Ion conductor, secondary battery, and manufacturing method of ion conductor

A secondary battery comprising:

• a solid electrolyte including an ion conductor;
• and
• a positive electrode and a negative electrode disposed to sandwich the solid electrolyte, wherein
• the ion conductor includes a plurality of cations, a plurality of anions, and vacancies in a crystal,
• among the plurality of anions, an anion having a largest isotropic atomic displacement parameter has an isotropic atomic displacement parameter of 2 Å2 or more,
• the plurality of cations includes conducting ions that are capable of migrating through the crystal and non-conducting ions that remain immobile in the crystal,
• at each of conduction sites through which the plurality of cations migrates, at least one of the conducting ions, the non-conducting ions, and the vacancies is present,
• a sum of occupancies of the conducting ions, the non-conducting ions, and the vacancies at the conduction sites is 100%, and
• at the conduction sites, the occupancy of the conducting ions is within a range of 10% to 70%, the occupancy of the non-conducting ions is within a range of 10% to 50%, and the occupancy of the vacancies is within a range of 8% to 50%.

A manufacturing method of an ion conductor that includes a plurality of cations, a plurality of anions, and vacancies in a crystal, wherein

• among the plurality of anions, an anion having a largest isotropic atomic displacement parameter has an isotropic atomic displacement parameter of 2 Å2 or more,
• the plurality of cations includes conducting ions that are capable of migrating through the crystal and non-conducting ions that remain immobile in the crystal,
• at each of conduction sites through which the plurality of cations migrates, at least one of the conducting ions, the non-conducting ions, and the vacancies is present,
• a sum of occupancies of the conducting ions, the non-conducting ions, and the vacancies at the conduction sites is 100%,
• at the conduction sites, the occupancy of the conducting ions is within a range of 10% to 70%, the occupancy of the non-conducting ions is within a range of 10% to 50%, and the occupancy of the vacancies is within a range of 8% to 50%, and
• the conducting ions are alkali metal cations,
• the manufacturing method comprising:
• a mixing process of mixing a composite oxide containing at least cations other than the alkali metal cations with an alkali metal compound containing the alkali metal cations;
• and
• a firing process of heating a mixture containing the composite oxide and the alkali metal compound at a predetermined temperature to produce a pyrochlore-type oxide, wherein
• in a case where the composite oxide contains the alkali metal cations, a composition ratio of the alkali metal cations in the composite oxide is less than a composition ratio of the alkali metal cations in the alkali metal compound,
• the mixture contains the alkali metal compound in an amount exceeding a stoichiometric ratio relative to the pyrochlore-type oxide, and
• in the firing process, the alkali metal compound is liquefied by heating at the predetermined temperature.