Synthesis Of Ether Functionalized Pyrazolium Ionic Liquid Electrolytes And Their Applications In Lithium Secondary Batteries

Ionic liquids have some superior properties, including negligiblevolatility, nonflammability, high chemical and thermal stability, highionic conductivity and electrochemical stability, which make it possiblefor ionic liquids to replace traditional organic electrolytes as safeelectrolytes for high energy density lithium batteries. In this thesis, aseries of pyrazolium ionic liquids, made up of six different pyrazoliumcations and TFSI-, have been synthesized. Physical and electrochemicalproperties of these ILs, including density, thermal stability, melting point,conductivity, viscosity and electrochemical window, were investigated.All these ether functionalized pyrazolium ILs were used as newelectrolytes for lithium secondary batteries.Six ILs, made up of pyrazolium cations and TFSI-, were synthesizedand their physical and electrochemical properties were investigated. All these ILs were liquids at room temperature. Their melting points werebelow-60oC, which allow them to work in a wide temperature range.The thermal decomposition temperatures were higher than338oC andone of them did not decompose until363oC. They showed low-viscositycharacteristic with viscosities under100mPa s at room temperature. Theviscosity of OEPZTFSI was41.2mPa s at25oC, which was the lowestviscosity reported among ILs used in lithium batteries with lithiumelectrode (without additives). The conductivities of OEPZTFSI andOMPZTFSI were higher than3mS cm-1, which were3.31mS cm-1and3.26mS cm-1, respectively. The relationships between temperature andviscosity, conductivity were also investigated and they fit with VTFequation very well (25oC to80oC). The electrochemical windows weremeasured through LSV test at25oC.2o1MPZTFSI (4.6V) andOEMPZTFSI (4.3V) had the widest and narrowest electrochemicalwindows, respectively.The six ionic liquid electrolytes with0.4mol kg-1LiTFSI were usedfor electrochemical impedance test and charge-discharge test. In theimpedance test, it was found that the bulk impedances of lithiumsymmetrical batteries were relatively stability all through the test, but theinterface impedances fluctuated seriously in the first eight hours. Then,they became stability. Li/LiFePO4cells were made for charge-dischargetests. At first, the discharge capacities went up gradually, then the capacities became stability and stayed around a certain number finally.The rate properties of OMPZTFSI electrolyte and OEPZTFSI electrolytewere better than other four electrolytes. The discharge capacities at therate of1.0C and2.0C retained about82%and58%of the capacities atthe rate of0.1C.