Regulation of Coordination Chemistry for Ultrastable Layered Oxide Cathode Materials of Sodium‐Ion Batteries

Abstract Layered transition‐metal (TM) oxide cathodes have attracted growing attention in sodium‐ion batteries (SIBs). However, their practical implementation is plagued by Jahn–Teller distortion and irreversible cation migration, leading to severe voltage decay and structure instability. Herein, O3‐Na 0.898 K 0.058 Ni 0.396 Fe 0.098 Mn 0.396 Ti 0.092 O 2 (KT‐NFM) is reported as an ultrastable cathode material via multisite substitution with rigid KO 6 pillars and flexible TiO 6 octahedra. The K pillars induce contracted TMO 2 slabs and their strong Coulombic repulsion to inhibit Ni/Fe migration; and Ti incorporation reinforces the hybridization of Ni(3deg*)‐O(2p) to mitigate the undesired O3–O'3 phase transition. These enable the reversible redox of Ni 2 +↔Ni 3 . 20 + and Fe 3 +↔Fe 3.69 + for 138.6 mAh g −1 and ultrastable cycles with >90% capacity retention after 2000 cycles in a pouch cell of KT‐NFM||hard carbon. This will provide insights into the design of ultrastable layered cathode materials of sodium‐ion batteries and beyond.