Solid-state synthesis of low-cost and high-energy-density sodium layered-tunnel oxide cathodes: Dynamic structural evolution, Na⁺/vacancy disordering, and prominent moisture stability

Manganese-based layered oxides show promise as cathode materials for sodium-ion batteries (SIBs). However, several challenges including sluggish Na+ kinetics, complex phase transitions, and poor air stability hinder their practical application. Herein, we proposed a dual-function strategy that not only precisely manipulates dynamic structural evolution from layered to tunnel structure, but also effectively suppresses Na+/vacancy and charge ordering by inhibiting electron delocalization. A series of Ti-substituted Na2/3Mn1-xTixO2 (x=0, 1/9, 2/9, 1/3) as proof of concept materials were designed to demonstrate the dual-function strategy. As a result, the optimized Na2/3Mn8/9Ti1/9O2 cathode material delivers a high specific capacity of 202.9 mAh g−1 at 0.1 C within 1.5−4.3 V, equivalent to 536.6 Wh kg−1 of energy density, and exhibits 71.0% of capacity retention after 300 cycles at 1 C. Meanwhile, a highly reversible P2/Tunnel-OP4/Tunnel phase transition process and interlocking effect between the layered and tunnel structure as well as prominent moisture stability even after soak water treatment are further confirmed by in-situ charge and discharge XRD and other advanced characterization techniques. Noting that the electrode assembled with water-solution binder still displays a high capacity retention of 85.4% after 400 cycles at 1 C. Our dual-function strategy provides valuable guidance for developing high energy density and water stable practical SIB cathode materials.