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

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_0.898K_0.058Ni_0.396Fe_0.098Mn_0.396Ti_0.092O₂ (KT-NFM) is reported as an ultrastable cathode material via multisite substitution with rigid KO₆ pillars and flexible TiO₆ octahedra. The K pillars induce contracted TMO₂ 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 Ni₂+↔Ni_{3 . 20}+ and Fe₃+↔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.