Strain Stabilization of Superionicity in Copper and Lithium Selenides

Superionic (SI) phases have utility as solid electrolytes for next generation battery technology, but these phases are typically not stable at room temperature. Our density functional theory calculations demonstrate that compressive lattice strain can stabilize SI phases of Cu₂Se and Li₂Se, two potential solid electrolytes. Electronic and bonding insights into this effect are obtained. In the ordered, non-SI phase, cations are localized primarily in tetrahedral (T) interstices with little access to the higher-energy octahedral (O) sites, but 1-2% compressive strain promotes attractive stabilization of the O cations with 6-fold coordination to Se anions, at the expense of the stability of 4-fold-coordinated T cations. In such compressed lattices, cations can access both T and O sites, resulting in a cation-disordered, SI phase. Thus, lattice strain is demonstrated as a handle for controlling ionic structure and transport and accomplishing ambient temperature superionicity.