Comparing aging of graphite/LiFePO 4 cells at 22  C and 55  C e Electrochemical and photoelectron spectroscopy studies Maria Hellqvist Kjell a, * , Sara Malmgren b , Katarzyna Ciosek b , Mårten Behm a , Kristina Edström b , Göran Lindbergh a a School of Chemical Science and Engineering, Department of Chemical Engineering and Technology, Applied Electrochemistry, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden b Department of Chemistry Ångström Laboratory, Uppsala University, Box 538, SE-75121 Uppsala, Sweden highlights Comparing aging of graphite/LiFePO 4 -based cells at 22  C and 55  C. Cycle-aging using a synthetic hybrid drive cycle. 22  C and 55  C cells show, e.g., different capacity loss and impedance at EOL. XPS spectra of electrodes cycle aged at the same temperature were similar. Degradation processes are accelerated in process specific ways. article info Article history: Received 26 March 2013 Received in revised form 30 May 2013 Accepted 3 June 2013 Available online 11 June 2013 Keywords: Aging XPS LiFePO 4 Electrolyte degradation Lithium-ion battery abstract Accelerated aging at elevated temperature is commonly used to test lithium-ion battery lifetime, but the effect of an elevated temperature is still not well understood. If aging at elevated temperature would only be faster, but in all other respects equivalent to aging at ambient temperature, cells aged to end-of-life (EOL) at different temperatures would be very similar. The present study compares graphite/LiFePO 4 - based cells either cycle- or calendar-aged to EOL at 22  C and 55  C. Cells cycled at the two temperatures show differences in electrochemical impedance spectra as well as in X-ray photoelectron spectroscopy (XPS) spectra. These results show that lithium-ion cell aging is a complex set of processes. At elevated temperature, the aging is accelerated in process-specific ways. Furthermore, the XPS results of cycle-aged samples indicate increased deposition of oxygenated LiPF 6 decomposition products in both the negative and positive electrode/electrolyte interfaces. The decomposition seems more pronounced at elevated temperature, and largely accelerated by cycling, which could contribute to the observed cell impedance increase. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Lithium-ion batteries, including graphite/LiFePO 4 -based ones, are attractive for energy storage in plug-in and hybrid electric ve- hicles. However, the vehicle lifetime today exceeds the lithium-ion battery lifetime [1]. Therefore, significant efforts are made to esti- mate and extend the lifetime of various batteries. Accelerated aging at elevated temperature is a commonly employed method to pre- dict battery lifetime at ambient temperature [2]. However, lithium- ion battery aging is a complex set of interacting processes. A range of different aging mechanisms have been proposed for lithium-ion batteries, including the deterioration of the active materials, elec- trolyte, separator, composite electrode structure, as well as evolu- tion of the solid electrolyte interphase (SEI) [3]. For LiFePO 4 -based systems, loss of cyclable lithium has been proposed as a major aging mechanism [4e13]. The observed loss of cyclable lithium has been attributed to, e.g., growth of SEI [4,5,7]. Furthermore, the available capacity of LiFePO 4 electrodes has been proposed to deteriorate due to, e.g., decreased wettability of the aged electrolyte [14] or particle isolation through cracking [15,16]. Electrolyte degradation may also result in clogging of separator pores [17]. Considering the variety of mechanisms contributing to lithium- ion battery aging, a range of different methods can be used to follow the degradation. Different methods are suitable for * Corresponding author. Tel.: þ46 8 790 81 74. E-mail address: mariakj@kth.se (M. Hellqvist Kjell). Contents lists available at SciVerse ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour 0378-7753/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jpowsour.2013.06.011 Journal of Power Sources 243 (2013) 290e298