Utilizing Co/Co Redox Couple in P2-Layered NaCoMnTiO Cathode for Sodium-Ion Batteries.

Developing sodium-ion batteries for large-scale energy storage applications is facing big challenges of the lack of high-performance cathode materials. Here, a series of new cathode materials NaCo Mn TiO for sodium-ion batteries are designed and synthesized aiming to reduce transition metal-ion ordering, charge ordering, as well as Na and vacancy ordering. An interesting structure change of NaCo Mn TiO from orthorhombic to hexagonal is revealed when Co content increases from = 0 to 0.33. In particular, NaCoMnTiO with a P2-type layered structure delivers a reversible capacity of 120 mAh g at 0.1 C. When the current density increases to 10 C, a reversible capacity of 63.2 mAh g can still be obtained, indicating a promising rate capability. The low valence Co substitution results in the formation of average Mn valence state in NaCoMnTiO, effectively suppressing the Mn-induced Jahn-Teller distortion, and in turn stabilizing the layered structure. X-ray absorption spectroscopy results suggest that the charge compensation of NaCoMnTiO during charge/discharge is contributed by Co/Co and Mn/Mn redox couples. This is the first time that the highly reversible Co/Co redox couple is observed in P2-layered cathodes for sodium-ion batteries. This finding may open new approaches to design advanced intercalation-type cathode materials.