The Application Of Several Organic Materials In Electrochemical Energy Storage Devices

Lithium-ion batteries(LIBs)have been extensively studied and widely used as electrochemical energy storage equipment in aerospace,communication,electric vehicles,and various portable electronic devices.Different from traditional inorganic electrodes,organic electrodes are featured with flexible,foldable,designable structure,low cost,large molecular volume,and super weight energy density,promoting the development of electrochemical energy storage systems.However,organic electrodes generally have the following problems in practical applications:inherent low conductivity and solubility in the liquid electrolyte,making the rate performance and long-term cycle performance far from the practical requirement.On the other hand,owing to the limited storage of Li resources on the earth(about 0.0065 wt.%)and the increasing consumption,it is urgent to develop alternative energy storage systems,such as potassium-ion batteries(PIBs)or potassium-ion-based dual-ion batteries(PDIBs).Thus,reducing the high solubility of organic electrode materials,overcoming the influence of low conductivity,and designing advanced organic electrode materials for LIBs,lithium-ion-based dual-ion batteries(LDIBs),PIBs,or PDIBs to achieve excellent electrochemical performance have great scientific significance and application value.This dissertation aimed at solving these typical problems of organic materials via reducing the solubility by screening and optimizing molecular species and molecular structure,polymerization,synthesizing metal-organic framework(MOF),conjugation,increasing the electrode conductivity by combining with high-specific-surface-area graphene(GR)and high-aspect-ratio carbon nanotubes(CNTs).The relationship between electrochemical energy storage characteristics and material parameters such as molecular species,molecular structure,crystal structure,and morphology of several organic electrode materials in LIBs,LDIBs,PIBs,and PDIBs have been fully studied to revealing the key to reducing the solubility and improving the rate performance and long-cycle stability.The highlights are summed up as follows:(1)To solve the solubility of organic quinone-based small-molecule electrode in liquid electrolyte,a benzoquinone-based polymer,poly(terephthalate-alt-benzoquinone),namely PTPBQ,with low solubility,was synthesized through a simple esterification reaction and employed in LIBs to study its lithium storage characteristics.Electrochemical performance,DFT theoretical calculation,SEM characterization,and ex-situ FT-IR test indicated that PTPBQ achieved an excellent long-term cycle performance but a low actual specific capacity(only 110 mAh g^-1 for 400 cycles at 40mA g^-1,the theoretical specific capacity of PTPBQ is 390 mAh g^-1)due to its low?conjugation and low conductivity,in which the active groups was partially reversibly oxidized or reduced.Moreover,just with a simple modification via increasing the content of the SP conductive additive from 30 wt.%to 40 wt.%,PTPBQ gained a significantly improved actual specific capacity of 180 mAh g^-1 at 80 mA g^-1.(2)In order to cancel the synthesis and purification processes,commercial polymers can be directly employed as electrode material for electrochemical energy storage devices.Here,the low soluble and redox-active PVK is selected and employed for novel PDIBs.The electrochemical performance test,ex-situ XPS,and ex-situ FT-IR characterization confirmed its redox reaction mechanism.The PVK cathode gains a higher voltage of 4.05 V vs.K⁺/K,dual-ion transport characteristics based on the N/N⁺couple in the carbazole group.Meanwhile,excellent rate performance and long-cycle performance were gained in PVK cathode,showing high average specific capacities of61 mAh g^-1 at 1000 mA g^-1 within 500 cycles and 117 mAh g^-1 at 20 mA g^-1,respectively.(3)Take advantage of the low solubility of transition MOF material,the ligand of 1,1'-Dicarboxyferrocene(DFc),with the electron-withdrawing carboxylate substituent on cyclopentadienyl rings in ferrocene and its unique redox reaction(Fe²⁺metal ions in ferrocene can be oxidized to Fe³⁺with a redox potential of 3.55 V vs.Li⁺/Li),was coordinated with different valence metal ions(Fe³⁺/Co²⁺)as novel electrodes.Based on the difference in valence metal ion(Fe³⁺/Co²⁺)as well as redox mechanism,Fe-MOF([Fe(DFc)1.5·3H₂O]m)and Co-MOF([Co^?2Co^?(DFc)2(OH)3·H₂O]n)were employed in LIBs and LDIBs,respectively.Both MOFs gained superior long-term cycle stability,good rate performance,and high energy density.(4)To effectively cure the solubility problem of the"universal ligand",pyridine-2,6-dicarboxylic acid(H₂PDA),and directly investigate the potassium storage characteristics of the organic H₂PDA ligands avoiding the interference of redox-active metal ions(Fe,Co,Ni,Cu,and other metal ions),a non-redox-metal-involved K-MOF([C7H3KNO4]n)coordinated by H₂PDA ligand and K⁺metal ion was designed and initially exploited as the promising anodes for organic PIBs.Single crystal X-ray diffraction and FT-IR were applied to confirm its molecular structure.Ex-situ FT-IR test and CASTEP theoretical calculation were innovatively used to reveal its potassium storage characteristics.With the aid of the N-K/O-K coordination bonds in[C7H3KNO4]n and the redox reaction of C=O/C-O couple,K⁺metal cations can be reversibly intercalated/deintercalated in[C7H3KNO4]n.(5)Based on the redox characteristics of K₂TP anode(two-electron redox behavior,the corresponding reduction potential is 0.5 V vs.K⁺/K)for PIBs.By extending the aromatic skeleton structure of K₂TP,two types of potassium dicarboxylates(namely potassium 1,1?-biphenyl-4,4?-dicarboxylate(K₂BPDC)and potassium 4,4?-e-stilbenedicarboxylate(K₂SBDC),respectively)were developed as the anodes for novel PIB to study the relationship between their reduction potentials and molecular configurations.Various measurements,such as electrochemical performance test,DFT calculation,and ex-situ FT-IR/XRD characterization demonstrated that K₂BPDC and K₂SBDC both gained a clear and highly reversible two-electron redox mechanism.Besides,different redox potentials were achieved because of the different molecular configurations(the reduction potential of K₂SBDC is 0.2 V higher than that of K₂BPDC).It means that the molecular structure can regulate the redox potential(the reduction potentials are related to the LUMO energies:the higher LUMO energy causing a lower reduction potential.The LUMO energies of K₂BPDC/K₂SBDC are0.51/-0.09 e V,respectively).Moreover,to overcome the inherent disadvantage of low conductivity of organic materials,K₂BPDC and K₂SBDC were in-situ combined with high-specific-surface-area GR to achieve excellent electrochemical performance.After hybridized with GR,at 1000 mA g^-1,the K₂BPDC obtained a high average specific capacity of 75 mAh g^-1 within 3000 cycles,compared to the capacity of 52 mAh g^-1 at500 mA g^-1 for pristine K₂BPDC.(6)Inspired by the fact that the dicarboxylates(K₂TP,K₂BPDC,and K₂SBDC)with different conjugated structures could show diverse redox potentials,the naphthalene-based dicarboxylate(K₂NDC)with a higher LUMO energy of 1.17 e V was synthesized as the PIB anodes to gain a lower reduction potential and a higher energy density.K₂NDC as the anode showed a lower reduction potential(?0.22 V vs.K⁺/K)compared with K₂TP,K₂BPDC,and K₂SBDC,which further proved that the high LUMO energy level of potassium dicarboxylate derivatives could achieve a low reduction potential,and the reduction potential can be determined by the molecular structure.Meanwhile,the low conductivity problem was addressed via K₂NDC in-situ compositing with the high-aspect-ratio CNTs,showing excellent electrochemical performance.