The Electrochemical Performance And Storage Mechanism Of Porphyrin-based Cathode Materials In Lithium And Magnesium Batteries

New technological revolution has prompted the development of energy storage technologies,and lithium-ion batteries have attracted much attention as an emerging energy storage system.In recent decades,lithium-ion batteries were widely commercialized and applied in a variety of electronic devices.However,the resource shortage of the traditional materials resulted in the increase of cost.Meanwhile,inorganic metal materials are also hard to recycling and decomposition,which have a negative effect on the environment.Hence,people have paid their attention to organic electrode materials,due to its advantages of low cost,adjustable structure,and environmental friendliness.However,organic electrode materials also faced some issues.On the one hand,small molecular organic compounds have high solubility in organic electrolytes,which affects long-term cycling performance.On the other hand,the molecular mass of macromolecular organics limits their theoretical capacity.The metalloporphyrin compounds could provide redox active sites and suitable molecular weight,which have applied in photovoltaic,catalysis and supercapacitor.Therefore,we designed and synthesized three kinds of metalloporphyrin compounds in this paper.The electrochemical properties of the three materials in different organic metal batteries were studied by various electrochemical test methods.Charge storage mechanism of porphyrin materials was analyzed by different characterization methods.The main contents of this paper are as follows:1)The metalloporphyrin organic material[5,15-bis（ethynyl）-porphinato]copper（II）（Cu DEP）was designed and synthesized as cathode material.Ex-situ fourier transform infrared spectroscopy（FTIR）test indicated that Cu DEP undergo electrochemical polymerization during cycling,which can enhance cycling stability in batteries.Coupled with lithium counter electrode,Cu DEP delivered a high reversible capacity of 106 m Ah g^-1 at 1.0 A g^-1 after 5000 cycles.Meanwhile,Cu DEP also showed the good rate capability of 83 m Ah g^-1 at 5 A g^-1 due to the fast reaction kinetic.The ex-situ X-ray photoelectron spectroscopy（XPS）test exhibited that nitrogen atoms are the active sites and the reaction is reversible.2)The metalloporphyrin organic materials[5,15-bis（ethynyl）-10,20-diphenylporphinato]nickel（Ni DEPP）and[5,15-bis（ethynyl）-10,20-dithienylporphinato]nickel（Ni DETP）were prepared and used as cathode materials in organic magnesium batteries.Different electrolytes were compared aiming to improve the charge storage performance.It was determined that 0.4 M Mg[B（hfip）₄]₂ in DME was the suitable electrolyte,in which Ni DEPP and Ni DETP exhibited good cycling performance.This indicates that the importance of matching electrolytes for the organic magnesium batteries system.At a current density of 1 A g^-1,Ni DEPP delivered a specific discharge capacity of 83 m Ah g^-1 after 100 cycles.Ni DETP also showed stable cyclic performance for 300 cycles.Ex-situ X-ray diffraction（XRD）test exhibited that the crystal structure of Ni DEPP showed reversible change during cycling,while the crystal structure of Ni DETP remained stable.FTIR and XPS also demonstrated that the nitrogen atoms act as active sites for reversible electrochemical reactions during charged and discharged processes.However,the characteristic peaks of the thiophene group of Ni DETP kept in original states,explaining its stable electrochemical performance.