U.S. patent application number 15/227135 was filed with the patent office on 2017-03-02 for sulfur-based positive-electrode active material, positive electrode and lithium-ion secondary battery.
This patent application is currently assigned to SUMITOMO RUBBER INDUSTRIES, LTD.. The applicant listed for this patent is NATIONAL INSTITUTE OF ADVANCED INDUSTRIAL SCIENCE AND TECHNOLOGY, SUMITOMO RUBBER INDUSTRIES, LTD.. Invention is credited to Toshikatsu KOJIMA, Tatsuya KUBO, Tetsuo SAKAI, Akihiro YAMANO, Masahiro YANAGIDA.
Application Number | 20170062809 15/227135 |
Document ID | / |
Family ID | 56618065 |
Filed Date | 2017-03-02 |
United States Patent
Application |
20170062809 |
Kind Code |
A1 |
KUBO; Tatsuya ; et
al. |
March 2, 2017 |
SULFUR-BASED POSITIVE-ELECTRODE ACTIVE MATERIAL, POSITIVE ELECTRODE
AND LITHIUM-ION SECONDARY BATTERY
Abstract
An object of the present invention is to provide a novel
sulfur-based positive-electrode active material which can largely
improve cyclability of a lithium-ion secondary battery, a positive
electrode comprising the positive-electrode active material and a
lithium-ion secondary battery comprising the positive electrode.
The sulfur-based positive-electrode active material is one
comprising: a carbon skeleton derived from a polymer composed of a
monomer unit having at least one hetero atom-containing moiety, and
sulfur incorporated into the carbon skeleton as the carbon skeleton
is formed from the polymer by heat treatment.
Inventors: |
KUBO; Tatsuya; (Kobe-shi,
JP) ; KOJIMA; Toshikatsu; (Ikeda-shi, JP) ;
SAKAI; Tetsuo; (Ikeda-shi, JP) ; YAMANO; Akihiro;
(Ikeda-shi, JP) ; YANAGIDA; Masahiro; (Ikeda-shi,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SUMITOMO RUBBER INDUSTRIES, LTD.
NATIONAL INSTITUTE OF ADVANCED INDUSTRIAL SCIENCE AND
TECHNOLOGY |
Kobe-shi
Tokyo |
|
JP
JP |
|
|
Assignee: |
SUMITOMO RUBBER INDUSTRIES,
LTD.
Kobe-shi
JP
NATIONAL INSTITUTE OF ADVANCED INDUSTRIAL SCIENCE AND
TECHNOLOGY
Tokyo
JP
|
Family ID: |
56618065 |
Appl. No.: |
15/227135 |
Filed: |
August 3, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C01B 32/182 20170801;
H01M 4/364 20130101; H01M 2004/028 20130101; C08G 65/38 20130101;
H01M 4/602 20130101; C08F 8/34 20130101; C08L 39/06 20130101; Y02P
70/50 20151101; H01M 10/0525 20130101; H01M 4/625 20130101; Y02E
60/10 20130101; H01M 10/052 20130101; C01B 32/20 20170801; H01B
1/04 20130101; H01M 4/137 20130101; H01B 1/10 20130101; C01B 32/158
20170801; H01M 4/136 20130101; H01M 4/133 20130101; C08K 3/04
20130101; H01M 4/587 20130101; C08K 3/04 20130101; C08L 39/06
20130101; C08F 8/34 20130101; C08F 126/06 20130101; C08F 8/34
20130101; C08F 112/30 20200201 |
International
Class: |
H01M 4/36 20060101
H01M004/36; H01M 4/62 20060101 H01M004/62; H01M 10/0568 20060101
H01M010/0568; H01M 10/0569 20060101 H01M010/0569; C08F 112/14
20060101 C08F112/14; C08F 126/06 20060101 C08F126/06; C08K 3/04
20060101 C08K003/04; C08F 265/04 20060101 C08F265/04; C08G 65/38
20060101 C08G065/38; H01M 10/0525 20060101 H01M010/0525; H01M 4/38
20060101 H01M004/38 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 2, 2015 |
JP |
2015-172845 |
Claims
1. A sulfur-based positive-electrode active material comprising: a
carbon skeleton derived from a polymer composed of a monomer unit
having at least one hetero atom-containing moiety, and sulfur
incorporated into the carbon skeleton as the carbon skeleton is
formed from the polymer by heat treatment, wherein the hetero
atom-containing moiety is a moiety having a group selected from the
group consisting of a monovalent functional group having at least
one hetero atom selected from the group consisting of O, S, P and
N, a heterocyclic group having at least one hetero atom selected
from the group consisting of O, S, P and N, and a group represented
by --S.sub.a-- ("a" is an integer of 2 to 4).
2. The sulfur-based positive-electrode active material of claim 1,
wherein the polymer composed of a monomer unit having at least one
hetero atom-containing moiety is one represented by the following
formula (1) or (2). ##STR00009## wherein R.sup.1 represents a
hydrogen atom or an alkyl group, X.sup.1 represents a group having
a monovalent functional group having a hetero atom selected from
the group consisting of O, S, P and N, or a group having a
heterocyclic group having a hetero atom selected from the group
consisting of O, S, P and N, "n" represents an integer.
##STR00010## wherein R.sup.2 represents an alkyl group, "a"
represents an integer of 2 to 4, "m" represents an integer of 2 to
12.
3. The sulfur-based positive-electrode active material of claim 1,
wherein the heterocyclic group is a 5- to 14-membered heterocyclic
group having 1 to 3 hetero atoms selected from the group consisting
of O, S, P and N.
4. The sulfur-based positive-electrode active material of claim 1,
wherein the monovalent functional group is at least one selected
from the group consisting of a hydroxyl group, a sulfo group, a
carboxyl group, a phosphate group and an ammonium group, and the
heterocyclic group is one selected from the group consisting of
pyrrolidine, pyrrole, pyridine, imidazole, pyrolidone,
tetrahydrofuran, triazine, thiophene, oxazole, thiazole, phosphole,
indole, benzimidazole, quinoline, carbazole, thianthrene,
phenoxazine, phenothiazine, xanthene, thieno[3,2-b]thiophene,
benzothiophene and phosphindole.
5. The sulfur-based positive-electrode active material of claim 1,
wherein the polymer is at least one selected from the group
consisting of polyvinylpyridine, phosphorylcholine polymer,
alkylphenol-sulfur chloride condensate, and polystyrene sulfonic
acid.
6. The sulfur-based positive-electrode active material of claim 1,
wherein a weight average molecular weight of the polymer is from
2000 to 1500000.
7. The sulfur-based positive-electrode active material of claim 1,
wherein at the heat treatment, an electrically conductive carbon
material is further mixed in addition to the polymer and the
sulfur.
8. The sulfur-based positive-electrode active material of claim 7,
wherein the electrically conductive carbon material is a carbon
material having a graphite structure.
9. The sulfur-based positive-electrode active material of claim 1,
wherein a total content of the sulfur is not less than 50% by
mass.
10. The sulfur-based positive-electrode active material of claim 1,
wherein the sulfur-based positive-electrode active material is one
prepared by a preparation process comprising a step of
heat-treating the polymer composed of a monomer unit having at
least one hetero atom-containing moiety and sulfur under a
non-oxidizing atmosphere, and a heat-treating temperature is from
250.degree. C. to 550.degree. C.
11. A sulfur-based positive-electrode active material prepared by a
preparation process comprising a step of heat-treating a polymer
composed of a monomer unit having at least one hetero
atom-containing moiety, and sulfur under a non-oxidizing
atmosphere, wherein the hetero atom-containing moiety is a moiety
having a group selected from the group consisting of a monovalent
functional group having at least one hetero atom selected from the
group consisting of O, S, P and N, a heterocyclic group having at
least one hetero atom selected from the group consisting of O, S, P
and N, and a group represented by --S.sub.a-- ("a" is an integer of
2 to 4), and a heat-treating temperature is from 250.degree. C. to
550.degree. C.
12. A positive-electrode comprising the sulfur-based
positive-electrode active material of claim 1.
13. A lithium-ion secondary battery comprising the positive
electrode of claim 12.
Description
TECHNICAL FIELD
[0001] The present invention relates to a novel sulfur-based
positive-electrode active material which can be used on a
lithium-ion secondary battery, a positive electrode comprising the
sulfur-based positive-electrode active material and a lithium-ion
secondary battery comprising the positive electrode.
BACKGROUND ART
[0002] Since a lithium-ion secondary battery, one type of
nonaqueous electrolyte secondary batteries, has a large charging
and discharging capacity, it has been used mainly as a battery for
portable electronic devices. Moreover, the number of lithium-ion
secondary batteries used as a battery for electric automobiles has
been increasing, and enhancement of performance thereof is
expected.
[0003] Generally, materials comprising a rare metal such as cobalt
or nickel are used as a positive-electrode active material of a
lithium-ion secondary battery. However, due to the fact that rare
metals are small in the distributed amount, not always easily
available and additionally expensive, a positive-electrode active
material using a material that replaces a rare metal has been
required. Further, in the case of a positive-electrode active
material comprising an oxidized compound, oxygen in the
positive-electrode active material is released due to overcharging,
or the like, and as a result, an organic electrolyte and a current
collector are oxidized and burnt, which may cause firing,
explosion, and the like.
[0004] On the other hand, a technique of using sulfur as a
positive-electrode active material is known. In the case where
sulfur is used as a positive-electrode active material, this sulfur
is easily available compared to rare metals and is inexpensive, and
has a further advantage that a charging and discharging capacity of
a lithium-ion secondary battery can be made larger than the present
state. For example, it is known that a lithium-ion secondary
battery using sulfur as a positive-electrode active material can
achieve about 6 times larger charging and discharging capacity than
a lithium-ion secondary battery using lithium cobalt oxide which is
a general positive-electrode material. Further, sulfur is low in
reactivity compared to oxygen, and there is a less risk of causing
firing, explosion, and the like due to overcharging. However, the
lithium-ion secondary battery using elemental sulfur as the
positive-electrode active material has a problem that a battery
capacity is deteriorated through repeated charging and discharging.
That is, elemental sulfur likely generates a compound with lithium
when discharging and since the generated compound is soluble into a
nonaqueous electrolyte (for example, ethylene carbonate and
dimethyl carbonate and the like) of the lithium-ion secondary
battery, the charging and discharging capacity is gradually reduced
through repeated charging and discharging due to the sulfur eluting
into the electrolyte.
[0005] In order to improve cyclability (a property of maintaining a
charging and discharging capacity in spite of repeated charging and
discharging) by preventing sulfur from eluting into an electrolyte,
a positive-electrode active material comprising sulfur and a
material other than sulfur (for example, a carbon material) has
been proposed. For example, Patent Document 1 discloses a technique
of using a specific carbon polysulfide comprising carbon and sulfur
as main component elements. Further, Patent Document 2 discloses a
sulfur-based positive-electrode active material obtained by
heat-treating a mixture of polyisoprene and sulfur powder.
PRIOR ART DOCUMENTS
Patent Documents
[0006] Patent Document 1: JP 2002-154815 A [0007] Patent Document
2: JP 2012-150933 A
SUMMARY OF THE INVENTION
Problem to be Solved by the Invention
[0008] However, there is still a room for improving cyclability of
a lithium-ion secondary battery. An object of the present invention
is to provide a novel sulfur-based positive-electrode active
material which can largely improve cyclability of a lithium-ion
secondary battery, a positive electrode comprising the
positive-electrode active material and a lithium-ion secondary
battery comprising the positive electrode.
Means to Solve the Problem
[0009] The present inventors have made intensive studies to solve
the above-mentioned problem and as a result, have found that a
sulfur-based positive-electrode active material exhibiting
excellent properties can be obtained by heat-treating a polymer
composed of a given monomer unit having at least one hetero
atom-containing moiety and sulfur under a non-oxidizing atmosphere.
The present inventors have made further studies and have completed
the present invention.
[0010] Namely, the present invention relates to: [0011] [1] a
sulfur-based positive-electrode active material comprising: [0012]
a carbon skeleton derived from a polymer composed of a monomer unit
having at least one hetero atom-containing moiety, and sulfur
incorporated into the carbon skeleton as the carbon skeleton is
formed from the polymer by heat treatment, [0013] wherein the
hetero atom-containing moiety is a moiety having a group selected
from the group consisting of a monovalent functional group having
at least one hetero atom selected from the group consisting of O,
S, P and N, a heterocyclic group having at least one hetero atom
selected from the group consisting of O, S, P and N, and a group
represented by --S.sub.a-- ("a" is an integer of 2 to 4), [0014]
[2] the sulfur-based positive-electrode active material according
to the above [1], wherein the polymer composed of a monomer unit
having at least one hetero atom-containing moiety is one
represented by the following formula (1) or (2):
##STR00001##
[0014] wherein R.sup.1 represents a hydrogen atom or an alkyl
group, the alkyl group is one having 1 to 4 carbon atoms, more
preferably methyl, X.sup.1 represents a group having a monovalent
functional group having hetero atom selected from the group
consisting of O, S, P and N, or a group having a heterocyclic group
having hetero atom selected from the group consisting of O, S, P
and N, "n" represents an integer, or
##STR00002##
wherein R.sup.2 represents an alkyl group, the alkyl group is one
having 5 to 12 carbon atoms, more preferably one having 6 to 10
carbon atoms, further preferably one having 7 to 9 carbon atoms,
most preferably one having 8 carbon atoms, "a" represents an
integer of 2 to 4, "m" represents an integer of 2 to 12, [0015] [3]
the sulfur-based positive-electrode active material according to
the above [1] or [2], wherein the heterocyclic group is a 5- to
14-membered heterocyclic group having 1 to 3 hetero atoms selected
from the group consisting of O, S, P and N, [0016] [4] the
sulfur-based positive-electrode active material according to any
one of the above [1] to [3], wherein the monovalent functional
group is at least one selected from the group consisting of a
hydroxyl group, a sulfo group, a carboxyl group, a phosphate group
and an ammonium group, and [0017] the heterocyclic group is one
selected from the group consisting of pyrrolidine, pyrrole,
pyridine, imidazole, pyrolidone, tetrahydrofuran, triazine,
thiophene, oxazole, thiazole, phosphole, indole, benzimidazole,
quinoline, carbazole, thianthrene, phenoxazine, phenothiazine,
xanthene, thieno[3,2-b]thiophene, benzothiophene and phosphindole,
[0018] [5] the sulfur-based positive-electrode active material
according to the above [1], wherein the polymer is at least one
selected from the group consisting of polyvinylpyridine,
phosphorylcholine polymer, alkylphenol-sulfur chloride condensate,
and polystyrene sulfonic acid, [0019] [6] the sulfur-based
positive-electrode active material according to any one of the
above [1] to [5], wherein a weight average molecular weight of the
polymer is from 2000 to 1500000, preferably from 2000 to 1300000,
more preferably from 2000 to 1200000, further preferably from 2000
to 1100000, further preferably from 2000 to 1000000, [0020] [7] the
sulfur-based positive-electrode active material according to any
one of the above [1] to [6], wherein at the heat treatment, an
electrically conductive carbon material is further mixed in
addition to the polymer and the sulfur, [0021] [8] the sulfur-based
positive-electrode active material according to the above [7],
wherein the electrically conductive carbon material is a carbon
material having a graphite structure, [0022] [9] the sulfur-based
positive-electrode active material according to any one of the
above [1] to [8], wherein a total sulfur content in the
sulfur-based positive-electrode active material is not less than
50% by mass, [0023] [10] the sulfur-based positive-electrode active
material according to any one of the above [1] to [9], wherein the
sulfur-based positive-electrode active material is one prepared by
a preparation process comprising a step of heat-treating the
polymer composed of a monomer unit having at least one hetero
atom-containing moiety and sulfur under a non-oxidizing atmosphere,
and a heat-treating temperature is from 250.degree. C. to
550.degree. C., preferably from 300.degree. C. to 450.degree. C.,
[0024] [11] a sulfur-based positive-electrode active material
prepared by a preparation process comprising a step of
heat-treating a polymer composed of a monomer unit having at least
one hetero atom-containing moiety, and sulfur under a non-oxidizing
atmosphere, wherein the hetero atom-containing moiety is a moiety
having a group selected from the group consisting of a monovalent
functional group having at least one hetero atom selected from the
group consisting of O, S, P and N, a heterocyclic group having at
least one hetero atom selected from the group consisting of O, S, P
and N, and a group represented by --S.sub.a-- ("a" is an integer of
2 to 4), and [0025] a heat-treating temperature is from 250.degree.
C. to 550.degree. C., preferably from 300.degree. C. to 450.degree.
C., [0026] [12] a positive electrode comprising the sulfur-based
positive-electrode active material according to any one of the
above [1] to [11], and [0027] [13] a lithium ion secondary battery
comprising the positive electrode of the above [12].
Effect of the Invention
[0028] According to the present invention, it is possible to
prepare a novel sulfur-based positive-electrode active material
which can largely improve a charging and discharging capacity and
cyclability of a lithium-ion secondary battery.
[0029] Herein "cyclability" means a property of maintaining a
charging and discharging capacity of a secondary battery in spite
of repeated charging and discharging. Therefore, while, as the
charging and discharging are repeated, a lithium-ion secondary
battery in which a degree of reduction of a charging and
discharging capacity is large and a capacity retention rate is low
is inferior in cyclability, a lithium-ion secondary battery in
which a degree of reduction of a charging and discharging capacity
is small and a capacity retention rate is high is excellent in
cyclability.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is a sectional view schematically illustrating a
reaction apparatus used for production of a sulfur-based
positive-electrode active material in Examples of the present
invention.
[0031] FIG. 2 is a graph showing a result of cyclic charging and
discharging in Comparative Example 1, and Examples 1 and 5.
DESCRIPTION OF EMBODIMENTS
[0032] Embodiments of the present invention are explained below in
detail.
[0033] The sulfur-based positive-electrode active material of the
present invention comprises a carbon skeleton derived from a
polymer (hereinafter optionally referred to as "a starting
compound") composed of a monomer unit having at least one hetero
atom-containing moiety and sulfur incorporated into the carbon
skeleton as the carbon skeleton is formed from the polymer by heat
treatment, and can be prepared, for example, by a preparation
process comprising a step of heat-treating the polymer composed of
a monomer unit having at least one hetero atom-containing moiety
and sulfur under a non-oxidizing atmosphere.
<Polymer (Starting Compound)>
(Hetero Atom-Containing Moiety)
[0034] In the polymer according to the present invention, "a hetero
atom-containing moiety" is a moiety having a group selected from
the group consisting of a monovalent functional group having at
least one hetero atom selected from the group consisting of O, S, P
and N, a heterocyclic group having at least one hetero atom
selected from the group consisting of O, S, P and N, and a group
represented by --S.sub.a-- ("a" is an integer of 2 to 4).
[0035] Example of "a monovalent functional group having hetero atom
selected from the group consisting of O, S, P and N" includes at
least one selected from the group consisting of a hydroxyl group, a
sulfo group, a carboxyl group, a phosphate group and an ammonium
group. The monovalent functional group may have a substituent
group.
[0036] In this case, examples of the substituent group include the
above-mentioned functional groups. Namely, these monovalent
functional groups may be further replaced with another monovalent
functional group or the same monovalent functional group as above,
and the replacement can be made plural times. In that case, a
spacer such as an alkylene group may be present between the
monovalent functional groups. Examples of the alkylene group
include those having 1 to 4 carbon atoms such as methylene,
ethylene and trimethylene.
[0037] Examples of the "heterocyclic group having hetero atom
selected from the group consisting of O, S, P and N" include 5- to
14-membered heterocyclic groups having 1 to 3 hetero atoms selected
from the group consisting of O, S, P and N. Here, a heterocyclic
ring constituting the heterocyclic group may be, for example, a
monocyclic ring such as pyrrolidine, pyrrole, pyridine, imidazole,
pyrolidone, tetrahydrofuran, triazine, thiophene, oxazole, thiazole
or phosphole, or a polycyclic ring such as indole, benzimidazole,
quinoline, carbazole, thianthrene, phenoxazine, phenothiazine,
xanthene, thieno[3,2-b]thiophene, benzothiophene or phosphindole,
and is selected from the group consisting thereof. These
heterocyclic groups may have a substituent group, or may be an
unsubstituted group. In the case where the heterocyclic group has a
substituent group, examples of the substituent group include the
above-mentioned monovalent functional groups.
(Polymer Composed of a Monomer Unit Having at Least One Hetero
Atom-Containing Moiety)
[0038] Preferred examples of the "polymer composed of a monomer
unit having at least one hetero atom-containing moiety" include
those represented by the following formula (1) or (2).
##STR00003##
wherein R.sup.1 represents a hydrogen atom or an alkyl group,
X.sup.1 represents a group having a monovalent functional group
having hetero atom selected from the group consisting of O, S, P
and N, or a group having a heterocyclic group having hetero atom
selected from the group consisting of O, S, P and N, "n" is an
integer.
##STR00004##
wherein R.sup.2 represents an alkyl group, "a" represents an
integer of 2 to 4, "m" represents an integer of 2 to 12.
[0039] In the formula (1), the alkyl group of R.sup.1 is preferably
one having 1 to 4 carbon atoms and is particularly preferably
methyl. In the formula (2), the alkyl group of R.sup.2 is
preferably one having 5 to 12 carbon atoms, more preferably one
having 6 to 10 carbon atoms, further preferably one having 7 to 9
carbon atoms, most preferably one having 8 carbon atoms.
[0040] Herein the alkyl group is either of one having a straight
chain and one having a branched chain, and the one having a
straight chain is preferred.
[0041] More preferred example of the polymer composed of a monomer
unit having at least one hetero atom-containing moiety is not
limited particularly and is at least one selected from the group
consisting of polyvinylpyridine, phosphorylcholine polymer,
alkylphenol-sulfur chloride condensate, and polystyrene sulfonic
acid. Further, a preferred polymer is one having hetero
atom-containing moiety in its side chain.
[0042] Polyvinylpyridine is a compound represented by the following
formula (3).
##STR00005##
wherein q.sup.1 represents an integer.
[0043] There exist, as the above-mentioned polyvinylpyridine, three
isomers such as poly (2-vinylpyridine), poly(3-vinylpyridine) and
poly(4-vinylpyridine), and among these, poly(4-vinylpyridine) is
preferred.
[0044] Example of the phosphorylcholine polymer includes a compound
(2-methacryloyloxyethyl phosphorylcholine polymer) represented by
the following formula (4).
##STR00006##
wherein q.sup.2 represents an integer.
[0045] Example of the alkylphenol-sulfur chloride condensate
includes a compound represented by the following formula (5).
##STR00007##
wherein R.sup.3 represents an alkyl group having 5 to 12 carbon
atoms, q.sup.3 represents an integer.
[0046] The alkyl group of R.sup.3 is preferably one having 6 to 10
carbon atoms, more preferably one having 7 to 9 carbon atoms,
further preferably one having 8 carbon atoms.
[0047] The compound represented by the formula (5) is preferably a
condensate of octylphenol and sulfur chloride (brand name Tackirol
V200 available from Taoka Chemical Co., Ltd.).
[0048] Example of the polystyrene sulfonic acid includes a compound
represented by the following formula (6).
##STR00008##
wherein q.sup.4 represents an integer.
[0049] There exist, as the above-mentioned polystyrene sulfonic
acid, three isomers such as poly(o-styrenesulfonic acid),
poly(m-styrenesulfonic acid) and poly(p-styrenesulfonic acid), and
among these, poly(p-styrenesulfonic acid) is preferred.
(Weight Average Molecular Weight of Polymer (Mw))
[0050] Mw of the polymer is preferably from 2000 to 1500000. Since
Mw is not less than 2000, there is a tendency that an amount of
sulfur to be incorporated into the carbon skeleton derived from the
polymer increases during the heat treatment. Meanwhile, there is a
tendency that even if Mw exceeds 1500000, the amount of sulfur
hardly increases and that since Mw is not more than 1500000, an
adequate sulfur content can be achieved. Further, Mw is not more
than 1500000, which makes process advantageous, for example, mixing
with sulfur is easier. Mw of the polymer is more preferably within
a range from 2000 to 1300000, more preferably within a range from
2000 to 1200000, further preferably within a range from 2000 to
1100000, still further preferably within a range from 2000 to
1000000. Mw is a value (calibrated based on polystyrene) measured
by gel permeation chromatography (GPC).
(Preparation of Polymer)
[0051] The polymer is commercially available or can be prepared by
a usual process within a scope of knowledge of a person ordinarily
skilled in the art.
<Step of Heat Treatment>
[0052] The heat treatment of the starting compound can be carried
out by mixing the compound with sulfur and heat-treating the
mixture under a non-oxidizing atmosphere. Here in the case of the
starting compound being a solid, its reactivity with sulfur can be
increased by pulverizing the compound and then mixing with sulfur.
By the heat treatment, the target sulfur-based positive-electrode
active material of the present invention can be prepared.
(Sulfur)
[0053] Sulfur in various forms such as powdery sulfur, insoluble
sulfur, precipitated sulfur, colloidal sulfur and the like may be
used. It is noted that from the viewpoint of uniform dispersion of
sulfur into the starting compound, colloidal sulfur which is fine
particles is preferred. The compounding ratio of sulfur is
preferably not less than 250 parts by mass, more preferably not
less than 300 parts by mass based on 100 parts by mass of the
starting compound. Since the compounding ratio is not less than 250
parts by mass, there is a tendency that a charging and discharging
capacity and cyclability can be increased. On the other hand, while
there is no upper limit of the compounding ratio of sulfur, the
compounding ratio is usually not more than 1500 parts by mass,
preferably not more than 1000 parts by mass. Even if the ratio
exceeds 1500 parts by mass, there is a tendency that a charging and
discharging capacity or cyclability can not be improved
sufficiently, and there is a tendency that the ratio of not more
than 1500 parts by mass is advantageous from the viewpoint of
cost.
(Electrically-Conductive Carbon Material)
[0054] When mixing the starting compound with sulfur, a carbon
material having electric conductivity may be further added for the
purpose of enhancing electric conductivity of the obtained
sulfur-based positive-electrode active material. A carbon material
having a graphite structure is preferable as such an
electrically-conductive carbon material. Examples of usable carbon
material include carbon materials having a fused aromatic ring
structure such as carbon black, graphite, carbon nanotube (CNT),
carbon fiber (CF), graphene, fullerene and the like. One or more
thereof can be used as the electrically conductive carbon
material.
[0055] Among them, carbon black is preferable since it is
inexpensive and excellent in dispersibility. Also, a small amount
of CNT or graphene may be combined with carbon black. In accordance
with such combination, cyclability of a lithium-ion secondary
battery can be further improved without largely increasing a cost.
The combined amount of CNT or graphene is preferably not less than
8% by mass and not more than 12% by mass based on the total amount
of electrically-conductive carbon material.
[0056] The compounding ratio of the electrically conductive carbon
material is preferably not less than 5 parts by mass, more
preferably not less than 10 parts by mass based on 100 parts by
mass of the starting compound. Since the compounding ratio is not
less than 5 parts by mass, a purpose of further enhancing a
charging and discharging capacity and cyclability tends to be
easily achieved. On the other hand, the compounding ratio is
preferably not more than 50 parts by mass, more preferably not more
than 30 parts by mass. Since the compounding ratio is not more than
50 parts by mass, there is a tendency that a purpose of further
enhancing a charging and discharging capacity and cyclability is
easily achieved without relatively lowering a ratio of a
sulfur-containing structure in the sulfur-based positive-electrode
active material.
(Conditions for Heat Treatment)
[0057] Heat treatment is performed by heating under a non-oxidizing
atmosphere. The non-oxidizing atmosphere means an atmosphere
substantially containing no oxygen and is used to prevent an
oxidative deterioration or an excess thermal decomposition of the
components. Specifically, the heat treatment is carried out under
an inert gas atmosphere in a silica tube filled with an inert gas
such as nitrogen or argon. The temperature of the heat treatment is
preferably within a range from 250.degree. C. to 550.degree. C.
Since the heat-treating temperature is not less than 250.degree.
C., there is a tendency that an insufficient sulfurizing reaction
is avoided and lowering of a charging and discharging capacity of
the target product can be prevented. On the other hand, when not
more than 550.degree. C., there is a tendency that decomposition of
the starting compound can be prevented and decrease in yield and
lowering of a charging and discharging capacity can be prevented.
The heat-treating temperature is preferably not less than
300.degree. C., more preferably not less than 450.degree. C. A
period of time for the heat treatment is preferably 2 to 6 hours.
When the heat-treating time is not less than 2 hours, there is a
tendency that the heat treatment can be advanced sufficiently, and
when the heat-treating time is not more than 6 hours, there is a
tendency that excessive thermal decomposition of the components can
be prevented. The sulfur-based positive-electrode active material
can also be produced by heat-treating while kneading the starting
compound, sulfur and the like in a continuous apparatus such as a
twin-screw extruder.
(Sulfur Removing Step)
[0058] In the treated product obtained after the heat treatment,
there remains a so-called unreacted sulfur which results from
cooling and deposition of sulfur sublimated at the heat treatment.
It is desirable to remove such unreacted sulfur as much as possible
since it causes deterioration of cyclability. Unreacted sulfur can
be removed by usual methods, for example, a removal by heating
under a reduced pressure, a removal by warm wind, a removal by
washing with a solvent and the like.
(Pulverization, and Classification)
[0059] The produced sulfur-based positive-electrode active material
is pulverized so as to be predetermined grain sizes and is
classified to be particles suitable for production of a positive
electrode. A preferred particle size distribution of the particles
is from about 5 to 20 .mu.m in a median size. It is noted that in
the above-explained heat treatment method using a twin-screw
extruder, the produced sulfur-based positive-electrode active
material can also be pulverized at the same time due to shearing at
kneading.
<Sulfur-Based Positive-Electrode Active Material>
[0060] The thus obtained sulfur-based positive-electrode active
material is mainly composed of carbon and sulfur and as the content
of sulfur increases, a charging and discharging capacity and
cyclability tend to be improved. Therefore, there is a tendency
that the content of sulfur as large as possible is preferable. The
content of sulfur in the sulfur-based positive-electrode active
material is preferably not less than 50% by mass. In the case where
an electrically conductive carbon material is compounded, even if
the sulfur content is below 50% by mass, an effect of enhancing a
charging and discharging capacity and cyclability can be expected
due to an influence of carbon constituting the electrically
conductive carbon material. In such a case, the sulfur content is
preferably not less than 45% by mass in the sulfur-based
positive-electrode active material.
[0061] By the heat treatment, hydrogen (H) in the starting compound
reacts with sulfur to be hydrogen sulfide, as a result, decreasing
in the resultant sulfide. It is preferable that the content of
hydrogen in the sulfur-based positive-electrode active material is
not more than 1.6% by mass. In the case of not more than 1.6% by
mass, there is a tendency that the heat treatment (sulfurization)
was carried out sufficiently. Therefore, in that case, a charging
and discharging capacity tends to be enhanced. The content of
hydrogen is more preferably not more than 1.0% by mass, further
preferably not more than 0.5% by mass, further preferably not more
than 0.1% by mass.
[0062] Herein contents of elements are measured by elemental
analysis in accordance with a usual method.
<Lithium-Ion Secondary Battery>
[0063] The sulfur-based positive-electrode active material of the
present invention can be used as the positive-electrode active
material of the lithium-ion secondary battery. The lithium-ion
secondary battery of the present invention using the sulfur-based
positive-electrode active material has a large charging and
discharging capacity and is excellent in cyclability.
[0064] The lithium-ion secondary battery of the present invention
can be produced by a usual method using a positive electrode
comprising the sulfur-based positive-electrode active material, a
negative electrode, an electrolyte, and further members such as a
separator as desired.
<Positive Electrode>
[0065] The positive electrode of the lithium-ion secondary battery
can be produced in the same manner as in a general positive
electrode of a lithium-ion secondary battery except that the above
sulfur-based positive-electrode active material is used as a
positive-electrode active material. For example, a particulate of
the sulfur-based positive-electrode active material is mixed with
an electrically-conductive additive, a binder and a solvent to
prepare a paste-like positive-electrode active material and the
positive-electrode active material is applied on a current
collector and dried to produce a positive electrode. Otherwise, it
is also possible that the sulfur-based positive-electrode active
material of the present invention is kneaded together with an
electrically-conductive additive, a binder and a small amount of
solvent using a mortar or the like, and the kneaded mixture is
formed into a film shape and then pressed against a current
collector using a pressing machine or the like to produce a
positive electrode.
(Electrically-Conductive Additive)
[0066] Examples of an electrically-conductive additive include
vapor grown carbon fibers (Vapor Grown Carbon Fibers: VGCF), carbon
powders, carbon black (CB), acetylene black (AB), KETJENBLACK (KB),
graphite, fine powders of metals being stable at positive-electrode
potentials, such as aluminum and titanium and the like. One or more
thereof can be used as the conductive additive.
(Binder)
[0067] Examples of a binder include polyvinylidene difluoride
(PVDF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber
(SBR), polyimide (PI), polyamide-imide (PAI), carboxymethyl
cellulose (CMC), polyvinyl chloride (PVC), methacryl resins (PMA),
polyacrylonitrile (PAN), modified polyphenylene oxide (PPO),
polyethylene oxide (PEO), polyethylene (PE), polypropylene (PP) and
the like. One or more thereof can be used as the binder.
(Solvent)
[0068] Examples of a solvent include N-methyl-2-pyrrolidone,
N,N-dimethylformaldehyde, alcohols, water and the like. One or more
thereof can be used as the solvent.
(Compounding Ratio)
[0069] The compounding ratio of each of the above components
constituting the positive electrode is not limited particularly but
for example, it is preferable to compound 20 to 100 parts by mass
of an electrically-conductive additive, 10 to 20 parts by mass of a
binder and an appropriate amount of a solvent based on 100 parts by
mass of the sulfur-based positive-electrode active material.
(Current Collector)
[0070] As for a current collector, those which have been used
commonly on positive electrodes for a lithium-ion secondary battery
may be used. Examples of a current collector include aluminum
foils, aluminum meshes, punched aluminum sheets, aluminum expanded
sheets, stainless-steel foils, stainless-steel meshes, punched
stainless-steel sheets, stainless-steel expanded sheets, foamed
nickel, nickel nonwoven fabrics, copper foils, copper meshes,
punched copper sheets, copper expanded sheets, titanium foils,
titanium meshes, carbon nonwoven fabrics, carbon woven fabrics and
the like. Among these, a carbon nonwoven fabric current collector
and a carbon woven fabric current collector, which are composed of
carbon with a high graphitization degree, are suitable for a
current collector in the case of using the sulfur-based
positive-electrode active material as a positive-electrode active
material because it does not include hydrogen and has low
reactivity to sulfur. As for a starting material for a carbon fiber
with a high graphitization degree, it is possible to use various
types of pitches (namely, the byproducts of petroleum, coal, coal
tar, and so on) that make a material for carbon fibers, or
polyacrylonitrile (PAN) fibers and the like.
(Negative Electrode)
[0071] Examples of a negative electrode material include known
metallic lithium, carbon-based materials such as graphite,
silicon-based materials such as a silicon thin film, alloy-based
materials such as copper-tin or cobalt-tin and the like. Among the
above-mentioned negative electrode materials, in the case where a
carbon-based material, a silicon-based material, an alloy-based
material or the like that does not include lithium is used, it is
advantageous from a point that short-circuiting between positive
and negative electrodes, which results from production of dendrite,
is less likely to arise. However, in the case where a negative
electrode material that does not include lithium is used in
combination with the positive electrode of the present invention,
neither the positive electrode nor the negative electrode includes
lithium and thus a pre-doping treatment, in which lithium is
inserted into either one of the negative electrode or positive
electrode, or into both of them, becomes necessary. For a method of
lithium pre-doping, a publicly known method can be used. For
example, in the case where a negative electrode is doped with
lithium, the following methods of inserting lithium can be given:
an electrolytically-doping method, in which a half-cell is
assembled using metallic lithium as the counter electrode and then
doping lithium electrochemically; and an application pre-doping
method, in which doping is done by a diffusion of lithium onto an
electrode by applying a metallic lithium foil onto the electrode
and then leaving the electrode with the metallic lithium foil
applied as it is within an electrolytic solution. Moreover, in
another case as well where the positive electrode is pre-doped with
lithium, it is possible to utilize the aforementioned
electrolytically-doping method. Silicon-based materials, which are
high capacity negative electrode materials, are preferred as a
negative electrode material that does not include lithium. Among
them, a silicon thin film that can make a thickness of the
electrode thinner and is advantageous in capacity per volume is
particularly preferable.
(Electrolyte)
[0072] As for an electrolyte to be used on the lithium-ion
secondary battery, it is possible to use those in which an
alkali-metal salt serving as an electrolyte is dissolved in an
organic solvent. Examples of a preferred organic solvent include at
least one selected from nonaqueous solvents, such as ethylene
carbonate, propylene carbonate, dimethyl carbonate, diethyl
carbonate, ethyl methyl carbonate, dimethyl ether,
.gamma.-butyrolactone, and acetonirile. Examples of a usable
electrolyte include LiPF.sub.6, LiBF.sub.4, LiAsF.sub.6,
LiCF.sub.3SO.sub.3, LiI, LiClO.sub.4 and the like. A concentration
of the electrolyte can be from about 0.5 mol/liter to 1.7
mol/liter. It is noted that the electrolyte is not limited to a
liquid form. For example, in the case where the lithium-ion
secondary battery is a lithium polymer secondary battery, the
electrolyte is a solid form (for example, a form of polymer
gel).
(Separator)
[0073] In addition to the above-described negative electrode,
positive electrode and electrolyte, the lithium-ion secondary
battery can be further equipped with the other members, such as
separators, as well. A separator intervenes between the positive
electrode and the negative electrode, thereby not only allowing the
movements of ions between the positive electrode and the negative
electrode but also functioning to prevent the positive electrode
and the negative electrode from internally short-circuiting one
another. When the lithium-ion secondary battery is a
hermetically-closed type, a function of retaining the electrolytic
solution is required for the separator. As for a separator, it is
preferable to use a thin-thickness and microporous or
nonwoven-shaped film that is made of a material, such as
polyethylene, polypropylene, polyacrylonitrile, aramid, polyimide,
cellulose, glass and the like.
(Shape)
[0074] A configuration of the lithium-ion secondary battery is not
limited particularly, and can be formed as a variety of
configurations, such as cylindrical types, laminated types, coin
types, button types and the like.
EXAMPLE
[0075] The present invention is explained by means of Examples, but
is not limited to the Examples.
[0076] Various chemicals used herein are collectively shown below.
The various chemicals were subjected to purification according to
necessity by a usual method.
<Materials Used for Test>
[0077] Rubber: Natural rubber (TSR20) [0078] Polymer 1:
Polyvinylpyridine (a reagent available from SIGMA-ALDRICH) [0079]
Polymer 2: 2-Methacryloyloxyethyl phosphorylcholine polymer [0080]
Polymer 3: Condensate of octylphenol and sulfur chloride (brand
name Tackirol V200 available from Taoka Chemical Co., Ltd.) [0081]
Polymer 4: Poly(p-styrenesulfonic acid) (a reagent available from
Wako Pure Chemical Industries, Ltd.) [0082] Carbon black: Acetylene
black (Denka black (registered trademark) available from DENKI
KAGAKU KOGYO KABUSHIKI KAISHA [0083] Sulfur: Colloidal sulfur
available from Tsurumi Chemical Industry Co., Ltd.
Comparative Example 1
<Production of Positive-Electrode Active Material>
(Preparation of Starting Compound)
[0084] To 100 parts by mass of natural rubber was compounded 500
parts by mass of sulfur and the compounded mixture was kneaded
using a kneading testing device [MIX-LABO manufactured by Moriyama
Company, Ltd.] to prepare a starting compound. The thus obtained
starting compound was cut into small pieces of not more than 3 mm
using scissors and then was subjected to heat treatment.
(Reaction Apparatus)
[0085] A reaction apparatus 1 as illustrated in FIG. 1 was used for
heat treatment of the starting compound. The reaction apparatus 1
comprises a reaction container 3, which has an outer diameter of 60
mm, an inner diameter of 50 mm and a height of 300 mm and is made
of quartz glass, that is formed as a bottomed cylindrical shape to
contain and heat-treat the starting compound 2; a silicone plug 4
for closing an upper opening of the reaction container 3; one
alumina protection tube 5 ("Alumina SSA-S" available from NIKKATO
CORPORATION, an outer diameter of 4 mm, an inner diameter of 2 mm
and a length of 250 mm) and two tubes, which are a gas introducing
tube 6 and a gas exhausting tube 7 (both are "Alumina SSA-S"
available from NIKKATO CORPORATION, an outer diameter of 6 mm, an
inner diameter of 4 mm and a length of 150 mm), these three tubes
penetrating through the plug 4; and an electric furnace 8 (crucible
furnace, width of an opening: 80 mm dia., heating height: 100 mm)
for heating the reaction container 3 from the bottom side.
[0086] The alumina protection tube 5 is formed in such a length
that the lower part below plug 4 reaches the starting compound 2
contained in the bottom of the reaction container 3 and a
thermocouple 9 is inserted through the inside of the alumina
protection tube 5. The alumina protection tube 5 is used as a
protective tube for the thermocouple 9. The leading end of the
thermocouple 9 is inserted into the starting compound 2 while being
protected by the closed leading end of the alumina protection tube
5 and functions to measure a temperature of the starting compound
2. Output of the thermocouple 9 is input in a temperature
controller 10 of the electric furnace 8 as shown by the solid arrow
in the drawing and the temperature controller 10 functions to
control a heating temperature of the electric furnace 8 based on
the input from the thermocouple 9.
[0087] The gas introducing tube 6 and the gas exhausting tube 7 are
formed such that the bottom end thereof projects in 3 mm downwardly
from the plug 4. Also, the upper part of the reaction container 3
projects from the electric furnace 8 to be exposed to atmosphere.
Therefore, steam of sulfur generating from the starting compound
due to heating of the reaction container 3 is raised to the upper
part of the reaction container 3 as shown by the long dashed short
dashed line arrow in the drawing, and transformed to a liquid drop
while being cooled to be dropped and refluxed as shown by the
broken line arrow in the drawing. Consequently, sulfur in the
reaction system does not leak to the outside through the gas
exhausting tube 7.
[0088] The gas introducing tube 6 is continuously supplied with Ar
gas from a gas supply system which is not shown. The gas exhausting
tube 7 is connected to a trapping bath 12 containing an aqueous
solution 11 of sodium hydroxide. The exhaust gas moving toward the
outside through the gas exhausting tube 7 from the reaction
container 3 is released to the outside after passing through the
aqueous solution 11 of sodium hydroxide in the trapping bath 12.
Therefore, even if hydrogen sulfide gas generated from a
vulcanization reaction is included in the exhaust gas, the hydrogen
sulfide gas is removed therefrom by being neutralized with the
aqueous solution of sodium hydroxide.
(Heat Treatment Step)
[0089] Heating with the electric furnace 8 was started 30 minutes
after starting a continuous supply of Ar gas to the reaction
container 3 holding the starting compound 2 in its bottom at a flow
rate of 80 ml/min from the gas supply system. The temperature
elevation rate was 5.degree. C./min. Since generation of gas was
started when the temperature of the starting compound became
200.degree. C., the heating was continued while adjusting the flow
rate of the Ar gas such that the flow rate of the exhaust gas
became as constant as possible. When the temperature of the
starting compound reached 400.degree. C., heat treatment was
conducted for two hours while maintaining the temperature of
400.degree. C. Then, the starting compound 2 was cooled naturally
under an Ar gas atmosphere to 25.degree. C. while adjusting the
flow rate of the Ar gas and a reaction product was taken out of the
reaction container 3.
(Removal of Unreacted Sulfur)
[0090] In order to remove the unreacted sulfur (free elemental
sulfur) remaining in the product after the heat treatment step, the
following step was carried out. Namely, the product was pulverized
in a mortar and 2 g of a pulverized product was put in a glass tube
oven and heated for three hours at 250.degree. C. while vacuum
suction was conducted to produce a sulfur-based positive-electrode
active material in which unreacted sulfur was removed (or only a
trace amount of unreacted sulfur was contained). The temperature
elevation rate was 10.degree. C./min.
<Preparation of Lithium-Ion Secondary Battery>
(Positive Electrode)
[0091] To 3 mg of the sulfur-based positive-electrode active
material as produced above were added 2.7 mg of acetylene black as
an electrically-conductive additive, 0.3 mg of
polytetrafluoroethylene as a binder and an appropriate amount of
hexane, and the mixture was kneaded in an agate mortar till the
mixture turned into a film shape. Then the entire amount of the
kneaded product in a film shape in the mortar was put on an
aluminum mesh as a current collector with #100 in mesh roughness
that had been punched out to a circle with 14 mm in diameter, and
after being press-fitted with a table pressing machine, the film
was dried for three hours at 100.degree. C. to form a positive
electrode.
(Negative Electrode)
[0092] A metallic lithium foil [manufactured by Honjo Metal Co.,
Ltd.] having a thickness of 0.5 mm was punched out to a circle with
14 mm in diameter to prepare a negative electrode.
(Electrolyte)
[0093] A nonaqueous electrolyte in which LiPF.sub.6 had been
dissolved in a mixed solvent of ethylene carbonate and diethyl
carbonate was used as an electrolyte. A volume ratio of ethylene
carbonate and diethyl carbonate was 1:1. A concentration of
LiPF.sub.6 was 1.0 mol/liter.
(Lithium-Ion Secondary Battery)
[0094] Using the above positive electrode, negative electrode and
electrolyte, a coin-type lithium-ion secondary battery was prepared
in a dry room. Specifically, a separator [Celgard (registered
trademark) 2400 manufactured by Celgard] consisted of a
polypropylene microporous film with 25 .mu.m in thickness and a
glass nonwoven filter with 500 .mu.m in thickness were sandwiched
between the positive electrode and the negative electrode to form
an electrode-assembly battery.
[0095] Then, the formed electrode-assembly battery was accommodated
in a battery case (e.g., a member for CR2032-type coin battery, a
product of HOSEN Co., Ltd.) made of a stainless-steel container and
the electrolyte solution was added thereto. After that, the battery
case was sealed hermetically with a crimping machine, thereby
obtaining a coin-type lithium-ion secondary battery.
Examples 1 to 5
[0096] Starting compounds, sulfur-based positive-electrode active
materials and lithium-ion secondary batteries were prepared in the
same manner as in Comparative Example 1 except that the starting
compounds were prepared in accordance with the formulations shown
in Table 1. Before subjecting the starting compounds to heat
treatment, they were pulverized for three minutes with a cutter
mill ("Labo Millser LM-Plus" available from Osaka Chemical Co.,
Ltd.) and used in a form of powder. The starting compounds were
also dried for two hours with a vacuum pump ("a small size single
stage vacuum pump (with a solenoid valve)" available from FUSO
CORPORATION) before use in order to remove moisture therefrom.
Comparative Examples 2 and 3
[0097] Starting compounds, sulfur-based positive-electrode active
materials and lithium-ion secondary batteries were prepared in the
same manner as in Comparative Example 1 except that the starting
compounds were prepared in accordance with the formulations shown
in Table 1 and the heat-treating temperature was changed as shown
in Table 1. Before subjecting the starting compounds to heat
treatment, they were pulverized for three minutes with a cutter
mill ("Labo Millser LM-Plus" available from Osaka Chemical Co.,
Ltd.) and used in a form of powder. The starting compounds were
also dried for two hours with a vacuum pump ("a small size single
stage vacuum pump (with a solenoid valve)" available from FUSO
CORPORATION) before use in order to remove moisture therefrom.
<Measurement of Discharging Capacity and Capacity Retention
Rate>
[0098] With respect to each coin-type lithium-ion secondary battery
prepared in Examples and Comparative Examples, charging and
discharging were carried out at an electric-current value
equivalent to 33.3 mA per 1 g of the positive-electrode active
material under a condition of a test temperature of 30.degree.
C.
[0099] The discharge termination voltage was 1.0 V and the charging
termination voltage was 3.0 V. Charging and discharging was
repeated 50 times. The test was started from discharging, and each
discharging capacity (mAh/g) was measured and a discharging
capacity (mAh/g) at the second discharging was regarded as an
initial capacity. The larger the initial capacity is, the larger
the charging and discharging capacity of the lithium-ion secondary
battery is, which is evaluated as preferable. Moreover, from a
discharging capacity DC.sub.10 (mAh/g) at the tenth discharging and
a discharging capacity DC.sub.20 (mAh/g) at the twentieth
discharging, a capacity retention rate (%) was calculated by the
formula (a).
Capacity retention rate (%)=(DC.sub.20/DC.sub.10).times.100 (a)
[0100] As explained above, it can be said that the higher the
capacity retention rate is, the more excellent cyclability of the
lithium-ion secondary battery is.
<Elemental Analysis>
[0101] An elemental analysis of sulfur-based positive-electrode
active materials produced in Examples and Comparative Examples was
carried out.
[0102] As for carbon, hydrogen and nitrogen, a mass ratio (%) based
on a total amount of a sulfur-based positive-electrode active
material was calculated from a mass amount measured with a full
automatic elemental analysis device vario MICRO cube manufactured
by Elementar Analysensysteme GmbH. As for sulfur, a mass ratio (%)
based on a total amount of a sulfur-based positive-electrode active
material was calculated from a mass amount measured with an ion
chromatograph device DX-320 manufactured by Dionex Corporation
using a column (IonPac AS12A) manufactured by the same Corporation.
For phosphorus, a mass ratio (%) based on a total amount of a
sulfur-based positive-electrode active material was calculated from
a mass amount measured with an inductively coupled plasma
spectrometer manufactured by Hitachi High-Tech Science
Corporation.
TABLE-US-00001 TABLE 1 Com. Ex. Example Com. Ex. 1 1 2 3 4 5 2 3
Formulation (part by mass) Rubber 100 -- -- -- -- -- -- -- Polymer
1 -- 100 -- -- -- 100 100 100 Polymer 2 -- -- 100 -- -- -- -- --
Polymer 3 -- -- -- 100 -- -- -- -- Polymer 4 -- -- -- -- 100 -- --
-- Carbon black -- -- -- -- -- 10 -- -- Sulfur 500 500 500 500 500
500 500 500 Mw of polymer 900000 160000 2000 50000 1000000 160000
160000 160000 Heat-treating temp. 400 400 400 400 400 400 200 600
Evaluation Discharging capacity (mAh/g) First time 693 711 891 502
556 765 303 216 Second time 344 405 572 273 301 442 162 135 Tenth
time 245 400 456 265 283 436 150 120 Twentieth time 211 400 432 263
278 435 121 108 Capacity retention rate (%) 86 100 95 99 98 100 81
90 Elemental analysis (%) C 44.1 39.8 38.2 39.6 42.1 46.2 56.2 65.3
H 0.2 0.5 0.5 0.3 0.3 0.5 1.6 0.1 N 0.4 6.2 0.0 1.3 0.1 5.6 7.2 4.2
S 54.4 52.7 58.4 50.1 54.5 50.3 38.7 29.6 O 0.1 0.0 2.1 7.1 0.0 0.1
0.2 0.1 P 0.0 0.0 0.0 2.5 0.0 0.0 0.0 0.0
[0103] Table 1 indicates that in Examples 1 to 5, larger initial
capacity (mAh/g) and capacity retention rate (%) are shown compared
to Comparative Example 1. A capacity retention rate of not less
than 95% is regarded as satisfactory.
[0104] A variation of an electric capacity resulting from cycle
charging and discharging in Comparative Example 1 and Examples 1
and 5 is shown in FIG. 2. In Examples 1 and 5, both of initial
capacity and a capacity retention rate are high compared to
Comparative Example 1. Particularly in Example 5 where carbon black
was added, the electric capacity was improved compared to Example
1. From the results of Example 1 and Comparative Examples 2 and 3,
it is found that high battery performance cannot be obtained in
either cases of too low and too high heat-treating
temperatures.
INDUSTRIAL APPLICABILITY
[0105] The present invention can provide a novel sulfur-based
positive-electrode active material which can largely improve a
charging and discharging capacity and cyclability of a lithium-ion
secondary battery.
EXPLANATION OF SYMBOLS
[0106] 1 Reaction apparatus
[0107] 2 Starting compound
[0108] 3 Reaction container
[0109] 4 Silicone plug
[0110] 5 Alumina protection tube
[0111] 6 Gas introducing tube
[0112] 7 Gas exhausting tube
[0113] 8 Electric furnace
[0114] 9 Thermocouple
[0115] 10 Temperature controller
[0116] 11 Aqueous solution of sodium hydroxide
[0117] 12 Trapping bath
* * * * *