The Synthesis And Study Of Multiple Active-site Modified Schiff-base Covalent Organic Frameworks Materials In Lithium-ion Batteries

Lithium-ion batteries（LIBs）have a crucial role in our life.However,the use of LIBs might cause exist two issues:（1）graphite-based anode has shortcomings including low capacity and non-renewable resources;（2）liquid LIBs have combustibility,explosibility and other safety hazard.The resolved method for the first one is that constructs novel organic porous polymer electrode materials due to their merits with environmentally friendly and sustainable features and high capacity than graphite.Organic porous polymer electrode materials cannot dissolve liquid electrolyte and have abundant active-site and performance than organic small molecules.The resolved method for second two is constructing solid-state batteries with solid-state electrolyte.By comparison with most inorganic electrolytes,organic conductors show some advantages such as facile preparation,low density,renewability,mechanical toughness and easy film-formation.They are potential campaigner for solid-state battery.Therefore,design and preparation an organic polymer material that can act as electrode material and solid electrolyte is significative.Covalent organic frameworks（COFs）are popular,porous and crystalline materials formed with strong covalent bonds.COFs-based anode materials have limited active center.Consequently,dual-active-site modified COFs can enhance the capacity of LIBs.Moreover,the COF-based solid-state electrolytes have been reported.Nevertheless,the Li salts or organic solvents are injected into the skeleton for Li-ion transportation,impede the Li-ion movement and conductivity.As a result,designing and constructing solvent-free single Li-ion conductor have greatly potential applications.The directional ion channel of COFs can accelerate Li-ion transportation and diffusion,enhance conductivity and interfacial stability.Therefore,COF materials are ideal platform for the electrode materials or electrolytes.For anode material,the dual-active-site of C=O and C=N modified Tp-Ta-COF,C=O and N=N modified Tp-Azo-COF are successfully prepared as the anode materials of LIBs in the thesis.The mechanism between active-site and Li⁺has been closely studied by density function theory（DFT）calculations and in（ex）situ fourier transform infrared（FT-IR）,Raman,and X-ray photoelectron spectroscopy（XPS）characterization.For the solid-state electrolyte,various single-ion conductors（carboxylic acid lithium modified LiOOC-COF3 and sulfonic acid lithium LiO₃S-COF2）are designed and synthesized.Single-ion conductors enhance conductivity,transference number,improve interfacial stability and weaken polarization effects.The studied contents of this thesis are as follows:（1）Dual-active-site of C=O and C=N modified Tp-Ta-COF is synthesized via the Schiff-base as the LIBs anode materials.Meanwhile,one-active-site of C=N modified Tf-Ta-COF and C=O modified Tp-Tb-COF are controllably prepared in order to prove the capacity contribution of dual-active-site groups.Under current density of 200 m A g^-1,Tf-Ta-COF has 264 m Ah g^-1capacity after 500 cycles,Tp-Tb-COF has 217 m Ah g^-1capacity after 500 cycles.However,Tp-Ta-COF has higher capacity with 413 m Ah g^-1 after 800cycles,which suggests short activation time of Tp-Ta-COF electrode materials.Electrochemical tests demonstrate that dual-active-site group obviously enhance capacity and improve Li⁺dynamics.（2）Comparison with C=O and C=N,N=N can receive two Li-ions.Herein,a unique covalent organic framework（Tp-Azo-COF）with a dual active site of N=N and C=O is designed and successfully applied as the anode material for LIBs.The assembled LIBs with Tp-Azo-COF deliver a higher specific initial capacity(802.2 m Ah g^-1/571.8 m Ah g^-1),and exhibits capability of 306 m Ah g^-1 at a current density of 1000 m A g^-1 after3000 cycles.Tp-Azo-COF has excellently electrochemical performance than other electrode materials.Moreover,relevant in-situ techniques and DFT imply that N=N and C=O reversibly interact with Li⁺.（3）In consideration of the battery safety system and superiorities of COFs as solid-state electrolyte,herein,we fully study the carboxylic acid lithium regulated diverse skeleton structure COFs（denoted as LiOOC-COFn,n=1,2 and 3）as the solvent-free and single Li-ion conductor in order to study the effect between various frameworks and performances.Among them,LiOOC-COF3 has higher Li-ion conductivity with 1.36×10^-5 S cm^-1,Li-ion transference number with 0.91 and lower action energy of 0.17 e V.（4）In this part,sulfonic acid lithium COFs（LiO₃S-COF1 and LiO₃S-COF2）are prepared via Schiff-base reaction.At temperature with 30℃,LiO₃S-COF2 shows high conductivity of 5.47×10^-5 S cm^-1 than LiO₃S-COF1(4.65×10^-5 S cm^-1)and LiOOC-COF3(1.36×10^-5 S cm^-1).Assembled solid-state battery with LiO₃S-COF2 as solid-state electrolyte,metal Li as anode and organic small molecule quinone as cathode displays excellent performances.And the mechanism of Li-ion migration is studied by DFT calculation.