Insights into the Improved Chemical Stability against Water of LiF-Incorporated Layered Oxide Cathodes for Sodium-Ion Batteries

Rechargeable Na-ion batteries are considered to be a promising alternative for grid energy storage applications because of their competitive cost benefits and sustainable resource supply. O3-type-layered oxide cathodes are regarded as promising candidates for sodium-ion batteries because of the facile synthesis procedure and high energy density. Unfortunately, the practical use of O3-type cathodes is still hindered by their low chemical stability against water/moisture. Here we report that incorporating LiF into the crystal structure of the O3-layered oxide cathodes could significantly improve their chemical stability against water. As revealed by neutron powder diffraction, nanobeam diffraction, and X-ray photoelectron spectroscopy results, the incorporation of Li+ ions in the tetrahedral sites in the Na layer notably inhibits the Na+/H3O+ exchange process and the strong oxyfluoride bonding stabilizes the crystal structure and inhibits the O loss, further suppressing the phase transitions from O3 to hydrated-O3, hydrated-P2, and rock salt phases and the formation of surface insulating layers. As a result, the LiF-doped sample shows improved electrochemical performance after being exposed to water, in terms of higher capacity retention, faster reaction kinetics, and lower interfacial resistance.