Utilizing Co²⁺/Co³⁺ Redox Couple in P2-Layered Na_0.66Co_0.22Mn_0.44Ti_0.34O₂ 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_0.66Co _x Mn_0.66-x Ti_0.34O₂ 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_0.66Co _x Mn_0.66-x Ti_0.34O₂ from orthorhombic to hexagonal is revealed when Co content increases from x = 0 to 0.33. In particular, Na_0.66Co_0.22Mn_0.44Ti_0.34O₂ 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 Mn^3.7+ valence state in Na_0.66Co_0.22Mn_0.44Ti_0.34O₂, 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 Na_0.66Co_0.22Mn_0.44Ti_0.34O₂ during charge/discharge is contributed by Co^2.2+/Co³⁺ and Mn^3.3+/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.