text: By 1971, solid-state cells and batteries based on rubidium silver iodide (RbAgI) have been designed and tested in a wide range of temperatures and discharge currents. Despite the relatively high conductivity of RbAgI, they have never been commercialized due to a low overall energy content per unit weight (ca. 5 W·h/kg).
  On the contrary, LiI, which had a conductivity of only ca. 1 S/cm at room temperature, found a wide-scale application in batteries for artificial pacemakers. The first such device, based on undoped LiI, was implanted into a human in March 1972 in Ferrara, Italy. Later models used as electrolyte a film of LiI, which was doped with alumina nanoparticles to increase its conductivity. LiI was formed in an in situ chemical reaction between the Li anode and iodine-poly(2-vinylpyridine) cathode, and therefore was self-healed from erosion and cracks during the operation.
  Sodium-sulfur cells, based on ceramic β-AlO electrolyte sandwiched between molten-sodium anode and molten-sulfur cathode showed high energy densities and were considered for car batteries in the 1990s, but disregarded due to the brittleness of alumina, which resulted in cracks and critical failure due to reaction between molten sodium and sulfur. Replacement of β-AlO with NASICON did not save this application because it did not solve the cracking problem, and because NASICON reacted with the molten sodium.
  Yttria-stabilized zirconia is used as a solid electrolyte in oxygen sensors in cars, generating voltage that depends on the ratio of oxygen and exhaust gas and providing electronic feedback to the fuel injector. Such sensors are also installed at many metallurgical and glass-making factories. Similar sensors of CO, chlorine and other gases based on solid silver halide electrolytes have been proposed in the 1980s–1990s. 
  Since mid-1980s, a Li-based solid electrolyte is used to separate the electrochromic film (typically WO) and ion-storing film (typically LiCoO) in the smart glass, a window whose transparency is controlled by external voltage.
  Solid-state ionic conductors are essential components of lithium-ion batteries, proton exchange membrane fuel cells (PEMFCs), supercapacitors, a novel class of electrochemical energy storage devices, and solid oxide fuel cells, devices that produces electricity from oxidizing a fuel. Nafion, a flexible fluoropolymer-copolymer discovered in the late 1960s, is widely used as a polymer electrolyte in PEMFCs.