We report a two-step synthesis of highly luminescent CdS/ZnSe core/shell nanocrystals (emission quantum yields up to 50%) that can produce efficient spatial separation of electrons and holes between the core and the shell (type-II localization regime). Our synthesis involves fabrication of cubic-singony CdS core particles that are subsequently overcoated with a layer of ZnSe in the presence of surfactant-ligands in a noncoordinating solvent. Studies of different growth regime of the ZnSe shell indicate that one approach to obtaining high emission efficiencies is through alloying the CdS/ZnSe interface with CdSe, which leads to the formation of an intermediate ZnCdSe layer with a graded composition. We perform theoretical modeling of these core/shell nanocrystals using effective mass approximation and applying first-order perturbation theory for treating both direct electron-hole coupling and the core/shell interface-polarization effects. Using this model we determine the range of geometrical parameters of the core/shell structures that result in a type-II localization regime. We further applied this model to evaluate the degree of electron-hole spatial separation (quantified in terms of the electron-hole overlap integral) based on measured emission wavelengths. We also discuss the potential applicability of these nanocrystals in lasing technologies and specifically the possibility of single-exciton optical gain in type-II nanostructures.

We describe in this report the electrochemistry of Mg deposition and dissolution from the magnesium aluminum chloride complex (MACC). The results define the requirements for reversible Mg deposition and definitively establish that voltammetric cycling of the electrolyte significantly alters its composition and performance. Elemental analysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy (SEM-EDS) results demonstrate that irreversible Mg and Al deposits form during early cycles. Electrospray ionization mass spectrometry (ESI-MS) data show that inhibitory oligomers develop in THF-based solutions. These oligomers form via the well-established mechanism of a cationic ring-opening polymerization of THF during the initial synthesis of the MACC and under resting conditions. In contrast, MACC solutions in 1,2-dimethoxyethane (DME), an acyclic solvent, do not evolve as dramatically at open circuit potential. From these results, we propose a mechanism describing how the conditioning process of the MACC in THF improves its performance by both tuning the Mg:Al stoichiometry and eliminating oligomers.

A materials-to-system analysis for the lithium-sulfur (Li-S) electric vehicle battery is presented that identifies the key electrode and cell design considerations from reports of materials chemistry. The resulting systems-level energy density, specific energy and battery price as a function of these parameters is projected. Excess lithium metal amount at the anode and useable specific capacity, electrolyte volume fraction, sulfur to carbon ratio and reaction kinetics at the cathode are all shown to be critical for the high energy density and low cost requirements. Electrode loading is determined as a key parameter to relate the battery price for useable energy to the investigated design considerations. The presented analysis proposes that electrode loadings higher than 8 mAh/cm 2 (∼7 mg S/cm 2 ) are necessary for Li-S systems to exhibit the high energy density and low cost required for transportation applications. Stabilizing the interface of lithium metal at the required current densities and areal capacities while simultaneously maintaining cell capacity with high sulfur loading in an electrolyte starved cathode are identified as the key barriers for ongoing research and development efforts to address. In the search for high energy density and inexpensive rechargeable batteries for the electric vehicles, Li-S batteries have gained significant attention due to the high specific capacity (1675 mAh/g), low cost, natural abundance and non-toxicity of elemental sulfur.1-6 Compared to the state-of-art Li-ion batteries, Li-S batteries have very high theoretical specific energy of 2567 Wh/kg.1-6 The Li-S battery is commonly composed of a sulfur-carbon composite cathode, an organic electrolyte and a lithium anode.1-6 The overall Li-S redox reaction is given in equation 1.with a standard potential of U 0 = 2.2 V (vs Li/Li + ). 1,2Despite these attractive features of the Li-S battery, multiple formidable challenges limit the cycle life significantly. [1][2][3][4][5][6] Firstly, precipitation of insulating reactants, sulfur and Li 2 S, in the cathode leads to poor electronic conductivity and passivation that could limit the active material utilization.1-6 Secondly, the soluble polysulfide reaction intermediates produced during charging can migrate to the anode where they react with Li to either precipitate on the anode surface or migrate back to the cathode causing infinite charging.1-6 This polysulfide shuttle mechanism leads to poor coulombic efficiency and significant self-discharge as well as corrosion of the Li-anode.1-6 Finally, the instability of the Li-anode is a major concern.2,4,5,7 Li surface area can increase significantly with cycling due to morphological changes, which accelerate Li and electrolyte depletion owing to the absence of a stable interphase. 2,4,5,7 While polysulfide migration may lead to the corrosion of dendritic or high surface area lithium reducing the risk of short circuiting, the resulting reactions typically result in a reduction in inventory of cyclable lithium. 4 All of these mechanisms ...