Unprecedented Cyclability and Moisture Durability of NaCrO2 Sodium-Ion Battery Cathode via Simultaneous Al Doping and Cr2O3 Coating

Although there are many cathode candidates for sodium-ion batteries (NIBs), NaCrO₂ remains one of the most attractive materials due to its reasonable level of capacity, nearly flat reversible voltages, and high thermal stability. However, the cyclic stability of NaCrO₂ needs to be further improved in order to compete with other state-of-the-art NIB cathodes. In this study, we show that Cr₂O₃-coated and Al-doped NaCrO₂, which is synthesized through a simple one-pot synthesis, can achieve unprecedented cyclic stability. We confirm the preferential formation of a Cr₂O₃ shell and a Na(Cr_1-2xAl_2x)O₂ core, rather than xAl₂O₃/NaCrO₂ or Na_1/1+2x(Cr_1/1+2xAl_2x/1+2x)O₂, through spectroscopic and microscopic methods. The core/shell compounds exhibit superior electrochemical properties compared to either Cr₂O₃-coated NaCrO₂ without Al dopants or Al-doped NaCrO₂ without shells because of their synergistic contributions. As a result, Na(Cr_0.98Al_0.02)O₂ with a thin Cr₂O₃ layer (5 nm) shows no capacity fading during 1000 charge/discharge cycles while maintaining the rate capability of pristine NaCrO₂. In addition, the compound is inert against humid air and water. We also discuss the reasons for the excellent performance of Cr₂O₃-coated Na(Cr_1-2xAl_2x)O₂.