Recently, Organodisulfide compounds have attracted extensive research interests as cathode materials due to the advantages of higher theoretical capacity, environmental friendness, lightweight and abundant resources for high specific energy lithium secondary batteries. The key moiety of high specific capacity in organodisulfide compounds is the reversible two-electron redox reaction of the disulfide bond (S-S). However, the exiting problem of organodisulfide compounds as electrodes materials in LIBs is the large capacity fading caused by the high solubility of polysulfides in liquid electrolytes. An Important strategy to solve this problem is to polymerize the small molecules into insoluble polymeric chains in order to improve the cyclic stability of the electrodes.

Based on this strategy, the electrochemical performance and energy storage mechanism of Poly [1, 2-dithiole-3-thion-4(5)-thio] (PDTT) and Poly [2-(1, 2-ethylenediamino)-1, 6, 6a, Δ⁴-trithia-1, 6-diaza-pentalen-5-yl] (PETDP) were designed, synthesized, and investigated. As shown in figure 1, based on the two-electron reaction, PDTT can provide theoretical capacity of 326 mAh g^-1, and the theoretical capacity of PETDP is calculated to be as high as 371 mAh g^-1 based on the three-electron reaction. Electrochemical performances of PDTT and PETDP were evaluated by cyclic voltammogram and galvanostatic discharge/charge measurements. CV results indicated that PDTT electrode has two pairs of redox peaks at 2.09/2.38 and 2.29/2.63 V. PETDP cathode also presents two pairs of well-defined redox peaks at 2.11/2.26 and 2.37/2.42 V, respectively. Cyclic performance of PDTT and PETDP was compared using two type of electrolytes, the ether based (1M LiTFSI in DME/DOL) and carbonate based (1M LiPF₆ in EC/DMC) electrolytes. Results show that both of them deliver best cyclic stability in ether based electrolyte. In addition, the discharge and charge profiles of PDTT electrode are consistent with its CV curves and having specific capacity of 321 mAh g^-1 at a current density of 0.1 C (1C=33 mA g^-1), this value is as high as 98.4% of the theoretical capacity. Even at 0.5C rate, high initial capacity of 318 mAh g^-1 (97.5% of theoretical value) was obtained, confirming the almost fully utilization of active materials. When cycled over 50 cycles, PDTT electrodes can still deliver high capacities of 291 mAh g^-1with capacity retentions of 90.6%, showing a high potential to be used as organodisulfide cathode materials for high capacity Li-S batteries.More details about the charge storage mechanism studies using synchrotron based X-ray absorption spectroscopy will also be presented at the meeting.

Acknowledgement

The work at Brookhaven National Lab. was supported by the U.S. Department of Energy, the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies (BMR and VTO Battery500 projects) under Contract Number DE-AC02-98CH10886.