Swastika Banerjee

Possible application of 2D-boron sheets as anode material in lithium ion battery: A DFT and AIMD study

Density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations have been employed to investigate the possibility of 2D boron sheets (BSs) as an anode material in lithium ion batteries (LIBs). Among α, α1 and η4/28 metallic BSs, planarity is retained for the α1 and η4/28 polymorphs after the formation of the layered structure. The optimum anodic nature of the α1 and α1-AA polymorphs has been suggested based on their electronic, structural and Li adsorption/desorption studies. The highly symmetric ‘H’ site is energetically favored for Li adsorption at both 0 and 298 K. Li migration occurs from one ‘H’ site to another via the top of a boron atom, with a 0.66 and 0.39 eV energy barrier at 0 and 298 K respectively. An increase in the lithium concentration, up to a 50% coverage of ‘H’ sites, decreases the diffusion barrier gradually and reaches the saturation point at 0.59 eV (at 0 K). The lithium saturation requires eight lithium atoms per 1.63 nm2 surface area of the α1 sheet, when all ‘H’ sites become occupied. This confers the theoretical estimate of the capacity as 383 mA h g−1, which is higher than that of the conventional graphitic electrode. Finally, the structural stability at the lithium saturation point is confirmed by increasing the number of layers up to four. All of these characteristics suggest the appropriateness of α1-AA as an anode material for LIBs.

Origin of the Order–Disorder Transition and the Associated Anomalous Change of Thermopower in AgBiS2 Nanocrystals: A Combined Experimental and Theoretical Study

Bulk AgBiS2 crystallizes in a trigonal crystal structure (space group, P3̅m1) at room temperature, which transforms to a cation disordered rock salt structure (space group, Fm3̅m) at ∼473 K. Surprisingly, at room temperature, a solution-grown nanocrystal of AgBiS2 crystallizes in a metastable Ag/Bi ordered cubic structure, which transforms to a thermodynamically stable disorded cubic structure at 610 K. Moreover, the order-disorder transition in nanocrystalline AgBiS2 is associated with an unusual change in thermopower. Here, we shed light on the origin of a order-disorder phase transition and the associated anomalous change of thermopower in AgBiS2 nanocrystals by using a combined experimental, density functional theory based first-principles calculation and ab initio molecular dynamics simulations. Positron-annilation spectroscopy indicates the presence of higher numbers of Ag vacancies in the nanocrystal compared to that of the bulk cubic counterpart at room temperature. Furthermore, temperature-dependent two-detector coincidence Doppler broadening spectroscopy and Doppler broadening of the annihilation radiation (S parameter) indicate that the Ag vacancy concentration increases abruptly during the order-disorder transition in nanocrystalline AgBiS2. At high temperature, a Ag atom shuttles between the vacancy and interstitial sites to form a locally disordered cation sublattice in the nanocrystal, which is facilitated by the formation of more Ag vacancies during the phase transition. This process increases the entropy of the system at higher vacancy concentration, which, in turn, results in the unusual rise in thermopower.

First-principles design of a borocarbonitride-based anode for superior performance in sodium-ion batteries and capacitors

Three fundamental challenges for the development of technologically relevant sodium-ion batteries (SIB) and sodium-ion capacitors (SIC) are the lower cell voltage, decreased ionic-diffusivity and larger volume of sodium-ions relative to their lithium-ion analogues. Using first-principles computation, we show that two-dimensional BxCyNz with nitrogen-excess trigonal BxNz-domain (TN) meets the requirements of a superior anode for SIB. Variation in the shape of the BxNz-domain and B–N charge-imbalance in BxCyNz results in tunable anodic properties. Monolayer TN-sheet can store Na(Li) up to Na2.2C6(Li1.8T6) composition, which corresponds to a specific capacity as high as 810(668) mA h g−1 for SIB(LIB). The average open circuit voltage is ∼1.25 V vs. Na/Na+ for a wide range of chemical stoichiometries of NaxTN, which is also beneficial to the overall cell-voltage. The enhanced electronic transport and fast diffusion kinetics of the Na-ions is particular for the TN-anode, which can result in high power efficiency in SIB, even better than that of graphite electrode in conventional LIB. Charge-storage upon layer-wise accumulation of Na-ions on the TN surface is also appealing for application to sodium-ion capacitors, as an alternative to lithium-ion capacitors. These features are in contrast to conventional layered materials, where the voltage drops quickly as Na-ions are removed from the matrix. Hence, this article may serve as a guide for designing borocarbonitride electrodes for SIB(SIC) with controlled experimental behaviour.

Synthetically tuned structural variations in CePdxGe2−x (x = 0.21, 0.32, 0.69) towards diverse physical properties

In this work, we have studied the structure and physical properties of a series of intermetallic compounds with the general formula CePdxGe2−x (where, x = 0.21, 0.32, 0.69). It was found that the compound crystallizes in three different phases with stoichiometries: CePd0.32Ge1.68, CePd0.21Ge1.79 and CePd0.69Ge1.31 by varying the Pd to Ge ratio. While CePd0.32Ge1.68 and CePd0.69Ge1.31 crystallize in the hexagonal AlB2 structure type with the space group P6/mmm, CePd0.21Ge1.79 crystallizes in the tetragonal α-ThSi2 structure type with the space group I41/amd. CePd0.69Ge1.31 is in fact an ordered superstructure of CePd0.32Ge1.68 with tripling of the c-lattice. Relative changes in the Pd/Ge ratio also impart substantial variation in their magnetic properties, although Ce is in the trivalent state in both the phases. CePd0.21Ge1.79 shows metamagnetic behavior below 10 K whereas CePd0.69Ge1.31 showed ferromagnetic behavior in the same temperature range. In addition to the metamagnetic behavior, CePd0.21Ge1.79 also shows spin glass behavior at low temperature. DFT calculations were used to obtain ulterior information on the CePd0.69Ge1.31 phase. Self-consistent calculations revealed that the ferromagnetic ordering of the ground state arises from the spins at the Ce and Pd sites. The observed sharp rise in the low temperature resistivity of CePd0.69Ge1.31 is an indication of a pseudo-gap formation or possible Kondo behavior in the electronic density of states, enhancing the scattering of the charge carriers. Heat capacity measurements on CePd0.69Ge1.31 suggest that it falls in the category of medium heavy fermion compounds.

Eu3Ir2In15: A Mixed-Valent and Vacancy-Filled Variant of the Sc5Co4Si10 Structure Type with Anomalous Magnetic Properties

A new compound, Eu3Ir2In15, has been synthesized using indium as an active metal flux. The compound crystallizes in the tetragonal P4/mbm space group with lattice parameters a = 14.8580(4) Å, b = 14.8580(4) Å, and c = 4.3901(2) Å. It was further characterized by SEM-EDX studies. The effective magnetic moment (μeff) of this compound is 7.35 μB/Eu ion with a paramagnetic Curie temperature (θp) of -28 K, suggesting antiferromagnetic interaction. The mixed-valent nature of Eu observed in magnetic measurements was confirmed by XANES measurements. The compound undergoes demagnetization at a low magnetic field (10 Oe), which is quite unusual for Eu-based intermetallic compounds. Temperature-dependent resistivity studies reveal that the compound is metallic in nature. A comparative study was made between Eu3Ir2In15 and hypothetical vacancy-variant Eu5Ir4In10, which also crystallizes in the same crystal structure. However, our computational studies along with control experiments suggest that the latter is thermodynamically less feasible compared to the former, and hence we propose that it is highly unlikely that an RE5T4X10 would exist with X as a group 13 element.