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.

Lithium-doped plastic crystal electrolytes exhibiting fast ion conduction for secondary batteries

Rechargeable lithium batteries have long been considered an attractive alternative power source for a wide variety of applications. Safety and stability1 concerns associated with solvent-based electrolytes has necessitated the use of lithium intercalation materials (rather than lithium metal) as anodes, which decreases the energy storage capacity per unit mass. The use of solid lithium ion conductors - based on glasses, ceramics or polymers - as the electrolyte would potentially improve the stability of a lithium metal anode while alleviating the safety concerns. Glasses and ceramics conduct via a fast ion mechanism, in which the lithium ions move within an essentially static framework. In contrast, the motion of ions in polymer systems is similar to that in solvent-based electrolytes - motion is mediated by the dynamics of the host polymer, thereby restricting the conductivity to relatively low values. Moreover, in the polymer systems, the motion of the lithium ions provides only a small fraction of the overall conductivity2, which results in severe concentration gradients during cell operation, causing premature failure3. Here we describe a class of materials, prepared by doping lithium ions into a plastic crystalline matrix, that exhibit fast lithium ion motion due to rotational disorder and the existence of vacancies in the lattice. The combination of possible structural variations of the plastic crystal matrix and conductivities as high as 2 3 1024 S cm21 at 60 8C make these materials very attractive for secondary battery applications.

Ftir study of ion-pairing effects in plasticized polymer electrolytes

The effect of plasticizer on the ubiquitous ion-pairing observed in polymer electrolytes has been investigated using FTIR as a probe of the local environment of the triflate ion in sodium and lithium triflate based electrolytes. Plasticizers having a range of properties, such as, propylene carbonate, and dimethyl formamide (DMF), have been investigated in the pure state for comparison with the polymer (a random copolymer of ethylene oxide at propylene oxide (mol ratio 3: 1)). The different plasticizers exhibited strikingly different effects on the triflate ion bands normally observed in polyether salt systems. In particular, the cation associated triflate ion bands at 1288 and 1248 cm⁻¹ and the band at 1272 cm⁻¹ which has variously been assigned to the free ion and also to the strongly aggregated anion, are different. PC produces a rapid disappearance of the “free” ion band in favour of the monodentate ion pair. On the other hand, DMF strongly enhances the band near 1270 cm⁻¹ at salt concentrations higher than 0.7 mol kg⁻¹. These observations are discussed in terms of recent ab initio calculations of the triflate vibrational bands.

Pyrrolidinium imides : a new family of molten salts and conductive plastic crystal phases

A new family of molten salts is reported, based on the N-alkyl, N-alkyl pyrrolidinium cation and the bis(trifluoromethane sulfonyl)imide anion. Some of the members of the family are molten at room temperature, while the smaller and more symmetrical members have melting points around 100 °C. Of the room-temperature molten salt examples, the methyl butyl derivative exhibits the highest conductivity; at 2 × 10^-3 S/cm this is the highest molten salt conductivity observed to date at room temperature among the ammonium salts. This highly conductive behavior is rationalized in terms of the role of cation planarity. The salts also exhibit multiple crystalline phase behavior below their melting points and exhibit significant conductivity in at least their higher temperature crystal phase. For example, the methyl propyl derivative (mp = 12 °C) shows ion conductivity of 1 × 10^-6 S/cm at 0 °C in its higher temperature crystalline phase.

Polymer electrolytes for electrochromic window applications

Polymer electrolytes suitable for use in electrochromic window devices for architectural applications have been developed. The electrolyte properties required for a large area, long lifetime, long cycle life and wide temperature range application such as this place some restrictions on the type of polymer system required. The electrolytes developed in this work are based on LiClO₄ in a polyether copolymer which is cured in place to produce a clear elastomer. The material properties and conductivity of electrolytes with and without the plasticizers are presented. Electrochromic device performance tests show that the electrolyte impedance is a factor in the device performance, but not the limiting factor.

Structure-property relationships in plasticized solid polymer electrolytes

The addition of various kinds of plasticizers can enhance the conductivity of polymer electrolyte systems, in some cases by many orders of magnitude. The plasticizer may be a low molecular weight solvent, or be a low molecular weight polymer. As the plasticizer concentration increases there is an inevitable deterioration in material properties. In this work we have investigated the effect of plasticizer on the conductivity, thermal properties and matrial properties of a number of systems including urethane cross-linked polyethers and polyacrylates. In some of the systems, in particular the polyether electrolytes, the plasticizer acts to enhance conduction by acting as a cosolvent for the salt as well as increasing chain flexibility. Its efficacy is dependent on its structure and characteristics as a solvent. Although T_g is lowered in a close to linear fashion with increasing plasticizer content and thereby conductivity increased rapidly, the elastic modulus changes more slowly. This reflects the coupling of conduction to the local mobility of the molecular units of the combined solvent system and the relative decoupling of the mobility and glass transition from the material properties. In these systems the latter are a function mainly of the longer range structure of the polymer network. The changes in conductivity and materials properties are interpreted in terms of a configurational entropy model of the solution.

A nuclear magnetic resonance study of 7Li dynamics in polyether-urethane based solid polymer electrolytes

Nuclear magnetic resonance (NMR) is a technique that allows the probing of the dynamics of specific magnetically active nuclei. In the present study a polyethylene glycol network containing varying concentrations of LiClO₄ have been studied using ⁷Li NMR relaxation techniques. A plasticiser, tetraglyme, has been added to several samples to improve the mobility of the polymer and thus of the ionic species. The effects of tetraglyme and salt concentration on the cationic mobility and environment have been investigated using T₁ and T₂ relaxation experiments, with the presence of two cationic species of differing relaxation times (and possibly mobility) reported. The results are discussed with relevance to conductivity measurements made on similar samples.

Composition effects in polyetherurethane-based solid polymer electrolytes

Nuclear magnetic resonance spectroscopy (n.m.r.), dynamic mechanical thermal analysis (d.m.t.a.) and AC impedance techniques have been used in combination to probe the effect of electrolyte composition in an archetypal solid polymer electrolyte (SPE). A series of solid polymer electrolytes (SPEs) based on a urethane-crosslinked trifunctional poly(ethylene glycol) polymer host containing dissolved ionic species (LiClO₄ and LiCF₃SO₃) have been studied. D.m.t.a. has established that increasing LiClO₄ concentration causes a decrease in the polymer segmental mobility, owing to the formation of transient crosslinks via cation-polymer interaction. Investigation of the distribution of mechanical/structural relaxation times for the LiClO₄/polymer complex with d.m.t.a. reveals that increasing LiClO₄ concentration causes a slight broadening of the distribution, indicating a more heterogeneous environment. Results of n.m.r. ⁷Li T₁ and T₂ relaxation experiments support the idea that higher salt concentrations encourage ionic aggregation. This is of critical importance in determining the conductivity of the material since it affects the number of charge carriers available. Introduction of the plasticiser tetraglyme into the LiClO₄-based SPEs suppresses the glass transition temperature of the SPE, and causes a significant broadening of the relaxation time distribution (as measured by d.m.t.a.).

Dynamic mechanical thermal analysis studies of factors influencing relaxation in polyetherurethane based solid electrolytes

Dynamic mechanical thermal analysis (DMTA) has been used to study the effects of plasticizers on the mobility and homogeneity of a series of solid polymer electrolytes (SPEs). With reference to previously published results on similar systems containing LiClO₄ salts and tetraglyme as plasticizer, the effects of propylene carbonate (PC) on the glass transition temperature (T_g) of the SPE and on the distribution of relaxation times within the sample are discussed; at low plasticizer concentration PC has little effect on T_g as measured by DMTA in comparison with tetraglyme, and at higher plasticizer concentrations PC significantly broadens the mechanical relaxation behaviour indicating a greater degree of dynamical heterogeneity within the sample. A second low temperature relaxation is evident at lower PC contents indicating that some regions of this plasticized SPE are distinctly more mobile than others or perhaps, on this length scale, that some degree of phase separation is present. Activation energies for the mechanical relaxation were also determined as a function of PC concentration and are significantly greater than those determined from conductivity measurements.

Simulations of structure and transport in polymer electrolytes

Simulations implementing both Monte Carlo (MC) and molecular dynamics (MD) techniques were used to explore various aspects of polymer electrolytes. Evidence is presented to support the conclusion that collective behavior of ions determines much of the behavior of these complex materials. Simple theories attributing ion transport to either single ions or clusters of three ions are inadequate to explain ion transport behavior; in particular, the Nernst-Einstein relation commonly used to discuss polymer electrolytes is almost certainly quantitatively inappropriate for these materials.