U.S. patent application number 11/024153 was filed with the patent office on 2005-06-02 for lithium secondary battery.
This patent application is currently assigned to NGK Insulators, Ltd.. Invention is credited to Kitoh, Kenshin, Nemoto, Hiroshi, Takahashi, Michio.
Application Number | 20050118505 11/024153 |
Document ID | / |
Family ID | 26510617 |
Filed Date | 2005-06-02 |
United States Patent
Application |
20050118505 |
Kind Code |
A1 |
Nemoto, Hiroshi ; et
al. |
June 2, 2005 |
Lithium secondary battery
Abstract
A lithium secondary battery has small internal resistance and
has good charge-discharge cycle characteristics, with a lithium
transition metal compound being used as a positive active material.
A portion of transition element Me, which comprises Ni, Co or both
Ni and Co, in a lithium transition metal compound LiMeO.sub.2 to be
used as a positive active material is substituted for with at least
two elements selected from among Ti, Li and Mn. The compound also
corresponds to the formula
LiM.sub.z(Ni.sub.X1Co.sub.X2).sub.1-ZO.sub.2, in which:
0.ltoreq.X1.ltoreq.1; 0.ltoreq.X2.ltoreq.1; X1+X2=1; M comprises at
least two elements selected from among Ti, Li and Mn; and
0<Z<1.
Inventors: |
Nemoto, Hiroshi;
(Nagoya-City, JP) ; Takahashi, Michio;
(Nagoya-City, JP) ; Kitoh, Kenshin; (Nagoya-City,
JP) |
Correspondence
Address: |
BURR & BROWN
PO BOX 7068
SYRACUSE
NY
13261-7068
US
|
Assignee: |
NGK Insulators, Ltd.
Nagoya-City
JP
|
Family ID: |
26510617 |
Appl. No.: |
11/024153 |
Filed: |
December 28, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11024153 |
Dec 28, 2004 |
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10071664 |
Feb 8, 2002 |
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10071664 |
Feb 8, 2002 |
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09348530 |
Jul 7, 1999 |
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6368750 |
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Current U.S.
Class: |
429/223 ;
429/224; 429/231.3; 429/231.5; 429/231.8 |
Current CPC
Class: |
C01P 2006/40 20130101;
H01M 4/485 20130101; H01M 4/525 20130101; H01M 4/0471 20130101;
C01P 2002/32 20130101; H01M 10/0525 20130101; H01M 2004/028
20130101; H01M 4/1391 20130101; C01G 53/42 20130101; C01G 51/42
20130101; H01M 4/587 20130101; C01P 2002/76 20130101; C01G 45/1242
20130101; Y02T 10/70 20130101; C01P 2002/52 20130101; C01P 2002/54
20130101; Y02E 60/10 20130101; C01G 45/1221 20130101; H01M 4/505
20130101 |
Class at
Publication: |
429/223 ;
429/231.3; 429/224; 429/231.5; 429/231.8 |
International
Class: |
H01M 004/52; H01M
004/50; H01M 004/58 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 13, 1998 |
JP |
10-197,853 |
Nov 20, 1998 |
JP |
10-331,449 |
Claims
What is claimed is:
1. A lithium secondary battery, comprising: a positive active
material comprising lithium transition metal compound corresponding
to the formula LiMeO.sub.2, wherein Me comprises Ni, Co or both Ni
and Co, a portion of which is substituted for with at least two
elements selected from among Ti, Li and Mn, said compound also
corresponding to the formula
LiM.sub.z(Ni.sub.X1Co.sub.X2).sub.1-ZO.sub.2, in which:
0.ltoreq.X1.ltoreq.1; 0.ltoreq.X2.ltoreq.1; X1+X2=1; M comprises at
least two elements selected from among Ti, Li and Mn; and
0<Z<1.
2. A lithium secondary battery as recited in claim 1, wherein said
lithium transition metal compound has a two-dimensionally layered
configuration.
3. A lithium secondary battery as recited in claim 1, wherein a
ratio of Z/(X1+X2) is not less than 0.05 and not more than 0.3.
4. A lithium secondary battery as recited in claim 1, further
comprising a negative active material which comprises carbon.
5. A lithium secondary battery as recited in claim 1, further
comprising a negative active material selected from the group
consisting of amorphous carbon material, artificial graphite and
natural graphite.
6. A lithium secondary battery as recited in claim 1, wherein the
lithium transition metal compound is composed by firing a mixed
compound comprising salts and/or oxides having been prepared with a
predetermined ratio in the presence of oxygen within a temperature
range of 600.degree. C. to 1000.degree. C. for 5 hours to 50
hours.
7. A lithium secondary battery as recited in claim 1, wherein the
lithium transition metal compound has been synthesized and obtained
by conducting at least first and second firing steps, with the
firing temperature of the second step being higher than that of the
first step.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation of U.S. application Ser.
No. 10/071,664, filed Feb. 8, 2002, which in turn is a Divisional
of U.S. application Ser. No. 09/348,530, filed Jul. 7, 1999, now
U.S. Pat. No. 6,368,750, the entireties of which are incorporated
herein by reference.
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT
[0002] The present invention relates to, among secondary batteries
to be used as an operational power source for portable electronic
equipment, or as a motor driving battery for an electric vehicle or
a hybrid electric vehicle, etc., a lithium secondary battery which
has small internal resistance and has good charge-discharge cycle
characteristics, with a lithium transition metal compound being
used as a positive active material.
[0003] In recent years, miniaturization to go with lighter weight
is being investigated in an accelerated fashion with respect to
electronic equipment such as a personal handy phone system, a video
tape recorder, a notebook-sized personal computer, etc., and a
secondary battery comprising a lithium transition metal compound as
a positive active material, with a carbon material as a negative
material, and an electrolyte obtained by dissolving a Li ion
electrolyte in an organic solvent, has become common as the power
source battery.
[0004] Such a battery is generally called a lithium secondary
battery or a lithium ion battery, and since they are provided with
larger energy density as well as with higher unit cell voltage of
approximately 4V, attention is being paid to these batteries not
only for the aforementioned electronic equipment but also as a
motor driving power source for an electric vehicle or a hybrid
electric vehicle which is under consideration for positive
proliferation to the general public as a low pollution vehicle, in
view of the recent environmental problems.
[0005] In such a lithium secondary battery, its battery capacity as
well as its charge-discharge cycle characteristics (hereinafter
called "cycle characteristics") heavily depends on the material
characteristics of the positive active material to be used. The
lithium transition metal compound to be used as the positive active
material includes lithium cobalt oxide (LiCoO.sub.2), lithium
nickel oxide (LiNiO.sub.2), or lithium manganese oxide
(LiMn.sub.2O.sub.4), etc. in particular.
[0006] Here, LiCoO.sub.2 as well as LiNiO.sub.2 comprises such
features as a large Li capacity, a simple configuration, and
excellent reversibility, and is provided with a two dimensionally
layered configuration that is excellent in ion diffusion. On the
other hand, however, as concerns LiCoO.sub.2, producing areas of Co
are limited and it hardly is true that output quantity is abundant.
Accordingly, these materials are expensive, and thus there is a
cost issue and a problem that output density is smaller compared
with LiMn.sub.2O.sub.4. In addition, as concerns LiNiO.sub.2,
synthesis of compounds of stoichiometric composition is difficult
since the trivalent status of Ni is comparatively unstable, and in
the case where detachment of Li becomes abundant, Ni will become
subject to transition to bivalent status, emitting oxygen to
constitute NiO, which creates problems such that the battery will
stop functioning as a battery and there is a risk of battery
explosion due to oxygen detachment.
[0007] On the contrary, LiMn.sub.2O.sub.4 has a feature that raw
materials are inexpensive and larger output density as well as
higher voltage is provided. However, in the case where
LiMn.sub.2O.sub.4 has been used as a positive active material,
there is a problem that repetition of charging-discharging cycle
gradually decreases discharge capacity and good cycle
characteristics will not become obtainable. It is deemed that the
major cause of this is reduction of the positive capacity since
crystal configuration changes irreversibly due to insertion and
detachment of Li.sup.+.
[0008] Thus, a lithium transition metal compound such as
LiCoO.sub.2, etc. respectively has both advantages and
disadvantages together as a positive active material, and
therefore, there are no rules which substances must be used, and it
is deemed advisable that a positive active material which can show
an appropriate feature for a particular purpose should be suitably
selected for use.
[0009] Incidentally, regardless of the kind of positive active
material, it is preferred in terms of characteristics of a battery
that the internal resistance of the battery is small, and it is a
common problem to all the positive active materials to be solved
that resistance in a positive active material (namely electronic
conduction resistance) should be reduced, or in other words,
electronic conductivity should be improved for this reduction of
the internal resistance. Particularly, in a lithium secondary
battery of large capacity used as a motor driving battery for an
electric vehicle, etc., it is very important to obtain large
current output necessary for acceleration and gradeability, etc. to
improve charging-discharging efficiency.
[0010] Under the circumstances, conventionally, trials to improve
electronic conductivity by adding to a positive active material
conductive fine grains such as acetylene black, etc. to reduce
internal resistance of a battery have been conducted. This is
because the above-described lithium transition metal compound is a
mixed conducting body comprising both lithium ion conductivity and
electronic conductivity together, but its electronic conductivity
is not always strong.
[0011] However, there is a problem that addition of acetylene black
causes reduction of filling quantity of a positive active material
to reduce battery capacity. In addition, it is deemed that
improvement of electronic conductivity is not unlimited since
acetylene black is a kind of carbon and is a semiconductor.
Moreover, acetylene black is voluminous and presents such a problem
that it is difficult to handle when an electrode plate is to be
produced. Accordingly, the volume of its addition is to be limited
to an appropriate quantity, comparing and considering the
advantageous effect of reduction of internal resistance, the
disadvantageous effect of reduction of battery capacity, and the
simplicity in production, etc.
[0012] Now, as described above, in the case where acetylene black
has been added, acetylene black exists only on surfaces of
particles of a positive active material, resulting in contributing
to improvement of electronic conductivity among particles of
positive active material, but not resulting in contributing to
improvement of electronic conductivity inside a particle of a
positive active material. Thus, conventionally, for improving
electronic conductivity of a positive active material, attention
was only paid to electronic conductivity among particles of a
positive active material, but the relationship between diffusion of
Li+ and electronic conductivity inside a particle of a positive
active material at the time of battery reaction was not regarded as
a problem.
[0013] In short, detachment of Li.sup.+ from a particle of a
positive active material as well as insertion of Li.sup.+ to a
particle of a positive active material is proceeded by diffusion of
Li.sup.+ inside a particle of a positive active material,
simultaneously accompanied by transfer of electrons taking place
inside a particle of a positive active material, and at this time,
if electronic conductivity inside a particle of a positive active
material is low, diffusion of Li.sup.+ hardly is apt to take place
and velocity of detachment and insertion of Li.sup.+, namely
velocity of battery reaction, becomes slow, resulting in an
increase in internal resistance, which was not taken into
consideration at all.
[0014] The present inventors paid attention to this point, and
considered in earnest to improve electronic conductivity of a
positive active material itself so that diffusion of Li.sup.+
inside a positive active material may proceed well, thus reducing
resistance of the positive active material itself, and at the same
time, when a battery has been assembled without increasing volume
of acetylene black to be added, internal resistance of that battery
may be reduced, and as a result the present invention has been
achieved.
SUMMARY OF THE INVENTION
[0015] According to an aspect of the present invention, there is
provided a lithium secondary battery, comprising a lithium
transition metal compound LiMe.sub.XO.sub.Y, in which a portion of
transition element Me is substituted by not less than two
additional elements selected from the group consisting of Li, Fe,
Mn, Ni, Mg, Zn, B, Al, Co, Cr, Si, Ti, Sn, P, V, Sb, Nb, Ta, Mo,
and W to constitute LiM.sub.ZMe.sub.X-ZO.sub.Y (herein M represents
substitution elements, and M.noteq.Me, and Z represents quantity of
substitution), the LiM.sub.ZMe.sub.X-ZO.sub.Y being used as a
positive active material.
[0016] In the present invention, not less than 2 kinds of elements
are preferably selected as the substitution elements M among the
above-described group of elements, particularly Li, Fe, Mn, Ni, Mg,
Zn, Si, Ti, Sn, P, V, Sb, Nb, Ta, Mo, and W, and it is especially
preferred that at least Ti is included. It is also preferred that a
portion of the remaining transition elements Me in
LiM.sub.ZMe.sub.X-ZO.sub.Y to include not less than two kinds of
substitution elements M to be obtained this way is also preferably
substituted further by at least one element selected among B, Al,
Co, and Cr. Also it is preferred that in a lithium transition metal
compound LiM.sub.ZMe.sub.X-ZO.sub.Y, Z/X, the ratio of the
substitution quantity Z of substitution elements M and Me quantity
X of the original transition elements, fulfills the condition of
0.005.ltoreq.Z/X.ltoreq.0.3.
[0017] Incidentally, as one of lithium transition metal compounds
to be suitably used in the present invention, lithium manganese
oxide, especially a lithium manganese oxide having a spinel
configuration of cubic system, may be nominated. The average
valence of substitution elements M to substitute a portion of
manganese in such lithium manganese oxide is set at not less than 3
but not more than 4. Here, an average valence is an average value
of ion valence of not less than two different substitution elements
M in a positive active material. Here, in the case where lithium
manganese oxide has been used, a substitution quantity Z preferably
remains within a range of 0.01.ltoreq.Z.ltoreq.0.5 and more
preferably fulfills a condition of 0.1.ltoreq.Z.ltoreq.0.3.
[0018] In addition, in the present invention, lithium cobalt oxide
or lithium nickel oxide is suitably used as a lithium transition
metal compound. In the case where such materials have been used, it
is preferred that the average valence of substitution elements M to
be substituted with a portion of cobalt or nickel in lithium cobalt
oxide or lithium nickel oxide is 3. However, the case where all the
substitution elements M have the ion valence of 3 is excluded.
Here, the substitution quantity Z preferably remains within the
range of 0.005.ltoreq.Z.ltoreq.0- .3, and further preferably
fulfills the condition of 0.05.ltoreq.Z.ltoreq.0.3.
[0019] LiM.sub.ZMe.sub.X-ZO.sub.Y to be used in the above-described
lithium secondary battery of the present invention is composed by
firing a mixed compound comprising salts and/or oxides having been
prepared with a predetermined ratio in an oxidation atmosphere in a
temperature range of 600.degree. C. to 1000.degree. C., for 5 hours
to 50 hours. At this time, also suitably adopted is such a method
that is conducted, dividing firing into not less than twice, with
the firing temperature for the forthcoming step to be set higher
than that for the previous step, and thus proceeding with the
composition. Here, in the case where a plurality of firing steps is
conducted, the final firing is to be conducted under a firing
condition involving an oxidation atmosphere in a temperature range
of 600.degree. C. to 1000.degree. C., for 5 hours to 50 hours.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
[0020] In a lithium secondary battery of the present invention, a
portion of transition element Me of a lithium transition metal
compound LiMe.sub.XO.sub.Y is substituted by not less than two
elements to constitute LiM.sub.ZMe.sub.X-ZO.sub.Y, the
LiM.sub.ZMe.sub.X-ZO.sub.Y being used as a positive active
material. Here, M represents substitution elements, and
substitution elements M are the one which are different from a
transition element Me (M.noteq.Me), and Z represents quantity of
substitution. Strictly, since not less than two kinds of
substitution elements M are involved, the chemical formula of the
positive active material is described as
Li((M.sub.1).sub.x1(M.sub.2).sub.x2 . . .
(M.sub.n).sub.xn).sub.ZMe.sub.X-ZO.sub.Y (herein, M.sub.1, M.sub.2,
. . . , and M.sub.n represent respectively different elements, and
the total sum of x.sub.1 to x.sub.n is 1) for substitution by
n-numbered kinds of elements. Incidentally, element substitution of
the present invention involving such plural elements will be
hereinafter called "complex substitution".
[0021] As substitution elements M, not less than two elements are
selected from the group consisting of Li, Fe, Mn, Ni, Mg, Zn, B,
Al, Co, Cr, Si, Ti, Sn, P, V, Sb, Nb, Ta, Mo, and W. These elements
were determined by applying Hume-Rothery's rule to an ionic radius
introduced by SHANNON, et al which has been described in Acta
Cryst. (1976). A32, 751, and for the ion radius of transition
element Me to be substituted in a space group R(-3).sub.m (herein
"-" represents rotation-inversion) or in Fd3m (a spinel
configuration), a condition that the coordination number for oxygen
is the same as that for the transition element Me and the average
ionic radius of the substitution elements M remains within .+-.15%
of the ionic radius of the transition element Me, and is not a
radioactive element nor a gas, and not strongly toxic having been
fulfilled so as to select a combination of elements. Here, as a
transition element Me, Mn, Co, and Ni to be suitably used in the
present invention are regarded as a standard.
[0022] An ionic radius of substitution elements M is referred to an
average value of ionic radius of not less than 2 kinds of elements,
and is determined in consideration of existence ratio of each
element. In the present invention, it is preferable that all the
ionic radii of the substitution elements M remains within .+-.15%
of the ionic radius of the transition element Me, but in the case
where such a condition may not be fulfilled, for example even in
the case of the substitution element M.sub.1 numbered 1 with it
ionic radius far larger than the range of +15% of the ionic radius
of the transition element Me, and the substitution element M.sub.2
numbered 2 with its ionic radius far smaller than the range of -15%
of the ionic radius of the transition element Me, if an average
ionic radius of the substitution elements M.sub.1 and M.sub.2 falls
in the range of +15% of the ionic radius of the transition element
Me, complex substitution is feasible.
[0023] However, in the case where Li is used, Li can be used as a
substitution element M, exceptionally, even when the
above-described conditions on ionic radius are not fulfilled. The
reasons of this are that other than the ionic radius of the
above-described version of SHANNON, et al, there is also a version
of Polling, et al, and there is a big difference in normal values
for these versions, thus limiting consideration on only the ionic
radius of Li is problematic in terms of character itself, and that
Li is an original constitutional element and particularly in the
LiMn.sub.2O.sub.4 system, Li is deemed to substitute the position
of Mn, and further that it is experimentally possible to
solid-solubilize Li.
[0024] Incidentally, as concerns substitution elements M, in
theory, Li is to become a +1 valence ion, Fe, Mn, and Ni, Mg, and
Zn are +2 valence ions, B, Al, Co, and Cr are +3 valence ions, Si,
Ti, and Sn are +4 valence ions, P, V, Sb, Nb, and Ta are +5 valence
ions, and Mo and W are +6 valence ions, and they all are elements
to be solid-solubilized in LiM.sub.ZMe.sub.X-ZO.sub.Y. However, for
Co and Sn, they can be +2 valence ions, and for Fe, Sb and Ti, they
can be +3 valence ions and for Mn they can be +3 and +4 valence
ions, and for Cr they can even be +4 and +6 valence ions.
[0025] Therefore, as seen in an actual positive active material, in
the case where there exists a part of ionic valence subject to
change in valence values due to various crystallographic
deficiencies, in some cases there is a possibility that an average
valence of substitution elements M might not coincide with
theoretic valence value, e.g. 3.5 for lithium manganese oxide and 3
for lithium cobalt oxide as well as lithium nickel oxide, of a
transition element Me prior to complex substitution.
[0026] For example, since Ti can exist comparatively stably under
+3 valence condition in addition to +4 valence condition, in the
case where Ti has been solid-solubilized in
LiM.sub.ZMe.sub.X-ZO.sub.Y under the condition having such mixed
atomic valence, the average valence of Ti falls in a range between
+3 to +4. And as concerns Fe, since Fe remains equally stable under
+2 and +3 valence condition and it is also known that the status of
+4 valence exists stably in a certain chemical compound, the
average valence of Fe in LiM.sub.ZMe.sub.X-ZO.sub.Y is to fall in a
range between +2 to +4. In addition, similarly, also as concerns
quantity of oxygen in LiM.sub.ZMe.sub.X-ZO.sub.Y, it may exist in
deficit or in excess within a range to sustain a crystal
configuration.
[0027] Incidentally, as a lithium transition metal compound to be
used in the present invention, lithium manganese oxide, lithium
cobalt oxide, and lithium nickel oxide may be nominated in
particular. Here as concerns lithium manganese oxide, a lithium
manganese oxide (LiMn.sub.2O.sub.4) having a spinel configuration
of cubic system is suitably used. In LiMn.sub.2O.sub.4, one Mn in
two units of Mn is in the state of +3 valence while the other Mn is
in the state of +4 valence state. Accordingly, in complex
substitution, two cases can be considered, namely a case where
substitution elements M are used for substitution of Mn in this +3
valence state, and a case involving substitution of Mn in +4
valence state.
[0028] An average valence value of the substitution elements M is 3
in the case where complex substitution of +3 valence Mn takes
place, but here at least elements to become ions with other than +3
valence is included in the substitution elements M. For example,
such cases that two units of +3 valence Mn undergo complex
substitution with one +2 valence Mg and +4 valence Ti, and two
units of +3 valence Mn undergo substitution with one +1 valence Li
and one +5 valence V can be nominated. And in the case where a +3
valence Mn undergoes complex substitution with such an element
having other than +3 valence, it is permitted that the remaining +3
valence Mn is substituted with another +3 valence ion. Here, an
average valence is referred to an average value of ion valence of
not less than two different substitution elements M in a positive
active material and is determined, putting their existence ratio
under consideration.
[0029] Likewise, for the purpose that +4 valence Mn undergoes
complex substitution, it is necessary that substitution has taken
place with at least an element to provide a valence value other
than +4 valence, and thereafter the remaining +4 valence Mn may be
substituted with an element to provide the same +4 valence. In
general, in complex substitution of LiMn.sub.2O.sub.4, at least it
is necessary that the ionic valence of the substitution elements M
numbered 1 is not more than 3 and the ionic valence of another
substitution elements M is not less than 4, consequently resulting
in the average valence of only substitution elements M to be ranged
from not less than 3 to not more than 4, and the average valence
value obtained from the substitution elements M after complex
substitution inclusive of Mn being 3.5.
[0030] On the other hand, since the substitution elements M to make
a portion of Co or Ni in lithium cobalt oxide (LiCoO.sub.2), and
lithium nickel oxide (LiNiO.sub.2) undergo complex substitution is
to provide an average valence value of 3, similarly in the
above-described substitution of +3 valence Mn, the substitution
elements M are to include elements to provide ions with at least
other than +3 valence. Therefore, the case where all the
substitution elements M have ionic valence value of 3 valence is
excluded from complex substitution of the present invention.
[0031] In the case where a battery has been assembled using a
positive active material which had undergone such complex
substitution, there reveals an effect with remarkable reduction of
internal resistance. This is deemed to be caused by that electronic
conductivity is improved in the frame of lithium transition
elemental composite compound (a portion exclusive of Li
attributable to ionic conduction), and thus velocity of detachment
and insertion of Li ions in battery reaction has become faster. And
considering that the lattice constant gets small due to complex
substitution, the improvement of electronic conductivity in this
frame is presumed to heavily depend on that in the case where
transition elements Me each other and/or substitution elements M
are transition metal elements, the d orbital between substitution
elements M and a transition element Me is apt to overlap, which
makes it easier to smoothly proceed with the movement of electrons
by use of this d orbital.
[0032] In addition, repeating charge and discharge of a battery
assembled by use of materials which have undergone complex
substitution, no deterioration is observed, compared with the case
involving use of materials which have not undergone complex
substitution, and therefore, it is deemed that complex substitution
does not negatively affect stability of the frame. Moreover, in
LiMn.sub.2O.sub.4, as shown in the below-described embodiments, the
cycle characteristics have been improved, thus it is deemed that
complex substitution attributes to improvement of reversibility of
crystal lattice associated with insertion and detachment of Li
ions.
[0033] Incidentally, compared with the case where a portion of the
transition element Me is substituted by another element
(hereinafter, such substitution involving one element is referred
to as "single element substitution"), according to complex
substitution, such a problem that positive capacity might be
reduced by larger volume of substitution in single element
substitution can be avoided. Next, this example is explained by use
of LiMn.sub.2O.sub.4, but it goes without saying that the
explanation may be made to LiCoO.sub.2 and LiNiO.sub.2.
[0034] In the case where Mn.sup.3+ in LiMn.sub.2O.sub.4 has
undergone single element substitution with an element having
valence value of not more than two valence, e.g. one valence ion
such as Li.sup.+, charge equivalent to +2 valence value, being a
difference of charge with Mn.sup.3+, will be in short, thus for the
purpose of maintaining electrical neutrality of materials, two
units of Mn.sup.3+ will be changed to Mn.sup.4+. Thus,
consequently, one Li.sup.+ will be substituted with Mn.sup.3+ and
solid-solubilized, resulting in reduction of approximately three
units of Mn.sup.3+.
[0035] Here, in LiMn.sub.2O.sub.4, it is deemed that, at the time
of charging, electrical neutrality of materials is maintained by
compensating shortage of charge due to detachment of Li.sup.+ with
Mn.sup.3+ being changed to Mn.sup.4+, and at the time of
discharging reverse reaction takes place. In short, the quantity of
Mn.sup.3+ in LiMn.sub.2O.sub.4 determines the positive capacity,
and a quantity of Li.sup.+ corresponding to Mn.sup.3+ attributes
charging and discharging reaction. Therefore, for the purpose that
Li.sup.+ is detached from a crystal lattice or inserted into a
crystal lattice, it will become necessary that a change in valence
value takes place in cations other than Li.sup.+, namely
substitution elements M and/or transition element Me.
[0036] However, in the previous embodiment, Li.sup.+ which was
substituted with Mn.sup.3+ has not undergone change in valence
value, consequently Mn.sup.3+ remains in short by three units.
Therefore, 3 units of Li.sup.+ will not attribute to charging and
discharging reaction. In short, consequently there arises a problem
that the positive capacity is reduced in excess of quantity of
substitution. Such a problem similarly takes place in single
element substitution involving +2 valence ions.
[0037] On the other hand, in complex substitution of the present
invention, substitution elements M are to be narrowed to Li, Fe,
Mn, Ni, Mg, Zn, Si, Ti, Sn, P, V, Sb, Nb, Ta, MO, and W
(hereinafter these substitution elements M are referred to as
"substitution elements group within a reduced range"), and not less
than two elements are arranged to be selected, then in addition to
an effect that improves electronic conductivity, the
above-described problem that the positive capacity is reduced in
excess of quantity of element substitution is avoided.
[0038] In short, when ions with +1 valence or +2 valence and ions
with +4 to +6 valence are combined, as concerns shortage of
positive charge caused by solid-solubilizing ions with +1 valence
or +2 valence, the charge is not compensated by change of Mn.sup.3+
to Mn.sup.4+, but ions with +4 to +6 valence are solid-solubilized
and compensated, thus without reducing the positive capacity as a
result of reducing the number of Mn.sup.3+ in excess of
substitution quantity, Mn can undergo substitution.
[0039] For example, in the case where two units of Mn.sup.3+ are
substituted by one Li.sup.+ and one V.sup.5+, reduction of positive
capacity is limited to a reduced volume of two units of Mn.sup.3+,
and it will become possible to make quantity of reduction of
Mn.sup.3+ lesser than reduction by three units of Mn.sup.3+ in the
case where one Mn.sup.3+ has undergone single element substitution
with one Li.sup.+. In addition, in the case where two units of
Mn.sup.3+ have been substituted with one Mg.sup.2+ and one
Ti.sup.4+, reduction of positive capacity is limited to reduction
covering two units of Mn.sup.3+, and is less than reduction of four
units of Mn.sup.3+ in the case where two units of Mn.sup.3+ have
been substituted with two units of Mg.sup.2+. Thus, reduction
quantity of Mn.sup.3+ is equivalent to substitution quantity of
elements, and accordingly such event that reduction in positive
capacity exceeds substitution quantity is to be avoided.
[0040] Here, in complex substitution, when at least Ti is arranged
to be included as substitution elements M, a remarkable effect of
improvement on electronic conductivity is obtainable and
preferable. In addition, Ti can be effectively used to prevent a
drop in positive capacity, which is preferable.
[0041] In LiM.sub.ZMe.sub.X-ZO.sub.Y including not less than two
kinds of substitution elements M obtainable when complex
substitution using elements among the above-described substitution
elements group within a reduced range, a portion of remaining
transition elements Me may further be substituted with at least not
less than one element selected from B, Al, Co, and Cr. In this
case, complex substitution involving at least three elements is to
take place.
[0042] These elements such as B and Al, etc. exist in
LiM.sub.ZMe.sub.X-ZO.sub.Y as ions with +3 valence in theory. But,
as described above, in actual positive active materials, the ion
valence value does not always have to correspond with the theoretic
valence values. Ions with +3 valence are substituted with Mn.sup.3+
one on one, therefore, decrease in positive capacity is the same as
the quantity of substitution, and decrease in positive capacity not
less than the quantity of substitution does not take place, and on
the other hand, the said ion attributes to improvement of electron
conductivity of a positive active material itself. Incidentally, in
the case where LiMn.sub.2O.sub.4 is used, an effect that its
crystal configuration is made reversible toward insertion and
detachment of Li.sup.+ is provided.
[0043] Next, substitution quantity Z in complex substitution is
explained. In the present invention, it is preferred that Z/X, the
ratio of the quantity Z to be substituted by substitution elements
M to the quantity X of the original transition element Me fulfills
the condition of 0.005.ltoreq.Z/X.ltoreq.0.3. When Z/X is less than
0.005, resistance of a positive active material does not drop, and
improvement in cycle characteristics rarely appears. In short, no
effects of complex substitution appear. On the other hand, when Z/X
is more than 0.3, in synthesis of a positive active material,
production of a different phase is admitted through powder X-ray
diffraction method (XRD), and a single phase material was not
obtainable. In a battery, such a different phase only increases the
weight of a positive active material and does not attribute to
battery reaction, thus it goes without saying that production of a
different phase at the time of synthesis together with entry to the
battery should be avoided.
[0044] Positive-active-material-wise, in particular, when
LiMn.sub.2O.sub.4 has been used, the substitution quantity Z
preferably falls within a range of 0.01.ltoreq.Z.ltoreq.0.5, and
further preferably falls in a range of 0.1.ltoreq.Z.ltoreq.0.3, and
when LiCoO.sub.2 as well as LiNiO.sub.2 is used, the substitution
quantity Z preferably falls within a range of
0.005.ltoreq.Z.ltoreq.0.3, and further preferably falls in a range
of 0.05.ltoreq.Z.ltoreq.0.3, and within the respective preferable
ranges of the substitution quantity Z, there remarkably appears an
effect of improvement of electronic conductivity of a positive
active material, which is preferable.
[0045] Incidentally, when elemental substitution by not less than
one element selected from B, Al, Co, and Cr further took place as
well after complex substitution, the total substitution quantity
(Z+W) of substitution quantity Z of substitution elements M
selected from a group of substitution elements within a reduced
range, and the substitution quantity (to be referred to as "w") of
B and Al, etc. is required to fulfill a relationship of
0.01.ltoreq.Z+w.ltoreq.0.5.
[0046] Incidentally, LiM.sub.ZMe.sub.X-ZO.sub.Y to be used in a
lithium secondary battery of the present invention, is composed by
firing a mixed compound comprising salts and/or oxides of each
element (substitution elements M as well as Li and transition
element Me) having been prepared with a predetermined ratio in an
oxidation atmosphere at a temperature range of 600.degree. C. to
1000.degree. C., for 5 hours to 50 hours, and thus a single phase
product can be obtained. Here, an oxidation atmosphere is referred
to as an atmosphere having partial pressure of oxygen with which
generally a sample inside a furnace is brought into oxidation
reaction. In synthesis of LiCoO.sub.2 as well as LiNiO.sub.2, it is
preferable that the partial pressure of oxygen is set at not less
than 10%, and, in particular, air atmosphere and oxygen atmosphere,
etc. are applicable.
[0047] Incidentally, when the firing temperature is as low as less
than 600.degree. C., a peak showing residue of raw material, e.g.
peak of lithium carbonate (Li.sub.2CO.sub.3) in the case where
Li.sub.2CO.sub.3 is used as a lithium source, is to be observed in
XRD chart of fired product, and no single phase products can be
obtained. On the other hand, when the firing temperature is as high
as more than 1000.degree. C., high temperature phase is produced in
other than a compound of the targeted crystal system, and a single
phase product will become no longer obtainable.
[0048] In addition, firing may be conducted, being divided into not
less than twice. In that case, it is preferable that the firing is
proceeded with the firing temperature for the forthcoming step to
be set higher than that for the previous step, and the final firing
is to be conducted under a firing condition involving an oxidation
atmosphere at a temperature range of 600.degree. C. to 1000.degree.
C., for 5 hours to 50 hours. Thus, in the case of firing taking
place twice, for example, applying this condition of second firing
temperature as well as firing period, the product obtainable when
synthesis has been conducted with the temperature for the second
firing to be set at not less than the temperature for the first
firing features steeper projection in the peak shape in the XRD
chart than with the product obtainable when a single firing yields,
and as a result improvement of crystallinity can be planned.
[0049] A salt for each element will not be limited in particular,
but it goes without saying that those having intensive purity and
further being inexpensive as raw materials are preferably to be
used. Accordingly, such carbonate, hydroxide, and organic acid/salt
that do not produce harmful decomposition gas at the times of
elevation of temperature or firing are preferably used. However,
nitrate, hydrochloride, and sulfate, etc. are not always unusable.
Generally, in synthesis of LiCoO.sub.2 and LiNiO.sub.2, it is known
that synthesis temperature goes down with usage of salts instead of
oxides as raw materials. Here, as concerns raw materials on Li,
usually an oxide Li.sub.2O is chemically unstable, and thus it is
rarely used.
[0050] As the foregoing, implementation of complex substitution of
the present invention will make improvement in electronic
conductivity of a positive active material easier to plan,
providing preferable electric characteristics, and resulting in
decrease in internal resistance of a battery. In addition, the
problem that positive capacity might be reduced by larger volume of
element substitution in single element substitution which
conventionally used to be problematic in single element
substitution, is to be solved, and reduction of positive capacity
is to be suppressed to the extent equivalent to quantity of element
substitution. At the same time, as concerns LiMn.sub.2O.sub.4,
reversibility of crystal configuration for insertion and detachment
of Li.sup.+ is improved, thus cycle characteristics as a battery
are improved. Accordingly, decrease with the passage of time in
battery capacity due to repetition of charging and discharging is
controlled.
[0051] Reduction of internal resistance and reservation of positive
capacity, and increase in cycle characteristics are planned in such
a battery, which is used as a motor driving power source for an EV
or an HEV in particular, consequently providing with an excellent
effect that predetermined running performance such as acceleration
performance as well as slope-climbing performance, etc. is
maintained, and continuous running distance per charging is kept
long.
[0052] Incidentally, other materials to be used for production of a
battery are not limited whatsoever, and conventionally known
various materials can be used. For example, as a negative active
material, an amorphous carbon material such as soft carbon or hard
carbon, or carbon material such as artificial graphite such as high
graphitized carbon material, etc. and natural graphite, etc. are
used.
[0053] And as an organic electrolyte a carbonic acid ester family
such as ethylene carbonate (EC), diethyl carbonate (DEC), and
dimethyl carbonate (DMC), and the one in which one or more kinds of
lithium fluoride complex compound such as LiPF.sub.6, and
LiBF.sub.4, etc. or lithium halide such as LiClO.sub.4 as an
electrolyte are dissolved in a single solvent or mixed solvent of
organic solvents such as propylene carbonate (PC),
.gamma.-butyrolactone, tetrahydrofuran, and acetonitrile, etc., can
be used.
EXAMPLE
[0054] Successively, taking as a major embodiment complex
substitution involving two kinds of elements as substitution
elements M including Ti which provides most remarkable effect in
the present invention, based on whose experimental results an
explanation is provided as follows:
[0055] Synthesis of Positive Active Material
LiM.sub.ZMn.sub.2-ZO.sub.4:
[0056] As the starting raw material, powder of commercially
available Li.sub.2CO.sub.3, MnO.sub.2, TiO.sub.2, MgO, and NiO was
used and was weighed and mixed so that the positive active material
composition of respective embodiments shown in Table 1 (positive
active materials for measurement of internal resistance ratio),
Table 2 (positive active materials for measurement of capacity of
the initial charging), and Table 3 (positive active materials for a
cycle test) might be obtained, and firing took place at 800.degree.
C. in an air atmosphere for 24 hours, and the positive active
materials were obtained. Here, when the combination of substitution
elements M took place involving Ti and Mg or Ni, the mixing ratio
of them was set at Ti:Mg or Ni=1:1, and for the case involving Li
and Ti, it was set at Li:Ti=1:2. Incidentally, for the purpose of
comparing the effects of complex substitution and single element
substitution, positive active materials where a portion of Mn
underwent single element substitution with Mg, Ti, Ni, and Li
respectively as well as LiMn.sub.2O.sub.4 which did not undergo
element substitution were formed under the similar conditions.
1 TABLE 1 Positive active material Internal resistance ratio
composition of coin cells (%) Comparative LiMn.sub.2O.sub.4 100
example 1 Embodiment 1
Li(Mg.sub.0.5Ti.sub.0.5).sub.0.01Mn.sub.1.99O.sub.4 54 Embodiment 2
Li(Mg.sub.0.5Ti.sub.0.5).sub.0.10Mn.sub.1.90O.sub.4 37 Embodiment 3
Li(Mg.sub.0.5Ti.sub.0.5).sub.0.15Mn.sub.1.85O.sub.4 35 Embodiment 4
Li(Mg.sub.0.5Ti.sub.0.5).sub.0.30Mn.sub.1.70O.sub.4 29 Embodiment 5
Li(Mg.sub.0.5Ti.sub.0.5).sub.0.50Mn.sub.1.50O.sub.4 41 Embodiment 6
Li(Ni.sub.0.5Ti.sub.0.5).sub.0.01Mn.sub.1.99O.sub.4 52 Embodiment 7
Li(Ni.sub.0.5Ti.sub.0.5).sub.0.10Mn.sub.1.90O.sub.4 36 Embodiment 8
Li(Ni.sub.0.5Ti.sub.0.5).sub.0.15Mn.sub.1.85O.sub.4 36 Embodiment 9
Li(Ni.sub.0.5Ti.sub.0.5).sub.0.30Mn.sub.1.70O.sub.- 4 30 Embodiment
10 Li(Ni.sub.0.5Ti.sub.0.5).sub.0.50Mn.sub.1.50O.su- b.4 45
Comparative LiMg.sub.0.15Mn.sub.1.85O.sub.4 80 example 2
Comparative LiTi.sub.0.15Mn.sub.1.85O.sub.4 69 example 3
Comparative LiNi.sub.0.15Mn.sub.1.85O.sub.4 71 example 4
[0057]
2 TABLE 2 Positive active material Capacity of the initial
composition charging (mAh/g) Embodiment 3
Li(Mg.sub.0.5Ti.sub.0.5).sub.0.15Mn.sub.1.85O.sub.4 102 Embodiment
12 Li(Li.sub.0.33Ti.sub.0.67).sub.0.15Mn.sub.1.85O.sub.4 102
Comparative LiMg.sub.0.15Mn.sub.1.85O.sub.4 85 example 2
Comparative LiTi.sub.0.15Mn.sub.1.85O.sub.4 105 example 3
Comparative LiLi.sub.0.15Mn.sub.1.85O.sub.4 70 example 5
[0058]
3 TABLE 3 Capacity ratio toward capacity of the initial Positive
active material charging of a battery composition after 100 cycles
Embodiment 2 Li(Mg.sub.0.5Ti.sub.0.5).sub.0.10Mn.sub.1.90O.sub.4
0.69 Embodiment 3
Li(Mg.sub.0.5Ti.sub.0.5).sub.0.15Mn.sub.1.85O.sub.4 0.84 Embodiment
4 Li(Mg.sub.0.5Ti.sub.0.5).sub.0.30Mn.sub.1.70O.sub.4 0.83
Embodiment 5 Li(Mg.sub.0.5Ti.sub.0.5).sub.0.50Mn.sub.1.50O.su- b.4
0.73 Embodiment 11 Li(Li.sub.0.33Ti.sub.0.67).sub.0.10Mn.sub.1.-
90O.sub.4 0.71 Embodiment 12
Li(Li.sub.0.33Ti.sub.0.67).sub.0.15Mn.- sub.1.85O.sub.4 0.85
Embodiment 13 Li(Li.sub.0.33Ti.sub.0.67).sub.0-
.30Mn.sub.1.70O.sub.4 0.82 Embodiment 14
Li(Li.sub.0.33Ti.sub.0.67)- .sub.0.50Mn.sub.1.50O.sub.4 0.70
Comparative LiMg.sub.0.15Mn.sub.1.85O.sub.4 0.68 example 2
Comparative LiTi.sub.0.15Mn.sub.1.85O.sub.4 0.66 example 3
Comparative LiLi.sub.0.15Mn.sub.1.85O.sub.4 0.69 example 5
[0059] Synthesis of Positive Active Materials
LiM.sub.ZCo.sub.1-ZO.sub.2:
[0060] As the starting raw material, commercially available
Li.sub.2CO.sub.3, CO.sub.3O.sub.4, NiO, MgO, and TiO.sub.2 were
used and were weighed and mixed so that the composition of
respective kinds of embodiments shown in Table 4 as well as Table 5
(positive active materials for measurement of internal resistance
ratio) might be obtained. And as concerns
LiM.sub.ZCo.sub.1-ZO.sub.2, firing took place at 900.degree. C. in
an air atmosphere for 20 hours, and on the other hand as concerns
LiM.sub.ZNi.sub.1-ZO.sub.2, firing took place at 750.degree. C. in
an oxygen atmosphere for 20 hours to proceed with synthesis. In
addition, as put down in Table 4 as well as Table 5, LiCoO.sub.2 as
well as LiNiO.sub.2 in which no addition elements were added, and
also samples related to Examples undergoing single element
substitution were synthesized under the similar conditions. The
formed respective kinds of positive active materials of Embodiments
as well as Comparative examples were confirmed to be in a single
phase through XRD.
4 TABLE 4 Positive Internal active material resistance ratio
composition of coin cells (%) Comparative LiCoO.sub.2 100 example 6
Embodiment 15 Li(Mg.sub.0.5Ti.sub.0.5).sub.0.005Co.sub.0.995O.sub.2
86 Embodiment 16
Li(Mg.sub.0.5Ti.sub.0.5).sub.0.05Co.sub.0.95O.sub.2 69 Embodiment
17 Li(Mg.sub.0.5Ti.sub.0.5).sub.0.25Co.sub.0.75O.sub.2 65
Embodiment 18 Li(Mg.sub.0.5Ti.sub.0.5).sub.0.3Co.sub.0.7O.sub.2 73
Embodiment 19 Li(Ni.sub.0.5Ti.sub.0.5).sub.0.005Co.sub.0.995O.s-
ub.2 88 Embodiment 20
Li(Ni.sub.0.5Ti.sub.0.5).sub.0.05Co.sub.0.95O- .sub.2 63 Embodiment
21 Li(Ni.sub.0.5Ti.sub.0.5).sub.0.25Co.sub.0.7- 5O.sub.2 59
Embodiment 22 Li(Ni.sub.0.5Ti.sub.0.5).sub.0.3Co.sub.0.- 7O.sub.2
67 Comparative LiMg.sub.0.05Co.sub.0.95O.sub.2 90 example 7
Comparative LiTi.sub.0.05Co.sub.0.95O.sub.2 87 example 8
Comparative LiNi.sub.0.5Co.sub.0.95O.sub.2 94 example 9
[0061]
5 TABLE 5 Positive active material Internal resistance composition
ratio of coin cells (%) Comparative LiNiO.sub.2 100 example 10
Embodiment 23
Li(Li.sub.0.33Ti.sub.0.67).sub.0.005Ni.sub.0.995O.sub.2 91
Embodiment 24 Li(Li.sub.0.33Ti.sub.0.67).sub.0.05Ni.sub.0.95O.sub.2
77 Embodiment 25
Li(Li.sub.0.33Ti.sub.0.67).sub.0.25Ni.sub.0.75O.sub.2 72 Embodiment
26 Li(Li.sub.0.33Ti.sub.0.67).sub.03Ni.sub.0.7O.sub.- 2 80
Comparative LiTi.sub.0.05Ni.sub.0.95O.sub.2 93 example 11
Comparative LiLi.sub.0.05Ni.sub.0.85O.sub.2 110 example 12
[0062] Forming of a Battery:
[0063] At first, using the formed various kinds of positive active
materials, and mixing a positive active materials, acetylene black
powder being conductive material, and polyvinylidene fluoride being
bonding material with a weight ratio of 50:2:3 to form a positive
material. A disk shape having diameter of 20 mm.phi. was prepared
as a positive pole by press-forming 0.02 g of the said positive
material under a pressure of 300 kg/cm.sup.2. Next, in accordance
with test purposes, as described below, two kinds of coin cells
were formed. In short, the coin cells for measuring internal
resistance set forth in Table 1, Table 4, and Table 5 as well as
the coin cells for cycle tests set forth in Table 3 were formed by
using a positive pole having been formed as described above, an
electrolyte having been formed by dissolving LiPF.sub.6 being an
electrolyte into an organic solvent with ethylene carbonate and
diethyle carbonate being mixed with a same volume ratio to
constitute a density of 1 mol/L, a negative pole made of carbon,
and a separator separating the positive electrode and the negative
pole.
[0064] On the other hand, the coin cells for measuring the capacity
for initial charging set forth in Table 2 were formed by using a
positive pole having been formed, an electrolyte having been formed
by dissolving LiClO.sub.4 being an electrolyte into propylene
carbonate to constitute a density of 1 mol/L, a negative pole made
of metal Li, and a separator separating the positive pole and the
negative pole.
[0065] Method to Measure a Battery's Internal Resistance and the
Results Thereof:
[0066] As concerns coin cells having been formed as described
above, using respective kinds of positive active materials set
forth in Table 1, Table 4 and Table 5, only one cycle of charging
and discharging test was conducted, involving charging constant
current of 1 C rate and constant voltage of 4.1 V in accordance
with the capacity of a positive active material, and similarly
discharging constant current of 1 C rate and constant voltage 2.5
V, and the battery's internal resistance was obtained by dividing
difference between the potential at a resting state after finishing
charging and the potential immediately after commencement of
discharging (potential difference) with discharging currency. And
the internal resistance of a battery using a positive active
material which underwent single element substitution and complex
substitution was divided by the internal resistance of a battery
using a conventional compound which did not undergo elemental
substitution respectively (LiMn.sub.2O.sub.4, LiCoO.sub.2, and
LiNiO.sub.2) to yield a value, which was stipulated as an internal
resistance ratio. Accordingly, as the value of internal resistance
ratio gets smaller, reduction effect on internal resistance gets
larger. The results have been put down in Table 1, Table 4, and
Table 5, respectively.
[0067] Based on Table 1, as concerns LiMn.sub.2O.sub.4, in the case
where positive active materials having undergone single element
substitution were used, in other words, in the case where the
embodiments 1 through 10 having involved positive active materials
having undergone complex substitution while internal resistance in
comparative examples 2 through 4 has halted at approximately 70% at
the best, it is obvious that the substitution quantity Z has fallen
in the range of 0.01.ltoreq.Z.ltoreq.0.5, and the internal
resistance ratio has been decreased to reach not more than
approximately 50%. In addition, as shown in embodiments 2 through 4
as well as embodiments 7 through 9, in the case where complex
substitution has taken place so that substitution quantity Z may
fall in the range of 0.1.ltoreq.Z.ltoreq.0.3, it is obvious that
remarkable reduction effect in internal resistance has been
obtained.
[0068] Based on Table 4, as concerns LiCoO.sub.2, compared with
comparative examples 7 through 9 where single element substitution
took place, it was confirmed that remarkable reduction in internal
resistance appeared in embodiments 15 through 22 where complex
substitution took place. And, as shown in embodiments 16 through 18
as well as embodiments 20 through 22, for the range of
0.1.ltoreq.Z.ltoreq.0.3, Z being substitution quantity, remarkable
reduction effect in internal resistance has appeared. Incidentally,
in the case where LiCoO.sub.2 is the basic material, reduction
effect in internal resistance has been limited to a small extent,
compared with the case involving LiMn.sub.2O.sub.4.
[0069] The value of internal resistance ratio obtained by single
element substitution as well as complex substitution having used
LiNiO.sub.2 as the basic material has been similar to the case
having involved LiCoO.sub.2, and compared with comparative examples
11 and 12 where single substitution took place, the internal
resistance ratio has been reduced to a large extent in embodiments
23 through 26 where complex substitution took place, and as shown
in embodiments 24 through 26, for the range of
0.05.ltoreq.Z.ltoreq.0.3, Z being substitution quantity, a
reduction effect in internal resistance has appeared to a large
extent. However, as in the case of LiCoO.sub.2, compared with the
case using LiMn.sub.2O.sub.4, the effect of decrease in internal
resistance is little.
[0070] From these results, complex substitution by not less than
two kinds selected from the group consisting of Li, Fe, Cr, Mn, Ni,
Mg, Zn, B, Al, Co, Cr, Si, Ti, Sn, P, V, Sb, Nb, Ta, Mo, and W has
been conducted, and forming of positive active materials through
measurement of internal resistance by the method similar to the one
described above, and as a result the tendency similar to the case
involving complex substitution having shown in Table 1 was
confirmed.
[0071] Measurement of a Battery's Internal Charging Capacity and
the Results Thereof:
[0072] As concerns coin cells having been formed as previously
described, using positive active materials set forth in Table 2,
the initial charging capacity (battery capacity) was measured,
involving charging to reach 4.2 V at a constant currency and
constant voltage of 0.2 C rate in accordance with the capacity of a
positive active material. The results have been put down in Table
2. Based on these results, it is obvious that in the case where
element substitution quantity as a whole is same, compared with
single element substitution by Li.sup.+ and Mg.sup.2+ respectively,
battery capacity has got large in the case where complex
substitution took place, however, in the case involving single
element substitution by Ti.sup.4+, battery capacity approximately
equivalent to that in the case involving complex substitution has
especially been obtained.
[0073] It is deemed that in the single element substitution
respectively by Li.sup.+ and Mg.sup.2+, as previously described,
reduction in Mn.sup.3+ in the quantity not less than element
substitution quantity has reduced Li.sup.+ attributable to charging
and discharging, and thus has reduced battery capacity,
nevertheless, complex substitution has shown that it has controlled
the said reduction in capacity. It is deemed on the other hand that
in the case involving single element substitution by Ti.sup.4+,
most portion of Ti.sup.4+ has undergone change in valence value to
Ti.sup.3+ at the time of firing, and thus, substitution between
Ti.sup.3+ and Mn.sup.3+ have made available the battery capacity
equivalent to that obtainable in complex substitution.
[0074] Having this result in hand, complex substitution by not less
than two elements selected from the group consisting of Li, Fe, Cr,
Mn, Ni, Mg, Zn, Si, Ti, Sn, P, V, Sb, Nb, Ta, Mo, and W has been
conducted, and forming of positive active materials through
assessment of battery capacity was conducted by the method similar
to the one described above, and as a result the characteristics
similar to the case involving complex substitution having shown in
Table 2 were obtained.
[0075] In addition, for the purpose of looking into a range of
composition where reduction controlling effect on battery capacity
by complex substitution appears, experiments similar to those
described above with variety of substitution quantity Z, it became
obvious that the substitution quantity Z preferably fell in the
range of 0.01.ltoreq.Z.ltoreq.0.5. In the case where the
substitution quantity Z exceeded 0.5, in any combination of
substitution elements M, production of compounds other than those
in the spiner phase was confirmed by XRD.
[0076] Cycle Operation Test and the Results Thereof:
[0077] Successively, further for the purpose of looking into cycle
characteristics in a substitution quantity Z where an effect of
complex substitution reveals, as concerns batteries having been
formed as previously described, using positive active materials
having respective compositions set forth in Table 3, a cycle
operation test was conducted, repeating charging constant current
of 1 C rate and constant voltage of 4.1 V and likewise discharging
constant current of 1 C rate and constant voltage of 2.5 V in
accordance with the capacity of a positive active material.
[0078] In Table 3, discharging capacity of a battery after the
consummation of 100 cycles has been put down in terms of ratio
toward the initial discharging capacity of a battery. Consequently,
as this ratio gets larger, reduction in battery's discharging
capacity is deemed to get less. As shown in Table 3, it became
obvious that in a battery where positive active materials having
undergone complex substitution were used, reduction quantity in
battery's discharging capacity is as a whole smaller than in the
case involving positive active materials having undergone single
element substitution, and the said reduction was little especially
within a range of 0.1.ltoreq.Z.ltoreq.0.3, Z being substitution
quantity, and it became obvious that positive active materials
having undergone complex substitution so as to comprise such
compositions showed good cycle characteristics as a battery.
[0079] As described above, according to a lithium secondary battery
of the present invention, sizable reduction in battery's internal
resistance is realized since materials with improved electronic
conductivity as well as low resistance which have been obtained
with transition elements in a lithium transition metal compound
having undergone complex substitution have been used as positive
active materials. In addition, according to the present invention,
reduction in positive capacity in excess of element substitution
quantity is controlled. As a result of this, a lithium secondary
battery according to the present invention serves to provide
extremely excellent effects such as large output, huge capacity as
well as improved and good charge-discharge cycle characteristics,
and further with less energy loss at the time of charging and
discharging. Incidentally, in the case where LiMn.sub.2O.sub.4 has
been used, such effect that reversibility of crystal configuration
associated with charging and discharging is improved and superior
endurance is provided is obtainable.
* * * * *