Paul Meister

Dual-graphite cells based on the reversible intercalation of bis(trifluoromethanesulfonyl)imide anions from an ionic liquid electrolyte

Recently, dual-ion cells based on the anion intercalation into a graphite positive electrode have been proposed as electrochemical energy storage devices. For this technology, in particular electrolytes which display a high stability vs. oxidation are required due to the very high operation potentials of the cathode, which may exceed 5 V vs. Li/Li+. In this work, we present highly promising results for the use of graphite as both the anode and cathode material in a so-called “dual-graphite” or “dual-carbon” cell. A major goal for this system is to find suitable electrolyte mixtures which exhibit not only a high oxidative stability at the cathode but also form a stable solid electrolyte interphase (SEI) at the graphite anode. As an electrolyte system, the ionic liquid-based electrolyte mixture Pyr14TFSI-LiTFSI is used in combination with the SEI-forming additive ethylene sulfite (ES) which allows stable and highly reversible Li+ ion and TFSI− anion intercalation/de-intercalation into/from the graphite anode and cathode, respectively. By addition of ES, also the discharge capacity for the anion intercalation can be remarkably increased from 50 mA h g−1 to 97 mA h g−1. X-ray diffraction studies of the anion intercalation into graphite are conducted in order to understand the influence of the electrolyte additive on the graphite structure and on the cell performance.

Anodic Behavior of the Aluminum Current Collector in Imide-Based Electrolytes: Influence of Solvent, Operating Temperature, and Native Oxide-Layer Thickness

The inability of imide salts to form a sufficiently effective passivation layer on aluminum current collectors is one of the main obstacles that limit their broad application in electrochemical energy-storage systems. However, under certain circumstances, the use of electrolytes with imide electrolyte salts in combination with the aluminum current collector is possible. In this contribution, the stability of the aluminum current collector in electrolytes containing either lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) or lithium fluorosulfonyl-(trifluoromethanesulfonyl) imide (LiFTFSI) as conductive salt was investigated by electrochemical techniques, that is, cyclic voltammetry (CV) and chronocoulometry (CC) in either room-temperature ionic liquids or in ethyl methyl sulfone. In particular, the influence of the solvent, operating temperature, and thickness of the native oxide layer of aluminum on the pit formation at the aluminum current collector surface was studied by means of scanning electron microscopy. In general, a more pronounced aluminum dissolution and pit formation was found at elevated temperatures as well as in solvents with a high dielectric constant. An enhanced thickness of the native aluminum oxide layer increases the oxidative stability versus dissolution. Furthermore, we found a different reaction rate depending on dwell time at the upper cut-off potential for aluminum dissolution in TFSI- and FTFSI-based electrolytes during the CC measurements; the use of LiFTFSI facilitated the dissolution of aluminum compared to LiTFSI. Overall, the mechanism of anodic aluminum dissolution is based on: i) the attack of the Al2 O3 surface by acidic species and ii) the dissolution of bare aluminum into the electrolyte, which, in turn, is influenced by the electrolytes dielectric constant.