Design And Energy Storage Mechanism Study On Carbon Materials For High Mass Loading Zinc Ion Capacitors

Zinc ion capacitors（ZICs）,as sustainable emerging electrochemical energy storage devices,combine the advantages of high power density of supercapacitors（SCs）and high energy density of aqueous zinc ion batteries（ZIBs）.However,ZICs still face the mismatch of capacity and kinetics between Zn anode and capacitive-type cathode.Meanwhile,carbon cathode materials also face some disadvantages such as low active site utilization,low specific capacity and poor rate performance.The carbon electrode materials for ZICs were designed and synthesized,and the electrochemical performance of ZICs was significantly improved by the regulation of micromorphological structure,pore structure,specific surface area and carbon defects.In addition,systematic ex-situ characterizations coupled with in-situ Raman spectroscopy and electrochemical quartz crystal microbalance（EQCM）were employed to deeply investigate the energy storage mechanism.The optimized carbonaceous material endows the corresponding ZICs with fast reaction kinetics and excellent zinc ion storage capacity under high mass loading.On this basis,the pore structure of the carbonaceous material is modified to match the pore size for the carrier transport and storage,thus enhancing the cyclic stability and the utilization of active sites.This thesis focuses on the study on carbonaceous cathode materials for ZICs,including pore size distribution regulation,fast kinetics at high mass loading,and energy storage mechanism to prepare high-performance ZICs.The details are as follows:（1）The conventional carbon cathode material presents slow kinetic response under high mass loading,resulting in precipitous drop in electrochemical performance of ZICs under high mass loading.In this thesis,spherical superstructure of N doped carbon nanorods（SSNCR-x）were constructed by hydrothermal synthesis strategy.The optimized SSNCR-800 features a gradient carbon nanorods framework with hierarchical porous structure,abundant oxygen functional groups and defects,which facilitates ion diffusion and electron transfer and enables impressive properties for Zn²⁺storage under high mass loading.Consequently,SSNCR-800 displays a highly fascinating property in terms of capacitance and cycling stability as the cathode of ZICs,delivering pronounced energy density of 181.2 Wh kg^-1 and power density of31.1 k W kg^-1,as well as long lifespan with 93.5%capacity retention after 50000 cycles.Moreover,the constructed ZICs showcase a high specific capacitance of 215 F g^-1 and91.7%capacitance retention after 10000 cycles under mass loading of 50 mg cm^-2.Systematic in situ Raman spectra and EQCM measurements testify that the excellent electrochemical properties are attributed to the synergistic effect of the Zn²⁺,H⁺,and CF3SO3^-co-adsorption mechanism and reversible chemical adsorption.（2）In ZICs,the matching degree of the pore size of the cathode material and the size of the carriers is a key factor affecting the charge storage and reaction kinetics.In this thesis,phenol-melamine resin was obtained by co-polymerization of phenol and melamine in formaldehyde,and carbon nanosphere pore size structure was effectively regulated by adjusting the concentration of phenol-melamine resin.The optimized carbon nanosphere structure（LVCR）has high specific surface area,large number of microporous and mesopores,moderate amounts of macroporous structures,fast charge transport channels and abundant active sites.The optimized internal channel of the LVCR can be reasonably matched with the size of the carriers to provide excellent zinc storage capacity.Therefore,the LVCR-based ZICs have high energy density(126.6 Wh kg^-1)and power density(31.4 k W kg^-1),especially with 97.7%capacity retention over50000 cycles at current density of 10 A g^-1.Further,systematic ex-situ test combination with in situ Raman spectra and EQCM measurements testify that large number of microporous and mesopores combined with moderate amounts of macroporous structures provide fast transport channels for ion transport,speed up the kinetic reactions and improved zinc ion storage capacity.