| Recently,three-dimensional layered V2AlC has attracted widespread attention owing to its high electrical and thermal conductivities,good chemical stability,excellent mechanical properties and so on.It is greatly potential to apply V2AIC in reinforced phase for composites,nuclear materials,anticorrosive coatings,etc.Meanwhile,V2AlC is a critical precursor for preparing two-dimensional(2D)layered V2C MXene/V2Al1-xC.V2C MXene has become the research hotspot as an anode for lithium-ion batteries(LIBs)due to its unique laminar microstructure,high electrical conductivity,large interlayer spacing,and good electrochemical activity,etc.Currently,preparation of V2C MXene with the small and uniform size distribution is anticipated to be realized urgently for energy storage applications.Particle size distribution and synthesis technique of V2AlC play a critical role in the morphology and electrochemical characteristic of as-etched product.Thus,preparation of V2AlC with the small and well-distributed particle size is of significance to improve electrochemical properties of V2C MXene.However,so far,the methods for synthesizing V2AlC is complicated,high-cost or energy-consuming,and as-obtained products are large-sized and agglomerated severely,which may affect the preparation and elecrochemical properties of V2C MXene negatively.Therefore,it is imperative to develop one simple technique to synthesize small-sized V2AlC powders with the uniform size distribution under a low temperature at a cheap cost.Molten salt synthesis method has received increasing attention on account of its simple process technique,low reaction temperature as well as controllable product size and morphology,etc.In this work,small-sized and well-distributed ternary V2AlC powders were prepared by a molten salt synthesis method.Then 3D nanosheet-like and 2D layered V2Al1-xCTz powders were obtained through the etching of V2AlC.Furthermore,binary nanosized VC and VN powders with high purity were synthesized via the molten salt synthesis method.These vanadium carbide/nitride powders above were applied as anode materials for lithium/sodium-ion batteries(LIBs/SIBs),and corresponding electrochemical performances were studied detailedly.The main research results are shown as follows:(1)V2C powders with the particle size of 100～200 nm were firstly prepared in(Na,K)Cl molten salts using 2V/C precursors as raw materials at 900～1000℃ holding for 2 h,and the formation mechanism is shown below.V could be oxidized into V(Ⅳ)-ions in molten salts,subsequently V(Ⅳ)-ions would diffuse towards carbon surface and disproportionate into V(Ⅴ)-ions and atomic V,thereinto V2C can be in-situ formed based on carbon as the template.Then V2AlC powders with the average particle size of 400 nm and plate-like morphology could be synthesized using V2C/Al precursors as raw materials at 900～1000γ/γ’ holding for 2～3 h in molten(Na,K)Cl.The aforementioned process contains the mutual migration of V2C and liquid Al,self-assembly,and intercalation reaction of Al into V2C in molten salts.The rapid self-assembly between Al and V2C is driven by the minimization of free energy in molten salts.Furthermore,based on one-step molten salt synthesis method,submicro-sized spherical-like V2AlC powders were prepared from the mixture and cold pressing of 2V/Al/C precursors in molten(Na,K)Cl at 1000℃ holding for 5 h.Molten salts could accelerate the synthesis of V2AlC at a low temperature.(2)Using V2AlC powders obtained by one-and two-step molten salt synthesis methods as precursors,nanosheet-like and 2D layered V2Al1-xCTz were prepared by etching V2AlC in HF solution for 96 h at room temperature,respectively.2D layered V2Al1-xCTz has displayed superior electrochemical performances to nanosheet-like counterpart,and achieved a lithium-ions storage capacity of 450 mAh·g-1 at 0.1 A·g-1 in the 120 cycles.Such prominent electrochemical properties are attributed to the large specific surface area,more electroactive sites and porous microstructures.Li-ions storage mechanism of V2Al1-xCTz is intercalation pseudocapacitive,accompanied with the electrochemical reaction of-O functional group with Li-ions and redox reactions of V5+/V4+ and V4+/V3+during the Li-ions insertion/extraction process.(3)Based on the disproportionation reaction of V-ions on carbon surface,nanosized VC powders with the average size of 40 nm have been synthesized successfully at 950℃ for 5 h in molten(Na,K)Cl.As an anode material for LIBs,VC maintains a discharge capacity of 595.4 mAh·g-1 at 0.1 A·g-1 in the consecutive 150 cycles.Nanosized VN wrapped onto carbon fibers powders were obtained by heating the mixture of melamine and V precursors at 800～1000℃ for 3 h in molten(Li,K)Cl.VN delivers the lithium-ions storage capacities of 377.2 mAh·g-1 at 0.5 A·g-1 after 500 cycles and 669.4 mAh·g-1 at 0.1 A·g-1 after 230 cycles without capacity attenuation.Furthermore,VN exhibits excellent electrochemical properties as an anode material for SIBs,and releases reversible discharge capacities of 132.6 mAh·g-1 after 6600 cycles at 1.0 A·g-1 and 204 mAh·g-1 after 500 cycles at 0.1 A·g-1.Meanwhile,Na-ions storage behavior of VN is a surface redox-related pseudocapacitive process. |
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