U.S. patent application number 14/452991 was filed with the patent office on 2014-11-27 for method of depositing silicon on carbon materials and forming an anode for use in lithium ion batteries.
The applicant listed for this patent is Applied Sciences, Inc.. Invention is credited to David J. Burton, Max L. Lake, Maryam Nazri.
Application Number | 20140349186 14/452991 |
Document ID | / |
Family ID | 39872537 |
Filed Date | 2014-11-27 |
United States Patent
Application |
20140349186 |
Kind Code |
A1 |
Burton; David J. ; et
al. |
November 27, 2014 |
METHOD OF DEPOSITING SILICON ON CARBON MATERIALS AND FORMING AN
ANODE FOR USE IN LITHIUM ION BATTERIES
Abstract
A method of modifying the surface of carbon materials such as
vapor grown carbon nanofibers is provided in which silicon is
deposited on vapor grown carbon nanofibers using a chemical vapor
deposition process. The resulting silicon-carbon alloy may be used
as an anode in a rechargeable lithium ion battery.
Inventors: |
Burton; David J.;
(Waynesville, OH) ; Lake; Max L.; (Yellow Springs,
OH) ; Nazri; Maryam; (Bloomfield Hills, MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Applied Sciences, Inc. |
Cedarville |
OH |
US |
|
|
Family ID: |
39872537 |
Appl. No.: |
14/452991 |
Filed: |
August 6, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12107254 |
Apr 22, 2008 |
8828481 |
|
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14452991 |
|
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60913321 |
Apr 23, 2007 |
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Current U.S.
Class: |
429/217 ;
427/113; 429/231.4; 429/231.8 |
Current CPC
Class: |
H01M 4/13 20130101; H01M
4/0423 20130101; H01M 4/623 20130101; H01M 4/38 20130101; Y02T
10/70 20130101; Y02E 60/10 20130101; B82Y 30/00 20130101; C23C
16/24 20130101; B82Y 10/00 20130101; H01M 4/622 20130101; H01M
10/0525 20130101; H01M 4/386 20130101; H01M 10/052 20130101; H01M
4/366 20130101; H01M 2004/027 20130101; H01M 4/0421 20130101; H01M
4/587 20130101; H01M 4/139 20130101 |
Class at
Publication: |
429/217 ;
429/231.8; 429/231.4; 427/113 |
International
Class: |
H01M 4/04 20060101
H01M004/04; H01M 4/38 20060101 H01M004/38; H01M 4/587 20060101
H01M004/587; H01M 4/36 20060101 H01M004/36; H01M 4/62 20060101
H01M004/62 |
Claims
1. An anode for use in a lithium ion battery, said anode formed by:
providing a carbon material selected from vapor grown carbon
fibers, vapor grown carbon nanofibers, and PAN or pitch derived
carbon fibers; heating said carbon material at a temperature
between about 100.degree. C. to about 1200.degree. C.; flowing a
silicon-containing precursor gas in contact with said carbon
material for a time sufficient for said gas to decompose and form a
coating on at least the surface of said carbon material; and adding
a binder to said silicon-coated carbon material.
2. The anode of claim 1 wherein said precursor gas comprises
silane, a blend of silane and hydrogen, or a blend of silane and an
inert gas.
3. The anode of claim 1 wherein said carbon material comprises
vapor grown carbon nanofibers.
4. The anode of claim 3 wherein said vapor grown carbon nanofibers
have been heated treated at a temperature above 700.degree. C.
5. The anode of claim 1 wherein said silicon coating comprises
crystalline silicon, amorphous silicon, or silicon compounds.
6. The anode of claim 1 wherein said carbon material further
includes a carbide material selected from metal carbides, silicon
carbides, and silicon oxides.
7. The anode of claim 1 wherein said carbon material further
includes a carbon or graphite additive selected from single-walled
carbon nanotubes, multi-walled carbon nanotubes, exfoliated
graphite flakes, graphite platelets, graphene particles, carbon
black, and mesocarbon microbeads.
8. The anode of claim 1 wherein said carbon material further
includes a conductive additive comprising macroscopic vapor grown
carbon nanofibers having a diameter of from about 500 nm to 10
micrometers.
9. The anode of claim 1 wherein said carbon material has a length
of from about 1 to about 500 micrometers.
10. The anode of claim 1 wherein said carbon material is in the
form of a composite or preform.
11. The anode of claim 1 wherein said silicon is coated onto said
carbon material at a thickness of about 0.001 and 100 microns.
12. The anode of claim 1 wherein said silicon is coated onto said
carbon material at a thickness of about 2 to 100 nm.
13. The anode of claim 1 wherein said silicon-coated carbon
material has a graded interface.
14. The anode of claim 1 wherein said binder is selected from
polyvinylidene difluoride, EPDM, and polystyrene.
15. The anode of claim 1 having an electrical conductivity of from
about 0.01 to about 0.5 ohm/cm.
16. The anode of claim 1 having an irreversible capacity of from
about 5% to 40% of total capacity.
17. The anode of claim 1 having a reversible capacity of at least
350 mAH/g.
18. The anode of claim 1 having a reversible capacity of at least
1000 mAH/g.
19. The anode of claim 1 having a thermal conductivity of at least
50 w/m-K up to 1000 w/m-K.
20. A lithiated carbon-silicon alloy formed by providing a carbon
material selected from vapor grown carbon fibers, vapor grown
carbon nanofibers, PAN or pitch derived carbon fibers, graphite
flakes, graphene platelets, and carbon nanotubes; heating said
carbon material at a temperature between about 100.degree. C. to
about 1200.degree. C.; flowing a silicon-containing precursor gas
in contact with said carbon material for a time sufficient for said
gas to decompose and form a coating on at least the surface of said
carbon material; and depositing lithium on the silicon-coated
carbon material by evaporation.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. application Ser.
No. 12/107,254, filed Apr. 22, 2008, which claims the benefit of
U.S. Provisional Application No. 60/913,321, filed Apr. 23, 2007.
The entire contents of said applications are hereby incorporated by
reference.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a method of depositing
silicon onto a carbon material such as carbon nanofibers or
composites formed from carbon nanofibers to form an alloy which can
undergo lithiation/delithiation and which may be used as an anode
in lithium ion batteries.
[0003] The use of lithium ion batteries as rechargeable power
sources represent a promising technology for use in the development
of consumer electronics and electric-based vehicles as they can
replace traditional aqueous batteries such as lead-acid, nickel
metal hydride, nickel-cadmium, and nickel hydride batteries.
[0004] Current lithium ion batteries typically use lithium cobalt
oxide as the cathode and carbon or graphite as the anode. Efforts
have been made to increase the energy density and power capability
of the anode in lithium-ion batteries to provide improved operating
features for electric and/or hybrid-type vehicles, cordless power
tools, and electronics. For example, recent research indicates that
anodes formed from nanocarbon materials can provide increases in
both energy storage and power capability. Some single- and
multi-walled carbon nanotubes have shown reversible intercalation
capacities in excess of LiC.sub.6.
[0005] Another area of interest in recent years has been the
investigation of metals or alloys that will form alloys with
lithium, as such materials are known to store as much as eleven
times the energy of current negative electrodes made of carbon
alone. Silicon, which has a theoretical capacity of up to 4200
mAh/g, is one such material. Carbon-silicon alloys have previously
been formed through various milling processes and through solution
deposition of siloxanes onto graphite.
[0006] However, the use of such carbon-silicon alloys has been
limited for use in lithium batteries as they undergo a significant
volume change as they incorporate and release lithium during charge
and discharge. As silicon undergoes an approximate 300% volume
expansion when fully charged, alloys containing silicon can
fragment and lose electrical contact with the anode as the result
of these volume changes. This phenomenon is particularly
destructive when the active materials are in the form of
particulates, frequently resulting in a rapid loss of capacity upon
cycling.
[0007] Furthermore, the development of batteries designed for high
charge/discharge rates show evidence of heat retention in the
battery cell, which can ultimately degrade the performance of the
battery cell. High thermal conductivity composites have been
fabricated which facilitate heat transfer through the composite.
See, for example, U.S. Pat. No. 5,837,081, which teaches the use of
vapor grown carbon fibers to fabricate high thermal conductivity
composites. Use of high thermal conductivity materials in the
fabrication of the anode would serve to eliminate heat retention or
heat build-up within the battery cell as it is subjected to high
charge and discharge rates.
[0008] More recently, surface modification of carbon fibers has
been achieved by coating with materials such as silicon to provide
a high thermal conductivity network and provide the ability to
survive repeated thermal cycling. See, for example, U.S. Pat. No.
6,988,304, which teaches modification of vapor grown carbon fibers
for the purpose of forming a composite structure for containing a
phase change material for use in aircraft brakes. It would be
desirable to use a surface modification process on carbon
substrates or composite preforms for use in lithium ion
batteries.
[0009] Accordingly, there is still a need in the art for a method
of modifying carbon materials which can be used to make an improved
anode for use in a lithium ion battery.
SUMMARY OF THE INVENTION
[0010] The present invention meets that need by providing a method
of depositing a durable silicon coating or layer on carbon
materials such as vapor grown carbon fibers, vapor grown carbon
nanofibers, conventional carbon fibers, graphite flakes, graphene
platelets, carbon nanotubes, or composites formed from these
materials. The resulting alloy may be used as an anode in a lithium
ion battery.
[0011] According to one aspect of the present invention, a method
is provided for depositing silicon onto a carbon material to form
an alloy for use in lithium ion batteries. The silicon coating may
comprise crystalline silicon, amorphous silicon, or silicon
compounds such as silicon carbide and silicon oxide.
[0012] The method utilizes a chemical vapor deposition process and
includes providing a carbon material selected from vapor grown
carbon fibers, vapor grown carbon nanofibers, PAN or pitch derived
carbon fibers, graphite flakes, graphene platelets, and carbon
nanotubes; heating the carbon material at a temperature between
about 100.degree. C. to about 1200.degree. C., and flowing a
silicon-containing precursor gas in contact with the carbon
material for a time sufficient for the gas to decompose and form a
coating on at least the surface of the carbon material.
[0013] The precursor gas is selected from silane, a blend of silane
and hydrogen, or a blend of silane and an inert gas.
[0014] The carbon material preferably comprises vapor grown carbon
nanofibers. The vapor grown carbon nanofibers are preferably heated
treated at a temperature above 700.degree. C. prior to use in the
chemical vapor deposition method.
[0015] Carbon or graphite additives may be added to the carbon
material prior to the deposition process to increase the electrical
conductivity of the resulting anode and provide additional
capacity. Such additives may be selected from single-walled carbon
nanotubes, multi-walled carbon nanotubes, exfoliated graphite
flakes, graphite platelets, graphene particles, carbon black, and
mesocarbon microbeads.
[0016] A conductive additive may also be added to the carbon
material to provide thermal conductivity and mechanical
reinforcement to the resulting anode. A preferred additive is
macroscopic vapor grown carbon fibers having a diameter of from
about 500 nm to 10 micrometers.
[0017] The carbon material may further include a carbide material
or silicon compound selected from metal carbides, silicon carbide,
or silicon oxides.
[0018] The carbon material, along with any additives, is preferably
fabricated into the form of a composite or preform prior to silicon
deposition.
[0019] The resulting silicon coating may comprise crystalline
silicon, amorphous silicon, or silicon compounds such as silicon
carbide or silicon oxides. The silicon is coated onto the carbon
substrate at a thickness of about 0.001 microns and 100 microns,
more preferably, at a thickness of about 2 to 100 nm, and most
preferably, at a thickness of about 10 to 30 nm.
[0020] The method preferably further includes forming an anode by
adding a binder to the silicon-coated carbon nanomaterial. The
binder is preferably selected from polyvinylidene difluoride, EPDM,
and polystyrene.
[0021] The resulting anode preferably has an electrical
conductivity of from 0.01 ohm/cm to 0.5 ohm/cm, and a thermal
conductivity of at least 50 W/m-K up to 1000 W/m-K. The anode has
an irreversible capacity of from less than about 5% to 40% of total
capacity, and a reversible capacity of at least 350 milliamp
hour/gram (mAH/g).
[0022] The anode produced from the method of the present invention
may be incorporated into lithium ion batteries for a number of
uses. For example, lithium-ion batteries containing the anode of
the present invention may be used to extend the range of hybrid and
electric vehicles to a more practical usage of up to 150 miles.
[0023] In another embodiment of the invention, a lithiated
carbon-silicon alloy is provided which is formed by providing a
carbon material selected from vapor grown carbon fibers, vapor
grown carbon nanofibers, PAN or pitch derived carbon fibers,
graphite flakes, graphene platelets, and carbon nanotubes, heating
the carbon material at a temperature between about 100.degree. C.
to about 1200.degree. C.; flowing a silicon-containing precursor
gas in contact with the carbon material for a time sufficient for
the gas to decompose and form a coating on at least the surface of
the carbon material; and evaporating lithium on the carbon-silicon
alloy.
[0024] Accordingly, it is a feature of the present invention to
provide a method of depositing silicon on the surface of carbon
materials such as carbon nanofibers and to an anode produced from
such modified nanofibers. Other features and advantages of the
invention will be apparent from the following description, the
accompanying drawings, and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1A is an electron microscope image of carbon nanofibers
including a silicon coating deposited in accordance with an
embodiment of the present invention;
[0026] FIG. 1B is an electron microscope image of carbon nanofibers
including a silicon coating deposited in accordance with another
embodiment of the present invention;
[0027] FIG. 1C is a transmission electron microscope image of a
carbon nanofiber which has a graded nanostructure useful for
adhering silicon; and
[0028] FIG. 2 is a graph illustrating the capacity of a
carbon-silicon anode after thermal cycling.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0029] We have found that deposition of silicon onto carbon
materials such as vapor grown carbon nanofibers allows the carbon
nanofibers to function as an insertion host as well as a high
surface area support for the silicon, connecting the carbon and
silicon electrically and accommodating volume changes resulting
from a lithiation-delithiation process when the resulting alloy is
used as an anode.
[0030] The silicon alloy material is bonded to the carbon
nanomaterial through an interphase region comprised of the alloy
material and carbon. By "interphase," it is meant the transition
region between pure carbon and pure silicon, which enhances the
adherence between these two phases. In one embodiment, the
interphase comprises silicon-carbon compounds such as silicon
carbide.
[0031] The resulting silicon-carbon alloy exhibits high energy
capacity and high power capability when used as an anode in a
lithium ion battery cell. The resulting anode provides a
significant increase in energy capacity up to 1000 mAh/g or higher
for greater than 100 charge-discharge cycles. The use of such an
anode provides a low irreversible capacity loss upon cycling.
[0032] While not wishing to be bound by theory, it is believed that
the successful retention of capacity at high cycle numbers is
indicative that the silicon coating is not fracturing and/or losing
adhesion to the carbon nanofibers. This is further indicated by the
fact that we have obtained efficiencies greater than 99.4% in
capacity retention in a half cell configuration for more than 20
cycles at rates of C/2 (charge or discharge of the cell at two
hours rate).
[0033] The method of the present invention also results in a boost
in reversible and total charge capacity over that observed with the
use of graphitic materials alone. The presence of the silicon
alloying element facilitates prelithiation by reducing the
irreversible capacity associated with the use of bare carbon
nanofibers. By "prelithiation," it is meant a process in which the
carbon material is charged with Li in a single-electrode
configuration and then transferred under inert atmosphere
conditions to be assembled into the final lithium-ion battery. This
process creates an electrode with the surface layer of tightly
bound Li which will not participate in battery cycling (the solid
electrolyte interface (SEI) layer) already in place, eliminating
the need for excess Li to compensate for the anode's irreversible
capacity.
[0034] The silicon used in the method of the present invention may
be derived from a variety of gas phase silicon bearing compounds
including, but not limited to, methyl trichlorosilane and
SiH.sub.4. Preferred precursor gases include silane, a blend of
silane and hydrogen, or blends of silane and an inert gas such as
helium, nitrogen or argon. Silicon deposition may be performed at
atmospheric pressure, at reduced pressure, or at an elevated
pressure which is raised with respect to atmospheric pressure in
order to control the rate and properties of the deposited silicon
coating.
[0035] The resulting silicon coating may comprise crystalline
silicon, amorphous silicon, or silicon compounds such as silicon
carbide or silicon oxides. Amorphous silicon is preferred as it is
believed to be deposited in a nanoscale domain such that nanoscale
particles of amorphous silicon are incorporated into a graded
interface of carbon to SiC or SiO.sub.x so that they are
mechanically bound to the surface of the fiber. Further, such
particles are sufficiently small that strain induced at the
interface with the carbon/Si compound layer during expansion and
contraction as the battery cell is subjected to charge/discharge
cycles does not cause the silicon particles to de-bond from the
fiber surface. Such a graded interface permits entrapment of
inclusions and small particles that result in a more durable
coating or layer. The silicon is coated onto the carbon substrate
at a thickness of about 0.001 to 100 microns, more preferably, at a
thickness of about 10 to 200 nm, and most preferably, about 10 to
30 nm. It should be appreciated that the nature of the coating can
vary from unconnected islands of silicon to a continuous coating.
Too little silicon will not adequately increase anode capacity
while a thick layer of silicon will exceed the strain limit for
expansion and contraction induced by charge/discharge cycling,
becoming friable and lacking durability during battery cycling.
[0036] Referring to FIG. 1A, an electron microscope image of carbon
nanofibers including a silicon coating in accordance with the
present invention are shown. In FIG. 1A, the silicon has been
deposited at low loadings such that it is deposited as small
islands or nodules on the surface of the nanofibers. At higher
loadings as illustrated in FIG. 1B, the silicon is deposited in a
manner which produces a high surface area coating for rapid
lithiation/delithiation for high power capability.
[0037] FIG. 1C is a transmission electron microscope image which
illustrates the nature of the graded interface transitioning from
carbon in a graphitic structure at the core (D) to a blend of
silicon compounds (C) and (B) and to a layer of amorphous silicon
at the surface (A). (EDS analysis showed the presence A: silicon;
B: silicon carbide; C: carbon with low amounts of silicon; and D:
carbon). We believe that this graded interface of the silicon
coating may contribute to retention of charge capacity of the anode
after over 100 charge/discharge cycles, i.e., little or no fading
occurs. This is in contrast to prior methods of coating silicon
which result in fading of the charge capacity which can be so high
as to render high charge/discharge cycling impractical.
[0038] Preferred nanofibers for use in the present invention are
vapor grown carbon nanofibers comprised of long filaments having a
graphitic nature. Suitable nanofibers include Pyrograf.RTM. III,
commercially available from Applied Sciences, Inc. and Pyrograf
Products, Inc. The preferred carbon nanofibers are essentially
comprised of a graphitic hollow tube, referred to as the catalytic
phase of the carbon nanofiber, and having essentially no
turbostratic or disordered carbon on the surface of the nanofiber.
This type of nanofiber is preferred as it is highly electrically
conductive and has a high surface area and surface energy. The
carbon nanofibers preferably have a length of from about 1 to about
500 micrometers in order to provide a sufficiently entangled and
durable anode.
[0039] Alternatively, graphitic composites comprised of vapor grown
carbon fibers, vapor grown carbon nanofibers, and graphene
platelets may also be used.
[0040] The vapor grown carbon nanofibers are preferably heat
treated prior to use in order to remove iron. Heat treatment is
preferably performed in an inert atmosphere at a temperature above
700.degree. C., and more preferably between about 1500.degree. C.
and 3,000.degree. C.
[0041] Carbon and graphite additives may be added to the carbon
material prior to the CVD silicon coating step to increase the
electrical conductivity and capacity of the resulting anode. Such
additives may be selected from single-walled carbon nanotubes,
multi-walled carbon nanotubes, exfoliated graphite flakes, graphite
platelets, graphene particles, carbon black, and mesocarbon
microbeads. The additives may be added as a dry powder or by
incorporation with a solvent to form a slurry. Such additives may
be added by conventional techniques for incorporating solids into
liquid solutions such as planetary and impeller type mixers.
[0042] A conductive additive may also be added to the carbon
material before the CVD coating step to provide thermal
conductivity and mechanical reinforcement to the resulting anode.
The conductive additive may be added to the carbon material by
blending as a dry powder or by incorporation with a solvent to form
a slurry. A preferred additive is macroscopic vapor grown carbon
fibers having a diameter of from about 500 nm to 10 micrometers.
Such vapor grown carbon fibers are highly graphitizable and may be
added in selected proportions of about 1 to 90% by weight and more
preferably, 10 to 30% by weight to provide the desired
reinforcement and thermal conductivity.
[0043] The carbon nanomaterial may further include a carbide
material or silicon compound selected from metal carbides, silicon
carbides, or silicon oxides. Such additives are incorporated by
conventional mixing techniques such as impeller or planetary mixing
prior to the silicon chemical vapor deposition step. The addition
of such carbide materials creates a strong interface with the
underlying graphite component (e.g., vapor grown carbon
nanofibers), with deposition of the nanoscale silicon particles
within inclusions and on the surface of the carbon nanomaterial,
thus improving the durability of the silicon coating.
[0044] The carbon material, along with any of the additives
described above, is preferably fabricated into the form of a
low-density composite or preform prior to the chemical vapor
deposition process. By "composite" or "preform," we mean that the
carbon materials are provided in a form which excludes the use of
carbon nanofiber paper. Preferably, the carbon material form used
is less than about 250 micrometers in thickness so that the silicon
will be uniformly deposited on the fibers. Where the carbon
materials are in the form of a preform, the preform may be produced
from an aerogel, foam, or composite structure. If the fibrous
carbon and graphitic materials are formed as a composite, the
chemical binder or matrix content is minimal, comprising less than
20 weight percent of the total composite, in order to allow the
silicon to be uniformly deposited throughout the preform.
[0045] Where vapor grown carbon nanofibers are used as the carbon
material, such nanofibers are preferably incorporated into preforms
from the as-grown state, which reduces the number of high
temperature annealing treatments needed as well as allowing
fabrication of the preforms while the fibers are in the "green"
non-graphitized state, resulting in less fiber damage through
handling. Alternatively, the carbon materials may be heat treated
prior to fabrication of the preform so that no further heat
treatment is required. This allows elastomeric binders or other
binders which will not survive heat treatment to be used to
fabricate the composite preforms.
[0046] Rigid preforms can be constructed using chemical binders to
hold the fibers in place within the composite preform.
Alternatively, elastomeric binders may be used to impart
flexibility if no further heat treatment is required, or
graphitizable binders such as polymerized furfuryl alcohol may be
used as a solvent suitable for dispersing carbon nanofibers. For
example, appropriate lengths of vapor grown carbon nanofibers may
be spread by hand on the base of a compression mold in the desired
fiber lay-ups. The thin layers of the aligned fibers are then
saturated with binder and placed in a mold, with the molding being
programmed for a specific time-temperature-pressure cycle. The
fiber volume in the preforms is controlled by compression to
prescribed volumes using mold stops. After molding, the resulting
panels are trimmed, measured, and weighed. Following densification
and heat treatment, the panels are machined to specimen size for
further processing. Carbonization of the panels is then performed
by framing the panels between graphite plates and slowly heating
the panels to 1000.degree. C. (1832.degree. F.) in a purified argon
atmosphere. This process is generally carried out over a 3 to 4 day
period.
[0047] In an alternative method of forming a preform comprised
mainly of carbon nanofibers, the carbon nanofibers (along with any
additives) are combined in solution by mixing and dispersing the
suspension using sonication or other low shear/high energy methods.
Following dispersion, the carbon material suspension is poured over
a vacuum-assisted filtration system. Preforms are allowed to dry in
the system and are then collected. Where the preforms are
fabricated with a binder, this may require additional processing
such as curing or compression molding.
[0048] In the CVD method of coating carbon materials such as
nanofibers with silicon, the nanofibers (in fiber form or in the
form of a preform), along with any additives, are preferably placed
in a vessel including at least one gas inlet and one gas outlet.
The vessel is then inserted into a heating chamber, and is heated
in an inert atmosphere or under vacuum at a temperature between
about 100.degree. C. to about 1200.degree. C. A silane gas or a
blend of silane gas and hydrogen, or a blend of silane gas and an
inert gas such as nitrogen or argon is then flowed over and through
the carbon material for about 15 seconds to about 60 minutes such
that it decomposes, leaving a silicon-based coating on at least the
exterior surface of the nanomaterial. The deposition may be
conducted at atmospheric pressure, reduced pressure, or elevated
pressure so as to control the deposition rate and properties of the
coating on the fibers or preform. The silane gas is then purged
from the vessel with an inert gas such as nitrogen or argon and
cooled.
[0049] It should be appreciated that the deposition temperature
varies depending on the source gas used. For example, when silane
is used, a deposition temperature of about 410.degree. C. or higher
is used to allow decomposition of the silane gas into hydrogen and
silicon components. Operation at a temperature range near the
formation temperature for crystalline silicon and silicon carbide
for a short time duration will allow formation of such silicon
compound coatings transitioning to amorphous silicon on the surface
of the fiber.
[0050] In another method of coating the carbon nanomaterials, a
free flowing carbon nanomaterial in the form of a powder is loaded
into a fluidized bed, and is fluidized in nitrogen and heated
between about 100 to 1200.degree. C. A silane gas or blend thereof
is passed through the fluidized bed with or without the aid of an
inert gas. The fluidized bed is then purged with nitrogen to remove
the silane, and the carbon nanomaterial is removed from the
fluidized bed while hot or after the fluidized bed cools to room
temperature.
[0051] In yet another method, the carbon nanomaterial is provided
as a preform which may comprise a veil of chopped carbon fibers
with a binder. The preform is passed through a heated chamber in a
box or belt furnace which includes a continuous flow of silane at a
temperature between about 100.degree. C. to 1200.degree. C. The
carbon nanomaterial preform remains in contact with the silane flow
for a time ranging from about 10 seconds to about 60 minutes to
achieve the desired coating.
[0052] Another suitable method for depositing silicon is described
in U.S. Pat. No. 6,988,304, the disclosure of which is hereby
incorporated by reference. The interior surface of a porous
nanofiber composite may be coated by chemical vapor infiltration.
Hydrogen is bubbled through liquid methyltrichlorosilane (MTS) to
form a coating precursor which is vaporized and transported into a
coating chamber heated between 900 and 1100.degree. C. The
composite to be coated is positioned a distance away from the
precursor inlet and in an orientation in the coating furnace so as
to maximize the deposition rate of the coating. The MTS is
thermally decomposed onto the interior surface of the composite to
create a silicon-based coating that is of sufficient surface energy
to wet and attract liquid phase change material (PCM). The
temperature and pressure inside the coating furnace, the total gas
flow rate, and the ratio of hydrogen to MTS are regulated to
control the composition of the coating, which ultimately affects
coating properties such as coefficient of thermal expansion (CTE),
density, strength, surface energy, thermal conductivity, heat
capacity, and other properties. Since the only function of the
coating is to increase the surface energy of the fiber, the volume
of the coating should be minimized to allow the void volume to be
maximized, to reduce the thermal resistance between the fiber and
PCM, and to reduce weight. While the coating thickness may range
from 1 to 1000 nm, the coating thickness should preferably be about
10 to 250 nm.
[0053] In the method of forming a lithiated carbon-silicon alloy, a
carbon-silicon alloy is formed using the method described above,
and lithium is then evaporated on the carbon-silicon alloy. Various
evaporation techniques may be used including heating, electron-gun
evaporation, or sputtering techniques. For example, Li.sub.xSi
alloy can be sputtered on the carbon-silicon alloy to deposit
lithiated silicon on carbon nanofibers. This is a more preferred
technique.
[0054] An anode may be formed from the silicon-coated nanofibers by
a number of methods. In one method, the anode is formed by adding a
binder to the silicon-coated carbon nanomaterial. Suitable binders
include fluorinated polymers such as polyvinylidene difluoride
(PVdF), non-fluorinated binders such as ethylene propylene diene
monomers (EPDM), and polystyrene. In a preferred method, the
polymeric binder comprises polyvinylidene difluoride and is
dissolved in an organic solvent at a 5 wt % concentration. A
mixture of C--Si composite fibers is then mixed with the binder and
conductive carbon particulates. The carbon fibers coated with
nanosize silicon are then mixed with a pre-dissolved elastomer
binder such as PVdF in an organic solvent to make an ink-type
slurry. The mixing is preferably performed by a milling process in
which the C--Si composite and the pre-dissolved binder are placed
in a ceramic jar containing ceramic balls and rolled for a period
of about 10 to 60 minutes.
[0055] Other mixing techniques can be employed to make an ink-type
slurry; for example, using a ratio of about 60-90 wt % C--Si
composite, about 2-10 wt % binder, and about 5-20 wt % conductive
carbon, respectively. The mixture is then milled for about 5 to 10
minutes to form a homogeneous slurry. The slurry paste is then
coated on a copper substrate such as copper foil using a blade
technique. The loading may be adjusted from 5 to 50 mg/cm.sup.2.
The coated sample is then dried under vacuum at about 100.degree.
C. for about 2 to 4 hours. An electrode is then cut from the coated
foil and placed in a bottom cell against a metallic lithium
electrode. A porous polypropylene-polyethylene separator soaked in
an electrolyte (ethylene carbonate:dimethyl carbonate 50:50
containing 1M LiPF.sub.6 salt) is used to separate the C--Si
electrode to form a Li electrode and function as an electrolyte for
lithium ion conduction.
[0056] In another method of forming an anode, the C--Si composite
is in the form of a thin preform and is used as an anode without
the use of current collectors (i.e., a metal foil such as copper
foil, with thickness in the range of 10-15 microns. The current
collector functions as a plate to carry the electric current out of
the batter and also as a support to keep the active material in the
cell). An electrode is cut from the preform and placed in the
bottom cell against a lithium electrode and the cell is then made
as described above.
[0057] The resulting anode material demonstrates high thermal
conductivity which will enhance heat removal from the battery cell,
thereby reducing the risk of overheating during rapid
charge/discharge cycles. The thermal conductivity of the anode may
be in the range of 25 w/m-K to 1000 w/m-K, and preferably in excess
of 1000 w/m-K, depending on the selection and respective loadings
of carbon nanofiber, vapor grown carbon fiber, and other graphitic
reinforcement materials.
[0058] Referring to FIG. 2, the capacity of a carbon-silicon anode
is illustrated after electrochemical charge-discharge cycling. The
electrochemical charge-discharge cycling is performed under
constant current. A cell containing the C--Si composite electrode
against a metallic lithium foil is constructed by separating the
two electrodes by a porous separator containing 1M of LiPF.sub.6 in
dimethyl carbonate-ethylene carbonate (50:50), and placing in a
coin cell. The lithium insertion into C--Si composite is performed
by applying a fixed current to dissolve lithium from the counter
electrode and depositing it on the C--Si electrode. When the
voltage of the cell reaches a value of 5 mV, the current sign is
switched to withdraw the deposited lithium from the C--Si electrode
and redeposit the lithium onto the Li counter electrode. This
process is then repeated to obtain the cycling results. The results
are reported in current (mA) multiplied by time (in hours)
accumulated during each half cycle divided by the weight of the
C--Si composite on the electrode.
[0059] Having described the invention in detail and by reference to
preferred embodiments thereof, it will be apparent that
modifications and variations are possible without departing from
the scope of the invention.
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