Design And Preparation Of Ultra-wide Temperature Ionic Liquids Based Electrolyte

Lithium ion batteries?LIBs?have been widely employed in various areas,however,due to the limitations of extreme conditions,LIBs have not been well applied in aerospace,polar detection,military fields or other extreme conditions.Electrolyte is the indispensable component of a battery as well as one of the decisive factors in battery performance at low and high temperature.On the one hand,HF and other acids yielded by the decomposed of LiPF₆ at high temperature,corrode the aluminum foil and attack the SEI film,leading to rapid deterioration of cycling performance and even thermal runaway.On the other hand,the viscosity of organic solvents in electrolyte increases or even solidifies at low temperature,resulting in a decrease in ionic conductivity and a slow migration of Li⁺,which may cause anion loss and lithium dendrites on the surface of the negative electrode,thereby causing capacity fading and safe issues.Therefore,it is the key for electrolyte to improve the performance of batteries.Solvate ionic liquids?SILs?are formed by the chelate of special solvates and metal salts,which have same characteristics as ionic liquids,such as consisting of anion and cation,non-volatile and so on.As a typical species,[Li?glyme?]⁺X^-,composed of lithium salt?LiX?and glyme,not only improves the electrochemical stability of glyme,resulting in a wide electrochemical window,but also holds the path for Li-ion to transportation without extra lithium salt added.However,the high viscosity renders a low conductivity at subzero environment,which impedes the applications of SILs.In this study,SILs can be diluted by introducing a solvent with low dielectric constant,aiming to hold the structure of[Li?glyme?]⁺not destroyed.At the same time,the viscosity of the electrolyte is decreased and Li migration is accelerated,resulting in the temperature range widen.The main research contents are as follows:?1?SILs are prepared and their structure are characterized by Raman analysis.The freezing points of the mixtures of[Li?G3?][TFSI]and[Li?G4?][TFSI]in different proportions are investigated,and the lowest freezing point?-71°C?is acquired when the equimolar mixing.At the same time,cycle tests at 60°C show good electrochemical stability.?2?The(1,1,2,2?tetrafluoro?3??1,1,2,2?tetrafluoroethoxy??propane?D2?is introduced to dilute the SILs,while Raman characterizes the structural change of the mixtures.The introduction of D2 significantly reduces the viscosity of the electrolyte,improves conductivity,and plays a key role in cycling performance at low temperature.At the same time,different lithium salt additives are introduced to optimize the wide temperature performance of electrolyte,and the cell with 1 wt.%LiFSI exhibits excellent performances.?3?A higher Li⁺migration number of 0.62 and a smaller polarization at low temperature is obtained result from the dilution of ethyl methyl carbonate?EMC?compared with carbonate electrolyte.The LTO||Li half-cell with diluted solvate ionic liquid?DSIL?electrolyte has a superior capacity retain above 90 mAh g^-1 at-40°C,which is almost 52%of specific capacity at 20°C.In addition,the DSIL electrolyte has good compatibility with the lithium electrode and can protect the LTO electrode.We have studied the electrochemical performance and protection mechanism to lithium anode of DSIL electrolyte at range of-40⁶0°C.Compared to carbonate electrolyte,DSIL can promote effectively the processes of Li⁺immigration and deposition,moreover,better compatibility with LTO electrode.Therefore,the batteries with DSIL show outstanding electrochemical performances.