Optimized Preparation And Hybrid Capacitors Performance Study Of Niobium Nitride Anode Materials

Lithium-ion hybrid supercapacitors(LHSCs)are an efficient,unique and practical new electrochemical energy storage device with high power density and energy density.However,the problem of kinetic imbalance caused by different reaction mechanisms between positive and negative electrodes prevents LHSCs from exerting their unique advantages.An effective way to overcome this problem is to use anode materials with pseudocapacitive properties to balance their reaction kinetic rates.Niobium nitride(NbN),as a typical representative material in the transition metal nitride family with both high conductivity and excellent pseudocapacitive properties,has made some progress in the application of LHSCs.Despite the excellent cycling performance and pseudocapacitive behavior,its limited lithium storage capacity and unclear lithium storage mechanism are the main issues hindering its adoption in real LHSC devices.Therefore,it is necessary to take reasonable design schemes and improvement measures of the niobium nitride material to further explore the lithium storage performance and mechanism.This dissertation focuses on the modification of niobium nitride anode material,structural design and construction of lithium-ion hybrid supercapacitor system.Using advanced material characterization technologies,electrochemical performance testing methods,and combined theoretical calculations,the niobium nitride material is studied for its lithium storage performance,mechanism,and the practical application of hybrid capacitors is explored in detail.The main research contents are as follows:1.The commercial micron-sized niobium nitride(c-NbN)was nano-modified by high-energy ball milling and ultrasonic pulverization,and NbN particles(n-NbN)with a diameter of about 200 nm were successfully obtained.It is found that the n-NbN material exhibits higher rate,cycling performance,and distinct pseudocapacitance contribution than that of c-NbN,on the basis that the material structure does not change during the electrochemical reaction.The n-NbN//AC LHSC constructed with n-NbN as the negative electrode and activated carbon(AC)as the positive electrode exhibited a maximum energy density of 93.8 Wh kg-1 and a maximum power density of 4681 W kg1.The high performance indicates the application potential of the n-NbN anode materials.This study shows that material nanoization is an important apporach to improve the performance of electrode materials.However,the improvement of material performance is limited by a single nano-modification strategy,and more diverse and effective optimization schemes are needed.2.A structural design scheme of defect-dominated,ultra-small single-crystal niobium nitride(NbN)composited with porous carbon anode material was explored.Specifically,two visible structural defects,i.e.,crystal vacancy and lattice distortion have been introduced in situ in ultrafine NbN monocrystals that are integrated into carbon(C)framework.HRTEM,XPS,XAS and other characterizations demonstrated the existence of abundant vacancies and defects in NbN crystals.Highly reversible Liion storage capacities up to 540 mAh g-1 are demonstrated in such a NbN@C composite anode,together with excellent rate capability and cycling stability.An extra vacancyinduced capacity contribution of the defective NbN component is further evidenced by first-principles density functional theory(DFT)simulations in contrast to a perfect modeling.This work provides a reference for multiple and efficient structural modification schemes for the development and application of lithium storage anode materials.3.On the basis of the successful exploration of NbN@C composite electrodes for Li-ion half-cells.NbN@C//AC Li-ion hybrid supercapacitors are assembled with NbN@C anode coupled with AC cathode.The NbN@C//AC LHSC can deliver up to 125.5 Wh kg-1 of energy density and a maximum power density of 7818 W kg-1,and maintained an energy density of 56.1 Wh kg-1 after 10,000 cycles at 1 A g-1,demonstrating excellent long-cycle performance.Finding from this study,particularly the demonstrated defects-induced extra capacity of pseudocapacitive materials,may inspire new structural designs enabling practical applications of LHSCs.