Ayyakkannu Manivannan

High energy mechanical milling (HEMM) of a stoichiometric mixture of molybdenum and metal chalcogenides (CuT and MoT₂; T = S, Se) followed by heat treatment at elevated temperatures was successfully applied to synthesize Chevrel phases (Cu₂Mo₆T₈; T = S, Se) as positive electrodes for rechargeable magnesium batteries. Differential scanning calorimetry (DSC), thermogravimetric analyses (TGA), X-ray diffraction (XRD), and scanning electron microscopy (SEM) were used to understand the phase formation following milling and heat treatment. CuS and Mo were observed to react at 714–800 K and formed an intermediate ternary Chevrel phase (Cu_1.83Mo₃S₄), which further reacted with residual Mo and MoS₂ to form the desired Cu₂Mo₆S₈. Quantitative XRD analysis shows the formation of a ∼96%–98% Chevrel phase at 30 min following the milling and heat treatment. The electrochemical performance of de-cuprated Mo₆S₈ and Mo₆Se₈ phases were evaluated by cyclic voltammetry (CV), galvanostatic cycling, and electrochemical impedance spectroscopy (EIS). The results of the CV and galvanostatic cycling data showed the expected anodic/cathodic behavior and a stable capacity after the first cycle with the formation of Mg_xMo₆T₈ (T = S, Se; 1 ≤ x ≤ 2). EIS at ∼0.1 V intervals for the Mo₆S₈ electrode during the first and second cycle shows that partial Mg-ion trapping resulted in an increase in charge transfer resistance R_e. In contrast, the interfacial resistance R_i remained constant, and no significant trapping was evident during the galvanostatic cycling of the Mo₆Se₈ electrode. Importantly, the ease of preparation, stable capacity, high Coulombic efficiency, and excellent rate capabilities render HEMM a viable route to laboratory-scale production of Chevrel phases for use as positive electrodes for rechargeable magnesium batteries.

The monoclinic Li₂MnO₃ is electrochemically inactive for lithium intercalation at voltages lower than 4.4 V vs. Li⁺/Li⁰. In order to activate Li₂MnO₃, magnesium has been incorporated in the lithium site of the structure which resulted in a mixture of active phases capable of cycling in the 2.8 to 4.8 V vs. Li⁺/Li⁰ voltage range. Li₂MnO₃, Li_1.5Mg_0.25MnO₃, Li_1.0Mg_0.50MnO₃ and Li_0.5Mg_0.75MnO₃ compositions were synthesized by the modified Pechini method and formulated based on the oxidation state of magnesium and lithium. The compounds were characterized using XRD, XPS, SEM, BET and tap density. The XRD patterns for Li_1.5Mg_0.25MnO₃, Li_1.0Mg_0.50MnO₃ and Li_0.5Mg_0.75MnO₃ indicated the presence of monoclinic/spinel phases. Li_0.5Mg_0.75MnO₃ showed ideal values of tap density vs. surface area among all the Mg-substituted compounds. X-ray photoelectron spectroscopy of Mn 2p orbital showed a decrease in the oxidation state of manganese for Li_1.0Mg_0.50MnO₃ and Li_0.5Mg_0.75MnO₃, while the change in binding energy for Li_1.5Mg_0.25MnO₃ was not significant. The cyclic voltammetry and charge/discharge tests showed an increase in capacity and better stability for the Li_1.0Mg_0.50MnO₃ (80 mAh g⁻¹) and Li_0.5Mg_0.75MnO₃ (112 mAh g⁻¹) compositions.

Electrodeposition of amorphous silicon thin films on Cu substrate from organic ionic electrolyte using pulsed electrodeposition conditions has been studied. Scanning electron microscopy analysis shows a drastic change in the morphology of these electrodeposited silicon thin films at different frequencies of 0, 500, 1000, and 5000 Hz studied due to the change in nucleation and the growth mechanisms. These electrodeposited films, when tested in a lithium ion battery configuration, showed improvement in stability and performance with an increase in pulse current frequency during deposition. XPS analysis showed variation in the content of Si and oxygen with the change in frequency of deposition and with the change in depth of these thin films. The presence of oxygen largely due to electrolyte decomposition during Si electrodeposition and the structural instability of these films during the first discharge–charge cycle are the primary reasons contributing to the first cycle irreversible (FIR) loss observed in the pulse electrodeposited Si–O–C thin films. Nevertheless, the silicon thin films electrodeposited at a pulse current frequency of 5000 Hz show a stable capacity of ~805 mAh·g⁻¹ with a fade in capacity of ~0.056% capacity loss per cycle (a total loss of capacity ~246 mAh·g⁻¹) at the end of 500 cycles.

The effect due to systematic substitution of cobalt by iron in La_0.6Ca_0.4CoO₃ perovskites on the oxygen evolution reaction (OER) in alkaline media has been investigated. These compounds were synthesized by a facile glycine-nitrate synthesis and the phase formation was confirmed by X-ray and neutron powder diffraction analysis. The apparent OER activity was evaluated by quasi-steady state current measurements in alkaline media using a traditional three-electrode system. X-ray photoelectron spectroscopy shows an increase in Fe substitution causes an increase in the surface concentration of various Co oxidation states. A Tafel slope in the vicinity of 60 mV/decade and electrochemical reaction order for OH⁻ near unity were found for the unsubstituted compounds. A decrease in the Tafel slope to 49 mV/decade was observed when iron is incorporated in high amounts in the perovskite structure. The area specific current density showed dependence on the Fe fraction; however, the dependence of specific current density with Fe fraction is not linear. We believe that the iron incorporation in the La_0.6Ca_0.4CoO₃ perovskites decreases the electron transfer barrier and facilitates the formation of cobalt-hydroxides.