Utilizing Co(2+)/Co(3+) Redox Couple in P2-Layered Na(0.66)Co(0.22)Mn(0.44)Ti(0.34)O(2) 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 Na(0.66)Co (x) Mn(0.66-)(x) Ti(0.34)O(2) 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 Na(0.66)Co (x) Mn(0.66-)(x) Ti(0.34)O(2) from orthorhombic to hexagonal is revealed when Co content increases from x = 0 to 0.33. In particular, Na(0.66)Co(0.22)Mn(0.44)Ti(0.34)O(2) with a P2-type layered structure delivers a reversible capacity of 120 mAh g(-1) at 0.1 C. When the current density increases to 10 C, a reversible capacity of 63.2 mAh g(-1) can still be obtained, indicating a promising rate capability. The low valence Co(2+) substitution results in the formation of average Mn(3.7+) valence state in Na(0.66)Co(0.22)Mn(0.44)Ti(0.34)O(2), effectively suppressing the Mn(3+)-induced Jahn-Teller distortion, and in turn stabilizing the layered structure. X-ray absorption spectroscopy results suggest that the charge compensation of Na(0.66)Co(0.22)Mn(0.44)Ti(0.34)O(2) during charge/discharge is contributed by Co(2.2+)/Co(3+) and Mn(3.3+)/Mn(4+) redox couples. This is the first time that the highly reversible Co(2+)/Co(3+) 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.