Structure Ordered Quasi-solid Polymer Composite Electrolytes:Preparation And Ionic Conductivity Study

Quasi-solid electrolytes（QPEs）combining the safety of all-solid electrolytes and the high ionic conductivity of liquid electrolytes,have gained attention in recent years for lithium-ion batteries.Polymer/inorganic composite QPEs usually consist of polymers,plasticizers,electrolytes and fillers.The electrochemical properties of QPEs depend mainly on the structure,composition,inorganic particle size,morphology and arrangement of each component.For example,the ordered arrangement of inorganic particles significantly improves the transport efficiency of lithium ions.In this thesis,three polymer-inorganic nanocomposite QPEs with ordered structure were designed and prepared.The effects of filler morphology,type（active/inert）,and arrangement in the matrix on the electrochemical properties of the as-prepared QPEs were investigated.The details of the study are shown below:Laminated montmorillonite（Li-MMT）was selected as the inorganic filler and polyethylene glycol diacrylate（PEGDA）/succinonitrile（SCN）as the polymeric phase.The Li-MMT/PEGDA/SCN ordered composite quasi-solid electrolyte was prepared by the electric field driven orientation-solution uptake method.The alignment effect of Li-MMT particles at different field strengths,frequencies and time was observed online by optical microscopy,and the optimized condition for best chain formation under AC electric field was confirmed（field strength of 600 V/mm,frequency of 10 Hz and action time of 5 min）.The particles in the cross sections of the prepared composite films were connected to long chains and parallel to the electric field direction supported by scanning electron microscopy（SEM）and EDS elemental analysis.The electrochemical impedance spectroscopy（EIS）was tested by an electrochemical workstation,showing that the ionic conductivity of the quasi-solid electrolyte film with an addition of 5 wt%Li-MMT after electric field-induced alignment is 1.39 times higher(1.39×10^-5S·cm^-1)than that of the unaligned quasi-solid electrolyte.Molybdenum selenide flake（MoSe₂）was chosen as the inorganic filler and PEGDA/SCN/polyurethane acrylate（PUA）as the polymeric phase.Ordered composite quasi-solid electrolytes were prepared by the electric field induction-Solution uptake method.The results showed that the co-blending of PEGDA/SCN with PUA significantly improved the mechanical properties and swelling rate of QPEs,which enhanced the conductivity up to4.82×10^-5S·cm^-1.The alignment effect of Mo Se₂particles at different field strengths,frequencies and concentrations was observed online by optical microscopy,and the relationship between electric field strength and Mo Se2 chain structure was characterized by electric-DHR,and the optimal conditions for electric field action were determined to be 600V/mm and 10 Hz,and the optimal concentration of Mo Se₂particles was 10 wt%.The cross-section of the prepared composite films was also characterized by SEM and EDS elemental analysis,which confirmed that the particles in the cross-section of the prepared composite films were connected into long chains and parallel to the electric field direction.The optical properties of the composite films were further characterized,indicating that the orientation of the the particles in the matrix driven by electric field greatly enhanced the optical transmittance.The EIS was tested by an electrochemical workstation,When the Mo Se₂content in the polymer matrix was 10 wt%,the conductivity of QPEs before alignment was 3.45×10^-5S·cm^-1and the ionic conductivity after alignment was 6.58×10^-5S·cm^-1,which was a 2-fold increase.Li_0.33La_0.557TiO₃nanotubes（LLTONTs）with hollow structure were selected as inorganic fillers and PEGDA/SCN/PUA as the organic polymer phase.Preparation of PEGDA/SCN/PUA/LLTONTs composite quasi-solid electrolytes,and characterization of their electrochemical properties.It was found that the conductivity was as high as 5.92×10^-5S·cm^-1at room temperature with an addition of 5 wt%randomly dispersed LLTONTs.In addition,LLTONTs have a significant improvement on the electrochemical window stability of the composite quasi-solid electrolyte also at this content,enhancing from 3.8 V to 4.8 V.Therefore,LLTONTs/PEGDA/SCN/PUA composite films with ordered structure were prepared by UV curing under the action of AC electric field（300V/mm,10Hz）with 5 wt%LLTONTs oriented in PEGDA/SCN/PUA co-blended solution and formed chain-like structure.Characterization of the cross sections of the prepared composite films by SEM and EDS elemental analysis confirmed that the particles in the cross sections of the prepared composite films were connected into long chains and parallel to the electric field direction.The electrochemical performance was tested by an electrochemical workstation,showing that the ionic conductivity of the quasi-solid electrolyte film after AC field-driven alignment increased by about two times(1.33×10^-4S·cm^-1)as compared with that of the quasi-solid electrolyte without electric field orientation alignment,and the lithium ion mobility number increased from 0.55 to 0.63.The LLTONTs/PEGDA/SCN/PUA ordered quasi-solid electrolytes were assembled into Li Fe PO4/Li half cells for long cycle and multiplier performance tests.The cells showed a discharge specific capacity of 152.5 m A h/g at 0.1C current density,a capacity retention of 89%after 200 cycles,and a discharge specific capacity of 119.7 m A h/g at 1C current density,exhibiting excellent capacity,multiplicity and cycling performance.