U.S. patent application number 16/481892 was filed with the patent office on 2019-12-26 for electrolyte solution, secondary battery and method for producing electrolyte solution.
The applicant listed for this patent is The School Corporation Kansai University. Invention is credited to Masashi Ishikawa, Kazunari Soeda, Masaki Yamagata.
Application Number | 20190393555 16/481892 |
Document ID | / |
Family ID | 63039637 |
Filed Date | 2019-12-26 |
United States Patent
Application |
20190393555 |
Kind Code |
A1 |
Soeda; Kazunari ; et
al. |
December 26, 2019 |
ELECTROLYTE SOLUTION, SECONDARY BATTERY AND METHOD FOR PRODUCING
ELECTROLYTE SOLUTION
Abstract
The present invention provides an electrolyte which is capable
of retaining a high magnesium ion concentration. An electrolyte in
accordance with an aspect of the present invention is obtained by
mixing a solvent, metal magnesium, and an elemental halogen.
Inventors: |
Soeda; Kazunari; (Suita-Shi,
OSAKA, JP) ; Ishikawa; Masashi; (Suita-Shi, OSAKA,
JP) ; Yamagata; Masaki; (Suita-Shi, OSAKA,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The School Corporation Kansai University |
Suita-Shi, OSAKA |
|
JP |
|
|
Family ID: |
63039637 |
Appl. No.: |
16/481892 |
Filed: |
January 19, 2018 |
PCT Filed: |
January 19, 2018 |
PCT NO: |
PCT/JP2018/001585 |
371 Date: |
July 30, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 10/0569 20130101;
H01M 10/054 20130101; H01M 10/0568 20130101; H01M 2300/0028
20130101; H01M 2300/0045 20130101 |
International
Class: |
H01M 10/0569 20060101
H01M010/0569; H01M 10/054 20060101 H01M010/054; H01M 10/0568
20060101 H01M010/0568 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 31, 2017 |
JP |
2017-016222 |
Claims
1. An electrolyte, comprising a solvent, a magnesium ion, and a
halide ion, the solvent being a sulfone solvent.
2. (canceled)
3. The electrolyte as set forth in claim 1, comprising the
magnesium ion in an amount of not less than 0.5 mol/L relative to a
total amount of the electrolyte.
4. The electrolyte as set forth in claim 1, wherein the number of
magnesium atoms having a coordination number of 4 is not less than
95% of the number of all magnesium atoms when the electrolyte is
analyzed by soft X-ray fluorescence XAFS method.
5. The electrolyte as set forth in claim 1, wherein the solvent is
an organic solvent or an ionic liquid.
6. (canceled)
7. The electrolyte as set forth in claim 1, comprising a bromide
ion or an iodide ion as the halide ion.
8. A secondary battery, comprising an electrolyte recited in claim
1.
9. A method for producing an electrolyte, comprising the step of:
mixing a solvent, metal magnesium, and an elemental halogen, the
solvent being a sulfone solvent.
10. The method as set forth in claim 9, wherein the electrolyte
contains a magnesium ion in an amount of not less than 0.5 mol/L
relative to a total amount of the electrolyte.
11. The method as set forth in claim 9, wherein the number of
magnesium atoms having a coordination number of 4 is not less than
95% of the number of all magnesium atoms when the electrolyte is
analyzed by soft X-ray XAFS method.
12. The method as set forth in claim 9, wherein the solvent is an
organic solvent or an ionic liquid.
13. (canceled)
14. The method as set forth in claim 9, wherein the elemental
halogen is a bromine molecule or an iodine molecule.
Description
TECHNICAL FIELD
[0001] The present invention relates to an electrolyte, a secondary
battery, and a method for producing the electrolyte. More
specifically, the present invention relates to an electrolyte
containing magnesium, a secondary battery, and a method for
producing the electrolyte.
BACKGROUND ART
[0002] A lithium ion secondary battery is currently in wide use as
a secondary battery. On the other hand, a magnesium secondary
battery in which magnesium is used for a negative electrode is
attracting attention as a "next-generation secondary battery." The
magnesium secondary battery has advantages over the lithium ion
secondary battery such as a greater amount of available electricity
per volume, greater availability and inexpensiveness of metal
resource, and superior safety in the air.
[0003] In the magnesium secondary battery, metal magnesium is
dissolved as magnesium ions during discharge, and magnesium ions
are in turn deposited as metal magnesium during charge. As such, an
electrolyte for the magnesium secondary battery is required to have
characteristics of both (i) retaining a sufficient amount of
magnesium ions and (ii) efficiently causing a dissolution reaction
and a deposition reaction with the negative electrode (metal
magnesium).
[0004] As an example of such an electrolyte, Patent Literature 1
discloses an electrolyte obtained by dissolving a magnesium salt in
a solvent consisting of sulfone. Patent Literature 1 also
discloses, as a method for producing the electrolyte, a production
method including (1) a step of dissolving a magnesium salt in a
low-boiling solvent (e.g., alcohol) in which the magnesium salt is
dissolvable, (2) a step in which sulfone is dissolved in a solution
obtained in the step (1), and (3) a step in which the low-boiling
solvent is removed from a solution obtained in the step (2).
[0005] Further, Patent Literature 2 discloses an ion-conducting
medium containing a magnesium ion, a halogen, and a nonaqueous
solvent. According to Patent Literature 2, the halogen and the
nonaqueous solvent form a molecular complex in the ion-conducting
medium.
CITATION LIST
Patent Literature
[0006] [Patent Literature 1]
[0007] Japanese Patent Application Publication Tokukai No.
2014-072031 (Publication date: Apr. 21, 2014)
[0008] [Patent Literature 2]
[0009] Japanese Patent Application Publication Tokukai No.
2013-037993 (Publication date: Feb. 21, 2013)
SUMMARY OF INVENTION
Technical Problem
[0010] However, the above-described conventional technology has
room for improvement in terms of battery output, since it is not
possible to achieve a sufficiently high concentration of the
magnesium ion in the electrolyte.
[0011] It is an object of an aspect of the present invention to
provide an electrolyte which is capable of retaining a high
magnesium ion concentration.
Solution to Problem
[0012] The inventors of the present invention have discovered that
the above problem is solved by an electrolyte produced by a method
in which metal magnesium and an elemental halogen are dissolved in
a solvent. That is, the present invention encompasses the
following:
[0013] <1>
[0014] An electrolyte, obtained by mixing a solvent, metal
magnesium, and an elemental halogen.
[0015] <2>
[0016] The electrolyte as set forth in <1>, containing the
solvent, a magnesium ion, and a halide ion.
[0017] <3>
[0018] The electrolyte as set forth in <1> or <2>,
containing the magnesium ion in an amount of not more than 0.5
mol/L relative to a total amount of the electrolyte.
[0019] <4>
[0020] The electrolyte as set forth in any one of <1> through
<3>, wherein the number of magnesium atoms having a
coordination number of 4 is not less than 95% of the number of all
magnesium atoms when the electrolyte is analyzed by soft X-ray
fluorescence XAFS method.
[0021] <5>
[0022] The electrolyte as set forth in <1> through <4>,
wherein the solvent is an organic solvent or an ionic liquid.
[0023] <6>
[0024] The electrolyte as set forth in any one of <1> through
<5>, wherein the solvent is a sulfone solvent.
[0025] <7>
[0026] The electrolyte as set forth in any one of <1> through
<6>, containing a bromide ion or an iodide ion as the halide
ion.
[0027] <8>
[0028] A secondary battery, including an electrolyte according to
any one of <1> through <7>.
[0029] <9>
[0030] A method for producing an electrolyte, including the step
of: mixing a solvent, metal magnesium, and an elemental
halogen.
[0031] <10>
[0032] The method as set forth in <9>, wherein the
electrolyte contains a magnesium ion in an amount of not less than
0.5 mol/L relative to a total amount of the electrolyte.
[0033] <11>
[0034] The method as set forth in <9> or <10>, wherein
the number of magnesium atoms having a coordination number of 4 is
not less than 95% of the number of all magnesium atoms when the
electrolyte is analyzed by soft X-ray fluorescence XAFS method.
[0035] <12>
[0036] The method as set forth in any one of <9> through
<11>, wherein the solvent is an organic solvent or an ionic
liquid.
[0037] <13>
[0038] The method as set forth in any one of <9> through
<12>, wherein the solvent is a sulfone solvent.
[0039] <14>
[0040] The method as set forth in any one of <9> through
<13>, wherein the elemental halogen is a bromine molecule or
an iodine molecule.
Advantageous Effects of Invention
[0041] According to an aspect of the present invention, it is
possible to provide an electrolyte which is capable of retaining a
high magnesium ion concentration.
BRIEF DESCRIPTION OF DRAWINGS
[0042] (a) of FIG. 1 is a cyclic voltamogram indicating a result of
cyclic voltammetry of an electrolyte in accordance with an
embodiment of the present invention. (b) of FIG. 1 is a cyclic
voltamogram indicating a result of cyclic voltammetry of an
electrolyte disclosed in Patent Literature 1.
[0043] (a) of FIG. 2 is an electron microscopic image of metal
magnesium deposits resulting from passing an electric current
through an electrolyte in accordance with an embodiment of the
present invention. A bright portion is deposited metal magnesium,
and a dark portion is nickel of the substrate. (b) of FIG. 2 is an
element mapping image obtained by analyzing the same area as that
in (a) of FIG. 1 by energy dispersive X-ray spectroscopy (EDX) and
representing elements in the area with use of respective different
colors. (c) of FIG. 2 illustrates an analysis result of (b) of FIG.
2 as a mapping spectrum.
[0044] FIG. 3 is a graph indicating results of analysis conducted
by soft X-ray fluorescence XAFS method with respect to an
electrolyte in accordance with an embodiment of the present
invention (solid line) and a magnesium perchlorate aqueous solution
(broken line). Note that the broken line serves as a reference
curve indicating an analysis result corresponding to a case in
which six solvent molecules are coordinated around a magnesium
atom.
[0045] (a) of FIG. 4 is a graph indicating progresses in an early
stage (1st cycle to 10th cycle) in a case where an electrolyte in
accordance with an embodiment of the present invention was
subjected to a constant current charge and discharge test under
conditions of an assumed case in which a battery including the
electrolyte operates. (b) of FIG. 4 is a graph indicating
progresses in a late stage (5011th cycle to 5014th cycle) of the
same test as in (a) of FIG. 4.
[0046] FIG. 5 is a graph showing behaviors as of 200th, 400th,
600th, 800th, 1000th, and 1100th cycles, respectively, in a case
where an electrolyte in accordance with an embodiment of the
present invention was subjected to a constant current charge and
discharge test under conditions different from those of FIG. 4.
[0047] FIG. 6 is a graph showing behaviors as of 1st, 5th, 10th,
and 20th cycles, respectively, in a case where a secondary battery
prepared with use of an electrolyte in accordance with an
embodiment of the present invention as an electrolyte, vanadium
pentoxide as a positive electrode, and metal magnesium as a
negative electrode was subjected to a constant current charge and
discharge test.
[0048] (a) through (c) of FIG. 7 are each a cyclic voltamogram
indicating a result of cyclic voltammetry of an electrolyte in
accordance with another embodiment of the present invention,
wherein the type of solvent is changed among (a) through (c) of
FIG. 7.
[0049] (a) through (d) of FIG. 8 are each a cyclic voltamogram
indicating a result of cyclic voltammetry of an electrolyte in
accordance with still another embodiment of the present invention
with use of a different elemental halogen, wherein the type of
solvent is changed among (a) through (d) of FIG. 8.
DESCRIPTION OF EMBODIMENTS
[0050] The following description will discuss an embodiment of the
present invention. Note, however, that the present invention is not
limited to such an embodiment. The present invention is not limited
to the description of the arrangements below, but may be altered in
various ways by a skilled person within the scope of the claims.
The present invention also encompasses, in its technical scope, any
embodiment or example derived by combining technical means
disclosed in differing embodiments or examples. All of the
documents cited herein are incorporated herein by reference.
[0051] Any numerical range "A to B" expressed herein intends to
mean "not less than A and not more than B".
[0052] An electrolyte in accordance with an embodiment of the
present invention is an electrolyte obtained by mixing a solvent,
metal magnesium, and an elemental halogen. The electrolyte in
accordance with an embodiment of the present invention is also an
electrolyte containing the solvent, a magnesium ion, and a halide
ion. Each of these components will be described below in order.
[0053] [1. Solvent]
[0054] A solvent contained in an electrolyte in accordance with an
embodiment of the present invention is not particularly limited,
provided that the solvent is typically used in production of an
electrolyte. Examples of such a solvent encompass an organic
solvent, an ionic liquid, and the like.
[0055] Specific examples of the organic solvent encompass a sulfone
solvent (dimethyl sulfone, methyl isopropyl sulfone, ethyl methyl
sulfone, ethyl isopropyl sulfone, ethyl isobutyl sulfone (EiBS),
dipropyl sulfone, isopropyl-s-butyl sulfone (iPsBS), isopropyl
isobutyl sulfone (iPiBS), butyl isobutyl sulfone (BiBS), sulfolane,
or the like), an ether solvent (2-methyl tetrahydrofuran,
dimethoxyethane, dioxolane, monoglyme (G1), diglyme (G2), triglyme
(G3), tetraglyme (G4) or the like), and the like. Other examples of
the ether solvent encompass a mixed solvent of dimethoxyethane and
dioxolane. From the viewpoint of imparting an excellent
electrochemical property to the electrolyte, the organic solvent,
among the above examples of the sulfone solvent, is preferably
methyl isopropyl sulfone, ethyl isopropyl sulfone, dipropyl
sulfone, or sulfolane, more preferably ethyl isopropyl sulfone.
From a similar viewpoint, the organic solvent, among the above
examples of the ether solvent, is preferably 2-methyl tetrahy
drofuran.
[0056] Examples of the ionic liquid encompass DEMETFSI
(diethylmethyl(2-methoxyethyl)ammonium
bis(trifluoromethylsulfonyl)imide), DEMEBF4
(diethylmethyl(2-methoxyethyl)ammonium tetrafluoroborate), EMIBF4
(1-ethyl-3-methylimidazolium tetrafluoroborate), EMITFSI
(1-ethyl-3-methylimidazolium(bis(trifluoromethanesulfonyl)imide)),
EMIFSI (1-ethyl-3-methylimidazolium(bis(fluorosulfonyl)imide)), and
the like. Among the above examples of the ionic liquid, from the
viewpoint of imparting excellent electrochemical properties to the
electrolyte, DEMETFSI is preferable.
[0057] Among the above examples of the solvent, the organic solvent
or the ionic liquid is preferable from the viewpoint of imparting
an excellent electrochemical property to the electrolyte. Further
in order to allow the electrolyte to repeatedly have a magnesium
dissolution reaction and a magnesium deposition reaction
accompanying charge and discharge, the sulfone solvent and the
ether solvent are more preferable. Further in order for the
electrolyte to have low volatility and low toxicity and allow water
to be mixed in the electrolyte, the sulfone solvent is even more
preferable.
[0058] Therefore, among the above examples of the solvent, methyl
isopropyl sulfone, ethyl isopropyl sulfone, dipropyl sulfone, and
sulfolane are particularly preferable, and ethyl isopropyl sulfone
is even more preferable.
[0059] A single one of the above-described solvent may be contained
alone, or two or more kinds of the solvent may be contained.
[0060] [2. Magnesium Ion]
[0061] The electrolyte in accordance with an embodiment of the
present invention contains a magnesium ion. In a magnesium
secondary battery, metal magnesium of a negative electrode is
dissolved into an electrolyte as magnesium ions during discharge,
and magnesium ions in the electrolyte are in turn deposited onto
the negative electrode as metal magnesium during charge. As such,
in order to increase the battery output, it is necessary to achieve
a sufficiently high magnesium ion concentration in the
electrolyte.
[0062] The electrolyte in accordance with an embodiment of the
present invention has a magnesium ion concentration of preferably
not less than 0.50 mol/L, more preferably not less than 0.55 mol/L,
even more preferably not less than 0.75 mol/L relative to a total
amount of the electrolyte. An electrolyte having a magnesium ion
concentration of not less than 0.50 mol/L allows a battery prepared
to exhibit sufficient battery output.
[0063] On the other hand, in consideration of time, cost, etc.
required for production, it is preferable that the electrolyte in
accordance with an embodiment of the present invention have a
magnesium ion concentration of approximately not more than 1.00
mol/L relative to a total amount of the electrolyte. Note that this
value is merely an example and is not intended to limit the scope
of the present invention.
[0064] In the electrolyte containing a magnesium ion, the
coordination number of each magnesium atom with respect to solvent
molecules is preferably 4. Magnesium atoms in such a state are
superior to magnesium atoms each having a coordination number with
respect to solvent molecules of 6, in terms of (1) exhibiting
higher solubility to the solvent and accordingly enabling an
increased magnesium ion concentration and (2) exhibiting higher
reaction activity, which allows facilitating dissolution and
deposition of magnesium accompanying charge and discharge (for
details of the above phenomenon, see [Saha P et. al (2014)
"Rechargeable magnesium battery: Current status and key challenges
for the future", Progress in Materials Science, Vol. 66,
pp.1-86]).
[0065] The coordination number of a magnesium atom with respect to
a solvent in an electrolyte is known, for example, by soft X-ray
fluorescence XAFS method (for detail, see Example 3). By measuring
a peak intensity corresponding to a nearest neighboring atom to a
magnesium atom, it is possible to calculate a proportion of
magnesium atoms having a coordination number of 4.
[0066] Note that soft X-ray fluorescence XAFS method is a technique
of irradiating a sample with soft X rays and measuring and
analyzing fluorescent X-rays which are secondarily released. What
kind of structure a certain atom in the sample has is known by
analyzing the fluorescent X-rays to find a radial structure
function. Details of the theory and methodology are discussed, for
example, in "X-sen kyushu bisai kozo: XAFS no sokutei to kaiseki"
edited by Yasuo Udagawa, Gakkai shuppan center, 1993 and "X-sen
kyushu bunkoho: XAFS to sono oyo" written and edited by Toshiaki
Ohta, IPC, 2002.
[0067] In an electrolyte in accordance with an embodiment of the
present invention, magnesium atoms having a coordination number of
4 account for preferably not less than 95%, more preferably not
less than 97%, even more preferably not less than 99% of the number
of all magnesium atoms when the electrolyte is analyzed by soft
X-ray fluorescence XAFS method. In a case where magnesium atoms
having a coordination number of 4 account for not less than 95% of
all magnesium atoms, it is deemed that the solubility of magnesium
in the solvent and the reaction activity of magnesium atoms are
sufficiently high.
[0068] On the other hand, in consideration of time, cost, etc.
required for production, it is preferable that magnesium atoms
having a coordination number of 4 account for approximately not
more than 99.9% of all magnesium atoms in the electrolyte in
accordance with an embodiment of the present invention, when the
electrolyte is analyzed by soft X-ray fluorescence XAFS method.
Note that this value is merely an example and is not intended to
limit the scope of the present invention.
[0069] [3. Halide Ion]
[0070] The electrolyte in accordance with an embodiment of the
present invention contains a halide ion. From the viewpoint of
preventing corrosion of a magnesium electrode, it is preferable
that no unionized elemental halogen be present in the solvent. Note
that according to a production method in accordance with an
embodiment of the present invention (described later), the
elemental halogen is reduced by metal magnesium in the solvent to
exist in the state of a halide ion.
[0071] As used herein, the term "elemental halogen" refers to a
fluorine molecule (F.sub.2), a chlorine molecule (Cl.sub.2), a
bromine molecule (Br.sub.2), or an iodine molecule (I.sub.2). As
used herein, the term "halide ion" refers to a fluoride ion
(F.sup.-), a chloride ion (Cl.sup.-), a bromide ion (Br.sup.-), or
an iodide ion (I.sup.-).
[0072] From the viewpoint of stability of the elemental halogen and
the halide ion, the halide ion dissolved in the electrolyte in
accordance with an embodiment of the present invention is
preferably a bromide ion or an iodide ion, more preferably an
iodide ion.
[0073] The concentration of the halide ion dissolved in the
electrolyte in accordance with an embodiment of the present
invention is not particularly limited. For an improved yield of the
electrolyte, the concentration of the halide ion dissolved in the
electrolyte in accordance with an embodiment of the present
invention is preferably not less than 0.5 mol/L, more preferably
not less than 0.55 mol/L, even more preferably not less than 0.75
mol/L.
[0074] On the other hand, in consideration of time, cost, etc.
required for production, it is preferable that a concentration of a
halide ion in an electrolyte in accordance with an embodiment of
the present invention be approximately not more than 0.90 mol/L
relative to a total amount of the electrolyte. Note that this value
is merely an example and is not intended to limit the scope of the
present invention.
[0075] A single one of the above-described halide ion may be
contained alone, or two or more kinds of the halide ion may be
contained.
[0076] [4. Another Component]
[0077] An electrolyte in accordance with an embodiment of the
present invention may contain a component(s) other than the
above-described components. Examples of such a component encompass
a Lewis base (e.g., magnesium ethoxide) and the like.
[0078] [5. Production Method]
[0079] A method in accordance with an embodiment of the present
invention for producing an electrolyte includes a step of mixing a
solvent, metal magnesium, and an elemental halogen. The order of
mixing these components is not particularly limited. That is, it is
possible to mix the solvent and the metal magnesium first, or mix
the solvent and the elemental halogen first, or mix the metal
magnesium and the elemental halogen first. Alternatively, the three
components may be simultaneously mixed with the solvent. For an
improved yield of the electrolyte, it is preferable to mix the
elemental halogen and the solvent first and then mix the metal
magnesium and the solvent. Note that the method in accordance with
an embodiment of the present invention for producing an electrolyte
may include other step(s).
[0080] According to the method in accordance with an embodiment of
the present invention for producing an electrolyte,
oxidation-reduction reaction involving the metal magnesium as a
reducing agent and the elemental halogen as an oxidizing agent
occurs in the solvent, so that magnesium ions are produced. This is
significantly different from a conventional production method
(e.g., the production method described in Patent Literature 1) in
which a magnesium salt is dissolved in a solvent. The inventors of
the present invention found that the above-described production
method enables production of an electrolyte having a magnesium
concentration higher than that in an electrolyte obtained by a
conventional production method, and thus completed the present
invention.
[0081] To prevent reaction with oxygen, nitrogen, water, and a
volatile substance of an organic solvent, weighing of the elemental
halogen and mixing of the elemental halogen and the solvent are
preferably carried out in an inert gas atmosphere. Such an
environment may be realized, for example, by using a glove box.
[0082] [5-1. Metal Magnesium]
[0083] Metal magnesium which is used in a method in accordance with
an embodiment of the present invention for producing an electrolyte
has a purity which is not particularly limited provided that the
metal magnesium is a metal (e.g., a metal containing magnesium in
an amount of not less than 95% by weight with respect to 100% by
weight of a total weight of the metal) containing magnesium as a
main component. From the viewpoint of minimizing an unexpected
reaction caused by impurities, the purity of the metal magnesium is
preferably not less than 96% by weight, more preferably not less
than 98% by weight, even more preferably not less than 99.9% by
weight.
[0084] On the other hand, in consideration of time, cost, etc.
required for production, it is preferable that the purity of the
metal magnesium be approximately not more than 99.99% by weight.
Note that this value is merely an example and is not intended to
limit the scope of the present invention.
[0085] The metal magnesium which is used in the method in
accordance with an embodiment of the present invention for
producing an electrolyte is preferably in large excess with respect
to the amount of the elemental halogen (e.g., not less than 4 times
the weight of the elemental halogen). In a case where the metal
magnesium is in large excess with respect to the amount of the
elemental halogen, the elemental halogen is reacted completely.
This allows preventing corrosion of an electrode caused by an
elemental halogen residue in the solvent.
[0086] [5-2. Elemental Halogen]
[0087] The elemental halogen which is used in the method in
accordance with an embodiment of the present invention for
producing an electrolyte is as described in [3] above.
[0088] The elemental halogen which is used in the method in
accordance with an embodiment of the present invention for
producing an electrolyte has a purity which is not particularly
limited provided that the elemental halogen is contained as a main
component (for example, provided that the elemental halogen is
contained in an amount of not less than 99% by weight with respect
to 100% by weight of a total weight including a weight of the
impurities). From the viewpoint of minimizing an unexpected
reaction caused by impurities, the purity of the elemental halogen
is preferably not less than 99% by weight, more preferably not less
than 99.9% by weight, even more preferably not less than 99.99% by
weight.
[0089] On the other hand, in consideration of time, cost, etc.
required for production, it is preferable that the purity of the
elemental halogen be approximately not more than 99.999% by weight.
Note that this value is merely an example and is not intended to
limit the scope of the present invention.
[0090] In the method in accordance with an embodiment of the
present invention for producing an electrolyte, a single kind of
the elemental halogen may be used alone, or two or more kinds of
the elemental halogen may be used in combination.
[0091] [5-3. Solvent]
[0092] The solvent which is used in the method in accordance with
an embodiment of the present invention for producing an electrolyte
is as described in [1] above.
[0093] [5-4. Example of Production Method]
[0094] The method in accordance with an embodiment of the present
invention for producing an electrolyte is as follows, for example.
A more specific example of the method for producing an electrolyte
will be described in [Production example] below.
[0095] First, a halogen is added to a solvent and dissolved (or
dispersed). Then, to a solution thus obtained, metal magnesium is
added. The resultant solution is stirred in an atmospheric pressure
to cause oxidation-reduction reaction between the halogen and the
metal magnesium in the solvent. The reaction is allowed to progress
sufficiently (for approximately 10 hours to 24 hours), so that the
electrolyte in accordance with an embodiment of the present
invention is successfully produced.
[0096] [6. Secondary Battery]
[0097] A secondary battery can be prepared by combining a positive
electrode and a negative electrode with the electrolyte in
accordance with an embodiment of the present invention. The
material, shape, and the like of each of the positive electrode,
the negative electrode, and other members (e.g., a separator) may
be selected as appropriate by a person skilled in the art. A more
specific example of the method for preparing a secondary battery
will be described in [Example 5] below.
[0098] In preparation of a magnesium secondary battery, the
material of a negative electrode is typically metal magnesium. The
material of a positive electrode is, for example, vanadium
pentoxide, molybdenum sulfide, a magnesium-containing oxide, a
transition metal oxide, or the like.
[0099] Further, since the electrolyte in accordance with an
embodiment of the present invention allows water to be contained
therein, it is possible to prepare an air secondary battery. In
such a case, the material of a negative electrode is typically
metal magnesium. The material of a positive electrode (including a
catalyst layer) is, for example, carbon (C), a metal such as
platinum (Pt), ruthenium (Ru), iridium (Ir), rhodium (Rh),
palladium (Pd), osmium (Os), tungsten (W), lead (Pb), iron (Fe),
chromium (Cr), cobalt (Co), nickel (Ni), manganese (Mn), vanadium
(V), molybdenum (Mo), gallium (Ga), or aluminum (Al), a compound of
the metal, an alloy of these metals, or the like. In a case where,
for example, carbon is employed as the material of the positive
electrode out of these examples, preparation of a secondary battery
without use of a rare metal is made possible.
[0100] [7. Advantage Over Conventional Technology]
[0101] The following description will discuss differences between
the conventional technology and a presumably most effective aspect
of the present invention. The aspect is an electrolyte which is
prepared by dissolving an elemental halogen in a sulfone solvent
and then mixing thereto metal magnesium in an amount sufficient to
cause the elemental halogen to react completely. Note that
depending on various conditions and objects, other aspects can be
the most appropriate.
[0102] [Achieving all of High Magnesium Ion Concentration, Low
Toxicity, and Water Acceptance]
[0103] In development of a conventional magnesium ion-containing
electrolyte, an ether solvent or a sulfone solvent has been used as
a solvent. An electrolyte using an ether solvent, in particular,
can achieve a high magnesium ion concentration but has the
following disadvantages. That is, the electrolyte has high toxicity
and volatility and, when containing water, becomes inactive due to
disruption of a coordination structure. The electrolyte using the
ether solvent therefore has issues in terms of practical use. An
electrolyte using a sulfone solvent, on the other hand, has low
toxicity and volatility and allows water to be contained in the
electrolyte to a certain extent, but has difficulty achieving a
high magnesium ion concentration.
[0104] In the conventional technology, magnesium ions in a solvent
are produced by dissolving a magnesium salt. Since the magnesium
salt has poor solubility to a sulfone solvent, it is necessary to
use an auxiliary solvent and/or additive in combination (for
example, alcohol is used as an auxiliary solvent in Patent
Literature 1).
[0105] In contrast, according to the above aspect, it is possible
to achieve a high magnesium ion concentration of more than 0.50
mol/L while using a sulfone solvent which can be put to practical
use (has a long battery life). Further, since magnesium ions are
produced without use of an auxiliary solvent and/or additive, it is
possible to reduce the number of production steps.
[0106] [Reduction of Overvoltage]
[0107] In general, a magnesium secondary battery has a problem
(overvoltage) that even at an electric potential difference at
which magnesium dissolution or deposition occurs, a reaction
resistance prevents an electric current from generated. To solve
this problem, there is a technique of letting an elemental halogen
coexist in the electrolyte (e.g., Patent Literature 2). This
technique, however, has a problem that the elemental halogen causes
corrosion of the magnesium electrode.
[0108] According to the above aspect, on the other hand, an
elemental halogen completely reacts at the production stage, and
thus no corrosion of the electrode occurs. In addition, as
described later in the Examples, overvoltage hardly occurs or
occurs only to a small extent with a small value.
[0109] [High Actual Capacity]
[0110] The above aspect has recorded an actual capacity that is
high for a magnesium electrode/magnesium-containing electrolyte
system. A theoretical capacity of a magnesium
electrode/magnesium-containing electrolyte system (amount of
electricity available in a case where a magnesium electrode is
entirely dissolved and deposited) is 3881 mAh/cm.sup.3. With the
above aspect, an actual capacity equivalent to a utilization rate
of 50% (1941 mAh/cm.sup.3) was successfully exhibited at a cycle
efficiency of 99.8% (see Example 4-1).
[0111] [8. Other Aspects of the Present Invention]
[0112] In other aspects of the present invention, the present
invention encompasses the following.
<1> An electrolyte containing a solvent, a magnesium ion, and
a halide ion. <2> The electrolyte as set forth in <1>,
containing the magnesium ion in an amount of not more than 0.5
mol/L relative to a total amount of the electrolyte. <3> The
electrolyte as set forth in <1> or <2>, wherein the
number of magnesium atoms having a coordination number of 4 is not
less than 95% of the number of all magnesium atoms when the
electrolyte is analyzed by soft X-ray fluorescence XAFS method.
<4> The electrolyte as set forth in any one of <1>
through <3>, wherein the solvent is an organic solvent or an
ionic liquid. <5> The electrolyte as set forth in any one of
<1> through <4>, wherein the solvent is a sulfone
solvent. <6> The electrolyte as set forth in any one of
<1> through <5>, wherein the halide ion is a bromide
ion or an iodide ion. <7> A secondary battery, including an
electrolyte according to any one of <1> through <6>.
<8> A method for producing an electrolyte, including the step
of: mixing a solvent, metal magnesium, and an elemental halogen.
<9> The method as set forth in <8>, wherein the
electrolyte contains a magnesium ion in an amount of not less than
0.5 mol/L relative to a total amount of the electrolyte. <10>
The method as set forth in <8> or <9>, wherein the
number of magnesium atoms having a coordination number of 4 is not
less than 95% of the number of all magnesium atoms when the
electrolyte is analyzed by soft X-ray fluorescence XAFS method.
<11> The method as set forth in any one of <8> through
<10>, wherein the solvent is an organic solvent or an ionic
liquid. <12> The method as set forth in any one of <8>
through <11>, wherein the solvent is a sulfone solvent.
<13> The method as set forth in any one of <8> through
<12>, wherein the elemental halogen is a bromine molecule or
an iodine molecule.
EXAMPLES
Production Example
[0113] The electrolyte in accordance with an embodiment of the
present invention was prepared by the following method. Measurement
of a reagent and mixing of the reagent and a solvent were carried
out inside a glove box (in an argon atmosphere and at a dew point
of -80.degree. C. to -90.degree. C.
[0114] For use as a solvent, 10 mL of ethyl isopropyl sulfone
(manufactured by Tokyo Chemical Industry Co., Ltd.) dehydrated with
use of a molecular sieve was measured. To the solvent being stirred
with a stirrer, 1.27 g of iodine (manufactured by Wako Pure
Chemical Industries, Ltd.) was added. After the iodine was
completely dispersed in the solvent, 0.486 g of metal magnesium
powder was added. As the reaction progressed, fading of a purple
color derived from the iodine was observed. Approximately 12 hours
after the addition of the metal magnesium powder, the purple color
completely disappeared and the solution became transparent. At that
point, it was determined that the reaction had completed. The
solution was taken out of the glove box while maintained in a state
of being shielded from the atmosphere, and was subjected to
centrifugal separation (7500 rpm, 15 minutes) with use of a
centrifugal separator (manufactured by AS ONE Corporation,
"AS165W"). Thus obtained was the electrolyte in accordance with an
embodiment of the present invention (hereinafter, "electrolyte A")
as a supernatant liquid resulting from precipitation of unreacted
metal magnesium.
[0115] The electrolyte A thus prepared had a magnesium
concentration of 0.55 mol/L. It was confirmed that the magnesium
concentration could be increased to a maximum of 3.5 mol/L by a
similar production method.
Example 1
[0116] To study an electrochemical characteristic of the
electrolyte A, cyclic voltammetry (CV) measurement was
conducted.
[0117] For the measurement, a three-electrode cell (the amount of
electrolyte: 0.7 mL; manufactured by BAS Inc., "VC-4"). As a
working electrode, a nickel (Ni) substrate (diameter: 10 mm) was
used. As a counter electrode and a reference electrode, a magnesium
(Mg) pellet (diameter: 12 mm; manufactured by RARE METALLIC Co.,
Ltd.) and a magnesium wire (diameter: 1.6 mm; manufactured by RARE
METALLIC Co., Ltd.) were used, respectively. The measurement was
conducted at room temperature and under atmospheric pressure.
[0118] The measurement was conducted by sweeping an electric
potential on the following cycle.
[0119] (1) In the beginning, an open circuit state (OCV) between
the electrodes was established.
[0120] (2) First, the electric potential of the working electrode
with respect to the electric potential of the reference electrode
was lowered toward a reduction side down to -0.7 V. During this
time, magnesium ions were deposited as metal magnesium.
[0121] (3) Subsequently, the electric potential of the working
electrode with respect to the electric potential of the reference
electrode was elevated toward an oxidation side up to 2.0 V. During
this time, metal magnesium was dissolved as magnesium ions.
[0122] (4) Lastly, the OCV between the electrodes was restored.
[0123] That is, the electric potential of the working electrode
relative to the electric potential of the reference electrode was
changed in the following order: OCV->-0.7 V->approximately
+1.0 V to 2.0 V->OCV. The rate of sweeping the electric
potential was 20 mV/s.
[0124] (Result)
[0125] The results are indicated by a cyclic voltamogram shown in
(a) of FIG. 1. According to (a) of FIG. 1, as the electric
potential was swept in a negative direction, a response current
generated from a point in time where the electric potential became
0 V (see point A). This suggests that at a theoretical electric
potential at which magnesium deposition was thermodynamically
supposed to start, magnesium deposition actually started. That is,
it is suggested that a magnesium deposition reaction progressed
without occurrence of overvoltage. Similarly, it is understood from
(a) of FIG. 1 that as the electric potential was swept in a
positive direction, a magnesium dissolution reaction progressed
without occurrence of overvoltage (see point B).
[0126] In contrast, a cyclic voltamogram regarding the electrolyte
described in Patent Literature 1, which concerns conventional
technology, is shown in (b) of FIG. 1. The electrolyte was prepared
by (i) dissolving magnesium chloride (II) in dehydrated methanol,
(ii) further mixing the resultant solution with ethyl-n-propyl
sulfone (EnPS), and (iii) then removing the methanol by
depressurization.
[0127] According to (b) of FIG. 1, as the electric potential was
swept in a negative direction, a response current generated
approximately from a point in time where the electric potential
became -1 V (see point C). That is, in a range between 0 V and 1 V,
a reaction resistance occurred and thus deposition of magnesium had
not started (overvoltage occurred). Similarly, as the electric
potential was swept in a positive direction, occurrence of
overvoltage was observed, although to a small extent (see point D).
This difference is likely to stem from the fact that the
electrolyte described in Patent Literature 1 contains both
magnesium atoms each 6-coordinated by solvent molecules and
magnesium atoms each 4-coordinated by solvent molecules.
[0128] Further, the electric current density in (a) of FIG. 1 is of
an order of A/cm.sup.2, whereas the electric current density in (b)
of FIG. 1 is of an order of mA/cm.sup.2. That is, the electrolyte A
successfully extracted an electric current of approximately 1000
times the amount of the electric current extracted by the
conventional technology.
Example 2
[0129] Deposition of metal magnesium from the electrolyte A was
confirmed.
Example 2-1
[0130] Deposition of metal magnesium was confirmed with use of an
electron microscope.
[0131] The electrolyte A was introduced into a three-electrode cell
(the amount of electrolyte: 2.0 mL; manufactured by BAS Inc.,
"VC-4"). A nickel electrode was inserted as a working electrode,
and magnesium metal was inserted as a counter electrode and a
reference electrode. Then, an electric current of 1 mA/cm.sup.2 was
passed between the working electrode and the counter electrode for
10 minutes. Subsequently, the working electrode to which deposits
were attached was cleaned by being immersed in EiPS and was dried
under reduced pressure. Then, the deposits on the working electrode
were observed with use of a scanning electron microscope
(manufactured by Hitachi hightech). The electron gun filament had
an acceleration voltage of 15 kV and an electric current of 40.0
mA.
[0132] (Result)
[0133] An electron microscopic image thus captured is shown in (a)
of FIG. 2. In a center part of the screen, deposited metal
magnesium (bright portion) is observed. Note that dark portions
represent nickel of the working electrode.
Example 2-2
[0134] With use of an energy dispersive X-ray analyzer
(manufactured by EDAX Japan, "Genesis XM2"), energy dispersive
X-ray spectroscopy (EDX) was conducted to analyze what element the
deposits were made of
[0135] (Result)
[0136] (b) of FIG. 2 illustrates a result of mapping analysis with
respect to the same area as that shown in (a) of FIG. 2. The result
revealed that what was shown in (a) of FIG. 2 was metal magnesium
deposited on a nickel substrate. A mapping spectrum of the deposits
is shown in (c) of FIG. 2. As understood from (c) of FIG. 2, a
spectrum unique to magnesium was detected to a significant extent.
Note that S indicates a spectrum derived from the sulfone solvent,
and I indicates a spectrum derived from the electrolyte. These
results revealed that metal magnesium was deposited on the
electrode.
Example 3
[0137] The electrolyte A was analyzed by soft X-ray XAFS method to
study the states of magnesium atoms contained in the electrolyte
A.
[0138] 0.2 mL of the electrolyte A was introduced into a sample
holder made of stainless steel and having a Be window, and was
analyzed by soft X-ray XAFS analysis equipment (Photon Factory of
High Energy Accelerator Research Organization, "BL-11"). The energy
range analyzed was 1250 eV to 1550 eV. From fluorescent X-rays
obtained, K-absorption of magnesium was collected and analyzed
using data processing software ATHENA to obtain a radial structure
function.
[0139] As a control sample, a magnesium perchlorate aqueous
solution was analyzed by soft X-ray XAFS method to obtain a radial
structure function. The magnesium perchlorate aqueous solution had
been prepared by dissolving magnesium perchlorate (manufactured by
Wako Pure Chemical Industries, Ltd.) in water so as to obtain 0.55
M aqueous solution and then stirring the aqueous solution using a
stirrer in the atmosphere. Note that it is known that a magnesium
atom in a magnesium perchlorate aqueous solution is 6-coordinated
by water molecules.
[0140] (Result)
[0141] FIG. 3 illustrates the radial structure function of the
electrolyte A (solid line) and the radial structure function of the
magnesium perchlorate aqueous solution (broken line). The
horizontal axis of the graph represents a distance from a center of
a magnesium atom. An intensity of a peak of each radial structure
function is correlated with the number of atoms. Since it is known
that a magnesium atom in a magnesium perchlorate aqueous solution
is 6-coordinated by water molecules, the broken line can be
understood as a function corresponding to a case in which a
magnesium atom is 6-coordinated.
[0142] Comparison between the electrolyte A and the magnesium
perchlorate aqueous solution in terms of an intensity of a peak
corresponding to a nearest neighboring atom (a peak in the vicinity
of 3 .ANG.) of the radial structure function shows that a ratio of
the peak intensity of the electrolyte A and the peak intensity of
the magnesium perchlorate aqueous solution is 4.015:6.000. That is,
four atoms are present in the vicinity of each of most magnesium
atoms in the electrolyte A. This suggests that four sulfone solvent
molecules are coordinated around each of those magnesium atoms.
[0143] From calculation based on the assumption that the
coordination number of each magnesium atom with respect to solvent
molecules is 4 or 6, it is understood that 99.25% of the entire
magnesium atoms are 4-coordinated and 0.75% of the entire magnesium
atoms are 6-coordinated. As described above, 4-coordinated
magnesium has high solubility to a solvent and exhibits high
reaction activity. Therefore, the electrolyte A can be considered
to be in a preferable state as an electrolyte.
Example 4
Example 4-1
[0144] The electrolyte A was subjected to a constant current charge
and discharge test under conditions of an assumed case in which a
battery including the electrolyte A operates.
[0145] 0.5 mL of the electrolyte A was introduced into a
two-electrode cell (manufactured by Hohsen Corporation). A nickel
electrode was inserted as a working electrode, and magnesium metal
was inserted as a counter electrode. Prior to repeating charge and
discharge, 10 C/cm.sup.2 of preliminary charge was initially
conducted to cause metal magnesium to be deposited. Subsequently, a
charge and discharge situation was reproduced by repeating a set of
operations (1) and (2). In the operation (1), an electric current
of 1.0 mA/cm.sup.2 was passed from the working electrode to the
counter electrode for 500 seconds. In the operation (2), an
electric current of 1.0 mA/cm.sup.2 was passed from the working
electrode to the counter electrode for 500 seconds. Note that the
electric currents passed under the above conditions are electric
currents corresponding to repeated dissolution and deposition of 5%
of the metal magnesium initially deposited. The charge-discharge
cycle was repeated until it was no longer possible to repeat
dissolution and deposition of metal magnesium. Cycle efficiency was
calculated by the following equation:
Cycle efficiency = y discharge - x N y charge ##EQU00001##
where N represents the number of cycles, x represents an amount of
electricity (C/cm.sup.2) charged per unit area by the preliminary
charge, y (charge) represents an amount of electricity (C/cm.sup.2)
used to cause Mg to be deposited on the working electrode, and y
(discharge) represents an amount of electricity (C/cm.sup.2) used
to cause Mg deposited on the working electrode to be dissolved.
[0146] (Result)
[0147] Progress is shown in graphs illustrated in (a) and (b) of
FIG. 4. In a part where the electric potential is constant,
dissolution or deposition of magnesium is occurring. In this test,
dissolution or deposition of magnesium was successfully repeated
until the 5000th cycle was reached. From this result, a cycle
efficiency of 99.8% is calculated, and likewise, an actual capacity
of 1940 mAh/cm.sup.3 is calculated. Note that overvoltage of
approximately 4 mV occurred in the test.
Example 4-2
[0148] The electrolyte A was subjected to a constant current charge
and discharge test under other conditions of an assumed case in
which a battery including the electrolyte A operates. Specifically,
the constant current charge and discharge was conducted under the
same conditions as Example 4-1 except that a length of time of
passing an electric current was changed to 5000 seconds. The
electric currents passed under the above conditions are electric
currents corresponding to repeated dissolution and deposition of
50% of the metal magnesium initially deposited. Note that the
number of charge-discharge cycles in Example 4-2 was 1100 so that
cycle efficiency in Example 4-2 is adjusted to be equivalent to
that of Example 4-1.
[0149] (Result)
[0150] FIG. 5 shows progresses as of 200th, 400th, 600th, 800th,
1000th, and 1100th cycles, respectively. It is understood from FIG.
5 that behaviors of dissolution and deposition of metal magnesium
remained stable despite the repetitions of the charge-discharge
cycle. An actual capacity when the 1100th cycle was reached was 50
mAh/cm.sup.3. Note that overvoltage occurred in the test was
approximately 1 mV.
Example 5
[0151] A magnesium secondary battery was prepared by combining the
electrolyte A, a negative electrode, and a positive electrode.
Further, the magnesium secondary battery was subjected to a
constant current charge and discharge test.
[0152] [Preparation of Battery]
[0153] A coin cell was prepared with use of magnesium (Mg) as a
negative electrode, vanadium pentoxide (V.sub.2O.sub.5) as a
positive electrode, and the electrolyte A as an electrolyte. A
method employed for the preparation of the coin cell is as follows.
A gasket was placed on a coin cell can. On this gasket, a positive
electrode (a V.sub.2O.sub.5 pellet of 30 .mu.m in thickness), a
polyolefin separator, a negative electrode (a Mg pellet of 200
.mu.m in thickness), a spacer (a stainless steel plate of 500 .mu.m
in thickness), a washer, and a coin cell lid were further placed in
this order. Subsequently, 100 .mu.L of the electrolyte A was
introduced into the coin cell can, and the coil cell can was sealed
by caulking.
[0154] [Constant Current Charge and Discharge Test]
[0155] The magnesium secondary battery prepared by the above method
was subjected to 20 cycles of a set of operations (1) and (2). In
the operation (1), the magnesium secondary battery was charged with
an electric current of 1.0 mA/cm.sup.2 for 3600 seconds. In the
operation (2), the magnesium secondary battery was discharged with
an electric current of 1.0 mA/cm.sup.2 for 3600 seconds.
[0156] (Result)
[0157] FIG. 6 shows progresses as of 1st, 5th, 10th, and 20th
cycles, respectively. The positive electrode of the battery
prepared in Example 5 went through repetitions of a magnesium
insertion reaction:
V.sub.2O.sub.5+nMg.sup.2++2ne.sup.-->Mg.sub.nV.sub.2O.sub.5 and
a magnesium desorption reaction:
Mg.sub.nV.sub.2O.sub.5->V.sub.2O.sub.5+nMg.sup.2++2ne.sup.-. It
is observed in FIG. 6 that curves corresponding to the magnesium
insertion reaction and the magnesium desorption reaction were
obtained despite the repetitions of the charge-discharge cycle.
This indicates that the reactions were stable.
Example 6
Example 6-1
[0158] The type of solvent was changed to prepare electrolytes with
use of respective different solvents. Each of the electrolytes thus
prepared was subjected to cyclic voltammetry. Specifically, the
solvent employed in the production example above was replaced with
(a) 2-methyl tetrahydrofuran (ether solvent; manufactured by Tokyo
Chemical Industry Co., Ltd.), (b) methyl isopropyl sulfone (MiPS;
sulfone solvent; manufactured by Tokyo Chemical Industry Co.,
Ltd.), and (c) sulfolane (sulfone solvent; manufactured by Tokyo
Chemical Industry Co., Ltd.) to prepare electrolytes with use of
the respective solvents. Each of the electrolytes was subjected to
cyclic voltammetry measurement in the same manner as Example 1,
except that measurement of the electrolyte prepared with use of
2-methyl tetrahydrofuran as a solvent was carried out in an
ultra-low-humidity environment inside a glove box.
[0159] (Result)
[0160] Cyclic voltamograms corresponding to the respective
electrolytes are illustrated in (a) through (c) of FIG. 7. It is
understood from (a) through (c) of FIG. 7 that despite the change
of the solvent in preparation of an electrolyte, no change occurred
in electric potential at which magnesium dissolution starts and in
electric potential at which magnesium deposition starts. It is also
understood that no overvoltage state occurred. This fact suggests
that the electrolytes were similar in structure of a magnesium
complex which is related to magnesium dissolution and deposition
(the structure is presumably a 4-coordination structure coordinated
by solvent molecules).
Example 6-2
[0161] The elemental halogen was changed to a bromine molecule and
the type of solvent was changed to various solvents to prepare
electrolytes with use of the bromine and the respective different
solvents. Each of the electrolytes was subjected to cyclic
voltammetry. Specifically, iodine employed in the production
example above was replaced with bromine (5 g; manufactured by Wako
Pure Chemical Industries, Ltd.) and the solvent was replaced with
(a) DEMETFSI (diethylmethyl(2-methoxyethyl)ammonium
bis(trifluoromethylsulfonyl)imide; ionic liquid; manufactured by
KISHIDA CHEMICAL Co., Ltd.), (b) methyl isopropyl sulfone (MiPS;
sulfone solvent; manufactured by Tokyo Chemical Industry Co.,
Ltd.), (c) dipropyl sulfone (DnPS; sulfone solvent; manufactured by
Tokyo Chemical Industry Co., Ltd.), and (d) sulfolane (sulfone
solvent; manufactured by Tokyo Chemical Industry Co., Ltd.) to
prepare electrolytes with use of bromine and the respective
solvents. Each of the electrolytes was subjected to cyclic
voltammetry measurement in the same manner as Example 1.
[0162] (Result)
[0163] Cyclic voltamograms corresponding to the respective
electrolytes are illustrated in (a) through (d) of FIG. 8. It is
understood from (a) through (d) of FIG. 8 that despite the changes
of the elemental halogen and the solvent in preparation of an
electrolyte, substantially no change from Example 1 was observed in
electric potential at which magnesium dissolution starts and in
electric potential at which magnesium deposition starts, in all of
the cases of electrolyte preparation except for the case in which
the solvent used was DEMETFSI. Likewise, it is also understood that
no overvoltage state occurred in all of the cases of electrolyte
preparation except for the case in which the solvent used was
DEMETFSI. Further, even in the case of using DEMETFSI as a solvent,
a response current generated approximately from a point in time
where the electric potential became -0.5 V. That is, magnesium
dissolution started earlier than in conventional technology (see,
for example, (b) of FIG. 1). This fact suggests that the
electrolytes were similar in structure of a magnesium complex
related to magnesium dissolution and deposition (the structure is
presumably a 4-coordination structure coordinated by solvent
molecules).
INDUSTRIAL APPLICABILITY
[0164] The present invention is applicable, for example, to a
magnesium secondary battery.
* * * * *