Anion intercalation into graphitic carbon from ionic liquid based electrolytes for high performance dual-ion batteries T. Placke*, O. Fromm, R. Klöpsch, G. Schmülling, P. Bieker, S. F. Lux, H.-W. Meyer, S. Passerini, M. Winter University of Münster, Institute of Physical Chemistry, MEET Battery Research Center, Corrensstraße 46, 48149 Münster, Germany *tobiasplacke@uni-muenster.de Graphite is a redox-amphoteric intercalation host and can be intercalated by cations or anions yielding to so-called donor-type or acceptor-type graphite intercalation compounds [1, 2]. Examples for anions capable to form acceptor-type GICs are hexa- or tetrafluoride guest species such as PF 6 - , AsF 6 - or BF 4 - [3] and even large carbon-based anions such as tris(trifluoromethane-sulfonyl) methide ((CF 3 SO 2 ) 3 C - ) or bis(trifluoromethane-sulfonyl) imide ((CF 3 SO 2 ) 2 N - ) [4, 5]. Graphite as the positive electrode in electrochemical energy storage systems has been first introduced by patents of McCullough and the publications of Carlin [6] et al. in the 1990s. In their work, they build a so-called “dual-carbon cell” using graphite as both the negative and positive electrode. This concept was further examined in the works of Seel and Dahn, where they investigated the intercalation of PF 6 - anions into graphite from organic solvent based electrolytes such as sulfones [7]. Recently, different types of anion intercalation based energy storage systems are in the focus of research. Examples are the so-called hybrid supercapacitors [8], or systems that are based on a graphite cathode and a metal oxide anode, such as TiO 2 or MoO 3 , working at potentials above 1 V vs. Li/Li + [9, 10], therefore proposed as safe energy storage systems as there is no possibility of oxygen generation at the cathode and no risk of lithium plating or dendrite formation at the anode. Nevertheless, one major drawback of these systems is the limited oxidative electrolyte stability at the high working potentials of the graphite positive electrode. As the cathode potential during anion intercalation approaches 5 V vs. Li/Li + or even beyond, the organic solvent based electrolytes suffer from these highly oxidizing conditions and electrolyte oxidation takes place, resulting in insufficient discharge/charge efficiencies and continuous electrolyte degradation. A further issue concerning the organic solvent based electrolytes is the fact that solvent co-intercalation reactions between the graphite interlayer gaps can occur and therefore eventually lead to graphite exfoliation reactions. In this contribution, we present promising results for anion intercalation into graphite from ionic liquid based electrolytes (ILs) to build up a cell, which we call “dual-ion cell”. As the compatibility of ILs with graphite negative electrodes is poor, we started with alternative anodes, namely metallic lithium and Li 4 Ti 5 O 12 , as they display good compatibilities with ILs. The electrochemical system is based on intercalation of the anion bis(trifluoromethanesulfonyl) imide (TFSI - ) into the graphite cathode and the lithium ion insertion in LTO or lithium deposition onto metallic lithium as anode. Both ions derive from an electrolyte containing lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) in the ionic liquid N-butyl-N-methylpyrroli-dinium bis(trifluorome- thanesulfonyl) imide (Pyr 14 TFSI). The performance has been studied in terms of cell cut-off voltage, temperature, rate and self discharge. Depending on the cut-off voltage and temperature, coulombic efficiencies of more than 99 % and discharge capacities exceeding 100 mAh g -1 can be achieved. In addition, this system provides an excellent cycling stability and capacity retention of more than 99 % after 200 cycles, outperforming any kind of organic solvent based dual-ion cell. 0 50 100 150 200 0 20 40 60 80 charge capacity discharge capacity efficiency cycle number anode: metallic Li cathode: KS6 graphite electrolyte: Pyr 14 TFSI - 1M LiTFSI Current: 50 mA g -1 Cell cut-off: 5.0 V capacity / mAh g -1 0 20 40 60 80 100 efficiency / % Figure 1. Charge/discharge cycling performance for the met. Li/ KS6-graphite dual-ion cell. 0 20 40 60 80 100 3.5 4.0 4.5 5.0 5.5 Cut-off 4.80 V Cut-off 5.00 V Cut-off 5.20 V cell voltage / V capacity / mAh g -1 anode/cathode:metallic lithium vs. KS6 graphite electrolyte: Pyr 14 TFSI, 1M LiTFSI Figure 2. Cell voltage vs. specific capacity profiles for different upper cell cut-off voltages for the met. Li/ KS6- graphite dual-ion cell; electrolyte: Pyr 14 TFSI-1M LiTFSI. [1] J. O. Besenhard and H. P. Fritz, Angewandte Chemie- International Edition in English 1983, 22, 950-975. [2] M. Noel and R. Santhanam, Journal of Power Sources 1998, 72, 53-65. [3] D. Billaud, A. Pron and F. L. Vogel, Synthetic Metals 1980, 2, 177-184. [4] X. R. Zhang, N. Sukpirom and M. M. Lerner, Materials Research Bulletin 1999, 34, 363-372. [5] M. Winter, Doctoral Thesis, WWU Münster April 1995, 156-166. [6] R. T. Carlin, H. C. Delong, J. Fuller and P. C. Trulove, Journal of the Electrochemical Society 1994, 141, L73- L76. [7] J. A. Seel and J. R. Dahn, Journal of the Electrochemical Society 2000, 147, 892-898. [8] T. Ishihara, Y. Yokoyama, F. Kozono and H. Hayashi, Journal of Power Sources 2011, 196, 6956-6959. [9] A. K. Thapa, G. Park, H. Nakamura, T. Ishihara, N. Moriyama, T. Kawamura, H. Y. Wang and M. Yoshio, Electrochimica Acta 2010, 55, 7305-7309. [10] N. Gunawardhana, G.-J. Park, N. Dimov, A. K. Thapa, H. Nakamura, H. Wang, T. Ishihara and M. Yoshio, Journal of Power Sources 2011, 196, 7886-7890. 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