Study On The Preparation And Electrochemical Performance Of Reticulated Nitrogen-doped Carbon Nanosheets

With the rapid development of portable electronic devices and electric vehicles,energy storage systems must be required to possess larger energy density and power density.Among the current energy storage systems,high energy density lithium-ion batteries(LIBs)and high power density supercapacitors(SCs)are the two major energy storage devices.However,LIBs are limited by poor power density and SCs are limited by a low energy density.Lithium ion capacitors(LICs),inheriting the superiorities of both LIBs and SCs devices and combining a LIBs battery-type of anode and a SCs capacitor-type of cathode,possess larger energy density and higher power density.However,the performance of LICs devices is always restricted by cathode and anode and the kinetics mismatching between them.Generally,the imbalance of electrode kinetics characteristics between anodes and cathodes(the Li+ cations inserted reaction process for anode is greatly slower than PF6-anions adsorbed process for cathode)leads to the difficulty of joint improvements in energy density and power density.As a result,we herein come up a facile and effective synthesis of 3D mesh-like nitrogen-doped carbon nanosheets(NCNs)with multiscale pore structure as both cathode and anode to assemble a novel dual-carbon LICs device.With the excellent electrode materials and rational design,the assembled dual-carbon symmetric LICs exhibit excellent electrochemical properties.Porous carbon materials are promising electrode materials for lithium ion capacitors due to their excellent electrical conductivity,large specific surface area and environmentally benign nature.Moreover,a doping carbon framework with heteroatoms has been regarded as an effective strategy with which to increase the active sites and defects,which should be an effective strategy with which to improve the electrochemical properties of the carbon materials.As a result,we herein come up a facile and effective synthesis of 3D mesh-like nitrogen-doped carbon nanosheets(NCNs)with multiscale pore structure as electrode materials.In general,the hydrothermal treated and calcinated MgO with polygonal shapes was used as template,furfuryl alcohol(FA)as carbon source and dicyandiamide(DCDA)as nitrogen source.The final product was synthesized by carbonizing the carbon and nitrogen precursor obtained from hydrothermal treatment process and removing the MgO template.Such unique dispersed nanosheets with highly porous structure could provide sufficient specific surface area(SSA)for ions adsorption and intercalation,and channels for the transportation of both ions and electrons.What is more,it was demonstrated that N doping into NCNs can increase active sites and defects,and induce rapidly the pseudocapacitive reaction,all of which contribute to the improved electrode performances.Specially,the NCNs-2 sample shows the highest BET surface area(1606 m2 g-1)and the largest pore volume(2.54 cm3 g-1)and possesses the highest N content(up to 12.75%)compared with the control group samples(CNs,NCNs-1 and NCNs-3 samples),which makes it a hopeful electrode material.Furthermore,to investigate the electrochemical performances of NCNs as both cathodic and anodic electrode for LICs,coin-type half-cells were fabricated and tested.NCNs-2 anode presents a high capacity of 1107 mAh g-1 in the charge/discharge cycle at 0.1 A g-1.NCNs-2 anode also presents excellent rate performance,even under 10 A g-1,the NCNs-2 anode still maintains a specific capacity of-243 mAh g-1.Besides,the NCNs-2 cathode also exhibits the good specific capacity of-115 mAh g-1 at 0.1 A g-1.Moreover,NCNs-2 cathode also reveals superior stability with a capacity retention of 83.2%after 5,000 cycles under a current density of 2 A g-1.All of above features indicate that NCNs-2 sample could be a prospective electrode for high performance of LICs.In this work,we adopt an electrochemical charge injection(ECI)strategy to tune the electrode potential window(EPW)for improving the energy density of NCNs-2//NCNs-2 LICs.With rational design,this dual-carbon LICs device exhibits a superior electrochemical performance,including a broad high working voltage window(0-4.5 V),an ultrahigh energy density of 218.4 Wh kg-1 electodes(229.8 Wh L-1 electrodes),the highest power density of 22.5 kW kg-1 electrodes(23.7 kW L-1 electrodes)even maintaining an ultrahigh energy density of 97.5 Wh kg-1 electrodes(102.6 Wh L-1 electrodes),as well as reasonably excellent cycling stability with capacity retention of 84.5%(only 0.0016%capacity loss per cycle)within 10,000 cycles under a high current density of 5 A g-1.This study provides an efficient method and option for the development of high performance of energy-storage devices.