Preparation Of Nitrogen-doped Soft Carbon Anode Material And Its Application In Potassium/Sodium Ion Capacitors

The rapid development of sustainable energy sources has led to extensive research on high-performance energy storage devices.Potassium/sodium ion capacitor as a new type of energy storage device is composed of a battery-type cathode and a capacitor-type anode,which takes the advantages of high energy density of battery,high power density of supercapacitors and long service life of supercapacitor.However,the battery-type anode often faces problems such as slow reaction kinetics and serious capacity attenuation caused by volume expansion of electrode material due to large ion size during charge and discharge in practical applications.To address these problems,a novel accordion-like soft carbon with hierarchical micro-meso-macro-porous carbon framework has been designed and prepared in this dissertation,which enables the electrode to be in full contact with the electrolyte and facilitates the diffusion of potassium/sodium ions.Moreover,the cavities between the soft carbon nanosheets can alleviate the volume expansion of the electrode material caused during charging and discharging,which can improve the rate performance and cycling performance of the anode effectively.Meanwhile,highly nitrogen-doped graphene quantum dots（N-GQDs）are loaded onto the soft carbon and the nitrogen doping type is adjusted by annealing treatment to enhance the kinetic performance and energy storage performance of the anode.Finally,we assembled potassium/sodium ion capacitors with nitrogen-doped soft carbon as the anode and porous carbon prepared from wasted cabbage leaves as the cathode to study their energy storage performance.Overall,the specific studies in this dissertation are as follows:（1）Preparation and characterization of nitrogen-doped soft carbon anode materials.Soft carbon（SC）has attracted much attention because of its simple synthesis method,stable structure and high K/Na storage capacity.However,the surface of soft carbon is coated with a skin-like carbon film,which prevents it from contacting with the electrolyte fully,impeding the transmission and penetration of the electrolyte,and limiting its application in energy storage.A novel accordion-like layered structure of soft carbon（ASC）is constructed by developing a simple method of strong oxidation and selective etching to remove the carbon film on the surface of soft carbon.Nitrogen-doped graphene quantum dots（N-GQDs）are loaded onto soft carbon by electrodeposition method,and the nitrogen doping type is adjusted by annealing treatment to prepare nitrogen-doped soft carbon anode material（N-GQD@ASC-500）.The morphology and structure of SC,ASC and N-GQD@ASC-500 were characterized by SEM,TEM,XRD,Raman,BET and XPS.The experimental results show that the nitrogen-doped soft carbon anode material presents accordion-like morphology,and there are cavities between the layered nanosheets,which can alleviate the volume expansion of the electrode during charge and discharge effectively.Compared with the traditional soft carbon,the specific surface area of N-GQD@ASC-500 increases about three times.Hierarchical micro-meso-macro-porous carbon framework can make the electrode contact with the electrolyte fully,which is conducive to the diffusion of potassium/sodium ions.Annealing treatment can adjust the nitrogen doping type of N-GQD@ASC-500,and part of pyrrole nitrogen is converted to pyridine nitrogen,thus improving the energy storage activity of the anode material.（2）Research on potassium and sodium storage properties of nitrogen-doped soft carbon anode materials.The results of the potassium storage performance tests show that prepared N-GQD@ASC-500 anode material has a reversible capacity of 359 m Ah g^-1 at a current density of 100 m A g^-1 and retains a capacity of 229 m Ah g^-1 after 1000cycles at a high current density of 1A g^-1,indicating that the anode material has high capacity,excellent rate performance and cycle performance.The results of sodium storage performance tests show that N-GQD@ASC-500 exhibited a reversible specific capacity of 268 m Ah g^-1 at a current density of 100 m A g^-1 and a capacity of 142 m Ah g^-1 after 1000 cycles at 1 A g^-1.In addition,kinetic analysis,in-situ Raman and in-situ impedance analysis are employed to reveal the potassium ion storage mechanism of the N-GQD@ASC-500 anode material,and DFT calculations show that the loading of N-GQDs（N-doping）and annealing treatment（regulation of doping types）can enhance K⁺adsorption,decrease the HOMO-LUMO energy gap,as well as increase the electronic conductivity,thereby improving the storage performance of K⁺.（3）Preparation and characterization of porous carbon cathode materials and their electrochemical performance testing.The porous carbon cathode material（PC）is prepared by KOH activation method using wasted cabbage leaves as raw material.The morphology and structure of porous carbon are characterized by SEM,XRD,Raman and BET.The results show that the prepared cathode materials are porous carbon materials with high specific surface area(2697.33 m² g^-1)and Internally connected porous three-dimensional network structure,which is favorable for the rapid transport and diffusion of electrolyte ions.The results of potassium storage performance show that the porous carbon has a capacity of 84.2 m Ah g^-1 at a current density of 100 m A g^-1 and retains a capacity of 58.1 m Ah g^-1 after 1000 cycles at a high current density of1A g^-1,indicating that the cathode material has excellent rate and cycle performance.The sodium storage performance show that the capacity retention rate of porous carbon reaches 98%after 1000 cycles at high current density of 1 A g^-1,and the coulombic efficiency is close to 100%.（4）Research on assembled potassium/sodium ion capacitors and their energy storage performance.The N-GQD@ASC-500||PC potassium/sodium ion capacitors are assembled using nitrogen-doped soft carbon as the anode and porous carbon as the cathode with 1 M KPF₆（EC:DMC=1:1 Vol%）and 1 M Na PF₆（EC:PC=1:1 Vol%）as the electrolyte,respectively.Their potassium and sodium storage performance have been studied.N-GQD@ASC-500||PC potassium ion capacitor achieves an energy density of 171 Wh kg^-1 at a power density of 101 W kg^-1 and a capacity retention rate of 84.2%after 3000 cycles.The N-GQD@ASC-500||PC sodium ion capacitor achieves an energy density of 136 Wh kg^-1 at a power density of 99 W kg^-1 and a capacity retention rate of 90.2%after 3000 cycles.The results show that the assembled N-GQD@ASC-500||PC potassium/sodium ion capacitors both has high energy density,high power density and long cycle life,and has a good practical application prospect.