Utilizing Co²⁺/Co³⁺ Redox Couple in P2‐Layered Na_0.66Co_0.22Mn_0.44Ti_0.34O₂ Cathode for Sodium‐Ion Batteries

Abstract

Abstract 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.