The Preparation And Electrochemical Performance Of The Cathode Materials In Li-X ?X=S And I₂? Batteries

The sulfur and iodine cathodes in Li-X?X=S,I₂?batteries have generated considerable interest in the field of lithium batteries due to the virtues of high specific capacities and low cost compared with the traditional lithium-ion battery cathodes.However,the intermediates?Li₂S_n,3?n?8;I₂;LiI_n,3?n?generated during charging and discharging dissolve into the organic electrolyte easily,resulting in severe shuttle effect,which leads to low Coulombic efficiency and poor cycle stability.Furthermore,the poor conductivities of the sulfur,iodine and their discharge products lead to low utilization of active materials.Therefore,to suppress the dissolution of the active materials and improve the conductivity of the cathode materials are the key points for improving the electrochemical performances of the Li-X batteries.Based on the above issues,we prepare a serious of sulfur and iodine cathodes.The electrochemical performances are evaluated and investigated.The main conclusions are as follows:?1?We employed the g-C₃N₄ nanosheets?GCN?with abundant N atoms and large surface area as sulfur host materials to prepared the GCN/S and its electrochemical performances were investigated.The abundant N atoms?53.3 at%?in GCN acted as the anchor sites of lithium polysulfides.The Li-N ineraction between the N atoms and the lithium polysulfides was proved by the XPS spectra,which enchanced the polysulfides adsorptivity and suppressed the polysulfides disso lution into the electrolyte.Furthermore,the GCN with large surface could disperse the S,Li₂S₂ and Li₂S uniformly and enhance their electric contact with the conductive additive,increasing the utilization of the active amterials.After 750 cycles,the GCN/S dispalyed a discharge capacity of 578.0 mA h g^-1 at 0.5 C.?2?We employed the carbon nitride based hollow sphere?HCN_x?with abundant N atoms?15.31 wt%?and hollow spherical structure as sulfur host material to prepared the S/HCN_x and its electrochemical performances were investigated.The abundant N atoms acted as the anchor sites of lithium polysulfides.The DFT calculation indicated the strong interaction between the N atoms in the HCN_x and the lithium polysulfides.Furthermore,the hollow structure suppressed the polysulfides dissolution with physical confinement.After 500 cycles,the S/HCN_x composite exhibited a discharge capacity of 579 mA h g^-1,the capacity decay rate was as low as 0.076%per cycle.Superior to g-C₃N₄,the HCN_x with enhanced electric conductivity could increase the utilization of the active material and improve the electrochemical performances at high current density.At 2 C,the discharge capacity remained 658 mA h g^-1.?3?The nickel-based hydroxide wrapped activated carbon cloth/sulfur composite with tree-bark-like structure was prepared and its electrochemical performances were investigated.During the preparation,the ACC/S was firstly prepared with the“ethanol-transfer-adsorption”method,which was simple and energy-saving.Then,the prepared composite was wrapped in a thin-layered nickel-based hydroxide?NNH?to form a tree-bark-like structure.The self-discharge analysis and the visual charge/discharge experiment proved that the NNH layer could suppress the dissolution of the polysulfides into the electrolyte by both the physical confinement and chemisorption.The excellent electric conductivity of the ACC promoted the sulfur utilization in the cathode with high sulfur loading.After 100 cycles,the electrode with sulfur loading of 4.3 mg cm^-2 exhibited an areal discharge capacity of 4mA h cm^-2.?4?The PVP-I₂ was employed as cathode in Li-I₂ batteries and its reactionmechanism and electrochemical performances were investigated.In this case,after combining with PVP,the vapor pressure of iodine was significantly reduced,resulting in thermostable complex.Thus,the sublimation of iodine was restrained effectively.The PVP could bond polyiodides by chemical interaction.The ex-situ Raman analysis indicated this chemical interaction could work during the whole charge/discharge process,which largely suppressed the polyiodides dissolution and retarded the shuttle effect and self-discharge.The cycle stability of the cathode was largely improved.After 300 cycles at 0.5 C,the PVP-I₂ cathode exhibited a discharge capacity of 220mA h g^-1 with little capacity decay.The in-situ Raman analysis also revealed that the evolution of iodine species in the cell was I₅^-?I₃^-?I^-.The existence of I₅^-was attributed to the strong chemical interaction between PVP and iodine,which enhanced the stability of I₅^-.Moreover,by using the“solvent-in-salt”?SIS?electrolytes or reducing the discharge/charge voltage range,the Coulombic efficiency of the cell was further improved?99.9%?,indicating that the polyiodide/iodide redox could be effectively tuned by optimizing the cathode and the electrolyte chemistry.With the ACC as host material,the PVP-I₂ exhibited a high discharge capacity of 210 mA h g^-1after 2400 cycles at 8 C.