TECHNICAL FIELD
[1] Cross-Reference to Related Application
[2] This application claims priority to and the benefit of Korean Patent
Application No. 10-2020-0083507, filed on July 7, 2020, the disclosure of which is
10 incorporated herein by reference in its entirety.
[3] Technical Field
[4] The present invention relates to a negative electrode whose rapid charging
performance and lifetime characteristics are excellent and a secondary battery
including the negative electrode, and specifically, to a negative electrode including a
15 negative electrode active material including both uncoated artificial graphite particles
which are single particles and uncoated artificial graphite particles which are
secondary particles formed by agglomerating a plurality of primary particles, and a
secondary battery including the negative electrode.
BACKGROUND ART
20 [5] With a dramatic increase in the use of fossil fuels, there is an increasing
demand for use of alternative energy or clean energy, and power generation and
accumulation fields using electrochemical reactions have been most actively studied
to meet the increasing demand.
[6] A representative example of the electrochemical device using electrochemical
25 energy comprises secondary batteries, and an application area thereof is gradually
2
expanding. Recently, as technology development and demand for portable devices
such as portable computers, mobile phones, cameras, and the like increase, demand
for secondary batteries as energy sources is sharply increasing. Also, to improve
convenience in use of a secondary battery, a reduction in charging time is required,
5 and accordingly, excellent rapid charging performance is required.
[7] A secondary battery generally consists of a positive electrode, a negative
electrode, an electrolyte, and a separator. The negative electrode comprises a
negative electrode active material that allows lithium ions released from the positive
electrode to be intercalated and deintercalated.
10 [8] As the negative electrode active material, a graphite-based active material, for
example, natural graphite, artificial graphite, or the like, is commonly used.
Conventionally used natural graphite is advantageous in that it is inexpensive and has
excellent cost-effectiveness, but has a problem such as the occurrence of an
irreversible reaction caused by the penetration or decomposition of an electrolyte
15 solution when applied to a battery due to its irregular structure. Also, conventionally
used artificial graphite is advantageous in that it exhibits excellent initial
charge/discharge efficiency, but has a problem in that it exhibits lower discharge
capacity than that of natural graphite, thereby degrading battery capacity and energy
density.
20 [9] In order to solve the above problems, a negative electrode active material
made by mixing typical natural graphite and artificial graphite, a negative electrode
active material in which an amorphous carbon coating layer is formed on typical
natural graphite or artificial graphite, and the like have been conventionally used.
However, in this case, rapid charging performance and lifetime characteristics at room
25 temperature are degraded.
3
[10] Therefore, there is a demand for a negative electrode including a negative
electrode material capable of realizing a secondary battery which has high energy
density and is excellent in rapid charging performance and lifetime characteristics.
[11] [Related-Art Documents]
5 [12] [Patent Documents]
[13] Japanese Laid-Open Patent Publication No. 2019-179687
DETAILED DESCRIPTION OF THE INVENTION
TECHNICAL PROBLEM
[14] The present invention is directed to providing a negative electrode having high
10 energy density, excellent rapid charging performance, and excellent lifetime
characteristics and a secondary battery including the negative electrode.
TECHNICAL SOLUTION
[15] One aspect of the present invention provides a negative electrode which
comprises a current collector and a negative electrode active material layer formed on
15 the current collector, wherein the negative electrode active material layer comprises a
negative electrode active material, the negative electrode active material comprises
first uncoated artificial graphite particles and second uncoated artificial graphite
particles in a weight ratio of 4:6 to 6:4, the first uncoated artificial graphite particles
are single particles having an average particle diameter (D50) of 5 μm to 7 μm, and the
20 second uncoated artificial graphite particles are secondary particles formed by
agglomerating a plurality of primary particles and having an average particle diameter
(D50) of 20 μm to 25 μm.
[16] Another aspect of the present invention provides a secondary battery including
the negative electrode.
25 ADVANTAGEOUS EFFECTS
4
[17] According to the present invention, since a negative electrode comprises a
negative electrode active material including first uncoated artificial graphite particles
which are single particles having an average particle diameter (D50) of 5 μm to 7 μm
and second uncoated artificial graphite particles which are secondary particles formed
5 by agglomerating a plurality of primary particles and having an average particle
diameter (D50) of 20 μm to 25 μm in a specific weight ratio, the single particles can fill
up the empty space of the secondary particles to reduce voids, the single particle, which
is soft, can absorb shock between the secondary particles to allow the electrode to be
relatively less physically damaged and structurally changed when roll-pressed, and the
10 electrode can be smoothly roll-pressed to achieve a thin electrode and eventually a thin
battery, thereby improving the energy density of a battery. Also, the rapid charging
performance and lifetime characteristics of a battery can be improved.
MODES OF THE INVENTION
[18] Hereinafter, the present invention will be described in more detail to facilitate
15 understanding of the present invention.
[19] Terms and words used in the specification and the claims should not be
interpreted as limited to commonly used meanings or meanings in dictionaries and
should be interpreted with meanings and concepts which are consistent with the
technological scope of the present invention based on the principle that the inventors
20 can appropriately define concepts of terms in order to describe the invention in the best
way.
[20] The terminology used in the specification is merely used for the purpose of
describing exemplary embodiments, and is not intended to be limiting of the present
invention. The singular forms “a,” “an,” and “the” are intended to include the plural
25 forms as well, unless the context clearly indicates otherwise.
5
[21] It should be understood that the terms “comprises,” “comprising,” “comprises,”
“including,” “has,” and/or “having,” when used herein, specify the presence of stated
features, integers, steps, operations, elements, components and/or combinations
thereof, but do not preclude the presence or addition of one or more other features,
5 integers, steps, operations, elements, components and/or combinations thereof.
[22] In the specification, D50 may be defined as a particle diameter corresponding
to 50% of the cumulative volume in a particle size distribution curve (curve on the
graph of a particle size distribution). The D50 may be measured, for example, using
a laser diffraction method. The laser diffraction method generally allows the
10 measurement of a particle diameter ranging from a submicron level to several
millimeters and may produce a result having high reproducibility and high resolution.
[23]
[24] A negative electrode according to an embodiment of the present invention
15 comprises a current collector and a negative electrode active material layer formed on
the current collector, wherein the negative electrode active material layer comprises a
negative electrode active material, the negative electrode active material comprises
first uncoated artificial graphite particles and second uncoated artificial graphite
particles in a weight ratio of 4:6 to 6:4, the first uncoated artificial graphite particles
20 are single particles having an average particle diameter (D50) of 5 μm to 7 μm, and the
second uncoated artificial graphite particles are secondary particles formed by
agglomerating a plurality of primary particles and having an average particle diameter
(D50) of 20 μm to 25 μm.
6
[25] In the specification, a “primary particle” means a single particle, and a
“secondary particle” means an agglomerate formed by agglomerating a plurality of the
primary particles through a deliberate assembly or bonding process.
5 [26] The negative electrode comprises a current collector and a negative electrode
active material layer formed on the current collector.
[27] The current collector serves to support the negative electrode active material
layer.
[28] The current collector is not particularly limited as long as it does not cause a
10 chemical change in the battery and has conductivity. For example, copper, stainless
steel, aluminum, nickel, titanium, calcined carbon, aluminum or stainless steel whose
surface has been treated with carbon, nickel, titanium, silver, or the like, or the like
may be used as the current collector. Specifically, a transition metal that easily
adsorbs carbon, such as copper or nickel, may be used as the current collector. The
15 current collector may have a thickness of 6 μm to 20 μm, but the thickness of the
current collector is not limited thereto.
The negative electrode active material layer is formed on the current collector.
The negative electrode active material layer may be disposed on at least one surface
of the current collector, specifically, on one surface or both surfaces of the current
20 collector.
[29] The negative electrode active material layer comprises a negative electrode
active material. The negative electrode active material layer may further include a
conductive material, a binder, and the like.
7
[30] The negative electrode active material may include first uncoated artificial
graphite particles and second uncoated artificial graphite particles. Specifically, the
negative electrode active material may consist of first uncoated artificial graphite
particles and second uncoated artificial graphite particles. That is, the negative
5 electrode active material may consist only of the first uncoated artificial graphite
particles and the second uncoated artificial graphite particles.
[31] In the specification, an uncoated artificial graphite particle means an artificial
graphite particle that has been not coated and means that the entire surface of the
artificial graphite is exposed.
10 [32] Since the negative electrode active material is not cracked, that is, less
physically damaged, when roll-pressed by including only an uncoated active material,
the structure of a negative electrode can be maintained well.
[33] Since the negative electrode active material comprises single particles and
secondary particles, which have specific sizes, in a specific weight ratio, roll-pressing
15 may smoothly proceed in the production of a negative electrode to not only reduce the
thickness of a negative electrode and eventually the thickness of a battery but also
achieve a battery having high energy density, and the rapid charging performance and
lifetime characteristics of a battery may also be improved.
[34] The first uncoated artificial graphite particles may be single particles having
20 an average particle diameter (D50) of 5 μm to 7 μm. When the average particle
diameter (D50) of the single particles is less than 5 μm, an increase in specific surface
area of the active material and a decrease in capacity may be caused due to the
excessively small particle diameter, and when the average particle diameter (D50) of
the single particles exceeds 7 μm, the particle diameter is too large to densely fill up
8
the empty space of the secondary particles, and thus rolling performance may be
degraded.
[35] The second uncoated artificial graphite particles may be secondary particles
having an average particle diameter (D50) of 20 μm to 25 μm. When the average
5 particle diameter (D50) of the secondary particles is less than 20 μm, the specific
surface area of the active material increases, and thus high-temperature performance
may be degraded, and when the average particle diameter (D50) of the secondary
particles exceeds 25 μm, battery performance such as rapid charging and output may
be degraded. The secondary particles may specifically have an average particle
10 diameter (D50) of 21 μm to 24 μm, and more specifically, 22 μm to 23 μm. When the
average particle diameter of the secondary particles falls within the above-described
range, the secondary particles can be blended with the single particles in an appropriate
range to maximize the rolling performance of the electrode.
[36] The second uncoated artificial graphite particles are secondary particles
15 formed by agglomerating a plurality of primary particles, and the primary particles
constituting the secondary particle may have an average particle diameter (D50) of 7
μm to 9 μm. When the average particle diameter (D50) of the primary particles
constituting the second uncoated artificial graphite particle falls within the abovedescribed range, sufficiently high capacity and a high level of battery performance
20 such as rapid charging and output can be maintained due to the appropriate primary
particle size.
[37] In the second uncoated artificial graphite particles, a ratio of the average
particle diameter (D50) of the primary particles constituting the secondary particle and
the average particle diameter (D50) of the secondary particles formed by agglomerating
9
a plurality of primary particles may be 1:2 to 1.5, and specifically, 1:2 to 1:3 in view
of improving energy density and charging performance.
[38] A ratio of the average particle diameter (D50) of the first uncoated artificial
graphite particles and the average particle diameter (D50) of the second uncoated
5 artificial graphite particles may be 1:3 to 1:5, specifically 1:3 to 1:4.5, and more
specifically 1:3 to 1:4. When the ratio of the average particle diameter (D50) of the
first uncoated artificial graphite particles and the average particle diameter (D50) of the
second uncoated artificial graphite particles falls within the above-described range, the
first uncoated artificial graphite particles can fill up the voids of the second uncoated
10 artificial graphite particles to prepare a high-density negative electrode active material,
and thus the rapid charging performance and/or lifetime characteristics of a battery can
be improved.
[39] A nip pressure when the negative electrode active material is roll-pressed in a
15 tandem manner may be 8 Ton/cm or less, specifically 4 Ton/cm to 8 Ton/cm, and
specifically 4 Ton/cm to 6 Ton/cm. In the present invention, a nip pressure in rollpressing in a tandem manner is measured by a physical roll-pressing method after
coating a negative electrode that has not been roll-pressed and drying the same under
vacuum. A case where the nip pressure when the negative electrode active material
20 is roll-pressed in a tandem manner falls within the above-described range can be
advantageous for receiving less physical force during roll-pressing for production of
an electrode and achieving a thin electrode. The nip pressure in roll-pressing in a
tandem manner may be measured in a roll press machine equipped with a nip pressure
sensor.
10
[40] The negative electrode active material may have a tap density ranging from
1.00 g/cc to 1.20 g/cc, and specifically, 1.14 g/cc to 1.18 g/cc. When the tap density
of the negative electrode active material falls within the above-described range,
electrode coating with a low thickness can be made due to the sufficiently high tap
5 density.
[41] The tap density is a mass per volume of powder consisting of particles and
refers to a density in which voids between particles are filled by constant tapping or
vibration. Factors affecting the tap density include particle size distribution, moisture
content, particle shape, cohesiveness, and the like. The fluidity and compressibility
10 of a material may be predicted through the tap density. The tap density may be
measured based on ASTM D4781 and may be calculated using the equation TD=W/V
(TD: tap density, W: sample weight (g), V: sample volume after tapping).
[42] The negative electrode active material may have a BET specific surface area
ranging from 1.0 m2
/g to 2.5 m2
/g. The negative electrode active material may
specifically have a BET specific surface area of 1.4 m2
/g to 2.0 m2 15 /g, and more
specifically, 1.5 m2
/g to 1.9 m2
/g. The BET specific surface area may be measured
using a BEL Sorption instrument (BEL Japan Inc.). When the BET specific surface
area of the negative electrode active material falls within the above-described range,
high-temperature performance can be maintained at a high level.
20 [43] The negative electrode active material layer may have a pore volume ranging
from 10 cm3
/g to 20 cm3
/g. The negative electrode active material layer may
specifically have a pore volume of 10 cm3
/g to 15 cm3
/g, and more specifically, 15
cm3
/g to 20 cm3
/g. When the pore volume of the negative electrode active material
layer falls within the above-described range, high-temperature battery performance
11
such as high-temperature cycle characteristics, high-temperature storage, and the like
can be maintained at high levels.
[44] The negative electrode active material may be included in an amount of 90
wt% to 99 wt%, and specifically, 92 wt% to 97 wt% in the negative electrode active
5 material layer. When the content of the negative electrode active material falls within
the above-described range, desired battery capacity can be achieved.
[45] The first uncoated artificial graphite particles may be prepared by mixing coke
and a binder pitch and thermally treating (i.e., graphitizing) the mixture at a high
10 temperature. The coke may be needle coke and/or isotropic coke, and the coke and
the binder pitch may be mixed in a weight ratio of 80:20 to 95:5.
[46] The second uncoated artificial graphite particles may be prepared by mixing
coke and a binder pitch, thermally treating the mixture at a high temperature to prepare
primary particles, mixing the primary particles with a binder pitch, and thermally
15 treating the mixture at a high temperature, specifically, at 2,500 °C to 3,200 °C.
Since the binder pitch is graphitized by the thermal treatment at a high temperature, it
is possible to prepare second uncoated artificial graphite particles in which a coating
material such as amorphous carbon and the like is not present. The coke may be
needle coke and/or isotropic coke, the coke and the binder pitch may be mixed in a
20 weight ratio of 80:20 to 95:5, and the primary particles and the binder pitch may be
mixed in a weight ratio of 80:20 to 95:5.
[47] The conductive material is not particularly limited as long as it does not cause
a chemical change in the battery and has conductivity. For example, graphite such
25 as natural graphite, artificial graphite, or the like; carbon black such as acetylene black,
12
Ketjen black, channel black, furnace black, lamp black, thermal black, or the like; a
conductive fiber such as carbon fiber, metal fiber, or the like; a conductive tube such
as carbon nanotubes or the like; fluorocarbon; metal powder such as aluminum powder,
nickel powder, or the like; a conductive whisker such as zinc oxide, potassium titanate,
5 or the like; a conductive metal oxide such as titanium oxide or the like; or a conductive
material such as a polyphenylene derivative or the like may be used as the conductive
material.
[48] The conductive material preferably comprises at least one selected from
among carbon black and carbon nanotubes and more preferably comprises carbon
10 nanotubes.
[49] The conductive material may be included in an amount of 0.1 wt% to 1.0 wt%,
and specifically, 0.3 wt% to 0.7 wt% in the negative electrode active material layer.
A case where the content of the conductive material satisfies the above-described range
can be advantageous for maintaining a sufficient electric contact and preventing
15 degradation of capacity.
[50] The binder may include at least one selected from the group consisting of a
polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP),
polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol,
20 carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose,
polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene,
polyacrylic acid, an ethylene-propylene-diene monomer (EPDM), a sulfonated EPDM,
styrene butadiene rubber (SBR), fluoro-rubber, polyacrylic acid, and materials in
which hydrogens thereof have been substituted with Li, Na, Ca, or the like, and also
25 include various copolymers thereof.
13
[51] The binder may be included in an amount of 30 wt% or less, and specifically,
0.1 wt% to 30 wt% in the negative electrode active material layer. When the content
of the binder satisfies the above-described range, an adhesive effect resulting from the
use of the binder can be exhibited, and desired capacity per volume of a negative
5 electrode can be maintained.
[52] The negative electrode may have a pore resistance of 9 ohm or less by
including the negative electrode active material including the first uncoated artificial
graphite particles which are single particles having an average particle diameter (D50)
10 of 5 μm to 7 μm and the second uncoated artificial graphite particles which are
secondary particles formed by agglomerating a plurality of primary particles and
having an average particle diameter (D50) of 20 μm to 25 μm in a specific weight ratio.
The negative electrode preferably has a pore resistance of 6 ohm to 9 ohm. When the
pore resistance of the negative electrode falls within the above-described range, a pore
15 resistance in an electrolyte solution can be minimized, and a lithium ion diffusion route
can be minimized.
[53] The pore resistance may be defined as a resistance value obtained by injecting
a lithium ion-containing electrolyte solution into a symmetric cell produced using the
negative electrode for a lithium secondary battery as both a working electrode and a
20 counter electrode and then performing electrochemical impedance spectroscopy (EIS)
analysis. Since the pore resistance is measured by the EIS analysis of a symmetric
cell, only lithium ions derived from the electrolyte solution may be present, and
accordingly, resistance to lithium ion diffusion in a negative electrode may be
objectively measured.
25
14
[54]
[55] A secondary battery according to another embodiment of the present invention
comprises a negative electrode, and the negative electrode is the same as the abovedescribed negative electrode.
5 [56] Specifically, the secondary battery may include: the above-described negative
electrode; a positive electrode; a separator interposed between the negative electrode
and the positive electrode; and an electrolyte, and the negative electrode is the same as
the above-described negative electrode. Since the negative electrode has been
described above, detailed descriptions thereof will be omitted.
10
[57] The positive electrode comprises a positive electrode current collector and a
positive electrode active material layer formed on the current collector, wherein the
positive electrode active material layer comprises a positive electrode active material,
and the positive electrode active material may be a layered compound such as a lithium
15 cobalt oxide (LiCoO2), a lithium nickel oxide (LiNiO2), or the like or a compound
substituted with one or more transition metals; a lithium iron oxide such as LiFe3O4; a
lithium manganese oxide such as Li1+c1Mn2-c1O4 (0≤c1≤0.33), LiMnO3, LiMn2O3,
LiMnO2, or the like; a lithium copper oxide (Li2CuO2); a vanadium oxide such as
LiV3O8, V2O5, Cu2V2O7, or the like; a Ni-site-type lithium nickel oxide represented by
20 the chemical formula LiNi1-c2Mc2O2 (here, M is at least one selected from the group
consisting of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, and 0.01≤c2≤0.3 is satisfied); a
lithium manganese composite oxide represented by the chemical formula LiMn2-
c3Mc3O2 (here, M is at least one selected from the group consisting of Co, Ni, Fe, Cr,
Zn, and Ta, and 0.01≤c3≤0.1 is satisfied) or Li2Mn3MO8 (here, M is at least one
25 selected from the group consisting of Fe, Co, Ni, Cu, and Zn); or LiMn2O4 in which
15
some Li ions in the chemical formula have been substituted with alkaline earth metal
ions.
[58] The positive electrode active material is preferably one or more selected from
among a lithium cobalt oxide, a lithium nickel oxide, and LiaNix1Coy1Mnz1O2
5 (0.9≤a≤1.1, 0.6≤x1<1.0, 0