Sodium-based solid electrolyte, method for producing sodium-based solid electrolyte, modified positive electrode active material, modified negative electrode active material, solid-state secondary battery, electrode sheet for solid-state secondary battery, solid electrolyte sheet, and electrode for solid-state secondary battery

A sodium-based solid electrolyte containing Na, B, and O,

• wherein the sodium-based solid electrolyte further contains one or more specific elements selected from the group consisting of an element of Group 4 of the periodic table, an element of Group 15 of the periodic table, an element of Group 16 of the periodic table, an element of Group 17 of the periodic table, Si, C, Sc, and Y, and
• in a case where molar amounts of Na, O, and the specific elements in the sodium-based solid electrolyte are represented by setting a molar amount of B to 4.0, the molar amount of Na is 1.6 to 3.5, the molar amount of O is 6.2 to 25.0, and each molar amount of the specific elements is 0.01 to 10.0.