Heteroatom-Doping Mediated Regulation Of The Porous Carbon Materials For The Cathode Of Lithium-Ion Capacitors

Lithium ion capacitors（LICs）,as a new generation of potential energy storage devices,combine the high energy density of lithium ion batterys（LIBs）and the high power density of electric double layer capacitors（EDLCs）.However,due to the different storage mechanisms of the cathodes and anodes,the dynamic imbalance and capacity mismatch of the cathodes and anodes still exist in LICs,which affect the overall performance of LICs.In order to further improve the performance of LICs,researchers are committed to developing new electrode materials.As for cathodes,porous carbon is one of the most widely used materials at present.High conductivity and specific capacity are the key indicators of its performance,which have a decisive influence on improving the energy density of the device.However,the systematic exploration of the influence of heteroatom on porous carbon as cathode material for lithium ion capacitors has not been reported.In this paper,ordered mesoporous carbon was used as the carbon matrix precursor,controlled oxygen,nitrogen and phosphorus doping of mesoporous carbon were carried out through a series of post-processing,and the performance of different doped mesoporous carbon as the cathodes of lithium ion capacitor was explored.The main research conclusions are as follows:（1）By using the SBA-15 as the template and sucrose as the carbon source,the ordered mesoporous carbon CMK-3 was synthesized by the template method.CMK-3 has a high specific surface area(1208 m² g^-1),a large pore volume(1.35 cm³ g^-1)and an open and ordered pore structure.The material（CMK-3）was used as carbon precursor to investigate the electrochemical performance of the doped ordered mesoporous carbon material.Firstly,CMK-3 was oxygen functionalized with different concentrations of H₂O₂ to obtain oxygen doped ordered mesoporous carbon without changing the chemical composition.（OC-X,the X represents the volume ratio of H₂O₂）.When X was 75%,OC-75 had the highest oxygen content of 9.96 at%,and the C=O functional group formed by oxygen atom doping through oxygen functionalization could provide abundant lithium storage active sites and had stable configuration and good conductivity,so it had strong ion adsorption capacity.When OC-X was applied to the cathode of LICs,the specific capacity was improved to some extent compared with that of the CMK-3 after 3000 cycles in the voltage range of 2-4.5 V（vs.Li⁺/Li）and the current density of 1 A g^-1,Among them,the OC-75 increased by nearly 75%compared with CMK-3 performance.（2）Phosphorus atom doping could effectively adjust the surface properties of carbon materials to improve their electrochemical performance.By activating the CMK-3 with H₃PO₄,the high specific surface phosphorus doped ordered mesoporous carbon（PC-X,the X represents the mass ratio of H₃PO₄ to CMK-3）was prepared.PC-X retains short-range ordered mesoporous that facilitate the rapid diffusion of lithium ions,and the doping of phosphorus significantly inhibited the formation of unstable groups,which could enhance the oxidation stability of mesoporous carbon.When PC-X was used as the cathode of LICs,it had better cycle stability than CMK-3 at the potential range of 2-4.5 V（vs.Li⁺/Li）and the current density of 1A g^-1,and the capacity of PC-3 had increased by 70%.This research provided important significance for H₃PO₄ activated carbon materials in the industrial application of electrochemical energy storage.（3）By using the post-treatment method,the pre-prepared ordered mesoporous carbon CMK-3 was treated at a high temperature in 20%ammonia-argon mixed atmosphere to prepare a high specific surface nitrogen-doped ordered mesoporous carbon（NC-T,T stands for calcined temperature）.This experiment explored the effects of different calcination temperatures on the doping amount and the defects,chemical bonds and electrochemical properties of ordered mesoporous carbon.In addition,theoretical calculations were used to study the lithium ion adsorption energy and electronic conductivity of NC-T.At the calcination temperature of700°C,the nitrogen content of the obtained NC-700 was 3.76 at%,and it exhibited excellent electrochemical performance as the cathode of LICs.The capacity of NC-700 remains basically unchanged after 5000 cycles at a potential range of 2-4.5 V（vs.Li⁺/Li）and a current density of1 A g^-1,which is nearly 70%higher than that of CMK-3.In order to improve the application value of heteroatom-doped ordered mesoporous carbon,NC-700 was used as the cathode and anode materials to assemble symmetrical lithium ion capacitors.When the mass ratio of anode to cathode was 1:3,the LICs had excellent electrochemical performance.The LICs still had a capacitance retention rate of 73%after 3000 cycles at a current density of 1 A g^-1,and the potential range of 0.5-4.3 V.And the LICs shown the highest energy density of 225 Wh kg^-1 at the power density of 230 W kg^-1.At the same time,the energy density could reach 138 Wh kg^-1 at a high power density close to 10 k W kg^-1.