Abstract
Electrolytes based on ionic liquids (IL) are promising candidates to replace traditional liquid electrolytes in electrochemical systems, particularly in combination with carbon-based porous electrodes. Insight into the dynamics of such systems is imperative for tailoring electrochemical performance. In this work, 1-Methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide and 1-Hexyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide were studied in a carbon black (CB) host using spectrally resolved Carr-Purcell-Meiboom-Gill (CPMG) and 13-interval Pulsed Field Gradient Stimulated Echo (PFGSTE) Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR). Data were processed using a sensitivity weighted Laplace inversion algorithm without non-negativity constraint. Previously found relations between the alkyl length and the aggregation behavior of pyrrolidinium-based cations were confirmed and characterized in more detail. For the IL in CB, a different aggregation behavior was found compared to the neat IL, adding the surface of a porous electrode as an additional parameter for the optimization of IL-based electrolytes. Finally, the suitability of MAS was assessed and critically discussed for investigations of this class of samples.
Highlights
The investigation of ionic mobility plays a crucial role for the understanding and quantification of electrochemical systems such as batteries, fuel cells, or electrolyzers [1].Important parameters include transference numbers, diffusion, and aggregation in liquidand ionic liquid-based electrolytes or ion mobility in ion-conducting solid electrolytes [2].The investigation of species inside porous matrices, which are of importance in metal-air batteries, or in solid ceramic or hybrid ceramic-polymer electrolytes for solid-state batteries is challenging [3]
We investigated the dynamics of 1-Methyl-1-propylpyrrolidiniumbisimide ([Pyr13 ][Tf2 N]) confined to carbon black (CB) using PGSTE-Nuclear magnetic resonance (NMR) in combination with T1 relaxation time measurements [59]
NMR measurements of the neat ionic liquids (IL) were recorded statically, while the IL-loaded CB samples were measured under magic angle spinning (MAS) at 5 kHz
Summary
The investigation of species inside porous matrices, which are of importance in metal-air batteries, or in solid ceramic or hybrid ceramic-polymer electrolytes for solid-state batteries is challenging [3]. In both cases, ions move in different environments or across interfaces; multiple environments must be distinguished to obtain a comprehensive description of dynamic processes. Spectroscopic information provides structural and electronic information about the immediate molecular surroundings of a nucleus, while relaxation measurements describe local dynamics over a range of time scales, averaged over milliseconds to seconds. Mobility on length scales of μm can be quantified using complementary pulsed field gradient (PFG)
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