New insight into the oxidation mechanism of solid Li 2 S is presented by investigating a specially designed cell in which the Li 2 S particles are electrically isolated from the carbon cathode. Surprisingly, the cell containing the isolated Li 2 S particles delivered considerable charge and discharge capacities despite the prevention of any charge transfer process between the Li 2 S particles and the carbon cathode. This fact directly indicates that the electrochemical oxidation of Li 2 S occurs not through a direct charge (electron) transfer between solid Li 2 S 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 Li 2 S cathodes as well as a paradigm shift toward an innovative and rational design of Li-S batteries.

Phosphorus-doped soft carbons were successfully prepared by carbonizing petroleum cokes treated with phosphorus acids, such as H 3 PO 2 , H 3 PO 3 , and H 3 PO 4 . The effect of the type of phosphorus acids used on the composition, structure, and electrochemical performance of the soft carbons was extensively investigated. The soft carbons modified with H 3 PO 3 and H 3 PO 4 exhibited dramatically improved reversible capacities and outstanding rate capabilities.

The electrochemical performance of Li-S cells was investigated in various ternary electrolyte solutions composed of 1,2-dimethoxyethane (DME), tetra(ethylene glycol) dimethyl ether (TGM), and 1,3-dioxolane (DOX). The discharge capacity values and cycle data obtained at each composition were statistically treated with the Minitab program to obtain mixture contour plots, from which the optimal composition of the ternary solvent systems was predicted. The discharge capacities and capacity retention were quite dependent on the electrolyte composition. It was estimated from the contour plots of the capacity at 1.0 C that the discharge capacity sharply increased with a decrease in the TGM content. High capacities greater than 900 mAh/g at 1.0 C were expected for the electrolyte composition with a volume ratio of DME/TGM/DOX = 1/0/1. In contrast, it was predicted from the mixture contour plot of the capacity retention that the cycle performance would significantly increase with an increase in the DME content.

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 (S 8 ) is dissolved in the electrolyte solution to form sulfur molecules, which can be electrochemically reduced to polysulfides on the carbon surface.

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This paper examines the influence of the sulfur concentration on the electrochemical properties of Li-S cells over a very wide sulfur concentration range of 1.5 to 14.2 m. The sulfur utilization decreased gradually from 74 to 40% with increasing sulfur concentration in this concentration range. Surprisingly, the Li-S cells successfully operated with a considerable capacity (675 mAh/g) even under an extremely high sulfur concentration of 14.2 m. At a high rate of 5.0 C, the cells with 9.8 and 11.5 m sulfur showed much poorer rate capabilities than those with 2.3 and 5.6 m sulfur. This feature was attributed mainly to the significant ohmic drop and large reaction overpotential due to chemical reactions coupled with the electrode reactions of sulfur and polysulfides. In addition, there was a notable variation in the cycle performance with a change in the sulfur concentration. Interestingly, higher capacity retention was observed in 5.6 and 9.8 m sulfur than in low (2.3 m) and high (11.5 m) sulfur concentrations.

We examine the potential use of disordered mesoporous carbon as a functional additive for confining dissolved Li-polysulfides and improving the cycling performance of Li-S batteries. To promote a better understanding of the correlation between the total pore volume of disordered mesoporous carbon and the cycling performance of Li-S batteries, a series of disordered mesoporous carbons with different total pore volumes are successfully synthesized using a commercial silica template. Based on the electrochemical and structural analyses, we suggest that the total pore volume of disordered mesoporous carbon is a predominant factor in determining its capability for either the absorption or adsorption of Li-polysulfides, which is primarily responsible for enhancing the cycling performance. The addition of disordered mesoporous carbon is also effective in enhancing the homogeneous distribution of active sulfur in the cathode, thereby affecting the cycling performance.

We present a new and facile design of a high-performance Li-S cell by integrating a Li 2 S-impregnated glass fiber separator together with a common sulfur cathode. We find that a considerable amount of Li 2 S is consumed amidst the first charge, and most of Li 2 S disappears at the end of the second charge. During the charge process, additional sulfur material is formed and contributes to a significant enhancement of the discharge capacity (~1400 mAh/g), compared with a control cell (~1260 mAh/g) without Li 2 S. Moreover, the Li 2 S containing cell exhibits much higher cycling stability (a 31% increase from ~840 to ~1100 mAh/g in the 100th cycle) and rate capability (a 30% increase from ~580 to ~750 mAh/g at 2 C) than the control cell. Our results indicate that adopting Li 2 S-containing separator is highly effective to improving the electrochemical performances of Li-S cells.