Hierarchical Structure for Use as Electrode

Description

Cross-Reference to Related Applications

[01] This patent application claims the benefit of U.S. patent application 62/781,579, filed December 18, 2018, which is incorporated by reference along with all other references cited in this application.

Background of the Invention

[02] This invention relates to the field of metal electrodes, and more specifically to techniques of manufacturing a large-area copper nanofoam with hierarchical structure for use as advanced electrode in energy devices including batteries and energy storage cells.

[03] Techniques for the manufacture of dealloy ed nanoporous copper have not been realized for practical applications. This is because of any results have small size and poor mechanical properties caused by the use of metal“powders” to create a precursor alloy prior to dealloying.

[04] Therefore, there is a need for improved electrode material with carefully designed structure for lithium-ion battery to enhance their capacity and power performance for use in modern mobile and electronic devices.

Brief Summary of the Invention

[05] A facile synthesis is based on a pack-cementation process using copper foil instead of copper powder. Depending on the pack-cementation time and the amount of aluminum, a hierarchical microporous or nanoporous copper is created and can be coated with tin for use as lithium-ion battery anode. The coin-cell test of the nanocopper foam anode exhibited a four-fold higher areal capacity (7.4 milliamp-hours per square centimeter without any performance degradation up to 20 cycles) than traditional graphite anode owing to its considerably higher surface area.

[06] A technique utilizes pack cementation process for the manufacture of precursor alloy in the form of foil and dealloying process for forming nanoscale copper struts and pores throughout the specimen, results in hierarchical microporous or nanoporous or full nanoporous copper (NPC). Additionally, this method can be used to manufacture large-area nanocopper foam on the basis of a new foil-based process with high reproducibility and decent mechanical properties. This method is a much simpler manufacturing process compared to any conventional methods.

[07] As a representative example, the aluminum-copper alloy precursor foil was selected and processed with a aluminum concentration which can vary from about 20 atomic percent to about 85 atomic percent. The aluminum concentration can represent a porosity when it is subsequently etched away. The alloy precursor can be reacted in a dealloying solution (HC1). The ligament size can be modified from about 50 nanometers to about 500 nanometers and the pore size can be controlled from about 10 nanometers to about 10 microns due to different corrosion behavior for different the aluminum-copper phases.

[08] The pack cementation temperature can be varied from about 400 degrees Celsius to about 900 degrees Celsius in order to form the aluminum-copper precursor alloy foil. The dealloying solution can be about a 0.01 molar to about 20 molar hydrochloric acid (HC1) solution at about 20 degrees Celsius to about 100 degrees Celsius. The pack cementation can contain the mixed powder pack of one or more metal powders, filler, and halide salt activator.

[09] The dealloying process can be carried out on the fabricated precursor alloy based on the chemical corrosion potential difference in reference to the standard hydrogen electrode. Therefore, aluminum can be replaced with another element that possesses greater

corrosiveness than copper. For example, the other element can be magnesium (Mg), silicon (Si), chromium (Cr), niobium (Nb), zinc (Zn), titanium (Ti), molybdenum (Mo), tin (Sn), or manganese (Mn), or any combination. The halide salt can be sodium chloride (NaCl), sodium fluoride (NaF), or ammonium chloride (NH4C1), or a combination.

[10] The pack cementation temperature can be set below the melting temperature of metal precursor. The dealloying solution can be one of the following solutions: hydrogen chloride (HC1), sodium hydroxide (NaOH), nitric acid (NaOH), phosphoric acid (H3P04), or perchloric acid (HC104), or a combination.

[11] The manufactured hierarchical microporous or nanoporous or full nanoporous copper (NPC) can be used for various energy devices due to its large surface area and unique three- dimensional structure. As an example, it has been used as lithium-ion battery anode current collector after coated with tin active material, which reacts with and stores lithium ions and well accommodates the volume expansion during charging and discharging cycling processes.

[12] An additional anode active material can be filled in the nanocopper foam anode such as graphite-based material, metal-based material, or oxide-based material. It can also be selected from one of the following: artificial graphite, natural graphite, soft carbon, hard carbon, tin-lithium based alloys, silicon-lithium based alloys, indium-lithium based alloys, antimony-lithium based alloys, germanium-lithium based alloys, bismuth-lithium based alloys, gallium-lithium based alloys, and oxide based materials comprising at least one of tin dioxide (Sn02), cobalt oxide (Co304), copper oxide (CuO), nickel oxide (NiO), or iron oxide (Fe304), or a combination.

[13] Other objects, features, and advantages of the present invention will become apparent upon consideration of the following detailed description and the accompanying drawings, in which like reference designations represent like features throughout the figures.

Brief Description of the Drawings

[14] Figure 1 shows a schematic of the proposed novel synthesis method to create nanoporous or microporous or nanoporous copper using a pack-cementation aluminum coating process on copper foil to create large-area nanocopper foam. More specifically, the area of the final nanocopper foam product can be enlarged relatively easily, depending on the size of the initial copper foil used.

[15] Figures 2A-2F show x-ray diffraction patterns of the p3h and p6h samples after dealloying and their corresponding SEM images on the surface of nanoporous copper foam sample.

[16] Figures 3A-3F show x-ray diffraction patterns of the pl2h and pl5h samples after dealloying and their corresponding SEM images on the surface of the nanoporous copper foam with hierarchical structure.

[17] Figures 4A-4B show cycle performance of the Sn0/Cu ₃ Sn/Cu ₂ 0/Cu lithium-ion battery anode at about 1 milliamp per square centimeter in about 0.01-3.0 volts and comparison of the areal capacity of the Sn0/Cu3Sn/Cu20/Cu anode with those of similar nanoscale anode materials.

[18] Figure 5 shows plots of aluminum composition change and the coating thickness of the aluminum-copper alloy precursor foils with increasing time of the pack cementation process carried out at about 800 degrees Celsius.

[19] Figures 6A-6F show scanning electron microscopt (SEM) cross-sectional images of the copper foil samples during the pack-cementation aluminum coating process (a: pi 5m, b: p30m, c: p3h, d: p6h, e: pl2h, and f: pl5h), which compare the increased thickness of the copper foil specimens. [20] Figures 7A-7D show SEM images of the surface of the pack-cemented specimens (a: pl5m and b: p30m) after dealloying and the corresponding x-ray diffraction patterns. The x- ray diffraction patterns of both the pi 5m and p30m samples indicate the presence of only the copper and CU ₉ A1 ₄ phases.

[21] Figure 8 shows back-scattered SEM image of the pl5h sample showing a continuous network of the solid-solution a-Al (darker area: A) and Al ₂ Cu (brighter area: B). Upon deploying, uniform three dimensionally connected a-Al and Al ₂ Cu phases created micropores and nanopores, respectively.

[22] Figure 9 shows a comparison plot of strut size distribution in the nanoporous and microporous or nanoporous copper samples using a standard metallographic method.

[23] Figure 10 shows schematic showing the dealloying mechanism of the hierarchical microporous or nanoporous copper sample manufactured according to a technique described by this patent.. The solid-solution a-Al phase creates micropores whereas the intermetallic A1 ₂ CU phase creates nanopores upon dealloying.

[24] Figure 11 shows x-ray diffraction patterns of the pl5h microporous or nanoporous copper sample after tin coating using an electroless plating method with a heat treatment at about 150 degrees Celsius for about 1 hour.

Detailed Description of the Invention

[25] Nanoporous metallic structures can provide beneficial properties such as outstanding specific surface area, low density, and efficient catalytic reactions due to their unique three- dimensional structure. Therefore, nanoporous metallic structures have potential for use in energy device applications such as electrocatalysts, actuators, and energy storage (e.g., batteries and secondary batteries). Various syntheses of nanoporous metals may be created using different methods such as chemical dealloying, metallic melt dealloying, and oxygen plasma dealloying. The chemical dealloying method is the most commonly used method for manufacturing nanoporous metals, which are generally composed of pores with a mean diameter of less than 100 nanometers. In this case, chemical dealloying selectively dissolves one or more element (the less noble metal) from an alloy using various acid or base solutions.

[26] Recently, nanoporous copper (NPC) with interconnected nanosized pore structures have been discussed. A few processing methods have been developed to prepare an alloy precursor prior to dealloying such as powder-metallurgy, electrolytic plating, and ingot casting methods. All of these precursor methods have drawbacks to be overcome for use in practical applications due to their complexity and limitation in producing large-sized samples. In particular, the common powder-metallurgy method requires high pressure and temperature to completely consolidate two or more different powders. Consequently, the nanoporous copper produced using this method generally exhibits micro-sized defects, which eventually lead to cracks during the dealloying process (or in the final nanoporous copper).

[27] To improve the mechanical properties and enhance the area of the final nanoporous copper sample, a new facile dealloying technique is described in combination with a pack- cementation aluminum coating process for a copper foil. In other words, a uniform thin layer of aluminum is coated onto the prepared copper foil, using a pack-cementation method to create a copper-aluminum alloy precursor with a subsequent heat-treatment process;

according to the technique, large-area nanocopper foam sample can be produced by preparing large initial copper foil. Depending on the desired final porosity and pore structure, the thickness of aluminum coating layer can be controlled precisely by controlling the pack cementation time and temperature.

[28] By slight manipulation of the phases present in the aluminum-copper alloy precursor, it was possible to create a hierarchically structured microporous or nanoporous copper along with the traditional nanoporous copper. More specifically, the unique hierarchical microporous or nanoporous copper was created by optimizing the composition and heat- treatment process. This was based on the phase diagram between aluminum and copper such that a dual phase can be formed consisting of a solid-solution a-Al and intermetallic Al ₂ Cu.

In this case, the solid-solution phase of a-Al can be preferentially etched away in an acid solution to create micropores between the intermetallic Al ₂ Cu cellular phase, which is later also etched away to create nanopores. It is expected that the hierarchical architecture with a combination of nanopores and micropores can have promising applications for use in various electrocatalysts and other energy-related areas requiring high electrochemical efficiency such as lithium-ion batteries (LIBs) (due to its large surface area).

[29] To demonstrate the promising energy device applications of the microporous or nanoporous copper manufactured in this technique, it was used as an advanced anode of a lithium-ion battery after being coated with tin as an anode active material (through an electroless plating process), completely free of binder and conductive agent. Tin, which can form various alloys with lithium, has recently attracted significant attention as a high- capacity anode active material for advanced lithium-ion battery. This is because it has a higher theoretical capacity (e.g., 991 milliamp-hours per gram) than graphite (e.g.,

372 milliamp-hours per gram). However, an important drawback remains when it is applied to traditional lithium-ion battery anode. Tin suffers from severe volumetric expansion (up to 300 percent) during insertion or extraction of lithium ions during charging or discharging processes that eventually lead to premature cycling failure on copper foil anode design.

[30] This study has demonstrated that hierarchical microporous or nanoporous copper coated with tin could help to resolve the issues caused by use of tin during cycling. The structural characterization was investigated and the lithium-ion battery anode performance of the microporous or nanoporous structured three-dimensional (3-D or 3D) porous copper-tin anode was compared with those of the tin-based anodes with nanostructures. The

microstructure examination was carried out using x-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). In addition, the capacity and cyclic performance of the microporous or nanoporous copper anode coated with tin was analyzed.

[31] Example 1 : Nanoporous Copper Sample Preparation

[32] Aluminum-copper alloy precursors were synthesized using a pack cementation method prior to dealloying. Figure 1 shows the schematic of the overall processing route. The first step shows the mixing of powders used in the pack-cementation process. The powders were composed of 3 weight percent NH C1 powder (100 micron, Alfa Aesar, USA) as an activator, 15 weight percent pure aluminum powder (99.8 percent, +325 mesh, Alfa Aesar, USA) as a coating metal source, and 82 weight percent AI ₂ O ₃ powder (60 pm, Alfa Aesar, USA) as filler. Mechanical mixing (8000-D Mixer Mill, SPEX Sample Prep, USA) was carried out for 30 min to obtain uniformly mixed powders. After mixing, packing and sealing (figure lb) was carried out using a stainless-steel envelope for the subsequent heat-treatment step; pack cementation (figure lc) was then conducted at a constant 800 degrees Celsius for 15 minutes, 30 minutes, 3 hours, 6 hours, 12 hours, or 15 hours to form different aluminum copper alloy precursor foils for comparison in an air tube furnace. The pack-cemented aluminum copper alloy precursor foils are hereinafter referred to as pl5m, p30m, p3h, p6h, pl2h, and pl5h. At the end of the pack cementation process, the sealed envelope was inserted into water. Subsequently, an additional heat-treatment process (figure Id) was carried out for homogenization at 500 degrees Celsius for 9 hours or at 700 degrees Celsius for 6 hours in a tube furnace in an argon atmosphere. The dealloying process (figure le) of the aluminum copper alloy precursors was then carried out in a 3 weight percent HC1 solution at 50 degrees Celsius to etch away aluminum atoms from the aluminum copper alloy. The final nanoporous copper was rinsed with dehydrated alcohol. The microstructures of the pack-cemented aluminum-copper alloy precursor foils and the dealloyed nanoporous copper samples were characterized and analyzed using SEM (JSM7401F, JEOL) combined with an EDS analyzer. XRD (Rigaku Ultima III Xray diffractometer) with copper K-alpha radiation (wavelength of 1.5406 Angstroms) was also used to determine alloy phases between aluminum and copper.

[33] Example 2: Electroless Tin Coating Process

[34] To demonstrate the performance of the synthesized nanoporous copper as a lithium- ion battery anode, a high-capacity anode active material (tin) was coated onto the nanoporous copper through electroless plating. Nanoporous copper was immersed in a tin plating solution at 60 degrees Celsius for 1 minute. The tin plating solution consisted of 200 milliliters deionized water containing 2 grams of tin (II) chloride dehydrate (SnCl ₂ 2H ₂ 0), 2 grams of sodium phosphate monohydrate (NaH ₂ P0 ₂ 2H ₂ 0), 10.5 grams of thiourea (CS(NH ₂ )2), and 0.84 milliliters of concentrated hydrochloric acid. Subsequently, the tin-coated nanoporous copper anode samples were heat-treated at 150 degrees Celsius for 1 hour in a tube furnace in an argon atmosphere.

[35] Example 3: Lithium-Ion Battery Coin-Cell Cycling Test

[36] A copper disk was prepared with dimensions of 11 millimeters in diameter and 250 microns in thickness. A CR2032-type coin-cell was assembled in a glove box in a dry argon atmosphere using the tin-coated nanoporous copper anode coupon as the working electrode and a lithium metal foil for both the counter and reference electrodes. The electrolyte was a traditional 1.3 molar LiPF ₆ solution of ethylene carbonate (EC) and di ethylene carbonate (DEC) in a 3:7 volume ratio. Galvanostatic tests were carried out on the assembled coin cells containing the tin-coated nanoporous copper anode coupon at a current density of 1 milliamps per square centimeter in the voltage range of 3.0 volts to 0.01 volts (versus Li-ion/Li) at 25 degrees Celsius.

[37] Results of the Examples: Processing of Aluminum-Copper Precursor Based on

Pack Cementation

[38] The pack cementation time was varied from 15 minutes to 15 hours to yield different aluminum-copper alloy precursor foils with compositions ranging from 9.7 atomic percent aluminum to 79.6 atomic percent aluminum. The thickness of the foil specimen increased from 255.0 plus or minus 0.5 to 1139.6 plus or minus 26.6 microns with increasing pack cementation time up to 15 hours (table SI, figures 5 and 6A-6F). [39] Table SI provides a list of aluminum-copper alloy precursor samples with varied pack cementation time from 15 minutes to 15 hours. Based on the weight measurements before and after the pack cementation process, the relative aluminum and copper compositions were estimated as atomic percentages. The major phases present after the homogenization process were identified by x-ray diffraction.

[40] Table SI

[41] Three dimensionally connected nanoporous copper can be obtained through dealloying only when the composition of aluminum is sufficiently high. The pl5m and p30m samples contain only the copper and CU9AI4 phases. They cannot react with the dealloying solution through the interior of the specimen due to the low content of aluminum (figures 7A-7D). The type of dealloying solution (e.g., hydrochloric acid orsodium hydroxide) also affects the corrosion behavior of each constituent phase present in the alloy precursor. For example, the aluminum copper phase possesses good stability in hydrochloric acid (HC1) solution, whereas aluminum and Al ₂ Cu phases do not. Therefore, the Al ₂ Cu and a-Al phases become unstable and tend to produce nanoporous copper only when using a hydrochloric acid solution.

[42] Figures 2A-2F show the x-ray diffraction (XRD) patterns of the p3h and p6h samples with all the peaks corresponding to only the Al ₂ Cu phase, which can create uniform three- dimensional nanopores upon dealloying. As a result, both the p3h (figures 2C-2D) and p6h (figures 2E-2F) samples were able to dissolve the entire aluminum in the hydrochloric acid solution and form pure nanoporous copper ligaments throughout the whole thickness of the sample. The mean ligament size of the p3h (figures 2C-2D) and p6h (figures 2E-2F) samples was estimated to be 150 plus or minus 64 nanometers and 125 plus or minus 26 nanometers, respectively (after dealloying). The ligament size of the nanoporous copper after dealloying decreased with increasing aluminum content due to the formation of the finer Al ₂ Cu phase.

[43] Figures 3A-3F also show x-ray diffraction patterns of pl2h (figures 3C-3D) and pl5h (figures 3E-3F) specimens containing only the phases of a solid-solution a-Al and Al ₂ Cu. It is interesting to note that both the pl2h and pl5h samples are composed of hierarchically structured micropores or nanopores throughout the whole specimen. The mean micropore diameter of the pl2h and pl5h samples is calculated to be 5.5 plus or minus 2.2 microns and 6.5 plus or minus 2.4 microns, respectively. The mean micropore diameter of the pl5h sample is 18 percent larger than that of the pl2h sample. This is because the amount of solid- solution a-Al is greater in the pl5h sample, which creates larger micropores between the copper struts.

[44] Along with the micropores, each copper strut consists of nanoscale pores between randomly structured nano-sized ligaments. The mean thickness of the nano-sized ligament is estimated to be 209.8 plus or minus 100.7 nanometers and 98.7 plus or minus 46.7 nanometers for the pl2h and pl5h samples, respectively (figures 9). Furthermore, a back- scatted SEM image of the pl5h sample clearly shows a uniform, continuous network of the a-Al (darker area) and the Al ₂ Cu (brighter area) phases (figures 8). During the dealloying process, the a-Al phase dissolves first to create micropores. The Al ₂ Cu phase dissolves later to create nanopores in each of the microscale Al ₂ Cu struts, as schematically shown in figure 10.

[45] Lithium-Ion Battery Cycling Performance of the Hierarchical Porous Copper

as the Anode

[46] For potential use as an advanced lithium-ion battery anode, the hierarchical microporous or nanoporous copper sample (pl5h) was applied as both the anode current collector and the porous substrate for tin coating. The integrated, hierarchical electrode structure with the well-developed porosity can be a rational design to alleviate the large volume change of tin during repeated charging or discharging processes.

[47] Figure 11 shows x-ray diffraction patterns of the tin-coated microporous or nanoporous copper with an additional heat-treatment at 150 degrees Celsius for 1 hour. The tin-coated copper foam anode after heat treatment reveals diffraction peaks of the

Sn0/Cu ₃ Sn/Cu ₂ 0/Cu phases. The Cu ₂ 0 phase generally forms at temperatures between 70 degrees Celsius and 130 degrees Celsius. Additionally, the intermetallic compound phase of CU3S11 is known as being inactive to the reaction with lithium ions and the uniform presence of such inactive material (as in a composite material) can favorably buffer the volume change during charge or discharging processes.

[48] Figure 4A shows the cyclic performance of the tin-coated microporous or nanoporous copper anode at 1 milliamps per square centimeter in range from 0.01-3 volts. The first discharge capacity of this sample is up to 10.9 milliamp-hours per square centimeter. The second and third discharge capacities are 8.3 and 8.0 milliamp-hours per square centimeter, respectively. The slightly degraded capacity of the initial cycles is due to the irreversible reactions associated with the SnO and CU2O oxide active material. Additionally, the discharge capacity was stabilized to be 7.4 milliamp-hours per square centimeter on the twentieth cycle without any performance degradation being observed. In contrast, previous reports revealed severe capacity degradation and the premature failure of the tin-based anode during initial cycling.

[49] Figure 4B shows the performance comparison between the tin-coated microporous or nanoporous copper anode and similar nanoscale anode materials such as nanoparticles, nanosheets, and nanowires that have been developed in recent years. The tin-coated microporous or nanoporous copper anode (manufactured using a new copper foil-based pack cementation process in this approach) shows superior areal capacity performance compared with other similar nanostructured anode materials under similar current density conditions. For example, it shows approximately four-fold higher areal capacity than the traditional graphite anode.

[50] This patent shows that the tin-coated hierarchical microporous or nanoporous copper anode can achieve stable and improved capacity by effectively reducing the stress caused by the large volume expansion in tin active material during the charging/discharging processes. More specifically, the microporous or nanoporous copper or tin anode, which was integrated as both the current collector and the active anode material without the addition of binder and conductive agent, delivered the remarkable reversible capacity of 7.4 milliamp-hours per square centimeter after 20 cycles. The practical use of the hierarchical microporous or nanoporous copper is not limited to the lithium-ion battery anode application; however, it should also be applicable to other energy areas that can utilize its extremely large surface area and the unique pore structure.

[51] Figure 5 shows plots of aluminum composition change and the coating thickness of the aluminum-copper alloy precursor foils with increasing time of the pack cementation process carried out at 800 degrees Celsius. [52] Figures 6A-6F show SEM cross-sectional images of the copper foil samples during the pack-cementation aluminum coating process (a: pl5m, b: p30m, c: p3h, d: p6h, e: pl2h, and f: pl5h), which compare the increased thickness of the copper foil specimens.

[53] Figure 7 shows SEM images of the surface of the pack-cemented specimens (a: pi 5m and b: p30m) after dealloying and the corresponding x-ray diffraction patterns. The x-ray diffraction patterns of both the pi 5m and p30m samples indicate the presence of only the copper and CU ₉ AI ₄ phases. Note that the presence of only the two phases cannot make dealloying complete through the entire interior of the sample due to its low aluminum content. However, only the outer surfaces of the samples were dealloyed which created some irregular surface pores (as shown in the SEM images).

[54] Figure 8 shows back-scattered SEM image of the pl5h sample showing a continuous network of the solid-solution a-Al (darker area: A) and Al ₂ Cu (brighter area: B). Upon deploying, uniform three dimensionally connected a-Al and Al ₂ Cu phases created micropores and nanopores, respectively.

[55] Figure 9 shows a comparison plot of strut size distribution in the nanoporous and microporous or nanoporous copper samples using a standard metallographic method.

[56] Figure 10 shows schematic showing the dealloying mechanism of the hierarchical microporous or nanoporous copper sample manufactured as described. The solid-solution a- A1 phase creates micropores whereas the intermetallic Al ₂ Cu phase creates nanopores upon dealloying.

[57] Figure 11 shows x-ray diffraction patterns of the pl5h microporous or nanoporous copper sample after tin coating using an electroless plating method with a heat-treatment at 150 degrees Celsius for 1 hour.

[58] Conclusion

[59] A facile dealloying method was successfully developed in combination with a pack- cementation aluminum coating processing for a copper foil to prepare copper-aluminum alloy precursors with about 10-80 atomic percent aluminum. A pack-cementation time of 15 hours at 800 degrees Celsius resulted in a dual-phase of solid-solution a-Al and an intermetallic Al ₂ Cu phase, which could create a hierarchically structured microporous or nanoporous copper upon dealloying. In this case, the solid-solution a-Al phase could be preferentially etched away in an acid solution leaving micropores behind. Subsequently, the Al ₂ Cu phase could create nanopores in the Al ₂ Cu microscale struts. [60] To demonstrate the promising energy applications of the microporous or nanoporous copper, it was applied as an advanced anode of lithium-ion battery after coating it with metallic tin as an anode active material (through an electroless plating process). This resulted in a combination of Sn0/Cu ₃ Sn/Cu ₂ 0/Cu phases. In addition, the tin-coated hierarchical microporous or nanoporous copper anode exhibited a four-fold higher capacity as compared to traditional graphite. This was achieved with successful adoption of tin active material coating by effectively reducing the stress caused by the large volume expansion during the charging or discharging processes.

[61] This description of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form described, and many modifications and variations are possible in light of the teaching above. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications. This description will enable others skilled in the art to best utilize and practice the invention in various embodiments and with various modifications as are suited to a particular use. The scope of the invention is defined by the following claims.