Construction And Interfacial Regulation Of Polyethylene Oxide Based Polymer Electrolyte

Lithium?Li?secondary batteries are considered one of the most promising energy storage devices because of their high energy density and excellent cycling performance,which can be widely used in electronic devices and electric vehicles field.However,the flammability and explosion of the liquid electrolyte used in commercial Li secondary batteries make the great potential safety hazard.Polyethylene oxide?PEO?based polymer electrolytes are safe,foldable and easy to manufacture,which are promising candidates for substituting liquid electrolytes to solve the security problem and improve energy density of batteries.But PEO based electrolyte has many problems such as low ionic conductivity,poor mechanical properties and unstable electrolyte/electrode interface,which greatly limit its application in batteries.Aiming at resolving the above problems,this paper designs a variety of PEO-based polymer electrolytes with high ionic conductivity at room temperature and good mechanical properties through electrolyte modification strategy,meanwhile stabilizing the interface between Li metal anode and polymer electrolyte through surface treatment.By means of the state-of-the-art characterization techniques including pulsed gradient field nuclear magnetic resonance?PFG NMR?,cryo-transmission electron microscopy?cryo-TEM?,scanning electron microscopy?SEM?and X-ray photoelectron spectroscopy?XPS?,the mechanism of interface regulation and material evolution is analyzed.The main research contents are concluded as follows:?1?The synthesized magnesium borate?Mg₂B₂O₅?nanowires were used to modify PEO-Li TFSI electrolyte followed by systematic investigation over the changes of ionic conductivity,mechanical properties and flame retardant properties of PEO-Li TFSI electrolyte,which aimed to achieve the excellent PEO based polymer electrolyte.The results showed that ionic conductivity of PEO-Li TFSI-Mg₂B₂O₅ magnitude higher than that of PEO-Li TFSI.Additionally,the main reason for the obviously enhanced ionic conductivity is attributed to the accelerating mobility of PEO chain segment and the increasing migration channel of Li⁺at the two-phase interface between PEO-Li TFSI and Mg₂B₂O₅.As for the PFG NMR test,it confirmed m² s^-1,consequently improving the ionic conductivity of electrolyte.Mechanical property tests and combustion tests further showed that the presence of Mg₂B₂O₅could improve the tensile resistance of the electrolyte as well as prevent the aggressively combustion of PEO electrolyte in extreme cases by forming isolating carbon layer on the surface.In the electrochemical evaluation of the configuration paring the Li Fe PO₄ cathode with Li metal anode,the PEO-Li TFSI-Mg₂B₂O₅ enabled full cell delivered superior capacity of 150,106 and 50 m Ah g^-1 at 50,40 and 30 ^oC,respectively.?2?The ionic liquid grafted oxides?IL@NPs:IL@Zr O₂,IL@Ti O₂,IL@Si O₂?were used to optimize PEO-Li TFSI electrolyte.And the effect of such IL@NPs on the ionic conductivity of polymer electrolyte at room temperature as well as the electrochemical performance of all-solid-state lithium-sulfur batteries were systematically investigated and revealed.The experimental results showed that polymer electrolyte modified by IL@Zr O₂ had the highest ionic conductivity of the large anion group of TFSI^-in IL@NPs binds to the PEO chain,weakening the interaction between PEO and Li⁺,and the cationic group can provide Li vacancy.Moreover,Zr O₂ can coordinate with O atoms in the PEO chain to reduce the conductivity of electrolyte is significantly improved.Electrochemical results showed that when using carbon/sulfur as the cathode,Li metal as anode,and PEO-Li TFSI-IL@Zr O₂ as the electrolyte,the cycling capacity of the battery at 50 and37 ^oC was as high as 986 and 600 m Ah g^-1,respectively.?3?The lithium sulfide?Li₂S?nano-additive was introduced into PEO-Li TFSI electrolyte,and comprehensive studies were devoted to revealing the effect of Li₂S on the structure,morphology and components in Li/PEO interface,as well as analyzing the microscopic mechanism of Li₂S for the stabilized Li/PEO interface.Cryo-TEM results showed that nanocrystals of Li,Li₂O,Li OH and Li₂CO₃ were randomly distributed on the Li/PEO interface,which were proved to be Mosaic structures composed of inorganic nanocrystals and organic components.However,the presence of Li₂S can promote the generation of Li F nanocrystals at the interface and increase the conduction and migration.Two-dimensional XPS and content analysis further confirmed the existence and increase of Li F on the interface.Molecular dynamics simulation and first-principles calculation showed that the Li₂S can accelerate the decomposition of TFSI^-?N?CF₃SO₂?₂^-?and promote the in-situ generation of Li F.The relevant XPS analysis further showed that the generation of Li F can inhibit the C-O bond breaking on the polymer chain and prevent the continuous interfacial reaction between Li and PEO.The results of electrochemical studies showed that the Li F at the interface can make the cycle life of Li-Li half cell exceed 1800 h,which was significantly improved compared with 300 h lifespan of pure PEO-Li TFSI.When the all-solid-state battery was assembled by pairing Li Ni_0.8Co_0.1Mn_0.1O₂ cathode and Li metal anode,the full cell capacity retention after 150 cycles reached 91.2%at 50 ^oC.As for the full cell with Li Fe PO₄ cathode,the capacity retention was abouts 93%after150 cycles,and the coulombic efficiency was close to 100%,significantly higher than that of the pristine PEO-Li TFSI electrolyte based battery.?4?The platinum?Pt?nanolayer was embedded between the PEO-Li TFSI electrolyte and Li metal anode by magnetron sputtering,and the influence of such a Pt embedded layer on the components and morphology of Li/PEO interface was systematically studied.The results show that Pt can react with Li metal to produce conductive Li-Pt alloy,which significantly improves the Li/PEO interface.TEM results confirmed that Pt uniformly covered the PEO-Li TFSI electrolyte surface,and the presence of Pt would affect the deposition of Li,making the plated Li metal denser.The cryo-TEM results showed that the inorganic nanocrystals Li OH,Li₂O and Li were evenly distributed on the interface,presenting a Mosaic structure.The electrochemical results showed that when the interface was modified by Li-Pt alloy,Li-Li half cell could stably cycle for more than 2000 h.In the test of all-solid-state battery composed of Li Fe PO₄ cathode and Li metal anode,the charge/discharge capacity of the battery with the Pt nanolayer was about 150 m Ah g^-1 and the capacity retention was more than 98%after 270 cycles.Besides,the rate performance of battery was better than the control group.