Unprecedented Cyclability and Moisture Durability of NaCrO Sodium-Ion Battery Cathode via Simultaneous Al Doping and CrO Coating.

Although there are many cathode candidates for sodium-ion batteries (NIBs), NaCrO 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 NaCrO needs to be further improved in order to compete with other state-of-the-art NIB cathodes. In this study, we show that CrO-coated and Al-doped NaCrO, which is synthesized through a simple one-pot synthesis, can achieve unprecedented cyclic stability. We confirm the preferential formation of a CrO shell and a Na(CrAl)O core, rather than AlO/NaCrO or Na(CrAl)O, through spectroscopic and microscopic methods. The core/shell compounds exhibit superior electrochemical properties compared to either CrO-coated NaCrO without Al dopants or Al-doped NaCrO without shells because of their synergistic contributions. As a result, Na(CrAl)O with a thin CrO layer (5 nm) shows no capacity fading during 1000 charge/discharge cycles while maintaining the rate capability of pristine NaCrO. In addition, the compound is inert against humid air and water. We also discuss the reasons for the excellent performance of CrO-coated Na(CrAl)O.