Metal-organic Frameworks (MOFs) For Lithium-ion Batteries And Sodium-ion Batteries

Metal-organic frameworks,are a novel class of crystalline porous materials which are composed of coordinated metal ions（or clusters）and organic ligands.Compared with traditional porous materials,MOFs have received much attention due to its larger specific surface area and porousity.Based on its tunable porosity and structural diversity,MOFs have been widely used in many fields,such as chemical sensors,gas separation and storage and proton conductivity.Recently,its applications have been extended to lithium-ion batteries（LIBs）and sodium-ion batteries（SIBs）.However,the research focused on LIBs and SIBs is very limited and should be further explored.At present,MOFs have been applied to LIBs and SIBs fields in below two ways: directly as electrode materials and indirectly as templates to fabricate a series of anode materials,such as metal oxides and carbon materials.In the first case,most reported MOFs electrode materials suffer from low specific capacity,poor capacity retention and weak rate performance.In the second case,too much work focus on introducing MOFs as precursors to fabricate stuffy anode materials rather than cathode materials.In view of above research status,the exploration of novel MOFs electrode materials with improved electrochemical performances and use of MOFs to synthesize cathode materials are very valuable work to implement.In this thesis,we explore two novel MOFs（MOF-Ni and Ca₂BTEC）as anode materials for LIBs and SIBs,respectively,which show improved electrochemical performances.Moreover,we also utilize a MOF as precursor to synthesize novel Na₂Mn₃O₇ as cathode material for SIBs.The main research works are summarized as follow:（1）Pure phase MOF-Ni were synthesized by a hydrothermal method and applied as an anode material for LIBs for the first time.The results reveal MOF-Ni can be crystallized by mixing moderate nickel nitrate and terephthalic acid in a mixture solvent of absolute ethanol and deionized water under 180 ℃ for 18 h.The product is analyzed by various methods,such as XRD、SEM、FT-IR、TGA,and determined as MOF-Ni crystal（CCDC code: 638866）.Evaluated as an anode material for LIBs,the as-prepared MOF-Ni electrodes deliver a high capacity of 620 mA h g^-1 at 100 m A g^-1 after 100 cycles and also show excellent rate performance.（2）We successfully fabricate Ca₂ BTEC by a facile hydrolysis-cation exchange method and introduce it to SIBs as an anode material.Ca₂ BTEC can be obtained by hydrolysis of pyromellitic dianhydride（PMDA）and further cation-exchange with CaCl2.Because the crystal structure of Ca₂ BTEC has never been analyzed,we determine its phase by comparing the XRD patterns with its hydrate counterpart（Ca₂BTEC·6H₂O）and using other assistant analysis（TGA and FT-IR）.Evaluated as an anode material for SIBs,Ca₂ BTEC electrodes display excellent cyclic performance(140 m A h g^-1 after 300 cycles)and good rate performance.To explain the improved electrochemical performances of Ca₂ BTEC,we also compare it with those of Ca₂BTEC·6H₂O and Na₄ BTEC.The results show the improved electrochemical performances of Ca₂ BTEC could be attributed to its superior structural stability and low solubility（in electrolytes）.Moreover,a electrochemical redox mechanism of Ca₂ BTEC also be proposed.（3）Utilizing the adsorption characteristic of MOF,we synthesized a novel sodium manganese oxide Na₂Mn₃O₇ and introduced it to SIBs as a cathode material.The Na₂Mn₃O₇ phase was synthesized by a two-step method including solvothermal formation of precursor（MOF-73 with adsorbed Na+ and NO3-）and subsequent in-situ calcination process.Moreover,we further investigated the influence of calcination temperature on the micro-morphologies and electrochemical performances of Na₂Mn₃O₇ for the first time.The results reveal that Na₂Mn₃O₇ with micron-sized plates calcinated at 1100 ℃ delivers a high capacity of 130.2 mA h g^-1 after 50 cycles and shows good rate performance in the absence of any coating carbon.