Synthesis Of Non-stoichiometric Scandium Oxide In Porous Carbon Materials And Study On The Catalytic Conversion Mechanism Of Sulfur Cathode

Lithium-sulfur（Li-S）batteries have become a research hotspot for secondary batteries due to numerous advantages such as its energy density up to 2600 Wh kg^-1 and the theoretical specific capacity of elemental sulfur up to1675 mA h g^-1.However,Li-S batteries confront many challenges,such as the"shuttle effect",the sluggish sulfur cathode reaction kinetics,and the volume changes of 80%in the cathodes during the charge-discharge process.Porous carbon materials are widely used as sulfur host materials due to their good electrical conductivity and rich pore structure.However,its non-polar characteristics make it difficult to anchor the polysulfides by chemical interactions,making the utilization of the active material poor.The polarity of the carbon surfaces can be increased as such a transformation effectively enhances the interfacial properties between the carbon substrates and the electroactive sulfur species.Non-stoichiometric metal oxides are often used to modify porous carbon materials due to their excellent properties such as high surface polarity,unique electronic structure,and high electrical conductivity.However,the preparation process of non-stoichiometric metal oxides by conventional methods is more complicated and uncontrollable,which greatly limits their deeper development in various fields.In this thesis,we report a one-pot strategy for synthesizing a non-stoichiometric scandium oxide that is hybridized in a N-doped porous graphitic carbon(denoted as ScO_0.95@N-PGC)and the composite shows high electrocatalytic activity in promoting sulfur cathode kinetics.ScO_0.95@N-PGC composites were synthesized by simply carbonizing the raw materials（glucose as C source,dicyandiamide as the N source,and Sc（NO₃）₃·H₂O as the Sc source）to form porous dry gels.The significance of this strategy is that the hypoxia state of the ScO_0.95 as well as N doping of the carbon matrix are simultaneously achieved in one step.It avoids the use of stoichiometric oxides as precursors and dangerous reducing agents（e.g.,H₂,NH₃）that are commonly used in conventional methods.Density functional theory data indicate that the non-stoichiometric ScO_0.95 crystals are more capable of binding to sulfur species and can reduce the activation energies of the sulfur cathode reactions more drastically compared to the stoichiometric Sc₂O₃ crystals.The Li-S batteries prepared from ScO_0.95@N-PGC composites as sulfur host materials exhibit excellent electrochemical performance in terms of excellent rate capability(438mA h g^-1 of specific capacity at 8 C),high initial specific capacity(1046 mA h g^-1 at 0.5 C)and outstanding cycling stability(641 mA h g^-1 remaining after1000 cycles at 0.5 C with a capacity decay rate of only 0.038%per cycle).This work provides a new guiding perspective for the design of sulfur host materials for Li-S batteries.