SOLID ELECTROLYTE, ELECTROLYTE COMPOSITION, ELECTROLYTE SHEET AND POWER STORAGE DEVICE

To provide a solid electrolyte, an electrolyte composition, an electrolyte sheet, and a power storage device, in which gelling is suppressed. The solid electrolyte (18) has a garnet-type structure which contains Li, La, Zr, and O, in which, with respect to particles (21) of the electrolyte having a particle size equal to or greater than the particle size at which the cumulative value of frequency in a volume-based particle size distribution reaches 10%, a number average of degrees of envelope of particles is 0.8 or greater, the degree of envelope being defined as a ratio (area within a contour (22) of a particle/area within an envelope (23) of the particle). The electrolyte composition includes a solid electrolyte, an ionic liquid containing an imidazolium cation, a lithium salt, and a polymer having -CHCF-. The electrolyte sheet is formed of the electrolyte composition. The power storage device has an electrolyte layer formed of the electrolyte composition.