U.S. patent application number 13/614425 was filed with the patent office on 2013-01-10 for non-aqeous electrolyte secondary battery.
This patent application is currently assigned to SANYO ELECTRIC CO., LTD.. Invention is credited to Masahisa Fujimoto, Motoharu Saito, Sho Tsuruta.
Application Number | 20130011741 13/614425 |
Document ID | / |
Family ID | 42737939 |
Filed Date | 2013-01-10 |
United States Patent
Application |
20130011741 |
Kind Code |
A1 |
Saito; Motoharu ; et
al. |
January 10, 2013 |
NON-AQEOUS ELECTROLYTE SECONDARY BATTERY
Abstract
A non-aqueous electrolyte secondary battery has a positive
electrode containing a positive electrode active material
containing a lithium-containing oxide active material, a negative
electrode, and a non-aqueous electrolyte. The lithium-containing
oxide active material is represented by the general formula
Li.sub.aMg.sub.bMO.sub.2.+-..alpha. where
0.65.ltoreq.a.ltoreq.1.05, 0<b.ltoreq.0.3,
0.ltoreq..alpha..ltoreq.0.3, and M is at least one of manganese and
cobalt.
Inventors: |
Saito; Motoharu; (Kobe-shi,
JP) ; Tsuruta; Sho; (Kobe-shi, JP) ; Fujimoto;
Masahisa; (Osaka-shi, JP) |
Assignee: |
SANYO ELECTRIC CO., LTD.
Osaka
JP
|
Family ID: |
42737939 |
Appl. No.: |
13/614425 |
Filed: |
September 13, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12728718 |
Mar 22, 2010 |
8287606 |
|
|
13614425 |
|
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Current U.S.
Class: |
429/224 ;
429/231.6 |
Current CPC
Class: |
Y10T 29/49115 20150115;
H01M 10/052 20130101; H01M 10/058 20130101; Y02T 10/70 20130101;
Y02E 60/10 20130101; Y10T 29/49108 20150115; H01M 4/505 20130101;
Y10T 29/49112 20150115; H01M 4/1391 20130101; H01M 4/525
20130101 |
Class at
Publication: |
429/224 ;
429/231.6 |
International
Class: |
H01M 4/131 20100101
H01M004/131 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 23, 2009 |
JP |
2009-069497 |
Claims
1. A non-aqueous electrolyte secondary battery comprising: a
positive electrode containing a positive electrode active material
comprising a lithium-containing oxide active material; a negative
electrode; and a non-aqueous electrolyte, wherein: the
lithium-containing oxide active material is represented by the
general formula Li.sub.aMg.sub.bMO.sub.2.+-..alpha. where
0.65.ltoreq.a.ltoreq.1.05, 0<b.ltoreq.0.3,
0.ltoreq..alpha..ltoreq.0.3, and M is at least one of manganese and
cobalt; and the lithium-containing oxide active material has a main
peak at 2.theta.=17.95.degree. to 18.15.degree., as determined by
an X-ray powder crystal diffraction measurement (Cuk.alpha.).
2. The non-aqueous electrolyte secondary battery according to claim
1, wherein the lithium-containing oxide active material is
represented by the general formula
Li.sub.aMg.sub.bMn.sub.xCo.sub.yO.sub.2.+-..alpha., where
0.65.ltoreq.a.ltoreq.1.05, 0<b.ltoreq.0.3,
0.45.ltoreq.x.ltoreq.0.55, 0.45.ltoreq.y.ltoreq.0.55,
0.90.ltoreq.x+y.ltoreq.1.10, and 0.ltoreq..alpha..ltoreq.0.3.
3. The non-aqueous electrolyte secondary battery according to claim
1, wherein the lithium-containing oxide active material has a
crystal structure belonging to an O2 structure, a T2 structure, an
O6 structure, or a mixed structure thereof.
4. The non-aqueous electrolyte secondary battery according to claim
2, wherein the lithium-containing oxide active material has a
crystal structure belonging to an O2 structure, a T2 structure, an
O6 structure, or a mixed structure thereof.
5. The non-aqueous electrolyte secondary battery according to claim
1, wherein 0<b.ltoreq.0.2.
6. The non-aqueous electrolyte secondary battery according to claim
2, wherein 0<b.ltoreq.0.2.
7. The non-aqueous electrolyte secondary battery according to claim
3, wherein 0<b.ltoreq.0.2.
8. The non-aqueous electrolyte secondary battery according to claim
4, wherein 0<b.ltoreq.0.2.
9-16. (canceled)
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of U.S. application Ser.
No. 12/728,718, filed on Mar. 22, 2010, which claims benefit of
priority from the prior Japanese Patent Application No.
2009-069497, filed on Mar. 23, 2009, the entire contents of both of
which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a non-aqueous electrolyte
secondary battery that achieves high capacity and to a method of
manufacturing the battery.
[0004] 2. Description of Related Art
[0005] Mobile information terminal devices such as mobile
telephones, notebook computers, and PDAs have become smaller and
lighter at a rapid pace in recent years. This has led to a demand
for higher capacity batteries as the drive power source for the
mobile information terminal devices. With their high energy density
and high capacity, non-aqueous electrolyte secondary batteries,
which perform charge and discharge by transferring lithium ions
between the positive and negative electrodes, have been widely used
as a driving power source for the mobile information terminal
devices.
[0006] As the mobile information terminal devices tend towards
having greater numbers of functions, such as moving picture playing
functions and gaming functions, the power consumption of the
devices tends to increase. It is therefore strongly desired that
the non-aqueous electrolyte secondary batteries used for the power
sources of such devices have further higher capacities and higher
performance to achieve longer battery life and improved output
power. In addition, applications of the non-aqueous electrolyte
secondary batteries are expected to expand from just the
above-described applications but to power tools, power assisted
bicycles, and moreover hybrid electric vehicles (HEVs) and electric
vehicles (EVs). In order to meet such expectations, it is strongly
desired that the capacity and the performance of the battery be
improved further.
[0007] In order to increase the capacity of the non-aqueous
electrolyte secondary battery, it is necessary to increase the
capacity of the positive electrode. In particular, layered
compounds are viewed as promising materials for positive electrode
active materials. To date, many lithium-containing layered
compounds have been studied. Among the materials that have been
developed are LiCoO.sub.2, LiNiO.sub.2,
LiNi.sub.1/3Co.sub.1/3Mn.sub.1/3O.sub.2, and
Na.sub.xCo.sub.yMn.sub.1-yO.sub.2 where 0.6.ltoreq.x.ltoreq.0.8 and
0.4.ltoreq.y.ltoreq.0.6 (see Japanese Published Unexamined Patent
Application No. 2002-220231).
[0008] In addition, a technique for synthesizing a lithium compound
from a sodium compound has been studied as a method for
synthesizing a novel lithium compound (see Japanese Published
Unexamined Patent Application No. 2007-220650). According to this
method a layered compound, which is difficult to synthesize with
lithium, can be easily obtained. In particular, Na.sub.0.7CoO.sub.2
and NaCo.sub.1/2Mn.sub.1/2O.sub.2 can be used as positive electrode
active materials for lithium-ion batteries by ion-exchanging sodium
for lithium. Therefore, much research has been conducted on
synthesis methods and ion-exchange methods by chemical techniques
using Na.sub.0.7CoO.sub.2 and NaCo.sub.1/2Mn.sub.1/2O.sub.2.
[0009] The positive electrode active materials using sodium-based
oxides are promising materials that are expected to yield high
capacity, and by adding lithium thereto, further high capacity can
be obtained. However, the addition of lithium causes the average
discharge potential to decrease. Moreover, it causes formation of
an impurity similar to Li.sub.2MnO.sub.3, resulting in the problem
of side reactions during charge and discharge.
BRIEF SUMMARY OF THE INVENTION
[0010] Accordingly, it is an object of the present invention to
provide a non-aqueous electrolyte secondary battery and a
manufacturing method of the battery that can inhibit side reactions
during charge and discharge by suppressing formation of an impurity
similar to Li.sub.2MnO.sub.3 and that can preventing the discharge
potential from decreasing even when lithium is added.
[0011] The present inventors studied various materials that may
suppress formation of Li.sub.2MnO.sub.3 impurity and prevent the
discharge voltage from decreasing, and as a result found that the
foregoing object can be accomplished by using magnesium as an
additive metal.
[0012] Accordingly, the present invention provides a non-aqueous
electrolyte secondary battery comprising: a positive electrode
containing a positive electrode active material comprising a
lithium-containing oxide active material; a negative electrode; and
a non-aqueous electrolyte, wherein: the lithium-containing oxide
active material is represented by the general formula
Li.sub.aMg.sub.bMO.sub.2.+-..alpha. where
0.65.ltoreq.a.ltoreq.1.05, 0<b.ltoreq.0.3,
0.ltoreq..alpha..ltoreq.0.3, and M is at least one of manganese and
cobalt; and the lithium-containing oxide active material has a main
peak at 2.theta.=17.95.degree. to 18.15.degree., as determined by
an X-ray powder crystal diffraction measurement (Cuk.alpha.).
[0013] According to the present invention, formation of a
lithium-containing impurity that is caused when adding lithium is
suppressed so that side reactions are inhibited during charge and
discharge, and at the same time, the discharge potential is
prevented from decreasing, because magnesium is added to the
positive electrode active material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a cross-sectional view of a test cell used for the
embodiments of the present invention;
[0015] FIG. 2 is a graph showing the result of an XRD analysis for
a present invention oxide a1;
[0016] FIG. 3 is a graph showing the result of an XRD analysis for
a present invention oxide a2;
[0017] FIG. 4 is a graph showing the result of an XRD analysis for
a present invention oxide a3;
[0018] FIG. 5 is a graph showing the result of an XRD analysis for
a comparative reference oxide x;
[0019] FIG. 6 is a graph showing the result of an XRD analysis for
a comparative oxide z1;
[0020] FIG. 7 is a graph showing the result of an XRD analysis for
a comparative oxide z2;
[0021] FIG. 8 is a graph showing the result of an XRD analysis for
a comparative oxide z3;
[0022] FIG. 9 is a graph showing the result of an XRD analysis for
Li.sub.2MnO.sub.3;
[0023] FIG. 10 is a graph showing the result of an XRD analysis for
a present invention active material a1-i;
[0024] FIG. 11 is a graph showing the result of an XRD analysis for
a present invention active material a2-i;
[0025] FIG. 12 is a graph showing the result of an XRD analysis for
a present invention active material a3-i;
[0026] FIG. 13 is a graph showing the result of an XRD analysis for
a comparative reference active material x-i;
[0027] FIG. 14 is a graph showing the result of an XRD analysis for
a comparative active material z1-i;
[0028] FIG. 15 is a graph showing the result of an XRD analysis for
a comparative active material z2-i; and
[0029] FIG. 16 is a graph showing the result of an XRD analysis for
a comparative active material z3-i.
DETAILED DESCRIPTION OF THE INVENTION
[0030] The present invention provides a non-aqueous electrolyte
secondary battery comprising: a positive electrode containing a
positive electrode active material comprising a lithium-containing
oxide active material; a negative electrode; and a non-aqueous
electrolyte, wherein: the lithium-containing oxide active material
is represented by the general formula
Li.sub.aMg.sub.bMO.sub.2.+-..alpha. where
0.65.ltoreq.a.ltoreq.1.05, 0<b.ltoreq.0.3,
0.ltoreq..alpha..ltoreq.0.3, and M is at least one of manganese and
cobalt; and the lithium-containing oxide active material has a main
peak at 2.theta.=17.95.degree. to 18.15.degree., as determined by
an X-ray powder crystal diffraction measurement (Cuk.alpha.).
[0031] When magnesium is added to the lithium-containing oxide, a
lithium-containing impurity layer (a substance with a structure
similar to Li.sub.2MnO.sub.3) that is formed when adding lithium is
not formed. As a result, the side reactions that are caused by the
lithium-containing impurity (decomposition of the impurity itself,
decomposition of the electrolyte, and so forth) are inhibited.
[0032] Moreover, when magnesium is added, the average discharge
potential increases. Therefore, it is possible to prevent the
decrease of the average discharge potential caused by adding
lithium. Therefore, in the battery that has the foregoing
configuration, the energy density can be increased.
[0033] Note that although almost the whole amount of sodium is
ion-exchanged by lithium, a small amount of sodium may remain, as
will be described later. However, even if a small amount of sodium
remains, the remaining amount will be very small and
negligible.
[0034] It is desirable that the amount b of magnesium is preferably
within the range 0<b.ltoreq.0.2. The reason is as follows.
Addition of a large amount of magnesium may cause substantial
structural change or formation of impurity, resulting in more side
reactions. Consequently, battery deterioration (such as gas
formation, capacity loss, and storage performance deterioration)
may occur.
[0035] It is desirable that the lithium-containing oxide active
material be represented by the general formula
Li.sub.aMg.sub.bMn.sub.xCo.sub.yO.sub.2.+-..alpha., where
0.65.ltoreq.a.ltoreq.1.05, 0<b.ltoreq.0.3,
0.45.ltoreq.x.ltoreq.0.55, 0.45.ltoreq.y.ltoreq.0.55,
0.90.ltoreq.x+y.ltoreq.1.10, and 0.ltoreq..alpha..ltoreq.0.3. It is
also desirable that the lithium-containing oxide active material
have a crystal structure belonging to an O2 structure, a T2
structure, an O6 structure, or a mixed structure thereof.
[0036] The present invention also provides a method of
manufacturing a non-aqueous electrolyte secondary battery,
comprising the steps of subjecting a sodium-magnesium-containing
oxide represented by the general formula
Na.sub.cMg.sub.bMO.sub.2.+-..alpha., where
0.65.ltoreq.c.ltoreq.0.75, 0<b.ltoreq.0.3,
0.ltoreq..alpha..ltoreq.0.3, and M is at lease one of manganese and
cobalt, to ion-exchange of sodium for lithium by using a molten
salt, an aqueous solution, or an organic solvent, to prepare a
positive electrode active material; preparing a positive electrode
active material slurry containing the positive electrode active
material and a binder, and thereafter applying the positive
electrode active material slurry to a positive electrode current
collector to prepare a positive electrode; placing a separator
between the positive electrode and the negative electrode to
prepare a power-generating element; and encasing the
power-generating element in a battery case and filling a
non-aqueous electrolyte in the battery case.
[0037] When ion-exchanging the sodium-magnesium-containing oxide
using an organic solvent or the like in the step of preparing the
positive electrode active material, almost the whole amount of
sodium is ion-exchanged for lithium. As a result, the positive
electrode active material comprising the lithium-containing oxide
is synthesized. The magnesium may or may not be ion-exchanged for
lithium partially or entirely.
[0038] It is desirable that the sodium-magnesium-containing oxide
be represented by the general formula
Na.sub.cMg.sub.bMn.sub.xCo.sub.yO.sub.2.+-..alpha. where
0.65.ltoreq.c.ltoreq.0.75, 0<b.ltoreq.0.3,
0.45.ltoreq.x.ltoreq.0.55, 0.45.ltoreq.y.ltoreq.0.55,
0.90.ltoreq.x+y.ltoreq.1.10, and 0.ltoreq..alpha..ltoreq.0.3.
Other Embodiments
[0039] (1) As for the conductive agent used in preparing the
electrode, the electrode can function even without adding any
conductive agent in the case of using an active material having
high electrical conductivity. However, when using an active
material having low electrical conductivity, it is desirable to add
a conductive agent. The conductive agent may be any material as
long as it has electrical conductivity. It is desirable to use at
least one substance selected from oxides, carbides, nitrides, and
carbon materials that have particularly high conductivity. Examples
of such oxides include tin oxide and indium oxide. Examples of such
carbides include tungsten carbide and zirconium carbide. Examples
of such nitrides include titanium nitride and tantalum nitride. In
the case of adding a conductive agent, if the amount of the
conductive agent added is too small, the conductivity in the
positive electrode cannot be improved sufficiently. On the other
hand, if the amount of the conductive agent added is too large, the
relative proportion of the active material in the positive
electrode will be low, and consequently a high energy density
cannot be obtained. For this reason, it is desirable that the
amount of the conductive agent be from 0 mass % to 30 mass %, more
preferably from 0 mass % to 20 mass %, and still more preferably
from 0 mass % to 10 mass %, with respect to the total amount of the
positive electrode.
[0040] (2) Examples of the binder used for the electrode include
polytetrafluoroethylene, polyvinylidene fluoride, polyethylene
oxide, polyvinyl acetate, polymethacrylate, polyacrylate,
polyacrylonitrile, polyvinyl alcohol, styrene-butadiene rubber,
carboxymethylcellulose, and combinations thereof.
[0041] When the amount of the binder is too large, the relative
proportion of the active material contained in the positive
electrode will be small, so the battery will not have a high energy
density. For this reason, it is desirable that the amount of the
binder be from 0 mass % to 30 mass %, more preferably from 0 mass %
to 20 mass %, and still more preferably from 0 mass % to 10 mass %,
with respect to the total amount of the positive electrode.
[0042] (3) The material for the negative electrode may be any
material that is capable of absorbing and deintercalating lithium.
Examples include lithium, silicon, carbon, tin, germanium,
aluminum, lead, indium, gallium, lithium-containing alloys,
lithium-intercalated carbon materials, and lithium-intercalated
silicon materials.
[0043] (4) Examples of the non-aqueous solvent used in the present
invention include cyclic carbonic esters, chain carbonic esters,
esters, cyclic ethers, chain ethers, nitriles, and amides.
[0044] Examples of the cyclic carbonic esters include ethylene
carbonate, propylene carbonate, and butylenes carbonate. It is also
possible to use a cyclic carbonic ester in which part or all of the
hydrogen groups of the just-mentioned cyclic carbonic esters is/are
fluorinated. Examples of such include trifluoropropylene carbonate
and fluoroethyl carbonate. Examples of the chain carbonic esters
include dimethyl carbonate, ethyl methyl carbonate, diethyl
carbonate, methyl propyl carbonate, ethyl propyl carbonate, and
methyl isopropyl carbonate. It is also possible to use a chain
carbonic ester in which part or all of the hydrogen groups of one
of the foregoing chain carbonic esters is/are fluorinated. Examples
of the esters include methyl acetate, ethyl acetate, propyl
acetate, methyl propionate, ethyl propionate, and
.gamma.-butyrolactone. Examples of the cyclic ethers include
1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran,
2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide,
1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineol, and
crown ether. Examples of the chain ethers include
1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl
ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl
ether, methyl phenyl ether, ethyl phenyl ether, butylphenyl ether,
pentylphenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl
ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane,
1,2-dibutoxy ethane, diethylene glycol dimethyl ether, diethylene
glycol diethyl ether, diethylene glycol dibutyl ether,
1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol
dimethyl ether, and tetraethylene glycol dimethyl ether. Examples
of the nitriles include acetonitrile. Examples of the amides
include dimethylformamide. These substances may be used either
alone or in combination.
[0045] (5) The lithium salt to be added to the non-aqueous solvent
may be any lithium salt that is commonly used in conventional
non-aqueous electrolyte lithium-ion secondary batteries. For
example, it is possible to use at least one substance selected from
LiBF.sub.4, LiPF.sub.6, LiCF.sub.3SO.sub.3,
LiC.sub.4F.sub.9SO.sub.3, LiN(CF.sub.3SO.sub.2).sub.2,
LiN(C.sub.2F.sub.5SO.sub.2).sub.2, LiAsF.sub.6, and lithium
difluoro(oxalate)borate.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0046] Hereinbelow, preferred embodiments of the non-aqueous
electrolyte secondary battery according to the invention will be
described with reference to FIG. 1. It should be construed,
however, that the non-aqueous electrolyte secondary battery
according to this invention is not limited to the following
embodiments and examples but various changes and modifications are
possible without departing from the scope of the invention.
Preparation of Working Electrode
[0047] First, sodium nitrate (NaNO.sub.3), magnesium nitrate
[Mg(NO.sub.3).sub.2], magnesium carbonate (MgCO.sub.3), manganese
oxide (Mn.sub.2O.sub.3), and cobalt oxide (Co.sub.3O.sub.4) were
used as the starting materials, and they were mixed so that the
molar ratio of Na, Mg, Mn, and Co became 0.7:0.05:0.5:0.5. Next,
the mixed powder was pre-sintered in the air at 700.degree. C. for
10 hours, and then sintered in the air at 800.degree. C. for 20
hours. This yielded a sodium-magnesium-containing oxide represented
by the compositional formula
Na.sub.0.7Mg.sub.0.05Mn.sub.0.5Co.sub.0.5O.sub.2.
[0048] Next, a lithium oxide was prepared by ion-exchanging the
sodium-magnesium-containing oxide as follows. A mixture of lithium
nitrate and lithium chloride (88:12 mol %) was used as an
ion-exchange bed. To 10 g of this mixture, 3 g of the
sodium-magnesium-containing oxide to be ion-exchanged was added,
and the mixture was kept at 280.degree. C. for 10 hours to cause
the reaction. Thereafter, the reaction product was washed with
water to wash away nitrates, chlorides, and the unreacted products
of the starting materials, and thereafter vacuum dried at
100.degree. C. This yielded a positive electrode active material
represented by the compositional formula
Li.sub.0.7Mg.sub.0.05Mn.sub.0.5Co.sub.0.5O.sub.2.
[0049] Next, 80 mass % of the positive electrode active material,
10 mass % of acetylene black as a conductive agent, and 10 mass %
of polyvinylidene fluoride as a binder agent were mixed with
N-methyl-2-pyrrolidone to obtain a positive electrode active
material slurry. Lastly, the resultant positive electrode active
material slurry was applied onto a positive electrode current
collector surface, thereafter vacuum dried at 110.degree. C., and
shaped into a positive electrode.
Preparation of Counter Electrode and Reference Electrode
[0050] A metallic lithium plate was cut into a predetermined size,
and a tab was attached thereto, to thereby prepare a counter
electrode 2 (negative electrode) and a reference electrode 4.
Preparation of Non-Aqueous Electrolyte
[0051] A non-aqueous electrolyte solution was prepared by
dissolving lithium hexafluorophosphate (LiPF.sub.6) at a
concentration of 1 mole/liter in a mixed solvent of 3:7 volume
ratio of ethylene carbonate (EC) and diethyl carbonate (DEC) were
mixed in a volume ratio of 3:7.
Preparation of Test Cell
[0052] Under an inert atmosphere, a counter electrode 2, a
separator 3, a working electrode 1, a separator 3, and a reference
electrode 4 were placed in a test cell container 5 made of a
laminate film. Then, the above-described non-aqueous electrolyte
was filled in the test cell container 5. Thus, a test cell as shown
in FIG. 1 was prepared. Leads 6 were disposed in such a manner that
a portion of each of the leads 6 protrudes from the test cell
container 5.
EXAMPLES
Example 1
[0053] A sodium-magnesium-containing oxide, a positive electrode
active material, and a test cell were fabricated in the same manner
as described in the just-described embodiment.
[0054] The sodium-magnesium-containing oxide, the positive
electrode active material, and the test cell prepared in the
foregoing manner are hereinafter referred to as a present invention
oxide a1, a present invention active material a1-i, and a present
invention cell A1, respectively.
Example 2
[0055] A test cell was prepared in the same manner as described in
Example 1 above except for the following. The mixture ratio of
sodium nitrate, magnesium nitrate, magnesium carbonate, manganese
oxide, and cobalt oxide as the starting materials was varied to
prepare a sodium-magnesium-containing oxide represented by the
compositional formula
Na.sub.0.7Mg.sub.0.1Mn.sub.0.5Co.sub.0.5O.sub.2. The resultant
sodium-magnesium-containing oxide was ion-exchanged to synthesize a
positive electrode active material represented by the compositional
formula Li.sub.0.7Mg.sub.0.1Mn.sub.0.5Co.sub.0.5O.sub.2.
[0056] The sodium-magnesium-containing oxide, the positive
electrode active material, and the test cell prepared in the
foregoing manner are hereinafter referred to as a present invention
oxide a2, a present invention active material a2-i, and a present
invention cell A2, respectively.
Example 3
[0057] A test cell was prepared in the same manner as described in
Example 1 above except for the following. The mixture ratio of
sodium nitrate, magnesium nitrate, magnesium carbonate, cobalt
oxide, and manganese oxide as the starting materials was varied to
prepare a sodium-magnesium-containing oxide represented by the
compositional formula
Na.sub.0.7Mg.sub.0.2Mn.sub.0.5Co.sub.0.5O.sub.2. The resultant
sodium-magnesium-containing oxide was ion-exchanged to synthesize a
positive electrode active material represented by the compositional
formula Li.sub.0.7Mg.sub.0.2Mn.sub.0.5Co.sub.0.5O.sub.2.
[0058] The sodium-magnesium-containing oxide, the positive
electrode active material, and the test cell prepared in the
foregoing manner are hereinafter referred to as a present invention
oxide a3, a present invention active material a3-i, and a present
invention cell A3, respectively.
Comparative Reference Example
[0059] A test cell was prepared in the same manner as described in
Example 1 above except for the following. Sodium nitrate
(NaNO.sub.3), manganese oxide (Mn.sub.2O.sub.3), and cobalt oxide
(CO.sub.3O.sub.4) were used as the starting materials. The starting
materials were mixed together so that the composition ratio of Na,
Mn, and Co became 0.7:0.5:0.5 to prepare a sodium-containing oxide
represented by the compositional formula
Na.sub.0.7Mn.sub.0.5Co.sub.0.5O.sub.2. The resultant
sodium-containing oxide was ion-exchanged to synthesize a positive
electrode active material represented by the compositional formula
Li.sub.0.7Mn.sub.0.5Co.sub.0.5O.sub.2.
[0060] The sodium-containing oxide, the positive electrode active
material, and the test cell prepared in the foregoing manner are
hereinafter referred to as a comparative reference oxide x, a
comparative reference active material x-i, and a comparative
reference cell X, respectively.
Comparative Example 1
[0061] A test cell was prepared in the same manner as described in
Example 1 above except for the following. Sodium nitrate
(NaNO.sub.3), potassium acid carbonate (KHCO.sub.3), manganese
oxide (Mn.sub.2O.sub.3), and cobalt oxide (Co.sub.3O.sub.4) were
used as the starting materials, and the starting materials were
mixed together so that the composition ratio of Na, K, Mn, and Co
became 0.7:0.1:0.5:0.5, to prepare a sodium-potassium-containing
oxide represented by the compositional formula
Na.sub.0.7K.sub.0.1Mn.sub.0.5Co.sub.0.5O.sub.2. The resultant
sodium-potassium-containing oxide was ion-exchanged to synthesize a
positive electrode active material represented by the compositional
formula Li.sub.0.7K.sub.0.1Mn.sub.0.5Co.sub.0.5O.sub.2.
[0062] The sodium-potassium-containing oxide, the positive
electrode active material, and the test cell prepared in the
foregoing manner are hereinafter referred to as a comparative oxide
z1, a comparative active material z1-i, and a comparative cell Z1,
respectively.
Comparative Example 2
[0063] A test cell was prepared in the same manner as described in
Comparative Example 1 above except for the following. The mixture
ratio of sodium nitrate, potassium acid carbonate, manganese oxide,
and cobalt oxide as the starting materials was varied to prepare a
sodium-potassium-containing oxide represented by the compositional
formula Na.sub.0.7K.sub.0.2Mn.sub.0.5Co.sub.0.5O.sub.2. The
resultant sodium-potassium-containing oxide was ion-exchanged to
synthesize a positive electrode active material represented by the
compositional formula
Li.sub.0.7K.sub.0.2Mn.sub.0.5Co.sub.0.5O.sub.2.
[0064] The sodium-potassium-containing oxide, the positive
electrode active material, and the test cell prepared in the
foregoing manner are hereinafter referred to as a comparative oxide
z2, a comparative active material z2-i, and a comparative cell Z2,
respectively.
Comparative Example 3
[0065] A test cell was prepared in the same manner as described in
Comparative Example 1 above except for the following. The mixture
ratio of sodium nitrate, potassium acid carbonate, manganese oxide,
and cobalt oxide as the starting materials was varied to prepare a
sodium-potassium-containing oxide represented by the compositional
formula Na.sub.0.7K.sub.0.3Mn.sub.0.5Co.sub.0.5O.sub.2. The
resultant sodium-potassium-containing oxide was ion-exchanged to
synthesize a positive electrode active material represented by the
compositional formula
Li.sub.0.7K.sub.0.3Mn.sub.0.5Co.sub.0.5O.sub.2.
[0066] The sodium-potassium-containing oxide, the positive
electrode active material, and the test cell prepared in the
foregoing manner are hereinafter referred to as a comparative oxide
z3, a comparative active material z3-i, and a comparative cell Z3,
respectively.
Experiment 1
[0067] An XRD measurement (radiation source: CuK.alpha.,
measurement range: 2.theta.=10.degree. to 80.degree.) was conducted
for the present invention oxides a1 to a3, the comparative
reference oxide x, and the comparative oxides z1 to z3. The results
are shown in FIGS. 2 to 8. For reference, the XRD profile of
Li.sub.2MnO.sub.3 is shown in FIG. 9.
[0068] The XRD profiles of the present invention oxides a1 to a3,
the comparative reference oxide x, and the comparative oxides z1 to
z3 were compared with the XRD profile of Li.sub.2MnO.sub.3. As a
result, it was observed that all the oxides a1 to a3, x, and z1 to
z3 showed no peak attributed to Li.sub.2MnO.sub.3, and they did not
contain a substance having a similar structure to
Li.sub.2MnO.sub.3, which is observed when lithium is added.
[0069] In addition, the present invention oxides a1 to a3 were
compared with the comparative reference oxide x. As a result, it
was observed that although most of their XRD profiles were similar
to each other, but the present invention oxides a1 to a3, in which
magnesium was added, showed additional peaks at
2.theta.=36.5.degree., 37.1.degree., and 38.2.degree.. Accordingly,
it is believed that the structures of the present invention oxides
a1 to a3 are different from the comparative reference oxide x.
[0070] In addition, the comparative oxides z1 to z3 were also
compared with the comparative reference oxide x. As a result, it
was observed that their XRD profiles almost matched. Accordingly,
it is believed that these oxides x and z1 to z3 have a P2 structure
of the space group P6.sub.3/mmc. Although the peaks that are not
attributed to the P2 structure are observed in the XRD profiles of
the comparative oxides z2 and z3, these peaks are attributed to
impurities, which can be removed by washing with water.
Experiment 2
[0071] Lattice constants of the present invention oxides a1 to a3,
the comparative reference oxide x, and the comparative oxides z1 to
z3 were determined assuming the space group to be P6.sub.3/mmc. The
results are shown in Table 1 below.
TABLE-US-00001 TABLE 1 Additive a-axis c-axis Oxide Type Amount
({acute over (.ANG.)}) ({acute over (.ANG.)}) Present invention
oxide a1 Mg 0.05 2.8410 11.2075 Present invention oxide a2 0.1
2.8518 11.1566 Present invention oxide a3 0.2 2.8643 11.1481
Comparative reference oxide x -- -- 2.8301 11.2375 Comparative
oxide z1 K 0.1 2.8273 11.2160 Comparative oxide z2 0.2 2.8268
11.1905 Comparative oxide z3 0.3 2.8251 11.1566
[0072] As shown in Table 1, the relationship between the amount of
magnesium added and the lattice constant was studied for the
present invention oxides a1 to a3, in which magnesium was added. As
a result, it was observed that when the amount of magnesium added
was greater, the a-axis was longer and the c-axis was shorter
accordingly. In comparison, the relationship between the amount of
potassium added and the lattice constant was studied for the
comparative oxides z1 to z3, in which potassium was added. As a
result, it was observed that when the amount of potassium added was
greater, both the a-axis and the c-axis were shorter.
[0073] From these results, it is understood that the addition of
magnesium results in a structural change that is different from
when potassium is added.
Experiment 3
[0074] An XRD measurement (radiation source: CuK.alpha.,
measurement range: 2.theta.=10.degree. to 80.degree.) was conducted
for the present invention active materials a1-i to a3-i, the
comparative reference active material x-i, and the comparative
active materials z1-i to z3-i (these active materials were obtained
by subjecting the present invention oxides a1 to a3, the
comparative reference oxide x, and the comparative oxides z1 to z3
to the ion-exchange). The results are shown in FIGS. 10 to 16.
[0075] As clearly seen from FIGS. 10 to 13, the XRD profiles are
different between the comparative reference active material x-i, in
which no magnesium or potassium is added, and the present invention
active materials a1-i to a3-i, in which magnesium is added. This
clearly demonstrates that the addition of magnesium causes a change
in the structure.
[0076] Table 2 below shows the representative peak positions of the
present invention active materials a1-i to a3-i, the comparative
reference active material x-i, and the comparative active materials
z1-i to z3-i.
TABLE-US-00002 TABLE 2 Peak in the Main vicinity of Additive peak
37.4.degree. Additive Type Amount 2.theta. (deg.) 2.theta. (deg.)
Present invention active Mg 0.05 18.00 37.38 material a1-i Present
invention active 0.1 18.06 37.30 material a2-i Present invention
active 0.2 18.12 37.26 material a3-i Comparative reference -- --
17.96 37.40 active material x-i Comparative oxide z1 K 0.1 17.96
37.38 Comparative oxide z2 0.2 17.98 37.38 Comparative oxide z3 0.3
18.00 37.38
[0077] Generally, it is known that in an XRD measurement, a shift
of a peak position corresponds to a change of a lattice constant.
When studying Table 2, it is observed that, in the present
invention active materials a1-i to a3-i, the main peak shifts
toward a greater angle as the amount of magnesium added increases.
This is believed to correspond to the fact that the crystal lattice
becomes smaller because of the addition of magnesium.
[0078] In addition, as clearly seen from FIGS. 10 to 12, an
additional peak is observed at 2.theta.=18.4.degree. for the
present invention active materials a1-i to a3-i, in which magnesium
is added, and it is observed that the peak intensity of this peak
is greater when the amount of magnesium added is greater.
[0079] This peak is believed to originate from the lattice defects
in the crystal structure.
[0080] On the other hand, it is observed that the peak at
2.theta.=37.4.degree. observed for the comparative reference active
material x-i shifts toward a low-angle side when the amount of
magnesium added is greater.
[0081] From the foregoing, it is understood that the present
invention active materials a1-i to a3-i, in which magnesium is
added, has a different structure from that of the comparative
reference active material x-i, and moreover, the structural change
is greater when the amount of magnesium added is greater.
[0082] On the other hand, the XRD profiles of the comparative
active materials z1-i to z3-i, in which potassium is added, are
similar to that of the comparative reference active material x-i,
so they are believed to have the same structure.
[0083] From the foregoing results of the experiment, it is clear
that the crystal structure does not change when adding potassium
alone to the reference active material, but the crystal structure
changes when adding magnesium to the reference active material.
Thus, it is demonstrated that when adding a metal to the reference
active material, the effect on the crystal structure varies
depending on the type of the metal added.
[0084] Next, the relationship between the differences in the
crystal structure and battery performance was investigated. The
results will be described in the following experiment 4.
Experiment 4
[0085] Each of the present invention cells A1 to A3, the
comparative reference cell X, and the comparative cells Z1 to Z3
was charged and discharged one time under the following conditions
to determine the average discharge potential. The results are shown
in Table 3 below.
--Charge Conditions
[0086] The cells were charged at a charge current of 0.06
mA/cm.sup.2 (equivalent to 0.05 It) to an end-of-charge potential
of 5.0 V (vs. Li/Li.sup.+).
--Discharge Conditions
[0087] The cells were discharged at a discharge current of 0.06
mA/cm.sup.2 (equivalent to about 0.05 It) to an end-of-discharge
potential of 2.0 V (vs. Li/Li.sup.+).
TABLE-US-00003 TABLE 3 Average discharge potential [V Cell
Composition ratio (vs. Li/Li.sup.+) Present invention cell A1
Li.sub.0.7Mg.sub.0.05Mn.sub.0.5Co.sub.0.5O.sub.2 3.68 Present
invention cell A2 Li.sub.0.7Mg.sub.0.1Mn.sub.0.5Co.sub.0.5O.sub.2
3.69 Present invention cell A3
Li.sub.0.7Mg.sub.0.2Mn.sub.0.5Co.sub.0.5O.sub.2 3.70 Comparative
reference cell X Li.sub.0.7Mn.sub.0.5Co.sub.0.5O.sub.2 3.66
Comparative cell Z1 Li.sub.0.7K.sub.0.1Mn.sub.0.5Co.sub.0.5O.sub.2
3.58 Comparative cell Z2
Li.sub.0.7K.sub.0.2Mn.sub.0.5Co.sub.0.5O.sub.2 3.55 Comparative
cell Z3 Li.sub.0.7K.sub.0.3Mn.sub.0.5Co.sub.0.5O.sub.2 3.52
[0088] As clearly seen from Table 3, the present invention cells A1
to A3, in which magnesium was added, exhibited higher average
discharge potentials than the comparative reference cell X, in
which the additives were not contained. In addition, among the
present invention cells A1 to A3, the one containing a greater
amount of magnesium showed a higher average discharge potential. In
contrast, it was observed that the comparative cells Z1 to Z3, in
which potassium was added, showed lower average discharge
potentials than the comparative reference cell X, in which the
additives were not contained. In addition, among the comparative
cells Z1 to Z3, the one containing a greater amount of potassium
showed a lower average discharge potential. From these results, it
is demonstrated that the average potential can become either higher
or lower than the sample without the additives, depending on the
type of the metal to be added. This is believed to be because of
the crystal structures.
[0089] As clearly seen from Table 3, the present invention cell A3,
which contains a large amount of magnesium, can obtain a voltage
about 0.15 V higher than that of Z2, which contains the same amount
of potassium.
[0090] The present invention is applicable to, for example, driving
power sources for mobile information terminal devices such as
mobile telephones, notebook computers, PDAs, power tools, power
assisted bicycles, EVs and HEVs.
[0091] While detailed embodiments have been used to illustrate the
present invention, to those skilled in the art, however, it will be
apparent from the foregoing disclosure that various changes and
modifications can be made therein without departing from the spirit
and scope of the invention. Furthermore, the foregoing description
of the embodiments according to the present invention is provided
for illustration only, and is not intended to limit the
invention.
* * * * *