U.S. patent application number 15/387225 was filed with the patent office on 2017-07-13 for metal sulfide electrodes and energy storage devices thereof.
This patent application is currently assigned to Massachusetts Institute of Technology. The applicant listed for this patent is Massachusetts Institute of Technology. Invention is credited to Nir Baram, W. Craig Carter, Yet-Ming Chiang, Frank Y. Fan, Ahmed Helal, Zheng Li, Gareth H. McKinley, Kyle C. Smith, William Henry Woodford.
Application Number | 20170200966 15/387225 |
Document ID | / |
Family ID | 50150820 |
Filed Date | 2017-07-13 |
United States Patent
Application |
20170200966 |
Kind Code |
A1 |
Chiang; Yet-Ming ; et
al. |
July 13, 2017 |
METAL SULFIDE ELECTRODES AND ENERGY STORAGE DEVICES THEREOF
Abstract
The present invention generally relates to energy storage
devices, and to metal sulfide energy storage devices in particular.
Some aspects of the invention relate to energy storage devices
comprising at least one flowable electrode, wherein the flowable
electrode comprises an electroactive metal sulfide material
suspended and/or dissolved in a carrier fluid. In some embodiments,
the flowable electrode further comprises a plurality of
electronically conductive particles suspended and/or dissolved in
the carrier fluid, wherein the electronically conductive particles
form a percolating conductive network. An energy storage device
comprising a flowable electrode comprising a metal sulfide
electroactive material and a percolating conductive network may
advantageously exhibit, upon reversible cycling, higher energy
densities and specific capacities than conventional energy storage
devices.
Inventors: |
Chiang; Yet-Ming; (Weston,
MA) ; Woodford; William Henry; (Cambridge, MA)
; Fan; Frank Y.; (Cambridge, MA) ; Li; Zheng;
(Arlington, MA) ; Baram; Nir; (Ancaster, CA)
; Smith; Kyle C.; (Champaign, IL) ; Carter; W.
Craig; (Jamaica plain, MA) ; McKinley; Gareth H.;
(Acton, MA) ; Helal; Ahmed; (Cambridge,
MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Massachusetts Institute of Technology |
Cambridge |
MA |
US |
|
|
Assignee: |
Massachusetts Institute of
Technology
Cambridge
MA
|
Family ID: |
50150820 |
Appl. No.: |
15/387225 |
Filed: |
December 21, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14172648 |
Feb 4, 2014 |
9583779 |
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15387225 |
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61903574 |
Nov 13, 2013 |
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61903739 |
Nov 13, 2013 |
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61789964 |
Mar 15, 2013 |
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61760436 |
Feb 4, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 4/625 20130101;
H01M 4/5825 20130101; H01M 12/08 20130101; H01M 8/188 20130101;
H01M 8/20 20130101; Y02E 60/50 20130101; Y02E 60/10 20130101; H01M
8/225 20130101; H01M 4/382 20130101 |
International
Class: |
H01M 8/18 20060101
H01M008/18; H01M 4/58 20060101 H01M004/58; H01M 4/62 20060101
H01M004/62; H01M 4/38 20060101 H01M004/38; H01M 8/22 20060101
H01M008/22 |
Goverment Interests
GOVERNMENT FUNDING
[0002] The United States Government has rights in this invention
pursuant to DOE-FOA-0000559, Energy Innovation Hub--Batteries and
Energy Storage, and [ANL Subcontract No. 3F-31144], issued under
DOE Prime Contract No. DE-ACO2-06CH11357 between the United States
Government and UChicago Argonne, LLC representing Argonne National
Laboratory and under Grant Nos. DE-AR0000065 and DE-SC0002633
awarded by the Department of Energy. The Government has certain
rights in the invention.
Claims
1. (canceled)
2. An electrode, comprising: an electroactive material comprising
sulfur or a metal sulfide; and a percolating electronically
conductive network comprising a plurality of electronically
conductive particles, wherein the electrode is substantially fluid
prior to first use.
3. The electrode of claim 2, further comprising a fluid.
4. The electrode of claim 3, wherein the sulfur or metal sulfide is
suspended and/or dissolved in the fluid.
5. The electrode of claim 2, wherein the electronically conductive
particles are suspended in the fluid.
6. An energy storage device, comprising: a first electrode
comprising an electrode composition comprising a first
electroactive material comprising sulfur, wherein the first
electroactive material is suspended and/or dissolved in a fluid,
wherein the concentration of sulfur is at least about 2 M; and a
second electrode in electrochemical communication with the first
electrode, wherein the second electrode comprises a second
electroactive material comprising an electroactive metal or metal
alloy comprising a non-sulfur element.
7. The energy storage device of claim 6, wherein at least one of
the electrodes of the energy storage device is substantially fluid
prior to first use.
8. The energy storage device of claim 7, wherein the fluid
comprises TEGDME, DME, diglyme, triglyme, DOL, THF, methyl-TFH,
GBL, and/or mixtures thereof.
9. The energy storage device of claim 6, further comprising a layer
adjacent the second electrode of the energy storage device.
10. The energy storage device of claim 9, wherein the layer
comprises LiNO.sub.3.
11. The energy storage device of claim 9, wherein the layer
comprises a ceramic and/or a polymer.
12. The energy storage device of claim 6, wherein the second
electrode comprises lithium metal.
13. The energy storage device of claim 6, further comprising an
ionically conductive membrane separating the first and second
electrodes.
14. The energy storage device of claim 6, further comprising a
metal salt.
15. The energy storage device of claim 14, wherein the metal salt
comprises LiTFSI, lithium triflate, sodium triflate, lithium
perchlorate, sodium perchlorate, lithium tetrafluoroborate, and/or
sodium tetrafluoroborate.
16. The energy storage device of claim 6, wherein the
electronically conductive particles comprise carbon black
particles.
17. The energy storage device of claim 6, wherein the concentration
of the first electroactive material in the fluid is at least about
1 M.
18. The energy storage device of claim 6, wherein the viscosity of
the electrode composition is less than about 100 Pa-s at a shear
rate of about 100 s.sup.-1.
19. The energy storage device of claim 6, wherein the yield stress
of the electrode composition is less than about 1000 Pa.
20. The energy storage device of claim 6, wherein the electronic
conductivity of the first electrode is at least about 0.1
mS/cm.
21. A method of operating an energy storage device, comprising:
reversibly cycling an energy storage device containing a flowable
electrode such that a metal sulfide is precipitated and
subsequently dissolved in the flowable electrode during the
cycling.
22. The method of claim 21, wherein the precipitated and dissolved
metal sulfide comprises Li.sub.2S.
23. The method of claim 21, wherein the reversible cycling occurs
over a voltage range of from about 0.45 V to about 2.7 V.
24. The method of claim 21, wherein the flowable electrode of the
energy storage device is in motion during at least a portion of the
cycling.
25. The method of claim 21, wherein reversible cycling of the
energy storage device comprises: transporting the flowable
electrode through an electrode compartment; inhibiting the flow of
the flowable electrode; at least partially charging or discharging
a portion of the flowable electrode; and transporting the flowable
electrode out of the electrode compartment.
26. The method of claim 21, wherein the flowable electrode
comprises an electroactive material comprising sulfur or a metal
sulfide suspended and/or dissolved in a fluid, and a percolating
electronically conductive network comprising electronically
conductive particles suspended in the fluid.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 14/172,648, entitled "Metal Sulfide Electrodes
and Energy Storage Devices Thereof," filed Feb. 4, 2014, which
claims priority under 35 U.S.C. .sctn.119(e) to U.S. Provisional
Patent Application Ser. No. 61/760,436, entitled "Magnesium
Polysulfide Half-Flow Battery," filed Feb. 4, 2013; U.S.
Provisional Patent Application Ser. No. 61/789,964, entitled "High
Energy Density Semi-Solid Storage Electrodes and Batteries
Thereof," filed Mar. 15, 2013; U.S. Provisional Patent Application
Ser. No. 61/903,574, entitled "Metal-Polysulfide Batteries," filed
Nov. 13, 2013; and to U.S. Provisional Patent Application Ser. No.
61/903,739, entitled "Metal-Polysulfide Batteries," filed Nov. 13,
2013. Each of these applications is incorporated herein by
reference in its entirety.
FIELD
[0003] The present invention generally relates to energy storage
devices and, in particular, to sulfur or metal sulfide energy
storage devices.
BACKGROUND
[0004] A battery stores electrochemical energy by separating an ion
source and an ion sink at differing ion electrochemical potentials.
A difference in electrochemical potential produces a voltage
difference between the positive and negative electrodes; this
voltage difference will produce an electric current if the
electrodes are connected by a conductive element. In a battery, the
negative electrode and positive electrode are connected by two
conductive elements in parallel. Generally, the external element
conducts electrons, and the internal element (electrolyte) conducts
ions. Because a charge imbalance cannot be sustained between the
negative electrode and positive electrode, these two flow streams
supply ions and electrons at the same rate. In operation, the
electronic current can be used to drive an external device. A
rechargeable battery can be recharged by application of an opposing
voltage difference that drives electronic current and ionic current
in an opposite direction as that of a discharging battery in
service. Thus, the active materials of rechargeable batteries need
to be able to accept and provide ions.
[0005] Redox flow batteries are energy storage devices in which the
positive and negative electrode active materials are soluble metal
ions in liquid solution that are oxidized or reduced during the
operation of the cell. A redox flow battery typically has tanks for
separately storing the positive and negative electrode active
materials, along with a current-extracting stack that comprises at
least positive and negative electrode compartments separated by an
ionically conductive membrane, and positive and negative current
collectors that facilitate the transfer of electrons to the
external circuit but do not participate in the redox reaction
(i.e., the current collector materials themselves do not undergo
Faradaic activity). In a redox flow battery, the total stored
energy may be increased by simply increasing the size of the tanks
holding the electrode active materials without increasing the
amount of other components such as the separator, current
collectors, and the like. However, while conventional redox flow
batteries possess many attractive features, such as scalability and
the ability to decouple stored energy (tanks) from power (stack),
they typically have relatively low energy densities. Thus, there is
a need for improved, high-energy-density energy storage
devices.
SUMMARY
[0006] The present invention generally relates to energy storage
devices and, in particular, to metal sulfide energy storage
devices. The subject matter of the present invention involves, in
some cases, interrelated products, alternative solutions to a
particular problem, and/or a plurality of different uses of one or
more systems and/or articles.
[0007] In one aspect, the invention is related to an electrode
composition. The electrode composition comprises a sulfur or metal
sulfide electroactive material suspended and/or dissolved in a
fluid; and electronically conductive particles suspended in the
fluid, the electronically conductive particles forming a
percolating conductive network.
[0008] In another aspect, the invention is related to an electrode
comprising an electrode composition comprising a sulfur or metal
sulfide electroactive material suspended and/or dissolved in a
fluid; and electronically conductive particles suspended in the
fluid, the electronically conductive particles forming a
percolating conductive network.
[0009] In another aspect, the invention is related to an electrode
comprising an electroactive material comprising sulfur or a metal
sulfide; and a percolating electronically conductive network
comprising a plurality of electronically conductive particles,
wherein the electrode is substantially fluid prior to first
use.
[0010] In another set of embodiments, the invention is related to
an energy storage device comprising a first electrode and a second
electrode in electrochemical communication with the first
electrode. The first electrode comprises an electrode composition
comprising a first electroactive material comprising sulfur or a
metal sulfide, wherein the first electroactive material is
suspended and/or dissolved in a fluid; and electronically
conductive particles suspended in the fluid, the electronically
conductive particles forming a percolating conductive network.
[0011] In another set of embodiments, the invention is related to
an energy storage device comprising a first electrode comprising an
electrode composition comprising a first electroactive material
comprising sulfur, wherein the first electroactive material is
suspended and/or dissolved in a fluid, wherein the concentration of
sulfur is at least about 2 M; and a second electrode in
electrochemical communication with the first electrode, wherein the
second electrode comprises a second electroactive material
comprising an electroactive metal or metal alloy comprising a
non-sulfur element.
[0012] Another aspect of the invention is related to a method of
operating an energy storage device, comprising reversibly cycling
the energy storage device over a voltage range of from about 0.45 V
to about 2.7 V, wherein the energy storage device comprises a first
electrode comprising an electroactive material comprising sulfur or
a metal sulfide suspended and/or dissolved in a fluid, and a
percolating electronically conductive network comprising
electronically conductive particles suspended in the fluid; and a
second electrode in electrochemical communication with the first
electrode.
[0013] Another aspect of the invention is related to a method of
operating an energy storage device, comprising reversibly cycling
an energy storage device containing a flowable electrode such that
a metal sulfide is precipitated and subsequently dissolved in the
flowable electrode during the cycling.
[0014] Other advantages and novel features of the present invention
will become apparent from the following detailed description of
various non-limiting embodiments of the invention when considered
in conjunction with the accompanying figures. In cases where the
present specification and a document incorporated by reference
include conflicting and/or inconsistent disclosure, the present
specification shall control. If two or more documents incorporated
by reference include conflicting and/or inconsistent disclosure
with respect to each other, then the document having the later
effective date shall control.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] Non-limiting embodiments of the present invention will be
described by way of example with reference to the accompanying
figures, which are schematic and are not intended to be drawn to
scale. In the figures, each identical or nearly identical component
illustrated is typically represented by a single numeral. For
purposes of clarity, not every component is labeled in every
figure, nor is every component of each embodiment of the invention
shown where illustration is not necessary to allow those of
ordinary skill in the art to understand the invention. In the
figures:
[0016] FIGS. 1A-1B provide schematic illustrations of energy
storage devices in accordance with certain embodiments of the
invention;
[0017] FIGS. 2A-2D provide plots of viscosity and stress as
functions of shear rate for various embodiments of the
invention;
[0018] FIGS. 3A-3B provide cross-sectional illustrations and plots
comparing a conventional energy storage device and an energy
storage device according to some embodiments of the invention;
[0019] FIG. 4 provides images showing the microstructure of a
current collector from a conventional energy storage device;
[0020] FIG. 5 provides images showing the microstructure of
conductive particles in an energy storage device according to some
embodiments of the invention;
[0021] FIG. 6 shows a plot of current density as a function of
potential for an energy storage device according to one or more
embodiments of the invention;
[0022] FIG. 7 provides a comparison of impedance for energy storage
devices according to some embodiments of the invention;
[0023] FIGS. 8A-8D provide illustrations of a conventional energy
storage device and an energy storage device according to one or
more embodiments of the invention;
[0024] FIGS. 9A-9D provide plots of voltage as a function of
specific capacity and plots of impedance for a conventional energy
storage device and an energy storage device according to one or
more embodiments of the invention;
[0025] FIG. 10 provides a plot of specific capacity as a function
of cycle number for various embodiments of the invention;
[0026] FIG. 11 provides plots of voltage, current, and specific
capacity as a function of time for an energy storage device under
continuous flow, in accordance with some embodiments of the
invention;
[0027] FIGS. 12A-12C provide plots of voltage as a function of
specific capacity for an energy storage device under intermittent
flow, in accordance with certain embodiments of the invention;
[0028] FIG. 13 provides a periodic table showing the relative
abundance of elements in the Earth's crust, along with the relative
electronegativity of elements; and
[0029] FIG. 14 provides graphs comparing the theoretical energy
density and raw materials cost floor for Li/S, Mg/S, Al/S, Zn/S,
Mn/S, and Fe/S couples.
DETAILED DESCRIPTION
[0030] The present invention generally relates to energy storage
devices, and to sulfur or metal sulfide energy storage devices in
particular. Some aspects of the invention relate to energy storage
devices comprising at least one flowable electrode, wherein the
flowable electrode comprises a sulfur or metal sulfide
electroactive material suspended and/or dissolved in a fluid. In
some embodiments, the flowable electrode further comprises a
plurality of electronically conductive particles suspended in the
fluid, wherein the electronically conductive particles form a
percolating conductive network. An energy storage device comprising
a flowable electrode comprising a sulfur or metal sulfide
electroactive material and a percolating conductive network may
advantageously exhibit, upon reversible cycling, higher energy
densities and specific capacities than conventional energy storage
devices.
[0031] In many conventional energy storage devices with flow-based
systems (e.g., flow batteries, half-flow batteries, redox flow
devices, etc.), an electrode comprising an electroactive material
is flowed through or adjacent a stationary current collector. In
these conventional energy storage devices, the charge transfer
required for redox reactions takes place only when redox species
contact the current collector surface through diffusion or fluid
convection. By contrast, some embodiments of the present invention
include an electrode comprising a percolating conductive network,
and charge transfer may occur throughout the volume of the
electrode. As a result, the available charge transfer area may
significantly increase and may lead to a reduction in charge
transfer resistance. Additionally, in some embodiments, an energy
storage device comprising a sulfur or metal sulfide electroactive
material and a percolating conductive network may be reversibly
cycled into regimes where a metal sulfide is precipitated from and
subsequently dissolved and/or suspended in the fluid. The ability
to participate in precipitation regimes may result in higher energy
densities and specific capacities than conventional redox
solution-based energy storage devices, which generally cannot be
reversibly cycled through such precipitation regimes.
[0032] An exemplary energy storage device 100 is illustrated in
FIG. 1A. Energy storage device 100 may include a first electrode
compartment 102 and a second electrode compartment 104, separated
by an ion-permeable separator 106. First electrode compartment 102
may be fluidly connected to a first storage tank 108 via conduit
112. A first electrode composition comprising a first electroactive
material may be stored in first storage tank 108. Second electrode
compartment 104 may be fluidly connected to a second storage tank
110 via conduit 114. A second electrode composition comprising a
second electroactive material may be stored in second storage tank
110. First electrode compartment 102 may be in electronic
communication with a first current collector 116, and second
electrode compartment 104 may be in electronic communication with a
second current collector 118. Current collectors 116 and 118 may be
electronically connected via external circuit 120. In some
embodiments, the first and second electrode compositions are
flowable redox compositions that can be transported to and from the
electroactive compartments, where an electrochemical reaction
occurs. The first electrode composition may be a positive electrode
composition, and the second electrode composition may be a negative
electrode composition. Alternatively, the first electrode
composition may be a negative electrode composition, and the second
electrode composition may be a positive electrode composition.
[0033] In operation, the first electrode composition may be flowed
from first storage tank 108 to first electrode compartment 102 via
conduit 112, and the second electrode composition may be flowed
from second storage tank 110 to second electrode compartment 104
via conduit 114. A transporting device may be used in some
embodiments to transport the first and second electrode
compositions through conduits 112 and 114. Non-limiting examples of
suitable transporting devices include peristaltic pumps, piston
pumps, gear pumps, gravity feed devices, and any other device for
fluid transport. During discharge of energy storage device 100, in
some embodiments where the first electrode composition is positive
and the second electrode composition is negative, the first,
positive electrode composition may undergo reduction in first
electrode compartment 102, and the second, negative electrode
composition may undergo oxidation in second electrode compartment
104. Ions, which may be referred to as working ions, may flow
across ion-permeable membrane 106, and electrons may flow through
external circuit 120 to generate current. An opposing voltage
difference may be applied to energy storage device 100 to drive
electronic current and ionic current in a direction opposite to
that of discharging and reverse the electrochemical reaction of
discharging, thus charging the first and second electrode
compositions. The flow of the first and second electrode
compositions through the first and second electrode compartments
may be continuous or intermittent. In some embodiments, the first
and second electrode compositions may be continuously renewed and
replaced from first storage tank 108 and second storage tank
110.
[0034] In the embodiment illustrated in FIG. 1A, both the first and
second electrode compositions of energy storage device 100 are
flowable. In alternative embodiments, energy storage device 100 may
be a half-flow battery (also known as a semi-flow battery), with
one flowable electrode composition and one non-flowable electrode
composition. For example, FIG. 1B illustrates an exemplary
embodiment in which energy storage device 100 comprises a first
electrode compartment 102, through which a first electrode
composition can flow, in ionic communication with a second
electrode 122. Second electrode 122 may be substantially solid in
some embodiments and, therefore, substantially non-flowable.
[0035] Referring to FIG. 1B, in a particular embodiment of energy
storage device 100, the first, flowable electrode composition,
which may flow through first electrode compartment 102, comprises a
positive electroactive material comprising elemental sulfur, and
second electrode 122 comprises a negative electroactive material
comprising lithium metal. The elemental sulfur may be suspended
and/or dissolved in a fluid. One of ordinary skill in the art would
understand the term "elemental sulfur" to refer to sulfur in its
ground oxidation state (S.sup.0). In some cases, elemental sulfur
is S.sub.8. Similarly, one of ordinary skill in the art would
understand the term "lithium metal" to refer to lithium in its
ground oxidation state (Li.sup.0). In this particular embodiment,
discharge of energy storage device 100 may proceed by the elemental
sulfur S.sub.8 of the first electrode composition first being
lithiated/reduced to form Li.sub.2S.sub.8, which is typically
soluble in the fluid. The lithium polysulfide Li.sub.2S.sub.8 may
then be further lithiated/reduced to form Li.sub.2S.sub.6,
Li.sub.2S.sub.4, and Li.sub.2S.sub.2, and/or Li.sub.2S, etc. During
charge of energy storage device 100, the reactions may be reversed
to form Li.sub.2S, Li.sub.2S.sub.2, Li.sub.2S.sub.3,
Li.sub.2S.sub.4, Li.sub.2S.sub.6, Li.sub.2S.sub.8, and S.sub.8,
etc. Generally, the discharge curve for the Li-polysulfide system
exhibits at least a high voltage plateau, through which solid
sulfur exists with soluble lithium polysulfides, a solution regime,
in which sulfur is fully dissolved as higher-order polysulfides
(e.g., Li.sub.2S.sub.4, Li.sub.2S.sub.6, Li.sub.2S.sub.8, etc.),
and a low voltage plateau (precipitation regime), in which the
discharge reaction proceeds via precipitation of lower-order
polysulfides (e.g., Li.sub.2S.sub.2, Li.sub.2S, etc.). In this
embodiment, the first electrode composition may further comprise a
percolating conductive network comprising carbon black. The weight
ratio of carbon to sulfur may be at least about 0.1, at least about
0.2, at least about 0.3, at least about 0.4, or at least about 0.5,
and/or up to about 0.6, or more. In some embodiments, the weight
ratio of carbon to sulfur is less than about 0.6, less than about
0.5, less than about 0.4, less than about 0.3, or less than about
0.2, and/or down to about 0.1, or less. Combinations of any of
these are also possible. In some embodiments, for example, the
weight ratio of carbon to sulfur is in the range of about 0.1 to
about 0.4, about 0.1 to about 0.5, about 0.1 to about 0.6, or about
0.2 to about 0.5.
[0036] Some embodiments of the invention, like the particular
embodiment described above, may provide advantages over
conventional prior art devices in both the solution and
precipitation regimes. Without being bound by a particular theory,
cycling of an energy storage device in the solution regime may be
rate-limited by charge-transfer kinetics, and embodiments
comprising a percolating conductive network may experience lower
charge-transfer resistance than conventional energy storage devices
comprising electronically-insulating redox flow compositions and
stationary current collectors. In the precipitation regime, the
percolating conductive network may provide greater surface area
over which to deposit insoluble polysulfides, allowing an
equivalent volume of polysulfide precipitate to be more thinly
deposited. These advantages may allow embodiments of the invention
comprising a sulfur or metal sulfide electroactive material and a
lithium metal anode to achieve higher specific capacities and
energy densities than prior art devices. For example, in some
embodiments, the specific capacity may be five times greater than
the specific capacity of a conventional flow battery. As used
herein, the term "specific capacity" refers to the amount of charge
that can be delivered per unit mass. The term "energy density," as
used herein, refers to the amount of energy stored per unit mass or
per unit volume.
[0037] In some embodiments of the invention, the positive
electroactive material comprises sulfur or a metal sulfide. The
term "metal sulfide," as used herein, refers to chemical compounds
having the formula M.sub.xS.sub.y, where M is at least one metal
element, S is sulfur, x is a number between 1 and 2, and y is a
number between 1 and 8. In certain cases, M represents 2 or more
metal elements. Exemplary, non-limiting examples of suitable metal
elements include lithium, sodium, magnesium, aluminum, zinc,
manganese, titanium, and iron. In some embodiments, the metal
sulfide comprises a lithium sulfide, a sodium sulfide, a magnesium
sulfide, an aluminum sulfide, a zinc sulfide, a manganese sulfide,
a titanium sulfide, and/or an iron sulfide. In certain cases, the
metal sulfide is Li.sub.2S, Li.sub.2S.sub.2, Li.sub.2S.sub.3,
Li.sub.2S.sub.4, Li.sub.2S.sub.6, or Li.sub.2S.sub.8. In some
embodiments, the positive electroactive material comprises
elemental sulfur.
[0038] In some embodiments, the positive electroactive material
comprises selenium, a mixture of selenium and sulfur, metal
selenides, or mixtures of solid solutions of metal sulfides and
metal selenides. As used herein, the term "metal selenide" refers
to chemical compounds having the formula M.sub.xSe.sub.y, where M
is at least one metal element, Se is selenium, x is a number
between 1 and 2, and y is a number between 1 and 8. In certain
cases, M represents 2 or more metal elements. Exemplary,
non-limiting examples of suitable metal elements include lithium,
sodium, magnesium, aluminum, zinc, manganese, titanium, and iron.
In some embodiments, the metal selenide comprises a lithium
selenide, a sodium selenide, a magnesium selenide, an aluminum
selenide, a zinc selenide, a manganese selenide, a titanium
selenide, and/or an iron selenide.
[0039] In some embodiments, the sulfur or metal sulfide substance
present in the electrode composition is an electroactive material.
As used herein, "electroactive material" refers to a material
capable of participating in a reduction reaction and/or an
oxidation reaction, or forming an electrical double layer in order
to store capacitive charge such as in an electrochemical capacitor,
or both, in order to store energy.
[0040] In some embodiments, at least a portion of the sulfur or
metal sulfide electroactive material is suspended and/or dissolved
in a fluid, resulting in a mixture. In certain embodiments, the
electrode composition comprises a homogeneous mixture, such as a
solution. In some embodiments, the electrode composition comprises
a heterogeneous mixture, such as a suspension (e.g., a particle
suspension, a colloidal suspension), a slurry, and/or an emulsion.
Typically, a suspension comprises a dispersion of particles in a
dispersion medium. In some embodiments, the colloid chemistry and
rheology of the electrode composition is adjusted to produce a
stable suspension from which the solid particles settle only slowly
or not at all, in order to improve flow ability of the electrode
composition and to minimize any stirring or agitation needed to
avoid settling of the electroactive particles. The stability of the
suspension can be evaluated by monitoring a static suspension for
evidence of solid-liquid separation due to particle settling. The
electroactive material particle suspension may reach a stable state
when there is no observable particle settling in the suspension,
and/or no other macroscopic changes that can be observed.
Typically, the stability of the electroactive material particle
suspension increases with decreasing suspended particle size. In
some embodiments, the electrode composition includes thickeners or
binders to reduce settling and improve suspension stability. In
some embodiments, the shear flow produced by pumps provide
additional stabilization of the suspension. In some embodiments,
the electroactive material particle suspension is stable for at
least 1 day, at least 5 days, at least 10 days, or at least 30
days, etc.
[0041] The concentration of a substance in a mixture can be
expressed, for example, as molarity, volume percent, and/or weight
percent. In a suspension comprising particles suspended in a fluid,
the molar concentration of the particles would be the number of
moles of the particles present in the suspension divided by the
volume of the suspension. In some embodiments, the molar
concentration of sulfur in the electrode composition is at least
about 1 M, at least about 2.5 M, at least about 5 M, at least about
7 M, or at least about 15 M (and/or up to about 20 M). In some
cases, the molar concentration of sulfur in the electrode
composition is in the range of about 1 M to about 20 M, about 2 M
to about 10 M, about 5 M to about 20 M, about 10 M to about 20 M,
about 15 M to about 20 M, or about 1 M to about 10 M.
[0042] In some embodiments, an electrode composition comprising a
sulfur or metal sulfide electroactive material suspended and/or
dissolved in a fluid further comprises a plurality of
electronically conductive particles suspended in the fluid, the
electronically conductive particles forming a percolating
conductive network. As used herein, an "electronically conductive"
material is one that is capable of transporting electronic charge
carriers (e.g., electrons) and has a conductivity of at least about
0.1 mS/cm, or at least about 0.5 mS/cm in some cases. A
"percolating conductive network," as used herein, refers to
particles that are electronically connected, such that electronic
charge carriers can be transported throughout the network. The
particles themselves may be in actual physical contact with each
other and/or some of the particles may not necessarily be in actual
physical contact, but the particles may be positioned near enough
to each other (e.g., as in a suspension) such that the particles
are electronically connected and electronic charge carriers can be
transported between the particles. Without being bound by a
particular theory, a percolating conductive network may be formed
in some embodiments by electronically conductive particles
undergoing diffusion-limited aggregation (DLA). Diffusion-limited
aggregation refers to a process where particles undergoing a random
walk due to Brownian motion exhibit "hit-and-stick" behavior--that
is, they stick to other particles they hit--and thereby aggregate
to form fractal networks. Such networks may have a self-similar
structure when observed at varying magnifications. The mass of a
cluster of particles may vary with the characteristic size of the
cluster according to a power-law relationship M.about.R.sup.df
where M is the mass of the cluster, R is the characteristic size of
the cluster, and d.sub.f is the fractal dimension. The fractal
dimension may be measurable by means of radiative scattering
techniques such as small angle x-ray scattering, as well as other
methods. For example, the shear elastic modulus of a suspension
containing a fractal network may scale as
G'.about.f.sup.(3+x/3-df), where G' is the shear modulus, f is the
volume fraction of the solid, x is the fractal dimension of the
primary particles, and d.sub.f is the fractal dimension of the
network. In some embodiments, the fractal dimension may be
determined by the scaling of the shear modulus of the networks in
the present invention. In some embodiments, the fractal dimension
may be at least about 1.5, at least about 1.7, at least about 1.8,
at least about 2.1, or at least about 2.2 (and/or up to about 2.5,
or more). In some embodiments, the fractal dimension may be less
than about 2.5, less than about 2.2, less than about 2.1, less than
about 1.8, or less than about 1.7 (and/or down to about 1.5, or
less). In some embodiments, the fractal dimension may be in the
range of about 1.5 to about 2.5, about 1.7 to about 2.2, and/or
about 1.8 to about 2.1 Typically, a percolating network has an
associated percolating threshold, which may also be referred to as
a percolation limit or a critical concentration, below which a
percolating conductive network is not formed. For example, in some
embodiments, the electrode composition is not electronically
conductive below the percolation threshold, and the electrode
composition is electronically conductive at or above the
percolation threshold. In general, the percolation threshold may be
reduced by reducing the size of the particles. Additionally, the
percolation threshold may be reduced by increasing the ionic
strength of the medium in which the particles are dispersed.
Without being bound by a particular theory, a fluid with high ionic
strength (e.g., a liquid electrolyte) may provide a particularly
favorable environment for the formation of percolating networks
through DLA. In a suspension, Debye-Huckel electrostatic double
layers may form around each particle, which may result in
interparticle repulsion. A high ionic strength fluid may screen the
electrostatic double layers, thereby strengthening attractive DLA
interactions (i.e., allowing van der Waals attraction to become
dominant). Surprisingly, it has been found that the percolation
threshold for electronically conductive particles in the fluid may
be very low.
[0043] In some embodiments, the percolation threshold, as expressed
in volume percent of electronically conductive particles in the
electrode composition, is less than about 10 vol %, less than about
7 vol %, less than about 5 vol %, less than about 3 vol %, less
than about 1 vol %, and/or down to about 0.5 vol %. In some
embodiments, the percolation threshold is at least about 0.5 vol %,
at least about 1 vol %, at least about 3 vol %, at least about 5
vol %, at least about 7 vol %, and/or up to about 10 vol % of
electronically conductive particles. Combinations of any of these
are also possible. In some embodiments, for instance, the
percolation threshold is in the range of about 0.5 vol % to about 3
vol %, about 0.5 vol % to about 5%, about 0.5 vol % to about 10%,
about 1 vol % to about 3%, about 1 vol % to about 5%, about 1 vol %
to about 10%, about 3 vol % to about 5%, about 3 vol % to about
10%, about 5 vol % to about 7 vol %, about 5 vol % to about 10 vol
%, or about 7 vol % to about 10 vol % of electronically conductive
particles.
[0044] In some embodiments, the electronically conductive particles
make up less than about 10 vol %, less than about 7 vol %, less
than about 5 vol %, less than about 3 vol %, less than about 1.5
vol %, less than about 1 vol %, and/or down to about 0.5 vol % of
the electrode composition. In some embodiments, the electronically
conductive particles make up at least about 0.5 vol %, at least
about 1 vol %, at least about 1.5 vol %, at least about 3 vol %, at
least about 5 vol %, at least about 7 vol %, and/or up to about 10
vol % of the electrode composition. Combinations of any of these
are also possible. In some embodiments, for example, the
electronically conductive particles make up between about 0.5 vol %
to about 1.5 vol %, about 0.5 vol % to about 3 vol %, about 0.5 vol
% to about 5%, about 0.5 vol % to about 10%, about 1 vol % to about
3%, about 1 vol % to about 5%, about 1 vol % to about 10%, about
1.5 vol % to about 5 vol %, about 1.5 vol % to about 10 vol %,
about 3 vol % to about 5%, about 3 vol % to about 10%, about 5 vol
% to about 7 vol %, about 5 vol % to about 10 vol %, or about 7 vol
% to about 10 vol % of the electrode composition.
[0045] In some embodiments, the weight ratio of the electronically
conductive particles to sulfur is at least about 0.1, at least
about 0.2, at least about 0.3, at least about 0.4, or at least
about 0.5, and/or up to about 0.6, or more. In some embodiments,
the weight ratio of the electronically conductive particles to
sulfur is less than about 0.6, less than about 0.5, less than about
0.4, less than about 0.3, or less than about 0.2, and/or down to
about 0.1, or less. Combinations of any of these are also possible.
In some embodiments, for example, the weight ratio of the
electronically conductive particles to sulfur is in the range of
about 0.1 to about 0.4, about 0.1 to about 0.5, about 0.1 to about
0.6, or about 0.2 to about 0.5.
[0046] In some embodiments, the percolating conductive network may
render the electrode composition both ionically and electronically
conductive. Advantageously, the percolating network may allow the
electrode to function as a "liquid wire" and allow charge transfer
to occur throughout the volume of the electrode composition. This
may, for example, increase the charge transfer rate of the
electrode composition when used in an electrode in an energy
storage device. By contrast, in conventional flow batteries, the
electrode compositions typically comprise electronically-insulating
redox fluids that only undergo charge transfer reactions upon
contact with a stationary current collector.
[0047] The electronically conductive particles may comprise any
electronically conductive material. In some embodiments, the
electronically conductive particles comprise carbon-based
particles. For example, non-limiting examples of suitable
carbon-based, electronically conductive particles include carbon
black particles, graphitic carbon, carbon fibers, carbon
microfibers, vapor-grown carbon fibers, fullerenic carbons
including "buckyballs," carbon nanotubes, multi-wall carbon
nanotubes, single-wall carbon nanotubes, graphene sheets or
aggregates of graphene sheets, and materials comprising fullerenic
fragments. One of ordinary skill in the art would understand
"carbon black" to refer to a form of carbon produced by partial
combustion of hydrocarbons. In some embodiments, the electronically
conductive particles comprise a metal, a metal carbide, a metal
nitride, and/or a metal oxide. In some embodiments, the metal
comprises platinum, palladium, iridium, gold, silver, ruthenium,
tantalum, tin, aluminum, a first-row transition metal, and/or
alloys comprising one or more of these elements. In some
embodiments, the metal carbide comprises a carbide of a first-row
transition metal, of silicon, of tin, of tantalum, a mixed-metal
carbide comprising one or more of these metals, and/or a mixture of
different metal carbides. In one embodiment, the metal carbide is
titanium carbide. In some embodiments, the metal nitride comprises
a nitride of a first-row transition metal, a mixed-metal nitride
comprising one or more of these metals, and/or a mixture of
different metal nitrides. In one embodiment, the metal nitride is
titanium nitride. In some embodiments the metal oxide comprises an
oxide of a first-row transition metal, ruthenium oxide, tin oxide,
or zinc oxide. In some embodiments the conductive oxide comprises
an oxide usable as a transparent conducting oxide (TCO), including
but not limited to indium tin oxide (no). In some embodiments, the
nanoscale particles are substantially spherical, and/or have an
aspect ratio (largest to smallest cross-sectional dimension of the
particle) of less than about 3, less than about 2, less than about
1.5, less than about 1.2, etc. In other embodiments, the
nanoparticles have an aspect ratio greater than about 3 and include
nanotubes, nanorods, nanowires, and nanoplatelets. The nanoscale
particles may be prepared by a variety of methods including
mechanical grinding, chemical precipitation, vapor phase reaction,
laser-assisted reactions, and bio-assembly.
[0048] In some embodiments, the electronically conductive particles
are nanoscale particles. As noted above, the small size of the
electronically conductive particles may facilitate the formation of
stable suspensions and/or may lower the percolation threshold. In
some embodiments, the electronically conductive particles have a
primary particle size of less than about 1 micrometer, less than
about 500 nm, less than about 100 nm, less than about 50 nm, less
than about 30 nm, or less than about 20 nm (and/or, in certain
embodiments, down to about 10 nm, or less). In some embodiments,
the electronically conductive particles have a primary particle
size of at least about 10 nm, at least about 20 nm, at least about
30 nm, at least about 50 nm, at least about 100 nm, or at least
about 500 nm (and/or, in certain embodiments, up to about 1
micrometer, or more). Combinations of these are also possible. In
some embodiments, for instance, the electronically conductive
particles have a primary particle size in the range of about 10 nm
to about 30 nm, about 10 nm to about 50 nm, about 10 nm to about
100 nm, about 10 nm to about 500 nm, about 10 nm to about 1
micrometer, about 50 nm to about 100 nm, about 50 nm to about 500
nm, about 50 nm to about 1 micrometer, about 100 nm to about 500
nm, or about 100 nm to about 1 micrometer. The primary particle
size of a non-spherical particle may be taken as the diameter of a
perfect sphere having the same volume as the particle.
[0049] In some embodiments, the primary particles of the
electronically conductive material are aggregated into larger
agglomerates. In some embodiments, the electronically conductive
particles are present in a porous aggregate with an average
diameter of at least about 0.1 micrometers, at least about 0.2
micrometers, at least about 0.5 micrometers, at least about 1
micrometer, at least about 10 micrometers, at least about 50
micrometers, or at least about 100 micrometers (and/or, in certain
embodiments, up to about 500 micrometers, or more). In some
embodiments, the electronically conductive particles are present in
a porous aggregate with an average diameter of less than about 500
micrometers, less than about 100 micrometers, less than about 50
micrometers, less than about 10 micrometers, less than about 1
micrometer, or less than about 0.5 micrometers (and/or, in certain
embodiments, down to about 0.2 micrometers, or less). Combinations
of these are also possible. In some embodiments, for example, the
electronically conductive particles are present in a porous
aggregate with an average diameter in the range of about 1
micrometer to about 10 micrometers, about 1 micrometer to about 50
micrometers, about 1 micrometer to about 100 micrometers, about 1
micrometer to about 500 micrometers, about 50 micrometers to about
100 micrometers, about 50 micrometers to about 500 micrometers, or
about 100 micrometers to about 500 micrometers.
[0050] In some embodiments, the electronically conductive particles
may have relatively high specific surface areas. This feature may,
advantageously, increase the area available for charge transfer
when electrode compositions comprising the electronically
conductive particles are used in electrodes in energy storage
devices. As used herein, "specific surface area" refers to surface
area per unit mass or per unit volume. One method of measuring
specific surface area of the electronically conductive particles is
by using the Brunauer, Emmett, and Teller (BET) method. The BET
method may include the steps of flowing gas over a sample, cooling
the sample, and subsequently measuring the volume of gas adsorbed
onto the surface of the sample at specific pressures. In some
embodiments, the electronically conductive particles have a BET
specific surface area of at least about 500 m.sup.2 g.sup.-1, at
least about 1000 m.sup.2 g.sup.-1, of at least about 1400 m.sup.2
g.sup.-1, at least about 2000 m.sup.2 g.sup.-1, etc.
[0051] The fluid of the electrode composition may be any ionically
conductive liquid that can suspend and/or dissolve and transport
the sulfur and/or metal sulfide electroactive material and the
electronically conductive particles of the electrode composition.
In an energy storage device comprising a first electrode comprising
the electrode composition, a second electrode, and an ion-permeable
separator separating the first and second electrodes, the working
ions generally are the ions that are transported through the
ion-permeable separator between the first and second electrodes.
Examples of working ions may include, but are not limited to,
H.sup.+, OH.sup.-, Cl.sup.-, Br.sup.-, Li.sup.+, Na.sup.+,
Ca.sup.2+, Mg.sup.2+, Al.sup.3+, Zn.sup.2+, Mn.sup.2+, Mn.sup.3+,
Ti.sup.3+, Ti.sup.4+, Fe.sup.2+, and/or Fe.sup.3+. Ionically
conductive liquids typically permit transport of the working ion
and have an ionic conductivity of at least about 0.1 mS/cm, or at
least about 0.5 mS/cm in some cases. The ionically conductive fluid
may generally be referred to as an electrolyte. When a positive
electroactive material is suspended and/or dissolved in the fluid,
the fluid may be referred to as a catholyte. When a negative
electroactive material is suspended and/or dissolved in the fluid,
the fluid may be referred to as an anolyte. The fluid may be either
aqueous or non-aqueous. Suitable non-aqueous fluids include, but
are not limited to, tetraethylene glycol dimethyl ether (TEGDME),
dimethyl ether (DME), diglyme, triglyme, dioxolane (DOL),
tetrahydrofuran (THF), methyl-tetrahydrofuran (methyl-THF),
gamma-butyrolactone (GBL), and mixtures thereof.
[0052] In some embodiments, the electrode composition may further
comprise a supporting electrolyte. One of ordinary skill in the art
would understand the term "supporting electrolyte" to refer to a
non-electroactive ionic species. A supporting electrolyte may be
added, for example, to increase the conductivity of the electrode
composition. In some embodiments, the supporting electrolyte
comprises a metal salt. Examples of suitable metal salts include,
but are not limited to, bis(trifluoromethane)sulfonimide lithium
salt (LiTFSI), lithium triflate (LiCF.sub.3SO.sub.3), sodium
triflate (NaCF.sub.3SO.sub.3), lithium perchlorate (LiClO.sub.4),
sodium perchlorate (NaClO.sub.4), lithium tetrafluoroborate
(LiBF.sub.4), and/or sodium tetrafluoroborate (NaBF.sub.4). In some
embodiments, the molar concentration of the metal salt in the
electrode composition is at least about 0.1 M, at least about 0.2
M, at least about 0.5 M (and/or, in certain embodiments, up to
about 1 M, or more).
[0053] Some aspects of the invention relate to an electrode
comprising the electrode composition described above. In some
embodiments, the electrode is flowable. That is, in some
embodiments, the electrode may be substantially fluid and/or easily
deformed prior to first use and/or when substantially fully
charged. For example, in some embodiments, the electrode may have a
measurable viscosity, and/or the electrode may tend to flow and to
conform to the outline of its container, and/or the electrode may
have the consistency of a paste. In some cases, the flowable
electrode, after being left undisturbed for a day or less, may be
observably deformed from its original shape, and in some cases,
such observable deformations may occur on the time scale of minutes
or seconds.
[0054] In some embodiments, the electrode exhibits the behavior of
a Newtonian fluid, e.g., a fluid in which shear stress is directly
proportional to shear strain rate (also referred to as shear rate)
and viscosity is independent of shear rate. For example, in some
cases, an electrode comprising a sulfur or metal sulfide
electroactive material dissolved and/or suspended in a fluid may,
in the absence of electronically conductive particles, act as a
Newtonian fluid. FIG. 2A shows an exemplary log-log plot of
viscosity as a function of shear rate for solutions of
Li.sub.2S.sub.8 dissolved in TEGDME with 0.5 M LiTFSI. As shown in
FIG. 2A, a solution comprising 2.5 M Li.sub.2S.sub.8 (indicated as
squares in FIG. 2A) has a viscosity of 26 mPa-s that is constant
across shear rates ranging from 1 to 100 s.sup.-1, a solution
comprising 5 M Li.sub.2S.sub.8 (indicated as circles in FIG. 2A)
has a constant shear viscosity of 50 mPa-s, and a solution
comprising 7 M Li.sub.2S.sub.8 (indicated as triangles in FIG. 2A)
has a constant shear viscosity of 127 mPa-s. The viscosity of the
electrode may be measured, for example, by using a viscometer, a
torsional rheometer, and/or a capillary rheometer. In some
embodiments, the viscosities may be measured at a temperature of
about 25.degree. C. In some embodiments, the viscosity of the
electrode may be less than about 10.sup.7 mPa-s, less than about
10.sup.6 mPa-s, less than about 10.sup.5 mPa-s, less than about
10.sup.4 mPa-s, less than about 1000 mPa-s, less than about 800
mPa-s, less than about 500 mPa-s, less than about 200 mPa-s, less
than about 100 mPa-s, less than about 50 mPa-s, or less than about
20 mPa-s, or less than about 15 mPa-s (and/or down to about 10
mPa-s, or less).
[0055] In some embodiments, the electrode exhibits the behavior of
a non-Newtonian fluid, e.g., a fluid whose viscosity is dependent
on shear rate. For example, an electrode comprising a sulfur or
metal sulfide electroactive material dissolved and/or suspended in
a fluid and electronically conductive particles suspended in the
fluid may act as a non-Newtonian fluid. In some cases, the addition
of even small amounts of a conductive particle may result in
non-Newtonian behavior. FIG. 2B shows exemplary log-log plots of
viscosity as a function of shear rate for electrolytes comprising
2.5 M Li.sub.2S.sub.8 dissolved in TEGDME with 0.5 M LiTFSI and
varying concentrations of carbon black. As shown in FIG. 2B, while
a solution comprising 0 vol % carbon black (squares) has a
viscosity that is constant over a range of shear rates, solutions
comprising 0.5 vol % (circles) or 1.5 vol % (triangles) carbon
black have viscosities that vary with shear rate. In some
embodiments of the invention, at a shear rate of 100 s.sup.-1, the
viscosity of the electrode may be less than about 10.sup.7 mPa-s,
less than about 10.sup.6 mPa-s, less than about 10.sup.5 mPa-s,
less than about 10.sup.4 mPa-s, less than about 1000 mPa-s, less
than about 800 mPa-s, less than about 500 mPa-s, less than about
200 mPa-s, less than about 100 mPa-s, less than about 50 mPa-s,
less than about 20 mPa-s, or less than about 15 mPa-s (and/or down
to about 10 mPa-s, or less). In some embodiments, the electrode may
exhibit characteristics of a Bingham plastic or fluid, e.g., a
material that behaves as a rigid body below a finite shear stress
(also referred to as the yield point) and as a viscous fluid above
the finite shear stress. That is, a Bingham plastic material
requires a shear stress exceeding the yield point before beginning
to flow. The yield point may be obtained, for example, using a
torsional rheometer and/or a capillary rheometer. In some cases,
the yield point may be obtained from a shear stress versus shear
rate flow curve using either a torsional rheometer or a capillary
rheometer. In some cases, the yield point may be obtained from an
oscillatory strain sweep conducted on a torsional rheometer. In
some embodiments, the yield point of the electrode composition may
be less than about 1 kPa, less than about 500 Pa, or less than
about 100 Pa, less than about 50 Pa, or less than about 10 Pa
(and/or down to about 1 Pa, or less). For example, yield points can
be observed in FIG. 2C, which shows exemplary log-log plots of
stress as a function of shear rate for electrolytes comprising 2.5
M Li.sub.2S.sub.8 dissolved in TEGDME with 0.5 M LiTFSI and varying
concentrations of carbon black. From FIG. 2C, the yield points of a
solution comprising 0.5 vol % carbon black (labeled as yield point
1) and 1.5 vol % carbon black (labeled as yield point 2). In some
embodiments, the fluid may behave in a substantially Newtonian
fashion above the yield point, exhibiting a constant viscosity
(also referred to as "plastic viscosity"). Plastic viscosity may be
a parameter of the Bingham model describing the rheology of Bingham
materials. In some cases, the plastic viscosity may be less than
about 2000 mPa-s, less than about 1000 mPa-s, less than about 800
mPa-S, less than about 500 mPa-s (and/or down to about 100 mPa-s,
or less). In some embodiments, the fluid may exhibit shear thinning
above the yield point (e.g., the viscosity of the fluid may
decrease with increasing shear rate). Shear thinning may be
observed, for example, in FIG. 2B, which shows that for
electrolytes comprising 0.5 vol % or 1.5 vol % carbon black,
viscosity decreases with increasing shear rate. FIGS. 2C and 2D,
which respectively show log-log and linear plots of stress as a
function of shear rate for electrolytes comprising 2.5 M
Li.sub.2S.sub.8 dissolved in TEGDME with 0.5 M LiTFSI and varying
concentrations of carbon black, show the behavior of electrolytes
comprising 0.5 vol % or 1.5 vol % carbon black above their
respective yield points.
[0056] The electrode may be electronically conductive while in its
flowing and/or non-flowing states. Electronic conductivity may be
measured, for example, by measuring direct current resistance.
Alternatively, electronic conductivity may be measured by measuring
alternating current impedance. The alternating current impedance
may be measured, for example, using a cylindrical cell capped by
metal plates, where the metal comprises stainless steel, nickel,
gold-coated stainless steel, and/or gold-coated nickel. In some
embodiments, the electronic conductivity of the electrode is at
least about 0.1 mS/cm, at least about 1 mS/cm, at least about 2
mS/cm, or at least about 5 mS/cm (and/or up to about 10 mS/cm). In
some cases, the electronic conductivity of the electrode is in the
range of about 0.1 mS/cm to about 5 mS/cm, about 0.1 mS/cm to about
10 mS/cm, about 1 mS/cm to about 5 mS/cm, about 1 mS/cm to about 10
mS/cm, or about 5 mS/cm to about 10 mS/cm.
[0057] The electrode may be ionically conductive while in its
flowing and/or non-flowing states. Ionic conductivity may be
determined, for example, by measuring high-frequency impedance
using an alternating current method, taking the real component of
the high-frequency impedance, and correcting by a geometric cell
factor. The geometric cell factor may be determined analytically
and/or by experimental calibration. In some embodiments, the ionic
conductivity of the electrode is at least about 0.1 mS/cm, at least
about 1 mS/cm, at least about 2 mS/cm, or at least about 5 mS/cm
(and/or up to about 10 mS/cm). In some cases, the ionic
conductivity of the electrode is in the range of about 0.1 mS/cm to
about 5 mS/cm, about 0.1 mS/cm to about 10 mS/cm, about 1 mS/cm to
about 5 mS/cm, about 1 mS/cm to about 10 mS/cm, or about 5 mS/cm to
about 10 mS/cm.
[0058] In some embodiments, the electrode further comprises a
binder. In other embodiments, the electrode is substantially free
of a binder. For example, in some cases, the concentration of a
binder in the electrode may be less than about 5 wt % or less than
about 2 wt % (and/or down to about 1 wt %, or less). As used
herein, a binder may refer to a non-electroactive material that is
neither electronically conductive nor ionically conductive.
[0059] Some aspects of the invention relate to an energy storage
device comprising a first electrode and a second electrode in
electrochemical communication with the first electrode. The first
electrode may, in some cases, be flowable. In some embodiments, the
first electrode is a positive electrode (i.e., a cathode). In
certain cases, the positive electrode comprises a sulfur or metal
sulfide electroactive material suspended and/or dissolved in a
fluid, and electronically conductive particles suspended and/or
dissolved in the fluid, the electronically conductive particles
forming a percolating conductive network. In some embodiments, the
second electrode is a negative electrode (i.e., an anode). The
negative electrode may comprise, in some cases, an electroactive
metal or a metal alloy. Non-limiting examples of suitable metals
include lithium, sodium, magnesium, calcium, aluminum, zinc,
manganese, titanium, and/or iron. In some cases, the negative
electrode comprises a non-sulfur element of the positive electrode.
The negative electrode may store the working ion(s) of the energy
storage device at a more negative electrical potential than the
same ion(s) in the first electrode.
[0060] In some embodiments, the energy storage device may comprise
first and second current collectors for the first and second
electrodes. The current collectors may be made of materials
selected to be stable at the operating potentials of the first and
second electrodes. Non-limiting examples of suitable current
collector materials include aluminum, titanium, nickel, copper,
gold, and carbon, including fibrous carbon and/or carbon felt.
[0061] The energy storage device may, additionally, comprise an
ion-permeable separator between the first and second electrode
compartments. The ion-permeable separator can include any suitable
medium capable of allowing the working ion(s) of the energy storage
device to be passed through it. In some embodiments, the
ion-permeable medium comprises a membrane. The membrane can be any
conventional membrane that is capable of ion transport. In some
embodiments, the membrane may be a liquid-impermeable membrane that
permits the transport of ions therethrough, namely a solid or gel
ionic conductor. In other embodiments the membrane is a porous
polymer membrane infused with a liquid that allows for the
shuttling of ions between the first and second electroactive
materials, while preventing the transfer of electrons. In some
embodiments, the membrane is a microporous membrane that prevents
particles forming the positive and negative electrode flowable
compositions from crossing the membrane. Exemplary membrane
materials include polyethyleneoxide (PEO) polymer in which a
lithium salt is complexed to provide lithium conductivity, or
Nafion membranes, which are proton conductors. For example,
PEO-based electrolytes can be used as the membrane, which is
pinhole-free and a solid ionic conductor, optionally stabilized
with other membranes such as glass fiber separators as supporting
layers. PEO can also be used as a slurry stabilizer, dispersant,
etc. in the positive or negative flowable redox compositions.
[0062] The energy storage device may further comprise a layer
adjacent at least one of the electrodes. The layer may, for
example, prevent one of the electrodes from coming into direct
contact with an electrolyte, and the layer may permit the passage
of ions but not electrons. Advantageously, the layer may provide
controlled passivation of the electrode against oxidation reactions
(at the positive electrode) or reduction reactions (at the negative
electrode) that could potentially consume working active material,
increase the impedance of the electrodes, introduce safety issues,
and/or degrade the electrolyte. In some embodiments, the layer
comprises a metal salt. A non-limiting example of a metal salt is
LiNO.sub.3. In some embodiments, the layer comprises a ceramic. One
of ordinary skill in the art would understand a ceramic to comprise
an inorganic solid, such as an oxide, a nitride, and/or a carbide.
In some embodiments, the ceramic is a solid electrolyte, an
ion-conducting glass, and/or an ion-conducting crystal. In some
embodiments, the ceramic is a lithium-phosphorus-oxynitride
compound. In some cases, the layer comprises a polymer. One of
ordinary skill in the art would understand a polymer to refer to a
molecule comprising two or more monomer units covalently linked
together. In some embodiments, the polymer is a solid polymer
electrolyte (SPE). In some embodiments, the SPE is a
block-copolymer.
[0063] Certain aspects of the invention may relate to methods of
operating the energy storage device. In some embodiments, the
energy storage device may be reversibly cycled over a voltage range
of from about 0.45 V to about 2.7 V, from about 0.7 V to about 2.7
V, and from about 1.6 V to about 2.5 V. During the reversible
cycling, a metal sulfide may be precipitated and subsequently
dissolved. In particular embodiments, the metal sulfide that is
precipitated and dissolved may be Li.sub.2S. During cycling, the
electrochemical cell may have a specific capacity (per gram of
sulfur) of at least about 500 mAh/g, at least about 1000 mAh/g, or
at least about 1200 mAh/g.
[0064] In some cases, the flowable electrode of the energy storage
device may be in motion during at least a portion of the cycling.
The flowable electrode may, in some embodiments, be subjected to
continuous flow. In some cases, the flowable electrode may be
subjected to intermittent flow. It has been discovered that the
performance of energy storage devices can be enhanced by
transporting multiple portions of an electrode composition (e.g.,
"plugs" of electrode composition) to the energy storage device, and
charging and/or discharging the portions of the electrode
composition in succession. Such an arrangement can be achieved, for
example, by intermittently transporting first, second, and/or more
portions of electrode compositions to an electrode compartment and
operating the device such that the portion of the electrode
composition proximate the electrode compartment is
electrochemically isolated from upstream and downstream portions of
the electrode composition.
[0065] Accordingly, in some embodiments, a first portion of an
electrode composition can be transported into an electrode
compartment, after which, the flow of the electrode composition can
be inhibited. During and/or after inhibiting the flow of the
electrode composition, the first portion of electrode composition
can be at least partially charged and/or discharged while, at the
same time, a second portion of the electrode composition,
fluidically connected to the first portion via an open flow
pathway, is not substantially charged and/or discharged. In some
embodiments, after at least partially charging and/or discharging
the first portion of electrode composition, the flow rate of the
electrode composition can be increased. Increasing the flow rate of
the electrode composition can result in the first portion of
electrode composition being transported out of the electrode
compartment and the second portion of electrode composition being
transported into the electrode compartment. In some embodiments,
after the second portion of electrode composition has been
transported into the electrode compartment, flow of the electrode
composition can be inhibited a second time. During and/or after
inhibiting flow a second time, the second portion of electrode
composition can be charged and/or discharged while the first
portion (and/or a third portion) of the electrode composition is
not substantially charged and/or discharged. Operation of the
device can continue in this manner for any number of portions of
the electrode composition. When operated in this manner, multiple
"plugs" of electrochemically isolated, electrode composition can be
locally charged and/or discharged and, during each charging and/or
discharging step, other portions of the electrode composition are
not substantially charged and/or discharged.
[0066] The systems, articles, and methods described herein can
provide a variety of advantages over other energy storage devices.
As one example, the use of intermittent flow (as opposed to
continuous flow) can reduce the amount of energy required to
transport the fluid through the electrode compartment to achieve a
given state of charge. For example, in some embodiments,
substantially all of the electrode composition can be charged
and/or discharged to a given state of charge by intermittently
transporting the electrode composition once through the electrode
compartment. In some such cases, the use of a continuous flow
regime might require the same fluid to be circulated through the
electrode compartment multiple times (e.g., 2, 3, 4, 5, or more
times) in order to achieve the same state of charge.
[0067] An international PCT application entitled "Metal Sulfide
Electrodes and Energy Storage Devices Thereof," filed Feb. 4, 2014,
is incorporated herewith in its entirety.
[0068] The following examples are intended to illustrate certain
embodiments of the present invention, but do not exemplify the full
scope of the invention.
Example 1
[0069] In high ionic strength solutions such as liquid
electrolytes, nanoscale conductor particles were observed to
exhibit diffusion-limited aggregation (DLA) and form percolating
networks at remarkably low concentrations, as low as 1 vol %. In
several such electrolyte-nanoconductor suspensions using both
nonaqueous and aqueous electrolytes, electronic conductivities of
5-10 mS/cm were obtained, providing good mixed electronic-ionic
conductivity in a fluid of relatively low viscosity. A new design
concept for flow battery electrodes used such suspensions to form
an "infinite current collector" network in flowable redox
solutions, allowing electrochemical reaction to be carried out
throughout the volume of a flow electrode (FIG. 3b) rather than
only at the interface between flowing redox solution and stationary
porous current collector as in a conventional flow battery (FIG.
3a). This principle was applied to lithium polysulfide redox
electrodes, with surprising results, an example of which is shown
in FIG. 3 (lower right). For identical solutions (2.5 M
Li.sub.2S.sub.8) and cell geometry and electrochemical test
conditions (C/4), a nanoconductor suspension (1.5 vol % carbon)
exhibited about 5 times the capacity of the conventional flow cell
design (6 vol % carbon felt current collector) and reached the
theoretical capacity of the solution (1200 mAh/g). The
nanoconductor suspension also exhibited reduced polarization
compared to the conventional flow cell.
[0070] It was noted that the discharge curve of the Li--S system
has two distinct regions (FIG. 3, upper right): a higher voltage
region with sloping voltage (2.5-2.1 V) over which sulfur is fully
dissolved as soluble polysulfides, and a lower voltage plateau
(.about.2.0 V) where the discharge reaction proceeds via
precipitation of insoluble lower order polysulfides Li.sub.2S.sub.2
and Li.sub.2S. FIG. 3 shows that the conventional porous carbon
felt current collector delivered capacity primarily in the solution
region. However, the capacity and energy density of the flow
battery can be increased if the plateau region is utilized as well.
Although many literature studies showed that the reaction is poorly
reversible in this regime, the experiments and analysis below
demonstrated that the "infinite current collector" approach allowed
both higher capacity and significantly improved reversibility when
cycled to include the precipitation regime.
"Infinite Current Collector" had 10.sup.3 Times Higher
Charge-Transfer Area
[0071] FIG. 4 shows the microstructure of the carbon felt (SGL, a
typical current collector for aqueous flow batteries) at low (left)
and high (right) magnification. The individual carbon fibers had
diameters of about 6 micrometers, giving a geometrically estimated
specific surface area of 0.33 m.sup.2 g.sup.-1. The felt was
comprised of 6 vol % carbon, thus the total carbon surface area per
unit volume was approximately 40.times.10.sup.-3 m.sup.2 mL.sup.-1.
FIG. 5 shows the microstructure of a Ketjenblack suspension at 1.5
vol % loading in a polysulfide solution in TEGDME, viewed in a
scanning electron microscope (SEM) using a "wet cell" (left), and a
representative primary aggregate in the dry state (right). The
Ketjen carbon black had a BET surface area of 1400 m.sup.2g.sup.-1,
and for a 1.5 vol % suspension, the carbon surface area per unit
volume was 42 m.sup.2 mL.sup.-1. Thus the maximum interfacial area
available for charge transfer was about 3 orders of magnitude
greater for the suspension than for the carbon felt.
[0072] The ability to increase electroactive area by 10.sup.3 while
providing sufficient electronic conductivity (discussed below) and
retaining flowability benefitted electrochemical kinetics in both
the solution and precipitation regimes of the Li--S reaction. In
the solution regime, where both the suspension and felt were
rate-limited by charge-transfer kinetics, the suspension had better
rate capability because the required interfacial current density
was three orders of magnitude less than that needed for the
stationary current collector. On the precipitation plateau, the
greater surface area afforded by the suspension allowed an
equivalent volume of precipitated Li.sub.2S.sub.2 and Li.sub.2S to
deposit as a thinner layer, theoretically 10.sup.-3 the thickness,
on the nanoscale network than on the carbon felt. The poor
reversibility in this regime had been widely attributed to the
highly insulating nature of Li.sub.2S.sub.2 and Li.sub.2S. As shown
below, impedance analysis was consistent with markedly reduced
charge transfer resistance. In addition, the thinner deposition of
precipitate did not inhibit flow in the suspension case, since the
current collector and redox solution flowed together. This is
especially true as solution concentrations and energy densities
increase.
Electrochemical Kinetics for Solution-Phase Polysulfides at Carbon
Surfaces
[0073] In order to interpret the electrochemical kinetics of both
types of devices, the exchange current density at the
carbon-solution interface should be determined. These kinetics were
measured at "model" carbon surfaces using three-electrode cells,
carrying out three types of measurements: 1) steady-state cyclic
voltammetry at a carbon fiber microelectrode, 2) galvanostatic
polarization at a glassy carbon macroelectrode, and 3)
electrochemical impedance spectroscopy with a glassy carbon
macroelectrode. For concentrated Li.sub.2S.sub.8 solutions in
TEGDME, all three methods gave closely agreeing exchange current
densities of 0.01-0.02 mA cm.sup.-2. Table 1 summarizes the
exchange current densities measured in Li.sub.2S.sub.8 and
Li.sub.2S.sub.6 solutions by each of these measurement
techniques.
TABLE-US-00001 TABLE 1 Summary of measured exchange current
densities in units of mA cm.sup.-2 Macroelectrode Microelectrode
Macroelectrode Galvanostatic Steady-State Impedance Composition
Polarization Voltammetry Spectroscopy 2.5M Li.sub.2S.sub.8 0.011
0.011 0.022 2.5M Li.sub.2S.sub.6 0.017 Not measured 0.030
[0074] FIG. 6 shows Tafel plots obtained from galvanostatic
polarization and steady-state voltammetry experiments on 2.5 mol S
L.sup.-1 Li.sub.2S.sub.8 solutions in TEGDME with 0.5 M LiTFSI
supporting electrolyte. The exchange current density was determined
by extrapolating the linear portion of the Tafel plot back to the
intercept at the equilibrium potential.
[0075] FIG. 7 shows Nyquist (Cole-Cole) plots of the measured
impedance in Li.sub.2S.sub.8 and Li.sub.2S.sub.6 polysulfide
solutions prepared as 2.5 mol S L.sup.-1 in TEGDME with 0.5 M
LiTFSI supporting electrolyte. The solution resistances were nearly
identical, while the charge-transfer resistances differed by about
40%. The exchange current density was estimated from the
charge-transfer resistance using the relationship
R ct = RT nFAi 0 . ##EQU00001##
The impedance measurements were done in three-electrode cells with
a 3 mm glassy carbon working electrode and Li metal reference and
counter electrodes. Determining Rate-Limiting Mechanisms from
Impedance Spectroscopy
[0076] Assuming that the exchange current densities measured above
also applied to both the conventional carbon current collector and
the "infinite current collector" design, we were able to determine
the rate-limiting mechanisms using impedance spectroscopy.
Two-electrode Swagelok cells were used, containing the same 2.5 M
solution of Li.sub.2S.sub.8 in TEGDME with 0.5 M LiTFSI supporting
electrolyte, with one case having the 1.5 vol % Ketjen black
suspension and the other having the SGL carbon felt filling the
cell cavity as a current collector.
TABLE-US-00002 TABLE 2 Summary of impedance measurements on
Li/polysulfide Swagelok cells with carbon fiber and Ketjenblack
suspension cathodes. Solution Interfacial Charge Transfer
Resistance Resistance Resistance Sample (Ohms) (Ohms) (Ohms)
Suspension 16 79 85 Felt 11 79 430
As shown in Table 2, the solution resistance and interfacial
resistance had similar values in the two cells. The difference in
total cell impedance lay in the charge transfer resistance, which
in this case included impedance contributions from both the finite
electronic conductivity of the carbon network (whether felt or
suspension) and the Butler-Volmer kinetics of the electrochemical
reaction at the carbon surface. Taking exchange current density to
be 0.01 mA cm.sup.-2 for a 2.5 M Li.sub.2S.sub.8 solution as was
independently measured, the charge-transfer resistance due to
Butler-Volmer kinetics was expected to be 0.70 Ohms for the
suspension and 73 Ohms for the felt. These were much smaller than
the charge transfer resistances observed by impedance spectroscopy
in both cases (Table 2). Clearly, there were additional
contributions to the charge transfer resistance, which we believe
to be the electronic resistivity of the carbon current collector.
This interpretation was supported by independent measurements of
the electronic conductivity of the nanoconductor suspension in a
two-electrode cell with blocking (gold) electrodes, i.e., in the
absence of electrochemistry. The .about.11.5 mS cm.sup.-1
conductivity measured in this manner was close to the 10.41 mS
cm.sup.-1 conductivity obtained from the charge transfer arc in the
electrochemical cell. This indicated that the charge-transfer
resistance of the nanoscale suspension was almost entirely due to
the electronic conductivity of the network, even though this
conductivity was of similar magnitude to the ionic conductivity of
the electrolyte (i.e., we had an ideal mixed conductor). This in
turn implied that still higher rate capability could be obtained by
further increasing the electronic conductivity of the
suspension.
[0077] When discharging at a C/15 rate from composition
Li.sub.2S.sub.8 to Li.sub.2S, the nanoscale suspension required an
average current density at the carbon surface of only
1.7.times.10.sup.-5 mA cm.sup.-2, while the carbon felt needed to
draw 1.7.times.10.sup.-2 mA cm.sup.-2. Again using an exchange
current density of 0.01 mA cm.sup.-2, the suspension and
conventional cells required Butler-Volmer overpotentials of 0.04
and 41.5 mV, respectively. If a conformal Li.sub.2S of uniform
thickness is assumed to form on the carbon surface during discharge
from Li.sub.2S.sub.8 to Li.sub.2S, at present polysulfide
concentrations the layer thickness for the nanoscale suspension
would be 17 nm, while that for the carbon felt current collector
would be 17 micrometers.
Example 2
[0078] A new approach, broadly applicable to flowable redox
chemistries--including those which undergo
precipitation-dissolution reactions--whereby percolating networks
of nanoscale conductor particles are incorporated within fluid
electrodes, forming an embedded, self-healing current collector
enabling highly distributed electrochemical reactivity throughout
the electroactive zone of flow batteries, was demonstrated. In
contrast to conventional flow battery designs wherein the
electronically-insulating redox fluids undergo charge-transfer
reactions only upon contact with the stationary porous current
collector (FIG. 8c), the new scheme produced an electronically
conductive fluid (i.e., a "liquid wire", FIG. 8d) that acted as its
own current collector, providing for charge transfer to the
external circuit. The continuous nanoconductor network vastly
increased the available charge-transfer area while also reducing
the molecular diffusion length between electroactive sites. In the
high ionic strength environment of liquid electrolytes, nanoscale
carbon conductor particles underwent diffusion-limited aggregation
leading to electronic percolation at remarkably low volume
concentrations of <1%, and reached 5-20 mS/cm electronic
conductivity for only 1-3 vol % solids in non-aqueous electrolytes,
providing excellent mixed electronic-ionic conduction with
negligible impact on energy density. Lithium-polysulfide (Li--PS)
was used as a test system, and the new electrode architecture
allowed electrochemical utilization that was increased fivefold
over the same solutions used in a conventional flow cell
architecture, reaching near-theoretical reversible capacities at
practical current rates (e.g., corresponding to 4-15 h duration
stationary storage). A lithium-polysulfide flow battery operating
in both continuous-flow and intermittent-flow modes was
demonstrated for the first time.
Results
[0079] The last few years have seen renewed interest in
lithium-sulfur (Li--S) batteries, long considered an attractive
energy storage chemistry for its high theoretical energy density
(2567 Wh/kg and 2199 Wh/L for elemental Li and S as reactants) and
low active materials cost. However, only recently has the
adaptation of Li--S chemistry to flow batteries been considered.
The high solubility of polysulfides Li.sub.2S.sub.x in nonaqueous
electrolytes has historically been detrimental in Li--S batteries
as it provides a "shuttle" mechanism for internal self-discharge as
well as capacity loss due to incidental Li.sub.2S precipitation
within electrochemical cells. It has been suggested that the same
attributes could be exploited in flow batteries, with analysis
indicating low storage cost ($45/kWh for the raw materials) even
when electrochemical cycling is limited to the solution regime
between Li.sub.2S.sub.8 and Li.sub.2S.sub.4 where no precipitation
occurs (FIG. 9a). In contrast to the need to minimize polysulfide
solubility in a conventional Li--S electrolyte to decrease
self-discharge, a high polysulfide solubility is desirable to
increase the energy density and reduce the system-level cost of
flow batteries. Reversible cycling was demonstrated for 2.5 M to 5
M Li--PS catholyte solutions infiltrated into carbon paper current
collectors against self-passivated Li metal negatives electrodes in
a non-flowing "membraneless" cell configuration. In the present
work, we used similar Li--PS solutions in a half-flow cell against
Li metal negative electrodes, the catholyte solutions being
composed of Li.sub.2S.sub.8 dissolved in tetraethylene glycol
dimethyl ether (TEGDME) containing 0.5 M
bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) and 1 wt %
LiNO.sub.3 as the lithium electrode passivation additive. By
starting with Li.sub.2S.sub.8, the sulfur precipitation regime at
high charge voltage (FIG. 9a) was excluded, and the starting flow
electrodes had Li--PS purely in solution form. Only a microporous
separator film (Tonen Chemical Corporation) separated this
catholyte and the Li metal negative electrode. The theoretical
specific capacity of the solution upon discharging Li.sub.2S.sub.8
to the Li.sub.2S.sub.4 liquid solution limit was 210 mAh/g, while
that for discharging to Li.sub.2S precipitation was 1460 mAh/g
(FIG. 9a); at 2.5 mol S/L concentration, as used in our flow
experiments, the catholyte energy density (vs.) Li.sup.+/Li.sup.o)
was 34 Wh/L and 234 Wh/L for discharge to the solution and
precipitation limits, respectively. Clearly, it was desirable to
utilize storage capacity in the precipitation regime. (Herein, all
specific capacities given in mAh/g refer to capacity per gram of
sulfur.)
[0080] To implement the nanoconductor suspension approach, several
nanoscale carbons including carbon blacks and single- and
multi-wall carbon nanotubes (SWNTs and MWNTs) were dispersed in the
Li--PS solution, and their conductivities and rheological
properties measured, following methodologies developed in previous
work on semi-solid flow batteries.sup.5,18. For our purposes, the
ideal nanoscale conductor provides the highest electronic
conductivity at the lowest volume fraction and with the lowest
yield stress and viscosity. Amongst the nano-carbons tested, one
particular carbon black (Ketjenblack EC-600JD, AzkoNobel, hereafter
referred to as KB) met these criteria best. KB is a 1400 m.sup.2/g
nanoscale carbon with .about.30 nm primary particle size (TEM
images in FIG. 8). Surprisingly, electrical percolation producing 2
mS/cm electronic conductivity was observed at as low as 1 vol % KB,
increasing to 9 mS/cm by 1.5 vol % carbon and 18 mS/cm by 2 vol %
KB (inset of FIG. 9b). Since the room temperature ionic
conductivity of the Li--PS solution was 1.5 mS/cm, the suspensions
had the unusual characteristic of being mixed electronic-ionic
conductors in fluid form. Jamming behavior in suspensions of small
particles has been studied theoretically and experimentally; for
instance small amplitude oscillatory shear rheometry detects
mechanical percolation of micron-scale carbon particles in
nonaqueous media at volume concentrations of .about.5 vol %. This
is already much lower than the .about.30 vol % percolation
threshold for non-interacting like-sized spheres in
three-dimensions, and has been attributed to diffusion limited
aggregation (DLA) of strongly attracting particles into fractal
networks (i.e., "hit and stick" behavior). The still-lower
electrical percolation threshold observed here was attributed to
the combination of nanoscale particle size and high ionic strength
(true of any liquid electrolyte), which will further strengthen the
attractive DLA interactions by quenching Debye-Huckel electrostatic
double-layers that could produce interparticle repulsion. Limited
tests of solutions with and without LiTFSI showed a factor of two
higher dc conductivity when salt is used. Two-dimensional sections
observed in wet-cell SEM (FIG. 8b) were consistent with a
low-density three-dimensional solids network. Viscometry showed
that the 2.5 mol S/L solution had Newtonian rheology with 15 mPa s
viscosity, whereas the nanoparticle suspension based on the same
solvent with 1.5 vol % KB had Bingham plastic rheology with 64.6 Pa
yield stress and 710 mPa-s plastic viscosity. The nanoparticle
suspension had a rheological response at room temperature that was
qualitatively similar to ketchup, and it was readily pumped in the
flow battery experiments described later.
Electrode Tests in Non-Flowing Cells.
[0081] Electrochemical testing in half-cells of membraneless
configuration (i.e., using only the separator film to prevent
electrical contact of the two electrodes) showed that the in situ
nanoparticle conductor network markedly enhanced the
electrochemical utilization of the polysulfide solution. FIGS. 9c
and 9d show direct comparisons of the same 2.5 mol S/L polysulfide
solution in non-flowing half-cells with a carbon fiber current
collector, and with a carbon nanoparticle suspension. The carbon
fiber current collector used here and in the flow cells was a
low-flow-resistance non-woven "felt" (SGL GFD3, FIG. 8c) widely
used in aqueous flow batteries. For 3.0 mm-thick electrodes tested
at C/50 galvanostatic rate, the capacity of the nanoparticle
suspension was a factor of 4 greater than that of the conventional
configuration, reaching 1000 mAh/g specific capacity. For thinner
0.5 mm electrodes tested at a higher C/4 rate, the capacity of the
suspension electrode reached 1200 mAh/g (vs. 1460 mAh/g theoretical
capacity), about a factor of 5 greater than with the fiber current
collector. Note that the discharge curve for the Li--S system has
three distinct regions (FIG. 9a). There is a high voltage plateau
(.about.2.5 V) through which solid sulfur coexists with soluble
lithium polysulfides. This is followed by a solution regime with
sloping voltage (2.5-2.1 V) over which sulfur is fully dissolved as
soluble polysulfides, then a lower voltage plateau (.about.2.0 V)
where the discharge reaction proceeds via precipitation of the
insoluble Li.sub.2S. FIGS. 9c and 9d (showing the second cycle in
all cases) show that the conventional cell architecture delivered
capacity that was primarily in the solution regime. In contrast,
the nanoscale suspension had both higher capacity and significantly
improved reversibility when cells were cycled to include the
precipitation regime. This electrode format provided
correspondingly higher capacity and energy density in the same
cell. It is also possible to limit cycling to the solution regime.
As shown in FIG. 10, cycling of the nanoparticle suspension over
the 2.55 V-2.00 V solution regime gave 34% higher initial capacity
than the same solution used with the carbon fiber current
collector. For the suspension cycled over the solution regime,
stable cycling with 56% capacity retention after 100 cycles and 50%
retention after 500 cycles was seen. Cycling over the 1.90-2.50 V
range to include capacity enhancement from Li.sub.2S precipitation
gave fourfold higher initial capacity of 1200 mAh/g (FIG. 10). The
fade rate was also greater, yielding 610 mAh/g after 100 cycles, or
about 50% retention, but this level of capacity retention was in
fact superior to many published results for Li--S batteries using
solid sulfur cathodes. The coulombic efficiency exceeded 95% for
cycling within both the solution and precipitation regimes.
Reaction Kinetics and Contributions to Impedance.
[0082] Contributions to impedance, and the origin of the highly
facile reaction in the suspension electrodes, were deconvolved
using electrochemical impedance spectroscopy (EIS). The Nyquist
plot in FIG. 9b shows the large difference in cell impedance for
the two approaches, which was systematically investigated as
follows. We first measured the exchange current density at the
interface between carbon and the present Li--PS solutions, using
carbon fiber microelectrodes and glassy carbon macroelectrodes as
model current collectors. Three types of measurements, each with
three-electrode cells, were conducted: 1) steady-state cyclic
voltammetry at a carbon fiber microelectrode, 2) galvanostatic
polarization at a glassy carbon macroelectrode, and 3)
electrochemical impedance spectroscopy using a glassy carbon
macroelectrode. For the present Li.sub.2S.sub.8--TEGDME solutions,
the three methods gave exchange current densities in the range
0.011-0.030 mA cm.sup.-2. Measurements of otherwise identical
Li.sub.2S.sub.6 solutions (i.e., chemically prepared in a partially
discharged state) showed .about.40% higher exchange current
densities, while the solution conductivity was essentially
unchanged. This shows that as Li--S cells are discharged within the
solution regime, the exchange current density increases. To our
knowledge these were the first exchange current density
measurements for lithium-polysulfides on carbon electrodes.
[0083] Next, the EIS results in FIG. 9b were separated into three
major impedance contributions with the help of independent
measurements, using impedance spectroscopy in the absence of
electrochemistry with ion-blocking (gold) electrodes, of the ionic
conductivity of the Li--PS solution and the electronic conductivity
of the nanoscale suspensions (inset in FIG. 9b). As shown in Table
1, the solution resistance (obtained from the high frequency
intercept with the Z' axis) and interfacial resistance (the
left-most arc) had similar values in the two types of cells, as
expected. The difference in cell impedance was dominated by the
charge transfer resistance (right-most arc), which in this case
included impedance from the finite electronic conductivity of the
carbon network (whether fiber or suspension), as well as the
exchange current density (Butler-Volmer kinetics) at the carbon
surface. Based on the exchange current density of -0.01 mA
cm.sup.-2, the actual current densities and overpotentials can be
estimated as follows. The theoretically available
carbon-electrolyte interfacial area of the carbon fiber current
collector (FIG. 8c) was .about.0.33 m.sup.2g.sup.-1 based on the
.about.6 .mu.m fiber diameter. The fiber current collector occupied
6 vol % of the cell, giving a collector surface area per cell
volume of .about.40.times.10.sup.-3 m.sup.2 mL.sup.-1. FIG. 8c
shows the microstructure of the 1.5 vol % KB suspension, viewed in
an SEM using a "wet cell" with an electron-transparent window, and
a representative primary aggregate in the dry state. The carbon
nanoparticles comprised a BET area of .about.1400 m.sup.2 g.sup.-1,
which at 1.5 vol % provided carbon area per unit volume of -42
m.sup.2 mL.sup.-1. Thus the available interfacial area for charge
transfer was more than 10.sup.3 greater for the suspension than for
the carbon fiber collector, even though there was only one-fourth
as much carbon. Assuming all of the carbon-electrolyte interfacial
area to be active, and taking the exchange current density to be
0.01 mA cm.sup.-2 as independently measured, discharging at a C/15
rate from composition Li.sub.2S.sub.8 to Li.sub.2S corresponds to
an average current density at the carbon surface of only
1.7.times.10.sup.-5 mA cm.sup.-2 for the nanoscale suspension,
while the carbon fiber collector will need to draw
1.7.times.10.sup.-2 mA cm.sup.-2. The corresponding Butler-Volmer
overpotentials were 0.04 and 41.5 mV, respectively.
[0084] Further analysis of the charge transfer resistance for the
nanoscale suspension suggests that at such low exchange current
densities, the surface reaction kinetics may not be rate-limiting
at all. If Butler-Volmer kinetics were limiting, the charge
transfer resistance was computed to be 0.22 Ohm cm.sup.2 for the
suspension, which is much lower than the 3.8 Ohm cm.sup.2 observed
in EIS (Table 1), suggesting that the charge-transfer resistance of
the nanoscale suspension is dominated by the electronic
conductivity of the network. The electronic conductivity of the
nanoscale suspension is .about.10 mS cm.sup.-1 conductivity for the
1.5 vol % KB suspension (FIG. 9b), within a factor of two of that
computed from the EIS charge transfer resistance assuming
electronic conductivity to be the only contribution. This in turn
implies that further improvements in the electronic conductivity of
the nanoscale conductor suspensions could yield higher rate
capability. Improvements in solution-phase ionic conductivity may
also be expected to yield enhanced rate capability.
[0085] Within the precipitation regime, the poor reversibility of
conventional Li--S batteries has been widely attributed to the
highly insulating nature of Li.sub.2S. Here, we note that if a
conformal Li.sub.2S layer of uniform thickness is assumed to form
on the carbon surface during full discharge from Li.sub.2S.sub.8 to
Li.sub.2S, at 2.5 mole S/L concentration the layer thickness (based
on a molar volume of 2.768.times.10.sup.-2 L/mol for Li.sub.2S) is
1.7 nm for the nanoscale suspension, compared to 1.7 .mu.m for the
carbon felt current collector. Thus the superior capacity and
reversibility of the nanoscale suspension can be attributed to a
combination of two effects: ultra-low current densities at the
catholyte-carbon interface, and a much finer-scale precipitation of
Li.sub.2S. Topologically, the embedded, self-healing current
collector is in some ways the inverse of the mesoporous carbon
hosts for sulfur that have shown promise in conventional Li--S
batteries. The current conductive, flowable electrode approach may
have benefits for non-flowing metal-sulfide batteries as well, as
illustrated by the cell results in FIGS. 9c and 9d.
Flow Cell Tests.
[0086] Tests performed in a lab-scale half-flow cell (FIG. 8b)
showed that the vastly improved charge transfer kinetics of the
nanoscale suspensions were retained under two types of flowing
conditions: 1) continuous flow, which is the typical operating mode
of solution-based flow batteries; and 2) intermittent flow, where
fluid is pumped in discrete volumes with complete charging or
discharging of the electroactive region occurring in between.
Intermittent flow has been shown to produce higher round-trip
energy efficiency for significantly non-Newtonian redox fluids.
[0087] To demonstrate cycling under continuous flow, automated
digitally-controlled syringe pumps were used with the cell in FIG.
8b. The ratio of cell volume to pumped volume was 1:4, and the
total volume (80 microliters) passed through the cell in 100 min.
(i.e., for a flow battery with a single cathode tank, the entire
tank volume circulated once every 100 min.). Results for the
nanoscale suspensions appear in FIG. 11. During the first 1.5
cycles, the cell was discharged and charged potentiostatically
between 2.1 V and 2.5 V (FIG. 11) to stay in the solution regime.
Since the Li--PS battery was assembled in the charged state, the
first half-cycle discharged the cell, whereas the relevant
coulombic and energy efficiency for a storage battery was that
occurring upon charging and discharging. Efficiencies were
therefore only reported for subsequent cycles where there was a
full charge/discharge sequence. During the potentiostatic
charge/discharge cycle in the solution regime, the discharge
capacity was 181 mAh/g, the coulombic efficiency was 101% (the
higher-than-theoretical value is due to diffusive exchange of
charged cathode at the edges of the electroactive zone), and the
round-trip energy efficiency was 84.7%. Operating in this regime,
the energy density of the flow cathode alone was 30.4 Wh/L
(specific energy 380 Wh/kg). To evaluate the energy density of the
electrochemical couple, a reasonable excess of Li metal must be
assumed. Herein we will assume 100% excess of Li relative to the
actual capacity of the cell, in which case the
active-materials-only energy density in the solution regime is 26.7
Wh/L (specific energy 334 Wh/kg).
[0088] A stepped potentiostatic protocol was then applied between
voltage limits of 2.5 V-1.6 V, as shown for the intermediate cycles
in FIG. 11. This cycled the flow cathode into the precipitation
regime, more than doubling the capacity compared to the solution
regime. The step sequence was selected to provide approximately
constant overpotential relative to the equilibrium cell voltage;
more sophisticated feedback-based control algorithms that control
overpotential were possible. For the third complete
charge/discharge cycle, the discharge capacity was 393 mAh/g, and
coulombic and energetic efficiencies were 95.6% and 75.7%,
respectively. Under this potentiostatic protocol, the net cycling
rate for the entire system was C/15 on charge and C/22 on
discharge. The cathode discharge energy density was 61 Wh/L (760
Wh/kg), and the cathode plus lithium energy density was 46.7 Wh/L
(584 Wh/kg). Finally, the cell was cycled potentiostatically
between the same voltage limits, but without intermediate potential
steps. This increased the system-level C-rate to C/10 for charge
and C/15 for discharge, and correspondingly, the capacity was
slightly lower at 376 mAh/g in cycle 5, and energy efficiency
decreased to 63%. The reduced energy efficiency was to be expected
for this potentiostatic protocol due to the high overpotential
relative to equilibrium cell voltage that was applied across most
of the state of charge range. The active-materials-only discharge
energy density was 44.1-48.1 Wh/L (552-602 Wh/kg). During cycles
5-14, despite some capacity fade as shown in the inset in FIG. 11,
the coulombic efficiency remained above 99.5%. This suggests that
impedance growth rather than loss of storage capacity in the flow
cathode was responsible for the capacity fade. It would not be
surprising for impedance growth to take place at the stationary Li
metal negative electrode, given the large areal capacity that was
reversibly plated throughout this experiment. Upon increasing
charge time by .about.10% in cycle 15 (not shown), we found that
the discharge capacity was restored, consistent with an impedance
limitation.
[0089] Intermittent flow cycling was then demonstrated using the
same cell geometry and nanocarbon suspension, with results
appearing in FIG. 12. A total of five cell volumes was flowed
through the cell during a complete charge-discharge cycle, with
each flow pulse having a volume that was one-half the cell volume
(i.e., ten intermittent pumping pulses were used during each
discharge and charge). Fluid-mechanical analysis suggested that the
velocity profile in the flow cell was highly non-uniform, based on
the suspension's rheology, pulse flow rate (30 .mu.L/s), and
channel design. Accordingly, the volume of intermittent flow pulses
was chosen to minimize the coulombic and energetic inefficiencies
that were induced by flow non-uniformity. After each pumping step,
the cell was galvanostatically discharged or charged, testing the
solution and precipitation regimes with voltage limits of 2.55
V-1.95 V and 2.8 V-1.6 V, respectively. The capacity axis in FIG.
12 gives the specific capacity with respect to all sulfur in the
system, assuming a uniform state-of-charge.
[0090] FIG. 12a shows the discharge-charge curve for the
solution-only case, where the discharge and charge capacities were
162 mAh/g and 172 mAh/g, respectively, and the total discharge rate
for the tank was C/5. The voltage-efficiency under these conditions
(84% at C/5) exceeded that of potentiostatic, continuous-flow
cycling at lower effective rate (78% at .about.C/10). This
performance demonstrated the ability of the intermittent flow mode
to cycle more efficiently than continuous flow. The simulated
charge-discharge curve under these conditions is shown in FIG. 12b.
The discharge voltage trends of the experiment and simulation
agreed well. An instantaneous jump in discharge voltage occurred
after each intermittent flow pulse, because fresh suspension, rich
in Li.sub.2S.sub.8, flowed into the cell's electroactive region.
The value of the voltage after the pulse (2.1-2.2 V) was
substantially less than the open-circuit potential of the
Li.sub.2S.sub.8 suspension (2.42 V). Voltage was reduced below this
value because the electroactive region was incompletely replenished
by the flow pulse. As a result, the solution phase of the
suspension was more highly charged near the cell's inlet
(nominally, Li.sub.2S.sub.8) than near the outlet (nominally,
Li.sub.2S.sub.4). The average discharge voltage of the experiment
(2.06 V) was in excellent agreement with the simulation (2.07 V),
suggesting that Li--PS s were locally in equilibrium during
discharge, as assumed in the model. The experimental charge-voltage
trends differed markedly from the idealized model, though, with a
70 mV deviation on average. While the simulation predicted linear
variation of voltage with time after an intermittent pulse, the
experiment exhibited sigmoidal variation of voltage. Recent in
operando analyses of speciation in Li--S systems suggested that
charge and discharge processes in the Li--S system proceed through
different reaction pathways. The present deviation of experimental
charge voltage from the simulation was consistent with these
observations.
[0091] When the voltage window was widened to include
precipitation, the discharge capacity increased about four-fold to
653 mAh/g (FIG. 12c), with the discharge rate decreasing to C/18.
These initial results demonstrated what we believe to be a higher
efficiency alternative to continuous flow mode operation that
reduces pumping losses, especially for higher energy density redox
electrodes with substantial yield stresses and strongly
shear-thinning rheology.
[0092] Thus we showed that by using nanoscale conductor suspensions
to form flowable electrically percolating networks, the
electrochemical reactivity of solution-based redox electrodes could
be greatly enhanced compared to that obtained with a conventional
carbon fiber current collector. In addition, this novel flow
electrode architecture allows redox solutions to be reversibly
cycled into composition regimes where solid precipitation occurs.
The nanoscale suspensions retain their electronic conductivity and
enhanced reactivity under flow, and can be regarded as
"self-healing." They may be used in flow cells that operate in
continuous or intermittent flow modes. Just 1.5 vol % added
nanoscale carbon allows Li--PS flow electrodes to operate in the
precipitation regime where the specific energy and energy density
increase by up to a factor of 5 compared to that obtained using
only the solution regime. One of the great attractions of the Li--S
system is the very low cost of the elemental reactants on an energy
basis. For a reversible capacity of 600 mAh/g and average discharge
voltage of 1.9V representing the present 2.5 mol S/L system, the
raw materials cost of the flow cathode was $21/kWh, and that of the
electrochemical couple (assuming 100% excess of Li metal) was only
$37/kWh, leaving ample room for other component cost while meeting
the $100/kWh target. We believe that with further development,
higher capacity utilization in higher molarity solutions is
achievable, which would further drive down the cost of storage. The
nanoconductor suspension architecture can clearly also be used for
anolytes, allowing a full flow battery as well as the half-flow
system demonstrated here.
Methods:
Preparation of Flow Electrodes
[0093] To prepare the lithium polysulfide solution, Li.sub.2S,
sulfur, LiTFSI, and LiNO.sub.3 were dried under vacuum for 24 h at
100.degree. C. and added to TEGDME and stirred for 24 h in an
Ar-filled glovebox, at 60.degree. C. To prepare the suspension,
carbon black (KetjenblackEC-600JD, AzkoNobel) was added to the
aforementioned solution, manually stirred, and then sonicated for
30 minutes. LiTFSI and LiNO.sub.3 were obtained from Sigma-Aldrich,
while the sulfur, Li.sub.2S, and TEGDME were obtained from Alfa
Aesar.
Microscopy Characterization
[0094] Scanning electron microscopy of the carbon felt was carried
out using a FEI/Philips XL30 environmental SEM with secondary
electron detector operating at 25 kV accelerating voltage.
Suspensions were imaged in QuantomiX WETSEM.RTM. QX-102 capsules
using the FEI/Philips XL30 with a backscattered electron detector
and accelerating voltages of 20-30 kV. The dry Ketjenblack powder
was imaged on a JEOL 2010F transmission electron microscope with an
accelerating voltage of 200 kV. Samples were prepared by dispersing
the carbon black on a Cu grid coated with an amorphous carbon
film.
Exchange Current Density Measurements
[0095] In the galvanostatic polarization experiment, the
polysulfide solution was vigorously stirred to ensure no
mass-transport limitation, while a specified current was drawn from
the cell using a 3 mm glassy carbon working electrode (CH
Instruments, Inc.) for 30 minutes. The corresponding potential was
determined as the average potential during the 30 minute
galvanostatic step. In the steady-state voltammetry experiments, an
11 .mu.m carbon-fiber microelectrode (BASi, Inc) was used as a
working electrode and the potential was swept cathodically at 20
mV/s from 3.24 V vs. Li.sup.+/Li to 1.24 V.
Cycling Experiments in Non-Flowing Half-Cells
[0096] Swagelok.RTM. type cells with 0.5 mm deep wells were used. A
Tonen separator soaked with electrolyte was used. All cell assembly
was performed inside an argon-filled glovebox.
Flow Cell Experiments
[0097] The lab-scale flow cell used for intermittent and continuous
flow experiments was machined from polyvinylidene fluoride (PVDF),
with an electroactive region machined from nickel and
sputter-coated with gold. The electroactive region was 20 mm long,
with square cross sections ranging from 1 mm to 2.8 mm width. For a
1 mm channel, the channel volume was 20 microliters, with an active
membrane area of 0.2 cm.sup.2. The cell was connected with
polytetrafluoroethylene (PTFE) tubing to gastight glass syringes
(Hamilton Model 1005), which were driven using syringe pumps
(neMESYS, Cetoni GmbH). Electrochemical testing was performed using
a Bio-Logic potentiostat.
EIS and IS Measurements
[0098] Electrochemical impedance measurements were made in
Swagelok-type cells (Swagelok Co.) with a PTFE body, cathode well
6.35 mm in diameter and 2.8 mm deep, and with Li metal as a counter
and reference electrode. The measurements were taken with
Li.sub.2S.sub.8 solutions, 2.5 mol S L.sup.-1 in TEGDME with 0.5 M
LiTFSI supporting electrolyte. The suspension had 1.5 vol %
Ketjenblack. The well was Au coated Ni. When characterizing the
solution electrode, 3 mm thick carbon felt was cut to fit the
cathode well and compressed to 2.8 mm thickness upon assembly. When
measuring the suspension electrodes, the suspensions were
spatula-loaded into the well and covered with a circular piece of
separator film. A circular film of lithium metal, attached to a
stainless steel current collector, was applied against the well by
light spring force. IS electronic and ionic conductivity
measurements were made in a similar two-electrode cells using
gold-sputtered current collectors on both sides. Measurements were
taken over the course of one to two hours.
[0099] The EIS measurements used the Solartron 1400 system with a
Solartron 1455 frequency response analyzer. The experiments were
performed with cells in the fully charged (as-assembled) state.
Sinusoidal voltage oscillations of 10 mV amplitude were applied
about the cell's open circuit voltage between the two electrodes.
Oscillation frequencies were swept logarithmically from 0.01 Hz to
1 MHz. The measured impedance data were fit to an equivalent
circuit by a complex nonlinear least squares regression. For the IS
measurements, a Solartron Analytical model 1470 potentiostat and
model 1455 frequency response analyzer were used to apply either a
+/-50 mV AC or DC bias. The DC measurements were taken at a steady
state value after a 15 minute hold. Resistances were converted to
conductivities by dividing by the experimental cell factor,
calibrated to be 1.2 cm.sup.-1 using a 15 mS/cm conductivity
standard produced by Oakton Instruments.
Rheological Characterization
[0100] The viscometric behavior of the different solutions and
semi-solid suspensions was measured using a Malvern Kinexus Pro
torsional rheometer enclosed in a glovebox with an Argon atmosphere
(H.sub.2O & O.sub.2 levels <0.1 ppm). Steady shear
viscometry tests, as well as, small amplitude oscillatory frequency
sweeps, were performed using a smooth parallel plate geometry (D=40
mm; mean roughness R.sub.q=0.36 micrometers). All tests were
performed at T=25.degree. C., and the temperature was regulated
with a Peltier plate system. Steady shear tests were performed with
decreasing applied shear rates to insure the existence of a simple
yield stress for the material and avoid possible transient shear
banding. In addition, the same sample was tested at three different
gaps (H=1 mm, 0.75 mm, 0.5 mm respectively) to probe and correct
for slip effects. If the flow curves at different gaps superimpose,
the material does not slip. If gap-dependent rheology is observed,
a correction needs to be applied to extract the true shear rate
applied on the sample at each value of the applied stress.
TABLE-US-00003 TABLE 1 Comparison of impedance contributions for
non-flowing Li/polysulfide cells using the same Li--PS solution
with a nanoscale conductor suspension and with a conventional
carbon fiber current collector. Solution Interfacial Charge
Transfer Resistance Resistance Resistance Sample (Ohm cm.sup.2)
(Ohm cm.sup.2) (Ohm cm.sup.2) Nanoconductor 5.1 16 3.8 Suspension
Carbon Fiber 3.5 31 125 Current Collector
Example 3
High Energy Density Semi-Solid Electrodes and Batteries Thereof
[0101] The energy density of storage electrodes and of batteries
using such electrodes were increased by combining redox active
solution electrodes with redox active solid electrodes, forming
suspensions of the latter in the former. In contrast to
conventional battery types such as the lithium ion battery, wherein
solid storage particles such as intercalation compounds are used
with a liquid electrolyte that provides for ion transport but
negligible storage, having typically only about 1M Li salt
concentration in the electrolyte, or batteries using solution
electrodes such as aqueous redox flow batteries, here the energy
density of electrodes was increased by using solid electrode
materials suspended in a redox-active solution electrode, forming a
semi-solid. Such electrodes were used in flow batteries, wherein
the semi-solid electrode was flowed through a stack in which the
electrochemical reaction took place, and in non-flowing battery
designs, where the semi-solid was substantially stationary during
use.
[0102] In some embodiments, the electrochemical activity of the
semi-solid electrode was improved by rendering it electronically
conductive. Electronic conductivity was provided to the semi-solid
electrode by incorporating within a percolating network of
electronically conductive species. Such species may have been the
storage particles themselves, and/or additive materials that were
electronically conductive. In some embodiments an electronically
conductive network was obtained by co-suspending carbon conductive
additives with the storage materials particles. In another
embodiment, electronic conductivity was obtained by carbon coating
the redox active solid particles, which then formed a percolating
network in the semi-solid.
[0103] In some embodiments, the solution electrode was a solution
of metal polysulfides, such as polysulfides of lithium, magnesium,
zinc, or other metals. In some embodiments the suspended solid
particles comprised sulfur, or were composites of sulfur and an
electronically conductive solid. Said electronically conductive
solids may have included but were not limited to carbon, a metal,
metal nitride, metal carbide, or metal oxide.
Lithium Metal/Polysulfide Electrolyte/Sulfur-Carbon (S--C)
Composite Cell
[0104] The cathode comprised a suspension of high performance S--C
composite (e.g., made by infiltrating molten sulfur into a
mesoporous carbon) and a conductive additive such as carbon
particulates, the solids being suspended in a polysulfide
redox-active solution. The polysulfide solution was made by adding
Li.sub.2S and S at a molar ratio of 1:7 into a mixture of dioxolane
(DOL) and dimethoxyethane (DME) at 1:1 ratio by volume. The mixture
was stirred at 50.degree. C., then optionally 0.5M lithium
bis(Trifluoromethanesulfonyl)imide (LITFSI) was added to increase
ionic conductivity. This semi-solid served as a high energy density
electrode in a battery.
[0105] In some embodiments, the anode of the battery was lithium
metal, which was pretreated by soaking in 0.5M LITFSI in DOL/DME
solution with 1 wt % LiNO.sub.3 added. In some embodiments, the
semi-solid cathode was non-flowing. In other embodiments, the
semi-solid cathode was a flow electrode in a flow battery. Such a
flow battery may have been a half-flow cell in which the anode was
a stationary electrode, including but not limited to lithium metal,
or a full flow cell, in which the anode was also a flow electrode
of solution or semi-solid type.
[0106] The energy densities of cathode/Li metal couples according
to Example 1 are summarized in Table 1. The capacity for Sulfur was
assumed to be 1664 mAh/g,k or 3327 mAh/cc. The energy density of
theoretical Li-Sulfur cell was 2550 Wh/kg, or 2792 Wh/L, assuming
the capacity of Li was 3842 mAh/g, or 2502 mAh/cc. 2 vol % carbon
conductive additive with a density of 2.27 g/cc was included for
computing the values in Table 1, and the sulfur density was assumed
to be 2 g/cc. The polysulfide redox electrode was taken to be a 5M
sulfur solution in DOL/DME=1:1 vol with a density of 1:12 g/cc. In
Table 1, it is seen that with no added S--C composite electrode,
the 5M Li polysulfide solution electrode alone provided a specific
energy of 418 Wh/kg and energy density of 460 Wh/L for the
polysulfide-Li metal couple. Upon adding just 20 volume % of the
solid storage material, the specific energy nearly tripled. Such
semi-solids could be easily prepared so as to be highly flowable,
and could therefore by used in flow battery architectures.
TABLE-US-00004 TABLE 1 The energy density of cathode-Li metal
couples in which the cathode had the specified volume percent of
S--C composite. "Semi-Solid" indicates a suspension electrode in
which the liquid phase did not include redox storage. Semi- Solid +
polysulfide indicate cathodes that were suspensions of the S--C
composite in a 5M polysulfide redox solution. S--C composite Volume
Semi-Solid + Semi-Solid + percent Semi-Solid Semi-Solid Polysulfide
polysulfide (%) (Wh/kg) (Wh/L) (Wh/kg) (Wh/L) 0 0 0 418 460 20 981
1105 1210 1324 40 1590 1775 1718 1884 60 2005 2226 2072 2278 80
2306 2549 2332 2568 98 2513 2770 2513 2772
Na Metal/NASICON Membrane/Na.sup.+, Fe(Cn).sub.6.sup.4+ Aqueous
Electrolyte+Na.sub.xMN(CN).sub.6 (M or N.dbd.Fe, Ni, Co, Mn, Cu,
Zn, Ti, V, Cr, Zr) Solid Storage Particles+Carbon Additive
[0107] In this example, Na metal was the anode, a NASICON membrane
was used to provide Na ion conduction while separating the anode
and cathode, and the cathode was a semi-solid that comprised a
redox active solution electrode of Na.sup.+, Fe(CN).sub.6.sup.4+,
in which was suspended particles of solid storage compound in the
family Na.sub.xMN(CN).sub.6 (M or N.dbd.Fe, Ni, Co, Mn, Cu, Zn, Ti,
V, Cr, Zr). Carbon nanoparticles were added in order to form a
percolating electronically conductive network.
Example 4
[0108] The next generation of low-cost, high-performance batteries
is designed and demonstrated by developing the structure processing
property performance relations for new battery couples. Two central
principles are that batteries should be cheaper (<$100/kWh) and
more energy dense (>500 Wh/L, >400 Wh/kg).
[0109] Several new battery chemistries were identified based on
low-cost couples of earth-abundant materials (FIG. 13).
Specifically, the Mg--S, Al--S, and Zn--S electrochemical couples
are attractive for low-cost, high energy density batteries. Sulfur
is cheap, abundant and light, providing these couples with high
theoretical energy density. Additionally, Mg, Al, and Zn metals can
all be used as negative electrodes, eliminating dilutive effects at
the negative electrode.
[0110] Sulfur is cheap and abundant, and many metal sulfide
formation reactions have high energy density, characteristics which
make sulfur a desirable positive electrode material. FIG. 14 shows
the theoretical energy density and raw materials costs of selected
sulfide formation reactions xM+yS.fwdarw.M.sub.xS.sub.y. The top
figures compare electrochemical couples requiring non-aqueous
electrolytes, while the bottom figures compare three couples which
could potentially use an aqueous electrolyte.
[0111] Among the non-aqueous couples, the Mg--S couple is
particularly attractive due to its high theoretical energy density
(>3000 Wh/L), low raw materials cost (<$1.50/k Wh), and the
demonstrated reversibility of metallic Mg electrodes in certain
electrolyte solutions. To contextualize this, the Mg--S couple has
lower raw materials cost than a lead acid battery and theoretical
energy density similar to the most advanced lithium-ion
couples.
[0112] The Li/S couple has been extensively researched for over 20
years, and is beginning to see limited commercialization, thanks in
large part to its tremendous specific energy (gravimetric energy).
While many challenges remain, several are particular to the lithium
metal electrode, and not applicable to the Mg--S system. Most
importantly, reversible and safe Mg metal electrodes have been
demonstrated in certain electrolyte solutions. The viability of a
metallic negative electrode opens the possibility of achieving high
cell-level energy density and also reduces the number of supporting
components used in a cell.
[0113] One challenge to developing a rechargeable Mg--S battery is
identifying an electrolyte solution that is stable with Mg metal
and sulfur. Certain components of the baseline Mg electrolytes are
chemically incompatible with sulfur; however, the solvents and the
electrochemically active salt are stable with sulfur. Two
additional challenges common to all sulfur-based couples include
shutting off the polysulfide shuttle, employing any of the
strategies developed for Li--S batteries, including use of a solid
(polymer or ceramic) electrolyte or surface coatings; and providing
an electronically conductive network in the sulfur electrode.
[0114] Equimolar mixtures of Mg[N(SO.sub.2CF.sub.3F).sub.2].sub.2
(also known as Mg(TFSI).sub.2) and a linear ether (glymes, e.g.
triglyme or tetraglyme) can be used as electrolyte solutions for
Mg--S batteries. These equimolar mixtures form complexes
[Li(glyme)].sup.+, and no free glyme is available to solvate
polysulfide species. There is evidence that the TFSI anion can be
stabilized against Mg metal electrodes, and glyme solvents do not
form a passivating layer of Mg metal. Alternatively, the prospects
are good for developing solid electrolytes; Mg-conducting gel
polymer electrolytes have been demonstrated with conductivities up
to 2.67 mS cm.sup.-1.
[0115] There are several other possible polyvalent sulfur systems
with low-cost and high energy density, as shown in FIG. 14. Recent
developments efforts toward Zn-ion, Zn-air, and Al-ion batteries
have demonstrated renewed interest in the zinc and aluminum metal
electrodes for secondary batteries; sulfur is an attractive
positive electrode that pairs with these metals to yield low-cost,
high energy density couples. Stable cycling of a metallic zinc
anode has been demonstrated in mild ZnSO.sub.4 or
Zn(NO.sub.3).sub.2 aqueous electrolytes. As the potential of a
sulfur electrode falls within the electrochemical stability window
of these aqueous environments (1.06 V vs. Zn.sup.2+/Zn), an aqueous
Zn--S system is a viable target. A non-aqueous Al--S system may be
with reach: metallic aluminum negative electrodes can cycle
reversibly in an ionic liquid electrolyte
(1-ethyl-3-methylimidazolium chloride w/AlCl.sub.3 salt).
[0116] Flow batteries use flowable electrodes ("catholyte" and
anolyte") to decouple the power (rate-capability) and energy
storage of a battery chemistry. This architecture provides greater
flexibility in designing large-scale energy storage applications
and reduces the relative fraction of cost coming from power-related
components. Aqueous flow batteries have been of interest for
stationary applications due to their scalability, safety, and
relatively low cost ($250-$450/k Wh depending on the specific
chemistry). However, the chemistries used in conventional aqueous
flow batteries have low energy density of .about.40 Wh L.sup.-1 for
the fluids (catholyte and anolyte) alone. New chemistries can
enable high-energy density flow batteries.
[0117] The high solubility of reduced polysulfide species
(S.sub.n.sup.-2 2 less than or equal to n less than or equal to 8)
in many non-aqueous solvents is a long-standing problem. Lithium
polysulfides Li.sub.2S.sub.nn can be dissolved at concentrations up
to 10 mol S/L in THF and DMSO. At a 10 M concentration, the
Mg-polysulfide system has a theoretical energy density of 833 WH/L,
more than twenty times greater than conventional aqueous flow
electrolytes. Even in a semi-flow configuration (one flowable
electrode, one stationary), the Mg-polysulfide system is attractive
for battery vehicle application: the active materials required for
a 23 k Wh pack--enough for 300 miles all-electric range--occupy 33
L, approximately half the volume of a gasoline fuel tank in a
passenger vehicle. These energy densities are several times higher
than those of the lower concentration (2-4 M) aqueous polysulfide
solutions which were explored as the anolyte of sodium
bromide--sodium polysulfide flow batteries.
Metal Sulfide Formation Reactions:
[0118] xM+yS.fwdarw.M.sub.xS.sub.y
Energetics estimated from reaction enthalpies (more accessible than
free energies). Cost analysis following the
Wadia/Albertus/Srinivasan methodology, and raw material prices from
most recent USGS Mineral commodity survey.
TABLE-US-00005 Theoretical Theoretical Raw Average specific energy
materials cell voltage energy density cost floor Chemistry (V)
(Wh/Kg) (Wh/L) ($/k Wh) Li.sub.2S 2.32 2700 4490 10.633 Na.sub.2S
1.90 1300 2420 0.297 Al.sub.2S.sub.3 1.12 1200 2430 0.959 ZnS 1.06
580 2400 2.803 MgS 1.79 1710 4570 1.381 MnS 1.11 680 2730 0.137
TiS.sub.2 1.05 1010 3250 1.165 FeS.sub.2 0.45 400 1950 0.856
Mg--S
[0119] Advantages over Li--S. 1) Mg metal electrode is more stable
than Li metal electrode. In principle, do not need to develop a
Mg-ion sulfur cell. 2) As a result, can achieve higher practical
energy density (Wh/L). 3) Potentially much lower cost--raw
materials.about.1/8 as costly.
[0120] Challenges: 1) Identify an electrolyte solution that is
stable with both Mg metal and sulfur (Aurbach's baseline
electrolytes are incompatible with sulfur); 2) Address polysulfide
solubility; 3) Provide electronic conductivity to make sulfur
electrode active.
[0121] Possible electrolyte solutions: equimolar Mg(TFSI).sub.2
salt-glyme complex solutions. TFSI anion is stable with respect to
Mg metal. Glyme solutions do not form passivating layer on Mg
metal.
Zn--S
[0122] Zn electrode can cycle in a number of different
electrolytes, including a mild acidic aqueous electrolyte
(pH.about.4). The potential window for a hypothetical aqueous Zn--S
battery sits within the stability window of an aqueous
electrolyte.
Al--S
[0123] It may be possible to couple the Al anode to a S
cathode.
Polysulfide Catholyte Flow Battery
[0124] Polysulfide solubility has been a key problem preventing
practical implementation of Li/S batteries. The idea here is to
flip that problem on its head and embrace that high solubility. A
back of the envelope calculation goes as follows: Assume you can
get 7/8 of the energy density of the full metal-sulfur reaction by
cycling the polysulfides (sulfur starts as S.sub.8 rings which are
reduced downward to the eventual M.sub.xS product) but you don't go
all the way to insoluble products (e.g. Li.sub.2S or MgS). If you
can get 10 M polysulfide concentrations, this puts you at 833 Wh/L
flowable catholyte energy density for the hypothetical Mg system.
It would be advantageous to do this with a stable cycling metallic
anode (i.e. not Li).
Sulfide Displacement/Conversion Cathodes
[0125] M.sub.xS.sub.y+yN.fwdarw.xM+yNS
With, e.g., Mg working ion FeS.sub.2+2Mg.fwdarw.Fe+2 MgS -520
kJ/mol Average voltage=1.35 V MnS.sub.2+2Mg.fwdarw.Mn+2 MgS -485
kJ/mol Average voltage=1.26 V MnS+Mg.fwdarw.Mn+MgS -132 kJ/mol
Average voltage=0.68 V ZnS+Mg.fwdarw.Zn+MgS -141 kJ/mol Average
voltage=0.73 V Li/FeS.sub.2 is a primary cell chemistry that
operates by a two-step reaction
2Li+FeS.sub.2.fwdarw.Li.sub.2FeS.sub.2 intercalation
2Li+Li.sub.2FeS.sub.2.fwdarw.Fe+2Li.sub.2S conversion
Example 5
[0126] A Mg-sulfide electrode and battery according certain aspects
of the invention was demonstrated as follows. The electrode was
made from the following starting materials: magnesium sulfide (MgS)
and elemental sulfur (S) were mixed in a stoichiometric ratio of
1:5 and dissolved in 10 mL of triglyme by stirring for 12 h in an
argon-filled glovebox. The resulting concentration of dissolved
sulfur in the solution was 1M. Next, supporting electrolyte salt
was dissolved in the solution, the salt being a 1:1 molar mixture
of MgCl.sub.2 and AlCl.sub.3, i.e. a salt complex of MgAlCl.sub.5.
This salt was added to achieve a supporting electrolyte
concentration of 0.5M. To the resulting catholyte solution, 2 vol %
carbon black was added to achieve high electronic conductivity. The
resulting suspension was mixed by hand and sonicated for a period
of 1 h. The resulting electrode was electrochemically tested in a
Swagelok cell, using stainless steel current collectors. The
electrode was contained in a well with a diameter of 8 mm and a
depth of 500 microns. The negative electrode was a disk of
elemental magnesium metal with a diameter of 12 mm. The cell was
cycled at constant C-rates of C/20, C/10, C/5, C/2, etc. between
voltage limits of 0.5 V and 2.2 V.
[0127] While several embodiments of the present invention have been
described and illustrated herein, those of ordinary skill in the
art will readily envision a variety of other means and/or
structures for performing the functions and/or obtaining the
results and/or one or more of the advantages described herein, and
each of such variations and/or modifications is deemed to be within
the scope of the present invention. More generally, those skilled
in the art will readily appreciate that all parameters, dimensions,
materials, and configurations described herein are meant to be
exemplary and that the actual parameters, dimensions, materials,
and/or configurations will depend upon the specific application or
applications for which the teachings of the present invention
is/are used. Those skilled in the art will recognize, or be able to
ascertain using no more than routine experimentation, many
equivalents to the specific embodiments of the invention described
herein. It is, therefore, to be understood that the foregoing
embodiments are presented by way of example only and that, within
the scope of the appended claims and equivalents thereto, the
invention may be practiced otherwise than as specifically described
and claimed. The present invention is directed to each individual
feature, system, article, material, kit, and/or method described
herein. In addition, any combination of two or more such features,
systems, articles, materials, kits, and/or methods, if such
features, systems, articles, materials, kits, and/or methods are
not mutually inconsistent, is included within the scope of the
present invention.
[0128] All definitions, as defined and used herein, should be
understood to control over dictionary definitions, definitions in
documents incorporated by reference, and/or ordinary meanings of
the defined terms.
[0129] The indefinite articles "a" and "an," as used herein in the
specification and in the claims, unless clearly indicated to the
contrary, should be understood to mean "at least one."
[0130] The phrase "and/or," as used herein in the specification and
in the claims, should be understood to mean "either or both" of the
elements so conjoined, i.e., elements that are conjunctively
present in some cases and disjunctively present in other cases.
Multiple elements listed with "and/or" should be construed in the
same fashion, i.e., "one or more" of the elements so conjoined.
Other elements may optionally be present other than the elements
specifically identified by the "and/or" clause, whether related or
unrelated to those elements specifically identified. Thus, as a
non-limiting example, a reference to "A and/or B", when used in
conjunction with open-ended language such as "comprising" can
refer, in one embodiment, to A only (optionally including elements
other than B); in another embodiment, to B only (optionally
including elements other than A); in yet another embodiment, to
both A and B (optionally including other elements); etc.
[0131] As used herein in the specification and in the claims, "or"
should be understood to have the same meaning as "and/or" as
defined above. For example, when separating items in a list, "or"
or "and/or" shall be interpreted as being inclusive, i.e., the
inclusion of at least one, but also including more than one, of a
number or list of elements, and, optionally, additional unlisted
items. Only terms clearly indicated to the contrary, such as "only
one of" or "exactly one of," or, when used in the claims,
"consisting of," will refer to the inclusion of exactly one element
of a number or list of elements. In general, the term "or" as used
herein shall only be interpreted as indicating exclusive
alternatives (i.e. "one or the other but not both") when preceded
by terms of exclusivity, such as "either," "one of," "only one of,"
or "exactly one of." "Consisting essentially of," when used in the
claims, shall have its ordinary meaning as used in the field of
patent law.
[0132] As used herein in the specification and in the claims, the
phrase "at least one," in reference to a list of one or more
elements, should be understood to mean at least one element
selected from any one or more of the elements in the list of
elements, but not necessarily including at least one of each and
every element specifically listed within the list of elements and
not excluding any combinations of elements in the list of elements.
This definition also allows that elements may optionally be present
other than the elements specifically identified within the list of
elements to which the phrase "at least one" refers, whether related
or unrelated to those elements specifically identified. Thus, as a
non-limiting example, "at least one of A and B" (or, equivalently,
"at least one of A or B," or, equivalently "at least one of A
and/or B") can refer, in one embodiment, to at least one,
optionally including more than one, A, with no B present (and
optionally including elements other than B); in another embodiment,
to at least one, optionally including more than one, B, with no A
present (and optionally including elements other than A); in yet
another embodiment, to at least one, optionally including more than
one, A, and at least one, optionally including more than one, B
(and optionally including other elements); etc.
[0133] It should also be understood that, unless clearly indicated
to the contrary, in any methods claimed herein that include more
than one step or act, the order of the steps or acts of the method
is not necessarily limited to the order in which the steps or acts
of the method are recited.
[0134] In the claims, as well as in the specification above, all
transitional phrases such as "comprising," "including," "carrying,"
"having," "containing," "involving," "holding," "composed of," and
the like are to be understood to be open-ended, i.e., to mean
including but not limited to. Only the transitional phrases
"consisting of" and "consisting essentially of" shall be closed or
semi-closed transitional phrases, respectively, as set forth in the
United States Patent Office Manual of Patent Examining Procedures,
Section 2111.03.
* * * * *