Solid electrolyte and secondary battery including the same

A secondary battery comprising:

• an electrolyte layer containing a solid electrolyte;
• and
• a positive electrode and a negative electrode sandwiching the electrolyte layer therebetween, wherein
• the solid electrolyte includes a core phase having a particle shape and a shell phase covering at least a part of the core phase,
• the shell phase includes one or more phases,
• a constituent material of the core phase includes a pyrochlore solid electrolyte represented by a composition formula of Aa2−αAb(1+α)/3B2O7−βXβ, where Aa is an alkali metal, Ab is a lanthanoid, B is a cationic metal, and X is an anion that is substitutable with O,
• a constituent material of the shell phase includes a material having a chemical composition including Li and different from a chemical composition of the pyrochlore solid electrolyte, and having a melting point lower than a melting point of the pyrochlore solid electrolyte,
• in the composition formula, α is in a range of 0.6<α<2.0, β is in a range of 0<β≤1, and a sum of valences of cations composed of Aa, Ab and B and anions composed of O and X is negative, and
• the pyrochlore solid electrolyte has a defective structure.