Artificial Solid-Electrolyte Interphase Layer Material and Uses Thereof

Abstract

Li or Na based battery having an anode (or current collector) at least partially covered on its side facing the electrolyte by at least one artificial solid-electrolyte interphase layer with at least one layer of porous graphene of a thickness of less than 25 nm with pores having an average characteristic width in the range of 1-1000 nm.

Claims

1. A Li or Na based battery comprising an anode at least partially covered on its side facing the electrolyte by at least one artificial solid-electrolyte interphase layer with at least one layer of porous graphene of a thickness of less than 25 nm with pores having an average characteristic width in the range of 1-1000 nm.

2. The battery according to claim 1, wherein the artificial solid-electrolyte interphase layer has a thickness in the range of 1-15 nm, and/or wherein the porous graphene layer has an areal porosity in the range of at least 10%, and/or wherein said porous graphene has pores having an average characteristic width in the range of 5-900 nm, and/or wherein the battery is a solid-state battery and the electrolyte is a solid electrolyte.

3. The battery according to claim 1, wherein the artificial solid-electrolyte interphase layer comprises or consists of said at least one porous graphene layer and at least one additional selective graphene layer.

4. The battery according to claim 3, wherein the selective graphene layer is a defective graphene layer.

5. The battery according to claim 1, wherein the anode is an elemental metal layer.

6. The battery according to claim 1, wherein the at least one layer of porous graphene is a layer grown directly on an elemental metal layer forming the anode.

7. The battery according to claim 1, wherein said at least one layer of porous graphene is at least partially N-doped.

8. A layer of porous graphene as an artificial solid-electrolyte interphase layer for a battery, the layer of porous graphene comprising a thickness of less than 25 nm with pores having an average characteristic width in the range of 1-1000 nm.

9. The layer of porous graphene according to claim 8, wherein the layer of porous graphene has a thickness in the range of 1-15 nm, and/or wherein the porous graphene layer has an areal porosity in the range of at least 10%, and/or wherein said porous graphene has pores having an average characteristic width in the range of 5-900 nm.

10. A method for making a battery according to claim 1, wherein a catalytically active substrate is provided to catalyse the graphene formation under chemical vapour deposition conditions, said catalytically active substrate on its surface being provided with a plurality of catalytically inactive domains having a nanostructure essentially corresponding to the shape of the pores in the resultant porous graphene layer; chemical vapour deposition using a carbon source in the gas phase and formation of the porous graphene layer on the surface of the catalytically active substrate, the pores in the porous graphene layer being formed in situ due to the presence of the catalytically inactive domains, and wherein the catalytically active substrate with said porous graphene layer is used as an anode with an artificial solid-electrolyte interphase layer in the form of said porous graphene layer.

11. The method according to claim 10, wherein, before use of the catalytically active substrate with said porous graphene layer as the anode of the solid-state battery, said porous graphene layer is N-doped by subjecting the graphene layer to treatment with non-inert nitrogen-containing gas.

12. The method according to claim 10, wherein, before use of the catalytically active substrate with said porous graphene layer as the anode of the battery, on top of said porous graphene layer an additional selective, non-porous graphene layer is deposited.

13. The method according to claim 10, wherein the catalytically active substrate has a nickel content in the range of 0.06-1% by weight or 0.08-0.8% by weight complemented to 100% by weight by the copper content, and/or wherein the catalytically active substrate is prepared by applying a nickel film of a thickness in the range of 10 nm to 2.2 ?m on a pure copper foil, and by annealing.

14. The method according to claim 10, wherein the catalytically active substrate is provided on its surface with a plurality of catalytically inactive domains by applying, essentially contiguous tungsten layer, and by subsequently annealing at a pressure below normal pressure, to convert the tungsten film into a plurality of catalytically inactive domains.

15. The method according to claim 10, wherein the catalytically inactive domains have an average characteristic width in the range of 1-1000 nm.

16. The battery according to claim 1, wherein the artificial solid-electrolyte interphase layer in the form of a porous graphene layer has a thickness in the range of 1-15 nm or in the range of 5-10 nm, and/or wherein the porous graphene layer has an areal porosity in the range of at least 15%, or of at least 20% or at least 25% or at least 30% or at least 40%.

17. The battery according to claim 1, wherein the artificial solid-electrolyte interphase layer comprises or consists of said at least one porous graphene layer and at least one additional selective graphene layer, and wherein said at least one porous graphene layer is facing said anode and the at least one additional selective graphene layer is facing said electrolyte.

18. The battery according to claim 3, wherein the selective graphene layer is a defective graphene layer, having atomic defects, and wherein the selective graphene layer is a non-porous layer, or wherein said selective graphene layer has a thickness in the range of 0.34-5 nm, or in the range of 0.34-1 nm.

19. The battery according to claim 1, wherein the anode is an elemental metal layer, wherein the metal is selected from the group consisting of lithium, copper, nickel, gold, silver, aluminium, or an alloy or layered composite thereof, including where the anode is an elemental metal layer of a nickel copper alloy or a ternary or quaternary alloy of nickel copper and at least one further metal selected from the group consisting of gold, silver and/or aluminium.

20. The battery according to claim 1, wherein the at least one layer of porous graphene is a layer grown directly on an elemental metal layer forming the anode, wherein the metal of said anode is selected from copper or copper nickel alloy or layered structure or an alloy or layered structure based on copper and/or nickel and at least one further metal selected from the group consisting of gold, silver and/or aluminium.

21. The battery according to claim 1, wherein said at least one layer of porous graphene is at least partially N-doped, wherein the N-doping is in the form of at least one surficial N-doping and/or in the form of an N-doping of the pore boundaries.

22. The method according to claim 8, wherein the porous graphene is used as an artificial solid-electrolyte interphase layer for a lithium-based or sodium-based battery, or a solid-state battery.

23. The method according to claim 8, wherein the porous graphene layer has a thickness in the range of 1 5-10 nm, and/or wherein the porous graphene layer has an areal porosity in the range of at least 15%, or of at least 20% or at least 25% or at least 30% or at least 40%.

24. The method according to claim 10, wherein the catalytically active substrate is a copper-nickel alloy substrate with a copper content in the range of 98 to less than 99.96% by weight and a nickel content in the range of more than 0.04 to 2% by weight, the copper and nickel contents complementing to 100% by weight of the catalytically active substrate.

25. The method according to claim 10, wherein, before use of the catalytically active substrate with said porous graphene layer as the anode of the solid-state battery, said porous graphene layer is N-doped by subjecting the graphene layer to treatment with non-inert nitrogen-containing gas, including in the form of ammonia gas.

26. The method according to claim 10, wherein, before use of the catalytically active substrate with said porous graphene layer as the anode of the battery on top of said porous graphene layer an additional selective, non-porous graphene layer is deposited, in the form of a contiguous graphene layer having atomic defects.

27. The method according to claim 10, wherein the catalytically active substrate is prepared by applying, using electrochemical plating, e-beam evaporation, PVD or sputtering, a nickel film of a thickness in the range of 10 nm to 2.2 ?m or in the range of 25-300 or 20-500 nm, or in the range of 50-300 nm on a pure copper foil, including the pure copper foil having a thickness in the range of 0.01-0.10 mm, or in the range of 0.02-0.04 mm, and/or having a purity of more than 99.5%, and by annealing, at a temperature in the range of 800-1200? C., or in the range of 900-1100? C., during a time span of 10 minutes-120 minutes, or during a time span in the range of 30 minutes-90 minutes.

28. The method according to claim 10, wherein the catalytically active substrate is provided on its surface with a plurality of catalytically inactive domains by applying, using sputtering, e-beam evaporation or PVD, an essentially contiguous tungsten layer, with a thickness in the range of more than 1 nm, or more than 3 nm, or more than 5 nm, or in the range of 1-10 nm, or in the range of 5-10 nm, and by subsequently annealing at a pressure below normal pressure, or of less than 100 mTorr, under a reducing atmosphere, including in the presence of an inert gas such as argon or nitrogen gas, combined with hydrogen gas, to convert the tungsten film into a plurality of catalytically inactive domains, wherein the annealing takes place at a temperature in the range of 700-1100? C., or in the range of 750-950? C. or 800-900? C., during a time span in the range of 10-180 minutes, or in the range of 50-100 minutes.

29. The method according to claim 10, wherein the catalytically inactive domains have an average characteristic width in the range of 10-100 nm, or in the range of 10-50 nm, or having an average characteristic width in the range between 5-900 nm, or in the range of 10-200 nm, or in the range of 10-100 nm

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0084] Preferred embodiments of the invention are described in the following with reference to the drawings, which are for the purpose of illustrating the present preferred embodiments of the invention and not for the purpose of limiting the same. In the drawings,

[0085] FIG. 1 shows a schematic representation of a battery according to the invention;

[0086] FIG. 2 shows in a) from top to bottom a schematic representation of a charging process in a battery according to the prior art with formation of dendritic structures; in b) from top to bottom a schematic representation of a charging process in a battery according to the invention with an artificial solid electrolyte interphase layer;

[0087] FIG. 3 shows an SEM image of a porous graphene layer atop a metal alloy, in which after a pre-leaching process, a surface of the metal alloy was exposed through pores in the porous graphene layer;

[0088] FIG. 4 shows in a) a schematic representation of an artificial solid electrolyte interphase layer consisting of a porous graphene layer and a defective graphene layer in a top (top representation) and cut (bottom representation) view, and in b) an SEM image of such a structure, in which a selective graphene layer is covering a porous graphene layer;

[0089] FIG. 5 shows, in each case in a top (top representation) and cut (bottom representation) view, in a) a layer of N-doped highly porous graphene with surficial doping and in b) a layer of N-doped highly porous graphene with N-doping on the pore boundaries;

[0090] FIG. 6 shows in a) a copper metal current collector, in b) a current collector with a copper-base layer and surficial nickel layer, in c) a current collector with a copper/nickel alloy base layer and a surficial layer of gold, silver or aluminium or an alloy thereof, in d) a current collector of copper/nickel alloy and in e) a ternary metal alloy current collector, e.g. based on copper, nickel and gold or silver;

[0091] FIG. 7 schematically shows in a) the charging process for an anode in a battery with a single artificial solid-electrolyte interphase layer from left to right and in b) another charging process for an anode in a battery with an artificial solid-electrolyte layer comprising a porous graphene layer and a selective graphene layer;

[0092] FIG. 8 schematically shows in a) the charging process for an anode in a battery with a single artificial solid-electrolyte interphase layer having an N-doped bottom part from left to right and in b) another charging process for an anode in a battery with a single artificial solid-electrolyte layer on a ternary elemental metal layer;

[0093] FIG. 9 shows in a) a SEM image of free-standing highly porous graphene on SiNx membrane, clearly showing the planar porous structure and in b) an AFM image of highly porous graphene on SiO.sub.2/Si substrate, indicating greater than 10-nm thick film;

[0094] FIG. 10 shows a SEM image of a porous graphene layer transferred on Cu foil, showing bi-continuous graphene and planar porous structure;

[0095] FIGS. 11a-11e show in a) galvanostatic Li plating/stripping voltage profiles for the Li?Cu and Li?highly porous graphene/Cu asymmetric cells at a fixed current density of 0.5 mA/cm.sup.2 and a capacity of 0.5 mAh/cm.sup.2; in b) magnified cycling performance of the Li?Cu and Li?highly porous graphene/Cu asymmetric cells (from 0 to 400 hours); in c) magnified cycling performance of Li?highly porous graphene/Cu asymmetric cell from 300 to 500 hours, in d) from 1000 to 1200 hours, and in e) from 2400 to 2600 hours.

DESCRIPTION OF THE INVENTION

[0096] FIG. 1 shows a schematic representation of a anode-free battery 1 according to the invention. The cathode 2, which for example can be, but not limited to, a lithium sulphide (Li.sub.2S)y, an air, a lithium iron phosphate (LiFePO.sub.4), a lithium nickel cobalt aluminum oxide (LiNiCoAlO.sub.2), a lithium nickel manganese cobalt oxide (LiNi.sub.xMn.sub.yCo.sub.zO.sub.2), is followed by the electrolyte 3. This electrolyte 3 may be a liquid electrolyte comprising or consisting of lithium salt such as, but not limited to, a lithium hexafluorophosphate (LiPF.sub.6) or a lithium tetrafluoroborate (LiBF.sub.4), in an organic solvent, such as, but not limited to, ethylene carbonate, dimethyl carbonate, or diethyl carbonate, or it may be a solid electrolyte, which for example can be a polymer, a oxide-based, or sulphide-based solid electrolyte material. On the bottom there is provided the actual anode or current collector 5 in the form of a metal layer. Between the current collector 5 and the electrolyte layer 3 having either liquid electrolyte with separator or solid-state electrolyte there is provided an artificial solid-electrolyte interphase layer 4 in the form of a graphene layer having the desired properties, in particular the porosity as discussed above. So in this graphene layer 4 there are provided pores 9.

[0097] FIG. 2 shows in a) from top to bottom a schematic representation of a charging process in an anode-free battery according to the prior art with formation of dendritic structures. In this case, the current collector illustrated in the top representation during the charging process will be covered on the upper side facing the electrolyte material by a layer of deposited elemental lithium in case of a lithium battery. As described above, and as illustrated in the bottommost representation, over time this deposition does not take the form of a stratified layer deposition but it forms dendritic structures 7 which can reach considerable height so as to even penetrate separator elements and/or the solid electrolyte to short-circuit the whole battery.

[0098] FIG. 2 shows in b) from top to bottom a schematic representation of a charging process in an anode-free battery according to the invention with an artificial solid-electrolyte interphase layer. In this case on top of the current collector 5 there is provided the porous graphene layer 4 as an artificial solid-electrolyte interphase layer. During the charging process the lithium is selectively deposited in the interface between this artificial solid-electrolyte interphase layer and the current collector 5, so that a stratified elemental lithium deposition in the form of layer 6 takes place. This behaviour could experimentally be verified over a large number of cycles, so using the graphene layers produced as detailed above batteries were assembled using these layers as artificial solid-electrolyte interphase layers and no dendritic structure formation could be observed even after several hundreds of charge and discharge cycles.

[0099] FIG. 3 shows an SEM image of a porous graphene layer 4 atop current collector 5; wherein a CuNi alloy was used as the current collector 5. In the SEM image, after the pre-leaching process. The catalytically inactive W nanostructures material was completely removed, exposing the surface of the CuNi alloy substrate 5. During the charging process, the lithium can penetrate the porous graphene layer 4 through pores 9 and subsequently be deposited in the interface between the porous graphene layer 4 and the current collector 5.

[0100] FIG. 4 shows in a) a schematic representation of an artificial solid-electrolyte interphase layer of a different embodiment consisting of a porous graphene layer 4 and an additional defective graphene layer 8 in a top (top representation) and cut (bottom representation) view. The defective graphene layer 8 is not porous, so it does not have the pores 9 as present in the layer 4, and it is covering in a contiguous matter layer 4. This defective graphene layer 8 prevents a natural solid-electrolyte interphase from forming, so that no lithium salt in an electrolyte will be consumed, maintaining an initial ionic conductivity; however, atomic defects such as point and/or line defects in the defective graphene layer 8 can allow passage of lithium ions through an artificial solid-electrolyte interphase consisting of a porous graphene layer 4 and an additional defective graphene layer 8. Therefore, dense and flat morphology of metallic lithium deposits appear.

[0101] FIG. 4 shows in b) an SEM picture onto such a structure. The SEM image is an example of selective graphene layer 8 atop a porous graphene 4. Most of the pores in the porous graphene 4 are covered by an additional defective graphene layer 8.

[0102] FIG. 5 shows, in each case in a top (top representation) and cut (bottom representation) view, in a) a layer of N-doped highly porous graphene with surficial doping and in b) a layer of N-doped highly porous graphene with N-doping on the pore boundaries. N-doped graphene, in particular Pyridinic N and Pyrrolic N, exhibits larger binding energy with lithium atom than bare graphene, so Li ion tends to be attracted by a N-doped site in graphene. If a bottommost layer of graphene was doped by nitrogen, a lithium ion will be guided toward the surface of the metal alloy by a gradient of lithiophilicity. Furthermore, the edge of pore was doped by nitrogen, then a lithium ion can move along the pathway of N-doped edge. Therefore, the lithium ion can arrive and be deposited on the surface of the metal alloy. This can happen because the metal alloy produced here includes more lithiophilic surface than porous graphene and N-doped porous graphene (as discussed later in the document).

[0103] As detailed above, the anode of a battery according to this invention can take the form of a lithium layer but also of another metal layer (anode free solid-state battery). In particular for the case where the anode metal layer is non-lithium and at the same time is used as the catalytic substrate for the making of the porous graphene layer, several possibilities are given for such a catalytic substrate layer which then also forms the metal anode (or current collector) of the final battery.

[0104] FIG. 6 correspondingly shows in a) a copper metal current collector which can be used as the catalytic substrate for the porous graphene interlayer synthesis process and as the current collector in the anode-free battery, in b) a current collector with a copper-base layer and surficial nickel layer taking both functions, in c) a current collector with a copper/nickel alloy base layer and a surficial layer of gold, silver or aluminum or an alloy thereof, in d) a current collector of copper/nickel alloy and in e) a ternary metal alloy current collector, e.g. based on copper, nickel and gold, silver, or aluminum. Cu is the most widely used current collector at the anode side, but overpotential for lithium nucleation is high, causing dendritic lithium formation. Although nickel is slightly favorable for a lithium ion to nucleate compared with copper, high overpotential still exists. In contrast, gold, silver, or aluminum have a solubility in a lithium metal and show negligible or low overpotential for lithium nucleation. These metals at the surface of the metal alloy produced here provide a more lithiophilic environment, thereby a lithium ion tends to be attracted and uniformly deposited on the surface leading to a dense and flat morphology of metallic Li deposits.

[0105] FIG. 7 schematically shows in a) the charging process for an anode in a battery with a single artificial solid electrolyte interphase layer from left to right. As schematically illustrated, the lithium ions will penetrate through the interphase layer pores in the charging process and will only deposit at the interface between the porous graphene layer 4 and the anode layer 5 forming the elemental lithium layer 6 between those 2 layers.

[0106] FIG. 7 schematically shows in b) another charging process for an anode in a battery with an artificial solid electrolyte layer comprising a porous graphene layer and a selective graphene layer. As illustrated here, one can see that the lithium ions penetrate through defects in the rather thin selective graphene layer and then deposit in the same way as illustrated in figure a) selectively in the interface region between the anode layer 5 and the porous graphene layer 4 to form the elemental lithium layer 6 without dendritic structures;

[0107] FIG. 8 schematically shows in a) the charging process for an anode in a battery with a single artificial solid electrolyte interphase layer having an N-doped bottom part 10 from left to right. As could be verified experimentally, this N-doped bottom part of the layer fosters selective deposition of elemental lithium 6 at the interface between the porous graphene layer 4 and the anode layer 5.

[0108] FIG. 8 schematically shows in b) another charging process for an anode in a battery with a single artificial solid electrolyte layer on a ternary elemental metal layer. Such a ternary metal alloy having low overpotential for a lithium ion to nucleate could allow a lithium ion to smoothly form a dense and flat metallic lithium deposit layer.

Experimental Section

[0109] Aas-Grown Highly Porous Graphene

[0110] 1. Preparation of CuNi Alloy: [0111] a. A metal catalyst (e.g. Copper foil, 0.035 mm, 99.9%, JX Nippon mining & metals) is used; a Ni film with a varied thickness from 10 nm to 2.2 ?m or 50 to 300 nm is deposited on as-received Cu catalyst by E-beam evaporator or sputtering in vacuum (e.g. FHR, Pentaco 100, Ni purity 99.95%, 3?10.sup.?3 mbar); pressure of the sputtering is about 0.006 mbar with 200 sccm of Ar; a bi-layered structure of Ni/Cu catalyst is annealed at e.g. 1000? C. for e.g. 1 hour to convert to a binary metal alloy (CuNi alloy) under low pressure (e.g. 200 mTorr) with e.g. 50 sccm of H.sub.2 in a chemical vapor deposition (CVD) system (e.g. Graphene Square. Inc, TCVD-RF100CA). [0112] b. The concentration of Ni is more than 0.04% to 10% or preferably in the range of more than 0.04 to 2% by weight, or also in the range of 0.1-10% preferably in the range of 0.2-8% or 0.3-5%, typically in the range of 0.4-3%. Particularly preferably, the catalytically active substrate has a nickel content in the range of 0.06-1% by weight or 0.08-0.8% by weight complemented to 100% by weight by the copper content. The balance is Cu (for the broadest range it is thus 99.96-90%, for a typical range it is 99.94 less than 99% or 99.6-97%, the balance does not include very minor impurities which can be present in the starting Cu foil or in the starting Ni, and which in the final substrate make up less than 0.05% or less than 0.02% by weight in total). The range of Ni content depends on the initial Ni thickness. The typical working content of Ni is preferably in the range of 0.5-2%.

[0113] 2. Conversion of W Thin Film into W Nanostructures: [0114] a. A thin film of W (thickness 1-10 nm) is deposited on the CuNi alloy according to the preceding paragraph by sputtering or E-beam evaporator in vacuum (e.g. FHR, Pentaco 100, W purity 99.95%) with e.g. E-beam evaporator or sputtering in vacuum (e.g. 3?10.sup.?3 mbar); the pressure of the sputtering is e.g. 0.003 mbar with e.g. 100 sccm of Ar; the thin film of W is deposited from 1 to 10 nm with e.g. 0.25 kW of DC plasma; a W/CuNi alloy is mounted in the center of a 4-inch quartz tube chamber positioned in the furnace of the CVD system (e.g. Graphene Square. Inc, TCVD-RF100CA); the chamber is evacuated to reach a pressure of e.g. 45 mTorr and then purged with inert gas, e.g. N.sub.2 (e.g. 100 sccm) for e.g. 5 min normally at room temperature; after purging, the chamber is put under vacuum (e.g. 45 mTorr) again and then the pressure is increased e.g. with a gas mixture of Ar and H.sub.2 (800 sccm and 40 sccm, respectively); to convert the W thin film into W nanostructures. The nanostructures are based variously on symmetric W nanoparticles and asymmetric W nanowalls with various degrees of interparticle agglomeration. The W/CuNi alloy is carefully annealed at elevated temperature (e.g. 750-950? C. or 800-900? C.) for an extended period of time, e.g. 1 hour including ramping with the continuous supply of e.g. 800 sccm of Ar and 40 sccm of H.sub.2 under 4 Torr.

[0115] 3. Synthesis of Highly Porous Graphene [0116] a. Once W nanostructures appear in the process according to the preceding paragraph, a hydrocarbon source for example 40 sccm of CH.sub.4 is introduced in the chamber with e.g. 300 sccm of Ar and 40 sccm of H.sub.2 under 4 Torr in the low-pressure CVD system; depending on the desired level of porosity or thickness, a growth duration is carefully controlled from e.g. 5 to 60 min; afterwards, the furnace is programmed to cool to room temperature under flow of Ar and H.sub.2. Under these conditions, a total CVD time of 60 minutes leads to a graphene layer thickness of approximately 10 nm. CVD time of 5 minutes leads to a graphene layer thickness of approximately below 1 nm, but this may also depend on further parameters.

[0117] BSelective Layer on Highly Porous Graphene [0118] 1. Selective graphene layer is defined as top-most graphene layer, covering porous structure in highly porous graphene, yet including point and/or line defects, for example, grain boundaries. [0119] 2. A synthesis of selective layer of graphene on highly porous graphene is performed in the CVD system. A bi-layered W/CuNi alloy is annealed at an elevated temperature with e.g., 800 sccm of Ar and 40 sccm of H.sub.2, in which a thin film of W is converted into W nanostructure according to the preceding paragraph. Once the temperature is reached (750-950? C.) and W nanostructures appear on the surface of the alloy, a hydrocarbon source, for example 40 sccm of CH.sub.4, is introduced in the chamber with e.g. 300 sccm of Ar and 40 sccm of H.sub.2 under 4 Torr in the low-pressure CVD system; unlike the synthesis of highly porous graphene, a growth duration is slightly prolonged, for example, for 60 min to obtain highly porous graphene plus additional 10 min to acquire a selective layer of graphene atop, but this may also depend on other parameters.

[0120] CN-Doped Highly Porous Graphene

[0121] 1. Post-TreatmentHeat Treatment [0122] a. After the synthesis of highly porous graphene, a removal of W nanostructure is required by a pre-leaching process because W nanostructures can etch the graphene during elevated temperature annealing. The pre-leaching process is such that the as-grown sample is dipped in 0.1 M NaOH at 40? C. for 10-20 min to remove W nanostructures. Required duration of the process depends on an initial thickness of W thin film. [0123] b. After rinsing in deionized water and drying with N.sub.2, the pre-leached sample is re-inserted in the CVD system (e.g. Graphene Square. Inc, TCVD-RF100CA); the chamber is evacuated to reach a pressure of e.g. 45 mTorr and then purged with inert gas, e.g. N.sub.2 (e.g. 100 sccm) for e.g. 5 min normally at room temperature; after purging, the chamber is put under vacuum (e.g. 45 mTorr) again and then the pressure is increased with H.sub.2 (e.g. 50 sccm). The sample is carefully annealed at elevated temperature (500-1000? C.) with H.sub.2 (e.g. 10-100 sccm) for an extended period of time, e.g. 1 hour including ramping, in which the slightly oxidized CuNi surface is reduced. [0124] c. Nitrogen-containing gas, e.g. ammonia gas (NH.sub.3, 10-100 sccm), is introduced into the CVD system. A duration of doping step can vary depending on the amount of N-doping in highly porous graphene from 10 to 60 min. Afterwards, the furnace is programmed to cool to room temperature under flow of H.sub.2.

[0125] 2. Post TreatmentPlasma Treatment [0126] a. After the synthesis of highly porous graphene, as-grown highly porous graphene or pre-leached highly porous graphene sample according to the preceding paragraph is placed in a plasma-equipped CVD system. The chamber is evacuated to reach a pressure of e.g. 45 mTorr and then purged with inert gas, e.g. N.sub.2 (e.g. 100 sccm) for e.g. 5 min normally at room temperature; after purging, the chamber is put under vacuum (e.g. 45 mTorr) again and then the pressure is increased with H.sub.2 (e.g. 50 sccm). [0127] b. N-doped highly porous graphene can be prepared as following; 1. As-grown highly porous graphene is directly treated by nitrogen plasma, or 2. As-grown sample is treated by a combination of heat and plasma. [0128] i. The plasma equipped CVD system is employed in which highly porous graphene is placed in an induced coupled plasma (ICP) system; nitrogen-containing gas, for example, N.sub.2 gas or NH.sub.3 gas e.g., 10-100 sccm is introduced with plasma (RF power from 10 to 200 W). A duration of doping step can vary depending on the amount of N-doping in highly porous graphene from 10 to 60 min. [0129] ii. In order to increase the amount of N-doping level, heat and plasma treatment, which can expedite a doping process, are applied, simultaneously. The plasma equipped CVD system is employed in which an induced coupled plasma (ICP) system and a furnace are set side by side. As-grown highly porous graphene is placed in the furnace and annealed at an elevated temperature (300-1000? C.). Once the furnace is heated up, plasma is turned on (RF power from 10 to 200 W). A duration of doping step can vary depending on the amount of N-doping in highly porous graphene from 10 to 60 min. Afterwards, the furnace is programmed to cool to room temperature under flow of H.sub.2.

[0130] 3. In-Situ Treatment [0131] a. In-situ treatment of nitrogen doping is carried out in the CVD system. A bi-layered W/CuNi alloy is annealed at an elevated temperature with e.g., 800 sccm of Ar and 40 sccm of H.sub.2, in which a thin film of W is converted into W nanostructures according to the preceding paragraph. Once the temperature is reached (750-950? C.), methane (e.g., 10-40 sccm) and ammonia (e.g., 5-40 sccm) as carbon and nitrogen source, respectively, are introduced in the chamber with e.g. 300 sccm of Ar and 40 sccm of H.sub.2 under 4 Torr in the low-pressure CVD system; depending on the desired level of porosity, thickness, or N-doping, a growth duration and a flow rate of ammonia gas are carefully controlled from e.g. 10 to 60 min; afterwards, the furnace is programmed to cool to room temperature under flow of Ar and H.sub.2.

[0132] D. Ternary Metal Alloy [0133] 1. Cu catalyst (e.g. Copper foil, 0.035 mm, 99.9%, JX Nippon mining & metals) is used; a Ni film with a varied thickness from 10 nm to 2.2 ?m or 50 to 300 nm is deposited on as-received commercial Cu catalyst by E-beam evaporator or sputtering in vacuum (e.g. FHR, Pentaco 100, Ni purity 99.95%, 3?10.sup.?3 mbar); pressure of the sputtering is about 0.006 mbar with 200 sccm of Ar; the resulting film of Ni is deposited from 10 nm to 2.2 ?m or 50 to 300 nm with DC plasma whose power is 0.25 kW; a Ag (or Ag, or Al) thin film with a varied thickness from 1 to 100 nm or 3.5 to 35 nm is deposited on top of the bi-layer Ni/Cu or in between Ni and Cu; the resulting film of Ag is deposited from 1 to 100 nm or 3.5 to 35 nm with E-beam evaporator (e.g. Ag purity 99.95%, 2?10.sup.?6 mbar); a tri-layered structure of Ag/Ni/Cu catalyst is annealed at e.g. 1000? C. for e.g. 1 hour to convert to a ternary metal alloy (CuNiAg alloy) under low pressure (e.g. 200 mTorr) with e.g. 50 sccm of H.sub.2 in a chemical vapor deposition (CVD) system (e.g. Graphene Square. Inc, TCVD-RF100CA). [0134] 2. A thin film of W (thickness 1-10 nm) is deposited on the CuNiAg alloy according to the preceding paragraph by sputtering or E-b earn evaporator in vacuum (e.g. FHR, Pentaco 100, W purity 99.95%) with e.g. E-beam evaporator or sputtering in vacuum (e.g. 3?10.sup.?3 mbar); the pressure of the sputtering is e.g. 0.003 mbar with e.g. 100 sccm of Ar; the thin film of W is deposited from 1 to 10 nm with e.g. 0.25 kW of DC plasma; a W/CuNiAg alloy is mounted in the center of a 4-inch quartz tube chamber positioned in the furnace of the CVD system (e.g. Graphene Square. Inc, TCVD-RF100CA); the chamber is evacuated to reach a pressure of e.g. 45 mTorr and then purged with inert gas, e.g. N.sub.2 (e.g. 100 sccm) for e.g. 5 min normally at room temperature; after purging, the chamber is put under vacuum (e.g. 45 mTorr) again and then the pressure is increased e.g. with a gas mixture of Ar and H.sub.2 (800 sccm and 40 sccm, respectively); to convert the W thin film into W nanostructures. The nanostructures are based variously on symmetric W nanoparticles and asymmetric W nanowalls with various degrees of interparticle agglomeration. The W/CuNiAg alloy is carefully annealed at elevated temperature (e.g. 750-950? C. or 800-900? C.) for an extended period of time, e.g. 1 hour including ramping with the continuous supply of e.g. 800 sccm of Ar and 40 sccm of H.sub.2 under 4 Torr. [0135] 3. Once W nanostructures appear in the process according to the preceding paragraph, a hydrocarbon source for example 40 sccm of methane is introduced in the chamber with e.g. 300 sccm of Ar and 40 sccm of H.sub.2 under 4 Torr in the low-pressure CVD system; depending on the desired level of porosity or thickness, a growth duration is carefully controlled from e.g. 5 to 60 min; afterwards, the furnace is programmed to cool to room temperature under flow of Ar and H.sub.2. Under these conditions, a total CVD time of 60 minutes leads to a graphene layer thickness of approximately 10 nm. CVD time of 5 minutes leads to a graphene layer thickness of approximately below 1 nm, but this may also depend on further parameters.

FURTHER EXAMPLES

[0136] Catalyst Substrate Preparation:

[0137] A CuNi alloy catalyst substrate was prepared to synthesize highly porous graphene. A thin film of Ni (70 nm thickness) on bare Cu foil (JX Nippon Mining & Metals) without any treatment was deposited by sputtering (FHR, Pentaco 100, Ni purity 99.95%). The deposition of Ni thin film was performed with 0.25 kW of DC power and 200 sccm of Ar under 6?10.sup.?3 mbar for 10 mins. The bi-layered Ni/Cu was then annealed by chemical vapor deposition (CVD) to convert it into a binary CuNi alloy. The annealing process was as follows: (1) The CVD system was ramped up to 1000? C. for 60 mins with 50 sccm of H.sub.2, (2) The temperature was sustained at 1000? C. for 15 min to complete converting the Ni/Cu into the binary metal alloy (CuNi) in the presence of 50 sccm of H.sub.2, and (3) the whole system was cooled down to room temperature at a cooling rate of 50? C./min with the same level of H.sub.2. Subsequently, a thin film of W (6 nm) was deposited on the CuNi alloy by sputtering (FHR, Pentaco 100, W purity 99.95%). The deposition of W thin film was carried out with 0.25 kW of DC power and 100 sccm of Ar under 3?10.sup.?3 mbar for 45 secs. The as-prepared sample including a thin film of W atop CuNi alloy was inserted in the CVD system. The reactor chamber was pumped out until 45 mTorr to remove residual gases. After the pressure arrived at the base pressure, the chamber was purged out with 100 sccm of N.sub.2 for 2 min and vacuumed down to 45 mTorr for 2 min.

[0138] Highly Porous Graphene Synthesis:

[0139] The growth process falls into two parts: (1) the W annealing step and (2) the growth step. In the annealing process, the furnace was ramped up at 750? C. for 50 mins with the continuous supply of 800 sccm of Ar and 40 sccm of H.sub.2 under 4 Torr, followed by an additional 15-min annealing step, resulting in the conversion of W thin film into desired W nanostructures due to solid-state dewetting behavior. In the second phase of the process, the synthesis of highly porous graphene took place. Hydrocarbon precursors, such as CH.sub.4 (40 sccm) were issued into the CVD system for 30 mins, along with 40 sccm of H.sub.2 and 300 sccm of Ar under the same level of the process pressure. Afterward, the furnace was immediately shifted to rapidly cool down the CVD system at a cooling rate of 50? C./min in the presence of 40 sccm of H.sub.2.

[0140] Transfer of Highly Porous Graphene onto Test Substrates:

[0141] For the preparation of highly porous graphene on various substrates (SiNx, SiO.sub.2, and Cu foil), highly porous graphene was transferred onto a substrate of interest with the help of PMMA (950k, AR-P 672.03). A PMMA was spin-coated on the as-synthesized highly porous graphene at 4000 rpm for 60 secs. Afterward, the sample was baked at 110? C. to evaporate the solvent in the PMMA film for 1 min. The sample was then floated onto a solution of ammonium persulfate (0.5 M APS) for 3 hours to remove the metal alloy substrate, followed by a rinsing process with deionized water. The highly porous graphene supported by PMMA film was transferred onto the desired substrate and dried at room temperature. Finally, the PMMA layer was dissolved in acetone for 1 hour and the highly porous graphene on the substrate underwent a heat treatment at 350? C. for 1 hour under H.sub.2 to remove residual PMMA and residual water molecules which can cause parasitic reactions during a battery operation. In the case of highly porous graphene transferred on SiNx membrane, the PMMA film was directly removed by a heat treatment at 400? C. in the presence of 100 sccm of H.sub.2 and 900 sccm of Ar for 2 hours.

[0142] FIG. 9a shows an SEM image of the highly porous graphene transferred on SiNx. After the transfer process and subsequent heat treatment to remove the PMMA film, the planar porous structures in highly porous graphene were well maintained.

[0143] FIG. 9b an AFM image that indicates that the thickness of highly porous graphene synthesized in the abovementioned conditions is around 11 nm on average.

[0144] Battery Cell Characterization:

[0145] Li?highly porous graphene/Cu asymmetric cells (coin cell, diameter 13 mm) were assembled, consisting of lithium metal as a reference and counter electrode and highly porous graphene/Cu foil as a working electrode.

[0146] FIG. 10 is an SEM image of the highly porous graphene/Cu electrode. The electrolyte can vary depending on the battery systems, but for the experiments discussed herein, 1,2-Dimethoxyethane and 1,3-Dioxolane (DME/DOL, v/v: 1/1) solvent with 1M Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt and 2 wt. % LiNO.sub.3 additive was used. A separator (Celgard 2325, 25-?m thick, polypropylene/polyethylene/polypropylene) that was soaked with the liquid electrolyte was employed between the working electrode and the counter electrode. As a reference test, Li?Cu asymmetric cells were prepared with bare Cu foil as a working electrode, instead of highly porous graphene/Cu, and the rest of the conditions were identical. Li plating/stripping experiments were carried out on test cells constructed with either Cu or highly porous graphene/Cu. Li was plated on the corresponding working electrode at a rate of 0.5 mAh/cm.sup.2.

[0147] FIGS. 11a-11e show the long-term cycling properties of Li?Cu asymmetric cells with a capacity of 0.5 mAh/cm.sup.2 and a current density of 0.5 mA/cm.sup.2.

[0148] As shown in FIG. 11a, the cell with the Cu foil exhibited nearly stable overpotential in the first few cycles but after 50 hours, the overpotential of the cell vastly increased with cycling and short-circuited only after 300 hours.

[0149] For the cell with the highly porous graphene/Cu, remarkably low overpotential (9 mV) remained stable over the cycles (compared to 41 mV for the bare Cu cell, shown in FIG. 11b) and this cell achieves significantly better battery performance in terms of cycle lifespan (2600 hours Vs 300 hours for the Li?Cu asymmetric cell).

[0150] The zoomed-in plots (at 300-500, 1000-1200, and 2400-2600 hours, respectively) of Li?Cu cell, implementing the highly porous graphene/Cu electrode, reveal negligibly increased overpotential from 7 mV to 9 mV over 2600 hours (FIG. 11c-e). The long-term stability and low overpotential suggest the highly porous graphene/Cu electrode as a stable platform of Li plating/stripping, indicating the stabilizing effect of the highly porous graphene as an ASEI layer.