Research On Organic Liquid Electrolyte For Low Temperature Lithium Ion Battery

Since the electrolyte composition is one of the key factors in influencing the low temperature performance of lithium ion battery,it is important to develop the design of electrolyte for lithium ion battery.Searching electrolyte low temperature co-solvent and investigating the effect of it on low temperature electrochemical performance of lithium ion batteries are the intentions of this research.Research includes:(1)searching electrolyte low temperature co-solvent for lithium ion battery by Quantitative Structure-Activity Relationship(QSAR)method,(2)stud ying the effect of the co-solvent on the low temperature electrochemical behaviors of lithium ion battery,(3)studying the composition effect of the co-solvent and the film-forming additive on the low temperature electrochemical behaviors of lithium ion battery.Studying the theory design of electrolyte functional composition by QSAR method,and found the internal and quantitative relation between the molecule structure and its properties.By quantitative mathematic model,we can find some new electrolyte compositions and forecast their performance,after that we can select the optimum one. Through reading the literatures and analysis the data,we find that some parameters have significant influences on the performance of electrolyte.These parameters are highest occupied molecular orbital(HOMO),low unoccupied molecular orbital(LUMO),dipole moment,molecular topological index,and so on.Based on the calculation results of these parameters,we can choose electrolyte functional composition of lithium ion battery.In this thesis,we choose a sort of low temperature co-solvent,PA.The effect of electrolyte containing PA on the low temperature electrochemical behaviors of lithium ion battery was studied.PA,with low viscosity,low melting point and high dielectric constant,could effectively improve the ionic conductivities of electrolytes at lower temperatures,and improve the low temperature performance of lithium ion battery. For example,the lithium ion batteries with the optimized electrolyte of 1M LiPF₆ EC:DEC:PA(1:1:4)showed the capacity retentions about 99.8%,100.3%,99.9%,99.2%, 96.9%and 89.0%of its charge capacity obtained at room temperature when discharged at room temperature,0℃,-10℃,-20℃,-30℃and -40℃,respectively.The lithium ion battery displayed excellent cycle ability at -20℃.During cycles,the discharge capacities of the battery showed a stable value at about 72mAh,corresponding to 90%of the nominal capacity.The results of EIS,SEM,FTIR and XPS for electrodes after formation indicate that lithiated electrode has greater effect on the low temperature performance than delithiated electrode.In other words,for a full charged lithium ion battery,the resistance of the SEI film covered the surface of MCMB electrode is greater than that of LiCoO₂ electrode,and for a full discharged lithium ion battery,the resistance of the SEI film covered the surface of LiCoO₂ electrode is greater than that of MCMB electrode.During the first charge process of lithium ion battery,PA participates in the formation of SEI film, and the SEI film has lower impedance.Low impedance reduce the polarization at low temperature,so,the operation temperature of the lithium ion battery containing the optimized electrolytes can down to -40℃.In order to further reduce the operation temperature of lithium ion battery,we studied the composition effect of the co-solvent and the film-forming additive on the low temperature electrochemical behaviors of lithium ion battery.The results indicate that the lithium ion battery with electrolyte containing co-solvent and additive has excellent low temperature electrochemical behaviors,and can operate at -60℃.The lithium ion batteries with this electrolyte showed the capacity retentions about 99.6%,98.0%,91.1%,81.3% and 58.0%of its charge capacity obtained at room temperature when discharged at-20℃, -30℃,-40℃,-50℃and -60℃,respectively.Through analysis the electrodes after formation by EIS,SEM,FTIR and XPS,a conclusion can be drew that the additive of higher reductive voltage first decompose during the first charge process of lithium ion battery,and PA has no participation in the formation of SEI film.The SEI film after cycled in electrolyte containing additive and co-solvent is more stable and lower resistive,which extend the operation temperature range of the lithium ion battery.