Because of their high theoretical energy density and low cost, lithium–sulfur (Li–S) batteries are promising next-generation energy storage devices. The electrochemical performance of Li–S batteries largely depends on the efficient reversible conversion of Li polysulfides to Li2S in discharge and to elemental S during charging. Here, we report on our discovery that monodisperse cobalt atoms embedded in nitrogen-doped graphene (Co–N/G) can trigger the surface-mediated reaction of Li polysulfides. Using a combination of operando X-ray absorption spectroscopy and first-principles calculation, we reveal that the Co–N–C coordination center serves as a bifunctional electrocatalyst to facilitate both the formation and the decomposition of Li2S in discharge and charge processes, respectively. The S@Co–N/G composite, with a high S mass ratio of 90 wt %, can deliver a gravimetric capacity of 1210 mAh g–1, and it exhibits an areal capacity of 5.1 mAh cm–2 with capacity fading rate of 0.029% per cycle over 100 cycles at 0.2 C at S loading of 6.0 mg cm–2 on the electrode disk.

Lithium-sulfur batteries are widely seen as a promising next-generation energy-storage system owing to their ultrahigh energy density. Although extensive research efforts have tackled poor cycling performance and self-discharge, battery stability has been improved at the expense of energy density. We have developed an interlayer consisting of two-layer chemical vapor deposition (CVD)-grown graphene supported by a conventional polypropylene (PP) separator. Unlike interlayers made of discrete nano-/microstructures that increase the thickness and weight of the separator, the CVD-graphene is an intact film with an area of 5 × 60 cm and has a thickness of ∼0.6 nm and areal density of ∼0.15 μg cm, which are negligible to those of the PP separator. The CVD-graphene on PP separator is the thinnest and lightest interlayer to date and is able to suppress the shuttling of polysulfides and enhance the utilization of sulfur, leading to concurrently improved specific capacity, rate capability, and cycle stability and suppressed self-discharge when assembled with cathodes consisting of different sulfur/carbon composites and electrolytes either with or without LiNO additive.

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