Vertically aligned carbon nanotubes with interfacial carbon sublayer for increased adhesion

Abstract

An electrochemical cell includes an electrode in which a carpet of carbon nanotubes (CNTs) is grown from and bonded to a conductive substrate. A deposition process forms a catalyst layer over the substrate. A carbon growth process then decomposes a carbon source gas over the catalyst layer to different allotropes of carbon on the interfacial surface of the catalyst layer. CNTs grow from the surface of the catalyst layer and away from the current collector to form a CNT carpet. At the same time, a layer of amorphous carbon forms between the catalyst layer and the current collector. When an electrode thus formed is incorporated into an electrochemical cell, the surface area of the CNT carpet provides ample storage for active material, such as metallic lithium.

Description

VERTICALLY ALIGNED CARBON NANOTUBES WITH INTERFACIAL CARBON SUBLAYER FOR INCREASED ADHESIONBACKGROUND

[0001] An electric battery includes one or more electric cells. Each cell includes a positive electrode (cathode) and a negative electrode (anode) physically separated by an ion conductor (electrolyte). When a cell is discharged to power an external circuit, the anode supplies negative charge carriers (electrons) to the cathode via the external circuit and positive charge carriers (cations) to the cathode via the internal electrolyte. During charging, an external power source reverses this process, driving electrons from the cathode toward the anode via the power source and cations from the cathode to the anode via the electrolyte.

[0002] Lithium-ion (Li-ion) batteries store charge in the anode as Li cations (aka Li ions, or Li+). Li-ion batteries are rechargeable and ubiquitous in mobile communications devices and electric vehicles due to their high energy density, lack of memory effect, and low selfdischarge rate. Lithium-metal (Li-metal) batteries store charge in the anode as Li metal (aka Li or pure Li). Lithium-metal electrodes exhibit considerably higher theoretical capacity than lithium-ion electrodes. Unfortunately, rechargeable Li-metal batteries have yet to be commercialized, mainly due to the growth of electrically conductive Li dendrites that can extend from anode to cathode providing a destructive and potentially dangerous internal short. Anodes that use carbon nanotubes (CNTs) for lithium storage suppress dendrite formation. Commercially viable CNT anodes and methods of forming them are therefore in development.BRIEF DESCRIPTION OF THE DRAWINGS

[0003] The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:

[0004] Figure 1 is a flowchart 100 outlining a method of forming an electrode that can be used as an anode in an electrochemical cell.

[0005] Figure 2 depicts an electrode 200 in which a copper foil is treated to the method of Figure 1 to grow a CNT carpet 205 that extends from a catalyst layer 210.

[0006] Figure 3 is a cross-sectional scanning-electron microscope (SEM) image, or photomicrograph, of an electrode 300 like electrode 200 of Figure 2, with like-identified elements being the same or similar.

[0007] Figure 4 is another photomicrograph of electrode 300 of Figures 2 and 3 but taken at a higher level of magnification.

[0008] Figure 5A is a photomicrograph of a cross section of an electrode 500 formed using the process of Figure 1.

[0009] Figure 5B is a photomicrograph of electrode 500 of Figure 5 A at a higher level of magnification.

[0010] Figure 6 is a photomicrograph of a cross section of an electrode 600 formed using a process similar to the process of Figure 1 but omitting pre-activation step 119.

[0011] Figure 7 is a cross section of an electrochemical cell 700 that incorporates electrode 200 as an anode.DETAILED DESCRIPTION

[0012] An electrochemical cell includes an electrode in which a carpet of carbon nanotubes (CNTs) is grown from and bonded to a conductive substrate. A deposition process forms a catalyst layer over the substrate. A carbon growth process then decomposes a carbon source gas over the catalyst layer to different allotropes of carbon on the interfacial surface of the catalyst layer. CNTs grow from the surface of the catalyst layer and away from the current collector to form a CNT carpet. At the same time, a layer of amorphous carbon forms between the catalyst layer and the current collector. When an electrode thus formed is incorporated into an electrochemical cell, the surface area of the CNT carpet provides ample storage for active material, such as metallic lithium. The adhesive carbon layer under the CNT carpet prevents interfacial delamination and the formation of lithium islands at the bases of the CNTs during cycling, and thus prolongs cell life. The carbon layer is an amorphous carbon, comprising a short-range crystalline order as confirmed with Raman spectroscopy,which showed a weak 2D peak from the carbon layer after multiple rounds of peeling off the CNT using a tape with a strong adhesive.

[0013] Figure 1 is a flowchart 100 outlining a method of forming an electrode that can be used as an anode in an electrochemical cell. The surfaces of a copper foil are etched (105) using a dry etching technique to remove any native oxide or corrosion protection layer, such as chromium, from the copper surface or surfaces to leave exposed copper. Dry etching can be performed using e.g. sputter etching, ion-beam etching, or plasma etching.

[0014] Next, a catalyst-support layer of e.g. 3-40 nm of a buffer material (e.g. aluminum oxide) is deposited over the etched copper surface (110). A catalyst layer of 0.5-10 nm of iron, a continuous film or islands, is then deposited over the aluminum oxide (115). The aluminum oxide and iron layers can be deposited to a collective thickness of <= 50 nm in a physical vapor deposition (PVD) chamber using PVD processes such as e-beam evaporation, thermal evaporation, sputter deposition, or a combination. The copper and deposited materials are referred to collectively as the "catalyzed copper." The catalyst-support and catalyst layers can be deposited directly on the etched copper foil immediately after etch 105 and in the same vacuum chamber to prevent re-oxidation of the copper surface. The aluminum oxide passivates the fresh, non-contaminated copper surface from re-oxidation.

[0015] The catalyzed copper is moved from the PVD process environment to a low-pressure chemical-vapor-deposition (CVD) chamber (117). The iron catalyst from step 115 may be partially or fully oxidized, particularly if the catalyst is exposed to the room environment when the catalyzed electrode from step 115 is transferred between the PVD and CVD chambers. The aluminum oxide catalyst-support layer protects the copper surfaces during such transfer.

[0016] A pre-activation step 119 treats the catalyzed copper to a hydrogen process gas. The hydrogen reacts with the oxidized iron to partially or fully reduce to metallic iron, causing the iron to be more catalytically active. One embodiment employs a two-inch tubular reactor at 12 Torr and 650 °C with a hydrogen flow rate of 500 seem.

[0017] Thin iron catalyst films (less than about 3 nm) are in the form of islands or particles separated by space. During catalyst pre-activation, iron films can break up to form islandsthrough a thermodynamic process known as "dewetting." These islands can then coalesce to form larger islands, with island size controllable with the pre-activation time. Thicker iron catalyst films (more than about 3 nm) tend to form more continuous films, either fully continuous or films with fewer or smaller gaps.

[0018] A carbon growth process (120) commences after e.g. five minutes of catalyst preactivation. A carbon source gas, in one embodiment acetylene (C2H2), is added to the CVD chamber with accompanying hydrogen and argon at temperatures ranging between 500 °C and 750 °C. The hydrogen further activates the oxidized iron catalyst, and the activated catalyst decomposes the acetylene into different allotropes of carbon. Carbon nanotubes (CNTs) grow from the catalyst layer away from the current collector while amorphous carbon forms between the catalyst layer and the current collector. Growth step 120 takes e.g. five minutes. The CNTs can be vertically aligned with more or less disordered roots on the CNT side of the catalyst layer. Reactant gases other than or in addition to acetylene can be used.

[0019] The dual-carbon material from growth step 120 is removed to cool outside the hot zone of the CVD reactor (125). The cooled, catalyzed copper now bears a 100 pm thick CNT carpet with a sub-micron-thick amorphous-carbon sublayer. Total growth or residence time of the catalyzed copper inside the reactor can be tuned as low as 10 seconds to make 2 pm CNT and as high as 30 minutes to achieve 500 pm CNT.

[0020] In pre-activation step 119, catalyst-island or particle size is a function of preactivation time. CNT diameter is a function of catalyst-island or particle size and so can be tuned by controlling catalyst pre-activation time. The amorphous-carbon sublayer formed during the CNT growth process strongly binds the CNTs to the copper substrate to form a thermally and electrically conductive interface between the CNTs and the copper current collector.

[0021] The character of the interfacial carbon layer depends in part on the extent of removal of the copper oxide and contaminant layers, which can be controlled by ion-beam etching time and power. Less complete removal results in a thinner carbon layer due to lower activity of the copper surface to catalyze the carbon sublayer deposition. In one example, a carbonsublayer with an average layer thickness of about 100 nm was grown by short exposure of copper foil to an ion beam without complete removal of copper oxide and protection layers.

[0022] Figure 2 depicts an electrode 200 in which a copper foil is treated to the method of Figure 1 to grow a CNT carpet 205 that extends from a catalyst layer 210. When electrode 200 is incorporated into an electrochemical cell or capacitor, the copper foil serves as a current collector 215.

[0023] CNTs 225 can extend from catalyst layer 210 more than ten times the thickness of the catalyst layer. The areal density of CNT carpet 205 can be on the order of hundreds or thousands of square meters per gram, providing ample storage for alkali-metal plating or ion intercalation. These properties make electrode 200 an excellent candidate for energy storage, e.g. in electrochemical cells, supercapacitors, or hybrid supercapacitors. Catalyst layer 210 is bonded to copper current collector 215 via an interfacial carbon layer 235, which may include voids 240 with an average maximum dimension less than 0.5 um in this example, less than one micron in other embodiments.

[0024] Interfacial carbon layer 235 has an average thickness of less than 1 um in this embodiment and conforms to the etched surface of copper current collector 215. The thickness of layer 235 varies considerably with growth parameters. For example, catalyst preactivation produces thickness ranges between 100 and 500 nm, whereas skipping the preactivation step produces thickness ranges between 20 and 100 nm. Roughness between carbon layer 235 and current collector 215 increases the surface area and strength of the bond. Carbon layer 235 consists essentially of carbon but may include metals used in the catalyst and / or substrate, e.g., iron, aluminum, and / or copper, embedded in the carbon layer by diffusion during the growth process.

[0025] Figure 3 is a cross-sectional scanning-electron microscope (SEM) image, or photomicrograph, of an electrode 300 like electrode 200 of Figure 2, with like-identified elements being the same or similar. Bright spots 305 in catalyst layer 210 and at the tips of some nanotubes 225 are iron catalyst particles. This cross section shows that carbon layer 235 smooths catalyst layer 210 relative to the underlying copper surface. The surface roughness (Ra) of copper current collector 215 is greater than 0.2 um; the surface roughness of catalystlayer 210 is relatively smooth, less than 0.1 um. That is, the metal-side surface roughness of carbon layer 235 is more than twice the catalyst-side surface roughness. This level of magnification shows CNT carpet 205 includes a lower portion of relatively disordered CNTs. Continued growth produces CNTs that tend to extend vertically away from the underlying layers.

[0026] Figure 4 is another photomicrograph of electrode 300 of Figures 2 and 3 but taken at a higher level of magnification. A mark 400 at the interface of carbon layer 235 and catalyst layer 210 indicates a point of Energy Dispersive Spectroscopy (EDS) analysis, the results of which are depicted in an EDS diagram 405. K and L lines identify carbon, oxygen, iron, copper, and aluminum by their unique energy signatures. These data indicate that the interfacial carbon layer is formed underneath the catalyst layer.

[0027] Figure 5A is a photomicrograph of a cross section of an electrode 500 formed using the process of Figure 1. A recessed area 505 milled by focused ion beam (FIB) clarifies the interface between a copper substrate 510 and a CNT carpet 515. An interfacial carbon layer 520 is especially visible within area 505. A rectangle 525 encompasses an area magnified for presentation in Figure 5B. The rippled texture within area 505, the so-called "curtain effect," is an artifact of FIB milling. The CNTs of carpet 515 are relatively evenly distributed but can be grouped in clumps in other embodiments. When incorporated into an electrochemical cell, a solid-electrolyte interphase forms over the CNTs or clumps of CNTs.

[0028] Turning to Figure 5B, a photomicrograph of electrode 500 of Figure 5 A at a higher level of magnification, carbon layer 520 is shown to be about 0.8 um. Though difficult to see in the photomicrograph of Figures 5 A and 5B, electrode 500 includes a catalyst layer between CNTs 515 and carbon layer 520. Carbon layer 520 provides a strong, electrically and thermally conductive interface that anchors CNT carpet 515 to current collector 510. This example lacks significant voids within carbon layer 520 or at the interface between layer 520 and substrate 510. When electrode 500 is incorporated into an electrochemical cell, the adhesion due to carbon layer 520 resists interfacial delamination and the formation of lithium islands at the bases of the CNTs during cycling, and thus prolongs cell life.

[0029] Figure 6 is a photomicrograph of a cross section of an electrode 600 formed using a process similar to the process of Figure 1 but omitting pre-activation step 119. An interfacial carbon layer 605 bonds a CNT carpet 610 to a copper current collector 615. The absence of pre-activation stunts the growth of carbon layer 605, which is a relatively thin 45 nm as a result.

[0030] Figure 7 is a cross section of an electrochemical cell 700 that incorporates electrode 200 as an anode. Anode 705 (a first electrode) is matched with a sulfur-based cathode 710 (a second electrode). The electrodes are separated by an electrolyte 715 with a separator 717 of e.g. a porous polymer. The elements of anode 705 are as detailed above in connection with Figure 2.

[0031] Cathode 710 includes a current collector 730 of, e.g., aluminum physically and electrically connected to a porous cathode layer 735 that can be saturated with electrolyte 715. An alkali-metal layer 737 (e.g. of lithium) in electrolyte 715 and between electrodes 705 and 710, on either or both sides of the separator 717, is in contact with the external surface of at least one of porous layers 725 and 735 but is initially separated from the internal surfaces (in the pores or interstices) of both porous layers. The lithium metal of layer 737 is ionized and moved between anode 705 and cathode 710 when cell 700 is charged and discharged.

[0032] Cathode layer 735 is a nanoporous carbon-sulfur composite, a mixture of porous carbon and sulfur. The porous carbon collectively forms a matrix that improves thermal and electrical conductivity, traps harmful polysulfides that would otherwise migrate away from the cathode 710 and accommodates expansion and contraction that accompanies the addition and depletion of lithium. Detailed treatments of cathode materials suitable for cathode layer 735 are detailed in PCT application PCT / US24 / 37961 filed 14 July 2024 to Salvatierra et al. entitled "SULFUR-BASED ELECTRODES MECHANICALLY STABILIZED BY LONG CARBON NANOTUBE FILAMENTS" and incorporated herein by reference.

[0033] Lithium layer 737 can be a continuous or perforated lithium foil, the metal of which becomes the active material in cell 700. The mass of layer 737 is selected such that each of anode 705 and cathode 710 has the capacity to store the entire amount, between twenty and forty microns thick in one non-porous embodiment. Because essentially all the lithium isemployed, cell 700 exhibits improved specific capacity and energy density relative to cells with conventional lithium-ion anodes in which the amount of lithium is generally lower than the amount of carbon that stores the lithium ions. Metal layers can be introduced as e.g. perforated sheets, screens, or loose or agglomerated particles, wires, or rods that assemble into a layer during device assembly. A slurry of metal particles and electrolyte can be used in lieu of or with the electrolyte. Other embodiments omit layer 737 in favor of another source of active material. In some embodiments, for example, cathode layer 735 is formed using a lithiated nanoporous carbon-sulfur composite.

[0034] While the invention has been described with reference to specific embodiments thereof, various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. For example, carbon nanomaterials other than vertically aligned CNTs can be grown or attached to a current collector via a carbon sublayer. Moreover, CNT carpets can be used in anodes for electrochemical cells or in a variety of sensors, such as pressure sensors, temperature sensors, and chemical sensors. They can also be used to create nanoscale transistors, which can be used in computer chips and other electronic devices. Due to their high thermal conductivity, CNT carpets make excellent thermal interface materials. CNT carpets can also be effective field emitters because of their sharp tips and high electrical conductivity, and their optical properties make them highly effective at absorbing a wide range of light wavelengths, including visible and infrared light. Other applications will be evident to those of skill in the art. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description. Only those claims specifically reciting “means for” or “step for” should be construed in the manner required under the sixth paragraph of 35 U.S.C. §112.

Claims

CLAIMSWhat is claimed is:

1. An electrode comprising: a current collector of a metal; a carbon layer over and in contact with the metal of the current collector; a catalyst layer over and in contact with the carbon layer; and a layer of carbon nanomaterial extending from the catalyst layer.

2. The electrode of claim 1, wherein the carbon layer has a metal-side surface roughness and a catalyst-side surface roughness less than the metal-side surface roughness.

3. The electrode of claim 2, wherein the metal-side surface roughness is more than twice the catalyst-side surface roughness.

4. The electrode of claim 3, wherein the metal-side surface roughness is greater than 0.2 um and the catalyst-side surface roughness is less than 0.1 um.

5. The electrode of claim 1, wherein the metal consists primarily of copper.

6. The electrode of claim 1, the catalyst layer including catalyst particles disposed between the carbon layer and the carbon nanomaterial.

7. The electrode of claim 6, wherein the catalyst particles comprise iron.

8. The electrode of claim 1, wherein the carbon nanomaterial consists primarily of carbon nanotubes.

9. The electrode of claim 1, wherein the catalyst layer includes a buffer material.

10. The electrode of claim 9, wherein the buffer material comprises aluminum.

11. The electrode of claim 1, wherein the carbon layer comprises voids.

12. The electrode of claim 11, wherein the voids have an average maximum dimension less than one micron.

13. The electrode of claim 1, wherein the carbon layer has an average layer thickness of less than one micron.

14. The electrode of claim 1, wherein the carbon layer consists essentially of carbon.

15. The electrode of claim 1, further comprising lithium metal over and between the carb on nanom ateri al .

16. The electrode of claim 15, wherein the carbon nanomaterial is grouped in clumps, the electrode further comprising a solid-electrolyte interphase over the clumps.

17. The electrode of claim 1, wherein the carbon layer is of a thickness and the carbon nanomaterial extends from the catalyst layer more than ten times the thickness.

18. A method comprising: etching a copper surface; depositing catalyst-support layer over the copper surface; depositing a catalyst on the catalyst-support layer; and simultaneously forming a carbon layer between the catalyst-support layer and the copper surface and growing carbon nanotubes from the catalyst.

19. The method of claim 18, wherein the forming and growing comprises exposing the catalyst to reactant gases, including a carbon source gas.

20. The method of claim 19, wherein the carbon source gas comprises acetylene.

21. The method of claim 19, wherein the reactant gases include hydrogen.

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