Rapid Synthesis Of Graphene Based Materials And Their Applications In Lithium-Sulfur Batteries

With the aggravation of environmental pollution and the growth in energy demand for portable electronic devices and the electric vehicles will call for the use of low cost and higher energy density and longer cycle energy storage devices.Sulfur is characteristic of the features of excellent thermodynamic stability,rich oxidation,high theoretical specific capacity(1680m A·h/g),abundant resouces and low cost,while lithium has the lower standard potential,density and high theoretical specific capacity.Lithium sulfur battery is considered to be the most promising low cost and high energy density of energy storage equipment.At present,the challenges of the commercialization of lithium sulfur batteries are mainly derived from the insulation of elemental sulfur,the intermediate products are easy to dissolve in the electrolyte and the positive electrode volume changes during the cycles.Graphene has many excellent properties such as high electron conductivity(2×105 cm2/V·s),large specific surface area(2630 m2/g)and high mechanical strength,which can effectively improve the conductivity of the cathode material,accommodate the volume expansion and prevent the aggregation of nanoparticles.In this paper,graphene and modified graphene were prepared by oxidation reduction method,and then a series of graphene based carbon/sulfur composites were prepared by chemical precipitation method.Firstly,graphene oxide has been synthesized by improved Hummer’s method.Graphene has been synthesized from graphene oxide through microwave method,and then graphene/sulfur composite(MRGO/S)has been synthesized by chemical method,in which the size of sulfur is~20 nm.The excellent conductivity of graphene and nano sulfur is conducive to increase the electronic conductivity and utilizationof the anode materials The electrochemical performance test showed that the discharge capacity of MRGO-5/S composites is delivered at 1247 m A·h/g after the 1st cycle,and remains at 559 m A·h/g after 50 cycles at 0.1 C.The specific capacity of MRGO-5/S composites is 680 m A·h/g at 1 C and 430 m A·h/g at 2 C,respectively.It is obvious that the MRGO/S composite has showed good rate stability.Secondly,nitrogen and sulfur co-doped graphene(NSG)has been synthesized by microwave method with thiourea as nitrogen and sulfur source.The electrochemical activity,adsorption of the graphene can be improved by the doping of the N,S heteroatom.The affinity between the conductive substrate and the active sulfur let the electrochemical reaction more uniform,thereby improving the cycle life of the lithium sulfur battery.The content of nitrogen and sulfur is 2.47wt% and 6.31 wt% respectively in the NSG.And it has the similar structure as graphene through the observation of TEM and SEM,which the film layer is distorted and the surface appears lots of fold.The electrochemical performance tests showed that the NSG-4/S composite has displayed the initial discharge capacity of 1581 m A·h/g,and the discharge capacity remains 819 m A·h/g after 100 cycles at 0.1 C.Here,NSG-4/S presents a high reversible capacity comparing to RGO/S composite.The graphene/carbon nanotube/sulfur composite(GC/S)has been preparad by chemical method after ultrasonic mixing carbon nanotubes and graphene oxide.The three-dimensional network structure formed by the combination of carbon nanotubes and graphene can effectively inhibit the agglomeration of graphene and slow down the volume change of sulfur electrode during charge and discharge process.The GC/S composite is three-dimensional structure with pore,and the carbon nanotubes are dispersed uniformly between the graphene layers,and the sulfur particles are wrapped in the graphene sheets.Through the electrochemical performance test,the discharge capacity of GC/S is delivered at 1405 m A·h/g after 1st cycle,and remains at593 m A·h/g after 50 cycles at 0.1 C.GC/S composite can still maintain a three-dimensional network skeleton after cycles,showing good cycling stability.