Low-Viscous Functionalized Imidazolium Ionic Liquids Electrolyte For Lithium-Ion Batteries

A new series of ether-or alkenyl-functionalized imdazolium ionic liquids based on bis（fluorosulfonyl）imide anion have been synthesized and their physicochemical properties（including melting point,thermal stability,viscosity,conductivity and electrochemical stability）were analyzed and compared with corresponding bis（trifluoromethanesulfonyl）imide-based ILs.These ionic liquids based on functionalized imidazolium cation and bis（fluorosulfonyl）imide anion were selected as neat ionic liquid electrolytes and investigated in MCMB/LiFePO₄ full cells.All of synthesized samples were liquids at ambient temperature with low melting point.The melting points of most ionic liquids were lower than-60℃,and their decomposition temperatures were all above 220℃,revealing good thermal stability.It was confirmed that introduction of ether or alkenyl groups and bis（fluorosulfonyl）imide anions jointly could reduce viscosity and enhance conductivity effectively.These bis（fluorosulfonyl）imide-based ionic liquids owned viscosities lower than30 mPa s and conductivities higher than 7 mS cm^-1.AEI-FSI had the lowest viscosity（17.4 mPa s）among all the reported FSI-based ILs and it had relatively higher conductivity(12.8 mS cm^-1)as well.The temperature dependence of viscosity and conductivity were fitted well by VTF model in the range of 25℃to 80℃.The electrochemical windows of most ionic liquids were wider than 3.7 V,indicating their promising application for electrochemical devices.These ether-or alkenyl-functionalized imidazolium ionic liquids based on bis（fluorosulfonyl）imide anion were proposed as new electrolytes for MCMB/LiFePO₄ full cells.The commercial electrodes were used to evaluate the performance in the study,which is a pioneering work.It was proved that ether functionalization could bring the prominent improvement of performance.After sufficient activation at 0.02C for 2 cycles,the MCMB/LiFePO₄ cell using ether-functionalized imidazolium ionic liquids electrolyte could present excellent cycle performance at 0.1C.The capacity retention after 100 cycles was very close to that of conventional organic carbonate electrolyte.The mechanism of initial capacity loss was also explored deeply in view of full cells.We inferred that,during the charge process,the reduction products on anode diffused to the cathode and then were oxidized,which might result in the serious loss of initial capacity.