Two-dimensional Transition Metal Carbide/Nitride Based Separators For Lithium-sulfur Batteries

Lithium-sulfur batteries（LSBs）with low cost and high energy density are considered to be one of the most promising candidates for portable energy storage devices;however,there are many critical issues in LSBs,including shuttle effect,self-discharge,sluggish reaction kinetics,lithium dendrite growth,and so on.To solve these issues,each component including cathode,anode,separator,and electrolyte in LSBs have been optimized.Separator,located in the center of the system,is the bridge connecting cathode and anode,which plays a particularly key role in solving the above problems.Two-dimensional（2D）transition metal carbide/nitride（MXene）shows great application prospects in separator modification for LSBs because of its high conductivity,rich polar functional groups,large specific surface area,and mechanical flexibility.Therefore,aiming at the problem of LSBs,the following two works around MXene based composite separators have been carried out:（1）MXene/MOF double-sided anisotropic separators for lithium-sulfur batteriesIn LSBs,rapid capacity fading and poor safety caused by the shuttle effect of sulfur cathodes and the serious dendritic growth of lithium anodes hinder its practical application.At present,more research is focused on solving problems of sulfur cathode or lithium anode,but less attention is paid to solving challenges of both electrodes simultaneously.Therefore,an ultrathin double-sided anisotropic separator was proposed to address both sulfur dissolution and lithium dendrite growth.Such a delicate structure was obtained by loading Cu-TCPP and MXene nanosheets on both side of commercial PP separator respectively.On the one hand,conductive MXene layer towards the S cathode surface can be considered as expanded sulfur hosts to promote dissolved sulfur reduction,so as to achieve high sulfur utilization.MXene with polar functional groups on the surface can chemisorb soluble polysulfides and inhibit shuttle,so as to improve the Coulombic efficiency.On the other hand,the insulating and porous MOF layer with a narrow pore size of 1.2 nm on the anode-facing surface serving as the“ionic sieve”not only further intercepts polysulfides but also guides uniform Li deposition to block Li dendrites.This double-sided asymmetric design endows LSBs with significantly enhanced discharge capacity,rate capacity,cycling performance,and safety.（2）Au quantum dots modified MXene separators for catalytic lithium-sulfur batteriesAlthough preliminary designed MXene/MOF double-sided anisotropic separator shows a certain effect in limiting the diffusion of lithium polysulfide and the growth of lithium dendrites,the reduction of lithium polysulfide adsorption/conversion sites and the obstruction of electron transfer caused by the restacking of MXene nanosheet and insulating MOF are not conducive to efficient adsorption and rapid conversion of sulfur.In addition,under the action of vacuum filtration,the dense stacking of nanosheets further slows down ion transport.Herein,an Au quantum dots modified MXene separator was designed for LSBs.Au quantum dots featured high catalytic activity,high conductivity,size effect,large specific surface area,and rich catalytic sites at the edge,can accelerate electron transfer,promote rapid reaction,and improve sulfur utilization.Besides,Au quantum dots as spacer alleviate the restacking of MXene nanosheets,so as to expose more active sites,enhance the adsorbability of MXene toward Li₂S_x,and accelerate ion conduction.Thanks to the strong adsorption of MXene and the catalytic ability of Au quantum dots,the assembled LSBs delivered a high reversible specific capacity,excellent rate performance,and cycle stability.