Preparation And Investigation Of Electrolyte For High Temperature And High Voltage Lithium-ion Battery

Rechargeable lithium ion batteries?LIBs?have been widely utilized in electrical vehicles?EV?,hybrid electrical vehicles?HEV?and consumer electronics owing to high specific energy,high rate capacity and long cycle life.However,current application of LIBs still exists issues such as higher energy density and stability under high voltage and temperature.Among these issues,the design of electrolytes with high operating voltage and temperature is quite challenging.Nowadays,commercial electrolyte composed of LiPF₆ and carbonate solvent cannot meet the requirements of practical application to lithium ion batteries as a result of bad oxidation stability and thermal decomposition at elevated temperature?for instance,55??,the decomposition product can corrode the structure of cathode material,leading to poorer cell performance.Considering these issues,we adopted ionic liquid,organic solvent with high boiling point and lithium difluoro?oxalato?borate?LiDFOB?with excellent film forming ability on surface of electrodes to prepare ionic liquid electrolytes,and investigated physical properties and electrochemical performance of electrolytes at room temperature and elevated temperatures.1?PP₁₄TFSI-sulfolane/LiDFOB based electrolytes with inherent thermal stability and low flammability are investigated as promising alternatives of conventional LiPF₆/carbonate electrolyte in high voltage Li/Li_1.15(Ni_0.36Mn_0.64)_0.85O₂?Li/Li-rich?cells at elevated temperatures.The safe and stable electrolyte candidates are selected by optimizing relative composition and represent high oxidative stability?higher than 5.2 V vs Li/Li⁺?above 55?.The cells with electrolyte containing 60wt%PP₁₄TFSI?E60?achieved 172.5 and 238.8 mAh g^-1 after 50 cycles between 2.0-4.6 V at 0.5 C at 55 and 70?.Moreover,the Li/Li-rich cell with E60 electrolyte obtains a superior discharge capacity of 97.9 mAh g^-1 at high rate of 3 C at 70?.At extremely high temperature of 85?,the cell with E60 electrolyte delivers 218.7mAh g^-1 after 50 cycles at 1 C.The excellent cycling performance in high voltage Li/Li-rich cells at elevated temperatures is attributed to the intrinsic oxidative stability and the compact and stable cathode-electrolyte interface?CEI?film derived from E60 electrolyte,which can effectively prevent the structure of Li-rich cathode material from being destroyed during cycling,leading to excellent cycling performance,even at elevated temperatures.2?Ionic liquid?IL?PP₁₄TFSI possess inherent high thermal stability,non-flammability and wide electrochemical window even at high temperature,but its conductivity is so poor that practical application is unsatisfactory.To address this issue,we added carbonate and fluorinated carbonate solvent to ionic liquid to prepare electrolytes,and the addition of solvent did not affect the flammability and electrochemical window.Linear scanning voltammetry?LSV?tests clarified that the oxidative stability of IL/fluorinated carbonate based electrolyte is much higher than conventional LiPF₆/org electrolyte at 55?and 70?.The cycling performance of IL/fluorinated carbonate based electrolyte is much better than conventional LiPF₆/org electrolyte,especially the cell cycled with IL/fluorinated carbonate based electrolyte display the prominent discharge capacity of 187.9 mAh g^-1 at 55?,and a maximum value of 220.1 mAh g^-1 at 70?with a high capacity retention of 81.9%after 100cycles.Moreover,the CEI film derived from IL/fluorinated carbonate based electrolyte is more compact than that of IL/carbonate based electrolyte,which protects cathode material structure at high voltage and temperature.