Cubic Sodium Cobalt Metaphosphate [NaCo(PO)] as a Cathode Material for Sodium Ion Batteries.

Cubic-framework sodium cobalt-based metaphosphate NaCo(PO) 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(PO) assuming a cubic Pa3̅ framework. Herein, synergizing experimental and computational tools, we present the first report on the electrochemical activity and Na diffusion pathway analysis of cubic NaCo(PO) prepared via conventional solid-state route. The electrochemical analyses reveal NaCo(PO) to be an active sodium insertion material with well-defined reversible Co/Co 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(PO). BVSE analyses suggest feasible Na migration with moderate energy barrier of 0.68 eV. Cubic NaCo(PO) forms a 3.3 V sodium insertion material.