U.S. patent application number 11/526876 was filed with the patent office on 2007-03-29 for lithium-sulphur battery with high specific energy.
Invention is credited to Elena Karaseva, Vladimir Kolosnitsyn.
Application Number | 20070072076 11/526876 |
Document ID | / |
Family ID | 37441316 |
Filed Date | 2007-03-29 |
United States Patent
Application |
20070072076 |
Kind Code |
A1 |
Kolosnitsyn; Vladimir ; et
al. |
March 29, 2007 |
Lithium-sulphur battery with high specific energy
Abstract
A battery or chemical source of electric energy may include a
positive electrode including sulphur or sulphur-based organic
compounds, sulphur-based polymeric compounds or sulphur-based
inorganic compounds as a depolarizer, a negative electrode made of
metallic lithium or lithium-containing alloys, and an electrolyte
including a solution of at least one salt in at least one aprotic
solvent. In order to increase the specific energy, the chemical
source of electric energy may be configured to generate soluble
polysulphides in the electrolyte during discharge, and the
quantities of sulphur in the depolariser and the volume of
electrolyte may be selected such that, after discharge of the
cathode in a first stage (to a potential of 2.1-1.9V), the
concentration of soluble lithium polysulphides in the electrolyte
is at least 70% of a saturation concentration of the lithium
polysulphides in the electrolyte.
Inventors: |
Kolosnitsyn; Vladimir; (Ufa,
RU) ; Karaseva; Elena; (Ufa, RU) |
Correspondence
Address: |
PEARL COHEN ZEDEK, LLP;PEARL COHEN ZEDEK LATZER, LLP
1500 BROADWAY 12TH FLOOR
NEW YORK
NY
10036
US
|
Family ID: |
37441316 |
Appl. No.: |
11/526876 |
Filed: |
September 26, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60734320 |
Nov 8, 2005 |
|
|
|
60721062 |
Sep 28, 2005 |
|
|
|
Current U.S.
Class: |
429/218.1 ;
429/213; 429/231.95 |
Current CPC
Class: |
H01M 4/136 20130101;
H01M 4/134 20130101; H01M 4/60 20130101; H01M 10/0569 20130101;
H01M 10/052 20130101; H01M 10/05 20130101; H01M 4/581 20130101;
H01M 4/58 20130101; H01M 4/5815 20130101; H01M 4/137 20130101; H01M
10/056 20130101; Y02E 60/10 20130101; H01M 4/133 20130101; H01M
10/0568 20130101 |
Class at
Publication: |
429/218.1 ;
429/231.95; 429/213 |
International
Class: |
H01M 4/58 20060101
H01M004/58; H01M 4/60 20060101 H01M004/60; H01M 4/40 20060101
H01M004/40 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 26, 2005 |
GB |
0519491.5 |
Claims
1. A chemical source of electric energy comprising: a positive
electrode comprising sulphur or sulphur-based organic compounds,
sulphur-based polymeric compounds or sulphur-based inorganic
compounds as a depolarizer; a negative electrode comprising
metallic lithium or lithium-containing alloys; an electrolyte
comprising a solution of at least one salt in at least one aprotic
solvent, the chemical source of electric energy being configured to
generate soluble polysulphides in the electrolyte during a first
stage of a two stage discharge process, wherein the quantity of
sulphur in the depolariser and the volume of electrolyte are
selected such that, after first stage discharge of the cathode, the
concentration of the soluble polysulphides in the electrolyte is at
least 70% of a saturation concentration of the polysulphides in the
electrolyte at operating temperature and pressure.
2. A chemical source of electric energy as claimed in claim 1,
wherein the quantity of sulphur in the depolariser and the volume
of electrolyte are selected such that, after complete discharge of
the cathode, the concentration of the soluble lithium polysulphides
in the electrolyte is from 70 to 90% of a saturation concentration
of the polysulphides in the electrolyte at operating temperature
and pressure.
3. A chemical source of electric energy as claimed in claim 1,
wherein the depolarizer comprises sulphur, carbon black and
polyethylene oxide.
4. A chemical source of electric energy as claimed in claim 1,
wherein the electrolyte comprises a solution of one or several
lithium salts selected from the group consisting of: lithium
trifluoromethanesulphonate, lithium perchlorate, lithium
trifluoromethanesulfonimide, lithium hexafluorophosphate, lithium
hexafluoroarsenate, lithium tetrachloroaluminate,
tetraalkylammonium salt, lithium chloride, lithium bromide and
lithium iodide; in one or several solvents selected from the group
consisting of: dioxolane, tetrahydrofuran, dimethoxyethane,
diglyme, triglyme, tetraglyme, dialkyl carbonates, sulfolane and
butyrolactone.
Description
PRIOR APPLICATION DATA
[0001] The present application claims benefit from prior U.S.
provisional patent application No. 60/734,320, filed Nov. 8, 2005,
entitled "LITHIUM-SULPHUR BATTERY WITH HIGH SPECIFIC ENERGY", from
prior U.S. provisional patent application No. 60/721,062, filed
Sep. 28, 2005, entitled "LITHIUM-SULPHUR BATTERY WITH HIGH SPECIFIC
ENERGY", and from UK patent application no. 0519491.5, filed Sep.
26, 2005 each of which being incorporated by reference herein in
its entirety.
FIELD OF THE INVENTION
[0002] The present invention relates to electrochemical power
engineering, in particular it relates to chemical sources of
electric energy (cells or batteries) comprising negative electrodes
made of alkali metals and positive electrodes comprising sulphur
and/or sulphur-based non-organic or organic (including polymeric)
compounds as an electrode depolarizer substance.
BACKGROUND
[0003] The lithium-sulphur electrochemical system has a high
theoretical specific energy of 2600 Wh/kg (D. Linden, T. B. Reddy,
Handbook of batteries, third ed., McGraw-Hill, New-York, 2001), and
is therefore of great interest at present. Specific energy is
defined as the ratio of the energy output of a cell or battery to
its weight, and is expressed in Wh/kg. The term specific energy is
equivalent to the term gravimetric energy density.
[0004] It has been proposed to use various materials as a
depolarizer substance for the positive electrode in lithium-sulphur
batteries: elemental sulphur (U.S. Pat. No. 5,789,108; U.S. Pat.
No. 5,814,420), sulphur-organic compounds (U.S. Pat. No.
6,090,504), sulphur-containing polymers (U.S. Pat. No. 6,201,100;
U.S. Pat. No. 6,174,621; U.S. Pat. No. 6,117,590), and solutions of
sulphur or lithium polysulphides in aprotic electrolyte systems
(Rauh R. D., Abraham K. M., Pearson G. F., Surprenant J. K.,
Brummer S. B.: "A lithium/dissolved sulphur battery with an organic
electrolyte", J. Electrochem. Soc. 1979, vol. 126, no. 4, pp
523-527; Yamin H., Peled E.: "Electrochemistry of a nonaqueous
lithium/sulphur cell", J. of Power Sources, 1983, vol. 9, pp
281-287).
[0005] Solutions of lithium salts in aprotic dipolar solvents
(typically linear or cyclic ethers) or their mixtures have been
used as electrolytes in lithium-sulphur batteries (Yamin H.,
Penciner J., Gorenshtain A., Elam M., Peled E.: "The
electrochemical behavior of polysulphides in tetrahydrofuran", J.
of Power Sources, 1985, vol. 14, pp 129-134; Yamin H., Gorenshtein
A., Penciner J., Sternberg Y., Peled E.: "Lithium sulphur battery
oxidation/reduction mechanisms of polysulphides in THF solution",
J. Electrochem. Soc., 1988, vol 135, no. 5, pp 1045-1048; Duck-Rye
Chang, Suck-Hyun Lee, Sun-Wook Kim, Hee-Tak Kim: "Binary
electrolyte based on tetra(ethylene glycol) dimethyl ether and
1,3-dioxolane for lithium-sulphur battery", J. of Power Sources,
2002, vol. 112, pp 452-460).
[0006] The practical specific energy of a typical chemical source
of electric energy usually reaches 20-30% of the theoretical
maximum value of the specific energy of the electrochemical system
that is employed. This is because various auxiliary elements (the
separator, the current collectors of the electrodes, the
electrolyte and other components) of the battery contribute to its
total weight in addition to the electrode depolarizers. The
auxiliary elements of the battery design do not themselves take
part in the electrochemical reaction itself, but are provided so as
to facilitate the reaction process and to promote normal
functioning of the battery.
[0007] The value of the practical specific energy for laboratory
lithium-sulphur cells generally reaches only 10-15% of its
theoretical value, and is typically around 250-350 Wh/kg (J.
Broadhead, T. Skotheim: "A safe, fast-charge, two-volt
lithium/polymer cathode `AA`-size cell with a greater than 250 Wh
kg-1 energy density", Journal of Power Sources, 65 (1997), 1-2,
213-218; Peled E., Gorenshtein A., Segal M., Sternberg Y.:
"Rechargeable lithium-sulphur battery (extended abstract)", J. of
Power Sources, 1989, vol. 26, pp 269-271).
SUMMARY
[0008] Disregarding the weight of the auxiliary battery elements,
the difference between the theoretical and the practical values for
the specific energies of laboratory lithium-sulphur cells is due to
insufficient utilization of the positive electrode depolarizer
(sulphur or sulphur-based compounds) and the excess quantity of
electrolyte that is generally provided.
[0009] Embodiments of the present invention seek at least
substantially to optimize the electrolyte quantity in
lithium-sulphur cells, and thereby to improve their practical
specific energy.
[0010] One embodiment includes a chemical source of electric energy
comprising a positive electrode (cathode) including sulphur or
sulphur-based organic compounds, sulphur-based polymeric compounds
or sulphur-based inorganic compounds as a depolarizer, a negative
electrode (anode) made of metallic lithium or lithium-containing
alloys, and an electrolyte comprising a solution of at least one
salt in at least one aprotic solvent, the chemical source of
electric energy being configured to generate soluble polysulphides
in the electrolyte during a first stage of a two stage
discharge-process, characterised in that the quantity of sulphur in
the depolariser and the volume of electrolyte are selected such
that, after first stage discharge of the cathode, the concentration
of the soluble polysulphides in the electrolyte is at least 70% of
a saturation concentration of the polysulphides in the
electrolyte.
[0011] In another embodiment the quantity of sulphur in the
depolariser and the volume of electrolyte are selected such that,
after complete discharge of the cathode, the concentration of the
soluble lithium polysulphides in the electrolyte is from 70 to 90%
of a saturation concentration of the polysulphides in the
electrolyte.
[0012] In another embodiment the depolarizer includes sulphur,
carbon black and polyethylene oxide.
[0013] In another embodiment the electrolyte includes a solution of
one or several lithium salts selected from the group consisting of:
lithium trifluoromethanesulphonate, lithium perchlorate, lithium
trifluoromethanesulfonimide, lithium hexafluorophosphate, lithium
hexafluoroarsenate, lithium tetrachloroaluminate,
tetraalkylammonium salt, lithium chloride, lithium bromide and
lithium iodide; in one or several solvents selected from the group
consisting of: dioxolane, tetrahydrofuran, dimethoxyethane,
diglyme, triglyme, tetraglyme, dialkyl carbonates, sulfolane and
butyrolactone.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] 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 drawing, in which:
[0015] FIG. 1 is a plot showing the two stage discharge process of
a lithium-sulphur battery of an embodiment of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0016] In the following detailed description, numerous specific
details are set forth in order to provide a thorough understanding
of the present invention. However, it will be understood by those
skilled in the art that the present invention may be practiced
without these specific details. In other instances, well-known
methods, procedures, and components have not been described in
detail so as not to obscure the present invention.
[0017] The specific energy of a chemical source of electric energy
is determined by the theoretical specific energy of the selected
electrochemical system, as well as by the weight of auxiliary
components required to ensure the proper operation of the chemical
source of electric energy (e.g. a separator, current collectors of
electrodes, a binder, current conducting additives, an electrolyte
and other components), and also by the degree (efficiency) of
utilization of the depolarizer. The weight of the auxiliary
components generally makes up 70-80% of the total weight of the
cell. In order to achieve improved specific energy characteristics,
the weight of the auxiliary components must be reduced.
[0018] The weight of the electrolyte is a significant part of the
total weight of the chemical source of electric energy. The
electrolyte performs supplementary functions in chemical sources of
electric energy with solid depolarizers, for example supporting the
electrochemical reaction process and providing ion transport
between the electrodes. Therefore in such systems it is desirable
to minimize the quantity of the electrolyte.
[0019] However, in chemical sources of electric energy comprising
liquid cathodes, the electrolyte may consist of a salt solution in
a liquid depolarizer (for example a solution of lithium
tetrachloroaluminate in thionyl chloride), or a salt solution in a
mixture of a liquid depolarizer and an aprotic solvent (for example
a solution of lithium bromide in a mixture of sulphurous anhydride
and acetonitrile), or a salt solution in a solution of a liquid
depolarizer in an aprotic solvent (for example a lithium
perchlorate solution in a solution of lithium polysulphide in
tetrahydrofuran) (D. Linden, T. B. Reddy: "Handbook of batteries",
third ed., McGraw-Hill, New York, 2001).
[0020] The electrolyte in chemical sources of electric energy
comprising liquid cathodes performs a wider range of functions than
the electrolyte used in systems having solid cathodes. The
electrolyte not only supports the electrochemical reaction and ion
transport between the electrodes, but serves as a solvent for a
depolarizer of the positive electrode. Accordingly, when aprotic
solvents are used as a component of a liquid cathode, the specific
power characteristics of chemical sources of electric energy with
liquid cathodes depend on the content of the aprotic solvents and
hence on the content of the liquid cathode.
[0021] Despite the fact that sulphur and lithium sulphide are
poorly soluble in aprotic solvents, lithium-sulphur batteries are
classified as batteries with liquid cathodes. This is because of
the formation of well-soluble products, lithium polysulphides, that
occur during charge and discharge of such batteries.
[0022] The liquid cathode is formed in lithium-sulphur batteries
during discharge of the sulphur electrode. The electrochemical
oxidation of sulphur is realised by way of two stages. In the first
stage, long-chain lithium polysulphides (which are well-soluble in
aprotic electrolytes) are generated during the electrochemical
oxidation of elemental sulphur, which is non-soluble or poorly
soluble in most electrolyte systems (Equation 1).
S.sub.8+2e.sup.-.fwdarw.S.sub.8.sup.2-
S.sub.8.sup.2-.fwdarw.S.sub.6.sup.2-+1/4S.sub.8
S.sub.6.sup.2-.fwdarw.S.sub.4.sup.2-+1/4S.sub.8 Equation 1
[0023] The solution of lithium polysulphides in electrolyte which
is formed in the initial discharge phase is known to be a liquid
cathode.
[0024] In the second stage, an electrochemical reduction of soluble
lithium polysulphides occurs (Equation 2). During this phase,
sulphide or disulphide anions are formed, which, by reacting with
lithium cations, form non-soluble products: lithium sulphide and
lithium disulphide. S.sub.n.sup.2-+2(n-1)e.sup.-.fwdarw.nS.sup.2-
S.sub.n.sup.2-+(n-2)e.sup.-.fwdarw.(1/2n)S.sub.2.sup.2- Equation
2
[0025] The reduction of soluble lithium polysulphides is
accompanied by disproportionation reactions (Equation 3).
2S.sub.n.sup.2-.fwdarw.S.sub.n-1.sup.2-+S.sub.n+1.sup.2-
2S.sub.n+1.sup.2-.fwdarw.S.sub.n+2.sup.2-+S.sub.n.sup.2-
2S.sub.n-1.sup.2-.fwdarw.S.sub.n-2.sup.2-+S.sub.n.sup.2- Equation
3
[0026] The two stage mechanism of sulphur reduction is clearly seen
in the discharge curves for lithium-sulphur batteries as shown in
FIG. 1. It is represented by two discharge regions: a first region
in the voltage range from 2.5-2.4V to 2.1-1.9V corresponds to the
first discharge phase (Equation 1); and a second region in the
voltage range from 2.1-1.9V to 1.8-1.5V corresponds to the second
(Equation 2).
[0027] The efficiency of sulphur utilization in lithium-sulphur
batteries is determined by the quantitative ratio of sulphur to
electrolyte.
[0028] When the amount of electrolyte is relatively low, not all of
the sulphur and long-chain lithium polysulphides undergo subsequent
electrochemical reduction because of the formation of highly
viscous saturated solutions of polysulphides. This results in a
decrease in the practical specific energy of lithium-sulphur
batteries.
[0029] When there is an excess of electrolyte, the specific energy
of lithium-sulphur batteries is still lower than the maximum
possible because of the quantitative excess of electrolyte which
contributes to the overall weight of a cell. A particular ratio of
sulphur to electrolyte should be maintained for each type of
electrolyte so as to reach or at least approach the maximum
possible practical specific energy.
[0030] This ratio depends on the properties of the electrolyte
system. In particular, the ratio depends on the solubility of
initial, intermediate and final compounds.
[0031] To achieve the best practical specific energy
characteristics of lithium-sulphur batteries, the electrolyte
content in the batteries should be chosen in a way that provides
complete dissolution of lithium polysulphides (formed at the first
stage) with formation of liquid cathodes with moderate viscosity.
The present applicant has found that such condition is provided
when, during discharge of the sulphur electrode, the concentration
of soluble polysulphides in the electrolyte is at least 70%, and
preferably from 70 to 90%, of the saturation concentration at
operating temperature and pressure.
[0032] 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.
[0033] 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.
[0034] Embodiments of the present invention may operate at standard
temperature and pressure, for example at approximately 25 degrees
C. and 1 atm.
[0035] Embodiments of the present invention may operate at other
temperature and pressure ranges.
[0036] According to the present invention, there is provided a
chemical source of electric energy comprising a positive electrode
(cathode) including sulphur or sulphur-based organic compounds,
sulphur-based polymeric compounds or sulphur-based inorganic
compounds as a depolarizer, a negative electrode (anode) made of
metallic lithium or lithium-containing alloys, and an electrolyte
comprising a solution of at least one salt in at least one aprotic
solvent, the chemical source of electric energy being configured to
generate soluble polysulphides in the electrolyte during a first
stage of a two stage discharge process, characterised in that the
quantity of sulphur in the depolariser and the volume of
electrolyte are selected such that, after first stage discharge of
the cathode, the concentration of the soluble polysulphides in the
electrolyte is at least 70% of a saturation concentration of the
polysulphides in the electrolyte.
[0037] Preferably, the quantity of sulphur in the positive
electrode and the volume of electrolyte are selected such that,
after first stage discharge of the cathode, the concentration of
the soluble polysulphides in the electrolyte is from 70 to 90% of a
saturation concentration of the polysulphides in the
electrolyte.
[0038] In one particular embodiment, the depolarizer includes
sulphur, carbon black and polyethylene oxide.
[0039] The electrolyte may comprise a solution of one or several
lithium salts selected from the group consisting of: lithium
trifluoromethanesulphonate, lithium perchlorate, lithium
trifluoromethanesulfonimide, lithium hexafluorophosphate, lithium
hexafluoroarsenate, lithium tetrachloroaluminate,
tetraalkylammonium salt, lithium chloride, lithium bromide and
lithium iodide; in one or several solvents selected from the group
consisting of: dioxolane, tetrahydrofuran, dimethoxyethane,
diglyme, triglyme, tetraglyme, dialkyl carbonates, sulfolane and
butyrolactone.
[0040] Throughout the description and claims of this specification,
the words "comprise" and "contain" and variations of the words, for
example "comprising" and "comprises", means "including but not
limited to", and is not intended to (and does not) exclude other
moieties, additives, components, integers or steps.
[0041] Throughout the description and claims of this specification,
the singular encompasses the plural unless the context otherwise
requires. In particular, where the indefinite article is used, the
specification is to be understood as contemplating plurality as
well as singularity, unless the context requires otherwise.
[0042] Features, integers, characteristics, compounds, chemical
moieties or groups described in conjunction with a particular
aspect, embodiment or example of the invention are to be understood
to be applicable to any other aspect, embodiment or example
described herein unless incompatible therewith.
EXAMPLES
Example 1
[0043] A positive electrode comprising 70% elemental sublimed
sulphur (available from Fisher Scientific, Loughborough, UK), 10%
electroconductivity carbon black (Ketjenblack.RTM. EC-600JD,
available from Akzo Nobel Polymer Chemicals BV, Netherlands) and
20% polyethylene oxide (PEO, 4,000,000 molecular weight, available
from Sigma-Aldrich Company Ltd., Gillingham, UK) was produced by
the following procedure.
[0044] A dry mixture of these components was ground in a high-speed
Microtron.RTM. MB550 mill for 10-15 minutes. Then acetonitrile was
added as a solvent to the dry mixture and the suspension was mixed
for 15-20 hours with a DLH laboratory stirrer. The solids content
of the suspension was 10-15%. The suspension thus produced was
applied by an automatic film applicator Elcometer.RTM. SPRL to one
side of a 12 .mu.m thick aluminium foil with an electroconductive
carbon coating (Product No. 60303 available from Rexam Graphics,
South Hadley, Mass.) as a current collector.
[0045] The coating was dried at ambient conditions for 20 hours and
then in a vacuum at 50.degree. C. for 5 hours. The resulting dry
cathode active layer had a thickness of 19 .mu.m and contained 2.01
mg/cm.sup.2 of cathode mixture. The specific surface capacity of
the electrode was 2.35 mA*h/cm.sup.2.
Example 2
[0046] The positive electrode from the Example 1 was used in a
small assembly cell made of stainless steel. The cathode surface
area was 5.1 cm.sup.2. A pressure of 400 kg/cm.sup.2 was applied to
the electrode before it was used in the cell. The cathode thickness
after pressing was 16 .mu.m. A 1.0 M solution of lithium
trifluoromethanesulphonate (available from 3M Corporation, St.
Paul, Minn.) in sulfolane was used as an electrolyte. Celgard.RTM.
2500 (a trade mark of Tonen Chemical Corporation, Tokyo, Japan, and
also available from Mobil Chemical Company, Films Division,
Pittsford, N.Y.) was used as a separator. These components were
assembled to form a laminated structure comprising positive
electrode/separator/anode together with the liquid electrolyte
filling up the free volume of the separator and the positive
electrode. The cells were assembled as follows. The positive
electrode was inserted into the cell. Then 4 microlitres of
electrolyte were deposited onto the electrode by using a constant
rate syringe CR-700 (Hamilton Co). The separator was placed on top
of the wetted electrode and 3 microlitres of electrolyte were
deposited onto the separator. Then a lithium electrode made of 38
.mu.m thick lithium foil was placed on top of the separator. After
the electrode stack was assembled, the cell was hermetically sealed
by a lid containing a Teflon.RTM. coating or sealing. The ratio of
sulphur to electrolyte was 1 ml of electrolyte to 1 g of sulphur.
After complete dissolution of the sulphur in the form of lithium
polysulphide during discharge of the cell, the maximum sulphur
concentration in the electrolyte was determined as 31.25
mole/litre.
[0047] Charge-discharge cycling of the cell was carried out at a
current of 1.5 mA which was equivalent to a current density of 0.3
mA/cm.sup.2 with a discharge cut-of voltage at 1.5V and charge
termination at 2.8V. The total weight of the cell and the weight
distribution between elements of the cell are given in the Table 1,
with properties of the cell being shown in Table 2.
[0048] The specific energy of the cell was calculated from the
capacity at the second cycle by dividing the capacity by the weight
of the electrode stack including the electrolyte. TABLE-US-00001
TABLE 1 Weight distribution between components of lithium-sulphur
cell from Example 2 Component of the cell Weight, g Part, % Lithium
0.00551 13.1 Sulphur 0.00715 17 Binder 0.00204 4.8 Carbon 0.00102
2.4 Substrate (+) 0.01021 24.3 Separator 0.00613 14.6 Electrolyte
0.01001 23.8 Total weight of the cell 0.04207 100.0
[0049] TABLE-US-00002 TABLE 2 Parameters of lithium-sulphur cell
from Example 2 Total quantity of electrolyte, ml 0.007 Electrolyte
quantity per 1 g 1 of sulphur, ml Electrolyte quantity per 1 mAh
0.00058 of positive electrode capacity, ml/mAh Calculated lithium
polysulphide 31.25 concentration in electrolyte after the 1st
stage, mole/litre Specific energy, Wh/kg 119 Energy density, Wh/l
130
Example 3
[0050] A lithium-sulphur cell was assembled in the same way as
described in Example 2, except in that 11 microlitres of
electrolyte were deposited onto the positive electrode and 3
microlitres of electrolyte were deposited onto the separator. The
total electrolyte content in the cell was 14 microlitres, which
amounts to 2 ml of electrolyte per 1 g of sulphur. Cycling of the
cell was performed in the same way as in Example 2. The parameters
of the cell are shown in Tables 3 and 4. TABLE-US-00003 TABLE 3
Weight distribution between components of lithium-sulphur cell from
Example 3 Component of the cell Weight, g Part, % Lithium 0.00551
10.6 Sulphur 0.00715 13.7 Binder 0.00204 3.9 Carbon 0.00102 2
Substrate (+) 0.01021 19.6 Separator 0.00613 11.8 Electrolyte
0.02002 38.4 Total weight of the cell 0.05208 100
[0051] TABLE-US-00004 TABLE 4 Parameters of lithium-sulphur cell
from Example 3 Total electrolyte quantity, ml 0.014 Electrolyte
quantity per 1 g 1.96 of sulphur, ml Electrolyte quantity per 1 mAh
0.00117 of a positive electrode, ml/mAh Calculated lithium
polysulphide 15.62 concentration in electrolyte after the 1st
stage, mole/litre Specific energy, Wh/kg 265 Energy density, Wh/l
301
Example 4
[0052] A lithium-sulphur cell was assembled in the same way as
described in Example 2, except in that 22 microlitres of the
electrolyte were deposited onto the positive electrode and 3
microlitres of electrolyte were deposited onto the separator. The
total electrolyte content of the cell was 25 microlitres, which
corresponds to 3.5 ml of electrolyte per 1 g of sulphur. Cycling of
the cell was performed in the same way as in Example 2. The
parameters of the cell are shown in Tables 5 and 6. TABLE-US-00005
TABLE 5 Weight distribution between components of lithium-sulphur
cell from Example 4 Component of the cell Weight, g Part, % Lithium
0.00551 8.2 Sulphur 0.00715 10.8 Binder 0.00204 3 Carbon 0.00102
1.5 Substrate (+) 0.01021 15.2 Separator 0.00613 9.1 Electrolyte
0.03504 52.2 Total weight of the cell 0.0671 100.0
[0053] TABLE-US-00006 TABLE 6 Parameters of lithium-sulphur cell
from Example 4 Total electrolyte quantity, ml 0.025 Electrolyte
quantity for 1 g 3.5 of sulphur, ml Electrolyte quantity for 1 mAh
of 0.0021 positive electrode capacity, ml/mAh Calculated lithium
polysulphide 8.928 concentration in electrolyte after the 1st
stage, mole/litre Specific energy, Wh/kg 300 Energy density, Wh/l
355
Example 5
[0054] A lithium-sulphur cell was assembled in the same way as
described in Example 2, except in that 49 microlitres of
electrolyte were deposited onto the positive electrode and 3
microlitres of electrolyte were deposited onto the separator. The
total electrolyte content of the cell was 52 microlitres, which is
5.2 ml of electrolyte per 1 g of sulphur. Cycling of the cell was
performed in the same way as in Example 2. The parameters of the
cell are shown in Tables 7 and 8. TABLE-US-00007 TABLE 7 Weight
distribution between components of the lithium-sulphur cell from
Example 5 Component of the cell Weight, g Part, % Lithium 0.00551
5.3 Sulphur 0.00715 6.9 Binder 0.00204 2 Carbon 0.00102 1 Substrate
(+) 0.01021 9.8 Separator 0.00613 5.8 Electrolyte 0.07207 69.2
Total weight of the cell 0.10413 100.0
[0055] TABLE-US-00008 TABLE 8 Parameters of the lithium-sulphur
cell from Example 5 Total electrolyte quantity, ml 0.052
Electrolyte quantity per 1 g 7.2 of sulphur, ml Electrolyte
quantity per 1 mAh 0.0043 of positive electrode, ml/mAh Calculated
lithium polysulphide 4.344 concentration in electrolyte after the
1st stage, mole/litre Specific energy, Wh/kg 193 Energy density,
Wh/l 242
Example 6
[0056] A lithium-sulphur cell was assembled in the same way as
described in Example 2, except in that 69 microlitres of
electrolyte were deposited onto the positive electrode and 3
microlitres of electrolyte were deposited onto the separator. The
total electrolyte content of the cell was 72 microlitres, which
amounts to 7.2 ml of electrolyte per 1 g of sulphur. Cycling of the
cell was performed in the same way as in Example 2. The parameters
of the cell are shown in Tables 9 and 10. TABLE-US-00009 TABLE 9
Weight distribution between components of lithium-sulphur cell from
Example 6 Component of the cell Weight, g Part, % Lithium 0.00551
4.2 Sulphur 0.00715 5.5 Binder 0.00204 1.5 Carbon 0.00102 0.8
Substrate (+) 0.01021 7.7 Separator 0.00613 4.6 Electrolyte 0.1001
75.7 Total weight of the cell 0.13216 100.0
[0057] TABLE-US-00010 TABLE 10 Parameters of lithium-sulphur cell
from Example 6 Total electrolyte quantity, ml 0.072 Electrolyte
quantity for 1 g 10 of sulphur, ml Electrolyte quantity for 1 mAh
of 0.006 positive electrode capacity, ml/mAh Calculated lithium
polysulphide 3.128 concentration in electrolyte after the 1st
stage, mole/litre Specific energy, Wh/kg 162 Energy density, Wh/l
207
Example 7
[0058] The ultimate or saturation solubility of sulphur in the form
of lithium octasulphide in 1.0 M solution of lithium
trifluoromethanesulphonate in sulfolane was evaluated. The
evaluation of solubility was carried out in the following way: 1.0
g of a mixture of lithium sulphide and sulphur (the content of
sulphur in the mixture was 0.86 g) was taken in a molar ratio 1:7
and placed in a sealed glass reactor in an air thermostat, the
reactor being fitted with a mechanical blender and a metering
device. The thermostat temperature was set to 30.degree. C. A 1.0 M
solution of lithium trifluoromethanesulphonate in sulfolane was
added in small portions to the reactor under constant mixing. After
addition of each new portion the reaction mixture was thoroughly
mixed for 5-6 hours so as to establish a thermodynamic equilibrium.
The solubility of the solid phase was evaluated visually. If the
reaction mixture contained some solid phase residues, then a
further portion of electrolyte was added. The experiment was
carried out until the solid phase dissolved completely. The results
showed that 1.0 g of a mixture of lithium sulphide and sulphur
taken in the molar ratio 1:7 was fully dissolved in 3.3 ml of the
electrolyte solution; in other words the solubility of sulphur in
form of lithium octasulphide was established to be about 0.96
M/litre of electrolyte.
Example 8
[0059] The results of Examples 2-7 are summarised in FIG. 1 which
shows a non-limiting example of the dependence of specific energy
of a bare lithium-sulphur cell versus the ratio of
electrolyte:sulphur. Other curves may be relevant to other
embodiments. It can be seen that this dependence has a maximum
which is reached at a ratio of electrolyte:sulphur close to 3. In
other words, the maximum capacity of a lithium-sulphur cell is
reached at a volume-weight ratio of electrolyte:sulphur that is
close to the ultimate or saturation solubility of lithium
octasulphide in electrolyte.
[0060] The presence of the maximum is explained by the fact that at
low values of the electrolyte: sulphur ratio the efficiency of
sulphur utilization is low, while at higher ratios the excess of
electrolyte contributes just an additional mass to the cell thus
resulting in a decrease of the specific energy.
[0061] It will be appreciated by persons skilled in the art that
the present invention is not limited to what has been particularly
shown and described hereinabove.
* * * * *