Modification And Investigation On Sodium Vanadium Fluoride Phosphate As A Cathode Material For Sodium-Ion Batteries

The prosperity of new energy industry has been urged by environmental issues and the prostration of energy resource,and the high-efficient utilization of new energy needs the cooperation from proper energy storage technique.Sodium-ion batteries?SIBs?have appealed to researchers because of the abundant resource.Though SIBs can succeed the research experiment from lithium-ion batteries?LIBs?since Na and Li situate in the same main family,the SIBs still require an exclusive electrochemical theory system for large ionic radius and higher redox potential.Among various cathode materials for SIBs,polyanionic compounds possess open stable crystal structure and higher working potential.Especially,NASICON type Na₃V₂?PO₄?₂F₃?NVPF?with 3D structure and?3.95 V average discharge potential holds superiority in ion-diffusion kinetics and thermal stability,which is considered as one of the promising cathode materials.Whereas,in order to meet the satisfaction of high-performance cathode materials for SIBs,the main challenge for NVPF about the intrinsic inferior electronic conductivity needs further settlement.Base on the above background,advanced NVPF is chosen to be the research target and two modification methods have been proposed to optimize the electronic conductivity and Na-storage capability of NVPF.Conclusive research points are listed as following.At first,adulterating element Al for element V in the spatial site of NVPF provides non-electrochemical reactive pillars to stabilize the crystal structure.With the support from Al,the reaction kinetics of Al doped NVPF?NVPF-Al?has been promoted.Deformation and inactivation due to repeating Na⁺?de-?intercalation is suppressed,and the polarization is decreased by accelerating the speed of Na⁺diffusion.NVPF-Al shows a more conspicuous?3.4plateau and a beneficial Na+diffusion coefficient(2.03×10^-13 cm² s^-1,which is one magnitude higher than that of pure NVPF).NVPF-Al maintains 96.5%capacity(?90.0 m A h g^-1)after 110cycles at 1 C and delivers 84.2 m A h g^-1 reversible capacity at 20 C?212%surpassing that of pure NVPF?,which demonstrate the excellent cyclic and rate stability of NVPF-Al and validates the positive value of Al doping to improve the dynamics performance.Afterwards,taking the advantage of micro-morphology design and carbon coating,nano-cubic NVPF encapsulated by in-situ generated N-doped carbon?NVPF-NC?is synthesized by the solid state method with selected specific surfactant and proper dosage.On the one hand,the hierarchical morphology formed by nanoparticles supplies Na⁺with adequate shortened diffusion paths and enhances the permeability of electrolyte in to the bulk NVPF.On the other hand,the in-situ carbon scaffold improves the electronic conductivity of NVPF-NC composite.Therefore,NVPF-NC delivers delighting initial discharging capacity(120.3 m A h g^-1 at o.5 C)and maintains 107.7 m A h g^-1 capacity even at higher current rate of 30 C.Besides,the extraordinary cycling stability and structural integrity of NVPF-NC are also proved by the capacity retention of 98.7%after 200 cycles at 10 C and 84.8%after ultra-long 2500 cycles at extremely high current rate of 50 C.Last but not least,the NVPF-NC||hard carbon sphere?HCS?full cell is constructed,using suitable NVPF-NC as cathode and HCS as anode.The full cell exhibits an amazing energy density of 396.6 W h kg^-1 and retains 98.5%capacity after 100 cycles at 0.5 C.By lighting up the module with 38 light bulbs,the feasibility of NVPF-NC in practical utilizations is attested.To sum up,aiming at improving the electrochemical performance of NVPF,this dissertation has put forward two modification methods with succeeding experiments to analyze the Na-storage capability and mechanism of the optimized NVPF qualitatively and quantitatively.The research outcome emphasized that Al adulteration,nanotechnology and morphology regulation as well as compositing with conduction carbonaceous materials are beneficial to improve the transport efficiency?electrons and Na⁺?and to stabilize the crystallographic structure.The NVPF delivers enhanced capacity,prolonged life span and high rate tolerance.Such outstanding performance of NVPF in half and full cells validates its promising potential in large-scale manufacturing,and provides the essential evidence for the commercialization of NVPF.