Utilizing Co2+/Co3+ Redox Couple in P2‐Layered Na0.66Co0.22Mn0.44Ti0.34O2 Cathode for Sodium‐Ion Batteries

摘要

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 Na0.66CoxMn0.66– xTi0.34O2 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 Na0.66CoxMn0.66– xTi0.34O2 from orthorhombic to hexagonal is revealed when Co content increases from x = 0 to 0.33. In particular, Na0.66Co0.22Mn0.44Ti0.34O₂ 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.