Study On Solid Polymer Electrolyte Containing Imidazole Group

Solid lithium ion electrolyte is a critical component for the development of all solid state lithium ion battery.In this thesis,PVO-based and PVO-Me-based solid polymer electrolytes（SPE）and PVO-LLZTO-based composite solid polymer electrolyte are successfully prepared using the monomers of 1-vinylimidazole（1-Vm）and Poly（ethylene glycol）methyl ether methacrylate（OE）.The effects of polymer structure,types and contents of lithium salts,and LLZTO content on the electrochemical performance of the electrolytes are studied in detail.The main results are as follows,（1）Random copolymer PVO is successfully synthesized via conventional free radical polymerization.The lithium salts of LiTFSI and LiClO₄ are used to compare the effects.Their concentrations are regulated from 5 wt%to 20 wt%.The highest lithium ion conductivity of 1.30×10^-6 S·cm^-1 at 25℃ is obtained when the content of LiTFSI is 15 wt%.Generally,when LiClO₄ is used,the ionic conductivity of SPE is lower than when LiTFSI is used.But interestingly,the lithium ion transference number of the SPE using LiClO₄ is higher than LiTFSI.Both types of lithium salts-doped SPEs show similar and relatively high electrochemical stability up to 4.7 V versus Li⁺/Li,and good cycling performances in lithium deposition/stripping experiments.However,the charge/discharge results show that the specific capacity of the LiFePO₄/SPE/Li full battery is low(about 38 mAh g^-1 at 25℃,about 90 mAh g^-1 at 60℃),which may be due to the low ionic conductivity of SPE and poor contact with the electrode.（2）PVO is successfully quaternized on the imidazole groups via a room temperature reaction using methyl iodide,and the resulting polymer is named PVO-Me.A series of xLiTFSI-PVO-Me SPE electrolyte membranes are prepared by solution casting,and doped with 5-20 wt%LiTFSI.At room temperature,the ionic conductivity of xLiTFSI-PVO-Me first increases with the increase of LiTFSI content,and reaches the maximum value of 8.96×10^-7 S·cm^-1 at 15 wt%LiTFSI.The t_Li⁺（0.71）of 15LiTFSI-PVO-Me at room temperature is much higher than the t_Li⁺（0.20）of 15LiTFSI-PVO.At the same time and the same lithium salt content,the electrochemical stability of 15LiTFSI-PVO-Me（4.5 V vs.Li/Li⁺）is slightly lower than 15LiTFSI-PVO（4.7 V vs.Li/Li⁺）.（3）A series of 15PVO-yLLZTO composite SPE membranes are prepared by solution casting using 15LiTFSI-PVO（containing 15 wt%LiTFSI）doped with 10～40 wt%LLZTO.At room temperature,the ionic conductivity of 15PVO-yLLZTO does not change significantly with the increase of LLZTO contents,but fluctuats around 10^-7 S·cm^-1.The possible reason can be due to the poor interface contact.After adding 30 wt%propylene carbonate（PC）,the ionic conductivity increases significantly to 2.07×10^-4 S·cm^-1,which improves the contact between LLZTO and PVO polymer matrix.The t_Li⁺（0.44）of 15PVO-40LLZTO is higher than the t_Li⁺（0.20）of 15LiTFSI-PVO without LLZTO.At the same time,the addition of nano-LLZTO is also beneficial to the electrochemical stability of SPE（4.9 V vs.Li/Li⁺）.