U.S. patent application number 10/479975 was filed with the patent office on 2004-09-02 for positive electrode active material for secondary cell and nonaqueous electrolyte secondary cell using the same, and method for analysis of positive electrode active material for secondary cell.
Invention is credited to Amemiya, Kazuki, Endou, Shouta, Ooya, Yasumasa, Sakai, Ryo, Shirakawa, Yasuhiro, Takeuchi, Hajime, Tanaka, Koshin.
Application Number | 20040170894 10/479975 |
Document ID | / |
Family ID | 19033203 |
Filed Date | 2004-09-02 |
United States Patent
Application |
20040170894 |
Kind Code |
A1 |
Sakai, Ryo ; et al. |
September 2, 2004 |
Positive electrode active material for secondary cell and
nonaqueous electrolyte secondary cell using the same, and method
for analysis of positive electrode active material for secondary
cell
Abstract
A positive electrode active material for use in a non-aqueous
electrolyte secondary cell comprises a powdery metal oxide
(LiCoO.sub.2, LiNiO.sub.2, LiMn.sub.2O.sub.4 or the like). When the
positive electrode active material is classified with a
classification precision index .kappa. of 0.7 or greater so as to
obtain a coarse powder having a classification ratio in a range of
0.1% to 5%, a ratio (B/A) of the content (B) of an impurity metal
element in the coarse powder obtained by the classification to the
content (A) of the impurity metal element in the powder before the
classification is 1.5 or less. The contents of the impurity metal
elements are compared with respect to Ca, Mn, Fe, Cr, Cu, Zn and
the like (exclusive of the metal element constituting the powdery
metal oxide). The positive electrode active material for a
secondary cell serves to improve cell performance capabilities and
production yields.
Inventors: |
Sakai, Ryo; (Yokohama-shi,
JP) ; Shirakawa, Yasuhiro; (Yokohama-shi, JP)
; Takeuchi, Hajime; (Yokohama-shi, JP) ; Ooya,
Yasumasa; (Haibara-gun, JP) ; Tanaka, Koshin;
(Yokohama-shi, JP) ; Amemiya, Kazuki;
(Fujieda-shi, JP) ; Endou, Shouta; (Yokohama-shi,
JP) |
Correspondence
Address: |
Richard L Schwaab
Foley & Lardner
Suite 500
3000 K Street NW
Washington
DC
20007-5109
US
|
Family ID: |
19033203 |
Appl. No.: |
10/479975 |
Filed: |
December 12, 2003 |
PCT Filed: |
June 25, 2002 |
PCT NO: |
PCT/JP02/06326 |
Current U.S.
Class: |
429/218.1 ;
429/223; 429/224; 429/231.1; 436/127 |
Current CPC
Class: |
H01M 4/131 20130101;
H01M 2004/021 20130101; Y02E 60/10 20130101; H01M 4/525 20130101;
H01M 10/0525 20130101; Y10T 436/20 20150115; H01M 4/505
20130101 |
Class at
Publication: |
429/218.1 ;
429/223; 429/224; 429/231.1; 436/127 |
International
Class: |
H01M 004/48; H01M
004/52; H01M 004/50; G01N 033/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 27, 2001 |
JP |
2001-195174 |
Claims
What is claimed is:
1. A positive electrode active material for a non-aqueous
electrolyte secondary cell, comprising: a powdery metal oxide,
wherein, when the powdery composite metal oxide is classified
through the use of a difference in particle diameter and density of
its component particles to obtain coarse powder having a
classification ratio in a range of 0.1 to 5%, a ratio (B/A) of the
content B of an impurity metal element in the coarse powder
obtained by the classification to the content A of an impurity
metal element in the powdery metal oxide before the classification
is 1.5 or below.
2. The positive electrode active material for a secondary cell
according to claim 1, wherein the powdery metal oxide is classified
to have a classification precision index .kappa. of 0.7 or
more.
3. The positive electrode active material for a secondary cell
according to claim 1, wherein the powdery metal oxide is classified
by a classifier based on a balance between centrifugal force and
fluid resistance by a forced vortex.
4. The positive electrode active material for a secondary cell
according to claim 1, wherein the impurity metal element is at
least one kind of element (excepting the metal elements
constituting the powdery metal oxide) selected from Mg, Ca, Ba, Sr,
Sc, Y, Ti, Zr, Hf, V, Cr, Nb, Mo, Ta, W, Mn, Fe, Co, Ni, Cu, Zn,
Ga, Ge, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Re, Os, Ir, Tl, Pb and
Bi.
5. The positive electrode active material for a secondary cell
according to claim 4, wherein the impurity metal element is at
least one kind of element (excepting the metal elements
constituting the powdery metal oxide) selected from Fe, Cr, Cu, Zn,
Mg and Ca.
6. The positive electrode active material for a secondary cell
according to claim 1, wherein the positive electrode active
material comprises a composite metal oxide containing lithium and
at least one kind of element selected from cobalt, nickel and
manganese.
7. The positive electrode active material for a secondary cell
according to claim 6, wherein the positive electrode active
material comprises at least one kind of composite metal oxide
selected from the following general
formulae:LiA.sub.aO.sub.x(where, A denotes at least one kind of
element selected from Co, Ni and Mn, and a and x denote numerals
falling in a range of 0.8.ltoreq.a.ltoreq.1.1,
1.6.ltoreq.x.ltoreq.2.4), andLiB.sub.bO.sub.y(where, B denotes an
element containing at least Mn selected from Mn, Co and Ni, and b
and y denote numerals falling in a range of
1.5.ltoreq.b.ltoreq.2.1, 3.6.ltoreq.y4.4).
8. A non-aqueous electrolyte secondary cell, comprising: a positive
electrode containing a positive electrode active material
consisting essentially of a powdery metal oxide, a negative
electrode disposing with the positive electrode through a
separator, a cell casing for accommodating the positive electrode,
the separator and the negative electrode, and a non-aqueous
electrolyte filling in the cell casing, wherein, when the positive
electrode active material is classified through the use of a
difference in particle diameter and density of component particles
of the powdery metal oxide to obtain coarse powder having a
classification ratio in a range of 0.1 to 5%, a ratio (B/A) of the
content B of an impurity metal element in the coarse powder
obtained by the classification to the content A of an impurity
metal element in the powdery metal oxide before the classification
is 1.5 or below.
9. The non-aqueous electrolyte secondary cell according to claim 8,
wherein the impurity metal element is at least one kind of element
(excepting the metal elements constituting the powdery metal oxide)
selected from Mg, Ca, Ba, Sr, Sc, Y, Ti, Zr, Hf, V, Cr, Nb, Mo, Ta,
W, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn,
Sb, Re, Os, Ir, Tl, Pb and Bi.
10. The non-aqueous electrolyte secondary cell according to claim
8, wherein the positive electrode active material comprises a
composite metal oxide containing lithium and at least one kind of
element selected from cobalt, nickel and manganese.
11. The non-aqueous electrolyte secondary cell according to claim
8, wherein the positive electrode active material comprises at
least one kind of composite metal oxide selected from the following
general formulae:LiA.sub.aO.sub.x(where, A denotes at least one
kind of element selected from Co, Ni and Mn, and a and x denote
numerals falling in a range of 0.8.ltoreq.a.ltoreq.1.1,
1.6.ltoreq.x.ltoreq.2.4), andLiB.sub.bO.sub.y(where, B denotes an
element containing at least Mn selected from Mn, Co and Ni, and b
and y denote numerals falling in a range of
1.5.ltoreq.b.ltoreq.2.1, 3.6.ltoreq.y4.4).
12. The non-aqueous electrolyte secondary cell according to claim
8, wherein the secondary cell is a lithium-ion secondary cell.
13. A method for analysis of a positive electrode active material
comprising a powdery metal oxide to be used for a non-aqueous
electrolyte secondary cell, comprising: obtaining coarse powder
having a classification ratio in a range of 0.1 to 5% by
classifying the powdery metal oxide through the use of a difference
in particle diameter and density of its component particles;
measuring the content A of an impurity metal element in the powdery
metal oxide before the classification and the content B of an
impurity metal element in the coarse powder obtained by the
classification; and evaluating the amount of the particulate metal
impurity contained in the powdery metal oxide before the
classification based on a ratio (B/A) of the content B of the
impurity metal element to the content A of the impurity metal
element.
14. The method for analysis of a positive electrode active material
for a secondary cell according to claim 13, wherein the powdery
metal oxide is classified to have a classification precision index
.kappa. of 0.7 or more.
15. The method for analysis of a positive electrode active material
for a secondary cell according to claim 13, wherein the powdery
metal oxide is classified by a classifier based on a balance
between centrifugal force and fluid resistance by a forced vortex.
Description
TECHNICAL FIELD
[0001] The present invention relates to a positive electrode active
material to be used for a non-aqueous electrolyte secondary cell
such as a lithium-ion secondary cell, a non-aqueous electrolyte
secondary cell using it, and a method for analysis of a positive
electrode active material for a secondary cell.
BACKGROUND ART
[0002] In recent years, portable electronic equipment such as
notebook computers, personal digital assistants (PDAs), cellular
phones and camcorders are becoming popular increasingly.
Accordingly, a secondary cell used as the power supply for the
portable electronic equipment is highly demanded to be small and
have high capacity, high cyclic lifetime and the like. A known
secondary cell satisfying such demands is, for example, a
lithium-ion secondary cell using a non-aqueous electrolyte
containing lithium salt.
[0003] For such a lithium-ion secondary cell, a lithium-containing
transition metal composite oxide such as LiCoO.sub.2, LiNiO.sub.2,
LiMn.sub.2O.sub.4 are used as a positive electrode active material.
A carbon-based material is used for a negative electrode, and an
electrolyte having lithium salt such as LiPF.sub.6 or LiBF.sub.4
dissolved into a nonaqueous solvent is used as a non-aqueous
electrolyte.
[0004] The lithium-ion secondary cell has characteristics that its
energy density is higher as compared with a conventionally used
Ni--Cd cell or Ni--MH secondary cell and far superior in view of
safety to a secondary cell using lithium metal. Thus, a large
quantity of lithium-ion secondary cell is being used as the power
supply for portable electronic equipment.
[0005] For example, a positive electrode using a positive electrode
active material such as LiCoO.sub.2 or LiNiO.sub.2 is produced as
follows. First, a mixture of cobalt oxide or nickel hydroxide and
lithium carbonate, lithium hydroxide or the like is calcined in the
air or oxygen at a temperature of about 600 to 1000.degree. C. to
produce a composite oxide. The bulk composite oxide is pulverized
into sizes of several micrometers to several tens of micrometers
and further classified by screening or the like, if necessary. The
composite metal oxide powder obtained as described above is
suspended together with a conductive agent and a binder into an
appropriate solvent to prepare a slurry, which is then applied onto
a collector (metal foil) and dried to form a thin plate so to
produce a positive electrode (see Japanese Patent Laid-Open
Applications No. 11-135119 and No. 11-149925).
[0006] But, the lithium-ion secondary cell using the
above-described conventional positive electrode has a disadvantage
that a faulty voltage drop tends to occur at the time of initial
charging, resulting in that a production yield drops, battery
performance lowers, or the like. Such phenomena were studied to
find that a particulate metal impurity is often included in the
positive electrode active material produced by a conventional
production method, resulting in causing problems. The particulate
metal impurity was mingled in a quite small amount, which was such
a small amount not causing a problem even when the content of an
impurity metal element in the positive electrode active material as
a whole was analyzed. Therefore, such inclusion was not found by a
conventional production process or analyzing method.
[0007] An object of the invention provides a positive electrode
active material for a secondary cell that allows to increase a
production yield of a non-aqueous electrolyte secondary cell and
improves cell performance by establishing a method for analysis and
evaluation of factors (such as a particulate metal impurity) that
lower the cell performance and production yield. Another object of
the invention is to provide a non-aqueous electrolyte secondary
cell using such a positive electrode active material. Still another
object of the invention is to provide a method for analysis of a
positive electrode active material for a secondary cell that can
analyze and evaluate factors which lower the cell performance and
production yield.
SUMMARY OF THE INVENTION
[0008] The inventors of the present application have made research
and study on a relationship between a particulate metal impurity
mingled into the above-described positive electrode active material
and a fraction defective (especially, an infant mortality rate) of
a secondary cell, and found that a correlation between the content
of an impurity metal element analyzed in the entire positive
electrode active material and a fraction defective of the secondary
cell can not be found because the mingled amount of the particulate
metal impurity is very small in the positive electrode active
material as a whole.
[0009] Meanwhile, it was found that the content of the impurity
metal element in the coarse powder is closely related to a fraction
defective by separating coarse powder from the positive electrode
active material with high precision and analyzing the content of
the impurity metal element contained in the coarse powder. In other
words, the particulate metal impurity is concentrated into the
coarse powder by classifying the positive electrode active material
with high precision through the use of a difference in particle
diameter and density of the particles constituting it. And, it was
found that the content of the concentrated metal impurity element
(the content of the impurity metal element in the coarse powder) is
closely related to a fraction defective of the secondary cell.
[0010] The present invention is based on the above findings. The
positive electrode active material for a secondary cell of the
invention is a positive electrode active material comprising a
powdery metal oxide used for a non-aqueous electrolyte secondary
cell and characterized in that, when the powdery metal oxide is
classified through the use of a difference in particle diameter and
density of its component particles to obtain coarse powder having a
classification ratio in a range of 0.1 to 5%, a ratio (B/A) of the
content B of an impurity metal element in the coarse powder
obtained by the classification to the content A of an impurity
metal element in the powdery metal oxide before the classification
is 1.5 or below.
[0011] As described above, the positive electrode active material
for a secondary cell has, for example, metal impurities mingled
therein, resulting in occurrence of problems. Especially, a
particulate metal impurity (e.g., high density particles) having a
relatively large particle diameter is easily eluted by a high
potential of the positive electrode when the secondary cell is
initially charged. When the eluted metal ions are reduced and
deposited on the negative electrode side, the deposit breaks
through the separator to cause a micro-short circuit with the
positive electrode.
[0012] The particulate metal impurity, which tends to dissolve and
deposit in the electrolyte, becomes a cause of a failure when it is
contained in the positive electrode even if its content is on the
order of several ppm. But, according to the present method for
analysis of metal impurities, an analysis error is mostly on the
order of several ppm and buried in the background of impurities
(e.g., impurities contained at an atomic level into a crystal)
inherently contained uniformly in a material for the positive
electrode, such as Co material, so that the particulate metal
impurity cannot be detected. Conversely, even if the total amount
of the contents of an impurity metal element is enormous, there is
a possibility that a failure does not occur if coarse impurity
particles (a particulate metal impurity) are not contained.
[0013] According to the present invention, as a method for analysis
and evaluation of the content of the particulate metal impurity
which is hardly detected by an ordinary method for analysis, there
is applied a method which classifies a positive electrode active
material (powdery composite metal oxide) with high precision and
compares the content B of an impurity metal element (metal element
adversely affecting on the operation and characteristics of a
secondary cell) in the obtained coarse powder with the content A of
an impurity metal element in the positive electrode active material
before the classification. Specifically, the particulate metal
impurity is concentrated on the coarse powder side by classifying
the positive electrode active material with high precision.
Therefore, the content of the particulate metal impurity in the
positive electrode active material can be evaluated by comparing
the impurity content B in the coarse powder having concentrated the
particulate metal impurity with the impurity content (impurity
content before the classification) A as the whole positive
electrode active material.
[0014] Specifically, when the ratio (B/A) of the impurity content B
in the coarse powder to the impurity content (impurity content
before the classification) A as the whole positive electrode active
material is 1.5 or below, the production yield and cell performance
of the non-aqueous electrolyte secondary cell produced by using
such a positive electrode active material can be enhanced. In other
words, when the ratio B/A of the content of the impurity metal
element is 1.5 or below, it means that the content of the
particulate metal impurity in the positive electrode active
material is adequately reduced. Therefore, when the positive
electrode active material is used to produce a non-aqueous
electrolyte secondary cell, the occurrence of a micro-short circuit
resulting from the deposition of impurity metal ions at the initial
charging can be prevented. Thus, the non-aqueous electrolyte
secondary cell excelling in cell performance and having high
production yield can be provided with a high reproducibility.
[0015] The positive electrode active material for a secondary cell
of the invention is characterized in that the powdery metal oxide
is classified to have a classification precision index .kappa. of
0.7 or greater. The classification precision index .kappa. will be
described in detail later. The powdery metal oxide is classified in
the classification precision index .kappa. to obtain coarse powder
having a classification ratio in a range of 0.1 to 5%, and it
becomes possible to concentrate the particulate metal impurity into
the coarse powder with high precision. Therefore, the content of
the impurity metal element in the coarse powder becomes having a
significant effect on a fraction defective of the secondary cell,
so that the production yield and cell performance of the
non-aqueous electrolyte secondary cell can be enhanced more
effectively by suppressing the impurity content low.
[0016] In the positive electrode active material for a secondary
cell of the present invention, a metal element adversely affecting
on the operation and characteristics of the secondary cell is
selected for the impurity metal element whose content is compared
between the powdery composite metal oxide before the classification
and the coarse powder obtained by the classification. Specifically,
it is preferable to compare at least one kind of element (excepting
the metal elements constituting the powdery metal oxide) selected
from Mg, Ca, Ba, Sr, Sc, Y, Ti, Zr, Hf, V, Cr, Nb, Mo, Ta, W, Mn,
Fe, Co, Ni, Cu, Zn, Ga, Ge, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Re,
Os, Ir, Tl, Pb and Bi. These impurity metal elements are compared
as the content of each single element between the powdery metal
oxide before the classification and the coarse powder obtained by
the classification.
[0017] The non-aqueous electrolyte secondary cell of the invention
comprises a positive electrode containing the positive electrode
active material for a secondary cell of the invention, a negative
electrode disposed with the positive electrode through a separator,
a cell casing for housing the positive electrode, the separator and
the negative electrode, a non-aqueous electrolyte charged into the
cell casing. The production yield and cell performance can be
improved by the above non-aqueous electrolyte secondary cell.
[0018] The method for analysis of a positive electrode active
material for a secondary cell of the invention is a method for
analysis of a positive electrode active material comprising a
powdery metal oxide to be used for a non-aqueous electrolyte
secondary cell, having a step of obtaining coarse powder having a
classification ratio in a range of 0.1 to 5% by classifying the
powdery metal oxide through the use of a difference in particle
diameter and density of its component particles, a step of
measuring the content A of an impurity metal element in the powdery
metal oxide before the classification and the content B of an
impurity metal element in the coarse powder obtained by the
classification, and a step of evaluating the amount of the
particulate metal impurity contained in the powdery metal oxide
before the classification based on a ratio (B/A) of the content B
of the impurity metal element to the content A of the impurity
metal element.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a diagram showing an accumulation frequency and
partial classification efficiency of each powder before and after
classification evaluation of the positive electrode active material
(sample 1) according to Embodiment 1 of the invention.
[0020] FIG. 2 is a diagram showing particle size distribution of
each powder before and after the classification evaluation of the
positive electrode active material (sample 1) according to
Embodiment 1 of the invention.
[0021] FIG. 3 is a diagram showing a relationship between a partial
classification efficiency and a particle diameter prepared for
determination of a classification precision index by the
classification evaluation of the positive electrode active material
(sample 1) according to Embodiment 1 of the invention.
[0022] FIG. 4 is a sectional view showing a structure of the
non-aqueous electrolyte secondary cell according to one embodiment
of the invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0023] Embodiments for conducting the invention will be described
below.
[0024] The positive electrode active material for a secondary cell
of the invention is used for a positive electrode of a non-aqueous
electrolyte secondary cell such as a lithium-ion secondary cell.
For example, a metal oxide such as a lithium-containing composite
metal oxide is used for the positive electrode active material. The
lithium-containing composite metal oxide includes a lithium-cobalt
composite oxide, a lithium-nickel composite oxide, a
lithium-manganese composite oxide, and their composite or mixed
oxides.
[0025] The lithium-cobalt composite oxide and the lithium-nickel
composite oxide is basically denoted by LiAO.sub.2 (A is at least
one kind of element selected from Co, Ni and Mn), but a ratio
between Li and Co or Ni may be deviated from a stoichiometric
composition, and an amount of oxygen is also not limited to a
stoichiometric composition. Part (e.g., 10 atomic % or below) of
the A element such as Co or Ni can be substituted by a transition
metal element such as Sn, Al, V, Cr or Fe.
[0026] The lithium-manganese composite oxide is basically indicated
by LiB.sub.2O.sub.4 (B is an element containing at least Mn
selected from Mn, Co and Ni), but a ratio between Li and Mn may be
deviated slightly from the stoichiometric composition, and the
amount of oxygen is also not limited to the stoichiometric
composition. Part (e.g., 10 atomic % or below) of the B element
such as Mn may be substituted by a transition metal element such as
Fe, Sn, Al, V, Cr or Ni.
[0027] Specifically, as a positive electrode active material, at
least one kind of composite metal oxide selected from the following
general formulae:
LiA.sub.aO.sub.x
[0028] (where, A denotes at least one kind of element selected from
Co, Ni and Mn, and a and x are numerals falling in a range of
0.8.ltoreq.a.ltoreq.1.1, 1.6.ltoreq.x.ltoreq.2.4. But, 10 atomic %
or below of the A element can be substituted by a transition metal
element such as Sn, Al, V, Cr or Fe); and
LiB.sub.bO.sub.y
[0029] (where, B denotes an element containing at least Mn selected
from Mn, Co and Ni, and b and y are numerals falling in a range of
1.5.ltoreq.b.ltoreq.2.1, 3.6.ltoreq.y.ltoreq.4.4. But, 10 atomic %
or below of the B element can be substituted by a transition metal
element such as Fe, Sn, Al, V, Cr or Ni.)
[0030] The above-described composite metal oxide is pulverized and
used as a positive electrode active material. In the positive
electrode active material made of the powdery composite metal oxide
(composite metal oxide powder), the positive electrode active
material for a secondary cell of this embodiment has
characteristics satisfying the impurity content described below
when the content of the particulate metal impurity is evaluated
according to the analysis and evaluation method to be described
later in detail. Specifically, as the results of conducting the
analysis and evaluation according to the invention, when the
content A of the impurity metal element in the positive electrode
active material (a powdery composite metal oxide as a whole) before
the classification and the content B of the impurity metal element
in coarse powder with a classification ratio of 0.1 to 5% are
compared, it is a positive electrode active material made of a
powdery composite metal oxide with a ratio (B/A) of the impurity
content of 1.5 or below.
[0031] The method for analysis and evaluation of a particulate
metal impurity in the positive electrode active material of the
invention will be described in detail below. The analysis method of
the invention first classifies a positive electrode active material
made of a powdery composite metal oxide through the use of a
difference in particle diameter and density of the component
particles to obtain coarse powder with a classification ratio in a
range of 0.1 to 5%. This classification is strictly performed for
analysis and evaluation of the content of the particulate metal
impurity and different from the classification conventionally
performed for adjustment of a particle diameter of the positive
electrode active material, namely the classification for removal of
coarse powder or fine powder from the pulverized positive electrode
active material. The positive electrode active material is formed
of the whole powdery composite metal oxide before the
classification. In other words, the positive electrode active
material of this embodiment is made of the whole powder containing
the classified coarse powder and other particles (formally called
as the fine powder).
[0032] For the method of analyzing and evaluating the positive
electrode active material, a classification method capable of
classifying powder with high precision is applied. Specifically,
sieving is used for simple classification, but a sieve cannot
classify a fine powder material having an average particle diameter
in a range of approximately several micrometers to several tens
micrometers, which is used for the positive electrode active
material, with high precision. Therefore, the analysis and
evaluation method of the invention classifies through the use of a
fact that the resistance of particles against a physical force such
as gravity, inertia force or centrifugal force is variable
depending on a particle diameter or a density of the particles.
Specifically, it is desirable to apply airflow classification for
classifying on the basis of a balance between gravity, inertia
force or centrifugal force and fluid resistance.
[0033] The airflow classification can treat powder in a large
amount, it is industrially suitable and can classify on the basis
of a particle diameter and a density (mass) of the particles.
Therefore, it is preferably used for the analysis and evaluation
method of a particulate metal impurity of the invention. Here,
typical examples of the airflow classification include the
classification utilizing a balance between centrifugal force due to
a free vortex and fluid resistance, the classification utilizing a
balance between the centrifugal force due to a forced vortex and
fluid resistance, and the like.
[0034] A classification device utilizing the centrifugal force of a
free vortex such as a cyclone may not enhance classification
precision to an adequate level because a dispersion force is weak.
Meanwhile, a classifier, such as a Micron Separator, a Turboplex,
an AccuCut or a Turbo-classifier, which classifies based on a
balance between the centrifugal force by a forced vortex and fluid
resistance, is preferable as the classifier to be used particularly
for the analysis and evaluation method of a particulate metal
impurity of the invention because a dispersion force is high,
damage to the particles is small, and the classification precision
is remarkable.
[0035] According to the method for analysis and evaluation of a
positive electrode active material, the above-described classifier
(particularly, a classifier using a balance between the centrifugal
force by a forced vortex and the fluid resistance) is used to
classify the positive electrode active material so to have, for
example, a classification precision index .kappa. of 0.7 or more so
as to obtain coarse powder with a classification ratio (mass ratio)
in a range of 0.1 to 5%. When the coarse powder with a
classification ratio in a range of 0.1 to 5% is obtained, the
particulate metal impurity can be effectively and practically
concentrated to the coarse powder side. By comparing the content of
the metal impurity element between the coarse powder having the
particulate metal impurity concentrated and the positive electrode
active material before the classification, the content of the
particulate metal impurity in the entire positive electrode active
material can be practically evaluated with high precision.
[0036] Specifically, the particulate metal impurity which is easily
dissolved and deposited into the electrolyte becomes a cause of a
failure when it is contained in the positive electrode even if its
content is on the order of several ppm. But, an ordinary metal
impurity analysis method (quantitative analysis) according to the
ICP method or the like cannot accurately detect only the amount of
the particulate metal impurity because an analysis error is on the
order of several ppm. Conversely, even if the total sum of the
contents of the impurity metal element is large, a failure may not
occur when the particulate metal impurity is not contained.
Meanwhile, when the positive electrode active material is
classified to obtain coarse powder with a classification ratio in a
range of 0.1 to 5%, the particulate metal impurity which is hardly
detected by the ordinary analysis method can be concentrated into
the classified coarse powder. Therefore, the content B of the metal
impurity element in the classified coarse powder can be compared
with the content A of the metal impurity element in the positive
electrode active material before the classification to evaluate the
content of the particulate metal impurity in the entire positive
electrode active material.
[0037] When the positive electrode active material is classified as
described above, the fine powder is contained in a large amount in
the coarse powder if the coarse powder has a classification ratio
exceeding 5%, and the particulate metal impurity cannot be
concentrated adequately. Therefore, even if the content B of the
metal impurity element in the coarse powder and the content A of
the metal impurity element in the entire positive electrode active
material are compared, the ratio (B/A ratio) of the impurity
content closely related to a fraction defective of a secondary cell
cannot be obtained. Meanwhile, when the classification ratio is
less than 0.1%, it is not preferable because a great deal of
classification treatments are required to obtain a sample to be
measured for determination of a ratio (B/A ratio) of the impurity
content closely related to a fraction defective of the secondary
cell, namely a sample to be measured in an amount required for the
analysis of composition. It is further desirable that the
classification ratio of the coarse powder falls in a range of 1 to
3% to enhance the practicality of comparing the impurity content B
in the coarse powder and the impurity content A in the entire
positive electrode active material.
[0038] The positive electrode active material is preferably
classified so to have a classification precision index .kappa. of
0.7 or more. If the classification precision index .kappa. is less
than 0.7, it means that separation precision of the coarse powder
is low, and the particle size distribution of the coarse powder
becomes broad. Such coarse powder cannot be used to adequately
concentrate the particulate metal impurity, and the
interrelationship between the ratio (B/A ratio) of the impurity
content and the fraction defective of the secondary cell becomes
weak. Therefore, even the positive electrode active material having
the B/A ratio in a prescribed range may not able to adequately
reduce the fraction defective of the secondary cell. It is more
desirable that the classification precision index .kappa. is 0.8 or
higher. When the classification precision index .kappa. is high, it
means that the particulate metal impurity is concentrated to a
higher level, so that the particulate metal impurity amount in the
entire positive electrode active material is small as the impurity
content in such a highly concentrated state is lower.
[0039] Here, the classification precision index .kappa. is
determined as follows. Specifically, partial classification
efficiency .eta.(d) is first determined according to the following
expression (1) below. The partial classification efficiency
.eta.(d) is determined from a particle size distribution of each
powder and indicates a recovery rate in individual sections which
are obtained by dividing continuously variable particle diameters
into such sections. 1 ( d ) = c { Rc ( di ) - Rc ( di + 1 ) } Ro (
di ) - Ro ( di + 1 )
[0040] where,
[0041] di, di+1: i-st, i+1st particle diameter (.mu.m)
[0042] R.sub.c(di), R.sub.c(di+1): mass cumulative frequency of
coarse powder (%)
[0043] R.sub.0(di), R.sub.0(di+1): mass cumulative frequency of
powder before classification (%)
[0044] .eta..sub.c: yield of coarse powder (a classification ratio)
(%)
[0045] .eta.(d): partial classification efficiency (%)
[0046] The expression (1) is shown as a partial classification
efficiency curve indicating that classification precision is high
as the curve has a sharp (large) inclination. The classification
precision index .kappa. is a value of quantified classification
precision and determined from a particle diameter
(D.sub.p25(.mu.m)) when the partial classification efficiency is
25% and a particle diameter (D.sub.p75(.mu.m)) when the partial
classification efficiency is 75% by the following expression
(2).
.kappa.=D.sub.p25/D.sub.p75(2)
[0047] In this case, the classification precision index .kappa.
becomes a value smaller than 1, and it means that the
classification precision becomes high as the value becomes closer
to 1.
[0048] Examples of the results (the classified results of sample 1
of Example 1 described later) of actual classification of a
positive electrode active material (LiCoO.sub.2) will be shown in
Table 1, Table 2, FIG. 1, FIG. 2 and FIG. 3. The positive electrode
active material was classified by using the Turbo-classifier as an
air classifier, and the classification condition was adjusted so
that the ratio of coarse powder to fine powder became 2:98 (a
classification ratio of the coarse powder=2%). At this time, the
volume of air was maximum of the device in order to enhance a
dispersion force, and a classification ratio was adjusted by a
rotor speed.
1TABLE 1 Partial Particle Accumulation frequency (%) classification
diameter A B C D efficiency (.mu.m) 100 2 98 (100) (%) 0.5-0.9 0
0.53 0 0.01 0 0.9-1.1 0.34 0.78 0.27 0.28 1.85 1.1-1.3 1.02 1.03
0.95 0.95 0.74 1.3-1.5 2.00 1.30 1.97 1.96 0.54 1.5-1.8 3.96 1.76
4.06 4.01 0.45 1.8-2.2 7.31 2.54 7.66 7.56 0.44 2.2-2.6 11.38 3.53
12.02 11.85 0.46 2.6-3.1 17.20 5.06 18.23 17.97 0.50 3.1-3.7 24.90
7.25 26.35 25.97 0.54 3.7-4.3 32.95 9.69 34.74 34.24 0.59 4.3-5
42.23 12.63 44.30 43.67 0.62 5-6 54.48 16.76 56.75 55.95 0.67 6-7.5
69.69 22.86 72.00 71.02 0.81 7.5-9 80.89 29.69 83.02 81.95 1.25
9-10.5 88.53 38.18 90.33 89.29 2.32 10.5-12.5 94.67 51.72 95.96
95.08 4.68 12.5-15 98.42 68.74 99.07 98.46 10.05 15-18 99.91 84.55
100.00 99.69 25.76 18-21 100.00 93.85 100.00 99.88 100 21-25 100.00
98.96 100.00 99.98 100 25-30 100.00 100.00 100.00 100.00 100 30-36
100.00 100.00 100.00 100.00 -- D10 2.46 4.37 2.41 -- D50 5.63 12.25
5.56 -- D90 10.98 19.76 10.43 -- D99 16.17 25.17 14.94 -- A: Before
classification B: Classified coarse powder C: Classified fine
powder D: Coarse powder + fined powder
[0049]
2TABLE 2 Central particle Frequency (%) diameter (.mu.m) A B C D
0.7 0.0 0.0 0.0 0.0 1 0.3 0.2 0.2 0.2 1.2 0.7 0.3 0.7 0.7 1.4 1.2
0.3 1.3 1.2 1.65 1.9 0.4 2.0 2.0 2 2.9 0.7 3.2 3.1 2.4 4.3 1.0 4.6
4.5 2.85 5.8 1.5 6.2 6.1 3.4 7.7 2.2 8.1 8.0 4 9.4 2.8 9.8 9.7 4.65
10.8 3.4 11.2 11.0 5.5 11.9 4.0 12.0 11.9 6.75 12.0 4.8 12.1 11.9
8.25 10.8 6.6 10.7 10.6 9.75 8.7 9.7 8.4 8.4 11.5 6.2 13.6 5.7 5.8
13.75 3.6 16.4 3.0 3.3 16.5 1.4 15.2 0.9 1.2 19.5 0.1 10.6 0.0 0.2
23 0.0 5.1 0.0 0.1 27.5 0.0 1.0 0.0 0.0 33 0.0 0.0 0.0 0.0 Total
100.0 100.0 100.0 100.0 A: Before classification B: Classified
coarse powder C: Classified fine powder D: Coarse powder + fined
powder
[0050] Table 1 shows mass cumulative frequency (%) of powder before
the classification (a positive electrode active material),
classified coarse powder, classified fine powder and coarse
powder+fine powder. According to the calculation formula of the
partial classification efficiency .eta.(d) of the expression (1),
mass cumulative particle sizes of raw material R.sub.0(di),
R.sub.0(di+1) are used, but when the partial classification
efficiency .eta.(d) is calculated based on the expression (1)
considering a partial loss of particles at the time of
classification or the like, R.sub.0(di) and R.sub.0(di+1) are
assumed to use the mass cumulative frequency (%) of coarse
powder+fine powder for convenience. Thus, the partial
classification efficiency of Table 1 was obtained. FIG. 1 is a
graph of each cumulative frequency (%) and partial classification
efficiency (%) of Table 1. Table 2 shows the particle size
distribution (each particle size frequency (%)) of powder before
the classification (positive electrode active material), classified
coarse powder, classified fine powder and coarse powder+fine
powder, and FIG. 2 is a particle size distribution graph based on
the particle size and frequency of each powder.
[0051] The classification precision index .kappa. of this
classification example was calculated according to the
above-described expression (2) to find that the classification
precision index .kappa. was 0.90, indicating that the coarse powder
was classified with high precision. It is also apparent from FIG. 1
and FIG. 2 that the classification precision is high. To calculate
the classification precision index .kappa., the relationship
between the partial classification efficiency (%) and the particle
size is graphed as shown in FIG. 3, and D.sub.p25 and D.sub.p75 are
determined from the graph. In this classification example,
D.sub.p25 is 17.9 .mu.m, and D.sub.p75 is 19.8 .mu.m. Therefore,
the classification precision index .kappa. becomes
D.sub.p25/D.sub.p75=17.9/19.8=0.90.
[0052] The particle size distribution of the powder before the
classification and the classified powder according to the invention
indicates the values measured by means of a MICROTRAC II
PARTICLE-SIZE ANALYZER manufactured by LEEDS & NORTHRUP using a
light scattering phenomenon which occurs when a laser beam is
exposed to the particles. And, the content of the impurity metal
element in each powder indicates the value measured by ICP analysis
method after dissolving a sample to be measured in hydrochloric
acid or the like.
[0053] The positive electrode active material for a secondary cell
of this embodiment is characterized in that, when the positive
electrode active material is classified under the above-described
conditions, a ratio (B/A) of the content B of an impurity metal
element in the coarse powder obtained by the classification to the
content A of an impurity metal element in the positive electrode
active material (the whole powdery composite metal oxide before the
classification) is 1.5 or below. The positive electrode active
material of the invention is not limited to its production method,
but when the ratio (B/A ratio) of the impurity content based on the
above-described analysis and evaluation method is 1.5 or below, the
structure of the invention is satisfied.
[0054] When the ratio (B/A ratio) of the impurity content based on
the above-described analysis and evaluation method is 1.5 or below,
it means that the content of the particulate metal impurity in the
positive electrode active material is reduced appropriately.
Therefore, the production of a non-aqueous electrolyte secondary
cell by using such a positive electrode active material makes it
possible to suppress the occurrence of a micro-short circuit
resulting from the deposition of impurity metal ions at the time of
initial charging. It is more desirable that the ratio (B/A ratio)
of the impurity content in the positive electrode active material
when the analysis and evaluation are performed under the
above-described conditions is 1.1 or below.
[0055] For the impurity metal element whose content is compared
between the positive electrode active material before the
classification and the coarse powder obtained by the
classification, a metal element which adversely affects on the
operation and characteristics of a non-aqueous electrolyte
secondary cell is selected. The particulate metal impurity
containing a metal element other than the metal element
constituting the positive electrode active material is eluted as
metal ions when the secondary cell is charged for the first time,
the eluted metal ions are reduced and deposited on the side of the
negative electrode, and the deposit passes through the separator to
come into contact with the positive electrode and causes a
micro-short circuit or the like.
[0056] Therefore, various kinds of metal elements are compared, but
it is especially preferable to compare metal elements which tend to
be impurity ions, and more specifically at least one kind of
element (excepting the metal elements constituting the positive
electrode active material) selected from Mg, Ca, Ba, Sr, Sc, Y, Ti,
Zr, Hf, V, Cr, Nb, Mo, Ta, W, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Tc,
Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Re, Os, Ir, Tl, Pb and Bi.
[0057] The above-described contents A, B of the impurity metal
elements are determined on each single element, and the B/A ratio
is calculated from the contents A, B of each single metal.
Specifically, the content A in the positive electrode active
material before the classification and the content B in the coarse
powder are determined on the subject impurity metal element, the
B/A ratio is calculated from the contents A, B, and when the B/A
ratio is 1.5 or below, the positive electrode active material for
secondary cell of the present invention is configured.
[0058] It is most desirable that the measurement and comparison of
the contents A, B of the impurity metal element are performed on
all the above-described metal elements and the B/A ratio of all the
metal elements satisfy 1.5 or below. But, the cell characteristics
and production yield can be improved when the above-described
conditions are met by at least the elements which are significantly
contained. Besides, simply when the elements such as powder from
the worn pulverizer, e.g. Fe, Cr, Cu, Zn, Mg and Ca, which may be
mingled in the production process satisfy the above-described
conditions, the cell characteristics and production yield can be
improved.
[0059] As described above, the particulate metal impurity becomes a
cause of a faulty voltage drop or the like at the time of initial
charging of the secondary cell, so that the positive electrode
active material having the content of the particulate metal
impurity adequately reduced is provided with reference to the fact
that the ratio (B/A ratio) of the impurity content is 1.5 or below
according to the invention. Such a positive electrode active
material can be used to produce the non-aqueous electrolyte
secondary cell to prevent the occurrence of a micro-short circuit
due to the deposition of the impurity metal ions at the time of
initial charging, so that the battery characteristics and
production yield can be improved. In other words, the non-aqueous
electrolyte secondary cell is substantially prevented from having
the occurrence of a defect in production or an initial failure, and
the cell performance can be improved.
[0060] The positive electrode active material for a secondary cell
having the above-described ratio (B/A ratio) of the impurity
content of 1.5 or below can be obtained by, for example, using a
raw material having the content of a coarse particulate metal
impurity in a low level, adopting a process which retards the
mixing of the particulate metal impurity in the production process
or removing the coarse impurity particles mingled in the final
process. As described above, however, the method of producing the
positive electrode active material according to the invention is
not limited to a particular one, but one having a ratio (B/A ratio)
of the impurity content of 1.5 or below based on the
above-described analysis and evaluation method can be used as the
positive electrode active material of the invention.
[0061] One example of the production method of the positive
electrode active material will be described. First, a composite
metal oxide containing lithium, such as LiCoO.sub.2, LiNiO.sub.2 or
LiMn.sub.2O.sub.4, is composed by a common calcination method. The
process of composing the Li-containing composite metal oxide uses a
lithium compound and a compound such as cobalt, nickel or manganese
as the raw materials, mixes them at a prescribed ratio, and
calcines at temperatures of 650 to 950.degree. C. in the atmosphere
for example. The compound used as the raw material includes oxide,
carbonate, sulfate, nitrate and hydroxide. They are desired to
contain little amount of metal impurity. Before the calcination,
refinement or the like may be performed in order to remove metal
impurities. Besides, it is also effective that cobalt oxide powder
and individual raw material powders such as lithium carbonate are
mixed at a prescribed ratio after the used raw material is dried at
a temperature of 100.degree. C. or higher for one hour or more.
Thus, the flowability of the raw material powder is improved, and
inclusion of the impurity particles due to abrasion of the mixing
device can be retarded.
[0062] The Li-containing composite metal oxide (calcined material)
obtained by the composing step is pulverized by a pulverizer into,
for example, an average particle diameter of 0.5 to 15 .mu.m, more
preferably 1 to 10 .mu.m. At this time, it is desirable to use a
material which is not worn heavily or a material which does not
cause a defect even if worn out for the particle contact portion of
the pulverizer so to prevent mixing of a particulate metal
impurity. Specifically, it is effective to coat the particle
contact portion with ceramics or a resin. It is not necessary to
coat all the parts with such a material, but it is effective by
applying the coating to only the portions where the particles are
contacted at a high speed and the portions where metals make
collision with each other.
[0063] To pulverize the calcined material, it is effective to use a
pulverizer such as a jet mill for pulverizing by mutually colliding
the particles. It is also effective to perform the classification
and the removal by dissolving with an acid or the like so to remove
coarse particles containing the particulate metal impurity after
the pulverization and screening for controlling the particles.
Then, the content of particulate metal impurity is examined
according to the above-described analysis and evaluation method.
And, the positive electrode active material for a secondary cell
having an impurity content ratio (B/A ratio) of 1.5 or below
according the analysis and evaluation method for the particulate
metal impurity is mixed with a conductive agent, and a binder and a
solvent are added to it to prepare a slurry. The slurry is applied
onto a collector (such as a metal foil) and dried by heating to
form a thin plate, which is then cut into a prescribed size to
obtain a positive electrode of a non-aqueous electrolyte secondary
cell.
[0064] As described above, the typical steps of producing the
positive electrode active material includes four steps, namely a
raw material mixing step, a mixture calcining step, a calcined
material pulverizing step and a pulverized material classifying
step. As one means for preventing an impurity from being mingled,
the particle contact portion of each production device is formed of
a nonmetal material. But, when all the particle contact portions
are made of nonmetal or coated with nonmetal, the cost of the
production device becomes high, resulting in the increase of the
production cost of the positive electrode active material in
industrial viewpoint. And, it is hard to thoroughly prevent the
metal impurities from mingling because some metal portions remain
exposed. Especially, a typical process of producing the positive
electrode active material uses a production device made of a metal
material such as stainless steel (SUS) and is in the environment
that a metal impurity such as Fe is easily mingled.
[0065] Basically, the positive electrode active material for a
secondary cell of the invention can be obtained by selecting one
having an impurity content ratio (B/A ratio) of 1.5 or below
according to the above-described analysis and evaluation method. A
preferable example of the production process to positively attain
the impurity content ratio (B/A ratio) of 1.5 or below is described
below. First, it is desirable to perform the above-described raw
material powder drying step. The drying temperature is advised to
be 100.degree. C. or more but, if the temperature is excessively
high, drying equipment is heavily loaded, so that the drying
temperature is preferably set to about 100 to 300.degree. C. The
drying time is advised to be one hour or more, and if it is
excessively long, the drying equipment is heavily loaded, so that
the drying time is desired to be about 1 to 10 hours.
[0066] Second, the particle contact portion of the production
device is coated with nonmetal. The nonmetal coating material to be
used is preferably a ceramic material such as glass, nitride, oxide
or carbide or a resin material such as urethane resin,
fluorine-based resin, epoxy resin or liquid crystal resin.
Especially, the resin coating is suitable.
[0067] The positive electrode active material has high hardness,
and there are steps such as a pulverizing step and a classifying
step where the positive electrode active material collides heavily.
Therefore, the coating material such as the ceramics material
having low elasticity and high hardness is heavily worn, and it is
highly possible that it is scraped off in a short time. When the
coating material is worn to reveal the metal parts, there is a
possibility that the impurity metal is mixed into the positive
electrode active material through the revealed portion, and the
scraped ceramics coating material is also mixed into it to cause an
adverse effect. Therefore, the elastic resin coating is preferable
to the ceramics coating for coating the particle contact portion.
Besides, the coating material is preferably heat resistant because
some production steps are conducted at a high temperature of
100.degree. C. or higher.
[0068] The above-described analysis method of the present invention
exerts the effects on analysis and evaluation of the positive
electrode active material for a secondary cell, but it is not
always limited to the positive electrode active material for a
secondary cell but can be applied for analysis and evaluation of
the particulate metal impurity contained in various types of
powdery materials (e.g., a positive electrode active material for a
secondary cell, a powdery electronic functional material containing
powder of fluorescent substance).
[0069] Then, embodiments of the non-aqueous electrolyte secondary
cell according to the invention will be described.
[0070] FIG. 4 is a partially sectional diagram showing the
structure of an embodiment applying the non-aqueous electrolyte
secondary cell of the invention to a lithium-ion secondary cell. In
the drawing, 1 is a cell casing (cell can) formed of stainless
steel for example. An insulator 2 is disposed on the bottom of the
cell casing 1. The cell casing 1 is formed to have a shape, for
example, a bottomed cylindrical shape or a bottomed square tubular
shape. The present invention can be applied to both of a
cylindrical secondary cell and a square secondary cell. The cell
casing 1 also serves as a negative electrode terminal. The cell
casing 1 houses an electrode group 3 as an electricity generating
element in it.
[0071] The electrode group 3 has a structure that a strip-form
portion having a positive electrode 4, a separator 5 and a negative
electrode 6 laminated in this order is wound to have, for example,
a spiral shape, so to have the negative electrode 6 on the
outermost portion. The electrode group 3 is not limited to the
spiral shape but may have multiple of the positive electrode 4, the
separator 5 and the negative electrode 6 laminated in this order.
The cell casing 1 accommodating the electrode group 3 in it is
filled with a non-aqueous electrolyte. Insulating paper 7 having an
opening formed in its center is disposed on the top of the
electrode group 3 in the cell casing 1. An insulating sealing plate
8 is disposed in an opening on the top of the cell casing 1. The
insulating sealing plate 8 is fluid-tightly fixed to the cell
casing 1 by inwardly caulking the vicinity of the top end of the
cell casing 1.
[0072] A positive electrode terminal 9 is fitted to the center of
the insulating sealing plate 8. One end of a positive electrode
lead 10 is connected to the positive electrode terminal 9 through a
safety valve 11. The other end of the positive electrode lead 10 is
connected to the positive electrode 4. The negative electrode 6 is
connected to the cell casing 1 as the negative electrode terminal
through an unshown negative electrode lead. Thus, a lithium-ion
secondary cell 12 is configured as a non-aqueous electrolyte
secondary cell.
[0073] Then, the positive electrode 4, the separator 5 and the
negative electrode 6, which configure the electrode group 3, and
the non-aqueous electrolyte will be described in further detail.
The positive electrode 4 is produced by suspending the positive
electrode active material for a secondary cell of the invention, a
conductive agent and a binder in an appropriate solvent, applying
the prepared suspended substance onto a collector and drying to
have a thin plate.
[0074] The conductive agent and the binder to be mixed with the
positive electrode active material can be various kinds of
materials conventionally used for the non-aqueous electrolyte
secondary cell. As the conductive agent, acetylene black, carbon
black, graphite or the like is used. As the binder,
polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF),
ethylene-propylene-diene copolymer (EPDM), styrene-butadiene rubber
(SBR) or the like is used. A mixing ratio of the positive electrode
active material, the conductive agent and the binder is preferably
80 to 95% by mass of the positive electrode active material, 3 to
20% by mass of the conductive agent and 2 to 7% by mass of the
binder. As the collector onto which the suspended substance
containing the positive electrode active material, the conductive
agent and the binder is applied, for example, an aluminum foil, a
stainless steel foil or a nickel foil is used.
[0075] Various types of materials and configurations which are
conventionally used for the non-aqueous electrolyte secondary cell
can be applied to the other cell components such as the separator
5, the negative electrode 6, and the non-aqueous electrolyte. For
example, a synthetic resin nonwoven fabric, a polyethylene porous
film, a polypropylene porous film or the like is used as the
separator 5. The negative electrode 6 is produced by suspending the
negative electrode active material and the binder into an
appropriate solvent, applying the obtained suspension onto a
collector and drying to form a thin plate.
[0076] As the negative electrode active material, pyrolytic
carbons, pitch/cokes, graphites, vitreous carbons, a calcined
material of an organic high polymer such as phenol resin or furan
resin, a carbon material such as carbon fiber or activated carbon,
a lithium alloy such as metallic lithium or Li--Al alloy, or a
polymer such as polyacetylene or polypyrrole which is capable of
occluding/releasing lithium ions is used. For the binder, one
similar to the positive electrode 5 is used. A mixing ratio of the
negative electrode active material and the binder is preferably 90
to 95% by mass of the negative electrode active material and 2 to
10% by mass of the binder. As the collector onto which a suspended
substance containing the negative electrode active material and the
binder is applied, for example, a foil, mesh, punched metal or lath
metal of copper, stainless steel or nickel is used.
[0077] Besides, the non-aqueous electrolyte is prepared by
dissolving an electrolyte into a non-aqueous solvent. As the
non-aqueous solvent, for example, various types of non-aqueous
solvents known as a solvent for a lithium-ion secondary cell can be
used. The non-aqueous solvent for the non-aqueous electrolyte is
not particularly limited, but, for example, a mixture solvent of
propylene carbonate or ethylene carbonate with dimethyl carbonate,
methylethyl carbonate, diethyl carbonate, .gamma.-butyrolactone,
1,2-dimethoxyethane, 1,2-diethoxy ethane or ethoxy methoxyethane is
used. As an electrolyte, lithium salt such as LiPF.sub.6,
LiBF.sub.4, LiClO.sub.4, LiAsF.sub.6, LiCF.sub.3SO.sub.3 is
exemplified. An amount of the electrolyte dissolved into the
non-aqueous solvent is preferably in a range of 0.5 to 1.5 mol/L
(liter).
[0078] According to a lithium-ion secondary cell 12 to which the
above-described invention is applied, an amount of the particulate
metal impurity in the positive electrode active material is
reduced, so that the occurrence of a micro-short circuit at the
initial charging can be suppressed effectively. Therefore, a
production yield of the lithium-ion secondary cell 12 can be
enhanced substantially. Besides, the lithium-ion secondary cell 12,
in which the particulate metal impurity content is reduced because
it becomes a factor of lowering the cell performance, exerts
remarkable cell performance.
[0079] Then, specific examples of the invention and the evaluated
results thereof will be described.
EXAMPLE 1
Comparative Example 1
[0080] First, cobalt oxide powder and lithium carbonate were mixed
at a prescribed ratio and calcined in the air at 900.degree. C. for
four hours. The calcined material was pulverized by an appropriate
pulverizer, and sieving was performed to remove massive coarse
particles and fine particles. Thus, LiCoO.sub.2 powder having an
average particle diameter (D50) of 1 to 20 .mu.m was obtained as
positive electrode active materials (samples 1 to 3). To produce
the positive electrode active material powders, a particulate metal
impurity amount was reduced by adjusting a material for a
pulverizer, a molding machine, a mixer, a classifier and the like
and the operation conditions of the device for producing them.
[0081] Then, for the analysis and evaluation of the amount of a
particulate metal impurity contained in the above-described
individual positive electrode active materials were classified
according to the above-described method and conditions. For
classification of each positive electrode active material, the
conditions were adjusted so that a classification ratio of coarse
powder became 2% and the classification precision index .kappa.
became about 0.9. The classification was performed by the
Turbo-classifier as described above.
[0082] Then, each particle size distribution of the powder before
the classification, the classified coarse powder and the classified
fine powder of each positive electrode active material was measured
as described below. First, 0.5 g of each sample was collected,
charged into 100 ml of water and stirred. Ultrasonic distribution
was performed under conditions of 100 W and 3 min, and a particle
size distribution was measured by a MICROTRAC II PARTICLE-SIZE
ANALYZER TYPE 7997-10 manufactured by LEEDS & NORTHRUP. The
classification precision index .kappa. of each positive electrode
active material was determined from the particle size distribution.
The classification precision index .kappa. of each sample is
specifically shown in Table 3. Specific classified results of
sample 1 of Example 1 are shown in Table 1, Table 2, FIG. 1, FIG. 2
and FIG. 3 as described above.
[0083] And, as to the positive electrode active materials (samples
1 to 3), the Fe content in the positive electrode active material
before the classification and the Fe content in the classified
coarse powder were measured as the contents A, B of the impurity
metal elements. The contents of the impurity metal elements were
measured according to the above-described method. A B/A ratio was
determined from the contents A, B of the impurity metal elements
and their values. The impurity contents A, B of the individual
positive electrode active materials and the B/A ratio are as shown
in Table 3.
[0084] Meanwhile, as Comparative Example 1 of the present
invention, positive electrode active materials (samples 4 to 5) of
LiCoO.sub.2 powder were produced in the same way as in Example 1
except that the pulverizing conditions and the sieving conditions
after the calcination were changed. They were analyzed and
evaluated for an amount of the particulate metal impurity in the
same way as in Example 1. The results are shown in Table 3.
[0085] Using the individual positive electrode active materials
according to Example 1 and Comparative Example 1 described above,
individual lithium-ion secondary cells were produced. To produce
the lithium-ion secondary cells, powder before the classification
(coarse powder+fine powder) was used as the positive electrode
active material. First, 90% by mass of the positive electrode
active material, 6% by mass of graphite as a conductive agent and
4% by mass of polyvinylidene fluoride as a binder were mixed to
prepare a positive electrode mixture. The positive electrode
mixture was dispersed into N-methyl-2-pyrrolidone to prepare a
slurry, which was then applied onto an aluminum foil and dried, and
the resultant was compression-molded by a roller pressing machine.
The resultant was cut into a prescribed size to obtain a sheet
positive electrode.
[0086] Meanwhile, 93% by mass of a carbon material and 7% by mass
of polyvinylidene fluoride as a binder were mixed to prepare a
negative electrode mixture. A sheet negative electrode was produced
in the same way as the positive electrode excepting that the
prepared negative electrode mixture was used. And, the sheet
positive electrode, a separator formed of a microporous
polyethylene film and the sheet negative electrode were laminated
in this order, and the obtained laminate was wound into a spiral
shape to have the negative electrode on the outside so to produce
an electrode group. A lead was fitted to the electrode group before
housing into a bottomed cylindrical container (cell can), into
which a non-aqueous electrolyte was charged to assemble a
cylindrical lithium-ion secondary cell. The non-aqueous electrolyte
was prepared by dissolving LiPF.sub.6 in a concentration of 1 mol/L
into a solvent of ethylene carbonate and methylethyl carbonate
mixed in 1:1.
[0087] Individual cylindrical lithium-ion secondary cells of
Example 1 and Comparative Example 1 produced as described above
were measured and evaluated for their characteristics as described
below. First, as the first charging of the assembled cells, a
constant voltage of 4.2 V was charged for eight hours in an
environment of 20.degree. C. with current limited to 0.6 A. The
charged secondary cells were stored at room temperature for ten
days, and their voltages were measured. The voltages after the
10-day standing are shown in Table 3.
3 TABLE 3 Analyzed and evaluated results of particulate metal
impurity Voltage Classifi- Fe content Fe content of 10 days Sam-
cation before classified after ple precision classification coarse
powder B/A charging No. index .kappa. A (%) B (%) ratio (V) E1 1
0.90 0.1 0.1 1.00 4.18 2 0.92 0.1 0.11 1.10 4.18 3 0.93 0.1 0.13
1.30 4.16 CE1 4 0.92 0.1 0.3 3.00 3.20 5 0.91 0.1 0.6 6.00 0.20 E1:
Example 1 CE1: Comparative Example 1
[0088] It is apparent from Table 3 that the individual positive
electrode active materials according to Example 1 and Comparative
Example 1 have substantially the same value of Fe content in the
powders before the classification. However, the lithium-ion
secondary cells produced from such positive electrode active
materials had a little drop of voltage after the 10-day standing in
Example 1 but a large drop of voltage in Comparative Example 1. It
is because each of the individual positive electrode active
materials of Example 1 has a B/A ratio of 1.5 or below and a small
content of the particulate metal impurity, but each of the
individual positive electrode active materials of Comparative
Example 1 has a B/A ratio of exceeding 1.5 and contains a large
amount of particulate metal impurity.
[0089] Thus, the application of the analysis and evaluation method
of the present invention allows to concentrate the particulate
metal impurity, which is hardly detected by an ordinary analysis
method, on the coarse powder side. As a result, the content of the
particulate metal impurity in the whole positive electrode active
material can be evaluated effectively by the B/A ratio determined
from the impurity content B in the classified coarse powder and the
impurity content A in the positive electrode active material before
the classification. And, by using the positive electrode active
material having a B/A ratio of 1.5 or below as an evaluated result,
a lithium-ion secondary cell, which prevents an initial failure,
can be obtained with a high reproducibility.
EXAMPLES 2-6
Comparative Examples 2-6
[0090] Positive electrode active materials (LiCoO.sub.2 powder)
were produced as samples of Examples 2 to 6 in the same way as in
Example 1 described above. At this time, the material for a
pulverizer, a molding machine, a mixer, a classifier and the like
and the operation conditions of the devices for producing them were
adjusted to reduce the amount of the particulate metal
impurity.
[0091] The amount of the particulate metal impurity contained in
the above-described individual positive electrode active material
was analyzed and evaluated in the same way as in Example 1.
Classification for analysis and evaluation was also conducted under
the same conditions as in Example 1. For the contents A, B of the
impurity metal elements in the powder before the classification and
the classified coarse powder, a Cu content in Example 2, a Zn
content in Example 3, a Cr content in Example 4, a Ca content in
Example 5 and an Mg content in Example 6 were measured. The
measured results of the individual Examples are shown in Tables 4
to 8.
[0092] Meanwhile, as Comparative Examples 2 to 6 of the present
invention, positive electrode active materials of LiCoO.sub.2
powder were produced in the same way as in Examples 2 to 6 except
that the pulverizing conditions and the sieving conditions after
the calcinations were changed. They were analyzed and evaluated on
an amount of the particulate metal impurity in the same way as in
Examples 2 to 6. The results are as shown in Table 4 to Table
8.
[0093] The individual positive electrode active materials of
Examples 2 to 6 and Comparative Examples 2 to 6 were used to
produce lithium-ion secondary cells in the same way as in Example
1. The individual lithium-ion secondary cells were charged under
the same conditions as in Example 1 and left standing for ten days
in the same way, and their voltages were measured. The measured
results are shown in Table 4 to Table 8
4 TABLE 4 Analyzed and evaluated results of particulate metal
impurity Voltage Classifi- Cu content Cu content of 10 days Sam-
cation before classified after ple precision classification coarse
powder B/A charging No. index .kappa. A (ppm) B (ppm) ratio (V) E2
6 0.91 25 24 0.96 4.18 7 0.89 24 25 1.04 4.16 8 0.90 25 33 1.32
4.13 CE2 9 0.91 23 38 1.65 2.20 10 0.91 25 40 1.60 1.00 E2: Example
2 CE2: Comparative Example 2
[0094]
5 TABLE 5 Analyzed and evaluated results of particulate metal
impurity Voltage Classifi- Zn content Zn content of 10 days Sam-
cation before classified after ple precision classification coarse
powder B/A charging No. index .kappa. A (ppm) B (ppm) ratio (V) E3
11 0.92 15 16 1.06 4.19 12 0.91 14 15 1.07 4.17 13 0.89 15 21 1.40
4.12 CE3 14 0.91 14 28 2.00 3.20 15 0.92 16 33 2.06 2.00 E3:
Example 3 CE3: Comparative Example 3
[0095]
6 TABLE 6 Analyzed and evaluated results of particulate metal
impurity Voltage Classifi- Cr content Cr content of 10 days Sam-
cation before classified after ple precision classification coarse
powder B/A charging No. index .kappa. A (ppm) B (ppm) ratio (V) E4
16 0.92 12 12 1.00 4.19 17 0.88 13 14 1.08 4.18 18 0.89 12 15 1.25
4.15 CE4 19 0.91 12 22 1.83 3.10 20 0.90 13 28 2.15 2.50 E4:
Example 4 CE4: Comparative Example 4
[0096]
7 TABLE 7 Analyzed and evaluated results of particulate metal
impurity Voltage Classifi- Ca content Ca content of 10 days Sam-
cation before classified after ple precision classification coarse
powder B/A charging No. index .kappa. A (ppm) B (ppm) ratio (V) E5
21 0.91 210 220 1.04 4.18 22 0.89 220 220 1.00 4.17 23 0.92 220 240
1.09 4.15 CE5 24 0.91 200 410 2.05 2.80 25 0.92 210 580 2.76 1.00
E5: Example 5 CE5: Comparative Example 5
[0097]
8 TABLE 8 Analyzed and evaluated results of particulate metal
impurity Voltage Classifi- Mg content Mg content of 10 days Sam-
cation before classified after ple precision classification coarse
powder B/A charging No. index .kappa. A (ppm) B (ppm) ratio (V) E6
26 0.92 8 8 1.00 4.19 27 0.90 7 8 1.14 4.17 28 0.88 10 9 0.90 4.14
CE6 29 0.87 9 18 2.00 2.50 30 0.91 7 37 5.29 0.80 E6: Example 6
CE6: Comparative Example 6
[0098] It is apparent from Table 4 to Table 8 that the lithium-ion
secondary cell prepared from each positive electrode active
material with the B/A ratio of 1.5 or below according to Examples 2
to 6 had a little voltage drop after the 10-day standing. Thus,
even when the positive electrode active materials having Cu, Zn,
Cr, Ca or. Mg with the B/A ratio of 1.5 or below as an impurity
metal element were used, a lithium-ion secondary cell, which
prevents an initial failure, can be obtained with a high
reproducibility.
[0099] Fe, Cu, Zn, Cr, Ca and Mg were used as the impurity metal
elements in Examples 1 to 6 described above. Such elements were
used as the impurity metal elements which were especially easily
mingled because the device for producing the positive electrode
active material was often made of an iron alloy such as stainless
steel as described above. The same effects can be obtained by using
other metal elements when the ratio (B/A ratio) of the content of
the impurity metal element is adjusted to 1.5 or below.
EXAMPLE 7,
Comparative Example 7
[0100] In the same way as in Example 1 and Comparative Example 1
described above, the positive electrode active materials
(LiCoO.sub.2 powder) were produced as the samples of Example 7 and
Comparative Example 7. The amount of the particulate metal impurity
contained in each positive electrode active material was analyzed
and evaluated in the same way as in Example 1. Classification was
adjusted so to have the classification precision index .kappa. of
about 0.8. The contents A, B of the impurity metal elements in the
powder before the classification and the classified coarse powder
were measured for an Fe content in the same way as in Example 1.
The measured results in Example 7 and Comparative Example 7 are
shown in Table 9.
[0101] The respective positive electrode active materials of
Example 7 and Comparative Example 7 were used to produce individual
lithium-ion secondary cells in the same way as in Example 1. The
individual lithium-ion secondary cells were charged under the same
conditions as in Example 1, and their voltages were measured after
having left standing for ten days. The measured results are shown
in Table 9.
9 TABLE 9 Analyzed and evaluated results of particulate metal
impurity Voltage Classifi- Fe content Fe content of 10 days Sam-
cation before classified after ple precision classification coarse
powder B/A charging No. index .kappa. A (%) B (%) ratio (V) E7 31
0.79 0.1 0.10 1.00 4.17 32 0.81 0.1 0.11 1.10 4.19 33 0.82 0.1 0.10
1.00 4.18 CE7 34 0.80 0.1 0.2 2.00 2.80 35 0.81 0.1 0.3 3.00 0.50
E7: Example 7 CE7: Comparative Example 7
[0102] It is apparent from Table 9 that the lithium-ion secondary
cell can be prevented from an initial failure by classifying to
have the classification precision index .kappa. of 0.7 or more and
using the positive electrode active material having the B/A ratio
of 1.5 or below determined according to the results. It is seen
that the Fe content in the classified coarse powder is slightly low
because the classification precision index .kappa. in Example 7 and
Comparative Example 7 is set to be somewhat lower than in Example
1. It is because the concentrated level of the particulate metal
impurity into the classified coarse powder was lowered to some
extent. It is seen that an initial failure of the lithium-ion
secondary cell can be prevented securely by using the positive
electrode active material having the B/A ratio of 1.5 or below
based on the classification with the classification precision index
.kappa. of 0.7 or more.
EXAMPLE 8,
Comparative Example 8
[0103] Nickel hydroxide powder and lithium hydroxide powder were
mixed in a prescribed ratio and calcined in the air at 700.degree.
C. for six hours. This calcined material was pulverized by an
appropriate pulverizer, and sieving was performed to remove massive
coarse particles and fine particles. Thus, a positive electrode
active material of LiNiO.sub.2 powder was obtained. To produce the
positive electrode active material, the material for a pulverizer,
a molding machine, a mixer, a classifier and the like and the
operation conditions of the devices for producing them were
adjusted to reduce the amount of the particulate metal
impurity.
[0104] The amount of the particulate metal impurity contained in
the positive electrode active material described above was analyzed
and evaluated in the same way as in Example 1. Classification for
analysis and evaluation was also performed under the same
conditions as in Example 1. The contents A, B of the impurity metal
elements in the powder before the classification and the classified
coarse powder were determined on Fe in the same way as in Example
1. The results are shown in Table 10.
[0105] Meanwhile, as Comparative Example 8 of the present
invention, a positive electrode active material of LiNiO.sub.2
powder was produced in the same way as in Example 8 except that the
pulverizing conditions and the sieving conditions after the
calcination were changed. This positive electrode active material
was also analyzed and evaluated on the amount of the particulate
metal impurity in the same way as in Example 8. The results are
shown in Table 10.
[0106] The individual positive electrode active materials
(LiNiO.sub.2 powder) according to Example 8 and Comparative Example
8 were used to produce lithium-ion secondary cells in the same way
as in Example 1. The individual lithium-ion secondary cells were
charged in the same way as in Example 1 and their voltages after
ten-day standing were measured. The measured results are shown in
Table 10.
10 TABLE 10 Analyzed and evaluated results of particulate metal
impurity Voltage Classifi- Fe content Fe content of 10 days Sam-
cation before classified after ple precision classification coarse
powder B/A charging No. index .kappa. A (%) B (%) ratio (V) E8 36
0.91 0.1 0.11 1.10 4.19 CE8 37 0.91 0.1 0.30 3.00 2.85 E8: Example
8 CE8: Comparative Example 8
[0107] It is apparent from Table 10 that even when the positive
electrode active material of LiNiO.sub.2 is used, an initial
failure of the lithium-ion secondary cell can be retarded when the
LiNiO.sub.2 powder has a B/A ratio of 1.5 or below.
EXAMPLE 9,
Comparative Example 9
[0108] Manganese dioxide powder and lithium carbonate were mixed in
a prescribed ratio and calcined in the air at 800.degree. C. for
six hours. The calcined material was pulverized by an appropriate
pulverizer, and sieving was performed to remove massive coarse
particles and fine particles. Thus, the positive electrode active
material of LiMn.sub.2O.sub.4 powder was obtained. To produce the
positive electrode active material, the material for a pulverizer,
a molding machine, a mixer, a classifier and the like and the
operation conditions of the devices for producing them were
adjusted to reduce the amount of the particulate metal
impurity.
[0109] The amount of the particulate metal impurity contained in
the positive electrode active material described above was analyzed
and evaluated in the same way as in Example 1. Classification for
analysis and evaluation was also performed under the same
conditions as in Example 1. The contents A, B of the impurity metal
elements in the powder before the classification and the classified
coarse powder were determined on Fe in the same way as in Example
1. The results are shown in Table 11.
[0110] Meanwhile, as Comparative Example 9 of the present
invention, a positive electrode active material of
LiMn.sub.2O.sub.4 powder was produced in the same way as in Example
9 except that the pulverizing conditions and the sieving conditions
after the calcination were changed. This positive electrode active
material was also analyzed and evaluated on the amount of the
particulate metal impurity in the same way as in Example 9. The
results are shown in Table 11.
[0111] The individual positive electrode active materials
(LiMn.sub.2O.sub.4 powder) according to Example 9 and Comparative
Example 9 were used to produce lithium-ion secondary cells in the
same way as in Example 1. The individual lithium-ion secondary
cells were charged in the same way as in Example 1 and their
voltages after ten-day standing were measured. The measured results
are shown in Table 11.
11 TABLE 11 Analyzed and evaluated results of particulate metal
impurity Voltage Classifi- Fe content Fe content of 10 days Sam-
cation before classified after ple precision classification coarse
powder B/A charging No. index .kappa. A (%) B (%) ratio (V) E9 38
0.92 0.1 0.11 1.10 4.18 CE9 39 0.92 0.1 0.40 4.00 1.92 E9: Example
9 CE9: Comparative Example 9
[0112] It is apparent from Table 11 that even when the positive
electrode active material of LiMn.sub.2O.sub.4 is used, an initial
failure of the lithium-ion secondary cell can be retarded when the
LiMn.sub.2O.sub.4 powder has a B/A ratio of 1.5 or below.
[0113] In the above-described Examples, the lithium-ion secondary
cells were produced by using the positive electrode active material
before the classification, but the same effects can be obtained
when the lithium-ion secondary cell is produced by using the fine
powder obtained by the classification (removing the classified
coarse powder from the positive electrode active material before
the classification).
Industrial Applicability
[0114] It is apparent from the above-described embodiments that the
present invention has established the analysis and evaluation
method on factors (such as a particulate metal impurity) of
degrading the cell performance and production yield. And, the
amount of the particulate metal impurity in the positive electrode
active material is evaluated according to the analysis and
evaluation method. Therefore, the production yield and cell
performance of the non-aqueous electrolyte secondary cell can be
improved with a high reproducibility by using the positive
electrode active material for a secondary cell satisfying the
conditions of the invention.
* * * * *