Li<sup>+</sup>/Na<sup>+</sup> Hybrid Ion Conduction Mechanism in the Superionic Conductor Li<sub>3–x</sub>Na<sub>x</sub>Zr<sub>2</sub>Si<sub>2</sub>PO<sub>12</sub>

Hybrid ion conductors that transport multiple ionic conductive species provide a useful platform for understanding how mixed-ion transport governs ionic conductivity within a single phase. However, the controlled introduction of multiple mobile ions into solid-state electrolytes and a mechanistic understanding of their migration within the framework remain challenging. Herein, a skeleton-retained Li+↔Na+ cationic exchange was used to simultaneously induce Li+ and Na+ cations into the NASICON-type framework of Li3–xNaxZr2Si2PO12 (0 < x < 3). We show that the interpenetration of NaO6 and NaO8 coordination polyhedra significantly influences the ionic conductivity of hybrid ion conductors. Computational analysis indicates that Na+ transfer from octahedral NaO6 sites to octa-coordinated NaO8 sites is thermodynamically favorable, accompanied by Li+ relocation from NaO8 to tetrahedral LiO4 environments at former NaO6 sites, thereby promoting Li+/Na+ site segregation. The increased occupation of Na+ at NaO8 sites not only suppresses Na+ mobility due to bottleneck limitations but also hinders the formation of a continuous Li+ migration network, thereby reducing the room-temperature ionic conductivity from 1.78 to 0.50 mS·cm–1. Upon re-exchange, Na+ in the NaO8 sites is replaced by Li+ in penta-coordinated LiO5, which re-establish percolating ion-transport pathways for Li+ and enable reversible recovery of the overall conductivity. These results reveal a fast dual-ion conduction mechanism enabled by the interpenetrating occupation of Li+ and Na+ across the available sites. This work opens a new avenue for the development of hybrid ion conductors.