Introduction MOF-based electrolytes exhibit excellent chemical and thermal stability, which can improve the service life and safety of the whole battery. The research of MOF electrolytes is expected to promote the progress and development of solid-state battery technology with high energy density and high safety.The MOF electrolytes has more grain boundaries, which tend to cause interfacial problems such as dendrite growth. In order to eliminate the grain boundaries of MOF, vitrification is a more effective strategy. However, the strong coordination bonds between the metal and organic ligands in MOF lead to high melting points and higher energy consumption required for the MOF glass preparation. In addition, the melting point and decomposition temperature of MOF are very close, and only several MOF can melt when heated, and most of MOF will decompose before melting, accompanied by carbonization of organic ligands, and oxidation of metal ions. Therefore, for most MOF crystals, it is challenging to transform them into glass state by the conventional melt-quenching method. To expand the variety of MOF glasses, researchers have increased the entropy of melting by introducing functional groups with electron-absorbing ability and modified MOFs by strategies such as ion exchange and surface engineering, thereby lowering the melting temperature of MOFs. However, there is no clear mechanistic guidance for the melting point regulation of MOF crystals, and the relationship between its coordination bonds and its melting process is still unclear.
Methods By introducing different functional groups with different electron-withdrawing/donating effects on organic ligands, the strength of coordination bonds between metal ions and organic ligands can be adjusted, through which the mechanism and influence law of MOF melting point regulation is investigated. The ligand and metal ions were dissolved in 15 mL of DMF, and then the above solutions were heated and held at 130 ℃ for 48 h. After cooling, the products were collected by centrifugation, washed out with DMF (dimethylformamide) for three times, and finally dried under vacuum at 120 ℃ for 12 h. The obtained TIF-4 crystals were placed into a tube furnace and further held at 380 ℃ for 5 min under argon gas atmosphere, then cooled to room temperature to obtain TIF-4 glass (agTIF-4). A series of TIF-4 crystals using different halogen groups with same structure were prepared via solvothermal method at 130 ℃ in DMF. The strength of the coordination bonds was investigated via FTIR and DSC, from which the influence of the coordination bonds on the melting behavior of MOF were investigated systematically.
Results and discussion With the enhancement of the electronegativity of functional groups or the increase of the content of strong electronegative functional groups, i.e. the enhancement of the electron-withdrawing effect of functional groups, it can weaken the coordination bond between the ligand and the metal centre and lower the melting point of the MOF; on the contrary, through the introduction of electron-donating groups (methyl groups), it can enhance the coordination bond through the electron-donating effect and increase the melting point of the MOF. A series of TIF-4 crystals using different halogen groups and same structure were prepared,and the Tm of TIF-4 (from 375 ℃ to 295 ℃) gradually decreased with the enhancement of the electron-withdrawing ability of the halogen functional groups. The strength of the coordination bond was regulated by the electron-withdrawing/donating and the low-temperature preparation of MOF glass was realized. Meanwhile, it provides more theoretical guidance for the preparation of MOF glass, effectively avoids the decomposition of MOF in the melting process, improves the quality of MOF glass, reduces the preparation cost of MOF glass, and provides the theoretical basis and technical support for the application of MOF glass in the field of electrolytes and other fields, and also has a certain role in promoting the development and application of MOF.
Conclusions The relationship between the electron-withdrawing/donating of functional groups on organic ligands and the strength of coordination bonds is clarified, and its influence on the melting point of MOF is revealed, which facilitate the preparation of MOF glass with low melting temperature and high thermal stability. This work shows that the electron-withdrawing/donating effect could be utilized to regulate the melting point of MOF, which provides an effective strategy for the preparation of more different types of MOF glasses. This work could promote the large-scale preparation of low-cost and high-quality MOF glasses and promote the practical application of MOF glasses in different fields.