The Zircon Structure As a Prototype Host for Fast Monovalent and Di-Valent Ionic Conduction

Abstract

"Beyond Li-ion" energy storage solutions based on ions such as Na, Mg, Ca and Zn-ion 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 framework 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. Nudged elastic band (NEB) calculations show very low diffusion barriers for Na/Ca/Mg/Zn in pristine YPO4 and ab initio molecular dynamics (AIMD) simulations confirm the 1D percolating diffusion channel in the zircon-type framework. These 1D percolating channels of adjacent, distorted octahedral sites result in a smoothly varying coordination environment thus correspondingly low activation barriers. The phase stability of doped compositions is examined when both carrier ion and subvalent dopants are introduced into the system and confirm good stability through low decomposition energy of compositions with different carrier-dopant combinations. Among these compositions, we found Na0.0625YSi0.0625P0.9375O4 exhibiting a high Na+ conductivity of 0.99 mS/cm at 300 K with an activation energy of 220 meV. Finally, we identify dopant-carrier association in doped YPO4 as a key challenge for long-range diffusion in this structure family.