surface of the porous polymer substrate, and including a plurality of inorganic particles and a first binder polymer disposed totally or partially 15 on the surfaces of the inorganic particles so that the inorganic particles may be interconnected and fixed; and
a conductive coating layer formed on the first surface of the porous coating layer and including a conductive material and a second binder polymer,
wherein the loading amount of the conductive material in the conductive coating layer is 20 0.01-0.5 g/m2, and
the conductive material includes single-walled carbon nanotubes (SWCNTs) in an amount of 90 wt% or more based on 100 wt% of the conductive material.
According to the second embodiment, there is provided the separator for a lithium secondary battery as defined in the first embodiment, wherein the conductive material includes 25
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single-walled carbon nanotubes (SWCNTs) in an amount of 99 wt% or more based on 100 wt% of the conductive material.
According to the third embodiment, there is provided the separator for a lithium secondary battery as defined in the first or the second embodiment, wherein the conductive coating layer is a thin film and has a thickness of 2 μm or less. 5
According to the fourth embodiment, there is provided the separator for a lithium secondary battery as defined in any one of the first to the third embodiments, wherein the single-walled carbon nanotubes have a diameter of 0.1-10 nm.
According to the fifth embodiment, there is provided the separator for a lithium secondary battery as defined in any one of the first to the fourth embodiments, wherein each of the first binder 10 and the second binder independently is any one selected from the group consisting of polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethyl methacrylate, polyethylhexyl acrylate, polybutyl acrylate, polyacrylonitrile, polyvinyl pyrro1idone, polyvinyl acetate, polyethylene-co-vinyl acetate, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, 15 cyanoethylpolyvinyl alcho1, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, and carboxymethyl cellulose, or a mixture of two or more of them.
According to the sixth embodiment, there is provided the separator for a lithium secondary battery as defined in any one of the first to the fifth embodiments, wherein the conductive coating layer further includes a particle-type binder polymer. 20
According to the seventh embodiment, there is provided the separator for a lithium secondary battery as defined in the sixth embodiment, wherein the particle-type binder polymer has a glass transition temperature of 80°C or less.
According to the eighth embodiment, there is provided the separator for a lithium secondary battery as defined in the sixth embodiment, wherein the particle-type binder polymer has 25
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an average diameter (D50) of 100-500 nm.
According to the ninth embodiment, there is provided the separator for a lithium secondary battery as defined in the sixth embodiment, wherein the particle-type binder polymer includes any one selected from the group consisting of styrene butadiene rubber (SBR), acrylonitrile-butadiene rubber, acrylonitrile-butadiene-styrene rubber, acrylic copolymer, 5 polyacrylonitrile, polyvinyl chloride, polyvinylidene fluoride, polyvinyl alcohol, polystyrene and polycyanoacrylate, or a mixture of two or more of them.
According to the tenth embodiment, there is provided the separator for a lithium secondary battery as defined in any one of the first to the ninth embodiments, wherein the weight ratio of the conductive material to the second binder polymer is 40 : 60-99 : 1. 10
According to the eleventh embodiment, there is provided the separator for a lithium secondary battery as defined in any one of the first to the tenth embodiments, wherein the porous coating layer has a thickness of 1-10 μm.
In another aspect of the present disclosure, there is provided a lithium secondary battery according to any one of the following embodiments. 15
According to the twelfth embodiment, there is provided a lithium secondary battery including a positive electrode, a negative electrode and a separator interposed between the positive electrode and the negative electrode, wherein the separator is the same as defined in any one of the first to the eleventh embodiments, and the conductive coating layer in the separator faces the positive electrode. 20
According to the thirteenth embodiment, there is provided the lithium secondary battery as defined in the twelfth embodiment, wherein the positive electrode is a high-loading positive electrode having a positive electrode active material loading amount of 5 mAh/cm2 or more.
Advantageous Effects 25
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Since the separator for a lithium secondary battery according to an embodiment of the present disclosure includes a conductive coating layer, it is possible to improve the electroconductivity and to ensure the ion conductivity of the separator.
In addition, since the conductive coating layer includes a binder polymer, it is possible to improve the adhesion between the separator and an electrode. 5
DESCRIPTION OF DRAWINGS
The accompanying drawings illustrate a preferred embodiment of the present disclosure and together with the foregoing disclosure, serve to provide further understanding of the technical features of the present disclosure, and thus, the present disclosure is not construed as being limited 10 to the drawing. Meanwhile, shapes, sizes, scales or proportions of some constitutional elements in the drawings may be exaggerated for the purpose of clearer description.
FIG. 1 is a scanning electron microscopic (SEM) image illustrating the slurry dispersion for forming a single-walled carbon nanotube conductive coating layer prepared according to an embodiment of the present disclosure. 15
FIG. 2 is an SEM image illustrating the slurry dispersion for forming a multi-walled carbon nanotube conductive coating layer prepared according to Comparative Example.
FIG. 3 is a graph illustrating the electroconductivity of each of the separators according to an embodiment of the present disclosure and Comparative Example depending on carbon nanotube coating amount. 20
FIG. 4 is an SEM image illustrating the surface of the separator having a conductive coating layer according to Example 6.
FIG. 5 is an SEM image illustrating the surface of the separator having a conductive coating layer according to Comparative Example 6.
FIG. 6 is a schematic sectional view illustrating the separator according to an embodiment 25
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of the present disclosure.
BEST MODE
Hereinafter, preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Prior to the description, it should be understood 5 that the terms used in the specification and the appended claims should not be construed as limited to general and dictionary meanings, but interpreted based on the meanings and concepts corresponding to technical aspects of the present disclosure on the basis of the principle that the inventor is allowed to define terms appropriately for the best explanation. Therefore, the description proposed herein is just a preferable example for the purpose of illustrations only, not 10 intended to limit the scope of the disclosure, so it should be understood that other equivalents and modifications could be made thereto without departing from the scope of the disclosure.
Throughout the specification, the expression ‘a part includes an element’ does not preclude the presence of any additional elements but means that the part may further include the other elements. 15
As used herein, the terms ‘about’, ‘substantially’, or the like, are used as meaning contiguous from or to the stated numerical value, when an acceptable preparation and material error unique to the stated meaning is suggested, and are used for the purpose of preventing an unconscientious invader from unduly using the stated disclosure including an accurate or absolute numerical value provided to help understanding of the present disclosure.

WHAT IS CLAIMED IS:
1. A separator for a lithium secondary battery, comprising:
a porous polymer substrate;
a porous coating layer formed merely on the first surface of the porous polymer substrate 5 or formed on both the first surface and the second surface of the porous polymer substrate, and comprising a plurality of inorganic particles and a first binder polymer disposed totally or partially on the surfaces of the inorganic particles so that the inorganic particles are interconnected and fixed; and
a conductive coating layer formed on the first surface of the porous coating layer and 10 comprising a conductive material and a second binder polymer,
wherein the loading amount of the conductive material in the conductive coating layer is 0.01-0.5 g/m2, and
the conductive material comprises single-walled carbon nanotubes (SWCNTs) in an amount of 90 wt% or more based on 100 wt% of the conductive material. 15
2. The separator for a lithium secondary battery according to claim 1, wherein the conductive material comprises single-walled carbon nanotubes (SWCNTs) in an amount of 99 wt% or more based on 100 wt% of the conductive material.
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3. The separator for a lithium secondary battery according to claim 1, wherein the conductive coating layer is a thin film and has a thickness of 2 μm or less.
4. The separator for a lithium secondary battery according to claim 1, wherein the single-walled carbon nanotubes have a diameter of 0.1-10 nm. 25
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5. The separator for a lithium secondary battery according to claim 1, wherein each of the first binder and the second binder independently is any one selected from the group consisting of polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethyl methacrylate, polyethylhexyl acrylate, polybutyl acrylate, 5 polyacrylonitrile, polyvinyl pyrrolidone, polyvinyl acetate, polyethylene-co-vinyl acetate, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, cyanoethylpolyvinyl alcho1, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, and carboxymethyl cellulose, or a mixture of two or more of them.
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6. The separator for a lithium secondary battery according to claim 1, wherein the conductive coating layer further comprises a particle-type binder polymer.
7. The separator for a lithium secondary battery according to claim 6, wherein the particle-type binder polymer has a glass transition temperature of 80°C or less. 15
8. The separator for a lithium secondary battery according to claim 6, wherein the particle-type binder polymer has an average diameter (D50) of 100-500 nm.
9. The separator for a lithium secondary battery according to claim 6, wherein the 20 particle-type binder polymer comprises any one selected from the group consisting of styrene butadiene rubber (SBR), acrylonitrile-butadiene rubber, acrylonitrile-butadiene-styrene rubber, acrylic copolymer, polyacrylonitrile, polyvinyl chloride, polyvinylidene fluoride, polyvinyl alcohol, polystyrene and polycyanoacrylate, or a mixture of two or more of them.
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10. The separator for a lithium secondary battery according to claim 1, wherein the weight ratio of the conductive material to the second binder polymer is 40 : 60-99 : 1.
11. The separator for a lithium secondary battery according to claim 1, wherein the porous coating layer has a thickness of 1-10 μm. 5
12. A lithium secondary battery comprising a positive electrode, a negative electrode and a separator interposed between the positive electrode and the negative electrode, wherein the separator is the same as defined in any one of claims 1 to 11, and the conductive coating layer in the separator faces the positive electrode. 10
13. The lithium secondary battery according to claim 12, wherein the positive electrode is a high-loading positive electrode having a positive electrode active material loading amount of 5 mAh/cm2 or more.