Yuri G. Andreev

Increasing the conductivity of crystalline polymer electrolytes

Polymer electrolytes consist of salts dissolved in polymers (for example, polyethylene oxide, PEO), and represent a unique class of solid coordination compounds. They have potential applications in a diverse range of all-solid-state devices, such as rechargeable lithium batteries, flexible electrochromic displays and smart windows. For 30 years, attention was focused on amorphous polymer electrolytes in the belief that crystalline polymer:salt complexes were insulators. This view has been overturned recently by demonstrating ionic conductivity in the crystalline complexes PEO6:LiXF6 (X = P, As, Sb); however, the conductivities were relatively low. Here we demonstrate an increase of 1.5 orders of magnitude in the conductivity of these materials by replacing a small proportion of the XF6 - anions in the crystal structure with isovalent N(SO2CF3)2 - ions. We suggest that the larger and more irregularly shaped anions disrupt the potential around the Li+ ions, thus enhancing the ionic conductivity in a manner somewhat analogous to the AgBr1-x I x ionic conductors. The demonstration that doping strategies can enhance the conductivity of crystalline polymer electrolytes represents a significant advance towards the technological exploitation of such materials.

Structure of the polymer electrolyte poly(ethylene oxide)6:LiAsF6

Polymer electrolytes—salts (such as LiCF3SO3) dissolved in solid, high-molar-mass polymers (for example, poly(ethylene oxide), PEO),,—hold the key to the development of all-solid-state rechargeable lithium batteries. They also represent an unusual class of coordination compounds in the solid state. Conductivities of up to 10−4 S cm−1 may be obtained, but higher levels are needed for applications in batteries,,. To achieve such levels requires a better understanding of the conduction mechanism, and crucial to this is a knowledge of polymer-electrolyte structure. Crystalline forms of polymer electrolytes are obtained at only a few discrete compositions. The structures of 3 : 1 and 4 : 1 complexes (denoting the ratio of ether oxygens to cations) have been determined,,. But the 6 : 1 complex is of greater interest as the conductivity of polymer electrolytes increases significantly on raising the polymer content from 3 : 1 to 6 : 1 (refs 10, 11). Furthermore, many highly conducting polymer-electrolyte systems form crystalline 6 : 1 complexes whereas those with lower conductivities do not. Here we report the structure of the PEO:LiAsF6 complex with a 6 : 1 composition. Determination of the structure was carried out abinitio by employing a method for flexible molecular structures, involving full profile fitting to the X-ray powder diffraction data by simulated annealing. Whereas in the 3 : 1 complexes the polymer chains form helices, those in the 6 : 1 complex form double non-helical chains which interlock to form a cylinder. The lithium ions reside inside these cylinders and, in contrast to other complexes, are not coordinated by the anions.

Polymer electrolyte structure and its implications

There remains intense interest in developing solid polymer electrolytes, free from low molecular weight plasticiser and with a sufficiently high ionic conductivity for application in all-solid-state rechargeable lithium batteries. For such applications, conductivities above the present maximum of 10−4 S cm−1 are required. Innovative designs of polymers and salts which suppress crystallinity and yield amorphous polymer electrolytes with a low Tg have led to substantial improvements in the level of ionic conductivity compared with early systems, but the above mentioned barrier remains. We promote the view that it is important now to change our thinking concerning how to optimise ionic conductivity. We emphasise the importance of understanding the structure of polymer electrolytes in order to better understand the ion transport mechanism. Such studies indicate the importance of organising polymer chains while preserving chain dynamics. Recent evidence is presented from the work of others, supporting the view that more structured polymer electrolytes can lead to enhanced ionic conductivity. En route to this view, we present the crystal structures of several polymer salt complexes including the first structure of a 6:1 complex PEO6:LiAsF6.

Alkali metal crystalline polymer electrolytes

Polymer electrolytes have been studied extensively because uniquely they combine ionic conductivity with solid yet flexible mechanical properties, rendering them important for all-solid-state devices including batteries, electrochromic displays and smart windows. For some 30 years, ionic conductivity in polymers was considered to occur only in the amorphous state above Tg. Crystalline polymers were believed to be insulators. This changed with the discovery of Li(+) conductivity in crystalline poly(ethylene oxide)(6):LiAsF(6). However, new crystalline polymer electrolytes have proved elusive, questioning whether the 6:1 complex has particular structural features making it a unique exception to the rule that only amorphous polymers conduct. Here, we demonstrate that ionic conductivity in crystalline polymers is not unique to the 6:1 complex by reporting several new crystalline polymer electrolytes containing different alkali metal salts (Na(+), K(+) and Rb(+)), including the best conductor poly(ethylene oxide)(8):NaAsF(6) discovered so far, with a conductivity 1.5 orders of magnitude higher than poly(ethylene oxide)(6):LiAsF(6). These are the first crystalline polymer electrolytes with a different composition and structures to that of the 6:1 Li(+) complex.

Factors influencing the conductivity of crystalline polymer electrolytes

Crystalline polymer electrolytes conduct, in contrast to the established view for 30 years. The crystalline polymer poly(ethylene oxide)6:LiXF6, X = P, As, Sb is composed of tunnels formed from pairs of (CH2-CH2-O)n chains, within which the Li+ ions reside and along which they may migrate. The anions are located outside the tunnels. PEO6:LiXF6 formed from PEO of average molecular weight 1000 Da has an average chain length of 40 A compared with a typical crystallite size of 2500 angstroms, hence low molecular weight materials have many chain ends within a crystallite. More chain ends increase conductivity. Materials composed of polydispersed PEO (chains of different lengths) of average molecular weight 1000 Da exhibit a conductivity one order of magnitude greater than monodispersed materials of the same molecular weight. Replacing the -OCH3 groups on the chain ends with -OC2H5 increases the conductivity by a further order of magnitude. Conductivity may also be increased by isovalent or aliovalent doping of the 6:1 complexes in which XF6- is replaced by N(SO2CF3)2- or SiF6(2-), respectively.

Using crystallography to understand polymer electrolytes

Crystallography provides a unique insight into the structural properties of polymer electrolytes. Knowledge of the crystal structures helps in the understanding of the structural chemistry of such electrolytes and facilitates the design of electrolytes with a higher conductivity for applications in new devices such as all-solid-state batteries. In the present paper we present a detailed survey of the structural properties of all the crystalline phases of poly(ethylene oxide)-based polymer electrolytes along with various approaches to structure elucidation from powder diffraction data. The inter-relationship between the crystal structure and conductivity of the polymer electrolytes is also discussed.

The Shape of TiO2-B Nanoparticles

The shape of nanoparticles can be important in defining their properties. Establishing the exact shape of particles is a challenging task when the particles tend to agglomerate and their size is just a few nanometers. Here we report a structure refinement procedure for establishing the shape of nanoparticles using powder diffraction data. The method utilizes the fundamental formula of Debye coupled with a Monte Carlo-based optimization and has been successfully applied to TiO2-B nanoparticles. Atomistic modeling and molecular dynamics simulations of ensembles of all the ions in the nanoparticle reveal surface hydroxylation as the underlying reason for the established shape and structural features.

Demonstrating Structural Deformation in an Inorganic Nanotube

There is much current interest in nanostructured materials (nanotubes, nanobelts, nanospheres, etc.). Their crystal structures can differ from those of the equivalent bulk materials. Determining these differences is important in understanding how the properties of nanomaterials differ from those of the bulk. Established methods of X-ray structure determination become increasingly difficult or impossible to apply on reducing the dimensions to a few nanometers. Here we show that, by combining the Debye equation for X-ray scattering (which relates an ensemble of atoms to their diffraction pattern without recourse to symmetry) with a model of the crystal structure, generated by folding the ideal crystal structure into a nanotube, the severely broadened/distored powder diffraction pattern may be described. This procedure reveals the significant structural deformations necessary to accommodate the nanotube shape. The importance of knowing the (deformed) crystal structure is discussed.

Synthesis of Poly(ethylene oxide) Approaching Monodispersity

Polydispersity in polymers hinders fundamental understanding of their structure-property relationships and prevents them from being used in fields like medicine, where polydispersity affects biological activity. The polydispersity of relatively short-chain poly(ethylene oxide) [(CH2CH2O2)n; PEO] affects its biological activity, for example, the toxicity and efficacy of PEOylated drugs. As a result, there have been intensive efforts to reduce the dispersity as much as possible (truly monodispersed materials are not possible). Here we report a synthetic procedure that leads to an unprecedented low level of dispersity. We also show for the first time that it is possible to discriminate between PEOs differing in only 1 ethylene oxide (EO) unit, essential in order to verify the exceptionally low levels of dispersity achieved here. It is anticipated that the synthesis of poly(ethylene oxide) approaching monodispersity will be of value in many fields where the applications are sensitive to the distribution of molar mass.

Solving crystal structures of molecular solids without single crystals: a simulated annealing approach

The ab initio determination of relatively complex crystal structures of flexible molecules without the need for single crystals is discussed. A method is described based on simulated annealing in which the powder diffraction patterns of randomly generated trial structures are calculated and compared with the observed powder diffraction pattern in order to identify the model which provides the best fit and therefore the true structure. By employing simulated annealing both downhill (improved fit) and uphill (reduced fit) moves are possible ensuring escape from local minima in order to find the global minimum in the goodness-of-fit, i.e. the true structure. Key to the successful solution of flexible molecules is the introduction of a geometrical description which specifies atomic positions within the unit cell in terms of bond lengths and angles. In this way only those random structures which are chemically plausible are generated, greatly reducing the number of trial structures and rendering tractable the otherwise impossible task of ab initio determination. It is shown that structures with 37 variable parameters can be solved from only a few milligrams of powder. The limits of structural complexity for this method should be similar to those for refinement using powder data, i.e. around 200 variables. The variables may be those of position, or orientation of the molecule(s) in the unit cell as well as bond lengths, bond angles or torsion angles.