Preparation And Properties Of End Group Modified PEO-based Solid Polymer Electrolytes And Its Composite Polymer Electrolytes

The increasing energy demand has promoted the vigorous development of lithium-ion batteries（LIBs）.However,the application of traditional liquid LIBs is limited by safety issues.The basic way to solve the safety problems in lithium-ion batteries is to replace liquid electrolytes with solid electrolytes.In a variety of solid electrolytes,solid-state polymer electrolytes（SPEs）have unique inherent advantages,including lightweight,flexibility,and non-flammability.Polyethylene oxide（PEO）-based polymer electrolytes have been widely studied for their excellent ability to dissociate lithium salts,good thermal and chemical stability.As a semi-crystalline polymer,PEO has a high ionic conductivity above its melting temperature（65℃）,but has insufficient ionic conductivity and poor mechanical properties at room temperature,which hinder the application of PEO in all-solid-state lithium batteries.In order to solve the problems of the application of PEO-based polymer electrolytes at room temperature,the following researches had done in this paper:（1）The end groups of PEO were modified by introducing-CS-NH-and NH₂ functional groups at both ends of the PEO chain.And a new polymer named PTBAE was synthesized.The introduced functional groups can form hydrogen bonds,disrupt the regular arrangement of PEO,inhibit the crystallization of PEO,and improve its ionic conductivity at room temperature.The structure and physicochemical properties of PTBAE-SPE were characterized by NMR,FTIR,XRD,TGA,SEM,etc.The room temperature ionic conductivity of PTBAE-SPE was calculated by EIS test(1.99×10^-5 S cm^-1),which is two orders of magnitude higher than that of pure PEO-SPE.The electrolyte membrane was assembled into a Li||Li symmetrical battery,which could run stably for 800 h at a current densities of 0.05 m A cm^-2 and 0.1 m A cm^-2.And the overpotential is less than 0.4 V.These indicate that the interface between the polymer electrolyte and the metal lithium electrode is stable.And the polymer electrolyte can effectively inhibit the growth of lithium dendrites.We assembled LiFePO₄||Li all-solid-state battery for cycle performance and rate performance tests at room temperature.The discharge specific capacity could reach up to 153.0 mAhg^-1at a rate of 0.1 C.After 200 cycles,the capacity retention rate was 93%,and the average coulombic efficiency was 99%,showing excellent cycle stability at room temperature.（2）In order to further improve the ionic conductivity and mechanical strength of PTBAE-SPE at room temperature.The composite solid electrolyte（PL_x-CPE）was prepared by mixing active inorganic filler LAGP with PTBAE.The microscopic morphology and crystallinity of PL₂₀-CPE were analyzed by XRD and SEM tests.The room temperature ionic conductivity of PL₂₀-CPE was obtained by EIS testing at 3.18×10^-5 S cm^-1,and the electrochemical stability window was increased to 5.0 V.The Li|PL₂₀-CPE|Li symmetrical battery could cycle stably over 500 hours at the current density of 0.05 m A cm^-2 in the charge-discharge test showing the good interface stability between the PL₂₀-CPE and metal lithium electrode.The CPE was assembled into LiFePO₄||Li all-solid-state battery,which showed good cycle stability and rate performance.The initial discharge specific capacity could reach 135 mAhg^-1 at a rate of 0.2 C,the capacity retention rate after 180 cycles was87%,and the average coulombic efficiency was close to 99%;the discharge specific capacity was up to 67 mAhg^-1at a rate of 2 C.