Will Sodium Layered Oxides Ever Be Competitive for Sodium Ion Battery Applications?

The Na-ion battery technology is rapidly developing as a possible alternative to Li-ion for massive electrochemical energy storage applications because of sustainability and cost reasons. Two types of technologies based either on sodium layered oxides NaxMO2 (x <= 1, M = transition metal ion(s)) or on polyanionic compounds such as Na3V2(PO4)(2)F-3 as positive electrode and carbon as negative electrode are presently being pursued. Herein, we benchmark the performance of full Na-ion cells based on several sodium layered oxide materials against Na3V2(PO4)(2)F-3/hard carbon cells. Although several studies report more attractive capacities for sodium layered oxides vs. Na metal (similar to 200 mAh g(-1)) than for polyanionic phases (similar to 120 mAh g(-1)), we find that such advantages are not maintained when assembling practical full Na-ion cells; the opposite of what is found for Li-ion technology. The reasons for such a loss of supremacy of the layered oxides against polyanionic compounds are discussed in terms of materials structural stability and composition so as to identify fundamental challenges that impede their practical applications. Finally, a few perspectives are given to design better sodium layered oxide electrode materials that could outweigh the performance of today's stellar Na3V2(PO4)(2)F-3. (C) The Author(s) 2018. Published by ECS.