Crystalline electrolytes conduction mechanisms

Crystalline solid electrolytes have been subdivided into soft ionic crystals such as P-Pbp2 and hard covalent crystals such as P-alumina. The conduction mechanism can be pictured as involving a liquid-like charge carrier array moving in the vibrating potential energy profile set up by the immobile counterions. 

W. A. Henderson, D. M. Seo, Q. Zhou, P. D. Boyle, J.-H. Shin, H. C. De Long, P. C. Trulove, S. Passerini, Adv. Energy Mater. 2012,2,8014-8019. An alternative ionic conductivity mechanism for plastic crystalline salt-lithium salt electrolyte mixtures. 

The ionic conductivity in polymer electrolytes is related to the segmental motion of the polymeric chains thus the low degree of crystallinity for polymer electrolyte may be propitious for high ionic conductivity. The conductive mechanism can be stated as follows lithium ions first bond with the... 

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. 

According to Tarascon and co-workers, the swelling of PVdF—HFP by liquid electrolytes was never complete due to the semicrystalline nature of the copolymer, which tends to microphase-separate after the activation by electrolyte. On the other hand, it is those crystalline domains in the gelled PVdF—HFP that provide mechanical integrity for the resultant GPE. Thus, a dual phase structure was proposed for the Bellcore GPE by some authors, wherein the amorphous domain swollen by a liquid electrolyte serves as the ion conduction phase, while tiny crystallites act as dimensional stabilizer. 

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. 

Electrodes The anodes of SOFC consist of Ni cermet, a composite of metallic Ni and YSZ, Ni provides the high electrical conductivity and catalytic activity, zirconia provides the mechanical, thermal, and chemical stability. In addition, it confers to the anode the same expansion coefficient of the electrolyte and renders compatible anode and electrolyte. The electrical conductivity of such anodes is predominantly electronic. Figure 14 shows the three-phase boundary at the interface porous anode YSZ and the reactions which take place. The cathode of the SOFC consists of mixed conductive oxides with perovskite crystalline structure. Sr doped lanthanum manganite is mostly used, it is a good /7-type conductor and can contain noble metals. 

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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