A carbon sandwich electrode with graphene filling coated by N-doped porous carbon layers for lithium–sulfur batteries

Abstract

A sheet-like carbon sandwich, which contains a graphene layer as the conductive filling with N-doped porous carbon layers uniformly coated on both sides, is designed as a novel sulfur reservoir for lithium–sulfur batteries and experimentally obtained by a hydrothermal process of a mixture of graphene oxide, glucose and pyrrole, followed by KOH activation. In the hydrothermal process, graphene oxide is both employed as the precursor for the central graphene filling and a sheet-like template for both-side formation of N-doped porous carbon layers, resulting in an N-doped carbon sandwich structure (N-CS). This carbon sandwich is about 50–70 nm in thickness and has a high specific surface area (∼2677 m² g⁻¹) and a large pore volume (∼1.8 cm³ g⁻¹), making it a promising high capacity reservoir for sulfur and polysulfide in a lithium–sulfur cell. The sheet-like morphology and the interconnected pore structure of the N-CS, together with a nitrogen content of 2.2%, are transformed to the assembled N-CS/sulfur cell with a high rate performance and excellent cycling stability because of fast ion diffusion and electron transfer. At a 2C rate, the reversible capacity is up to 625 mA h g⁻¹ and remains at 461 mA h g⁻¹ after 200 cycles with only 0.13% capacity fading per cycle. More interestingly, the sheet-like structure helps the N-CS materials form a tightly stacked coating on an electrode sheet, guaranteeing a volumetric capacity as high as 350 mA h cm⁻³.