Preparation Of Graphene-based Materials And Electrochemical Energy Storage For Lithium-sulfur Batteries

Recently,with the rapid development of electric vehicle and large scale energy storage devices,industry proposes higher requirement on the energy density of energy storage batteries.The traditional lithium-ion battery can hardly meet the development requirements due to its low energy density.Therefore,the development of battery systems with high energy density becomes the main direction of future development in the field of lithium batteries.The lithium-sulfur battery（LSB）has a high theoretical specific capacity(1675 m Ah g^-1)for the sulfur cathode,and sulfur is abundant in nature,environmentally friendly and inexpensive.When assembled with lithium metal,the battery can achieve high energy density(2600 Wh kg^-1,which is 6-13 times higher than the energy density of conventional lithium-ion batteries),which is considered to be one of the most promising next-generation high energy density secondary batteries.However,there are some issues make the application of LSB are still hindered:（1）The poor electrical conductivity of sulfur and its discharge product Li₂S/Li₂S₂.（2）The intermediate lithium polysulfides（Li PSs）generated during the charging/discharging process is easily dissolved into the electrolyte,causing a serious"shuttle effect".（3）There is～80%volume expansion of sulfur during electrochemical process,which leads to unstable electrode structure.These serious problems will lead to a low utilization of sulfur,serious capacity fading,limited cycle life,and overcharge phenomenon,which eventually affect the electrochemical performance.To address the problems in LSB and based on the literatures,several carbon-based materials are prepared and applied to sulfur host or separator modified layer to suppress the"shuttle effect"and improve the utilization of sulfur.The conformational relationship between the structure and electrochemical properties of the material is investigated in detail.In-depth analysis of the inhibition of shuttle effect by physical chemisorption and material nucleation site facilitation on electrochemical conversion kinetics.The details are as follows:N and O co-doped graphene block material（NOGB）with the abundant dents structure is prepared as sulfur host material.The unique layered structure with abundant dents and the co-doped N and O atoms provides a buffer for sulfur expansion and a physical confinement for sulfur species,and provides physical/chemisorption sites for Li PSs.The problems of poor cycle performance and loss of active material due to shuttle effect and volume effect have been successfully solved,so that the material has a high specific capacity and long cycle stability.N and S co-doped wrinkled porous graphene block（NSPG）is synthesized as sulfur host material.Based on buffering volume expansion of sulfur species and suppressing the shuttle effect,the solid-liquid conversion process of polysulfide is accelerated by doping with heteroatoms that can induce uniform nucleation of lithium sulfide on the surface of carbon materials.The kinetics of the electrochemical reaction is accelerated and the capacity of the cell is increased while solving the loss of active material,so that the cell has not only a high capacity but also an outstanding rate performance.In addition to the design of the cathode material to suppress the"shuttle effect"and improve the electrochemical kinetics of the lithium-sulfur battery,the modification of the separator can also be effective in achieving these results.Meanwhile,the use of commercial sulfur as the active material can effectively increase the sulfur content in the electrode.RGO-PANI/Mo S₂（RPM）with Mott-Schottky heterogeneous structure is prepared as separator modified layer.Since the material has a unique reservoir structure and abundant polysulfide adsorption and conversion sites,a successive"capture-intercept-conversion"effect can be generated toward sulfur species.The reuse and rapid conversion of polysulfides are realized,effectively solving the low-utilization of sulfur and slow kinetics problems in LSB.By using commercial sulfur as the anode,the assembled battery has high energy density in addition to high sulfur utilization and long cycle life at high rate.