ABSTRACT
The reversible formation of chemical bonds has potential for tuning multi-electron redox reactions in emerging energy-storage applications, such as lithium-sulfur batteries. The dissolution of polysulfide intermediates, however, results in severe shuttle effect and sluggish electrochemical kinetics. In this study, quinonoid imine is proposed to anchor polysulfides and to facilitate the formation of Li2 S2 /Li2 S through the reversible chemical transition between protonated state (NH+ ) and deprotonated state (N). When serving as the sulfur host, the quinonoid imine-doped graphene affords a very tiny shuttle current of 2.60 × 10-4 mA cm-2 , a rapid redox reaction of polysulfide, and therefore improved sulfur utilization and enhanced rate performance. A high areal specific capacity of 3.72 mAh cm-2 is achieved at 5.50 mA cm-2 on the quinonoid imine-doped graphene based electrode with a high sulfur loading of 3.3 mg cm-2 . This strategy sheds a new light on the organic redox mediators for reversible modulation of electrochemical reactions.
ABSTRACT
Lithium-sulfur (Li-S) batteries with intrinsic merits in high theoretical energy density are the most promising candidate as the next-generation power sources. The strategy to achieve a high utilization of active materials with high energy efficiency is strongly requested for practical applications with less energy loss during repeated cycling. In this contribution, a metal/nanocarbon layer current collector is proposed to enhance the redox reactions of polysulfides in a working Li-S cell. Such a concept is demonstrated by coating graphene-carbon nanotube hybrids (GNHs) on routine aluminum (Al) foil current collectors. The interfacial conductivity and adhesion between the current collector and active material are significantly enhanced. Such novel cell configuration with metal/nanocarbon layer current collectors affords abundant Li ions for rapid redox reactions with small overpotential. Consequently, the Li-S cells with nanostructured current collectors exhibit an initial discharge capacity of 1,113mAhg-1 at 0.5C, which is â¼300mAhg-1 higher than those without a GNH coating layer. The capacity retention is 73% for cells with GNH after 300 cycles. A reduced voltage hysteresis and a high energy efficiency of ca. 90% are therefore achieved. Moreover, the Al/GNH layer current collectors are easily implanted into current cell assembly process for energy storage devices based on complex multi-electron redox reactions (e.g., Li-S batteries, Li-O2 batteries, fuel cells, and flow batteries).
