U.S. patent application number 16/746503 was filed with the patent office on 2020-05-14 for high voltage window electrolyte for supercapacitors.
The applicant listed for this patent is UT-Battelle, LLC. Invention is credited to Frank M. Delnick, Jagjit Nanda, Rose E. Ruther.
Application Number | 20200152397 16/746503 |
Document ID | / |
Family ID | 62064076 |
Filed Date | 2020-05-14 |
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United States Patent
Application |
20200152397 |
Kind Code |
A1 |
Ruther; Rose E. ; et
al. |
May 14, 2020 |
HIGH VOLTAGE WINDOW ELECTROLYTE FOR SUPERCAPACITORS
Abstract
A supercapacitor according to the present invention includes a
negative carbon-comprising electrode which does not intercalate
sodium, and a positive carbon-comprising electrode. An electrolyte
composition comprises sodium hexafluorophosphate and a non-aqueous
solvent comprising at least one selected from the group consisting
of ethylene glycol dimethyl ether, diethylene glycol dimethyl
ether, triethylene glycol dimethyl ether, and tetraethylene glycol
dimethyl ether. The supercapacitor has an electrochemical voltage
window of from +0.0 V to 3.5 V (full cell voltage). The electrolyte
has an electrochemical voltage window of from +0.05 V to 3.9 V vs.
Na/Na+. A method of making and a method of operating a
supercapacitor is also disclosed.
Inventors: |
Ruther; Rose E.; (Oak Ridge,
TN) ; Delnick; Frank M.; (Maryville, TN) ;
Nanda; Jagjit; (Knoxville, TN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
UT-Battelle, LLC |
Oak Ridge |
TN |
US |
|
|
Family ID: |
62064076 |
Appl. No.: |
16/746503 |
Filed: |
January 17, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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15806693 |
Nov 8, 2017 |
10559431 |
|
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16746503 |
|
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62419141 |
Nov 8, 2016 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01G 11/38 20130101;
H01G 11/24 20130101; H01G 11/60 20130101; H01G 11/04 20130101; H01G
11/62 20130101; H01G 11/06 20130101; Y02E 60/13 20130101; H01G
11/84 20130101; H01G 11/32 20130101 |
International
Class: |
H01G 11/60 20060101
H01G011/60; H01G 11/04 20060101 H01G011/04; H01G 11/62 20060101
H01G011/62; H01G 11/24 20060101 H01G011/24; H01G 11/84 20060101
H01G011/84; H01G 11/32 20060101 H01G011/32 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] This invention was made with government support under
contract no. DE-AC05-000R22725 awarded by the US Department of
Energy. The government has certain rights in this invention.
Claims
1. A method of making a supercapacitor, comprising the steps of:
providing a negative carbon-comprising electrode which does not
intercalate sodium on a negative electrode current collector;
providing a positive carbon-comprising electrode on a positive
electrode current collector; providing an electrolyte composition
comprising sodium hexafluorophosphate and a non-aqueous solvent
comprising at least one selected from the group consisting of
ethylene glycol dimethyl ether, diethylene glycol dimethyl ether,
triethylene glycol dimethyl ether, and tetraethylene glycol
dimethyl ether; positioning the electrolyte between the positive
electrode and the negative electrode; wherein the electrolyte has
an electrochemical voltage window of from +0.05 V to 3.9 V vs.
Na/Na.sup.+.
2. The method of making a supercapacitor according to claim 1,
wherein the electrolyte composition consists essentially of sodium
hexafluorophosphate and a non-aqueous solvent comprising at least
one selected from the group consisting of ethylene glycol dimethyl
ether, diethylene glycol dimethyl ether, triethylene glycol
dimethyl ether, and tetraethylene glycol dimethyl ether.
3. The method of making a supercapacitor according to claim 1,
wherein the electrolyte composition consists of sodium
hexafluorophosphate and a non-aqueous solvent comprising at least
one selected from the group consisting of ethylene glycol dimethyl
ether, diethylene glycol dimethyl ether, triethylene glycol
dimethyl ether, and tetraethylene glycol dimethyl ether.
4. The method of making a supercapacitor according to claim 1,
wherein the positive carbon-comprising electrode and the negative
carbon-comprising electrode comprise between 80 and 95 wt % carbon,
and between 5 and 20 wt % binder.
5. The method of making a supercapacitor according to claim 1,
wherein the negative carbon-comprising electrode is comprised of
carbon black, wherein the carbon black is high surface area carbon
black Black Pearls.RTM. 2000.
6. A method of operating a supercapacitor, comprising the steps of:
providing a negative carbon-comprising electrode which does not
intercalate sodium on a negative electrode current collector;
providing a positive carbon-comprising electrode on a positive
electrode current collector; providing an electrolyte composition
comprising sodium hexafluorophosphate and a non-aqueous solvent
comprising at least one selected from the group consisting of
ethylene glycol dimethyl ether, diethylene glycol dimethyl ether,
triethylene glycol dimethyl ether, and tetraethylene glycol
dimethyl ether; positioning the electrolyte between the positive
electrode and the negative electrode to form a supercapacitor; and,
operating the supercapacitor within an electrochemical voltage
window of from +0 V to 3.5 V (full cell voltage).
7. The method of operating a supercapacitor according to claim 6,
wherein the electrolyte composition consists essentially of sodium
hexafluorophosphate and a non-aqueous solvent comprising at least
one selected from the group consisting of ethylene glycol dimethyl
ether, diethylene glycol dimethyl ether, triethylene glycol
dimethyl ether, and tetraethylene glycol dimethyl ether.
8. The method of operating a supercapacitor according to claim 6,
wherein the electrolyte composition consists of sodium
hexafluorophosphate and a non-aqueous solvent comprising at least
one selected from the group consisting of ethylene glycol dimethyl
ether, diethylene glycol dimethyl ether, triethylene glycol
dimethyl ether, and tetraethylene glycol dimethyl ether.
9. The method of operating a supercapacitor according to claim 6,
wherein the positive carbon-comprising electrode and the negative
carbon-comprising electrode comprise between 80 and 95 wt % carbon,
and between 5 and 20 wt % binder.
10. The method of operating a supercapacitor according to claim 6,
wherein the negative carbon-comprising electrode is comprised of
carbon black, wherein the carbon black is high surface area carbon
black Black Pearls.RTM. 2000.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional patent application of U.S.
Non-Provisional patent application Ser. No. 15/806,693 filed Nov.
8, 2017, which claims priority to U.S. Provisional Patent
Application No. 62/419,141 filed Nov. 8, 2016, entitled "HIGH
VOLTAGE WINDOW ELECTROLYTE FOR SUPERCAPACITORS", the entireties of
which are all hereby incorporated herein by reference.
FIELD OF THE INVENTION
[0003] The present invention relates to supercapacitors, and more
particularly to high voltage supercapacitors.
BACKGROUND OF THE INVENTION
[0004] Supercapacitors store ionic charge electrostatically at the
interface of high surface area electrodes, such as carbon
electrodes, in a liquid electrolyte composition. Supercapacitors
are also referred to interchangeably as ultracapacitors or electric
double-layer capacitors (EDLC). Efforts to increase the energy
density of supercapacitors have focused mainly on developing higher
surface area electrodes and controlling electrode pore size. Energy
density of supercapacitors can also be increased through faradaic
mechanisms commonly known as pseudocapacitance, which arises from
the introduction of redox active groups through functionalization
of the carbon electrode surface or the incorporation of metal
oxides.
[0005] Despite significant improvements in electrode materials
design, most non-aqueous electrochemical capacitors use the same
electrolyte compositions: either a mixture of tetraethylammonium
tetrafluoroborate (TEABF.sub.4) in acetonitrile (AN) or TEABF.sub.4
in propylene carbonate (PC). These electrolyte compositions have a
high specific conductivity that minimizes resistive losses and
enables capacitors to operate at high power. However, these
electrolyte compositions typically exhibit a practical voltage
window around only 2.5-3.0 V, beyond which the capacitor lifetime
is significantly reduced. Ionic liquids and other organic solvents
(adiponitrile, sulfones, and carbonates) have been considered for
high voltage electrolytes for capacitors. Of these, ionic liquids
have generated the most interest due to their high stability, but
remain limited by high cost, low purity, and low conductivity.
Since the energy stored in a capacitor increases quadratically with
voltage, extending the electrochemical window of the electrolyte
composition could significantly improve the energy density of the
capacitor.
[0006] With prolonged cycling, the capacitance of EDLCs decreases
and the resistance increases. The performance degrades more rapidly
at elevated temperature or higher voltage. Degradation is typically
attributed to decomposition of the electrolyte, and is very
sensitive to the electrolyte composition, electrode polarity,
carbon surface functionality, and trace moisture. The long-term
performance of EDLCs can also be limited by the stability of other
components in the cell including the carbon, polymer binders, and
current collectors (typically aluminum). Commercial EDLCs with
organic electrolytes operate over a voltage window between
approximately 1.5 and 4.5 V vs. Na/Na.sup.+. Developing higher
voltage electrolytes for EDLCs requires careful consideration and
control of all possible side reactions. For example, extending the
positive voltage limit beyond 4.5 V vs. Na/Na.sup.+ likely requires
strategies to effectively suppress corrosion of the aluminum
current collector. Carbon oxidation may also occur at high
voltage.
[0007] Extending the negative voltage limit below 1.5 V vs.
Na/Na.sup.+ also presents certain challenges. The solvents most
commonly used in lithium-ion batteries and EDLCs (carbonates and
ACN) passivate electrodes at potentials below about 1.2 V vs.
Na/Na.sup.+. Effective passivation of the negative electrode is
critically important for the operation of lithium-ion batteries,
but detrimental for double-layer capacitors. Even very thin
insulating surface films can reduce the double layer capacitance
and block small pores. Binders based on polytetrafluoroethylene
(PTFE), which are commonly used in commercial EDLC electrodes, are
also reduced below about 1.0 V vs. Na/Na.sup.+. Finally, the
stability of the carbon itself with respect to reduction and/or
intercalation of cations must be considered. Accordingly, there
remains a continued need for an electrolyte composition that can
extend the operating voltage window of a supercapacitor.
SUMMARY OF THE INVENTION
[0008] A supercapacitor includes a negative carbon-comprising
electrode which does not intercalate sodium, and a positive
carbon-comprising electrode. An electrolyte composition comprises
sodium hexafluorophosphate and a non-aqueous solvent comprising at
least one selected from the group consisting of ethylene glycol
dimethyl ether, diethylene glycol dimethyl ether, triethylene
glycol dimethyl ether, and tetraethylene glycol dimethyl ether. The
supercapacitor has an electrochemical voltage window of from +0.0 V
to 3.5 V (full cell voltage). The electrolyte has an
electrochemical voltage window of from +0.05 V to 3.9 V vs.
Na/Na+.
[0009] The electrolyte composition can consist essentially of
sodium hexafluorophosphate and a non-aqueous solvent comprising at
least one selected from the group consisting of ethylene glycol
dimethyl ether, diethylene glycol dimethyl ether, triethylene
glycol dimethyl ether, and tetraethylene glycol dimethyl ether. The
electrolyte composition can consist of sodium hexafluorophosphate
and a non-aqueous solvent comprising at least one selected from the
group consisting of ethylene glycol dimethyl ether, diethylene
glycol dimethyl ether, triethylene glycol dimethyl ether, and
tetraethylene glycol dimethyl ether.
[0010] The positive carbon-comprising electrode and the negative
carbon-comprising electrode can have a specific surface area of
between 500 and 2000 m.sup.2/g. The positive carbon-comprising
electrode and the negative carbon-comprising electrode can comprise
between 80 and 95 wt % carbon, and between 5 and 20 wt %
binder.
[0011] A method of making a supercapacitor can include the steps of
providing a negative carbon-comprising electrode which does not
intercalate sodium on a negative electrode current collector, and
providing a positive carbon-comprising electrode on a positive
electrode current collector. An electrolyte composition is provided
which comprises sodium hexafluorophosphate and a non-aqueous
solvent comprising at least one selected from the group consisting
of ethylene glycol dimethyl ether, diethylene glycol dimethyl
ether, triethylene glycol dimethyl ether, and tetraethylene glycol
dimethyl ether. The electrolyte is positioned between the positive
electrode and the negative electrode. The electrolyte has an
electrochemical voltage window of from +0.05 V to 3.9 V vs.
Na/Na.sup.+. The supercapacitor has an electrochemical voltage
window of from +0.0 V to 3.5 V (full cell voltage).
[0012] An electrolyte for a supercapacitor comprises sodium
hexafluorophosphate and a non-aqueous solvent comprising at least
one selected from the group consisting of ethylene glycol dimethyl
ether, diethylene glycol dimethyl ether, triethylene glycol
dimethyl ether, and tetraethylene glycol dimethyl ether. The
electrolyte has an electrochemical voltage window of from +0.05 V
to 3.9 V vs. Na/Na+. The electrolyte can consist essentially of
sodium hexafluorophosphate and a non-aqueous solvent comprising at
least one selected from the group consisting of ethylene glycol
dimethyl ether, diethylene glycol dimethyl ether, triethylene
glycol dimethyl ether, and tetraethylene glycol dimethyl ether. The
electrolyte can consist of sodium hexafluorophosphate and a
non-aqueous solvent comprising at least one selected from the group
consisting of ethylene glycol dimethyl ether, diethylene glycol
dimethyl ether, triethylene glycol dimethyl ether, and
tetraethylene glycol dimethyl ether.
[0013] A method of operating a supercapacitor can include the steps
of providing a negative carbon-comprising electrode which does not
intercalate sodium on a negative electrode current collector, and
providing a positive carbon-comprising electrode on a positive
electrode current collector. An electrolyte composition is provided
which comprises sodium hexafluorophosphate and a non-aqueous
solvent comprising at least one selected from the group consisting
of ethylene glycol dimethyl ether, diethylene glycol dimethyl
ether, triethylene glycol dimethyl ether, and tetraethylene glycol
dimethyl ether. The electrolyte is positioned between the positive
electrode and the negative electrode to form a supercapacitor. The
supercapacitor is operated within an electrochemical voltage window
of from +0 V to 3.5 V (full cell voltage).
[0014] These and other features and advantages of the present
invention will become apparent from the following description of
the invention, when viewed in accordance with the accompanying
drawings and appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] There are shown in the drawings embodiments that are
presently preferred it being understood that the invention is not
limited to the arrangements and instrumentalities shown,
wherein:
[0016] FIG. 1 is a schematic cross-sectional illustration of a
three-electrode cell which is used to evaluate separately the
individual electrodes of a supercapacitor.
[0017] FIG. 2 is a schematic cross-sectional illustration of a
supercapacitor fabricated as a button cell stack.
[0018] FIG. 3 shows the complex impedance (-Im(Z)(.OMEGA.) vs.
Re(Z)(.OMEGA.)) of a single carbon BP2000 electrode with the
electrolyte of the invention (1.0 m NaPF.sub.6 in DME). The
measurements were made in the three-electrode configuration of FIG.
1.
[0019] FIG. 4 is a plot of capacitance (F/g) vs. voltage (V vs.
Na/Na.sup.+) of the carbon BP2000 electrode in the electrolyte of
the invention (1.0 m NaPF.sub.6 in DME). The capacitance values
were derived from complex impedance measurements using the
three-electrode cell configuration of FIG. 1. Example impedance
curves are shown in FIG. 3.
[0020] FIG. 5 shows the leakage current (.mu.A/cm.sup.2) vs. time
(h) at the carbon BP2000 electrode when it is used as an anode
(negative electrode) of a supercapacitor. The measurements were
made in the three-electrode configuration of FIG. 1.
[0021] FIG. 6 is a plot of the leakage current (.mu.A/cm.sup.2) vs.
time (h) at the carbon BP2000 electrode when it is used as a
cathode (positive electrode) of a supercapacitor. The measurements
were made in the three-electrode configuration of FIG. 1.
[0022] FIG. 7 shows the charge and discharge curves (cell voltage
vs. capacity (mAh)) for a supercapacitor consisting of a carbon
BP2000 anode and carbon BP2000 cathode in the electrolyte of the
invention (1.0 m NaPF.sub.6 in DME). Representative
charge-discharge cycles are shown (cycles 1, 50, 200, 500, 1000,
2000, and 3000). The measurements were made in the two-electrode
configuration of FIG. 2.
[0023] FIG. 8 shows the coulombic efficiency (%) vs. cycle number
for the supercapacitor of FIG. 7, over 3000 charge-discharge
cycles. The measurements were made in the two-electrode
configuration of FIG. 2.
[0024] FIG. 9 shows the complex impedance (-Im(Z)(.OMEGA.) vs.
Re(Z)(.OMEGA.)) of a supercapacitor consisting of a carbon BP2000
anode and carbon BP2000 cathode in the electrolyte of the invention
(1.0 m NaPF.sub.6 in DME). The measurements were made in the
two-electrode configuration of FIG. 2.
[0025] FIG. 10 is a plot of capacitance (F/g) vs. voltage for a
supercapacitor consisting of a carbon BP2000 anode and carbon
BP2000 cathode in the electrolyte of the invention (1.0 m
NaPF.sub.6 in DME). The capacitance values were derived from
complex impedance measurements using the two-electrode cell
configuration of FIG. 2. Example impedance curves are shown in FIG.
9.
[0026] FIG. 11 is a schematic diagram of one duty cycle of the
protocol used to generate the data shown in FIGS. 12, 13, and 14.
Supercapacitors were tested in the two-electrode configuration of
FIG. 2. The maximum cell voltage was chosen to be either 3.0 (FIG.
13) or 3.5 V (FIGS. 12 and 14).
[0027] FIG. 12 shows the complex impedance (-Im(Z)(.OMEGA.) vs.
Re(Z)(.OMEGA.)) of a supercapacitor consisting of a carbon BP2000
anode and carbon BP2000 cathode in the electrolyte of the invention
(1.0 m NaPF6 in DME). The measurements were made in the
two-electrode configuration of FIG. 2. The supercapacitor was
cycled using the duty cycle shown in FIG. 11. The maximum cell
voltage reached 3.5 V. The time durations in the figure legend
represent the cumulative time at 3.5 V before each impedance
measurement.
[0028] FIG. 13 shows the complex impedance (-Im(Z)(.OMEGA.) vs.
Re(Z)(.OMEGA.)) of a supercapacitor consisting of a carbon BP2000
anode and carbon BP2000 cathode in a conventional electrolyte (1.0
m tetraethylammonium tetrafluoroborate in acetonitrile). The
measurements were made in the two-electrode configuration of FIG.
2. The supercapacitor was cycled using the duty cycle shown in FIG.
11. The maximum cell voltage reached 3.0 V. The time durations in
the figure legend represent the cumulative time at 3.0 V before
each impedance measurement.
[0029] FIG. 14 shows the complex impedance(-Im(Z)(.OMEGA.) vs.
Re(Z)(.OMEGA.)) of a supercapacitor consisting of a carbon BP2000
anode and carbon BP2000 cathode in a conventional electrolyte (1.0
m tetraethylammonium tetrafluoroborate in acetonitrile). The
measurements were made in the two-electrode configuration of FIG.
2. The supercapacitor was cycled using the duty cycle shown in FIG.
11. The maximum cell voltage reached 3.5 V. The time durations in
the figure legend represent the cumulative time at 3.5 V before
each impedance measurement.
[0030] FIG. 15 is a plot of capacitance (F/g) vs. hours at voltage
maximum for supercapacitors consisting of a carbon BP2000 anode and
carbon BP2000 cathode in different electrolytes. The
supercapacitors were cycled using the duty cycle shown in FIG. 11
with different values for the maximum cell voltage.
[0031] FIG. 16 shows the resistance (.OMEGA.) vs. hours at maximum
voltage for supercapacitors consisting of a carbon BP2000 anode and
carbon BP2000 cathode in different electrolytes. The
supercapacitors were cycled using the duty cycle shown in FIG. 11
with different values for the maximum cell voltage.
DETAILED DESCRIPTION OF THE INVENTION
[0032] The invention is directed to high voltage electrolytes for
supercapacitors. A supercapacitor and an electrolyte composition
for use in a supercapacitor are described herein. The
supercapacitor includes a negative carbon electrode having a
current collector, a positive carbon electrode having a current
collector, an ion-permeable separator disposed between the negative
and positive electrodes, and an electrolyte composition disposed
between the negative and positive electrodes. A new combination of
solvent and salt compositions is stable up to 3.9 V electrochemical
window. The electrolyte composition includes a conductive sodium
salt component including sodium hexafluorophosphate (NaPF.sub.6)
and a non-aqueous solvent component including dimethoxyethane
(DME).
[0033] High voltage electrolytes are common to the lithium battery
and lithium-ion battery industries. These electrolytes typically
consist of a lithium salt (or mixture of lithium salts) dissolved
in a solvent or mixture of solvents. The solvents for these high
voltage applications are typically selected from the list:
Acetonitrile (AN), .gamma.-Butyrolactone (BL), 1,2-Dimethoxyethane
(DME,monoglyme), 1.2-Diethoxyethane (DEE, diglyme), Triglyme,
Tetraglyme, Diethyl Carbonate (DEC), Dimethyl Carbonate (DMC),
Dimethylformamide (DMF), Dimethylsulfite (DMS), Dimethylsulfoxide
(DMSO), Dioxolane (DL), Ethylene Carbonate (EC), Ethylene Glycol
Sulfite (EGS), Ethyl Propyl Carbonate (EPC), Methyl Acetate (MA),
Methyl Formate (MF), Methyl Ethyl Carbonate (MEC),
3-Methyl-2-Oxazolidinone (3-Me-20X), N-Methyl Pyrrilidinone (NMP),
Methyl Propyl Carbonate (MPC), 2-Methyltetrahydrofuran (2-MeTHF),
Nitromethane (NM), Propylene Carbonate (PC), Tetrahydrofuran (THF),
Thionyl Chloride (SOCl.sub.2, also a cathode material), Sulfolane
(SL), Sulfuryl Chloride (SO.sub.2Cl.sub.2, also a cathode
material), Sulfur Dioxide (SO.sub.2, also a cathode material).
[0034] The salt (or salt mixture) for high voltage battery
electrolytes is typically selected from the list: Lithium Bromide
(LiBr), Lithium Perchlorate (LiClO.sub.4), Lithium
Tetrafluoroborate (LiBF.sub.4), Lithium Tetrachloroaluminate
(LiAlCl.sub.4), Lithium Hexafluorophosphate (LiPF.sub.6), Lithium
Hexafluoroarsenate (LiAsF.sub.6), Lithium Trifluoromethanesulfonate
(LiTFS, LiCF.sub.3SO.sub.3), Lithium Perfluoromethylsulfonyl Imide
(LiTFSI, LiN(CF.sub.3SO.sub.2).sub.2), Lithium
Perfluoromethylsulfonyl Methide (LiC((CF.sub.3S0.sub.2).sub.3),
Lithium Bis(oxalato) borate (LiBOB, LiC.sub.4B0.sub.8).
[0035] These battery electrolytes are not used in supercapacitors
because (at negative potentials) most of these solvents are reduced
to yield insoluble compounds which passivate the electrode surface.
These passive films are essential for the safe operation of lithium
and lithium-ion batteries, but they reduce the capacitance at
carbon electrodes (often by several orders of magnitude) and are
therefore not useful in supercapacitors which rely on high
capacitance at the carbon electrode surface. Furthermore, lithium
ions and solvent-coordinated lithium ions intercalate into many
carbon electrode materials (lithium intercalation is the preferred
reaction for lithium-ion batteries) and this can lead to
exfoliation of the carbon and destruction of the electrode.
Therefore, for supercapacitors, the lithium salt is replaced with
an organic cation salt typically selected from the list:
Tetraethylammonium Hexafluorophosphate (Et.sub.4NPF.sub.6),
Tetraethylammonium Tetrafluoroborate (Et.sub.4NBF.sub.4),
Tetraethylammonium Trifluoromethanesulfonate
(Et.sub.4NCF.sub.3SO.sub.3), Tetraethylammonium
Perfluoromethylsulfonyl Imide (Et.sub.4N(CF.sub.3SO.sub.2).sub.2N),
Tetraethylammouinm Perchlorate (Et.sub.4NClO.sub.4),
Tetrabutylammonium Perchlorate (Bu.sub.4NClO.sub.4),
Trimethylethylammonium Hexafluorophosphate (Me.sub.3EtNPF.sub.6),
Tripropylmethylammonium Hexapfuorophosphate (Pr.sub.3MeNPF.sub.6),
Triisopropylmethylammonium Hexafluorophosphate
(iPr.sub.3MeNPF.sub.6), Diisopropylethylmethylammonium
Hexafluorophosphate (iPr.sub.2EtMeNPF.sub.6),
Tributylmethylammonium Hexafluorophosphate (Bu.sub.3MeNPF.sub.6),
Triisobutylmethylammonium Hexafluorophosphate
(iBu.sub.3MeNPF.sub.6), Triethylmethylphosphonium
Hexafluorophosphate (Et.sub.3MePPF.sub.6),
Tributylmethylphosphonium Hexafluorophosphate
(Bu.sub.3MePPF.sub.6).
[0036] Virtually all commercial nonaqueous supercapacitors utilize
either Et.sub.4NBF.sub.4 in acetonitrile or Et.sub.4NBF.sub.4 in
propylene carbonate. Acetonitrile and propylene carbonate are each
oxidatively stable to >4.0 V vs Li/Li.sup.+. Both solvents,
however are reductively unstable and this leads to a working
voltage range of .about.2.7 V for supercapacitors with these
electrolytes.
[0037] Glyme solvents and in particular DME are oxidatively stable
to >3.9 V vs Na/Na.sup.+, and they are reductively stable to
<0.05 vs Na/Na.sup.+. This voltage stability range enables a
possible working voltage window of approximately 3.9 V for
supercapacitors. However, organic cation salts exhibit relatively
low solubility in glyme solvents. Nevertheless, monovalent alkali
cation salts exhibit much higher solubility because the cations
coordinate with the ether oxygen linkages on the glyme molecules.
Specifically, DME has a high donor number (20) and it strongly
coordinates with alkali cations. Li.sup.+ and K.sup.+ cations
readily intercalate into carbon electrodes at negative potentials,
thereby limiting the use of lithium and potassium salts for
supercapacitors. However, Na.sup.+ cations exhibit anomalous
intercalation behavior into carbon which depends very much on the
crystallographic structure of the carbon and on the solvent. Some
solvents co-intercalate into some carbons with the Na.sup.+ cations
to yield C-Na-solvent ternary compounds. In particular, Na.sup.+
salts in THF, 2MeTHF and diglyme solvents exhibit this
behavior.
[0038] Other side reactions may also limit the working range of the
supercapacitor. In addition to strong alkali cation coordination,
the glyme solvents also strongly coordinate to other cations such
as Al.sup.+3. These solvents dramatically accelerate the corrosion
of aluminum current collectors. Conventional supercapacitor
electrodes utilize Polytetrafluoroethylene (PTFE) as a binder in
the formation of the electrodes from carbon powders. PTFE, however,
is reduced at potentials approximately <0.7 V vs Na/Na.sup.+.
Therefore, PTFE-bonded carbon electrodes will exhibit a working
voltage range approximately 0.65 V lower than the voltage window of
the DME solvent. In order to enable the utilization of the 3.9 V
working voltage range of the electrolyte of the current invention,
it is necessary to identify the precise sodium salt and the precise
carbon structure that does not permit intercalation of Na.sup.+ or
the cointercalation of the solvent to yield ternary intercalation
compounds. And, a new binder for the carbon electrode must be
identified and the aluminum corrosion reaction must be
eliminated.
[0039] The NaPF.sub.6 salt minimizes corrosion of the aluminum
current collectors. DME has a low viscosity, which promotes high
electrolyte conductivity. Glymes with higher molecular weight also
can be capable of providing a similar voltage window. Sodium
carboxymethyl cellulose can be used as an electrode binder
primarily for its excellent stability over a wide voltage window.
This binder also has the advantage of being water-soluble and
environmentally benign. The DME-based electrolyte shows an
electrochemical window up to 3.5 V in full cells with
high-surface-area carbon electrodes. The high voltage performance
could significantly increase the overall energy density of
EDLCs.
[0040] The invention represents an unexpected discovery that
combination of NaPF.sub.6 in DME does not intercalate (or
co-intercalate solvent) or form ternary intercalation compounds in
BP2000 carbon. Furthermore, the PF.sub.6.sup.- anion inhibits
aluminum corrosion, and the use of sodium carboxymethyl cellulose
as the binder for the BP2000 carbon electrode (in place of PTFE)
enables the utilization of the approximately 3.9V window of the
electrolyte of the current invention in supercapacitor whose
electrodes are comprised of sodium carboxymethyl cellulose bonded
BP2000 carbon.
[0041] An electrolyte consisting of NaPF.sub.6 salt in
1,2-dimethoxyethane (DME) can extend the voltage window of electric
double-layer capacitors (EDLCs) to >3.5 V. DME does not
passivate carbon electrodes at very negative potentials (near
Na/Na.sup.+), extending the practical voltage window by about 1.0 V
compared to standard, non-aqueous electrolytes based on
acetonitrile. The voltage window is demonstrated in two- and
three-electrode cells using a combination of electrochemical
impedance spectroscopy (EIS), charge-discharge cycling, and
measurements of leakage current. DME-based electrolytes cannot
match the high conductivity of acetonitrile solutions, but they can
satisfy applications that demand high energy density at moderate
power. The conductivity of NaPF.sub.6 in DME is comparable to
commercial lithium-ion battery electrolytes and superior to most
ionic liquids.
[0042] A supercapacitor according to the present invention includes
a negative carbon-comprising electrode which does not intercalate
sodium, and a positive carbon-comprising electrode. An electrolyte
composition comprises sodium hexafluorophosphate and a non-aqueous
solvent comprising at least one selected from the group consisting
of ethylene glycol dimethyl ether, diethylene glycol dimethyl
ether, triethylene glycol dimethyl ether, and tetraethylene glycol
dimethyl ether. The electrolyte has an electrochemical voltage
window of from +0.05 V to 3.9 V vs. Na/Na.sup.+. The upper limit of
the voltage window can be 3.5 V, 3.6 V, 3.7 V, 3.8 V, and 3.9 V vs.
Na/Na.sup.+, or within a range of any high and low value selected
from these.
[0043] The electrolyte composition can consist essentially of
sodium hexafluorophosphate and a non-aqueous solvent comprising at
least one selected from the group consisting of ethylene glycol
dimethyl ether, diethylene glycol dimethyl ether, triethylene
glycol dimethyl ether, and tetraethylene glycol dimethyl ether. The
electrolyte composition can consist of sodium hexafluorophosphate
and a non-aqueous solvent comprising at least one selected from the
group consisting of ethylene glycol dimethyl ether, diethylene
glycol dimethyl ether, triethylene glycol dimethyl ether, and
tetraethylene glycol dimethyl ether.
[0044] The positive carbon-comprising electrode and the negative
carbon-comprising electrode can have a specific surface area of
between 500 and 2000 m.sup.2/g. The positive carbon-comprising
electrode and the negative carbon-comprising electrode can comprise
between 80 and 95 wt % carbon, and between 5 and 20 wt %
binder.
[0045] A method of making a supercapacitor according to the
invention includes the steps of providing a negative
carbon-comprising electrode which does not intercalate sodium on a
negative electrode current collector, and providing a positive
carbon-comprising electrode on a positive electrode current
collector. An electrolyte composition is provided which comprises
sodium hexafluorophosphate and a non-aqueous solvent comprising at
least one selected from the group consisting of ethylene glycol
dimethyl ether, diethylene glycol dimethyl ether, triethylene
glycol dimethyl ether, and tetraethylene glycol dimethyl ether. The
electrolyte is positioned between the positive electrode and the
negative electrode. The electrolyte has an electrochemical voltage
window of from +0.05 V to 3.9 V vs. Na/Na.sup.+.
[0046] A method of operating a supercapacitor according to the
invention includes the steps of providing a negative
carbon-comprising electrode which does not intercalate sodium on a
negative electrode current collector, and providing a positive
carbon-comprising electrode on a positive electrode current
collector. An electrolyte composition is provided which comprises
sodium hexafluorophosphate and a non-aqueous solvent comprising at
least one selected from the group consisting of ethylene glycol
dimethyl ether, diethylene glycol dimethyl ether, triethylene
glycol dimethyl ether, and tetraethylene glycol dimethyl ether. The
electrolyte is positioned between the positive electrode and the
negative electrode. The supercapacitor is operated within an
electrochemical voltage window of from +0.0 V to 3.5 V (full cell
voltage).
[0047] An electrolyte for a supercapacitor according to the
invention includes sodium hexafluorophosphate and a non-aqueous
solvent comprising at least one selected from the group consisting
of ethylene glycol dimethyl ether, diethylene glycol dimethyl
ether, triethylene glycol dimethyl ether, and tetraethylene glycol
dimethyl ether. The electrolyte has an electrochemical voltage
window of from +0.05 V to 3.9 V vs. Na/Na.sup.+.
[0048] In one embodiment, a method of forming a supercapacitor
includes providing a negative electrode consisting of sodium
carboxymethyl cellulose bonded carbon Black Pearls 2000 (BP2000,
Cabot Corporation) and having an aluminum current collector; and
providing a positive electrode consisting of carbon BP2000 or other
high surface area carbon and having a current collector; providing
an ion-permeable separator in a space between the negative and
positive electrodes, and providing an electrolyte composition in
the space between the negative and positive electrodes. The
electrolyte composition includes a conductive sodium salt component
including sodium hexafluorophosphate (NaPF.sub.6) and a non-aqueous
solvent component including dimethoxyethane (DME).
[0049] In another embodiment, the electrolyte composition may
further contain additional salts such as: sodium
trifluoromethanesulfonate (NaTFS), sodium perchlorate
(NaClO.sub.4), tetrabutylammonium perchlorate (TBAClO.sub.4),
tetrabutylammonium hexafluorophosphate (TBAPF.sub.6), or
combinations thereof. The electrolyte composition may also further
contain additional solvents such as: diglyme, triglyme, tetraglyme,
tetrahydrofuran, 2-methyltetrahydrofuran, or combinations
thereof.
[0050] Experimental Procedure
[0051] Electrolyte Preparation
[0052] All procedures were carried out in a glove box filled with
high-purity argon. Sodium hexafluorophosphate (NaPF.sub.6, 98%
Sigma Aldrich) was dissolved in 1,2-dimethoxyethane (DME, battery
grade, Mitsubishi Chemical Company) at a concentration of 1 molal
(m). Some control experiments were performed with a standard
acetonitrile-based electrolyte. Tetraethylammonium
tetrafluoroborate (TEABF.sub.4, electrochemical grade, Sigma
Aldrich) was dissolved in acetonitrile (ACN, 99.8%, anhydrous,
Sigma Aldrich) at 1 m concentration. Both electrolytes were dried
over 4 A molecular sieve for several days. After drying, the
DME-based electrolyte was stored over sodium metal to remove other
impurities.
[0053] Electrode Preparation:
[0054] High-surface-area carbon electrodes were prepared using 85
wt. % Black Pearls 2000 (BP2000, Cabot Corporation, BET surface
area 1500 m.sup.2/g) and 15 wt. % sodium carboxymethyl cellulose
(CMC, Sigma Aldrich, molecular weight 700,000). BP2000 and CMC were
ultrasonicated in water to produce a homogenous suspension.
Electrodes were deposited by spray-coating onto aluminum foil.
Electrode thickness ranged from 25 to 130 .mu.m with loadings from
0.4 to 2.2 mg/cm.sup.2.
[0055] Electrochemical Testing:
[0056] Two-electrode button cells (316L stainless steel, size
CR2032, Hohsen. Corp. Japan) were prepared using high-surface-area
carbon electrodes and a polymer separator (Celgard 2325). Some
measurements were performed using a three-electrode cell (EL-Cell
GmbH) with sodium metal for the reference and counter electrodes,
using the configuration shown in FIG. 1. The two-electrode button
cell configuration is shown in FIG. 2. Potentials measured with the
three-electrode cell are referenced to the Na/Na+ potential
(E.sub.Na/Na+.apprxeq.+0.13 V vs. E.sub.Li/Li+). One polymer
separator and one glass fiber separator were used in the
three-electrode cells to provide sufficient electrode separation to
accommodate the reference electrode. Unless stated otherwise, 1 m
NaPF.sub.6 in DME was used as the electrolyte. Cyclic voltammetry
(CV), electrochemical impedance spectroscopy (EIS),
charge-discharge cycling, float tests, and leakage current
measurements were acquired using Bio-Logic instruments (VSP and
MPG2). All tests were conducted at room temperature.
[0057] FIG. 3 shows EIS (20 kHz-10 mHz) of the carbon electrode in
1 m NaPF.sub.6/DME electrolyte at different voltages measured in
the three-electrode cell of FIG. 1 with sodium counter and
reference electrodes. The carbon electrode was held at each voltage
step for 2 h prior to measurement, and EIS was taken between 0.05
and 3.9 V vs. Na/Na+.
[0058] FIG. 3 shows the complex impedance of a single carbon BP2000
electrode in the electrolyte of the current invention (1.0 m NaPF6
in DME). The measurements were made in the three-electrode
configuration of FIG. 1. The negative voltage limit extends almost
to the sodium potential. FIG. 3 shows EIS spectra collected between
0.05 and 3.9 V vs. Na/Na.sup.+. The low-frequency EIS data form
vertical lines over this entire voltage range, indicating that no
side-reactions occur within this window. Importantly, the electrode
is not passivated even within 50 mV of the sodium potential.
[0059] FIG. 4 shows the capacitance of the carbon BP2000 electrode
in the electrolyte of the current invention. The capacitance values
were derived from complex impedance measurements using the
three-electrode cell configuration of FIG. 1. Example impedance
curves are shown in FIG. 3.
[0060] FIG. 4 shows the electrode capacitance taken from the EIS
data. A minimum in the capacitance occurs near 3.0 V vs. Na/Na+,
which also corresponds to the open circuit potential. This minimum
reflects the potential of zero charge of the carbon electrode.
Moving away from the potential of zero charge, the capacitance
steadily increases, confirming the electrodes do not passivate over
this voltage range. The capacitance values measured by EIS range
from 50 to 250 F/g, which is comparable to what is obtained with
high surface area electrodes in ACN-based electrolyte with
three-electrode cells.
[0061] To verify that no side-reactions occur between 0.05 and 3.9
V vs. Na/Na.sup.+ leakage currents were also measured in the
three-electrode cells. FIG. 5 shows the leakage current at the
carbon BP2000 electrode when it is used as an anode (negative
electrode) of a supercapacitor. The measurements were made in the
three-electrode configuration of FIG. 1. FIG. 6 shows the leakage
current at the carbon BP2000 electrode when it is used as a cathode
(positive electrode) of a supercapacitor. The measurements were
made in the three-electrode configuration of FIG. 1.
[0062] The carbon electrode was held at different potentials for 12
h while the decay in the current was monitored. FIG. 5 shows that
the leakage currents are <2 .mu.A/cm.sup.2 at the low limit of
the voltage window. FIG. 6 shows that the leakage currents are
<1 .mu.A/cm.sup.2 at the high limit of the voltage window.
Impedance spectra were collected before and after the leakage
current measurements to verify that the low currents were not due
to electrode passivation (data not shown). Together, the data
obtained from the three-electrode cells indicate that the
NaPF.sub.6/DME electrolyte has a voltage window from 0.05-3.9 V vs.
Na/Na.sup.+.
[0063] FIG. 7 shows the charge and discharge curves for a
supercapacitor consisting of a carbon BP2000 anode and carbon
BP2000 cathode in the electrolyte of the current invention (1.0 m
NaPF.sub.6 in DME). Representative charge-discharge cycles are
shown. The measurements were made in the two-electrode
configuration of FIG. 2.
[0064] FIG. 8 shows the coulombic efficiency of the supercapacitor
of FIG. 7 over 3000 charge-discharge cycles. The measurements were
made in the two-electrode configuration of FIG. 2.
[0065] FIG. 9 shows the complex impedance of a supercapacitor
consisting of a carbon BP2000 anode and carbon BP2000 cathode in
the electrolyte of the current invention (1.0 m NaPF6 in DME). The
measurements were made in the two-electrode configuration of FIG.
2.
[0066] The voltage window for the full cells was checked by EIS.
FIG. 9 shows the EIS response of a full cell charged to different
voltages. Up to 3.8 V the EIS spectra are vertical lines,
indicating the cell behaves as an ideal capacitor with no
measurable side-reactions. At 4.0 V the EIS spectrum is no longer
vertical, demonstrating that the voltage window has been
exceeded.
[0067] FIG. 10 shows the capacitance of a supercapacitor consisting
of a carbon BP2000 anode and carbon BP2000 cathode in the
electrolyte of the current invention (1.0 m NaPF6 in DME). The
capacitance values were derived from complex impedance measurements
using the two-electrode cell configuration of FIG. 2. Example
impedance curves are shown in FIG. 9. The capacitance of the full
cell with the DME-based electrolyte is comparable to what can
typically be achieved with ACN-based electrolytes in similar
two-electrode cells.
[0068] FIG. 11 shows the schematic of one duty cycle of the
protocol used to generate the data shown in FIGS. 12, 13, and 14.
Supercapacitors were tested in the two-electrode configuration of
FIG. 2. The maximum cell voltage was chosen to be either 3.0 (FIG.
13) or 3.5 V (FIGS. 12 and 14). Cells were cycled four times at 1
mA/cm.sup.2 followed by a two hour hold at the maximum voltage.
Cells were then discharged prior to EIS analysis (20 kHz-100 mHz).
EIS results for up to 25 consecutive duty cycles are shown
corresponding to a total float time of 50 h at the maximum
voltage.
[0069] FIG. 12 shows the complex impedance of a supercapacitor
consisting of a carbon BP2000 anode and carbon BP2000 cathode in
the electrolyte of the current invention (1.0 m NaPF6 in DME). The
measurements were made in the two-electrode configuration of FIG.
2. The supercapacitor was cycled using duty cycle shown in FIG. 11.
For each duty cycle the cell was cycled four times at 1 mA/cm.sup.2
followed by a two hour hold at 3.5 V. The supercapacitor was then
fully discharged and the complex impedance was measured. Complex
impedance spectra are shown for up to 25 consecutive duty cycles
corresponding to a total float time of 50 h at the maximum voltage.
The impedance spectra are stable and reproducible after 50 h
confirming that the electrolyte is stable under these
conditions.
[0070] FIG. 13 shows the complex impedance of a supercapacitor
consisting of a carbon BP2000 anode and carbon BP2000 cathode in a
conventional electrolyte (1.0 m tetraethylammonium
tetrafluoroborate in acetonitrile). The measurements were made in
the two-electrode configuration of FIG. 2. The supercapacitor was
cycled using duty cycle shown in FIG. 11. For each duty cycle the
cell was cycled four times at 1 mA/cm.sup.2 followed by a two hour
hold at 3.0 V. The supercapacitor was then fully discharged and the
complex impedance was measured. Complex impedance spectra are shown
for up to 25 consecutive duty cycles corresponding to a total float
time of 50 h at the maximum voltage. The impedance spectra are
stable and reproducible after 50 h confirming that the electrolyte
is stable under these conditions.
[0071] FIG. 14 shows the complex impedance of a supercapacitor
consisting of a carbon BP2000 anode and carbon BP2000 cathode in a
conventional electrolyte (1.0 m tetraethylammonium
tetrafluoroborate in acetonitrile). The measurements were made in
the two-electrode configuration of FIG. 2. The supercapacitor was
cycled using duty cycle shown in FIG. 11. For each duty cycle the
cell was cycled four times at 1 mA/cm.sup.2 followed by a two hour
hold at 3.5 V. The supercapacitor was then fully discharged and the
complex impedance was measured. Complex impedance spectra are shown
for up to 20 consecutive duty cycles corresponding to a total float
time of 40 h at the maximum voltage. The changes in the impedance
spectra show that the electrolyte is not stable under these
conditions.
[0072] FIG. 15 shows the capacitance of supercapacitors consisting
of a carbon BP2000 anode and carbon BP2000 cathode in different
electrolytes. The supercapacitors were cycled using the duty cycle
shown in FIG. 11 with different values for the maximum cell
voltage. The capacitance values were derived from complex impedance
measurements using the two-electrode cell configuration of FIG. 2.
Example impedance curves are shown in FIGS. 12-14.
[0073] FIG. 16 shows the resistance of supercapacitors consisting
of a carbon BP2000 anode and carbon BP2000 cathode in different
electrolytes. The supercapacitors were cycled using the duty cycle
shown in FIG. 11 with different values for the maximum cell
voltage. The capacitance values were derived from complex impedance
measurements using the two-electrode cell configuration of FIG. 2.
Example impedance curves are shown in FIGS. 12-14.
[0074] This invention can be embodied in other forms without
departing from the essential attributes thereof. Accordingly,
reference should be made to the following claims to determine the
scope of the invention.
* * * * *