ABSTRACT
Cubic-framework sodium cobalt-based metaphosphate NaCo(PO3)3 was recently demonstrated to be an attractive Na+ cationic conductor. It can be potentially used in the next-generation rechargeable Na ion batteries. The crystal structure and electrical conductivity were studied and found to have a three-dimensional framework with interconnecting tunnels for Na+ migration ( J. Solid State Electrochem. , 2016 , 20 , 1241 ). This inspired us to study the electrochemical (de)intercalation behavior of Na+ in the NaCo(PO3)3 assuming a cubic Pa3Ì framework. Herein, synergizing experimental and computational tools, we present the first report on the electrochemical activity and Na+ diffusion pathway analysis of cubic NaCo(PO3)3 prepared via conventional solid-state route. The electrochemical analyses reveal NaCo(PO3)3 to be an active sodium insertion material with well-defined reversible Co3+/Co2+ redox activity centered at 3.3 V (vs Na/Na+). Involving a solid-solution redox mechanism, close to 0.7 Na+ per formula unit can be reversibly extracted. This experimental finding is augmented with bond valence site energy (BVSE) modeling to clarify Na+ migration in cubic NaCo(PO3)3. BVSE analyses suggest feasible Na+ migration with moderate energy barrier of 0.68 eV. Cubic NaCo(PO3)3 forms a 3.3 V sodium insertion material.
