Graphene Materials Used In Lithium Sulfur Battery

The severe capacity decay and low sulfur mass loading of sulfur cathode,as well as the poor Coulombic efficiency upon cycling hinder the commercial application of Li-S batteries.These issues derive from the low conductivity of elemental sulfur and its final discharge product Li2S,the volume expansion of sulfur during discharge,and the dissolution and shuttle effect of highly soluble polysulfides(Li2Sx,x =4,6,and 8)generated during cycling.Graphene,a sp2-hybridized carbon,has high electrical conductivity,flexibility,mechanical strength,and excellent chemical resistance.The introduction of graphene into the cathode or separator would engender the properties of graphene to the Li-S batteries,which may enhance the electrochemical performance of Li-S batteries.However,the relevant of graphene structure to the electrochemical performance of sulfur/carbon composite as a cathode of Li-S battery is still unclear.Furthermore,the low sulfur content in the cathode decreases the areal capacity of Li-S battery.Finally,the severe increase of thickness and weight for the separator induced by the interlayers made of graphene powder undermines the energy density of Li-S batteries.Based on this,the main research results are summarized as follows:(1)Understanding the effect of carbon structure on the electrochemical properties of sulfur/carbon composites is of great importance to guide future rational design of the sulfur/carbon composite for optimized cathode of Li-S batteries.Based on the fact that graphene has a tunable pore structure,and is ready for functional groups decoration,reduced graphene oxide(RGO),cysteine-modified RGO(RGO-CYS),microwave-expanded graphite oxide(MEGO)and activated-MEGO(aMEGO)with specific surface areas(SSA)of 58-3010 m2 g-1,C/O atomic ratios of 2.41-10.81,pore sizes of 0.9-5.6 nm and pore volumes of 0.14-2.03 cm3 g-1 were prepared and used as the sulfur host materials.The key factors affecting the electrochemical properties of S/C composite were obtained by comparing the discharge capacity,ratio performance and cycle stability of S/RGO,S/RGO-CYS,S/MEGO and S/aMEGO.(2)The slow liquid-solid phase transformation of soluble Li2S4 to solid Li2S leads to the severe shuttle effect of polysulfides and restricts the sulfur mass loading in cathode.Based on the high catalytic activity and maximum utilization efficiency.of single-atom catalyst,Co single atom supported on nitrogen-doped graphene(Co-NG)was prepared from the thermo reduction of graphene oxide using NH3 and CoCl2 as the dopants.The S/Co-NG composite with a high sulfur content of 90 wt.%exhibits a high discharge capacity of 618 mAh g-1 at a high rate of 4 C.Even when the areal sulfur mass loading was increased to 6.0 mg cm-2,the S/Co-NG still exhibits a high areal capacity of 5.1 mAh cm-2 with 0.029%capacity fading per cycle over 100 discharge-charge cycles.Symmetric cell and in-site XANES test results demonstrate that atomic Co-N sites in Co-NG serve as the bifunctional electro-catalysts participating in the formation and decomposition of solid Li2S in discharge and charge processes,respectively,to accelerate the kinetics of polysulfides,thus inhibiting the shuttle effect of polysulfides.(3)The large amount of binder needed in the slurry-coating method reduces the electrical conductivity of the interlayer,and the unsatisfactory thickness and high loading of the interlayer undermines the energy density.Based on the high electrical conductivity and large single-layer coverage of CVD-graphene film,an interlayer consisting of atom-thick CVD-graphene supported by a polypropylene separator was developed.The CVD-graphene is able to suppress the shuttling of polysulfides and enhance the utilization of sulfur.The optimized number of CVD-graphene layers was found to be two.Unlike interlayers made of discrete nano/micro-structures that increase the thickness and weight of the separator,the CVD-graphene has a thickness of-0.6 nm and areal density of-0.15 ?g cm 2,which is the thinnest and lightest interlayer to-date.When assembled with different sulfur/carbon composites and electrolytes,the CVD-graphene modified separator exhibits improved electrochemical performance.