Arup Chakraborty

Rechargeable magnesium batteries (RMBs) are a promising alternative to lithium‐ion batteries because of the high capacity and crustal abundance of magnesium. However, RMBs are primarily plagued by low‐energy‐density of traditional cathode materials, which utilize transition‐metal cationic redox chemistry. Herein, a new family of layered Ti _{1‐ x} Fe _x S ₂ (0 < x < 1) materials is proposed to assess anionic redox and boost the energy density of RMBs. The structural stability and redox activity are systematically investigated using first‐principles calculations and operando experimental techniques. It is found that layered Mg _n Ti _{1‐ x} Fe _x S ₂ materials are stable below x = 0.5, forming the solid‐solution sulfides. The Mg _n Ti _0.75 Fe _0.25 S ₂ sample is experimentally shown to offer a high operating voltage of 1.8 V, a competitive energy density of 103 mAh g ⁻¹ , and better cycling stability compared to prototype TiS ₂ material. Both cations (Ti and Fe) and anion (S) are revealed to take part in the redox reaction. The new family of layered Ti _{1‐ x} Fe _x S ₂ cathodes with joint cationic and anionic redox chemistry proposed in this work are promising high‐energy cathode candidates for RMBs.

Layered oxide cathodes for Na-ion batteries containing Mn or Fe are of considerable interest for sustainable energy storage applications largely due to cost issues, relative abundance and high capacities.

Lithiated oxides like Li[Ni _x Co _y Mn _z ]O ₂ ( x + y + z = 1) with high nickel content ( x ≥ 0.8) can possess high specific capacity ≥200 mA h g ⁻¹ and have attracted extensive attention as perspective cathode materials for advanced lithium-ion batteries.

Semiconductor coupled quantum dots provide a unique opportunity for tuning band gaps by tailoring band offsets, making them ideal for photovoltaic and other applications. Here, we have studied stability, trends in the band gap, band offsets, and optical properties for a series of coupled quantum dots comprised of II–VI semiconductors using a hybrid functional method. We have shown how the quantum confinement and interfacial strain considerably affect the band gap and band offsets for these heterostructures at the nanoscale. We show that the trend in band offsets obtained from our first‐principles electronic structure calculations agrees with that obtained from the method of average electrostatic potential. It is found that a common anion rule for band offset is followed for these heterostructures at the nanoscale. Further, the calculated optical absorption spectra for these coupled quantum dots reveal that absorption peaks lie in the ultra‐violet (UV) region, whereas absorption edges are in the visible region. In addition to electronic and optical properties, we have also explored transport properties for two representative coupled quantum dots, either having common cations or common anions, which revealed asymmetric nature in current–voltage characteristics. Therefore, these semiconductor coupled quantum dots may be useful for photovoltaic, light‐emitting diode, and optoelectronic devices.

In this work, we continued our systematic investigations on synthesis, structural studies, and electrochemical behavior of Ni-rich materials Li[Ni_xCo_yMn_z]O₂ (x + y + z = 1; x ≥ 0.8) for advanced lithium-ion batteries (LIBs). We focused, herein, on LiNi_0.85Co_0.10Mn_0.05O₂ (NCM85) and demonstrated that doping this material with high-charge cation Mo⁶⁺ (1 at. %, by a minor nickel substitution) results in substantially stable cycling performance, increased rate capability, lowering of the voltage hysteresis, and impedance in Li-cells with EC-EMC/LiPF₆ solutions. Incorporation of Mo-dopant into the NCM85 structure was carried out by in-situ approach, upon the synthesis using ammonium molybdate as the precursor. From X-ray diffraction studies and based on our previous investigation of Mo-doped NCM523 and Ni-rich NCM811 materials, it was revealed that Mo⁶⁺ preferably substitutes Ni residing either in 3a or 3b sites. We correlated the improved behavior of the doped NCM85 electrode materials in Li-cells with a partial Mo segregation at the surface and at the grain boundaries, a tendency established previously in our lab for the other members of the Li[Ni_xCo_yMn_z]O₂ family.

Electronic structure and transport characteristics of coupled CdS and ZnSe quantum dots are studied using density functional theory and non equilibrium Greens function method respectively. Our investigations show that in these novel coupled dots, the Frontier occupied and unoccupied molecular orbitals are spatially located in two different parts, thereby indicating the possibility of asymmetry in electronic transport. We have calculated electronic transport through the coupled quantum dot by varying the coupling strength between the individual quantum dots in the limits of weak and strong coupling. Calculations reveal asymmetric current vs voltage curves in both the limits indicating the rectifying properties of the coupled quantum dots. Additionally we discuss the possibility to tune the switching behavior of the coupled dots by different gate geometries.

Li transition metal olivine phosphates are proposed as good cathode materials and electrocatalysts for OER. This first principles study provides new insights into the catalytic performance of olivine phosphates with mixed alkali, mixed transition metal, and Li vacancies.

We have studied a Fe-based di-nuclear molecular complex having the chemical formula [{Fe(bpp)(NCS) ₂ } ₂ ( ’-bipy)]·2MeOH (where bpp = -bis(pyrazol-3-yl) pyridine and ’-bipy = ’-bipyridine, 1) using density functional theory and model Hamiltonian approach. Our study provides insight to the pressure driven spin-crossover (SCO) phenomena observed experimentally in these systems. Upon increasing the pressure, the spin state of Fe(II) cation gradually changes from a high spin state (S =2) to a low spin (LS) state (S =0) accompanied by volume contraction. The gradual increase in pressure shrinks Fe–N bond length and also causes angular deviation of the FeN ₆ octahedron leading to full conversion to the LS state without global structural phase transition. We have carried out exact diagonalization study of an effective single site Hamiltonian and confirmed the importance of intramolecular interaction for SCO phenomena. We have investigated the cooperativity of the observed SCO phenomena. We have also studied the effect of Co doping on the spin state of Fe and find that the spin state of Fe has a subtle dependency on the concentration of dopant atoms. Excess Co doping pave the way towards the possibility of an intermediate spin state for Fe and can give rise to a bistable spin transition process.