Capacity Degradation Mechanism and Cycling Stability Enhancement of AlF-Coated Nanorod Gradient Na[NiCoMn]O Cathode for Sodium-Ion Batteries.

O3-type Na[Ni Co Mn ]O 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 Co Mn ]O 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 AlF-coated nanorod gradient Na[NiCoMn]O cathode with enhanced electrochemical performance in both half-cells and full cells. AlF-coated nanorod gradient Na[NiCoMn]O 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 with 90% capacity retention in half cells (50 cycles) and 132 mAh g with 90% capacity retention in full cells (200 cycles) at 75 mA g (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 Co Mn ]O from a structural perspective with emphasis on high-resolution transmission electron microscopy, providing valuable information on improving O3-type Na[Ni Co Mn ]O cathode performance.