Patent Application
20160254528
Dimensionality can play a role in determining the fundamental properties of nanomaterials. Due to physical and chemical properties, such as quantum confinement and surface effects, two-dimensional (2D) nanosheet materials can show potential in a wide range of applications, such as electronics, optics, catalysis, energy storage, and environmental technologies. This has been highlighted over the past decade in graphene materials, which can exhibit enhanced properties compared to bulk graphite and other carbon nanomaterials.
Transition metal oxides are a family of materials that can be used in a broad range applications, for example catalysis, energy storage, and energy conversion technologies. Transition metal oxide nanomaterials are typically obtained in the form of zero-dimensional (0D) nanoparticles, 1D nanotubes or nanowires, and 3D nanoclusters or microspheres. In contrast, 2D transition metal oxide nanostructures have remained a challenge.
Disclosed herein are two-dimensional (2D) nanosheets comprising a continuous transition metal oxide phase permeated by a plurality of pores. The average characteristic dimension of the plurality of pores can, for example, be from 1 nm to 30 nm.
The transition metal oxide can comprise, for example, a metal selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Hf, Ta, W, Re, Os, Ir, Pt, Au, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and combinations thereof. In some embodiments, the transition metal oxide can comprise a catalytically active metal oxide. In some embodiments, the transition metal oxide can comprise a mixed metal oxide. In some embodiments, the transition metal oxide can be selected from the group consisting of ZnMn2O4, ZnCo2O4, NiCo2O4, CoFe2O4, Mn2O3, Co3O4, NiO, and combinations thereof.
Characteristics of the 2D nanosheets, including chemical composition, thickness, aspect ratio, surface area, pore size (e.g., average characteristic dimension of the plurality of pores), and surface porosity, can be varied in view of the desired application for the 2D nanosheet. In some embodiments, the thickness of the 2D nanosheet can be 50 nm or less. In some embodiments, the 2D nanosheet can have an aspect ratio of 10:1 or more. The surface area of the 2D nanosheet can, in some embodiments, be 20 m2/g or more. The 2D nanosheets described herein can, for example, have a surface porosity of 10% or more. The 2D nanosheets described herein can, in some examples, be substantially free of carbon.
Also disclosed herein are methods of making the 2D nanosheets described herein. The 2D nanosheets can be prepared by reacting a graphene template with a transition metal compound to form a nanosheet precursor and calcining the nanosheet precursor to form the 2D nanosheet.
Reacting the graphene template with the transition metal compound can, in some embodiments, comprise contacting the graphene template with the transition metal compound and reducing the transition metal compound. Reducing the transition metal compound can comprise, for example, heating the transition metal compound, contacting the transition metal compound with a reducing agent, or a combination thereof.
Reacting the transition metal compound with the graphene template can, in some embodiments, comprise depositing a transition metal oxide onto the graphene template. As such, in some embodiments, the nanosheet precursor can comprise a transition metal oxide-graphene hybrid material.
Calcining the nanosheet precursor can, for example, comprise heating the nanosheet precursor at a temperature at which the graphene template decomposes, thereby forming the 2D nanosheet. In some embodiments, calcining the nanosheet precursor can comprise heating the nanosheet precursor at a temperature of 400° C. or more.
The 2D nanosheets described herein can be used in applications including, but not limited to, catalysis, sensors, electronics, optoelectronics, energy conversion (e.g., fuel cells, thermoelectrics, solar cells, etc.), and energy storage (e.g., batteries, supercapacitors, etc.).
In some embodiments, the 2D nanosheets described herein can be used as electrodes. In other words, also disclosed herein are electrodes comprising the 2D nanosheets described herein. The electrode can, for example, have a larger specific capacity than that of graphite under the same conditions. In some embodiments, the electrode can have a specific capacity of 250 mA h g−1 or more at a current density of 1000 mA g−1 over 1000 charge/discharge cycles. The electrodes described herein can, for example, have a capacity retention of 85% or more after 1000 charge/discharge cycles. In some embodiments, the electrode can have a Coulombic efficiency of 99% or more over 1000 charge/discharge cycles.
Also disclosed herein are batteries comprising a first electrode comprising a 2D nanosheet described herein, a second electrode, and an electrolyte electrochemically connecting the first electrode and the second electrode. In some embodiments, the battery can further comprise a separator disposed between the first electrode and the second electrode. In some embodiments, the electrolyte can comprise a Li+ electrolyte, a Mg+ electrolyte, a Na+ electrolyte, or combinations thereof. In some embodiments, the electrolyte can comprise a Li+ electrolyte.
The materials, methods and devices described herein may be understood more readily by reference to the following detailed description of specific aspects of the disclosed subject matter, figures and the examples included therein.
Before the present materials, devices and methods are disclosed and described, it is to be understood that the aspects described below are not intended to be limited in scope by the specific devices and methods described herein, which are intended as illustrations. Various modifications of the materials, devices and methods in addition to those shown and described herein are intended to fall within the scope of that described herein. Further, while only certain representative materials, devices and method steps disclosed herein are specifically described, other combinations of the materials, devices and method steps also are intended to fall within the scope of that described herein, even if not specifically recited. Thus, a combination of steps, elements, components, or constituents may be explicitly mentioned herein, however, other combinations of steps, elements, components, and constituents are included, even though not explicitly stated.
“Phase,” as used herein, generally refers to a region of a material having a substantially uniform composition which is a distinct and physically separate portion of a heterogeneous system. The term “phase” does not imply that the material making up a phase is a chemically pure substance, but merely that the chemical and/or physical properties of the material making up the phase are essentially uniform throughout the material, and that these chemical and/or physical properties differ significantly from the chemical and/or physical properties of another phase within the material. Examples of physical properties include density, thickness, aspect ratio, specific surface area, porosity and dimensionality. Examples of chemical properties include chemical composition.
“Continuous,” as used herein, generally refers to a phase such that all points within the phase are directly connected, so that for any two points within a continuous phase, there exists a path which connects the two points without leaving the phase.
The term “two-dimensional nanosheet” or 2D nanosheet, as used herein, refers to a material that has an ultrathin thickness of 50 nm or less, and lateral dimensions (e.g., a length and a width) that are each larger than the thickness of the material, such that the nanosheet has an aspect ratio of 10:1 or more. The term “aspect ratio,” as used herein, refers to the ratio of the shortest lateral dimension of the nanosheet to its thickness.
The term “characteristic dimension,” as used herein, refers to the largest cross-sectional dimension of a pore in a plane perpendicular to the longitudinal axis of the pore. The longitudinal axis of the pore refers to the axis of a pore extending from a first face of the 2D nanosheet into the 2D nanosheet towards or to the second face of the 2D nanosheet. For example, in the case of a substantially cylindrical pore formed in the 2D nanosheet, the characteristic dimension of the pore would be the diameter of the pore.
The characteristic dimension of a pore can be determined, for example, using electron microscopy (e.g., scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), scanning transmission electron microscopy (STEM)), Brunauer-Emmett-Teller (BET) measurements, or a combination thereof.
The term “graphene,” as used herein, refers to materials that include from one to several atomic monolayers of sp2-bonded carbon atoms. Graphene can have a thickness of from about 1 to about 100 carbon layers (e.g., from about 1 to about 80 graphene layers, from about 1 to about 60 graphene layers, from about 1 to about 40 graphene layers, or from about 1 to about 20 graphene layers). The graphene can have an average thickness, for example, of from about 0.3 nm to about 55 nm (e.g., from about 0.3 nm to about 50 nm, from about 0.3 nm to about 45 nm, from about 0.3 nm to about 40 nm, from about 0.3 nm to about 35 nm, from about 0.3 nm to about 30 nm, from about 0.3 nm to about 25 nm, from about 0.3 nm to about 20 nm, from about 0.3 nm to about 15 nm, from about 0.3 nm to about 10 nm, or from about 0.3 nm to about 5 nm). The term “graphene,” as used herein can thus include a wide range of graphene-based materials including, for example, graphene oxide, graphite oxide, chemically converted graphene, functionalized graphene, functionalized graphene oxide, functionalized graphite oxide, functionalized chemically convened graphene, and combinations thereof.
The term “comprising” and variations thereof as used herein is used synonymously with the term “including” and variations thereof and are open, non-limiting terms. Although the terms “comprising” and “including” have been used herein to describe various examples, the terms “consisting essentially of” and “consisting of” can be used in place of “comprising” and “including” to provide for more specific examples of the invention and are also disclosed. Other than in the examples, or where otherwise noted, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood at the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, to be construed in light of the number of significant digits and ordinary rounding approaches.
As used in the description and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a composition” includes mixtures of two or more such compositions, reference to “an agent” includes mixtures of two or more such agents, reference to “the component” includes mixtures of two or more such components, and the like.
“Optional” or “optionally” means that the subsequently described event or circumstance can or cannot occur, and that the description includes instances where the event or circumstance occurs and instances where it does not.
It is understood that throughout this specification the identifiers “first” and “second” are used solely to aid in distinguishing the various components and steps of the disclosed subject matter. The identifiers “first” and “second” are not intended to imply any particular order, amount, preference, or importance to the components or steps modified by these terms.
Also, throughout this specification, various publications are referenced. The disclosures of these publications in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art to which the disclosed matter pertains. The references disclosed are also individually and specifically incorporated by reference herein for the material contained in them that is discussed in the sentence in which the reference is relied upon.
Two-Dimensional Nanosheets
Disclosed herein are two-dimensional (2D) nanosheets comprising a continuous transition metal oxide phase permeated by a plurality of pores.
The 2D nanosheets can be described as porous. The term “porous,” as used herein, refers to materials that include openings and spacings (e.g., pores) which are present as a surface characteristic or a bulk material property, partially or completely penetrating the material. As such, the 2D nanosheets can possess a plurality of pores, voids, holes and/or channels, each of which may or may not extend through the entire thickness of the 2D nanosheet.
The 2D nanosheets comprise a plurality of pores. The average characteristic dimension of the plurality of pores can, for example, be 30 nm or less (e.g., 28 nm or less, 26 nm or less, 24 nm or less, 22 nm or less, 20 nm or less, 18 nm or less, 16 nm or less, 14 nm or less, 12 nm or less, 10 nm or less, 8 nm or less, 6 nm or less, 4 nm or less, or 2 nm or less). In some embodiments, the average characteristic dimension of the plurality of pores can be 1 nm or more (e.g., 2 nm or more, 4 nm or more, 6 nm or more, 8 nm or more, 10 nm or more, 12 nm or more, 14 nm or more, 16 nm or more, 18 nm or more, 20 nm or more, 22 nm or more, 24 nm or more, 26 nm or more, or 28 nm or more).
The average characteristic dimension of the plurality of pores can range from any of the minimum values described above to any of the maximum values described above, for example from 1 nm to 30 nm (e.g., from 1 nm to 16 nm, from 16 nm to 30 nm, from 1 nm to 10 nm, from 10 nm to 20 nm, from 20 nm to 30 nm, from 4 nm to 20 nm, from 4 nm to 10 nm, from 6 nm to 12 nm, or from 14 nm to 20 nm).
The plurality of pores can, in some examples, have a substantially constant characteristic dimension along their length. In some embodiments, the characteristic dimension of the plurality of pores is substantially constant from pore to pore throughout the 2D nanosheet, such that substantially all (e.g., 75% or more, 80% or more, 85% or more, 90% or more, or 95% or more) of the pores in the 2D nanosheet have a characteristic dimension that is within 40% of the average characteristic dimension of the plurality of pores (e.g., within 35% of the average characteristic dimension of the plurality of pores, within 30% of the average characteristic dimension of the plurality of pores, within 25% of the average characteristic dimension of the plurality of pores, within 20% of the average characteristic dimension of the plurality of pores, within 15% of the average characteristic dimension of the plurality of pores, or within 10% of the average characteristic dimension of the plurality of pores).
The walls of the plurality of pores are formed from the continuous transition metal oxide phase. The transition metal oxide can comprise, for example, a metal selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Hf, Ta, W, Re, Os, Ir, Pt, Au, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and combinations thereof. In certain embodiments, the transition metal oxide can comprise a metal selected from the group consisting of Zn, Mn, Co, Ni, Fe, and combinations thereof
In some embodiments, the transition metal oxide can comprise a catalytically active metal oxide. In some embodiments, the transition metal oxide can comprise a mixed metal oxide. In some embodiments, the transition metal oxide can comprise a transition metal oxide selected from the group consisting of ZnMn2O4, ZnCo2O4, NiCo2O4, CoFe2O4, Mn2O3, Co3O4, NiO, and combinations thereof. The nature of the transition metal oxide can be determined, for example, using X-Ray powder diffraction (XRD), selected area electron diffraction (SAED), elemental analysis, or a combination thereof.
The amount of organic carbon present in a 2D nanosheet can be estimated by measuring the material's loss-on-ignition (LOI). The LOI of a filler refers to the percent weight loss of a sample of the 2D nanosheet upon ignition at 750° C. for 2 hours, and then further heating at 750° C. to a constant mass to consume any organic carbon present in the 2D nanosheet, as described, for example in ASTM C618-12a.
In some embodiments, the 2D nanosheet can have an LOI of less than 10% (e.g., less than 9.75, less than 9.5%, less than 9.25%, less than 9.0%, less than 8.75, less than 8.5%, less than 8.25%, less than 8.0%, less than 7.75, less than 7.5%, less than 7.25%, less than 7.0%, less than 6.75%, less than 6.5%, less than 6.25%, less than 6.0%, less than 5.75%, less than 5.5%, less than 5.25%, less than 5.0%, less than 4.75%, less than 4.5%, less than 4.25%, less than 4.0%, less than 3.75%, less than 3.5%, less than 3.25%, less than 3.0%, less than 2.75%, less than 2.5%, less than 2.25%, less than 2.0%, less than 1.9%, less than 1.8%, less than 1.7%, less than 1.6%, less than 1.5%, less than 1.4%, less than 1.3%, less than 1.2%, less than 1.1%, less than 1.0%, less than 0.95%, less than 0.90%, less than 0.85%, less than 0.80%, less than 0.75%, less than 0.70%, less than 0.65%, less than 0.60%, or less than 0.55%). In certain embodiments, the 2D nanosheets described herein can be substantially free of carbon (i.e., the 2D nanosheet can have an LOI of less than 0.50%).
Characteristics of the 2D nanosheets, including thickness, aspect ratio, surface area, pore size (e.g., average characteristic dimension of the plurality of pores), and surface porosity, can be varied in view of the desired application for the 2D nanosheet.
In some embodiments, the thickness of the 2D nanosheet can be 50 nm or less (e.g., 45 nm or less, 40 nm or less, 35 nm or less, 30 nm or less, 25 nm or less, 20 nm or less, 15 nm or less, or 10 nm or less). In some embodiments, the thickness of the 2D nanosheet can be 5 nm or more (e.g., 10 nm or more, 15 nm or more, 20 nm or more, 25 nm or more, 30 nm or more, 35 nm or more, 40 nm or more, or 45 nm or more).
The 2D nanosheet can have a thickness ranging from any of the minimum values described above to any of the maximum values described above. For example, the 2D nanosheet can have a thickness of from 5 nm to 50 nm (e.g., from 5 nm to 30 nm, from 30 nm to 50 nm, from 5 nm to 15 nm, from 15 nm to 30 nm, from 30 nm to 40 nm, or from 40 nm to 50 nm). The thickness of the 2D nanosheet can be determined, for example, via atomic force microscopy (AFM).
The thickness of the 2D nanosheet can be varied based on the intended application for the 2D nanosheet. In some applications (e.g., for use as a catalyst), a thinner 2D nanosheet (e.g., a 2D nanosheet having a thickness of from 5 nm to 15 nm) can be desirable. For other applications (e.g., for use as an electrode in a battery) a thinner 2D nanosheet (e.g., a 2D nanosheet having a thickness of from 5 nm to 15 nm) can be desirable.
In some embodiments, the 2D nanosheet can have an aspect ratio of 10:1 or more (e.g., 15:1 or more, 20:1 or more, 25:1 or more, 30:1 or more, 35:1 or more, 40:1 or more, 45:1 or more, 50:1 or more, 60:1 or more, 70:1 or more, 80:1 or more, 90:1 or more, 100:1 or more, 150:1 or more, 200:1 or more, 250:1 or more, 300:1 or more, 350:1 or more, 400:1 or more, 450:1 or more, 500:1 or more, 600:1 or more, 700:1 or more, 800:1 or more, or 900:1 or more). In some embodiments, the 2D nanosheet can have an aspect ratio of 1000:1 or less (e.g., 900:1 or less, 800:1 or less, 700:1 or less, 600:1 or less, 500:1 or less, 450:1 or less, 400:1 or less, 350:1 or less, 300:1 or less, 250:1 or less, 200:1 or less, 150:1 or less, 100:1 or less, 90:1 or less, 80:1 or less, 70:1 or less, 60:1 or less, 50:1 or less, 45:1 or less, 40:1 or less, 35:1 or less, 30:1 or less, 25:1 or less, 20:1 or less, or 15:1 or less).
The 2D nanosheet can have an aspect ratio ranging from any of the minimum values described above to any of the maximum values described above. For example, the 2D nanosheet can have an aspect ratio of from 10:1 to 1000:1 (e.g., from 10:1 to 500:1, from 500:1 to 1000:1, from 10:1 to 250:1, from 250:1 to 500:1, or from 20:1 to 100:1).
The surface area of the 2D nanosheet can, in some embodiments, be 20 m2/g or more (e.g., 25 m2/g or more, 50 m2/g or more, 75 m2/g or more, 100 m2/g or more, 125 m2/g or more, 150 m2/g or more, or 175 m2/g or more). In some embodiments, the surface area of the 2D nanosheet can be 200 m2/g or less (e.g., 175 m2/g or less, 150 m2/g or less, 125 m2/g or less, 100 m2/g or less, 75 m2/g or less, 50 m2/g or less, or 25 m2/g or less).
The 2D nanosheet can have a surface area ranging from any of the minimum values described above to any of the maximum values described above. For example, the 2D nanosheet can have a surface area of from 20 m2/g to 200 m2/g (e.g., from 20 m2/g to 100 m2/g, from 100 m2/g to 200 m2/g, or from 50 m2/g to 175 m2/g). The surface area of the 2D nanosheets described herein can be determined by any suitable method, such as the Brunauer-Emmett-Teller (BET) method.
The 2D nanosheets described herein can, for example, have a surface porosity of 10% to 50%. The term “surface porosity,” as used herein, refers to the percentage of a surface of the 2D nanosheet that comprises pores. For example, the surface porosity of a 2D nanosheet can be determined by capturing an image of the 2D nanosheet (e.g., by electron microscopy), and determining the percent of the surface area of the 2D nanosheet that comprises pores (i.e., the surface porosity) from that image
The 2D nanosheet can, in some embodiments, have a surface porosity of 10% or more (e.g., 15% or more, 20% or more, 25% or more, 30% or more, 35% or more, 40% or more, or 45% or more). In some embodiments, the 2D nanosheet can have a surface porosity of 50% or less (e.g., 45% or less, 40% or less, 35% or less, 30% or less, 25% or less, 20% or less, or 15% or less).
The 2D nanosheet can have a surface porosity ranging from any of the minimum values described above to any of the maximum values described above. For example, the 2D nanosheet can have a surface porosity of from 10% to 50% (e.g., from 10% to 30%, from 30% to 50%, from 10% to 20%, from 20% to 30%, from 30% to 40%, from 40% to 50%, from 15% to 45%, or from 20% to 40%).
Methods of Making
Also disclosed herein are methods of making the 2D nanosheets described herein. The 2D nanosheets can be prepared by (i) reacting a graphene template with a transition metal compound to form a nanosheet precursor, and (ii) calcining the nanosheet precursor to form the 2D nanosheet.
Any suitable graphene template can be used. For example the graphene template can comprise synthetic graphene, natural graphene, or combinations thereof. The graphene template can, for example, comprise graphene flakes, graphene sheets, graphene ribbons, graphene particles, or combinations thereof. Suitable graphene templates are known in the art, and can be obtained commercially or prepared according to known methods.
A ready source of graphene is bulk graphite, which consists of a large number of graphene sheets held together through van der Waals forces. Single- and few-layer graphene sheets have been prepared in microscopic quantities by mechanical exfoliation of bulk graphite (commonly referred to as the “Scotch-tape” method) and by epitaxial chemical vapor deposition.
To date, methods for preparing bulk quantities of graphene have centered on chemical exfoliation of graphite. The most common approach for exfoliation of graphite has been to use a strong oxidizing agent to produce graphene oxide, a non-conductive and hydrophilic carbon material. Although the exact chemical structure of graphene oxide is difficult to conclusively determine, it is at least qualitatively evident that the regular sp2 structure is disrupted in graphene oxide with epoxides, alcohols, carbonyls and carboxylic acid groups. The disruption of the lattice in bulk graphite is reflected in an increase in interlayer spacing from 0.335 nm in bulk graphite to more than 0.625 nm in graphene oxide.
Graphene oxide was first prepared in 1859 by adding potassium chlorate to a slurry of graphite in fuming nitric acid. The synthesis was improved in 1898 by including sulfuric acid in the reaction mixture and adding the potassium chlorate portionwise over the course of the reaction. The most common method used today is that reported by Hummers in which bulk graphite is oxidized by treatment with KMnO4 and NaNO3 in concentrated H2SO4 (Hummers' method).
In some embodiments, the graphene template can comprise graphene oxide. In certain embodiments, the graphene template can comprise graphene oxide prepared by Hummers' method.
The transition metal compound can comprise any compound comprising a transition metal. In some embodiments, the transition metal compound can comprise a metal selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Hf, Ta, W, Re, Os, Ir, Pt, Au, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and combinations thereof. In some embodiments, the transition metal compound can comprise a transition metal salt. The counterion of the transition metal salt can be, for example, a nitrate, phosphate, acetate, sulfate, or chloride. Other suitable counterions include organic or inorganic ions, such as carbonate, bromide, iodide, sulfite, phosphite, nitrite, and combinations thereof. In some embodiments, the transition metal compound can comprise a transition metal acetate. The transition metal compounds suitable for use herein can be readily obtained from commercial suppliers or synthesized by methods known in the art.
Reacting the graphene template with the transition metal compound can, in some embodiments, comprise contacting the graphene template with the transition metal compound and reducing the transition metal compound. Contacting the graphene template with the transition metal compound can be performed by, for example, adding the graphene template to the transition metal compound or by adding the transition metal compound to the graphene template. Contacting can also be performed by slowing mixing one component with the other or by drop-wise addition of one component into the other. Agitation (e.g., stirring, shaking, or ultrasonic agitation) can be used to facilitate the contacting of the graphene template with the transition metal compound. Reducing the transition metal compound can comprise, for example, heating the transition metal compound, contacting the transition metal compound with a reducing agent, or a combination thereof.
In some embodiments, reacting the graphene template with the transition metal compound can comprise contacting the transition metal compound with a reducing agent. The reducing agent can be added to the mixture by any method known in the art or described herein. Suitable reducing agents include, but are not limited to, hydrogen gas, alcohols (e.g., methanol, ethanol), polyols, polyethers (e.g., ethylene glycol), carboxylic acids (e.g., acetic acid), aldehydes, hydrazines, hydrides, ketones, boranes, and the like, and combinations thereof. In some examples, the reducing agent is ethylene glycol.
In some embodiments, reacting the graphene template with the transition metal compound can comprise heating. In some embodiments, reacting the graphene template with the transition metal compound can comprise heating at a temperature of 200° C. or more (e.g., 225° C. or more, 250° C. or more, 275° C. or more, 300° C. or more, 325° C. or more, 350° C. or more, or 375° C. or more). In some embodiments, reacting the graphene template with the transition metal compound can comprise heating at a temperature of 400° C. or less (e.g., 375° C. or less, 350° C. or less, 325° C. or less, 300° C. or less, 275° C. or less, 250° C. or less, or 225° C. or less). The temperature at which the graphene template reacts with the transition metal compound can range from any of the minimum values described above to any of the maximum values described above. For example, in some embodiments, reacting the graphene template with the transition metal compound can comprise heating at a temperature of from 200° C. to 400° C. (e.g., from 200° C. to 300° C., from 300° C. to 400° C., from 200° C. to 250° C., from 250° C. to 300° C., from 300° C. to 350° C. from 350° C. to 400° C., or from 250° C. to 350° C.).
Reacting the transition metal compound with the graphene template can, in some embodiments, comprise depositing a transition metal oxide onto the graphene template. As such, in some embodiments, the nanosheet precursor can comprise a transition metal oxide-graphene hybrid material (e.g., a transition metal oxide deposited on the graphene template).
Calcining the nanosheet precursor can, for example, comprise heating the nanosheet precursor at a temperature at which the graphene template decomposes, thereby forming the 2D nanosheet. The decomposition of the graphene template can be determined, for example, using thermogravimetric (TG) analysis.
In some embodiments, calcining the nanosheet precursor can comprise heating the nanosheet precursor at a temperature of 400° C. or more (e.g., 425° C. or more, 450° C. or more, 475° C. or more, 500° C. or more, 525° C. or more, 550° C. or more, or 575° C. or more). In some embodiments, calcining the nanosheet precursor can comprise heating the nanosheet precursor at a temperature of 600° C. or less (e.g., 575° C. or less, 550° C. or less, 525° C. or less, 500° C. or less, 475° C. or less, 450° C. or less, or 425° C. or less). The temperature at which the nanosheet precursor is calcined can range from any of the minimum values described above to any of the maximum values described above. For example, in some embodiments, calcining the nanosheet precursor can comprise heating the nanosheet precursor at a temperature of from 400° C. to 600° C. (e.g., from 400° C. to 500° C., from 500° C. to 600° C., from 400° C. to 450° C. from 450° C. to 500° C., from 500° C. to 550° C., from 550° C. to 600° C., or from 450° C. to 550° C.).
Applications
The 2D nanosheets described herein can be used in applications including, but not limited to, catalysis, sensors, electronics, optoelectronics, energy conversion (e.g., fuel cells, thermoelectrics, solar cells, etc.), and energy storage (e.g., batteries, supercapacitors, etc.).
The utility of the 2D nanosheets for a particular application will depend on several factors, including the nature of the continuous transition metal oxide phase, as well as the morphology of the 2D nanosheet. Appropriate 2D nanosheets for a particular application can be selected in view of the type of application.
In some embodiments, the 2D nanosheets described herein can be used as electrodes. The electrode can, for example, have a larger specific capacity than that of graphite under the same conditions. In some embodiments, the electrode can have a specific capacity of 250 mA h g−1 or more at a current density of 1000 mA g−1 over 1000 charge/discharge cycles (e.g., 300 mA h g−1 or more, 350 mA h g−1 or more, 400 mA h g−1 or more, 450 mA h g−1 or more, 500 mA h g−1 or more, 550 mA h g−1 or more, 600 mA h g−1 or more, 650 mA h g−1 or more, 700 mA h g−1 or more, 750 mA h g−1 or more, 800 mA h g−1 or more, 850 mA h g−1 or more, 900 mA h g−1 or more, or 950 mA h g−1 or more). In some embodiments, the electrode can have a specific capacity of 1000 mA h g−1 or less at a current density of 1000 mA g−1 over 1000 charge/discharge cycles (e.g., 950 mA h g−1 or less, 900 mA h g−1 or less, 850 mA h g−1 or less, 800 mA h g−1 or less, 750 mA h g−1 or less, 700 mA h g−1 or less, 650 mA h g−1 or less, 600 mA h g−1 or less, 550 mA h g−1 or less, 500 mA h g−1 or less, 450 mA h g−1 or less, 400 mA h g−1 or less, 350 mA h g−1 or less, or 300 mA h g−1 or less).
The specific capacity of the electrode can range from any of the minimum values described above to any of the maximum values described above. For example, the electrode can have a specific capacity of from 250 mA h g−1 to 1000 mA h g−1 at a current density of 1000 mA g−1 over 1000 charge/discharge cycles (e.g., from 250 mA h g−1 to 650 mA h g−1, from 650 mA h g−1 to 1000 mA h g−1, from 250 mA h g−1 to 750 mA h g−1, from 500 mA h g−1 to 1000 mA h g−1, from 250 mA h g−1 to 500 mA h g−1, from 500 mA h g−1 to 750 mA h g−1, from 750 mA h g−1 to 1000 mA h g−1, or from 450 mA h g−1 to 800 mA h g−1).
The electrodes described herein can, for example, retain most of their specific capacity after several charge/discharge cycles. For example, in some embodiments, the electrode can have a capacity retention of 85% or more after 1000 charge/discharge cycles (e.g., 86% or more, 87% or more, 88% or more, 89% or more, 90% or more, 91% or more, 92% or more, 93% or more, or 94%0 or more). In some embodiments, the electrode can have a capacity retention of 95% or less after 1000 charge/discharge cycles (e.g., 94% or less, 93% or less, 92% or less, 91% or less, 90% or less, 89% or less, 88% or less, 87% or less, or 86% or less).
The capacity retention of the electrode can range from any of the minimum values described above to any of the maximum values described above. For example, in some embodiments the capacity retention of the electrode can be from 85% to 95% after 1000 charge/discharge cycles (e.g., from 85% to 90%, from 90% to 95%, or from 88% to 92%).
The electrodes described herein can, in some embodiments, have a high Coulombic efficiency. For example, in some embodiments, the electrode can have a Coulombic efficiency of 99% or more over 1000 charge/discharge cycles (e.g., 99.1% or more, 99.2% or more, 99.3% or more, 99.4% or more, 99.5% or more, 99.6% or more, 99.7% or more, 99.8% or more, or 99.9% or more). In some embodiments, the electrode can have a Coulombic efficiency of 100% or less over 1000 charge/discharge cycles (e.g., 99.9% or less, 99.8% or less, 99.7% or less, 99.6% or less, 99.5% or less, 99.4% or less, 99.3% or less, 99.2% or less, or 99.1% or less).
The Coulombic efficiency of the electrode can range from any of the minimum values described above to any of the maximum values described above. For example, in some embodiments the electrode can have a Coulombic efficiency of from 99% to 100% over 1000 charge/discharge cycles (e.g., from 99% to 99.5%, from 99.5% to 100%, from 99% to 99.3%, from 99.3% to 99.6%, from 99.6% to 100%, or from 99.2% to 99.8%).
Also disclosed herein are methods of use of the 2D nanosheets described herein, or the electrodes comprising the 2D nanosheets described herein, in batteries. As such, disclosed herein are batteries comprising a first electrode comprising any of the 2D nanosheets described herein, a second electrode, and an electrolyte in electrochemical connect with the first electrode and the second electrode. Also disclosed herein are methods of use of the 2D nanosheets described herein as electrodes for lithium-ion batteries and for new-generation batteries even beyond lithium-ion, such as sodium ion batteries and magnesium ion batteries.
The electrolyte can comprise any electrolyte consistent with the methods described herein. In some embodiments, the electrolyte can comprise a Li+ electrolyte, a Mg+ electrolyte, a Na+ electrolyte, or combinations thereof. In some embodiments, the electrolyte can comprise a Li+ electrolyte.
In some embodiments, the battery can further comprise a separator disposed between the first electrode and the second electrode.
The examples below are intended to further illustrate certain aspects of the systems and methods described herein, and are not intended to limit the scope of the claims.
The following examples are set forth below to illustrate the methods and results according to the disclosed subject matter. These examples are not intended to be inclusive of all aspects of the subject matter disclosed herein, but rather to illustrate representative methods and results. These examples are not intended to exclude equivalents and variations of the present invention which are apparent to one skilled in the art.
Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperature, etc.) but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, temperature is in ° C. or is at ambient temperature, and pressure is at or near atmospheric. There are numerous variations and combinations of reaction conditions, e.g., component concentrations, temperatures, pressures and other reaction ranges and conditions that can be used to optimize the product purity and yield obtained from the described process.
Two-dimensional (2D) nanomaterials, such as graphene and transition metal dichalcogenides, can be desirable for many applications but the preparation of 2D transition metal oxide nanostructures can be challenging. Herein, a template-directed self-assembly strategy for synthesis of 2D holey transition metal oxide nanosheets is discussed. This route can be used to generate 2D holey nanosheets of various transition metal oxides, including mixed oxides such as ZnMn2O4, ZnCo2O4, NiCo2O4, and CoFe2O4, and simple oxides such as Mn2O3, Co3O4, and NiO. The synthesis strategy discussed herein can also be used to design 2D holey nanostructures with adjustable hole size. Unlike conventional nanosheets with flat and smooth surfaces, the 2D holey nanosheets possess tunable porosity that can enhance charge/mass transport properties, which can be important for many energy devices. It is shown herein that these 2D holey nanosheet structures can exhibit excellent rate capability and cycling stability when functioning as lithium-ion battery anodes. The approach presented herein can be used to design and synthesize 2D holey nanostructures that can synergize features of both 2D nanostructures and controlled porosity. These types of 2D holey nanostructures can be of interest in a broad range of technological areas from electronics and optoelectronics to energy and environmental technologies.
Dimensionality can play a role in determining the fundamental properties of nanomaterials (Huang X et al. Adv. Mater. 2014, 26, 2185-2204; Jariwala D et al. Chem. Soc. Rev. 2013, 42, 2824-2860). For example, electrons can interact differently in three-, two-, one- and zero-dimensional nanostructures (Jariwala D et al. Chem. Soc. Rev. 2013, 42, 2824-2860). Recent research on the two-dimensionalization of materials has discussed tuning the fundamental physical and chemical properties of such two-dimensional materials (Huang X et al. Adv. Mater. 2014, 26, 2185-2204). Due to physical and chemical properties, such as quantum confinement and surface effects, two-dimensional (2D) nanosheet materials can show potential in a wide range of applications, such as catalysis, energy storage, and electronics (Huang X et al. Adv. Mater. 2014, 26, 2185-2204). This has been highlighted over the past decade in graphene materials, which can exhibit enhanced properties compared to bulk graphite and other low-dimensional carbon nanostructures (Geim A K and Novoselov K S. Nat. Mater. 2007, 6, 183-191; Zhu Y et al. Adv. Mater. 2010, 22, 3906-3924). Beyond graphene, 2D nanosheets of hexagonal boron nitride (h-BN), metal dichalcogenides (TMDs), and metal phosphates have also been investigated and have exhibited properties that can be distinct from their bulk counterparts (Chhowalla M et al. Nat. Chem. 2013, 5, 263-275; Wang Q H et al. Nat. Nanotechnol. 2012, 7, 699-712; Butler S Z et al. ACS Nano. 2013, 7, 2898-2926: Renzhi M and Takayoshi S. Adv. Mater. 2010, 22, 5082-5104).
Transition metal oxides, including simple transition metal oxides (e.g., with one type of transition metal element) and mixed transition metal oxides (e.g., with different transition metal elements), are a family of materials that can be used in a broad range applications, for example catalysis, energy storage, and energy conversion technologies (Cheng F et al. Nat Chem. 2011, 3, 79-84: Liang Y et al. J. Am. Chem. Soc. 2012, 134, 3517-3523; Xiong P et al. J. Mater. Chem. 2012, 22, 17485-17493: Liang Y et al. Nat. Chem. 2011, 10, 780-786: Jiang J et al. Adv. Mater. 2012, 24, 5166-5180; Yuan C et al. Angew. Chem. Int. Ed 2014, 53, 1488-1504; Xiong P et al. J. Power Sources 2014, 245, 937-946; Xiong P et al. ACS Nano 2014, 8, 8610-8616). Transition metal oxide nanomaterials can be obtained in the form of zero-dimensional (0D) nanoparticles (Zeng H et al. J. Am. Chem. Soc. 2004, 126, 11458-11459: Niederberger M. Acc. Chem. Res. 2007, 40, 793-800), 1D nanotubes or nanowires (Devan R et al. Adv. Funct. Mater. 2012, 22, 3326-3370), and 3D nanoclusters or microspheres (Hu L et al. Sci. Rep. 2012, 2, 986: Nakashima T and Kimizuka N. J. Am. Chem. Soc. 2003, 125, 6386-6387: Jin Z et al. Angew. Chem. Int. Ed. 2012, 51, 6406-6410). In contrast, 2D nanostructures, such as free-standing nanosheets with confined thickness, are less common in transition metal oxide material systems.
General methods that can be used to prepare 2D nanomaterials include mechanical exfoliation (Novoselov K S et al. Proc. Natl. Acad. Sci. USA 2005, 102, 10451-10453; Yin Z et al. ACS Nano 2011, 6, 74-80) and direct liquid exfoliation (Coleman J N et al. Science 2011, 331, 568-571: Zhou K G et al. Angew. Chem. Int. Ed. 2011, 50, 10839-10842) of their layered crystals. The bulk material forms of graphene, TMDs, and h-BN are layered structures with strong covalent bonding within each layer and weak van der Waals forces between the layers. Thus, single or few-layer nanosheets of these materials can be obtained via mechanical cleavage or direct ultrasonication in solvents. It has been reported that almost all bulk layered TMD crystals can be exfoliated in common solvents, such as N-methylpyrrolidone (NMP), dimethylformamide (DMF), and isopropyl alcohol (IPA), to give mono- and few-layer nanosheets (Coleman J N et al. Science 2011, 331, 568-571). Unfortunately, based on these top-down synthesis strategies, only a few kinds of 2D nanomaterials (e.g., those possessing a suitably layered crystal matrix) can be obtained (Coleman J N et al. Science 2011, 331, 568-571; Nicolosi V et al. Science 2013, 340, 1226419). Moreover, scalable synthesis can be challenging. Although some simple transition metal oxide nanosheets, such as MnO2 nanosheets, have been prepared via exfoliation of their layered matrices (Omomo Y et al. J. Am. Chem. Soc. 2003, 125, 3568-3575), most bulk transition metal oxides do not have layered structures and therefore cannot be exfoliated via the general exfoliation method to obtain a 2D nanostructure. Therefore, scalable synthetic strategies for generating transition metal oxide nanosheets for materials with non-layered bulk crystal structures are still needed.
Template-directed strategies can be used to prepare nanomaterials with controllable structure (Liang H W et al. Adv. Mater. 2010, 22, 3925-3937). Recently, self-assembly of transition metal sulfides (Du Y et al. Nat. Commun. 2012, 3, 1177) and oxides (Liu Q et al. Small 2014, 10, 48-51) on laminar templates have been applied to synthesize well-defined 2D features with confined thickness. Graphene oxide, an oxidized 2D carbon sheet, has been used as a template to prepare various graphene-transition metal oxide nanosheets owing to the oxygen-contained active sites on the surface of the graphene oxide (Xiong P et al. ACS Nano 2014, 8, 8610-8616: Huang X et al. Chem. Soc. Rev. 2012, 41, 666-686; Wang H et al. J. Am. Chem. Soc. 2010, 132, 13978-13980; Liang Y et al. Nat. Mater. 2011, 10, 780-786). However, in these previous studies the structures of the graphene oxide substrates remain unchanged, such that the resulting material is a graphene-based composite/hybrid nanosheet, rather than single component 2D nanostructure.
Herein, a general template-directed self-assembly strategy for the synthesis of 2D holey transition metal oxide nanosheets by employing graphene oxide as a sacrificial template is discussed. Graphene oxide was employed as a template to grow various transition metal oxide precursors on its surface. The transition metal oxide precursors were transformed into 2D holey transition metal oxide nanosheets due to the interconnection of the transition metal oxide nanoparticles and the decomposition of the graphene oxide during thermal post-treatment. This strategy was also used to synthesize various 2D holey nanosheets of transition metal oxides, including mixed transition metal oxides (such as ZnMn2O4, ZnCo2O4, NiCo2O4, and CoFe2O4), and simple transition metal oxides (such as Mn2O3, Co3O4, and NiO). In addition, 2D holey nanosheets with adjustable hole size were obtained through control of calcination temperatures. Compared with exfoliation methods, this strategy can extend the 2D nanomaterial family to include 2D nanosheets for those materials not having a layered bulk structure. This strategy can also make scalable synthesis possible. Unlike conventional nanosheets with smooth surfaces and/or porous micro-scaled materials, these resulting 2D holey nanosheets can possess both 2D nanostructure and porosity, which can result in the 2D holey nanosheets exhibiting superior properties compared to conventional nanosheets and/or porous micro-scaled materials. 2D nanosheets can be used in areas ranging from electronics to catalysis (Osada M and Sasaki T. Adv. Mater. 2012, 24, 210-228; Gunjakar J L et al. J. Phys. Chem. C 2014, 118, 3847-3863). For example, 2D nanostructures can potentially bring not only effective electron transport, but also enhanced host capabilities, which can arise from the enlarged surface areas and improved diffusion processes (Seo J W et al. Angew. Chem. Int. Ed. 2007, 46, 8828-8831). 2D nanostructures have also been employed to increase the surface area of total catalysis and improve catalytic activities (Gunjakar J L et al. J. Am. Chem. Soc. 2011, 133, 14998-15007; Shin S I et al. Energy Environ. Sci. 2013, 6, 608-617). Nanomaterials with porosity have been involved in advanced energy storage and conversion systems, owing to their interfacial transport properties, shortened diffusion paths, reduced diffusion effects, and enhanced structure integrity (Li Y et al. Adv. Funct. Mater. 2012, 22, 4634-4667; Ge M et al. Nano Lett. 2013, 14, 261-268).
Experimental
Synthesis of Graphene Oxide.
Graphene oxide was prepared from purified natural graphite by a modified Hummers method. Simply, 10 g of graphite powder was first added to 15 mL of concentrated H2SO4. Then, 5 g of K2S2O8 and 5 g of P2O5 were slowly added. The as-obtained mixed solution was heated to 80° C. and maintained at this temperature for 6 h. After cooling to room temperature, the resultant mixture was carefully diluted with distilled water, filtered, and washed on the filter until the pH of the rinse water became neutral. The product was dried in air at ambient temperature overnight. The preoxidized graphite was then added to 230 mL of concentrated H2SO4 cooled in an ice-water bath. To this mixture, 30 g of KMnO4 was added very slowly with stirring and cooling. All operations were carried out very slowly in a fume hood. The mixture was then stirred at 35° C. for 30 min. Then, 460 mL of distilled water was slowly added to increase the temperature to 98° C. and the mixture was maintained at that temperature for 15 min. The reaction was terminated by adding 1.4 L of distilled water, followed by 10 mL of 30% H2O2 solution. The solid product was separated by centrifugation, washed repeatedly with 5% HCl solution, and then dialyzed for a week.
Synthesis of 2D Holey Transition Metal Oxide Nanosheets.
The 2D holey transition metal oxide nanosheets were prepared via a template-directed self-assembly method as illustrated in
Characterizations.
The structures of the as-synthesized samples were characterized by powder X-ray diffraction (XRD) performed on a Philips Vertical Scanning diffractometer. The morphology of the samples was investigated using scanning transmission electron microscopy (STEM) (Hitachi $5500), and transmission electron microscopy (TEM) (JEOL 2010F). Thermogravimetric (TG) analysis was performed on a TGA/SDTA851e thermogravimetric analyzer under an air atmosphere from 25° C. to 850° C. at a heating rate of 10° C. min−1. Atomic force microscopy (AFM) (ParkAFM XE-70) was used to determine the thicknesses of the nanosheets.
Electrochemical Measurements.
The working electrodes were prepared by mixing active materials (2D holey ZnMn2O4 nanosheets) and polyvinylidene difluoride (PVDF) at a weight ratio of 90:10 in N-methyl-2-pyrrolidinone (NMP). The slurries were then coated onto a copper foil. The as-prepared electrodes were dried under vacuum at 110° C. for 10 h. The loading of active materials was ˜0.8-1.0 mg cm−2. After being sealed, the electrodes were assembled into coin cells (CR2032) in an argon-filled glovebox using Celgard 2320 as a separator, 1 mol L−1 LiPF6 in ethylenecarbonate (EC) and diethylenecarbonate (DEC) (1:1, v/v) as the electrolyte and Li metal as the counter electrode. The electrolyte used for the sodium-ion battery was 1 M NaClO4 dissolved in propylene carbonate (PC) with 2% fluoroethlyene carbonate (FEC) additive. The assembled coin cells were tested on an Arbin battery test system with a voltage range of ˜0.01-3.0 V. For comparison, the free ZnMn2O4 samples were used as the active materials (denoted as control ZnMn2O4). Additionally, the free ZnMn2O4 samples were physically mixed with Super-P carbon in a weight ratio of 75:25 and used as the active materials (denoted as control ZnMn2O4+SP).
Results
Synthesis and Characterization of 2D Holey Transition Metal Oxide Nanosheets.
The general synthesis of the 2D holey transition metal oxide nanosheets via the template-directed self-assembly strategy is illustrated in
Post-calcination of the as-prepared ZnMn2O4 precursors/reduced graphene oxide at 400° C. in air induced the transformation of the amorphous precursors into crystalline ZnMn2O4 (
The reduced graphene oxide templates can play a role in the formation of the 2D holey ZnMn2O4 nanosheets. First, reduced graphene oxide is a 2D template with sufficient oxygen-containing groups to ensure the template-directed growth of ZnMn2O4 precursors on its surface. Second, unlike most conventional template processes, where weak interactions exist between the precursors and template, herein the ZnMn2O4 precursors can be anchored covalently on the reduced graphene oxide through residual functional groups, such as carboxyl, hydroxyl, and epoxy groups (Wang H et al. J. Am. Chem. Soc. 2010, 132, 13978-13980; Liang Y et al. Nat. Mater. 2011, 10, 780-786). Thanks to the strong coupling interaction between ZnMn2O4 and reduced graphene oxide, the flexible reduced graphene oxide template can accommodate the structure and volume changes of the ZnMn2O4 nanoparticles and ensure that the ZnMn2O4 partially agglomerated and linked together to form the holey nanosheets during the thermal treatment. When free ZnMn2O4 was synthesized via the same method, without any graphene oxide added, as a control experiment, only an aggregated flower-like structure of assembled spinel ZnMn2O4 discs was obtained (
It can be desirable to control the hole size in the prepared holey nanomaterials, since the hole size can affect the performance of the prepared holey nanomaterials (Ren Y et al. J. Am. Chem. Soc. 2009, 132, 996-1004. Largeot C et al. J. Am. Chem. Soc. 2008, 130, 2730-2731). The hole size of 2D holey ZnMn2O4 nanosheets prepared by the strategy discussed herein can be controlled via the annealing temperature during the post-calcination process. The 2D holey nanosheet structure can be maintained at higher annealing temperatures (
The strategy discussed herein was also used to prepare other 2D holey mixed transition metal oxide nanosheets.
2D Holey Transition Metal Oxide Nanosheets as Anodes for Lithium-Ion Battery.
Recently, transition metal oxides (especially mixed transition metal oxides) have been studied as anode materials for rechargeable lithium-ion batteries owing to their larger specific capacities than conventional graphite (Yuan C et al. Angew. Chem. Int. Ed. 2014, 53, 1488-1504; Xiong P et al. ACS Nano 2014, 8, 8610-8616). As mentioned above, the 2D holey nanostructures discussed herein can have both 2D nanostructure and porosity, which can result in enhanced performance compared to conventional nanosheets with smooth surfaces or porous microscale materials. As such, the 2D holey mixed transition metal oxide nanosheets discussed herein were tested as anodes for lithium-ion batteries.
To examine the specific capacity of the 2D holey nanosheet based anodes, the 2D holey ZnMn2O4 nanosheets were cycled at a current density of 800 mA g−1 for 50 cycles (after an initial 2 cycles for activation). A stable specific capacity of ca. 510 mA h g−1 (all specific capacities estimated based on mass of active materials) was observed after 50 cycles for the 2D holey nanosheet anode (
The rate performance of anodes based on 2D holey ZnMn2O4 nanostructures prepared at different temperatures was also examined (
The long-term cycling stability of the 2D holey ZnMn2O4 nanosheet based anodes was measured at a current rate of 1000 mA g−1 for 1000 charge/discharge cycles (
2D holey CoFe2O4, ZnCo2O4, and NiCo2O4 nanosheets were also examined as anodes for lithium storage. All these 2D holey nanosheet anodes displayed high cycling stability (
Interfaces 2013, 5,
Mater. 2007, 17, 2855-2861.
Interfaces 2013, 5, 981-988.
Sources 2014, 256, 153-159.
A 2014, 2, 149-154.
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A 2014, 2, 3741-3748.
A 2014, 2, 4449-4456.
A 2013, 1, 7444-7450.
2D Holey Transition Metal Oxide Nanosheets as Anodes for New-Generation Batteries Beyond Lithium-Ion.
Transition metal oxides have also been explored as electrode materials for beyond lithium-ion batteries, such as sodium-ion and lithium air batteries, owing to their large specific capacities and electrochemical activity (Jiang Y et al. Nano Energy 2014, 5, 60-66; Alcántara R et al. Chem. Mater. 2002, 14, 2847-2848; Chen L et al. J. Mater. Chem. A, 2015, 3, 3620-3626). The 2D holey transition metal oxide nanosheets discussed herein can have both 2D nanostructure and porosity, which can result in improved electrochemical performance compared to conventional nanosheets with smooth surfaces and porous microscale materials. As such, the 2D holey transition metal oxide nanosheets were tested as anodes for sodium-ion batteries.
To examine the cycling performance of the 2D holey nanosheet based anodes, the 2D holey Co3O4 nanosheets were cycled at a current density of 800 mA g−1 for 100 cycles. A stable specific capacity of ca. 200 mA h g−1 (all specific capacities estimated based on mass of active materials) was observed after 100 cycles for the 2D holey Co3O4 nanosheet anode (
The high rate capability and cycling stability observed for the 2D holey ZnMn2O4 nanosheets can be attributed to the combinative merits of both the 2D nanostructure and variable porosity. Firstly, the interconnected holes on the surfaces of 2D nanosheets can enable diffusion of liquid electrolyte into the electrode materials and can reduce the Li+ ion diffusion length (Ren Y et al. J. Am. Chem. Soc. 2009, 132, 996-1004; Fang Y et al. J. Am. Chem. Soc. 2013, 135, 1524-1530). The diffusion of Li+ ions through the nanoholes and the transport of electrons along the interconnected nanocrystals of the 2D holey nanosheets are shown schematically in
In conclusion, a general strategy employing graphene oxide as a sacrificial template to prepare various 2D holey transition metal oxide nanosheets, including mixed transition metal oxides and simple transition metal oxides, was discussed herein. These 2D holey nanosheets exhibited enhanced performance for lithium storage.
Other advantages which are obvious and which are inherent to the invention will be evident to one skilled in the art. It will be understood that certain features and sub-combinations are of utility and may be employed without reference to other features and sub-combinations. This is contemplated by and is within the scope of the claims. Since many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.
This application claims the benefit of priority to U.S. Provisional Application No. 62/121,245, filed Feb. 26, 2015, which is hereby incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62121245 | Feb 2015 | US |
