Comparative Study of the Solid Electrolyte Interphase on Graphite in Li-ion Battery Cells Using XPS, TOF-SIMS, and Electron Microscopy
Graphite is the most commonly used anode material for commercial lithium ion batteries and has been very well studied. The excellent properties of graphite are due primarily to the robust solid electrolyte interphase (SEI). Consequently, numerous studies have been conducted to study these properties utilizing various techniques. Many of these techniques such as EDS, XRD, and TGA, are unable to resolve subtle changes in these thin SEI films due to their large information depths. [1,2] Due to these problems analyses using high spatial resolution have been applied using XPS and TOF-SIMS to observe the chemical and concentration gradients of these complex SEI films on the nanometer scale and as a function of their cycle life. This research analyzed commercially available electrodes composed of a graphite anode, nickel manganese oxide cathode, and PC:EC:DEC 1:1:3 with LiPF6 electrolyte at three stages of cycling. Cell 1 experienced only formation process to form a protective solid electrolyte interface film on the surface of graphite electrode. Cell 2 stored 14 days at 10 % of state of charge (SOC) at 25°C and then 700 cycles with 50-100% of SOC at 45°C and Cell 3 had 495 cycles with 0-50% of SOC at 45°C and had additional 1317 cycles with 50-100% of SOC at 45°C. Once cycled, these cells were opened in an argon glovebox and the anode material was prepared without cleaning to avoid removal of the SEI or dissolution of ionic species. Samples were mounted using carbon tape and transferred to the analysis chamber with < 1 minute of exposure to air. XPS depth profiling was performed using a Thermo K-Alpha (Al Kα peak) with an 1000keV Ar+ ion beam. TOF-SIMS depth profiling was performed using an Ion- ToF-SIMS5-300 configured with a 25keV Bi+ primary liquid metal ion gun. The sputter rates for TOF-SIMS and XPS were calibrated vs. a 100 nm layer of SiO2 grown on a Si wafer. SEI thickness was measured by FIB/SEM as well as the C+ profile and graphitic carbon in TOF-SIMS and XPS respectively and was found to be ~150 nm thick and grow by 50nm after cycling. The ionic concentration of Mn+ was observed to be inhomogeneously distributed within the SEI which may be used to estimate the Mn+ dissolution and migration behavior in anode materials. Acknowledgement: This work was partially supported by the American Honda Motor Co., Inc. References  E. Peled, D. Golodnitsky et al, “Composition, depth profiles and lateral distribution of materials in the SEI built on HOPG-TOF SIMS and XPS studies,” Journal of Power Sources, vol. 97-98, pp. 52-57, Jul. 2001.  H. Ota, Y. Sakata, A. Inoue, and S. Yamaguchi, “Analysis of Vinylene Carbonate Derived SEI Layers on Graphite Anode,” J. Electrochem. Soc., vol. 151, no. 10, p. A1659-A1669, Oct. 2004.  J. Benson, N. Nitta, J. T. Lee, A. Magasinski, I. Kovalenko, T. Fuller, and G. Yushin, “Comparative Study of the SEI on Graphite in Full Li-ion Battery Cells Using XPS, SIMS, and Electron Microscopy,” submitted 2012.