Unprecedented Cyclability and Moisture Durability of NaCrO 2 Sodium-Ion Battery Cathode via Simultaneous Al Doping and Cr 2 O 3 Coating.

Although there are many cathode candidates for sodium-ion batteries (NIBs), NaCrO 2 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 2 needs to be further improved in order to compete with other state-of-the-art NIB cathodes. In this study, we show that Cr 2 O 3 -coated and Al-doped NaCrO 2 , which is synthesized through a simple one-pot synthesis, can achieve unprecedented cyclic stability. We confirm the preferential formation of a Cr 2 O 3 shell and a Na(Cr 1-2 x Al 2 x )O 2 core, rather than x Al 2 O 3 /NaCrO 2 or Na 1/1+2 x (Cr 1/1+2 x Al 2 x /1+2 x )O 2 , through spectroscopic and microscopic methods. The core/shell compounds exhibit superior electrochemical properties compared to either Cr 2 O 3 -coated NaCrO 2 without Al dopants or Al-doped NaCrO 2 without shells because of their synergistic contributions. As a result, Na(Cr 0.98 Al 0.02 )O 2 with a thin Cr 2 O 3 layer (5 nm) shows no capacity fading during 1000 charge/discharge cycles while maintaining the rate capability of pristine NaCrO 2 . In addition, the compound is inert against humid air and water. We also discuss the reasons for the excellent performance of Cr 2 O 3 -coated Na(Cr 1-2 x Al 2 x )O 2 .