U.S. patent application number 10/630501 was filed with the patent office on 2004-07-08 for amorphous electrode compositions.
This patent application is currently assigned to 3M Innovative Properties Company, a Delaware corporation. Invention is credited to Courtney, Ian A., Dahn, Jeffrey R., Fredericksen, Brian D., Krause, Larry J., Larcher, Dominique C., Mao, Ou, Turner, Robert L..
Application Number | 20040131936 10/630501 |
Document ID | / |
Family ID | 26871668 |
Filed Date | 2004-07-08 |
United States Patent
Application |
20040131936 |
Kind Code |
A1 |
Turner, Robert L. ; et
al. |
July 8, 2004 |
Amorphous electrode compositions
Abstract
An electrode composition that includes an electrode material
consisting essentially of at least one electrochemically inactive
elemental metal and at least one electrochemically active elemental
metal in the form of an amorphous mixture at ambient temperature.
The mixture remains amorphous when the electrode composition is
incorporated into a lithium battery and cycled through at least one
full charge-discharge cycle at ambient temperature.
Inventors: |
Turner, Robert L.;
(Woodbury, MN) ; Fredericksen, Brian D.;
(Watertown, MN) ; Krause, Larry J.; (Stillwater,
MN) ; Dahn, Jeffrey R.; (Hubley, CA) ;
Larcher, Dominique C.; (Amiens, FR) ; Courtney, Ian
A.; (St. Francis, MN) ; Mao, Ou; (New Milford,
CT) |
Correspondence
Address: |
3M INNOVATIVE PROPERTIES COMPANY
P.O. BOX 33427
ST. PAUL
MN
55133-3427
US
|
Assignee: |
3M Innovative Properties Company, a
Delaware corporation
|
Family ID: |
26871668 |
Appl. No.: |
10/630501 |
Filed: |
July 30, 2003 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10630501 |
Jul 30, 2003 |
|
|
|
09751169 |
Dec 29, 2000 |
|
|
|
6699336 |
|
|
|
|
60175893 |
Jan 13, 2000 |
|
|
|
Current U.S.
Class: |
429/218.1 ;
429/231.95 |
Current CPC
Class: |
C22C 45/08 20130101;
H01M 4/44 20130101; H01M 4/38 20130101; C22C 45/00 20130101; H01M
4/134 20130101; C22C 21/00 20130101; C23C 14/022 20130101; C23C
14/0682 20130101; C23C 4/123 20160101; H01M 4/364 20130101; H01M
4/0471 20130101; H01M 10/0525 20130101; H01M 4/405 20130101; H01M
4/46 20130101; H01M 4/624 20130101; C22C 21/02 20130101; H01M
10/052 20130101; H01M 4/366 20130101; C22C 21/14 20130101; Y02E
60/10 20130101; C23C 4/10 20130101; H01M 4/386 20130101; H01M 4/387
20130101; H01M 4/621 20130101 |
Class at
Publication: |
429/218.1 ;
429/231.95 |
International
Class: |
H01M 004/58; H01M
004/40 |
Claims
What is claimed is:
1. An electrode composition comprising: an electrode material
consisting essentially of at least one electrochemically inactive
elemental metal and at least one electrochemically active elemental
metal in the form of an amorphous mixture at ambient temperature
that remains amorphous when said electrode composition is
incorporated into a lithium battery and cycled through at least one
full charge-discharge cycle at ambient temperature.
2. An electrode composition according to claim 1 wherein said
electrode material consists essentially of at least one
electrochemically inactive elemental metal and a plurality of
electrochemically active elemental metals.
3. An electrode composition according to claim 1 wherein said
electrode material consists essentially of plurality of
electrochemically inactive elemental metals and at least one
electrochemically active elemental metal.
4. An electrode composition according to claim 1 wherein said
electrochemically active elemental metal is selected from the group
consisting of aluminum, silicon, tin, antimony, lead, germanium,
magnesium, zinc, cadmium, bismuth, and indium.
5. An electrode composition according to claim 1 wherein said
electrochemically inactive elemental metal is selected from the
group consisting of molybdenum, niobium, tungsten, tantalum, iron,
nickel, manganese, and copper.
6. An electrode composition according to claim 1 wherein said
electrochemically active elemental metal is aluminum.
7. An electrode composition according to claim 1 wherein said
electrochemically active elemental metal is silicon.
8. An electrode composition according to claim 1 wherein said
electrochemically active elemental metal is tin.
9. An electrode composition according to claim 1 where said
electrochemically active elemental metals are aluminum and
silicon.
10. An electrode composition according to claim 1 wherein said
electrochemically active elemental metals are silicon and tin.
11. An electrode composition according to claim 1 wherein said
electrode material consists essentially of aluminum, silicon, and
manganese.
12. An electrode composition according to claim 1 wherein said
electrode material consists essentially of germanium, nickel,
silicon, and aluminum.
13. An electrode composition according to claim 1 wherein said
electrode material consists essentially of aluminum, silicon, and
copper.
14. An electrode composition according to claim 1 wherein said
electrode material consists essentially of silicon, tin, and
copper.
15. An electrode composition according to claim 1 wherein said
composition is in the form of a thin film.
16. An electrode composition according to claim 1 wherein said
composition is in the form of a powder.
17. A lithium ion battery comprising: (a) a first electrode
comprising an electrode material consisting essentially of at least
one electrochemically inactive elemental metal and at least one
electrochemically active elemental metal in the form of an
amorphous mixture at ambient temperature; (b) a counterelectrode;
and (c) an electrolyte separating said electrode and said
counterelectrode, wherein said electrode material remains amorphous
after said battery has been cycled through at least one full
charge-discharge cycle.
Description
STATEMENT OF PRIORITY
[0001] This application derives priority from a provisional
application filed on Jan. 13, 2000 bearing serial No. 60/175,893
which is hereby incorporated by reference in its entirety.
TECHNICAL FIELD
[0002] This invention relates to electrode materials useful in
secondary lithium batteries.
BACKGROUND OF THE INVENTION
[0003] Two classes of materials have been proposed as anodes for
secondary lithium batteries. One class includes materials such as
graphite and other forms of carbon, which are capable of
intercalating lithium. While the intercalation anodes generally
exhibit good cycle life and coulombic efficiency, their capacity is
relatively low. A second class includes metals that alloy with
lithium metal. Although these alloy-type anodes generally exhibit
higher capacities relative to intercalation-type anodes, they
suffer from relatively poor cycle life and coulombic
efficiency.
SUMMARY OF THE INVENTION
[0004] The invention provides electrode compositions suitable for
use in secondary lithium batteries in which the electrode
compositions have high initial capacities that are retained even
after repeated cycling. The electrode compositions, and batteries
incorporating these compositions, are also readily
manufactured.
[0005] To achieve these objectives, the invention features an
electrode composition that includes an electrode material
consisting essentially of at least one electrochemically inactive
elemental metal and at least one electrochemically active elemental
metal in the form of an amorphous mixture at ambient temperature.
The electrode material is essentially free of intermetallic
compounds. The mixture of elemental metals remains amorphous when
the electrode composition is incorporated into a lithium battery
and cycled through at least one full charge-discharge cycle at
ambient temperature. Preferably, the mixture remains amorphous
after cycling through at least 10 cycles, more preferably at least
100 cycles, and even more preferably at least 1000 cycles.
[0006] An "electrochemically active elemental metal" is a metal
that reacts with lithium under conditions typically encountered
during charging and discharging in a lithium battery. An
"electrochemically inactive elemental metal" is a metal that does
not react with lithium under those conditions.
[0007] An "amorphous mixture" is a mixture that lacks the long
range atomic order characteristic of crystalline material. The
existence of an amorphous mixture can be confirmed using techniques
such as x-ray diffraction, transmission electron microscopy, and
differential scanning calorimetry.
[0008] When incorporated in a lithium battery, the electrode
composition preferably exhibits (a) a specific capacity of at least
about 100 mAh per gram of active metal for 30 full charge-discharge
cycles and (b) a coulombic efficiency of at least 99% (preferably
at least 99.5%, more preferably at least 99.9%) for 30 full
charge-discharge cycles when cycled to realize about 100 mAh per
gram of active metal of the composition. Preferably, this level of
performance is realized for 500 cycles, more preferably for 1000
cycles.
[0009] In another preferred embodiment, the electrode composition,
when incorporated in a lithium battery, exhibits (a) a specific
capacity of at least about 500 mAh per gram of active metal for 30
full charge-discharge cycles and (b) a coulombic efficiency of at
least 99% (preferably at least 99.5%, more preferably at least
99.9%) for 30 full charge-discharge cycles when cycled to realize
about 500 mAh per gram of active metal of the composition.
Preferably, this level of performance is realized for 200 cycles,
more preferably for 500 cycles.
[0010] The electrode composition can be in the form of a thin film
or a powder. Thin films can be prepared using a number of
techniques, including sputtering and melt spinning. Examples of
suitable electrochemically active elemental metals include
aluminum, silicon, tin, antimony, lead, germanium, magnesium, zinc,
cadmium, bismuth, and indium. Examples of suitable
electrochemically inactive elemental metals include Group IB
through Group VIIB elemental metals, as well as group VIII and rare
earth elemental metals. Specific examples include Ti, V, Cr, Mn,
Fe, Co, Ni, Cu, Y, Zr, Nb, Mo, La, Hf, Ta, W, Ce, Pr, Nd, Eu, Gd,
Tb, Dy, Ho, Er, Yb, Lu, Be, and Sm. Of this group, molybdenum,
niobium, tungsten, tantalum, iron, nickel, manganese, and copper
are preferred.
[0011] Lithium-batteries including the above-described electrode
compositions may be used as power supplies in a variety of
applications. Examples include power supplies for motor vehicles,
computers, power tools, and telecommunications devices.
[0012] Other features and advantages of the invention will be
apparent from the following description of the preferred
embodiments thereof, and from the claims,
DESCRIPTION OF DRAWINGS
[0013] FIG. 1 is a 1 is an x-ray diffraction profile for the
aluminum-silicon-manganese sample described in Example 1, obtained
prior to cycling.
[0014] FIG. 2A illustrates the cycling performance, in terms of
reversible specific capacity, of the aluminum-silicon-manganese
sample described in Example 1.
[0015] FIG. 2B illustrates the cycling performance, in terms of
coulombic efficiency, of the aluminum-silicon-manganese sample
described in Example 1.
[0016] FIG. 3 is a differential voltage curve obtained for the
aluminum-silicon-manganese sample described in Example 1.
[0017] FIG. 4 is an x-ray diffraction profile for the
germanium-nickel-silicon-aluminum sample described in Example 2,
obtained prior to cycling.
[0018] FIG. 5 is a TEM diffraction pattern for the
germanium-nickel-silico- n-aluminum sample described in Example 2,
obtained prior to cycling.
[0019] FIG. 6 is a TEM photomicrograph of the
germanium-nickel-silicon-alu- minum sample described in Example 2,
obtained prior to cycling.
[0020] FIG. 7 illustrates the cycling performance, in terms of
reversible specific capacity, of the
germanium-nickel-silicon-aluminum sample described in Example
2.
[0021] FIGS. 8A and 8B are differential voltage curves obtained for
the germanium-nickel-silicon-aluminum sample described in Example
2.
[0022] FIG. 9 illustrates the results of a differential scanning
calorimetry test performed using the
germanium-nickel-silicon-aluminum sample described in Example
2.
[0023] FIG. 10 is an x-ray diffraction profile for the
aluminum-silicon-copper sample described in Example 3, obtained
prior to cycling.
[0024] FIG. 11 is a differential voltage curve obtained for the
aluminum-silicon-copper sample described in Example 3.
[0025] FIG. 12 illustrates the cycling performance, in terms of
reversible specific capacity, of the aluminum-silicon-copper sample
described in Example 3.
[0026] FIGS. 13(a)-(d) are in-situ x-ray diffraction scans obtained
from a cell constructed using the silicon-tin-copper sample
described in Example 4.
[0027] FIG. 14 illustrates the cycling performance, in terms of
charge rest voltage and trickle charge of the silicon-tin-copper
sample having an electrically conductive layer described in Example
5.
[0028] FIG. 15 is an x-ray diffraction profile for the
silicon-tin-copper sample having an electrically conductive layer
described in Example 4, obtained prior to cycling.
[0029] FIG. 16 illustrates the cycling performance, in terms of
irreversible capacity, of the annealed and unannealed
silicon-tin-copper samples described in Example 6.
DETAILED DESCRIPTION
[0030] The electrode compositions have the chemical composition and
microstructure described in the Summary of the Invention, above.
They may be in the form of thin films or powders. The thin films
may be prepared using techniques such as chemical and vapor
deposition, vacuum deposition (i.e., sputtering), vacuum
evaporation, melt spinning, splat cooling, spray atomization,
electrochemical deposition, and the like. The powders may be
prepared directly using techniques such as ball-milling or chemical
reduction of active metal. Alternatively, the powders may be
prepared in the form of thin films and then pulverized to form
powders.
[0031] The electrode compositions are particularly useful in
secondary lithium batteries. To prepare a battery, the film
containing the active material is used directly as the electrode.
Alternatively, if the active material is in the form of a powder,
the powder is combined with a binder (e.g., a polyvinylidene
fluoride binder) and solvent to form a slurry which is then coated
onto a backing using conventional coating techniques and dried to
form the electrode. The electrode is then combined with an
electrolyte and a counterelectrode.
[0032] The electrolyte may be a solid or liquid electrolyte.
Examples of solid electrolytes include polymeric electrolytes such
as polyethylene oxide, polytetrafluoroethylene, fluorine-containing
copolymers, and combinations thereof. Examples of liquid
electrolytes include ethylene carbonate, diethyl carbonate,
propylene carbonate, and combinations thereof. The electrolyte is
provided with a lithium electrolyte salt. Examples of suitable
salts include LiPF.sub.6, LiBF.sub.4, and LiClO.sub.4.
[0033] Examples of suitable counterelectrode compositions for
liquid electrolyte-containing batteries include LiCoO.sub.2,
LiCo.sub.0.2Ni.sub.0.8O.sub.2, and LiMn.sub.2O.sub.4, Examples of
suitable counterelectrode compositions for solid
electrolyte-containing batteries include LiV.sub.3O.sub.8 and
LiV.sub.2O.sub.5.
[0034] The invention will now be described further by way of the
following examples.
EXAMPLES
[0035] A. Electrode Preparation
[0036] Electrode materials were prepared in the form of thin films
either by sputtering or by melt spinning according to the following
procedures.
[0037] Sputtering Procedure #1
[0038] Electrodes in the form of thin films were prepared by
sequential or single target sputtering using a modified
Perkin-Elmer Randex Model 2400-8SA Sputtering System. The original
8 inch diameter of sputter sources were replaced with 6 inch
diameter dc magnetron sputtering sources commercially available
from Materials Science of San Diego Calif. The sputtering sources
were powered using Advanced Energy Model MDX-10 dc sputtering power
supplies operating in constant current mode. The turntable drive
unit of the Randex System was replaced with a stepper motor to
improve rotation speed range and control. The System was pumped
with an untrapped oil diffusion pump backed by a conventional
rotary vane pump.
[0039] Sputtering was performed at argon pressures in the range of
3-30 mTorr. The pressure was maintained by controlling the argon
flow in combination with a venetian blind-style conductance limiter
placed over the diffusion pump.
[0040] A copper foil (thickness=0.001 inch) was bonded to the
water-cooled substrate turntable of the Randex System using double
sided adhesive tape (3M Brand Y9415, commercially available from 3M
Company of St. Paul, Minn.). The system was closed and pumped down,
typically to base pressures below 1.times.10.sup.-5 Torr (the base
pressure prior to deposition is not critical). The samples were
etched prior to deposition using the "Sputter Etch" mode of the
Randex System with 13.56 MHz power applied to the substrate
turntable and an argon pressure of 8 mTorr in the sputtering
chamber. This procedure caused the copper foil surface to be
bombarded with moderate energy argon ions (100-150 eV) to further
clean the copper and insure good adhesion of the sputtered film to
the copper surface. A typical cleaning cycle was 150W for 30
minutes, with the substrate table rotating during the cycle.
[0041] Following etching, the sputtering sources were started up
using a mechanical shutter between the sources and the copper
substrate. This removed contaminants from the source surface
without depositing them on the substrate surface. Next, both
sources were started up at pre-determined current levels and
deposition initiated. After a suitable deposition time, one or both
sources were turned off.
[0042] Sputtering Procedure #2
[0043] Films were prepared using a sputtering coater consisting of
a conventional web handling system driving a 6 inch wide web over a
water-cooled chill roll opposing three 6 inch long water-cooled
sputtering targets. A multitude of water-cooled shields confined
the sputtering plasma and defined the exposed area of the moving
web. The coater was evacuated using a CTI-CRYOGENICS ON-BORD.RTM.
high vacuum cryo pump and a Leybold high vacuum turbo pump model
220 backed by a conventional roughing pump. The targets were
powered by an ADVANCED ENERGY MDX II dc power supply operated in
constant power mode. Sputtering was performed at 30 mTorr in an
argon atmosphere. Adjusting the sputtering power and the web speed
controlled the amount-of deposited material (coating weight).
[0044] Melt Spinning Procedure
[0045] In preparation for melt spinning, ingots of the metal
mixture were prepared as follows. A mixture of metal pieces was
placed in a 50 mL alumina crucible (Vesuvius McDaniel, Beaver
Falls, Pa.). The crucible was placed into a graphite fiber-wound
susceptor and the crucible-containing susceptor was placed into the
copper coil of a Model 2030 GCA Vacuum Induction Furnace (Centor
Vacuum Industries, Nashua, N.H.). The furnace was evacuated to a
vacuum of about 0.05 mTorr, the radio frequency power supply was
turned on, and the mixture was heated and melted for about 1 hour
to ensure alloying. After cooling, the resulting ingot was removed
from the crucible and broken with a hammer into smaller pieces to
form sample pieces for melt spinning.
[0046] The melt spinning process was conducted as follows. A
standard quartz nozzle for melt spinning was ground using 1000 grit
sandpaper to create an orifice at the tip measuring 0.030 inch in
diameter. A number of sample pieces were inserted into the nozzle
and suspended in a copper coil inside a vacuum chamber. The quartz
tube was connected to tubing that supplied pressurized nitrogen to
the nozzle. The nozzle was adjusted so that a height of 0.048 inch
was obtained between the nozzle tip and the surface of an 8 inch
diameter Cu/Be wheel. The chamber was evacuated to 66 mTorr and an
overpressure of 30 mm Hg was obtained between a nitrogen storage
tank and the inside of the vacuum chamber. The motor/belt driven
Cu/Be wheel was rotated at a speed of 2500 rpm and a sufficient
radio frequency power was supplied to the copper coil to melt the
alloy pieces in the induction field. When the pieces had liquefied,
nitrogen gas was applied to the nozzle to cause the molten metal to
be ejected onto the surface of the rotating Cu/Be wheel. The
process produced a ductile ribbon having a width of 1-2 mm and a
thickness of 0.0012 inch.
[0047] B. Transmission Electron Microscopy
[0048] Transmission electron microscopy ("TEM") was used to examine
the microstructure of the electrode samples before cycling. This
technique produces images of the microstructure using spatial
variations in transmitted intensity associated with spatial
variations in the structure, chemistry, and/or thickness of the
sample. Because the radiation used to form these images consists of
high energy electrons of very short wavelength, it is possible to
obtain information at the atomic scale under high resolution
electron microscopy (HREM) imaging conditions. Moreover, the
interaction of these electrons with the sample produces information
about the crystal structure (electron diffraction) and local
chemistry (x-ray microanalysis) that is complementary to the
information contained in the image.
[0049] Prior to cycling, samples were prepared from melt spun films
by cutting the film in random directions. The cut samples were then
embedded in 3M Scotchcast.TM. Electrical Resin #5 (commercially
available from 3M Company of St. Paul, Minn.) and ultramicrotomed
to obtain slices thin enough for TEM examination. Slice thickness
was nominally 24 nm.
[0050] The TEM instrumentation used to obtain microstructural data
was a HITACHI H9000-NAR transmission electron microscope which
operates at an accelerating voltage of 300 kV. It is capable of a
point-to-point resolution of 1.75 angstroms and a microprobe
resolution of 16 angstroms for x-ray microanalysis. The
microanalysis instrumentation consisted of a NORAN VOYAGER III.
Direct-to-digital image acquisition and quantitative length
measurements were performed by a GATAN slow-scan CCD
(charged-couple device) camera. Z-contrast images were generated
using a JEOL 2010-F field emission TEM/STEM having a resolution
limit for both imaging and microanalysis of 1.4 angstroms.
[0051] C. X-Ray Diffraction
[0052] Diffraction patterns were collected using a Siemens Model
Kristalloflex 805 D500 diffractometer equipped with a copper or
molybdenum target x-ray tube and a diffracted beam monochromator.
Approximately 2 cm.sup.2 samples of the thin film were mounted on
the sample holder. All the sputter-deposited samples were on a
copper substrate which gives rise to a series of identifiable
diffraction peaks at particular scattering angles. Specifically,
the copper substrate gives rise to peaks at scattering angles of
43.30 degrees, 50.43 degrees, and 74.13 degrees, corresponding to
Cu(111), Cu(200), and Cu(220), respectively.
[0053] To examine the electrode material during cycling, in-situ
x-ray diffraction experiments were performed at room temperature
using a 2325 coin cell. The cell was constructed using a 50
micrometer thick microporous polypropylene separator and a lithium
negative electrode. The electrolyte was 1 molal LiPF.sub.6 in a 1:1
v/v mixture of ethylene carbonate and diethyl carbonate. The coin
cell can was further provided with a circular hole measuring 18 mm
in diameter. A 21 mm diameter beryllium window (thickness=250
micrometers) was affixed to the inside of the hole using a pressure
sensitive adhesive (Roscobond from Rosco of Port Chester. N.Y.).
The electrode material was prepared by combining 85 wt. % active
powder, 10 wt. % Super-S carbon black (MMM Carbon, Belgium), and 5
wt.% polyvinylidene fluoride to form a coatable composition, and
then coating this composition directly onto the window before it
was attached to the can.
[0054] The cell was assembled and crimped closed in an argon-filled
glove box. It was tested with constant charge and discharge
currents (30 mA/g) and cycled between fixed capacity limits using a
MACCOR cycler. The first discharge was to a limit of 660 mAh/g. The
first charge was to 1.3V, and over 600 mAh/g of lithium was
extracted. The next discharge was to 720 mAh/g.
[0055] The cell was mounted in a Siemens D5000 diffractometer, and
slowly discharged and charged between 0.0V and 1.3V. The x-ray
diffractometer was repeatedly scanned every three hours. The
testing current was selected so that the discharge to 600 mAh/g
would take about 20 hours. Two hour x-ray diffraction patterns were
collected sequentially during the charge and discharge.
[0056] We now describe the preparation and characterization of
specific electrode samples.
Example 1
[0057] An amorphous film containing 54 wt. % aluminum, 28 wt. %
silicon, and 18 wt. % manganese was prepared by sputter deposition
according to Sputtering Procedure #1 described above from a single
target using a current of 1 amp for 120 minutes under 15 mTorr of
argon and a sample rotation rate of 38 rpm. The ternary single
target material was sputter deposited at a rate of 230
angstroms/minutes. The sputter-deposited film had a thickness of
2.8 microns and a density of about 2.9 g/cm.sup.3. There were no
pre- or post-layers.
[0058] The x-ray diffraction profile of the film was measured
according to the procedure described above using a molybdenum
target x-ray tube and is set forth in FIG. 1 The pattern shows no
peaks for crystalline aluminum, silicon, or manganese, or
crystalline intermetallic compounds of AlSiMn. All peaks present
originate from copper used in the sample backing.
[0059] The cycling behavior of the film was tested as follows. An
electrode was cut from the sputtered film with a die measuring 7.1
mm. The test cell was a half cell in which the film formed the
cathode and a lithium foil (about 300 micrometers thick, available
from Aldrich Chemical Co. of Milwaukee, Wis.) formed the anode of a
1225 coin cell.
[0060] The cell was constructed using a 50 micrometer thick
polyethylene separator. The electrolyte was 1 molal LiPF.sub.6 in a
1:1 v/v mixture of ethylene carbonate and diethyl carbonate. Copper
spacers were used as current collectors and to fill void areas in
the cell.
[0061] The electrochemical performance of the cell was measured
using a MACCOR cycler. The first discharge of the cell was a
constant current discharge at 0.5 mA/cm.sup.2 down to 5 mV and then
a constant voltage (5 mV) discharge until the current fell to 50
microamps/cm.sup.2. The initial discharge (lithiation) specific
capacity was about 1400 mAh/g. The cell was then cycled under
conditions set for constant current charge and discharge at
approximately a C/3 rate (0.5 mA/cm.sup.2) with cutoff voltages of
5 mV and 1.4 V. The reversible specific capacity and coulombic
efficiency of the cell are shown in FIGS. 2A and 2B, respectively.
The results demonstrate that the electrode film will reversibly
cycle at greater than 450 mAh/g for at least 450 cycles with a
coulombic efficiency that is greater than 99.0%.
[0062] The differential voltage curve for the electrode is shown in
FIG. 3. The curve shows that there are no significant changes in
the electrochemical behavior of the electrode during cycling,
indicating that no large crystalline regions developed upon
cycling.
Example 2
[0063] An amorphous melt-spun film containing 20 wt. % germanium,
10 wt. % nickel, 10 wt. % silicon, and 60 wt. % aluminum was
prepared according to the procedure described above. The x-ray
diffraction profile of the film was measured according to the
procedure described above using a copper target x-ray tube. A step
size of 0.05 degrees and a scan time of 5 seconds were used. The
results are set forth in FIG. 4. As shown in FIG. 4, the profile
lacks sharp peaks characteristic of a crystalline material.
[0064] The film was also subjected to TEM and electron diffraction
analysis prior to cycling according to the procedure described
above. The TEM diffraction pattern of the film, set forth in FIG.
5, lacks sharp rings or spots characteristic of a crystalline
material. A TEM photomicrograph, shown in FIG. 6, likewise lacks
features characteristic of a crystalline material.
[0065] The cycling behavior of the film was tested as follows. An
electrode was prepared from two strips of the melt-spun film, one
measuring 15.11 mm long by 1.15 mm wide and the other measuring
7.76 mm by 1.15 mm wide. The test cell was a half cell in which the
film formed the cathode and a lithium foil (about 0.015 inch thick
and 17 mm in diameter) formed the anode of a 2325 coin cell. The
cell was constructed using a 0.001 inch thick Celgard LLC separator
(Celgard of Charlotte, N.C.). The electrolyte was 1 molal
LiPF.sub.6 in a 1:1 v/v mixture of ethylene carbonate and diethyl
carbonate.
[0066] The electrochemical performance of the cell was measured
using a MACCOR cycler. The first discharge of the cell was a
constant current discharge at 0.5 mA/cm.sup.2 down to 5 mV and then
a constant voltage (5 mV) discharge until the current fell to 50
microamps/cm.sup.2. The initial discharge (lithiation) specific
capacity was about 800 mAh/g. The cell was then cycled under
conditions set for constant current charge and discharge at
approximately a C/3 rate (0.5 mA/cm.sup.2) with cutoff voltages of
5 mV and 1.4 V. The reversible specific capacity is shown in FIG.
7. The results demonstrate that the electrode film will reversibly
cycle at greater than 400 mAh/g for at least 20 cycles.
[0067] The differential voltage curves for the electrode are shown
in FIGS. 8A (cycles 0-5) and 8B (cycles 6-10). The curves show that
there are no significant changes in the electrochemical behavior of
the electrode during cycling, indicating that no crystalline
regions developed upon cycling.
[0068] The crystallization temperature of the film was determined
by a differential scanning calorimetry (DSC) using a Seiko
Instruments DSC220C model calorimeter. A 1.58 mg sample of the film
was used. The calorimeter was programmed to stabilized at, 25 EC
for 20 minutes, then ramp from 25 EC to 450 EC at a rate of 5
EC/minute, and finally to ramp from 450 EC to 25 EC at a rate of 10
EC/minute. During the test, the sample chamber was flooded with
argon gas. The results are shown in FIG. 9 and demonstrate that the
sample has a crystallization temperature of greater than 150
EC.
Example 3
[0069] An amorphous film containing 74 wt. % aluminum-silicon (50
wt. % aluminum and 24 wt. % silicon) and 26 wt. % copper was
prepared by sputter deposition according to Sputtering Procedure #1
described above from an aluminum-silicon target and a copper
target. Based upon elemental analysis, the aluminum-silicon target
contained 68 wt. % aluminum and 32 wt. % silicon. Deposition was
accomplished under 12 mTorr of argon using a substrate rotation
rate of 38 rpm. The sputter rates were 180 angstroms/minute for the
aluminum-silicon target and 18 angstroms/minute for the copper
target. The sputter-deposited film had a thickness of 4.61 microns
and a density of about 3.13 g/cm.sup.3. The sample also had a 300
angstrom thick pre-layer of copper and a 300 angstrom thick
post-layer of aluminum-silicon.
[0070] The x-ray diffraction profile of the film was measured
according to the procedure described above using a copper target
x-ray tube and is set fourth in FIG. 10. All peaks present
originate from copper used in the sample backing. This is
demonstrated by the fact that an x-ray diffraction profile taken of
a film prepared according to the same procedure but with the copper
backing removed does not exhibit these peaks.
[0071] The cycling behavior of the film was tested following the
procedure described in Example 1. FIG. 11 illustrates the
differential capacity of the sample measured during the first 6
cycles of charge. The differential capacity is smooth and
featureless, consistent with the absence of crystalline material.
The reversible capacity of the film, measured at C/40, was about
700 mAh/g for the first two recharge cycles. As shown in FIG. 12,
the sample retained a capacity of about 600 mAh/g for over 100
cycles at C10.
Example 4
[0072] An amorphous film containing 30 wt. % silicon, 66 wt. % tin,
and 4 wt. % copper was prepared by sputter deposition according to
Sputtering Procedure #2 described above using 11 kW total power for
the three individual targets. The sputtering was conducted under 30
mTorr of argon using a web speed of 0.24 ft/min. Three targets of
identical silicon/tin/copper composition were sputter deposited at
a rate of about 3 grams/kwh. A 10 micron thick copper foil (Japan)
coated with a binder was used as the backing. The backing was
prepared by coating the foil with a 6 wt. % solids dispersion of 40
wt. % Super P carbon and 60 wt. % polyvinylidene fluoride in
N-methyl-2-pyrrolidinone using an 8 mil notch bar, followed by
drying under vacuum at 60.degree. C. for 4 hours to remove residual
solvent. The dry binder thickness was about 8 microns. The sputter
deposited film had a thickness of about 5 microns and a density of
about 4 g/cm.sup.3.
[0073] The x-ray diffraction profile of the silicon-tin-copper
material was obtained using a molybdenum target x-ray tube and is
set forth in FIG. 15. It is characterized by the absence of
crystalline tin and silicon. The large peaks are due to the copper
foil current collector.
[0074] With the help of acetone, the film was scraped from the
copper backing using a razor blade, pulverized, and sieved using a
270-mesh sieve (U.S. standard sieve size; ASTM E-11-61). This
material was then used to construct a 2325 coin cell for in situ
x-ray diffraction measurements. To prepare the coin cell, a
dispersion was prepared having 86 wt. % of the material, 7 wt. %
Super, P carbon (MMM Carbon, Belgium), and 7 wt. % polyvinylidene
fluoride binder in N-methyl-2-pyrrolidinone. The dispersion was
then coated onto a copper foil and dried under vacuum for several
hours to remove residual solvent. The resulting coated foil was to
construct the 2325 coin cell using a lithium foil (about 300
micrometers thick, available from Aldrich Chemical Co. of
Milwaukee, Wis.) as the counterelectrode. The cell was constructed
using a 50 micrometer thick polyethylene separator. The electrolyte
was 1 molal LiPF.sub.6 in a 1:1 v/v mixture of ethylene carbonate
and diethyl carbonate.
[0075] In situ x-ray diffraction measurements were performed as
described above using a copper target x-ray tube. The results are
shown in FIGS. 13(a)-(d). FIG. 13(d) shows the initial pattern of
the electrode before the discharge current was initiated. All sharp
peaks in the pattern originate from components of the cell (e.g.,
beryllium, beryllium oxide on the window, etc.). These peaks do not
change during charge and discharge. The broad peaks centered near
26 and 43 degrees are due to the silicon-tin-copper electrode.
[0076] FIG. 13(c) shows the x-ray diffraction pattern measured
after 660 mAh/g of lithium has been incorporated with the
electrode. As shown in the figure, the broad peaks have shifted in
position due to the reaction of lithium. The peaks remain broad. No
evidence of crystallization is observed.
[0077] FIG. 13(b) shows the state of the electrode after the first
removal of all the lithium. The pattern returns to that of the
original material shown in FIG. 13(d). Again, no evidence of
crystallization is observed.
[0078] FIG. 13(a) shows the state of the electrode after lithium
has been inserted again to the level of 720 mAh/g. Once again, no
evidence of crystallization is observed.
Example 5
[0079] An electrode was prepared following the procedure of Example
4 except that the final dispersion used to prepare the electrode
was made by mixing 1 gram of the sieved powder and 16 grams of a
4.5% solids dispersion of Super P carbon and polyvinylidene
chloride (70:30) in N-methyl-2-pyrrolidinone. The final dried
coating contained 50 wt. % active silicon-tin-copper, 35 wt. %
Super P carbon, and 15 wt. % polyvinylidene fluoride. The electrode
was used to construct a 2325 coin cell as described in Example
4.
[0080] The electrochemical performance of the cell, in terms of
trickle charge capacity and charge rest voltage, was measured using
a MACCOR cycler. The cell was first discharged at a high rate of
350 mA/g to a fixed capacity of 700 mAh/g to lithiate the
electrode. The cell was then charged at a rate of 350 mA/g to a
voltage of 1.2V to extract lithium from the electrode. Next, the
cell was allowed to rest (zero current) for 15 minutes, after which
the cell voltage may drop below 1.0V. The potential at the end of
this rest period was recorded as the "charge rest voltage." It
provides a measure of the amount of lithium remaining in the
electrode. In general, the higher the charge rest voltage and the
more stable it is versus cycle number, the more effectively lithium
is being removed.
[0081] At the end of the rest period, the cell was charged at a low
rate ("trickle charge") of 35 mA/g to 1.2V to remove any lithium
not removed at the higher rate (350 mA/g). The trickle charge
capacity is a measure of the extent of lithium removal and is
analogous to coulombic efficiency. In general, the more lithium
removed during application of the trickle charge, the less
effective the electrode is at giving up lithium during the high
rate charge. Accordingly, it is desirable to minimize the trickle
charge capacity for a given cycle, and to maintain a low trickle
charge capacity after repeated cycling.
[0082] The results for the sample are shown in FIG. 14. The results
demonstrate that the electrode performs well, both in terms of
trickle charge and charge rest voltage,
Example 6
[0083] An amorphous film containing 30 wt. % silicon, 66 wt. % tin,
and 4 wt. % copper was prepared as described in Example 4, and
cycled according to the protocol described in Example 5. The
irreversible capacity of this film was calculated as the difference
between the discharge and charge capacity after each cycle. For the
sake of comparison, three other films were prepared but they were
annealed at 150.degree. C. for 24 hours in a vacuum oven prior to
measuring its irreversible capacity as a function of cycle number.
Annealing results in the production of a semi-crystalline film. The
results for all four films are shown in FIG. 16. The amorphous film
is labeled "A" in FIG. 16 and was cycled at a four hour rate. The
annealed films were labeled "B", "C", and "D", and were cycled at
rates of six, two, and four hours, respectively. The results
demonstrate that the amorphous film had an irreversible capacity
that was significantly lower that that of the annealed films.
[0084] Other embodiments are within the scope of the following
claims.
* * * * *