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.