U.S. patent application number 12/565500 was filed with the patent office on 2011-03-24 for method of fabricating electrodes including high-capacity, binder-free anodes for lithium-ion batteries.
This patent application is currently assigned to Alliance for Sustainable Energy, LLC. Invention is credited to Chunmei Ban, Anne C. Dillon, Zhuangchun Wu.
Application Number | 20110070495 12/565500 |
Document ID | / |
Family ID | 43756902 |
Filed Date | 2011-03-24 |
United States Patent
Application |
20110070495 |
Kind Code |
A1 |
Ban; Chunmei ; et
al. |
March 24, 2011 |
METHOD OF FABRICATING ELECTRODES INCLUDING HIGH-CAPACITY,
BINDER-FREE ANODES FOR LITHIUM-ION BATTERIES
Abstract
An electrode (110) is provided that may be used in an
electrochemical device (100) such as an energy storage/discharge
device, e.g., a lithium-ion battery, or an electrochromic device,
e.g., a smart window. Hydrothermal techniques and vacuum filtration
methods were applied to fabricate the electrode (110). The
electrode (110) includes an active portion (140) that is made up of
electrochemically active nanoparticles, with one embodiment
utilizing 3d-transition metal oxides to provide the electrochemical
capacity of the electrode (110). The active material (140) may
include other electrochemical materials, such as silicon, tin,
lithium manganese oxide, and lithium iron phosphate. The electrode
(110) also includes a matrix or net (170) of electrically
conductive nanomaterial that acts to connect and/or bind the active
nanoparticles (140) such that no binder material is required in the
electrode (110), which allows more active materials (140) to be
included to improve energy density and other desirable
characteristics of the electrode. The matrix material (170) may
take the form of carbon nanotubes, such as single-wall,
double-wall, and/or multi-wall nanotubes, and be provided as about
2 to 30 percent weight of the electrode (110) with the rest being
the active material (140).
Inventors: |
Ban; Chunmei; (Littleton,
CO) ; Wu; Zhuangchun; (Lakewood, CO) ; Dillon;
Anne C.; (Boulder, CO) |
Assignee: |
Alliance for Sustainable Energy,
LLC
Golden
CO
|
Family ID: |
43756902 |
Appl. No.: |
12/565500 |
Filed: |
September 23, 2009 |
Current U.S.
Class: |
429/221 ;
429/209; 977/750; 977/773; 977/948 |
Current CPC
Class: |
B82Y 30/00 20130101;
Y02T 10/70 20130101; H01B 1/08 20130101; H01M 4/52 20130101; H01M
10/0525 20130101; H01B 1/00 20130101; H01M 4/48 20130101; B01J
19/08 20130101; H01M 4/0404 20130101; H01M 4/0471 20130101; Y02E
60/10 20130101; H01M 4/625 20130101; H01M 4/485 20130101; H01M
4/131 20130101; H01M 4/02 20130101; H01B 1/04 20130101 |
Class at
Publication: |
429/221 ;
429/209; 977/948; 977/773; 977/750 |
International
Class: |
H01M 4/52 20100101
H01M004/52; H01M 4/02 20060101 H01M004/02 |
Goverment Interests
CONTRACTUAL ORIGIN
[0001] The United States Government has rights in this invention
under Contract No. DE-AC36-08GO28308 between the United States
Department of Energy and the Alliance for Sustainable Energy, LLC,
the Manager and Operator of the National Renewable Energy
Laboratory.
Claims
1. An electrode for an electrochemical device, comprising: an
active portion comprising an electrochemically active
nanoparticles; a matrix of electrically conductive nanomaterial
connecting the electrochemically active particles, wherein the
electrically conductive material of the matrix comprises less than
about 30 percent by weight of the electrode.
2. The electrode of claim 1, wherein the electrochemically active
particles comprise active nanoparticles or active
non-nanoparticles.
3. The electrode of claim 1, wherein the active portion provides a
remaining material make up of the electrode after consideration of
the matrix, whereby the electrode is binder-free.
4. The electrode of claim 1, wherein the electrically conductive
nanomaterial of the matrix comprises at least one of carbon
nanoparticles, graphene, a carbon-based nanostructured material, a
doped carbon nanostructure, boron-doped nanotubes, nitrogen-doped
nanotubes, and BCN nanostructures.
5. The electrode of claim 1, wherein the electrically conductive
nanomaterial of the matrix provides 2 to 10 percent by weight of
the electrode.
6. The electrode of claim 5, wherein the electrode is a cathode or
an anode depending upon a material used for the electrochemically
active nanoparticles.
7. The electrode of claim 5, wherein the electrically conductive
nanomaterial of the matrix comprises carbon single-wall
nanotubes.
8. The electrode of claim 7, wherein the carbon single-wall
nanotubes are about 5 to about 10 percent by weight of the
electrode.
9. The electrode of claim 1, wherein the electrochemically active
nanoparticles comprise metal oxide nanoparticles.
10. The electrode of claim 9, wherein the metal oxide nanoparticles
comprise iron oxide nanorods and provide at least about 70 percent
by weight of the electrode and wherein the iron oxide nanorods are
bound within the electrode by the matrix.
11. An electrochemical device, comprising: a cathode layer; and an
anode or cathode layer proximate to the cathode layer comprising an
electrochemically active material and a connective net binding the
electrochemically active nanomaterial within the anode or cathode
layer, wherein the connective net comprises at least semiconducting
nanoparticles.
12. The electrochemical device of claim 11, wherein the
nanoparticles comprise at least one of semiconducting or metallic
carbon nanotubes, fullerenes, graphene, a carbon-based
nanostructured material, a doped carbon nanostructure, boron-doped
nanotubes, nitrogen-doped nanotubes, and BCN nanostructures.
13. The electrochemical device of claim 12, wherein the carbon
nanotubes comprise single-wall nanotubes and the carbon nanotubes
provide about 2 to about 10 percent by weight of the anode
layer.
14. The electrochemical device of claim 13, wherein the carbon
nanotubes are substantially uniformly distributed within the anode
layer.
15. The electrochemical device of claim 11, wherein the
electrochemically active material comprises metal oxide
nanoparticles.
16. The electrochemical device of claim 15, wherein the metal oxide
nanoparticles comprise iron oxide nanorods that provide at least
about 90 percent by weight of the anode layer.
17. A battery, comprising: a lithium-ion cathode; an electrolyte;
and an anode comprising a binder-free electrode layer comprising
about 2 to 30 percent by weight carbon nanoparticles and at least
about 70 percent by weight metal oxide nanoparticles.
18. The battery of claim 17, wherein the carbon particles comprise
carbon single-wall nanotubes, double-wall nanotubes, multi-wall
nanotubes, carbon fiber, or fullerenes providing 5 to 10 percent by
weight of the binder-free electrode layer with remaining material
consisting of the metal oxide nanoparticles.
19. The battery of claim 17, wherein the metal oxide nanoparticles
comprise iron oxide nanorods.
20. The battery of claim 19, wherein the iron oxide nanorods
provide at least about 90 percent by weight of the binder-free
electrode layer.
21. The battery of claim 17, wherein the anode has a reversible
capacity of at least about 1000 mAh/g at C rate over at least about
100 cycles.
22. The battery of claim 17, wherein the carbon nanoparticles
comprise carbon SWNTs providing less than about 10 percent weight
of the binder-free electrode layer and wherein the metal oxide
particles comprise nanorods of at least one 3d-transitional metal
that are substantially uniformly mixed with the carbon SWNTs,
whereby the metal oxide particles are bound by a connective matrix
formed by the carbon SWNTs free of additional binder material.
Description
BACKGROUND
[0002] Energy storage requirements continue to grow as the
electronic, portable power, and energy infrastructure industries
expand and transition away from more historic non-renewable energy
supplies. For example, there has been a renewed interest in
batteries and other energy storage devices for use in electric and
hybrid automobiles, and this has been caused, in part, by volatile
oil costs and the possibility of catastrophic climate change that
has greatly pushed scientific attention toward the development of
electrical and hybrid vehicles powered by rechargeable batteries,
e.g., rechargeable lithium-ion (Li-ion) batteries that may be
powered with electricity from renewable sources. Similarly, there
is ongoing research in ways to make lighter and more efficient
batteries for electronic devices ranging from portable computers to
cellular phones and other wireless communication devices.
[0003] General goals for battery manufacturers include providing
long life and significant power levels with the least amount of
weight while also providing a recharging functionality. More
specifically, one of the most critical parameters for new energy
storage technologies and designs is the demand for higher energy
densities (i.e., energy storage per unit of battery or storage
device weight). Additionally, there is growing concern over
potential long term environmental impacts of product manufacture
and use, and, the energy storage industry continues to search for
storage devices that can make use of environmentally benign or
green materials while still providing desirable energy densities.
Unfortunately, many existing electrode materials that have high
durable capacities and good rate capability are expensive and/or
are toxic. Furthermore, improved energy density and rate
capabilities are still demanded by the battery and other energy
storage industries such as for battery designs facilitating a
successful deployment of a fleet of electric vehicles. Hence, there
remains a need for electrodes fabricated from abundant and nontoxic
elements with durable high-reversible capacity and highly improved
rate capability.
[0004] In the search for electrode materials for electrochemical
devices such as batteries, smart windows, and the like, many
efforts have centered on materials with structures that can
intercalate small cations without major structural changes
occurring. For example, lithium-on batteries are one of the most
prevalent energy storage devices for portable electronics and for
vehicles because these batteries offer relatively high energy
densities and longer lifespans than comparable technologies.
Lithium-ion batteries utilizing existing technologies and electrode
design have sufficient specific energy and power densities to meet
some targets for hybrid electric vehicles (HEVs) and plug-in hybrid
electric vehicles (PHEVs) for up to a 40 mile range.
[0005] However, significant improvements in lifetimes of batteries
along with reductions in costs and use of less toxic electrode
materials are needed before lithium-ion batteries are employed
fully in the transportation industry. Increasing energy density of
electrode materials, for example, is desirable to support use of
electrochemical devices such as lithium-ion batteries being used in
fully electric vehicles. Note, also, that materials for
electrochromic applications and devices are required to meet many
of the same criteria as called for in batteries, and the following
description may use the word electrochemical device to apply to
nearly any electric device with an electrode such as a battery or
an electrochromic device.
[0006] With reference to some exemplary electrode research or
design efforts, three-d-transition metal oxides (Fe.sub.2O.sub.3,
Fe.sub.3O.sub.4, MoO.sub.3, CoO, NiO, and the like) are capable of
Li.sup.+ insertion/extraction in excess of 6 Li.sup.+ per formula
unit, resulting in a larger reversible capacity than commercially
employed graphite. For example, the specific capacity of metal
oxide anodes can be over 1000 mAh/g, which is approximately three
times higher than that of graphitic carbons. Differing from the
intercalation mechanism occurring with graphite, the 3d transition
metal oxides are reduced in a conversion reaction to small metal
clusters, and the oxygen reacts with the lithium to form Li.sub.2O.
In general, this leads to volumetric expansion and destruction of
the structure upon electrochemical cycling, which, for bulk
particles, typically results in capacity loss during cycling, even
at very low rates.
[0007] It has also been reported, for example, that MoO.sub.3
nanoparticles that react with approximately 5.7 Li ions may lead to
an electrode with a durable reversible capacity as high as 1050
mAh/g. Additionally, an Fe.sub.3O.sub.4-based Cu nano-architectured
electrode has been developed that allowed for small diffusion paths
and better electrical and mechanical contact by using a
Cu-nanopillar current collector, enabling improved rate capability.
Various groups have also reported the use of metal oxides with
optimal sizes and carbon nanostructures or nanostructures with
carbon-modified surfaces to improve reversible capacity and rate
capability. Highly dispersed Fe.sub.3O.sub.4 nanocrystals have been
used in a carbon matrix that provided an electrode that had a
reversible capacity of about 600 mAh/g at 0.1 C rate. "C"
represents "charge rate" signifying a charge or discharge rate
equal to the capacity of a battery divided by one (1) hour. Further
studies have shown electrodes formed with carbon/Fe.sub.3O.sub.4
composite nanofibers fabricated with an electro-spinning technique
had a reversible capacity of 1007 mAh/g at 0.1 C and 623 mAh/g at 2
C rate. While these efforts have shown improvements in electrode
technologies, these designs have not been widely adopted as there
remains a need for even higher energy densities and other
improvements in electrodes before such electrodes will be
implemented by the transportation and other industries. For
example, electrodes formed of more green materials are needed with
high reversible capacities and improved rate capabilities as well
as desirable energy densities.
[0008] The foregoing examples of the related art and limitations
related therewith are intended to be illustrative and not
exclusive. Other limitations of the related art will become
apparent to those of skill in the art upon a reading of the
specification and a study of the drawings.
SUMMARY
[0009] The following embodiments and aspects thereof are described
and illustrated in conjunction with systems, tools and methods that
are meant to be exemplary and illustrative, not limiting in scope.
In various embodiments, one or more of the above-described problems
have been reduced or eliminated, while other embodiments are
directed to other improvements.
[0010] An electrode is provided that may be used in an
electrochemical device such as an energy storage/discharge device
(e.g., a lithium-ion battery or the like) or an electrochromic
device (e.g., a smart window). The electrode includes an active
portion that is made up of electrochemically active nanoparticles,
with one embodiment utilizing 3d-transition metal oxides to provide
the electrochemical capacity of the electrode. The electrode also
includes a matrix or net of electrically conductive nanomaterial
that acts to connect and/or bind the active nanoparticles such that
no binder material is required (which allows more active materials
to be included to improve energy density and other desirable
characteristics of the electrode).
[0011] The matrix material may take the form of carbon nanotubes
(such as single-wall, double-wall, and/or multi-wall nanotubes),
carbon fibers, fullerenes, grapheme, and/or any carbon based
nanostructured material including doped carbon nanostructures,
e.g., boron or nitrogen-doped nanotubes and/or BCN nanostructures
(e.g., any hybrid nanotubes constructed of boron (B), carbon (C),
and/or nitrogen (N) elements or other nanostructures of the
so-called BCN material system) or the like. The matrix material may
be provided as about 2 to 30 percent weight of the electrode with
the rest being the active material. For example, the electrode may
be formed by substantially uniformly mixing/combining 5 to 10
percent by weight carbon SWNTs with 90 to 95 percent by weight iron
oxide (or another active material such as silicon, lithium iron
phosphate, lithium manganese phosphate, or a combination of these
materials and/or metal oxides) in the form of nanorods,
nanoparticles, or the like.
[0012] According to another aspect, an electrochemical device is
provided with a cathode layer and an anode layer (with an optional
electrolyte therebetween). The anode layer includes an
electrochemically active nanomaterial and a connective net that
binds the active nanomaterial within the anode layer. The
connective net may include electrically conductive nanoparticles
such as carbon SWNTs or the like to provide at 2 to 30 percent by
weight. The active nanomaterial may take the form of nanorods of
metal oxide such as an iron oxide, and since no binder is required,
the metal oxide nanorods may make up 70 or more percent by weight
of the electrode.
[0013] In addition to the exemplary aspects and embodiments
described above, further aspects and embodiments will become
apparent by reference to the drawings and by study of the following
descriptions.
BRIEF DESCRIPTION OF THE DETAILED DRAWINGS
[0014] Exemplary embodiments are illustrated in referenced figures
of the drawings. It is intended that the embodiments and figures
disclosed herein are to be considered illustrative rather than
limiting.
[0015] FIG. 1A illustrates in simplified form an electrochemical
device such as a battery, electrochromic device, or the like with
one or more electrode as described herein;
[0016] FIG. 1B illustrates a sectional view showing schematically
or functionally two significant portions of the electrode including
an electrically or electrochemically active material portion (e.g.,
iron oxide or other active material nanorods) and a connective
matrix (e.g., an additive that may be made up of, for example,
carbon nanotubes);
[0017] FIG. 2 illustrates a battery such as a lithium-ion battery
(i.e., an exemplary electrochemical device) including an electrode
(i.e., an anode) with a high-capacity, binder-free electrode formed
as taught herein;
[0018] FIG. 3 illustrates an X-ray diffraction (XRD) spectra of
test materials including nanorod precursor materials,
processed/heated active or nanorod materials, and electrode
materials including single wall nanotubes (SWNTs);
[0019] FIG. 4A are photographs showing surface results of scanning
electroscope microscopy (SEM) of an electrode formed as taught
herein showing mixing of active material in the form of nanorods
and a connective matrix in the form of nanotubes or nanotube
bundles;
[0020] FIG. 4B is a photograph of an SEM providing a sectional view
of the electrode of FIG. 4A;
[0021] FIG. 5 illustrates voltage composition curves for various
electrodes including electrodes formed with SWNTs as a connective
matrix for nanorod active material;
[0022] FIG. 6 is a graph comparing specific capacities of several
electrodes including ones formed according to the methods taught
herein with a nano-connective matrix;
[0023] FIG. 7 illustrates an AC impedance spectra of several tested
lithium cells fabricated with an electrode described herein;
[0024] FIG. 8 shows rate capabilities of electrodes with
nano-connective matrices as taught herein with differing amounts of
SWNTs (or differing weight percentages of the connective matrix
material); and
[0025] FIG. 9 is a flow chart of a method of forming an
electrochemical device with an electrode with a connective matrix
in which an active material is supported.
DESCRIPTION
[0026] The following description is directed generally toward
methods of manufacturing or providing electrodes with high
reversible capacity and enhanced rate capability and to
electrochemical devices that include such electrodes (e.g., as a
binderless anode of an Li-ion battery or the like). It may be
useful to provide a relatively specific example and then to
describe the electrode in a more general manner and its use in a
battery with reference to FIGS. 1A to 2. The description then
proceeds to more specific test results achieved in testing several
exemplary electrode implementations with reference to FIGS. 3 to 8.
FIG. 9 then provides an overview of a process for fabricating an
electrochemical device with one or more of the electrodes taught
herein that is useful for achieving significantly higher energy
capacities through the use of an electrically-conductive connective
matrix formed of nanomaterials such as carbon single-wall nanotubes
(SWNTs) or the like. In other words, the connective matrix is a
smaller weight percentage component of the electrode (e.g.,
typically, less than about 30 percent by weight of the electrode
material and, often, less than about 10 percent such as about 5
percent by weight) that may be thought of as a conductive additive
while the active material portion such as iron oxide contributes
the main electrochemical capacity of the electrode.
[0027] In one exemplary electrode, highly improved electrochemical
performance was observed by using a nano iron (II, III) oxide
(Fe.sub.3O.sub.4) binder-free electrode, which was synthesized with
a hydrothermal process and vacuum filtration method. The electrode
may be used in nearly any electrochemical device and contained
Fe.sub.3O.sub.4 nanorods as the active material for lithium storage
and carbon single-wall nanotubes (SWNTs) as a conductive additive
(or electrically conductive net or matrix. The inclusion of SWNTs
improves both mechanical integrity and electrical conductivity as
well as allowing a high volumetric energy density to be achieved
with the electrode. In some cases, the matrix or net makes up about
30 percent or less by weight of the electrode or electrode layer.
For example, the highest, or at least a relatively high, reversible
capacity was obtained using only about 5 percent by weight of SWNTs
in the binder-free electrode.
[0028] The reversible capacity of the electrode when coupled with a
lithium metal electrode in a Li-ion battery for example reaches
1000 mAh/g at C rate and was sustained over 100 cycles with an
acceptable or desirable volumetric capacity. Furthermore, testing
of this exemplary electrode showed a high rate capability and a
stable capacity of about 800 mAh/g at 5 C and a stable capacity of
about 600 mAh/g at 10 C. Scanning electron microscopy (SEM) of the
electrode revealed that the Fe.sub.3O.sub.4 nanorods were
substantially uniformly suspended in a conductive matrix of only 5
percent by weight SWNTs, which, in part, provides the improved rate
capability and durability. Raman spectroscopy was also employed
during testing to characterize the SWNTs in the electrode and
explain the Li-insertion process. Further, AC impedance
spectroscopy of the electrode indicated the Li charge/discharge
after the fifth cycle was highly reversible.
[0029] The electrodes taught in this description with reference to
the attached figures may be utilized in nearly any electrochemical
device including electrodes/layers in batteries ultra capacitors,
fuel cells, water-splitting electrodes, and other energy
storage/discharge devices and in electrochromic devices such as
smart windows and the like. FIG. 1A illustrates simplistically and
generally such an electrochemical device 100 that may be in the
form of a stack or otherwise include a number of layers of
materials that provide particular functions. For example, the
device may be an energy storage device with a pair of electrodes
(an anode and a cathode) separated by an electrolyte layer. As
shown, the device 100 includes a layer of electrode material or
electrode 110 that may be configured to be binder-free. In
contrast, a conventional electrode for a battery or storage device
may have an active material (e.g., about 80 percent by weight), a
conductive additive (e.g., 10 percent by weight of carbon black or
the like), and a binder (e.g., 10 percent by weight of a polymer)
that holds the active material and conductive additive together and
assists in binding with adjoining layers/substrates.
[0030] FIG. 1B illustrates a functional section view of the
electrode 110 of electrochemical device 100 that shows that the
main two components/portions of the electrode are an electrically
conductive, connective matrix/net 170 (with some impurities
typically also being present in relatively small amounts) and an
electrochemically active material 140. The active material 140
contributes the electrochemical capacity of the device 100 while
the connective matrix 170 acts to electrically connect and support
the active material 140 and eliminates the need for binder
materials, and, as a result, substantially more active material 140
may be provided in the electrode 110 increasing, for example, its
energy density.
[0031] In one example, the connective matrix 170 is provided as
about 2 to about 30 percent weight of the electrode material and is
an electrically conductive material or additive such in
nanomaterial form. For example, but not as a limitation, the
connective matrix 170 may take the form of carbon nanomaterial. In
some cases, the nanomaterial may be fullerenes (such as buckyballs
or cylindrical fullerenes) and/or nanotubes (e.g., SWNTs,
double-wall nanotubes, multi-wall nanotubes, or the like). In some
specific implementations, the connective matrix 170 is formed of
carbon SWNTs provided at 2 to 30 percent by weight (with some more
particular examples using 5 to 30 percent by weight carbon SWNTs
such as about 5 to 10 percent by weight).
[0032] The active material 140 may also take a variety of forms to
provide the electrode 110 such as a metal oxide nanomaterial
provided as 70 to 98 percent by weight of the electrode material
(e.g., all or substantially all of the material of the electrode
110 not provided or made up of the matrix 170 such as about 90 to
95 percent by weight when the matrix 170 provides about 5 to 10
percent by weight of the electrode 110). In one example, the
nanomaterial of the active material is provided in the form of a
metal oxide nanoparticles, and, more specifically, nanorods of iron
oxide. In other examples, though, the nanomaterials are
nanoparticles (such as but not limited to nanorods) of silicon,
tin, molybdenum oxide, vanadium oxide, manganese oxide, nickel
oxide, cobalt oxide, lithium cobalt oxide, lithium manganese oxide,
lithium iron phosphate, lithium manganese phosphate, graphite,
carbon, nanographite, mixed metal oxide, mixed metal, and/or a
combination thereof.
[0033] FIG. 2 illustrates a particular device 200, e.g., a battery
that may take the form of a lithium-ion battery in practice. The
battery 200 includes a container 250, an anode 210, a cathode 220,
an electrolyte 230, and a separator 240 (optional and may be a
polyethylene or the like component). The anode 210 may include a
negative current collector layer 212 such as metal foil (e.g.,
copper foil or the like) and an electrode according to the present
description may be provided as shown at 214, e.g., a layer of
material including a nanomaterial connective matrix and a
nanoparticle active material portion substantially uniformly
distributed about the matrix. The anode/electrode 214 is adhered to
the surface of the collector 212 with the matrix material rather
than an additional binder.
[0034] The cathode 220 may include a positive current collector
layer 222 and an active material 224 disposed thereon (which may be
formed in the manner of the electrodes described herein or in other
ways as known in the battery industry such as using a lithium foil
or lithium transition metal oxide when the battery 200 is a
lithium-ion battery). The anode 210, the cathode 220, the
electrolyte 230, and the separator 240 are positioned within the
container 250, and a negative terminal 216 and a positive terminal
226 are disposed on or electrically connected to the negative
current collector layer 216 of the anode 210 and the positive
current collector layer 222 of the cathode 220 to allow connection
of the battery 200 to an electrical circuit/power use or storage
circuit (not shown).
[0035] At this point, it may be useful to provide a more specific
example of a design for an electrode (e.g., electrode 110 or 214 of
FIGS. 1A to 2) for use in an electrochemical device. In one
example, an electrode was provided by suspending Fe.sub.3O.sub.4
nanorods into a conductive and durable matrix made with long,
crystalline SWNTs. There was no polymer binder provided in the
electrode, and this enabled or allowed a significantly higher
loading of active material (i.e., the 10 percent or so by weight of
the electrode that would be typically taken up by binder could be
replaced by active material). The elimination of binder also
allowed high volumetric capacities to be achieved with the
electrode design. Further, due to the ballistic conductance and
high strength of SWNTs, the electrode had the ability to
accommodate large volume changes and also provided very significant
improvements in rate capability, with stable capacity of 600 mAh/g
at 10 C in one example.
[0036] In one electrode formation process, a suspension containing
FeOOH nanorods and carbon SWNTs was employed to make a
nano-Fe.sub.3O.sub.4 electrode via vacuum filtration. The FeOOH
nanostructured precursor (e.g., with widths of 40 nm, lengths of
250 nm, and thicknesses of 20 nm) was formed from the reaction of
FeCl.sub.3 and NaOH in a hydrothermal process. An XRD spectra of
the as-prepared nanorods and reference .alpha.-FeOOH phase
(goethite, JCPDS 81-0463) are shown in the graph 300 of FIG. 3 at
310. "JCPDS" signifies "Joint Committee on Power Diffraction
Standards." In the XRD spectra of 330, all of the reflection peaks
can be indexed to the tetragonal .alpha.-FeOOH phase. After heating
the FeOOH nanorods to 450.degree. C. in an argon atmosphere, a
mixture of .alpha.-Fe.sub.2O.sub.3 (hematite) and Fe.sub.3O.sub.4
(magnetite) was observed. The spectra or graph portion shown at 320
shows the XRD patterns of the heated product and reference pattern
of .alpha.-Fe.sub.2O.sub.3 (JCPDS 33-0664). Some reflection peaks
(shown with "*" symbols) from the heated product can be indexed to
the Fe.sub.3O.sub.4 phase (JCPDS 88-0315) shown at 330 of graph
300. The electrode made with the hydrothermal precursor (FeOOH) and
carbon SWNTs was then heated identically (or substantially
identically) to the Fe.sub.3O.sub.4 nanorods. Complete reduction to
Fe.sub.3O.sub.4 was obtained as indicated in the XRD spectra 330 of
graph 300 of FIG. 3.
[0037] For one of the SWNT purification methods employed to form
the electrodes described herein a small amount of non-nanotube
carbon and metal catalyst still remained after processing. Any
nanotube purification process may be employed prior to or after the
claimed/described inventive fabrication processes or steps
described herein. It is speculated that any remaining non-nanotube
impurities are oxidized and actually may enable the complete
reduction to the pure Fe.sub.3O.sub.4 phase observed at this
relatively low temperature.
[0038] The Fe.sub.3O.sub.4 nanorod/SWNT electrode was characterized
with scanning electron microscopy (SEM). FIG. 4A shows a surface
SEM photograph/image 400 of the electrode which includes bundles or
sets of carbon nanotubes 410 forming a conductive net or connective
matrix for active material in the form of iron oxide nanorods 120
(not shown in FIG. 4A). FIG. 4B shows a cross-section SEM
photograph/image 440 of the electrode. Note, the Fe.sub.3O.sub.4
nanorods 420 with an average width of 100 nm are dispersed with a
regular pore structure. We believe that the removal of H.sub.2O and
reduction of Fe (III) to Fe (II) contribute to the formation of the
porous structure. This porosity then aids the diffusion of Li.sup.+
ions in the electrode. The SEM images 400, 440 also show that small
bundles of SWNTs 410 are interlaced with the Fe.sub.3O.sub.4
nanorods 420. As shown, the Fe.sub.3O.sub.4 nanorods 420 are
substantially uniformly suspended in the SWNT connective matrix
provided by the carbon SWNTs 410. A few sub-10 nm metal particles
(Ni, Co) from the laser vaporization process may also be detected
in the SEM images 400, 440.
[0039] Raman spectroscopy was also employed to characterize the
Fe.sub.3O.sub.4 nanorod/SWNT electrode. As part of the
characterization, pure SWNTs were found in the electrode with Raman
excitation at 632.8 nm. Raman spectra of the SWNT materials or
connective matrix revealed resonantly enhanced tangential bands
between 1500-1600 cm.sup.-1 (G-bands) as well as a broad band at
.about.1350 cm.sup.-1 attributed to a convolution of the
disorder-induced band (D-band) of carbon impurities and the D-band
of the SWNTs themselves. The carbon tangential vibrations were
strong first-order bands, which included six components with
2A+2E.sub.1+2E.sub.2 symmetries arising from curvature-induced
splitting of the tangential E.sub.2g mode of graphite. The line
shapes and widths of these modes may vary significantly, depending
on how close the laser excitation energy is to the nanotube
resonance and whether the nanotube is semiconducting or metallic
(e.g., by a semi-conducting material provided as the matrix is
considered to be electrically conductive in this description and in
the following claims).
[0040] Typically the semi-conducting bands are fit with multiple
Lorentzians to describe the six Raman active modes, and the
metallic tubes are fit with only two peaks, e.g., a Lorentzian line
shape describing the dominant higher-frequency feature and a
Breit-Wigner-Fano (BWF) line describing the dominant lower
frequency feature. The G-band features in the spectra of the pure
SWNTs indicate that both semiconducting and metallic nanotubes were
present in the purified sample (e.g., the sample includes
electrically conductive nanomaterial in the form of semiconducting
and metallic carbon SWNTs). It is believed that the inventive
electrode and manufacturing processes may be implemented with
nanotube samples that are enriched with metallic or semiconducting
nanotubes and/or with nanotubes that are functionalized or have
modified electronic properties. Also, the intensity of the D-band
in the purified sample suggested the presence of some non-nanotube
carbon. Changes in the Raman spectrum of the SWNTs in the electrode
after annealing to 450.degree. C. were clearly observed. The loss
of intensity of the D-band relative to the G-bands was consistent
with the oxidation of some non-nanotube carbon. The change in shape
for the G-bands suggested that there was some charge transfer
between the nanotubes and the Fe.sub.3O.sub.4 nanorods and that the
charge transfer preferentially occurred with the semiconducting
nanotubes in the electrode. The quenching of the Raman lines in the
cycled electrodes was consistent with charge transfer from Li.sup.+
that was inserted irreversibly. In situ Raman measurements were
also made where the Raman nanotube lines return upon charging the
battery to a particular voltage and allowing for the removal of
irreversibly inserted Li.sup.+.
[0041] In another testing process for a prototyped electrode and
electrochemical device, the electrochemical performance of the
electrodes was characterized using galvanostatic cycling in a coin
cell (not shown) with Li metal as the negative electrode. A portion
of the testing/analysis of the electrode design is shown in the
graph 500 of FIG. 5. The Li.sup.+ insertion process in the first
two cycles was examined for: (1) a pure SWNT electrode that is
labeled "SWNT" and shown at 510; (2) a conventional Fe.sub.3O.sub.4
electrode that is labeled "micro1" and shown at 520 (e.g., the
micro1 electrode may include 5 .mu.m Fe.sub.3O.sub.4 nanorods,
which may be obtained commercially from Aldrich or other
distributors and be made by mixing the commercial Fe.sub.3O.sub.4
with acetylene black (AB) and poly vinylidene fluoride (PVDF) at a
weight ratio of 80: 8:12); (3) a nano-Fe.sub.3O.sub.4 electrode
fabricated according to this description with about 5 percent by
weight carbon SWNTs and about 95 percent Fe.sub.3O.sub.4 nanorods
as the active material that is labeled as "nano" and shown at 530;
and (4) to further study the effect of particle size on the
electrochemical performance, the same fabrication procedure for the
nano-Fe.sub.3O.sub.4 electrode was also applied to commercial
Fe.sub.3O.sub.4 (e.g., 5 .mu.m, bought from Aldrich), with is
labeled as "micro2" and shown at 540 in FIG. 5.
[0042] All the electrodes were tested under identical conditions in
a lithium coin cell. FIG. 5 displays the voltage-composition curves
510, 520, 530, 540 of these four electrodes in graph 500. The cells
are cycled here at a low rate of 0.1 C (8 Li.sup.+ insertion per
formula unit in 10 hours) within a 0.005-3.0 V voltage limit. A
well-defined plateau is observed at 0.8V, attributed to the
reduction process of Fe.sub.3O.sub.4 into Fe (II) and Fe (0), for
the first discharge curves of "micro1" 520, "micro2" 540, and
"nano" 530. The voltage of the SWNT electrode shown at 510 rapidly
reaches a plateau at 1.0 V before a smooth and long slope where
Li.sup.+ intercalation into carbon occurs. The large surface area
and the solid electrolyte interphase (SET) formation on SWNTs are
the major causes of the large irreversible capacity. During
charging, there is no obvious plateau below 0.2 V where Li
extraction from graphitic carbons usually generally occurs. The
Raman data collected discussed above is consistent with
irreversible Li ion insertion in the SWNT bundles. Thus, different
Li.sup.+ insertion/extraction mechanisms appear to be operating in
the SWNTs, which will be discussed later.
[0043] Compared to the potential curves 520, 540 of the
micro-Fe.sub.3O.sub.4 electrodes (micro1, and micro2), the voltage
in the first discharge curve 530 of the nano-Fe.sub.3O.sub.4
electrode drops stepwise before the conversion plateau is observed
at 0.8V. Lr consumption for the formation of the SET layer on the
SWNTs and Fe.sub.2O.sub.3 nanorods may account for the short
plateau around 1 V. Li.sup.+ intercalation into the spinel
structure of nano-Fe.sub.3O.sub.4 and was considered as potentially
explaining the smooth slope plus a plateau-like step at 1.2V (e.g.,
which may be an effect driven by the particle size on the reduction
process). The first discharge capacities for the
nano-Fe.sub.3O.sub.4 electrode and commercial Fe.sub.3O.sub.4
electrodes (micro 1 and 2) appear to be higher than the theoretical
capacity of Fe.sub.3O.sub.4 expected for the reduction of
Fe.sub.3O.sub.4 by 8 Li.sup.+. The initial coulombic efficiency of
the nano-Fe.sub.3O.sub.4 electrode is .about.75%, which is higher
than the .about.55% coulombic efficiency observed for the
commercial Fe.sub.3O.sub.4 electrodes (micro2). SET formation on
SWNTs and amphorization of microsized Fe.sub.3O.sub.4 account for
the large irreversible capacity of the microsized Fe.sub.3O.sub.4
electrode (micro2). After the first discharge, the
nano-Fe.sub.3O.sub.4 electrode reaches the theoretical capacity and
has a similar voltage profile to the micro Fe.sub.3O.sub.4
electrodes, indicating a similar conversion reaction during
charge/discharge.
[0044] Further, the subsequent 50 cycles were tested at a high
current rate of 1 C (8 Li per formula unit in 1 hour). FIG. 6 shows
with graph 600 the Li.sup.+ insertion and extraction capacity per
unit mass of the Fe.sub.3O.sub.4 particles versus cycle number for
the nano electrode at 610, for the micro2 electrode at 630, and for
the micro1 electrode at 620. The graph 600 provides a comparison of
the specific capacity for various Fe.sub.3O.sub.4 electrodes (the
proposed nano electrode 610, the micron-size Fe.sub.3O.sub.4
electrode made by the new method/micro2 shown at 630, and the
micron-size Fe.sub.3O.sub.4 electrode made with AB and PVDF as
binder/micro1 shown at 620). The capacity of the conventional
Fe.sub.3O.sub.4 electrode (micro 1) with AB and PVDF deteriorates
after the first cycle and loses approximately 50 percent of the
initial charge capacity after only 50 cycles. The volume expansion
due to the conversion reaction mitigates the structural integrity
of the electrode made with AB and PVDF, and results in a
degradation of capacity. Although the capacity of the second
commercial Fe.sub.3O.sub.4 electrode (micro 2) decreases to about
600 mAh/g after cycling at 1 C rate, the capacity remains at 600
mAh/g for 50 cycles. This demonstrates that the new fabrication
method greatly improves the cycling performance even for the
micro-size material and that it may be important to any battery
electrode technology.
[0045] The best result in this test was obtained in the
nano-Fe.sub.3O.sub.4/SWNT electrode (nano shown at 610). After the
first 3 cycles at the low cycling rate of 0.1 C, the capacity still
remains constant and then slightly increases while cycling at 1 C
rate. The rise in capacity is not surprising for the
nano-Fe.sub.3O.sub.4 electrode, although it is very rarely observed
when an intercalation mechanism is occurring. One reason may be
that a gel-like film from the decomposition of the electrolyte at
low voltage plays a crucial role in the capacity increase. The
intrinsic properties of SWNTs coupled with the "conductive SWNT
net" or electrically conductive, connective matrix/net allow for
volume expansion and improved conductivity. During testing, it was
found that the conductivity was increased from 1500 ohms/square for
the commercial Fe.sub.3O.sub.4 electrode (micro1) to 50 ohms/square
for the nano-Fe.sub.3O.sub.4 electrode with 5 wt. % SWNTs
(nano).
[0046] As shown in the graph 700 of FIG. 7, impedance measurements
on the fresh and cycled cells are presented in Nyquist polts (Z' vs
-Z'') at 710 for a fresh cell, at 720 for a cell after 5 cycles,
and at 730 for a cell after 10 cycles. The profiles 710, 720, and
730 were obtained at open circuit voltage in the frequency range
from 10 mHz to 100 kHz. The depressed semicircle in the high
frequency range followed by a straight line in the low-frequency
range is observed for the cell. Differing from the Warburg
impedance that usually exhibits a 45.degree. slope straight line,
the straight line observed here indicates capacitive behavior. The
capacitive behavior may be attributed to lithium intercalation in
the SWNTs. The size of the semicircle for the cycled cell is
smaller than that for the initial cell, indicating that the barrier
to the charge transfer process is reduced. The Nyquist plot 730
after the 10.sup.th cycle resembles that of the fifth cycle. This
appears to confirm that the Li.sup.+ insertion/extraction in the
electrode is reversible.
[0047] The truly high rate capability of the
nano-Fe.sub.3O.sub.4/SWNT electrode taught by this description is
displayed in FIG. 8 with graph 800 showing plot 810, 820, and 830
for electrodes having differing amounts of matrix/net material
(e.g., 5, 10, and 30 percent by weight, respectively, of carbon
SWNTs in this example). The cells were cycled at different rates
from 0.1 C to 10 C in the voltage range from 0.005 V to 3 V. The
electrodes were fabricated with various ratios of nano active
material and carbon SWNTs as shown and were tested to establish the
effect of the SWNTs for high-rate capability. The cycling behavior
for nano-Fe.sub.3O.sub.4 electrodes with 5% SWNTs, 10% SWNTs, and
30% SWNTs are shown in FIG. 8 at 810, 820, and 830, respectively.
The best performance in this particular test or implementation was
obtained by using the nano-Fe.sub.3O.sub.4 electrode with 5% SWNTs.
This cell had a capacity of 550 mAh/g at 10 C (i.e., 8 Li per
formula unit in 6 minutes), while the cell with 30% SWNTs had a
capacity of 200 mAh/g after 60 cycles with increased rate. The
electrode with 30% SWNTs also had a larger irreversible
capacity.
[0048] Based on these test results, there appears to be a
significant ratio between the SWNTs and the nano active material.
At high SWNT loadings, the nanotubes agglomerate into larger
bundles in providing the connective matrix, leading to an uneven
distribution of SWNTs and Fe.sub.3O.sub.4 nanorods that results in
a loss of electrical conductivity and a reduction in the rate
capability. The SEI formation on the electrode containing 30% SWNTs
accounts for the higher irreversible capacity. The durability of a
nano-Fe.sub.3O.sub.4 electrode was also tested with 5% SWNTs at a
high rate of 5 C (4360 mAg.sup.-1) and was found acceptable. The
capacity at 5 C rate is 850 mAh/g in the initial cycle and slowly
decreases to 790 mAh/g after 60 cycles.
[0049] To summarize these testing results, a uniform suspension of
Fe.sub.3O.sub.4 nanorods in a "conductive net" made with carbon
SWNTs was fabricated via simple vacuum filtration. The highest
reversible capacity was obtained using about 5 percent by weight
carbon SWNTs in the binder-free electrode. The reversible capacity
of the anode reaches 1000 mAh/g at C rate and is sustained over 100
cycles with a useful volumetric capacity. Furthermore, the
electrodes provide a high rate capability and a stable capacity of
approximately 800 mAh/g at 5 C (60 cycles) and a stable capacity of
approximately 600 mAh/g at 10 C. These results suggest that the
described and suggested nano-Fe.sub.3O.sub.4/SWNT electrode is a
promising candidate for an anode in high-performance Li-ion
batteries for electric vehicles as well as an electrode layer for
many other electrochemical devices.
[0050] FIG. 9 illustrates one method 900 for fabricating an
electrochemical device with a nano-metal oxide active material,
binder-less electrode described herein. The method 900 begins at
904 such as by designing of a particular device to be fabricated
such as lithium-ion battery or the like, and this step 904 may
include defining energy densities desired, durabilities, rates, and
other physical and/or operating parameters for the device. With
this design information in mind, the method 900 continues at 910
with selection of an electrochemically active material and/or its
precursor. For example, it may be desirable to provide a particular
metal oxide in the form of nanorods as the active material of the
electrode, and step 910 may include selecting this material and
also the precursor to obtain the active material (e.g., iron
oxyhydroxide nanorods may be used as a precursor for forming iron
oxide nanorods). At 916, the electrochemically active material may
be produced (such as by producing iron oxide nanorods from iron
oxyhydroxide nanorods) or simply provided as the output of another
process or as obtained commercially from a distributor.
[0051] At 920, the method 900 may include selecting a material for
use as the connective matrix of the electrode. Typically, this may
be nearly any semiconductor or conductor nanomaterial or
nanoparticles such as carbon in the form of single, double, or
multi-wall nanotubes or in the form of fullerenes (e.g., buckyballs
or the like), carbon fiber, grapheme, and/or any carbon based
nanostructured material including doped carbon nanostructures,
e.g., boron or nitrogen-doped nanotubes and/or BCN nanostructures
or the like, and it may be selected to suit the chosen active
material (or vice versa). The carbon nanotubes or other
nanoparticles may be formed at 924 (or simply obtained from a
distributor or as the output of a separate process) in a variety of
ways such as via laser vaporization, chemical vapor deposition
(CVD), plasma enhanced CVD, wet chemical synthesis, arc generated,
hot-wire CVD, and/or other techniques. The processing may result in
a variety of impurities that may be allowed to remain for use in
the electrode (such as some amount of carbon MWNTs and fullerenes
in a volume of carbon SWNTs) or be removed as undesirable
impurities (e.g., to obtain relatively high purities such as
greater than 90 percent SWNTs or even up to 99.5 percent or higher
purity carbon SWNTs in some cases).
[0052] At 930, the active material and the connective matrix
material are combined in predefined weight ratios so as to provide
a substantially uniform mixing or distribution of the carbon SWNTs
or other active material in the net or matrix formed by the
conductive additive. For example, the matrix material may be
provided at step 930 at 2 to 30 percent by weight while the active
material would make up the rest of the electrode material (e.g., 98
to 70 percent by weight). In some embodiments, the matrix material
makes up about 5 to 10 percent by weight of the electrode such as
in some carbon SWNT implementations useful for lithium-ion
batteries and other electrochemical devices. At 940, the mixed
electrode material is applied such as via vacuum filtration and
then transfer to a substrate such as a conductive foil or the like.
At 950, the method 900 includes performing additional processing,
such as heating and/or cutting/shaping to form an electrode layer
or electrode of particular size and shape to suit a device stack or
device. At 960, the method 900 includes providing or installing the
electrode in an electrochemical device such as in an energy storage
stack on abutting an electrolyte or the like. The method 900 then
may end at 990.
[0053] It may be useful at this time to more fully describe
fabrication of a nano-Fe.sub.3O.sub.4 electrode that was tested as
described above. It will be understood, though, that the
fabrication techniques may be modified to allowing scaling up to
support commercial fabrication of electrodes and electrochemical
devices with such electrodes. In one particular implementation,
iron oxyhydroxides (FeOOH) nanorods were employed as a precursor to
make iron (II, III) oxide (Fe.sub.3O.sub.4) electrodes with a
hydrothermal process. Iron (III) chloride (FeCl.sub.3, 2.5 mmol)
was dissolved in 7 ml of distilled water and heated to 50.degree.
C. Sodium hydroxide (NaOH, 10 mmol) was dissolved in 5 ml of
distilled water and then added to the above iron chloride solution
"dropwise." The mixture was stirred for 30 minutes to form a
homogeneous gel before it was transferred into a Teflon-lined
stainless steel autoclave. The reaction was maintained at
160.degree. C. for 24 hours. After the reaction was completed, the
resulting yellow solid product was rinsed with distilled water and
dried at 80.degree. C. under vacuum.
[0054] Raw material containing single-wall carbon nanotubes was
produced by a known laser vaporization techniques. Large
agglomerations of amorphous and non-nanotube carbon and metal
nanoparticles (Ni, Co) were observed in the as-produced raw SWNTs.
Most of the non-nanotube carbon and metal catalyst particles were
simply removed by an HNO.sub.3 reflux/air oxidation procedure.
Vacuum filtration, e.g., techniques known for use to prepare a
carbon nanotube film, was used to fabricate the
nano-Fe.sub.3O.sub.4/SWNT electrode. FeOOH nanorods from the
hydrothermal process and SWNTs were suspended in 1% sodium dodecyl
sulfate (SDS) solution and sonicated for 15 minutes before vacuum
filtration. The mixture was rinsed three times with deionized water
and then transferred to copper foil pretreated with a water and
ethanol rinse. The electrode was baked in an argon atmosphere at
450.degree. C. for 1 hour to complete the conversion from FeOOH to
Fe.sub.3O.sub.4.
[0055] Regarding characterization of the formed electrode, the
analysis to confirm the uniform distribution of Fe.sub.3O.sub.4 was
investigated by a scanning electron microscope (FEI NOVA 630).
X-ray diffraction data were collected on a powder Scintag X-ray
Diffractometer operating at 45 kV and 36 mA and using Cu-K.alpha.
radiation. Raman spectroscopy was performed using 632.8 nm (1.96
eV) laser excitation. The back-scattered light was analyzed with a
Jobin Yvon 270M spectrometer equipped with a liquid-nitrogen-cooled
Spectrum One CCD and holographic notch filters. Averaging three
30-second scans was sufficient to obtain high intensity,
well-resolved Raman spectra. Coin cells were assembled in an
argon-filled dry box using the binder-free electrode as the
positive electrode and Li metal as the negative electrode. A
Celgard separator and 1 M LiPF.sub.6 electrolyte solution in
EC:DEC/1:1 (mass ratio) purchased/available from Nanolyte were used
to fabricate the coin cells. The cells were first galvanostatically
cycled between 3 and 0.005 V for three cycles at a rate of 0.1 C (8
Li.sup.+ per formula unit in 10 hours), which was used to complete
the conversion reaction with lithium. Subsequently, the cells were
cycled at different rates. All electrochemical impedance spectra
were obtained using the computer-interfaced VMP3 (Biologic Claix
France) potentiostat with a 5 mV AC signal ranging from 10 mHz to
100 kHZ.
[0056] While a number of exemplary aspects and embodiments have
been discussed above, those of skill in the art will recognize
certain modifications, permutations, additions, and
sub-combinations thereof. It is therefore intended that the
following appended claims and claims hereafter introduced are
interpreted to include modifications, permutations, additions, and
sub-combinations to the exemplary aspects and embodiments discussed
above as are within their true spirit and scope. The term
non-nanoparticles is intended to include at least micron-sized or
scale particles and non-nano sized single crystals (such as
millimeter-sized particle single crystals and the like).
* * * * *