Dibakar Datta

Defect-induced plating of lithium metal within porous graphene networks

Lithium metal is known to possess a very high theoretical capacity of 3,842 mAh g(-1) in lithium batteries. However, the use of metallic lithium leads to extensive dendritic growth that poses serious safety hazards. Hence, lithium metal has long been replaced by layered lithium metal oxide and phospho-olivine cathodes that offer safer performance over extended cycling, although significantly compromising on the achievable capacities. Here we report the defect-induced plating of metallic lithium within the interior of a porous graphene network. The network acts as a caged entrapment for lithium metal that prevents dendritic growth, facilitating extended cycling of the electrode. The plating of lithium metal within the interior of the porous graphene structure results in very high specific capacities in excess of 850 mAh g(-1). Extended testing for over 1,000 charge/discharge cycles indicates excellent reversibility and coulombic efficiencies above 99%.

Defective Graphene As a High-Capacity Anode Material for Na- and Ca-Ion Batteries

Because of their abundance, sodium and calcium can be attractive in ion batteries for large-scale grid storage. However, many of the anode materials being pursued have limitations including volume expansion, lack of passivating films, and slow kinetics. Here, we investigate the adsorption of Na and Ca on graphene with divacancy and Stone-Wales defects in graphene. Our results show that although adsorption of Na and Ca is not possible on pristine graphene, enhanced adsorption is observed on defective graphene because of increased charge transfer between the adatoms and defects. We find that the capacity of graphene increases with the density of the defects. For the maximum possible divacancy defect densities, capacities of 1450 and 2900 mAh/g for Na- and Ca-ion batteries, respectively, can be achieved. For Stone-Wales defects, we find maximum capacities of 1071 and 2142 mAh/g for Na and Ca, respectively. Our results provide guidelines to create better high-capacity anode materials for Na- and Ca-ion batteries.

Aerosol synthesis of cargo-filled graphene nanosacks

Water microdroplets containing graphene oxide and a second solute are shown to spontaneously segregate into sack-cargo nanostructures upon drying. Analytical modeling and molecular dynamics suggest the sacks form when slow-diffusing graphene oxide preferentially accumulates and adsorbs at the receding air-water interface, followed by capillary collapse. Cargo-filled graphene nanosacks can be nanomanufactured by a simple, continuous, scalable process and are promising for many applications where nanoscale materials should be isolated from the environment or biological tissue.

Graphene-Based Environmental Barriers

Many environmental technologies rely on containment by engineered barriers that inhibit the release or transport of toxicants. Graphene is a new, atomically thin, two-dimensional sheet material, whose aspect ratio, chemical resistance, flexibility, and impermeability make it a promising candidate for inclusion in a next generation of engineered barriers. Here we show that ultrathin graphene oxide (GO) films can serve as effective barriers for both liquid and vapor permeants. First, GO deposition on porous substrates is shown to block convective flow at much lower mass loadings than other carbon nanomaterials, and can achieve hydraulic conductivities of 5 × 10(-12) cm/s or lower. Second we show that ultrathin GO films of only 20-nm thickness coated on polyethylene films reduce their vapor permeability by 90% using elemental mercury as a model vapor toxicant. The barrier performance of GO in this thin-film configuration is much better than the Nielsen model limit, which describes ideal behavior of flake-like fillers uniformly imbedded in a polymer. The Hg barrier performance of GO films is found to be sensitive to residual water in the films, which is consistent with molecular dynamics (MD) simulations that show lateral diffusion of Hg atoms in graphene interlayer spaces that have been expanded by hydration.

Mechanical properties of amorphous LixSi alloys: a reactive force field study

Silicon is a high-capacity anode material for lithium-ion batteries. Electrochemical cycling of Si electrodes usually produces amorphous LixSi (a-LixSi) alloys at room temperature. Despite intensive investigation of the electrochemical behaviors of a-LixSi alloys, their mechanical properties and underlying atomistic mechanisms remain largely unexplored. Here we perform molecular dynamics simulations to characterize the mechanical properties of a-LixSi with a newly developed reactive force field (ReaxFF). We compute the yield and fracture strengths of a-LixSi alloys under a variety of chemomechanical loading conditions, including the constrained thin-film lithiation, biaxial compression, uniaxial tension and compression. Effects of loading sequence and stress state are investigated to correlate the mechanical responses with the dominant atomic bonding, featuring a transition from the covalent to the metallic glass characteristics with increasing Li concentration. The results provide mechanistic insights for interpreting experiments, understanding properties and designing new experiments on aLixSi alloys, which are essential to the development of durable Si electrodes for high-performance lithium-ion batteries.

Plastic deformation drives wrinkling, saddling, and wedging of annular bilayer nanostructures.

We describe the spontaneous wrinkling, saddling, and wedging of metallic, annular bilayer nanostructures driven by grain coalescence in one of the layers. Experiments revealed these different outcomes based on the dimensions of the annuli, and we find that the essential features are captured using finite element simulations of the plastic deformation in the metal bilayers. Our results show that the dimensions and nanomechanics associated with the plastic deformation of planar nanostructures can be important in forming complex three-dimensional nanostructures.

Surface hydrogenation regulated wrinkling and torque capability of hydrogenated graphene annulus under circular shearing

Wrinkles as intrinsic topological feature have been expected to affect the electrical and mechanical properties of atomically thin graphene. Molecular dynamics simulations are adopted to investigate the wrinkling characteristics in hydrogenated graphene annulus under circular shearing at the inner edge. The amplitude of wrinkles induced by in-plane rotation around the inner edge is sensitive to hydrogenation, and increases quadratically with hydrogen coverage. The effect of hydrogenation on mechanical properties is investigated by calculating the torque capability of annular graphene with varying hydrogen coverage and inner radius. Hydrogenation-enhanced wrinkles cause the aggregation of carbon atoms towards the inner edge and contribute to the critical torque strength of annulus. Based on detailed stress distribution contours, a shear-to-tension conversion mechanism is proposed for the contribution of wrinkles on torque capacity. As a result, the graphane annulus anomalously has similar torque capacity to pristine graphene annulus. The competition between hydrogenation caused bond strength deterioration and wrinkling induced local stress state conversion leads to a U-shaped evolution of torque strength relative to the increase of hydrogen coverage from 0 to 100%. Such hydrogenation tailored topological and mechanical characteristics provides an innovative mean to develop novel graphene-based devices.

Effect of cobalt content on the electrochemical properties and structural stability of NCA type cathode materials

At present, the most common type of cathode materials, NCA (Li1-xNi0.80Co0.15Al0.05O2, x = 0 to 1), have a very high concentration of cobalt. Since cobalt is toxic and expensive, the existing design of cathode materials is neither cost-effective nor environmentally benign. We have performed density functional theory (DFT) calculations to investigate electrochemical, electronic, and structural properties of four types of NCA cathode materials with the simultaneous decrease in Co content along with the increase in Ni content. Our results show that even if the cobalt concentration is significantly decreased from 16.70% (NCA_I) to 4.20% (NCA_IV), variation in intercalation potential and specific capacity is not significant. For example, in the case of 50% Li concentration, the voltage drop is only ∼17% while the change in specific capacity is negligible. Moreover, we have also explored the influence of sodium doping in the intercalation site on the electrochemical, electronic, and structural properties. By considering two extreme cases of NCAs (i.e., with highest and lowest Co content: NCA_I and NCA_IV, respectively), we have demonstrated the importance of Na doping from the structural and electronic point of view. Our results provide insight into the design of environmentally benign, low-cost cathode materials with reduced cobalt concentration.

Amorphous germanium as a promising anode material for sodium ion batteries: a first principle study

The abundance of sodium (Na), its low-cost, and low reduction potential provide a lucrative inexpensive, safe, and environmentally benign alternative to lithium ion batteries (LIBs). The significant challenges in advancing sodium ion battery (NIB) technologies lie in finding the better electrode materials. Experimental investigations revealed the real potency of germanium (Ge) as suitable anode materials for NIBs. However, a systematic atomistic study is necessary to understand the fundamental aspects of capacity–voltage correlation, microstructural changes of Ge, as well as diffusion kinetics. We, therefore, performed the Density Functional Theory (DFT) and Ab Initio Molecular Dynamics (AIMD) simulation to investigate the sodiation–desodiation kinetics in germanium–sodium system (Na64Ge64). We analyzed the intercalation potential and capacity correlation for intermediate equilibrium structures and compared our data with the experimental results. Effect of sodiation on inter-atomic distances within Na–Ge system is analyzed by means of Pair Correlation Function (PCF). This provides insight into possible microstructural changes taking place during sodiation of amorphous Ge (a-Ge). We further investigated the diffusivity of sodium in a-Ge electrode material and analyzed the volume expansion trend for Na64Ge64 electrode system. Our computational results provide the fundamental insight into the atomic scale and help experimentalists design Ge-based NIBs for real-life applications.

Studies on the development of membrane-based plasmapheresis device

The present work deals with the indigeneous development and characterization of cellulose acetate membranes, in both flat sheet and hollow fibre forms, for plasmapheresis application. Using these membranes, blood filtration studies are carried out and the results are presented.