Crystalline polymer electrolyte conductivity

Table 1. Some imide ions and carbanions used in salts to enhance polymer electrolyte conductivity and reduce crystallinity...

Polymer electrolytes conduct cations by segmental motions of the polymer backbone that carry the cations from one complexation site to the next.60-62 Since this requires significant fluidity, the polymers are conductive only in the amorphous state, i.e. above the crystalline to gel transition temperature. For the bulk polymer of PHB, this temperature is in the range of 0° to 10 °C. Accordingly, single molecules of PHB dissolved in fluid lipid bilayers should be capable of considerable segmental motions at physiological temperatures. 

Gadjourova Z, Andreev Y, Tunstall D, Bruce P (2001) Ionic conductivity in crystalline polymer electrolytes. Nature 412 520-523... 

Christie AM, Lilley SJ, Staunton E et al (2005) Increasing the conductivity of crystalline polymer electrolytes. Nature 433 50-53... 

Cyclodexfrins have various configurations. When the a-form is added to a crystalline polymer electrolyte consisting of PEO and LiAsF, nanochannels are formed by the a-cyclodextrin, confining the PEO/Li+ complexes. The nanochannels provide pathways for the directional motion of LP ions and at the same time prevent the access of anions by size exclusion. As a result, the ionic conductivity of the electrolyte is 30 times higher than that of the comparable PEO/L1+ complex crystal at room temperature [24]. 

Figure 1.14 Temperature variation of the conductivity for selected crystalline polymer electrolytes. Unless stated otherwise, all complexes were prepared with 1000 Da PEO end-capped with -CH3. G4 is CH3-(CH2CH20)4-CH3...

There is increasing interest in sodium-based batteries, because of the greater abundance and lower costs of sodium compared with lithium. Recently the first examples of crystalline polymer electrolytes that support Na" " conduction have been reported.Potassium and rubidium based crystalline polymer electrolytes have also been described. " This result is significant not only because these are the first Na, K" ", Rb, crystalline polymer electrolytes but also because they are the first example with a crystal structure that differs from the PEOg LiXF6 complexes. In the former case, the ratio of ether oxygens to cations is 8 1 rather than 6 1 and only one polymer chain wraps around the cations (Figure 1.15). 

GADJOUROVA z, ANDREEV Y G, TUNSTALL D p and BRUCE p G, lonlc Conductivity in crystalline polymer electrolytes . Nature, 2001, 412, 520-523... 

The polymer electrolytes discussed so far suffer from a number of disadvantages. Firstly, they exhibit low conductivities in comparison with liquid or solid (crystalline or glassy) electrolytes at or below room temperature. The best all-amorphous systems have conductivities less than 10"4 S cm-1 at room temperature. These ambient... 

In polymer electrolytes (even prevailingly crystalline), most of ions are transported via the mobile amorphous regions. The ion conduction should therefore be related to viscoelastic properties of the polymeric host and described by models analogous to that for ion transport in liquids. These include either the free volume model or the configurational entropy model . The former is based on the assumption that thermal fluctuations of the polymer skeleton open occasionally free volumes into which the ionic (or other) species can migrate. For classical liquid electrolytes, the free volume per molecule, vf, is defined as ... 

These materials are introduced in Chapter 5 and only brief mention of them is necessary here. It is important to appreciate that polymer electrolytes, which consist of salts, e.g. Nal, dissolved in solid cation coordinating polymers, e.g. (CH2CH20) , conduct by quite a different mechanism from crystalline or glass electrolytes. Ion transport in polymers relies on the dynamics of the framework (i.e. the polymer chains) in contrast to hopping within a rigid framework. Intense efforts are being made to make use of these materials as electrolytes in all solid state lithium batteries for both microelectronic medical and vehicle traction applications. 

One method of reducing crystallinity in PEO-based systems is to synthesize polymers in which the lengths of the oxyethylene sequences are relatively short, such as through copolymerization. The most notable hnear copolymer of this type is oxymethylene-linked poly(oxyethylene), commonly called amorphous PEO, or aPEO for short. Other notable polymer electrolytes are based upon polysiloxanes and polyphosphazenes. Polymer blends have also been used for these applications, such as PEO and poly (methyl methacrylate), PMMA. The general performance characteristics of the polymer electrolytes are to have ionic conductivities in the range of cm) or (S/cm). 

There are two classes of materials which may be used as electrolytes in all-solid-state cells polymer electrolytes, materials in which metal salts are dissolved in high molar mass coordinating macromolecules or are incorporated in a polymer gel, and ceramic crystalline or vitreous phases which have an electrical conductance wholly due to ionic motion within a lattice structure. The former were described in Chapter 7 in this... 

Solvent-free polymer-electrolyte-based batteries are still developmental products. A great deal has been learned about the mechanisms of ion conductivity in polymers since the discovery of the phenomenon by Feuillade et al. in 1973 [41], and numerous books have been written on the subject. In most cases, mobility of the polymer backbone is required to facilitate cation transport. The polymer, acting as the solvent, is locally free to undergo thermal vibrational and translational motion. Associated cations are dependent on these backbone fluctuations to permit their diffusion down concentration and electrochemical gradients. The necessity of polymer backbone mobility implies that noncrystalline, i.e., amorphous, polymers will afford the most highly conductive media. Crystalline polymers studied to date cannot support ion fluxes adequate for commercial applications. Unfortunately, even the fluxes sustainable by amorphous polymers discovered to date are of marginal value at room temperature. Neat polymer electrolytes, such as those based on poly(ethyleneoxide) (PEO), are only capable of providing viable current densities at elevated temperatures, e.g., >60°C. 

Because the conductivity of polymer electrolytes is generally low, thin batteries are assembled (50-200 pm) with electrolyte thickness ranging from 20 to 50 pm. Conventional polymer electrolytes based on PEO and lithium salts, owing to their high crystallinity, reach useful conductivity values only at temperatures above 60 °C, i.e., above the melting temperature of the crystalline phase. If the low conductivity at room temperature prevents their application for consumer electronics, this does not represent an obstacle for electric vehicles, for which an operating temperature higher than that of the transition in the amorphous phase is expected. 

Although PEO is an excellent solvent for the solvation of alkali metal ions, polymer electrolytes derived from pure PEO-metal salt complexes do not show high ionic conductivities at ambient temperatures, due to the partial crystalline nature of PEO [27,29,37,59,79] (vide supra). 

In general the effect of the added organic plasticizer appears to increase the free volume of the polymer thereby decreasing the Tg [93] and or reducing the content of the crystalline phase in PEO [83] and also to effect the ionic association in the polymer electrolytes [81]. Many of these effects have been studied by use of a variety of experimental methods such as IR spectroscopy, DSC, EXAFS, X-ray diffraction, NMR, conductivity studies, viscosity measurements etc. [81, 90,93-103]. The effects of the plasticizers on the conductivity behavior of PEO polymer electrolytes along with the conductivity data of other PEO-polymer electrolytes discussed above are summarized in Table 2. 

Various methods have been employed to find out about the structure of polymer electrolytes. These include thermal methods such as differential scanning calorimetry (DSC), differential thermal analysis (DTA), X-ray methods such as X-ray diffraction and X-ray absorption fine structure (XAFS), solid state NMR methods particularlyusing7LiNMR,andvibrationalspectroscopicmethodssuch as infrared and Raman [27]. The objective of these various studies is to establish the structural identity of the polymer electrolyte at the macroscopic as well as the molecular levels. Thus the points of interest are the crystallinity or the amorphous nature of materials, the glass transition temperatures, the nature and extent of interaction between the added metal ion and the polymer, the formation of ion pairs etc. Ultimately the objective is to understand how the structure (macroscopic and molecular) of the polymer electrolyte is related to its behavior particularly in terms of ionic conductivity. Most of the studies have been carried out, quite understandably, on PEO-metal salt complexes. In comparison, there has been no attention on the structural aspects of the other polymers particularly at the molecular level. 

Other polymers such as MEEP, poly MEEM A and polysiloxanes are completely amorphous polymers and remain amorphous even in the polymer-metal salt complexes at least upto certain concentrations of the metal salt. The question of the amorphous nature of the polymer electrolytes is important in view of Berthi-er s demonstration in the PEO-polymer electrolyte system that the ionic conductivity occurs mostly in the amorphous phase [59]. Thus the room temperature ionic conductivities of the completely amorphous MEEP-LiX complexes is at least three orders of magnitude higher than the corresponding PEO-LiX complexes. This has led to the use of plasticizers and other modifications to suppress crystallinity and increase free volume as discussed above. 

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