Electrochimica Acta 92 (2013) 102–107 Contents lists available at SciVerse ScienceDirect Electrochimica Acta jou rn al h om epa ge: www.elsevier.com/locate/electacta Lithium difluoro(oxalato)borate: A promising salt for lithium metal based secondary batteries? T. Schedlbauer, S. Krüger, R. Schmitz, R.W. Schmitz, C. Schreiner, H.J. Gores ∗,1 , S. Passerini 1 , M. Winter ∗∗,1 Institute of Physical Chemistry, University of Münster (Westfälische Wilhelms-Universität (WWU) Münster), MEET Battery Research Center, Corrensstr. 46, 48149 Münster, Germany a r t i c l e i n f o Article history: Received 5 December 2012 Received in revised form 30 December 2012 Accepted 4 January 2013 Available online 10 January 2013 Keywords: Lithium metal anodes Lithium difluoro(oxalato)borate Lithium hexafluorophosphate SEI-formation Coulombic efficiency Surface analysis a b s t r a c t This work is a comparative study on lithium cycling on copper in solutions based on ethylene carbon- ate (EC) and diethyl carbonate (DEC) (3:7, by wt.) and two lithium salts, lithium hexafluorophosphate (LiPF 6 ) and lithium difluoro(oxalato)borate (LiDFOB). Coulombic efficiencies of the long term lithium deposition/dissolution experiments and dissolution-rate (D-rate) tests on copper demonstrated clearly the superior behavior of the LiDFOB-based electrolyte. To clarify the impact of the formed solid elec- trolyte interphase (SEI) on the Coulombic efficiencies achieved in the electrolytes, voltage drop values of the D-rate tests were compared with measured values of conductivities and AC impedance measure- ments of the electrolytes. The formed SEI has a larger influence on voltage drop values of a cell than the conductivity of the electrolyte. The correlation between surface chemistry, morphology, and Coulom- bic efficiencies of lithium deposition on copper was investigated by scanning electron microscopy (SEM) and Raman spectroscopy. All methods demonstrated the strong influence of the investigated lithium salts on the lithium deposition/dissolution performance on copper substrates. The LiDFOB-based electrolyte showed superior SEI properties than the LiPF 6 -based electrolyte. © 2013 Elsevier Ltd. All rights reserved. 1. Introduction Rechargeable batteries using a lithium metal anode are still a promising alternative to rechargeable lithium ion batteries with graphite as anode material [1,2]. The theoretical specific capacity of lithium is ten times larger (3860 mAh g -1 ) than the theoretical specific capacity of lithiated graphite (372 mAh g -1 ) [3–5]. Further- more, there are less or even no requirements for an anode current collector and complicated slurry preparation [3]. These advantages can result in lower costs and weight of the lithium metal tech- nology in comparison to lithium ion technologies. Unfortunately, the lithium metal technology suffers from low lithium cycling effi- ciency and a dendritic morphology of deposited lithium, resulting in tremendous safety problems [6–8]. It is widely accepted that these problems are caused by the solid electrolyte interphase (SEI) [9], which is built during the first cycle when freshly deposited lithium reacts with electrolyte components [6,10]. In general, it is known, that the SEI formation behavior, the SEI composition and ∗ Corresponding author. Tel.: +49 9402 8040; fax: +49 9402 8035. ∗∗ Corresponding author. Tel.: +49 251 83 36031; fax: +49 251 83 36032. E-mail addresses: hgore 01@uni-muenster.de, w.heitzer h.j.gores@t-online.de (H.J. Gores), mwint 01@uni-muenster.de (M. Winter). 1 ISE member. the SEI performance in the cell depends on the anode material and on the electrolyte composition [11–13]. The lithium salt is a key component for the SEI formation on lithium metal. Previous studies were mostly focused on electrolytes with common salts such as lithium hexafluorophosphate (LiPF 6 ), lithium perchlorate (LiClO 4 ), lithium tetrafluoroborate (LiBF 4 ), and lithium hexafluoroarsenate (LiAsF 6 ) [14–18]. The investigations were primarily focused on solvent composition or additives. The most promising salt among these salts is LiAsF 6 [6,10]. However, because of the high toxicity of its reaction products this salt has no future in consumer applications. Therefore, recent studies were mainly focused on LiPF 6 [19] or LiClO 4 [2,20] as lithium salts in liquid electrolytes for lithium metal batteries. Despite the devel- opment of new additives and new solvent mixtures, there are still no liquid electrolytes including abovementioned salts that could be used in commercial lithium batteries [2,5,21]. Therefore, the demand to find alternative electrolytes which form an appropriate and efficient SEI on lithium metal still exists. LiDFOB as a salt for lithium and lithium ion batteries was at first investigated by Zhang [22]. There are only two properties, conductivity and electrochemical stability window in non-aqueous solutions, where LiDFOB electrolytes cannot reach the performance of LiPF 6 containing electrolytes [23]. Despite the lower conductiv- ity values of LiDFOB electrolytes the cationic transference number of 1 M LiDFOB in solvent blends of ethylene carbonate (EC) and 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.01.023