Capacity Degradation Mechanism and Cycling Stability Enhancement of AlF3-Coated Nanorod Gradient Na[Ni0.65Co0.08Mn0.27]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[Ni0.65Co0.08Mn0.27]O2 cathode with enhanced electrochemical performance in both half-cells and full cells. AlF3-coated nanorod gradient Na[Ni0.65Co0.08Mn0.27]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.