Application Of Zirconium-based MOF-808 Membranes And Their Derivatives In Interlayers For Lithium-sulfur Batteries

Lithium-sulfur batteries have high theoretical specific capacity(1675 m Ah g^-1)and high energy density(2600 Wh kg^-1),and are considered as one of the most promising next-generation rechargeable batteries.However,there are still some problems to be solved,among which the"shuttle effect"of polysulfides in lithium-sulfur batteries is considered to be the main reason for the rapid decay of battery capacity.Current studies show that metal-organic framework（MOF）materials have become a hot topic in the research of material modification for lithium-sulfur batteries due to their inorganic-organic properties and highly ordered and adjustable crystal structure.In this paper,zirconium based MOF-808 and its derivatives were combined with carbon based conductive substrate to construct a lithium sulfur battery interlayer material.The physical characterization of the synthesized interlayer material was investigated,and the electrochemical performance of the assembled lithium sulfur battery was tested.The main work is divided into two parts as follows:（1）In this work,MOF-808/CNT membrane was constructed by hydrothermal growth of continuous dense MOF-808 membrane on the surface of carbon nanotube（CNT）film as the interlayer of lithium sulfur battery.MOF-808 membrane with microporous structure can inhibit the diffusion of polysulfides through size effect.A large number of Lewis acid Zr⁴⁺centers and oxygen atoms in organic ligands can be used as ion traps to anchor polysulfides and promote the dynamic transformation of polysulfides.The use of trifluoroacetic acid as a modulator along with functionalized ligands of MOF-808 provides a highly electronegative environment to achieve charge shielding of polysulfides and facilitate rapid transport of lithium ions.MOF-808/CNT membrane based on sieving-catch-charge shielding mechanism can work stably all the time and inhibit the"shuttle effect".Due to these properties,MOF-808/CNT membranes show excellent cycling performance,with a reversible capacity of1085.2 m Ah g^-1 after 100 cycles at 0.2 C current density.At 1 C,its initial discharge capacity is 898.1 m Ah g^-1 after 500 cycles,the capacity decay rate of the cell is as low as 0.032%.（2）In this work,MOF-808 film was in-situ grown on the surface of carbon cloth by hydrothermal method,and FC@Ce_1-xZr_xO_2-yF_δ@CC was prepared as the interlayer material of lithium sulfur battery by introducing cerium nitrate into the pore and heat treating.Porous carbon layer and carbon cloth provide electronic access to improve the poor electrical conductivity of metal oxides.Most importantly,Ce⁴⁺/Ce³⁺redox pairs,active ionic/semi-ionic fluorine and rich oxygen vacancy in the materials can not only improve the trapping capacity of polysulfide,but also catalyze the rapid transformation of polysulfide.Due to these characteristics,FC@Ce_1-xZr_xO_2-yF_δ@CC interlayer shows excellent rate and cycling performance,with capacity decay rate of only 0.026%for 500 cycles at 1 C.In addition,it can still provide 746.1 m Ah g^-1 high discharge specific capacity at 5 C.The FC@Ce_1-xZr_xO_2-yF_δ@CC interlayer based on adsorption-catalytic mechanism has made great contributions to the inhibition of"shuttle effect"and the acceleration of the redox kinetics of polysulfides.