Zircon Structure as a Prototype Host for Fast Monovalent and Divalent Ionic Conduction

“Beyond Li-ion” energy storage solutions based on ions such as Na, Mg, Ca, and Zn have attracted increasing attention due to growing concerns about the cost, resource availability, and safety of the currently dominant Li-ion batteries. One of the greatest challenges for beyond-Li systems, especially multivalent ones, is the lack of materials with high ionic mobility. In this study, we find that zircon-type YPO4 presents a unique structural environment that enables superior conduction of multiple species including Na+, Ca2+, Mg2+, and Zn2+, even in the dilute carrier concentration regime. This highly unusual capability originates from one-dimensional (1D) percolating channels of adjacent, distorted octahedral sites, which results in a smoothly varying coordination environment and correspondingly low activation barriers. Low decomposition energy of multiple compositions of doped YPO4, where the carrier ions are introduced into the system along with subvalent doping into P sites, confirmed good stability and synthesizability. Among these compositions, we found Na0.0625YSi0.0625P0.9375O4 exhibiting good Na+ conductivity of 0.99 mS/cm at 300 K with an activation energy of 220 meV. Zircon-structured Na0.05YSi0.05P0.95O4 was successfully synthesized; however, the highest density achieved (78%) was insufficient to conclusively establish its conductivity. Finally, we identify dopant–carrier association in doped YPO4 as a key challenge for long-range diffusion in this structure family.