O3-Type Na_0.95Ni_0.40Fe_0.15Mn_0.3Ti_0.15O₂ Cathode Materials with Enhanced Storage Stability for High-Energy Na-Ion Batteries

O3-type layered oxides with high initial sodium content are promising cathode candidates for Na-ion batteries. However, affected by the undesired transition metal slab sliding and reaction with H₂O/CO₂, their further application is typically hindered by unsatisfactory cycling stability upon charging to high voltage and poor storage stability under humid air. Herein, we demonstrate a Fe/Ti cosubstitution strategy to simultaneously enhance the electrochemical performance and storage stability of pristine O3-NaNi_0.5Mn_0.5O₂ cathode material, via employing high redox potential and inactive stabilized dopants. The resultant Fe/Ti cosubstituted Na_0.95Ni_0.40Fe_0.15Mn_0.3Ti_0.15O₂ undergoes highly reversible O3-P3-OP2 phase transitions with a small cell volume change of 2.8%, instead of complex O3-O'3-P3-P'3-P3'-O1 phase transitions in NaNi_0.5Mn_0.5O₂. Consequently, the cathode displays a high specific capacity of 161.6 mAh g^-1 with an average working voltage of 3.28 V and 81.8% capacity retention after 200 cycles at 5C. Furthermore, the cathode material remains very stable after exposure to air for 7 days and even after soaking in water for 1 h, owing to the prohibition of sodium losing by elevating redox potential and contracting sodium layer spacing. This work proposes an effective method to enhance the electrochemical performance and storage stability of O3-type layered oxide cathodes and promises advancing Na-ion batteries toward large-scale industrialization.