U.S. patent application number 11/332471 was filed with the patent office on 2006-08-10 for electrolyte compositions for batteries using sulphur or sulphur compounds.
Invention is credited to Elena Karaseva, Vladimir Kolosnitsyn.
Application Number | 20060177741 11/332471 |
Document ID | / |
Family ID | 36692600 |
Filed Date | 2006-08-10 |
United States Patent
Application |
20060177741 |
Kind Code |
A1 |
Kolosnitsyn; Vladimir ; et
al. |
August 10, 2006 |
Electrolyte compositions for batteries using sulphur or sulphur
compounds
Abstract
There are disclosed electrolytes comprising solutions of lithium
salts with large anions in polar aprotic solvents with a particular
concentration of background salts. The concentration of the
background salts is selected to be equal or close to the
concentration of a saturated solution of these salts in the aprotic
solvents used. The electrolytes disclosed can be used in chemical
sources of electric energy such as secondary (rechargeable) cells
and batteries comprising sulphur-based positive active materials.
The use of such electrolytes increases cycling efficiency and cycle
life of the cells and batteries.
Inventors: |
Kolosnitsyn; Vladimir; (Ufa,
RU) ; Karaseva; Elena; (Ufa, RU) |
Correspondence
Address: |
Pearl Cohen Zedek Latzer, LLP
Suite 1001
10 Rockefeller Plaza
New York
NY
10020
US
|
Family ID: |
36692600 |
Appl. No.: |
11/332471 |
Filed: |
January 17, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60652769 |
Feb 15, 2005 |
|
|
|
Current U.S.
Class: |
429/324 ;
429/231.4; 429/231.95; 429/337; 429/338; 429/342; 429/343 |
Current CPC
Class: |
H01M 2300/0025 20130101;
H01M 10/0565 20130101; H01M 10/056 20130101; H01M 10/0569 20130101;
H01M 10/0568 20130101; H01M 10/3918 20130101; H01M 4/40 20130101;
Y02E 60/10 20130101; H01M 4/136 20130101; H01M 2010/4292 20130101;
H01M 4/58 20130101; H01M 4/13 20130101 |
Class at
Publication: |
429/324 ;
429/337; 429/342; 429/343; 429/338; 429/231.95; 429/231.4 |
International
Class: |
H01M 10/40 20060101
H01M010/40; H01M 4/58 20060101 H01M004/58; H01M 4/40 20060101
H01M004/40 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 18, 2005 |
GB |
0501001.2 |
Claims
1. An electrolyte composition for a sulphur-based chemical source
of electric energy, the electrolyte composition comprising: a
nonaqueous aprotic solvent and an alkali metal salt, wherein said
electrolyte composition is chosen in a way that a concentration of
the alkali metal salt is substantially equal to or close to a
saturation concentration of the alkali metal salt in the solvent at
operating temperature and pressure.
2. An electrolyte as claimed in claim 1, wherein the solvent is:
tetrahydrofurane, 2-methyltetrahydrofurane, dimethylcarbonate,
diethylcarbonate, ethylmethylcarbonate, methylpropylcarbonate,
methylpropylpropyonate, ethylpropylpropyonate, methylacetate,
ethylacetate, propylacetate, dimethoxyethane, 1,3-dioxalane,
diglyme (2-methoxyethyl ether), tetraglyme, ethylenecarbonate,
propylenecarbonate, y-butyrolactone, sulfolane, or a sulfone.
3. An electrolyte as claimed in claim 1, wherein the alkali metal
salt is: lithium hexafluorophosphate (LiPF.sub.6), lithium
hexafluoroarsenate (LiAsF.sub.6), lithium perchlorate
(LiClO.sub.4), lithium bis(trifluoromethane sulfonyl)imide
(LiN(CF.sub.3SO.sub.2).sub.2)), lithium trifluorosulfonate
(LiCF.sub.3SO.sub.3) or other lithium salts or salts of another
alkali metal or a mixture thereof.
4. An electrolyte as claimed in claim 1, wherein the concentration
of alkali metal salt in the solvent is at least 90% of its
saturation concentration at operating temperature and pressure.
5. An electrolyte as claimed in claim 1, wherein the concentration
of the alkali metal salt in the solvent is at least 95% of its
saturation concentration at operating temperature and pressure.
6. An electrolyte as claimed in claim 1, wherein the concentration
of the at least one alkali metal salt in the at least one solvent
is at least 99% of its saturation concentration at operating
temperature and pressure.
7. A chemical source of electrical energy comprising: a negative
electrode (anode) comprising an anode active material for providing
ions; a positive electrode (cathode) comprising a cathode active
material comprising sulphur, organic or inorganic compounds based
on sulphur; and an intermediate separator element containing a
liquid or a gel electrolyte solution through which ions from the
negative electrode move to the positive electrode during charge and
discharge cycles of the chemical source of electrical energy,
wherein the gel electrolyte solution comprises the composition of
claim 1.
8. A chemical source of electric energy as claimed in claim 7,
wherein the cathode active material comprising sulphur is:
elemental sulphur, lithium polysulphides, inorganic and organic
compounds based on sulphur, or mixtures thereof.
9. A chemical source of electric energy as claimed in claim 7,
wherein the cathode active material comprising sulphur further
comprises a binder and an electrically conductive material.
10. A chemical source of electric energy as claimed in claim 7,
wherein said anode active material is: metallic lithium, alloys of
lithium, metallic sodium, alloys of sodium, alkali metals or alloys
thereof, metal powders, alkali metal-carbon and alkali
metal-graphite intercalates, compounds capable of reversibly
oxidizing and reducing with an alkali metal ion, or mixtures
thereof.
11. A chemical source of electric energy as claimed in claim 8,
wherein the polysulfide is represented by the general formula LiSn,
where n.gtoreq.1.
12. A chemical source of electric energy as claimed in claim 8,
wherein the organic compounds based on sulfur are comprised of
polymeric and oligomeric compounds.
Description
PRIOR APPLICATION DATA
[0001] This application claims benefit from prior U.S. provisional
application Ser. No. 60/652,769, filed Feb. 15, 2005, entitled
"ELECTROLYTE COMPOSITIONS FOR BATTERIES USING SULPHUR OR SULPHUR
COMPOUNDS", and claims benefit from prior UK patent application
number 0501001.2 filed on 18th Jan. 2005, both of which being
incorporated herein by reference in their entirety.
FIELD OF THE INVENTION
[0002] The present invention relates to electrolyte compositions
for chemical sources of electric energy comprising positive
electrodes (cathodes) and negative electrodes (anodes). In
particular, embodiments of the invention relate to rechargeable
(secondary) battery cells comprising a negative electrode (made of
lithium, sodium or another active material or composition)
providing ions (anode), an intermediate separator element
containing a liquid or gel electrolyte solution through which ions
from a source electrode material move between cell electrodes
during charge and discharge cycles of the cell, and a positive
electrode (cathode) comprising sulphur, organic or inorganic
compounds based on sulphur as an electrode depolarizer substance
(cathode active material). Embodiments of the invention also relate
to chemical sources of electric energy comprising said
electrolytes. Further embodiments of the invention relate to the
composition of electrolyte systems comprising nonaqueous aprotic
solvents, lithium salts and modifying additives and designed for
use in lithium-sulphur batteries.
BACKGROUND OF THE INVENTION
[0003] Throughout this application various patents and published
patent applications are referred to by an identifying citation. The
disclosures of the patents and published patent applications
referred to in this application are hereby incorporated by
reference into the present disclosure to more fully describe the
state of the art to which this invention pertains.
[0004] An electroactive material that has been fabricated into a
structure for use in a battery is referred to as an electrode. Of a
pair of electrodes used in a battery, herein referred to as a
chemical source of electrical energy, the electrode on the side
having a higher electrochemical potential is referred to as the
positive electrode, or the cathode, while the electrode on the side
having a lower electrochemical potential is referred to as the
negative electrode, or the anode.
[0005] An electrochemically active material used in the cathode or
positive electrode is referred to hereinafter as a cathode active
material. An electrochemically active material used in the anode or
negative electrode is hereinafter referred to as an anode active
material. Multi-component compositions possessing electrochemical
activity and comprising an electrochemically active material and
optional electron conductive additive and binder, as well as other
optional additives, are referred to hereinafter as electrode
compositions. A chemical source of electrical energy or battery
comprising a cathode with the cathode active material in an
oxidized state and an anode with the anode active material in a
reduced state is referred to as being in a charged state.
Accordingly, a chemical source of electrical energy comprising a
cathode with the cathode active material in a reduced state, and an
anode with the anode active material in an oxidized state, is
referred to as being in a discharged state.
[0006] A lithium, sodium or other alkali metal salt or mixture of
such salts dissolved in a solvent or mixture of solvents so as to
maintain conductivity in the solution is referred to hereinafter as
a supporting salt.
[0007] There is a wide variety of electroactive materials that may
be utilized in the cathode active layers of chemical sources of
electrical energy. For example, a number of these are described in
U.S. Pat. No. 5,919,587 to Mukherjee et al. These electroactive
materials vary widely in their specific densities (g/cm.sup.3) and
in their specific capacities (mAh/g) so the desired volumetric
densities in mg/cm.sup.3 of the electroactive material in the
cathode active layer correspondingly vary over a wide range.
Lithium and sulphur are highly desirable as the electrochemically
active materials for the anode and cathode, respectively, of
chemical sources of electrical energy because they provide nearly
the highest energy density possible on a weight or volume basis of
any of the known combinations of active materials. To obtain high
energy densities, the lithium may be present as the pure metal, in
an alloy, or in an intercalated form, and the sulphur may be
present as elemental sulphur or as a component in an organic or
inorganic material with high sulphur content, preferably above 75
weight percent sulphur. For example, in combination with a lithium
anode, elemental sulphur has a specific capacity of 1680 mAh/g.
This high specific capacity is particularly desirable for
applications such as portable electronic devices and electric
vehicles, where low weight of the battery is important.
[0008] Solutions of lithium salts with large anions in individual
aprotic dipole solvents and their mixtures are widely used as
electrolytes in lithium and lithium-ion rechargeable batteries. The
main requirements of these electrolytes are: [0009] high
conductivity; [0010] capability to stay in a liquid or gel (for gel
electrolytes) state over a wide temperature region; [0011] high
stability against electrode active materials; [0012] chemical and
electrochemical stability (wide electrochemical stability region);
[0013] fire and explosion safety; [0014] nontoxicity.
[0015] High conductivity over a wide temperature range is the main
of the above mentioned requirements. The electrolyte conductivity
is determined by the physical and chemical properties of the
solvents and salts. To obtain high conductivity, it is preferred to
use solvents having high donor characteristics, a high dielectric
constant, and low viscosity, thus providing a high dielectric
dissociation degree for the lithium salts. Lithium salts with large
anions are preferably used since these have a high dissociation
ability.
[0016] The conductivity of the salt solutions is determined by
their concentration. With an increase of salt concentration, the
conductivity at first increases, then reaches a maximum and finally
decreases. The salt concentration is usually chosen to provide
maximum conductivity of the resulting electrolyte [Lithium
batteries: Science and Technology; Gholam-Abbas Nazri and
Gianfranco Pistoia (Eds.); Kluwer Academic; published 2004; pp.
509-573].
[0017] Solutions of one or several lithium salts in individual
solvents or their mixtures are also used as electrolytes in
lithium-sulphur batteries [U.S. Pat. No. 6,030,720, Chu et al.].
The choice of solvents is the main concern when designing
electrolytes for lithium-sulphur batteries because the nature (the
physical and chemical properties) of the solvents has the principal
influence on the battery properties.
[0018] The electrolyte salts that are used in the main prior art
lithium and lithium-ion batteries can be used as supporting salts
in lithium-sulphur batteries. As a rule, prior art patent
disclosures of which the present applicant is aware do not provide
recommendations for specific preferable salt concentrations, but
instead give a very wide range of possible concentrations.
[0019] The nearest closest prior art to the present invention is
currently believed to be described in U.S. Pat. No. 6,613,480 to
Hwang, et al. The text of the patent discloses the information that
electrolyte salts for lithium-sulphur batteries can be chosen from
a list containing: lithium hexafluorophosphate (LiPF.sub.6),
lithium hexafluorarsenate (LiAsF.sub.6), lithium perchlorate
(LiClO.sub.4), lithium sulfonylimid trifluoromethane
(LiN(CF.sub.3SO.sub.2).sub.2)) and lithium trifluorosulfonate
(CF.sub.3SO.sub.3Li). The electrolyte salt concentration should be
taken from the range of 0.5 to 2.0M.
[0020] High conductivity over a wide temperature range (together
with electrochemical stability) is the main requirement of the
electrolyte compositions used in lithium and lithium-ion batteries
with traditional hard cathode active materials. The choice of the
electrolyte composition for lithium-sulphur batteries is much
harder because the sulphur may dissolve in the electrolyte solvents
and react with their components, with this having a major influence
on the battery properties.
[0021] Despite the numerous electrolyte solvents and electrolyte
salts proposed for use in rechargeable cells, there remains a need
for improved non-aqueous electrolyte compositions that provide
beneficial effects during the useful life of the chemical sources
of electric energy comprising sulphur-based positive electrode
active material.
SUMMARY OF THE INVENTION
[0022] Embodiments of the present invention may provide an improved
non-aqueous electrolyte composition which is suitable for use in
rechargeable cells comprising sulphur-based positive electrode
active material and which has greater temperature stability and
conductivity and provides a higher cycling efficiency and long
cycle life of the battery.
[0023] Embodiments of the present invention relate to electrolytes
for lithium-sulphur batteries, such as electrolytes comprising
solutions of lithium salts with large anions in aprotic polar
solvents with predetermined concentrations of supporting salts. In
particular, embodiments of the present invention may provide the
use of lithium salts or mixtures of lithium salts in an electrolyte
at a concentration substantially equal to or at least close to a
concentration of saturated solution of the lithium salt (or salts)
in the solvent (or mixture of solvents). The use of such
electrolytes in lithium-sulphur batteries provides improved
efficiency and cycling duration.
[0024] According to a first aspect of the present invention, there
is provided an electrolyte composition for a sulphur-based chemical
source of electric energy, the electrolyte composition comprising
at least one nonaqueous aprotic solvent, at least one alkali metal
salt, and optional modifying additives, wherein said electrolyte
composition is chosen in a way that a concentration of the at least
one salt is substantially equal to or close to a saturation
concentration of the at least one alkali metal salt in the at least
one solvent.
[0025] Preferably, the concentration of the at least one salt is at
least 90%, preferably at least 95%, and even more preferably at
least 99% of the saturation concentration.
[0026] The at least one salt can be a single salt or a mixture of
alkali metal salts. Lithium salts are particularly preferred, but
sodium and other alkali metal salts and mixtures thereof may also
be used.
[0027] Examples of lithium salts include lithium
hexafluorophosphate (LiPF.sub.6), lithium hexafluoroarsenate
(LiAsF.sub.6), lithium perchlorate (LiClO.sub.4), lithium
bis(trifluoromethane sulfonyl)imide (LiN(CF.sub.3SO.sub.2).sub.2))
and lithium trifluorosulfonate (LiCF.sub.3SO.sub.3).
[0028] The at least one aprotic solvent can be a single solvent or
a mixture of solvents selected from a group comprising:
tetrahydrofurane, 2-methyltetrahydrofurane, dimethylcarbonate,
diethylcarbonate, ethylmethylcarbonate, methylpropylcarbonate,
methylpropylpropyonate, ethylpropylpropyonate, methylacetate,
ethylacetate, propylacetate, dimethoxyethane, 1,3-dioxalane,
diglyme (2-methoxyethyl ether), tetraglyme, ethylenecarbonate,
propylenecarbonate, .gamma.-butyrolactone, sulfolane, and at least
one sulfone.
[0029] According to a second aspect of the present invention, there
is provided a chemical source of electrical energy comprising a
negative electrode (anode) including an anode active material for
providing ions, a positive electrode (cathode) including a cathode
active material comprising sulphur or organic or inorganic
compounds based on sulphur, and an intermediate separator element
containing a liquid or gel electrolyte solution through which ions
from the negative electrode move to the positive electrode during
charge and discharge cycles of the chemical source of electrical
energy, wherein the electrolyte solution comprises an electrolyte
composition according to the first aspect of the present
invention.
[0030] The chemical source of electrical energy may be a cell or
battery.
[0031] The anode active material may comprise an alkali metal such
as lithium or sodium or another active material or composition.
[0032] Particularly preferred anode active materials include
metallic lithium, alloys of lithium, metallic sodium, alloys of
sodium, alkali metals or alloys thereof, metal powders, alkali
metal-carbon and alkali metal-graphite intercalates, compounds
capable of reversibly oxidizing and reducing with an alkali metal
ion, and mixtures thereof.
[0033] The cathode active material containing sulphur may be
selected from a group comprising: elemental sulphur, lithium
polysulphides (Li.sub.2Sn with n.gtoreq.1), non-organic and organic
(including oligomeric and polymeric) compounds based on sulphur,
and mixtures thereof.
[0034] The cathode active material may additionally include a
binder and an electrically conductive material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] For a better understanding of the present invention and to
show how it may be carried into effect, reference shall now be made
by way of example to the accompanying drawings, in which:
[0036] FIG. 1 is a graph showing charge and discharge capacity fade
during cycling of a standard lithium-sulphur cell;
[0037] FIG. 2 is a graph showing changes in the cycling efficiency
and capacity fade rate for the standard lithium-sulphur cell;
[0038] FIG. 3 is a graph showing charge and discharge capacity fade
during cycling of a second lithium-sulphur cell with a more
concentrated electrolyte;
[0039] FIG. 4 is a graph showing changes in the cycling efficiency
and capacity fade rate for second lithium-sulphur cell;
[0040] FIG. 5 is a graph showing charge and discharge capacity fade
during cycling of a third lithium-sulphur cell with a saturated
electrolyte solution in accordance with an embodiment of the
invention;
[0041] FIG. 6 is a graph showing changes in the cycling efficiency
and capacity fade rate for the third lithium-sulphur cell;
[0042] FIG. 7 is a graph showing charge and discharge capacity fade
during cycling of a fourth lithium-sulphur cell with a different,
non-saturated electrolyte;
[0043] FIG. 8 is a graph showing changes in the cycling efficiency
and capacity fade rate for the fourth lithium-sulphur cell;
[0044] FIG. 9 is a graph showing charge and discharge capacity fade
during cycling of a fifth lithium-sulphur cell with a saturated
electrolyte solution in accordance with an embodiment of the
invention, where the electrolyte is a 1.7 M solution of LiClO4 in
methylpropylsulfone and the charge rate is 0.25 C and the discharge
rate is 0.25 C. and
[0045] FIG. 10 is a graph showing changes in the cycling efficiency
and capacity fade rate for the fifth lithium-sulphur cell, where
the electrolyte is a 1.7 M solution of LiClO.sub.4 in
methylpropylsulfone.
DETAILED DESCRIPTION OF THE INVENTION
[0046] Fast capacity fade and relatively low cycling efficiency are
the main problems encountered when designing lithium-sulphur
batteries. Irreversible transfer of sulphur from the positive
electrode (cathode) to the surface of the negative electrode
(anode) and its accumulation there in the form of lithium sulphide
or disulphide is one of the major reasons for capacity fade during
cycling of lithium-sulphur cells. The low cycling efficiency of
lithium-sulphur batteries is caused by the reversible transfer of
sulphur in the middle of the charge and discharge process. This
transfer results in what is known as the sulphide cycle, i.e. the
energy transfer inside the battery (in its internal circuit).
[0047] Elemental sulphur and the end products of sulphur reduction
(lithium sulphide or disulphide) are known to be poorly soluble in
most organic solvents. In contrast, lithium polysulphides
(intermediate forms produced during the reduction of elemental
sulphur or during oxidation of lithium sulphide and disulphide) are
well soluble in many organic solvents.
[0048] The rate of sulphur transfer between the positive and
negative electrodes of lithium-sulphur batteries is determined by
the form of sulphur present in the electrolyte solution. The form
of sulphur and sulphur-lithium compounds present in the
electrolytes of lithium-sulphur batteries depends on the
electrolyte system composition and the properties thereof. In
particular, it depends on the polarity and donor properties of the
solvents used and by the concentration of the supporting salts.
[0049] Lithium polysulphides may be present in electrolyte systems
in three forms: molecular, mono-anionic, and di-anionic. Hence
sulphur in the electrolyte can be transferred either in molecular
(neutral) or in ionic (anionic) form. The diffusion of elemental
sulphur and non-dissociated lithium polysulphides dissolved in the
electrolyte contributes to the molecular transfer of sulphur. The
diffusion and electromigration of the mono- and di-anions of
polysulphides, as well as sulphur anion-radicals, contributes to
the ionic form of sulphur transfer. The existence of two mechanisms
increases the overall sulphur transfer. The sulphur transfer will
be higher in the case of a diffusion-migration process as compared
to a pure diffusion mechanism. As a result, the rate of capacity
fade and the cycling efficiency of lithium-sulphur batteries are
dependent on the form of the sulphur present in the electrolyte
solution and the form of sulphur transfer from the positive
electrode to the interelectrode space and thence to the surface of
the negative electrode. The rate of capacity fade for
lithium-sulphur batteries will be much lower and their cycling
efficiency will be much higher if the sulphur is present as neutral
particles (molecular form) as opposed to charged particles (ionic
form).
[0050] The degree of electrolytic dissociation of each salt in the
electrolyte solution will be determined by their respective
concentrations and dissociation constants in the presence of two or
more different salts in the electrolyte composition (here, for
example, lithium polysulphides and the supporting salts). Based on
the nature of the relevant anions, the present applicant believes
that the electrolytic dissociation constants of lithium
polysulphides are much lower than those of most lithium salts that
may be used as supporting salts. In this case, with an increase in
the supporting salt concentration, the equilibrium in the
dissociation reaction of lithium polysulphides will shift towards a
greater presence of the molecular form rather than the ionic
form.
[0051] Accordingly, the dissociation degree of lithium
polysulphides will decrease with an increase in the concentration
of the supporting salts. Hence, a decrease should be found in the
rate of sulphur transfer between the electrodes and,
correspondingly, in the rate of the capacity fade of a
lithium-sulphur cell during cycling thereof.
[0052] Moreover, the cycling efficiency should increase as a result
of the rate decrease of the sulphide cycle. This is clearly shown
in the examples that follow.
[0053] When forming the electrolyte compositions of embodiments of
the present invention, the following considerations may be taken
into account:
1) The electrolyte composition should comprise a non-aqueous
aprotic solvent, lithium or another alkali metal salt and optional
modifying additives.
2) Said salt can be an individual salt or a number of different
salts.
3) Said salt or number of salts are dissolved in an individual
aprotic polar solvent or a mixture of solvents.
[0054] 4) Said electrolyte composition should be chosen in a way
that the concentration of the lithium salt or the mixture of salts
is equal (or close) to the concentration of a saturated solution of
the salt or salts used in the solvent or mixture of solvents.
[0055] In generally, batteries or other devices, or compositions
such as electrolyte compositions, or chemical sources of electric
energy, according to embodiments of the present invention operate
at certain temperature and pressure ranges.
[0056] For example, in one embodiment, the operating temperature
may be approximately -40 to +150 degrees Celsius. In another
embodiment, the operating temperature may be approximately -20 to
+110 degrees C., or -10 to +50 degrees C.
[0057] In one embodiment, the operating pressure may be
approximately 5 mmHg to 76000 mmHg (0.0066 to 100 atm). In another
embodiment, the operating pressure may be approximately 20 mmHg to
38000 mmHg (0.026 to 50 atm), or for example approximately 1
atm.
[0058] Embodiments of the present invention may operate at standard
temperature and pressure, for example at approximately 25 degrees
C. and 1 atm.
[0059] Embodiments of the present invention may operate at other
temperature and pressure ranges.
EXAMPLE 1
[0060] A lithium-sulphur cell was produced by assembling an anode
made of metal lithium foil; a porous separator Celgard 2500 (a
registered trademark of Celgard Inc., available from Celgard K.K.,
Tokyo, Japan, and also available from Celgard Inc. South Lakes,
N.C. USA.); and a sulphur cathode comprising elemental sulphur as a
depolariser (70% by weight), a carbon electro-conducting additive
(10% by weight) Ketjenblack EC-600JD (available from Akzo Nobel
Polymer Chemicals BV, Netherlands), and a binder (polyethyleneoxide
with molecular mass 4000000-20% by weight). The sulphur cathode was
deposited by an automatic film applicator Elcometer SPRL onto one
side of an 18 micrometer thick conductive carbon coated aluminium
foil (available from InteliCoat.RTM., South Hadley, Mass.) as a
current collector and substrate. A specific surface capacity of the
cathode was 1 mAh/cm.sup.2. The assembled cell was filled with an
electrolyte comprising a 0.1M solution of LiClO.sub.4 in
sulpholane. All stages of the cell assembling and filling were
performed in a "Jacomex Type BS531" glove box. The cell was cycled
at a charge and discharge rate of 0.25 C and at a temperature of
25.degree. C. The change in the charge and discharge capacity of
the cell during the cycling is shown in FIG. 1. FIG. 1 depicts
curves of the sulphur electrode capacity change in a
lithium-sulphur battery during cycling, according to one embodiment
of the invention. In FIG. 1, the electrolyte is 0.1 M LiClO.sub.4
solution in sulpholane, the charge rate is 0.25 C, and the
discharge rate is 0.25 C.
[0061] The change of the cycling efficiency and the rate of the
capacity fade during cycling are shown in FIG. 2. In FIG. 2, the
electrolyte is 0.1 M LiClO.sub.4 solution and the average capacity
fade rate is 4.5%.
[0062] The cycling efficiency is calculated as the ratio between
the discharge capacity and the charge capacity expressed as a
percentage. The rate of the capacity fade is calculated as the
difference of the capacity at two cycles, following each other,
divided by the mean capacity at these cycles and expressed as a
percentage. As can be seen in FIG. 2, the efficiency of cycling and
the rate of capacity fade initially change after the beginning of
cycling, but later on they stabilize. The mean cycling efficiency
between the 10.sup.th and 20.sup.th cycles was 68%, and the rate of
the capacity fade was 4.5%.
EXAMPLE 2
[0063] A lithium-sulphur cell was produced as described in the
Example 1, but this time the assembled cell was filled with an
electrolyte comprising a 1 M solution of LiClO.sub.4 in sulpholane.
The cell was cycled at a charge and discharge rate of 0.25 C and at
a temperature of 25.degree. C. The change in the charge and
discharge capacity of the cell during the cycling is shown in
Figure, showing the capacity fade of the sulphur electrode in
lithium-sulphur cell during cycling. In FIG. 3, the electrolyte is
a 1 M solution of LiClO.sub.4 in sulpholane, the charge rate is
0.25 C, and the discharge rate is 0.25 C.
[0064] The change in the cycling efficiency and the rate of the
capacity fade during cycling are shown in FIG. 4. In FIG. 4 the
electrolyte is 1 M solution of LiClO.sub.4 in sulpholane.
[0065] As can be seen in FIG. 4, the efficiency of cycling and the
rate of capacity fade initially change after the beginning of
cycling, but later on they stabilize. The mean cycling efficiency
between the 10.sup.th and 20.sup.th cycles was 90%, and the rate of
the capacity fade was 2.25%. This is a marked improvement over the
cell of Example 1.
EXAMPLE 3
[0066] A lithium-sulphur cell was produced as described in the
Example 1, but this time the assembled cell was filled with an
electrolyte comprising a 2M saturated solution of LiClO.sub.4 in
sulpholane in accordance with an embodiment of the present
invention. The cell was cycled at a charge and discharge rate of
0.25 C and at a temperature of 25.degree. C. The change in the
charge and discharge capacity of the cell during the cycling is
shown in FIG. 5, showing the capacity fade of a sulphur electrode
in a lithium-sulphur cell during cycling. In FIG. 5, the
electrolyte is a 2 M solution of LiClO.sub.4 in sulpholane, the
charge rate is 0.25 C, and the discharge rate is 0.25 C.
[0067] The change in the cycling efficiency and the rate of the
capacity fade during cycling are shown in FIG. 6. In FIG. 6, the
electrolyte is 2 M solution of LiClO4 in sulpholane. As can be seen
in FIG. 6, the efficiency of cycling and the rate of capacity fade
initially change after the beginning of cycling, but later on they
stabilize. The mean cycling efficiency between the 10.sup.th and
20.sup.th cycles was 96%, and the rate of the capacity fade was
1.75%. This is a marked improvement over the cells of Examples 1
and 2.
EXAMPLE 4
[0068] A lithium-sulphur cell was produced as described in the
Example 1, but this time the assembled cell was filled up with an
electrolyte comprising a 0.1M solution of LiClO.sub.4 in
methylpropylsulfone. The cell was cycled at a charge and discharge
rate of 0.25 C and at a temperature of 25.degree. C. The change in
the charge and discharge capacity of the cell during the cycling is
shown in FIG. 7, showing the capacity fade of a sulphur electrode
in lithium-sulphur cell during cycling. The electrolyte is a 0.1 M
solution of LiClO4 in methylpropylsulfone, the charge rate is 0.25
C, and the discharge rate is 0.25 C.
[0069] The change in the cycling efficiency and the rate of
capacity fade during cycling are shown in FIG. 8. In FIG. 8, the
electrolyte is 0.1 M solution of LiClO4 in methylpropylsulfone. As
can be seen in FIG. 8, the efficiency of cycling and the rate of
capacity fade initially change after the beginning of cycling, but
later on they stabilize. The mean cycling efficiency between the
10.sup.th and 20.sup.th cycles was 55%, and the rate of the
capacity fade was 3.1%.
EXAMPLE 5
[0070] A lithium-sulphur cell was produced as is described in the
Example 1, but this time the assembled cell was filled with an
electrolyte comprising a 1.7M solution of LiClO.sub.4 in
methylpropylsulfone (the concentration close to the saturated
solution). The cell was cycled at a charge and discharge rate of
0.25 C and at a temperature of 25.degree. C. The change in the
charge and discharge capacity of the cell during the cycling is
shown in FIG. 7.
[0071] The change in the cycling efficiency and the rate of
capacity fade during cycling are shown in FIG. 8. As can be seen in
FIG. 8, the efficiency of cycling and the rate of capacity fade
initially change after the beginning of cycling, but later on they
stabilize. The mean cycling efficiency between the 10.sup.th and
20.sup.th cycles was 90%, and the rate of the capacity fade was
1.15%, which is a marked improvement over the cell of Example
4.
[0072] Examples 4 and 5 illustrate that the improvement in cycling
efficiency and rate of capacity fade is independent of the chemical
identity of the solvent, but instead depends on the electrolyte
concentration.
[0073] While some embodiments of the invention have been
illustrated and described, it is clear that the invention is not
limited to these specific embodiments. Numerous modifications,
changes, variations, substitutions, and equivalents will occur to
those skilled in the art without departing from the scope of the
present invention.
[0074] The preferred features of the invention are applicable to
all aspects of the invention and may be used in any possible
combination.
[0075] Throughout the description and claims of this specification,
the words "comprise" and "contain" and variations of the words, for
example "comprising" and "comprises", mean "including but not
limited to", and are not intended to (and do not) exclude other
components, integers, moieties, additives or steps.
* * * * *