Raman Spectroscopy Study Of Solid Electrolyte Interface On Ag Electrode And Cathode Material LiCoO₂ For Lithium Ion Batteries

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. 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 ideal test method to study the SEI film,which can provide the informations of the component and variety of the film.At present the mainly research of the SEI film has been concentrated on the anode material. Reserachers study the SEI film of cathode materials and found that the SEI film exist on the cathode materials surface through TEM, XPS, FTIR, Raman in recent years. Our group has been concentrated on the cathode material (LiMn2O4) for Lithium Ion Batteries, and the SERS spectrum of LiMn2O4 with nanometer gold has been investigated. On this basis, I studied the composition and evolution rule of the SEI film on the LiCoO2. The advantages of LiCoO2 are high energy density, excellent cycle performance. LiCoO2 is the most common commercial material because of its advantages of high discharge voltage, stable discharge, long cycle life, good thermal stability, and little irreversible capacity in the first cycle. Study on the composition and evolution process of SEI film has an importance meaning for stability and cycling life of the lithium ion cathode material.We have mainly studied the contents as follows:1. Using the method of Raman spectrum study the SEI characteristic on the lithium ion battery materials, I should firstly understand the knowledge of Raman. So I study codeine phosphate by Raman spectrum and SERS spectrum, I get that analyzing the components of SEI film through theoretical calculated results is feasible.2. Noble metals and carbon material at lower potentials have similar surface film (SEI film), so our groups have observed the SEI film of silver electrode by ex situ Raman spectrum at discharging process. On this basis, high quality in suit Raman spectrum of the SEI film was perfectly observed by SERS, and the results indicated that except Li2CO3,LiOH·H2O reported before, the species 1,2,4,15 exist in the SEI film.3. In order to enhance specific capacity, and improve the cycle performance and high-rate charge-discharge ability, nm- LiCoO2 material has been prepared by molten salt synthesis method. We study this material by XRD, Raman, SEM et al. and found that the nm- LiCoO2 is hexagonal structure and it suits for lithium ion cathode material4. The ex-situ Raman spectroscopy of the SEI film on LiCoO2 varied with the voltages distinctly in the first cycle was investigated. The results of the theoretical calculation and the test indicated LiCOOCH2CH2OLi (species15) is the stable composition. The SEI signals were the richest and strongest on the condition of 4.1V, which indicated that the components of the film are the most complex at 4.1V.