Structure and dynamics in solid state sodium ion electrolytes based on organic ionic plastic crystals

 The investigation of solid state sodium ion electrolytes based on Organic Ionic Plastic Crystals were carried out for potential use in the electrochemical devices such as batteries.

Composite cell components for elevated temperature all-solid-state Li-ion batteries

Application of Li-ion batteries with liquid electrolytes at elevated temperatures (above 60°C) is limited due to the decomposition of the electrolyte. Stable solid state electrolytes can solve this problem, but the conductivity of these electrolytes are relatively low, the interfacial contacts with the electrodes are poor, and the charge transfer kinetics in the electrodes are limited. Solutions for these problems by using composite electrodes and electrolytes have been investigated and the results are described. A new concept for making all-solid-state Li-ion batteries that can be applied in the temperature range between room temperature and about 150°C will be presented.

Solid state lithium ion conduction in pyrrolidinium imide–lithium imide salt mixtures

The properties of the binary salt system based on mixtures of methyl ethyl pyrrolidinium bis(trifluoromethane sulfonyl) imide (P₁₂) and lithium bis(trifluoromethane sulfonyl) imide (Li imide) are reported. The lithium containing mixtures were found to be more than two orders of magnitude more conductive than the parent P₁₂ phase and the 33 mol% Li imide systems showed a solid state conductivity around 1×10⁻⁴ S/cm at 20°C. This solid state conductivity is believed to take place in plastic crystal phases of the P₁₂ compound.

Sodium ion dynamics in a sulfonate based ionomer system studied by 23Na solid-state nuclear magnetic resonance and impedance spectroscopy

A poly(2-acrylamido-2-methyl-1-propane-sulphonate) (PAMPS) ionomer containing both sodium and quaternary ammonium cations functionalised with an ether group, has been characterised in terms of its thermal properties, ionic conductivity and sodium ion dynamics. The ether oxygen was incorporated to reduce the Na+ association with the anionic sulfonate groups tethered to the polymer backbone, thereby promoting ion dissociation and ultimately enhancing the ionic conductivity. This functionalised ammonium cation led to a significant reduction in the ionomer Tg compared to an analogue system without an ether group, resulting in an increase in ionic conductivity of approximately four orders of magnitude. The sodium ion dynamics were probed by 23Na solid-state NMR, which allowed the signals from the dissociated (mobile) and bound Na+ cations to be distinguished. This demonstrates the utility of 23Na solid-state NMR as a probe of sodium dynamics in ionomer systems.

Probing ion exchange in the triflic acid-guanidinium triflate system: a solid-state nuclear magnetic resonance study

Knowledge of ion exchange and transport behavior in electrolyte materials is crucial for designing and developing novel electrolytes for electrochemical device applications such as fuel cells or batteries. In the present study, we show that, upon the addition of triflic acid (HTf) to the guanidinium triflate (GTf) solid-state matrix, several orders of magnitude enhancement in the proton conductivity can be achieved. The static 1H and 19F solid-state NMR results show that the addition of HTf has no apparent effect on local molecular mobility of the GTf matrix at room temperature. At higher temperatures, however, the HTf exhibits fast ion exchange with the GTf matrix. The exchange rate, as quantified by our continuum T2 fitting analysis, increases with increasing temperature. The activation energy for the chemical exchange process was estimated to be 58.4 kJ/mol. It is anticipated that the solid-state NMR techniques used in this study may be also applied to other organic solid-state electrolyte systems to investigate their ion-exchange processes.

In situ MRI of operating solid-state Lithium metal cells based on ionic plastic crystal electrolytes

Solid-state ion conductors based on organic ionic plastic crystals (OIPCs) are a promising alternative to conventional liquid electrolytes in lithium battery applications. The OIPC-based electrolytes are safe (nonflammable) and flexible in terms of design and operating conditions. Magnetic resonance imaging (MRI) is a powerful noninvasive method enabling visualization of various chemical phenomena. Here, we report a first quantitative in situ MRI study of operating solid-state lithium cells. Lithium ion transfer into the OIPC matrix during the ongoing discharge of the anode results in partial liquefaction of the electrolyte at the metal interface. The developed liquid component enhances the ion transport across the interface and overall battery performance. Displacement of the liquefaction front is accompanied by a faster Li transfer through the grain boundaries and depletion at the cathode. The demonstrated solid-liquid hybrid properties, inherent in many OIPCs, combine benefits of highly conductive ionic liquids with safety and flexibility of solids.

Oligomethylene-bridged dinuclear triorganotin triflates and diphenylphosphinates. Ion pairing in the solid state and electrolytic dissociation in solution of [Ph2Sn(CH2)nSnPh2X](O3SCF3) (X = OH, O2PPh2; n = 1-3)

The condensation of [Ph₂(OH)Sn(CH₂)_nSn(OH)Ph₂] (1-3; n = 1-3) with HO₃SCF₃ and HO₂PPh₂ provided [Ph₂Sn(CH₂)_nSnPh₂(OH)](O₃SCF₃) (4-6; n = 1-3) and [Ph₂(O₂PPh₂)Sn(CH₂)_nSn(O₂PPh₂)Ph₂] (10-12; n = 1-3), respectively. The reaction of [Ph₂Sn(CH₂)_nSnPh₂(OH)](O₃SCF₃) (4-6; n = 1-3) with HO₂PPh₂ and NaO₂PPh₂ gave rise to the formation of [Ph₂Sn(CH₂)_nSnPh₂(O₂PPh₂)](O₃SCF₃) (7-9; n = 1-3) and [Ph₂(OH)Sn(CH₂)_nSn(O₂PPh₂)Ph₂] (13-15; _n = 1-3), respectively. In the solid state, compounds 4-9 comprise ion pairs of cationic cyclo-[Ph₂SnCH₂SnPh₂(OH)]₂²⁺, cyclo-[Ph₂Sn(CH₂)_nSnPh₂(OH)]⁺ (n = 2, 3), and cyclo-[Ph₂Sn(CH₂)_nSnPh₂(O₂PPh₂)]⁺ (n = 1-3) and triflate anions. In MeCN, the eight-membered-ring system cyclo-[Ph₂SnCH₂SnPh₂(OH)]₂²⁺ appears to be in equilibrium with the four-membered-ring system cyclo-[Ph₂SnCH₂SnPh₂(OH)]⁺. In contrast, compounds 10-15 show no ionic character. Compounds 1-15 were characterized by multinuclear NMR spectroscopy in solution and in the solid state, IR spectroscopy, conductivity measurements, electrospray mass spectrometry, osmometric molecular weight determinations, and X-ray crystallography (4, 5, 7, and 12).

Cation dynamics in dimethyl-pyrrolidinium-based solid-state ion conductors

Dimethyl-pyrrolidinum-based salts have been investigated by means of DSC, conductivity, NMR and Raman spectroscopy. The investigation aims to study the effect of the anion on the behaviour of the salt, in terms of plastic properties as well as rotational degrees of freedom of the cation. The materials range from the non-plastic iodide salt to the highly plastic BF₄ salt, which flows under its own weight at elevated temperatures. The different rotational and translational motions of the cations, and the difference between rotator and plastic phases are discussed.

Solid state ion transport and phase behaviour in composites of N,N-methyl propylpyrrolidinium tetrafluoroborate and amorphous polyethylene oxide

The binary and ternary addition of 2 wt.% LiBF₄ and 2 wt.% amorphous polyethylene oxide (aPEO) respectively to the plastic crystal forming salt P₁₃BF₄ (where P₁₃⁺=methylpropyl pyrrolidinium cation) was investigated with specific focus on the phase behaviour and evaluation of transport characteristics. Differential scanning calorimetry (DSC), optical thermomicroscopy, solid state nuclear magnetic resonance (NMR), and AC impedance spectroscopy were used to develop an understanding of the conduction process in the pure and mixed systems. The morphology of the ternary compound appeared as hexagonal spherulites upon solidification. Multinuclear NMR Pulsed Field Gradient measurements (¹H,¹⁹F,⁷Li) to probe both cation and anion diffusion coefficients are reported. The anion is shown to be the most diffusive (at 320 K:¹⁹F=2.5×10⁻¹¹ m² s⁻¹; 1H: 1.8×10⁻¹¹ m² s⁻¹; ⁷Li: 1.1×10⁻¹¹ m² s⁻¹) in the ternary compound, with enhanced conductivity (2.7×10⁻⁵ S cm⁻¹ at 310 K) just below the melt.

Plastic crystal electrolyte materials : new perspectives on solid state ionics

Plastic crystal materials have long been known but have only relatively recently become of interest as solid–state ion conductors. Their properties are often associated with dynamic orientational disorder or rotator motions in the crystalline lattice. This paper describes recent work in the field including the range of organic ionic compounds that exhibit ion conduction at room temperature. Conductivity in some cases is high enough to render the compounds of interest as electrolyte materials in all solid state electrochemical devices. Doping of the plastic crystal phase with a small ion such as Li+ in some cases produces an even higher conductivity. In this case the plastic crystal acts as a solid state “solvent” for the doped ion and supports the conductive motion of the dopant via motions of the matrix ions. These doped materials are also described in detail.

Solid state actuators based on polypyrrole and polymer-in-ionic liquid electrolytes

Novel polymer-in-ionic liquid electrolytes (PILEs) have been developed for solid state electrochemical actuators based on polypyrrole. The active polymer electrodes are readily oxidized/reduced without degradation in the PILE. It was found that the actuator cycle life is significantly enhanced in the PILE as is the ‘shelf life’ of the device.

Lithium doped N,N-dimethyl pyrrolidinium tetrafluoroborate organic ionic plastic crystal electrolytes for solid state lithium batteries

The organic ionic plastic crystal material N,N-dimethyl pyrrolidinium tetrafluoroborate ([C1mpyr][BF4]) has been mixed with LiBF4 from 0 to 8 wt% and shown to exhibit enhanced ionic conductivity, especially in the higher temperature plastic crystal phases (phases II and I). The materials retain their solid state well above 100 °C with the melt not being observed up to 300 °C. Interestingly the conductivity enhancement is highest with the lowest level of LiBF4 addition in phase II, but then the order of enhancement is reversed in phase I. In all cases, a conductivity drop is observed at the II → I phase transition (105 °C) which is associated with increased order in the pure matrix, as previously reported, although the conductivity drop is least for the highest LiBF4 amount (8 wt%). The 8 wt% sample displays different conductivity behaviours compared to the lower LiBF4 concentrations, with a sharp increase above 50 °C, which is apparently not related to the formation of an amorphous phase, based on XRD data up to 120 °C. Symmetric cells, Li/OIPC/Li, were prepared and cycled at 50 °C and showed evidence of significant preconditioning with continued cycling, leading to a lower over-potential and a concomitant decrease in the cell resistivity as measured by EIS. An SEM investigation of the Li/OIPC interfaces before and after cycling suggested significant grain refinement was responsible for the decrease in cell resistance upon cycling, possibly as a result of an increased grain boundary phase.

Organic ionic plastic crystal electrolytes; a new class of electrolyte for high efficiency solid state dye-sensitized solar cells

Dye-sensitized solar cells are an increasingly promising alternative to conventional silicon solar cells as a method of converting solar energy to electricity and thus providing an effectively inexhaustible energy source. However, the most efficient of these devices currently utilize liquid electrolytes, which suffer from the associated problems of leakage and evaporation. Hence, significant research is currently focused on the development of solid state alternatives. Here we report a new class of solid state electrolyte for these devices, organic ionic plastic crystal electrolytes, that allow relatively rapid diffusion of the redox couple through the matrix, which is critical to the cell performance. A range of different organic ionic plastic crystal materials, utilizing different cation and anion structures, have been investigated and the conductivities, diffusion rates and photovoltaic performance of the electrolytes are reported. The best material, utilizing the dicyanamide anion, achieves efficiencies of more than 5%.

Thin and flexible solid-state organic ionic plastic crystal-polymer nanofibre composite electrolytes for device applications

All solid-state organic ionic plastic crystal–polymer nanofibre composite electrolytes are described for the first time. The new composite materials exhibit enhanced conductivity, excellent thermal, mechanical and electrochemical stability and allow the production of optically transparent, free-standing, flexible, thin film electrolytes (10’s lms thick) for application in electrochemical devices. Stable cycling of a lithium cell incorporating the new composite electrolyte is demonstrated, including cycling at lower temperatures than previously possible with the pure material.