Study On The Effect Of Pore Effect Of MOF On The Performance Of Battery Separators

Lithium metal electrodes are considered as one of the ideal anode materials for the next generation due to their very high theoretical capacity(3860 m Ah g^-1),extremely low density(0.53 g cm^-3)and lowest electrochemical potential（-3.04 V vs.standard hydrogen electrode）.However,lithium metal anodes in lithium metal batteries（LMBs）inevitably undergo swelling and pulverization,lithium dendrite growth,and dead lithium formation during cycling,thus limiting their further commercial application.As one of the four key materials for batteries,separators not only serve as reservoirs for electrolyte,but also prevent contact between positive and negative electrodes in the battery,and provide internal ion transport channels to regulate ion transport.Polyolefin separators are one of the most commonly used commercial battery separators due to their excellent chemical stability and mechanical strength,but their hydrophobic properties prevent them from being completely infiltrated by the electrolyte,resulting in low ionic conductivity and lithium ion transfer number,and polyolefin separators are prone to shrinkage under high temperature conditions,which can cause the risk of fire or explosion when the positive and negative electrodes come into contact.Therefore,there is a need to find functional separators with good thermal stability,high ionic conductivity,high lithium ion transfer number and controlled ion transport to replace polyolefin separators.Metal organic frameworks（MOFs）materials have unique structure,large porosity,high specific surface area and high chemical and thermal stability,which make them more advantageous in the field of energy storage.In addition,the pore structure of MOFs can be precisely designed and adjusted,which provides an ideal platform for the functionalized and precise regulation of ion transport in separators.In this thesis,Zr-based MOFs with excellent structural properties and stability are selected as the object of study,and the organic ligand length and chemical structure are changed to precisely regulate the pore environment of Zr-MOFs to obtain MOFs with different pore effects,which are used as coating materials for conventional commercial polypropylene separators to precisely regulate the ion transport in battery systems by controlling the performance of battery separators,mainly as follows:First,based on the size effect of MOFs,Zr-MOFs with different pore sizes（Ui O-66 and Ui O-67）were prepared with organic ligands of different lengths,and both were coated onto polypropylene（PP）separators to obtain separators with different pore sizes（PU6 and PU7）.MOF separators exhibit excellent electrolyte wettability due to the presence of abundant nano-pores,with larger pores of Ui O-67 exposes more-OH,-COOH groups and organic molecules in the cage,and the rich hydrogen bonding network formed between them can interact with the anions in the electrolyte,which in turn promotes lithium ion transport,and the PU7 diaphragm exhibits excellent cell performance in lithium metal batteries with lithium iron phosphate and ternary 811 as the cathode.Secondly,the introduction of functional groups in the MOF structure can not only modulate the properties of the MOF surface,but also create a specific pore environment to achieve a special pore effect.Ui O-66 and Ui O-66-NH₂ were synthesized and used as coating materials for PP separators,and tests revealed that the-NH₂ functional groups grafted in the pores can interact with the anions in the electrolyte through hydrogen bonding,and-NH₂ occupies the site resistance in the pores and has a blocking effect on the anions,which promotes the migration of lithium ions and achieves excellent electrolyte wettability,ionic conductivity,lithium ion transfer number,electrochemical stability window,and long-term ultra-stable constant-current cycling,and also exhibits excellent electrochemical performance in the full cell.Finally,based on the coordination effect of MOF,unsaturated pyridine nitrogen was introduced into MOF by N-heterocyclic ligands to change the chemical environment of the pore channel without occupying the pore size and changing the specific surface area.Ui O-67 and Ui O-67-BPY nanoparticles with the same pore size were coated on the surface of PP separator to prepare MOF diaphragm,and the test results showed that the functional separator（PUB）with unsaturated pyridine nitrogen could interact with the anion in the electrolyte to promote the migration of lithium ions and achieve high electrolyte wettability,electrolyte retention,ionic conductivity,lithium ion transfer number and ultra-stable long-term cycling,and promote lithium uniform deposition and inhibit lithium dendrite growth significantly.The functional separator application in the full battery also shows excellent electrochemical performance.