In electrodes the insertion of the "ions" = atoms occurs under conditions in which this insertion is thermodynamically favorable, playing a crucial role in determining the voltages and capacities of electrochemical cells. Often, when talking about electrolytes, it is assumed that they are liquids. However, it's worth noting that there are also various solid materials capable of fulfilling the main functions of an electrolyte. These solids enable the movement of charged ionic species while selectively impeding the transport of electronic species. Although there are numerous materials that effectively insulate against electronic currents, what sets apart materials capable of acting as a solid electrolyte is the high mobility of ionic species within their crystal structures.