3-D Edge-Oriented Electrocatalytic NiCo2S4 nanoflakes on Vertical Graphene for Lithium Sulfur Batteries

Abstract

Polysulfide shuttling between the sulfur cathode and the lithium anode leads to low Columbic efficiency and cycling stability, which hinders the development of practical lithium sulfur batteries (LSBs). Introducing catalytic nanostructures to stabilize the otherwise soluble polysulfides and promote their conversion to solids has been proved to be an effective strategy in addressing this problem, but the heavy mass of catalysts often results in a low specific energy of the whole electrode.In this work, nickel cobalt sulfide (NiCo2S4, NCS), one of the promising catalytic materials, was investigated to enhance the LSB performance by grafting it to vertical graphene (VG) grown on carbon nanofiber film(CNF) substrate. Different from previous studies, here NCS nanosheets possess ultrathin and edge-oriented structure, inherited from the underlying VG. As a result, this NCS/VG material not only significantly reduces the needed mass of NCS catalyst but also enormously improves the conductivity of the hybrid structure. When using this free-standing film as catalytic overlayer on top of the sulfur cathode, the polysulfide shuttle effect is largely alleviated, as revealed by the enhanced electrochemical performance and the catalytic function demonstration. The assembled LSB exhibited attractive electrochemical performances of 1000 mAh g-1 specific capacity at 0.3C and 800 mAh g-1 at 1C within 400 cycles for 2 mg cm-2of sulfur loading and 850 mAh g-1 specific capacity at 0.25C for 6 mg cm-2 of sulfur loading. An areal capacity of 4.5 mAh cm-2 was demonstrated at the 100th cycle at 0.3 C.