Design And Properties Of Carbon-based Electrode Materials Of Supercapacitors For Compact Energy Storage

With the development of society,and the miniaturized trend of electronic and electrical equipments,Electrical energy storage devices must be required to have high volumetric energy and power density,and good cyclic stability.Supercapacitor,is a high-power energy storage device with the advantages of fast charge/discharge,high efficiency,no pollution,long cycle life,and high safety.But the volumetric energy density is still relatively low,which goes against its practical application.Therefore,it is particularly important to develop new electrode materials and design new device structures to increase the volumetric energy density of supercapacitor while maintaining high power density,which is of great significance for the miniaturization of electronic and power equipment.To improve the volumetric energy density of supercapacitor,the electrode materials with high capacitance should be developed.For example,reduced graphene oxide?rGO?has been considered as one of the excellent electrode materials.In general,rGO materials are usually designed to be porous and fluffy structures to reduce the resistance of ion transport and diffusion in the electrode,as well as increase the ion accessible surface areas of the electrodes,which effectively improve the gravimetric energy density of the rGO electrode.However,the porous and fluffy rGO electrodes have low packing density and insufficient contact between the rGO nanosheets,resulting in increased electron resistance of electrode and lowered volumetric energy and power densities.It is generally believed that highly dense rGO electrode has a small specific surface area and a large ion transport resistance in the electrode,thus leading to a small capacitance and a low power density.In this thesis,three electrodes with different compactness were designed and prepared,including three dimensional hydrogels?3DrGO-H?,rGO hydrogel films?rGO-HF?and dense rGO hydrogel films?DrGO-HF?,and their electrochemical performances were also studied.Amazingly,the DrGO-HF achieved by simply pressing filtrated rGO films,show excellent electrochemical performance,with a high volumetric specific capacitance of 170.8 F cm^-3,a high volumetric energy of 3.8 mWh cm^-3 at about 535.3mW cm^-3 and an excellent cycle stability?92%capacitance retention after 250000 cycles?.These excellent performances are attributed to the dense electrode(1.60 g cm^-3)and the close contact between the rGO nanosheets,thus enhance the structural stability and improve the conductivity of the electrode.The scientific fact is clarified that the re-stacking of rGO sheets in dense electrodes would not degrade the electrochemical energy storage performances,which is of great significance for the design and preparation of supercapacitor electrodes with high volumetric energy density.When evaluating the electrochemical performance of the electrode materials,the results produced by different mass loadings vary greatly.The electrode materials usually show excellent performances at low mass loadings.However,at device scale,including all components,such as electrolyte,current collector and separator,the small proportion of active materials would limit the improvement of their electrochemical performance.Therefore,it is particularly important to increase the loadings of active materials to effectively improve the electrochemical performance of the supercapacitor device.However,with the increase of mass loading of the active material,the ion transport path in the electrode will gradually become longer and complex,which will reduce the power performance of the supercapacitor and limit the effective improvements of their volumetric electrochemical performances.In the thesis,a new type of supercapacitors with alternating stacked conductive Ti₃C₂T_x films as electrodes,and using thin layers of gel electrolyte as ionic carrier and separators,is proposed.This device structure eliminates the current collectors and separator,increases the mass loading of active materials of the whole device without increasing the mass loading of a single electrode,shortens the ion transport path,and effectively improves the areal and volumetric energy densities without sacrificing power density.Especially,the supercapacitor with 33-layer interdigital electrodes can achieve a high areal capacitance of 11.5 F cm^-2 at the scan rate of 0.5 mV s^-1,and ultrahigh energy density of about 8.3 mWh cm^-3 in an aqueous electrolyte system,much higher than that of the supercapacitor with two-electrode configuration under the same ultra-high loadings of the active materials.The novel design of the electrodes towards next-generation high dense energy storage devices,not limited to supercapacitors,would offer distinctive opportunities for the energy storage systems.