Research On The Structure Regulation And Sodium Storage Performance Of Sodium Superion Conductor Sodium Vanadium Phosphat

Globally fluctuating global oil prices and battery raw materials have led to growing expectations for low-cost power storage systems in recent years.The many advantages of lithium-ion batteries make them widely used in mobile electronic devices.However,the large-scale application of lithium-ion batteries is limited by the high price of lithium-containing materials.In the short term,safety and environmental issues are also challenges.There is an urgent need to develop new types of power storage devices.Sodium-ion batter-ies have attracted much attention due to the earth’s wide distribution of sodium resources,which can reduce the difficulty of battery production.Its electrochemical mechanism is also similar to some of the theories of lithium-ion batteries.However,the safety and cost of organic electrolytes are still major issues in the practical application of organic ion batter-ies in commercial ion batteries.Therefore,the application prospect of aqueous sodium-ion batteries using sodium sulfate aqueous solution as an electrolyte in supplementary lithium batteries has great potential.Aqueous sodium-ion batteries have the advantages of low cost,less environmental pollution,non-flammability,and high ion mobility.Among the many host materials of aqueous sodium-ion batteries,sodium superionic conductors are materials with sodium ion conductivity comparable to liquid electrolytes under certain conditions.Among them,sodium vanadium phosphate is a typical sodium superionic conductor structure,and sodium vanadium phosphate has been successfully developed as an excellent cathode mate-rial for sodium-ion batteries.However,when sodium vanadium phosphate is applied in an aqueous electrolyte,its capacity will drop soon.The main research content of this paper is as follows:First,carbon-coated sodium vanadium phosphate’s charge storage and capacity decay mechanisms were studied in detail through systematic material characterization and den-sity functional theory calculations.The results show that during the charge-discharge cycle,the aqueous electrolyte’s protons diffuse to the sodium vanadium phosphate’s surface,oc-cupy the sodium sites and attack the nearby phosphate,resulting in the deformation and fracture of the phosphorus-oxygen-vanadium bond.The twisted and damaged phosphate groups on the surface of sodium vanadium phosphate gradually dissolve into the electrolyte,resulting in a decrease in capacity.In order to stabilize the phosphate on the surface of sodium vanadium phosphate,density functional theory calculations show that iron doping on the vanadium site of sodium vanadium phosphate can effectively relieve the deformation of the phosphorus-oxygen-vanadium bond and inhibit capacity fading.Experiments show that sodium vanadium phosphate only retains 55%of the initial capacity in the first 10 cy-cles,while iron-doped sodium vanadium phosphate increases the cycle capacity from 55%to 95%.Secondly,to further study the effect of vanadium doping on the stability of the crystal structure,based on the previous content,it was newly discovered that the transition metal tungsten could also improve the stability of sodium vanadium phosphate.Although the ef-fects of the two metals introduced alone are not satisfactory,the synergistic incorporation of the bimetals greatly improves the electrochemical performance.A novel bimetallic-doped sodium vanadium phosphate cathode was designed and synthesized.The initial capacity of the cathode was 95%at the 50th cycle,and the cycle stability was good.The electrochemical behaviour and charge storage mechanism of bimetallic doped sodium vanadium phosphate were systematically studied through in-situ and ex-situ characterization.Iron and tungsten co-doping can stabilize the Na superionic conductor framework and inhibit the attack of pro-tons on Na sites in aqueous electrolytes,resulting in good cycle stability.Combined with density functional theory calculations,the mechanism of bimetallic doping improving the structural stability of sodium vanadium phosphate was demonstrated.In addition,a full aque-ous cell prepared with bimetallic doped sodium vanadium phosphate cathode and titanium sodium phosphate anode can provide an initial capacity of 64 m Ah g-~1at room temperature,50 cycles(1 A g-~1),the capacity remains at 95%.Finally,in order to study the effect of the electrode/electrolyte interface on the cycle stability of the material,it is necessary to modify the interface of the thin film cathode ma-terial.This study selected titanium sheets with physical and chemical stability and good electrical conductivity as the research object.However,the high-temperature synthesis of sodium vanadium phosphate on the surface of current titanium collectors is prone to side reactions.It cannot be synthesized in situ on the surface.The titanium substrate coated with a pencil and then sintered at high temperature to obtain a graphite-coated titanium substrate is an excellent in-situ synthesis substrate.The experimental results show that the surface of titanium-graphite is smooth,and there are few defects in the graphite layer.In addition,titanium carbide is formed at the interface between graphite and titanium matrix,improving titanium-graphite’s physicochemical stability.An in-depth understanding of the evolution mechanism during the synthesis process was carried out by molecular dynamics simula-tion research.The results show that after sintering and cooling treatment,the porosity and stress at the model interface are significantly reduced,indicating that titanium-graphite has a smoother surface,a more stable interface and fewer defects at the atomic scale.Based on the above research results,a binder-free sodium vanadium phosphate film was designed for the cathode of aqueous sodium-ion batteries.In this work,a sodium vanadium phosphate film was synthesized in situ on a titanium-graphite substrate,simplifying the slurry coating process in preparing traditional electrodes.In addition,the film maintains the same electro-chemical performance as conventional electrodes and is more resistant to high temperatures.On this basis,a graphite layer was coated to protect the sodium vanadium phosphate.Af-ter high-temperature annealing,the dissolution of the sodium vanadium phosphate in the aqueous electrolyte was successfully inhibited.Experiments showed that the graphite-coated sodium vanadium phosphate film retained 50%of its initial capacity after 50 cycles.The uncoated sodium vanadium phosphate film retains only 20%of its initial capacity.Therefore,doping and surface modification are effective ways to stabilize the crystal structure stability.Studying how to suppress the failure of sodium vanadium phosphate in aqueous sodium ions can develop promising aqueous sodium-ion batteries and provide a theoretical basis for the next step in developing all-solid-state sodium-ion batteries.