Sourindra Mahanty

MoS2–MWCNT hybrids as a superior anode in lithium-ion batteries

Molybdenum disulfide (MoS(2))-multiwalled carbon nanotube (MWCNT) hybrids have been prepared by simple dry grinding. Excellent initial charge capacity (1214 mA h g(-1)) and ~85% retention after 60 discharge-charge cycles at different current densities (100-500 mA g(-1)) make MoS(2)-MWCNT (1 : 1) hybrids a superior anode in Li-ion batteries.

Reversible Lithium Storage in Manganese 1,3,5-Benzenetricarboxylate Metal–Organic Framework with High Capacity and Rate Performance

Metal organic frameworks (MOFs) with diverse structural chemistry are being projected as futuristic electrode materials for Li-ion batteries. In this work, we report synthesis of Mn-1,3,5-benzenetricarboxylate MOF by a simple solvothermal method and its application as an anode material for the first time. Scanning electron microscopy of the synthesized MOF shows a bar shaped morphology where these bars, about 1 μm wide and of varied lengths between 2 and 20 μm, are made of porous sheets containing mesoporous walls and macroporous channels. The MOF anode, when examined in the potential window of 0.01-2.0 V versus Li/Li(+), shows high specific capacities of 694 and 400 mAh g(-1) at current densities of 0.1 and 1.0 A g(-1) along with good cyclability, retention of capacity, and sustenance of the MOF network. Ex situ X-ray diffraction, Fourier transform infrared, and X-ray photoelectron spectroscopy studies on the electrode material at different states of charge suggest that the usual conversion reaction for Li storage might not be applicable in this case. Conjugated carboxylates being weakly electron withdrawing ligands with a stronger π-π interaction, a probable alternative Li storage mechanism has been proposed that involves the organic moiety. The present results show promise for applying Mn-1,3,5-benzenetricarboxylate MOF as high performance <2 V anode.

Structural characterization of CuIn2Se3.5, CuIn3Se5 and CuIn5Se8 compounds

Abstract Polycrystalline CuIn2Se3.5, CuIn3Se5 and CuIn5Se8 have been synthesized in vacuum under different cooling conditions resulting in different phases. X-ray diffraction and Rietveld refinement of the diagrams have been used to characterize the different structures obtained. The two former compounds were refined using two different types of structure: stannite (S.G.: I42m) and the proposed P–chalcopyrite (S.G.: P42c). The determined bond distances after refinements indicate a higher tetragonal distortion for stannite type structure than P–chalcopyrite structure. These results seem to indicate that the Cu–In–Se structure with these two stoichiometries belongs to the P42c S.G. On the other hand, for the CuIn5Se8 compound a far different XRD diagram was obtained where all the reflections have been assigned to two structures, named γ-phases, one trigonal (γ) and one hexagonal (γH).

COMPOSITION EFFECTS ON THE CRYSTAL STRUCTURE OF CUINSE2

By x‐ray powder diffraction and the Rietveld refinement method, the atomic positions in CuInSe2 were determined for compositions close to stoichiometry. The Se position, x(Se), was found to be correlated to the Cu content. According to a model proposed by Jaffe and Zunger [Phys. Rev. B 29, 1882 (1984)], changes in x(Se) induce a variation in the optical band gap, Eg. This could explain the spread in energy gap, Eg, found by many authors for this compound. The increase in the lattice parameter, a, correlated with the difference between the energy dispersive analysis of x rays and x‐ray diffraction determined Cu contents, suggests the presence of a fraction of Cu atoms as interstitials.

V-shaped defects in InGaN/GaN multiquantum wells

InGaN/GaN multiquantum well (MQW) structures have been grown on (0001) sapphire substrate by metalorganic chemical vapor deposition. From cross-sectional transmission electron microscopy (TEM), a number of V-shaped defects has been observed on the surface which are associated with mixed or pure-edge screw dislocations, as well as with inversion domains. Atomic force microscopy (AFM) reveals that these are hexagonal pits with a width of about 70 nm. The mechanism of formation of these defects has been discussed in terms of stress induced by lattice mismatch and reduced In incorporation on the {1011} planes in comparison to the (0001) surface. A decrease in In concentration and also in well thickness during growth has been found. No optical emission has been observed from these defects by cathodoluminescence (CL) studies.

Improved Electrochemical Performance of Li2MnSiO4 / C Composite Synthesized by Combustion Technique

Li(2)MnSiO(4) cathode powders were synthesized by a simple combustion technique using citric acid as a chelating agent. The as-synthesized powder was ballmilled with acetylene black (0-20 wt %) and heated at 700 degrees C in argon atmosphere to form a Li(2)MnSiO(4)/C composite. X-ray powder diffraction indicated the formation of Li(2)MnSiO(4) possessing an orthorhombic crystal structure along with manganese oxide as a minor impurity phase. Field-emission-scanning electron microscopy showed that pristine Li(2)MnSiO(4) consists of large agglomerates of similar to 500 to 800 nm. The addition of acetylene black resulted in a drastic change in morphology for Li(2)MnSiO(4)/C composites consisting of uniform grains of similar to 50 nm. The electrochemical discharge capacity as well as the rate capability of Li(2)MnSiO(4) also improved dramatically with an increasing amount of conducting carbon (acetylene black) in the matrix, and a value as high as 164 mAh g(-1) was obtained at a current density of 0.01 mA/cm(2). Impedance spectroscopy showed that the addition of acetylene black decreases the charge-transfer impedance and checks the growth of cell impedance during cycling. Cyclic voltammetry showed two oxidation/reduction couples at 3.6/2.9 and 4.5/4.3 V with good reversibility.

Electrochemical energy storage in Mn2O3 porous nanobars derived from morphology-conserved transformation of benzenetricarboxylate-bridged metal–organic framework

Tailoring the morphology of mesoporous nanostructures toward performance enhancement plays a key role in developing efficient energy storage devices. Herein, we report the formation of well crystallized Mn2O3 mesoporous nanobars through simple exo-templating of a manganese 1,3,5-benzenetricarboxylate metal–organic framework (Mn-BTC MOF) by thermal treatment whereby the general morphology of the parent MOF is conserved, but with more voids and spaces. The parent Mn-BTC MOF was synthesized by solvothermal reaction of trimesic acid with manganese nitrate in alcoholic solution. The MOF-derived Mn2O3 was characterized by XRD, field-emission SEM, high-resolution TEM, and N2 adsorption/desorption isotherm measurements. When examined as an anode material for lithium-ion batteries in the potential windows of 0.01–3.0 V and 0.01–2.0 V, high reversible specific capacities of 849 and 778 mAh g−1 were obtained. It was found that the electrochemical processes are more reversible when cycled in the 2 V window. A steady capacity of ∼410 mAh g−1 was observed after 300 continuous cycles at ∼C/5.5 exhibiting good cycling stability in the 2 V window. When tested as a pseudocapacitor electrode in a three-electrode configuration, a specific capacitance of 250 F g−1 at 0.2 A g−1 could be achieved. Further, to demonstrate practical applicability, two-electrode asymmetric supercapacitor pouch cells were assembled with Mn2O3 as the positive electrode and commercial activated carbon as the negative electrode which showed an ultrahigh energy density of 147.4 W h kg−1 at a power density of 1004 W kg−1. The present work shows the potential of a MOF derived route for obtaining metal oxides with desired nano-architectures for electrochemical applications with high performance.

Influence of imidazolium-based ionic liquid electrolytes on the performance of nano-structured MnO2 hollow spheres as electrochemical supercapacitor

Use of room temperature ionic liquids (ILs) as electrolytes with a wider potential window offers scope for developing high energy density supercapacitors for efficient energy storage. In this work, a comparative study on the performance of nanostructured MnO2 hollow spheres as electrochemical supercapacitor was made by fabricating activated carbon (AC)//MnO2 asymmetric supercapacitor cells using imidazolium-based ILs as electrolytes. Mesoporous MnO2 hollow spheres were synthesized through a simple low temperature (80 °C) solvothermal method by reduction of KMnO4 under acidic condition in presence of Cu3(1,3,5-benzenetricarboxylate)2 metal organic framework (MOF) in the precursor solution, which acts as the source of Cu2+ playing a crucial role for the formation of the hollow spheres. Four different ILs were investigated from combinations of two different cations [1-ethyl-3-methylimidazolium (EMI+) and 1-butyl-3-methylimidazolium (BMI+)] and four different anions [hexaflurophosphate (PF6−), tetrafluoroborate (BF4−), trifluromethanesulfonate (Otf−) and bis(trifluromethylsulfonyl)imide (TFSI−)]. Influences of the physico-chemical properties such as ionic size, nucleophilicity, viscosity of the IL on the electrochemical properties are discussed. A high energy density of 163 W h kg−1, which is comparable to the energy density of a lithium-ion battery, could be achieved with EMIMBF4 as electrolyte. The present findings would help in further research for developing IL-based supercapacitors.

Morphology-mediated tailoring of the performance of porous nanostructured Mn2O3 as an anode material

Tailoring of functional properties by varying the size and shape of porous nanostructured materials is an important frontier area of research. Herein, we report the successful synthesis of nanostructured Mn2O3 with desired 3D architectures such as porous hollow spheres, lotus shapes and tubular shapes, as well as aggregated nanoparticles, through the calcination of corresponding MnCO3 with the same architecture. Porous structures were formed upon evolution of CO2 during decomposition of the carbonate intermediate, and hollow structures were formed through a nonequilibrium interdiffusion process, i.e., the Kirkendall effect. The bare MnCO3 structures were synthesized using the chelating agents citric acid (CA), tartaric acid (TA), oxalic acid (OA), and ethylenediaminetetraacetic acid (EDTA), which mediated the growth of these MnCO3 structures by hydrothermal treatment of a precursor solution containing MnCl2, ammonium carbonate and chelating agent. A systematic evaluation of the effect of the morphology of the synthesized Mn2O3 on its performance as an anode material in Li-ion batteries reveals that the shape and the nature of pores of Mn2O3 strongly influence its Li-ion storage capacity. A superior specific capacity of 478 mAh g−1 is obtained for hollow spheres with 38% retention after 30 cycles compared to other shapes due its high accessible surface area and inner hollow architecture.

Filter paper templated interconnected nanocrystalline LiMn2O4 with high coulombic efficiency and rate capability

A novel filter paper templating method has been developed to prepare nanocrystalline LiMn2O4. Whatman 42 filter paper impregnated with the precursor gel produces ultrafine powder (80–100 nm) when calcined at 800 °C. The structure and morphology of the powder and the template have been studied in detail by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). A single phase cubic spinel with interconnected powder morphology is found. The electrochemical properties have been studied using 2032 coin type cells in the potential window of 3.4 to 4.5 V versusLi/Li+. Sharp oxidation–reduction peaks at 4.09/3.90V and 4.22/4.04V in the cyclic voltagram indicate high crystallinity and good reversibility of the cathode. A typical cell shows an initial discharge capacity of 97 mAh/g with ∼98% columbic efficiency. Rate capability study of the synthesized cathode has been performed by cycling the cell for 130 cycles at different current densities (0.1–1.5 mA/cm2) and it is observed that 80–100% retention of initial capacity is possible when cycled at 0.1–0.6 mA/cm2. A capacity fading of only 0.03 mAh/g per cycle is observed even at 1.5 mA/cm2 (8C). Thus, we have shown that simple filter paper templating technique can produce nanocrystalline LiMn2O4 with high coulombic efficiency (100–95%) and rate capability.