U.S. patent application number 13/736191 was filed with the patent office on 2013-05-16 for lithium secondary battery.
The applicant listed for this patent is Kazushige Kohno, Tatsuya Toyama. Invention is credited to Kazushige Kohno, Tatsuya Toyama.
Application Number | 20130122371 13/736191 |
Document ID | / |
Family ID | 41013426 |
Filed Date | 2013-05-16 |
United States Patent
Application |
20130122371 |
Kind Code |
A1 |
Toyama; Tatsuya ; et
al. |
May 16, 2013 |
LITHIUM SECONDARY BATTERY
Abstract
To provide a lithium secondary battery excellent in the life
characteristic and the power density. A lithium secondary battery,
comprising: a positive electrode capable of intercalating and
deintercalating lithium; and an negative electrode capable of
intercalating and deintercalating lithium, wherein the positive
electrode contains a manganese-containing positive electrode active
material of a spinel structure and an oxide that coats the surface
of this positive electrode active material, wherein the oxide
contains a metallic element, wherein the metallic element forms a
solid solution with the positive electrode active material, and
wherein the atomic concentration of the metallic element is
approximately 0 at depths of from 50 to 100 nm from an external
surface of the negative electrode.
Inventors: |
Toyama; Tatsuya; (Tokai,
JP) ; Kohno; Kazushige; (Hitachi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Toyama; Tatsuya
Kohno; Kazushige |
Tokai
Hitachi |
|
JP
JP |
|
|
Family ID: |
41013426 |
Appl. No.: |
13/736191 |
Filed: |
January 8, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12372751 |
Feb 18, 2009 |
8357467 |
|
|
13736191 |
|
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Current U.S.
Class: |
429/220 ;
429/221; 429/223; 429/224; 429/231.6 |
Current CPC
Class: |
H01M 4/366 20130101;
H01M 10/0525 20130101; H01M 4/525 20130101; H01M 4/131 20130101;
H01M 4/505 20130101; Y02E 60/10 20130101; Y02T 10/70 20130101 |
Class at
Publication: |
429/220 ;
429/221; 429/223; 429/224; 429/231.6 |
International
Class: |
H01M 4/131 20060101
H01M004/131 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 29, 2008 |
JP |
2008-049866 |
Claims
1. A lithium secondary battery comprising: a positive electrode
capable of intercalating and deintercalating lithium; and a
negative electrode capable of intercalating and deintercalating
lithium, wherein the positive electrode comprises a
manganese-containing positive electrode active material of a spinel
structure represented by a general formula
Li.sub.aMn.sub.bM.sub.cO.sub.4, where, 1.0.ltoreq.a.ltoreq.1.15,
1.8.ltoreq.b.ltoreq.1.94, 0.01.ltoreq.c.ltoreq.0.10, a+b+c=3, and M
is a substitutional element, and is one or more elements selected
from the group consisting of Mg, Ni, and Cu, and an oxide coating a
surface of the positive electrode active material, wherein the
oxide coating the surface of the positive electrode active material
contains first atoms of at least one metallic element selected from
the group consisting of Al, Co, and Fe, and wherein second atoms of
the at least one metallic element form a solid solution with the
positive electrode active material.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation application of U.S.
application Ser. No. 12/372,751, filed Feb. 18, 2009, the contents
of which are incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to lithium secondary batteries
using a manganese-based spinel compound as a positive electrode
active material.
BACKGROUND OF THE INVENTION
[0003] In recent years, the lithium secondary battery is widely
used as the power supply of a personal computer, a portable device,
and the like because it has a high power density. Furthermore, the
lithium secondary battery has been studied for applications to the
power supplies for environmentally-friendly electric vehicles and
hybrid vehicles, and also applications to stationary power supplies
and the like for absorbing the output fluctuation due to natural
phenomena in combination with renewable energy power generation,
such as photovoltaic power generation, or wind power generation. In
the field of such large-sized lithium batteries, inexpensiveness
and long-life as well as high-performance are required.
[0004] The examples of the positive electrode active material of
the lithium secondary battery include LiCoO.sub.2, LiFePO.sub.4,
and LiMn.sub.2O.sub.4. Although LiCoO.sub.2 is most promising in
terms of the battery performance, cobalt as the raw material is
expensive and thus it is difficult to reduce cost. Moreover, if the
lithium secondary battery is held in high voltage state, Co will
dissolve from the positive electrode active material and the
battery life will decrease significantly. For LiFePO.sub.4, the raw
material cost is inexpensive because iron is used but its
manufacturing cost is high. Furthermore, in terms of the battery
performance, LiFePO.sub.4 has a problem that the power density is
low because the electron conductivity is low or the true density is
low. On the other hand, LiMn.sub.2O.sub.4 is advantageous in terms
of cost because the deposit of manganese as the raw material is 60
or more times as compared with that of cobalt, and furthermore the
electron conductivity and the true density are approximately equal
to those of LiCoO.sub.2.
[0005] However, LiMn.sub.2O.sub.4 has a problem that when the
temperature thereof is increased, Mn will dissolve from the
positive electrode active material, resulting in a decrease in the
battery life. For such problems, for example, Patent Document 1
proposes that a part of manganese in the surface of the active
material particle is substituted with a transition metal to form a
surface layer, thereby suppressing the dissolution of manganese and
achieving a long battery life. Although the thickness of the
substitution layer in this method is not clear, judging from the
description of claim 6 and the like the thickness of this
substitution layer is estimated as on the order of 1 to 2
.mu.m.
[0006] Moreover, Patent Document 2 proposes that a volume change of
the positive electrode active material associated with charge and
discharge is suppressed with a surface layer having a high fracture
toughness value, thereby improving the cycle life. [0007] [Patent
Document 1] JP-A-2000-030709 [0008] [Patent Document 2]
J-A-2003-178759
BRIEF SUMMARY OF THE INVENTION
[0009] An object of the present invention is to provide a lithium
secondary battery having excellent cycle-life characteristic as
well as a high initial capacity by specifying the coating state of
an oxide, the oxide being obtained by treating the surface of a
spinel-type positive electrode active material, and thereby
suppressing the dissolution of Mn from the positive electrode
active material.
[0010] The present invention relates to a lithium battery
comprising: a positive electrode capable of intercalating and
deintercalating lithium; and an negative electrode capable of
intercalating and deintercalating lithium, wherein the positive
electrode contains a manganese-containing positive electrode active
material of a spinel structure and an oxide that coats the surface
of this positive electrode active material, wherein the oxide
contains one or more kinds of metallic elements, wherein the
metallic element forms a solid solution with the positive
electrode, and wherein the atomic concentration of the metallic
element is approximately 0 at an average depth of from 50 to 100 nm
from an external surface of the positive electrode active
material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 shows a schematic cross section of a lithium
secondary battery.
[0012] FIG. 2 is a graph showing the atomic concentration versus
the sputtering depth at the time of surface treatment of a positive
electrode active material to which aluminium oxide is applied.
[0013] FIG. 3 is a graph showing a relationship of the power
density with respect to the depth (solution depth) at which the
atomic concentration of a metallic element contained within a metal
oxide used in the surface treatment becomes approximately 0%.
[0014] FIG. 4 is a graph showing a relationship between the
solution depth and the cycle characteristic.
[0015] FIG. 5 is a block diagram showing the outline of a secondary
battery system.
TABLE-US-00001 [0016] (Description of Reference Numerals) 1
positive plate, 2 negative plate, 3 separator, 4 battery can, 5
negative lead, 6 lid portion, 7 positive lead, 8 packing, 9
insulating plate, 10 lithium secondary battery, 11 cell controller,
12 battery controller, 13 signal line, 111 input section, 112
output section
DETAILED DESCRIPTION OF THE INVENTION
[0017] In the present invention, the positive electrode active
material comprises a manganese-containing positive electrode active
material of a spinel structure, and an oxide containing a metallic
element that coats the surface of this positive electrode active
material. This positive electrode active material is an assembly of
particles, and a metallic element containing oxide layer formed in
this surface coats the particle surface of this positive electrode
active material. Then, the metallic element within the oxide forms
the solid solution with the positive electrode active material. The
average depth of the solid solution layer is preferably in a range
of 50 to 100 nm.
[0018] For the dissolution of Mn from the positive electrode active
material, it is thought that a trivalent Mn existing near the
surface of the positive electrode active material transforms into a
divalent Mn and a tetravalent Mn by disproportionation reaction and
then the divalent Mn will dissolve. In order to suppress the
dissolution of Mn from the positive electrode active material
without reducing the initial capacity of the battery, the present
invention focused on the state of a surface treatment of the
positive electrode active material. Generally, the objective of the
positive electrode active material whose surface is coated with
other compound is to reduce the contact area between an electrolyte
and the positive electrode active material, thereby contributing to
suppress the disproportionation reaction of Mn due to activating
components such as hydrogen fluoride contained in the electrolyte.
Some coating compounds contribute to the intercalation and
deintercalation of lithium, while others do not. In the case of a
compound contributing to the intercalation and deintercalation of
lithium, the battery capacity will not decrease by coating,
however, when the coefficients of expansion and contraction due to
charge-and-discharge reaction differ between the coating compound
and the positive electrode active material, a distortion will occur
at the interface therebetween and the coating effect cannot be
obtained.
[0019] On the other hand, in the case of a compound that does not
contribute to the intercalation and deintercalation of lithium, the
battery capacity will decrease depending on the coating amount, and
moreover, when the volume change due to the charge-and-discharge
reaction of the positive electrode active material is large, a
distortion will occur. Then, in order not to be adversely affected
by this distortion while minimizing the capacity decrease, it is
important to use a minimum amount of coating metallic element that
causes an effect on the dissolution of Mn and to dissolve this
metallic element with the positive electrode active material. In
the present invention, a solid solution comprising a positive
electrode active material and a metallic element oxide used in
treating the surface of the positive electrode active material is
formed so that the atomic concentration of this metallic element
may become approximately 0 at the average depth of from 50 to 100
nm from the external surface of the positive electrode. In other
words, this is substantially synonymous with that the average
thickness of the solid solution layer is in a range of 50 to 100
nm.
[0020] The "solid solution" used in the present invention is formed
from a positive electrode active material and a metallic element
oxide used in treating the surface of the positive electrode active
material. Moreover, one or more kinds of metallic elements
contained in the oxide in the present invention are elements that
are not contained in the positive electrode active material, and do
not include an element within the positive electrode active
material that may move by the heat treatment in the coating process
or by the charge-and-discharge reaction.
[0021] Discrimination between a metallic element contained in the
positive electrode active material and a metallic element contained
in the oxide used in the surface treatment becomes apparent from
the concentration gradient in the depth direction from the positive
electrode active material surface. A metallic element whose
concentration gradient increases from the surface toward the depth
direction is the metallic element contained in the positive
electrode active material, while a metallic element whose
concentration gradient decreases from the surface toward the depth
direction is the metallic element contained in the oxide used in
coating.
[0022] Most preferably, the present invention provides a lithium
secondary battery having excellent cycle-life characteristic as
well as a high initial capacity by specifying the coating state of
an oxide, the oxide being obtained by treating the surface of a
spinel-type positive electrode active material, and thereby
suppressing the dissolution of Mn from the positive electrode
active material.
[0023] Embodiments for implementing the present invention will be
shown hereinafter. FIG. 1 shows a schematic cross section of the
lithium secondary battery. In the lithium secondary battery, a
separator 3 is interposed between a positive plate 1 and a negative
plate 2. These positive plate 1, negative plate 2, and separator 3
are rolled up and then enclosed together with a nonaqueous
electrolyte into a battery can 4 made of stainless steel or
aluminium. A positive lead piece 7 is formed in the positive plate
1 and a negative lead piece 5 is formed in the negative plate 2,
respectively, and current is taken out therethrough. Between the
positive plate 1 and the negative lead piece 5 and between the
negative plate 2 and the positive lead piece 7, an insulating plate
9 is formed, respectively. Moreover, between the battery can 4 in
contact with the negative lead piece 5 and a sealing lid portion 6
in contact with the positive lead piece 7, a packing 8 for
preventing leakage of the electrolyte and separating a positive
pole from a negative pole is formed.
[0024] The positive plate 1 is formed applying a positive electrode
mixture to a collector comprising aluminium and the like. The
positive electrode mixture contains an active material contributing
to the intercalation and deintercalation of lithium, a conductive
material, a binder, and the like.
[0025] The negative plate 2 is formed applying a negative electrode
mixture to a collector comprising copper and the like. The negative
electrode mixture contains an active material contributing to the
intercalation and deintercalation of lithium, a conductive
material, a binder, and the like. As the negative electrode active
material, metal lithium, a carbon material, or a material capable
of inserting lithium or forming a compound thereof can be used,
while the carbon material is suitable in particular.
[0026] Examples of the carbon material include graphites, such as
natural graphite and artificial graphite, and amorphous carbons,
such as carbides of coal-based coke and coal-based pitch, carbides
of petroleum-based coke and petroleum-based pitch, and carbides of
pitch coke. Preferably, the above-described carbon materials
subjected to various kinds of surface treatments are desirable.
These carbon materials can be used alone as well as in combination
of two or more kinds thereof.
[0027] Moreover, examples of the materials capable of inserting
lithium or forming a compound thereof include the metals, such as
aluminium, tin, silicon, indium, gallium, and magnesium, an alloy
containing these elements, and a metal oxide containing tin,
silicon, or the like. Furthermore, the examples of the materials
also include a composite material of the above-described metal,
alloy, or metal oxide, and carbon material such as graphite or
amorphous carbon.
[0028] As the active material of the positive plate 1, a lithium
manganese complex oxide having a crystal structure of a spinel
structure (hereinafter, referred to as "spinel manganese") is used.
Then, in the active material of the positive plate 1, primary
particles gather to form a secondary particle, and the average
particle size of the secondary particle is in a range of 10 to 40
.mu.m, and the one having the average particle size in a range of
10 to 30 .mu.m is particularly preferable.
[0029] As such spinel manganese, specifically, the one represented
by a general formula Li.sub.aMn.sub.bM.sub.cO.sub.4 (where,
1.0.ltoreq.a.ltoreq.1.15, 1.8.ltoreq.b.ltoreq.1.94,
0.01.ltoreq.c.ltoreq.0.10, and a+b+c=3, M is one or more elements
selected from a group consisting of Mg, Ni, and Cu) is used.
[0030] Here, the content "a" of Li is 1.0.ltoreq.a.ltoreq.1.15,
however, if a<1.0, then other element will enter the Li site,
preventing diffusion of lithium ions and degrading the cycle
characteristic. Moreover, if 1.15<a, then the average valence of
Mn will increase significantly so as to maintain electrical
neutrality. In the spinel manganese, electrons are deintercalationd
by a trivalent Mn being transformed into the quadrivalent during
charging, and therefore, the lower the ratio of the trivalent Mn,
the further decrease in the battery capacity is caused. It is
therefore not preferable that the average valence of Mn exceed
3.75. Then, by setting the content "a" of Li in the spinel
manganese to 1.0.ltoreq.a.ltoreq.1.15, a longer battery life and an
increase in the power density can be achieved.
[0031] As the metallic element contained in the oxide, a trivalent
metallic element is preferable. In order to reduce the trivalent
Mn, which causes the dissolution of Mn, near the surface of the
spinel manganese, it is necessary to dissolve the trivalent or
lower valent metallic elements and increase the content of the
tetravalent Mn. Furthermore, in order to substitute over a large
area near the surface, it is effective to distribute the
substitutional elements over a large area. From such viewpoint, it
is preferable to dissolve the trivalent element rather than the
divalent or lower valence elements that can even in small
quantities increase the ratio of the tetravalent Mn.
[0032] As the trivalent metallic element to dissolve, Al, Co, and
Fe are preferable. Here, it turned out that when the solid solution
state of a metallic element is specified using the atomic
concentration distribution of the metallic element, the atomic
concentration of the metallic element needs to decrease from the
external surface toward the inside and become approximately 0 at
depths of from 50 to 100 nm from the external surface.
[0033] According to the present invention, there is provided a
lithium secondary battery comprising: a positive electrode capable
of intercalating and deintercalating lithium; and a negative
electrode capable of intercalating and deintercalating lithium,
wherein the positive electrode contains a manganese-containing
positive electrode active material of a spinel structure and an
oxide containing a metallic element that coats the surface of this
positive electrode active material, wherein the metallic element
forms a solid solution with the positive electrode active material,
wherein the average thickness of this solid solution layer is in a
range of 50 to 100 nm from the external surface of the positive
electrode active material, wherein the capacity retention after
1000 cycles performed in a range of 2.7 to 4.2 V at 0.2 C under
environment of 50.degree. C. is no less than 85%, and wherein the
power density is no less than 90 Wh/kg. When the average thickness
of the solid solution layer is in a range of 50 to 100 nm, it is
possible to provide a lithium battery, whose capacity retention is
no less than 85% and whose power density is no less than 90 Wh/kg,
having a long-life characteristic and a high capacity. According to
the present invention, it is possible to provide a lithium
secondary battery wherein the dissolution amount of MN from the
positive electrode active material measured with a predetermined
method described later is no more than 2 Wt ppm.
[0034] When the atomic concentration of a metallic element becomes
0 at a depth less than 50 nm from the external surface, the
solution is insufficient and thus the capacity retention cannot be
improved. Moreover, when the atomic concentration of a metallic
element exceeds 0 at a depth of 100 nm from the external surface,
the atomic concentration of the metallic element near the external
surface will decrease and the metallic element cannot serve to
reduce the contact area between the positive electrode active
material and the electrolyte, resulting in a decrease in the
capacity retention. The existence form of the metal element is
thought to be a metal oxide in the external surface of the positive
electrode active material and be in solid solution with the spinel
manganese thereinside. In order for the atomic concentration of a
metallic element to become 0 at depths of 50 to 100 nm from the
external surface, the thickness of the metal oxide is from 1 to 20
nm, more preferably from 3 to 10 nm.
[0035] Moreover, if the atomic concentration of a metallic element
in the external surface is less than 10%, no effect of the solution
can be seen, and if it exceeds 40%, the electron conductivity will
decrease and the battery characteristic will degrade significantly.
Accordingly, this atomic concentration is preferably in a range of
10 to 40%.
[0036] It turned out that in order for a metallic element to be in
solid solution with the spinel manganese, and for the atomic
concentration of the metallic element to become approximately 0 at
the average depth of from 50 to 100 mm from the external surface
and furthermore to become 10 to 40% in the external surface, the
metallic element contained in the metal oxide may be 0.05 to 0.5 wt
% relative to Mn contained in the spinel manganese.
[0037] On the other hand, in order to suppress the dissolution of
Mn just by dissolving the trivalent metallic element, a plenty of
trivalent metallic elements need to be dissolved, which would then
reduce the initial capacity of the battery. Therefore, for the
spinel manganese, the average valence of the Mn within the spinel
manganese needs to be increased in advance by element
substitution.
[0038] In order to substitute an element for the spinel manganese,
the kind and content of the substitutional element is important.
That is, if the substitutional element is trivalent, then the
required substitutional amount will increase and the crystal
structure is likely to collapse due to a difference in the ion
radius. Therefore, a divalent element which even in small
quantities can increase the average valence of the Mn is preferably
selected as the substitutional element.
[0039] As the divalent metallic element to substitute, Mg, Ni,
and/or Cu are preferable. The substitutional amount "c" of the
metal substitutional element is 0.01.ltoreq.c.ltoreq.0.10, however,
if c<0.01, the increase of the average valence of Mn due to the
substitution is very small and the effect of substitution will not
be exhibited. On the other hand, if 0.10<c, the average valence
of Mn will increase significantly, resulting in a decrease of the
battery capacity. Note that, taking into consideration the atomic
ratio of these Li, substitutional element M, and O, the atomic
ratio "b" of Mn is preferably 1.8.ltoreq.b.ltoreq.1.94.
[0040] The lithium secondary battery using the spinel manganese as
described above has the capacity retention no less than 85% after
1000 cycles performed in a range of 2.7 to 4.2 V at 0.2 C under
environment of 50.degree. C.
[0041] Now, a manufacturing method using the spinel manganese as
the positive electrode active material is described.
[0042] As the raw material of the positive electrode active
material, the following ones can be used. As the lithium compound,
lithium hydroxide, lithium carbonate, lithium nitrate, lithium
acetate, lithium chloride, lithium sulfate, or the like can be
used, while lithium hydroxide or lithium carbonate is
preferable.
[0043] As the manganese compound, manganese hydroxide, manganese
carbonate, manganese nitrate, manganese acetate, manganese sulfate,
manganese oxide, or the like can be used, while manganese carbonate
or manganese oxide is preferable.
[0044] Examples of the compounds of the substitutional element M
include hydroxide, carbonate, nitrate, acetate, sulfate, and
oxide.
[0045] The material serving as the raw material is supplied as a
powder having a predetermined composition ratio, and this is ground
and mixed using a mechanical method, such as a ball mill. In
grinding and mixing, either of a dry type method and a wet type
method may be used. The maximum particle size of the ground raw
powder is preferably no greater than 1 .mu.m, more preferably no
greater than 0.5 .mu.m.
[0046] Then, the obtained powder is calcinated at temperature of
from 800 to 1000.degree. C., preferably from 850 to 950.degree. C.
The atmosphere in calcinating is preferable an oxidative gas
atmosphere containing oxygen, air, or the like.
[0047] Next, a surface treatment is carried out using the positive
electrode active material obtained this way. As the method of
surface treatment, principally a solid phase synthesis and a liquid
phase synthesis are enumerated, while the liquid phase synthesis is
preferable in order to prepare a solid solution having an arbitrary
depth. The surface treatment method using the liquid phase
synthesis is shown below.
[0048] A specified quantity of nitrate, acetate, and sulfate
containing one or more metallic elements selected from a group
consisting of Al, Co, and Fe is dissolved into water or an organic
solvent. In this case, a compound containing a nonmetallic element,
such as phosphorus or boron, may be dissolved.
[0049] Then, the resultant aqueous solution is adjusted using a pH
adjuster so as to be weak alkaline (PH=8 to 10). Examples of the pH
adjuster include ethanolamine, sodium hydroxide, lithium hydroxide,
citric acid, and nitric acid. The positive electrode active
material is mixed into the thus adjusted solution. For the mixing
ratio, the mass ratio between the metallic element used in the
surface treatment and the Mn within the spinel manganese is in a
range of 0.05 to 0.5 wt %, more preferably 0.2 to 0.4 wt %.
[0050] Next, the solvent is evaporated from the obtained solution
so as to apply the compound containing the metallic element to the
surface of the spinel manganese compound particle. The evaporation
of the solvent is preferably carried out by heating and stirring or
spray drying.
[0051] The thus obtained powder is heat-treated at temperature from
400 to 700.degree. C., preferably 500 to 600.degree. C. It is
important to carry out heat treatment in a short time in order to
control the diffusion of the coated metallic element, and therefore
a microwave heating or plasma heating treatment is preferably
carried out. Heat-treatment time is in a range of 5 minutes to 1
hour, preferably 10 minutes to 40 minutes.
(Method of Measuring the Atomic Concentration of an Element Near
the Surface of an Active Material)
[0052] For the measurement of the atomic concentration of an
element near the surface of an active material, an element detected
in the sample surface was analyzed in the depth direction using an
Auger Electron Spectrometer (Model 650, manufactured by ULVAC-PHI
Incorporated) in conjunction with Ar-ion etching. The depth profile
was determined based on the depth profile of each element by
separating noise using waveform analysis. Then, the resultant depth
profile was converted into the atomic concentration using a
relative sensitivity coefficient described in the Augier Handbook,
thereby calculating the element concentration in the depth
direction.
(Method of Measuring the Weight Ratio of Elements)
[0053] The weight ratio between the Mn within the spinel manganese
and the metallic element used in the surface treatment as well as
the weight ratio of the Mn dissolved into the electrolyte were
measured using an Inductively Coupled Plasma-Atomic Emission
Spectrometer (p-4000, manufactured by Hitachi Ltd.). First, 5 g of
positive electrode active material, and 2 ml of nitric acid or 5 ml
of electrolyte were put in 45 ml of ion exchange water contained in
a beaker, and was stirred by a stirrer for 30 minutes. After
standing for 5 minutes, a filtrate filtered through a filter paper
was sprayed together with an argon gas into radio frequency
atmosphere, and the intensity of light unique to each excited
element was measured to calculate the weight ratio of an
element.
(Method of Measuring the Average Particle Size of an Active
Material)
[0054] The average particle size was measured with a laser
diffraction/scattering method using a Laser Diffraction,
Scattering, Particle Size Distribution Analyzer (LA-920,
manufactured by Horiba Ltd.) as follows. First, pure water mixed
with 0.2 wt % of hexametaphosphoric acid sodium was used as a
dispersing agent, and an active material was put therein. An
ultrasonic wave was applied thereto for 5 minutes in order to
suppress condensation of the active material, and afterward the
median diameter (particle size of a particle whose relative
particle weight is 50%) was measured and defined as the average
particle size.
[0055] An example of a method of manufacturing the lithium
secondary battery is shown below. A positive electrode active
material is mixed with a conductive material made of a carbon
material powder and a binder such as polyvinylidene fluoride to
prepare a slurry. The mixing ratio of the conductive material
relative to the positive electrode active material is preferably
from 3 to 10 wt %. Moreover, the mixing ratio of the binder
relative to the positive electrode active material is preferably
from 2 to 10 wt %.
[0056] In this case, in order to uniformly distribute the positive
electrode active material within the slurry, it is preferable to
carry out sufficiently kneading using a kneading machine.
[0057] The obtained slurry is applied to both sides of an aluminum
foil of from 15 to 25 .mu.m thickness using a roll transcription
machine or the like, for example. After applying the slurry to both
sides, the resultant aluminum foil was pressed and dried to form an
electrode plate of the positive plate 1. The thickness of a mixture
portion where the positive electrode active material, the
conductive material, and the binder are mixed together is
preferably in a range of 200 to 250 .mu.m.
[0058] For the negative electrode, as in the positive electrode,
the negative electrode active material is mixed with a binder, and
then applied and pressed and dried to form the electrode. Here, the
thickness of the negative electrode mixture is preferably in a
ranged of 120 to 170 .mu.m. For the negative plate 2, a copper foil
of from 7 to 20 .mu.m thickness is used as the collector. The
mixing ratio for coating is preferably about from 90:10 to 98:2,
for example, at the weight ratio between the negative electrode
active material and the binder.
[0059] The obtained electrode plate is cut into a predetermined
length to form the electrode, and then a tab portion of a current
drawing portion is formed by spot welding or ultrasonic welding.
The tab portion comprises a metallic foil of the same quality of
the material as that of the rectangular-shaped collector, and is
attached in order to draw current from the electrode, and serves as
the positive lead 7 and the negative lead 5, respectively.
[0060] Between the tabbed positive plate 1 and negative plate 2, a
microporous membrane, for example, a microporous separator 3
comprising polyethylene (PE), polypropylene (PP), or the like is
sandwiched and laminated, and these are cylindrically rolled up to
serve as a group of electrodes, which is then houses into the
battery can 4 comprising a cylindrical container. Or, as the
separator, a bag-shaped one may be used to house each of the
positive and negative electrodes thereinside, and these resultant
bag-shaped separators may be sequentially laminated and housed into
a square-shaped container. As the quality of the material of the
container, stainless steel or aluminium is preferable.
[0061] After housing the group of batteries into the battery can 4,
a nonaqueous electrolyte is injected therein, and then the battery
can 4 is sealed using the lid portion 6 and the packing 8. As the
nonaqueous electrolyte, it is preferable to use the one obtained by
dissolving a lithium salt, such as lithium hexafluorophosphate
(LiPF.sub.6), lithium tetrafluoroborate (LiBF.sub.4), lithium
perchlorate (LiClO.sub.4), or lithium bis(oxalato)borate (LiBOB),
as the electrolyte, into a solvent, such as ethylene carbonate
(EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl
carbonate (DEC), methyl-ethyl carbonate (MEC), methyl acetate (MA),
methylpropyl carbonate (MPC), or vinylene carbonate (VC). The
concentration of the electrolyte is preferably from 0.7 to 1.5
M.
[0062] A lithium secondary battery prepared in this manner
comprises a pair of positive electrode and negative electrode
facing to each other via a separator and a nonaqueous electrolyte,
wherein the positive electrode contains a positive electrode active
material of a spinel structure represented by a general formula
Li.sub.aMn.sub.bM.sub.cO.sub.4 (where, 1.0.ltoreq.a.ltoreq.1.15,
1.8.ltoreq.b.ltoreq.1.94, 0.01.ltoreq.c.ltoreq.0.10, and a+b+c=3, M
is one or more elements selected from a group consisting of Mg, Ni,
and Cu) and an oxide that coats the surface of the positive
electrode active material, wherein the oxide contains one kind of
metallic element, wherein the metallic element forms a solid
solution with the positive electrode, and wherein the atomic
concentration of the metallic element becomes 0 at the average
depth of from 50 to 100 nm from the external surface of the
positive electrode. By using such positive electrode, a lithium
secondary battery having a long-life characteristic and a high
power density can be provided.
[0063] Hereinafter, the present invention will be described in
detail using examples, but the present invention is not limited to
these examples.
Example 1
[0064] Preparation of the positive electrode active material is
described. In Example 1, as the raw material for preparing the
positive electrode active material, lithium hydroxide, manganese
oxide, and nickel oxide were used and weighed so that Li:Mn:Ni may
become 1.07:1.90:0.03 as the raw material ratio, and then the
resultant material was wet-ground and mixed with a grinder. The
obtained powder was dried and then placed in a high-purity alumina
container and temporarily calcinated at 600.degree. C. for 12 hours
in order to improve the degree of sintering, and was then cracked
after air cooling. Next, in order to form the spinel structure, the
cracked powder was again placed in the high-purity alumina
container and finally-calcinated at 900.degree. C. for 12 hours,
and was then cracked after air cooling.
[0065] The composition of the positive electrode active material
obtained in this case was Li.sub.1.07Mn.sub.1.90Ni.sub.0.03O.sub.4.
The valence of each atom is Li:1, Ni:2, and O:-2, and it turned out
that the valence of Mn is 3.62 from the charge neutrality
condition.
[0066] The surface treatment step is described. 2.2 g of aluminium
nitrate was dissolved in 400 ml of ion exchange water, and
furthermore 0.8 g of lithium hydroxide was dissolved therein to set
the pH of the aqueous solution to 9.0. At this instance the aqueous
solution became clouded, which confirms the presence of the
microparticle of aluminium hydroxide. 100 g of positive electrode
active material was put into this aqueous solution and stirred at
room temperature for 1 hour, thereby causing aluminium hydroxide to
adhere to the surface of the positive electrode active material.
Next, this solution was dried with a spray dryer. The obtained
powder was placed in a high-purity alumina container, and heated at
600.degree. C. for 20 minutes using a microwave heating device in
order to transform the aluminium hydroxide into aluminium oxide and
also dissolve Al with the spinel manganese. In this case, Al was
0.26 wt % relative to Mn.
[0067] Evaluation on the concentration gradient of a metallic
element is described. The atomic concentrations of Al, O, and Mn
were measured using Auger Electron Spectroscopy with an electron
gun of a heat-radiation type under the conditions of accelerating
voltage 5.0 kV and beam current of 90 nA. The depth direction
analysis was conducted under the conditions of an ion gun: the
accelerating voltage of 3.0 kV, the ionic-species of Ar.sup.+, the
raster size of 3 mm.times.3 mm, and the sample gradient of 30
deg.
[0068] The obtained atomic concentration versus depth profile is
shown in FIG. 2. FIG. 2 shows the concentration (atomic %) of each
element with respect to the sputtering depth (nm). In FIG. 2, a
curve 1 represents the concentration distribution of Al, a curve 2
represents the concentration distribution of Mn, and a curve 3
represents the concentration distribution of O. This graph reveals
that the atomic concentration of Al occupies 30% in the external
surface, decreases gradually toward the depth direction and becomes
approximately 0% at a depth of 70 nm. On the other hand, the atomic
concentration of Mn increases gradually toward the depth direction
from the external surface and becomes approximately constant after
exceeding the depth of 20 nm. In other words, the layer of
Al.sub.2O.sub.3 may be dominant from the external surface to 20 nm
in depth while from 20 to 70 nm in depth Al may be present in a
solid solution with the spinel manganese.
[0069] The evaluation on the dissolution amount of Mn is described.
Into a fluororesin (PFA) container within a glove box under argon
atmosphere, 0.5 g of surface-treated positive electrode active
material and 6 cc of electrolyte, the electrolyte being made by
dissolving 1 mol/l of LiPF.sub.6 into an organic solvent liquid, in
which EC and MEC are mixed so that the volume ratio thereof may be
1:2, were placed and sealed. This container was taken out from the
glove box and placed in an oven at 80.degree. C. and left for one
week. After one week, the container was taken out from the oven,
and 5 cc of electrolyte was taken out so that the positive
electrode active material might not mix inside the glove box under
argon atmosphere. The Mn dissolved into the taken-out electrolyte
was quantified using the Inductively Coupled Plasma-Atomic Emission
Spectrometer. The calculated dissolution amount of Mn was 1.2 wt
ppm.
[0070] The characteristics of the positive electrode active
material prepared in Example 1 are shown in Table 1.
TABLE-US-00002 TABLE 1 Amount of Positive surface Dissolution
electrode Composition in treatment Solution amount active
Li.sub.aMn.sub.bM.sub.cO.sub.4 Valence Surface metal depth of Mn
material a b M c of Mn layer (wt %) (nm) (wtppm) Example 1 1.07
1.90 Ni 0.03 3.62 Al.sub.2O.sub.3 0.26 70 1.2
[0071] Preparation of a 18650 (diameter 18 mm.times.height 650 mm)
type battery is described. The 18650 type battery was prepared
using the obtained positive electrode active material. First, the
surface-treated positive electrode active material, the conductive
material made of a carbon material powder, and the binder made of
PVdF were mixed so as to be 90:4.5:5.5 at weight ratio, and then an
adequate amount of NMP was added therein to prepare a slurry. The
prepared slurry was stirred and kneaded with a planetary mixer for
3 hours.
[0072] Next, the kneaded slurry was applied to both sides of an
aluminium foil with 20 .mu.m thickness using a roll transcription
type coating machine. This foil was pressed with a roll press
machine so that the mixture density may become 2.70 g/cm.sup.3,
thereby obtaining the positive electrode.
[0073] Graphite used as the negative electrode active material,
carbon black used as the conductive material, and PVdF used as the
binder were mixed so as to be 92.2:1.6:6.2 at weight ratio, and
this was stirred and kneaded with a slurry mixer for 30 minutes.
The kneaded slurry was applied to both sides of a copper foil with
10 .mu.m thickness using the coating machine, and was pressed with
a roll press method after drying, thereby obtaining the negative
electrode.
[0074] The electrodes of the positive electrode and negative
electrode were cut into a predetermined size, respectively, and a
current-collector tab was attached to an uncoated portion of the
slurry in the electrode by ultrasonic welding.
[0075] A microporous polyethylene film was sandwiched between the
electrodes of the positive electrode and negative electrode, and
these were cylindrically rolled up and then inserted in an 18650
type battery can. After connecting the current-collector tab to the
lid portion of the battery can, the lid portion of the battery can
and the battery can were welded by laser welding, thereby sealing
the battery.
[0076] Finally a nonaqueous electrolyte was injected from a liquid
discharge plug provided in the battery can, thereby obtaining the
18650 type battery. The battery weight was 39 g.
[0077] Evaluation on the power density is described. The power
density of the prepared 18650 type battery was evaluated using the
following procedure. First, the battery was charged to 4.2 V at a
charging rate 0.2 C using the constant current/constant voltage
method. After one hour rest was interposed, the battery was
constant-current discharged to 2.5 V using the current of the same
current value.
[0078] The battery capacity at this point was measured, and the
power density was calculated from a product of this value and the
average voltage 3.7 V. The result is shown in Table 2.
[0079] Evaluation on the cycle characteristic is described. The
cycle characteristic of the prepared 18650 type battery was
evaluated using the following procedure. The battery was
constant-current charged to an end-of-charge voltage 4.2 V by
feeding a current of 0.3 mA/cm.sup.2, and after one hour rest was
interposed, the battery was constant-current discharged to 2.7 V
using the same current value. This was repeated 1000 cycles, and
the capacity retention of 1000th cycle/first cycle was calculated.
Test environment temperature was set to 50.degree. C. The result is
shown in Table 2.
TABLE-US-00003 TABLE 2 Positive electrode Power Capacity active
density retention material (Wh/kg) (%) Example 1 105 88
Example 2
[0080] In Example 2, except that 1.1 g of cobalt nitrate instead of
2.2 g of aluminium nitrate was used in the surface treatment step,
a surface-treated positive electrode active material was prepared
as in Example 1. In this case, in the surface of the positive
electrode active material, Co.sub.2O.sub.3 is dominant.
[0081] Co within the obtained positive electrode active material
was 0.36 wt % relative to Mn, and the depth at which the atomic
concentration of Co becomes approximately 0 was 70 nm. Moreover,
the dissolution amount of Mn was 0.9 wt ppm.
[0082] The characteristics of the positive electrode active
material prepared in Example 2 are shown in Table 3. As in Example
1, a 18650 type batter was prepared, and the weight energy density
and the cycle characteristic were evaluated, and the results are
show in Table 4. It can be seen that the positive electrode
prepared in Example 2 also shows high performance.
Example 3
[0083] In Example 3, except that 1.1 g of iron nitrate instead of
2.2 g of aluminium nitrate was used in the surface treatment step,
a surface-treated positive electrode active material was prepared
as in Example 1. In this case, in the surface of the positive
electrode active material, Fe.sub.2O.sub.3 is dominant.
[0084] Fe within the obtained positive electrode active material
was 0.35 wt % relative to Mn, and the depth at which the atomic
concentration of Fe becomes approximately 0 was 85 nm. Moreover,
the dissolution amount of Mn was 1.6 wt ppm.
[0085] The characteristics of the positive electrode active
material prepared in Example 3 are shown in Table 3. As in Example
1, a 18650 type batter was prepared, and the weight energy density
and the cycle characteristic were evaluated, and the results are
show in Table 4. It can be seen that the positive electrode
prepared in Example 3 also shows high performance.
Example 4
[0086] In Example 4, except that 1.8 g of aluminium nitrate, 0.7 g
of diammonium hydrogenphosphate, and 2 g of diethanolamine were
used instead of 2.2 g of aluminium nitrate and 0.8 g of lithium
hydroxide and heated for 30 minutes using a microwave heating
device in the surface treatment step, a surface-treated positive
electrode active material was prepared as in Example 1. In this
case, in the surface of the positive electrode active material,
AIPO.sub.4 is dominant.
[0087] Al within the obtained positive electrode active material
was 0.22 wt % relative to Mn, and the depth at which the atomic
concentration of Al becomes approximately 0 was 65 nm. Moreover,
the dissolution amount of Mn was 0.6 wt ppm.
[0088] The characteristics of the positive electrode active
material prepared in Example 4 are shown in Table 3. As in Example
1, a 18650 type batter was prepared, and the weight energy density
and the cycle characteristic were evaluated, and the results are
show in Table 4. It can be seen that the positive electrode
prepared in Example 4 also shows high performance.
Example 5
[0089] In Example 5, except that 1.1 g of cobalt nitrate, 0.4 g of
diammonium hydrogenphosphat, and 2 g of diethanolamine were used
instead of 2.2 g of aluminium nitrate and 0.8 g of lithium
hydroxide and heated for 30 minutes using a microwave heating
device in the surface treatment step, a surface-treated positive
electrode active material was prepared as in Example 1. In this
case, in the surface of the positive electrode active material,
Co.sub.3(PO.sub.4).sub.2 was dominant.
[0090] Co within the obtained positive electrode active material
was 0.24 wt % relative to Mn, and the depth at which the atomic
concentration of Co becomes approximately 0 was 50 nm. Moreover,
the dissolution amount of Mn was 1.0 wt ppm.
[0091] The characteristics of the positive electrode active
material prepared in Example 5 are shown in Table 3. As in Example
1, a 18650 type batter was prepared, and the weight energy density
and the cycle characteristic were evaluated, and the results are
show in Table 4. It can be seen that the positive electrode
prepared in Example 5 also shows high performance.
Example 6
[0092] In Example 6, except that 3.3 g of iron nitrate, 1.2 g of
diammonium hydrogenphosphate, and 2 g of diethanolamine were used
instead of 2.2 g of aluminium nitrate and 0.8 g of lithium
hydroxide in the surface treatment step, a surface-treated positive
electrode active material was prepared as in Example 1. In this
case, in the surface of the positive electrode active material,
FePO.sub.4 is dominant.
[0093] Fe within the obtained positive electrode active material
was 0.24 wt % relative to Mn, and the depth at which the atomic
concentration of Fe becomes approximately 0 was 65 nm. Moreover,
the dissolution amount of Mn was 1.3 wt ppm.
[0094] The characteristics of the positive electrode active
material prepared in Example 6 are shown in Table 3. As in Example
1, a 18650 type battery was prepared, and the weight energy density
and the cycle characteristic were evaluated, and the results are
show in Table 4. It can be seen that the positive electrode
prepared in Example 6 also shows high performance.
Example 7
[0095] In Example 7, except that magnesium oxide instead of nickel
oxide was used as the raw material of the positive electrode active
material, a surface-treated positive electrode active material was
prepared as in Example 1. In this case, in the surface of the
positive electrode active material, Al.sub.2O.sub.3 was
dominant.
[0096] Al within the obtained positive electrode active material
was 0.26 wt % relative to Mn, and the depth at which the atomic
concentration of Al becomes approximately 0 was 95 nm. Moreover,
the dissolution amount of Mn was 1.4 wt ppm.
[0097] The characteristics of the positive electrode active
material prepared in Example 7 are shown in Table 3. As in Example
1, a 18650 type batter was prepared, and the weight energy density
and the cycle characteristic were evaluated, and the results are
show in Table 4. It can be seen that the positive electrode
prepared in Example 7 also shows high performance.
Example 8
[0098] In Example 8, except that copper oxide instead of nickel
oxide was used as the raw material of the positive electrode active
material, a surface-treated positive electrode active material was
prepared as in Example 1.
[0099] Al within the obtained positive electrode active material
was 0.26 wt % relative to Mn, and the depth at which the atomic
concentration of Al becomes approximately 0 was 100 nm. Moreover,
the dissolution amount of Mn was 1.0 wt ppm.
[0100] The characteristics of the positive electrode active
material prepared in Example 8 are shown in Table 3. As in Example
1, a 18650 type batter was prepared, and the weight energy density
and the cycle characteristic were evaluated, and the results are
show in Table 4. It can be seen that the positive electrode
prepared in Example 8 also shows high performance.
Example 9
[0101] In Example 9, except that lithium hydroxide, manganese
oxide, and nickel oxide were used and weighed so that Li:Mn:Ni may
become 1.15:1.80:0.05 at the raw material ratio instead of that
Li:Mn:Ni may become 1.07:1.90:0.03, a surface-treated positive
electrode active material was prepared as in Example 1.
[0102] Al within the obtained positive electrode active material
was 0.26 wt % relative to Mn, and the depth at which the atomic
concentration of Al becomes approximately 0 was 60 nm. Moreover,
the dissolution amount of Mn was 0.5 wt ppm.
[0103] The characteristics of the positive electrode active
material prepared in Example 9 are shown in Table 3. As in Example
1, a 18650 type batter was prepared, and the weight energy density
and the cycle characteristic were evaluated, and the results are
show in Table 4. It can be seen that the positive electrode
prepared in Example 9 also shows high performance.
Example 10
[0104] In Example 10, except that lithium hydroxide, manganese
oxide, and nickel oxide were used and weighed so that Li:Mn:Ni may
become 1.07:1.83:0.10 at the raw material ratio instead of that
Li:Mn:Ni may become 1.07:1.90:0.03, a surface-treated positive
electrode active material was prepared as in Example 1.
[0105] Al within the obtained positive electrode active material
was 0.26 wt % relative to Mn, and the depth at which the atomic
concentration of Al becomes approximately 0 was 70 nm. Moreover,
the dissolution amount of Mn was 0.9 wt ppm.
[0106] The characteristics of the positive electrode active
material prepared in Example 10 are shown in Table 3. As in Example
1, a 18650 type batter was prepared, and the weight energy density
and the cycle characteristic were evaluated, and the results are
show in Table 4. It can be seen that the positive electrode
prepared in Example 10 also shows high performance.
Example 11
[0107] In Example 11, except that lithium hydroxide, manganese
oxide, and nickel oxide were used and weighed so that Li:Mn:Ni may
become 1.00:1.94:0.06 at the raw material ratio instead of that
Li:Mn:Ni may become 1.07:1.90:0.03, a surface-treated positive
electrode active material was prepared as in Example 1.
[0108] Al within the obtained positive electrode active material
was 0.26 wt % relative to Mn, and the depth at which the atomic
concentration of Al becomes approximately 0 was 75 nm. Moreover,
the dissolution amount of Mn was 0.9 wt ppm.
[0109] The characteristics of the positive electrode active
material prepared in Example 11 are shown in Table 3. As in Example
1, a 18650 type batter was prepared, and the weight energy density
and the cycle characteristic were evaluated, and the results are
show in Table 4. It can be seen that the positive electrode
prepared in Example 11 also shows high performance.
Example 12
[0110] In Example 12, except that lithium hydroxide, manganese
oxide, and nickel oxide were used and weighed so that Li:Mn:Ni may
become 1.12:1.87:0.01 at the raw material ratio instead of that
Li:Mn:Ni may become 1.07:1.90:0.03, a surface-treated positive
electrode active material was prepared as in Example 1.
[0111] Al within the obtained positive electrode active material
was 0.26 wt % relative to Mn, and the depth at which the atomic
concentration of Al becomes approximately 0 was 60 nm. Moreover,
the dissolution amount of Mn was 1.2 wt ppm.
[0112] The characteristics of the positive electrode active
material prepared in Example 12 are shown in Table 3. As in Example
1, a 18650 type batter was prepared, and the weight energy density
and the cycle characteristic were evaluated, and the results are
show in Table 4. It can be seen that the positive electrode
prepared in Example 12 also shows high performance.
TABLE-US-00004 TABLE 3 Amount of Positive surface Dissolution
electrode Composition in treatment Solution amount active
Li.sub.aMn.sub.bM.sub.cO.sub.4-dF.sub.d Valence Surface metal depth
of Mn material a b M c of Mn layer (wt %) (nm) (wtppm) Example 2
1.07 1.90 Ni 0.03 3.62 Co.sub.2O.sub.3 0.36 70 0.9 Example 3 1.07
1.90 Ni 0.03 3.62 Fe.sub.2O.sub.3 0.35 85 1.6 Example 4 1.07 1.90
Ni 0.03 3.62 AlPO.sub.4 0.22 65 0.6 Example 5 1.07 1.90 Ni 0.03
3.62 Co.sub.3(PO.sub.4).sub.2 0.24 50 1.0 Example 6 1.07 1.90 Ni
0.03 3.62 FePO.sub.4 0.24 65 1.3 Example 7 1.07 1.90 Mg 0.03 3.62
Al.sub.2O.sub.3 0.26 95 1.4 Example 8 1.07 1.90 Cu 0.03 3.62
Al.sub.2O.sub.3 0.26 100 1.0 Example 9 1.15 1.80 Ni 0.05 3.75
Al.sub.2O.sub.3 0.26 60 0.5 Example 10 1.07 1.83 Ni 0.10 3.68
Al.sub.2O.sub.3 0.26 70 0.9 Example 11 1.00 1.94 Ni 0.06 3.55
Al.sub.2O.sub.3 0.26 75 0.9 Example 12 1.12 1.87 Ni 0.01 3.67
Al.sub.2O.sub.3 0.26 60 1.2
TABLE-US-00005 TABLE 4 Positive electrode Power Capacity active
density retention material (Wh/kg) (%) Example 2 98 90 Example 3 92
85 Example 4 111 91 Example 5 108 89 Example 6 102 86 Example 7 106
88 Example 8 97 86 Example 9 95 87 Example 10 93 91 Example 11 121
86 Example 12 92 85
[0113] The following examples show the experiments that were
conducted to find out the preferable conditions about each of the
surface treatment temperature, the kind of a salt used in the
surface treatment, and the salt concentration used in the surface
treatment in the present invention. For the preferable conditions,
in comparison with the characteristic of the positive electrode
active material of Example 1, the following examples show the
conditions that might provide characteristic poorer than that of
the positive electrode active material of Example 1. Accordingly,
this does not mean that the following examples were known prior to
this application.
Example 13
[0114] In Example 13, except that the heating time was set to 3
minutes in the surface treatment step, a surface-treated positive
electrode active material was prepared as in Example 1.
[0115] Al within the obtained positive electrode active material
was 0.26 wt % relative to Mn, and the average depth at which the
atomic concentration of Al becomes approximately 0 was 20 nm.
Moreover, the dissolution amount of Mn was 4.2 wt ppm.
[0116] The characteristics of the positive electrode active
material prepared in Example 13 are shown in Table 3. As in Example
1, a 18650 type batter was prepared, and the weight energy density
and the cycle characteristic were evaluated, and the results are
shown in Table 6. Table 5 and Table 6 revealed that the cycle
characteristic is poor as compared with the one prepared in Example
1.
Example 14
[0117] In Example 14, except that the heating time was set to one
hour in the surface treatment step, a surface-treated positive
electrode active material was prepared as in Example 1.
[0118] Al within the obtained positive electrode active material
was 0.26 wt % relative to Mn, and the atomic concentration of Al
was 10% even at a depth of 100 nm. In this case, the dissolution
amount of Mn was 5.0 wt ppm. Table 5 shows the characteristics of
the positive electrode active materials prepared in Examples 1, 4.
As in Example 1, a 18650 type batter was prepared, and the weight
energy density and the cycle characteristic were evaluated, and the
results are shown in Table 6. Table 5 and Table 6 revealed that the
power density and the cycle characteristic are poor as compared
with the one prepared in Example 1.
Example 15
[0119] In Example 16, except that 4.4 g of aluminium nitrate and
1.5 g of lithium hydroxide instead of 2.2 g of aluminium nitrate
and 0.8 g of lithium hydroxide were used in the surface treatment
step, a surface-treated positive electrode active material was
prepared as in Example 1.
[0120] Al within the obtained positive electrode active material
was 0.52 wt % relative to Mn, and the average depth at which the
atomic concentration of Al becomes approximately 0 from the
external surface was 95 nm. Moreover, the dissolution amount of Mn
was 2.3 Wt ppm.
[0121] Table 5 shows the characteristics of the positive electrode
active material prepared in Example 15. As in Example 1, a 18650
type batter was prepared, and the weight energy density and the
cycle characteristic were evaluated, and the results are shown in
Table 6. Table 5 and Table 6 revealed that the power density and
the cycle characteristic are poor as compared with the one prepared
in Example 1.
Example 16
[0122] In Example 16, except that 0.8 g of aluminium nitrate and
0.3 g of lithium hydroxide instead of 2.2 g of aluminium nitrate
and 0.8 g of lithium hydroxide were used in the surface treatment
step, a surface-treated positive electrode active material was
prepared as in Example 1.
[0123] Al within the obtained positive electrode active material
was 0.09 wt % relative to Mn, and the depth at which the atomic
concentration of Al becomes approximately 0 was 50 nm. Moreover,
the dissolution amount of Mn was 5.7 wt ppm.
[0124] Table 5 shows the characteristics of the positive electrode
active material prepared in Example 16. As in Example 1, a 18650
type battery was prepared, and the weight energy density and the
cycle characteristic were evaluated, and the results are shown in
Table 6. Table 5 and Table 6 revealed that the cycle characteristic
is poor as compared with the one prepared in Example 1.
Example 17
[0125] In Example 17, except that lithium hydroxide, manganese
oxide, and nickel oxide were used and weighed so that Li:Mn may
become 1.10:1.90 where Ni=0 instead of that Li:Mn:Ni may become
1.07:1.90:0.03, a surface-treated positive electrode active
material was prepared as in Example 1.
[0126] Al within the obtained positive electrode active material
was 0.26 wt % relative to Mn, and the depth at which the atomic
concentration of Al becomes approximately 0 was 55 nm from the
external surface. Moreover, the dissolution amount of Mn was 28.6
wt ppm.
[0127] Table 5 shows the characteristics of the positive electrode
active material prepared in Example 17. As in Example 1, a 18650
type battery was prepared, and the weight energy density and the
cycle characteristic were evaluated, and the results are shown in
Table 6. Table 5 and Table 6 revealed that the cycle characteristic
is poor as compared with the one prepared in Example 1.
TABLE-US-00006 TABLE 5 Amount of Positive surface Dissolution
electrode Composition in treatment Solution amount active
Li.sub.aMn.sub.bM.sub.cO.sub.4 Valence Surface metal depth of Mn
material a b M c of Mn layer (wt %) (nm) (wtppm) Example 13 1.07
1.90 Ni 0.03 3.62 Al.sub.2O.sub.3 0.26 20 4.2 Example 14 1.07 1.90
Ni 0.03 3.62 Al.sub.2O.sub.3 0.26 >100 5.0 Example 15 1.07 1.90
Ni 0.03 3.62 Al.sub.2O.sub.3 0.52 95 2.3 Example 16 1.07 1.90 Ni
0.03 3.62 Al.sub.2O.sub.3 0.09 50 5.7 Example 17 1.10 1.90 -- --
3.63 Al.sub.2O.sub.3 0.26 55 28.6
TABLE-US-00007 TABLE 6 Positive electrode Power Capacity active
density retention material (Wh/kg) (%) Example 13 102 81 Example 14
83 78 Example 15 73 74 Example 16 106 80 Example 17 110 65
[0128] FIG. 3 and FIG. 4 show the evaluation results with regard to
Example 1 to Example 17 described above. FIG. 3 shows the power
density versus the average depth (average thickness of the solid
solution layer) at which the atomic concentration of a metallic
element contained within the metal oxide used in the surface
treatment becomes approximately 0%. FIG. 4 shows the cycle
characteristic versus solution depth. The cycle characteristic was
obtained by measuring the residual capacity. From these data, it
can be seen that the batteries whose average depth, at which the
atomic concentration of a metallic element contained within the
metal oxide became approximately 0%, is in a range of 50 to 100 nm
exhibit excellent characteristics, such as the power density of no
less than 90 Wh/kg and the cycle characteristic of no less than
85%.
[0129] According to the present embodiment, the surface of the
spinel manganese substituted with an element is treated, and the
average depth, at which the atomic concentration of a metallic
element contained within the metal oxide used in the surface
treatment becomes approximately 0%, is specified to 50 to 100 nm,
whereby a positive electrode active material excellent in cycle
characteristic can be formed and a long-life lithium secondary
battery using this positive electrode active material can be
provided.
[0130] Moreover, FIG. 5 shows a secondary battery system with the
lithium secondary batteries that were prepared in the present
embodiment. A plurality of lithium secondary batteries 10 (e.g., 4
to 8 pieces) are connected in series to form a lithium secondary
battery. Furthermore, a plurality of these lithium batteries are
connected to configure a group of lithium secondary batteries.
[0131] A cell controller 11 is formed corresponding to such a group
of lithium secondary batteries, respectively, and controls the
lithium secondary batteries 10. The cell controller 11 monitors the
over-charge or over-discharge of the lithium secondary battery 10
or monitors the remaining capacity of the lithium secondary battery
10.
[0132] A battery controller 12 provides a signal to the cell
controller 11 using a communication means, for example, and also
acquires a signal from the cell controller 11 using a communication
means, for example. The battery controller 12 controls the
input/output of electric power with respect to the cell controller
11.
[0133] The battery controller 12 provides a signal to an input
section 111 of the first cell controller 11, for example. Such a
signal is transmitted from an output section 112 of the cell
controller 11 to the input section 111 of other cell controller 11,
in series. This signal is provided from the output section 112 of
the last cell controller 11 to the battery controller 12. Thus, the
battery controller 12 can monitor the cell controllers 11.
INDUSTRIAL APPLICABILITY
[0134] The lithium secondary battery of the present invention is
particularly useful as a large-sized stationary power supply.
ADVANTAGES OF THE INVENTION
[0135] According to the present invention, a lithium secondary
battery having a long-life characteristic and a high power density
can be provided.
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