Design And Development Of Low Temperature Electrolyte For Lithium Battery

With the development of economy,more attention is paid to environmental protection and green energy.Lithium-ion batteries(LIBs)are one of the most popular chemical power sources for their high energy density,environmental friendliness and memory-free effect.However,the performance of LIBs is significantly reduced in low temperature environment,which severely limits its scale application.As a medium for ion transport in batteries,electrolytes not only affect the charging and discharging performance and safety of batteries,but also determine the migration resistance of lithium ions at low temperature.Therefore,an important direction to improve the low temperature performance of LIBs is to choose the appropriate electrolyte solvent system and additives to improve the electrolyte lithium ion migration number(t_Li⁽¹⁰⁾)and the conductivity at low temperature.In this paper,a low-temperature electrolyte system is prepared,which includes base solvents optimization and synthesis of inorganic-organic combined nanostructured ionic liquid additives,Al₂O₃tethered 1-methyl-3-propylpyrrolidine bis(trifluoromethanesulfonate)imide(Al₂O₃-PY-TFSI).The effects of nanostructured ionic liquid additives on the physical properties of electrolyte were discussed.The action mechanism of additives was studied from the angle of compatibility between electrolyte and electrode.The detailed research contents are given below:(1)The nanostructured ionic liquid is synthesized by a three-step method.The product is analyzed by thermogravimetry,FT-IR,and BET specific surface area.The graft rate of the nanostructured ionic liquid is 3.2/nm².(2)Two kinds of low temperature basic electrolytes are prepared to study the charging and discharging performance of batteries with different electrolytes at-40?.Due to the low freezing point and high conductivity of propylene carbonate organic solvent electrolyte(40%PC),it is selected as the low temperature basic electrolyte.Taking electrolyte with different Al₂O₃-PY-TFSI content as the research object,it is achieved that the conductivity of the 1%Al₂O₃-PY-TFSI electrolyte is 1.25m S/cm at-40°C.The lithium ion transference number(t_Li⁽¹⁰⁾)is 0.47 and the electrochemical window is 5.4V at room temperature.The analysis of the results shows that Al₂O₃-PY-TFSI with a mass fraction of 1%can reduce the activation energy of ion transport and improve the conductivity and low temperature performance of the electrolyte.(3)The electrolyte containing 1%Al₂O₃-PY-TFSI is used as the research object to test the electrolyte/electrode compatibility.At-20?,0.5C discharge capacity of NMC622||LTO full battery is 149m Ah/g and capacity retention rate in 185 cycles is93.1%.The 150 cycles capacity retention rate of 1Ah LCO||LTO pouch battery is86.91%at-20?.The electrolyte reduces the charge transfer resistance of the battery at low temperature,protects the cathode from corrosion by HF,inhibits the dissolution of transition metal ions,and improves the low-temperature cycling performance of the three-element positive electrode materials.In conclusion,we explored the application of nanostructured ionic liquid as electrolyte additive for low temperature LIBs in this work,and promising results were obtained.We believe that this work can not only expands the application area of ionic liquid,but also,provides new way of thinking for the development of low temperature LIBs.