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 Na0.66 Co x Mn0.66- x Ti0.34 O2 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 Na0.66 Co x Mn0.66- x Ti0.34 O2 from orthorhombic to hexagonal is revealed when Co content increases from x = 0 to 0.33. In particular, Na0.66 Co0.22 Mn0.44 Ti0.34 O2 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 Co2+ substitution results in the formation of average Mn3.7+ valence state in Na0.66 Co0.22 Mn0.44 Ti0.34 O2 , effectively suppressing the Mn3+ -induced Jahn-Teller distortion, and in turn stabilizing the layered structure. X-ray absorption spectroscopy results suggest that the charge compensation of Na0.66 Co0.22 Mn0.44 Ti0.34 O2 during charge/discharge is contributed by Co2.2+ /Co3+ and Mn3.3+ /Mn4+ redox couples. This is the first time that the highly reversible Co2+ /Co3+ 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.