Controllable Preparation Of Carbon-Based Functional Nanofibers By Electrospinning For Lithium-Sulfur Full Battery

Lithium sulfur（Li-S）batteries have been widely studied due to their high energy density(2600 Wh kg^-1 and 2800 Wh L^-1),abundant sulfur on earth,environmentally friendly and low cost,making them a promising candidate for next-generation energy storage systems.However,the commercial application of the Li-S batteries is still hampered by some wicked issues,such as the poor conductivity of the sulfur and Li₂S,slow conversion kinetics and shuttle effect of the lithium polysulfides（Li PS）,the instability of the electrode structure in cathode;and the dendrite overgrowth,side reaction of the electrolyte and highly active lithium,and the fragile solid electrolyte interface（SEI）in anode.These issues lead to low sulfur utilization,rapid capacity fading,short cycle life,and safety concerns.Therefore,in this paper,a series of nanofibers@vertical graphene composites were synthesized by combining electrospinning with chemical vapor deposition（CVD）or plasma-enhanced chemical vapor deposition（HPECVD）technique to resolve the cathode/anode issues in high energy density Li-S batteries.The main research contents of this work are as follows:（1）To improve the electrical conductivity of cathode and enhance the conversion kinetics of Li PS,a novel strategy was developed for the growth of vertical graphene with the simultaneous conversion of the substrate semiconductor TiO₂ nanofibers into TiC ones by electrospinning coupled with CVD technique,serving as multi-functional electrocatalyst for high-performance Li-S batteries.The VG@TiC electrocatalyst not only makes up for the low conductivity of the semiconductor TiO₂ nanofibers but also retains the strong adsorption ability of Li PS.Meanwhile,combining TiC nanofiber with high surface area and conductivity vertical graphene enhances the electrocatalytic performance.Consequently,the VG@TiC electrocatalyst helps Li-S batteries to deliver a high specific capacity of 855 mAh g^-1 at 2 C.Even at a high sulfur loading of 7.3 mg cm^-2,the battery still delivers a high areal capacity of 8.7 mAh cm^-2.These results confirm the superior electrocatalysis of VG@TiC.（2）To augment the sulfur areal loading in the cathode,a 3 D self-support sulfur host of VG@TiC@CNF（CFTG-1）was prepared by electrospinning coupled with the CVD technique.CFTG-1 integrates the peculiarities of a unique 3 D carbon fiber skeleton structure,electrocatalytic active TiC site,and high-quality vertical graphene.When served as sulfur host in cathode,the unique 3 D carbon nanofiber skeleton structure can alleviate the volume fluctuation during discharge/charge process;the TiC electrocatalyst embedded on the carbon nanofiber skeleton provides abundant active sites to inhibit shuttle effect,improve conversion kinetics of Li PS,induce the uniform deposition of Li₂S,and reduce the impedance in the cathode;the vertical graphene grown on the carbon nanofibers can provide sufficient electrons passways and more surface area for Li₂S deposition,and reduce the impedance increasing caused by Li₂S deposition.As a result,Li-S batteries with CFTG-1 as sulfur host deliver a high areal capacity of 9.3 mg cm^-2 at a high sulfur loading of 10.5 mg cm^-2,which confirm the superiority of the design of CFTG-1.（3）To improve the electrochemical performance of Li-S full batteries,a novel VNCNFs composite with asymmetric structure was prepared by lab built HPECVD system.The VNCNFs could simultaneously protect the cathode and anode in Li-S batteries,for high-performance of Li-S full batteries.VNCNFs are composed of a vertical graphene layer on the upper surface and a nitrogen-doped carbon fiber layer on the bottom,presenting a typical asymmetric structure.This composite not only retains the surface active sites of the nitrogen-doped carbon nanofiber but also introduces the special structure of vertical graphene,giving a full play of synergistic effect for the two materials.When used as sulfur cathode interlayers in Li-S batteries,VNCNFs could inhibit the shuttle effect and promote the conversion kinetics of Li PS,improving the electrochemical performance of the cathode.When served as anode interlayer,VNCNFs could avoid the side reaction and inhibit the lithium dendrite growth,significantly protecting the lithium during the cycling and improving the cycle life of the batteries.Benefiting from these characteristics,the excellent electrochemical performance of Li-S full batteries can be achieved,such as the high areal capacity of17.49 mAh cm^-2 at a high sulfur loading of 15.27 mg cm^-2 and low ratios of N/P（2:1）and E/S（4:1）,and a total capacity of 43 mAh with 50 stable cycles for the pouch cell.These results fully verify the superiority of the design of VNCNFs in Li-S full batteries.（4）To further solve the problems in lithium anode and improve the cycle stability of Li-S full batteries,an asymmetric structure of VG@ZnTiO₃ composite with lithiophilic ZnTiO₃ nanofibers and lithiophobic vertical graphene was prepared.The 3D skeleton of lithiophilic ZnTiO₃ nanofibers is firstly introduced in the composite,which could improve the lithiophiliticty of the composite and alleviate the volume fluctuation of lithium anode in the plating/stripping process.Subsequently,lithiophobic high-quality vertical graphene is introduced to minimize the side reactions and improve the stability of SEI.This work has designed an asymmetric structure of VG@ZnTiO₃composite with lithiophilic ZnTiO₃ nanofibers and lithiophobic vertical graphene,serving as lithium host to protect lithium in the plating/stripping process.As a result,the symmetrical batteries assembled with VG@ZnTiO₃@Li electrodes can cycle stably for 1000 h at 10 mA cm^-2 with a plating/stripping capacity of 10 mAh cm^-2.Importantly,Li-S full batteries with VG@ZnTiO₃@Li anode deliver a high areal capacity of 19.5mAh cm^-2 at a sulfur loading of 16.5 mg cm^-2,a E/S ratio of 3.5:1 and a N/P ratio of1.5:1,The VG@ZnTiO₃@Li based pouch cell delivers a high total capacity of 59 mAh with 100 stable cycles.In this work,a novel lithium host with asymmetric lithiophiliticty and lithiophobicity structure was prepared,which could effectively improve the stability of the lithium anode and enhance the electrochemical performance of Li-S full batteries.