Lithium Ion Capacitors:Study On Modification Preparation And Electrochemical Performance Of Nb₂O₅ Anode

Recently,lithium ion capacitors?LICs?constructed by a Faraday-type anode of lithium ion battery and a capacitive cathode of supercapacitor are becoming a very promising energy storage technology.Anode and cathode electrodes with different charge storage mechanisms can work reversibly in different potential windows in lithium ion capacitors,providing high energy density and power density as well as long cycle life.However,the kinetics of the imbalance between the Faraday-type anode and the capacitive cathode limits the practical application of the lithium ion capacitor.Therefore,the development of anode materials with excellent kinetic behavior is the key to achieving high performance lithium ion capacitors.The kinetic behavior of the orthorhombic Nb₂O₅?T-Nb₂O₅?intercalation pseudocapacitance lithium storage mechanism is not affected by solid state ion diffusion,and is considered to be an ideal anode material for lithium ion capacitors.This thesis focuses on Nb₂O₅ anode material controllable preparation,electrode structure design,surface modification,charge storage mechanism and construction of lithium ion capacitors with high energy density,high power density and long cycle life.The specific research contents are as follows:1.Controllable preparation and intercalation pseudocapacitance mechanism of Nb₂O₅microspheres:Nb₂O₅ microspheres?Nb₂O₅ MSs?were controllable prepared by a mild hydrothermal method.As the heat treatment temperature increases,the crystal growth of Nb₂O₅ MSs undergone a transition from pseudohexagonal?H-Nb₂O₅ MSs-5?to orthorhombic?T-Nb₂O₅ MSs-7?,and the specific surface area of the material decreases.Then,we assembled lithium-ion batteries to test the electrochemical performance of three materials.Overall,T-Nb₂O₅ MSs-7 exhibits better rate performance and cycle performance.We performed kinetic analysis on H-Nb₂O₅ MSs-5 and T-Nb₂O₅MSs-7 electrode materials.The critical scanning rate of H-Nb₂O₅ MSs-5 and T-Nb₂O₅ MSs-7 is 1 mV s^-1 and 4 mV s^-1,respectively.T-Nb₂O₅ MSs-7 has a higher contribution of intercalation pseudocapacitance at the sweep rate range of 0.2-4 mV s^-1.Most of the current in the electrochemical reaction comes from the rapid capacitance-controlled process,which has a better kinetic behavior.At the same time,we found that the contribution of pseudocapacitance independent of the specific surface area,thus confirming the intercalation pseudocapacitance lithium storage mechanism from the kinetic point of view.2.Controllable preparation of 2D T-Nb₂O₅/NC NSs and its application for lithium ion capacitors:2D Nb₂O₅ nanosheets were prepared by a two-step method,forming a fast and continuous electronic/ion conductive network structure.Most of the charge storage during electrochemical reaction is derived from the intercalation pseudocapacitance of the capacitance-control process by analyzing the b value and the pseudocapacitance contribution ratio of T-Nb₂O₅/NC NSs.Therefore,T-Nb₂O₅/NC NSs have good kinetic behavior.T-Nb₂O₅/NC NSs exhibits excellent electrochemical properties:such as the discharge capacity is still 142.3 mA h g^-1even at 20 C;the discharge capacity of the T-Nb₂O₅/NC NSs after 1000 cycles at 5 C is 147.6 mA h g^-1,and the capacity retention is 88%.A T-Nb₂O₅/NC NSs//PSC LIC was constructed using T-Nb₂O₅/NC NSs as the anode electrode and PSC as the chathode electrode.T-Nb₂O₅/NC NSs//PSC LIC exhibits high energy density and power density:at the power density of 320 W kg^-1,the corresponding energy density is 70.3 W h kg^-1;at the energy density of 38.7 W h kg^-1,the corresponding power density is 16014 W kg^-1.The T-Nb₂O₅/NC NSs//PSC LIC also exhibits excellent cycle performance:the capacity retention rate is approximately 84.0%after 3000 cycles at the current of 1 A g^-1.3.Controllable preparation of 3D T-Nb₂O₅/NC NWs/NSs and its application for lithium ion capacitors:Firstly,a one-step method was developed to prepare in-situ carbon-coated self-assembled3D T-Nb₂O₅ nanowires/nanosheets?C-3D T-Nb₂O₅ NWs,C-3D T-Nb₂O₅ NSs?.Most of the charge storage during electrochemical reaction is derived from the intercalation pseudocapacitance of the capacitance-control process by analyzing the b value and the pseudocapacitance contribution ratio of C-3D T-Nb₂O₅ NWs and C-3D T-Nb₂O₅ NSs.C-3D T-Nb₂O₅ NWs and C-3D T-Nb₂O₅ NSs exhibit excellent rate performance and cycle life:the discharge capacity at 20 C is still 150 mA h g^-1 and159.1 mA h g^-1;the discharge capacity after 3000 cycles at 2 A g^-1 is 121 mA h g^-1 and 122 mA h g^-1,capacity retention is 74.8%and 73.7%,respectively.In addition,the assembled C-3D T-Nb₂O₅NWs//AC and C-3D T-Nb₂O₅ NSs//AC lithium ion capacitors exhibit high energy density(the energy density is 65.92 and 66.63 W h kg^-1 at a power density of 350 W kg^-1)and high power density(power density is 17500 W kg^-1,the correspongding energy density is 34.51 and 37.33 W h kg^-1)and ultalong cycle life(the energy density retention is 78.2%and 88.6%after 20000 cycles at the current density of2 A g^-1).