A Comprehensive Study of the Intercalation Behavior of Anions from Ionic-Liquid Based Electrolytes into a Graphite Positive Electrode for Dual-Ion Batteries

The increasing demand for large scale energy storage capabilities causes a big field of research to make the battery systems safer and environmentally benign. A promising approach is the replacement of conventional volatile and highly flammable organic solvent-based electrolytes by ionic liquids (ILs) as the main component of the electrolyte. ILs exhibit excellent properties such as a broad electrochemical stability window, a high thermal stability, non-flammability and a high ionic conductivity.[1] A further prevention of pollution is the application of carbon-based, transition metal-free electrode materials. Recently, “dual-ion” and “dual-graphite” cells were introduced which are composed of a lithium-incorporating anode and an anion incorporating graphite-based cathode.[2][3] During charge the ions intercalate into the electrode active materials and are released back into the electrolyte during discharge (see Figure 1). Since the cathode potential reaches and may even exceed 5 V vs. Li/Li+, electrolytes that exhibit a high stability vs. oxidation are necessary. For that reason, ILs with their versatile properties were introduced and investigated. The state of the art electrolyte for this system is composed of a solution of lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) in N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (Pyr14TFSI). In order to increase the specific capacity of this system, one main strategy is to focus on the intercalation of smaller anions than TFSI into the graphite structure. In this context, the major challenge is the synthesis of ILs or mixtures of ILs with organic electrolytes consisting of small anions to provide an electrochemically stable electrolyte. Interestingly, not only the size and shape of the intercalated anions plays a major role in view of onset potential for anion uptake as well as discharge capacity. Instead, also more complex coherences such as ion association, self-aggregation of ions as well as solvent co-intercalation have to be taken into consideration. In this work, we investigate the electrochemical intercalation of anions with different shapes and sizes from IL based electrolytes into a graphite-based cathode. Against this background, the influence of side chain modifications within the anion structure as well as the influence of the used cation within the conductive salt (Li+/K+) on the intercalation behavior was studied using different electrochemical analysis techniques. Therefore, in this contribution we will give new insights to the question if the anion size is the main issue to tailor the reversible capacity for anion intercalation. Furthermore, the influence of lithium plating behavior in different electrolyte systems on the overall cell performance as well as the effect of adding small amounts of different additives such as foreign cations and different conductive salts into the baseline electrolyte is discussed. Figure 1: Schematic illustration of the operating principle of a "dual-ion cell”.[2] 1] A. Lewandowski, A. Swiderska-Mocek, Journal of Power Sources, 194 (2009) 601-609. [2] T. Placke, P. Bieker, S.F. Lux, O. Fromm, H.W. Meyer, S. Passerini, M. Winter, Zeitschrift für Physikalische Chemie, 226 (2012) 391-407. [3] K. Beltrop, P. Meister, S. Klein, A. Heckmann, M. Grünebaum, H.-D. Wiemhöfer, M. Winter, T. Placke, Electrochimica Acta, 209 (2016) 44-55. Figure 1