Reversible M-M Bonding by Alkaline Earth Metals (Mg, Ca, Sr, Ba) in Graphite Intercalation Compounds.

The alkaline earth metals (M=Mg, Ca, Sr, and Ba) exhibit a +2 oxidation state in nearly all known stable compounds, but MI dimeric complexes with M-M bonding, [M2 (en)2 ]2+ , (en=ethylenediamine) of all these metals can be stabilized within the galleries of donor-type graphite intercalation compounds (GICs). These metals can also form GICs with more conventional metal (II) ion complexes, [M(en)2 ]2+ . Here, the facile interconversion between dimeric-MI and monomeric-MII intercalates upon the addition/removal of en are reported. Thermogravimetry, powder X-ray diffraction, and pair distribution function analysis of total scattering data support the presence of either [M2 (en)2 ]2+ or [M(en)2 ]2+ guests. This phase conversion requires coupling graphene and metal redox centers, with associated reversible M-M bond formation within graphene galleries. This chemistry allows the facile isolation of unusual oxidation states, reveals M0 →M2+ reaction pathways, and present new opportunities in the design of hybrid conversion/intercalation materials for applications such as charge storage.

Reversible Insertion of Mg-Cl Superhalides in Graphite as a Cathode for Aqueous Dual-Ion Batteries.

Oxidative anion insertion into graphite in an aqueous environment represents a significant challenge in the construction of aqueous dual-ion batteries. In dilute aqueous electrolytes, the oxygen evolution reaction (OER) dominates the anodic current before anions can be inserted into the graphite gallery. Herein, we report that the reversible insertion of Mg-Cl superhalides in graphite delivers a record-high reversible capacity of 150 mAh g-1 from an aqueous deep eutectic solvent comprising magnesium chloride and choline chloride. The insertion of Mg-Cl superhalides in graphite does not form staged graphite intercalation compounds; instead, the insertion of Mg-Cl superhalides makes the graphite partially turbostratic.

Graphite Intercalation by Mg Diamine Complexes.

The first structural and compositional details of a low-stage graphite interaction compound (GIC) containing Mg are reported, with the GIC obtained by combining magnesium metal and graphite powder in ethylenediamine (en) at 100 °C under an inert atmosphere. Thermal analyses indicate the bottle-green stage 1 product has a composition of [Mg(en)1.0]C13. X-ray diffraction shows a c-axis expansion of 0.55 nm, indicating the presence of intercalate monolayers with the en cointercalate oriented perpendicular to the encasing graphene layers. Redox titration indicates two electrons are transferred per Mg. A structural model is proposed with dimeric [Mg2(en)2]2+ intercalate species.

Pillared graphite anodes for reversible sodiation.

There has been a major effort recently to develop new rechargeable sodium-ion electrodes. In lithium ion batteries, LiC6 forms from graphite and desolvated Li cations during the first charge. With sodium ions, graphite only shows a significant capacity when Na+ intercalates as a solvated complex, resulting in ternary graphite intercalation compounds (GICs). Although this chemistry has been shown to be highly reversible and to support high rates in small test cells, these GICs can require >250% volume expansion and contraction during cycling. Here we demonstrate the first example of GICs that reversibly sodiate/desodiate without any significant volume change. These pillared GICs are obtained by electrochemical reduction of graphite in an ether/amine co-solvent electrolyte. The initial gallery expansion, 0.36 nm, is less than half of that in diglyme-based systems, and shows a similar capacity. Thermal analyses suggest the pillaring phenomenon arises from stronger co-intercalate interactions in the GIC galleries.

Mg-Ion Battery Electrode: An Organic Solid's Herringbone Structure Squeezed upon Mg-Ion Insertion.

We report that crystalline 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), an organic solid, is highly amenable to host divalent metal ions, i.e., Mg2+ and Ca2+, in aqueous electrolytes, where the van der Waals structure is intrinsically superior in hosting charge-dense ions. We observe that the divalent nature of Mg2+ causes unique squeezing deformation of the electrode structure, where it contracts and expands in different crystallographic directions when hosting the inserted Mg-ions. This phenomenon is revealed experimentally by ex situ X-ray diffraction and transmission electron microscopy, and is investigated theoretically by first-principles calculations. Interestingly, hosting one Mg2+ ion requires the coordination from three PTCDA molecules in adjacent columns of stacked molecules, which rotates the columns, thus reducing the (011) spacing but increasing the (021) spacing. We demonstrate that a PTCDA Mg-ion electrode delivers a reversible capacity of 125 mA h g-1, which may include a minor contribution of hydronium storage, a good rate capability by retaining 75 mA h g-1 at 500 mA g-1 (or 3.7 C), and a stable cycle life. We also report Ca2+ storage in PTCDA, where a reversible capacity of over 80 mA h g-1 is delivered.

Preparation of Graphite Intercalation Compounds Containing Crown Ethers.

Crown ethers are well established as cointercalates in many layered hosts, but there are no reports of crown ethers incorporated into graphite. Here, we describe the preparation of the first graphite intercalation compounds (GICs) containing crown ethers. These GICs are obtained either by reductive intercalation of an alkali metal-amine complex followed by cointercalate exchange or by the direct reaction of graphite with a crown ether, alkali metal, and an electrocatalyst. Structural and compositional characterization of these new GICs using powder X-ray diffraction, thermal analysis, and GC/MS indicates the formation of well-ordered, stage-1 bilayer galleries.

Preparation of a homologous series of tetraalkylammonium graphite intercalation compounds.

Graphite intercalation compounds (GICs) of a series of symmetric or asymmetric tetraalkylammonium (TAA) intercalates are obtained from stage-1 [Na(en)1.0]C15 via cation exchange. The prepared dull-black TAA-GICs contain either flattened monolayer or bilayer galleries, with significant cointercalation of the dimethylsulfoxide (DMSO) solvent in the bilayer galleries. The TAA-GIC products obtained are characterized by X-ray diffraction and related structural modeling, compositional analyses, and Raman spectroscopy. [(C4H9)4N]C43 is obtained as a pure stage-1 GIC with the flattened monolayer structure. The larger symmetric TAA cations, (C6H13)4N, (C7H15)4N, (C8H17)4N, and the asymmetric TAA cations, (C12H25)(CH3)3N, (C18H37)(CH3)3N, (C18H37)2(CH3)2N, all form pure stage-1 GICs with flattened bilayer conformations. Thermogravimetric analyses combined with mass spectrometry and elemental analyses indicate the presence of ∼1-2 DMSO cointercalates per bilayer cation. The intercalate layers in these TAA-GICs have expansions along the stacking direction of ∼0.40 nm. Raman data confirm the low graphene sheet charge densities in the obtained TAA-GICs.

Synthesis and characterization of low-generation polyamidoamine (PAMAM) dendrimer-sodium montmorillonite (Na-MMT) clay nanocomposites.

Polymer-inorganic nanocomposites are a recently developed class of materials that have altered physical or chemical properties with respect to the pure polymer, inorganic host, or their micro- and macrocomposites. Lower generation (G0.0-2.0) polyamidoamine (PAMAM) dendrimer/sodium montmorillonite (Na-MMT) nanocomposites were synthesized in a solution-phase exfoliation adsorption reaction. These are the first reports of the G0.0/ and G1.0/Na-MMT nanocomposites and of a structurally-ordered G2.0/Na-MMT. The materials were characterized using powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), and Fourier transform infrared spectroscopy (FTIR). PAMAM characteristics at acidic and basic aqueous media were studied using capillary zone electrophoresis (CZE). Pseudospherical PAMAM dendrimers in aqueous medium attain a highly flattened conformation within the confined space between MMT sheets upon nanocomposite formation. The nanocomposite structure depends on the PAMAM generation and the starting dendrimer/organic composition. G0.0 always forms monolayer structures (d = 0.42 nm), while G2.0 forms monolayer structure, mixed phase, and bilayer structures (d = 0.84 nm) at lower, intermediate, and higher organic content, respectively, showing an interesting monolayer to bilayer transition. G1.0 showed an intermediate behavior, with monolayer to mixed-phase transition at the reactant ratios studied. This monolayer arrangement of PAMAM/clay nanocomposites is reported for the first time. Maximum organic contents of G0.0 monolayer and G2.0 bilayer nanocomposites were ∼7% and ∼14%, respectively. Gallery expansions were similar to those observed with linear polymer intercalates, but the packing fractions (0.31-0.32) were 2-3 times lower. At acidic pH, the nanocomposites forming only monolayer structures are obtained, indicating a stronger electrostatic attraction between MMT and protonated PAMAM, and these nanocomposites formed more slowly and were more ordered. Na(+) ions play a significant role in nanocomposite formation. At high pH, PAMAMs show high mobility, ζ potential, and surface charge densities due to Na(+) complexation in solution. FTIR data indicates that both Na-MMT and PAMAM structural units are preserved in the nanocomposites obtained.

Preparation and characterization of a tetrabutylammonium graphite intercalation compound.

The intercalation of tetrabutylammonium (TBA) cations into graphite by cation exchange from a sodium-ethylenediamine graphite intercalation compound yields a single-phase first-stage product, C(44)TBA, with a gallery expansion of 0.47 nm. The gallery dimension requires an anisotropic "flattened" cation conformation.

Synthesis of ternary and quaternary graphite intercalation compounds containing alkali metal cations and diamines.

A series of ternary graphite intercalation compounds (GICs) of alkali metal cations (M = Li, Na, K) and diamines [EN (ethylenediamine), 12DAP (1,2-diaminopropane), and DMEDA (N,N-dimethylethylenediamine)] are reported. These include stage 1 and 2 M-EN-GIC (M = Li, d(i) = 0.68-0.84 nm; M = Na, d(i) = 0.68 nm), stage 2 Li-12DAP-GIC (d(i) = 0.83 nm), and stage 1 and 2 Li-DMEDA-GIC (d(i) = 0.91 nm), where d(i) is the gallery height. For M = Li, a perpendicular-to-parallel transition of EN is observed upon evacuation, whereas for M = Na, the EN remains in parallel orientation. Li-12DAP-GIC and Li-DMEDA-GIC contain chelated Li(+) and do not show the perpendicular-to-parallel transition. We also report the quaternary compounds of mixed cations (Li,Na)-12DAP-GIC and mixed amines Na-(EN,12DAP)-GIC, with d(i) values in both cases between those of the ternary end members. (Li,Na)-12DAP-GIC is a solid solution with lattice dimensions dependent on composition, whereas for Na-(EN,12DAP)-GIC, the lattice dimension does not vary with amine content.

Derivitization of pristine graphene with well-defined chemical functionalities.

Covalent functionalization of pristine graphene poses considerable challenges due to the lack of reactive functional groups. Herein, we report a simple and general method to covalently functionalize pristine graphene with well-defined chemical functionalities. It is a solution-based process where solvent-exfoliated graphene was treated with perfluorophenylazide (PFPA) by photochemical or thermal activation. Graphene with well-defined functionalities was synthesized, and the resulting materials were soluble in organic solvents or water depending on the nature of the functional group on PFPA.

Preparation of graphite intercalation compounds containing oligo and polyethers.

Layered host-polymer nanocomposites comprising polymeric guests between inorganic sheets have been prepared with many inorganic hosts, but there is limited evidence for the incorporation of polymeric guests into graphite. Here we report for the first time the preparation, and structural and compositional characterization of graphite intercalation compounds (GICs) containing polyether bilayers. The new GICs are obtained by either (1) reductive intercalation of graphite with an alkali metal in the presence of an oligo or polyether and an electrocatalyst, or (2) co-intercalate exchange of an amine for an oligo or polyether in a donor-type GIC. Structural characterization of products using powder X-ray diffraction, Raman spectroscopy, and thermal analyses supports the formation of well-ordered, first-stage GICs containing alkali metal cations and oligo or polyether bilayers between reduced graphene sheets.

Electrochemically Expandable Soft Carbon as Anodes for Na-Ion Batteries.

Na-ion batteries (NIBs) have attracted great attention for scalable electrical energy storage considering the abundance and wide availability of Na resources. However, it remains elusive whether carbon anodes can achieve the similar scale of successes in Na-ion batteries as in Li-ion batteries. Currently, much attention is focused on hard carbon while soft carbon is generally considered a poor choice. In this study, we discover that soft carbon can be a high-rate anode in NIBs if the preparation conditions are carefully chosen. Furthermore, we discover that the turbostratic lattice of soft carbon is electrochemically expandable, where d-spacing rises from 3.6 to 4.2 Å. Such a scale of lattice expansion only due to the Na-ion insertion was not known for carbon materials. It is further learned that portions of such lattice expansion are highly reversible, resulting in excellent cycling performance. Moreover, soft carbon delivers a good capacity at potentials above 0.2 V, which enables an intrinsically dendrite-free anode for NIBs.

Efficient fabrication of nanoporous si and Si/Ge enabled by a heat scavenger in magnesiothermic reactions.

Magnesiothermic reduction can directly convert SiO2 into Si nanostructures. Despite intense efforts, efficient fabrication of highly nanoporous silicon by Mg still remains a significant challenge due to the exothermic reaction nature. By employing table salt (NaCl) as a heat scavenger for the magnesiothermic reduction, we demonstrate an effective route to convert diatom (SiO2) and SiO2/GeO2 into nanoporous Si and Si/Ge composite, respectively. Fusion of NaCl during the reaction consumes a large amount of heat that otherwise collapses the nano-porosity of products and agglomerates silicon domains into large crystals. Our methodology is potentially competitive for a practical production of nanoporous Si-based materials.

A simple and scalable route to wafer-size patterned graphene.

Producing large-scale graphene films with controllable patterns is an essential component of graphene-based nanodevice fabrication. Current methods of graphene pattern preparation involve either high cost, low throughput patterning processes or sophisticated instruments, hindering their large-scale fabrication and practical applications. We report a simple, effective, and reproducible approach for patterning graphene films with controllable feature sizes and shapes. The patterns were generated using a versatile photocoupling chemistry. Features from micrometres to centimetres were fabricated using a conventional photolithography process. This method is simple, general, and applicable to a wide range of substrates including silicon wafers, glass slides, and metal films.