Preparation And Properties Of Nb₂O₅-Based Composite Anode Materials For Sodium-Ion Capacitors

Sodium-ion capacitors（SICs）with high energy and power density have emerged as promising energy storage technology.However,the practical application of typical SICs is hindered by several limitations,such as sluggish reaction kinetics and low initial Coulombic efficiency（ICE）of the battery-type anode materials.The anode material of Nb₂O₅ exhibits pseudocapacitive sodium-storage behaviors,which facilitates rapid insertion and extraction of sodium ions during the charge-discharge process.Nevertheless,the practical implementation of Nb₂O₅ in SICs is impeded by its poor electronic conductivity,low ionic conductivity,and low ICE.In this dissertation,a series of Nb₂O₅-based composite electrode materials were synthesized,and their electronic and ionic conductivities were enhanced through different construction strategies to boost the sodium-storage reaction kinetics.Meanwhile,a simple and efficient pre-sodiation method was developed to improve the ICE of the Nb₂O₅-based electrode,achieving high-performance SICs with increased energy/power density and long cycle life.The main results of this dissertation are as follows:（1）A N-doped carbon（outside）coating,S-doped（inside）Nb₂O₅（S-Nb₂O₅@C）electrode material based on low-cost commercial Nb₂O₅ nanoparticles（<200 nm）was prepared by using ionic liquids enriched with carbon,nitrogen,and sulfur.The effects of the modified structure on the electrochemical properties of the Nb₂O₅-based composite electrodes were systematically investigated.The synergistic incorporation of N-doped carbon（surface）and S doping（bulk）in the S-Nb₂O₅@C,resulted in a remarkable enhancement of electronic conductivity and a modest improvement for the Na⁺diffusion in S-Nb₂O₅@C.Consequently,the electrode material of S-Nb₂O₅@C,exhibited a remarkable capacity of 218.4 mAh g^-1 at 0.5 C and 94.1 mAh g^-1 at 50 C.The Na-insertion/extraction kinetics of the S-Nb₂O₅@C anode has been significantly enhanced,enabling it to match that of the nonfaradic capacitive cathode（activated carbon）,thereby achieving high-performance SICs with high energy density(59.0 Wh g^-1)and long cycle life over 10,000 cycles.（2）Carbon-coated/carbon-bridged Nb₂O₅ mesocrystals（meso-Nb₂O₅@C）were synthesized as an anode material with ultrafast kinetics through a rapid microwave-assisted method（1 min）and subsequent heat treatment.A systematic investigation was conducted to elucidate the conformational relationship between the structure and electrochemical performance of the synthesized anode materials.Benefiting from superior electronic conductivity by carbon coating/bridging and fast kinetics by mesocrystalline design,the meso-Nb₂O₅@C electrode exhibited good pseudocapacitive electrochemical behavior and high sodium ion diffusion coefficient up to 5.88×10^-12 cm² s^-1.Meanwhile,meso-Nb₂O₅@C with mere 7.8 wt%carbon displayed outstanding rate capability(133.4 mAh g^-1even at 50 C).Anchored by the carbon bridge,meso-Nb₂O₅@C exhibited exceptional stability at 20 C,i.e.a capacity of 112.4 mAh g^-1 with 100%average Coulombic efficiency after 10,000 cycles.The as-assembled SICs presented outstanding electrochemical performance with a high energy density of 28.4 Wh kg^-1 at 15,600 W kg^-1.（3）A simple bifunctional chemical pre-sodiation method was developed to precisely compensate the Nb₂O₅ anodes for its electrolyte decomposition losses during the initial sodium insertion process,as well as irreversible sodium uptake originated from the necessary activation of Nb₂O₅ anode（i.e.amorphous Na_xNb₂O₅ formation）,by using sodium naphthalene/2-methyltetrahydrofuran with an ultralow redox potential（<100 mV vs.Na/Na⁺）as a pre-sodiated agent.The pre-sodiated electrode displayed a high ICE of up to 100%and exhibited enhanced electrochemical performance.Meanwhile,the pre-sodiated Nb₂O₅ electrode surface with a robust solid electrolyte interface（SEI）riched with NaF/NaCl inorganics,significantly improved the cycling stability of the electrodes in SICs.The energy density and cycling stability of the SICs were effectively improved by the synergistic compensation of the surface and bulk electrode.The constructed pouch-type SICs exhibited a high energy density of up to 53.9 Wh kg^-1 and long cycle life（90%capacity retention over 10,000 cycles）.