Preparation And Characterization Of Polymer/Metal Organic Framework Composite Quasi Solid Electrolyte By Photoinitiated In Situ Polymerization

With the growing demand for high-energy-density batteries,graphite anodes with lower theoretical specific capacity(372 m Ah g^-1)have been unable to meet the current needs in recent years.Compared with graphite anodes,lithium metal anodes with ultra-high theoretical specific capacity(3860 m Ah g^-1)are considered to be one of the most promising alternatives to anode materials for lithium-ion batteries.In addition,compared with liquid electrolytes,solid-state polymer electrolytes（SPEs）have gained more and more attention due to their outstanding advantages such as good mechanical ductility,low price,non-flammability,good stability with lithium metal anodes,and inhibition of lithium dendrite growth.However,the development of SPEs is limited due to the low ionic conductivity at room temperature and the inability to fundamentally solve the problem of lithium dendrite growth.Although a large number of reports have confirmed the preparation of composite polymer electrolytes（CPEs）by combining inorganic nanoparticles with organic polymers method can reduce the crystallinity of the polymer,thereby improving the ionic conductivity,but there is still the problem of poor interface contact between the electrolyte and the electrode material,and the growth of lithium dendrites is also a big challenge for SPEs.Therefore,in this thesis,we choose the photo-induced in situ polymerization approach to improve the contact between electrode materials and SPEs,while introducing porous and active-site metal-organic frameworks（MOFs）materials to stabilize the deposition/stripping of Li ions.In this thesis,the purpose of preparing CPEs with excellent performance is to study the application of CPEs in lithium metal batteries through different ways.The main research contents of this paper are as follows:（1）Although polyethylene oxide（PEO）has good solubility for lithium salts,the low mechanical strength of flexible PEO-based SPE results in limited inhibition of lithium dendrite growth.Rigid and porous MOF-808 is selected as inorganic Nanoparticles,a quasi-solid composite polymer electrolyte（Q-SPCE-MOF）was prepared by photo-initiated in-situ polymerization with ethoxylated trimethylolpropyl triacrylic acid（EMPTA）as the polymerized monomer.On the one hand,the introduction of MOF can reduce the crystallinity of PEO and improve the ionic conductivity;on the other hand,the purpose of inhibiting lithium dendrites can be achieved by regulating the deposition of lithium ions.The experimental results confirm that the Q-SPCE-MOF has a high room temperature ionic conductivity(1.25×10^-4 S cm^-1),a wide electrochemical stability window（5.04 V）,and a high lithium-ion transference number(₄)+=0.72),the Li/Li symmetric cell results show stable Li deposition/stripping up to 1500 h even at a current density of 1 m A cm^-2.And the assembled Li Fe PO4/Q-SPCE-5 wt.%MOF/Li battery can have 110.4 m Ah g^-1 even at 5 C,and the long-cycle performance at 0.5 C shows the discharge specific capacity 160 m Ah g^-1.,after 120 cycles,the capacity retention rate exceeds90%.（2）Based on the advantages of single-ion polymer electrolytes that can weaken the concentration polarization phenomenon of batteries.In this chapter,the precursor solution of PEGMEA-AMPSLi was prepared by rational design,and the spinning membrane of polymer/MOF was prepared by electrospinning technology.A single-ion quasi-solid composite polymer electrolyte containing MOF was prepared on the surface of the cathode material by photo-initiated in-situ polymerization.The experimental results confirm that the prepared single-ion polymer electrolyte has a high lithium-ion transference number(₄)+=0.8),a wide electrochemical stability window up to 5.68 V,the ionic conductivity is 1.14×10^-5 S cm^-1 and2.23×10^-5 S cm^-1 at 30°C and 60°C,respectively.Furthermore,the lithium symmetric cell with SIPCE-MOF shows a polarization voltage of 120 m V and stable cycling for over 2200 h at a current density of 2 m A cm^-2.As a result,the assembled solid-state Li Fe PO₄/SIPCE-MOF/Li battery achieves unparalleled rate capability(105 m Ah g^-1 at 2 C)with a capacity retention of98.3%after 120 cycles at 0.5 C and ultra-high coulombic efficiency（99%）.（3）For traditional polymer electrolytes,due to the opposite migration of lithium ions and anions of lithium salts of polymer electrolytes during the charge and discharge process,the anions hinder the migration of lithium ions,which leads to a low lithium-ion transference number.Here,we prepared MOF-SO₃Li grafted with lithium sulfonate by post-modification of MOF structure.The MOF-SO₃Li and PVDF binder were mixed uniformly by grinding and then scraped on the surface of cellulose porous film.Monomer PEGMEA was dropped on the MOF-SO₃Li film covering the cathode material,and the composite quasi-solid polymer electrolyte（Q-SPCE-MOF-Li）containing MOF-SO₃Li was prepared by photo-initiated in-situ polymerization.The experimental results show that the as-prepared Q-SPCE-MOF-Li exhibits an improved Li-ion transference number due to the as-prepared MOF-SO₃Li combining the advantages of MOF porosity,opening metal sites,and providing additional Li-ion conduction.At the same time,the ionic conductivity,electrochemical stability window,and cycling stability of the lithium symmetric battery are improved(over 1000 h at a current density of 1 m A cm^-2and a capacity of 1 m A h cm^-2).When assembled with Li Fe PO₄ into a battery,the battery performance at high rates was improved,and the capacity retention rate was still 88.5%after150 cycles at 1 C,which was higher than that of Q-SPE with pure cellulose films（60.5%）.