| In order to meet the demand of chemical power in the future,the study of electrochemical energy storage devices has become an important direction.Electrode materials with both high energy and high power densities are the research focus.Pseudocapacitance,a faradaic process involving fast redox reactions at surface or sub-surface of the material,offers a means of achieving high energy density at high charge-discharge rates.Recently,as a new type of intercalation pseudocapacitive material,orthorhombic(T-)Nb2O5 could provide a high specific capacity close to that of battery level even at high charge-discharge rates.However,like most transition metal oxides,the low conductivity of Nb2O5 will limit its high rate-performance.Therefore,we decided to solve this problem from two perspectives.Firstly,constructing composites of porous carbon and Nb2O5,the synergistic effect between them can improve specific capacity and promote electron transfer rate.Secondly,modifying the surface chemistry and crystal structure of Nb2O5 in order to obtain high rate-capability.These methods are of great importance for achieving high energy-high power density supercapacitors and forming a new concept of energy storage.This paper mainly includes the following conclusions:(1)A high-rate hybrid supercapacitor has been constructed based on T-Nb2O5@C hollow core-shell nanostructured electrode.Due to the unique orthorhombic crystal structure,T-Nb2O5 exhibits pseudocapacitive behavior with almost no kinetics limitations from solid-state diffusion.However,when prepared into a relatively thick electrode,its low resistivity makes the electron transfer rate slow,resulting in poor rate-capability.In order to solve this problem,we proposed a one-step polymerization method to confine T-Nb2O5 nanoparticles within mesoporous carbon hollow shells,which could facilitate ion diffusion and charge transfer,thus obtaining excellent pseudo capacitance properties with high-rate performance.Furthermore,a high-performance hybrid supercapacitor device is successfully constructed by employing the as-prepared Nb2O5@MC as anode and mesoporous carbon hollow spheres as cathode,delivering a maximum energy density of 38 Wh kg-1 at 0.2 A g-1 and a maximum power density of 32 kW kg-1 at 20 A g-1.The high-rate capacitive behaviors of both the positive electrode and negative electrode make it possible to combine high energy density and ultrahigh power density in hybrid supercapacitors.(2)The ultrafine T-Nb2O5/G nanocomposite has been prepared with excellent Li+intercalation pseudocapacitive properties.The size of Nb2O5 particles in T-Nb2O5/G nanomaterials synthesized by simple hydrothermal-heat treatment method according to our previous report is 20-40 nm.To prevent particle agglomeration,we have developed a novel strategy for the facile preparation of graphene-nanosheet-supported untrafine Nb2O5 nanoparticles encapsulated by thin mesoporous silica(mSiO2)layers.The thermally stable mSiO2 outer layer can restrain the restacking of graphene nanosheets and a great number of mesoporous in the mSiO2 layers provide spaces for confining Nb2O5 nanoparticles with specific sizes.Moreover,the sintering of Nb2O5 can be avoided,even under harsh reaction conditions,because of the confinement effects in porous materials.It revealed that the specific capacity of ultrafine T-Nb2O5/G was higher(652 C g-1 at 1 mV s-1),because Nb2O5 nanodots provided more electro active sites.Furthermore,ultrafine nanocrystals shorten ion diffusion paths,and graphene promotes electron diffusion rates,resulting in excellent rate capability(from 1 A g-1 to 50 A g-1,with a capacity retention of 58.1%)and long cycling stability.Using mSiO2/rGO template to prepare ultrafine metal oxide nanodots/graphene composites is a novel and effective method to improve their electrochemical properties.(3)The Li+intercalation pseudocapacitive properties of T-Nb2O5 has been improved by introducing oxygen vacancies.In addition to the synthesis of Nb2O5/C composites,the surface-chemistry modification of T-Nb2O5 based on its structural characteristics is another method to obtain excellent electrochemical properties.Hydrogen reduction pre-treatment is a simple and efficient method to introduce oxygen defects into the T-Nb2O5 lattice.The H-Nb2O5 electrode hydrogenated at 750? for 10 min presented the highest amount of oxygen vacancies,delivering a remarkable specific capacity of 516 C g-1 at 1 mV s-1.This electrode also exhibited excellent rate performance(48.2%retention from 0.5 A g-1 to 50 A g-1)and outstanding long-term cycling stability(>86%retention after 2000 cycles).The superior electrochemical performance of H-Nb2O-10 can be attributed to the formation of oxygen vacancies,which serve as active sites.Meanwhile,the creation of oxygen vacancies leads to an increased donor density and improved conductivity,which promotes charge transfer kinetics.Furthermore,the H-Nb2O5-10 electrode with a high thickness of 150 ?m delivers excellent electrochemical properties.Therefore,hydrogenation can be considered as an effective approach to incorporate oxygen vacancies into various transition metal oxides towards boosting their electrochemical performance for pseudocapacitive charge storage.(4)The origin of rapid Li+ intercalation pseudocapacitive response in Nb2O5 and NbO2 crystals has been explored.Based on our previous studies,we found that tetragonal(t-)NbO2 and tetragonal(M-)Nb2O5 exhibit higher rate-capability than T-Nb2O5.In order to better understand the intrinsic causes,we prepared T-Nb2O5,t-NbO2 and M-Nb2O5 nanocrystals.By comparing the electrochemical properties of these three crystals,it can be found that M-Nb2O5 has the highest specific capacity(1 mV s-1,498 C g-1),t-NbO2 and M-Nb2O5 nanocrystals have excellent rate performance(from 0.5 A g-1 to 50 A g-1,with capacity retention rate of 39.4%and 37.3%,respectively)and long cycle stability.In addition to the influence of crystal structure,that is,the crystal structure of t-NbO2 and M-Nb2O5 is more ordered in the long range,which is beneficial for Li+intercalation.Through DFT calculation,it can also be found that the diffusion energy barrier of Li+ in t-NbO2 and M-Nb2O5 crystals is lower than that of t-NbO2,so that to give rise to rapid energy storage reaction.Investigating the origin of rapid Li+intercalation pseudocapacitive materials is favorable for the development of new high-rate performance electrodes and could improve the research efficiency. |
PDF Full Text Request