Preparation And Property Of MOFs-PEO Composite Polymer Electrolyte

With the development of large-scale equipment with battery as energy source,higher requirements are put forward for the energy density and safety of lithium secondary battery.In recent years,solid-state battery with advantages of high safety,high energy density and long cycle life,constructed with solid-state electrolyte and metal Li anode,has attracted much research.As the vital component of solid-state batteries,solid-state electrolytes play a decisive role in the performance of batteries.Therefore,the design and preparation of solid-state electrolyte with high ionic conductivity,wide electrochemical window and good stability and compatibility against electrode have become an urgent problem.Solid polymer electrolytes?SPEs?are widely concerned by scientific research and industry owing to the good flexibility and processability,which can help to achieve varied geometries and industrial scale production.However,SPEs usually suffer from low ionic conductivity,poor mechanical property and poor ability to suppress lithium dendrite.In this paper,PEO-Li TFSI polymer electrolytes used as matrix were compounded with metal-organic framework?MOF?materials,MOF-based ionic conductors and the graded ionic conductors composed of MOF-based ionic conductors and Li₇La₃Zr₂O₁₂?LLZO?,respectively,preparing varied type of composite polymer electrolytes.Then the effect of fillers on ion transport and electrochemical performance was explored.And the main results are summarized as the following:?1?UiO-66 nano materials with the size of 80-150 nm were synthesized by two-step hydrothermal method and compounded with PEO-Li TFSI polymer electrolytes to prepare the Ui O-66/PEO composite polymer electrolytes.The results show that the composite polymer electrolyte with 10 wt%Ui O-66 exhibit higher ion conductivity(3.0×10^-5 S/cm at 25?,5.8×10^-4 S/cm at 60?),wide electrochemical window?4.9 V v.s.Li/Li⁺?,high transfer number of Li⁺?0.36?,good stability against lithium metal and excellent ability to inhibit the growth of Li dendrites.Therefore,the assembled Li|Ui O-66/PEO|Li battery can stably cycle for more than 1000 h under the current density of 0.15 m A cm^-2 at 60?.?2?Based on the nano-porous MOF material,Li-IL was encapsulated into the pores of MOF host by capillary adsorption to obtain MOF-based ionic conductor?MOF@Li-IL,LIM?.The LIM was compounded with PEO-Li TFSI polymer electrolyte to prepare the LIM/PEO composite electrolyte.In the composite polymer electrolyte,Li⁺can be transported by LIM,and thereby an increase of LIM fillers is accompanied by an increasing of ionic conductivity.The composite polymer electrolyte with 70 wt%LIM shows high ionic conductivity at room temperature(1.0×10^-4 S/cm).Due to the high solid content,the LIM/PEO electrolyte with 70 wt%LIM shows good ability to inhibit the growth of lithium dendrites,and the assembled symmetric Li cell can continuously operate for 1000 h at 0.15m A cm^-2 at 60?without short circuit.?3?Benefited from the nano properties and wettability,LIM was acted as an assistant filler and compound with LLZO to constructed a graded filler.The graded filler was added into PEO-Li TFSI polymer electrolyte to prepare the LLZO/LIM/PEO composite polymer electrolyte with high solid material content.Owing to the nano effect,LIM particles are concentrated around LLZO particles,and the Li-IL encapsulated in the pore of MOF can directly contact with LLZO particle,boosting fast interfacial Li⁺transport.And the solid-state nuclear magnetic resonance spectroscopy shows that the Li⁺is mainly transported along the LLZO-LIM conductive channel.The composite polymer electrolyte with 70 wt%LIM/LLZO possesses high ionic conductivity(5.8×10^-5 S/cm at 25?,5.2×10^-4 S/cm at60?),high mechanical strength?4.95 MPa?and high transference number of Li⁺?0.59?.Moreover,the composite polymer electrolyte shows good interface stability against metal Li electrode and excellent ability to inhibit the growth of lithium dendrites.And at 60?the assembled Li|LIM/LLZO/PEO|Li cell can achieve excellent cycling stability for 4800h at the current density of 0.15 m A cm^-2 and 550 h at 0.30 m A cm^-2.At a rate of 0.1 C(1 C corresponding to 170 m Ah g^-1),the Li Fe PO₄|Li cell can discharge 127.7 m Ah g^-1 after 50cycles with the capacity retention of 88.71%.