Water-Stable Cathode for High Rate Na-Ion Batteries.

Most of sodium-layered oxide cathodes are unstable under moisture conditions. As a unique water-stable cathode, NaNiMnO (NNM) usually becomes vulnerable to water molecules after element substitution treatment to suppress the Na vacancy ordering arrangement, which causes limited Na diffusion kinetics. Herein, we show that these issues can be addressed simultaneously by rational designing the transition-metal (TM) layer to achieve both water-stable and Na vacancy disordering structures. Density functional theory calculations reveal that the water-stability of the layered oxide cathode can be correlated to the surface adsorption energy of HO molecules. In the TM layer, the Co/Mn and Fe/Mn units exhibit a much lower adsorption energy than that of the Li/Mn unit, and hence the HO molecule prefers to be absorbed on Co/Mn and Fe/Mn units rather than Li/Mn. Moreover, the Li/Mn unit in the TM layer can suppress the Na vacancy ordering structure in NNM to improve the Na diffusion kinetics. As a consequence, the well-designed NaLiNiMnO cathode can not only maintain its original crystal structure and electrochemical property after water soaking treatment but also exhibit high rate capability (78% capacity retention at 20 C) and excellent cycling stability (87% capacity retention after 1000 cycles).