Carbon nanotube composite structures and methods of manufacturing the same

A current conductor for an electrochemical power device that includes an array of carbon nanotubes (CNT) anchored in a carbon nanotube metal composite layer and a structure that may incorporate nanoscale particles or thin film onto the current conductor is described. Additionally, a process for creating the structure using electrochemical plating of the metal layer onto the CNT array followed by separation of the structure from the substrate is provided. Another process includes creating the structure using co-electrodeposition of the CNT and metal from an electroplating bath using surfactants, physical energy, and a magnetic and/or electric field to orient the CNT and enhance the CNT density in the composite.

BACKGROUND

This invention relates generally to structures and devices including an anchored array of carbon nanotubes (CNT) that extend from a conductive composite for application as a current conductor and fully or part of the electrode for an electrochemical power device such as a battery, supercapacitor, fuel cell, or the like. Additionally, the invention also generally relates to methods of manufacture of the structures and devices described above.

2. Related Art

Electrochemical power devices described herein generally include devices that can: store electrical energy in chemical form and release it back in electrical form on demand (for example, a Li ion battery that is used in mobile phones; convert chemical energy into electrical energy (for example, a fuel cell that can use a chemical fuel such as hydrogen or methanol and convert it into electrical energy); and/or store electrical energy and release it on demand (for example, a supercapacitor).

While the mechanism of energy storage and conversion maybe different in these devices, one common aspect of all these devices is the need for current conductors and/or electrodes. Each device typically has two electrodes, an anode, through which current flows into the device, and a cathode, through which current flows out of the device. Sometimes, a current collector, typically a metal such as copper or aluminum, is used in addition to or as part of an electrode for improved electrical conduction.

Carbon nanotubes (CNT) are generally known to have excellent electrical conductivity, thermal conductivity, mechanical strength, and chemical resistance. Although CNT have been observed over half a century ago, recent predictions of the remarkable physical properties of CNT sparked widespread interest in this material and CNT have been touted as the new material of the 21stcentury. CNT have been studied widely and various groups have proposed potential applications for this material in composites for higher strength and thermal conductivity, nano-probes and nano-pipettes for biomedical applications such as targeted drug delivery, field emission devices such as light emitting diodes, energy generation devices such as solar cells, nanoscale contact probes, nanoscale semiconductor device applications, electrodes for electrochemical energy devices such batteries, fuel cells, supercapacitors, and so on.

In the area for electrochemical energy devices, for example, CNT based structures have been suggested as potential candidates for electrodes for batteries, fuel cells, and supercapacitors. In particular, in the Lithium Ion battery field, there are several examples of utilizing CNT in electrodes. U.S. Pat. No. 6,709,471 discloses a CNT-Boron Nitride battery, which includes a structure utilizing the walls of the CNT as electrodes with Boron Nitride as an intermediate dielectric layer. Further, U.S. Pat. No. 5,879,836 describes using carbon fibrils as lithium intercalation sites in an electrode. Yet another example includes U.S. Pat. No. 7,442,284, which describes the use of an array of CNT coated with a conductive polymer as an electrode for various devices including energy storage devices.

In the field of fuel cells, U.S. Pat. No. 7,585,584 describes utilizing CNT grown on a carbon substrate with catalyst particles deposited on the CNT. Further, in the field of supercapacitors, U.S. Pat. Nos. 6,665,169 and 6,205,016, describe utilizing carbon nanofibers as electrodes to increase performance.

BRIEF SUMMARY

According to one aspect of the present invention, structures for an electrochemical power device are provided. In one example, a structure includes a conductive composite layer and an array of carbon nanotubes anchored within, such that at least a portion of the carbon nanotubes extend from the composite conductive layer. The structure may further include nanoscale particles or a thin film disposed on the carbon nanotubes. The exemplary structures may be used as electrodes in various electrochemical power devices, such as batteries, fuel cells, or capacitors.

The portion the carbon nanotubes extending from the composite layer may be aligned in a common direction, more (or less) aligned than the portion anchored in the composite layer, or tangled. In some examples, the carbon nanotubes may extend from both sides of the composite layer. Further, the portion of the carbon nanotubes anchored in the composite layer may be tangled or aligned. In some examples, the conductive composite layer includes a carbon-metal composite material.

According to another aspect of the present invention, an exemplary process for forming a structure for an electrochemical power device is provided. In one example, the method includes forming or arranging an array of carbon nanotubes embedded or anchored in a conductive composite layer, where at least a first portion of the length of the array of carbon nanotubes extend from the conductive composite layer. In some examples, the first portion of the carbon nanotubes is further aligned along a common direction.

The array of carbon nanotubes may be formed at least partially aligned according to deposition processes, electroplating processes, a template, or the like. In one example, the structures are formed by passing a substrate through an electrochemical bath, the electrochemical bath having carbon nanotubes in solution, and electroplating a conductive layer having the carbon nanotubes to the substrate. A magnetic and/or electric field may be used to assist in aligning the carbon nanotubes during formation of the conductive composite layer. Further, the exemplary process for forming such structures may be part of a roll-to-roll manufacturing process.

DETAILED DESCRIPTION

According to one aspect of the present invention, and broadly speaking, structures including carbon nanotubes (CNT) anchored in a conductive composite, and extending therefrom, are described. The exemplary structures may be used as a conductor for an electrochemical power device such as a battery, capacitor, fuel cell, or the like. Further, exemplary processes for forming such structures and devices are provided. Initially, exemplary structures are described, followed by exemplary process and manufacturing methods.

Exemplary Devices and Structures:

FIG. 1illustrates an exemplary electrochemical power device, which may include one or more of the exemplary current conductor structures described herein. In particular,FIG. 1schematically illustrates a battery device10, e.g., a Li-ion battery, generally comprising a cathode20and anode28, separated by a separator24. Cathode20is typically made of a metal oxide material and separator24is typically made of polymers such as polyethylene, polypropylene, laminates of such polymers, or the like. In this example, anode28comprises a CNT structure including CNT102anchored in a conductive composite layer110, the CNT extending therefrom and within an electrolyte solution30such as a lithium salt or an organic solvent. Additionally, it will be recognized by those of skill in the art that the region of electrolyte solution30and/or anode28may include additional materials such as carbon or graphite powder, silicon powder or alloyed silicon powder, or the like, which forms the lithium ion intercalating part of the electrode.

FIG. 2Aillustrate a first embodiment of a current conductor structure100for an electrochemical power device. The exemplary structure100includes an aligned array of CNT102anchored in a conductive composite layer110. In this particular example, CNT102are anchored within and extend from a first side of the conductive composite110and are generally aligned, that is, extending in a substantially parallel fashion from conductive composite110. CNT102may further extend from a second side of conductive composite110, e.g., on a side opposite of the first side. The extension of CNT102on the second side may or may not be aligned. CNT102may further form a mesh or be intertwined within conductive composite110as schematically illustrated, providing a mechanical anchor for CNT102therein.

The conductive composite may include various materials, such as conductive metals, metal alloys, conductive polymers, and the like. For example, a metal alloy including Ni, Zn, Cu, Al, Au, Ag, and/or other metals may be used.

CNT102may include single or multi-walled structures, may vary in height, width, and so on. Further, catalysts or particles may be deposited on CNT102. In one example, CNT102may include a four wall structure to match the atomic spacing of a single crystal silicon lattice. CNT102may be aligned during deposition, e.g., via a physical vapor deposition or through the use of a template or the like (the template may include an anodized aluminum oxide (AAO) template, polymer-based template, ceramic based template, or the like). Exemplary processes for forming the structure of this and other embodiments will be described in greater detail below with references toFIGS. 6A-11.

Structure100may be used as a component or substructure of an electrochemical power device; particularly, for use as a current conductor in an electrochemical device. As one illustrative example, structure100, and other exemplary structures described, may be used as an anode for a battery device, for example, as illustrated inFIG. 1.

FIG. 2Billustrates a second embodiment of a current conductor structure200for an electrochemical power device. Structure200is similar to that ofFIG. 2A, but in this example, conductive composite layer110fully covers one end of CNT102and extends beyond the conductive composite region. As such, CNT102only extends from one surface or side of the conductive composite110.

FIG. 3illustrates another exemplary embodiment of a current conductor structure300for an electrochemical power device. The current conductor and electrode structure of this example is similar to that ofFIG. 2B, including an array of CNT102anchored in the conductive composite layer110; however, in this example, CNT102are not necessarily aligned with respect to each other or aligned in a common direction. In one example, the portion of CNT102extending from composite layer110is more aligned along a direction perpendicular to the composite layer110than the portion embedded or anchored within composite layer110. In one example, the portion of CNT102extending from composite layer110is less tangled than the portion embedded or anchored within composite layer110.

FIG. 4illustrates another exemplary embodiment of a current conductor structure400for an electrochemical power device. In this example, structure400is similar to that ofFIGS. 2A and 2B, but further includes nanoparticles106disposed or attached to the aligned array of CNT102anchored in the conductive composite layer110. Nanoparticles106, for use in a battery device, may form part of the electrode and act as sites for lithium ion intercalation. For use in fuel cells, for example, nanoparticles106may serve as catalysts for driving chemical reaction

Nanoparticles106may be deposited by any suitable process. In one example, nanoparticles are deposited by electrodeposition from a chemical bath having the particles dispersed in solution. Another exemplary process includes electroplating or electroless plating of nanoparticles to the structure. Yet another exemplary processing includes dispersing a nanoparticle powder with a solvent and an adhesive agent onto the structure and evaporating the solvent, thereby leaving the particles randomly adhered to the CNT102and conductive composite layer110.

FIG. 5illustrates another exemplary embodiment of a current conductor structure500for an electrochemical power device. In this example, structure500is similar to that ofFIG. 2A,2B, or4, but further includes a thin film or material108deposited on CNT102within the array; for example, at least on portions of the CNT102extending from composite layer110. Material108may function as a lithium ion intercalation layer, for example.

Material108may include silicon. In one example, material108includes a graded silicon carbon layer that ranges from 100% C at the surface of each of the tubes of CNT102to 100% Silicon at the outer surface of the layer of material108, where the gradation may assist in managing the stress associated in transitioning from one material to another. Material108may be deposited by any suitable process. In one example, material108is deposed by Plasma Enhanced Chemical Vapor Deposition (PECVD) and Atomic Layer Deposition (ALD) using precursor gases for C (such as CH4), Si (such as SiH4), and an inert gas (such as Ar or N2).

It will be recognized that other combinations of the examples shown herein are possible and contemplated. For example, nanoparticles and/or a thin film(s) may be included with any of the examples, extending CNT102on both sides of conductive composite layer110may be included with any of the examples, and any of the examples may extend from composite layer110as illustrated inFIG. 3.

Broadly speaking, and in embodiment for manufacturing exemplary structures described herein, an exemplary process includes forming an array of CNT anchored or embedded within a composite layer and extending therefrom. The CNT extending from the composite layer are at least partially aligned in a common direction. The portion of the array CNT anchored within the composite layer may be intertwined or positioned in non-aligned orientations.

FIGS. 6A-6Eillustrate an exemplary process for forming a structure for an electrochemical power device. This exemplary process begins inFIG. 6Awith a substrate620and a seed layer622formed thereon for subsequent CNT growth (and/or template deposition) as described below. Substrate620may include, e.g., silicon, germanium silicon dioxide, aluminum oxide, stainless steel, or the like. Seed layer622may include, e.g., cobalt, iron, nickel, or the like. Seed layer622may be deposited on substrate layer622in a variety of suitable processes, including various deposition processes (such as physical vapor deposition), electroplating, or the like. For example, a deposition of a Ni seed layer by physical vapor deposition to a thickness of less than 10 nm on an oxidized silicon substrate may be performed.

A template624is deposited on seed layer622as shown inFIG. 6B. In one example, template624is formed over seed layer622and a plurality of pores, having a high aspect ratio, are etched or otherwise formed therein for subsequent CNT growth. Template624may be formed of anodized aluminum oxide, anodized Al—Fe, anodized Ti, silicon dioxide, or the like. In one example, template624has a thickness of approximately 50 nm to 50 μm, and pores formed therein may have a diameter of 1 nm to 200 nm and a separation distance of 1 nm to 200 nm from each other. Of course, other dimensions are possible and contemplated.

The pores of template624may be formed through a variety of processes. In one example, an anodization technique may be used, where a block or film of material forming template624is anodized in a chemical bath and high voltage is used to etch out a pattern on the template. Additionally, patterning using photolithography, followed by dry etch of the pores may be used, for example.

FIG. 6Cillustrates CNT growth within the pores of template624. In particular, CNT630are grown from seed layer622, growing within the pores of template624and extending therefrom. CNT630are grown aligned within the pores of template624, but once the CNT630growth exceeds the height of template624, CNT630generally grow freely and become tangled as schematically shown. In one example, CNT630are grown by Plasma Enhanced Chemical Vapor Deposition (PECVD) using acetylene and ammonia at about 1 m Torr pressure and temperatures between 450° C. and 850° C. In another example, CNT630may be deposited using Atomic Layer Deposition (ALD) at temperatures ranging between from 20° C. to 450° C. CNT630may be grown single or multi-walled, and may vary in the number of walls along their height.

A composite layer640is deposited onto the tangled CNT as shown inFIG. 6D. In one example, a carbon-metal composite material is deposited over the tangled portions of CNT630, above template624. Composite layer640may include a metal alloy comprised of Ni, Zn, Cu, Al, Au, Ag, in any combination or ratio. Composite layer640may be deposited by Physical Vapor Deposition (PVD), electroplating, electroless plating, evaporation, or the like. Further, composite layer640may be deposited to a thickness of approximately 1 nm to 20 μm, which may or may not encapsulate or exceed the height of the tangled portion of CNT630. Composite layer640may further be etched back after deposition for a desired thickness or surface characteristic. Further, a polymer may be used to coat composite layer640.

Composite layer640and CNT630are detached from substrate620, seed layer622, and template624as illustrated inFIG. 6E. For example, template624may be etched away, e.g., in a wet or dry chemical etch, resulting in detachment of the structure. In other example, substrate620may be mechanically peeled away from composite layer640and CNT630. Further, in some examples, substrate620can be re-used in a subsequent process to form another structure.

FIGS. 7A-7Dillustrate another exemplary process for forming a structure for an electrochemical power device. This example is similar to that ofFIGS. 6A-6D; however, in this example, a template is not used. This exemplary process begins inFIG. 7Awith a substrate720and a seed layer722formed thereon for subsequent CNT growth.

CNT730are grown having a first portion thereof aligned and a second portion thereof un-aligned or tangled, as shown inFIG. 7B. For example, CNT730may be grown using Chemical Vapor Deposition (CVD) or Plasma Enhanced Chemical Vapor Deposition (PECVD), without the use of a template, where once the growth of CNT730reaches a certain height, e.g., 50 μm to 500 μm, they generally grow freely and become tangled. The tubes of CNT730generally have a tendency to tangle due to surface forces, where the likelihood of tangling increases as the aspect ratio (height/diameter) exceeds about 100-1000. In PECVD processes, the presence of a plasma, and hence an electric field, typically keeps the tubes aligned, but shutting off the plasma while keeping the gases flowing can lead to continuation of the growth in a unaligned manner. In one example, the CNT730are disposed by PECVD using acetylene and ammonia at about 1 m Torr pressure and temperature ranging from 450° C. to 850° C.

As illustrated inFIG. 7C, a composite layer740is then deposited over the tangled portion of CNT730. Composite layer740may be deposited by Physical Vapor Deposition (PVD), electroplating, electroless plating, evaporation, or the like. Further, composite layer740may include a thickness of 1 nm to 20 μm, which may or may not encapsulate or exceed the height of the tangled portion of CNT730.

In one example, a top down deposition is used to form composite layer740, such that the tangled structure of CNT730receives the deposition of composite layer740and effectively seals off most of the aligned portion of CNT730from deposition. Of course, in some examples, some of the aligned portion of CNT730may be covered by the material as well. The deposition conditions of composite layer740can be controlled in an attempt to not fill beyond a certain depth, e.g., using lower deposition rates and adding bias in the PVD process, and/or removing or reducing additives such as polyalkylene glycol and organic sulfides in electroplating, which may reduce the tendency to fill deep into the structure.

The structure of the CNT730and composite layer740is detached from the substrate720and seed layer722as illustrated inFIG. 7D. For example, composite layer740and CNT730may be peeled or mechanically removed from substrate720and seed layer722. In some examples, substrate720can be re-used in a subsequent process to form another structure.

FIGS. 8A-8Fillustrate another exemplary process for forming a structure for an electrochemical power device. This example is similar to that ofFIGS. 7A-7D; however, in this example, CNT830are detached from the substrate820and seed layer822before the addition of a composite material layer. This exemplary process begins inFIG. 8Awith a substrate820and a seed layer822formed thereon for subsequent CNT growth.

CNT830are grown having a first portion thereof aligned and a second portion thereof tangled, as shown inFIG. 8B. For example, CNT830may be grown using Chemical Vapor Deposition (CVD), for example, without the use of a template. Once the growth of CNT830reaches a certain height, e.g., 50 μm to 500 μm depending on the particular process, they generally grow freely and become tangled as shown.

As illustrated inFIGS. 8C-8D, CNT830are then detached from substrate820and seed layer822. In one example, a conductive adhesive tape860is pressed on the tangled portion of CNT830and then moved to detach or peal CNT830from substrate820. Of course, other processes may be used for detaching CNT830.

As illustrated inFIG. 8E, a composite840is then deposited onto the tangled portion of CNT830. Composite layer840may be deposited by Physical Vapor Deposition (PVD), electroplating, electroless plating, evaporation, or the like. Further, composite layer840may include a thickness of 1 nm to 100 nm, which may or may not encapsulate or exceed the height of the tangled portion of CNT830.

The structure is further detached from tape860as illustrated inFIG. 8F. For example, composite840and CNT830may be peeled or mechanically removed from tape860. In other examples, tape860may be omitted, and a metal may be deposited on the tangled portion of CNT830by evaporation or spin on a liquid polymer glue layer that solidifies subsequently, and used in a manner similar that shown for tape860.

According to another exemplary embodiment of the present invention, a co-electrodeposition process for forming exemplary structures is provided. In particular, a co-electrodeposition process of CNT and conductive composite layers is described, where the CNT are at least partially extend from the conductive composite layer.

FIG. 9schematically illustrates a first exemplary roll-to-roll process and system for forming exemplary structures as described herein. In particular, the exemplary process begins with a spool of substrate material. The substrate material may include a metal or alloys of copper, nickel, aluminum, zinc, gold, silver, stainless steel, or the like, ranging in thickness from 5 μm to 5 mm.

The substrate material passes through an electroplating solution with CNT dispersed therein, for example, by passing around a drum at least partially submerged in the electroplating solution. The electroplating solution generally contains a solution of the metal or metal ion being deposited as the composite layer and CNT dispersed therein (after appropriate surface treatment for decontamination, for example). The electroplating solution may include a copper-plating electrolyte based on copper sulfate acidified with sulfuric acid. In one example, a bath may include a copper sulfate concentration of between 0.1 and 1.0 moles and a sulfuric acid concentration of between 0.2 and 4 moles. Further, the CNT content of the bath may be between 1 and 50 percent by weight in one example. In another example, the CNT content may be between 1 and 50 percent by volume.

Of course, various other concentrations, compositions, and/or electroplating solutions are possible and contemplated depending on the desired composite layer and characteristics, CNT characteristics, material thicknesses, and so on. Additionally, the electroplating bath may include various other additives such as chloride ions, polyether, organic sulfide, nitrogen compounds, and so on. Other examples of electroplating with CNT in solution, including various electroplating solution characteristics, are described in U.S. Pat. No. 7,651,766 and U.S. Patent Publication No. 2010/0122910, both of which are incorporated herein by reference in their entirety.

In this example, the drum provides the negative electrode and a positive electrode made of the metal being deposited (not shown in figure) is used along with an electrical field to drive the deposition process. For example, an electric field may be used to attract the CNT to the metal substrate such that they are at least partially encased by the metal composite during the electroplating process and extend from the deposited metal composite layer. The chemical solution may also be subjected to physical energy such as agitation, bubbling of gas, ultrasonic or megasonic waves to improve mixing, the addition of non-ionic surfactants to prevent adhering of the CNT, and so on.

After deposition, this example illustrates an optional intermediate set of rollers to reduce the thickness of the sheet. Further, in one example, the substrate layer may be peeled off from the composite layer prior to passing through the set of rollers or otherwise being wound on the final spool. The formed structures, e.g., including a composite layer, including anchored CNT extending at least partially therefrom, may be used to form various electrochemical power devices as described. Further, the formed structures may be cut in various sizes, e.g., dimensions ranging from 1 mm by 1 mm to 1 m by 1 m or larger.

In other examples, the composite layer, including anchored CNT, may be formed in an electroplating process without the use of a substrate. For example, the composite layer may be formed on the drum (or other element of the electroplating apparatus in the solution) and then peeled or removed therefrom.

FIG. 10illustrates another exemplary co-electrodeposition process of CNT and metal composite layers. This example is similar to the process described forFIG. 9, however, in this example, a magnet field is used to orient the carbon nanotubes in the solution and potentially increase the density of CNT within the composite formed on the substrate. For instance, a magnet may be placed within the drum to attract and orient the CNT in solution such that they are captured within in and extend out of the metal being deposited on the substrate as illustrated in the CNT metal composite portion. This example may provide for greater alignment of the CNT extending from the metal composite layer than without such a magnetic field.

FIG. 11illustrates another exemplary co-electrodeposition process of CNT and metal composite layers. This example is similar to the process described forFIG. 10, however, in this example, an inert electrode is used within the bath to create an electric field for orienting the CNT in the solution and potentially increasing the density of CNT within the composite formed on the substrate, similar to that described with reference toFIG. 10. Further, this example may provide for greater alignment of the CNT extending from the metal composite layer than without such an electric field.

FIG. 12illustrates an exemplary SEM image of a structure in accordance with certain embodiments described herein. In particular,FIG. 12illustrates a SEM image of CNT before and after a metallization process, i.e., the addition of a metal composite layer to anchor the CNT. Particular, to anchor the tangled portion of the CNT. Further, the figure includes a schematic depiction of the various components of the structure after metallization, e.g., an array of aligned CNT, which may be disposed in Li-ions as part of a Li-ion battery, the array of CNT anchored in a metal layer. The structure illustrates in the SEM images was formed according to the process illustrated inFIGS. 7A-7D.

The above-described embodiments of the present invention are merely meant to be illustrative and not limiting. Various changes and modifications may be made without departing from the invention in its broader aspects. The appended claims encompass such changes and modifications within the spirit and scope of the invention.