A Janus MXene/MOF separator for the all-in-one enhancement of lithium-sulfur batteries

Rapid capacity fading and poor safety originating from the shuttle effect of S cathodes and the dendritic growth of Li anodes hinder the practical applications of lithium-sulfur batteries. Herein, we designed a thin (MXene/PP/ Cu-TCPP) Janus separator with anisotropic spatial heterogeneity regarding the sulfophile for the cathode-facing surface, while maintaining the opposite side as a multifunctional ionic sieve. Asymmetry was realized by plating nanometer-thick MXene and MOF layers onto the two different sides of a commercial separator. The highlyconductive MXene, rich with polar surface functional groups, acted as an expanded S host that confined sulfur and its intermediates on the cathode-facing surface, which boosted the conversion. The insulating and porous MOF layer on the anode-facing surface served as an "ionic sieve" that further intercepted polysulfides and guided the uniform deposition of Li and blocked Li dendrite growth. This design endowed lithium-sulfur batteries with all-in-one enhanced cycling performance, with a high discharge capacity and low self-discharge shuttle constant of 0.0004 h-1, as well as a low polarization voltage of 20 mV at 0.5 mA cm-2 for a Li||Li symmetrical cell.