Effect of Li⁺ and Mg²⁺ on the Electrochemical Decomposition of the Ionic Liquid 1‐Butyl‐1‐ methylpyrrolidinium bis(trifluoromethanesulfonyl)imide and Related Electrolytes

Abstract

We investigated the decomposition of 1‐butyl‐1‐methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (BMP‐TFSI) based electrolytes on Au and glassy carbon (GC) in the absence and presence of Li(TFSI) and Mg(TFSI)2. Detecting the volatile reaction products via differential electrochemical mass spectrometry (DEMS) measurements allowed us to gain insight into the decomposition mechanisms. In neat ionic liquid (IL) and on Au electrodes both ions are decomposed at reductive potentials. The TFSI− anion mainly decomposes by releasing CF3, while for the BMP+ cation ring opening and scission of either the methyl or the butyl side chain occur in parallel. At oxidative potentials the decomposition of the anion is the predominant process. Changing to the GC electrode increases the fraction of TFSI− decomposition products at both cathodic and anodic potentials. The addition of Li+ largely hinders the formation of volatile BMP‐TFSI decomposition. In contrast, addition of Mg2+ promotes the TFSI− decomposition at the expense of the BMP+ decomposition.

Surface Science and Electrochemical Model Studies on the Interaction of Graphite and Li‐Containing Ionic Liquids

“…It is not clear if, especially on the graphite powder electrode, the latter process is also accompanied by Li + or [BMP] + intercalation and whether it results in the formation of a surface layer that passivates against further insertion processes. In recent differential electrochemical mass spectrometry (DEMS) studies, we found indirect evidence for the formation of adsorbed decomposition products in Li + ‐containing [BMP][TFSI] . This was concluded from the lack of volatile decomposition products during the reduction in Li + ‐containing electrolyte, whereas [BMP] + and [TFSI] − fragments were found in neat and Mg 2+ ‐containing IL …”

“…In recent differential electrochemical mass spectrometry (DEMS) studies, we found indirect evidence for the formation of adsorbed decomposition products in Li + ‐containing [BMP][TFSI] . This was concluded from the lack of volatile decomposition products during the reduction in Li + ‐containing electrolyte, whereas [BMP] + and [TFSI] − fragments were found in neat and Mg 2+ ‐containing IL …”

“…Examples include Li (Li‐coated Cu), Pt, Au, and glassy carbon as well as HOPG and graphite composite electrodes . These studies revealed that the comparatively large stability window of [BMP][TFSI] is greatly influenced and diminished by the presence of traces of moisture and other contaminants . Electrolyte degradation has been reported to progress through reductive [TFSI] − decomposition and subsequent SEI formation/surface passivation .…”

Designing a Safe Electrolyte Enabling Long‐Life Li/S Batteries

“…Additionally, the atomic concentration of all elements is summarized in Figure f, which further highlights the reduction in oxygen‐containing species at the Li surface and an increase in carbon species upon addition of the IL. This points to a mechanism in which the decomposition of Py 1,4 TFSI leads to the formation of a polymeric SEI, rather than inorganic species, with the added benefit of a reduction of sulfur deposition …”

“…This points to amechanism in which the decompositiono fP y 1,4 TFSI leads to the formation of ap olymeric SEI, rather than inorganic species, with the added benefit of ar eductiono fsulfur deposition. [41] To further confirmt he effect of Py 1,4 TFSI as an additive to the ether-based electrolyte solution, the IL50 mixture, that is, the best-performing in terms of cycling stability and coulombic efficiency,w as compared with as tandard DOL/DME1 m LiTFSI electrolyte using LiNO 3 (0.4 m)a sa na dditive in Li/S cells. [16] The voltage profiles of the Li/S cellsw ith the two electrolytes for different cycles are reported in Figure 5a.T he cell containing the IL additive (left panel)h ad the highest discharge capacity during the first cycle, 20 %h igher than the cell containing the LiNO 3 additive.…”