U.S. patent application number 12/829410 was filed with the patent office on 2011-01-13 for positive electrode active material.
This patent application is currently assigned to SEMICONDUCTOR ENERGY LABORATORY CO., LTD.. Invention is credited to Junichi KOEZUKA, Akiharu MIYANAGA, Masahiro TAKAHASHI.
Application Number | 20110008233 12/829410 |
Document ID | / |
Family ID | 43427621 |
Filed Date | 2011-01-13 |
United States Patent
Application |
20110008233 |
Kind Code |
A1 |
MIYANAGA; Akiharu ; et
al. |
January 13, 2011 |
POSITIVE ELECTRODE ACTIVE MATERIAL
Abstract
A highly effective positive electrode is obtained by using a
material such as Na which is an inexpensive abundant resource. A
positive electrode active material of sodium transition metal
phosphate of olivine structure in which the sodium transition metal
phosphate of olivine structure includes, a phosphorus atom that is
located at the center of a tetrahedron having an oxygen atom in
each vertex, a transition metal atom that is located at the center
of a first octahedron having an oxygen atom in each vertex; and a
sodium atom that is located at the center of a second octahedron
having an oxygen atom in each vertex, and adjacent sodium atoms are
arranged one-dimensionally in a <010> direction.
Inventors: |
MIYANAGA; Akiharu; (Hadano,
JP) ; KOEZUKA; Junichi; (Atsugi, JP) ;
TAKAHASHI; Masahiro; (Atsugi, JP) |
Correspondence
Address: |
NIXON PEABODY, LLP
401 9TH STREET, NW, SUITE 900
WASHINGTON
DC
20004-2128
US
|
Assignee: |
SEMICONDUCTOR ENERGY LABORATORY
CO., LTD.
Atsugi-shi
JP
|
Family ID: |
43427621 |
Appl. No.: |
12/829410 |
Filed: |
July 2, 2010 |
Current U.S.
Class: |
423/306 |
Current CPC
Class: |
H01M 4/045 20130101;
C01B 25/45 20130101; H01M 2004/028 20130101; H01M 4/5825 20130101;
Y02E 60/10 20130101; H01M 10/054 20130101; H01M 4/136 20130101 |
Class at
Publication: |
423/306 |
International
Class: |
C01B 25/30 20060101
C01B025/30 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 10, 2009 |
JP |
2009-164159 |
Claims
1. A positive electrode active material comprising sodium
transition metal phosphate having an olivine structure, wherein
adjacent sodium atoms in the positive electrode active material are
arranged in one direction and in a <010> direction without
being inhibited by other atoms.
2. The positive electrode active material according to claim 1,
wherein the sodium transition metal phosphate is sodium iron
phosphate.
3. The positive electrode active material according to claim 1,
wherein the sodium transition metal phosphate is sodium nickel
phosphate.
4. The positive electrode active material according to claim 1,
wherein the sodium transition metal phosphate is sodium cobalt
phosphate.
5. The positive electrode active material according to claim 1,
wherein the sodium transition metal phosphate is sodium manganese
phosphate.
6. A secondary battery comprising the positive electrode active
material according to claim 1.
7. A positive electrode active material comprising sodium
transition metal phosphate having an olivine structure, wherein
adjacent sodium atoms in the positive electrode active material are
arranged in one direction and in a b-axis direction without being
inhibited by other atoms.
8. The positive electrode active material according to claim 7,
wherein the sodium transition metal phosphate is sodium iron
phosphate.
9. The positive electrode active material according to claim 7,
wherein the sodium transition metal phosphate is sodium nickel
phosphate.
10. The positive electrode active material according to claim 7,
wherein the sodium transition metal phosphate is sodium cobalt
phosphate.
11. The positive electrode active material according to claim 7,
wherein the sodium transition metal phosphate is sodium manganese
phosphate.
12. A secondary battery comprising the positive electrode active
material according to claim 7.
13. A positive electrode active material comprising sodium
transition metal phosphate having an olivine structure; wherein a
phosphorus atom is located at a center of a tetrahedron having an
oxygen atom in each vertex, wherein a transition metal atom is
located at a center of a first octahedron having an oxygen atom in
each vertex, wherein a sodium atom is located at a center of a
second octahedron having an oxygen atom in each vertex, and wherein
adjacent sodium atoms in the positive electrode active material are
arranged in a <010> direction without being inhibited by
other atoms.
14. The positive electrode active material according to claim 13,
wherein the transition metal atom is an iron atom and the sodium
transition metal phosphate is sodium iron phosphate.
15. The positive electrode active material according to claim 13,
wherein the transition metal atom is a nickel atom and the sodium
transition metal phosphate is sodium nickel phosphate.
16. The positive electrode active material according to claim 13,
wherein the transition metal atom is a cobalt atom and the sodium
transition metal phosphate is sodium cobalt phosphate.
17. The positive electrode active material according to claim 13,
wherein the transition metal atom is a manganese atom and the
sodium transition metal phosphate is sodium manganese
phosphate.
18. A secondary battery comprising the positive electrode active
material according to claim 13.
19. A positive electrode active material comprising sodium
transition metal phosphate having an olivine structure; wherein a
phosphorus atom is located at a center of a tetrahedron having an
oxygen atom in each vertex, wherein a transition metal atom is
located at a center of a first octahedron having an oxygen atom in
each vertex, wherein a sodium atom is located at a center of a
second octahedron having an oxygen atom in each vertex, and wherein
adjacent sodium atoms in the positive electrode active material are
arranged in a b-axis direction without being inhibited by other
atoms.
20. The positive electrode active material according to claim 19,
wherein the transition metal atom is an iron atom and the sodium
transition metal phosphate is sodium iron phosphate.
21. The positive electrode active material according to claim 19,
wherein the transition metal atom is a nickel atom and the sodium
transition metal phosphate is sodium nickel phosphate.
22. The positive electrode active material according to claim 19,
wherein the transition metal atom is a cobalt atom and the sodium
transition metal phosphate is sodium cobalt phosphate.
23. The positive electrode active material according to claim 19,
wherein the transition metal atom is a manganese atom and the
sodium transition metal phosphate is sodium manganese
phosphate.
24. A secondary battery comprising the positive electrode active
material according to claim 19.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an active material used for
an electrode of a secondary battery.
[0003] 2. Description of the Related Art
[0004] In recent years, with an increase of environmental
engineering, development of power generating technologies which
pose less burden on the environment (e.g., solar power generation)
than conventional power generation methods has been actively
conducted. Concurrently with the development of power generation
technology, development of power storage technology has also been
underway.
[0005] A power storage technology includes, for instance, a lithium
ion secondary battery. Lithium ion secondary batteries are widely
prevalent since their energy density is high and because they are
well suited for miniaturization. As an active material used for a
positive electrode of the lithium ion secondary battery, there is
olivine structure LiFePO.sub.4, for example.
[0006] Olivine structure LiFePO.sub.4 (lithium iron phosphate) has
favorable characteristics since the lithium atoms (Li) are arranged
in one direction without being inhibited by other atoms. However,
since Li is a rare metal, its reserves are few and it is expensive.
Therefore, sodium (Na), which is plentiful at low cost, is being
considered as a substitute material for Li.
[0007] Conventional NaMPO.sub.4 (M is Mn, Fe, Co or Ni) takes a
maricite structure (for reference, see Patent Document 1 and Patent
Document 2). In the maricite structure, since the sodium atoms
contributing to electrical conduction are not arranged in one
direction without being inhibited by other atoms, the drift of the
applied field effect state of sodium ions (Na ions) is small, thus
there is the problem that favorable characteristics can not be
obtained.
REFERENCE
[0008] [Patent Document 1] Japanese Published Patent Application
No. 2008-260666
[0009] [Patent Document 2] Japanese Published Patent Application
No. 2009-104970
SUMMARY OF THE INVENTION
[0010] It is an object of an embodiment of the present invention to
provide a highly efficient positive electrode using Na which is a
low cost abundant resource.
[0011] An embodiment of the present invention is a positive
electrode active material which includes sodium transition metal
phosphate having the olivine type structure, sodium atoms being
arranged in one direction without being inhibited by other
atoms.
[0012] An embodiment of the present invention is a positive
electrode active material which includes sodium transition metal
phosphate having an olivine structure, a phosphorus atom located at
the center of a tetrahedron having an oxygen atom in each vertex, a
transition metal atom located at the center of a first octahedron
having an oxygen atom in each vertex, and a sodium atom located at
the center of a second octahedron having an oxygen atom in each
vertex, and adjacent sodium atoms arranged in one direction
(<010> direction) without being inhibited by other atoms.
[0013] In the abovementioned structure for the positive electrode
active material, the aforesaid transition metal may be iron,
nickel, cobalt, or manganese, and the aforesaid sodium transition
metal phosphate may be sodium iron phosphate, sodium nickel
phosphate, sodium cobalt phosphate, and sodium manganese
phosphate.
[0014] A highly efficient positive electrode material using Na,
which is a low cost abundant resource, can be provided.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a schematic diagram of a crystal structure for
sodium iron phosphate having an olivine structure.
[0016] FIG. 2 is a schematic diagram of a crystal structure for
sodium lithium iron phosphate having an olivine structure.
[0017] FIG. 3 is a schematic diagram of a crystal structure for
sodium iron phosphate having a maricite structure.
[0018] FIG. 4 is a schematic diagram of a structure for a secondary
battery.
DETAILED DESCRIPTION OF THE INVENTION
[0019] Embodiments of the present invention are described with
reference to the drawings. However, the present invention is not
limited to the following description. The present invention can be
implemented in various different ways and it will be readily
appreciated by those skilled in the art that various changes and
modifications are possible without departing from the spirit and
the scope of the present invention. Therefore, unless such changes
and modifications depart from the scope of the invention, they
should be construed as being included therein. Note that reference
numerals denoting the same portions are commonly used in different
drawings.
Embodiment 1
[0020] In this embodiment, there is an embodiment of the present
invention regarding a positive electrode active material which will
be described using FIG. 1 and FIG. 3.
[0021] An embodiment of the positive electrode active material
shown in this embodiment is sodium transition metal phosphate
(NaMPO.sub.4) in which a transition metal (M) such as iron, cobalt,
nickel, or manganese can be used. Herein is a description using
sodium iron phosphate (NaFePO.sub.4) in which iron is used for
example as the transition metal (M).
[0022] In FIG. 1, a unit cell 101 of sodium iron phosphate
(NaFePO.sub.4) of olivine structure is shown. Sodium iron phosphate
of olivine structure is an orthorhombic crystal structure, and
includes four formula units of sodium iron phosphate (NaFePO.sub.4)
within a unit cell. The basic framework of the olivine structure is
a hexagonal close-packed structure of an oxide ion, in which a
sodium atom, an iron atom and a phosphorus atom are located in the
gaps of the hexagonal close-packed structure.
[0023] Further, the olivine structure of sodium iron phosphate
(NaFePO.sub.4) has a tetrahedral site and two kinds of octahedral
sites. The tetrahedral site has four oxygen atoms in the vertices.
The octahedral sites have six oxygen atoms in the vertices.
Phosphorus atom 107 is located at the center of the tetrahedral
site, and sodium atom 103 or iron atom 105 is located at the center
of the octahedral sites. The octahedral site with the sodium atom
103 located at the center is called a M1 site, and the octahedral
site with the iron atom 105 located at the center is called a M2
site. The adjacent M1 sites are arranged in a b-axis direction
without being inhibited by other atoms. That is, sodium atoms each
located in each of the adjacent M1 sites are arranged in one
direction (<010> direction) without being inhibited by other
atoms. Note here, bonds between the sodium atoms 103 and other
atoms or ions are not shown with a line in FIG. 1.
[0024] The iron atoms 105 of adjacent M2 sites are bonded in a
zigzag shape with an oxygen atom 109 interposed therebetween. Then,
the oxygen atom 109 bonded between the iron atoms 105 of the
adjacent M2 sites, is also bonded to the phosphorus atom 107 of the
tetrahedral site. Thus, the bond between the iron atom and the
oxygen atom and the bond between the oxygen atom and the phosphorus
atom are continuous.
[0025] Note that the sodium iron phosphate of olivine structure may
have distortion. Further, regarding the sodium iron phosphate, the
composition ratio of sodium, iron, phosphorus, and oxygen is not
limited to 1:1:1:4. Also, as the transition metal (M) of the sodium
transition metal phosphate (NaMPO.sub.4), a transition metal which
has an ionic radius that is larger than that of a Na ion may be
used.
[0026] In a positive electrode active material shown in FIG. 1,
since even iron phosphate alone is stable, diffusion of sodium is
easy. For this reason, the sodium capable of diffusion contribute
to an electrical conduction. Furthermore, since the sodium atoms
which contribute to an electrical conduction is arranged in one
direction and in a b-axis direction without being inhibited by
other atoms, the diffusibility of the Na ions in the b-axis
direction is high. That is, since the diffusive resistance of the
Na ions can be reduced, the drift of the Na ions is large. Also,
since sodium is used, the positive electrode active material is
highly practical at low cost. For this reason, by using sodium iron
phosphate in the positive electrode active material, the internal
resistance of a secondary battery can be reduced, and its output
power can be increased.
[0027] Here, as a comparative example, the sodium iron phosphate of
maricite structure will be described. In FIG. 3, a unit cell 121
for sodium iron phosphate of maricite structure (NaFePO.sub.4) is
shown. The sodium iron phosphate of maricite structure includes an
octahedral site having a sodium atom 103 in the center, an
octahedral site having an iron atom 105 in the center, and a
tetrahedral site having a phosphorus atom 107 in the center.
Further, iron atoms 105 are arranged in one direction and in a
b-axis direction without being inhibited by other atoms, and sodium
atoms 103 and oxygen atoms 109 are alternately arranged. Here,
since sodium atoms contributing to electrical conduction are not
arranged in one direction without being inhibited by other atoms, a
diffusibility of the Na ion is low. That is to say, a diffusive
resistance of the Na ion is high, and a drift of the Na ion is
small.
[0028] From the above, as shown in FIG. 1, by sodium atoms which
contribute to an electrical conduction being arranged in one
direction and in a <010> direction (b-axis direction) without
being inhibited by other atoms, at least, the diffusibility of the
Na ions is increased. Namely, since the diffusive resistance of the
Na ions can be reduced, the drift of the Na ions becomes larger. In
addition, since at least the Na ions are used as ions which
contribute to an electrical conduction, a positive electrode active
material is highly practical at low cost. For this reason, by using
sodium iron phosphate or sodium lithium iron phosphate in the
positive electrode active material, an internal resistance of the
secondary battery is reduced, and a high output power can be
achieved.
[0029] Next, a manufacturing method for the positive electrode
active material of the secondary battery of the present embodiment
will be explained.
[0030] First, a transition metal phosphate of the olivine structure
is prepared. Here, as an example, the case of manufacturing an iron
phosphate of olivine structure is explained but is not limited
thereto; thus, if of olivine structure, another transition metal
(e.g., nickel, cobalt, or manganese) may be substituted for the
iron.
[0031] The iron phosphate of olivine structure can be manufactured
by mixing, for example, iron or a material including iron with a
phosphate or a material including a phosphate, and then causing a
reaction.
[0032] As the material including iron, for example, an iron
oxyhydroxide, iron(II) oxide, iron(III) oxide, iron(II) oxalate
dihydrate, iron chlorides and the like can be used. Alternatively,
a material including iron that has a microcrystal structure can be
used. By using the material including iron that has a microcrystal
structure, a particle size of the formed lithium iron phosphate can
be approximately several nanometers.
[0033] As the material including a phosphate, for example,
phosphorus pentoxide, diammonium hydrogen phosphate, or ammonium
dihydrogen phosphate can be used. For example, it is preferred to
use the phosphate or the phosphorus pentoxide since a strong acid
condition can be maintained in a process of dissolving iron, and
since the generation of the ammonia gas can be suppressed. For
instance, when using an iron powder as the iron material, the iron
powder is mixed with phosphorus pentoxide, pure water is added to
the obtained mixture which is then left still and a reaction is
caused, a first heat treatment is performed on the material that
has been left still and underwent a reaction, and then the heat
treated material is ground (mixed by grinding). Additionally, by
performing a second heat treatment, iron phosphate of olivine
structure is manufactured. Here, the first heat treatment may be
performed until drying is completed; for example, the first heat
treatment may be performed at 100.degree. C. in atmospheric air for
24 hours, and a second heat treatment, for example, may be
performed at 100.degree. C. to 650.degree. C. in atmospheric air
for 12 hours.
[0034] Additionally, a third heat treatment is performed as a
manufacturing step for an iron phosphate of olivine structure. The
third heat treatment is, for example, a one stage temperature
process which is from room temperature to a heat treatment "end
temperature" (e.g., 100.degree. C. to 800.degree. C., more
preferably 300.degree. C. to 650.degree. C.), in other words, it is
preferable to increase a temperature continuously from room
temperature until the "end temperature" is achieved. However,
without being limited thereto, a two stage temperature process
(pre-bake and main-bake) can also be performed. In the case of a
two stage temperature process, for instance, as a first stage
(pre-bake), a heat treatment is performed from room temperature to
300.degree. C., and as a second stage (main-bake), a heat treatment
of 300.degree. C. to 800.degree. C. is performed. In this way, the
iron phosphate of olivine structure can be manufactured.
[0035] Next, Na ions are introduced to the iron phosphate of
olivine structure which is manufactured as explained above.
[0036] Methods for introducing Na ions which can be used are, for
example, while not particularly limited hereto, a method of
impregnating the aforesaid iron phosphate of olivine structure with
a solution including Na ions, or a method in which a sodium sheet
is provided to a surface of the aforesaid iron phosphate and then
left still and heated, or a voltage is applied. Note that according
to the present embodiment, the sodium sheet comprises metallic
sodium spread in a sheet form so as to have a thickness of 0.01 mm
to 0.1 mm (e.g., a thickness of 0.05 mm). However, the thickness of
the sodium sheet is not limited thereby, and the sodium sheet may
have a suitable thickness as necessary.
[0037] Here, when the aforesaid iron phosphate compound of olivine
structure is impregnated with the solution including Na ions, it is
preferred that the solution including Na ions has a Na ion
concentration in a range of 1 mol % to 10 mol %, and particularly
in a range of 4 mol % to 6 mol %. As the solution including Na
ions, for example, a solution including NaClO.sub.4 can be
used.
[0038] First, as described above, the transition metal phosphate of
olivine structure is prepared, and by introducing Na ions to the
prepared transition metal phosphate, the positive electrode active
material of sodium transition metal phosphate can be manufactured
while maintaining the olivine structure.
Embodiment 2
[0039] In the present embodiment, an embodiment of the present
invention regarding a positive electrode active material, which is
different from that of Embodiment 1, will be explained. A positive
electrode active material shown in the present embodiment is
sodium-lithium transition metal phosphate
(Na.sub.xLi.sub.(1-x)MPO.sub.4 (0.ltoreq.x.ltoreq.1)), and as the
transition metal (M), iron, cobalt, nickel, manganese, and the like
can be used. In other words, an aspect of differentiation from
Embodiment 1 is that besides sodium, lithium is also included.
Herein, sodium-lithium iron phosphate
(Na.sub.xLi.sub.(1-x)FePO.sub.4 (0.ltoreq.x.ltoreq.1)) using iron
as the transition metal (M) is used for explanation.
[0040] FIG. 2 shows a unit cell 111 for sodium-lithium iron
phosphate (Na.sub.xLi.sub.(1-x)FePO.sub.4 (0.ltoreq.x.ltoreq.1)) of
olivine structure. Sodium-lithium iron phosphate of olivine
structure is an orthorhombic crystal structure, and includes four
formula units of sodium-lithium iron phosphate
(Na.sub.xLi.sub.(l-x)FePO.sub.4 (0.ltoreq.x.ltoreq.1)) within a
unit cell.
[0041] Sodium atom 103 and lithium atom 113 of the sodium-lithium
iron phosphate (Na.sub.xLi.sub.(1-x)FePO.sub.4
(0.ltoreq.x.ltoreq.1)) are arranged in one direction and in a
b-axis direction without being inhibited by other atoms. That is to
say, the sodium atom 103 and the lithium atom 113 are arranged in
one direction (<010> direction) without being inhibited by
other atoms. Here, bonds between the sodium atoms 103 and other
atoms and between the lithium atoms 113 and other atoms are not
shown by lines in FIG. 2.
[0042] Note that the sodium-lithium iron phosphate of olivine
structure may be distorted. Furthermore, regarding the
sodium-lithium iron phosphate, the composition ratio of the sodium
and the lithium, the iron, the phosphorus, and the oxygen is not
limited to 1:1:1:4. As the transition metal (M) of the
sodium-lithium transition metal phosphate
(Na.sub.xLi.sub.(1-x)MPO.sub.4 (0.ltoreq.x.ltoreq.1)), a transition
metal which has an ionic radius that is larger than that of the Na
ion and the lithium ion (Li ion) may be used.
[0043] In the positive electrode active material shown in FIG. 2,
since even an iron phosphate alone is stable, diffusion of sodium
and lithium is easy. For this reason, sodium and lithium capable of
diffusion contribute to an electrical conduction. Furthermore,
since the sodium atoms and the lithium atoms which contribute to
the electrical conduction are arranged in one direction and in a
b-axis direction without being inhibited by other atoms, the
diffusibility of the Na ions and the Li ions in the b-axis
direction is high. That is, since the diffusive resistance of the
Na ions and the Li ions can be reduced, the drift of the Na ions
and the Li ions is large. Also, since lithium is used with the
sodium, the amount of lithium used can be reduced; thus, the
positive electrode active material is highly practical at low cost.
For this reason, by using sodium-lithium iron phosphate in the
positive electrode active material, the internal resistance of a
secondary battery is reduced, and a high output power can be
obtained.
[0044] Next, a manufacturing method for the secondary battery
positive electrode active material of the present embodiment will
be explained.
[0045] First, a lithium transition metal phosphate of olivine
structure is manufactured. Here, as an example, the case of
manufacturing a lithium iron phosphate of olivine structure is
explained, but not limited thereto; thus, if of olivine structure,
a material including another transition metal (e.g., nickel,
cobalt, and manganese) instead of the iron or a material including
plural transition metals may be used.
[0046] The lithium iron phosphate of olivine structure can be
manufactured by mixing, for example, lithium or a material
including lithium, iron or a material including iron, and a
phosphate or a material including a phosphate, and performing a
heat treatment.
[0047] As the material including iron, for example, an iron
oxyhydroxide, iron(II) oxide, iron(III) oxide, iron(II) oxalate
dihydrate, iron chlorides, and the like can be used. Alternatively,
a material including iron that has a microcrystal structure can be
used. By using the material including iron that has a microcrystal
structure, a particle size of the formed lithium iron phosphate can
be approximately several nanometers.
[0048] As the material including lithium, for instance, lithium
carbonate, lithium hydroxide, lithium hydroxide hydrate, lithium
nitrite, and the like can be used. For example, the lithium
carbonate is preferred for its low hygroscopic property.
[0049] As the material including a phosphate, for example,
phosphorus pentoxide, diammonium hydrogen phosphate, or ammonium
dihydrogen phosphate can be used.
[0050] For example, lithium iron phosphate is manufactured by
mixing lithium carbonate, iron(II) oxalate dihydrate, and ammonium
hydrogen phosphate, performing a first heat treatment on the
obtained mixture, and additionally performing a second heat
treatment. Here for instance, a ball mill is used to mix the
materials. The first heat treatment is performed, for example at
350.degree. C. for 10 hours, and the second heat treatment is
performed, for example in an argon atmosphere at 600.degree. C. for
10 hours.
[0051] Furthermore, the lithium iron phosphate of olivine structure
can be manufacture by a first method having steps of: dissolving
lithium or a material including lithium, iron or a material
including iron, and phosphate or a material including phosphate in
a solution; evaporating or nebulizing and drying the solution; and
performing a heat treatment under a reducing atmosphere, or a
second method having steps of: dissolving lithium or a material
including lithium, iron or a material including iron, and phosphate
or a material including phosphate in a solution; and performing a
hydrothermal treatment. By manufacturing the lithium iron phosphate
of olivine structure using the first or second method, a particle
size can be approximately several tens of nanometers to several
hundreds of nanometers. For example, by impregnating and mixing the
material including iron with the solution including Li ions and
phosphate ions and then performing a hydrothermal treatment, the
lithium iron phosphate of olivine structure is manufactured using
the second method. Here, the hydrothermal treatment is, for
example, performed within a range of 150.degree. C. to 200.degree.
C. Also, a heat treatment may be performed after the hydrothermal
treatment. At such a time, the heat treatment is performed, for
example, in a reducing atmosphere within a range of 500.degree. C.
to 700.degree. C.
[0052] Next, a process for replacing at least a portion of Li ions
with Na ions (hereafter, referred to as a sodium-lithium ion
replacement process) is performed on the manufactured lithium iron
phosphate of olivine structure.
[0053] Methods for the sodium-lithium ion replacement process,
which can be used are a method of impregnating the manufactured
lithium iron phosphate of olivine structure with the solution
including Na ions as described above, and replacing the Li ions
which form the lithium iron phosphate of olivine structure with the
Na ions, or introducing Na ions by providing a sodium sheet to a
surface of the lithium iron phosphate of olivine structure, which
is then left still and heated, or a voltage is applied. However, as
long as a method can replace at least a portion of the Li ions
which form the lithium iron phosphate of olivine structure with the
Na ions, the method is not particularly limited.
[0054] Here, when impregnating the aforesaid lithium iron phosphate
of olivine structure with the solution including Na ions, it is
preferred that the solution including Na ions has a Na ion
concentration in a range of 1 mol % to 10 mol %, and particularly
in a range of 4 mol % to 6 mol %. As the solution including Na
ions, for example, a solution including NaClO.sub.4 can be
used.
[0055] Also, a heat treatment may be performed in the
sodium-lithium ion replacement process. By performing the heat
treatment, at least a portion of the Li ions can be more
efficiently replaced with the Na ions. The heat treatment
temperature at this time, for example, is preferably in a range of
300.degree. C. to 400.degree. C., and particularly preferable in a
range of 330.degree. C. to 350.degree. C. Note that a heating time,
for instance, is preferably 1 hour to 10 hours, and particularly
preferable for 2 hours to 5 hours.
[0056] Also, in a method of manufacturing the positive electrode
active material of the present embodiment, while not limited
hereby, the sodium-lithium ion replacement process may be performed
once after a battery is assembled. Here, the method of performing
the sodium-lithium ion replacement process after a battery is
assembled is explained next.
[0057] First, a lithium iron phosphate of olivine structure is
manufactured in the similar manner as described above, and using
the obtained lithium iron phosphate of olivine structure, a
positive electrode is manufactured. Also, other than the positive
electrode, a negative electrode and an electrolyte are prepared,
and then combined with the manufactured positive electrode to
manufacture a battery. Additionally, after at least a portion of
the Li ions is extracted from the lithium iron phosphate of olivine
structure by applying a voltage to the manufactured battery, by
performing the sodium-lithium replacement process, at least a
portion of the Li ions which forms the manufactured lithium iron
phosphate can be replaced with Na ions.
[0058] As described above, first, the lithium iron phosphate of
olivine structure is manufactured, and by replacing at least the
portion of the Li ions which forms the manufactured lithium iron
phosphate with Na ions, the positive electrode active material of
the sodium-lithium iron phosphate of olivine structure can be
manufactured. Moreover, replaced Li may be collected and
reused.
Embodiment 3
[0059] A secondary battery using the positive electrode active
material which is one embodiment of the present invention described
in the above embodiments, is described in the present
embodiment.
[0060] A structure of a secondary battery 130 is shown in FIG. 4.
The secondary battery 130 has a housing 141, a positive electrode
148 including a positive electrode current collector 142 and a
positive electrode active material 143, a negative electrode 149
including a negative electrode current collector 144 and a negative
electrode active material 145, a separator 146 between the positive
electrode 148 and the negative electrode 149, and an electrolyte
147.
[0061] As a material of the positive electrode current collector
142 of the secondary battery 130, an element such as aluminum (Al)
and titanium (Ti), may be used alone or in a compound thereof.
[0062] The positive electrode active material in an embodiment of
the present invention described in Embodiment 1 or Embodiment 2 is
used as a material for the positive electrode active material 143
of the secondary battery 130.
[0063] As a material of the negative electrode current collector
144 of the secondary battery 130, an element such as copper (Cu),
aluminum (Al), nickel (Ni), and titanium (Ti), may be used alone or
in a compound thereof.
[0064] As a material of the negative electrode active material 145
of the secondary battery 130, a material capable of Na ion
insertion and extraction or a compound of Na may be used. As the
material capable of Na ion insertion and extraction, there is
carbon, silicon, silicon alloy, and the like. As the carbon capable
of Na ion insertion and extraction, there is a carbon material such
as a fine graphite powder or a graphite fiber.
[0065] Note that when using silicon as the material for the
negative electrode active material 145 of the secondary battery
130, microcrystalline silicon (microcrystal silicon) is deposited,
and the microcrystalline silicon with its amorphous silicon removed
by etching can be used. When amorphous silicon is removed from
microcrystalline silicon, the surface area of the remaining
microcrystalline silicon is increased.
[0066] Additionally, as the negative electrode active material 145
of the secondary battery 130, an alloy including tin (Sn) can be
used.
[0067] Na ions are taken in and react with a layer formed of the
aforesaid material capable of Na ion insertion and extraction, and
the negative active material 145 is formed.
[0068] As the separator 146, paper, nonwoven fabric, a glass fiber,
or a synthetic fiber such as nylon (polyamide), vinylon (also
called vinalon) (a polyvinyl alcohol based fiber), polyester,
acrylic, polyolefin, polyurethane, and the like may be used.
However, a material which does not dissolve in the electrolyte 147,
described later, should be selected.
[0069] More specific examples of materials for the separator 146
are high-molecular compounds based on fluorine-based polymer,
polyether such as polyethylene oxide and polypropylene oxide,
polyolefin such as polyethylene and polypropylene,
polyacrylonitrile, polyvinylidene chloride, polymethyl
methacrylate, polymethylacrylate, polyvinyl alcohol,
polymethacrylonitrile, polyvinyl acetate, polyvinylpyrrolidone,
polyethyleneimine, polybutadiene, polystyrene, polyisoprene, and
polyurethane, derivatives thereof, cellulose, paper, and nonwoven
fabric, all of which can be used either alone or in a
combination.
[0070] Also, the electrolyte 147 of the secondary battery 130
includes Na ions, and these Na ions are responsible for electrical
conduction. The electrolyte 147 includes, for example, a solvent
and a sodium salt dissolved in the solvent. The sodium salt, for
example, can be a sodium salt such as sodium chloride (NaCl),
sodium fluoride (NaF), sodium perchlorate (NaClO.sub.4), and sodium
fluoroborate (NaBF.sub.4), which may be used alone or in
combination in the electrolyte 147. Note that in the present
embodiment, an electrolyte including a solvent and a sodium salt is
used; however, a solid electrolyte may be used as necessary.
[0071] Examples of the solvent for the electrolyte 147 include
cyclic carbonates such as ethylene carbonate (hereinafter
abbreviated as EC), propylene carbonate (PC), butylene carbonate
(BC), and vinylene carbonate (VC); acyclic carbonates such as
dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl
carbonate (EMC), methylpropyl carbonate (MPC), methylisobutyl
carbonate (MIBC), and dipropyl carbonate (DPC); aliphatic
carboxylic acid esters such as methyl formate, methyl acetate,
methyl propionate, and ethyl propionate; .gamma.-lactones such as
.gamma.-butyrolactone; acyclic ethers such as 1,2-dimethoxyethane
(DME), 1,2-diethoxyethane (DEE), and ethoxymethoxy ethane (EME);
cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran;
dimethylsulfoxide; 1,3-dioxolane and the like; alkyl phosphate
esters such as trimethyl phosphate, triethyl phosphate, and
trioctyl phosphate and fluorides thereof, all of which can be used
either alone or in combination.
[0072] As described above, the secondary battery using the
secondary battery positive electrode active material of an
embodiment of the present invention can be manufactured.
[0073] This application is based on Japanese Patent Application
serial no. 2009-164159 filed with Japan Patent Office on Jul. 10,
2009, the entire contents of which are hereby incorporated by
reference.
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