Insight into sulfur reactions in Li-S batteries.

Understanding and controlling the sulfur reduction species (Li2Sx, 1 ≤ x ≤ 8) under realistic battery conditions are essential for the development of advanced practical Li-S cells that can reach their full theoretical capacity. However, it has been a great challenge to probe the sulfur reduction intermediates and products because of the lack of methods. This work employed various ex situ and in situ methods to study the mechanism of the Li-S redox reactions and the properties of Li2Sx and Li2S. Synchrotron high-energy X-ray diffraction analysis used to characterize dry powder deposits from lithium polysulfide solution suggests that the new crystallite phase may be lithium polysulfides. The formation of Li2S crystallites with a polyhedral structure was observed in cells with both the conventional (LiTFSI) electrolyte and polysulfide-based electrolyte. In addition, an in situ transmission electron microscopy experiment observed that the lithium diffusion to sulfur during discharge preferentially occurred at the sulfur surface and formed a solid Li2S crust. This may be the reason for the capacity fade in Li-S cells (as also suggested by EIS experiment in Supporting Information ). The results can be a guide for future studies and control of the sulfur species and meanwhile a baseline for approaching the theoretical capacity of the Li-S battery.

Paving the way for using Li2S batteries.

In this work, a novel lithium-sulfur battery was developed comprising Li2S as the cathode, lithium metal as the anode and polysulfide-based solution as the electrolyte. The electrochemical performances of these Li2S-based cells strongly depended upon the nature of the electrolytes. In the presence of the conventional electrolyte that consisted of lithium bis(trifluoromethanesulfonyl)-imide (LiTFSI) salt dissolved in a solvent combination of dimethoxyethane (DME)/1,3-dioxolane (DOL), the Li/Li2S cells showed sluggish kinetics, which translated into poor cycling and capacity retention. However, when using small amounts of polysulfides in the electrolyte along with a shuttle inhibitor the Li2S cathode was efficiently activated in the cell with the generation of over 1000 mAh g(-1) capacity and good cycle life.

Platinum nanowires produced by electrospinning.

A method of making long (cm) Pt nanowires of a few nanometers diameter from electrospinning is described. A major problem of avoiding bead formation along the nanofibers is analyzed, and the conditions under which the bead formation is minimized are investigated. Our ultimate purpose is to make free-standing fuel cell electrodes from these nanowires.

Mechanical grain growth in nanocrystalline copper.

Nanograined materials have some unusual properties. To maintain the small size of the grains, grain growth should be avoided. But recently grain growth has been observed under an indenter at liquid-nitrogen temperatures. Such grain growth has never been reported before. How can this happen and how can it be prevented? These questions are answered here using a simple tilt boundary. It is found that high purity and nonequilibrium structure are necessary conditions for mechanical grain growth. The material must be pure enough so that free dislocations are available to move out of the boundary. But the boundary should not be in the lowest-energy state so that extra dislocations are available to be emitted by stress. Based on these conditions, methods can be devised to avoid low temperature grain growth.