SECONDARY BATTERY OF IMPROVED LITHIUM ION
MOBILITY AND CELL CAPACITY
FIELD OF THE INVENTION
The present invention relates to a secondary battery with improved lithium ion
mobility characteristics and increased cell capacity. More specifically, the present
invention relates to a secondary battery having improved discharge characteristics in a
range of high-rate discharge without degradation of general characteristics of the
battery, by fabricating a cathode using two or more active materials having different
oxidation-reduction (hereinafter, simply referred to as "redox") levels so as to exert
superior discharge characteristics in the range of high-rate discharge via sequential
action of cathode active materials in a discharge process, and having maximized cell
capacity via increased electrode density and loading amounts.
BACKGROUND OF THE INVENTION
With recent development of mobile communication and the Information-
Electronic Industry, higher capacity, smaller and lighter lithium secondary batteries are
increasingly in demand. However, with diversification of functions of the portable or
mobile electronic equipment, which is thereby concomitantly accompanied by increased
energy consumption of the equipment, there is also a strong need for realization of
higher power and capacity of the batteries. Therefore, a great deal of research and study
-1-
has been widely conducted to increase C-rate characteristics and capacity of the battery
cells.
However, there is the presence of reciprocal relationship between C-rate
characteristics and capacity of the battery cell. That is, when a loading amount or
electrode density of the cell is increased in order to improve cell capacity, this attempt
usually results in deterioration of C-rate characteristics of the battery cell.
Upon taking into consideration ionic conductivity of active materials, lithium
secondary batteries, as shown in FIG. 1, are needed to maintain the electrode porosity
over a predetermined level. Whereas, if the electrode is rolled at a high-rolling
reduction rate in order to achieve increased loading amount or electrode density, the
electrode porosity is excessively decreased, as shown in FIG. 2, which in turn leads to a
rapid decrease in the C-rate. Further, when the same active materials having different
particle diameters are used as an electrode active material, it is possible to accomplish
a high electrode density by moderate rolling, but the electrode porosity is strikingly
decreased as shown in FIG. 3, thereby leading to significant decreases in the C rate.
Therefore, although it is important to maintain appropriate porosity in order to
meet a proper level of C rate characteristics, the thus-maintained void remains as a dead
volume where the electrode is free of the active materials.
Secondary batteries must maintain a given level of C-rate suited for the
corresponding uses thereof. In particular, secondary batteries for use in electricallydriven
tools that require elevated power or secondary batteries for use in electric
vehicles (EVs) and hybrid electric vehicles (HVs) require significantly higher C-rate.
Consequently, in order to increase the battery power, there is a strong need for the
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development of a lithium secondary battery having improved C-rate characteristics in
conjunction with maximized cell capacity.
SUMMARY OF THE INVENTION
Therefore, the present invention has been made to solve the above problems
and other technicaJ problems that have yet to be resolved.
As a result of a variety of extensive and intensive studies and experiments to
solve the problems as described above, the inventors of the present invention have
surprisingly discovered that, when a mixture of two or more active materials having a
redox potential difference with given conditions is used as a cathode active material, it
is possible to prepare a lithium secondary battery having improved discharge
characteristics in a range of high-rate discharge while minimizing a dead volume as
described hereinbefore, and at the same time, having increased cell capacity via
increased electrode density and loading amounts. The present invention has been
completed based on these findings.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and other advantages of the present
invention will be more clearly understood from the following detailed description
taken in conjunction with the accompanying drawings, in which:
FIG, 1 is a schematic view of a cathode composed of one active material
having the same particle diameter in accordance with a conventional art;
FIG, 2 is a schematic view showing a rolled active material of FIG. 1 at a
high-rolling reduction rate;
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FIG 3 is a schematic view of a cathode composed of one active material
having a different particle diameter in accordance with a conventional art;
FIG. 4 is a schematic view of a cathode composed of two active materials
having different redox potentials in accordance with one embodiment of the present
invention;
FIG. 5 is a schematic view showing rolled active materials of FIG. 4 with a
high-rolling reduction rate;
FIG. 6 is a schematic view of a cathode composed of two active materials
having different particle diameters and redox potentials in accordance with another
embodiment of the present invention;
FIG. 7 is a graph showing electric potential changes versus discharge
capacity (discharge rate) of active materials used in some experiments of the present
invention;
FIG. 8 is a comparison graph of discharge capacity corresponding to each Crate,
obtained in Experimental Example 1 for battery cells of Examples 5 and 6 and
Comparative Examples 4 and 5;
FIG. 9 is a comparison graph of discharge capacity corresponding to each Crate,
obtained in Experimental Example 2 for battery cells of Comparative Examples 6
through 8;
FIG. 10 is a comparison graph of discharge capacity corresponding to each Crate,
obtained in Experimental Example 2 for battery cells of Examples 7 through 9;
-4-
FIG. 11 is a comparison graph of discharge capacity corresponding to each Crate,
obtained in Experimental Example 3 for battery cells of Examples 10 through 12
and Comparative Examples 9 through 11;
FIG. 12 is a comparison graph of discharge capacity corresponding to the
respective C-rates, obtained in Experimental Example 4 for battery cells of Examples
6 and 13, and Comparative Examples 12 and 13;
FIG. 13 is a comparison graph of discharge capacity corresponding to each Crate,
obtained in Experimental Example 5 for battery cells of Examples 14 and 15;
FIG. 14 is a graph showing electric potential changes versus discharge rate of
active materials used for preparing battery cells of some Comparative Examples of the
present invention; and
FIGS. 15 and 16 are graphs showing electric potential changes versus
discharge rate of active materials used for preparing battery cells of some
Comparative Examples of the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
In accordance with an aspect of the present invention, the above and other
objects can be accomplished by the provision of a cathode active material for a lithium
secondary battery, characterized in that the cathode active material is comprised of two
or more active materials having different redox levels so as to exert superior discharge
characteristics in a range of high-rate discharge by sequential action of cathode active
materials in a discharge process.
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As used herein, the term "redox level" refers to an electric potential of the
plateau range in the discharge process or an electric potential at a discharge rate of
approximately 50%. Preferably, the active materials in accordance with the present
invention have the redox level in the range of 3.5 to 4.5 V. The difference of the redox
level between active materials will also be referred to as "a redox potential difference"
As used herein, the phrase "sequential action of cathode active materials in a
discharge process" means that a cathode active material having a relatively high redox
level (hereinafter, referred to as a high-potential cathode active material) preferentially
acts in the discharge process, followed by the action of a cathode active material having
a relatively low redox level (hereinafter, referred to as low-potential cathode active
material).
As used herein, the term "sequential action" means that relatively large
amounts of the high-potential cathode active material preferentially act, but does not
mean that action of low-potential cathode active material is initiated after all of the
high-potential cathode active material has acted. Therefore, this term also encompasses
the condition in which the low-potential cathode active material acts in the discharge
process at a time point where a substantial amount, for example more than 50%, of the
high-potential cathode active materials has acted in the discharge process.
In accordance with the present invention, when the cathode is formed of active
materials having different redox levels, for example when the redox level of the active
material A is 3.8 V and that of active material B is 4.0 V, the active material B does not
undergo oxidation-reduction in an electric potential region of 3.8 V where oxidationreduction
of active material A takes place, and serves as an electrolyte carrier, i.e., a
void. In contrast, active material A serves as a void in 4.0 V electric potential region
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where oxidation-reduction of active material B takes place. As a result, substantially
higher porosity is attained in the redox level regions of each active material A and B
whose oxidation-reduction occurs under the conditions in which the apparent porosity
possessed by the electrode is generally the same.
The reason why such phenomena occur is because lithium (Li) ions should be
smoothly supplied during oxidation-reduction of the active materials, while a higher Crate
leads to higher consumption of lithium (Li) ions for the same period of time.
Therefore, when one active material is singly used and sufficient porosity is not secured
due to increased rolling reduction rate of the active material, as shown in the abovedescribed
conventional art of FIG. 2, smooth supply of lithium ions is not effected and
the higher C-rate leads to rapidly decreased capacity and service life of the electrode
active material.
In contrast, when the cathode is formed of two or more active materials having
a predetermined redox potential difference according to the present invention,
electrical conductivity of the electrode can be increased even when a rolling reduction
rate is increased as shown in FIG. 5, and as will be described hereinafter, it is therefore
possible to achieve increased cell capacity via improved C rate characteristics and
increased electrode density.
The above-mentioned "range of high-rate discharge" may be affected by a
variety of factors and for example, may be set to a range in which a significant decline
of the discharge capacity occurs. Typically, such a discharge range may be flexibly
determined depending upon supply state of an electrolyte inside the battery. As can be
seen from Experimental Example 6 which will be described hereinafter, the discharge
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range may be set at a relatively low discharge rate in a battery having a full-cell
structure in which supply of an electrolyte is limited.
Such discharge characteristics in the range of high-rate discharge will be often
referred to hereinafter as "C-rate characteristics". As described hereinbefore, the
cathode active materials in accordance with the present invention exert superior C-rate
characteristics via sequential action of each active material. The phrase "superior C-rate
characteristics" as used herein means that the actual C-rate values measured for mixed
active materials are significantly large, as compared to calculated values (predicted
values) with respect to a mixing ratio in mixed active materials, based on C-rate values
measured independently for each active material. Such facts are results that were
completely unpredictable prior to the present invention.
Therefore, even when the apparent porosity is lessened via high-rolling
reduction rate so as to increase capacity, as shown in FIG. 5, the cathode active material
in accordance with the present invention can exhibit superior C rate characteristics that
were completely unpredictable before. Even though the redox potential differences of
active materials, which exhibit sequential action as described above and consequently
exert superior C rate characteristics, are not particularly defined as critical values, it was
confirmed, as will be seen in Experimental Examples hereinafter, that desired results are
not obtained when potential differences between active materials used in experiments
are less than 0.03 V.
The different active materials in accordance with the present invention, i.e.
heterogeneous active materials, may be selected from active materials represented by
Formulae I through IV below. Different redox levels are obtained depending upon kinds
of transition metals contained in each active material, which are involved in oxidation-
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reduction, and oxidation numbers thereof. In addition, even when the same transition
metals take part in oxidation-reduction, active materials may exhibit different redox
levels, depending upon the composition and chemical structure thereof.
Specifically, the active materials that are used in the present invention may
include active materials represented by Formulae I through IV below:
[Formula I]
wherein,
-0.2 Co4+ oxidation-reduction in layered structures thereof.
[Formula II]
wherein,
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0 Ni4+ oxidation-reduction in layered structures thereof.
[Formula III]
Li1+xNi1-y-zMyM' ZO2 Aa
wherein,
-0.2 Mn4+ oxidation-reduction in spinel structures thereof.
In one specific embodiment, electrodes including the cathode active materials
in accordance with the present invention may be composed of two active materials
selected from Formulae I through IV above. Specific examples may include the
following combinations, and contents of either of active materials in combinations may
be in the range of 15 to 50%, based on the total weight of the active materials.
- Active material (A): LiCoO2 Active material (B):
- Active material (A): LiCoO2 Active material (B):
- Active material (A): LiCoO2 Active material (B):
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- Active material (A): LiCoO2 Active material (B): LiMn2O4
- Active material (A): LiMn2O4 Active material (B): LiNi1/3Mn1/3Co1/3O2
- Active material (A): LiMn2O4 Active material (B): LiNi0.7Co0.25Mn0.05O2
- Active material (A): LiMn2O4 Active material (B): LiNi0.8Co0.1Mn0.1O2
In particular, when the above active materials have different average particle
diameters, it is possible to provide high-electrode densities and to increase loading
amounts of electrodes, For example, from FIG. 6 schematically showing an electrode
composed of active material A having a relatively large particle diameter and active
material B having a relatively small particle diameter, it can be seen that it is possible to
increase the electrode density and electrode loading amounts while maintaining
inherently high porosity, from the viewpoint of characteristics in that the present
invention uses heterogeneous active materials.
Regarding differences in average particle diameters between active materials,
the size of active material B having a relatively small particle diameter may be less than
50%, preferably in the range of 10 to 35%, of that of active material A having a
relatively large particle diameter, upon taking into consideration actual porosity and
electrode density. From theoretical calculation on the assumption that all of the active
materials are spherical, the particle diameter size of small-particle diameter active
material B capable of being filled into empty spaces which are formed by large-particle
diameter active material A, should be less than or equal to a product from a factor of
0.225 X the particle diameter of large-particle diameter active material A. However,
since active materials A and B generally are not of perfect spherical shapes, it is
possible to achieve increased density even within the above-specified range. Absolute
size differences may be preferably more than 10 μm.
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Where the electrode is composed of two active materials, the content of active
material B having a relatively small particle diameter may be preferably in the range of
15 to 50%, more preferably 20 to 35%, based on the total weight of the active material
mixture (A+B). From experiments conducted by the present inventors, it was confirmed
that addition of less than 15% content of small-particle diameter active material B
exhibits essentially no addition effects or has insignificant effects on improvement of C
rate characteristics. In contrast, when the content of small-particle diameter active
material B is too high, it is difficult to accomplish improvement of the electrode density.
For example, two active materials having different average particle diameter to
each other may be used by any combination of two or more active materials selected
from Formulae I through IV. Examples of preferred combinations may include, but are
not limited to, the following combinations. The particle diameter of active materials A
is defined as being larger than that of active materials B.
- Active material (A): LiCoO2 Active material (B): LiNi1/3Mn1/3Co1/3O2
- Active material (A): LiNi1/3Mn1/3Co1/3O2 Active material (B): LiCoO2
- Active material (A): LiCoO2 Active material (B): LiNi0.7Co0.25Mn0.05O2
- Active material (A): LiNi0.8Co0.15Mn0.05O2 Active material (B): LiCoO2
- Active material (A): LiCoO2 Active material (B):LiNi0.8Co0.1Mn0.1O2
- Active material (A): LiNi0.8Co0.1Mn0.1O2 Active material (B): LiCoO2
- Active material (A): LiCoO2 Active material (B): LiMn2O4
- Active material (A); LiMn2O4 Active material (B): LiCoO2
- Active material (A): LiNi1/3Mn1/3Co1/3O2 Active material (B):LiMn2O4
- Active material (A): LiMn2O4 Active material (B): LiNi1/3Mn1/3Co1/3O2
- Active material (A); LiNi0.7Co0.25Mn0.05O2 Active material (B): LiMn2O4
- Active material (A): LiMn2O4 Active material (B): LiNi0.7Co0.25Mn0.05O2
-13-
In accordance with another aspect of the present invention, there is provided a
lithium secondary battery comprising the above-mentioned cathode active material. In
general, the lithium secondary battery is comprised of a cathode, an anode, a separator,
and a non-aqueous electrolyte containing a lithium salt.
The cathode is, for example, fabricated by applying a mixture of the abovementioned
cathode active material, a conductive material and a binder to a cathode
current collector, followed by drying. If desired, a filler may be added to the above
mixture.
The cathode current collector is generally fabricated to have a thickness of 3 to
500 μm. There is no particular limit to the cathode current collector, so long as it has
high conductivity without causing chemical changes in the battery. As examples of the
cathode current collector, mention may be made of stainless steel, aluminum, nickel,
titanium, sintered carbon and aluminum or stainless steel which was surface-treated
with carbon, nickel, titanium or silver. The current collector may be fabricated to have
fine irregularities on the surface thereof so as to enhance adhesiveness to the cathode
active material. In addition, the current collector may take various forms including
films, sheets, foils, nets, porous structures, foams and non-woven fabrics.
The conductive material utilized in the present invention is typically added in
an amount of 1 to 50% by weight, based on the total weight of the mixture including the
cathode active material. There is no particular limit to the conductive material, so long
as it has suitable conductivity without causing chemical changes in the battery. As
examples of conductive materials, mention may be made of conductive materials,
including graphite such as natural or artificial graphite; carbon blacks such as carbon
black, acetylene black, Ketjen black, channel black, furnace black, lamp black and
-14-
thermal black; conductive fibers such as carbon fibers and metallic fibers; metallic
powders such as carbon fluoride powder, aluminum powder and nickel powder;
conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides
such as titanium oxide; and polyphenylene derivatives.
The binder is an ingredient assisting in bonding between the active material
and conductive material, and in binding to current collectors. The binder utilized in the
present invention is typically added in an amount of 1 to 50% by weight, based on the
total weight of the mixture including the cathode active material. As examples of the
binder, mention may be made of polyvinylidene fluoride, polyvinyl alcohols,
carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose,
polyvinyl pyrollidone, tetrafluoroethylene, polyethylene, polypropylene, ethylenepropylene-
diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber,
fluoro rubber and various copolymers.
The filler is an optional ingredient that inhibits cathode expansion. There is no
particular limit to the filler, so long as it does not cause chemical changes in the
battery and is a fibrous material. As examples of the filler, there may be used olefm
polymers such as polyethylene and polypropylene; and fibrous materials such as glass
fiber and carbon fiber.
The anode is fabricated by applying an anode active material to an anode
current collector, followed by drying. If necessary, other components, as described
above, may be further added.
The anode current collector is generally fabricated to have a thickness of 3 to
500 μm. There is no particular limit to the anode current collector, so long as it has
suitable conductivity without causing chemical changes in the battery. As examples of
-15-
the anode current collector, mention may be made of copper, stainless steel,
aluminum, nickel, titanium, sintered carbon, copper or stainless steel having a surface
treated with carbon, nickel, titanium or silver, and aluminum-cadmium alloys. Similar
to the cathode current collector, the anode current collector may also be fabricated to
form fine irregularities on the surface thereof so as to enhance adhesiveness to the
anode active material. In addition, the anode current collector may take various forms
including films, sheets, foils, nets, porous structures, foams and non-woven fabrics.
As examples of the anode active materials utilizable in the present invention,
mention may be made of carbon such as non-graphitizing carbon and graphite-based
carbon; metal composite oxides such as LixFe203 (0
Active material Thickness Electrode density Electrode porosity
-19-
Comp. Example 1
Comp. Example 2
Comp. Example 3
Example 1
Example 2
Example 3
Example 4
(cm)
0.39
0.38
0.379
0.384
0.38
0.38
0.388
(g/cc)
3.629
3.724
3.737
3.685
3.724
3.724
3.646
(%)
18.6
16.4
16.1
16.9
15.8
15.6
18.2
As can be seen from Table 1, when the active materials having a different
particle diameter to each other were mixed (Comparative Examples 2 and 3, and
Examples 1 through 4), a density between active materials was increased and the void
was decreased, as compared to when the active material having the same particle
diameter was used alone.
[Example 5]
An active material mixed in a weight ratio as in Example 3, a conductive
material and a binder were mixed in a ratio of 95:2.5:2.5 to prepare a slurry. The slurry
thus obtained was coated on aluminum (Al) foil having a thickness of 20 μm to prepare
a cathode. Thereafter, a coin-type cell was manufactured using the thus-prepared
cathode, a lithium metal as an anode, and 1M LiPF6 in EC:EMC (1:2) as an electrolyte.
[Example 6]
An active material mixed in a weight ratio as in Example 4, a conductive
material and a binder were mixed in a ratio of 95:2.5:2.5 to prepare a slurry. The slurry
thus obtained was coated on aluminum (Al) foil having a thickness of 20 μm to prepare
a cathode. Thereafter, a coin-type cell was manufactured using the thus-prepared
cathode, a lithium metal as an anode, and 1M LiPF6 in EC:EMC (1:2) as an electrolyte.
-20-
[Comparative Example 4]
An active material of Comparative Example 1, a conductive material and a
binder were mixed in a ratio of 95:2.5:2.5 to prepare a slurry. The slurry thus obtained
was coated on aluminum (Al) foil having a thickness of 20 μm to prepare a cathode.
Thereafter, a coin-type cell was manufactured using the thus-prepared cathode, a
lithium metal as an anode, and 1M LiPF6 in EC:EMC (1:2) as an electrolyte.
[Comparative Example 5]
An active material mixed in a weight ratio as in Comparative Example 3, a
conductive material and a binder were mixed in a ratio of 95:2.5:2.5 to prepare a slurry.
The slurry thus obtained was coated on aluminum foil (Al) having a thickness of 20 (am
to prepare a cathode. Thereafter, a coin-type cell was manufactured using the thusprepared
cathode, a lithium metal as an anode, and 1M LiPF6 in EC:EMC (1:2) as an
electrolyte.
FIG. 7 is a graph showing electric potential changes versus discharge
capacity (discharge rate) of active materials used in some experiments. As can be seen
from FIG. 7, the respective active materials exhibit plateau ranges having substantially
no changes of a slope at a discharge rate of about 10 to 90%. In the following
experiments, redox levels of active materials concerned are established as the
magnitude of electric potentials at a 50% discharge rate. For example, the redox level
of LiCoO2 is 3.92 V while that of LiNi1/3Mn1/3Co1/3O2 is 3.77 V, and therefore two
active materials exhibit a redox potential difference of about 0.15 V therebetween.
[Experimental Example 1 ]
-21-
For battery cells prepared in Examples 5 and 6 and battery cells prepared in
Comparative Examples 4 and 5, discharge capacity (charged at 0.2 C rate) thereof was
measured at 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C and 3 C rates, and a ratio of discharge capacity
at the respective C-rates relative to 0.2 C discharge capacity was calculated. The results
thus obtained are shown in FIG. 8. As shown in FIG. 8, it can be seen that use of two
active materials having different redox potentials as in Examples 5 and 6 provides
gradually better results starting from the C-rate of more than 1 C, as compared to use of
one active material having the same particle diameter as in Comparative Example 4 or
use of one active material having the different particle diameter as in Comparative
Example 5. In particular, it can be seen that an increasing ratio of LiNi1/3Mn1/3Co1/3O2
results in superior results.
Upon considering the fact that LiCoO2 is known to have C rate characteristics
superior to LiNi1/3Mn1/3Co1/3O2(see FIG. 7), the results of Examples 5 and 6, exhibiting
improved C rate characteristics by addition of LiNi1/3Mn1/3Co1/3O2 having inferior C
rate characteristics to LiCoO2 as contrary to the expectation, are extraordinarily new
results that were completely unpredictable from conventional arts.
[Example 7]
An active material composed of a mixture of LiCoO2 having an average
particle size of 20 jam and LiNi0.8Co0.1Mn0.1O2 having an average particle size of 5 urn
in a weight ratio of 1:1, a conductive material and a binder were mixed in a ratio of
95:2.5:2.5 to prepare a slurry. The slurry thus obtained was coated on aluminum foil
(Al) having a thickness of 20 μm to prepare a cathode. Thereafter, a coin-type cell was
manufactured using the thus-prepared cathode, a lithium metal as an anode, and 1M
LiPF6 in EC:EMC (1:2) as an electrolyte.
-22-
[Example 8]
An active material composed of a mixture of LiNi0.8Co0.15Mn0.05O2 having an
average particle size of 18 μm and LiMn2O4 having an average particle size of 5 um in
a weight ratio of 7:3, a conductive material and a binder were mixed in a ratio of
95:2.5:2.5 to prepare a slurry. The slurry thus obtained was coated on aluminum foil
(Al) having a thickness of 20 μm to prepare a cathode. Thereafter, a coin-type cell was
manufactured using the thus-prepared cathode, a lithium metal as an anode, and 1M
LiPF6 in EC:EMC (1:2) as an electrolyte.
[Example 9]
An active material composed of a mixture of LiNi1/3Mn1/3Co1/3O2 having an
average particle size of 18 μm and LiMn2O4 having an average particle size of 5 um in
a weight ratio of 7:3, a conductive material and a binder were mixed in a ratio of
95:2.5:2.5 to prepare a slurry. The slurry thus obtained was coated on aluminum foil
(Al) having a thickness of 20 μm to prepare a cathode. Thereafter, a coin-type cell was
manufactured using the thus-prepared cathode, a lithium metal as an anode, and 1M
LiPF6 in EC:EMC (1:2) as an electrolyte.
[Comparative Example 6]
An active material composed of LiCoO2 having an average particle size of 20
μm, a conductive material and a binder were mixed in a ratio of 95:2.5:2.5 to prepare a
slurry. The slurry thus obtained was coated on aluminum (Al) foil having a thickness of
20 um to prepare a cathode. Thereafter, a coin-type cell was manufactured using the
thus-prepared cathode, a lithium metal as an anode, and 1M LiPF6 in EC:EMC (1:2) as
an electrolyte.
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[Comparative Example 7]
An active material composed of LiNi0.8Co0.15Mn0.05O2 having an average
particle size of 18 μm, a conductive material and a binder were mixed in a ratio of
95:2.5:2.5 to prepare a slurry. The slurry thus obtained was coated on aluminum (Al)
foil having a thickness of 20 um to prepare a cathode. Thereafter, a coin-type cell was
manufactured using the thus-prepared cathode, a lithium metal as an anode, and 1M
LiPF6 in EC:EMC (1:2) as an electrolyte.
[Comparative Example 8]
An active material composed of LiNi1/3Mn1/3Co1/3O2 having an average particle
size of 20 μm, a conductive material and a binder were mixed in a ratio of 95:2.5:2.5 to
prepare a slurry. The slurry thus obtained was coated on aluminum (Al) foil having a
thickness of 20 urn to prepare a cathode. Thereafter, a coin-type cell was manufactured
using the thus-prepared cathode, a lithium metal as an anode, and 1M LiPF6 in EC:EMC
(1:2) as an electrolyte.
[Experimental Example 2]
For respective battery cells prepared in Comparative Examples 6 through 8,
discharge capacity (charged at 0.2 C rate) thereof was measured at discharge rates of 0.1
C, 0.2 C, 0.5 C, 1 C and 2 C, and was then calculated as a ratio relative to 0.2 C
discharge capacity. The results thus obtained are shown in FIG. 9. In addition, for
battery cells prepared in Examples 7 through 9, discharge capacity thereof was
measured at discharge rates of 0.1 C, 0.2 C, 0.5 C, 1 C and 2 C, and was then calculated
as a ratio relative to 0.2 C discharge capacity. The results thus obtained are shown in
FIG. 10. Similar to FIG. 8, it can be seen from two graphs that C rate characteristics
-24-
were improved, in spite of the fact that LiCoO2 having superior C rate characteristics
was mixed with LiNi0.8Co0.1Mn0.1O2 having relatively poor C rate characteristics.
Further, it can be seen that LiNi0.8Co0.15Mn0.05O2 and LiNi1/3Mn1/3Co1/3O2 also exhibit
improvement in C rate characteristics, when they are used in admixture with LiNn2O4
having a different redox level, as compared to when they are used alone. For
reference, LiNi0.8Co0.1Mn0.1O2 has a redox level of 3.80 V and that of LiMn2O4 is 4.06
V.
[Example 10]
An active material mixed in a weight ratio as in Example 4, a conductive
material and a binder were mixed in a ratio of 95:2.5:2.5 to prepare a slurry. The slurry
thus obtained was coated on aluminum (Al) foil having a thickness of 20 um to prepare
a cathode having a loading amount of 2.5 mAh/cm2. Thereafter, a coin-type cell was
manufactured using the thus-prepared cathode, a lithium metal as an anode, and 1M
LiPF6 in EC:EMC (1:2) as an electrolyte.
[Example 11]
An active material mixed in a weight ratio as in Example 4, a conductive
material and a binder were mixed in a ratio of 95:2.5:2.5 to prepare a slurry. The slurry
thus obtained was coated on aluminum (Al) foil having a thickness of 20 μm to prepare
a cathode having a loading amount of 3.0 mAh/cm2. Thereafter, a coin-type battery cell
was manufactured using the thus-prepared cathode, a lithium metal as an anode, and 1M
LiPF6 in EC:EMC (1:2) as an electrolyte.
[Example 12]
-25-
An active material mixed in a weight ratio as in Example 4, a conductive
material and a binder were mixed in a ratio of 95:2.5:2.5 to prepare a slurry. The slurry
thus obtained was coated on aluminum (Al) foil having a thickness of 20 μm to prepare
a cathode having a loading amount of 3.5 mAh/cm2. Thereafter, a coin-type battery cell
was manufactured using the thus-prepared cathode, a lithium metal as an anode, and 1M
LiPF6 in EC:EMC (1:2) as an electrolyte.
[Comparative Example 9]
An active material of Comparative Example 1, a conductive material and a
binder were mixed in a ratio of 95:2.5:2.5 to prepare a slurry. The slurry thus obtained
was coated on aluminum (Al) foil having a thickness of 20 μm to prepare a cathode
having a loading amount of 2.5 mAh/cm2. Thereafter, a coin-type battery cell was
manufactured using the thus-prepared cathode, a lithium metal as an anode, and 1M
LiPF6 in EC:EMC (1:2) as an electrolyte.
[Comparative Example 10]
An active material of Comparative Example 1, a conductive material and a
binder were mixed in a ratio of 95:2.5:2.5 to prepare a slurry. The slurry thus obtained
was coated on aluminum (Al) foil having a thickness of 20 μm to prepare a cathode
having a loading amount of 3.0 mAh/cm2. Thereafter, a coin-type battery cell was
manufactured using the thus-prepared cathode, a lithium metal as an anode, and 1M
LiPF6 in EC:EMC (1:2) as an electrolyte.
[Comparative Example 11]
-26-
An active material of Comparative Example 1, a conductive material and a
binder were mixed in a ratio of 95:2.5:2.5 to prepare a slurry. The slurry thus obtained
was coated on aluminum (Al) foil having a thickness of 20 urn to prepare a cathode
having a loading amount of 3.5 mAh/cm2. Thereafter, a coin-type battery cell was
manufactured using the thus-prepared cathode, a lithium metal as an anode, and 1M
LiPF6 in EC:EMC (1:2) as an electrolyte.
[Experimental Example 3]
For respective battery cells prepared in Comparative Examples 9 through 11
and Examples 10 through 12, discharge capacity thereof was measured at discharge
rates of 0.1 C, 0.2 C, 0.5 C and 1 C, and was then calculated as a ratio relative to the
reference discharge capacity at 0.2 C rate. The results thus obtained are shown in FIG.
11. As shown in FIG, 11, it can be seen that the cells of Comparative Example 9 and
Example 10, having low-loading amounts, exhibited similar C rate characteristics
therebetween, whereas the cells of Comparative Examples 10 and 11, and Examples 11
and 12, which use a mixture of two active materials having different redox levels with
increasing loading amounts, exhibited improvement in C rate characteristics.
[Example 13]
A mixed active material of Example 4 (a mixture of LiCoO2 having an average
particle size of 20 μm and LiNi1/3Mn1/3Co1/3O2 having an average particle size of 5 μm),
a conductive material and a binder were mixed in a weight ratio of 95:2.5:2.5 to prepare
a slurry. The slurry thus obtained was coated on aluminum (Al) foil having a thickness
of 20 μm to prepare a cathode with a loading amount of 2.4 mAh/cm2 (based on a
discharge loading). In addition, artificial graphite, a conductive material and a binder
-27-
were mixed in a weight ratio of 94:1:5 to prepare a slurry. The slurry thus obtained was
coated on copper (Cu) foil having a thickness of 10 μm to prepare an anode with a
loading amount of 2.4 mAh/cm2 (based on discharge loading). The thus-prepared
cathode and anode were stacked with intercalation of a separator therebetween, thereby
preparing an electrode assembly. The electrode assembly was built in a pouch-type case
made up of an aluminum laminate sheet to which 1 M LiPFe impregnated in EC:EMC
(1:2) as an electrolyte was then introduced, thereby preparing a pouch-type cell (full
cell).
[Comparative Example 12]
An active material composed of LiCoO2 having an average particle size of 5
μm, a conductive material and a binder were mixed in a weight ratio of 95:2.5:2.5 to
prepare a slurry. The slurry thus obtained was coated on aluminum (Al) foil having a
thickness of 20 μm to prepare a cathode with a loading amount of 2.4 mAh/cm2 (based
on discharge loading). In addition, artificial graphite, a conductive material and a binder
were mixed in a ratio of 94:1:5 to prepare a slurry. The slurry thus obtained was coated
on copper (Cu) foil having a thickness of 10 (am to prepare an anode with a loading
amount of 2.4 mAh/cm2 (based on discharge loading). The thus-prepared cathode and
anode were stacked with intercalation of a separator therebetween, thereby preparing an
electrode assembly. The electrode assembly was built in a pouch-type case made up of
an aluminum laminate sheet to which 1 M LiPF6 impregnated in EC:EMC (1:2) as an
electrolyte was the introduced, thereby preparing a pouch-type cell (full cell).
[Comparative Example 13]
-28-
An active material composed of LiCoO2 having an average particle size of 5
μm, a conductive material and a binder were mixed in a ratio of 95:2.5:2.5 to prepare a
slurry. The slurry thus obtained was coated on aluminum (Al) foil having a thickness of
20 μm to prepare a cathode. Thereafter, a coin cell was prepared using the thus-prepared
cathode, a lithium metal as an anode, and 1M LiPF6 in EC:EMC (1:2) as an electrolyte.
[Experimental Example 4]
For cells prepared in Examples 6 and 13 and cells prepared in Comparative
Examples 12 and 13, discharge capacity (charged at 0.2 C rate) thereof was measured at
0.1 C, 0.2 C, 0.5 C, 1 C and 2 C rates. The results thus obtained are shown in FIG. 12.
Although the cells of Examples 6 and 13 have the same composition of active materials
(using a mixed active material of LiCoO2 having an average particle size of 20 μm and
LiNi1/3Mn1/3Co1/3O2 having an average particle size of 5 μm), the cell of Example 13 is
a cell having a full-cell structure while the cell of Example 6 is a cell having a coin-type
structure. Similarly, although the cells of Comparative Examples 12 and 13 have the
same composition of active materials (using an active material LiCoO2 having an
average particle size of 5 μm), the cell of Comparative Example 12 is of a full-cell
structure while the cell of Comparative Example 13 is of a coin-type structure.
Generally, in active materials belonging to the same class, a smaller size of the
active materials results in superior C rate characteristics. Therefore, as compared to the
cell of Comparative Example 6 (see FIG. 9) using LiCoO2 having an average particle
size of 20 μm as the active material, the cell of Comparative Example 13 using LiCoO2
having an average particle size of 5 μm as the active material, as shown in FIG. 12,
generally exhibit superior discharge characteristics even in the range of high-rate
discharge when the cell is of a coin-type structure.
-29-
However, as discussed hereinbefore, as large consumption of the electrolyte
occurs in the range of high-rate discharge, there is a tendency of sharply decreased
discharge characteristics in a full-cell structure of the cell in which the electrolyte acts
as a limiting factor, as compared to the coin type structure of the cell exhibiting no
limitation to the electrolyte. As a result, it was confirmed that the cell using 5 μm -sized
LiCoO2 having superior C rate characteristics as the active material, when it was
manufactured in the form of a full-cell structure as in Comparative Example 13(12?),
also exhibited a rapid decrease of discharge characteristics in the range of high-rate
discharge as shown in FIG. 12.
On the other hand, it was confirmed that use of the mixed active material in
accordance with the present invention leads to improvement in discharge
characteristics even in a full-cell structure of the cell in which the electrolyte acts as a
limiting factor. As discussed hereinbefore, LiNi1/3Mn1/3Co1/3O2 exhibits C-rate
characteristics inferior to LiCoO2. Whereas, it can be confirmed from FIG. 12 that even
though a cell having a full-cell structure was manufactured by addition of
LiNi1/3Mn1/3Co1/3O2 to 20 μm -sized LiCoO2 having C-rate characteristics inferior to 5
μm -sized LiCoO2 in Example 13, this cell exhibits superior discharge characteristics in
the range of high-rate discharge, as compared to the cell of Example 6 which was
manufactured in the coin-type structure having the same composition.
These are results that were completely unpredictable from conventional arts.
Based on these facts, it can be seen that even active materials having superior C-rate
characteristics, but unfortunately showing limitation of application thereof to the battery
in which the electrolyte acts as a limiting factor, can essentially overcome such
problems via inventive constitution in accordance with the present invention.
-30-
[Example 14]
An active material mixed in a weight ratio as in Example 1, a conductive
material and a binder were mixed in a ratio of 95:2.5:2.5 to prepare a slurry. The slurry
thus obtained was coated on aluminum (Al) foil having a thickness of 20 um to prepare
a cathode. Thereafter, a coin-type cell was prepared using the thus-prepared cathode, a
lithium metal as an anode, and 1M LiPF6 in EC:EMC (1:2) as an electrolyte.
[Example 15]
An active material mixed in a weight ratio as in Example 2, a conductive
material and a binder were mixed in a ratio of 95:2.5:2.5 to prepare a slurry. The slurry
thus obtained was coated on aluminum (Al) foil having a thickness of 20 um to prepare
a cathode. Thereafter, a coin-type cell was prepared using the thus-prepared cathode, a
lithium metal as an anode, and 1M LiPF6in EC:EMC (1:2) as an electrolyte.
[Experimental Example 5]
For cells prepared in Examples 14 and 15, experiments were carried out in the
same manner as in Experimental Example 1. The results thus obtained are shown in
FIG. 13. For comparison, experimental results of Experimental Example 1 are also
given in FIG. 13. As can be seen from FIG. 13, the battery in which 10% by weight of
LiNi1/3Mn1/3Co1/3O2 having a particle diameter of 5 μm was added to LiCoO4 having a
particle diameter of 20 um did not exhibit a significant difference in addition effects, as
compared to the cell of Comparative Example 4 in which LiCoO4 having a particle
diameter of 20 um was used alone, but addition of 15% by weight of
LiNi1/3Mn1/3Co1/3O2 having a particle diameter of 5 um began to result in pronounced
improvement in C rate characteristics.
-31-
[Comparative Examples 14 through 21]
Using LiMn2O4 (Comparative Example 14), LiNi1/3Mn1/3Co1/3O2 (Comparative
Example 15), LiNi0.8Mn0.1Co0.1O2 (Comparative Example 16), LiNi0.7Mn0.05Co0.25O2
(Comparative Example 17), LiNi0.8Co0.2O2 (Comparative Example 18), LiNi0.5Mn0.5O2
(Comparative Example 19), LiNi0.45Mn0.45Co0.1O2 (Comparative Example 20) and
LiNi0.425Mn0.425Co0.15O2 (Comparative Example 21), having various particle diameters
as set forth in Table 2 below, the corresponding battery cells were respectively
manufactured in the same manner as in Example 13.
[Examples 16 and 17]
Using a mixed active material of LiMn2O4 having a particle diameter of 15 μm
and LiNi0.8Mn0.1Co0.1O2 having a particle diameter of 6 μm (Example 16), and a mixed
active material of LiMn2O4 having a particle diameter of 15 μm and
LiNi1/3Mn1/3Co1/3O2 having a particle diameter of 5 μm (Example 17), respectively, as
set forth in Table 2 below, the corresponding cells were manufactured in the same
manner as in Example 13.
[Comparative Examples 22 through 24]
Using a mixed active material of LiNi0.7Mn0.05Co0.25O2 having a particle
diameter of 12 μm and LiNi0.8Co0.2O2 having a particle diameter of 6 μm (Comparative
Example 22), a mixed active material of LiNi0.425Mn0.425Co0.15O2 having a particle
diameter of 6 μm and LiNi0.45Mn0.45Co0.1O2 having a particle diameter of 6 μm
(Comparative Example 23) and a mixed active material of LiNi0.425Mn0.425Co0.15O2
having a particle diameter of 6 μm and LiNi0.5Mn0.5O2 having a particle diameter of 6
-32-
μm (Comparative Example 24), respectively, as set forth in Table 2 below, the
corresponding cells were manufactured in the same manner as in Example 13.
[Experimental Example 6]
First, electrical potential changes versus a discharge rate for LiMn2O4,
LiNi1/3Mn1/3Co1/3O2, LiNi0.8Mn0.1Co0.1O2, LiNi0.7Mn0.05Co0.25O2 and
LiNi0.425Mn0.425Co0.15O2, which were active materials used to prepare cells of
Comparative Examples 14-17 and 21, were measured. The results thus obtained are
shown in FIG. 14. As can be seen from FIG. 14, the respective active materials exhibit
different redox levels therebetween.
In addition, for LiNi0.7Mn0.05Co0.25O2, LiNi0.8Co0.2O2,
LiNi0.45Mn0.45Co0.01O2 and LiNio.425Mno.425Co0.1502, which were active materials used to
prepare cells of Comparative Examples 22 through 24, electrical potential changes
versus a discharge rate were measured. The results thus obtained are shown in FIGS. 15
and 16. As can be seen from there, active materials used to prepare cells of Comparative
Examples 22 through 24 were composed of combinations of active materials having
very small redox potential difference therebetween.
Meanwhile, for the respective cells prepared in Comparative Examples 14
through 24 and Examples 16 and 17, discharge capacity thereof was measured at 0.1 C,
0.2 C, 0.5 C, 1.0 C, 2.0 C, 3.0 C and 5.0 C rates. The results thus obtained are shown in
Table 2 below. Calculated values given in Table 2 below are values in which C rates
(observed values) individually measured for the respective active materials of
Comparative Examples 14 through 21 were calculated as a mixing ratio between the
-33-
respective components upon constituting cathodes in Examples 16 and 17 and
Comparative Examples 22 through 24.

Materials
LiMn2O4(15^m)
LiNiI/3Mn1/3Coi/3O2
(5 (am)
LiNio.gMno.iCoo.iO2
(6 jam)
LiNio.vMno.osCoojsO
2 (12 urn)
LiNio.gCoo.2O2 (6
(am)
LiNio.5Mno.sO2 (6
urn)
LiNio.4sMno.45Coo.iO
2 (6 urn)
Nio.42sMno.425CoCoo
i5O2(6|am)
LiMn204(15 (im)+
LiNio.sMnoiCoo.iOi
(6 ^m)
LiMn204(15 um)+
LiNii/3Mni/3Coi/302
(5 jxm)
LiNio.vMno.osCoo^sO
2(l2nm)+
LiNi0.8Coo.2O2 (6
^im)
LiNio.42sMno.42sCoo i j
502(6
|am)-fLiNio.4sMno.45
Co0.i02(6(xm) |
Rate
Ob*
Ob
Ob
Ob
Ob
Ob
Ob
Ob
Ob
Th*+
Ob
Th
Ob
Th
Ob
Th
C-rate(150mAh/glC)
0.1
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
0.2
100.2
98.0
97.2
97.2
96.7
96.5
97.1
97.7
98.5
99.3
98.9
99.5
97.1
97.1
97.3
97.5
0.5
100.1
95.2
93.7
93.1
91.8
91.6
92.6
93.0
96.6
98.2
97.4
98.6
92.3
92.7
92.4
92.9
1.0
99.6
92.4
90.7
89.6
87.2
84.2
88.1
88.5
95.2
96.9
96.0
97.4
87.5
88.9
87.9
88.4
2,0
97.0
88.8
87.4
85.5
83.4
76.3
81.9
82.6
92.9
94.1
94.0
94.5
83.4
84.9
81.5
82.4
3.0
86.4
85.1
84.3
81.8
79.2
65.4
76.2
77.9
89.1
85.7
89.0
86.0
81.2
81.0
76.1
77.4
5.0
71.5
78.2
75.1
71.3
67.1
50.3
63.2
65.7
82.2
72.6
83.3
73.5
70.4
70.0
62.9
65.0
-34-
LiNio.425Mno.42.5COo.!
502(6um)+
LiNi0.5Mno.502(6
Mm)
Ob
Th
100.0
100.0
96.6
97.3
91.2
92.6
86.0
87.2
78.5
80.7
72.4
74.1
59.7
61.1
Note
Ob : Observed value
Th": Theoretical value
As can be seen from Table 2, it was confirmed that cells of Examples 16 and
17 in which cathodes were formed of active materials having different redox levels to
each other exhibited the observed values greater than the theoretical values at more than
3.0 C rate and in particular, a higher discharge rate leads to increases in such a deviation
and 5.0 C rate leads to occurrence of considerable deviation.
In contrast, it was also confirmed that even though the cathode of the cell was
formed of active materials having different redox levels, there were substantially no
increases or even decreases in the observed values as compared to the theoretical values
when the redox potential difference between active materials was not large as shown in
FIGS. 15 and 16 (Comparative Examples 22 through 24).
In conclusion, it is difficult to achieve desired effects of the present invention
by simply mixing heterogeneous active materials and therefore it can be seen that such
combinations of heterogeneous active materials can provide desired effects when active
materials have the redox potential difference meeting conditions specified in the present
invention.
INDUSTRIAL APPLICABILITY
-35-
As apparent from the above description, in accordance with the present
invention, when a mixture of two or more active materials having different redox
potentials to each other is used as a cathode active material and preferably the active
materials have different particle diameters, it is possible to prepare a lithium secondary
battery having improved discharge characteristics in a range of high-rate discharge
while minimizing a dead volume, and at the same time, having increased cell capacity
via increased electrode density and electrode loading amounts.
Although the preferred embodiments of the present invention have been
disclosed for illustrative purposes, those skilled in the art will appreciate that various
modifications, additions and substitutions are possible, without departing from the
scope and spirit of the invention as disclosed in the accompanying claims.

CLAIMS:We Claim:
1. A cathode active material for a lithium secondary battery, comprising an active material B and an active material A having different redox levels
wherein the active material B has a relatively small particle diameter and the active material A has a relatively large particle diameter,
wherein the content of active material B having the relatively small particle diameter is in the range of 20 to 35% by weight,
wherein a redox potential difference between the redox levels of the active material B and the active material A is at least 0.15 V, and
the active material A is represented by the following Formulae IV:
Li1+xMn2-yMyO4Aa (IV)

(wherein -0.2