Extraction materials, solid electrolytes

Hitherto we have dealt with model FICs that are mostly useful as solid electrolytes. The other class of compounds of importance as electrode materials in solid state batteries is mixed electronic-ionic conductors (with high ionic conductivity). The conduction arises from reversible electrochemical insertion of the conducting species. In order for such a material to be useful in high-energy batteries, the extent of insertion must be large and the material must sustain repeated insertion-extraction cycles. A number of transition-metal oxide and sulphide systems have been investigated as solid electrodes (Murphy Christian, 1979). 

Exposure of the n-type films to either liquid (styrene, methyl methacrylate) or gaseous (ethylene oxide, isoprene) monomers resulted in polymerization. Much of our initial work has focused on grafting of poly(ethylene oxide) (PEO) to (CH)X in an effort to render the (CH)X surface more hydrophilic and to provide covalent attachment of a material capable of functioning as a solid electrolyte (12). Films of n-type (CH)X were exposed to dry (CaH2-treated), gaseous ethylene oxide in the range 55-75°C with initial pressures being ca. 500 torr. Reaction times were typically 5 hours. The films were washed with dry, 02-free methylene chloride to remove non-covalently bound PEO and then with deaerated H2O to protonate oxyanions and remove the NaOH byproduct. The presence of bound PEO after extraction was confirmed by IR spectroscopy. 

Once the functional relation between the electric quantity and the concentration of a gas constituent is well known for a solid/gas system, numerous development steps remain until a sensor can be put to practical use. The determining factor is the goal of application, for example the gas analysis in the laboratory, industrial gas analysis, its use in medical devices or in motor vehicles. In any case, long-term stable physico-chemical systems have to be established which have to fulfill certain conditions. These concern for example the sample extraction and treatment, the temperature, the stability of total pressure and of material qualities, and the construction in a miniature or mechanically and thermally robust form. With solid electrolytes of the same kind one has come across to completely different sensor designs, depending on the application (Figure 25-1). 

Equation (13.40) is always valid provided the ealalyst and reference electrodes are made of the same material.It is also valid when other types of solid electrolytes are used, e.g., P"-Al203, aNa+ conductor. Equation (13.40) is mnch more general and fundamental than Equation (13.35), as it does not require the establishment of ai r specific electrochemical equilibrium. It shows that solid electrolyte cells are work function probes for their gas-exposed electrode surfaces. It also shows that SEP is essentially a work function measuring technique and that several aspects of the SEP literature reviewed in References 92,97-99 must be reexamined in the light of these findings. One can still use SEP to extract information about surface activities, provided the nature of the electrocataly tic reaction at the tpb is well known, but, even when this is not the case, the cell emf still provides a direct measure of the work function difference between the two gas-exposed electrode surfaces. Equation (13.40) also holds under closed-circuit conditions, as is further discussed in Section III.B. 

Raman spectroscopy is sensitive to both the chemical and the stmctural variations of a material, liquid or solid/ As an in situ technique, Raman spectroscopy has been used to characterize the crystalline structural variation of graphite anodes and Li vPj and LiMn O cathodes in lithium ion batteries during lithium ion insertion and extraction. In the authors laboratory, Raman spectroscopy was used to extensively study the strong interactions between the components of polyacrylonitrile (PAN)-based electrolytes, the competition between the polymer and the solvent on association with the Li ions, the ion transport mechanisms of both salt-in-polymer and polymer-in-salt electrolytes. Based on the Raman spectroscopic study, Li ion insertion and extraction mechanisms in low-temperature pyrolytic carbon anode have also been proposed. " In many cases, Raman spectroscopy is used as compensation to the IR spectroscopy to give a complete understanding to the structure of a substance though there are as many cases that Raman spectroscopy is used independently. 

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