Mixed Ionic-Elecronic Conductive Materials For Alkali Metal Transport During Battery Cycling, and Batteries Incorporating Same

A mixed ionic-electronic conductor (MIEC) in contact with a solid electrolyte includes a material having a bandgap less than 3 eV. The material includes an end-member phase directly connected to an alkali metal by a tie-line in an equilibrium phase diagram. The material is thermodynamically stable with a solid electrolyte. The MIEC includes plurality of open pores, formed within the MIEC, to facilitate motion of the alkali metal to at least one of store the alkali metal in the plurality of open pores or release the alkali metal from the plurality of open pores. The solid electrolyte has an ionic conductivity to ions of the alkali metal greater than 1 mS cm, a thickness less than 100 μm, and comprises at least one of a ceramic or a polymer.