Engineering Pore Architecture into 2D Morphology: Synthesis of Multifunctional Nitrogen-Doped Porous Carbon Nanosheets Towards Long Calendar Life Lithium-Sulfur Battery
Authors: Feng Shuo , Song Junhua , Du Dan , Lin Yuehe 
Published: February 2020

Owing to its high theoretical capacity, lithium-sulfur battery (Li-S) is considered as one of the leading alternatives to conventional lithium-ion batteries (LIBs) for electrical vehicles and grid-scale energy storage systems. The sulfur cathode has a theoretical capacity of 1675 mAh/g, accounting for an energy density as high as 2600 Wh/kg when coupled with lithium metal anode.1 Nevertheless, Li-S still need to overcome several challenges before being implemented in practical applications. The major issues include the low conductivity of sulfur, large volume change and severe polysulfide shuttling.2 In the recent reports, carbon material have been considered as an efficient sulfur host material to promote Li-S’s performance. However, it is difficult to solve all the problems simultaneously by a simple carbon material design. For instance, carbon nanotube has the advantages of its excellent conductivity, large surface area to volume ratio and prominent mechanical strength. However, it lacks porous sulfur reservoir to store the active sulfur, which makes their dissolution into electrolyte inevitable.3 Therefore, it is of great importance to develop multi-functional carbon materials for high-performance Li-S.

Porous carbon materials have been intensively studied as the sulfur host due to its tunable porosity, large pore volume and controllable morphologies.4 The micro/meso pores can not only provide enough void space for sulfur volume expansion but also squeeze the cyclic sulfur molecules into short chain sulfur species and hence physically immobilize them from detrimentally shuttling to the anode side. To these aspects, we applied a dual-template strategy to synthesize nitrogen-doped two-dimensional porous carbon nanosheets (N-PCS). During the synthesis, we successfully integrating carbon pores into rational-designed secondary structure. The 2D carbon skeleton can effectively form conductive pathway for rapid electron/ion transfer. Moreover, when further doped with nitrogen, the carbon matrix enables a significantly improved conductivity and stronger interaction with polysulfides species due to the chemical bonding. With the advantages, when using N-PCS and sulfur composite as active material, an initial discharge capacity of 1360 mAh/g can be achieved at 0.1 C, showing a sulfur utilization as high as 81%. After 1000 cycles at 0.5 C, the cell can still retain ~50% of its initial capacity. At 1 C rate, N-PCS/S cathode is capable of delivering a high discharge capacity of 996 mAh/g, indicating its excellent electron conductivity and rate capability.

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Figure 1