Metallic Foam Anode Coated with an Active Oxide Material

Abstract

A three-dimensional metallic foam is fabricated with an active oxide material for use as an anode for lithium batteries. The porous metal foam, which can be fabricated by a freeze-casting process, is used as the anode current collector of the lithium battery. The porous metal foam can be heat-treated to form an active oxide material to form on the surface of the metal foam. The oxide material acts as the three-dimensional active material that reacts with lithium ions during charging and discharging.

Claims

1. A method comprising: placing a mold on a copper rod into liquid nitrogen and pouring a cobalt metal slurry in the mold; freezing the cobalt metal slurry, wherein cobalt metal particles of the slurry are coupled to ice crystals; forming a green-body with directional pores by drying the frozen slurry at a sufficiently low temperature at or below freezing, leaving pores in their places with physical attachment; and constructing a porous metal foam by reducing and sintering the porous green-body at a sufficiently high temperature under an atmosphere comprising hydrogen.

2. The method of claim 1 wherein the reducing and sintering the porous green-body comprises: sintering at about 550 degrees Celsius for about 4 hours; and sintering at about 1000 degrees Celsius for about 9 hours.

3. The method of claim 1 wherein the reducing and sintering the porous green-body comprises: reducing the porous green-body in a hydrogen atmosphere, wherein the cobalt oxide is reduced to cobalt.

4. The method of claim 1 wherein the reducing and sintering the porous green-body comprises: sintering at a first temperature for a T1 time period; and sintering at a second temperature for a T2 time period.

5. The method of claim 4 wherein the second temperature is greater than the first temperature.

6. The method of claim 4 wherein the T2 time period is greater than the T1 time period.

7. The method of claim 6 wherein the T2 time period is greater than the T1 time period.

8. The method of claim 1 wherein the reducing and sintering the porous green-body comprises: reducing the porous green-body in a hydrogen atmosphere; and sintering at about 550 degrees Celsius for about 4 hours.

9. The method of claim 8 comprising sintering at about 1000 degrees Celsius for about 9 hours.

10. The method of claim 1 wherein the reducing and sintering the porous green-body comprises: reducing the porous green-body in a hydrogen atmosphere; and sintering at about 1000 degrees Celsius for about 9 hours.

11. The method of claim 1 wherein the cobalt metal slurry is based on about 30 milliliters of deionized water comprising of about 7 volume percent cobalt oxide powder and about 8 weight-percent polyvinyl alcohol binder.

12. The method of claim 1 comprising: dissolving the cobalt metal slurry using a combination of stirring and sonication.

13. The method of claim 1 comprising: maintaining a temperature of about −10 degrees Celsius by way of liquid nitrogen and a heater coupled to an end of the copper rod.

14. The method of claim 1 comprising: after freezing the cobalt metal slurry, sublimating at −88 degrees Celsius for 24 hours in a freeze dryer in a vacuum.

15. A device comprising a three-dimensional porous cobalt foam made according to the method of claim 1, wherein an initial discharge capacity of the three-dimensional porous cobalt is 8.7 milliamp-hours per square centimeter.

16. A device comprising a three-dimensional porous cobalt foam made according to the method of claim 1, wherein a Coulombic efficiency of the three-dimensional porous cobalt foam maintains about 99.8 percent charge after a thirtieth cycle.

17. A method comprising: placing a mold on a copper rod into liquid nitrogen and pouring an iron metal slurry in the mold; freezing the iron metal slurry, wherein iron metal particles of the slurry are coupled to ice crystals; forming a green-body with directional pores by drying the frozen slurry at a sufficiently low temperature at or below freezing, leaving pores in their places with physical attachment; and constructing a porous metal foam by reducing and sintering the porous green-body at a sufficiently high temperature under an atmosphere comprising hydrogen.

18. The method of claim 17 wherein the reducing and sintering the porous green-body comprises: sintering at about 300 degrees Celsius for about 2 hours; and sintering at about 950 degrees Celsius for about 14 hours.

19. The method of claim 17 wherein the reducing and sintering the porous green-body comprises: reducing the porous green-body in a hydrogen atmosphere, wherein the iron oxide is reduced to iron.

20. The method of claim 17 wherein the reducing and sintering the porous green-body comprises: sintering at a first temperature for a T1 time period; and sintering at a second temperature for a T2 time period.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] FIG. 1A is an illustration of an anode electrode made of three-dimensional metallic foam with an active oxide material.

[0030] FIG. 1B shows a flow diagram of fabricating an electrode made of a metal foam with an active oxide material.

[0031] FIG. 2 is a scanning electron microscope (SEM) micrograph of three-dimensional iron foam with an active iron oxide layer formed on the surface of the iron.

[0032] FIG. 3 is an X-ray diffraction (XRD) spectrum of the three-dimensional iron foam confirming that pure iron oxide layer is formed on the surface of the iron foam.

[0033] FIG. 4 is an SEM micrograph of three-dimensional titanium foam with an active titanium oxide layer formed on the surface of the titanium.

[0034] FIG. 5 is an XRD spectrum of the three-dimensional titanium foam confirming that pure titanium oxide layer is formed on the surface of the titanium foam.

[0035] FIG. 6 shows optical micrographs of mounted and polished top- and cross-sections of three-dimensional cobalt foam.

[0036] FIGS. 7A-7C show SEM micrographs and an energy dispersive X-ray (EDX) spectrum of the three-dimensional cobalt foam with an active cobalt oxide layer formed on the surface.

[0037] FIG. 8 shows an XRD spectrum of the three-dimensional cobalt foam.

[0038] FIG. 9 shows an XRD spectrum of the three-dimensional cobalt foam with an active cobalt oxide layer formed on the surface.

[0039] FIG. 10 shows the cycle performance and Coulombic efficiency of the three-dimensional cobalt foam with an active oxide layer formed on its surface.

DETAILED DESCRIPTION OF THE INVENTION

[0040] A metal foam structure is fabricated for use as the anode of lithium-ion battery. A method includes: fabricating a porous metal foam with pore size ranging from several nanometers to several hundred microns as the current collector; and forming an active oxide material layer through a heat-treatment, which charges and discharges lithium ions.

[0041] FIG. 1A shows an embodiment of the present invention which describes the formation of an active oxide material layer with nanoscale surface roughness on the surface of porous metal foam for use as a current collector of a lithium-ion battery. In a specific implementation, the current collector of the lithium-ion battery is an anode current collector. The current collector is made of multiple porous metal foam plates with an active oxide material layer surrounding the multiple plates. The surface area of the anode current collector is thus increased and can expedite electrochemical reactions at a greater rate as well as store more lithium ions than that of a conventional anode current collector.

[0042] A technique of fabricating a three-dimensional porous anode electrode is described in U.S. patent application 62/194,564, filed Jul. 20, 2015, which is incorporated by reference along with all other references cited in this application.

[0043] Porous metal can be fabricated according to a number of techniques. In an implementation, the porous metal foam has a three-dimensional porous structure fabricated through a freeze-casting method. For example, three-dimensional copper foam is fabricated through a freeze-casting method as an example manufacturing process. U.S. patent application Ser. No. 13/930,887 describes a freeze-casting technique and is incorporated by reference. This process features a simple, low-cost processing method to fabricate porous structures. An active oxide material with nanoscale surface roughness is formed on the surface of the porous metal foam, which can be used as the current collector.

[0044] Specific flow implementations are presented in this patent, but it should be understood that the invention is not limited to the specific flows and steps presented. A flow of the invention may have additional steps (not necessarily described in this application), different steps which replace some of the steps presented, fewer steps or a subset of the steps presented, or steps in a different order than presented, or any combination of these. Further, the steps in other implementations of the invention may not be exactly the same as the steps presented and may be modified or altered as appropriate for a particular application or based on other factors.

[0045] FIG. 1B shows a detailed method 130 of fabricating the porous metal foam and forming the metal oxide layer includes: [0046] (a) Referring to a step 132, immersing a copper rod (with high thermal conductivity) into liquid nitrogen and pouring a metal slurry in a mold with a freeze-cast setup. [0047] (b) Referring to a step 135, freezing the metal slurry, where the metal particles are piled up between the growing ice crystals. [0048] (c) Referring to a step 138, forming a porous structure by drying the ice crystals of the frozen sample at low temperature and reduced pressure, leaving pores in their place. [0049] (d) Referring to a step 141, forming the three-dimensionally connected porous metal foam by sintering the porous structure under hydrogen atmosphere. [0050] (e) Referring to a step 144, cutting the porous metal foam into a thin layer to be applied as the current collector for the anode of a lithium-ion battery. [0051] (f) Referring to a step 147, forming an active oxide material layer on the surface of metal foam for use as the current collector of the anode of lithium-ion battery through heat-treatment. The oxide material that is formed can be Co3O4, CoO, Fe2O3, Fe3O4, CuO, Cu2O, NiO, or TiO2 as the anode active material.

[0052] In the present invention, three example embodiments are provided: cobalt, titanium, and iron foams with oxide layers formed on their surfaces. The choice of the metallic foam, however, is not limited to them, but is open to other metallic materials, such as copper, nickel foams, and others. The metal foam can be used as a three-dimensional current collector with high electrical conductivity and mass transport efficiency.

[0053] Some specific embodiments are presented below. These embodiments are provided only to describe some examples of detailed implementations, and it will be apparent to those skilled in the art to that the scope of the present invention is not limited by the embodiments.

Exemplary Embodiment 1 (Three-Dimensional Porous Iron Oxide or Iron Anode)

[0054] Iron foam is selected as a model material for the confirmation of this implementation and is fabricated by a freeze-casting process. A metal slurry is prepared by mixing iron oxide powder with deionized water and binder. The iron oxide powder is well dispersed in the slurry by a combination of stirring and sonication processes. The slurry is then poured into a fluoropolymer resin or Teflon mold onto the copper rod, which is cooled using liquid nitrogen (N2). The temperature of the top of the copper rod is controlled by a heater and is fixed at −15 degrees Celsius.

[0055] After freezing, the frozen slurry is freeze-dried at −90 degrees Celsius for about two days (e.g., about 48 hours), forming a porous green-body. The green-body is reduced and sintered in a tube furnace in hydrogen (H2)-95 percent argon gas mixture. The reduction is performed step-by-step both at 300 degrees Celsius for 2 hours and at 500 degrees Celsius for 2 hours, and the sintering is performed at 950 degrees Celsius for 14 hours.

[0056] FIG. 2 shows an SEM micrograph of three-dimensional iron foam with an active iron oxide layer formed on the surface of the iron foam. FIG. 3 shows an XRD spectrum of the three-dimensional iron foam confirming that pure iron oxide layer is formed on the surface of the iron foam and has higher peak intensities than that of iron foam without the iron oxide layer.

Exemplary Embodiment 2 (Three-Dimensional Porous Titanium Oxide or Titanium Anode)

[0057] Titanium foam is selected as a model material and is fabricated by a freeze-casting process. Prior to freeze-casting, polyvinyl alcohol (PVA) is dissolved in distilled water, and titanium powder is added to the prepared solution to complete the slurry. The slurry is then poured directly onto the top of a copper chiller rod standing in a stainless steel vessel under liquid nitrogen (N2).

[0058] A frozen green-body is lyophilized to remove ice through sublimation at a subzero temperature (e.g., less than 0 degrees Celsius) for about a day (e.g., about 24 hours). The lyophilized green-body is then sintered in a vacuum furnace via a two-step heat-treatment process: at 300 degrees Celsius for 3 hours and then at 1100 degrees Celsius for 7 hours.

[0059] Finally, for the formation of anatase oxidation layer, titanium foam is presoaked in hydrogen peroxide (H2O2) at 100 degrees Celsius for 3 hours. Titanium foam is then oxidized in an air furnace at 400 degrees Celsius for 6 hours.

[0060] FIG. 4 shows SEM micrograph of three-dimensional titanium foam with an active titanium oxide layer formed on the surface of the titanium foam. FIG. 5 is an XRD spectrum of the three-dimensional titanium foam confirming that pure titanium oxide layer is formed on the surface of the titanium foam and has higher peak intensities than that of titanium foam without the titanium oxide layer.

Exemplary Embodiment 3 (Three-Dimensional Porous Cobalt Oxide or Cobalt Anode)

[0061] Cobalt powder slurry based on 30 milliliters of deionized water consists of 7 volume percent cobalt oxide powder and 8 weight percent PVA binder. The slurry is dissolved by using a combination of stirring and sonication to improve the degree of dispersion. The slurry is then poured into a fluoropolymer resin or Teflon mold onto a copper rod. The temperature of the top of the copper rod is fixed at −10 degrees Celsius by liquid nitrogen and a heater.

[0062] After the slurry is completely frozen, the frozen sample is sublimated at −88 degrees Celsius for 24 hours in a freeze dryer in vacuum, resulting in the removal of ice crystals, forming a green-body with directional pores. The green body is then reduced from cobalt oxide to cobalt in hydrogen atmosphere and then sintered. The reduction and sintering processes consist of presintering at 550 degrees Celsius for 4 hours and actual sintering at 1000 degrees Celsius for 9 hours in a tube furnace under a 5 percent hydrogen mixture gas.

[0063] FIG. 6 shows optical micrographs of mounted and polished top- and cross-sections of three-dimensional cobalt foam (radial and longitudinal views). More specifically, FIG. 6 shows that the cobalt foam is indeed a three-dimensional architecture with regularly distributed lamellar structured pores on the orders of several tens of microns. FIGS. 7A-7C also show that a cobalt oxide layer with nanoscale surface roughness is formed on the surface of the cobalt foam as confirmed by both SEM images and energy dispersive X-ray mapping.

[0064] FIG. 8 is an XRD spectrum of the three-dimensional cobalt foam. FIG. 9, shows an XRD pattern for the three-dimensional porous cobalt foam heat-treated at 600 degrees Celsius and verifies the formation of both Co3O4 and CoO phases on the surface of cobalt foam and that both cobalt oxides have higher peak intensities than that of pure cobalt foam as shown in FIGS. 7A-7C. In particular, cobalt demonstrates that the anode system, which consists of a cobalt foam current collector and a cobalt oxide active material, can show superior coin-cell performance.

[0065] FIG. 10 shows the cyclic performance of the three-dimensional porous cobalt foam heat-treated at 600 degrees Celsius, confirming that the three-dimensional porous cobalt foam fabricated in this invention is indeed applicable for use as the anode of a lithium-ion battery. The initial discharge capacity of the three-dimensional porous cobalt foam heat-treated at 600 degrees Celsius is 8.7 milliamp-hours per square centimeter, which is higher than the conventional anode. Furthermore, the Coulombic efficiency of the three-dimensional porous cobalt foam heat-treated at 600 degrees Celsius is also high, maintaining near 99.8 percent charge after the 30th cycle.

[0066] 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.