TECHNICAL FIELD
The present application claims priority to Korean Patent Application No. 10-2020-
10 0132199 filed on October 13, 2020 in the Republic of Korea.
The present disclosure relates to a negative electrode for a lithium secondary battery,
a lithium secondary battery including the same, and a method for manufacturing the lithium
secondary battery.
15 BACKGROUND ART
Recently, energy storage technology has been given an increasing attention. 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, there has been an
increasing need for providing batteries used as power sources for such electronic instruments
20 with higher energy density. Lithium secondary batteries are those satisfying such a need
best, and thus active studies have been made about such lithium secondary batteries.
In general, a lithium secondary battery includes a positive electrode including a
lithium metal oxide, a negative electrode including a carbonaceous material, etc., an
3
electrolyte containing a lithium salt and an organic solvent, and a separator interposed
between the positive electrode and the negative electrode so that both electrodes may be
insulated electrically from each other.
Carbonaceous materials have been used frequently as negative electrode materials
5 forming the negative electrode of a lithium secondary battery. However, as the use of a
lithium secondary battery has been extended, a high-capacity lithium secondary battery has
been increasingly in demand. Therefore, there is a need for a high-capacity negative
electrode active material capable of substituting for a carbonaceous material having low
capacity. To meet such a need, there has been an attempt to use Si, having higher
10 charge/discharge capacity as compared to carbonaceous materials and capable of
electrochemical alloying with lithium, as a negative electrode active material.
However, the Si-based negative electrode active material has a serious problem in
that it undergoes a significant change in volume due to lithium-ion intercalation and
deintercalation during charge/discharge. The Si-based negative electrode active material
15 undergoes volumetric swelling to 300% or more by charging, and the mechanical stress
applied herein generates cracks inside of and on the surface of the active material. In
addition, when lithium ions are deintercalated by discharging, the Si-based negative
electrode active material is shrunk. Since the cracks are not recovered again, repetition of
charge/discharge cycles causes pulverization of the active material, and thus the negative
20 electrode active material may be detached from the negative electrode current collector, or
the negative electrode active material particles are detached from one another to form a dead
volume causing an electrical short-circuit. Therefore, it is known that the Si-based negative
electrode active material causes a rapid decrease in charge/discharge capacity, as
4
charge/discharge cycles proceed. In addition, side reactions with an electrolyte occur,
while the interface where the active material is exposed is increased by the cracks, resulting
in continuous consumption of lithium ions and electrolyte.
Under these circumstances, there is an imminent need for technology capable of
5 preventing the problem caused by a volumetric change of the Si-based negative electrode
active material.

WHAT IS CLAIMED IS:
1. A negative electrode for a lithium secondary battery, comprising:
a negative electrode current collector; and
5 a negative electrode active material layer disposed on at least one surface of the
negative electrode current collector, and comprising a Si-based negative electrode active
material, a conductive material and a first binder polymer,
wherein the Si-based negative electrode active material has cracks formed after
activating,
10 a second binder polymer is coated in the cracks, and
the first binder polymer and the second binder polymer are heterogeneous.
2. The negative electrode for a lithium secondary battery according to claim 1,
wherein the second binder polymer comprises a copolymer of a first monomer derived from
15 vinylidene fluoride (VDF) and a second monomer derived from hexafluoropropylene (HFP),
and
the second monomer is present in an amount of 20 wt% or more based on 100 wt%
of the copolymer.
20 3. The negative electrode for a lithium secondary battery according to claim 1,
wherein the Si-based negative electrode active material comprises Si, SiOx (1≤x≤2), Si/C,
or two or more of them.
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4. The negative electrode for a lithium secondary battery according to claim 1,
wherein the first binder polymer comprises polyvinyl alcohol, carboxymethyl cellulose
(CMC), starch, hydroxypropyl cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene,
styrene butadiene rubber (SBR), or two or more of them.
5
5. A lithium secondary battery, comprising:
a positive electrode;
a negative electrode comprising a negative electrode current collector, and a
negative electrode active material layer, which is disposed on at least one surface of the
10 negative electrode current collector, and comprises a Si-based negative electrode active
material having cracks formed after activating, a conductive material and a first binder
polymer, wherein a second binder polymer is coated in the cracks; and
a separator interposed between the positive electrode and the negative electrode, and
comprising a porous polymer substrate, and a porous coating layer disposed on at least one
15 surface of the porous polymer substrate and comprising a plurality of inorganic particles and
the second binder polymer.
6. A method for manufacturing a lithium secondary battery, comprising the
steps of:
20 (S1) preparing a positive electrode, and a preliminary negative electrode comprising
a Si-based negative electrode active material and a first binder polymer;
(S2) coating a slurry for forming a porous coating layer, comprising inorganic
particles, a second binder polymer and a solvent for the second binder polymer, on at least
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one surface of a porous polymer substrate, followed by drying, to obtain a separator;
(S3) interposing the separator obtained from step (S2) between the positive electrode
and the preliminary negative electrode prepared from step (S1), and carrying out lamination
to obtain an electrode assembly;
5 (S4) introducing the electrode assembly obtained from step (S3) into a battery casing,
and injecting an electrolyte thereto to obtain a preliminary battery;
(S5) activating the preliminary battery of step (S4);
(S6) heating the preliminary battery of step (S5) so that the second binder polymer
is dissolved in the electrolyte, and allowing the preliminary battery to stand; and
10 (S7) cooling the resultant product of step (S6),
wherein the first binder polymer is not dissolved in the electrolyte at the heating
temperature of step (S6).
7. The method for manufacturing a lithium secondary battery according to claim
15 6, wherein the heating temperature of step (S6) is 70-90°C.
8. The method for manufacturing a lithium secondary battery according to claim
6, wherein the second binder polymer comprises a copolymer of a first monomer derived
from vinylidene fluoride (VDF) and a second monomer derived from hexafluoropropylene
20 (HFP), and
the second monomer is present in an amount of 20 wt% or more based on 100 wt%
of the copolymer.