U.S. patent application number 16/303181 was filed with the patent office on 2019-07-04 for method for producing positive electrode active material for lithium ion secondary batteries, positive electrode active material .
This patent application is currently assigned to HITACHI METALS, LTD.. The applicant listed for this patent is HITACHI METALS, LTD.. Invention is credited to Akira GUNJI, Takashi NAKABAYASHI, Shin TAKAHASHI, Shuichi TAKANO, Hisato TOKORO, Tatsuya TOOYAMA.
Application Number | 20190207215 16/303181 |
Document ID | / |
Family ID | 60783445 |
Filed Date | 2019-07-04 |
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United States Patent
Application |
20190207215 |
Kind Code |
A1 |
TOKORO; Hisato ; et
al. |
July 4, 2019 |
METHOD FOR PRODUCING POSITIVE ELECTRODE ACTIVE MATERIAL FOR LITHIUM
ION SECONDARY BATTERIES, POSITIVE ELECTRODE ACTIVE MATERIAL FOR
LITHIUM ION SECONDARY BATTERIES, AND LITHIUM ION SECONDARY
BATTERY
Abstract
Making a positive electrode active material for lithium ion
secondary batteries includes: weighting and mixing lithium
carbonate and a compound containing respective metallic elements
other than Li in a composition formula
Li.sub..alpha.Ni.sub.xCo.sub.yM1.sub.1-x-y-zM2.sub.zO.sub.2+.beta-
. so as to have a metallic constituent ratio of the formula to
obtain a mixture, and firing the mixture to obtain a lithium
composite compound. Performing, on the mixture, a first heat
treatment at 200.degree. C. to 400.degree. C. for 0.5 to 5 hours to
obtain a first precursor. A step of performing a heat treatment on
the first precursor under an oxidizing atmosphere at 450.degree. C.
to 800.degree. C. for 0.5 to 50 hours, and reacting 92 mass % or
more of the lithium carbonate to obtain a second precursor, and a
finishing step of performing a heat treatment on the second
precursor under an oxidizing atmosphere at 755.degree. C. to
900.degree. C. for 0.5 to 50 hours to obtain the lithium composite
compound.
Inventors: |
TOKORO; Hisato; (Tokyo,
JP) ; NAKABAYASHI; Takashi; (Tokyo, JP) ;
TAKANO; Shuichi; (Tokyo, JP) ; GUNJI; Akira;
(Tokyo, JP) ; TOOYAMA; Tatsuya; (Tokyo, JP)
; TAKAHASHI; Shin; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HITACHI METALS, LTD. |
Tokyo |
|
JP |
|
|
Assignee: |
HITACHI METALS, LTD.
Tokyo
JP
|
Family ID: |
60783445 |
Appl. No.: |
16/303181 |
Filed: |
April 28, 2017 |
PCT Filed: |
April 28, 2017 |
PCT NO: |
PCT/JP2017/016995 |
371 Date: |
November 20, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 4/505 20130101;
H01M 10/0525 20130101; C01P 2002/77 20130101; C01G 53/50 20130101;
H01M 10/052 20130101; H01M 4/525 20130101; H01M 2004/028
20130101 |
International
Class: |
H01M 4/505 20060101
H01M004/505; H01M 4/525 20060101 H01M004/525; H01M 10/0525 20060101
H01M010/0525 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 23, 2016 |
JP |
2016-124594 |
Claims
1. A method for producing positive electrode active material for
lithium ion secondary batteries, the method comprising: a mixing
step of weighting and mixing a lithium carbonate and a compound
containing respective metallic elements other than Li in Formula
(1), Li.sub..alpha.Ni.sub.xCo.sub.yM1.sub.1-x-y-zM2.sub.zO.sub.2+B,
so as to have a metallic constituent ratio of a composition formula
in accordance with the Formula (1) to obtain a mixture, where
values in the Formula (1) meet: 0.97.ltoreq..alpha..ltoreq.1.08,
-0.1.ltoreq..beta..ltoreq.0.1, 0.7<x .ltoreq.0.9,
0.03.ltoreq.y.ltoreq.0.3, 0.ltoreq.z.ltoreq.0.1, and 0 <1-x-y-z,
M1 is at least one kind of an element selected from the group
consisting of Mn and Al, and M2 is at least one kind of an element
selected from the group consisting of Mg, Ti, Zr, Mo, and Nb; and a
firing step of firing the mixture to obtain a lithium composite
compound expressed by the following Formula (1), wherein the firing
step includes: a first precursor forming step of performing a heat
treatment on the mixture at a heat treatment temperature of
200.degree. C. or more and 400.degree. C. or less for 0.5 hours or
more and 5 hours or less to obtain a first precursor; a second
precursor forming step of performing a heat treatment on the first
precursor under an oxidizing atmosphere at a heat treatment
temperature of 450.degree. C. or more and 800.degree. C. or less
for 0.5 hours or more and 50 hours or less, the second precursor
forming step reacting 92 mass % or more of the lithium carbonate to
obtain a second precursor; and a finishing heat treatment step of
performing a heat treatment on the second precursor under an
oxidizing atmosphere at a heat treatment temperature of 755.degree.
C. or more and 900.degree. C. or less for 0.5 hours or more and 50
hours or less to obtain the lithium composite compound.
2. The method for producing positive electrode active material for
lithium ion secondary batteries according to claim 1, wherein the
heat treatment temperature in the finishing heat treatment step is
840.degree. C. or more and 890.degree. C. or less.
3. The method for producing positive electrode active material for
lithium ion secondary batteries according to claim 1, wherein the
second precursor forming step forms the second precursor such that
97 mass % or more of the lithium carbonate has reacted, the lithium
carbonate being contained in the mixture weighted and mixed so as
to have the metallic constituent ratio of the composition formula
in the Formula (1).
4. The method for producing positive electrode active material for
lithium ion secondary batteries according to claim 1, wherein the
heat treatment temperature in the second precursor forming step is
600.degree. C. or more and 700.degree. C. or less.
5. The method for producing positive electrode active material for
lithium ion secondary batteries according to claim 1, wherein the
M1 in the Formula (1) is Mn, 0.04.ltoreq.1-x-y-z.ltoreq.0.18 being
met.
6. The method for producing positive electrode active material for
lithium ion secondary batteries according to claim 1, wherein the
lithium composite compound is not water-washed after the finishing
heat treatment step.
7. The method for producing positive electrode active material for
lithium ion secondary batteries according to claim 1, the method
comprising a step of performing a water washing on the lithium
composite compound after the finishing heat treatment step.
8. The method for producing positive electrode active material for
lithium ion secondary batteries according to claim 7, the method
comprising a step of performing a drying at least once or more
after the step of performing the water washing.
9. A positive electrode active material for lithium ion secondary
batteries expressed by the following Formula (1),
Li.sub..alpha.Ni.sub.xCo.sub.yM1.sub.1-x-y-zO.sub.2+.beta., where
values in the Formula (1) meet: 0.97.ltoreq.a.ltoreq.1.08,
-0.1.ltoreq..beta..ltoreq.0.1, 0.7<x.ltoreq.0.9,
0.03.ltoreq.y.ltoreq.0.3, 0.ltoreq.z.ltoreq.0.1, and 0<1-x-y-z,
M1 is at least one kind of an element selected from the group
consisting of Mn and Al, and M2 is at least one kind of an element
selected from the group consisting of Mg, Ti, Zr, Mo, and Nb; and
wherein a specific surface area is 0.10 m.sup.2/g or more, and an
amount of dissolution (B-A) is 0.33 mass % or less, the amount of
dissolution (B-A) being a difference between an amount of lithium
hydroxide A and an amount of lithium hydroxide B, the amount of
lithium hydroxide A being detected by a neutralization titration
after an immersion into a pure water at a solid content percentage
of 1.6 mass % for 30 minutes, the amount of lithium hydroxide B
being detected by the neutralization titration after an immersion
into a pure water at a solid content percentage of 1.6 mass % for
120 minutes.
10. A positive electrode active material for lithium ion secondary
batteries expressed by the following Formula (1),
Li.sub..alpha.Ni.sub.xCo.sub.yM1.sub.1-x-y-zM2.sub.zO.sub.2+.beta.,
where values in the Formula (1) meet:
0.97.ltoreq..alpha..ltoreq.1.08, -0.1.ltoreq..beta..ltoreq.0.1,
0.7<x.ltoreq.0.9, 0.03.ltoreq.y.ltoreq.0.3,
0.ltoreq.z.ltoreq.0.1, and 0<1-x-y-z, M1 is at least one kind of
an element selected from the group consisting of Mn and Al, and M2
is at least one kind of an element selected from the group
consisting of Mg, Ti, Zr, Mo, and Nb, and wherein a molding density
at a press pressure of 5 MPa is 2.5 g/cm.sup.3 or more.
11. The positive electrode active material for lithium ion
secondary batteries according to claim 10, wherein the M1 in the
Formula (1) is Mn, 0.04.ltoreq.1-x-y-z.ltoreq.0.18 being met.
12. A lithium ion secondary battery, wherein the lithium ion
secondary battery uses the positive electrode active material for
lithium ion secondary batteries according to claim 9.
13. A lithium ion secondary battery, wherein the lithium ion
secondary battery uses the positive electrode active material for
lithium ion secondary batteries according to claim 10.
14. A lithium ion secondary battery, wherein the lithium ion
secondary battery uses the positive electrode active material for
lithium ion secondary batteries according to claim 11.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for producing a
positive electrode active material used for a positive electrode of
a lithium ion secondary battery, the positive electrode active
material, and a lithium ion secondary battery.
BACKGROUND ART
[0002] Conventionally, as typified by, for example, lithium ion
secondary batteries, non-aqueous secondary batteries in which a
non-aqueous electrolyte mediates an electrical conduction between
electrodes have been used as secondary batteries. The lithium ion
secondary battery is a secondary battery in which lithium ions are
in charge of electrical conduction between electrodes in
charge/discharge reactions. The lithium ion secondary battery
features a high energy density and a small memory effect compared
with other secondary batteries such as a nickel-hydrogen storage
battery and a nickel-cadmium storage battery. Therefore, the
lithium ion secondary battery has been expanding its application
ranging from small power supplies for, for example, mobile
electronic devices and household electrical equipment, stationary
power supplies for, for example, electric power storage devices,
uninterruptible power supply systems, and power leveling devices to
medium and large power supplies such as driving power supplies for,
for example, ships, railways, hybrid vehicles, and electric
vehicles.
[0003] Especially, in the use of the lithium ion secondary
batteries as the medium and large power supplies, a high energy
density is required for the batteries. Positive electrodes and
negative electrodes are required to have a high energy density to
achieve the high energy density of the batteries; therefore, active
materials used for the positive electrode and the negative
electrode are required to have a high capacity. As a positive
electrode active material having a high charge/discharge capacity,
powder of a lithium composite compound expressed by a chemical
formula of LiM'O.sub.2 (M' indicates an element such as Ni, Co, and
Mn) having an .alpha.-NaFeO.sub.2 type layered structure has been
known. Since exhibiting a trend of increasing the capacity as
especially a proportion of Ni increases, this positive electrode
active material is expected as the positive electrode active
material achieving the high-energy batteries.
[0004] There has been disclosed powder of a lithium-containing
compound expressed by
Li.sub.aNi.sub.bM1.sub.cM2.sub.d(O).sub.2(SO.sub.4).sub.X as such
positive electrode active material and a method for producing the
positive electrode active material (see the following Patent
Literature 1). An object of the invention described in Patent
Literature 1 is to provide lithium-mixed metal oxide with which
secondary particles are not broken in a battery production (a
positive electrode) and do not turn into powder. As means to
achieve the object, the invention configures a difference in D10
values between initial powder and powder after compression at 200
MPa measured in accordance with ASTM B 822, the standard formulated
by American Society for Testing and Materials, to be 1.0 .mu.m or
less.
[0005] A production step of the powder of the lithium-containing
compound described in Patent Literature 1 includes a step of
preparing a coprecipitated nickel-containing precursor having a
predetermined voidage and a step of mixing the nickel-containing
precursor with a lithium-containing compound to obtain a precursor
mixture. Example of this lithium-containing compound includes
lithium carbonate, lithium hydroxide, lithium hydroxide
monohydrate, lithium oxide, lithium nitrate, or a mixture of these
substances. The production step further includes a step of heating
the obtained precursor mixture to 200 to 1000.degree. C. in
multiple stages using carrier gas containing
CO.sub.2-non-containing (CO.sub.2 content proportion: 0.5 ppm or
less) oxygen to produce a powder product and a step of
disintegrating the powder by ultrasonic wave and screening the
disintegrated powder.
[0006] According to Patent Literature 1, a reaction control
pertaining to a temperature holding stage in the above-described
production step allows obtaining a product of no aggregation of
secondary particles firmly sintered to one another. Patent
Literature 1 discloses that this allows an elimination of a
pulverization step that forms square-shaped particles with squares,
which cause particles to break in a material floor under a high
pressure in an electrode production.
[0007] There has been disclosed a production method that allows
obtaining a high-capacity Li.sub.yNi.sub.(1-x)Mn.sub.xO.sub.2
(Here, the numbers of moles of x and y are 0.ltoreq.x .ltoreq.0.3,
1.0.ltoreq.y.ltoreq.1.3.) regarding a positive electrode active
material for non-aqueous electrolyte secondary battery (see the
following Patent Literature 2). The production method described in
Patent Literature 2 employs a manganese compound equivalent to the
number of moles of atoms of Mn indicated by x, a nickel compound
equivalent to the number of moles of atoms of Ni indicated by 1-x,
and a lithium compound equivalent to the number of moles of atoms
of Li indicated by y as starting materials. This production method
is a synthesis method that performs a first heat treatment after
preliminarily drying these starting materials, obtains an
intermediate through a temperature decrease process, and after that
performs a second heat treatment again at a temperature different
from that of the first heat treatment. The production method
features that a processing atmosphere at firing is an oxidizing
atmosphere (see claim 1 or a similar description). Patent
Literature 2 discloses that the use of the above-described
synthesis method ensures obtaining a positive electrode active
material for non-aqueous electrolyte secondary battery having a
high charge/discharge capacity (paragraph 0020).
[0008] Patent Literature 2 describes a LiNO.sub.3 hydrate and
Li.sub.2CO.sub.3 as examples of the lithium compounds as the
starting materials (paragraph 0019). The production method
described in Patent Literature 2 does not obtain the synthesis of
LiNiO.sub.2 having a space group R-3m structure directly from the
starting materials, Ni and Li compounds, by a heat treatment, but
obtains a final object via the intermediate. Since this
intermediate has an oxygen close packing type similar to a NiO type
mainly having a rhombohedral structure and further contains Li
sites at positions close to Ni and O atoms, the intermediate is
considered to be facilitated to change to the R-3m structure (see
paragraphs 0022 and 0023 and a similar description). In a
determination from X-ray diffraction diagrams illustrated in FIG. 7
and FIG. 9, in the case where, for example, an unreacted Li
compound indicative of a crystalline structure different from that
of the intermediate is contained, it is determined as improper (see
paragraphs 0040 and 0050).
CITATION LIST
Patent Literature
[0009] Patent Literature 1: JP-T 2010-505732
[0010] Patent Literature 2: JP-A H6-96768 A
SUMMARY OF INVENTION
Technical Problem
[0011] A method for producing a positive electrode active material
that fires a mixture of lithium carbonate with a compound
containing Ni and produces a lithium composite compound with a high
Ni concentration has the following problems. The lithium composite
compound with the high Ni concentration here means, for example, a
lithium composite compound having a layered structure in which an
atom ratio (Ni/M') of Ni to M' in a chemical formula LiM'O.sub.2
(M' is a metallic element containing Ni) in excess of 0.7.
[0012] For industrial mass production of the above-described
lithium composite oxide with the high Ni concentration, the
synthesis reaction needs to be progressed by the large amount and
uniformly. However, it has been found that heating a mixture of
lithium carbonate and a compound containing Ni generates a large
amount of carbonic acid gas from the lithium carbonate; therefore,
the uniform synthesis reaction by the large amount is inhibited.
This is because a reverse reaction of the carbonic acid gas with
the lithium composite compound with the high Ni concentration
containing Ni.sup.3+ much progresses easily. Additionally, an
oxygen partial pressure lowers and a reaction that oxidizes an
oxidation number of Ni from Ni.sup.2+ to Ni.sup.3+ is
inhibited.
[0013] Especially, a problem such as the following has been found
out. The lithium composite compound with the high Ni concentration
in which an oxidization of Ni is insufficient substantially lowers
the capacity in a secondary battery using a positive electrode
containing the lithium composite compound as a positive electrode
active material. It has been found out that also in the case where,
for example, an unreacted Li compound indicative of a crystalline
structure different from that of the intermediate is not observed
in an X-ray diffraction diagram, there may be a case where a
property as the positive electrode active material cannot be
sufficiently obtained, and the oxidation reaction of Ni needs to be
progressed with more certainty.
[0014] The present invention has been made in consideration of the
problems, and an object of the present invention is to provide a
method for producing a positive electrode active material that
ensures industrially mass-producing positive electrode active
materials containing lithium composite oxide with a high Ni
concentration, the positive electrode active material, and a
lithium ion secondary battery.
Solution to Problem
[0015] To achieve the object, a method for producing positive
electrode active material for lithium ion secondary batteries of
the present invention is a method for producing the positive
electrode active material used for positive electrodes of the
lithium ion secondary batteries. The method includes: a mixing step
of weighting and mixing a lithium carbonate and a compound
containing respective metallic elements other than Li in the
following Formula (1) so as to have a metallic constituent ratio of
a composition formula in the following Formula (1) to obtain a
mixture; and a firing step of firing the mixture to obtain a
lithium composite compound expressed by the following Formula (1).
The firing step includes: a first precursor forming step of
performing a heat treatment on the mixture at a heat treatment
temperature of 200.degree. C. or more and 400.degree. C. or less
for 0.5 hours or more and 5 hours or less to obtain a first
precursor; a second precursor forming step of performing a heat
treatment on the first precursor under an oxidizing atmosphere at a
heat treatment temperature of 450.degree. C. or more and
800.degree. C. or less for 0.5 hours or more and 50 hours or less,
the second precursor forming step reacting 92 mass % or more of the
lithium carbonate to obtain a second precursor; and a finishing
heat treatment step of performing a heat treatment on the second
precursor under an oxidizing atmosphere at a heat treatment
temperature of 755.degree. C. or more and 900.degree. C. or less
for 0.5 hours or more and 50 hours or less to obtain the lithium
composite compound.
Li.sub..alpha.Ni.sub.xCo.sub.yM1.sub.1-x-y-zM2.sub.zO.sub.2+.beta.
(1)
[0016] Note that values in the Formula (1) meet:
0.97.ltoreq..alpha..ltoreq.1.08, -0.1.ltoreq..beta..ltoreq.0.1 ,
0.7<x.ltoreq.0.9, 0.03.ltoreq.y.ltoreq.0.3,
0.ltoreq.z.ltoreq.0.1, and 0<1-x-y-z, M1 is at least one kind of
an element selected from the group consisting of Mn and Al, and M2
is at least one kind of an element selected from the group
consisting of Mg, Ti, Zr, Mo, and Nb.
Advantageous Effects of Invention
[0017] With the present invention, a positive electrode active
material containing lithium composite oxide with a high Ni
concentration can be industrially mass-produced and further a
service life of the positive electrode active material can be
longer than those of conventional positive electrode active
materials.
BRIEF DESCRIPTION OF DRAWINGS
[0018] FIG. 1 is a flowchart of a method for producing a positive
electrode active material according to an embodiment of the present
invention.
[0019] FIG. 2 is a schematic partial cross-sectional view of a
secondary battery containing a positive electrode active material
according to the embodiment of the present invention.
[0020] FIG. 3 is a graph illustrating compression properties of
positive electrode active materials according to working examples
of the present invention.
Description of Embodiments
[0021] The following describes embodiments of a method for
producing a positive electrode active material and the positive
electrode active material of the present invention in detail with
reference to the drawings.
[0022] The method for producing a positive electrode active
material of this embodiment is a method to produce the positive
electrode active material used for a positive electrode of a
non-aqueous secondary battery such as a lithium ion secondary
battery. First, the following describes the positive electrode
active material produced by the method for producing the positive
electrode active material of this embodiment in detail, and next
describes the method for producing the positive electrode active
material of this embodiment in detail.
(Positive Electrode Active Material)
[0023] The positive electrode active material produced by the
production method of this embodiment is a lithium composite
compound having an .alpha.-NaFeO.sub.2 type layered structure and
expressed by the following Formula (1). The positive electrode
active material contains, for example, the above-described powdery
lithium composite compound with a specific surface area of 0.10
m.sup.2/g or more.
Li.sub..alpha.Ni.sub.xCo.sub.yM1.sub.1-x-y-zM2.sub.zO.sub.2+.beta.
(1)
[0024] Note that the values in the above-described Formula (1)
meet: 0.97.ltoreq.a.ltoreq.1.08, -0.1.ltoreq..beta..ltoreq.0.1,
0.7<x<0.9, 0.03.ltoreq.y.ltoreq.0.3, 0.ltoreq.z.ltoreq.0.1,
and 0<1-x-y-z, M1 is at least one kind of an element selected
from the group consisting of Mn and Al, and M2 is at least one kind
of an element selected from the group consisting of Mg, Ti, Zr, Mo,
and Nb.
[0025] The positive electrode active material made of the lithium
composite compound having the .alpha.-NaFeO.sub.2 type layered
structure expressed by the above-described Formula (1) allows
repetitive reversible insertion and detachment of lithium ions in
association with charge and discharge and has a low resistance.
[0026] Particles of the lithium composite compound constituting the
positive electrode active material may be primary particles where
individual particles are separated, may be secondary particles
where the plurality of primary particles are bonded by sintering or
a similar method, or may be primary particles or secondary
particles containing a free lithium compound.
[0027] The primary particles of the positive electrode active
material preferably have an average grain diameter of, for example,
0.1 .mu.m or more and 2 .mu.m or less. Designing the average grain
diameter of the primary particles of the positive electrode active
material to be 2 .mu.m or less improves a filling property of the
positive electrode active material in the positive electrode when
the positive electrode containing the positive electrode active
material is produced, thereby ensuring producing the positive
electrodes with a high energy density. From a similar aspect, the
secondary particles of the positive electrode active material
preferably have the average grain diameter of, for example, 3 .mu.m
or more and 50 .mu.m or less.
[0028] By granulating the primary particles or the secondary
particles of the positive electrode active material produced by the
method for producing the positive electrode active material
described later by dry granulation or wet granulation, the average
grain diameter of the secondary particles can be adjusted. For
example, a granulator such as a spray dryer and a tumbling
fluidized bed device is available as granulation means.
[0029] In the above-described Formula (1), .alpha.indicates a
content ratio of Li. The higher the content ratio of Li is, the
higher a valence of transition metal before charge is, and the rate
of change in the valence of the transition metal at Li detachment
is lowered, ensuring improving charge/discharge cycle
characteristics of the positive electrode active material. On the
contrary, the higher the content ratio of Li is, the lower the
charge/discharge capacity of the positive electrode active material
is. Accordingly, the range of .alpha. indicative of an amount of
excess/deficiency of Li in the above-described Formula (1) is
designed to be 0.97 or more and 1.08 or less. This ensures
improving the charge/discharge cycle characteristics of the
positive electrode active material and reducing the decrease in the
charge/discharge capacity.
[0030] More preferably, the range of a indicative of the content
ratio of Li in the above-described Formula (1) can be designed to
be 0.98 or more and 1.05 or less. As long as .alpha. in the
above-described Formula (1) is 0.98 or more, the amount of Li
sufficient to contribute to the charge and discharge is secured and
the high-capacity positive electrode active material can be
achieved. Additionally, as long as a in the above-described Formula
(1) is 1.05 or less, a charge compensation can be sufficiently
secured in case of the change in valence of the transition metal,
and both the high capacity and the high charge/discharge cycle
characteristics can be satisfied.
[0031] With x indicative of the content ratio of Ni in the
above-described Formula (1) larger than 0.7, the amount of Ni
preferable to contribute to the charge and discharge can be secured
in the positive electrode active material, thereby ensuring
achieving the high capacity. Meanwhile, with x in the
above-described Formula (1) in excess of 0.9, a part of Ni is
replaced by a Li site. This fails to secure the amount of Li
sufficient to contribute to the charge and discharge and possibly
lowers the charge/discharge capacity of the positive electrode
active material. Accordingly, by designing x indicative of the
content ratio of Ni in the above-described Formula (1) in the range
of larger than 0.7 to 0.9 or less and more preferably in a range of
larger than 0.75 to 0.85 or less, the positive electrode active
material can have the high capacity and also the decrease in
charge/discharge capacity can be reduced.
[0032] Additionally, as long as y indicative of the content ratio
of Co in the above-described Formula (1) is 0.03 or more, this
ensures contributing to stabilization of the layered structure of
the positive electrode active material. Stably maintaining the
layered structure allows reducing a cation mixing that mixes, for
example, Ni into the Li sites; therefore, the excellent
charge/discharge cycle characteristics can be obtained. Meanwhile,
y in the above-described Formula (1) in excess of 0.3 relatively
increases the ratio of Co, which is limited in the supply amount
and has the high cost, being disadvantageous in terms of industrial
production of the positive electrode active materials. Accordingly,
by designing y indicative of the content ratio of Co in the
above-described Formula (1) in the range of 0.03 or more to 0.3 or
less, and more preferably in the range of larger than 0.05 to 0.2
or less, the charge/discharge cycle characteristics of the positive
electrode active material can be improved, being advantageous in
terms of the industrial mass production of the positive electrode
active materials.
[0033] M1 in the above-described Formula (1) is at least one kind
or more of elements selected from the group consisting of Mn and
Al. Adding Mn or Al, or Mn and Al together provides an effect of
stably maintaining the layered structure even when Li is detached
by charging. However, 1-x-y-z indicative of a content ratio of at
least one kind or more of elements selected from the group
consisting of Mn and Al in the above-described Formula (1) of 0.30
or more lowers the capacity of the positive electrode active
material.
[0034] M1 in the above-described Formula (1) is preferably Mn. This
is because, with the M1 in the above-described Formula (1) being
Mn, the layered structure can be further stably maintained even
when Li is detached by charging and the capacity higher than the
case of M1 being Al can be obtained. Designing 1-x-y-z indicative
of the content ratio of Mn when M1 is Mn to be 0.04 or more ensures
decreasing an average valence of Ni in LiM'O.sub.2. Therefore, even
when the oxidation reaction of Ni does not sufficiently progress,
the reaction indicated in the following Formula (2) progresses, a
reaction temperature lowers, and a reaction of lithium carbonate in
a second precursor forming step described later is promoted. Since
Al can take tetravalent similarly to Mn, an effect similar to that
of Mn can be expected.
Li.sub.2CO.sub.3+2M'O+0.5O.sub.2.fwdarw.2LiM'O.sub.2+CO.sub.2
(2)
[0035] Note that M' in the above-described Formula (2) indicates an
element such as Ni, Co, and Mn. As described above, the promotion
of the reaction of the lithium carbonate in the second precursor
forming step lowers lithium carbonate melted and becoming a liquid
phase in a finishing heat treatment step described later. This
lowers an amount of liquid phase in the finishing heat treatment
step and reduces a growth of crystal grains, making a
high-temperature firing possible.
[0036] Furthermore, with M1 being Mn, designing 1-x-y-z indicative
of the content ratio of Mn to be 0.04 or more allows the
charge/discharge reactions even when a crystallite diameter and a
primary particle diameter of the positive electrode active material
are large, making the high heat treatment temperature in the
finishing heat treatment step possible. Consequently, the Ni
oxidation reaction is promoted in the finishing heat treatment
step, a lithium compound remaining on a surface can be reduced, and
lithium ions in the layered structure are stabilized.
[0037] With M1 being Mn, 1-x-y-z indicative of the content ratio of
Mn in excess of 0.18 lowers the capacity of the positive electrode
active material. Additionally, with M1 being Mn, designing 1-x-y-z
indicative of the content ratio of Mn to be 0.10 or more allows the
heat treatment temperature in the finishing temperature treatment
step to be further high, and therefore is preferable. Accordingly,
with M1 being Mn, 1-x-y-z indicative of the content ratio of Mn
preferably meets 0.04 .ltoreq.1-x-y-z.ltoreq.0.18.
[0038] Additionally, with M1 being Mn, y/(1-x-y-z) indicative of
the ratio of the content ratios of Co to Mn is preferably 0.1 or
more and 3 or less. y/(1-x-y-z) of 0.1 or more maintains the
content ratio of Co in the preferable range and contributes to the
stabilization of the layered structure. As long as y/(1-x-y-z) is 3
or less, the high reaction temperature of the above-described
Formula (2) can be reduced and the reaction of the lithium
carbonate can be sufficiently progressed in the second precursor
forming step described later.
[0039] With M2 in the above-described Formula (1) containing at
least one kind of a metallic element selected from the group
consisting of Mg, Ti, Zr, Mo, and Nb, an electrochemical activity
of the positive electrode active material can be enhanced.
Replacing metal sites of the positive electrode active material
with these metallic elements ensures improving stability of the
crystalline structure of the positive electrode active material and
an electrochemical property (such as the charge/discharge cycle
characteristics) of the layered positive electrode active material.
With z indicative of the content ratio of M2 in the above-described
Formula (1) in excess of 0.1 lowers the capacity of the positive
electrode active material. Accordingly, designing the range of z in
the above-described Formula (1) to be 0 or more and 0.1 or less
ensures further reducing the decrease in capacity of the positive
electrode active material.
[0040] .beta.in the above-described Formula (1) indicates an
allowable range of a layered structure compound belonging to a
space group R-3m and excess or deficiency amount of oxygen. The
range of .beta. in the above-described Formula (1) being the range
of -0.1 or more and 0.1 or less ensures maintaining the layered
structure of the positive electrode active material.
[0041] Additionally, a weight of lithium carbonate remaining on the
surface of the positive electrode active material after the
finishing heat treatment step is preferably 0.2 mass % or less.
Designing the weight of the lithium carbonate remaining on the
surface to be 0.2 mass % or less ensures reducing an amount of
carbonic acid gas generated by lithium carbonate degradation caused
by the charge/discharge cycles and ensures improving the
charge/discharge cycle characteristics. The weight of the lithium
carbonate remaining on the surface of the positive electrode active
material after the finishing heat treatment step can be adjusted
by, for example, performing a water washing step.
[0042] The weight of the lithium hydroxide remaining on the surface
of the positive electrode active material is preferably 0.7 mass %
or less. The lithium hydroxide generates hydrofluoric acid (HF)
that exhibits strong acid through reaction to fluorine-based
electrolyte contained in the electrolyte of the secondary battery.
Further, the lithium hydroxide promotes an oxidative decomposition
of the electrolyte by a high voltage. This deteriorates the
performance of the secondary battery and makes it difficult to
obtain the satisfactory charge/discharge cycle characteristics.
Therefore, by designing the lithium hydroxide remaining on the
surface of the positive electrode active material to be 0.7 mass %
or less ensures obtaining the secondary battery having the
satisfactory charge/discharge cycle characteristics.
[0043] The lithium compound remaining on the surface of the
positive electrode active material can be quantitated by, for
example, a Titration Method, a Temperature Programmed
Desorption-Mass Spectrometry (TPD-MS), and Ion Chromatography (IC).
The crystalline structure of the particles of the positive
electrode active material can be confirmed by, for example, an
X-ray diffraction method (XRD). An average composition of the
particles of the positive electrode active material can be
confirmed by, for example, an Inductively Coupled Plasma (ICP) and
an Atomic Absorption Spectrometry (AAS).
[0044] The positive electrode active material of this embodiment is
the positive electrode active material used for the positive
electrode of the lithium ion secondary battery and features the
following. The specific surface area is formed of 0.10 m.sup.2/g or
more of the lithium composite compound expressed by the
above-described Formula (1). An amount of dissolution of the
lithium hydroxide is 0.33 mass % or less. In other words, the
positive electrode active material of this embodiment is the
positive electrode active material used for the positive electrode
of the lithium ion secondary battery and features the following.
The specific surface area is formed of 0.10 m.sup.2/g or more of
the lithium composite compound expressed by the above-described
Formula (1). An dissolution speed of the lithium hydroxide is 0.22
mass %/h or less. Here, the amount of dissolution of the lithium
hydroxide is a difference between an amount of lithium hydroxide A
and an amount of lithium hydroxide B, which is (B-A) mass %. The
amount of lithium hydroxide A is detected after immersion of the
positive electrode active material into pure water for 30 minutes
at a solid content percentage of 1.6 mass % by neutralization
titration. The amount of lithium hydroxide B is detected after
immersion of the positive electrode active material into pure water
for 120 minutes at a solid content percentage of 1.6 mass % by
neutralization titration. The dissolution speed corresponding to
this amount of dissolution is (B-A) mass %/1.5 h. The amount of
dissolution of the lithium hydroxide is preferably less than 0.30
mass % and the dissolution speed of the lithium hydroxide is
preferably less than 0.2 mass %/h.
[0045] By designing the specific surface area of the positive
electrode active material to be 0.10 m.sup.2/g or more, the average
grain diameters of the primary particles and the secondary
particles of the positive electrode active material can fall within
the above-described preferable ranges. The specific surface area of
the positive electrode active material is preferable to be 2.0
m.sup.2/g or less. This ensures improving the filling property of
the positive electrode active material in the positive electrode
and producing the positive electrode with the high energy
density.
[0046] The positive electrode active material having the specific
surface area of 0.8 m.sup.2/g or more and 1.2 m.sup.2/g or less is
more preferable. The specific surface area of the positive
electrode active material is a specific surface area that can be
measured using, for example, an automatic specific surface area
measuring apparatus and calculated by a BET method.
[0047] As long as the amount of dissolution (B-A) of the lithium
hydroxide in the positive electrode active material of 0.33 mass %
or less or the dissolution speed of the lithium hydroxide is 0.22
mass %/h or less, the layered structure is stabilized, thus
ensuring obtaining the satisfactory charge/discharge cycle
characteristics. Meanwhile, with the amount of dissolution (B-A) of
the lithium hydroxide in the positive electrode active material in
excess of 0.33 mass % or the dissolution speed of the lithium
hydroxide in excess of 0.22 mass %/h, the layered structure is
destabilized, making it difficult to obtain the satisfactory
charge/discharge cycle characteristics.
[0048] A particle fracture strength of the positive electrode
active material is preferably 10 MPa or more and 200 MPa or less.
This does not fracture the particles of the positive electrode
active material in the process of manufacturing the electrodes and
reduces a poor coating such as peeling when slurry containing the
positive electrode active material is coated over the surface of a
positive electrode current collector to form a positive electrode
mixture layer. The particle fracture strength per particle of the
positive electrode active material can be measured using, for
example, a microcompression testing machine.
[0049] A molding density of the positive electrode active material
at a press pressure of 5 MPa is preferably 2.5 g/cm.sup.3 or more.
How much the electric energy can be accumulated per unit volume
depends on how the electrode density is configured to be a high
density. When the molding densities, which are densities when
powder molding is performed at the identical pressure, are
compared, as the molding density becomes high, the powder is better
in compressibility. The compressibility of the positive electrode
active material can be evaluated by, for example, performing a
compression test with an autograph and measuring the molding
density.
(Method for Producing Positive Electrode Active Material)
[0050] Next, the following describes the method for producing the
positive electrode active material of this embodiment that produces
the above-described positive electrode active material. FIG. 1 is a
flowchart illustrating respective steps included in the method for
producing the positive electrode active material of this
embodiment.
[0051] The method for producing the positive electrode active
material of this embodiment is a method for producing the positive
electrode active material used for the positive electrode in the
lithium ion secondary battery. The method for producing the
positive electrode active material of this embodiment mainly
includes a mixing step S1 and a firing step S2. The mixing step S1
weights and mixes the lithium carbonate and the compound containing
the respective metallic elements other than Li in the
above-described Formula (1) so as to be the metallic constituent
ratio of the composition formula of the above-described Formula (1)
to obtain a mixture. The firing step S2 fires the mixture obtained
at the mixing step Si to obtain the lithium composite compound
expressed by Formula (1).
[0052] The mixing step S1 can employ a compound containing the
metallic elements other than Li in the above-described Formula (1),
for example, a Ni-containing compound, a Co-containing compound, an
Mn-containing compound, an Al-containing compound, and an
M2-containing compound as starting materials of the positive
electrode active material, in addition to the lithium carbonate.
The M2-containing compound is a compound containing at least one
kind of a metallic element selected from the group consisting of
Mg, Ti, Zr, Mo, and Nb.
[0053] At the mixing step S1, the starting material weighted at the
proportion so as to be the predetermined element composition
corresponding to the above-described Formula (1) is mixed to
prepare raw material powder. The method for producing the positive
electrode active material of this embodiment uses the lithium
carbonate as the starting material containing Li. Compared with
other Li-containing compounds, such as lithium acetate, lithium
nitrate, lithium hydroxide, lithium chloride, and lithium sulfate,
the lithium carbonate is excellent in stability in supply, a low
cost, and weak alkali; therefore, damage to manufacturing equipment
is little and industrial utilization and usefulness are
excellent.
[0054] As the Ni-containing compound, the Co-containing compound,
the Mn-containing compound, and the Al-containing compound as the
starting materials of the positive electrode active material, for
example, oxide, hydroxide, carbonate, hydrosulfate, or acetate is
available, and the use of oxide, hydroxide, or carbonate is
especially preferable. As the M2-containing compound, for example,
acetate, nitrate, carbonate, hydrosulfate, oxide, or hydroxide is
available, and the use of carbonate, oxide, or hydroxide is
especially preferable.
[0055] At the mixing step S1, the starting material is preferably
pulverized by, for example, a pulverizer and mixed. This ensures
preparing a uniformly-mixed powdery solid mixture. The starting
material is preferably pulverized to have the average grain
diameter of 0.3 .mu.m or less, and further preferably pulverized to
have the average grain diameter of 0.15 .mu.m or less. As the
pulverizer, which pulverizes the compound of the starting material,
a general precision pulverizer such as a ball mill, a jet mill, and
a sand mill is available. A granulation step can employ, for
example, a spray drying method. Various kinds of methods such as a
two-fluid or a four-fluid nozzle and a disk type are employable as
a spray method in the spray drying method.
[0056] The firing step S2 is a step of obtaining the lithium
composite compound expressed by the above-described Formula (1)
through firing of the mixture obtained at the mixing step Si and
includes a first precursor forming step S21, a second precursor
forming step S22, and a finishing heat treatment step S23.
[0057] The first precursor forming step S21 obtains a first
precursor through a heat treatment of the mixture at a heat
treatment temperature of 200.degree. C. or more and 400.degree. C.
or less for 0.5 hours or more and 5 hours or less. The main purpose
of the first precursor forming step S21 is to remove a vaporized
component such as water content, which inhibits a synthesis
reaction of the positive electrode active material, from the
mixture obtained at the mixing step S1. That is, the first
precursor forming step S21 is a heat treatment step for dehydration
that removes the water content in the mixture.
[0058] At the first precursor forming step S21, the vaporized
component contained in the heat-treated mixture such as water
content, impurities, and a volatile component in association with a
pyrolysis, for example, evaporates, burns, and volatilizes, and gas
is generated. Since the heat-treated mixture contains carbonate
such as the lithium carbonate, carbonic acid gas in association
with a pyrolysis of the carbonate is also generated in the first
precursor forming step S21.
[0059] At the first precursor forming step S21, the heat treatment
temperature of less than 200.degree. C. possibly results in
insufficient burning reaction of the impurities and insufficient
pyrolysis reaction of the starting material. At the first precursor
forming step S21, the heat treatment temperature in excess of
400.degree. C. possibly forms the layered structure of the lithium
composite compound under an atmosphere containing the gas generated
from the mixture by the heat treatment. Accordingly, the heat
treatment of the mixture at the heat treatment temperature of
200.degree. C. or more and 400.degree. C. or less in the first
precursor forming step S21 ensures sufficiently removing the
vaporized component such as the water content and obtaining the
first precursor in which the layered structure has not been formed
yet.
[0060] As long as the heat treatment temperature is in the range of
250.degree. C. or more and 400.degree. C. or less and more
preferably 250.degree. C. or more and 380.degree. C. or less in the
first precursor forming step S21, the removal effect of the
vaporized component such as the water content and the effect of
reducing the formation of the layered structure can be further
improved. A heat treatment period in the first precursor forming
step S21 can be appropriately changed according to, for example,
the heat treatment temperature, a degree of removal of the
vaporized component, and a degree of reduction of the formation of
the layered structure.
[0061] The first precursor forming step S21 preferably performs the
heat treatment in an air current of atmosphere gas and under
exhaust with a pump in order to exhaust the gas generated from the
heat-treated mixture. A flow rate of the atmosphere gas per minute
or an amount of exhaust per minute with the pump is preferably
larger than the volume of the gas generated from the mixture. The
volume of the gas generated from the heat-treated mixture in the
first precursor forming step S21 can be calculated based on, for
example, a ratio of the vaporized component to the mass of the
starting material contained in the mixture.
[0062] The first precursor forming step S21 may be performed under
a reduced pressure at an atmospheric pressure or less. Since the
main purpose of the first precursor forming step S21 is not the
oxidation reaction, an oxidizing atmosphere in the first precursor
forming step S21 may be the atmosphere. The use of the atmosphere
as the oxidizing atmosphere in the first precursor forming step S21
allows simplifying a configuration of a heat treatment apparatus,
facilitating the supply of the atmosphere, improving the
productivity of the positive electrode active materials, and
lowering the production cost. The atmosphere for the heat treatment
in the first precursor forming step S21 is not limited to the
oxidizing atmosphere and may be a non-oxidizing atmosphere such as
inert gas.
[0063] While the firing step S2 performs the second precursor
forming step S22 after the termination of the first precursor
forming step S21, the oxidizing atmosphere used at the first
precursor forming step S21 may be exhausted after the termination
of the first precursor forming step S21, and the second precursor
forming step may be performed by introducing a new oxidizing
atmosphere. Thus performing the gas replacement allows preventing
the gas generated from the mixture of the starting material by the
heat treatment at the first precursor forming step S21 from
affecting the second precursor forming step S22. The first
precursor may be once taken out from the heat treatment apparatus
after the first precursor forming step S21, and the first precursor
may be put into the heat treatment apparatus again. In the case
where the exhaust is performed at the heat treatment or after the
heat treatment in the first precursor forming step S21, the first
precursor forming step S21 and the second precursor forming step
S22 may be consecutively performed.
[0064] The second precursor forming step S22 performs the heat
treatment on the first precursor obtained at the first precursor
forming step S21 at the heat treatment temperature of 450.degree.
C. or more and 800.degree. C. or less for 0.5 hours or more and 50
hours or less under the oxidizing atmosphere and causes 92 mass %
or more of the lithium carbonate to react, thus obtaining a second
precursor. The main purpose of the second precursor forming step
S22 is to transform the lithium carbonate in the first precursor
into lithium oxide, cause the lithium carbonate to react to
transition metal, synthesize a compound having a layered structure
expressed by a composition formula LiM'O.sub.2, and remove a
carbonic acid component. That is, the second precursor forming step
S22 is a heat treatment step that removes the carbonic acid
component in the first precursor.
[0065] To develop the positive electrode active material with the
high Ni concentration in which the range of x indicative of the
content ratio of Ni in the above-described Formula (1) is larger
than 0.7 and 0.9 or less into the high capacity, the valence of Ni
needs to be oxidized from a bivalence to a trivalent in the firing
step S2. The bivalent Ni easily occupies the Li positions in the
layered structure LiM'O.sub.2, causing a decrease in the capacity
of the positive electrode active material. Therefore, the firing
step S2 fires the mixture obtained at the mixing step Si under the
oxidizing atmosphere to change an oxidation number of Ni from
Ni.sup.2+ to Ni.sup.3+. The carbonic acid gas inhibits the progress
of the reaction of the above-described Formula (2), casing the low
capacity of the positive electrode active material. Therefore, the
firing step S2 preferably performs the firing under the atmosphere
not containing the carbonic acid gas as much as possible.
[0066] To promote the Ni oxidation reaction at the finishing heat
treatment step S23, the second precursor forming step S22 degrades
the lithium carbonate as the main carbonic acid gas source to lower
an amount of generated carbonic acid gas in the finishing heat
treatment step S23 as much as possible. To promote the reaction of
the above-described Formula (2), the atmosphere for the heat
treatment in the second precursor forming step S22 is an oxidizing
atmosphere containing oxygen and the oxygen concentration is
preferably 80% or more, the oxygen concentration of 90% or more is
more preferable, the oxygen concentration of 95% or more is further
preferable, and the oxygen concentration of 100% is yet further
preferable. The carbonic acid gas concentration under the
atmosphere for the heat treatment in the second precursor forming
step S22 is preferably 5% or less, 1% or less is more preferable,
and 0.1% or less is further preferable. For successive progress of
the reaction of the above-described Formula (2), consecutively
supplying the oxygen at the heat treatment in the second precursor
forming step S22 is preferable, and performing the heat treatment
in the air current of the oxidizing atmosphere gas is
preferable.
[0067] To smoothly progress the Ni oxidation reaction in the second
precursor in the finishing heat treatment step S23, the second
precursor forming step S22 needs to sufficiently lower a residue
derived from the starting material. Accordingly, the second
precursor forming step S22 reacts 92 mass % or more of the lithium
carbonate contained in the mixture weighed and mixed so as to have
the metallic constituent ratio of the composition formula of the
above-described Formula (1). When the second precursor forming step
S22 reacts 92 mass % or more of the lithium carbonate contained in
the mixture, the amount of generated carbonic acid gas in the
finishing heat treatment step S23 can be lowered, and the reaction
of the above-described Formula (2) and the oxidation reaction of Ni
can be promoted.
[0068] Furthermore, when the second precursor forming step S22
reacts 92 mass % or more of the lithium carbonate contained in the
mixture, an amount of liquid phase of the lithium carbonate being
melted and becoming the liquid phase is lowered in the finishing
heat treatment step S23, and a growth of crystal grains is reduced,
making the high temperature firing possible. Performing the
finishing heat treatment step S23 at a higher temperature promotes
the Ni oxidation reaction; therefore, the lithium compound
remaining on the surface can be reduced, and the lithium ions in
the layered structure are stabilized. Consequently, the positive
electrode active material having the satisfactory charge/discharge
cycle characteristics is obtained. The second precursor forming
step S22 preferably reacts 97 mass % or more of the lithium
carbonate contained in the mixture. When the second precursor
forming step S22 reacts 97 mass % or more of the lithium carbonate
contained in the mixture, the amount of generated carbonic acid gas
can be further lowered in the finishing heat treatment step S23 and
the positive electrode active material having the more satisfactory
charge/discharge cycle characteristics can be obtained.
[0069] In the case where lithium salt other than the lithium
carbonate is used as a part of the starting material of the lithium
contained in the positive electrode active material, a proportion
of the lithium present as the lithium carbonate is preferably less
than 7 mole % among the lithium components in the second precursor.
This allows the amount of generated carbonic acid gas to be lowered
in the finishing heat treatment step S23, and the reaction of the
above-described Formula (2) and the oxidation reaction of Ni can be
promoted. Additionally, in this case, the amount of liquid phase of
the lithium carbonate is lowered and the growth of the crystal
grains is reduced, making the high temperature firing possible in
the finishing heat treatment step S23. As described above, the
positive electrode active material having the satisfactory
charge/discharge cycle characteristics can be obtained.
[0070] In the case where lithium salt other than the lithium
carbonate is used as a part of the starting material of the lithium
contained in the positive electrode active material, a proportion
of the lithium present as the lithium carbonate is more preferably
less than 3 mole % among the lithium components in the second
precursor. Accordingly, the amount of generated carbonic acid gas
can be further lowered in the finishing heat treatment step S23 and
the positive electrode active material having the more satisfactory
charge/discharge cycle characteristics can be obtained.
[0071] The heat treatment temperature in the second precursor
forming step S22 of less than 450.degree. C. results in
considerably slow progress of the formation reaction of the layered
structure and excessively remaining lithium carbonate while the
first precursor is heat-treated to form the second precursor having
the layered structure. Meanwhile, the heat treatment temperature in
the second precursor forming step S22 in excess of 800.degree. C.
excessively progresses the grain growth, failing to obtain the
high-capacity positive electrode active material. Accordingly, the
heat treatment temperature in the second precursor forming step S22
is preferably higher than 550.degree. C., more preferably
600.degree. C. or more and 700.degree. C. or less, and further
preferably a high temperature, 650.degree. C. or more and
680.degree. C. or less. Since the reaction of 92 mass % or more of
the lithium carbonate and preferably 97 mass % or more of the
lithium carbonate can be further promoted, the heat treatment
temperature and/or the ratio of Mn in the second precursor forming
step S22 is preferably set to high. Specifically, setting the heat
treatment temperature in the second precursor forming step S22 to
be higher than 550.degree. C. ensures the further promoted reaction
of the lithium carbonate. In the case where M1 in the
above-described Formula (1) is Mn and 1-x-y-z is larger than 0 and
smaller than 0.075, the heat treatment temperature is preferably
600.degree. C. or more, and in the case where 1-x-y-z is 0.075 or
more, the heat treatment temperature is preferably higher than
550.degree. C. The high ratio of Mn allows decreasing the average
valence of Ni in LiM'O.sub.2. Even when the oxidation reaction of
Ni does not sufficiently progress, the reaction indicated by the
above-described Formula (2) progresses and the reaction temperature
lowers; therefore, the reaction of the lithium carbonate in the
second precursor forming step S22 is promoted. Therefore, in the
case where M1 in the above-described Formula (1) is Mn and 1-x-y-z
is larger than 0 and smaller than 0.075, the heat treatment
temperature is set to 600.degree. C. or more, and in the case where
1-x-y-z is 0.075 or more, the heat treatment temperature is set to
be higher than 550.degree. C. This ensures the reaction of 92 mass
% or more of the lithium carbonate contained in the mixture and
therefore is preferable. Meanwhile, setting the heat treatment
temperature in the second precursor forming step S22 to be
700.degree. C. or less ensures reducing generation of the liquid
phase and further improving the reduction effect of the growth of
the crystal grains.
[0072] To fully react the first precursor to the oxygen in the
temperature range of the heat treatment in the second precursor
forming step S22, the period of the heat treatment can be set to
0.5 hours or more and 50 hours or less. From an aspect of promoting
the reaction of the lithium carbonate, the period of the heat
treatment in the second precursor forming step S22 is preferably
two hours or more and 50 hours or less. From an aspect of improving
the productivity, the period of the heat treatment in the second
precursor forming step S22 is more preferably two hours or more and
15 hours or less.
[0073] While the firing step S2 performs the finishing heat
treatment step S23 after the termination of the second precursor
forming step S22, the oxidizing atmosphere used at the second
precursor forming step S22 may be exhausted after the termination
of the second precursor forming step S22, and the finishing heat
treatment step S23 may be performed by introducing a new oxidizing
atmosphere.
[0074] This allows preventing the gas generated by the heat
treatment in the second precursor forming step S22 from affecting
the finishing heat treatment step S23. The second precursor may be
once taken out from the heat treatment apparatus after the
termination of the second precursor forming step S22, and the
second precursor may be put into the heat treatment apparatus
again. In the case where the exhaust is performed at the heat
treatment or after the heat treatment in the second precursor
forming step S22, the second precursor forming step S22 and the
finishing heat treatment step S23 may be consecutively performed.
The second precursor forming step can use, for example, a
continuous conveyance furnace and a rotary kiln.
[0075] The finishing heat treatment step S23 performs the heat
treatment on the second precursor obtained in the second precursor
forming step S22 at the heat treatment temperature of 755.degree.
C. or more and 900.degree. C. or less for 0.5 hours or more and 50
hours or less under the oxidizing atmosphere, thus obtaining the
lithium composite compound. The lithium composite compound obtained
in this finishing heat treatment step S23 constitutes the positive
electrode active material of this embodiment. The main purpose of
the finishing heat treatment step S23 is to grow the crystal grains
to fully progress the Ni oxidation reaction, which oxidizes Ni in
the second precursor obtained at the second precursor forming step
S22 from the bivalence to the trivalent, and to develop electrode
performance of the lithium composite compound obtained by the heat
treatment. That is, the finishing heat treatment step S23 is a heat
treatment step that performs the Ni oxidation reaction in the
second precursor and the crystal grain growth.
[0076] To fully progress the Ni oxidation reaction in the second
precursor in the finishing heat treatment step S23, the atmosphere
for the heat treatment in the finishing heat treatment step S23 is
an oxidizing atmosphere containing oxygen.
[0077] In the oxidizing atmosphere in the finishing heat treatment
step 23, the oxygen concentration is preferably 80% or more, the
oxygen concentration of 90% or more is more preferable, the oxygen
concentration of 95% or more is further preferable, and the oxygen
concentration of 100% is yet further preferable. The carbonic acid
gas concentration under the atmosphere for the heat treatment in
the second precursor forming step S22is preferably 5% or less, 1%
or less is more preferable, and 0.1% or less is further
preferable.
[0078] The heat treatment temperature in the finishing heat
treatment step S23 of less than 755.degree. C. possibly makes the
progress of the crystallization of the second precursor difficult.
The heat treatment temperature in excess of 900.degree. C. fails to
reduce the degradation of the layered structure in the second
precursor and generates the bivalent Ni, thereby lowering the
capacity of the obtained lithium composite compound. Accordingly,
setting the heat treatment temperature in the finishing heat
treatment step S23 to 755.degree. C. or more and 900.degree. C. or
less promotes the grain growth of the second precursor.
Additionally, the degradation of the layered structure is reduced
and the capacity of the obtained lithium composite compound can be
improved. When the heat treatment temperature in the finishing heat
treatment step S23 is set to be higher than 800.degree. C.,
preferably 840.degree. C. or more and 890.degree. C. or less, and
more preferably in excess of 850.degree. C. and 890.degree. C. or
less, the promotion effect of the grain growth and the degradation
reduction effect of the layered structure can be further
improved.
[0079] A low oxygen partial pressure in the finishing heat
treatment step S23 promotes the Ni oxidation reaction and therefore
heat is required. Accordingly, in the case where the oxygen supply
to the second precursor is insufficient in the finishing heat
treatment step S23, the heat treatment temperature needs to be
increased. The increase in the heat treatment temperature cannot
avoid the degradation of the layered structure in the obtained
lithium composite compound, failing to obtain the satisfactory
electrode property of the positive electrode active material.
Accordingly, to fully supply the oxygen to the second precursor in
the finishing heat treatment step S23, the period of the heat
treatment in the finishing heat treatment step S23 can be set to
0.5 hours or more and 50 hours or less. From an aspect of improving
the productivity of the positive electrode active material, the
period of the heat treatment in the finishing heat treatment step
S23 is preferably 0.5 hours or more and 15 hours or less.
[0080] From an aspect of the production process, the obtained
lithium composite compound is preferably not washed with water
after the finishing heat treatment step S23. While performing the
water washing allows decreasing the lithium compound remaining on
the surface, the water possibly extracts the lithium in the
positive electrode active material other than the lithium remaining
on the positive electrode active material surface, leading to
deterioration of the property of the positive electrode active
material. The water content remained after the water washing
degenerates a binder during the positive electrode production and
possibly causes the poor coating. Entering the remaining water
content in the battery causes the water content to react to the
electrolyte. This generates hydrogen fluoride and possibly
deteriorates the battery property.
[0081] However, it has been found that, from an aspect of
performance improvement in the positive electrode active material,
the water washing of the obtained lithium composite compound after
the finishing heat treatment step S23 is advantageous. That is, it
has been found out that providing the step of water washing after
the finishing heat treatment step S23 improves the compression
property of the powder of the positive electrode active material.
This will be described later.
[0082] As described above, in the method for producing the positive
electrode active material of this embodiment, the firing step S2,
which fires the mixture obtained at the mixing step S21, includes
the first precursor forming step S21, the second precursor forming
step S22, and the finishing heat treatment step S23. This ensures
obtaining the first precursor in which the vaporized component,
mainly such as the water content, has been removed from the mixture
in the first precursor forming step S21. Then, the second precursor
forming step S22 performs the heat treatment on the first precursor
to fully generate the carbonic acid gas, and reacts 92 mass% or
more of the lithium carbonate in the second precursor, thereby
ensuring obtaining the second precursor in which the generation of
the carbonic acid gas by heating is reduced.
[0083] The reaction of 92 mass % or more of the lithium carbonate
in the second precursor allows the high heat treatment temperature
in the finishing heat treatment step S23. Consequently, the
oxidation reaction of Ni is promoted, the oxidation number of Ni
changes from Ni.sup.2+ to Ni.sup.3+, the lithium compound remaining
on the surface of the positive electrode active material can be
lowered, and the lithium ions in the layered structure are
stabilized. Therefore, the positive electrode active material
having the satisfactory charge/discharge cycle characteristics can
be obtained.
[0084] Furthermore, the high heat treatment temperature becomes
possible in the finishing heat treatment step S23, and the
generation of the carbonic acid gas from the second precursor is
reduced. Accordingly, the low oxygen partial pressure under the
oxidizing atmosphere is suppressed, the large amount of Ni
oxidation reaction in the second precursor uniformly progresses,
and the growth of the crystal grains progresses. Accordingly, the
method for producing the positive electrode active material of this
embodiment can decrease the bivalent Ni remaining in the lithium
composite compound with the high Ni concentration having the
layered structure, change the bivalent Ni to the trivalent Ni, and
obtain the high-capacity positive electrode active material
excellent in a capacity retention rate.
[0085] The method for producing the positive electrode active
material of this embodiment brings the significant effect when the
weight of the produced positive electrode active material becomes a
large amount, for example, several hundred g or more. This is
because, when the weight of the produced positive electrode active
material is several g, the influence from the gas generated from
the starting material in the firing step S2 is small; however, when
the positive electrode active materials are mass-produced in an
industrial scale, the volume of the gas generated from the starting
material in the firing step S2 increases, and this is likely to
lower the oxygen partial pressure under the oxidizing
atmosphere.
[0086] When the first precursor forming step S21 is omitted in the
firing step S2, the oxygen partial pressure lowers in the second
precursor forming step S22 and the finishing heat treatment step
S23. As a result, in order to sufficiently progress the formation
reaction of the layered structure of the lithium composite
compound, which involves the oxidation of Ni, the finishing heat
treatment would need to be performed at a higher temperature, and
therefore the preferable temperature range would be exceeded.
Additionally, when the second precursor forming step S22 is
omitted, the grain growth in the lithium composite compound
progresses with the insufficient oxidation reaction of Ni and
therefore the omission is not preferable. Further, omitting the
finishing heat treatment step S23 fails to obtain the appropriate
electrode property.
(Positive Electrode and Lithium Ion Secondary Battery)
[0087] The following describes a positive electrode for a
non-aqueous secondary battery using the positive electrode active
material produced by the above-described method for producing the
positive electrode active material and a configuration of the
non-aqueous secondary battery that includes the positive electrode.
FIG. 2 is a schematic partial cross-sectional view of a positive
electrode 111 of this embodiment and a non-aqueous secondary
battery 100 including the positive electrode 111.
[0088] The non-aqueous secondary battery 100 of this embodiment is,
for example, a cylindrical lithium ion secondary battery. The
non-aqueous secondary battery 100 includes a cylindrical battery
can 101 with a closed bottom that houses non-aqueous electrolyte, a
wound electrode group 110 housed in the battery can 101, and a
circular plate-shaped battery lid 102 that seals an upper opening
of the battery can 101. The battery can 101 and the battery lid 102
are, for example, manufactured of a metallic material such as
stainless steel and aluminum. The battery lid 102 is fixed to the
battery can 101 via a sealing material 106 made of a resin material
having an insulating property by a crimping or a similar method.
This seals the battery can 101 with the battery lid 102, and the
battery can 101 and the battery lid 102 are mutually electrically
insulated. The shape of the non-aqueous secondary battery 100 is
not limited to the cylindrical shape, and any other shapes such as
a square shape, a button shape, and a laminated sheet shape are
employable.
[0089] The wound electrode group 110 is manufactured by winding a
long strip-shaped positive electrode 111 and negative electrode
112, which are opposed via a long strip-shaped separator 113,
around the winding center axis. In the wound electrode group 110, a
positive electrode current collector 111a is electrically connected
to the battery lid 102 via a positive electrode lead piece 103, and
a negative electrode current collector 112a is electrically
connected to a bottom portion of the battery can 101 via a negative
electrode lead piece 104. An insulating plate 105 that prevents
short-circuit is located between the wound electrode group 110 and
the battery lid 102 and between the wound electrode group 110 and
the bottom portion of the battery can 101. The positive electrode
lead piece 103 and the negative electrode lead piece 104 are
members for current extraction manufactured of materials similar to
those of the positive electrode current collector 111a and the
negative electrode current collector 112a, respectively, and are
joined to the positive electrode current collector 111a and the
negative electrode current collector 112a by a spot welding, an
ultrasonic pressure welding, or a similar method, respectively.
[0090] The positive electrode 111 of this embodiment includes the
positive electrode current collector 111a and a positive electrode
mixture layer 111b, which is formed on the surface of the positive
electrode current collector 111a. As the positive electrode current
collector 111a, for example, a metal foil, an expanded metal, and a
perforated metal made of, for example, aluminum or an aluminum
alloy are available. The metal foil can be configured to have a
thickness of, for example, around 15 .mu.m or more and 25 .mu.m or
less. The positive electrode mixture layer 111b contains the
positive electrode active material produced by the above-described
method for producing the positive electrode active material. The
positive electrode mixture layer 111b may contain a conductive
material, a binder, or a similar material.
[0091] The negative electrode 112 includes the negative electrode
current collector 112a and a negative electrode mixture layer 112b,
which is formed on the surface of the negative electrode current
collector 112a. As the negative electrode current collector 112a,
for example, a metal foil, an expanded metal, and a perforated
metal made of, for example, copper or a copper alloy or nickel or a
nickel alloy are available. The metal foil can be configured to
have a thickness of, for example, around 7 .mu.m or more and 10
.mu.m or less. The negative electrode mixture layer 112b contains a
negative electrode active material used for the general lithium ion
secondary batteries. The negative electrode mixture layer 112b may
contain a conductive material, a binder, or a similar material.
[0092] As the negative electrode active material, for example, one
kind or more of materials such as a carbon material, a metallic
material, and a metal oxide material are available. As the carbon
material, for example, graphites such as natural graphite and
artificial graphite, carbides such as coke and pitch, amorphous
carbon, and carbon fiber are available. As the metallic material,
lithium, silicon, tin, aluminum, indium, gallium, magnesium, and an
alloy of these substances, and as the metal oxide material, metal
oxide containing, for example, tin, silicon, lithium, and titanium
are available.
[0093] As the separator 113, for example, a polyolefin-based resin
such as polyethylene, polypropylene, and polyethylene-polypropylene
copolymer, a microporous film such as polyamide resin and aramid
resin, and nonwoven fabric are available.
[0094] The positive electrode 111 and the negative electrode 112
can be produced through, for example, a mixture preparing step, a
mixture coating step, and a molding step. The mixture preparing
step uses, for example, stirring means such as a planetary mixer, a
dispersion mixer, and a rotating and revolving mixer to stir and
homogenize a positive electrode active material or a negative
electrode active material together with solution containing, for
example, a conductive material and a binder to prepare mixture
slurry.
[0095] As the conductive material, a conductive material used for
the general lithium ion secondary batteries is available.
Specifically, for example, carbon particles such as graphite
powder, acetylene black, furnace black, thermal black, and channel
black, and carbon fiber are available as the conductive material.
The conductive material by the amount of, for example, around 3
mass % or more and 10 mass % or less with respect to the entire
mass of the mixture is available.
[0096] As the binder, a binder used for the general lithium ion
secondary batteries is available. Specifically, for example,
polyvinylidene fluoride (PVDF), polytetrafluoroethylene, poly
hexafluoropropylene, styrene-butadiene rubber, carboxymethyl
cellulose, polyacrylonitrile, and modified polyacrylonitrile are
available as the binder. The binder by the amount of, for example,
around 2 mass % or more and 10 mass % or less with respect to the
entire mass of the mixture is available. The mixing ratio of the
negative electrode active material to the binder is desirable to
be, for example, 95:5 by the weight ratio.
[0097] A solvent of the solution is selectable from, for example,
N-methyl pyrrolidone, water, N,N-dimethylformamide,
N,N-dimethylacetamide, methanol, ethanol, propanol, isopropanol,
ethylene glycol, diethylene glycol, glycerin, dimethylsulfoxide,
and tetrahydrofuran according to the kind of the binder.
[0098] The mixture coating step first applies the mixture slurry
containing the positive electrode active material and the mixture
slurry containing the negative electrode active material prepared
at the mixture preparing step over the surfaces of the positive
electrode current collector 111a and the negative electrode current
collector 112a, respectively, by coating means such as a bar
coater, a doctor blade, and a roll transfer machine. Next, the
respective positive electrode current collector 111a and negative
electrode current collector 112a over which the mixture slurries
have been applied are heat-treated to volatile or vaporize the
solvents in the solutions contained in the mixture slurries to
remove the solvents. Thus, the positive electrode mixture layer
111b and the negative electrode mixture layer 112b are formed on
the surfaces of the positive electrode current collector 111a and
the negative electrode current collector 112a, respectively.
[0099] The molding step first performs compression molding on the
respective positive electrode mixture layer 111b, which is formed
on the surface of the positive electrode current collector 111a,
and negative electrode mixture layer 112b, which is formed on the
surface of the negative electrode current collector 112a, for
example, using pressurizing means such as a roll press. This
ensures configuring the positive electrode mixture layer 111b so as
to have a thickness around, for example, 100 .mu.m or more and 300
.mu.m or less, and the negative electrode mixture layer 112b so as
to have a thickness around, for example, 20 .mu.m or more and 150
.mu.m or less. Afterwards, the positive electrode current collector
111a and the positive electrode mixture layer 111b, and the
negative electrode current collector 112a and the negative
electrode mixture layer 112b are each cut out into the long strip
shape, thus ensuring producing the positive electrode 111 and the
negative electrode 112.
[0100] Here, as described above, in the case where the step of the
water washing of the obtained lithium composite compound is
provided after the finishing heat treatment step S23 in the
production step of the positive electrode active material,
compressibility of the positive electrode mixture layer 111b, which
is formed on the surface of the positive electrode current
collector 111a, can be improved. More specifically, water-washing
and drying the lithium composite compound allows modifying the
surface of the lithium composite compound and improving the
compressibility of the positive electrode active material. This
allows improving the density of the positive electrode mixture
layer 111b and increasing electric energy accumulated per unit
volume.
[0101] The following describes a water washing/drying step that
performs the water washing and the drying of the lithium composite
compound in more details.
[0102] The water washing/drying step immerses the lithium composite
compound into pure water, removes the liquid by a solid-liquid
separation, and dries the remaining solid material. These
substances may be stirred when the lithium composite compound is
immersed into the pure water. Adding the pure water such that the
solid content percentage falls within the range of 33 mass % to 77
mass % is preferable for such water washing. The solid content
percentage higher than 77 mass % makes the uniform water washing
difficult. The solid content percentage lower than 33 mass %
results in an excessive decreased amount of the lithium in the
positive electrode active material, possibly deteriorating the
property of the positive electrode active material. Since the
effect appears in an extremely short period, the immersion period
during which the lithium composite compound is immersed into the
pure water is preferably within 20 minutes and more preferably
within ten minutes. The immersion period in excess of 20 minutes
decreases the lithium in the positive electrode active material,
possibly deteriorating the property of the positive electrode
active material.
[0103] As the solid-liquid separation, various kinds of methods
such as a filtration under reduced pressure, a filtration under
pressure, a filter press, a roller press, and a centrifuge are
available. A moisture percentage of the lithium composite compound
after the solid-liquid separation is preferably 20 mass % or less
and 10 mass % or less is more preferable. The moisture percentage
of the lithium composite compound in excess of 20 mass % increases
an amount of reprecipitation of the lithium compound dissolved into
the water content on the positive electrode active material and
increases the water content remaining after the drying. This loses
coatability when the positive electrode active material is applied
over the positive electrode current collector and worsens the
battery property, and therefore is not preferable. The moisture
percentage of the lithium composite compound after the solid-liquid
separation can be measured with, for example, an infrared moisture
meter.
[0104] In the drying step of the lithium composite compound after
the water washing, the atmosphere in the drying step is selectable
from, for example, in the air in which partial pressures of the
water vapor and carbon dioxide are lowered, in nitrogen, in oxygen,
or in vacuum. The drying step is especially preferable to be
performed in the vacuum. A drying temperature in the drying step is
preferably 150.degree. C. or more and 300.degree. C. or less and
more preferably 190.degree. C. or more and 250.degree. C. or less.
The drying temperature in the drying step of lower than 150.degree.
C. makes it difficult to fully remove the water content, and the
drying temperature higher than 300.degree. C. makes a side reaction
that worsens the property of the positive electrode active material
remarkable.
[0105] The drying step preferably dividedly dries the lithium
composite compound after the water washing twice or more. For
example, before the temperature of the lithium composite compound
after the water washing is increased to 150.degree. C. or more,
removing the most water content at a temperature around 60.degree.
C. to 100.degree. C. is preferable. By removing the most water
content at the low temperature, the side reaction in the subsequent
drying at a high temperature is lowered, ensuring reducing a
negative effect to the property of the lithium composite compound.
The moisture percentage of the lithium composite compound after the
drying is preferably 400 ppm or less, more preferably 300 ppm or
less, and further preferably 250 ppm or less. The moisture
percentage of the lithium composite compound after the drying can
be measured by a Karl Fischer's method.
[0106] The positive electrode 111 and the negative electrode 112
produced as described above are opposed via the separator 113 and
wound around the winding center axis to be the wound electrode
group 110. In the wound electrode group 110, the negative electrode
current collector 112a is coupled to the bottom portion of the
battery can 101 via the negative electrode lead piece 104, the
positive electrode current collector 111a is coupled to the battery
lid 102 via the positive electrode lead piece 103, the insulating
plate 105 or a similar member prevents the battery can 101 and the
battery lid 102 from short-circuiting, and the wound electrode
group 110 is housed in the battery can 101. Afterwards, the
non-aqueous electrolyte is injected to the battery can 101, the
battery lid 102 is fixed to the battery can 101 via the sealing
material 106, and the battery can 101 is sealed, thus ensuring
producing the non-aqueous secondary battery 100.
[0107] As the electrolyte injected to the battery can 101, the use
of electrolyte produced by dissolving lithium hexafluorophosphate
(LiPF.sub.6), lithium tetrafluoroborate (LiBF.sub.4), lithium
perchlorate (LiCLO.sub.4), or a similar substance as the
electrolyte into a solvent such as diethyl carbonate (DEC),
dimethyl carbonate (DMC), ethylene carbonate (EC), propylene
carbonate (PC), vinylene carbonate (VC), methyl acetate (MA), ethyl
methyl carbonate (EMC), and methyl propyl carbonate (MPC) is
desirable. The electrolyte desirably has the concentration of 0.7 M
or more and 1.5 M or less. A compound having a carboxylic acid
anhydride group, a compound containing elemental sulfur such as
propanesultone, and a compound containing boron may be mixed with
these electrolytes. The objects to add these compounds are to
reduce reductive decomposition of the electrolyte on the surface of
the negative electrode, prevent reduction precipitation at the
negative electrode of the metallic element such as manganese eluted
from the positive electrode, improve an ion conductive property of
the electrolyte, provide incombustibility of the electrolyte, and a
similar object; therefore, the compound only needs to be
appropriately selected according to the object.
[0108] The non-aqueous secondary battery 100 having the
above-described configuration includes the battery lid 102 as a
positive electrode external terminal and the bottom portion of the
battery can 101 as a negative electrode external terminal. The
non-aqueous secondary battery 100 can accumulate electric power
supplied from the outside at the wound electrode group 110 and the
electric power accumulated to the wound electrode group 110 can be
supplied to an external device or a similar device. Thus, the
non-aqueous secondary battery 100 of this embodiment is, for
example, available as a small power supply for, for example, a
mobile electronic device and household electrical equipment, a
stationary power supply for, for example, an uninterruptible power
supply and a power leveling device, and a driving power supply for,
for example, a ship, a railway, a hybrid vehicle, and an electric
vehicle.
[0109] The following describes Working Examples based on the method
for producing the positive electrode active material, the positive
electrode active material, and the lithium ion secondary battery of
the present invention and Comparative Examples different from the
method for producing the positive electrode active material, the
positive electrode active material, and the lithium ion secondary
battery of the present invention.
Working Example 1
(Mixing Step)
[0110] First, lithium carbonate, nickel hydroxide, cobalt
carbonate, and manganese carbonate were prepared as the starting
materials of the positive electrode active material. Next, the
respective starting materials were weighted so as to meet:
Li:Ni:Co:Mn=1.04:0.80:0.10:0.10 by an atom ratio, pulverized by a
pulverizer, and a wet blending was performed to prepare slurry.
(Firing step: First Precursor Forming Step, Second Precursor
Forming Step)
[0111] Next, the slurry (mixture) obtained at the mixing step was
dried by a spray dryer, and the dried mixture was fired to obtain
fired powder. Specifically, the mixture of 300 g produced by drying
the slurry obtained at the mixing step was filled in an alumina
container with 300 mm in length, 300 mm in width, and 100 mm in
height. A heat treatment was performed on the mixture under an air
atmosphere at a heat treatment temperature of 350.degree. C. for
one hour in a continuous conveyance furnace (a first precursor
forming step). Next, a heat treatment was performed on the powder
(the first precursor) obtained in the first precursor forming step
in an oxygen air current at the heat treatment temperature of
575.degree. C. for ten hours in the continuous conveyance furnace
whose atmosphere was replaced by an atmosphere with an oxygen
concentration in the furnace of 99% or more (a second precursor
forming step).
(Measurement of Amount of Reacted Lithium Carbonate in Second
Precursor)
[0112] The amount of reacted lithium carbonate in the powder (the
second precursor) obtained in the second precursor forming step was
analyzed by a neutralization titration as follows. First, the
second precursor of 0.2 g was dispersed into pure water of 30 ml
bubbled with argon gas, and the pure water was stirred for 60
minutes. Afterwards, the pure water into which the second precursor
had been dispersed was suctioned and filtered to obtain filtrate.
The obtained filtrate was titrated with hydrochloric acid.
[0113] The titration curve goes through two stages, a curve up to a
first equivalence point indicates a total amount of hydroxide ion
in lithium hydroxide and carbonate ion in lithium carbonate, and a
curve from the first equivalence point to a second equivalence
point indicates an amount of carbonic acid hydrogen ion generated
from the carbonate ion. Therefore, the amount of lithium carbonate
was calculated from a titer from the first equivalence point to the
second equivalence point. The amount of the lithium hydroxide was
calculated from a difference between the titer up to the first
equivalence point and the titer from the first equivalence point to
the second equivalence point. Based on the calculated amount of
lithium carbonate, the amount of reacted lithium carbonate in the
second precursor was obtained by the following Formula (3).
{(Q.sub.0-Q.sub.2)/Q.sub.2}.times.100=Q.sub.R (3)
[0114] In the above-described Formula (3), Q.sub.0 indicates the
amount of lithium carbonate in the starting material, Q.sub.2
indicates the amount of lithium carbonate in the second precursor,
and Q.sub.R indicates the amount of reacted lithium carbonate in
the second precursor. Here, the amount of lithium carbonate Q.sub.0
in the starting material can be calculated, for example, from the
mixing ratio of the starting material.
(Firing Step: Finishing Heat Treatment Step)
[0115] Next, the heat treatment was performed on the second
precursor obtained in the second precursor forming step in
oxidation air current at the heat treatment temperature of
865.degree. C. for ten hours in the continuous conveyance furnace
whose atmosphere was replaced by the atmosphere with an oxygen
concentration in the furnace of 99% or more to obtain fired powder
(a lithium composite compound). The obtained fired powder was
classified by openings of 53 .mu.m or less to produce positive
electrode active materials. This Working Example did not perform
the water washing on the obtained lithium composite compound after
the finishing heat treatment step.
(Measurements of Amount of Remaining Lithium Hydroxide and Amount
of Remaining Lithium Carbonate in Positive Electrode Active
Material)
[0116] The amount of remaining lithium hydroxide and the amount of
remaining lithium carbonate on the positive electrode active
material of Working Example 1 obtained by the above-described steps
were analyzed by the neutralization titration as follows. First,
similarly to the measurement of the amount of reacted lithium
carbonate in the second precursor, the positive electrode active
material of 0.5 g was dispersed into pure water of 30 ml bubbled
with argon gas, and the pure water was stirred for 60 minutes.
Afterwards, the pure water into which the second precursor had been
dispersed and stirred was suctioned and filtered to obtain
filtrate. The obtained filtrate was titrated with hydrochloric
acid. Similarly to the amount of lithium hydroxide and the amount
of lithium carbonate in the second precursor, the amount of
remaining lithium hydroxide and the amount of remaining lithium
carbonate on the positive electrode active material were calculated
by a titration of the filtrate produced by immersing the positive
electrode active material into the pure water and stirring the pure
water for 60 minutes.
(Measurement of Amount of Dissolution of Lithium Hydroxide in
Positive Electrode Active Material)
[0117] The amount of dissolution of the lithium hydroxide in the
positive electrode active material was measured by the following
procedure. First, the positive electrode active material with the
solid content percentage of 1.6 mass % was immersed and stirred
into pure water for 30 minutes, and then an amount of lithium
hydroxide A was detected by a neutralization titration of the
filtrate. The identical positive electrode active material with the
solid content percentage of 1.6 mass % was immersed and stirred
into pure water for 120 minutes, and then an amount of lithium
hydroxide B was detected by the neutralization titration of the
filtrate. Then, an amount of dissolution (B-A), which is a
difference between the amount of lithium hydroxide A and the amount
of lithium hydroxide B, was obtained. The dissolution speed can be
obtained by dividing the amount of dissolution by the immersion
period.
(Measurements of Lattice Constant and Crystallite Diameter of
Positive Electrode Active Material)
[0118] The crystallite diameter of the positive electrode active
material was measured by the following procedure. First, the
crystalline structure of the positive electrode active material was
measured by X-ray diffraction (XRD) to obtain the lattice constant
of the positive electrode active material. The XRD measurement was
performed by a concentration method using an XRD measurement device
manufactured by Rigaku Corporation, RINT-2000. The CuKa line was
used for the X-ray, and the output was set to 48 kV and 28 mA.
[0119] With the measuring conditions, a step width was set to
0.02.degree., the measurement period per step was set to one
second, and the measurement result was smoothened by a
Savitzky-Golay method. Afterwards, the background and the
K.alpha..sub.2 line were removed to obtain a (003) peak at the time
and a half-value width .beta..sub.exp p of (104). Furthermore, a
half-value width .beta..sub.i when a standard Si sample (NIST
Standard Material 640d) was measured in the identical device and
under the identical conditions was obtained, and the half-value
width .beta. was defined by the following Formula (4).
[Expression 1]
.beta.= {square root over
(.beta..sub.exp.sup.2-.beta..sub.l.sup.2)} (4)
[0120] Using this half-value width .beta., the crystallite diameter
was obtained using the Scherrer formula expressed by the following
Formula (5).
[ Expression 2 ] D = K .lamda. .beta. cos .theta. ( 5 )
##EQU00001##
[0121] In the above-described Formula (5), .lamda. indicates the
wavelength of the X-ray, .theta. indicates the reflection angle,
and K indicates the Scherrer constant, and K=0.9 was met. Then, an
average value of the crystallite diameters at the (003) peak and
the (104) peak was set as the crystallite diameter of the positive
electrode active material of Working Example 1.
(Composition of Positive Electrode Active Material and Measurement
of Specific Surface Area)
[0122] Furthermore, the composition of the positive electrode
active material was analyzed by ICP using an ICP emission
spectrophotometer manufactured by PerkinElmer Inc., OPTIMA8300. As
the pretreatment, the positive electrode active material was
dissolved into aqua regia, and quantitative analysis was performed
on the dissolved solution by the ICP. Furthermore, the measurement
was performed using an automatic specific surface area measuring
apparatus BELCAT manufactured by BEL JAPAN, INC., and the specific
surface area of the positive electrode active material was
calculated by the BET method.
(Evaluation for Compressibility of Positive Electrode Active
Material)
[0123] The compressibility of the positive electrode active
material was evaluated by measurement with the autograph AGS-X
manufactured by Shimadzu Corporation. Positive electrode active
material powder of 0.5 g was filled in a mold with .phi.10 mm, and
the mold was installed to the autograph. The positive electrode
active material was measured at a crosshead speed of a speed of 1.0
mm/min and up to a test force of 4000 N to obtain a compression
curve of the powder from the test force at the time, the amount of
stroke, the cross-sectional area of the mold, and the weight of the
positive electrode active material. The compressibility was
evaluated from the molding density at the press pressure of 5 MPa
in this compression curve.
Working Example 2
[0124] Except that the heat treatment temperature in the second
precursor forming step was set to 600.degree. C. and the heat
treatment temperature in the finishing heat treatment step was set
to 842.degree. C., a positive electrode active material of Working
Example 2 was produced similarly to the positive electrode active
material of
[0125] Working Example 1, and the measurements were performed
similar to those of the positive electrode active material of
Working Example 1.
From Working Example 3 to Working Example 6
[0126] Except that the heat treatment temperature in the finishing
heat treatment step was set to 850.degree. C. for Working Example
3, 865.degree. C. for Working Example 4, 880.degree. C. for Working
Example 5, and 895.degree. C. for Working Example 6 in the
production steps of the positive electrode active materials, the
positive electrode active materials of Working Example 3 to Working
Example 6 were produced similarly to the positive electrode active
material of Working Example 2, and the measurements were performed
similar to those of the positive electrode active material of
Working Example 1. Additionally, when measured using a
microcompression testing machine (MCT-510 manufactured by Shimadzu
Corporation), the particle fracture strength was 66 MPa in Working
Example 3, 61 MPa in Working Example 4, 58 MPa in Working Example
5, and 57 MPa in Working Example 6. The particle fracture strength
with the microcompression testing machine was measured by sparging
the positive electrode active material on a pressure plate by a
minute amount and compressing the positive electrode active
material in units of one particle at the test force of 49 mN and
the load rate of 0.4747 mN/sec.
Working Example 7
[0127] In the mixing step, the respective starting materials were
weighted so as to meet Li:Ni:Co:Mn =1.04:0.80:0.125:0.075 by the
atom ratio. In the firing step, the heat treatment temperature in
the second precursor forming step was set to 600.degree. C. and the
heat treatment temperature in the finishing heat treatment step was
set to 850.degree. C. Except that, a positive electrode active
material of Working Example 7 was produced similarly to the
positive electrode active material of Working
[0128] Example 1 and the measurements were performed similar to
those of the positive electrode active material of Working Example
1.
Working Example 8
[0129] In the mixing step, the respective starting materials were
weighted so as to meet Li:Ni:Co:Mn =1.08:0.80:0.05:0.15 by the atom
ratio. In the firing step, the heat treatment temperature in the
second precursor forming step was set to 600.degree. C. and the
heat treatment temperature in the finishing heat treatment step was
set to 865.degree. C. Except that, a positive electrode active
material of Working Example 8 was produced similarly to the
positive electrode active material of Working Example 1 and the
measurements were performed similar to those of the positive
electrode active material of Working Example 1.
Working Example 9
[0130] In the mixing step, the respective starting materials were
weighted so as to meet Li:Ni:Co:Mn =1.04:0.80:0.15:0.05 by the atom
ratio. In the firing step, the heat treatment temperature in the
finishing heat treatment step was set to 755.degree. C. Except
that, a positive electrode active material of Working Example 9 was
produced similarly to the positive electrode active material of
Working Example 2 and the measurements were performed similar to
those of the positive electrode active material of Working Example
1.
Working Example 10
[0131] Except that the heat treatment temperature in the finishing
heat treatment step was set to 770.degree. C., a positive electrode
active material of Working Example 10 was produced similarly to the
positive electrode active material of Working Example 9 and the
measurements were performed similar to those of the positive
electrode active material of Working Example 1.
From Working Example 11 to Working Example 13
[0132] In the mixing step, the respective starting materials were
weighted so as to meet Li:Ni:Co:Mn =1.08:0.80:0.15:0.05 by the atom
ratio. In the firing step, the heat treatment temperature in the
second precursor forming step was set to 690.degree. C. Except that
the heat treatment temperature in the finishing heat treatment step
was set to 820.degree. C. for Working Example 11, 850.degree. C.
for Working Example 12, and 880.degree. C. for Working Example 13,
positive electrode active materials of Working Example 11 to
Working Example 13 were produced similarly to the positive
electrode active material of Working Example 1, and the
measurements were performed similar to those of the positive
electrode active material of Working Example 1. Additionally, when
measured using the microcompression testing machine (MCT-510
manufactured by Shimadzu Corporation), the particle fracture
strength was 71 MPa in Working Example 9, 132 MPa in Working
Example 10, and 125 MPa in Working Example 11.
Working Example 14 and Working Example 15
[0133] Working Example 14 and Working Example 15 were obtained by
performing the water washing and the drying processes on the
positive electrode active materials obtained in Working Example 13
and Working Example 5, respectively. The water washing and the
drying processes were performed by the following procedure. The
positive electrode active materials were water-washed by being
immersed in pure water with the solid content percentage of 43 mass
% and stirred at room temperature for 20 minutes. The positive
electrode active materials were filtered and then were dried in
vacuum at 190.degree. C. for ten hours.
Working Example 16
[0134] A positive electrode active material of Working Example 16
was obtained by performing the water washing and the drying
processes on the positive electrode active material obtained in
Working Example 13. The water washing and the drying processes of
the positive electrode active material were performed by the
following procedure. First, the positive electrode active material
obtained in Working Example 13 was immersed into pure water with
the solid content percentage of 66 mass % at room temperature for
ten seconds and was water-washed. Next, the positive electrode
active material that had been immersed into the pure water and on
which the water washing had been completed was depressurized and
filtered. Afterwards, the drying was performed in vacuum in two
stages: drying at the drying temperature of 80.degree. C. for 14
hours and further drying at the drying temperature of 190.degree.
C. for 14 hours.
Working Example 17
[0135] Except that the drying temperature at the second stage was
set to 240.degree. C. in Working Example 16, a positive electrode
active material of Working Example 17 was manufactured similarly to
the positive electrode active material of Working Example 16.
Working Example 18
[0136] Except that the drying at the first stage in Working Example
17 was not performed and only the drying at the second stage was
performed, a positive electrode active material of Working Example
18 was manufactured similarly to the positive electrode active
material of Working Example 17.
Comparative Example 1
[0137] In the mixing step, the respective starting materials were
weighted so as to meet Li:Ni:Co:Mn=1.04:0.80:0.15:0.05 by the atom
ratio. In the firing step, the heat treatment temperature in the
second precursor forming step was set to 550.degree. C. and the
heat treatment temperature in the finishing heat treatment step was
set to 755.degree. C. Except that, a positive electrode active
material of Comparative Example 1 was produced similarly to the
positive electrode active material of Working Example 9 and the
measurements were performed similar to those of the positive
electrode active material of Working Example 1.
Comparative Example 2
[0138] Except that the heat treatment temperature in the second
precursor forming step was set to 575.degree. C. and the heat
treatment temperature in the finishing heat treatment step was set
to 755.degree. C., a positive electrode active material of
Comparative Example 2 was produced similarly to the positive
electrode active material of Comparative Example 1, and the
measurements were performed similar to those of the positive
electrode active material of Working Example 1.
Comparative Example 3
[0139] Except that the heat treatment temperature in the finishing
heat treatment step was set to 906.degree. C., a positive electrode
active material of Comparative Example 3 was produced similarly to
the positive electrode active material of Working Example 2 and the
measurements were performed similar to those of the positive
electrode active material of Working Example 1.
[0140] The following Table 1 shows the composition formulae of the
positive electrode active materials, the heat treatment
temperatures in the second precursor forming step, the heat
treatment temperatures in the finishing heat treatment step, and
presences/absences of the water washing-drying steps from Working
Example 1 to Working Example 18 and from Comparative Example 1 to
Comparative Example 3. As the results from the XRD measurement, a
diffraction pattern corresponding to an .alpha.-NaFeO.sub.2 type
layered structure was obtained from Working Example 1 to Working
Example 18 and from Comparative Example 1 to Comparative Example
3.
TABLE-US-00001 TABLE 1 Heat treatment Heat treatment Composition
formula temperature in temperature in of positive electrode second
precursor finishing heat Water washing and active material forming
step [C. .degree.] treatment step [C. .degree.] drying steps
Working Example 1
Li.sub.1.01Ni.sub.0.80Co.sub.0.10Mn.sub.0.10O.sub.2 575 865 Absent
Working Example 2
Li.sub.1.01Ni.sub.0.80Co.sub.0.10Mn.sub.0.10O.sub.2 600 842 Absent
Working Example 3
Li.sub.1.01Ni.sub.0.80Co.sub.0.10Mn.sub.0.10O.sub.2 600 850 Absent
Working Example 4
Li.sub.1.01Ni.sub.0.80Co.sub.0.10Mn.sub.0.10O.sub.2 600 865 Absent
Working Example 5
Li.sub.1.01Ni.sub.0.80Co.sub.0.10Mn.sub.0.10O.sub.2 600 880 Absent
Working Example 6
Li.sub.1.01Ni.sub.0.80Co.sub.0.10Mn.sub.0.10O.sub.2 600 895 Absent
Working Example 7
Li.sub.1.01Ni.sub.0.80Co.sub.0.125Mn.sub.0.075O.sub.2 600 850
Absent Working Example 8
Li.sub.1.05Ni.sub.0.80Co.sub.0.05Mn.sub.0.15O.sub.2 600 865 Absent
Working Example 9
Li.sub.1.00Ni.sub.0.80Co.sub.0.15Mn.sub.0.05O.sub.2 600 755 Absent
Working Example 10
Li.sub.1.05Ni.sub.0.80Co.sub.0.15Mn.sub.0.05O.sub.2 600 770 Absent
Working Example 11
Li.sub.1.00Ni.sub.0.80Co.sub.0.15Mn.sub.0.05O.sub.2 690 820 Absent
Working Example 12
Li.sub.1.00Ni.sub.0.80Co.sub.0.15Mn.sub.0.05O.sub.2 690 850 Absent
Working Example 13
Li.sub.1.00Ni.sub.0.80Co.sub.0.15Mn.sub.0.05O.sub.2 690 880 Absent
Working Example 14
Li.sub.1.01Ni.sub.0.80Co.sub.0.15Mn.sub.0.05O.sub.2 690 880 Present
Working Example 15
Li.sub.1.01Ni.sub.0.80Co.sub.0.10Mn.sub.0.10O.sub.2 600 880 Present
Working Example 16
Li.sub.1.00Ni.sub.0.80Co.sub.0.15Mn.sub.0.5O.sub.2 690 880 Present
Working Example 17
Li.sub.1.00Ni.sub.0.80Co.sub.0.15Mn.sub.0.05O.sub.2 690 880 Present
Working Example 18
Li.sub.1.00Ni.sub.0.80Co.sub.0.15Mn.sub.0.05O.sub.2 690 880 Present
Comparative Example 1
Li.sub.1.01Ni.sub.0.80Co.sub.0.15Mn.sub.0.05O.sub.2 550 755 Absent
Comparative Example 2
Li.sub.1.01Ni.sub.0.80Co.sub.0.15Mn.sub.0.05O.sub.2 575 755 Absent
Comparative Example 3
Li.sub.1.00Ni.sub.0.80Co.sub.0.10Mn.sub.0.10O.sub.2 600 906
Absent
[0141] The following Table 2 shows the amounts of reacted lithium
carbonate in the second precursor forming step in the production
step of the positive electrode active materials, the amounts of
dissolution of the lithium hydroxide in the positive electrode
active materials, and the specific surface areas from Working
Example 1 to Working Example 18 and from Comparative Example 1 to
Comparative Example 3. The following Table 3 shows the amounts of
remaining lithium hydroxide and the amounts of remaining lithium
carbonate on the positive electrode active materials from Working
Example 1 to Working Example 18 and from Comparative Example 1 to
Comparative Example 3.
TABLE-US-00002 TABLE 2 Amount of reacted lithium carbonate Amount
of elution of lithium hydroxide Specific surface area in second
precursor forming step [mass %] in positive electrode active
material [mass %] [m.sup.2/g] Working Example 1 94 0.10 0.38
Working Example 2 97.4 0.14 0.78 Working Example 3 97.4 0.16 0.54
Working Example 4 97.4 0.09 0.32 Working Example 5 97.4 0.07 0.24
Working Example 6 97.4 0.06 0.17 Working Example 7 97.5 0.31 0.80
Working Example 8 97.2 0.17 0.51 Working Example 9 92.5 0.44 1.19
Working Example 10 92.5 0.33 0.94 Working Example 11 99.2 0.13 1.50
Working Example 12 99.2 0.08 0.47 Working Example 13 99.2 0.07 0.25
Working Example 14 99.2 0.07 0.65 Working Example 15 97.4 0.07 0.64
Working Example 16 99.2 0.07 0.53 Working Example 17 99.2 0.07 0.46
Working Example 18 99.2 0.07 0.47 Comparative Example 1 91.5 0.45
0.91 Comparative Example 2 91.9 0.43 0.99 Comparative Example 3
97.4 0.10 0.09
TABLE-US-00003 TABLE 3 Amount of remaining lithium hydroxide Amount
of remaining lithium carbonate on positive electrode active
material on positive electrode active material Moisture percentage
(mass %) (mass %) (ppm) Working Example 1 0.40 0.13 Working Example
2 0.66 0.18 Working Example 3 0.48 0.15 Working Example 4 0.36 0.12
Working Example 5 0.28 0.11 Working Example 6 0.17 0.10 Working
Example 7 1.14 0.35 Working Example 8 0.52 0.15 Working Example 9
0.80 0.22 Working Example 10 0.72 0.22 Working Example 11 0.59 0.27
Working Example 12 0.46 0.21 Working Example 13 0.27 0.18 200
Working Example 14 0.03 0.08 230 Working Example 15 0.04 0.09 240
Working Example 16 0.08 0.06 250 Working Example 17 0.03 0.06 190
Working Example 18 0.04 0.07 200 Comparative Example 1 0.83 0.27
Comparative Example 2 0.82 0.25 Comparative Example 3 0.25 0.11
[0142] As shown in Table 2, in the positive electrode active
material of Working Example 1, the amount of reacted lithium
carbonate in the second precursor forming step was about 94 mass %.
In the positive electrode active materials from Working Example 2
to Working Example 6, the amounts of reacted lithium carbonate in
the second precursor forming step were about 97.4 mass %. In the
positive electrode active material of Working Example 7, the amount
of reacted lithium carbonate in the second precursor forming step
was about 97.5 mass %. In the positive electrode active material of
Working Example 8, the amount of reacted lithium carbonate in the
second precursor forming step was about 97.2 mass %. In the
positive electrode active materials of Working Example 9 and
Working Example 10, the amounts of reacted lithium carbonate in the
second precursor forming step were about 92.5 mass %. In the
positive electrode active materials from Working Example 11 to
Working Example 14 and Working Example 16 to Working Example 18,
the amounts of reacted lithium carbonate in the second precursor
forming step were about 99.2 mass %. In the positive electrode
active material of Working Example 15, the amount of reacted
lithium carbonate in the second precursor forming step was about
97.4 mass %.
[0143] Table 2 shows that the smaller the amount of dissolution of
the lithium hydroxide in the positive electrode active material is,
the higher the amount of reacted lithium carbonate in the positive
electrode active material is and the higher the crystal stability
is. Table 3 shows that the smaller the amount of remaining lithium
carbonate is, the higher the amount of reacted lithium carbonate in
the positive electrode active material is and the higher the
crystal stability is. The amounts of dissolution of the lithium
hydroxide in the positive electrode active materials from Working
Example 1 to Working Example 8 and from Working Example 10 to
Working Example 18 shown in Table 2 were all low values of 0.33
mass % or less, the amounts of reacted lithium carbonate in the
positive electrode active materials were high, and the crystal
stability was high.
[0144] As shown in Table 2, the amounts of dissolution of the
lithium hydroxide in the positive electrode active materials from
Working Example 14 to Working Example 18 on which the water washing
had been performed exhibited the low values of 0.07 mass %.
Furthermore, as shown in Table 3, the amounts of remaining lithium
carbonate in the positive electrode active materials from Working
Example 14 to Working Example 18 on which the water washing had
been performed exhibited the low values of 0.09 or less.
Accordingly, it has been found that performing the water washing
ensures reducing the amount of remaining lithium carbonate in the
positive electrode active material, reducing the amount of carbonic
acid gas generated by lithium carbonate degradation caused by the
charge/discharge cycles, and improving the charge/discharge cycle
characteristics.
[0145] Among the positive electrode active materials from Working
Example 14 to Working Example 18 on which the water washing was
performed, the positive electrode active material of Working
Example 17 on which the drying was performed in two stages was able
to reduce the moisture amount to be the least, 190 ppm. Although
the drying at the first stage was not performed on the positive
electrode active material of Working Example 18, the moisture
amount was able to be reduced to 200 ppm. Accordingly, it has been
found that, although the drying needs not to be performed in two
stages, the drying at a high temperature of 240.degree. C. for 14
hours is effective in that the moisture amount decreases.
[0146] In contrast, the positive electrode active materials of
Comparative Example 1 and Comparative Example 2 exhibited the
amounts of reacted lithium carbonate in the second precursor
forming step of about 91.5 mass % and about 91.9 mass %,
respectively. Additionally, both of Comparative Example 1 and
Comparative Example 2 exhibited the amounts of dissolution of the
lithium hydroxide in the positive electrode active materials shown
in Table 2 higher than 0.33 mass %. Since the heat treatment
temperature was low in the second precursor forming step and also
the heat treatment temperature was low in the finishing heat
treatment step, the amounts of reacted lithium carbonate lowered in
these Comparative Examples.
[0147] The amount of reacted lithium carbonate of Comparative
Example 3 was 97.4 mass %. Although this Comparative Example had
the high heat treatment temperature in the finishing heat treatment
step and therefore satisfied the amount of reacted lithium
carbonate, the heat treatment temperature in the finishing heat
treatment step was high and the decrease in the specific surface
area decreased a reaction area with Li. Consequently, as described
later, the resistance increase rates of the lithium ion secondary
batteries that used the positive electrode active materials from
Comparative Example 1 to Comparative Example 3 became high and
therefore they became unmeasurable. Additionally, the discharge
capacities of the lithium ion secondary batteries using the
positive electrode active materials from Comparative Example 1 to
Comparative Example 3 were all low.
[0148] The following Table 4 shows crystallite diameters of the
positive electrode active materials, lattice constants of an
a-axis, and lattice constants of a c-axis from Working Example 1 to
Working Example 18 and from Comparative Example 1 to Comparative
Example 3.
TABLE-US-00004 TABLE 4 Crystallite Lattice constant Lattice
constant diameter of a-axis of c-axis [nm] [.ANG.] [.ANG.] Working
Example 1 135 2.87 14.22 Working Example 2 138 2.87 14.21 Working
Example 3 124 2.87 14.22 Working Example 4 142 2.87 14.22 Working
Example 5 177 2.87 14.22 Working Example 6 128 2.87 14.22 Working
Example 7 144 2.87 14.20 Working Example 8 118 2.88 14.23 Working
Example 9 69 2.87 14.17 Working Example 10 79 2.87 14.20 Working
Example 11 112 2.87 14.19 Working Example 12 104 2.87 14.21 Working
Example 13 115 2.87 14.21 Working Example 14 115 2.87 14.21 Working
Example 15 177 2.87 14.22 Working Example 16 115 2.87 14.21 Working
Example 17 115 2.87 14.21 Working Example 18 115 2.87 14.21
Comparative Example 1 79 2.87 14.21 Comparative Example 2 74 2.87
14.22 Comparative Example 3 -- 2.88 14.21
[0149] In Table 4, the crystallite diameter indicates the degree of
the grain growth in the positive electrode active material. As long
as the ratio (1-x-y-z) of Mn in the composition of the positive
electrode active material indicated by the above-described Formula
(1) is larger than 0, or more preferably
0.05.ltoreq.(1-x-y-z).ltoreq.0.18 is met, the charge and discharge
are possible even when the crystal grains in the positive electrode
active material grow. The lattice constant indicates that the
positive electrode active material is correctly produced.
(Evaluation for Compressibility of Positive Electrode Active
Material)
[0150] Using the autograph, the molding densities at the press
pressure of 5 MPa were measured as the compressibility of the
powder of the positive electrode active materials of Working
Examples 5, 9, 13, 14, and 15. The results were that the positive
electrode active material of Working Example 5 was 2.3 g/cm.sup.3,
the positive electrode active material of Working Example 9 was 1.7
g/cm.sup.3, the positive electrode active material of Working
Example 13 was 2.3 g/cm.sup.3, the positive electrode active
material of Working Example 14 was 2.6 g/cm.sup.3, and the positive
electrode active material of Working Example 15 was 2.6 g/cm.sup.3.
The following Table 5 shows the results of evaluation for
compressibility of the positive electrode active materials of
Working Examples 5, 9, 13, 14, and 15.
TABLE-US-00005 TABLE 5 Molding density at press pressure of 5 MPa
(g/cm.sup.3) Working Example 5 2.3 Working Example 9 1.7 Working
Example 13 2.3 Working Example 14 2.6 Working Example 15 2.6
[0151] With the positive electrode active materials of Working
Examples 5 and 14 on which the heat treatment was performed at the
high temperature in the finishing heat treatment step, the grain
growth was promoted, unevennesses on the secondary particle
surfaces decreased, and a friction between the secondary particles
was lowered; therefore, the compressibility was satisfactory.
Further, in the positive electrode active materials of Working
Examples 14 and 15 on which the water washing and the drying steps
were performed, further improvement in compressibility was
observed. It is inferred that this occurs because the surfaces were
modified through the water washing. When the molding density at the
press pressure of 5 MPa is 2.5 g/cm.sup.3 or more, a density of a
positive electrode mixture layer can be improved, and an effect to
increase the accumulated electric energy per unit volume can be
sufficiently obtained.
[0152] FIG. 3 is a graph illustrating a relationship between the
press pressure and the molding density of the positive electrode
active materials of Working Examples 5, 9, 13, 14, and 15 taking
the press pressure [MPa] on the horizontal axis and the molding
density [g/cm.sup.3] on the vertical axis. In the positive
electrode active materials of all the embodiments, an approximately
rectilinear proportional relationship was observed between the
press pressure and the molding density at the press pressure of 5
MPa or more; therefore, the compressibility was satisfactory.
(Manufacturing Lithium Ion Secondary Batteries)
[0153] Using the positive electrode active materials from Working
Example 1 to Working Example 18 and from Comparative Example 1 to
Comparative Example 3, the respective lithium ion secondary
batteries from Working Example 1 to Working Example 18 and from
Comparative Example 1 to Comparative Example 3 were manufactured by
the following procedure.
[0154] First, the positive electrode active material, a binder, and
a conductive material were mixed to prepare positive electrode
mixture slurry. Then, the prepared positive electrode mixture
slurry was applied over an aluminum foil as a positive electrode
current collector with a thickness of 20 and the positive electrode
mixture slurry was dried at 120.degree. C. After that, a
compression molding was performed with a press such that the
electrode density became 2.6 g/cm.sup.3, and this product was
punched into a disk shape with a diameter of 15 mm to manufacture
positive electrodes. Additionally, negative electrodes were
manufactured using metallic lithium or lithium titanate (LTO) as
negative electrode active materials.
[0155] Next, lithium ion secondary batteries were manufactured
using the manufactured positive electrodes, negative electrodes,
and non-aqueous electrolytes. As the non-aqueous electrolytes,
solution produced by dissolving LiPF.sub.6 so as to be the
concentration of 1.0 mol/L into a solvent in which ethylene
carbonate and dimethyl carbonate were mixed such that the volume
ratio became 3: 7 was used.
[0156] Next, the metallic lithium was used as the negative
electrode active materials for the respective lithium ion secondary
batteries from Working Example 1 to Working Example 18 and from
Comparative Example 1 to Comparative Example 3, a charge/discharge
test was conducted, and the first discharge capacity was measured.
With a charging current of 0.2 CA, the charge was performed at a
constant current and a constant voltage up to a charge cutoff
voltage of 4.3 V. With a discharge current of 0.2 CA, the discharge
was performed at a constant current up to a discharge cutoff
voltage of 3.3 V. The following Table 6 shows the measurement
results of the discharge capacities of the lithium ion secondary
batteries from Working Example 1 to Working Example 13 and from
Comparative Example 1 to Comparative Example 3.
[0157] The charge/discharge test was conducted on the respective
lithium ion secondary batteries from Working Example 1 to Working
Example 18 and from Comparative Example 1 to Comparative Example 3
using lithium titanate (LTO) as the negative electrode active
materials. With a charging current of 0.2 CA, the charge was
performed at a constant current and a constant voltage up to a
charge cutoff voltage of 2.75 V. With a discharge current of 0.2
CA, the discharge was performed by the charge and discharge by two
cycles at a constant current up to a discharge cutoff voltage of
1.7 V. Afterwards, an initial DC resistance value was measured at a
State of Charge (SOC) of 50%. Furthermore, the charge and discharge
were repeated by 100 cycles with the charge and discharge currents
of 3.0 CA, the charge cutoff voltage of 2.85 V, and the discharge
cutoff voltage of 1.7 V. After 100 cycles, a DC resistance value
with an electric potential at which the initial DC resistance value
was measured was measured. A percentage of a value found by
dividing the DC resistance value measured at the 100th cycle by the
initial DC resistance value was calculated and defined as the
resistance increase rate.
[0158] The following Table 6 shows the measurement results of the
resistance increase rate of the lithium ion secondary batteries
from Working Example 1 to Working Example 13 and from Comparative
Example 1 to Comparative Example 3.
TABLE-US-00006 TABLE 6 Discharge capacity Resistance increase rate
[Ah/kg] [%] Working Example 1 196 30 Working Example 2 197 27
Working Example 3 195 27 Working Example 4 195 26 Working Example 5
193 23 Working Example 6 191 22 Working Example 7 189 30 Working
Example 8 192 27 Working Example 9 200 35 Working Example 10 195 33
Working Example 11 198 24 Working Example 12 197 29 Working Example
13 192 27 Comparative Example 1 178 47 Comparative Example 2 182 41
Comparative Example 3 168 --
[0159] As described above, the positive electrode active materials
from Working Example 1 to Working Example 13 are produced through
the above-described mixing step and firing step, that is, the first
precursor forming step, the second precursor forming step, and the
finishing heat treatment step. The second precursor forming step
reacts 92 mass % or more of the lithium carbonate in the first
precursor to obtain the second precursor. With the lithium ion
secondary batteries from Working Example 1 to Working Example 13
that used the thus produced positive electrode active materials
from Working Example 1 to Working Example 13 as the positive
electrodes, the discharge capacity was 189 Ah/kg or more and the
resistance increase rate was 35% or less; therefore, all Working
Example 1 to Working Example 13 obtained the satisfactory results.
With the lithium ion secondary batteries from Working Example 14 to
Working Example 18 that used the positive electrode active
materials from Working Example 14 to Working Example 18 on which
the water washing was performed as the positive electrodes as well,
the discharge capacity was 185 Ah/kg or more and the resistance
increase rate was 35% or less; therefore, all Working Example 14 to
Working Example 18 obtained the satisfactory results.
[0160] In contrast, the lithium ion secondary batteries of
Comparative Example 1 and Comparative Example 2 use the positive
electrode active materials of Comparative Example 1 and Comparative
Example 2 with the amount of reacted lithium carbonate in the first
precursor of less than 92 mass % as the positive electrodes in the
second precursor forming step. Consequently, although the lithium
ion secondary batteries of Comparative Example 1 and Comparative
Example 2 exhibited the comparatively high discharge capacity, the
resistance increase rate became 40% or more. Thus, the properties
worsened compared with the results of the lithium ion secondary
batteries from Working Example 1 to Working Example 13. It has been
found that since the discharge capacity of Comparative Example 3
was the capacity lower than those of Working Example 1 to Working
Example 13, adjusting only the temperature at the finishing heat
treatment step cannot obtain the excellent positive electrode
active material.
[0161] As described above, it was able to be confirmed that Working
Example 1 to Working Example 18 based on the method for producing
the positive electrode active material and the positive electrode
active material of the present invention can obtain the positive
electrode active material that features the high capacity, the low
resistance increase rate, and the excellent charge/discharge cycle
characteristics.
[0162] While the embodiments of the present invention have been
described in detail with reference to the drawings, the specific
configuration is not limited to the embodiments. Design changes and
the like within a scope not departing from the gist of the present
invention are included in the present invention.
Reference Signs List [0166]
[0163] 100 Non-aqueous secondary battery (lithium ion secondary
battery) [0164] 111 Positive electrode [0165] S1 Mixing step [0166]
S2 Firing step [0167] S21 First precursor forming step [0168] S22
Second precursor forming step [0169] S23 Finishing heat treatment
step
* * * * *