U.S. patent application number 16/953588 was filed with the patent office on 2021-08-19 for vertically aligned carbon nanotube arrays as electrodes.
This patent application is currently assigned to William Marsh Rice University. The applicant listed for this patent is William Marsh Rice University. Invention is credited to Abdul-Rahman O. Raji, Rodrigo V. Salvatierra, James M. Tour.
Application Number | 20210257616 16/953588 |
Document ID | / |
Family ID | 1000005568538 |
Filed Date | 2021-08-19 |
United States Patent
Application |
20210257616 |
Kind Code |
A1 |
Tour; James M. ; et
al. |
August 19, 2021 |
VERTICALLY ALIGNED CARBON NANOTUBE ARRAYS AS ELECTRODES
Abstract
Embodiments of the present disclosure pertain to electrodes that
include a plurality of vertically aligned carbon nanotubes and a
metal associated with the vertically aligned carbon nanotubes. The
vertically aligned carbon nanotubes may be in the form of a
graphene-carbon nanotube hybrid material that includes a graphene
film covalently linked to the vertically aligned carbon nanotubes.
The metal may become reversibly associated with the carbon
nanotubes in situ during electrode operation and lack any dendrites
or mossy aggregates. The metal may be in the form of a
non-dendritic or non-mossy coating on surfaces of the vertically
aligned carbon nanotubes. The metal may also be infiltrated within
bundles of the vertically aligned carbon nanotubes. Additional
embodiments pertain to energy storage devices that contain the
electrodes of the present disclosure. Further embodiments pertain
to methods of forming said electrodes by applying a metal to a
plurality of vertically aligned carbon nanotubes.
Inventors: |
Tour; James M.; (Bellaire,
TX) ; Raji; Abdul-Rahman O.; (Houston, TX) ;
Salvatierra; Rodrigo V.; (Houston, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
William Marsh Rice University |
Houston |
TX |
US |
|
|
Assignee: |
William Marsh Rice
University
Houston
TX
|
Family ID: |
1000005568538 |
Appl. No.: |
16/953588 |
Filed: |
November 20, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16514184 |
Jul 17, 2019 |
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16953588 |
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15568186 |
Oct 20, 2017 |
10403894 |
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PCT/US16/29184 |
Apr 25, 2016 |
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16514184 |
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62151941 |
Apr 23, 2015 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 4/663 20130101;
H01M 4/661 20130101; H01M 4/625 20130101; H01G 11/06 20130101; H01M
10/0525 20130101; H01M 4/587 20130101; H01M 2004/027 20130101; H01G
11/36 20130101; H01G 11/68 20130101; H01M 10/052 20130101; H01G
11/86 20130101; H01M 4/66 20130101; H01M 4/382 20130101; H01G 11/28
20130101; Y02E 60/13 20130101; H01M 4/366 20130101; H01M 4/045
20130101 |
International
Class: |
H01M 4/587 20060101
H01M004/587; H01M 4/62 20060101 H01M004/62; H01M 4/66 20060101
H01M004/66; H01M 10/0525 20060101 H01M010/0525; H01G 11/36 20060101
H01G011/36; H01G 11/28 20060101 H01G011/28; H01G 11/86 20060101
H01G011/86; H01G 11/68 20060101 H01G011/68; H01M 4/04 20060101
H01M004/04; H01M 4/36 20060101 H01M004/36; H01M 4/38 20060101
H01M004/38; H01M 10/052 20060101 H01M010/052 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] This invention was made with government support under Grant
No. FA9550-12-1-0035, awarded by the U.S. Department of Defense;
and Grant No. FA9550-14-1-0111, awarded by the U.S. Department of
Defense. The government has certain rights in the invention.
Claims
1-5. (canceled)
6. A method comprising: wetting carbon nanotubes with an
electrolyte, the electrolyte having a concentration of ions of a
metal, to form electrolyte-wetted carbon nanotubes; providing a
counter electrode of the metal in contact with the electrolyte; and
applying a voltage between the electrolyte-wetted carbon nanotubes
and the counter electrode, the voltage inducing a current between
the electrolyte-wetted carbon nanotubes and the counter electrode;
wherein the current between the electrolyte-wetted carbon nanotubes
and the counter electrode electrochemically strips the metal from
the counter electrode and plates the metal stripped from the
counter electrode onto the electrolyte-wetted carbon nanotubes to
form a coating of the metal over and between the carbon
nanotubes.
7. The method of claim 6, wherein the metal consists essentially of
lithium.
8. The method of claim 6, further comprising applying the current
between the carbon nanotubes and the counter electrode at a current
density of from one to ten milliamps per centimeter squared.
9. The method of claim 6, further comprising holding the current
constant while inducing the current between the carbon nanotubes
and the counter electrode.
10. The method of claim 6, further comprising separating the
counter electrode from the carbon nanotubes using a membrane and
the electrolyte.
11. The method of claim 10, further comprising replacing the
counter electrode with a cathode.
12. The method of claim 11, wherein the cathode comprises
sulfur.
13. The method of claim 11, further comprising assembling the
cathode and the carbon nanotubes coated with the metal into an
electrochemical cell.
14. The method of claim 13, wherein the electrochemical cell
includes the membrane.
15. The method of claim 13, wherein the electrochemical cell
includes the electrolyte.
16. The method of claim 6, wherein the concentration of the ions of
the metal is 4M lithium.
17. The method of claim 6, wherein the carbon nanotubes are
aligned.
18. The method of claim 6, further comprising growing the carbon
nanotubes from a substrate.
19. A method comprising: wetting carbon nanotubes with an
electrolyte, the electrolyte having a concentration of ions of a
metal, to form electrolyte-wetted carbon nanotubes; and plating the
metal from the electrolyte onto the electrolyte-wetted carbon
nanotubes to form a coating of the metal over and between the
carbon nanotubes.
20. The method of claim 19, further providing a counter electrode
in contact with the electrolyte.
21. The method of claim 20, wherein the counter electrode comprises
the metal.
22. The method of claim 20, wherein the counter electrode comprises
a surface in contact with the electrolyte, and wherein the surface
consists essentially of lithium.
23. The method of claim 20, further comprising a separator between
the carbon nanotubes and the counter electrode.
24. The method of claim 20, further comprising inducing a current
between the electrolyte-wetted carbon nanotubes and the counter
electrode, the current electrochemically stripping the metal from
the counter electrode and plating the metal stripped from the
counter electrode onto the electrolyte-wetted carbon nanotubes to
plate the metal over and between the carbon nanotubes.
25. The method of claim 24, further comprising applying the current
between the carbon nanotubes and the counter electrode at a current
density of from one to ten milliamps per centimeter squared.
26. The method of claim 19, further comprising forming a solid
electrolyte interphase over the coating of the metal.
27. The method of claim 19, wherein the coating of the metal lacks
dendrites or mossy aggregates.
28. The method of claim 19, wherein the metal consists essentially
of lithium.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. Non-provisional
patent application Ser. No. 16/514,184, filed on Jul. 17, 2019,
which is a continuation of U.S. Non-provisional patent application
Ser. No. 15/568,186, filed on Oct. 20, 2017, which is a 35 U.S.C.
371 national stage entry of PCT/US2016/02918, filed on Apr. 25,
2016, which claims priority to U.S. Provisional Patent Application
No. 62/151,941, filed on Apr. 23, 2015. The entirety of the
aforementioned applications is incorporated herein by
reference.
BACKGROUND
[0003] Current electrodes suffer from numerous limitations,
including limited metal storage capacities, and the formation of
dendritic materials during operation. Various aspects of the
present disclosure address the aforementioned limitations.
SUMMARY
[0004] In some embodiments, the present disclosure pertains to
electrodes that include a plurality of vertically aligned carbon
nanotubes and a metal associated with the vertically aligned carbon
nanotubes. In some embodiments, the vertically aligned carbon
nanotubes include vertically aligned single-walled carbon nanotubes
that are in the form of an array. In some embodiments, the
vertically aligned carbon nanotubes are associated with a
substrate. In some embodiments, the substrate serves as a current
collector. In some embodiments, the vertically aligned carbon
nanotubes and the substrate serve as a current collector.
[0005] In some embodiments, the vertically aligned carbon nanotubes
are in the form of a graphene-carbon nanotube hybrid material,
where the vertically aligned carbon nanotubes are covalently linked
to the graphene film through carbon-carbon bonds at one or more
junctions between the carbon nanotubes and the graphene film. In
some embodiments, the graphene film is also associated with a
substrate, such as a copper or nickel substrate.
[0006] The vertically aligned carbon nanotubes of the present
disclosure may be associated with various metals. For instance, in
some embodiments, the metal includes, without limitation, alkali
metals, alkaline earth metals, transition metals, post transition
metals, rare-earth metals, and combinations thereof. In some
embodiments, the metal includes, without limitation, Li, Na, K, Mg,
Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, Sn, Sb, Pb, and
combinations thereof. In some embodiments, the metal includes
lithium.
[0007] In some embodiments, the metal becomes reversibly associated
with the vertically aligned carbon nanotubes in situ during
electrode operation. In some embodiments, the metal associated with
the vertically aligned carbon nanotubes lacks any dendrites or
aggregates (e.g., mossy aggregates). In some embodiments, the metal
is in the form of a non-dendritic or non-mossy coating on surfaces
of the vertically aligned carbon nanotubes. In some embodiments,
the metal is infiltrated within bundles of the vertically aligned
carbon nanotubes.
[0008] In some embodiments, the vertically aligned carbon nanotubes
serve as the active layer of the electrode. In some embodiments,
the metals serve as the active layer of the electrode while the
vertically aligned carbon nanotubes serve as a current collector
(either alone or in conjunction with a substrate). In some
embodiments, the electrode is an anode or a cathode. In some
embodiments, the electrode is a component of an energy storage
device, such as a lithium-ion battery or a lithium-ion
capacitor.
[0009] Additional embodiments of the present disclosure pertain to
energy storage devices that contain the electrodes of the present
disclosure. Further embodiments of the present disclosure pertain
to methods of forming the electrodes of the present disclosure.
DESCRIPTION OF THE FIGURES
[0010] FIGS. 1A-1C illustrate the formation of electrodes (FIG.
1A), a structure of a formed electrode (FIG. 1B), and the use of
the formed electrodes in a battery (FIG. 1C).
[0011] FIGS. 2A-2G illustrate the growth and structural
characterization of graphene-carbon nanotube hybrid materials
(GCNTs). FIG. 2A provides a schematic of GCNT growth. E-beam
deposited 1 nm iron nanoparticles were non-continuous and they
served as the catalysts for the carbon nanotube (CNT) growth while
a 3 nm layer of aluminum oxide provided the support for a vertical
growth. FIGS. 2B-D provide scanning electron microscopy (SEM)
images of GCNT showing a CNT carpet grown vertically from a
graphene-covered copper (Cu) substrate. FIG. 2E shows a Raman
spectrum of graphene as-grown on Cu. The graphene is conformally
connected to its native Cu substrate upon which it is grown. The G
band appears at 1589 cm.sup.-1 while the 2D band appears at 2705
cm.sup.-1 to provide an I.sub.G/I.sub.2D ratio of more than 1. A
trace D band appears at .about.1360 cm.sup.-1. The Raman scattering
signatures signify a high quality multilayer graphene. The skewed
baseline occurred because the spectrum is obtained atop Cu. FIG. 2F
provides a Raman spectrum of CNTs grown on the Cu-graphene
substrate with the G band at 1587 cm.sup.-1, the 2D band at 2652
cm.sup.-1, and the D band at 1336 cm.sup.-1. FIG. 2G provides a
Raman radial breathing mode (RBM) spectrum of the CNTs in expanded
format.
[0012] FIGS. 3A-3N illustrate the morphology of GCNT associated
with lithium (GCNT-Li). FIG. 3A provides a schematic of GCNT-Li
formation. FIG. 3B provides voltage vs. time of lithiation and
delithiation processes of GCNT-Li. FIG. 3C provides a photograph of
GCNTs, GCNT-Li, and delithiated GCNT-Li (scale bar corresponds to 1
cm). SEM images of GCNT-Li (0.7 mAh cm.sup.-2 at 2 mA cm.sup.2)
after 250 cycles are shown through a top-view (FIG. 3D), side-view
(FIG. 3E), expanded top-view (FIG. 3F), and expanded side-view
(FIG. 3G). SEM images of de-lithiated GCNT-Li are also shown
through a top-view (FIG. 3H) and an expanded top-view (FIG. 3I).
Transmission electron microscopy (TEM) images of a CNT from GCNT-Li
(FIG. 3J) and its higher magnification (FIG. 3K) are also shown.
FIG. 3L shows a schematic of Li deposited on graphene grown on Cu.
FIG. 3M provides an SEM image of Li deposited directly on graphene
grown on Cu foil (0.7 mAh cm.sup.-2 at 2 mA cm.sup.-2) without
GCNT, showing the mossy and dendritic Li deposition, especially at
higher magnification (FIG. 3N).
[0013] FIGS. 4A-4D provide electrochemical characteristics of
GCNT-Li anodes. FIG. 4A shows the charge/discharge profile of
GCNT-Li. Gravimetric capacity is based on the mass of GCNT,
measured on a microbalance after CNT growth. FIG. 4B shows a
voltage profile of GCNT over 200 hours, corresponding to 300
charge-discharge cycles. FIG. 4C provides a voltage profile of
Cu--Li over 160 hours, corresponding to 250 charge-discharge
cycles. FIG. 4D provides cycle performance and coulombic efficiency
of GCNT-Li. The current density is 2 mA cm.sup.-2 (12 A
g.sup.-1.sub.GCNT).
[0014] FIG. 5 shows the first cycle charge/discharge profile of Li
metal deposited on copper-graphene (CuG) materials (CuG-Li).
[0015] FIG. 6 shows the charge/discharge profile of GCNT-Li.
[0016] FIG. 7 shows the voltage characteristics of GCNT-Li anodes.
Charge/discharge voltage profiles of GCNT-Li for the 6th and 300th
cycles are shown. The slightly higher Li extraction time for the
300th cycle corresponds to a slightly higher capacity and increased
coulombic efficiency of 99.83% compared to 94.3% for the 6th cycle.
The current density is 2 mA cm.sup.2 (12 A g.sup.-1.sub.GCNT).
[0017] FIGS. 8A-8B compare the electrochemical characteristics of
GCNT-based anodes with horizontal CNT-based anodes. FIG. 8A shows
the schematics and voltage profiles of vertical and seamless GCNT
grown on Cu. FIG. 8B shows the schematics and voltage profiles of
horizontal CNT deposited on graphene-covered Cu.
[0018] FIGS. 9A-9E show data relating to Li storage and rate
capabilities of GCNT-Li anodes. FIG. 9A shows the Li storage
capacities of GCNTs from 0.4 to 4 mAh cm.sup.-2. Comparison of the
gravimetric capacity of GCNTs with other anode materials with
respect to the mass of the anode (FIG. 9B) and the mass of the
anode and Li inserted (FIG. 9C) are also shown. The areal
capacities of GCNT-Li from 0.4 to 4 mAh cm.sup.-2 are represented
by GCNT-Li-0.4 to GCNT-Li-4.
[0019] FIG. 9D shows the charge-discharge profiles measured at
different current densities expressed in current density per area
and per mass of electrode. FIG. 9E shows the cycle performance of
GCNT-Li measured at different current densities.
[0020] FIG. 10 shows the volumetric capacities of GCNT-Li anodes
with areal capacity from 0.4 to 4 mAh cm.sup.-2. Despite the very
low density of GCNTs (35 mg/cm.sup.3), the GCNT is capable of
storing large amounts of Li on the surfaces of the CNTs without Li
particulate formation in the large (micrometer-scale) pores of the
material.
[0021] FIGS. 11A-11D show the electrochemical characteristics of
prelithiated GCNTs. FIG. 11A shows the voltage profile of GCNTs
during Li insertion. FIG. 11B shows the voltage profile of GCNTs
during Li extraction followed by Li insertion up to 1 mAh
cm.sup.-2. The excess Li remains in the GCNT. FIG. 11C shows cycle
performance of GCNT-Li with excess Li. FIG. 11D shows coulombic
efficiency of GCNT-Li with and without excess Li.
[0022] FIG. 12 shows the electrochemical performance of a full
battery that contains GCNT-Li as the anode and sulfur/carbon black
as the cathode. The charge-discharge profiles of the first three
cycles of the battery were measured. The electrochemical
performance of the battery is expressed in terms of gravimetric
capacity (mass of S and mass of inserted Li). The two plateau are
related to high order and low order lithium polysulfide
(Li.sub.xS.sub.y) formation.
[0023] FIG. 13 shows the electrochemical performance of a full
battery that contains GCNT-Li as the anode and lithium cobalt oxide
(LiCoO.sub.2) as the cathode. The charge-discharge profiles of the
first two cycles of the full battery were measured.
DETAILED DESCRIPTION
[0024] It is to be understood that both the foregoing general
description and the following detailed description are illustrative
and explanatory, and are not restrictive of the subject matter, as
claimed. In this application, the use of the singular includes the
plural, the word "a" or "an" means "at least one", and the use of
"or" means "and/or", unless specifically stated otherwise.
Furthermore, the use of the term "including", as well as other
forms, such as "includes" and "included", is not limiting. Also,
terms such as "element" or "component" encompass both elements or
components comprising one unit and elements or components that
comprise more than one unit unless specifically stated
otherwise.
[0025] The section headings used herein are for organizational
purposes and are not to be construed as limiting the subject matter
described. All documents, or portions of documents, cited in this
application, including, but not limited to, patents, patent
applications, articles, books, and treatises, are hereby expressly
incorporated herein by reference in their entirety for any purpose.
In the event that one or more of the incorporated literature and
similar materials defines a term in a manner that contradicts the
definition of that term in this application, this application
controls.
[0026] Due to the increased use of energy storage devices in
various electronics, there has been a need for the development of
energy storage devices with high power densities, high energy
densities, and fast charge/discharge rates. For instance,
lithium-ion batteries have been utilized as energy storage devices
due to their high energy and power capabilities.
[0027] In particular, lithium-ion batteries contain high capacity
lithium host materials that serve as anodes. Such host materials
can include silicon, tin, graphite, and transition metal compounds
(e.g., iron oxide). Generally, lithium ions intercalate into the
host materials to form an alloy. The lithium ions can also become
integrated into the host materials by a conversion reaction.
[0028] However, the theoretical capacity of lithium ion batteries
is limited by the amount of lithium that can be stored in or
reacted with the host materials. For instance, the theoretical
capacity of lithium-ion batteries that contain graphite-based
anodes is limited to about 372 mAh/g. Likewise, the theoretical
capacity of lithium-ion batteries that contain iron oxide-based
anodes is limited to about 1,007 mAh/g. Similarly, the theoretical
capacity of lithium-ion batteries that contain silicon-based anodes
is limited to about 3,579 mAh/g.
[0029] Furthermore, major safety concerns exist when lithium is
utilized as an anode component in lithium ion batteries and other
energy storage devices. In particular, safety hazard issues arise
due to the formation of dendritic and related structures by the
lithium ions, especially at high current densities. Such dendritic
structures are usually non-uniform crystals that grow in the form
of fiber-like, needle-like, moss-like, or tree-like structures.
[0030] The dendritic structures can generate significant volume
expansion during cycling. The volume expansions can in turn
diminish an energy storage device's coulombic efficiency and cycle
life by blocking the separator pores and inducing continuous
electrolyte decomposition. Such effects can in turn lead to
internal short circuits. This is especially dangerous because of
the presence of organic solvent components in batteries.
[0031] Various approaches have been utilized to address issues
arising from dendritic growth in energy storage devices. Such
approaches have included: (a) new additives and electrolyte
salt/solvent combinations to enable formation of a strong and
stable solid electrolyte interphase (SEI); (b) coating the
electrode with a mechanically strong porous polymer, solid
membrane, or ionic conductor as a separator in order to suppress or
prevent dendritic growth and penetration; and (c) forming a
protective shell on the current collector to encapsulate the
lithium and prevent dendritic growth. However, since dendrite
formation is more rapid and severe at higher current densities, the
aforementioned approaches can limit lithium storage capacity per
unit electrode area and cycle life. For the same reasons, the
aforementioned approaches can restrict electrode current
density.
[0032] As such, a need exists for electrodes that exhibit optimal
metal storage capacities and minimal dendrite formation. Various
aspects of the present disclosure address this need.
[0033] In some embodiments, the present disclosure pertains to
methods of making electrodes that contain vertically aligned carbon
nanotubes. In some embodiments illustrated in FIG. 1A, the methods
of the present disclosure include applying a metal to a plurality
of vertically aligned carbon nanotubes (step 10) such that the
metal becomes associated with the vertically aligned carbon
nanotubes (step 12). In some embodiments, the methods of the
present disclosure also include a step of incorporating the formed
electrode as a component of an energy storage device (step 14).
[0034] In additional embodiments, the present disclosure pertains
to the formed electrodes. In some embodiments, the electrodes of
the present disclosure include a plurality of vertically aligned
carbon nanotubes and a metal that is associated with the vertically
aligned carbon nanotubes. In more specific embodiments illustrated
in FIG. 1B, the electrodes of the present disclosure can be in the
form of electrode 30, which includes metal 32, vertically aligned
carbon nanotubes 34, graphene film 38, and substrate 40. In this
embodiment, vertically aligned carbon nanotubes 34 are in the form
of array 35. The vertically aligned carbon nanotubes are covalently
linked to graphene film 38 through seamless junctions 36. In
addition, metal 32 is associated with vertically aligned carbon
nanotubes 34 in the form of non-dendritic or non-mossy films.
[0035] Further embodiments of the present disclosure pertain to
energy storage devices that contain the electrodes of the present
disclosure. For instance, as illustrated in FIG. 1C, the electrodes
of the present disclosure can be utilized as components of battery
50, which contains cathode 52, anode 56, and electrolytes 54. In
this embodiment, the electrodes of the present disclosure can serve
as cathode 52 or anode 56.
[0036] As set forth in more detail herein, the present disclosure
can utilize various types of vertically aligned carbon nanotubes.
Moreover, various metals may be associated with the vertically
aligned carbon nanotubes in various manners. Furthermore, the
electrodes of the present disclosure can be utilized as components
of various energy storage devices.
[0037] Vertically Aligned Carbon Nanotubes
[0038] The electrodes of the present disclosure can include various
types of vertically aligned carbon nanotubes. For instance, in some
embodiments, the vertically aligned carbon nanotubes include,
without limitation, single-walled carbon nanotubes, double-walled
carbon nanotubes, triple-walled carbon nanotubes, multi-walled
carbon nanotubes, ultra-short carbon nanotubes, small diameter
carbon nanotubes, pristine carbon nanotubes, functionalized carbon
nanotubes, and combinations thereof. In some embodiments, the
vertically aligned carbon nanotubes include vertically aligned
single-walled carbon nanotubes.
[0039] In some embodiments, the vertically aligned carbon nanotubes
of the present disclosure include pristine carbon nanotubes. In
some embodiments, the pristine carbon nanotubes have little or no
defects or impurities.
[0040] In some embodiments, the vertically aligned carbon nanotubes
of the present disclosure include functionalized carbon nanotubes.
In some embodiments, the functionalized carbon nanotubes include
sidewall-functionalized carbon nanotubes. In some embodiments, the
functionalized carbon nanotubes include one or more functionalizing
agents. In some embodiments, the functionalizing agents include,
without limitation, oxygen groups, hydroxyl groups, carboxyl
groups, epoxide moieties, and combinations thereof.
[0041] In some embodiments, the sidewalls of the vertically aligned
carbon nanotubes of the present disclosure contain structural
defects, such as holes. In some embodiments, carbons at the edges
of the structural defects (e.g., holes) are terminated by one or
more of atoms or functional groups (e.g., hydrogen, oxygen groups,
hydroxyl groups, carboxyl groups, epoxide moieties, and
combinations thereof).
[0042] The vertically aligned carbon nanotubes of the present
disclosure can be in various forms. For instance, in some
embodiments, the vertically aligned carbon nanotubes are in the
form of an array (e.g., array 35 in FIG. 1B). In some embodiments,
the array is in the form of a carpet or a forest. In some
embodiments, the array is in the form of superlattices held
together by van der Waals interactions.
[0043] In some embodiments, the vertically aligned carbon nanotubes
of the present disclosure are in the form of carbon nanotube
bundles that include a plurality of channels. In some embodiments,
the carbon nanotube bundles have inter-tube spacings ranging from
about 3 .ANG. to about 20 .ANG.. In some embodiments, the carbon
nanotube bundles have inter-tube spacings of about 3.4 .ANG.. In
some embodiments, the carbon nanotube bundles have channels with
sizes that range from about 5 .ANG. to about 20 .ANG.. In some
embodiments, the carbon nanotube bundles have channels with sizes
of about 6 .ANG..
[0044] The vertically aligned carbon nanotubes of the present
disclosure can have various angles. For instance, in some
embodiments, the vertically aligned carbon nanotubes of the present
disclosure have angles that range from about 45.degree. to about
90.degree.. In some embodiments, the vertically aligned carbon
nanotubes of the present disclosure have angles that range from
about 75.degree. to about 90.degree.. In some embodiments, the
vertically aligned carbon nanotubes of the present disclosure have
an angle of about 90.degree..
[0045] The vertically aligned carbon nanotubes of the present
disclosure can also have various thicknesses. For instance, in some
embodiments, the vertically aligned carbon nanotubes of the present
disclosure have a thickness ranging from about 10 .mu.m to about 2
mm. In some embodiments, the vertically aligned carbon nanotubes of
the present disclosure have a thickness ranging from about 10 .mu.m
to about 1 mm. In some embodiments, the vertically aligned carbon
nanotubes of the present disclosure have a thickness ranging from
about 10 .mu.m to about 500 .mu.m. In some embodiments, the
vertically aligned carbon nanotubes of the present disclosure have
a thickness ranging from about 10 .mu.m to about 100 .mu.m. In some
embodiments, the vertically aligned carbon nanotubes of the present
disclosure have a thickness of about 50 .mu.m.
[0046] Substrates
[0047] In some embodiments, the vertically aligned carbon nanotubes
of the present disclosure may be associated with a substrate (e.g.,
substrate 40 in FIG. 1B). In some embodiments, the substrate also
includes a graphene film (e.g., graphene film 38 in FIG. 1B). In
some embodiments, the substrate serves as a current collector. In
some embodiments, the substrate and the vertically aligned carbon
nanotubes serve as a current collector.
[0048] Various substrates may be utilized in the electrodes of the
present disclosure. For instance, in some embodiments, the
substrate includes, without limitation, nickel, cobalt, iron,
platinum, gold, aluminum, chromium, copper, magnesium, manganese,
molybdenum, rhodium, ruthenium, silicon, tantalum, titanium,
tungsten, uranium, vanadium, zirconium, silicon dioxide, aluminum
oxide, boron nitride, carbon, carbon-based substrates, diamond,
alloys thereof, and combinations thereof. In some embodiments, the
substrate includes a copper substrate. In some embodiments, the
substrate includes a nickel substrate.
[0049] In some embodiments, the substrate includes a carbon-based
substrate. In some embodiments, the carbon-based substrate
includes, without limitation, graphitic substrates, graphene,
graphite, buckypapers (e.g., papers made by filtration of carbon
nanotubes), carbon fibers, carbon fiber papers, carbon papers
(e.g., carbon papers produced from graphene or carbon nanotubes),
graphene papers (e.g., graphene papers made by filtration of
graphene or graphene oxide with subsequent reduction), carbon
films, metal carbides, silicon carbides, and combinations
thereof.
[0050] The vertically aligned carbon nanotubes of the present
disclosure may be associated with a substrate in various manners.
For instance, in some embodiments, the vertically aligned carbon
nanotubes of the present disclosure are covalently linked to the
substrate. In some embodiments, the vertically aligned carbon
nanotubes of the present disclosure are substantially perpendicular
to the substrate. Additional arrangements can also be
envisioned.
[0051] Graphene-Carbon Nanotube Hybrid Materials
[0052] In some embodiments, the vertically aligned carbon nanotubes
of the present disclosure are in the form of graphene-carbon
nanotube hybrid materials. In some embodiments, the graphene-carbon
nanotube hybrid materials include a graphene film (e.g., graphene
film 38 in FIG. 1B) and vertically aligned carbon nanotubes
covalently linked to the graphene film (e.g., vertically aligned
carbon nanotubes 34 in FIG. 1B). In some embodiments, the
vertically aligned carbon nanotubes are covalently linked to the
graphene film through carbon-carbon bonds at one or more junctions
between the carbon nanotubes and the graphene film (e.g., junction
36 in FIG. 1B). In some embodiments, the vertically aligned carbon
nanotubes are in ohmic contact with a graphene film through the
carbon-carbon bonds at the one or more junctions. In some
embodiments, the one or more junctions include seven-membered
carbon rings. In some embodiments, the one or more junctions are
seamless.
[0053] In some embodiments, the graphene-carbon nanotube hybrid
materials of the present disclosure can also include a substrate
that is associated with the graphene film (e.g., substrate 40 in
FIG. 1B). Suitable substrates were described previously. For
instance, in some embodiments, the substrate can include a metal
substrate, such as copper. In some embodiments, the substrate
includes a carbon-based substrate, such as a graphitic substrate.
In some embodiments, the carbon-based substrate can work both as a
current collector and a carbon source for the growth of carbon
nanotubes.
[0054] The graphene-carbon nanotube hybrid materials of the present
disclosure can include various graphene films. For instance, in
some embodiments, the graphene film includes, without limitation,
monolayer graphene, few-layer graphene, double-layer graphene,
triple-layer graphene, multi-layer graphene, graphene nanoribbons,
graphene oxide, reduced graphene oxide, graphite, and combinations
thereof. In some embodiments, the graphene film includes reduced
graphene oxide. In some embodiments, the graphene film includes
graphite.
[0055] The vertically aligned carbon nanotubes of the present
disclosure may also be associated with graphene films in various
manners. For instance, in some embodiments, the vertically aligned
carbon nanotubes are substantially perpendicular to the graphene
film (e.g., vertically aligned carbon nanotubes 34 in FIG. 1B). In
some embodiments, the vertically aligned carbon nanotubes of the
present disclosure are associated with graphene films at angles
that range from about 45.degree. to about 90.degree..
[0056] The vertically aligned carbon nanotubes of the present
disclosure can be prepared by various methods. For instance, in
some embodiments, the vertically aligned carbon nanotubes of the
present disclosure can be made by: (1) associating a graphene film
with a substrate; (2) applying a catalyst and a carbon source to
the graphene film; and (3) growing carbon nanotubes on the graphene
film.
[0057] In some embodiments, catalysts may include a metal (e.g.,
iron) and a buffer (e.g., alumina). In some embodiments, the metal
(e.g., iron) and buffer (e.g., alumina) can be grown from
nanoparticles (e.g., iron alumina nanoparticles).
[0058] In some embodiments, the metal and buffer are sequentially
deposited onto a graphene film by various methods, such as electron
beam deposition. In some embodiments, various carbon sources (e.g.,
ethene or ethyne) may be deposited onto the graphene film by
various methods, such as chemical vapor deposition. In some
embodiments, the graphene film can be grown on a substrate from
various carbon sources, such as gaseous or solid carbon
sources.
[0059] Additional embodiments of graphene-carbon nanotube hybrid
materials and methods of making the hybrid materials are described
in an additional PCT application by Applicants, which has been
published as WO 2013/119,295. The entirety of the aforementioned
application is incorporated herein by reference.
[0060] Metals
[0061] The vertically aligned carbon nanotubes of the present
disclosure may become associated with various metals. For instance,
in some embodiments, the metals include, without limitation, alkali
metals, alkaline earth metals, transition metals, post transition
metals, rare-earth metals, and combinations thereof.
[0062] In some embodiments, the metals include alkali metals. In
some embodiments, the alkali metals include, without limitation,
Li, Na, K, and combinations thereof. In some embodiments, the
metals include Li.
[0063] In some embodiments, the metals include alkaline earth
metals. In some embodiments, the alkaline earth metals include,
without limitation, Mg, Ca, and combinations thereof.
[0064] In some embodiments, the metals include transition metals.
In some embodiments, the transition metals include, without
limitation, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and combinations
thereof.
[0065] In some embodiments, the metals include post transition
metals. In some embodiments, the post transition metals include,
without limitation, Al, Sn, Sb, Pb, and combinations thereof.
[0066] Application of Metals to Vertically Aligned Carbon
Nanotubes
[0067] Various methods may be utilized to apply metals to
vertically aligned carbon nanotubes. For instance, in some
embodiments, the applying occurs by filtration, ultrafiltration,
coating, spin coating, spraying, spray coating, patterning, mixing,
blending, thermal activation, electro-deposition, electrochemical
deposition, doctor-blade coating, screen printing, gravure
printing, direct write printing, inkjet printing, mechanically
pressing, melting, and combinations thereof. In some embodiments,
the applying occurs by electrochemical deposition.
[0068] The application of metals to vertically aligned carbon
nanotubes can occur at various times. For instance, in some
embodiments, the applying occurs during electrode fabrication. In
some embodiments, the applying occurs after electrode
fabrication.
[0069] In some embodiments, the applying occurs in situ during
electrode operation. For instance, in some embodiments, electrodes
that contain the vertically aligned carbon nanotubes of the present
disclosure are placed in an electric field that contains metals.
Thereafter, the metals become associated with the vertically
aligned carbon nanotubes during the application of the electric
field.
[0070] In some embodiments, the applying occurs by melting a metal
(e.g., a pure metal, such as lithium) over a surface of vertically
aligned carbon nanotubes. Thereafter, the metals can become
associated with the vertically aligned carbon nanotubes during the
wetting of the vertically aligned carbon nanotubes by the liquid
metal.
[0071] In some embodiments, the applying occurs by
electro-depositing a metal (e.g., a pure metal or a
metal-containing solid material, such as lithium or lithium-based
materials) over a surface of vertically aligned carbon nanotubes.
Thereafter, the metals can become associated with the vertically
aligned carbon nanotubes during the electro-deposition. In some
embodiments, the metal may be dissolved in an aqueous or organic
electrolyte during electro-deposition.
[0072] Association of Metals with Vertically Aligned Carbon
Nanotubes
[0073] The metals of the present disclosure can become associated
with vertically aligned carbon nanotubes in various manners. For
instance, as set forth previously, the metal can become associated
with the vertically aligned carbon nanotubes in situ during
electrode operation. In some embodiments, the metal can become
reversibly associated with the vertically aligned carbon nanotubes.
In some embodiments, the metal can become reversibly associated
with the vertically aligned carbon nanotubes during electrode
operation by association during charging and dissociation during
discharging.
[0074] In some embodiments, the metals of the present disclosure
can become associated with vertically aligned carbon nanotubes in a
uniform manner. For instance, in some embodiments, the metal
becomes associated with the vertically aligned carbon nanotubes
without forming dendrites. In some embodiments, the metal becomes
associated with the vertically aligned carbon nanotubes without
forming aggregates (e.g., metal particulates or mossy
aggregates).
[0075] The metals of the present disclosure can become associated
with various regions of vertically aligned carbon nanotubes. For
instance, in some embodiments, the metal becomes associated with
surfaces of the vertically aligned carbon nanotubes. In some
embodiments, the metal forms a non-dendritic or non-mossy coating
on the surfaces of the vertically aligned carbon nanotubes. In some
embodiments, the metal becomes infiltrated within the bundles of
the vertically aligned carbon nanotubes.
[0076] In some embodiments, the metal becomes associated with the
vertically aligned carbon nanotubes in the form of a film. In some
embodiments, the film is on the surface of the vertically aligned
carbon nanotubes (e.g., film 32 in FIG. 1B). Additional modes of
associations can also be envisioned.
[0077] Electrode Structures and Properties
[0078] The electrodes of the present disclosure can have various
structures. For instance, in some embodiments, the electrodes of
the present disclosure are in the form of films, sheets, papers,
mats, scrolls, conformal coatings, and combinations thereof. In
some embodiments, the electrodes of the present disclosure have a
three-dimensional structure.
[0079] The electrodes of the present disclosure can serve various
functions. For instance, in some embodiments, the electrodes of the
present disclosure can serve as an anode. In some embodiments, the
electrodes of the present disclosure can serve as a cathode.
[0080] Different components of the electrodes of the present
disclosure can serve various functions. For instance, in some
embodiments, the vertically aligned carbon nanotubes serve as the
active layer of the electrodes (e.g, active layers of cathodes and
anodes). In other embodiments, the metals serve as the electrode
active layer while vertically aligned carbon nanotubes serve as a
current collector. In some embodiments, vertically aligned carbon
nanotubes serve as a current collector in conjunction with a
substrate (e.g., a copper substrate associated with a graphene
film). In some embodiments, the vertically aligned carbon nanotubes
of the present disclosure also serve to suppress dendrite
formation.
[0081] In more specific embodiments, the graphene-carbon nanotube
hybrid materials of the present disclosure serve as a current
collector while the metal serves as an active material. In some
embodiments, the graphene-carbon nanotube hybrid materials of the
present disclosure serve as a current collector in conjunction with
a substrate.
[0082] The electrodes of the present disclosure can have various
advantageous properties. For instance, in some embodiments, the
electrodes of the present disclosure have surface areas that are
more than about 650 m.sup.2/g. In some embodiments, the electrodes
of the present disclosure have surface areas that are more than
about 2,000 m.sup.2/g. In some embodiments, the electrodes of the
present disclosure have surface areas that range from about 2,000
m.sup.2/g to about 3,000 m.sup.2/g. In some embodiments, the
electrodes of the present disclosure have surface areas that range
from about 2,000 m.sup.2/g to about 2,600 m.sup.2/g. In some
embodiments, the electrodes of the present disclosure have a
surface area of about 2,600 m.sup.2/g.
[0083] The electrodes of the present disclosure can also have high
metal storage capacities. For instance, in some embodiments, the
electrodes of the present disclosure have metal storage capacities
that are more than about 50 wt %. In some embodiments, the
electrodes of the present disclosure have metal storage capacities
that range from about 75 wt % to about 2,000 wt %. In some
embodiments, the electrodes of the present disclosure have metal
storage capacities ranging from about 600 wt % to 700 wt %. In some
embodiments, the electrodes of the present disclosure have metal
storage capacities of about 650 wt %. In some embodiments, the
aforementioned weight percentages are represented as the mass of
deposited metal divided by the mass of the vertically aligned
carbon nanotubes.
[0084] The electrodes of the present disclosure can also have high
specific capacities. For instance, in some embodiments, the
electrodes of the present disclosure have specific capacities of
more than about 400 mAh/g. In some embodiments, the electrodes of
the present disclosure have specific capacities of more than about
2,000 mAh/g. In some embodiments, the electrodes of the present
disclosure have specific capacities ranging from about 1,000 mAh/g
to about 4,000 mAh/g. In some embodiments, the electrodes of the
present disclosure have specific capacities ranging from about
3,000 mAh/g to about 4,000 mAh/g. In some embodiments, the
electrodes of the present disclosure have specific capacities
ranging from about 3,500 mAh/g to about 3,900 mAh/g.
[0085] The electrodes of the present disclosure can also have high
areal capacities. For instance, in some embodiments, the electrodes
of the present disclosure have areal capacities ranging from about
0.1 mAh/cm.sup.2 to about 20 mAh/cm.sup.2. In some embodiments, the
electrodes of the present disclosure have areal capacities ranging
from about 0.4 mAh/cm.sup.2 to about 4 mAh/cm.sup.2. In some
embodiments, the electrodes of the present disclosure have areal
capacities of more than about 2 mAh/cm.sup.2
[0086] Incorporation into Energy Storage Devices
[0087] The methods of the present disclosure can also include a
step of incorporating the electrodes of the present disclosure as a
component of an energy storage device. Additional embodiments of
the present disclosure pertain to energy storage devices that
contain the electrodes of the present disclosure.
[0088] The electrodes of the present disclosure can be utilized as
components of various energy storage devices. For instance, in some
embodiments, the energy storage device includes, without
limitation, capacitors, batteries, photovoltaic devices,
photovoltaic cells, transistors, current collectors, and
combinations thereof.
[0089] In some embodiments, the energy storage device is a
capacitor. In some embodiments, the capacitor includes, without
limitation, lithium-ion capacitors, super capacitors, micro
supercapacitors, pseudo capacitors, two-electrode electric
double-layer capacitors (EDLC), and combinations thereof.
[0090] In some embodiments, the energy storage device is a battery
(e.g., battery 50 in FIG. 1C). In some embodiments, the battery
includes, without limitation, rechargeable batteries,
non-rechargeable batteries, micro batteries, lithium-ion batteries,
lithium-sulfur batteries, lithium-air batteries, sodium-ion
batteries, sodium-sulfur batteries, sodium-air batteries,
magnesium-ion batteries, magnesium-sulfur batteries, magnesium-air
batteries, aluminum-ion batteries, aluminum-sulfur batteries,
aluminum-air batteries, calcium-ion batteries, calcium-sulfur
batteries, calcium-air batteries, zinc-ion batteries, zinc-sulfur
batteries, zinc-air batteries, and combinations thereof. In some
embodiments, the energy storage device is a lithium-ion
battery.
[0091] The electrodes of the present disclosure can be utilized as
various components of energy storage devices. For instance, in some
embodiments, the electrodes of the present disclosure are utilized
as a cathode in an energy storage device (e.g., cathode 52 in
battery 50, as illustrated in FIG. 1C). In some embodiments, the
electrodes of the present disclosure are utilized as anodes in an
energy storage device (e.g., anode 56 in battery 50, as illustrated
in FIG. 1C).
[0092] In some embodiments, the electrodes of the present
disclosure include a graphene-carbon nanotube hybrid material that
is utilized as an anode in an energy storage device. In some
embodiments, the anodes of the present disclosure may be associated
with various cathodes. For instance, in some embodiments, the
cathode is a transition metal compound. In some embodiments, the
transition metal compound includes, without limitation,
Li.sub.xCoO.sub.2, Li.sub.xFePO.sub.4, Li.sub.xNiO.sub.2,
Li.sub.xMnO.sub.2, Li.sub.aNi.sub.bMn.sub.cCo.sub.dO.sub.2,
Li.sub.aNi.sub.bCo.sub.cAl.sub.dO.sub.2, NiO, NiOOH, and
combinations thereof. In some embodiments, integers a,b,c,d, and x
are more than 0 and less than 1.
[0093] In some embodiments, cathodes that are utilized along with
the anodes of the present disclosure include sulfur. In some
embodiments, the cathode includes oxygen, such as dioxygen,
peroxide, superoxide, and combinations thereof. In some
embodiments, the cathode contains metal oxides, such as metal
peroxides, metal superoxides, metal hydroxides, and combinations
thereof. In some embodiments, the cathode includes lithium cobalt
oxide. In some embodiments, the cathode includes a sulfur/carbon
black cathode.
[0094] In some embodiments, the electronic devices that contain the
electrodes of the present disclosure may also contain electrolytes
(e.g., electrolytes 54 in battery 50, as illustrated in FIG. 1C).
In some embodiments, the electrolytes include, without limitation,
non-aqueous solutions, aqueous solutions, salts, solvents,
additives, composite materials, and combinations thereof. In some
embodiments, the electrolytes include, without limitation, lithium
hexafluorophosphate (LiPF6), lithium (trimethylfluorosulfonyl)
imide (LITFSI), lithium (fluorosulfonyl) imide (LIFSI), lithium
bis(oxalate)borate (LiBOB), hexamethylphosphoustriamide (HMPA), and
combinations thereof. In some embodiments, the electrolytes are in
the form of a composite material. In some embodiments, the
electrolytes include solvents, such as ethylene carbonate, diethyl
carbonate, dimethyl carbonate, ethyl methyl carbonate,
1,2-dimethoxyl methane, and combinations thereof.
[0095] The energy storage devices of the present disclosure can
have various advantageous properties. For instance, in some
embodiments, the energy storage devices of the present disclosure
have high specific capacities. In some embodiments, the energy
storage devices of the present disclosure have specific capacities
of more than about 100 mAh/g. In some embodiments, the energy
storage devices of the present disclosure have specific capacities
ranging from about 100 mAh/g to about 2,000 mAh/g. In some
embodiments, the energy storage devices of the present disclosure
have specific capacities ranging from about 100 mAh/g to about
1,000 mAh/g. In some embodiments, the energy storage devices of the
present disclosure have specific capacities of about 800 mAh/g.
[0096] The energy storage devices of the present disclosure can
also have high energy densities. For instance, in some embodiments,
the energy storage devices of the present disclosure have energy
densities of more than about 300 Wh/kg. In some embodiments, the
energy storage devices of the present disclosure have energy
densities ranging from about 300 Wh/kg to about 3,000 Wh/kg. In
some embodiments, the energy storage devices of the present
disclosure have energy densities ranging from about 1,000 Wh/kg to
about 2,000 Wh/kg. In some embodiments, the energy storage devices
of the present disclosure have energy densities of about 1,840
Wh/kg.
Additional Embodiments
[0097] Reference will now be made to more specific embodiments of
the present disclosure and experimental results that provide
support for such embodiments. However, Applicants note that the
disclosure below is for illustrative purposes only and is not
intended to limit the scope of the claimed subject matter in any
way.
Example 1. Carbon Nanotube-Based Electrodes for Lithium-Ion
Batteries
[0098] In this Example, Applicants report a seamless
graphene-carbon nanotube (GCNT) electrode that is capable of
reversibly storing large amounts of lithium (Li) metal with
complete suppression of dendrite formation. The GCNT serves as a
host material to insert and form Li as a thin coating over its high
surface area (.about.2600 m.sup.2 g.sup.-1). With a Li storage
capacity of up to 4 mAh cm.sup.-2 (823 mAh cm.sup.-3) and 25.3 Ah
g.sup.-1.sub.G-CNT, the GCNT stores 6.6 times its weight in Li,
which is 6.6 times greater than silicon (Si). The capabilities,
reversibility, and dendrite-free nature of the GCNT bode well for
its use as a model structure for metal-based anodes in secondary
batteries.
[0099] Graphene was first grown via chemical vapor deposition (CVD)
on a copper (Cu) substrate, followed by deposition of iron
nanoparticles and aluminum oxide and subsequent CVD growth of
carbon nanotubes (CNTs) at 750.degree. C. using acetylene as the
carbon source (FIG. 2A). This method was previously shown by
Applicants to produce CNTs that were covalently and seamlessly
connected to the underlying graphene (FIG. 2B), providing ohmic
conductance between Cu and CNTs. See WO 2013/119,295.
[0100] CNTs were grown vertically from the Cu-graphene substrate as
a 50 .mu.m thick carpet (FIG. 2B). They exist in bundles (FIGS.
2C-2D), which are superlattices held together by van der Waals
interactions. In addition to an inter-tube spacing of .about.3.4
.ANG., the CNT bundles have 6 .ANG. channels. The presence of
formed CNTs were confirmed (FIG. 2F). In addition, the radial
breathing modes (RBM) at 100 to 300 cm.sup.-1 indicate single- to
few-walled CNTs (FIG. 2G).
[0101] Li is inserted into the highly porous and high surface area
GCNT, where the morphology of the CNTs induce formation of Li on
the CNT surfaces as a film or non-dendritic coating (FIG. 3A),
slightly below 0 V vs Li/Li.sup.+ (FIG. 3B). Reversible Li
insertion and extraction from the GCNT are observed (FIG. 3B).
These are confirmed by GCNT color change from black to silver,
indicating formation of Li metal (FIG. 3C), and back to black upon
Li extraction.
[0102] Scanning electron microscopy (SEM) images of the lithiated
GCNT (GCNT-Li) (FIGS. 3D-3E) show that Li is not deposited atop the
GCNT as a separate film, but is rather inserted into the pillared
CNT structure. The absence of Li aggregation or particulates
deposited in the micrometer-sized pores of the GCNT-Li (FIG. 3F)
suggests either Li formation on the surface of CNT bundles or
penetration into the CNT bundles to form on individual CNTs.
Moreover, the relatively rough surface of the CNT bundles shows the
presence of a thin layer of film, clearly indicating that Li is
formed on the CNT surfaces.
[0103] The base-view SEM image (FIG. 3G) also indicates a similarly
rough surface of the CNT bundles and the presence of a deposited
film, which underscores the significance of the micrometer-sized
pores in Li ion diffusion through the GCNT. No discernable
exfoliation of the CNT bundles in the delithiated GCNT (FIGS.
3H-3I) is observed.
[0104] In FIGS. 3J-3K, the transmission electron microscopy (TEM)
images of the lithiated CNTs show deposition in the form of
nanoparticles on the surface of the CNTs. The SEM images of the
GCNT-Li presented in FIGS. 3D-3I were recorded after 250 cycles and
they show no evidence of formation of dendritic, mossy, and related
structures that have hindered application of Li metal anodes.
[0105] In contrast, deposition over flat substrates
(graphene-covered copper foil, CuG) as shown in FIG. 3L produces
irregular deposits of Li (FIGS. 3M-3N). Mossy structures are
observed in less than 10 cycles. In the three-dimensional, high
surface area GCNT, there is enormous surface area for Li to deposit
without dendritic/mossy Li formation. The porosity facilitates Li
ion diffusion in and out of the GCNT.
[0106] FIG. 4A shows representative curves of the Li insertion and
extraction from the 6th cycle. The discharge capacity of the
GCNT-Li is 3920 mAh g.sup.-1 with a coulombic efficiency of 94.3%.
An areal capacity of 2 mAh cm.sup.-2 is obtained from 50 .mu.m
thick GCNT. The first cycle coulombic efficiency is .about.60%. The
discharge and charge curves are characterized by remarkably flat
voltages at .about.50 mV and 50 mV, respectively (FIG. 4A). The
voltage profile of the GCNT-Li resembles that of Li metal directly
plated on a current collector, having a characteristic flat
charge/discharge profile close to 0 V (FIG. 5).
[0107] It is evident that the inserted Li in the GCNT is metallic
in contrast with Li-intercalated graphite where the Li forms a
well-defined intercalation compound (LiC.sub.6) with graphite and
exists as an ion. Additionally, previously reported insertion of Li
into CNTs have had limited promise toward developing practical LIBs
because the voltage profile was not flat and the electrode needed
to be charged above 3 V to reversibly extract much of the inserted
Li (FIG. 6). The flat voltage here is observed over 200 hours of
continuous cycling (300 cycles) (FIGS. 4B and 7).
[0108] In comparison, Li deposited directly on Cu-graphene shows
oscillating coulombic efficiency and increased polarization (FIG.
4C), in addition to the problematic morphology of Li formed on the
bare Cu-graphene substrate (FIGS. 3L-3N). After 300 cycles, there
is no capacity fading, and the coulombic efficiency is 99.83% (FIG.
4D). The concentrated electrolyte, 4 M lithium
bis(fluorosulfonyl)imide in 1,2-dimethoxyethane, was reported to
promote high coulombic efficiency in Li metal anodes due to
decreased reactive solvent amount and increased Li.sup.+
concentration.
[0109] A control experiment was carried out to compare the seamless
monolithic GCNT grown on Cu relative to CNTs randomly dispersed on
Cu. While the GCNT maintains a flat voltage profile over many
cycles, the horizontally deposited CNT exhibits oscillating,
unstable voltage cycles (FIGS. 8A-8B and 9A-9E).
[0110] The specific capacity of the GCNT-Li is tunable by a
time-controlled constant current Li insertion up to 4 mAh cm.sup.-2
(25.3 Ah g.sup.-1.sub.G-CNT) (FIG. 9A). GCNT-Li electrodes with
capacities from 0.4 to 4 mAh cm.sup.-2 (2 to 25.3 Ah
g.sup.-1.sub.G-CNT) are shown with flat voltage profiles and
dendrite-free Li insertion (FIGS. 9A and 10). The large areal
capacity demonstrates the high volumetric capacity (FIG. 10). A
small voltage gap of 100 mV between the Li insertion and extraction
curves is observed for 0.7 mAh cm.sup.-2 (4.4 Ah
g.sup.-1.sub.G-CNT), increasing to 200 mV at 4 mAh cm.sup.-2 (25.3
Ah g.sup.-1.sub.G-CNT), likely due to the thicker inserted Li or
possible thicker solid electrolyte interphase (SEI) layer.
[0111] With a capacity of 25.3 Ah g.sup.-1.sub.G-CNT (FIG. 9B), the
GCNT stores 6.6 times its weight in Li, 68 times greater than does
graphite (372 mAh g.sup.-1.sub.C), and 6.6 times greater than does
Si (3859 mAh g.sup.-1.sub.Si). The capacity also exceeds other Li
storage materials. With the mass of Li included in computing the
capacity, the GCNT-Li has a capacity of 3351 mAh
g.sup.-1.sub.GCNT-Li, which is very close to the theoretical
capacity of Li (3860 mAh g.sup.-1.sub.Li). In this regard, the
GCNT-Li (3351 mAh g.sup.-1.sub.GCNT+Li) has 1.8 times higher Li
content than Li.sub.15Si.sub.4 (1857 mAh g.sup.-1.sub.Li15Si4), and
9.9 times higher Li content than LiC.sub.6 (339 mAh
g.sup.-1.sub.LiC6) (FIG. 9C).
[0112] The GCNT-Li electrode exhibits high specific capacity, both
areal and gravimetric, under increased current densities. In FIG.
9D, the GCNT is shown to insert and extract Li to a rate as high as
10 mA cm.sup.-2 (58 A g.sup.-1.sub.G-CNT), producing a capacity of
.about.0.7 mAh cm.sup.-2 (4.4 Ah g.sup.-1.sub.G-CNT), which is
independent of the current density. The flatness of the curves is
still maintained up to 4 mA cm.sup.-2 (23 A g.sup.-1.sub.G-CNT).
However, during the GCNT-Li cycling at 10 mA cm.sup.-2 (58 A
g.sup.-1), a significant polarization is observed from the Li
insertion/extraction curves with loss of the characteristic
flatness at lower current densities.
[0113] As shown in FIG. 9E, the GCNT-Li maintains a very high
coulombic efficiency and good cycle stability at high current
densities. The high current capability supersedes values reported
on other LIB electrodes. Moreover, the optimal electrical
conductivity of the GCNT monolith facilitates electron transport
without the need for conductive additives. The seamless growth of
CNTs on graphene, where the graphene is grown in intimate contact
with the Cu, eliminates the electrode-current collector resistance.
The vertical carpet nature of the CNTs would enhance Li-ion
diffusion through non-tortuous Li insertion and extraction with
flexible CNT movements.
[0114] In a further experiment, excess Li was inserted into the
GCNT until 5 mAh cm.sup.-2 was attained (FIG. 11A). The electrode
was then delithiated and lithiated for 5 cycles to stabilize the
coulombic efficiency. The GCNT-Li was then allowed to undergo Li
insertion/extraction cycles up to a capacity of 1 mAh cm.sup.-2
(FIG. 11B), yielding an excess Li equivalent of 4 mAh cm.sup.-2.
This significantly improved the cycle life of the electrode with no
sign of decline after 500 cycles and a coulombic efficiency of 100%
(FIGS. 11C-D).
[0115] In addition, the GCNT-Li anode was combined with a sulfur
cathode to produce a full Li-sulfur battery. The areal capacity of
the GCNT-Li was matched with that of the sulfur cathode. As shown
in FIG. 12, the two characteristic plateaus of sulfur lithiation
appear at 2.3 and 2.1 V. The resulting sulfur lithiation products
(lithium polysulfides) are known to diminish the cycle life of
Li-sulfur batteries because they react with the Li metal anode,
such as those inserted in the GCNT-Li.
[0116] Thus, a layer of graphene nanoribbons was deposited on the
separator to restrain the polysulfides to the cathodic side,
thereby improving the stability of the battery. Additionally, a
small voltage gap of 190 mV between the charge and discharge of the
full-cell is observed. The battery delivers a specific capacity of
800 mAh g.sup.-1 (2 mAh cm.sup.-2), which far exceeds the
theoretical capacity of .about.100 mAh g.sup.-1 in a
graphite/LiCoO.sub.2 system. This high capacity, despite the
relatively low voltage feature of the sulfur cathode, enables a
full battery with a high energy density of .about.1840 Wh
kg.sup.-1, more than 6 times higher than 300 Wh kg.sup.-1 for
graphite/LiCoO2 cells.
[0117] In addition, a full battery made from GCNT-Li and LiCoO2 is
demonstrated (FIG. 13). The gravimetric energy density is 310 Wh
kg.sup.-1 for the first cycle discharge.
Example 1.1. GCNT Preparation
[0118] The preparation of GCNT was similar to the previously
reported methods. See WO 2013/119295. First, Bernal-stacked
multilayer graphene was grown on copper foil (25 .mu.m) using the
CVD method, as reported elsewhere. The catalysts for CNT growth
were deposited by e-beam evaporation over the graphene/Cu foil to
form graphene/Fe (1 nm)/Al.sub.2O.sub.3 (3 nm). The CNT growth was
conducted under reduced pressure using a water-assisted CVD method
at 750.degree. C. First, the catalyst was activated by using atomic
hydrogen (H.) generated in situ by H.sub.2 decomposition on the
surface of a hot filament (0.25 mm W wire, 10 A, 30 W) for 30
seconds under 25 Torr (210 sccm H.sub.2, 2 sccm C.sub.2H.sub.2 and
water vapor generated by bubbling 200 sccm of H.sub.2 through
ultra-pure water). After the activation of the catalyst for 30
seconds, the pressure was reduced to 8.3 Torr and the growth was
carried out for 15 minutes.
Example 1.2. Electrochemical Insertion (and Extraction) of Li into
GCNT
[0119] The electrochemical reaction was performed in 2032 coin-type
cells using GCNT substrates and Li foil as both counter and
reference electrodes. The GCNT substrates are circular with total
area of .about.2 cm.sup.2. The electrolyte used was 4 M lithium
bis(fluorosulfonyl)imide (LiFSI) (Oakwood Inc.) in
1,2-dimethoxyethane (DME). The LiFSI salt was vacuum dried (<20
Torr) at 100.degree. C. for 24 hours and DME was distilled over Na
strips. All the experiments were conducted inside a glove box with
oxygen levels below 5 ppm. The separator was Celgard membranes
K2045.
[0120] Previous to the coin cell assembly, the GCNT substrate was
prelithiated by putting one drop of electrolyte on the surface of
GCNT, pressing a Li coin gently against the GCNT and leaving it
with the Li coin on top for 3 hours. Adding excessive amounts of
the electrolyte solution during the pretreatment was found to yield
ineffective prelithiation due to poor contact between the GCNT and
the Li. After the prelithiation, the GCNT was assembled in a coin
cell using the same Li chip used in the prelithiation. The current
density for the electrochemical measurements (insertion/extraction
and cycling) ranges from 1 to 10 mA cm.sup.-2, all performed at
room temperature. For the Li plating (discharging process), a
time-controlled process with a constant current regime was applied
with no cut-off voltage limit. The stripping process (charge
process) was set to a constant current regime with a cut-off
voltage of 1 V (vs Li.sup.+/Li). A control experiment was carried
out using a copper foil upon which graphene is grown by CVD.
Example 1.3. Materials Characterization
[0121] Coin cells were dissembled inside a glove box to check the
morphology of the GCNT electrodes after Li insertion/extraction.
SEM images of the GCNT electrodes were obtained with an FE-SEM
(JEOL-6500F) at an accelerating voltage of 20 kV. High resolution
TEM (HRTEM) images (JEOL FEG-2100F) were obtained after preparing
the samples by sonicating the GCNT substrate in acetonitrile and
dropping the dispersion over TEM grids.
[0122] Without further elaboration, it is believed that one skilled
in the art can, using the description herein, utilize the present
disclosure to its fullest extent. The embodiments described herein
are to be construed as illustrative and not as constraining the
remainder of the disclosure in any way whatsoever. While the
embodiments have been shown and described, many variations and
modifications thereof can be made by one skilled in the art without
departing from the spirit and teachings of the invention.
Accordingly, the scope of protection is not limited by the
description set out above, but is only limited by the claims,
including all equivalents of the subject matter of the claims. The
disclosures of all patents, patent applications and publications
cited herein are hereby incorporated herein by reference, to the
extent that they provide procedural or other details consistent
with and supplementary to those set forth herein.
* * * * *