Does Size really Matter? New Insights into the Intercalation Behavior of Anions into a Graphite-Based Positive Electrode for Dual-Ion Batteries

There are many reports on electrochemical anion intercalation into graphite using different types of electrolyte mixtures for application in dual-graphite or dual-ion cells, showing promising results in terms of cycling stability, reversible capacity and Coulombic efficiency. However, there is no clear understanding of the influence of the anion/electrolyte characteristics on the intercalation properties. In this work, we present a comprehensive study of the intercalation behavior of a series of imide-based ionic liquid (IL) electrolytes into a graphite positive electrode with special emphasis on the influence of anion size on the electrochemical parameters such as the onset potential for anion uptake and the reversible capacity. The onset potentials of anion intercalation into graphite ranged between 4.42 V to 4.53 V vs. Li/Li+ with the following descending order for the studied anions: BETI > FSI > FTFSI > FSI/TFSI (molar ratio = 11:1) > TFSI > TFSI/FSI (molar ratio = 10:1). The electrochemical results support the assumption that electrolyte effects such as ion pair formation and self-aggregation in the electrolyte overrule the influence of the anion size (up to a certain point) in terms of the onset potential for anion uptake. The charge/discharge cycling performance was studied in view of reversible capacity and Coulombic efficiency. In this context, the BETI system shows only very poor intercalation ability whereas the quaternary mixture TFSI/FSI displays a very promising cycling behavior providing a specific capacity of similar to 54 mAh g(-1) with a Coulombic efficiency exceeding 99%. Furthermore, the characteristics of the different imide-based electrolytes such as the oxidative stability (TFSI > BETI > FTFSI > TFSI/FSI > FSI/TFSI > FSI) as well as the influence on aluminum current collector dissolution were studied to draw further conclusions about impact on the Coulombic efficiency. (C) 2016 Elsevier Ltd. All rights reserved.