U.S. patent application number 12/329670 was filed with the patent office on 2009-05-21 for carbon-coated silicon particle powder as the anode material for lithium ion batteries and method of making the same.
This patent application is currently assigned to CONOCOPHILLIPS COMPANY. Invention is credited to Bharat Chahar, Zhenhua Mao.
Application Number | 20090130562 12/329670 |
Document ID | / |
Family ID | 34678135 |
Filed Date | 2009-05-21 |
United States Patent
Application |
20090130562 |
Kind Code |
A1 |
Mao; Zhenhua ; et
al. |
May 21, 2009 |
Carbon-Coated Silicon Particle Powder as the Anode Material for
Lithium Ion Batteries and Method of Making the Same
Abstract
A process for the production of coated silicon/carbon particles
comprising: providing a carbon residue forming material; providing
silicon particles; coating said silicon particles with said carbon
residue forming material to form coated silicon particles;
providing particles of a carbonaceous material; coating said
particles of carbonaceous material with said carbon residue forming
material to form coated carbonaceous particles; embedding said
coated silicon particles onto said coated carbonaceous particles to
form silicon/carbon composite particles; coating said
silicon/carbon composite particles with said carbon residue forming
material to form coated silicon/carbon composite particles; and
stabilizing the coated composite particles by subjecting said
coated composite particles to an oxidation reaction. The coated
composite particles will have a substantially smooth coating. The
particles may be coated with multiple layers of carbon residue
forming material/
Inventors: |
Mao; Zhenhua; (Ponca City,
OK) ; Chahar; Bharat; (Houston, TX) |
Correspondence
Address: |
ConocoPhillips Company - IP Services Group;Attention: DOCKETING
600 N. Dairy Ashford, Bldg. MA-1135
Houston
TX
77079
US
|
Assignee: |
CONOCOPHILLIPS COMPANY
Houston
TX
|
Family ID: |
34678135 |
Appl. No.: |
12/329670 |
Filed: |
December 8, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10741381 |
Dec 19, 2003 |
|
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12329670 |
|
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Current U.S.
Class: |
429/231.8 ;
29/623.1; 428/405 |
Current CPC
Class: |
C25D 17/10 20130101;
Y10T 29/53135 20150115; Y10T 29/49108 20150115; H01M 4/13 20130101;
H01M 10/0525 20130101; Y02E 60/10 20130101; H01M 4/38 20130101;
H01M 2004/021 20130101; H01M 4/133 20130101; H01M 4/583 20130101;
H01M 2004/027 20130101; Y10T 428/2995 20150115; H01M 4/134
20130101; H01M 4/0471 20130101; H01M 4/366 20130101; H01M 4/0416
20130101 |
Class at
Publication: |
429/231.8 ;
428/405; 29/623.1 |
International
Class: |
H01M 4/58 20060101
H01M004/58; B32B 9/00 20060101 B32B009/00; H01M 10/00 20060101
H01M010/00; H01M 6/00 20060101 H01M006/00 |
Claims
1-36. (canceled)
37. Coated silicon/carbon composite particles comprising a core of
silicon and carbonaceous particles wherein each of the silicon
particles and carbonaceous particles are each coated with a carbon
residue forming material, combined together and further coated with
a layer of carbon residue forming material.
38. The coated composite particles according to claim 37, wherein
the composite particles comprise a pulvurent carbonaceous material
selected from the group consisting of petroleum pitches, calcined
petroleum cokes, uncalcined petroleum cokes, highly crystalline
cokes, coal tar cokes, synthetic graphites, natural graphites, soft
carbons derived from organic polymers, and soft carbons derived
from natural polymers.
39. The coated carbonaceous particles according to claim 37,
wherein the composite particles are a pulvurent carbonaceous
material selected from the group consisting of calcined petroleum
cokes, uncalcined petroleum cokes, highly crystalline cokes,
synthetic graphites, and natural graphites.
40. The coated carbonaceous particles of claim 37 wherein the
coating layer is graphitic.
41. A method for the production of a Li-ion battery wherein the
coated carbonaceous particles of claim 37 are used as the anode
material, and wherein such Li-ion battery exhibits a first cycle
charge efficiency greater than 90% at the cut-off potential of 1
volt versus Li when tested with electrolyte containing no propylene
carbonate solvent.
42. An electrical storage cell comprising the coated carbonaceous
particles of claim 37.
43. An electrical storage cell according to claim 42, wherein the
electrical storage cell is a rechargeable electrical storage
cell.
44. A method for the manufacture of an electrical storage cell
which comprises incorporating the coated composite particles of
claim 37 into an anode of the electrical storage cell.
45-52. (canceled)
53. A method for the manufacture of an electrical storage cell,
wherein the method comprises incorporating into an anode of the
electrical storage cell coated, silicon/carbon composite materials
comprising coated silicon particles and coated carbonaceous
particles having a coating layer formed of an oxidized, carbon
residue forming material.
54-64. (canceled)
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates to silicon/carbon composite
materials that are useful as electrode active materials in
batteries. More particularly, the present invention relates to
carbon-coated silicon particles that find particular use as
electrode materials, as well as methods for the manufacture of said
carbon-coated silicon particles.
BACKGROUND OF THE INVENTION
[0002] Synthetic graphites are widely used as standard negative
electrode materials in lithium ion batteries. Other carbonaceous
materials are also widely used in such batteries due to their
efficiency and reasonable cost. Lithium ion batteries are primarily
used as power sources in portable electronic devices. Compared to
other classes of rechargeable batteries such as nickel-cadmium and
nickel-metal hydride storage cells, lithium ion cells have become
increasingly popular due to relatively high storage capacity and
rechargeability.
[0003] Due to increased storage capacity per unit mass or unit
volume over similarly rated nickel-cadmium and nickel-metal hydride
storage cells, the smaller space requirements of lithium ion cells
allow production of cells that meet specific storage and delivery
requirements. Consequently, lithium ion cells are popularly used in
a growing number of devices, such as digital cameras, digital video
recorders, computers, etc., where compact size is particularly
desirable from a utility standpoint.
[0004] Nonetheless, rechargeable lithium ion storage cells are not
without deficiencies. These deficiencies may be minimized with the
use of improved materials of construction. Commercial lithium ion
batteries which use synthetic graphite electrodes are expensive to
produce and have low relatively lithium capacities. Additionally,
graphite products currently used in lithium ion electrodes are near
their theoretical limits for energy storage (372 mAhr/g).
Accordingly, there is a need in the art for improved electrode
materials that reduce the cost of rechargeable lithium batteries
and provide improved operating characteristics, such as higher
energy density, greater reversible capacity and greater initial
charge efficiency. There also exists a need for improved methods
for the manufacture of such electrode materials.
[0005] Silicon has been investigated as an anode material for
lithium ion batteries because silicon can alloy with a relatively
large amount of lithium, providing greater storage capacity. In
fact, silicon has a theoretical lithium capacity of more than ten
times that of graphite. However, pure silicon is a poor electrode
material because its unit cell volume can increase to more than
300% when lithiated. This volume expansion during cycling destroys
the mechanical integrity of the electrode and leads to a rapid
capacity loss during battery cycling. Although silicon can hold
more lithium than carbon, when lithium is introduced to silicon,
the silicon disintegrates and results in less electrical contact
which ultimately results in decreased ability to recharge the
storage cell.
[0006] Continuous research efforts in solving silicon volume
expansion problems have yielded limited results. Silicon/carbon
composite particles or powders have good cycle life compared to
mechanical mixtures of carbon and silicon powders made by milling
or other mechanical methods. Thin film silicon-coated carbon
particles or carbon-coated silicon powders are potential
replacements for graphite powders as the anode material for next
generation lithium ion batteries. However, chemical vapor
deposition methods typically used to apply silicon coatings or
carbon coatings have intrinsic shortcomings that include slow
deposition rates and/or expensive precursors for deposition. Vapor
deposited silicon films may be extremely expensive relative to the
cost of bulk silicon powders. Therefore, another method of
manufacturing coated silicon particles is needed.
SUMMARY OF THE INVENTION
[0007] The present invention provides processes for the manufacture
of the silicon/carbon composite materials. The silicon/carbon
composite materials comprise coated silicon particles that are
combined with coated carbon particles; wherein the resulting
silicon/carbon composite particle is further coated with a layer of
oxidized, carbon residue-forming material. These carbon-coated
silicon/carbon composite particles are useful in the manufacture of
electrodes in electrical storage cells, particularly in
rechargeable lithium ion electrical storage cells.
[0008] The compositions of the invention provide high capacity and
high efficiency carbon-coated silicon/carbon composite particles
that can be derived from a wide variety of carbon sources. In a
further aspect of the invention, the silicon/carbon composite
particle may be coated with multiple layers of carbon residue
forming material. In a still further aspect of the invention, the
coating layer(s) of the composite particle may be optionally
carbonized.
[0009] The compositions of the present invention provide
carbon-coated silicon/carbon composite particles with substantially
smooth coatings. Additionally, the compositions feature good powder
flowability, which is particularly beneficial during the handling
or manufacturing steps necessary to form these materials into
useful electrodes or into other articles not specifically described
herein.
[0010] In further aspects of the invention there are provided
methods for the manufacture of such carbon-coated silicon/carbon
composite particles. The carbon-coated powders prepared in
accordance with the invention not only increase charge efficiency
but also provide excellent processability for electrode
fabrication. In a yet further aspect of the invention there are
provided methods for the manufacture of electrical storage cells,
particularly rechargeable batteries that include said carbon-coated
composite particles. A still further aspect of the invention
relates to the use of said carbon-coated composite particles in
electrical storage cells, particularly in rechargeable
batteries.
[0011] These and other aspects and features of the invention will
become apparent from the following description of the invention and
preferred embodiments thereof.
DESCRIPTION OF DRAWINGS
[0012] FIG. 1 shows a schematic view of a composite carbon-silicon
particle according to the present invention.
[0013] FIG. 2 shows a comparison of charge and discharge potential
profiles on the first cycle for different low cut-off potentials
for silicon/carbon composite particles and uncoated silicon
particles.
[0014] FIG. 3 shows a scanning electron microscopy image of
silicon/carbon composite particles as prepared in Example 2.
[0015] FIG. 4 shows the discharge capacity and charge efficiency
within the charge/discharge potential window between 0.09 and 1.5
volts during the first 5 cycles for the silicon/carbon composite
particles produced in Example 2.
[0016] FIG. 5 shows the capacity and columbic efficiency during
charge/discharge cycles between 0.09 and 1.5 volts for the
composite silicon/carbon particles as prepared in Example 3.
[0017] Like reference symbols in the various drawings indicate like
elements.
DETAILED DESCRIPTION
[0018] The present invention provides processes for the manufacture
of silicon/carbon composite particles, which particles exhibit
improved operating characteristics when used as electrodes in
electrical storage cells, particularly in rechargeable electrical
storage cells. Generally, the process contemplates combining coated
fine silicon powders with coated carbonaceous particles to form a
silicon/carbon composite particle and further coating the composite
particle with a layer or layers of carbon residue-forming
material.
[0019] More specifically, particles of a carbonaceous material
substrate are coated with a fusible, carbon residue-forming
material. Particles of fine silicon powders, which have been coated
with a fusible, carbon residue-forming material, are embedded onto
the coated carbonaceous particle to form a composite particle of
silicon and carbonaceous materials. The silicon/carbon composite
particle is further provided with at least one coating of a
fusible, carbon residue-forming material. The coated silicon/carbon
composite particle is thereafter stabilized by subjecting said
coated composite particle to an oxidation reaction using an
oxidizing agent. The stabilized coated composite particle is
thereafter carbonized.
[0020] While it is possible to embed uncoated silicon particles
onto a coated carbonaceous substrate material, it is preferable
that the silicon particles be coated prior to embedding the silicon
onto the carbonaceous substrate material to achieve an enhancement
in cycling ability and mechanical strength over that of composite
particle comprising uncoated silicon powder.
[0021] The silicon/graphite composite particle may be further
coated with additional layer(s) of carbon residue-forming material
following stabilization or optional carbonization.
[0022] It is preferable to apply a coating onto the carbonaceous
particle prior to applying the silicon particles. It is preferable
to embed coated silicon particles onto the coated carbonaceous
substrate. Alternatively, uncoated silicon particles may be
embedded onto the coated carbonaceous substrate. Further, it is
preferable to coat the silicon/carbon composite particle to enhance
the mechanical strength of the composite, resulting in longer
lasting silicon composite electrodes. Preferably, the process
provides carbon-coated silicon/carbon composite particles having
substantially smooth coatings. Optionally, the composite particles
may be coated repeatedly with carbon residue forming material to
further increase the mechanical strength of the particles.
[0023] In the preferred embodiment, particles of carbonaceous
substrate material are required for the practice of the invention.
These may be obtained from a variety of sources, examples of which
include petroleum and coal tar cokes, synthetic and natural
graphite, or pitches as well as other sources of carbonaceous
materials that are known in the manufacture of prior art
electrodes, although these sources are not elucidated here.
Preferred sources of carbonaceous materials include calcined or
uncalcined petroleum cokes as well as synthetic graphite. Preferred
sources of carbonaceous materials also include calcined or
uncalcined, highly crystalline "needle" cokes. Particularly
preferred sources of carbonaceous material include natural graphite
and flake coke. Thus, preferred carbonaceous materials are either
graphitic materials or materials which form graphite on heating to
graphitization temperatures of 2200.degree. C. or higher.
[0024] Fine particles of such carbonaceous substrate material are
conveniently provided by milling, crushing, grinding or by any
other means that can be used to provide a pulverant carbonaceous
substrate material having particles of dimensions that are suitable
for use in the formation of electrodes. Although the principles of
the present invention are believed to be applicable to carbonaceous
substrate particles of varying sizes and particle size
distributions, preferred carbonaceous substrate particles having
average particle sizes up to about 50 .mu.m, more preferably from
about 1 to about 30 .mu.m.
[0025] Particles of silicon are required for the practice of the
invention; such particles may be used alone or in conjunction with
the carbonaceous substrate material. The purity of the silicon may
be of ordinary industrial strength, i.e., 97-98 wt. %. Although the
principles of the present invention are believed to be applicable
to silicon particles of varying sizes and particle size
distributions, preferred silicon particles having average particle
sizes up to about 50 .mu.m, more preferably from about 0.03 to
about 20 .mu.m.
[0026] According to steps in the inventive process, the silicon
particles, carbonaceous substrate particles and silicon/carbon
composite particles are provided with a fusible, carbon
residue-forming material as a coating material. Preferred for use
as coating materials are carbon residue-forming materials that are
capable of being reacted with an oxidizing agent. Preferred
compounds include those with a high melting point and a high carbon
yield after thermal decomposition. Exemplary useful coating
materials include heavy aromatic residues from petroleum, chemical
process pitches; lignin from pulp industry; phenolic resins; and
carbohydrate materials such as sugars and polyacrylonitriles.
Especially preferred for use as coating materials are petroleum and
coal tar pitches and lignin that are readily available and have
been observed to be effective as fusible, carbon residue-forming
materials.
[0027] It is to be understood that the carbon residue-forming
material provided as the coating for the carbonaceous, silicon or
silicon/carbon composite particles, as the case may be, may be any
material which, when oxidized and then thermally decomposed in an
inert atmosphere to a carbonization temperature of 850.degree. C.
or an even greater temperature forms a residue which is
"substantially carbon". It is to be understood that "substantially
carbon" indicates that as the residue is at least 95% by weight
carbon, it is also preferred that the carbon residue-forming
material form at least 10%, and preferably at least 40% and more
preferably at least 60% carbon residue on carbonization, based on
the original mass of the carbon residue-forming coating for the
carbonaceous substrate, silicon or silicon/carbon composite
particle.
[0028] It should be understood that the coatings used for one type
of particle may vary significantly from the coatings used for
another type of particle. By way of non-limiting examples, the
carbon residue forming material provided as a coating for the
carbonaceous substrate particles may be composed of a completely
different carbon residue forming material as that provided as a
coating for the silicon particles or for that provided as a coating
for composite particles. Further, subsequent coatings provided for
composite particles may be composed of carbon residue forming
materials that differ from coatings applied to the carbonaceous or
silicon particles, or from the previous coatings on the composite
particles.
[0029] Any organic compound that can be oxidized and then thermally
decomposed to yield carbon residue can be used as the coating
material. However, in coating processes in which the organic
compounds are dissolved in solvent, aromatics compounds that
include various molecular weights are preferred because of the
mutual dissolution of the compound with the solvent. Preferred
compounds include those with a high melting point and a high carbon
yield after thermal decomposition (e.g., petroleum and coal tar
pitches).
[0030] Any useful technique for coating the carbonaceous, silicon
or composite particles may be used. By way of non-limiting
examples, useful techniques include the steps of: liquefying the
carbon residue-forming material by a means such as melting or
forming a solution with a suitable solvent combined with a coating
step such as spraying the liquefied carbon residue-forming material
onto the subject particle, or dipping the particle in the liquefied
carbon residue-forming material and subsequently drying out any
solvent.
[0031] A particularly useful method of forming a uniform coating of
a carbon residue-forming material by precipitating the material
onto the surface of the carbonaceous, silicon or silicon/carbon
composite particles is provided according to the following process.
First, a concentrated solution of the carbon residue-forming
material in a suitable solvent is formed. The solution of carbon
residue-forming material is prepared by combining the carbon
residue-forming material with a solvent or a combination of
solvents. The solvent should be compatible with the carbon
residue-forming material and should dissolve all or a substantial
portion of the coating material. Solvents include pure organic
compounds or a mixture of different solvents. The choice of
solvent(s) depends on the particular coating material used.
[0032] Suitable solvents for dissolving the carbon residue-forming
material include, e.g., benzene, toluene, xylene, quinoline,
tetrahydrofuran, naphthalene, acetone, cyclohexane, and
tetrahydronaphthalene (sold by Dupont under the trademark
Tetralin), ether, water and methyl-pyrrolidinone, etc. When
petroleum or coal tar pitch is used as the carbon residue-forming
material or coating material, e.g., solvents such as toluene,
xylene, quinoline, tetrahydrofuran, Tetralin, or naphthalene are
preferred. The ratio of the solvent(s) to the carbon
residue-forming material for the carbonaceous, silicon or composite
particle in the solution and the temperature of the solution is
controlled so that the carbon residue-forming material completely
or almost completely dissolves into the solvent. Typically, the
solvent to carbon residue-forming material ratio is less than 2,
and preferably about 1 or less, and the carbon residue-forming
material is dissolved in the solvent at a temperature that is below
the boiling point of the solvent.
[0033] Concentrated solutions wherein the solvent-to-solute ratio
is less than 2:1 are commonly known as flux solutions. Many
pitch-type materials form concentrated flux solutions wherein the
pitch is highly soluble when mixed with the solvent at
solvent-to-pitch ratios of 0.5 to 2.0. Dilution of these flux
mixtures with the same solvent or a solvent in which the carbon
residue-forming material is less soluble results in partial
precipitation of the carbon residue-forming coating material. When
this dilution and precipitation occurs in the presence of a
suspension of carbonaceous, silicon or composite particles, the
particles act as nucleating sites for the precipitation. The result
is an especially uniform coating of the carbon residue material on
the particles.
[0034] The coating layer of the subject particle, whether
carbonaceous substrate, silicon, or silicon/carbon composite, can
be applied by mixing the particles into a solution of carbon
residue-forming material directly. When the particles are added to
the solution of carbon residue-forming material directly,
additional solvent(s) is generally added to the resulting mixture
to effect partial precipitation of the carbon residue-forming
material. The additional solvent(s) can be the same as or different
than the solvent(s) used to prepare the solution of the carbon
residue-forming materials.
[0035] An alternative method to precipitation would require a
suspension of carbonaceous substrate, silicon or silicon/carbon
composite particles be prepared by homogeneously mixing the
particles in the same solvent used to form the solution of carbon
residue-forming material, in a combination of solvent(s) or in a
different solvent to a desired temperature, preferably below the
boiling point of the solvent(s). The suspension of the target
particles is then combined with the solution of carbon
residue-forming material causing a certain portion of the carbon
residue-forming material to deposit substantially uniformly on the
surface of the particles.
[0036] The total amount and morphology of the carbon
residue-forming material that precipitates onto the surface of a
particle depends on the portion of the carbon residue-forming
material that precipitates out from the solution, which in turn
depends on the difference in the solubility of the carbon
residue-forming material in the initial solution and in the final
solution. When the carbon residue-forming material is a pitch, wide
ranges of molecular weight species are typically present. One
skilled in the art would recognize that partial precipitation of
such a material would fractionate the material such that the
precipitate would be relatively high molecular weight and have a
high melting point, and the remaining solubles would be relatively
low molecular weight and have a low melting point compared to the
original pitch.
[0037] The solubility of the carbon residue-forming material in a
given solvent or solvent mixture depends on a variety of factors
including, for example, concentration, temperature, and pressure.
As stated earlier, dilution of concentrated flux solutions causes
solubility to decrease since the solubility of the carbon
residue-forming material in an organic solvent increases with
temperature, precipitation of the coating is further enhanced by
starting the process at an elevated temperature and gradually
lowering the temperature during the coating process. The carbon
residue-forming material can be deposited at either ambient or
reduced pressure and at a temperature of about -5.degree. C. to
about 400.degree. C. By adjusting the total ratio of the solvent to
the carbon residue-forming material and the solution temperature,
the total amount and hardness of the precipitated carbon
residue-forming material on the carbonaceous, silicon or composite
particles can be controlled.
[0038] The suspension of carbonaceous substrate, silicon or
silicon/carbon composite particles in the final diluted solution of
carbon residue-forming material generally has a ratio of solvent to
carbon residue-forming material of greater than about 2; and
preferably greater than about 4. It would be understood by one
skilled in the art that the specific solvent to carbon
residue-forming pitch ratio at the conclusion of the coating
process depends on the carbon residue-forming material and solvent
selected for the process. On one hand, it is desirable to use as
little solvent as possible because of the cost of solvent, while on
the other hand, enough solvent is required so that the particles
can be dispersed in the solvent.
[0039] Upon completion of the precipitation step, the coated
particles are separated from the mixture of solvent, particles, and
carbon residue-forming material using conventional methods, such
as, for example, centrifugal separation, or filtration. The
particles are optionally washed with solvent to remove residual
pitch (or other carbon forming residue forming material) solution
and dried using conventional methods.
[0040] According to an inventive step of the process, the
silicon/carbon composite particle is produced by co-precipitating
pitch onto a mixture of uncoated fine silicon powder particles and
coated, relatively coarse carbonaceous particles simultaneously,
thereby effectively embedding silicon particles onto the coating
layer of the relatively large carbonaceous substrate particles. The
resulting silicon/carbon composite particle is thereafter coated
with pitch.
[0041] Alternatively, the silicon/carbon composite particle may be
produced by separately coating silicon particles and carbonaceous
substrate particles with pitch in separate containers; thereafter
the coated particles are mixed together in a solution of pitch and
solvent to embed the coated silicon particle onto the coated
carbonaceous substrate particle.
[0042] According to a further step of the invention process, the
coating layer of the silicon, carbon and silicon/carbon composite
particles are rendered partly or completely infusible, preferably
by oxidative stabilization. The coating of the particles are
stabilized by subjecting said particles to an oxidation reaction
using an oxidizing agent under appropriate reaction conditions.
Generally, only mild-to-moderate reaction conditions are required.
Typically, contacting the coated particles with an oxidizing agent
at mild conditions and activating the oxidizing agent at elevated
conditions satisfactorily perform the oxidation reaction. Contact
with the oxidizing agent can occur at ambient temperatures
(approximately 20.degree. C.) or at moderately elevated
temperatures (up to approximately 400.degree. C.). Activation of
the oxidizing agent would typically occur at moderately elevated
temperatures up to 400.degree. C. Preferably the temperature of the
oxidation reaction is maintained below the instantaneous melting
point of the coating material so as to insure that melting point of
the coating material is not exceeded during the oxidation
reaction.
[0043] According to a further step of the inventive process, the
stabilized coated silicon, carbonaceous substrate particles or
silicon/carbon composite particles may be optionally carbonized.
The degree to which the surface of the coating is rendered
infusible by stabilization is dependent upon the type of pitch used
as well as the solvents or combination of solvents used. Further,
if multiple layers of coating are desired, it is preferable to
apply additional layers of coating following stabilization or
carbonization. The final coating on a composite particle with
multiple coatings is preferably carbonized.
[0044] The stabilization step of the current invention is carried
out to render the surface of the coating layer infusible to the
subsequent carbonization. Oxidative stabilization allows the smooth
surface produced in the coating process to be preserved in the
coated composite particles of the instant invention, as the
oxidative stabilization renders the surface of the coating
infusible to the subsequent processing steps.
[0045] Heat treatment of the stabilized coated particles is
desirably conducted in a controlled manner in order to minimize
fusion of the particles. One skilled in the art will recognize that
highly stabilized, infusible, coated particles can be heated
relatively aggressively and quickly during carbonization. In
contrast, relatively mildly stabilized coated particles require
slower heating in order to avoid excessive melting of the coating
and fusion of the particles. Use of a fluidized bed during
stabilization and heat treatment is especially beneficial in
preventing clumping and fusion of the coated particles.
[0046] With regard to the temperature required to insure
carbonization for coated particles, desirably this is achieved by
raising the temperature in a controlled manner from a starting
temperature, usually ambient temperature, to the final
carbonization temperature which falls within the above-identified
range of about 400.degree. C. to about 1500.degree. C., preferably
within the range of about 800.degree. C. to about 1300.degree. C.,
and more preferably within the range of about 900.degree. C. and
1200.degree. C.
[0047] With regard to the atmospheric conditions for the
carbonization process for the stabilized coated particles, the
atmosphere may be ambient air up to about 850.degree. C. but an
inert atmosphere is preferred at temperatures above about
400.degree. C. Ambient air is an acceptable atmosphere when the
oxygen is largely displaced during heating or during heating under
vacuum. Suitable inert atmospheres include nitrogen, argon, helium,
etc., which are non-reactive with the heated coated particles.
[0048] It is understood that during the heating of the coated
particles, particular attention must be paid to ensure that neither
the temperatures attained during this heating process, nor the rate
of the temperature rise during any part of the heating process be
such that the instantaneous melting point of the coating on the
particles is exceeded. More simply stated, the thermal degradation
of the coating is to be effected by a controlled temperature rise
wherein the process temperature is maintained at or below the
instantaneous melting point of the coating where said melting point
is generally increasing with time during the process. In view of
this requirement, preferred heating processes are those that
exhibit slower rates of temperature rise.
[0049] The most preferred aspects of the invention result in the
provision of a smooth coating upon the silicon/carbon composite
particles. Preferably the stabilization of the coating of the
silicon/carbon composite particle is followed by controlled heating
of the coated stabilized silicon/carbon composite particles so as
to effect carbonization of the coated particles with little or no
clumping or self-adhesion of the individual particles. The desired
results are coated particles with little or no broken fracture
surfaces of the type which are characteristically formed when the
separate particles fuse and must be crushed or broken apart in
order to provide a free flowing powder. Such fracture surfaces are
desirably minimized or avoided, as they are believed to contribute
to low electrochemical efficiency when the particles are used as an
anode material in rechargeable electrical storage cells,
particularly in rechargeable lithium ion batteries.
[0050] According to a particularly preferred embodiment of the
inventive process taught herein, the carbon residue forming
material is provided in a fluid form. It has been observed by the
inventors that when the carbon residue forming material is
precipitated from a liquid, a smooth coating forms at the interface
of the individual carbonaceous particles and the surrounding
liquid. A smooth coating is retained when subsequently
carbonized.
[0051] Although less advantageous, when the carbon residue-forming
coating is supplied as a solid, it is desirably fused on the
surface of the particles in order to form a smooth coating thereon.
Especially preferred embodiments of the present invention produce a
free-flowing powder of coated particles after the carbonization,
which particles exhibit little or no fusion among the particles,
but can generally be broken into a free-flowing powder by simple
mechanical agitation, such as by use of a stirring rod, or by
rubbing between the thumb and forefinger. Where some fusion may
have occurred between particles, and mechanical agitation is used
to separate these particles which may result in the formation of
new fracture surfaces, in the preferred embodiments of the
invention these fracture surfaces do not comprise more than 10%,
preferably no more than 2% of the total surface area of the
particles. Such are considered as being substantially smooth
coatings.
[0052] A preferred aspect of the present invention is in the pitch
coating process, or carbon residue-forming material coating
process. This coating process provides uniform carbon
residue-forming coating on particles regardless of particle size.
The coating can be accomplished in a number of ways but it is
especially advantageous to precipitate the coating material in the
presence of a suspension of the targeted particles, whether
silicon, carbonaceous substrate material or silicon/carbon
composite particles. This coating method yields a uniform coating
of controlled composition and produces a loose particle powder, so
that the pitch-coated particles do not agglomerate and no further
milling process is required in the subsequent process steps.
[0053] Another preferred aspect of the present invention is in an
oxidation reaction that is carried out on the coated particles
prior to carbonization of the coating. The oxidation reaction is
believed to provide certain technical benefits. First, it is
believed that the reacted coated particles are relatively infusible
following oxidation, which is particularly desirable in view of
subsequent process steps, and subsequent handling of the particles.
Second, it is believed that the reacted coated particles are
endowed with a surface which yields high efficiency when used as an
electrode, particularly when the reacted coated particles are used
in an anode material in a rechargeable storage cell, particularly
in a rechargeable lithium ion cell.
[0054] A further aspect of the invention contemplates the use of
coated silicon or coated silicon/carbon composite particles in
electrodes, particularly anodes, of electrical storage cells,
particularly in rechargeable batteries. According to this aspect of
the invention, there is contemplated a method for the manufacture
of an electrical storage cell which comprises the steps of:
incorporating into an anode of the electrical storage cell silicon
materials comprising silicon/carbon composite particles having a
coating layer formed of an oxidized, carbon residue forming
material.
[0055] According to this aspect of the invention, the coated
silicon/carbon composite particles produced from the processes
described above are formed using the conventional techniques into
electrodes, particularly anodes. While not described with
particularity herein, it is contemplated that known-art
manufacturing techniques for the assemblage of such electrodes, as
well as known-art devices which facilitate in the formation of such
electrodes can be used. A particular advantage which is obtained by
the use of the coated particles taught herein lies in the fact that
due to their coating, they rarely fuse together thus resulting in a
flowable powder.
[0056] Aspects of the present invention, including certain
preferred embodiments are described in the following Examples.
EXAMPLE 1
Material Preparation
[0057] The silicon powder used in this example had an average
particle size of 5 .mu.m (from Johnson Matthey Company). The pitch
used for the coating layer was a petroleum pitch from Conoco, Inc.
that was approximately 27% insoluble in xylene. The procedure for
coating pitch on silicon powder is as follows. First, 20 grams of
the silicon powder was mixed with about 100 ml of xylene so that
silicon particles were uniformly dispersed in xylene in a glass
flask. Concurrently, 14 grams of the pitch was mixed with an equal
amount of xylene in another flask so that the pitch was completely
dissolved in xylene. Both the solutions were heated to
approximately 110.degree. C. and the pitch solution was added into
the silicon solution while being continuously mixed. The resulting
solution was then heated to 140.degree. C. and continuously stirred
for about 15 minutes. The solution was removed from the heater and
the solution gradually cooled to ambient temperature
(.about.25.degree. C.). While the solutions were mixed and cooled,
the insoluble pitch precipitated out of solution and coated
uniformly onto the silicon particles. The resulting solid particles
in the solution are pitch-coated silicon powder. The powder was
then separated from the liquid by filtration and washed with 50 ml
of xylene.
[0058] The pitch-coated silicon powders were then dried under
vacuum at .about.100.degree. C. The total weight of the dried
powder was about 23.8 g, resulting in a 16 wt % pitch coating on
the silicon. The powders were then transferred into a tube furnace
and heated at 1.degree. C./minute to 300.degree. C. and heated
further for 10 hours at 300.degree. C. under a reduced air pressure
(typically .about.-22'' Hg). During such heat treatment
(stabilization), the weight of the pitch on silicon particles
increased by about 5%. Following stabilization, the powders were
heated at 5.degree. C./minute to a temperature higher than
1150.degree. C. in nitrogen gas for 2 hours. Typically, the weight
of the stabilized pitch decreased by about 25% during
carbonization. Based on the amount of initial pitch prior to
stabilization, the overall weight of the pitch decreased by about
20% or about 80% of the pitch remains as carbon coating after
carbonization.
[0059] The resulting powder was then evaluated as the anode
material for a lithium-ion battery, as described below in the
section "Evaluation of Electrical Capacity". FIG. 2 shows a
comparison of the potential profiles during the first cycle charge
and discharge for different cut-off potentials. For comparison, the
potential profiles of a mechanic mixture of plain silicon and
graphite powders were also shown in the figure. In this figure, the
y-axis is the electrical potential of the silicon electrode versus
lithium metal during the charging and discharging, the x-axis
represents the charge stored into and removed from the electrode
based on unit weight of the composite material. The electrical
potential of the material is an indicator of the saturation level
of lithium alloying; the lower the potential, the closer the
material is to saturation. It can be seen that the ratio of
coulombic efficiency is fairly high (>90%) for the composite
carbon/silicon particles, whereas it is very low (<30%) for the
mechanic mixture of graphite and plain silicon. In addition, the
capacity as defined in the next section is very large for the
carbon-coated silicon powder.
Evaluation of Electrical Capacity
[0060] The electrical reversible capacity and the coulombic
efficiency of the powder particles according to Examples 1-3 as
well as Comparative Examples were evaluated by the following
techniques.
[0061] A uniform slurry was formed by thoroughly mixing powder (5
grams) with 3.82 grams of a solution containing 0.382 grams of
polyvinylidene fluoride (PVDF, ex. Aldrich Chemical Co., Inc.),
3.44 g of 1-methyl-pyrrolidinone (NMP, ex. Aldrich Chemical Co.,
Inc.), and 0.082 grams of acetylene black (having an effective
surface area of 80 m.sup.2/g, ex. Alfa Aesar). The slurry was then
manually cast utilizing a doctor blade to form a thin film having a
loading of about 6 mg/cm.sup.2 onto the rough side of an
electrodeposited copper foil (10 .mu.m, ex. Fuduka Metal Foil &
Powder Co., Ltd.). The cast film was then dried on a hot plate at
approx. 100.degree. C. and pressed to a desired density (approx.
1.4 g/cm.sup.2) with a roll press. A disc having an area of 1.6
cm.sup.2 was then punched out from the film and weighed to
determine the exact amount of the mass on the copper foil.
Subsequently this disc was further dried under vacuum at a
temperature of 80.degree. C. for approximately 15 minutes and
transferred into a sealed box without exposing the disc to ambient
air. The sealed box was filled with ultra-pure argon gas having
oxygen and moisture levels of less than 1 ppm.
[0062] Subsequently the disc was cast as the positive electrode in
the manufacture of a standard coin cell (2025 size) which was
subsequently used as the test cell. The other electrode of the test
cell was a foil of pure lithium (100 .mu.m, ex. Alfa Aesar). A two
layer separator was used in the test cell: a glass mat (GF/B Glass
Microfibre Filter, Whatman International Ltd.) was used as the
first layer on the composite Carbon/silicon powder and a porous
polypropylene film (available as Celgard.RTM. 2300, ex. Celgard
Inc.) was used as the second layer on the lithium foil. The
electrolyte of the test cell was a 1 M LiPF.sub.6 in ethylene
carbonate (EC)/diethyl carbonate (DEC)/dimethyl carbonate (DMC)
solvent mixture (40/30/30) (purchased from EM Industrial). Test
cells were produced utilizing the component described above
according to conventional techniques, although the samples of
powder particles were varied to ensure that at least one sample
coin cell was produced incorporating a powder particle sample
according to either one of the demonstrative examples, or according
to one of the comparative examples. These powders were tested as
the anode material in a coin cell configuration of
carbon/separator/lithium metal at room temperature
(.about.25.degree. C.). Two or three cells were made for each
sample; the reported charge capacity and charge efficiency were the
average value of the cells.
[0063] The capacity and charging efficiency of a specific powder
particle sample was determined according to the following protocol.
Utilizing a standard electrochemical test station (Model BT-2043,
Arbin Instrument Corp.), an assembled test cell was first
discharged (equivalently alloyed with lithium) at 0.5 mA (approx.
52 mA/g) to a given voltage on the first cycle. Thereafter, the
assembled test cell was charged (de-alloyed) at 0.5 mA to 1.5 volts
during which time the charge passed during charging was used to
calculate the specific capacity of the composite powder, while the
ratio of the total charge passed during charging to the total
charge passed during discharging was used to determine the charge
efficiency.
EXAMPLE 2
[0064] Twenty grams of a natural flake graphite powder (average
particle size 5 .mu.m from China) were coated with 10 wt %
petroleum pitch according to the procedure as described in Example
1. The coated graphite powder was stabilized, carbonized, and
graphitized at 3000.degree. C. in argon. Concurrently, a silicon
powder (average particle size 2 .mu.m, purchased from Johnson
Matthey company) was coated with 10 wt % pitch as described in
Example 1, stabilized, and carbonized at 1050.degree. C. A mixture
of the coated natural graphite powder and the coated silicon powder
were combined in the proportion of 6 parts coated graphite and 4
parts of coated silicon powder and coated with 15 wt % solution of
the same pitch using the same method. After stabilization in air,
the resulting composite powder was carbonized at 1050.degree. C. in
nitrogen atmosphere. The resulting graphite/silicon/carbon
composite particle powder has morphology as shown in FIG. 3. It can
be seen that small silicon particles are embedded in the carbon
coating on large graphite particles, a structure similarly
illustrated in FIG. 1.
[0065] The composite powder was then evaluated as the anode
material for a lithium-ion battery, as described above in the
section entitled "Evaluation of Electrical Capacity". The cycling
potential window was between 0.09 and 1.5 volts. The results are
shown in FIG. 4. It can be noted that the material has a capacity
of about 850 mAh/g and is fairly reversible from cycle to
cycle.
EXAMPLE 3
[0066] Twenty grams of a natural flake graphite powder (average
particle size 5 .mu.m from China) were coated with 7 wt % petroleum
pitch according to the procedure as described in example 1. The
coated graphite powder was stabilized and carbonized at
1200.degree. C. The coated graphite powder was mixed with the
coated silicon powder as described in Example 2, in the same
proportions. The mixture was then coated with 15 wt % pitch as
described in Example 1 and stabilized. Subsequently, the resulting
composite particle powder was coated again with 10 wt % pitch,
stabilized, and carbonized at 1050.degree. C. in nitrogen
atmosphere. The material was evaluated as the anode material for
Li-ion batteries in the same manner as described previously. The
capacity and efficiency of this material are shown in FIG. 5 for
the first five cycles. A significant increase in the
rechargeability of the silicon powder is displayed.
COMPARATIVE EXAMPLE
[0067] To compare the carbon-coated silicon powder with uncoated
silicon powder at the same carbon coating level, electrodes were
made by adding 20% graphite to uncoated silicon and 7% of the same
graphite to the carbon-coated silicon. The graphite used was
natural graphite based composite graphite powder.
[0068] FIG. 2 shows the charge and discharge cell voltage profiles
for the carbon-coated silicon and uncoated silicon powders. It
should be noted that "charging" means that the lithium is being
electrochemically inserted into the electrode and "discharging"
denotes that the lithium is being removed from the electrode. The
charge and discharge capacity was calculated based on total
electrode material except for the binding material. As shown in the
figure, the cell voltage rapidly drops to the low cut-off voltage
on the charging and the discharged capacity and the efficiency are
very small for the silicon/graphite mixture electrode.
* * * * *