U.S. patent application number 11/831530 was filed with the patent office on 2007-12-20 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 | 20070292761 11/831530 |
Document ID | / |
Family ID | 46328153 |
Filed Date | 2007-12-20 |
United States Patent
Application |
20070292761 |
Kind Code |
A1 |
PARK; Hong-Kyu ; et
al. |
December 20, 2007 |
MATERIAL FOR LITHIUM SECONDARY BATTERY OF HIGH PERFORMANCE
Abstract
Provided is a lithium mixed transition metal oxide having a
composition represented by Formula I of Li.sub.xM.sub.yO.sub.2 (M,
x and y are as defined in the specification) having mixed
transition metal oxide layers ("MO layers") comprising Ni ions and
lithium ions, wherein lithium ions intercalate into and
deintercalate from the MO layers and a portion of MO layer-derived
Ni ions are inserted into intercalation/deintercalation layers of
lithium ions ("reversible lithium layers") thereby resulting in the
interconnection between the MO layers. The lithium mixed transition
metal oxide of the present invention has a stable layered structure
and therefore exhibits improved stability of the crystal structure
upon charge/discharge. In addition, a battery comprising such a
cathode active material can exhibit a high capacity and a high
cycle stability. Further, such a lithium mixed transition metal
oxide is substantially free of water-soluble bases, and thereby can
provide excellent storage stability, decreased gas evolution and
consequently superior high-temperature stability with the
feasibility of low-cost mass production.
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: |
46328153 |
Appl. No.: |
11/831530 |
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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11831530 |
Jul 31, 2007 |
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Current U.S.
Class: |
429/223 ;
420/400; 429/224; 429/231.95 |
Current CPC
Class: |
C01P 2002/77 20130101;
C01P 2004/03 20130101; C01P 2006/37 20130101; C01P 2002/88
20130101; H01M 4/525 20130101; H01M 4/505 20130101; H01M 10/052
20130101; H01M 4/485 20130101; C01P 2004/84 20130101; H01M 4/131
20130101; Y02E 60/10 20130101; Y02P 70/50 20151101; C01G 45/1228
20130101; C01P 2006/10 20130101; C01P 2006/80 20130101; C01P
2002/54 20130101; C01P 2006/40 20130101; C01P 2002/72 20130101;
H01M 2004/028 20130101; C01G 51/50 20130101; C01G 53/50
20130101 |
Class at
Publication: |
429/223 ;
420/400; 429/224; 429/231.95 |
International
Class: |
H01M 4/40 20060101
H01M004/40; C22C 24/00 20060101 C22C024/00; H01M 4/50 20060101
H01M004/50; H01M 4/52 20060101 H01M004/52 |
Claims
1. A lithium mixed transition metal oxide having a composition
represented by Formula I and comprising mixed transition metal
oxide (MO) layers comprising Ni ions, and reversible lithium layers
which allow intercalation and deintercalation of lithium ions,
wherein the MO layers and the reversible lithium layers are
disposed alternately and repeatedly to form a layered crystal
structure, and a portion of Ni ions derived from the MO layer are
inserted into reversible lithium layers to interconnect the MO
layers and the reversible lithium layers: 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.ltoreq.0.05; and x+y 2 and
0.95.ltoreq.x.ltoreq.1.05.
2. The lithium mixed transition metal oxide according to claim 1,
wherein the Ni ions comprise Ni.sup.2+ ions and Ni.sup.3+ ions
coexisting in the MO layers, and the portion of Ni.sup.2+ ions are
inserted and bound into the reversible lithium layers.
3. The lithium mixed transition metal oxide according to claim 1,
wherein in Formula I a mole fraction of Ni is 0.4 to 0.7, a mole
fraction of Mn is 0.05 to 0.4, and a mole fraction of Co is 0.1 to
0.4.
4. The lithium mixed transition metal oxide according to claim 2,
wherein the mole fraction of Ni.sup.2+ ions is 0.05 to 0.4, based
on the total content of Ni ions.
5. The lithium mixed transition metal oxide according to claim 4,
wherein the portion as a mole fraction of Ni.sup.2+ ions inserted
and bound into the reversible lithium layer is 0.03 to 0.07, based
on the total content of Ni.
6. The lithium mixed transition metal oxide according to claim 1,
wherein the molar ratio of Li:M is 0.95 to 1.04:1.
7. The lithium mixed transition metal oxide according to claim 1,
wherein the lithium mixed transition metal oxide is substantially
free of Li.sub.2CO.sub.3 as impurities.
8. The lithium mixed transition metal oxide according to claim 7,
wherein Li.sub.2CO.sub.3 is contained in an amount such that less
than 20 mL of a 0.1 M HCl titrant solution is added during pH
titration of a solution of water-soluble bases extracted from the
lithium mixed transition metal oxide to reach a value of less than
5, wherein the solution of the water-soluble bases is prepared by
repeatedly soaking and decanting 10 g of the lithium mixed
transition metal oxide with water such that the resulting solution
contains all of the water soluble bases of the lithium mixed
transition metal oxide, and wherein the total volume of the
solution of water-soluble bases is 200 mL.
9. The lithium mixed transition metal oxide according to claim 8,
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.
10. A cathode active material for a secondary battery, comprising
the lithium mixed transition metal oxide of claim 1.
11. A lithium secondary battery comprising the cathode active
material of claim 10.
12. A lithium mixed transition metal oxide comprising mixed
transition metals of Ni, Mn and Co, and comprising mixed transition
metal oxide (MO) layers containing Ni ions, and reversible lithium
layers which allow intercalation and deintercalation of lithium
ions, wherein the MO layers and the reversible lithium layers are
disposed alternately and repeatedly to form a layered crystal
structure, and a portion of Ni ions derived from the MO layers are
inserted into the reversible lithium layers to interconnect the MO
layers.
13. A lithium mixed transition metal oxide having a composition
represented by Formula II, wherein a molar cation mixing ratio for
a portion of Ni located in lithium sites of the lithium mixed
transition metal oxide is 0.03 to 0.07 of the total content of Ni,
and is sufficient to provide a structurally stable layered crystal
structure: Li.sub.x'M''.sub.y'A'.sub.z'O.sub.2 (II) wherein
0.95.ltoreq.x'.ltoreq.1.05, 0.ltoreq.z'<0.05, x'+y'.apprxeq.2,
and y'+z'=1; M'' is a mixed transition metal of Ni, Mn and Co; and
A' is at least one element selected from the group consisting of B,
Ca, Zr, S, F, P, Bi, Al, Mg, Zn, Sr, Cu, Fe, Ga, In, Cr, Ge and
Sn.
14. A lithium mixed transition metal oxide having the composition
of Formula II of claim 13, wherein a molar ratio of lithium to a
mixed transition metal (M) is 0.95:1 to 1.04:1, and a portion of Ni
ions derived from the MO layer are inserted into the reversible
lithium layers to interconnect the MO layers and the reversible
lithium layers.
15. The lithium mixed transition metal oxide according to claim 14,
wherein the portion of Ni ions as a mole fraction inserted into the
lithium sites is 0.03 to 0.07 based on the total content of Ni.
16. A lithium mixed transition metal oxide comprising mixed
transition metal oxide (MO) layers of Ni, Mn and Co, and reversible
lithium layers which allow intercalation and deintercalation of
lithium ions, wherein the MO layers and the reversible lithium
layers are disposed alternately and repeatedly to form a layered
crystal structure, the MO layers contain Ni.sup.3+ ions and
Ni.sup.2+ ions, and a portion of Ni.sup.2+ ions derived from the MO
layers are inserted into the reversible lithium layers.
17. A lithium mixed transition metal oxide having a composition of
Formula I of claim 1 and comprising mixed transition metal oxide
(MO) layers containing Ni ions, and reversible lithium layers which
allow intercalation and deintercalation lithium ions, wherein the
MO layers contain Ni.sup.3+ ions and Ni.sup.2+ ions, and a portion
of Ni.sup.2+ ions derived from the MO layers are inserted into the
reversible lithium layers during preparation of the lithium mixed
transition metal oxide by a reaction of lithium mixed transition
metal oxide precursor materials under an O.sub.2-deficient
atmosphere.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a Ni-based lithium mixed
transition metal oxide and a cathode active material for a
secondary battery comprising the same. More specifically, the
Ni-based lithium mixed transition metal oxide according to the
present invention has a given composition and exhibits
intercalation/deintercalation of lithium ions into/from mixed
transition metal oxide layers ("MO layers") and interconnection of
MO layers via the insertion of a portion of MO layer-derived Ni
ions into intercalation/deintercalation layers (reversible lithium
layers) of lithium ions, thereby improving the structural stability
of the crystal structure upon charge/discharge to provide an
excellent sintering stability. In addition, a battery comprising
such a cathode active material can exert a high capacity and a high
cycle stability. Further, with substantially no water-soluble bases
present, such a lithium mixed transition metal oxide exhibits
excellent storage stability and chemical resistance, and decreased
gas evolution, thereby resulting in an excellent high-temperature
stability and the feasibility of mass production at low cost.
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
evolution of excess of 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 gelation of slurries by polymerization of an
N-methylpyrrolidone/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 LiNiMeO.sub.2 (Me=Co, Mn or Al) 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] Further, 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).
However, it was found through various experiments conducted by the
inventors of the present invention that the aforementioned oxides
include large amounts of impurities such as lithium carbonates, and
suffer from significant problems associated with severe gas
evolution at high temperatures and structural instability.
[0012] Therefore, there is an urgent need in the art for the
development of a technology which is capable of achieving a
structurally stable crystal structure while utilizing a
lithium/nickel-based cathode active material which can allow for
high charge capacity and is capable of securing high-temperature
safety.
SUMMARY OF THE INVENTION
[0013] Therefore, the present invention has been made to solve the
above problems and other technical problems that have yet to be
resolved.
[0014] 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 lithium mixed
transition metal oxide, as will be illustrated hereinafter, having
a given composition and a specific atomic-level structure, with
which it is possible to realize superior thermal stability, and
high cycle stability in conjunction with a high capacity, due to
improvements in the stability of the crystal structure upon
charge/discharge. Further, such a lithium mixed transition metal
oxide can be prepared in a substantially water-soluble base-free
form and therefore exhibits excellent storage stability, reduced
gas evolution and thereby excellent high-temperature safety in
conjunction with the feasibility of industrial-scale production at
low production costs. The present invention has been completed
based on these findings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The above and other features and advantages of the present
invention will be more clearly understood from the following
detailed description taken in conjunction with the accompanying
drawings, in which:
[0016] FIG. 1 is a schematic view showing a crystal structure of a
conventional Ni-based lithium transition metal oxide;
[0017] FIG. 2 is a schematic view showing a crystal structure of a
Ni-based lithium mixed transition metal oxide according to an
embodiment;
[0018] FIGS. 3 and 4 are graphs showing a preferred composition
range of a Ni-based lithium mixed transition metal oxide according
to an embodiment;
[0019] FIG. 5 is an FESEM (Field Emission Scanning Electron
Microscope) image (.times.2,000) showing LiNiMO.sub.2 according to
Example 1 of the present invention. 5A: 850.degree. C.; 5B:
900.degree. C.; 5C: 950.degree. C.; and 5D: 1000.degree. C.;
[0020] 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;
[0021] 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;
[0022] 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;
[0023] 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;
[0024] FIG. 10 is an SEM image of a sample according to Example
3;
[0025] FIG. 11 shows the Rietveld refinement on X-ray diffraction
patterns of a sample according to Example 3;
[0026] FIG. 12 is a graph showing electrochemical properties of
LiNiMO.sub.2 according to Example 3 in Experimental Example 1. 12A:
Graph showing voltage profiles and rate characteristics at room
temperature (1 to 7 cycles); 7B: Graph showing cycle stability at
25.degree. C. and 60.degree. C. and a rate of C/5 (3.0 to 4.3V);
and 7C: 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.;
[0027] FIG. 13 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 LiMO.sub.2
(M=Ni) of Comparative Example 3, and B: Commercial
AlPO.sub.4-coated LiMO.sub.2 (M=Ni) of Comparative Example 4;
[0028] FIG. 14 is a graph showing DSC values for LiNiMO.sub.2
according to Example 3 in Experimental Example 2;
[0029] FIG. 15 is a graph showing electrophysical properties of a
polymer cell according to one embodiment in Experimental Example
3;
[0030] FIG. 16 is a graph showing swelling of a polymer cell during
high-temperature storage in Experimental Example 3; and
[0031] FIG. 17 is a graph showing lengths of a-axis and c-axis of
crystallographic unit cells of samples having different molar
ratios of Li:M in Experimental Example 4.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0032] In accordance with an aspect of the present invention, the
above can be accomplished by the provision of a lithium mixed
transition metal oxide having a composition represented by Formula
I below, wherein lithium ions undergo intercalation/deintercalation
into/from intercalation/deintercalation layers which are disposed
alternately and repeatedly with mixed transition metal oxide layers
("MO layers") containing Ni ions, and a portion of MO layer-derived
Ni ions are inserted into the intercalation/deintercalation layers
of lithium ions (also referred to herein as "reversible lithium
layers") thereby resulting in the interconnection of and between
the MO layers and reversible lithium layers. Li.sub.xM.sub.yO.sub.2
(I)
[0033] wherein:
[0034] 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;
[0035] A is a dopant;
[0036] 0.ltoreq.k.ltoreq.0.05; and
[0037] x+y.apprxeq.2 and 0.95.ltoreq.x.ltoreq.1.05.
[0038] A conventional lithium transition metal oxide has a layered
crystal structure as shown in FIG. 1, and performs charge/discharge
processes through lithium insertion and desertion into/from the
reversible lithium layers. However, when the oxide having such a
structure is used as a cathode active material, deintercalation 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, and thus lithium transition metal oxide suffers from
problems associated with sharp decreases in the capacity and cycle
characteristics, resulting from the structural changes in the
crystal structure due to repeated charge/discharge cycles.
[0039] 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 found that the
crystal structure in the lithium mixed transition metal oxides
disclosed herein stabilizes contrary to conventionally known or
accepted ideas in the related art that
intercalation/deintercalation of lithium ions will be hindered when
a portion of the nickel ions present transfer to and are
immobilized in the reversible lithium layers as shown in FIG. 2, so
it is possible to prevent problems associated with collapse of the
crystal structure caused by the intercalation/deintercalation of
lithium ions. As a result, since the lifespan characteristics and
safety of the lithium mixed transition metal oxide are
simultaneously improved due to a lack of occurrence of additional
structural collapse by oxygen desorption and prevention of further
formation of Ni.sup.2+ ions, the battery capacity and cycle
characteristics can be significantly improved and a desired level
of rate characteristics can be exerted. Therefore, 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
ideas of structural stability in such systems.
[0040] That is, due to the insertion of a portion of Ni ions into
the reversible lithium layers, the lithium mixed transition metal
oxide in accordance with the present invention does not undergo
disintegration of the crystal structure 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, thereby being capable of maintaining a well-layered
structure. Hence, a battery comprising the lithium mixed transition
metal oxide having such a structure as a cathode active material
can exert a high capacity and a high-cycle stability.
[0041] Further, the lithium mixed transition metal oxide in
accordance with the present invention has excellent thermal
stability as the crystal structure is stably maintained even upon
sintering at a relatively high temperature during a production
process.
[0042] Generally, when conventional high-nickel LiMO.sub.2 is
subjected to high-temperature sintering in air containing a trace
amount of CO.sub.2, LiMO.sub.2 decomposes with a decrease of
Ni.sup.3+ ions as shown in the following reaction below, and during
which amounts of impurities Li.sub.2CO.sub.3 increase.
LiM.sup.3+O.sub.2+CO.sub.2.fwdarw.aLi.sub.1-xM.sub.1+x.sup.3+,2+O.sub.2+b-
Li.sub.2CO.sub.3+cO.sub.2
[0043] Further, continuous degradation of the crystal structure
occurs with increased cation mixing, an increased lattice constant
and a decreased c:a ratio, molten Li.sub.2CO.sub.3 impurities
segregate particles of the LiMO.sub.2, and primary particles lose a
contact state therebetween, thereby resulting in the disintegration
of secondary particles.
[0044] However, unlike the conventional high-nickel LiMO.sub.2, the
lithium mixed transition metal oxide in accordance with the present
invention, due to the stability of the atomic-level structure, does
not include Li.sub.2CO.sub.3 impurities resulting from oxygen
deficiency due to a decrease in and decomposition of Ni.sup.3+ ions
as shown in the above reaction, and therefore exhibits no
degradation of the crystal structure, even when subjected to
high-temperature sintering under an air atmosphere. Further, there
are substantially no water-soluble bases present, as the lithium
mixed transition metal oxide of the present invention can be
prepared without addition of an excess of a lithium source.
Accordingly, the lithium mixed transition metal oxide of the
present invention exhibits excellent storage stability, decreased
gas evolution and thereby excellent high-temperature stability
simultaneously with the feasibility of industrial-scale production
at low production costs.
[0045] In contrast to the desirable properties of the lithium mixed
transition metal oxide of the present invention, the inventors
hereof have found that the above-referenced oxides disclosed in
Japanese Unexamined Patent Publication Nos. 2004-281253,
2005-150057 and 2005-310744, and which have a similar composition,
nonetheless do not have the same structure as that of the lithium
mixed transition metal oxides of the present invention, include
large amounts of impurities such as lithium carbonates, and suffer
from severe gas evolution at high temperatures and structural
instability.
[0046] 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", and the term "Ni ions inserted and bound into the
reversible lithium layers" is used interchangeably with the term
"inserted Ni". Therefore, NiM in LiNiMO.sub.2 is a suggestive
expression representing a complex composition of Ni, Mn and Co in
Formula I.
[0047] In one specific embodiment, the lithium mixed transition
metal oxide have a structure wherein Ni.sup.2+ and Ni.sup.3+ ions
coexist in the MO layers and a portion of Ni.sup.2+ ions are
inserted into the reversible lithium layers. That is, in such a
structure of the metal oxide, all of Ni ions inserted into the
reversible lithium layers are Ni.sup.2+ ions and the oxidation
number of Ni ions is not changed in the charge process.
[0048] Specifically, when Ni.sup.2+ and Ni.sup.3+ ions coexist in a
Ni-excess lithium transition metal oxide, an oxygen atom-deficient
state is present under given conditions (reaction atmosphere, Li
content, and the like) and therefore insertion of a portion of the
Ni.sup.2+ ions into the reversible lithium layers may occur with
changes in the oxidation number of Ni.
[0049] The composition of the lithium mixed transition metal oxide
should satisfy the following specific requirements as defined in
Formula I:
[0050] (i) Ni.sub.1-(a+b)(N.sub.1/2Mn.sub.1/2).sub.aCo.sub.b and
0.65.ltoreq.a+b.ltoreq.0.85
[0051] (ii) 0.1.ltoreq.b.ltoreq.0.4, and
[0052] (iii) x+y.apprxeq.2 and 0.95.ltoreq.x.ltoreq.1.05
[0053] Regarding the aforementioned requirement (i), Ni.sub.1-(a+b)
means a content of ions 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 precursor material, so the lithium transition
metal oxide should be produced using LiOH as a precursor 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.
[0054] Meanwhile, taking into consideration both of the above
requirements (i) and (ii), the total mole fraction of Ni ions
including Ni.sup.2+ ions and Ni.sup.3+ ions in LiNiMO.sub.2 of the
present invention is preferably of a relative nickel-excess
compared to manganese and cobalt and may be 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 may 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.
[0055] Further, if the mole fraction of Ni.sup.2+ ions is too high
relative to the Ni content, the cation mixing increases, resulting
in formation of a "rock salt" like crystalline structure that is
excessively stable to the point of being locally and
electrochemically non-reactive, and such a rock salt structure not
only hinders charge/discharge and but also may bring about a
decrease in a discharge capacity. On the other hand, if the mole
fraction of Ni.sup.2+ ions is too low, this may lead to an increase
in the structural instability to thereby lower the cycle stability.
Therefore, the mole fraction of Ni.sup.2+ ions should be
appropriately adjusted taking into consideration such problems that
may occur. Preferably, within the range as shown in FIG. 3, the
mole fraction of Ni.sup.2+ ions may be 0.05 to 04, based on the
total content of Ni.
[0056] Therefore, since Ni.sup.2+ is inserted into the reversible
lithium layers and serves to support the MO layers, Ni.sup.2+ is
contained in an amount sufficient to provide a stable structural
support between MO layers such that the charge stability and cycle
stability can be improved to a desired level, and at the same time
it is inserted in an amount not so as to hinder
intercalation/deintercalation of lithium ions into/from the
reversible lithium layers such that rate characteristics are not
deteriorated. Taken altogether, the mole fraction of Ni.sup.2+ ions
inserted and bound into the reversible lithium layers may be
specifically 0.03 to 0.07, based on the total molar content of
Ni.
[0057] The content of Ni.sup.2+ ions or the content of inserted
Ni.sup.2+ ions may be determined by controlling, for example, a
sintering atmosphere, the content of lithium, and the like. For
example, when a concentration of oxygen in the sintering atmosphere
is high, the content of Ni.sup.2+ will be relatively low.
[0058] With regard to the aforementioned condition (ii), a content
of cobalt (b) is 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 a 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.
[0059] 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.
[0060] 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 enough 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% (k<0.05), as defined in
Formula I. In addition, the dopants may be preferably added in an
amount of <1%, within a range that can improve the stability
without causing deterioration of the reversible capacity.
[0061] As a molar ratio (Li/M) of Li to the transition metal (M)
decreases, the amount of Ni ions present in the MO layers gradually
increases. Therefore, if excessive amounts of Ni ions transfer into
the reversible lithium layers, a movement of Li.sup.+ during
charge/discharge processes is hampered to thereby lead to problems
associated with a decrease in the reversible capacity or
deterioration of the rate characteristics. On the other hand, the
Li/M ratio is excessively high, the amount of Ni present in the MO
layer is excessively low, which may undesirably lead to structural
instability, thereby presenting decreased safety of the battery and
poor lifespan characteristics. Further, at an excessively 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 chemical resistance and
high-temperature stability may deteriorate. Therefore, in one
preferred embodiment, the ratio of Li:M in LiNiMO.sub.2 may be 0.95
to 1.04:1.
[0062] In one embodiment, the lithium mixed transition metal oxide
in accordance with the present invention is substantially free of
water-soluble base impurities, particularly Li.sub.2CO.sub.3.
[0063] Usually, Ni-based lithium 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 may 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.
[0064] The former water-soluble bases are formed primarily due to
unreacted lithium raw materials upon sintering. This is because as
a conventional Ni-based lithium transition metal oxide employs
relatively large amounts of lithium and low-temperature sintering
so as to prevent the collapse of a layered crystal structure, the
resulting metal oxide has relatively large amounts of grain
boundaries as compared to the cobalt-based oxides and lithium ions
do not sufficiently react upon sintering.
[0065] On the other hand, since LiNiMO.sub.2 in accordance with the
present invention, as discussed hereinbefore, maintains a
structurally stable layered crystal structure, it is possible to
carry out a sintering process at a relatively high-temperature
under an air atmosphere and thereby LiNiMO.sub.2 has relatively
small amounts of grain boundaries. In addition, as retention of
unreacted lithium species on the surfaces of particles is
prevented, the particle surfaces are substantially free of lithium
salts such as lithium carbonates, lithium sulfates, and the like.
Further, there is no need to add an excess of a lithium source upon
production of LiNiMO.sub.2, so it is possible to fundamentally
prevent a problem associated with the formation of impurities due
to the unreacted lithium source remaining in the powder.
[0066] As such, it is possible to fundamentally solve various
problems that may occur due to the presence of the water-soluble
bases, particularly the problem that may damage the battery safety
due to evolution of gas arising from acceleration of electrolyte
decomposition at high temperatures.
[0067] As used herein, the phrase "is (are) substantially free of
water-soluble bases" refers to an extent that upon titration of 200
mL of a solution containing the lithium mixed transition metal
oxide with 0.1M HCl, the amount of HCl solution used to reach a pH
of less than 5 is less than 20 mL, more preferably less than 10 mL.
Herein, 200 mL of the aforementioned solution contains
substantially all of the water-soluble bases in the lithium mixed
transition metal oxide, and is prepared by repeatedly soaking and
decanting 10 g of the lithium mixed transition metal oxide.
[0068] More specifically, first, 5 g of a cathode active material
powder is added to 25 mL of water, followed by brief stirring.
About 20 mL of a clear solution is separated and pooled from the
powder by soaking and decanting. 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, total 100 mL of the clear solution
containing water-soluble bases is pooled. A 0.1M HCl solution is
added to titrate the thus-pooled solution, where pH titration is
done with stirring. The pH profile is recorded as a function of
time. Experiments are terminated when the pH reaches a value of
less than about 3, and a flow rate may be selected within a range
that titration takes about 20 to about 30 min. The content of the
water-soluble bases is given as an amount of acid that was used
until the pH reaches a value of less than about 5.
[0069] In this manner, it is possible to determine the content of
the water-soluble bases contained in the aforesaid powder. At this
time, there are no significant influences of parameters such as a
total soaking time of the powder in water.
[0070] A method of preparing the lithium mixed transition metal
oxide in accordance with the present invention may be preferably
carried out under an oxygen-deficient atmosphere. The
oxygen-deficient atmosphere may be an atmosphere with an oxygen
concentration of preferably 10% to 50% by volume, more preferably
10% to 30% by volume. In an embodiment, the lithium mixed
transition metal oxide can be prepared by a solid-state reaction of
Li.sub.2CO.sub.3 and mixed transition metal precursors in the air
atmosphere. Therefore, it is also possible to solve various
problems associated with increased production costs and the
presence of large amounts of water-soluble bases, which the
conventional arts suffer from when making the lithium transition
metal oxide involving adding, mixing and sintering of LiOH and each
transition metal precursor under an oxygen atmosphere. That is, the
present invention enables production of the lithium mixed
transition metal oxide having a given composition and a high
content of nickel via a simple solid-state reaction in air, using a
raw material that is cheap and easy to handle. Therefore, it is
possible to realize a significant reduction of production costs and
high-efficiency mass production.
[0071] That is, due to thermodynamic limitations in the
conventional art, it was impossible to produce high-nickel lithium
mixed transition metal oxide in the air containing trace amounts of
carbon dioxide. In addition, the conventional art suffered from
problems in that utilization of Li.sub.2CO.sub.3 as a precursor
material brings about evolution of CO.sub.2 due to decomposition of
Li.sub.2CO.sub.3, which then thermodynamically hinders additional
decomposition of Li.sub.2CO.sub.3 even at a low partial pressure,
consequently resulting in no further progression of the reaction.
For these reasons, it was impossible to use Li.sub.2CO.sub.3 as the
precursor in the practical production process.
[0072] On the other hand, the lithium mixed transition metal oxide
in accordance with the present invention employs Li.sub.2CO.sub.3
and the mixed transition metal precursor as the reaction materials,
and can be prepared by reacting reactants in an oxygen-deficient
atmosphere, preferably in air. As a result, it is possible to
achieve a significant reduction of production costs, and a
significant increase of the production efficiency due to the
feasibility to produce a desired product by a relatively simple
process. This is because the lithium mixed transition metal oxide
in accordance with the present invention is very excellent in the
sintering stability at high temperatures, i.e. the thermodynamic
stability, due to a stable crystal structure.
[0073] Further, since the lithium source material and the mixed
transition metal material may be preferably added in a ratio of
0.95 to 1.04:1 when producing the lithium mixed transition metal
oxide in accordance with the present invention, there is no need to
add an excess of a lithium source and it is possible to
significantly reduce the possibility of retention of the
water-soluble bases which may derive from the residual lithium
source.
[0074] In order to carry out a smooth reaction via high air
circulation upon production of the lithium mixed transition metal
oxide by the large-scale mass production process, preferably at
least 2 m.sup.3 (a volume at room temperature) of air, per kg of
the final lithium mixed transition metal oxide, may be pumped into
or out of a reaction vessel. For this purpose, a heat exchanger may
be employed to further enhance the efficiency of energy utilization
by pre-warming the in-flowing air before it enters the reactor,
while cooling the out-flowing air.
[0075] In accordance with a further aspect of the present
invention, there is provided a cathode active material for a
secondary battery comprising the aforementioned lithium
nickel-based oxide.
[0076] 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 and the specific
atomic-level structure, or where appropriate, it may be comprised
of the lithium mixed transition metal oxide in conjunction with
other lithium-containing transition metal oxides.
[0077] 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.
[0078] In accordance with a still further 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 known and therefore
detailed description thereof will be omitted herein.
[0079] The cathode is, for example, fabricated by applying a
mixture of the above-mentioned cathode active material, a
conductive material and a binder to a cathode current collector,
followed by drying. If necessary, a filler may be further added to
the above mixture.
[0080] The cathode current collector is generally fabricated to
have a thickness of 3 to 500 .mu.m. There is no particular limit to
materials for the cathode current collector, so long as they have
high conductivity without causing chemical changes in the
fabricated battery. As examples of the materials for 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
adhesion 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.
[0081] The conductive material 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 fabricated battery. As
examples of conductive materials, mention may be made of conductive
materials, including graphite such as natural or artificial
graphite; carbon black such as carbon black, acetylene black,
Ketjen black, channel black, furnace black, lamp black and 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.
[0082] The binder is a component assisting in binding between the
electrode active material and the conductive material, and in
binding of the electrode active material to the current collector.
The binder 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, ethylene-propylene-diene terpolymer (EPDM),
sulfonated EPDM, styrene butadiene rubber, fluoro rubber and
various copolymers.
[0083] The filler is an optional ingredient used to inhibit cathode
expansion. There is no particular limit to the filler, so long as
it does not cause chemical changes in the fabricated battery and is
a fibrous material. As examples of the filler, there may be used
olefin polymers such as polyethylene and polypropylene; and fibrous
materials such as glass fiber and carbon fiber.
[0084] 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 included.
[0085] The anode current collector is generally fabricated to have
a thickness of 3 to 500 .mu.m. There is no particular limit to
materials for the anode current collector, so long as they have
suitable conductivity without causing chemical changes in the
fabricated battery. As examples of materials for 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 processed to
form fine irregularities on the surfaces thereof so as to enhance
adhesion to the anode active material. In addition, the anode
current collector may be used in various forms including films,
sheets, foils, nets, porous structures, foams and non-woven
fabrics.
[0086] 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 Li.sub.yFe.sub.2O.sub.3 (0.ltoreq.y.ltoreq.1),
Li.sub.yWO.sub.2 (0.ltoreq.y.ltoreq.1) and
Sn.sub.xMe.sub.1-xMe'.sub.yO.sub.z (Me: Mn, Fe, Pb or Ge; Me': Al,
B, P, Si, Group I, Group II and Group III elements of the Periodic
Table of the Elements, or halogens; 0.ltoreq.x.ltoreq.1;
1.ltoreq.y.ltoreq.3; and 1.ltoreq.z.ltoreq.8); lithium metals;
lithium alloys; silicon-based alloys; tin-based alloys; metal
oxides such as SnO, SnO.sub.2, PbO, PbO.sub.2, Pb.sub.2O.sub.3,
Pb.sub.3O.sub.4, Sb.sub.2O.sub.3, Sb.sub.2O.sub.4, Sb.sub.2O.sub.5,
GeO, GeO.sub.2, Bi.sub.2O.sub.3, Bi.sub.2O.sub.4, and
Bi.sub.2O.sub.5; conductive polymers such as polyacetylene; and
Li--Co--Ni based materials.
[0087] The separator is interposed between the cathode and anode.
As the separator, an insulating thin film having high ion
permeability and mechanical strength is used. The separator
typically has a pore diameter of 0.01 to 10 .mu.m and a thickness
of 5 to 300 .mu.m. As the separator, sheets or non-woven fabrics
made of an olefin polymer such as polypropylene and/or glass fibers
or polyethylene, which have chemical resistance and hydrophobicity,
are used. When a solid electrolyte such as a polymer is employed as
the electrolyte, the solid electrolyte may also serve as both the
separator and electrolyte.
[0088] The lithium salt-containing non-aqueous electrolyte is
composed of an electrolyte and a lithium salt. As the electrolyte,
a non-aqueous organic solvent, an organic solid electrolyte, or an
inorganic solid electrolyte may be utilized.
[0089] As the non-aqueous organic solvent that can be used in the
present invention, for example, mention may be made of aprotic
organic solvents such as N-methyl-2-pyrrolidone, propylene
carbonate, ethylene carbonate, butylene carbonate, dimethyl
carbonate, diethyl carbonate, gamma-butyro lactone, 1,2-dimethoxy
ethane, tetrahydrofuran, 2-methyl tetrahydrofuran,
dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide,
dioxolane, acetonitrile, nitromethane, methyl formate, methyl
acetate, phosphoric acid triester, trimethoxy methane, dioxolane
derivatives, sulfolane, methyl sulfolane,
1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives,
tetrahydrofuran derivatives, ether, methyl propionate, and ethyl
propionate.
[0090] As examples of the organic solid electrolyte utilized in the
present invention, mention may be made of polyethylene derivatives,
polyethylene oxide derivatives, polypropylene oxide derivatives,
phosphoric acid ester polymers, alginate/poly-1-lysine, polyester
sulfide, polyvinyl alcohols, polyvinylidene fluoride, and polymers
containing ionic dissociation groups.
[0091] As examples of the inorganic solid electrolyte utilized in
the present invention, mention may be made of nitrides, halides and
sulfates of lithium such as Li.sub.3N, LiI, Li.sub.5NI.sub.2,
Li.sub.3N--LiI--LiOH, LiSiO.sub.4, LiSiO.sub.4--LiI--LiOH,
Li.sub.2SiS.sub.3, Li.sub.4SiO.sub.4, Li.sub.4SiO.sub.4--LiI--LiOH,
and Li.sub.3PO.sub.4--Li.sub.2S--SiS.sub.2.
[0092] The lithium salt is a material that is readily soluble in
the above-mentioned non-aqueous electrolyte and may include, for
example, LiCl, LiBr, LiI, LiClO.sub.4, LiBF.sub.4,
LiB.sub.10Cl.sub.10, LiPF.sub.6, LiCF.sub.3SO.sub.3,
LiCF.sub.3CO.sub.2, LiAsF.sub.6, LiSbF.sub.6, LiAlCl.sub.4,
CH.sub.3SO.sub.3Li, CF.sub.3SO.sub.3Li,
(CF.sub.3SO.sub.2).sub.2NLi, chloroborane lithium, lower aliphatic
carboxylic acid lithium, lithium tetraphenyl borate, and imide.
[0093] Additionally, in order to improve charge/discharge
characteristics and flame retardancy, for example, pyridine,
triethylphosphite, triethanolamine, cyclic ether, ethylenediamine,
n-glyme, hexaphosphoric triamide, nitrobenzene derivatives, sulfur,
quinone imine dyes, N-substituted oxazolidinone, N,N-substituted
imidazolidine, ethylene glycol dialkyl ether, ammonium salts,
pyrrole, 2-methoxy ethanol, aluminum trichloride, or the like may
be added to the non-aqueous electrolyte. If necessary, in order to
impart incombustibility, the non-aqueous electrolyte may further
include halogen-containing solvents such as carbon tetrachloride
and ethylene trifluoride. Further, in order to improve
high-temperature storage characteristics, the non-aqueous
electrolyte may additionally include carbon dioxide gas.
[0094] In accordance with yet another aspect of the present
invention, there is provided a lithium mixed transition metal oxide
comprising a mixed transition metal of Ni, Mn and Co and comprising
mixed-transition metal oxide layers ("MO layers") containing Ni
ions, and reversible lithium layers which allow intercalation and
deintercalation of lithium ions, wherein the MO layers and the
reversible lithium layers are disposed alternately and repeatedly
to form a layered crystal structure, and a portion of Ni ions
derived from the MO layer are inserted into the reversible lithium
layers, thereby resulting in interconnection of the MO layers.
[0095] Generally, the use of nickel-based lithium mixed transition
metal oxide as the cathode active material may bring about the
collapse of the crystal structure upon
intercalation/deintercalation of lithium ions. However, the lithium
mixed transition metal oxide in accordance with the present
invention exhibits stabilization of the crystal structure due to
nickel ions inserted into the reversible lithium layers and
therefore does not undergo additional structural collapse that may
result from oxygen desorption, thereby simultaneously improving the
lifespan characteristics and safety.
[0096] Further, the present invention provides a lithium mixed
transition metal oxide having a composition represented by Formula
II below, wherein a molar cation mixing ratio for a portion of Ni
located in lithium sites of the lithium mixed transition metal
oxide is 0.03 to 0.07 of the total content of Ni, and is sufficient
to provide the structurally stable, layered crystal structure.
Li.sub.x'M''.sub.y'A'.sub.z'O.sub.2 (II)
[0097] wherein
[0098] 0.95.ltoreq.x'.ltoreq.1.05, 0.ltoreq.z'.ltoreq.0.05,
x'+y'.apprxeq.2, and y'+z'=1;
[0099] M'' is a mixed transition metal of Ni, Mn and Co; and
[0100] A' is at least one element selected from the group
consisting of B, Ca, Zr, S, F, P, Bi, Al, Mg, Zn, Sr, Cu, Fe, Ga,
In, Cr, Ge and Sn.
[0101] According to the experimental results by the present
inventors, the cation mixing ratio is particularly preferably 0.03
to 0.07, upon considering of both the charge and cycle stability,
and the rate characteristics. When the cation mixing ratio is
within the above-specified range, the crystal structure can be
further stably maintained and a hindrance of Li.sup.+ migration can
be minimized.
[0102] Further, the present invention provides a lithium mixed
transition metal oxide having a composition of Formula II, wherein
a molar ratio of lithium to a mixed transition metal (M) is 0.95:1
to 1.04:1, and a portion of Ni ions are inserted into lithium sites
in the reversible lithium layers to interconnect the MO layers and
the reversible lithium layers and to provide a structurally stable
layered crystal structure.
[0103] Therefore, there is no need to add an excess of a lithium
source and it is also possible to significantly reduce the
possibility of retention of the water-soluble base impurities which
may derive from the lithium source.
[0104] When the molar ratio of lithium to the mixed transition
metal (M) is 0.95:1 to 1.04:1, this may lead to appropriate cation
mixing to thereby further enhance the structural stability and
minimize a hindrance of Li.sup.+ migration during the charge and
discharge process. Therefore, the lithium mixed transition metal
oxide exhibits excellent stability, lifespan characteristics and
rate characteristics. Further, this metal oxide has a high
capacity, as well as high chemical resistance and high-temperature
stability due to substantially no water-soluble bases.
[0105] In one preferred embodiment, the lithium mixed transition
metal oxide has a composition of Formula II and a cation mixing
ratio for the amount of Ni ions located in lithium sites is 0.03 to
0.07 based on the total content of Ni to thereby stably support the
layered crystal structure. Further, the present invention provides
a lithium mixed transition metal oxide comprising mixed transition
metal oxide layers ("MO layers") comprising Ni, Mn and Co, and
reversible lithium layers which allow lithium ion
intercalation/deintercalation, wherein the MO layers and the
reversible lithium layers are disposed alternately and repeatedly
to form a layered crystal structure, the MO layers contain
Ni.sup.3+ ions and Ni.sup.2+ ions, and a portion of Ni.sup.2+ ions
derived from the MO layers are inserted into the reversible lithium
layers.
[0106] Such a structure is not known in the art and makes a
significant contribution to the stability and lifespan
characteristics of the lithium mixed transition metal oxide
disclosed herein.
[0107] Further, the present invention provides a lithium mixed
transition metal oxide having a composition of Formula I and
comprising mixed transition metal oxide layers ("MO layers"),
wherein the MO layers contain Ni.sup.3+ ions and Ni.sup.2+ ions,
and a portion of Ni.sup.2+ ions derived from the MO layers are
inserted into the reversible lithium layers, during preparation of
the lithium mixed transition metal oxide by a reaction of lithium
mixed transition metal oxide precursor materials under an
O.sub.2-deficient atmosphere.
[0108] In comparison therefore, a conventional lithium transition
metal oxide has a crystal structure with no insertion of Ni.sup.2+
ions into the MO layer as effected by the reaction of identical
precursor materials under an oxygen atmosphere, whereas the lithium
mixed transition metal oxide in accordance with the present
invention has a structure wherein relatively large amounts of
Ni.sup.2+ ions are produced by carrying out the reaction under an
oxygen-deficient atmosphere and a portion of the thus-produced
Ni.sup.2+ ions are inserted into the reversible lithium layers.
Therefore, as illustrated hereinbefore, the crystal structure is
stably maintained to thereby exert superior sintering stability,
and a secondary battery comprising the same exhibits excellent
cycle stability.
EXAMPLES
[0109] 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.
Example 1
[0110] A mixed hydroxide of Formula MOOH
(M=Ni.sub.4/15(Mn.sub.1/2Ni.sub.1/2).sub.8/15Cu.sub.0.2) as a mixed
transition metal precursor and Li.sub.2CO.sub.3 were mixed in a
stoichiometric ratio (Li:M=1.02:1), and the mixture was sintered in
air at various temperatures of 850 (Ex. 1A), 900 (Ex. 1B), 950 (Ex.
1C), and 1,000.degree. C. for 10 hours, to preparing 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.
[0111] 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.
[0112] The crystallographic data for the thus-prepared lithium
mixed transition metal oxide are given in Table I 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 a 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, proper
amounts of Ni.sup.2+ ions were inserted into the lithium layer,
thereby achieving the desired structural stability.
[0113] Particularly, Sample B, sintered at 900.degree. C.,
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 1.0118 c:a/24{circumflex over ( )}0.5
Cation mixing 4.5% 3.9% 4.3% 4.5% (Rietveld refinement)
Comparative Example 1
[0114] 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.
[0115] 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)
[0116] Therefore, it can be seen that it is impossible to produce a
conventional lithium mixed transition metal oxide having the
above-specified composition in the air containing trace amounts of
carbon dioxide, due to thermodynamic limitations. In addition, upon
producing the lithium transition metal oxide having the above
composition 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 additional decomposition of
Li.sub.2CO.sub.3, 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
[0117] 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
[0118] 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
[0119] 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. 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
[0120] A high-nickel LiNiO.sub.2 sample having a composition of
LiNi.sub.0.8Mn.sub.0.05Cu.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 a higher content of Li.sub.2CO.sub.3 was
formed than in the sample 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
[0121] A mixture of Li.sub.2CO.sub.3 with mixed hydroxide of
Formula MOOH
(M=Ni.sub.4/15(Mn.sub.1/2Ni.sub.1/2).sub.8/15Co.sub.0.2) was
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.
[0122] 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 showed that a ratio of Li and M is very close to 1.00,
and the unit cell volume was within the target range. FIG. 10 shows
an SEM image of the thus-prepared cathode active material and FIG.
11 shows results of Rietveld refinement. Referring to these
drawings, it was found that the sample exhibits high crystallinity
and well-layered structure, a mole fraction of Ni.sup.2+ inserted
into a reversible lithium layer is 3.97%, and the calculated value
and the measured value of the mole fraction of Ni.sup.2+ are
approximately the same.
[0123] 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
Li.sub.2CO.sub.3-free LiNiMO.sub.2 having a stable crystal
structure from the mixed hydroxide and Li.sub.2CO.sub.3 by a
solid-state reaction.
Experimental Example 1
Test of Electrochemical Properties
[0124] Coin cells were fabricated using the lithium mixed
transition metal oxide of Example 3, LiNiMO.sub.2 of Comparative
Examples 2 to 4, and commercial LiMO.sub.2 with
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 at 25.degree. C. and 60.degree. C., primarily a
charge rate of C/5 and a discharge rate of C/5 (1 C=150 mA/g)
within a range of 3 to 4.3 V.
[0125] Experimental results of the electrochemical properties for
the coin cells of Comparative Examples 2 to 4 are given in Table 3
below. Referring to Table 3, 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 4 (Sample
C) 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 the conventional nickel-based active materials suffer
from poor cycle stability and low stability against exposure to
air, in conjunction with a high level of Li.sub.2CO.sub.3
impurities and high production costs. TABLE-US-00003 TABLE 3
Examples 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 impurities High High
Low Capacity at 193, 175 mAh/g 195, 175 mAh/g 185, 155 25.degree.
C. mAh/g C/10, C/1 Capacity loss 30% per 100 11% per 100 >30%
per 100 cycles cycles cycles
[0126] 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.
[0127] Whereas, 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.
[0128] Table 4 below summarizes electrochemical results of coin
cells using LiNiMO.sub.2 in accordance with Example 3 as a cathode,
and FIG. 12 depicts voltage profiles, discharge curves and cycle
stability. TABLE-US-00004 TABLE 4 Capacity retention after 100
Primary charge cycles (extrapolated) capacity C/5-C/5 cycle,
3.0-4.3 V 3.0-4.3 V, C/10 Discharge capacity 25.degree. C.
60.degree. C. -- 25.degree. C., C/1 25.degree. C., 60.degree. C.,
C/20 C/20 >96% >90% >190 mAh/g 152 mA/g 173 mAh/g 185
mAh/g
Experimental Example 2
Determination of Thermal Stability
[0129] 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. 13 and
14. 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 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.
[0130] 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.
13).
[0131] 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 (about 260.degree. C.) as compared to Comparative
Examples 3 and 4 (about 200.degree. C.) (see FIG. 14). 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
[0132] 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
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.
[0133] FIG. 15 shows the cycle stability of the battery of the
present invention (0.8 C charge, 1 C 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 result thus obtained are given in FIG. 16. 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 in accordance with the present invention exhibits very
high stability and chemical resistance.
Experimental Example 4
[0134] Samples with different Li:M molar ratios were prepared from
Formula 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. 7 samples each of
about 50 g with Li:M ratios ranging from 0.925 to 1.125 were
prepared by a sintering process in air at a temperature of 910 to
920.degree. C. Then, electrochemical properties were tested.
[0135] Table 5 below provides the obtained crystallographic data.
The unit cell volume changes smoothly according to the Li:M ratio.
FIG. 17 shows its crystallographic map. All samples are located on
a straight line. According to the results of pH titration, the
content of soluble base increases slightly with an increase of the
Li:M ratio. The soluble base probably originates from the surface
basicity (ion exchange) but not from the dissolution of
Li.sub.2CO.sub.3 impurities as observed in Comparative Example
1.
[0136] 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.
[0137] That is, as the Li/M ratio decreases, the amount of
Ni.sup.2+ 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 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 ratio of Li:M is
particularly preferably in a range of 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 in a range of 3 to 7%. TABLE-US-00005 TABLE 5
Samples A B C D E F G Li:M ratio 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 titration 3 3.5 5 9 15 19 25
Example 4
[0138] A mixed hydroxide of Formula MOOH
(M=Ni.sub.4/15(Mn.sub.1/2Ni.sub.1/2).sub.8/15Cu.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.
[0139] 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
[0140] A lithium transition metal oxide was prepared in the same
manner as in Example 4, 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. 33.921 .ANG..sup.3 33.798 .ANG..sup.3 Normalized c:a ratio
1.0122 1.0124 c:a/24{circumflex over ( )}0.5 Cation mixing 4.6%
1.5%
[0141] As can be seen from Table 6, the lithium transition metal
oxide of Comparative Example 7 exhibited a significantly 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 5
[0142] 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 4, 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 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 6
[0143] 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 4, 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
[0144] 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
[0145] As apparent from the above description, a lithium mixed
transition metal oxide in accordance with the present invention has
a specified composition and interconnection of MO layers via the
insertion of a portion of MO layer-derived Ni ions into reversible
lithium layers, whereby the crystal structure can be stably
supported to thereby prevent the deterioration of cycle
characteristics. In addition, such a lithium mixed transition metal
oxide exhibits a high battery capacity and is substantially free of
impurities such as water-soluble bases, thereby providing excellent
storage stability, decreased gas evolution and consequently
superior high-temperature stability.
[0146] 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.
* * * * *