LITHIUM-BASED SOLID ELECTROLYTE, METHOD FOR PRODUCING LITHIUM-BASED SOLID ELECTROLYTE, MODIFIED POSITIVE ELECTRODE ACTIVE MATERIAL, MODIFIED NEGATIVE ELECTRODE ACTIVE MATERIAL, ALL-SOLID-STATE SECONDARY BATTERY, ELECTRODE SHEET FOR ALL-SOLID-STATE SECONDARY BATTERY, SOLID ELECTROLYTE SHEET, AND ELECTRODE FOR ALL-SOLID-STATE SECONDARY BATTERY

An object of the present invention is to provide a lithium-based solid electrolyte having excellent ion conductivity, a method for producing a lithium-based solid electrolyte, a modified positive electrode active material, a modified negative electrode active material, an all-solid-state secondary battery, an electrode sheet for an all-solid-state secondary battery, a solid electrolyte sheet, and an electrode for an all-solid-state secondary battery. The lithium-based solid electrolyte of the present invention contains amorphous lithium tetraborate, water, and a lithium salt, in which a content of the water is 45% by mass or less with respect to a total mass of the lithium-based solid electrolyte.