High-Purity Lithium Metal Films from Aqueous Mineral Solutions.

Lithium metal is a leading candidate for next-generation electrochemical energy storage and therefore a key material for the future sustainable energy economy. Lithium has a high specific energy, low toxicity, and relatively favorable abundance. The majority of lithium production originates from salt lakes and is based on long (>12 months) periods of evaporation to concentrate the lithium salt, followed by molten electrolysis. Purity requires separation from base metals (Na, K, Ca, Mg, etc.), which is a time-consuming, energy-intensive process, with little control over the microstructure. Here, we show how a membrane-mediated electrolytic cell can be used to produce lithium thin films (5-30 µm) on copper substrates at room temperature. Purity with respect to base metals content is extremely high. The cell design allows an aqueous solution to be a continuous feedstock, advocating a quick, low-energy-consumption, one-step-to-product process. The film morphology is controlled by varying the current densities in a narrow window (1-10 mA/cm2), to produce uniform nanorods, spheres, and cubes, with significant influence over the physical and electrochemical properties.

Solution-processed titanium carbide MXene films examined as highly transparent conductors.

MXenes, a new family of two-dimensional structures, have recently gained significant attention due to their unique physical properties suitable for a wide range of potential applications. Here we introduce Ti3C2Tx delaminated monolayers as ultrathin transparent conductors with properties exceeding comparable reduced graphene oxide films. Solution processed Ti3C2Tx films exhibit sheet resistances as low as 437 Ω sq-1 with 77% transmittance at 550 nm. Field effect transistor measurements confirm that these films have a metallic nature, which makes them suitable as electrodes. We show using Kelvin Probe Atomic Force Microscopy that the work function of delaminated Ti3C2Tx flakes (with OH terminal groups) is 5.28 ± 0.03 eV. These results demonstrate that solution-processed Ti3C2Tx conducting films could open up a new direction for the next generation of transparent conductive electrodes.

Anodized Ti3SiC2 As an Anode Material for Li-ion Microbatteries.

We report on the synthesis of an anode material for Li-ion batteries by anodization of a common MAX phase, Ti3SiC2, in an aqueous electrolyte containing hydrofluoric acid (HF). The anodization led to the formation of a porous film containing anatase, a small quantity of free carbon, and silica. By varying the anodization parameters, various oxide morphologies were produced. The highest areal capacity was achieved by anodization at 60 V in an aqueous electrolyte containing 0.1 v/v HF for 3 h at room temperature. After 140 cycles performed at multiple applied current densities, an areal capacity of 380 µAh·cm(-2) (200 µA·cm(-2)) has been obtained, making this new material, free of additives and binders, a promising candidate as a negative electrode for Li-ion microbatteries.

Amine-Assisted Delamination of Nb2C MXene for Li-Ion Energy Storage Devices.

2D Nb2CTx MXene flakes are produced using an amine-assisted delamination process. Upon mixing with carbon nanotubes and filtration, freestanding, flexible paper is produced. The latter exhibits high capacity and excellent stability when used as the electrode for Li-ion batteries and capacitors.

Synthesis of carbon/sulfur nanolaminates by electrochemical extraction of titanium from Ti2SC.

Herein we electrochemically and selectively extract Ti from the MAX phase Ti2SC to form carbon/sulfur (C/S) nanolaminates at room temperature. The products are composed of multi-layers of C/S flakes, with predominantly amorphous and some graphene-like structures. Covalent bonding between C and S is observed in the nanolaminates, which render the latter promising candidates as electrode materials for Li-S batteries. We also show that it is possible to extract Ti from other MAX phases, such as Ti3AlC2, Ti3SnC2, and Ti2GeC, suggesting that electrochemical etching can be a powerful method to selectively extract the "M" elements from the MAX phases, to produce "AX" layered structures, that cannot be made otherwise. The latter hold promise for a variety of applications, such as energy storage, catalysis, etc.

One-step synthesis of nanocrystalline transition metal oxides on thin sheets of disordered graphitic carbon by oxidation of MXenes.

Herein we show that heating 2D Ti3C2 in air results in TiO2 nanocrystals enmeshed in thin sheets of disordered graphitic carbon structures that can handle extremely high cycling rates when tested as anodes in lithium ion batteries. Oxidation of 2D Ti3C2 in either CO2 or pressurized water also resulted in TiO2-C hybrid structures. Similarly, other hybrids can be produced, as we show here for Nb2O5/C from 2D Nb2C.

Structure of nanocrystalline Ti3C2 MXene using atomic pair distribution function.

The structures of nanocrystalline pristine, potassium hydroxide and sodium acetate intercalated new two-dimensional materials Ti3C2 MXenes were studied using the x-ray atomic pair distribution function technique. Pristine MXene has a hexagonal structure with a=b=3.0505(5) Å, c=19.86(2) Å (S.G. P63/mmc No. 194). Both hydroxyl and fluoride terminating species are present. The intercalation of K+ or Na+ ions expands the Ti3C2 layers perpendicular to the planes but shrinks the in-plane a and b lattice parameters.

Carbon pipette-based electrochemical nanosampler.

Sampling ultrasmall volumes of liquids for analysis is essential in a number of fields from cell biology to microfluidics to nanotechnology and electrochemical energy storage. In this article, we demonstrate the possibility of using nanometer-sized quartz pipettes with a layer of carbon deposited on the inner wall for sampling attoliter-to-picoliter volumes of fluids and determining redox species by voltammetry and coulometry. Very fast mass-transport inside the carbon-coated nanocavity allows for rapid exhaustive electrolysis of the sampled material. By using a carbon pipette as the tip in the scanning electrochemical microscope (SECM), it can be precisely positioned at the sampling location. The developed device is potentially useful for solution sampling from biological cells, micropores, and other microscopic objects.

Cation intercalation and high volumetric capacitance of two-dimensional titanium carbide.

The intercalation of ions into layered compounds has long been exploited in energy storage devices such as batteries and electrochemical capacitors. However, few host materials are known for ions much larger than lithium. We demonstrate the spontaneous intercalation of cations from aqueous salt solutions between two-dimensional (2D) Ti3C2 MXene layers. MXenes combine 2D conductive carbide layers with a hydrophilic, primarily hydroxyl-terminated surface. A variety of cations, including Na(+), K(+), NH4(+), Mg(2+), and Al(3+), can also be intercalated electrochemically, offering capacitance in excess of 300 farads per cubic centimeter (much higher than that of porous carbons). This study provides a basis for exploring a large family of 2D carbides and carbonitrides in electrochemical energy storage applications using single- and multivalent ions.

Intercalation and delamination of layered carbides and carbonitrides.

Intercalation and delamination of two-dimensional solids in many cases is a requisite step for exploiting their unique properties. Herein we report on the intercalation of two-dimensional Ti3C2, Ti3CN and TiNbC-so called MXenes. Intercalation of hydrazine, and its co-intercalation with N,N-dimethylformamide, resulted in increases of the c-lattice parameters of surface functionalized f-Ti3C2, from 19.5 to 25.48 and 26.8 Å, respectively. Urea is also intercalated into f-Ti3C2. Molecular dynamics simulations suggest that a hydrazine monolayer intercalates between f-Ti3C2 layers. Hydrazine is also intercalated into f-Ti3CN and f-TiNbC. When dimethyl sulphoxide is intercalated into f-Ti3C2, followed by sonication in water, the f-Ti3C2 is delaminated forming a stable colloidal solution that is in turn filtered to produce MXene 'paper'. The latter shows excellent Li-ion capacity at extremely high charging rates.

Two-dimensional transition metal carbides.

Herein we report on the synthesis of two-dimensional transition metal carbides and carbonitrides by immersing select MAX phase powders in hydrofluoric acid, HF. The MAX phases represent a large (>60 members) family of ternary, layered, machinable transition metal carbides, nitrides, and carbonitrides. Herein we present evidence for the exfoliation of the following MAX phases: Ti(2)AlC, Ta(4)AlC(3), (Ti(0.5),Nb(0.5))(2)AlC, (V(0.5),Cr(0.5))(3)AlC(2), and Ti(3)AlCN by the simple immersion of their powders, at room temperature, in HF of varying concentrations for times varying between 10 and 72 h followed by sonication. The removal of the "A" group layer from the MAX phases results in 2-D layers that we are labeling MXenes to denote the loss of the A element and emphasize their structural similarities with graphene. The sheet resistances of the MXenes were found to be comparable to multilayer graphene. Contact angle measurements with water on pressed MXene surfaces showed hydrophilic behavior.