Ariel Rosenman

The Use of Redox Mediators for Enhancing Utilization of Li2S Cathodes for Advanced Li–S Battery Systems

The development of Li2S electrodes is a crucial step toward industrial manufacturing of Li-S batteries, a promising alternative to Li-ion batteries due to their projected two times higher specific capacity. However, the high voltages needed to activate Li2S electrodes, and the consequent electrolyte solution degradation, represent the main challenge. We present a novel concept that could make feasible the widespread application of Li2S electrodes for Li-S cell assembly. In this concept, the addition of redox mediators as additives to the standard electrolyte solution allows us to recover most of Li2S theoretical capacity in the activation cycle at potentials as low as 2.9 VLi, substantially lower than the typical potentials >4 VLi needed with standard electrolyte solution. Those novel additives permit us to preserve the electrolyte solution from being degraded, allowing us to achieve capacity as high as 500 mAhg(-1)Li2S after 150 cycles with no major structural optimization of the electrodes.

The effect of a solid electrolyte interphase on the mechanism of operation of lithium–sulfur batteries

Composite sulfur–carbon electrodes were prepared by encapsulating sulfur into the micropores of highly disordered microporous carbon with micrometer-sized particles. The galvanostatic cycling performance of the obtained electrodes was studied in 0.5 M Li bis(fluorosulfonyl)imide (FSI) in methylpropyl pyrrolidinium (MPP) FSI ionic-liquid (IL) electrolyte solution. We demonstrated that the performance of Li–S cells is governed by the formation of a solid electrolyte interphase (SEI) during the initial discharge at potentials lower than 1.5 V vs. Li/Li+. Subsequent galvanostatic cycling is characterized by a one plateau voltage profile specific to the quasi-solid-state reaction of Li ions with sulfur encapsulated in the micropores under solvent deficient conditions. The stability of the SEI thus formed is critically important for the effective desolvation of Li ions participating in quasi-solid-state reactions. We proved that realization of the quasi-solid-state mechanism is controlled not by the porous structure of the carbon host but rather by the nature of the electrolyte solution composition and the discharge cut off voltage value. The cycling behavior of these cathodes is highly dependent on sulfur loading. The best performance at 30 °C can be achieved with electrodes in which the sulfur loading was 60% by weight, when sulfur filled micropores are not accessible for N2 molecules according to gas adsorption isotherm data. A limited contact of the confined sulfur with the electrolyte solution results in the highest reversible capacity and initial coulombic efficiency. This insight into the mechanism provides a new approach to the development of new electrolyte solutions and additives for Li–S cells.

High performance of thick amorphous columnar monolithic film silicon anodes in ionic liquid electrolytes at elevated temperature

The cycling performance of thick (about 7 μm) amorphous columnar monolithic film silicon anodes was studied in ionic liquid based electrolyte solutions. Cycling results obtained for these Si anodes in 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide-based electrolyte solutions are superior to those demonstrated in LiPF6/fluoroethylene carbonate/dimethyl carbonate electrolyte solution under identical conditions.