Optimising organic ionic plastic crystal electrolyte for all solid-state and higher than ambient temperature lithium batteries

For the first time, practical lithium cell performance is achieved at ambient temperature with an organic ionic plastic crystal solid electrolyte.

A novel organic ionic plastic crystal (OIPC) electrolyte based on a quaternary ammonium cation and the triflate anion has been synthesized, which shows fast proton transport and high thermal stability in the solid state when doped with triflic acid.

Organic ionic plastic crystal, 1-propyl-1-methylpyrrolidinium iodide (P₁₃I), which possesses a broad plastic phase from -36 to 135 °C, was doped with silica nanoparticles (SiO₂ NPs) and 1-ethyl-3-methylimidazolium iodide (EMII), for the preparation of SiO₂/EMII/P₁₃I solid-state electrolytes for dye-sensitized solar cells (DSSCs). The thermal properties of all the electrolytes, including solid-solid phase transitions and melting temperatures, were investigated by differential scanning calorimetry (DSC). The effect of silica particles on the ionic conductivity, diffusion of I⁻/I₃⁻ redox couple in electrolytes, and photovoltaic performance for solid-state DSSCs were investigated. The fabricated solid-state DSSCs yielded a high power conversion efficiency of 5.25% under simulated air mass 1.5 solar spectrum illuminations at 50 mW cm⁻². Furthermore, the DSSCs based on SiO₂/EMII/P₁₃I solid-state electrolytes show good stability after an accelerating aging test, demonstrating potential practical applications.

Organic Ionic Plastic Crystals (OIPCs) are increasingly drawing attention as a new type of solid-state electrolyte for lithium rechargeable batteries, because they can essentially relieve the safety concerns of the present commercial lithium batteries based on organic and flammable solvents and thus pave the way towards more robust energy storage solutions for electric vehicles and power grids. However, although significant progress has been made towards the realisation of practical OIPC-based solid electrolytes, the lack of thorough fundamental understanding of their properties, especially the nature of their ionic motion and transport and the effects of Li salt addition, has limited the development of prototype lithium cells using OIPC-based electrolytes. This PhD thesis therefore has three goals: 1) to obtain more insights into the fundamentals of the materials, especially their phase-dependent plastic crystal behaviour; 2) to under- stand the effects of the addition of Li ions on the structure and dynamics of OIPC matrices; 3) to demonstrate prototype Li cells with OIPC-based electrolytes operating at ambient temperature. For Goal 1, two recently synthesised phosphonium cation-based OIPCs, i. e. diethyl(methyl)(isobutyl)phosphonium hexafluorophosphate ([P1224][PF6]) and triethyl(methyl)phosphonium bis(fluorosulfonyl)imide ([P1222][FSI]) were investigated by a combination of analytical methods and theoretical simulations/calculations. The evolution of the motions of the cations and the anions in different phases of both plastic crystals was examined, and models proposed to explain the strongly phase-dependent ionic conductivities of these OIPCs. Cooperative motion of cations and anions was proposed for [P1222][FSI] at a particular temperature range, in which some unusual phase behaviour was observed. For Goal 2, molecular dynamics simulations of the effects of lithium salt addition to a model plastic crystal, tetramethylammonium dicyanamide ([N1111][DCA]) were performed. Cluster formation between Li ions and anions was found: Li-DCA clusters enhance the mobility of the ions that are not in the clusters. This is a somewhat surprising atomic-level explanation of how doped Li ions in the OIPC enhance its ion transport properties. For Goal 3, the electrochemical properties of triisobutyl(methyl) phosphonium bis(fluorosulfonyl) imide ([P1444][FSI]) were examined, and this plastic crystal was used to build prototypical Li metal cells. These cells showed, for the first time, practical cell performance (ca. 160 mA h g-1 discharge capacity achieved at 0.1 C) at ambient temperature. The main contributions of this thesis to the field are 1) proposing new models of the ionic motions of OIPCs with respect to their phase behaviour 2) showing the value of the combined analytical and theoretical methodology in studying OIPCs; 3) demonstrating promising Li cell performance with an OIPC-based electrolyte at ambient temperature.

Probing disorder and dynamics in composite electrolytes of an organic ionic plastic crystal and lithium functionalised acrylic polymer nanoparticles

Protic dialkylammonium-based ionic liquids as promising solid-solid phase change materials for thermal energy storage: Synthesis and thermo-physical characterization

Organic ionic plastic crystals (OIPCs) are a unique class of materials that exhibit a short-range disorder on the molecular level but are ordered at higher length scales. Recent experiments in our group have shown that the OIPC methyl(diethyl)isobutylphosphonium hexafluorophosphate ([P122i4][PF6]) exhibits a high ideal selectivity of 30 with respect to CO2 and N2 at 35 °C. Here, we employ classical molecular dynamics simulations for studying gas uptake in the OIPC [P122i4][PF6] at different temperatures. Both adsorption and absorption of the gases CO2, N2, O2, and CH4 were estimated using a gas/solid interface model. The observed trend in gas uptake was CO2 > CH4 > O2 > N2. The CO2 uptake was found to be dependent on both the OIPC structure and temperature. Owing to phase transitions and intermolecular motions, the solid gets disordered with increasing temperatures. The study finds that such disordering effects can also be caused by interface effects, which can enhance gas absorption. The results qualitatively confirmed that the OIPC can be effective in separating CO2 not only from N2, as per the previous experimental study, but also from O2 and CH4. However, strategies to improve the free volume in the material become important.

PEDOT:PSS was combined with OIPCs (C2mpyrFSI and C2mpyrTFSI) which gave high ionic and electronic conductivities and when employed for the first time in a solid-state Li|LiFePO4cell as an OMIEC binder it showed an outstanding performance.

This research demonstrates the cation and anion transfer of an organic ionic plastic crystal (OIPC), quinuclidinium hexafluorophosphate ([HQ]PF6), in the pure state and mixtures doped with lithium or chloride ions. The pure material and the mixtures show relatively simple thermal behaviors. Powder X-ray diffraction determined a cubic crystal structure, a common feature of OIPCs, in the high-temperature phase of the pure material. Solid-state nuclear resonance spectra of the pure material reveal rotation-assisted ion mobility and the presence of the plastic crystal phase. The ionic conductivity of the mixtures doped with lithium ions and chloride ions increases significantly. Arrhenius conduction behaviors are observed in both the pure material and ion-doped mixtures, with effective activation energies of ∼60–110 kJ mol–1. Ionic conductivity also implies that ions transfer either through the bulk crystal or via grain boundaries. This is the first time that the cation and anion transfer in a quinuclidinium-based OIPC is reported, and the anion (here Cl–) is transferred in plastic crystal systems where solid solutions are likely formed. Different ion conduction behaviors of the pure and doped materials are closely associated with shapes and relative sizes of constituent ions. The results provide useful information to improve the understanding of structure–property relationships in OIPCs.

Organic ionic plastic crystals (OIPCs) are a ductile soft material where the composing ions are in isotropic free rotation, while their positions are aligned in order. The rotational motion in its plastic phase promotes ion conduction by decreasing the activation energy. Here, we report novel OIPCs comprised of tetracyanoborate ([TCB]-) and various organic cations. In particular, the OIPC composed of [TCB]- and spiro-(1,1')-bipyrrolidinium ([spiropyr]+) cations can transform into its plastic phase at ultralow temperature (Tp = -55 °C) while maintaining a high melting point (Tm = 242 °C). Replacement of the cation with either tetraalkylammonium or phosphonium and comparing their phase behavior, the high Tm was attributed to the relatively small interionic distance between [spiropyr]+ and [TCB]-. At the same time, the low Tp was realized by the restricted vibrational mode of the spirostructure, allowing the initiation of isotropic rotational motion with less thermal energy input.

Organic ionic plastic crystals (OIPCs) are molecularly disordered solids, and their potential for the development of gas separation membranes has recently been demonstrated. Here, the gas absorption capability of the OIPC, diethyl(methyl)(isobutyl)phosphonium hexafluorophosphate ([P122i4][PF6]), for four gases is predicted through potential of mean force (PMF) calculations based on two methods – average force method and adaptive biasing force method. Both methods correctly predicted the different trends of adsorption and absorption of these gases across the OIPC–gas interface. The distinct energy barriers of the PMF profiles of CO2 and N2 near the interface directly reflect the good selectivity of OIPC to these two gases. However, the selectivity of CH4 and O2 cannot be accurately reflected by the PMF curve near the interface, because the relative energy varies greatly at different positions inside the OIPC. Thus the average free energy change should be calculated over the entire OIPC box to evaluate the difference in selectivity between the two gases. This also suggests that gas absorption in OIPCs is greatly affected by the structural order and chemical environment. The adaptive biasing force method overall outperforms the average force method. The method should be able to provide a prediction of gas selectivity for a wider range of organic ionic plastic crystals and other solid materials.

Organic ionic plastic crystal as electrolyte for lithium-oxygen batteries

Mixed ionic-electronic conductors, such as poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) are postulated to be the next generation materials in energy storage and electronic devices. Although many studies have aimed to enhance the electronic conductivity and mechanical properties of these materials, there has been little focus on ionic conductivity. In this work, blends based on PEDOT stabilized by the polyelectrolyte poly(diallyldimethylammonium) (PolyDADMA X) are reported, where the X anion is either chloride (Cl), bis(fluorosulfonyl)imide (FSI), bis(trifluoromethylsulfonyl)imide (TFSI), triflate (CF3SO3) or tosylate (Tos). Electronic conductivity values of 0.6 S cm−1 were achieved in films of PEDOT:PolyDADMA FSI (without any post-treatment), with an ionic conductivity of 5 × 10−6 S cm−1 at 70 °C. Organic ionic plastic crystals (OIPCs) based on the cation N-ethyl-N-methylpyrrolidinium (C2mpyr+) with similar anions were added to synergistically enhance both electronic and ionic conductivities. PEDOT:PolyDADMA X / [C2mpyr][X] composites (80/20 wt%) resulted in higher ionic conductivity values (e.g., 2 × 10−5 S cm−1 at 70 °C for PEDOT:PolyDADMA FSI/[C2mpyr][FSI]) and improved electrochemical performance versus the neat PEDOT:PolyDADMA X with no OIPC. Herein, new materials are presented and discussed including new PEDOT:PolyDADMA and organic ionic plastic crystal blends highlighting their promising properties for energy storage applications.

Organic ionic plastic crystals (OIPCs) have attracted attention as novel organic solid electrolyte materials, but their insufficient mechanical strength and ionic conductivity have prevented their application. In this study, a lithium salt, lithium bis(fluorosulfonyl)amide (LiFSA), and an inorganic solid electrolyte, Li7La3Zr2O12 (LLZO), were added to an OIPC, N,N-diethylpyrrolidinium bis(fluorosulfonyl)amide ([C2epyr][FSA]). The fabricated organic-inorganic hybrid solid electrolytes were evaluated thermally, mechanically, and electrochemically to reveal which factors affect the properties of the electrolytes. All samples showed excellent thermal stability regardless of LiFSA or LLZO concentration, and they were found to be highly plastic and ion-conductive solids at a wide range of temperatures. It was also revealed that the addition of LLZO raised the nanoindentation stiffness (HIT) of the [C2epyr][FSA]/LiFSA composites. The ionic conductivity of the hybrid electrolytes was higher than that of the pristine OIPC, reaching a value of 2.1 × 10-4 S cm-1 at 25 °C upon addition of appropriate amounts of LiFSA and LLZO. Overall, samples with higher LiFSA concentration and moderate LLZO concentration exhibited higher ionic conductivity. Cyclic voltammetry results showed that the [C2epyr][FSA]/LiFSA/LLZO composites were lithium-ion conductors. These findings indicate that by optimizing the concentrations of lithium salt and LLZO, it would be possible to realize their applications as solid electrolytes.

Organic ionic plastic crystals (OIPCs) are gaining attention as next-generation solid electrolytes owing to their excellent thermal and electrochemical stability and high ionic conductivity. However, reports on the application of OIPCs to Mg electrolytes are limited. In this study, Mg-ion conductors based on the bis(fluorosulfonyl)amide (FSA) anion were prepared by adding Mg(FSA)2 to the pyrrolidinium-based OIPC N,N-diethylpyrrolidinium bis(fluorosulfonyl)amide ([C2epyr][FSA]). We then evaluated the Mg transference number (tMg2+), ionic conductivity, phase transition behaviour, and Mg-ion solvation number (with FSA anions) of the resultant materials to investigate how the Mg-salt concentration influences their ionic conduction characteristics and thermal stability. The [C2epyr][FSA]/Mg(FSA)2 composites were solid at room temperature when the Mg-salt concentration was below 10 mol% but became liquid at concentrations above 10 mol%. Among the composites investigated in this study, [C2epyr][FSA]/Mg(FSA)2 (10 mol%) showed the highest ionic conductivity, which reached 1.78 × 10-3 S cm-1 at 25 °C. When the Mg-ion concentration was above 10 mol%, the tMg2+ of [C2epyr][FSA]/Mg(FSA)2 was below 0.16. By contrast, when the Mg-salt concentration was below 10 mol%, the tMg2+ of [C2epyr][FSA]/Mg(FSA)2 was 0.26 or higher, surpassing those of the liquid-state composites. Cyclic voltammetry measurements revealed that [C2epyr][FSA]/Mg(FSA)2 (5 mol%) exhibited the redox behaviour of Mg at 60 °C. These results indicate that FSA-anion-based composites function more effectively as Mg-ion conductors in the solid state than in the liquid state.