U.S. patent application number 11/831516 was filed with the patent office on 2007-12-27 for material for lithium secondary battery of high performance.
This patent application is currently assigned to LG CHEM, LTD.. Invention is credited to Hong-Kyu PARK, Sin young PARK, Jens M. PAULSEN, Ho suk SHIN, Sun Sik SHIN.
Application Number | 20070298512 11/831516 |
Document ID | / |
Family ID | 46328154 |
Filed Date | 2007-12-27 |
United States Patent
Application |
20070298512 |
Kind Code |
A1 |
PARK; Hong-Kyu ; et
al. |
December 27, 2007 |
MATERIAL FOR LITHIUM SECONDARY BATTERY OF HIGH PERFORMANCE
Abstract
Provided is a cathode active material containing a Ni-based
lithium mixed transition metal oxide. More specifically, the
cathode active material comprises the lithium mixed transition
metal oxide having a composition represented by Formula I of
Li.sub.xM.sub.yO.sub.2 wherein M, x and y are as defined in the
specification, which is prepared by a solid-state reaction of
Li.sub.2CO.sub.3 with a mixed transition metal precursor under an
oxygen-deficient atmosphere, and has a Li.sub.2CO.sub.3 content of
less than 0.07% by weight of the cathode active material as
determined by pH titration. The cathode active material in
accordance with the present invention and substantially free of
water-soluble bases such as lithium carbonates and lithium sulfates
and therefore has excellent high-temperature and storage
stabilities and a stable crystal structure. A secondary battery
comprising such a cathode active material exhibits a high capacity
and excellent characteristics, and can be produced by an
environmentally friendly method with low production costs and high
production efficiency.
Inventors: |
PARK; Hong-Kyu; (Daejeon,
KR) ; SHIN; Sun Sik; (Daejeon, KR) ; PARK; Sin
young; (Daejeon, KR) ; SHIN; Ho suk; (Daejeon,
KR) ; PAULSEN; Jens M.; (Daejeon, KR) |
Correspondence
Address: |
CANTOR COLBURN, LLP
55 GRIFFIN ROAD SOUTH
BLOOMFIELD
CT
06002
US
|
Assignee: |
LG CHEM, LTD.
LG Twin Towers 20, Yeouido-dong, Yeongdeungpo-gu
Seoul
KR
150-721
|
Family ID: |
46328154 |
Appl. No.: |
11/831516 |
Filed: |
July 31, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11104734 |
Apr 13, 2005 |
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11831516 |
Jul 31, 2007 |
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Current U.S.
Class: |
436/163 ;
429/231.1 |
Current CPC
Class: |
H01M 4/505 20130101;
C01P 2002/54 20130101; Y02P 70/50 20151101; H01M 10/052 20130101;
C01G 53/006 20130101; H01M 4/485 20130101; C01P 2002/88 20130101;
C01P 2004/03 20130101; C01P 2006/40 20130101; Y10T 436/15 20150115;
C01P 2006/10 20130101; C01P 2002/72 20130101; C01P 2004/84
20130101; C01G 51/50 20130101; C01P 2006/37 20130101; C01G 53/50
20130101; C01G 45/1228 20130101; C01P 2006/80 20130101; H01M 4/525
20130101; C01P 2002/77 20130101; Y02E 60/10 20130101 |
Class at
Publication: |
436/163 ;
429/231.1 |
International
Class: |
G01N 31/16 20060101
G01N031/16; H01M 4/48 20060101 H01M004/48 |
Claims
1. A cathode active material comprising a lithium mixed transition
metal oxide having a composition represented by Formula I below,
wherein the lithium mixed transition metal oxide is prepared by a
solid-state reaction of Li.sub.2CO.sub.3 and a mixed transition
metal precursor under an oxygen-deficient atmosphere, and having a
Li.sub.2CO.sub.3 content of less than 0.07% by weight of the
cathode active material as determined by pH titration:
Li.sub.xM.sub.yO.sub.2 (I) wherein: M=M'.sub.1-kA.sub.k, wherein M'
is Ni.sub.1-a-b(Ni.sub.1/2Mn.sub.1/2).sub.aCo.sub.b,
0.65.ltoreq.a+b.ltoreq.0.85 and 0.1.ltoreq.b.ltoreq.0.4; A is a
dopant; 0.ltoreq.k<0.05; and x+y.apprxeq.2 and
0.95.ltoreq.x.ltoreq.1.05.
2. The cathode active material according to claim 1, wherein
Li.sub.2CO.sub.3 is contained in an amount such that less than 20
mL of a 0.1M HCl titrant solution is added during pH titration of a
solution of water-soluble bases extracted from the cathode active
material to reach a value of less than 5, wherein the solution of
water-soluble bases is prepared by repeatedly soaking and decanting
10 g of the cathode active material with water such that the
resulting solution contains all of the water-soluble bases in the
cathode active material, and wherein the total volume of solution
of water-soluble base is 200 mL.
3. The cathode active material according to claim 2, wherein the
amount of the 0.1M HCl solution added during pH titration to reach
the pH of less than 5 is less than 10 mL.
4. The cathode active material according to claim 1, wherein the
oxygen-deficient atmosphere has an oxygen concentration of 10 to
50% by volume.
5. The cathode active material according to claim 4, wherein the
oxygen concentration is from 10% to 30% by volume.
6. The cathode active material according to claim 4, wherein the
oxygen-deficient atmosphere is an air atmosphere.
7. The cathode active material according to claim 1, wherein a
mixing ratio of Li.sub.2CO.sub.3 and the mixed transition metal
precursor for the solid-state reaction is from 0.95 to 1.04:1,
wherein the ratio of Li.sub.2CO.sub.3:mixed transition metal
precursor is by w/w.
8. The cathode active material according to claim 1, wherein the
mixed transition metal precursor is one or more selected from the
group consisting of M(OH).sub.2 and MOOH wherein M is as defined in
Formula I.
9. The cathode active material according to claim 8, wherein the
mixed transition metal precursor is MOOH, and is prepared by an
ammonia-free process.
10. The cathode active material according to claim 1, wherein the
mixed transition metal precursor has a tap density of 1.1 to 1.6
g/cm.sup.3.
11. The cathode active material according to claim 1, wherein the
solid-state reaction includes a sintering process at 600 to
1100.degree. C. for 3 to 20 hours.
12. The cathode active material according to claim 11, wherein an
amount of air exceeding 2 m.sup.3/kg LiMO.sub.2 during the
sintering process is supplied to a reaction vessel equipped with a
heat exchanger for pre-warming of the air.
13. The cathode active material according to claim 1, wherein the
lithium mixed transition metal oxide is prepared by a large-scale
process under a high rate of air circulation.
14. The cathode active material according to claim 13, wherein for
the high rate of air circulation during the sintering process, at
least 2 m.sup.3 of air at room temperature per 1 kg of the final
lithium mixed transition metal oxide is pumped into or out of the
reaction vessel.
15. The cathode active material according to claim 14, wherein at
least 10 m.sup.3 of air at room temperature per 1 kg of the final
lithium mixed transition metal oxide is pumped into or out of the
reaction vessel.
16. The cathode active material according to claim 12, wherein the
heat exchanger pre-warms the in-flowing air before the in-flowing
air enters the reaction vessel, while cooling the out-flowing
air.
17. A lithium secondary battery comprising the cathode active
material of claim 1.
18. A method for determining an amount of water-soluble base
contained in a cathode active material comprises determining an
amount of a 0.1 M HCl titrant solution neutralized by pH titration
of a solution containing the water-soluble base until the pH of the
solution reaches a value of less than 5, wherein the solution
containing the water-soluble base is prepared by repeatedly soaking
and decanting 10 g of the cathode active material with water until
the resulting solution contains all of the water-soluble base in
the cathode active material, and the total volume of the solution
of water-soluble base is 200 mL.
19. The method according to claim 18, wherein pH titration with an
addition of 0.1M HCl is carried out for 5 hours or less.
20. The cathode active material of claim 1, wherein a comparative
lithium transition metal oxide that is not prepared by the
solid-state reaction of Li.sub.2CO.sub.3 and a mixed transition
metal precursor under an oxygen-deficient atmosphere, has a
Li.sub.2CO.sub.3 content of greater than or equal to 0.07% as
determined by pH titration.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a cathode active material
containing a Ni-based lithium mixed transition metal oxide. More
specifically, the present invention relates to a cathode active
material which comprises a lithium mixed transition metal oxide
having a given composition, in which the lithium mixed transition
metal oxide is prepared by a solid-state reaction of
Li.sub.2CO.sub.3 with a mixed transition metal precursor under an
oxygen-deficient atmosphere, and has a Li.sub.2CO.sub.3 content of
less than 0.07% by weight of the cathode active material as
determined by pH titration.
BACKGROUND OF THE INVENTION
[0002] Technological development and increased demand for mobile
equipment have led to a rapid increase in the demand for secondary
batteries as an energy source. Among other things, lithium
secondary batteries having a high-energy density and voltage, a
long cycle lifespan and a low self-discharge rate are commercially
available and widely used.
[0003] As cathode active materials for the lithium secondary
batteries, lithium-containing cobalt oxide (LiCoO.sub.2) is largely
used. In addition, consideration has been made to using
lithium-containing manganese oxides such as LiMnO.sub.2 having a
layered crystal structure and LiMn.sub.2O.sub.4 having a spinel
crystal structure, and lithium-containing nickel oxides
(LiNiO.sub.2).
[0004] Of the aforementioned cathode active materials, LiCoO.sub.2
is currently widely used due to superior general properties
including excellent cycle characteristics, but suffers from low
safety, expensiveness due to finite resources of cobalt as a raw
material, and limitations in practical and mass application thereof
as a power source for electric vehicles (EVs) and the like.
[0005] Lithium manganese oxides, such as LiMnO.sub.2 and
LiMn.sub.2O.sub.4, are abundant resources as raw materials and
advantageously employ environmentally-friendly manganese, and
therefore have attracted a great deal of attention as a cathode
active material capable of substituting LiCoO.sub.2. However, these
lithium manganese oxides suffer from shortcomings such as low
capacity and poor cycle characteristics.
[0006] Whereas, lithium/nickel-based oxides including LiNiO.sub.2
are inexpensive as compared to the aforementioned cobalt-based
oxides and exhibit a high discharge capacity upon charging to 4.3
V. The reversible capacity of doped LiNiO.sub.2 approximates about
200 mAh/g which exceeds the capacity of LiCoO.sub.2 (about 165
mAh/g). Therefore, despite a slightly lower average discharge
voltage and a slightly lower volumetric density, commercial
batteries comprising LiNiO.sub.2 as the cathode active material
exhibit an improved energy density. To this end, a great deal of
intensive research is being actively undertaken on the feasibility
of applications of such nickel-based cathode active materials for
the development of high-capacity batteries. However, the
LiNiO.sub.2-based cathode active materials suffer from some
limitations in practical application thereof, due to the following
problems.
[0007] First, LiNiO.sub.2-based oxides undergo sharp phase
transition of the crystal structure with volumetric changes
accompanied by repeated charge/discharge cycling, and thereby may
suffer from cracking of particles or formation of voids in grain
boundaries. Consequently, intercalation/deintercalation of lithium
ions may be hindered to increase the polarization resistance,
thereby resulting in deterioration of the charge/discharge
performance. In order to prevent such problems, conventional prior
arts attempted to prepare a LiNiO.sub.2-based oxide by adding an
excess of a Li source and reacting reaction components under an
oxygen atmosphere. However, the thus-prepared cathode active
material, under the charged state, undergoes structural swelling
and destabilization due to the repulsive force between oxygen
atoms, and suffers from problems of severe deterioration in cycle
characteristics due to repeated charge/discharge cycles.
[0008] Second, LiNiO.sub.2 has shortcomings associated with the
evolution of excess gas during storage or cycling. That is, in
order to smoothly form the crystal structure, an excess of a Li
source is added during manufacturing of the LiNiO.sub.2-based
oxide, followed by heat treatment. As a result, water-soluble bases
including Li.sub.2CO.sub.3 and LiOH reaction residues remain
between primary particles and thereby they decompose or react with
electrolytes to thereby produce CO.sub.2 gas, upon charging.
Further, LiNiO.sub.2 particles have an agglomerate secondary
particle structure in which primary particles are agglomerated to
form secondary particles and consequently a contact area with the
electrolyte further increases to result in severe evolution of
CO.sub.2 gas, which in turn unfortunately leads to the occurrence
of battery swelling and deterioration of desirable high-temperature
safety.
[0009] Third, LiNiO.sub.2 suffers from a sharp decrease in the
chemical resistance of a surface thereof upon exposure to air and
moisture, and the gelation of slurries by polymerization of an
N-methyl pyrrolidone/poly(vinylidene fluoride) (NMP-PVDF) slurry
due to a high pH value. These properties of LiNiO.sub.2 cause
severe processing problems during battery production.
[0010] Fourth, high-quality LiNiO.sub.2 cannot be produced by a
simple solid-state reaction as is used in the production of
LiCoO.sub.2, and LiNiMO.sub.2 cathode active materials containing
an essential dopant cobalt and further dopants manganese and
aluminum are produced by reacting a lithium source such as
LiOH.H.sub.2O with a mixed transition metal hydroxide under an
oxygen or syngas atmosphere (i.e., a CO.sub.2-deficient
atmosphere), which consequently increases production costs.
Further, when an additional step, such as intermediary washing or
coating, is included to remove impurities in the production of
LiNiO.sub.2, this leads to a further increase in production
costs.
[0011] Many prior arts focus on improving properties of
LiNiO.sub.2-based cathode active materials and processes to prepare
LiNiO.sub.2. However, various problems, such as high production
costs, swelling due to gas evolution in the fabricated batteries,
poor chemical stability, high pH and the like, have not been
sufficiently solved. A few examples will be illustrated
hereinafter.
[0012] U.S. Pat. No. 6,040,090 (T. Sunagawa et al., Sanyo)
discloses a wide range of compositions including nickel-based and
high-Ni LiMO.sub.2, the materials having high crystallinity and
being used in Li-ion batteries in ethylene carbonate (EC)
containing an electrolyte. Samples were prepared on a small scale,
using LiOH.H.sub.2O as a lithium source. The samples were prepared
in a flow of synthetic air composed of a mixture of oxygen and
nitrogen, free of CO.sub.2.
[0013] U.S. Pat. No. 5,264,201 (J. R. Dahn et al.) discloses a
doped LiNiO.sub.2 substantially free of lithium hydroxides and
lithium carbonates. For this purpose, lithium hydroxide and
LiOH.H.sub.2O as a lithium source are employed and heat treatment
is performed under an oxygen atmosphere free of CO.sub.2,
additionally with a low content of H.sub.2O. An excess of lithium
"evaporates"; however, "evaporation" is a lab-scale effect and not
an option for large-scale preparation. That is, when applied to a
large-scale production process, it becomes difficult to evaporate
excess lithium, thereby resulting in problems associated with the
formation of lithium hydroxides and lithium carbonates.
[0014] U.S. Pat. No. 5,370,948 (M. Hasegawa et al., Matsushita)
discloses a process for the production of Mn-doped
LiNi.sub.1-xMn.sub.xO.sub.2 (x<0.45), wherein the manganese
source is manganese nitrate, and the lithium source is either
lithium hydroxide or lithium nitrate.
[0015] U.S. Pat. No. 5,393,622 (Y. Nitta et al., Matsushita)
discloses a process to prepare LiNi.sub.1-xMn.sub.xO.sub.2 by a
two-step heating, involving pre-drying, cooking and the final
heating. The final heating is done in an oxidizing gas such as air
or oxygen. This patent focuses on oxygen. The disclosed method uses
a very low temperature of 550 to 650.degree. C. for cooking, and
less than 800.degree. C. for sintering. At higher temperatures,
samples deteriorate dramatically. Excess lithium is used such that
the final samples contain a large amount of water-soluble base
(i.e., lithium compounds). According to the research performed by
the inventors of the present invention, the observed deterioration
is attributable to the presence of lithium salts as impurities
which melt at about 700 to about 800.degree. C., thereby detaching
the crystallites.
[0016] WO 9940029 A1 (M. Benz et al., H. C. Stack) describes a
complicated preparation method very different from that disclosed
in the present invention. This preparation method involves the use
of lithium nitrates and lithium hydroxides and recovering the
evolved noxious gasses. The sintering temperature never exceeds
800.degree. C. and typically is far lower.
[0017] U.S. Pat. No. 4,980,080 (Lecerf, SAFT) describes a process
to prepare LiNiO.sub.2-based cathodes from lithium hydroxides and
metal oxides at temperatures below 800.degree. C.
[0018] In prior arts including the above, LiNiO.sub.2-based cathode
active materials are generally prepared by high cost processes, in
a specific reaction atmosphere, especially in a flow of synthetic
gas such as oxygen or synthetic air, free of CO.sub.2, and using
LiOH.H.sub.2O, Li nitrate, Li acetate, etc., but not the
inexpensive, easily manageable Li.sub.2CO.sub.3. Furthermore, the
final cathode active materials have a high content of soluble
bases, originating from carbonate impurities present in the
precursors, which remain in the final cathode because of the
thermodynamic limitation. Further, the crystal structure of the
final cathode active materials per se is basically unstable even
when the final cathode active materials are substantially free of
soluble bases. Consequently, upon exposure to air containing
moisture or carbon dioxide during storage of the active materials,
lithium is released to surfaces from the crystal structure and
reacts with air to thereby result in continuous formation of
soluble bases.
[0019] Meanwhile, Japanese Unexamined Patent Publication Nos.
2004-281253, 2005-150057 and 2005-310744 disclose oxides having a
composition formula of Li.sub.aMn.sub.xNi.sub.yM.sub.zO.sub.2 (M=Co
or Al, 1.ltoreq.a.ltoreq.1.2, 0.ltoreq.x.ltoreq.0.65,
0.35.ltoreq.y.ltoreq.1, 0.ltoreq.z.ltoreq.0.65, and x+y+z=1). These
inventions provide a method of preparing the oxide involving mixing
each transition metal precursor with a lithium compound, grinding,
drying and sintering the mixture, and re-grinding the sintered
composite oxide by ball milling, followed by heat treatment. In
addition, working examples disclosed in the above prior art employ
substantially only LiOH as a lithium source. Further, it was found
through various experiments conducted by the inventors of the
present invention that the aforesaid prior art composite oxide
suffers from significant problems associated with a
high-temperature safety, due to production of large amounts of
impurities such as Li.sub.2CO.sub.3.
[0020] Alternatively, encapsulation of high Ni--LiNiO.sub.2 by
SiO.sub.x protective coating has been proposed (H. Omanda, T.
Brousse, C. Marhic, and D. M. Schleich, J. Electrochem. Soc. 2004,
151, A922.), but the resulting electrochemical properties are very
poor. In this connection, the inventors of the present invention
have investigated the encapsulation by LiPO.sub.3 glass. Even where
a complete coverage of the particle is accomplished, a significant
improvement of air-stability could not be made and electrochemical
properties were poor.
[0021] Therefore, there is a strong need for the development of a
LiNiO.sub.2-based cathode active material that can be produced at a
low cost from inexpensive precursors, and which show improved
properties such as low swelling when applied to commercial lithium
secondary batteries, improved chemical stability and improved
structural safety, and high capacity.
SUMMARY OF THE INVENTION
[0022] Therefore, the present invention is provided herewith in
view of the above problems and other technical problems that have
yet to be resolved.
[0023] As a result of a variety of extensive and intensive studies
and experiments and in view of the problems as described above, the
inventors of the present invention provide herewith a cathode
active material, containing a lithium mixed transition metal oxide
having a given composition, prepared by a solid-state reaction of
Li.sub.2CO.sub.3 with a mixed transition metal precursor under an
oxygen-deficient atmosphere, and being substantially free of
Li.sub.2CO.sub.3, exhibits a high capacity, excellent cycle
characteristics, significantly improved storage and
high-temperature stability, and can be produced with low production
costs and improved production efficiency. The present invention has
been completed based on these findings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] 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:
[0025] FIG. 1 is a schematic view showing a crystal structure of a
conventional Ni-based lithium transition metal oxide;
[0026] FIG. 2 is a schematic view showing a crystal structure of a
Ni-based lithium mixed transition metal oxide prepared by a method
according to one embodiment;
[0027] FIGS. 3 and 4 are graphs showing a preferred composition
range of a Ni-based lithium mixed transition metal oxide prepared
by a method according to one embodiment;
[0028] FIG. 5 is an FESEM (Field Emission Scanning Electron
Microscope) image (x 2,000) showing LiNiMO.sub.2 according to
Example 1. 5A: 850.degree. C.; 5B: 900.degree. C.; 5C: 950.degree.
C.; and 5D: 1,000.degree. C.;
[0029] FIG. 6 is an FESEM image showing commercial LiMO.sub.2
(M=Ni.sub.0.8Co.sub.0.2) according to Comparative Example 1. 6A:
FESEM image of a sample as received, and 6B: FESEM image of a
sample after heating to 850.degree. C. in air;
[0030] FIG. 7 is an FESEM image showing the standard pH titration
curve of commercial high-Ni LiNiO.sub.2 according to Comparative
Example 2. A: Sample as received, B: After heating of a sample to
800.degree. C. under an oxygen atmosphere, and C: Copy of A;
[0031] FIG. 8 is a graph showing a pH titration curve of a sample
according to Comparative Example 3 during storage of the sample in
a wet chamber. A: Sample as received, B: After storage of a sample
for 17 hrs, and C: After storage of a sample for 3 days;
[0032] FIG. 9 is a graph showing a pH titration curve of a sample
according to Example 2 during storage of the sample in a wet
chamber. A: Sample as received, B: After storage of a sample for 17
hrs, and C: After storage of a sample for 3 days;
[0033] FIG. 10 is a graph showing lengths of a-axis and c-axis of
crystallographic unit cells of samples having different ratios of
Li:M in Experimental Example 3;
[0034] FIG. 11 is an SEM image of a sample according to Example
4;
[0035] FIG. 12 shows the Rietveld refinement on X-ray diffraction
patterns of a sample according to Example 4;
[0036] FIG. 13 is an SEM image (x 5000) of a precursor in Example
5, which is prepared by an inexpensive ammonia-free process and has
a low density;
[0037] FIG. 14 is a graph showing electrochemical properties of
LiNiMO.sub.2 according to the present invention in Experimental
Example 1. 14A: Graph showing voltage profiles and rate
characteristics at room temperature (1 to 7 cycles); 14B: Graph
showing cycle stability at 25.degree. C. and 60.degree. C. and a
rate of C/5 (3.0 to 4.3V); and 14C: Graph showing discharge
profiles (at C/10 rate) for Cycle 2 and Cycle 31, obtained during
cycling at 25.degree. C. and 60.degree. C.;
[0038] FIG. 15 is a graph showing DSC (differential scanning
calorimetry) values for samples of Comparative Examples 3 and 4 in
Experimental Example 2. A: Commercial Al/Ba-modified LiNiO.sub.2 of
Comparative Example 3, and B: Commercial AlPO.sub.4-coated
LiNiO.sub.2 of Comparative Example 4;
[0039] FIG. 16 is a graph showing DSC values for LiNiMO.sub.2
according to Example 3 in Experimental Example 2;
[0040] FIG. 17 is a graph showing electrophysical properties of a
polymer cell according to one embodiment in Experimental Example 3;
and
[0041] FIG. 18 is a graph showing swelling of a polymer cell during
high-temperature storage in Experimental Example 3.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0042] In accordance with an aspect of the present invention, the
above can be accomplished by the provision of a cathode active
material comprising a lithium mixed transition metal oxide having a
composition represented by Formula I below, prepared by a
solid-state reaction of Li.sub.2CO.sub.3 and a mixed transition
metal precursor under an oxygen-deficient atmosphere, and having a
Li.sub.2CO.sub.3 content of less than 0.07% by weight of the
cathode active material as determined by pH titration:
Li.sub.xM.sub.yO.sub.2 (I)
[0043] wherein:
[0044] M=M'.sub.1-kA.sub.k, wherein M' is
Ni.sub.1-a-b(Ni.sub.1/2Mn.sub.1/2).sub.aCo.sub.b,
0.65.ltoreq.a+b.ltoreq.0.85 and 0.1.ltoreq.b.ltoreq.0.4;
[0045] A is a dopant;
[0046] 0.ltoreq.k<0.05; and
[0047] x+y.apprxeq.2 and 0.95.ltoreq.x.ltoreq.1.05.
[0048] Therefore, owing to a low Li.sub.2CO.sub.3 content of less
than 0.07% by weight, the cathode active material comprising a
high-Ni lithium mixed transition metal oxide having a the above
composition so provided in accordance with the present invention
has excellent sintering and storage stability, excellent
high-temperature stability including decreased gas evolution, and a
high capacity and excellent cycle characteristics due to a stable
crystal structure. The cathode active material is prepared by a
simple solid-state reaction in air, using a raw material that is
environmentally-friendly, cheap and easy to handle, so the present
invention can be applied to industrial-scale production of the
cathode active material, at low production cost and high production
efficiency.
[0049] As used herein, the term "high-Ni" means that a content of
nickel is high relative to the other transition metals present
which constitute the lithium mixed transition metal oxide, such as
nickel, manganese, cobalt, and the like. Hereinafter, where
appropriate throughout the specification, the term "lithium mixed
transition metal oxide in accordance with the present invention" is
used interchangeably with the term "LiNiMO.sub.2". Therefore, NiM
in LiNiMO.sub.2 is a suggestive expression representing a complex
composition of Ni, Mn and Co and a high-Ni content in Formula
I.
[0050] The composition of the lithium mixed transition metal oxide
should satisfy the following specific requirements as defined in
Formula I or as shown in FIG. 3:
[0051] (i) Ni.sub.1-(a+b)(Ni.sub.1/2Mn.sub.1/2).sub.aCo.sub.b and
0.65.ltoreq.a+b.ltoreq.0.85
[0052] (ii) 0.1.ltoreq.b.ltoreq.0.4, and
[0053] (iii) x+y z.apprxeq.2 and 0.95.ltoreq.x.ltoreq.1.05
[0054] Regarding the aforementioned requirement (i), Ni.sub.1-(a+b)
means a content of Ni.sup.3+. Therefore, if a mole fraction of
Ni.sup.3+ exceeds 0.35 (a+b<0.65), it is impossible to implement
an industrial-scale production in air, using Li.sub.2CO.sub.3 as a
raw material, so the lithium mixed transition metal oxide should be
produced using LiOH as a raw material under an oxygen atmosphere,
thereby presenting a problems associated with decreased production
efficiency and consequently increased production costs. On the
other hand, if a mole fraction of Ni.sup.3+ is lower than 0.15
(a+b>0.85), the capacity per volume of LiNiMO.sub.2 is not
competitive as compared to LiCoO.sub.2.
[0055] With regard to the aforementioned requirement (ii), a
content of cobalt (b) is from 0.1 to 0.4. If the content of cobalt
is excessively high (b>0.4), the overall cost of a raw material
increases due to high content of cobalt, and the reversible
capacity decreases. On the other hand, if the content of cobalt is
excessively low (b<0.1), it is difficult to achieve sufficient
rate characteristics and a high powder density of the battery at
the same time.
[0056] Meanwhile, taking into consideration both of the above
requirements (i) and (ii), the total mole fraction of Ni including
Ni.sup.2+ and Ni.sup.3+ in LiNiMO.sub.2 of the present invention is
specifically a relatively nickel-excess as compared to manganese
and cobalt and can be from 0.4 to 0.7. If a content of nickel is
excessively low, it is difficult to achieve a high capacity.
Conversely, if a content of nickel is excessively high, the safety
can be significantly lowered. In conclusion, the lithium transition
metal oxide (LiNiMO.sub.2) exhibits a large volume capacity and low
raw material costs, as compared to lithium cobalt-based oxides.
[0057] Further, if the mole fraction of Ni.sup.2+ is too high
relative to the Ni content, the cation mixing increases to thereby
result in formation of an excessively stable "rock salt" type
structure that is locally and electrochemically non-reactive, where
such a rock salt structure not only hinders charge/discharge and
but also can bring about a decrease in a discharge capacity. On the
other hand, if the mole fraction of Ni.sup.2+ is too low, this can
lead to an increase in the structural instability which thereby
lowers the cycle stability. Therefore, the mole fraction of
Ni.sup.2+ should be appropriately adjusted taking into
consideration such problems that can occur. Specifically, within
the range as shown in FIG. 3, the mole fraction of Ni.sup.2+ can be
from 0.05 to 04, based on the total content of Ni.
[0058] With regard to the aforementioned condition (iii), if a
content of lithium is excessively high, i.e. x>1.05, this may
result in a problem of decreased stability during charge/discharge
cycling, particularly at T=60.degree. C. and a high voltage (U=4.35
V). On the other hand, if a content of lithium is excessively low,
i.e. x<0.95, this may result in poor rate characteristics and a
decreased reversible capacity.
[0059] In an embodiment, LiNiMO.sub.2 may further comprise trace
amounts of dopants. Examples of the dopants may include aluminum,
titanium, magnesium and the like, which are incorporated into the
crystal structure. Further, other dopants, such as B, Ca, Zr, F, P,
Bi, Al, Mg, Zn, Sr, Ga, In, Ge, and Sn, may be included via the
grain boundary accumulation or surface coating of the dopants
without being incorporated into the crystal structure. These
dopants are included in amounts sufficient to increase the safety,
capacity and overcharge stability of the battery while not causing
a significant decrease in the reversible capacity. Therefore, a
content of the dopant is less than 5% by mole (k<0.05), as
defined in Formula I. In addition, the dopants may be specifically
added in an amount of <1% by mole, within a range that can
improve the stability without causing deterioration of the
reversible capacity.
[0060] Typically, Ni-based lithium mixed transition metal oxides
contain large amounts of water-soluble bases such as lithium
oxides, lithium sulfates, lithium carbonates, and the like. These
water-soluble bases may be bases, such as Li.sub.2CO.sub.3 and
LiOH, present in LiNiMO.sub.2, or otherwise can be bases produced
by ion exchange (H.sup.+ (water) .rarw. .fwdarw. Li.sup.+ (surface,
an outer surface of the bulk)), performed at the surface of
LiNiMO.sub.2. The bases of the latter case are usually present at a
negligible level.
[0061] The former water-soluble bases may be formed due to the
presence of unreacted lithium raw materials primarily upon
sintering. This is because as production of conventional Ni-based
lithium mixed transition metal oxides involves an addition of
relatively large amounts of lithium and a low-temperature sintering
process so as to prevent the disintegration of a layered crystal
structure, the resulting particles have relatively large amounts of
grain boundaries as compared to the cobalt-based oxides, and a
sufficient reaction of lithium ions is not realized upon
sintering.
[0062] In addition, even when an initial amount of Li.sub.2CO.sub.3
is low, Li.sub.2CO.sub.3 may also be produced during fabrication of
the battery or storage of electrode active materials. These
water-soluble bases react with electrolytes in the battery to
thereby cause gas evolution and battery swelling, which
consequently result in severe deterioration of the high-temperature
safety.
[0063] On the other hand, since the cathode active material in
accordance with the present invention, as defined above, stably
maintains the layered crystal structure by a specific composition
of transition metal elements and a reaction atmosphere, despite the
use of Li.sub.2CO.sub.3 as a raw material, it is possible to carry
out the sintering process at a high-temperature, thereby resulting
in small amounts of grain boundaries. In addition, as retention of
unreacted lithium on surfaces of particles is prevented, the
particle surfaces are substantially free of water-soluble bases
such as lithium carbonates, lithium sulfates, and the like.
Accordingly, the present invention is characterized in that
Li.sub.2CO.sub.3 is contained in a trace amount of less than 0.07%
by weight of the cathode active material.
[0064] In the present invention, the content of Li.sub.2CO.sub.3
includes all of Li.sub.2CO.sub.3 remaining upon production of the
lithium mixed transition metal oxide, or Li.sub.2CO.sub.3 produced
during fabrication of the battery or storage of electrode active
materials.
[0065] The content of Li.sub.2CO.sub.3 refers to an extent that
upon titration of 200 mL of a solution containing a cathode active
material powder with an acid titrant solution, e.g., 0.1M HCl, the
acid titrant solution used to reach a pH of less than 5 is
specifically an amount of less than 20 mL, more specifically less
than 10 mL. Herein, 200 mL of the aforementioned solution contains
substantially all kinds of the water-soluble bases in the cathode
active material, and is prepared by repeatedly soaking and
decanting 10 g of the cathode active material with water. There are
no significant influences of parameters such as a total soaking
time of the cathode active material powder in water on the amount
of water-soluble base extracted.
[0066] Therefore, the content of Li.sub.2CO.sub.3 can be determined
in terms of an amount of acid solution titrant (e.g., HCl) used to
reach pH of less than 5, according to the following method. To
accomplish this, 5 g of a cathode active material powder is added
to 25 mL of water, followed by brief stirring to effect a soaking
process. About 20 mL of the clear solution is separated from the
powder after soaking by decanting, and the separated solutions are
pooled. Again, about 20 mL of water is added to the powder and the
resulting mixture is stirred, followed by decanting and pooling.
The soaking and decanting are repeated at least 5 times. In this
manner, a total of 100 mL of the clear solution containing
water-soluble bases is pooled. In another exemplary embodiment,
this process can be scaled such that 10 g of the cathode active
material is extracted with water by the above soaking and decanting
process at least 5 times to provide a total of 200 mL of a solution
containing the water-soluble base. A 0.1M HCl solution is added to
the thus-pooled solution, followed by pH titration with stirring.
The pH is recorded as a function of time. Experiments are
terminated when the pH reaches a value of less than 3, and the flow
rate may be selected such that titration takes about 20 to about 30
min.
[0067] One of important features of the present invention is that a
desired cathode active material is prepared by a solid-state
reaction of Li.sub.2CO.sub.3 and a mixed transition metal precursor
under an oxygen-deficient atmosphere.
[0068] In this way, it was found through various experiments
conducted by the inventors of the present invention that when
conventional high-nickel LiMO.sub.2 is sintered in air containing a
trace amount of CO.sub.2, LiMO.sub.2 decomposes with a decrease of
Ni.sup.3+ as shown in the following reaction below, during which
amounts of Li.sub.2CO.sub.3 impurities increase.
LiM.sup.3+O.sub.2+CO.sub.2.fwdarw.a
Li.sub.1-xM.sub.1+x1.sup.3+,2+O.sub.2+b
Li.sub.2CO.sub.3+cO.sub.2
[0069] This is believed to be due to that the decomposition of some
Ni.sup.3+ into Ni.sup.2+ upon sintering results in destabilization
of the crystal structure, which consequently leads to an oxide form
having excessive cation mixing, i.e. Li-deficient form of
Li.sub.1-aNi.sub.1+aO.sub.2 having transition metal cations
misplaced on lithium sites of the crystal structure, and lithium
ions, released from partial collapse of the crystal structure,
react with CO.sub.2 in air.
[0070] The conventional methods of preparing high-nickel LiMO.sub.2
thus suffer from the use of Li.sub.2CO.sub.3 as a raw material,
which brings about the evolution of CO.sub.2 due to decomposition
of Li.sub.2CO.sub.3, and which then thermodynamically hinders
further decomposition of Li.sub.2CO.sub.3 necessary for the
reaction even at a low partial pressure of CO.sub.2, consequently
resulting in no further progression of the reaction. In addition,
excessive addition of Li.sub.2CO.sub.3 is accompanied by a problem
of residual Li.sub.2CO.sub.3 after the reaction.
[0071] Therefore, in order to prevent such problems associated with
the lithium-deficiency and cation mixing and in order to increase a
relative amount of Ni.sup.3+ increased over that of the
conventional methods, the production reaction for lithium mixed
transition metal oxides could be carried out using an excessive
amount of LiOH.H.sub.2O as a lithium source, with a ratio of
M(OH).sub.2 and Li of 1:1.05 to 1.15 (M(OH).sub.2:Li-compound)
under a high-oxygen atmosphere.
[0072] However, LiOH.H.sub.2O (technical grade) contains primarily
>1% by weight Li.sub.2CO.sub.3 impurities that are not
decomposed or removed during the sintering process under an oxygen
atmosphere and therefore remain in the final product. Further, an
excess of the residual Li.sub.2CO.sub.3 accelerates the electrolyte
decomposition to thereby result in the evolution of gas. Therefore,
the conventional method suffered from various problems such as
disintegration of secondary particles into single primary
crystallites, lowered storage stability, and deterioration of the
high-temperature safety resulting from the gas evolution due to the
reaction of the residual Li.sub.2CO.sub.3 with the electrolyte in
the battery.
[0073] Further, the lithium mixed transition metal oxide prepared
by a conventional method has a layered crystal structure as shown
in FIG. 1, and desertion of lithium ions from the reversible
lithium layers in the charged state brings about swelling and
destabilization of the crystal structure due to the repulsive force
between oxygen atoms in the MO layers (mixed-transition metal oxide
layers), thus suffering from the problems associated with sharp
decreases in the capacity and cycle characteristics, resulting from
changes in the crystal structure due to repeated charge/discharge
cycles.
[0074] As a result of a variety of extensive and intensive studies
and experiments, the inventors of the present invention discovered
that when the lithium mixed transition metal oxide is prepared by a
solid-state reaction of Li.sub.2CO.sub.3 with the mixed transition
metal precursor under an oxygen-deficient atmosphere, it is
possible to produce a cathode active material containing the
lithium mixed transition metal oxide substantially free of
Li.sub.2CO.sub.3, i.e., having a Li.sub.2CO.sub.3 content of less
than 0.07% by weight, of the cathode active material as determined
by pH titration. In a specific embodiment, the Li.sub.2CO.sub.3
content is less than 0.05% by weight of the cathode active material
as determined by pH titration. In a more specific embodiment, the
Li.sub.2CO.sub.3 content is less than 0.035% by weight of the
cathode active material as determined by pH titration. In another
embodiment, a cathode active material comprising a comparative
lithium transition metal oxide that is not prepared by the
solid-state reaction of Li.sub.2CO.sub.3 and a mixed transition
metal precursor under an oxygen-deficient atmosphere, has a
Li.sub.2CO.sub.3 content of greater than or equal to 0.07% by
weight of the cathode active material.
[0075] Specifically, under the oxygen-deficient atmosphere,
desorption of some oxygen atoms takes place from the MO layers,
which leads to a decrease in an oxidation number of Ni, thereby
increasing amounts of Ni.sup.2+ ions. As a result, a portion of the
Ni.sup.2+ ions are inserted into the reversible lithium layers, as
shown in FIG. 2. However, contrary to conventionally known or
accepted ideas in the related art that
intercalation/deintercalation of lithium ions will be hindered due
to such insertion of Ni.sup.2+ ions into the reversible lithium
layers, an insertion of an effective amount of Ni.sup.2+ ions
prevents destabilization of the crystal structure that may occur
due to the repulsive force between oxygen atoms in the MO layers,
upon charge. As used herein, "an effective amount of Ni.sup.2+
ions", can include about 3 to about 7 mole percent of the total
amount of Ni ions present. Therefore, stabilization of the crystal
structure is achieved to result in no occurrence of further
structural collapse by oxygen desorption. Further, it is believed
that the lifespan characteristics and safety are simultaneously
improved, due to no further formation of Ni.sup.2+ ions with
maintenance of the oxidation number of Ni ions inserted into the
reversible lithium layers, even when lithium ions are released
during a charge process. Hence, it can be said that such a concept
of the present invention is a remarkable one which is completely
opposite to and overthrows the conventional idea.
[0076] Thus, the present invention can fundamentally prevent the
problems that may occur due to the presence of the residual
Li.sub.2CO.sub.3 in the final product (i.e., the cathode active
material), and provides a highly economical process by performing
the production reaction using a relatively small amount of
inexpensive Li.sub.2CO.sub.3 as a reactant and an oxygen-deficient
atmosphere such as air. Further, the sintering and storage
stabilities are excellent due to the stability of the crystal
structure, and thereby the battery capacity and cycle
characteristics can be significantly improved simultaneously with a
desired level of rate characteristics.
[0077] However, under an atmosphere with excessive
oxygen-deficiency, an excessive amount of Ni.sup.2+ ions transfer
to the reversible lithium layers during the synthesis process,
thereby resulting in hindrance of the intercalation/deintercalation
of lithium ions, and therefore the performance of the battery
cannot be exerted sufficiently. On the other hand, if the oxygen
concentration is excessively high, the desired amount of Ni.sup.2+
cannot be inserted into the reversible lithium layers. Taking into
consideration such problems, the synthetic reaction may be carried
out under an atmosphere with an oxygen concentration of
specifically 10% to 50% by volume, and more specifically 10% to 30%
by volume. In an exemplary embodiment, he reaction can be carried
out under an air atmosphere.
[0078] Another feature of the present invention is that raw
materials produced by an inexpensive or economical process and
being easy to handle can be used, and particularly Li.sub.2CO.sub.3
which is difficult to employ in the prior art can be used itself as
a lithium source.
[0079] As an added amount of Li.sub.2CO.sub.3 as the lithium source
decreases, that is, a molar ratio (Li/M) of lithium to the mixed
transition metal source (M) decreases, an amount of Ni inserted
into the MO layers gradually increases. Therefore, if excessive
amounts of Ni ions are inserted into the reversible lithium layers,
the movement of Li.sup.+ ions during charge/discharge processes is
hampered, which thereby leads to problems associated with a
decrease in the capacity or deterioration of the rate
characteristics. On the other hand, if an added amount of
Li.sub.2CO.sub.3 is excessively large, that is, the Li/M molar
ratio is excessively high, the amount of Ni inserted into the
reversible lithium layers is excessively low, which may undesirably
lead to structural instability, thereby presenting decreased safety
of the battery and poor lifespan characteristics. Further, at a
high Li/M value, amounts of unreacted Li.sub.2CO.sub.3 increase to
thereby result in a high pH-titration value, i.e., production of
large amounts of impurities, and consequently the high-temperature
safety may deteriorate.
[0080] Therefore, in one embodiment, an added amount of
Li.sub.2CO.sub.3 as the lithium source can be from 0.95 to 1.04:1
where the ratio of Li.sub.2CO.sub.3:mixed transition metal raw
material is a w/w ratio, based on the weight of the mixed
transition metal as the other raw material.
[0081] As a result, the product is substantially free of impurities
due to a lack of surplus Li.sub.2CO.sub.3 in the product (the
cathode active material) by adding only a stoichiometric amount
(i.e., by not adding an excess) of the lithium source, so that
there are no problems associated with residual Li.sub.2CO.sub.3,
and thereby a relatively small amount of inexpensive
Li.sub.2CO.sub.3 is used to provide a lithium mixed transition
metal compound in a highly economical process.
[0082] As the mixed transition metal precursor, M(OH).sub.2 or MOOH
(M is as defined in Formula I) can be used specifically. As used
herein, the term "mixed" means that several transition metal
elements are well mixed at the atomic level.
[0083] In conventional processes, as the mixed transition metal
precursors, mixtures of Ni-based transition metal hydroxides are
generally employed. However, these materials commonly contain
carbonate impurities. This is because Ni(OH).sub.2 is prepared by
co-precipitation of a Ni-based salt such as NiSO.sub.4 with a base
such as NaOH in which the technical grade NaOH contains
Na.sub.2CO.sub.3 and the CO.sub.3.sup.2- anion is more easily
inserted into the Ni(OH).sub.2 structure than the OH anion.
[0084] Further, in order to increase an energy density of the
cathode active material, conventional prior art processes employed
MOOH having a high tap density of 1.5 to 3.0. However, the use of
such a high tap density precursor makes it difficult to achieve the
incorporation of the reactant (lithium) into the inside of the
precursor particles during the synthetic process, which then lowers
the reactivity to thereby result in production of large amounts of
impurities. Further, for preparation of MOOH having a high tap
density, co-precipitation of MSO.sub.4 and NaOH should be carried
out in the presence of excess ammonia as a complexing additive.
However, ammonia in waste water causes environmental problems and
thus is strictly regulated. It is, however, generally not possible
to prepare the mixed oxyhydroxide having high density by an
ammonia-free process that is less expensive, is more
environmentally friendly and is easier to carry out than this
process.
[0085] However, according to the research performed by the
inventors of the present invention, it was found that even though
the mixed transition metal precursor prepared by the ammonia-free
process exhibits a relatively low tap density, a lithium mixed
transition metal oxide prepared using the thus-prepared precursor
which has an excellent sintering stability makes it is possible to
prepare a mixed transition metal oxide having a superior
reactivity.
[0086] In this way, the cathode active material in accordance with
the present invention, as discussed hereinbefore, can maintain a
well-layered structure due to the insertion of some Ni ions into
the reversible lithium layers, thus exhibiting excellent sintering
stability. Accordingly, the present invention can employ the mixed
transition metal precursor having a low tap density, as the raw
material.
[0087] Therefore, since the raw material, i.e., the mixed
transition metal precursor, is environmentally friendly, can be
easily prepared at low production costs and also has a large volume
of voids between primary particles, e.g. a low tap density, it is
possible to easily realize the introduction of the lithium source
into the inside of the precursor particles, thereby improving the
reactivity, and it is also possible to prevent production of
impurities and reduce an amount of the lithium source
(Li.sub.2CO.sub.3) to be used, so the method of the present
invention is highly economical.
[0088] As used herein, the term "ammonia-free process" means that
only NaOH without the use of aqueous ammonia is used as a
co-precipitating agent in a co-precipitation process of a metal
hydroxide. That is, the transition metal precursor is obtained by
dissolving a metal salt such as MSO.sub.4 and MNO.sub.3 (M is a
metal of a composition to be used) in water, and gradually adding a
small amount of a precipitating agent NaOH with stirring. The
introduction of ammonia lowers the repulsive force between
particles to thereby result in densification of co-precipitated
particles, which then increases a density of particles. However,
when it is desired to obtain a hydroxide having a low tap density
as in the present invention, there is no need to employ ammonia. In
addition to the above-exemplified sulfates and nitrates, other
materials may be employed as the metal salt.
[0089] In one specific embodiment, the tap density of the mixed
transition metal precursor may be from 1.1 to 1.6 g/cm.sup.3. If
the tap density is excessively low, a chargeable amount of the
active material decreases, so the capacity per volume may be
lowered. On the other hand, if the tap density is excessively high,
the reactivity with the lithium source material is lowered and
therefore impurities may be undesirably formed.
[0090] The solid-state reaction includes a sintering process
specifically at 600 to 1,100.degree. C. for 3 to 20 hours, and more
specifically 800 to 1,050.degree. C. for 5 to 15 hours. If the
sintering temperature is excessively high, this may lead to
non-uniform growth of particles, and reduction of the volume
capacity of the battery due to a decreased amount of particles that
can be contained per unit area, arising from an excessively large
size of particles. On the other hand, if the sintering temperature
is excessively low, an insufficient reaction leads to the retention
of the raw materials in the particles, thereby presenting the risk
of damaging the high-temperature safety of the battery, and it may
be difficult to maintain a stable structure, due to the
deterioration of the volume density and crystallinity. Further, if
the sintering time is too short, it is difficult to obtain a
lithium nickel-based oxide having high crystallinity. On the other
hand, if the sintering time is too long, this may undesirably lead
to excessively large particle diameter and reduced production
efficiency.
[0091] Meanwhile, various additional parameters may arise as the
process for preparation of the lithium mixed transition metal oxide
is scaled up. A few grams of samples in a furnace behave very
differently from a few kg of samples, because the gas transport
kinetics at a low partial pressure is completely different.
Specifically, in a small-scale process, Li evaporation occurs and
CO.sub.2 transport is fast, whereas in a large-scale process, these
processes are retarded. Where the Li evaporation and CO.sub.2
transport are retarded, a gas partial pressure in the furnace
increases, which in turn hinders further decomposition of
Li.sub.2CO.sub.3 necessary for the reaction, consequently resulting
in retention of the unreacted Li.sub.2CO.sub.3, and the resulting
LiNiMO.sub.2 decomposes to result in the destabilization of the
crystal structure.
[0092] Accordingly, when it is desired to prepare the lithium mixed
transition metal oxide in accordance with the present invention on
a large-scale, the preparation process is specifically carried out
under a high rate of air circulation. As used herein, the term
"large scale" means that a sample has a size of 5 kg or more
because similar behavior is expected in 100 kg of sample when the
process has been correctly scaled-up, i.e., a similar gas flow
(m.sup.3/kg of sample) reaches 100 kg of sample.
[0093] In order to achieve high air circulation upon the production
of the lithium transition metal oxide by the large-scale mass
production process, specifically at least 2 m.sup.3 (volume at room
temperature) of air, and more specifically at least 10 m.sup.3 of
air, per kg of the final product (active material), i.e., lithium
mixed transition metal oxide, may be pumped into or out of a
reaction vessel. As such, even when the present invention is
applied to a large-scale production process, it is possible to
prepare the cathode active material which is substantially free of
impurities including water-soluble bases.
[0094] In an embodiment of the present invention, a heat exchanger
may be employed to minimize energy expenditure upon air circulation
by pre-warming the in-flowing air before it enters the reaction
vessel, while cooling the out-flowing air.
[0095] In a specific example, air flow of 2 m.sup.3/kg corresponds
to about 1.5 kg of air at 25.degree. C. The heat capacity of air is
about 1 kJ/kg.degree.K and the temperature difference is about
800K. Thus, at least about 0.33 kWh is required per kg of the final
sample for air heating. Where the air flow is 10 m.sup.3, about 2
kWh is then necessary. Thus, the typical additional energy cost
amounts to about 2 to about 10% of the total cathode sales price.
The additional energy cost can be significantly reduced when the
air-exchange is made by using a heat exchanger. In addition, the
use of the heat exchanger can also reduce the temperature gradient
in the reaction vessel. To further decrease the temperature
gradient, it is recommended to provide several air flows into the
reaction vessel simultaneously.
[0096] The cathode active material in accordance with the present
invention may be comprised only of the lithium mixed transition
metal oxide having the above-specified composition or, where
appropriate, it may be comprised of the lithium mixed transition
metal oxide in conjunction with other lithium-containing transition
metal oxides.
[0097] Examples of the lithium-containing transition metal oxides
that can be used in the present invention may include, but are not
limited to, layered compounds such as lithium cobalt oxide
(LiCoO.sub.2) and lithium nickel oxide (LiNiO.sub.2), or compounds
substituted with one or more transition metals; lithium manganese
oxides such as compounds of Formula Li.sub.1+yMn.sub.2-yO.sub.4
(0.ltoreq.y.ltoreq.0.33), LiMnO.sub.3, LiMn.sub.2O.sub.3, and
LiMnO.sub.2; lithium copper oxide (Li.sub.2CuO.sub.2); vanadium
oxides such as LiV.sub.3O.sub.8, V.sub.2O.sub.5, and
Cu.sub.2V.sub.2O.sub.7; Ni-site type lithium nickel oxides of
Formula LiNi.sub.1-yM.sub.yO.sub.2 (M=Co, Mn, Al, Cu, Fe, Mg, B, or
Ga, and 0.01.ltoreq.y.ltoreq.0.3); lithium manganese composite
oxides of Formula LiMn.sub.2-yM.sub.yO.sub.2 (M=Co, Ni, Fe, Cr, Zn,
or Ta, and 0.01.ltoreq.y.ltoreq.0.1), or Formula
Li.sub.2Mn.sub.3MO.sub.8 (M=Fe, Co, Ni, Cu, or Zn);
LiMn.sub.2O.sub.4 wherein a portion of Li is substituted with
alkaline earth metal ions; disulfide compounds; and
Fe.sub.2(MoO.sub.4).sub.3, LiFe.sub.3O.sub.4, and the like.
[0098] In accordance with another aspect of the present invention,
there is provided a lithium secondary battery comprising the
aforementioned cathode active material. The lithium secondary
battery is generally comprised of a cathode, an anode, a separator
and a lithium salt-containing non-aqueous electrolyte. Methods for
preparing the lithium secondary battery are well-known in the art
and therefore detailed description thereof will be omitted
herein.
[0099] In accordance with a further aspect of the present
invention, there is provided a method for determining the total
amount of water-soluble base contained in a cathode active material
from the amount of an aqueous acid solution titrant neutralized by
pH titration of a solution containing the water-soluble base until
the pH of the solution reaches a value of less than 5, wherein the
solution of water-soluble base is prepared by repeatedly soaking
and decanting 10 g of the cathode active material with water until
the resulting solution contains all of the water-soluble bases in
the cathode active material. In an embodiment, the acid solution
titrant is 0.1M HCl solution. In another embodiment, the total
volume of the solution of water-soluble base is 200 mL
[0100] That is, all of the water-soluble bases contained in a
cathode active material are readily dissolved by repeated soaking
and decanting of the cathode active material, so the amount of the
water-soluble bases can be precisely determined in a reproducible
manner. Therefore, it is possible using this method to predict
probable deterioration of high-temperature safety or cycle
characteristics that may occur due to the presence of impurities in
the battery fabricated using the cathode active material. Knowledge
about the content of the water-soluble bases can be a potent method
for use in the development of a cathode active material having
superior storage stability as disclosed herein.
[0101] Upon pH titration with addition of 0.1M HCl, this process is
generally negligible at normal speed (i.e., about 30 min), but is
carried out for 5 hours or less. This is because deviations of the
pH profile may occur in a slow ion-exchange process (H.sup.+ in the
solution .rarw. ! Li.sup.+ in the powder). Such deviations of the
pH profile would occur mostly at pH of less than about 5.
[0102] Upon only measuring pH, for example, as described in EP 1
317 008 A2, even a small amount of LiOH-type impurities can give a
higher pH than that obtained for a significantly harmful
Li.sub.2CO.sub.3 impurities. Therefore, it is important to measure
the pH profile in order to characterize which soluble bases are
present. Accordingly, in one preferred embodiment, it is possible
to understand the properties of the water-soluble bases from pH
profile, by recording of the pH profile upon pH titration.
[0103] Appropriate modifications may be made with kinds and
concentrations of acids used for pH titration, a reference pH and
the like, and it should be understood that those modifications are
apparent to those skilled in the art and fall within the scope of
the invention.
EXAMPLES
[0104] Now, the present invention will be described in more detail
with reference to the following Examples. These examples are
provided only for illustrating the present invention and should not
be construed as limiting the scope and spirit of the present
invention. For reference, the content of water-soluble bases
contained in the powder in the working examples was measured
according to the following method.
[0105] Contents and Characterization of Water-Soluble Bases (pH
Titration)
[0106] First, 5 g of a cathode active material powder was added to
25 mL of water, followed by brief stirring. About 20 mL of a clear
solution was separated after this soaking from the powder by
decanting and pooling the supernatant. Again, about 20 mL of water
was added to the powder and the resulting mixture was soaked and
stirred, followed by decanting and pooling. The soaking and
decanting were repeated at least 5 times. In this manner, a total
of 100 mL of the clear solution containing water-soluble bases was
pooled. A 0.1M HCl solution was added to the thus-pooled solution,
followed by pH titration with stirring. The pH was recorded as a
function of time. Experiments were terminated when the pH reached a
value of less than about 3, and a flow rate was appropriately
selected within a range in which titration takes about 20 to about
30 min. The content of the water-soluble bases was measured as an
amount of acid that was used until the pH reaches a value of less
than about 5. Characterization of water-soluble bases was made from
the pH profile.
Example 1
[0107] A mixed oxyhydroxide of Formula MOOH
(M=Ni.sub.4/15(Mn.sub.1/2Ni.sub.1/2).sub.8/15Co.sub.0.2) as a mixed
transition metal precursor and Li.sub.2CO.sub.3 were mixed in a
stoichiometric molar ratio (Li:M=1.02:1), and the mixture was
sintered in air at temperatures of 850 (Ex. 1A), 900 (Ex. 1B), 950
(Ex. 1C), and 1,000.degree. C. (Ex. 1D) for 10 hours, to prepare a
lithium mixed transition metal oxide. Herein, secondary particles
were maintained intact without being collapsed, and the crystal
size increased with an increase in the sintering temperature.
[0108] X-ray analysis showed that all samples have a well-layered
crystal structure. Further, a unit cell volume did not exhibit a
significant change with an increase in the sintering temperature,
thus representing that there was no significant oxygen-deficiency
and no significant increase of cation mixing, in conjunction with
essentially no occurrence of lithium evaporation.
[0109] The crystallographic data for the thus-prepared lithium
mixed transition metal oxide are given in Table 1 below, and FESEM
images thereof are shown in FIG. 5. From these results, it was
found that the lithium mixed transition metal oxide is LiNiMO.sub.2
having a well-layered crystal structure with the insertion of
nickel at a level of 3.9 to 4.5% into the reversible lithium layer.
Further, it was also found that even though Li.sub.2CO.sub.3 was
used as a raw material and sintering was carried out in air, a
sufficient amount of Ni.sup.2+ ions was inserted into the lithium
layer, thereby achieving the desired structural stability.
[0110] Particularly, Sample B, sintered at 900.degree. C. (Ex. 1B),
exhibited a high c:a ratio and therefore excellent crystallinity, a
low unit cell volume and a reasonable cation mixing ratio. As a
result, Sample B showed the most excellent electrochemical
properties, and a BET surface area of about 0.4 to about 0.8
m.sup.2/g. TABLE-US-00001 TABLE 1 Example 1 (A-D) (A) (B) (C) (D)
Sintering temp. 850.degree. C. 900.degree. C. 950.degree. C.
1,000.degree. C. Unit cell vol. 33.902 .ANG..sup.3 33.921
.ANG..sup.3 33.934 .ANG..sup.3 33.957 .ANG..sup.3 Normalized c:a
ratio 1.0123 1.0122 1.0120 1.0117 c:a/24{circumflex over ( )}0.5
Cation mixing 4.5% 3.9% 4.3% 4.5% (Rietveld refinement)
Comparative Example 1
[0111] 50 g of a commercial sample having a composition of
LiNi.sub.0.8Co.sub.0.1Mn.sub.0.1O.sub.2 represented by Formula
LiNi.sub.1-xM.sub.xO.sub.2 (x=0.3, and
M=Mn.sub.1/3Ni.sub.1/3Co.sub.1/3) was heated in air to 750.degree.
C. (CEx. 1A), 850.degree. C. (CEx. 1B), 900.degree. C. (CEx. 1C)
and 950.degree. C. (CEx. 1D) (10 hrs), respectively.
[0112] X-ray analysis was carried out to obtain detailed lattice
parameters with high resolution. Cation mixing was observed by
Rietveld refinement, and morphology was analyzed by FESEM. The
results thus obtained are given in Table 2 below. Referring to
Table 2, it can be seen that all of the samples heated to a
temperature of T.gtoreq.750.degree. C. (CEx. 1A-D) exhibited
continuous degradation of a crystal structure (increased cation
mixing, increased lattice constant and decreased c:a ratio). FIG. 6
shows a FESEM image of a commercial sample as received and a FESEM
image of the same sample heated to 850.degree. C. (CEx. 1B) in air;
and it can be seen that the sample heated to a temperature of
T.gtoreq.850.degree. C. (CEx. 1B-D) exhibited structural collapse.
This is believed to be due to that Li.sub.2CO.sub.3, formed during
heating in air, melts to thereby segregate particles.
TABLE-US-00002 TABLE 2 Comp. Ex. 1 (A-D) (A) (B) (C) (D) Sintering
temp. 750.degree. C. 850.degree. C. 900.degree. C. 950.degree. C.
Unit cell vol. 33.902 .ANG..sup.3 33.920 .ANG..sup.3 33.934
.ANG..sup.3 33.957 .ANG..sup.3 Normalized c:a ratio 1.0103 1.0100
1.0090 1.0085 c:a/24{circumflex over ( )}0.5 Cation mixing 10% 12%
15% 18% (Rietveld refinement)
[0113] Therefore, it can be seen that it is impossible to produce a
conventional lithium mixed transition metal oxide in the air
containing trace amounts of carbon dioxide, due to thermodynamic
limitations. In addition, upon producing the lithium mixed
transition metal oxide according to a conventional method, the use
of Li.sub.2CO.sub.3 as a raw material is accompanied by evolution
of CO.sub.2 due to decomposition of Li.sub.2CO.sub.3, thereby
leading to thermodynamic hindrance of further decomposition of
Li.sub.2CO.sub.3 necessary for the reaction, consequently resulting
in no further progression of the reaction. For these reasons, it
was shown that such a conventional method cannot be applied to the
practical production process.
Comparative Example 2
[0114] The pH titration was carried out at a flow rate of >2
L/min for 400 g of a commercial sample having a composition of
LiNi.sub.0.8Co.sub.0.2O.sub.2. The results thus obtained are given
in FIG. 7. In FIG. 7, Curve A (CEx. 2A) represents pH titration for
the sample as received, and Curve B (CEx. 2B) represents pH
titration for the sample heated to 800.degree. C. in a flow of pure
oxygen for 24 hours. From the analysis results of pH profiles, it
can be seen that the contents of Li.sub.2CO.sub.3 before and after
heat treatment were the same therebetween, and there was no
reaction of Li.sub.2CO.sub.3 impurities. That is, it can be seen
that the heat treatment under an oxygen atmosphere resulted in no
additional production of Li.sub.2CO.sub.3 impurities, but
Li.sub.2CO.sub.3 impurities present in the particles were not
decomposed. Through slightly increased cation mixing, a slightly
decreased c:a ratio and a slightly decreased unit cell volume from
the X-ray analysis results, it was found that the content of Li
slightly decreased in the crystal structure of LiNiO.sub.2 in
conjunction with the formation of a small amount of Li.sub.2O.
Therefore, it can be seen that it is impossible to prepare a
stoichiometric lithium mixed transition metal oxide with no
impurities and no lithium-deficiency in a flow of oxygen gas or
synthetic air.
Comparative Example 3
[0115] LiAl.sub.0.02Ni.sub.0.78Co.sub.0.2O.sub.2 containing less
than 3% aluminum compound, as commercially available
Al/Ba-modified, high-nickel LiNiO.sub.2, was stored in a wet
chamber (90% relative humidity, abbreviated "RH") at 60.degree. C.
in air. The pH titration was carried out for a sample prior to
exposure to moisture, and samples wet-stored for 17 hrs and 3 days,
respectively. The results thus obtained are given in FIG. 8.
Referring to FIG. 8, an amount of water-soluble bases was low
before storage, but substantial amounts of water-soluble bases,
primarily comprising Li.sub.2CO.sub.3, were continuously formed
upon exposure to air. Therefore, even when an initial amount of
Li.sub.2CO.sub.3 impurities was low, it was revealed that the
commercially available high-nickel LiNiO.sub.2 is not stable in air
and therefore rapidly decomposes at a substantial rate, and
substantial amounts of Li.sub.2CO.sub.3 impurities are formed
during storage.
Example 2
[0116] The pH titration was carried out for a sample of the lithium
mixed transition metal oxide in accordance with Example 2 prior to
exposure to moisture, and samples stored in a wet chamber (90% RH)
at 60.degree. C. in air for 17 hours and 3 days, respectively. The
results thus obtained are given in FIG. 9.
[0117] Upon comparing the lithium mixed transition metal oxide of
Example 2 (see FIG. 9) with the sample of Comparative Example 3
(see FIG. 8), the sample of Comparative Example 3 (stored for 17
hours) exhibited consumption of about 20 mL of HCl, whereas the
sample of Example 2 (stored for 17 hours) exhibited consumption of
10 mL of HCl, thus showing an about two-fold decrease in production
of the water-soluble bases. Further, in 3-day-storage samples, the
sample of Comparative Example 3 exhibited consumption of about 110
mL of HCl, whereas the sample of Example 2 exhibited consumption of
26 mL of HCl, which corresponds to an about five-fold decrease in
production of the water-soluble bases. Therefore, it can be seen
that the sample of Example 2 decomposed at a rate about five-fold
slower than that of the sample of Comparative Example 3. Then, it
can be shown that the lithium mixed transition metal oxide of
Example 2 exhibits superior chemical resistance even when it is
exposed to air and moisture.
Comparative Example 4
[0118] A high-nickel LiNiO.sub.2 sample having a composition of
LiNi.sub.0.8Mn.sub.0.05Co.sub.0.15O.sub.2, as a commercial sample
which was surface-coated with AlPO.sub.4 followed by gentle heat
treatment, was subjected to pH titration before and after storage
in a wet chamber. As a result of pH titration, 12 mL of 0.1M HCl
was consumed per 10 g cathode, an initial content of
Li.sub.2CO.sub.3 was low, and the content of Li.sub.2CO.sub.3 after
storage was slightly lower (80 to 90%) as compared to the sample of
Comparative Example 3, but the content of Li.sub.2CO.sub.3 was
higher than that of Example 2. Consequently, it was found that the
aforementioned high-Ni LiNiO.sub.2 shows no improvements in the
stability against exposure to the air even when it was
surface-coated, and also exhibits insignificant improvements in the
electrochemical properties such as the cycle stability and rate
characteristics.
Example 3
[0119] Samples with different Li:M molar ratios were prepared from
MOOH (M=Ni.sub.4/15(Mn.sub.1/2Ni.sub.1/2).sub.8/15Co.sub.0.2).
Li.sub.2CO.sub.3 was used as a lithium source. Specifically, 7
samples each of about 50 g with Li:M ratios ranging from 0.925 to
1.12 were prepared by a sintering process in air at a temperature
of 910 to 920.degree. C. Then, electrochemical properties were
tested.
[0120] Table 3 below provides the obtained crystallographic data.
The unit cell volume changes smoothly according to the Li:M ratio.
FIG. 10 shows its crystallographic map. All samples are located on
a straight line. According to the results of pH titration, the
content of soluble base slightly increased with an increase of the
Li:M ratio, but the total amount thereof was small. Accordingly,
the soluble base probably originates from the surface basicity
(i.e., is present by an ion exchange mechanism) but not from the
dissolution of Li.sub.2CO.sub.3 impurities as observed in
Comparative Example 1.
[0121] Therefore, this experiment clearly shows that the lithium
mixed transition metal oxide prepared by the method in accordance
with the present invention is in the Li stoichiometric range and
additional Li is inserted into the crystal structure. In addition,
it can be seen that stoichiometric samples without Li.sub.2CO.sub.3
impurity can be obtained even when Li.sub.2CO.sub.3 is used as a
precursor and the sintering is carried out in air.
[0122] That is, as the Li/M molar ratio decreases, the amount of
Ni.sup.2+ ions inserted into the reversible lithium layer gradually
increases. Insertion of excessively large amounts of Ni.sup.2+ into
the reversible lithium layer hinders the movement of Li.sup.+
during the charge/discharge process, thereby resulting in decreased
capacity or poor rate characteristics. On the other hand, if the
Li/M molar ratio is excessively high, the amount of Ni.sup.2+
inserted into the reversible lithium layer is too low, which may
result in structural instability leading to deterioration of the
battery safety and lifespan characteristics. Further, at the high
Li/M value, amounts of unreacted Li.sub.2CO.sub.3 increase to
thereby result in a high pH-titration value. Therefore, upon
considering the performance and safety of the battery, the molar
ratio of Li:M is specifically from 0.95 to 1.04 (Samples B, C and
D) to ensure that the value of Ni.sup.2+ inserted into the lithium
layer is from 3 to 7%. TABLE-US-00003 TABLE 3 Samples A B C D E F G
Li:M ratio (molar) 0.925 0.975 1.0 1.025 1.05 1.075 1.125 Unit cell
vol. 34.111 .ANG..sup.3 34.023 .ANG..sup.3 33.923 .ANG..sup.3
33.921 .ANG..sup.3 33.882 .ANG..sup.3 33.857 .ANG..sup.3 33.764
.ANG..sup.3 c:a ratio 1.0116 1.0117 1.0119 1.0122 1.0122 1.0123
1.0125 Cation mixing 8.8% 6.6% 4.7% 4.0% 2.1% 2.5% 1.4% pH 3 3.5 5
9 15 19 25
Example 4
[0123] Li.sub.2CO.sub.3 and a mixed oxyhydroxide of Formula MOOH
(M=Ni.sub.4/15(Mn.sub.1/2Ni.sub.1/2).sub.8/15Co.sub.0.2) were
introduced into a furnace with an about 20 L chamber and sintered
at 920.degree. C. for 10 hours, during which more than 10 m.sup.3
of air was pumped into the furnace, thereby preparing about 5 kg of
LiNiMO.sub.2 in one batch.
[0124] After sintering was complete, a unit cell constant was
determined by X-ray analysis, and a unit cell volume was compared
with a target value (Sample B of Example 1: 33.921 .ANG..sup.3).
ICP analysis found that the molar ratio of Li and M is very close
to 1.00, and the unit cell volume was within the target range. FIG.
11 shows an scanning electron microscope (SEM) image of the
thus-prepared cathode active material and FIG. 12 shows results of
Rietveld refinement. Referring to these drawings, it can be seen
that the sample exhibits high crystallinity and well-layered
structure, a mole percentage of Ni.sup.2+ ions inserted into a
reversible lithium layer is 3.97%, and the calculated value and the
measured value of the mole percentage of Ni.sup.2+ ions is
approximately the same.
[0125] Meanwhile, upon performing pH titration, less than 10 mL of
0.1M HCl was consumed to titrate 10 g of a cathode to achieve a pH
of less than 5, which corresponds to a Li.sub.2CO.sub.3 impurity
content of less than about 0.035 wt %. Hence, these results show
that it is possible to achieve mass production of substantially
Li.sub.2CO.sub.3-free LiNiMO.sub.2 having a stable crystal
structure from the mixed oxyhydroxide and Li.sub.2CO.sub.3 by a
solid-state reaction.
Example 5
[0126] More than 1 kg of MOOH
(M=Ni.sub.4/15(Mn.sub.1/2Ni.sub.1/2).sub.8/15Co.sub.0.2) was
prepared by ammonia-free coprecipitation of MSO.sub.4 and NaOH at
80.degree. C. under the pH-adjustment condition. FIG. 13 shows an
SEM micrograph of the thus-prepared precursor hydroxide. The
aforementioned MOOH exhibited a narrow particle diameter
distribution, and a tap density of about 1.2 g/cm.sup.3. A lithium
mixed transition metal oxide was prepared using MOOH as a
precursor. Sintering was carried out at 930.degree. C. The lithium
mixed transition metal oxide prepared using such a precursor did
not exhibit the disintegration of particles as shown in Comparative
Example 2. Therefore, from the excellent sintering stability of
LiMO.sub.2, it can be seen that LiMO.sub.2 can be prepared from the
mixed oxyhydroxide having a low tap density.
Experimental Example 1
Test of Electrochemical Properties
[0127] Coin cells were fabricated using the lithium mixed
transition metal oxide of Examples 3 and 5, and LiNiMO.sub.2 of
Comparative Examples 2 to 4
(M=(Ni.sub.1/2Mn.sub.1/2).sub.1-xCo.sub.x and x=0.17 (Comparative
Example 5) and x=0.33 (Comparative Example 6), respectively, as a
cathode, and a lithium metal as an anode. Electrochemical
properties of the thus-fabricated coin cells were tested. Cycling
was carried out primarily at 25.degree. C. and 60.degree. C., a
charge rate of C/5 and a discharge rate of C/5 (1 C=150 mA/g) from
3 to 4.3 V.
[0128] Experimental results of the electrochemical properties for
the coin cells of Comparative Examples 2 to 4 are given in Table 4
below. Referring to Table 4, the cycle stability was poor with the
exception of Comparative Example 3 (Sample B). It is believed that
Comparative Example 4 (Sample C) exhibits the poor cycle stability
due to the lithium-deficiency of the surface. Whereas, even though
Comparative Example 2 (Sample A) and Comparative Example 3 (Sample
B) were not lithium-deficient, only Comparative Example 3 (Sample
B) exhibited a low content of Li.sub.2CO.sub.3. The presence of
such Li.sub.2CO.sub.3 may lead to gas evolution and gradual
degradation of the performance (at 4.3 V, Li.sub.2CO.sub.3 slowly
decomposes with the collapse of crystals). That is, there are no
nickel-based active materials meeting both the excellent cycle
stability and the low-impurity content, and therefore it can be
shown that no commercial product is available in which the
nickel-based active material has excellent cycle stability and high
stability against exposure to air, in conjunction with a low level
of Li.sub.2CO.sub.3 impurities and low production costs.
TABLE-US-00004 TABLE 4 Sample (A) Sample (B) Sample (C)
LiNi.sub.0.8Co.sub.0.2O.sub.2 Al/Ba-modified AlPO.sub.4-coated
Substrate Comp. Ex. 2 Comp. Ex. 3 Comp. Ex. 4 Stoichiometric
Stoichiometric Stoichiometric Li-deficient at Li:M surfaces
Li.sub.2CO.sub.3 High High Low impurities Capacity at 193, 175
mAh/g 195, 175 mAh/g 185, 155 mAh/g 25.degree. C. C/10, C/1
Capacity loss 30% per 100 11% per 100 >30% per cycles cycles 100
cycles
[0129] On the other hand, the cells of Comparative Examples 5 and 6
exhibited a crystallographic density of 4.7 and 4.76 g/cm.sup.3,
respectively, which were almost the same, and showed a discharge
capacity of 157 to 159 mAh/g at a C/10 rate (3 to 4.3 V). Upon
comparing with LiCoO.sub.2 having a crystallographic density of
5.04 g/cm.sup.3 and a discharge capacity of 157 mAh/g, a volume
capacity of the cell of Comparative Example 5 is equal to a 93%
level of LiCoO.sub.2, and the cell of Comparative Example 6
exhibits a crystallographic density corresponding to a 94% level of
LiCoO.sub.2. Therefore, it can be seen that a low content of Ni
results in a poor volume capacity.
[0130] Table 5 below summarizes electrochemical results of coin
cells using LiNiMO.sub.2 in accordance with Example 3 as a cathode,
and FIG. 14 depicts voltage profiles, discharge curves and cycle
stability. A crystallographic density of LiNiMO.sub.2 in accordance
with Example 3 was 4.74 g/cm.sup.3 (cf. LiCoO.sub.2: 5.05
g/cm.sup.3). A discharge capacity was more than 170 mAh/g (cf.
LiCoO.sub.2: 157 mAh/g) at C/20, thus representing that the volume
capacity of LiNiMO.sub.2 was much improved as compared to
LiCoO.sub.2. Electrochemical properties of LiNiMO.sub.2 in
accordance with Example 5 were similar to those of Example 3.
TABLE-US-00005 TABLE 5 Capacity retention after Primary 100 cycles
charge (extrapolated) capacity C/5-C/5 cycle, 3.0-4.3 V, Discharge
capacity 3.0-4.3 V C/10 25.degree. C., 25.degree. C. 60.degree. C.
-- C/1 25.degree. C., C/20 60.degree. C., C/20 >96% >90%
>190 mAh/g 152 mA/g 173 mAh/g 185 mAh/g
Experimental Example 2
Determination of Thermal Stability
[0131] In order to examine the thermal stability for the lithium
mixed transition metal oxide of Example 3 and LiNiMO.sub.2 in
accordance with Comparative Examples 3 and 4, DSC analysis was
carried out. The thus-obtained results are given in FIGS. 15 and
16. For this purpose, coin cells (anode: lithium metal) were
charged to 4.3 V, disassembled, and inserted into hermetically
sealed DSC cans, followed by injection of an electrolyte. A total
weight of the cathode was from about 50 to about 60 mg, A total
weight of the electrolyte was approximately the same. Therefore, an
exothermic reaction is strongly cathode-limited. The DSC
measurement was carried out at a heating rate of 0.5 K/min.
[0132] As a result, Comparative Example 3 (Al/Ba-modified
LiNiO.sub.2) and Comparative Example 4 (AlPO.sub.4-coated
LiNiO.sub.2) showed the initiation of a strong exothermic reaction
at a relatively low temperature. Particularly, Comparative Example
3 exhibited a heat evolution that exceeds the limit of the device.
The total accumulation of heat generation was large, i.e. well
above 2,000 kJ/g, thus indicating a low thermal stability (see FIG.
15).
[0133] Meanwhile, LiNiMO.sub.2 of Example 3 in accordance with the
present invention exhibited a low total heat evolution, and the
initiation of an exothermic reaction at a relatively high
temperature as compared to Comparative Examples 3 and 4 (see FIG.
16). Therefore, it can be seen that the thermal stability of
LiNiMO.sub.2 in accordance with the present invention is
excellent.
Experimental Example 3
Test of Electrochemical Properties of Polymer Cells with
Application of Lithium Mixed Transition Metal Oxide
[0134] Using the lithium mixed transition metal oxide of Example 3
as a cathode active material, a pilot plant polymer cell of 383562
type was fabricated. For this purpose, the cathode was mixed with
17% by weight LiCoO.sub.2, and the cathode slurry was an
NMP/PVDF-based slurry. No additives for the purpose of preventing
gelation were added. The anode was a mesocarbon microbead (MCMB)
anode. The electrolyte was a standard commercial electrolyte free
of additives known to reduce excessive swelling. Experiments were
carried out at 60.degree. C. and charge and discharge rates of C/5.
A charge voltage was from 3.0 to 4.3 V.
[0135] FIG. 17 shows the cycle stability of the battery of the
present invention (0.8 C charge, 1C discharge, 3 to 4 V, 2 V) at
25.degree. C. An exceptional cycle stability (91% at C/1 rate after
300 cycles) was achieved at room temperature. The impedance build
up was low. Also, the gas evolution during storage was measured.
The results thus obtained are given in FIG. 18. During a 4
h-90.degree. C. fully charged (4.2 V) storage, a very small amount
of gas was evolved and only a small increase of thickness was
observed. The increase of thickness was within or less than the
value expected for good LiCoO.sub.2 cathodes tested in similar
cells under similar conditions. Therefore, it can be seen that
LiNiMO.sub.2 prepared by the method in accordance with the present
invention exhibits very high stability and chemical resistance.
Example 6
[0136] A mixed hydroxide of Formula MOOH
(M=Ni.sub.4/15(Mn.sub.1/2Ni.sub.1/2).sub.8/15Co.sub.0.2) as a mixed
transition metal precursor and Li.sub.2CO.sub.3 were mixed in a
molar ratio of Li:M=1.01:1, and the mixture was sintered in air at
900.degree. C. for 10 hours, thereby preparing 50 g of a lithium
mixed transition metal oxide having a composition of
LiNi.sub.0.53Co.sub.0.2Mn.sub.0.27O.sub.2.
[0137] X-ray analysis was carried out to obtain detailed lattice
parameters with high resolution. Cation mixing was observed by
Rietveld refinement. The results thus obtained are given in Table 6
below.
Comparative Example 7
[0138] A lithium mixed transition metal oxide was prepared in the
same manner as in Example 6, except that a molar ratio of Li:M was
set to 1:1 and sintering was carried out under an O.sub.2
atmosphere. Then, X-ray analysis was carried out and the cation
mixing was observed. The results thus obtained are given in Table 6
below. TABLE-US-00006 TABLE 6 Ex. 4 Comp. Ex. 7 Li:M 1.01:1 1:1
Unit cell vol. (.ANG..sup.3) 33.921 33.798 Normalized c:a ratio
1.0122 1.0124 c:a/24{circumflex over ( )}0.5 Cation mixing 4.6%
1.5%
[0139] As can be seen from Table 6, the lithium mixed transition
metal oxide of Example 6 in accordance with the present invention
exhibited a larger unit cell volume and a smaller c:a ratio, as
compared to that of Comparative Example 7. Therefore, it can be
seen that the lithium mixed transition metal oxide of Comparative
Example 7 exhibited an excessively low cation mixing ratio due to
the heat treatment under the oxygen atmosphere. This case suffers
from deterioration of the structural stability. That is, it can be
seen that the heat treatment under the oxygen atmosphere resulted
in the development of a layered structure due to excessively low
cation mixing, but migration of Ni.sup.2+ ions was hindered to an
extent that the cycle stability of the battery is arrested.
Example 7
[0140] A lithium mixed transition metal oxide having a composition
of LiNi.sub.0.4Co.sub.0.3Mn.sub.0.3O.sub.2 was prepared in the same
manner as in Example 6, except that a mixed hydroxide of Formula
MOOH (M=Ni.sub.1/10(Mn.sub.1/2Ni.sub.1/2).sub.6/10Co.sub.0.3) was
used as a mixed transition metal precursor, and the mixed hydroxide
and Li.sub.2CO.sub.3 were mixed in a molar ratio of Li:M=1:1. The
cation mixing was observed by X-ray analysis and Rietveld
refinement. The results thus obtained are given in Table 7 below.
TABLE-US-00007 TABLE 7 Li:M 1:1 Unit cell vol. 33.895 .ANG..sup.3
Normalized c:a ratio 1.0123 c:a/24{circumflex over ( )}0.5 Cation
mixing 3% Capacity (mAh/g) 155
Example 8
[0141] A lithium mixed transition metal oxide having a composition
of LiNi.sub.0.65Co.sub.0.2Mn.sub.0.15O.sub.2 was prepared in the
same manner as in Example 6, except that a mixed hydroxide of
Formula MOOH
(M=Ni.sub.5/10(Mn.sub.1/2Ni.sub.1/2).sub.3/10Co.sub.0.2) was used
as a mixed transition metal precursor, and the mixed hydroxide and
Li.sub.2CO.sub.3 were mixed in a molar ratio of Li:M=1:1. The
cation mixing was observed by X-ray analysis and Rietveld
refinement. The results thus obtained are given in Table 8 below.
TABLE-US-00008 TABLE 8 Li:M 1:1 Unit cell vol. 34.025 .ANG..sup.3
Normalized c:a ratio 1.0107 c:a/24{circumflex over ( )}0.5 Cation
mixing 7% Capacity (mAh/g) 172
[0142] From the results shown in Tables 7 and 8, it can be seen
that the lithium mixed transition metal oxide in accordance with
the present invention provides desired effects, as discussed
hereinbefore, in a given range.
INDUSTRIAL APPLICABILITY
[0143] As apparent from the above description, a cathode active
material in accordance with the present invention comprises a
lithium mixed transition metal oxide having a given composition,
prepared by a solid-state reaction of Li.sub.2CO.sub.3 with a mixed
transition metal precursor under an oxygen-deficient atmosphere,
and has a Li.sub.2CO.sub.3 content of less than 0.07% by weight of
the cathode active material as determined by pH titration.
Therefore, the thus-prepared cathode active material exhibits
excellent high-temperature stability and stable crystal structure,
thereby providing a high capacity and excellent cycle stability,
and also can be produced by an environmentally friendly method with
low production costs and high production efficiency.
[0144] 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.
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