U.S. patent application number 13/293165 was filed with the patent office on 2013-05-16 for lithium-ion battery electrodes with shape-memory-alloy current collecting substrates.
This patent application is currently assigned to GM GLOBAL TECHNOLOGY OPERATIONS LLC. The applicant listed for this patent is Anil K. Sachdev, Mark W. Verbrugge, Xingcheng Xiao. Invention is credited to Anil K. Sachdev, Mark W. Verbrugge, Xingcheng Xiao.
Application Number | 20130122374 13/293165 |
Document ID | / |
Family ID | 48225426 |
Filed Date | 2013-05-16 |
United States Patent
Application |
20130122374 |
Kind Code |
A1 |
Verbrugge; Mark W. ; et
al. |
May 16, 2013 |
LITHIUM-ION BATTERY ELECTRODES WITH SHAPE-MEMORY-ALLOY CURRENT
COLLECTING SUBSTRATES
Abstract
Lithium-ion battery electrode constructions use an array of
nanowires (or of other long thin shapes) of active electrode
material anchored at one end to a surface of a metal current
collector sheet in an electrode assembly or construction. This form
of active electrode material permits good contact with a liquid
lithium-ion containing electrolyte that infiltrates the
closely-spaced thin shapes. Stresses arising from volume changes in
the long shapes with lithiation and de-lithiation of the active
material is mitigated by strategic placement of shape memory apply
forms between attachment surfaces of the active electrode material
and other members of the electrode assembly.
Inventors: |
Verbrugge; Mark W.; (Troy,
MI) ; Xiao; Xingcheng; (Troy, MI) ; Sachdev;
Anil K.; (Rochester Hills, MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Verbrugge; Mark W.
Xiao; Xingcheng
Sachdev; Anil K. |
Troy
Troy
Rochester Hills |
MI
MI
MI |
US
US
US |
|
|
Assignee: |
GM GLOBAL TECHNOLOGY OPERATIONS
LLC
DETROIT
MI
|
Family ID: |
48225426 |
Appl. No.: |
13/293165 |
Filed: |
November 10, 2011 |
Current U.S.
Class: |
429/231.8 ;
429/218.1 |
Current CPC
Class: |
H01M 4/134 20130101;
H01M 4/70 20130101; H01M 4/661 20130101; H01M 4/667 20130101; H01M
10/0585 20130101; H01M 4/133 20130101; Y02E 60/10 20130101; H01M
4/0421 20130101; H01M 4/662 20130101; B82Y 30/00 20130101; Y02T
10/70 20130101; H01M 4/1395 20130101; H01M 10/052 20130101; H01M
4/1393 20130101 |
Class at
Publication: |
429/231.8 ;
429/218.1 |
International
Class: |
H01M 4/583 20100101
H01M004/583 |
Claims
1. A lithium-ion electrochemical cell comprising a negative
electrode comprising a current collector carrying a negative
electrode active material; a positive electrode comprising a
current collector carrying a positive electrode active material; a
liquid electrolyte containing lithium ions, the liquid electrolyte
contacting both electrode materials and providing transport of
lithium ions between the negative electrode active material and
positive electrode active material; and a separator preventing
electrical contact between the positive and negative electrodes
while providing transport of lithium ions between the electrode
materials; the negative electrode active material and positive
electrode active material being formed of different compositions
with each electrode active material composition being subject to
lithiation and de-lithiation during discharge and charge cycling of
the electrochemical cell; at least one of the negative electrode
and positive electrode further comprising: a metallic current
collector sheet having opposing sheet surfaces and being formed of
a metal composition that is not subject to lithiation, or volume
change due to lithiation, in the cycling of the electrochemical
cell; electrode active material formed on at least one of the
current collector sheet surfaces; the electrode active material
being formed as one or more bodies that extend outwardly from the
sheet surface a distance of at least one micrometer with transverse
body sections in contact with the current collector sheet and
having thicknesses less than five hundred nanometers, the one or
more bodies of electrode active material experiencing repeated
volume changes during lithiation and de-lithiation of cell cycling;
and a body of an elastic shape memory alloy formed against
transverse body sections of the electrode active material in
contact with a current collector sheet surface.
2. A lithium-ion electrochemical cell as recited in claim 1 in
which the shape memory alloy is NiTi.
3. A lithium-ion electrochemical cell as recited in claim 1 in
which the shape memory alloy also serves as a current collector for
the electrode material.
4. A lithium-ion electrochemical cell as recited in claim 1 in
which the negative electrode active material comprises at least one
element selected from the group consisting of silicon, tin, and
graphite.
5. A lithium-ion electrochemical cell as recited in claim 1 in
which the negative electrode active material comprises at least one
element selected from the group consisting of silicon, tin, and
graphite, the metallic current collector is high conductivity
copper, and the shape memory alloy is NiTi.
6. A lithium-ion electrochemical cell comprising a negative
electrode comprising a current collector carrying a negative
electrode active material; a positive electrode comprising a
current collector carrying a positive electrode active material; a
liquid electrolyte containing lithium ions, the liquid electrolyte
contacting both electrode active materials and providing transport
of lithium ions between the negative electrode active material and
positive electrode active material; and a separator preventing
electrical contact between the positive and negative electrodes
while providing transport of lithium ions between the electrode
materials; the negative electrode active material and positive
electrode active material being formed of different compositions
with each electrode material composition being subject to
lithiation and de-lithiation during discharge and charge cycling of
the electrochemical cell; the negative electrode further
comprising: a metallic current collector sheet having opposing
sheet surfaces and a thickness up to about thirty micrometers, the
current collector sheet being formed of a metal composition that is
not subject to lithiation, or volume change due to lithiation, in
the cycling of the electrochemical cell; negative electrode active
material formed on at least one of the current collector sheet
surfaces; the negative electrode material being formed as one or
more bodies that extend outwardly from the sheet surface a distance
of at least one micrometer with transverse body sections in contact
with the current collector sheet and having transverse thicknesses
less than five hundred nanometers, the one or more bodies of
electrode active material experiencing repeated volume changes
during lithiation and de-lithiation of cell cycling; and a body of
an elastic shape memory alloy formed against transverse body
sections of the electrode active material in contact with a current
collector sheet surface.
7. A lithium-ion electrochemical cell as recited in claim 6 in
which the shape memory alloy is NiTi.
8. A lithium-ion electrochemical cell as recited in claim 6 in
which the shape memory alloy also serves as a current collector for
the electrode active material.
9. A lithium-ion electrochemical cell as recited in claim 6 in
which the negative electrode active material comprises at least one
element selected from the group consisting of silicon, tin, and
graphite.
10. A lithium-ion electrochemical cell as recited in claim 6 in
which the negative electrode active material comprises at least one
element selected from the group consisting of silicon, tin, and
graphite, the metallic current collector is high conductivity
copper, and the shape memory alloy is NiTi.
11. A lithium-ion electrochemical cell as recited in claim 6 in
which the negative electrode active material is formed as nanowires
each attached at an end to a base of shape memory alloy which in
turn is formed on a surface of the metal current collector
sheet.
12. A lithium-ion electrochemical cell as recited in claim 11 in
which the negative electrode active material consists of about one
million to about one trillion nanowires per square millimeter of
metal current collector sheet and the lengths of the nanowires
extends to a height up to about 100 micrometers from the surface of
the current collector sheet.
13. A lithium-ion electrochemical cell as recited in claim 6 in
which the negative electrode active material is formed as nanowires
attached at their ends to a layer of shape memory alloy which also
is the metallic current collector sheet.
14. A lithium-ion electrochemical cell as recited in claim 13 in
which the negative electrode active material consists of about one
million to about one trillion nanowires per square millimeter of
metal current collector sheet and the lengths of the nanowires
extends to a height up to about 100 micrometers from the surface of
the current collector sheet.
15. A lithium-ion electrochemical cell as recited in claim 6 in
which a base layer of shape memory alloy is deposited on a surface
of the metal collector sheet with an array of spaced nanoposts of
shape memory alloy extending to uniform heights from the base layer
and particles of negative electrode active material filling the
space on the base layer of shape memory alloy and between the
spaced nanoposts of shape memory alloy.
16. A lithium-ion electrochemical cell as recited in claim 15 in
which height of the array of spaced nanoposts of shape memory alloy
is up to about one hundred micrometers.
17. A lithium-ion electrochemical cell as recited in claim 6 in
which a sheet of shape memory alloy is the negative electrode
current collector and at least one surface of the shape memory
alloy sheet carries an array of spaced nanoposts of shape memory
alloy extending to uniform heights from the base layer and a
continuous film of negative electrode active material is deposited
over, around, and between each nanopost and on the surface of the
shape memory alloy sheet.
18. A lithium-ion electrochemical cell as recited in claim 17 in
which the height of the nanoposts is up to about ten micrometers
and the thickness of the film of negative electrode active material
is up to about fifty micrometers.
Description
TECHNICAL FIELD
[0001] This invention pertains to electrode materials and
structures for the cells of lithium-ion batteries in which lithium
atoms are repeatedly inserted into and removed from small,
wire-like shapes of negative electrode active material during
charging and discharging of the battery. More specifically, this
invention pertains to the formation of such shapes of electrode
materials, anchored in an elastic shape memory alloy substrate, to
enable longer and more effective cycling of such electrode
materials.
BACKGROUND OF THE INVENTION
[0002] Lithium-ion batteries are used as electric storage systems
for powering electric and hybrid electric vehicles. These batteries
comprise a plurality of suitably interconnected electrochemical
cells arranged to provide a predetermined electrical current at a
specified electrical potential. In each such cell, lithium is
transported as lithium ions from a negative electrode through a
non-aqueous, lithium-containing, electrolyte solution to a lithium
ion-accepting positive electrode as an electronic current is
delivered from the battery to an external load, such as an electric
traction motor. A suitable porous separator material, infiltrated
with the electrolyte solution and permeable to the transport of
lithium ions in the electrolyte, is employed to prevent
short-circuiting physical contact between the electrodes. Graphite
has been used as a negative electrode material and bonded in a thin
electrode layer on a copper current collector. During charging of
the cells, lithium is inserted into the graphite (lithiation,
forming LiC.sub.6, about 372 mAh/g) and extracted from the
graphitic carbon during discharging (de-lithiation). A suitable
particulate material for receiving and storing inserted lithium
during discharge of each cell is used as the positive electrode
material. Examples of such positive electrode materials include
lithium cobalt oxide (LiCoO.sub.2), a spinel lithium transition
metal oxide such as spinel lithium manganese oxide
(LiMn.sub.xO.sub.y), a lithium polyanion such as a
nickel-manganese-cobalt oxide
[Li(Ni.sub.xMn.sub.yCo.sub.z)O.sub.2], lithium iron phosphate
(LiFePO.sub.4), or lithium fluorophosphate (Li.sub.2FePO.sub.4F),
or a mixture of any of these materials. Suitable positive electrode
materials are often bonded as a thin layer to an aluminum current
collector. The electrochemical potential of such lithium ion cells
is typically in the range of about 2 to 4.5 volts.
[0003] The use of lithium-ion batteries to power electric motors in
automotive vehicles has led to the need for higher gravimetric
and/or volumetric capacity batteries. While graphitic carbon is a
durable and useful lithium-intercalating, negative electrode
material for lithium-ion cells, it has a relatively low capacity
for such lithium insertion. Other potential negative electrode
materials such as silicon (theoretical capacity, 3578 mAh/g for
Li.sub.15Si.sub.4) and tin (theoretical capacity, 994 mAh/g for
Li.sub.22Sn.sub.5) have much higher theoretical capacities than
graphite for lithium insertion. However, the volume change of up to
300 volume percent for silicon during lithiation and de-lithiation
processes leads to fracture of the active silicon material and/or
loss of electrical contact with the conductive additives or the
current collectors. And tin has the same problem of a large volume
expansion upon lithiation, leading to rapid capacity
degradation.
[0004] The basic mechanism responsible for the loss of capacity of
a battery due to fracture of the electrode materials in its cells
is the loss of electrical contact with conductive material and the
creation of new surfaces, which irreversibly consume the active
lithium to form new solid electrolyte interfaces. Both problems
shorten the effective cycling capacity of a battery. There remains
a need for a more effective way or material form of utilizing
silicon or tin in negative electrodes of lithium-ion cells.
SUMMARY OF THE INVENTION
[0005] This invention pertains to lithium-ion electrochemical cells
that use electrode materials with relatively high capacities for
inserting and de-inserting lithium in repeated cycling of the cell.
While embodiments of the invention may be used advantageously with
electrode materials in general, the invention is particularly
useful with respect to materials for negative electrodes of
lithium-ion cells.
[0006] In lithium-ion electrochemical cells, negative electrode
materials typically experience significant volume change as lithium
is absorbed or released. In accordance with embodiments of the
invention, such electrode materials are used in the form of
nanowires or in other relatively long, thin (nanometer size
thicknesses) shapes for providing abundant surface area for contact
with a liquid lithium-ion containing electrolyte. The nanowires or
thin shapes may have a diameter or cross-section thickness of, for
example, 2 nm to 200 nm and a length (or height) of five to fifty
micrometers. Ends of the nanowires or long, thin shapes of
electrode material are in contact with a metal current collector
for conduction of electrons from and to the active material. In
further accordance with embodiments of the invention, structural
members of a suitable super-elastic shape memory alloy are placed
or formed so as to mitigate stress in the long members of active
material, particularly in portions of the members where they are
fixed to a current collector. The shape memory alloy members of the
negative electrode construction are, thus, shaped and placed to
release the stress of volume changes in the electrode material as
it is repeatedly lithiated and de-lithiated in the cycling of the
electrochemical cell.
[0007] Suitable negative electrode materials include silicon or
tin, and may even include carbon (graphite) or other
lithium-accepting material when used in wire-like or other long,
thin shapes. When the active negative electrode material is in the
form of nanowires, one end of each wire-shape may be attached to a
negative electrode current collector through a super-elastic shape
memory alloy base member. Each wire-end and SMA interfacial base
member may, for example, be fixed to (or in) a surface of a copper
current collector. In this embodiment, many closely spaced
nanowires of active material extend side-by-side from their SMA
attachment members and the surface of the copper current collector.
A representative negative electrode may have, for example, a
million to a trillion upstanding wires or wire shapes per square
millimeter of current collector surface. In an assembled
lithium-ion battery, many interconnected cells containing these
wire shapes of negative electrode active material are infiltrated
with a non-aqueous liquid solution of a lithium ion containing
electrolyte. And in each cell a suitable electrolyte permeable
separator material isolates the negative electrode material from
complementary positive electrolyte material. Such closely spaced
wires, with nanometer-size diameters, may have lengths of, for
example, up to fifty or one hundred micrometers. The nanowires
provide abundant surface areas for absorbing lithium ions from the
electrolyte and releasing lithium ions to the electrolyte, with
concurrent flow of electrons, during charging and discharging of
the lithium-ion battery.
[0008] In other embodiments of the invention a SMA material may be
used as the current collector and nanowires of active material may
be formed with their ends attached to the SMA surface, or with the
ends of the active material nanowires anchored in shallow cavities
in the SMA current collector surface. Again, the nanowires may have
lengths of up to fifty to one hundred nanometers.
[0009] In still other embodiments of the invention, SMA material
may be deposited on a metal current collector as a base layer
having many upstanding nanometer-diameter posts for confining a
continuous porous body of particles (bonded or unbonded) of active
electrode material adjacent the collector surface and surrounding
the regularly spaced posts. The posts of SMA would be placed to
form many wire-like sections in the continuous phase of active
material, with the SMA posts and surrounding active material
extending fifty to one hundred micrometers from the surface of the
current collector. In still another embodiment of the invention, a
like base layer of SMA with nanometer posts is formed and a
continuous nanometer-thick coating of active electrode material is
applied over and between the posts and against the SMA base layer.
Again, the generally continuous film of active material has
nanometer size thicknesses but with wall lengths overlying the
posts that extend for many micrometers.
[0010] Such electrode material forms, whether of wire-shapes or
other long, thin shapes, with both nanometer and micrometer
dimensions, expand as lithium atoms enter the long thin shapes
(lithiation) during cell charging and shrink as lithium leaves the
wire shapes (as lithium ions, de-lithiation) during cell discharge.
While much of the lengthy regions of such electrode material shapes
may accommodate such size changes, the fixed thin sections, such as
wire ends anchored to a metal current collector surface, experience
much strain which may break the connection between the active
material and the current collector.
[0011] In accordance with embodiments of the invention, vulnerable
thin sections of electrode material nanometer shapes are grown on,
or embedded in a shape memory alloy (SMA) material. This SMA
material in the electrode structure is placed and formed so as to
accommodate growth and shrinkage of the thin shapes of active
electrode material during cell operation, and to prevent them from
breaking at critical locations and reducing the energy capacity of
the cell. A nickel-titanium alloy, composed of 53 atomic percent
titanium, is an example of a shape memory alloy that that displays
superelastic properties. Nickel-titanium alloy (sometimes NiTi in
this specification) is commercially available. NiTi and like
superelastic metal alloys are used in practices of this invention
to provide a stress buffer between the base ends of the many
negative electrode material nanowires or other material shapes and
the metal current collector bodies to which they are anchored. As
will be described in more detail, the shape memory alloy buffer
material displays large reversible deformation in response to the
changing size of the ends of the nanowires and reduces the stress
on them.
[0012] Masking techniques and material deposition techniques are
used to selectively place SMA material on one or both surfaces of a
strip of copper current collector to provide a stress-relieving
bond between the wire-shapes of electrode material and the current
collector. In some cases the SMA material is located at the bonded
end of the wire-shapes (which may be embedded in the surface of the
current collector), and in other embodiments the SMA material also
encloses the active material along some or much of the length of
the wire-shape.
[0013] In other embodiments, the SMA also serves as the current
collector for the nanowire shapes as it carries and supports them
for stress relief. Practices are disclosed in this specification
for forming suitable bases and shapes of the SMA with nanometer
size holes for growth and support of the nanowires of active
electrode material.
[0014] Other objects and advantages of the invention will be
apparent from detailed descriptions of practices for forming the
SMA and nanowire structures and arrangements. Reference will be had
to illustrative drawing figures which are described in the
following section of this specification.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is an enlarged schematic illustration of a few
electrochemical cells of an exemplary lithium ion battery
arrangement. Each cell includes a thin, rectangular negative
electrode, a like-shaped positive electrode, and a separator
sandwiched between the electrodes.
[0016] FIG. 2 is a further enlarged and partially-exploded
cross-sectional view of one of the electrochemical cells shown in
FIG. 1. FIG. 2 also illustrates the metallic current collectors
that support the respective electrode materials. FIG. 2 also
illustrates a negative electrode with wire-shapes of negative
electrode material of this invention Some elements of the cell are
shown separated for more clear illustration, but they are actually
in compressed face-to-face contact and the electrode materials are
formed on, or bonded to, their respective current collectors.
[0017] FIGS. 3(a)-3(f) are a series of schematic illustrations in
cross-section of progressively formed elements of a negative
electrode for describing a method of forming nanowires of negative
electrode material on a conventional copper current collector with
a layer of NiTi shape memory alloy interposed between the ends of
the nanowires and the surface of the copper. Six steps of the
process are illustrated, from the top to the bottom of the Figure,
starting with the provision of the copper current collector
layer.
[0018] FIGS. 4(a)-4(g) are a series of schematic illustrations in
cross-section of progressively-formed elements of a negative
electrode for describing another method of forming nanowire-like
shapes of negative electrode material on a conventional copper
current collector with an interposed layer of NiTi shape memory
alloy on the surface of the copper current collector. The SMA
material is shaped to confine a mass of particles of negative
electrode material in wire-like shapes. Seven steps of the process
are illustrated, from the top to the bottom of the Figure, starting
with the provision of the copper current collector layer with a
layer of NiTi shape memory alloy applied to one surface of the
copper layer.
[0019] FIGS. 5(a)-5(g) are a series of schematic illustrations in
cross-section of progressively-formed elements of a negative
electrode for describing another method of forming nanowire-like
shapes of negative electrode material on a layer of NiTi shape
memory alloy which also serves as the current collector for the
applied nanowires.
[0020] FIGS. 6(a)-6(f) are a series of schematic illustrations in
cross-section of progressively formed elements of a negative
electrode for describing another method of forming a thin layer of
negative electrode material on a layer of NiTi shape memory alloy
which also serves as the current collector and provides NiTi
supportive posts for the applied active material.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0021] An exemplary and generalized illustration of a lithium-ion
battery 10 is depicted in FIG. 1. The lithium-ion battery 10 shown
here includes several thin rectangular-shaped electrochemical
battery cells 12 that are each bracketed by metallic current
collectors. The electrochemical battery cells 12 are stacked
side-by-side in a modular configuration and, in this example,
connected in parallel. A lithium-ion battery 10 may be formed of
many like electrochemical cells in electrical series or in parallel
connection to form a lithium ion battery that exhibits the voltage
and current capacity demanded for a particular application. It
should be understood the lithium ion battery 10 shown here is only
a schematic illustration. FIG. 1 is presented to show the relative
position and physical interactions of the various components that
constitute the electrochemical battery cells 12 (i.e., the
electrodes and the separator); it is not intended to inform the
relative sizes of the electrochemical battery cells' components, to
define the number of electrochemical battery cells 12 in the
lithium ion battery 10, or to limit the wide variety of structural
configurations the lithium ion battery 10 may assume.
[0022] The electrochemical cell 12 (one indicated) contained in the
lithium ion battery 10 includes a negative electrode 14 (also
referred as negative electrode material), a positive electrode 16
(also referred to a positive electrode material), and the separator
18 situated between the two opposing electrodes 14, 16. Each of the
negative electrode 14, the positive electrode 16, and the separator
18 is wetted with a liquid electrolyte solution that enables the
transport of lithium ions between the electrodes 14, 16. A
negative-electrode metallic current collector 20 (typically,
copper) that includes a negative polarity tab 22 is located between
the back-to-back negative electrodes 14 of adjacent electrochemical
cells 12. Likewise, a positive-side metallic current collector 24
(typically, aluminum) that includes a positive polarity tab 26 is
located between neighboring positive electrodes 16. The negative
polarity tab 22 is electrically coupled to a negative terminal 28
and the positive polarity tab 26 is electrically coupled to a
positive terminal 30. Each electrode material 14, 16 is typically
formed on or bonded to its respective metallic current collector
20, 24. An applied compressive force usually presses the metallic
current collectors 20, 24 and their electrodes 14, 16 against the
separator 18 to achieve intimate interfacial contact between
adjacent contacting components. Negative terminal 28 and positive
terminal 30 are connected to an electrical power consuming load 50.
For example, a suitable battery pack comprising many like
individual cells may be provided to power an electric traction
motor to drive the wheels in an automotive vehicle. In such a
battery pack many cells are connected in groups in electrical
parallel arrangement for providing suitable energy capacity and
many groups are connected in series to provide a suitable
electrical voltage potential.
[0023] A cross-sectional view (partially presented as an exploded
view) of the electrochemical battery cell 12 and its associated
metallic current collectors 20, 24 is generally illustrated in FIG.
2. In many battery cells, the negative electrode 14 includes a
distinguishable inner face surface and an outer face surface
relative to the location of the separator 18. In this invention the
negative electrode material 14 consists essentially of nanowires or
wire-like shapes of silicon, tin, carbon or other suitable material
susceptible to lithiation and de-lithiation. In the embodiment of
FIG. 2, the inner ends of nanowires are bonded, using a shape
memory alloy to the negative metallic current collector 20. The
outer ends of the nanowires making up the negative electrode
material present an outer surface or face 32 (outer with respect to
the negative electrode current collector).
[0024] The positive electrode 16 often includes a distinguishable
inner face surface 38 lying against the positive current collector
24 and an outer face surface 38. The negative electrode 14 may, but
is not required to, encompass a larger two-dimensional surface area
than the corresponding area of the positive electrode 16, as shown.
When assembled into the electrochemical battery cell 12, the outer
face surfaces 32, 36 of the negative and positive electrodes 14, 16
confront one another and are pressed against a negative-side major
face surface 40 and a positive-side major face surface 42 of the
separator 18, respectively. Such pressed-engagement occurs
generally uniformly along the entire interface of the major face
surfaces 40, 42 of the separator 18 and the corresponding portions
of the inner face surfaces 32, 36 of the electrodes 14, 16. Both of
the metallic current collectors 20, 24 engage their respective
electrode materials 14, 16 over an appreciable interfacial surface
area to facilitate the efficient collection and conduction of free
electrons.
[0025] In many lithium-ion batteries the elements of
electrochemical cells 12 are made of materials such that they are
generally thin and flexible. By way of illustrative example, a
typical thickness (T in FIG. 2) of the electrochemical cell 12
including the negative electrode current collector 20, the negative
electrode material 12, the separator 18, the positive electrode 16
and its current collector 26 may about 80 .mu.m to about 350 .mu.m.
By way of further example, each electrode material 14, 16 may be
about 20 .mu.m to 200 .mu.m thick and the separator 18 may be about
10 .mu.m to 50 .mu.m thick. The metallic current collectors 20, 24
are often about 5 .mu.m to 20 .mu.m thick. The relatively thin and
flexible nature of the elements of the electrochemical cell 12 and
its associated metallic current collectors 20, 24 allows them to be
rolled, folded, bent, or otherwise maneuvered into a variety of
lithium-ion battery configurations depending on design
specifications and spatial constraints. The lithium ion battery 10
may, for example, include a number of distinct electrochemical
cells 12 that have been fabricated, cut, aligned, and laid-up next
to one another or, in an alternative embodiment, the cells 12 may
be derived from a continuous layer that is folded back-and-forth
over itself many times.
[0026] In accordance with this invention, negative electrode
materials for a lithium-ion battery are provided. The negative
electrode materials consist essentially of a suitable metallic
current collector (such as a copper current collector) to which the
ends of many nanowires of active negative electrode material or
wire-shapes of negative electrode material are attached using a
shape memory metal alloy (preferably a shape memory alloy
displaying super elastic properties) in anchoring the attached ends
of the nanowires to the current collector. In some embodiments of
the invention, the SMA material may also support or confine
negative electrode active material along a portion or entirety of
the length of wire-shape of the material. And in some embodiments
of the invention the shape memory alloy may serve as a major
portion of the metal current collector, or all of the current
collector material.
[0027] In many lithium-ion cells and batteries substantially pure
high conductivity copper is used as the current collector because
of its stability during discharging and charging of the negative
electrode materials of the cell. As stated above, some shape memory
metal material is used to anchor the nanowires or wire shapes of
active negative electrode material to the copper so that there is
an interface of NiTi or the like between the ends of the nanowires
and the copper current collector layer of the cell. In some
embodiments of the invention, a relatively thin layer of NiTi
metal, or like suitable SMA material, may serve as the current
collector metal for the nanowires anchored in it.
[0028] Before describing methods of forming the SMA material and
electrode material constructions of this invention, it will be
useful to describe how the SMA materials are employed in the
electrode structures of a lithium-ion cell. Super-elastic shape
memory alloys (SE-SMAs) possess the ability to reversibly change
shape at large strains compared to conventional metallic alloys
upon the application and relaxation of an applied stress. These
alloys can accommodate such large and reversible strains due to
reversible, stress-induced phase transformations. These phase
transformations typically involve, but are not limited to the
phases: austenite and martensite, with austenite being stable in
the low stress state and martensite in the high stress state.
Because the phase transformations are stress assisted, the overall
stress-strain response is non-linear and discontinuous. At low
stresses, the austenite remains stable and obeys a linearly elastic
(Hookean) behavior until a critical stress is reached, above which,
the austenitic phase begins to transform to martensite. Upon
further deformation, the SE alloy continues to transform to
martensite and deform at a constant stress until all the austenite
has been fully transformed. Once the phase transformation has been
completed, stress increases again with strain until the material
yields plastically.
[0029] The region of constant stress is known as the plateau
stress, and corresponds to the beginning and end of the phase
transformation. Theoretically, the degree of phase transformation
is proportional to the fractional position of strain so that zero
percent transformation exists at one end of the stress plateau, and
one hundred percent transformation at the other. It is this stress
plateau that provides the most significant strain in the
elastic-super-elastic stress/strain response, and is responsible
for the majority of the reversible transformation strain and, thus,
for the recoverable transformation strain back to the material's
prescribed shape at low stress.
[0030] For the purposes of this specification, one can view the SE
alloy as a non-linear stress spring that lies within the active
negative electrode of the battery. When the electrode expands due
to lithiation, the SE material stretches imposing a reverse stress
that is in opposition to that of the active electrode. Upon
de-lithiation both the electrode and that SE alloy contract, but
with the same reverse stress acting to drive the active electrode
material back to its de-lithiated shape and volume.
[0031] Following are descriptions of some examples of preferred
practices for preparation of nanowire negative electrode materials
on metal current collectors in accordance with this invention.
[0032] FIG. 3(a) is an enlarged schematic cross-sectional
illustration of a thin sheet of copper current collector material
300 for application of nanowires of negative electrode material
where the nanowires are connected to the upper surface 302 of
copper sheet 300 by a layer of NiTi super elastic shape memory
alloy. The thickness of the copper sheet 300 may, for example, be
five to ten or twenty micrometers (.mu.m). In the six successive
images of FIG. 3(a) through (f), material layers are formed on the
copper sheet 300 and processed to form anchored nanowires of
negative electrode material. The illustrations in FIGS. 3(a)-3(f)
are presented to illustrate processing steps and are not drawn to
dimensional scale. In this and following examples it may be
preferred to start with a suitably large sheet of copper for
preparation of negative electrode material with the intention of
later cutting the prepared material into smaller pieces for
assembly into many individual lithium-ion cell packages or
arrangements. Practices of the invention will be illustrated by
formation of negative electrode active material on one side of a
current collector sheet. But it is to be understood that negative
electrode material may be applied to both sides of a current
collector strip or body.
[0033] A porous layer of aluminum 303 (FIG. 3(b)) is formed on
surface 302 and generally coextensive with the surface of the
current collector sheet 300. The aluminum layer 303 is oxidized by
anodizing to form a porous layer of alumina 304 in a suitable
acidic or alkaline anodizing bath, a commercially available
technology. The pores formed in the alumina during anodization of
the aluminum are very small; they are nanopores. The thickness of
the nanoporous alumina layer 304 will typically be equal to the
intended lengths of the nanowires or wire shapes of active material
for the negative electrode, for example, often about 20 to 200
.mu.m. Where the current collector is an aluminum sheet, the
surface may simply be anodized.
[0034] As illustrated in FIG. 3(c), pores 306 with generally
straight walls of alumina are formed by the conversion of aluminum
to alumina (Al.sub.2O.sub.3) during the anodization process by
controlling the processing parameters of the anodization step in
acidic or alkaline baths in a known manner. These walls when formed
to surround or define cylindrical pores 306, fifty to five hundred
nanometers in diameter, through the alumina layer at least to the
surface 302 of the copper current collector. Although not
illustrated in FIG. 3(c), it may be preferred to form pores 306
through the aluminum layer 303 (as it is being anodized to
alumina), past the surface 302 and a micrometer or so into the body
of the current collector 300. These many pores 306 in alumina layer
304 will serve as templates for the deposition of many bases 308 of
SMA for the growth of many nanowires of active negative electrode
material. For example, it is suitable to form an alumina template
304 with one million to one trillion pores 306 per square
millimeter on the surface 302 of the metal current conductor
300.
[0035] There are alternatives to the illustrated practice of
depositing and anodizing aluminum to form an alumina layer 304 that
has an array of vertical nanopores 306 that form a template for
formation of nanowires of negative electrode active material. One
alternative is to use electron or light based lithography with a
suitable thick polymer film resist in the formation of a template
for the formation of nanowires on the current collector surface
302. Another alternative approach in forming a template for SMA
bases and nanowires of active material is to use an available
anodized nanoporous alumina membrane or to use another available
nano-porous membrane such as a track-etched polycarbonate membrane.
For example, anodized aluminum membrane templates with a pore size
of 20 to 200 nm or polycarbonate templates with like pore size, may
be tightly applied to the current collector metal for formation of
an SMA base and nanowires of active material. At the completion of
nanowire formation using a suitable template (as described in
following paragraphs), the template material is removed. For
example, an alumina template may be dissolved in 5% to 30% aqueous
potassium hydroxide and a polycarbonate template may be dissolved
in acetone.
[0036] After the etched alumina layer 304 with its pores 306 (or an
equivalent template as described above) has been prepared, shape
memory alloy, such as super-elastic NiTi, is deposited through each
of the template pores 306 in the alumina 304 and onto surface 302
of current collector 300 as SMA bases 308 for the subsequent growth
of nanowires of active material. The result is illustrated in FIG.
3(d). The many SMA bases 308 are suitably about one to about ten
micrometers in height. The NiTi material or its suitable equivalent
may be deposited in template pores 306 by a suitable existing
physical vapor deposition process or a chemical vapor deposition
process. In this example the SMA bases 308 are deposited to lie on
the surface 302 of the copper current collector 300. In other
embodiments of the invention, shallow holes may have been etched in
surface 302 and the SMA bases 308 will fill such holes (not
illustrated in FIGS. 3(a)-3f)) and still extend above surface
302.
[0037] As illustrated in FIG. 3(e), active negative electrode
material is then deposited into template holes 306 on the SMA bases
308 to form nanowires 310 of silicon, tin, carbon, or other
suitable active material into which lithium may be inserted in the
operation of a lithium-ion cell. As stated, the length of the
nanowires 310 is often up to about 100 micrometers.
[0038] There are known practices for growing nanowires of active
negative electrode material in the holes of a suitable template
material (such as the nanometer-size holes 306 in the alumina film
304) fixed to a surface 302 of a current collector 300. For
example, physical vapor deposition (including magnetron sputtering,
cathode arc, e-beam evaporation, or pulsed laser ablation
practices) may be used to place a catalyst, if required, in the
holes for promoting subsequent growth of a nanowire material. A
catalyst deposit of gold and iron, about 2-50 nm thick may be
deposited on the SMA bases 308 for growing silicon nanowires.
Catalytic amounts of iron, nickel, or cobalt and their alloys may
be used for growing carbon nanowires or nanotubes. Then, chemical
vapor deposition may be used to grow silicon nanowires, carbon
nanofibers, or the like, to lengths of fifty micrometers or so
depending on the catalyst deposit.
[0039] Following formation of nanowires 310, the alumina template
material may be removed with a caustic solution, as described
above, to leave a negative electrode structure, as illustrated in
FIG. 3(f), consisting essentially of a metal current collector 300
with nanowires of active negative electrode material 310 connected
to the current collector 300 by SMA bases 308. The SMA supported
nanowires of active material are suitably spaced to permit suitable
penetration of the nanowires 310 with a suitable lithium-ion
containing and transporting liquid electrolyte for operation of a
lithium-ion cell. As the nanowire electrode materials repeatedly
experience lithiation and de-lithiation, the corresponding
expansion and contraction of the nanowires are accommodated by the
super-elastic properties of the SMA base material connecting the
nanowires to the current collector. The SMA base material deforms
sufficiently to permit the expansion and contraction of the
nanowires without breakage and separation of the nanowires from the
current collector. And the SMA bases permit suitable electron flow
between the nanowires and the current collector.
[0040] FIG. 4, with its seven illustrative schematic drawings
4(a)-4(g), is now used to describe a second example of utilizing
super-elastic SMA material to bond active negative electrode
material with nanowire-shaped portions to a current collector in
making a negative electrode for a lithium-ion cell and battery. In
this example, and referring to FIG. 4(a), a sheet of copper current
collector material 400 is coated on one surface 402 (or both
surfaces in some embodiments) with a co-extensive layer of NiTi 404
or other suitable SMA material. A coating of aluminum film 405 is
formed coextensively over the SMA layer 404 as illustrated in FIG.
4(b). The aluminum film 405 is oxidized by anodization to form a
nanoporous alumina layer 406 as illustrated in FIG. 4(c). Many
straight vertical walls of alumina are formed by controlling the
anodization process in either acidic or alkaline baths. The
vertical walls of alumina 406 surround cylindrical nanopores 408 as
illustrated in FIG. 4(c). Nanopores 408 extend substantially
parallel through the alumina layer to the underlying SMA layer 404
and are aligned generally perpendicular to the SMA layer 404. The
total volume of the nanopores 408 will be greater than about five
percent and less than about fifty percent of the volume of the
alumina layer 406 formed from aluminum.
[0041] The nanopores 408 in the alumina layer 406 are filled with
SMA material by any suitable process such as physical vapor
deposition or chemical vapor deposition. This step is illustrated
in FIG. 4(d). The remaining alumina material is removed by a
suitable caustic etch to leave SMA nanowires 410 that are bonded to
and upstanding from the base 404 of SMA material. The result of
this processing step is illustrated in FIG. 4(e). The many
upstanding, parallel, SMA nanowires 410 are anchored at their lower
ends to underlying SMA layer 404 which lies on the upper surface
402 of current collector strip 400. The continuous open space 412
between the many generally round SMA nanowires 410 will now be
filled with active material for the negative electrode.
[0042] In this example of a practice of the invention, the active
material for the negative electrode will be in the form of small
particles so as to be placed in the open space 412 of the partially
prepared electrode as illustrated in FIG. 4(e). The particles of
active material may be, for example, carbon, silicon, tin, or other
composition of absorbing and releasing lithium. The particles of
active material may be mixed if necessary with particles of a
binder (e.g., a polymeric binder), and/or particles of an
electrical conductor. The particles of active material may be
prepared in the form of a liquid-containing slurry for spin coat
loading or other flow process for placing particles of active
electrode material into open space 412 between the SMA nanowires
410, The solvent or liquid carrier for the particles is then
evaporated form the electrode structure to leave a generally
continuous porous body of electrode particles 414 as illustrated in
FIG. 4(f). The height of the continuous body of active material and
SMA nanowires 410 above the surface 402 of current collector 400
may be, for example, about fifty to one hundred micrometers. The
many SMA nanowires serve to anchor the porous body of active
material particles 414 and to absorb stresses produced in the body
414 during lithiation and de-lithiation of the active material.
[0043] In some embodiments of the invention it may be desired to
form an additional layer 414' (a few micrometers in thickness) of
the active material over the upper ends of the SMA nanowires 410
and continuous body of active particles 414 (see FIG. 4(g). In the
assembled negative electrode structure illustrated in FIG. 4(g),
SMA material is the nanowire portion of the structure instead of
the active material.
[0044] In still other embodiments of the invention, shape memory
alloy material may serve as the strip of current collector material
to serve as a stress relieving, absorbing anchor layer for
nanowires of active material for the negative electrode of
lithium-ion electrochemical cells and batteries.
[0045] As illustrated in FIG. 5(a), a strip of SMA (e.g., NiTi
super-elastic alloy) is formed as a layer 500 having a surface area
for the assembly of one or more electrodes. A thin layer of
suitable polymeric photo resist material 504 is spin coated or
otherwise suitably applied to at least one surface 502 of the SMA
current collector strip 500. A suitable mask (not illustrated) is
prepared and placed over the photo resist composition. The goal of
the masking process is to expose a large number of small round
areas in surface 502 for later formation of nanowires or tubes of
active material. The surface areas could also be square,
triangular, or of other suitable shape. The photo resist layer is
exposed to UV radiation through the mask and the undeveloped
masking material is removed to leave many, regularly spaced holes
506 (e.g., 2 to 500 nm in diameter) in the masking material 504
overlying surface 502 of the SMA current collector strip 500, FIG.
5(c).
[0046] An etchant liquid is used to flood into holes 506 and form a
corresponding number of holes or cavities 508 in surface 502 of the
SMA current collector strip 500. The etching process is practiced
to form holes 508 (after rinsing, etc.) to a depth of about fifty
to one hundred nanometers. If necessary for subsequent formation of
nanowires of active negative electrode material in holes 508,
particles of a suitable catalyst are placed through mask hole 506
into the holes 508 in the SMA layer, as illustrated in FIG. 5(e).
The remaining photo resist material is removed, (FIG. 5(f), and
nanowires or nanotubes of active negative electrode material 512
are grown in the holes 508 in surface 502 of the SMA current
collector strip 500, FIG. 5(g). The total length of the active
material nanowires 512 from their anchored ends to their upper ends
is usually in the range of about one micrometer to about fifty
micrometers. Again, the density of the nanowires of active material
is suitably in the range of about a million to a trillion wires per
square millimeter. The nanowires are preferably closely and
regularly spaced for the infiltration of lithium-containing
electrolyte liquid in the assembly and operation of the cell.
[0047] A further embodiment of the invention for supporting
negative electrode active material on stress relieving SMA current
collector material is illustrated in the sequence of schematic,
cross-section views of FIGS. 6(a)-6(f). Again, a super-elastic
shape memory alloy composition is used as the current collector for
the negative electrode of a lithium-ion electrochemical cell. And
in this embodiment a procedure using a suitable photo resist
polymer coating and masking is employed to provide for the
formation of a pattern of SMA supported, spaced-apart, upstanding
bodies of negative electrode material on the current collector.
[0048] A sheet of SMA current collector 600 having a surface layer
602 is coated with a thin layer of photo resist material 604 as
illustrated in FIGS. 6(a) and 6(b). The thickness of the current
collector 600 is suitably about five to fifty micrometers and the
thickness of the photo resist layer is, for example, about one to
five micrometers. The area of the sheet of SMA material 600 may be
sized for the formation of a single electrode or for the formation
of several electrodes which may be cut from the sheet.
[0049] A mask (not illustrated) is used in the exposure of the
photo resist with UV frequency light and development and removal of
portions of the photo resist to form many regularly and uniformly
spaced, round nanowire-like posts 606 in the photo resist layer
604. In a plan view of the structure of FIG. 6(c) the posts of
photo resist would be seen as located over the entirety of the
boundaries of the surface 602 of SMA current collector sheet
600.
[0050] A suitable wet etchant is then applied to the photo resist
layer posts 606, the etchant penetrating around photo resist posts
606 to remove an abundance of exposed SMA material from surface 602
to leave shallow posts 608 of SMA material in the etched and
lowered surface 602' of SMA current collector sheet 600. As seen in
FIG. 6(d), SMA posts 608 extend above etched surface 602' and
underlie the photo resist layer posts 606. The liquid etchant is
used to form posts 608 to a height (or length) of, for example, one
to five micrometers before it is rinsed from the etched current
collector 600. The remaining photo resist material 606 is removed
from the upper ends of posts 608 of SMA current collector material
as illustrated in the cross-section of FIG. 6(e).
[0051] Active negative electrode material, such as silicon, is then
deposited as a generally continuous layer 612 on and between SMA
posts 608. The layer 612 of negative electrode material may be
deposited, for example, by chemical vapor deposition and to a
thickness of about 100 nm to about 5 micrometers. As illustrated in
FIG. 6(f), the deposited layer of active electrode material 612
covers etched surface 602' of SMA current collector 600 and it's
upstanding SMA posts 608. Thus, the deposited layer 612 of silicon,
or other suitable active negative electrode material, is carried on
stress-relieving, super-elastic SMA material in a configuration of
active material 612 that provides continuous and abundant surfaces
for lithiation and de-lithiation on upstanding SMA posts 608 and on
SMA surface 602' lying between the posts 608.
[0052] In the several above illustrated practices of the invention,
active negative electrode material is formed in a variety of
upstanding shapes that are supported on and connected to a negative
electrode current collector. In each of these upstanding shaped
bodies of active material, the electrode material is buffered with
a super elastic SMA support structure that better enables the
active material to experience repeated growth and shrinkage without
breaking and separating from the current collector.
[0053] The above descriptions of embodiments of the invention are
intended to illustrate the invention and not intended to limit the
claimed scope of the invention.
* * * * *