Study On Design And Electrochemical Performance Of Sulfur Cathode For Lithium-sulfur Battery

The energy density of the conventional lithium-ion batteries is close to their theoretical value with the improvement and perfection of the manufacturing technique.To better meet the requirement of mobile electronic devices and electric vehicle,Lithium-Sulfur?Li-S?battery is considered to be a promising candidate as the next generation secondary battery system due to its high theoretical capacity of 1675 mAh g^-1,non-toxic and low cost.However,the current Li-S batteries are still hindered by many unfavorable factors of the sulfur electrode,such as the low ionic and electronic conductivity of sulfur,the soluble intermediate Li polysulfides and the irreversible lithium sulfide by-products and the volume of sulfur cathode changing greatly during charging-discharging,which would greatly lower the utilization of active materials,cause low discharge capacity and short cycle life and restrict the practical application of Li-S battery.Aiming at solving the above problems of lithium sulfur battery cathode,our research starts with focusing on the structural design of sulfur cathode,constructing a stable and reliable carbon network as an effective carrier of the active material sulfur.We firstly synthesize nitrogen and phosphorus co-doped disordered mesoporous carbon as a carrier for sulfur.This kind of disordered mesoporous carbon with large specific surface area and pore volume can effectively accommodate sulfur discharge reaction.It is found that the defect sites produced by heteroatom doping change the electronic structure of carbon surface are easier to form strong chemical adsorption with polysulfides,thus alleviating the"shuttle effect".This nitrogen phosphorus co-doped disordered mesoporous carbon/sulfur composite exhibites superior electrochemical activity and performance,delivers an initial discharge capacity of948.1 mAh g^-1 at 0.5 C rate,with capacity retention of 71.4%after 400 cycles.The carbon material has good electrical conductivity,by combining sulfur with carbon it can improve the ion diffusion rate and electron transmission rate of sulfur cathode.However,the polysulfides ions adsorption of carbon carrier is weak,which cannot effectively avoid the diffusion and loss of soluble polysulfide ions,while metal compounds have strong interaction materials with polar polysulfide,and can capture polysulfide more effectively.Based on this,we synthesis CeO2@G composite benefiting from both high conductivity carbon materials and chemical adsorption metal compounds as the carrier for sulfur.It was confirmed that CeO₂ nanoparticles were uniformly distributed on graphene,and CeO₂@G/S composite possessed both high conductivity and good catalytic activity.The results of electrochemical performance test show that the presence of CeO₂ nanoparticles can promote the reaction from soluble long-chain polysulfides to short-chain polysulfides and lithium sulfide.Composite electrodes exhibit better cyclic stability,its initial discharge capacity is 1173.3 mAh g^-1 at 0.1 C rate,with capacity retention of 62.4%after 100cycles.In common carbon/sulfur composite cathode,sulfur is generally introduced into the pre-synthesized carbon conductive framework through the capillary action under the high-temperature fusion.In this paper,under the condition of liquid phase,the reduction degree of partial reduced GO was regulated to optimize the thickness of sulfur layer deposited on it.Under the combined effect of both electrostatic repulsion and surface energy,composite materials with sulfur uniformly dispersed in regular layered prGO conductive carbon network carrier were prepared.The electrical conductivity of partially reduced GO obviously improved.The layered conductive carbon network is highly ordered and sulfur is evenly distributed in it.The electrical conductivity and the distribution and types of surface oxygen functional groups of prGO/S composites with different reduction time were studied.It was confirmed that chemical adsorption existed between sulfur and residual oxygen functional groups,which was helpful to alleviate the"shuttle effect"and improve the cycle performance of lithium-sulfur batteries.The orderly layered carbon network structure remained stable during charge-discharge process.The initial discharge capacity of this composite is up to 1107.1 mAh g^-1,and the capacity retention rate is 64.7%after 100cycles.On the basis of this,considering the conductivity of the partial reduced graphene oxide rising and the residual oxygen functional groups adsorption of polysulfides,we extend the application of partially reduced graphene oxide to the cathode coating material and interlayer of lithium sulfur battery.By depositing a layer of reduced graphene oxide on the battery separator as an interlayer,it can effectively relieve the“shuttle effect”and improve cyclic stability of sulfur cathode.After coating by partial reduced graphene oxide and using it as interlayer,the discharge capacity of the first cycle rise to 1218.1 mAh g^-1 and the capacity retention ratio is still 80.5%after 100cycles.