U.S. patent application number 16/814734 was filed with the patent office on 2020-11-05 for process for metal-sulfur battery cathode containing humic acid-derived conductive foam.
This patent application is currently assigned to Global Graphene Group, Inc.. The applicant listed for this patent is Global Graphene Group, Inc.. Invention is credited to Bor Z. Jang, Aruna Zhamu.
Application Number | 20200350559 16/814734 |
Document ID | / |
Family ID | 1000004974669 |
Filed Date | 2020-11-05 |
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United States Patent
Application |
20200350559 |
Kind Code |
A1 |
Zhamu; Aruna ; et
al. |
November 5, 2020 |
PROCESS FOR METAL-SULFUR BATTERY CATHODE CONTAINING HUMIC
ACID-DERIVED CONDUCTIVE FOAM
Abstract
Provided is a process for producing a sulfur cathode for a
metal-sulfur battery. The process comprises: (a) Preparing a humic
acid-derived foam or combined humic acid/graphene-derived foam
composed of multiple pores and pore walls, wherein the pore walls
contain one or a plurality of hexagonal carbon atomic planes; and
(b) Impregnating the foam with sulfur or sulfide in a form of thin
particles or coating, having a diameter or thickness less than 500
nm, which are lodged in the pores or deposited on the pore walls of
the foam.
Inventors: |
Zhamu; Aruna; (Springboro,
OH) ; Jang; Bor Z.; (Centerville, OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Global Graphene Group, Inc. |
Dayton |
OH |
US |
|
|
Assignee: |
Global Graphene Group, Inc.
Dayton
OH
|
Family ID: |
1000004974669 |
Appl. No.: |
16/814734 |
Filed: |
March 10, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15270868 |
Sep 20, 2016 |
10593932 |
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16814734 |
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15251849 |
Aug 30, 2016 |
10584216 |
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15270868 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 4/0416 20130101;
H01M 10/054 20130101; H01M 4/139 20130101; H01M 10/052 20130101;
H01M 4/13 20130101; H01M 4/808 20130101; H01M 2004/028 20130101;
H01M 4/1397 20130101; H01M 4/663 20130101; H01M 4/136 20130101;
H01M 4/38 20130101; H01M 4/5815 20130101 |
International
Class: |
H01M 4/1397 20060101
H01M004/1397; H01M 4/04 20060101 H01M004/04; H01M 4/66 20060101
H01M004/66; H01M 4/13 20060101 H01M004/13; H01M 4/38 20060101
H01M004/38; H01M 4/139 20060101 H01M004/139; H01M 4/136 20060101
H01M004/136; H01M 4/80 20060101 H01M004/80; H01M 4/58 20060101
H01M004/58 |
Claims
1. A roll-to-roll process for producing a continuous-length sheet
of the humic acid-derived foam, said process comprising: (a)
preparing a humic acid dispersion having humic acid molecules and
optional graphene sheets dispersed in a liquid medium, wherein said
dispersion contains a blowing agent, wherein said humic acid has a
content of non-carbon elements less than 5% by weight; (b)
continuously or intermittently dispensing and depositing said humic
acid dispersion onto a surface of a supporting substrate to form a
wet layer of humic acid, wherein said supporting substrate is a
continuous thin film supplied from a feeder roller and collected on
a collector roller; (c) partially or completely removing said
liquid medium from the wet layer of humic acid to form a dried
layer of humic acid in a heating zone or multiple heating zones;
and (d) heat treating the dried layer of humic acid in one of said
heating zones containing a heating temperature from 80.degree. C.
to 500.degree. C. at a desired heating rate sufficient to activate
said blowing agent for producing said humic acid-derived foam,
which is composed of multiple pores and pore walls, wherein said
pore walls contain single-layer or few-layer humic acid-derived
hexagonal carbon atomic planes, said few-layer hexagonal carbon
atomic planes are chemically bonded to create electron-conducting
pathways, have 2-10 layers of stacked hexagonal carbon atomic
planes having an inter-plane spacing d.sub.002 from 0.3354 nm to
0.60 nm as measured by X-ray diffraction, and contain 0.01% to 5%
by weight of non-carbon elements.
2. The process of claim 1, wherein said humic acid-derived foam has
a density from 0.005 to 1.7 g/cm.sup.3, a specific surface area
from 50 to 3,200 m.sup.2/g, a thermal conductivity of at least 100
W/mK per unit of specific gravity, and/or an electrical
conductivity no less than 500 S/cm per unit of specific
gravity.
3. A The process of claim 1, wherein said dispensing and depositing
procedure includes subjecting said humic acid dispersion to an
orientation-inducing stress.
4. The process of claim 1, further including a step of
heat-treating the humic acid-derived foam at a second heat
treatment temperature higher than said first heat treatment
temperature for a length of time sufficient for obtaining a
graphitic foam wherein said pore walls contain stacked hexagonal
carbon atomic planes having an inter-planar spacing d.sub.002 from
0.3354 nm to 0.36 nm and a content of non-carbon elements less than
2% by weight.
5. The process of claim 4, wherein said humic acid-derived foam has
a density from 0.005 to 1.7 g/cm.sup.3, a specific surface area
from 50 to 3,200 m.sup.2/g, a thermal conductivity of at least 100
W/mK per unit of specific gravity, and/or an electrical
conductivity no less than 500 S/cm per unit of specific
gravity.
6. The process of claim 1, wherein said blowing agent is a physical
blowing agent, a chemical blowing agent, a mixture thereof, a
dissolution-and-leaching agent, or a mechanically introduced
blowing agent.
7. The process of claim 1, wherein said first heat treatment
temperature is from 100.degree. C. to 1,500.degree. C.
8. The process of claim 4, wherein said second heat treatment
temperature includes at least a temperature selected from (A)
300-1,500.degree. C., (B) 1,500-2,100.degree. C., and/or (C)
2,100-3,200.degree. C.
9. The process of claim 4, wherein said second heat treatment
temperature includes a temperature in the range of
300-1,500.degree. C. for at least 1 hour and then a temperature in
the range of 1,500-3,200.degree. C. for at least 1 hour.
10. The process of claim 4, wherein said non-carbon elements
include an element selected from the group consisting of oxygen,
fluorine, chlorine, bromine, iodine, nitrogen, hydrogen, and
boron.
11. The process of claim 4, wherein said step (c) of heat treating
the dried layer of humic acid at said first heat treatment
temperature is conducted under a compressive stress.
12. The process of claim 1, further comprising a compression step
to reduce a thickness, a pore size, or a porosity level of said
foam.
13. The process of claim 4, wherein said first and/or second heat
treatment temperature contains a temperature in the range of
300.degree. C-1,500.degree. C. and the foam has an oxygen content
or non-carbon content less than 1%, and pore walls having an
inter-planar spacing less than 0.35 nm, a thermal conductivity of
at least 250 W/mK per unit of specific gravity, and/or an
electrical conductivity no less than 2,500 S/cm per unit of
specific gravity.
14. The process of claim 4, wherein said first and/or second heat
treatment temperature contains a temperature in the range of
1,500.degree. C.-2,100.degree. C. and the foam has an oxygen
content or non-carbon content less than 0.01%, pore walls having an
inter-planar spacing less than 0.34 nm, a thermal conductivity of
at least 300 W/mK per unit of specific gravity, and/or an
electrical conductivity no less than 3,000 S/cm per unit of
specific gravity.
15. The process of claim 4, wherein said first and/or second heat
treatment temperature contains a temperature greater than
2,100.degree. C. and the foam has an oxygen content or non-carbon
content no greater than 0.001%, pore walls having an inter-planar
spacing less than 0.336 nm, a mosaic spread value no greater than
0.7, a thermal conductivity of at least 350 W/mK per unit of
specific gravity, and/or an electrical conductivity no less than
3,500 S/cm per unit of specific gravity.
16. The process of claim 4, wherein said first and/or second heat
treatment temperature contains a temperature no less than
2,500.degree. C. and the foam has pore walls containing stacked
hexagonal carbon planes having an inter-planar spacing less than
0.336 nm, a mosaic spread value no greater than 0.4, a thermal
conductivity greater than 400 W/mK per unit of specific gravity,
and/or an electrical conductivity greater than 4,000 S/cm per unit
of specific gravity.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation of U.S. patent
application Ser. No. 15/270,868 filed, Sep. 20, 2016 and U.S.
patent application Ser. No. 15/251,849 filed, Aug. 30, 2016 the
contents of each are hereby incorporated by reference for all
purposes.
FIELD OF THE INVENTION
[0002] The present invention is directed at a unique cathode
composition and structure in a metal-sulfur battery, including the
lithium-sulfur battery, sodium-sulfur battery, magnesium-sulfur
battery, and aluminum-sulfur battery. The cathode of such a battery
contains sulfur- or sulfide-impregnated conductive foam derived
from humic acid. The invention also provides a process for
producing such a cathode.
BACKGROUND OF THE INVENTION
[0003] Rechargeable lithium-ion (Li-ion) and lithium metal
batteries (including Li-sulfur and Li metal-air batteries) are
considered promising power sources for electric vehicle (EV),
hybrid electric vehicle (HEV), and portable electronic devices,
such as lap-top computers and mobile phones. Lithium as a metal
element has the highest capacity (3,861 mAh/g) compared to any
other metal or metal-intercalated compound as an anode active
material (except Li.sub.44Si, which has a specific capacity of
4,200 mAh/g). Hence, in general, Li metal batteries have a
significantly higher energy density than lithium ion batteries.
[0004] Historically, rechargeable lithium metal batteries were
produced using non-lithiated compounds having relatively high
specific capacities, such as TiS.sub.2, MoS.sub.2, MnO.sub.2,
CoO.sub.2, and V.sub.2O.sub.5, as the cathode active materials,
which were coupled with a lithium metal anode. When the battery was
discharged, lithium ions were transferred from the lithium metal
anode through the electrolyte to the cathode, and the cathode
became lithiated. Unfortunately, upon repeated charges/discharges,
the lithium metal resulted in the formation of dendrites at the
anode that ultimately grew to penetrate through the separator,
causing internal shorting and explosion. As a result of a series of
accidents associated with this problem, the production of these
types of secondary batteries was stopped in the early 1990's,
giving ways to lithium-ion batteries.
[0005] In lithium-ion batteries, pure lithium metal sheet or film
was replaced by carbonaceous materials as the anode. The
carbonaceous material absorbs lithium (through intercalation of
lithium ions or atoms between graphene planes, for instance) and
desorbs lithium ions during the re-charge and discharge phases,
respectively, of the lithium ion battery operation. The
carbonaceous material may comprise primarily graphite that can be
intercalated with lithium and the resulting graphite intercalation
compound may be expressed as Li.sub.xC.sub.6, where x is typically
less than 1.
[0006] Although lithium-ion (Li-ion) batteries are promising energy
storage devices for electric drive vehicles, state-of-the-art
Li-ion batteries have yet to meet the cost and performance targets.
Li-ion cells typically use a lithium transition-metal oxide or
phosphate as a positive electrode (cathode) that de/re-intercalates
Li.sup.+ at a high potential with respect to the carbon negative
electrode (anode). The specific capacity of lithium transition
metal oxide- or phosphate-based cathode active material is
typically in the range of 140-170 mAh/g. As a result, the specific
energy of commercially available Li-ion cells is typically in the
range of 120-220 Wh/kg, most typically 150-180 Wh/kg. These
specific energy values are two to three times lower than what would
be required for battery-powered electric vehicles to be widely
accepted.
[0007] With the rapid development of hybrid (HEV), plug-in hybrid
electric vehicles (HEV), and all-battery electric vehicles (EV),
there is an urgent need for anode and cathode materials that
provide a rechargeable battery with a significantly higher specific
energy, higher energy density, higher rate capability, long cycle
life, and safety. One of the most promising energy storage devices
is the lithium-sulfur (Li-S) cell since the theoretical capacity of
Li is 3,861 mAh/g and that of S is 1,675 mAh/g. In its simplest
form, a Li--S cell consists of elemental sulfur as the positive
electrode and lithium as the negative electrode. The lithium-sulfur
cell operates with a redox couple, described by the reaction
S.sub.8+16Li8Li.sub.2S that lies near 2.2 V with respect to
Li.sup.+/Li.sup.o. This electrochemical potential is approximately
2/3 of that exhibited by conventional positive electrodes (e.g.
LiMnO.sub.4). However, this shortcoming is more than offset by the
very high theoretical capacities of both Li and S. Thus, compared
with conventional intercalation- based Li-ion batteries, Li--S
cells have the opportunity to provide a significantly higher energy
density (a product of capacity and voltage). Assuming complete
reaction to Li.sub.2S, the values of energy densities can approach
2,500 Wh/kg and 2,800 Wh/l, respectively, based on the combined Li
and S weight or volume. If based on the total cell weight or
volume, the energy densities can reach approximately 1,000 Wh/kg
and 1,100 Wh/l, respectively. However, the current Li-sulfur cells
reported by industry leaders in sulfur cathode technology have a
maximum cell specific energy of 250-400 Wh/kg (based on the total
cell weight), which is far below what is possible.
[0008] In summary, despite its considerable advantages, the Li--S
cell is plagued with several major technical problems that have
thus far hindered its widespread commercialization: [0009] (1)
Conventional lithium metal cells still have dendrite formation and
related internal shorting issues. [0010] (2) Sulfur or
sulfur-containing organic compounds are highly insulating, both
electrically and ionically. To enable a reversible electrochemical
reaction at high current densities or charge/discharge rates, the
sulfur must maintain intimate contact with an electrically
conductive additive. Various carbon-sulfur composites have been
utilized for this purpose, but only with limited success owing to
the limited scale of the contact area. Typical reported capacities
are between 300 and 550 mAh/g (based on the cathode carbon-sulfur
composite weight) at moderate rates. [0011] (3) The cell tends to
exhibit significant capacity decay during discharge--charge
cycling. This is mainly due to the high solubility of the lithium
polysulfide anions formed as reaction intermediates during both
discharge and charge processes in the polar organic solvents used
in electrolytes. During cycling, the lithium polysulfide anions can
migrate through the separator to the Li negative electrode
whereupon they are reduced to solid precipitates (Li.sub.2S.sub.2
and/or Li.sub.2S), causing active mass loss. In addition, the solid
product that precipitates on the surface of the positive electrode
during discharge becomes electrochemically irreversible, which also
contributes to active mass loss. [0012] (4) More generally
speaking, a significant drawback with cells containing cathodes
comprising elemental sulfur, organosulfur and carbon-sulfur
materials relates to the dissolution and excessive out-diffusion of
soluble sulfides, including polysulfides, organo-sulfides,
carbon-sulfides, and carbon-polysulfides (hereinafter referred to
as anionic reduction products) from the cathode into the rest of
the cell. This phenomenon is commonly referred to as the Shuttle
Effect. This process leads to several problems: high self-discharge
rates, loss of cathode capacity, corrosion of current collectors
and electrical leads leading to loss of electrical contact to
active cell components, fouling of the anode surface giving rise to
malfunction of the anode, and clogging of the pores in the cell
membrane separator which leads to loss of ion transport and large
increases in internal resistance in the cell.
[0013] In response to these challenges, new electrolytes,
protective films for the lithium anode, and solid electrolytes have
been developed. Some interesting cathode developments have been
reported recently to contain lithium polysulfides; but, their
performance still falls short of what is required for practical
applications.
[0014] For instance, Ji, et al reported that cathodes based on
nanostructured sulfur/meso-porous carbon materials could overcome
these challenges to a large degree, and exhibit stable, high,
reversible capacities with good rate properties and cycling
efficiency [Xiulei Ji, Kyu Tae Lee, & Linda F. Nazar, "A highly
ordered nanostructured carbon-sulphur cathode for lithium-sulphur
batteries," Nature Materials 8, 500-506 (2009)]. However, the
fabrication of the proposed highly ordered meso-porous carbon
structure requires a tedious and expensive template-assisted
process. It is also challenging to load a large proportion of
sulfur into these meso-scaled pores using a physical vapor
deposition or solution precipitation process.
[0015] Zhang, et al. (US Pub. No. 2014/0234702; 08/21/2014) makes
use of a chemical reaction method of depositing S particles on
surfaces of isolated graphene oxide (GO) sheets. But, this method
is incapable of creating a large proportion of S particles on GO
surfaces (i.e. typically <66% of S in the GO-S nanocomposite
composition). The resulting Li--S cells also exhibit poor rate
capability; e.g. the specific capacity of 1,100 mAh/g (based on S
weight) at 0.02 C rate is reduced to <450 mAh/g at 1.0 C rate.
It may be noted that the highest achievable specific capacity of
1,100 mAh/g represents a sulfur utilization efficiency of only
1,100/1,675=65.7% even at such a low charge/discharge rate (0.02 C
means completing the charge or discharge process in 1/0.02=50
hours; 1 C=1 hour, 2 C=1/2 hours, and 3 C =1/3 hours, etc.)
Further, such a S-GO nanocomposite cathode-based Li--S cell
exhibits very poor cycle life, with the capacity typically dropping
to less than 60% of its original capacity in less than 40
charge/discharge cycles. Such a short cycle life makes this Li--S
cell not useful for any practical application. Another chemical
reaction method of depositing S particles on graphene oxide
surfaces is disclosed by Wang, et al. (US Pub. No. 2013/0171339;
07/04/2013). This Li--S cell still suffers from the same
problems.
[0016] A solution precipitation method was disclosed by Liu, et al.
(US Pub. No. 2012/0088154; Apr. 12, 2012) to prepare
graphene-sulfur nanocomposites (having sulfur particles adsorbed on
GO surfaces) for use as the cathode material in a Li--S cell. The
method entails mixing GO sheets and S in a solvent (CS.sub.2) to
form a suspension. The solvent is then evaporated to yield a solid
nanocomposite, which is then ground to yield nanocomposite powder
having primary sulfur particles with an average diameter less than
approximately 50 nm. Unfortunately, this method does not appear to
be capable of producing S particles less than 40 nm. The resulting
Li--S cell exhibits very poor cycle life (a 50% decay in capacity
after only 50 cycles). Even when these nanocomposite particles are
encapsulated in a polymer, the Li--S cell retains less than 80% of
its original capacity after 100 cycles. The cell also exhibits a
poor rate capability (specific capacity of 1,050 mAh/g(S wt.) at
0.1 C rate, dropped to <580 mAh/g at 1.0 C rate). Again, this
implies that a large proportion of S did not contribute to the
lithium storage, resulting in a low S utilization efficiency.
[0017] Furthermore, all of the aforementioned methods involve
depositing S particles onto surfaces of isolated graphene sheets.
The presence of S particles or coating (one of the most insulating
materials) adhered to graphene surfaces would make the resulting
electrode structure non-conducting when multiple S-bonded graphene
sheets are packed together. These S particles prevent graphene
sheets from contacting each other, making it impossible for
otherwise conducting graphene sheets to form a 3-D network of
electron-conducting paths in the cathode. This unintended and
unexpected outcome is another reason why these prior art Li--S
cells have performed so poorly.
[0018] Despite the various approaches proposed for the fabrication
of high energy density Li--S cells, there remains a need for
cathode materials, production processes, and cell operation methods
that retard the out-diffusion of S or lithium polysulfide from the
cathode compartments into other components in these cells, improve
the utilization of electro-active cathode materials (S utilization
efficiency), and provide rechargeable Li--S cells with high
capacities over a large number of cycles.
[0019] Most significantly, lithium metal (including pure lithium,
lithium alloys of high lithium content with other metal elements,
or lithium-containing compounds with a high lithium content; e.g.
>80% or preferably >90% by weight Li) still provides the
highest anode specific capacity as compared to essentially all
other anode active materials (except pure silicon, but silicon has
pulverization issues). Lithium metal would be an ideal anode
material in a lithium-sulfur secondary battery if dendrite related
issues could be addressed.
[0020] Sodium metal (Na), potassium metal (K), magnesium metal
(Mg), and aluminum metal (Al) have similar chemical characteristics
to Li, and the sulfur cathode in room temperature sodium-sulfur
cells (RT Na-S batteries), potassium-sulfur cells (K--S),
magnesium-sulfur cell, and aluminum-sulfur cell face the same
issues observed in Li--S batteries, such as: (i) low active
material utilization rate, (ii) poor cycle life, and (iii) low
Coulombic efficiency. Again, these drawbacks arise mainly from
insulating nature of S, dissolution of S and metal polysulfide
intermediates in liquid electrolytes (and related Shuttle effect),
and large volume changes during charge/discharge.
[0021] Hence, an object of the present invention is to provide a
rechargeable metal-sulfur battery (e.g. Li--S, Na--S, K--S, Mg--S,
or Al--S battery) that exhibits an exceptionally high specific
energy density or high volumemetric energy density. One particular
technical goal of the present invention is to provide a
metal-sulfur or metal ion-sulfur cell with a cell specific energy
greater than 400 Wh/Kg, preferably greater than 500 Wh/Kg, and more
preferably greater than 600
[0022] Wh/Kg (all based on the total cell weight).
[0023] Another object of the present invention is to provide a
metal-sulfur cell that exhibits a high cathode specific capacity
(higher than 1,200 mAh/g based on the sulfur weight, or higher than
1,000 mAh/g based on the cathode composite weight, including
sulfur, conducting additive or substrate, and binder weights
combined, but excluding the weight of cathode current collector).
The specific capacity is preferably higher than 1,400 mAh/g based
on the sulfur weight alone or higher than 1,200 mAh/g based on the
cathode composite weight. This must be accompanied by a high
specific energy, good resistance to dendrite formation, and a long
and stable cycle life.
[0024] It may be noted that in most of the open literature reports
(scientific papers) and patent documents, scientists or inventors
choose to express the cathode specific capacity based on the sulfur
or lithium polysulfide weight alone (not the total cathode
composite weight), but unfortunately a large proportion of
non-active materials (those not capable of storing lithium, such as
conductive additive and binder) is typically used in their Li--S or
Na--S cells. For practical use purposes, it is more meaningful to
use the cathode composite weight-based capacity value.
[0025] A specific object of the present invention is to provide a
rechargeable metal-sulfur cell based on rational materials and
battery designs that overcome or significantly reduce the following
issues commonly associated with conventional metal-S cells: (a)
dendrite formation (internal shorting); (b) extremely low electric
and ionic conductivities of sulfur, requiring large proportion
(typically 30-55%) of non-active conductive fillers and having
significant proportion of non-accessible or non-reachable sulfur or
metal polysulfides); (c) dissolution of S and metal polysulfide in
electrolyte and migration of polysulfides from the cathode to the
anode (which irreversibly react with Li, Na, K, Mg, or Al at the
anode), resulting in active material loss and capacity decay (the
shuttle effect); and (d) short cycle life.
[0026] Additionally, the production of graphene sheets typically
involves the use of undesirable chemicals, such as sulfuric acid
and potassium permanganate, and the efflux of regulated gases, such
as SO.sub.2 and NO.sub.2. FIG. 1 illustrates a commonly used
process for graphene production. Thus, an urgent need exists to
have a new class of carbon nano materials that are comparable or
superior to graphene in terms of properties, but can be produced
more cost-effectively, faster, more scalable, and in a more
environmentally benign manner. The production process for such a
new carbon nano material requires a reduced amount of undesirable
chemical (or elimination of these chemicals all together),
shortened process time, less energy consumption, reduced or
eliminated effluents of undesirable chemical species into the
drainage (e.g., sulfuric acid) or into the air (e.g., SO.sub.2 and
NO.sub.2). Furthermore, one should be able to readily make this new
nano material into a foam structure that is essentially a 3D
network of electron-conducting pathways and, hence, thermally and
electrically conductive.
[0027] Generally speaking, a foam or foamed material is composed of
pores (or cells) and pore walls (a solid material). The pores can
be interconnected to form an open-cell foam. As an example,
graphene foam is composed of pores and pore walls that contain a
graphene material. We envision that graphene, when made into a foam
structure, may be a good protective material for sulfur. However,
most of the methods of producing graphene foams are all tedious,
energy-intensive, and slow. Every prior art method or process for
producing graphene and graphene foams has major deficiencies. Thus,
it is an object of the present invention to provide a new class of
foam material that is thermally and electrically conducting,
mechanically robust, and chemically compatible with sulfur or
polysulfide. Another object is to provide a cost-effective method
of producing this class of foam to protect sulfur or
polysulfide.
[0028] Humic acid (HA) is an organic matter commonly found in soil
and coal products. HA can be extracted from the soil using a base
(e.g. KOH). HA can also be extracted, with a high yield, from a
type of coal called leonardite, which is a highly oxidized version
of lignite coal. HA extracted from leonardite contains a number of
oxygenated groups (e.g. carboxyl groups) located around the edges
of the graphene-like molecular center (SP.sup.2 core of hexagonal
carbon structure).
[0029] This material is slightly similar to graphene oxide (GO)
which is produced by strong acid oxidation of natural graphite. HA
has a typical oxygen content of 5% to 42% by weight (other major
elements being carbon and hydrogen). HA, after chemical or thermal
reduction, has an oxygen content of 0.01% to 5% by weight. For
claim definition purposes in the instant application, humic acid
(HA) refers to the entire oxygen content range, from 0.01% to 42%
by weight. The reduced humic acid (RHA) is a special type of HA
that has an oxygen content of 0.01% to 5% by weight.
[0030] The present invention is directed at a new class of
graphene-like 2D materials (i.e. humic acid) that surprisingly can
be converted into a foamed structure of high structural integrity.
Thus, another object is to provide a cost-effective process for
producing such a nano carbon foam (specifically, humic acid-derived
foam) in large quantities. This process does not involve the use of
an environmentally unfriendly chemical. This method enables the
flexible design and control of the porosity level and pore
sizes.
[0031] It is another object of the present invention to provide a
humic acid-derived foam that exhibits a thermal conductivity,
electrical conductivity, elastic modulus, and/or strength
comparable to or greater than those of the conventional graphite
foams, carbon foams, or graphene foams. Yet another object of the
present invention is to provide a humic acid-derived foam that
preferably has a meso-scaled pore size range (2-50 nm). The
HA-derived foams must also be capable of retaining, confining, or
protecting sulfur or sulfide to solve metal-sulfur cell issues.
SUMMARY OF THE INVENTION
[0032] The invention provides a process for producing sulfur
cathode for a metal-sulfur battery. The process comprises: (a)
Preparing a humic acid-derived foam or combined humic
acid/graphene-derived foam composed of multiple pores and pore
walls, wherein the pore walls contain one or a plurality of
hexagonal carbon atomic planes; and (b) Impregnating the foam with
sulfur or sulfide in a form of thin particles or coating, having a
diameter or thickness less than 500 nm, which are lodged in the
pores or deposited on the pore walls.
[0033] The present invention also provides a sulfur cathode for a
metal-sulfur battery (a primary battery or secondary battery). The
sulfur cathode contains a humic acid-derived foam or combined humic
acid/graphene-derived foam, composed of multiple pores and pore
walls, and sulfur or sulfide impregnated into the pores or
deposited on the pore walls, wherein the pore walls contain
single-layer or few-layer humic acid-derived hexagonal carbon
atomic planes or sheets. The few-layer hexagonal carbon atomic
planes or sheets have 2-10 layers of stacked hexagonal carbon
atomic planes having an inter-plane spacing d.sub.002 from 0.3354
nm to 0.60 nm as measured by X-ray diffraction. The single-layer or
few-layer hexagonal carbon atomic planes contain 0.01% to 25% by
weight of non-carbon elements, wherein the humic acid is selected
from oxidized humic acid, reduced humic acid, fluorinated humic
acid, chlorinated humic acid, brominated humic acid, iodized humic
acid, hydrogenated humic acid, nitrogenated humic acid, doped humic
acid, chemically functionalized humic acid, or a combination
thereof. The sulfide may be selected from a polysulfide,
organo-sulfide, carbon-sulfide, metal polysulfide,
carbon-polysulfide, or a combination thereof. The graphene may be
selected from pristine graphene, graphene oxide, reduced graphene
oxide, graphene fluoride, nitrogenated graphene, doped graphene, or
chemically functionalized graphene.
[0034] Preferably, the sulfur or sulfide is chemically bonded to
the humic acid-derived or combined humic acid/graphene-derived
hexagonal carbon atomic planes. The sulfur or sulfide impregnated
into the pores or deposited on the pore walls are preferably in a
particle or coating form having a diameter or thickness less than
20 nm, more preferably less than 10 nm, and further preferably less
than 5 nm, and can be as thin as 0.5-2 nm.
[0035] Preferably, the sulfur or sulfide occupies a weight fraction
of at least 70% of the total weight of the foam and the sulfur or
sulfide combined. This weight fraction is preferably at least 80%,
more preferably at least 90%, and most preferably at least 95%.
[0036] In the sulfur cathode, the sulfide preferably contains a
metal polysulfide selected from lithium polysulfide, sodium
polysulfide, potassium polysulfide, magnesium polysulfide, aluminum
polysulfide, or a combination thereof. In some embodiments,
polysulfide contains a metal polysulfide, M.sub.xS.sub.y, wherein x
is an integer from 1 to 3 and y is an integer from 1 to 10, and
wherein M is a metal element selected from an alkali metal, an
alkaline metal selected from Mg or Ca, a transition metal, a metal
from groups 13 to 17 of the periodic table, or a combination
thereof.
[0037] The humic acid-derived foam in the sulfur cathode herein
invented can be divided into three types: (a) humic acid (HA) foams
that contain at least 10% by weight (typically from 10% to 42% by
weight and most typically from 10% to 25%) of non-carbon elements
that can be used for a broad array of applications (wherein the
original humic acid molecules remain substantially the same, but
some chemical linking between HA molecules has occurred); (b) a
chemically merged and reduced humic acid-based foam wherein
extensive linking and merging between original HA molecules has
occurred to form incipient graphene-like hexagonal carbon sheets
constituting pore walls, resulting in evolution of chemical species
containing non-carbon elements originally attached to HA molecules
(hence, non-carbon element content reduced to generally between 2%
and 10% by wt.); and (c) humic acid-derived graphitic foam that
contains essentially all carbon only (<2% by weight of
non-carbon content, preferably <1%, and further preferably
<0.1%), wherein the pore walls contain single-layer or few-layer
(2-10) graphene-like sheets that are hexagonal carbon atomic
planes. In each and every one of these types, a graphene material
can be added to humic acid and both humic acid and graphene are
subsequently subjected to essentially the same heat treatments.
This graphene material may be selected from pristine graphene,
graphene oxide, reduced graphene oxide, graphene fluoride,
nitrogenated graphene, doped graphene, or chemically functionalized
graphene.
[0038] Preferably and typically, the HA-derived foam, when measured
without the sulfur or sulfide, has a density from 0.005 to 1.7
g/cm.sup.3, a specific surface area from 50 to 3,200 m.sup.2/g, a
thermal conductivity of at least 100 W/mK per unit of specific
gravity, and/or an electrical conductivity no less than 500 S/cm
per unit of specific gravity. More typically, the humic
acid-derived foam has a density from 0.01 to 1.5 g/cm.sup.3 or an
average pore size from 2 nm to 50 nm.
[0039] In an embodiment, the foam has a specific surface area from
200 to 2,000 m.sup.2/g or a density from 0.1 to 1.3 g/cm.sup.3.
[0040] Typically, if the HA-derived foam is produced from a process
that does not contain a heat treatment temperature (HTT) higher
than 300.degree. C., the foam has a content of non-carbon elements
in the range of 10% to 42% by weight. The pore walls can still
contain identifiable humic acid molecules that are sheet-like
hexagonal carbon atomic structures. The non-carbon elements can
include an element selected from oxygen, fluorine, chlorine,
bromine, iodine, nitrogen, hydrogen, or boron. In a specific
embodiment, the pore walls contain fluorinated humic acid and the
foam contains a fluorine content from 0.01% to 25% by weight. In
another embodiment, the foam contains an oxygen content from 0.01%
to 25% by weight.
[0041] With a HTT higher than 300.degree. C., neighboring HA
molecules that are closely packed and well-aligned can be
chemically linked together to form multi-ring aromatic structures
that resemble incipient graphene-like hexagonal carbon atomic
structures. As heat treatment continues, these highly aromatic
molecules are merged together in an edge-to-edge manner to increase
the length and width of graphene-like hexagonal planes and,
concurrently, several hexagonal carbon planes can be stacked
together to form multi-layer carbon atomic structures, similar to
few-layer graphene structures. The inter-planar spacing is
typically reduced to <<0.60 nm, more typically <0.40 nm.
If the HTT is from 300.degree. C. up to 1,500.degree. C., one
typically produces chemically merged and reduced humic acid-based
foam, wherein extensive linking and merging between original HA
molecules has occurred to form incipient graphene-like hexagonal
carbon sheets that constitute pore walls. The non-carbon content in
the foam is typically reduced to from 2% to 10%.
[0042] If the HTT is from 1,500.degree. C. to 3,200.degree. C. and
the foam can become essentially a graphitic foam wherein the pore
walls contain single-layer or few-layer graphene-like hexagonal
carbon planes and the non-carbon content is reduced to less than 2%
by wt.
[0043] In a preferred embodiment, the foam is made into a
continuous-length roll sheet form (a roll of a continuous foam
sheet) having a thickness no greater than 200 .mu.m and a length of
at least 1 meter long, preferably at least 2 meters, further
preferably at least 10 meters, and most preferably at least 100
meters. This sheet roll is produced by a roll-to-roll process.
There has been no prior art HA-derived graphene-like foam that is
made into a sheet roll form.
[0044] In a preferred embodiment, the HA-derived foam has an oxygen
content or non-carbon content less than 1% by weight, and the pore
walls have stacked graphene-like planes having an inter-planar
spacing less than 0.35 nm, a thermal conductivity of at least 200
W/mK per unit of specific gravity, and/or an electrical
conductivity no less than 1,000 S/cm per unit of specific
gravity.
[0045] In a further preferred embodiment, the HA-derived foam has
an oxygen content or non-carbon content less than 0.1% by weight
and said pore walls contain stacked graphene-like hexagonal carbon
atomic planes having an inter-planar spacing less than 0.34 nm, a
thermal conductivity of at least 250 W/mK per unit of specific
gravity, and/or an electrical conductivity no less than 1,500 S/cm
per unit of specific gravity.
[0046] In yet another preferred embodiment, the HA-derived foam has
an oxygen content or non-carbon content no greater than 0.01% by
weight and said pore walls contain stacked graphene-like planes
having an inter-graphene spacing less than 0.336 nm, a mosaic
spread value no greater than 0.7, a thermal conductivity of at
least 300 W/mK per unit of specific gravity, and/or an electrical
conductivity no less than 2,000 S/cm per unit of specific
gravity.
[0047] In still another preferred embodiment, the HA-derived foam
has pore walls containing stacked graphene-like atomic planes
having an inter-planar spacing less than 0.336 nm, a mosaic spread
value no greater than 0.4, a thermal conductivity greater than 400
W/mK per unit of specific gravity, and/or an electrical
conductivity greater than 3,000 S/cm per unit of specific
gravity.
[0048] In a preferred embodiment, the pore walls contain stacked
graphene-like hexagonal carbon atomic planes having an inter-planar
spacing less than 0.337 nm and a mosaic spread value less than 1.0.
In a preferred embodiment, the foam exhibits a degree of
graphitization no less than 80% (preferably no less than 90%)
and/or a mosaic spread value less than 0.4. In a preferred
embodiment, the pore walls contain a 3D network of interconnected
graphene-like hexagonal carbon atomic planes.
[0049] In a preferred embodiment, the HA-derived foam contains
meso-scaled pores having a pore size from 2 nm to 50 nm. The solid
foam can also be made to contain micron-scaled pores (1-500
.mu.m).
[0050] Preferably, the presently invented HA-derived foam may be
produced by a process comprising: (a) preparing a humic acid
dispersion having multiple humic acid molecules or sheets dispersed
in a liquid medium, wherein the humic acid is selected from
oxidized humic acid, reduced humic acid, fluorinated humic acid,
chlorinated humic acid, brominated humic acid, iodized humic acid,
hydrogenated humic acid, nitrogenated humic acid, doped humic acid,
chemically functionalized humic acid, or a combination thereof and
wherein the dispersion contains an optional blowing agent having a
blowing agent-to-humic acid weight ratio from 0/1.0 to 1.0/1.0; (b)
dispensing and depositing the graphene dispersion onto a surface of
a supporting substrate (e.g. plastic film, rubber sheet, metal
foil, glass sheet, paper sheet, etc.) to form a wet layer of humic
acid; (c) partially or completely removing the liquid medium from
the wet layer of humic acid to form a dried layer of humic acid;
and (d) heat treating the dried layer of humic acid at a first heat
treatment temperature from 80.degree. C. to 3,200.degree. C. at a
desired heating rate sufficient to induce formation and releasing
of volatile gas molecules from the non-carbon elements (e.g. 0, H,
N, B, F, Cl, Br, I, etc.) or to activate the blowing agent for
producing humic acid-derived foam. Preferably, the dispensing and
depositing procedure includes subjecting the humic acid dispersion
to an orientation-inducing stress.
[0051] This optional blowing agent is not required if the HA
material has a content of non-carbon elements (e.g. 0, H, N, B, F,
Cl, Br, I, etc.) no less than 5% by weight (preferably no less than
10%, further preferably no less than 20%, even more preferably no
less than 30%). The subsequent high temperature treatment serves to
remove a majority of these non-carbon elements from the edges of HA
molecules, generating volatile gas species that produce pores or
cells in the solid foam structure. In other words, quite
surprisingly, these non-carbon elements play the role of a blowing
agent. Hence, an externally added blowing agent is optional (not
required). However, the use of a blowing agent can provide added
flexibility in regulating or adjusting the porosity level and pore
sizes for a desired application. The blowing agent is typically
required if the non-carbon element content in the humic acid is
less than 5%.
[0052] The blowing agent can be a physical blowing agent, a
chemical blowing agent, a mixture thereof, a
dissolution-and-leaching agent, or a mechanically introduced
blowing agent.
[0053] The process may further include a step of heat-treating the
solid foam at a second heat treatment temperature higher than the
first heat treatment temperature for a length of time sufficient
for obtaining a graphene-like foam wherein the pore walls contain
stacked hexagonal carbon atomic planes having an inter-planar
spacing d.sub.002 from 0.3354 nm to 0.40 nm and a content of
non-carbon elements less than 5% by weight (typically from 0.001%
to 2%). When the resulting non-carbon element content is from 0.1%
to 2.0%, the inter-plane spacing d.sub.002 is typically from 0.337
nm to 0.40 nm.
[0054] If the original HA molecules in the dispersion contains a
non-carbon element content higher than 5% by weight, the hexagonal
carbon atomic planes in the solid foam (after the heat treatment)
contain structural defects that are induced during the step (d) of
heat treating. The liquid medium can be simply water and/or an
alcohol, which is environmentally benign.
[0055] In a preferred embodiment, the process is a roll-to-roll
process wherein steps (b) and (c) include feeding the supporting
substrate from a feeder roller to a deposition zone, continuously
or intermittently depositing the HA dispersion onto a surface of
the supporting substrate to form the wet layer of HA material
thereon, drying the wet layer of HA material to form the dried
layer of HA material, and collecting the dried layer of HA material
deposited on the supporting substrate on a collector roller. Such a
roll-to-roll or reel-to-reel process is a truly industrial-scale,
massive manufacturing process that can be automated.
[0056] In one embodiment, the first heat treatment temperature is
from 100.degree. C. to 1,500.degree. C. In another embodiment, the
second heat treatment temperature includes at least a temperature
selected from (A) 300-1,500.degree. C., (B) 1,500-2,100.degree. C.,
and/or (C) 2,100-3,200.degree. C. In a specific embodiment, the
second heat treatment temperature includes a temperature in the
range of 300-1,500.degree. C. for at least 1 hour and then a
temperature in the range of 1,500-3,200.degree. C. for at least 1
hour.
[0057] There are several surprising results of conducting first
and/or second heat treatments to the dried HA layer, and different
heat treatment temperature ranges enable us to achieve different
purposes, such as (a) removal of non-carbon elements from the HA
material (e.g. thermal reduction of fluorinated humic acid to
obtain reduced humic acid) which generate volatile gases to produce
pores or cells in the HA foam, (b) activation of the chemical or
physical blowing agent to produce pores or cells, (c) chemical
linking or merging of humic acid molecules into highly aromatic
molecules and edge-to-edge merging of aromatic ring structures or
hexagonal carbon planes to significantly increase the lateral
dimensions (length and width) of graphene-like hexagonal carbon
sheets in the foam walls (solid portion of the foam), (d) healing
of defects naturally existing in HA or created during fluorination,
oxidation, or nitrogenation of humic acid molecules, and (e)
re-organization and perfection of graphitic domains or graphite
crystals. These different purposes or functions are achieved to
different extents within different temperature ranges. The
non-carbon elements typically include an element selected from
oxygen, fluorine, chlorine, bromine, iodine, nitrogen, hydrogen, or
boron. Quite surprisingly, even under low-temperature foaming
conditions, heat-treating induces chemical linking, merging, or
chemical bonding between sheet-like HA molecules, often in an
edge-to-edge manner (some in face-to-face manner).
[0058] In one embodiment, the HA-derived foam has a specific
surface area (when measured without the presence of sulfur or
sulfide) from 200 to 2,000 m.sup.2/g. In one embodiment, the solid
foam has a density from 0.1 to 1.5 g/cm.sup.3. In an embodiment,
step (d) of heat treating the layer of HA material at a first heat
treatment temperature is conducted under a compressive stress. In
another embodiment, the process comprises a compression step to
reduce a thickness, pore size, or porosity level of the film of
HA-derived foam. In some applications, the foam has a thickness no
greater than 200 .mu.m.
[0059] In an embodiment, the HA dispersion has at least 5% by
weight of HA dispersed in the liquid medium to form a liquid
crystal phase. In an embodiment, the first heat treatment
temperature contains a temperature in the range of 80.degree.
C-300.degree. C. and, as a result, the HA foam has an oxygen
content or non-carbon element content less than 5%, and the pore
walls have an inter-planar spacing less than 0.40 nm, a thermal
conductivity of at least 150 W/mK (more typically at least 200
W/mk) per unit of specific gravity, and/or an electrical
conductivity no less than 1,000 S/cm per unit of specific gravity.
Unless otherwise specified, all these properties are measured when
no sulfur or sulfide is present in the pores.
[0060] In a preferred embodiment, the first and/or second heat
treatment temperature contains a temperature in the range of
300.degree. C-1,500.degree. C. and, as a result, the HA-derived
foam has an oxygen content or non-carbon content less than 2%, and
the pore walls have an inter-planar spacing less than 0.35 nm, a
thermal conductivity of at least 250 W/mK per unit of specific
gravity, and/or an electrical conductivity no less than 1,500 S/cm
per unit of specific gravity.
[0061] When the first and/or second heat treatment temperature
contains a temperature in the range of 1,500.degree. C.-
2,100.degree. C., the HA-derived foam has an oxygen content or
non-carbon content less than 1% and pore walls have an
inter-graphene spacing less than 0.34 nm, a thermal conductivity of
at least 300 W/mK per unit of specific gravity, and/or an
electrical conductivity no less than 3,000 S/cm per unit of
specific gravity.
[0062] When the first and/or second heat treatment temperature
contains a temperature greater than 2,100.degree. C., the
HA-derived foam has an oxygen content or non-carbon content no
greater than 0.1% and pore walls have an inter-planar spacing less
than 0.336 nm, a mosaic spread value no greater than 0.7, a thermal
conductivity of at least 350 W/mK per unit of specific gravity,
and/or an electrical conductivity no less than 3,500 S/cm per unit
of specific gravity.
[0063] If the first and/or second heat treatment temperature
contains a temperature no less than 2,500.degree. C., the
HA-derived foam has pore walls containing stacked graphene-like
hexagonal carbon planes having an inter-planar spacing less than
0.336 nm, a mosaic spread value no greater than 0.4, and a thermal
conductivity greater than 400 W/mK per unit of specific gravity,
and/or an electrical conductivity greater than 4,000 S/cm per unit
of specific gravity.
[0064] In one embodiment, the pore walls contain stacked
graphene-like hexagonal carbon planes having an inter-planar
spacing less than 0.337 nm and a mosaic spread value less than 1.0.
In another embodiment, the solid wall portion of the HA-derived
foam exhibits a degree of graphitization no less than 80% and/or a
mosaic spread value less than 0.4. In yet another embodiment, the
solid wall portion of the HA-derived foam exhibits a degree of
graphitization no less than 90% and/or a mosaic spread value no
greater than 0.4.
[0065] Typically, after a heat treatment at a HTT higher than
2,500.degree. C., the pore walls in the HA-derived graphitic foam
contain a 3D network of interconnected hexagonal carbon atomic
planes that are electron-conducting pathways. The cell walls
contain graphitic domains or graphite crystals having a lateral
dimension (L.sub.a, length or width) no less than 20 nm, more
typically and preferably no less than 40 nm, still more typically
and preferably no less than 100 nm, still more typically and
preferably no less than 500 nm, often greater than 1.mu.m, and
sometimes greater than 10 .mu.m. The graphitic domains typically
have a thickness from 1 nm to 20 nm, more typically from 1 nm to 10
nm, and further more typically from 1 nm to 4 nm.
[0066] Preferably, the HA-derived foam contains meso-scaled pores
having a pore size from 2 nm to 50 nm (preferably 2 nm to 25
nm).
[0067] In a preferred embodiment, the present invention provides a
roll-to-roll process for producing a solid HA foam or HA-derived
foam composed of multiple pores and pore walls The process
comprises: (a) preparing a humic acid dispersion having multiple
humic acid molecules or sheets dispersed in a liquid medium,
wherein the humic acid is selected from oxidized humic acid,
reduced humic acid, fluorinated humic acid, chlorinated humic acid,
brominated humic acid, iodized humic acid, hydrogenated humic acid,
nitrogenated humic acid, doped humic acid, chemically
functionalized humic acid, or a combination thereof and wherein the
dispersion contains an optional blowing agent having a blowing
agent-to-humic acid weight ratio from 0/1.0 to 1.0/1.0; (b)
continuously or intermittently dispensing and depositing the HA
dispersion onto a surface of a supporting substrate to form a wet
layer of HA material, wherein the supporting substrate is a
continuous thin film supplied from a feeder roller and collected on
a collector roller; (c) partially or completely removing the liquid
medium from the wet layer of humic acid to form a dried layer of
humic acid in a heating zone or multiple heating zones; and (d)
heat treating the dried layer of humic acid in one of these heating
zones containing a heating temperature from 80.degree. C. to
500.degree. C. at a desired heating rate sufficient to activate the
blowing agent for producing the humic acid-derived foam having a
density from 0.01 to 1.7 g/cm.sup.3 or a specific surface area from
50 to 3,000 m.sup.2/g. In this process, heat treatments occur in
situ during the roll-to-roll procedure. This is a highly
cost-effective process amenable to mass production of HA-derived
graphitic foam sheets that are wrapped around on a roller for ease
of shipping and handling and, subsequently, ease of cutting and
slitting.
[0068] The orientation-inducing stress may be a shear stress. As an
example, the shear stress can be encountered in a situation as
simple as a "doctor's blade" that guides the spreading of HA
dispersion over a plastic or glass surface with a sufficiently high
shear rate during a manual casting process. As another example, an
effective orientation-inducing stress is created in an automated
roll-to-roll coating process in which a "knife-on-roll"
configuration dispenses the graphene dispersion over a moving solid
substrate, such as a plastic film at a sufficiently high speed. The
relative motion between this moving film and the coating knife can
act to effect orientation of graphene sheets along the shear stress
direction. Comma coating and slot-die coating are particularly
effective methods for this function.
[0069] This orientation-inducing stress is a critically important
step in the production of the presently invented HA-derived foams
due to the surprising observation that the shear stress enables the
HA molecules or sheets to align along a particular direction (e.g.
X-direction or length-direction) to produce preferred orientations
and facilitate contacts between HA molecules or sheets along foam
walls. Further surprisingly, these preferred orientations and
improved HA-to-HA contacts facilitate chemical merging or linking
between HA molecules or sheets during the subsequent heat treatment
of the dried HA layer. Such preferred orientations and improved
contacts are essential to the eventual attainment of exceptionally
high thermal conductivity, electrical conductivity, elastic
modulus, and mechanical strength of the resulting HA-derived foam.
In general, these great properties could not be obtained without
such a shear stress-induced orientation control.
[0070] The HA-derived foam is then impregnated with sulfur or
sulfide using any well-known impregnation procedure, such as sulfur
vapor impregnation, solution deposition, electrochemical
deposition, chemical deposition of sulfur or sulfide.
[0071] The present invention also provides a metal-sulfur battery
containing the aforementioned sulfur cathode as an active cathode
layer, an anode, and a metal ion-conducting electrolyte in ionic
contact with the cathode and the anode. The metal-sulfur battery
may be a lithium-sulfur battery, sodium-sulfur battery,
potassium-sulfur battery, magnesium-sulfur battery, or
aluminum-sulfur battery. In certain embodiments, the anode of the
metal-sulfur battery contains a metal, metal alloy, or metal
compound of Li, Na, K, Mg, or Al metal as an anode active
material.
[0072] It may be noted that the humic acid-derived foam itself also
plays the role of a cathode current collector due to its high
electrical conductivity. This foam layer can be directly connected
to an external circuit load via a terminal tab, obviating the need
to have a separate layer of cathode current collector (e.g.
typically an Al foil). This feature significantly reduces the
weight and volume of a battery, thereby further increasing the
energy density of the battery. This is an unexpected, added
advantage of the presently invented sulfur cathode.
[0073] However, optionally, one could still use a separate
(additional) current collector. Also optionally, one can use an
anode current collector (e.g. Cu foil, Ti foil, or stainless steel
foil) in electronic contact with the anode of the metal-sulfur
cell. Thus, another embodiment of the instant invention is a
metal-sulfur battery that further comprises an anode current
collector in electronic contact with the anode and/or a cathode
current collector in electronic contact with the sulfur
cathode.
[0074] In certain embodiments, the electrolyte in the metal-sulfur
battery is selected from polymer electrolyte, polymer gel
electrolyte, composite electrolyte, ionic liquid electrolyte,
non-aqueous liquid electrolyte, soft matter phase electrolyte,
solid-state electrolyte, or a combination thereof.
[0075] In certain embodiments, the battery electrolyte contains a
salt selected from lithium perchlorate (LiClO.sub.4), lithium
hexafluorophosphate (LiPF.sub.6), lithium borofluoride
(LiBF.sub.4), lithium hexafluoroarsenide (LiAsF.sub.6), lithium
trifluoro-metasulfonate (LiCF.sub.3SO.sub.3), bis-trifluoromethyl
sulfonylimide lithium (LiN(CF.sub.3SO.sub.2).sub.2, Lithium
bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate
(LiBF.sub.2C.sub.2O.sub.4), lithium oxalyldifluoroborate
(LiBF.sub.2C.sub.2O.sub.4), Lithium nitrate (LiNO.sub.3),
Li-Fluoroalkyl-Phosphates (LiPF3(CF.sub.2CF.sub.3).sub.3), lithium
bisperfluoroethysulfonylimide (LiBETI), an ionic liquid salt,
sodium perchlorate (NaClO.sub.4), potassium perchlorate
(KClO.sub.4), sodium hexafluorophosphate (NaPF.sub.6), potassium
hexafluorophosphate (KPF.sub.6), sodium borofluoride (NaBF.sub.4),
potassium borofluoride (KBF.sub.4), sodium hexafluoroarsenide,
potassium hexafluoroarsenide, sodium trifluoro-metasulfonate
(NaCF.sub.3SO.sub.3), potassium trifluoro-metasulfonate
(KCF.sub.3SO.sub.3), bis-trifluoromethyl sulfonylimide sodium
(NaN(CF.sub.3SO.sub.2).sub.2), sodium trifluoromethanesulfonimide
(NaTFSI), bis-trifluoromethyl sulfonylimide potassium
(KN(CF.sub.3SO.sub.2).sub.2), Mg(AlCl.sub.2EtBu).sub.2,
MgCl.sub.2/AlCl.sub.3, Mg(ClO.sub.4).sub.2, Mg(OH).sub.2,
Al(OH).sub.3, or a combination thereof.
[0076] In certain embodiments, the battery electrolyte contains a
solvent selected from ethylene carbonate (EC), dimethyl carbonate
(DMC), methylethyl carbonate (MEC), diethyl carbonate (DEC), ethyl
propionate, methyl propionate, propylene carbonate (PC),
gamma.-butyrolactone (y-BL), acetonitrile (AN), ethyl acetate (EA),
propyl formate (PF), methyl formate (MF), toluene, xylene or methyl
acetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate
(VC), allyl ethyl carbonate (AEC), 1,3-dioxolane (DOL),
1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether
(TEGDME), Poly(ethylene glycol) dimethyl ether
[0077] (PEGDME), diethylene glycol dibutyl ether (DEGDBE),
2-ethoxyethyl ether (EEE), sulfone, sulfolane,
1-ethyl-methyl-imidazolium chloride (EMIC), tetrahydrofuran (THF),
room temperature ionic liquid, or a combination thereof.
[0078] In certain embodiments, the anode of the metal-sulfur
battery contains an anode active material selected from lithium
metal, sodium metal, potassium metal, magnesium metal, aluminum
metal, a lithium metal alloy, a sodium metal alloy, a potassium
metal alloy, a magnesium metal alloy, an aluminum alloy, a lithium
intercalation compound, a sodium intercalation compound, a
potassium intercalation compound, a lithium-containing compound, a
sodium-containing compound, a potassium-doped compound, a
magnesium-doped compound, a magnesium-intercalated compound, an
aluminum-doped compound, an aluminum-containing compound, or a
combination thereof.
[0079] In certain preferred embodiments, the metal-sulfur battery
is a lithium ion-sulfur cell and the anode contains an anode active
material selected from the group consisting of: (a) silicon (Si),
germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi),
zinc (Zn), aluminum (Al), nickel (Ni), cobalt (Co), manganese (Mn),
titanium (Ti), iron (Fe) and cadmium (Cd), and lithiated versions
thereof; (b) alloys or intermetallic compounds of Si, Ge, Sn, Pb,
Sb, Bi, Zn, Al, or Cd with other elements, and lithiated versions
thereof, wherein said alloys or compounds are stoichiometric or
non-stoichiometric; (c) oxides, carbides, nitrides, sulfides,
phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi,
Zn, Al, Fe, Ni, Co, Ti, Mn, or Cd, and their mixtures or
composites, and lithiated versions thereof; (d) salts and
hydroxides of Sn and lithiated versions thereof; (e) carbon or
graphite materials and prelithiated versions thereof; and
combinations thereof.
[0080] In certain preferred embodiments, the metal-sulfur battery
is a sodium ion-sulfur cell or potassium ion-sulfur cell and the
anode contains an anode active material selected from the group
consisting of: (a0 Sodium- or potassium-doped silicon (Si),
germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi),
zinc (Zn), aluminum (Al), titanium (Ti), cobalt (Co), nickel (Ni),
manganese (Mn), cadmium (Cd), and mixtures thereof; (b) Sodium- or
potassium-containing alloys or intermetallic compounds of Si, Ge,
Sn, Pb, Sb, Bi, Zn, Al, Ti, Co, Ni, Mn, Cd, and their mixtures; (c)
Sodium- or potassium-containing oxides, carbides, nitrides,
sulfides, phosphides, selenides, tellurides, or antimonides of Si,
Ge, Sn, Pb, Sb, Bi, Zn, Al, Fe, Ti, Co, Ni, Mn, Cd, and mixtures or
composites thereof: (d) Sodium or potassium salts; (e) particles of
graphite, hard carbon, soft carbon or carbon particles and
pre-sodiated versions thereof; and combinations thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0081] FIG. 1 Schematic drawing illustrating the processes for
producing graphene sheets from natural graphite particles.
[0082] FIG. 2 A possible mechanism of chemical linking and merging
between humic acid molecules and between "linked HA molecules." Two
or three original HA molecules can get chemically linked together
to form longer or wider HA molecules, called "linked HA molecules".
Multiple "linked HA molecules" can be merged to form graphene-like
hexagonal carbon atomic planes.
[0083] FIG. 3(A) Thermal conductivity values vs. specific gravity
of the HA-derived foam produced by the presently invented process,
meso-phase pitch-derived graphite foam, and Ni foam-template
assisted CVD graphene foam;
[0084] FIG. 3(B) Thermal conductivity values of the HA-derived
foam, sacrificial plastic bead-templated GO foam, and the
hydrothermally reduced GO graphene foam.
[0085] FIG. 4 Electrical conductivity data from the HA-derived foam
produced by the presently invented process and the hydrothermally
reduced GO graphene foam.
[0086] FIG. 5 Thermal conductivity values of the foam samples,
derived from HA and fluorinated HA, plotted as a function of the
specific gravity.
[0087] FIG. 6 Thermal conductivity values of foam samples derived
from HA and pristine graphene as a function of the final (maximum)
heat treatment temperature.
[0088] FIG. 7 The specific capacities vs. number of
charge/discharge cycles for three Li--S cells: one featuring a
HA-derived foam cathode containing solution deposited
Li.sub.2S.sub.8 coating, one featuring a cathode of physical vapor
deposited sulfur in HA-derived foam, and one containing a cathode
containing RGO and sulfur ball-milled together
[0089] FIG. 8 The specific capacities vs. number of
charge/discharge cycles for 3 Na--S cells: one featuring a cathode
made of HA-derived foam containing solution deposited
Na.sub.2S.sub.8 coating in the pores, one containing vapor
deposited sulfur in the pores of HA-derived foam, and one
containing a cathode containing carbon black and sulfur ball-milled
together
[0090] FIG. 9 The cycling behaviors of a Li--S cell featuring a
Li.sub.2S .sub.1-loaded HA-derived foam structure and a Li--S cell
featuring a Li.sub.2S.sub.9-loaded HA-derived foam structure.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0091] The present invention provides a sulfur cathode for a
metal-sulfur battery, which can be a primary battery or secondary
battery (rechargeable battery). The sulfur cathode contains a humic
acid-derived foam, composed of multiple pores and pore walls, and
sulfur or sulfide impregnated into the pores or deposited on the
pore walls.
[0092] Humic acid (HA) is an organic matter commonly found in soil
and can be extracted from the soil using a base (e.g. KOH). HA can
also be extracted from a type of coal called leonardite, which is a
highly oxidized version of lignite coal. HA extracted from
leonardite contains a number of oxygenated groups (e.g. carboxyl
groups) located around the edges of the graphene-like molecular
center (SP.sup.2 core of hexagonal carbon structure). This material
is slightly similar to graphene oxide (GO) which is produced by
strong acid oxidation of natural graphite. HA has a typical oxygen
content of 5% to 42% by weight (other major elements being carbon,
hydrogen, and nitrogen). An example of the molecular structure for
humic acid, having a variety of components including quinone,
phenol, catechol and sugar moieties, is given in Scheme 1 below
(source: Stevenson F. J. "Humus Chemistry: Genesis, Composition,
Reactions," John Wiley & Sons, New York 1994).
##STR00001##
[0093] It is generally believed that the basic molecular structure
of humic acid derived from coal products (e.g. leonardite) has
fused benzene rings as schematically shown in Scheme 2 below (as an
example for illustration purposes), wherein the number of fused
rings can be varied from approximately 5 to several thousands, but
more typically from 10 to several hundreds. There are typically
functional groups, such as --COOH, --OH, and >O, attached to
edges of the fused-ring or aromatic structure.
##STR00002##
[0094] The present application is directed at all humic acid
species that can be represented by either Scheme 1 or Scheme 2.
Some of lower molecular weight humic acid molecules having --COOH
or --OH groups can be dissolved in water and alcohol. Non-aqueous
solvents for humic acid include polyethylene glycol, ethylene
glycol, propylene glycol, an alcohol, a sugar alcohol, a
polyglycerol, a glycol ether, an amine based solvent, an amide
based solvent, an alkylene carbonate, an organic acid, or an
inorganic acid.
[0095] The present invention provides a humic acid-derived foam
composed of multiple pores and pore walls and a process for
producing same. These pores are impregnated with sulfur or
polysulfide, preferably in a thin coating or fine nano particle
form. The pores in the foam are formed during or after sheet-like
humic acid molecules are (1) chemically linked/merged together
(edge-to-edge and/or face-to-face) typically at a temperature from
100 to 1,500.degree. C. and/or (2) organized into larger graphite
crystals or domains (herein referred to as graphitization) along
the pore walls at a high temperature (typically >2,100.degree.
C. and more typically >2,500.degree. C.).
[0096] The invention also provides a production process for the
impregnated foam. The process comprises: (a) preparing a humic acid
dispersion having multiple humic acid molecules or sheets dispersed
in a liquid medium (and, optionally, graphene sheets), wherein the
humic acid is selected from oxidized humic acid, reduced humic
acid, fluorinated humic acid, chlorinated humic acid, brominated
humic acid, iodized humic acid, hydrogenated humic acid,
nitrogenated humic acid, doped humic acid, chemically
functionalized humic acid, or a combination thereof and wherein the
dispersion contains an optional blowing agent having a blowing
agent-to-humic acid weight ratio from 0/1.0 to 1.0/1.0; (b)
dispensing and depositing the graphene dispersion onto a surface of
a supporting substrate (e.g. plastic film, rubber sheet, metal
foil, glass sheet, paper sheet, etc.) to form a wet layer of humic
acid; (c) partially or completely removing the liquid medium from
the wet layer of humic acid to form a dried layer of humic acid;
(d) heat treating the dried layer of humic acid at a first heat
treatment temperature from 80.degree. C. to 3,200.degree. C. at a
desired heating rate sufficient to induce volatile gas molecules
from the non-carbon elements (e.g. O, H, N, B, F, Cl, Br, I, etc.)
or to activate the blowing agent for producing humic acid-derived
foam; and (e) impregnating the pores with sulfur or polysulfide.
Preferably, the dispensing and depositing procedure includes
subjecting the humic acid dispersion to an orientation-inducing
stress. These non-carbon elements, when being removed via
heat-induced decomposition, produces volatile gases that act as a
foaming agent or blowing agent. The resulting humic acid-derived
foam or HA/graphene-derived foam, when measured without the
presence of sulfur or sulfide, typically has a density from 0.005
to 1.7 g/cm.sup.3 (more typically from 0.01 to 1.5 g/cm.sup.3, and
even more typically from 0.1 to 1.0 g/cm.sup.3, and most typically
from 0.2 to 0.75 g/cm.sup.3), or a specific surface area from 50 to
3,000 m.sup.2/g (more typically from 200 to 2,000 m.sup.2/g, and
most typically from 500 to 1,500 m.sup.2/g).
[0097] A blowing agent or foaming agent is a substance which is
capable of producing a cellular or foamed structure via a foaming
process in a variety of materials that undergo hardening or phase
transition, such as polymers (plastics and rubbers), glass, and
metals. They are typically applied when the material being foamed
is in a liquid state. It has not been previously known that a
blowing agent can be used to create a foamed material while in a
solid state. More significantly, it has not been previously taught
or hinted that an aggregate of humic acid molecules can be
converted into a graphene-like foam via a blowing agent. The
cellular structure in a matrix is typically created for the purpose
of reducing density, increasing thermal resistance and acoustic
insulation, while increasing the thickness and relative stiffness
of the original polymer.
[0098] Blowing agents or related foaming mechanisms to create pores
or cells (bubbles) in a matrix for producing a foamed or cellular
material, can be classified into the following groups: [0099] (a)
Physical blowing agents: e.g. hydrocarbons (e.g. pentane,
isopentane, cyclopentane), chlorofluorocarbons (CFCs),
hydrochlorofluorocarbons (HCFCs), and liquid CO.sub.2. The
bubble/foam-producing process is endothermic, i.e. it needs heat
(e.g. from a melt process or the chemical exotherm due to
cross-linking), to volatize a liquid blowing agent. [0100] (b)
Chemical blowing agents: e.g. isocyanate, azo-, hydrazine and other
nitrogen-based materials (for thermoplastic and elastomeric foams),
sodium bicarbonate (e.g. baking soda, used in thermoplastic foams).
Here gaseous products and other by-products are formed by a
chemical reaction, promoted by process or a reacting polymer's
exothermic heat. Since the blowing reaction involves forming low
molecular weight compounds that act as the blowing gas, additional
exothermic heat is also released. Powdered titanium hydride is used
as a foaming agent in the production of metal foams, as it
decomposes to form titanium and hydrogen gas at elevated
temperatures. Zirconium (II) hydride is used for the same purpose.
Once formed the low molecular weight compounds will never revert to
the original blowing agent(s), i.e. the reaction is irreversible.
[0101] (c) Mixed physical/chemical blowing agents: e.g. used to
produce flexible polyurethane (PU) foams with very low densities.
Both the chemical and physical blowing can be used in tandem to
balance each other out with respect to thermal energy
released/absorbed; hence, minimizing temperature rise. For
instance, isocyanate and water (which react to form CO.sub.2) are
used in combination with liquid CO.sub.2 (which boils to give
gaseous form) in the production of very low density flexible PU
foams for mattresses. [0102] (d) Mechanically injected agents:
Mechanically made foams involve methods of introducing bubbles into
liquid polymerizable matrices (e.g. an unvulcanized elastomer in
the form of a liquid latex). Methods include whisking-in air or
other gases or low boiling volatile liquids in low viscosity
lattices, or the injection of a gas into an extruder barrel or a
die, or into injection molding barrels or nozzles and allowing the
shear/mix action of the screw to disperse the gas uniformly to form
very fine bubbles or a solution of gas in the melt. When the melt
is molded or extruded and the part is at atmospheric pressure, the
gas comes out of solution expanding the polymer melt immediately
before solidification. [0103] (e) Soluble and leachable agents:
Soluble fillers, e.g. solid sodium chloride crystals mixed into a
liquid urethane system, which is then shaped into a solid polymer
part, the sodium chloride is later washed out by immersing the
solid molded part in water for some time, to leave small
inter-connected holes in relatively high density polymer products.
[0104] (f) We have found that the above five mechanisms can all be
used to create pores in the HA-derived materials while they are in
a solid state. Another mechanism of producing pores in a HA
material is through the generation and vaporization of volatile
gases by removing those non-carbon elements in a high-temperature
environment. This is a unique self-foaming process that has never
been previously taught or suggested.
[0105] The pore walls (cell walls) in the presently invented foam
contain chemically bonded and merged graphene-like hexagonal carbon
atomic planes. These planar aromatic molecules or hexagonal
structured carbon atoms are well interconnected physically and
chemically. The lateral dimensions (length or width) of these
planes are huge (from 20 nm to >10 .mu.m), typically several
times or even orders of magnitude larger than the maximum
length/width of the starting humic acid molecules. The hexagonal
carbon atomic planes are essentially interconnected to form long
electron-conducting pathways with low resistance. This is a unique
and new class of material that has not been previously discovered,
developed, or suggested to possibly exist.
[0106] In step (b), a HA suspension (or HA/graphene suspension) is
formed into a wet layer on a solid substrate surface (e.g. PET film
or glass) preferably under the influence of a shear stress. One
example of such a shearing procedure is casting or coating a thin
film of HA suspension (or HA/graphene suspension) using a coating
machine. This procedure is similar to a layer of varnish, paint,
coating, or ink being coated onto a solid substrate. The roller,
"doctor's blade", or wiper can create a shear stress when the film
is shaped at a high rate, or when there is a relative motion
between the roller/blade/wiper and the supporting substrate at a
high relative motion speed sufficient for achieving a high shearing
rate. (It may be noted that the mere use of a roller/blade/wiper
normally is not sufficient to enable a sufficient level of shearing
stress for HA molecular alignment.) Quite unexpectedly and
significantly, such a shearing action enables the planar HA
molecules to well align along, for instance, the shearing
direction. Further surprisingly, such a molecular alignment state
or preferred orientation is not disrupted when the liquid
components in the HA suspension are subsequently removed to form a
well-packed layer of highly aligned sheet-like HA molecules that
are at least partially dried. The dried HA film has a high
birefringence coefficient between an in-plane direction and the
normal-to-plane direction.
[0107] In an embodiment, this HA or HA/graphene layer is then
subjected to a heat treatment to activate the blowing agent and/or
the thermally-induced reactions that remove the non-carbon elements
(e.g. F, O, etc.) from the HA molecules to generate volatile gases
as by-products. These volatile gases generate pores or bubbles
inside the solid HA material, pushing sheet-like HA molecules into
a wall structure, forming a HA foam. If no blowing agent is added,
the non-carbon elements in the HA material preferably occupy at
least 10% by weight of the HA material (preferably at least 20%,
and further preferably at least 30%). The first (initial) heat
treatment temperature is typically greater than 80.degree. C.,
preferably greater than 100.degree. C., more preferably greater
than 300.degree. C., further more preferably greater than
500.degree. C. and can be as high as 1,500.degree. C. The blowing
agent is typically activated at a temperature from 80.degree. C. to
300.degree. C., but can be higher. The foaming procedure (formation
of pores, cells, or bubbles) is typically completed within the
temperature range of 80-1,500.degree. C. Quite surprisingly, the
chemical linking or merging between hexagonal carbon atomic planes
in an edge-to-edge and face-to-face manner (FIG. 2) can occur at a
relatively low heat treatment temperature (e.g. as low as from 150
to 300.degree. C.).
[0108] The HA- or HA/graphene-derived foam may be subjected to a
further heat treatment that involves at least a second temperature
that is significantly higher than the first heat treatment
temperature.
[0109] A properly programmed heat treatment procedure can involve
just a single heat treatment temperature (e.g. a first heat
treatment temperature only), at least two heat treatment
temperatures (first temperature for a period of time and then
raised to a second temperature and maintained at this second
temperature for another period of time), or any other combination
of heat treatment temperatures (HTT) that involve an initial
treatment temperature (first temperature) and a final HTT (second),
higher than the first. The highest or final HTT that the dried HA
layer experiences may be divided into four distinct HTT regimes:
[0110] Regime 1 (80.degree. C. to 300.degree. C.): In this
temperature range (the initial chemical linking regime and also the
activation regime for a blowing agent, if present), HA layer
primarily undergoes thermally-induced chemical linking of
neighboring HA molecules, as schematically illustrated in the upper
portion of FIG. 2. This also involves removal of some non-carbon
atoms, such as 0 and H, leading to a reduction of oxygen content
from typically 20-42% (of O in HA) to approximately 10-25%. This
treatment results in a reduction of inter-planar spacing in foam
walls from approximately 0.6-1.2 nm (as dried) down to
approximately 0.4-0.6 nm, and an increase in thermal conductivity
to 100 W/mK per unit specific gravity and/or electrical
conductivity to 2,000 S/cm per unit of specific gravity. (Since one
can vary the level of porosity and, hence, specific gravity of a
graphene foam material and, given the same graphene material, both
the thermal conductivity and electric conductivity values vary with
the specific gravity, these property values must be divided by the
specific gravity to facilitate a fair comparison.) Even with such a
low temperature range, some chemical linking between HA molecules
occurs. The inter-planar spacing remains relatively large (0.4 nm
or larger). Many O-containing functional groups survive (e.g. --OH
and --COOH). [0111] Regime 2 (300.degree. C-1,500.degree. C.): In
this chemical linking and merging regime, extensive chemical
combination, polymerization, and cross-linking between adjacent HA
molecules or linked HA molecules occur to form incipient
graphene-like hexagonal carbon atomic planes, as illustrated in
lower portion of FIG. 2. The oxygen content is reduced to typically
from 2% to 10% (e.g. after chemical linking and merging), resulting
in a reduction of inter-planar spacing to approximately 0.345 nm.
This implies that some initial graphitization has already begun at
such a low temperature, in stark contrast to conventional
graphitizable materials (such as carbonized polyimide film) that
typically require a temperature as high as 2,500.degree. C. to
initiate graphitization. This is another distinct feature of the
presently invented graphene foam and its production processes.
These chemical linking reactions result in an increase in thermal
conductivity to >250 W/mK per unit of specific gravity, and/or
electrical conductivity to 2,500-4,000 S/cm per unit of specific
gravity. Regime 3 (1,500-2,500.degree. C.): In this ordering and
graphitization regime, extensive graphitization or merging of
graphene-like planes occurs, leading to significantly improved
degree of structural ordering in the foam walls. As a result, the
oxygen content is reduced to typically 0.1%-2% and the
inter-graphene spacing to approximately 0.337 nm (achieving degree
of graphitization from 1% to approximately 80%, depending upon the
actual HTT and length of time). The improved degree of ordering is
also reflected by an increase in thermal conductivity to >350
W/mK per unit of specific gravity, and/or electrical conductivity
to >3,500 S/cm per unit of specific gravity. [0112] Regime 4
(higher than 2,500.degree. C.): In this re-crystallization and
perfection regime, extensive movement and elimination of grain
boundaries and other defects occur, resulting in the formation of
nearly perfect single crystals or poly-crystalline graphene-like
crystals with huge grains in the foam walls, which can be orders of
magnitude larger than the original sizes of HA molecules. The
oxygen content is essentially eliminated, typically 0%-0.01%. The
inter-planar spacing is reduced to down to approximately 0.3354 nm
(degree of graphitization from 80% to nearly 100%), corresponding
to that of a perfect graphite single crystal. The foamed structure
thus obtained exhibits a thermal conductivity of >400 W/mK per
unit of specific gravity, and electrical conductivity of >4,000
S/cm per unit of specific gravity.
[0113] The presently invented HA- or HA/graphene-derived foam
structure can be obtained by heat-treating the dried HA or
HA/graphene layer with a temperature program that covers at least
the first regime (typically requiring 1-4 hours in this temperature
range if the temperature never exceeds 500.degree. C.), more
commonly covers the first two regimes (1-2 hours preferred), still
more commonly the first three regimes (preferably 0.5-2.0 hours in
Regime 3), and can cover all the 4 regimes (including Regime 4 for
0.2 to 1 hour, may be implemented to achieve the highest
conductivity).
[0114] X-ray diffraction patterns were obtained with an X-ray
diffractometer equipped with CuKcv radiation. The shift and
broadening of diffraction peaks were calibrated using a silicon
powder standard. The degree of graphitization, g, was calculated
from the X-ray pattern using the Mering's Eq, d.sub.002=0.3354
g+0.344 (1-g), where .sub.002 is the interlayer spacing of
graphite- or graphene-type crystal in nm. This equation is valid
only when d.sub.002 is equal or less than approximately 0.3440 nm.
The HA-derived foam walls having a d.sub.002 higher than 0.3440 nm
reflects the presence of oxygen-containing functional groups (such
as --OH, >O, and --COOH on graphene-like molecular plane
surfaces or edges) that act as a spacer to increase the
inter-planar spacing.
[0115] Another structural index that can be used to characterize
the degree of ordering of the stacked and bonded hexagonal carbon
atomic planes in the foam walls of HA-derived graphene-like and
conventional graphite crystals is the "mosaic spread," which is
expressed by the full width at half maximum of a rocking curve
(X-ray diffraction intensity) of the (002) or (004) reflection.
This degree of ordering characterizes the graphite or graphene
crystal size (or grain size), amounts of grain boundaries and other
defects, and the degree of preferred grain orientation. A nearly
perfect single crystal of graphite is characterized by having a
mosaic spread value of 0.2-0.4. Most of our graphene walls have a
mosaic spread value in this range of 0.2-0.4 (if produced with a
heat treatment temperature (HTT) no less than 2,500.degree. C.).
However, some values are in the range of 0.4-0.7 if the HTT is
between 1,500 and 2,500.degree. C., and in the range of 0.7-1.0 if
the HTT is between 300 and 1,500.degree. C.
[0116] Illustrated in FIG. 2 is a plausible chemical linking and
merging mechanism where only 2 aligned HA molecular segments are
shown as an example, although a large number of HA molecules can be
chemically linked together and multiple "linked HA molecules" can
be chemically merged to form a foam wall. Further, chemical linking
could also occur face-to-face, not just edge-to-edge for HA
molecules or sheets. These linking and merging reactions proceed in
such a manner that the molecules are chemically merged, linked, and
integrated into one single entity. The resulting product is not a
simple aggregate of individual HA sheets, but a single entity that
is essentially a network of interconnected giant molecules with an
essentially infinite molecular weight. All the constituent
hexagonal carbon planes are very large in lateral dimensions
(length and width) and, if the HTT is sufficiently high (e.g.
>1,500.degree. C. or much higher), these planes are essentially
bonded together with one another.
[0117] In-depth studies using a combination of SEM, TEM, selected
area diffraction , X-ray diffraction, AFM, Raman spectroscopy, and
FTIR indicate that the HA-derived foam walls are composed of
several huge hexagonal carbon atomic planes (with length/width
typically >>20 nm, more typically >>100 nm, often
>>1 .mu.m, and, in many cases, >>10 .mu.m, or even
>>100 .mu.m). These giant graphene-like planes are stacked
and bonded along the thickness direction (crystallographic c-axis
direction) often through not just the van der Waals forces (as in
conventional graphite crystallites), but also covalent bonds, if
the final heat treatment temperature is lower than 2,500.degree. C.
In these cases, wishing not to be limited by theory, but Raman and
FTIR spectroscopy studies appear to indicate the co-existence of
sp.sup.2 (dominating) and sp.sup.a (weak but existing) electronic
configurations, not just the conventional sp.sup.2 in graphite. p1
(1) This HA-derived graphitic foam wall is not made by gluing or
bonding discrete flakes/platelets together with a resin binder,
linker, or adhesive. Instead, HA molecules are merged through
joining or forming of covalent bonds with one another, into an
integrated graphene-like crystal entity, without using any
externally added linker or binder molecules or polymers. p1 (2) The
foam wall is typically a poly-crystal composed of large grains
having incomplete grain boundaries. This entity is derived from
multiple HA molecules and these aromatic HA molecules have lost
their original identity. Upon removal of the liquid component from
the suspension, the resulting HA molecules form an essentially
amorphous structure. Upon heat treatments, these HA molecules are
chemically merged and linked into a unitary or monolithic graphitic
entity that constitutes the foam wall. This foam wall is highly
ordered. p1 (3) Due to these unique chemical composition (including
oxygen or non-carbon content), morphology, crystal structure
(including inter-planar spacing), and structural features (e.g.
high degree of orientations, few defects, incomplete grain
boundaries, chemical bonding and no gap between graphene sheets,
and substantially no interruptions in hexagonal carbon planes), the
HA-derived foam has a unique combination of outstanding thermal
conductivity, electrical conductivity, mechanical strength, and
stiffness (elastic modulus).
[0118] It may be further noted that a certain desired degree of
hydrophilicity can be imparted to the pore walls of the humic acid
-derived foam if the non-carbon element content (H and O) is from 2
to 20% by weight. These features impart different type of bonding
between sulfur (or sulfide) and hexagonal carbon atomic planes of
the pore walls.
[0119] If a high electrical or thermal conductivity is desired, the
HA-carbon foam can be subjected to graphitization treatment at a
temperature higher than 2,500.degree. C.
[0120] It may be noted that the HA-derived foam may be subjected to
compression during and/or after the graphitization treatment. This
operation enables us to adjust the orientation of hexagonal carbon
atomic planes and the degree of porosity.
[0121] In order to characterize the structure of the graphitic
materials produced, X-ray diffraction patterns were obtained with
an X-ray diffractometer equipped with CuKcv radiation. The shift
and broadening of diffraction peaks were calibrated using a silicon
powder standard. The degree of graphitization, g, was calculated
from the X-ray pattern using the Mering's Eq, d.sub.002=0.3354
g+0.344 (1-g), where d.sub.002 is the interlayer spacing of
graphite or graphene crystal in nm. This equation is valid only
when d.sub.002 is equal or less than approximately 0.3440 nm. In
the present study, the graphene-like (HA or RHA) foam walls having
a d.sub.002 higher than 0.3440 nm reflects the presence of
oxygen-containing functional groups (such as --OH, >O, and
--COOH on graphene molecular plane surfaces or edges) that act as a
spacer to increase the inter-graphene spacing.
[0122] Another structural index that can be used to characterize
the degree of ordering of the stacked and bonded RHA planes in the
foam walls of graphene-like and conventional graphite crystals is
the "mosaic spread," which is expressed by the full width at half
maximum of a rocking curve (X-ray diffraction intensity) of the
(002) or (004) reflection. This degree of ordering characterizes
the graphite or graphene crystal size (or grain size), amounts of
grain boundaries and other defects, and the degree of preferred
grain orientation. A nearly perfect single crystal of graphite is
characterized by having a mosaic spread value of 0.2-0.4. Most of
our RHA walls have a mosaic spread value in this range of 0.2-0.4
(if produced with a heat treatment temperature (HTT) no less than
2,500.degree. C.). However, some values are in the range of 0.4-0.7
if the HTT is between 1,500 and 2,500.degree. C., and in the range
of 0.7-1.0 if the HTT is between 300 and 1,500.degree. C.
[0123] In-depth studies using a combination of SEM, TEM, selected
area diffraction , X-ray diffraction, AFM, Raman spectroscopy, and
FTIR indicate that the humic acid-derived foam walls are composed
of several large graphene-like hexagonal carbon atomic planes (with
length/width typically >>20 nm, more typically >>100
nm, often >>1.mu.m, and, in many cases, >>10 .mu.m).
This is quite unexpected since the lateral dimensions (length and
width) of original humic acid sheets or molecules, prior to being
heat treated, are typically <20 nm and more typically <10 nm.
This implies that a plurality of HA sheets or molecules can be
merged edge to edge through covalent bonds with one another, into a
larger (longer or wider) sheet.
[0124] These large graphene-like planes also can be stacked and
bonded along the thickness direction (crystallographic c-axis
direction) often through not just the van der Waals forces (as in
conventional graphite crystallites), but also covalent bonds, if
the final heat treatment temperature is lower than 2,500.degree. C.
In these cases, wishing not to be limited by theory, but Raman and
FTIR spectroscopy studies appear to indicate the co-existence of
sp.sup.2 (dominating) and sp.sup.3 (weak but existing) electronic
configurations, not just the conventional sp.sup.2 in graphite.
[0125] The integral HA-derived foam is composed of multiple pores
and pore walls, wherein the pore walls contain single-layer or
few-layer HA sheets chemically bonded together, wherein the
few-layer HA sheets have 2-10 layers of stacked graphene-like
merged HA planes having an inter-plane spacing d.sub.002 from
0.3354 nm to 0.36 nm as measured by X-ray diffraction and the
single-layer or few-layer graphene-like HA sheets contain 0.01% to
25% by weight of non-carbon elements (more typically <15%).
[0126] The integral HA-derived foam typically has a density from
0.001 to 1.7 g/cm.sup.3, a specific surface area from 50 to 3,000
m.sup.2/g, a thermal conductivity of at least 200 W/mK per unit of
specific gravity, and/or an electrical conductivity no less than
2,000 S/cm per unit of specific gravity. In a preferred embodiment,
the pore walls contain stacked graphene-like RHA planes having an
inter-planar spacing d.sub.002 from 0.3354 nm to 0.40 nm as
measured by X-ray diffraction. All these properties were measured
without the presence of sulfur or sulfide.
[0127] Many of the HA sheets can be merged edge to edge through
covalent bonds with one another, into an integrated reduced HA
(RHA) entity. Due to these unique chemical composition (including
oxygen or hydrogen content, etc.), morphology, crystal structure
(including inter-planar spacing), and structural features (e.g.
degree of orientations, few defects, chemical bonding and no gap
between graphene-like sheets, and substantially no interruptions
along hexagonal plane directions), the HA-derived foam has a unique
combination of outstanding thermal conductivity, electrical
conductivity, mechanical strength, and stiffness (elastic
modulus).
[0128] Once a layer of HA-derived foam (preferably having pores of
1-100 nm in size, more preferably 2-50 nm, and most preferably 2-20
nm) is prepared, this layer can be impregnated with a desired
amount of sulfur or sulfide, particularly metal polysulfide,
M.sub.xS.sub.y, using several techniques:
[0129] The dip-coating technique is simple and effective and can be
fully automated. In an embodiment, a proper amount of sulfur or
M.sub.xS.sub.y is dissolved in a suitable solvent up to 0.1-10% by
weight (typically <5%) to form a solution. A continuous sheet of
HA-derived foam can be fed from a feeder roller and immersed into a
bath containing such solution and emerged from this path, allowing
the solvent to be removed before the impregnated porous sheet of
foam is wound on a winding roller. With a proper pore size range
(preferably 2-50 nm) and surface chemical state of the hexagonal
carbon atomic planes, species of sulfur or M.sub.xS.sub.y readily
migrate into the pores and deposit, as a coating or nano particles,
onto pore internal wall surfaces, or simply precipitates out as
nano sulfur or M.sub.xS.sub.y particles residing in the pores of
the porous structure.
[0130] This is a roll-to-roll or reel-to-reel process and is highly
scalable. In other words, the active cathode layer can be mass
produced cost-effectively.
[0131] The liquid dispensing and coating technique is also simple
and effective, and can be automated as well. Again, a layer of
porous structure can be fed from a feeder roller and collected on a
winding roller. Between these two ends, a solution or suspension
(containing sulfur or M.sub.xS.sub.y dissolved/dispersed in a
liquid solvent) is dispensed and deposited on one or both surfaces
of a porous structure, allowing solution or suspension to permeate
into pores of the foam structure. Heating and/or drying provisions
are also installed to help remove the solvent, allowing the sulfur
or M.sub.xS.sub.y species to permeate into the porous structure and
precipitate out as a nano coating or nano particles. A broad array
of dispensing/depositing techniques can be used;
[0132] e.g. spraying (aerosol spraying, ultrasonic spraying,
compressed air-driven spraying, etc.), printing (inkjet printing,
screen printing, etc.), and coating (slot-die coating, roller
coating, etc.). Alternatively, sulfur may be sublimed or vaporized
in a chamber and the foam structure is allowed to pass through this
chamber, enabling permeation of sulfur into pores. This
roll-to-roll process is highly scalable.
[0133] The processing conditions can be readily adjusted to deposit
sulfur or M.sub.xS.sub.y particles or coating that have a thickness
or diameter smaller than 20 nm (preferably <10 nm, more
preferably <5 nm, and further preferably <3 nm). The
resulting nano-scaled metal polysulfide particles or coating occupy
a weight fraction of from 1% to 99%, but preferably at least 50%
(preferably >70%, further preferably 80%, more preferably
>90%, and most preferably >95%) based on the total weights of
the sulfur particles or coating and the graphene material
combined.
[0134] A range of polysulfide, M.sub.xS.sub.y, can be selected,
wherein x is an integer from 1 to 3 and y is an integer from 1 to
10, and M is a metal element selected from an alkali metal, an
alkaline metal selected from Mg or Ca, a transition metal, a metal
from groups 13 to 17 of the periodic table, or a combination
thereof. In a desired embodiment, the metal element M is selected
from Li, Na, K, Mg, Zn, Cu, Ti, Ni, Co, Fe, or Al. In a
particularly desired embodiment, M.sub.xS.sub.y is selected from
Li.sub.2S.sub.6, Li.sub.2S.sub.7, Li.sub.2S.sub.8, Li.sub.2S.sub.9,
Li.sub.2S.sub.10, Na.sub.2S.sub.4, Na.sub.2S.sub.5,
Na.sub.2S.sub.6, Na.sub.2S.sub.7, Na.sub.2S.sub.8, Na.sub.2S.sub.9,
Na.sub.2S.sub.10, K.sub.2S.sub.4, K.sub.2S.sub.5, K.sub.2S.sub.6,
K.sub.2S.sub.7, K.sub.2S.sub.8, K.sub.2S.sub.9, or
K.sub.2S.sub.10.
[0135] Depending upon the intended type of M.sub.xS.sub.y used, the
solvent may be selected from 1,3-dioxolane (DOL),
1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether
(TEGDME), poly(ethylene glycol) dimethyl ether (PEGDME), diethylene
glycol dibutyl ether (DEGDBE), 2-ethoxyethyl ether (EEE), sulfone,
sulfolane, ethylene carbonate (EC), dimethyl carbonate (DMC),
methylethyl carbonate (MEC), diethyl carbonate (DEC), ethyl
propionate, methyl propionate, propylene carbonate (PC),
gamma-butyrolactone (y-BL), acetonitrile (AN), ethyl acetate (EA),
propyl formate (PF), methyl formate (MF), toluene, xylene, methyl
acetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate
(VC), allyl ethyl carbonate (AEC), a hydrofluoroether, a room
temperature ionic liquid solvent, or a combination thereof. The
M.sub.xS.sub.y deposition is conducted before the cathode active
layer is incorporated into an intended alkali metal-sulfur battery
cell (e.g. a Li--S).
[0136] The solution or suspension may optionally contain some metal
ion salts (e.g. Li salt if the cathode layer is intended for use in
a Li--S cell, Na salt if Na-S cell, etc.). After battery cell
fabrication, this salt can become part of the electrolyte system of
the intended battery cell. This alkali metal salt may be selected
from lithium perchlorate (LiClO.sub.4), lithium hexafluorophosphate
(LiPF.sub.6), lithium borofluoride (LiBF.sub.4), lithium
hexafluoroarsenide (LiAsF.sub.6), lithium trifluoro-metasulfonate
(LiCF.sub.3SO.sub.3), bis-trifluoromethyl sulfonylimide lithium
(LiN(CF.sub.3SO.sub.2).sub.2), lithium bis(oxalato)borate (LiBOB),
lithium oxalyldifluoroborate (LiBF.sub.2C.sub.2O.sub.4), lithium
oxalyl-difluoroborate (LiBF.sub.2C.sub.2O.sub.4), lithium nitrate
(LiNO.sub.3), Li-Fluoroalkyl-Phosphates
(LiPF.sub.3(CF.sub.2CF.sub.3).sub.3), lithium
bisperfluoro-ethysulfonylimide (LiBETI), lithium
bis(trifluoromethanesulphonyl)imide, lithium
bis(fluorosulphonyl)imide, lithium trifluoromethanesulfonimide
(LiTFSI), an ionic liquid-based lithium salt, sodium perchlorate
(NaClO.sub.4), potassium perchlorate (KClO.sub.4), sodium
hexafluorophosphate (NaPF.sub.6), potassium hexafluorophosphate
(KPF.sub.6), sodium borofluoride (NaBF.sub.4), potassium
borofluoride (KBF.sub.4), sodium hexafluoroarsenide, potassium
hexafluoroarsenide, sodium trifluoro-metasulfonate
(NaCF.sub.3SO.sub.3), potassium trifluoro-metasulfonate
(KCF.sub.3SO.sub.3), bis-trifluoromethyl sulfonylimide sodium
(NaN(CF.sub.3SO.sub.2).sub.2), sodium trifluoromethanesulfonimide
(NaTFSI), and bis-trifluoromethyl sulfonylimide potassium
(KN(CF.sub.3SO.sub.2).sub.2), or a combination thereof.
[0137] After an extensive and in-depth research effort, we have
come to realize that such a M.sub.xS.sub.y pre-loading strategy
surprisingly solves several most critical issues associated with
current Li--S, Na--S, and K--S cells. For instance, this method
enables the metal sulfide to be deposited in a thin coating or
ultra-fine particle form, thus, providing ultra-short diffusion
paths for Li/Na/K ions and, hence, ultra-fast reaction times for
fast battery charges and discharges. This is achieved while
maintaining a relatively high proportion of metal sulfide, which is
later converted into sulfur in the battery cell. Since sulfur is
the active material responsible for storing Li, Na, or K, this high
loading of M.sub.xS.sub.y implies a high specific Li/Na/K storage
capacity of the resulting cathode active layer in terms of mAh/g,
based on the total weight of the cathode layer, including the
masses of the active material, supporting conductive material such
as HA-derived graphene-like walls, optional binder resin, and
optional conductive filler).
[0138] It is of significance to note that one might be able to use
a prior art procedure to deposit small particles of S or select
lithium polysulfide, but not a high S or lithium polysulfide
proportion, or to achieve a high proportion but only in large
particles or thick film form. But, the prior art procedures have
not been able to achieve both high S or lithium polysulfide
proportion and ultra-thin coating/particles at the same time. It is
highly advantageous to obtain a high lithium polysulfide loading
and yet, concurrently, maintaining an ultra-thin/small
thickness/diameter of lithium polysulfide for significantly
enhanced energy density and power density. This has not been
possible with any prior art sulfur loading techniques. For
instance, we have been able to deposit nano-scaled metal
polysulfide particles or coating that occupy a >90% weight
fraction of the cathode layer and yet maintaining a coating
thickness or particle diameter <3 nm. This is quite a feat in
the art of metal-sulfur batteries. As another example, we have
achieved a >95% S loading at an average polysulfide coating
thickness of 4.0-6 nm.
[0139] Electrochemists or materials scientists in the art of Li--S
and Na--S batteries would expect that a greater amount of highly
conducting carbon or graphite materials (hence, a smaller amount of
S or polysulfide) in the cathode active layer should lead to a
better utilization of S, particularly under high charge/discharge
rate conditions. Contrary to these expectations, we have observed
that the key to achieving a high S utilization efficiency is
minimizing the S or metal polysulfide coating or particle size and
this is independent of the amount of S or polysulfide loaded into
the cathode provided the coating or particle thickness/diameter is
small enough (e.g. <10 nm, or even better if <5 nm). The
problem here is that it has not been previously possible to
maintain a thin S or metal polysulfide coating or small particle
size if S or polysulfide is higher than 50% by weight. Here we have
further surprisingly observed that the key to enabling a high
specific capacity at the cathode under high charge/discharge rate
conditions is to maintain a high S or polysulfide loading and still
keep the coating or particle size as small as possible, and this is
accomplished by using the presently invented polysulfide
pre-loading method.
[0140] The electrons coming from or going out through the external
load or circuit must go through the conductive additives (in a
conventional sulfur cathode) or a conductive framework (e.g.
conductive meso-porous structure as herein disclosed) to reach the
cathode active material. Since the cathode active material (e.g.
sulfur or metal polysulfide) is a poor electronic conductor, the
active material particle or coating must be as thin as possible to
reduce the required electron travel distance.
[0141] Furthermore, the cathode in a conventional metal-sulfur cell
typically has less than 70% by weight of sulfur in a composite
cathode composed of sulfur and the conductive additive/support.
Even when the sulfur content in the prior art composite cathode
reaches or exceeds 70% by weight, the specific capacity of the
composite cathode is typically significantly lower than what is
expected based on theoretical predictions. For instance, the
theoretical specific capacity of sulfur is 1,675 mAh/g. A composite
cathode composed of 70% sulfur (S) and 30% carbon black (CB),
without any binder, should be capable of storing up to
1,675.times.70%=1,172 mAh/g. Unfortunately, the observed specific
capacity of sulfur cathode is typically less than 879 mAh/g
(<75% of S being utilized) and often less than 586 mAh/g (or
<50% in this example) of what could be achieved. In other words,
the active material (S) utilization rate is typically less than 75%
(or even <50%). This has been a major issue in the art of
metal-sulfur cells and there has been no effective solution to this
problem. Most surprisingly, the implementation of a porous
structure (HA-derived foam) as a conductive supporting material for
polysulfide has made it possible to achieve an active material
utilization rate of typically >>80%, more often greater than
90%, and, in many cases, close to 95%-99%.
[0142] Still another unexpected result of the instant invention is
the observation that thinner polysulfide coating leads to more
stable charge/discharge cycling with significantly reduced
shuttling effect that has been a long-standing impediment to full
commercialization of metal-sulfur batteries. We overcome this
problem yet, at the same time, achieving a high specific capacity.
In all prior art Li--S cells, a higher S loading leads to a faster
capacity decay. The shuttling effect is related to the tendency for
sulfur or alkali metal polysulfide that forms at the cathode to get
dissolved in the solvent and for the dissolved metal polysulfide
species to migrate from the cathode to the anode, where they
irreversibly react with metal anode (e.g. lithium or sodium) to
form species that prevent sulfide from returning back to the
cathode during the subsequent discharge operation of the Li--S cell
(the detrimental shuttling effect). It seems that the presence of
massive hexagonal carbon atomic plane surfaces have been able to
prevent or reduce such a dissolution and migration issue.
[0143] Further significantly, we have unexpectedly discovered that
a M.sub.xS.sub.y-preloaded cathode layer is more robust than a
S-preloaded cathode layer in terms of maintaining the specific
capacity of the cathode. This is likely due to the notion that a
M.sub.xS.sub.y-preloaded cathode layer has already naturally built
in some expanded volume and hence is less prone or more resistant
to sulfur volume expansion-induced damage upon repeated
charges/discharges.
[0144] In some embodiments, the cathode layer can contain a
conductive filler, such as carbon black (CB), acetylene black (AB),
graphite particles, activated carbon, meso-porous carbon,
meso-carbon micro bead (MCMB), carbon nano-tube (CNT), carbon
nano-fiber (CNF), carbon fiber, or a combination thereof. These
materials (not meso-porous) are merely for use as a conductive
filler, not as a support for polysulfide.
[0145] The present invention also provides a rechargeable
metal-sulfur cell comprising an anode active material layer, an
optional anode current collector, a porous separator and/or an
electrolyte, an S-- or M.sub.xS.sub.y-preloaded active cathode
layer herein disclosed, and an optional cathode current collector.
The metal-sulfur cell can be a lithium-sulfur cell (including a
lithium metal-sulfur cell or lithium ion-sulfur cell),
sodium-sulfur cell (including a sodium metal-sulfur cell or sodium
ion-sulfur cell), potassium-sulfur cell (including potassium
metal-sulfur cell or potassium ion-sulfur cell), magnesium
metal-sulfur or magnesium-ion sulfur cell, and aluminum-sulfur or
aluminum ion-sulfur cell.
[0146] In the rechargeable metal-sulfur cell, the electrolyte maybe
selected from polymer electrolyte, polymer gel electrolyte,
composite electrolyte, ionic liquid electrolyte, non-aqueous liquid
electrolyte, soft matter phase electrolyte, solid-state
electrolyte, or a combination thereof.
[0147] The electrolytic salts to be incorporated into a non-aqueous
electrolyte may be selected from a metal salt such as lithium
perchlorate (LiClO.sub.4), lithium hexafluorophosphate
(LiPF.sub.6), lithium borofluoride (LiBF.sub.4), lithium
hexafluoroarsenide (LiAsF.sub.6), lithium trifluoro-metasulfonate
(LiCF.sub.3SO.sub.3), bis-trifluoromethyl sulfonylimide lithium
[LiN(CF.sub.3SO.sub.2).sub.2], lithium bis(oxalato)borate (LiBOB),
lithium oxalyldifluoroborate (LiBF.sub.2C.sub.2O.sub.4), lithium
oxalyldifluoroborate (LiBF.sub.2C.sub.2O.sub.4), lithium nitrate
(LiNO.sub.3), Li-Fluoroalkyl-Phosphates
(LiPF3(CF.sub.2CF.sub.3).sub.3), lithium
bisperfluoroethysulfonylimide (LiBETI), an ionic liquid salt,
sodium perchlorate (NaClO.sub.4), potassium perchlorate
(KClO.sub.4), sodium hexafluorophosphate (NaPF.sub.6), potassium
hexafluorophosphate (KPF.sub.6), sodium borofluoride (NaBF.sub.4),
potassium borofluoride (KBF.sub.4), sodium hexafluoroarsenide,
potassium hexafluoroarsenide, sodium trifluoro-metasulfonate
(NaCF.sub.3SO.sub.3), potassium trifluoro-metasulfonate
(KCF.sub.3SO.sub.3), bis-trifluoromethyl sulfonylimide sodium
(NaN(CF.sub.3SO.sub.2).sub.2), sodium trifluoromethanesulfonimide
(NaTFSI), and bis-trifluoromethyl sulfonylimide potassium
(KN(CF.sub.3SO.sub.2).sub.2), Mg(AlCl.sub.2EtBu).sub.2,
MgCl.sub.2/AlCl.sub.3, Mg(ClO.sub.4).sub.2, Mg(OH).sub.2,
Al(OH).sub.3. Among them, LiPF.sub.6, LiBF.sub.4 and
LiN(CF.sub.3SO.sub.2).sub.2 are preferred for Li--S cells,
NaPF.sub.6 and LiBF.sub.4 for Na--S cells, KBF.sub.4 for K--S
cells, and Mg(AlCl.sub.2EtBu).sub.2, MgCl.sub.2/AlCl.sub.3,
Mg(ClO.sub.4).sub.2, Mg(OH).sub.2, and Al(OH).sub.3 for Mg--S or
Al--S cells. The content of aforementioned electrolytic salts in
the non-aqueous solvent is preferably 0.5 to 3.0 M (mol/L) at the
cathode side and 2.0 to >10 M at the anode side.
[0148] The ionic liquid is composed of ions only. Ionic liquids are
low melting temperature salts that are in a molten or liquid state
when above a desired temperature. For instance, a salt is
considered as an ionic liquid if its melting point is below
100.degree. C. If the melting temperature is equal to or lower than
room temperature (25.degree. C.), the salt is referred to as a room
temperature ionic liquid (RTIL). The IL salts are characterized by
weak interactions, due to the combination of a large cation and a
charge-delocalized anion. This results in a low tendency to
crystallize due to flexibility (anion) and asymmetry (cation).
[0149] A typical and well-known ionic liquid is formed by the
combination of a 1-ethyl-3-methylimidazolium (EMI) cation and an
N,N-bis(trifluoromethane)sulphonamide (TFSI) anion. This
combination gives a fluid with an ionic conductivity comparable to
many organic electrolyte solutions and a low decomposition
propensity and low vapor pressure up to .about.300-400.degree. C.
This implies a generally low volatility and non-flammability and,
hence, a much safer electrolyte for batteries.
[0150] Ionic liquids are basically composed of organic ions that
come in an essentially unlimited number of structural variations
owing to the preparation ease of a large variety of their
components. Thus, various kinds of salts can be used to design the
ionic liquid that has the desired properties for a given
application. These include, among others, imidazolium,
pyrrolidinium and quaternary ammonium salts as cations and
bis(trifluoromethanesulphonyl) imide, bis(fluorosulphonyl)imide,
and hexafluorophosphate as anions. Based on their compositions,
ionic liquids come in different classes that basically include
aprotic, protic and zwitterionic types, each one suitable for a
specific application.
[0151] Common cations of room temperature ionic liquids (RTILs)
include, but not limited to, tetraalkylammonium, di-, tri-, and
tetra-alkylimidazolium, alkylpyridinium, dialkyl-pyrrolidinium,
dialkylpiperidinium, tetraalkylphosphonium, and trialkylsulfonium.
Common anions of RTILs include, but not limited to, BF.sub.4.sup.-,
B(CN).sub.4.sup.-, CH.sub.3BF.sub.3.sup.-, CH2CHBF.sub.3.sup.-,
CF.sub.3BF.sub.3.sup.-, C.sub.2F.sub.5BF.sub.3.sup.-,
n-C.sub.3F.sub.7BF.sub.3.sup.-, n-C.sub.4F.sub.9BF.sub.3.sup.-,
PF.sub.6.sup.-, CF.sub.3CO.sub.2.sup.-, CF.sub.3SO.sub.3.sup.-,
N(SO.sub.2CF.sub.3).sub.2.sup.-, N(COCF.sub.3)(SO.sub.2CF.sub.3)
N(SO.sub.2F)2, N(CN).sub.2.sup.-, C(CN).sub.3.sup.-, SCN.sup.-,
SeCN.sup.-, CuCl.sub.2.sup.-, AlCl.sub.4, F(HF).sub.2.3.sup.-, etc.
Relatively speaking, the combination of imidazolium- or
sulfonium-based cations and complex halide anions such as
AlCl.sub.4, BF.sub.4.sup.-, CF.sub.3CO.sub.2.sup.-,
CF.sub.3SO.sub.3.sup.-, NTf.sub.2.sup.-, N(SO.sub.2F).sub.2.sup.-,
or F(HF).sub.2.3.sup.-results in RTILs with good working
conductivities.
[0152] RTILs can possess archetypical properties such as high
intrinsic ionic conductivity, high thermal stability, low
volatility, low (practically zero) vapor pressure,
non-flammability, the ability to remain liquid at a wide range of
temperatures above and below room temperature, high polarity, high
viscosity, and wide electrochemical windows. These properties,
except for the high viscosity, are desirable attributes when it
comes to using an RTIL as an electrolyte ingredient (a salt and/or
a solvent) in a Li--S cell.
[0153] In the present metal-sulfur cell or metal ion-sulfur cell,
the anode active material may contain, as an example, lithium metal
foil (Li particles, Na metal foil, K metal foil, Mg foil, Al foil,
etc.) or a high-capacity anode (e.g. Si, Sn, or SnO.sub.2) capable
of storing a great amount of lithium (or Na, or K).
[0154] At the anode side, when lithium metal or sodium metal is
used as the sole anode active material in a Li--S or Na-S cell,
there is concern about the formation of lithium dendrites, which
could lead to internal shorting and thermal runaway. Herein, we
have used two approaches, separately or in combination, to address
this dendrite formation issue: one involving the use of a
high-concentration electrolyte at the anode side and the other the
use of a nano-structure composed of conductive nano-filaments. For
the latter, multiple conductive nano-filaments are processed to
form an integrated aggregate structure, preferably in the form of a
closely packed web, mat, or paper, characterized in that these
filaments are intersected, overlapped, or somehow bonded (e.g.,
using a binder material) to one another to form a network of
electron-conducting paths. The integrated structure has
substantially interconnected pores to accommodate electrolyte. The
nano-filament may be selected from, as examples, a carbon nano
fiber (CNF), graphite nano fiber (GNF), carbon nano-tube (CNT),
metal nano wire (MNW), conductive nano-fibers obtained by
electro-spinning, conductive electro-spun composite nano-fibers,
nano-scaled graphene platelet (NGP), or a combination thereof. The
nano-filaments may be bonded by a binder material selected from a
polymer, coal tar pitch, petroleum pitch, meso-phase pitch, coke,
or a derivative thereof.
[0155] Nano fibers may be selected from the group consisting of an
electrically conductive electro-spun polymer fiber, electro-spun
polymer nanocomposite fiber comprising a conductive filler, nano
carbon fiber obtained from carbonization of an electro-spun polymer
fiber, electro-spun pitch fiber, and combinations thereof. For
instance, a nano-structured electrode can be obtained by
electro-spinning of polyacrylonitrile (PAN) into polymer
nano-fibers, followed by carbonization of PAN. It may be noted that
some of the pores in the structure, as carbonized, are greater than
100 nm and some smaller than 100 nm.
[0156] The presently invented cathode active layer may be
incorporated in one of at least four broad classes of rechargeable
lithium-sulfur cells (or, similarly, for sodium-sulfur,
potassium-sulfur, magnesium-sulfur, and aluminum-sulfur cells):
[0157] (A) Lithium metal-sulfur with a conventional anode
configuration: The cell contains an optional cathode current
collector, a presently invented cathode active layer, a
separator/electrolyte, and an anode current collector. Potential
dendrite formation may be overcome by using the high-concentration
electrolyte at the anode. [0158] (B) Lithium metal-sulfur cell with
a nano-structured anode configuration: The cell contains an
optional cathode current collector, a cathode herein invented, a
separator/electrolyte, an optional anode current collector, and a
nano-structure to accommodate lithium metal that is deposited back
to the anode during a charge or re-charge operation. This
nano-structure (web, mat, or paper) of nano-filaments provide a
uniform electric field enabling uniform Li metal deposition,
reducing the propensity to form dendrites. This configuration can
provide a dendrite-free cell for a long and safe cycling behavior.
[0159] (C) Lithium ion-sulfur cell with a conventional anode: For
instance, the cell contains an anode composed of anode active
graphite particles bonded by a binder, such as polyvinylidene
fluoride (PVDF) or styrene-butadiene rubber (SBR). The cell also
contains a cathode current collector, a cathode of the instant
invention, a separator/electrolyte, and an anode current collector;
and [0160] (D) Lithium ion-sulfur cell with a nano-structured
anode: For instance, the cell contains a web of nano-fibers coated
with Si coating or bonded with Si nano particles. The cell also
contains an optional cathode current collector, an active cathode
layer herein invented, a separator/electrolyte, and an anode
current collector. This configuration provides an ultra-high
capacity, high energy density, and a safe and long cycle life.
[0161] In the lithium-ion sulfur cell (e.g. as described in (C) and
(D) above), the anode active material can be selected from a wide
range of high-capacity materials, including (a) silicon (Si),
germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi),
zinc (Zn), aluminum (Al), nickel (Ni), cobalt (Co), manganese (Mn),
titanium (Ti), iron (Fe) and cadmium (Cd), and lithiated versions
thereof; (b) alloys or intermetallic compounds of Si, Ge, Sn, Pb,
Sb, Bi, Zn, Al, or Cd with other elements, and lithiated versions
thereof, wherein said alloys or compounds are stoichiometric or
non-stoichiometric; (c) oxides, carbides, nitrides, sulfides,
phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi,
Zn, Al, Fe, Ni, Co, Ti, Mn, or Cd, and their mixtures or
composites, and lithiated versions thereof; (d) salts and
hydroxides of Sn and lithiated versions thereof; (e) carbon or
graphite materials and prelithiated versions thereof; and
combinations thereof. Non-lithiated versions may be used if the
cathode side contains lithium polysulfides or other lithium sources
when the cell is made.
[0162] A possible lithium metal cell may be comprised of an anode
current collector, an electrolyte phase (optionally but preferably
supported by a porous separator, such as a porous
polyethylene-polypropylene co-polymer film), a cathode of the
instant invention, and an optional cathode collector. This cathode
current collector is optional because the presently invented layer
of porous HA-derived foam structure, if properly designed, can act
as a current collector or as an extension of a current collector
due to its high electrical and thermal conductivity.
[0163] For a sodium ion-sulfur cell or potassium ion-sulfur cell,
the anode active material layer can contain an anode active
material selected from the group consisting of: (a) Sodium- or
potassium-doped silicon (Si), germanium (Ge), tin (Sn), lead (Pb),
antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), titanium
(Ti), cobalt (Co), nickel (Ni), manganese (Mn), cadmium (Cd), and
mixtures thereof; (b) Sodium- or potassium-containing alloys or
intermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Co,
Ni, Mn, Cd, and their mixtures; (c) Sodium- or potassium-containing
oxides, carbides, nitrides, sulfides, phosphides, selenides,
tellurides, or antimonides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Fe,
Ti, Co, Ni, Mn, Cd, and mixtures or composites thereof; (d) Sodium
or potassium salts; (e) particles of graphite, hard carbon, soft
carbon or carbon particles and pre-sodiated versions thereof
(pre-doped or pre-loaded with Na), and combinations thereof.
[0164] The following examples are used to illustrate some specific
details about the best modes of practicing the instant invention
and should not be construed as limiting the scope of the
invention.
EXAMPLE 1
Humic Acid and Reduced Humic Acid from Leonardite and Production of
HA- and HA/Graphene-Derived Foams
[0165] Humic acid can be extracted from leonardite by dispersing
leonardite in a basic aqueous solution (pH of 10) with a very high
yield (in the range of 75%). Subsequent acidification of the
solution leads to precipitation of humic acid powder. In an
experiment, 3 g of leonardite was dissolved by 300 ml of double
deioni zed water containing 1M KOH (or NH.sub.4OH) solution under
magnetic stirring. The pH value was adjusted to 10. The solution
was then filtered to remove any big particles or any residual
impurities. The resulting humic acid dispersion, containing HC
alone or with the presence of a blowing agent, was cast onto a
glass substrate to form a series of films for subsequent heat
treatments.
[0166] In some samples, a chemical blowing agent (hydrazo
dicarbonamide) was added to the suspension just prior to casting.
In some samples, graphene oxide (GO) was added into the suspension.
The resulting suspension was then cast onto a glass surface using a
doctor's blade to exert shear stresses, inducing HA molecular
orientations. The resulting HA coating films, after removal of
liquid, have a thickness that can be varied from approximately 10
nm to 800 .mu.m (preferably and typically from 50 .mu.m to 500
.mu.m for metal-sulfur battery cathodes).
[0167] For making an HA-derived or HA/graphene-derived foam
specimen, the HA or HA/graphene coating film was then subjected to
heat treatments that typically involve an initial thermal reduction
temperature of 80-350.degree. C. for 1-8 hours, followed by
heat-treating at a second temperature of 1,500-2,850.degree. C. for
0.5 to 5 hours. It may be noted that we have found it essential to
apply a compressive stress to the coating film sample while being
subjected to the first heat treatment. This compress stress seems
to have helped maintain good contacts between the HA molecules or
sheets so that chemical merging and linking between HA molecules or
sheets can occur while pores are being formed. Without such a
compressive stress, the heat-treated film was typically excessively
porous with constituent hexagonal carbon atomic planes in the pore
walls being very poorly oriented/positioned, and incapable of
chemical merging and linking with one another. As a result, the
thermal conductivity, electrical conductivity, and mechanical
strength of the graphene foam were severely compromised.
EXAMPLE 2
Various Blowing Agents and Pore-Forming (Bubble-Producing)
Processes
[0168] In the field of plastic processing, chemical blowing agents
are mixed into the plastic pellets in the form of powder or pellets
and dissolved at higher temperatures. Above a certain temperature
specific for blowing agent dissolution, a gaseous reaction product
(usually nitrogen or CO.sub.2) is generated, which acts as a
blowing agent. However, a chemical blowing agent cannot be
dissolved in a graphene material, which is a solid, not liquid.
This presents a challenge to make use of a chemical blowing agent
to generate pores or cells in a graphene material. After extensive
experimenting, we have discovered that practically any chemical
blowing agent (e.g. in a powder or pellet form) can be used to
create pores or bubbles in a dried layer of graphene when the first
heat treatment temperature is sufficient to activate the blowing
reaction. The chemical blowing agent (powder or pellets) may be
dispersed in the liquid medium to become a second component in the
suspension, which can be deposited onto the solid supporting
substrate to form a wet layer. This wet layer of HA material may
then be dried and heat treated to activate the chemical blowing
agent. After a chemical blowing agent is activated and bubbles are
generated, the resulting foamed HA structure is largely maintained
even when subsequently a higher heat treatment temperature is
applied to the structure. This is quite unexpected, indeed.
[0169] Chemical foaming agents (CFAs) can be organic or inorganic
compounds that release gasses upon thermal decomposition. CFAs are
typically used to obtain medium- to high-density foams, and are
often used in conjunction with physical blowing agents to obtain
low-density foams. CFAs can be categorized as either endothermic or
exothermic, which refers to the type of decomposition they undergo.
Endothermic types absorb energy and typically release carbon
dioxide and moisture upon decomposition, while the exothermic types
release energy and usually generate nitrogen when decomposed. The
overall gas yield and pressure of gas released by exothermic
foaming agents is often higher than that of endothermic types.
Endothermic CFAs are generally known to decompose in the range of
130 to 230.degree. C. (266-446.degree. F.), while some of the more
common exothermic foaming agents decompose around 200.degree. C.
(392.degree. F.). However, the decomposition range of most
exothermic CFAs can be reduced by addition of certain compounds.
The activation (decomposition) temperatures of CFAs fall into the
range of our heat treatment temperatures. Examples of suitable
chemical blowing agents include sodium bi-carbonate (baking soda),
hydrazine, hydrazide, azodicarbonamide (exothermic chemical blowing
agents), nitroso compounds (e.g. N, N-Dinitroso pentamethylene
tetramine), hydrazine derivatives (e.g. 4.4'-Oxybis
(benzenesulfonyl hydrazide) and Hydrazo dicarbonamide), and
hydrogen carbonate (e.g. Sodium hydrogen carbonate). These are all
commercially available in plastics industry.
[0170] In the production of foamed plastics, physical blowing
agents are metered into the plastic melt during foam extrusion or
injection molded foaming, or supplied to one of the precursor
materials during polyurethane foaming. It has not been previously
known that a physical blowing agent can be used to create pores in
a HA material, which is in a solid state (not melt). We have
surprisingly observed that a physical blowing agent (e.g. CO.sub.2
or N.sub.2) can be injected into the stream of HA suspension prior
to being coated or cast onto the supporting substrate. This would
result in a foamed structure even when the liquid medium (e.g.
water and/or alcohol) is removed. The dried layer of HA or
HA/graphene material is capable of maintaining a controlled amount
of pores or bubbles during liquid removal and subsequent heat
treatments.
[0171] Technically feasible blowing agents include Carbon dioxide
(CO.sub.2), Nitrogen (N.sub.2), Isobutane (C.sub.4H.sub.10),
Cyclopentane (C.sub.5H.sub.10), Isopentane (C.sub.5H.sub.12),
CFC-11 (CFCI.sub.3), HCFC-22 (CHF.sub.2CI), HCFC-142b
(CF.sub.2CICH.sub.3), and HCFC-134a (CH.sub.2FCF.sub.3). However,
in selecting a blowing agent, environmental safety is a major
factor to consider. The Montreal Protocol and its influence on
consequential agreements pose a great challenge for the producers
of foam. Despite the effective properties and easy handling of the
formerly applied chlorofluorocarbons, there was a worldwide
agreement to ban these because of their ozone depletion potential
(ODP). Partially halogenated chlorofluorocarbons are also not
environmentally safe and therefore already forbidden in many
countries. The alternatives are hydrocarbons, such as isobutane and
pentane, and the gases such as CO.sub.2 and nitrogen.
[0172] Except for those regulated substances, all the blowing
agents recited above have been tested in our experiments. For both
physical blowing agents and chemical blowing agents, the blowing
agent amount introduced into the suspension is defined as a blowing
agent-to-HA material weight ratio, which is typically from 0/1.0 to
1.0/1.0.
EXAMPLE 3
Preparation of Humic Acid from Coal and HA-Derive Foams
[0173] In a typical procedure, 300 mg of coal was suspended in
concentrated sulfuric acid (60 ml) and nitric acid (20 ml), and
followed by cup sonication for 2 h. The reaction was then stirred
and heated in an oil bath at 100 or 120.degree. C. for 24 h. The
solution was cooled to room temperature and poured into a beaker
containing 100 ml ice, followed by a step of adding NaOH (3M) until
the pH value reached 7.
[0174] In one experiment, the neutral mixture was then filtered
through a 0.45-mm polytetrafluoroethylene membrane and the filtrate
was dialyzed in 1,000 Da dialysis bag for 5 days. For the larger
humic acid sheets, the time can be shortened to 1 to 2 h using
cross-flow ultrafiltration. After purification, the solution was
concentrated using rotary evaporation to obtain solid humic acid
sheets. These humic acid sheets alone and their mixtures with a
blowing agent were re-dispersed in a solvent (ethylene glycol and
alcohol, separately) to obtain several dispersion samples for
subsequent casting or coating.
[0175] Various amounts (1%-30% by weight relative to HA material)
of chemical bowing agents (N,N-Dinitroso pentamethylene tetramine
or 4.4'-Oxybis (benzenesulfonyl hydrazide) were added to a
suspension containing HA sheets. The suspension was then cast onto
a glass surface using a doctor's blade to exert shear stresses,
inducing orientation and proper positioning of HA molecules or
sheets. Several samples were cast, including one that was made
using CO.sub.2 as a physical blowing agent introduced into the
suspension just prior to casting. The resulting HA films, after
removal of liquid, have a thickness that can be varied from
approximately 1 to 500 .mu.m.
[0176] The HA films were then subjected to heat treatments that
involve an initial (first) thermal reduction temperature of
80-1,500.degree. C. for 1-5 hours. This first heat treatment
generated a HA foam (if HTT is <300.degree. C.) and a foam of
large sheet-like HA molecules or domains of hexagonal carbon atomic
planes in the pore walls (if HTT is from 300 to 1,500.degree. C.).
Some of the foam samples were then subjected to a second
temperature of 1,500-2,850.degree. C. to determine if the
graphene-like domains of hexagonal carbon atomic planes in the foam
wall could be further perfected (graphitized to become more ordered
or having a higher degree of crystallinity).
COMPARATIVE EXAMPLE 3-a
CVD Graphene Foams on Ni Foam Templates
[0177] The procedure was adapted from that disclosed in open
literature: Chen, Z. et al. "Three-dimensional flexible and
conductive interconnected graphene networks grown by chemical vapor
deposition," Nat. Mater. 10, 424-428 (2011). Nickel foam, a porous
structure with an interconnected 3D scaffold of nickel was chosen
as a template for the growth of graphene foam. Briefly, carbon was
introduced into a nickel foam by decomposing CH.sub.4 at
1,000.degree. C. under ambient pressure, and graphene films were
then deposited on the surface of the nickel foam. Due to the
difference in the thermal expansion coefficients between nickel and
graphene, ripples and wrinkles were formed on the graphene films.
In order to recover (separate) graphene foam, Ni frame must be
etched away. Before etching away the nickel skeleton by a hot HCl
(or FeCl.sub.3) solution, a thin layer of poly(methyl methacrylate)
(PMMA) was deposited on the surface of the graphene films as a
support to prevent the graphene network from collapsing during
nickel etching. After the PMMA layer was carefully removed by hot
acetone, a fragile graphene foam sample was obtained. The use of
the PMMA support layer is critical to preparing a free-standing
film of graphene foam; only a severely distorted and deformed
graphene foam sample was obtained without the PMMA support layer.
This is a tedious process that is not environmentally benign and is
not scalable.
COMPARATIVE EXAMPLE 3-b
Conventional Graphitic Foam from Pitch-Based Carbon Foams
[0178] Pitch powder, granules, or pellets are placed in a aluminum
mold with the desired final shape of the foam. Mitsubishi ARA-24
meso-phase pitch was utilized. The sample is evacuated to less than
1 ton and then heated to a temperature approximately 300.degree. C.
At this point, the vacuum was released to a nitrogen blanket and
then a pressure of up to 1,000 psi was applied. The temperature of
the system was then raised to 800.degree. C. This was performed at
a rate of 2 degree C/min. The temperature was held for at least 15
minutes to achieve a soak and then the furnace power was turned off
and cooled to room temperature at a rate of approximately 1.5
degree C/min with release of pressure at a rate of approximately 2
psi/min. Final foam temperatures were 630.degree. C. and
800.degree. C. During the cooling cycle, pressure is released
gradually to atmospheric conditions. The foam was then heat treated
to 1050.degree. C. under a nitrogen blanket and then heat treated
in separate runs in a graphite crucible to 2500.degree. C. and
2800.degree. C. in Argon.
[0179] Samples from the foam were machined into specimens for
measuring the thermal conductivity. The bulk thermal conductivity
ranged from 67 W/mK to 151 W/mK. The density of the samples was
from 0.31-0.61 g/cm.sup.3. When weight is taken into account, the
specific thermal conductivity of the pitch derived foam is
approximately 67/0.31 =216 and 151/0.61 =247.5 W/mK per specific
gravity (or per physical density).
[0180] The compression strength of the samples having an average
density of 0.51 g/cm.sup.3 was measured to be 3.6 MPa and the
compression modulus was measured to be 74 MPa. By contrast, the
compression strength and compressive modulus of the presently
invented HA-derived graphitic foam having a comparable physical
density are 5.7 MPa and 103 MPa, respectively.
[0181] Shown in FIG. 3(A) are the thermal conductivity values vs.
specific gravity of the HA-derived foam, meso-phase pitch-derived
graphite foam, and Ni foam template-assisted CVD graphene foam.
These data clearly demonstrate the following unexpected results:
[0182] 1) HA-derived foams produced by the presently invented
process exhibit significantly higher thermal conductivity as
compared to both meso-phase pitch-derived graphite foam and Ni foam
template-assisted CVD graphene, given the same physical density.
[0183] 2) This is quite surprising in view of the notion that CVD
graphene is essentially pristine graphene that has never been
exposed to oxidation and should have exhibited a much higher
thermal conductivity compared to HA-derived hexagonal carbon atomic
planes, which are highly defective (having a high defect population
and, hence, low conductivity) after the oxygen-containing
functional groups are removed via conventional thermal or chemical
reduction methods. These exceptionally high thermal conductivity
values observed with the HA-derived graphitic foams herein produced
are much to our surprise. [0184] 3) Given the same amount of solid
material, the presently invented HA-derived foam after a heat
treatment at a HTT>1,500.degree. C. is intrinsically most
conducting, reflecting a high level of graphitic crystal perfection
(larger crystal dimensions, fewer grain boundaries and other
defects, better crystal orientation, etc.). This is also
unexpected. [0185] 4) The specific conductivity values of the
presently invented HA-derived foam and fluorinated HA-derived foams
(FIG. 5) exhibit values from 250 to 490 W/mK per unit of specific
gravity; but those of the other two foam materials are typically
lower than 250 W/mK per unit of specific gravity.
[0186] Other experimental data also indicate that the HA-derived
foams and HA/graphene-derived foams are not only highly conducting
(overcoming the low conductivity issues of sulfur or sulfide) but
also compatible with sulfur and sulfide, which can adhere pore
walls very well. This feature is important for preventing excessive
dissolution of sulfur and sulfide in liquid electrolyte; hence,
reducing or eliminating the shuttle effect.
COMPARATIVE EXAMPLE 3-c
Preparation of Pristine Graphene Foam (0% Oxygen)
[0187] Recognizing the possibility of the high defect population in
HA sheets acting to reduce the conductivity of individual graphene
plane, we decided to study if the use of pristine graphene sheets
(non-oxidized and oxygen-free, non-halogenated and halogen-free,
etc.) can lead to a graphene foam having a higher thermal
conductivity. Pristine graphene sheets were produced by using the
direct ultrasonication process (also known as the liquid-phase
exfoliation in the art).
[0188] In a typical procedure, five grams of graphite flakes,
ground to approximately 20 .mu.m or less in sizes, were dispersed
in 1,000 mL of deionized water (containing 0.1% by weight of a
dispersing agent, Zonyl.RTM. FSO from DuPont) to obtain a
suspension. An ultrasonic energy level of 85 W (Branson 5450
Ultrasonicator) was used for exfoliation, separation, and size
reduction of graphene sheets for a period of 15 minutes to 2 hours.
The resulting graphene sheets are pristine graphene that have never
been oxidized and are oxygen-free and relatively defect-free. There
are no other non-carbon elements.
[0189] Various amounts (1%-30% by weight relative to graphene
material) of chemical bowing agents (N, N-Dinitroso pentamethylene
tetramine or 4.4'-Oxybis (benzenesulfonyl hydrazide) were added to
a suspension containing pristine graphene sheets and a surfactant.
The suspension was then cast onto a glass surface. Several samples
were cast, including one that was made using CO.sub.2 as a physical
blowing agent introduced into the suspension just prior to casting.
The resulting HA or HA/graphene films, after removal of liquid,
have a thickness that can be varied from approximately 10 to 500
.mu.m. The films were then subjected to heat treatments at a
temperature of 80-1,500.degree. C. for 1-5 hours, which generated a
HA- or HA/graphene-derived foam.
[0190] Summarized in FIG. 6 are thermal conductivity data for a
series of HA-derived foams and a series of pristine graphene
derived foams, both plotted over the same final (maximum) heat
treatment temperatures. These data indicate that the thermal
conductivity of the HA-derived foams is highly sensitive to the
final heat treatment temperature (HTT). Even when the HTT is very
low, clearly some type of HA molecular linking and merging or
crystal perfection reactions have already been activated. The
thermal conductivity increases monotonically with the final HTT. In
contrast, the thermal conductivity of pristine graphene foams
remains relatively constant until a final HTT of approximately
2,500.degree. C. is reached, signaling the beginning of a
re-crystallization and perfection of graphite crystals. There are
no functional groups in pristine graphene, such as --COOH and --OH
in HA, that enable chemical linking of molecules at relatively low
HTTs. With a HTT as low as 1,250.degree. C., HA molecules and
resulting hexagonal carbon atomic planes can merge to form
significantly larger graphene-like hexagonal carbon sheets with
reduced grain boundaries and fewer electron transport path
interruptions. Even though HA-derived sheets are intrinsically more
defective than pristine graphene, the presently invented process
enables the HA molecules to form graphitic foams that outperform
pristine graphene foams. This is another unexpected result.
COMPARATIVE EXAMPLE 3-d
Preparation of Graphene Oxide (GO) Suspension from Natural Graphite
and Graphene Foams from Hydrothermally Reduced Graphene Oxide
[0191] Graphite oxide was prepared by oxidation of graphite flakes
with an oxidizer liquid consisting of sulfuric acid, sodium
nitrate, and potassium permanganate at a ratio of 4:1:0.05 at
30.degree. C. When natural graphite flakes (particle sizes of 14
.mu.m) were immersed and dispersed in the oxidizer mixture liquid
for 48 hours, the suspension or slurry appears and remains
optically opaque and dark. After 48 hours, the reacting mass was
rinsed with water 3 times to adjust the pH value to at least 3.0. A
final amount of water was then added to prepare a series of
GO-water suspensions. We observed that GO sheets form a liquid
crystal phase when GO sheets occupy a weight fraction >3% and
typically from 5% to 15%.
[0192] A self-assembled graphene hydrogel (SGH) sample was then
prepared by a hydrothermal method. In a typical procedure, the SGH
can be easily prepared by heating 2 mg/mL of homogeneous graphene
oxide (GO) aqueous dispersion sealed in a Teflon-lined autoclave at
180.degree. C. for 12 h. The SGH containing about 2.6% (by weight)
graphene sheets and 97.4% water has an electrical conductivity of
approximately 5.times.10.sup.-3 S/cm. Upon drying and heat treating
at 1,500.degree. C., the resulting graphene foam exhibits an
electrical conductivity of approximately 1.5.times.10.sup.-1 S/cm,
which is 2 times lower than those of the presently invented
HA-derived foams produced by heat treating at the same
temperature.
COMPARATIVE EXAMPLE 3-e
Plastic Bead Template-Assisted Formation of Reduced Graphene Oxide
Foams
[0193] A hard template-directed ordered assembly for a macro-porous
bubbled graphene film (MGF) was prepared. Mono-disperse poly methyl
methacrylate (PMMA) latex spheres were used as the hard templates.
The GO liquid crystal prepared in Comparative Example 3-d above was
mixed with a PMMA spheres suspension. Subsequent vacuum filtration
was then conducted to prepare the assembly of PMMA spheres and GO
sheets, with GO sheets wrapped around the PMMA beads. A composite
film was peeled off from the filter, air dried and calcinated at
800.degree. C. to remove the PMMA template and thermally reduce GO
into RGO simultaneously. The grey free-standing PMMA/GO film turned
black after calcination, while the graphene film remained
porous.
[0194] FIG. 3(B) shows the thermal conductivity values of the
presently invented HA-derived foam, GO foam produced via
sacrificial plastic bead template-assisted process, and
hydrothermally reduced GO graphene foam. Most surprisingly, given
the same HTTs, the presently invented HA-derived foam exhibits the
highest thermal conductivity. Electrical conductivity data
summarized in FIG. 4 are also consistent with this conclusion.
These data further support the notion that, given the same amount
of solid material, the presently invented HA suspension deposition
(with stress-induced orientation) and subsequent heat treatments
give rise to a HA-derived foam that is intrinsically most
conducting, reflecting a highest level of graphitic crystal
perfection (larger crystal dimensions, fewer grain boundaries and
other defects, better crystal orientation, etc. along the pore
walls).
[0195] It is of significance to point out that all the prior art
processes for producing graphite foams or graphene foams appear to
provide macro-porous foams having a physical density in the range
of approximately 0.2-0.6 g/cm.sup.3 only with pore sizes being
typically too large (e.g. from 20 to 300 .mu.m) for most of the
intended applications. In contrast, the instant invention provides
processes that generate HA-derived foams having a density that can
be as low as 0.01 g/cm.sup.3 and as high as 1.7 g/cm.sup.3. The
pore sizes can be varied between meso-scaled (2-50 nm) up to
macro-scaled (1-500 .mu.m) depending upon the contents of
non-carbon elements and the amount/type of blowing agent used. This
level of flexibility and versatility in designing various types of
graphitic foams is unprecedented and un-matched by any prior art
process.
EXAMPLES 4
Preparation of Fluorinated HA Foams
[0196] In a typical procedure, a sheet of HA-derived foam was
fluorinated by vapors of chlorine trifluoride in a sealed autoclave
reactor to yield fluorinated HA-carbon hybrid film. Different
durations of fluorination time were allowed for achieving different
degrees of fluorination. Sheets of fluorinated HA-derived foam were
then separately immersed in containers each containing a metal
polysulfide solution. On a separate basis, several sheets of foam
were exposed to sulfur vapor for physical vapor infiltration of
sulfur into pores of HA-derived foam.
EXAMPLE 5
Preparation of Nitrogenated HA Foams
[0197] Several pieces of HA-derived foam prepared in Example 3 were
immersed in a 30% H.sub.2O.sub.2-water solution for a period of
2-48 hours to obtain oxidized HA-derived foams, having a controlled
oxygen content of 2-25% by weight.
[0198] Some oxidized HA-derived foam samples were mixed with
different proportions of urea and the mixtures were heated in a
microwave reactor (900 W) for 0.5 to 5 minutes. The products were
washed several times with deionized water and vacuum dried. The
products obtained were nitrogenated HA foam. The nitrogen contents
were from 3% to 17.5 wt. %, as measured by elemental analysis.
[0199] It may be noted that different functionalization treatments
of the HA-derived foam were for different purposes. For instance,
oxidized HA foam structures are particularly effective as an
absorber of polar solvent containing metal salt dissolved therein.
Nitrogenated foams were more effective in up-taking other types of
solutions.
Example 6
Characterization of Various HA-Derived Foams and Conventional
Graphite Foam
[0200] The internal structures (crystal structure and orientation)
of several series of HA-carbon foam materials were investigated
using X-ray diffraction. The X-ray diffraction curve of natural
graphite typically exhibits a peak at approximately
2.theta.=26.degree. , corresponds to an inter-graphene spacing
(d.sub.002) of approximately 0.3345 nm. The RHA walls of the hybrid
foam materials exhibit a d.sub.002 spacing typically from 0.3345 nm
to 0.40 nm, but more typically up to 0.34 nm.
[0201] With a heat treatment temperature of 2,750.degree. C. for
the foam structure under compression for one hour, the d.sub.002
spacing is decreased to approximately to 0.3354 nm, identical to
that of a graphite single crystal. In addition, a second
diffraction peak with a high intensity appears at 20 =55.degree.
corresponding to X-ray diffraction from (004) plane. The (004) peak
intensity relative to the (002) intensity on the same diffraction
curve, or the I(004)/(002) ratio, is a good indication of the
degree of crystal perfection and preferred orientation of
graphene-like planes. The (004) peak is either non-existing or
relatively weak, with the I(004)/(002) ratio <0.1, for all
graphitic materials heat treated at a temperature lower than
2,800.degree. C. The I(004)I(002) ratio for the graphitic materials
heat treated at 3,000-3,250.degree. C. (e.g., highly oriented
pyrolytic graphite, HOPG) is in the range of 0.2-0.5. In contrast,
a graphene foam prepared with a final HTT of 2,750.degree. C. for
one hour exhibits a I(004)/(002) ratio of 0.78 and a Mosaic spread
value of 0.21, indicating the pore walls being a practically
perfect graphite single crystal with a good degree of preferred
orientation (if prepared under a compression force).
[0202] The "mosaic spread" value is obtained from the full width at
half maximum of the (002) reflection in an X-ray diffraction
intensity curve. This index for the degree of ordering
characterizes the graphite or graphene crystal size (or grain
size), amounts of grain boundaries and other defects, and the
degree of preferred grain orientation. A nearly perfect single
crystal of graphite is characterized by having a mosaic spread
value of 0.2-0.4. Some of our HA-derived foams have a mosaic spread
value in this range of 0.3-0.6 when produced using a final heat
treatment temperature no less than 2,500.degree. C.
[0203] It is of significance to point out that a heat treatment
temperature as low as 500.degree. C. is sufficient to bring the
average inter-planar spacing between hexagonal carbon atomic planes
along the pore walls to below 0.4 nm, getting closer and closer to
that of natural graphite or that of a graphite single crystal. The
beauty of this approach is the notion that this HA suspension
coating and heat treating strategy has enabled us to organize,
orient/align, and chemically merge the planar HA molecules into a
unified structure with all the graphene-like hexagonal carbon
atomic planes now being larger in lateral dimensions (significantly
larger than the length and width of the original HA molecules). A
potential chemical linking and merging mechanism is illustrated in
FIG. 3. This has given rise to exceptional thermal conductivity and
electrical conductivity values.
EXAMPLE 7
Deposition of Metal Polysulfide in Various HA-Derived Foams
Prepared in Previous Examples for Metal-Sulfur Batteries
[0204] The deposition of metal polysulfide was conducted before the
cathode active layer was incorporated into a metal-sulfur battery
cell (Li--S, Na--S, K--S, Mg--S, or Al--S cell).
[0205] In a typical procedure, sulfur or a metal polysulfide
(M.sub.xS.sub.y) is dissolved in a solvent (e.g. mixture of DOL/DME
in a volume ratio from 1:3 to 3:1) to form an electrolyte solution.
Several types of metal polysulfide materials are commercially
available. A wide variety of solvents can be utilized for this
purpose and there is no theoretical limit to what type of solvents
can be used; any solvent can be used provided that there is some
solubility of the elemental sulfur, polymeric sulfur,
carbon-sulfide, or metal polysulfide in this desired solvent. A
greater solubility would mean a larger amount of sulfur or sulfide
can be precipitated out from the electrolyte solution and deposited
in the porous structure.
[0206] For those commercially unavailable metal polysulfide
materials, one can readily prepare them in a lab setting. As a
series of examples, lithium polysulfide (Li.sub.xS.sub.y) and
sodium polysulfide (Na.sub.xS.sub.y) with desired x and y values
(e.g. x=2, and y=6-10) dissolved in solvent were prepared by
chemically reacting stoichiometric amounts of sulfur and Li.sub.2S
or Na.sub.2S in polysulfide free electrolyte of 0.5 M LiTFSI +0.2 M
LiNO.sub.3 (or 0.5 M NaTFSI +0.2 M NaNO.sub.3) in DOL/DME (1:1,
v:v). The electrolyte was stirred at 75.degree. C. for 3-7 hours
and then at room temperature for 48 hours. The resulting
electrolytes contain different Li.sub.xS.sub.y or Na.sub.xS.sub.y
species (e.g. x=2, and y=6-10, depending upon reaction times and
temperatures), which are intended for use as a sulfur source in a
battery cell.
[0207] Several methods were utilized to introduce
polysulfide-solvent solution into the pores of the conductive
porous structure. One method entailed drawing a desired amount of
solution into a syringe, which was then discharged and dispensed
onto the surface of a HA-derived foam. In most cases, the solution
naturally permeates into the pores. Another method involved using a
lab-scale liquid sprayer to spray the solution over the porous
structure. Yet another method included dipping the entire porous
structure (foam) into the solution for a desired period of time. In
all methods, precipitation of metal polysulfide occurred upon
removal of the solvent. This drying procedure allows the
precipitated polysulfide to deposit onto the internal walls of the
pores in a thin coating form, or to form nano particles that simply
lodge in the pores of the porous structure.
[0208] Some examples of the metal polysulfide (M.sub.xS.sub.y)
materials, solvents, porous foams used in the present study are
presented in Table 1 below.
TABLE-US-00001 TABLE 1 Selected examples of the metal polysulfide
materials, solvents used for forming polysulfide solution, and
conductive foam structures used in the present study.
M.sub.xS.sub.y Solvent Type of foam structure in the cathode
Li.sub.2S.sub.6 DOL/DME HA-derived foam Li.sub.2S.sub.9 DOL/DME
HA-derived foam Li.sub.2S.sub.10 DOL/DME HA-derived foam
Na.sub.2S.sub.2 Tetra ethylene glycol HA/GO-derived foam dimethyl
ether (TEGDME) Na.sub.2S.sub.4 TEGDME HA/GO-derived foam
Na.sub.2S.sub.6 TEGDME HA/GO-derived foam K.sub.2S.sub.6 TEGDME
HA/graphene-derived foam K.sub.2S.sub.4 Diglyme/tetraglyme
HA-derived foam, nitrogenated K.sub.2S Diglyme/tetraglyme
HA-derived foam, fluorinated MgS.sub.6 Diglyme/tetraglyme
HA-derived foam; for Mg--S and Al--S cells MgS.sub.4
Diglyme/tetraglyme HA-derived foam; for Mg--S and Al--S cells
CuS.sub.2 NH.sub.4OH or HCl or H.sub.2SO.sub.4 HA-derived foam
Cu.sub.8S.sub.5 NH.sub.4OH or HCl or H.sub.2SO.sub.4 HA-derived
foam ZnS H.sub.2SO.sub.4 solution HA/GO-derived foam: for Al--S
cells Al.sub.2S.sub.3 H.sub.2SO.sub.4 HA/GO-derived foam; for Al--S
cells SnS.sub.2 HNO.sub.3 and HCl HA/GO-derived foam, nitrogenated
SnS HCl HA/GO-derived foam, fluorinated
[0209] In a metal-sulfur cell, a proper electrolyte was selected to
combine with an anode current collector (Cu foil), an anode layer
(e.g. Li metal foil, Na particles, K particles, Mg foil, or Al foil
chemically cleaned), a porous separator, a layer of conductive
porous structure, and a cathode current collector (Al foil) to form
a Li--S cell, a room temperature Na--S cell, a K--S cell, a Mg--S
cell, and a Al--S cell, respectively. The cell was then subjected
to a first discharge or charge procedure using a current density
preferably ranging from 50 mA/g to 5 A/g.
[0210] Sulfur vapor was also introduced into a chamber wherein
pieces of HA-derived foam were properly positioned to receive
sulfur vapor, which naturally permeates into pores of the foam
structures.
[0211] For comparison purposes, several prior art methods were used
to incorporate sulfur (the cathode active material) in the cathode
layer; e.g. direct mixing of S powder with carbon black particles,
physical vapor deposition of S in a carbon paper (e.g. carbon
nano-fiber, CNF), direct mixing lithium polysulfide particles with
a conductive filler (e.g. carbon nanotubes), etc.
EXAMPLE 8
Chemical Reaction-Induced Deposition of Sulfur Particles or
Coating
[0212] A chemical deposition method was also herein utilized to
prepare S-impregnated HA-derived foam structures. In one typical
experiment, the procedure began with adding 0.58 g Na.sub.2S into a
flask that had been filled with 25 ml distilled water to form a
Na.sub.2S solution. Then, 0.72 g elemental S was suspended in the
Na.sub.2S solution and stirred with a magnetic stirrer for about 2
hours at room temperature. The color of the solution changed slowly
to orange-yellow as the sulfur dissolved. After dissolution of the
sulfur, a sodium polysulfide (Na.sub.2S.sub.x) solution was
obtained (x=4-10).
[0213] Subsequently, a sulfur-impregnated foam sample was prepared
by a chemical deposition method in an aqueous solution. First,
pieces of HA-derived foam were dipped into the Na.sub.2S.sub.x
solution, in the presence of 5 wt. % surfactant cetyl
trimethyl-ammonium bromide (CTAB). Then, 100 ml of 2 mol/L HCOOH
solution was added into the Na.sub.2S.sub.x solution at a rate of
30-40 drops/min. Finally, the product (basically sulfur-impregnated
foam) was washed with acetone and distilled water several times to
eliminate salts and impurities. The resulting product was dried at
50.degree. C. in a drying oven for 48 hours. The reaction may be
represented by the following reaction:
S.sub.x.sup.2-+2H.sup.+.fwdarw.(x-1) S+H.sub.2S.
EXAMPLE 9
Redox Chemical Reaction-Induced Deposition of Sulfur Particles or
Coating
[0214] In this chemical reaction-based deposition process, sodium
thiosulfate (Na.sub.2S.sub.2O.sub.3) was used as a sulfur source
and HCl as a reactant. The two reactants (HCl and
Na.sub.2S.sub.2O.sub.3) were then dispersed and dissolved in water
to form a solution. A piece of HA-derived foam was then immersed
into this solution. A chemical reaction was allowed to proceed at
25-75.degree. C. for 1-3 hours, leading to the precipitation of S
particles deposited in pores of the foam. The reaction may be
represented by the following reaction: 2HC1
+Na.sub.2S.sub.2O.sub.3.fwdarw.2NaCl+S.dwnarw.+SO.uparw.+H.sub.2O.
COMPARATIVE EXAMPLE 9A
Preparation of S/MC and S/CB Nanocomposites via Solution
Deposition
[0215] Meso-porous carbon (MC) and, separately, carbon black
particles and S were mixed and dispersed in a solvent (CS.sub.2) to
form a suspension. After thorough stirring, the solvent was
evaporated to yield a solid nanocomposite, which was then ground to
yield nanocomposite powder. The primary sulfur particles in these
nanocomposite particles have an average diameter of approximately
10-30 nm.
Comparative Examples 9B
Preparation of Sulfur-Deposited Webs and Foams
[0216] The step involves deposition of elemental sulfur on
meso-porous structures through, for instance, a sublimation-based
physical vapor deposition. Sublimation of solid sulfur occurs at a
temperature greater than 20.degree. C., but a significant
sublimation rate typically does not occur until the temperature is
above 40.degree. C. In a typical procedure, a meso-porous
structure, a nano-filament web, and a HA-derived foam were sealed
in a glass tube with the solid sulfur positioned at one end of the
tube and the web near another end at a temperature of 40-75.degree.
C. The sulfur vapor exposure time was typically from several
minutes to several hours for a sulfur coating of several nanometers
to several microns in thickness. A sulfur coating thickness lower
than 100 nm is preferred, but more preferred is a thickness lower
than 20 nm, most preferred lower than 10 nm or even 5 nm.
[0217] Several series of alkali metal and alkali metal-ion cells
were prepared using the presently prepared cathode. For instance,
for the Li--S cells, the first series is a Li metal cell containing
a copper foil as an anode current collector and the second series
is also a Li metal cell having a nano-structured anode of
conductive filaments (based on electro-spun carbon fibers) plus a
copper foil current collector. The third series is a Li-ion cell
having a nano-structured anode of conductive filaments (based on
electro-spun carbon fibers coated with a thin layer of Si using
CVD) plus a copper foil current collector. The fourth series is a
Li-ion cell having a graphite-based anode active material as an
example of the more conventional anode.
EXAMPLE 10
Evaluation of Electrochemical Performance of Various Metal-Sulfur
Cells
[0218] Charge storage capacities were measured periodically and
recorded as a function of the number of cycles. The specific
discharge capacity herein referred to is the total charge inserted
into the cathode during the discharge, per unit mass of the
composite cathode (counting the weights of the cathode active
material, conductive additive or foam structure, binder, and any
optional additive combined). The specific charge capacity refers to
the amount of charges per unit mass of the composite cathode. The
specific energy and specific power values presented in this section
are based on the total cell weight. The morphological or
micro-structural changes of selected samples after a desired number
of repeated charging and recharging cycles were observed using both
transmission electron microscopy (TEM) and scanning electron
microscopy (SEM).
[0219] Active material utilization efficiency data from many
samples or cells investigated are summarized in Table 2 and Table 3
below:
TABLE-US-00002 TABLE 2 Sulfur utilization efficiency data for
alkali metal-sulfur cell cathodes containing various S contents,
polysulfide coating thicknesses or particle diameters, porous
structure materials. Equivalent S % (assuming 100% conversion from
M.sub.xS.sub.y Cathode Discharge Active to S) and polysulfide
discharge capacity, material Sample Cathode active layer thickness
or diameter capacity mAh/g, based utilization ID material (nm)
(mAh/g) on S weight efficiency HA-1 HA-derived foam
Li.sub.2S.sub.10; 90% S; 7.3 nm 1388 1542 92.07% HA-2 HA-derived
foam Li.sub.2S.sub.10; 90% S; 13.3 nm 1305 1450 86.57% HA-3
HA-derived foam Li.sub.2S.sub.10; 75% S; 13.4 nm 1052 1403 83.74%
HA-C-1 CNT mat 75% S (PVD) + CNT 660 880 52.54% HA-C-2 CNT mat 75%
S; Li.sub.2S.sub.10 + CNT 690 920 54.93% HA-C-3 Carbon black 75% S;
Li.sub.2S.sub.10 + CB 415 553 33.03% HGO-1 HA/GO-derived foam
Li.sub.2S.sub.6; 85% S; 13.4 nm 1185 1394 83.23% HGO-2
HA/GO-derived foam 85% S, Chem. reaction 1033.7 1299 75.58% HGO-3
HA/GO-derived foam Na.sub.2S.sub.6; 85% S; 13.4 nm 1107.8 1303
77.81% RGO-C RGO Na.sub.2S.sub.6; 85% S; ball-milled 983.3 1157
69.06% NGO-1 HA-derived, nitrogenated Na.sub.2S.sub.4; 65% S; 13.4
nm 893 1374 82.02% NGO-2 HA-derived, nitrogenated K.sub.2S.sub.6;
65% S; 10.2 nm 877 1349 80.55% f-GO-1 f-GO (fluorinated)
K.sub.2S.sub.8; 70% S; 10.2 nm 989.3 1413 84.38% f-HA-1 f-HA
(fluorinated) Li.sub.2S.sub.8; 85% S; 7.6 nm 1297 1526 91.10% HG-1
HA/graphene-derived Na.sub.2S.sub.6; 85% S; 15.4 nm 1198 1409
84.14% HG-2 HA/graphene-derived K.sub.2S.sub.6; 85% S; 14.4 nm 1167
1373 81.97% EG-3C CNT Na.sub.2S.sub.6; 85% S; 34 nm 944 1111
66.30%
TABLE-US-00003 TABLE 3 Active material utilization efficiency data
for alkali metal-sulfur cell cathodes containing various S
contents, polysulfide coating thicknesses or particle diameters,
porous structure materials. Discharge Equivalent S % (assuming
Cathode capacity, Active 100% conversion from M.sub.xS.sub.y
discharge mAh/g, material Sample Cathode active to S) and
polysulfide capacity based on S utilization ID layer material
thickness or diameter (nm) (mAh/g) weight efficiency CSC-1
HA-derived Li.sub.2S.sub.10; 90% S; 8.4 nm 1365 1517 90.6% CSC-2
HA-derived Li.sub.2S.sub.10; 90% S; 15.3 nm 1293 1437 85.8% CSC-3
HA-derived Li.sub.2S.sub.10; 75% S; 16.2 nm 1038 1384 82.6% CSC-c1
HA-derived 75% S (PVD) 926 1235 .sup. 74% CSC-c2 CB 75% S + CB;
ball-milled 668 891 53.25% HOG-v HA/GO-derived Li.sub.2S.sub.6; 85%
S; 13.4 nm 1263 1506 90.1% HOG-ch HA/GO-derived 85% S, Chem.
reaction 1033 1454 85.7% HG-1 HA/G-derived Na.sub.2S.sub.6; 85% S;
13.4 nm 1022 1460 87.16% HG-2 HA/G-derived Na.sub.2S.sub.6; 85% S;
Chem. 987 1390 82.5% HA-CN1 HA + CNT Na.sub.2S.sub.4; 65% S; 13.4
nm 1010 1342 81.2% HA-CN2 HA + CNT K.sub.2S.sub.6; 65% S; 11.2 nm
1025 1362 82.4% C-CNF C-CNF K.sub.2S.sub.8; 65% S; 14.4 nm 913 1288
.sup. 76% Al-1 HA/GO-derived Li.sub.2S.sub.8; 85% S; 7.6 nm 1254
1475 88.08% Al-2 HA/GO-derived Na.sub.2S.sub.6; 85% S; 15.4 nm 1202
1414 84.42% Mg-1 HA-derived K.sub.2S.sub.6; 85% S; 26.4 nm 1088
1280 76.42% Mg-2 HA-derived Na.sub.2S.sub.6; 85% S; 34 nm 1138 1237
73.85%
[0220] The following observations can be made from the data of
Table 2 and Table 3: [0221] 1) Compared to other means of
protecting cathode active materials and facilitating a higher
sulfur utilization efficiency in a metal-sulfur cell, both
HA-derived and HA/graphene-derived foams are the most effective.
[0222] 2) Both HA-derived and HA/graphene-derived foams are
conducive to deposition of a high M.sub.xS.sub.y proportion while
maintaining a thin M.sub.xS.sub.y coating (hence, high active
material utilization efficiency) for alkali metal-sulfur cells.
Other materials, such as CNT-based mats, are not capable of
achieving both. [0223] 3) Thinner M.sub.xS.sub.y coatings on pore
walls in HA-derived foam prepared according to the instant
invention lead to higher active material utilization efficiency
given comparable S proportion.
[0224] 4) For all alkali metal-sulfur cells and aluminum-sulfur
cells, the sulfur utilization efficiency is typically in the range
of 81-91%. This has not been possible for prior art sulfur
cathodes.
[0225] Shown in FIG. 7 are the specific capacities vs. number of
charge/discharge cycles for three Li--S cells: one featuring a
HA-derived foam cathode containing solution deposited
Li.sub.2S.sub.8 coating of the present invention, one featuring a
sulfur cathode of physical vapor deposited sulfur in HA-derived
foam, and one containing a cathode containing RGO and sulfur
ball-milled together.
[0226] These data indicate that, as a non-limiting example, the
presently invented Li--S cell featuring a HA-derived foam
impregnated with solution-deposited metal sulfide as a sulfur
source does not exhibit any significant decay (only 8.3%) after 275
cycles. The cell containing a cathode of sulfur vapor deposited S
coating-infiltrated HA foam experiences a 16.4% capacity decay
after 275 cycles. The cathode containing ball-milled mixture of RGO
and Li.sub.2S.sub.8 suffers a 68.3% capacity decay after 250
cycles. The cycle life of a lithium battery cell is usually defined
as the number of cycles when the cell reaches 80% of its original
capacity. With this definition, the prior art Li--S cell featuring
a cathode containing ball-milled S/RGO shows a life of 50 cycles.
These results are quite unexpected considering that the same amount
of sulfur was incorporated in these three cell cathodes.
[0227] The cycling stability of the cathode featuring nano
Li.sub.2S.sub.8-deposited HA-derived foam might be due to the
effectiveness of the presently invented foam and deposition method
to uniformly deposit ultra-thin sulfur coating in the meso-pores in
the porous structure and to retain the outstanding ability of the
pore walls to retain thin sulfur coating, preventing dissolution of
sulfur and polysulfide during battery operations. Additionally, as
compared to pure S, the Li.sub.2S.sub.8 coating appears to be more
resistant to electrode disintegration caused by cathode volume
changes. This is likely due to the notion that a
M.sub.xS.sub.y-preloaded cathode layer has already naturally built
in some expanded volume and hence is less prone to sulfur volume
expansion-induced damage upon repeated charges/discharges
[0228] Similarly, FIG. 8 shows the specific capacities vs. number
of charge/discharge cycles for 3 Na--S cells: one featuring a
cathode made of HA-derived foam containing solution deposited
Na.sub.2S.sub.8 coating in the pores, one containing vapor
deposited sulfur in the pores of HA-derived foam, and one
containing a cathode containing carbon black and sulfur ball-milled
together. The presently invented M.sub.xS.sub.y deposition and
HA-derived form approach provides the most cycling-stable Na--S
cell.
[0229] Although one might be able to use Li.sub.2S.sub.1,
Li.sub.2S.sub.2, Li.sub.2S.sub.3, and Li.sub.2S.sub.4, in the
presently invented cathode active layer, we have found some
unexpected disadvantages or limitations of using Li.sub.2S.sub.y,
where y=1-4. For instance, there is limited solubility of
Li.sub.2S.sub.1 and Li.sub.2S.sub.2 in most of the solvents and,
hence, it is difficult to incorporate any significant proportion of
Li.sub.2S.sub.1 and Li.sub.2S.sub.2 in the porous structure.
Further, there are limited sulfur contents in the resulting cathode
when Li.sub.2S.sub.1 and Li.sub.2S.sub.2 are used to load the pores
of the porous structure. This implies that the theoretical
capacities of Li.sub.2S.sub.1 and Li.sub.2S.sub.2 are 1,167 and
1,377 mAh/g, respectively, even though pure sulfur provides the
theoretical capacity of 1,675 mAh/g. Furthermore, quite
unexpectedly and significantly, there is a significant degree of
irreversibility of Li.sub.2S.sub.1 and Li.sub.2S.sub.2 when they
are deposited in the porous structure. These issues, in
combination, have surprisingly led to relatively low sulfur content
and low sulfur utilization efficiency in the cathode, as well as
poor cycling stability. In contrast, for instance, Li.sub.2S.sub.9
has a theoretical capacity of 1,598 mAh/g, is highly soluble in
several desirable solvents (yet, well confined by the pores of the
invented conductive pore), enables highly reversible reaction of
the cathode active material, and is conducive to cycling
stability.
[0230] The advantages of the instant invention are further
demonstrated in FIG. 9, which indicates the cycling behaviors of a
Li--S cell featuring a Li.sub.2S.sub.1-loaded HA-derived foam
structure and a Li--S cell featuring a Li.sub.2S.sub.9-loaded
HA-derived foam structure. We have attempted to incorporate as much
Li.sub.2S.sub.1 as we can into the cathode foam structure, but the
resulting composite cathode delivers a best specific capacity of
only 786 mAh/g (based on the total composite cathode weight). This
capacity rapidly decays to 667 mAh/g (a loss of 15.2%) after 275
cycles. In contrast, the Li--S cell featuring a
Li.sub.2S.sub.9-loaded graphene porous structure at the cathode
delivers a capacity of 1,430 mAh/g, which decays by 9.7% to 1,292
mAh/g after 275 cycles.
[0231] Further unexpectedly, Na.sub.2S.sub.1, Na.sub.2S.sub.2,
Na.sub.2S.sub.3, and Na.sub.2S.sub.4 do not have these
irreversibility and cycling instability issues as in their lithium
counterparts.
[0232] In summary, the present invention provides an innovative,
versatile, and surprisingly effective platform materials technology
that enables the design and manufacture of superior metal-sulfur
rechargeable batteries. The metal-sulfur cell featuring a cathode
containing a conductive, HA-derived foam with ultra-thin cathode
active sulfur or M.sub.xS.sub.y deposited thereon exhibits a high
cathode active material utilization rate, high specific capacity,
little or no shuttling effect, and long cycle life.
[0233] We have successfully developed an absolutely new, novel,
unexpected, and patently distinct class of HA foam or HA-derived
graphitic foam materials for accommodating sulfur or polysulfide
and related processes of production. The chemical composition (% of
oxygen, fluorine, and other non-carbon elements), structure
(crystal perfection, grain size, defect population, etc), crystal
orientation, morphology, process of production, and properties of
this new class of foam materials are fundamentally different and
patently distinct from meso-phase pitch-derived graphite foam, CVD
graphene-derived foam, and graphene foams from hydrothermal
reduction of GO, and sacrificial bead template-assisted RGO foam.
The thermal conductivity, electrical conductivity, elastic modulus,
and flexural strength exhibited by the presently invented foam
materials are much higher than those of prior art foam
materials.
* * * * *