Effects of Lithium Polysulfides on the Formation of Solid Electrolyte Interfaces in Silicon Anodes.

Lithium-ion batteries are one of the most important energy storage devices of the future and pave the way for a greener society. In this context, the demand for batteries with high energy density is increasing significantly and is reaching the limits of the technology currently in use. Therefore, intensive research is being conducted to utilize a new class of materials for energy storage. The most promising alternatives to today's nickel-based cathode and graphite anode materials are silicon and sulfur. Both silicon and sulfur are abundant and cheap and possess extremely high theoretical specific capacities of 4200 mAh/gSi and 1675 mAh/gS, respectively. One of the biggest challenges with sulfur-based batteries is the polysulfide shuttle effect, which occurs with sulfur cathodes, leading to an insulating passivation layer, especially on the commonly used lithium metal anodes. Therefore, to replace lithium metal anodes with silicon, it is of major importance to understand the reactivity of polysulfides with silicon. To investigate the effect of lithium polysulfides on the performance of the anodes in the critical formation cycles, mesoporous silicon anodes were galvanostatically cycled in electrolytes containing different concentrations of polysulfides. In this process, the anodes were analyzed after one, five and ten cycles by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy to determine the composition of the SEI. Higher concentrations of polysulfides in the electrolyte result in more inorganic, oxide-containing species in the SEI. Silicon anodes with lower amounts of surface oxide show little or negative effect on the performance in the presence of polysulfides, while anodes with large amounts of surface oxide show higher impedance during cycling, an effect that is enhanced with increasing polysulfide content.

Temperature-Dependent Vapor Infiltration of Sulfur into Highly Porous Hierarchical Three-Dimensional Conductive Carbon Networks for Lithium Ion Battery Applications.

Hierarchical, conductive, porous, three-dimensional (3D) carbon networks based on carbon nanotubes are used as a scaffold material for the incorporation of sulfur in the vapor phase to produce carbon nanotube tube/sulfur (CNTT/S) composites for application in lithium ion batteries (LIBs) as a cathode material. The high conductivity of the carbon nanotube-based scaffold material, in combination with vapor infiltration of sulfur, allows for improved utilization of insulating sulfur as the active material in the cathode. When sulfur is evenly distributed throughout the network via vapor infiltration, the carbon scaffold material confines the sulfur, allowing the sulfur to become electrochemically active in the context of an LIB. The electrochemical performance of the sulfur cathode was further investigated as a function of the temperature used for the vapor infiltration of sulfur into the carbon scaffolds (155, 175, and 200 °C) in order to determine the ideal infiltration temperature to maximize sulfur loading and minimize the polysulfide shuttle effect. In addition, the nature of the incorporation of sulfur at the interfaces within the 3D carbon network at the different vapor infiltration temperatures will be investigated via Raman, scanning electron microscopy/energy dispersive X-ray, and X-ray photoelectron spectroscopy. The resulting CNTT/S composites, infiltrated at each temperature, were incorporated into a half-cell using Li metal as a counter electrode and a 0.7 M LiTFSI electrolyte in ether solvents and characterized electrochemically using cyclic voltammetry measurements. The results indicate that the CNTT matrix infiltrated with sulfur at the highest temperature (200 °C) had improved incorporation of sulfur into the carbon network, the best electrochemical performance, and the highest sulfur loading, 8.4 mg/cm2, compared to the CNTT matrices infiltrated at 155 and 175 °C, with sulfur loadings of 4.8 and 6.3 mg/cm2, respectively.

Low-Temperature Solution Synthesis of Au-Modified ZnO Nanowires for Highly Efficient Hydrogen Nanosensors.

In this research, the low-temperature single-step electrochemical deposition of arrayed ZnO nanowires (NWs) decorated by Au nanoparticles (NPs) with diameters ranging between 10 and 100 nm is successfully demonstrated for the first time. The AuNPs and ZnO NWs were grown simultaneously in the same growth solution in consideration of the HAuCl4 concentration. Optical, structural, and chemical characterizations were analyzed in detail, proving high crystallinity of the NWs as well as the distribution of Au NPs on the surface of zinc oxide NWs demonstrated by transmission electron microscopy. Individual Au NPs-functionalized ZnO NWs (Au-NP/ZnO-NWs) were incorporated into sensor nanodevices using an focused ion bean/scanning electron microscopy (FIB/SEM) scientific instrument. The gas-sensing investigations demonstrated excellent selectivity to hydrogen gas at room temperature (RT) with a gas response, Igas/Iair, as high as 7.5-100 ppm for Au-NP/ZnO-NWs, possessing a AuNP surface coverage of ∼6.4%. The concentration of HAuCl4 in the electrochemical solution was observed to have no significant impact on the gas-sensing parameters in our experiments. This highlights the significant influence of the total Au/ZnO interfacial area establishing Schottky contacts for the achievement of high performances. The most significant performance of H2 response was observed for gas concentrations higher than 500 ppm of H2 in the environment, which was attributed to the surface metallization of ZnO NWs during exposure to hydrogen. For this case, an ultrahigh response of about 32.9 and 47 to 1000 and 5000 ppm of H2 was obtained, respectively. Spin-polarized periodic density functional theory calculations were realized on Au/ZnO bulk and surface-functionalized models, validating the experimental hypothesis. The combination of H2 gas detection at RT, ultralow power consumption, and reduced dimensions makes these micro-nanodevices excellent candidates for hydrogen gas leakage detection, including hydrogen gas monitoring (less than 1 ppm).