Title: Positive active material for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery comprising same
Abstract: A positive active material for a non-aqueous electrolyte secondary battery includes a lithium-nickel composite oxide represented by the compositional formula LiaNi1-b-cCObMncO2 (a≦1.09, 0.05≦b≦0.35, 0.15≦c≦0.35, and 0.25≦b+c≦0.55). By X-ray diffractometry with a CuKα ray, the lithium-nickel composite oxide exhibits an intensity ratio R ((I012+I006)/I101) of not greater than 0.50, wherein R is the ratio of the sum of the diffraction peak intensity I012 on the 012 plane and the diffraction peak intensity I006 on the 006 plane to the diffraction peak intensity I101 on the 101 plane. The crystallinity of the positive active material of the compositional formula LiaNi1-b-cCobMncO2 can be kept high and it is possible to secure good capacity density and cycle life performance.
Patent Number: 6,893,776 Issued on 05/17/2005 to Naruoka, et al.
| Inventors:
|
Naruoka; Yoshinori (Kyoto, JP);
Toriyama; Junichi (Kyoto, JP);
Terasaki; Masanao (Kyoto, JP)
|
| Assignee:
|
Japan Storage Battery Co., Ltd. (Kyoto, JP)
|
| Appl. No.:
|
986431 |
| Filed:
|
November 8, 2001 |
Foreign Application Priority Data
| Nov 14, 2000[JP] | P.2000-346973 |
| Current U.S. Class: |
429/231.3; 252/521.2; 429/223; 429/224; 429/231.1; 429/231.4; 429/231.8 |
| Intern'l Class: |
H01M 004/58 |
| Field of Search: |
429/2313,231.1,223,224,231.4,231.8
252/521.2
|
References Cited [Referenced By]
U.S. Patent Documents
| 5707756 | Jan., 1998 | Inoue et al.
| |
| 5718989 | Feb., 1998 | Aoki et al.
| |
| 5795558 | Aug., 1998 | Aoki et al.
| |
| Foreign Patent Documents |
| 5-242891 | Sep., 1993 | JP.
| |
| 8-37007 | Feb., 1996 | JP.
| |
| 8-213015 | Aug., 1996 | JP.
| |
| 9-171824 | Jun., 1997 | JP.
| |
| 10-289731 | Oct., 1998 | JP.
| |
| 11-307094 | Nov., 1999 | JP.
| |
| 2000/-133262 | May., 2000 | JP.
| |
Primary Examiner: Weiner; Laura
Attorney, Agent or Firm: Sughrue Mion, PLLC
Claims
1. A positive active material for a non-aqueous electrolyte secondary battery,
comprising a lithium-nickel composite oxide represented by the compositional formula
Li
aNi
1-b-cCo
bMn
cO
2 (in which
the suffix a is not greater than 1.09, the suffix b is from not smaller than 0.05
to not greater than 0.25, and the suffix c is from not smaller than 0.2 to not
greater than 0.35, and the sum of b and c is from not smaller than 0.25 to not
greater than 0.55) having a hexagonal structure, wherein
when subjected to X-ray diffractometry with a CuKα ray, said lithium-nickel
composite oxide exhibits an intensity ratio R ((I
012+I
006)I
101)
of not greater than 0.50 and not less than 0.422, R being the ratio of the sum
of the diffraction peak intensity I
012 on the 012 plane and the diffraction
peak intensity I
006 on the 006 plane to the diffraction peak intensity
I
101 on the 101 plane, said positive active material has a mean particle
diameter D
50 of from 4 μm to 25 μm and a BET specific surface
area of from 0.2 to 1.5 m
2/g.
2. A non-aqueous electrolyte secondary battery comprising a positive electrode
comprising said positive active material defined in claim 1, a negative electrode
comprising a carbon-based material, and a non-aqueous electrolyte.
3. The positive active material for the non-aqueous electrolyte secondary battery
according to claim 1, wherein said positive active material has a mean particle
diameter D
50 of from 10.1 to 25 μm.
4. The positive active material for the non-aqueous electrolyte secondary battery
according to claim 3, wherein said positive active material has a BET specific
surface area of from 0.37 to 0.69 m
2/g.
5. The positive active material for the non-aqueous electrolyte secondary battery
according to claim 3, wherein said positive active material has a mean particle
diameter D
50 of from 11.5 to 25 μm.
6. The positive active material for the non-aqueous electrolyte secondary battery
according to claim 5, wherein said positive material has a BET specific surface
area of from 0.37 to 0.58 m
2/g.
7. The positive active material for the non-aqueous electrolyte secondary battery
according to claim 1, wherein said positive active material has a BET specific
surface area of from 0.37 to 0.69 m
2/g.
8. The positive active material for the non-aqueous electrolyte secondary battery
according to claim 7, wherein said positive active material has a BET specific
surface area of from 0.37 to 0.58 m
2/g.
9. A positive active material for a non-aqueous electrolyte secondary battery,
comprising a lithium-nickel composite oxide represented by the compositional formula
Li
aNi
1-b-c-dCo
bMn
cM
dO
2
(in which M is at least one metal element selected from the group consisting of
Al, Ti, W, Nb and Mo, the suffix a is not greater than 1.09, the suffix b is from
not smaller than 0.05 to not greater than 0.35, the suffix c is from not smaller
than 0.15 to not greater than 0.35, and the suffix d is from greater than 0 to
not greater than 0.35, and the sum of b, c and d is from not smaller than 0.25
to not greater than 0.55) having a hexagonal structure, wherein
when subjected to X-ray diffractometry with a CuKα ray, said lithium-nickel
composite oxide exhibits an intensity ratio R ((I
012+I
006)/I
101)
of not greater than 0.50, R being the ratio of the sum of the diffraction peak
intensity I
012 on the 012 plane and the diffraction peak intensity I
006
on the 006 plane to the diffraction peak intensity I
101 on the
101 plane.
10. The positive active material for the non-aqueous electrolyte secondary battery
according to claim 9, wherein said positive active material has a mean particle
diameter D
50 of from 4 μm to 25 μm and a BET specific surface
area of from 0.2 to 1.5 m
2/g.
11. A non-aqueous electrolyte secondary battery comprising a positive electrode
comprising said positive active material defined in any one of 10, a negative electrode
comprising a carbon-based material, and a non-aqueous electrolyte.
Description
FIELD OF THE INVENTION
The present invention relates to a positive active material for the non-aqueous
electrolyte secondary battery and a non-aqueous electrolyte secondary battery comprising same.
BACKGROUND OF THE INVENTION
In recent years, reduction in the size and weight of the portable electronic
apparatus
is remarkable. With this tendency, there has been a growing demand for the reduction
of the size and weight of the secondary battery as a power supply. In order to
meet this requirement, various secondary batteries have been developed. Nowadays,
a lithium ion battery comprising a positive electrode made of lamellar lithium-cobalt
composite oxide as a positive active material has a high working voltage and a
high energy density and thus is useful for the foregoing purpose and has been widely
used. Lithium-cobalt composite oxide occurs scarcely and thus is expensive. Therefore,
as a positive active material substituting for lithium-cobalt composite oxide,
there has been proposed lithium-manganese composite oxide or lithium-nickel composite oxide.
However, lithium-manganese composite oxide is disadvantageous in that it
has a low theoretical capacity density and shows a great capacity drop with charge
and discharge cycles. Further, lithium-nickel composite oxide has the highest theoretical
capacity density but is disadvantageous in that it exhibits deteriorated cycle
life performance and thermal stability. A lithium-nickel composite oxide comprising
lithium in a molar ratio which is not completely stoichiometrical can easily have
an incomplete hexagonal structure having the Ni element incorporated in Li layer
sites and thus easily cause deterioration of the cycle life performance.
In the case of large-sized battery, when a large current flows due to shortcircuiting,
misuse, etc., the battery temperature suddenly rises, making it likely that a combustible
liquid electrolyte or its decomposition gas can flow out and further can be ignited.
In particular, when a lithium-nickel composite oxide is used as a positive active
material, it releases oxygen at high temperatures while being charged because of
the deteriorated thermal stability. Thus, there is a fear of causing a sudden reaction
of the electrode with the liquid electrolyte that leads to thermal runaway and
ignition/rupture of the battery.
The method for evaluating the safety of these batteries include the nail penetrating
test defined in "Guideline for Criterion on Evaluation of Safety of Lithium Secondary
Battery (SBA G101)" published by
Nihon Chikudenchi Kogyokai (Japan Society
of Storage Battery Industry). In accordance with this method, a nail having a diameter
of from 2.5 mm to 5 mm is allowed to pierce through a fully charged battery at
a substantially central portion at room temperature perpendicularly to the plane
of the electrode. The battery is then allowed to stand for 6 hours or longer. This
test is designed on the supposition that the battery can encounter misuse such
as accidental penetration of a nail or the like during the packaging (e.g., in
a wood box). When a nail pierces through the battery, the positive electrode and
the negative electrode come in direct contact with each other in the battery to
cause internal shortcircuiting. Accordingly, this method is also used to evaluate
the possibility of ignition or rupture due to heat generation by sudden reaction
in the battery.
In the foregoing nail penetrating test, it has been confirmed that the existing
lithium secondary battery can undergo rupture/ignition. Therefore, it has been
desired to develop a technique for enhancing the thermal stability of the battery
without impairing the high performance thereof.
In order to provide the battery with high resistance to internal shortcircuiting
or high safety, various mechanisms have heretofore been proposed. For example,
a technique has been proposed which is designed to fuse a separator made of a porous
membrane to close its pores and hence cause shutdown. Another technique involves
the attachment of a PTC element which raises its resistivity with the temperature
rise to the exterior of the battery. In this arrangement, when any abnormality
occurs, flowing current gradually decreases.
However, it is essentially necessary that the safety of the secondary battery
should be enhanced to prevent the occurrence of dangerous conditions even upon
unforeseen accident. At present, it is difficult to say that the safety of the
battery can be sufficiently established. In particular, a large-sized secondary
battery having a capacity of 3 Ah or higher has an increased chemical energy stored
in the battery. Thus, it is more important for this secondary battery to have a
sufficient safety.
Under these circumstances, an object of the present invention is to provide
a lithium-nickel composite oxide having a high capacity density and improved charge
and discharge cycle life performance and thermal stability and provide a non-aqueous
electrolyte secondary battery having a higher safety comprising such a lithium-nickel
composite oxide as a positive active material.
SUMMARY OF THE INVENTION
It has been found that these problems can be solved by arranging the composition,
crystallinity, mean particle diameter and BET surface area of lithium-nickel composite
oxide within respective predetermined ranges to provide a positive active material
having a high capacity density, and the excellent cycle life performance and thermal stability.
In other words, the present invention relates to a positive active material for
the non-aqueous electrolyte secondary battery comprising a lithium-nickel composite
oxide represented by the compositional formula Li
aNi
1-b-cCo
bMn
cO
2
(in which the suffix a is not greater than 1.09 (a≦1.09), the suffix b is
from not smaller than 0.05 to not greater than 0.35 (0.05≦b≦0.35),
and the suffix c is from not smaller than 0.15 to not greater than 0.35 (0.15≦c≦0.35),
with the proviso that the sum of b and c is from not smaller than 0.25 to not greater
than 0.55 (0.25≦b+c≦0.55)) having a hexagonal structure. When subjected
to the X-ray diffractometry with the CuKα ray, the lithium-nickel composite
oxide exhibits an intensity ratio R [=(I
012+I
006)/I
101]
of not greater 0.50, wherein R is the ratio of the sum of the diffraction peak
intensity I
102 on the 012 plane and the diffraction peak intensity I
006
on 006 plane to the diffraction peak intensity I
101 on the 101 plane.
In this arrangement, the crystallinity of the positive active material represented
by the compositional formula Li
aNi
1-b-cCo
bMn
cO
2
can be kept high. At the same time, the adhesivity of the positive active material
to the electrically conductive material and the binder in the positive electrode
material can be kept to inhibit the rise of internal resistance, making it possible
to secure the good capacity density and cycle life performance.
In other words, by predetermining the compositional ratio a of Li to a range
of
not greater than 1.09, the proportion of the Li element in the Li layer site in
the hexagonal structure can be increased to obtain a positive active material having
a high crystallinity. When the compositional ratio "a" exceeds 1.09, Li layer site
is filled with Li element, but Li element may exist also in other sites, lowering
the crystallinity of the positive active material.
By replacing some of the Ni element by the Co element and the Mn element, the
resulting positive active material can be provided with an enhanced thermal stability.
By predetermining the compositional ratio b and c to the range of from not smaller
than 0.05 to not greater than 0.35 and from not smaller than 0.15 to not greater
than 0.35, with the proviso that the sum of b and c is from not smaller than 0.25
to not greater than 0.55, the resulting positive active material can be provided
with an excellent thermal stability without lowering the capacity density thereof.
Referring to the crystallinity of the lithium-nickel composite oxide, the
data of the diffraction peak intensity on various crystalline planes obtained by
X-ray diffractometry is used as an important parameter from which the crystallinity
of lithium-nickel composite oxide can be presumed. In other words, the intensity
ratio R [=(I
012+I
006)/I
101] of the sum of the
diffraction peak intensity I
012 on the 012 plane and the diffraction
peak intensity I
006 on the 006 plane to the diffraction peak intensity
I
101 on the 101 plane observed when the lithium-nickel composite oxide
is subjected to the X-ray diffractometry with the CuKα ray can be used as
a parameter from which the crystallinity thereof can be presumed. It is considered
that the smaller this intensity ratio is, the higher is the crystallinity of lithium-nickel
composite oxide. In the present invention, it was found that, when R is not greater
than 0.50, the resulting lithium-nickel composite oxide has a high crystallinity
and thus gives an excellent cycle life performance.
The positive active material preferably has a mean particle diameter D
50
of from 4 μm to 25 μm and a BET specific surface area of from 0.2 to
1.5 m
2/g.
The mean particle diameter D
50 of the lithium-nickel composite oxide
indicates the particle diameter corresponding to the volume of 50% on the volume
distribution of particles measured by the laser diffraction scattering method.
By using a lithium-nickel composite oxide having a mean particle diameter D
50
of from 4 μm to 25 μm as a positive active material, the capacity
density can be kept high. When the mean particle diameter D
50 of the
lithium-nickel composite oxide falls below 4 μm, a part of the composite
oxide powder may not come in contact with the electrically conductive material.
On the contrary, when the mean particle diameter D
50 of the lithium-nickel
composite oxide exceeds 25 μm, the electrolyte may not penetrate deep into
the composite oxide powder, presumably producing portions which cannot make sufficient
contribution to charge and discharge reaction.
By using a lithium-nickel composite oxide having a BET specific surface area
of
from 0.2 to 1.5 m
2/g as determined by the N
2 gas absorption
method as a positive active material, the capacity density can be kept high. When
BET specific surface area falls below 0.2 m
2/g, the reaction area on
the electrode in contact with the liquid electrolyte is reduced, raising the reaction
resistance. On the contrary, when BET specific surface area exceeds 1.5 m
2/g,
the repetition of charge and discharge causes expansion/shrinkage that reduces
the adhesivity of the positive active material to the binder, raising the internal
resistance and hence making it impossible to obtain a sufficient capacity density.
Further, the composition of the positive active material is preferably arranged
such that the suffixes b and c are from not smaller than 0.05 to not greater than
0.25 and from not smaller than 0.2 to not greater than 0.35, respectively. By controlling
b and c to the above defined range, the amount of Mn to be replaced in the lithium-nickel
composite oxide as the positive active material can be controlled further more
preferably, making it possible to obtain a positive active material having a high
thermal stability without lowering the capacity density too much.
The present invention also relates to a positive active material for the non-aqueous
electrolyte secondary battery comprising a lithium-nickel composite oxide represented
by the compositional formula Li
aNi
1-b-c-dCo
bMn
cM
dO
2
(in which M is at least one metal element selected from the group consisting of
Al, Ti, W, Nb and Mo, the suffix a is not greater than 1.09(a≦1.09), the
suffix b is from not smaller than 0.05 to not greater than 0.35 (0.05≦b≦0.35),
the suffix c is from not smaller than 0.15 to not greater than 0.35 (0.15≦c≦0.35),
and the suffix d is from greater than 0 to not greater than 0.35 (0≦d≦0.35),
with the proviso that the sum of b, c and d is from not smaller than 0.25 to not
greater than 0.55 (0.25≦b+c+d≦0.55)) having a hexagonal structure.
When subjected to the X-ray diffractometry with the CuKα ray, the lithium-nickel
composite oxide exhibits an intensity ratio R (=(I
012+I
006)/I
101)
of not greater 0.50, R being the ratio of the sum of the diffraction peak intensity
I
012 on the 012 plane and the diffraction peak intensity I
006 on
the 006 plane to the diffraction peak intensity I
101 on the 101 plane.
By this constitution, the crystallinity of the positive active material represented
by the compositional formula Li
aNi
1-b-c-dCo
bMn
cM
dO
2
an be kept high. At the same time, the adhesivity of the positive active material
to the electrically conductive material and the binder in the positive electrode
compound can be kept to inhibit the increase of internal resistance, making it
possible to secure the excellent capacity density and cycle life performance.
In this case, too, the positive active material preferably has a mean particle
diameter D
50 of from 4 μm to 25 μm and a BET specific surface
area of from 0.2 to 1.5 m
2/g.
By using a lithium-nickel composite oxide having a mean particle diameter D
50
of from 4 μm to 25 μm as a positive active material, the capacity density
can be kept high. When the mean particle diameter D
50 of the lithium-nickel
composite oxide falls below 4 μm, a part of the composite oxide powder may
not come in contact with the electrically conductive material. On the contrary,
when the mean particle diameter D
50 of the lithium-nickel composite
oxide exceeds 25 μm, the liquid electrolyte may not penetrate deep into the
composite oxide powder, presumably producing portions which cannot make sufficient
contribution to charge and discharge reaction.
By using a lithium-nickel composite oxide having a BET specific surface area
of
from 0.2 to 1.5 m
2/g as determined by the N
2 gas absorption
method as a positive active material, the capacity density can be kept high. When
the BET specific surface area falls below 0.2 m
2/g, the reaction area
on the electrode in contact with the liquid electrolyte is reduced, raising the
reaction resistance. On the contrary, when BET specific surface area exceeds 1.5
m
2/g, the repetition of charge and discharge causes expansion/shrinkage
that reduces the adhesivity of the positive active material to the binder, raising
the internal resistance and hence making it impossible to obtain sufficient capacity density.
By satisfying all the composition, the crystallinity and powder properties of
lithium-nickel composite oxide in the predetermined requirements, a lithium-nickel
composite oxide having a high capacity density and improved cycle life performance
and thermal stability can be obtained as a positive active material.
A non-aqueous electrolyte secondary battery comprising a positive electrode containing
the foregoing positive active material, a negative electrode containing a carbon-based
material and a non-aqueous electrolyte exhibits an improved cycle life performance
and a drastically enhanced safety. Even when the foregoing positive active material
comprises other active materials incorporated therein, the effect of the foregoing
positive active material can be exerted as a matter of course, making it possible
to obtain a non-aqueous electrolyte secondary battery having the same excellent
performance as mentioned above.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view illustrating an embodiment of the cylindrical lead
acid battery according to the invention.
FIG. 2 is a perspective view illustrating the electricity-generating element
of the cylindrical lead acid battery of FIG. 1.
FIG. 3 is a graph illustrating the relationship between the mean particle diameter
D
50 of a positive active material and the capacity retention of the
positive active material after 50 cycles of charge and discharge.
FIG. 4 is a graph illustrating the relationship between the BET surface area
of a positive active material and the capacity retention of the positive active
material after 50 cycles of charge and discharge.
FIG. 5A to 5F are graphs illustrating the results of the measurement
of heat flow of positive active material (compound) by a differential scanning calorimeter.
DETAILED DESCRIPTION OF THE INVENTION
One embodiment of the present invention will be described in connection with
the attached drawings.
The positive active material for the non-aqueous electrolyte secondary battery
of the present invention can be obtained by, for example, the method described
in the following Examples.
As the positive active material for the non-aqueous electrolyte secondary battery
of the present invention, there is used a lithium-nickel composite oxide having
a hexagonal structure, which is represented by the compositional formula Li
aNi
1-b-cCo
bMn
cO
2
or Li
aNi
1-b-c-dCo
bMn
cM
dO
2
(in which M is at least one metal element selected from the group consisting
of Al, Ti, W, Nb and Mo), wherein the compositional ratio of elements and the physical
properties of the positive active material are specified. The resulting non-aqueous
electrolyte secondary battery exhibits almost the same capacity density as that
of lithium-cobalt composite oxide, i.e., not smaller than 150 mAh/g, an excellent
cycle life performance and a drastically enhanced the safety of the battery. As
compared with the lithium-cobalt composite oxide, the foregoing positive active
material has a small cobalt content, making it possible to provide a non-aqueous
electrolyte secondary battery at a low cost.
The non-aqueous electrolyte secondary battery of the invention comprises, as
a positive active material, a lithium-nickel composite oxide having a hexagonal
structure represented by the compositional formula Li
aNi
1-b-cCo
bMn
cO
2
or Li
aNi
1-b-c-dCo
bMn
cM
dO
2
(in which M is at least one metal element selected from the group consisting
of Al, Ti, W, Nb and Mo), and the foregoing lithium-nickel composite oxide may
be used in admixture with other positive active materials.
The non-aqueous electrolyte secondary battery
1 according to the present
embodiment comprises a flat electrode block
2 and a non-aqueous electrolyte
containing an electrolyte salt received in a battery case
3a, wherein
the flat electrode block
2 comprises a positive electrode
2a having
an aluminum current collector coated with a positive electrode compound and a negative
electrode
2b having a copper current collector coated with a negative
electrode compound wound with a separator
2c interposed therebetween,
as shown in FIGS. 1 and 2.
A case cover
3a provided with a safety valve
6 is laser-welded
to the battery case
3a. A positive electrode terminal
4 is
connected to the positive electrode
2a via a positive electrode lead
wire. A negative electrode terminal
5 is electrically connected to the negative
electrode
2b by causing the negative electrode
2b to
contact the inner wall of the battery case
3a.
The negative electrode, separator
2c, electrolyte, etc. to be used
in the non-aqueous electrolyte secondary battery
1 are not specifically
limited. Materials known as these components may be used.
The negative electrode material to be used herein is not specifically limited.
For example, known carbon-based materials such as coke, glass-like carbon, graphite,
hardly graphitizable carbon, pyrolytic carbon and carbon fiber, or metallic lithium,
lithium alloy, polyacene, etc. may be used singly or in admixture or two or more.
Alternatively, a transition metal may be used in the form of oxide or nitrite.
As the separator to be incorporated in the non-aqueous electrolyte secondary
battery
of the invention, there may be used a microporous membrane made of polyolefin resin
such as polyethylene. A laminate of a plurality of microporous membranes made of
different materials having different weight-mean molecular weights and porosities
may be used. Alternatively, these microporous membranes may comprise various additives
such as a plasticizer, an antioxidant and a fire retardant incorporated therein
in a proper amount.
The organic solvent for the liquid electrolyte to be used in the non-aqueous
electrolyte secondary battery of the invention is not specifically limited. Examples
of the organic solvents employable herein include ethers, ketones, lactones, nitrites,
amines, amides, sulfur compounds, halogenated hydrocarbons, esters, carbonates,
nitro compounds, phosphate compounds, and sulfolan-based hydrocarbons. Preferred
among these organic solvents are ethers, ketones, esters, lactones, halogenated
hydrocarbons, carbonates, and sulfolan-based compounds. Specific examples of these
organic solvents include tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane,
anisole, monoglyme, 4-methyl-2-pentanone, ethyl acetate, methyl acetate, methyl
propionate, ethyl propionate, 1,2-dichloroethane, γ-butyrolactone, dimethoxyethane,
methyl formate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate,
propylene carbonate, ethylene carbonate, vinylene carbonate, dimethylformamide,
dimethyl sulfoxide, dimethylthioformamide, sulfolan, 3-methyl-sulfolan, trimethyl
phosphate, triethyl phosphate, and mixture thereof. However, the present invention
is not limited to these compounds. Preferred among these compounds are cyclic carbonates
and cyclic esters. Even more desirable among these compounds are ethylene carbonate,
propylene carbonate, methyl ethyl carbonate, and diethyl carbonate. These compounds
may be used singly or in combination of two or more thereof.
The electrolyte salt to be used in the non-aqueous electrolyte secondary battery
of the invention is not specifically limited. In practice, however, LiClO
4,
LiBF
4, LiAsF
6, CF
3SO
3Li, LiPF
6,
LiPF
3 (C
2F
5)
3, LiN (CF
3SO
2)
2,
LiN(C
2F
5SO
2) 2, LiI, LiAlCl4, and mixture thereof
may be used. Preferably, lithium salt such as LiBF
4 and LiPF
6 may
be used singly or in admixture.
As the electrolyte for, the present invention, a solid ionically-conductive polymer
electrolyte may be used auxiliary. In this case, the structure of the non-aqueous
electrolyte secondary battery may be a combination of a positive electrode, a negative
electrode, a separator, an organic or inorganic solid electrolyte membrane as a
separator, and the foregoing non-aqueous liquid electrolyte, or a combination of
a positive electrode, a negative electrode, an organic or inorganic solid electrolyte
membrane as a separator, and the aforementioned non-aqueous liquid electrolyte.
A polymer electrolyte membrane made of polyethylene oxide, polyacrylonitrile, polyethylene
glycol or modification product thereof has a light weight and flexibility and thus
can be used as an electrode to be wound to advantage. Besides the polymer electrolyte,
an inorganic solid electrolyte or a mixture of an organic polymer electrolyte and
an inorganic solid electrolyte may be used.
Other battery constituents include the current collector, the terminals, the
insulating plate, the battery case, etc. Conventionally known materials as they
are may be used as these constituents.
Taking into account the effect of the present invention to enhance the safety,
the present invention is preferably applied to a large-sized, non-aqueous electrolyte
secondary battery having a capacity of 3 Ah or more.
The present invention will be further described in the following examples.
(Preparation of Positive Active Material)
As the starting materials for the positive active material, a mixed carbonates
represented by the compositional formula Li
aNi
1-b-cCo
bMn
cCO
2
(in which b and c vary from greater than 0 to smaller than 1 (0<b<1;
0<c<1)) and lithium hydroxide were mixed. The mixture was calcined at
the temperature set forth in Table 1 in an oxygen atmosphere for 24 hours, and
then ground to obtain a lithium-nickel composite oxide represented by the compositional
formula Li
aNi
1-b-cCo
bMn
cO
2 set
forth in Table 1. As a result of analysis by the X-ray diffraction, it was confirmed
that many of these composite oxides have a hexagonal structure. The composition
of these composite oxides were then quantitatively analyzed by the ICP emission
spectroscopy. The results are set forth in terms of compositional formula of composite
oxide in Table 1.
Subsequently, as the starting materials, lithium carbonate and cobalt
tetraoxide were mixed. The mixture was calcined at a temperature of 800° C.
in the atmosphere for 24 hours, and then ground to obtain a lithium-cobalt composite
oxide represented by the compositional formula LiCoO
2 (Comparative Example
12). As a result of the powder X-ray diffractometry, it was confirmed that the
lithium-cobalt composite oxide has a hexagonal structure.
All the positive active materials thus prepared exhibited a mean particle diameter
D
50 of 11.0 μm and a BET surface area of 0.60 m
2/g.
For the determination of the mean particle diameter D
50, the distribution
of volume of particles was measured by the laser diffraction scattering method.
The mean particle diameter D
50 corresponding to the volume of 50% was
then determined. The BET surface area was measured by the N
2 gas absorption
method. (Preparation of positive electrode and test battery) To a positive electrode
compound obtained by mixing 87% of the aforementioned positive active material,
5% by weight of acetylene black and 8% by weight of a polyvinylidene fluoride was
added N-methyl-2-pyrrolidone to prepare a viscous material. A foamed aluminum having
a porosity of 90% was filled with this viscous material, dried in vacuo at a temperature
of 150° C. to cause N-methyl-2-pyrrolidone to evaporate thoroughly, and then pressure-molded.
The positive electrode having an electrode area of 2.25 cm
2 thus pressure-molded,
a counter electrode and a reference electrode were put in a glass cell. The glass
cell was then filled with a non-aqueous, liquid electrolyte obtained by dissolving
1 mol/L of LiClO
4 in a 1/1 (vol/vol) mixture of ethylene carbonate and
diethyl carbonate to form a test battery.
(Measurement of Discharge Capacity of Positive Active Material)
This test battery was charged to 4.3 V (with respect to lithium metal) at a
current of 1.0 mA/cm
2, and then discharged to 3.0 V at a current of
1.0 mA/cm
2. The discharge capacity at this point was then measured.
The capacity density per 1 g of positive active material was then calculated. The
results of evaluation are set forth in Table 1.
The test battery was charged to 4.3 V (with respect to lithium metal) at a current:
of 1.0 mA/cm
2, and then discharged to 3.0 V at a current of 1.0 mA/cm
2.
The discharge capacity at this point was then measured. Under these conditions,
charge and discharge were then repeated. After 50 cycles of charge and discharge,
the test battery was then measured for discharge capacity. The capacity retention
was then calculated by dividing the discharge capacity by the initial discharge capacity.
| TABLE 1 |
| |
| Kind of |
|
Diffraction |
Calcining |
|
Discharge |
| positive |
|
peak intensity |
temper- |
Capacity |
capacity |
| active |
LiaNil - b - cCobMncO2 |
ratio |
ature |
density |
retention |
| material |
a |
b |
c |
b + c |
R |
(° C.) |
(mAh/g) |
(%) |
| |
| Ex. 1 |
1.00 |
0.09 |
0.18 |
0.27 |
0.499 |
800 |
170 |
82 |
| Ex. 2 |
1.00 |
0.20 |
0.18 |
0.38 |
0.496 |
800 |
170 |
91 |
| Ex. 3 |
1.00 |
0.30 |
0.19 |
0.49 |
0.496 |
800 |
160 |
90 |
| Ex. 4 |
1.00 |
0.09 |
0.29 |
0.38 |
0.493 |
800 |
155 |
88 |
| Ex. 5 |
1.00 |
0.20 |
0.29 |
0.49 |
0.491 |
800 |
156 |
91 |
| Ex. 6 |
1.02 |
0.15 |
0.30 |
0.45 |
0.495 |
800 |
160 |
80 |
| Ex. 7 |
1.04 |
0.14 |
0.31 |
0.45 |
0.489 |
1,000 |
161 |
85 |
| Ex. 8 |
1.05 |
0.15 |
0.30 |
0.45 |
0.489 |
900 |
160 |
89 |
| Ex. 9 |
1.07 |
0.16 |
0.29 |
0.45 |
0.454 |
1,000 |
161 |
91 |
| Ex. 10 |
1.08 |
0.15 |
0.30 |
0.45 |
0.469 |
900 |
161 |
93 |
| Ex. 11 |
1.09 |
0.05 |
0.15 |
0.20 |
0.493 |
900 |
185 |
81 |
| Ex. 12 |
1.08 |
0.05 |
0.25 |
0.30 |
0.482 |
900 |
165 |
83 |
| Ex. 13 |
1.04 |
0.35 |
0.15 |
0.50 |
0.491 |
900 |
161 |
84 |
| Ex. 14 |
1.03 |
0.25 |
0.15 |
0.40 |
0.497 |
900 |
167 |
92 |
| Ex. 15 |
1.02 |
0.15 |
0.35 |
0.50 |
0.462 |
900 |
151 |
84 |
| Ex. 16 |
1.06 |
0.10 |
0.15 |
0.25 |
0.473 |
900 |
174 |
85 |
| Ex. 17 |
1.05 |
0.20 |
0.35 |
0.55 |
0.486 |
900 |
152 |
84 |
| Comp. Ex. 1 |
1.09 |
0.05 |
0.15 |
0.20 |
0.511 |
800 |
148 |
81 |
| Comp. Ex. 2 |
1.03 |
0.05 |
0.15 |
0.30 |
0.549 |
800 |
142 |
64 |
| Comp. Ex. 3 |
1.04 |
0.35 |
0.15 |
0.50 |
0.520 |
800 |
141 |
84 |
| Comp. Ex. 4 |
1.05 |
0.25 |
0.15 |
0.40 |
0.531 |
800 |
146 |
79 |
| Comp. Ex. 5 |
1.03 |
0.15 |
0.35 |
0.50 |
0.509 |
800 |
139 |
76 |
| Comp. Ex. 6 |
1.03 |
0.10 |
0.15 |
0.25 |
0.508 |
800 |
147 |
70 |
| Comp. Ex. 7 |
1.04 |
0.20 |
0.35 |
0.55 |
0.514 |
800 |
140 |
83 |
| Comp. Ex. 8 |
1.10 |
0.05 |
0.15 |
0.20 |
0.498 |
1,000 |
146 |
83 |
| Comp. Ex. 9 |
1.07 |
0.04 |
0.26 |
0.30 |
0.472 |
1,000 |
143 |
71 |
| Comp. Ex. 10 |
1.05 |
0.15 |
0.37 |
0.52 |
0.484 |
1,000 |
136 |
75 |
| Comp. Ex. 11 |
1.06 |
0.30 |
0.30 |
0.60 |
0.476 |
1,000 |
141 |
84 |
| Comp. Ex. 12 |
1.0 |
1.0 |
0.0 |
1.0 |
0.473 |
800 |
150 |
79 |
| |
(X-ray Diffractometry Test and Physical Property Determination Test on Positive
Active Material)
The aforementioned lithium-nickel composite oxides were each subjected to the
powder X-ray diffractometry with the cuKα ray to determine the diffraction
peak intensity I
101 on the 101 plane, the diffraction peak intensity
I
012 on the 012 plane and the diffraction peak intensity I
006 on
the 006 plane. From these measurements was then calculated the intensity R defined
by (I
101+I
006)/I
101.
As can be seen in Table 1, the lithium-nickel composite oxide LiNi
1-b-cCo
bMn
cO
2
giving a capacity density of not smaller than that of the conventional LiCoO
2
(150 mA/g) and a capacity retention as good as not smaller than 80% has a
formulation such that the suffix a is not greater than 1.09, the suffix b is from
not smaller than 0.05 to not greater than 0.35, the suffix c is from not smaller
than 0.15 to not greater than 0.35, the sum of b and c is from not smaller than
0.25 to not greater than 0.55, and R is not greater than 0.50.
Positive active materials having the same formulation as in Example 10 but
having different mean particle diameters D
50 and BET surface areas were
prepared as shown in Table 2. These positive active materials were each then used
to prepare positive electrodes in the same manner as mentioned above. These positive
electrodes were each then used to prepare test batteries.
(Charge and Discharge Cycle Life Test on Positive Active Material)
The test batteries were each charged to 4.3 V (with respect to lithium metal)
at a current of 1.0 mA/cm
2, and then discharged to 3.0 V at a current
of 1.0 mA/cm
2. At this point, the test batteries were each measured
for discharge capacity. Under these conditions, charge and discharge were then
repeated. After 50 cycles of charge and discharge, the test battery was then measured
for discharge capacity. The capacity retention was then calculated by dividing
the discharge capacity by the initial discharge capacity.
The relationship between the capacity retention and the average particle diameter
D
50 and BET surface area is plotted in FIGS. 3 and 4.
| TABLE 2 |
| |
| |
|
Mean |
|
|
| Kind of |
|
particle |
BET |
Discharge |
| positive |
|
diameter |
surface |
capacity |
| active |
LiaNil - b - cCObMncO2 |
D50 |
area |
retention |
| material |
a |
b |
c |
b + c |
(μm) |
(m2) |
(%) |
| |
| Ex. 18 |
1.08 |
0.15 |
0.30 |
0.45 |
7.6 |
1.20 |
80 |
| Ex. 19 |
1.08 |
0.15 |
0.30 |
0.45 |
10.1 |
0.69 |
85 |
| Ex. 20 |
1.08 |
0.15 |
0.30 |
0.45 |
21.0 |
0.37 |
89 |
| Ex. 21 |
1.08 |
0.15 |
0.30 |
0.45 |
11.5 |
0.45 |
90 |
| Ex. 22 |
1.08 |
0.15 |
0.30 |
0.45 |
14.5 |
0.58 |
91 |
| Ex. 23 |
1.08 |
0.15 |
0.30 |
0.45 |
12.2 |
0.50 |
93 |
| |
As can be seen in these figures, when the mean particle diameter D
50
is from 4 μm to 25 μm and the BET surface area is from 0.2 to 1.5,
the resulting positive active material exhibits, particularly, a high capacity
retention and a good charge and discharge cycle life performance.
(Thermal Stability Test on Positive Active Material)
Positive active materials having the formulation set forth in Table 3 were
prepared in the same manner as mentioned above.
| TABLE 3 |
| |
| |
|
Diffraction |
|
Exotherm |
|
| Kind of |
|
peak |
Calcining |
starting |
Dissi- |
| positive active |
LiaNil - b - cCObMncO2 |
intensity ratio |
temperature |
temperature |
pated heat |
| material |
a |
b |
c |
b + c |
R |
(° C.) |
(° C.) |
(J/g) |
| |
| Comp. Ex. 13 |
1.00 |
0.35 |
0.10 |
0.45 |
0.491 |
900 |
231.2 |
662 |
| Ex. 13 |
1.04 |
0.35 |
0.15 |
0.50 |
0.491 |
900 |
232.8 |
435 |
| Ex. 14 |
1.03 |
0.25 |
0.15 |
0.40 |
0.497 |
900 |
235.0 |
603 |
| Ex. 24 |
1.00 |
0.25 |
0.20 |
0.45 |
0.462 |
900 |
236.9 |
649 |
| Ex. 25 |
1.00 |
0.15 |
0.30 |
0.45 |
0.422 |
900 |
251.2 |
405 |
| Comp. Ex. 14 |
1.00 |
1.0 |
0.0 |
1.0 |
0.473 |
900 |
210.1 |
489 |
| |
The specimen to be tested for thermal stability was prepared as follows.
A positive electrode compound was prepared by mixing 94% by weight of each of
the
positive active materials set forth in Table 3, 2% by weight of acetylene black
and 4% by weight of a polyvinylidene fluoride. To the positive electrode compound
thus prepared was then added N-methyl-2-pyrrolidone to prepare a viscous material.
The viscous material was applied to an aluminum foil, and then dried at a temperature
of 150° C. in vacuo to cause N-methyl-2-pyrrolidone to evaporate thoroughly.
The coated aluminum foil was roll-pressed in such a manner that the electrode area
and porosity reached 3 cm
2 and 30%, respectively. The aluminum foil
thus processed was used as a positive electrode. Lithium metal was used as counter
electrode and reference electrode. As a liquid electrolyte, there was used a mixture
of ethylene carbonate containing 1M LiPF
6 and diethyl carbonate. Thus,
a test battery was prepared.
The test batteries of Comparative Example 13, Example 13, Example 14, Example
24 and Example 25 were each charged at a current of 0.5 mA/cm
2 until
the state of Li reached Li
0.3. The test battery of Comparative Example
14 was charged at a current of 0.5 mA/cm
2 until the state of Li reached
Li
0.5. The positive electrode compound thus charged was withdrawn from
the battery, and then heated with the liquid electrolyte present therein by means
of a differential scanning calorimeter (DSC). At this point, the exotherm and endotherm
were measured.
The exothermic and endothermic chart of positive electrode compounds of Comparative
Example 13, Example 13, Example 14, Example 24, Example 25 and Comparative Example
14 are shown in FIG.
5. The exotherm starting temperature and exotherm read
from these charts are set forth in Table 3.
All the compounds comprising the positive active materials of Example 13, Example
14, Example 24 and Example 25 exhibit a higher exotherm starting temperature and
a less exotherm than that of Comparative Example 13 and thus were confirmed to
have an excellent thermal stability. Further, all the compounds comprising the
positive active materials of Example 13, Example 14, Example 24 and Example 25
exhibit a higher exotherm starting temperature than that of Comparative Example
14 and thus was confirmed to have an excellent thermal stability.
Moreover, the compounds comprising the positive active materials of Example
13, Example 14, Example 24 and Example 25 had an increased content of manganese
and thus showed an exotherm starting temperature shifted toward higher temperature
and a reduced exotherm. This is presumably because the manganese element inhibited
the elimination of oxygen in the crystal structures and suppressed the exotherm.
Among these positive electrode compounds, those of Examples 24 and 25 exhibited
a high exotherm starting temperature and a small exotherm. In particular, the positive
electrode compound of Example 25 exhibited a better thermal stability than that
of Comparative Example 14.
As can be seen in the aforementioned results, the positive active material of
the invention exhibits an excellent thermal stability. It can be seen that the
formulation of lithium-nickel composite oxide represented by the compositional
formula LiNi
1-b-cCo
bMn
cO
2 where the
thermal stability can be judged particularly excellent can be represented by the
relationships 0.05≦b≦0.25 and 0.2≦c≦0.35.
(Preparation of Large-sized Battery)
Subsequently, the positive active materials of Comparative Example
13, Example 24, Example 25 and Comparative Example 14 were each used to prepare
a large-sized battery.
This battery was a non-aqueous electrolyte secondary battery
1 having
a designed capacity of 10 Ah as shown in FIG.
1. The positive electrode
2a was prepared by mixing the aforementioned positive active material
with a polyvinylidene fluoride and acetylene black, adding NMP to the mixture to
prepare a paste, applying the paste to an aluminum foil, and then drying the coated
aluminum to form a positive electrode compound layer thereon. The negative electrode
2b was prepared by mixing a carbon-based material (graphite) with
a polyvinylidene fluoride, adding NMP to the mixture to prepare a paste, applying
the paste to a copper foil, and then drying the coated copper foil to form a negative
electrode compound layer thereon.
(Safety Test on Large-sized Battery (the Nail Penetrating Test))
The large-sized battery having a designed capacity of 10 Ah thus prepared was
charged, and then subjected to the nail penetrating test according to the method
defined in SBA G1101. The results are set forth in Table 4.
| TABLE 4 |
| |
| Kind of |
|
|
|
| positive |
|
Diffraction |
| active |
LiaNi1-b-cCobMncO2 |
peak intensity |
| material |
a |
b |
c |
b + c |
ratio R |
Explosion |
| |
| Comp. Ex. |
1.00 |
0.35 |
0.10 |
0.45 |
0.491 |
Yes |
| 13 |
| Ex. 24 |
1.00 |
0.25 |
0.20 |
0.45 |
0.462 |
Not so much |
| Ex. 25 |
1.00 |
0.15 |
0.30 |
0.45 |
0.422 |
No |
| Comp. Ex. |
1.00 |
1.0 |
0.0 |
1.0 |
0.473 |
No |
| 14 |
| |
