Capacity Degradation Mechanism and Cycling Stability Enhancement of AlF₃-Coated Nanorod Gradient Na[Ni_0.65Co_0.08Mn_0.27]O₂ Cathode for Sodium-Ion Batteries

O3-type Na[Ni _xCo _yMn _z]O₂ materials are attractive cathodes for sodium-ion batteries because of their full cell fabrication practicality, high energy density, and relatively easy technology transfer arising from their similarity to Li[Ni _xCo _yMn _z]O₂ materials, yet their performance viability with Ni-rich composition ( x ≥ 0.6) is still doubtful. More importantly, their capacity degradation mechanism remains to be established. In this paper, we introduce an O3-type Ni-rich AlF₃-coated nanorod gradient Na[Ni_0.65Co_0.08Mn_0.27]O₂ cathode with enhanced electrochemical performance in both half-cells and full cells. AlF₃-coated nanorod gradient Na[Ni_0.65Co_0.08Mn_0.27]O₂ particles were synthesized through a combination of dry ball-mill coating and columnar composition gradient design and deliver a discharge capacity of 168 mAh g^-1 with 90% capacity retention in half cells (50 cycles) and 132 mAh g^-1 with 90% capacity retention in full cells (200 cycles) at 75 mA g^-1 (0.5C, 1.5-4.1 V). Through analysis of the cycled electrodes, the capacity-degradation mechanism was unraveled in O3-type Ni-rich Na[Ni _xCo _yMn _z]O₂ from a structural perspective with emphasis on high-resolution transmission electron microscopy, providing valuable information on improving O3-type Na[Ni _xCo _yMn _z]O₂ cathode performance.