Two step charge transfer reactions

When an ion is transferred between two phases an intermediate valency species is not possible. However, in the transfer of Cu from a metal electrode into an electrolyte it is possible that the reaction mechanism is  

(1987) in Polymer Electrolyte Reviews - 1, Eds. J. R. MacCallum and C. A. Vincent, Elsevier, London. 

In the early part of this century, many types of solid electrolyte had already been reported. High conductivity was found in a number of metal halides. One of the first applications of solid electrolytes was to measure the thermodynamic properties of solid compounds at high temperatures. Katayama (1908) and Kiukkola and Wagner (1957) made extensive measurements of free enthalpy changes of chemical reactions at higher temperatures. Similar potentiometric measurements of solid electrolyte cells are still made in the context of electrochemical sensors which are one of the most important technical applications for solid electrolytes. 

Insertion electrodes also represent an important group of compounds 

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