Controllable Preparation Of Niobium-based Nanocomposites For Lithium Ion Capacitors

In recent years,the excessive use of non-renewable energy has led to the exhaustion of fossil energy and serious environmental pollution.Countries are actively seeking sustainable clean energy and new technologies related to energy storage.Supercapacitors?SCs?and lithium-ion batteries?LIBs?are the most widely studied of the new energy storage devices.SCs have the advantages of fast charge/discharge,long cycle life and high power density,and have been widely concerned in recent years.However,its low energy density limits its commercial applications.While LIBs have the advantages of wide operating potential range and high energy density,but LIBs also have the disadvantages of serious self-discharge,short cycle life and low power density.Therefore,searching for a new type of energy storage device that can combine the advantages of SCs and LIBs and make up for the disadvantages of both is crucial for the rapidly developing energy storage industry.Fortunately,lithium ion capacitors?LICs?have retained the advantages of high energy density of LIBs and the characteristics of high power density of SCs,making them become the focus of current research.However,it is found that there are still some problems to be solved,such as the mismatch of reaction kinetics between cathode and anode materials.The purpose of this dissertation is to prepare electrode materials with different structures,morphologies and conductivity by different methods to solve the problems of reaction kinetics mismatching between cathode and anode of LIC,and to explore the"structure-activity relationship"and electrochemical energy storage mechanism between structures and properties through morphological structure characterization and electrochemical performance testing.The main research contents are as follows:1.The three-dimensional H-Nb₂O₅ nanomaterial was prepared by one-step hydrothermal method.The GO and H-Nb₂O₅ nanocomposites were compounded and the interface between H-Nb₂O₅ and rGO was constructed through thermal reduction to improve the electrical conductivity of the H-Nb₂O₅/rGO nanocomposite.The"structure-activity relationship"of electrode materials was studied through material characterization and electrochemical performance test,and the influence of the introduction of rGO on ion diffusion rate and capacitance contribution ratio was studied through calculation.H-Nb₂O₅/rGO//SCCB devices with energy density and power density up to 100.2 Wh kg^–11 and 20 kW kg^–11 were obtained by electrochemical pre-lithium treatment of H-Nb₂O₅/rGO//SCCB electrode materials when matching with the capacitance of cathode materials.However,further study found that the H-Nb₂O₅ electrode material with three-dimensional lamellar structure was prone to collapse when charging and discharging at higher current.In order to solve this problem,NaNbO₃ nanometer cube with a phase lamellar structure was designed and synthesized as LIC electrode material.2.The orthorhombic NaNbO₃ nanocube was prepared by hydrothermal method.The NaNbO₃ nanocube with bulk phase layered structure is more stable,which can solve the problem that the structure of H-Nb₂O₅ electrode material is easy to be collapsed when measured at higher current.However,through calculation and analysis,it was found that the ion diffusion rate of NaNbO₃/rGO electrode material was significantly lower than that of H-Nb₂O₅/rGO electrode material,and the maximum energy density(166.8 Wh kg^–1)and maximum power density(33.75 kW kg^–1)of NaNbO₃/rGO//SCCB LIC device were not significantly improved.The main reason for this phenomenon is the poor carbon coating effect,caused by the large size of NaNbO₃ nanocube;In addition,large size of NaNbO₃ nanocube hinders the diffusion of lithium ions in the phase,which is not conducive to the improvement of energy density and power density of LIC devices.In order to further improve the electrochemical properties of LIC device and solve the problem of poor electrochemical properties caused by large size electrode materials,Nb₄N₅nanoparticles with porous structure were designed and synthesized as LIC electrode materials.3.The Nb₄N₅ nanomaterials with porous structure were prepared by ammonia-nitridationinhightemperaturegas-solidphasereaction.The electrochemical performance test results showed that Nb₄N₅/rGO electrode material had better rate performance and cycling stability than H-Nb₂O₅/rGO and NaNbO₃/rGO electrode material under the same test conditions.In addition,it was found that lithium ions had the fastest diffusion rate in Nb₄N₃/rGO electrode materials by electrochemical calculation,and the maximum power density and energy density of Nb₄N₅/rGO//SCCB devices were significantly increased to 295.1 Wh kg^-11 and41.25 kW kg^-1,respectively.The excellent electrochemical properties of Nb₄N₅/rGO//SCCB devices benefit from the existence of porous structure of Nb₄N₅nanomaterials,which greatly reduces the lithium ion diffusion path and solves the problem that the electrode material structure is prone to collapse when charging and discharging at large current,making Nb₄N₅/rGO//SCCB devices have good magnification performance and cycling stability.