Resolving the degradation pathways of the O3-type layered oxide cathode surface through the nano-scale aluminum oxide coating for high-energy density sodium-ion batteries

A surface-modified O3-type Na[Ni0.6Co0.2Mn0.2]O-2 cathode was synthesized by Al2O3 nanoparticle coating using a simple dry ball-milling route. The nanoscale Al2O3 particles (similar to 15 nm in diameter) densely covering the spherical O3-type Na[Ni0.6Co0.2Mn0.2]O-2 cathode particles effectively minimized parasitic reactions with the electrolyte solution while assisting Na+ migration. The proposed Al2O3 coated Na [Ni0.6Co0.2Mn0.2]O-2 cathode exhibited a high specific capacity of 151 mA h g(-1), as well as improved cycling stability and rate capability in a half cell. Furthermore, the Al2O3 coated cathode was scaled up to a pouch-type full cell using a hard carbon anode that exhibited a superior rate capability and capacity retention of 75% after 300 cycles with a high energy density of 130 W h kg(-1). In addition, the postmortem surface characterization of the cathodes from the long-term cycled full cells helped in identifying the exact mechanism of the surface reaction with the electrolyte and the reason for its subsequent degradation and showed that the nano-scale Al2O3 coating layer was effective at resolving the degradation pathways of the cathode surface from hydrogen fluoride (HF) attack.