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

"Beyond Li-ion" energy storage solutionsbased onions such as Na, Mg, Ca, and Zn have attracted increasing attentiondue to growing concerns about the cost, resource availability, andsafety of the currently dominant Li-ion batteries. One of the greatestchallenges for beyond-Li systems, especially multivalent ones, isthe lack of materials with high ionic mobility. In this study, wefind that zircon-type YPO4 presents a unique structuralenvironment that enables superior conduction of multiple species includingNa(+), Ca2+, Mg2+, and Zn2+, even in the dilute carrier concentration regime. This highly unusualcapability originates from one-dimensional (1D) percolating channelsof adjacent, distorted octahedral sites, which results in a smoothlyvarying coordination environment and correspondingly low activationbarriers. Low decomposition energy of multiple compositions of dopedYPO(4), where the carrier ions are introduced into the systemalong with subvalent doping into P sites, confirmed good stabilityand synthesizability. Among these compositions, we found Na0.0625YSi0.0625P0.9375O4 exhibiting goodNa(+) conductivity of 0.99 mS/cm at 300 K with an activationenergy of 220 meV. Zircon-structured Na0.05YSi0.05P0.95O4 was successfully synthesized; however,the highest density achieved (78%) was insufficient to conclusivelyestablish its conductivity. Finally, we identify dopant-carrierassociation in doped YPO4 as a key challenge for long-rangediffusion in this structure family.