Abstract
Here, we investigate the physicochemical and electrochemical properties of fluorine-free ionic liquid (IL)-based electrolytes with two different cations, tetrabutylphosphonium, (P4,4,4,4)+, and tetrabutylammonium, (N4,4,4,4)+, coupled to a new anion, 2-[2-(2-methoxyethoxy)ethoxy]acetate anion (MEEA)−, for both neat and (P4,4,4,4)(MEEA) also doped with 10–40 mol % of Li(MEEA). We find relatively weaker cation–anion interactions in (P4,4,4,4)(MEEA) than in (N4,4,4,4)(MEEA), and for both ILs, the structural flexibility of the oligoether functionality in the anion results in low glass transition temperatures, also for the electrolytes made. The pulsed field gradient nuclear magnetic resonance (PFG NMR) data suggest faster diffusion of the (MEEA)− anion than (P4,4,4,4)+ cation in the neat IL, but the addition of a Li salt results in slightly lower mobility of the former than the latter and lower ionic conductivity. This agrees with the combined 7Li NMR and attenuated total reflection–Fourier transform infrared (ATR–FTIR) spectroscopy data, which unambiguously reveal preferential interactions between the lithium cations and the carboxylate groups of the IL anions, which also increased as a function of the lithium salt concentration. In total, these systems provide a stepping stone for further design of fluorine-free and low glass transition temperature IL-based electrolytes and also stress how crucial it is to control the strength of ion–ion interactions.
Highlights
The liquid electrolytes used in conventional lithium-ion batteries (LIBs) are most often composed of the salt lithium hexafluorophosphate (LiPF6), or sometimes lithium tetrafluoroborate (LiBF4), salts that dissolve in flammable carbonatebased organic solvents.[1,2] Such electrolytes have a number of drawbacks adversely affecting the battery performance when used and cause difficulties at the recycling stage
For both ionic liquid (IL) and electrolytes, we start by first assessing the phase, thermal, and electrochemical stabilities before addressing more direct LIB relevant performance parameters such as ionic conductivity and diffusion together with ion−ion interactions
The Λimp/ΛNMR ratio being lower than unity indicates the presence of ionic association and that only a fraction of the diffusing species contributes to the ionic conduction.[62]
Summary
The liquid electrolytes used in conventional lithium-ion batteries (LIBs) are most often composed of the salt lithium hexafluorophosphate (LiPF6), or sometimes lithium tetrafluoroborate (LiBF4), salts that dissolve in flammable carbonatebased organic solvents.[1,2] Such electrolytes have a number of drawbacks adversely affecting the battery performance when used and cause difficulties at the recycling stage. As touched upon briefly above, ILs/RTILs possess a combination of properties making them excellent bases for electrolytes such as a nonflammability, negligible vapor pressures, high chemical and thermal stabilities, inherent high ionic conductivities, and wide electrochemical stability windows (ESWs) sometimes quoted up to 6.0 V.19−21. The working frequencies were 400.21 MHz for 1H, 100.64 MHz for 13C, 162.01 MHz for 31P, and 155.53 MHz for 7Li. The 7Li spectra of the neat electrolytes were recorded by placing the samples in a 5 mm standard NMR tube, which was further placed inside a 10 mm standard NMR tube containing CDCl3. Pulsed field gradient (PFG) NMR measurements were performed on a Bruker Avance III (Bruker BioSpin AG, Fallanden, Switzerland) NMR spectrometer. NMR selfdiffusion measurements were performed on 1H and 7Li with a PFG NMR probe Diff[50] (Bruker) with a maximum amplitude of the magnetic field gradient pulse of 29.73 T m−1.
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