Lithium ion conductor, sheet and power storage device

The present invention provides: a lithium ion conductor which is capable of increasing the Li ion transference number; a sheet; and a power storage device. This lithium ion conductor comprises: a solid electrolyte which contains Li, La, Zr and O and has a garnet-type or garnet-like crystal structure; and an electrolyte solution which is obtained by dissolving a lithium salt in an organic solvent. The organic solvent contains sulfolane or a sulfolane derivative; and the electrolyte solution has a mass molarity of the lithium salt of 1.4 mol/kg or more. The ratio of the volume of the solid electrolyte to the sum of the volume of the solid electrolyte and the volume of the electrolyte solution is not less than 52% but less than 100%. The sheet and the power storage device each comprise this lithium ion conductor.