Structural Optimization Of Order Mesoporous Carbon Microspheres And Their Energy Storage Mechanism As Cathode Material For Dual-ion Batteries

In the past few years,the rapid development of battery-driven devices has put higher requirements on the power density,energy density and stability of electrochemical energy storage medium.The traditional lithium-ion battery has been recognized and widely adopted by the public through continuous research and improvement.But it is currently facing the shortage of resource metals such as Li,Co and the high mining costs.Therefore,the development of economical and high quality energy storage system is imminent.Dual-ion battery（DIBs）based on the co-intercalation/deintercalation mechanism of cation and anion for energy storage is an emerging energy storage system.The high operating voltage of the cathode（>4.5 V vs Li/Li⁺）endow it with higher energy density,while the graphite-carbon electrode reduces manufacturing costs.This is consistent with the future development direction of energy storage systems.However,this emerging energy storage system also has its drawbacks.On the one hand,the intercalation of large-sized anions requires a larger interlayer structure to accommodate them,which results in a relatively low storage capacity.On the other hand,the mismatch between the large-sized anions and the graphite layered structure causes irreversible damage to the cathode material during the intercalation/de-intercalation process.This greatly affects the safety and electrochemical performance of the DIBs.So it is crucial to select and optimize the structure of the cathode material.To address the problems discussed above,this paper selects ordered mesoporous carbon with high specific surface area and relatively stable structure as the cathode material for DIBs.By studying the influence of hydrothermal process conditions on OMC materials,the structure of the cathode material is continuously optimized to obtain excellent electrochemical performance.The specific research progress of this paper is as follows:（1）Ordered mesoporous carbon（OMC）was synthesized by a one-step hydrothermal synthesis method in acidic conditions with formaldehyde and resorcinol as carbon source precursors,F127 of high turbidity point as the structure-oriented template.The effects of the resorcinol/formaldehyde（R/F）ratio in the carbon source composition and the amount of F127 on the microscopic morphology and mesoscopic structure of OMC were explored.Combined with the comprehensive results,the increase of R/F ratio was beneficial to the improvement of OMC orderliness and specific surface area.Only when the amount of F127 in the system was 0.3 mmol,the obtained OMC could combine the regular spherical shape,high specific surface area(613.95 m²g^-1)and pore volume(0.51 cm³g^-1).Comparing with the electrochemical test results,the increase of specific surface area was beneficial to enhance the storage capacity of PF₆^-,and the regular micromorphology,ordered mesoscopic structure and pore volume were all beneficial to the stability of cycling performance.Furthermore,combined with ex-situ XPS analysis,the working mechanism of PF₆^-intercalation/extraction in the OMC cathode was confirmed.（2）Urotropine（HMT）was chosen to control the formaldehyde at a low concentration in the system.Mesoporous carbon materials（HOMCs）with high specific surface area(1349.55 m²g^-1)were obtained by varying the amount of HMT only.The effect of homotrimethylbenzene（TMB）introduction on the morphological structure of the HOMCs was explored.The results showed that TMB could effectively improve the mesoscopic structure orderliness of HOMCs,while the pore volume increased by nearly 32.9%compared with that of HOMCs-B-0 prepared in the absence of TMB.A process step of pre-polymerization at different temperatures was proposed to optimize the morphological structure of the HOMCs.Under comparative analysis,the HOMCs-T-70 prepared by pre-polymerization at 70℃had the most regular spherical shape and higher specific surface area(1754.91 m²g^-1)and pore volume(0.83 cm³g^-1).The two different polymerization mechanisms were then analyzed.The results showed that separate pre-polymerization can avoid the effect of carbon source polymerization on the micelle self-assembly process,and the synthesized HOMCs-T-70-S obtained extremely high specific surface area(2315.09 m²g^-1)and pore volume(1.16 cm³g^-1)while maintaining the regular spherical morphology and mesoscopic structural orderliness.This was comparable to the OMCs activated by complex processes（such as alkali,and water vapor）.Combined with the analysis of electrochemical test results,the increase in the specific surface area of the HOMCs cathode provided more PF₆^-intercalated active sites,which effectively improved the storage performance.And the increase of mesoscopic structural orderliness and pore structure volume facilitated the mutual infiltration of electrolyte and HOMCs cathode,which promoted the rapid transfer of PF₆^-into the cathode structure,and then exhibited good cycling performance and multiplicative performance.Finally,the changes of the electrode structure under different charging/discharging states were analyzed,and the process of the interaction between PF₆^-and HOMCs cathode was investigated in depth.（3）Based on the optimized system of HOMCs,m-aminophenol（MAP）was used as the nitrogen source and carbon precursors.Then nitrogen-doped micro-mesoporous coexistence materials（N-HOMCs）with different contents were prepared by varying the amount of MAP.Combined with the analysis of the thermogravimetric characterization results,MAP affected the nitrogen-containing carbon precursor-micellar assembly process,resulting in the poor thermal stability of the micellar complex precursors.This in turn led to the poor micromorphological regularity of the obtained N-HOMCs,and the content of specific surface area,pore volume gradually decreased with the increase of nitrogen doping.Despite the significant reduction in specific surface area and pore volume,N-HOMCs-20 prepared with a small amount of nitrogen doping still exhibited good electrochemical performance.This is related to the fact that the nitrogen introduced by functionalized doping can combine with PF₆^-in the electrolyte to form[N⁺][PF₆^-]compounds,which in turn contributed to the enhancement of its electrochemical performance.