U.S. patent application number 15/550874 was filed with the patent office on 2018-01-25 for positive electrode active substance for non-aqueous electrolyte secondary batteries, and non-aqueous electrolyte secondary battery.
The applicant listed for this patent is TODA KOGYO CORP.. Invention is credited to Shoichi FUJINO, Akihisa KAJIYAMA, Tetsuya KASHIMA, Ryuta MASAKI, Kazutoshi MATSUMOTO, Osamu SASAKI, Tsuyoshi WAKIYAMA.
Application Number | 20180026265 15/550874 |
Document ID | / |
Family ID | 56688902 |
Filed Date | 2018-01-25 |
United States Patent
Application |
20180026265 |
Kind Code |
A1 |
KAJIYAMA; Akihisa ; et
al. |
January 25, 2018 |
POSITIVE ELECTRODE ACTIVE SUBSTANCE FOR NON-AQUEOUS ELECTROLYTE
SECONDARY BATTERIES, AND NON-AQUEOUS ELECTROLYTE SECONDARY
BATTERY
Abstract
In accordance with the present invention, there are provided
positive electrode active substance particles for non-aqueous
electrolyte secondary batteries which is excellent in life
characteristics of a battery with respect to a repeated charging
and discharging performance thereof, as well as a non-aqueous
electrolyte secondary battery. The present invention relates to a
positive electrode active substance for non-aqueous electrolyte
secondary batteries comprising lithium transition metal layered
oxide having a composition represented by the formula:
Li.sub.a(Ni.sub.xCo.sub.yMn.sub.1-x-y)O.sub.2 wherein a is
1.0.ltoreq.a.ltoreq.1.15; x is 0<x<1; and y is 0<y<1,
in which the positive electrode active substance is in the form of
secondary particles formed by aggregating primary particles
thereof, and a coefficient of variation of a compositional ratio:
Li/Me wherein Me is a sum of Ni, Co and Mn (Me=Ni+Co+Mn) as
measured on a section of the secondary particle is not more than
25%.
Inventors: |
KAJIYAMA; Akihisa; (Sanyo
Onoda-shi, JP) ; MASAKI; Ryuta; (Sanyo Onoda-shi,
JP) ; WAKIYAMA; Tsuyoshi; (Sanyo Onoda-shi, JP)
; KASHIMA; Tetsuya; (Sanyo Onoda-shi, JP) ;
FUJINO; Shoichi; (Sanyo Onoda-shi, JP) ; SASAKI;
Osamu; (Sanyo Onoda-shi, JP) ; MATSUMOTO;
Kazutoshi; (Sanyo Onoda-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TODA KOGYO CORP. |
Hiroshima-shi, Hiroshima-ken |
|
JP |
|
|
Family ID: |
56688902 |
Appl. No.: |
15/550874 |
Filed: |
February 15, 2016 |
PCT Filed: |
February 15, 2016 |
PCT NO: |
PCT/JP2016/054278 |
371 Date: |
August 14, 2017 |
Current U.S.
Class: |
429/223 |
Current CPC
Class: |
Y02E 60/10 20130101;
H01M 4/0471 20130101; H01M 4/525 20130101; H01M 10/0525 20130101;
C01P 2004/61 20130101; H01M 2004/028 20130101; C01G 53/006
20130101; H01M 4/505 20130101; C01G 53/50 20130101 |
International
Class: |
H01M 4/505 20060101
H01M004/505; H01M 4/04 20060101 H01M004/04; H01M 10/0525 20060101
H01M010/0525; H01M 4/525 20060101 H01M004/525 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 17, 2015 |
JP |
2015-029055 |
Claims
1. A positive electrode active substance for non-aqueous
electrolyte secondary batteries comprising lithium transition metal
layered oxide having a composition represented by the formula:
Li.sub.a(Ni.sub.xCo.sub.yMn.sub.1-x-y)O.sub.2 wherein a is
1.0.ltoreq.a.ltoreq.1.15; x is 0<x<1; and y is 0<y<1,
in which the positive electrode active substance is in the form of
secondary particles formed by aggregating primary particles
thereof, and a coefficient of variation of a compositional ratio:
Li/Me wherein Me is a sum of Ni, Co and Mn (Me=Ni+Co+Mn) as
measured on a section of the secondary particle is not more than
25%.
2. The positive electrode active substance for non-aqueous
electrolyte secondary batteries according to claim 1, wherein an
average secondary particle diameter of the active substance is 3.0
to 16 .mu.m.
3. The positive electrode active substance for non-aqueous
electrolyte secondary batteries according to claim 1, wherein an
average particle diameter (crystallite size) of primary particles
of the active substance is 100 to 600 nm.
4. A non-aqueous electrolyte secondary battery using the positive
electrode active substance for non-aqueous electrolyte secondary
batteries as defined in claim 1.
5. A process for producing the positive electrode active substance
for non-aqueous electrolyte secondary batteries as defined in claim
1, comprising the steps of: obtaining spherical
nickel-cobalt-manganese-based composite compound particles as a raw
material; mixing the composite compound particles with lithium
hydroxide such that a molar ratio of Li to a sum of Ni, Co and Mn
(Li/(Ni+Co+Mn)) is in the range of 1.00 to 1.20 to obtain a mixture
thereof; calcining the thus obtained mixture at a temperature of
600 to 900.degree. C. in an oxygen-containing atmosphere; and
subjecting the calcined product to annealing treatment at a
temperature of 500 to 750.degree. C. which is lower than the
calcination temperature, without subjecting the calcined product to
water-washing treatment.
Description
TECHNICAL FIELD
[0001] The present invention relates to a positive electrode
(cathode) active substance for non-aqueous electrolyte secondary
batteries, and more particularly, to a positive electrode active
substance that is capable of conducting stable charging and
discharging operations without significant deterioration in
characteristics thereof even when subjected to repeated charging
and discharging cycles.
BACKGROUND ART
[0002] With the recent rapid development of portable and cordless
electronic devices such as audio-visual (AV) devices and personal
computers, there is an increasing demand for secondary batteries
having a small size, a light weight and a high energy density as a
power source for driving these electronic devices. Also, in
consideration of global environments, electric cars and hybrid cars
have been recently developed and put into practice, so that there
is an increasing demand for lithium ion secondary batteries for
large size applications which exhibit excellent durability. Under
these circumstances, the lithium ion secondary batteries that are
excellent in service life when subjected to repeated charging and
discharging cycles as well as high output characteristics have been
noticed.
[0003] As the method of meeting the aforementioned needs, there has
been usually used the method of controlling an interface reaction
between an electrode active substance and an electrolyte solution
in association with insertion and desorption of lithium ions upon
charging and discharging operations. An example of the method is
the method of subjecting the active substance to various surface
treatments, and the advantageous effects of the surface treatments
have also been validated.
[0004] In addition, for the purpose of improving output
characteristics and durability of the active substance, the method
of atomizing crystallites of the active substance and designing a
particle form of the active substance in the form of secondary
particles constituted of an aggregate of the crystallites as a
behaving unit thereof has become predominant and actually exhibited
good effects. However, the active substance that acts in the form
of the secondary particles as a behaving unit thereof tends to
still have peculiar problems to be improved such as degradation of
the aggregated form during charging and discharging cycles, i.e.,
occurrence of cracks in the behaving particles around a grain
boundary thereof. The occurrence of cracks in the particles tends
to induce reduction in conductive path or deterioration in
electrode density, and further induce rapid deterioration in
battery characteristics. Therefore, in order to further improve
performance of the battery, it is necessary to overcome such a
problem that characteristics of the active substance are gradually
deteriorated owing to the separation along a crystal interface
thereof, etc.
[0005] As an example of the conventional particles acting in the
form of secondary particles as a behaving unit in which attention
is paid to the control of a composition of the crystal grain
boundary formed inside the behaving unit of the aggregate-based
active substance, there has been present such a report that a
coating film is formed even on a crystal interface inside the
aggregated particles.
[0006] For example, as a positive electrode active substance formed
of a Ni-containing layered oxide, there are mentioned those active
substances in which Ti is allowed to be present along a grain
boundary thereof (Patent Literature 1), those active substances in
which Nb is allowed to be present along a grain boundary thereof
(Patent Literature 2), those active substances in which at least
one element selected from the group consisting of Ti, Zr, Hf, Si,
Ge and Sn is allowed to be present along a grain boundary thereof
(Patent Literature 3) and the like.
[0007] As a result of the present inventors' study on designing of
compositions of these grain boundaries, it has been found that only
by allowing the different kinds of compounds to be present along
the grain boundary, it is difficult to sufficiently improve
properties of the active substances, and deposition of an Li
component as a raw material of the active substance on the grain
boundary rather tends to occur so that a service life of the
resulting battery is shortened. Meanwhile, the deposition of the Li
component is caused by local segregation of Li due to addition of a
surplus amount of Li or poor mixing of the raw materials upon
synthesis of the active substance, or thermal decomposition of the
active substance owing to reduction of Ni during calcination
thereof.
[0008] In the present invention, special attention has been paid to
the aforementioned composition of the grain boundary, in
particular, the surplus Li component therein, and the present
invention aims at inhibiting formation and growth of resistive
components in the grain boundary which are formed due to the
surplus Li component, as well as obtaining a battery having high
output characteristics and prolonged service life.
CITATION LIST
Patent Literature
[0009] Patent Literature 1: Japanese Patent Application Laid-open
(KOKAI) No. 2012-28163
[0010] Patent Literature 2: Japanese Patent Application Laid-open
(KOKAI) No. 2002-151071
[0011] Patent Literature 3: Japanese Patent Application Laid-open
(KOKAI) No. 2007-317576
SUMMARY OF INVENTION
Technical Problem
[0012] The present invention provides a positive electrode active
substance used in non-aqueous electrolyte secondary batteries, and
more specifically, a material capable of meeting continuously
increasing requirements for a quality thereof, in particular, a
material capable of improving life characteristics of the battery
with respect to repeated charging and discharging performance.
[0013] That is, by utilizing only technologies of the
aforementioned Patent Literatures 1 to 3, it may be difficult to
obtain an electrode that is capable of conducting stable charging
and discharging operations without significant deterioration in
characteristics thereof when subjected to repeated charging and
discharging cycles.
[0014] In addition, the Patent Literatures 1 to 3 fail to
specifically describe the variation of a concentration of Li in the
grain boundary and crystals.
[0015] In the present invention, special attention has been paid to
the composition of the grain boundary, in particular, the surplus
Li component therein, and the present invention aims at inhibiting
formation and growth of resistive components in the grain boundary
which are formed due to the surplus Li component, as well as
obtaining a battery having high output characteristics and
prolonged service life. Thus, the object or technical task of the
present invention is to provide a positive electrode active
substance that is capable of conducting stable charging and
discharging operations without significant deterioration in
characteristics thereof even when subjected to repeated charging
and discharging cycles.
Solution to Problem
[0016] That is, according to the present invention, there is
provided a positive electrode active substance for non-aqueous
electrolyte secondary batteries comprising lithium transition metal
layered oxide having a composition represented by the formula:
Li.sub.a(Ni.sub.xCo.sub.yMn.sub.1-x-y)O.sub.2
wherein a is not less than 1.0 and not more than 1.15
(1.0.ltoreq.a.ltoreq.1.15); x is more than 0 and less than 1
(0<x<1); and y is more than 0 and less than 1
(0<y<1),
[0017] in which the positive electrode active substance is in the
form of secondary particles formed by aggregating primary particles
thereof, and a coefficient of variation of a compositional ratio:
Li/Me wherein Me is a sum of Ni, Co and Mn (Me=Ni+Co+Mn) as
measured on a section of the secondary particle is not more than
25% (Invention 1).
[0018] Also, according to the present invention, there is provided
the positive electrode active substance for non-aqueous electrolyte
secondary batteries as defined in the above Invention 1, wherein an
average secondary particle diameter of the active substance is 3.0
to 16 .mu.m (Invention 2).
[0019] Also, according to the present invention, there is provided
the positive electrode active substance for non-aqueous electrolyte
secondary batteries as defined in the above Invention 1 or 2,
wherein an average particle diameter (crystallite size) of primary
particles of the active substance is 100 to 600 nm (Invention
3).
[0020] In addition, according to the present invention, there is
provided a non-aqueous electrolyte secondary battery using the
positive electrode active substance for non-aqueous electrolyte
secondary batteries as defined in any one of the above Inventions 1
to 3 (Invention 4).
[0021] Furthermore, according to the present invention, there is
provided a process for producing the positive electrode active
substance for non-aqueous electrolyte secondary batteries as
defined in any one of the above Inventions 1 to 3, comprising the
steps of:
[0022] obtaining spherical nickel-cobalt-manganese-based composite
compound particles as a raw material;
[0023] mixing the composite compound particles with lithium
hydroxide such that a molar ratio of Li to a sum of Ni, Co and Mn
(Li/(Ni+Co+Mn)) is in the range of 1.00 to 1.20 to obtain a mixture
thereof;
[0024] calcining the thus obtained mixture at a temperature of 600
to 900.degree. C. in an oxygen-containing atmosphere; and
[0025] subjecting the calcined product to annealing treatment at a
temperature of 500 to 750.degree. C. which is lower than the
calcination temperature, without subjecting the calcined product to
water-washing treatment (Invention 5).
Advantageous Effects of Invention
[0026] The positive electrode active substance according to the
present invention is capable of conducting stable charging and
discharging operations without significant deterioration in
characteristics thereof even when subjecting a non-aqueous
electrolyte secondary battery using the positive electrode active
substance to repeated charging and discharging cycles, and
therefore can be suitably used as a positive electrode active
substance for non-aqueous electrolyte secondary batteries.
BRIEF DESCRIPTION OF THE DRAWING
[0027] FIG. 1 is a conceptual view of measurement of a
compositional ratio on a section of a secondary particle.
DESCRIPTION OF EMBODIMENTS
[0028] The construction of the present invention is described in
more detail below.
[0029] The positive electrode active substance according to the
present invention has a coefficient of variation of a ratio of a
concentration of Li to a transition metal as a main bulk component
of not more than 25%, and is in the form of a layered oxide
represented by the chemical formula:
Li.sub.a(Ni.sub.xCo.sub.yMn.sub.1-x-y)O.sub.2
wherein a is not less than 1.0 and not more than 1.15
(1.0.ltoreq.a.ltoreq.1.15); x is more than 0 and less than 1
(0<x<1); and y is more than 0 and less than 1
(0<y<1).
[0030] The layered oxide having such a crystal structure has a very
small Li solid solution range unlike an all proportional solid
solution such as, for example, LiMn.sub.2O.sub.4 spinel oxides. For
this reason, the ratio of Li to the transition element (Me) (Li/Me)
in the crystals immediately after synthesized is not largely
deviated from 1.0. On the other hand, in the case where a portion
having a low transition metal concentration is present inside of
respective aggregated behaving particles, it is meant that a grain
boundary of the crystals is present in the portion. In the present
invention, it has been found that the variation of Li/Me is
increased by reduction in concentration of Me in the grain boundary
portion and deposition of Li therein, and the object of the present
invention is to control the variation of Li/Me to a predetermined
range. When the coefficient of variation of Li/Me in the present
invention is controlled to not more than 25%, it is shown that the
variation of Li/Me is reduced and the deviation of the local
composition is suppressed, so that the aggregated particles as a
whole exhibit an average composition.
[0031] In the preferred composition having the chemical formula of
Li.sub.a(Ni.sub.xCo.sub.yMn.sub.1-x-y)O.sub.2, a (Li/Me) is in the
range of 1.0 to 1.15; it is more preferred that a is in the range
of 1.02 to 1.12, x is in the range of 0.1 to 0.8, y is in the range
of 0.1 to 0.4; and it is even more preferred that the abundance
ratios of Ni, Co and Mn are identical to each other (i.e., x=1/3,
y=1/3), or x is 0.5 (x=0.5) and y is 0.2 (y=0.2).
[0032] In addition, the positive electrode active substance
according to the present invention may comprise different kinds of
elements such as F, Mg, Al, P, Ca, Ti, Y, Sn, Bi, Ce and the
like.
[0033] The lithium transition metal oxide constituting the positive
electrode active substance according to the present invention has a
coefficient of variation of Li/Me of not more than 25%, so that it
is possible to reduce an initial resistance inside the secondary
particles and prevent formation of a resistive component therein
during the charging and discharging cycles, whereby occurrence of
cracks in the aggregated form during repeated charging and
discharging cycles as well as deterioration in the battery
performance in association therewith can be prevented. The
coefficient of variation of Li/Me in the positive electrode active
substance is more preferably not more than 20% and even more
preferably not more than 18%. The lower limit of the coefficient of
variation of Li/Me is zero except for the case where the ratio
Li/Me in the grain boundary is lower than that inside the
crystals.
[0034] The average secondary particle diameter of the positive
electrode active substance according to the present invention is
preferably 3.0 to 16 .mu.m. When the upper limit of the average
secondary particle diameter of the positive electrode active
substance is more than 16 .mu.m, diffusion of Li with the charging
and discharging cycles tends to be disturbed, so that input and
output powers of the battery tend to be deteriorated. The lower
limit of the average secondary particle diameter of the positive
electrode active substance according to the present invention is
preferably 3.0 .mu.m. When the average secondary particle diameter
of the positive electrode active substance is less than 3.0 .mu.m,
the interface between the active substance and the electrolyte
solution tends to be increased so that undesirable side reactions
tend to be caused. The average secondary particle diameter of the
positive electrode active substance according to the present
invention is more preferably 4.0 to 14 .mu.m.
[0035] The average particle diameter (crystallite size) of primary
particles of the positive electrode active substance according to
the present invention is preferably 100 to 600 nm. When the average
primary particle diameter of the positive electrode active
substance is more than 600 nm, the secondary particles of the
positive electrode active substance tend to be deteriorated in
mechanical aggregation strength and thereby tend to suffer from
occurrence of cracks in the aggregate. When the lower limit of the
average primary particle diameter of the positive electrode active
substance is less than 100 nm, the area of the grain boundary
inside the secondary aggregated structure tends to be increased, so
that the deterioration in battery performance owing to side
reactions tend to become predominant. The average primary particle
diameter (crystallite size) of the positive electrode active
substance according to the present invention is more preferably 150
to 500 nm.
[0036] Next, the process for producing the positive electrode
active substance according to the present invention is
described.
[0037] The process for producing the positive electrode active
substance according to the present invention is not particularly
limited. For example, in the production process of the present
invention, first, a mixed sulfuric acid aqueous solution comprising
cobalt, nickel and manganese is continuously fed to an aqueous
solution whose pH value is adjusted to an optimum value to thereby
obtain spherical nickel-cobalt-manganese-based composite compound
particles as a raw material. The nickel-cobalt-manganese-based
composite compound particles are preferably in the form of a
composite hydroxide. Next, the composite compound particles are
mixed with lithium hydroxide to obtain a mixture thereof in which a
molar ratio of Li to a sum of Ni, Co and Mn (Li/(Ni+Co+Mn)) is in a
predetermined range. The thus obtained mixture is calcined at a
temperature of 600 to 900.degree. C. in an oxygen-containing
atmosphere to produce the positive electrode active substance.
Meanwhile, after calcining the mixture, the resulting calcined
product is preferably subjected to annealing treatment at a
temperature of 500 to 750.degree. C. either while cooling the
calcined product or after once cooling the calcined product.
[0038] The nickel-cobalt-manganese-based composite compound
particles have an average particle diameter (crystallite size) of
their primary particles of 100 to 600 .mu.m, an average secondary
particle diameter of 3 to 20 .mu.m and a BET specific surface area
of 0.2 to 1.0 m.sup.2/g.
[0039] The molar ratio Li/Me in the aforementioned mixture is
preferably 1.00 to 1.20. When the molar ratio Li/Me is less than
1.00, Li tends to be included in an Ni site of the crystal
structure, so that the obtained calcined product tends to fail to
have a single crystal phase and therefore tends to fail to satisfy
a coefficient of variation of Li/Me of not more than 25% in some
cases, whereby there tends to occur deterioration in performance of
the resulting battery. When the molar ratio Li/Me is more than
1.20, a surplus amount of Li exceeding an amount of Li in a
stoichiometric composition of the resulting calcined product tends
to form a resistive component therein to thereby cause
deterioration in performance of the resulting battery. The molar
ratio Li/Me in the aforementioned mixture is more preferably 1.02
to 1.12, and even more preferably 1.05 to 1.08.
[0040] The atmosphere used upon calcining the mixture is an
oxygen-containing atmosphere. The oxygen content of the
oxygen-containing atmosphere is preferably not less than 20% by
volume. When the oxygen content of the oxygen-containing atmosphere
is less than the aforementioned range, Li ions tend to be included
in a transition metal site of the crystal structure in the calcined
product, so that the resulting battery tends to be deteriorated in
performance thereof. The upper limit of the oxygen content of the
oxygen-containing atmosphere is not particularly limited.
[0041] The temperature used upon calcining the mixture is
preferably 600 to 900.degree. C. When the calcination temperature
is lower than 600.degree. C., the resulting calcined product tends
to fail to have a crystal structure having the aimed thermal
equilibrium conditions and therefore tends to fail to form a single
crystal phase owing to shortage of a diffusion energy of elements
therein. For this reason, in the aforementioned condition, the
resulting positive electrode active substance tends to fail to
satisfy a coefficient of variation of Li/Me of not more than 25% in
some cases. On the other hand, when the calcination temperature is
higher than 900.degree. C., the resulting calcined product tends to
suffer from oxygen deficiency in the crystals thereof owing to
reduction of the transition metal therein, so that it is not
possible to form a single crystal phase having the aimed crystal
structure. Therefore, in the aforementioned condition, the
resulting positive electrode active substance tends to fail to
satisfy a coefficient of variation of Li/Me of not more than 25% in
some cases.
[0042] In the case where the calcined product is subjected to
annealing treatment, the temperature used in the annealing
treatment is preferably in the range of 500 to 750.degree. C., and
the atmosphere used therein is preferably an oxygen-containing
atmosphere. When the annealing temperature is lower than
500.degree. C., surplus lithium present in the grain boundary tends
to be hardly diffused into the crystals owing to shortage of a
diffusion energy of elements therein, so that it is not possible to
achieve the aimed object of reducing the variation of the
composition. Therefore, in the aforementioned condition, the
resulting positive electrode active substance tends to fail to
satisfy a coefficient of variation of Li/Me of not more than 25% in
some cases. When the annealing temperature is higher than
750.degree. C., the oxygen tends to be insufficient in activity
thereof, and a transition metal oxide having a rock salt-type
structure as an impurity phase tends to be produced. For this
reason, in the aforementioned condition, the resulting positive
electrode active substance tends to fail to satisfy a coefficient
of variation of Li/Me of not more than 25% in some cases. The
annealing temperature is more preferably 550 to 730.degree. C., and
even more preferably 580 to 700.degree. C.
[0043] Meanwhile, the annealing temperature is preferably lower
than the calcination temperature, and more preferably lower by
30.degree. C. or more than the calcination temperature.
[0044] Even in the case where the calcination prior to the
annealing treatment is incapable of satisfying a coefficient of
variation of Li/Me of not more than 25% owing to the aforementioned
various reasons, by subjecting the calcined product to the
annealing treatment, it becomes possible to satisfy a coefficient
of variation of Li/Me of not more than 25% in some cases.
[0045] In the present invention, it is preferred that the calcined
product is subjected to no water-washing treatment between the
calcination and the annealing treatment. If the the calcined
product is subjected to any water-washing treatment before the the
annealing treatment, elution of Li from the surface of the
secondary particles tends to be caused, so that the variation of
the composition of the resulting product tends to be increased.
[0046] In the present invention, when the mixture comprising the
raw materials at the predetermined compositional ratio is subjected
to calcination and heat treatments under the desired conditions, it
is possible to obtain a positive electrode active substance having
a coefficient of variation of Li/Me of not more than 25%.
[0047] Next, the non-aqueous electrolyte secondary battery
according to the present invention is described.
[0048] The non-aqueous electrolyte secondary battery according to
the present invention comprises a positive electrode comprising the
aforementioned positive electrode mixture, a negative electrode and
an electrolyte. The non-aqueous electrolyte secondary battery
according to the present invention can be used even under such a
condition that the operation voltage or the voltage in association
with an initial crystal phase transition is not more than 4.5 V
based on lithium.
[0049] Next, the positive electrode mixture according to the
present invention is described.
[0050] The positive electrode mixture according to the present
invention is not particularly limited, and may be obtained, for
example, by kneading an active substance, a conducting agent and a
binder at a mixing ratio of 90:5:5.
[0051] As a negative electrode active substance, there may be used
metallic lithium, lithium/aluminum alloys, lithium/tin alloys,
silicon, silicon/carbon composite materials, graphite and the
like.
[0052] In addition, as a solvent for the electrolyte solution,
there may be used not only a combination of ethylene carbonate (EC)
and diethyl carbonate (DEC), but also an organic solvent comprising
at least one compound selected from the group consisting of
carbonates comprising propylene carbonate (PC), dimethyl carbonate
(DMC), etc., as a basic structure, and ethers such as
dimethoxyethane (DME).
[0053] As an electrolyte, there may be used a solution prepared by
dissolving lithium phosphate hexafluoride (LiPF.sub.6) as well as
at least one lithium salt such as lithium perchlorate
(LiClO.sub.4), lithium borate tetrafluoride (LiBF.sub.4) and the
like in the aforementioned solvent.
<Function>
[0054] The important point of the present invention resides in such
a fact that the non-aqueous electrolyte secondary battery obtained
using the positive electrode active substance according to the
present invention is capable of conducting stable charging and
discharging operations with less deterioration in capacity thereof
when subjected to repeated charging and discharging cycles at a
temperature ranging from a low temperature to a high
temperature.
[0055] In the present invention, it is estimated that when
subjecting a lithium transition metal oxide that acts in the form
of aggregated secondary particles as a behaving unit to repeated
charging and discharging cycles, occurrence of side reactions on
the surface of the crystals is suppressed, so that it is possible
to prevent deterioration in capacity of the resulting battery.
Examples of the side reactions include a reaction between the
surplus lithium and fluorine ions in the active substance or the
electrolyte solution, a reaction between the surplus lithium and
sulfur ions in the electrolyte solution, and further a side
reaction occurring owing to growth of an electric double layer
caused by high resistance of an Li-deficient phase, etc. As
undesirable side effects derived from these side reactions, there
may be mentioned delamination of the grain boundary due to side
reaction by-products generated in the grain boundary and further
deterioration in conductivity within secondary particles as a
behaving unit due to the delamination of the grain boundary,
decomposition of organic impurities, dissolution and deposition of
metal impurities, as well as swelling of the electrode from the
macroscopic viewpoint.
[0056] In the present invention, it has been found that deposition
of the Li component derived from the raw materials on the grain
boundary causes a factor for disturbing a long service life of the
battery, and special attention has bee paid to a composition of the
grain boundary, in particular, surplus Li component therein. As a
result, the molar ratio between Li and a transition metal (Li/Me)
inside the aggregated secondary particles (on a broken section of
the aggregated secondary particles shown in the below-mentioned
Examples) is controlled to as uniform a value as possible, so that
it is possible to reduce a surplus local Li component. For this
reason, the present inventors have estimated that the amount of a
resistive component formed on the grain boundary can be reduced,
and the resulting battery is capable of conducting stable charging
and discharging operations with less deterioration in capacity
thereof when subjected to repeated charging and discharging cycles
at a temperature ranging from a low temperature to a high
temperature.
EXAMPLES
[0057] Typical examples of the present invention are described
below.
[0058] In order to confirm positions of grain boundaries of the
crystals as well as determine a crystal structure inside crystal
particles in the vicinity of the grain boundaries, the section of
the crystals obtained by Ar ion milling method was identified by
TEM Image multi-wave interference images at an acceleration voltage
of 300 keV and selected area electron diffraction patterns.
[0059] The positions of grain boundaries of the crystals as well as
the distribution of ions in a section of secondary particles
including the grain boundaries were determined by secondary ion
mass spectrometry. More concretely, using a secondary ion mass
spectrometer "Nano-SIMS50L" manufactured by AMETEK CAMECA, Cs.sup.+
ions were accelerated at 8 keV, contracted and converged into a
diameter of not more than 100 nm, and irradiated on a cut section
to be observed at intervals of 60 nanometers to thereby identify
secondary ions emitted from a sample. By using the aforementioned
method, the distribution condition of main elements such as Ni
including Li having a fine space resolution with the order of 60 to
100 nm was measured.
[0060] Meanwhile, the observation surface of the aggregated
particles was formed by cutting a positive electrode active
substance embedded in a resin by ion milling method. At this time,
the diameter of the section to be cut was controlled to at least 3
.mu.m, and the compositional ratio of the active substance was
continuously measured along the linear diametrical portion having a
length of at least 3 .mu.m from one end of the aggregated particles
to the other end thereof to calculate a standard deviation and an
average value of the compositional ratio, thereby determining a
coefficient of variation (standard deviation/average value)
thereof.
[0061] FIG. 1 shows a conceptual view of the aforementioned
measurement. The positive electrode active substance according to
the present invention was in the form of a secondary particle 2
formed by aggregating a number of primary particles (crystal
particles) 1. On the observation section of the secondary particle
2 embedded in the resin, a linear portion 3 having a predetermined
length was selected, and the compositional ratio thereof was
measured along the linear portion 3.
[0062] Furthermore, as a supplemental analysis, FIB-SIM image and
Ni distribution of the aforementioned Nano-SIMS were previously
compared with each other to confirm that the Ni distribution
obtained by Nano-SIMS was consistent with actual positions of the
grain boundaries.
[0063] Similarly, the analysis of the state of the transition metal
in the vicinity of the grain boundary, i.e., in the vicinity of the
surface of the crystals was carried out using STEM-EELS under the
condition that an acceleration voltage was 200 key, a beam diameter
was 0.2 nm, and an electric current for irradiation was 1.00
nA.
[0064] A coin cell having a 2032 size was used in the measurement
of repeated charging and discharging characteristics of the
positive electrode mixture according to the present invention. In
the measurement, 100 charging and discharging cycles were carried
out at a charging rate of 0.5 C and a discharging rate of 1 C.
[0065] The coin cell used for evaluation of the battery was
produced as follows. That is, 90% by weight of a composite oxide as
positive electrode active substance particles, 6% by weight of
carbon black as a conducting material and 4% by weight of
polyvinylidene fluoride dissolved in N-methyl pyrrolidone as a
binder were mixed with each other, and the resulting mixture was
applied onto an Al metal foil and then dried at 110.degree. C. The
thus obtained sheets were blanked into 16 mm.phi. and then
compression-bonded to each other under a pressure of 3.0 t/cm.sup.2
to produce a positive electrode used for the evaluation. A metallic
lithium foil was used as a negative electrode, and a 1 mol/L
LiPF.sub.6 solution of a mixed solvent comprising EC and DMC at a
volume ratio of 1:2 was used as an electrolyte solution, thereby
producing a coin cell having the aforementioned size.
[0066] In the measurement of the repeated charging and discharging
characteristics, the coin cell was charged at 0.5 C until reaching
4.3 V (CC-CV), and then discharged at 1 C until reaching 3.0 V
(CC), and 100 cycles of the charging and discharging operations
were repeated to calculate a capacity retention rate of the coin
cell. Meanwhile, the aforementioned test was conducted in a
thermostat adjusted to 60.degree. C.
Example 1
[0067] In a reaction vessel equipped with a blade-type stirrer, a
sodium hydroxide aqueous solution having a pH value of 12.0 was
prepared, and an ammonia aqueous solution was added dropwise into
the sodium hydroxide aqueous solution such that the obtained
reaction solution had an ammonia concentration of 0.80 mol/L.
Furthermore, a mixed solution comprising cobalt sulfate, nickel
sulfate and manganese sulfate was continuously fed to the reaction
vessel. During the aforementioned procedure, a sodium hydroxide
aqueous solution and an ammonia aqueous solution were continuously
fed to the reaction vessel so as to control a pH value of the
resulting reaction solution to 12 and an ammonia concentration
thereof to 0.8 mol/L, so that the particles in the reaction
solution were grown to those having an average secondary particle
diameter as aimed, and further by applying a mechanical shear force
to the resulting suspension, a precipitate comprising a spherical
composite transition metal was obtained.
[0068] After completion of the reaction, the thus obtained
suspension was taken out from the reaction vessel and washed with
water using a filter press, and then the resulting filter cake was
dried at 150.degree. C. for 12 hours, thereby obtaining
nickel-cobalt-manganese-based compound particles
(nickel-cobalt-manganese-based composite hydroxide particles). The
thus obtained composite hydroxide and lithium hydroxide monohydrate
were mixed with each other such that a molar ratio of Li to a sum
of Ni, Co and Mn in the resulting mixture was 1.01
(Li/Ni+Co+Mn=1.01).
[0069] The thus obtained mixture was calcined in an oxygen
atmosphere at 750.degree. C. for 10 hours. Thereafter, the calcined
product was subjected to heat treatment (annealing treatment) at
600.degree. C. for 4 hours and then deaggregated. As a result of
ICP analysis, the resulting calcined product had a chemical
composition represented by the formula of
Li.sub.1.00Ni.sub.0.5Co.sub.0.2Mn.sub.0.3O.sub.2, an average
secondary particle diameter of 10 .mu.m and a primary particle
diameter (crystallite size) of 462 nm.
[0070] The section of the thus obtained particles was subjected to
Nano-SIMS element distribution analysis, so that it was confirmed
that a coefficient of variation of Li/Me in the composition
including the crystals and grain boundaries was 24.6%.
[0071] As a supplemental measurement, using high resolution TEM,
multi-wave interference images and selected area electron
diffraction patterns as well as STEM-EELS analysis were conducted
from the grain boundaries to an inside of the crystals at intervals
of 20 nm. As a result, it was confirmed that the crystal structure
in the vicinity of the grain boundaries was the same R-3m structure
as that of a bulk thereof, and no reduction of the transition
metals was caused.
[0072] The resulting positive electrode active substance was used
to produce a coin cell. As a result of subjecting the thus produced
coin cell to the measurement of charging and discharging cycles,
the capacity retention rate of the coin cell was 98.7%.
Example 2
[0073] The same procedure as in Example 1 was conducted except that
the ratio of Ni/Co/Mn was changed to 1.0/1.0/1.0, and a mixture
comprising the Li raw material and the transition metal mixed
spherical oxide was calcined in an oxygen atmosphere at 750.degree.
C. for 10 hours, and then the resulting calcined product was
deaggregated to produce positive electrode active substance
particles, thereby obtaining a positive electrode active
substance.
[0074] The section of the thus obtained particles was subjected to
Nano-SIMS element distribution analysis, so that it was confirmed
that a coefficient of variation of Li/Me in the composition
including the crystals and grain boundaries was 18.7%.
[0075] As a supplemental measurement, using high resolution TEM,
multi-wave interference images and selected area electron
diffraction patterns as well as STEM-EELS analysis were conducted
from the grain boundaries to an inside of the crystals at intervals
of 20 nm. As a result, it was confirmed that the crystal structure
in the vicinity of the grain boundaries was the same R-3m structure
as that of a bulk thereof, and no reduction of the transition
metals was caused.
[0076] The resulting positive electrode active substance was used
to produce a coin cell. As a result of subjecting the thus produced
coin cell to the measurement of charging and discharging cycles,
the capacity retention rate of the coin cell was 99.5%.
Example 3
[0077] The same procedure as in Example 2 was conducted except that
the ratio of Ni/Co/Mn was changed to 1.0/1.0/1.0, and the ratio of
Li/Me was changed to 1.00 (Li/Me=1.00), thereby obtaining a
positive electrode active substance.
[0078] The section of the thus obtained particles was subjected to
Nano-SIMS element distribution analysis, so that it was confirmed
that a coefficient of variation of Li/Me in the composition
including the crystals and grain boundaries was 7.1%.
[0079] As a supplemental measurement, using high resolution TEM,
multi-wave interference images and selected area electron
diffraction patterns as well as STEM-EELS analysis were conducted
from the grain boundaries to an inside of the crystals at intervals
of 20 nm. As a result, it was confirmed that the crystal structure
in the vicinity of the grain boundaries was the same R-3m structure
as that of a bulk thereof, and no reduction of the transition
metals was caused.
[0080] The resulting positive electrode active substance was used
to produce a coin cell. As a result of subjecting the thus produced
coin cell to the measurement of charging and discharging cycles,
the capacity retention rate of the coin cell was 100.3%.
Comparative Example 1
[0081] The same procedure as in Example 1 was conducted except that
the calcination was conducted in an oxygen atmosphere at
750.degree. C. for 10 hours, and then the resulting calcined
product was deaggregated (without being subjected to annealing
treatment), thereby obtaining a positive electrode active
substance.
[0082] The section of the thus obtained particles was subjected to
Nano-SIMS element distribution analysis, so that it was confirmed
that a coefficient of variation of Li/Me in the composition
including the crystals and grain boundaries was 26.1%.
[0083] As a supplemental measurement, using high resolution TEM,
multi-wave interference images and selected area electron
diffraction patterns as well as STEM-EELS analysis were conducted
from the grain boundaries to an inside of the crystals at intervals
of 20 nm. As a result, it was confirmed that the crystal structure
in the vicinity of the grain boundaries was the same R-3m structure
as that of a bulk thereof, and no reduction of the transition
metals was caused. However, only in the nearest vicinity of the
grain boundaries, inclusion of the transition metals into Li sites
was recognized, and at the same tine, EELS energy shift suggesting
reduction of the transition metals was confirmed.
[0084] The resulting positive electrode active substance was used
to produce a coin cell. As a result of subjecting the thus produced
coin cell to the measurement of charging and discharging cycles,
the capacity retention rate of the coin cell was 95.5%.
[0085] The coefficient of variation of Li/Me and the charging and
discharging characteristics of the resulting positive electrode
active substance are shown in Table 1.
TABLE-US-00001 TABLE 1 Average secondary Examples and Coefficient
particle Comparative of variation diameter Crystallite Cycle
Examples of Li/Me (.mu.m) size (nm) 101st/1st % Example 1 24.6 10.4
462 98.7 Example 2 17.6 9.5 500 99.5 Example 3 7.1 9.13 556 101.3
Comparative 26.1 10.5 667 95.5 Example 1
[0086] From the aforementioned results, it was confirmed that the
secondary battery produced using the positive electrode active
substance particles according to the present invention was
excellent in repeated charging and discharging characteristics and
therefore effective as a positive electrode active substance for
non-aqueous electrolyte secondary batteries.
INDUSTRIAL APPLICABILITY
[0087] The positive electrode active substance particles according
to the present invention has a large discharge capacity and is
excellent in cycle characteristics, and therefore can be suitably
used as positive electrode active substance particles for
non-aqueous electrolyte secondary batteries.
EXPLANATION OF REFERENCE NUMERALS
[0088] 1: Primary particles.
[0089] 2: Secondary particles.
[0090] 3: Line as a reference for measuring a compositional
ratio.
* * * * *