The increasing concerns regarding the rapid depletion of fossil fuels and their environmental footprint have prompted a strong worldwide interest in the exploration of inexpensive and eco-friendly energy storage technologies.¹ Layered sodium oxides, particularly O3-type layered compounds, have been extensively studied due to their adequate capacity and well-established chemistry which was inherited from LIB technologies. So far, various chemical compositions have been introduced through different transition metal substitutions (Ni, Co, Mn, and Fe) such as Na[Ni_xFe_yMn_z]O₂ or Na[Ni_xCo_yMn_z]O₂.^2-4 A surface-modified O3-type Na[Ni_0.6Co_0.2Mn_0.2]O₂ cathode was covered by Al₂O₃ nanoparticle 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 while assisting Na⁺ migration. The proposed Al₂O₃ coated Na[Ni_0.6Co_0.2Mn_0.2]O₂ cathode showed a high specific capacity of 151 mA h g⁻¹, as well as improved cycling performance 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 cooperated with identifying the specific mechanism of the surface reaction with the electrolyte and the reason for its subsequent degradation and showed that the nano-size Al₂O₃ coating layer was effective at resolving the degradation pathways of the cathode surface from hydrogen fluoride (HF) attack.

References

1. J.-Y. Hwang, S.-T. Myung and Y.-K. Sun, Soc. Rev., 2017, 46, 3529–3614.
2. J.-Y. Hwang, S.-T. Myung, D. Aurbach and Y.-K. Sun, Power Sources, 2016, 324, 106–112.
3. J.-Y. Hwang, S.-M. Oh, S.-T. Myung, K. Y. Chung, I. Belharouak and Y.-K. Sun, Nat. Commun., 2014, 6, 6856.
4. J.-Y. Hwang, C. S. Yoon, I. Belharouak and Y.-K. Sun, J. Mater. Chem. A, 2016, 4, 17952–17959.