A battery in which both electrodes and the electrolyte are solids. Solid electrolytes are a class of materials also known as superionic conductors and fast ion conductors, and their study belongs to an area of science known as solid-state ionics. As a group, these materials are very good conductors of ions but are essentially insulating toward electrons, properties that are prerequisites for any electrolyte. The high ionic conductivity minimizes the internal resistance of the battery, thus permitting high power densities, while the high electronic resistance minimizes its self-discharge rate, thus enhancing its shelf life. Examples of such materials include Ag₄RbI₅ for Ag⁺ conduction, LiI/Al₂O₃ mixtures for Li⁺ conduction, and the clay and β-alumina group of compounds (NaAl₁₁O₁₇) for Na⁺ and other mono- and divalent ions. At room temperature the ionic conductivity of a single crystal of sodium β-alumina is 0.035 S/cm, comparable to the conductivity of a 0.1 M HCl solution. This conductivity, however, is reduced in a battery by a factor of 2–5, because of the use of powdered or ceramic material rather than single crystals. Of much interest are glassy and polymeric materials that can be readily made in thin-film form, thus enhancing the rate capability of the overall system.