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

Abstract

A surface-modified O3-type Na[Ni_0.6Co_0.2Mn_0.2]O₂ cathode was synthesized by Al₂O₃ nanoparticle coating using a simple dry ball-milling route. The nanoscale Al₂O₃ particles (∼15 nm in diameter) densely covering the spherical O3-type Na[Ni_0.6Co_0.2Mn_0.2]O₂ cathode particles effectively minimized parasitic reactions with the electrolyte solution while assisting Na⁺ migration. The proposed Al₂O₃ coated Na[Ni_0.6Co_0.2Mn_0.2]O₂ 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 post-mortem 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.