Synthesis And Properties Of ZIF-67 Separator For Lithium-ion Battery

With the advantages of high energy and power density,no memory effect,as well as environmentally friendliness,lithium ion batteries?LIBs?have been widely used in hybrids,electric vehicles,and grid storages.Although LIBs have high energy density and long lifetime,significant improvements in energy density and safety characteristics are still desired.It is generally recognized that the safety of batteries is closely related to electrolyte,electrode,and especially the separator.Previous studies on LIBs have been mostly focused on the design and synthesis of novel electrodes and electrolyte materials.As an essential component of LIBs,separator plays a crucial role in preventing internal short-circuit and offering ion-conducting routes simultaneously.Due to excellent strength and chemical stability,separators based on polyolefins are very popular.However,large shrinkages of the separators occur at a high temperature because of the low glass transformation temperature and poor thermal stability of the polyolefins,and it might trigger internal short-circuit failure,which will cause fire or even explosion.Furthermore,the liquid electrolyte cannot completely fill the separator due to the hydrophobic nature of the polyolefins,and it will increase ionic resistance of LIBs.Therefore,high performance separators with superior electrolyte wettability and enhanced thermal stability are compelling,particularly in energy storage market.Metal organic frameworks?MOFs?are formed by metal ions and electron-donating organic ligands.Zeolitic imidazolate frameworks?ZIFs?,a promising class of MOFs,have attracted great attention due to their large surface areas,tunable porosities,excellent thermal and chemical stabilities.Based on the excellent performance of ZIF-67,the electrochemical properties of LIBs were studied.The main research contents are as follows:Firstly,ZIF-67 was prepared by hydrothermal method at room temperature.The composite separator of PP-ZIF-67 was obtained by coating the surface of PP separator with ZIF-67.The wettability,porosity and thermal stability of PP-ZIF-67 separator are better than PP separator,and the electrochemical stability of PP-ZIF-67 cell is better than PP cell.Secondly,in order to further explore the impact of ZIFs on battery performance,ZIFs with different pore sizes and crystal defects were synthesized by solvents control strategy.The surface of PP separator is fully covered by a thin layer of ZIF-67 through a direct coating method to form PP-ZIF-67 composite separator.Interestingly,the existence of crystal defects arising from missing of Co nodes has profound effects on the performance of the battery.What's more,the cell with ZIF-67-CH₃OH coated separator exhibits higher capacity retention value of 61.5%than that of the PP separator?0.0%?after 100 cycles at 1.0 C at a high temperature of 55 ^oC.The method of producing PP-ZIF-67-CH₃OH separator in this work will pave a new avenue for enhancing the high-safety LIBs.Finally,ZIF-67 was employed as separator via direct coating onto the surface of LiNi_0.5Co_0.2Mn_0.3O₂ electrode through a facile blade-coating technique.Comparing with the conventionally used PP separator,Electrode-supported ZIF-67 separator?short for ZIF-67 separator?features superior electrolyte uptake?154.00%?,higher ionic conductivity(0.00164 S cm^-1)and thermal stability.Notably,at a high temperature of55 ^oC,the cell with ZIF-67 separator exhibits nearly 2 times higher capacity retention than the PP separator?ZIF-67:61.20%vs PP:35.09%?.In view of its advantages,the ZIF-67 separator shows significant potential in the design of safe lithium ion batteries.