Preparation Of MOFs-Based Solid Electrolyte And The Application Research In Sodium Ion Battery

With the surge in global electric vehicle production and the implementation of energy-saving and emission-reduction policies,there is a growing demand for sustainable energy storage technologies.The cost of lithium-ion batteries has gradually increased due to the lack of lithium resources,so in addition to lithium-based energy storage systems,sodium-ion batteries are favored by researchers.Electrolyte is an important part of the battery,not only has high energy density and high safety,but also realizes diversified processing,which makes solid electrolyte a key research object.In this paper,MOF-74（Mg）solid electrolytes were prepared by MOF-74（Mg）loaded sodium salts in metal-organic frameworks（MOFs）,and the two-ligand MOF-74（Mg）solid electrolyte and bimetallic MOF-74（Mg）solid electrolyte were prepared by compounding the organic ligand terephthalic acid（BDC）in the MOF-74（Mg）backbone and doping Cu²⁺at the metal active site for modification,respectively.Based on these three solid electrolytes,an all-solid-state sodium-ion battery was assembled,and its physicochemical properties and electrochemical properties were analyzed by corresponding characterization and testing methods.（1）MOF-74（Mg）was synthesized by solvothermal method,and NaClO₄was stirred in N,N-dimethylformamide solution for 12 h,so that NaClO₄fully entered the MOF-74（Mg）pores to prepare a solid electrolyte membrane Na/MOF-74（Mg）loaded with sodium salt.FT-IR and XPS showed that NaClO₄successfully entered the pores,and ClO₄^-anions were anchored by the Mg²⁺metal active center to provide a transport site for Na⁺ions.The electrochemical test results showed that the Na⁺ion mobility number of the electrolyte membrane Na/MOF-74（Mg）was 0.58,the ion conductivity reached 3.48×10^-4S cm^-2,and the wide electrochemical window range was 1-4.3 V.It showed excellent performance with an initial discharge specific capacity of 104 mAh/g at 0.2 C.（2）Organic ligands terephthalic acid（BDC）with molar ratios of 0.9:0.1,0.8:0.2 and0.7:0.3 were synthesized with the original ligand 2,5-dihydroxyterephthalic acid in the MOF-74（Mg）backbone,and the double ligand MOF-74（Mg）(BMOF-X（B=BDC,X composite amount=10%,20%,30%）,were synthesized.The N₂adsorption and desorption test and XPS showed that the pore size,specific surface area and pore volume of BMOF-X were significantly increased compared with MOF-74（Mg）,which effectively increased the load of NaClO₄.When the BDC composite amount was 20%,the electrochemical performance was better,the ionic conductivity of the electrolyte membrane Na/BMOF-2increased to 1.71×10^-3S cm^-2at room temperature,and the Na⁺ion migration number increased to 0.88.At 0.2 C,the initial charging specific capacity reaches 112.7 mAh/g,which is 95.8%of the theoretical capacity,and the electrochemical performance is improved.（3）Doped Cu²⁺at the MOF-74（Mg）metal active center site,the molar ratio with the original Mg²⁺metal active center was 0.1:0.9,0.2:0.8 and 0.3:0.7,respectively,and the bimetallic active center MOF-74（Mg）(CMOF-X(C=Cu²⁺,X doping amount=10%,20%,30%)),it can be seen from EDS that Cu elements are evenly distributed in the sample and occupy a certain mass ratio,indicating Cu²⁺The successful incorporation of.Cu（ClO₄）₂was observed in the XPS Cl 2p and Cu 2p spectra of the electrolyte membrane,indicating that Cu²⁺also plays a role in understanding the off-pore NaClO₄.When the Cu²⁺doping amount was 20%,the electrochemical performance was better,the ionic conductivity of the electrolyte membrane Na/CMOF-2 at room temperature was as high as 3.18×10^-3S cm^-2,and the electrolyte membrane Na/CMOF-1,Na/CMOF-2 and Na/CMOF-3 all obtained high Na⁺ion migration numbers,0.77、0.86 and 0.71,respectively.At 0.2 C,the number of cycles increased to 110 cycles,and the capacity retention rate increased significantly to75.88%.