Gaetan M. A. Girard

Preparation and characterization of gel polymer electrolytes using poly(ionic liquids) and high lithium salt concentration ionic liquids

Polymerized ionic liquids or poly(ionic liquids) (polyILs) have been considered as promising hosts for fabrication of gel polymer electrolytes (GPEs) containing ionic liquids. In this work, a novel GPE based on a polyIL, poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl)imide (PDADMA TFSI), and a high lithium-concentration phosphonium ionic liquid, trimethyl(isobutyl)phosphonium bis(fluorosulfonyl)imide (P111i4FSI), is prepared. The composition-dependent behaviour of the GPEs is investigated by differential scanning calorimetry (DSC), electrochemical impedance spectroscopy (EIS) and solid-state nuclear magnetic resonance (solid-state NMR). The effects of Al2O3 nano-particles on the polymer electrolyte properties are also discussed. It is shown that the introduction of high lithium-concentration ionic liquids into the polyIL can effectively decrease the glass transition temperature (Tg) of the resulting GPE, leading to improved ion dynamics and higher ionic conductivity. The Al2O3 nano-particles effectively enhanced the mechanical stability of the GPEs. Most importantly, although adding PDADMA TFSI to the ionic liquids decreases the diffusion coefficient of both Li+ and anions, a greater decrease in the anion diffusion is observed, resulting in a higher Li+ transport number (as evaluated by NMR) than that seen in the original ILs. Finally, a highly conductive free-standing GPE membrane is fabricated, and extremely stable lithium symmetrical cell performance is demonstrated.

Spectroscopic Characterization of the SEI Layer Formed on Lithium Metal Electrodes in Phosphonium Bis(fluorosulfonyl)imide Ionic Liquid Electrolytes

The chemical composition of the solid electrolyte interphase (SEI) layer formed on the surface of lithium metal electrodes cycled in phosphonium bis(fluorosulfonyl)imide ionic liquid (IL) electrolytes are characterized by magic angle spinning nuclear magnetic resonance (MAS NMR), X-ray photoelectron spectroscopy (XPS), fourier transformed infrared spectroscopy, and electrochemical impedance spectroscopy. A multiphase layered structure is revealed, which is shown to remain relatively unchanged during extended cycling (up to 250 cycles at 1.5 mA·cm-2, 3 mA h·cm-2, 50 °C). The main components detected by MAS NMR and XPS after several hundreds of cycles are LiF and breakdown products from the bis(fluorosulfonyl)imide anion including Li2S. Similarities in chemical composition are observed in the case of the dilute (0.5 mol·kg-1 of Li salt in IL) and the highly concentrated (3.8 mol·kg-1 of Li salt in IL) electrolyte during cycling. The concentrated system is found to promote the formation of a thicker and more uniform SEI with larger amounts of reduced species from the anion. These SEI features are thought to facilitate more stable and efficient Li cycling and a reduced tendency for dendrite formation.

Passivation behaviour of aluminium current collector in ionic liquid alkyl carbonate (hybrid) electrolytes

The compatibility of current collectors with the electrolyte plays a major role in the overall performance of lithium batteries, critical to obtain high storage capacity as well as excellent capacity retention. In lithium-ion batteries, in particular with cathodes that operate at high voltage such as lithium nickel cobalt manganese oxide, the cathodic current collector is aluminium and it is subjected to high oxidation potentials (>4 V vs. Li/Li+). As a result, the composition of the electrolyte needs to be carefully designed in order to stabilise the battery performance as well as to protect the current collectors against corrosion. This study examines the role of a hybrid electrolyte composed of an ionic liquid (N-methyl-N-propyl pyrrolidinium bis(trifluoromethanesulfonyl)imide or N-methyl-N-propyl pyrrolidinium bis(fluorosulfonyl)imide) and a conventional electrolyte mixture (LiPF6 salt and alkyl carbonate solvents) with correlation to their electrochemical behaviour and corrosion inhibition efficiency. The hybrid electrolyte was tested against battery grade aluminium current collectors electrochemically in a three-electrode cell configuration and the treated aluminium surface was characterised by SEM/EDXS, optical profilometry, FTIR, and XPS analysis. Based on the experimental results, the hybrid electrolytes allow an effective and improved passivation of aluminium and lower the extent of aluminium dissolution in comparison with the conventional lithium battery electrolytes and the neat ionic liquids at high anodic potentials (4.7 V vs. Li/Li+). The mechanism of passivation behaviour is also further investigated. These observations provide a potential direction for developing improved hybrid electrolytes, based on ionic liquids, for higher energy density devices.Battery electrolytes: helping with hybridsThe use of ionic liquids in hybrid electrolytes has reduced the corrosion of an important Li-ion battery component. Aluminium current collectors in Li-ion batteries are susceptible to corrosion when subjected to high oxidation potentials and thus the composition of the electrolyte is critical to both battery performance and stability. The addition of ionic liquids to conventional carbonate-based solvents has previously demonstrated corrosion inhibition. Now, Sowmiya Theivaprakasam and colleagues, as part of an international collaboration between researchers at Deakin University and Monash University in Australia and the Indian Institute of Technology Bombay in India, have confirmed the corrosion inhibition properties of hybrid electrolytes, specifically those featuring a pyrrolidinium ionic liquid. Using surface characterisation techniques they show that bis(fluorosulfonyl)imide and bis(trifluoromethanesulfonyl)imide anions enable improved passivation at the aluminium surface.