M. V. Reddy

Li storage and impedance spectroscopy studies on Co3O4, CoO, and CoN for Li-ion batteries.

The compounds, CoN, CoO, and Co3O4 were prepared in the form of nano-rod/particles and we investigated the Li-cycling properties, and their use as an anode material. The urea combustion method, nitridation, and carbothermal reduction methods were adopted to prepare Co3O4, CoN, and CoO, respectively. X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), and the Brunauer-Emmett-Teller (BET) surface and density methods were used to characterise the materials. Cyclic voltammetry (CV) was performed and galvanostatic cycling tests were also conducted up to 60-70 cycles. The observed reversible capacity of all compounds is of the increasing order CoO, Co3O4, CoN and all compounds showed negligible capacity fading. CoO allows for Li2O and Co metal to form during the discharge cycle, allowing for a high theoretical capacity of 715 mA h g(-1). Co3O4 allows for 4 Li2O and 3Co to form, and has a theoretical capacity of 890 mAhg(-1). CoN is the best anode material of the three because the nitrogen allows for Li3N and Co to form, resulting in an even higher theoretical capacity of 1100 mAhg(-1) due to the Li3N and Co metal formation. Irrespective of morphology the charge profiles of all three compounds showed a major plateaux ~2.0 V vs. Li and potential values are almost unchanged irrespective of crystal structure. Electrochemical impedance spectroscopy (EIS) was performed to understand variation resistance and capacitance values.

Electrospun α-Fe2O3 nanostructures for supercapacitor applications

Herein, we report the facile synthesis of two α-Fe2O3 nanostructures with different morphologies via an electrospinning technique using ferric acetyl acetonate as a precursor and polyvinyl acetate and polyvinyl pyrrolidone as the respective polymers. The as-electrospun metal oxide–polymer composite fibers were sintered at 500 °C to obtain two distinct nanostructures, denoted as nanograins and porous fibers throughout this manuscript. These crystalline nanostructures were characterized using powder X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDAX) and transmission electron microscopy (TEM). The characterization results elucidated the predominance of hematite (α-Fe2O3) with particle sizes of 21 and 53 nm, for the respective nanostructures. Electrophoretic deposition was carried out in order to fabricate thin film electrodes, which were then subjected to electrochemical analysis. Electrochemical characterization revealed that both of the fabricated electrodes exhibited excellent performance in 1 M LiOH electrolyte with specific capacitance values of 256 and 102 F g−1 for the porous fiber and nanograin structures, respectively, at a scan rate of 1 mV s−1 and excellent capacitance retention, even after 3000 cycles, thus making them promising electrode materials for energy storage devices.

Molten salt synthesis and energy storage studies on CuCo2O4 and CuO·Co3O4

CuCo2O4 and CuO·Co3O4 compounds were prepared by a one-pot simple molten salt method (MSM) at 280 °C to 750 °C. Changes in morphology, crystal structure and electrochemical properties of CuCo2O4 as a function of preparation temperatures were investigated using X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Brunauer–Emmett–Teller absorption isotherm. XRD patterns of the sample prepared at 280 °C show a crystalline cubic structure with a lattice parameter value of a = 8.131 A and a surface area value of 9.8 m2 g−1. The sample prepared at temperatures >510 °C shows the presence of CuO·Co3O4 phases. Energy storage properties are evaluated using cyclic voltammetry (CV) and galvanostatic cycling studies. CV studies show a main anodic peak at ∼2.1 V and cathodic peak at ∼1.2 V. At a current rate of 60 mA g−1 and in the voltage range of 0.005–3.0 V vs. Li, CuCo2O4 composite prepared at 510 °C shows a high and stable capacity of ∼680 (quenched) and 740 (slow cooling) mAh g−1 at the end of the 40th cycle.

Lithium Storage Properties of Pristine and (Mg, Cu) Codoped ZnFe2O4 Nanoparticles

ZnFe2O4 and MgxCu0.2Zn0.82-xFe1.98O4 (where x = 0.20, 0.25, 0.30, 0.35, and 0.40) nanoparticles were synthesized by sol-gel assisted combustion method. X-ray diffraction (XRD), FTIR spectroscopy, Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Brunauer-Emmett-Teller (BET) surface area studies were used to characterize the synthesized compounds. ZnFe2O4 and the doped compounds crystallize in Fd3m space group. The lattice parameter of ZnFe2O4 is calculated to be a = 8.448(3) Å, while the doped compounds show a slight decrease in the lattice parameter with an increase in the Mg content. The particle size of all the compositions are in the range of ∼50-80 nm, and the surface area of the compounds are in the range of 11-12 m(2) g(-1). Cyclic voltammetry (CV), galvanostatic cycling, and electrochemical impedance spectroscopy (EIS) studies were used to investigate the electrochemical properties of the different compositions. The as-synthesized samples at 600 °C show large-capacity fading, while the samples reheated at 800 °C show better cycling stability. ZnFe2O4 exhibits a high reversible capacity of 575 mAh g(-1) after 60 cycles at a current density of 100 mA g(-1). Mg0.2Cu0.2Zn0.62Fe1.98O4 shows a similar capacity of 576 mAh g(-1) after 60 cycles with better capacity retention.

Exfoliated Graphene Oxide/MoO2 Composites as Anode Materials in Lithium-Ion Batteries: An Insight into Intercalation of Li and Conversion Mechanism of MoO2

Exfoliated graphene oxide (EG)/MoO2 composites are synthesized by a simple solid-state graphenothermal reduction method. Graphene oxide (GO) is used as a reducing agent to reduce MoO3 and as a source for EG. The formation of different submicron sized morphologies such as spheres, rods, flowers, etc., of monoclinic MoO2 on EG surfaces is confirmed by complementary characterization techniques. As-synthesized EG/MoO2 composite with a higher weight percentage of EG performed excellently as an anode material in lithium-ion batteries. The galvanostatic cycling studies aided with postcycling cyclic voltammetry and galvanostatic intermittent titrations followed by ex situ structural studies clearly indicate that Li intercalation into MoO2 is transformed into conversion upon aging at low current densities while intercalation mechanism is preferably taking place at higher current rates. The intercalation mechanism is found to be promising for steady-state capacity throughout the cycling because of excess graphene and higher current density even in the operating voltage window of 0.005-3.0 V in which MoO2 undergoes conversion below 0.8 V.

Synthesis and electrochemical studies of layer-structured metastable αI-LiVOPO4

The layer structured αI-LiVOPO4 was obtained via a two step chemical synthesis. In the first step, a hydrated phase, LiVOPO4·2H2O, was obtained by a simple hydrothermal route at 120 °C. Single crystal X-ray diffraction analysis revealed the structure of LiVOPO4·2H2O to be orthorhombic with lattice parameters: a = 8.9454(7) A, b = 9.0406(7) A and c = 12.7373(10) A. Dehydration of the parent compound led to its structural transformation to tetragonal αI-LiVOPO4, which was only identified previously during the lithium insertion in VOPO4. We have investigated the solid-state dehydration of LiVOPO4·2H2O and proposed a possible mechanism for the crystal structure transformation. Electrochemical characterization of this rarely studied tetragonal phase revealed its good lithium cycling at high operating voltage. Galvanostatic charge–discharge cycling of αI-LiVOPO4 was studied in a voltage window of 2.5–4.5 V, which shows a stable reversible capacity of 103(±3) mA h g−1 at a current density of 16 mA g−1 (0.1 C). At higher current rates, although it exhibited good cyclability, the capacity was found to decrease with increasing current rates. The long term cycling stability of the above material was demonstrated at a current rate of 0.5 C up to 200 cycles.

Li-storage and cycling properties of Sn–Sb-mixed oxides, (M1/2Sb1/2Sn)O4, M = In, Fe

The mixed oxide compounds, (M1/2Sb1/2Sn)O4, M = In and Fe are prepared by the high-temperature solid-state reaction, at 800xa0°C for M = In and at 1,150xa0°C for M = Fe. High-energy ballmilling is used to reduce the particle size to nm-range. The compounds are characterized by X-ray diffraction, Rietveld refinement, scanning electron microscopy, and Brunauer–Emmett–Teller surface area methods. The Li-storage and cycling properties of the bare and ballmilled compounds are evaluated by galvanostatic cycling at ~0.15xa0C and in the voltage ranges 0.005–1.0 and 0.005–1.2xa0V vs. Li up to 50 (or 100) cycles and by cyclic voltammetry (CV) at room temperature. Effect of electrode heat treatment and carbon nanotube (CNT) addition is also studied. Initial reversible capacities in the range 425–550xa0mAhxa0g−1 are observed depending on the metal (M) upper cut-off voltage, CNT content and electrode heat treatment. Ballmilled (In1/2Sb1/2Sn)O4 showed a stable capacity of 445xa0mAhxa0g-1 up to 30 cycles and 5xa0% capacity fading after 50 cycles. In all other cases, capacity fading is observed ranging from 9 to 60xa0%. The CV showed that the main cathodic and anodic peaks occur at 0.15–0.25xa0V and ~0.5xa0V vs. Li, respectively, for both M. The reaction mechanism involves alloying–de-alloying reactions of Sn and In with Li3Sb or Fe acting as conducting matrix, and corroborated by the ex-situ X-ray diffraction data on (In1/2Sb1/2Sn)O4.

A disc-like Mo-metal cluster compound, Co2Mo3O8, as a high capacity anode for lithium ion batteries

The disc-like Mo-metal cluster compound, Co2Mo3O8 has been prepared by a one-step carbothermal reduction method from CoMoO4 and characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution-transmission electron microscopy (HR-TEM), Fourier transform-infrared (FT-IR) and micro-Raman spectroscopy at room temperature. The Li-cycling behaviour was investigated by galvanostatic cycling and cyclic voltammetry (CV) in the voltage range of 0.005–3.0 V vs. Li at room temperature. It is inferred that the as prepared Co2Mo3O8 delivered a high first cycle reversible capacity of 900 (±5) mA h g−1 (17.9 moles of Li). The compound showed slow capacity degradation upon cycling and reached 425 (±5) mA h g−1 (8.5 Li) at the end of the 40th cycle. The ball milled Co2Mo3O8 (Co2Mo3O8-B) showed similar cycling behaviour and a reversible capacity of 400 (±5) mA h g−1 (7.8 Li) was observed at the end of the 30th cycle. Interestingly, the heat-treated Co2Mo3O8 (Co2Mo3O8-H) showed improved Li-storage behaviour when cycled under similar conditions. It showed almost a stable capacity of 790 (±5) mA h g−1 (15.7 Li) at the end of 60th cycle. All the compounds showed excellent coulombic efficiency in the range of 97–99%. The electrochemical impedance spectroscopy (EIS) measurement is performed on a Li/Co2Mo3O8 system at selected voltages during first discharge and charge cycling. The possible reaction mechanism is proposed based on CV, ex situ XRD, ex situ TEM and electrochemical impedance spectroscopy (EIS) data on the Li/Co2Mo3O8 system.

Facile one pot synthesis and Li-cycling properties of MnO2

For the first time, a molten salt method was attempted to prepare MnO2 with three different precursors of Mn(CH3COO)2, Mn(NO3)2 and MnSO4·H2O by using 0.375 M LiNO3, 0.18 M NaNO3u2006:u20060.445 M KNO3 as a molten salt heated at 380 °C for the application of lithium batteries. The prepared compounds were characterized by various techniques such as, X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), a surface area analyzer and scanning electron microscopy (SEM), respectively. XRD results revealed the cubic phase of λ-MnO2 and tetragonal phase of α-MnO2 and the morphology of the compounds shows spherical particles as well as rod shaped nano-sized particles. The electrochemical performance of the compounds has been evaluated by, galvanostatic cycling (GC), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The Li-cycling results indicated that the performance of nano-rod shaped α-MnO2 prepared by MnSO4·H2O exhibits a stable and high reversible capacity of 845 mA h g−1 (87.5% capacity retention) at the end of the 50th cycle, cycled at a constant current density of 60 mA g−1, in the potential range of 0.005–3.0 V vs. Li. The MnO2 compound prepared by using three different precursors shows a stable coulombic efficiency of 99% after a few cycles.

Room temperature large-scale synthesis of layered frameworks as low-cost 4 V cathode materials for lithium ion batteries

Li-ion batteries (LIBs) are considered as the best available technology to push forward the production of eco-friendly electric vehicles (EVs) and for the efficient utilization of renewable energy sources. Transformation from conventional vehicles to EVs are hindered by the high upfront price of the EVs and are mainly due to the high cost of LIBs. Hence, cost reduction of LIBs is one of the major strategies to bring forth the EVs to compete in the market with their gasoline counterparts. In our attempt to produce cheaper high-performance cathode materials for LIBs, an rGO/MOPOF (reduced graphene oxide/Metal-Organic Phosphate Open Framework) nanocomposite with ~4u2009V of operation has been developed by a cost effective room temperature synthesis that eliminates any expensive post-synthetic treatments at high temperature under Ar/Ar-H2. Firstly, an hydrated nanocomposite, rGO/K2[(VO)2(HPO4)2(C2O4)]·4.5H2O has been prepared by simple magnetic stirring at room temperature which releases water to form the anhydrous cathode material while drying at 90u2009°C during routine electrode fabrication procedure. The pristine MOPOF material undergoes highly reversible lithium storage, however with capacity fading. Enhanced lithium cycling has been witnessed with rGO/MOPOF nanocomposite which exhibits minimal capacity fading thanks to increased electronic conductivity and enhanced Li diffusivity.