Structure Evolution And Ion Transport Properties Of Polyethylene Oxide/Flake Ceramic Composite System Under Electric Field

In recent years,wearable electronic devices need to be flexible and foldable.Lithium-ion battery（LIB）is the main component of electronic devices because of its high working voltage,strong energy density,fast charge and discharge rate and long cycle life.The liquid electrolyte in lib is easy to cause heating out of control and explosion due to the flammable solvent,causing safety problems.Polymer based solid electrolytes have potential applications in flexible electronic devices because of their good flexibility,good thermal stability and high safety.There are two kinds of polymer based solid electrolytes:polymer based and polymer/nano ceramic composite electrolytes.The ionic conductivity of pure polymer electrolyte is related to the aggregation structure of polymer.Lithium ions are mainly transported in the amorphous region of polymer.For polymer/ceramic electrolyte,lithium ion can be transported not only in the amorphous region of polymer,but also in the interior of ceramic and the interface region of polymer/ceramic.The effect of electric field will change the aggregation structure of polymer and the phase structure（texture structure）of ceramic phase in polymer.In this paper,polymer polyethylene oxide（PEO）and flake ceramic molybdenum selenide（MoSe₂）were selected as the basic components of solid electrolyte.The relationship between phase structure and ion transport performance was studied by adjusting the phase structure of the system through non-contact electric field.The main contents are as follows:System 1:PEO/lithium trifluoromethane sulfonimide（LiTFSI）was used as solid electrolyte to study the effect of AC electric field on the crystallization and electrochemical properties of PEO in PEO/lithium trifluoromethane sulfonimide（LiTFSI）.The crystallization of PEO was characterized by polarizing microscope（POM）,X-ray diffraction（XRD）and differential scanning calorimetry（DSC）.The electrochemical performance of PEO solid electrolyte was studied by electrochemical workstation.The AC impedance,lithium-ion migration number and electrochemical stability were measured.The results show that the particle size of PEO spherulites decreases after AC electric field.The results of DSC and XRD show that the crystallinity of PEO increases under electric field.When the field strength is 600V/mm,the grain size of PEO is 40.82μm and the crystallinity of the film is48.41%.With the increase of electric field,the conductivity of PEO/LiTFSI decreases,and the lithium-ion migration number and electrochemical stability will decrease.The conductivity of the film under the electric field of 600V/mm is 8.12×10^-7S cm^-1,migration number 0.18 and electrochemical window 5.37V.System 2:MoSe₂/PEO/LiTFSI system:MoSe₂is orderly assembled in PEO through non-contact electric field.The effect of electric field on the phase structure of MoSe₂/PEO and the relationship between the phase structure and ion transport performance are emphatically explored.The structural evolution of MoSe₂in electric field was observed online by microscope.The changes of shear viscosity and stress of the system under the action of electric field were measured by electrorheological instrument.Using the relationship between MoSe₂microstructure and viscosity stress,the relationship between MoSe₂ordered structure construction speed,ordered structure stability and electric field intensity was discussed.Based on the above research,MoSe₂and PEO/LiTFSI composite electrolyte membranes were prepared.The cross-sectional structure and surface morphology of the composite films were characterized by scanning electron microscope（SEM）and nano indentation instrument.The arrangement of MoSe₂in PEO was further characterized by light transmittance test.The changes of ion conductivity and lithium-ion migration number were measured by electrochemical workstation.The results show that under the action of non-contact electric field,when the load of MoSe₂is 3wt%,MoSe₂/LiTFSI/PEO system can form a stable ordered structure under the AC electric field with field strength of 600V/mm;The results of Er show that with the increase of voltage,the orderly construction speed is accelerated.Under the field strength of 600V/mm,MoSe₂completes the orderly arrangement along the electric field direction in 191s;UV-Vis spectrum analysis shows that the transmittance of the composite film along the thickness direction increases with the increase of field strength.SEM results show that the electric field changes the distribution of MoSe2 and forms a continuous ordered structure.Under the field strength of 600V/mm,the ionic conductivity of 3wt%MoSe₂/LiTFSI/PEO composite electrolyte increased from 5.67×10^-7S cm^-1increased to 1.33×10^-6S cm^-1,the number of lithium-ion migration increased from 0.186 to 0.221.the number of lithium-ion migration increased from 0.186 to 0.221.