Study Of Cathode Material And Solid Electrolyte Interface Film On Electrode For Lithium Ion Batteries

In the first cycle of lithium ion batteries,solvent decomposition reaction on the surface of the electrode will lead to formation of a passivating layer film,which is commonly named the solid electrolyte interface(SEI) film.It is an ionic conductor but electronic insulator.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 the SEI film on anode or cathode.Investigation on the SEI film is of great importance.However,accurate analysis on the chemical composition of the SEI film is experimentally challengable due to the coexistence of inorganic,organic and polymer species.The SEI film has been extensively investigated by FT-IR,HRTEM,SEM,XPS and AFM,which are the methods of surface analysis with rather high sensitivity.However,those methods cannot differentiate all species in the SEI film.But surface enhanced Raman scattering(SERS) is a new test method to study the SEI film,which can provide the information of the component and variety of the SEI film.The cathode materials for lithium ion batteries have an important influence on each performance of lithium ion batteries.Therefore,people have paid a lot of attention to study and exploit cathode materials.The advantages of LiCoO₂ are high energy density, excellent cycle performance,high discharge voltage,stable discharge,long cycle life, good thermal stability and little irreversible capacity and so on in the first cycle.LiCoO₂ is also the most common commercial material.Study on the cathode material LiCoO₂ and the components in the SEI film of LiCoO₂ has an importance meaning for stability and cycling life of the lithium ion batteries.Recently,transitional metal compounds have attracted much attention as high capacity anode materials for Li-ion batteries.Among many candidates,Cr₂O₃ is quite attractive due to its low emf value,low cost,high density and nontoxic.However,being different from graphitic carbon anode materials,there is a thick SEI film on Cr₂O₃ and decomposed during the de-lithiated process,which has significant influence on its electrochemical performances.Therefore,accurate analysis on the components in the SEI film of Cr₂O₃ is essential to improve the electrochemical performance of the electrode.We have mainly studied the contents as follows:1.The physical characteristic and structural change of LiCoO₂ were investigated by scanning electron microscope(SEM),X-ray diffraction(XRD) and Normal Raman spectroscopy.2.Firstly,the Normal Raman spectroscopy was used to analyze electrolyte and the solvent of electrolyte,and so on.In order to gain active substrate of SERS,we deposited Ag grain on the surface of commercial Cr₂O₃ powder.We selected different lithiated and delithiated states to study the SEI film of Cr₂O₃ anode.EIS and CV suggected that SEI film was formed in the first cycle of lithium ion batteries.The ex-situ and in-situ Surface Enhanced Raman spectra of SEI film were studied after Ag deposited.The Raman peaks at low voltages were richer than those at high voltages,which accorded with the FT-IR results.It also demonstrated the dynamic transformation of SEI film in the first cycle.6 components were detected in ex-situ SERS study in the first cycle with 11 components in in-situ SERS study without considering Li₂CO₃ and LiOH·H₂O reported before.The differences derived from the dissolution of some certain components into solvent in the sample preparation process.Our work at the same time demonstrated that SERS was an appropriate detection method in the study of interfaces in lithium secondary batteries,and in-situ study was an intuitionistic and effective method for its sincerity and accuracy.3.In order to gain active substrate of SERS,we deposited Au grain on the surface of commercial LiCoO₂ powder.Put the cathode electrode and anode electrode into the the lithium ion simulate battery.We selected different lithiated and delithiated states to study the SEI film of LiCoO₂ cathode.EIS and CV suggected that SEI film was formed in the first cycle of lithium ion batteries.The charge-discharge apparatus accurately control the state of battery.We can obtain the information of the evolvement of the SEI film with the co-focus micro-Raman spectroscopy.The Raman peaks at high voltages were richer than those at low voltages.It also demonstrated the dynamic transformation of SEI film in the first cycle.The structural changes of LiCoO₂ after charge-discharge were investigated by X-ray diffraction(XRD).