U.S. patent application number 12/915580 was filed with the patent office on 2011-05-05 for lithium-oxygen electrochemical cells and batteries.
This patent application is currently assigned to UCHICAGO ARGONNE, LLC. Invention is credited to Christopher S. JOHNSON, Vilas G. POL, Zhengcheng ZHANG.
Application Number | 20110104576 12/915580 |
Document ID | / |
Family ID | 43925786 |
Filed Date | 2011-05-05 |
United States Patent
Application |
20110104576 |
Kind Code |
A1 |
JOHNSON; Christopher S. ; et
al. |
May 5, 2011 |
LITHIUM-OXYGEN ELECTROCHEMICAL CELLS AND BATTERIES
Abstract
A lithium-oxygen electrochemical cell of the invention comprises
a lithium-containing anode, an oxygen-permeable cathode, a
non-aqueous electrolyte comprising a lithium salt in a non-aqueous
liquid between the anode and the cathode, and a source of gaseous
oxygen in fluid communication with the cathode; the cathode
comprising an oxygen-permeable support bearing carbon nanotubes
having at least one open end. In some embodiments, the cell is
rechargeable and the cathode includes a nanoparticulate catalyst in
contact with the carbon nanotubes; wherein the catalyst is adapted
to facilitate the reversible interconversion between oxygen gas and
an oxygen anion e.g., oxide ion, peroxide ion, or a combination
thereof, during charge and discharge of the cell.
Inventors: |
JOHNSON; Christopher S.;
(Naperville, IL) ; POL; Vilas G.; (Willowbrook,
IL) ; ZHANG; Zhengcheng; (Naperville, IL) |
Assignee: |
UCHICAGO ARGONNE, LLC
Chicago
IL
|
Family ID: |
43925786 |
Appl. No.: |
12/915580 |
Filed: |
October 29, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61360027 |
Jun 30, 2010 |
|
|
|
61280025 |
Oct 29, 2009 |
|
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Current U.S.
Class: |
429/405 ;
977/742; 977/948 |
Current CPC
Class: |
H01M 4/136 20130101;
B82Y 30/00 20130101; H01M 4/485 20130101; H01M 4/625 20130101; H01M
4/131 20130101; H01M 4/366 20130101; H01M 4/5825 20130101; H01M
4/133 20130101; H01M 4/587 20130101; Y02E 60/10 20130101 |
Class at
Publication: |
429/405 ;
977/742; 977/948 |
International
Class: |
H01M 12/06 20060101
H01M012/06 |
Goverment Interests
CONTRACTUAL ORIGIN OF THE INVENTION
[0002] The United States Government has rights in this invention
pursuant to Contract No. DE-ACO2-06CH11357 between the United
States Government and UChicago Argonne, LLC representing Argonne
National Laboratory.
Claims
1. A lithium-oxygen electrochemical cell comprising a
lithium-containing anode, an oxygen-permeable cathode, a
non-aqueous electrolyte comprising a lithium salt in a non-aqueous
liquid between the anode and the cathode, and a source of gaseous
oxygen in fluid communication with the cathode; the cathode
comprising an oxygen-permeable support bearing carbon nanotubes
having at least one open end.
2. The electrochemical cell of claim 1 wherein the source of oxygen
is ambient air.
3. The electrochemical cell of claim 1 wherein the gas-permeable
support comprises a nickel mesh.
4. The electrochemical cell of claim 1 wherein the gas-permeable
support comprises a glass fiber mat.
5. The electrochemical cell of claim 1 wherein the cathode
comprises a binder to adhere the carbon nanotubes to the
support.
6. The electrochemical cell of claim 1 wherein the cathode
comprises a nanoparticulate catalyst in contact with the carbon
nanotubes; the catalyst being adapted to facilitate the reversible
interconversion between oxygen gas and an oxygen anion selected
from oxide ion, peroxide ion, and a combination thereof, during
charge and discharge of the cell.
7. The electrochemical cell of claim 6 wherein the catalyst
comprises nanoparticles of a metal, a metal oxide, or a combination
thereof.
8. The electrochemical cell of claim 7 wherein the catalyst
comprises nanoparticles of at least one material selected from the
group consisting of cobalt, manganese, iron, and an oxide of any of
the foregoing.
9. The electrochemical cell of claim 7 wherein the catalyst
comprises Li.sub.4Ti.sub.5O.sub.12.
10. The electrochemical cell of claim 7 wherein at least some of
the nanoparticles are located at the open end of a carbon nanotube,
within the carbon nanotubes, or both.
11. The electrochemical cell of claim 1 wherein the carbon
nanotubes have an average tube diameter in the range of about 80 to
about 90 nm.
12. The electrochemical cell of claim 1 wherein the carbon
nanotubes have an average length in the range of about 200 nm to
about 7 gm.
13. The electrochemical cell of claim 1 wherein the carbon
nanotubes have an average length-to-diameter aspect ratio in the
range of about 20 to about 100.
14. The electrochemical cell of claim 1 wherein the non-aqueous
liquid comprises an organic carbonate.
15. The electrochemical cell of claim 1 wherein the lithium salt
comprises LiPF.sub.6.
16. The electrochemical cell of claim 1 wherein the
lithium-containing anode comprises metallic lithium.
17. The electrochemical cell of claim 1 wherein the
lithium-containing anode comprises lithium metal.
18. A rechargeable lithium-oxygen electrochemical cell comprising a
lithium-containing anode, an oxygen-permeable cathode in fluid
communication with ambient air, and a non-aqueous electrolyte
comprising a lithium salt in a non-aqueous liquid between the anode
and the cathode; the cathode comprising an oxygen-permeable support
bearing carbon nanotubes having at least one open end, and a
nanoparticulate catalyst in contact with the carbon nanotubes;
wherein the catalyst is adapted to facilitate the reversible
interconversion between oxygen gas and an oxygen anion selected
from oxide ion, peroxide ion, and a combination thereof, during
charge and discharge of the cell.
19. The electrochemical cell of claim 18 wherein the catalyst
comprises nanoparticles of a metal, a metal oxide, or a combination
thereof.
20. The electrochemical cell of claim 18 wherein the catalyst
comprises nanoparticles of at least one material selected from the
group consisting of cobalt, manganese, iron, and an oxide of any of
the foregoing.
21. The electrochemical cell of claim 18 wherein the catalyst
comprises Li.sub.4Ti.sub.5O.sub.12.
22. The electrochemical cell of claim 18 wherein the carbon
nanotubes have an average tube diameter in the range of about 50 to
about 100 nm.
23. The electrochemical cell of claim 18 wherein the carbon
nanotubes have an average length in the range of about 200 nm to
about 7 .mu.m.
24. The electrochemical cell of claim 18 wherein the carbon
nanotubes have an average length-to-diameter aspect ratio in the
range of about 20 to about 100.
25. The electrochemical cell of claim 18 wherein the non-aqueous
liquid comprises an organic carbonate.
26. The electrochemical cell of claim 18 wherein the lithium salt
comprises LiPF.sub.6.
27. The electrochemical cell of claim 18 wherein the
lithium-containing anode comprises metallic lithium.
28. The electrochemical cell of claim 18 wherein the
lithium-containing anode comprises lithium metal.
29. A battery comprising a plurality of electrochemical cells of
claim 1 arranged in parallel, in series, or both.
30. A battery comprising a plurality of electrochemical cells of
claim 18 arranged in parallel, in series, or both.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application Ser. No. 61/360,027, filed on Jun. 30, 2010, and of
U.S. Provisional Application Serial No. 61/280,025, filed on Oct.
29, 2009, each of which is incorporated herein by reference in its
entirety.
FIELD OF THE INVENTION
[0003] This invention relates to energy storage devices, notably
electrochemical cells and batteries and, more particularly,
lithium-oxygen electrochemical cells. The present invention
provides electrochemical cells and batteries that include a carbon
nanotube-based cathode.
BACKGROUND
[0004] Metal-oxygen (e.g., metal-air) batteries combine a metal
anode, similar to that used in conventional primary batteries, and
an oxygen (air) gas-diffusion cathode similar to that used in fuel
cells. During operation, the metal anode typically based on Zn, Al,
Mg, Ca, or Li is electrochemically oxidized for the expense of the
oxygen from air, which is reduced on the gas-diffusion cathode.
Some properties of metal-oxygen electrochemical cells are presented
in Table 1. A classic example of a household zinc-air system is a
so-called "hearing aid" cell. Besides that, these power sources
occupy several other niches in the energy market.
TABLE-US-00001 TABLE 1 Properties of metal-air batteries Metal/
Calcu- Theoretical specific Theoretical specific O.sub.2 lated
energy, Wh/kg energy, Wh/kg battery OCV, V (including oxygen)
(excluding oxygen) Li/O.sub.2 2.91 5200 11140 Na/O.sub.2 1.94 1677
2260 Ca/O.sub.2 3.12 2990 4180 Mg/O.sub.2 2.93 2789 6462 Zn/O.sub.2
1.65 1090 1350
[0005] In the case of lithium-oxygen cells, the possible discharge
cell reactions and the associated cell voltages are:
2Li+O.sub.2.fwdarw.Li.sub.2O.sub.2;G.sub.0=-145Kcal(E.sub.0=3.1V)
and
4Li+O.sub.2.fwdarw.2Li.sub.2O;G.sub.0=-268Kcal(E.sub.0=2.91V);
based on experimental discharge data from a Li/O.sub.2 test cell.
It has been reported that the main discharge reaction is the
reduction of oxygen to form Li.sub.2O.sub.2 based on Raman
spectroscopic analysis of the products in a non-aqueous cell.
[0006] A lithium-oxygen (Li--O.sub.2) battery can be rechargeable
when the carbon cathode contains catalysts derived from complexes
or oxides of metals such as cobalt. The catalyst can be viewed as
lowering the over-voltage for the oxidation of Li.sub.2O.sub.2 or
Li.sub.2O to form metallic Li and oxygen. The first example of a
Li--O.sub.2 rechargeable cell utilized non-aqueous conducting gel
polymer electrolytes that were used to construct polymer-Li-air
cells based on poly(acrylonitrile)(PAN) and poly(vinylidene
difluoride) (PVdF). Non-aqueous lithium-air batteries represent a
class of potentially ultrahigh energy density power sources useful
for a variety of civilian applications. However, such systems
generally exhibit a limited number of recharge cycles.
[0007] There are a number of factors that can limit the
rechargeablity of Li--O.sub.2 cells. These include the structural
and chemical design of the air-diffusion cathode, the kinetics of
the redox reactions, the solubility and rates of dissolution of the
reaction products during charging, and the need for high oxygen
solubility in the non-aqueous electrolyte.
[0008] The cathode in all known types of metal-oxygen cells and
batteries is an gas-diffusion electrode, in which oxygen (either
pure or diluted in another gas, e.g., air) diffuses through the
cathode, generally on a continuous basis. Such gas-diffusion
electrodes typically comprise a porous, thin plate or sheet, which
serves as a wall for the metal-oxygen cell and separates the
electrolyte in the cell from the surrounding oxygen-containing gas
(e.g., air). The dual purpose of the porous plate poses some
conflicting requirements for cathode design. For example, the
cathode must be highly porous and permeable to gaseous oxygen while
simultaneously preventing leakage of the electrolyte through the
porous cathode. The cathode must be electrically conductive and
must possess enough mechanical strength to withstand the
hydrostatic pressure of the electrolyte and any hydrodynamic shocks
that may eventually occur. If the Li-oxygen cell is to be
rechargeable, the porous cathode must contain an active catalyst
for electrochemical oxidation of oxide and peroxide ions to form
oxygen in contact with the electrolyte. Stable operation of the
oxygen (air) cathode with time is also needed. Cathodes for
metal-oxygen cells can contain various forms of carbon, for
example, in the electrochemically active material, as conductive
additive, and as part of the current collector.
[0009] Lithium-oxygen (e.g., Li-air) electrochemical cells and
batteries have been targeted as the next generation energy storage
system, possessing a very high theoretical specific energy that
renders such batteries attractive for a number of power source
applications including electric vehicles (EVs). Based on the high
electropositive voltage of lithium and its low atomic weight, and
the ready source of atmospheric oxygen, the theoretical specific
energy for a lithium-air cell is 5200 Wh/kg, including oxygen. This
is much higher than is achievable with a typical Li-ion battery,
which has a specific energy value near 600 Wh/kg. In practice,
oxygen is not stored in the battery, but is rather supplied by
ambient air. The theoretical specific energy of a lithium-air cell,
excluding oxygen, is 11140 Wh/kg. Oxygen from the ambient
atmosphere enters the pores of the carbon cathode to serve as the
cathode active material. In the discharge of a Li-air battery
utilizing a porous carbon electrode, this oxygen is reduced, and
the products are stored in the pores of the carbon electrode. As a
result, the cell capacity is expressed as ampere-hour per kilogram
of the carbon in the cathode. The specific characteristics, such as
Ah/kg and AWL of metal-air batteries are significantly higher than
that of the classical electrochemical systems with the same metal
anode. Theoretical data show that Li and Ca possess very high
energy densities of 13172 and 4560 Ah/kg, respectively, but these
metals are not suitable to be used as anodes when aqueous
electrolytes are utilized. Consequently, non-aqueous electrolytes
must be used in such systems. When fully developed, lithium-oxygen
batteries could exhibit practical specific energies of 1000-3000
Wh/kg.
[0010] In a primary Li--O.sub.2 cell, Li metal is electrochemically
oxidized at expense of oxygen from air, which is reduced at the
cathode, producing either Li.sub.2O.sub.2 (3.1 V vs.
Li.sup.+/VLi.sup.0) or Li.sub.2O (2.91 V vs. Li.sup.+/Li.sup.0) or
a combination of both. In the discharge of the Li-air battery, this
oxygen is reduced and the products are stored in the cathode
(typically high surface area carbon black). The Li--O.sub.2 battery
may also be charged in the presence of a high-surface area carbon
black and a manganese oxide catalyst such as .alpha.-MnO.sub.2
(hollandite structure) that is used to assist in the oxidation of
the discharge product, Li.sub.2O.sub.2 and/or Li.sub.2O to oxygen.
While high values of energy density have been realized for the
Li--O.sub.2 cell as a primary power source, the utilization of the
device as a rechargeable cell have been hindered by many
obstacles.
[0011] Electrodes that utilize nanostructures for energy storage
are increasingly prevalent. An example is synthesized
nanostructured carbon-free LiFePO.sub.4 olivine, which reportedly
has a high-rate for Li-ion battery applications. Nanostructured
materials that feature high-surface areas, fast ion diffusion and
high electronic conductivity due to their very small particle size
have led to higher rates of electrochemical reactions in various
applications. The evolution and improvement of Li-air batteries
ultimately will involve tailored nanostructured materials that can
support the rigors of the air-cathode during operation.
SUMMARY OF THE INVENTION
[0012] A lithium-oxygen electrochemical cell of the present
invention comprises a lithium-containing anode, an oxygen-permeable
cathode, a non-aqueous electrolyte comprising a lithium salt in a
non-aqueous liquid between the anode and the cathode, and a source
of gaseous oxygen in fluid communication with the cathode. The
cathode comprises an oxygen-permeable support bearing carbon
nanotubes having at least one open end.
[0013] A rechargeable lithium-oxygen electrochemical cell of the
present invention comprises a lithium-containing anode, an
oxygen-permeable cathode in fluid communication with an oxygen
source (e.g., ambient air), and a non-aqueous electrolyte
comprising a lithium salt in a non-aqueous liquid between the anode
and the cathode. The cathode comprises an oxygen-permeable support
bearing carbon nanotubes having at least one open end and a
nanoparticulate catalyst in contact with the carbon nanotubes. The
catalyst is adapted to facilitate the reversible interconversion
between oxygen gas and an oxygen anion (e.g., oxide ion, peroxide
ion, or both), during charge and discharge of the cell.
[0014] Batteries of the invention comprise two or more
electrochemical cells connected in series, in parallel, or
both.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The invention includes certain novel features and a
combination of parts hereinafter fully described, illustrated in
the accompanying drawings, and particularly pointed out in various
aspects of the invention, it being understood that various changes
in the details may be made without departing from the spirit, or
sacrificing any of the advantages of the described invention.
[0016] FIG. 1 depicts (a) a scanning electron micrograph (SEM) and
(b) a transmission electron micrograph (TEM) of carbon nanotubes
(CNTs) containing a nanoparticulate cobalt metal catalyst within
the tubes and/or at the open ends of the tubes (referred to herein
as "Co-CNT" for convenience); Panel (c) provides a SEM of a cathode
formed from a Ni mesh coated with the Co-CNTs shown in panels (a)
and (b); Panel (d) shows energy dispersive X-ray spectroscopy (EDS)
mapping of the cathode shown in Panel (c), which confirms the
Co-CNT composition, namely the amounts of carbon, cobalt and
oxygen.
[0017] FIG. 2 depicts electrochemical discharge-charge voltage
profiles of a Li--O.sub.2 cell having a cathode comprising the CNTs
and a cobalt catalyst; Panel (a) shows the first cycle, and Panel
(b) shows the second and third cycles.
[0018] FIG. 3 depicts the electrochemical first and fourth
discharge-charge voltage profiles of a Li--O.sub.2 cell having a
cathode comprising the CNTs and a cobalt catalyst.
[0019] FIG. 4 depicts the electrochemical first, second and third
discharge-charge voltage profiles of a Li--O.sub.2 cell having a
cathode comprising CNTs combined with nanoparticulate MnO.sub.2 and
cobalt catalysts.
[0020] FIG. 5 depicts the electrochemical discharge-charge voltage
profiles of a Li--O.sub.2 cell having a cathode comprising the CNTs
combined with a Li.sub.4Ti.sub.5O.sub.12 catalyst.
[0021] FIG. 6 depicts the electrochemical discharge-charge voltage
profiles of a Li--O.sub.2 cell having a cathode comprising about 50
wt % of CNTs and about 50 wt % of a Vinyl fluoride resin (2801)
binder.
[0022] FIG. 7 depicts in Panel (a) the SEM and EDS of a
nanoparticulate Fe.sub.3O.sub.4 catalyst; Panel (b) depicts the
X-ray powder diffraction pattern confirming the Fe.sub.3O.sub.4
phase; and Panel (c) depicts the electrochemical discharge-charge
voltage profiles of a Li--O.sub.2 cell having a cathode comprising
CNTs and a Fe.sub.3O.sub.4 nanoparticle catalyst.
[0023] FIG. 8 schematically illustrates an embodiment of an
electrochemical cell of the invention.
[0024] FIG. 9 schematically illustrates an embodiment of cathode
for use in an electrochemical cell of the invention.
[0025] FIG. 10 schematically illustrates an embodiment of a battery
of the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0026] The present invention provides lithium-oxygen (e.g.,
lithium-air) electrochemical cells and batteries formed from such
cells. A lithium-oxygen electrochemical cell of the present
invention comprises a lithium-containing anode, an oxygen-permeable
cathode, a non-aqueous electrolyte comprising a lithium salt in a
non-aqueous liquid between the anode and the cathode, and a source
of gaseous oxygen in fluid communication with the cathode. The
cathode comprises an oxygen-permeable support bearing carbon
nanotubes having at least one open end.
[0027] A rechargeable lithium-oxygen electrochemical cell of the
present invention comprises a cathode that also contains a
nanoparticulate catalyst in contact with the CNTs, which is adapted
to facilitate the reversible interconversion between oxygen gas and
an oxygen anion e.g., oxide ion, peroxide ion, or a combination
thereof, during charge and discharge of the cell. One preferred
catalyst comprises cobalt metal. Another preferred catalyst
comprises a cobalt oxide. Yet other preferred catalysts include
nanoparticulate manganese dioxide (MnO.sub.2), ferric oxide
(Fe.sub.2O.sub.3), WO.sub.3, CeO.sub.2, MoO.sub.2, MoO.sub.3,
ferric/ferrous oxide (Fe.sub.3O.sub.4), Li.sub.4Ti.sub.5O.sub.12
(2Li.sub.2O.5TiO.sub.2), TiO.sub.2 or their mixtures with noble
metals such as Pt, Au, Ru and the like. The catalyst can be located
at the tip of the carbon nanotubes, within the nanotubes, or
intermixed with the carbon nanotubes, nanofibers and grapheme
sheets.
[0028] Suitable carbon nanotubes can be obtained by any method
known to produce open-ended carbon nanotubes. One such method is
described in U.S. Patent Application Publication No. 2010/0178232,
which is incorporated herein by reference in its entirety, and
which involves thermal decomposition of low density polyethylene in
an autogenic pressure reactor in the presence of a metal salt such
as a cobalt salt. This method provides CNTs containing
nanoparticulate cobalt. Another method of producing CNTs, e.g.,
from recycled polymeric materials (e.g., LDPE and HDPE) and a
catalyst such as Ni, Co, Fe, and oxides thereof, is described in
U.S. 2010/0178232. The carbon nanotubes of the electrochemical
cells of the invention typically have a average tube diameter in
the range of about 50 to about 100 nm, preferably in the range of
about 80 to about 90 nm. Typically, the carbon nanotubes have an
average length in the range of about 200 nm to about 7 .mu.m,
preferably about 1000 nm to about 10 .mu.m. The carbon nanotubes
preferably have an average aspect ratio (length-to-diameter) in the
range of about 20 to about 100.
[0029] The carbon nanotubes can be intermixed with an inert
material, or with another active cathode material, if desired. In
some preferred embodiments, the carbon nanotubes are intermixed
another porous carbon material, such as carbon black.
[0030] The carbon nanotube-based cathode can include a binder to
help maintain the cathode in a particular shape or configuration
and to adhere the carbon nanotubes and catalyst, when present, to
the support. Any binder that is stable to the electrochemical
conditions of a lithium-oxygen battery can be utilized in the
present invention. Many such materials are known in the art. One
preferred binder is polyvinylidene difluoride (PVdF).
[0031] The support for the cathode can be any gas-permeable
material suitable for use in metal-oxygen cells (e.g., materials
that are not chemically reactive with oxygen and with typical
electrolyte materials under the electrochemical conditions of cell
charge and discharge), many of which are well known in the art. In
some preferred embodiments, the support comprises a metal mesh
(e.g., a nickel mesh). In other embodiments, the support comprises
a non-woven fibrous material, such as a glass fiber mat or paper.
The carbon nanotubes and optional catalyst can be applied to the
support in any way that will provide a thin, even coating on the
support (e.g., by spraying, dipping, painting, etc.). For example,
the carbon nanotubes, catalyst, and a binder can be suspended in a
volatile liquid (e.g., water, an organic solvent, and the like) and
then painted onto the surface of the support. The liquid is then
evaporated to leave a coating of carbon nanotubes and catalyst on
the support surface.
[0032] The anode of the electrochemical cell can be any
lithium-containing material suitable for use in lithium-oxygen
cells and batteries. Some exemplary materials include metallic
lithium, and lithium oxide containing materials that are known to
be useful as anodes in lithium-oxygen or lithium ion
electrochemical cells.
[0033] The electrochemical cells can include any non-aqueous
electrolyte suitable for use in lithium-oxygen (e.g., lithium-air)
cells and batteries, many of which are well known in the art.
Typically, these non-aqueous electrolytes comprise an inert polar
organic liquid, such as an organic carbonate (e.g., dimethyl
carbonate, ethylmethyl carbonate, ethylene carbonate, propylene
carbonate, and the like), or a polymer-based gel. The liquid or gel
contains a lithium salt (e.g., LiPF.sub.6,
LiB(C.sub.2O.sub.4).sub.2, LiBF.sub.2C.sub.2O.sub.4, LiBF.sub.4,
LiPF.sub.2(C.sub.2O.sub.4).sub.2, LiPF.sub.4C.sub.2O.sub.4, and the
like) that is soluble in the liquid or gel. The following examples
describe the principles of the invention as contemplated by the
inventors, but they are not to be construed as limiting
examples.
Example 1
[0034] Carbon nanotubes containing a nanoparticulate cobalt
catalyst (Co-CNTs) were prepared via thermal decomposition of low
density polyethylene (LDPE) containing about 20 percent by weight
(wt %) cobalt acetate (CoAc; Co(C.sub.2H.sub.4O.sub.2).sub.2)
catalyst in a sealed autoclave (made up of Haynes 230 alloy) under
a nitrogen atmosphere. The autoclave was heated at about
700.degree. C. to about 750.degree. C. for about 2 to 3 hours
followed by gradual cooling. The mixture of LDPE and catalyst in
the autoclave generated about 50 pounds-per-square inch (psi) of
pressure upon heating up to about 680.degree. C., which increased
to about 1000 psi at about 700.degree. C. A chemical reaction took
place under the autogenic pressure generated in the autoclave
during the thermolysis of LDPE in the presence of CoAc, leading to
the growth of Co-CNTs in about 40% yield.
[0035] Panel (a) of FIG. 1 shows a SEM of these carbon nanotubes
(CNTs). The dark dots at the tip of the carbon-nanotubes in the SEM
are the Co catalyst that was converted to nano-sized cobalt metal
particles. The SEM demonstrates that the diameters of the
multiwalled carbon nanotubes are about 80 nm. A nanotube length of
more than a micron was obtained within about 2 hours of initiating
the heating of the autoclave, demonstrating that the growth of the
Co-CNTs is a function of reaction time. In comparison with the
known methods for the fabrication of CNTs, this particular method
appears to be one of the easiest, least expensive and most
environmentally friendly ways to produce CNTs. The Co-CNTs are
grown randomly and the cobalt nanoparticles are trapped at the tip
or inside of the nanotubes. The as-prepared CNTs possessed about 11
wt % of encapsulated cobalt, confirmed by EDS analysis.
[0036] Although the SEMs taken using secondary electrons
demonstrate the one-dimensional fiber-like nanotubes, a
transmission electron micrograph (TEM) further confirmed the hollow
tubular structures of the CNTs, as shown in FIG. 1, Panel (b),
which also confirmed that the one ends of the carbon nanotubes are
open. Panel (c) of FIG. 1 provides a TEM of a cathode formed by
painting a slurry of another sample of Co-CNTs prepared in the same
manner) and PVdF binder on a Ni mesh. N-Methyl-2-pyrrolidone was
the solvent used to make the slurry of Co-CNT and PVdF. The cathode
coating comprised about 75 wt % carbon nanotubes and about 14 wt %
Co catalyst, and about 12 wt % PVdF binder. Panel (d) of FIG. 1
shows the EDS signal obtained along the arrow shown in Panel (c),
confirming the carbon and cobalt composition of the nanotubes.
Example 2
[0037] The coated cathode described in Example 1 was evaluated in a
lithium-oxygen electrochemical cell with a lithium metal foil as
the anode, and an electrolyte consisting of 1.2M LiPF.sub.6 in a
3:7 (w/w) mixture of ethylene carbonate (EC) and ethylmethyl
carbonate (EMC), between the anode and the cathode. The cells were
assembled in helium-filled glove box. After bringing the
lithium-air cell out of the glove box, it was purged with oxygen,
and then filled with O.sub.2 at about 20 psi pressures. FIG. 2
depicts the electrochemical voltage profiles that were obtained by
galvanostatically cycling the cells between about 1.5 V (discharge)
and 4.7 V (charge). The observed current density was about 35 mA/g,
at a C/20 rate. Panel (a) shows the first cycle, while Panel (b)
shows the second and third cycles. The observed discharge capacity
was about 300 mAh/g, and the observed charge capacity was about 800
mAh/g. The average discharge voltage was about 2.5 V, and the
average charge voltage was about 4.4 V. The electrochemical
charge/discharge voltage profiles of the cell exhibited
reversibility, and demonstrated the utility of the
cobalt-containing CNT cathode. With further cycling, the discharge
and charge capacities show better coulombic efficiency. The second
discharge capacity was about 470 mAh/g, and the second charge
capacity was about 500 mAh/g.
Example 3
[0038] A charge-discharge cycling evaluation was also carried out
on cells of the same construction described in Example 2. The cell
was galvanostatically discharged and charged between 1.5 V and 4.7
V, respectively. The observed current density was about 75 mA/g at
a C/4 rate, as shown in FIG. 3. The date in FIG. 3 were obtained
using a cathode analogous to the one described in Example 2 with
C/4 fast cycling rate; the figure depicts discharge and charge
cycle numbers two and three.
Example 4
[0039] Another cathode was prepared on a nickel mesh, with a
coating containing about 15 wt % electrolytic MnO.sub.2 catalyst,
about 35.5 wt % Co-CNTs comprising 4.5 wt % Co nanoparticles, about
30 wt % carbon black, and about 15 wt % PVdF binder. The CNTs
preparation and dimensions are discussed in Example 1. The cathode
was evaluated in a cell of the same design as described in Example
1, as well. FIG. 4 depicts the observed electrochemical voltage
profiles, galvanostatically discharged and charged between 1.5 and
4.7 V, respectively. The observed current density was about 10
mA/g, at a C/90 rate. The first cycle discharge capacity was about
730 mAh/g at an average voltage of about 2.8 V, and the first
charge capacity was about 400 mAh/g at an average voltage of about
4.3 V.
Example 5
[0040] Another cathode was prepared comprising a nickel mesh coated
with about 24 wt % of CNTs (purchased from Stream Chemicals), about
33 wt % KYNAR.RTM. 2801 vinyl fluoride resin binder, and about 42
wt % of Li.sub.4Ti.sub.5O.sub.12 catalyst (40 nm), with a propylene
carbonate (PC) plasticizer. The CNTs had a diameter of about 50 nm
and a length of several micrometers. FIG. 5 depicts the
electrochemical voltage profiles that were carried out in a cell of
similar design to that described in Example 2, but with an
electrolyte consisting of 1M LiPF.sub.6 in PC. The cell was cycled
between 1.5 V and 4.7 V. The observed current density was about 35
mA/g, at a C/20 rate.
Example 6
[0041] A cathode comprising a coating of 50 wt % CNTs (purchased
from Stream Chemicals) and 50 wt % KYNAR.RTM. 2801 vinyl fluoride
resin binder coated on glass fiber filter paper was evaluated in a
cell of similar design to that described in Example 2, with a
lithium foil anode and an electrolyte comprising 1M LiPF.sub.6 in a
PC (pressurized with 20 psi of oxygen). The CNTs had a diameter of
about 50 nm and a length of several micrometers. FIG. 6 depicts the
electrochemical voltage profiles that were obtained with this cell.
An average voltage of about 2.6 V vs. Li'/Li was observed. This
value is at a higher potential than the previously reported value
of 2.33 V for an electrode utilizing carbon black, thus
demonstrating a higher specific energy or less overpotential for
the cathodes comprising CNTs versus conventional high-surface area
carbon black. The discharge process was stopped at about 2.2 V. The
specific capacity per gram carbon in this example was about 1352
mAh/g at a C/25 rate or 53 mA/g at C/1.
Example 7
[0042] A cathode was composed of about 33 wt % CNTs, about 33 wt %
Fe.sub.3O.sub.4 nanoparticles, and about 33 wt % KYNAR.RTM. 2801
vinyl fluoride resin binder, coated on a glass fiber filter paper
was prepared as described above. Panel (a) of FIG. 7 depicts the
SEM of nanoparticulate Fe.sub.3O.sub.4 catalyst, well dispersed
among carbon nanotubes. The EDS in Panel (a) confirms the presence
of Fe and carbon in the material. The XRD measurement at Panel (b)
confirms the purity of Fe.sub.3O.sub.4 nanoparticles. Panel (c)
depicts the electrochemical voltage profiles that were obtained in
lithium air cells using a lithium metal foil as anode. The cell
contained an electrolyte composed of 1M LiPF.sub.6 in a PC, and was
pressurized with about 20 psi of oxygen prior to discharge and
charge cycling between 1.5 to 4.7 V. The observed current density
was about 25 mA/g at a C/25 rate.
[0043] FIG. 8 provides schematic representation of a typical
electrochemical cell of the present invention. The cell comprises
an anode 12 and an oxygen-permeable cathode 16 with a non-aqueous
electrolyte 14 there between. Cathode 16 comprises carbon nanotubes
that optionally include a catalyst material. Electrolyte 14
comprises a non-aqueous liquid, such as an organic carbonate (e.g.,
dimethyl carbonate, ethylene carbonate, propylene carbonate, and
the like) containing a lithium salt (e.g., LiPF.sub.6).
[0044] FIG. 9 illustrates one form of oxygen-permeable cathode 16,
which comprises a porous support 18 (e.g., a nickel metal mesh or a
glass fiber mat or paper) having a coating 20 comprising the carbon
nanotubes and any catalyst that may be included in the cathode.
[0045] FIG. 10 illustrates a battery 30 comprising a plurality of
electrochemical cells 10 arranges in series and in parallel.
[0046] The use of the terms "a" and "an" and "the" and similar
referents in the context of describing the invention (especially in
the context of the following claims) are to be construed to cover
both the singular and the plural, unless otherwise indicated herein
or clearly contradicted by context. The terms "comprising,"
"having," "including," and "containing" are to be construed as
open-ended terms (i.e., meaning "including, but not limited to,")
unless otherwise noted. Recitation of ranges of values herein are
merely intended to serve as a shorthand method of referring
individually to each separate value falling within the range,
unless otherwise indicated herein, and each separate value is
incorporated into the specification as if it were individually
recited herein. All methods described herein can be performed in
any suitable order unless otherwise indicated herein or otherwise
clearly contradicted by context. The use of any and all examples,
or exemplary language (e.g., "such as") provided herein, is
intended merely to better illuminate the invention and does not
pose a limitation on the scope of the invention unless otherwise
claimed. No language in the specification should be construed as
indicating any non-claimed element as essential to the practice of
the invention.
[0047] Preferred embodiments of this invention are described
herein, including the best mode known to the inventors for carrying
out the invention. Variations of those preferred embodiments may
become apparent to those of ordinary skill in the art upon reading
the foregoing description. The inventors expect skilled artisans to
employ such variations as appropriate, and the inventors intend for
the invention to be practiced otherwise than as specifically
described herein. Accordingly, this invention includes all
modifications and equivalents of the subject matter recited in the
claims appended hereto as permitted by applicable law. Moreover,
any combination of the above-described elements in all possible
variations thereof is encompassed by the invention unless otherwise
indicated herein or otherwise clearly contradicted by context.
* * * * *