Design And Preparation Of Vanadium Oxide/Sulfide Anode Nanomaterials And Their Sodium Ion Storage Performance

Large-scale energy storage systems（ESSs）can store unstable electrical energy and serve as load-levelling devices,which are great significant for building the security of energy system and achieving the goal of carbon neutrality.Sodium ion batteries（SIBs）with the advantages of abundant raw materials,low price,and high safety,are generally considered as an ideal choice for large-scale ESSs.However,traditional lithium ion battery（LIB）anode materials are not suitable for Na⁺storage,due to the larger radius and heavier mass of Na⁺compared to Li⁺.The lack of high-performance anode materials is currently one of the key factors restri cting the commercialization of SIBs.Vanadium-based compounds are widely researched as anode materials of SIBs,taking into account their wide varieties,abundant resources,good thermal stability,multi-electron reactions,and adjustable crystal structure.However,there are still some limitations in their practical applications,such as sluggish reaction kinetics and serious volume expansion.In this thesis,vanadium-based oxides and sulfides are selected as the research objects.On the basis of nano-structures,the reaction kinetics and structural robustness are enhanced by the strategies of crystal structure tuning,carbon coating,defect introducing,hollow structure and anion intercalation,so as to improve the Na-ion storage performance.Furthermore,theoretical calculations and electrochemical analysis are used to study the structure-activity relationship of the modified materials.Based on the spatial confinement effect of the molten salt,a metastable cubic V₂O₃ embedded in 2D N-doped carbon matrix（c-V₂O₃@NC）was successfully synthesized by a simple heterogeneous molten salt method with urea assisted.The added urea provides nitrogen source for N-doping and induces the formation of metastable V₂O₃ during the synthesis process.Ab initio molecular dynamics calculations demonstrate that cubic V₂O₃ possesses reduced Na⁺diffusion activation energy compared with the common rhombohedral phase.The 2D carbon matrix provides a continuous conductive base,limits the self-aggregation of V₂O₃nanoparticles,and improves the structural stability.In addition,N-doping also improves the conductivity of the carbon matrix and optimizes the electrochemical activity.As a potential anode material for SIBs,c-V₂O₃@NC manifests superior Na⁺storage properties including high specific capacity,ultralong cycle life and ultrahigh rate capability.The c-V₂O₃@NC provides a specific capacity of 220 m Ah g^-1 at a current density of 10 A g^-1,and maintains a high specific capacity of 400 m Ah g^-1after 1150 cycles at 1 A g^-1.Using the phase separation in the electrospinning process,Co₃V₂O₈ nanotubes（CVO-NTs）were designed and prepared by blend-electrospinning two incompatible polymers（PVP and PAN）combining with appropriate heat treatment.The formation mechanism of this nanotube structure was explored and promoted to prepare other transition metal oxide nanotubes.The nanotube structure can not only shorten the diffusion path of Na⁺,but also alleviate the volume expansion during charging and discharging.The prepared CVO-NTs show enhanced cycle stability and improved rate performance,with a specific capacity of 115 m Ah g^-1 at 5 A g^-1,and a 169 m Ah g^-1 remaining specific capacity at 1 A g^-1 after 1600 cycles.Employing VO（acac）₂ as the vanadium source,a novel honeycomb-like amorphous Zn₂V₂O₇（ZVO-AH）nanofibers was fabricated by a simple electrospinning with subsequent calcination.The volatilization of the vanadium during the phase formation causes the amorphous of the material,and its expansion during decomposition creates a honeycomb cavity.The as-fabricated ZVO-AH possesses large specific surface area,mesoporous,rich oxygen defect,and smaller band gap.Originating from the synergies of amorphous nature and honeycomb-like cavities,ZVO-AH shows increased electrochemical activity,accelerated Na-ion diffusion and robust structure,resulting in significant rate performance and ultra-long cycle stability.Impressively,the ZVO-AH anode delivers a high specific capacity of 150 m Ah g^-1 at a current density of 10 A g^-1,and almost no attenuation capacity after 5000 cycles at 5 A g^-1.A novel and efficient low-temperature solid-state method was developed to synthesize interlayer engineered VS₄（IE-VS₄）with thiourea as sulfur source.The intercalation of[NCN]^2-anion expands the interlayer of VS₄ to accelerate the Na-ion diffusion.Furthermore,the intercalated anion also reduces the band gap of VS₄ and enhances the electronic conductivity.In addition,the IE-VS₄ also exhibits a loose 3D cross-linked nanowire morphology,which is conducive to enhance mechanical stability and promotes electrolyte penetration.Thanks to these merits,the IE-VS₄ enables standout Na-ion storage performance in terms of ultrafast rate capability,high capacity and superior cycle stability.Specifically,IE-VS₄ delivers a specific capacity of 500 m Ah g^-1 at a current density of 20 A g^-1 whit discharge time less than 90 s,and a remaining specific capacity of 550 m Ah g^-1 after 2500 cycles at of 5 A g^-1.