TECHNICAL FIELD
The present application claims priority to Korean Patent Application No. 10-2020-
0134283 filed on October 16, 2020 in the Republic of Korea.
10 The present disclosure relates to a separator applicable to an electrochemical
device, such as a lithium secondary battery, and a method for manufacturing the same.
BACKGROUND ART
Recently, energy storage technology has been given an increasing attention.
15 Efforts into research and development for electrochemical devices have been actualized
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
20 focused. More recently, active studies have been conducted about designing a novel
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
3
batteries developed in the early 1990’s have been spotlighted, since they have a higher
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 such electrochemical devices have 5 been produced from many production
companies, safety characteristics thereof show different signs. Evaluation and
securement of safety of such electrochemical devices are very important. For example, a
separator prevents a short-circuit between a positive electrode and a negative electrode and
provides a channel for transporting lithium ions. Therefore, such a separator is an
10 important element affecting the safety and output characteristics of a battery. Particularly,
a polyolefin-based porous polymer substrate used conventionally as a separator for an
electrochemical device shows a severe heat shrinking behavior at a temperature of 100°C
or higher due to its material property and a characteristic during its manufacturing process,
including orientation, thereby causing a short-circuit between a positive electrode and a
15 negative electrode.
To solve the above-mentioned safety problem of electrochemical devices, there
has been suggested a separator having a porous coating layer which is formed by coating a
mixture of an excessive amount of inorganic particles with a binder polymer on at least one
surface of a porous polymer substrate having a plurality of pores.
20 Such a porous coating layer may have pores typically formed by vapor induced
phase separation or immersed phase separation. Particularly, a porous coating layer
formed by vapor induced phase separation shows better quality in terms of heat shrinkage
as compared to immersed phase separation. However, vapor induced phase separation
4
has a problem in that the binder polymer contained in slurry block the pores of the porous
polymer substrate during the formation of a porous coating layer.
On the other hand, immersed phase separation has an advantage in that the coating
layer is solidified, while slurry is applied to a substrate at the same time, and thus provides
low resistance before/after coating of a porous 5 polymer substrate, as compared to vapor
induced phase separation. In other words, there is an advantage in that the pores of the
porous polymer substrate are not blocked by the binder polymer. However, the porous
coating layer formed through immersed phase separation shows poor quality, particularly
in terms of high-temperature heat shrinkage, as compared to vapor induced phase
10 separation, and provides lower adhesion (Lami strength) between a separator and an
electrode.
DISCLOSURE
Technical Problem
15 The present disclosure is designed to solve the problems of the related art, and
therefore the present disclosure is directed to providing a separator, which reduces
blocking of pores in a porous polymer substrate by taking an advantage of immersed phase
separation, while providing improved adhesion to an electrode and improved heat
shrinkage, and a method for manufacturing the separator.
20
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.
5
According to the first embodiment, there is provided a separator for a lithium
secondary battery, including:
a porous polymer substrate; and
a porous coating layer formed on at least one surface of the porous polymer
substrate, and including inorganic particles, 5 a fluorine-based binder polymer and a
polyvinyl acetate (PVAc) binder polymer,
wherein the porous coating layer has a structure in which the inorganic particles
are dispersed in a matrix formed by the fluorine-based binder polymer and the polyvinyl
acetate binder polymer, and the fluorine-based binder polymer, the polyvinyl acetate
10 binder polymer and the inorganic particles are distributed homogeneously in the thickness
direction of the porous coating layer,
the content of the inorganic particles is 70 parts by weight or more based on 100
parts by weight of the porous coating layer, and
the content of the polyvinyl acetate binder polymer is less than 80 parts by weight
15 based on 100 parts by weight of the total content of the binder polymers.
According to the second embodiment, there is provided a separator for a lithium
secondary battery, including:
a porous polymer substrate; and
a porous coating layer formed on at least one surface of the porous polymer
20 substrate, and including inorganic particles, a fluorine-based binder polymer and a
polyvinyl acetate (PVAc) binder polymer,
wherein the pores in the porous coating layer are formed by mass interchange
between a solvent and a non-solvent for the binder polymers,
6
the content of the inorganic particles is 70 parts by weight or more based on 100
parts by weight of the porous coating layer, and
the content of the polyvinyl acetate binder polymer is less than 80 parts by weight
based on 100 parts by weight of the total content of the binder polymers.
According to the third embodiment, there 5 is provided the separator for a lithium
secondary battery as defined in the first or the second embodiment, wherein the content of
the polyvinyl acetate binder polymer may be 5-75 parts by weight based on 100 parts by
weight of the total content of the binder polymers.
According to the fourth embodiment, there is provided the separator for a lithium
10 secondary battery as defined in the second or the third embodiment, wherein the solvent
may include at least one selected from N-methyl-2-pyrrolidone, dimethyl acetamide and
dimethyl formamide.
According to the fifth embodiment, there is provided the separator for a lithium
secondary battery as defined in any one of the second to the fourth embodiments, wherein
15 the non-solvent may be water.
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
fluorine-based binder polymer may have a weight average molecular weight of 100,000-
1,500,000.
20 According to the seventh embodiment, there is provided the separator for a lithium
secondary battery as defined in any one of the first to the sixth embodiments, wherein the
polyvinyl acetate binder polymer may have a weight average molecular weight of 100,000-
1,000,000.
7
According to the eighth embodiment, there is provided the separator for a lithium
secondary battery as defined in any one of the first to the seventh embodiments, which may
show an adhesion to an electrode (Lami strength) of 60 gf/25 mm or more.
According to the ninth embodiment, there is provided the separator for a lithium
secondary battery as defined in any 5 one of the first to the eighth embodiments, which may
show a heat shrinkage of 10% or less in at least one of the machine direction and the
transverse direction, as determined after heating the separator at 150°C for 30 minutes.
In another aspect of the present disclosure, there is provided an electrochemical
device according to any one of the following embodiments.
10 According to the tenth embodiment, there is provided an electrochemical device
including a positive electrode, a negative electrode and a separator interposed between the
negative electrode and the positive electrode, wherein the separator is the same as defined
in any one of the above-described embodiments.
According to the eleventh embodiment, there is provided the electrochemical
15 device as defined in the tenth embodiment, which may be a lithium secondary battery.
Advantageous Effects
According to an embodiment of the present disclosure, it is possible to provide a
separator which includes inorganic particles at a higher content as compared to the
20 conventional separator, and uses a fluorine-based binder polymer and a polyvinyl acetate
binder polymer at the same time. In this manner, it is possible to improve the adhesion to
an electrode (Lami strength) and to enhance the heat resistance of the separator. In
addition, it is possible to reduce blocking of the pores in a porous polymer substrate, and
8
thus to reduce resistance in the separator.
DESCRIPTION OF DRAWINGS
The accompanying drawings illustrate a preferred embodiment of the present
disclosure and together 5 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 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.
10 FIG. 1 is a schematic view illustrating the separator obtained by vapor induced
phase separation according to Comparative Example.
FIG. 2 is a scanning electron microscopic (SEM) image illustrating the separator
obtained by vapor induced phase separation according to Comparative Example.
FIG. 3 is a schematic view illustrating the separator obtained by immersed phase
15 separation according to an embodiment of the present disclosure.
FIG. 4 is an SEM image illustrating the separator obtained by immersed phase
separation according to an embodiment of the present disclosure.

WHAT IS CLAIMED IS:
1. A separator for a lithium secondary battery, comprising:
a porous polymer substrate; and
a porous coating layer formed on at least 5 one surface of the porous polymer
substrate, and comprising inorganic particles, a fluorine-based binder polymer and a
polyvinyl acetate (PVAc) binder polymer,
wherein the porous coating layer has a structure in which the inorganic particles
are dispersed in a matrix formed by the fluorine-based binder polymer and the polyvinyl
10 acetate binder polymer, and the fluorine-based binder polymer, the polyvinyl acetate
binder polymer and the inorganic particles are distributed homogeneously in the thickness
direction of the porous coating layer,
the content of the inorganic particles is 70 parts by weight or more based on 100
parts by weight of the porous coating layer, and
15 the content of the polyvinyl acetate binder polymer is less than 80 parts by weight
based on 100 parts by weight of the total content of the binder polymers.
2. A separator for a lithium secondary battery, comprising:
a porous polymer substrate; and
20 a porous coating layer formed on at least one surface of the porous polymer
substrate, and comprising inorganic particles, a fluorine-based binder polymer and a
polyvinyl acetate (PVAc) binder polymer,
wherein the pores in the porous coating layer are formed by mass interchange
44
between a solvent and a non-solvent for the binder polymers,
the content of the inorganic particles is 70 parts by weight or more based on 100
parts by weight of the porous coating layer, and
the content of the polyvinyl acetate binder polymer is less than 80 parts by weight
5 based on 100 parts by weight of the total content of the binder polymers.
3. The separator for a lithium secondary battery according to claim 1 or 2,
wherein the content of the polyvinyl acetate binder polymer is 5-75 parts by weight based
on 100 parts by weight of the total content of the binder polymers.
10
4. The separator for a lithium secondary battery according to claim 2, wherein
the solvent comprises at least one selected from N-methyl-2-pyrrolidone, dimethyl
acetamide and dimethyl formamide.
15 5. The separator for a lithium secondary battery according to claim 2, wherein
the non-solvent is water.
6. The separator for a lithium secondary battery according to claim 1 or 2,
wherein the fluorine-based binder polymer has a weight average molecular weight of
20 100,000-1,500,000.
7. The separator for a lithium secondary battery according to claim 1 or 2,
wherein the polyvinyl acetate binder polymer has a weight average molecular weight of
45
100,000-1,000,000.
8. The separator for a lithium secondary battery according to claim 1 or 2,
which shows an adhesion to an electrode (Lami strength) of 60 gf/25 mm or more.
5
9. The separator for a lithium secondary battery according to claim 1 or 2,
which shows a heat shrinkage of 10% or less in at least one of the machine direction and
the transverse direction, as determined after heating the separator at 150°C for 30 minutes.
10 10. An electrochemical device comprising a positive electrode, a negative
electrode and a separator interposed between the negative electrode and the positive
electrode, wherein the separator is the same as defined in claim 1 or 2.
11. The electrochemical device according to claim 10, which is a lithium
15 secondary battery.