Defects, diffusion, dopants and encapsulation of Na in NaZr2(PO4)3

Abstract

NaZr2(PO4)3 is an attractive ceramic material in the application of Na-ion batteries due to its high ionic conductivity and low thermal expansion. Here we employ classical simulation techniques to examine the defect processes, Na-ion migration and solution of various dopants in NaZr2(PO4)3. We find that the Na Frenkel (0.70 eV) is the most favourable defect process ensuring the formation of Na vacancies required for the vacancy assisted Na-ion migration. Long-range sodium vacancy migration was considered and it is calculated that Na-ions diffuse fast with a low activation energy of migration of 0.26 eV. The most favourable isovalent dopants on the Na and Zr sites are the K and Ge, respectively. The formation of Na interstitials and oxygen vacancies can be favoured by doping of Yb, Y, Ga and In on the Zr site. Encapsulation of Na was considered using density functional theory simulations and it is found that the incorporation is exoergic. Furthermore, the formation of Na+ ions and electrons simultaneously constitutes this material metallic together with high concentration of Na+ ions. The final encapsulated composite can be suitable for the preparation of electrodes in rechargeable Na-ion batteries.