Preparation Of Yolk-shell Structured Nb₂O₅ And Nb₄N₅ Microspheres And Their Performances For Li-ion Capacitors

Recently,lithium-ion capacitors?LICs?appear in the scientific community due to its high energy/power densities as well as long life-span.Currently,LICs suffer from limited power delivery which caused by the nonequilibrium reaction kinetics between the anode and cathode.The development of more suitable anode materials with rapid Li-insertion kinetics is highly desirable.As an important Nb-based oxide,T-Nb₂O₅ delivers enhanced pseudocapacitive behavior to bridge the huge gap between the diffusion-limited Li-insertion process and the surface-controlled physical adsorption/desorption.Unfortunately,the electrochemical performances of bulk Nb2O5 are usually unsatisfactory.Nanostructuring can shorten the ion-transfer path and increase ion-accessible surface.Nevertheless,low electrical conductivity vastly limits the performance of Nb2O5.Nanostructured Nb4N5 is regarded as a more promising anode material due to the excellent electrical conductivity.Herein,yolk-shell Nb₂O₅ microspheres?YS-Nb₂O₅?were synthesized.On this basis,Nb4N5 were prepared by the followed nitridation process.Detailed characterizations were performed to investigate the crystallographic phase and microstructure information.Electrochemical testing and kinetics study were used to evaluate the Li⁺storage mechanism of YS-Nb₂O₅ and Nb₄N₅.Moreover,LICs based on YS-Nb₂O₅/Nb₄N₅ anode and activated carbon?AC?cathode were assembled.The obtained results are as follows:?1?Yolk-shell Nb₂O₅ microspheres?YS-Nb₂O₅?were successfully synthesized via a scalable and continuous spray drying method with niobium oxalate hydrate and sucrose as the precursors.When used as anode for LIBs,YS-Nb2O5 exhibited excellent cycling stabilities.YS-Nb₂O₅ delivered a capacity of 175 mAh g^-1 after 200 cycles at0.1 A g^-1.YS-Nb₂O₅ also displayed outstanding rate performance(118 mAh g^-1 at 5 A g^-1).Kinetics study and in-situ XRD indicated the intercalation pseudocapacitive mechanism of YS-Nb₂O₅.The LICs based on YS-Nb₂O₅ anode and AC cathode were assembled.The LIC devices delivered excellent cycling stability?⁹8%capacity retention for 1000 cycles?and ultrahigh energy/power densities(173 Wh kg^-1/10.8 kW kg^-1).This work not only confirmed the ideal performances of YS-Nb₂O₅ for LICs,but also proved the enhanced effects of nanostructure for Nb₂O₅.Besides,this work provides a new method for the design and synthesis of high-performance anode materials.?2?On this basis,Nb₄N₅ were prepared by the followed nitridation process.When used as anode for LIBs,Nb₄N₅ exhibited excellent cycling stabilities(180 mAh g^-1 after2000 cycles at 1 A g^-1).The capacitive contribution ratio of Nb₄N₅ is 86.4%at the scan rate of 1 mV s^-1.Kinetics study demonstrated that Nb₄N₅ have obviously pseudocapacitive behavior.The Nb4N5//AC LIC devices exhibited outstanding cycling stability?81.1%capacity retention for 600 cycles?.This work discovered the potential of Nb₄N₅ for LICs for the first time.It is certain that Nb₄N₅ is an ideal material for LICs due to its lower electrochemical reaction potential and pseudocapacitive behavior.