Design And Research Of Metal-Organic-Frameworks-Based Anode Materials With High Specific Capacity For Lithium-Ion Batteries

The demand for rechargeable batteries is increasing as a result of rapid scientific and technological innovation.As the most important secondary battery,lithium-ion batteries are widely used in various fields,such as portable devices,electric vehicles,and smart grids.Although the commercialization of lithium-ion batteries has matured,lighter,lower cost lithium-ion batteries are required.Therefore,the study of high energy density electrode materials is of great importance for cost reduction and weight reduction.In this thesis,we propose to design a high capacity anode material based on metal-organic frameworks(MOFs)containing fused aromatic rings.In addition,the electron cloud densities of aromatic rings are regulated by the coordination between metal ions and ligands of fused aromatic rings,resulting in a series of high capacity anode materials.Research contents presented in this thesis can be summarized as follows:A coordination-induced electron redistribution mechanism is proposed by comparing the differences in electronic structure and lithium storage performances of three compounds derived from perylene ring,PTCDA/Li-PTCA/Zn-PTCA.PTCDA and Li-PTCA exhibit relatively low reversible lithium storage capacity due to the low electron cloud density and low affinity to Li⁺ions of the perylene rings.Whereas,in Zn-PTCA,the electron cloud density of perylene ring and Li⁺affinity is greatly increased due to Zn-O coordination,exhibiting a reversible specific capacity of the storage of 14 lithium ions.At the same time,the molecular structure of Zn-PTCA is stable and no chemical bonds broken after the insertion of a large amount of Li⁺ions.The improved reversible Li⁺insertion into perylene rings demonstrates a coordination-induced electron redistribution mechanism,which can improve the lithium storage capacity and cyclic stability of metal-organic frameworks by regulating the electron cloud density.Based on the coordination-induced electron redistribution mechanism,we further designed a novel MOFs material of Co-PTCA,a lithium-ion battery anode material with ultra-high specific capacity.As the electronegativity of the metal ions increases,the coordination between metal ions and carboxyl group increases the?-conjugated electron cloud density in fused aromatic rings more significantly.With a reversible specific capacity of up to 995 m A h g^-1,Co-PTCA can reversibly host 20 Li⁺ions.Co-PTCA has excellent cycling performance with stable cycling of 300 and 500 cycles at current densities of 200 and 1000 m A g^-1 and the capacity retentions maintain at 99.5%and 90.4%,respectively.The excellent electrochemical performances of Co-PTCA further confirm the feasibility of coordination-induced electron redistribution mechanisms in the design of metal-organic frameworks-based electrode materials.In this thesis,the coordination-induced electron redistribution mechanism is discovered by comparing the electrochemical performances between a series of MOFs of metal ions with different electronegativity.Finally,the effect of electronegativity of metallic elements on ligand electron clouds is revealed,and a MOF material with a super-high specific capacity is further designed.This molecular design approach can be further extended to Covalent Organic Frameworks(COFs),providing a certain experimental and theoretical basis for the design of new generation of lithium-ion battery electrode materials.