Preparation Of Flexible Niobium-based Oxides Nanoarray Electrodes And Their Properties For Lithium-ion Storage

The continuous development of flexible and wearable portable electronic devices in the recent years brings about the need for flexible energy-storage systems with excellent electrochemical performances.Lithium-ion capacitors(LICs),combining the advantages of Lithium-ion batteries(LIBs)and supercapacitors(SCs),are able to exhibit relatively high power density and long cycling stability.The LICs show promising applications in the field of fast charge/discharge.However,the high-rate performances of LICs are limited by the kinetic imbalance between the battery-type anode and the capacitor-type cathode.Therefore,the research and development of anode materials with excellent Li-ion storage performances and mechanical flexibility becomes the crucial research topic.In this paper,firstly with respect to the pseudocapacitive property of T-Nb₂O₅materials,they were prepared in3D nanowire array which were directly grown on flexible carbon cloth.The as-prepared materials show superior Li-ion storage performances,due to the dramatically reduced Li-ion diffusion pathway and the fast electronic transport along the single-crystalline nanowire of T-Nb₂O₅.Secondly,a new niobium oxide material(Nb₃O₇F)in Nb₃O₇F nanosheet array is prepared and its electrochemical lithium-storage performances and application in flexible lithium-ion capacitors are also studied.The main content of this paper is summarized as follows:(1)T-Nb₂O₅ material in vertically aligned nanowire array is directly grown on the flexible carbon cloth by a facile solvothermal reaction and post-annealing treatment at high temperatures.Material characterizations indicate that the nanowires grow with a preferred orientation along the[001]direction,which ensures the greatly reduced Li-ion diffusion path between the(001)planes.Accordingly,superior high-rate and cycling performances are resulted:even at an ultra-high rate of 50 C,the capacity retention rate of 69%is attained;while cycling at 5 C,a capacity retention rate is 87.2%after 5000 cycles is achieved.The flexible LICs assembled with this kind of T-Nb₂O₅ anode and the active carbon cathode,is able to show a high gravimetric energy and power densities(111.5 Wh·kg^-1 and 4335.8 W·kg^-1,respectively).Besides,the flexibility of the anode was demonstrated by driving three LED lights in parallel to work normally at bending or folding states.(2)Carbon-coated Nb₃O₇F nanosheet array is prepared by a two-step method:first,Nb₃O₇F nanosheet array was directly growth on the flexible carbon cloth by a facile hydrothermal process;second,coating with dopamine and then post-annealing at high temperatures in Ar to form a carbon layer on the surface.The charge/discharge testing results indicate that the carbon-coated Nb₃O₇F nanosheet array electrode is able to show high-rate performances exceeding that of most nanostructured niobium oxide materials previously reported so far.A reversible capacity of?130 m Ah·g^-1 is retained at an ultra-high rate of 100 C(full charge/discharge in 36 s).Besides,the Nb₃O₇F nanosheet array shows remarkable long cycling stability,capacity retention of 98.8%can be obtained after 5000 cycles at 20 C.Model LICs assembled with the nanosheet array anode and a commercial AC(YEC-8B)cathode,can deliver high gravimetric energy and power densities(125.94 Wh·kg^-1 and 12093.66 W·kg^-1,respectively).In addition,capacity retention of 97.5%after 2000 cycles at 20 C is also observed.Kinetic investigations indicate that the superior high-rate capability of the carbon-coated Nb₃O₇F nanosheet array can be attributed to the carbon coating and the nanosheet structure,which provides significantly reduced Li-ion diffusion length and enhanced electronic transport.