Preparation Of All Graphene Aerogel-based Electrode Material And Study On The Performance Of Li-ion Capacitor

Lithium-ion hybrid capacitor is a novel type of energy-storage device,as compared with the symmetric supercapacitor,it exhibits high energy density,and could deliver high power density simultaneously.Therefore,it is considered as an important developing direction of energy-storage devices in the future.Graphene,relying on the excellent intrinsically physical properties,shows great application prospect in the filed of electrode materials for hybrid capacitors.However,due to the van der Waals andπ-πinteraction,graphene nanosheets easily aggregate,which seriously affects the electrochemical properties.On the one hand,the packing density and porous structure of graphene were boosted after the combination of graphene aerogel（GA）and modified activated carbon（AC）in this work,leading to the improved capacitance.As compared with monolithic GA,the introduced AC in the GA/AC composite not only served as“spacers”that mitigate the restacking of graphene nanosheets effectively,maintain the microstructure of the GA,and increase the specific surface area,but also acted as conducting bridges to improve the conductivity of the whole aerogel.The packing density of the GA+3AC composite sample could reach to 0.67 g cm^-3,and it could deliver maximum gravimetric and corresponding volumetric capacitances of 128.2 F g^-1 and 85.9 F cm^-3 respectively,in 6 M KOH aqueous electrolyte.Meanwhile,it showed no apparent degradation of the specific capacitance at a current density of 10 A g^-1 after 20000 cycles,indicating the excellent capacitance performance and cycle stability.The gravimetric and corresponding volumetric capacitances of 116.6 F g^-11 and 78.1 F cm^-3 with a decent cyclic stability were also achieved in 1 M EMIMBF₄ ionic liquid electrolyte.On the other hand,after mild pore-forming and post nitrogen-doping processes,the high-level nitrogen-doped defective graphene aerogel（N-DGA）was prepared,and the performance of lithium-ion insertion was also explored.As compared with GA and N-GA,the graphene nanosheets of N-DGA present more obvious defects,and the porous structures were also boosted.The N atoms mainly combined with the broken bonds of the edges and defect areas.The abundance of defects can also provide more activated sites to accommodate extraneous N atoms,which can imporve the contents of N atoms drastically.N atoms and the defects throughout the aerogel may increase the lithium-ion storage sites,meanwhile,the porous structures could shorten the diffusion path and provide more channels that facilitate the ion transport and adsorption at the interface between electrode and electrolyte.The specific capacities of 1387 and 143 mAh g^-1 could be achieved at current densities of 50 and 1000 mA g^-1,and the samples exihibit superior capacitance retention of 90%after 300 cycles at the current density of 500 mA g^-1,demonstrating excellent reversibility of the lithium ion insertion/extraction process,rate capability and cyclic stability.The LIC was also evaluated using the GA+3AC cathode and N-DGA anode after balancing mass ratio.The all graphene-based LICs with decent energy/power densities(the maximum energy/power densities were 39 Wh kg^-1 and 1.2 kW kg^-1)show a promising application perspective.