Ionic conduction in solids has been a subject of interest since the beginning of the 19th century. Michael Faraday established in 1839 that the laws of electrolysis are also obeyed in ionic solids like lead(II) fluoride (PbF 2) and silver sulfide (Ag 2 S). In 1921, solid silver iodide (AgI) was found to have had extraordinary high ionic conductivity at temperatures above 147 °C, AgI changes into a phase that has an ionic conductivity of ~ 1 –1 cm. This high temperature phase of AgI is an example of a superionic conductor. The disordered structure of this solid allows the Ag ions to move easily. The present record holder for ionic conductivity is the related material Ag 2 [HgI 4 ]. β''-alumina was developed at the Ford Motor Company in the search for a storage device for electric vehicles while developing the sodium–sulfur battery.