Spectroscopic Study On The Cathode Material And Solid Electrolyte Interface For Lithium Secondary Batteries

In the first cycle of lithium secondary batteries, solvent decomposition reaction on the surface of the electrode will lead to formation of a passivating layer surface film, which is commonly named the solid electrolyte interface(SEI). It is an ionic conductor but electron insulator. SEI plays a key role in the electrochemical processes occurring during battery operation. At one hand, SEI affects the Li intercalation and deintercalation process, thus, decreasing the cycling efficiency of the battery, but on the other, it prevents the electrode material from continuous reaction with electrolyte components, thus assuring shelf life. The cathode material surface-modifying and SEI film on both cathode and anode were studied in this paper.In order to gain active substrate of SERS, we deposited Ag grain on the surface of commercial LiCoO₂ particles. The suitable size of Ag island on LiCoO₂ was taken as the active substrate of SERS. After soaked in electrolyte(1M LiPF6 EC/DMC) for 24h , naked-LiCoO₂ and Ag-coated LiCoO₂ were taken Raman spectra. It can be approved from the spectra that there existed enhancement of Raman scattering on Ag-coated LiCoO₂. Meantime, the Raman peaks of SEI components in Raman spectra had been analyzed. The components of SEI film were assignmented as Li2CO3,LiOH·H2O,LiF,ROCO3Li,COC group,LiO2Li et al., which were all enhanced by SERS. Commercial cathode material LiCoO₂ for lithium ion batteries, naked and Al2O3-coated nano-LiCoO₂ particles are soaked in 1M EC/DMC for a period of time. The influences of solvent soakage on the structure and electrochemical performance of the material were characterized by inductively coupled plasma (ICP) technique, X-raydiffraction(XRD), Raman spectroscopy and electrochemical cycling. It was found that solvent soakage had significant impact on the structure and electrochemical properties of naked and Al2O3-coated nano-LiCoO₂ materials. Li+ can dissolve from nano-scale LiCoO₂, which led to the descent of effective capacity of LiCoO₂, and this proved the importance of electrolyte choice. Ag grain was also deposited on anode material HCS particles of lithium secondary batteries. Comparing the Raman spectra of HCS with those of HCS coated with Ag island soaked in electrolyte(1M LiPF6 EC/DMC), it can be confirmed that there existed enhancement of Raman scattering on Ag-coated HCS. Then, test cells were assembled with Ag-coated HCS as the working electrode, fresh lithium foil as the counter electrode and 1M LiPF6 EC/DMC as the electrolyte. The cells were discharged and charged to a series of different voltages for FTIR and Raman study.In conclusion, we expanded the practical scope of SERS by depositing silver islands on useful electrode powder materials, which was different from the common SERS active substrates such as the smooth surfaces of glass or silicon that were chemically deposited by silver. The study of the SEI film on electrolyte for lithium secondary batteries using SERS effect lay a foundation of the in-situ investigation of SERS film by Raman spectroscopy, which was very helpful for exploring the formation mechanism of SEI film in later reaserch. Our work at the same time demonstrates that SERS was an appropriate detect method in the study of interfaces in lithium secondary batteries.