Most of sodium-layered oxide cathodes are unstable under moisture conditions. As a unique water-stable cathode, Na2/3Ni1/3Mn2/3O2 (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 H2O 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 H2O 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 Na2/3Li1/9Ni5/18Mn2/3O2 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).
Keywords: Na+/vacancy disordering; cathode; sodium ion battery; transition metal oxides; water-stable.