Surface Reaction of LiCoO 2 /Li System under High-Voltage Conditions by X-ray Spectroscopy and Two-Dimensional Correlation Spectroscopy (2D-COS) YEONJU PARK, NAM HOON KIM, JONG MYONG KIM, YOUNG CHUL KIM, YEON UK JEONG, SUNG MAN LEE, HYUN CHUL CHOI,* and YOUNG MEE JUNG* Department of Chemistry, and Institute for Molecular Science and Fusion Technology, Kangwon National University, Chunchon 200-701, Korea (Y.P., N.H.K., Y.M.J.); Agency for Defense Development, Daejeon 305-152, Korea (J.M.K., Y.C.K.); School of Materials Science and Engineering, Kyungpook National University, Daegu 702-701, Korea (Y.U.J.); Department of Advanced Materials Science & Engineering, Kangwon National University, Chunchon 200-701, Korea (S.M.L.); and Department of Chemistry and Institute of Basic Science, Chonnam National University, Gwangju 500-757, Korea (H.C.C.) We studied the surface reactions of a LiCoO 2 /Li cell under high-voltage conditions using X-ray photoelectron spectroscopy (XPS), X-ray absorp- tion spectroscopy (XAS), and two-dimensional correlation spectroscopy (2D-COS). 2D XPS correlation spectra show that Li 2 CO 3 is formed first by decomposition of the organic solvents, and then polycarbonate, which is formed by polymerization of the electrolytes, is produced on the cathode surface of the LiCoO 2 /Li system under high-voltage conditions. XAS measurements also confirm that the solid electrolyte interface (SEI) layer is formed on the LiCoO 2 electrode by decomposition of the organic solvents. The thickness of the SEI layer is less than 100 A ˚ . Index Headings: Solid electrolyte interface; SEI; LiCoO 2 /Li; High voltage; X-ray photoelectron spectroscopy; XPS; Two-dimensional correlation spectroscopy; 2D-COS; Lithium-ion battery. INTRODUCTION Lithium (Li)-ion batteries are currently considered the most competitive power sources for portable electronic instruments and electric vehicles because of their high energy density and excellent cycling stability compared to other currently used battery systems. 1–3 In order to enhance the capacity of Li-ion batteries, the charge cut-off voltage has to be increased to 4.5 V from 4.2 V. However, there are safety concerns regarding high- voltage conditions because a number of exothermic reactions with the electrolyte components take place on the charged cathode surface, such as electrolyte oxidation and decompo- sition of the cathode material. 4–7 These reactions can produce harmful byproducts such as insulating films, gases, and heat, which will cause self-discharge, capacity fading, and safety problems within the Li-ion batteries. Therefore, an understand- ing of the surface reactions between charged cathode materials and organic electrolytes is very important in order to improve battery performance. The surface reactions of Li-ion batteries have been studied using various surface analysis tools, such as Fourier transform infrared (FT-IR) spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS). However, these reac- tions are very complicated and are still not well understood. 8–14 Generalized two-dimensional correlation spectroscopy (2D- COS) has attracted a high level of interest in the analytical science community as it provides considerable utility and benefit in many fields of spectroscopic study. 15–17 2D correlation spectra are calculated from the dynamic spectra obtained during the measurement of the spectra under an external perturbation. 2D correlation spectroscopy has various advantages such as the enhancement of the spectral resolution, establishment of unambiguous assignments, and determination of the sequence of emergence of the spectral peaks. With these advantages, 2D correlation spectroscopy has been successfully used to study the characterization of electrode materials. 18–20 We have demonstrated the surface reaction of LiCoO 2 /Li cells using XPS survey spectra and 2D Raman correlation spectra. 18 In this study, we present XPS and XAS measurements for a LiCoO 2 /Li cell under high-voltage conditions. The results obtained show that the thin layer is formed on the charged LiCoO 2 cathode by various decomposition reactions of the organic electrolyte. The detailed compositions and formation sequences of the thin layer were investigated by 2D XPS correlation spectroscopy. EXPERIMENTAL The electrochemical behavior of LiCoO 2 was investigated by using coin cells (2016 type). Slurries were prepared consisting of 95 wt. % LiCoO 2 (Nippon Chemical Co.) powder, 3 wt. % acetylene black, and 2 wt. % polyvinylidene fluoride (PVdF) dissolved in 1-methyl-2-pyrrolidinone. Cath- odes were made by coating the slurry onto an aluminum foil substrate. Test cells were fabricated using these cathodes, metallic Li anodes, and polypropylene separators (Celgard 2400) in a glove box filled with Ar gas. The electrolyte used in this study was a 1.0 M solution of LiPF 6 in ethylene carbonate- diethyl carbonate (EC : DEC ¼ 1 : 1 by volume) purchased from Merck. The test cells were aged for 4 h at 40 8C in a vacuum oven. The cell performance was evaluated by galvanostatically discharging and charging the cell at a constant current density of 0.2 C-rate at room temperature with a WBCS 3000 battery tester system (Won A Tech Corp., Korea). To produce delithiated Li 1-x CoO 2 electrodes, in this study, we controlled the cut-off voltage at a constant current density. The delithiation content (x) in Li 1-x CoO 2 is calculated from the amount of electricity that has passed through the electrode. The values of x in the cut-off range of 4.2 to 4.5 V are 0.50 at 4.2 V, 0.53 at 4.3 V, and 0.55 at 4.5 V. X-ray diffraction analysis was performed using an XPERT- PRO high-resolution X-ray diffractometer (HRXRD, PANa- Received 20 July 2010; accepted 13 December 2010. * Authors to whom correspondence should be sent. E-mail: chc12@ chonnam.ac.kr and ymjung@kanwon.ac.kr. DOI: 10.1366/10-06073 320 Volume 65, Number 3, 2011 APPLIED SPECTROSCOPY 0003-7028/11/6503-0320$2.00/0 Ó 2011 Society for Applied Spectroscopy