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Nonaqueous redox flow batteries (RFBs) with an organic electrolyte can work at voltages higher than 2.1 V, thanks to the excellent electrochemical stability of such electrolytes.However, the biggest challenge related to nonaqueous RFBs is that the electrochemically active salts used in these batteries exhibit lower solubilities in organic electrolytes compared to those in aqueous electrolytes. In this study, bis(trifluoromethanesulfonyl)imide (TFSI) is investigated as the counter anions for the complexes of iron and nickel tris(2,2'-bipyridine) ((Bpy)3) for use as electrolytes in high-energy RFBs. The synthesized salts were characterized systematically through ab-initio calculations, nuclear magnetic resonance (NMR), inductively coupled plasma mass spectrometry (ICP-MS) and electrochemical analyses. Ni(Bpy)3(TFSI)2 and Fe(Bpy)3(TFSI)3 containing bulky imide anions exhibit the improved solubility by weakening inter-anion Coulombic interactions, via charge delocalization. [ABSTRACT FROM AUTHOR]

New insight into the oxidation mechanism of solid Li2S is presented by investigating a specially designed cell in which the Li2S particles are electrically isolated from the carbon cathode. Surprisingly, the cell containing the isolated Li2S particles delivered considerable charge and discharge capacities despite the prevention of any charge transfer process between the Li2S particles and the carbon cathode. This fact directly indicates that the electrochemical oxidation of Li2S occurs not through a direct charge (electron) transfer between solid Li2S and conducting materials but through chemical reactions coupled with the charge transfer process. We believe that these unexpected results will greatly contribute to a deep understanding of the exact working mechanism of Li-S batteries and Li2S cathodes as well as a paradigm shift toward an innovative and rational design of Li-S batteries. [ABSTRACT FROM AUTHOR]

This work presents a new insight into the reduction mechanism of solid sulfur during the first step of cell discharge, that is, a 2.4 V plateau in Li-S batteries, by testing a specially designed cell with the solid sulfur electrically isolated from the carbon cathode and comparing it with a conventional cell. Importantly, the cell with the electrically isolated sulfur particles confined between two separators shows very normal operation even during the first cycle and provides the same result as a conventional cell after several cycles. Based on the controlled potentiostatic and galvanostatic experiments, we propose a reasonable reaction route: a portion of the electrically isolated solid sulfur (S8) is dissolved in the electrolyte solution to form sulfur molecules, which can be electrochemically reduced to polysulfides on the carbon surface. [ABSTRACT FROM AUTHOR]

We investigated a novel approach for utilizing LiPF6 as the lithium salt for Li–S batteries and verifying its chemical reactivity with the main solvent. It is found that the main obstacle for the adoption of LiPF6 is the undesired acid-catalyzed, cascade-type polymerization reaction between cyclic ether components in the solvent and LiPF6. Therefore, several kinds of acid scavengers are proposed to enhance the chemical stability between the main solvent and LiPF6. Simple storage tests indicate that polymerization occurred as acid residue is removed from the electrolyte. Consequently, the cell with a modified electrolyte shows excellent discharge capacity and moderate retention based on its improved chemical stability. These results indicate that assuring the chemical stability is the most important factor to utilizing LiPF6 as the main lithium salt for a Li–S cell. Additionally, it is believed that an understanding of the nature of chemical reactivity will be beneficial to constructing more efficient electrolyte systems owing to enhanced electrochemical performance of many kinds of energy storage systems including Li–S, Li–air, and metal–air batteries. [ABSTRACT FROM AUTHOR]