Application Of Ionic Liquid PP₁₃TFSI As Electrolyte For Lithium-ion Battery And Conductivity Measurement Of Its Binary Mixtures

Li-ion battery is one of the most promising battery technologies for its high workingvoltage, high energy density and long life. However, the widely used electrolytes in Li-ionbattery are flammable organic solvent, which result in the poor safety of Li-ion battery. Ionicliquids （ILs） may be an ideal candidate of electrolytes for their outstanding physicochemicalproperties, such as low melting temperature, wide electrochemical stability window,negligible vapor pressure, nonflammable property, high electrical conductivity, and goodthermal stability, etc.. Thus, the application of ionic liquid as electrolyte for Li-ion batterywill not only improve the safety of battery but also reduce environmental pollution.Ionic liquid N-propyl-N-methylpiperidinium bis（trifluoromethyls ulfonyl）imide（PP13TFSI） was successfully synthesized and characterized by1H NMR. The mixtures ofIL（PP13TFSI） and organic solvent of propylene carbonate （PC） were prepared as electrolytesfor Li-ion battery. The electrolytes with weight ratio （IL/PC） of1:1and3:2were denoted aselectrolyte1and electrolyte2, respectively. The electrolyte2containing60%IL（PP13TFSI）can not be lit in the flammability test.Li/LiFePO₄, Li/Li₄Ti₅O₁₂and Li₄Ti₅O₁₂/LiFePO₄CR2016cells were used to evaluatethe electrochemical properties of the prepared electrolytes. All test cells showed internalresistance lower than10under the EIS test. Li-LiFePO₄cells with both electrolyte1andelectrolyte2showed specific capacity about145mAh/g under1C rate, which is comparablewith conventional LB-315electrolyte. For Li/electrolyte/LiFePO₄cell, electrolyte1exhibitedbetter rate property than electrolyte2because its lower viscosity. The electrolyte2wasfurther evaluated by the Li-Li₄Ti₅O₁₂half cell and Li₄Ti₅O₁₂-LiFePO₄whole cell. The thedischarge specific capacity of Li/electrolyte2/Li₄Ti₅O₁₂cell approached120mAh/g at1C,while the discharge specific capacity of Li₄Ti₅O₁₂/electrolyte2/LiFePO₄cell is99mAh/g at1C after100th cycle and the capacity retention is80%of the first discharge capacity.In addition, the conductivity of binary mixtures of ILs(PI₁₃TFSI) in ethanol or1-propanol was measured at different temperature in detail. For a given concentration of ionicliquid, the conductivity in ethanol is higher than that in1-propanol. With the concentration ofPI₁₃TFSI increasing, the conductivity of binary mixtures of ILs(PI₁₃TFSI) in ethanol or1-propanol initially increased and reached the maximum followed by decreased, which isconsistent with the fitting results of empirical Casteel-Amis （CA） equation. The conductivity of binary mixtures of ILs(PI₁₃TFSI) in ethanol or1-propanol increased with the increase ofthe temperature, which can be satisfactorily explained by the Vogel-Fulcher-Tamman （VFT）equation.