TECHNICAL FIELD
The present application claims priority to Korean Patent Application No. 10-2020-
0113902 filed on September 7, 2020 in the Republic of Korea, the disclosures of which are
incorporated herein by reference.
The present disclosure relates to a separator for an electrochemical device and a
10 method for manufacturing the same.
BACKGROUND ART
Recently, energy storage technology has been given an increasing attention.
Efforts into research and development for electrochemical devices have been actualized
15 more and more, as the application of energy storage technology has been extended to
energy for cellular phones, camcorders and notebook PC and even to energy for electric
vehicles. In this context, electrochemical devices have been most spotlighted. Among
such electrochemical devices, development of rechargeable secondary batteries has been
focused. More recently, active studies have been conducted about designing a novel
20 electrode and battery in order to improve the capacity density and specific energy in
developing such batteries.
Among the commercially available secondary batteries, lithium secondary
batteries developed in the early 1990’s have been spotlighted, since they have a higher
3
operating voltage and significantly higher energy density as compared to conventional
batteries, such as Ni-MH, Ni-Cd and sulfuric acid-lead batteries using an aqueous
electrolyte.
Although electrochemical devices, such as lithium secondary batteries, have been
5 produced from many production companies, safety characteristics thereof show different
signs. Evaluation and securement of safety of such electrochemical devices are very
important.
Meanwhile, ‘overcharge’ refers to the behavior of continuing to charge a cell to a
level beyond the upper cut-off voltage (e.g. 4.2-4.3 V) in a certain charging process so that
10 the cell may exceed the normal capacity thereof. Herein, side reactions occur in the
electrodes, electrolyte, or the like, in the cell to cause an increase in internal temperature of
the cell, resulting in shrinking of the separator and an internal short-circuit. Due to the
momentary short-circuit generated herein, the cell temperature is increased rapidly and
reactions may occur with combustible gases inside of the cell, resulting in explosion.
15 Such a rapid increase in cell temperature is further amplified due to the low heat
conductivity of the cell.

WHAT IS CLAIMED IS:
1. A separator for an electrochemical device comprising:
a porous polymer substrate; and
5 a porous coating layer positioned on at least one surface of the porous polymer
substrate and comprising inorganic particles and a binder polymer,
wherein the porous coating layer comprises a first porous coating layer that is in
contact with the porous polymer substrate and a second porous coating layer positioned on
the opposite side of the first porous coating layer that is not in contact with the porous
10 polymer substrate,
each of the binder polymer contained in the first porous coating layer and the
second porous coating layer has a concentration gradient increasing from the porous
polymer substrate toward the outermost the porous coating layer, and
the concentration gradient of the binder polymer contained in the second porous
15 coating layer has a larger slope than the slope of the concentration gradient of the binder
polymer contained in the first porous coating layer.
2. The separator for an electrochemical device according to claim 1, wherein
the content of the binder polymer contained in the first porous coating layer is the same as
20 the content of the binder polymer contained in the second porous coating layer.
3. The separator for an electrochemical device according to claim 1, wherein
the ratio of the thickness of the first porous coating layer to the thickness of the second
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porous coating layer is 4:6-1:9.
4. The separator for an electrochemical device according to claim 1, wherein
the binder polymer comprises polyvinylidene fluoride (PVDF), polyvinylidene fluoride-co5 hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethyl
methacrylate, polybutyl acrylate, polyacrylonitrile, polyvinyl pyrrolidone, polyvinyl
acetate, polyethylene-co-vinyl acetate, polyethylene oxide, polyarylate, cellulose acetate,
cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl pullulan, cyanoethyl
polyvinylalcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, carboxymethyl
10 cellulose, acrylonitrile-styrene-butadiene copolymer, polyimide, or two or more of them.
5. An electrochemical device which comprises an electrode assembly
comprising two electrodes having opposite polarities and a separator interposed between
the two electrodes, and received in a battery casing, wherein the separator is the separator
15 for an electrochemical device as defined in any one of claims 1 to 4.
6. The electrochemical device according to claim 5, wherein the porous
coating layer faces the positive electrode.
20 7. The electrochemical device according to claim 6, which shows an adhesion
strength of 30 gf/25 mm or more at the interface between the separator and the positive
electrode, as determined by heating and pressurizing the separator and the positive
electrode at 80°C under a pressure of 1000 kgf for 1 second, and then applying force at
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180° and a rate of 100 mm/min.
8. A method for manufacturing a separator for an electrochemical device,
comprising the steps of:
5 (S1) preparing a composition for forming a first porous coating layer and a
composition for forming a second porous coating layer;
(S2) coating and drying the composition for forming a first porous coating layer on
at least one surface of the porous polymer substrate; and
(S3) coating and drying the composition for forming a second porous coating layer
10 on the product of step (S2),
wherein the composition for forming a second porous coating layer is dried at a
higher drying rate than the drying rate of composition for forming a first porous coating
layer.
15 9. The method for manufacturing a separator for an electrochemical device
according to claim 8, wherein each of the drying rate of the composition for forming a first
porous coating layer and the drying rate of the composition for forming a second porous
coating layer is controlled to an amount of the solvent dried per second of 1-50 mg/sec,
and the drying rate of the composition for forming a second porous coating layer is higher
20 than the drying rate of the composition for forming a first porous coating layer.
10. The method for manufacturing a separator for an electrochemical device
according to claim 8, wherein the drying rate of the composition for forming a second
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porous coating layer is higher than the drying rate of the composition for forming a first
porous coating layer by 1-20 mg/sec.
11. The method for manufacturing a separator for an electrochemical device
5 according to claim 8, wherein the drying rate of the composition for forming a first porous
coating layer is 10-50% based on the drying rate of the composition for forming a second
porous coating layer.