Design And Preparation Of Metallic Compound/N-doped Carbon Composites And Their Lithium(Sodium)Storage Properties

Nowadays,the development of high energy efficiency and long life lithium ion batteries(LIBs)and sodium ion batteries(SIBs)is more and more attractive.However,the development of LIBs is greatly limited due to the shortage of lithium resources,safety issues,especially the improvement of energy density(the theoretical specific capacity of commercial graphite is only 372 mA h g-1).SIBs has a broad development prospect,but it is more likely to cause serious volume expansion and structure collapse during repeated cycles with a larger radius of Na+,leading to rapid attenuation of battery capacity.Therefore,SIBs needs higher requirements for electrode materials.In recent years,the modification of the morphology and structure of anode materials has been regarded as an effective means to improve the energy density and cycle life of lithium/sodium ion batteries.In this paper,we focus on the problems of the electrode materials in the application process,i.e.the insufficient conductivity,low specific capacity,poor circulation/rate performance caused by the serious volume effect.Start with the composition and structure of material,we designed two kind metal matrix/nitrogen doped carbon conductive support composites(ZnNb2O6/NC and CO2P/NC)with good micro structure and then systematically explored its reaction mechanism and electrochemical performance while applied in LIBs and SIBs.The main contents are summarized as follows:(1)In view of the shortcomings of Nb2O5,such as low theoretical specific capacity and poor conductivity,the Nb-based alloy oxide/porous conductivity N-doped carbon(ZnNb2O6/NC)composites were successfully prepared by creatively combining T-Nb2O5 with ZIF-8 which has large specific surface area and good adaptability.The combination of niobium alloy oxides with nitrogen-doped carbon supports effectively improves the electrical conductivity and structural stability of Nb-based materials and shows excellent lithium storage performance(the specific capacity can reach 470.5 mA h g-1 after 800 cycles at the current density of 2 A g-1).(2)The synthesis process of common phosphorous sources(triphenylphosphine,NaH2PO2,red P,PH3 etc.)for transition metal phosphates(TMPs)are mostly complicated,high pollution with uneven products.Moreover,the synthesized TMPs are prone to structure collapse and powder in repeated cycles.Based on that,phytic acid which has unique structure and green renewable characteristics was utilized as phosphorus source to efficiently synthesize porous flexible Co2P/NC composites with highly dispersed Co2P nanoparticles and open network structure supported by N-doped carbon as conductive support.The method can be easily applied to the synthesis of other TMPs(Ni2P/NC,Cu3P/NC and Fe2P/NC have been synthesized successfully),which proves that this method has good universality.The as-obtained Co2P/NC has good conductivity and flexibility,and can be directly used as self-supporting flexible binder-free electrode material,which can avoid the adverse effects brought by the traditional electrode material preparation process.The results indicate that when the flexible Co2P/NC electrode applied to LIBs and SIBs,it can still show very good structural stability and excellent electrochemical performance after the large current and long cycle(LIBs:retain a steady reversible capacity of 563.6 mA h g-1 after 1000 cycles at 10 A g-1;SIBs:retains 132.3 mA h g-1 after 2500 cycles at 1.0 A g-1).