Preparation Of Imidazolium PIL-Based Polymer Electrolyte And Its Application In Lithium Battery

Lithium-ion batteries（LIBs）have increasingly received attention due to their high energy density,wide operating voltage,and long service life with the rapid growth of consumer electronics and electric vehicles.In particular,as the ionic transport partway,the electrolytes are of vital importance for improving LIBs performance such as cell capacity and working efficiency.However,these liquid electrolytes generally cause a variety of safety problems such as electrolyte leakage,even fire and explosion,which restrain the further practical application of LIBs.Replacing liquid electrolytes with solid polymer electrolytes（SPEs）appears to be an effective solution to address these safety concerns.However,SPEs normally exhibit lower ionic conductivity at room temperature because the migration of lithium ions often occurs in the amorphous region of polymer matrixes.In addition,in the growing practical application of flexible,stretchable energy storage devices,SPEs are often prone to cracking due to fatigue embrittlement during cyclic deformation.In the charge/discharge cycling of LIBs,the breakage of SPEs tends to lead to direct contact between the anode and cathode electrodes,resulting in catastrophic failures and serious safety problems.These problems present a great challenge for researchers to find appropriate solutions.Recently,the polymerization products of ionic liquids（ILs）monomers,poly（ionic liquid）s（PILs）,have drawn much attention in the field of electrochemical energy.This structural characteristic of anions and cations endows ILs with unique properties such as high ionic conductivity,wide electrochemical stability window and non-flammability.More importantly,they can reduce electrode/electrolyte interfacial resistance,facilitate ion transport by forming highly connected ion clusters or well-defined ions channels,and hence,remarkably increase ionic conductivities.Furthermore,PIL-based SPEs can be enabled with self-healing capability by introducing repairable functional units in PILs,hence preventing the failure of device performance.In this dissertation,we designed and synthesized novel imidazolium-based PILs and prepared PILs-based SPEs,The electrochemical properties such as room temperature ionic conductivity of SPEs and the cycling stability of assembled Li Fe PO₄/Li cells were systematically studied.The main research works are as follows:（1）For the common poly（ethylene oxide）（PEO）electrolytes with low ionic conductivity at room temperature,the poly-Radziszewsk reaction was used to introduce EO chain segments under simple conditions and synthesize the imidazolium PIL（PILEO（TFSI^-））after anion exchange.SPEs were prepared by co-blending PILEO（TFSI^-）and polyvinylidene fluoride-hexafluoropropylene（PVDF-HFP）.The results show that PILEO-60 electrolyte film have room temperature ionic conductivity of 1.78×10^-4 S cm^-1,electrochemical window of 4.9 V vs.Li/Li⁺,and Li⁺transference number of 0.31.Furthermore,the assembled Li Fe PO₄/PILEO-60/Li cell shows an excellent cycling durability（90.3%capacity retention after 100 cycles at 0.1 C）and desirable charge/discharge property(140 m Ah g^-1 discharge specific capacity at 0.1 C).More importantly,the industrially feasible synthesis process of PILEO makes it a potential candidate for the fabrication of high-performance LIBs at a large scale.（2）To improve the durability of PIL-based SPEs and the lifetime of LIBs,hydrogen-bonded self-healing functional units（UPy）were introduced into PILs by radical polymerization,and self-healing solid polymer electrolytes（SHSPEs）were prepared by adding lithium bis（triflouromethane sulfonimide）（Li TFSI）.The introduced UPy units form a reversible hydrogen bonding network in the copolymer,which confers ideal self-healing properties to SPEs（the cut can heal within 24 h at 25℃）.The results show that the prepared PILUPy-7 electrolyte film has a room temperature ionic conductivity of 8.41×10^-5 S cm^-1,as well as an excellent electrochemical window（5.1 V vs.Li/Li⁺）and Li⁺transference number(t_Li⁺=0.55).In addition,the good adhesion and protection conferred by the hydrogen bonding network in the SPEs results in a Li/PILUPy-7/Li symmetric cell with an excellent interfacial stability up to 800 h at a current density of 0.2 m A cm^-2.The assembled Li Fe PO₄/PILUPy-7/Li cell has an outstanding rate performance(130.7 m Ah g^-1discharge specific capacity at 0.2 C)and cycling stability（96.5%capacity retention after 100 cycles at 0.2 C）,as well as withstand multiple cut/repair processes（93.2%of original discharge specific capacity at 0.2 C after 5 cycles）.