Preparation And Properties Of Calcium Vanadate/C Composites For Sodium-Ion Battery Anode Materials

The world is becoming more industrialized,and researchers working for the world's energy systems have been looking for more efficient,environmentally friendly energy sources to cope with surging energy demand.Lithium ion batteries(LIBs)based on lithium metal batteries have become major energy storage devices in the fields of communications,transportation and renewable energy.However,the scarcity of lithium resources in nature and the low taste of li-ore restrict the development and application of LIBs.Sodium ion batteries(SIBs)attracts researchers'attention because of its abundant natural resources and low cost.In addition,the SIBs have similar energy storage mechanism with LIBs,and it is expected to show higher stability and safety.It is a good candidate for the next generation of secondary battery.SIBs transfer charge through metal ions de-intercalation between positive and negative electrodes,but the radius of sodium ion is larger than that of lithium ion.Therefore,exploring the suitable electrode material is one of the key breakthrough points in constructing the energy storage system for SIBs.Vanadoxy compounds with rich redox state and composed of many coordination polyhedrons.The open lamellar structure composed of the vanadium oxide skeleton can provide enough spacing and redox reaction sites,thus giving the vanadium oxide compound material excellent sodium storage capacity.However,vanadoxy compounds have some natural disadvantages as electrode materials.On the one hand,the poor electrical conductivity of most vanadoxy compounds makes it difficult to achieve theoretical specific capacity.On the other hand,vanadoxy compounds tend to change crystalline phase in the process of sodium ion de-intercalation,and form irreversible by-products,which cause the structure destruction and capacity decay.In order to improve the stability of the layered crystal structure,the method of pre-intercalation of alkaline earth metal ions was used.Then,the combined methods of graphene doping and carbon coating were used to increase the overall conductivity of vanadium-based materials and reduce the effects of side reactions on their properties.Herein,CaV₈O₂₀·3H₂O monoclinic crystal was synthesized by simple hydrothermal method.CaV₈O₂₀·3H₂O is a double layer structure formed by the interconnection of nearly pentavalent Vanadium with oxygen.Ca²⁺and crystallized water can effectively strengthen the stability of crystal structure.The nano-thin microstructure of CaV₈O₂₀·3H₂O can shorten the diffusion distance of sodium ions and increase the contact area between solid phase and electrolyte.In addition,the 3D graphene network composites enhance the efficiency of the redox reaction by increasing conductivity.The ultra-thin carbon coated composites improve the electrical conductivity of this material,buffer the lattice stress and avoiding the capacity attenuation caused by the side reaction in the process of solid liquid mass transfer.In particular,CaV₈O₂₀·3H₂O@RGO and CaV₈O₂₀·3H₂O@C have a capacity retention rate of 72%and 92.7%respectively after 200 cycles.The retention of capacity still about 72%after 450 cycles at current density of 5.0 A/g.Secondly,we adjusted the hydrothermal conditions and added a dispersant to control the crystal structure to form an orthorhombic crystal CaV₃O₇ based on above hydrothermal method.The single layer CaV₃O₇ is formed by interconnecting regular tetrahedrons composed of positive tetravalent vanadium.Calcium ions serve as intercalation ions to strengthen the stability of the layer structure.Its microstructure is a nano-flower-like structure composed of nano-sheets,which maintains the advantages of nano-structures while effectively preventing agglomeration.In addition,the three-dimensional network structure with graphene doping can improve the conductivity and structural stability of the material to accelerate the electronic transmission efficiency and electrochemical properties.Carbon coating on the surface of the material by in-situ polymerization can improve the electrical conductivity of this material and buffers the lattice stress generated during the de-intercalation of sodium ions.In particular,Specifically,the specific discharge capacity of CaV₃O₇@RGO and CaV₃O₇@C can be maintained at 168.1 m Ah/g and 257.3 m Ah/g at a low current of 0.2 A/g for 350 cycles.After1000 cycles at 5.0 A/g,the capacity retention rate of these two materials at can still reach 94.7%and 82.8%,respectively.