Polymer electrolytes cation solvation

Quasi-solid-state electrolytes include gel polymer electrolytes, ionic liquids, and plastic crystal systems. It is important to distinguish polymer electrolytes and gel polymer electrolytes. In polymer electrolytes, charged cationic or anionic groups are chemically bonded to a polymer chain, while gel polymer electrolytes are solvated by a high dielectric constant solvent and are free to move. In a classical gel electrolyte, polymer and salts are mixed with a solvent, usually having a concentration above 50 wt%, and the role of the polymer is to act as a stiffener for the solvent, creating a three-dimensional network, where cations and anions move freely in the liquid phase [88]. The solid polymer electrolyte includes poly(ethylene oxide) (PEO)-based lithium ion conductors that typically show conductivities of 10 S cm while the gel polymer electrolytes have semisolid character with much higher ionic conductivities of the order 10 —10 S cm . ... 

PEO is found to be an ideal solvent for alkali-metal, alkaline-earth metal, transition-metal, lanthanide, and rare-earth metal cations. Its solvating properties parallel those of water, since water and ethers have very similar donicites and polarizabilities. Unlike water, ethers are unable to solvate the anion, which consequently plays an important role in polyether polymer-electrolyte formation. 

Therefore, an ideal polymer electrolyte must be flexible (associated with a low Tg), completely amorphous, and must have a high number of cation-coordination sites to assist in the process of salt solvatation and ion pair separation (see Table 11). A review on this subject has been recently published by Inoue [594]. 

The direct evidence on which our view of cation solvation in polymer electrolytes is based comes mainly from spectroscopic techniques. IR and Raman studies have been carried out on a variety of systems (see Chapter 5, Torell and Schantz, 1989 and Freeh, Manning, Teeters and Black, 1988). Low frequency vibrational modes, around 860-870 cm associated with the cation-ether oxygen interactions in PEG based systems have been observed they are absent in PEO itself... 

There is also a large group of ion-conducting organic polymer electrolytes (typically containing solvent or low-molecular-weight plasticizer) that are capable of solvating dissolved alkali metal cations (e.g., Li+). 

W. A. Henderson, N. R. Brooks, W. W. Brennessel, V. G. Young Jr., Chem. Mater. 2003,15, 4679-4684. Triglyme-li cation solvate structures Models for amorphous concentrated liquid and polymer electrolytes (1). 

The cations are linked to the polymer chain matrix as shown schematically in Figure 1.5. As a consequence, for a solvent- and moisture-free polymer electrolyte, solvated cations are not mobile. By contrast, in an aqueous electrolyte it is comparatively usual for a cation to be surrounded by a solvation sheath, often containing four or six water molecules, and for this entire entity to be mobile. Solvation does not arise in dry solid electrolytes such as alpha silver iodide. 

The similarity in the ionic transport mechanism in organic liquid electrolytes and solid polymer electrolytes is reflected in the ionic transport numbers measured in the two media. Table 3.5 lists the transport numbers for Li in LiC104 solutions in propylene carbonate (PC) and propylene carbonate/dimethoxy ethane (PC/DME) mixtures [26]. The t+ in PC/LiC104 is 0.28 which increases to between 0.40 and 0.50 with the addition of DME. This increase in t+ in PC/DME mixtures may reflect a change in the solvation characteristics of Li, and/or ionic species present, with the addition of DME. It is then possible that a range of cation transference numbers between 0.2 and 0.6 measured in polymer electrolytes is a reflection of the coordination properties of the particular polymer host with Li" and the nature of the ionic species present. 

The development and the application of polymeric composite materials filled with nanosized rigid particles (essentially inorganic) has attracted both scientific and industrial interest. The development of new polymer electrolytes is needed for many kinds of electrochemical applications such as separators in high-energy density lithium batteries. Poly(oxyethylene) (POE)-based polymer electrolytes are the most commonly studied, due to their cationic solvation ability [188]. 

Inherent properties of polymer electrolytes such as ease of elaboration with large surface/thickness ratio, absence of convection, specific mechanisms for ion solvatation, eventual control of the charge carriers anionic or cationic transport... shall certainly result in a multitude of electrochemical devices beyond the sole aspect of power generation. A particularly promising area will probably be the exploration of possible applications resulting from coupling ionically conductive polymers with numerous electronically conductive materials such as polypyrroles. 

Early work on polymer electrolytes indicated that polyethylene oxide (PEO) is a promising candidate as a host for lithium salt in relation to its ability to solvate Lithium cation [64-66]. This is attributed to its flexible ethylene oxide segments and ether oxygen atoms, which have a strong donor character and solvate lithium... 

In polymer electrolyte research one of the desired properties is high selective mobility of the cations (i.e. LT). A multitude of approaches for increasing this selective mobihty has been published, " including decreasing crystalhne content, inhibition of the anion mobility, solvating of cation in a mobile medium or introduction of inorganic fillers as referenced above. 

Most of the solid polymer electrolytes used can be classified as follows (1) polymer or gel matrixes swollen with liquid electrolyte solutions (e.g. ethylene carbonate (EC)/PAN/sodium perchlorate (NaC104)) (2) singleion systems in which only one ionic species is mobile within a polymer matrix (e.g. perfiuorosulphonate ionomer Nafion ) (3) solvent-free ion-coupled systems consisting of ion-solvating polymers mixed with salts, so that cations and ions become mobile within the polymer network, e.g. PEO mixed with salts. 

HPC exhibited a notable increase in adsorption with increasing NaCl concentration. Entrapment in the interlayer of recovered sodium montmorillonite did not vary with salinity the extent of entrapment was greater with the 4 M.S. HE and HP celluloses than either of the 2.0 M.S. polymers. Mixed ethers of HEC (2 M.S.) containing an anionic (carboxymethyl) or cationic (3-0-2-hydroxypropyltrimethylaramonium chloride) group at 0.4 M.S. levels did not adsorb from fresh water. Adsorption of these polar mixed ethers increased with increasing electrolyte until electrostatic and solvation effects were negated in 0.54N NaCl solutions and the adsorbed amounts typical of a 2 M.S. HEC were observed. Interlayer entrapments comparable to the equivalent M.S. HEC were observed at lower (0.18N) electrolyte concentrations. 

These cationic and anionic exchange polymers require that the mobile ions be well solvated with a polar solvent such as water. For applications such as the electrolyte phase in lithium batteries, an ionic conducting polymer is needed in which ionic mobility is obtained without the ions being solvated by water or some other solvent. This has been... 

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