Density Functional Theory Study For Raman Spectra Of Components Of SEI Film For Lithium Ion Batteries And Experimental Study Of Positive Electrode Material

Lithium-ion battery,as one of the most advanced technologies of chemical battery, has several advantages,such as high energy density,long service life and no pollution. Lithium-ion battery has been applied to more and more fields.Electrochemical performances of lithium ion batteries,such as coulombic efficiency,cyclic performance, rate performance,callendar life and safety,are influenced strongly by the physical and chemical properties of solid electrolyte interphase(SEI) on anode or cathode. Investigation on SEI film is of great importance to lithium-ion battery.The SEI film has been extensively investigated by FT-IR,HRTEM,SEM,XPS and TPD-GC/MS and so on,which can offer ex-situ information.FT-IR could offer in-situ study for lithium surface and inactivite electrode(such as platinum,gold and nickel). FT-IR is restricted for many reasons.For example,because of reflection method, collected reflection signal could be decreased sharply when the reflecting surface is rough electrode(such as carbon,oxide and alloy).In addition,those methods mentioned above cannot differentiate all species in the SEI film and cannot offer absorption information.As a result,more work needs to be done.Surface Enhanced Raman Scattering(SERS) is a ideal test method to study the SEI film,which can provide the information of the component and absorption of the film. Raman spectroscopy,as a method of in-situ study,is sensitive and effective to detect different phases and to characterize the components of samples,so it has been widely applied in many fields in recent yeas.Our group has firstly obtained successful SERS of SEI film on silver electrode surface,and has also studied SEI film in-situ change under different discharge states.It is concluded that components of SEI film is different under different discharge states.Main components include Li₂CO₃,LiF and LiOH·H₂O,other peaks could be assigned to RLi,ROLi,ROCO₂Li. We have mainly studied three contents as follows:1.Since we use the method of Raman spectrum to study the SEI characteristic on the lithium ion battery materials,we need to understand some knowledge with regard to Raman spectroscopy.So we firstly do the Raman spectroscopy study of in-situ change in laser irradiated spinel LiMn₂O₄ in this thesis.2.CH₃OLi and CH₃CH₂OLi are repectively reduction products of popular solvent DMC and DEC of lithium-ion battery.It is known that CH₃OLi and CH₃CH₂OLi are the simplest molecules of possible components of SEI film.Molecular configurations of CH₃OLi and CH₃CH₂OLi were structured based on the previous study that Lithium atom and Oxygen atom are directly joined by O-Li bond in alkoxy lithium(ROLi).DFT method was employed to calculate vibration frequencies and Raman Spectra of these two molecules.Besides,the Raman vibration modes of CH₃OLi and CH₃CH₂OLi were assigned according to potential energy distribution of each vibration frequency,which will provide theoretical basis for experimental workers to analyze the components of solid electrolyte interface film(SEI film) of lithium ion battery.3.It is believed that many kinds of RLi,ROLi and RCO₃Li exist in the SEI film besides Li₂CO₃,LiOH·H₂O and LiF,while the lithium salts are difficult to be affirmed directly from the experiment for its complexity and instability.In this paper,eleven unstable compounds,possibly existing in SEI film under EC-DMC-DEC/LiPF₆,were summarized.Corresponding molecular configurations with the lowest energy were chose.Quantum chemistry method was used to calculate vibration frequencies and the Raman vibration modes were assigned.