U.S. patent application number 12/944576 was filed with the patent office on 2011-05-12 for intermediate layers for electrode fabrication.
This patent application is currently assigned to AMPRIUS INC.. Invention is credited to Eugene M. Berdichevsky, Yi Cui, William S. DelHagen, Rainer J. Fasching, Song Han, Ghyrn E. Loveness, Mark C. Platshon, Constantin I. Stefan.
Application Number | 20110111300 12/944576 |
Document ID | / |
Family ID | 43974404 |
Filed Date | 2011-05-12 |
United States Patent
Application |
20110111300 |
Kind Code |
A1 |
DelHagen; William S. ; et
al. |
May 12, 2011 |
INTERMEDIATE LAYERS FOR ELECTRODE FABRICATION
Abstract
Provided are novel electrodes for use in lithium ion batteries.
An electrode includes one or more intermediate layers positioned
between a substrate and an electrochemically active material.
Intermediate layers may be made from chromium, titanium, tantalum,
tungsten, nickel, molybdenum, lithium, as well as other materials
and their combinations. An intermediate layer may protect the
substrate, help to redistribute catalyst during deposition of the
electrochemically active material, improve adhesion between the
active material and substrate, and other purposes. In certain
embodiments, an active material includes one or more high capacity
active materials, such as silicon, tin, and germanium. These
materials tend to swell during cycling and may loose mechanical
and/or electrical connection to the substrate. A flexible
intermediate layer may compensate for swelling and provide a robust
adhesion interface. Provided also are novel methods of fabricating
electrodes containing one or more intermediate layers.
Inventors: |
DelHagen; William S.; (Menlo
Park, CA) ; Fasching; Rainer J.; (Mill Valley,
CA) ; Loveness; Ghyrn E.; (Menlo Park, CA) ;
Han; Song; (Foster City, CA) ; Berdichevsky; Eugene
M.; (Menlo Park, CA) ; Stefan; Constantin I.;
(San Jose, CA) ; Cui; Yi; (Sanford, CA) ;
Platshon; Mark C.; (Menlo Park, CA) |
Assignee: |
AMPRIUS INC.
Menlo Park
CA
|
Family ID: |
43974404 |
Appl. No.: |
12/944576 |
Filed: |
November 11, 2010 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61260297 |
Nov 11, 2009 |
|
|
|
Current U.S.
Class: |
429/223 ; 427/77;
427/78; 429/209; 429/218.1; 977/700 |
Current CPC
Class: |
H01M 10/052 20130101;
H01M 4/1395 20130101; Y02E 60/10 20130101; H01M 4/134 20130101;
H01M 4/366 20130101; H01M 4/661 20130101; H01M 4/13 20130101 |
Class at
Publication: |
429/223 ;
429/209; 429/218.1; 427/77; 427/78; 977/700 |
International
Class: |
H01M 4/134 20100101
H01M004/134; H01M 4/13 20100101 H01M004/13; H01M 4/139 20100101
H01M004/139; B05D 5/12 20060101 B05D005/12 |
Claims
1. An electrode for use in a lithium ion battery, the electrode
comprising: a substrate; one or more intermediate layers formed on
the substrate; and an electrochemically active material in the form
of nanostructures formed over the one or more intermediate layers
and operable for inserting and removing lithium ions during battery
cycling, wherein the electrochemically active material is in
electrical communication with the substrate.
2. The electrode of claim 1, wherein the substrate comprises one or
more materials selected from the group consisting of copper,
nickel, aluminum, stainless steel, and titanium.
3. The electrode of claim 1, wherein the active material comprises
one or more materials selected from the group consisting of
silicon, tin, germanium, a silicon-germanium combination, tin
oxide, silicon oxycarbide (SiOC), and their compounds.
4. The electrode of claim 3, wherein the active material comprises
silicides.
5. The electrode of claim 4, wherein the active material comprises
nickel silicides.
6. The electrode of claim 1, wherein at least one of the one or
more intermediate layers comprises one or more elements selected
from the group consisting of chromium, titanium, tantalum,
tungsten, nickel, molybdenum, iron, and lithium.
7. The electrode of claim 1, wherein a thickness of the one or more
intermediate layers is between about 1 nanometer and 2000
nanometers.
8. The electrode of claim 1, wherein an electrical resistance over
a unit of surface area of the one or more intermediate layers is
less than about 1 Ohm-centimeter squared.
9. The electrode of claim 1, wherein the nanostructures comprise
substrate-rooted nanowires.
10. The electrode of claim 1, wherein the one or more intermediate
layers comprise a diffusion barrier layer configured to shield the
substrate during formation of the electrochemically active
material.
11. The electrode of claim 1, wherein the one or more intermediate
layers comprise an adhesion layer configured to maintain mechanical
connection between the substrate and the electrochemically active
material during battery cycling.
12. The electrode of claim 1, wherein the one or more intermediate
layers has a surface tension configured for depositing a catalyst
layer and forming catalyst islands from the catalyst layer during
formation of the active material.
13. The electrode of claim 1, wherein the one or more intermediate
layers are configured to separate catalyst particles from a carrier
fluid.
14. The electrode of claim 1, wherein the one or more intermediate
layers comprise an exposed surface having a roughness that enables
distribution of a catalyst in discreet patches.
15. The electrode of claim 1, wherein the one or more intermediate
layers comprise a surface condition providing nucleation sites for
facilitating deposition of the electrochemically active
material.
16. A method of manufacturing a battery electrode for use in a
lithium ion battery, the method comprising: receiving a substrate
for the battery electrode; forming a conductive intermediate layers
on the substrate; and depositing an electrochemically active
material comprising nanowires on the one or more intermediate
layers, wherein the electrochemically active material is configured
for inserting and removing lithium ions during battery cycling.
17. The method of claim 16, wherein depositing the
electrochemically active material comprises a vapor-solid-solid
chemical (VLS) vapor deposition (CVD) process.
18. The method as in one of claims 16, wherein depositing the
active material includes depositing a catalyst on the one or more
intermediate layers.
19. The method as in one of claims 16, wherein the formation of the
conductive intermediate layers comprises depositing at least two
intermediate layers.
20. The method as in one of claims 16, wherein the intermediate
layer comprises a surface condition that enhances nucleation of the
active material during the deposition of the active material.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 61/260,297, filed Nov. 11, 2009, entitled
"INTERMEDIATE LAYERS FOR ELECTRODE FABRICATION," which is
incorporated herein by reference in its entirety for all
purposes.
BACKGROUND
[0002] The demand for high capacity rechargeable batteries is
strong. Many areas of application, such as aerospace, medical
devices, portable electronics, and automotive, require high
gravimetric and/or volumetric capacity cells. Lithium ion battery
technology represents a significant improvement in this regard.
However, to date, application of this technology has been primarily
limited to graphite electrodes, and graphite has a theoretical
capacity of only about 372 mAh/g during lithiation.
[0003] Silicon, germanium, tin, and many other materials are
attractive active materials because of their high electrochemical
capacity. For example, the theoretical capacity of silicon in
lithium ion cells has been estimated at about 4200 mAh/g. Yet many
of these materials not been widely adopted in commercial batteries.
One reason is the substantial change in volume they undergo during
cycling. For example, silicon swells by as much as 400% when
charged to a level at or near its theoretical capacity
(Li.sub.4.4Si). Volume changes of this magnitude can cause
substantial stresses in active material structures resulting in
fractures and pulverization, loss of electrical connections within
the electrode, and capacity fading of the battery.
[0004] Conventional methods of electrode fabrication using
slurries, where slurries include high capacity active material
particles and polymer binders, typically result in electrochemical
cells with poor cycle life. Most polymer binders are not
sufficiently elastic to accommodate active material's swelling,
which results in separation between polymers and active material
particles during the discharge and loss of electrical connection
between the active material particles and the current
collector.
[0005] Overall, there is a need for improved application of high
capacity active materials in battery electrodes that minimize the
drawbacks described above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 illustrates three general stages a Vapor Liquid Solid
(VLS) deposition process in accordance with certain
embodiments.
[0007] FIG. 2 is a schematic representation of an electrode
cross-section containing active materials, a substrate, and an
intermediate layer in accordance with certain embodiments.
[0008] FIG. 3 is an expanded schematic representation of a portion
of an electrode cross-section further illustrating certain details
of an intermediate layer in accordance with certain
embodiments.
[0009] FIG. 4 is a flow chart of a general process for fabricating
an electrode containing an intermediate layer in accordance with
certain embodiments.
[0010] FIGS. 5A-B are a top schematic view and a side schematic
view of an illustrative electrode arrangement in accordance with
certain embodiments.
[0011] FIGS. 6A-B are a top schematic view and a perspective
schematic view of an illustrative round wound cell in accordance
with certain embodiments.
[0012] FIG. 7 is a top schematic view of an illustrative prismatic
wound cell in accordance with certain embodiments.
[0013] FIGS. 8A-B are a top schematic view and a perspective
schematic view of an illustrative stack of electrodes and separator
sheets in accordance with certain embodiments.
[0014] FIG. 9 is a schematic cross-section view of an example of a
wound cell in accordance with embodiments.
SUMMARY
[0015] Provided are novel electrodes for use in lithium ion
batteries. An electrode includes one or more intermediate layers
positioned between a substrate and an electrochemically active
material. Intermediate layers may be made from chromium, titanium,
tantalum, tungsten, nickel, molybdenum, lithium, as well as other
materials and their combinations. An intermediate layer may protect
the substrate, help to redistribute catalyst during deposition of
the electrochemically active material, improve adhesion between the
active material and substrate, and other purposes. In certain
embodiments, an active material includes one or more high capacity
active materials, such as silicon, tin, and germanium. These
materials tend to swell during cycling and may loose mechanical
and/or electrical connection to the substrate. A flexible
intermediate layer may compensate for swelling and provide a robust
adhesion interface. Provided also are novel methods of fabricating
electrodes containing one or more intermediate layers.
[0016] In certain embodiments, an electrode for use in a lithium
ion battery includes a substrate, one or more intermediate layers
formed on the substrate, and an electrochemically active material
in the form of nanostructures formed over the one or more
intermediate layers and operable for inserting and removing lithium
ions during battery cycling. The electrochemically active material
is in electrical communication with the substrate. In certain
embodiments, a substrate includes one or more of the following
materials: copper, nickel, aluminum, stainless steel, and titanium.
In the same or other embodiments, the active material includes one
or more of the following materials: silicon, tin, germanium, a
silicon-germanium combination, tin oxide, silicon oxycarbide
(SiOC), and their compounds. In more specific embodiments, the
active material includes silicides or, even more specifically,
nickel silicides. For example, an active material may include
nickel silicide nanowires with an amorphous silicon layer formed
over the nanowires. In certain embodiments, the active material
nanostructures are substrate-rooted nanowires.
[0017] In certain embodiments, one or more intermediate layers
include one or more of the following elements: chromium, titanium,
tantalum, tungsten, nickel, molybdenum, iron, and lithium. A
thickness of the intermediate layers may be between about 1
nanometer and 2000 nanometers. An electrical resistance over a unit
of surface area of the intermediate layers may be less than about 1
Ohm-centimeter squared.
[0018] In certain embodiments, one or more intermediate layers
include a diffusion barrier layer configured to shield the
substrate during formation of the electrochemically active
material. In the same or other embodiments, the intermediate layers
include an adhesion layer configured to maintain mechanical
connection between the substrate and the electrochemically active
material during battery cycling. An intermediate layer may have a
surface tension configured for depositing a catalyst layer and
forming catalyst islands from the catalyst layer during formation
of the active material. In the same or other embodiments, one or
more intermediate layers are configured to separate catalyst
particles from a carrier fluid. An intermediate layer may include
an exposed surface having a roughness that enables distribution of
a catalyst in discreet patches. An intermediate layers may have a
surface condition providing nucleation sites for facilitating
deposition of the electrochemically active material.
[0019] Provided also a method of manufacturing a battery electrode
for use in a lithium ion battery. A method may involve receiving a
substrate for the battery electrode, forming a conductive
intermediate layers on the substrate, and depositing an
electrochemically active material comprising nanowires on the one
or more intermediate layers. The electrochemically active material
is configured for inserting and removing lithium ions during
battery cycling. Depositing the electrochemically active material
may involve a vapor-solid-solid chemical (VLS) vapor deposition
(CVD) technique. In certain embodiments, depositing the active
material involves first depositing a catalyst on the one or more
intermediate layers. Two or more intermediate layers may be
deposited. In certain embodiments, an intermediate layer includes a
surface condition that enhances nucleation of the active material
during the deposition of the active material.
[0020] These and other aspects of the invention are described
further below with reference to the figures.
DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS
[0021] In the following description, numerous specific details are
set forth in order to provide a thorough understanding of the
present invention. The present invention may be practiced without
some or all of these specific details. In other instances, well
known process operations have not been described in detail to not
unnecessarily obscure the present invention. While the invention
will be described in conjunction with the specific embodiments, it
will be understood that it is not intended to limit the invention
to the embodiments.
Introduction
[0022] Instead of binding active materials to a substrate using a
polymer binder as is conventionally done in lithium ion battery
manufacturing, active materials may be attached directly to the
current collecting substrate either during their fabrication by
deposition or otherwise (thereby producing "growth rooted" active
materials) or after their fabrication (by, e.g., sintering or
otherwise fusing). In certain embodiments, a surface of the
substrate may need to be protected during the fabrication or
attachment process. The reasons for needing such protection, as
well as techniques for applying such protection are described
below. For now, it should be understood that the protection is
provided by one or more "intermediate layers" interposed between
the electrode substrate and the active materials. It should also be
understood that the active material is often in the form of a small
particles or "nanostructures," which will be described in more
detail below.
[0023] High capacity active materials generally experience
substantial volume change during electrochemical cycling of the
cell. Such active materials may loose electrical and mechanical
connection with the substrate leading to cell degradation. One way
to address this issue is by bonding the active materials, which may
be in the form of nanostructures, to the substrate. In some cases,
the active material attaches to the substrate in a manner referred
to as "substrate-rooting." This arrangement provides direct
mechanical support and electrical communication between the
substrate and active materials; often this will provide a
metallurgical connection (which does not necessarily mean that the
connection is lattice matched) and/or electrical coupling (and/or
connection) between the substrate and the active materials. Various
examples of the substrate-rooted nanostructures and corresponding
fabrication methods are described in U.S. patent application Ser.
No. 12/437,529 filed on May 7, 2009, which is incorporated by
reference herein in its entirety for purposes of describing
substrate rooted nanostructures.
[0024] In certain embodiments, nanostructures have one dimension
that is substantially larger than the other two. The largest
dimension is referred to as a length. Some nanostructures,
especially ones with high aspect ratios, may have curved shapes. In
these cases, the length of the nanostructure is the length of the
representative curve. A cross-section is defined as a profile of a
nanostructure in a plane perpendicular to the length.
Nanostructures may have many varying cross-sectional (transverse)
dimensions along their lengths. Further, an active layer may have
nanostructures with different cross-sections, both shapes and
dimensions (e.g., tapered nanostructures). Examples of
nanostructure shapes include spheres, cones, rods, wires, arcs,
saddles, flakes, ellipsoids, tapes, etc.
[0025] Cross-sectional shapes are generally dependent on
compositions, crystallographic structures (e.g., crystalline,
poly-crystalline, amorphous), sizes, deposition process parameters,
and many other factors. Shapes may also change during cycling.
Irregularities of cross-sectional shapes require a special
dimensional characterization. For the purposes of this application,
a cross-section dimension is defined as a distance between the two
most separated points on a periphery of a cross-section that is
transverse to the principal dimension, such as length. For example,
a cross-section dimension of a cylindrical nano-rod circle is the
diameter of the circular cross-section. In certain embodiments, a
cross-section dimension of nanostructures is between about 1
nanometer and 10,000 nanometers. In more specific embodiments, a
cross-section dimension is between about 5 nanometers and 1000
nanometers, and more specifically between 10 nanometers and 400
nanometers. Typically, these dimensions represent an average or
mean across the nanostructures employed in an electrode.
[0026] In certain embodiments, nanostructures are hollow. They may
be also described as tube or tube-like structures. Therefore, the
cross-sectional profile of these hollow nanostructures includes
void regions surrounded by annular solid regions. An average ratio
of the void regions to the solid regions may be between about 0.01
and 100, more specifically between about 0.01 and 10. The
cross-section dimension of the hollow nanostructures may be
substantially constant along the principal dimension (e.g.,
typically the axis). Alternatively, the hollow nanostructures may
be tapered along the principal dimension. In certain embodiments,
multiple hollow nanostructures may form a core-shell arrangement
similar to multiwall nanotubes. Additional examples of hollow
nanostructures are provided in U.S. patent application Ser. No.
12/787,138, entitled "INTERCONNECTED HOLLOW NANOSTRUCTURES
CONTAINING HIGH CAPACITY ACTIVE MATERIALS FOR USE IN RECHARGEABLE
BATTERIES" filed on May 10, 2010, which is incorporated herein by
reference in its entirety for purposes of describing hollow
nanostructures.
[0027] In certain embodiments, a "nanowire" is defined as a
structure that has, on average, an aspect ratio of at least about
four. In certain examples, the average aspect ratio may be at least
about ten, at least about one hundred, or even at least about one
thousand. In some cases, the average nanowire aspect ratio may be
at least about ten thousand, and can even reach about one hundred
thousand. Nanowire active materials can undergo substantial
swelling without disrupting the overall structure of the active
layer, provide better electrical and mechanical connections with
the layer, and can be easily realized using the vapor-liquid-solid
and vapor-solid template free growth methods or other templated
methods.
[0028] Substrate-rooted nanostructures may be deposited on a
substrate using various methods. One such method is a
chemical-vapor deposition (CVD) that employs a vapor-liquid-solid
(VLS) phase transformation of a deposited material. This approach
will be referred to herein as a "VLS" technique. Another method
includes a CVD with a vapor-solid-solid (VSS) phase transformation,
referred to herein as a "VSS" technique.
[0029] In various embodiments, an intermediate layer is positioned
between the substrate and the active material to facilitate the
fabrication or use of a lithium negative electrode. In one example,
an intermediate layer serves to protect the substrate from
reactants used to deposit the active material. Such intermediate
layer may also (or alternatively) facilitate formation of the
active material by VLS or other suitable process. It may accomplish
this by, e.g., preventing a deposition catalyst from being
contaminated by materials diffusing from the substrate or prevent
catalyst defusing into the substrate.
[0030] A general description of a VLS process is provided here to
better understand certain functions and structures of intermediate
layers and other components of the electrode, in accordance with
certain embodiments. VLS is a mechanism for the growth of
one-dimensional structures, such as nanowires, from CVD. The VLS
process introduces a catalytic liquid alloy phase, which can
rapidly adsorb a precursor vapor to super-saturation levels, and
thereby facilitate crystal growth at the liquid-solid
interface.
[0031] FIG. 1 illustrates three general stages of a typical VLS
deposition process in accordance with certain embodiments. During
the initial stage 100, discrete catalyst islands 104 are formed on
the surface of the substrate 102. The surface typically has an
intermediate layer, not shown, which is described below in more
detail.
[0032] A substrate may be a metallic foil, an open structure
substrate (e.g., mesh, foam), a composite that include structural
and conductive materials, and other forms. Substrate materials for
electrodes used in various lithium ion cells may include copper
and/or copper dendrite coated metal oxides, stainless steel,
titanium, aluminum, nickel (also used as a diffusion barrier),
chromium, tungsten, metal nitrides, metal carbides, metal oxides,
carbon, carbon fiber, graphite, graphene, carbon mesh, conductive
polymers, or combinations of above including multi-layer
structures. It will be understood by one having ordinary skills in
the art that selection of the materials also depends on
electrochemical potentials of the materials. The substrate material
may be formed as a foil, films, mesh, laminate, wires, tubes,
particles, multi-layer structure, or any other suitable
configurations. For example, the substrate 102 may be a stainless
steel foil having thickness of between about 1 micrometer and 50
micrometers. In other embodiments, the substrate 102 is a copper
foil with thickness of between about 5 micrometers and 50
micrometers. Certain substrate examples are described in U.S.
patent application Ser. No. 12/437,529 filed on May 7, 2009 and
U.S. patent application Attorney Docket No. AMPRP005P filed
herewith, which are incorporated by reference herein in their
entirety for purposes of describing substrates.
[0033] The catalyst islands 104 may be formed by first depositing a
continuous layer containing catalyst and then removing parts of the
layer (by, e.g., using lithographic etching, ablating) or breaking
the continuous layer by thermal annealing. An intermediate layer
may be used to protect the substrate during this removal. In other
embodiments, a continuous or partial layer containing catalyst
(typically, a eutectic alloy containing metallic catalyst, such as
gold) is heated, which leads to formation of discrete droplets due
to the surface tension. An intermediate layer may be used to change
the surface properties of the substrate, to form a eutectic alloy
with catalyst containing material, prevent catalyst losses into the
substrate (e.g., a gold catalyst over the copper substrate), and
other purposes. Some properties that may impact formation of
discontinuous catalyst islands include surface roughness, grain
structure and porosity, magnetic orientation, and electronic
structure.
[0034] An intermediate layer may contain a catalyst, which may be,
e.g., plated, sputtered, and/or evaporated on the intermediate
layer. In certain embodiments, materials of the intermediate layer
and catalyst may be deposited together and then subjected to phase
separation to control distribution of the materials in this
combined layer. Nano and/or micro crystals may occur near or at the
exposed surface of the intermediate layer. The size of the crystals
may be controlled during the deposition process. For example, the
power level, chamber pressure, and/or temperature may be controlled
during sputtering. If plating is used for deposition of the
materials, then plating currents and bath composition can be
controlled. Furthermore, certain post deposition treatment
parameters (e.g., temperature and/or duration for annealing) may be
controlled. As resulting distribution of the catalyst on the
surface effect density and size of nanowires in some of these
embodiments.
[0035] In certain embodiments, roughness of the intermediate layer
and formation of catalyst islands is established by chemical
etching. Etchant may be introduced after the deposition of the
intermediate later or during such deposition (e.g., close the end
of the deposition) and react with the intermediate layer to create
rougher surface and form catalyst islands.
[0036] Further, an intermediate layer may have portions with
different chemical or physical properties (e.g., polarization,
binding sites, magnetic properties), which can be used to
distribute catalysts particles or to form islands during a
deposition process.
[0037] In certain embodiments, catalyst containing materials may be
deposited on a substrate as discrete catalyst islands without first
forming a continuous layer. For example, a slurry solution with
catalyst particles and/or a catalyst suspension (e.g., a colloid
suspension) may be used to coat the substrate surface. The slurry
is then dried to form catalyst islands. Certain details of these
embodiments are described in U.S. patent application Ser. No.
11/103,642 filed on Apr. 12, 2005, which is incorporated herein in
its entirety for purposes of describing process examples of forming
catalyst islands. In these embodiments, intermediate layers mat be
used to provide desirable surface properties for slurry flow and
drying and protect the substrate from the slurry. In other
embodiments, catalytic materials are embedded onto the intermediate
layer such that only a portion of the catalyst material is exposed.
Other methods for depositing a catalyst include electroless
deposition and mixing a salt precursor with catalyst elements
followed by heating or annealing the mixtures with a presence of
hydrogen.
[0038] Materials suitable for the catalyst include any materials
capable of reacting and forming a compound with the process gas in,
for example, VLS or VSS types of deposition processes. Examples
include gold, nickel, cobalt, aluminum, copper, gallium, indium,
silver, titanium, carbon, carbides, alloys, and mixtures of
thereof. Catalysts can be deposited using thermal evaporation,
sputtering, electroplating, and filtration methods, etc. Depending
on the deposition condition, either a continuous film or discrete
catalyst islands form on the intermediate layer
[0039] In certain embodiments, deposition using evaporation or
sputtering of catalyst over a rough surface of the substrate or an
intermediate layer creates shadowing effects, which results in
clustered deposition. This may eliminate a need for a separate
post-deposition treatment to create catalyst islands. Further,
platting on rough surfaces may result in preferential deposition on
the extending tips of the rough surface structure caused by uneven
field distribution.
[0040] In certain embodiments, depositing small amounts of certain
catalyst materials on certain surfaces (e.g. gold on silicon oxides
and/or silicon) do not form a homogenous single atomic layer and
instead deposit in clusters. Without being restricted to any
particular theory, it is believed that thermodynamic driving forces
of the surface tension effect such distribution). Clustering may be
controlled by controlling deposition process conditions, such as
temperature and deposition rate.
[0041] In certain embodiments, plating on partially oxidized
surfaces or surfaces with a porous template (e.g., porous polymers)
on the top of it (porous polymer) is used to form catalyst islands.
For example, a substrate or an intermediate layer may be partially
oxidized by heating at ambient conditions or by introducing an
oxidizing agent into the deposition chamber. Alternatively or in
addition to this method, a surface may be then coated with a
polymer that forms a porous structure during the deposition or
during subsequent treatment (e.g., heating).
[0042] In certain embodiments, pulse plating of the catalyst may
results in catalyst islands formed on the surfaced. For example,
relatively short pulse duration can be used to form a discontinuous
film. The duration of the pulse depends on the plating bath
configuration, plating currents, plating bath composition,
deposition surface materials and geometry (e.g., surface
roughness), and other process parameters.
[0043] In certain embodiments, a partial electrochemical
dissolution of the catalyst layer is used to form islands of the
metal. For example, a pulsed current, a template, roughening or
oxidizing the surface may be used to establish selective
dissolution.
[0044] Various criteria may be taken into account in selecting
catalyst materials. Such criteria include melting and eutectic
points with nanostructure materials, wetting properties such as
surface tension on the intermediate layer (to form catalyst islands
upon melting), bulk diffusivity in the intermediate layer, impact
on electrochemical and electrical properties of deposited
nanostructures, and others. For example, aluminum has lower
diffusivity in crystalline silicon than gold but also makes a
eutectic with silicon, though at higher temperature than gold
(about 577.degree. C.). Copper, in turn, diffuses very fast in
silicon but has even higher eutectic (about 802.degree. C.). Copper
may be used to grow silicon nanowires in a Vapor-Solid-Solid mode.
Further, gallium has both low melting temperature and low
diffusivity in silicon in comparison to gold.
[0045] In the next stage 110 of the VLS process, one or more
precursor gases 106 are provided to the surface of the substrate
102 containing catalyst islands 104a. These precursor gases can
decompose or otherwise react to form electrochemically active
materials, such as silicon, germanium, silicon-germanium alloys
(SiGe), silicon oxycarbide (SiOC), tin, tin oxide, titanium oxide,
carbon, a variety of metal hydrides (e.g., MgH.sub.2), silicides,
phosphides, carbides and nitrides, that later form nanostructures
112. The precursor gases 106 react at the surface of the catalyst
islands 104 releasing certain materials 108 that are adsorbed by
the islands 104a and other materials 119 and then released to the
environment. This process is sometimes referred to as a
dissociative chemisorption. For example, silane (SiH.sub.4)
decomposes at high temperatures or with an assist of plasma to
produce silicon, silane radical, and hydrogen. Deposited silicon or
silicon-containing material then diffuses into the catalyst islands
and form alloys with the catalysts. Another example is chloride
based silane such as dichloride, trichloride, and tetrachloride
silane. Chlorosilanes (H.sub.xSiCl.sub.4-x) may react with hydrogen
(H.sub.2) on the surface of a gold containing catalyst island and
release silicon (Si) into the islands and hydrogen chloride (HCl)
into environment of the processing chamber.
[0046] As the dissociative chemisorption process continues, the
catalyst islands 104a increase the concentration of the adsorbed
materials 108 until it reaches the saturation level. At this point,
shown in the next stage 120, further adsorption of the material 108
causes precipitation of this material at the substrate interface
leading to formation of a solid nanostructure 112. This
nanostructure 112 contains active materials and, in certain
embodiments, other materials configured to enhance conductivity
(e.g., dopants), structural integrity, adhesion to the substrate,
and other properties of the nanostructures. The nanostructures may
be functionalized during or after deposition, e.g., forming
core-shell arrangement with other materials, pre-loaded with
lithium.
[0047] In these and other embodiments, an intermediate layer may be
used to protect the substrate 102 from interacting with materials
in the catalyst islands 104a and, as well, the nanostructures 112,
precursors 106 and released reaction products 119 during the
VLS-type deposition process, and during functionalization. For
example, a gold-containing catalyst may be used to deposit silicon
nanowires. However, depositing gold on a copper substrate leads to
formation of a gold-copper alloy, which may negatively impact the
catalytic effect and require more gold to be deposited. Further,
copper may form silicides when exposed to silane, silicon
tetrachloride, or other silicon containing precursor gas. Copper
silicides are generally not desirable in silicon based electrodes
due to it poor mechanical and undesired electrochemical
properties.
[0048] Providing an intermediate layer allows using various
substrate materials that otherwise would react or with precursor
gases (e.g., silane) or form alloys with the deposited materials
during or after the deposition. For example, depositing silicon
nanowires directly on a copper or nickel substrate may result in
formation of undesirable silicides. An intermediate layer serves as
a barrier during deposition and prevents contact between such
substrates and precursors gases. As a result, a number of possible
material alternatives for substrates is greatly increased.
[0049] In certain embodiments, pre-fabricated nanostructures are
bonded (e.g., fused or sintered) to the substrate surface using a
combination of heat and pressure or other techniques. An
intermediate layer may enhance the bonding formed by these
techniques. In other embodiments, substrate-rooted nanostructures
are formed by depositing a bulk layer of the active material onto
the substrate and then selectively etching parts of the layer
forming substrate-rooted nanostructures. A substrate may need to be
protected from etchants in this embodiments, e.g., using an
intermediate barrier layer.
[0050] In certain other embodiments, high capacity materials may be
bound to the substrate using polymeric binders. An intermediate
layer deposited on the substrate may allow using binders to
accommodate for excessive swelling of high capacity materials yet
to maintain a sufficient electrical and mechanical communication
with the substrate. For example, an intermediate layer may be used
to increase substrate surface roughness. In other embodiments, an
intermediate layer includes functional groups on its surface that
provide better adhesion of the polymer to the substrate. It should
be understood that embodiments relying on binders will not
typically provide a substrate-rooted structure nor will they
provide a metallurgical bond between the substrate and the active
materials nanostructures.
[0051] In certain embodiments, pre-synthesized (e.g., preformed)
nanoparticles are deposited on the substrate followed by thermal
annealing steps to form metallurgical connections between the
nanoparticles and substrates. An intermediate layer may be used to
assist during this bond formation or other parts of the overall
process.
Structure and Materials of Intermediate Layer
[0052] An intermediate layer may be used as a diffusion barrier.
For example, an intermediate layer may prevent substrate materials
from diffusing into (and thereby degrading the performance of)
catalysts used to grow active material nanostructures.
Additionally, in some cases, the intermediate layer may prevent
interaction between the substrate and active material precursors
and/or other reagents used during active material fabrication and
other processing operations. Further, the intermediate layer may
enhance adhesion of the active material to the substrate,
especially when nanostructures undergo substantial volume change
during cycling. Still further, an intermediate layer may provide,
e.g., an epitaxial or chemical-bond connection between the
substrate and active material nanostructures (to address a lattice
mismatch and reduce strain), and/or a thermal expansion coefficient
that allows electrode sub-assemblies to be brought from processing
temperatures (e.g., deposition temperature, post-deposition
treatment temperatures) to the room temperature without causing
fractures at the substrate-active material interface, and be
electrically conductive. An intermediate layer could facilitate or
accelerate nanowire growth since surface roughness and wetability
between the intermediate layer and catalyst islands can be
optimized by choosing different deposition process and different
intermediate materials. An intermediate layer may be also used to
promote mechanical integrity during a roll to roll or other method
of fabrication (e.g., prevent deformation because of a high
temperature, high tension environment).
[0053] Selection of materials for an intermediate layer depends on
substrate materials, active materials, contact/attachment
conditions, targeted functionality of the intermediate layer, and
other parameters. Examples of intermediate layer materials include
refractory metals, such as tungsten, molybdenum, niobium, tantalum,
rhenium, tungsten nitride, tungsten carbide, titanium, titanium
oxide, titanium nitride, titanium carbide, zirconium, zirconium
nitride, tantalum, tantalum nitride, cobalt, ruthenium, indium
oxide, cadmium, hafnium, tellurium, tellurium oxide, tellurium
nitride, chromium, iron, chromium oxide, a titanium-tungsten
combination, an iron-tungsten combination, a cobalt-tungsten
combination, molybdenum, nickel, lithium and others. A thickness of
the intermediate layer may be between about 1 nanometer and 5
micrometers, more specifically between about 5 nanometers and 1
micrometer, even more specifically between about 25 nanometers and
100 nanometers. Introducing certain materials into the layer, such
as copper nickel, chromium, and titanium may improve adhesion of
deposited nanostructures to the substrate surface. The thickness
generally depends on functionality required from the layer and
corresponding properties of the materials included in the layer. In
certain embodiments, the intermediate layer has a contact
resistance per unit surface area of the layer that is less than
about 10 Ohm-centimeter squared or in more specific embodiments
less about 5 Ohm-centimeter squared. A resistance over a unit of
surface area is defined as a resistivity of the intermediate layer
materials multiplied by a thickness of the layer.
[0054] In certain embodiments, an intermediate layer includes
tungsten having a thickness of between about 150 nanometers and 250
nanometers. Tungsten does not form alloys with many materials that
can be used as a catalyst to deposit high capacity nanostructured
materials. In other embodiments, a composite intermediate layer is
used containing a sub-layer of tungsten containing material (e.g.,
between about 150 nanometer and 250 nanometer thick) and a
sub-layer of titanium containing material (e.g., between about 1
nanometers and 50 nanometers thick). The titanium sub-layer may be
used to enhance adhesion of the intermediate layer to the
substrate. Intermediate layers described above may be used with
copper and nickel substrates.
[0055] In certain embodiments, an intermediate layer includes
chromium and has a thickness of between about 500 nanometers and
1,500 nanometers. While chromium forms an alloys with gold (and
possible can not be used with this type of catalyst), it can be
successfully used with other catalyst and be deposited over copper,
nickel, and silver substrate layers.
[0056] In certain embodiments, the electrode includes multiple
intermediate layers that form a stack. Each of these layers may
contain the same or different materials. A stack of the
intermediate layers may also be referred to as a "barrier system".
For examples, FIG. 2 illustrates a schematic cross-section of an
electrode 200 with a stack 204 that is positioned between the
substrate 202 and the active material nanostructures 206. FIG. 3
illustrates an expanded view of an electrode portion 210 with a
stack 204 containing three layers 208, 210, and 212. It should be
understood that a stack may include any number of intermediate
layers (e.g., 1, 2, 3, 4, 5, or 6). A number of layers may depend
on materials used, deposition techniques, and targeted
functionality. In certain embodiments, a composite intermediate
layer is used, for example, as a combination of an adhesion layer
and a diffusion barrier, as a combination of a diffusion barrier
and a nucleation surface layer, as a combination of an adhesion
layer, a diffusion barrier, and a nucleation surface layer, and in
a case where two materials provide better diffusion barrier than
just one (e.g., synergistic diffusion barrier effects).
[0057] In certain embodiments, one layer in a stack may be used to
improve adhesion of the nanostructures to the substrate. In more
specific examples, one layer (e.g., layer 207 in FIG. 3) may be
used to improve adhesion of the substrate to the stack, while
another layer (e.g., layer 209 in FIG. 3) may be used to improve
adhesion of the nanostructures to the stack. Examples of materials
for such layers include chromium, titanium, tungsten, tantalum,
nickel, and molybdenum. An adhesion layer may be chosen to
accommodate substantial swelling of the nanostructure base, while
the substrate remains substantially static.
[0058] A layer, for example layer 208 in FIG. 3, may be used as a
diffusion barrier. This layer may prevent the substrate from
interacting with catalyst islands, precursors, and reaction
products. In certain embodiments, an intermediate layer (e.g.,
layer 209 in FIG. 3), may be used to assist in formation of
catalyst islands during the VLS and/or VSS deposition process. For
example, a layer may be used to modify surface properties of the
substrate to provide adequate surface tension so that the catalyst
islands are agglomerated or aggregated. In other embodiments, the
layer may be used to prevent substrate damage during lithographic
etching, ablation, and other methods of forming catalyst
islands.
[0059] In certain embodiments, an intermediate layer or a portion
of the intermediate layer (e.g., a top sub-layer, such as layer 209
in FIG. 3) has a surface condition that facilitates nucleation of
the nanostructures deposited onto the substrate, or more
specifically onto the intermediate layer of the substrate. The
surface condition may be created as a part of an overall catalyst
island formation operation or a separate operation. As understood,
in VLS or VSS deposition methods a solution containing an active
material (or a precursor thereof) precipitates a solid phase
containing the active material when the concentration of the active
material in the solution reaches a certain high level. Initiation
of this precipitation can be controlled, to a certain degree, by
controlling the surface properties of the intermediate layer in the
contact with the solution. Examples of these properties include
surface roughness, surface polarization, surface tension, and
morphology of the surface materials (e.g., crystalline, amorphous,
lattice size and orientation). These properties can be controlled
by selecting certain materials for an intermediate layer (or
portions thereof). Examples of such materials include chrome,
tungsten, nickel, molybdenum, iron, as well as mixtures and alloys
containing one or more of these materials. Further, these
properties can be controlled by using certain deposition methods
and controlling process parameters during the deposition. Examples
of deposition methods include sputtering, electrodeposition (e.g.,
electroless deposition, pulse-plating), evaporation, chemical vapor
deposition (CVD), physical vapor deposition (PVD), and atomic layer
deposition (ALD). In certain embodiments, a post-deposition
treatment, such as back-plating, electro-etching, resputtering,
CVD, annealing, plasma etching, and oxidation, is used to further
control properties of the intermediate layer. While the specific
surface condition(s) chosen to facilitate nucleation will depend on
the composition of the intermediate layer, the active material, and
the catalyst (if any), generally a surface having a surface
roughness of at least about 0.01 .mu.m R, more specifically at
least about 0.05 .mu.m R, or at least about 0.1 .mu.m R.
Frequently, surface conditions that match properties of the active
material will be preferred. For example, substantially similar
lattice constants, polarizations, surface tensions, etc. for the
intermediate layer and active material are preferred.
Process
[0060] FIG. 4 illustrates a flow chart of a general process 400 for
depositing active material on a substrate with one or more
intermediate layer positioned between the substrate and the active
material(s). The process 400 may start with providing a substrate
(block 402). In certain embodiments, a substrate may be provided
into a processing chamber, such a CVD apparatus, in a roll-to-roll
format. A deposition area of the substrate is generally preheated
to a predetermined temperature established by deposition conditions
of the intermediate layer and/or active materials.
[0061] The process 400 may continue with depositing one or more
intermediate layers (block 404). In certain embodiments,
intermediate layer materials are deposited using a Physical Vapor
Deposition, a Chemical Vapor Deposition, electrodeposition, or any
other suitable deposition technique. For example, a layer of
titanium and/or titanium nitride may be deposited using a
sputtering target containing titanium as well as evaporation,
sputtering, plating, laser ablation, Atomic Layer Deposition, and
Chemical Vapor Deposition. This deposition operation (block 404)
may be followed by one or more post deposition treatment
operations, such as back-plating/electro-etching, resputtering, CVD
treatment, annealing, plasma etching, and oxidation. For example,
surface properties of the intermediate layer may need to be
controlled to allow formation of the catalyst islands and/or
nucleation of the active material during active material deposition
operation 406. In certain embodiments, intermediate layer
deposition operation 404 and/or post-deposition treatment may be
repeated a number of times to build an intermediate layer stack as,
for example, shown in FIG. 4.
[0062] The process 400 may continue with depositing active
materials (block 406). Details of some embodiments of this
operation are described in U.S. patent application Ser. No.
12/437,529 filed on May 7, 2009, which is incorporated by reference
herein in its entirety for purposes of describing an operation for
depositing active materials.
[0063] In certain embodiments, particularly those involving a VLS
or VSS process, the deposition operation 406 starts with depositing
catalyst islands on the substrate surface. In addition to single
material catalyst embodiments, catalyst islands may include two or
more materials (e.g., binary catalysts, tertiary catalysts, etc.).
Besides modifying catalytic functions, a combination of catalyst
may lead to changes in eutectic properties, rheological properties
(e.g., viscosity, surface tension), and other described above.
[0064] In certain embodiments, deposition processes other than a
VLS may be used to deposit active materials in operation 406.
Certain examples are described above.
[0065] It should be noted that the above mentioned operations could
be implemented on a single apparatus or a series of apparatus such
that operations are performed soon after completion of the previous
operation. For example, an apparatus may include one or more
sputtering stations for adding intermediate layer and catalyst
materials and one or more CVD stations for depositing active
material nanostructures onto the moving web. In other embodiments,
different apparatuses may be used for one or more of theses. A
period of time may pass before two sequential operations, in which
case, a partially manufactured electrodes may need to be protected
from the storage environment by adding a protective layer.
Sub-Assembly: Electrodes with Separators
[0066] Two common arrangements of the electrodes in lithium ion
cells are wound and stacked. One goal is to position and align the
surfaces of active layers of the two electrodes surfaces as close
as possible without causing an electrical short. Close positioning
allows lithium ions to travel more rapidly and more directly
between the two electrodes leading to better performance.
[0067] FIG. 5A illustrates a side view of an aligned stack
including a positive electrode 502, a negative electrode 504, and
two sheets of the separator 506a and 506b in accordance with
certain embodiments. The positive electrode 502 may have a positive
active layer 502a and a substrate portion 502b not coated with a
positive active material (but may include an intermediate layer
coating), i.e., an uncoated portion. Similarly, the negative
electrode 504 may have a negative active layer 504a and a negative
substrate portion 504b not coated with a negative active material
(but may include an intermediate layer coating), i.e., an uncoated
portion. In many embodiments, the exposed area of the negative
active layer 504a is slightly larger that the exposed area of the
positive active layer 502a to ensure trapping of the lithium ions
released from the positive active layer 502a by intercalation
material of the negative active layer 504a. In one embodiment, the
negative active layer 504a extends at least between about 0.25 and
5 mm beyond the positive active layer 502a in one or more
directions (typically all directions). In a more specific
embodiment, the negative layer extends beyond the positive layer by
between about 1 and 2 mm in one or more directions. In certain
embodiments, the edges of the separator sheets 506a and 506b extend
beyond the outer edges of at least the negative active layer 504a
to provide electronic insulation of the electrode from the other
battery components. The positive uncoated portion 502b may be used
for connecting to the positive terminal and may extend beyond
negative electrode 504 and/or the separator sheets 506a and 506b.
Likewise, the negative uncoated portion 504b may be used for
connecting to the negative terminal and may extend beyond positive
electrode 502 and/or the separator sheets 506a and 506b.
[0068] FIG. 5B illustrates a top view of the aligned stack. The
positive electrode 502 is shown with two positive active layers
512a and 512b on opposite sides of the flat positive current
collector 502b. Similarly, the negative electrode 504 is shown with
two negative active layer 514a and 514b on opposite sides of the
flat negative current collector. Any gaps between the positive
active layer 512a, its corresponding separator sheet 506a, and the
corresponding negative active layer 514a are usually minimal to
non-existent, especially after the first cycle of the cell. The
electrodes and the separators are either tightly would together in
a jelly roll or are positioned in a stack that is then inserted
into a tight case. The electrodes and the separator tend to swell
inside the case after the electrolyte is introduced and the first
cycles remove any gaps or dry areas as lithium ions cycle the two
electrodes and through the separator.
[0069] A wound design is a common arrangement. Long and narrow
electrodes are wound together with two sheets of separator into a
sub-assembly, sometimes referred to as a jellyroll, shaped and
sized according to the internal dimensions of a curved, often
cylindrical, case. FIG. 6A shows a top view of a jelly roll
comprising a positive electrode 606 and a negative electrode 604.
The white spaces between the electrodes represent the separator
sheets. The jelly roll is inserted into a case 602. In some
embodiments, the jellyroll may have a mandrel 608 inserted in the
center that establishes an initial winding diameter and prevents
the inner winds from occupying the center axial region. The mandrel
608 may be made of conductive material, and, in some embodiments,
it may be a part of a cell terminal. FIG. 6B presents a perspective
view of the jelly roll with a positive tab 612 and a negative tab
614 extending from the jelly roll. The tabs may be welded to the
uncoated portions of the electrode substrates.
[0070] The length and width of the electrodes depend on the overall
dimensions of the cell and thicknesses of active layers and current
collector. For example, a conventional 18650 cell with 18 mm
diameter and 65 mm length may have electrodes that are between
about 300 and 1000 mm long. Shorter electrodes corresponding to low
rate/higher capacity applications are thicker and have fewer
winds.
[0071] A cylindrical design may be desirable for some lithium ion
cells because the electrodes swell during cycling and exert
pressure on the casing. A round casing may be made sufficiently
thin and still maintain sufficient pressure. Prismatic cells may be
similarly wound, but their case may bend along the longer sides
from the internal pressure. Moreover, the pressure may not be even
within different parts of the cells and the corners of the
prismatic cell may be left empty. Empty pockets may not be
desirable within the lithium ions cells because electrodes tend to
be unevenly pushed into these pockets during electrode swelling.
Moreover, the electrolyte may aggregate and leave dry areas between
the electrodes in the pockets negative effecting lithium ion
transport between the electrodes. Nevertheless, in certain
applications, such as those dictated by rectangular form factors,
prismatic cells are appropriate. In some embodiments, prismatic
cells employ stacks rectangular electrodes and separator sheets to
avoid some of the difficulties encountered with wound prismatic
cells.
[0072] FIG. 7 illustrates a top view of a wound prismatic
jellyroll. The jelly roll comprises a positive electrode 704 and a
negative electrode 706. The white space between the electrodes is
representative of the separator sheets. The jelly roll is inserted
into a rectangular prismatic case. Unlike cylindrical jellyrolls
shown in FIGS. 6A and 6B, the winding of the prismatic jellyroll
starts with a flat extended section in the middle of the jelly
roll. In one embodiment, the jelly roll may include a mandrel (not
shown) in the middle of the jellyroll onto which the electrodes and
separator are wound.
[0073] FIG. 8A illustrates a side view of a stacked cell including
a plurality of sets (801a, 801b, and 801c) of alternating positive
and negative electrodes and a separator in between the electrodes.
One advantage of a stacked cell is that its stack can be made to
almost any shape, and is particularly suitable for prismatic cells.
However, such cell typically requires multiple sets of positive and
negative electrodes and a more complicated alignment of the
electrodes. The current collector tabs typically extend from each
electrode and connected to an overall current collector leading to
the cell terminal.
Housing
[0074] FIG. 9 illustrates a cross-section view of the wound
cylindrical cell in accordance with one embodiment. A jelly roll
comprises a spirally wound positive electrode 902, a negative
electrode 904, and two sheets of the separator 906. The jelly roll
is inserted into a cell case 916, and a cap 918 and gasket 920 are
used to seal the cell. In some cases, cap 912 or case 916 includes
a safety device. For example, a safety vent or burst valve may be
employed to break open if excessive pressure builds up in the
battery. Also, a positive thermal coefficient (PTC) device may be
incorporated into the conductive pathway of cap 918 to reduce the
damage that might result if the cell suffered a short circuit. The
external surface of the cap 918 may used as the positive terminal,
while the external surface of the cell case 916 may serve as the
negative terminal. In an alternative embodiment, the polarity of
the battery is reversed and the external surface of the cap 918 is
used as the negative terminal, while the external surface of the
cell case 916 serves as the positive terminal. Tabs 908 and 910 may
be used to establish a connection between the positive and negative
electrodes and the corresponding terminals. Appropriate insulating
gaskets 914 and 912 may be inserted to prevent the possibility of
internal shorting. For example, a Kapton.TM. film may used for
internal insulation. During fabrication, the cap 918 may be crimped
to the case 916 in order to seal the cell. However prior to this
operation, electrolyte (not shown) is added to fill the porous
spaces of the jelly roll.
[0075] A rigid case is typically required for lithium ion cells,
while lithium polymer cells may be packed into a flexible,
foil-type (polymer laminate) case. A variety of materials can be
chosen for the case. For lithium-ion batteries, Ti-6-4, other Ti
alloys, Al, Al alloys, and 300 series stainless steels may be
suitable for the positive conductive case portions and end caps,
and commercially pure Ti, Ti alloys, Cu, Al, Al alloys, Ni, Pb, and
stainless steels may be suitable for the negative conductive case
portions and end caps.
CONCLUSION
[0076] Although the foregoing invention has been described in some
detail for purposes of clarity of understanding, it will be
apparent that certain changes and modifications may be practiced
within the scope of the appended claims. It should be noted that
there are many alternative ways of implementing the processes,
systems, and apparatus of the present invention. Accordingly, the
present embodiments are to be considered as illustrative and not
restrictive, and the invention is not to be limited to the details
given herein.
* * * * *