The Design Of Carbon-Based Cathode Materials And System Optimization For Lithium-Ion Capacitors

Lithium-ion capacitors(LICs)combine the energy storage mechanisms of lithium-ion batteries and supercapacitors.Theoretically,LICs with the advantages of high power,high energy,long life and high safety are the development direction of the next generation of supercapacitors.However,the energy density of LICs cannot be fully exploited due to the low specific capacity of the capacitive cathode mismatched with that of the battery-type cathode.Low energy density is still the key factor restricting the large-scale application of LICs.Due to the "bucket effect" of the cathode and anode in full capacitors,the key to improve the energy density is to develop carbon-based capacitive materials with high specific capacity.Furthermore,system optimization can improve the practical electrochemical performance of the cathode and anode.Therefore,in this thesis,the design of carbon-based capacitive cathode materials and the system optimization are investigated for building the LICs with high energy density.Specifically,it includes the following three parts:(1)Using coal tar pitch as the soft carbon source and flaky phenolic resin as the hard carbon source,a hard@soft composited porous carbon was synthesized by the thermal coating effect of coal tar pitch.The coal tar pitch with a highly aromatic ring structure can improve the graphitization degree and electrical conductivity of the porous carbon.The flaky phenolic resin can work as a structure-directing agent for the coal tar pitch melting at the high temperature to improve the porosity and specific surface area of the porous carbon after activation.Hence,the composited porous carbon cathode material can make high electrical conductivity and high porosity compatible and improve the specific capacity and rate performance of the capacitive cathode,simultaneously.For achieving a high energy density of the full capacitor,a porous MnO@C anode with high specific capacity and low Li-insertion potential was selected to make the energy density as high as 104.7 Wh/kg at the power density of 410 W/kg and maintaining 21.7 Wh/kg at a high power density of 10250 W/kg.In addition,the working potential evolutions of the anode and cathode during the long cycle revealed that the specific capacitance of the cathode is almost no lost,and the capacity loss of the anode is the main reason for the capacity decay of the full capacitor.(2)Using the precursor of the coordination compound of CuCl2 and L-glutamic acid,a series of porous carbons with precisely controllable pore size distribution ranging of 0.5～4 nm were synthesized.Through controlling the gas velocity during carbonization and the ratio of copper salt,the pore size distribution can be regulated by the CuCl with the dual pore-forming mechanism of template volatilization and etching activation.By comparing the double-layer capacitive performances of this series of porous carbons,the relationship between pore size and desolvation of organic lithium-ion electrolyte was investigated.The electrochemical results showed that the micropores at 0.56 nm can store desolvated ions with high specific capacitance but sluggish mass transfer kinetics.Meanwhile,by cooperating the larger pores at 1～4 nm,the problem of sluggish mass transfer kinetics can be effectively alleviated.Considering the effect of the electrolyte filling in pores on the energy density,an appropriate pore size distribution of capacitive carbon cathode is suggested that to be dominated with 0.56 nm micropores and cooperated with an appropriate amount of the micropores at 1～1.5 nm with low pore volume.The optimized porous carbon can reach a high specific capacitance of 130.3 F/g and maintain 78%of specific capacitance at the high current density of 10 A/g.The optimized porous carbon cathode can endow the full capacitor with a balanced energy density and power density,showing a high energy density up to 128.7 Wh/kg at the power density of 450 W/kg and 62.5 Wh/kg at a high power density of 22500 W/kg.(3)By using mesoporous carbon as a carrier,a highly developed carbon wall can block the crystallization of the loaded LiFePO4,constructing an amorphous LiFePO4@mesoporous carbon composited cathode.Based on the pseudocapacitive Li-insertion mechanism in the bulk phase,amorphous LiFePO4 can significantly enhance the specific capacity of the cathode limited by the specific surface area of porous carbon.The solid solution reaction of amorphous LiFePO4 removes the constraint of Li-insertion kinetics by the phase transition reaction,increasing the Li+diffusion by more than two orders of magnitude.Hence,the composited cathode can retain 82%of the specific capacity at the current density of 5 A/g and hold an excellent cyclic stability.After matching the composited cathode with a highly amorphous carbon anode that also shows a pseudocapacitive Li-insertion,the energy density of the full capacitor can reach 131 Wh/kg at the power density of 500 W/kg and maintain 92 Wh/kg at the high power density of 25000 W/kg.Meanwhile,its energy density is 62%higher than the traditional configuration using pure activated carbon as the cathode and shows a similar energy efficiency.On the contrary,if the porous carbon cathode combining with crystalline LiFePO4 based on the two-phase reaction,the energy efficiency and power performance of the full capacitor are decimated.Therefore,amorphous LiFePO4 with a pseudocapacitive Li-insertion can substantially improve the energy density of the full capacitor without sacrificing energy efficiency and other superior characteristics.