Study On The Performance Of Cathodes For Lithium Sulfur Batteries Based On Freestanding Carbon Nanomaterials

With the development of portable electronic devices and electric vehicles,traditional lithium-ion batteries have been unable to satisfy the increasing demands for energy storage due to their limited energy densities and higher prices.Lithium-sulfur batteries are considered to be one of the most promising alternatives to lithium-ion batteries because of high theoretical specific capacity(1675 mAh g^-1)and energy density(2567 Wh kg^-1).However,lithium-sulfur batteries also meet the problems of the poor conductivity of sulfur and discharge products,the large volume change during charge and discharge process,together with the shuttle effect of soluble polysulfides,which result in poor cycling stability and low utilization of sulfur,and seriously restrict the actual application of lithium-sulfur batteries.To solve these problems and improve the specific capacity of the whole electrode,our works focus on free-standing carbon nanomaterials as sulfur host for lithium-sulfur batteries,which efficiently increase the conductivity of cathode materials and suppress the dissolution of polysulfides,as well as lead to the better electrochemical performance of the Li-S batteries.It can be summarized as follows:Based on the continuous-grown carbon nanotube film,a carbon nanotube film-sulfur composite electrode has been prepared at first.The free-standing composite has excellent electrical conductivity and shows a high initial specific capacity when directly used as the cathode of a lithium-sulfur battery.Furthermore,a layer of partially reduced graphene oxide is deposited on the surface of the carbon nanotube film-sulfur composite.This layer can effectively not only adsorb polysulfides to inhibit their dissolution loss,but also increase the utilization rate of the sulfur and improve the cycle stability of the battery.In addition,the electrochemical performance of the composite electrodes under different sulfur loading per unit area are further studied.A kind of free-standing and curled reduced graphene oxide network and its preparation method are proposed.Such materials can maintain well the specific surface area and mechanical properties of graphene in macroscopic scale.Using the curled and partially reduced graphene oxide network-sulfur composite as cathode,we can effectively increase the specific capacity,reduce the shuttle effect,and enhance the cycling stability of the battery.With a sulfur content of 60 wt%,the decay rate per cycle is as low as 0.03%at 0.2 C rate.In addition,it is discussed that the effects of stacking degree and thermal reduction degree of curled graphene sheets on the battery performance.We explored the fabrication and optimization of cathode materials with high sulfur loading.Firstly,the surface of the carbonized cotton fibers are coated with partially reduced graphene oxide,which improves the conductivity of the electrode materials and the binding ability to the polysulfides,leading to the good electrochemical performance.Then,the cotton fibers can be further modified by etching via the potassium hydroxide solution to obtain hierarchical pores in the fibers,which can effectively faciliate the infiltration of electrolyte and diffusion of lithium ions.More importantly,after coated with a layer of partially reduced graphene oxide,the composites greatly suppress the outward migration of dissolved polysulfides and the loss of active loss.When the sulfur loading reaches as high as 30.9 mg cm^-2 with sulfur content of 70 wt%,the composite achieves a capacity per unit area of about 20mAh cm^-2,demonstrating its great potential in commercial applications.