Na2.32Co1.84(SO4)3 as a new member of the alluaudite family of high-voltage sodium battery cathodes

Electrochemical energy storage has recently seen tremendous emphasis being placed on the large-scale (power) grid storage. Sodium-ion batteries are capable of achieving this goal with economic viability. In a recent breakthrough in sodium-ion battery research, the alluaudite framework (Na2Fe2(SO4)(3)) has been reported, with the highest Fe3+/Fe2+ redox potential (ca. 3.8 V, Barpanda, et al., Nat. Commun., 2014, 5, 4358). Exploring this high-voltage sodium insertion system, we report the discovery of Na2+2xCo2-x(SO4)(3) (x = 0.16) as a new member of the alluaudite class of cathode. Stabilized by low-temperature solid-state synthesis (T <= 350 degrees C),this novel Co-based compound assumes a monoclinic structure with C2/c symmetry, which undergoes antiferromagnetic ordering below 10.2 K. Isotypical to the Fe-homologue, it forms a complete family of solid-solution Na2+2x(Fe1-yCoy)(2-x)(SO4)(3) [ y = 0-1]. Ab initio DFT analysis hints at potential high voltage operation at 4.76-5.76 V (vs. Na), depending on the degree of desodiation involving a strong participation of the oxygen sub-lattice. With the development of safe organic electrolytes, Na2+2xCo2-x(SO4)(3) can work as a cathode material (similar to 5 V) for sodium-ion batteries.