TECHNICAL FIELD
Cross-reference to Related Applications
[0001] This application claims the benefit of Korean Patent
Application No. 10-2020-0120429, filed on September 18, 2020,
in the Korean Intellectual Property Office, the disclosure of
10 which is incorporated herein in its entirety by reference.
Technical Field
[0002] The present invention relates to a negative electrode
and a secondary battery including the same.
BACKGROUND ART
15 [0003] Rising prices of energy sources due to fossil fuel
depletion and ever-growing awareness of environmental
pollution have driven eco-friendly alternative energy sources
to be placed as a first priority for life in the future.
[0004] In particular, the demand for secondary batteries as
20 an eco-friendly alternative energy source is sharply
increasing as mobile device technology continues to develop
and the demand for the mobile devices continues to increase.
[0005] On top of that, in recent years, growing concern over
environmental issues has given rise to extensive research into
25 electric vehicles (EV) and hybrid electric vehicles (HEV),
2
which may replace vehicles powered by fossil fuels, such as
gasoline vehicle and diesel vehicle, one of the main culprits
for air pollution. Lithium secondary batteries having high
energy density, high discharge voltage, and high output
5 stability have been predominantly studied and applied as power
sources for these electric vehicles (EV) and hybrid electric
vehicles (HEV).
[0006] In the secondary batteries, lithium metal has been
typically used as a negative electrode, but due to a battery
10 short circuit caused by formation of dendrites and risk of
explosion resulting therefrom, carbon-based active materials,
which serve to reversibly intercalate and deintercalate
lithium ions and maintain structural and electrical properties
have emerged as a replacement.
15 [0007] Various types of carbon-based active materials, such
as artificial graphite, natural graphite, and hard carbon,
have been used as the carbon-based active materials, and, among
these materials, a graphite-based active material, which may
ensure life characteristics of lithium secondary batteries due
20 to excellent reversibility, has been most widely used. Since
the graphite-based active material has a low discharge voltage
versus lithium of -0.2 V, a battery using the graphite-based
active material may exhibit a high discharge voltage of 3.6 V,
and thus, the graphite-based active material provides a number
25 of benefits in regards to energy density of lithium batteries.
3
[0008] Among the carbon-based active materials, natural
graphite, in particular, is known to exhibit higher output and
capacity than other carbon-based active materials such as
artificial graphite. However, the natural graphite generally
5 has a low particle strength, and accordingly, causes internal
deformation of particles upon rolling involved in the
manufacture of a negative electrode, and reduces porosity
inside the natural graphite or the negative electrode, thereby
leading to deterioration in the diffusion rate of lithium ions
10 and output characteristics.
[0009] Therefore, in the application of natural graphite to
the negative electrode, there is a need for developing a
negative electrode enabling high output.
[0010] Japanese Patent No. 4403327 discloses graphite powder
15 for a negative electrode of a lithium ion secondary battery,
but does not provide an alternative to the above-described
tasks.
[0011] [Related Art Document]
[0012] [Patent Document]
20 [0013] Japanese Patent No. 4403327.
DISCLOSURE OF THE INVENTION
TECHNICAL PROBLEM
[0014] An aspect of the present invention provides a negative
electrode with high output characteristics and high
25 temperature storage performance.
4
[0015] Another aspect of the present invention provides a
method for manufacturing the negative electrode described
above.
[0016] Another aspect of the present invention provides a
5 secondary battery including the negative electrode described
above.
TECHNICAL SOLUTION
[0017] According to an aspect of the present invention, there
is provided a negative electrode including: a negative
10 electrode current collector; and a negative electrode active
material layer disposed on the negative electrode current
collector and including a negative electrode active material,
wherein the negative electrode active material includes
natural graphite particles, and has a particle strength of 40
15 MPa to 200 MPa when being plastically deformed.
[0018] In addition, according to another aspect of the present
invention, there is provided a secondary battery including the
above-described negative electrode; a positive electrode
facing the negative electrode; a separator disposed between
20 the negative electrode and the positive electrode; and an
electrolyte.
ADVANTAGEOUS EFFECTS
[0019] A negative electrode of the present invention includes
a negative electrode active material containing natural
25 graphite particles and having a specific range of particle
5
strength when being plastically deformed. The negative
electrode active material has a desirable level of particle
strength upon plastic deformation, and accordingly, internal
deformation of the active material in the negative electrode
5 may be prevented and pores of natural graphite may be
maintained. Therefore, in the negative electrode according to
the present invention, the excellent capacity characteristics
of the natural graphite particles may well be presented,
reduction in the diffusion path of lithium may be prevented,
10 and the output characteristics and high temperature storage
performance of the negative electrode and a secondary battery
may be enhanced.
MODE FOR CARRYING OUT THE INVENTION
[0020] It will be understood that words or terms used in the
15 specification and claims shall not be interpreted as the
meaning defined in commonly used dictionaries, and it will be
further understood that the words or terms should be
interpreted as having a meaning that is consistent with their
meaning in the context of the relevant art and the technical
20 idea of the invention, based on the principle that an inventor
may properly define the meaning of the words or terms to best
explain the invention.
[0021] The terminology used herein is for the purpose of
describing particular example embodiments only and is not
25 intended to be limiting of the present invention. In the
6
specification, the terms of a singular form may include plural
forms unless referred to the contrary.
[0022] It will be further understood that the terms “include”,
“comprise”, or "have" when used in this specification, specify
5 the presence of stated features, numbers, steps, elements, or
combinations thereof, but do not preclude the presence or
addition of one or more other features, numbers, steps,
elements, or combinations thereof.
[0023] The term “average particle diameter (D50)” as used
10 herein may be defined as a particle diameter at a cumulative
volume of 50% in a particle size distribution curve of
particles. The average particle diameter (D50), for example,
may be measured by using a laser diffraction method. The laser
diffraction method may generally measure a particle diameter
15 ranging from a submicron level to a few mm, and may obtain
highly repeatable and high-resolution results.
[0024] Hereinafter, the present invention will be described
in more detail.
20 [0025] Negative Electrode
[0026] The present invention relates to a negative electrode,
and particularly, to a negative electrode for a lithium
secondary battery.
[0027] Specifically, the negative electrode includes a
25 negative electrode current collector, and a negative electrode
7
active material layer disposed on the negative electrode
current collector and including a negative electrode active
material, wherein the negative electrode active material
includes natural graphite particles, and has a particle
5 strength of 40 MPa to 200 MPa when being plastically deformed.
[0028] Typically, it is known that natural graphite particles
have an excellent capacity compared to artificial graphite
particles, but have a low particle strength. In general, the
negative electrode requires the involvement of a rolling
10 process in forming a negative electrode active material layer
on the negative electrode current collector, and the rolling
process causes internal deformation of the particles and
reduces porosity in the negative electrode when typical natural
graphite particles are used as a negative electrode active
15 material. Accordingly, the negative electrode using typical
natural graphite particles as an active material fails to
exhibit excellent capacity characteristics of natural graphite
particles, and has a capacity decrease at high output due to
reduction in the diffusion path of lithium.
20 [0029] In order to overcome such limitations, the negative
electrode of the present invention includes a negative
electrode active material containing natural graphite
particles and having the above-described range of particle
strength when being plastically deformed. The negative
25 electrode active material has a desirable level of particle
8
strength upon plastic deformation, and accordingly, internal
deformation of the active material may be prevented and
porosity of natural graphite may be maintained even when
rolling is performed in the manufacture of a negative electrode.
5 Therefore, in the negative electrode according to the present
invention, the excellent capacity characteristics of the
natural graphite particles may well be presented, reduction in
the diffusion path of lithium may be prevented, and the output
characteristics and high temperature storage performance of
10 the negative electrode and a secondary battery may be enhanced.
[0030] A negative electrode current collector generally used
in the art may be used without limitation as the negative
electrode current collector, and, for example, the negative
15 electrode current collector is not particularly limited so
long as it has high conductivity without causing adverse
chemical changes in the lithium secondary battery. For example,
the negative electrode current collector may include at least
one selected from copper, stainless steel, aluminum, nickel,
20 titanium, fired carbon, and an aluminum-cadmium alloy,
preferably, copper.
[0031] The negative electrode current collector may have fine
irregularities on the surface thereof to improve the bonding
strength of a negative electrode active material, and the
25 negative electrode current collector may be used in various
9
shapes such as a film, a sheet, a foil, a net, a porous body,
a foam body, and a non-woven fabric body.
[0032] The negative electrode current collector generally may
have a thickness of 3 μm to 500 μm.
5
[0033] The negative electrode active material layer is
disposed on the negative electrode current collector.
[0034] The negative electrode active material includes
natural graphite particles. In general, natural graphite
10 particles contain more pores than artificial graphite
particles, and thus have high output characteristics.
Meanwhile, the natural graphite particles according to the
present invention have a high level of particle strength upon
plastic deformation, as will be described later, and thus, the
15 internal deformation in the negative electrode is prevented
and a pore structure in the negative electrode is maintained
to effectively ensure the diffusion path of lithium, and to
enhance output characteristics and high temperature storage
characteristics.
20 [0035] The negative electrode active material may further
include, in addition to natural graphite particles, a carbon
coating layer positioned on the natural graphite particles.
The carbon coating layer may contribute to enhancing structural
stability of the natural graphite particles, increasing
25 particle strength, and preventing side reactions between the
10
negative electrode active material and an electrolyte solution.
[0036] The carbon coating layer may be included in an amount
of 1 wt% to 15 wt%, preferably, 2 wt% to 5 wt% in the negative
electrode active material. The presence of the carbon coating
5 layer may improve the particle strength of the negative
electrode active material, but since there is a concern that
excessive formation of the carbon coating layer rather causes
reduction of pores inside the negative electrode active
material to increase the diffusion resistance of lithium ions,
10 and deterioration in thermal stability and output
characteristics due to an increase in electrolyte side
reactions, it is desirable to form the carbon coating layer
within the above-described amount range.
[0037] The carbon coating layer may include amorphous carbon.
15 For example, the carbon coating layer may be formed by
providing at least one carbon coating layer precursor selected
from the group consisting of coal-tar pitch, rayon, and
polyacrylonitrile-based resin to the natural graphite
particles, and then heat-treating the obtained. The heat
20 treatment for the forming of the carbon coating layer may be
performed at 900°C to 1,500°C in terms of promoting uniform
formation of the carbon coating layer.
[0038] The negative electrode active material has a particle
strength of 40 MPa to 200 MPa when being plastically deformed.
25 The negative electrode active material according to the present
11
invention has a particle strength range described above upon
plastic deformation to maintain a pore structure without
internal deformation of the negative electrode active material
in the negative electrode, thereby reducing the lithium ion
5 diffusion resistance of the negative electrode, improving
output characteristics and high temperature storage
performance thereof.
[0039] When the negative electrode active material has a
particle strength of less than 40 MPa upon plastic deformation,
10 the internal deformation of the negative electrode active
material becomes significant in a rolling process essentially
involved in manufacturing the negative electrode, and
accordingly, the internal pores of the negative electrode are
reduced to remarkably deteriorate output characteristics, and
15 lithium ions are not effectively diffused to lead to Li-plating,
thereby causing a concern over life characteristics and storage
performance. When the negative electrode active material has
a particle strength of greater than 200 MPa upon plastic
deformation, particle breakage may occur due to contact or
20 collision of particles upon rolling of the negative electrode
to cause deterioration of the pore structure in the negative
electrode, and thus the output characteristics of lithium may
be impaired.
[0040] The negative electrode active material may have a
25 particle strength of preferably 70 MPa to 150 MPa, more
12
preferably 90 MPa to 130 MPa upon plastic deformation, and
within the range described above, the structural deformation
of the active material may be prevented and the pore structure
may be maintained, and particle breakage caused by contact of
5 particles may be prevented as well, and thus the output
characteristics and lifespan characteristics of the negative
electrode may be remarkably improved.
[0041] The above-described particle strength of the negative
electrode active material upon plastic deformation may be
10 implemented by, in the manufacture of the natural graphite
particles, performing cold isostatic pressing (CIP) and
controlling conditions, or by controlling the amount of a
carbon coating layer, specifically, performing cold isostatic
pressing (CIP) and controlling conditions.
15 [0042] As used herein, the term “plasticity” refers to a
characteristic of an object to be permanently altered in shape
even with the removal of external force when the object is
altered in external shape due to the external force applied
thereto without a change in motion state. Meanwhile, the term
20 “elasticity” refers to a characteristic of an object to return
to an original state thereof with the removal of external force
when the object is altered in external shape due to the
external force applied thereto without a change in motion state,
and is a concept distinct from the plasticity. As used herein,
25 the term “particle strength upon plastic deformation” may be
13
defined as particle strength at a point where an object is
permanently altered in shape even with the removal of external
force when the object is altered in external shape due to the
external force applied thereto without a change in motion state.
5 Specifically, when a stress-strain curve is plotted by applying
a load to the negative electrode active material in a powder
resistance measuring device or the like, it may be defined as
an elastic region in a case where stress and strain have a
proportional relationship, and it may be defined as a plastic
10 region when only the strain changes without a change in stress,
and the particle strength upon plastic deformation may be
defined as stress at the point of changing from an elastic
region to the plastic region in the stress-strain curve.
[0043] The negative electrode active material may have a
15 compressive fracture strength of 350 MPa to 1,000 MPa,
preferably 400 MPa to 900 MPa, more preferably 480 MPa to 900
MPa, and even more preferably 650 MPa to 800 MPa. As used
herein, the compressive fracture strength may be defined as a
maximum compressive stress applied to an object without object
20 breakage. The compressive fracture strength is a concept
distinct from “particle strength upon plastic deformation”,
which measures the particle strength for permanent deformation
of the outer shape of an object. When the negative electrode
active material according to the present invention has a
25 compressive fracture strength in the range described above,
14
preventing structural deformation of the active material and
maintaining the pore structure are maximized, the particle
breakage caused by contact of particles is prevented, and
accordingly, the output characteristics and lifespan
5 characteristics of the negative electrode may be remarkably
improved.
[0044] The above-described compressive fracture strength of
the negative electrode active material may be implemented by,
in the manufacture of the natural graphite particles,
10 performing cold isostatic pressing (CIP) and controlling
conditions, or by controlling the amount of a carbon coating
layer, specifically, performing cold isostatic pressing (CIP)
and controlling conditions.
[0045] The negative electrode active material may be
15 spherical. When the negative electrode active material is
spherical, the negative electrode active materials effectively
maintain a pore structure inside the negative electrode,
thereby securing a lithium ion diffusion path and improving
output characteristics of the negative electrode. As used
20 herein, the term “spherical” refers to a concept including a
substantially spherical shape even when slightly distorted, in
addition to a perfect spherical shape.
[0046] The negative electrode active material may have an
average particle diameter (D50) of 10 μm to 25 μm, preferably
25 15 μm to 20 μm. When the average particle diameter (D50) of
15
negative electrode active material is in the above range, it
is preferable in terms of improving both output characteristics
and lifespan characteristics.
[0047] The negative electrode active material may be included
5 in an amount of 80 wt% to 99 wt%, preferably, 88 wt% to 98 wt%
in the negative electrode active material layer.
[0048] The negative electrode active material layer may
further include a binder, a conductive agent, and/or a
10 thickener in addition to the above-described negative
electrode active material.
[0049] The binder is a component that assists in the binding
between the active material and/or the current collector,
wherein the binder may commonly be included in an amount of 1
15 wt% to 30 wt%, preferably, 1 wt% to 10 wt% in the negative
electrode active material layer.
[0050] The binder may include at least one selected from the
group consisting of polyvinylidene fluoride (PVDF), polyvinyl
alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl
20 cellulose, regenerated cellulose, polyvinylpyrrolidone,
polytetrafluoroethylene, polyethylene, polypropylene, an
ethylene-propylene-diene polymer (EPDM), a sulfonated EPDM, a
styrene-butadiene rubber, and a fluorine rubber, preferably,
at least one selected from polyvinylidene fluoride and a
25 styrene-butadiene rubber.
16
[0051] Any thickener used in a typical lithium secondary
battery may be used as the thickener, and an example thereof
is carboxymethyl cellulose (CMC).
[0052] The conductive agent is a component for further
5 improving the conductivity of the negative electrode active
material, wherein the conductive agent may be included in an
amount of 1 wt% to 30 wt%, for example, 1 wt% to 10 wt% in the
negative electrode active material layer.
[0053] Any conductive agent may be used without particular
10 limitation so long as it has conductivity without causing
adverse chemical changes in the battery, and, for example, a
conductive material, such as: graphite such as natural graphite
or artificial graphite; carbon black such as acetylene black,
Ketjen black, channel black, furnace black, lamp black, and
15 thermal black; conductive fibers such as carbon fibers or metal
fibers; fluorocarbon; metal powder such as aluminum powder,
and nickel powder; conductive whiskers such as zinc oxide
whiskers and potassium titanate whiskers; conductive metal
oxide such as titanium oxide; or polyphenylene derivatives,
20 may be used. Specific examples of a commercial conductive
agent may include acetylene black-based products (Chevron
Chemical Company, Denka black (Denka Singapore Private
Limited), or Gulf Oil Company), Ketjen black, ethylene
carbonate (EC)-based products (Armak Company), Vulcan XC-72
25 (Cabot Company), and Super P (Timcal Graphite & Carbon).
17
[0054] A negative electrode slurry is prepared by mixing the
above-described negative electrode active material and at
least one selected from the binder, the conductive agent, and
the thickener in a solvent, and the negative electrode active
5 material layer may be prepared by coating the negative
electrode current collector with the negative electrode slurry,
and rolling and drying the coated negative electrode current
collector.
[0055] The solvent may include water or an organic solvent,
10 such as N-methyl-2-pyrrolidone (NMP), and may be used in an
amount such that desirable viscosity is obtained when the
negative electrode active material as well as optionally the
binder and the conductive agent are included. For example,
the solvent may be included in an amount such that a
15 concentration of a solid content including the negative
electrode active material as well as optionally at least one
selected from the binder, the thickener, and the conductive
agent is in the range of 50 wt% to 95 wt%, preferably, 70 wt%
to 90 wt%.
20
[0056] The negative electrode may have a porosity of 20% to
45%, preferably 25% to 35%. When the porosity of the negative
electrode is in the above range, it is preferable in terms of
improving both lifespan characteristics and output
25 characteristics.
18
[0057] In the present description, the porosity of the
negative electrode may be calculated using Equation 1 below.
[0058] [Equation 1]
Porosity of negative electrode (%) = ｛ 1-(electrode
5 density of negative electrode / true density of negative
electrode)｝ × 100
[0059] In Equation 1 above, the true density of the negative
electrode is density of a negative electrode active material
layer measured when the negative electrode is taken to a
10 predetermined size and pressed with press equipment until the
change in thickness of the negative electrode stops, and the
electrode density of the negative electrode is density of a
negative electrode active material layer measured when the
negative electrode is taken to a predetermined size.
15 [0060] The negative electrode may have a pore resistance of
15 Ω or less, preferably 3 Ω to 14 Ω, and more preferably 9.5
Ω to 11.5 Ω.
[0061] The pore resistance may be defined as a resistance
20 value obtained by injecting an electrolyte containing lithium
ions into a symmetric cell prepared using the negative
electrode as a working electrode and a counter electrode in
the same manner, and performing electrochemical impedance
spectroscopy (EIS). The pore resistance is EIS-analyzed with
25 the symmetric cell, and thus only lithium ions derived from
19
the electrolyte may be present, and accordingly, the lithium
ion diffusion resistance in the negative electrode may be
objectively measured.
[0062] The negative electrode has pore resistance in the
5 above-described range, and thus, the lithium ion diffusion
path may be minimized in the negative electrode, thereby
improving the output performance of a battery. In addition,
within the above-described pore resistance range, the lithium
ion diffusion path may be minimized, Li-plating on a surface
10 that may occur upon high-rate charging may be effectively
prevented, and side reactions on the surface of the negative
electrode may be prevented. Accordingly, the negative
electrode of the present invention having the pore resistance
in the above-described range may be improved to an excellent
15 level in output characteristics and lifespan characteristics.
[0063] The above-described pore resistance range may be
adjusted, for example, by controlling particle strength upon
plastic deformation, compressive fracture strength, structure,
size, and the like of the negative electrode active material.
20
[0064] Secondary Battery
[0065] Furthermore, the present invention provides a
secondary battery including the above-described negative
electrode, more particularly, a lithium secondary battery.
25 [0066] The secondary battery may include the above-described
20
negative electrode; a positive electrode facing the negative
electrode; a separator disposed between the negative electrode
and the positive electrode; and an electrolyte.
[0067] The positive electrode may include a positive
5 electrode current collector; and a positive electrode active
material layer disposed on the positive electrode current
collector.
[0068] A positive electrode current collector generally used
in the art may be used without limitation as the positive
10 electrode current collector, and, for example, the positive
electrode current collector is not particularly limited so
long as it has high conductivity without causing adverse
chemical changes in the secondary battery. For example, the
positive electrode current collector may include at least one
15 selected from copper, stainless steel, aluminum, nickel,
titanium, fired carbon, and an aluminum-cadmium alloy,
preferably, aluminum.
[0069] The positive electrode current collector may have fine
irregularities on the surface thereof to improve the bonding
20 strength of a positive electrode active material, and the
positive electrode current collector may be used in various
shapes such as a film, a sheet, a foil, a net, a porous body,
a foam body, and a non-woven fabric body.
[0070] The positive electrode current collector generally may
25 have a thickness of 3 μm to 500 μm.

CLAIMS
1. A negative electrode comprising:
a negative electrode current collector; and
5 a negative electrode active material layer disposed on
the negative electrode current collector and comprising a
negative electrode active material,
wherein the negative electrode active material comprises
natural graphite particles, and
10 the negative electrode active material has a particle
strength of 40 MPa to 200 MPa when being plastically deformed.
2. The negative electrode of claim 1, wherein the negative
electrode active material has a compressive fracture strength
15 of 350 MPa to 1,000 MPa.
3. The negative electrode of claim 1, wherein the negative
electrode active material has an average particle diameter (D50)
of 10 μm to 25 μm.
20
4. The negative electrode of claim 1, wherein the negative
electrode active material is spherical.
5. The negative electrode of claim 1, wherein the negative
25 electrode active material further comprises a carbon coating
39
layer positioned on the natural graphite particles.
6. The negative electrode of claim 5, wherein the carbon
coating layer is included in an amount of 1 wt% to 15 wt% in
5 the negative electrode active material.
7. The negative electrode of claim 5, wherein the carbon
coating layer comprises amorphous carbon.
10 8. The negative electrode of claim 1, wherein the negative
electrode has a pore resistance of 15 Ω or less.
9. The negative electrode of claim 1, wherein the negative
electrode active material layer has a porosity of 20% to 45%.
15
10. A secondary battery comprising:
a negative electrode according to claim 1;
a positive electrode facing the negative electrode;
a separator disposed between the negative electrode and
20 the positive electrode; and
an electrolyte.