Crystallinity highly conductive polymer electrolyte

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. 

The idea that ions can diffuse as rapidly in a solid as in an aqueous solution or in a molten salt may seem astonishing. However, since the 1960s, a variety of solids that include crystalline compounds, glasses, polymers, and composite materials with exceptionally high ionic conductivities have been discovered. Materials that conduct anions (e.g. and 0 ) and cations including monovalent (e.g. H+, Fi+, Na+, Cu+, Ag+), divalent, and even trivalent and tetravalent ions have been synthesized. A variety of names that have been used for these materials include solid electrolytes, superionic conductors, and fast-ionic conductors. Solid electrolytes arguably provides the least misleading and broadest description for this class of materials. 

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). 

Atomic force microscopy (AFM) and electrochemical atomic force microscopy (ECAFM) have proven usefiil for the study of nucleation and growth of electrodeposited CP films on A1 alloy [59]. AFM was used to study adhesion between polypyrrole and mild steel [60], whereas electric force microscopy (EFM) has been used to study local variations in the surface potential (work function) of CP films [61]. AFM with a conductive tip permits a nanoscale AC impedance measurement of polymer and electrolyte interfaces, permitting differentiation between highly conductive amorphous regions and less-conductive crystalline regions of the CP film [62]. 

In terms of crystalline structure, this means that a channel of sufficient size must link the accessible sites in order that ion diffiision is not hindered if channels are too large with respect to the ion radius, ions can be trapped. In polymers, the region of high conductivity is found in the homogeneous, elastomeric amorphous phase and the presence of partial crystallinity inhibits conduction. The motion of mobile ions is strongly linked to that of polymer segments and ionic conductivity drops to extremely low values below 7 where the chain segment motions are frozen. Both anions and cations are mobile in many polymer/salt electrolytes. 

Partly because of the crystallinity problem, the room-temperature conductivity of polymer electrolytes in which the only polymer component is PEO is too low for high current- and power-density applications such as vehicle power sources. The conductivity can be raised to about 10" Scm" at 25°C by use of plasticizers or irradiation cross-linking as described above but this is still insufficient for many purposes. 

To achieve high conductance, both reasonable conductivity and mechanical stability in a thin film form are required. Semi-crystalline polymers have superior mechanical characteristics but vastly inferior conductivity properties to those which are fully amorphous (and well above their Tg), since ionic motion does not occur in the crystalline regions. The design of an optimized electrolyte for battery use is in fact more strongly dictated by morphological considerations (which are affected by the choice of dissolved salt) than by the selection of a system containing a solute with a high cationic transference number. 

PEO can coordinate alkali metal ions strongly and is used as a solid polymer electrolyte [20-22]. However, conventional PEO-Li salt complexes show conductivities of the order of 10 S/cm, which is not sufficient for battery, capacitor and fuel-cell applications. A high crystalline phase concentration limits the conductivity of PEO-based electrolytes. Apart from high crystallinity, PEO-based electrolytes suffer from low cation transport number (t ), ion-pair formation and inferior mechanical properties. Peter and co-workers [23] reported the modification of PEO with phenolic resin for improvement in mechanical properties and conductivity. 

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