a solid electrolyte

Inside an ordinary battery, the two electrodes are separated by an electrolyte — the ion-conducting layer that lets charged atoms shuttle between the plates while forcing the electrons to take the long way round through your device (that detour IS the useful current). Normally the electrolyte is a liquid or a soaked-in gel. A solid electrolyte does the same job as a single, rigid, ceramic wall: it conducts ions freely, blocks electrons, and blocks any gas or liquid from crossing.

To earn the name, a solid electrolyte must do three things well. First, conduct its chosen ion fast (high ionic conductivity — it must be a good ionic or superionic conductor). Second, refuse to conduct electrons (a transference number for ions near 1), or the cell short-circuits inside itself. Third, be dense, gas-tight and chemically stable against the electrodes and the gases or liquids it separates. The classic examples each specialise in one ion: yttria-stabilized zirconia moves oxygen ions (O2-), beta-alumina moves sodium ions (Na+), and lithium garnet (Li7La3Zr2O12) moves lithium ions (Li+).

Solid electrolytes make possible devices that a puddle of liquid never could: they run red-hot without boiling away (the solid-oxide fuel cell, the exhaust oxygen sensor), they cannot leak or catch fire (the promise of solid-state batteries), and they can be shaped into thin, gas-tight ceramic membranes. The honest weak points are at the edges and joins: the resistance of grain boundaries inside the ceramic, and of the interfaces where the solid electrolyte meets a solid electrode, often dominates the total resistance and is harder to reduce than the bulk conductivity itself.