Three-dimensional Integrated Electrode And Interlayer For Lithium-sulfur Batteries

With the high theoretical energy density of 2600 Wh kg-1,lithium sulfur(Li-S)batteries is one of the promising next-generation secondary battery systems.However,the shuttle effect from the migration of dissolved polysulfides results in the serious capacity fading,inhibiting the practical application of Li-S batteries.Inserting an interlayer between the cathode and separator,and using small S2-4 molecules,instead of traditional S8,as active materials are two effective methods to enhance the cycle performance of Li-S batteries.In this paper,a three-dimensional(3D)conductive carbon interlayer with high electrolyte adsorption capability is prepared to mitigate the shuttle effect;Furthermore,focusing on the electrode fabrication,three methods are employed to prepare the 3D integrated ultra-microporous carbon(UMC)/S2-4 electrodes with high capacity and excellent cycle stability,including using 3D carbon foam as current collector of S2-4/UMC electrode,constructing a graphene-bonded S2-4/UMC electrode by self-propagating method,and assembling a flexible S2-4/UMC electrode with MXene as multi-functional conductive binder.(1)A 3D conductive carbon interlayer with high electrolyte adsorption capability was prepared.A 3D interwoven interlayer(CCF)with hollow structure was simply prepared by carbonization of cotton fabric.Inserting the CCF interlayer between cathode and separator can inhibit the shuttle effect by block the migration of soluble polysulfides.The effect of the interlayer structure and electrolyte adsorption capability on the electrochemical performance of Li-S batteries was confirmed by comparing the electrochemical performance of the cells with the 3D CCF interlayer and CCF coating interlayer with destroyed 3D structure.It is found that the 3D CCF interlayer with more higher electrolyte adsorption capability(9.64 g g-1)can absorb nearly all the electrolyte and the dissolved polysulfides to inhibit its migration.Furthermore,the 3D network can form a continuous network for the electron transfer and provide more active sites to the conversion and utilization of the polysulfides.As a result,by inserting the 3D CCF interlayer,the capacity of the Li-S batteries reaches to 1346.9 mAh g-1 at 0.1 C,and retains 1076.6 mAh g-1 after 100 cycles,a capacity of 553.2 mAh g-1 is achieved even at 4 C rate.Even for the cathodes with 5 mg cm-2 high sulfur loading,the cell with 3D CCF interlayer performs 1085 mAh g-1 high capacity.These results indicate that using the 3D conductive carbon interlayer with high electrolyte adsorption capability can significantly improve the capacity,cycle stability and rate performance of Li-S batteries.(2)A S2-4/UMC electrode with high areal sulfur loading was constructed with 3D carbon foam(MFC)as current collector.An ultramicroporous carbon(UMC)with 0.55 nm pore size was prepared by directly carbonization of polyvinylidene fluoride,which can only encapsulate the S2-4 molecules and avoid the appearance of S5-8 by its pore size,in favor of fundamentally eradicating the formation and dissolution of polysulfides and the generated shuttle effect.Thus,the S2-4/UMC is a promising cathode material for Li-S batteries with excellent cycle stability.Based on the S2-4/UMC elecrode,a 3D MFC carbon foam is prepared and employed as 3D current collector to enhance the areal sulfur loading of S2-4/UMC electrode.The 3D MFC current collector can hold massive S2-4/UMC composites and build a conductive network for fast electron transfer.Moreover,enough free space is reserved even after holding the S2-4/UMC composites,which favors the electrolyte infiltration and ion transport,and provids effective contact between the S2-4 and Li+to enhance the utilization of active S2-4.As a result,with the 3D MFC foam current collector,the S2-4 electrode reaches an areal sulfur loading of 4.2 mg cm-2,and performs a capacity of 837.8 mAh g-1 with 82.7%retention after 100 cycles.The 3D MFC current collector provides a new way to prepare the S2-4/UMC electrode with high sulfur loading.(3)A 3D freestanding rGO-bonded S2-4/UMC electrode was prepared by self-propagating reduction of graphene oxide(GO)-S2-4/UMC film.The S2-4/UMC was firstly mixed with GO then underwent a vacuum-assistant filtration to get the freestanding GO-S2-4/UMC film.After a simple,fast self-propagating reduction process,the rGO-bonded S2-4/UMC film is achieved,which can avoid the loss of active S2-4 in the pores of UMC.Moreover,the gas generated during the self-propagating reduction process formed a fluffy film structure,facilitating the infiltration of electrolyte and transfer of Li+.Furthermore,the conductive rGO nanosheets construct a continuous conductive network for the fast electron transfer.As a result,the freestanding rGO-bonded S2-4/UMC electrode presents better electrochemical performance than the traditional electrode with a high capacity of 1095.7 mAh g-1 at 0.1 C,and achieves 597.4 mAh g-1 high capacity at 2 C rate.Moreover,when the sulfur loading increased to 2 mg cm-2,a capacity of 890 mAh g-1 is achieved.The self-propagating reduction method is an efficient method to prepare rGO-bonded sulfur electrode for Li-S batteries by avoiding the loss of sulfur.(4)A flexible S2-4/UMC electrode was prepared by introducing 2D Ti3C2Tx(MXene)nanosheets as the conductive binder and flexible substrate.The flexible MXene-bonded S2-4/UMC(MSC)electrode was constructed with 2D conductive MXene nanosheets as conductive binder and flexible substrate.The MXene nanosheets was firstly mixed with S2-4/UMC to make a solution,then underwent a vacuum filtration process to get the flexible MSC film.The S2-4/UMC particles were uniformly embed in the 3D MXene framework,inhibiting the restack of MXene nanosheets and facilitating the ion transport.MXene nanosheets acting as the flexible substrate,the conductive agent and the the binder simultaneously,provid good flexibility and construct a continuous network for the fast electron transfer.The MSC electrode can be directly used as flexible cathode of Li-S batteries,which has a capacity of 1029.7 mAh g-1 at 0.1 C and a capacity retention of 91.9%after 200 cycles.Besides,the freestanding MSC electrode performs a capacity of 502.3 mAh g-1 at 2 C.This work provides the foundation for preparing flexible electrode of Li-S batteries with excellent cycle stability.(5)A 3D porous MXene foam is prepared via sulfur-template method to prevent the aggregation of MXene nanosheets.The 3D porous MXene foam provides sufficient active sites for lithium ion storage,facilitates the electrolyte infiltration and Li+transfer,and constructs continuous channels for fast ion transport.Furthermore,the porous MXene foam performs excellent conductivity and flexibility,which can be directly used as the anode of Li-ion batteries.As a result,the 3D porous MXene foam exhibits a high capacity(455.5 mAh g-1 at 50 mA g-1),excellent rate property(101 mAh g-1 even at 18 A g-1)and an ultralong-term cycle stability(220 mAh g-1 at 1 A g-1 after 3500 cycles).This study provides a reference for the MXene’s application in other fields such as energy storage and catalysis.