Preparation And Properties Of Ionic Liquid Polymer Composite Solid Electrolyte

Lithium ion batteries have gradually become the most important energy storage device since the first commercialization.Although organic electrolytes have high ion conductivity,the inherent flammability may result in the safety hazards of the battery.Because of solid electrolytes have high electrochemical stability,good thermal stability and excellent mechanical strength,solid batteries with solid electrolytes instead of organic liquid electrolytes can fundamentally solve the safety problem.Among them,composite solid electrolytes have been widely studied due to the combination of good interface contact and high ionic conductivity of inorganic electrolytes.However,the low ion migration number and electrochemical stability window limit the large-scale application.Based on the above reasons,we introduced ionic liquid polymers as electrolyte matrix for research.Solid electrolyte was constructed using poly（vinylidene fluoride-co-hexafluoropropylene）（PVDF-HFP）,meanwhile,vinyl ionic liquid（VBIm NTF2）and initiator azodiisobutyronitrile（AIBN）were polymerized inside the electrolyte.The ionic conductivity of polymer electrolyte IP21（IL:PVDF-HFP=2:1）is 2.76×10^-4S cm^-1at room temperature.In order to further improve the performance of the electrolyte,inorganic electrolyte powder Li_6.4La₃Zr_1.4Ta_0.6O₁₂（LLZTO）was added,resulting in a composite electrolyte with high ion conductivity(9.26×10^-4S cm^-1),wider electrochemical window（5.2 V）,higher lithium ion transfer number（0.57）and excellent mechanical strength.To verify the interface stability of the composite electrolyte,the Li/IPL21/Li symmetric battery was assembled.It was stably cycled for 800 hours at 25℃and 0.1 m A cm^-2.The specific discharge capacity of the Li/IPL21/LFP solid-state battery is 162.1 m A h g^-1at 0.1 C,and the capacity retention rate is 94.1%after 100 cycles at 0.5C.At the same time,we also assembled a soft pack battery,which can still light LED lights after folding and shearing.It indicated that the prepared IPL21 electrolyte has good safety and reliability.On this basis,a polymer electrolyte was constructed through in-situ polymerization,which significantly improved the interface contact between the electrode and the electrolyte.The experimental results showed that the room temperature ion conductivity reached 4×10^-4S cm^-1.In addition,it also has high lithium ion transfer number（0.54）and wide electrochemical window（5.3V）.In order to investigate the effect of in-situ polymerization on the interface,a lithium symmetric battery was assembled.It exhibited lower polarization voltage and better stability over 1000 hours at 25℃and 0.1 m A cm^-2.We also assembled Li/PIL/LFP and Li/PIL/NCM811 batteries,with capacity retention rates of 92.1%and 93.1%at room temperature and the current density of 0.5 C,respectively.It proved that polymerized ionic liquids can have good cycling performance in high energy density batteries.