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-Na0.898K0.058Ni0.396Fe0.098Mn0.396Ti0.092O2 (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+↔Ni3 . 20+ and Fe3+↔Fe3.69+ 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.