Water-Stable Cathode for High Rate Na-Ion Batteries

Abstract

Most of sodium-layered oxide cathodes are unstable under moisture conditions. As a unique water-stable cathode, Na_2/3Ni_1/3Mn_2/3O₂ (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 Na_2/3Li_1/9Ni_5/18Mn_2/3O₂ 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).