Electrohydrodynamically formed structures of carbonaceous material

A method for the electrohydrodynamic deposition of carbonaceous materials utilizing an electrohydrodynamic cell comprising two electrodes comprised of a conductive material, by first combining a solid phase comprising a carbonaceous material and a suspension medium, placing the suspension between the electrodes, applying an electric field in a first direction, varying the intensity of the electric field sufficiently to drive lateral movement, increasing the electrical field to stop the lateral transport and fix the layers in place, then removing the applied field and removing the electrodes. Among the many different possibilities contemplated, the method may advantageously utilize: varying the spacing between the electrodes; removing the buildup from one or both electrodes; placing the electrodes into different suspensions; adjusting the concentration, pH, or temperature of the suspension(s); and varying the direction, intensity or duration of the electric fields.

BACKGROUND OF THE INVENTION

The present invention relates generally to carbon structures and specifically to electrohydrodynamically formed structures of carbonaceous material. Coatings, such as physically coherent films, coatings, membranes, or tapes made from high carbon content materials, such as graphene sheets, can be assembled using electrophoretic deposition, tape casting, spin casting, drop casting, or filtration. Cast or filtered structures typically have to be at least 400 nm thick to provide continuity and mechanical stability. Such structures contain flaws created by removing the liquid through drying or filtration. In addition, such structures can have a reduced flexibility and compliance, which can result in an increase in susceptibility to damage during transfer and/or fitting to the item to be covered. Similar to electrohydrodynamic deposition, electrophoretic deposition uses an applied electric field to attract particles or sheets to a surface having an overall charge opposite to the charge intrinsic to or induced on the particles or sheets, thereby coating the surface, as described in U.S. Pat. No. 2,894,888 to Shyne, et al., and U.S. Pat. No. 3,932,231 to Hara, et al., and many others. However, in electrophoretic deposition the particles or sheets adhere at the point of initial contact to the substrate or previously deposited layers and remain fixed in position, which leaves defects or gaps between the particles or sheets comprising the layers that constitute the coating, membrane, or film. A fully dense covering requires several layers, resulting in increased thickness of the coating, membrane, or film which limits its applications.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to a method for depositing carbonaceous materials utilizing an electrohydrodynamic cell. In the present invention methods are provided in which carbonaceous materials in a suspension medium are deposited on electrodes by applying an electric field having a first direction, varying the intensity to drive lateral movement, building up layers, fixing the layers in place, then reducing the field and removing the electrodes. The method of claim1, wherein at least one of the electrodes further comprises a substrate in contact with or electrical communication with the outer surface of the conductive material.

Among the many different possibilities contemplated, the method may advantageously utilize electrodes positioned either parallel or perpendicular to a base surface of the electrohydrodynamic cell. It is further contemplated that the method may also vary the spacing between the electrodes, the temperature, pH, or concentration of the suspension, or the duration or intensity of the applied field. The method may be utilized to create graded structures, or to deposit at least one carbonaceous material and at least one other material. It is further contemplated that the method may also involve reversing the field direction to allow deposition of materials having an overall charge opposite that of the carbonaceous materials, and this process of reversing the field may be repeated. It is also contemplated that one or more of the coated electrodes may be removed from the electrohydrodynamic cell and placed into a different electrohydrodynamic cell containing a suspension comprising a different carbonaceous material, and utilizing a similar process, which may be repeated if desired. It is contemplated that the method may also utilize a suspension of a non-carbonaceous material that can be deposited onto previously deposited coatings via electrophoretic or electrohydrodynamic deposition. It is also contemplated that at least one of the coated electrodes is covered by a material prior to electrohydrodynamic deposition, the material being selected so as to be impermeable to atoms, molecules, ions, oligomers, and polymers; or to have an intrinsic porosity in which the average channel diameter, accessibility, tortuosity, and length is selected to facilitate the passage of targeted agents such as atoms, molecules, ions, oligomers, and polymers. It is still further contemplated that the method may involve removing the layered structure from at least one of the electrodes, and that the electrodes may be dense or have an intrinsic porosity such that it may be completely or partially filled by the suspended materials in order to create a dense or a porous coating on the at least one of the anode or cathode. It is contemplated that the removed structure may be dried or exposed to EM radiation, and may also involve the reduction of the carbonaceous materials or intercalation of the layered structure.

DETAILED DESCRIPTION

As used herein, the term “carbonaceous materials” includes any solid material, other than an inorganic carbonate, which is comprised of carbon, including mixtures or compounds comprising carbon. This includes, but is not limited to: graphite, graphite oxide, pristine graphene, graphene oxide, functionalized graphene sheets—whether those sheets are composed of single sheets, multilayered stacks of single sheets, or some other multi-sheet arrangement—graphitic materials, and mixtures of the same.

Coatings, such as physically coherent films, coatings, membranes, or tapes made from high carbon content materials, such as graphene sheets, can be assembled using electrophoretic deposition, drop casting, spin casting, tape casting, or filtration. Such structures typically have to be at least 400 nm thick to provide continuity and mechanical stability. Such structures contain flaws created by removing the liquid through drying or filtration. In addition, such structures can have a reduced flexibility and/or compliance, which can result in an increase in susceptibility to damage during transfer and/or fitting to the item to be covered. Electrophoretic deposition (“EPD”) as described in U.S. Pat. No. 2,894,888 to Shyne, et al., and U.S. Pat. No. 3,932,231 to Elara, et al., and many others, brings particles or sheets to the surface under an applied field but these adhere to the surface upon contact and cannot move from the original point of contact. Embodiments of the present invention seek to provide coatings that are formed using electrohydrodynamic deposition (“EHD”), in which mutually repulsive sheets or particles rearrange after deposition to eliminate defects and from which liquid is excluded during the layering process (FIGS. 1A and 1B). Other aspects of the present invention seek to provide a method of forming electrohydrodynamically deposited coatings.

FIGS. 1A, B, and C depict an EHD method, in accordance with an embodiment of the present invention.FIG. 1Aillustrates a horizontal EHD method, which includes electrodes100and105suspended in suspension125. For a suspension of particles or sheets bearing an overall negative charge and under a field applied in the direction (150) shown, electrodes100and105represent a cathode and anode, respectively. The standard definition for the direction of an electric field is that direction in which a positively charged object will move under the influence of the field. Thus, negatively charged materials will deposit on the anode and positively charged materials will deposit on the cathode. The direction of field can be reversed by reversing the charges on the electrodes100and105, for example, or by applying an alternating current (“AC”) instead of the direct current (“DC”) shown inFIG. 1A. Despite the charged states of the electrodes, substrate110can be positioned in contact with or electrical communication with electrode100. When present, substrate110can be an electrically conductive (such as a metal), semiconducting (such as silicon), or insulating material (such as a metal oxide ceramic); in addition, substrate110may be fully dense or porous (further description below). Substrate110can include flexible or rigid films, composed of hydrocarbon polymers. To initiate the EHD process of the current invention (“the EHD process”), an electrical field is applied between electrodes100and105to initiate the formation of coating115. In the case of electrically insulating materials, the applied voltage is set below the breakdown voltage of the material.

In certain embodiments, substrate110is not present and coating115is formed directly on electrode100. Substrate110can be a thin porous insulating membrane, wherein the formation of coating115thereon creates a carbonaceous film on a porous insulating film. Where the carbonaceous material is graphene oxide, substrate110can serve as a compliant, insulating substrate for coating115. Where the carbonaceous material is a conductive material, for example, reduced graphene oxide or graphene, the process creates an electronically asymmetric composite consisting of an insulator on one side and a conductor on the other.

Coating115can comprise one or more layers of individual sheets of carbonaceous material. The carbonaceous material can have a sheet- or plate-like structure. Applicable carbonaceous materials include, but are not limited to, graphene sheets, graphite, graphite oxide, and carbon precursors formed through vapor deposition. Suspension125comprises carbonaceous material suspended in a liquid. To facilitate coatings preparation, suspension125can be prepared using any suitable method for agitating, including but not limited to, ultrasonication, stirring, milling, and shear mixing, with or without additional heating and/or cooling.

Variations of the liquid's pH can facilitate coating formation depending on the nature of the carbonaceous material used.

FIG. 1Bis a magnification of a region (140) inFIG. 1Ato illustrate how the EHD process is initiated by the attraction of particles and/or molecular sheets of carbonaceous material toward the deposition surface when the particles are distant from the substrate surface, converting to EHD in close proximity of substrate110(when present) or electrode100, where the particles or sheets are repelled by the surface and/or each other. In the EHD process, the well-dispersed particles or sheets in suspension125are mutually repulsive, thereby remaining dispersed in the liquid as a stable suspension and deposit on the substrate110only under the application of a field of sufficient intensity. The initial deposition has the particles or sheets maintaining separation, but under the influence of electrohydrodynamic forces, as described in U.S. Pat. No. 6,033,547 to Trau et al. and U.S. Pat. No. 5,855,753 again to Trau et. al., the particles or sheets are forced closer together via lateral transport130on the surface (FIG. 1B). The result is a closely packed layer of particles or sheets onto which a subsequent layer is deposited through the same EHD process (FIG. 1C). EHD is essential to the success of the present invention as it is desirable to reduce and/or inhibit the immediate adherence of the carbonaceous material to itself and/or to the substrate surface but instead it is desirable to promote the lateral movement130of the carbonaceous material in near proximity of the substrate (110) to produce fully dense, well stacked, and staggered layers in which potential gaps and defects have been filled through transport130(FIG. 1A). Stiction, or adherence between layers, can then be induced by overcoming the repulsion, for instance, with an interim potential pulse of sufficient intensity to adhere one layer to another. Whereas in the typical EPD process the particles or sheets stick to the surface upon first contact and cannot move following this initial contact.

The EHD process is a rapid process that can be used either in batch or continuous operation. In some applications, coating115can be deposited directly to the surface of a device or structure (such as a surface). In others, coating115can be formed separately and then applied and/or fitted to the device or structure. The structure may be electronically conductive, semiconducting, or insulating, providing a generic method for coating any surface of interest. Coating115can be a flexible, thin, and free-standing membrane that can be applied to surfaces with complex shapes or patterning.

Coating115can have a thickness of 100 nm or less.FIG. 1Billustrates that coating115can be formed into multi-layered structures having a highly connected and/or contiguous microstructure with well-defined spacing between the layers. In this embodiment, coating115can be formed by applying the electric field for about 3 seconds to about 30 minutes. Electrode100or substrate110may include, but are not limited to, any metal suitable for use as a current collector in batteries, for example lithium ion batteries, such as gold, nickel, and/or stainless steel.

Coating115can be thermally stable, electrochemically stable, resistant to abrasion, and/or chemically inert, except, in the latter case, to the loss of oxygen (e.g., reduction) from oxygen-containing materials. The oxygen content of the carbonaceous material can be adjusted, which can allow the electrical conductivity of coating115to be adjusted by thermal, radiative, electrochemical, and/or chemical reduction of the carbonaceous material, either before being suspended, while in suspension, or after aggregation (discussed further below).

As the electric field is applied to electrodes100and105, sheets of carbonaceous material are layered and the liquid of suspension125can be excluded from between the layers in the direction opposite to the deposition direction of the carbonaceous material during the formation of coating115(dashed arrows120,FIG. 1B). Liquid is also excluded as the sheets or particles move together during lateral translation130caused by the electrohydrodynamic forces (FIG. 1B). The exclusion of liquid during deposition reduces the amount of entrained liquid within coating115(FIG. 1C), which eliminates or reduces the need to dry coating115before use. The exclusion of the liquid combined with the electrohydrodynamic translation typically allows coating115to be a homogeneous structure without filtration channels or other defects as the lateral movement130acts to repair or fill such defects. These defects can be inherent in the filtration processes of previously described EHD processes such as U.S. Pat. No. 6,533,903 to Hayward et al., U.S. Pat. No. 6,033,547 to Trau et al. and U.S. Pat. No. 5,855,753 again to Trau et al. Coating115can be formed in a manner to comprise designed patterns by, for example, inducing current density variations on the surface of substrate110(when present) or electrode100. The thickness of coating115can be controlled by modifying the intensity and/or duration of the applied electrical field. The conditions under which the EHD process can be performed, such as temperature, suspension concentration and the type of carbonaceous sheet in suspension, may be varied to control the areal density of connected sheets within separate layers (thereby determining the size, number and distribution of gaps within the layer). Coating115can be formed in a manner to be compliant and exhibit an improved resistance to damage during subsequent application and processing.

The carbonaceous material can be prepared from a variety of graphene sources including but not limited to graphite, graphite oxide or oxidized graphite, and/or carbon precursors formed via vapor deposition. The carbonaceous material can be dispersed in an appropriate liquid prior to coating production. Examples of liquids used as a suspension medium can include, but are not limited to, water, ammoniated water, liquid hydrocarbons, alcohols (such as ethanol), water/alcohol mixtures (such as ethanol/water), esters and carbonates (such as ethylene carbonate, propylene carbonate), dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), acetonitrile, dimethylsulfoxide (DMSO), and ionic liquids. Appropriate liquids can also include melts of organic compounds such as waxes, fatty acids, and ionic liquids with melting points above room temperature. Ionic, non-ionic and/or polymer surfactants can be added to suspension125to facilitate processing.

Suspension125can be used as made, concentrated, purified, and/or treated with additives. Undesirable large aggregates that may form in suspension125can sediment out of suspension, requiring no special apparatus or filtration to be removed before, during or after coating115formation. The carbonaceous material can be modified to improve the dispersion in different liquids and to control the spacing between layers of coating115. Other properties of the suspension, including but not limited to the ionic concentration or pH of the liquid, may be modified to control the degree of dispersion and suspension stability, the mobility of the suspended materials and/or the surface potential of the suspended materials. This may be accomplished in a variety of ways, including but not limited to changing the concentration of the dispersed material and/or adding ionic salts, acids, bases, ionic liquid, or other substances having advantageous properties such as bearing a charge. Further, the suspension may include a mixture of the carbonaceous material and other material, such as particles, polymers, ions, clusters, and the like, which allows the construction of composite layered materials. In this manner the permeability of coating115can be adjusted to permit the transport of targeted agents across the membrane, while excluding others. An immediate application of a size-exclusion membrane is in the development of sensing devices and arrays. The functional groups that serve to separate the sheets may also be active agents, reacting with, immobilizing, or retarding the transport of specified agents across the membrane.

Coating115can be a multilayer structure comprised of stacked and overlapping sheets of carbonaceous material rendered impermeable to liquids and/or vapors. Coating115can protect against corrosion, while remaining permeable to smaller moieties, such as ions. The EHD layering of individual sheets of carbonaceous material can create a structure composed of separate layers having edge-to-edge and partially overlapping sheets, reducing if not completely eliminating the number and size of gaps (voids) in the layer. Stacked layers are separated by spaces between the layers, the breadth of which depend on and can be controlled by the extent of oxidation and/or functional groups attached to the sheets of carbonaceous material. The lateral movement of the particles or sheets coupled with the stacking of the layers eliminates any gaps that may exist in each layer, and remaining gaps or open areas in one layer are unlikely to register with those in neighboring layers, above or below, allowing the extent of permeability to be controlled, as described in the previous section. The EHD process requires lateral movement of the deposited sheets or particles which, as noted, ensures that gaps or spaces between the edges of sheets or the surfaces of particles are minimized in each layer.

Coating115can be formed in a manner to be selective to ion intercalation and be ionically conductive to ionic species that can intercalate the spaces between the sheet layers. Coating115can be an impermeable membrane utilized as a protective coating on lithium metal anodes in lithium ion batteries, in which the carbonaceous material prevents contact between the corrosive electrolyte solution and the lithium metal while permitting the transport of lithium ions, thereby slowing down significantly the formation of a porous lithium layer that can degrade battery performance and/or cycle life. Coating115can suppress the growth of dendritic lithium deposits that can lead to shorting and overheating of the battery.

The electrical conductivity of coating115can be tailored by modifying the extent of interconnections between, the level of oxidation of, and/or the nature and presence of functional groups on the sheets of carbonaceous material. Such embodiments may be used in applications such as the current collectors or conductive backings in solar cells. The wetting of a carbonaceous surface by polar liquids (e.g., water) can be controlled by the degree of oxidation of the surface and/or the attachment of functional groups to either encourage or reduce wetting. Surfaces can then be rendered lyophobic or -philic depending on the desired characteristic. Coating115can be formed in a manner to be partially transparent.

In this manner coating115can be used as semitransparent protective coatings and/or radiative barriers (infrared or thermal, ultraviolet) on transparent materials, such as window glass. When rendered electrically conductive, coating115can be used, for example, as transparent electrodes for electrochromic windows and infrared reflectors for solar-control or low-emittance windows to improve the thermal efficiency of buildings or vehicles. Combining protection against radiation with electronic conductivity, coating115can protect against electronic interference and electromagnetic fields in electronic devices, and so is suitable for use in the electrostatic shielding of sensitive materials, the packaging of circuit boards and other sensitive electronics, or to shield sensitive electronics in, for example, aerospace applications. The thermal conductivity of coating115can be utilized as thermal transfer materials for better thermal management of temperature-sensitive devices such as electronic components. Coating115can, due to inherent layered structuring, reduce friction between bodies in contact and may be used to modify surfaces to protect against abrasion, friction (adhesion and cohesion), erosion, and/or corrosion.

FIG. 2depicts an EHD method, in accordance with an embodiment of the present invention. Specifically,FIG. 2illustrates an alternative embodiment of the EHD process, wherein electrodes200and210are vertically oriented. This alternative embodiment utilizes the same components and materials as discussed above; hence, similar elements are comprised of similar materials and perform similar functions as discussed above. Electrodes200and210can be at least partially submerged in suspension215, which comprises carbonaceous material222suspended in a liquid. Electrodes200and210can be separated by width225. An electrical field can be formed between electrodes200and210to initiate the formation of coating220upon the surface of electrode200, in the case depicted where negative charges are on the carbonaceous material.

FIG. 3depicts the operation steps of the EHD process, in accordance with an embodiment of the present invention. Carbonaceous material is synthesized (step300). For example, graphite oxide can be synthesized by oxidizing graphite powder using an applicable procedure. The carbonaceous material is exfoliated to individual sheets (step305). For example, the carbonaceous material can be exfoliated to individual sheets using rapid heating, ultrasonication, chemical intercalation or a combination of these methods, and suspended in a polar liquid, including water and aprotic liquids such as tetrahydrofuran, ethyl acetate, acetone, dimethylformamide, acetonitrile or dimethyl sulfoxide. For example, carbonaceous material can be formed as disclosed in U.S. Pat. No. 7,658,901 B2 to Prud'Homme et al.

When appropriate, the carbonaceous material can be reduced or chemically functionalized while in suspension. This can change the stability of the functionalized sheets in the suspending liquid, and may require replacing the original liquid with a more chemically compatible liquid, such as a less polar liquid or changing the suspension pH, to maintain suspension stability and sheet separation. In some applications, it is desirable to work with aggregates composed of layered sheets. In such cases, the suspension pH or polarity can be varied to encourage sheet aggregation whilst in suspension. As noted above, large aggregates can sediment out of suspension.

Thermal exfoliation of the carbonaceous material can reduce the oxygen content. The degree of reduction can be determined by the temperature at which exfoliation is performed and the residence time of the material in the exfoliating chamber. The two electrodes are immersed into the suspension (step310). For example, the electrodes can have a vertical or horizontal orientation. One or both electrodes may be covered with a substrate, which may be the item to be coated or a sacrificial substrate used to remove the EHD layer from the electrode. Alternatively, the object to be coated can be immersed in the suspension in lieu of one of the electrodes, although this requires that the object have a sufficiently conductive surface to act as an electrode.

Apply the desired electrical field across the electrodes (step315). For example, in most suspensions, the sheets of carbonaceous material are negatively charged and deposit on the positive electrode. Field intensity can be varied during deposition to change the rate of deposition, create graded structures, and/or overcome the increasing electrical resistance of the coated caused by the increasing thickness of the deposited membrane. The field polarity can be flipped to alternate deposition of the carbonaceous sheets with that of another component to form a bilayer structure. The field intensity can be increased to electrochemically reduce the carbonaceous material in the coating.

Remove the electrical field and the now-coated substrate (step320). For example, after a desirable interval, the electrical field may be removed and the now-coated electrode, substrate, or object removed from the suspension. In some cases the adhesion of the coating to the surface will be strong enough to withstand washing; otherwise the coating may be removed from the substrate prior to washing and/or subsequent processing to make a free-standing membrane. The coating may then be reduced or subjected to additional processing to render it more suitable for the intended application. Subsequent processing may involve thermal, radiative (including, but not limited to, exposure to ionizing radiation), or chemical processes. This additional processing can be used to modify the layered structure, including but not limiting to changing the nature of the exposed surfaces of the coating, changing the electrical conductivity of the coating, or introducing active agents into the spaces between the layers (intercalation). If fabricated as a free-standing film, the coating can then be applied to a surface. For example, the coating may be layered onto an anode and subsequently assembled into a battery.

As the thickness of the coating increases with deposition, the electrical resistance of the coating may increase, which can require an increase in the intensity of the applied field for continued deposition. However, as field intensity increases, the suspension can be electrolyzed and bubbles formed therein. For example, for certain water-based suspensions an applied voltage above 3-4 V is avoided. Bubbling at the solid/liquid interface may disrupt deposition and damage the existing coating. This limitation can be overcome using suitable organic or ionic liquids as the suspending medium, allowing higher voltages to be applied either for longer time periods to increase thickness or to increase the deposition rate.

Following deposition, the coating may swell when in contact with a liquid that wets the surfaces of the coating, penetrating between the layers of the coating and reducing its effectiveness as a filter, separator, and/or barrier. This may be countered by reducing the oxygen content of the coating to increase the van der Waals attraction binding carbonaceous sheets together, changing the composition of the liquid to reduce wetting (for example, changing the composition of the electrolyte in a battery assembly), and/or connecting the layers using intercalating linking agents that bind parallel sheet surfaces while maintaining the separation between the carbonaceous sheets. Intercalants can include difunctionalized linear molecules (such as diamines), nanoparticles (metal, metal oxide, or polymer), carbon nanotubes, fullerenes, polyaromatic hydrocarbons (PAHs), or similar materials or molecules. Linkages may be through covalent bonding between functionalities on the sheet surfaces and the intercalant, or via7C-7C stacking of smaller planar molecules (such as PAHs) between the carbonaceous sheets.

FIGS. 4A-Dare a series of optical microscopy (“OM”) images taken at different times (FIG. 4A=1 second,FIG. 4B=14 seconds,FIG. 4C=24 seconds, andFIG. 4D=54 seconds) after an electric field has been established between two vertically positioned electrodes (not visible). In the images shown, graphene oxide particles in suspension move towards a transparent electrode to the right of each image, an electrically conductive indium tin oxide (ITO) coated silica glass substrate, becoming fixed onto the surface of the electrode, then moving via hydrodynamic forces to form a continuous dense layer. In the example shown, the lateral transport of graphene oxide sheets on the electrode surface is from left to right within the field of view. The film density increases with time, which can be seen as the darker portions on the right side of each image grow in size fromFIG. 4AthroughFIG. 4D.

FIG. 5is an OM image of a continuous film of carbonaceous material formed on a substrate, in accordance with an embodiment of the present invention. Specifically,FIG. 5is an OM image of a continuous film of functionalized graphene sheets formed on a Celgard® 2320 microporous trilayer (PP/PE/PP) membrane, a non-conductive polymer film commonly used as an electrode separator in batteries. In the embodiment shown inFIG. 5, the Celgard® 2320 microporous trilayer (PP/PE/PP) membrane layer is 20 micrometers thick and is 39% porous, with average pore diameter of 27 nanometers.

FIG. 6is an atomic force microscopy (“AFM”) image of the coated polymer film, in accordance with an embodiment of the present invention. The AFM image reflects that the coating on the polymer film is 100 nm or less.

FIG. 7illustrates3surface profiles (710,720,730) of the coating made using the AFM image ofFIG. 6to check surface smoothness and film continuity.

The EHD coatings were tested as barriers to corrosion of lithium metal, an application vital to the development of long-life lithium ion batteries. In the testing procedure, the graphene oxide coated Celgard® 2320 microporous trilayer (PP/PE/PP) membrane is placed on a lithium metal anode, with the graphene oxide membrane in direct contact with the metal. The “protected anode” is assembled into CR2032 type coin cell batteries composed of the anode assemblage, liquid electrolyte (applied to the Celgard® side of the membrane), and capped with a lithium iron phosphate (LFP) cathode. Control batteries were constructed in the same manner, but without the graphene oxide coating on the Celgard® 2320 microporous trilayer (PP/PE/PP) membrane (that is, having an “unprotected anode”). By comparing the electrochemical performance of the batteries containing protected anodes against that of batteries with unprotected anodes, the effectiveness of an EHD membrane as a lithium ion conductor and as a barrier preventing contact between the lithium anode and the corrosive electrolyte were demonstrated.

FIG. 8depicts a discharge capacity versus cycle index graph, in accordance with an embodiment of the present invention, along with an indication of the 80% discharge capacity (810). A rate of 1.5 mA·cm−2for both charge and discharge was chosen as an appropriate rate for evaluating membrane performance within a suitable timeframe. Under these parameters, the presence of the graphene oxide membrane (820) (the protected anode) increased the number of charge/discharge cycles the battery could withstand by about 75-100 cycles, for cycle life based on 50% initial discharge capacity, over the cycle life of the controls (830) (unprotected anode). The effect at 80% and 90% initial discharge capacity was smaller, although still evident. As shown inFIG. 9, the slower rise in the internal resistance of the batteries containing the graphene oxide membrane (910), with respect to that of the controls (920), indicates that the graphene oxide membrane delays the corrosion of the lithium anode.

FIGS. 10A, B, C, D, E, and F depict scanning electron microscope (“SEM”) images, in accordance with an embodiment of the present invention. Specifically,FIGS. 10A-Fillustrates cross-sectional SEM images of cells cycled 5, 25, and 94 times at a rate of 1.5 mA·cm−2for an unprotected lithium electrode (FIG. 10A=5 cycles,FIG. 10B=25 cycles,FIG. 10C=94 cycles), and a lithium electrode protected with an EHD graphene oxide membrane (FIG. 10D=5 cycles,FIG. 10E=25 cycles,FIG. 10F=94 cycles). The presence of the graphene oxide membrane retards the development of the mossy lithium layer, caused by the corrosion of the lithium anode through contact with the electrolyte. The images taken at the membrane/anode interface show that lithium corrosion (1010,1020,1030) of the electrode (1015,1025,1035) is apparent within the first 5-25 cycles of testing when no graphene oxide membrane is present. After 5 cycles, the thickness of the mossy lithium layer (1010) is approximately 50 μm. Under the high current density used in the tests (1.5 mA·cm−2), the thickness of the mossy lithium layer (1020) continues to grow to 60 μm after 25 cycles on the unprotected anodes, and after almost100charge/discharge cycles, the mossy lithium layer (1030) is 200 μm.

However, when a graphene oxide membrane deposited on the Celgard® 2320 microporous trilayer (PP/PE/PP) is used to protect the anode (protected anode), little or no mossy lithium is visible on the anode (1045) surface after 5 cycles, as seen inFIG. 10D. This indicates that the graphene oxide-coated Celgard® 2320 microporous trilayer (PP/PE/PP) membrane protects the anode from corrosive attack by the electrolyte.FIG. 10Eillustrates that after 25 cycles, only patches of mossy lithium (1050), no more than 25 μm thick, are visible on the protected anode (1055).FIG. 10Fillustrates that after 94 cycles, the layer of mossy lithium (1060) present between the Celgard® 2320 microporous trilayer (PP/PE/PP) membrane and graphene oxide membrane is roughly equivalent to that observed on the unprotected anode. The increase in the lifetime of the batteries (by about 75 to 100 cycles) is due to the initial gains from protecting the lithium anode using a graphene oxide membrane deposited on Celgard® 2320 microporous trilayer (PP/PE/PP). Battery life is extended in the protected anode samples by this delay in the growth of the mossy lithium layer on the protected anode.