O3-NaFe(1/3–x)Ni1/3Mn1/3AlxO2 Cathodes with Improved Air Stability for Na-Ion Batteries

Na-ion batteries (NIBs) have been considered as potential candidates for large-scale energy storage, where O3-type Na-based layered oxide cathodes have attracted great attention due to their high capacity and low cost. However, O3-Na_xTMO₂ materials still suffer from insufficient air stability, which could lead to deteriorative electrochemical properties and thus hinder their practical application. In this work, a series of Al-doped O3-NaFe_(1/3-x)Ni_1/3Mn_1/3Al_xO₂ cathodes prepared by a co-precipitation method were investigated to enhance their electrochemical performance and air stability through stabilizing their structural and interface chemical properties. The Al-doped O3-NaFe_(1/3-0.01)Ni_1/3Mn_1/3Al_0.01O₂ (NFNMA_0.01) cathode delivers a comparable capacity of 138 mAh g^-1 and keeps a capacity retention of 85.88% after 50 cycles at 0.2 C, while the undoped O3-NaFe_1/3Ni_1/3Mn_1/3O₂ (NFNM) can only keep a capacity retention of 71.02%, although with an initial capacity of 141 mAh g^-1 at 0.2 C. For the air stability, the capacity decay rates are 58.87 and 5.07% for the undoped NFNM and Al-doped NFNMA_0.01 after the air exposure for 30 days, respectively. The greatly decaying electrochemical performance could be due to the formation of carbonates during air exposure, which can be efficiently suppressed by Al doping. The doped Al³⁺ has been confirmed to be inserted into the NFNM crystal lattice, inducing the reduced values of lattice parameters a and c due to the smaller ionic radius of Al³⁺ (53.5 pm) vs Fe³⁺ (55.0 pm). This study demonstrates that Al doping plays an important role in the air stability and cycling stability for layered cathode materials, which offers an efficient strategy to optimize the material design for their practical application in NIBs.