Prashanth Hanumantha Jampani

Increased demand for efficient portable energy storage systems has promoted researches to focus on identifying better alternatives to the conventional insertion-based lithium-ion battery cathodes. With a theoretical specific capacity of 1672 mAh/g, sulfur has emerged as a promising candidate for high energy density lithium battery cathodes^{1 , 2}. Favorable economics, abundance and ease of availability are additional arguments favoring the use of sulfur cathodes in comparison to transition metal – based cathodes. However, elemental sulfur has poor electronic conductivity (~10^-15 S/cm)³ at room temperature which restricts the active material utilization in Li-S batteries. The formation of soluble non-intercalation based polysulfide intermediates (Li₂S_n; n=2-8)⁴ during electrochemical cycling further leads to active material loss by coating onto the anode leading to eventual battery failure. Numerous sulfur encapsulation strategies have been reported in the literature of which, utilizing a mesoporous⁵ and hollow carbonaceous materials^{6, 7} as sulfur hosts has shown to reduce polysulfide dissolution by avoiding direct contact with the liquid electrolyte. However the pore size of these materials are in the order on microns and hence the problem of polysulfide dissolution has not been completely addressed. Porous materials with pore sizes comparable or less than the size of polysulfide species (PSS) need to devised in order to completely overcome this problem. In this work, several highly ordered nanoporous complex framework materials (CFM) were synthesized and engineered for use as effective hosts for sulfur. These nanoporous CFM architectures act as effective hosts to bind and trap the polysulfide species (PSS) serving to effectively prevent their dissolution into the electrolyte during cycling. The CFMs with three different functional groups were effectively crafted and synthesized. The resulting materials were then infiltrated with sulfur using vapor-phase infiltration techniques. The surface structure of the CFMs play a major role in efficiently trapping the polysulfide species and controlling their dissolution. The effects of functional groups created on these CFMs specifically on their polysulfide (PS) trapping properties and the corresponding electrochemical cycling performance of the sulfur impregnated CFM based cathodes were studied in detail and the results will be presented and discussed. References J. R. Akridge, Y. V. Mikhaylik and N. White, Solid State Ionics, 2004, 175, 243-245. M. M. Thackeray, C. Wolverton and E. D. Isaacs, Energy & Environmental Science, 2012, 5, 7854-7863. M. Edeling, R. W. Schmutzler and F. Hensel, Philosophical Magazine Part B, 1979, 39, 547-550. S. S. Zhang, Journal of Power Sources, 2013, 231, 153-162. S.-R. Chen, Y.-P. Zhai, G.-L. Xu, Y.-X. Jiang, D.-Y. Zhao, J.-T. Li, L. Huang and S.-G. Sun, Electrochimica Acta, 2011, 56, 9549-9555. W. Ahn, K.-B. Kim, K.-N. Jung, K.-H. Shin and C.-S. Jin, Journal of Power Sources, 2012, 202, 394-399. J. Guo, Y. Xu and C. Wang, Nano Letters, 2011, 11, 4288-4294.

Earth abundant and economical rock salt (NaCl) particles of different sizes (NF) comprising largely amorphous Si (a-Si) with a small volume fraction of nanocrystalline Si (nc-Si), and Si nanorods (Si_NR) composed of a larger volume fraction of crystalline Si (c-Si) and a small volume fraction of a-Si resulted from modification of the NaCl crystals. Si_NF yielded first-cycle discharge and charge capacities of ∼2,830 and 2,175 mAh·g⁻¹, respectively, at a current rate of 50 mA·g⁻¹ with a first-cycle irreversible loss (FIR loss) of ∼15%–20%. Si_NR displayed first-cycle discharge and charge capacities of ∼2,980 and ∼2,500 mAh·g⁻¹, respectively, at a current rate of 50 mA·g⁻¹ with an FIR loss of ∼12%–15%. However, at a current rate of 1 A·g⁻¹, Si_NF exhibited a stable discharge capacity of ∼810 mAh·g⁻¹ at the end of 250 cycles with a fade rate of ∼0.11% loss per cycle, while Si_NR showed a stable specific discharge capacity of ∼740 mAh·g⁻¹ with a fade rate of ∼0.23% loss per cycle. The morphology of the nanostructures and compositions of the different phases/phase of Si influence the performance of Si_NF and Si_NR, making them attractive anodes for lithium-ion batteries.

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.

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.

Identification of low cost, highly active, durable completely noble metal-free electro-catalyst for oxygen reduction reaction (ORR) in proton exchange membrane (PEM) fuel cells, oxygen evolution reaction (OER) in PEM based water electrolysis and metal air batteries remains one of the major unfulfilled scientific and technological challenges of PEM based acid mediated electro-catalysts. In contrast, several non-noble metals based electro-catalysts have been identified for alkaline and neutral medium water electrolysis and fuel cells. Herein we report for the very first time, F doped Cu_1.5Mn_1.5O₄, identified by exploiting theoretical first principles calculations for ORR and OER in PEM based systems. The identified novel noble metal-free electro-catalyst showed similar onset potential (1.43 V for OER and 1 V for ORRvsRHE) to that of IrO₂and Pt/C, respectively. The system also displayed excellent electrochemical activity comparable to IrO₂for OER and Pt/C for ORR, respectively, along with remarkable long term stability for 6000 cycles in acidic media validating theory, while also displaying superior methanol tolerance and yielding recommended power densities in full cell configurations.