Research On G-C₃N₄-based Composites As Modified Layers On Polyolefin Separator Towards High-performance Lithium-sulfur Batteries

Lithium-sulfur(Li-S)batteries have been deemed as one of the most promising next generation energy storage equipment for their high theoretical specific energy density(?2600Wh·kg^-1)and the low-cost and abundant reserve of sulfur.However,many troubles,such as sluggish redox kinetic and shuttle effect of polysulfides which result in low practical specific capacity and coulombic efficiency as well as poor cycle stability,obstruct their commercial applications.It has been indicated that the modification of pristine separator with various functional coating layers is a feasible method to further boost the electrochemical performance for Li-S batteries.The modified layer can not only act as a secondary collector to enhance the reutilization of sulfur-based species,but also can act as a physical barrier to block the polysulfides and thus decrease their shuttle effect.Various nanomaterials deposited onto one surface of original separator to improve the electrochemical performance of Li-S batteries by adsorbing the soluble polysulfides have been reported.Notably,graphitic carbon nitride(g-C₃N₄),with the rich N content(?57.1 atom%)can provide abundant chemical adsorption sites for polysulfides.However,the low electron conductivity and poor catalytic effect for the conversion of the polysulfides hinder its practical application.Herein,g-C₃N₄-based composites with unique structural features and chemical compositions were firstly synthesized and then used as functional layers onto polyolefin separator in Li-S batteries.The specific works are as follows:(1)A hybrid material consisting of well-dispersed and needle-like shape Co S anchored on g-C₃N₄ nanosheets(denoted as Co S@g-C₃N₄)was firstly prepared by a facile solvothermal strategy.Then it was integrated on the pristine polypropylene(PP)membrane combined with highly conductive KB(KB=Ketjen black)by simple flirtation.A multifunctional and ultrathin interlayer with a thickness of only?2.1 um and areal mass loading of?0.07 mg·cm^-2 firstly can capture polysulfides by Li-N bond as well as Lewis acid-base interaction between Co²⁺and S_x^2-,and more importantly,it also plays a positive effect on catalyzing the redox conversion of intermediate polysulfides.As expected,a Li-S cell assembled with this modified separator and high sulfur content cathode(70 wt%)displays a high specific capacity of?1290 m Ah·g^-1 at 0.2 C rate and a very low fading rate of 0.03%per cycle after 500 cycles at 1.0 C rate,showing an excellent cycle stability.(2)A composite consisting of ultra-fine Co particles anchored on g-C₃N₄ and graphene nanosheets(denoted as Co@g-C₃N₄/G)was firstly prepared successfully by a facile one-step calcination.Then it was deposited on the surface of PP separator by vacuum filtration to obtain a functional separator with a Co@g-C₃N₄/G interlayer.The functional layer plays three important roles for improvement the electrochemical performance in a Li-S cell assembled by this modified separator.Firstly,an efficient conductive network formed by the graphene nanosheets can be used as a secondary collector to promote the conversion of polysulfide.Secondly,the mesoporous in the interlayer can offer space for accommodation of the adsorbed polysulfide.And thirdly,the obtained highly dispersed and ultrafine Co particles,achieved from the effective prevent of the agglomeration of Co nanoparticles by the formation of Co-N_x bonds,can provide abundant active catalytic sites for the acceleration of lithium polysulfide conversions.Thus,a Li-S cell with this functional layer and S/KB cathode(70wt%)exhibits a high specific capacity of?1458 m Ah·g^-1 at 0.1 C and a low capacity fading rate of 0.08%per cycle in 400 cycles at 1 C rate.