Preparation Of Vanadium-based Compounds And Their Application In Lithium-sulfur Batterie

Lithium-sulfur batteries have a high theoretical specific capacity and energy density,and is a new generation of rechargeable battery energy storage system with great application potential and prospects.However,in practice,it is difficult to realize the advantages of high specific energy and long cycle of lithium-sulfur batteries:dissolvable lithium polysulfide constantly shuttles back and forth between positive and negative electrodes during charging and discharging,lithium dendrites pierce the diaphragm causing short circuit and the non-conductivity and volume expansion of sulfur,etc.,which seriously hinder the step of lithium-sulfur batteries towards practicalization.To address these problems,in this paper,two vanadium-based heterojunction materials were successfully prepared based on vanadium-based oxides,sulfides and carbides,combining the strong adsorption ability of vanadium-based oxides for polysulfides and the high electrical conductivity of sulfides and carbides,and material modification at the cathode body and diaphragm,and the physical phase characterization and electrochemical performance tests of the materials were conducted,with the following main research:（1）By a two-step hydrothermal method,VS₂ in nanoparticles was grown on VO₂ nanowires by adding surfactant sodium citrate on the basis of VO₂synthesis,and it was seen by the physical phase characterization that VO₂exhibited a nanowire structure and VS₂ particles partially wrapped around VO₂ nanowires,forming a VO₂-VS₂ heterostructure.This heterogeneous structure takes into account both the adsorption properties of VO₂ on polysulfides and the rapid anchoring and catalytic conversion of VS₂.This dual adsorption-catalytic conversion mechanism allows the VO₂-VS₂heterogeneous structure to exhibit better and cycling performance than single components.At a of 0.2 C,the initial capacity of VO₂-VS₂@S positive electrode is 1236 m Ah g^-1,and it still has 432 m Ah g^-1 discharge capacity after 500 cycles at a rate of 1 C.（2）V₂O₅-VC modified separator materials were successfully synthesized by high temperature calcination and hydrothermal method using ammonium metavanadate and sodium metavanadate as vanadium source and glucose as carbon source,respectively.The microstructure of V₂O₅-VC exhibited a mixed morphology of rods and lamellae with significantly more surface defects and voids,which facilitated the electrolyte penetration and polysulfide adsorption.The modified layer of V₂O₅-VC@PP was in good contact with PP separator is in good contact,the surface is relatively flat,and the separator has abundant voids,which is conducive to the penetration of electrolyte.The electrochemical tests such as lithium ion diffusion coefficient,symmetric cell and lithium sulfide deposition demonstrated that the V₂O₅-VC heterojunction modified diaphragm can effectively accelerate the kinetics of polysulfide catalytic conversion.The V₂O₅-VC@PP electrode prepared by the above method has a capacity performance of 498.7 m Ah g^-1when it is cycled at 0.2 C,and the capacity performance of 814.2 m Ah g^-1 is still maintained after 300 cycles,even after 500 long cycles at 2 C high rate,there is still a capacity performance of 498.7 m Ah g^-1,the capacity attenuation rate is only 0.23%,and the coulombic efficiency is basically maintained at 100%.It is proved that the synergistic mechanism of adsorption-catalytic conversion of V₂O₅-VC heterojunction modified separator adsorbs can accelerate the catalytic conversion of polysulfides,increase the reaction kinetics of electrodes,and improve the overall electrochemical stability.