N-Doped Hierarchically Porous CNT@C Membranes for Accelerating Polysulfide Redox Conversion for High-Energy Lithium-Sulfur Batteries.

To improve the structural design of electrodes and interlayers for practical applications of Li-S batteries, we report two scalable porous CNT@C membranes for high-energy Li-S batteries. The asymmetric CNT@C (1:2) membrane with both dense and macroporous layers can act as an Al-free cathode for current collection and high sulfur loading, while the symmetric CNT@C (1:1) membrane with hierarchically porous networks can be used as an interlayer to trap lithium polysulfides (LiPSs), thus weakening the shuttle effect by strong adsorption of the N atoms toward LiPSs. The doped N sites in carbon membranes are identified as bifunctional active centers that electrocatalytically accelerate the oxidation of Li2S and polysulfide conversion. First-principles calculations reveal that the pyridinic and pyrrolic N sites exhibit favorable reactivity for strong adsorption/dissociation of polysulfide species. They lead to greatly reduced energy and kinetic barrier for polysulfide conversion without weakening the polysulfide adsorption on the membrane. Using the synergistic circulation groove with the two membranes, the practical S loading can be tailored from 1.2 to 6.1 mg cm-2. The Li-S battery can deliver an areal capacity of 4.6 mA h cm-2 (684 mA h g-1) at 0.2 C even at an ultrahigh S loading of 6.1 mg cm-2 and a lean electrolyte to sulfur ratio of 5.3 μL mg-1. Our work for scalable membrane fabrication and structural design provides a promising strategy for practical applications of high-energy Li-S batteries.