TECHNICAL FIELD
[0001] The present invention relates to an anode active
material and a lithium secondary battery, and more
particularly, to an anode active material including natural
􀁘􀁗􀁇 graphite and mosaic coke-based artificial graphite and a
lithium secondary battery including the anode active material.􀁇
[0002] 􀁇
BACKGROUND ART
[0003] Recent developments in information and communication
􀁘􀁜􀁇 industry enable compact, lightweight, thin, and portable
electronic devices, and thus high energy densification of
batteries used as power supplies of such electronic devices
are increasingly demanded. A lithium secondary battery may
be a most suitable one to satisfy such demands, and thus
􀁙􀁗􀁇 studies on the lithium secondary battery are being actively
carried out.
[0004] Carbonaceous materials are generally used as an anode
material of a lithium secondary battery, and the carbonaceous
materials include crystalline carbon and amorphous carbon.
􀁙􀁜􀁇 Representative examples of crystalline carbon may include
3
􀁇
graphite carbon, such as natural graphite or artificial
graphite; and examples of amorphous carbon may include nongraphitizable
carbon (hard carbon) obtained by carbonization
of a polymer resin and graphitizable carbon (soft carbon)
obtained 􀁇 d by heat treatment of pitch.
[0005] In general, soft carbon is made by applying 1,000
levels of heat to coke which is a by-product produced during
crude oil refining, and exhibits high output and short
charging time unlike a conventional graphite anode active
􀁘􀁗􀁇 material or hard carbon-based anode active material.
[0006] On the other hand, hard carbon may be produced by
carbonization of a material such as resin, thermosetting
polymer, or wood. When such hard carbon is used as an anode
material of a lithium secondary battery, it has a high
􀁘􀁜􀁇 reversible capacity of 400 mAh/g or more due to micropores
but has low initial efficiency of approximately 70%. Thus,
it is disadvantageous in that when hard carbon is used for an
electrode of a lithium secondary battery, irreversible
consumption of lithium is significant.
􀁙􀁗􀁇 [0007] Such irreversibility is observed because solid
electrolyte interphase (SEI) as a surface film is created by
dissociation of electrolyte on the surface of an electrode
during charging, or because lithium stored in carbon
particles during charging is prevented from being discharged
􀁙􀁜􀁇 during discharging. Of these two cases, the former case is
4
􀁇
more problematic and the creation of a surface film is known
as a major cause of irreversibility.
[0008] Moreover, it is known that most of high-capacity
graphite materials have a highly developed layer structure,
and thus have a high degree 􀁇 ee of graphitization and a flake
shape. In the case of such flake-shaped graphite, regions
where Li ions are intruded between the layers thereof, that
is edge surfaces, are small. Thus, when the flake-shaped
graphite is used as an anode active material of a lithium
􀁘􀁗􀁇 secondary battery, a high rate discharge characteristic,
which is a characteristic in the case of discharge with high
current, is deteriorated.
[0009] Furthermore, spherical natural graphite is
disadvantageous in that it has a limited ionic conductivity,
􀁘􀁜􀁇 and empty spaces are created between active materials to
increase the resistance of an electrode when only the
spherical natural graphite is used as an anode active
material, thereby bringing about a decrease in rate
performance.
􀁙􀁗􀁇 [0010] Therefore, it is necessary to develop an anode active
material capable of replacing typical anode active materials,
and reducing interfacial resistance and improving rate
performance when applied to a lithium secondary battery.
[0011]
􀁙􀁜􀁇 DISCLOSURE OF THE INVENTION
5
􀁇
TECHNICAL PROBLEM
[0012] It is an object of the present invention to provide
an anode active material capable of reducing interfacial
resistance and having improved rate performance as well as
improving 􀁇 ng conductivity.
[0013] It is another object of the present invention to
provide an anode having a specific orientation ratio and
electrode density by including the anode active material, and
a resultant lithium secondary battery having improved
􀁘􀁗􀁇 performance.
[0014] The object of the present invention is not limited to
the above-described objects, and other objects which are not
described herein will be clearly understood to those skilled
in the art from the description below.
􀁘􀁜􀁇 [0015]
TECHNICAL SOLUTION
[0016] According to an embodiment of the present invention,
an anode active material including natural graphite and
mosaic coke-based artificial graphite is provided.
􀁙􀁗􀁇 [0017] Also, according to another embodiment of the present
invention, an anode including the above-described anode
active material is provided.
[0018] Furthermore, the present invention provides a lithium
secondary battery using the above-described anode, the
􀁙􀁜􀁇 battery including a cathode, an anode and a separator
6
􀁇
disposed between the cathode and the anode.
[0019]
ADVANTAGEOUS EFFECTS
[0020] According to an embodiment of the present invention,
since an anode active material including natural graphite 􀁇 e and
mosaic coke-based artificial graphite is used,
intercalation/deintercalation of lithium ions is more
facilitated and conductivity of an electrode is improved even
if no or little conductive material is used. Furthermore,
􀁘􀁗􀁇 the increase in conductivity can lead to not only a further
improvement in rate performance of a lithium secondary
battery but also a reduction in interfacial resistance.
[0021]
BRIEF DESCRIPTION OF THE DRAWINGS
􀁘􀁜􀁇 [0022] The accompanying drawings herein illustrate exemplary
embodiments of the present invention and, together with the
description, serve to provide a further understanding of the
inventive concept, and thus the present invention should not
be construed as being limited to only the drawings.
􀁙􀁗􀁇 [0023] FIG. 1 illustrates a schematic diagram of an anode
active material according to an embodiment of the present
invention.
[0024] FIG. 2 illustrates a structure of a graphite particle.
[0025] FIG. 3 is a graph showing an XRD measurement result
􀁙􀁜􀁇 of mosaic coke-based artificial graphite used according to an
7
􀁇
embodiment of the present invention.
[0026] FIG. 4 is a graph showing measurement results for
rate performance of lithium secondary batteries prepared in
Example 3 according to an embodiment of the present invention
and 􀁇 Comparative Example 4.
[0027] FIG. 5 is a graph showing measurement results for
resistance of an electrode, as measured for anodes in lithium
secondary batteries prepared in Example 3 according to an
embodiment of the present invention and Comparative Example 4.
􀁘􀁗􀁇 [0028]
MODE FOR CARRYING OUT THE INVENTION
[0029] Hereinafter, the present invention will be described
in more detail to facilitate understanding the present
invention.
􀁘􀁜􀁇 [0030] Terms or words used in the description and claims
should not be restrictively interpreted as ordinary or
dictionary meanings, but should be interpreted as meanings
and concepts conforming to the inventive concept on the basis
of a principle that an inventor can properly define the
􀁙􀁗􀁇 concept of a term to explain his or her own invention in the
best ways.
[0031] An anode active material according to an embodiment
of the present invention may include natural graphite and
mosaic coke-based artificial graphite.
􀁙􀁜􀁇 [0032] More specifically, the anode active material
8
􀁇
according to an embodiment of the present invention, as shown
in FIG. 1, includes natural graphite and mosaic coke-based
artificial graphite as being mixed together. Thus, when
compared with the case where only natural graphite is used,
mosaic coke-based artificial graphite 􀁇 fills empty spaces
between active materials so that conductivity may increase
and therefore rate performance of a secondary battery may be
improved and interfacial resistance may be reduced.
[0033] Also, the mosaic coke-based artificial graphite has
􀁘􀁗􀁇 its own unique random crystal structure, and thus further
facilitates intercalation and deintercalation of lithium ions
to thereby further improve performance of a secondary battery.
In addition, the mosaic coke-based artificial graphite is
included in an anode active material together with natural
􀁘􀁜􀁇 graphite, and can thus serve as a conductive material.
Therefore, it is possible to obtain the conductivity which is
equal to or higher than that of an anode active material
using a typical conductive material even if no or little
conductive material is used.
􀁙􀁗􀁇 [0034] Furthermore, since the mosaic coke-based artificial
graphite may express characteristics due to the
aforementioned random crystal structure that a mosaic texture
has, generally available needle coke-based artificial
graphite having a plate or needle shape may be difficult to
􀁙􀁜􀁇 facilitate intercalation and deintercalation of lithium ions,
9
􀁇
and thus difficult to obtain advantages such as an
improvement of rate performance or a reduction in interfacial
resistance.
[0035] Specifically, mosaic coke-based artificial graphite
included in an anode active material according to 􀁇 an
embodiment of the present invention, which is made of, for
example, coal and coke as a raw material, may have an
anisotropic structure which is identified as a mosaic texture
when a polished surface of a carbonized material is observed
􀁘􀁗􀁇 using a polarization microscope. Also, the anisotropic
structure of the mosaic texture has a random crystal
structure, and thus may further facilitate intercalation and
deintercalation of lithium ions when applied to a lithium
secondary battery.
􀁘􀁜􀁇 [0036] Mosaic coke-based artificial graphite being usable
according to an embodiment of the present invention may have
an average major axis length of, for example 5 to 30 􀻃, and
preferably 10 to 25 􀻃.
[0037] When the major axis length of the mosaic coke-based
􀁙􀁗􀁇 artificial graphite is less than 5 􀻃, initial efficiency of
a battery is reduced due to an increase in a specific surface
area, and thus battery performance may be degraded. When the
major axis length is greater than 30 􀻃, it may cause a short
circuit due to penetration of the mosaic coke-based
􀁙􀁜􀁇 artificial graphite into a separator, and may cause low
10
􀁇
capacity retention due to low packing density.
[0038] Also, it is preferable that the mosaic coke-based
artificial graphite according to an embodiment of the present
invention has a specific surface area of 3.0 to 4.0 /g, and
compressed density of 1.5 to 2.1 g/cc under the pressure of 􀁜􀁇 8
to 25 mPa.
[0039] When the compressed density is less than 1.5 g/cc,
energy density per unit volume may be reduced. When the
compressed density is greater than 2.1 g/cc, it may cause a
􀁘􀁗􀁇 reduction in initial efficiency and deterioration in hightemperature
properties, and may also cause a reduction in
adhesive strength of an electrode.
[0040] Also, the mosaic coke-based artificial graphite
preferably has such a crystal habit that Lc (002) is 21.6 to
􀁘􀁜􀁇 21.9 nm, and d002 is 0.3377 nm or less, preferably 0.3357 to
0.3377 nm, and most preferably 0.3376 nm, wherein Lc (002) is
a crystallite size in C-axis direction and d002 is interplanar
spacing of (002) plane during XRD measurement.
[0041] The d002 of mosaic coke-based artificial graphite can
􀁙􀁗􀁇 be calculated by Equation 1 below using Bragg’s law after
determining peak positions by integration from a graph of two
values measured using XRD.
[0042]
[0043] d002 = 􀁏/2sin􀁔
􀁙􀁜􀁇 [0044] Also, Lc (002), which is a crystallite size in C-axis
11
􀁇
direction of a particle, can be calculated by Equation 2
below as Scherrer equation which calculates a crystallite
size Lc of mosaic coke-based artificial graphite.
[0045]
􀁜􀁇 [0046]
[0047] where, K is a Scherrer constant (K=0.9),
[0048] 􀁅 is a half value width,
[0049] 􀁏 is a wavelength (0.154056 nm), and
[0050] 􀁔 is an angle at a maximum peak.
􀁘􀁗􀁇 [0051] According to an embodiment of the present invention,
the mosaic coke-based artificial graphite may have Lc (002)
of 21.6 to 21.9 nm, wherein Lc (002) is a crystallite size in
C-axis direction during XRD measurement using CuK. When the
Lc of the mosaic coke-based artificial graphite falls within
􀁘􀁜􀁇 the above range, diffusion velocity of lithium ions becomes
higher due to a high electrical conductivity, and thus
intercalation and deintercalation of lithium ions may be
performed more easily. On the other hand, when the Lc is
greater than 21.9 nm, an increase in moving distance of
􀁙􀁗􀁇 lithium ions may act as resistance to cause deterioration in
output characteristics; and when the Lc is less than 21.6 nm,
it may be difficult to express the unique capacity of
graphite.
12
􀁇
[0052] It is preferable that an anode active material
according to an embodiment of the present invention includes
natural graphite together with the mosaic coke-based
artificial graphite.
[0053] In the case of artificial graphite, 􀁇 phite, charge/discharge
efficiency is high but cost is high, and also dispersibility
in aqueous slurry is very low, so that the artificial
graphite has difficulties in an aspect of processibility and
it is difficult to obtain desired physical properties of a
􀁘􀁗􀁇 battery due to a low capacity.
[0054] On the contrary, since natural graphite is
inexpensive and also shows a high voltage flatness and a high
capacity close to a theoretical capacity, natural graphite is
very useful as an active material.
􀁘􀁜􀁇 [0055] According to an embodiment of the present invention,
either plate-shaped or spherical natural graphite may be used
as the natural graphite, but spherical natural graphite may
be preferable.
[0056] In an anode active material according to an
􀁙􀁗􀁇 embodiment of the present invention, it is preferable that a
weight ratio of the natural graphite to the mosaic coke-based
artificial graphite is 1:0.1 to 1:1, and preferably 1:0.3 to
1:1.
[0057] When the weight of the mosaic coke-based artificial
􀁙􀁜􀁇 graphite is greater than the above range, the mosaic coke13
􀁇
based artificial graphite covers natural graphite in an
excess amount to increase a specific surface area, and thus
dissociation of an electrolytic solution may be dominant; on
the other hand, when the weight is less than the above range,
the mosaic coke-based artificial graphite may not 􀁇 t completely
fill empty spaces between natural graphite particles, and
thus conductivity may be reduced.
[0058] According to an embodiment of the present invention,
the natural graphite may have an average particle size D50 of
􀁘􀁗􀁇 5 to 30, and preferably 20 to 25. When the average particle
size D50 of the spherical natural graphite is less than 5,
initial efficiency of a secondary battery is reduced due to
an increase in a specific surface area, and thus battery
performance may be reduced. On the other hand, when the
􀁘􀁜􀁇 average particle size D50 is greater than 30, it may cause a
short circuit by penetration of the natural graphite into a
separator, and may cause low capacity retention due to low
packing density.
[0059] The average particle size of the natural graphite
􀁙􀁗􀁇 according to an embodiment of the present invention may be
measured, for example, using a laser diffraction method. The
laser diffraction method is generally capable of measuring
particle sizes in a range of submicron to a few millimeters,
and obtaining results with high reproducibility and high
􀁙􀁜􀁇 resolution. The average particle size D50 of the natural
14
􀁇
graphite may be defined as a particle size based on 50% of
particle size distribution.
[0060] The average particle size D50 of the natural graphite
according to an embodiment of the present invention may be
measured, for example, in such a way that natural graphite i􀁜􀁇 s
dispersed in a solution of ethanol/water, the resultant
solution is introduced into a commercially available particle
size measuring apparatus using laser diffraction (for example,
Microtrac MT 3000) and irradiated with ultrasonic wave having
􀁘􀁗􀁇 a frequency of about 28 kHz at power of 60W, and then the
average particle size D50 is calculated based on 50% of
particle size distribution in the measuring apparatus.
[0061] According to an embodiment of the present invention,
spherical natural graphite satisfying the average particle
􀁘􀁜􀁇 size range of the natural graphite, may be obtained in such a
way, but not limited to, that natural graphite particles are
introduced into a spheroidizing apparatus (Nara Hybridization
System, NHS-2), and then spheroidized, for example, for 10 to
30 minutes at a rotor speed of about 30 to 100 m/sec.
􀁙􀁗􀁇 [0062] Furthermore, according to an embodiment of the
present invention, it is preferable that the natural graphite
has specific surface area (BET-SSA) of 2 to 8 m2/g. When the
specific surface area of the natural graphite is less than 2
m2/g, adhesive strength between electrodes may be reduced.
􀁙􀁜􀁇 The specific surface area of the natural graphite greater
15
􀁇
than 8 m2/g causes an increase in initial irreversible
capacity during charge/discharge, and is thus undesirable.
[0063] According to an embodiment of the present invention,
the specific surface area may be measured using a Brunauer-
Emmett-Teller (BET) method. For example, it may 􀁇 ay be measured
with BET six-point method by means of a nitrogen gas
adsorption method using a porosimetry analyzer (Bell Japan
Inc., Belsorp-II mini).
[0064] Meanwhile, a method of preparing the anode active
􀁘􀁗􀁇 material according to an embodiment of the present invention
may include mixing natural graphite and mosaic coke-based
artificial graphite.
[0065] In the method of preparing the anode active material
according to an embodiment of the present invention, the
􀁘􀁜􀁇 mixing for preparing an anode active material may be
performed by simple mixing or mechanical milling using an
ordinary method known in the art. For example, the mixing
may be performed simply using a mortar, or a carbon composite
may be formed by compressive stress which is mechanically
􀁙􀁗􀁇 applied by rotating the mixture with revolutions of 100 to
1,000 rpm using a blade or a ball mill.
[0066] According to an embodiment of the present invention,
an anode may be provided using the anode active material,
wherein the anode includes a current collector and the anode
􀁙􀁜􀁇 active material formed on at least one surface of the current
16
􀁇
collector.
[0067] An anode according to an embodiment of the present
invention includes both natural graphite and mosaic cokebased
artificial graphite in an anode active material, and
thus an orientation ratio I110/I004 may 􀁇 ay be 0.08 to 0.086, and
preferably 0.0819 to 0.0836, under the compressed density of
1.40 to 1.85 g/cc.
[0068] According to an embodiment of the present invention,
an orientation ratio of an anode is adjusted by using the
􀁘􀁗􀁇 mosaic coke-based artificial graphite, and thus performance
of a lithium secondary battery may be further improved.
[0069] According to an embodiment of the present invention,
the orientation ratio of an anode may be dependent on a
compressive force which is applied when the anode active
􀁘􀁜􀁇 material is coated and rolled onto an anode current collector.
[0070] In an anode according to an embodiment of the present
invention, the orientation ratio may be measured, for example,
by X-ray diffraction (XRD).
[0071] An orientation ratio of an anode according to an
􀁙􀁗􀁇 embodiment of the present invention is an area ratio
(110)/(004) which is obtained by measuring (110) and (004)
planes of an anode, more specifically, of an anode active
material included in the anode using XRD, and then
integrating peak intensities of (110) and (004) planes. More
􀁙􀁜􀁇 specifically, XRD measurement conditions are as follows.
17
􀁇
[0072] – Target: Cu(K􀈻 line) graphite monochromator
[0073] – Slit: divergence slit= 1o, receiving slits= 0.1,
scattering slit= 1o
[0074] – Measuring range and step angle/measuring time:
[0075] (110) plane: 76.5o < 2􀁔 < 78.5o, 0.01o 􀁜􀁇 /3sec
[0076] (004) plane: 53.5o < 2􀁔 < 56.0o, 0.01o /3sec, where,
2􀁔 is a diffraction angle. The above XRD measurement is an
example, thus other measuring methods are also used, and the
orientation ratio of the anode may be measured through the
􀁘􀁗􀁇 method as described above.
[0077] An anode according to an embodiment of the present
invention may be prepared by an ordinary method known in the
art. For example, the anode active material is mixed with a
solvent, and a binder (if necessary), and the mixture is
􀁘􀁜􀁇 agitated to prepare slurry, then the slurry is applied
(coated) onto a current collector made of a metallic material,
and thereafter the current collector coated with the slurry
is compressed and dried to prepare an anode.
[0078] According to an embodiment of the present invention,
􀁙􀁗􀁇 the anode active material slurry may further include a
conductive material. The available conductive material may
be any one or a mixture of two or more selected from the
group consisting of natural graphite, artificial graphite,
carbon black, acetylene black, ketjenblack, channel black,
􀁙􀁜􀁇 furnace black, lamp black, thermo-black, carbon nanotubes,
18
􀁇
fullerene, carbon fiber, metal fiber, fluorocarbon, aluminum,
nickel powder, zinc oxide, potassium titanate, titanium oxide,
and polyphenylene derivatives, and preferably carbon black.
[0079] As similar to the anode, a cathode according to the
present invention may also be prepared 􀁇 ed by an ordinary method
in the art.
[0080] For example, a cathode active material is mixed with
a binder and a solvent, and a conductive material and a
dispersant (if necessary), and the mixture is agitated to
􀁘􀁗􀁇 prepare slurry, then the slurry is applied onto a current
collector, and thereafter the current collector coated with
the slurry is compressed to prepare an electrode.
[0081] A binder used in the present invention is used to
maintain a green body by binding particles in cathode and
􀁘􀁜􀁇 anode active materials, and a binder such as
polytetrafluoroethylene (PTFE), polyvinylidene fluoride
(PVDF), or styrene-butadiene rubber (SBR) is used. A binder
includes a solvent-based binder (that is, a binder using an
organic solvent as a solvent) represented by PVDF, and a
􀁙􀁗􀁇 water-based binder (that is a binder using water as a
solvent) which is any one or a mixture of two or more
selected from the group consisting of acrylonitrile-butadiene
rubber, SBR, and acrylic rubber. The water-based binder is
economical, environmentally friendly, and also harmless to
􀁙􀁜􀁇 health of workers contrary to the solvent-base binder, and
19
􀁇
has a high binding effect as compared with the solvent-based
binder, so that the proportion of an active material per unit
volume can be increased to achieve high capacity. SBR may be
preferably used as the water-based binder.
[0082] A lithium-containing transition metal 􀁇 etal oxide, which is
generally used in the art, may be preferably used as a
cathode active material. Also, the lithium-containing
transition metal oxide may be coated with a metal such as
aluminum (Al), or a metal oxide. Sulfides, selenides,
􀁘􀁗􀁇 halides, and the like may also be used in addition to the
lithium-containing transition metal oxide.
[0083] Once the electrodes were prepared, a lithium
secondary battery may be prepared which is generally used in
the art and includes an electrolytic solution and a separator
􀁘􀁜􀁇 disposed between the cathode and the anode, together with the
electrodes.
[0084] In the electrolytic solution used in the present
invention, any material which is generally used in the
electrolytic solution for lithium secondary batteries may be
􀁙􀁗􀁇 unrestrictedly used as a lithium salt which may be included
as an electrolyte, and an anion of the lithium salt may be,
for example, any one selected from the group consisting of F-,
Cl-, Br-, I-, NO3-, N(CN)2-, BF4-, ClO4-, PF6-, (CF3)2PF4-,
(CF3)3PF3-, (CF3)4PF2-, (CF3)5PF-, (CF3)6P-, CF3SO3-, CF3CF2SO3-,
􀁙􀁜􀁇 (CF3SO2)2N-, (FSO2)2N-, CF3CF2(CF3)2CO-, (CF3SO2)2CH-, (SF5)3C-,
20
􀁇
(CF3SO2)3C-, CF3(CF2)7SO3-, CF3SO2-, CH3CO2-, SCN- and
(CF3CF2SO2)2N-.
[0085] In the electrolytic solution used in the present
invention, any material which is generally used in the
electrolytic solution for lithium secondary batteries may b􀁜􀁇 e
unrestrictedly used as an organic solvent included in the
electrolytic solution.
[0086] Furthermore, a general porous polymer film which has
been conventionally used as a separator may be used as the
􀁘􀁗􀁇 separator, and for example, a porous polymer film which is
made of polyolefin polymer such as ethylene homopolymer,
propylene homopolymer, ethylene/butene copolymer,
ethylene/hexene copolymer, and ethylene/methacrylate
copolymer, may be used alone or in a laminated form thereof.
􀁘􀁜􀁇 Alternatively, general porous non-woven fabrics such as nonwoven
fabrics made of glass fiber having a high melting point,
and polyethyleneterephthalate fiber may be used as the
separator, but the separator is not limited thereto.
[0087] A lithium secondary battery of the present invention
􀁙􀁗􀁇 is not particularly limited in terms of a shape, but may have,
for example, a cylinder shape using a can, a square shape, a
pouch shape, or a coin shape.
[0088] Hereinafter, the present invention will be described
in detail with reference to Examples in order to concretely
􀁙􀁜􀁇 describe the present invention. The invention may, however,
21
􀁇
be embodied in many different forms and should not be
construed as being limited to the embodiments set forth
herein; rather, these embodiments are provided to more
completely describe the concept of the invention to those of
ordinary 􀁇 y skill in the art.
[0089]
[0090] Examples
[0091] Hereinafter, the present invention is described with
reference to Examples and Experimental Examples, but not
􀁘􀁗􀁇 limited thereto.
[0092]
[0093]
[0094] Example 1
[0095] Natural graphite particles having an average particle
􀁘􀁜􀁇 size of 100 􀻃 were introduced into a spheroidizing apparatus
(Nara Hybridization System, NHS-2), and then spheroidized for
10 minutes at a rotor speed of 65 m/sec to obtain spherical
natural graphite particles having an average particle size
D50 of 20 􀻃, FWHM of 7.0 􀻃, and BET specific surface area of
􀁙􀁗􀁇 2.60 m2/g.
[0096] Mosaic coke-based artificial graphite (Hitachi
chemical, MAGE3) having a major axis length of about 20 􀻃, a
specific surface area of 3 to 4 m2/g, and a compressive
density of 1.7 to 1.8 g/cc under the pressure of 12 to 16 mPa,
􀁙􀁜􀁇 was used.
22
􀁇
[0097] The spherical natural graphite and the mosaic cokebased
artificial graphite were mixed at a weight ratio of
1:0.3, and homogeneously agitated using a mortar to prepare
an anode active material.
􀁜􀁇 [0098]
[0099] Example 2
[00100] An anode active material was prepared by the same
method as Example 1 except that the spherical natural
graphite and the mosaic coke-based artificial graphite were
􀁘􀁗􀁇 mixed at a weight ratio of 1:1.
[00101]
[00102] Comparative Example 1
[00103] An anode active material was prepared by the same
method as Example 1 except that 100% of spherical natural
􀁘􀁜􀁇 graphite was used without mosaic coke-based artificial
graphite.
[00104]
[00105] Comparative Example 2
[00106] An anode active material was prepared by the same
􀁙􀁗􀁇 method as Example 1 except that the spherical natural
graphite and the mosaic coke-based artificial graphite were
mixed at a weight ratio of 1:0.05.
[00107]
[00108] Comparative Example 3
􀁙􀁜􀁇 [00109] An anode active material was prepared by the same
23
􀁇
method as Example 1 except that the spherical natural
graphite and the mosaic coke-based artificial graphite were
mixed at a weight ratio of 1:1.2.
[00110]
[00111]
[00112] Example 3
[00113] Preparation of Anode
[00114] The anode active material obtained in Example 1, SBR
as a binder, carboxy methyl cellulose (CMC) as a thickener,
􀁘􀁗􀁇 and acetylene black as a conductive material were mixed at a
weight ratio 95:2:2:1, and then the mixture was mixed with
water (H2O) which was a solvent, to prepare homogeneous anode
slurry. The prepared anode slurry was coated onto one
surface of a copper current collector to have a thickness of
􀁘􀁜􀁇 65 􀻃, and the coated current collector is dried and rolled
and then punched into a desired size to prepare an anode.
[00115]
[00116] Preparation of Lithium Secondary Battery
[00117] Ethylene carbonate (EC) and diethyl carbonate (DEC)
􀁙􀁗􀁇 were mixed at volume ratio of 30:70, and then LiPF6 was added
into the non-aqueous electrolyte solvent to prepare a nonaqueous
electrolytic solution of 1M LiPF6.
[00118] Also, a lithium metal foil was used as a counter
electrode, that is a cathode, and a polyolefin separator was
􀁙􀁜􀁇 disposed between both electrodes, and then the electrolytic
24
􀁇
solution was injected to prepare a coin-shaped half-cell.
[00119]
[00120] Example 4
[00121] An anode and a lithium secondary battery were
prepared by the same method as Example 􀁇 mple 3 except that the
anode active material prepared in Example 2 was used.
[00122]
[00123] Comparative Examples 4 to 6
[00124] Anodes and lithium secondary batteries were prepared
􀁘􀁗􀁇 by the same method as Example 3 except that the anode active
materials prepared in Comparative Examples 1 to 3 were used.
[00125]
[00126] Example 5
[00127] An anode and a lithium secondary battery were
􀁘􀁜􀁇 prepared by the same method as Example 3 except that the
anode active material prepared in Example 2 was used and any
conductive material was not added during preparation of the
anode.
[00128]
􀁙􀁗􀁇 [00129] Comparative Example 7
[00130] An anode and a lithium secondary battery were
prepared by the same method as Example 5 except that needle
coke-based artificial graphite instead of mosaic coke-based
artificial graphite was mixed with natural graphite at a
􀁙􀁜􀁇 weight ratio of 1:1, then the mixture was used as an anode
25
􀁇
active material, and any conductive material was not added
during preparation of the anode.
[00131]
[00132] Experimental Example 1: Measurement of Orientation
􀁜􀁇 Ratio
[00133] XRD diffraction measurements using Cu(K line)were
performed on the anodes prepared in Examples 3 and 4.
Orientation ratios were calculated by an area ratio
(110)/(004) which was obtained by measuring (110) and (004)
􀁘􀁗􀁇 planes of an anode active material included in the anode
using XRD, and then integrating peak intensities of (110) and
(004) planes. More specifically, XRD measurement conditions
were as follows.
[00134] 􀋀 Target: Cu(K􀄮 line) graphite monochromator
􀁘􀁜􀁇 [00135] 􀋀 Slit: divergence slit= 1o, receiving slits= 0.1,
scattering slit= 1o
[00136] 􀋀 Measuring range and step angle/measuring time:
[00137] (110) plane: 76.5o < 2􀁔 < 78.5o, 0.01o /3sec
[00138] (004) plane: 53.5o < 2􀁔 < 56.0o, 0.01o /3sec, where,
􀁙􀁗􀁇 2􀁔 is a diffraction angle.
[00139] XRD measurements were performed on the mosaic cokebased
artificial graphite used in Examples 1 and 2, and the
results thereof were shown in FIG. 3. Lc (002) and d002 of
the mosaic coke-based artificial graphite were calculated
􀁙􀁜􀁇 using Equations 1 and 2 below.
26
􀁇
[00140]
[00141] d002 = 􀁏/2sin􀁔
[00142] Also, Lc (002), which is a crystallite size in C-axis
direction of a particle, can be calculated by Equation 2
below as Scherrer equation which calculates 􀁜􀁇 a crystallite
size Lc of mosaic coke-based artificial graphite.
[00143]
[00144]
[00145] where, K is a Scherrer constant (K=0.9),
􀁘􀁗􀁇 [00146] 􀁅 is a half value width,
[00147] 􀁏 is a wavelength (0.154056 nm), and
[00148] 􀁔 is an angle at a maximum peak.
[00149] As shown in FIG. 3, the mosaic coke-based artificial
graphite showed such a crystal habit that Lc (002) was 21.6
􀁘􀁜􀁇 to 21.9 nm and d002 was 0.3376 nm, wherein Lc (002) is a
crystallite size in C-axis direction and d002 is interplanar
spacing of (002) plane during XRD measurement.
[00150]
[00151] Experimental Example 2: Rate Performance Evaluation A
􀁙􀁗􀁇 [00152] Rate performances of the lithium secondary batteries
obtained in Examples 3 and 4, and Comparative Examples 4 to 6
were measured in a voltage range of 0 to 1.5V at room
temperature. Batteries were charged under 0.1C constant27
􀁇
current/constant-voltage (CC/CV) condition up to 1.5V, and
then discharged in a constant current mode until current
reached 0.1C at 5 mV. Thereafter, the measurement was
finished.
[00153] As shown in FIG. 4, from comparative 􀁇 ative analysis of each
rate performance for 0.2C, 0.5C, and 1C in a voltage range of
0 to 1.5V at room temperature, it can be seen that the
lithium secondary battery in Example 3, using the anode
active material in Example 1 where the natural graphite and
􀁘􀁗􀁇 the mosaic coke-based artificial graphite are mixed, has the
improved rate performance which is higher by about 5-10% than
that of the lithium secondary battery in Comparative Example
4 using the anode active material in Comparative Example 1
where the mosaic coke-based graphite is not used.
􀁘􀁜􀁇 [00154] Furthermore, in order to investigate rate performance
according to a mixing ratio by weight of natural graphite to
mosaic coke-based artificial graphite, each rate performance
for 0.2C, 0.5C, and 1C in a voltage range of 0 to 1.5V at
room temperature was compared for the lithium secondary
􀁙􀁗􀁇 batteries in Examples 3 and 4, and Comparative Examples 5 and
6. The results are shown in Table 1 below.
[00155] [Table 1]
Anode active
material
(weight ratio of
natural graphite
to mosaic coke-
Rate performance
0.2C 0.5C 1C
28
􀁇
based artificial
graphite)
Example 3 1:0.3 100% 97.5% 81.2%
Example 4 1:1 100% 98.6% 89.7%
Comparative
Example 6
1:0.05 100% 95.9% 76.7%
Comparative
Example 7
1:1.2 100% 94.8% 75.8%
[00156] As shown in Table 1, it can be found that the anode
active materials in Examples 3 and 4, in which spherical
natural graphite and mosaic coke-based artificial graphite
are mixed at a weight ratio of 1:0.3 to 1:1 and used as the
anode active material, has improved rate performance􀁜􀁇 ,
particularly for 0.5C and 1C, as compared with the anode
active material in Comparative Example 5 in which natural
graphite and mosaic coke-based artificial graphite are mixed
at a weight ratio 1:0.05, that is a small amount of mosaic
􀁘􀁗􀁇 coke-based artificial graphite is used, and the anode active
material in Comparative Example 6 in which natural graphite
and mosaic coke-based artificial graphite are mixed at a
weight ratio 1:1.2, that is an excess amount of mosaic cokebased
artificial graphite is mixed.
􀁘􀁜􀁇 [00157]
[00158] Experimental Example 3: Rate Performance Evaluation B
[00159] In order to investigate rate performance according to
the type of artificial graphite mixed with natural graphite,
29
􀁇
rate performance during charge/discharge was evaluated for
the lithium secondary batteries in Example 5 and Comparative
Example 8. Each rate performance during charging up to
0.005V was measured under 0.2C CC/CV, 0.2C constant-current,
0.5C constant-current, and 1.0C constant-current conditions􀁜􀁇 .
Each rate performance during discharging was measured for
0.2C, 0.5C, 1C, and 2C in a voltage range of 0.005 to 1.5V at
room temperature. The results are shown in Table 2 below.
[00160] [Table 2]
Rate during charging Rate during discharging
0.2C 0.2C 0.5C 1.0C 0.2C 0.5C 1C 2C
Example 5 100 76.0 39.3 15.1 100 99.6 94.6 70.8
Comparative
Example 8
100 70.0 33.6 10.0 100 86.5 86.5 56.8
􀁘􀁗􀁇 [00161] As shown in Table 2, it can be found that the lithium
secondary battery in Example 5 in which spherical natural
graphite and mosaic coke-based artificial graphite are mixed
at a weight ratio of 1:1 and used as the anode active
material, has improved rate performance during charging by
􀁘􀁜􀁇 about 5 to 10%, and improved rate performance during
discharging by about 10 to 15%, than that those of the
lithium secondary battery in Comparative Example 8 in which
natural graphite and needle coke-based artificial graphite
are mixed at a weight ratio of 1:1.
􀁙􀁗􀁇 [00162]
30
􀁇
[00163] Experimental Example 4: Evaluation of Electrode
Resistance
[00164] The lithium secondary battery in Example 3 where the
anode including spherical natural graphite and mosaic cokebased
artificial graphite was used, and the lithium 􀁇 ithium secondary
battery in Comparative Example 4 where the anode including
only spherical natural graphite was used, were charged and
discharged for a long period, and then resistance of each
electrode was measured. The results are shown in FIG. 5.
􀁘􀁗􀁇 Measurement conditions are as follows.
[00165] – Sample preparation: Lithium secondary batteries in
Example 3 and Comparative Example 4 were charged and
discharged during 200 cycles, and then disassembled to obtain
only anodes, and then symmetric cells were made using the
􀁘􀁜􀁇 obtained anodes to measure impedances.
[00166] – Impedance frequency range: 100,000 to 0.005 Hz
[00167] As shown in FIG. 5, it can be seen that when the
lithium secondary battery in Example 3 is subjected to
charge/discharge for a long period, half circle shown in the
􀁙􀁗􀁇 graph is smaller and thus interfacial resistance is reduced
in the case of using the anode including spherical natural
graphite and mosaic coke-based artificial graphite, such as
in Example 3, as compared with Comparative Example 4 in which
the anode including only spherical natural graphite is used.
􀁙􀁜􀁇 [00168] While this invention has been particularly shown and
31
􀁇
described with reference to preferred embodiments thereof and
drawings, it will be understood by those skilled in the art
that various changes in form and details may be made therein
without departing from the spirit and scope of the invention
as defined
32
􀁇
I/We Claim:
1. An anode active material, comprising natural graphite
and mosaic coke-based artificial graphite.
􀁜􀁇
2. The anode active material of claim 1, wherein a weight
ratio of the natural graphite to the mosaic coke-based
artificial graphite is 1:0.1 to 1:1.
􀁘􀁗􀁇 3. The anode active material of claim 1, wherein the
mosaic coke-based artificial graphite has an average major
axis length of 5 to 30.
4. The anode active material of claim 1, wherein the
􀁘􀁜􀁇 mosaic coke-based artificial graphite has Lc (002) of 21.6 to
21.9 nm, wherein the Lc (002) is a crystallite size in C-axis
direction during XRD measurement.
5. The anode active material of claim 4, wherein the
􀁙􀁗􀁇 mosaic coke-based artificial graphite has d002 of 0.3377 nm or
less, wherein the d002 is interplanar spacing of (002) plane
during XRD measurement.
6. The anode active material of claim 1, wherein the
􀁙􀁜􀁇 mosaic coke-based artificial graphite has a specific surface
33
􀁇
area of 3.0 to 4.0 m2/g, and a compressed density of 1.5 to
2.1 g/cc under the pressure of 8 to 25 mPa.
7. The anode active material of claim 1, wherein the
natural graphite has 􀁇 a spherical shape.
8. The anode active material of claim 7, wherein the
natural graphite has an average particle size D50 of 5 to 30
􀻃.
􀁘􀁗􀁇
9. The anode active material of claim 1, wherein the
natural graphite has a specific surface area (BET) of 2 to 8
m2/g.
􀁘􀁜􀁇 10. An anode, comprising:
a current collector; and
the anode active material of claim 1, disposed on at
least one surface of the current collector.
􀁙􀁗􀁇 11. The anode of claim 10, wherein an orientation ratio
I110/I004 is 0.08 to 0.086 under the compressed density of
1.40 to 1.85 g/cc.
34
􀁇
12. A lithium secondary battery, comprising the anode of
claim 10.