Preparation,Characterization And Electrochemical Properties Of Niobium-based MXene And Their Derived Oxides

The rapid development of electric vehicles,large power grids,aerospace requires electrochemical energy storage devices（EESDs）to have both high energy/power densities and long-duration lifespan.However,commercial lithium-ion batteries（LIBs）and supercapacitors（SCs）can not meet these needs due to their respective limitations.Lithium-ion capacitors（LICs）are promising next-generation EESDs,which incorporating both merits of LIBs and ECs in one device with high energy/power density and long cycle life.But the kinetics of battery-type anode materials are several orders of magnitude slower than that of capacitor-type cathodes,leading to an imbalance in the kinetics between anode and cathode,which seriously affects the practical applications of the devices.Therefore,the development of high-rate battery-type anode materials is the key to solve this problem.Now,niobium-based oxides stand out due to their high theoretical capacity(Nb₂O₅≈200 m Ah g^-1 and Na Nb O₃≈260m Ah g^-1)and very small volume expansion（less than 5%）among the various battery-type anode materials.Here,we take niobium-based MXene as an entry point to design and prepare niobium-based oxide anode materials with high-rate performance to solve the problem of imbalance electrode kinetics in LICs.The results of research are as follows.（1）Nb₂O₅/Nb₂CT_x composites were in-situ constructed by a simple hydrothermal method using multilayer Nb₂CT_x（m-Nb₂CT_x）MXene as niobium source.On the one hand,MXene as a conductive substrate significantly improves the kinetics of the composites and the two-dimensional block structure is preserved,which is more favorable for the migration of Li⁺.On the other hand,Nb₂O₅ nanorods in-situ grown on the surface or between the layers of m-Nb₂CT_xcan effectively prevent the structural collapse and stacking of MXene nanosheets.When the electrochemical performance of the prepared Nb₂O₅/Nb₂CT_x composites were tested,it showed excellent high rate performance(a reversible capacity of～65 m Ah g^-1 at 10 A g^-1).（2）One-dimensional（1D）single-crystalline pseudohexagonal Nb₂O₅（TT-Nb₂O₅）nanorods（NRs）were fabricated via a simple hydrothermal strategy by using the two-dimensional few-layered Nb₂CTx（f-Nb₂CT_x）MXene nanosheets（NSs）as the niobium-based precursor,the TT-Nb₂O₅ NRs were successfully transformed into single-crystal orthorhombic Nb₂O₅ nanorods（T-Nb₂O₅ NRs）through subsequent annealing treatment.Based on the results of products with different hydrothermal time,the growth mechanism of the NRs was revealed.The resultant single-crystalline T-Nb₂O₅ NRs grew along the[001]crystal orientation possessing the minimum Li⁺-transport barrier,and thanks to the unique pseudocapacitance properties and the robust crystal skeleton,the single-crystal T-Nb₂O₅ electrodes exhibit superior rate performance(～147 m Ah g^-1 at 2.0 A g^-1).More importantly,the Li⁺-storage mechanism of the 1D single-crystal T-Nb₂O₅ NRs was revealed by in-situ X-ray diffraction analysis,and demonstrated that the（001）crystal plane is the main channel for Li⁺embedding.The constructed LICs exhibited a maximum energy density of about 92.1 Wh kg^-1(at 80 W kg^-1)and a maximum power density of 8 k W kg^-1(at 35.6 Wh kg^-1)with an ultra-high capacity retention rate of 95%after 4000 continuous charge/discharge cycles.（3）The orthogonal single-crystalline perovskite-type Na Nb O₃ nanocubes（S-P-NNO NCs）was fabricated via a simple hydrothermal alkalization strategy by using 2D f-Nb₂CT_x NSs as a precursor,and the S-P-NNO NCs and f-Nb₂CT_x NSs were further assembled into S-P-NNO/f-Nb₂CT_x hybrids by freeze-drying method.By analyzing the products of different hydrothermal alkalization duration,the formation mechanism of S-P-NNO NCs was analyzed in detail.Thanks to the synergistic effect between the S-P-NNO NCs and the f-Nb₂CT_x MXene NSs,the hybrids electrodes have superior high-rate performance.More importantly,for the first time,the unique Li⁺-storage mechanism of the hybrids electrode was revealed by in/ex situ X-ray diffraction techniques.The assembled S-P-NNO/f-Nb₂CT_x NSs//AC LIC can provide an ultra-high energy density of 240.5 Wh kg^-1 and 12.5 W h kg^-1 at an energy density of 55.6 W h kg^-1with excellent long-duration lifespan（75%capacity retention after 4000 cycles）.