Ruhamah Yunis

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.

The influence of the size and symmetry of cations and anions on the physicochemical behavior of organic ionic plastic crystal electrolytes mixed with sodium salts

The phase behaviour, ionic conductivity, electrochemical stability and diffusion coefficients of mobile components in three organic ionic plastic crystals (OIPCs): triisobutylmethylphosphonium bis(fluorosulphonyl)amide (P1i444FSI), triisobutylmethylphosphonium bis(trifluromethanesulphonyl)amide (P1i444NTf2) and trimethylisobutylphosphonium bis(trifluoromethanesulphonyl)amide (P111i4NTf2) are compared to study the effect of the anions and cations on phase behaviour and dynamics. The FSI-based OIPC shows lower melting point and higher conductivity values most likely because of the higher degree of charge distributions and weaker ion-ion interactions compared to NTf2 anion-based OIPCs. Cyclic voltammetry of electrolytes consisting of these OIPCs with 70 mol% sodium salt incorporated indicates stable sodium plating/stripping behaviour at 70 and 50 °C for all samples. The magnitude of the peak currents, however, are much higher for the FSI-based electrolyte.

Probing Ionic Liquid Electrolyte Structure via the Glassy State by Dynamic Nuclear Polarization NMR Spectroscopy

Dynamic nuclear polarization (DNP)-enhanced solid-state NMR spectroscopy has been used to study an ionic liquid salt solution (N-methyl-N-propyl-pyrrolidinium bis(fluorosulfonyl)imide, C3mpyrFSI, containing 1.0 m lithium bis(fluorosulfonyl)imide, 6LiFSI) in its glassy state at a temperature of 92 K. The incorporation of a biradical to enable DNP signal enhancement allowed the proximities of the lithium to the individual carbon sites on the pyrrolidinium cation to be probed using a 13C-6Li REDOR pulse sequence. Distributions in Li-C distances were extracted and converted into a 3D map of the locations of the Li+ relative to the C3mpyr that shows remarkably good agreement with a liquid-phase molecular dynamics simulation.

A symmetrical ionic liquid/Li salt system for rapid ion transport and stable lithium electrochemistry

Contrary to the accepted wisdom that avoids cation symmetry for the sake of optimum electrolyte properties, we reveal outstanding behaviour for the diethylpyrrolidinium cation ([C2epyr]), in combination with the bis(fluorosulfonyl)imide (FSI) anion and Li[FSI]. The equimolar [C2epyr][Li][FSI]2 is a liquid with high conductivity, high Li transference number and >90% lithium metal cycling efficiency. The high level of performance for these electrolytes invites consideration of a new class of electrolytes for lithium batteries.

Ionic liquids and plastic crystals with a symmetrical pyrrolidinium cation

Solid and liquid salts utilising the N-methyl-N-alkyl pyrrolidinium cation, [Cnmpyr]+, and their use in electrochemical devices, are well established. However, new materials with enhanced properties, such as higher conductivity, lower viscosity or more favourable thermal phase behaviour, are still required. Here we report the synthesis and characterisation of new ionic liquids and plastic crystals using the N,N-diethylpyrrolidinium cation ([C2epyr]+) with six different anions. With the fluorosulfonyl(trifluoromethanesulfonyl)imide ([FTFSI]−) and dicyanamide ([DCA]−) anions, room temperature ionic liquids with low viscosity are formed. With the bis(trifluoromethanesulfonyl)imide ([NTf2]−), bis(fluorosulfonyl)imide ([FSI]−), hexafluorophosphate ([PF6]−) and tetrafluoroborate ([BF4]−) anions, organic ionic plastic crystals are produced. Of the new solid salts, [C2epyr][FSI] has the highest conductivity, higher than the well-established methyl-substituted analogue, giving 1.9 × 10−5 S cm−1 at 30 °C. Thermal analysis, conductivity and the Walden relationship are used to compare the new N,N-diethylpyrrolidinium salts across the different anions and with some previously reported N-methyl-N-alkylpyrrolidinium salts.

Efficient and Stable Solid-State Dye-Sensitized Solar Cells by the Combination of Phosphonium Organic Ionic Plastic Crystals with Silica

Remarkably efficient quasi-solid-state dye-sensitized solar cells (DSSCs) have been fabricated using organic ionic plastic crystal electrolytes based on a small triethyl(methyl)phosphonium [P1222] cation and two types of sulfonamide anions, bis(fluorosulfonyl)amide (FSA) and bis(trifluoromethanesulfonyl)amide (TFSA), in combination with varying amounts of silica (SiO2). Solar cell efficiencies of up to 7.4% were obtained, which is comparable to our benchmark efficiencies of liquid (acetonitrile) electrolyte-based devices. Such a high efficiency for DSSCs using quasi-solid-state electrolytes is attributed to improved ionic conductivity, enhanced redox couple transport, improved interfacial interaction between the electrolyte and the electrode as well as decreased resistance at both electrode interfaces. Notably, the devices with the silica-containing electrolytes displayed excellent stability after 5 months of storage, with the most stable devices, formed with either plastic crystal electrolyte containing 2% silica, showing no decrease in efficiency.