U.S. patent application number 13/612493 was filed with the patent office on 2013-03-14 for encapsulated sulfur cathodes for rechargeable lithium batteries.
This patent application is currently assigned to The Board of Trustees of the Leland Stanford Junior University. The applicant listed for this patent is Yi Cui, Weiyang Li, Zhi Wei Seh, Yuan Yang, Guangyuan Zheng. Invention is credited to Yi Cui, Weiyang Li, Zhi Wei Seh, Yuan Yang, Guangyuan Zheng.
Application Number | 20130065128 13/612493 |
Document ID | / |
Family ID | 47830116 |
Filed Date | 2013-03-14 |
United States Patent
Application |
20130065128 |
Kind Code |
A1 |
Li; Weiyang ; et
al. |
March 14, 2013 |
ENCAPSULATED SULFUR CATHODES FOR RECHARGEABLE LITHIUM BATTERIES
Abstract
A battery includes an anode, a cathode, and an electrolyte
disposed between the anode and the cathode. The cathode includes a
hollow structure defining an internal volume and a sulfur-based
material disposed within the internal volume. A characteristic
dimension of the internal volume is at least 20 nm, and the
sulfur-based material occupies less than 100% of the internal
volume to define a void.
Inventors: |
Li; Weiyang; (Sunnyvale,
CA) ; Cui; Yi; (Stanford, CA) ; Seh; Zhi
Wei; (Stanford, CA) ; Zheng; Guangyuan;
(Stanford, CA) ; Yang; Yuan; (Cambridge,
MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Li; Weiyang
Cui; Yi
Seh; Zhi Wei
Zheng; Guangyuan
Yang; Yuan |
Sunnyvale
Stanford
Stanford
Stanford
Cambridge |
CA
CA
CA
CA
MA |
US
US
US
US
US |
|
|
Assignee: |
The Board of Trustees of the Leland
Stanford Junior University
|
Family ID: |
47830116 |
Appl. No.: |
13/612493 |
Filed: |
September 12, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61533740 |
Sep 12, 2011 |
|
|
|
61693677 |
Aug 27, 2012 |
|
|
|
Current U.S.
Class: |
429/218.1 ;
977/755 |
Current CPC
Class: |
H01G 11/24 20130101;
Y02E 60/10 20130101; H01M 4/765 20130101; H01M 4/136 20130101; H01M
4/622 20130101; H01M 10/0525 20130101; H01G 11/30 20130101; H01M
10/052 20130101; H01M 4/76 20130101; H01M 4/626 20130101; H01M
4/621 20130101; B82Y 30/00 20130101; H01M 4/5815 20130101; H01M
4/625 20130101; H01M 4/1397 20130101; Y02E 60/13 20130101 |
Class at
Publication: |
429/218.1 ;
977/755 |
International
Class: |
H01M 4/58 20100101
H01M004/58 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] This invention was made with Government support under
contract DE-AC02-76SF00515 awarded by the Department of Energy. The
Government has certain rights in this invention.
Claims
1. A battery comprising: an anode; a cathode; and an electrolyte
disposed between the anode and the cathode, wherein the cathode
includes a hollow structure defining an internal volume and a
sulfur-based material disposed within the internal volume, a
characteristic dimension of the internal volume is at least 20 nm,
and the sulfur-based material occupies less than 100% of the
internal volume to define a void.
2. The battery of claim 1, wherein the characteristic dimension of
the internal volume is at least 50 nm.
3. The battery of claim 1, wherein the hollow structure includes a
wall, and the wall is configured as a barrier to spatially
segregate the sulfur-based material from the electrolyte.
4. The battery of claim 3, wherein an interior of the wall is
substantially devoid of the sulfur-based material.
5. The battery of claim 1, wherein, in a substantially de-lithiated
state of the sulfur-based material, a volume of the void is at
least 1/3 of the internal volume.
6. The battery of claim 1, wherein, in a substantially de-lithiated
state of the sulfur-based material, a volume of the void is at
least 2/3 of the internal volume.
7. The battery of claim 1, wherein the hollow structure is a
hollow, elongated structure having an aspect ratio of at least 5,
and the sulfur-based material is disposed adjacent to an inner
surface of the hollow, elongated structure.
8. The battery of claim 7, wherein the hollow, elongated structure
corresponds to one of a hollow, carbon fiber; a hollow, metal
fiber; a hollow, metal oxide fiber; a hollow, metal nitride fiber;
a hollow, metal sulfide fiber; and a hollow, composite fiber.
9. The battery of claim 1, wherein the hollow structure is a
hollow, spheroidal structure.
10. The battery of claim 9, wherein the hollow, spheroidal
structure is configured as an outer shell, and the sulfur-based
material is configured as at least one nanostructure disposed
within the outer shell.
11. The battery of claim 10, wherein the sulfur-based material is
configured as a hollow nanostructure, and the void is disposed
within an interior of the hollow nanostructure.
12. The battery of claim 9, wherein the hollow, spheroidal
structure corresponds to one of a hollow, polymer shell; a hollow,
metal shell; a hollow, metal oxide shell; a hollow, metal nitride
shell; a hollow, metal sulfide shell; and a hollow, composite
shell.
13. The battery of claim 1, wherein, after 200 cycles at a rate of
C/2, at least 85% of an initial discharge capacity is retained.
14. The battery of claim 1, wherein, after 500 cycles at a rate of
C/2, at least 80% of an initial discharge capacity is retained.
15. A battery comprising: an anode; a cathode; and an electrolyte
disposed between the anode and the cathode, wherein the cathode
includes an encapsulating structure defining an internal volume and
a sulfur-based material disposed within the internal volume, the
sulfur-based material occupies less than 100% of the internal
volume to define a void, and a ratio of a volume of the void
relative to a volume of the sulfur-based material is at least 1/5
for at least one cycling state of the sulfur-based material.
16. The battery of claim 15, wherein, in a substantially
de-lithiated state of the sulfur-based material, the ratio of the
volume of the void relative to the volume of the sulfur-based
material is at least 1/3.
17. The battery of claim 15, wherein, in a substantially
de-lithiated state of the sulfur-based material, the ratio of the
volume of the void relative to the volume of the sulfur-based
material is in the range of 1/2 to 3/1.
18. The battery of claim 15, wherein a characteristic dimension of
the internal volume is at least 20 nm.
19. The battery of claim 15, wherein, after 30 cycles at a rate of
C/2, no greater than 23% by weight of the sulfur-based material is
present in the electrolyte.
20. The battery of claim 15, wherein, after 100 cycles at a rate of
C/2, no greater than 25% by weight of the sulfur-based material is
present in the electrolyte.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application Ser. No. 61/533,740, filed on Sep. 12, 2011, and the
benefit of U.S. Provisional Application Ser. No. 61/693,677, filed
on Aug. 27, 2012, the disclosures of which are incorporated herein
by reference in their entireties.
BACKGROUND
[0003] Rechargeable batteries with high specific energy are
desirable for solving imminent energy and environmental issues.
Lithium-ion batteries have one of the highest specific energy among
rechargeable batteries, but state-of-the-art technology based on
intercalation mechanism has a theoretical specific energy of about
400 Wh/kg for both LiCoO.sub.2/graphite and LiFePO.sub.4/graphite
systems. To achieve higher specific energy, new materials in both
the cathode and anode are desired. Despite significant progress in
the development of high capacity anode materials such as
nanostructured silicon, the relatively low charge capacity of
cathodes remains a limiting factor for commercializing rechargeable
batteries with high specific energy. Current cathode materials,
such as transition metal oxides and phosphates, typically have an
inherent limit of about 300 mAh/g. On the other hand, sulfur-based
cathodes have a theoretical capacity of about 1,673 mAh/g. Although
its voltage is about 2.2 V vs Li/Li.sup.+, which is about 60% of
conventional lithium-ion batteries, the theoretical specific energy
of a lithium-sulfur cell is about 2,600 Wh/kg, which is about five
times higher than a LiCoO.sub.2-graphite system. Sulfur also has
many other advantages such as low cost and non-toxicity. However,
the poor cycle life of lithium-sulfur batteries has been a
significant hindrance towards its commercialization. The fast
capacity fading during cycling may be due to a variety of factors,
including the dissolution of intermediate lithium polysulfides
(e.g., Li.sub.2S.sub.x, 4.ltoreq.x.ltoreq.8) in the electrolyte,
large volumetric expansion of sulfur (about 80%) during cycling,
and the insulating nature of Li.sub.2S. In order to improve the
cycle life of lithium-sulfur batteries, the dissolution of
polysulfides is one of the problems to tackle. Polysulfides are
soluble in the electrolyte and can diffuse to the lithium anode,
resulting in undesired parasitic reactions. The shuttle effect also
can lead to random precipitation of Li.sub.2S, and Li.sub.2S on the
positive electrode, which can change the electrode morphology and
result in fast capacity fading.
[0004] Other approaches have been attempted to address material
challenges of sulfur, such as surface coating, conductive matrix,
improved electrolytes, and porous carbon. For example,
graphene/polymer coating has been shown to yield a smaller capacity
decay. Porous carbon is another approach to trap polysulfides and
provide conductive paths for electrons. Nevertheless, in the case
of porous carbon, for example, a large surface area of sulfur can
still be exposed to the electrolyte, which exposure can cause
undesired polysulfide dissolution. Moreover, lesser emphasis has
been placed on dealing with the large volume expansion of sulfur
during lithiation. This volume expansion of sulfur can cause a
surrounding material, such as a coating, to crack and fracture,
rendering the surrounding material ineffective in trapping
polysulfides.
[0005] It is against this background that a need arose to develop
the sulfur-based cathodes and related methods and electrochemical
energy storage devices described herein.
SUMMARY
[0006] Embodiments of the invention relate to improved sulfur-based
cathodes and the incorporation of such cathodes in electrochemical
energy storage devices, such as batteries and supercapacitors.
[0007] Sulfur has a high specific capacity of about 1,673 mAh/g as
lithium battery cathodes, but its rapid capacity fading due to
polysulfide dissolution presents a significant challenge for
practical applications. Certain embodiments provide a hollow carbon
nanofiber-encapsulated sulfur cathode for effective trapping of
polysulfides and exhibiting high specific capacity and excellent
electrochemical cycling of battery cells. The hollow carbon
nanofiber arrays are fabricated using an anodic aluminum oxide
("AAO") template through thermal carbonization of polystyrene. The
AAO template also facilitates sulfur infusion into the hollow
fibers and substantially prevents sulfur from coating onto the
exterior carbon wall. The high aspect ratio of carbon fibers
provides a desirable structure for trapping polysulfides, and the
thin carbon wall allows rapid transport of lithium ions. The
dimension and shape of these nanofibers provide a large surface
area per unit mass for Li.sub.2S deposition during cycling and
reduce pulverization of active electrode materials due to
volumetric expansion. In some embodiments, a stable discharge
capacity of at least about 730 mAh/g can be observed at C/5 rate
after 150 cycles of charge/discharge. The introduction of
LiNO.sub.3 additive to the electrolyte can improve the coulombic
efficiency to at least about 99% at C/5 rate. The hollow carbon
nanofiber-encapsulated sulfur structure can be useful as a cathode
design for rechargeable lithium-sulfur batteries with high specific
energy, such as at least about 500 Wh/kg, at least about 700 Wh/kg,
at least about 900 Wh/kg, at least about 1,100 Wh/kg, at least
about 1,300 Wh/kg, at least about 1,500 Wh/kg, at least about 1,700
Wh/kg, at least about 1,900 Wh/kg, or at least about 2,100 Wh/kg,
and up to about 2,300 Wh/kg, up to about 2,400 Wh/kg, up to about
2,500 Wh/kg, or up to about 2,600 Wh/kg.
[0008] Other embodiments provide substantially monodisperse,
polymer-encapsulated hollow sulfur nanoparticles, presenting a
rational design to address various materials challenges. Some
embodiments demonstrate high specific discharge capacities of at
least about 1,179 mAh/g, at least about 1,018 mAh/g, and at least
about 990 mAh/g at C/10, C/5, and C/2 rates, respectively.
Excellent capacity retention can be attained, with at least about
80.3% retention after 500 cycles and at least about 60% retention
after 1,000 cycles at C/2 rate. Together with the high abundance of
sulfur, embodiments provide a room-temperature, one-stage aqueous
solution synthesis, which is highly scalable for manufacturing of
low-cost and high-energy batteries.
[0009] Further embodiments demonstrate the design of a
sulfur-TiO.sub.2 yolk-shell nanoarchitecture with internal void
space for stable and prolonged cycling over 1,000 charge/discharge
cycles in lithium-sulfur batteries. Compared to bare sulfur and
sulfur-TiO.sub.2 core-shell nanoparticles, the yolk-shell
nanostructures can exhibit high capacity retention due to the
presence of sufficient empty space to accommodate the volume
expansion of sulfur, resulting in a structurally intact TiO.sub.2
shell to mitigate against polysulfide dissolution. Using the
yolk-shell nanoarchitecture, an initial specific capacity of at
least about 1,030 mAh/g at C/2 rate and a Coulombic efficiency of
at least about 98.4% over 1,000 cycles can be achieved. Moreover,
the capacity decay after 1,000 cycles can be 0.033% or lower per
cycle.
[0010] Other aspects and embodiments of the invention are also
contemplated. The foregoing summary and the following detailed
description are not meant to restrict the invention to any
particular embodiment but are merely meant to describe some
embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] For a better understanding of the nature and objects of some
embodiments of the invention, reference should be made to the
following detailed description taken in conjunction with the
accompanying drawings.
[0012] FIG. 1. Schematic of design and fabrication process of
hollow carbon nanofiber-encapsulated sulfur cathode structure. (a)
The design principle showing the high aspect ratio of the carbon
nanofiber for effective trapping of polysulfides and (b) the
fabrication process of cathode structure. (c) Digital camera images
showing the contrast of AAO template before and after carbon
coating and sulfur infusion.
[0013] FIG. 2. Schematic of sulfur nanoparticle with empty space
and inside an outer shell, illustrating inward expansion during
lithiation to accommodate volume expansion and confinement of
polysulfides by the shell.
[0014] FIG. 3A. Schematic of yolk-shell morphology to provide
internal void space to accommodate volume expansion of sulfur
during lithiation, resulting in structural intact shell for
effective trapping of polysulfides.
[0015] FIG. 3B. Schematic of multi-yolk-shell morphology.
[0016] FIG. 4. Schematic of a battery including an encapsulated
sulfur cathode.
[0017] FIG. 5. Scanning electron microscopy ("SEM")
characterizations of hollow carbon nanofiber-encapsulated sulfur.
(a) AAO template after carbon coating. (b) Carbon
nanofiber-encapsulated sulfur after etching away AAO template. (c)
Cross-sectional image of hollow carbon/sulfur nanofiber arrays and
elemental mapping of carbon (d) and sulfur (e) of FIG. 5c.
[0018] FIG. 6. Transmission electron microscopy ("TEM")
characterizations of hollow carbon nanofiber-encapsulated sulfur.
(a) Bright field TEM image of an individual nanofiber. The jagged
line represents counts of sulfur signal along the dashed horizontal
line. (b) Dark field scanning TEM image (up) and energy-dispersive
X-ray spectroscopy mapping of sulfur (down) of the nanofiber. (c)
Zoom-in image of another sulfur-filled carbon nanofiber, showing
the thin carbon wall. (d) The corresponding average
energy-dispersive X-ray spectrum obtained from the nanofiber in
(c). Scale bars in FIG. 6a and b are both 500 nm.
[0019] FIG. 7. Auger electron spectroscopy of the AAO/carbon
template filled with sulfur, before and after sputtering with Ar
ions. (a) SEM image of the top view of carbon coated AAO template
after infusion of sulfur. The scale bar is 200 nm. Elemental
mapping of sulfur (b) before Ar sputtering, (c) 1.5 hours after Ar
sputtering, (d) 4.5 hours after Ar sputtering and (e) 7.5 hours
after sputtering. The sputtering rate is about 3.3 .mu.m/hour.
[0020] FIG. 8. Electrochemical performance of the carbon
nanofiber-encapsulated sulfur cathode. (a) Typical charge/discharge
voltage profiles at C/5 and C/2. (b) Cycle life at C/5 and C/2, as
compared to a control sample in which the AAO was not etched away.
The voltage range is 1.7-2.6 V vs Li/Li.sup.+.
[0021] FIG. 9. Electrochemical performance of the carbon
nanofiber-encapsulated sulfur electrode in electrolyte with
LiNO.sub.3 additive. (a) Capacities for charge/discharge cycling at
C/5. (b) Comparison of coulombic efficiencies for samples with and
without LiNO.sub.3 additive in the electrolyte, for cycling at C/5
and C/2.
[0022] FIG. 10. SEM images of the two sides of AAO template.
[0023] FIG. 11. Raman spectra of four samples: pure sulfur, carbon
coated AAO template, hollow carbon nanofiber-encapsulated sulfur,
and pure AAO template. No noticeable sulfur signal was detected in
the hollow carbon nanofiber-encapsulated sulfur.
[0024] FIG. 12. Comparison of the X-ray diffraction spectra for
pristine sulfur and hollow carbon nanofiber-encapsulated sulfur.
Inset is the zoom-in image of the encapsulated sulfur between
22.degree. and 24.degree.
[0025] FIG. 13. X-ray diffraction spectra of AAO template after
carbon coating at 750.degree. C. and 780.degree. C.
[0026] FIG. 14. Charge/discharge voltage profiles of hollow carbon
nanofiber-encapsulated sulfur at C/10 and C/5 rates.
[0027] FIG. 15. Fabrication, characterization and lithiation of
polymer-encapsulated hollow sulfur nanoparticles. (a) Schematic of
the formation mechanism for polymer-encapsulated hollow sulfur
nanoparticles. (b) SEM image of the as-prepared
polymer-encapsulated hollow sulfur nanoparticles. (c) SEM image of
the hollow sulfur nanoparticles after washing them with water to
remove the polymer on the particle surface. Inset in (c): TEM image
of an individual hollow sulfur nanoparticle. (d) Schematic diagram
illustrating the subliming process of the polymer-encapsulated
hollow sulfur nanoparticles. (e-g) SEM images of the sulfur
nanoparticles (e) before, (f) during, and (g) after sulfur
sublimation, respectively. (h) X-ray photoelectron spectroscopy
("XPS") spectra of polymer-encapsulated hollow sulfur nanoparticles
(top trace) and pure elemental sulfur (bottom trace). (i, j)
Typical SEM images of sulfur nanoparticles on a conducting
carbon-fiber paper (i) before and (j) after lithiation. The
particles after lithiation were marked with circles in (j). (k) A
comparison of the size distribution of sulfur nanoparticles before
and after lithiation.
[0028] FIG. 16. Electrochemical characteristics of
polymer-encapsulated hollow sulfur nanoparticles. (a) Typical
discharge-charge voltage profiles of cells made from the
polymer-encapsulated hollow sulfur nanoparticles at different
current rates (C/10, C/5 and C/2, 1C=1673 mA/g) in the potential
range of 2.6-1.5 V at room temperature. (b) Cycling performance and
Coulombic efficiency of the cell at a current rate of C/5 for 300
cycles. (c) Cycling performance and Coulombic efficiency of the
cell at a current rate of C/2 for 1,000 cycles. (d) Rate capability
of the cell discharged at various current rates.
[0029] FIG. 17. Electrode thickness evaluation of a hollow sulfur
nanoparticle cathode. (a) Schematic illustrating that an electrode
thickness undergoes little or no change owing to the inward
expansion of each of the hollow sulfur nanoparticles upon
lithiation. Typical SEM images of the cross-sections of the hollow
sulfur nanoparticle cathode, showing the thickness of (b) the
pristine electrode and (c) the electrode after 20 charge/discharge
cycles (at fully discharged (lithiated) state). (d) A comparison of
the thickness at 20 different locations for the cross-sections of
the pristine electrode and the electrode after 20 charge/discharge
cycles.
[0030] FIG. 18. Size distribution of hollow sulfur nanoparticles,
based on counting over 200 particles from SEM images.
[0031] FIG. 19. Thermal gravimetric analysis ("TGA") curve of
as-prepared polymer-encapsulated hollow sulfur nanoparticles
recorded in the range of 40-400.degree. C. in argon at a heating
rate of about 2.degree. C./min, showing that an amount of elemental
sulfur in the sample is about 70.4 wt %. Another curve is for a
control sample of pure polymer powder under the same experimental
condition.
[0032] FIG. 20. Voltage profile of a pouch cell assembled in an
argon-filled glovebox using a carbon-fiber paper with
polymer-encapsulated hollow sulfur nanoparticles as cathode and
lithium foil as anode. The pouch cell was discharged at a current
rate of C/5 to 1.5 V, and then the voltage was held at 1.5 V for
about 18 h.
[0033] FIG. 21. Typical discharge/charge profiles of a cell made
from polymer-encapsulated hollow sulfur nanoparticles at 1C
rate.
[0034] FIG. 22. Synthesis and characterization of sulfur-TiO.sub.2
yolk-shell nanostructures. (a) Schematic of the synthetic process
which involves coating of sulfur nanoparticles with TiO.sub.2 to
form sulfur-TiO.sub.2 core-shell nanostructures, followed by
partial dissolution of sulfur in toluene to achieve the yolk-shell
morphology. (b) SEM image and (c) TEM image of as-synthesized
sulfur-TiO.sub.2 yolk-shell nanostructures. Through large-ensemble
measurements, the average nanoparticle size and TiO.sub.2 shell
thickness were determined to be about 800 nm and about 15 nm,
respectively.
[0035] FIG. 23. Morphology of sulfur-TiO.sub.2 yolk-shell
nanostructures after lithiation. (a-c) SEM images of
sulfur-TiO.sub.2 yolk-shell nanostructures (a) before and (b) after
lithiation and (c) their respective particle size distributions.
(d) TEM image of a sulfur-TiO.sub.2 yolk-shell nanostructure after
lithiation, showing the presence of an intact TiO.sub.2 shell
(highlighted by arrow). (e) Energy-dispersive X-ray spectrum and
electron energy loss spectrum (inset) of the nanostructure in (d),
showing the presence of lithiated sulfur and TiO.sub.2. The Cu peak
arises due to the use of a copper TEM grid.
[0036] FIG. 24. Electrochemical performance of sulfur-TiO.sub.2
yolk-shell nanostructures. (a) Charge/discharge capacity and
Coulombic efficiency over 1,000 cycles at 0.5C. (b) Capacity
retention of sulfur-TiO.sub.2 yolk-shell nanostructures cycled at
0.5C, in comparison with bare sulfur and sulfur-TiO.sub.2
core-shell nanoparticles. (c) Charge/discharge capacity and (d)
voltage profiles of sulfur-TiO.sub.2 yolk-shell nanostructures
cycled at various C-rates from 0.2C to 2C. Specific capacity values
were calculated based on the mass of sulfur.
[0037] FIG. 25. (a) SEM image, (b) TEM image, and (c)
Energy-dispersive X-ray spectrum of as-synthesized bare sulfur
nanoparticles.
[0038] FIG. 26. (a) SEM image, (b) TEM image, and (c)
Energy-dispersive X-ray spectrum of as-synthesized sulfur-TiO.sub.2
core-shell nanoparticles. The TiO.sub.2 shell is not clearly
visible in this case because it is relatively thin compared to the
size of the entire nanoparticle.
[0039] FIG. 27. X-ray diffraction patterns of bare sulfur,
sulfur-TiO.sub.2 core-shell nanoparticles, and sulfur-TiO.sub.2
yolk-shell nanoparticles. In the sulfur-TiO.sub.2 core-shell and
yolk-shell particles, the diffraction peaks of sulfur (marked with
asterisks) were observed but not those of TiO.sub.2, indicating the
amorphous nature of TiO.sub.2.
[0040] FIG. 28. Voltage profile of a pouch cell assembled using
sulfur-TiO.sub.2 yolk-shell nanoparticles on carbon-fiber paper as
the working electrode and lithium foil as the counter electrode.
The cell was discharged at 0.1C to a voltage of 1.7 V, and the
voltage was maintained for over 20 h.
[0041] FIG. 29. SEM images of (a) bare sulfur and (b)
sulfur-TiO.sub.2 core-shell nanoparticles after lithiation, showing
random precipitation of irregularly-shaped Li.sub.2S.sub.2 and
Li.sub.2S particles on the electrodes due to dissolution of lithium
polysulfides into the electrolyte.
[0042] FIG. 30. TGA of bare sulfur, sulfur-TiO.sub.2 core-shell
nanoparticles, and sulfur-TiO.sub.2 yolk-shell nanoparticles. The
wt % of sulfur in these 3 samples were determined to be about 99%,
about 79%, and about 71%, respectively.
[0043] FIG. 31. Charge/discharge capacity and Coulombic efficiency
of sulfur-TiO.sub.2 yolk-shell nanostructures in terms of mAh/g of
the electrode mix over 1,000 cycles at 0.5C.
[0044] FIG. 32. SEM images of the electrode cross-section of
sulfur-TiO.sub.2 yolk-shell nanoparticles (a) before and (b) after
70 cycles at the various C-rates shown in FIG. 24c. (c) Their
corresponding distributions in electrode thickness, showing little
change in thickness before and after 70 cycles.
DETAILED DESCRIPTION
Encapsulated Sulfur Cathodes
[0045] Embodiments of the invention relate to improved sulfur-based
cathode materials and the incorporation of such cathode materials
in electrochemical energy storage devices, such as batteries and
supercapacitors. Embodiments of the invention can effectively
address the materials challenges of sulfur-based cathode materials
that otherwise can lead to rapid capacity fading in lithium-sulfur
batteries. In some embodiments, the materials design and synthesis
of sulfur-based cathode materials can realize superior performance
for high capacity sulfur-based cathodes. Lithium-sulfur batteries
incorporating such cathodes can show high specific discharge
capacities and high capacity retention over long cycling. Together
with the high abundance of sulfur, manufacturing of sulfur-based
cathode materials can be carried out in a highly scalable and
low-cost manner.
[0046] Some embodiments relate to an encapsulating structure for a
sulfur-based electrode having at least one or any combination or
sub-combination of the following characteristics: 1) a largely or
substantially closed structure for efficient containment of a
sulfur-based material (e.g., one or more of elemental sulfur, a
metal sulfide, and a metal polysulfide); 2) a reduced surface area
for sulfur-electrolyte contact; 3) sufficient empty space to
accommodate sulfur volumetric expansion to avoid or reduce
pulverization of a sulfur-based active material; 4) a short
transport pathway for either, or both, electrons and Li ions to
achieve high capacity at a high power rate; 5) a large conductive
surface area in contact with a sulfur-based material; and 6) a set
of suitable electrolyte additives to passivate a lithium surface to
minimize or reduce the shuttle effect.
[0047] Some embodiments implement a cathode structure with a
sulfur-based material contained or disposed within largely or
substantially continuous, hollow encapsulating structures to
mitigate against dissolution of the sulfur-based material. The
sulfur-based material can include one or more of elemental sulfur,
a metal sulfide, a metal polysulfide, or a mixture thereof. For
example, the sulfur-based material can include one or more of
elemental sulfur, Li.sub.2S, Li.sub.2S.sub.2, Li.sub.2S.sub.3,
Li.sub.2S, with 4.ltoreq.x.ltoreq.8, or a mixture thereof. The
relatively thin walls of the encapsulating structures can enhance
the electrical conductivity of the cathode while allowing for the
transfer of Li ions through the walls, thereby also affording
enhanced ionic conductivity. In conjunction, the walls of the
encapsulating structures can serve as effective barriers against
polysulfide leakage and dissolution, which barriers are disposed
between at least a portion of the sulfur-based material and an
electrolyte. In such manner, the walls of the encapsulating
structures can form a largely or substantially closed structure for
efficient containment of the sulfur-based material, thereby
spatially separating or segregating the sulfur-based material as
one or more active material domains in an interior of the
encapsulating structures and apart from an electrolyte in an
exterior of the encapsulating structures. In some embodiments, the
presence of barriers and the separation of a sulfur-based material
from an electrolyte by the barriers can be demonstrated using, for
example, microscope images as explained in the Examples further
below. In addition, the presence of voids or empty spaces inside
the encapsulating structures allows for sulfur expansion during
electrochemical cycling.
[0048] Referring to an embodiment of FIG. 1a, a hollow carbon
nanofiber-encapsulated sulfur cathode 100 is provided, including a
substantially vertical array of hollow carbon nanofibers 102
partially filled with a sulfur-based active material 104 (FIG. 1a).
The array of hollow carbon nanofibers 102 can be an ordered or
disordered array. AAO membranes are used as templates for the
fabrication of hollow carbon nanofibers 102, through a polystyrene
carbonization process. The AAO membranes serve both as a template
for carbon nanofiber formation and a barrier to inhibit the
sulfur-based material 104 from coating onto the exterior carbon
fiber walls. In such manner, the sulfur-based material 104 is
selectively coated onto the inner surfaces of the hollow nanofibers
102, with any gaps or spaces between the nanofibers 102
substantially devoid of the sulfur-based material 104. By way of
example, the nanofiber diameters (e.g., outer diameter) can range
from about 200 nm to about 300 nm, while the length is up to about
60 .mu.m or more, corresponding to the AAO template structure. The
sulfur-based material 104 is effectively contained in the high
aspect ratio carbon nanofibers 102, and its contact with an
electrolyte is limited to the two openings at the ends of the
nanofibers 102. It is also contemplated that the ends of the
nanofibers 102 can be capped to further reduce contact with the
electrolyte, such as by depositing or otherwise applying a suitable
material to the ends of the nanofibers 102. The hollow structure
provides a large space for sulfur expansion during cycling. As
lithium can readily penetrate the thin carbon wall, rapid ionic
transport is also possible. The one-dimensional nature of
conductive carbon allows facile transport of electrons and a large
area for contact with the sulfur-based material 104. It is also
contemplated that the inner surfaces of the carbon nanofibers 102
can be chemically modified to facilitate coating of the
sulfur-based material 104. These attributes of the hollow carbon
nanofiber structure allow high specific capacity and stable cycle
life of the sulfur-based cathode 100 in lithium-sulfur
batteries.
[0049] More generally for some embodiments, carbon nanofibers (or
other types of hollow, elongated encapsulating structures) can have
an outer lateral dimension (e.g., an outer diameter, an outer
lateral dimension along a major axis, an averaged outer lateral
dimension along a major axis and a minor axis, or another
characteristic outer lateral dimension) in the range of about 10 nm
to about 5 .mu.m, such as about 20 nm to about 5 .mu.m, from about
30 nm to about 2 .mu.m, about 30 nm to about 1 .mu.m, about 30 nm
to about 900 nm, about 50 nm to about 800 nm, about 50 nm to about
700 nm, about 50 nm to about 600 nm, about 50 nm to about 500 nm,
about 100 nm to about 500 nm, about 50 nm to about 400 nm, about
100 nm to about 400 nm, or about 200 nm to about 300 nm, a
longitudinal dimension (e.g., a length or another characteristic
longitudinal dimension) in the range of about 500 nm to about 500
.mu.m, such as about 800 nm to about 400 .mu.m, about 1 .mu.m to
about 300 .mu.m, about 1 .mu.m to 200 .mu.m, about 1 .mu.m to about
150 .mu.m, about 1 .mu.m to about 100 .mu.m, or about 10 .mu.m to
about 100 .mu.m, and an aspect ratio (e.g., specified as a ratio of
its longitudinal dimension and its outer lateral dimension) that is
greater than about 1, such as at least or greater than about 5, at
least or greater than about 10, at least or greater than about 20,
at least or greater than about 50, at least or greater than about
100, at least or greater than about 300, from about 2 to about
2,000, about 5 to about 1,000, about 10 to about 900, about 10 to
about 800, about 50 to about 700, about 50 to about 600, about 50
to about 500, about 100 to about 500, about 100 to about 400, or
about 200 to about 400. Also, the carbon nanofibers (or other types
of hollow, elongated encapsulating structures) can have an inner
lateral dimension (e.g., an inner diameter, an inner lateral
dimension along a major axis, an averaged inner lateral dimension
along a major axis and a minor axis, or another characteristic
inner lateral dimension defining an internal volume to accommodate
a sulfur-based material) that is at least about 10 nm, such as at
least about 15 nm, at least about 20 nm, at least about 30 nm, at
least about 40 nm, at least about 50 nm, at least about 100 nm, at
least about 150 nm, or at least about 200 nm, and up to an outer
lateral dimension while accounting for a thickness of the walls of
the carbon nanofibers. The walls of the carbon nanofibers (or other
types of hollow, elongated encapsulating structures) can be in the
range of about 0.5 nm to about 100 nm, such as about 1 nm to about
90 nm, about 1 nm to about 80 nm, about 1 nm to about 70 nm, about
1 nm to about 60 nm, about 1 nm to about 50 nm, about 1 nm to about
40 nm, about 1 nm to about 30 nm, about 5 nm to about 30 nm, about
10 nm to about 30 nm, about 1 nm to about 20 nm, about 5 nm to
about 20 nm, about 10 nm to about 20 nm, about 1 nm to about 10 nm,
or about 5 nm to about 20 nm. The above specified values for
dimensions, thicknesses, and aspect ratios can apply to an
individual carbon nanofiber (or another type of hollow, elongated
encapsulating structure), or can represent an average or a median
value across a population of carbon nanofibers.
[0050] Diameters of carbon nanofibers (or other types of hollow,
elongated encapsulating structures) can be substantially constant
or can vary along the lengths of the nanofibers, such as in
accordance with a pore morphology of an AAO template structure.
Hollow, elongated structures can be formed of other types of
conductive materials in place of, or in combination with carbon,
such as titanium oxide (doped or undoped) and other types of metal
oxides. Examples of other hollow, elongated structures include
hollow, metal nanofibers; hollow, metal oxide nanofibers; hollow,
metal nitride nanofibers; hollow, metal sulfide nanofibers; and
hollow, composite nanofibers. Hollow, elongated structures can be
single-shelled or multi-shelled, with different shells formed of
the same material or different materials, and surfaces of the
hollow, elongated structures can be smooth or rough. Hollow,
elongated structures can be electrically conductive, ionically
conductive (e.g., with respect to one or more of Li ions, Na ions,
K ions, Mg ions, Al ions, Fe ions, and Zn ions), or both.
[0051] Advantageously, a sulfur-based material is selectively
coated in a well-controlled and reproducible manner onto the inner
surfaces of hollow carbon nanofibers, instead of their exterior
surfaces, and instead of incorporation within walls of the carbon
nanofibers. In such manner, the sulfur-based material can form one
or more active material domains that are spaced apart from an
electrolyte by the walls of the carbon nanofibers, and exposure of
the sulfur-based material to an electrolyte can be reduced, thereby
addressing the dissolution issue. To tackle this issue, a
template-assisted method is used to fabricate a cathode structure
with sulfur selectively coated on the inner wall of the carbon
fibers, as shown in FIG. 1b. AAO template (e.g., Whatman, pore size
of about 200 nm, thickness of about 60 .mu.m) is used as the
template for making hollow carbon nanofibers. Typically, about 120
mg of AAO membrane is placed inside an alumina boat, and about 2 ml
of about 10 wt % polystyrene ("PS") (or another suitable
carbon-containing polymer) suspended in dimethylformamide ("DMF")
(or another suitable solvent) is dropped onto the template as the
carbon precursor. The carbonization is performed by heating the
AAO/PS/DMF mixture at about 750.degree. C. (or another suitable
temperature, such as in the range of about 500.degree. C. to about
1,000.degree. C.) for about four hours (or another suitable time
period, such as in the range of about 1 hour to about 10 hours)
under a slow flow of N.sub.2 gas. After cooling down, the
carbon-coated AAO template is loaded into a small glass vial,
together with a controlled amount of about 1% sulfur solution in
toluene (or another suitable solvent). The sample is dried in a
vacuum oven, before being heated up to about 155.degree. C. (or
another suitable temperature, such as in the range of about
100.degree. C. to about 200.degree. C.) and kept for about 12 hours
(or another suitable time period, such as in the range of about 5
hours to about 20 hours) to ensure uniform sulfur diffusion into
the carbon fibers. In this fabrication process, the AAO membrane
not only provides a template for hollow carbon nanofiber formation,
but also prevents or mitigates against sulfur from coating onto the
external surface of the fiber wall. To remove the AAO template, the
AAO/carbon nanofiber/S composite is immersed in about 2 M
H.sub.3PO.sub.4 solution (or another suitable acidic solution) for
about 10 hours (or another suitable time period, such as in the
range of about 5 hours to about 20 hours). FIG. 1c shows digital
camera images of a pristine AAO template before (lighter shade) and
after (darker shade) carbon coating and sulfur infusion, indicating
that sulfur was absorbed into the hollow carbon fibers. Other types
of porous template structures can be used in place of, or in
combination with, the AAO template structure.
[0052] Referring to another embodiment of FIG. 2, encapsulating
structures 200 are in the form of outer shells having a spherical
or spheroidal shape. The encapsulating structures 200 can have an
outer lateral dimension (e.g., an outer diameter, an outer lateral
dimension along a major axis, an averaged outer lateral dimension
along a major axis and a minor axis, or another characteristic
outer lateral dimension) in the range of about 10 nm to about 10
.mu.m, such as about 10 nm to about 5 .mu.m, about 50 nm to about 2
.mu.m, about 100 nm to about 1 .mu.m, about 100 nm to about 900 nm,
about 200 nm to about 800 nm, about 300 nm to about 700 nm, about
300 nm to about 600 nm, or about 400 nm to about 500 nm, and an
aspect ratio (e.g., specified as a ratio of outer lateral
dimensions along a major axis and a minor axis) that is less than
about 5, such as no greater than about 4.5, no greater than about
4, no greater than about 3.5, no greater than about 3, no greater
than about 2.5, no greater than about 2, no greater than about 1.5,
or about 1. In some embodiments, the hollow, spheroidal
encapsulating structures are largely or substantially monodisperse,
such that at least about 50%, at least about 60%, at least about
70%, at least about 80%, at least about 90%, or at least about 95%
of the hollow, spheroidal encapsulating structures are within one
or more of the ranges of dimensions specified above. Also, the
hollow, spheroidal encapsulating structures can have an inner
lateral dimension (e.g., an inner diameter, an inner lateral
dimension along a major axis, an averaged inner lateral dimension
along a major axis and a minor axis, or another characteristic
inner lateral dimension defining an internal volume to accommodate
a sulfur-based material) that is at least about 10 nm, such as at
least about 15 nm, at least about 20 nm, at least about 30 nm, at
least about 40 nm, at least about 50 nm, at least about 100 nm, at
least about 150 nm, at least about 200 nm, at least about 250 nm,
at least about 300 nm, at least about 350 nm, or at least about 400
nm, and up to an outer lateral dimension while accounting for a
thickness of the walls of the encapsulating structures. The walls
of the encapsulating structures can have a thickness in the range
of about 1 nm to about 100 nm, such as about 5 nm to about 90 nm,
about 10 nm to about 80 nm, about 10 nm to about 70 nm, about 10 nm
to about 60 nm, about 10 nm to about 50 nm, about 10 nm to about 40
nm, or about 10 nm to about 30 nm. As shown in FIG. 2, the hollow,
spheroidal encapsulating structures are formed of a polymer, such
as poly(vinyl pyrrolidone) or another conductive polymer having
polar groups, non-polar groups, or both (e.g., an amphiphilic
polymer). Hollow, spheroidal encapsulating structures can be formed
of other types of conductive materials in place of, or in
combination with, a polymer, such as carbon, metals, titanium oxide
(doped or undoped), and other types of metal oxides, metal
nitrides, and metal sulfides. Examples of other hollow, spheroidal
structures include hollow, metal shells; hollow, metal oxide
shells; hollow, metal nitride shells; hollow, metal sulfide shells;
and hollow, composite shells. Hollow, spheroidal encapsulating
structures can be electrically conductive, ionically conductive
(e.g., with respect to Li ions or other types of ions), or
both.
[0053] As shown in FIG. 2, a sulfur-based material 202 is disposed
within the encapsulating structures 200, with a void or an empty
space disposed within an interior of each encapsulating structure
200. In the illustrated embodiment, the sulfur-based material 202
is provided as inner shells or as hollow nanoparticles, with a void
or an empty space disposed within an interior of each hollow
nanoparticle. In other embodiments, a sulfur-based material 302 or
306 can be disposed within each encapsulating structure 300 or 304
as one or more substantially solid nanoparticles or other types of
substantially solid nanostructures (FIGS. 3A and 3B). The
embodiment of FIG. 3A can be referred to as having a yolk-shell
morphology, with the sulfur-based material 302 corresponding to a
"yolk" surrounded by an outer "shell" of the encapsulating
structure 300, and the embodiment of FIG. 3B can be referred to as
having a multi-yolk-shell morphology, with the sulfur-based
material 306 corresponding to multiple "yolks" surrounded by an
outer "shell" of the encapsulating structure 304. Although two
nanoparticles of the sulfur-based material 306 are shown in FIG.
3B, more or fewer nanoparticles of the sulfur-based material 306
can be included in the encapsulating structure 304, and the number
of nanoparticles can be substantially uniform or can vary across a
population of the encapsulating structures 304.
[0054] More generally, sulfur-based nanoparticles (or other types
of nanostructures formed of a sulfur-based material) can have an
outer lateral dimension (e.g., an outer diameter, an outer lateral
dimension along a major axis, an averaged outer lateral dimension
along a major axis and a minor axis, or another characteristic
outer lateral dimension) that is at least about 1 nm, such as at
least about 5 nm, at least about 10 nm, at least about 15 nm, at
least about 20 nm, at least about 30 nm, at least about 40 nm, at
least about 50 nm, at least about 100 nm, at least about 150 nm, at
least about 200 nm, at least about 250 nm, at least about 300 nm,
at least about 350 nm, or at least about 400 nm, and up to an inner
lateral dimension of encapsulating structures, while leaving
sufficient room for volume expansion during cycling. A sulfur-based
nanoparticle can be largely or substantially solid, or can have a
void or an empty space disposed within an interior of, and at least
partially surrounded by, the sulfur-based nanoparticle. In some
embodiments, sulfur-based nanoparticles have a spherical or
spheroidal shape. Typically, a sulfur-based nanoparticle has an
aspect ratio that is less than about 5, such as no greater than
about 4.5, no greater than about 4, no greater than about 3.5, no
greater than about 3, no greater than about 2.5, no greater than
about 2, no greater than about 1.5, or about 1. Other types of
sulfur-based nanostructures can be used in place of, or in
combination with, sulfur-based nanoparticles, such as elongated
nanostructures having an aspect ratio that is at least about 5,
whether solid or hollow.
[0055] Referring, for example, to the embodiments of FIGS. 1
through 3B, an exterior of encapsulating structures can be largely
or substantially devoid of a sulfur-based material. Also, the walls
of the encapsulating structures can be largely or substantially
devoid of the sulfur-based material. Stated in another way, the
sulfur-based material can be selectively positioned in a
well-controlled and reproducible manner within an interior of the
encapsulating structures, instead of their exterior surfaces, and
instead of infiltration or impregnation of the sulfur-based
material within walls of the encapsulating structures. In some
embodiments, at a portion of a sulfur-based material is spatially
separated or segregated as one or more active material domains in
an interior of each encapsulating structure and apart from an
electrolyte in an exterior of the encapsulating structure, such as
at least about 10% (by weight or volume), at least about 20% (by
weight or volume), at least about 30% (by weight or volume), at
least about 40% (by weight or volume), at least about 50% (by
weight or volume), at least about 60% (by weight or volume), at
least about 70% (by weight or volume), at least about 75% (by
weight or volume), at least about 80% (by weight or volume), at
least about 85% (by weight or volume), at least about 90% (by
weight or volume), at least about 95% (by weight or volume), or at
least about 98% (by weight or volume), and up to about 99% (by
weight or volume), or up to about 99.9% (by weight or volume), up
to about 99.99% (by weight or volume), or more, relative to any
remaining portion of the sulfur-based material on an exterior and
within a wall of the encapsulating structure.
[0056] Still referring, for example, to the embodiments of FIGS. 1
through 3B, each encapsulating structure defines an internal
volume, and a sulfur-based material is disposed within the internal
volume and occupies less than 100% of the internal volume, thereby
leaving a void or an empty space inside the encapsulating structure
to allow for expansion of the sulfur-based material. In some
embodiments, such as for the case of the sulfur-based material is
its substantially de-lithiated state, a ratio of the volume of the
void inside the encapsulating structure relative to the volume of
the sulfur-based material inside the encapsulating structure is in
the range of about 1/20 to about 20/1, such as from about 1/10 to
about 10/1, from about 1/10 to about 5/1, from about 1/10 to about
3/1, from about 1/10 to about 2/1, from about 1/10 to about 1/1,
from about 1/5 to about 3/1, from about 1/5 to about 2/1, from
about 1/5 to about 1/1, from about 1/3 to about 3/1, from about 1/3
to about 2/1, from about 1/3 to about 1/1, from about 1/2 to about
3/1, from about 1/2 to about 2/1, from about 1/2 to about 1/1, from
about 2/3 to about 3/1, from about 2/3 to about 2/1, or from about
2/3 to about 1/1. In some embodiments, such as for the case of the
sulfur-based material is its substantially de-lithiated state, the
volume of the void can be at least about 1/20 of the total internal
volume inside the encapsulating structure, such as at least about
1/10, at least about 1/5, at least about 1/3, at least about 1/2,
or at least about 2/3, with a remainder of the internal volume
inside the encapsulating structure taken up by the sulfur-based
material. The loading of the sulfur-based material within the
encapsulating structures can be controlled so that there is enough
empty space for sulfur to expand during lithiation. In some
embodiments, a weight ratio of the sulfur-based material relative
to a combined mass of the sulfur-based material and the
encapsulating structures is in the range of about 1% to about 99%,
such as from about 5% to about 95%, from about 10% to about 90%,
from about 20% to about 90%, from about 30% to about 80%, from
about 40% to about 80%, from about 50% to about 80%, or from about
60% to about 80%.
Electrochemical Energy Storages Including Encapsulated Sulfur
Cathodes
[0057] The electrodes described herein can be used for a variety of
batteries and other electrochemical energy storage devices. For
example, the electrodes can be substituted in place of, or used in
conjunction with, conventional electrodes for lithium-sulfur
batteries or other types of batteries. As shown in an embodiment of
FIG. 4, a resulting battery 400 can include a cathode 402, an anode
404, and a separator 406 that is disposed between the cathode 402
and the anode 404. The battery 400 also can include an electrolyte
408, which is disposed between the cathode 402 and the anode 404.
The cathode 402 can be an encapsulated sulfur cathode as described
herein, and the anode 404 can be a lithium-based anode, a
silicon-based anode, a germanium-based anode, or another suitable
anode.
[0058] Resulting batteries, such as the battery 400, can exhibit a
maximum discharge capacity at a current rate of C/10 (or at C/5,
C/2, 1C, or another higher or lower reference rate and as evaluated
relative to Li/Li.sup.+ or another counter/reference electrode)
that is at least about 400 mAh/g, such as at least about 500 mAh/g,
at least about 600 mAh/g, at least about 700 mAh/g, at least about
800 mAh/g, at least about 900 mAh/g, at least about 1,000 mAh/g, at
least about 1,100 mAh/g, at least about 1,200 mAh/g, at least about
1,300 mAh/g, at least about 1,400 mAh/g, or at least about 1,500
mAh/g, and up to about 1,670 mAh/g or more, such as up to about
1,600 mAh/g or up to about 1,560 mAh/g.
[0059] Resulting batteries, such as the battery 400, also can
exhibit excellent retention of discharge capacity over several
cycles, such that, after 100 cycles at a rate of C/10 (or at C/5,
C/2, 1C, or another higher or lower reference rate), at least about
50%, at least about 60%, at least about 70%, at least about 75%, at
least about 80%, at least about 85%, or at least about 88%, and up
to about 90%, up to about 95%, or more of an initial, maximum, or
other reference discharge capacity (e.g., at the 14.sup.th cycle)
is retained. And, after 200 cycles at a rate of C/10 (or at C/5,
C/2, 1C, or another higher or lower reference rate), at least about
45%, at least about 55%, at least about 65%, at least about 70%, at
least about 75%, at least about 83%, at least about 85%, or at
least about 87%, and up to about 90%, up to about 95%, or more of
an initial, maximum, or other reference discharge capacity (e.g.,
at the 14.sup.th cycle) is retained. And, after 500 cycles at a
rate of C/10 (or at C/5, C/2, 1C, or another higher or lower
reference rate), at least about 40%, at least about 50%, at least
about 60%, at least about 65%, at least about 70%, at least about
75%, at least about 80%, or at least about 81%, and up to about
90%, up to about 95%, or more of an initial, maximum, or other
reference discharge capacity (e.g., at the 14.sup.th cycle) is
retained. And, after 1,000 cycles at a rate of C/10 (or at C/5,
C/2, 1C, or another higher or lower reference rate), at least about
30%, at least about 40%, at least about 50%, at least about 55%, at
least about 60%, at least about 63%, at least about 65%, or at
least about 67%, and up to about 80%, up to about 85%, or more of
an initial, maximum, or other reference discharge capacity (e.g.,
at the 14.sup.th cycle) is retained.
[0060] Also, in terms of coulombic efficiency (e.g., an initial or
a maximum coulombic efficiency or one that is averaged over a
certain number of cycles, such as 100, 200, 500, or 1,000 cycles)
at a rate of C/10 (or at C/5, C/2, 1C, or another higher or lower
reference rate), resulting batteries, such as the battery 400, can
have an efficiency that is at least about 75%, at least about 80%,
at least about 85%, at least about 90%, at least about 95%, or at
least about 98%, and up to about 99%, up to about 99.5%, up to
about 99.9%, or more.
[0061] Moreover, in some embodiments, the effectiveness of
containment of a sulfur-based material and the structural integrity
of encapsulation structures can be assessed in terms of a weight
percentage of sulfur (e.g., whether in elemental or another form)
present in an electrolyte after a certain number of cycles,
relative to a total weight of sulfur (e.g., whether in elemental or
another form) as initially included in a cathode. In some
embodiments, after 30 cycles at a rate of C/10 (or at C/5, C/2, 1C,
or another higher or lower reference rate), no greater than about
23% of sulfur is present in the electrolyte, such as no greater
than about 21% or no greater than about 19%. After 100 cycles at a
rate of C/10 (or at C/5, C/2, 1C, or another higher or lower
reference rate), no greater than about 25% of sulfur is present in
the electrolyte, such as no greater than about 23% or no greater
than about 21%. After 500 cycles at a rate of C/10 (or at C/5, C/2,
1C, or another higher or lower reference rate), no greater than
about 30% of sulfur is present in the electrolyte, such as no
greater than about 28% or no greater than about 26%.
EXAMPLES
[0062] The following examples describe specific aspects of some
embodiments of the invention to illustrate and provide a
description for those of ordinary skill in the art. The examples
should not be construed as limiting the invention, as the examples
merely provide specific methodology useful in understanding and
practicing some embodiments of the invention.
Example 1
Hollow Carbon Nanofiber-Encapsulated Sulfur
[0063] This example describe the synthesis of hollow carbon
nanofiber-encapsulated sulfur electrode structures, including a
substantially vertical array of hollow carbon nanofibers filled
with melted sulfur. AAO membranes are used as templates for the
fabrication of hollow carbon nanofibers, through a polystyrene
carbonization process.
[0064] Scanning electron microscopy ("SEM") images of designed
structures of some embodiments at different stages of fabrication
are shown in FIG. 5. After carbon coating at about 750.degree. C.,
substantially continuous hollow carbon nanofibers are formed inside
the AAO template (FIG. 5a). The outer diameters of the nanofibers
are about 200 nm to about 300 nm, corresponding to the pore size of
AAO template (FIG. 10). The weight gain after carbon coating was
about 2% of the AAO template. FIG. 5b shows the image of hollow
carbon nanofibers after sulfur infusion and AAO etching. Typically,
the weight ratio of sulfur to carbon was 3:1 in the final electrode
structure, corresponding to about 75 vol % of sulfur content in the
composite, although other suitable weight ratios are contemplated,
such as from about 1:1 to about 10:1 or about 1.5:1 to about 5:1.
The sulfur loading is controlled so that there is enough free space
(e.g., in the form of gaps or voids within and at least partially
extending through the nanofibers after sulfur infusion) for sulfur
to expand during the formation of Li.sub.2S. To confirm the
presence of carbon and sulfur, energy-dispersive X-ray spectroscopy
("EDS") mappings are performed over the cross section of the whole
carbon nanofiber array, with the corresponding SEM image in FIG.
5c, Carbon (FIG. 5d) and sulfur (FIG. 5e) signals are detected
substantially uniformly over the whole cross section, validating
the structural design and indicating that sulfur was well
distributed within the hollow carbon nanofibers.
[0065] Further evidence of sulfur containment within the carbon
nanofiber was provided by transmission electron microscopy ("TEM")
images of some embodiments. FIG. 6a shows a hollow carbon nanofiber
with sulfur encapsulated inside. Sulfur appears darker under TEM as
it is heavier than carbon. An EDS line-scan (dashed line) across
the carbon nanofiber further confirms the presence of sulfur. The
spectrum represents the counts of sulfur signal along the dashed
line. The spectrum shows that sulfur is present inside the hollow
carbon nanofibers, but not outside. This is also verified by the
sulfur EDS mapping in FIG. 6b. The full EDS spectrum over the whole
tube (FIG. 6d) shows the carbon and sulfur peaks but not any
aluminum signal, indicating that there is little or no alumina
residue left from the AAO template. The zoom-in image (FIG. 6c) of
another carbon nanofiber shows the fiber wall has a small thickness
of about 8 nm to about 9 nm, which is desirable in allowing fast
kinetics of lithium ion diffusion.
[0066] Spatial distribution of sulfur inside the hollow carbon
nanofiber arrays is further demonstrated by auger electron
spectroscopy ("AES") with Ar ion sputtering, according to some
embodiments. FIG. 7a shows the top view SEM image of the nanofiber
array, revealing the hexagonal packing. The elemental mapping of
sulfur before sputtering (FIG. 7b) also yields a similar hexagonal
pattern, suggesting that sulfur is present in the hollow channels.
FIG. 7c-e show the sulfur elemental mappings after 1.5 hours, 4.5
hours and 7.5 hours of Ar ion sputtering respectively. Around 25
.mu.m of the sample is etched away after 7.5 hours of sputtering.
The variation in the sulfur mapping patterns is due to the change
in the AAO channel morphology at different depths (FIG. 10). The
hexagonal packing becomes clearer at regions closer to the center
of the hollow nanofiber array. The AES mappings show that globally
sulfur is well distributed from the top to deep inside the hollow
carbon nanofibers. A variety of other regular or irregular patterns
are contemplated, such as square patterns, rectangular patterns,
triangular patterns, octagonal patterns, and so forth.
[0067] The above characterizations show that hollow carbon
nanofiber-encapsulated sulfur can be formed with the assist of AAO
template. To further understand the crystal structure of carbon and
sulfur in the final structure, Raman spectroscopy and X-ray
diffraction ("XRD") are performed to study the as-fabricated sulfur
electrode of some embodiments. The Raman measurement shows a
typical spectrum of partially graphitized carbon, indicated by the
G band (about 1600 cm.sup.-1) and D band (about 1360 cm.sup.-1) in
FIG. 11. G band features the in-plane vibration of sp.sup.2 carbon
atoms, and D band originates from the defects. The coexistence of
the two bands indicates that the carbon was partially graphitized
with some amount of defects and disorders. Further optimizations
can be implemented to reduce the amount of defects and disorders.
The absence of sulfur peak in the Raman spectrum of carbon
nanofibers/S composite indicates that sulfur is well encapsulated
within the carbon nanofibers.
[0068] XRD spectrum (FIG. 12) of the carbon/sulfur composites shows
a weak peak at about 23.05.degree., corresponding to the strongest
(222) peak of orthorhombic sulfur (PDF 00-001-0478). This indicates
that sulfur in the hollow nanofiber was less crystalline, although
further optimizations can be implemented to enhance or otherwise
adjust the degree of crystallinity. In some embodiments, there is
no peak related to crystalline Al.sub.2O.sub.3 phase in the XRD
pattern, indicating that the AAO template was still amorphous after
carbonization at about 750.degree. C. This is desirable for the
etching of Al.sub.2O.sub.3. In contrast, AAO template heated to
about 780.degree. C. can be more difficult to remove, and extra
peaks appear in the XRD pattern, suggesting that AAO transformed
into a crystalline phase (FIG. 13).
[0069] To evaluate the electrochemical performance of hollow carbon
nanofiber-encapsulated sulfur, 2032-type coin cells are fabricated
according to some embodiments. The prepared sample is pressed onto
aluminum substrate (or another type of current collector) as the
working electrode without any binder or conductive additives.
Lithium is used as the counter electrode. The electrolyte is about
1 M lithium bis(trifluoromethanesulfonyl)imide ("LiTFSI") in
1,3-dioxolane and 1,2-dimethoxyethane (volume ratio of about 1:1),
although other Li-containing salts and organic solvents can be
used. The typical mass loading is about 1.0 mg sulfur/cm.sup.2
(although another suitable mass loading is contemplated, such as
from about 0.1 mg sulfur/cm.sup.2 to about 10 mg sulfur/cm.sup.2),
and the specific capacities are calculated based on the sulfur mass
alone in some embodiments.
[0070] The voltage profiles of hollow carbon nanofiber-sulfur
composites at different current rates are shown in FIG. 8a
according to some embodiments. The discharge/charge profile of both
C/5 and C/2 show the typical two-plateau behavior of sulfur-based
cathodes, corresponding to the formation of long chain polysulfides
(e.g., Li.sub.2S.sub.x, 4.ltoreq.x.ltoreq.8) at about 2.3 V and
short chain Li.sub.2S.sub.2 and Li.sub.2S at about 2.1 V. Moreover,
the second plateau is substantially flat, suggesting a uniform
deposition of Li.sub.2S with little kinetic barriers. It is also
observed that the cycling capacity drop was small (about 5%) when
current rate increases from C/10 to C/5 after four cycles (FIG.
14), indicating good kinetics of the working electrode. This can be
attributed to the high quality of carbon and the thin carbon fiber
wall, which significantly improved electronic and ionic transport
at the cathode.
[0071] Cycling performance at C/5 and C/2 is presented in FIG. 8b,
together with that of the same carbon hollow fiber/S composite
without removing AAO template, according to some embodiments. With
AAO etched away, the cathode structure shows impressive capacity
retention. At C/5, the reversible capacity is higher than about 900
mAh/g after 30 cycles of charge/discharge. A small decay of about
7% is observed in the next 30 cycles, and the capacity is about 730
mAh/g after 150 cycles. The discharge capacity at C/2 also shows
good stability, and the reversible capacity is about 630 mAh/g
after 150 cycles. In the control sample in which the AAO template
was not etched away, the electrode has a lower stable capacity of
about 380 mAh/g. Interestingly, the cycling stability of the
non-etched sample is slightly better, as the capacity stabilized
after 15 cycles of charge/discharge, and the decay is about 3% for
the next 30 cycles before leveling off. This shows that the removal
of AAO template can improve charge transfer through the sidewall of
the carbon fibers to achieve high cycling capacity, but, at the
same time, alumina can potentially help trap polysulfides to
improve the cycle life. The mechanical support provided by the AAO
template can also enhance the stability of the cathode structure.
Further optimizations of the etching time can provide a sulfur
electrode with even higher specific capacity and more stable cycle
life.
[0072] To further improve the battery performance, about 0.1 mol/L
LiNO.sub.3 is added to the electrolyte as an additive in some
embodiments. LiNO.sub.3 can passivate the surface of a lithium
anode and thus reduce the shuttle effect. FIG. 9a shows that, in
the presence of LiNO.sub.3, the initial discharge capacity is about
1,560 mAh/g, approaching the theoretical capacity of sulfur. The
cycling stability is similar to the samples without the LiNO.sub.3
additive. More importantly, the average coulombic efficiency
increases from about 84% to over about 99% at C/5 and from about
86% to about 98% at C/2 (FIG. 9b). The improvement in coulombic
efficiency confirms that the LiNO.sub.3 additive can significantly
reduce polysulfides reaction at the lithium anode and thus the
shuttle effect. The combination of rational design of cathode
structure and electrolyte additives can achieve high specific
capacity sulfur-based cathodes with stable cycling performance and
high efficiency.
[0073] By way of summary, some embodiments provide a hollow carbon
nanofiber-encapsulated sulfur cathode to achieve high performance
lithium-sulfur batteries. In this rational design, sulfur is
selectively coated onto the inner wall of carbon nanofibers by
utilizing an AAO template. The high aspect ratio of hollow carbon
nanofibers reduces the random diffusion of polysulfides in the
organic electrolyte, while the thin carbon wall allows fast
transport of lithium ions. In some embodiments, a stable discharge
capacity of about 730 mAh/g is retained after more than 150 cycles
of charge/discharge at C/5. Addition of LiNO.sub.3 to the
electrolyte can further improve the coulombic efficiency to about
98% and about 99% at C/2 and C/5, respectively.
[0074] Synthesis of hollow carbon nanofiber-encapsulated sulfur:
AAO membrane (Whatman, pore size of about 200 nm, thickness of
about 60 .mu.m) was used as the template for making carbon
nanofibers. Typically, about 120 mg AAO was placed inside an
alumina boat, and about 2 ml polystyrene ("PS") suspended in
dimethylformamide ("DMF," about 0.1 g/ml) was dropped onto the
template as the carbon precursor. The carbonization was carried out
by heating AAO/PS/DMF at about 750.degree. C. for about 4 hours
under a slow flow of N.sub.2 gas. After cooling down, about 15 mg
of carbon-coated AAO template was loaded into a small glass vial,
and about 300 .mu.l of about 1% sulfur solution in toluene was
dropped onto the template. The amount of sulfur solution was
controlled so that the final loading of sulfur in the AAO template
was about 1 mg. After drying, the mixture was heated up to about
155.degree. C. and kept for about 12 hours to ensure sufficient
sulfur diffusion into the hollow nanofibers. The AAO template
helped prevent sulfur coating onto the outer surface of the carbon
nanofibers. Sulfur residue sticking on the surface of the template
was washed away using methanol. Total sulfur loading in the sample
was calculated by weighing the sample before and after sulfur
infusion. The AAO template was removed by immersing in a solution
of 2 M H.sub.3PO.sub.4 for about 10 hours.
[0075] Characterizations: FEI XL30 Sirion SEM with field emission
guns ("FEG") source was used for SEM characterizations. FEI Tecnai
G2 F20 X-TWIN Transmission Electron Microscope was used for TEM
characterizations. A Renishaw RM1000 Raman microscope at the
Extreme Environments Laboratory at Stanford University was used for
the Raman spectroscopy.
[0076] Electrochemical Measurement: To evaluate the electrochemical
performance of hollow carbon nanofiber/sulfur composite, 2032-type
coin cells (MTI Corporation) were fabricated. The prepared samples
were pressed onto aluminum substrate as the working electrode
without any binder or conductive additives. Lithium foil (Alfa
Aesar) was used as the counter electrode. The electrolyte is about
1 M lithium bis(trifluoromethanesulfonyl)imide ("LiTFSI") in
1,3-dioxolane and 1,2-dimethoxyethane (volume ratio of about 1:1).
For electrolyte with LiNO.sub.3 additive, LiNO.sub.3 (Sigma
Aldrich) was first dried at about 100.degree. C. under vacuum over
night, before being added to the electrolyte to reach a
concentration of about 0.1 mol/L. The mass loading of sulfur in the
working electrode was about 1.0 mg/cm.sup.2. Batteries testing were
performed using a 96-channel battery tester (Arbin Instrument). The
voltage range is 1.7-2.6 V vs Li/Li.sup.+
[0077] SEM characterization of AAO template: AAO template has
different morphology on the two sides (FIG. 10). The pore sizes are
about 200 nm to about 300 nm.
[0078] Raman spectroscopy measurement: FIG. 11 shows Raman spectra
of four samples.
[0079] X-ray Diffraction characterizations: FIG. 12 shows a
comparison of the XRD spectra for pristine sulfur and hollow carbon
nanofiber-encapsulated sulfur. A series of peaks are observed in
XRD spectra for AAO template heated to about 780.degree. C., while
AAO template heated to about 750.degree. C. shows no diffraction
peaks (FIG. 13). This indicates a phase transition of AAO between
about 750.degree. C. and about 780.degree. C.
[0080] Voltage profiles: FIG. 14 shows charge/discharge voltage
profiles of hollow carbon nanofiber-encapsulated sulfur at C/10 and
C/5.
Example 2
Polymer-Encapsulated Hollow Sulfur Nanoparticles
[0081] This example describes the implementation of a monodisperse,
polymer-encapsulated hollow sulfur nanoparticle cathode through a
scalable, one-stage, room-temperature synthesis. The cathode
incorporates features to largely or fully address various
challenges of sulfur-based materials. The synthesis is based on a
reaction between sodium thiosulfate and hydrochloric acid in an
aqueous solution in the presence of poly(vinyl pyrrolidone)
("PVP"). The reaction can be represented as the following:
Na.sub.2S.sub.2O.sub.3+2HCl.fwdarw.S.dwnarw.+SO.sub.2.uparw.+NaCl+H.sub.-
2O. (1)
[0082] Compared with other possible approaches for sulfur cathode
synthesis, the fabrication of sulfur nanoparticles offers one or
more of the following advantages. 1) Neither time-consuming
procedures nor high temperatures are involved. The synthesis is
carried out at about room temperature within about two hours in one
stage. 2) The synthesis is low-cost, environmentally benign, and
highly reproducible. The synthesis can produce hollow sulfur
nanoparticles with high quality on a scale of grams per batch. Such
a synthesis also can be readily scaled up for industrial
applications. 3) The use of sulfur nanoparticles in battery
electrodes is compatible with traditional lithium-ion battery
manufacturing techniques by allowing the use of conventional
conductive additives, binders, and electrolytes.
[0083] FIG. 15a schematically shows the formation mechanism for
polymer-encapsulated hollow sulfur nanoparticles. PVP molecules can
form hollow microspheres due to a self-assembly process, and can be
used as a soft template for synthesizing hollow spheres of
conductive polymers. In aqueous solution, both the polymer backbone
and the methylene groups in the five-membered ring of PVP can allow
the association of PVP molecules through hydrophobic interaction,
while the electronegative amide groups (dots) are effectively
linked together through the hydrogen bond network of water.
Therefore, it is expected that the PVP molecules can self-assemble
into a hollow spherical vesicular micelle with a double-layer
structure, having their hydrophobic alkyl backbones pointed toward
the interior of the micelle wall and the hydrophilic amide group
facing outward (FIG. 15a). The hydrophobic nature of sulfur
promotes its preferential grow onto the hydrophobic portion of the
PVP micelles, and thus these micelles serve as a soft template to
direct the growth of hollow sulfur nanoparticles. PVP can also
absorb on a sulfur nanoparticle surface if sulfur is exposed to
water during growth, forming a dense layer of polymer coating. That
is, sulfur is located in the interior of the hollow PVP wall and is
isolated from the water by PVP.
[0084] FIGS. 15b-c show SEM images of the PVP-encapsulated hollow
sulfur nanoparticles. These SEM images show several features of
these nanoparticles. First, the nanoparticles are substantially
monodisperse in size. A statistic counting over 200 sulfur
nanoparticles in a synthesis batch (FIG. 18) shows that the
diameters of about 95% of nanoparticles are in the range of about
400 nm to about 460 nm. Between different batches of synthesis, a
similar monodispersity is reproducible with the average diameter
shifted slightly within the window of about 400 nm to about 500 nm.
Second, the nanoparticles appear to be hollow. This was also
confirmed by the distinct contrast shown in the TEM image (inset of
FIG. 15c). It is noted that many of the sulfur nanoparticles have
small pores in their walls besides the large empty space or void in
the middle. This could be due to SO.sub.2 bubbles generated during
the nanoparticle synthesis accompanying with sulfur precipitation
(Eq. 1). However, despite the pores inside the sulfur wall, sulfur
is still largely or substantially isolated from the outside
solution by PVP since sulfur is hydrophobic.
[0085] To reveal PVP shells on sulfur nanoparticles, a sulfur
nanoparticle suspension is drop-casted on a silicon substrate,
followed by heating in vacuum and analysis under SEM. FIG. 15d
shows a schematic illustrating the sulfur subliming process of the
PVP-encapsulated hollow sulfur nanoparticles. FIG. 15e-g show the
corresponding SEM images of the sample before, during, and after
sulfur sublimation, respectively. After substantially all of the
sulfur had been sublimed (as confirmed by Energy-dispersive X-ray
analysis showing no detectable sulfur signal), the PVP shells
surrounding the nanoparticles can be readily resolved (FIG. 15g).
Thermal gravimetric analysis (FIG. 19) reveals that the amount of
elemental sulfur in the sample is about 70.4 wt %. From the weight
percentage of sulfur, nanoparticle size, and a thickness of a PVP
shell, it can be estimated that the void volume inside each
nanoparticle is about 56% of the sulfur wall volume. Assuming the
volume expansion is linearly dependent on the degree of lithiation,
this void volume would allow about 70% of the theoretical capacity
or about 1,170 mAh/g to be used if only inward volume expansion is
considered. FIG. 15h shows the variations in the XPS spectra of the
PVP-encapsulated hollow sulfur nanoparticles and pure elemental
sulfur. For pure sulfur, two peaks positioned at about 163.9 eV and
about 165.1 eV can be assigned to the S 2p3/2 and S 2p1/2,
respectively. As to hollow sulfur nanoparticles, the sulfur peak
shifts to a higher binding energy with its core level located at
about 167 eV, and exhibits broader full widths at half maximum,
indicating partial charge transfer of sulfur to PVP. This result
shows the interaction between PVP and sulfur, which can contribute
to the effective trapping of polysulfides and thus result in
excellent capacity retention.
[0086] To evaluate whether there is any volume expansion outwards
of the PVP-encapsulated hollow sulfur nanoparticles after
lithiation, a sulfur nanoparticle suspension is drop-casted on a
piece of conducting carbon-fiber paper (used as substrate), and
then dried in vacuum overnight. A pouch cell was assembled in an
argon-filled glovebox using the carbon-fiber paper with sulfur
nanoparticles as cathode and lithium foil as anode. The pouch cell
was discharged at a current rate of about C/5 to about 1.5 V, and
then the voltage was held at about 1.5 V for about 18 h. A typical
two-plateau voltage profile of the sulfur cathode can be observed
(FIG. 20), indicating that lithium ions can penetrate through the
PVP shell and react with the interior sulfur during lithiation.
After the first lithiation, the carbon-fiber paper cathode from the
cell was retrieved and washed with 1,3-dioxolane. Typical SEM
images of the PVP-encapsulated hollow sulfur nanoparticles on the
carbon-fiber paper substrate before and after lithiation are shown
in FIGS. 15i-j, while FIG. 15k presents the size distribution of
the sulfur nanoparticles before and after lithiation. It can be
observed that the nanoparticles after lithiation still preserved
nearly a spherical shape (FIG. 15j, marked by circles), and no
noticeable size difference was observed between the nanoparticles
before and after lithiation (average diameter: about 483 nm
(before) and about 486 nm (after)). This indicates that sulfur
expands inwardly into the hollow space, and the polymer shell is
mechanically rigid enough to impede outward expansion or breakage.
Thus, the polymer shell can effectively impede polysulfides from
diffusing into an electrolyte.
[0087] The inward expansion of PVP-encapsulated hollow sulfur
nanoparticles during lithiation opens up opportunities for high
performance sulfur-based battery cathodes. To test electrochemical
performance, Type 2032 Coin cells were assembled using a metallic
lithium foil as anode. LiNO.sub.3 was added to an electrolyte as an
additive to passivate the lithium anode surface. The specific
capacities were calculated based on sulfur mass alone.
[0088] FIG. 16a shows the typical discharge-charge voltage profiles
of the cells made from the PVP-encapsulated, hollow sulfur
nanoparticles at different current rates (C/10, C/5 and C/2, where
1C=1,673 mA/g) in the potential range of 2.6-1.5 V at room
temperature. At C/10, a capacity of about 1,179 mAh/g can be
obtained, consistent with the estimation of capacity based on the
available internal available space inside hollow nanoparticles. At
higher discharge rates of C/5 and C/2, the electrode delivered a
capacity of about 1,018 mAh/g and about 990 mAh/g, respectively.
The discharge profiles of all three current densities were
characterized by a two-plateau behavior of a typical sulfur
cathode. FIG. 16b shows the cycling performances of the cells made
from the PVP-encapsulated hollow sulfur nanoparticles at C/5 rate
for 300 cycles. The discharge capacity exhibited a gradual increase
during the first several cycles, indicating an activation process
for the electrodes. This activation may be due to the polymer
coating on the sulfur nanoparticle surface, and may relate to an
amount of time for the electrolyte to wet an outer surface of the
nanoparticles and become electrochemically active. At C/5 rate, an
initial capacity of about 792 mAh/g was measured. After several
cycles of activation, the discharge capacity reached its highest,
about 1,018 mAh/g. A capacity of about 931 mAh/g was retained after
100 cycles of charge/discharge, showing excellent capacity
retention of about 91.5% (of its highest discharge capacity of
about 1,018 mAh/g). A reversible capacity of about 790 mAh/g was
still retained after 300 cycles, corresponding to a capacity
retention of about 77.6% of its highest capacity, and corresponding
to a decay of about 7.8% per 100 cycles. The average Coulombic
efficiency of the cell at C/5 rate for 300 cycles is about 98.08%
(FIG. 16b).
[0089] When discharged/charged at C/2 rate (FIG. 16c), the cell
also exhibits excellent cycling stability. After reaching its
highest capacity, the discharge capacity stabilized at about 905
mAh/g after 10 more cycles (at the 14.sup.th cycle). A discharge
capacity of about 857 mAh/g and about 773 mAh/g can be obtained
after 100 and 300 cycles, corresponding to a capacity retention of
about 94.7% and about 85.4% of its stabilized capacity at the
14.sup.th cycle, respectively. After 500 and 1,000 cycles, the cell
delivered a reversible discharge capacity of about 727 mAh/g and
535 mAh/g, respectively, corresponding to a capacity retention of
about 80.3% and about 59.1% (of its stabilized capacity at the
14.sup.th cycle). The capacity decay was as low as about 0.04%
(about 0.37 mAh/g) per cycle. The cell also maintained a high
Coulombic efficiency even after 1,000 cycles (FIG. 16c), and the
average over 1,000 cycles is about 98.5%.
[0090] The excellent cycling performance of the hollow sulfur
nanoparticles was reproducible over many coin cells. Another
example of the electrochemical performance of the hollow sulfur
nanoparticle electrode is demonstrated in FIG. 16d. The cell
reached its highest capacity of about 1,099 mAh/g after 18 cycles
at C/5 rate, and showed a stable reversible capacity of about 1,026
mAh/g after 36 cycles. Further cycling at different rates (C/2 and
1C, each for 10 cycles) showed a reversible capacity of about 800
mAh/g at C/2 rate and about 674 mAh/g at 1C rate (a typical
discharge/charge curve at 1C rate is shown in FIG. 21). When the
cell was discharged at C/5 rate again, a reversible capacity of
about 989 mAh/g can be obtained after 10 more cycles, suggesting
the high stability of the electrode. Even after another round of
cycling at various current rates, a reversible capacity of about
953 mAh/g can still be retained at C/5 rate after 100 cycles,
indicating superior capacity reversibility and good rate
performance.
[0091] For battery materials with a relatively large volume change,
the associated volume change propagating to the macroscopic scale
of the whole electrode can be a challenge. This macroscopic
expansion problem can be overcome via the PVP-encapsulated hollow
sulfur nanoparticles since the volume expansion is mitigated
locally at each particle toward an inside hollow space. FIG. 17a
presents a schematic showing that the whole electrode thickness
undergoes little or no change. FIG. 17b shows typical SEM images of
the cross-sections of the cathode before and after 20
charge/discharge cycles (at C/5 rate, fully discharged (lithiated)
state). FIG. 17d shows the electrode thickness at 20 different
locations for the cross-sections of the pristine electrode and the
electrode after cycling, showing no noticeable signs of volume
expansion at the whole electrode level. These results are
noteworthy for the design of a full battery.
[0092] By way of summary, substantially monodisperse,
polymer-encapsulated hollow sulfur nanoparticles are synthesized
through a cost-effective, one-stage method in aqueous solution at
room temperature. This example demonstrates excellent performance
of battery electrodes formed of these hollow nanoparticles. As a
highlight, battery electrodes exhibit excellent cycle life at or
beyond 500 cycles with about 80% or more capacity retention, which
is a standard industrial specification for portable
electronics.
[0093] Methods: For the synthesis of PVP-encapsulated hollow sulfur
nanoparticles, about 50 mL of about 40 mM sodium thiosulfate
(Na.sub.2S.sub.2O.sub.3, Aldrich) aqueous solution was mixed with
about 50 mL of about 0.2 M PVP (MW of about 55,000, Aldrich) at
room temperature. Then, about 0.2 mL of concentrated hydrochloric
acid (HCl) was added to the Na.sub.2S.sub.2O.sub.3/PVP solution
under magnetic stirring. After the reaction had proceeded at room
temperature for about 2 h, the solution was centrifuged at about
8,000 rpm for about 10 min to isolate the precipitate. In the
washing process, the precipitate was washed with about 0.8 M of PVP
aqueous solution once and centrifuged at about 6,000 rpm for about
15 min.
[0094] For SEM and TEM characterization, SEM images were taken
using FEI XL30 Sirion SEM operated at an accelerating voltage of
about 5 kV. TEM imaging was performed on a FEI Tecnai G2 F20 X-TWIN
TEM operated at about 200 kV.
[0095] For electrochemical measurement, the PVP-encapsulated hollow
sulfur nanoparticle powder was mixed with Super-P carbon black and
polyvinylidene fluoride (PVDF) binder, with mass ratio of about
60:25:15, in N-Methyl-2-pyrrolidone (NMP) solvent to produce an
electrode slurry. The slurry was coated onto an aluminum foil
current collector using doctor blade and then dried to form the
working electrode. The typical mass loading of active sulfur was in
the range of about 0.8-1.8 mg/cm.sup.2. 2032-type coin cells (MTI
Corporation) were fabricated using the working electrode and
lithium metal foil as the counter electrode. The electrolyte was
about 1.0 M lithium bis(trifluoro methanesulfonyl)imide (LiTFSI)
and about 0.1 M LiNO.sub.3 in 1,3-dioxolane and 1,2-dimethoxyethane
(volume ratio of about 1:1). The coin cells were assembled inside
an argon-filled glovebox. Galvanostatic measurements were made
using MTI battery analyzers. The specific_capacities were all
calculated based on the mass of active sulfur.
[0096] Calculation of the void volume inside each nanoparticle:
[0097] Density of sulfur: .rho..sub.S=2 g/cm.sup.3 [0098] Density
of PVP: .rho..sub.PVP=1.2 g/cm.sup.3 [0099] The thickness of PVP
shell on the sulfur particle (based on FIG. 2g): 28.5 nm. [0100]
The diameter of the PVP-encapsulated sulfur nanoparticles: D=483
nm. [0101] The outer diameter of the sulfur wall is
d.sub.out=483-28.5.times.2=426 nm [0102] The volume of PVP shell
(V.sub.PVP) is:
[0102] V PVP = 4 / 3 .times. .pi. .times. ( ( D / 2 ) 2 - ( d out /
2 ) 3 ) = 4 / 3 .times. .pi. .times. ( 483 / 2 ) 3 - ( 426 / 2 ) 3
) ##EQU00001## [0103] The weight of PVP (m.sub.PVP) is:
[0103] m.sub.PVP=V.sub.PVP.times..rho..sub.PVP [0104] Since the
weight of PVP is 30% of the weight of the PVP-encapsulated sulfur
particles. [0105] So the volume of sulfur (Vs) is
[0105] V S = m PVP / 30 % .times. 70 % / .rho. S = 2.59 .times. 10
7 nm 3 ##EQU00002## [0106] So the inner diameter of the sulfur wall
(d.sub.in) is:
[0106]
Vs=4/3.times..pi..times.((d.sub.out/2).sup.3-(d.sub.m/2).sup.3)
d.sub.m=303 nm
[0107] Thus, the void volume inside each hollow nanoparticle is
represented as:
(d.sub.in/2).sup.3/((d.sub.out/2).sup.3-(d.sub.in/2).sup.3)=56%
(2)
[0108] Size distribution of hollow sulfur nanoparticles: FIG. 18
shows a size distribution of hollow sulfur nanoparticles, based on
counting over 200 nanoparticles from SEM images.
[0109] Thermal gravimetric analysis: FIG. 19 shows a TGA curve of
as-prepared PVP-encapsulated hollow sulfur nanoparticles recorded
in the range of 40-400.degree. C. in argon at a heating rate of
about 2.degree. C./min, showing that the amount of elemental sulfur
in the sample is about 70.4 wt %.
[0110] Voltage profile: FIG. 20 shows a voltage profile of a pouch
cell assembled in an argon-filled glovebox using a carbon-fiber
paper with PVP-encapsulated hollow sulfur nanoparticles as cathode
and lithium foil as anode.
[0111] Discharge/charge profile: FIG. 21 shows a typical
discharge/charge profiles of a cell made from PVP-encapsulated
hollow sulfur nanoparticles at 1C rate.
Example 3
Sulfur-TiO.sub.2 Yolk-Shell Nanostructures
[0112] This example describes the implementation of a
sulfur-TiO.sub.2 yolk-shell nanoarchitecture for stable and
prolonged cycling over 1,000 charge/discharge cycles in
lithium-sulfur batteries. An advantage of the yolk-shell morphology
lies in the presence of an internal void space to accommodate a
relatively large volumetric expansion of sulfur during lithiation,
thus preserving a structural integrity of a shell to mitigate
against polysulfide dissolution. In comparison with bare sulfur and
sulfur-TiO.sub.2 core-shell nanoparticles, the yolk-shell
nanostructures are found to exhibit a high capacity retention due
to the effectiveness of the intact TiO.sub.2 shell in restricting
polysulfide dissolution. Using the yolk-shell nanoarchitecture, an
initial specific capacity of about 1,030 mAh/g at 1/2C rate and a
Coulombic efficiency of about 98.4% over 1,000 cycles was achieved.
Moreover, the capacity decay at the end of 1,000 cycles was found
to be as small as about 0.033% per cycle (3.3% per 100 cycles).
[0113] The sulfur-TiO.sub.2 yolk-shell morphology was
experimentally realized as shown schematically in FIG. 22a. First,
substantially monodisperse sulfur nanoparticles were prepared using
the reaction of sodium thiosulfate with hydrochloric acid (FIG.
25). The sulfur nanoparticles were then coated with TiO.sub.2
through controlled hydrolysis of a sol-gel precursor, titanium
diisopropoxide bis(acetylacetonate), in an alkaline
isopropanol/aqueous solution, resulting in the formation of
sulfur-TiO.sub.2 core-shell nanoparticles (FIG. 26; a TEM image was
taken immediately after an electron beam was turned on to avoid
sublimation of sulfur under the beam). This was followed by partial
dissolution of sulfur in toluene to create an empty space between
the sulfur core and the TiO.sub.2 shell, resulting in the
yolk-shell morphology. The ability of toluene to diffuse through
the TiO.sub.2 shell to partially dissolve sulfur indicates its
porous nature. A SEM image in FIG. 22b shows relatively uniform
spherical nanoparticles of about 800 nm in size. A TEM image in
FIG. 22c, taken immediately after the electron beam was turned on,
shows sulfur nanoparticles encapsulated within TiO.sub.2 shells
(about 15 nm thick) with internal void space. Due to the
two-dimensional projection nature of TEM images, the void space
appears as an empty area or an area of lower intensity depending on
the orientation of the particles (FIG. 22c). The TiO.sub.2 in the
yolk-shell nanostructures were determined to be amorphous using
X-ray diffraction (FIG. 27).
[0114] Next, the effectiveness of the yolk-shell morphology was
investigated in terms of accommodating the volume expansion of
sulfur and restricting polysulfide dissolution. The
sulfur-TiO.sub.2 yolk-shell nanostructures were drop-cast onto
conducting carbon-fiber papers to form working electrodes, and
pouch cells were assembled using lithium foil as the counter
electrode. The cells were discharged at 0.1C rate (1C=1,673 mA/g)
to a voltage of 1.7 V vs. Li+/Li, during which a capacity of about
1,110 mAh/g was attained (FIG. 28), and the voltage was maintained
for over 20 h. The as-obtained discharge profile shows the typical
two-plateau behavior of sulfur cathodes, indicating the conversion
of elemental sulfur to long-chain lithium polysulfides
(Li.sub.2S.sub.n, 4.ltoreq.n.ltoreq.8) at about 2.3 V, and the
subsequent formation of Li.sub.2S.sub.2 and Li.sub.2S at about 2.1
V (FIG. 28). After the lithiation process, the contents of the
cells (cathode, anode, and separator) were washed with
1,3-dioxolane solution for further characterization. This
polysulfide-containing solution was then oxidized with concentrated
HNO.sub.3 and diluted with deionized water for analysis of sulfur
content using inductively coupled plasma spectroscopy ("ICP"). For
comparison, electrode materials were also prepared using bare
sulfur and sulfur-TiO.sub.2 core-shell nanoparticles and subjected
to the same treatment.
[0115] There was little change in morphology and size distribution
of the sulfur-TiO.sub.2 yolk-shell nanostructures before and after
lithiation (FIGS. 23a-c). A TEM image of a lithiated yolk-shell
nanostructure shows a structurally intact TiO.sub.2 coating (FIG.
23d), indicating the ability of the yolk-shell morphology in
accommodating the volume expansion of sulfur. The presence of
lithiated sulfur and TiO.sub.2 in the yolk-shell nanostructure was
confirmed using energy-dispersive X-ray spectroscopy and electron
energy loss spectroscopy (FIG. 23e). In the case of bare sulfur and
sulfur-TiO.sub.2 core-shell nanoparticles, random precipitation of
irregularly-shaped Li.sub.2S.sub.2 and Li.sub.2S particles was
observed on the electrodes due to dissolution of lithium
polysulfides into the electrolyte (FIG. 29). ICP analysis showed a
loss of about 81% and about 62% of the total sulfur mass into the
electrolyte for the bare sulfur and sulfur-TiO.sub.2 core-shell
nanoparticles, respectively. In comparison, about 19% of the total
sulfur mass was found to be dissolved in the electrolyte in the
case of the yolk-shell nanostructures, which indicates the
effectiveness of the intact TiO.sub.2 shell in restricting
polysulfide dissolution.
[0116] To further evaluate the electrochemical cycling performance
of the sulfur-TiO.sub.2 yolk-shell nanoarchitecture, 2032-type coin
cells were fabricated. The working electrodes were prepared by
mixing the yolk-shell nanostructures with conductive carbon black
and polyvinylidene fluoride binder in N-methyl-2-pyrrolidinone to
form a slurry, which was then coated onto an aluminum foil and
dried under vacuum. Using a lithium foil as the counter electrode,
the cells were cycled from 1.7-2.6 V vs. Li+/Li. The electrolyte
used was lithium bis(trifluoromethanesulfonyl)imide in
1,2-dimethoxyethane and 1,3-dioxolane, with LiNO.sub.3 (1 wt %) as
an additive to passivate a surface of the lithium anode. Specific
capacity values were calculated based on the mass of sulfur, which
was determined using TGA (FIG. 30). The sulfur content was found to
be about 71 wt % in the yolk-shell nanostructures, accounting for
about 53 wt % of the electrode mix, and with a typical sulfur mass
loading of about 0.4-0.6 mg/cm.sup.2. The contribution of TiO.sub.2
to the total capacity is relatively small in the voltage range of
this example.
[0117] The sulfur-TiO.sub.2 yolk-shell nanoarchitecture exhibited
stable cycling performance over 1,000 charge/discharge cycles at
1/2C rate (1C=1,673 mA/g) as displayed in FIG. 24a (see also FIG.
31). After an initial discharge capacity of about 1,030 mAh/g, the
yolk-shell nanostructures achieved capacity retentions of about
88%, about 87%, and about 81% at the end of 100, 200, and 500
cycles, respectively (FIGS. 24a-b). Moreover, after prolonged
cycling over 1,000 cycles, the capacity retention was found to be
about 67%, which corresponds to a small capacity decay of about
0.033% per cycle (about 3.3% per 100 cycles). The average Coulombic
efficiency over the 1,000 cycles was calculated to be about 98.4%
(FIG. 24a), which shows little shuttle effect due to polysulfide
dissolution. In comparison, cells based on bare sulfur and
sulfur-TiO.sub.2 core-shell nanoparticles suffered from rapid
capacity decay, showing capacity retentions of about 48% and about
66% respectively after 200 cycles (FIG. 24b), indicating a greater
degree of polysulfide dissolution into the electrolyte.
[0118] Next, the sulfur-TiO.sub.2 yolk-shell nanostructures were
subjected to cycling at various C-rates to evaluate their
robustness (FIGS. 24c-d). After an initial discharge capacity of
about 1,215 mAh/g at 0.2C rate, the capacity was found to stabilize
at about 1,010 mAh/g. Further cycling at 0.5C, 1C, and 2C showed
high reversible capacities of about 810 mAh/g, about 725 mAh/g, and
about 630 mAh/g, respectively (FIGS. 4c-d). When the C-rate was
switched abruptly from 2C to 0.2C again, the original capacity was
largely recovered (FIG. 24c), indicating robustness and stability
of the cathode material. Moreover, there was little change in the
thickness of the cathode before and after 70 cycles at these
various C-rates (FIG. 32), which further confirms the ability of
the yolk-shell nanostructures in accommodating the volume expansion
of sulfur.
[0119] There are at least two characteristics of a yolk-shell
design that impart the sulfur-TiO.sub.2 nanostructures with stable
cycling performance over 1,000 charge/discharge cycles. First,
sufficient empty space is present to allow for volume expansion of
sulfur. Using image processing software on the yolk-shell
nanostructures (FIG. 22c), sulfur was determined to occupy about
62% of the volume within the TiO.sub.2 shell, which corresponds to
about 38% internal void space. This value is supported by TGA of
the relative wt % of sulfur vs. TiO.sub.2 (FIG. 30), from which the
volume of empty space in the yolk-shell nanostructures was
estimated to be about 37%. This volume of void space can
accommodate about 60% volume expansion of the sulfur present within
the shell, allowing for about 1,250 mAh/g, namely about 75% of the
maximum theoretical capacity of sulfur (assuming volume expansion
is linearly dependent on the degree of lithiation). Experimentally,
a maximum discharge capacity of about 1,215 mAh/g has been achieved
(FIG. 24c); therefore, there is sufficient void space for volume
expansion without causing the shell to crack and fracture. Second,
the intact TiO.sub.2 shell is effective in mitigating against
polysulfide dissolution. The ability of toluene to diffuse through
the TiO.sub.2 shell to partially dissolve sulfur (FIG. 22a)
indicates the porous (<2 nm) nature of the shell, which is
typical of amorphous TiO.sub.2 prepared using sol-gel methods. The
stable cycling performance demonstrated in this example indicates
that the TiO.sub.2 shell is effective in restricting polysulfide
dissolution due to its small pore size and the presence of
hydrophilic Ti--O groups that can bind favorably with polysulfide
anions.
[0120] By way of summary, this example demonstrates the design of a
sulfur-TiO.sub.2 yolk-shell nanoarchitecture for long cycling
capability over 1,000 charge/discharge cycles, with a capacity
decay as small as about 0.033% per cycle. Compared to bare sulfur
and sulfur-TiO.sub.2 core-shell counterparts, the yolk-shell
nanostructures exhibited the highest capacity retention due to the
presence of internal void space to accommodate the volume expansion
of sulfur during lithiation, resulting in an intact shell to
restrict polysulfide dissolution.
[0121] Synthesis of Sulfur-TiO.sub.2 Yolk-Shell Nanostructures:
First, sulfur nanoparticles were synthesized by adding concentrated
HCl (0.8 mL, 10 M) to an aqueous solution of Na.sub.2S.sub.2O.sub.3
(100 mL, 0.04 M) including a low concentration of
polyvinylpyrrolidone (PVP, MW of about 55,000, 0.02 wt %). After
reaction for about 2 h, the sulfur nanoparticles (100 mL) were
washed by centrifugation and redispersed into an aqueous solution
of PVP (20 mL, 0.05 wt %). For TiO.sub.2 coating, the solution of
sulfur nanoparticles (20 mL) was mixed with isopropanol (80 mL) and
concentrated ammonia (2 mL, 28 wt %). Titanium diisopropoxide
bis(acetylacetonate) (50 mL, 0.01 M in isopropanol) was then added
in five portions (5.times.10 mL) at about half-hour intervals.
After reaction for about 4 h, the solution of sulfur-TiO.sub.2
core-shell nanoparticles was washed by centrifugation to remove
freely-hydrolyzed TiO.sub.2, followed by redispersion into
deionized water (20 mL). To prepare the sulfur-TiO.sub.2 yolk-shell
nanostructures, the solution of core-shell particles (20 mL) was
stirred with isopropanol (20 mL) and toluene (0.4 mL) for about 4 h
to achieve partial dissolution of sulfur. The as-synthesized
sulfur-TiO.sub.2 yolk-shell nanostructures were then recovered
using centrifugation and dried under vacuum overnight.
[0122] Characterization: SEM and TEM images were taken using a FEI
XL30 Sirion SEM (accelerating voltage 5 kV) and a FEI Tecnai G2 F20
X-TWIN (accelerating voltage 200 kV), respectively. Elemental
analysis was performed using energy-dispersive X-ray spectroscopy
and electron energy loss spectroscopy equipped in the TEM device.
X-ray diffraction patterns were obtained on a PANalytical X'Pert
Diffractometer using Cu K.alpha. radiation. TGA was carried out
using a Netzsch STA 449 at a heating rate of about 2.degree. C./min
under argon atmosphere. ICP optical emission spectroscopy was
performed using a Thermo Scientific ICAP 6300 Duo View
Spectrometer.
[0123] Electrochemical Measurements: To prepare working electrodes,
various sulfur-based materials were mixed with carbon black (Super
P) and polyvinylidene fluoride (PVDF) binder (75:15:10 by weight)
in N-methyl-2-pyrrolidinone (NMP) to form a slurry. This slurry was
then coated onto an aluminum foil using doctor blade and dried
under vacuum to form a working electrode. 2032-type coin cells were
assembled in an argon-filled glove box using a lithium foil as the
counter electrode. The electrolyte used was a solution of lithium
bis(trifluoromethanesulfonyl)imide (LiTFSI, 1 M) in 1:1 v/v
1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) including
LiNO.sub.3 (1 wt %). Galvanostatic cycling was carried out using a
96-channel battery tester (Arbin Instruments) from 1.7-2.6 V vs.
Li+/Li. Specific capacity values were calculated based on the mass
of sulfur in the samples, which was determined using TGA (FIG. 30).
The sulfur content was found to be about 71 wt % in the yolk-shell
nanostructures, accounting for about 53 wt % of the electrode mix,
with a typical sulfur mass loading of about 0.4-0.6
mg/cm.sup.2.
[0124] While the invention has been described with reference to the
specific embodiments thereof, it should be understood by those
skilled in the art that various changes may be made and equivalents
may be substituted without departing from the true spirit and scope
of the invention as defined by the appended claims. In addition,
many modifications may be made to adapt a particular situation,
material, composition of matter, method, or process to the
objective, spirit and scope of the invention. All such
modifications are intended to be within the scope of the claims
appended hereto. In particular, while the methods disclosed herein
have been described with reference to particular operations
performed in a particular order, it will be understood that these
operations may be combined, sub-divided, or re-ordered to form an
equivalent method without departing from the teachings of the
invention. Accordingly, unless specifically indicated herein, the
order and grouping of the operations are not limitations of the
invention.
* * * * *