A single-phase crystalline Na 3 V 2 O 2 (PO 4 ) 2 F material has been prepared by the solvothermal method. Partial ion exchange between Na and Li was then used to form Na 3−x Li x V 2 O 2 (PO 4 ) 2 F. The two materials were studied as positive cathodes by physical characterization, electrochemical measurements, and simulation. With density functional theory calculations, four stable phases of Na x V 2 O 2 (PO 4 ) 2 F were identified at the Na concentrations of x = 0, 1, 2, 3. The transitions between these phases give rise to three values of the Na chemical potential and three voltage plateaus for Na intercalation. The lower two voltages, corresponding to removal of the first two Na per formula unit, agree well with the corresponding experimental electrochemical measurements. Removal of the third Na, however, is not observed experimentally, because it is outside of the (4.8 V) stability window of the electrolyte. This observation is consistent with our calculations that show that the last Na will only be removed at 5.3 V, owing to the stability of the V−O bonding state and a strong Coulomb attraction between the Na and the anions. Computational modifications of the material were considered to activate the third Na with an oxidation energy in the electrolyte stability window, including swapping the anions from O and F to less-electronegative Cl and Br. The most promising material, Na 3 V 2 Cl 2 (PO 4 ) 2 F, is found to be stable and a good candidate as a Na cathode because all three Na ions can be reversibly removed without significant reduction in the cell potential or energy density of the material. Finally, we show that Li can partially replace Na and that these Li intercalate into the material with a higher rate owing to a lower diffusion barrier as compared to Na.

Composites of nitrogen-doped reduced graphene oxide (NRGO) and nanocrystalline tin sulfides were synthesized, and their performance as lithium ion battery anodes was evaluated. Following the first cycle the composite consisted of LiS/LiSn/NRGO. The conductive NRGO cushions the stress associated with the expansion of lithiation of Sn, and the noncycling LiS increases the residual Coulombic capacity of the cycled anode because (a) Sn domains in the composite formed of unsupported SnS expand only by 63% while those in the composite formed of unsupported SnS expand by 91% and (b) Li percolates rapidly at the boundary between the LiS and LiSn nanodomains. The best cycling SnS/NRGO-derived composite retained a specific capacity of 562 mAh g at the 200th cycle at 0.2 A g rate.

one of the most promising lithium-based batteries, the Li-S batteries are appealing as both the sulfur cathode and the lithiummetal anode offer an order of magnitude higher charge-storage capacity compared to the currently used insertion-compound electrodes. [ 16,17 ] In addition, sulfur is abundant and environmentally benign while lithium metal offers a desirable low negative electrochemical potential. Unfortunately, the lithium-metal anode suffers from nonuniform metal redeposition and unstable surface chemistry in organic electrolytes, which lead to a continuous breakdown and reformation of the solid electrolyte interphase (SEI) layer during cycling. [ 18 ] To stabilize the lithium-metal anode, various electrolyte systems that target the reduction of the amount of free solvent molecules causing unwanted side reactions have been proposed. [19][20][21] Stable high-rate performance with lithium-metal anode has been reported by employing electrolytes with high lithium-salt concentrations, but the practical application of these electrolyte systems in Li-S batteries has not been verifi ed. [ 22 ] Alternatively, the concept of artifi cial SEI layers has been proposed, e.g., isolation of the lithium anode by hollow carbon nanosphere or lithiated graphite fi lms. [ 23,24 ] Albeit increased cycling effi ciency, the stability of the artifi cial SEIs in Li-S cells and their large-scale production remain a challenge for industrial applications. [ 25 ] One key issue hampering the rational design of Li-S battery architecture is the limited understanding of the full chemical composition of the reacted lithium-metal anode and its connection to the structure and cell performance. Extensive studies employing X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) have revealed the basic surface chemical composition of the reacted lithium-metal anodes. [ 26 ] The microstructure formation at the lithium-metal surface has also been investigated by optical microscopy, [ 27 ] secondary electron microscopy (SEM), [ 28 ] atomic force microscopy (AFM), [ 29 ] nuclear magnetic resonance (NMR) spectroscopy, [ 30 ] and hard X-ray microtomography. [ 31 ] To date, however, a clear correlation among the composition, long-range crystallinity and thickness of the reacted lithium region, and the cell performance has not been established.Lithium-sulfur batteries offer high energy density, but their practical utility is plagued by the fast decay of lithium-metal anode upon cycling. To date, a fundamental understanding of the degradation mechanisms of lithium-metal anode is lacking. It is shown that (i) by employing a specifi cally designed electrolyte, the lithium-metal anode degradation can be signifi cantly reduced, resulting in a superior, high-rate battery performance and (ii) by combining advanced, 3D chemical analysis with X-ray diffraction, the properties of the lithium-metal anode can be effectively monitored as a function of cycling, which is critical in understanding its degradation mechanisms. These fi ...

Exciton-delocalizing ligands (EDLs) are of interest to researchers due to their ability to allow charge carriers to spread into the ligand shell of semiconductor nanocrystals (NCs). By increasing charge carrier surface accessibility, EDLs may facilitate the extraction of highly photoexcited carriers from NCs prior to their relaxation to the band edge, a process that can boost the performance of NCbased photocatalysts and light harvesting systems. However, hot carrier extraction must compete with carrier cooling, which could be accelerated by the stronger interaction of charge carriers and EDLs. This report describes the influence of the EDL phenyldithiocarbamate (PTC) on the electron and hole cooling rates of CdSe NCs. Using state-resolved transient absorption spectroscopy, we find that PTC treatment accelerates hole cooling by a factor of 1.7. However, upon further treatment of these NCs with cadmium(II) acetate, the hole cooling rate reverts to the value measured prior to PTC treatment, yet these NCs maintain a red-shifted absorption spectrum indicative of PTC bound to the NC surface. This result provides strong evidence for the existence of two distinct surface-bound PTC species: one that traps holes before they cool and can be removed by cadmium(II) acetate, and a second species that facilitates exciton delocalization. This conclusion is supported by both DFT calculations and photoluminescence measurements. The outlook from our work is that EDLs do not necessarily lead to an acceleration of carrier cooling, suggesting that they may provide a path for hot carrier extraction.

Sodium Super Ionic Conductor (NASICON) structured NaxTi2(PO4)3 phosphate framework compounds represent a very attractive class of materials for their use as Na-ion battery electrodes.

Weak chemisorption of ethylene has been shown to be an important characteristic in the use of metals for the separation of ethylene from ethane. Previously, density functional theory (DFT) has been used to predict the binding energies of various metals and alloys, with Ag having the lowest chemisorption energy amongst the metals and alloys studied. Here Au/Cu alloys are investigated by a combination of DFT calculations and experimental measurements. It is inferred from experiments that the binding energy between a Au/Cu alloy and ethylene is lower than to either of the pure metals, and DFT calculations confirm this is the case when Au segregates to the particle surface. Implications of this work suggest that it may be possible to further tune the binding energy with ethylene by compositional and morphological control of films produced from Au-surface segregated alloys.

The synthesis and characterization of Sn nanoparticles in organic solvents using sixth-generation dendrimers modified on their periphery with hydrophobic groups as stabilizers are reported. Sn(2+):dendrimer ratios of 147 and 225 were employed for the synthesis, corresponding to formation of Sn147 and Sn225 dendrimer-stabilized nanoparticles (DSNs). Transmission electron microscopy analysis indicated the presence of ultrasmall Sn nanoparticles having an average size of 3.0-5.0 nm. X-ray absorption spectroscopy suggested the presence of Sn nanoparticles with only partially oxidized surfaces. Cyclic voltammetry studies of the Sn DSNs for Li alloying/dealloying reactions demonstrated good reversibility. Control experiments carried out in the absence of DSNs clearly indicated that these ultrasmall Sn DSNs react directly with Li to form SnLi alloys.

Li 1+x Ti 2 O 4 spinel structures are used as model systems to study the complex environment of electrode/electrolyte interfaces in lithium-ion batteries. The lithiation pathways and the potential dependence of delithiation on the corresponding Li 1+x Ti 2 O 4 surfaces were explored using the density functional theory. Low-index surfaces are found to be highly reactive, with Li forming a fully lithiated phase (Li 2 Ti 2 O 4 ) before more Li can penetrate farther into the bulk. The calculated activation energies for the formation of Li 2 Ti 2 O 4 at the surface are found to be much lower than those for Li diffusion through LiTi 2 O 4 , suggesting that a two-phase lithiation process takes place during cycling. Additionally, the delithiation reaction mechanism in Li 2 Ti 2 O 4 is studied by evaluating the free energies for Li + transfer to an ethylene carbonate electrolyte by employing a Born−Haber thermodynamic cycle. The effects of an applied (external) potential are effectively incorporated into the thermodynamic cycle and provides the means to calibrate the bias potential to the experimentally known scale. Finally, the effects of an applied electrode potential are studied on the Li 2 Ti 2 O 4 delithiation energetic pathways in various environments, emphasizing the different contributions to the charge-transfer energetics in these electrode materials.