Preparation Of Vanadium Nitride Integrated Sulfur Host And Research Of Energy Storage In Lithium-Sulfur Battery

Lithium-sulfur batteries are considered to be one of the most promising candidates for the next generation of efficient and economical energy storage systems due to their ultra-high theoretical energy density(2600 Wh kg^-1),low material prices,and excellent environmental friendliness.However,the low sulfur loading,the shuttle effect of lithium polysulfides and the slow redox reaction result in the rapid attenuation of capacity and reduced energy density,which make the commercialization process still face huge challenges.The existence of no current collector,conductive agent and binder in the integrated self-support electrode is an effective strategy for developing high sulfur utilization and high energy density lithium-sulfur battery.Lightweight carbon-based materials have been widely reported for the construction of integrated electrodes,but their limited adsorption capacity and electrocatalytic activity,large specific surface area causing difficulties in wetting the electrolyte,and low energy density caused by low tap density restrict their practical applications.As polar non-carbon materials,metal nitrides have high conductivity,strong chemisorption of lithium polysulfides and high catalytic activity;while boron oxides have weak conductivity but stronger chemisorption,which can effectively reduce the reaction barrier and accelerate the redox reaction kinetics.Therefore,based on the advantages of metal nitrides and boron oxides,two integrated porous ceramics,carbon/vanadium nitride（C/VN）and carbon/diboron trioxide/vanadium nitride（C/B₂O₃/VN）,were designed and prepared for sulfur hosts of lithium-sulfur battery,achieving excellent electrochemical performance under high sulfur loading,and providing a feasible and large-scale preparation strategy for the design of high energy density sulfur cathode.The specific research results are shown as follows:（1）C/VN integrated porous ceramics were successfully prepared by powder pressing and high-temperature sintering using VN obtained from vanadium pentoxide（V₂O₅）nitriding as raw material and polyvinylidene fluoride（PVDF）as the ceramic forming agent.The three-dimensional conductive network formed by C/VN can promote electron transport and ion diffusion,realize effective anchoring and adsorption of lithium polysulfides,accelerate the catalytic conversion of lithium polysulfides and alleviate the shuttle effect.At the same time,the integrated structure reduces the contact resistance of the electrode interface and expands the storage space of active substances effectively,which is conducive to the improvement of the utilization rate of sulfur.The C/VN@S cathode can still provide 709.6 m Ah g^-1discharge specific capacity after 500 cycles at 0.5 C.The discharge specific capacity of 200 cycles at 3 C is 843.2 m Ah g^-1,and the capacity decay rate per cycle is only 0.03%.（2）Through the powder pressing-high temperature sintering method,on the basis of the C/VN,C/B₂O₃/VN integrated porous ceramics were prepared.Based on the three-dimensional porous structure of integrated ceramics,the high electrical conductivity of VN and the high catalytic activity for lithium polysulfides,the enhanced chemical adsorption of B₂O₃,and the advantages of improving the affinity of VN catalyst for lithium polysulfides,its synergistic effect can shorten the ion/electron transmission paths,effectively inhibit the shuttle effect of lithium polysulfides,reduce the electrochemical reaction polarization,so as to improve the electrode capacity and cycle stability.The2-C/B₂O₃/VN@S cathode with 10%B₂O₃ doping ratio shows the best electrochemical performance,and can still provide 1187.2 m Ah g^-1 discharge specific capacity after 200 cycles at 0.5 C.Compared with the C/VN@S cathode,the specific capacity of the electrode is increased by 52.9%and the capacity retention rate is increased by 24.5%.Under the high sulfur loading of 4.0 mg cm^-2,it can still provide 557.9 m Ah g^-1 discharge specific capacity after 150 cycles at 0.5 C.