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Extending the life-span of O3-type layered oxide cathode enabled by the nanoscale aluminum oxide coating for high-energy density sodium-ion batteries

Sodium-ion batteries (SIBs) appear to be a promising alternative due to the abundance of sodium in the Earth's crust. [1] Also, the chemistry of SIBs is similar to that of wellestablished lithium batteries, which adds additional merit. So far, various chemical compositions have been introduced through different transition metal substitutions (Ni, Co, Mn, and Fe) such as Na[NixFeyMnz]O2 or Na[NixCoyMnz]O2 compound. [2-4] However, the O3-type layered oxides in SIBs exhibit a low reversible capacity and poor cycle retention due to the structural instability that arises from multi-phase transitions. A surface-modified O3-type Na[Ni0.6Co0.2Mn0.2]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[Ni0.6Co0.2Mn0.2]O2 cathode particles effectively minimized parasitic reactions [5] with the electrolyte solution while assisting Na migration. The proposed Al2O3 coated Na[Ni0.6Co0.2Mn0.2]O2 cathode exhibited a high specific capacity of 151 mA h g, 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. 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.